Next Article in Journal
Corrosion Protection of Monel Alloy Coated with Graphene Quantum Dots Starts with a Surge
Previous Article in Journal
Assessing Environmental Sustainability Using Ecological Limits Expressed as Mass Flowrates with the Inclusion of a Sustainable Time Perspective
Previous Article in Special Issue
Novel Photocatalytic NH3 Synthesis by NO3 Reduction over CuAg/TiO2
Open AccessArticle

The Effect of Cu and Ga Doped ZnIn2S4 under Visible Light on the High Generation of H2 Production

1
Global Environment Center for Education & Research, Mie University, Mie 514-8507, Japan
2
Department of Chemistry for Materials, Graduate School of Engineering, Mie University, Mie 514-8507, Japan
*
Author to whom correspondence should be addressed.
ChemEngineering 2019, 3(4), 79; https://doi.org/10.3390/chemengineering3040079
Received: 28 June 2019 / Revised: 13 September 2019 / Accepted: 25 September 2019 / Published: 29 September 2019
(This article belongs to the Special Issue Advances in Metal-Based Catalysts)

Abstract

A Cu+ and Ga3+ co-doped ZnIn2S4 photocatalyst (Zn(1−2x)(CuGa)xIn2S4) with controlled band gap was prepared via a simple one-step solvothermal method. Zn(1−2x)(CuGa)xIn2S4 acted as an efficient photocatalyst for H2 evolution under visible light irradiation (λ > 420 nm; 4500 µW/cm2). The effects of the (Cu and Ga)/Zn molar ratios of Zn(1−2x)(CuGa)xIn2S4 on the crystal structure (hexagonal structure), morphology (microsphere-like flower), optical property (light harvesting activity and charge hole separation ability), and photocatalytic activity have been investigated in detail. The maximum H2 evolution rate (1650 µmol·h−1·g−1) was achieved over Zn0.84(CuGa)0.13In2S4, showing a 3.3 times higher rate than that of untreated ZnIn2S4. The bandgap energy of Zn(1−2x)(CuGa)xIn2S4 decreased from 2.67 to 1.90 eV as the amount of doping Cu+ and Ga3+ increased.
Keywords: photocatalytic hydrogen generation; copper; gallium; ZnIn2S4 photocatalytic hydrogen generation; copper; gallium; ZnIn2S4

1. Introduction

Global energy shortage and the environmental pollution associated with burning fossil fuels have stimulated people’s interest in clean and sustainable energy. Photocatalytic hydrogen evolution through water splitting and photocatalytic CO2 reduction on semiconductor photocatalysts, are of interest due to their intriguing application to converting solar energy to chemical energy [1,2,3,4,5,6]. Among the two processes, the photocatalytic hydrogen production plays an important role, producing hydrogen for society. Sunlight contains ultraviolet light, visible light, and infrared light, and it is ideal to use all the wavelengths when using sunlight. It is known that the wavelength ranges of light in which the photocatalyst reacts are caused by the size of the band gap of the photocatalyst [7,8,9]. Metal sulfides have a relatively negative valence band (VB) edge and a narrower band gap derived from sulfide ions than metal oxides. Therefore, photocatalytic activity can be exhibited even with long wavelength light [10,11,12]. The addition of foreign elements, which is one of the ways to change these factors, greatly affects the characteristics of the catalyst. Specific requirements to improve the activity of the photocatalytic material include efficient light absorption, effective separation of photogenerated charge carriers, and better efficiency to the interface for direct release of hydrogen and/or oxygen from water [13,14,15]. In efficient light absorption, the size of the band gap formed by the conduction band (CB) and the valence band of the semiconductor’s photocatalyst is the most important issue [16,17,18]. In regard to the effective separation of photogenerated charge carriers, many elements, such as defects in the crystal structure and band structure, are involved in a complex manner [19,20,21]. Sulfide photocatalysts are advantageous for visible light driven photocatalysts because they have narrow band gaps and negative valence bands, due to the S electron orbital, than oxide-based photocatalysts [22,23,24]. ZnIn2S4 has been reported to have a suitable band gap corresponding to the visible-light absorption region, high photocatalytic activity, and considerable chemical stability for photocatalytic H2 evolution. However, bare ZnIn2S4 can show inferior performance for photocatalytic hydrogen evolution due to the short lifetime of the photo-induced carriers, and is expected to be further improved for the visible light response [25,26,27]. In recent years, there have been various studies for improving the photocatalytic activity of ZnIn2S4 [28,29]. Cu species, such as Cu+ and Cu2+, affected the valence band of ZnIn2S4 and constituted an advantageous band structure for photocatalysts [30,31,32]. It has been found that Ga doping into TiO2 produces an oxygen vacancy and an incomplete level conduction band, and electron traps enhance the separation of photogenerated electron-hole pairs [33,34,35]. Additionally, since Zn2+ (88 pm), Cu+ (91 pm), and Ga3+ (76 pm) have relatively similar radii, it is not difficult to replace the Zn2+ part of ZnIn2S4 with Cu+ and Ga3+. Since the solid solution changes structural, mechanical, optical, and physical properties more dramatically than simple phase mixtures, the properties can be controlled [36]. Ga doping into ZnGa2S4 improved the photocatalytic activity [37]. Since ZnIn2S4 and ZnGa2S4 have a similar chalcopyrite structure, the insertion of Ga into ZnIn2S4 was hypothesized to improve the photocatalytic activity. Therefore, it is reasonable to think that doping Ga into ZnIn2S4 improves the hydrogen generation activity. There are a few reports that ZnIn2S4 is co-doped Cu+ and Ga3+. In this study, we investigated photocatalytic activity, optical properties, and surface morphology of ZnIn2S4 simultaneously co-doped with Ga3+ and Cu+.

2. Materials and Methods

2.1. Preparation of Photocatalysts

All chemicals were analytical grade and used as received without further purification. Zn(1−2x)(CuGa)xIn2S4 compounds were prepared by a simple hydrothermal method. In total, 3.76 mmol of cetyltrimethylammonium bromide (CTAB) (Wako Pure Chemical Industries, Ltd., Japan), a stoichiometric ratio of ZnSO4·7H2O (Nacalai Tesque, Inc., Japan), InCl3 4H2O, CuCl (I), gallium chloride (III), and the excess of thioacetamide (TAA) (Wako Pure Chemical Industries, Ltd., Japan) were dissolved in 50 mL of distilled water. The starting chemical regents are shown in Table S1. In order to keep Cu monovalent, nitrogen was purged for 10 min to remove dissolved oxygen. The mixed solution was then transferred into a 100 mL Teflon autoclave. The autoclave was sealed and kept at 160 °C for 1 h and cooled to room temperature naturally. After cooling, the product was dried in a vacuum at 40 °C for 4 h and was ground for 30 min. ZnIn2S4 (no doped), Zn0.87Cu0.13In2S3.935 (only Cu+ doped), and Zn0.87Ga0.13In2S4.065 (only Ga3+ doped) were also prepared by the same method as the reference material. The prepared photocatalysts are shown in the Table S2.

2.2. Characterization of Samples

X-ray powder diffraction (XRD) measurements were performed using a Rigaku RINT Ultima-IV diffractometer. That was carried out using Cu radiation at a scan rate of 0.04°/s in a scan range of 10–80°. X-ray photoelectron spectroscopy (XPS) measurements were carried out with a PHI Quantera SXM photoelectron spectrometer using Al Kα radiation. To compensate for surface charges effects, binding energies were calibrated using the C1s peak at 284 eV as the reference. Scanning electron microscope (SEM) observations were performed using a Hitachi S-4000 SEM. The transmission electron microscopy (TEM) images were taken on JEOL product JEM1011. The nitrogen adsorption and desorption isotherm and the Brunaure Emmett Teller (BET) specific surface area were measured from BELSORP miniII (MicrotracBEL Corp). The UV–vis diffuse reflectance spectra (DRS) of the photocatalysts were recorded using a Shimadzu UV-2450 spectrophotometer equipped with an integral sphere assembly, using BaSO4 as a reflectance standard. Photoluminescence (PL) spectra were obtained at an excitation wavelength of 350 nm using a Shimadzu RF-5300PC spectrofluorophotometer. The band gap was calculated from Equation (1). In addition, valence bands and conduction bands were calculated from XPS results and band gap values.
αhν = A(hν − Eg)n
where h, α, ν, A, and Eg represent Planck’s constant, optical absorption coefficient, photon frequency, a constant, and the photonic energy band gap; and n = 1/2 for a direct band gap semiconductor.

2.3. Photocatalytic Hydrogen Generation

The side-irradiation-type pyrex batch reactor (inner volume: 123 mL) was used for the photocatalytic production of hydrogen from aqueous sulfide solution. A 300 W Xe lamp was used as the light source, and the UV part of the light was removed by a cutoff filter (λ ≥ 420 nm). In all experiments, 40 mL of solutiom (pH12) containing 40 mg of catalyst, 0.01 ug Pt/mL of H2PtCl6, and 0.25 M Na2SO3/0.35 M Na2S mixed sacrificial agent was added into the reaction cell. The light source was a 4000–4500 µW/cm2 Xe-lamp, used for 6 h with a cut-off filter (λ ≥ 420 nm). Nitrogen purged through the system for 30 min before the reaction to remove oxygen. The concentrations of H2 were measured with a gas chromatograph (GC). The injection, column, and detector in the GC were 50 °C. A thermal conductivity detector (TCD) was used as the detector. The hydrogen generation experimental conditions are shown in Table 1. In the experiment on the stability of the photocatalyst, one cycle of photocatalytic hydrogen generation was 6 h, and 4 cycles were performed under the same conditions. At the end of each cycle, a nitrogen purge was performed in the reaction vessel to remove residual hydrogen.

3. Results and Discussion

3.1. Structural Characterization

The XRD patterns of Zn(1−2x)(CuGa)xIn2S4 are shown in Figure 1. The XRD pattern of ZnIn2S4 could be indexed as the hexagonal structure (JCPDS number 65-2023). The four diffraction peaks at around 20.8°, 27.5°, 47.2°, and 55.9° could be assigned to the (006), (101), and (110) and (202) planes, respectively [38,39]. The XRD patterns of other composition ratio of Zn(1−2x)(CuGa)xIn2S4 also showed the similar results. This XRD patterns were in agreement with those reported in previous studies, which revealed it that Zn(1−2x)(CuGa)xIn2S4 is a hexagonal structure and contained almost no impurities of ZnS. The reason for the fact that that the Cu and Ga derived peaks were not observed could be due to the very low doping amount. In addition, with respect to the peak of the (006) plane, a slight shift toward the high angle was observed when doping with Cu and Ga. This means it that the interplanar spacing is reduced by doping, suggesting that Cu and Ga may be incorporated into the crystal structure of ZnIn2S4 and exist as a solid solution. The shift of the peak was observed between x = 0 and 0.13 in the doping amount, and a further shift was not observed when x = 0.16 and 0.19. This result indicates that beyond x = 0.13, it was not incorporated into the crystal structure of ZnIn2S4.
The results of XPS measurement for further structural analysis of photocatalysts are shown in Figure S1. The elemental ratios determined from the XPS spectrum are shown in the Table S3. A series of XPS survey spectra matched the reference material ZnIn2S4. This indicates that the impurities are not present, regardless of the doping amounts of Cu and Ga. Furthermore, from the result of elemental ratio analysis of XPS, it was able to be confirmed that the elemental ratio of the prepared catalysts was substantially in agreement with the theoretical ratio. Typical narrow spectra of Zn(1−2x)(CuGa)xIn2S4 are shown in Figure 2. In the XPS spectrum of Zn 2p, two peaks located at Zn 2p3/2 (1020.4 eV) and Zn 2p1/2 (1043.6 eV) were observed. Binding energies of 443.7 (In 3d5/2) and 451.0 eV (In 3d3/2) were observed in the XPS spectrum of In 3d. The peaks of S 2p were at 161.3 (S 2p3/2) and 162.6 eV (S 2p1/2) [40,41]. In addition, since the peak position of Cu 2p3/2 was observed only at 932 eV, and Cu 2p3/2 satellite peak derived from Cu2+ was not present at 942 eV, it can be seen that Cu was doped in a monovalent state [42,43]. The Ga 2p1/2 and Ga 2p3/2 peaks derived from Ga3+ were located at around 1144 eV and 1117 eV, respectively [44]. These results of XRD and XPS show that the basic structure of Zn(1−2x)(CuGa)xIn2S4 is hexagonal ZnIn2S4, co-doped with Cu+ and Ga3+.

3.2. Morphological Analysis

In order to investigate the influence of Cu and Ga doping on the characteristic surface morphology of Zn(1−2x)(CuGa)xIn2S4, SEM images of the prepared photocatalysts were observed. The results are shown in Figure 3. In pure ZnIn2S4, a microsphere-structure-like flower formed by overlapping of nanosheets was observed [45]. Co-doped with Ga and Cu, the spherical structure was destroyed, and the destruction was promoted as the doping amount increased. The nanosheet structure also collapsed and aggregated as the doping amount of Ga and Cu increased. The shapes of pure ZnIn2S4 and Zn0.62(CuGa)0.19In2S4 were very different. Cu+, Ga3+, and ZnIn2S4 are not composites; solid solutions may have been formed.
The nitrogen adsorption and desorption isotherm and BET specific surface area are shown in Figure S2 and Table S4, respectively. According to the results, co Cu+ and Ga3+ doping increased the specific surface area. The specific surface area of Zn0.74(CuGa)0.13In2S4 was six times larger than that of ZnIn2S4. This result could be related to the surface morphology observed by SEM.

3.3. Optical Analysis

Figure 4 shows the relationship between the doping amount of Cu and Ga to ZnIn2S4 and the absorption wavelength region of light using the UV-visible diffuse reflectance spectrum. Figure S3 shows the fitting curve of (αhν)1/n versus hν, for various Zn(1−2x)(CuGa)xIn2S4 photocatalysts; the curve was best fitted when n = ½, meaning that Zn(1−2x)(CuGa)xIn2S4 photocatalysts have a direct band gap transition. As shown in the literature, the absorption edge of pure ZnIn2S4 was 480 nm, and the size of the band gap was 2.67 eV. For Zn(1−2x)(CuGa)xIn2S4, as the doping amount was increased, the absorption edge of photocatalyst was red-shifted, because the 3d orbital derived from Cu+ affects the valence band edge of ZnIn2S4, and the 4s orbital from Ga3+ influences the conduction band edge for ZnIn2S4. The absorption edge reached about 700 nm when the doping amount was x = 0.13, but a further increase in the doping amount (x = 0.16 and 0.19) had scarcely any influence on the absorption edge. The results could be explained by the same doping amount over x = 0.13. Additionally, the band gap of Zn(1−2x)(CuGa)xIn2S4 similarly decreased as the doping amount of Cu+ and Ga3+ increased. In addition, to evaluate the band structure, the valence band edge measurements for ZnIn2S4 and Zn0.74(CuGa)0.13In2S4 were performed by XPS. From Figure S4, the valence band edge of Zn0.74(CuGa)0.13In2S4 is visibly 0.7 V lower than that of ZnIn2S4.
We investigated the photoluminescence spectra in order to investigate the electron-hole pair separation efficiency due to the doping of Cu and Ga, and the results are shown in the Figure 5. Luminescence in ultraviolet and visible regions was observed in the photoluminescence spectrum. In general, ultraviolet emission is associated with exciton transition and recombination from the conduction band level to the valence band, and visible light emission is mainly associated with intrinsic or extrinsic defects in the catalyst. Both peaks in the spectrum were lowered by increasing the Cu+ and Ga3+ doping. This may be due to the reduction of the recombination rate between the photogenerated holes and the electrons photogenerated in the valence band being trapped in the oxygen vacancies generated in the photocatalyst by doping.

3.4. Photocatalytic Activity

A hydrogen generation was conducted using the photocatalyst Zn(1−2x)(CuGa)xIn2S4. ZnIn2S4, Zn0.87Cu0.13In2S3.935, and Zn0.87Ga0.13In2S4.065 were used as comparative objects. All catalysts were loaded with 1 wt% Pt as a cocatalyst. The results are shown in Figure S5. The photocatalyst mono-doped with Ga or Cu showed higher hydrogen generation activity than pure ZnIn2S4. Furthermore, the photocatalyst co-doped with Cu and Ga showed the highest hydrogen generation activity. The optimum doping amounts of Cu and Ga were investigated, as shown in Figure 6.
The hydrogen generation increased when the doping amount of Cu and Ga was from x = 0 to 0.13. The largest H2 production rate was obtained with Zn(1−2x)(CuGa)xIn2S4 (x = 0.13), and it was 3.3 times better than that of ZnIn2S4. However, the amount of hydrogen decreased from x = 0.16 to 0.19. The reason for the photocatalytic activity from increase to decrease may be due to the shielding by Cu and Ga. The dependence of photocatalytic H2 generation on doping levels has been reported by other studies [31,37]. In the stability test of the photocatalyst in Figure 7, The decrease in the amount of hydrogen production could not be observed in four cycles. A slight change in the amount of hydrogen produced in the 3rd and 4th cycles may have been due to the deterioration of the sacrificial agent. The TEM images before and after the hydrogen generation of Zn(1−2x)(CuGa)xIn2S4 photocatalyst are shown in the Figure S6. Only after hydrogen generation, a 3–4 nm spot was observed on the sample surface. The spot deposition was Pt, which was produced by the reduction during the hydrogen production.

3.5. Proposed Hydrogenation Mechanism

The reaction mechanism is shown in Scheme 1. Irradiation of light having a wavelength corresponding to the band gap energy of the Zn(1−2x)(CuGa)xIn2S4 photocatalyst excites electrons in the valence band of Zn(1−2x)(CuGa)xIn2S4 to the conduction band, in order to produce an electron hole pair (Equation (2)). The band gap in the case of Cu+ and Ga3+ co-doping was narrowed by forming impurity levels derived from the Cu 3d and Ga 4s orbitals on the negative side of the valence band and the positive side of the conduction band of ZnIn2S4, respectively. The tuned band gap promotes charge separation of the photocatalyst and improves the visible light response. In Equation (3), some of electrons excited in the conduction band of Zn(1−2x)(CuGa)xIn2S4 are consumed for the photodeposition of Pt. The hydrogen was produced from H2O and electrons which were transferred from conduction band of Zn(1−2x)(CuGa)xIn2S4 to Pt metal (Equation (4)). On the other hand, with regard to holes, sulfite ions and sulfide ions consume the holes and promote a hydrogen generation reaction. The presence of Na2S is very important for enhancing the photocatalytic activity; on the other hand, Na2S stabilizes the surface of the metal sulfide by suppressing the formation of surface defects as a scavenger of holes. However, when the concentration of Na2S is high, the pH becomes high. High pH values are thermodynamically disadvantageous in the reaction represented by Equation (4). As described in the Equations (5)–(7), SO32− and S2− consume holes, and promote hydrogen generation reaction. According to Equation (6), S22− ions are generated and act as an optical filter. If S22− is not consumed, it interferes with light absorption. As shown in Equation (8), the reaction between S22− and SO32− forms S2O32−, is colorless and does not compete with light absorption.
Photocatalyst + hν → e + h+
Pt2+ + 2eCB → Pt
2H2O + 2eCB → H2 + 2OH
SO32− + H2O + 2h+VB → SO42− + 2H+
2S2− + 2h+VB → S22−
SO32− + S2− + 2h+VB → S2O32−
S22− + SO32− → S2O32− + S2−

4. Conclusions

The novel Zn(1−2x)(CuGa)xIn2S4 photocatalyst, in which ZnIn2S4 is co-doped with Cu+ and Ga3+ was prepared by a simple one-pot solvothermal method. The amount of doping with Cu+ and/or Ga3+ into ZnIn2S4 has effects on the crystal structures, morphologies, optical properties, and photocatalytic activities of the obtained Zn(1−2x)(CuGa)xIn2S4 photocatalysts. From the SEM image, doping Cu+ and Ga3+ breaks microsphere-like flowers. That is due to the insertion of Cu+ and Ga3+ into Zn2+ parts of ZnIn2S4. A shifted XRD pattern showed that Zn(1−2x)(CuGa)xIn2S4 has a high probability for a solid solution of having a hexagonal ZnIn2S4 as its basic structure. XPS spectra suggests the presence of Cu+ and Ga3+ in Zn(1−2x)(CuGa)xIn2S4. DRS and PL results show that the control of the band gap and suppression of electron-hole recombination were confirmed by doping ZnIn2S4 with Cu+ and Ga3+. In addition, the doping of Cu+ and Ga3+ improved the photocatalytic activity. The optimum doping amount was Zn: (CuGa) = 0.74:0.13. The hydrogen generation rate by Zn(1−2x)(CuGa)xIn2S4 at the optimum doping amount was 1650 μmol·g−1·h−1, which was 3.3 times higher than that of ZnIn2S4 (500μmol·g−1·h−1). Furthermore, this photocatalyst showed good stability during 24 h of hydrogen generation.

Supplementary Materials

The following are available online at https://www.mdpi.com/2305-7084/3/4/79/s1: Table S1: Starting material for the preparation of photocatalyst; Table S2: Expected composite components of photocatalyst (molar ratio); Table S3: Elemental ratios of Zn(1−2x)(CuGa)xIn2S4 from XPS result; Table S4: Surface area of photocatalysts; Figure S1: XPS survey spectra of Zn(1−2x)(CuGa)xIn2S4; Figure S2: Adsorption/desorption Isotherm of Zn(1−2x)(CuGa)xIn2S4; Figure S3: Tauc plots of Zn(1−2x)(CuGa)xIn2S4; Figure S4: Valence-band edge XPS spectra of (a) ZnIn2S4, (b) Zn0.74(CuGa)0.13In2S4. Figure S5: Photocatalytic hydrogen production with (a) ZnIn2S4 (rhombus), (b) Zn0.87Ga0.13In2S4.065 (cross), (c) Zn0.87Cu0.13In2S3.935 (triangle), and (d) Zn0.74(CuGa)0.13In2S4 (square). Pt doping: 1.0 wt%. Figure S6: TEM images of Zn0.74(CuGa)0.13In2S4.

Author Contributions

I.T. and H.K. conceived and designed the experiments. I.T. performed the experiments and wrote the paper. I.T., M.F., S.K., and H.K. analyzed the results and advised the project.

Conflicts of Interest

The authors declare no conflict of interest.

Note

All experiments were conducted at Mie University. Any opinions, findings, conclusions or recommendations expressed in this paper are those of the authors and do not necessarily reflect the view of the supporting organizations.

References

  1. Kudo, A. Development of photocatalyst materials for water splitting. Int. J. Hydrogen Energy 2006, 31, 197–202. [Google Scholar] [CrossRef]
  2. Kato, H.; Kudo, A. Photocatalytic water splitting into H2 and O2 over various tantalate photocatalysts. Catal. Today 2003, 78, 561–569. [Google Scholar] [CrossRef]
  3. Chen, X.; Shen, S.; Guo, L.; Mao, S.S. Semiconductor-based Photocatalytic Hydrogen Generation. Chem. Rev. 2010, 110, 6503–6570. [Google Scholar] [CrossRef] [PubMed]
  4. Sorcar, S.; Hwang, Y.; Lee, J.; Kim, H.; Grimes, K.M.; Grimes, C.A.; Jung, J.-W.; Cho, C.-H.; Majima, T.; Hoffmann, M.R.; et al. CO2, water, and sunlight to hydrocarbon fuels: A sustained sunlight to fuel (Joule-to-Joule) photoconversion efficiency of 1%. Energy Environ. Sci. 2019, 12, 2685–2696. [Google Scholar] [CrossRef]
  5. Schreier, M.; Héroguel, F.; Steier, L.; Ahmad, S.; Luterbacher, J.S.; Mayer, M.T.; Luo, J.; Grätzel, M. Solar conversion of CO2 to CO using Earth-abundant electrocatalysts prepared by atomic layer modification of CuO. Nat. Energy 2017, 2, 17087. [Google Scholar] [CrossRef]
  6. Kumar, B.; Asadi, M.; Pisasale, D.; Ray, S.S.; Rosen, B.A.; Haasch, R.; Abiade, J.; Yarin, A.L.; Khojin, A.S. Renewable and metal-free carbon nanofiber catalysts for carbon dioxide reduction. Nat. Commun. 2013, 4, 2819. [Google Scholar] [CrossRef]
  7. Tsuji, I.; Kato, H.; Kobayashi, H.; Kudo, A. Photocatalytic H2 evolution reaction from aqueous solutions over band structure-controlled (Agln)xZn2(1−x)S2 solid solution photocatalysts with visible-light response and their surface nanostructures. J. Am. Chem. Soc. 2004, 126, 13406–13413. [Google Scholar] [CrossRef] [PubMed]
  8. Peng, S.; Zhu, P.; Thavasi, V.; Mhaisalkar, S.G.; Ramakrishna, S. Facile solution deposition of ZnIn2S4 nanosheet films on FTO substrates for photoelectric application. Nanoscale 2011, 3, 2602–2608. [Google Scholar] [CrossRef]
  9. Zhang, X.; Yu, L.; Zhuang, C.; Peng, T.; Li, R.; Li, X. Highly Asymmetric Phthalocyanine as a Sensitizer of Graphitic Carbon Nitride for Extremely Efficient Photocatalytic H2 Production under Near-Infrared Light. ACS Catal. 2014, 4, 162–170. [Google Scholar] [CrossRef]
  10. Dai, W.W.; Zhao, Z.Y. DFT study on the interfacial properties of vertical and in-plane BiOI/BiOIO3 hetero-structures. Phys. Chem. Chem. Phys. 2017, 19, 9900–9911. [Google Scholar] [CrossRef] [PubMed]
  11. Vaiano, V.; Iervolino, G.; Rizzo, L. Cu-doped ZnO as efficient photocatalyst for the oxidation of arsenite to arsenate under visible light. Appl. Catal. B 2018, 238, 471–479. [Google Scholar] [CrossRef]
  12. Shafaee, M.; Goharshadi, E.K.; Mashreghi, M.; Sadeghinia, M. TiO2 nanoparticles and TiO2@graphene quantum dots nancomposites as effective visible/solar light photocatalysts. J. Photochem. Photobiol. A Chem. 2018, 357, 90–102. [Google Scholar] [CrossRef]
  13. Chang, C.J.; Wang, C.W.; Wei, Y.H.; Chen, C.Y. Enhanced photocatalytic H2 production activity of Ag-doped Bi2WO6-graphene based photocatalysts. Int. J. Hydrogen Energy 2018, 43, 11345–11354. [Google Scholar] [CrossRef]
  14. Jia, Y.; Zhao, D.; Li, M.; Han, H.; Li, C. La and Cr Co-doped SrTiO3 as an H2 evolution photocatalyst for construction of a Z-scheme overall water splitting system. Chin. J. Catal. 2018, 39, 421–430. [Google Scholar] [CrossRef]
  15. Yan, X.; Xue, C.; Yang, B.; Yang, G. Novel three-dimensionally ordered macroporous Fe3+-doped TiO2 photocatalysts for H2 production and degradation applications. Appl. Surf. Sci. 2017, 394, 248–257. [Google Scholar] [CrossRef]
  16. Singh, J.; Sharma, S.; Sharma, S.; Singh, R.C. Effect of tungsten doping on structural and optical properties of rutile TiO2 and band gap narrowing. Optik 2019, 182, 538–547. [Google Scholar] [CrossRef]
  17. Sharma, P.K.; Cortes, M.A.L.; Hamilton, J.W.; Han, Y.; Byrne, J.A.; Nolan, M. Surface modification of TiO2 with copper clusters for band gap narrowing. Catal. Today 2018, 321–322, 9–17. [Google Scholar] [CrossRef]
  18. Liu, C.; Chai, B.; Wang, C.; Yan, J.; Ren, Z. Solvothermal fabrication of MoS2 anchored on ZnIn2S4 microspheres with boosted photocatalytic hydrogen evolution activity. Int. J. Hydrogen Energy 2018, 43, 6977–6986. [Google Scholar] [CrossRef]
  19. Gao, P.; Li, A.; Sun, D.D.; Ng, W.J. Effects of various TiO2 nanostructures and graphene oxide on photocatalytic activity of TiO2. J. Hazard. Mater. 2014, 279, 96–104. [Google Scholar] [CrossRef] [PubMed]
  20. Guo, J.; Liao, X.; Lee, M.-H.; Hyett, G.; Huang, C.C.; Hewak, D.W.; Mails, S.; Zhou, W.; Jiang, Z. Experimental and DFT insights of the Zn-doping effects on the visible-light photocatalytic water splitting and dye decomposition over Zn-doped BiOBr photocatalysts. Appl. Catal. B 2019, 243, 502–512. [Google Scholar] [CrossRef]
  21. Behzadifard, Z.; Shariatinia, Z.; Jourshabani, M. Novel visible light driven CuO/SmFeO3 nanocomposite photocatalysts with enhanced photocatalytic activities for degradation of organic pollutants. J. Mol. Liq. 2018, 262, 533–548. [Google Scholar] [CrossRef]
  22. Li, T.L.; Cai, C.D.; Yeh, T.F.; Teng, H. Capped CuInS2 quantum dots for H2 evolution from water under visible light illumination. J. Alloys Compd. 2013, 550, 326–330. [Google Scholar] [CrossRef]
  23. You, D.; Pan, B.; Jiang, F.; Zhou, Y.; Su, W. CdS nanoparticles/CeO2 nanorods composite with high-efficiency visible-light-driven photocatalytic activity. Appl. Surf. Sci. 2016, 363, 154–160. [Google Scholar] [CrossRef]
  24. Jin, X.; Chen, F.; Jia, D.; Cao, Y.; Duan, H.; Long, M.; Yang, L. Influences of synthetic conditions on the photocatalytic performance of ZnS/graphene composites. J. Alloys Compd. 2019, 780, 299–305. [Google Scholar] [CrossRef]
  25. Zhang, S.; Wang, L.; Liu, C.; Luo, J.; Crittenden, J.; Liu, X.; Ca, T.; Yuan, J.; Pei, Y.; Liu, Y. Photocatalytic wastewater purification with simultaneous hydrogen production using MoS2 QD-decorated hierarchical assembly of ZnIn2S4 on reduced graphene oxide photocatalyst. Water Res. 2017, 121, 11–19. [Google Scholar] [CrossRef] [PubMed]
  26. Fan, B.; Chen, Z.H.; Liu, Q.; Zhang, Z.G.; Fang, X.M. One-pot hydrothermal synthesis of Ni-doped ZnIn2S4 nanostructured film photoelectrodes with enhanced photoelectrochemical performance. Appl. Surf. Sci. 2016, 370, 252–259. [Google Scholar] [CrossRef]
  27. Ding, Y.; Gao, Y.; Li, Z. Carbon quantum dots (CQDs) and Co(dmgH)2PyCl synergistically promote photocatalytic hydrogen evolution over hexagonal ZnIn2S4. Appl. Surf. Sci. 2018, 462, 255–262. [Google Scholar] [CrossRef]
  28. Zeng, C.; Huang, H.; Zhang, T.; Dong, F.; Zhang, Y.; Hu, Y. Fabrication of Heterogeneous-Phase Solid-Solution Promoting Band Structure and Charge Separation for Enhancing Photocatalytic CO2 Reduction: A Case of ZnXCa1–XIn2S4. ACS Appl. Mater. Interfaces 2017, 9, 27773–27783. [Google Scholar] [CrossRef]
  29. Sun, M.; Zhao, X.; Zeng, Q.; Yan, T.; Ji, P.G.; Wu, T.T.; Wei, D.; Du, B. Facile synthesis of hierarchical ZnIn2S4/CdIn2S4 microspheres with enhanced visible light driven photocatalytic activity. Appl. Surf. Sci. 2017, 407, 328–336. [Google Scholar] [CrossRef]
  30. Liu, A.; Yu, C.; Lin, J.; Sun, G.; Xu, G.; Huang, Y.; Liu, Z.; Tang, C. Construction of CuInS2@ZIF-8 nanocomposites with enhanced photocatalytic activity and durability. Mater. Res. Bull. 2019, 112, 147–153. [Google Scholar] [CrossRef]
  31. Shen, S.; Zhao, L.; Zhou, Z.; Guo, L. Enhanced Photocatalytic Hydrogen Evolution over Cu-Doped ZnIn2S4 under Visible Light Irradiation. J. Phys. Chem. C 2008, 112, 16148–16155. [Google Scholar] [CrossRef]
  32. Zhang, G.; Zhang, W.; John, C.C.; Chen, Y.; Minakata, D.; Wang, P. Photocatalytic hydrogen production under visible-light irradiation on (CuAg)0.15In0.3Zn1.4S2 systhesized by precipitation and calcination. Chin. J. Catal. 2013, 34, 1926–1935. [Google Scholar] [CrossRef]
  33. Liu, X.; Khan, M.; Liu, W.; Xiang, W.; Guan, M.; Jiang, P.; Cao, W. Synthesis of nanocrystalline Ga–TiO2 powders by mild hydrothermal method and their visible light photoactivity. Ceram. Int. 2015, 41, 3075–3080. [Google Scholar] [CrossRef]
  34. Khallaf, H.; Chai, G.; Lupan, O.; Chow, C.; Park, S.; Schulte, A. Characterization of gallium-doped CdS thin films grown by chemical bath deposition. Appl. Surf. Sci. 2009, 255, 4129–4134. [Google Scholar] [CrossRef]
  35. Sola, A.C.; Gösser, M.B.; de la Piscina, P.R.; Homs, N. Promoter effect of Ga in Pt/Ga-TiO2 catalysts for the photo-production of H2 from aqueous solutions of ethanol. Catal. Today 2017, 287, 85–90. [Google Scholar] [CrossRef]
  36. Da Costa, R.C.; Rodrigues, A.D.; Pizani, P.S. Phase mixture, solid solution or composite: Raman scattering analyses of NixPb1-xTiO3 and (NiTiO3)xþ (PbTiO3)1−x. J. Alloys Compd. 2017, 697, 68–71. [Google Scholar] [CrossRef]
  37. Kaga, H.; Kudo, A. Cosubstituting effects of copper(I) and gallium(III) for ZnGa2S4 with defect chalcopyrite structure on photocatalytic activity for hydrogen evolution. J. Catal. 2014, 310, 31–36. [Google Scholar]
  38. Chai, B.; Liu, C.; Wang, C.; Yan, J.; Ren, Z. Photocatalytic hydrogen evolution activity over MoS2/ZnIn2S4 microspheres. Chin. J. Catal. 2017, 38, 2067–2075. [Google Scholar] [CrossRef]
  39. Cui, W.; Guo, D.; Liu, L.; Hu, J.; Rana, D.; Liang, Y. Preparation of ZnIn2S4/K2La2Ti3O10 composites and their photocatalytic H2 evolution from aqueous Na2S/Na2SO3 under visible light irradiation. Catal. Commun. 2014, 48, 55–59. [Google Scholar] [CrossRef]
  40. Du, C.; Zhang, Q.; Lin, Z.; Yan, B.; Xia, C.; Yang, G. Half-unit-cell ZnIn2S4 monolayer with sulfur vacancies for photocatalytic hydrogen evolution. Appl. Catal. B 2019, 248, 193–201. [Google Scholar] [CrossRef]
  41. Yang, X.; Li, Q.; Lv, K.L.; Li, M. Heterojunction construction between TiO2 hollowsphere and ZnIn2S4 flower for photocatalysis application. Appl. Surf. Sci. 2017, 398, 81–88. [Google Scholar]
  42. Yue, W.; Han, S.; Peng, R.; Shen, W.; Geng, H.; Wu, F.; Tao, S.; Wang, M. CuInS2 quantum dots synthesized by a solvothermal route and their application as effective electron acceptors for hybrid solar cells. J. Mater. Chem. 2010, 20, 7570–7578. [Google Scholar] [CrossRef]
  43. Sudip, K.B.; Tian, L.; Venkatram, N.; Wei, J.; Vittal, J.J. Phase-selective synthesis of CuInS2 nanocrystals. J. Phys. Chem. C 2009, 113, 15037–15042. [Google Scholar]
  44. Zepeda, T.A.; Pawelec, B.; De Leon, J.D.; De los Reyes, J.A.; Olivas, A. Effect of gallium loading on the hydrodesulfurization activity of unsupported Ga2S3/WS2 catalysts. Appl. Catal. B 2012, 111–112, 10–19. [Google Scholar] [CrossRef]
  45. Chen, S.; Li, S.; Xiong, L.; Wang, G. In-situ growth of ZnIn2S4 decorated on electrospun TiO2 nanofibers with enhanced visible-light photocatalytic activity. Chem. Phys. Lett. 2018, 706, 68–75. [Google Scholar] [CrossRef]
Figure 1. XRD patterns of Zn(1−2x)(CuGa)xIn2S4. (a) x = 0, (b) x = 0.07, (c) x = 0.10, (d) x = 0.13, (e) x = 0.16, and (f) x = 0.19.
Figure 1. XRD patterns of Zn(1−2x)(CuGa)xIn2S4. (a) x = 0, (b) x = 0.07, (c) x = 0.10, (d) x = 0.13, (e) x = 0.16, and (f) x = 0.19.
Chemengineering 03 00079 g001
Figure 2. XPS narrow and survey spectra of Zn0.62(CuGa)0.19In2S4. (a) survey, (b) Zn, (c) In, (d) Cu, (e) Ga, and (f) S.
Figure 2. XPS narrow and survey spectra of Zn0.62(CuGa)0.19In2S4. (a) survey, (b) Zn, (c) In, (d) Cu, (e) Ga, and (f) S.
Chemengineering 03 00079 g002
Figure 3. SEM images of Zn(1−2x)(CuGa)xIn2S4. (a) x = 0, (b) x = 0.07, (c) x = 0.10, (d) x = 0.13, (e) x = 0.16, and (f) x = 0.19.
Figure 3. SEM images of Zn(1−2x)(CuGa)xIn2S4. (a) x = 0, (b) x = 0.07, (c) x = 0.10, (d) x = 0.13, (e) x = 0.16, and (f) x = 0.19.
Chemengineering 03 00079 g003
Figure 4. UV-visible spectra of Zn(1−2x)(CuGa)xIn2S4.
Figure 4. UV-visible spectra of Zn(1−2x)(CuGa)xIn2S4.
Chemengineering 03 00079 g004
Figure 5. Photoluminescence spectra for Zn(1−2x)(CuGa)xIn2S4 excitation: 350 nm.
Figure 5. Photoluminescence spectra for Zn(1−2x)(CuGa)xIn2S4 excitation: 350 nm.
Chemengineering 03 00079 g005
Figure 6. Photocatalytic hydrogen production rate with Zn(1−2x)(CuGa)xIn2S4.
Figure 6. Photocatalytic hydrogen production rate with Zn(1−2x)(CuGa)xIn2S4.
Chemengineering 03 00079 g006
Figure 7. Cycling runs of Zn0.74(CuGa)0.13In2S4.
Figure 7. Cycling runs of Zn0.74(CuGa)0.13In2S4.
Chemengineering 03 00079 g007
Scheme 1. H2 production mechanism.
Scheme 1. H2 production mechanism.
Chemengineering 03 00079 sch001
Table 1. Hydrogen generation experimental conditions.
Table 1. Hydrogen generation experimental conditions.
PhotocatalystZn(1−2x)(CuGa)xIn2S(4−1.5x) (x = 0, 0.07, 0.10, 0.13, 0.16, 0.19)
Zn0.87Cu0.13In2S3.935, Zn0.87Ga0.13In2S4.065
CocatalystH2PtCl6 (1.0 wt%)
Medium0.25 M Na2SO3/0.35 M Na2S 40 mL, (pH 12)
ReactorPyrex glass vessel (volume: 123 mL)
TemperatureRoom Temperature (25 °C)
Light sourceXenon lamp (λ ≧ 420 nm, 4500 µW/cm2)
Irradiation time6 h
AnalysisGas chromatography (TCD)
Back to TopTop