Recent Achievements in Development of TiO₂-Based Composite Photocatalytic Materials for Solar Driven Water Purification and Water Splitting

Abstract

Clean water and the increased use of renewable energy are considered to be two of the main goals in the effort to achieve a sustainable living environment. The fulfillment of these goals may include the use of solar-driven photocatalytic processes that are found to be quite effective in water purification, as well as hydrogen generation. H₂ production by water splitting and photocatalytic degradation of organic pollutants in water both rely on the formation of electron/hole (e⁻/h⁺) pairs at a semiconducting material upon its excitation by light with sufficient photon energy. Most of the photocatalytic studies involve the use of TiO₂ and well-suited model compounds, either as sacrificial agents or pollutants. However, the wider application of this technology requires the harvesting of a broader spectrum of solar irradiation and the suppression of the recombination of photogenerated charge carriers. These limitations can be overcome by the use of different strategies, among which the focus is put on the creation of heterojunctions with another narrow bandgap semiconductor, which can provide high response in the visible light region. In this review paper, we report the most recent advances in the application of TiO₂ based heterojunction (semiconductor-semiconductor) composites for photocatalytic water treatment and water splitting. This review article is subdivided into two major parts, namely Photocatalytic water treatment and Photocatalytic water splitting, to give a thorough examination of all achieved progress. The first part provides an overview on photocatalytic degradation mechanism principles, followed by the most recent applications for photocatalytic degradation and mineralization of contaminants of emerging concern (CEC), such as pharmaceuticals and pesticides with a critical insight into removal mechanism, while the second part focuses on fabrication of TiO₂-based heterojunctions with carbon-based materials, transition metal oxides, transition metal chalcogenides, and multiple composites that were made of three or more semiconductor materials for photocatalytic water splitting.

1. Introduction

Nowadays, accessible clean water and energy resources are among the highest priorities for sustainable economic growth and societal wellbeing. Water supports life and is a crucial resource for humanity; it is also at the core of natural ecosystems and climate regulation. Water stress is primarily a water quantity issue, but it also occurs as a consequence of a deterioration of water quality and a lack of appropriate water management [1]. Environmental problems that are associated with water pollution have been a persistently important issue over recent decades, correlated negatively with the health and ecosystem. Activities of the Water JPI’s Strategic Research and Innovation Agenda focus on, among others, new materials and processes, energy efficiency, thus supporting key enabling technologies for clean water and wastewater treatment [2]. EU Energy Strategies 2020, 2030, and 2050 set increasing standards for the reduction of greenhouse gas emissions by 20, 40, and 80–95%, respectively, which is achievable by significant investments in the development and application of new low-carbon and renewable energy technologies [3]. In light of increased energy demands and the need to reduce greenhouse gas emissions, the focus has been turned from the fossil fuels toward renewable energy resources and vectors: solar, wind, tides, waves, geothermal, biomass, biofuels, and hydrogen (H₂) [4]. Alternative fuels are required to have as small environmental footprint, and be storable and economical, whereas H₂ satisfies the first two conditions. The research over the last decades has been focused on fulfilling the third requirement, which triggers its production by solar energy, a largely available and intrinsically renewable energy resource, through water splitting. It should be emphasized that H₂, as a fuel, possesses higher heat content than gasoline (per unit mass) [5].

The pioneering work of Fujishima and Honda [6] for H₂ production by photoelectrochemical water splitting while using TiO₂ photoanode and Pt cathode opened the potential possibilities for generating this energy vector, i.e., fuel, directly from water and solar energy. Works by Bard and Frank in 1977 [7], exhibiting photocatalytic oxidation of CN to CNO⁻, and by Ollis et al. [8], studying the photocatalytic degradation of organic contaminants in water, practically opened a new research field within new water purification technologies. H₂ production by water splitting and photocatalytic degradation of organic pollutants in water both rely on the formation of electron/hole (e^-/h⁺) pairs at a semiconducting material upon its excitation by light with sufficient photon energy [9,10,11,12]. These processes, which can be conducted under environmentally friendly and mild conditions, are economically viable, possessing a potential of becoming effective methods to produce clean energy and water, owing to their low-cost, long-term stability, and usage of solar energy [13].

A well-suited model catalyst for photocatalytic studies is TiO₂. Its wide application has been promoted, due to: (i) high photocatalytic activity under the incident photon wavelength of 300 < λ <3 90 nm and (ii) multi-faceted functional properties, such as chemical and thermal stability, resistance to chemical breakdown, and attractive mechanical properties [14,15]. However, harvesting a broader spectrum of solar irradiation involves the lowering of the band gap of semiconducting material, whilst inhibiting the recombination of photogenerated charges. Strategies, including doping with non-metals, incorporation or deposition of noble metals (ions), and material engineering solutions that are based on composites formation using transition metals, carbon nanotubes, dye sensitizers, conductive polymers, graphene (oxide), and semiconducting materials, present viable solutions for set tasks [9,10,15,16]. It is of great importance to combine TiO₂ with narrow band gap semiconductors with visible light response to obtain an effective composite for photocatalytic applications. The obtained synergistic effect between two or more semiconductors will then promote efficient charge separation, sufficient visible light response, and high photocatalytic performance. With the dramatic increase of the papers published related to these topics, a comprehensive review is desirable, providing a general overview on processes occurring while using TiO₂-based heterojunction (semiconductor) systems for photocatalytic water purification and water splitting. Despite reviews focusing on TiO₂–based semiconductor composites [17,18], those focusing on the removal of contaminants of emerging concern (CECs) are quite scarce. In addition, this review also summarizes TiO₂-based nanocomposites for photocatalytic water splitting providing insight into effectiveness of a variety of materials groups representing the alternative for replacing the utilization of expensive, toxic, and non-abundant materials. The first part of the review focuses on TiO₂–based heterojunction (semiconductor-semiconductor coupling) composites, being selected on the basis of band gap energies suitable to make heterojunctions with documented applications providing promising results in CECs treatment and stability of prepared materials, and also respecting their most recent applications for the photocatalytic degradation of CECs (i.e., demonstrating the current focus within the field), such as pharmaceuticals and pesticides, with critical insight into the pollutants removal mechanism. The second part targets the most recent achievements in the field of fabrication of TiO₂-based heterojunctions with carbon based materials, transition metal oxides, transition metal chalcogenides, and multiple composites that were made of three or more semiconductor materials for photocatalytic water splitting.

2. Photocatalytic Water Treatment

The general mechanism of semiconductor photocatalysis (Figure 1) is composed of three main steps: 1. e⁻/h⁺ pairs are generated on the surface of the semiconductor under illumination with the required wavelength or energy; then, 2.) photogenerated charges (i.e., e⁻/h⁺) migrate to the surface of the semiconductor; and lastly, 3.) e⁻ and h⁺ induce redox reactions on the surface that facilitate destruction of organic pollutants [19,20]. As stressed above, TiO₂ is still the most studied and widely used material for photocatalytic degradation reactions. However, TiO₂ suffers from fast e⁻/h⁺ pair recombination and large band gap (E_g = 3.1–3.2 eV), which can only be excited under UV light irradiation. The strategies for improving these issues are provided above, while, among them, semiconductor-coupling presents a viable structure-properties engineering solution for the enhancement of TiO₂ photocatalytic activity due to the simultaneously reduced e⁻/h⁺ recombination rate and enhanced visible light absorption [21].

Figure 1.

Photocatalytic reaction mechanism over semiconductor material.

Three main types of heterojunction architectures are reported for TiO₂/semiconductor composites [22]. In Type I heterojunction, the conduction band (CB) of TiO₂ is higher in energy (more negative potential) when compared to the CB of semiconductor 2 and the valence band (VB) of TiO₂ is lower in energy (more positive potential) as compared to the VB of semiconductor 2 [23,24]. This leads to the accumulation of photogenerated h⁺ and e⁻ in semiconductor 2. In Type II heterojunction (where TiO₂ can be semiconductor 1 or 2), the CB of semiconductor 2 is higher than the CB position of semiconductor 1 leading to facile transfer of photogenerated e⁻ from CB of semiconductor 2 to CB of semiconductor 1 [25]. Meanwhile, photogenerated h⁺ in VB of semiconductor 1 can travel to the VB of semiconductor 2, which facilitates efficient charge separation. Type III heterojunction (also known as broken gap situations) [26] shares the same charge transfer mechanism, like Type II heterojunction. In this case, the CB and VB of semiconductor 2 are higher than CB and VB of TiO₂ [27,28]. These heterojunction types are explained in detail in the context of particular material combinations in the further text and graphically represented through Figures 2, 3 and 5–10.

2.1. Coupling of TiO₂ with Metal Oxides

2.1.1. TiO₂/WO₃

Tungsten oxide (WO₃), which is a visible light active photocatalyst with band gap of 2.4–2.8 eV, is a promising candidate for photocatalytic applications, due to its oxidative properties, nontoxicity, low cost, and stability in acidic solutions. In addition, WO₃ directly matches the band positions of TiO₂ to form a heterojunction (Type II Heterojunction) [29,30,31,32]. Several authors studied the application of TiO₂/WO₃ composites for the degradation of various CECs; either pesticides or pharmaceuticals (Table 1). Hence, Macias et al. [24] studied the photocatalytic degradation of herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) while using TiO₂/WO₃ composites under natural sunlight. They reported the rather high effectiveness of the studied system: 94.6% degradation of 2,4-D and 88.6% mineralization of overall organic content under two and four hours of natural sunlight irradiation, respectively.

Table 1.

Photocatalytic degradation of contaminants of emerging concern (CECs) over TiO₂/WO₃ composites.

Catalyst	Target Pollutant	Initial Concentration/Working Volume ((mg L−1) /mL)	Experimental Conditions	Reaction Time	Removal Extent (%)	Reference
TiO2 - WO3 (0.5 g/L )	2,4-dichlorophenoxy acetic acid	50 (in 250 mL)	Light Source: natural sunlight 11AM-4PM; pH = 4	120 min	94.6 (TOC = 88.6)	[24]
TiO2 - WO3 (0.6 g/L)	Diclofenac	25 (in 100 mL)	Light Source: 400 W Metal Halide Lamp; pH = 5	240 min	TOC = 91	[30]
TiO2 - WO3 (0.1 g/L)	Amoxicillin	100 (in 25,000 mL)	CPC Reactor with accumulated energy 550,000 J/m2	NA	64.4 (@ 550 kJ/m2)	[35]
(WO3/TiO2-C) (1.0 g/L)	Diclofenac	10 (in 300 mL)	Light Source: 1500 W Xenon Lamp with filter(λ > 290 nm) ; pH = 7	NA	100 (@ 250 kJ/m2) (TOC = 82.4 @ 400 kJ/m2)	[32]
(WO3/TiO2-N) (1.0 g/L)	Diclofenac	10	Light Source: 1500 W Xenon Lamp with ID65 solar filter; pH = 6.5	NA	100 (@ 250 kJ/m2) (TOC = 100 @400 kJ/m2)	[31]

NA—not available.

Besides, they studied the mechanisms that are responsible for forming reactive species within the system and, based on their findings, proposed that, upon forming e⁻/h⁺ pairs under solar irradiation, photogenerated e⁻ from CB of TiO₂ are transferred to CB of WO₃. Consequently, W⁶⁺ was first reduced to W⁵⁺ on WO₃ surface, while the W⁵⁺ ions are then oxidized to W⁶⁺ by adsorbed O₂ producing superoxide anion radical (O₂^●‒). The photogenerated h⁺ in VB of WO₃ are transferred to VB of TiO₂ where they reacted with water (or hydroxyl ions, HO⁻) forming hydroxyl radicals (•OH) (Figure 2). The generated reactive oxygen species (ROS) promoted the degradation of 2,4-D and its intermediates, eventually yielding rather high mineralization extents, while their occurrence in the system was confirmed through tests with common scavenging agents (e.g., tert-butanol (TB) for •OH, formic acid (FA) for h⁺, and p-benzoquinone (BQ) for O₂^●‒) [24].

Figure 2.

Photocatalytic degradation mechanism over TiO₂/WO₃ composite.

The same composite type was used in the degradation of pharmaceuticals. Hence, Mugunthan et al. [30] treated diclofenac (DCF) while using TiO₂/WO₃ composites under 4 hrs of visible light irradiation and reported a maximum of 92% mineralization of overall organic content. They also elucidated the DCF degradation pathway by LC/MS measurements, which included C-N cleavage in the DCF molecule forming benzene-ring based intermediates at the first stage, and open-ring intermediates at the later stage, which were eventually mineralized. Such findings were quite similar to other studies employing •OH based processes in the degradation of DCF ([33,34]), thus implying the important role of formed ROS, primarily •OH, in the case of TiO₂/WO₃ solar driven photocatalysis as well. Arce-Sarria et al. [35] studied the performance of TiO₂/WO₃ composite for the degradation of another pharmaceutical, Amoxicillin (AMX), in pilot scale reactor, where they achieved 64.4% degradation.

Besides “pure” TiO₂/WO₃ composite, several authors studied the performance of its enriched analogues (Table 1). Hence, Cordero-García et al. [32] studied DCF degradation by WO₃/C-doped TiO₂ and reported 100% DCF degradation and 82.4% mineralization of the overall organic content under 250 kJ/m² and 400 kJ/m² of solar-accumulated energy, respectively. They also stated that the WO₃/C-doped TiO₂ composite showed superior photocatalytic activity for the complete degradation and mineralization of DCF when comparing to the pristine TiO₂, used as benchmark material. Based on the findings on elucidated mechanisms within the studied composite and DCF degradation pathway provided, the authors concluded that the incorporation of elemental carbon to TiO₂ crystal structure promoted the formation of a C2p-hybridized valence band that lowered the band gap of TiO₂ by mixing with O2p orbitals. As a result, upon visible light irradiation, TiO₂ generates e⁻/h⁺ pairs, where the photogenerated e⁻ are promoted to the Ti 3d states (VB), thus reducing Ti⁴⁺ to Ti³⁺. Ti³⁺ can be easily oxidized by WO₃ due to the differences in the reduction potential between TiO₂ (−0.70 V vs NHE) and WO₃ (−0.03 V vs NHE). Subsequently, W⁶⁺ traps photogenerated e⁻ to form its reduced state W⁵⁺, while the redox reaction occurs further by returning to its original oxidation state in reaction with adsorbed O₂ on the composite catalyst surface (similarly as discussed above in the case of “pure” TiO₂/WO₃), thus leading to improved charge separation and the formation of ROS, which contributed in DCF degradation and mineralization of formed intermediates. The same authors studied the degradation of DCF with another enhanced WO₃/TiO₂ composite (N-doped TiO₂), and again reported high degradation and mineralization rates; 100% according to both indicators under 250 kJ/m² and 400 kJ/m² of solar-accumulated energy, respectively [31]. They stressed that the same mechanism that was responsible for the enhancement of photocatalyst activity in C-doped WO₃/TiO₂ composite [32] also improved the performance of N-doped WO₃/TiO₂ [31].

2.1.2. TiO₂/Fe₂O₃

Iron oxide (α-Fe₂O₃) is a promising candidate for photocatalytic applications, due to its abundance, nontoxicity, low cost, stability in aqueous solutions (pH > 3), and narrow band gap (2.0–2.2 eV), which directly matches the band positions of TiO₂ to form heterojunction (Type I Heterojunction) [23,36].

Several authors report the photocatalytic degradation of CECs using TiO₂/Fe₂O₃ composites (Table 2). Hence, Mirmasoomi et al. [37] used TiO₂/Fe₂O₃ as a catalyst for photocatalytic degradation of Diazinon (DZ). The authors reported an optimized system with maximum degradation of DZ equal to 95.07% within 45 min. under visible light irradiation. In another study by Moniz et al. [23], photocatalytic degradation of 2,4-D while using TiO₂/Fe₂O₃ composites was investigated, reporting 100% 2,4-D degradation and 100% mineralization of overall organic content within 2 h and 3 h, respectively, using irradiation from a 300 W Xenon Lamp. The authors found out that, when compared to the benchmark TiO₂ (P25), the TiO₂/Fe₂O₃ composite shows superior photocatalytic activity. Based on photoluminescence and photocurrent studies, the TiO₂/Fe₂O₃ composite exhibits enhanced separation of e⁻/h⁺ pairs due to the formed heterojunction. The proposed mechanism was supported with DFT studies, which firstly involved the transfer of photogenerated e⁻ from TiO₂ CB to Fe₂O₃ CB. In addition, Fe₂O₃ binds strongly with (dissolved) oxygen, thus aiding the photoelectron transfer. This in-situ second stage mechanism facilitates the facile migration of h⁺ from the VB of TiO₂ [23]. Macías et al. [24] studied the same system, TiO₂/Fe₂O₃ composites for photocatalytic degradation of 2,4-D, but while using natural sunlight. The authors reported 96.8% 2,4-D degradation and 90.0% mineralization of overall organic content under two hours and four hours, respectively. Contrary to the presented mechanism of Moniz et al. [23], Macias et al. [24] proposed that the incorporation of Fe₂O₃ causes the photogenerated e⁻ in CB of TiO₂ to be transferred to CB of Fe₂O₃, promoting the reduction of Fe³⁺ to Fe²⁺. Photogenerated h⁺ in VB of TiO₂ are transferred to VB of Fe₂O₃, which leads to the regeneration of Fe³⁺ and avoids the recombination of e⁻/h⁺ pairs at TiO₂ surface. In addition, Fe₂O₃ (E_g = 2.2 eV) can be excited by visible light irradiation producing photogenerated e⁻/h⁺ pairs. Photogenerated e⁻ in CB of Fe₂O₃ can be transferred to O₂ dissolved in water to form O₂^●‒, while photogenerated h⁺ in VB of Fe₂O₃ can facilitate generation of •OH eventually contributing to the degradation of present organics [24] (Figure 3). The formation of mentioned ROS and their involvement in degradation of targeted pollutant was confirmed through common scavenging tests using TB, FA, and BQ.

Table 2.

Photocatalytic degradation of CEC’s over TiO₂/Fe₂O₃ composites.

Catalyst	Target Pollutant	Initial Concentration/Working Volume ((mg L−1) /mL)	Experimental Conditions	Reaction Time	Removal Extent (%)	Reference
TiO2/Fe2O3 (0.1 g/L )	Diazinon	10 (in 300 mL)	Light Source: 60 W Philips Visible lamp; pH = natural	45 min	95.07	[37]
TiO2/Fe2O3 (10 mg)	2,4-dichlorophenoxy acetic acid	50 (in 100 mL)	Light Source: 300 W Xenon Lamp; pH = natural	120 min	100 (TOC = 100 @ 150 min.)	[23]
TiO2/Fe2O3 (0.5 g/L )	2,4-dichlorophenoxy acetic acid	50 (in 250 mL)	Light Source: natural sunlight 11AM-4PM; pH = 4	120 min	96.8 (TOC =90 @ 240 min.)	[24]
TiO2/Fe2O3 (70 mg)	Oxytetracycline Hydrochloride	60 (in 70 mL)	Light Source: 300 W Iodine Tungsten Lamp; pH = 5.5	300 min	75.6	[38]
TiO2/Fe2O3 (1.0 g/L)	Oxytetracycline	60	Light Source: 300 W Iodine Tungsten Lamp; pH = 5.5	300 min	~80	[39]
TiO2/Fe2O3/CNT (100 mg)	Tetracycline	20 (in 100mL)	Light Source: 300 W Xenon Lamp; pH = natural	90 min	89.41	[40]
TiO2-coated α-Fe2O3 core-shell (100 mg)	Tetracycline Hydrochloride	50 (in 200 mL)	Light Source: 300 W Xenon Lamp (λ > 420 nm) ; pH = 5. 45 Oxidant = 120 µL (30% H2O2)	90 min	100	[41]

Figure 3.

Photocatalytic degradation mechanism over TiO₂/Fe₂O₃ composite.

The photocatalytic degradation of the pharmaceutical tetracycline (TC) and its derivatives, such as oxytetracycline (OTC), using TiO₂/Fe₂O₃ materials has also been reported. Hence, it was found out that, under visible light irradiation (λ = 400–750 nm), α-Fe₂O₃ was activated and generated e⁻/h⁺ pairs, and then photogenerated e⁻ from CB of α-Fe₂O₃ moved to TiO₂ trapping sites for atmospheric O₂ to form O₂^●‒, which was proven to largely contribute to the degradation of OTC. On the other hand, the photogenerated h⁺ from VB of α-Fe₂O₃ primarily reacted with OH⁻, resulting in the generation of •OH, which also contributed to the degradation of OTC. When compared to TiO₂ reference material, the TiO₂/Fe₂O₃ composite exhibited an enhanced photocatalytic activity under visible light due to efficient e⁻/h⁺ separation, as stated above [38]. The same authors [38,39] also studied the degradation mechanism of OTC while using LC/MS TOF analysis and, based on the formed intermediates, established the OTC degradation pathway, and concluded that •OH mainly mediated degradation. Besides, “pure” TiO₂/Fe₂O₃, enriched analogue with carbon nanotubes (CNTs) was also studied (Table 2). Hence, TiO₂/Fe₂O₃/CNTs was used as the catalyst for photocatalytic degradation of TC, under visible light illumination [40]. It was found that the effectiveness of photocatalytic degradation of TC within 90 min. treatment using TiO₂/Fe₂O₃/CNTs was almost twice higher when comparing to that achieved by benchmark TiO₂; 89.41% and 47.64%, respectively. The authors attributed the improved photocatalytic efficiency to the presence of the CNT, which acted as a photogenerated e⁻ acceptor, thereby suppressing e⁻/h⁺ recombination [40]. In another study, the core-shell structured α-Fe₂O₃ (with TiO₂ shell of around 15 nm) exhibited 100% TC removal in 90 min. [41]. The degradation improvement was ascribed to the addition of H₂O₂ in the system, which generated more ROS than by the common photocatalytic mechanisms described above [41]. Hence, the contribution of H₂O₂ in such a system can be described through restraining e⁻/h⁺ recombination and increasing HO• generation in the system, as in Equation (1) [15]:

$${\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}\to \mathrm{HO}•+{\mathrm{H}}_2\mathrm{O}$$ (1)

2.1.3. TiO₂/Spinel Ferrite

Spinel ferrites (MFe₂O₄) are metal oxides, where M is a divalent ion (i.e., Mg²⁺, Ca²⁺, Sr²⁺, Ni²⁺, Zn²⁺, etc.), serving as promising candidates for photocatalytic applications due to their narrow band gap range (1.3–2.2 eV) and magnetic properties [42,43]. Spinel ferrites band positions match TiO₂, thus possessing compatibility to form a heterojunction (Type II Heterojunction) [44,45,46,47].

The literature provides applications of MFe₂O₄/TiO₂ materials as photocatalysts in treatment of CECs, as in the case of previously discussed TiO₂-based composites, however, it should be noted that authors within such composites used modified TiO₂ (Table 3). Hence, Chen et al. [44] studied photocatalytic degradation of TC and its derivatives using N-doped TiO₂/CaFe₂O₄/diatomite, and reported 91.7% removal of TC within 150 min. under visible light irradiation. The authors studied the composite stability and reusability; the results obtained after five cycles indicates that employed composite is rather stable, enabling 89.2% removal of TC. They also proposed the photocatalytic mechanism occurring within the composite; the excitation of both N-TiO₂ and CaFe₂O₄ by visible light leads to the formation of e⁻/h⁺ pairs (Figure 4). The photogenerated e⁻ in CB of N-TiO₂ can directly react to adsorbed O₂ generating O₂^●‒, while photogenerated h⁺ in VB of N-TiO₂ directly react with H₂O and OH⁻ producing •OH. Simultaneously, photogenerated e⁻ in CB of CaFe₂O₄ can undergo the same mechanism (i.e., reaction with O₂ to produce O₂^●‒). In addition, the formed heterojunction helps the migration of e⁻ from CB of CaFe₂O₄ to CB of N-TiO₂, and h⁺ from VB of N-TiO₂ to VB of CaFe₂O₄ (Figure 4). Such a transfer of charge carriers between the two semiconductors hinders the recombination process and enhances the photocatalytic activity of the composite, thus leading to more efficient generation of ROS (O₂^●‒ and •OH) [44]; the existence of formed ROS was confirmed through scavenging tests while using isopropyl alcohol (IPA), ammonium oxalate (AO), and BQ for •OH, h⁺ and O₂^●‒, respectively. Such behavior is confirmed by studying the degradation pathway of TC; it was found that the TC intermediates match those that formed through radical driven reactions undergoing in the first step demethylation and hydroxylation. The second step considered the removal of functional groups (amino, hydroxyl, and methyl) and further ring opening reactions that are mainly mediated by photogenerated h⁺, yielding small fragments that were eventually mineralized [44]. Such a pathway confirmed the dual role of photogenerated h⁺, as a promotor •OH generation and as sites for the direct oxidation of adsorbed organics. There are several other studies investigating the application of different MFe₂O₄/TiO₂ materials (N-doped TiO₂/SrFe₂O₄ diatomite [46]; Ce/N-co-doped TiO₂/NiFe₂O₄/ diatomite and ZnFe₂O₄/TiO₂ [47]) for the photocatalytic degradation of CECs, such as TC, OTC, and bisphenol A (BPA). Interestingly, the same mechanisms responsible for charge transfer and consequent generation of ROS were reported, regardless of the different M type within the spinel ferrite part of composite and/or TiO₂ (non-doped or doped with metal and/or non-metal ions).

Table 3.

Photocatalytic degradation of CEC’s over TiO₂/MFe₂O₄ composites.

Catalyst	Target Pollutant	Initial Concentration/ Working Volume ((mg L−1) /mL)	Experimental Conditions	Reaction Time	Removal Extent (%)	reference
N-TiO2/ CaFe2O4 /diatomite (2.0 g/L)	Tetracycline	10 (in 200 mL)	Light Source: 150 W Xenon Lamp with UV light filter	150 min	91.7 (TOC =~80 @ 2h)	[44]
N-TiO2/ SrFe2O4 /diatomite (2.0 g/L)	Tetracycline	10 (in 200 mL)	Light Source: 150 W Xenon Lamp with UV light filter	150 min	92 (TOC = ~80 @ 2h)	[46]
Ce/N co-doped TiO2 / NiFe2O4 diatomite (0.5 g/L)	Tetracycline	20 (in 200 mL)	Light Source: 150 W Xenon Lamp with UV light filter	180 min	98.2 (TOC = ~95)	[45]
ZnFe2O4 / TiO2 (1.0 g/L)	Bisphenol A	10 (in 200 mL)	Light Source: 300 W Xenon Lamp pH= 7	30 min	100	[47]

Figure 4.

Proposed mechanism for the tetracycline (TC) photodegradation process using N-doped TiO₂/CaFe₂O₄/ diatomite [44].

2.1.4. TiO₂/Cu₂O

Cu₂O, a p-type semiconductor (E_g = 2.0–2.2 eV), is also a good candidate for making heterojunctions with TiO₂ (Type II Heterojunction). Hence, the photocatalytic degradation of various CECs (TC [48], and tetrabromodiphenyl ethers [49]) using TiO₂/Cu₂O composite materials was reported under solar light irradiation (Table 4). Based on the findings in the mentioned studies, the photocatalytic mechanism of TiO₂/Cu₂O under solar light illumination involves the activation of both Cu₂O and TiO₂ to generate e⁻/h⁺ pairs (Figure 5). Photogenerated e⁻ in CB of Cu₂O then can migrate to CB of TiO₂ and, along with photogenerated e⁻ in CB of TiO₂, react with O₂ to form O₂•^_. Simultaneously, photogenerated h⁺ in VB of Cu₂O can be directly involved in the oxidation of adsorbed organics, while photogenerated h⁺ in VB of TiO₂ can directly oxidize adsorbed organics or react with H₂O (i.e., OH⁻) and generate •OH. Besides, these h⁺ can also directly migrate to the VB of Cu₂O, thus leading to effective charge separation that improves the overall photocatalytic activity of the composite [48].

Table 4.

Photocatalytic degradation of CEC’s over TiO₂/Cu₂O composites.

Catalyst	Target Pollutant	Initial Concentration/ Working Volume ((mg L−1) /mL)	Experimental Conditions	Reaction Time	Removal Extent (%)	Reference
Cu2O-TiO2 supported palygorskite (1.0 g/L)	Tetracycline Hydrochloride	30 (in 50mL)	Light Source: 500 Xe Lamp; pH = 8.7	240 min	88.81	[48]
TiO2-Cu2O film	Tetrabromodiphenyl Ethers	5 (in 100 mL)	Light Source: 300 W Xenon Lamp; pH = natural solvent CH3OH:H2O (50:50 v/v)	150 min	90	[49]

Figure 5.

Photocatalytic degradation mechanism over TiO₂/Cu₂O composite.

2.1.5. TiO₂/Bi₂O₃

Bi₂O₃, a semiconductor with band gap range in the visible region (2.5–2.8 eV), is also a good candidate for making heterojunctions with TiO₂ (Type II Heterojunction). Studies including its application in photocatalytic degradation of CECs (quinalphos [50] and ofloxacin [51]) under solar light irradiation (Table 5) revealed the occurring photocatalytic mechanism. Both of the composite phases can be activated under solar irradiation generating e⁻/h⁺ pairs (Figure 6). Accordingly, photogenerated h⁺ in VB of TiO₂ are involved in the production of •OH (through reactions with H₂O, i.e., OH⁻) as of e⁻/h⁺ pairs. In addition, h⁺ in VB of Bi₂O₃ can be transferred to VB of TiO₂ that contributes to the direct oxidation of adsorbed organics or the generation of •OH [51].

Table 5.

Photocatalytic degradation of CEC’s over TiO₂/Bi₂O₃ composites.

Catalyst	Target Pollutant	Initial Concentration/ Working Volume ((mg L−1) /mL)	Experimental Conditions	Reaction Time	Removal Extent (%)	Reference
Bi2O3–TiO2 (50 mg)	Quinalphos	25 (in 50 mL)	Light Source: Visible light with 1.56µmol/m2/s; pH = 8	100 min	92	[50]
Bi2O3–TiO2 (0.5 g/L)	Ofloxacin	25	Light Source: 70.3 K lux; pH = 7	120 min	92	[51]

Figure 6.

Photocatalytic degradation mechanism over TiO₂/Bi₂O₃ composite.

2.2. Coupling of TiO₂ with Metal Sulfides

Cadmium sulfide (CdS), a metal sulfide semiconductor with a visible light range band gap (E_g = 2.1–2.4 eV), has been proven to be compatible with TiO₂, due to its higher position of CB than that of TiO₂ (Type II Heterojunction) (Figure 7) [25,52]. However, one should be aware that its application can lead to adverse effects due to its instability, resulting in the leaching of toxic Cd²⁺ during treatment [53]. Although its CB and VB positions are thermodynamically favorable for photocatalytic application, CdS as a photocatalytic material faces serious problems. Next to the above-mentioned promotion of toxic effects, issues, like poor stability due to photocorrosion and limited separation efficiency of photogenerated charge carriers, do not speak in favor of CdS application [54,55]. Photocorrosion is not only related to the photogenerated h⁺ in semiconductor itself that oxidizes S^2– and release Cd²⁺ to the solution, but also with newly formed O₂, where higher solubility in water leads to more dramatic levels of photocorrosion of CdS [54,56]. However, CdS was widely investigated in photocatalytic purposes, even in recent studies that focused on the degradation of CECs (ofloxacin, ciprofloxacin, tetracycline, and 17α-ethynylestradiol), where it was used in various forms (nano-rods, nano-belts) [25,52,57,58] (Table 6). Generally, upon visible light illumination, CdS is excited and generates the e⁻/h⁺ pair, where photogenerated e⁻ in CB of CdS migrates to CB of TiO₂ and is consumed in reactions with O₂ to produce O₂^●‒, while h⁺ remain in the VB of CdS.

Figure 7.

General photocatalytic degradation mechanism over TiO₂/MS (M = Cd or Cu) or MS₂ (M = Mo and Sn) composite.

Table 6.

Photocatalytic degradation of CEC’s over TiO₂ /Metal sulfide composites.

Catalyst	Target Pollutant	Initial Concentration/ Working Volume ((mg L−1)/mL)	Experimental Conditions	Reaction Time	Removal Extent (%)	Reference
CdS –TiO2 (50 mg)	Tetracycline Hydrochloride	50 (in 50mL)	Light Source: 500 W Xenon Lamp with filter (λ > 400 nm); pH = natural	480 min	87.0	[58]
Au-CdS/TiO2 nanowire (20 mg)	Ciprofloxacin	20	Average solar light intensity = 100, 000	60 min	99	[57]
CdS/TiO2 (450 mg)	Ofloxacin	10 (in 100mL)	Light Source: 85 W Oreva bulb with 4150 lumens (λ = 450-650 nm); pH = natural	180 min	86	[52]
CdS nano-rod/TiO2 nano-belt ( 0.50 g/L)	17α-ethynylestradiol	3 (in 10 mL)	Light Source: 500 W Xenon Lamp with filter (λ > 420 nm); pH = natural	120 min	92	[25]
CuS/TiO2 nanobelts	Enrofloxacin	5 (in 35 mL)	Light Source: 35 W Xenon Lamp; pH = natural	120 min	85.5 (TOC = 27.7)	[59]
Au-CuS-TiO2 nanobelts	Oxytetracycline	5 ( in 35 mL)	Light Source: 35 W Xenon Lamp; pH = natural	60 min	96 (TOC = 68)	[60]
MoS2 /TiO2 (25 mg/L)	Acetaminophen	302	Light Source: Sunlight; pH = natural	25 min	40	[64]
N,S co-doped TiO2 @MoS2 (0.98g/L)	Diclofenac	0.15 ( in 100 mL)	Light Source: 60 W LED lamp; pH = 5.5	150 min	98	[61]
TiO2/SnS2 films	17β-estradiol	1.36 (in 90 mL)	Light Source: 450 W Xenon Arc Lamp	90 min	51.0	[66]
TiO2/SnS2 films	Diclofenac	31.8 ( in 90mL)	Light Source: 450 W Xenon Arc Lamp; pH = 4	60 min	76.21	[67]

Copper sulfide (CuS), which is another metal sulfide semiconductor with narrow band gap of 2.0 eV, has also been reported to be coupled with TiO₂ (Type II Heterojunction) [59,60]. Jiang et al. [59] reported a 85.5% degradation of enrofloxacin and 27.7% mineralization of overall organic content using immobilized CuS/TiO₂ nanobelts (Table 6). They elucidated the mechanisms occurring in the composite upon excitation by solar irradiation. Hence, such broad wavelengths excited both composite phases (CuS and TiO₂) and resulted in e⁻/h⁺ pairs, while the transfer of charges was analogous, as in the case of the CdS/TiO₂ composite. Photogenerated e⁻ in CB (−0.33 eV) of CuS underwent transfer to CB (−0.19 eV) of TiO₂ and were consumed in reactions with O₂ forming O₂^●‒. Photogenerated h⁺ in VB of CuS remained there and present potential active sites for direct degradation of organics that were adsorbed at the CuS surface, since they cannot be involved in generation of •OH due to too high energy band positioning. On the other hand, photogenerated h⁺ in VB of TiO₂ can directly react with adsorbed organics and OH⁻ generating •OH. Chen et.al [60], incorporated Au nanoparticles to CuS/TiO₂ nanobelts structure to enhance the photocatalytic degradation ability of the composite by capturing e⁻ and, consequently, suppressing the recombination of photogenerated charges. As a result, they obtained 96% degradation of OTC and 68% mineralization of the overall organic content within one hour under artificial sunlight illumination. Accordingly, the mechanism of such enriched CuS/TiO₂ composite involves, besides the above discussed mechanism, the path considering the transfer of e⁻ to Au, which leads to enhanced charge separation, thus delaying recombination. In such a case scenario, photogenerated h⁺ would have higher probability to react either with adsorbed organics or with HO^‒ in order to generate •OH (exclusively those in VB of TiO₂), thus contributing to the overall system efficiency. The involvement of formed ROS into reaction mechanisms for OTC degradation was confirmed by scavenging tests using TB, AO, and BQ.

Molybdenum disulfide (MoS₂), a two-dimensional (2D) layered metal chalcogenide with an indirect band gap of 1.1 eV and 1.9 eV direct band gap in monolayered form, with unique structure, low-cost, high thermal stability, and electrostatic integrity, is also a suitable candidate for forming heterojunction with TiO₂ (Type II Heterojunction) [61,62,63]. Hence, Kumar et al. [64] reported its application in the photocatalytic degradation of paracetamol. Furthermore, Irandost et al. [61] applied the modified MoS₂/TiO₂ composite (they used N,S-co-doped TiO₂) in the photocatalytic degradation of DCF under visible LED lamp irradiation (Table 6). Hence, the synergistic effect of dopants in TiO₂ promoted its visible light activity, yielding the formation of e⁻/h⁺ pairs in both composite phases. The mechanism of charge formation and consequent transfer was similar, as described above for CuS/TiO₂, which was excited by solar irradiation. Hence, photogenerated e⁻ in CB of N,S-co doped TiO₂ and CB of MoS₂ were able to undergo reactions with O₂ forming O₂^●‒, while h⁺ in VB of TiO₂ promoted •OH formation in reactions with HO^‒ and provide the direct oxidation of adsorbed organics, while, again, h⁺ in MoS₂ were able to do only the latter. The importance of •OH and h⁺ in DCF degradation was confirmed by trapping agents used in scavenging tests: TB and potassium iodide (KI), respectively.

Tin sulfide (SnS₂), which is a metal sulfide semiconductor with band gap of 2.2 eV [65], has also been reported to be coupled with TiO₂ (Type II Heterojunction) [66,67]. Hence, Kovačić et al. [66] reported improved the degradation of 17β-estradiol (E2), for 51%, using SnS₂/TiO₂ when comparing to the benchmark material (P25) TiO₂ under solar irradiation. A similar improvement was obtained by comparing performances of the same materials in the case of DCF degradation [67] (Table 6). The reason for such improvement relies on the potential of photogenerated e⁻ in CB of SnS₂ to migrate to CB of TiO₂, while h⁺ remained at the VB of SnS₂. In such case, the efficient separation of charges is achieved, thus facilitating the improved redox reactions, enabling effective degradation of adsorbed organics directly on the surface by h⁺, in spite of the limited ability of such a composite to generate •OH. Accordingly, the adsorption has been shown as an important step in the effectiveness of the SnS₂/TiO₂ composite. Kovačić et al. [67] utilized DFT calculations to study the surface interaction of polar compounds (DCF) and non-polar compounds (memantine) at SnS₂/TiO₂ composite and found that DCF was more efficiently degraded due to much higher adsorption ability in comparison to memantine, which is one of its structure feature limitations (amine functionality).

2.3. Coupling of TiO₂ with Silver- Based Semiconductors

Silver Phosphate (Ag₃PO₄), a promising semiconductor with narrow band gap (E_g ≥ 2.4 eV), showed good photocatalytic performance in the degradation of organic pollutants under visible light irradiation [68,69]. Namely, Ag₃PO₄ exhibits a quantum efficiency of up to 90% [68] and it can absorb wavelengths that are shorter than ~530 nm [69]. Despite the qualities of Ag₃PO₄ as a potential photocatalyst, it still suffers from limitations, such as photocorrosion, small but not negligible solubility in water (K_sp = 1.6 × 10⁻¹⁶), and particle agglomeration upon synthesis [70]. To overcome these limitations, constructing a heterojunction between Ag₃PO₄ and a compatible semiconductor has attracted attention due to the increase in charge separation and production of more ROS [71]. The positions of VB and CB in TiO₂ directly match the Ag₃PO₄ band positions, thus providing the compatibility to form a heterojunction.

Hence, Wang et al. [72] investigated the performance of TiO₂ nanotubes/Ag₃PO₄ quantum dots for the degradation of TC under visible light illumination, and reported a high removal rate within a short treatment period; 90% TC removal within 8 min (Table 7).

Table 7.

Photocatalytic degradation of CEC’s over TiO₂/Silver-Based Semiconductor composites.

Catalyst	Target Pollutant	Initial Concentration/ Working Volume ((mg L−1)/mL)	Experimental Conditions	Reaction Time	Removal Extent (%)	Reference
Ti3+ -doped TiO2 nanotubes/ Ag3PO4 quantum dots (0.5 g/L)	Tetracycline	10 (NA)	Light Source: 400 W Xenon Lamp; pH = natural	8 min	90	[72]
TiO2 nanotube/ Ag3PO4 nanoparticles (40 mg)	Ciprofloxacin	10 (in 40 mL)	Light Source: 300 W Xenon Lamp	60 min	85.3	[73]
TiO2-x / Ag3PO4 (100 mg)	Bisphenol A	10 (in 100 mL)	Light Source: 500 W Xenon Lamp with filter (λ = 420 nm); pH = natural	16 min	95	[74]
Ag2O/ TiO2 quantum dots (0.25 g/L)	Levofloxacin	10 (in 100 mL)	Light Source: 85 W Oreva CFL (4150 lumens) (λ = 380–700 nm) pH=4	90 min	81	[27]
Ag2O /TiO2 –zeolite (50 mg)	Norfloxacin	5 (in 100 mL)	Light Source: 35 W Xenon Lamp	60 min	98.7 (TOC = 83.1)	[28]

The conventional heterojunction transfer mechanism (Figure 8a) explains that the photogenerated h⁺ in the composite would be promoted from the VB of Ag₃PO₄ to VB of Ti³⁺-doped TiO₂ nanotubes, where can react with H₂O or HO⁻ forming •OH. Simultaneously, photogenerated e⁻ from the Ti³⁺-doped TiO₂ nanotubes CB can react with O₂ forming O₂^●‒ or may transfer to the CB of Ag₃PO₄. However, O₂^●‒ are not formed in Ag₃PO₄, due to the fact that the position of its CB is lower than the standard reduction potential of O₂^●‒/O₂. Wang et al. [72] concluded that TC was primarily degraded by O₂^●− and photogenerated h⁺ based on the results of the conducted electron trapping experiments. Accordingly, they have extended the study by proposing a Z-scheme heterojunction transfer mechanism (Figure 8b). Under this mechanism, Ag(0) acts a recombination center, “collecting” photogenerated e⁻ from CB of Ag₃PO₄, where they undergo recombination with the photogenerated h⁺ from VB of Ti³⁺-doped TiO₂ nanotubes. In such case, photogenerated h⁺ on VB of Ag₃PO₄ might participate in the direct oxidation reactions with adsorbed organics, while the photogenerated e⁻ in the CB of Ti³⁺-doped TiO₂ nanotubes can be involved in forming desired ROS, O₂^●‒, thus contributing to the enhanced performance of composite photocatalyst. Du et al. [73] applied analogue TiO₂/Ag₃PO₄ composite employing TiO₂ nanotube arrays for the degradation of ciprofloxacin (CIP) under solar irradiation and reported that 85.3% removal of CIP within 60 min. was facilitated through the above-mentioned mechanisms. Furthermore, Liu et al. [74] reported 95% degradation of BPA in 16 min. using TiO_2−X/Ag₃PO₄ under visible light irradiation (Table 7). They reported that both composite phases, TiO_2−X and Ag₃PO₄, were excited and generated e⁻/h⁺ pairs. Hence, photogenerated h⁺ in VB of TiO_2−X are promoted to VB of Ag₃PO₄ and contributed to the direct oxidation of adsorbed organics, similarly as reported in the study by Wang et al. [72]. Photogenerated e⁻ from the CB of Ag₃PO₄ are transferred to oxygen vacancies (V_o) of TiO₂ and contributed in reactions with adsorbed O₂ generating O₂^●‒ (Figure 9). They also investigated the role of these species in the degradation of BPA and found, based on monitoring BPA degradation pathway by LC/MS analysis, that intermediates are formed through two pathways: 1) hydroxylation, through reactions with O₂^●‒ yielding BPA-o-catechol; and, 2) direct oxidation by h⁺ forming isopropenylphenol and phenol, which was further oxidized by h⁺ yielding hydroquinone and its dehydrated form benzoquinone.

Figure 8.

Photocatalytic mechanisms Ti³⁺-TNTs/Ag₃PO₄ (a) conventional heterojunction, and (b) Z-scheme heterojunction.

Figure 9.

Photocatalytic reaction mechanism of TiO_2−X/Ag₃PO₄ under visible light irradiation.

Silver oxide (Ag₂O), a visible light active photocatalyst with band gap of 1.2 eV, is another silver-based compound with semiconducting properties. Based on the band positions (VB and CB), it represents a promising matching candidate to form heterojunctions with TiO₂ (Type III Heterojunction). Hence, photocatalytic degradation of levofloxacin (LEV) using Ag₂O/TiO₂ quantum dots is reported with the maximum of 81% LEV degradation within 90 min. of visible light irradiation [27]. Based on the proposed mechanism under visible light illumination (Figure 10), upon excitation of Ag₂O, e⁻/h⁺ pairs are formed, whereas TiO₂ is not activated due to its wide band gap. Photogenerated e⁻ in the CB of Ag₂O were transferred to CB of TiO₂ and involved in reactions with adsorbed O₂ forming O₂^●‒ that participated in LEV degradation. In addition, photogenerated h⁺ in VB of Ag₂O yielded the formation of •OH, through reactions with OH⁻, and participated in LEV degradation as well. The authors employed LC-MS analysis to elucidate LEV degradation pathway and, as such, establish the role of formed ROS. Hence, parent compound LEV underwent decarboxylation of the acetyl group; hydroxylation resulting in the formation of quinolone moieties; demethylation and the subsequent addition of hydrogen atom generating modifications at piperazine ring; while successive •OH attack resulted in multi-hydroxylated intermediates. Such findings confirmed the dominant role of •OH in LEV degradation.

Figure 10.

Photocatalytic degradation mechanism over TiO₂/Ag₂O composite.

In another study, Gou et al. [28] investigated the application of Ag₂O/TiO₂/zeolite composite for solar-driven degradation of norfloxacin (NOR) (Table 7). Besides high effectiveness (98.7% NOR degradation and 83.1% mineralization of organic content within 60 min. treatment), they elucidated the NOR degradation pathway, involving in the initial stage decarboxylation, defluorination or hydroxylation of parent compound (NOR), which confirmed the involvement of both formed ROS (O₂^●‒ and •OH).

2.4. Coupling of TiO₂ with Graphene and Graphene-Like Materials

2.4.1. TiO₂/Graphene Composites

Graphene is a zero bandgap semiconductor with a sheet-like structure (i.e., it is considered as a 2D monolayer material) consisting of sp² hybridized carbon atoms with excellent thermal conductivity, optical transmittance, high mechanical strength, large surface area (2600 m²/g), and appreciable charge carrier transport [75]. Under light illumination, it can achieve a reverse saturation state with high density (~ 10¹³ cm²) of hot electrons above the Fermi level, which can be used as a powerful agent in redox reactions [76]. It was also found that the incorporation of graphene-based materials (i.e., graphene oxide and its reduced form; GO and rGO, respectively) with TiO₂ might suppress e⁻/h⁺ pairs recombination. As such, TiO₂/graphene-based composites were employed in the photocatalytic degradation of CECs (Table 8).

Table 8.

Photocatalytic degradation of CEC’s over TiO₂/Semiconductor/graphene composites.

Catalyst	Target Pollutant	Initial Concentration/ Working Volume ((mg L−1)/mL)	Experimental Conditions	Reaction Time	Removal Extent (%)	Reference
TiO2/ WO3/GO (2 mg)	Bisphenol A	20 ( in 50 mL)	Light Source: sunlight Ph = 7	7 h	93.2	[77]
Graphene-WO3 /TiO2 nanotube (photoelectrodes )	Dimethyl Phthalate	10 (in 40 mL)	Light Source: 150W Xe lamps	120 min	75.9	[78]
TiO2 /ZnO/GO (0.5 g/L)	Bisphenol A	10 (in 50 mL)	Light Source: 18 UV lamps (λ =365 nm) ;Visible metal halide lamps(λ = 400–800 nm) pH = 6	120 min. (UV) 180 min. (Vis)	99.7 (UV) 94.9 (Vis)	[79]
TiO2 /ZnO/GO (0.5 g/L)	Ibuprofen	10 (in 50 mL)	Light Source: 18 UV lamps (λ = 365 nm) ;Visible metal halide lamps(λ = 400–800 nm) pH = 6	120 min. (UV) 180 min. (Vis)	98.5 (UV) 79.6 (Vis)	[79]
TiO2 /ZnO/GO (0.5 g/L)	Flurbiprofen	10 ( in 50 mL)	Light Source: 18 UV lamps (λ=365 nm) ;Visible metal halide lamps(λ= 400–800 nm) pH= 6	120 min. (UV) 180 min. (Vis)	98.1(UV) 82.2 (Vis)	[79]
ZnFe2O4/rGO/TiO2 (0.1 g)	Fulvic Acid	20 (in 50 mL)	Light Source: 300 W (λ=420 nm); Vol H2O2 = 0.8 mL, pH= 7	180 min	95.4%	[80]
TiO2 /MoS2 /rGO (0.5 g/L)	Bisphenol A	10 (in 50 mL)	Light Source: 20 W (λ = 254 nm);	300 min	62.4	[81]
TiO2/BiVO4/rGO	Tetracycline	10 µg/L (NA)	Light Source: 1000 W Xe Lamp (λ = 420 nm) with filter	120 min	96.2	[82]
TiO2/BiVO4/rGO	Chlorotetracycline	10 µg/L (NA)	Light Source: 1000 W Xe Lamp (λ = 420 nm) with filter	120 min	97.5	[82]
TiO2/BiVO4/rGO	Oxytetracycline	10 µg/L (NA)	Light Source: 1000 W Xe Lamp (λ = 420 nm) with filter	120 min	98.7	[82]
TiO2/BiVO4/rGO	Doxycycline	10 µg/L (NA)	Light Source: 1000 W Xe Lamp (λ = 420 nm) with filter	120 min	99.6	[82]

2.4.2. TiO₂/Semiconductor/Graphene Composites

As described in above sections, the coupling of TiO₂ with other semiconductors promotes efficient charge transfer, eventually yielding improved photocatalytic activity. However, in most cases, the recombination is still an existing issue that needs to be suppressed. Such a double effect can be obtained by combining composite concept involving two semiconductors (even “pure” TiO₂, which cannot be active under visible light) with graphene-based materials. For instance, Hao et al. [77] reported 93.2% degradation of BPA in seven hours of sunlight irradiation while using the TiO₂/WO₃/GO composite. The mechanism occurring in such combined composite involved the excitation of both TiO₂ and WO₃ under solar light irradiation (TiO₂ utilized UV-A fraction), yielding the generation e⁻/h⁺ in both semiconductors. Hence, photogenerated e⁻ in CB of TiO₂ can directly react with absorbed O₂, producing O₂^●−, or it can be transferred to CB of WO₃, and then further migrate to GO enhancing charge separation. Since the amount of adsorbed O₂ is quite limited, the tendency of e⁻ to recombine with h⁺ is rather favored; ~90% of pairs recombine rapidly after excitation [14]. Hence, the charge separation represents an important factor in the evaluation of photocatalyst performance. Accordingly, in the case of effective separation and recombination suppression, as in the case with GO, photogenerated h⁺ in VB of activated composite components, e.g., of TiO₂ and WO₃ in the case of TiO₂/WO₃/GO, can be involved in a larger amount, either directly or indirectly (through formation of •OH) in the degradation of present organics. It should be noted that, in composites with two semiconductors, GO could also act as redox site, attracting photogenerated e⁻ and h⁺, thus promoting improved surface migration of charges [77]. Table 8 summarizes several works regarding TiO₂/semiconductor/GO composites employed for the degradation of CECs with analogous mechanism, as mentioned above.

2.4.3. TiO₂/g-C₃N₄

Graphitic carbon nitride (g-C₃N₄), a two-dimensional, metal-free polymeric π-conjugated semiconductor material, which has attracted a lot of attention [83,84,85,86,87,88,89,90,91] since the pioneering work of Wang et al. [92] in 2009, due to its high stability, visible light response with the bandgap of 2.7 eV and non-toxicity [93], thus representing a viable candidate to be applied in photocatalytic water treatment [80], has certainly been one of the most investigated photocatalysts inside carbon-based nanomaterials. It can be easily synthesized through the direct pyrolysis of nitrogen-rich precursors, such as melamine, cyanamide, dicyandiamide, and urea, but its practical application and principle drawback is low specific surface area and high rate of electron-hole recombination [83,94,95]. Therefore, g-C₃N₄ modification to address shortcomings are needed, e.g., as an excellent candidate to form heterojunction with TiO₂ (Type II Heterojunction), due to their matched band positions (VB and CB). Hence, several studies employing g-C₃N₄/TiO₂ were focused on photocatalytic degradation of CECs (Table 9). For instance, Yang et al. [96] reported 88.1% degradation of CIP within 180 min. under visible light irradiation. The authors ascribed the improved photocatalytic activity to multiple effects: (i) an increase in the surface area of the composite; (ii) good dispersity of TiO₂ in g-C₃N₄ enabling the intimately coupling of composite phases; and, (iii) extension of light absorption of the composite due to low band gap of g-C₃N₄. Trapping experiments that were conducted revealed that photogenerated h⁺ were the major reactive site involved in CIP degradation.

Table 9.

Photocatalytic degradation of CEC’s over TiO₂ /g-C₃N₄ composites.

Catalyst	Target Pollutant	Initial Concentration/ Working Volume ((mg L−1)/mL)	Experimental Conditions	Reaction Time	Removal Extent (%)	Reference
g-C3N4/TiO2 (30 mg)	Ciprofloxacin	10 (in 80 mL)	Light Source: 300 W Xe Lamp with filter (λ > 400 nm) pH = natural	180 min	88.1	[96]
g-C3N4/TiO2 (30 mg)	Acyclovir	10 (in 100 mL)	Light Source: 300 W Xe Lamp with filter (λ > 420 nm) pH = natural	90 min	100	[97]
mpg-C3N4/TiO2 (membrane)	Sulfamethoxazole	10 (in 50 mL)	Light Source: 300 W Xe Lamp pH = natural Flow rate = 13 mL/min. Membrane flux = 918 L /m2 h	1800 min	69	[98]
TiO2@g-C3N4 core-shell (100 mg)	Tetracycline	20 (in 100 mL)	Light Source: Xenon Lamp with full spectrum pH = natural	9 min	(2.2 mg/min.)	[99]
g-C3N4 –shielding polyester/ TiO2 (130 mg)	sulfaquinoxaline	2 × 10−5 mol/L (30 mL)	Light Source: Q-Sun Xe-1 test, pH = 7	90 min	97	[101]
g-C3N4 –shielding polyester/ TiO2 (130 mg)	thiamethoxam	2 × 10−5 mol/L (30 mL)	Light Source: Q-Sun Xe-1 test, pH = 7	180 min	~95	[101]
g-C3N4/TiO2/kaolinite (200 mg)	Ciprofloxacin	10 (in 100 mL)	Light Source: Ave. light intensity =90 mW/cm2 ; Xe Lamp with filter (λ > 400 nm), pH = natural	240 min	92	[100]
S-Ag/ TiO2 @ g-C3N4 (0.20 g/L)	Triclosan	10 (in 100 mL)	Light Source: 250 W Xe Lamp with filter (λ > 420 nm), pH = 7.8	60 min	92.3 (Detoxification Efficiency= 64.3± 3.9)	[102]
Co-TiO2 @g-C3N4 (5 mg ; 2 × 2 cm2 membranes)	Tetracycline Hydrochloride	20 (in 10 mL)	Light Source: 300 W Xe Lamp with filter (λ > 420 nm), pH = 7	60 min.	90.8	[103]
D35-TiO2/g-C3N4 (0.5g/L)	Bisphenol A	10 (in 100 mL)	Light Source: 300 W Metal Halide pH = 7, Oxidant = 2mM Persulfate	15 min	100 (TOC= 50)	[104]
C dots decorated g-C3N4/ TiO2 (1.0 g/L)	Enrofloxacin	4 ( in 50 mL)	Light Source: 350 W Xe Lamp with filter (λ > 420 nm) pH = natural	60 min	91.6	[105]
graphene quantum dots/ Mn-N-TiO2 /g-C3N4 (45 mg)	Ciprofloxacin	10 (in 80 mL)	Light Source: 300 W Xe Lamp (320 ≤λ ≤ 780 nm), pH = 7	120 min	89	[107]
graphene quantum dots/ Mn-N-TiO2 /g-C3N4 (45 mg)	Diethyl Phthalate	10 (in 80mL)	Light Source: 300 W Xe Lamp (320 ≤ λ ≤ 780 nm), pH = 7	120 min	70.4	[107]
MoS2 supported TiO2/g-C3N4 (30 mg)	Atrazine	10 (in 100 mL)	Light Source: 500 W Xe Lamp (λ > 420 nm), pH = 7	300 min	86.5	[108]
WO3–TiO2 @g-C3N4	Acetylsalicylate	10 (in 100 mL)	Light Source: 500 W Metal Halide pH = natural	90 min	98	[109]
WO3–TiO2 @g-C3N4	Methyl-theobromine	10 (in 100 mL)	Light Source: 500 W Metal Halide pH = natural	90 min	97	[109]

In another study, Li et al. [97] reported the 100% degradation of Acyclovir in 90 min. using g-C₃N₄/TiO₂ under visible light irradiation. However, after seven hours of continuous irradiation, any TOC removal was not noticed, implying the formation of rather recalcitrant intermediates with high resistance to degradation by ROS that formed within the studied system. Trapping experiments for formed reactive species elucidated that g-C₃N₄/TiO₂ under visible light irradiation only produced h⁺ and O₂^●‒, and not the most reactive •OH, explaining limited oxidation capability and none TOC removal in the case of acyclovir degradation. This significant contribution proves that the use of g-C₃N₄/TiO₂ under visible light irradiation must undergo careful laboratory tests regarding the susceptibility of targeted organics and their intermediates to degradation by h⁺ and O₂^●‒ prior to considering real scale application [97].

Several studies also showed that the tailoring of composite morphology promotes improved photocatalytic efficiency. For instance, Yu et al. [98] prepared a mesoporous g-C₃N₄/TiO₂ that was applied to polysulfone ultrafiltration membranes for sulfamethoxazole (SMX) removal. It was found that mpg-C₃N₄/TiO₂ exhibit 69% SMX degradation within 30 hours of sunlight irradiation. On the other hand, TiO₂ nanosheets with exposed facets (001) (core)-g-C₃N₄ (shell) composite exhibit a higher degradation rate of 2.2 mg/min., which is 36% faster when compared to TiO₂ and g-C₃N₄ physically-mixed composite. The improved effect is ascribed to the close interaction of TiO₂ and g-C₃N₄ core-shell structure, whereas, in physically mixed composite the formed heterojunction is random and non-uniform [99].

The use of support materials, such as clays [100] and polymers [101], has been also utilized for improved adsorption capacity and the stability of g-C₃N₄/TiO₂ composites. For instance, Chen et al. [101] used g-C₃N₄–shielding polyester fiber (PET)/TiO₂ for photocatalytic degradation of sulfaquinoxaline and thiamethoxam. Interestingly, the composite removal efficiency for sulfaquinoxaline reached 97%, after 10 consequent cycles. Furthermore, the introduction of kaolinite with g-C₃N₄/TiO₂ improved the surface area and adsorption capacity of the composite, leading to 92% degradation of CIP in 240 min. of visible light irradiation [100].

An additional approach considers doping of metals and non-metals in TiO₂, enhancing its light absorption capacity from UV absorption to visible light absorption. Thus, incorporating doped TiO₂ with g-C₃N₄ structures has also attracted great attention for the degradation of CECs. For instance, S-Ag/TiO₂ @g-C₃N₄ [102] was employed for the degradation of Triclosan (TS) and yielded 92.3% degradation of TS within 60 min. under visible light irradiation. Song et al. [103] fabricated a nanofibrous Co-TiO₂ coated with g-C₃N₄, which was applied to TC removal; the authors reported a consistent stability of composite photocatalyst during five consecutive cycles.

Besides doping, sensitization with dyes [104] and carbon dots [105] was also found to enhance the light absorption capacity of g-C₃N₄/TiO₂ composite. For example, D35 organic dye was applied next to g-C₃N₄/TiO₂ and it was found that the light absorption range was enhanced up to 675 nm [104]. On the other hand, Su et al. [105] studied the application of C dots decorated/g-C₃N₄/TiO₂ for the degradation of enrofloxacin under visible light and assigned the observed enhancement to the upconversion photoluminescence properties of C dots, which convert near-infrared light wavelength into visible light wavelength [106]. As effective solutions for improving g-C₃N₄/TiO₂ performance, the incorporation of graphene quantum dots [107] and another semiconductor (i.e., MoS₂ [108], WO₃ [109]) is also reported; such systems resulted in enhanced separation of charges and the suppression of their recombination, thus leading to improved photocatalytic activity in the degradation of CECs.

3. Photocatalytic Water Splitting

Photocatalytic water splitting implies a non-spontaneous process, where the light photons are used to break the water molecules assisted by a photocatalyst, which generates photoexcited charge carriers, i.e., e⁻/h⁺ pairs, delivering them to the solid-liquid interface, where the redox half-reactions of water oxidation and reduction are catalyzed [110,111], analogously, as described above for photocatalytic water treatment. The difference in water splitting is that photogenerated charges (i.e., e⁻ and h⁺) need to react with H⁺ as the electron acceptor adsorbed on the photocatalyst surface or within the surrounding electrical double layer of the charged particles in order to generate H₂ [112], instead of O₂ generating O₂^●−, as in photocatalytic water treatment (Figure 1). The donors are the same; H₂O, however, desired the product of such reaction is O₂. Figure 11 shows the principal mechanism of photocatalytic water splitting with the use of TiO₂ semiconductor nanoparticle. The VB and CB of semiconductor or their composites have to have favorable positions in order to enable occurrence of such reactions.

Figure 11.

The principle photocatalytic water splitting mechanism over illuminated TiO₂ nanoparticle.

Generally speaking, there are two competitive processes that occur inside the photocatalyst and affect H₂ evolution. Similarly as in the case of water purification, first is the charge recombination process. Such a process reduces the excited charges for >90%, as mentioned above [14]; according to some authors, even less than 1% of photoexcited charge carriers are able to participate in the photo-redox reactions forming H₂ [111]. Such a negative tendency can be improved by controlling the recombination rate [75], as also described in detail in the case of the composite materials used for water purification. The second process is the separation of photogenerated charge carriers that favor H₂ evolution, also mentioned above in the case of water purification, but here with more important role [111].

The positions of CB and VB define the redox potential of photogenerated charge carriers. A CB minimum (CB_min) that is smaller than 0 V vs. standard hydrogen electrode (SHE) is required for H₂ generation, while the maximum of VB (VB_max) has to be higher than O₂/H₂O reduction potential, by definition, in order to enable O₂ evolution [112]. As mentioned above, H₂ generation through this process is non-spontaneous, needing the standard Gibbs free energy change of +237 kJ/mol or 1.23 eV, and to accomplish water splitting under visible light irradiation, the bandgap of the photocatalyst should be more than 1.23 eV and less than 3.0 eV [111]. The electronic structures of diverse semiconductors fulfill the necessary conditions for the water splitting reaction, as can be seen from Figure 12.

Figure 12.

Valence band (VB) and conduction band (CB) band positions of various (a) n-type semiconductors; (b) p-type semiconductors [111].

Within the scope of this review are recent achievements in TiO₂-heterojunction systems for photocatalytic H₂ generation. It is important to explain the separation mechanisms of charge carriers that occur in such hybrid materials: (i) Schottky junctions—photogenerated e⁻ migration from semiconductor to metal surface due to a higher work function of metal than those of semiconductor, thus forming a Schottky junction (Figure 13a); (ii) Type II heterojunction (represented in details in the case of water purification) (Figure 13b); and, (iii) p-n Heterojunction—supply of an additional electric field to accelerate the charge carrier transfer (Figure 13c); and, (iv) Direct Z-scheme heterojunction—e⁻ in the CB of second semiconductor recombined with the photogenerated h⁺ in the VB of the first semiconductor, leaving the photogenerated e⁻ in first semiconductor and the photogenerated h⁺ in second semiconductor for photocatalysis (Figure 13d) [112].

Figure 13.

Separation mechanisms of charge carriers in hybrid materials: (A) Schottky junction; (B) Type II Heterojunction; (C) p-n Heterojunction; and, (D) Direct Z-scheme Heterojunction [112].

The process efficiency is determined through the Quantum yield (QY) and Apparent Quantum Yield (AQY), as described with Equations (2) and (3) [93]. The overall quantum yield is predicted to be higher than the apparent one since the number of absorbed photons is usually less than that of incident photons [111].

$$\mathrm{QY}(\%)=\frac{Number of reacted electrons}{Number of absorbed photons}\times 100=\frac{2 x Nubmer of hydrogen molecules}{Number of absorbed photons}\times 10$$ (2)

$$\mathrm{AQY}(\%)=\frac{Nubmer of reacted electrons}{Number of incident photons}\times 100=\frac{2 x Numbmer of hydrogen molecules}{Number of incident photons}\times 100$$ (3)

H₂ generation can also be realized in the presence of sacrificial agents, which, in this case, serve as electron donors that accept photogenerated h⁺ of the VB, thus enhancing the separation of photogenerated charge carriers, which results in higher quantum efficiency [113]. Alcohols are generally used as a h⁺ scavenger, and the more α-H atoms the alcohol has, the higher H₂ production rate is achieved due to more efficient consumption of h⁺ in the photoreaction. The number of α-H atoms in the alcohols can serve as the reference when selecting an appropriate scavenger for photocatalytic reaction [112].

After briefly providing the basic principles to fully understand the H₂ evolution through photocatalytic water splitting, following sections are more focused on the recent achievements in fabrication and the evaluation of different TiO₂-based heterojunctions with different families of materials, including carbon-based, transition metal oxides and chalcogenides, and multiple-based composites consisting of three or more semiconductor materials for H₂ generation.

3.1. Carbon-Based/TiO₂

Among a variety of materials that are selected for the preparation of TiO₂-based nanocomposites to increase their photocatalytic efficiency, nanostructured carbon materials, such as carbon nanotubes and graphene family nanomaterials (e.g., GO, rGO, g-C₃N₄), are of particular interest [114]. The advantages, such as chemical stability, structural diversity with prominent light-absorptive, and electron transport properties, make them promising materials for use in photocatalytic H₂ generation by the water splitting processes [115].

3.1.1. TiO₂/g-C₃N₄

The advantages and limitations of g-C₃N₄ are already mentioned above in the case of water treatment. The limitations referring to low light utilization efficiency and insufficient surface area can be easily broken by the preparation of 2D nanomaterials, especially g-C₃N₄ nanosheets (CNNS) [84]. The self-assembly method of construction 2D/2D TiO₂/CNNS heterojunction composites achieved a hydrogen evolution rate (HER) of 350 µmol/h/g under visible light, in comparison with the produced H₂ with the use of pure TiO₂ nanosheets (20 µmol/h/g) and g-C₃N₄ nanosheets (130 µmol/h/g) [85].

Liu et al. recorded another use of CNNS [84], who synthesized partially reduced TiO_2−x through NaBH₄ treatments with the formation of an additional mid-gap band state (Ti³⁺ and oxygen vacancies—O_vs) to extend absorption edge. The implementation of novel design tactic in the form of a protective carbon layer that was coated onto TiO_2−x/CNNS hetero-junction photocatalyst enhanced the photocatalytic efficiency. The H₂ evolution was tested under visible and simulated solar light with the use of triethanolamine (TEOA) as a sacrificial agent and Pt as a co-catalyst. In the case of visible light irradiation, the highest HER was 417.24 µmol/h/g, while, under AM 1.5 irradiation, the obtained amount was 1830.93 µmol/h/g. The enhanced photocatalytic activity that was ascribed to the formation of Ti³⁺ defects was also noticed with the use of g-C₃N₄/Ti³⁺-doped TiO₂ Z-scheme system that was synthesized via the polycondensation of urea with TiO₂, followed by hydrogenation treatment [86]. UV-Vis diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR) have shown that hydrogenation treatment conferred Ti³⁺ defect states that were below the CB_min of TiO₂ and improved the visible light absorption of the composite with the obtained HER of 1938 µmol/h/g under solar light.

Although special efforts are being made to synthesize noble-metal free nanocomposites, there is still widespread use of Pt as a co-catalyst in H₂ evolution reactions. Except for the already mentioned TiO_2−x/CNNS photocatalyst [84], TiO₂/g-C₃N₄ composites with the use of photodeposited Pt as co-catalyst reached HER of 4128 µmol/h/g [87] and 1041 µmol/h/g [83] under solar and visible light irradiation, respectively. Pan et al. [88] also exhibited a high HER of 13800 µmol/h/m² by the use of Pt as a co-catalyst with g-C₃N₄/TiO₂ nanofilm. Enhanced activity is also attributed to the use of a magnetic-driven rotating frame, which was developed to enhance the mass transfer process during the photocatalytic reaction.

The charge transfer efficiency between TiO₂/g-C₃N₄ composite can be enhanced by the doping of different heteroatoms, like C and K atoms. Hence, Zou et al. [89] synthesized C-doped TiO₂@g-C₃N₄ core-shell hollow nanospheres with enhanced visible-light photocatalytic activity for H₂ evolution of 35.6 µmol/h/g, which was 22.7 and 10.5 times higher than that of C-TiO₂ and g-C₃N₄. The structure of TiO₂ hollow spheres resulted in the reflection of light within the interior cavity, thus increasing the utilization of the light energy. Ma et al. [90] prepared a series of K intercalated g-C₃N₄ modified TiO₂ nanobelts with enhanced light absorption, transfer efficiency, and H₂ evolution efficiency of 50 µmol/h, which is 6.4 times greater than that of pristine g-C₃N₄. The use of carbon atoms in the form of carbon quantum dots (CQDs) as electron reservoirs improves the efficiency of separating the photogenerated charge carriers. CQDs present an important class of carbon materials since their discovery in 2004 by Xu et al. [116], with varying sizes in the range of 1–10 nm. They are good materials for photocatalytic applications due to features, like superiority in chemical stability and low toxicity [117]. Pan et al. synthesized he 2D carbon quantum dots modified porous g-C₃N₄/TiO₂ nano-heterojunction [91] and reached 6.497 µmol/h/g of produced H₂ with the full spectrum absorption.

3.1.2. TiO₂-G/GO/rGO

Following above-mentioned hot-electron mechanism, which can promote redox reactions, Lu et al. explored 3D graphene materials (3DG) coupled with TiO₂ [76] for efficient photocatalytic H₂ production under UV-visible light. TiO₂/3DG with a 5 wt.% graphene loading that was annealed at 650 °C exhibited the highest H₂ evolution rate of 1205 µmol/h/g.

Yi et al. [118] synthesized a composite in which TiO₂ nanobelts were supported by N-doped graphene (NG) coordinated with a single Co atom to replace noble metals with a cost-effective photocatalyst. Under simulated solar irradiation (Figure 14), e⁻/h⁺ pairs are formed. The transfer of photogenerated e⁻ from the CB of TiO₂ to Co-NG was energetically favorable since the Fermi energy level of graphene (−0.08V vs. NHE) is lower than the CB of TiO₂ (−0.39 V vs. NHE). NG, with a large specific surface area, acted as “freeway” for e⁻ transportation, delivering e⁻ from TiO₂ to Co single-atom, where they were trapped catalyzing H⁺ reduction to form H₂ due to lower the overpotential needed for Co-NG when comparing to that of NG. Co-NG/TiO₂ showed HER of 677.44 µmol/h/g under the illumination of AM 1.5 G simulated sunlight.

Figure 14.

Schematic diagram of proposed photocatalytic mechanism in the CO-NG/TiO₂ system.

GO/TiO₂ nanocomposites have recently been used for H₂ production via photocatalytic water splitting under visible light through the formation of Ti-O-C bonding by unpaired π electron of GO with TiO₂ surface [114,119]. GO acts as an e⁻ acceptor, promoting the separation of the photogenerated e⁻/h⁺ pairs in TiO₂. These nanocomposites can be synthesized by photo assisted reduction via mixing or sonication and by sol-gel [114]. Hernández-Majalca et al. [114] enhanced the synthesis for the GO-TiO₂ nanocomposite using photoassisted anchoring and modifying GO oxidation method through the use of microwaves. The obtained nonporous product had a specific surface area of 45 m²/g and absorption onset of 477 nm, which made it active under visible light. Finally, the photocatalytic activity of the nanocomposite was enhanced towards the production of H₂, reaching 6500 mol/g in 8 h, which was much higher amount when comparing to that obtained by TiO₂-P25 (460 mol/g) at the same irradiation time.

The reduced form of GO, rGO, is a two-dimensional carbon material with the role of an electron mediator that is much superior in chemical stability and morphological diversity than GO [120]. Iwase et al. published the very first report using rGO as a e⁻ mediator in 2011 [121]. Since then, a number of published works were recorded for the use of rGO-based composites in photocatalytic H₂ production [75,122,123]. Recent achievements in the synthesis of TiO₂/rGO composites for the purpose of H₂ generation include work from Reedy et al. [75] and Samal et al. [122]. They obtained rather high HERs while using TiO₂/rGO composites under solar and visible light: 24880 µmol/h/g and 2700 µmol/h/g of produced H₂, respectively. Ida et al. undertook further investigation on TiO₂/rGO composites [123], managing to enhance the photocatalytic activity of the obtained composite by the simultaneous doping of nitrogen on TiO₂ and rGO. The following values for the HER are obtained: TiO₂ (1585 µmol/h/g) < N-TiO₂ (6179 µmol/h/g) < TiO₂/RGO (12244 µmol/h/g) < N-TiO₂/N-RGO (15028 µmol/h/g).

3.1.3. TiO₂/CNT

Recently, TiO₂/carbon nanotubes (CNT) have been of great interest due to their high-quality active sites, large specific surface area, and retention of charge recombination, where CNTs can act as a p-type semiconductor, having a role as a powerful electron sink [119]. The coupling of CNT with TiO₂ forms an advanced nanocomposite with enhanced quantum efficiency that forms heterojunction acting as an impurity by forming Ti-O-C or Ti-C defects that enable visible light absorption and, consequently, the creation of e⁻/h⁺ pairs and hindering e⁻/h⁺ recombination [124]. CNTs, such as single-wall carbon nanotubes (SWCNTs) and multiwall carbon nanotubes (MWCNTs), have attracted much interest due to their unique chemical, electrical, and optical properties [125]. Olowoyo et al. [124] prepared a series of TiO₂ nanoparticles that were modified with MWCNTs by a combined sonothermal-hydrothermal method. The synthesized photocatalysts were examined for water splitting under batch conditions at different pH ranges. The highest rate of H₂ yield, amounting 69.41 µmol/h/g, was obtained using 2 wt.% CNT-TiO₂ under visible light at pH 2. Hence, the acidic medium improved the photocatalytic feasibility of the system due to a higher concentration of H⁺ ions, serving as the reactants, thus increasing the reaction rate.

Bellamkonda et al. [126] used a different approach in synthesizing CNT-G-TiO₂ composites and prepared nanocomposites via the solution-based method, in which nanocrystalline anatase TiO₂ was grown onto graphene nanosheets and carbon nanotubes. Spectroscopic and photocatalytic studies revealed that graphene acts as an electron reservoir, while the role of CNTs is to prevent the restacking of graphene nanosheets and provide additional electron transport channels, thereby suppressing the recombination rate of e⁻/h⁺ pairs in the obtained composite. The combination of all these factors resulted in increasing the HER from 19000 µmol/h/g (obtained by anatase TiO₂) to 22000 µmol/h/g (obtained by G-TiO₂), and finally to 29000 µmol/h/g (obtained by CNT-G-TiO₂), which is 8-fold higher than obtained by the commercial TiO₂ (Degussa P25).

The photocatalytic performance of TiO₂ under visible light can be promoted by coupling both MWCNTs and SWCNTs, as presented by Umer et al. [125]. Such an effect occurs due to their dual natural behavior, such as reducing rapid recombination of e⁻/h⁺ pairs and providing support in harvesting visible light. The maximum H₂ evolution rate of 5486 µmol/h/g was achieved over MWCNT/TiO₂/SWCNT, which is 1.24– and 1.42–fold higher than using single CTN-TiO₂ composites (SWCNT/TiO₂ and MWCNT/TiO₂, respectively).

3.2. Transition-Metal Oxides/TiO₂

Excellent chemical stability has opened the possibility of the application of transition metal oxides (TMO) in the field of clean energy production. The above displayed Figure 12 contains main TMOs, like p-type (CuO, V₂O₅) and n-type (TiO₂, WO₃, MoO₃, ZnO, Fe₂O₃) semiconductors that are used in photocatalytic H₂ production with pertaining VB and CB energy levels. Visible-light driven TMOs with narrow band gaps are highly desired. The most used materials within this group, such as CuO, Fe₂O₃, and WO₃, have the bandgap energies that allow for them to be active in the visible light region, but the low energy levels of CB position disable them from consuming photoinduced electrons in reactions yielding H₂. By changing the morphologies of desired components and co-doping with different elements, their CB and VB edges can be shifted toward a H₂ reduction and O₂ oxidation potential [127].

Some TMOs, specifically WO₃, have been loaded with a different co-catalyst, like Rh, to effectively produce H₂ from water, to control the desired morphology in the form of nanorods, nanotubes, and nanowires. Camposeco et al. [128] focused on the use of Rh-WO₃ photocatalyst that was supported on TiO₂ nanotubes (Rh-WO₃/NT) for H₂ production via the water splitting process. WO₃ alone cannot take part in H₂ production since the CB energy level of WO₃ is lower than H₂ reduction potential. However, by loading with Rh nanoparticles, the enhancement in H₂ production was noticed. An analysis of energy band levels for the VB and CB that were determined by UV-Vis results and XPS spectra showed that the presence of WO₃ and Rh in the titanate nanotubes simultaneously shift the VB_max and CB_min, thus reducing the bandgap of titanate nanotubes. 0.5 wt.% Rh– 3 wt.% WO₃/NT nanocomposite under visible light irradiation yielded HER of 87 μmol/h, while 3 wt.% WO₃/NT showed much lower effectiveness (only 13 μmol/h).

In another work, Ren et al. [129] constructed cooperative Schottky and p-n (SPN) heterojunction by forming a NiO/Ni/TiO₂ heterostructure that showed a narrower band gap, higher photocurrent density, and ability to absorb light in the visible region. High HER is obtained for the observed composite, amounting 4653 μmol/h/g, which is approx. 2.3 times higher than obtained with NiO/TiO₂ with a p-n junction (2059 μmol/h/g), under visible light irradiation and by the use of Pt as a co-catalyst.

A representative example of TMOs is also hematite (Fe₂O₃), already referred above to as promising visible light active photocatalyst for water treatment. The coupling of Fe₂O₃ with TiO₂ efficiently inhibits the recombination of photogenerated charge carriers and enhances the absorption of solar light [130]. By the use of thermal decomposition of FeCl₃ and TiCl₄ as precursors, Bhagya et al. [113] synthesized the Fe₂O₃-TiO₂ composite and investigated its photocatalytic activity for H₂ production under the influence of different proton sources. Besides focusing on the use of TMOs for improving photocatalytic efficiency, this work also highlights the influence of different sacrificial agents as electron donors that consume photogenerated h⁺, yielding H₂ production. Under simulated solar irradiation, a very high H₂ rate of 880 μmol/h, with an apparent quantum efficiency of 19.39%, is achieved while using Fe₂O₃-TiO₂ and diethylamine hydrogen chloride (DAH), which is much higher than that obtained in the case without DAH (323 μmol/h). Madhumitha et al. [130] also explored the influence of different sacrificial reagents on H₂ production under a visible light source. With the optimization of parameters, i.e., catalyst dosage, flow rate, incident light irradiation, and type of sacrificial agent, they achieved the maximum of HER, amounting 2700 μmol/h. The increase in photoactivity was attributed to the effective charge transfer from TiO₂ to Fe₂O₃ and the use of EDTA, which suppressed the recombination of photogenerated charge carriers.

Easy preparation, environmental friendliness, and good re-utilization enable the wide use of ZnO/TiO₂-based composites. Xie et al. [131] achieved high rates of H₂ evolution while using ZnO/TiO₂ composites with Pt as a co-catalyst. Under visible light irradiation, 2150 µmol/h/g of H₂ is achieved. Additionally, high carrier mobility can be achieved by the use of ZnO in the form of quantum dots (QDs), which presents ideal heavy-metal free “green” modification of TiO₂ composites. Chen et al. obtained the fabrication of ZnO QDs decorated TiO₂ nanowires via a facile calcination method [132]. They used the obtained composite under solar irradiation next to Pt as a co-catalyst and achieved HER of 313.5 µmol/h.

The use of TMO QDs is also recorded in the work of Liu et al. [133], who decorated two-dimensional TiO₂ nanobelts with zero-dimensional Co₃O₄ quantum dots. When compared with bulk materials, 0D Co₃O₄ QDs have attracted considerable attention due to their small size (<10 nm), providing a large specific surface area with more active sites and shorter charges transport paths. Due to the decoration of Co₃O₄ QDs, the bandgap of the obtained composite was also narrowed, and in application next to Pt as co-catalyst, they obtained a rather high HER of 1735.1 μmol/h/g. In comparison, Zhang et al. [134], with the use of bulk p-Co₃O₄/n-TiO₂, achieved a smaller HER of 8.16 μmol/h/g.

Among the TMOs that appear as promising candidates for coupling with TiO₂, CuO is one of such, especially due to its narrow bandgap (1.4–1.6 eV) and the promotion of effective charge separation [5,135], as already reported as promising water treatment photocatalyst. Hasan et al. [5] conducted solar H₂ production using a TiO₂/CuO nanofiber composite that was synthesized by electrospinning technique. Fabricated nanofibers were annealed in different atmospheres to determine the crystalline phase and photocatalytic performance. For the nanofibers that were crystallized in the anatase phase, EPR and XRD analysis referred to the substitution of some Ti⁴⁺ ions by Cu²⁺ ions, leading to the formation of some defects below the CB of TiO₂, which led to a narrow band gap, yielding enhanced HER in the amount of 2715 µmol/h/g. For comparison, without annealing in a different atmosphere, Wang et al. [135] only achieved 47 µmol/h/g of produced H₂ while using the TiO₂/CuO composite that was irradiated by solar light. Other oxidation states of Cu inside the oxides are investigated for coupling with TiO₂, such as Cu₂O [136]. With all of the benefits, such as environmental compatibility, high visible light activity and earth abundance, wider applications of Cu₂O in water splitting are still limiting, since the redox potential of monovalent copper lies within its band gap, thus photogenerated charge carriers thermodynamically prefer the transformation of Cu₂O into CuO and Cu, rather than to be used in redox reactions with water constituents forming H₂ [136]. Wei et al. [136] stabilized the Cu₂O by modulating the defects in faceted Cu₂O/TiO₂ heterostructures to suppress this disproportionation process. Hence, Cu₂O was arranged onto 101-faceted TiO₂ and it was found that oxygen vacancies in {101}-faceted TiO₂ can create a unique channel for Z-scheme charge transfer in Cu₂O/TiO₂ heterostructures. Composite that was obtained by such an approach showed the maximum HER of 32600 µmol/h/g, with a quantum efficiency of 53.5% at an irradiation wavelength of 350 nm.

3.3. Transition Metal Chalcogenides-TiO₂

As already mentioned, various techniques have been applied to modify the TiO₂ photocatalysts with a purpose of wider application in the field of solar-driven H₂ production through water splitting. Sensitization with narrow bandgap semiconductors was found to be an effective method for enhancing all of the deficiencies that occur during the sole use of TiO₂. Regarding an appropriate band gaps, another group of semiconductors with great application potential in photocatalytic H₂ generation are transition metal chalcogenides. Recently, CdS is one of the most studied materials [54,55,137,138,139,140]; however, the toxic effects due to potential leaching of Cd²⁺ have to be strongly considered, followed by others (MoS₂, ZnS, ZnSe, CdSe) indicated in Figure 12, as mentioned in part related to water treatment [127].

3.3.1. TiO₂/CdS

CdS is the most important chalcogenides semiconductor as a hydrogen production catalyst due to its narrow bandgap (2.4 eV), which enables its visible light response [56]. Its drawbacks described above in Section 2.2. can be alleviated; susceptibility to photocorrosion can be suppressed by the use of sacrificial agents (sodium sulfite/sulfide) that effectively consume photogenerated h⁺, while the limited separation efficiency of photogenerated charge carriers can be solved either by using CdS in the form of QDs due to a shorter transportation path or by incorporating CdS onto support materials, such as TiO₂ [55,56].

Rao et al. [137] synthesized CdS/TiO₂ core/shell nanorods with tunable shell thickness to minimize charge carriers recombination and limit photocorrosion. The investigation of photocatalytic activity performed under UV-vis light irradiation confirmed that optimized concentration of sacrificial agents (0.3 M Na₂S and Na₂SO₄ aqueous solution), shell thickness of 6.3 nm, and solution pH of 8.0 enhance the H₂ production rate of 5791 mL/h/g. Du et al. [138], who fabricated pyramid-like CdS nanoparticles that were grown on porous TiO₂, obtained the same type of composite, but with the different morphology. Under UV-vis irradiation and without noble-metal co-catalysts, 5 mol% CdS-TiO₂ achieved an H₂ production rate of 1048.7 μmol/h/g, which is almost six times and 1.5 times higher than that of pure TiO₂ and CdS, respectively. Table 10 provides further examples of TiO₂/CdS-based composites with their respective photocatalytic activities for H₂ production.

Table 10.

The photocatalytic performance of H₂ generation in some related TiO₂/CdS-based nanocomposites.

Photocatalyst	Light Source	HER	Reference
CdS/Pt/TiO2 film	300 W Xe lamp	3.074 µmol/h/g	[54]
CdS/Pt/TiO2 nanosheets	350 W Xe arc lamp	265 µmol/h	[141]
CdS/Pt/TiO2 nanotubes	300 W Xe lamp	402 µmol/h	[142]
CdS/TiO2 nanotubes	350 W Xe lamp	2585 µL/h/g	[143]
CdS-Ti-MCM-48-21-25	300 W Xe lamp	2726 µmol/h	[139]

3.3.2. TiO₂/CuS

CuS has emerged as an alternative co-catalyst for H₂ production, which is abundant, cheap, and nonhazardous. With its VB and CB at positions of −1.56 eV vs. NHE, and −0.09 eV, CuS shows low reflectance in the visible and relatively high reflectance in the near-infrared region, which makes it a good candidate for solar energy absorption [144]. Chandra et al. [144] investigated the photocatalytic activity for H₂ generation of synthesized CuS/TiO₂ (CT) heterostructured nanocomposite under UV-vis and only visible light. Under irradiation, the highest HER of 12362 μmol/h/g was achieved while using 0.4 mol.% CuS/TiO₂, while only 155 μmol/h/g of H₂ was produced under visible light and comparable conditions.

In comparison, Dang et al. [145] produced a series of Cu_xS (x = 1 or 2) co-modified TiO₂ nanocomposites while using a one-step precipitation approach. The EDS, XPS, and XRF results confirmed the existence of three different phases: CuS, Cu₂S, and TiO₂. The results have shown that CuS and Cu₂S dual co-catalysts under simulated solar light exhibited high H₂ production of 5620 µmol/h/g, which is about 58 times higher than that of the unloaded TiO₂. The enhancement in H₂ production can be contributed to the co-deposition of CuS and Cu₂S onto the TiO₂ nanoparticle surface that efficiently extended visible light absorption and facilitated the separation of charge carriers. The Cu_xS/TiO₂ composite showed high stability; after three consecutive cycles the photocatalytic efficiency for H₂ production decreased for only 11.7%.

3.3.3. TiO₂/MoS₂

MoS₂ represents two-dimensional transition metal dichalcogenide (TMD) that can be prepared into ultrathin-layered structures and, thus, participates in the H₂ evolution reaction as an effective non-noble metal alternative [117,146]. Without the use of any sacrificial agent and co-catalyst, Huang et al. [146] observed photocatalytic H₂ generation while using MoS₂ quantum dots@TiO₂ nanotube arrays nanocomposite in pure water under visible light. The photocatalytic activity was influenced by the amount of MoS₂ QDs coated on TiO₂ NTAs. Hence, the maximum of 53.9 μmol/cm²/h of H₂ was produced, which was ascribed to the decreased bandgap and the surface plasmonic properties of the obtained composite promoting charge carrier separation and the absorption capacity to visible light. Du et al. [63] grew in situ two transition metal chalcogenides—MoS₂ and CdS—on porous TiO₂ by using the sol-gel method, followed by the calcination and hydrothermal method. Under visible light irradiation and without the use of noble metals as the co-catalyst, 3% MoS₂-CdS-TiO₂ produced 4146 μmol/h/g of H₂. In this ternary composite (Figure 15), the porous structure of TiO₂ accepts generated e⁻ from CdS and provides surface area for H₂ production, while MoS₂, as a conductive medium, enabled the transfer of e⁻ between CB of CdS and TiO₂, simultaneously inhibiting the photocorrosion of CdS as h⁺ collector.

Figure 15.

The proposed photocatalytic mechanism in MoS₂-CdS-TiO₂ photocatalyst.

3.4. Multiple TiO₂-Based Composites

This section is dedicated to multiple ternary and quaternary TiO₂-based composites. Recently, significant emphasis was laid on the formation of Z-scheme structured photocatalysts. In such materials, redox reactions occur in each semiconductor, allowing for the combination of a semiconductor with strong reduction power with another semiconductor with strong oxidation power [93].

All of the solid Z-scheme heterojunctions are usually composed of two photocatalysts and an electron mediator, which enable efficient H₂ production through the synergistic action between two isolated photosystems and electron mediator cleverly arranged in a nano-platform [147]. Reversible redox couples (e.g., IO₃⁻/I⁻, Fe³⁺/Fe²⁺) are usually applied as electron mediators in the Z-scheme system. Solid electron mediators are more suitable for the application. Noble metals (Au, Ag) and carbon-based materials (MWCNT, rGO, CQDs) are commonly used as solid electron mediators for photocatalytic H₂ generation [148].

Ng et al. [147], who synthesized Zn_0.5Cd_0.5S-MWCNT-TiO₂ ternary nanocomposite, where MWCNT acted as an electron mediator, fabricated a solid Z-scheme system. The obtained material efficiently suppressed charge recombination and promoted water reduction, achieving HER of 21.9 µmol/h. Furthermore, Liu et al. [149] have used carbon quantum dots (QDs) as an electron mediator between TiO₂ and Zn_0.5Cd_0.5S film and achieved 38740 µmol/h/m² of produced H₂ under solar light. Lv et al. investigated the use of rGO as an electron mediator [120], synthesizing sandwich-like TiO₂/rGO/LaFeO₃ ternary heterostructure, which, under solar light, obtained HER of 893 µmol/h/g, which is almost 3.2, 14.4, and 11.4 times superior to the direct Z-scheme components TiO₂/LaFeO₃ composite, pure TiO₂, and LaFeO₃, respectively. The photocatalytic mechanism of H₂ production is the same for all three above mentioned solid electron mediators: MWCNT, rGO, and CQDs (Figure 16). Upon excitation by light, e⁻/h⁺ are formed in Z-scheme semiconducting components, depending on the ability of each component to absorb emitted light. Hence, photogenerated e⁻ from semiconductor I can be easily recombined with h⁺ from semiconductor II through the electron mediator, leaving more oxidative holes and reductive electrons to participate in the redox reactions in the corresponding active sites [148].

Figure 16.

The type Z-heterojunction with the use of different solid electron mediators (multiwall carbon nanotubes (MWCNTs), carbon quantum dots (CQDs), rGO).

In Z-type heterojunction, noble metals can also perform the role of electron mediators. Zou et al. performed the construction of g-C₃N₄/Au/C-TiO₂ hollow spheres with Au nanoparticles (NPs) as the electron mediator [150]. The as-prepared composite showed high HER of 129 µmol/h/g under visible light, which can be attributed to the efficient charge separation in the constructed Z-scheme system, the broadened visible-light response range, owing to the surface plasmon resonance (SPR) effects on Au nanoparticles, and the hollow structure of C-TiO₂ that gives photocatalyst unique properties of low density and high light-harvesting efficiency. In another work presented by Yang et al. [151], Au NPs were applied as a solid electron mediator in the ternary urchin-like ZnIn₂S₄-Au-TiO₂ nanocomposite, which, at an optimal ratio of 24 wt.% Au NPs and 60 wt.% ZnIn₂S₄, achieved HER of 186.3 µmol/h/g under solar light irradiation.

Table 11 lists other multiple composites, except ones with the Z-scheme heterojunction that are already mentioned, as well as reaction conditions in the photocatalytic system and the obtained hydrogen evolution rates.

Table 11.

Multiple TiO₂-based composites for the use in photocatalytic hydrogen generation.

Photocatalyst	Reaction Conditions	HER	Reference
CdSQDs/WC/TiO2	Photocatalyst dispersed in 20 vol.% lactic acid as an electron donor under visible light.	624.9 µmol/h/	[55]
ZnO/ZnCr2O4@TiO2-NTA	Photocatalyst dispersed in aqueous methanol solution under simulated solar light.	1680 µmol/cm2	[152]
F-TiO2/CdSe-DETA	Photocatalyst was dispersed in a mixed solution of Na2S and Na2SO3 as a sacrificial agents under visible light with the use of Pt as a cocatalyst.	12381 µmol/h/g	[153]
g-C3N4/TiO2/RGO	5 mg of the g-C3N4-TiO2/RGO nano-composite was dispersed in 50 mL glycerol-water solution under UV-vis light.	19610 µmol/h/g	[13]
CDS/CNF/Pt-TiO2	Photocatalyst was dispersed in a mixed solution of Na2S and Na2SO3 as a sacrificial agents under visible light.	16.34 µmol for 3 h	[154]
F-TiO2/CdS-DETA, Pt as a cocatalyst	50 mg of the photocatalyst was dispersed in 100 mL of mixed aqueous solution containing 0.35 mg/L Na2S and 0.25 mg/L Na2SO3 with the use of Pt as a cocatalyst.	5342.86 µmol/h/g	[155]
CdS@TiO2@Au	20 mg of the photocatalyst was dispersed in 40 ml of aqueous solution containing 0.1 M Na2S and 0.1 M Na2SO3 as the sacrificial agents under visible light.	1720 µmol/h/g	[156]
TiO2-Au-CdS	0.1 g of the sample was immersed in an aqueous solution containing 0.1 M Na2S and 0.1 M Na2SO3 as the sacrificial agents under visible light.	1810 µmol/h/g	[157]
N-TiO2/g-C3N4@NixP	50 mg photocatalyst was suspended in a 100 mL solution containing 10 vol.% triethanolamine (TEOA) under 300 W Xe lamp irradiation.	5438 µmol/h/g	[158]
TiO2/CdS/CNT	0.1 g of photocatalyst was dispersed in solution containing 70 mL of distilled water or seawater and 30 mL of sacrificial agent. The photocatalyst were irradiated using three visible-light sunlamps of each 100 W and UV lamp of 8 W.	3502 µmol/h from pure water and 1373 µmol/h from seawater	[159]
WS2/ C-TiO2/ g-C3N4	50 mg of photocatalyst was added in to 80 mL TEOA aqueous solution with the use of Pt as a cocatalyst.	17726 for DI water and 29978 µmol/g for seawater	[148]
TiO2/La2O2CO3/rGO	0.05 g of powder catalyst was dispersed in 80 Ml ethylene-glycol water (5/95, v/v) solution under UV-vis light.	583 µmol/h	[160]
TiO2-Cu@C	Photocatalyst was dispersed in methanol aqueous solution under UV-vis light.	3911 µmol/g/h	[161]

4. Conclusions

TiO₂-based photocatalytic technology represents a promising up-coming technology for both renewable energy generation and water purification applications. It is necessary to work on developing TiO₂-based semiconductor materials, which are active under broader spectrum of solar light, overcoming the indicated disadvantages of the solely TiO₂ utilization, since TiO₂ as a photocatalytic material cannot be used alone due to its limitations such as activity only in UV light region and rapid recombination of photogenerated charge carriers.

TiO₂-semiconductor coupling offers promising results in water purification, particularly for the degradation and mineralization of CECs. However, it is necessary to evaluate the toxicities of degradation intermediates of CEC to check the real efficiency of such composites. Furthermore, the immobilization of TiO₂–Semiconductor composites to photocatalytic reaction membranes must be envisaged for further upscale opportunities.

Great progress has also been recorded in the development of TiO₂-based heterojunction for the application in solar-driven photocatalytic hydrogen generation. By morphological control of the obtained composites, higher H₂ generation, as well as better light harvesting can be achieved. It is noticed that still a great deal of research is being conducted by the use of different noble-metal co-catalysts and sacrificial agents, which, although increasing the efficiency of the process, reduces its environmental friendliness and increases the performance cost. Accordingly, to allow for the practical deployment of such units, as well as commercialization, it is necessary to produce cost-effective systems that will consider economic impact assessment, including operation cost and energy consumption, and that will not require the use of costly co-catalysts and other substances that promote system performance.

Author Contributions

Conceptualization, H.K., F.F., and A.L.B.; writing—original draft preparation, K.P., F.M.d.R., H.K.; writing—review and editing, M.K., U.L.Š., D.D.D., F.F. and A.L.B. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by projects: “Nano-sized Solar-active Catalysts for Environmental Technologies” (NaSCEnT, IP-2018-01-1982), Croatian Science Foundation, and “Water Purification and Energy Conversion using Novel Composite Materials and Solar Irradiation” (KK.01.1.1.04.0001), European Structural and Investment Funds (Croatia)

Conflicts of Interest

The authors declare no conflict of interest.