Graphene and Carbon Quantum Dots: Competing Carbons in Harmonized Photoelectrochemical Platforms

Abstract

Over a decade, a significant advancement in the development of nanohybrids has been observed to address the challenges related to sustainable energy systems. In this regard, the review critically examines the emerging functions of carbon‐based quantum dots (CQDs) and graphene‐based quantum dots (GQDs) and their nanohybrids in photoelectrochemical (PEC) applications, focusing on their different physicochemical properties. Nanohybrids constituting CQDs and GQDs can be breakthrough materials, which have evolved as emerging materials recently in PEC research owing to the mirrored properties of conventional quantum dots. Moreover, the surface chemistry and the electronic structure governed by the size of the quantum dots (QDs) facilitate wide tailoring opportunities in the nanohybrids, serving multifunctionality. A perspective on material design and probability is put forward. Understanding their individual contributions prior to exploring their impact on the PEC system is highly essential. The successful creation and functionality of CQDs/GQDs nanohybrids depend on the composite/nanohybrid processing of CQDs/GQDs while maintaining their intrinsic properties in the nanohybrid forms. The application of CQDs/GQDs in PEC applications remains largely experimental and lab‐scale at this stage. Establishing a detailed structure‐performance‐stability relationship is crucial while conducting permutation‐combination of different PEC experiments, integrating physicochemical characterization's, and understanding the QDs properties.

This review discusses the essential roles of carbon quantum dots (CQDs) and graphene quantum dots (GQDs) in photoelectrochemical (PEC) systems. It addresses specific aspects of their 1D properties, nanohybrid combinations, unique functionalization's, and other mechanisms to improve light absorption and charge separation, which provides new opportunities for developing sustainable systems for energy conversion.

1. Introduction

Catalysis, a fundamental mechanism that enables sustainable energy conversions by lowering the activation barriers, is a highly crucial process. As humanity confronts urgent issues such as climate change, energy instability, and resource exhaustion, catalysis presents feasible routes to renewable fuel generation, eco‐friendly chemical routes, and circular systems.^{[ 1} , ^{2 ]} Looking into the efficiency of the catalytic process, it depends on the factors such as active sites density and their nature that act as specific locations where the transformation occurs after adsorption of the reactant molecule.^{[ 3 ]} The kinetics of the reaction, including charge transfer speeds, adsorption/desorption processes, and stability of intermediates, play a crucial role in determining the performance and selectivity of catalysts.^{[ 4 ]}

In this aspect, electrocatalysis enhances this idea by concentrating on catalytic processes at the electrode interface powered by electrical bias, which are crucial for water splitting reactions and CO₂ reduction. Improving electron transport routes, surface chemistry, and electronic structure is crucial for reducing overpotentials and increasing turnover rates.^{[ 5 ]} Electrical conductivity and active sites can be manipulated through strategies such as doping, developing sophisticated nanostructures, and synergy interactions between components, promoting electrocatalytic characteristics. Another catalytic process, photocatalysis, that utilizes the light energy to activate the photochemical reactions, is extremely popular for sustainable hydrogen production and environmental purposes. Although, origin of the heterogeneous photocatalysis dates back to 1970′s since Fujishima and Honda's discovery. Since then, a plethora of research has been reported with the aim of sustainable energy and environmental solutions.^{[ 6} , ^{7 ]}

Fascinatingly, photoelectrocatalysis (PEC): integration of electrocatalysis with the photocatalytic process has opened new ventures in the materials chemistry field. It utilizes the dual energy input, boosting the charge separation, and reaction kinetics while reducing the recombination processes. PEC is also known to be studied in sensing, energy conversion, and wastewater treatment. One of the biggest bottlenecks is selectivity, as the photogenerated radicals non‐specifically oxidize the analytes in a given reaction system and add to the cost for developing PEC systems.^{[ 8 ]}

Emerging materials with high efficiency in PEC applications involve 2D materials such as graphene, 2.5D materials such as transition metal dichalcogenides (TMDs), and carbon‐based materials, especially quantum dots (QDs). These materials have unique properties to improve light absorption, charge separation, and catalytic activity, thereby improving the overall efficiency and sustainability of hydrogen production from solar energy.^{[ 9 ]}

Carbon quantum dots (CQDs), also known as carbon dots (C‐dots/CDs), are carbon‐based nanoparticles <10 nm. They are like graphene, graphene oxide, carbon nanotubes, and fullerene‐like structures. Since the accidental discovery of CQDs, various classifications have emerged to describe CDs, identifying four distinct types of carbon nanostructured quantum dots, such as carbon nanodots (CNDs), graphene quantum dots (GQDs), polymer dots (PDs), and CQDs. Carbon‐based QDs exhibit excellent properties, such as fluorescence, low toxicity, high water solubility, excellent biocompatibility, and tunable optical characteristics.^{[ 10 ]} In recent years, CQDs have been researched for use in drug delivery, biomedical imaging, light‐emitting diodes, battery electrodes, supercapacitors, chemical sensing, and photocatalysis due to their large surface area and high conductivity. Research highlights that CQDs are effective light absorbers in solar cells.^{[ 11 ]} Their ability to inhibit charge recombination and boost light absorption shows promise for enhancing power conversion efficiency when integrated into perovskite solar cells.^{[ 12 ]} CQDs function effectively as photoelectrodes in PEC systems by absorbing both visible and near‐infrared regions of the light spectrum. Studies show that QDs enhance PEC cell photocurrent density and stability by facilitating charge transfer and reducing recombination losses. For example, Shi al. report that nitrogen‐doped CQDs act as sensitizers on semiconductor surfaces, significantly enhancing photoelectrochemical performance.^{[ 13 ]}

Similarly, GQDs derived from graphene/graphene oxide are gaining prominence in advanced materials research.^{[ 14 ]} Graphene, a single layer of carbon atoms arranged in a 2D honeycomb lattice, possesses high electrical, mechanical, and thermal properties. In PEC water splitting, graphene and its derivatives function as conductive support, electron acceptors, and catalysts for hydrogen evolution reactions (HER). Graphene‐based materials enhance light absorption and charge separation in PEC structures, increasing solar‐to‐hydrogen conversion efficiency.^{[ 15 ]}

While CQDs and GQDs have many similar properties, such as significant visible‐light absorption, photostability, and surface functional groups that allow for very good charge transfer (photoexcited charge generation and separation), their differences provide unique opportunities and therefore require systematic investigation. Typically, GQDs have a higher specific surface area, higher carrier mobility, and smaller bandgaps than CQDs, allowing GQDs to extend their light absorption further into the visible spectrum and improve the dynamics of photogenerated charge carriers. GQD‐based nanohybrid materials demonstrate hydrogen evolution rates that are better than those of CQD‐based nanohybrid materials. Improvements to photocurrent densities on photoelectrodes when included as a co‐catalyst can improve up to 8.8 times compared to CQD co‐catalysts, as they have better cost separation and transport. Although CQDs are great for light harvesting and surface passivation in PEC systems, they do not have quite as good catalytic efficiencies or rates of hydrogen evolution compared to GQDs.^{[ 16 ]}

Besides these inherent physicochemical variations, doping techniques (e.g., incorporation of nitrogen and sulfur into CQDs' carbon frameworks) greatly alter both electrocatalytic activity and stability.^{[ 17 ]} For example, nitrogen‐doped CQDs on TiO₂ were able to enhance CO₂ photoreduction efficiencies (e.g., up to seven times increase in CO and CH₄ formation) through their acting as electron reservoirs and reducing recombination of photogenerated electron‐hole pairs.^{[ 18 ]} In another study, nitrogen‐doped GQDs display enhanced oxygen reduction reaction (ORR) electrocatalytic activity, since the pyridinic nitrogen species provide a greater number of active sites, inducing a more dominant four‐electron transfer pathway. The onset potentials improved from approximately −0.35 V in non‐doped graphene to −0.09 V for N‐GQDs, further confirming that nitrogen‐doped GQDs exhibit improved catalytic efficiency and stability is sustained for energy‐related applications.^{[ 19 ]}

With global demand for clean energy and a global need to reduce green house gas emissions, the development of safer, efficient, and less expensive solar energy conversion technologies is even more urgent. PEC systems have achieved considerable milestones, but their challenges still persist, as discussed in the above paragraphs. CQDs, GQDs, and their nanohybrids are promising materials to mitigate these challenges because of their properties mentioned in the above paragraphs. Conversely, even though there is an increasing body of work producing promising results with CQD and GQD‐based nanohybrids, the literature lacks a clear understanding of: the contributions of CQDs versus GQDs, the structure‐property‐performance relationships, and their synergistic effects, which makes even a direct comparison with GQDs difficult to track. Therefore, an effective evaluation of the CQDs and GQDs synthetic methods, doping effects, electronic properties, and reported PEC efficiencies is important for understanding the fundamental mechanisms and future materials design.

As a result, the objective of this review paper is to address this important knowledge gap by inclusively reviewing the exceptional roles of CQDs and GQDs in PEC systems, along with their unique ability as nanohybrid combinations. Finally, this work provides an inclusive framework for the rational design of next generation using CQD and GQDs and there nanohybrids, metal‐free, cost‐effective photocatalysts and photoelectrodes for sustainable hydrogen production, CO₂ reduction, and chemical sensing applications.

2. Carbon Quantum Dots Versus Graphene Quantum Dots‐ Uncovering Disparity

In the history of QDs, the emergence of CQDs marks a new era of QDs. Although traditional QDs exhibit exceptional entities inducing luminescent, electrical, and optical absorption properties, the new generation of CQDs not only retains these features but also offers enhanced biocompatibility, functionalization ability, fluorescence behavior, and energetically active catalytic properties.^{[ 1} , ² , ³ , ^{20 ]} Among QDs, GQDs and CQDs have garnered significant attention in catalytic water splitting research^{[ 4} , ⁵ , ⁶ , ^{7 ]} due to their diverse structural chemistry and optical properties. Since their discovery, GQDs/CQDs have been extensively studied to elucidate their mechanisms and functionalities across bio‐related, energy‐related, and catalytic‐related fields. However, the literature shows limited studies highlighting their distinct properties and mechanisms in nanohybrid forms. In this context, this review aims to provide a clear analysis of the structures and properties of GQDs and CQDs, relating them to their potential photoelectrochemical applications.

CQDs, a subclass of the C‐dot family, are quasi‐spherical structures typically 2–10 nm in size, known for their strong fluorescence (Figure 1–I ).^{[ 8} , ^{21 ]} The term “CQDs” was introduced after their initial discovery as carbon nanoparticles.^{[ 22} , ^{23 ]} Owing to the lack of specified terminology, some research still refers to them as carbon nanoparticles rather than CQDs. CQDs generally have a core‐shell structure, with functional groups attached to a carbon core. The core, ≈2–3 nm in size, is passivated by a shell of functional groups.^{[ 21} , ^{24 ]} The core could be amorphous or crystalline based on the structural unit arrangement, which may include a mixed sp²/sp³ or only sp² graphitic structure.^{[ 9 ]} The crystalline core with a graphitic structure was later segregated as GQDs. In this review, CQDs will be referred to as amorphous/weak‐crystalline moieties. CQDs present a strong alternative to the renowned QDs of CdSe and CdTe because of their cost‐effectiveness, less‐toxicity, and the absence of hazardous precursors in their synthesis.^{[ 10 ]} Several research teams have presented comprehensive critical reviews on the applicability of CQDs in specified fields.^{[ 11} , ^{12 ]} CQDs are easy to prepare, highly water‐soluble, compatible with major biological systems, stable under light exposure, and readily modifiable.^{[ 13 ]} Given these features, CQDs have applications across biomedical and energy fields.^{[ 15} , ^{16 ]} The size of CQDs can increase the energy gap due to alterations in surface energy levels. This occurs because the size of the QDs is smaller than the Bohr radius, causing the electron‐hole separation to become independent. The altered electron distribution makes the energy band gap size‐dependent.^{[ 17} , ^{18 ]} The quantum yield (QY) of CQDs is generally low (below 20%) as observed in Table 1 , but could be increased with modification strategies such as doping or functionalization.^{[ 12} , ¹⁹ , ^{25 ]} In addition to their photon‐induced luminescent properties, CQDs exhibit electrochemical properties due to increased electrical conductivity from a large surface area. Doping/surface functionalization can affect the electronic properties of CQDs.^{[ 26 ]} The CQD shell regulates electron‐donor/acceptor behavior, exhibiting remarkable properties.^{[ 27 ]} The stability of CQDs can be increased by decreasing the degree of electrostatic repulsion, as indicated by a small magnitude in zeta potential. Overall, the intrinsic property of CQDs is structure and size‐dependent.

Figure 1.

I) Chemical structure of CQDs, Reproduced (Adapted) with permission CCC‐ 6 013 591 453 075, Copyright Willey, 2025;^{[ 53 ]} II) Structural arrangements of GQDs (a. Hexagonal shape with an armchair edge, b. Triangular shape with an armchair edge, c. Hexagonal shape with zigzag edge, d. Triangular shape with zigzag edge), adapted from open access publications under CC BY 4.0;^{[ 54 ]} and III) CQDs versus GQDs: Differences in fluorescence intensity due to distinct exciton recombination mechanisms Reproduced (Adapted) with permission CCC‐ 6 013 591 453 075, Copyright Elsevier, 2025.^{[ 52 ]} (Abbreviations: CQDs, carbon quantum dots; GQDs, graphene carbon quantum dots).

Table 1.

Effects of Various Dopants on CQDs and GQDs Optical and Catalytic Behavior.

Dopant type	Material type	PLQY (%) range	Effect on sp2/sp3 hybridization	Impact on optical and catalytic properties	Refs.
Nitrogen (N)	GQDs	50–80	Increases sp2 graphitic do mains	Enhances electron donation, creates mid‐gap states; improved charge transfer and HER/OER	[28, 29, 30]
Sulfur (S)	CQDs	20–50	Alters edge functional groups, more sp3	Improves electrical conductivity and charge transfer efficiency; moderate PLQY	[31, 32, 33]
Nitrogen + Sulfur	GQDs / CQDs	60–75	Optimizes sp2/sp3 balance	Synergistic effects on electron mobility, surface binding sites, and photocurrent gains	[34, 35]
Boron (B)	GQDs	≈30–60	Modifies electronic density, stabilizes sp2 network	Increases catalytic sites and conductivity, enhances electrocatalytic reactions (ORR/OER/HER)	[36, 37]
Fluorine (F)	GQDs	Moderate (30–50)	Introduces semi‐ionic bonds, influences sp3 edge states	Enhances adsorption of reaction intermediates, improves catalytic stability and activity	[38, 39]
Co‐doping (N + F)	GQDs	60–80	Balance of sp2/sp3 and surface functionalization	Significantly boosts photocurrent and hydrogen evolution efficiencies	[40, 41]
Undoped CQDs	CQDs	10–30	Higher sp3 content with more surface traps	Lower PLQY, moderate catalytic activity due to defects and less efficient charge transfer	[7, 42]

GQDs, a derivative of carbon structures, are exceptionally unique in nature and exhibit remarkable properties due to their inherent graphene‐like structure and confinement effect.^{[ 43 ]} GQDs are 0D derivatives of graphene, consisting of graphene sheet fragments with an sp²‐hybridized atom arrangement in a 2D lattice.^{[ 44 ]} In GQDs, C‐atoms are closely stacked, with each atom sp²‐hybridized and bonded to three C‐atoms, forming d‐bonds with three of its four valence electrons. The unpaired p‐electrons form a large p‐p conjugated network, facilitating electronic movement resulting in higher electrical conductivity in the system.^{[ 45 ]} Figure 1‐IIa,b illustrates structural differences between CQDs and GQDs. These QDs typically range between 0 and 20 nm in size but can extend up to 60 nm, with a thickness of < 10 layers of graphene sheets.^{[ 46} , ⁴⁷ , ^{48 ]} Graphene, a carbon allotrope, exhibits a zero bandgap as its conduction and valence bands intersect at the Dirac point, where the density of electronic states is zero. Although this property limits the application of graphene in optoelectronics. In contrast, GQDs exhibit a tunable, nonzero bandgap, influenced by their size and edge structure. The photoluminescence in GQDs arises from quantum confinement, a structural arrangement such as zigzag and armchair edges, and surface flaws. GQDs possess molecular features that enable surface functionalization and tunability, promoting both their luminous and electrical characteristics.^{[ 43 ]} As carbon derivatives, GQDs exhibit low cytotoxicity, biocompatibility, chemical stability, environmental friendliness, high electronic conductivity, minimal photobleaching, and excellent water dispersibility and solubility due to their amphiphilic nature and large edge effects.^{[ 49 ]} Compared to semiconductor QDs, GQDs can achieve photoluminescence quantum yields of 30–80% through appropriate surface functionalization and heteroatom doping.^{[ 50} , ^{51 ]} In a study, GQDs exhibited stronger fluorescence (higher QY) due to shallow emissions occurring from the quantum confinement effect, crystalline nature, and surface edge effects. In contrast, the competition between depth traps and non‐radiative electron‐hole recombination restricts the fluorescence behavior of CQDs (Figure 1‐III).^{[ 52 ]}

In summary, GQDs surpass CQDs for photocatalytic hydrogen evolution because of their exposed surface area, higher carrier mobility, more extensive visible light absorption, and lower bandgap energies. CQDs lower to moderate PLQYs (20‐50% typically) but contribute towards function while not exclusively contributing to light harvesting charge separation, or by acting as multifunctional components, particularly for heterostructured nanocomposites with CQDs/TiO₂ strategies. CQDs show significantly enhanced CO₂ photoreduction efficiency due to better electron transfer and/or significantly extended exciton lifetime. The doping elements and degree of sp² hybridization govern the overall optical properties and catalytic performances of the GQDs. This is critically important when trying to strategically design materials for activity in sustainable energy applications.

2.1. Significance of Carbon/Graphene‐Quantum Dots in Photoelectrochemical applications

Understanding the mechanisms of CQDs/GQDs is essential before examining them in‐PEC systems and their exploration in extended hybrid structures. Generally, the mechanism of these carbon‐based QDs is like that of semiconductor QDs, with the key difference being that their emission colors arise from different trap sites or electron‐hole recombination. Upon photon excitation, carbon/‐graphene QDs generate electron‐hole pairs, which undergo efficient separation. Fluorescence arises from the radiative recombination of these charge carriers. The high surface‐to‐volume ratio resulting from the small size of these QDs introduces surface defects, which in turn alter their optical and electronic properties. This effect modifies the optical and electronic characteristics. Changes in particle size can influence structural energetics by modifying surface defects and radiative recombination pathways of charge carriers.^{[ 55 ]}

Over the past decades, CQDs/GQDs have been studied in various ways^{[ 56 ]} (Figure 2 ). In addition to acting as a standalone photocatalyst, they can also serve as photosensitizers in hybrid systems,^{[ 57 ]} electron mediators, heterojunction‐forming materials, and promoters of large‐/specific spectral absorption. In some cases, CQDs have been reported as growth regulators that control TiO₂ nanowire growth while also acting as cocatalysts.^{[ 58 ]} In 2010, Yan et al. utilized GQDs as sensitizers over TiO₂ in solar cells, achieving a conversion efficiency of 0.056%.^{[ 59 ]} GQDs are also known as “green sensitizers”.^{[ 60 ]} CQDs/GQDs, as photosensitizers or co‐sensitizers, can enhance the light absorption of material systems. For example, in a Bi₂O₃/CQD system, light absorption induces an electron transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) level in CQDs, followed by migration to the conduction band of Bi₂O₃. Under sufficient bias, these generated electrons can reach the counter electrode, thereby enhancing performance. Another interesting phenomenon is the upconversion photoluminescence (UCPL) property of CQDs, which uses the full spectrum of light to excite and generate electron‐hole pairs in Bi₂O₃, contributing to improved performance.^{[ 57 ]}

Figure 2.

Applied synthesis methods for various applications (PEC water splitting, PEC sensing, CO₂ reduction, and emerging PEC applications) for GQDs nanohybrids: Hydrothermal methods;^{[ 61} , ⁶² , ⁶³ , ⁶⁴ , ⁶⁵ , ⁶⁶ , ⁶⁷ , ⁶⁸ , ⁶⁹ , ⁷⁰ , ⁷¹ , ⁷² , ⁷³ , ⁷⁴ , ⁷⁵ , ⁷⁶ , ⁷⁷ , ⁷⁸ , ⁷⁹ , ^{80 ]} Simple pyrolysis;^{[ 30} , ⁶⁴ , ⁷⁸ , ^{81 ]} Flame spray pyrolysis; spin coating; ultrasonication/probe sonication;^{[ 76} , ⁷⁷ , ^{82 ]} Microwave assisted, and CQDs nanohybrids: Hydrothermal methods;^{[ 58} , ⁸³ , ⁸⁴ , ⁸⁵ , ⁸⁶ , ⁸⁷ , ⁸⁸ , ⁸⁹ , ⁹⁰ , ⁹¹ , ⁹² , ⁹³ , ⁹⁴ , ⁹⁵ , ⁹⁶ , ^{97 ]} Simple pyrolysis;^{[ 98} , ^{99 ]} Flame spray pyrolysis;^{[ 100 ]} spin coating;^{[ 101 ]} ultrasonication/probe sonication;^{[ 83} , ^{102 ]} Microwave assisted,^{[ 103} , ^{104 ]} drawn based on the number of reference reported in literature.

They also act as electronic media or serve as “mediators,” imparting photothermal effects in hybrid materials. This is due to their distinctive π–π conjugated structures and surrounding functional groups.^{[ 105 ]} GQDs effectively shuttle electrons between the working electrode and Doxycillin, facilitating electron regeneration and exchange.^{[ 106 ]} Similar to graphene or metals as electron mediators in catalytic systems,^{[ 107 ]} CQDs/GQDs can also channel electrons, serving as a bridge between two semiconducting materials^{[ 108 ]} or between a photoanode and an electrocatalyst for efficient water oxidation.^{[ 109 ]} In PEC sensors, GQDs facilitate charge transport kinetics at the metal‐semiconductor interface, reducing charge recombination rates as examined in electrochemical impedance measurements.^{[ 110 ]} N, S‐doped GQDs form a Schottky heterojunction with MoSe₂, playing a crucial role in charge carrier transport, restricting photocorrosion, and enhancing photocurrent.^{[ 61 ]} Additionally, Li et al. report the formation of a heterojunction with GQDs and surface state passivation of ZnO nanorods for utilization as PEC photodetectors.^{[ 111 ]}

The UPCL property in QDs enables full solar spectrum utilization via absorbing low‐energy photons, which are initially emitted as high‐energy photons, followed by an anti‐Stokes transition.^{[ 112} , ¹¹³ , ^{114 ]} The upconversion procedure allows a material to transfer photons from near‐infrared (NIR) to ultraviolet–visible (UV–VIS) through effective fluorescence resonance energy transfer (FRET). As a result, the upconverted photons are absorbed by the semiconductor, photo‐generating electron‐hole pairs that participate in the reaction.^{[ 115 ]} UV to VIS upconverted CQDs hybridized with TiO₂ enhance photocatalytic activity.^{[ 116} , ^{117 ]} CQDs as “spectral converters” are utilized in CdSe/TiO₂ photoanodes for water oxidation through NIR photon harvest and conversion.^{[ 118 ]}

From this perspective, CQDs/GQDs are highly valuable in photoelectrochemical applications, enhancing photogenerated charge transfer and separation, improving light absorption through their UCPL property, and offering stable performance due to chemical stability and photo‐resistance. Surface functionalization can induce active sites for photoelectrochemical reactions (Table 2 ). Hybrids formed with CQDs/GQDs are highly efficient for green energy and environmental applications.

Table 2.

Comparative Analysis of CQDs and GQDs Nanohybrids.

Characteristics	GQDs	CQDs
Structure	Crystalline sp2‐hybridized carbon with a honeycomb lattice	Amorphous or partially crystalline carbon with sp2/sp3 hybridization
Size	<10 nm (with well‐defined edges)	2‐10 nm (with tunable size and functional groups)
Bandgap[ 119 , 120 ]	1.5–3.0 eV, depending on size and edge configuration	2.0–4.0 eV (highly tunable)
Surface Properties	Fewer oxygen‐containing groups than CQDs, but highly conductive with π–π stacking interactions	Rich in oxygen‐containing groups
Photoluminescence[ 121 , 122 ]	Moderate to higher QY (30–60%)	Lower to moderate QY (20 to 50%)
Bandgap Tunability	Good	Excellent
Charge Transport	Excellent	Moderate
Electrical Conductivity	High	Low
Stability	High	Moderate

Abbreviations: GQDs, Graphene Quantum Dots; QY, quantum yield; CQDs, Carbon Quantum Dots.

3. Materials Nanohybrids: Modification and Strategies

3.1. Carbon Quantum Dots Nanohybrids

Photo‐electrocatalysts are crucial for developing application‐oriented solutions that deliver high energy output at low cost, compared to the prohibitively expensive “state of the art catalyst designed from Platinum Group Metals (PGMs)”. Recent development prompts researchers to explore non‐PGM catalysts, alteration with a lower weight percentage of PGMs, aiming to achieve catalytic efficiency like or higher than that of PGMs‐based catalysts. The development of efficient, long‐term, stable catalysts for oxidation and reduction reactions remains ongoing, highlighting the need for efficient and cost‐effective catalysts. Since the discovery of CQDs, several semiconductor conjugated materials (CQDs/BiVO₄,^{[ 98 ]} CQDs/SrTiO₃, CQDs/Ta₃N, CQDs/NiFe‐LDH/BiVO₄,^{[ 123 ]} CQDs/ZnO,^{[ 124 ]} CQDs/WO₃,^{[ 125 ]} CQDs/TiO₂,^{[ 87 ]} and CQDs/Fe₂O₃ ^{[ 109 ]}) have been extensively explored for various high‐efficiency applications.

In efficient PEC systems, nanohybrids are categorized into metal‐CQD and semiconducting materials‐CQD Nano systems, with the first combination exhibiting nano enzyme‐like behavior for biosensing applications. The combined effects of plasmonics and fluorescence quenching enhance the performance of photodetectors and LEDs. Although metals exhibit limited optical behavior, poor surface functionality, and often low biocompatibility, metal‐CQD nanohybrids offer unparalleled synergistic performance. Similarly, semiconducting material—CQD nanohybrids exhibit superior catalytic performance higher than their individual components, leveraging the high surface area, functionalization ability, and stability provided by the CQD systems.

A study on TiO₂ highlights the importance of designing CQDs in a nanohybrid form. Zhang et al. report improved visible‐light absorption and effective charge transfer in CQD/TiO₂/Pt nanocomposites. H₂ generation is optimized at 1% Pt loading, with reduced efficiency after 2%. Efficiency is greatly increased by preloading Pt before CQD deposition. The catalyst remains stable for 36 h and performs optimally at pH 4.0. The Pt‐free sample exhibited lower efficiency (6.2 µmol g⁻¹ h⁻¹), highlighting the essential role of platinum (Pt) in hydrogen (H₂) production. Although Pt‐preloaded TiO₂ (40 µmol g⁻¹ h⁻¹) and CQDs (6.0 µmol g⁻¹ h⁻¹) showed some catalytic activity, the CQD/TiO₂/Pt composites performed best. Catalytic rates increased from 895 to 2650 µmol g⁻¹ h⁻¹ as the Pt content was increased from 0.5% to 3%. Though CQDs assist in reducing TiO₂, the study reports that excessive CQD levels could impair performance by increasing light scattering^{[ 126 ]} (Figure 3a–d).

Figure 3.

a) Schematic illustration of the preparation of CQDs/TNWAs^{[ 58 ]} b) Schematic illustration of the fabrication of CQDs‐H/TiO₂ nanoarrays.^{[ 87 ]} Reprinted (adapted) with permission from^{[ 87 ]} Copyright 2019. American Chemical Society, c,d) HRTEM images of the CQDs/3%Pt@TiO₂ hybrid, e) Direct interaction between CQDs and TiO₂, facilitating charge transfer from CQDs to TiO_2, improving H₂ generation. Reprinted (adapted) with permission from.^{[ 126 ]} Copyright 2019. American Chemical Society, f) Schematic illustration of the preparation of CQDs/A/R‐TiO₂ Reprinted with permission from Elsevier with License number‐601366035739, copyright 2020, Elsevier.^{[ 127 ]} g) Schematic representation of the growth mechanism of CCFT NRs. Reprinted with permission from CCC with License number‐1636664‐1, copyright 2025, Royal Society of Chemistry.^{[ 128 ]} (Abbreviations: CQDs, carbon quantum dots; TNWAs, titanium dioxide nanowire arrays; H/TiO₂, hydrogenated titanium dioxide nanowire arrays; HRTEM, high‐resolution transmission electron microscopy; H₂, hydrogen; CCFT NRs, cobalt‐carbon fiber tube nanorods).

Similarly, considering the prominent benefits of CQDs, design strategies were improved. Zhou et al.^{[ 127 ]} enhanced the material by growing anatase TiO₂ nanothorns on rutile TiO₂ nanorods, followed by surface modifications with CQDs. A significant reduction in bulk recombination of photogenerated carriers was observed when used as a photoanode material in PEC. Compared to pristine TiO₂, a 240‐mV negative shift in starting potential and an 11.72‐fold increase in photocurrent at 0.6 V vs RHE were observed (Figure 3f).

The efficiency of surface charge injection (η _surface) and bulk charge separation (η_bulk) increased by factors of 5.74 and 1.69, respectively. Intensity‐modulated photocurrent/photovoltaic spectroscopy (IMPS/IMVS) revealed extended carrier lifetimes, while electrochemical impedance spectroscopy (EIS) verified a significant decrease in charge transfer resistance. After 5 h, the CQDs/A/R‐TiO₂ photoanode retained 92.3% of its photocurrent, demonstrating exceptional stability. This method overcomes the intrinsic limitations of TiO₂ and provides a viable method for effective solar‐powered water splitting and sustainable hydrogen generation with the introduction of CQDs. Unlike pure TiO₂, which primarily absorbs UV light, the incorporation of CQDs significantly broadens the absorption spectrum into the visible range, improving the photoelectrochemical efficiency.

Recent developments show that CQD‐decorated hydrogenated TiO₂ nanorod arrays significantly enhance solar‐to‐hydrogen conversion efficiency in PEC water splitting. Under simulated sunlight, these modified photoanodes achieve a high photocurrent density of approximately 3.0 mAcm^{− 2} at 1.23 V versus RHE, approximately six times higher than previous hydrogenated TiO₂ photoanodes. This improvement results from hydrogenation therapy, which introduces oxygen vacancies (VOs) and Ti³⁺ species, reducing charge recombination and enhancing conductivity. Additionally, CQDs serve as electron reservoirs through their up‐conversion effect, facilitating charge separation and extending light absorption into the visible spectrum. The incident photon‐to‐current conversion efficiency (IPCE) of these photoanodes is approximately 66.8%, significantly outperforming that of pristine TiO₂. This dual‐modification strategy offers a promising approach for achieving high‐efficiency PEC performance and sustainable hydrogen production (Figure 3b).^{[ 87 ]}

In some cases, CQDs alter the electrochemical characteristics of MoS₂, facilitating the design of HER catalysts for energy applications.^{[ 129 ]} Shi et al. (2016) also examined CQD‐decorated WO₃ nanoflakes, synthesized via a seed‐mediated solvothermal technique, as photoanodes for PEC water splitting. The addition of CQDs significantly enhances light absorption and charge transfer efficiency at the WO₃/electrolyte interface, resulting in a twofold increase in photocurrent density (1.46 mA cm^{− 2} at 1.0 V versus Ag/AgCl in 1 M H₂SO₄ under AM 1.5G illumination). Overall, CQDs served as effective electron mediators, as confirmed by EIS data, which showed a decreased charge transfer resistance (R _ct) upon their addition. Nevertheless, despite the significant enhancement in CQDs/WO₃ performance, it still lags behind advanced BiVO₄ or TiO₂‐based PEC systems, suggesting the need for further optimization, especially in tandem cell integration and long‐term stability.^{[ 125 ]} However, Zhao et al. (2015) investigated the CQDs/SnO₂‐Co₃O₄ composite for OER catalysis in alkaline media, where CQDs contribute to system stability by creating an insoluble protective layer that enhances electron transport and hinders deterioration in alkaline conditions. The addition of CQDs was promising, enhancing both catalytic activity and charge transport behavior, as reflected in the increased current density.^{[ 129 ]}

Using thermal polycondensation and hydrothermal techniques, a CQD‐modified porous g‐C₃N₄/TiO₂ 2D heterojunction was created. It produced 6.497 mmolg⁻¹·h⁻¹ H₂, more than twice the amount generated by unmodified g‐C₃N₄. CQDs minimize catalyst poisoning and enhance light absorption through up‐conversion photoluminescence by facilitating the breakdown of H₂O₂. The porous structure increases reactive sites, while the heterojunction enhances charge separation. However, problems with the TiO₂ bandgap, CQD leaching, and inconsistent synthesis raise concerns about stability and scalability.^{[ 130 ]} Loading noble metal nanoparticles onto semiconductor surfaces enhances light‐harvesting in the visible spectrum, promoting the generation of more eCB⁻‐hVB⁺ pairs due to the strong electromagnetic field induced by localized surface plasmon resonance (LSPR). Additionally, the Schottky junction at the interface helps prevent the recombination of photogenerated carriers and preserves the reducibility of transferred electrons. Among noble metals, Ag nanoparticles are most cost‐effective and exhibit the strongest effect.^{[ 131 ]}

Furthermore, to enhance photocatalytic performance, Ag nanoparticles typically serve as bridges, connecting to another semiconductor or serving as an electron sink within a three‐component photocatalyst system. In photocatalysis, CQDs can exhibit both down‐conversion and up‐conversion photoluminescence (PL), allowing them to function as electronic buffers, accepting or donating electrons to the contiguous semiconductor while also acting as photosensitizers to capture visible and NIR light. The π‐conjugated structure and oxygen‐containing groups on the surface of CQDs provide photostability and enable easy complexation with metal nanoparticles and semiconductors.^{[ 132} , ^{133 ]} Qin et al. report that integrating CQDs into g‐C₃N₄‐based photocatalysts resulted in a hydrogen generation rate of 626.93 µmol g⁻¹ h⁻¹ under visible light. This figure illustrates a substantial enhancement, 6.7 times higher than that of pure g‐C₃N₄ and 2.8 times greater than the highest‐performing CQDs/g‐C₃N₄ composite. These data validate the effective role of CQDs in increasing quantum efficiency and improving charge separation, both of which are crucial for photocatalytic processes and can be extended to PEC applications. Furthermore, the study shows that the silver (Ag) nanoparticles, when combined with CQDs, serve as electron reservoirs, stimulating the separation of photogenerated electron‐hole pairs and significantly boosting quantum efficiency. Studies highlight the significant improvements in hydrogen production rates achieved through CQD integration into nanocomposite systems. For example, hybrid CQD‐TiO₂ systems exhibit hydrogen evolution rates of 4.5 mmol h⁻¹, demonstrating how CQDs capture longer wavelengths of light to enhance charge transfer characteristics. The efficacy of such systems is further enhanced using noble metal nanoparticles, such as Ag, which amplify SPR effects to boost light absorption and facilitate improved charge separation. The detailed characterization of the Ag/CQDs/g‐C₃N₄ heterostructures discussed in the study indicates both their efficacy and the robustness and stability of these systems under visible light irradiation. Although, this study shows efficiency in photocatalytic hydrogen production, similar advantages of CQDs could also be expected in PEC processes. Stability is a pivotal issue, as photo‐corrosion and CQD degradation under prolonged illumination challenge consistent performance.^{[ 134 ]}

Anodic water oxidation via oxygen evolution, using a bifunctional catalyst—cerium oxide‐carbon quantum dots/reduced graphene oxide (CeOx‐HDCQDs/RGO) nanohybrid—modified on glassy carbon electrodes in an aqueous alkaline electrolyte (1 M KOH) at 1.0 V exhibits a high oxygen evolution reaction (OER) current density of 38 mA cm^{− 2}. This indicates the high efficiency of the nanohybrid catalyst in converting water into oxygen and hydroxyl ions. Moreover, the catalyst demonstrates a favorable onset potential of ≈+0.42 V, suggesting lower energy requirements for initiating water oxidation and enhancing its efficiency compared to other catalysts. The synergistic interaction among cerium oxide, heteroatom‐co‐doped CQDs, and reduced graphene oxide significantly enhances catalytic performance. Additionally, the small particle size of CeOx provides a higher density of oxygen vacancies, facilitating oxygen extraction from the electrolyte.^{[ 135 ]} In PEC water splitting, effective charge carrier separation and transport are crucial.^{[ 136 ]} Like TiO₂, Fe₂O₃ exhibits a short hole diffusion length, leading to carrier recombination and stability issues. Compared to pristine Fe₂O₃ (3.5 mAcm^{− 2}), Xin Xie et al. report that CQD decoration on Fe₂O₃ nanowires increased PEC activity, raising the photocurrent density to 94 mA cm^{− 2} at 0.23 V versus Ag/AgCl, a 27‐fold improvement. This enhancement is attributed to improved electron‐hole separation, greater charge transfer, and increased light absorption; however, CQD aggregation can hinder performance, necessitating careful tuning. The study emphasizes the importance of surface engineering in improving PEC and highlights the need to balance photonic efficiency and CQD loading for practical solar energy applications.^{[ 137 ]} Additional research highlights that CQDs improve Fe₂O₃‐based photoanodes for PEC water splitting, achieving photocurrent densities up to 3.0 mA cm^{− 2} at 1.23 V versus RHE. CQDs address the short hole diffusion length and slow OER kinetics of Fe₂O₃ by generating localized heating under NIR light, thereby enhancing charge transport and catalytic activity. Furthermore, heat activation of the co‐phosphate cocatalyst improves reaction kinetics. However, CQD stability, scalability, and long‐term efficiency remain challenging. Nevertheless, for effective solar‐to‐hydrogen conversion, optimizing Fe₂O₃‐based PEC systems requires balancing structural advantages with thermal effects (Figure 3g).^{[ 128 ]} Under 100 mW cm^{− 2} illumination, a TiO₂ photoanode with a Pt‐loaded carbon nanofiber counter electrode exhibited 95% stability over 10 h, achieving a photocurrent density of approximately 4 mA cm^{− 2} at 0.6 V versus RHE. Owing to their strong TiO₂ interaction and high crystallinity, CQDs improve charge separation and transport. However, challenges such as low solar‐to‐hydrogen conversion efficiency and scaling constraints persist.^{[ 138 ]} For sustainable PEC applications, optimizing CQD synthesis and developing environmentally friendly precursors are essential.^{[ 139 ]} Additionally, integrating CQDs with iron metal‐organic frameworks (Fe MOFs) and BiVO₄ enhances their function in PEC systems. The MW: BVO@Fe MOF@CQDs composite achieved a photocurrent density of 5.9 mA cm^{− 2}, nearly five times higher than that of pure BiVO₄. Dual‐layer structures boost light absorption and provide more active sites for oxygen evolution, whereas CQDs increase charge separation, decrease recombination, and introduce oxygen vacancies. Stability and long‐term performance issues with CQD persist. Therefore, future studies should concentrate on scalability, synthesis optimization, and substrate compatibility to optimize their influence on applications using renewable energy.^{[ 140 ]} Moreover, its study should also focus on physical and covalent engineered heterostructures.

While physical mixing processes such as solution blending or layered assembly achieve light absorption or charge separation by way of weaker van der Waals or electrostatic interactions, they are typically more simple, versatile, and can also come with the drawbacks of interface contact that is less stable as well as charge transport that is inefficient and unstable with continuous exposure to illumination. Covalent engineering could be achieved with direct covalent bonding of two materials or the use of linker molecules and can lead to strong heterostructures that provide a pathway for charge separation and transport with less interfacial resistance and less electron‐hole recombination, as well as an increase in photocatalytic activity.

In summary, clean energy production through photoelectrocatalysis is crucial for addressing the energy crisis. Recent research explores CQDs to enhance the efficiency of various catalytic systems. CQDs enhance charge transfer, stability, and light absorption, resulting in higher photocatalytic performance. Integrating CQDs with different semiconductor materials such as TiO₂, ZnS, Cu₂O, H‐g‐TaON, Bi₄O₅I₂, Ni‐complex, and non‐precious metals shows promise for sustainable hydrogen production and OERs. Ongoing studies focus on optimizing CQD synthesis, scalability, and integration into existing energy systems.

3.2. Graphene Quantum Dots Nanohybrids

CQDs enhance hydrogen production, but often under specific conditions or at lower efficiencies compared to GQDs. GQDs stand out from CQDs in photocatalysis due to their superior ability to significantly inhibit the recombination of photogenerated electron‐hole pairs and act as solid‐state electron reservoirs. Nevertheless, there are additional pathways to enhance photocatalytic activity beyond a structural assembly approach through strategic doping and surface functionalization. Specifically, adding heteroatoms into carbon quantum dots, such as nitrogen, sulfur, fluorine, or boron, introduces mid‐gap electronic states that can act as effective electron reservoirs and redox‐active sites. These dopants can help tune the band structure for better light absorption and charge carrier dynamics, while also stabilizing reactive intermediate species during catalytic cycles. Altogether, these strategies produce increased photocurrent densities and valuable products including hydrogen and reduced CO₂ species. Additionally, covalently bonded systems, such as 0D/2D or ternary composites, are expected to perform similarly, while also potentially offering better structural stability and long‐term operation compared to purely physically mixed systems, highlighting even more potential for sustainable photoelectrochemical applications. Few studies show that GQDs exhibit superior photocatalytic hydrogen evolution activity compared to CQDs when combined with metals/metal oxides.^{[ 7} , ^{141 ]} Although both GQDs and CQDs serve as effective photosensitizers and electron transporters, GQDs offer advantages such as a larger surface area, higher carrier mobility, and a lower bandgap due to quantum confinement effects in their 2D system, which enhances their electron transfer and broadens light absorption.^{[ 142 ]} Although studies on GQDs directly experimented in PEC applications are limited, extensive research on their electrocatalytic and photocatalytic activities highlights a valuable foundation for developing nanohybrids in PEC systems.

A study shows the significant role of GQDs in enhancing the photocatalytic hydrogen evolution activity of TiO₂. The GQDs/TiO₂ composite achieves a maximum H₂ production rate of 41.26 µmol, 2.2 times higher than the 18.74 µmol produced by pure TiO₂ under identical conditions in a methanol aqueous solution.^{[ 143 ]} This remarkable improvement is attributed to the critical mechanistic role of GQDs in enhancing charge transfer and reducing recombination losses. Photoexcited electrons in TiO₂ are transferred to GQDs, which act as solid‐state electron acceptors and transporters, significantly enhancing charge separation efficiency by up to 80% and reducing recombination losses. The accumulated electrons on GQDs subsequently drive proton reduction, enabling efficient hydrogen gas (H₂) production. Additionally, the robust chemical stability of GQDs ensures their regeneration within the photocatalytic cycle, supporting sustained catalytic activity. In addition to their functional advantages, GQDs expand the light absorption range of TiO₂ and provide an environmentally friendly alternative to metal‐based quantum dots, reducing contamination risks due to their nontoxic nature.^{[ 144} , ^{145 ]} However, challenges persist in optimizing synthesis methods, ensuring long‐term stability, and determining the optimal GQD loading on TiO₂. Although GQDs offer significant potential for sustainable hydrogen production, further research is needed to address scalability and integration into existing systems, ensuring their viability for large‐scale energy applications.^{[ 143 ]}

Studies show that a nitrogen atom adjacent to carbon induces a positive shift in Fermi energy, enhancing charge transfer. Recently, nitrogen‐functionalized GQDs have emerged as highly efficient metal‐free catalysts for PEC reactions. Theoretical calculations indicate that the electronic structure of carbon can be modulated through the synergistic coupling of heteroatoms such as nitrogen, boron, sulfur, and fluorine, enhancing adsorption and desorption sites for improved charge transfer. Nitrogen‐doped GQDs (N‐GQDs) have shown promise as a sustainable approach for hydrogen production through photocatalysis. N‐GQDs are synthesized by treating graphene oxide with ammonia at 500 °C, resulting in particles with an average size of 8.1 ± 1.8 nm. They exhibit both p‐type and n‐type conductivities, enhancing the efficiency of charge separation during the photocatalysis process. A key discovery of this research is the ability of N‐GQDs to catalyze water‐splitting, producing hydrogen and oxygen gases at a molar ratio of 2:1, with a significant bandgap of ≈2.2 eV. Although their photocatalytic activity is approximately half that of the more active metal‐containing catalyst Rh₂‐yCryO₃/GaN:ZnO,^{[ 146 ]} the metal‐free nature of N‐GQDs highlights their potential for sustainable energy applications.^{[ 30 ]} This emphasizes the benefits of using carbon‐based materials for renewable energy solutions. The research also introduces (N‐GQDs) anchored on thermally reduced graphene oxide (rGO) as a highly effective electrocatalyst for the oxygen reduction reaction (ORR). Despite these significant advancements, challenges such as limited performance in acidic media and synthesis uniformity still require further optimization for future catalytic applications.^{[ 147 ]}

Innovative doping concepts, for instance, nitrogen and fluorine co‐doped graphene quantum dots (N, F‐GQDs), have been explored for electrocatalytic water splitting and energy storage applications. The enhanced activity is attributed to the synergistic effects of the dopants, which modify the electronic structure, thus boosting charge transfer and facilitating electrocatalysis.^{[ 40 ]} Mechanistically, nitrogen incorporation into the N‐GQDs reduces the bandgap, effectively shifting their absorption into the visible light spectrum. This enhancement improves light harvesting and generates higher electrocatalytically active sites. The substrate architecture rGO coated on carbon fabric plays a pivotal role by providing a high surface area and facilitating charge mobility. This conductive substrate minimizes charge recombination, further enhancing the efficiency of the N‐GQDs during HER. The synthesis of N‐GQDs employed key strategies that enhanced their electrocatalytic and photocatalytic performance. A key approach was the use of a hydrothermal method, which enabled effective nitrogen doping into the graphene oxide (GO) framework. This method enabled the controlled formation of N‐GQDs with optimal structure and size, essential for enhancing their catalytic properties. Additionally, nitrogen doping significantly modified the electronic properties, resulting in a decreased bandgap, as discussed previously. This reduction enabled visible light absorption by N‐GQDs, which is critical for photocatalytic activities, including the (HER). Furthermore, the self‐assembled branched leaf‐like structure of N‐GQDs created a highly porous morphology with abundantly exposed edge surfaces.^{[ 28 ]} This structural configuration enhances the accessibility of reaction sites, increases surface area, and promotes better interactions with reactants, thus improving catalytic efficiency. Another important strategy was incorporating woven carbon fabric coated with rGO as a conductive substrate. This design provided excellent conductivity and a high surface area, facilitating rapid electron transfer, and supporting the physical stability of the electrode, which is crucial for maintaining performance in various catalytic applications.^{[ 148} , ^{149 ]}

Although preparing and modifying N‐GQDs can be complex and yield‐limited, various methods are available for their production. These include solution chemistry, electrochemical, and chemical scissoring methods. However, the hydrothermal synthesis method, which uses small molecules, is the most widely studied and promising approach. This is due to its simplicity in preparation and modification, high yield, and high quality. N‐GQDs significantly enhance performance due to the presence of pyridinic nitrogen, which improves catalytic efficiency by providing more active sites and improving the absorption of reaction intermediates. This leads to a dominant four‐electron transfer pathway and an improved onset potential. Another advantage is the tunability of nitrogen species. During synthesis, especially through hydrothermal methods, the types of nitrogen configurations (such as pyridinic‐N and graphitic‐N) can be adjusted. Lastly, the hydrothermal synthesis method produces high‐yield and high‐quality N‐GQDs, facilitating their practical application in electrocatalysis without the need for extensive modification. The robustness and stability of N‐GQDs during electrochemical testing, along with their enhanced conductivity, make them an excellent metal‐free alternative to traditional platinum‐based catalysts for ORR applications. N‐GQDs demonstrated excellent electrocatalytic activity, with an electron transfer number (n) ranging between 3.6 and 4.1, indicating a dominant four‐electron transfer pathway during the reaction. Furthermore, the onset potential of N‐GQDs was significantly improved compared to non‐doped counterparts, with values of approximately −0.09 V for N‐GQDs/G, compared to −0.35 V for blank graphene and −0.39 V for blank N‐GQDs. This enhancement demonstrates that pyridinic‐N boosts catalytic efficiency and improves the absorption and activation of reaction intermediates, significantly improving the overall performance of N‐GQDs in ORR applications.^{[ 150 ]} Furthermore, the exceptionally low charge transfer resistance at low overpotential is attributed to the enhanced electrode/electrolyte interface and multidimensional charge and electrolyte transport, which prevent p‐p restacking. These achievements result from strategies employed in designing N‐GQDs to induce 3D porous orientation of holey graphene oxide (hGO) nanosheets. Ali M. et al. explored the electrocatalytic potential of N‐GQDs, indicating that the 3D orientation enhances properties by preventing graphene sheet restacking—a common issue in stacked structures that limits ion access. This results in unobstructed channels for effective electrolyte flow. The phase separation behavior of N‐GQDs with different sizes was optimized to create porous support through the hydrothermal synthesis process. Furthermore, the interconnected porous network facilitates the movement of charge carriers, reducing charge transfer resistance and enhancing the efficiency of electrochemical processes. Lastly, the advancements in structural design, electron transport, and surface functionalization can significantly benefit photocatalytic systems if applied. The performance enhancements outlined could improve efficiency in PEC applications, aligning well with sustainable energy goals of utilizing solar energy for chemical transformations.^{[ 151 ]}

Lastly, the combination of photocatalytic and electrocatalytic processes, known as photo‐/electrocatalysis, enables effective harnessing of both light energy and external bias voltage. This synergy accelerated pollutant degradation while maximizing the overall efficiency of hydrogen production. By employing these strategies, the synthesized N‐GQDs displayed enhanced catalytic activity and stability, making them suitable for applications in environmental remediation and energy conversion technologies. The presence of semi‐ionic bonds improves the adsorption of reaction intermediates. These findings highlight the potential of N, F‐GQDs as cost‐effective and efficient alternatives to noble‐metal electrocatalysts, offering promising solutions for sustainable energy applications.^{[ 152} , ¹⁵³ , ^{154 ]}

Furthermore, the study introduces graphene hydrogel/boron‐doped graphene quantum dots (GH‐BGQD) composites as effective trifunctional electrocatalysts for ORR, OER, and HER. These composites feature a unique three‐dimensional structure that enhances electrolyte transport and offers numerous active sites through the integration of boron‐doped GQDs. The GH‐BGQD variant demonstrates impressive performance, with an onset potential of 0.93 V for ORR, an overpotential of 370 mV for OER at 10 mAcm^{− 2}, and a HER onset potential of −0.08 V. Although the structural benefits of GQDs enhance catalytic activity, however, achieving performance levels similar to commercial Pt/C catalysts remains a challenge. The GH‐BGQD nanohybrid is developed through a one‐step hydrothermal synthesis method that combines GO, glucose, and boric acid to produce a matrix in which B‐doped GQDs are anchored to the graphene hydrogel structure. Boron doping significantly improves the electronic properties of the graphene framework by increasing hole‐type charge carriers, enhancing conductivity, and catalytic activity. The 3D architecture of graphene hydrogel provides high porosity and a large specific surface area, optimizing reaction kinetics for ORR, OER, and HER. The synergy between GH and B‐doped GQDs boosts electrocatalytic performance. Various boron bonding structures, such as BC3, BC2O, and BCO2, enhance the adsorption of reaction intermediates, increasing overall reaction efficiency. Future research should focus on optimizing boron doping levels, ensuring long‐term stability, and assessing scalability for practical use. This study underscores the potential of graphene‐based materials in sustainable energy applications, bridging the gaps between cost, efficiency, and sustainability.^{[ 155 ]}

In summary, GQD nanohybrids outperform CQD nanohybrids in photocatalytic hydrogen evolution, exhibiting higher hydrogen production rates and improved charge separation efficiency. GQDs extend light absorption into the visible spectrum, are environmentally friendly, and are more cost‐effective than metal‐based catalysts. Their larger surface area, higher carrier mobility, and lower bandgap enhance electron transfer and catalytic processes. GQDs offer significant advantages in efficiency, environmental influence, and cost‐effectiveness, making them a promising material for advancing clean energy solutions.

4. Potential Photoelectrochemical Applications

4.1. Water Splitting

Previous studies report that GQDs are mostly used for their excellent electrical conductivity and boosted electron transfer capabilities, apart from their light‐harvesting properties. In contrast, CQDs also contribute effectively towards light harvesting and reduce charge recombination. However, their involvement in nanohybrid photoelectrodes for PEC applications depicts multifunctionality, which has been discussed and understood.

To mitigate charge carrier accumulation and the recombination process in PEC, a simple assembly of surface‐modified GQDs and over hollow TiO₂ wires was formed. GQDs in colloidal form functioned as effective sensitizers for TiO₂ hollow nanowires.^{[ 156 ]} The effective coupling of the GQDs and the TiO₂ during the one‐step hydrothermal process improves light absorption and enhances the charge separation efficiency. As a result, hydrogen evolution increases by sevenfold and photocurrent by threefold. This is achieved due to the GQDs, which act as both photosensitizers for improving light utilization and electron acceptors for suppressing charge recombination.^{[ 157 ]} Furthermore, to overcome limitations in charge carrier transport and transfer efficiency in spinel CuBi₂O₄ (CBO) photocathodes, Ag and N‐GQDs were introduced. This modification enhances charge separation, charge transfer, and light harvesting. Under AM1.5G at 0.3 V versus RHE, the N‐GQDs/Ag‐CBO submicron rods (SMRs)^{[ 158 ]} exhibit photocurrent densities that are 8.8, 2.3, 1.8, and 5.9 times higher than those of the individual CBO SMRs, Ag–CBO SMRs, NGQDs–CBO SMRs, and NGQDs–Ag‐CBO microspheres (MSs), respectively. Glutathione‐capped gold (Au_x) nanoclusters and GQDs were deposited onto hierarchically ordered nanoporous TiO₂ nanotube arrays. In this configuration, Au_x nanoclusters act as electron modulators for the GQDs.^{[ 159 ]} The GQDs were also decorated onto CdS nanoparticles, resulting in dot‐on‐particle formation. This integration enhances the light absorption edge of the nanocomposite beyond the band edge of CdS. In this study, GQDs function not as photosensitizers but as electron acceptors. The resulting nanohybrid exhibits HER 2.7 times greater than that of the bare CdS, with an AQE of approximately 4.2% at 420 nm.^{[ 74 ]} Additionally, Schottky‐like 0D/2D van der Waals heterojunctions were formed between the 0D NS co‐doped GQDs and 2D pristine graphene sheets, with GQDs acting as intercalations. This heterostructure results in 2.39‐fold and 1.75‐fold improvements in HER and OER, respectively, indicating effective utilization of photogenerated carriers. Nitrogen dopants promote water adsorption, whereas sulfur dopants facilitate charge transfer.^{[ 70 ]} An in‐situ growth method was employed to produce a ternary CdS‐GQDs‐titanium nanotubes (CdS‐GQDs‐TNTs) nanocomposite. The role of TNTs as a host for co‐loading CdS and GQDs was systematically investigated for photocatalytic applications. CdS and GQDS both act as photoinduced electron donors to TNTs, while the photoinduced holes on the surfaces of CdS and GQDs are consumed by sacrificial agents.^{[ 160 ]} Studies report the effects of nitrogen and fluorine co‐doping in GQDs for water splitting. This co‐doping was prepared via plasma treatment and resulted in covalent C─F bond and semi‐ionic bonds, enhancing both the electrocatalytic and electrochemical reactions.^{[ 40 ]} Similarly, NS‐GQDs were also investigated. Compared to the bare TiO₂ electrode, these co‐doped GQDs show a 4–5‐fold increase in photocurrent density.^{[ 35 ]} Different synthesis procedures were employed for preparing GQDs/ hierarchical porous TiO₂ nanocomposites, including sol–gel processing, spin coating, and electrophoretic deposition. Among these, spin coating yields the highest PEC activity, with a photocurrent density of 6.35 mA cm⁻². In this system, GQDs act as photosensitizers under visible light, transferring photoexcited electrons to the conduction band of TiO₂, which subsequently transfers the electrons to the Pt electrode for hydrogen generation.^{[ 161 ]}

A ternary composite including cobalt ferrite, graphitic carbon nitride, and N‐GQDs exhibits a low overpotential of 287 mV and 445 mV at 10 mAcm⁻² current density for HER and OER, respectively. The overall water‐splitting cell achieves a current density of 10 mA/cm² at a low cell voltage of 2.0 V. Cobalt ferrite enhances stability and charge transfer, while N‐GQDs significantly improve electrical conductivity.^{[ 162 ]} Zn‐doped BiVO₄/GQDs/Co‐Pi hybrid structure displays 57% IPCE and a photocurrent density of 3.01 mA cm⁻² at 0.6 V versus RHE—8.6 times greater than that of the pristine BiVO₄. The Zn doping, as well as oxygen vacancies, stimulate the Bi sites which can influence the H₂O adsorption, which is favorable for water splitting. The hybrid contributes to improved light absorption and electrode stability. The oxygen vacancies created through zinc doping function as electron donors, increasing carrier density and inducing favorable band position shifts in BiVO₄. The Zn‐doped BiVO₄/GQDs/Co‐Pi hybrid structure exhibits hydrogen and oxygen evolution amounts of 882 and 440 µmol, respectively.^{[ 163 ]}

Multilayer, layer‐by‐layer assembly of metal nanocrystals (Au, Ag, and Pt) and GQDs demonstrate optimal catalytic, electrocatalytic, and PEC performance. Among these, Au/GQDs exhibit the most favorable results. Tuning of the assembly cycle affects catalytic performances. These highly active multilayer thin films enable selective catalytic reduction of aromatic nitro compounds and efficient methanol oxidation^{[ 164 ]} (Figure 4a). In related studies, the electronic properties of GQDs are controlled via chemical modification. For instance, hydrazine treatment increases the bandgap of GQDs from 2.62 to 2.92 eV by reducing defect density. This widened band gap enhances visible‐light‐driven photoreduction of methyl viologen and ferricyanide ions. These reduced dots also facilitate the photochemical hydrogen evolution at 375 nm without requiring cocatalyst^{[ 165 ]} (Figure 4b,c).

Figure 4.

a) Schematic representation of LbL assembly for (M/GQDs).^{[ 164 ]} b) Hydrogen evolution using 100 mg of GQDs and hydrazine‐reduced GQDs. c) Energy diagram correlating to the energy levels of GQDs and hydrazine‐reduced GQDs. Reprinted with permission from.^{[ 165 ]} Copyright, 2018, American Chemical Society. (Abbreviations: LbL, layer‐by‐layer; GQDs, graphene quantum dots).

For the CQD nanohybrids, a one‐pot electrochemical deposition process results in the formation of CQDs/CuSCN composite thin films. The photocurrent intensity for the composite is 6.5 times higher than that of the pristine CuSCN. This composite could be considered as a hole transport material. The enhanced PEC performance likely stems from the effective charge separation, specifically the transport of photogenerated electrons from the CB of CuSCN to the CQD surface.^{[ 166 ]} A CQDs/WO₃ electrode was fabricated using a seed‐mediated solvothermal method along with subsequent impregnation and assembly. This results in a photocurrent density of 1.46 mA/cm⁻² at 1.0 V versus Ag/AgCl, approximately two times higher than that of the WO₃ electrode. The enhancement in the PEC property is due to the light‐harvesting and charge‐transfer capabilities of the CQDs.^{[ 125 ]} Dual modification of the photoanodes (WO₃) was conducted to further modulate the photocatalytic activity. Initially, β‐FeOOH was introduced as a shell structure around the WO₃ core. Subsequent plasma treatment facilitates effective transfer and hole injection along with the creation of oxygen vacancies by inducing the coexistence of multiple Fe oxidation states (Fe²⁺/ Fe³⁺). This was also conducted aiding in the formation of photoinduced holes.^{[ 167 ]} In another study, anatase TiO₂ nanothorns were grown over rutile‐phase TiO₂ nanorods, followed by CQDs, resulting in a CQD‐modified anatase/rutile TiO₂ photoanode. The heterojunction of anatase and rutile phases suppresses bulk recombination, leading to an 11.72‐fold increase in photocurrent density compared to that of the pristine TiO₂, along with a 240‐mV negative shift in onset potential. The bulk charge separation efficiency was 1.69 times higher than that of the pristine TiO₂. CQDs effectively utilize both ultraviolet and visible light spectra.^{[ 127 ]} In one study, a 5 nm thick Al₂O₃ layer was deposited as a passive layer by atomic layer deposition (ALD) to accelerate the charge separation and improve the adhesion between the CQDs and TiO₂ NRs. This process results in the formation of the CQDs/3D‐TiO₂ NRs covered with Al₂O₃. To trap more sunlight, a branched 3D‐TiO₂ structure was grown using the hydrothermal method. The result shows a 3.2 mAcm⁻² photocurrent density at 1.23 V versus RHE, which is five times greater than that of the pristine TiO₂. The ALD process stabilizes the interface and prevents oxygen from reaching the substrate surface^{[ 168 ]} (Figure 5a). To improve light harvesting and charge collection, a novel approach was developed, which involves hydrogenation treatment of TiO₂ nanoarrays and decoration with CQDs. The hydrogenation treatment leads to the formation of oxygen vacancies, which suppresses the recombination of photogenerated charges. This results in a photocurrent density of 3 mAcm⁻² at 1.23.^{[ 87 ]}

Figure 5.

a) Step‐by‐step process for synthesizing a 5 nm Al₂O₃ layer on Al₂O₃‐CQDs/3D‐TiO₂ NRs, Reproduced (Adapted) under the terms of the CCC license‐1636666, Royal Society of Chemistry, 2025.^{[ 168 ]} b) Proposed mechanism for photocatalytic water splitting in the hybrid Ni₄P₂‐CQDs@CdS system, Reproduced (Adapted) under the terms of the CCC license‐ 6 013 920 734 339, Copyright Elsevier, 2025.^{[ 169 ]} c) Synthesis process illustration of CuNi/CQDs, Reproduced (Adapted) under the terms of the CCC license‐ 6 013 940 750 039, Copyright Elsevier.^{[ 170 ]} d) Mechanism for enhanced PEC performance in BiVO₄ photoanodes through co‐modification with CQDs and FeOOH, Reproduced (Adapted) under the terms of the CCC license‐ 6 013 940 901 865, Copyright Elsevier.^{[ 101 ]} e) PEC device design showing a three‐electrode configuration for water splitting measurements. f) Illustration representing the separation of photo‐generated electrons and holes in the water splitting system. g) Illustration of the PEC water oxidation at the NFCB photoanode Reproduced (Adapted) under the terms of the CCC license‐ 1 636 669, Royal Society of Chemistry, 2025.^{[ 99 ]} (Abbreviations: PEC, photoelectrochemical; CQDs, carbon quantum dots; TiO₂, titanium dioxide; NRs, nanorods; CdS, carbon dots; HER, hydrogen evolution rate; OER, oxygen evolution reaction).

CQDs were decorated onto Fe₂O₃ nanowires to form CQDs/Fe₂O₃ composite. The inclusion of CQDs results in a 27‐fold increase in photocurrent density at 0.23 V versus Ag/AgCl compared to that in the pristine Fe₂O₃. As previously discussed, CQDs exhibit high light absorption, enhanced interfacial charge transfer, and other beneficial properties.^{[ 137 ]} In this system, CdS acts as the light harvester, while CQDs and Ni₄P₂ serve as the electron acceptor and the catalyst, respectively. The hydrogen evolution occurs at the Ni₄P₂ site, as the electrons get transferred from CdS to CQDs and then to Ni₄P₂, while holes transfer from the valence band of CdS to CQDs. The electrode was obtained via an in situ photoreaction of CQDs@CdS and the polyoxometalate [Ni₄(H₂O)₂(PW₉O₃₄)₂]₁₀ (Ni₄P₂) (Figure 5b).^{[ 169 ]} The surface plasmon resonance effect of the CuNi bimetal produces electrons for the photocatalytic water splitting in the dendrite‐like plasmonic CuNi bimetal (Figure 5c). CQDs (3 wt.%) act as a cocatalyst over the CuNi bimetal and possess high charge transfer properties. The hydrogen and oxygen evolution rates are 55.19 µmol gh⁻¹ and 27.51 µmolgh⁻¹, respectively, with excellent stability after 5 h of repeated cycling.^{[ 170 ]}

CQDs act as a charging bridge to promote hole transfer in (Co‐Pi)/Fe₂O₃ photoanode, improving their charge transfer kinetics. The carbon nitride nanosheets (CNSS)/CQDs exhibit an HER of 116.1 µmolh⁻¹. CQDs trap electrons and help promote the separation of photogenerated charges in CNSS.^{[ 171 ]} Additionally, CQDs chemically absorb water and promote photoexcited holes to participate in the oxidation process, forming ·OH from the O─H bond.^{[ 172 ]} A CQD/H‐ZnO_1‐x/ZnO photoanode was fabricated by combining high oxygen‐rich CQDs with oxygen‐deficient ZnO@H‐ ZnO_1‐x. The photoactivity of this electrode is five times higher than that of the pristine ZnO NR (nanorod). The oxygen vacancy in the ZnO@H‐ ZnO_1‐x creates sub‐band electronic states, while the carboxyl‐conjugated CQD acts as a reservoir for photogenerated holes, guiding the rate of charge transport.^{[ 173 ]}

The bottleneck associated with BiVO₄, namely inefficient hole extraction and transfer, is addressed by integrating oxygen vacancy‐rich CQDs (O_V‐CQDs) onto BiVO_4, forming the O_V‐CQDs/BiVO₄ composite. In this structure, the O_V‐CQDs act as ultrafast hole transporters. Both UV and visible light are harvested, resulting in a photocurrent density of 4.01 mAcm⁻² at 1.23 V versus RHE.^{[ 83 ]} Following a similar strategy, a composite of O_V‐CQDs along with electrochemically reduced TiO₂ was fabricated (CQDs/E‐TiO₂). The CQDs modification enhances the surface charge transfer by acting as catalysts for oxygen evolution and a visible‐light sensitizer. The bulk charge separation efficiency of E‐TiO₂ reaches ≈94.7%_, which is four times greater than that of pristine TiO₂. Additionally, the result shows a photocurrent density of 2.55 mAcm⁻² at 1.23 V versus RHE, which is 10 times higher than that of pristine TiO₂.^{[ 174 ]} A CQDs/FeOOH/BiVO₄ heterostructure was developed to address the same bottleneck in the BiVO_4. Co‐modification with CQDs and ultrathin β‐FeOOH introduces a high density of oxygen vacancies.^{[ 175 ]} The CQDs further enhance light harvesting, while the bulk recombination is suppressed through the CQDs/ BiVO₄ heterojunction (Figure 5d). This structure exhibits a photocurrent density 2.98 times higher than that of the FeOOH/BiVO₄ alone at 0 V versus RHE, along with a 255 mV shift in the onset potential.^{[ 101 ]} BiVO₄ has been investigated solely for oxygen evolution due to its low conduction band position and high reaction overpotential. These limitations are addressed by designing a structure consisting of BiVO₄ QDs deposited on nanostructured SnO_2, along with CQDs, to improve charge transfer and electrocatalytic performance. SnO₂ functions as a host material and a passivation layer. A type II heterojunction forms at the SnO₂/ BiVO₄ interface. The hydrophilic groups on the CQDs could act as electron bridges, accelerating hole injections from the CQDs into the electrolyte. This structure achieves a photocurrent density of 2.2 mAcm⁻² at 0.6 V versus Ag/AgCl along with a HER of 0.54 µmolh⁻¹.^{[ 176 ]} Another example of CQDs acting as photosensitizers is observed in NiOOH/FeOOH/CQD/BiVO₄ (NFCB) photoanode, which results in a photocurrent of 5.99 mAcm⁻² at 1.23 V versus RHE. In addition to enhancing light absorption and charge separation, the CQDs facilitate efficient hole transfer from BiVO₄ to the catalyst layers (Figure 5e–g).^{[ 99 ]} In the CQDs/CdIn₂S₄/BiVO₄ structure, the CdIn₂S₄ addresses the hole recombination issue by providing aligned band positions beneficial for hole transfer from BiVO₄ to CdIn₂S₄ and suppressing hole recombination. CQDs act as a charge transfer channel, as previously discussed. The dual role of CQDs as photosensitizers and catalysts is also recognized here, with the structure achieving a photocurrent density of 4.84 mAcm⁻².^{[ 177 ]}

The Co‐Pi/CQDs/Fe₂O₃/TiO₂ electrode shows the photothermal effect of CQDs. Hematite exhibits low minority carrier transport characteristics; therefore, elevating the temperature serves as an effective strategy to overcome this limitation. Under NIR light irradiation, the photothermal effect of CQDs simultaneously enhances surface oxidation kinetics and bulk charge transport by increasing the electrode temperature. This results in a photocurrent density of 3 mAcm⁻² at 1.23 V versus RHE.^{[ 128 ]} Silicon nanowires (SiNWs) decorated with the CQDs facilitate light absorption in the UV region, while Au‐CQDs are utilized to improve visible light absorption through their surface plasmon effect. Based on these principles, SiNW_Au‐CQDs photocathode exhibits a HER of 182.93 µmolh⁻¹, a photocurrent density of 1.7 mAcm^{− 2}, and high operational stability.^{[ 178 ]} Tables 3 and  4 show a list of several GQDs and CQDs nanohybrids used as photocathodes and photoanodes, along with their comparative HER. Through a comparative assessment of the performance of GQD and CQD‐based nanohybrids based on their electrode, we show tangible performance benefits for either GQD‐ or CQD‐based materials. For photocathodes, GQD‐based systems showed higher HER activity (ranging from the GQD‐based systems studied), for example, CdS‐GQDs‐TNTs recorded the greatest HER at 530.1µmol·h⁻¹, notable performance of a GQD‐based photocathode is attributed to the efficient interfacial charge separation/harvesting of light in 0D/2D heterostructures.^{[ 160 ]} On the contrary, CQD‐based photoanodes showed higher photocurrent densities, which have been observed in NiOOH/FeOOH/CQD/BiVO₄ at 1.23 eV, achieving photocurrent densities of 5.99 mA·cm^{− 2}.^{[ 99 ]} The performance enhancement is a result of the synergistic effects of CQDs and multicomponent oxides in promoting redox kinetics and charge transport they facilitated as photogenerated charges.

Table 3.

HER of GQDs and CQDs nanohybrids as photocathodes.

Sl. No.	Photocathode	HER	Electrolyte	Refs.
GQDs
1.	Graphene@Au	65.6 µmol h−1	0.1 M Na2SO4	[179]
2.	TiO2/GQDs	2.2 µmol h−1	0.5 M Na2SO4	[157]
3.	N‐GQDs‐ZnNb2O6/g‐C3N4	340.9 µmol h−1g−1	0.5 M Na2SO4	[180]
4.	CdS/GQDs	95.4 µmol·h−1	0.5 M Na2SO3	[74]
5.	2N‐GQDs	8.33 µmol h−1	0.5 M Na2SO4	[62]
6.	CdS‐GQDs‐TNTs	530.1 µmol h−1	0.1 M Na2SO4	[160]
7.	rGQDs	1 µmol h−1	0.5 mol L−1 K2SO4	[165]
CQDs
1.	Chl/CQDs_SiNW	100 µmol h−1 0.5 M H2SO4		[181]
2.	C‐QDs modified BVO‐NSO	0.54 µmol h−1	0.5 M Na2SO4	[176]
3.	Ni4P2‐CQDs@CdS	145.0 µmol h−1	0.1 M KPi buffer (pH 7)	[169]
4.	CNNS/CQDs	116.1 µmol h−1	1 M Na2SO4	[171]
5.	CuNi/CQDs	55.19 µmol g−1h−1	1 M Na2SO4	[170]
6.	CNNS/CQDs	116.1 µmol h−1	1 M Na2SO4	[171]

Abbreviations: HER, hydrogen evolution rate; GQDs, graphene quantum dots; CQDs, carbon quantum dots; TiO₂, titanium dioxide; CdS, carbon dots; N‐GDQs, nitrogen‐doped graphene quantum dots; TNT, titanium nanotubes.

Table 4.

HER of GQDs and CQDs nanohybrids as photoanodes.

Sl. No.	Photoanode	Photocurrent density at 1.23 eV [mA cm−2]	Electrolyte	Refs.
GQD nanohybrids
1.	HBL|FLGs|TiO2/ GQDs|Ni(OH)2	0.3	0.5 M Na2SO4	[182]
2.	Aux/GQDs/NP‐TNTA	1.3	0.5 M Na2SO4	[159]
3.	Au/GQDs	0.25	0.5 M Na2SO4	[164]
4.	BiVO4/GQDs/Co‐Pi	3.01	0.1 M phosphate buffer	[163]
5.	NGQDs/ZnO NT	2.0	0.1 M Na2SO4	[183]
6.	GQDs‐15/TiO2 NFs	0.28	1 M Na2SO4	[184]
7.	G‐QD/TiO2	0.19	0.5 M Na2SO4	[156]
CQD nanohybrids
1.	CQDs‐H/TiO2	3	1 M KOH	[87]
2.	PCSiNW_AuCQDs	1.7	0.5 M H2SO4	[178]
3.	CQDs/CdIn2S4/BiVO4	4.84	0.2 M Na2SO4	[177]
4.	CQDs/3D‐TiO2 NRs	3.20	0.1 M NaOH	[168]
5.	NiOOH/FeOOH/CQD/BiVO4	5.99	KH2PO4 (pH 7)	[99]
6.	OV‐ CQDs/BiVO4	4.01	KH2PO4 (pH 7)	[83]
7.	CQDs/E‐TiO2	2.55	0.2 M Na2SO4	[174]
8.	CQD/p‐FeOOH@WO3	2.18	0.5 M Na2SO4	[167]
9.	Co‐Pi/CQDs/Fe2O3/TiO2	3.0	1 M NaOH	[128]
10.	C‐QDs modified BVO‐NSO	2.2 (0.6 V vs Ag/AgCl)	0.5 M Na2SO4	[176]
11.	CQD/H‐ZnO 1–x /ZnO	4.5	0.5 M Na2SO4	[173]
12.	CQDs/A/R‐TiO2	2.76	0.2 M Na2SO4	[127]
13.	CQDs/Fe2O3	0.1	1 M NaOH	[137]
14.	CQDs/FeOOH/BiVO4	2.53	0.2 M Na2SO4	[101]
15.	C‐Co‐Pi/Fe2O3	4.9 (0.6 V vs Ag/AgCl)	1 M NaOH	[109]
16.	CQDs/WO3	1.46 (1.0 V vs Ag/AgCl)	1 M H2SO4	[125]

Abbreviations: HER, hydrogen evolution rate; GQDs, graphene quantum dots; CQDs, carbon quantum dots; TiO₂, titanium dioxide; OV, oxygen vacancy.

Overall, GQDs are shown to be more effective in facilitating photocathodic hydrogen generation, while CQDs act more as sensitizers in anodic configurations. However, when considering such direct comparativeness of both carbon dot types, CQDs acted properly in good ternary systems and co‐catalyst configurations. This study, therefore, highlights the relevance of carbon dot type and possible inclusion and resultant tailoring of the doped architecture or engineered interface that has been designed to characterize PEC performance based on carbon dots; and presents a cursory pathway for selecting materials in next‐generation photoelectrochemical devices. Furthermore, Figure  6a–d illustrates a comparative analysis of the PEC HER and photocurrent densities for GQD and CQD nanohybrids used as photocathodes and photoanodes. The analysis unambiguously clarifies that GQD‐based systems are more advantageous when utilized as photocathodes for standing HER, as they achieve higher average hydrogen evolution rates. Conversely, CQD‐based systems provide superior and steadier performance when used as photoanodes, as evidenced by the photocurrent normalized to their mass of the photoanodes being much higher. In conclusion, this dependency upon photoelectrochemical systems confirms the importance of different types of carbon quantum dots, depending on whether they are used in their cited role as a cathode or an anode.

Figure 6.

a) Comparison of PEC HER for GQD and CQDs nanohybrids photocathodes (Table 3). b) Photocurrent density data of GQD and CQDs nanohybrids photoanodes (Table 4). c) Photocurrent density data of photocathodes at 1.23 V versus RHE from Table 3. d) Photocurrent density data of photoanodes at 1.23 V versus RHE. (Abbreviations: GQD, graphene quantum dot; CQDs, carbon quantum dots; PEC, photochemical; HER, hydrogen evolution rate).

4.2. Co₂ Reduction

Artificial photosynthesis offers a promising method to transform carbon dioxide (CO₂) into valuable chemicals by utilizing solar energy, thereby promoting a sustainable carbon cycle and mitigating the growing energy crisis.^{[ 185 ]} In the long term, harnessing unlimited and clean solar energy is essential for meeting global energy needs.^{[ 186 ]} However, despite its potential, artificial photosynthesis faces significant challenges due to the high dissociation energy of the carbon‐oxygen bond (C═O) in CO₂, which requires ≈750 kJmol⁻¹ to break. This renders CO₂ thermodynamically stable and challenging to reduce.^{[ 85} , ^{187 ]}

Among the various CO₂ reduction products, methane (CH₄) has garnered considerable attention due to its high energy density as a fuel. The conversion of CO₂ to CH₄, however, is a complex, multi‐electron process involving the transfer of eight electrons, as shown in Equation (1):

$$\mathrm{C}{\mathrm{O}}_2+8{\mathrm{H}}^{+}+8{\mathrm{e}}^{-}\to \mathrm{C}{\mathrm{H}}_4+2{\mathrm{H}}_2\mathrm{O}$$ (1)

Various catalysts have been developed for converting CO₂ into CH₄. Since a report on artificial photosynthesis in 1979,^{[ 188 ]} which involves using sunlight to drive CO₂ conversion with water, these catalysts face challenges such as charge recombination, slow OER kinetics, limited visible light absorption, low conversion efficiencies, and poor product selectivity.^{[ 174 ]} Enhancing conversion efficiency and selectivity for the CO₂‐to‐CH₄ transformation thus remains a formidable challenge in advancing artificial photosynthesis.

Compared to traditional technologies, the photocatalytic reduction of CO₂ using photoactive inorganic semiconductors (such as TiO₂, BiVO₄, and ZnIn₂S₄) and transition metal complexes (including Ru‐tris‐bipyridine) operates without the need for high temperatures, high pressures, expensive materials, or external electrical input.^{[ 189} , ^{190 ]} Driven solely by solar light, this process produces solar chemicals and fuels—such as CO, HCOOH, HCHO, CH₄, and CH₃OH—under mild conditions, including room temperature and atmospheric pressure.^{[ 191 ]} Consequently, CO₂ photoreduction provides a straightforward and adaptable method for CO₂ valorization.^{[ 185 ]} Current research focuses on developing innovative methods and advanced materials to enhance efficiency under mild conditions, aiming to maximize CH₄ yield while minimizing by‐products. These efforts not only deepen our understanding of photocatalytic mechanisms but also advance the pursuit of a greener, more sustainable energy future.

Wang et al. report the design of an efficient catalyst,^{[ 85 ]} demonstrating the impressive performance of the CQDs‐25/NiMOF_V catalyst in CO₂ reduction. The catalyst achieves a CH₄ production rate of 1 mmolg⁻¹ h⁻¹ with an outstanding selectivity of 97.58%. After loading CQDs onto the surface of NiMOF_V, all CQDs‐X/NiMOF_V catalysts exhibit superior catalytic activity than that of pristine NiMOF_V, with CH₄ production following the order: CQDs‐25/NiMOF_V > CQDs‐10/NiMOF_V > CQDs‐50/NiMOF_V > CQDs‐75/NiMOF_V. The optimal photocatalyst, CQDs‐25/NiMOF_V, produces a maximum production (6.0 mmolg⁻¹) of CH₄ and a minimum production (0.24 mmolg⁻¹) of CO after 6 h of reaction. This high efficiency arises from the combined effects of CQDs and oxygen vacancies, which enhance electron transmission and facilitate the complex eight‐electron reduction pathway required for converting CO₂ to CH₄. The oxygen vacancies act as electron traps, capturing photogenerated electrons to react with adsorbed CO₂, while the CQDs improve charge separation. In situ diffuse reflectance infrared Fourier transform (in situ DRIFTS) analysis reveals prominent peaks associated with CH₄ conversion, with significant peaks at 1560, 1635, and 1700 cm⁻¹ corresponding to ≈COOH functional groups, intermediates widely recognized in CO or CH₄ formation. With elapsed irradiation time, the peak at 1869 cm⁻¹ weakens, while strong absorption bands attributed to CHO and CH₃O emerge at 1090 and 1151 cm⁻¹, respectively.

In the mechanistic pathway of pure NiMOF and NiMOF_V, CO emerges as the primary product of CO₂ reduction due to the weak ability to migrate photoelectrons. Loading CQDs onto the surface of NiMOF_V significantly accelerates the transmission of photogenerated electrons. The oxygen vacancies acting as the active sites can effectively capture multiple electrons to react with CO₂ and produce CH₄. The synergistic interaction between CQDs and oxygen vacancies, which enhances the separation of photogenerated charges, is a critical factor for enabling efficient conversion of CO₂ to CH₄ using the CQDs‐X/NiMOFy catalyst (Figure 7a). Even under environments with diluted CO₂ concentrations (10%, 20%, and 50%), the catalyst maintains stable production levels, highlighting its robustness and potential for real‐world applications in sustainable energy production.

Figure 7.

a) Catalytic mechanism of NiMOF and CQDs‐X/NiMOFV catalysts, Reproduced under the terms of the CCC license‐ 6 013 941 282 778, Copyright Elsevier, 2025.^{[ 121 ]} b) Mechanistic illustration of N‐CQDs/TiO₂ heterostructured nanocomposites, Reproduced under the terms of the CCC license‐6013960281668, Copyright Elsevier, 2025.^{[ 125 ]} c) Photochemical reaction pathway for CO₂ reduction in the Complex, Reproduced under the terms of the CCC license‐ 6 013 960 081 627, Copyright Elsevier, 2025.^{[ 126 ]} d) Proposed structure and mechanism of the TNT/NiO/CQD photocatalyst for CO₂ conversion, Reproduced under the terms of the CCC license‐ 6 013 951 470 778, Copyright Elsevier, 2025.^{[ 127 ]} (Abbreviations: CQDs, carbon quantum dots; N‐CQDs, nitrogen‐doped carbon quantum dots TiO₂, titanium dioxide; TNT, titanium nanotubes).

Furthermore, introducing elements such as boron, nitrogen, phosphorus, and sulfur into carbon materials improves their electrical conductivity and increases their surface area.^{[ 92 ]} This modification not only enhances electron mobility but also provides additional binding sites. Incorporating nitrogen and sulfur into sp²‐hybridized carbon frameworks further alters their electrical properties and chemical reactivity, resulting in improved catalytic efficiency. N‐CQDs anchored on TiO₂ form a novel heterostructured nanocomposite that significantly enhances CO₂ photoreduction efficiency.^{[ 103 ]} Under simulated solar light, the optimized nanocomposite (0.4‐P25) achieves maximum CO and CH₄ yields of 1.153 and 0.769 µmol, respectively, over seven times higher than those of pristine TiO₂ (Figure 7b). Additionally, this study underscores the importance of strategic photocatalyst modifications to improve CO₂ conversion efficiency, as NCQDs enhance light absorption and electron transfer by acting as electron reservoirs, thereby reducing the recombination of photogenerated electron‐hole pairs.^{[ 192 ]} The CNQDs/TiO₂ composite also demonstrates excellent stability during prolonged irradiation, highlighting its potential for sustainable CO₂ conversion without the need for noble‐metal co‐catalysts. However, these catalysts exhibit slow activity and low efficiency.

The potential of metal‐free catalysts to address the energetic challenges of CO₂ reduction has been explored. The primary obstacle lies in the high thermodynamic stability of the CO₂ molecule, which requires considerable energy to break its bonds. This process involves a complex redox reaction with electron transfer, especially during the initial activation of CO₂—a critical and challenging step. Moreover, the reduction process entails multiple proton transfer steps that ultimately determine the final oxidation state of the carbon atom. A significant feature of this process is the formation of hydrogen bonds among the photocatalyst, CO₂, and H₂O, commonly present in CO₂ photoreduction systems. These hydrogen bonds play a crucial role in facilitating electron and proton transfer, making them central to understanding the CO₂ reduction mechanism. Fully comprehending the role of excited‐state hydrogen bonds is essential for achieving efficient CO₂ conversion and for developing more effective and sustainable methods. Zhao et al. explored this by combining experimental and theoretical approaches.^{[ 102 ]} They report that 9‐hydroxyphenal‐1‐one (HPHN), CO₂, and H₂O molecules form different hydrogen‐bonded complexes at various binding sites, with CO₂ activation as the initial and key step. This step facilitates subsequent reactions, although the activation of ≈CO₂ does not occur during the photophysical process. Instead, the structure of the ≈CO₂ molecule bends slightly, forming hydrogen‐bonded complexes that enable proton‐coupled electron transfer. generation of ≈COOH radicals, C═O bond cleavage, and ≈OH radicals. The hydroxyl oxygen atom, determined through hydrogen bonding, acts as the active center. Electron spin density plots show that the unpaired electron is primarily localized on the carbon atom, suggesting CO₂ is reduced. The formation of the ≈COOH radical is identified as the rate‐limiting step with a calculated activation barrier of 38.3 kcal/mol. Improving the electron‐donating ability of the HPHN catalyst, thereby enhancing photocatalytic activity, can be achieved by introducing metal ions such as Zn²⁺. This addition significantly facilitates the rate‐limiting step, shifting the active center from the hydroxyl oxygen atom to the Zn atom and decreasing the activation barrier. Consequently, the activity of HPHN‐CQDs‐Zn²⁺ increases to 166.6 µmolg⁻¹‐cat, nearly three times higher than that of HPHN‐CQDs alone (Figure 7c). Moreover, the TNT/NiO/CQD ternary nanocomposites exhibit superior photocatalyst performance than that of previous systems, primarily due to several innovative features. These nanocomposites achieve significant photocatalytic activity, producing 1.47 µmol cm⁻² h⁻¹ of methane, which is five times higher than that of bare TiO₂ nanotubes and comparable to that of other TiO₂‐based photocatalysts. This enhanced performance is attributed to the effective junction formation between TNT and NiO nanoparticles, which facilitates a Z‐scheme charge transfer mechanism that improves the separation and transport of photogenerated charge carriers. Additionally, the incorporation of CQDs into the system plays a dual role: extending light absorption into the visible spectrum and acting as charge collectors, thereby improving charge transport at the TNT, NiO, and CQD interfaces (Figure 7d). This dual functionality is critical, as it not only increases the activation of the photocatalytic process but also optimizes charge carriers’ dynamics, leading to a higher concentration of carriers available for CO₂ reduction. Overall, these attributes enable the TNT/NiO/CQD photocatalysts to achieve outstanding performance without relying on expensive materials or complex modifications.^{[ 193 ]}

4.3. Advances in PEC Sensing

Considerable progress has been made in the development and modification of PEC techniques, especially using CQDs and GQDs. Studies have focused on enhancing the sensitivity and selectivity of PEC sensors to achieve more precise and reliable results.

The PEC method with CQDs has emerged as a new ultrasensitive analytical technique, offering advantages such as easy operation, fast response, automated instruments, and real‐time detection. In a PEC device, the detection signal depends upon the photocurrent, which determines the PEC performance, and it is influenced by the photoelectrode used for the analyte to be investigated. However, it lacks selectivity since most other environmental contaminants can be oxidized simultaneously by strong oxidants, such as superoxide (O₂⋅⁻), hydroxyl (OH⋅), and hydroperoxyl (HO₂⋅) radicals produced by light irradiation during PEC tests. Therefore, developing strategies to effectively improve the selectivity of the PEC method is essential.^{[ 194 ]} TiO2 (≈3.2 eV) and hematite (bandgap ≈2.1 eV), particularly highly ordered 1D TiO2, are appropriate for PEC sensing and first‐choice semiconductor materials. The material offers low cost, good biocompatibility, large surface area, nontoxicity, and chemical stability.^{[ 195 ]} However, the inherent drawbacks of these materials, such as charge recombination and reduced surface reaction kinetics, limit their practical sensing applications. Strategies such as elemental doping, defect engineering, heterojunction construction, and electrocatalyst modification using CQDs could improve the PEC performance of semiconductor photoelectrodes.^{[ 196} , ^{197 ]}

To detect hydrogen peroxide, which can cause harm to humans and other organisms, a PEC sensor was developed by integrating CQD‐mediated CoAl‐layered double hydroxide (CQDs/CoAl‐LDH) onto hematite (α‐Fe₂O₃) photoelectrode (Figure 8I). In this system, CQDs mediate the growth of CoAl‐LDH, promote the exposure of active sites, and improve the electroconductivity to facilitate the charge transfer.^{[ 88 ]} The sensor achieves high sensitivities of 92.10 and 39.27 µAmM⁻¹ cm⁻² within the linear detection ranges of 0.001−0.9 mM and 0.9−2 mM, respectively, with a detection limit of 0.02 µM at a signal‐to‐noise ratio of 3 at 0 V versus SCE was observed. CoAL‐LDH provides abundant active sites for charge trapping and H₂O₂ oxidation. CQDs, along with the CoAL‐LDH, enhance the PEC activity of the hematite photoelectrode for H₂O₂ oxidation.

Figure 8.

I) Schematic diagram of photoelectrode fabrication, Reprinted (adapted) with permission from.^{[ 88 ]} Copyright 2024, American Chemical Society. II) illustration of the synthesis procedure of CdS (A), synthesis of CdS/COFs (B), and the construction process and detection mechanism of the MIP‐PEC system (C), Reproduced under the terms of the CCC license‐ 6 013 961 511 071, Copyright Elsevier, 2025.^{[ 198 ]} Abbreviations: CdS, carbon dots; CQDs, carbon quantum dots; PEC, photoelectrochemical; CoAI‐LDH, CoAl‐layered double hydroxide.

Tetracycline is a widely used, low‐cost, broad‐spectrum antibiotic; however, its excessive use can lead to its accumulation in the food chain, soil contamination, and the development of antibiotic‐resistant bacteria. Therefore, to detect this antibiotic, a 2D covalent organic framework confined to 0D CQDs/COFs^{[ 198 ]} film was designed (Figure 8II). A rapid in situ polymerization method was employed for electrode preparation. CQDs are encapsulated within the COFs via aldehyde‐amine condensation at room temperature. The sensor exhibits a wide linear response across a concentration range of 5.00 × 10⁻¹² to 1.00 × 10⁻⁵ M, with a detection limit as low as 6.00 × 10⁻¹³ M.

The role of CQDs is to enhance the light absorption and electron conductivity in CdS/COFs through a type‐II heterojunction, which improves charge transfer. One study reports the detection of enrofloxacin, a potent antibiotic pollutant, using N‐CQDs/Bi₄O₅Br₂ as a PEC aptasensor. The N‐O and N‐Bi‐O linkages between the materials enhance sensitivity, resulting in a 17‐fold increase in photocurrent compared to that in Bi₄O₅Br₂ alone.^{[ 199 ]} CQDs have also been incorporated into a PEC immunosensor for detecting alpha‐fetoprotein (AFP), a glycoprotein in the albumin family. AFP can serve as a positive indicator for diverse tumors, particularly for liver cancer (Figure 9I).^{[ 200 ]} In this sensor, carbon dots and MnO₂ were deposited on a screen‐printed carbon electrode by conjugating with monoclonal anti‐AFP antibodies. Polyclonal anti‐AFP antibodies with alkaline phosphatase were used as the detection antibody. Upon addition of ascorbic acid two‐phosphate, ascorbic acid and phosphate are produced. The ascorbic acid then reduces MnO₂ to Mn²⁺ ions, leading to the dissolution of the nanosheets and the release of the CQDs, which causes a decrease in the photocurrent density. The limit of detection was found to be 9.3 pg mL⁻¹, with the photocurrent linearly decreasing as AFP concentration increased from 0.01–100 ng mL⁻¹. Another study utilizes SnO₂/NCQDs/BiOI for the detection of cardiac troponin I (cTnI), a positive indicator for myocardial injury, which can be fatal in higher quantities. N‐CQDs serve as both a sensitizer and a carboxyl linker. Owing to their small size, good water solubility, and excellent dispersion, N‐CQDs are promising candidates for immunosensing applications. When conjugated with SnO_2, N‐CQDs offer a large surface area and enhanced fluorescence, making them ideal for cTnI PEC detection. In the study, ascorbic acid was used as an electron donor to eliminate photogenerated holes, thereby improving the photocurrent density and enhancing immunosensor sensitivity.

Figure 9.

I) Fabrication electrode for the monitoring of (AFP), Reproduced under the terms of the CCC license‐1636672, Copyright, Royal Society of Chemistry, 2025.^{[ 200 ]} II) Schematic representation of CdS/Au/GQDs for the detection of Cu⁺² ions, Reproduced under the terms of the CCC license‐ 6 013 980 104 991, Copyright Elsevier, 2025.^{[ 110 ]} Abbreviations: AFP, alpha‐fetoprotein; CdS, carbon dots; GQDs, graphene quantum dots; RC, recombination center; PEC, photoelectrochemical; CQDs, carbon quantum dots.

The PEC immunosensor shows a large‐scale response (0.001–100 ng mL⁻¹) and a low detection limit (0.3 pg mL⁻¹) under visible light illumination. Additionally, NGQDs have been utilized to develop a sandwich‐type PEC immunosensor with dual sensitization using N‐GQDs/CdS over urchin‐like TiO₂ on an ITO substrate for prostate‐specific antigen (PSA) detection.^{[ 95 ]} Furthermore, the addition of Au/MWCNTs nanohybrids improves the performance of the sensor, yielding a detection limit of 6.16 fg mL⁻¹ with a linear response between 0.1 pg mL⁻¹ and 50 ng mL⁻¹. Such strategic systems offer great potential for detecting trace tumor markers with good sensitivity and stability.^{[ 201 ]} Similarly, PSA detection has been achieved using a PEC biosensor platform based on a CdS/CuInS₂/CQDs heterojunction, which outperforms the CdS/CuInS₂ system by 10 times owing to the upconversion luminescence properties of CQDs. This platform achieves a limit of detection of 0.07 pg/mL⁻¹ with a linear range from 0.0001–10,000 ngmL⁻¹.^{[ 202 ]}

PEC sensors for metal ion detection have gained significant attention in recent years. For instance, copper leaching into the drinking water remains a major concern in Canada, the United States, Malaysia, and several countries. The primary cause of leaching is metal pipe corrosion and structural damage. To address this issue, a PEC sensor based on CdS/Au/GQDs^{[ 110 ]} was fabricated for detecting Cu²⁺. The sensor achieves a detection limit of 2.27 nM. In this system, GQDs and CdS act as photosensitizers that generate charge carriers, while Au serves as an assistant catalyst, primarily facilitating resonant photon scattering. The conduction band of GQDs is more negative than that of the CdS and Au, resulting in the efficient separation of the photogenerated charge carriers and prolonging the lifetime of the electron‐hole pairs. Upon exposure to Cu²⁺ ions, the photocurrent signal decreases due to the formation of Cu_xS on the CdS surface through the chemical displacement of Cd²⁺ by Cu²⁺. This reaction also leads to the creation of a new energy level below the CB of CdS, introducing recombination centers (Figure 9II).

Phthalic acid esters, commonly used as plasticizers in plastic products, are known endocrine disruptors. Among them, di‐2‐ethylhexyl phthalate (DEHP) is of particular concern due to its high propensity to leach into surrounding environments. To detect DEHP, a PEC aptasensing approach was employed, where GQDs decorated TiO₂ nanotube arrays act as the transducer, and an anti‐DEHP aptamer provides biological recognition.^{[ 194 ]} The loading of GQDs on TiO₂ nanotubes through electrostatic interactions fosters visible light absorption and charge separation, thereby improving photocurrent response and PEC detection activity. The PEC aptasensor exhibits a low detection limit of 0.1 ng L⁻¹.

4.4. Emerging PEC Applications

Recently, CQD/GQD hybrids have been utilized as photodetectors, with significant efforts focused on enhancing the PL properties and photoresponsivity of the host materials. CQD/GQD nanohybrids increase carrier mobility through strong light absorption, broad bandwidth, and tunable emissions capability.^{[ 203} , ^{204 ]} As artificial intelligence and automation systems rapidly advance, photodetectors are becoming essential for communication and diagnostics.^{[ 205 ]} Hybrid UV‐photodetectors constructed with CQDs/ZnO nanorods effectively replace charge transport poly‐n‐vinylcarbazole (PVK) polymers, achieving a high detection ability of 8.33 × 10¹² Jones under low light intensity of 1 mWcm⁻².^{[ 206 ]} Recently, CQDs were exploited for deep‐UV (DUV) photodetection in combination with polyvinyl alcohol (PVA) to leverage different light absorption properties under short (254 nm) and long wavelength UV (265 nm) regions. The 254 nm wavelength showed the highest wavelength‐dependent photonic response.^{[ 207 ]} Recently, cost‐effective self‐powered DUV‐photodetectors consisting of polyvinylidene fluoride (PVDF)@Ga₂O₃ and polyethyleneimine (PEI)/CQDs incorporate in‐situ piezoelectricity in the photon‐active region.^{[ 208 ]} In contrast to CQDs, researchers have increasingly explored GQDs as both broad‐range and self‐powered photodetectors. In recent developments, both graphene and plant‐derived GQDs were incorporated into lead zirconate titanate (PZT) substrates to produce ultrasensitive UV‐based photodetectors. The sensitivity (4.06 × 10⁹ AW⁻¹) of the device was 120 times higher than that of other comparable devices.^{[ 209 ]} The polarization in the PZT substrate generates a built‐in electric field, guiding holes through the graphene layer, while GQD facilitates the broad absorption range of the device. Recently, self‐powered NIR photo detectors have gained significant attention due to their high photosensitivity and rapid switching speed, without depending on external power sources. Following this footprint, hybrids of QDs and 2D material (N‐doped GQDs with a WSe₂ monolayer) were developed to enhance light absorption and facilitate charge transfer from N‐doped GQDs to WSe₂, resulting in a 480% increase in photoresponsivity compared to the WSe₂ photodetector. The photodetector retained 46% of its performance after 30 days in ambient conditions, indicating partial stability.^{[ 203 ]} TiO₂, a widely used material in PEC applications, shows a 40% performance enhancement when sensitized with GQDs, achieving a peak photoresponsivity of 118 mAW⁻¹ under UV illumination.^{[ 210 ]} Subsequently, GQDs combined with TiO₂ nanotubes have been explored, with GQDs exhibiting dual functionality: enhancing light absorption property and preventing charge carrier recombination at the photoanode/electrolyte interface. This enhances photodetector performance by 473% under low‐intensity UV light with zero bias.^{[ 211 ]} Dey et al. report hybrids of lead chalcogenides with chalcogen‐doped GQDs where Se‐doped GQD/PbSe hybrid nanocomposites exhibited a high spectral response due to lower charge carrier recombination, enhancing photodetector performance. The extended carrier lifetime enables the effective collection of photon‐generated charge carriers. Under zero‐bias, peak responsivity at NIR wavelengths was 1.8 AW⁻¹, while a 2.5 AW⁻¹ peak responsivity was observed at UV‐visible wavelengths, enabling two‐color UV‐NIR photodetectors.^{[ 212 ]} PEC‐operated alcohol oxidation has recently gained significant attention due to its multi‐verse applications in addressing energy and environmental issues. In this context, N‐doped GQDs are decorated on CdS nanowires in a “dot‐on‐nanowire” configuration, facilitating charge transfer to Pt nanoparticles. Under visible light irradiation, N‐GQDs bridge the electrocatalytic and photocatalytic processes, thereby prolonging CdS lifetime.^{[ 213 ]} The study highlights N‐GQDs as key to efficient interface charge transfer, enhancing PEC oxidation activity.

Photocatalytic fuel cells (PFCs) are an emerging field like methanol fuel cells (MFC), with the key difference being that PFCs use PEC devices for photocatalytic degradation to produce electricity. In addition, an intrinsic bias is needed to direct electrons for independent and bias‐free functionalization.^{[ 214 ]} Developing bias‐free photoanodes or cathodes capable of undergoing oxygen reduction reactions would greatly benefit such systems. In this context, photoelectrocatalytic production of hydrogen peroxide (H₂O₂) emerges as a promising research area.^{[ 215 ]} Although extensive research on PEC‐H₂O₂ production exists, few studies have explored the use of CQDs/GQDs for this process. Although PEC‐H₂O₂ systems have been less explored, studies report on the photo‐production of H₂O₂. A hybrid composite of CQD and the organic semiconductors 9,10‐dibromoanthracene and trimethylsilyl acetylene (DAn TMS compound) has been identified as a metal‐free photocatalyst capable of producing both H₂ and H₂O₂ without the need for sacrificial agents/solvents/additives.^{[ 216 ]} Another study highlights the properties of CQDs in Ni₂P/CQDs, where CQDs effectively store photogenerated electrons and stabilize them on the surface, preventing recombination. Since photogenerated electrons lack stability, CQDs have been incorporated to enhance stability and facilitate the storage of photo‐electrons, benefiting the photo‐production of H₂ and H₂O₂.^{[ 217 ]} These concepts can be effectively applied in PFCs for performance enhancement.^{[ 218 ]}

Recently, Lv et al. proposed PEC‐based energy storage designs to support carbon neutrality and renewable device technology. These innovative systems are referred to as photoelectrochemical energy storage (PES) devices. These devices convert direct solar energy into stored electrochemical energy, a process different from the mechanisms of PEC and PV cells. Since these proposed systems are still in the early stages, with testing conducted on known PES materials, it is inevitable that CQDs will soon be experimented with and explored, supporting the goal of renewable and carbon‐neutral systems.

5. Summary, Challenges, and Future Prospects

PEC research spans over five decades with extensive publications and advancement in materials development, fundamental issues such as charge carrier recombination, stability, selectivity in multi‐electron reactions, scalability, and cost‐effectiveness persist as bottlenecks. The discovery of carbon‐based QDs has opened a new era for exploration in the field of photoelectrochemistry. Building on the success of semiconductor QDs, both CQDs/GQDs have expanded their scope through nanohybrid forms. Their efficient utilization has led to performance enhancements across different PEC sectors. A plethora of research on photocatalysis and electrocatalysis exists in the literature, but the implications of CQDs/GQDs in PEC remain in their early stages, relying on fundamental approaches/concepts from these applications. The enormous advantages of CQDs/GQDs, including low cost, less toxicity, and property tunability, have attracted significant research interest in their utilization. Both CQDs and GQDs possess unique traits. CQDs are more easily functionalized, making them efficient in variant sensing‐based applications and photo detectors, whereas GQDs excel in electronic conductivity and charge transport, making them ideal for energy conversion systems.

In PEC water splitting and CO₂ reduction, the significance of CQDs/GQDs lies in their role in photoelectrodes, where they function as photosensitizers, co‐catalysts, heterojunction/up conversion material, and mediators. In addition, doping and surface functionalization of CQDs/GQDs significantly affect their behavior and corresponding performance in nanohybrids. Though doping/surface functionalization can facilitate properties, uncontrolled dopant quantities/functional groups may introduce defects/flaws in the QDs, potentially altering the PEC reaction. Thus, controlling the amount of dopant in CQDs/GQDs to positively influence the PEC reaction in conjugated nanohybrid forms is challenging and requires extensive research. In this context, another concern is the stability of the modified CQDs/GQDs. Recently, a theoretical analysis revealed that defects, such as sp³‐hybridized carbon atoms, can be introduced during GQD processing. These defects can tune the HOMO‐LUMO gap, shifting the optical absorption of GQDs into the red to IR regions (600–900 nm) of the solar spectrum. Electron storage by CQDs for H₂/O₂ photoproduction and their contribution to photoelectrode stability opens new research opportunities. Dua et al. comprehensively discussed the photo‐/thermo‐/ion‐stability of CQDs, claiming that CQDs are more stable in a polymer matrix and salts.^{[ 9 ]} In conclusion, the stability of a photoelectrode consisting of CQDs/GQDs depends on the deposition method on the semiconductor conducting substrate, their interaction with the semiconducting material, energy level alignment, reaction electrolyte, and PEC application. For CO₂ reduction reactions by PEC, key challenges include selectivity, intermediate stabilization, and control over product distribution. In PEC‐based sensors, the role of CQDs varies across different nanohybrids. In some cases, CQDs enhance the exposure of active sites, improve electroconductivity, and facilitate charge transfer; in others, they serve to boost light absorption. However, in GQD‐based nanohybrids for Cu‐detection, GQDs primarily facilitate photogenerated charge carrier separation and extend carrier lifetime due to their low conduction band. In other hybrids, GQDs act as a transducer species.

The relative performance of CQDs and GQDs established from the PEC systems is still largely dependent on a selection of configurations of the electrode architecture. Although CQDs and GQDs have all shown the potential to enhance PEC performance, it was essential to closely regulate the dimension since it affects their relative bandgap, energy band alignment with the semiconductor materials, and their charge transfer efficiency. With surface modification and heteroatom doping of quantum dots (QDs), we can significantly enhance the quantum dot's (QD's) performance. Heteroatom (N, F, B, S, and P) doping alters the electronic and optical structures, generates defect states that can improve charge separation, and improves overall electrical conductivity. N‐ and F‐ doped GQDs improved photocurrent densities, electrocatalytic variations of performance, and stability in PEC devices. Precisely, under PEC conditions, CQDs tend to degrade at the surface, while GQDs undergo oxidation at edge sites, compromising consistency in the results. Charge transfer becomes complex when interfaced with other semiconductors due to weak bonds/defects between the materials. The successful formation and performance of CQDs/GQDs nanohybrids depend on the processing of the composite/nanohybrid and the preservation of their inherent properties in the nanohybrid forms. For instance, the optical properties of CQDs are quenched when coupled with g‐C₃N₄. The luminescence properties of CQDs/GQDs are greatly affected by the materials they form nanohybrids with and the alignment of energy levels with the conduction/valence bands of the corresponding semiconductor. Poor energy level alignment inevitably reduces quantum efficiency. When integrated with other semiconducting materials, interfacial contact with CQDs/GQDs is important for effective charge transfer. Indeed, multilayered structures like metal nanocrystals (Au, Ag, Pt) interacting with GQDs and semiconductors like TiO₂, CuSCN, and CdS, offer improved charge separation, increased light harvesting, and more available active surface sites. For example, multilayer Au/GQDs structures provide superior electrocatalytic properties, and GQDs in heterostructures with TiO₂ allow for light absorption in the visible light and improved charge transfer pathways, leading to much higher photocurrent and hydrogen production rates. Additionally, synergistic doping facilitates covalent bonding that promotes better adsorption of reaction intermediates, offers selectivity, and catalysis kinetics. These forms of engineering give rise to nanohybrids with improved stability and activity, imperative for long‐term PEC applications. However, the mechanism of CQD/GQD nanohybrids adapts to specific needs based on research findings and charge transfer at the interfaces remains indistinct, leaving uncertainty about its role (sensitization/electron mediator/passivation) in the PEC reaction.

The application of CQDs/GQDs in PEC applications remains largely experimentally oriented, confined to lab‐scale studies without fully addressing durability or real‐world applicability. Establishing a detailed structure‐performance‐stability relationship will be essential by systematically combining different PEC experiments and aligning them with physicochemical characterizations. Despite the existing challenges, a broad range of research opportunities exists, such as 1) Developing scalable, environmentally friendly synthesis methods that could manufacture high‐quality and defect‐controlled quantum dots, 2) Carrying out systematic stability and durability assessments to verify reliability in PEC performance, 3) Development in selectivity in multi‐electron reactions to uplift the interfacial stability under operation conditions, 4) Designing integrated photoelectrochemical systems with combined improvements, that can approach or exceed the efficacy of commercial noble metals‐based catalysts.

Conflict of Interest

The authors declare no conflict of interest.

Biographies

Sandip Mandal is a Post‐Doctoral Researcher at the College of Dentistry and the Convergence Research Centre for Treatment of Oral Soft Tissue Disease (MRC), Chosun University, South Korea. Since 2021, he has also served as a Full‐Time Post‐doctoral Researcher in the School of Earth Science and Environmental Engineering at the Gwangju Institute of Science and Technology, Gwangju, South Korea. He earned his master's degree and Ph.D. in Chemistry from the National Institute of Technology, Rourkela, India. His research interests lie in environmental chemistry and engineering, with a particular emphasis on the development of novel multifunctional nanomaterials, nanostructure‐supported metal catalysts, adsorbents, and photocatalyst materials for applications in energy and environmental remediation.

Sangeeta Adhikari is a Research Professor in Catalyst Research Institute within the School of Chemical Engineering at Chonnam National University, South Korea, on a Brain‐Pool fellowship from National Research Foundation, Korea. Prior to her current position, she worked at the Indian Institute of Science, Bangalore, India, on a prestigious UGC‐D.S. Kothari postdoctoral fellowship. She received her Ph.D. degree from the National Institute of Technology, Rourkela, India. Her research interests are the rational design of heterostructure‐based nanomaterials via surface structure control, thin‐film processing via atomic layer deposition, and the fabrication of devices for energy storage and conversion, sensing, and water purification applications.

Manasi Murmu received her bachelor's and master's degree (2020) in Department of Ceramic Engineering at National Institute of Technology, Rourkela, India. She is currently pursuing her doctoral research in School of Chemical Engineering at Chonnam National University since 2021. Her research interests focuses on the growth and characterizations of nanoscale metal oxide/nitride films via atomic layer deposition for supercapacitor, water splitting, and environmental applications.

Byung Hoon Kim is an eminent and Full Professor in the Department of Dental Materials, Chair of Pre‐Dentistry, and Director of the Convergence Research Centre for the Treatment of Oral Soft Tissue Diseases at the College of Dentistry, Chosun University, South Korea. He is internationally recognized for his contributions to the development and characterization of advanced dental materials and devices. His research spans the design and application of metals, ceramics, and polymers for restorative dentistry; the development of innovative implant and orthodontic materials; and the creation of biomaterials for soft and hard tissue regeneration. His work has been cited extensively, underscoring its influence on both academic research and clinical practice. He has served as a reviewer and editorial board member for leading journals in dental materials science and biomaterials engineering. Professor Kim's achievements have been recognized with numerous awards, and he has successfully led multiple national and international research grants, fostering collaborations across academia, industry, and clinical practice to translate laboratory innovations into real‐world dental solutions.

Do‐Heyoung Kim is an eminent and full professor and principal researcher and leads the Chemical Process Laboratory for Advanced Materials (CPLAM) in the School of Chemical Engineering at Chonnam National University in South Korea. He obtained his Ph.D. at the Department of Chemical Engineering at Rensselaer Polytechnic Institute (Troy, USA) in 1993. Prior to his joining in Chonnam National University, he worked as a senior researcher at the Central Research Center of LG Semicon after his postdoctoral research at North Carolina State University. His research interests are applications of atomic layer deposition for IC devices, supercapacitors, batteries, water splitting, and sensors. His work has been cited extensively, underscoring its influence on both academic research and teaching. He has served editorial in chief and board member for leading journals in materials science and chemical engineering. Professor Kim's achievements have been recognized with numerous awards, and he has successfully led multiple national and international research grants, fostering collaborations across academia, and industry transform laboratory innovations into real‐world solutions.

Mandal S., Adhikari S., Murmu M., Kim B.‐H., and Kim D.‐H., “Graphene and Carbon Quantum Dots: Competing Carbons in Harmonized Photoelectrochemical Platforms.” Small 21, no. 40 (2025): e05846. 10.1002/smll.202505846

Data Availability Statement

This review article does not present original experimental data. All data referenced and discussed are drawn from previously published sources, as cited. Figures 2 and 6 were adapted from established literature data.