CdIn2S4 microspheres with enhanced visible light driven photocatalytic activity

Accepted Manuscript Title: Facile synthesis of hierarchical ZnIn2 S4 /CdIn2 S4 microspheres with enhanced visible light driven photocatalytic activity Author: Meng Sun Xia Zhao Qi Zeng Tao Yan Pengge Ji Tingting Wu Dong Wei Bin Du PII: DOI: Reference:

S0169-4332(17)30554-8 http://dx.doi.org/doi:10.1016/j.apsusc.2017.02.181 APSUSC 35290

To appear in:

APSUSC

Received date: Revised date: Accepted date:

19-12-2016 13-2-2017 20-2-2017

Please cite this article as: M. Sun, X. Zhao, Q. Zeng, T. Yan, P. Ji, T. Wu, D. Wei, B. Du, Facile synthesis of hierarchical ZnIn2 S4 /CdIn2 S4 microspheres with enhanced visible light driven photocatalytic activity, Applied Surface Science (2017), http://dx.doi.org/10.1016/j.apsusc.2017.02.181 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Facile synthesis of hierarchical ZnIn2S4/CdIn2S4

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photocatalytic activity

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microspheres with enhanced visible light driven

†

School of Resources and Environment, University of Jinan, Jinan 250022, P. R. China.

Key Laboratory of Chemical Sensing & Analysis in Universities of Shandong, School of Chemistry

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‡

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Meng Sun,† Xia Zhao,† Qi Zeng,† Tao Yan,† Pengge Ji,† Tingting Wu,† Dong Wei,† and Bin Du,†,‡,*

and Chemical Engineering, University of Jinan, Jinan 250022, P. R. China

Highlights

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E-mail: [email protected]; Fax: +86 531-82765969; Tel: +86 531-82769235

1. ZnIn2S4/CdIn2S4 microspheres were prepared by chemical co-precipitation method. 2. The CZIS-3 heterojunction exhibits the best activity under visible light. 3. Suppressed recombination of photo-carriers lead to the activity enhancement.

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Abstract Hierarchical ZnIn2S4/CdIn2S4 microspheres have been synthesized by a facile chemical co-precipitation

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method at low temperature. The obtained samples were characterized by XRD, SEM, XPS, and UV–vis

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DRS. The XRD patterns of ZnIn2S4/CdIn2S4 showed the distinctive peaks of both ZnIn2S4 and CdIn2S4. The obtained ZnIn2S4 microspheres were composed of numerous large nanoplates, while CdIn2S4

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microspheres possessed a relatively smooth surface. As for ZnIn2S4/CdIn2S4 composites, it was observed that partial of the ZnIn2S4 nanoplates were grown on the surface of CdIn2S4 microspheres. The

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absorption edge of ZnIn2S4 move towards longer wavelengths with the increment of CdIn2S4 component. In the photocatalytic degradation of MO, the ZnIn2S4/CdIn2S4 (mole ratio = 1:3) photocatalyst exhibited

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the best activity compared with pure ZnIn2S4 and CdIn2S4. The degradation ratio of MO (10 ppm) was

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about 99.7% after 90 min of reaction, while RhB (4 × 10-5 M) could be completely degraded within 70 min over ZnIn2S4/CdIn2S4. The activity enhancement can be ascribed to the improvement of visible light adsorption and separation efficiency enhancement of photo-generated carriers. Controlled experiments proved that active species of •O2−, •OH, and h+ were produced in the degradation system, which played the major role in the degradation process. Keywords: Photocatalyst; ZnIn2S4/CdIn2S4; Degradation; Organic Pollutant; Heterojunction

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1. Introduction In the past many decades, natural and synthetic organic contaminants have been continuously

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discharged into the environment, such as textile dyes, benzene hydrocarbon, surfactants, herbicide,

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fertilizers, and insecticides [1-4]. Now, the environment protection and management are under tremendous pressure like never before. Most of these contaminants usually have stable structure, so it is

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very difficult for them to be biodegraded under natural conditions [5,6]. Conventional wastewater treatment methods such as adsorption, biological treatment, and physical-chemical treatment usually

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have the disadvantages of high cost, long period, or second pollutions. Thus, a novel and green advanced treatment method is greatly needed. In the past many years, advanced oxidation processes

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(AOPs) including photocatalysis, Fenton method, photo-fenton, and sonolysis have been employed as efficient methods to decompose various organic pollutants [7-10]. Especially for the photocatalytic

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light illumination.

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technology, most organic pollutants usually can be oxidized and mineralized over photocatalyst under

Today, among the numerous semiconductor materials, TiO2 is the most investigated functional material due to its advantages of chemical stability, low cost, and non-toxicity [11,12]. However, the wide band gap (3.2 eV for anatase) and fast recombination of photo-generated carriers greatly retarded its extensive application in decomposing organic pollutants [13-15]. The large band gap of TiO2 determined that it was only can be photo-activated by ultraviolet light (accounting for 3–5% of the whole solar spectrum energy). Thus, in order to make full use of the sunlight, numerous efforts have been done to explore novel visible light-responsive photocatalysts. Furthermore, various methods have been applied to modify these visible light-responsive catalysts for the purpose of suppressing the recombination of photo-generated carriers, which usually result in the enhancement of photocatalytic activity [16,17]. 3

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Ternary metal sulfide with unique optoelectronic and catalytic functions has attracted great attention in recent years. For example, AgIn5S8, CaIn2S4, CdIn2S4, AgGaS2, ZnIn2S4 SnIn4S8, Cu2WS4, and AgInS2 have been reported as visible-light-driven photocatalysts for H2 evolution or contaminants degradation [18-25]. Chen et al. reported a solvothermal synthesis of ZnIn2S4 with phase junctions and evaluated the activity for H2 evolution under visible light illumination [26]. Li et al. reported the

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thermal solution synthesis of ZnIn2S4 and its application in organic pollutant degradation [27]. Yu et al.

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have reported the hydrothermal synthesis of CdIn2S4 and its photocatalytic activity towards dyes [28]. CdIn2S4 has also been synthesized by ultrasonic spray pyrolysis method as a photocatalyst for bacterial

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inactivation [29]. However, due to the high recombination rate of photo-generated carriers, the activity of individually CdIn2S4 and ZnIn2S4 is very low. However, the constructing of heterojunction by

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semiconductor combination usually could suppress the recombination of photo-generated carriers [30,31]. For instance, hierarchical CdIn2S4/graphene nano-heterostructures with multi-functionality has

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been prepared as a photocatalyst for solar hydrogen production [32]. ZnIn2S4/g-C3N4 has been prepared with enhanced photocatalytic activity toward 2,4-dichlorophenoxyacetic acid [33].

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Herein, hierarchical ZnIn2S4/CdIn2S4 heterostructures was fabricated via a facile template-free

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chemical co-precipitation method at low temperature (80 °C). As expected, the formation of heterojunction at their interfaces greatly enhanced the separation efficiency of photo-generated electron/hole pairs. The photocatalytic performances of ZnIn2S4/CdIn2S4 heterostructures have been investigated by decomposing methyl orange (MO) and rhodamine B (RhB) under visible light illumination. In order to investigate the separation and migration of electron-hole pairs, the transient photocurrent responses test and electrochemical impedance spectroscopy (EIS) have also been performed. Finally, a possible degradation mechanism is proposed based on the experimental results.

2. Experimental section 2.1. Synthesis of ZnIn2S4/CdIn2S4 Hierarchical ZnIn2S4/CdIn2S4 heterostructures have been synthesized by a facile chemical coprecipitation method. In a typical procedure, ZnCl2 (0.25 mmol), Cd(Ac)2•2H2O (0.75 mmol), 4

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InCl3•4H2O (2.0 mmol), and thiacetamide (6.0 mmol) were dissolved into 60 mL distilled water to form a transparent solution. The solution was then transferred into a 100 mL conical flask and the pH value was adjusted to 3 by using 0.5 M HCl solution. Then, the conical flask was sealed by sealing film and heated at 80 °C for 6 h using an electric oven. Finally, the precipitate was collected, washed with deionized water for several times, and then dried at 60 °C for 8 h to get the hierarchical ZnIn2S4/CdIn2S4

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microspheres. By changing the amount of ZnCl2 and Cd(Ac)2•2H2O added, ZnIn2S4/CdIn2S4

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heterostructures with different mole ratios have been prepared. For comparison, pure ZnIn2S4 and CdIn2S4 have been fabricated by the same method. In the following text, ZnIn2S4/CdIn2S4

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heterostructures with mole ratio Cd/Zn =1:3, 2:2, and 3:1 would be denoted as CZIS-1, CZIS-2, and CZIS-3, respectively.

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2.2. Characterization of photocatalysts

The crystal structures of the as-obtained products were characterized by X-ray powder diffraction

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(XRD) patterns under a D8 Advance X-ray diffractometer employing Ni-filtered Cu Kα radiation. The morphology and composition of the samples were examined by a field-emission scanning electron

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microscope (FEI, NanoSEM 230) operated at an accelerating voltage of 10 kV. The chemical states of

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the obtained samples were analyzed with X-ray photoelectron spectroscopy (XPS) on an ESCALAB 250 apparatus (Thermo Fisher Scientific) at 3.0 × 10-10 mbar using Al Kα X-ray beam (1486.6 eV). UV–vis diffuse reflectance spectra (DRS) were acquired on a UV–vis spectrophotometer (Cary 500 Scan Spectrophotometers, Varian, and U.S.A) equipped with an integrating sphere attachment. The electrochemical and photo electrochemical measurements were performed in 0.02 M Na3PO4 electrolyte solution in a three-electrode quartz cell using a CHI-660D electrochemical workstation (CH Instruments, USA). The sample was deposited on ITO glass to serve as the working electrode. Pt sheet was used as a counter electrode, and Ag/AgCl was used as a reference electrode. 2.3. Photocatalytic activity experiments The activities of ZnIn2S4/CdIn2S4 heterostructures were mainly evaluated by the degradation of organic dyes (MO and RhB) under visible light illumination. The light source is a 500 W halogen lamp 5

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(Philips Electronics) equipped with cut-off filters (420 nm < λ < 800 nm). The ZnIn2S4/CdIn2S4 powder (40 mg) was added into a 100 mL Pyrex glass vessel containing 80 mL of the aqueous dye solution. Before illumination, the suspension was firstly magnetically stirred in dark for 60 min to reach an adsorption/desorption equilibrium. During irradiation, 3 mL aliquots containing the catalyst powder and dye were sampled at the given time intervals and centrifuged to remove the catalyst. The resulting clear

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absorbance of the target contaminant at their maximum absorption wavelength.

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liquor was analyzed on a Perkin-Elmer UV-vis spectrophotometer (model: lambda 35) to record the

3. Results and discussion

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3.1. Structural and morphology characterizations

XRD patterns of the as-synthesized ZnIn2S4, CdIn2S4, and ZnIn2S4/CdIn2S4 composites are shown in

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Figure 1. The crystal form of pure ZnIn2S4 was identified to the hexagonal phase (JCPDS 65-2023, top

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of Figure 1). The three distinctive diffraction peak located at 21.6, 27.7, and 47.2º matched well with the (006), (102), and (110) crystal planes of ZnIn2S4, respectively. As for CdIn2S, its crystal form was

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identified to the cubic phase (JCPDS 27-0060, bottom of Figure 1). Four distinctive diffraction peaks

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located at 27.0, 33.2, 44.2, and 47.5º matched well with the (311), (400), (511), and (440) crystal planes of CdIn2S4. There is no trace of any impurity phase under the instrument’s resolution. For the obtained ZnIn2S4/CdIn2S4 composites, the characteristic diffraction peaks of both ZnIn2S4 and CdIn2S4 can be obviously identified in the patterns of CZIS-1, CZIS-2, and CZIS-3 composites. More importantly, when the mole ratio of Cd/Zn increased from 1:3 to 3:1, the diffraction peak ascribed to ZnIn2S4 at 21.6º decreased gradually, while the peak belonging to CdIn2S4 located at 44.2º increased evidently. The morphology of the obtained samples was characterized by SEM. Figure 2 shows the typical SEM images of ZnIn2S4, CdIn2S4, and ZnIn2S4/CdIn2S4 heterostructures. It can be found that the asprepared ZnIn2S4 exhibited regular sphere-like morphology with diameters of 4-6 µm. A magnification of the microsphere surface shows that it was composed of large amount of ZnIn2S4 nanosheets. Figure 2c shows the typical SEM image of pure CdIn2S. As we can see, CdIn2S also exhibited sphere-like morphology but with relatively smooth surface compared with that of ZnIn2S4. In addition, CdIn2S4 6

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microspheres exhibited various diameters of 1-7µm, most of which were smaller than that of ZnIn2S4. The magnification image of CdIn2S microsphere was shown in Figure 2d. It shows that the CdIn2S4 microsphere was composed of large amount of relatively smaller nanosheets. Figure 2e shows the SEM image of ZnIn2S4/CdIn2S4 composite. As we can see, the ZnIn2S4 and CdIn2S4 mainly exhibited spherelike morphology, and all the microspheres were adnate to each other. It was noticeable that many large

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ZnIn2S4 nanoplates were grown on the surfaces of CdIn2S microspheres (Figure 2f), leading to the

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formation of heterostructures. This heterostructure would facilitate the transfer of photo-generated carriers and promote their separation efficiency, finally leading to the enhancement of photocatalytic

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activities. In addition, elemental mapping was employed to further delineate the combination and spatial distribution of ZnIn2S4 and CdIn2S4 in ZnIn2S4/CdIn2S4 composites. Figure 2 also shows the Zn, Cd, In,

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and S elemental maps with distinct color contrast. The common elemental In and S were detected and mainly distributed in spherical shapes. The elemental Cd distributed was also observed in spherical

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shape but with smaller diameters, while Zn was homogeneously distributed on the surface of the entire composite, illustrating that ZnIn2S4 and CdIn2S4 were closely contacted with each other. This cross-

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linking structure might provide enough contacting surface area between ZnIn2S4 and CdIn2S4,

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facilitating the transfer of photo-generated carriers over the composites. 3.2. Optical and XPS characterizations

Figure 3 shows the UV–vis diffuse reflectance spectra of ZnIn2S4, CdIn2S4, and ZnIn2S4/CdIn2S4 composites. It is obviously that the absorption edge of ZnIn2S4/CdIn2S4 composite move towards longer wavelengths compared with pure ZnIn2S4.  The absorption edge for pure ZnIn2S4 was about 460 nm, while that for pure CdIn2S4 was located at 515 nm. The photographs of the as-prepared samples were also shown in Figure 3 (inset). It clearly shows that the color of ZnIn2S4/CdIn2S4 composite gradually changed from light yellow to orange. That is to say, when increasing the mole ratio of Cd/Zn, the visible light absorption of ZnIn2S4/CdIn2S4 composite would be strengthened. XPS analysis of ZnIn2S4/CdIn2S4 composite was carried out to further investigate the surface compositions and chemical states. The binding energies obtained for different elements in the XPS 7

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analysis were corrected for specimen charging by referencing the C 1s line to 284.6 eV. Figure 4e presents the wide survey XPS spectra of CZIS-3 sample, which implied the presence of Zn, Cd, In, and S elements in the prepared sample without other element signals being detected. Figure 4a shows the typical peaks of Cd 3d locating at 405.6 and 412.3 eV, which can be respectively ascribed to Cd 3d5/2 and 3d3/2 that lightly shift (about 0.2 eV) toward higher binding energies as compared to pure CdIn2S4

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according to a previous literature [34 ]. The In 3d peak also included two peaks ascribed to In 3d5/2 and

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In 3d3/2, which was located at 445.1 and 452.7 eV. So, the Cd 3d peaks also shifted (about 0.3 eV) toward higher binding energies compared to pure CdIn2S4. The S 2p peak observed in Figure 4d was

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centered at 161.7 eV, which was higher than that of pure CdIn2S4 (161.3 eV) [34]. As for the element Zn, two peaks located at 1022.1 and 1045.2 eV have been observed, which was ascribed to the Zn 2p3/2

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and Zn 2p1/2, respectively. All in all, this binding energy shifts might be ascribed to the strong interaction between CdIn2S4 and ZnIn2S4 because different electronegativity could result in the shift of

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valence electrons and the change of electric screening of inner shells [35]. Combined with the XRD and SEM results, these as-prepared products could be determined to be the composite of ZnIn2S4 and

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current synthetic conditions.

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CdIn2S4. Thus, heterostructured ZnIn2S4/CdIn2S4 photocatalyst could be easily synthesized under these

3.3. Photocatalytic degradation of organic dyes Figure 5a shows the photocatalytic degradation of MO over ZnIn2S4, CdIn2S4, and ZnIn2S4/CdIn2S4 composites under visible light irradiation. As we can see, the naked ZnIn2S4 or CdIn2S4 only exhibited poor photocatalytic activity lower than that of ZnIn2S4/CdIn2S4 composites. After 60 min of reaction, the removal ratio of MO over ZnIn2S4 was about 63%, while that over pure CdIn2S4 was only about 40%. However, when ZnIn2S4 and CdIn2S4 combined together, its activity could be improved greatly. Furthermore, the mole ratio of Cd/Zn had a great influence on the photocatalytic activity of ZnIn2S4/CdIn2S4 composites. And the sample CZIS-3 (Cd/Zn = 3:1) exhibited the best activity with a degradation ratio of 94% after 60 min of irradiation. Figure 5b shows the temporal evolution of the spectral changes of MO solution mediated by CZIS-3 sample. The absorption peak of MO at 464 nm 8

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decreases gradually under visible light illumination, which was finally disappeared after 90 min of reaction. It has also been found that the photocatalytic degradation of MO over ZnIn2S4/CdIn2S4 composites followed the pseudo-first-order kinetics (shown in Figure 5c) by the formula below: ln (Ct/C0 ) = − kt In the above equation, k is kinetic constant. The k of MO removal in the photocatalytic process with

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ZnIn2S4 was 0.0207 min−1, while that over CdIn2S4 was only about 0.0087 min-1 (shown in Figure 5d).

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However, for the ZnIn2S4/CdIn2S4 composite, all of the kinetic constants (k) have been enhanced. Especially for the CZIS-3 sample, its kinetic constant k was up to 0.0460 min-1, which was about 5.3

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times higher than that of pure CdIn2S4. This result revealed that the heterogeneous combination between ZnIn2S4 and CdIn2S4 could severely enhance the photocatalytic activity under visible light irradiation.

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Besides MO dye, ZnIn2S4/CdIn2S4 composite has also exhibited superior activity in decomposing RhB under visible light irradiation. Figure 6a shows the degradation curves of RhB solution (4 × 10-5 M)

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over ZnIn2S4, CdIn2S4 and ZnIn2S4/CdIn2S4 composites with different contents. As we can see, pure CdIn2S4 only exhibited poor activity, while ZnIn2S4/CdIn2S4 composites all possessed higher activities,

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which were more or less the same as that of pure ZnIn2S4. Figure 6b shows the temporal evolution of

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the spectral changes of RhB solution mediated by CZIS-3 sample. With the increase of illumination time, the absorption peak of RhB located at 554 nm decreased gradually, and after only 70 min of irradiation, it became completely disappeared.

To determine the active species generated in photocatalytic degradation process, different trapping agents were added in the degradation process of MO, such as tert-butanol (TBA), p-benzoquinone (BQ), and ammonium oxalate (AO), which was commonly used as scavenger of hydroxyl radical, superoxide radical, and holes, respectively [36-40]. The degradation results are presented in Figure 7, when trapping agent TBA was added, the degradation rate only showed a slight change. However, with the presence of BQ or AO, the degradation rates of MO had exhibited a severe decrease. For example, after 75 min of reaction, the degradation ratio of MO dropped sharply to 20% when BQ was added. Based on

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the analysis above, it can be concluded that h+ and •O2- were the main active species generated in the organic contaminant photodegradation process over CZIS-3 sample. The stability of photocatalyst is another vital consideration for its application in environmental service [41]. Repeated experiments have been done to confirm the photocatalytic stability of ZnIn2S4/CdIn2S4 composite. As it can be seen in Figure 8a, after three consecutive runs of experiments,

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slight activity deactivation was observed in the degradation of MO under visible light irradiation. Figure

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8b shows the XRD patterns of the fresh and used CZIS-3 sample. It was obviously that the main peaks of the used sample were nearly the same as that of the fresh.

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3.4. Photocatalytic mechanism

The formation of heterojunction between different materials was usually beneficial for enhancing the

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separation of electron/hole pairs over photocatalysts [42]. Transient photocurrent responses test has been performed to investigate the separation and migration of electron-hole pairs over ZnIn2S4/CdIn2S4.

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Higher photocurrent intensity usually means a more efficient separation efficiency of photo-induced carriers, which commonly result in superior photocatalytic activity. Figure 9 shows the photocurrent–

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time (I-t) curves for pure ZnIn2S4, CdIn2S4, and CZIS-3 composite in the light on and off. Compared

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with pure ZnIn2S4 and CdIn2S4, the CZIS-3 sample displayed a much higher photocurrent response. That is to say, the photo-induced electrons and holes can be efficiently separated over ZnIn2S4/CdIn2S4 composites because of the formation of heterojunction at their interfaces. The result is also in accordance with the improved photocatalytic activity discussed above. Furthermore, the EIS was also conducted to investigate the migration and transfer processes of photo-generated electrons and holes [43]. Generally, the smaller radius of the arc in the EIS spectra reflects the lower interface layer resistance as well as the higher efficiency of charge transferring. Figure 10 displays the Nyquist plots of pure ZnIn2S4, CdIn2S4, and ZnIn2S4/CdIn2S4 heterojunction. As we can see, the arc radius of CZIS-3 is smaller than these of pure ZnIn2S4 and CdIn2S4. The EIS result implies that the transfer of photo-generated carriers would be much easier over ZnIn2S4/CdIn2S4 heterojunction. 10

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Based upon the experimental results a possible mechanism for the photocatalytic degradation of dyes over ZnIn2S4/CdIn2S4 heterojunction was predicted (Scheme 1). Under visible light irradiation, electrons would be respectively generated in the valance band (VB) of ZnIn2S4 and CdIn2S4, which were then transferred to their conduction band (CB) but leaving photo-induced holes in their VB. Because the CB band potential of ZnIn2S4 (−0.73 eV) [33] was more negative than that of CdIn2S4

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(−0.59 eV) [30], the photo-excited electrons over the CB of ZnIn2S4 could easily transfer into that of

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CdIn2S4 through the heterojunction formed at their interfaces. Meanwhile, the holes generated over the VB of CdIn2S4 could facilely transfer into that of ZnIn2S4 because of its negative band potential.

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Because the CB potential of CdIn2S4 was more negative than O2/•O2− (−0.28 V vs. NHE) [44], the electrons over the CB of CdIn2S4 would react with the dissolved O2 to produce •O2−, which could

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further react with H+ to produce •OH or oxidize organic pollutants directly. As for the holes that transferred onto the VB of ZnIn2S4, the potential (+1.37 eV) was too low for holes to react with OH− to

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4. Conclusions

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pollutants in water directly.

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produce •OH (•OH/OH− = +2.38 V vs. NHE) [33,45]. Alternatively, the holes could oxidize the organic

In summary, hierarchical ZnIn2S4/CdIn2S4 heterojunction photocatalysts have been synthesized by a facile chemical co-precipitation method at a low temperature. Because of the formation of heterojunction at the interfaces of ZnIn2S4/CdIn2S4, the separation ratio of photo-generated electron/hole pairs has been greatly enhanced. It was found that the mole ratio of Cd/Zn in ZnIn2S4/CdIn2S4 composites has great influence on the activities. The CZIS-3 sample exhibited the best activity in the degradation process of MO under visible light irradiation. After 60 min of irradiation, the degradation ratio of MO was up to be 94%. Besides MO, RhB also could be completely degraded over ZnIn2S4/CdIn2S4 composites within 70 min of reaction. Furthermore, controlled experiment proved that the •O2− and holes generated in the system have played the major role in the degradation process.

Acknowledgment 11

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This work was financially supported by the Shandong Provincial Natural Science Foundation, China

(No.

ZR2016BQ12,

ZR2014BL017),

Doctoral

Foundation

of

Shandong

Province

(BS2012HZ001), National Natural Science Foundation of China (No. 21103069, 21505051, 21175057, and 40672158), the Science and Technology Development Plan of Shandong Province (No. 2016GSF117002), and Scientific Research Foundation for Doctors of University of Jinan (XBS1037

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and XKY1043).

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photocatalyst under visible light irradiation, Int. J. Hydrogen Energy 38 (2013) 13153–13158. [19] J.K. Jang, S.H. Choi, N. Shin, C.J. Yu, J.S. Lee, AgGaS2-type photocatalysts for hydrogen

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Figure captions

Fig. 1. XRD patterns of the as-synthesized ZnIn2S4, CdIn2S4, and ZnIn2S4/CdIn2S4 heterostructures. Fig. 2. SEM images of (a,b) ZnIn2S4, (c,d) CdIn2S4, (e,f) CZIS-3 composites, and (g) elemental mapping scanning for CZIS-3 sample.

17

Page 17 of 29

Fig. 3. UV-vis diffuse reflectance absorption spectra of ZnIn2S4, CdIn2S4, and ZnIn2S4/CdIn2S4 heterostructures. Fig. 4. XPS patterns of CZIS-3 sample for Cd 3d, Zn 2p, In 3d, S 2p and full spectra scan. Fig. 5. (a) Photocatalytic degradation curves of MO over obtained samples under visible light irradiation; (b) UV-vis spectral changes of MO in aqueous CZIS-3 sample dispersions as a function of

ip t

irradiation time; (c) The kinetics of photocatalytic degradations of MO over obtained samples and (d) the corresponding reaction constant k.

cr

Fig. 6. (a) Photocatalytic degradation curves of RhB over obtained samples under visible light irradiation; (b) UV-vis spectral changes of RhB in aqueous CZIS-3 sample dispersions as a function of

us

irradiation time

Fig. 7. Effects of various scavengers on the photocatalytic efficiency of MO degradation over CZIS-3

an

sample.

Fig. 8. (a) Recycling runs of the photocatalytic degradation of MO under visible light irradiation; (b)

M

XRD patterns of CZIS-3 sample before and after the photocatalytic reaction. Fig. 9. Transient photocurrent responses of (a) CdIn2S4, (b) ZnIn2S4, and (c) ZnIn2S4/CdIn2S4

d

heterostructures coated FTO glass photoelectrodes in 0.02 M Na3PO4 under visible light illumination.

Ac ce pt e

Fig. 10. EIS plots of the pure ZnIn2S4, CdIn2S4, and ZnIn2S4/CdIn2S4 heterostructures under irradiation with visible light

Scheme titles

Scheme 1. Proposed photocatalytic mechanism for ZnIn2S4/CdIn2S4 heterostructures under visible light irradiation.

18

Page 18 of 29

Figures Figure 1.

ZnIn2S4

♥

ip t

CZIS-1 ♦

CZIS-2

cr

♥

♦

40

60

2Theta (degree)

80

Ac ce pt e

d

M

20

CdIn2S4

an

♦

CZIS-3

us

Intensity (a.u.)

♥

Page 19 of 29

Ac ce pt e

d

M

an

us

cr

ip t

Figure 2.

Page 20 of 29

Figure 3.

100

ZnIn2S4 CZIS-1 CZIS-2 CZIS-3 CdIn2S4

60

ip t

R%

80

cr

40

300

400

an

0 200

us

20

500

600

700

800

Ac ce pt e

d

M

Wavelength (nm)

Page 21 of 29

Figure 4. Cd 3d

417

414

411

408

1045.2 eV

405

1050 1044 1038 1032 1026 1020

Binding Energy (eV)

(c)

In 3d

(d)

Counts (a.u.)

Counts (a.u.)

445.1 eV

Binding Energy (eV)

S 2p

161.7 eV

455

450

445

(e)

Counts (a.u.)

InMN1

1200

440

d

Binding Energy (eV)

Ac ce pt e

460

M

an

452.7 eV

cr

420

1022.1 eV

ip t

412.3 eV

Zn2p

(b)

Counts (a.u.)

Counts (a.u.)

405.6 eV

us

(a)

172

168

164

Binding Energy (eV)

160

Survey Scan In3d

In3p3 In3p1

Zn2p

1000

176

800

O1s Cd3p3 Cd3d C1s

600

400

S2s

S2p

200

In4d

0

Binding Energy (eV)

Page 22 of 29

Figure 5. 1.0

(b) 0.8

C/C0

Abs. (a.u.)

(a)

0.6 ZnIn2S4

0.4

0 min 15 min 30 min 45 min 60 min 75 min 90 min

CdIn2S4

20

40

60

80

200

Time (min)

(c) (min )

an

-1

ZnIn2S4

3

CdIn2S4

k

obs

CZIS-1 CZIS-2 CZIS-3

2

0 30

45

60

Time (min)

75

Ac ce pt e

15

d

1

0

400

90

500

Wavelength (nm)

(d)

M

-ln(C/C0)

4

300

cr

0

us

0.0

ip t

CZIS-1 CZIS-2 CZIS-3 CZIS-3 In dark

0.2

600

700

0.0460

0.0353 0.0291

0.0207

0.0087

ZnIn2S4 CZIS-1

CZIS-2 CZIS-3

CdIn2S4

Page 23 of 29

Figure 6. 1.0

(a) ZnIn2S4 CZIS-1 CZIS-2 CZIS-3

0.4 0.2

(a)

0.0 15

30

Abs. (a.u.)

an

2.0

Ac ce pt e

d

1.0

300

400

500

Wavelength (nm)

(b)

0 min 10 min 20 min 30 min 40 min 50 min 60 min 70 min

M

1.5

0.0 200

60

70min

0 min

0.5

45

Time (min)

us

0

ip t

CdIn2S4

0.6

cr

C/C0

0.8

600

700

Page 24 of 29

Figure 7. 1.0

C/C0

0.8

ip t

0.6 0.4

0

15

30

45

Time (min)

60

75

Ac ce pt e

d

M

an

0.0

us

0.2

cr

No scavenger AO TBA BQ

Page 25 of 29

Figure 8. (a)

(b)

C/C0

Intensity (a.u.)

0.8 0.6 0.4

Used

Fresh

0.2

45

90

135

180

225

Time (min)

270

15

30

45

60

2Theta (degree)

75

Ac ce pt e

d

M

an

us

0

3rd run

2nd run

cr

1st run

0.0

ip t

1.0

Page 26 of 29

Light on

3.2

Light off

2.4

c

ip t

b

1.6

a 0.0

0

40

80

120

cr

0.8

160

us

Photocurrent(μA/cm2)

Figure 9.

200

Ac ce pt e

d

M

an

Time (S)

Page 27 of 29

Figure 10.

25 ZnIn2S4 CdIn2S4 CZIS-3

15

ip t

-Z'' (ohm)

20

10

10

20

30

us

0

Z' (ohm)

40

50

Ac ce pt e

d

M

an

0

cr

5

Page 28 of 29

Scheme 1.

•

-

ee

CB

-

e e-

hv

-

⋅OH

CB H+

hv

0 ZnIn2S4

CdIn2S4

1 VB Pollutant

+

h h+

+

+ h h

By-products

VB

O2/⋅O2 = -0.33 -

Ac ce pt e

d

M

an

2

O2

ip t

-

By-products

cr

-1

Pollutant

us

Potential vs NHE

O2

Page 29 of 29