Surface Modulation, Optics, and Electrochemical Hydrogen Evolution Studies on CdS‐Ag₂S Superlattice Heterostructures

ABSTRACT

Semiconductor superlattice heterostructures with periodic electronic states hold promise for catalysis, photodetectors, and electronic devices. CdS‐based superlattice quantum dots are particularly attractive due to exciton dynamics that enhance electron transport across heterogeneous energy states. In this study, CdS‐Ag₂S@octadecyl amine (ODA) random superlattices quantum dots were formed via ligand‐mediated decomposition of [(PPh₃)₂Ag(SCOPh)] and [Cd(SCOPh)₂] complexes. Conversely, CdS‐Ag₂S@dodecane thiol (DDT) ordered nanorod superlattices were produced through thiol‐mediated decomposition. High‐resolution scanning transmission electron microscopy and energy‐dispersive X‐ray spectroscopy confirmed the periodic interfaces. Photoluminescence (PL) studies of CdS‐Ag₂S@ODA superlattices showed dual‐band emissions, with fast band‐edge recombination (τ_av = 1.37 ± 0.03 ns) and delayed non‐radiative decay (54.44–99.31 µs) from surface trap states. In contrast, CdS‐Ag₂S@DDT nanorods exhibited a majority charge carrier recombination with band‐edge emission lifetimes (1.63–1.74 ns). Notably, CdS‐Ag₂S@DDT nanorod superlattices demonstrated enhanced electrochemical hydrogen evolution in neutral media, outperforming DDT/ODA‐capped type‐I/II heterostructures. They showed a slightly higher HER overpotential under illumination (1.572 V) than in the dark (1.38 V) at a current density of 10 mA cm⁻², consistent with the metallic character of Ag₂S and interfacial charge accumulation. However, ODA lowers atomic packing and disordered interfacial coupling, limiting HER activity with an overpotential of 1.636 V in CdS–Ag₂S@ODA superlattices.

We report the synthesis of CdS‐Ag₂S@ODA/DDT‐capped superlattice heterostructure assemblies using a molecular source approach. TEM and XPS confirm random and ordered domains within the CdS‐Ag₂S superlattices, influencing electronic properties. The ordered superlattice exhibits controlled charge carrier recombination, impacting electrochemical hydrogen evolution. These superlattices show promise for hydrogen production and optoelectronic applications like LEDs.

1. Introduction

Semiconductor heterostructures are an alignment of different energy band materials. These nanomaterial hybrids are widely used in photocatalysis, lasers, electronic devices, and hydrogen evolution reactions [1]. The synergic effect of energy bands in heterostructures is important for selective applications. For example, a Ni‐coated CdS frees the photoexcited electron from the holes and further transfers it to the Ni center, reducing the hydride to hydrogen [2]. However, the main challenge is photo corrosion and the fast e/h recombination rate of these materials [3]. Bandgap engineering via heterostructure interface controls the charge separation barrier in CdS nanomaterials and enhances device performance. The nanomaterial heterostructure engineering process was mainly focused on reducing surface charge recombination along with the surface defect, increasing the light‐matter interaction for better hydrogen evolution reaction and catalysis [4, 5, 6].

Conventionally semiconductor heterostructures such as core‐shell and patchy structures have inherent limitations. Poor surface passivation and delay in the carrier charge transfer prohibit its use at full potential [7]. The classic type‐1 heterostructures are narrow bandgap semiconductors in the core and wide bandgap semiconductors on the shell of the core‐shell nanostructures [7]. The highly confined narrow bandgap semiconductor core blocked the charge carrier extraction. Eventually, in the reverse type‐1 case, the core participates in interaction with light, but the narrow bandgap semiconductors have a weakly passivating surface charge. However, in another well‐known category of type‐II and patchy cocatalyst semiconductors, the narrow bandgap is in a higher proportion than wide bandgap semiconductors [7]. Bandgap engineering in type‐II heterostructures leads to greater separation of electron‐hole carriers, reducing the recombination rate and thereby enhancing photocatalytic efficiency [8]. The interface of a wide bandgap perovskite nanosheet with the one‐dimensional CdS nanomaterials resulted in a low electron‐hole recombination rate and enhanced charge transfer processes for the hydrogen evolution reaction [9]. The concept of superlattice (SL) was developed to improve charge‐carrier passivation at the heterojunction interface, band engineering, miniband structures, and quantum confinement effect. The atomic arrangement of nanocrystals in super‐particles and nano‐rod superlattices is highly ordered. These quantum‐confined ordered superlattice semiconductors possess excellent e/ h transport behaviors [10]. Introducing quantum confinement effect in the superstructure assemblies, intricate their optoelectronics [11, 12]. The optoelectronics of the superstructures are categorized into a continuum heterostructure type‐1 or type‐II bandgap model. Superlattices are alternate patterns of ultra‐thin, wide/ narrow (type I/ type II) bandgap semiconductors. This alternate fashion directs the charge carrier mobility. These quantum‐confined sections are fabricated in various morphologies to generate highly efficient superstructures. For example, the superlattice in spherical particles is established as “superparticles”. Moreover, axial superlattices form in the nanorods and deliver facet‐selective carrier charge transport. These axial superlattices are widely used in photonics and thermoelectrics [13]. The first quantum dot superlattice was reported by C. B. Murray [14] and grabbed major attention in the fabrication of light‐emitting devices [15].

Superlattice synthesis is a complex process, often involving noble techniques, e.g., molecular beam epitaxy, e‐beam lithography, vapor‐liquid‐solid (VLS), cation exchange, and annealing methods [16, 17, 18, 19, 20]. Moreover, the conventional approach for heterostructure nanomaterials synthesis is cation exchange [21], sol‐gel [22], and chemical deposition [23, 24]. In a colloidal path of super lattice synthesis, the Alivisatos group reported the spontaneous growth of CdS‐Ag₂S superlattice [25]. In the CdS‐Ag₂S superlattice, the strain field generated by the lattice mismatch directs the vertical deposition of semiconductor heterostructure layers in the quantum dots [26, 27, 28, 29]. Diffusion limit and bandwidth of these wide‐narrow bandgap semiconductors confined arrays can be controlled through the judicious choice of surface ligands.

We recently reported the phase‐controlled synthesis of air‐stable cationic AgCdS alloys [30]. The alloy hexagonal and monoclinic phase synthesis and isolation are controlled by the presence of octadecyl amine and dodecane thiol, respectively. The dodecane thiol‐capped AgCdS alloy monoclinic phase‐based device was actively involved in the electrochemical hydrogen evolution (HER) in neutral media. However, the hexagonal phase of this alloy failed as a device for the same. Considering the global push for decarbonization, neutral‐condition electrochemical water splitting plays a critical role in driving forward the green hydrogen economy [31, 32, 33]. However, electrochemical water spitting in neutral/basic media was not commonly performed due to sluggish reaction mechanisms. The splitting of water required energy ΔG°= +237.24 J/mol (water), with a minimum potential difference ΔE = 1.23 V vs reversible hydrogen electrode [34]. The recent report on dodecane thiol/ oleyl amine‐capped CdS/Ag₂S‐based devices is found suitable for electrochemical hydrogen evolution in neutral media [30].

To improve the electronic properties of the amine‐capped CdS‐Ag₂S devices, literature reports on the CdS/Ag₂S heterostructure established that the carrier concentration can be tuned by the presence of surface ligands [35]. The surface ligands also take the lead in creating different dimensional growth of the Ag₂S/CdS components and rationalize their integrations [36]. The dodecane thiol capping in the CdS/Ag₂S heterostructure slows down the e^–/h⁺ recombination. Moreover, the thiol interaction with the silver sulfide will generate a quasi‐type‐II band heterostructure useful for photovoltaic devices [37]. However, to the best of our knowledge, there are no reports on the octadecyl amine‐capped CdS‐Ag₂S@ODA superlattice heterostructures.

To elucidate the optical and electrochemical hydrogen evolution properties of CdS‐Ag₂S superlattice heterostructure, here we report one‐pot synthesis of the CdS‐Ag₂S superlattices. The ODA‐capped CdS‐Ag₂S@ODA superparticles and DDT‐capped nanorod superlattices structures were characterized, and CdS/Ag₂S longitudinal stacking in nanocrystals/ nanorods was confirmed by aberration‐corrected high‐resolution scanning transmission electron microscopy with energy dispersive x‐ray spectroscopy (HR STEM‐EDS), powder X‐ray diffraction (PXRD), and X‐ray photoelectron microscopy (XPS), respectively. The optical process in these long‐ordered alternate heterostructure superlattices was elaborated with the help of solution‐state UV‐visible absorption and photoluminescence spectra. We have illustrated the charge‐carrier e^–/h⁺ dynamics in these superlattice nanocrystals/nanorods (CdS‐Ag₂S@ODA SL‐1 nanocrystals, and CdS‐Ag₂S@DDT Nanorods SL‐2) by the time‐resolved single photon counting measurements (TCSPC). Superlattice energy band exciton dynamics path based on basic type‐I (CdS‐rich CdS‐Ag₂S@ODA, heterostructure type‐1) and Ag₂S‐rich patchy type‐II band structures were discussed. Exciton dynamics studied are further explained based on cathodoluminescence at variable voltage.

We elaborate on the electrochemical hydrogen evolution reaction (HER) process of the CdS‐Ag₂S@ODA SL‐1 quantum dot, silver sulfide‐rich CdS‐Ag₂S@DDT SL‐2 superlattices, along with the basic CdS/Ag₂S@ODA type‐I and CdS/Ag₂S@DDT type‐II heterostructures. The electrochemical HER studies in neutral media revealed that the CdS‐Ag₂S@DDT SL‐2 performance was better than the CdS‐Ag₂S@ODA SL‐1 nanocrystals in neutral media. Moreover, we performed the electrochemical surface area, charge transfer resistance, and Mott‐Schottky plot analysis in light and dark mode to establish the p/n‐type nature of superlattice interfaces.

2. Results and Discussion

2.1. Synthesis of CdS‐Ag₂S Superlattices

The semiconductor superlattice synthesis is followed by e‐beam lithography and vapor‐liquid solid methods [16, 17, 18, 19, 20]. These synthetic protocols required a higher class of instruments. Here, we have used one‐pot, dual‐source precursor methods for the synthesis of air‐stable CdS‐Ag₂S‐based semiconductor superlattices (Figure 1). We have controlled the shape and bandwidth of the CdS and Ag₂S heterostructures in the presence of octadecyl amine/ dodecane thiol. Experimental details are given in the experimental section of the Supporting Information. Metal thiocarboxylate precursor [Cd(SCOPh)₂] 1 [38], [(PPh₃)₂Ag(SCOPh)] 2 [39] reactions with octadecyl amine/dodecane thiol, in the ratio (1:50) at 165°C under argon for 3 h, led to superlattice formation (Figure 1a). The high affinity of octadecyl amine toward the CdS leads to the CdS‐rich dark green‐colored CdS‐Ag₂S@ODA quantum dot superlattice formation. These CdS‐Ag₂S@ODA superlattices are termed super‐particles QD SL‐1. However, dodecane thiol presence directs the decomposition of precursor 1 [Cd(SCOPh)₂] and [(PPh₃)₂Ag(SCOPh)] 2 toward the generation of gray‐colored superlattices (CdS‐Ag₂S@DDT NR‐SL2) (Figure 1b). The Inductively Coupled plasma‐emission (ICP‐OES) analysis further confirmed the elemental ratio of Ag: Cd in CdS‐Ag₂S@ODA QD SL‐1, which was formed to be 1:1.82 (Table S1). For the CdS‐Ag₂S@DDT nanorod superlattice, this ratio was formed to be 16.15:1. Scanning electron microscopy (SEM), along with energy dispersive X‐ray electron microscopy analysis (EDS), confirmed the Cadmium‐rich CdS‐Ag₂S@ODA SL‐1 quantum dot superlattice with Ag: Cd: S ratio, 2.05:2.80:1 (Figure S1A). However, CdS‐Ag₂S@DDT nanorod superlattice (NR SL‐2) SEM‐EDS elemental composition supports the Silver‐rich superlattice formation with the Ag: Cd: S ratio of 47.42:3.13:1 (Figure S1B). The cation exchange process in these semiconductor heterostructures is directed by the thiol/ amine/ phosphine ligands. The CdX‐ Ag₂X cation exchange reactions are energy‐efficient processes with high conversion rates [40, 41]. The triphenyl phosphine and dodecane thiol presence directs the CdS/Ag₂S heterostructure cation exchange reaction into the Silver sulfide‐rich CdS‐Ag₂S@DDT nanorod superlattice [26, 27, 28, 29]. This transformation is driven by the Ag─S bond affinity in the presence of dodecane thiol. As reported, the decomposition of metal thiocarboxylates in solid led to the metal sulfides along with high‐energy S─C bond cleavage [42]. Moreover, in the presence of amine, the metal thiocarboxylate decomposition started at low temperatures and produced the metal sulfide nanomaterials [43]. Here in this report, the amine/ thiol existence offers significant benefits to initiate the reaction at lower temperatures than their melting points. The presence of triphenylphosphine in a coordination atmosphere of precursor [(PPh₃)₂Ag(SCOPh)] 2 influences the diffusion of cations (Cd⁺²/Ag⁺¹) in the CdS/Ag₂S lattices. This is further reflected through the isolation of crystalline superlattice heterostructure formation [44, 45].

FIGURE 1.

(a, b) Synthesis of CdS‐Ag₂S@ODA super particle and CdS‐Ag₂S@DDT nanorod superlattices with dual source precursor [Cd(SCOPh)₂] (1), and [(PPh₃)₂Ag(SCOPh)] (2) triphenylphosphine precursor together with ODA/DDT. (c,d) CdS‐Ag₂S@ODA/DDT type‐I/II heterostructures nanocrystals with Ag(SCOPh) (3) precursor in the presence of dodecane thiol and octadecyl amine [precursor: capping agent‐ 1:50 ratio].

On the other hand, the decomposition of precursor [Ag(SCOPh)] 3 [38] and [Cd(SCOPh)₂] precursor 1 in the presence of ODA/DDT under argon atmosphere at 165°C for 3 h leads to the CdS‐Ag₂S@ODA/DDT heterostructures Figure 1c,d (Experimental Section). The ICP‐OES revealed an Ag to Cd elemental ratio of 1:2.36 in the CdS‐Ag₂S@ODA type‐I heterostructure (Table S1). Inversely, the silver‐enriched CdS‐Ag₂S@DDT heterostructures were formed by the dodecane thiol‐mediated decomposition of precursors 3 and 1. Further, ICP‐OES analysis confirmed that the elemental ratio of Ag to Cd was 5.01:1 in the CdS‐Ag₂S@DDT type‐II heterostructure (Table S1). Ag₂S‐rich, narrow bandgap‐rich CdS‐Ag₂S@DDT heterostructures suggest the type‐II bandgap semiconductors. Here, the absence of the triphenylphosphine ligand limits the diffusivity of the Cd⁺²/Ag⁺¹ cations in the lattices [44, 45]. The lattice mismatch in the CdS/Ag₂S system is 3.7%. Moreover, a high concentration of Ag⁺ (>36%) limits diffusivity at the CdS lattice with enhanced strain [26, 27, 28, 29]. This caused the CdS‐Ag₂S patchy heterostructure formation of the type I/ type II bandgap with poor crystallinity.

2.2. X‐Ray Photoelectron Spectroscopic Studies

The surface chemical nature of the CdS‐Ag₂S@ODA/ DDT heterostructures has been analysed by X‐ray photoelectron spectroscopy, and the results were displayed in Figure S3 and Table S2. The XPS spectra of Cd 3d spin‐orbital doublet for the CdS‐Ag₂S@ODA quantum dot superlattice (QD SL‐1) showed the + 0.21 eV shift in orbital separation energy (6.71 eV, CdS standard 6.51 eV, 6.7 eV CdS/Ag₂S core‐shell) and appeared with the binding energy of 3d orbitals energy 3d_5/2 = 404.86 eV and 3d_3/2 = 411.57 eV (Figure S31(A). The Cd 3d binding energy in the CdS‐Ag₂S core‐shell nanostructure was reported with no change at 3d_5/2 = 405.4 eV and 3d_3/2 = 412.3 eV, respectively [46]. The XPS spectra of ODA‐capped CdS‐Ag₂S@ODA (QD SL‐1) quantum dot superlattice showed an Ag 3d spin‐orbital doublet [3d_5/2 = 367.59 eV, 3d_3/2 = 373.61 eV] with a separation energy of 6.02 eV, which is a relatively lower energy shift (−0.08 eV) compared to the Ag₂S (6.1 eV) standard. The positive shift in the Cd 3d spin‐orbital doublet energy and a negative shift in the Ag 3d energy confirmed the electron transfer from the Cd to the Ag valence state. The positive binding energy shift (+1.13 eV) of S 2p spin orbitals established the existence of Cadmium sulfide‐rich strong metal‐sulfide bonding at the interface of the quantum dot superlattices. On the other hand, reported CdS‐Ag₂S core‐shell nanomaterials showed no significant change in the Ag 3d core level spin‐orbital pair [3d_5/2 = 368 eV and 3d_3/2 = 374 eV], revealing poor charge passivation at the CdS‐Ag₂S interface in the core‐shell [46]. Notably, CdS‐rich CdS‐Ag₂S@ODA type‐I heterostructures displayed the higher separation energy (7.81 eV) in between the Ag 3d spin‐orbital doublet binding energy [3d_5/2 = 366.27 eV, 3d_3/2 = 374.08 eV] (Figure S3 and Table S2). The relatively higher energy shift in the Ag 3d spin‐orbital energy (+ 1.71 eV) and minimal shift (+ 0.24 and 6.75 eV) at the Cd 3d spin‐orbital doublet established the electron transfer from Ag 3d to the Cd 3d valence state. This confirmed the weak confined interface between Silver sulfide and Cadmium sulfide. However, the XPS analysis confirmed the presence of the strongly confined CdS‐Ag₂S interface in the CdS‐Ag₂S@ODA quantum dot superlattice. The Cd LMM auger spectrum analysis of the CdS‐Ag₂S@ODA superlattices reveals more polarization of the valence band (+6.52 eV), confirming the change in the electronics of the superstructure compared to the pure CdS (6.0 eV).

The dodecane thiol‐capped CdS‐Ag₂S@DDT nanorod superlattice (NR SL‐2) XPS spectra revealed the positive shift in separation energy (separation energy shift = +2.28 eV, 3d_5/2 = 404.36 eV, 3d _3/2 = 414.14 eV) of Cd 3d spin‐orbit doublet (Figure S3 and Table S2). Nevertheless, no significant shift was demonstrated by the Ag 3d [3d_5/2 = 367.66, 3d _3/2 = 373.70 eV, separation energy = 6.04 eV] and S 2p spin‐orbital energy [2p_3/2 = 161.39, 2p_1/2 = 162.18, separation energy 0.79 eV]. On the other hand, the + 6.68 eV separation energy at the Cd LMM Auger spectra exhibited higher valence band polarization than the ODA‐capped QD‐SL‐1 nanomaterials. Interestingly, the more negative shift in the S 2p spin‐orbital energy confirmed substantial electron transfer from the Cd 3d and Ag 3d spin orbitals to the S 2p spin‐orbital.

The silver sulfide‐rich CdS‐Ag₂S@DDT type‐II heterostructure gives a broad S 2p spectrum with the separation energy shift of +0.8 eV. The Cd 3d and Ag 3d spin‐orbital state doublet spectra revealed a minor shift in the separation energy (+0.25 eV) and (−0.07 eV), respectively. The highest charge separation in the valence band energy of (+ 6.87 eV) Cd LMM Auger spectrum was observed in this type‐II band heterostructure. This established the CdS support over the Ag₂S nanostructures as reported in the case of CdS‐Ag₂S core‐shell structures [46].

2.3. Transmission Electron Microscopic Studies on the CdS‐Ag₂S Superlattices

The octadecyl amine mediated decomposition of precursor 1 [Cd(SCOPh)₂] and 2 [(PPh₃)₂Ag(SCOPh)] at 165°C gives the CdS rich CdS‐Ag₂S@ODA quantum dot super particles (CdS‐Ag₂S@ODA QD SL‐1). The aberration‐corrected scanning transmission electron microscopic analysis with high‐angle annular dark field imaging (STEM‐HAADF) confirms the formation of CdS‐rich CdS‐Ag₂S@ODA quantum dot superlattice. The single particle STEM‐energy dispersive X‐ray spectroscopy (EDS) area and line map analysis for Ag L, Cd L, and S K elements confirmed the formation of CdS‐rich CdS‐Ag₂S@ODA quantum dot superlattice (QD SL‐1) (Figure 2). TEM bright field (TEM‐BF) images were captured at an accelerating voltage of 150 kV, considering the sample's beam sensitivity. The quantum dot superlattice (QD SL‐1) showed an average particle size distribution of 1–1.5 nm (Figure S5). The STEM‐EDS color map Ag L (red color), Cd L (green color), and S K (yellow) together line scan revealed the CdS‐Ag₂S alternate lattice interface within the quantum dot heterostructure super particle (Figure 2b,d). The single particle HR STEM‐EDS vertical line scan 1, 2 confirms random distribution of (Figure 2c, Vertical line scan) CdS‐rich CdS‐Ag₂S interface in a single quantum dot (beam sensitive surface of the quantum dots prohibited the Cd, Ag, S STEM‐HR‐EDS atomic column mapping on TEM samples).

FIGURE 2.

(a,c) STEM HAADF image of the CdS‐Ag₂S@ODA QD SL‐1 super particles with Ag (red), Cd (green), S (yellow) elemental EDS map, and (b) HR‐ TEM image of a quantum dot super particle. (d) HR‐TEM micrograph of CdS‐Ag₂S@ODA quantum dot superlattice (QD SL‐1) with the selected area fast Fourier transformation (FFT) and inverse FFT images from the selected region of super particles (SL‐1, SL‐2, and SL‐3 from the red square) by masking the diffraction pattern.

The high‐resolution TEM bright field analysis, along with the selected area Fast Fourier transformation (FFT) pattern, confirms the presence of a CdS‐Ag₂S heterostructure unit in the super particles (Figure S4). Selected area FFT and inverse FFT (IFFT) patterns from region SL‐1(Figure 2d) reveal hexagonal CdS (JCPDS number‐ 41‐1049) patterns for lattice d spacings along a plane, 3.338 Å (002) and 1.270 Å (105). Moreover, the lattice d spacing value for 3.454 Å corresponds to the monoclinic Ag₂S (JCPDS‐14‐0072) (‐111) plane (Table S3). The phase contrast alternate‐waved lattice patterns in inverse FFT patterns were observed at a lattice d spacing of 2.445 Å, confirming the CdS‐Ag₂S heterostructure superlattice interface at the (121) plane of Silver sulfide (JCPDS 14‐0072) and (102) plane of CdS (JCPDS No. 41‐ 1049), respectively (Table S3). Interestingly, the SL‐2 and SL‐3 sections in a quantum dot also revealed the phase contrast superlattice IFFT (inverse FFT) pattern (Figure 2d). These patterns coexisted with the lattice d‐spacing 1.67 Å with the CdS‐Ag₂S as an altered heterostructure superlattice. The darker line, along with the lighter one in IFFT, corresponds to the monoclinic Ag₂S (041) plane, hexagonal CdS (004), and cubic CdS (041) plane.

The powder X‐ray diffraction pattern of the CdS‐Ag₂S@ODA quantum dot superlattice (QD SL‐1) showed the relatively high‐intensity peak of the (002) plane at the d‐spacing of 3.34 Å (100%). The lower d spacing shift (0.02 Å) of the highest intensity peak to the hexagonal CdS JCPDS No.‐ 41‐0149 (Figure S6a) was observed. The relative intensity pattern modification in the hexagonal CdS (100), (002), and (101) planes established the presence of stacking faults along with the distorted sulfur lattice in the quantum dot superlattice [47]. The highest intensity peak of the (002) plane, which also contains the (111) plane of CdS cubic (zinc blende‐ ZB JCPDS number‐ 75‐0581), appeared together as a sharp peak (Table S3). The CdS‐Ag₂S@ODA QD SL‐1 superlattice d‐ spacing 2.067 Å (2θ = 43.81°), recognized as an interface pattern between the CdS (040) [JCPDS‐41‐1049] and monoclinic Ag₂S (112) planes [JCPDS‐ 14‐0072]. The QD SL‐1 superlattice showed sharp PXRD planes of (112), (121), and (‐103) monoclinic Silver sulfide corresponding to the 2θ values 37.016° (d = 2.429 Å), 37.846° (d = 2.377 Å), and 43.773° (d = 2.068 Å). This PXRD pattern confirms the alternate CdS/Ag₂S lattice interface in the quantum dot and forms the QD superlattice structure. However, the CdS‐Ag₂S@ODA type‐I heterostructures have only one broad peak of the (121) plane at 2θ = 37. 922° (2.372 Å). Broad, low‐order Bragg reflections at (121), (‐103), and (311) planes confirmed the random CdS/Ag₂S interface in the CdS‐Ag₂S@ODA type‐I heterostructure (Figure S9).

The dodecane thiol‐mediated composition with precursors 1 and 2 at 165°C for 3 h resulted in the interconnected CdS‐Ag₂S@DDT nanorod superlattices (CdS‐Ag₂S@DDT NR SL‐2). The TEM analysis on the synthesized nanorod superlattice was carried out at 80 V due to the highly beam‐sensitive nature of the sample. The low kV STEM‐HAADF images (80 kV), along with the EDS line and area Ag L, Cd L, and S K elements map, confirm an ordered Ag₂S‐rich CdS‐Ag₂S@DDT nanorod superlattices (NR SL‐2) formation (Figure 3).

FIGURE 3.

STEM‐HAADF images of CdS‐Ag₂S@DDT nanorod superlattice (NR SL‐2) along with the Ag L (red color), Cd L (green color), S K (yellow) EDS elemental area and line scan map (at 80 kV). Vertical line scan 1 represents the silver sulfide (V line scans 1, 2), and horizontal line scans 1 and 2 confirmed the cadmium sulfide formation (H line scans 1 and 2).

STEM‐EDS analysis of the dodecane thiol‐capped NR SL‐2 revealed atomic contrast (Z‐contrast), with the brighter spherical segments corresponding to Ag₂S (9–10 nm), interconnected with the lower Z‐contrast CdS (2‐ 3 nm) rectangular regions. The STEM HAADF EDS line and area scan analysis for the Cd L, S K, and Ag L elements confirmed the presence of Ag and S elements on the spherical segment of the CdS‐Ag₂S@DDT NR SL‐2 nanorod superlattices (Figure 3, Area #1, and vertical line scan 2). On the other hand, Cd L (green) and S K (yellow) elements' presence on the rectangular interconnected zone was confirmed with the selective area horizontal line scan 1 and 2 (Figure 3, H line 1 and H line 2) in the CdS‐Ag₂S@DDT NR SL‐2 superlattices. The interconnected NR SL‐2 assemblies consist of 3 different CdS zones. In the first category, an elongated nano‐rod superlattice with an equal distance Ag₂S spherical zone. Interestingly, in the second category, nanorods of Ag₂S lattice end interconnected with CdS lattice and established as a triangle with a vertex of Silver sulfide. The interlinked Ag₂S‐CdS equilibrium state of nanorod superstructure frameworks was observed. These uniformly distributed interlinked nanorods fall under category 3 (Figure 4).

FIGURE 4.

STEM—Dark field HAADF image of an ordered CdS‐Ag₂S@DDT NR SL‐2 nanorod superlattice assemblies (acquired at 80 kV).

The PXRD pattern of the CdS‐Ag₂S@DDT NR SL‐2 showed the maximum intensity peak at lattice d‐spacing 2.356 Å corresponding to the monoclinic Ag₂S (‐103) plane (Figure S6). The CdS‐deficient kinetic ordering in the CdS‐Ag₂S@DDT NR SL‐2 showed a weak diffraction pattern against the hexagonal CdS (100), (002), and (101) planes with the relatable d‐spacings at 3.58, 3.359, and 3.163 Å, respectively. Furthermore, the stronger diffraction pattern of (203) plane with the d = 1.444 Å (2θ = 64.53°), (204) plane [d = 1.232 Å, 2θ = 77.47°] and (300) plane [d = 1.177 Å, 2θ = 81.861°] values respectively matched with hexagonal CdS phase (JCPDS No‐ 41‐1049). The weak PXRD diffraction patterns from the CdS lattice confirmed the Ag₂S‐enriched CdS‐Ag₂S@DDT NR SL‐2 superlattice.

Valence force field model‐based theoretical studies described the role of Cd⁺²/Ag⁺ ion diffusion‐limited growth and expitaxial strain at Cd‐S‐Ag interface, directing the CdS‐Ag₂S nanorod superlattices formation [26, 27, 28, 29]. Herein, DDT‐capped ordered CdS‐Ag₂S@DDT NR SL‐2 superlattices fall under the category of epitaxial superlattices, together with Ag₂S and CdS regions as reported in the literature [26, 27, 28, 29]. Moreover, Alivisatos' group performed the theoretical simulations on the CdS‐Ag₂S supercell model derived from the monoclinic Ag₂S unit cell, where sulfur atoms form a slightly disordered body‐centered cubic (bcc) lattice. The monoclinic cell axes of silver sulfide, attached along the a, b, and c directions, are related to the orthorhombic supercell by the following vectors: a [0, –0.5, 0.5], b [100], and c [020] [26, 27, 28, 29]. The supercell is aligned with either its [001] plane or (100) plane parallel to the (001) plane of the wurtzite CdS phase. This attachment results in the Ag₂S lattice constant being approximately 4% smaller along the [100] or [001] axes of the supercell, while a 14% larger lattice constant is observed along the [010] axis.

The strain fields created by this lattice mismatch at the interface, combined with repulsive interactions and the mobility of the segments, promote the ordered assembly within the thiol‐capped CdS‐Ag₂S@DDT nanorod superlattices [26, 27, 28, 29]. To analyze the lattice strain fields and elastic interactions, the classical valence force field (VFF) method, utilizing generalized gradient approximation (GGA), was employed in combination with ab initio calculations. These VFF relaxation and elastic energy calculations indicate that the elastic energy decreases with increasing separation between Ag₂S segments at distances of 6, 8, and 10 nm in nanorods with a 4.8 nm diameter [26, 27, 28, 29]. Furthermore, VFF elastic energy models demonstrate a decrease in elastic energy when the separation between Ag₂S segments is below 4 nm in nanorods with a 6 nm diameter along the z‐axis. This reduction in elastic energy induces Cd─S─Ag bond stretching in the x–y plane, which facilitates the ordered arrangement of nanorod superlattices. The presence of the Ag₂S segment of 9–10 nm in our DDT‐capped CdS‐Ag₂S@DDT NR SL‐2 superlattices system, with a separation of >4.86 nm, falls within the range of the reported theoretical data base for the decrease in the elastic energy [26, 27, 28, 29]. Moreover, VFF elastic energy models' theoretical calculations support the stability of DDT‐capped CdS‐Ag₂S@DDT NR SL‐2 superlattices [26, 27, 28, 29].

Reversible Cd⁺²/Ag⁺ cation‐exchange reactions were observed during CdS‐Ag₂S nanostructure synthesis [26, 27, 28, 29]. Notably, the Ag⁺¹ ion of Ag₂S was selectively replaced by Cd⁺² from CdS/Ag₂S owing to the favorable soft acid, soft base interaction with the triphenyl phosphine oxide ligands in comparison to the Cd⁺² cations [45]. Vittal et al. studied the lability of the triphenyl phosphine ligand of a molecular source [(PPh₃)₂AgSCOPh] (precursor 2) in organic solvents. The associative pathway of dodecane thiol/octadecyl amine and the lability of triphenyl phosphine led to the CdS‐Ag₂S @DDT/ ODA superlattice formation [39, 42]. Alivisatos' group reported that tributyl phosphine (TBA) mediated the synthesis of CdS‐Ag₂S superlattice structure by cation exchange reaction in ionic nanocrystals [26, 27, 28, 29]. This Lewis acid‐base interaction and better coordination of triphenyl phosphine, together with the Ag⁺¹ ion with greater sigma donation ability, improved the crystallinity and enhances the reaction rate.

Notably, octadecyl amine is an electron‐rich L‐type ligand which binds with the Cd center of the CdS‐Ag₂S@ODA superlattices [48]. The 18‐carbon chain length in unbranched octadecyl amine increases the colloidal stability in the CdSe/CdS nanocrystals [49]. Conversely, long‐chain, unbranched amines limit the atomic density and packing in the nanocrystals. This may further lead to more surface traps in the quantum dots. Moreover, this property of the octadecylamine surface ligand is accountable for the formation of a random superlattice interface in the CdS‐Ag₂S@ODA superlattice quantum dots [50] (Figure 2).

2.4. TEM Studies on the CdS‐Ag₂S@ODA/DDT Type‐1/ Type‐ II Heterostructures

The CdS‐Ag₂S@ODA type‐1 bandgap polydisperse heterostructure nanomaterials were obtained by the reaction of precursor 1 and precursor 3 [Ag (SCOPh)], at 165°C for 3 h under an argon atmosphere. In the absence of triphenylphosphine, the diffusion rate of the Silver sulfide in the CdS lattice declined. Due to this, CdS‐rich rods, along with the attached silver sulfide spherical particles, were isolated as heterostructures. The high‐resolution STEM‐HAADF EDS Ag L, Cd L, S K area map confirms the atomic layer of hexagonal CdS nanorods with the surface passivated silver sulfide nanoparticles (Figure S7C). Selected area diffraction FFT/ inverse FFT pattern analysis on single particle/nanorod area number 1, 11, 12, 13, and 16 confirmed the interaction of CdS hexagonal and Silver sulfide monoclinic phase at higher lattice d spacings at 3.577, 3.413, 2.049, 1.829, and 1.799 Å with the dislocation dipoles is present in the CdS‐Ag₂S@ODA type‐1 heterostructure (Figure S8). The higher d spacing value at 3.577 Å showed the perfect frequency of the lattices matched in IFFT at the (110) plane of hexagonal CdS (JCPDS No. 41‐1049) and (100) plane of monoclinic Silver sulfide (JCPDS No. 14‐ 0072). The dislocation dipoles were observed at the lattice d spacing of 1.724 Å. This lattice d‐spacing showed Shockley dislocation due to the hexagonal CdS (201) and monoclinic Ag₂S (041) plane interactions. In contrast to the lattice planes of CdS‐Ag₂S@ODA QD SL‐1, the @ODA type‐1 htcs single particle selected area FFT/IFFT patterns from the selected area 1, 11,12, 13, and 16, we only observed the higher lattice d‐spacing values at 3.759, 3.696, 3.577, and 3.431Å further confirms the CdS‐Ag₂S type‐ I heterostructures formation (Figure S8). The lattice d spacing value 3.577 Å corresponds to the (110) plane of CdS(JCPDS number‐ 41‐1049) and the (100) plane of Ag₂S (JCPDS No.14‐0072). The (‐110) and (022) planes with d spacing values 3.431 and 2.528 Å, corresponding to the Silver sulfide, were observed. In the HR‐TEM of fused heterostructure nanoparticles, a CdS (light)/ Ag₂S (dark) phase contrast was observed. The selected area FFT and inverse FFT patterns from the 2 and 3 sections, (002) plane of wurtzite CdS (d‐ 3.389 Å) and (‐111) (JCPDS No. 41‐1049), (114) plane of monoclinic Ag₂S (JCPDS No‐ 14‐0072) with the corresponding d‐ value 3.430 and 1.801 Å were spotted. In the PXRD pattern of the CdS‐Ag₂S@ODA type‐I heterostructure, we have spotted the (100), (002), (101), (112), (004), (204) plane of CdS (JCPDS No.‐41‐1049) with the d spacing at 3.565 Å (2θ = 24.979°), 3.356 Å (2θ = 26.559°), 3.125 Å (2θ = 28.561°), 1.762 Å (2θ = 51.90°), 1.674 Å (2θ = 54.817°) and 1.223 Å (2θ = 76.243 Å) respectively. Moreover, monoclinic Ag₂S (JCPDS No.‐14‐0072) (−121), (022), (121) planes correspond to the d‐spacings 2.8476 Å (2θ = 31.41°), 2.591 Å (2θ = 34.619°), and 2.454 Å (2θ = 36.621°), respectively (Figure S9).

Interestingly, CdS‐Ag₂S@DDT type‐II band heterostructures were isolated from the decomposition of precursor 1 and precursor 3 with DDT at 165°C for 3 h. The STEM‐DF EDS Ag L, Cd L, and S K elemental map at a 10 nm scale bar revealed the presence of an Ag₂S eutectoid structure, accompanied by the attachment of CdS nanoparticles on the surface (Figure S10). The high‐resolution TEM‐HR images analysis coupled with the selected area FFT and IFFT pattern from the surface region 1 and eutectoid area 2 of the heterostructure revealed the hexagonal CdS (JCPDS number‐ 41‐1049) phase on the surface and Ag₂S (JCPDS number‐14‐0072) monoclinic phase correlated to the eutectoid morphology (Figure 5). The selected FFT and inverse FFT (IFFT) pattern from the surface region 1 showed the d spacing values 1.766 Å, 1.842 Å, corresponding to the (112) plane with phase contrast Cd‐S inverse FFT, (103) plane with dislocation dipole at the right direction, and perfect lattice IFFT on the opposite side (position marked with *). The eutectoid shape Ag₂S nanoparticles contain d‐spacing values of 2.881 Å of the (‐112) plane and 1.439 Å of the (‐134) plane corresponding to the monoclinic phase (JCPDS No.‐14‐0072). The (‐134) plane of the d‐spacing 1.439 Å inverse FFT patterns revealed the Ag₂S Silver and sulfur elements phase contrast frequency domain in the plane.

FIGURE 5.

The high‐resolution HR‐TEM images of the CdS‐Ag₂S@DDT type‐II heterostructure, along with the false thermal color pattern. The selected area FFT and IFFT analysis from sections 1 (wurzite CdS) and 2 (Monoclinic Ag₂S).

In the PXRD pattern of the DDT‐capped CdS‐Ag₂S@DDT type‐II heterostructure, reflection of the hexagonal CdS (111), (002) plane along with (−121), (121), and (103) planes of the monoclinic Ag₂S (JCPDS No. 14‐0072) (Figure S11) are present. In contrast to the CdS‐Ag₂S@DDT nanorod superlattice (NR SL‐2), reflections from the Silver sulfide planes were broader, further supporting the Silver sulfide‐rich CdS‐Ag₂S@DDT type‐II heterostructure formation.

2.5. Optical Studies on the CdS‐Ag₂S@ODA/DDT Superlattices/Heterostructures

The optical properties of the semiconductor nanostructures can be tuned by the spatial variable in the compositions [48, 51]. Semiconductor quantum dots exhibit unique optical and electronic properties [52], e.g., size‐tunable bandgap, fluorescence, and charge carrier transfer rate [53]. In the quantum‐confined semiconductor heterostructure, multiple material phases along with the variable chemical compositions create different spatial distributions and charge carrier transport behaviors [54]. To understand the CdS‐Ag₂S heterostructure indirect bandgap optics and its electron‐hole exciton dynamics in detail, absorption and photoluminescence studies were performed in a solution state (in toluene). The exciton dynamics in the solid state were further elaborated by variable cathodoluminescence measurements.

The CdS‐Ag₂S@ODA QD SL‐1 quantum dot superlattice shows a broad absorption band at 492 nm, which is 22 nm red‐shifted compared to the CdS@ODA nanomaterial (470 nm) (Figure S12a). On the other hand, the absorption spectra of heterostructure (htcs) CdS‐Ag₂S@ODA type‐I is ∼ 10 nm blue‐shifted from that of CdS‐Ag₂S@ODA QD SL‐1 superlattice (Figure S12b). The red shift observed for the CdS‐Ag₂S heterostructure absorption bands in the superlattices is attributed to the metal sulfide band interactions in the ≤1 nm region in a continuum model. The TEM and PXRD analyses further validate the CdS‐Ag₂S QD‐SL‐1 random superlattice heterostructure interface width (Figure 3). As the CdS‐Ag₂S interface is localized on the surface of CdS nanorods (with 2‐3 nm) in the form of CdS‐Ag₂S type‐1 htcs, the absorption bands appeared at 481 nm.

The dodecane thiol‐capped CdS‐Ag₂S@DDT NR SL‐2 displayed an indirect bandgap with a broad UV‐absorption band at 495 nm, which is highly redshifted (∼ 90 nm) compared to that of the DDT‐capped CdS@DDT nanomaterial (404 nm). This red shift in the absorption band in the superlattice, compared to the nanostructure, established the different spatial carrier dynamics. However, the broad absorption band at 486 nm from the CdS‐Ag₂S@DDT type‐II heterostructure is due to the poor charge passivation on the surface of Silver sulfide.

The indirect bandgap of the CdS‐Ag₂S superlattices/ heterostructures was calculated by the Tauc, Cody, and baseline methods. The optical bandgap calculation results were summarized in Table S4, revealing the effect of the capping agent on the superlattices and heterostructures accurately. The Tauc relation was used to calculate the optical bandgap of the semiconductor nanomaterials, and the indirect bandgap is given by Equation (1) [55, 56]

$${\left(\alpha h\upsilon \right)}^{1/2}=\mathrm{A}\left(h\nu -{\mathrm{E}}_g\right)$$ (1)

where α is the energy‐dependent absorption coefficient, h = Planck's constant, ν= frequency of the emitted photon, Eg = bandgap of the material, and A is a constant.

The Cody method followed the least‐square fit for the (𝛼/E)^1/2 against E, while for the Tauc method, the plot is the linear least square fit for the (𝛼E)^1/2 versus E.

The indirect optical bandgap calculated by the Tauc method for the indirect bandgap semiconductor CdS‐Ag₂S@ODA quantum dot superlattice (CdS‐Ag₂S@ODA QD SL‐1) was 1.74 eV. Cody and the baseline method of optical bandgap calculation revealed that the optical bandgap was 1.57 and 2.33 eV (Figure S13a and Table S4). Moreover, octadecyl amine capped CdS‐Ag₂S@ODA type‐I heterostructure (CdS‐Ag₂S@ODA type‐I htcs) demonstrates the indirect optical bandgap Eg_Tauc 1.91 eV. The highest direct optical bandgap Eg _Tauc = 2.78 eV [Eg_cody = 2.37 eV, Eg_Baseline = 2.52 eV] was obtained for CdS@ODA nanocrystals among all (Table S4). This redshift (+1.04 eV) in the indirect optical band of CdS‐Ag₂S@ODA QD SL‐1 toward Silver sulfide (0.9–1.1 eV) energy band suggests the continuum CdS‐Ag₂S mini bands. Notably, limited CdS‐Ag₂S interface formation prohibited the electron‐hole charge transfer in the CdS‐Ag₂S@ODA type‐I htcs. The TEM studies established that the CdS‐Ag₂S@ODA type‐I htcs contain hexagonal CdS nanorods with surface‐passivated Silver sulfide quantum dots (Figure S7). This surface‐passivated Silver sulfide creates a weakly confined CdS‐Ag₂S interface in patch form. This phenomenon generates the blue shift in the CdS‐Ag₂S@ODA type‐1 htcs in comparison to the CdS‐Ag₂S@ODA QD SL‐1 quantum dot superlattice.

The indirect optical bandgap calculated for the dodecane thiol‐capped CdS‐Ag₂S@DDT NR SL‐2 revealed the superlattice bandgap Eg_Tauc = 1.58 eV inclined toward the Silver sulfide band [Eg_Tauc = 1.58 eV, Eg_cody = 1.67 eV, Eg_baseline = 2.20 eV] (Figure S13b, b^* and Table S4). The STEM‐EDS analysis established NR SL‐2 superlattice framework bears the interconnected spherical Ag₂S zones (9–10 nm) together with the highly confined CdS (lattice band ≤ 3 nm) zone (Figure 3). This created the p‐n junction in a continuum quantum well model with the Silver sulfide‐enriched e^–/h⁺ carrier dynamics in a very thin barrier. However, the CdS‐Ag₂S@DDT type‐II heterostructure indirect optical bandgap, Eg_Tauc = 1.33 eV, lies near the silver sulfide (1.2 eV) narrow bandgap. The indirect optical bandgap calculated by the Cody/baseline method shows similar trends with the heterostructures (Figure S14b and Table S4). The uneven distribution of CdS quantum dots on the Ag₂S eutectoid surface (Figure 5) altered the exciton dynamics at the CdS‐Ag₂S heterojunctions.

2.6. Photoluminescence and Cathodoluminescence Studies

The photoluminescence spectra (PL) of the CdS‐Ag₂S@ODA quantum dot superlattice (isolated powder QD SL‐1), recorded in toluene at different excitation wavelengths, show band edge emission from 390‐ 550 nm and surface trap emission in the 600–800 nm range (Figure 6a). Upon comparing the emission spectra with the CdS@ODA quantum dots, the broadband edge emission appeared at 480 nm, along with surface trap emission centered at 750 nm (Figure S15d). On the other hand, Ag₂S@ODA fused quantum dots showed the band edge emission from 420–523 nm (Figure S15c). The decrease in the surface‐to‐volume ratio leads to the broad emission of CdS and the blue shift in band‐edge emission of the Ag₂S quantum dots. The CdS‐Ag₂S@ODA QD SL‐1 superlattice demonstrated the strong intensity band edge spectra at 407, 420–430, and 460–490 nm range due to the CdS‐Ag₂S interface in the superlattice. Sudden decreases in the surface trap emission intensity in CdS‐Ag₂S@ODA QD SL‐1 are due to the CdS‐Ag₂S electron density overlap in the conduction band of the superlattice. Bi‐exponential decay was observed in the band edge emission (400–490 nm) with an average radiative lifetime of 1.46 ns (Figure 6b). These results show the anisotropic electron‐hole recombination directed toward the CdS‐Ag₂S superlattice with high quantum confinement. Moreover, the surface trap emission at 600, 670, and 800 nm possesses bi‐exponential decay with the average lifetime value of 54.44 ± 0.85, 78.85 ± 1.00, and 99.31 ± 1.76 µs (Figure 6c). A conduction/valence band interface of wide bandgap CdS (2.42 eV) with the narrow bandgap Ag₂S (0.9–1.2 eV) creates a superlattice type‐I p‐n junction. The heterojunction boundary in this junction is very thin. In the first step of carrier passivation, the first exciton transfer from the low‐lying CdS valence band (1S_h, 1n_h) to (1Se,1n_e) energy state of the conduction band generates the band edge spectra with bi‐exponential decay (Figure 6f). Nevertheless, the transition of electrons from the tightly bound, long‐lived exciton in the conduction band (CB) of the CdS/Ag₂S superlattice to the valence band (VB) recombination, facilitated by the mini superlattice band of Ag₂S, induces non‐radiative decay. In the non‐radiative decay process of exciton relaxation, capping agents play a crucial role. Octadecylamine delays recombination, resulting in relatively longer relaxation times in these donor–acceptor species [30], as shown in Figure 6f. Moreover, the long‐chain unbranched amines reduce atomic packing density in the nanocrystals, which can introduce additional surface traps in the quantum dots [48]. In the literature, exciton dynamics of CdS/Ag₂S heterostructures show the CdS‐centered edge emission at 470 nm and surface trap emission at 750 nm. The bi‐exponential decay was reported in these heterojunctions with the τ ₁ value of the excitonic band in the order of 100 ps [57]. The CdS‐Ag₂S@ODA type‐I heterostructure exhibited weak band‐edge emission at 450 nm and strong surface‐trap emission at 724 nm, with an overall low PL intensity in solution. Its defect‐state emissions at 630 and 735 nm showed lifetimes of 65.97 ± 0.84 µs and 59.88 ± 0.82 µs, respectively. In contrast, the CdS‐Ag₂S@ODA QD superlattice (SL‐1) displayed faster PL dynamics at surface‐trap emission (600 nm, τ_{a v} = 54.44 µs) compared to the heterostructure (630 nm, τ_{a v} = 65.97 µs). This acceleration is attributed to higher octadecylamine (ODA) ligand coverage on the superlattice (27.29%) than on the heterostructure (22.37%), as confirmed by TGA/DSC analysis (Figure S22) [48]. Moreover, at 600 nm emission, the weight of the faster (τ₁)/ slow (τ₂) decay component in the CdS‐Ag₂S@ODA QD SL‐1 superlattice, τ₁ = 18.22 µs (54.77%), τ₂ = 98.53 µs (45.23%), is significantly higher than the decay component τ₁ = 11.62 µs (47.27%) of the CdS‐Ag₂S@ODA type‐1 heterostructure. Notably, a higher population of weaker decay component τ₂ = 108.96 (52.73%) in the CdS‐Ag₂S@ODA type‐1 heterostructure suggested the slower electron transfer in the heterostructure.

FIGURE 6.

(a) Photoluminescence spectra at λ_ex = 375 and 400 nm and PL decay curve at different λ_em (b) 400–490 nm,(c) 600–800 nm for CdS‐Ag₂S@ODA quantum dot superlattice QD SL‐1 (quantum dots dispersed in toluene),(d,e) Cathodoluminescence spectra of the CdS‐Ag₂S@ quantum dot superlattice, (f) schematic for donor/acceptor recombination at  band edge/ surface state for the QD SL‐1 superlattice.

Solid state SEM‐cathodoluminescence (SEM‐CL) spectra of the CdS‐Ag₂S@ODA SL‐1 superlattice recorded at low voltage, 5 kV, revealed the weak intensity band edge emission at 420 and 508 nm (Figure 6d). However, the highest intensity surface trap emission appeared at 820 nm along with the weak band at 600 nm. Upon increasing the voltage to 10 kV, an enhancement in the superlattice surface trap emission intensity was observed with no change in the position. At the 25 kV voltage, red shift in the fast exciton band edge emission at 443, 484, and 532 nm was monitored. The red shift in the band edge emission at 25 kV established the increase in the number of fast exciton relaxations in the band edge emission e/h recombination through the superlattice Ag₂S (L_B, superlattice band) energy band [58]. Interestingly, an increase in the voltage of 25 kV, quenching was favoured, e.g., surface trap emission band intensity inclined, and a new single deep trap emission band appeared at 556 nm.

The CdS‐Ag₂S@ODA type‐I heterostructure cathodoluminescence spectra showed the weaker band edge emission at 506 nm along with the surface trap emission peak at 682 nm at 15 kV (Figure S15b). At 5 kV, only the 506 nm band emission appeared. This demonstrates that the limited electron/hole is primarily passivated through the CdS valence and conduction bands of the CdS‐Ag₂S@ODA type‐I heterostructure.

The PL spectra of CdS‐Ag₂S@DDT NR SL‐2 show band edge (400–490 nm) emission along with the surface trap emission at 725 nm (Figure 7a). Further, the lifetime measurements at band edge emission show biexponential decay with an average lifetime of 1.19 ns (Figure 7b). The dodecane thiol‐capped CdS‐Ag₂S@DDT NR SL‐2 possesses the interconnected Silver sulfide‐rich spherical zones with the rectangular CdS. These interconnected superlattices contain an 8–9 nm Ag₂S zone together with the quantum‐confined CdS (2–3 nm). This superlattice frame generates a wide superlattice continuum model where the CdS exciton traps in the Silver sulfide‐rich CdS‐Ag₂S conduction band. As a result, e^–/h⁺ donor‐acceptor recombination at the loosely bound 1n_h to 1n_e energy level was only observed at the band edge emission. The surface trap emission with a very low count was used to determine the lifetime [58].

FIGURE 7.

(a,a*) Photoluminescence spectra at λ_ex = 375, 380, and 400 nm and (b) PL decay curve for different λ_em = 345–490 nm range of CdS‐Ag₂S@DDT nanorod superlattice (recorded in toluene from 300 to 800 nm and 500 to 800 nm region). (c) Cathodoluminescence spectra of the CdS‐Ag₂S@ NR SL‐2 superlattice (in solid state), (d) schematic  for recombination pathways at the band‐edge of the NR SL‐2 superlattice.

The CL measurements on the solid CdS‐Ag₂S@DDT NR SL‐2 show no band at 5 kV. However, upon accelerating the voltage to 15 kV, band edge emission appeared at 411 and 481 nm. Deep trap emission appeared at 544 nm, along with surface trap emission evolving at 669, 765, and 792 nm. Anionic deficiency and the thiol presence, along with the wider Ag₂S zone (8–9 nm), created more surface traps in the superlattices [58]. The CL measurement at 5, 15 kV confirmed the enhanced superlattice band‐edge counts together with the low count surface traps 765 nm in the NR SL‐2 superlattice. At 15 kV operating voltage, counts enhancement at 669 and 765 nm confirmed the presence of surface traps in the nanorod superlattice.

The PL spectra of CdS‐Ag₂S@DDT type‐II htcs show a band edge at 400–490 nm upon 375 nm excitation (Figure S16a–c). The broad surface trap emission appeared at 800 nm. The lifetime decay on the band edge emission by using a 375 nm source laser revealed the bi‐exponential decay (Figure S16c). The presence of various donor‐acceptor CdS nanoparticles gives a fast e^–/h⁺ charge carrier recombination with an average lifetime of 1.12 ns. Due to the limited occurrence of surface trap emission counts toward surface trap emission, we could not record a record lifetime for the same.

The solid‐state CL spectra of the CdS‐Ag₂S@DDT type‐II htcs acquired at 10 kV showed weak intensity band‐edge emission peaks at 429 and 474 nm. Interestingly, the strong surface trap emission appeared at 820 nm with 10 kV (Figure S16d). The DDT‐capped type‐II htcs possess the eutectoid Ag₂S nanomaterial with the surface passivated CdS quantum dots (Figure S10). The type‐II band structure optical bandgap Eg_Tauc = 1.33 eV and PL carrier lifetime decay data support the fast electron‐hole D–A electron‐hole recombination at band edge/deep trap 537 at 429, and 537 nm respectively, with a lifetime of 1.18 ± 0.03 ns, and 1.19 ± 0.01 ns respectively (Figure S10 and Table S5).

The time‐resolved PL decay of DDT‐capped CdS‐Ag₂S@DDT NR superlattices shows slow e^–/h⁺ recombination with bi‐exponential band‐edge decay at 407 nm (τ_{a v} = 1.63 ± 0.04 ns), slightly longer than that of ODA‐capped CdS‐Ag₂S@ODA SL‐1 QDs (τ_{a v} = 1.42 ± 0.02 ns) (Table S5). The fitted average lifetime decays of DDT‐capped CdS‐Ag₂S@DDT NR and ODA‐capped CdS‐Ag₂S@ODA SL‐1 quantum dots exhibited similar bi‐exponential with no major change in fast decay components [τ_{1 DDT NR} 0.95 ns (90.89%), and τ_{1 ODA QD SL‐1} = 0.90 ns (91.54 %)]. However, the fitted PL lifetime decay (τ₂) minor species analysis indicates the slow electron transfer supports in the CdS‐Ag₂S@DDT NR superlattice [τ_{2DDT NR} = 8.47 ns (9.11%)] compared to the ODA‐capped CdS‐Ag₂S@ODA SL‐1 superlattices at 407 nm [τ_{2ODA QD SL‐1} = 7.04 ns (8.46%), (Table S5). This suggests that S^2– presence on the dodecane thiol‐capped CdS‐Ag₂S@DDT NR surface (DDT capping 34.76%, Figure S22) captured the photogenerated holes and enhanced the charge separation on the surface of the nanorod superlattice [59, 60, 61].

2.7. Raman Spectroscopic Studies on CdS‐Ag₂S Superstructures‐

Raman scattering spectra of the CdS‐Ag₂S superlattice/heterostructures were recorded by using a 532 nm laser source (Figure S17a). The CdS‐Ag₂S@ODA QD SL‐1 superlattice Raman spectra show no optical phonon in the amine‐capped superlattice. In the smaller‐sized semiconductor superlattices (0.9–3.8 nm), phonon energies lie below the phonon energy level of the characteristic individual nanomaterials. This highlights the novelty in phonon engineering due to the formation of superlattices [62, 63]. However, the wider optical bandgap CdS‐Ag₂S@ODA type‐I heterostructure shows the comparatively lower intensity Cd‐S first‐order longitudinal optical phonon vibration at 297 cm⁻¹ along with the second‐order longitudinal optical phonon at 607 cm⁻¹ (Figure S17c) [57, 63].

Interestingly, the DDT‐capped CdS‐Ag₂S@DDT NR SL‐2 superlattice Raman spectra revealed the high‐intensity first‐order longitudinal phonon peak 297 cm⁻¹ (Figure S17b). Moreover, the Stokes shift was observed in the second‐order optical phonon peak at 596 cm⁻¹. The vibrational mode of the CdS‐Ag₂S@DDT type‐II htcs shows comparatively weak intensity first and second‐order longitudinal optical phonon peaks at 293 and 595 cm⁻¹. Further, poor crystallinity and charge passivation in the DDT type‐II heterostructures are attributed to the low intensity first and second‐order optical phonon peaks.

2.8. Electrochemical Hydrogen Evolution Studies in Neutral Media

Electrochemical hydrogen evolution studies were conducted using CdS‐rich CdS‐Ag₂S@ODA quantum dots and Ag₂S‐rich CdS‐Ag₂S@DDT nanorod superlattices under neutral conditions. The HER performance of these superlattice‐based electrochemical devices was evaluated in 1.0 m Na₂SO₄ using a three‐electrode system. Linear sweep voltammetry (LSV) was performed at a 5 mV/s scan rate for CdS‐Ag₂S@ODA SL‐1, CdS‐Ag₂S@DDT NR SL‐2 superlattices, and CdS‐Ag₂S@ODA type‐I and CdS‐Ag₂S@DDT type‐II heterostructures.

In neutral media, white light illumination on CdS‐rich CdS‐Ag₂S@ODA SL‐1 and Ag₂S‐rich CdS‐Ag₂S@DDT NR SL‐2 electrochemical device exhibited onset potentials of 1.601 and 1.558 V, with current densities of 38.99 and 85.24 mA/cm² vs. RHE, at 1.79 V, respectively (Figure 8a). CdS‐Ag₂S@ODA type‐I and CdS‐Ag₂S@DDT type‐II heterostructures had onset potentials of 1.563 and 1.589 V, with maximum current densities of 71.69 and 39.12 mA/cm², at 1.79 V, respectively (Figure S18). The Ag₂S‐rich CdS‐Ag₂S@DDT NR SL‐2 showed superior HER activity with the lowest onset potential and highest current density compared to the other structures. The overpotentials for CdS‐Ag₂S@DDT NR SL‐2 and CdS‐Ag₂S@ODA SL‐1 were measured to be 1.572 and 1.636 V (vs. RHE) at a current density of 10 mA/cm² (Table S6).

FIGURE 8.

(a) LSV curve of the CdS‐Ag₂S@DDT/ ODA superlattices recorded in neutral media, (b,b#) double‐layer capacitance measurement at variable scan rates of the CdS‐Ag₂S@ODA quantum dot SL‐1, @DDT SL‐2 nanorod superlattices in light‐dark condition, (c) Pulsed chronoamperometry curve for the CdS‐Ag₂S@DDT SL‐2 nanorods superlattices at potential of 1.24 V (vs RHE), (d), and (d#) electrochemical impedance spectra (EIS) plot of the CdS‐Ag₂S@ODA quantum dot SL‐1 and @DDT SL‐2 nanorod superlattices in the light and dark mode respectively.

In the dark, CdS‐Ag₂S@DDT NR SL‐2 exhibited a lower onset potential (1.43 V vs. RHE, ΔV = −0.128 V) and a higher current density (109.25 mA/cm²) for HER, while CdS‐Ag₂S@ODA SL‐1 showed a higher onset potential (1.610 V vs. RHE, onset potential difference ΔV = +0.009 V) with a current density of 44.80 mA/cm². The CdS‐Ag₂S@DDT NR SL‐2 maintained the highest HER activity, with a current density of 109.2 mA/cm² at 1.79 V (vs. RHE), outperforming CdS‐Ag₂S@ODA SL‐1 (1.61 V, 44.8 mA/cm²), CdS‐Ag₂S@ODA type‐I (1.613 V, 32.4 mA/cm²), and CdS‐Ag₂S@DDT type‐II heterostructures (1.495 V, 47.2 mA/cm²). The lowest overpotential in dark conditions for CdS‐Ag₂S@DDT NR superlattice was 1.38 V (vs. 1.572 V under illumination, onset potential difference ΔV = −0.192 V) at 10 mA/cm². Strong DDT binding on the CdS–Ag₂S@DDT NR SL‐2 superlattice surfaces through Ag–S and Cd─S bonds enhances surface charge passivation and increases local electron density. Under illumination, partial Silver reduction and charge accumulation at the Ag₂S/CdS interface slightly increase the HER overpotential (η_light = 1.572 V) compared to dark conditions (η_dark = 1.38 V), giving overpotential difference ΔV = −0.192 V at 10 mA/cm². In contrast, under dark conditions, stable Ag─S/Cd─S surface bonding promotes efficient charge passivation, resulting in a lower overpotential for CdS–Ag₂S@DDT NR SL‐2 (Tables S6 and S7). Conversely, octadecylamine limits atomic packing density [50], disrupts the CdS–Ag₂S interface, and increases surface trap density in CdS–Ag₂S@ODA quantum dot superlattices (Table S5), leading to sluggish HER kinetics with a higher overpotential of 1.636 V at 10 mA/cm².

Notably, CdS‐Ag₂S@DDT type‐II heterostructures exhibited dark current with 47.2 mA/cm² at an onset potential of 1.495 V. Under illumination, photobleaching reduced the current density to 39.1 mA/cm², with an increased onset potential of 1.589 V.

Mott‐Schottky analysis of CdS‐Ag₂S@ODA/DDT superlattices determined flat band potentials under light and dark conditions (Figure S19). CdS‐Ag₂S@DDT NR SL‐2 exhibited flat band potentials of +0.70 V (light) and +1.45 V (dark), indicating minimal band bending in light mode suggest a predominant n‐type character. Faster hole transfer from the superlattice to the solution and electron withdrawing nature of thiols lower the flat band potential of DDT‐capped CdS‐Ag₂S@DDT NR SL‐2 in light mode. Moreover, excess hole carriers result in slower electron transfer compared to dark conditions [64].

Mott‐Schottky analysis of CdS‐Ag₂S@ODA SL‐1 confirmed n‐type behavior, with flat band potentials of +1.54 V (light) and +1.47 V (dark) (Table S7). The electron‐donating nature of the amine group increased the flat band potential values in the ODA‐capped CdS‐Ag₂S@ODA SL‐1 superlattices (Figure S15b and Tables S5 and S6). The marginal difference in the flat band potentials (Δf_b = 0.02 V) of CdS‐Ag₂S@DDT NR SL‐2 [V f_b = +1.45 V]/ CdS‐Ag₂S@ODA SL‐1 (V f_b = +1.47 V) in the dark confirmed the stable Ag─S/Cd─S bonds together with no charger accumulation at interfaces of the superlattices [64]. Notably, the ODA‐capped CdS‐Ag₂S@ODA type‐I heterostructures displayed flat band potentials of +1.485 V (light) and +1.52 V (dark) (Table S7). Under illumination, the photogenerated charge carrier reduced the trap state density and enhanced the efficient charge transport. In contrast, dodecanethiol‐capped CdS–Ag₂S@DDT type‐II heterostructures strongly passivate surface states. Moreover, fast photogenerated charge‐carrier recombination is observed, resulting in a decreased flat‐band potential under illumination (V_fb = 0.75 V). A higher flat‐band potential value in the dark (V_fb = 1.55 V) indicates a multistep charge‐transport behavior typical of p‐type materials. [64]. These findings highlight the dominant photo‐effect on charge carrier dynamics at the ODA/DDT‐capped type‐I and type‐II CdS‐Ag₂S interfaces due to poor charge carrier passivation.

Electrochemical impedance spectroscopy (EIS) analysis was performed under HER dark and illuminating conditions at a fixed applied bias (0.8 V), directly probing the interfacial charge transfer behavior of CdS‐Ag₂S@DDT NR SL‐2 nanorod and CdS‐Ag₂S@ODA SL‐1quantum dot superlattices. The Silver sulfide‐rich CdS‐Ag₂S@DDT nanorod SL‐2 superlattice under illumination revealed the lowest charge transfer resistance (R_ct) and series resistance (R_s) values of R_ct = 41 kΩ and R_s = 25.3 Ω (Figure 8d; Figure S18c,c# and Table S6). The improved electronic coupling lowers the value of charge transfer resistance in the ordered DDT‐capped CdS‐Ag₂S@DDT NR SL‐2 superlattices. In dark mode, the higher charge transfer and series resistance values (R_ct = 718 kΩ and R_s = 29.8 Ω) reflect sluggish electron‐hole carrier dynamics in the CdS‐Ag₂S@DDT nanorod SL‐2‐based electrochemical device for HER (Figure 8d #). In contrast, upon illumination, octadecyl amine‐capped CdS‐Ag₂S@ODA SL‐1 quantum dot superlattices showed significantly high charge transfer and series resistance values to R_ct = 214 kΩ and R_s = 25.8 Ω, reflecting carrier mobility limitations arising from trap mediated recomobinations (Figure 6d,e). Moreover, owing to the higher density of trap states, the CdS–Ag₂S@ODA QD SL‐1 quantum dot superlattice displayed increased charge‐transfer resistance (R_ct = 574 kΩ) and series resistance (R_s = 28.8 Ω) under dark conditions. The CdS‐Ag₂S@DDT NR SL‐2 nanorod superlattices exhibit lower charge transfer resistance than the ODA‐capped SL‐1 quantum dot superlattices (Table S5), indicating more efficient carrier charge transfer enabled by stronger Ag₂S/CdS interface coupling in thiol‐capped NR superlattices. Moreover, Ag₂S@DDT/ODA showed no response for electrochemical hydrogen evolution in neutral/basic/acid media [30].

Conversely, the patchy CdS‐Ag₂S@DDT type‐II heterostructure exhibited higher charge transfer resistance (R_ct) and series resistance (R_s) values of 569.9 kΩ and 27.4 Ω, respectively, in light mode. In contrast, the charge transfer and series resistance values in the dark mode were significantly lower, with R_ct = 285 kΩ and R_s = 26.6 Ω (Figure S18c,c## and Table S6). Meanwhile, the R_ct/R_s values for CdS‐Ag₂S@ODA type‐I heterostructures were 123 kΩ / 18.7 Ω in light mode and 350 kΩ / 28.8 Ω in dark mode (Figure S18c,c#,c## and Table S6).

The Nyquist curve, along with charge transfer and series resistance analysis, further establishes that dodecane thiol‐capped CdS‐Ag₂S@DDT nanorod SL‐2 ordered superlattice‐based electrochemical devices are well‐suited for light‐driven HER applications. EIS analysis confirmed that ODA‐capped CdS‐Ag₂S@ODA SL‐1 quantum dot carrier transport‐limited relative to thiol‐capped CdS‐Ag₂S@DDT NR SL‐ 2 nanorod superlattices. In contrast, CdS‐Ag₂S@ODA type‐I heterostructure‐based devices exhibited a selective response due to the high density of surface traps in a weakly confined space, performing effectively only under illumination. Photobleaching in CdS‐Ag₂S@DDT type‐II patchy heterostructures limits their HER performance, making them effective under dark conditions.

To gain deeper insight into the superior activity of CdS‐Ag₂S@DDT/ODA superlattices compared to CdS‐Ag₂S@ODA/DDT type‐I/II heterostructures, electrochemical surface area (ECSA) analysis was conducted (Table S7). Cyclic voltammetry (CV) at different scan rates in the non‐faradaic region, along with the double‐layer capacitance (C_dl) plot, is presented in Figure 8b,b #, and Figure S18. Double‐layer capacitance (C_dl) measurements were conducted to estimate the electrochemical surface area [65]. C_dl was obtained via CV tests in the non‐faradaic potential region at different scan rates. The cyclic voltammograms of CdS‐Ag₂S@ODA/DDT superlattices and type‐I/type‐II heterostructures within the voltage window of 1.4–1.5 V vs. RHE are provided in (Figure 8b,b #; Figure S18).

The Silver sulfide‐rich CdS‐Ag₂S@DDT nanorod superlattice exhibited an ECSA of 22.10 cm² in light mode, which decreased to 16.3 cm² in dark mode, indicating a reduction in active sites under dark conditions (Table S7). For octadecyl amine‐capped CdS‐rich CdS‐Ag₂S@ODA SL‐1 quantum dot superlattices, the ECSA was 22.65 cm² in light mode, higher than the 17.7 cm² recorded in dark conditions. Comparatively, CdS‐Ag₂S@ODA type‐I heterostructures exhibited even higher ECSA values in both light (28.5 cm²) and dark (26.5 cm²) modes. The presence of CdS nanorods and Ag₂S nanoparticles in type‐I heterostructures (Figure S7) contributed to an increased electrochemical surface area for HER compared to the CdS‐Ag₂S@ODA SL‐1 quantum dot superlattice (Figure 2A–C). In contrast, the CdS‐Ag₂S@DDT type‐II heterostructure showed a lower ECSA in light mode (18.35 cm²) compared to dark mode (38 cm²), confirming greater photobleaching of HER active sites than in the ordered nanorod superlattice.

In a neutral medium, the hydrogen evolution reaction (HER) begins with the Volmer step (H₂O + e⁻ → H* + HO⁻), followed by water dissociation, generating reactive hydrogen intermediates via either the Heyrovsky pathway (H₂O + e⁻ + H* → H₂ + OH⁻) or the Tafel recombination pathway (H* + H* → H₂). The Tafel slope values of 197 mV dec^{− 1} for the CdS‐Ag₂S@DDT SL‐2 nanorod superlattice and 289 mV dec^{− 1} for the CdS‐Ag₂S@DDT SL‐1 quantum dot superlattice under illumination indicate that HER in a neutral medium follows the Heyrovsky‐Volmer mechanism [66]^. In this regime, electrochemical desorption (Heyrovsky) governs the overall reaction kinetics.

Importantly, the significantly lower Tafel slope of the thiol‐capped nanorod superlattice reflects faster reaction kinetics enabled by more efficient charge transport to catalytically active sites compared to the ODA‐capped quantum dot superlattice.

A recent theoretical study on the HER performance of Ag@CdS nanotubes, based on calculated Gibbs free energy change (ΔG_H), proposed the Volmer‐Heyrovsky reaction pathway for electrochemical water splitting [67]. This mechanism involves the adsorption of generated H* species onto the CdS nanotube surface. The Gibbs free energy value (ΔG_H) for Ag‐doped CdS (0.52 eV) showed a significantly reduced overpotential compared to pristine CdS (1.60 eV). These theoretical findings on Ag@CdS nanotubes are consistent with the experimentally observed superior HER performance of Ag₂S‐rich CdS–Ag₂S@DDT SL‐2 nanorod superlattices relative to CdS‐rich CdS–Ag₂S@ODA SL‐1 quantum dot superlattices, where charge transport is constrained by the surface‐assisted recombinations.

Under dark conditions, the DDT‐capped CdS–Ag₂S@DDT SL‐2 nanorod superlattice exhibits a low Tafel slope of 181 mV dec⁻¹, while the corresponding quantum dot superlattice displays significantly higher values (289 mV dec⁻¹), confirming intrinsically faster HER kinetics in the ordered nanorod (Figure S21a, a# and Tables S6 and S7). The anisotropic geometry and strong Ag₂S/CdS interfacial coupling in the ordered nanorod superlattice facilitate directional carrier transport, thereby minimizing kinetic barriers associated with surface hydrogen adsorption and desorption.

Notably, the CdS–Ag₂S@ODA SL‐1 quantum dot superlattice and CdS–Ag₂S@ODA type‐I heterostructures exhibit higher Tafel slopes under illumination (289 and 255 mV dec⁻¹, respectively), indicative of charge transport‐limited HER kinetics dominated by trap‐mediated recombination (Figure S21 and Tables S6 and S7). The illumination‐selective behavior of CdS–Ag₂S@ODA type‐I heterostructures is further confirmed by the reduction in Tafel slope from 292 mV dec⁻¹ in dark mode to 255 mV dec^{− 1} under illumination. Conversely, the marginally lower Tafel slope of the CdS‐rich CdS–Ag₂S@ODA SL‐1 quantum dot superlattice in dark conditions reflects partial relaxation of short‐lived donor–acceptor recombination pathways (τ₁ = 0.90 ± 0.008 ns, 91.54%) yet remains inferior to the nanorod superlattice in terms of overall HER kinetics and charge‐transport efficiency.

The surface ligand‐mediated optical and electrochemical performance of CdS‐Ag₂S@ODA/DDT superlattice and heterostructure materials shows a strong correlation with their charge dynamics and catalytic efficiency (Table S6). The dodecane thiol‐capped CdS‐Ag₂S@DDT NR SL‐2 ordered superlattice [DDT capping = 34.76%, Figure S22] exhibits biexponential decay at band edge emission, with the charge carrier recombination lifetimes of τ₁ = 0.95 ± 0.007 ns (90.89%) and τ₂ = 8.47 ± 0.41 ns (9.11%), resulting in an average lifetime of τ (avg.) = 1.63 ± 0.04 ns. Additionally, XPS analysis reveals a significant negative shift in the S 2p spin‐orbital energy, confirming substantial electron transfer from the Cd 3d and Ag 3d spin orbitals to the S 2p spin‐orbital (Table S2). Moreover, S^2– presence on the dodecane thiol‐capped CdS‐Ag₂S@DDT NR superlattice surface, together with an ordered CdS‐Ag₂S interface (DDT capping 34.76%, Figure S22), captured the photogenerated holes and enhanced the charge separation on the surface [48]. The strong Ag─S/Cd─S bond in the thiol‐capped CdS‐Ag₂S@DDT superlattices lead significant surface charge passivation, and the lower Tafel slope value of 197 mV/dec in a neutral medium favors the proton adsorption and desorption on the ordered superlattice surface. The higher current density of 85.24 mA/cm² at 1.558 V onset potential and lower charge transfer resistance (R_ct = 41 kΩ) confirm the superior catalytic efficiency of the ordered CdS‐ Ag₂S@DDT NR superlattice.

Notably, octadecyl amine‐capped CdS‐Ag₂S@ODA SL‐1 quantum dot superlattice [ODA capping 27.29%, Figure S22] exhibited dual‐band emission. The bi‐exponential decay at band edge emission (407 nm), with donor‐acceptor (D‐A) recombination lifetimes of τ₁ = 0.90 ± 0.008 ns (91.54%) and τ₂ = 7.04 ± 0.37 ns (8.46%), resulted in an average lifetime of τ _(avg) = 1.42 ± 0.02 ns, indicating a high population of short‐lived of D‐A recombination species (91.54%) in the CdS‐Ag₂S@ODA SL‐1 quantum dot superlattice. Moreover, the CdS‐Ag₂S@ODA SL‐1 superlattice displays surface‐trap emissions centered at 670 nm, with biexponential decay yielding an average D‐A lifetime of τ (_avg) = 78.8 ± 1.00 µs. The octadecyl amine capping reduces packing density and introduces surface disorder together with a defect state [48]. These effects correlate well with the relatively high charge transfer resistance (R_ct = 214 kΩ) and series resistance (R_s = 25.8 Ω) (Tables S5 and S6), and a higher Tafel slope value of 289 mV dec⁻¹, explaining the sluggish carrier charge transport‐limited HER activity under illumination. XPS analysis further reveals a positive shift in the Cd 3d (+6.71 eV) and S 2p (+1.13 eV) spin‐orbital doublet energies, accompanied by a negative shift in the Ag 3d energy (−6.02 eV), confirming electron transfer from Cd and S toward Ag (Table S2). This surface ligand‐induced electronic transition contributes to the stable electrochemical performance of CdS‐Ag₂S@ODA SL‐1‐qautnum dots superlattice‐based devices under both light and dark conditions.

The superior HER activity of the CdS‐Ag₂S@DDT SL‐2 nanorod superlattice [ICP elemental ratio Cd: Ag = 1:16.15, Table S1), compared to the CdS‐Ag₂S@ODA SL‐1 superlattice (Cd: Ag ICP ratio = 1.82:1, Table S1, ODA capping = 27.29% Figure S22), is attributed to thiol‐capped ordered CdS‐Ag₂S interface, one‐dimensional nanorod growth, lower charge transfer resistance, reduced onset/overpotential.

Furthermore, chronoamperometry (i‐t) measurements, performed using a four‐step pulse sequence with a width of 4000 s in both dark and light conditions, demonstrated the remarkable stability of CdS‐Ag₂S@DDT SL‐2 nanorods for 3000 s at a potential of 1.24 V. The preserved catalytic activity is confirmed by similar LSV curves recorded before and after the stability test (Figure 8c; Figures S23 and S24).

In contrast, XPS analysis of the CdS‐Ag₂S@DDT type‐II heterostructure (DDT capping = 22.64%, Figure S22) confirms electron transfer from the Cd 3d and S 2p spin orbitals to the Ag 3d spin orbitals (Table S2). A higher population of short lived donor‐acceptor species is observed for this type‐II heterostructure (Cd: Ag ratio = 1: 5.01, Table S2), with carrier lifetimes of τ₁ = 0.92 ± 0.005 ns (96.31%) and τ₂ = 6.56 ± 0.50 ns (3.69%), yielding an average lifetime of τ _(avg) = 1.13 ± 0.02 ns (Table S5). This rapid recombination correlates with photobleaching effects and the suppressed HER performance in light mode [68]. Previous reports on citrate‐capped CdS‐Ag₂S hierarchically nanocomposite with an atomic weight ratio of Cd and Ag 2 % showed enhanced visible light‐driven photocatalytic H₂ evolution (375.6 µmol h⁻¹g⁻¹) [46, 69, 70]. Notably, the excess Silver (>2%) in the patchy heterostructures limits the charge carrier transport and photochemical HER performance among the CdS/Ag₂S@ type‐1/II and core‐shell nanostructures. Our experimental results confirm that the thiol‐capped, ordered nanorod superlattice exhibits enhanced HER activity for the CdS–Ag₂S@DDT NR SL‐2 system, particularly at a higher Silver content (ICP elemental ratio Cd: Ag = 1:16.15).

On the other hand, the octadecyl amine‐capped CdS‐Ag₂S@ODA type‐I heterostructure exhibits lower charge transfer resistance (R_ct = 123 kΩ), series resistance (R_s = 18.7 Ω), and sluggish hydrogen adsorption/desorption with Tafel slope value (255 mV dec⁻¹) under illumination compared to CdS‐Ag₂S@DDT SL‐2 nanorod superlattice devices (Tafel slope 197 mV dec⁻¹). The emergence of long‐lived D‐A electron‐hole recombination, with lifetimes of τ₁ = 11.62 ± 0.18 µs (47.27%) and τ₂ = 108.96 ± 1.58 µs (52.73%), resulting in an average lifetime of τ_(avg) = 65.97 ± 0.84 µs at surface states emission centered at 630 nm. This indicates that ODA‐capped CdS–Ag₂S@ODA type‐I heterostructure–based electrochemical devices exhibit poor surface charge‐carrier passivation for HER.

3. Conclusion

In this study, we report a one‐pot synthesis of CdS‐Ag₂S@ODA/DDT‐capped superlattice heterostructures using a molecular source approach. The structural morphology, phase, and surface chemical nature of CdS‐ Ag₂S superlattices were investigated through high‐resolution TEM, PXRD, and XPS analyses. High‐resolution TEM confirmed that the CdS‐rich CdS‐Ag₂S@ODA QD SL‐1 random superlattice framework features a narrow Silver sulfide/Cadmium sulfide interfacial region (≤1 nm) within the superlattice. Interestingly, STEM‐DF EDS analysis@80 kV revealed an ordered, dodecane thiol (DDT)‐capped CdS‐Ag₂S@DDT NR SL‐2 superlattice with a Silver sulfide‐rich wide domain. The energy band alignment shifted toward Silver sulfide, highlighting the impact of composition on electronic properties. The tunability between superlattices and type‐I/type‐II heterostructures is governed by triphenylphosphine, through the deliberate selection of precursors [(PPh₃)₂Ag(SCOPh)] 2 over [Ag(SCOPh)] 3.

The dodecane thiol‐capped, ordered CdS‐Ag₂S@DDT NR SL‐2 nanorod superlattice exhibits a high population of fast charge carrier recombination at the band edge, with minimal surface trap states [26, 27, 28, 29]. Notably, dual‐band emission was featured within the octadecylamine‐capped CdS‐Ag₂S@ODA SL‐1 superlattice. In contrast, CdS‐Ag₂S@ODA type‐I heterostructures display a bi‐exponential decay of surface state emissions originating from electronic dislocations within the CdS wurzite phases and ODA surface ligands [48]. Additionally, dodecane thiol‐capped CdS‐Ag₂S@DDT type‐II eutectoid structures exhibit fast electron‐hole recombination dynamics at both the band edge and surface states. Solid‐state variable‐voltage cathodoluminescence studies further captured and confirmed the surface traps within amine‐capped CdS‐Ag₂S@ODA superlattices and DDT‐ capped type‐II heterostructures.

The observed electronic properties and charge carrier dynamics significantly influence the electrochemical hydrogen evolution performance of CdS‐Ag₂S@ODA/DDT superlattice and heterostructure‐based devices. A comparative electrochemical HER study on CdS‐Ag₂S@ODA/DDT superlattices confirms that thiol‐capped CdS‐Ag₂S@DDT nanorod superlattices are highly efficient for electrolysis in neutral media, under both dark and illuminated conditions, owing to the ordered CdS‐Ag₂S interface within the nanorod superlattice. Meanwhile, the surface modification with L‐category surface ligand, e.g, octadecylamine‐capped CdS‐Ag₂S@ODA quantum dot superlattice, offered disordered crystal phase packing and comparatively lower charge transfer efficiency under illumination.

This study differs from previous work on CdS‐Ag₂S core–shell and conventional structures, where multiple energy levels caused multi‐exponential decay and poor photoelectrochemical performance at higher silver contents, with no prior reports on electrochemical water splitting in neutral media [46, 67]. We anticipate that our CdS‐Ag₂S superlattices will be utilized in a wide range of applications, including photo/electrochemical hydrogen production through hydrogen evolution reactions, as well as in optoelectronic devices such as light‐emitting diodes.

Conflicts of Interest

The authors declare no conflict of interest.

Supporting information

Supporting File: smll73427‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.