English

• 0.   Introduction

  In recent years, the exploration of clean energy has become a major trend in energy development^[1-3]. The more popular areas of research are the continuous development and application of hydrogen energy sources^[4]. However, traditional hydrogen energy production methods usually have the disadvantages of high cost, high pollution, and low efficiency, regretfully^[5-7]. Therefore, it is urgent to find a new type of pollution - free, low-cost, and high-efficiency hydrogen production strategy^[8-10]. Fortunately, photocatalytic hydrogen production technology has been recognized as a new type of high - efficiency hydrogen production technology with great potential^[11-15]. It is crucial to design and develop efficient photocatalysts for hydrogen production. In many challenges, developing efficient catalysts for photocatalytic hydrogen production is critical.

  ZnSnO₃ (ZSO) has been widely studied as a multifunctional material, but in the field of photocatalytic field, its large - scale application has been hampered due to the wide gap, low light utilization, and low efficiency^[16-18]. Therefore, researchers have used various methods such as noble metal modification, narrow bandgap semiconductor coupling, ion doping, and other means to improve photocatalytic activity^[19-22]. Among them, semiconductor recombination is a research hotspot to improve the photocatalytic efficiency of catalysts^[23]. Transition metal sulfides have been extensively studied for photocatalytic hydrogen production reactions. As an efficient narrow band gap photocatalytic material, CdIn₂S₄ (CIS) is usually combined with the broadband gap photocatalytic material to improve the photocatalytic activity^[24-26].

  There are very few studies of ZnSnO₃ (ZSO)-based semiconductors for photocatalytic hydrogen production^[27]. Our synthetic CdIn₂S₄/ZnSnO₃ (CIS/ZSO) heterostructure materials are relatively novel. The rationally designed Z-scheme heterostructure facilitates the separation of photogenerated electrons and holes and significantly increases the photocatalytic efficiency^[28-30]. This paper may create a new idea that belongs to the composite of ZSO-based photocatalyst and transition metal sulfide for photocatalytic hydrogen production.

  1.   Experimental

  1.1   Chemicals

  Sodium hydroxide (NaOH, 96.0%) and zinc chloride (ZnCl₂, 98.0%) were obtained from Tianjin Kemiou Chemical Reagent Co., Ltd. and Tianjin Tianli Chemical Reagent Co., Ltd. (Tianjin, China), respectively. Absolute alcohol (C₂H₅OH, 99.7%), cadmium nitrate tetrahydrate (CdN₂O₆·4H₂O, 99.0%), and trisodium citrate dihydrate (Na₃C₆H₅O₇·2H₂O, 99.0%) were obtained from Tianjin Zhiyuan Chemical Reagent Co., Ltd. (Tianjin, China). Thiourea (CH₄N₂S, 99.0%) was obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Hydrated stannic chloride (SnCl₄ ∙ 5H₂O, 99.0%) and indium nitrate hydrate (InN₃O₉· xH₂O) were obtained from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). The deionized water was used as the solvent. All chemicals were analytical grade and used as received.

  1.2   Sample preparation

  1.2.1   Preparation of ZSO

  ZSO was synthesized by a facile calcination method^[31]. First, 2 g of SnCl₄·5H₂O was dispersed in 25 mL of ethanol (EtOH) solution. After the solution was evenly stirred, and 0.5 g of ZnCl₂, 0.75 g of Na₃C₆H₅O₇·2H₂O were dispersed to 50 mL of deionized water. After the solution was evenly stirred, 100 mL of 0.4 mol·L^-1 NaOH was dispersed to form solution A. After magnetically stirring for 1 h, the resulting solution was washed several times, and dried overnight in an oven at 70 ℃ to obtain the precursor ZnSn(OH)₆-1. Then the previous steps were repeated to obtain solution A. The prepared precursor was dispersed and magnetically stirred for 0.5 h. Then 50 mL of 2 mol·L^-1 NaOH solution was dispersed and magnetically stirred for 0.5 h. The resulting solid was washed with deionized water and absolute EtOH several times, then placed in a 70 ℃ oven to dry overnight to obtain the ZnSn(OH)₆ - 2. Finally, the obtained sample was calcined in a muffle furnace at 300 ℃ for 3 h.

  1.2.2   Preparation of CIS/ZSO heterostructures

  The CIS/ZSO heterostructures were synthesized by a hydrothermal process^[32]. Typically, 0.079 g of CdN₂O₆·4H₂O, 0.077 g of InN₃O₉·xH₂O, and 0.019 g of CH₄N₂S were dispersed in 30 mL of mixed solution of water and absolute alcohol (1∶1, V/V). After stirring for 0.5 h, the mixture was transferred into a Teflon - lined stainless autoclave. Then, as - prepared ZSO (1 g) was dispersed and stirred for 0.5 h and the mixture maintained at 180 ℃ for 24 h hydrothermally. After the reaction, the mixture was cooled to room temperature, centrifuged, washed with water and EtOH three times, and the solid was finally dried overnight at 70 ℃ to obtain 12% CIS/ZSO (12% meant the molar ratio of CIS to ZSO). Similarly, a series of X% CIS/ZSO (X% meant molar ratios of CIS to ZSO, X=1, 4, 8, 16) can be obtained by changing the amount of CdN₂O₆·4H₂O, InN₃O₉·xH₂O, and CH₄N₂S. For comparison, pristine CIS and ZSO were also prepared.

  1.3   Characterization

  The crystal structures of the prepared catalysts were determined with powder X-ray diffraction (XRD, Shimadzu XRD-7000) (Cu Kα irradiation, λ=0.154 18 nm, U=40 kV, I=30 mA, 2θ =10° - 80°). The binding energies of elements can be tested by X-ray photoelectron spectroscopy (XPS, PHI - 5400 (America PE) 250 xi system). The morphological characteristics and microstructure of the catalysts were determined with scanning electron microscopy (SEM, JSM-6700F) under the accelerating voltage of 15 kV and transmission electron microscopy (TEM, JEM-2100) (Japan electronics) under the accelerating voltage of 200 kV. Optical properties were tested by the UV - Vis diffuse reflectance spectra (UV - Vis DRS) (Shimadzu, Japan). The electrochemical properties, including photocurrent density versus time (i-t) curves and electrochemical impedance spectroscopy (EIS), were measured on a CHI660E electrochemical workstation (Chenhua Instruments, China). Photoluminescence (PL) spectra were measured on an F-4600 fluorescence spectrophotometer (Hitachi, Japan). Time - resolved photoluminescence (TR - PL) spectra were conducted on an FLS920 fluorescence spectrometer (Edinburgh Analytical Instruments, UK). The in-situ electron spin resonance (in-situ ESR) signals of the ·OH and ·O₂^- radicals spin - trapped 5, 5 -dimethyl-1-pyrroline N-oxide (DMPO) as the spin-trap reagent (JEOL JES-FA200).

  1.4   Photocatalytic activity

  Photocatalytic H₂ evolution experiments were carried out on a sealed Pyrex top-irradiation photoreactor (LabSobar - ⅢAG, Beijing Pect Light Technology Co., Ltd.), using the Xenon lamp (300 W) as a light source for the excitation of photocatalytic materials. Before illumination, high-purity nitrogen (99.999%) was kept feeding and the solution was continuously stirred for 30 min to remove the dissolved oxygen in the photocatalytic system. The 30 mg catalyst was dissolved into 100 mL sacrificial agents, including 100 mL Na₂S (0.4 mol·L^-1) and Na₂SO₃ (0.4 mol·L^-1) solution, 100 mL triethanolamine (TEOA) aqueous solution (7∶3, V/V), 100 mL methanol (MeOH) aqueous solution (7∶3, V/V), 100 mL EtOH aqueous solution (7∶3, V/V), 100 mL triethylamine (TEA) aqueous solution (7∶3, V/V), and 100 mL deionized water for control experiment, respectively. The generated hydrogen was detected by gas chromatography, and nitrogen was used as carrier gas. The stability experiment was carried out. The apparent quantum yield (AQY) was calculated using the following equation (Eq.1):

  $ {\rm{AQY = }}{N_{\rm{r}}}{\rm{/}}{N_{\rm{i}}}{\rm{ \times 100\% }} $

  (1)

  Where N_r represents the number of reacted electrons and N_i represents the number of incident photons.

  2.   Results and discussion

  2.1   Construction and characterization of CIS/ZSO heterostructures

  The synthetic route of CIS/ZSO heterostructures was presented in Fig. 1. Firstly, the double-shelled ZSO cubes were synthesized according to the previous report^[31]. Then, the CIS nanoparticles were deposited on the surface of ZSO by a hydrothermal method. As shown in XRD patterns of catalysts (Fig. 2 and Fig.S1, Supporting information), the diffraction pattern for ZSO exhibited two broad peaks at 32.1° and 60.6°, respectively, corresponding to (110) and (300) planes of ZSO (PDF No. 52 - 1381). Compared with pristine ZSO, X% CIS/ZSO (X=1, 4, 8, 12, 16) heterostructures exhibited the extra peaks at 27.2°, 43.3°, and 47.4°, respectively, corresponding to (311), (511), and (440) characteristic lattice planes of CIS (PDF No. 27 - 0060) ^[33]. The XRD results proved that CIS had been successfully coupled with ZSO.

  Figure 1

  

  Figure 1.  Schematic diagram of the preparation process of the CIS/ZSO heterostructures

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  Figure 2

  

  Figure 2.  XRD patterns of the as-prepared samples

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  XPS was employed to investigate the surface chemical state and composition of the catalysts^[34]. Fig. S2 exhibited the survey spectra of the prepared catalysts, which can determine the elements contained in the samples. As shown in Fig. 3a, the binding energies of Zn2p_1/2 and Zn2p_3/2 for 12% CIS/ZSO located at 1 044.58 and 1 021.53 eV^[35], which are attributed to Zn²⁺ and slightly shifted compared with pristine ZSO. The binding energies of 494.88 and 486.43 eV, 494.83 and 486.38 eV corresponded to Sn3d_3/2 and Sn3d_5/2 for ZSO and 12% CIS/ZSO heterostructure, respectively, which are assigned to Sn^{2+ [36]}, as shown in Fig. 3b. For CIS/ZSO heterostructure, the binding energy of Sn²⁺ shifted to a certain extent after the coupling of CIS. As shown in Fig. 3c, the O1s binding energy of 12% CIS/ZSO may be fitted into 530.13, 531.33, and 532.03 eV, respectively, corresponding to lattice and absorbed oxygen species^[37]. The XPS peak of Cd3d_3/2 and Cd3d_5/2 for 12% CIS/ZSO were located at 411.78 and 405.03 eV, respectively, which can be assigned to Cd²⁺ (Fig. 3d) ^[38]. The In3d XPS binding energy was located at 444.48 and 452.03 eV (Fig. 3e), which can be assigned to In3d_5/2 and In3d_3/2 of 12% CIS/ZSO^[39]. The binding energy of S2p for 12% CIS/ZSO was also found at 163.08 and 161.83 eV (Fig. 3f), respectively, which belonged to S^{2- [40]}. The above XPS analysis proved that CIS and ZSO were successfully compounded, and the formed heterostructure promoted the interface charge transfer between the two pristine components.

  Figure 3

  

  Figure 3.  High-resolution XPS spectra of (a) Zn2p, (b) Sn3d, (c) O1s, (d) Cd3d, (e) In3d, and (f) S2p for the samples

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  The morphologies of the prepared samples were characterized by SEM, TEM, and high-resolution TEM (HR-TEM). Fig. 4a-4c are the SEM images of the prepared samples^[41-43]. The pristine ZSO exhibited a double-shell cubic structure, and the CIS exhibited an octahedral structure (Fig. 4a, 4b). In the CIS/ZSO heterostructure, CIS nanoparticles covered the surface of the ZSO cube, which proved the successful preparation of CIS and ZSO (Fig. 4c). The morphology of the prepared catalysts was further investigated by TEM (Fig. 4d-4f). As shown in Fig. 4d-4f, ZSO possessed a novel double - shell hollow cubic structure, and CIS exhibited a clear nanocrystalline structure. At the same time, the surface of ZSO was covered by CIS nanoparticles in the heterostructure (Fig. 4f), which was further confirmed by HR - TEM. The lattice fringes d- spacing of 0.208 and 0.279 nm corresponded to the (511) plane of CIS and (104) plane of ZSO, respectively (Fig. 4g), which are in good agreement with those calculated from the XRD results. Moreover, the EDS mapping images (Fig. 4h-4n) indicated that Zn, Sn, O, Cd, In, and S elements were uniformly distributed on the surface of CIS/ZSO.

  Figure 4

  

  Figure 4.  SEM images of (a) ZSO, (b) CIS, and (c) 12% CIS/ZSO; TEM images of (d) ZSO, (e) CIS, and (f) 12% CIS/ZSO; (g) HR-TEM image of 12% CIS/ZSO; (h) Spatial distribution of all the elements in 12% CIS/ZSO, and elemental mappings of (i) Zn, (j) Sn, (k) S, (l) O, (m) Cd, and (n) In

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  2.2   Optical properties and photocatalytic properties of CIS/ZSO

  The optical properties of the prepared photocatalysts were studied by UV - Vis DRS^[44]. As shown in Fig. 5a, the CIS/ZSO heterostructure had a red shift compared with pristine ZSO, which enhanced the light response range. And pristine ZSO had a basic absorption edge of 386 nm and poor light response capability. Compared with pristine ZSO, the prepared CIS/ZSO heterostructure had an expanded visible light absorption range and produced more photogenerated carriers. As shown in Fig. 5b, the band gap (E_g) was estimated from a plot of (αhν)² versus hν^[45]. The band gaps of CIS and ZSO were calculated as 2.11 and 3.21 eV, respectively. Fig. 5c and 5d showed the valence band (VB) - XPS results, which indicated that the valence band maximum (VBM) of ZSO and CIS were 2.62 and 1.34 V, respectively. Using the empirical formula, E_VBM= E_CBM+E_g, the conduction band minimum (CBM) of ZSO and CIS was estimated to be -0.04 and -0.78 V, respectively. Thus the band structure of the heterostructure can be determined.

  Figure 5

  

  Figure 5.  (a) UV-Vis DRS of ZSO, CIS, and 12% CIS/ZSO; (b) Plots of (αhν)² vs hν for ZSO and CIS; VB-XPS spectra of (c) ZSO and (d) CIS

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  As shown in Fig. 6a, the ZSO electrode showed negligible photocurrent response due to the rapid charge recombination, while the 12% CIS/ZSO electrode showed a considerably increased photocurrent response, indicating that the CIS/ZSO heterostructure can effectively suppress the charges recombination. The origin of the enhanced charge separation efficiency was further studied by EIS measurement. As depicted in Fig. 6b, 12% CIS/ZSO exhibited a much smaller semicircle diameter than ZSO, suggesting a reduced charge transfer resistance at the interface between ZSO and CIS, which is favorable to enhance the charge separation efficiency of the CIS/ZSO photocatalyst. The PL spectrum was used to evaluate the separation efficiency of photogenerated carriers of the photocatalyst. The PL intensity of 12% CIS/ZSO heterostructure was weaker than ZSO (Fig. 6c), indicating that the loading of CIS promotes the effective separation of photogenerated electrons and holes. Moreover, the average fluorescent lifetimes (τ_ave) of 12% CIS/ZSO and ZSO were determined to be 1.26 and 1.06 ns (Fig. 6d), evidencing that the construction of heterostructure can effectively accelerate the charges separation efficiency.

  Figure 6

  

  Figure 6.  (a) Transient photocurrent response, (b) EIS, (c) PL spectra, and (d) TR-PL spectra of ZSO and 12% CIS/ZSO

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  Photocatalytic hydrogen production of various related catalysts was evaluated. Whether compared to pristine CIS or ZSO, the heterostructures had better photocatalytic hydrogen production activity^[46-49]. Fig. 7a showed the increase in the amount of hydrogen generated by pristine ZSO, CIS, and X% CIS/ZSO heterostructures under simulated sunlight with the change of irradiation time. It is worth noting that when a load of CIS increased from 1% to 12%, the amount of hydrogen generated increased from 385.35 to 1 676.48 μmol·g^-1, which is owing to the addition of CIS significantly promotes the photogenic carrier separation ability of ZSO and improves the optical response range. However, the activity decreased significantly when the loading increased to 16%, which may be due to the recombination of photo-generated charge carriers that prevent the process of photocatalytic performance. Simultaneously, the influence of different sacrificial agents (Na₂S/ Na₂SO₃, TEOA, MeOH, EtOH, and TEA) on the photocatalytic hydrogen production activity was studied, which contained abundant proton sources. As shown in Fig. 7b, sulfate as the sacrificial agent had the best effect on hydrogen evolution. When S^2-/SO₃^2- was used as a sacrificial donor, it could be protonation. For example, the dissolved Na₂S formed SH^- (alkaline aqueous solution) (Eq.2). Simultaneously, SH^- was oxidized to form hydrogen (Eq.3) ^[50]. Therefore, in the choice of sacrificial agent, Na₂S/Na₂SO₃ has the advantage.

  Figure 7

  

  Figure 7.  (a) Hydrogen production activity of X% CIS/ZSO (X=1, 4, 8, 12, 16), ZSO, and CIS under simulated solar light; (b) Photocatalytic H₂ evolution of 12% CIS/ZSO in different sacrificial agents and Pt/CIS/ZSO using Na₂S/Na₂SO₃ as sacrificial agent

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  $ \mathrm{S}^{2-}+\mathrm{H}_2 \mathrm{O} \rightleftharpoons \mathrm{SH}^{-}+\mathrm{OH}^{-} $

  (2)

  $ \mathrm{SH}^{-}+\text {hole } \rightleftharpoons \mathrm{S}+\frac{1}{2} \mathrm{H}_2 $

  (3)

  When TEOA was used as a sacrificial donor, the HER activity was lower than using Na₂S/Na₂SO₃. Surprisingly, when MeOH, EtOH, and TEA were used as sacrificial agents, hydrogen production was almost negligible. Furthermore, the HER activity of the photocatalyst was related to the oxidation potential of the hole scavenger^[51]. The driving force of the hole scavenging was the largest for the hole scavenger with the greater oxidation potential. Among the selected hole scavengers, S^2-/SO₃^2- had the high oxidation potential with an electronic energy level at -3.0 V, followed by TEOA at -3.4 V, MeOH at -4.5 V, EtOH at -4.6 V, and TEA at -5.1 V vs vacuum^[52-53]. It is obvious that using S^2- or SO₃^2- as sacrificial reagents instead of alcohols is advantageous because they are more easily oxidizable than alcohols. Accordingly, using Na₂S/Na₂SO₃ as sacrificial reagents CdS/ZSO exhibited the best HER activity than others. Furthermore, the presence of H₂Pt₂Cl₆· 3H₂O significantly boosted the HER activity of CIS/ ZSO heterostructure, achieving higher HER activity (3 479.04 μmol·g^-1).

  Furthermore, using different light sources to evaluate the photocatalytic hydrogen production activity of the catalyst (Fig. 8a). Photocatalytic performance was optimal in simulated light. And the long-time test was carried out, which showed that the heterostructure had excellent stability (Fig. 8b). The amount of catalyst used to determine the optimal HER activity was 30 mg (Fig. 8c). As shown in Fig. 8d, 12% CIS/ZSO heterostructure photocatalyst presented excellent AQY of 1.18% at the wavelength of 420 nm. In addition, XRD, XPS, and SEM of 12% CIS/ZSO after irradiation were also investigated (Fig. S3). It showed that the phase structure and composition can be well maintained, confirming the excellent stability of 12% CIS/ZSO heterostructure.

  Figure 8

  

  Figure 8.  (a) Hydrogen production activity of ZSO, CIS, and 12% CIS/ZSO under visible light, simulated sunlight, and without light irradiation for 3 h; (b) 12 h hydrogen production test of 12% CIS/ZSO; (c) Optimization of 12% CIS/ZSO dosage; (d) Wavelength dependent AQY of H₂ evolution over 12% CIS/ZSO and UV-Vis absorption spectrum of 12% CIS/ZSO

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  2.3   Photocatalysis mechanism

  Based on the VBM and CBM positions of CIS and ZSO described above, the charge transfer between two semiconductors may follow a type Ⅱ or Z - scheme mechanism. To further demonstrate the catalytic mechanism of CIS/ZSO photocatalyst, the in - situ ESR was conducted to probe the main active species in the process of photogenerated carrier transfer by employing DMPO as a trapping agent. As expected, the prepared catalysts detected two signal peaks, which presented a standard four peak with a peak height ratio of 1∶2∶2∶1 and 1∶1∶1∶1, which were consistent with the literature reports^[54]. The ESR signal of CIS/ZSO was stronger than pristine ZSO, which indicated the formation of heterostructure enhanced effectively the separation efficiency of photogenerated electron and hole pairs, and the photocatalytic hydrogen production performance was improved (Fig. 9 and S4). According to the traditional type Ⅱ mechanism analysis, the holes in the VBM of ZSO were transferred to the VBM of CIS. H₂O or OH^- cannot be oxidized to ·OH by the holes in VBM of CIS (1.34 V vs NHE) due to it was more negative than E(·OH/H₂O) (2.27 V vs NHE) and E(·OH/OH^-) (1.99 V vs NHE). However, the results of ESR verified the obvious signals of ZSO and CIS/ZSO for producing ·OH, indicating that the charge transfer in CIS/ZSO prefers to be a Z-scheme rather than a type Ⅱ mechanism.

  Figure 9

  

  Figure 9.  ESR spectra of (a) DMPO-·O₂^- trapping and (b) DMPO-·OH trapping of the as-prepared catalysts under the illumination

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  Furthermore, according to the calculated work function (W) in Fig. S5, the work function of ZSO was larger than CIS, on the contrary, the Fermi level (E_F) of ZSO was lower than CIS (Fig. 10a). Therefore, after the contact between the two semiconductors, the electrons will transfer from CIS to ZSO until the Fermi level reaches equilibrium^[55-56]. As a result, CIS lost electrons and became positively charged, while ZSO gained electrons with negative charges, which can generate an internal electric field at the interface (Fig. 10b). Meanwhile, the energy band of CIS bent upward due to the loss of electrons, while that of ZSO bent downward caused by the accumulation of electrons^[57]. The VB electrons of CIS and ZSO were excited and transited to the conduction band under simulated sunlight. As mentioned above, the electrons transfer between ZSO and CIS does not follow the traditional type Ⅱ mechanism where holes in the VBM of ZSO are transferred to the VBM of CIS (Fig. 10c). Therefore, under the action of the built-in electric field, the electrons in the CBM of ZSO recombined with the holes in the VBM of CIS (Fig. 10d). Meanwhile, in the influence of these factors, the photogenerated carriers separation efficiency of CIS and ZSO was greatly improved. The electron and hole transfer process enables CIS/ZSO composite heterostructure to have strong redox ability and realized photocatalytic water splitting for hydrogen production. The above results further confirm that charge transfer in CIS/ZSO heterojunction follows the Z - scheme mechanism.

  Figure 10

  

  Figure 10.  Proposed photocatalytic HER mechanism by the CIS/ZSO Z-scheme heterostructure

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  3.   Conclusions

  In summary, CIS/ZSO heterostructures were synthesized by hydrothermal method, and the CIS/ZSO heterostructures improved the visible light response range, conspicuously. After 3 h, the hydrogen production of 12% CIS/ZSO can reach 1 676.48 μmol·g^-1 under the simulated sunlight irradiation, and the corresponding long-time test also proves the stability of the catalyst. The better photocatalytic hydrogen production performance of the prepared heterostructure compared with the pristine catalysts may be due to the accelerated separation of photogenic carriers and the wider range of visible light response. The work may open up new ways to synthesize photocatalysts that efficiently use solar energy to produce hydrogen.

  Supporting information is available at http://www.wjhxxb.cn

  
  
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  Figure 1  Schematic diagram of the preparation process of the CIS/ZSO heterostructures

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  Figure 2  XRD patterns of the as-prepared samples

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  Figure 3  High-resolution XPS spectra of (a) Zn2p, (b) Sn3d, (c) O1s, (d) Cd3d, (e) In3d, and (f) S2p for the samples

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  Figure 4  SEM images of (a) ZSO, (b) CIS, and (c) 12% CIS/ZSO; TEM images of (d) ZSO, (e) CIS, and (f) 12% CIS/ZSO; (g) HR-TEM image of 12% CIS/ZSO; (h) Spatial distribution of all the elements in 12% CIS/ZSO, and elemental mappings of (i) Zn, (j) Sn, (k) S, (l) O, (m) Cd, and (n) In

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  Figure 5  (a) UV-Vis DRS of ZSO, CIS, and 12% CIS/ZSO; (b) Plots of (αhν)² vs hν for ZSO and CIS; VB-XPS spectra of (c) ZSO and (d) CIS

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  Figure 6  (a) Transient photocurrent response, (b) EIS, (c) PL spectra, and (d) TR-PL spectra of ZSO and 12% CIS/ZSO

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  Figure 7  (a) Hydrogen production activity of X% CIS/ZSO (X=1, 4, 8, 12, 16), ZSO, and CIS under simulated solar light; (b) Photocatalytic H₂ evolution of 12% CIS/ZSO in different sacrificial agents and Pt/CIS/ZSO using Na₂S/Na₂SO₃ as sacrificial agent

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  Figure 8  (a) Hydrogen production activity of ZSO, CIS, and 12% CIS/ZSO under visible light, simulated sunlight, and without light irradiation for 3 h; (b) 12 h hydrogen production test of 12% CIS/ZSO; (c) Optimization of 12% CIS/ZSO dosage; (d) Wavelength dependent AQY of H₂ evolution over 12% CIS/ZSO and UV-Vis absorption spectrum of 12% CIS/ZSO

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  Figure 9  ESR spectra of (a) DMPO-·O₂^- trapping and (b) DMPO-·OH trapping of the as-prepared catalysts under the illumination

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  Figure 10  Proposed photocatalytic HER mechanism by the CIS/ZSO Z-scheme heterostructure

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