Synergistic role of self-induced CdIn₂S₄ nanopyramid sacrificial layer for the enhanced photocatalytic activity of TiO₂ nanotube-based chalcogenide heterostructures

Abstract

A simple treatment was employed to obtain a novel design of a highly stable visible light responsive In₂S₃/CdS/TiO₂ nanotube-based photocatalyst for H₂ production. At first, In₂S₃/CdS/TiO₂ nanotubes were fabricated by incorporating CdS and In₂S₃ quantum dots (QDs) onto TiO₂ nanotubes (TNTs) via a chemical bath deposition method. In the second step, a portion of the QDs layer (CdS and In₂S₃) was converted into a layer of CdIn₂S₄ pyramid-like structure by thermal treatment of In₂S₃/CdS/TNTs in an Ar environment (Ar-In₂S₃/CdS/TNTs). The photocatalytic performances of the different heterostructure assemblies (simple and Ar-treated samples) were tested. The incorporation of CdS and In₂S₃ QDs significantly improved the visible response, showing higher photocurrent density and H₂ production rate (0.5694 mL h⁻¹ cm⁻²) values. Furthermore, the outer layer of CdIn₂S₄ improved the photostability of In₂S₃/CdS/TNTs and further boosted the H₂ evolution rate (0.6281 mL h⁻¹ cm⁻²). The apparent quantum yields (AQYs) of Ar-In₂S₃/CdS/TNTs at 450 nm and 632 nm LED were 5.54% and 13.57%, respectively. The higher photocurrent density and lower charge transfer resistance suggest that the structure of the Ar-In₂S₃/CdS/TNTs photoanodes provides efficient charge carrier separation and interfacial charge transport properties. Dye degradation of methylene blue using Ar-In₂S₃/CdS/TNTs showed complete degradation of the dye within 22 min, which was 5 minutes lesser than that observed for In₂S₃/CdS/TNTs.

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Introduction

The dearth of fossil fuel-based energy resources has led the world to seek alternative ways to harvest energy. Since the ground-breaking work of Fujishima and Honda on water splitting using titanium dioxide (TiO₂),¹ photocatalysis has gained much interest for harnessing solar energy to produce abundant and environmentally friendly hydrogen fuel. Semiconductor-mediated photocatalysis offers remarkable potential to produce hydrogen due to their suitable band structures. To date, in addition to TiO₂, zinc oxide (ZnO) and tungsten oxide (WO₃) have been the most extensively explored photocatalyst materials.^2–6 However, TiO₂-based photocatalysts offer advantages over others due to their corrosion resistance, biocompatibility and higher chemical stability.^7–11 Furthermore, the one-dimensional (1D) morphology of TiO₂ (e.g., TiO₂ nanotubes (TNTs)) offers additional benefits over its bulk counterpart. For instance, the diffusion length of the photogenerated charge carriers is increased because the grain boundary effects, which typically act as recombination centers, are minimized due to the 1D, highly ordered structure of the nanotubes. The photogenerated electron–hole pair recombination is also prevented due to alternative charge transport pathways and differences in the diffusion lengths of electrons and holes.^12,13 The hollow interior of the nanotubes boosts photoharvesting due to multi-light scattering and encourages surface-activated redox reactions on both sides of the walls. Despite all these benefits, the photocatalytic activity of simple TNTs is not significant due to their large band gap of ∼3.2 eV. This limits the light absorption by TNTs to the UV range in the solar spectrum. To overcome this issue, co-sensitization of TNTs with a photoactive material with a suitable band gap and position is expected to produce the desired results.

Metal chalcogenides, such as CdS, have been seen as promising visible light responsive materials for coupling with TNTs. CdS offers complementary band edge positions for proton reduction. However, it tends to easily photo-corrode in aqueous electrolytes due to the capture of photogenerated holes. The photo-corrosion property of CdS hinders its extensive utilization for photocatalytic hydrogen generation and degradation of water contaminants. To resolve this problem, a corrosion-resistant passivation layer covering the sulfide surface was applied to prevent direct contact with the electrolyte. Efforts have also been made to facilitate hole transfer away from the sulfide surface. In some cases, a thin polymer coating has been used as a passivating layer to prevent photo-corrosion by shielding the sulfide nanostructures.^14–17 However, the thickness of the passivation layer is the critical parameter. Furthermore, such a layer reduces the available active sites for photocatalysis, resulting in inferior efficiency. Meanwhile, the utilization of CdS in heterostructures with other semiconductors can significantly minimize these issues. CdS-based heterostructures have been synthesized by introducing metals and low band gap semiconductors with relatively higher chemical stability and cascade band alignment to reduce the photo-corrosion problem of CdS.^18–20 Materials, such as CdS/rGO/MoS₂, CdS/ZnIn₂S₄ core–shell, AgVO₃/ZnIn₂S₄ and TiO₂ nanowire-supported In₂S₃, have been reported. In such hetero-structures, easy hole transfer kinetics is achieved to prevent hole accumulation at the sulfide surface.^21–24 CdS-based hetero-structures have been reported to show improved stability, with a subsequent increase in H₂ production.^{18,21,25–27} The established potential gradient among the nanostructure interfaces of the two functional components of the heterostructure causes the rapid transfer of charge carriers. This, in turn, decreases the possibility of carrier recombination in such hybrid materials.^28–31 Recently, sulfur vacancy-chalcogenide/Ti₃C₂ Schottky junctions have been studied to achieve a giant built-in electric field (BIEF) via defect-mediated heterocomponent anchorage.³² This technique yields a 13.34-fold higher efficiency than that of unmodulated chalcogenides. Other studies show that inclusion of In₂S₃ in CdS-based heterostructures is highly advantageous owing to its compatible band alignment. In addition, In₂S₃ is a more stable and environmentally friendly narrow band gap semiconductor compared to CdS.^33–37 Ternary heterostructures based on TNTs can be utilized for achieving enhanced photocatalytic activity and improved photo-corrosion resistance of CdS. Moreover, the cascade bandgap positions of TNTs with CdS and In₂S₃ may also lead to a well-separated electron–hole pair.

There is a need to devise synthesis strategies to overcome the problem of photo-corrosion and increase the functional characteristics of sulfide-based photocatalysts.³⁸ Due to the relatively unstable nature of binary chalcogenides, their conversion into highly stable ternary chalcogenides by chemical or physical means is highly useful to deal with the photo-corrosion issues.^39,40 Recently, enhanced photo-conduction and photocatalytic efficiency were reported for In₂S₃ nanosheets doped with Cd and Zn.⁴¹ It would be interesting to fabricate a passivation layer composed of CdIn₂S₄, which may favor photogenerated charge separation and increase the stability of the sulfide layer due to its hole-capturing ability.

In this work, a novel design for a highly stable visible light-responsive ternary hetero-structure (In₂S₃/CdS/TNTs) was explored. First, a ternary hetero-structure based on TNTs incorporating CdS and In₂S₃ QDs was fabricated. The photocatalytic performance of the different hetero-structure assemblies (e.g., TNTs, CdS/TNTs, In₂S₃/TNTs, and In₂S₃/CdS/TNTs) was investigated. The synergistic effect of CdS and In₂S₃ QDs dramatically increases the visible response of the hetero-structure electrode. Furthermore, a simple treatment procedure was utilized to improve its photostability by introducing an outer sacrificial layer of CdIn₂S₄. The passivation layer composed of CdIn₂S₄ enhanced the H₂ evaluation rate. In addition, the argon-treated hetero-structure (Ar-In₂S₃/CdS/TNTs) shows further enhancement in photocatalytic activity, as evidenced by the degradation of methylene blue (MB).

Materials and methods

Titanium (Ti) sheets (0.125 mm thickness and 99.6% purity) were acquired from ADVENT Research Materials Ltd, England. Ethylene glycol (EG, 99.9% purity), ammonium fluoride (NH₄F), cadmium chloride (CdCl₂, 99.99%), sodium sulfide (Na₂S, 99.5%), indium fluoride (InF₃) and methylene blue were obtained from Sigma-Aldrich, USA. Deionized (DI) water was used as per the requirements of the synthesis and photocatalytic experiments.

Anodization of Ti

Commercial Ti sheets were cleaned with acetone, ethanol, and DI water successively and dried under a nitrogen (N₂) stream. Anodization was performed in a two-electrode anodization cell (Ti sheet served as the anode and Pt sheet as the cathode) using a high-voltage potentiostat (Jaissle IMP 88 PC, Germany). Ti sheets were anodized at 50 V for 4 h in EG containing 0.25 wt% NH₄F and 5 wt% DI water. The digital images of different stages of the sample are provided in Fig. S1(a–g). The TNTs grown on the Ti sheet were then removed by sonication in DI water for 15 min, resulting in a honeycomb-like patterned morphology on the surface (Digital image: Fig. S1b). The patterned Ti sheets were then used to obtain uniform arrays of nanotubes via the second step anodization under identical conditions to those used previously.⁴² After the second-step anodization, the samples were washed with ethanol and dried in a N₂ stream (Digital image: Fig. S1c). The resulting amorphous TNTs were converted to anatase phase by annealing in air at 450 °C for 2 h, as shown in Scheme 1a (Digital image: Fig. S1d).

Scheme 1 Fabrication process of Ar-In₂S₃ QDs/CdS QDs/TNTs. (a) Anodized TiO₂ nanotubes (TNTs), (b) decoration of TNTs with CdS QDs, (c) decoration of In₂S₃ QDS on CdS/TNTs, and (d) self induced CdIn₂S₄ pyramid layer after Ar annealing of In₂S₃/CdS/TNTs at 400 °C.

Decoration of TNTs with CdS QDs

CdS QDs were decorated on TNTs using the conventional sonication-assisted successive ion layer adsorption and reaction (SILAR) method, as reported previously.⁴³ Briefly, aqueous solutions of 0.05 M CdCl₂ and 0.05 M Na₂S were prepared separately. The annealed TNTs were sequentially immersed in both precursor solutions, each for 1 min, aided with a sonication process at room temperature. The excess amount of the precursors was removed after each step by washing with DI water. This four-step process (soaking and rinsing) is considered as one cycle, as shown in Scheme 1b (Digital image: Fig. S1e). This process was repeated for 3, 6, 9, and 12 cycles to increase the density of CdS QDs on TNTs, producing greenish-yellow samples. The samples were named as CdS(3) QDs/TNTs, CdS(6) QDs/TNTs, CdS(9) QDs/TNTs, and CdS(12) QDs/TNTs, corresponding to the number of deposition cycles.

Fabrication of In₂S₃/CdS/TNTs photocatalysts

A layer of In₂S₃ QDS was deposited on CdS/TNTs using the facile sonication-assisted chemical bath deposition (CBD) method. The CdS deposition cycles were maintained at 9 cycles (CdS(9) QDs/TNTs). Briefly, the CdS/TNTs were sequentially dipped in a 0.025 M InF₃ aqueous solution for 1 min, followed by rinsing with DI water for 30 s. The samples were then immersed in 0.375 M Na₂S aqueous solution for another 1 min, followed by washing with DI water. The loading densities of In₂S₃ QDs were maintained by 3, 5, 7 and 9 deposition cycles, as shown in Scheme 1c (Digital image: Fig. S1f).

Fabrication of CuIn₂S₄ passivation layer on In₂S₃/CdS/TNTs

The prepared In₂S₃/CdS/TNTs hetero-structure samples were annealed in an Ar environment at 400 °C for 1 h in order to obtain highly stable photo-electrodes (Digital image: Fig. S1g). Annealing induced a passivation layer of CdIn₂S₄ on In₂S₃/CdS/TNTs by thermal diffusion of Cd ions towards the In₂S₃ crystal structure. At first, CdS and In₂S₃ QDs form a compact thin layer, which subsequently changes into larger pyramid-like structures⁴⁴ covering the nanotubes, as shown in Scheme 1d and can be seen from the TEM and SEM images (Fig. 3–5).

Results and discussion

X-ray diffraction (XRD, X'pert Philips PMD, CuK_α 1.54056 Å) was utilized for ascertaining the phase, purity, and crystal structure of the synthesized hetero-structures. Fig. 1a shows the XRD patterns of the pristine TNTs, CdS/TNTs, In₂S₃/TNTs, and In₂S₃/CdS/TNTs hetero-structures. The diffraction peaks for reflections from TNTs and CdS/TNTs crystal planes are indexed according to the tetragonal structure of anatase TiO₂ (ICCD card # 01-071-1166) and CdS (ICCD card # 42-1411). The diffraction pattern of In₂S₃/TNTs shows the main reflection peaks at 23.33°, 27.43°, 33.24°, and 43.62° from (220), (311), (400) and (511) crystal planes of β-In₂S₃ (ICCD card # 01-084-1385), respectively, besides the characteristic reflections from anatase TiO₂ (ref. 45) (Inset image). The qualitative analysis of the XRD data for the In₂S₃/CdS/TNTs hetero-structure shows all the characteristic peaks from its constituent materials without any significant changes in peak positions. The inset graph clearly shows the co-existence of reflections from both CdS and In₂S₃ crystal planes, designated by letters C and I, respectively, in their reflective planes, along with TNTs.

Fig. 1 XRD patterns of pristine TNTs, CdS/TNTS, In₂S₃/TNTs, and In₂S₃/CdS/TNTs (a) without and (b) with Ar-treatment at 400 °C. (Inset) (a) Shows the enlarge portion of XRD of In₂S₃/CdS/TNTs.

Fig. 1b presents the XRD pattern of Ar-treated TNTs, CdS/TNTs, In₂S₃/TNTs, and In₂S₃/CdS/TNTs to determine any changes in the crystal structure and phase of the samples. XRD patterns of Ar-TNTs, Ar-CdS/TNTs, and Ar-In₂S₃/TNTs do not show any significant change in peak positions; however, the sharp peaks indicate improved crystallinity. An interesting result has emerged from the XRD measurement of the Ar-In₂S₃/CdS/TNTs heterojunction. Along with the crystalline phases of TNTs, a new diffraction peak appeared at 23.1° and 43.3°, corresponding to (220) and (511) related to CdIn₂S₄ at the expense of CdS. Moreover, the intensity of the apparent diffraction peaks of In₂S₃ at 23.3° and 27.4° (311) increases after thermal annealing of In₂S₃/CdS/TNTs in Ar as compared to the XRD pattern of Ar-In₂S₃/TNTs. This fact can be ascribed to the presence of cubic CdIn₂S₄ (ICCD card # 00-027-0060) sacrificial layer whose peaks lie in close proximity to those of In₂S₃ and thus overlap.

Morphological studies of various fabricated samples were conducted using a field emission scanning electron microscope (Hitachi FE-SEM S4800, Japan). Fig. 2 shows FESEM images of pristine TNTs, CdS/TNTs, In₂S₃/TNTs, and In₂S₃/CdS/TNTs systems. The top-view image of pristine TNTs (Fig. 2a) shows a regular array of nanotubes with open mouths, with an inner tube diameter of about 100 nm. The corresponding cross-sectional image (Fig. 2b) shows highly smooth tube walls. SEM images of CdS/TNTs (Fig. 2c and d) and In₂S₃/TNTs (Fig. 2e and f) show the homogeneous deposition of the CdS QDs or In₂S₃ QDs on the top as well as on the walls of the tube, respectively. The dimensions of CdS and In₂S₃ QDs estimated from SEM images are 5 nm and 4 nm, respectively. In the In₂S₃/CdS/TNTs heterogeneous system (Fig. 2g and h), more particles are deposited on the tube walls, but the top of the nanotubes remains open. It can also be seen (Fig. 2h) that the tube walls become rougher after co-sensitization of TNTs with CdS and In₂S₃ QDs. No significant increase in the QD size is observed for the co-sensitized system. This shows that no agglomeration occurs at these optimized experimental parameters, that is, the deposition cycle and solution concentration. The effect of different deposition cycles of In₂S₃ QDs on the morphology of In₂S₃/CdS/TNTs is presented in Fig. S2(a–h). The top and cross-sectional SEM images show the increased loading density of QDs on TNTs by increasing the deposition cycles from 3 to 7 (Fig. S2(a–f)). However, the QDs become agglomerated by further increasing the deposition cycles up to 9 (almost covering the nanotube's top as shown in Fig. S2g). Therefore, 7 cycles are considered an optimized value for the deposition cycle for In₂S₃ on In₂S₃/CdS/TNTs.

Fig. 2 FESEM top and cross-sectional images of (a and b) TNTs, (c and d) CdS/TNTs, (e and f) In₂S₃/TNTs, and (g and h) In₂S₃/CdS/TNTs hetero-structure.

The argon (Ar) treatment of In₂S₃/CdS/TNTs heterogeneous structure was conducted in an Ar environment at 400 °C. The morphology of In₂S₃/CdS/TNTs was observed to change from uniformly deposited QDs on TNTs into randomly oriented three-dimensional pyramids aggregated on the surface of TNTs (Fig. 3a–h). To understand the formation process of CdIn₂S₄ pyramids on Ar-In₂S₃/CdS/TNTs (CdIn₂S₄/In₂S₃/CdS/TNTs), different loading densities of In₂S₃ QDs were utilized and controlled by the deposition cycles. FESEM images of Ar-treated In₂S₃/CdS/TNTs are presented in Fig. 3(a–h) for various deposition cycles of In₂S₃. For 3 cycles (Fig. 3(a and b)), after Ar annealing, a clearly rough surface appeared with a few small pyramids due to the insignificant amount of In₂S₃.

Fig. 3 FESEM top and cross-sectional images of Ar-In₂S₃/CdS/TNTs hetero-structures for (a and b) 3, (c and d) 5, (e and f) 7 and (g and f) 9 loading cycles of In₂S₃ on CdS/TNTs.

By increasing the deposition cycles to 5 and 7 (Fig. 3(e and f)), the formation of interconnected pyramids increased and covered the whole TNTs surface. Any further increase in the loading density up to 9 cycles of In₂S₃ led to the deterioration of the pyramids into large agglomerates (Fig. 3g and h). This fact is attributed to the thermal diffusion of Cd from CdS into the In₂S₃ crystal structure, forming a new CdIn₂S₄ phase. Moreover, the loading density of In₂S₃ plays a vital role in the formation of these pyramids, which can be controlled by the deposition cycles. For comparison, Fig. S3 shows the cross-sectional FESEM images of Ar-annealed TNTs, CdS(9)/TNTs, and In₂S₃(7)/TNTs. Apart from their increased crystallinity as confirmed by XRD, the Ar-treatment brings no significant morphological change into the TNTs (Fig. S3a), CdS(9)/TNTs (Fig. S3b) and In₂S₃(7)/TNTs (Fig. S3c). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images of In₂S₃/CdS/TNTs and Ar-In In₂S₃/CdS/TNTs are presented in Fig. 4 and 5, respectively. Fig. 4a and b reveal hollow TiO₂ nanotube (TNT) arrays with a diameter of about 100 nm and wall thickness of about 10–12 nm, which is in accordance with the FESEM findings. Fig. 4c and e show rough tube walls indicating the deposition of CdS and In₂S₃ on TNTs, which are uniformly decorated with CdS and In₂S₃ QDs, and appear as small spherical nanoparticles. The average sizes of CdS and In₂S₃ are 3 nm and 5 nm, respectively. Fig. 4d and f show the HRTEM images of In₂S₃/CdS/TNTs. The images clearly show well-defined lattice fringes. The interplanar spacing is determined using the ImageJ software and indexed in the figures. In Fig. 4d and f, the fringes corresponding to interplanar spacings of approximately 0.622 nm, 0.272 nm (attributed to (103) and (400) lattice planes of In₂S₃)^46–48 and 0.358 nm, 0.336 nm (for (100) and (002) planes of CdS),⁴⁹ confirm their existence and crystalline nature. The lattice fringes at 0.389 nm and 0.32 nm corresponding to the (220) and (311) crystal planes of CdIn₂S₄ are also observed, which indicate that the interdiffusion between Cd and In species occurred, leading to the formation of a homogeneous ternary CdIn₂S₄ phase on the TNTs surface after annealing in an Ar atmosphere.^47,50,51 Moreover, the TNT walls remained crystalline, showing characteristic (101) planes with a spacing of ∼0.355 nm (Fig. 4d).⁵²

Fig. 4 (a–c and e) TEM and (d and f) HRTEM images of Ar-In₂S₃/CdS/TNTs.

Fig. 5 (a and b) TEM and (c) HRTEM images of Ar-In₂S₃/CdS/TNTs.

Fig. 5(a–c) shows the TEM and HRTEM images of Ar-In₂S₃/CdS/TNTs. The TEM image in Fig. 5a shows a thin layer of CdS and In₂S₃ QDs on the TNT surface, indicating that the walls of TNTs are encapsulated by a thin layer of CdS and In₂S₃ QDS, forming a core-shell-like structure. In Fig. 5b, a continuous layer composed of deformed nano-pyramidal grains can be seen on the surface of TNTs, where the bottom of the nano-pyramids is interlaced with each other. The lattice fringes in the HRTEM image (Fig. 5c) show interplanar spacings of approximately 0.389 nm and 0.32 nm, which were indexed to the (220) and (311) crystal planes of CdIn₂S₄, respectively.

Energy-dispersive X-ray spectra (EDS) and elemental mapping of Ar-In₂S₃/CdS/TNTs further confirmed the structural evolution of the heterogeneous structure. Fig. 6a reveals the TEM image of the sample showing a rough surface between the walls of the nanotubes. This roughness of the walls can be due to the deposition of CdS and In₂S₃ QDS, along with a layer of deformed pyramids of CdIn₂S₄. The elemental mapping of the same image in Fig. (6b and c) shows Ti and O were localized within the tubular framework, while Fig. (6d–f) displays a homogeneous and overlapping distribution of Cd, In, and S elements along the nanotube walls, indicating core shell morphology along tube walls and interdiffusion of Cd into In₂S₃, leading to the formation of a non-uniform CdIn₂S₄ nano-pyramid layer. These observations confirm the partial transformation of CdS and In₂S₃ heterostructures into an outer layer of the crystalline CdIn₂S₄ phase with well-integrated interfaces on TNTs. To further validate the formation of CdIn₂S₄ after annealing In₂S₃/CdS/TNTs in the Ar environment, we obtained Raman spectra (LabRAM Odyssey, HORIBA France SAS, France) of the pristine TNTs, CdS/TNTs, In₂S₃/TNTs, In₂S₃/CdS/TNTs, and Ar-In₂S₃/CdS/TNTs, as shown in Fig. 6h. The Raman spectrum of the annealed TNTs displays five characteristic peaks at 143, 195, 398, 515, and 636 cm⁻¹, corresponding to the E_g, B_1g, A_1g, and E_g modes of the anatase phase, respectively.⁵⁰ The appearance of a sharp and broad peak at 297 cm⁻¹ in CdS/TNTs indicates the presence of CdS QDs on the TNTs surface.⁵³ Similarly, the In₂S₃/TNTs spectrum shows peaks at 306 and 933 cm⁻¹, confirming the formation of an In₂S₃ layer on TNTs.^46,48 For In₂S₃/CdS/TNTs, the peaks at 306 and 601 cm⁻¹, along with those of TNTs, suggest the co-existence of In₂S₃ and CdS QDs. After the Ar annealing of In₂S₃/CdS/TNTs, two small but broad peaks appeared at 185 and 353 cm⁻¹, attributed to the E_g and A_1g modes, respectively, along with the main CdS and In₂S₃ peaks, confirming the appearance of CdIn₂S₄ on the In₂S₃/CdS/TNTs heterostructure.^54,55 It is worth noting that the CdIn₂S₄/In₂S₃/CdS/TNTs electrode is referred to as Ar-In₂S₃/CdS/TNTs for comparison with other Ar-annealed samples, such as Ar-TNTs, Ar-CdS/TNTs, and Ar-In₂S₃/TNTs.

Fig. 6 (a)Transmission electron microscopy image and (b–g) energy-dispersive X-ray spectra (EDS) and elemental mapping of Ar-In₂S₃/CdS/TNTs. (h) Raman spectra of TNTs, CdS/TNTs, In₂S₃/TNTs, In₂S₃/CdS/TNTs and Ar-In₂S₃/CdS/TNTs.

To investigate the surface composition and valence states of Ar-In₂S₃/CdS/TNTs, the XPS core level spectra of Ti 2p, O 1s, Cd 3d, In 3d, and S 2p were evaluated (Fig. 7(a–e)). The deconvoluted Ti 2p spectrum (Fig. 7a) exhibits a strong peak around 458.98 and 464.96 eV, which can be assigned to Ti 2p_3/2 and Ti 2p_1/2, respectively. These states demonstrate the presence of Ti⁴⁺ states in the sample. The high-resolution spectrum of oxygen is deconvoluted into two peaks, as shown in Fig. 7b. The main peak of O 1s at ∼529.8 eV can be assigned to the anionic oxygen in the TiO₂ lattice. In addition, a small shoulder peak located at about 530.9 eV corresponds to the hydroxyl species. In Cd 3d spectra (Fig. 7c), the characteristic peaks located at 405.36 eV and 411.97 eV are attributed to Cd 3d_5/2 and Cd 3d_3/2, respectively, which verify the existence of Cd²⁺ in the system. In Fig. 7d (high scan spectrum of In 3d), the two peaks located at 445.16 eV and 452.76 eV are due to the In 3d_5/2 and In 3d_3/2 doublet states, respectively, which shows that In ions exist in the In³⁺ oxidation state. The S 2p spectrum is deconvoluted into three peaks. The peaks due to spin–orbit splitting for the S 2p core level (Fig. 7e) are located at 162.57 eV and 161.56 eV. Another small peak at 169.2 eV is attributed to S₄²⁻. Fig. S4 shows the comparison between high-resolution XPS spectra of In₂S₃/CdS/TNTs and Ar-In₂S₃/CdS/TNTs. Based on the high-resolution XPS elemental analysis of both annealed (treated) and un-annealed (un-treated) In₂S₃/CdS/TNTs samples, all the peaks (in Ti 2p, O 1s, Cd 3d, In 3d and S 2p spectra) match well with each other in both samples. However, a slight shift in the binding energies was observed, indicating the conversion of the binary chalcogenides CdS and In₂S₃ into the ternary chalcogenide CdIn₂S₄ crystal structure on the surface of TNTs.

Fig. 7 XPS core level spectra of (a) Ti 2p, (b) O 1s, (c) Cd 3d, (d) In 3d and (e) S 2p for Ar-In₂S₃/CdS/TNTs samples.

The light absorption characteristics of the untreated and Ar-treated hetero-structures were investigated by UV-vis diffuse reflectance spectroscopy (DRS). Fig. 8a shows the DRS spectra of the untreated TNTs, CdS/TNTs, In₂S₃/TNTs, and In₂S₃/CdS/TNTs. An obvious absorption band at 368 nm in the UV range is observed for the pristine TNTs, which depicts the excitation of the electron from the valence band (VB) of TiO₂ to the conduction band (CB) upon illumination with solar radiation. Incorporating CdS or In₂S₃ QDs on TNTs results in a red shift in the absorption edge of the TNTs. For a ternary heterogeneous structure, integrating In₂S₃ on the surface of CdS/TNTs, a well pronounced red shift in the absorption onset is observed. This can be due to the low band gap of In₂S₃ (about 2.3 eV), which further extends the absorption range of the heterogeneous sample (In₂S₃/CdS/TNTs) into the visible region. Fig. 8b shows the DRS spectra for Ar-TNTs, Ar-CdS/TNTs, Ar-In₂S₃/TNTs, and Ar-In₂S₃/CdS/TNTs. The Ar-TNTs, Ar-CdS/TNTs, and Ar-In₂S₃/TNTs samples do not show any significant changes in the spectra; however, Ar-In₂S₃/CdS/TNTs show an increase in the redshift with a flat tail, which depicts the emergence of a new phase of photoactive CdIn₂S₄via thermal treatment. These results indicate that the heterogeneous structure is also active in the visible range. The DRS data were employed to determine the optical bandgap of Ar-TNTs, Ar-CdS/TNTs, Ar-In₂S₃/TNTs, and Ar-In₂S₃/CdS/TNTs using Tauc analysis. The energy of the photons (hν) and absorption coefficient (α) were measured using the following relations, respectively,

where d is the thickness of the sample. Considering the direct allowed transition, the Tauc plot of (αhν)²vs. (hν) was constructed and is illustrated in Fig. S5. The linear region near the absorption edge was fitted and extrapolated to the energy axis. The intercept shows the value of the optical bandgap (E_g) of the material. Fig. S5(a–d) shows the values of optical bandgaps for Ar-TNTs, Ar-CdS/TNTs, Ar-In₂S₃/TNTs, and Ar-In₂S₃/CdS/TNTs, which are 3.2 eV, 2.6 eV, 2.4 eV and 1.9 eV, respectively. The obtained bandgap values are in close agreement with the values reported in the literature and confirm the expected band structure of Ar-In₂S₃/CdS/TNTs. CB and VB edge positions of all samples are measured by the following equations and are presented in Scheme S1.

E_VB = χ − 0.5E_g − E_c

E_CB = E_VB + E_g

E_CB, E_VB, E_g and E_c are the conduction band energy, valence band energy, bandgap energy and E_e is the energy of free electrons on the hydrogen scale (4.5 eV w.r.t NHE), respectively. χ is the electronegativity of TiO₂, CdS, In₂S₃, and CdIn₂S₄ and their values are estimated as the average of electron affinity and ionization potential of the material.^56,57

Fig. 8 (a and b) DRS spectra and (c and d) EIS plots of pristine TNTs, CdS/TNTs, In₂S₃/TNTs and In₂S₃/CdS/TNTs (a and c) without and (b and d) with Ar-treatment at 400 °C.

Electrochemical impedance spectroscopy (EIS) was used to explore charge transfer characteristics at the surface of the photo-electrodes. Fig. 8c shows the Nyquist plots of TNTs, CdS/TNTs, In₂S₃/TNTs, and In₂S₃/CdS/TNTs (in 0.1 M PBS at 100 mV). The Nyquist plots clearly depict a distinctive smaller semicircle for the ternary heterogeneous structure (In₂S₃/CdS/TNTs). On the other hand, pristine TNTs and CdS QDs-sensitized TNTs do not show a semicircle and exhibit higher impedance values. The EIS of the Ar-treated samples (Fig. 8d) shows a remarkable decrease in the impedance for Ar-TNTs, Ar-In₂S₃/TNTs, and Ar-CdS/TNTs compared to untreated samples. This indicates that the charge transfer resistance has decreased tremendously. In the case of Ar-treated In₂S₃/CdS/TNTs, an obvious decrease in impedance and semicircle diameter (a low charge transfer resistance) is accompanied by interface-controlled charge carrier transport at low frequency values. It is evident from the DRS and EIS results that In₂S₃/CdS/TNTs ternary hetero-structure and Ar-In₂S₃/CdS/TNTs support efficient charge carrier separation and interfacial charge transport characteristics.

In order to demonstrate photoelectrochemical (PEC) activity, photocurrent density versus time responses were recorded. Fig. 9a shows photocurrent density vs. time curves for TNTs, CdS/TNTs, In₂S₃/TNTs, and In₂S₃/CdS/TNTs in 1 M KOH aqueous solution using a three-electrode system where Pt and Ag/AgCl electrodes served as the counter and reference electrodes, respectively. Transient photocurrent time curves are measured by illuminating the photocatalysts using a solar simulator (AM 1.5, 1000 W m⁻²) at regular time intervals. During illumination (the on cycle), the TNTs show a stable but smaller current density value (0.39 mA cm⁻²) compared to CdS/TNTs (1.57 mA cm⁻²) and In₂S₃/TNTs (2.48 mA cm⁻²). The current density of CdS/TNTs deteriorates with illumination time due to the lower photostability of the CdS QDs. However, the In₂S₃/TNTs electrode shows a more stable photocurrent response than the CdS/TNTs electrode due to the high photostability of the β-In₂S₃ QDs. In the case of the In₂S₃/CdS/TNTs ternary hetero-structure, a photocurrent density of 3.22 mA cm⁻² (about 51% and 23% higher than those of CdS/TNTs and In₂S₃/TNTs electrodes, respectively) was observed. The enhancement of the PEC response is due to the staggered band structure of the constituent elements. On the other side, when illumination is cut off (the off cycle), a rapid drop in the photocurrent density to almost zero was observed. The photocurrent density regains its value for the In₂S₃/CdS/TNTs electrode and remains stable during several on–off cycles. In order to re-emphasize the choice of a particular number of deposition cycles for In₂S₃ QDs on the CdS/TNTs electrode, their PEC responses are recorded and shown in Fig. 9b. A sequentially higher photocurrent density was obtained by increasing the In₂S₃ deposition cycles, and an optimum value is achieved at 7 cycles. Furthermore, an abrupt decrease is observed when increasing the cycles to 9 due to the higher coverage of the QDs, which blocks the active sites for the photoelectrochemical process.

Fig. 9 (a) PEC response of (a) pristine TNTs, CdS/TNTs, In₂S₃/TNTs and In₂S₃/CdS/TNTs, (b) of In₂S₃/CdS/TNTs at different loading densities of In₂S₃ on CdS/TNTs. PEC response of (c) Ar-treated In₂S₃/CdS/TNTs for different thickness of CdIn₂S₄ passivation layer depending on the different loading densities of In₂S₃ on CdS/TNTs, (d) of Ar-TNTs, Ar-CdS/TNTs, Ar-In₂S₃/TNTs and Ar-In₂S₃/CdS/TNTs.

After heat treating In₂S₃/CdS/TNTs hetero-structures in an Ar-environment at 400 °C, photocurrent–time curves of the Ar-In₂S₃(3)/CdS/TNTs, Ar-In₂S₃(5)/CdS/TNTs, Ar-In₂S₃(7)/CdS/TNTs, and Ar-In₂S₃(9)/CdS/TNTs samples were recorded, as shown in Fig. 9c. The digit in parentheses indicates the number of different deposition cycles for In₂S₃ QDs on CdS/TNTs. These curves show a significant increase in the photocurrent density and stability of the treated samples. This is due to the formation of a stable layer (as a result of Ar annealing treatment) of ternary chalcogenide CdIn₂S₄ with compact pyramid-like nanostructures on the surface of the TNTs. Following the trend observed in Fig. 9b, the highest photocurrent density was observed for Ar-In₂S₃(7)/CdS/TNTs. Furthermore, this sample (Ar-In₂S₃(7)/CdS/TNTs) with the highest photo current density responses is compared with Ar-treated CdS/TNTs, In₂S₃/TNTs and pristine TNTs in Fig. 9d. It is evident from the figure that Ar-treatment of photo-electrodes leads to better crystallinity and stability.

The significantly enhanced photocurrent density of the ternary electrode system is due to the high surface area of the nanostructure, which offers more active sites for reactions. Integrating CdS and In₂S₃ simultaneously on TNTs gives the hetero-structure material a broader absorption window and increased photoresponse due to their reduced band gap energy. This fact was confirmed by the red shift in the DRS spectra. In parallel, the staggered band gap alignment formed at the interface of the conjugated active materials is desirable for the easy transfer and separation of photoexcited electron–hole pairs.

Photocatalytic H₂ evolution on In₂S₃/CdS/TNTs

The photocatalytic ability of TNTs, CdS/TNTs, In₂S₃/CdS/TNTs, and In₂S₃/TNTs was recorded by monitoring the H₂ evolution efficiency of the photocatalysts. The H₂ evolution experiments were performed by irradiating each catalyst with visible light using a solar simulator (AM 1.5, 1000 W m⁻²) while immersing it in a water–methanol mixture solution within a quartz tube. The quartz cell was purged with N₂ for 20 min before irradiation to remove the dissolved oxygen and air from the cell. The 200 µl gas was collected using a septum and injected into the gas chromatograph (GCMS-QO2010SE, Shimadzu, Japan) to analyze the amount of H₂ evolved during the course of the reaction. Fig. S6a shows the average H₂ evolution rate for different electrode configurations. The data clearly show a gradual increase in the H₂ evolution rate by introducing the active materials one after another. The highest H₂ generation rate (0.5694 mL h⁻¹ cm⁻²) is achieved for the ternary In₂S₃/CdS/TNTs hetero-structure. The increased H₂ generation activity is due to the existence of active heterojunctions between TNTs-CdS and CdS–In₂S₃. This allows efficient charge separation/transfer behavior at the interface of the junctions and increases light-harvesting ability.

The consistency and re-usability of In₂S₃/CdS/TNTs for H₂ production were tested for 8 consecutive cycles, as shown in Fig. S6b. After each cycle, the photocatalyst was thoroughly washed with DI water to remove any leftover residues of the reaction intermediates. These measurements (Fig. S6b) show a slight decline in photoresponse in the second cycle, and the H₂ evolution efficiency reduces by 40% after the eighth cycle. The photocorrosion of the exposed CdS and In₂S₃ interface is the limiting factor for the reduced H₂ evolution rate. Due to this reason, ternary chalcogenides formed by annealing In₂S₃/CdS/TNTs in an Ar atmosphere are highly useful to deal with the photo-corrosion issue.^39,40 Introducing the CdIn₂S₄ nanostructure as a sacrificial layer on In₂S₃/CdS/TNTs not only favors photogenerated charge separation but also increases the stability of the sulfide layer due to its hole in-taking ability.^58,59

Fig. 10a shows the H₂ evolution rate for Ar-annealed samples, namely Ar-TNTs, Ar-CdS/TNTs, Ar-In₂S₃/TNTs, and Ar-In₂S₃/CdS/TNTs. It can be seen that Ar-In₂S₃/CdS/TNTs exhibits a significant increase in H₂ evolution (0.6281 mL h⁻¹ cm⁻²) in comparison to the un-annealed In₂S₃/CdS/TNTs (0.5694 mL h⁻¹ cm⁻²). Ar annealing results in the formation of a thick layer of CdIn₂S₄ over TNTs due to the temperature-induced ion diffusion process. The band gap positions of CdIn₂S₄, along with those of In₂S₃, CdS, and TiO₂, also favor the photocatalytic process. Interestingly, Ar-TNTs (0.0346 mL h⁻¹ cm⁻²), Ar-CdS/TNTs (0.3808 mL h⁻¹ cm⁻²), and Ar-In₂S₃/TNTs (0.3948 mL h⁻¹ cm⁻²) also show a considerable increase in the H₂ evolution rate compared to the non-Ar-treated samples. This is also due to the increased crystallinity of CdS and In₂S₃ QDS. Meanwhile, the H₂ generation rate increases with increasing density of CdIn₂S₄ pyramids (Fig. 10b). The highest H₂ evolution rate is observed for Ar-In₂S₃(7)/CdS/TNTs. Further increasing the density of CdIn₂S₄ reduces the H₂ generation rate as CdIn₂S₄ no longer retains the pyramid nanostructure but is converted into big agglomerates (SEM Fig. 3g and h). Such agglomerates reduce the number of active interfaces and also inhibit the access of light to the photoactive sites. We have reported the average values of the H₂ evolution rate by repeating all photocatalytic tests three times to ensure the reproducibility of the catalyst, and the data are presented in Fig. S(7–9). Fig. S7 and S8 show the H₂ production rate for untreated and Ar-treated TNTs, CdS/TNTs, In₂S₃/TNTs and In₂S₃/CdS/TNTs. Fig. S9 shows the H₂ production rate for Ar-In₂S₃/CdS/TNTs for different loading densities of In₂S₃ QDs.

Fig. 10 (a) H₂ production rate for different Ar annealed samples and (b) for Ar-In₂S₃/CdS/TNTs of different thickness of the CdIn₂S₄ passivation layer controlled by In₂S₃ deposition cycles (c) stability response of Ar-In₂S₃/CdS/TNTs for H₂ production for different cycles, (d) H₂ production rate for Ar-In₂S₃/CdS/TNTs for different illumination sources.

Furthermore, to evaluate the stability of the photocatalysts for H₂ evolution, 8 cycles of measurements were conducted on the same electrode. After each measurement, the electrode was cleaned with DI water and dried. A slight decrease in the H₂ evolution rate is observed during the course of 3 cycles, and efficiency retention is 91% (Fig. 10c). The stability test was extended to eight consecutive cycles, showing decent retention of photocatalytic activity for Ar-In₂S₃/CdS/TNTs. The increased stability of the Ar-In₂S₃/CdS/TNTs sample is due to the considerably higher chemical and photostability of the outer CdIn₂S₄ layer. In order to reinforce the claim that CdIn₂S₄ suppresses photocorrosion, analysis using an inductively coupled plasma optical emission spectrometer (ICP-OES, LEEMAN, Prodigy 7, USA) of the reaction solution after H₂ production by In₂S₃/CdS/TNTs and Ar-In₂S₃/CdS/TNTs is presented in Table S1. The electrolyte solution was collected and examined for Cd, In, and S ion traces. The results show the presence of significantly lower concentrations of Cd, S, and In ions in CdIn₂S₄-containing samples compared to unannealed samples, confirming reduced photocorrosion due to the induction of a CdIn₂S₄ layer. Furthermore, post-reaction XPS of Ti 2p, S 2p, O 1s, In 3d, and Cd 3d spectra are also presented in Fig. S10. By comparing the results with the XPS spectra of pre-reaction samples (Fig. 7), no major changes in Ti 2p, Cd 3d, In 3d, O 1s, and S 2p spectra are observed. The results further show fewer amounts of oxidized sulfur species, supporting photocorrosion suppression by CdIn₂S₄ passivation. The post-reaction SEM images of Ar-In₂S₃/CdS/TNTs were also checked and compared with the pre-reaction samples (Fig. S11) to analyze the stability of the CdIn₂S₄ structure and morphology. No major differences in the SEM images are observed, confirming the stable nature of CdIn₂S_4.

Further experiments were conducted by illuminating the Ar-In₂S₃/CdS/TNTs using 450 nm blue (90 mW cm⁻²) and 632 nm red LEDs (40 mW cm⁻²). Fig. 10d shows the average value of the H₂ evolution rate obtained from 632 nm and 450 nm LED irradiation. The results show that the Ar-In₂S₃/CdS/TNTs heterostructure demonstrates wavelength-dependent photocatalytic behavior, with the highest H₂ evolution rate obtained under 632 nm LED illumination. This behavior suggests that the material responds more efficiently to the red light than to blue or broad-spectrum illumination. The activity under 450 nm and the solar simulator was comparatively lower. This trend can be attributed to the unique optical response and charge transfer dynamics of the multi-sulfide heterostructure. Under 632 nm irradiation, the excitations match well with the absorption bands of CdIn₂S₄ and In₂S₃, leading to the creation of charge carriers and reducing recombination due to active charge transfer between the cascade band edge positions. For simulated sunlight, the broad-spectrum spreads energy over a wide wavelength range, leading to effective absorption of only a small portion and shows comparatively lower H₂ production rates. These results indicate that CdS, In₂S₃, and CdIn₂S₄-based heterostructures have strong absorption ability towards a monochromatic visible source, as further confirmed by DRS measurements, which show higher absorption at 632 nm. AQY for photocatalytic H₂ evolution was calculated by exciting the sample using a 450 nm LED lamp for 6 h. The sample is immersed in a solution of DI water having 12% methanol. The amount of evolved H₂ production was measured using a gas chromatograph (GCMSQO2010SE, SHIMADZU, Japan) with a TCD detector. The power of the illumination source at the surface of the sample is 90 mW cm⁻². AQY is determined using the following standard expression.

where n_H2 is the amount of H₂ generated in moles and n_p is the total number of incident photons reaching the catalyst (incident photon flux from a monochromatic source of light) surface during the specified illuminated time. The incident photon flux is quantified experimentally by measuring the power density of the light source at the surface of the sample of a specific area. The obtained value of AQY is 5.54% at 450 nm. It is observed from the data that the rate of H₂ evolution is more efficient and higher for visible light-driven H₂ evolution under monochromatic irradiation compared to the H₂ evolution rate for samples illumined by a solar simulator. The calculated value of AQY for 632 nm LED illumination is 13.57%. These results can be ascribed to the fact that all power lines lie within the absorption band of the heterostructure.

The photocatalytic hydrogen evolution in the absence of a scavenger was conducted in pure water. Ar-In₂S₃/CdS/TNTs was immersed in DI water and illuminated using a solar simulator (AM 1.5, 1000 W m⁻²). The results are presented in Fig. S12, which shows that a negligible amount of H₂ is produced by Ar-annealed TNTs. For Ar-CdS/TNTs, Ar-In₂S₃/TNTs and Ar-In₂S₃/CdS/TNTs, about 24.5, 31.3 and 92.8 µL per h per cm² per H₂ evolution rates were obtained, respectively. In comparison to H₂ production with sacrificial agents, about a 7-fold decrease is observed for Ar-In₂S₃/CdS/TNTs in pure water, indicating the necessity of sacrificial agents for efficient charge separation. The small amount of H₂ production can be ascribed to the formation of a heterostructure, which effectively inhibit charge carrier recombination rates due to staggered band edge positions between CdIn₂S₄, In₂S₃, CdS, and TiO₂. To demonstrate the effect of the annealing environment, we synthesized a control sample and annealed it in a N₂ environment in a tube furnace under similar annealing conditions. Fig. S13 shows the cross-sectional SEM image of the N₂-annealed sample, which is compared with the sample annealed in the Ar environment. The results demonstrate that Ar treatment yields a more defined CdIn₂S₄ nanostructure, particularly the formation of a distinctive nanopyramid structure observed on the walls of TNTs. The main phenomena in the formation of the CdIn₂S₄ phase are controlled interdiffusion of Cd²⁺ ions from CdS and In³⁺ ions from In₂S₃ into the sulfide lattice through solid-state cation exchange and anion reorganization, which results in nucleation and growth of CdIn₂S₄.^47,54,60 An inert environment is required to prevent the oxidation of sulfide species that would otherwise disturb the stoichiometry. The localized interdiffusion at the In₂S₃/CdS interface promotes anisotropic crystal growth, resulting in the emergence of a faceted nano-pyramidal structure. These facets are related to low-energy crystallographic planes that are preferably stabilized under oxygen-free conditions provided by inert annealing. As shown in the figures, a noticeable difference in the morphology of CdIn₂S₄ formed on the surface of TNTs is observed for the sample annealed in a N₂ environment compared to that in Ar. Although both gases offer an inert environment essential for the protection of sulfide stoichiometry, their elusive physical and chemical differences can influence crystal growth. Ar, being a completely inert noble gas, encourages uniform heat distribution and allows controlled solid-state diffusion between CdS and In₂S₃, which results in the formation of well-faceted pyramid-like CdIn₂S₄ nanocrystals. The lack of the ability of Ar to interact with surfaces ensures solely thermodynamically controlled atomic rearrangements. In contrast, N₂ can interact weakly with surface defects and oxygen vacancies, particularly with TNTs, which can alter nucleation and growth kinetics. Consequently, the sample annealed in an N₂ environment exhibits a smaller, less faceted and less textured CdIn₂S₄ layer. The photocatalytic H₂ production for the N₂ annealed sample is also shown in Fig. S14. The results exhibit low efficiency of H₂ production for the N₂ annealed sample as compared to the sample annealed in Ar. These findings depict that Ar annealing favors highly crystalline and well-defined pyramid structures compared to the N₂ annealed sample.

The rate of H₂ evolution in the present work was further compared with those reported in the literature.^61–68 First, the H₂ evolution rate for Ar-In₂S₃/CdS/TNTs is converted from mL h⁻¹ cm⁻² to mmol h⁻¹ g⁻¹, where the mass of the sample is 0.38 mg. From Table 1, it can be seen that among other photocatalysts, Ar-In₂S₃/CdS/TNTs exhibits the highest H₂ evolution rate of 73.94 mmol h⁻¹ g⁻¹. This outstanding performance of Ar-In₂S₃/CdS/TNTs results from the separation of photogenerated charge carriers and higher utilization of the visible range of the solar spectrum, which are key for efficient photocatalysts. These two factors are supported by the staggered band alignment and low band gap of CdS and In₂S₃ and CdIn₂S₄.

Table 1 Comparison of H₂ evolution rates of Ar-In₂S₃/CdS/TNTs with the reported photocatalysts

Sr. no.	Sample name	H2 evolution rate mmol h^−1 g^−1	Illuminating source	Ref.
1	W-TiO2/Au hybrid	24.000	300 W Xe lamp	67
2	Pt/TiO2	11.200	AM 1.5 G solar simulator	68
3	F-TiO2	18.270	300 W Xe lamp	66
4	Co-TiO2	11.021	Solar and UV (400 W Hg vapour lamp)	65
5	Gd/N-TiO2	10.764	150 W Xe lamp	64
6	GQDs/TiO2 nanocomposites (CTNP-3)	29.548	Natural solar light	63
7	CdIn2S4–In2O3–In2S3	2.004	225 W Xe arc lamp (320-780 nm)	61
8	Dibenzo[b,d]thiophene sulfone (P10-e)	60.600	300 W Xe light source λ > 420 nm	62
9	P-Mo2CTx/CdS	7.365	Visible-light irradiation (λ = 420 nm)	69
10	F-doped TiO2/ZnIn2S4	1.58	300 W xenon lamp	70
11	CoP/ZIS-4	4.24	300 W xenon lamp with a 420 nm filter (λ ≥ 420 nm)	71
12	Ar-In2S3 QDs/CdS QDs/TNTs	73.94	1.5 AM solar simulator (300 W Xe lamp)	Present work

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Photodegradation of MB on In₂S₃/CdS/TNTs

To evaluate the merits of ternary heterogeneous photocatalysts for wastewater decontamination, we adopted MB as a model dye in aqueous media and studied its degradation under simulated visible light illumination generated by a 300 W PLS-SXE300UV xenon lamp equipped with a UV cut-off filter (UVCCUT400). Prior to each experiment, samples were immersed in 50 ml of a 30 ppm dye solution for 40 min under dark conditions (with continuous stirring) to attain equilibrium between dye adsorption/desorption on the surface of the photocatalyst. The dye solution, along with the catalysts, was irradiated, and adequate amounts of aliquots (2 ml) were collected after a time interval of 3 min. The photo-degradation efficiency was examined by quantifying the variations in the characteristic absorption peak of the dye solution using the following relation.

D (%) = 100 × (C₀ − C)/C₀ (1)

where C and C₀ are the concentrations of the dye solution at any time (t) and at the time of absorption equilibrium (t₀).

Analogous experiments were performed in the dark and in the absence of any photocatalyst in order to validate that the degradation is exclusively due to the photocatalytic process initiated by incident light on the surface of the photocatalyst. The characteristic absorption edge of MB (at 668 nm) was utilized to observe the degradation process over time (Fig. S15a). Fig. S15b shows the dye degradation rate (C/C₀) vs. the degradation time for pristine TNTs, CdS/TNTs, In₂S₃/TNTs, and In₂S₃/CdS/TNTs for MB. It is observed that the photocatalytic degradation performance of the In₂S₃/CdS/TNTs ternary heterostructure is much higher than that of other electrode configurations. In order to get a more appropriate representation of the dye degradation process, the data is fitted using a pseudo-first-order equation ln(C₀/C) vs.t (Fig. S15c). The estimated linear fitting of the curves for all electrodes shows that all photocatalysts follow pseudo-first-order reaction kinetics. The comparison of the photocatalytic activities, the photocatalysis rate constants k (calculated from the slope of the linear plots of ln(C₀/C) vs. irradiation time t) along with other experimental parameters for TNTs, CdS/TNTs, In₂S₃/TNTs, and In₂S₃/CdS/TNTs degradation of MB is listed in Table 2. The In₂S₃/CdS/TNTs heterogeneous electrode shows superior photocatalytic activity with a higher degradation rate constant than other electrodes. The photo-degradation efficiency block diagram for MB shown in Fig. S15d reveals that almost complete degradation of the dye was attained via heterogeneous nanostructures within 27 min, whereas for TNTs, about 25% of the dye remained for the same time interval and complete degradation occurred in 40 min.

Table 2 Photodegradation reaction kinetic parameters for MB on different samples

Sr. no.	Sample name	Photodegradation rate constant (k) min^−1	Photodegradation efficiency (%) for 27 min irradiation	Total degradation time (min)
1	TNTs	0.0647	74	40
2	CdS/TNTs	0.886	85	36
3	In2S3/TNTs	0.1225	92	33
4	In2S3/CdS/TNTs	0.1661	99	27

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The significantly enhanced photocatalytic efficiency of the ternary electrode system is attributed to the high surface area of the nanostructure, which offers more active sites and assists the adsorption of dye molecules on the catalyst surface. Another reason can be the utilization of appropriate active materials, which favor the utmost utilization of visible light. Integrating CdS and In₂S₃ on TNTs imparts an increased photoresponse due to their reduced band gap energy, as confirmed by the red shift in the DRS results. In parallel, the staggered band gap alignment formed at the interface of the conjugated active materials is desirable for facile transfer and separation of the photoexcited electron–hole pair.

To unfold the role of active species in the photodegradation of dyes under visible light illumination, a sequential addition of quenchers was utilized to determine the governing active species in photocatalysis. Benzoquinone (BQ), methanol (M), silver nitrate (AgNO₃) and isopropanol (IP) were added separately to the dye solution before irradiation, which act as scavengers for O₂⁻, h⁺, e⁻ and ˙OH, respectively. Fig. S15e shows the photodegradation of MB, along with different scavengers. The presence of AgNO₃, BQ and isopropanol considerably inhibits the photodegradation process for MB. However, the hole scavenger methanol slightly influences the photodegradation. The above findings illustrate that ˙OH radicals, which are also a subsequent product of photogenerated electrons and O₂⁻, are the primary key element in the photodegradation of the dye, while a secondary role is played by the holes.

The consistency and re-usability of In₂S₃/CdS/TNTs for dye degradation were tested for 3 successive cycles, as shown in Fig. S15f. After each cycle, the photocatalyst was thoroughly washed with DI water and ethanol to remove any leftover residues of reaction intermediates. The results specify a minor decline in the photoactivity of the catalyst after 3 cycles, which could be due to photocorrosion of the exposed CdS QDs in the aqueous dye solution. These findings reveal improved stability and re-usability of In₂S₃/CdS/TNTs for potential dye degradation applications. These results suggest the significance of the morphological architecture (structural design), which favors an appropriate band structure that allows the transfer of holes to In₂S₃, thus preventing CdS photo-corrosion. This captivated approach to electrode fabrication lowers the operational cost of the purification process and leads to a new approach for designing novel heterostructures.

Degradation of MB on Ar-In₂S₃/CdS/TNTs

The dye degradation abilities of Ar-TNTs, Ar-CdS/TNTs, Ar-In₂S₃/TNTs, and Ar-In₂S₃/CdS/TNTs were measured by monitoring changes in the UV absorption spectra of MB with irradiation time. Fig. 11a shows the characteristic UV absorption spectra of MB complying with Ar-In₂S₃/CdS/TNTs. The characteristic absorption peaks seem to reduce effectively and turn into a nearly flat spectrum within a short time. The dye degradation rate (Fig. 11b) for all prepared structures shows that complete degradation for MB is observed within 22 min with Ar-In₂S₃/CdS/TNTs compared to Ar-TNTs, Ar-CdS/TNTs, and Ar-In₂S₃/TNTs. All electrodes follow the pseudo-first-order reaction as verified from the linear fitting of ln(C₀/C) vs.t (Fig. 11c). Meanwhile, the corresponding rate constants derived from the slope of Fig. 11c for all electrodes are tabulated in Table 3. The Ar-In₂S₃/CdS/TNTs composite nanostructure displays the highest photocatalytic activity with higher photodegradation, where about 99.7% of MB is degraded in just 22 min (Fig. 11d). It can also be confirmed from the table that the value of the reaction rate constant for Ar-In₂S₃/CdS/TNTs is higher than that of In₂S₃/CdS/TNTs, while the values for annealed Ar-TNTs, Ar-CdS/TNTs, and Ar-In₂S₃/TNTs are better than that of the un-annealed samples. The higher degradation efficiency of Ar-In₂S₃/CdS/TNTs conjugated with ternary chalcogenides CdIn₂S₄ compared to the heterostructure of binary chalcogenides (In₂S₃/CdS/TNTs) can be ascribed to the higher catalytic sites on the surface of CdIn₂S₄ pyramids. Generally, the photocatalytic performance of a photocatalyst depends on its photoabsorption range, efficiency of electron–hole pair generation and higher separation to recombination ratio. In ternary chalcogenides, the metal ion insertion (Cd) in the structure of a binary chalcogenide (In₂S₃) leads to an increase in defect sites, which enables enhanced light harvesting and extra charge transport channels from the conduction band (CB).^39,40

Fig. 11 (a) Absorption spectra of MB for different irradiation time on Ar-In₂S₃/CdS/TNTs, (b) degradation rate, (c) degradation reaction kinetics, and (d) degradation efficiency of MB with respect to irradiation time for different Ar- treated samples. (e) Effect of different reactive species scavenges of photo-degradation of MB and (f) degradation ratio plots for different repeating cycle of Ar-In₂S₃/CdS/TNTs.

Table 3 Photodegradation reaction kinetic parameters for MB on different Ar-treated samples

Sr. No.	Sample name	Photodegradation rate constant (k) min^−1	Photodegradation efficiency (%) for 27 min irradiation	Total degradation time (min)
1	Ar-TNTs	0.0657	65	34
2	Ar-CdS/TNTs	0.1592	93.8	30
3	Ar-In2S3/TNTs	0.1724	94.9	24
4	Ar-In2S3/CdS/TNTs	0.2134	99.7	22

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To get insight into the photocatalytic process of Ar-In₂S₃/CdS/TNTs, a schematic representation of the degradation process is shown in Scheme S1. Under irradiation, photogenerated electrons were produced in the CB of CdIn₂S₄ (due to its low band gap energy) and rapidly transferred to the CB of TiO₂via In₂S₃ and CdS. This is due to the better interface between CdIn₂S₄–In₂S₃, In₂S₃–CdS, and CdS–TiO₂ and favorable band edges. Meanwhile, the transfer of photogenerated holes from the VB of CdIn₂S₄ towards the VB of TiO₂ is thermodynamically not favored. This is due to the more cathodic VB potential of CdIn₂S₄ as compared to those of In₂S₃, CdS and TiO₂, enabling the separation of the electron–hole pair. These holes may further react with H₂O molecules and produce high-activity hydroxyl radicals (˙OH). The electrons in the CB of TiO₂ can be captured by dissolved oxygen in water to form superoxide radicals (O₂⁻), which can effectively degrade the noxious molecules.

Various scavenging agents were used to test the predominant reactive species in the photodegradation of dyes. As evident from the results in Fig. 11e, a relatively small loss of photodegradation activity is observed for the hole scavenger, while others show a significant decrease. These results emphasize that the electrons transferred to the CB of TNTs are trapped by molecular oxygen in the electrolyte to form the reactive oxygen species (superoxide radical anion). They subsequently produce hydroxyl radicals, the dominant active species in the photodegradation process. The involvement of the photogenerated holes was also evident from the trapping experiments; thus, the combined effect of hydroxyl radicals and photogenerated holes determines the degradation of dyes. Furthermore, the photostability of Ar-In₂S₃/CdS/TNTs was tested by repeating the degradation experiments for MB. Fig. 11f shows the significant increase in the stability along with the degradation rate. These results show a comparatively enhanced anti-photocorrosion activity of the In₂S₃/CdS/TNTs due to induced CdIn₂S4 sacrificial layer on In₂S₃/CdS/TNTs.

Conclusions

A ternary heterostructure based on TNTs incorporating CdS and In₂S₃ QDs was prepared and employed for photocatalysis of water and H₂ generation (facile energy source). The photocatalytic performance of the different heterostructures was explored. The synergistic effect of CdS QDs on TNTs increases the visible light response of the heterostructure. Introducing In₂S₃ QDs on CdS/TNTs dramatically increases H₂ evolution, which is due to the charge separation ability and cascade band gap positions of In₂S₃ with CdS and TiO₂. A passivation layer composed of CdIn₂S₄ is formed by thermally treating In₂S₃/CdS/TNTs in Ar. This enhanced the chemical stability of the photoelectrode and significantly boosted the H₂ production rate to 0.6281 mL h⁻¹ cm⁻². The AQY of Ar-In₂S₃/CdS/TNTs at 450 nm and 632 nm LED are 5.54% and 13.57% respectively. The higher photocurrent density and lower charge transfer resistance suggest that the structure of the Ar-In₂S₃/CdS/TNTs photoanodes supports more efficient charge carrier separation and interfacial charge transport properties. Ar-In₂S₃/CdS/TNTs shows complete degradation of MB within 22 min, as compared to In₂S₃/CdS/TNTs, where the dye is degraded within 27 min.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included in the main text of the manuscript and in the supplementary information (SI). Supplementary information: the digital images of different stages of fabricated electrode, FESEM top and cross-sectional images of In₂S₃/CdS/TNTs hetero-structures for different loading cycles of In₂S₃ on CdS/TNTs, FESEM cross-sectional view of Ar-treated pristine TNTs, CdS/TNTs, and In₂S₃/TNTs, comparison of XPS core level spectra of Ti 2p, O 1s, Cd 3d, In 3d, and S 2p for In₂S₃/CdS/TNTs and Ar-In₂S₃/CdS/TNTs samples, Touc plots for Ar-TNTs, Ar-CdS/TNTs, Ar-In₂S₃/TNTs, and Ar-In₂S₃/CdS/TNTs. Band diagram with CB and VB edge positions of TNTs, CdS, In₂S₃ and CdIn₂S₄. H₂ production rate for different electrodes and (b) H₂ production rate of In₂S₃/CdS/TNTs for 8 consecutive cycles. H₂ production reproducibility measurements for unannealed samples. H₂ production reproducibility measurements for Ar annealed samples. H₂ production reproducibility measurements for Ar-In₂S₃/CdS/TNTs of different thickness of the CdIn₂S₄ passivation layer controlled by In₂S₃ deposition cycles. Table indicating traces of Cd, S and In ions for In₂S₃/CdS/TNTs and Ar-In₂S₃/CdS/TNTs using Inductively Coupled Plasma Optical Emission Spectrometer. XPS core level spectra of Ti 2p, O 1s, Cd 3d, S 2p and In 3d for Ar-In₂S₃/CdS/TNTs after photocatalytic reactions. FESEM cross-sectional view of Ar-In₂S₃/CdS/TNTs samples (a) pre and (b) post photocatalytic reaction. H₂ evolution rate by Ar-TNTs (Ar-T), Ar-CdS/TNTs (Ar-CT), Ar-In₂S₃/TNTs (Ar-IT) and Ar-In₂S₃/CdS/TNTs (Ar-ICT) in pure water in the absence of methanol. SEM cross-sectional image of In₂S₃/CdS/TNTs after N₂ annealing and Ar annealing at 400 °C for 1 hour. H₂ production rate for In₂S₃/CdS/TNTs after N₂ annealing and Ar annealing. Absorption spectra of MB for different irradiation time on In₂S₃/CdS/TNTs, degradation rate, degradation reaction kinetics and degradation efficiency of MB with respect to irradiation time for different samples. Effect of different reactive species scavenges of photo-degradation of MB and degradation ratio plots for different repeating cycle on In₂S₃/CdS/TNTs. See DOI: https://doi.org/10.1039/d5ta05742f.

Acknowledgements

The authors greatly acknowledge the Higher Education Commission (HEC) of Pakistan (grant no. IRSIP 36 PSc 31) for providing the financial support for this research work. Financial support from DFG, within the framework of its Excellence Initiative for the Cluster of Excellence “Engineering of Advanced Materials”, is thankfully acknowledged.

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