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Effects of indium contents on photocatalytic performance of ZnIn2S4 for hydrogen evolution under visible light
Journal of Solid State Chemistry 232 (2015) 138–143
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Effects of indium contents on photocatalytic performance of ZnIn2S4 for hydrogen evolution under visible light Kelin Song a, Rongshu Zhu a,b,n, Fei Tian a, Gang Cao a,b,n, Feng Ouyang a,b a Harbin Institute of Technology Shenzhen Graduate School, Shenzhen Key Laboratory of Water Resource Utilization and Environmental Pollution Control, Shenzhen 518055, PR China b Public Platform for Technological Service in Urban Waste Reuse and Energy Regeneration, Shenzhen 518055, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 14 May 2015 Received in revised form 8 September 2015 Accepted 22 September 2015 Available online 25 September 2015
A series of ZnInxS4 þ y (x ¼1.6, 2.0, 2.3, 2.6, 2.9, 3.1) photocatalysts were synthesized via a facile hydrothermal method and characterized by various analytical techniques, such as XRD, EDS, UV–vis DRS, SEM, TEM, BET and PL. The ZnInxS4 þ y photocatalysts had a similar crystal structure. With the increase of indium content, the absorption edges of ZnInxS4 þ y photocatalysts shifted to longer wavelength, their crystal sizes decreased firstly and then increased and the variation of the specific surface area and total pore volume was exactly opposite. Especially, when x ¼2.3, ZnIn2.3S4 þ y catalyst had smallest crystal size, largest specific surface area and total pore volume. Additionally, the morphology of ZnInxS4 þ y greatly depended on the contents of indium. The photocatalytic activity of ZnInxS4 þ y was evaluated by photocatalytic hydrogen production from water under visible light. The ZnIn2.3S4 þ y sample had the highest photocatalytic activity among these ZnInxS4 þ y photocatalysts and its hydrogen production rate was 363 μmol/g h. & 2015 Elsevier Inc. All rights reserved.
Keywords: ZnIn2S4 Indium content Water splitting Hydrogen evolution Visible light
1. Introduction Hydrogen as a clean renewable energy can effectively solve currently energy shortages and environmental pollution caused by fossil fuel consumption. Since the discovery of photoinduced decomposition of water on TiO2 electrodes in 1972 [1], many studies have been conducted on photocatalytic water splitting to produce hydrogen. In the past few decades, a lot of photocatalysts such as TiO2-based semiconductors [2–6], ZnGa2O4 [7], titanates [8,9], niobates [10,11], tantalates [12–14], metal sulfides [15–18], have been developed for water splitting. In these photocatalysts reported, ZnIn2S4 has recently been studied for its unique optoelectronic [19] and catalytic property [20]. ZnIn2S4 is the member of the AB2x4 family of semiconductors. ZnIn2S4, which has been proven to have good properties such as suitable band gap corresponding to visible light absorption region, high photocatalytic activity and substantial chemical stability, can act as a catalyst for photocatalytic H2 evolution [20]. Lei et al. [20] successfully synthesized a novel ZnIn2S4 photocatalyst and found that ZnIn2S4 could be used to produce H2 through water reduction under visible light irradiation, but its hydrogen production n Corresponding authors at: Harbin Institute of Technology Shenzhen Graduate School, Shenzhen Key Laboratory of Water Resource Utilization and Environmental Pollution Control, Shenzhen 518055, PR China. E-mail addresses: [email protected] (R. Zhu), [email protected] (G. Cao).
http://dx.doi.org/10.1016/j.jssc.2015.09.025 0022-4596/& 2015 Elsevier Inc. All rights reserved.
efficiency was low. In the past decade, adding additives or doping metals had been shown to improve ZnIn2S4’s photocatalytic activity. Shen et al. [21] synthesized a series of ZnIn2S4 photocatalysts via a cetyltrimethylammoniumbromide (CTAB)-assisted hydrothermal method, and found that appropriate amount of CTAB could improve the photocatalytic activity of ZnIn2S4, and the highest quantum yield at 420 nm of ZnIn2S4 photocatalyst, which was prepared through the CTAB (9.6 mmol)-assisted hydrothermal procedure for 1 h, was determined to be 18.4%. After that, Shen et al. [22] synthesized a series of alkaline-earth metal doped ZnIn2S4 photocatalysts and found that the photocatalytic results demonstrated that Ca doping could greatly enhance the activity of ZnIn2S4, with about two times higher than undoped ZnIn2S4. The previous study of our group on the rare earth doped catalysts revealed that the La3 þ showed the best effect [23,24]. Lazzez et al. [25] investigated the structural and optoelectronic properties of In–Zn–S sprayed layers with different [Zn2 þ ]/[In3 þ ] ratios, and found that the [Zn2 þ ]/[In3 þ ] ratios could greatly influence the composition of In–Zn–S thin layers. The film was mainly formed by the ternary compound ZnIn2S4 which crystallized in cubic phase when [Zn2 þ ]/[In3 þ ] ¼0.4 and the band gap energy increased with the increasing of [Zn2 þ ]/[In3 þ ] ratio. Shen et al. [26] studied the effect of excess zinc on photocatalytic performance of catalyst and synthesized a new series of ZnmIn2S3 þ m (m ¼1–5, integer) photocatalysts by a CTAB-assisted hydrothermal method, and found that photocatalytic performance of catalyst firstly increased and
K. Song et al. / Journal of Solid State Chemistry 232 (2015) 138–143
then decreased with the increasing of zinc content, especially when m ¼2, the best photocatalytic performance was obtained. However, the effects of excess indium on photocatalytic performance of ZnIn2S4 for hydrogen evolution under visible light have rarely been reported. In order to understand the effects of excess indium on photocatalytic performance of ZnIn2S4 for hydrogen evolution under visible light, a series of ZnInxS4 þ y (x ¼1.6, 2.0, 2.3, 2.6, 2.9, 3.1) photocatalysts have been successfully synthesized by a hydrothermal route via adjusting the amount of the indium content. We also discussed the characteristics of these photocatalysts, such as crystal size, morphology, optical property and so on in detail.
2. Experimental 2.1. Materials All chemicals were analytical grade and used as received without further purification. ZnSO4 7H2O, In(NO3)3 4H2O, TAA (Thioacetamide), absolute ethanol, Na2S and Na2SO3 (purity Z99.99%) were purchased from Sinopharm Chemical Reagent Co., Ltd., China. H2PtCl6 6H2O (purity Z99.99%) was purchased from Shanghai July Chemical Co., Ltd., China. 2.2. Preparation of photocatalyst The ZnInxS4 þ y samples were prepared by a hydrothermal method [27]. For the synthesis of ZnInxS4 þ y, 2.56 mmol ZnSO4 7H2O, N mmol In(NO3)3 4H2O (N ¼ 4.02, 5.10, 5.90, 6.70, 7.51, 8.04) and 20.4 mmol TAA were dissolved in 50 mL distilled water. The mixed solution was then transferred into a 100 mL Teflon-lined autoclave. The autoclave was sealed and kept at 160 °C for 1 h, and then cooled to room temperature naturally. A yellow precipitate was obtained, then filtered and washed with absolute ethanol and deionized water for three times. After dried in an oven at 80 °C, a series of catalysts ZnInxS4 þ y were obtained. These catalysts were assigned to ZnInxS4 þ y (x¼ 1.6, 2.0, 2.3, 2.6, 2.9, 3.1), respectively. 2.3. Characterization The UV–visible absorption spectra of the samples were recorded using a UV–vis diffuse reflectance spectroscopy (UV–vis DRS) (UV-2501PC, Shimadzu, Japan) in the spectral range of 300– 750 nm. The crystal phases and crystallite sizes of the as-prepared samples were characterized by X-ray diffraction (XRD) (Rigaku, D/ max 2500) at room temperature, and the patterns were recorded over the angular range 15–60°(2θ), using a scan rate of 5°/min and Cu-Kα1 radiation with working voltage and current of 40 kV and 20 mA, respectively. The surface morphology and energy dispersive spectroscopy (EDS) element mapping of the samples were observed by a field-emission scanning electron microscope with energy dispersive spectrometer (SEM, S-4800, Hitachi, Japan). Elemental analysis was also carried out on an inductively coupled plasma optical emission spectrometer (ICP, OPTIMA 2000DV, Perkin Elmer, USA). Transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) images were obtained by a Tecnai G2 F30 instrument at an accelerating voltage of 300 kV. The BET surface area was evaluated by N2 adsorption in a constant volume adsorption apparatus (Bel sorpll, Bayer Japan Co., Ltd., Japan). The analyses of photoluminescence (PL) spectra were carried out at room temperature using a Renishaw in Via raman microscope.
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2.4. Photocatalytic reaction Photocatalytic reactions were conducted in an 868 mL gasclosed stainless steel reactor [28]. The cross section area and height of the reactor were 72.3 cm2 and 12 cm, respectively. A quartz glass cover on the top of the reactor allowed irradiation to pass through. The light source was a 350-W Xe lamp (Shenzhen Stone-lighting Opto Device Co.,Ltd., China) and the UV part of the light was removed by a cut-off filter (λ 4 420 nm). The light beam was focused coherently on the surface of the 200 mL catalyst suspension (2.8 cm in depth). The suspension in the reactor was continuously stirred using a magnetic stirrer at a speed of 400 rpm. The light intensity was measured using a spectroradiometer (FZ-A, Photoelectric Instrument Factory of Beijing Normal University). In all experiments, 200 mL of deionized water containing 0.2 g of catalyst and 0.25 M Na2SO3/0.35 M Na2S mixed sacrificial agent was added into the reaction cell [21–24,26]. Here, a sacrificial agent was used to scavenge photo-generated holes. In order to remove air prior to the reaction, argon gas was bubbled through the reaction mixture for 30 min before the reaction started. Pt (1 wt%) as a cocatalyst for the promotion of H2 evolution was photodeposited in situ on the photocatalyst from the precursor of H2PtCl6 6H2O [27]. The temperature for all the photocatalytic reactions was kept at 257 1 °C. The concentrations of H2 were measured with a gas chromatograph (Shanghai Precision & Scientific Instrument Co., Ltd., GC-112A, High-performance carbon molecular sieve packed column (2 m 3 mm)), equipped with a thermal conductivity detector (TCD). All the measurements of the produced H2 concentrations at different irradiation times were performed three times to confirm their reproducibility.
3. Results and discussion 3.1. Characterization of catalysts 3.1.1. XRD analysis Fig. 1 shows the XRD patterns of the ZnInxS4 þ y (x ¼1.6, 2.0, 2.3, 2.6, 2.9, 3.1) samples. ZnInxS4 þ y could be assigned to the crystal phase of hexagonal ZnIn2S4 (JCPDS No. 65-2023). With the content of indium increasing, a new diffraction peak appears at 33° and gradually increases, which is the (0012) characteristic peak of In2S3 [29] (JCPDS No. 65-0459).
Fig. 1. XRD patterns of ZnInxS4 þ y samples (a: In2S3, b: ZnIn1.6S4 þ y, c: ZnIn2.0S4 þ y, d: ZnIn2.3S4 þ y, e: ZnIn2.6S4 þ y, f: ZnIn2.9S4 þ y, g: ZnIn3.1S4 þ y).
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Table 1 Absorption edge, crystal structure parameters, photocatalytic activities for H2 production over various Photocatalysts.
Crystal sizea (nm) Absorption edge (nm) Band gap (eV) ECB0 (eV) EvB0 (eV) BET (m2/g) Total pore volume (cm3/g) H2 production (μmol/g h) a
ZnS
ZnIn1.6S4 þ y
ZnIn2.0S4 þ y
ZnIn2.3S4 þ y
ZnIn2.6S4 þ y
ZnIn2.9S4 þ y
ZnIn3.1S4 þ y
In2S3
– 360 3.44 – – 48.6 0.0732 –
4.2 527 2.35 0.781 1.569 119.2 0.1185 154
4.0 535 2.32 0.766 1.554 122.3 0.1270 256
3.6 540 2.30 0.756 1.544 124.4 0.1306 363
3.8 550 2.25 0.731 1.519 109.0 0.1112 317
4.0 557 2.23 0.721 1.509 100.5 0.1024 303
4.2 552 2.25 0.731 1.519 97.3 0.1067 279
– 644 1.93 0.761 1.169 133.1 0.1751 –
Determined from XRD results by using the Scherrer equation.
Additionally, the crystal sizes of the catalysts are also listed in Table 1. As shown in Table 1, with the increase of indium contents, crystal size of the catalysts firstly decreases and then increases. When the value of x is 2.3, the crystal size of the catalyst reaches the minimum and is 3.6 nm. 3.1.2. EDS and ICP analysis Bulk elemental composition was measured by ICP and surface elemental composition was obtained from EDS. The Zn/In/S ratios of ZnInxS4 þ y (x ¼1.6, 2.0, 2.3, 2.6, 2.9, 3.1) catalysts were listed in Table 2. As can be seen from Table 2, the Zn/In/S ratios obtaine from the ICP and EDS are very close, indicating that the chemical homogeneity of ZnInxS4 þ y catalysts is very good. Additionally, with increase of indium contents, the mol ratio of In to S element increases when the mol of Zn is 1. After deduction of the corresponding atomic ratio ZnIn2S4, the mol ratio of In to S element is approximately 2:3. The diffraction peak of In2S3 can be observed clearly from Fig. 1, indicating that the balance of In and S elements mainly formed In2S3. According to the balance of In and S atom ratio measured by ICP in Table 2, the chemical composition of the prepared catalyst may be ZnIn2S4–mIn2S3 (m ¼0.01, 0.53, 0.77, 1.31, 1.45, 2.09). 3.1.3. Morphology analysis Fig. 2 shows the SEM images of ZnInxS4 þ y (x ¼1.6, 2.0, 2.3, 2.6, 2.9, 3.1) samples. As shown in Fig. 2a, c, e, g, i and k, ZnInxS4 þ y samples crystallites were self-organized into microsphere morphology with a wide distribution of diameters ranging from 3 mm to 6 mm. The morphology of these ZnInxS4 þ y samples greatly depended on the indium contents. As shown in Fig. 2b, when the value of x is 1.6, the shape of the ZnIn1.6S4 þ y sphere which accompanies by some of the notches shows a molten state. At the same time, its crystal gathers together, and its pore structure is not yet open. When the value of x is 2.0 (in Fig. 2d), the shape of the ZnIn2.0S4 þ y sphere surface remains a molten state and the pore structure is still not open, but the surface bump becomes more obvious. When the value of x is 2.3 (in Fig. 2f), the pore structure is opened completely and the microspheres show a regular morphology. The surface of the ZnIn2.3S4 þ y sphere is composed of numerous petals/sheets and assembled into rose-like microclusters. Agglomeration is hardly observed on the surface of the ZnIn2.3S4 þ y sphere and the width of the gap between the petals is
0.2–0.3 μm. When the value of x is 2.6 (in Fig. 2h), the majority of the ZnIn2.6S4 þ y sphere surface of the petal-like projection is wrapped up again, and the pore structure gradually disappears. When the value of x continues increasing to 2.9 and 3.1, as shown in Fig. 2j and Fig. 2l, the petal bumps on ZnIn2.9S4 þ y and ZnIn3.1S4 þ y surface disappear and become smooth. As can be seen from the above, with the value of x increasing, the pore structure of the catalyst surface is gradually opened. However, when the value of x is more than 2.3, the pore structure of the catalyst surface gradually disappears and becomes smooth again. In summary, the indium content has a great influence on the morphology of the catalyst. This phenomenon is similar to that reported by Shen et al. [26] and the growth tendency of lamellar structures might be related to the layered feature of hexagonal ZnIn2S4. The morphology of ZnIn2.3S4 þ y sample is examined using HRTEM measurements and also show in Fig. 2. As shown in Fig. 2n, the interplanar spacings were measured to be 0.622 and 0.324 nm, which could be assigned to the (111) plane of In2S3 and the (102) plane of ZnIn2S4, respectively. This result is in agreement with the XRD results and proves the presence of In2S3. 3.1.4. UV–vis DRS analysis Fig. 3 shows the UV–vis DRS of various ZnInxS4 þ y (x ¼1.6, 2.0, 2.3, 2.6, 2.9, 3.1), ZnS and In2S3 samples. The corresponding absorption edge and band gap are also listed in Table 1. As can been seen from Fig. 3, the shapes of the absorption edges are steep and the absorption in the visible region is strong. This steep absorption edge indicates that the visible-light absorption is due to a band gap transition, not due to the transition from impurity levels to the conduction band of ZnInxS4 þ y [30]. As can been seen from the spectrum of In2S3 and ZnS, their absorption edges locate at 644 nm and 360 nm, respectively, which is similar to the previous results [31,32], whereas the absorption edge of ZnInxS4 þ y catalysts exactly locates in between. As shown in Table 1, with the value of x increasing from 1.6 to 3.1, the absorption edge of these ZnInxS4 þ y samples shift to longer wavelength, indicating that the increase of the indium content is benefit to enhance the visible light adsorption of ZnInxS4 þ y. According to the characterization in Sections 3.1.1 and 3.1.2, the light absorption performance shifting to longer wavelength is related to the forming of In2S3.
Table 2 EDS and ICP characterization of each element in the catalyst sample (mmol). Element EDS
ICP
Zn In S Zn:In:S Zn:In:S
ZnIn1.6S4 þ y
ZnIn2.0S4 þ y
ZnIn2.3S4 þ y
ZnIn2.6S4 þ y
ZnIn2.9S4 þ y
ZnIn3.1S4 þ y
13.23 28.24 58.53 1:2.13:4.42 1:2.02:4.08
10.96 31.77 57.27 1:2.90:5.23 1:3.05:5.58
9.34 32.76 57.90 1:3.51:6.20 1:3.53:6.32
8.03 33.84 58.13 1:4.21:7.24 1:4.61:7.94
6.79 33.62 59.59 1:4.95:8.78 1:4.90:8.42
5.70 34.48 59.82 1:6.05:10.49 1:6.18:10.77
K. Song et al. / Journal of Solid State Chemistry 232 (2015) 138–143
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Fig. 3. UV–vis diffuse reflectance spectra of ZnInxS4 þ y samples.
3.1.5. BET analysis The BET surface area and total pore volume of ZnInxS4 þ y (x ¼1.6, 2.0, 2.3, 2.6, 2.9, 3.1) samples are also listed in Table 1. As shown in Table 1, the BET surface area and total pore volume of these ZnInxS4 þ y samples first increase rapidly when the value of x increases from 1.6 to 2.3. When the value of x is higher than 2.3, the BET surface area and total pore volume decrease gradually. That is to say, when the value of x is 2.3, the specific surface area and total pore volume of the catalyst are the maximum and are 124.35 m2/g and 0.1306 cm3/g, respectively. These results are coincided with those observed from SEM in Fig. 2. This is mainly due to the indium content at 2.3 which promotes the formation of the catalyst surface petal-like protrusions and let the pore structure fully open, so that the specific surface area and total pore volume of the catalyst can be increased. 3.1.6. PL analysis The PL spectra of ZnInxS4 þ y (x ¼1.6, 2.0, 2.3, 2.6, 2.9, 3.1) samples are shown in Fig. 4. In the PL spectra, all ZnInxS4 þ y samples have only one PL emission peak centered at 565 nm, which is longer than the ZnInxS4 þ y absorption edge. That is to say, the band gap energy (527–557 nm) is higher than the energy of the radiative photon (565 nm). Jing et al. [33] introduced the PL performance and the mechanism of nano-sized semiconductor materials. According to their study, the PL performance of the photocatalyst shown in Fig. 4 is an excitonic PL process, in which
Fig. 2. SEM images of ZnInxS4 þ y samples (a, b: ZnIn1.6S4 þ y; c, d: ZnIn2.0S4 þ y; e, f: ZnIn2.3S4 þ y; g, h: ZnIn2.6S4 þ y; i, j: ZnIn2.9S4 þ y; k, l: ZnIn3.1S4 þ y) and TEM(m), HRTEM(n) images of ZnIn2.3S4 þ y. Fig. 4. Photoluminescence spectra of ZnInxS4 þ y samples.
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Fig. 5. Photocatalytic activities of ZnInxS4 þ y samples.
the non-radiative transitions of excited electrons from the conduction band bottom to different sub-bands (or surface states) occur first and subsequent radiative transitions from the sub-band to the valence band top take place. Therefore, the energy of the radiative photon, which is the energy difference between the subband and valence band (VB) top, is lower than the band gap energy. As can be seen from Fig. 4, when the value of x is from 1.6 to 3.1, the photoluminescence intensity of the catalyst initially decreases and then increases. Especially, when the value of x is 2.6, the photoluminescence intensity of the catalyst is minimum. This phenomenon is consistent with the results that reported by Jing [34]. These results imply that the generation of a small amount of In2S3 can promote the separation of the photoproduction electronhole pairs, leading to a weak photoluminescence signal [33].
performance (as shown in Table 1). The light absorption performance is related to the forming of In2S3. Mei et al. [29] showed that In2S3 could improve ZnIn2S4’s photocatalytic activity and found that the content of In2S3 had an optimum value. In order to analyze the effect of In2S3 for the catalytic activity, the conduction band (CB) and valence band (VB) potential of ZnInx S4 þ y and In2S3 at the point of zero charge were calculated by the following Eqs. (1) and (2) [29]. Where, X is the absolute electronegativity of the semiconductors, Ec is the energy of free electrons on the hydrogen scale ( 4.5 eV) and Eg is the band gap of a semiconductor. The calculated values of X are 4.894 eV and 4.704 eV for ZnIn2S4 and In2S3, respectively. According to Eqs. (1) and (2), the potential of CB and VB of ZnInxS4 þ y and In2S3 are also listed in Table 1. As shown in Table 1, for ZnIn1.6S4 þ y and ZnIn2.0S4 þ y, their potential of CB are both more negative than that of In2S3, while their potential of VB are both more positive than that of In2S3. In this situation, the photoinduced electrons and holes of ZnIn2S4 can transfer to CB and VB of In2S3, respectively, which can suppress the separation ability of the photoexcited electrons and holes [35]. Thus, the hydrogen production decreases. For ZnIn2.3S4 þ y, ZnIn2.6S4 þ y, ZnIn2.9S4 þ y and ZnIn3.1S4 þ y, their potential of CB and VB are both more positive than that of In2S3. In this situation, the photoinduced electrons of In2S3 can transfer to CB of ZnIn2S4 and the holes of ZnIn2S4 will transfer to VB of In2S3, which can enhance the separation ability of the photoexcited electrons and holes [29,36,37]. Therefor, the hydrogen production can significantly improve. However, there should be an optimal band gap of catalyst. This is because the conduction band potential may be more positive than that of H þ /H2 when the band gap decreases enough, which will not be benifit for the hydrogen production [38]. So, the best photocatalyst can be obtained by keeping the balance between the conduction band potential and the band gap, and the best catalyst is ZnIn2.3S4 þ y.
ECB 0 = X − Ec − 0.5Eg
(1)
EVB 0 = Eg + ECB 0
(2)
3.2. Evaluation of photocatalytic activity 3.2.1. Photocatalytic activity Fig. 5 shows the time profile of photocatalytic H2 evolution over ZnInxS4 þ y (x ¼1.6, 2.0, 2.3, 2.6, 2.9, 3.1) samples under visible light irradiation. The H2 production rate is also listed in Table 1. As shown in Fig. 5, the efficiency of H2 evolution increases rapidly when the value of x increases from 1.6 to 2.3 and then decreases gradually when the value of x is higher than 2.3. The catalyst activity with the increase of the indium content is in the order ZnIn2.3S4 þ y 4ZnIn2.6S4 þ y 4ZnIn2.9S4 þ y 4ZnIn3.1S4 þ y 4ZnIn2.0S4 þ y 4 ZnIn1.6S4 þ y. The average H2 production rate of ZnIn2.3S4 þ y is the maximum and reaches 363 μmol/g h. These results indicate that the H2 production activity of ZnInxS4 þ y is influenced by the indium contents. In order to analyze the reasons for the catalytic activity, the correlations between the activity order and the characteristics of the catalyst were analyzed. For optimum catalyst ZnIn2.3S4 þ y, it has the smallest particle size, the largest surface area and pore volume (as shown in Table 1). For ZnIn2.6S4 þ y, although it has the highest hole–electron separation capability (as shown in Fig. 4), its activity is lower than ZnIn2.3S4 þ y since the crystal size increases, the specific surface area and pore volume decrease. Compared with ZnIn2.0S4 þ y, ZnIn2.9S4 þ y has the same crystal size (as shown in Table 1), ZnIn3.1S4 þ y even has greater crystal size (as shown in Table 1), weaker separation capability of hole–electron pairs (as shown in Fig. 4) and much smaller specific surface area (as shown in Table 1), the activities of ZnIn2.9S4 þ y and ZnIn3.1S4 þ y are higher than that of ZnIn2.0S4 þ y because they have better light absorption
Based on the above analysis, with the indium content increasing, the properties of the catalysts are changed, and thus their catalytic activities are altered correspondingly. Among those characteristics such as catalyst crystal size, light absorption properties, the surface area and pore volume, surface defects, the catalytic activity mainly associates with the crystal size and followed by light absorption properties. The suitable surface defects, large specific surface area and pore volume also have a beneficial effect on the activity. 3.2.2. Photostability Fig. 6 shows the photostability of ZnIn2.3S4 þ y sample in five consecutive runs for total 15 h. As shown in Fig. 6, no significant deactivation is observed during the photocatalytic reaction for at least 15 h, indicating a very good photostability of the catalyst.
4. Conclusions A series ZnInxS4 þ y (x ¼1.6, 2.0, 2.3, 2.6, 2.9, 3.1) photocatalysts were successfully synthesized using the hydrothermal method. With the increase of indium content, the crystal size decreases initially and then increases. The minimum value was 3.6 nm when x¼ 2.3. The morphology of these ZnInxS4 þ y catalysts was affected greatly by the indium contents. The band gap of ZnInxS4 þ y catalysts became small gradually, and its specific surface area and total pore volume of the catalyst increased. Catalytic activity experiments showed that the hydrogen production increased in the
K. Song et al. / Journal of Solid State Chemistry 232 (2015) 138–143
Fig. 6. Stability study of photocatalytic H2 evolution over ZnIn2.3S4 þ y under the visible-light irradiation from an aqueous Na2SO3 and Na2S solution.
beginning and then decreased under visible light with the increase of indium content. The best rate of hydrogen production reached 363 μmol/g h when the value of x was 2.3. The catalytic activity mainly associated with the crystal size and followed by light absorption properties. The suitable surface defects, large specific surface area and pore volume also had beneficial effect on the activity.
Acknowledgments All the authors gratefully acknowledge support from the Special fund for the development of Strategic and new industry in Shenzhen (No. JCYJ20120613114951217 and JCYJ20130329162012793), the Fund for the Research and Development of Science and Technology in Shenzhen (No. CXZZ20130516145955144).
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Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
Effects of indium contents on photocatalytic performance of ZnIn2S4 for hydrogen evolution under visible light Kelin Song a, Rongshu Zhu a,b,n, Fei Tian a, Gang Cao a,b,n, Feng Ouyang a,b a Harbin Institute of Technology Shenzhen Graduate School, Shenzhen Key Laboratory of Water Resource Utilization and Environmental Pollution Control, Shenzhen 518055, PR China b Public Platform for Technological Service in Urban Waste Reuse and Energy Regeneration, Shenzhen 518055, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 14 May 2015 Received in revised form 8 September 2015 Accepted 22 September 2015 Available online 25 September 2015
A series of ZnInxS4 þ y (x ¼1.6, 2.0, 2.3, 2.6, 2.9, 3.1) photocatalysts were synthesized via a facile hydrothermal method and characterized by various analytical techniques, such as XRD, EDS, UV–vis DRS, SEM, TEM, BET and PL. The ZnInxS4 þ y photocatalysts had a similar crystal structure. With the increase of indium content, the absorption edges of ZnInxS4 þ y photocatalysts shifted to longer wavelength, their crystal sizes decreased firstly and then increased and the variation of the specific surface area and total pore volume was exactly opposite. Especially, when x ¼2.3, ZnIn2.3S4 þ y catalyst had smallest crystal size, largest specific surface area and total pore volume. Additionally, the morphology of ZnInxS4 þ y greatly depended on the contents of indium. The photocatalytic activity of ZnInxS4 þ y was evaluated by photocatalytic hydrogen production from water under visible light. The ZnIn2.3S4 þ y sample had the highest photocatalytic activity among these ZnInxS4 þ y photocatalysts and its hydrogen production rate was 363 μmol/g h. & 2015 Elsevier Inc. All rights reserved.
Keywords: ZnIn2S4 Indium content Water splitting Hydrogen evolution Visible light
1. Introduction Hydrogen as a clean renewable energy can effectively solve currently energy shortages and environmental pollution caused by fossil fuel consumption. Since the discovery of photoinduced decomposition of water on TiO2 electrodes in 1972 [1], many studies have been conducted on photocatalytic water splitting to produce hydrogen. In the past few decades, a lot of photocatalysts such as TiO2-based semiconductors [2–6], ZnGa2O4 [7], titanates [8,9], niobates [10,11], tantalates [12–14], metal sulfides [15–18], have been developed for water splitting. In these photocatalysts reported, ZnIn2S4 has recently been studied for its unique optoelectronic [19] and catalytic property [20]. ZnIn2S4 is the member of the AB2x4 family of semiconductors. ZnIn2S4, which has been proven to have good properties such as suitable band gap corresponding to visible light absorption region, high photocatalytic activity and substantial chemical stability, can act as a catalyst for photocatalytic H2 evolution [20]. Lei et al. [20] successfully synthesized a novel ZnIn2S4 photocatalyst and found that ZnIn2S4 could be used to produce H2 through water reduction under visible light irradiation, but its hydrogen production n Corresponding authors at: Harbin Institute of Technology Shenzhen Graduate School, Shenzhen Key Laboratory of Water Resource Utilization and Environmental Pollution Control, Shenzhen 518055, PR China. E-mail addresses: [email protected] (R. Zhu), [email protected] (G. Cao).
http://dx.doi.org/10.1016/j.jssc.2015.09.025 0022-4596/& 2015 Elsevier Inc. All rights reserved.
efficiency was low. In the past decade, adding additives or doping metals had been shown to improve ZnIn2S4’s photocatalytic activity. Shen et al. [21] synthesized a series of ZnIn2S4 photocatalysts via a cetyltrimethylammoniumbromide (CTAB)-assisted hydrothermal method, and found that appropriate amount of CTAB could improve the photocatalytic activity of ZnIn2S4, and the highest quantum yield at 420 nm of ZnIn2S4 photocatalyst, which was prepared through the CTAB (9.6 mmol)-assisted hydrothermal procedure for 1 h, was determined to be 18.4%. After that, Shen et al. [22] synthesized a series of alkaline-earth metal doped ZnIn2S4 photocatalysts and found that the photocatalytic results demonstrated that Ca doping could greatly enhance the activity of ZnIn2S4, with about two times higher than undoped ZnIn2S4. The previous study of our group on the rare earth doped catalysts revealed that the La3 þ showed the best effect [23,24]. Lazzez et al. [25] investigated the structural and optoelectronic properties of In–Zn–S sprayed layers with different [Zn2 þ ]/[In3 þ ] ratios, and found that the [Zn2 þ ]/[In3 þ ] ratios could greatly influence the composition of In–Zn–S thin layers. The film was mainly formed by the ternary compound ZnIn2S4 which crystallized in cubic phase when [Zn2 þ ]/[In3 þ ] ¼0.4 and the band gap energy increased with the increasing of [Zn2 þ ]/[In3 þ ] ratio. Shen et al. [26] studied the effect of excess zinc on photocatalytic performance of catalyst and synthesized a new series of ZnmIn2S3 þ m (m ¼1–5, integer) photocatalysts by a CTAB-assisted hydrothermal method, and found that photocatalytic performance of catalyst firstly increased and
K. Song et al. / Journal of Solid State Chemistry 232 (2015) 138–143
then decreased with the increasing of zinc content, especially when m ¼2, the best photocatalytic performance was obtained. However, the effects of excess indium on photocatalytic performance of ZnIn2S4 for hydrogen evolution under visible light have rarely been reported. In order to understand the effects of excess indium on photocatalytic performance of ZnIn2S4 for hydrogen evolution under visible light, a series of ZnInxS4 þ y (x ¼1.6, 2.0, 2.3, 2.6, 2.9, 3.1) photocatalysts have been successfully synthesized by a hydrothermal route via adjusting the amount of the indium content. We also discussed the characteristics of these photocatalysts, such as crystal size, morphology, optical property and so on in detail.
2. Experimental 2.1. Materials All chemicals were analytical grade and used as received without further purification. ZnSO4 7H2O, In(NO3)3 4H2O, TAA (Thioacetamide), absolute ethanol, Na2S and Na2SO3 (purity Z99.99%) were purchased from Sinopharm Chemical Reagent Co., Ltd., China. H2PtCl6 6H2O (purity Z99.99%) was purchased from Shanghai July Chemical Co., Ltd., China. 2.2. Preparation of photocatalyst The ZnInxS4 þ y samples were prepared by a hydrothermal method [27]. For the synthesis of ZnInxS4 þ y, 2.56 mmol ZnSO4 7H2O, N mmol In(NO3)3 4H2O (N ¼ 4.02, 5.10, 5.90, 6.70, 7.51, 8.04) and 20.4 mmol TAA were dissolved in 50 mL distilled water. The mixed solution was then transferred into a 100 mL Teflon-lined autoclave. The autoclave was sealed and kept at 160 °C for 1 h, and then cooled to room temperature naturally. A yellow precipitate was obtained, then filtered and washed with absolute ethanol and deionized water for three times. After dried in an oven at 80 °C, a series of catalysts ZnInxS4 þ y were obtained. These catalysts were assigned to ZnInxS4 þ y (x¼ 1.6, 2.0, 2.3, 2.6, 2.9, 3.1), respectively. 2.3. Characterization The UV–visible absorption spectra of the samples were recorded using a UV–vis diffuse reflectance spectroscopy (UV–vis DRS) (UV-2501PC, Shimadzu, Japan) in the spectral range of 300– 750 nm. The crystal phases and crystallite sizes of the as-prepared samples were characterized by X-ray diffraction (XRD) (Rigaku, D/ max 2500) at room temperature, and the patterns were recorded over the angular range 15–60°(2θ), using a scan rate of 5°/min and Cu-Kα1 radiation with working voltage and current of 40 kV and 20 mA, respectively. The surface morphology and energy dispersive spectroscopy (EDS) element mapping of the samples were observed by a field-emission scanning electron microscope with energy dispersive spectrometer (SEM, S-4800, Hitachi, Japan). Elemental analysis was also carried out on an inductively coupled plasma optical emission spectrometer (ICP, OPTIMA 2000DV, Perkin Elmer, USA). Transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) images were obtained by a Tecnai G2 F30 instrument at an accelerating voltage of 300 kV. The BET surface area was evaluated by N2 adsorption in a constant volume adsorption apparatus (Bel sorpll, Bayer Japan Co., Ltd., Japan). The analyses of photoluminescence (PL) spectra were carried out at room temperature using a Renishaw in Via raman microscope.
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2.4. Photocatalytic reaction Photocatalytic reactions were conducted in an 868 mL gasclosed stainless steel reactor [28]. The cross section area and height of the reactor were 72.3 cm2 and 12 cm, respectively. A quartz glass cover on the top of the reactor allowed irradiation to pass through. The light source was a 350-W Xe lamp (Shenzhen Stone-lighting Opto Device Co.,Ltd., China) and the UV part of the light was removed by a cut-off filter (λ 4 420 nm). The light beam was focused coherently on the surface of the 200 mL catalyst suspension (2.8 cm in depth). The suspension in the reactor was continuously stirred using a magnetic stirrer at a speed of 400 rpm. The light intensity was measured using a spectroradiometer (FZ-A, Photoelectric Instrument Factory of Beijing Normal University). In all experiments, 200 mL of deionized water containing 0.2 g of catalyst and 0.25 M Na2SO3/0.35 M Na2S mixed sacrificial agent was added into the reaction cell [21–24,26]. Here, a sacrificial agent was used to scavenge photo-generated holes. In order to remove air prior to the reaction, argon gas was bubbled through the reaction mixture for 30 min before the reaction started. Pt (1 wt%) as a cocatalyst for the promotion of H2 evolution was photodeposited in situ on the photocatalyst from the precursor of H2PtCl6 6H2O [27]. The temperature for all the photocatalytic reactions was kept at 257 1 °C. The concentrations of H2 were measured with a gas chromatograph (Shanghai Precision & Scientific Instrument Co., Ltd., GC-112A, High-performance carbon molecular sieve packed column (2 m 3 mm)), equipped with a thermal conductivity detector (TCD). All the measurements of the produced H2 concentrations at different irradiation times were performed three times to confirm their reproducibility.
3. Results and discussion 3.1. Characterization of catalysts 3.1.1. XRD analysis Fig. 1 shows the XRD patterns of the ZnInxS4 þ y (x ¼1.6, 2.0, 2.3, 2.6, 2.9, 3.1) samples. ZnInxS4 þ y could be assigned to the crystal phase of hexagonal ZnIn2S4 (JCPDS No. 65-2023). With the content of indium increasing, a new diffraction peak appears at 33° and gradually increases, which is the (0012) characteristic peak of In2S3 [29] (JCPDS No. 65-0459).
Fig. 1. XRD patterns of ZnInxS4 þ y samples (a: In2S3, b: ZnIn1.6S4 þ y, c: ZnIn2.0S4 þ y, d: ZnIn2.3S4 þ y, e: ZnIn2.6S4 þ y, f: ZnIn2.9S4 þ y, g: ZnIn3.1S4 þ y).
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Table 1 Absorption edge, crystal structure parameters, photocatalytic activities for H2 production over various Photocatalysts.
Crystal sizea (nm) Absorption edge (nm) Band gap (eV) ECB0 (eV) EvB0 (eV) BET (m2/g) Total pore volume (cm3/g) H2 production (μmol/g h) a
ZnS
ZnIn1.6S4 þ y
ZnIn2.0S4 þ y
ZnIn2.3S4 þ y
ZnIn2.6S4 þ y
ZnIn2.9S4 þ y
ZnIn3.1S4 þ y
In2S3
– 360 3.44 – – 48.6 0.0732 –
4.2 527 2.35 0.781 1.569 119.2 0.1185 154
4.0 535 2.32 0.766 1.554 122.3 0.1270 256
3.6 540 2.30 0.756 1.544 124.4 0.1306 363
3.8 550 2.25 0.731 1.519 109.0 0.1112 317
4.0 557 2.23 0.721 1.509 100.5 0.1024 303
4.2 552 2.25 0.731 1.519 97.3 0.1067 279
– 644 1.93 0.761 1.169 133.1 0.1751 –
Determined from XRD results by using the Scherrer equation.
Additionally, the crystal sizes of the catalysts are also listed in Table 1. As shown in Table 1, with the increase of indium contents, crystal size of the catalysts firstly decreases and then increases. When the value of x is 2.3, the crystal size of the catalyst reaches the minimum and is 3.6 nm. 3.1.2. EDS and ICP analysis Bulk elemental composition was measured by ICP and surface elemental composition was obtained from EDS. The Zn/In/S ratios of ZnInxS4 þ y (x ¼1.6, 2.0, 2.3, 2.6, 2.9, 3.1) catalysts were listed in Table 2. As can be seen from Table 2, the Zn/In/S ratios obtaine from the ICP and EDS are very close, indicating that the chemical homogeneity of ZnInxS4 þ y catalysts is very good. Additionally, with increase of indium contents, the mol ratio of In to S element increases when the mol of Zn is 1. After deduction of the corresponding atomic ratio ZnIn2S4, the mol ratio of In to S element is approximately 2:3. The diffraction peak of In2S3 can be observed clearly from Fig. 1, indicating that the balance of In and S elements mainly formed In2S3. According to the balance of In and S atom ratio measured by ICP in Table 2, the chemical composition of the prepared catalyst may be ZnIn2S4–mIn2S3 (m ¼0.01, 0.53, 0.77, 1.31, 1.45, 2.09). 3.1.3. Morphology analysis Fig. 2 shows the SEM images of ZnInxS4 þ y (x ¼1.6, 2.0, 2.3, 2.6, 2.9, 3.1) samples. As shown in Fig. 2a, c, e, g, i and k, ZnInxS4 þ y samples crystallites were self-organized into microsphere morphology with a wide distribution of diameters ranging from 3 mm to 6 mm. The morphology of these ZnInxS4 þ y samples greatly depended on the indium contents. As shown in Fig. 2b, when the value of x is 1.6, the shape of the ZnIn1.6S4 þ y sphere which accompanies by some of the notches shows a molten state. At the same time, its crystal gathers together, and its pore structure is not yet open. When the value of x is 2.0 (in Fig. 2d), the shape of the ZnIn2.0S4 þ y sphere surface remains a molten state and the pore structure is still not open, but the surface bump becomes more obvious. When the value of x is 2.3 (in Fig. 2f), the pore structure is opened completely and the microspheres show a regular morphology. The surface of the ZnIn2.3S4 þ y sphere is composed of numerous petals/sheets and assembled into rose-like microclusters. Agglomeration is hardly observed on the surface of the ZnIn2.3S4 þ y sphere and the width of the gap between the petals is
0.2–0.3 μm. When the value of x is 2.6 (in Fig. 2h), the majority of the ZnIn2.6S4 þ y sphere surface of the petal-like projection is wrapped up again, and the pore structure gradually disappears. When the value of x continues increasing to 2.9 and 3.1, as shown in Fig. 2j and Fig. 2l, the petal bumps on ZnIn2.9S4 þ y and ZnIn3.1S4 þ y surface disappear and become smooth. As can be seen from the above, with the value of x increasing, the pore structure of the catalyst surface is gradually opened. However, when the value of x is more than 2.3, the pore structure of the catalyst surface gradually disappears and becomes smooth again. In summary, the indium content has a great influence on the morphology of the catalyst. This phenomenon is similar to that reported by Shen et al. [26] and the growth tendency of lamellar structures might be related to the layered feature of hexagonal ZnIn2S4. The morphology of ZnIn2.3S4 þ y sample is examined using HRTEM measurements and also show in Fig. 2. As shown in Fig. 2n, the interplanar spacings were measured to be 0.622 and 0.324 nm, which could be assigned to the (111) plane of In2S3 and the (102) plane of ZnIn2S4, respectively. This result is in agreement with the XRD results and proves the presence of In2S3. 3.1.4. UV–vis DRS analysis Fig. 3 shows the UV–vis DRS of various ZnInxS4 þ y (x ¼1.6, 2.0, 2.3, 2.6, 2.9, 3.1), ZnS and In2S3 samples. The corresponding absorption edge and band gap are also listed in Table 1. As can been seen from Fig. 3, the shapes of the absorption edges are steep and the absorption in the visible region is strong. This steep absorption edge indicates that the visible-light absorption is due to a band gap transition, not due to the transition from impurity levels to the conduction band of ZnInxS4 þ y [30]. As can been seen from the spectrum of In2S3 and ZnS, their absorption edges locate at 644 nm and 360 nm, respectively, which is similar to the previous results [31,32], whereas the absorption edge of ZnInxS4 þ y catalysts exactly locates in between. As shown in Table 1, with the value of x increasing from 1.6 to 3.1, the absorption edge of these ZnInxS4 þ y samples shift to longer wavelength, indicating that the increase of the indium content is benefit to enhance the visible light adsorption of ZnInxS4 þ y. According to the characterization in Sections 3.1.1 and 3.1.2, the light absorption performance shifting to longer wavelength is related to the forming of In2S3.
Table 2 EDS and ICP characterization of each element in the catalyst sample (mmol). Element EDS
ICP
Zn In S Zn:In:S Zn:In:S
ZnIn1.6S4 þ y
ZnIn2.0S4 þ y
ZnIn2.3S4 þ y
ZnIn2.6S4 þ y
ZnIn2.9S4 þ y
ZnIn3.1S4 þ y
13.23 28.24 58.53 1:2.13:4.42 1:2.02:4.08
10.96 31.77 57.27 1:2.90:5.23 1:3.05:5.58
9.34 32.76 57.90 1:3.51:6.20 1:3.53:6.32
8.03 33.84 58.13 1:4.21:7.24 1:4.61:7.94
6.79 33.62 59.59 1:4.95:8.78 1:4.90:8.42
5.70 34.48 59.82 1:6.05:10.49 1:6.18:10.77
K. Song et al. / Journal of Solid State Chemistry 232 (2015) 138–143
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Fig. 3. UV–vis diffuse reflectance spectra of ZnInxS4 þ y samples.
3.1.5. BET analysis The BET surface area and total pore volume of ZnInxS4 þ y (x ¼1.6, 2.0, 2.3, 2.6, 2.9, 3.1) samples are also listed in Table 1. As shown in Table 1, the BET surface area and total pore volume of these ZnInxS4 þ y samples first increase rapidly when the value of x increases from 1.6 to 2.3. When the value of x is higher than 2.3, the BET surface area and total pore volume decrease gradually. That is to say, when the value of x is 2.3, the specific surface area and total pore volume of the catalyst are the maximum and are 124.35 m2/g and 0.1306 cm3/g, respectively. These results are coincided with those observed from SEM in Fig. 2. This is mainly due to the indium content at 2.3 which promotes the formation of the catalyst surface petal-like protrusions and let the pore structure fully open, so that the specific surface area and total pore volume of the catalyst can be increased. 3.1.6. PL analysis The PL spectra of ZnInxS4 þ y (x ¼1.6, 2.0, 2.3, 2.6, 2.9, 3.1) samples are shown in Fig. 4. In the PL spectra, all ZnInxS4 þ y samples have only one PL emission peak centered at 565 nm, which is longer than the ZnInxS4 þ y absorption edge. That is to say, the band gap energy (527–557 nm) is higher than the energy of the radiative photon (565 nm). Jing et al. [33] introduced the PL performance and the mechanism of nano-sized semiconductor materials. According to their study, the PL performance of the photocatalyst shown in Fig. 4 is an excitonic PL process, in which
Fig. 2. SEM images of ZnInxS4 þ y samples (a, b: ZnIn1.6S4 þ y; c, d: ZnIn2.0S4 þ y; e, f: ZnIn2.3S4 þ y; g, h: ZnIn2.6S4 þ y; i, j: ZnIn2.9S4 þ y; k, l: ZnIn3.1S4 þ y) and TEM(m), HRTEM(n) images of ZnIn2.3S4 þ y. Fig. 4. Photoluminescence spectra of ZnInxS4 þ y samples.
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Fig. 5. Photocatalytic activities of ZnInxS4 þ y samples.
the non-radiative transitions of excited electrons from the conduction band bottom to different sub-bands (or surface states) occur first and subsequent radiative transitions from the sub-band to the valence band top take place. Therefore, the energy of the radiative photon, which is the energy difference between the subband and valence band (VB) top, is lower than the band gap energy. As can be seen from Fig. 4, when the value of x is from 1.6 to 3.1, the photoluminescence intensity of the catalyst initially decreases and then increases. Especially, when the value of x is 2.6, the photoluminescence intensity of the catalyst is minimum. This phenomenon is consistent with the results that reported by Jing [34]. These results imply that the generation of a small amount of In2S3 can promote the separation of the photoproduction electronhole pairs, leading to a weak photoluminescence signal [33].
performance (as shown in Table 1). The light absorption performance is related to the forming of In2S3. Mei et al. [29] showed that In2S3 could improve ZnIn2S4’s photocatalytic activity and found that the content of In2S3 had an optimum value. In order to analyze the effect of In2S3 for the catalytic activity, the conduction band (CB) and valence band (VB) potential of ZnInx S4 þ y and In2S3 at the point of zero charge were calculated by the following Eqs. (1) and (2) [29]. Where, X is the absolute electronegativity of the semiconductors, Ec is the energy of free electrons on the hydrogen scale ( 4.5 eV) and Eg is the band gap of a semiconductor. The calculated values of X are 4.894 eV and 4.704 eV for ZnIn2S4 and In2S3, respectively. According to Eqs. (1) and (2), the potential of CB and VB of ZnInxS4 þ y and In2S3 are also listed in Table 1. As shown in Table 1, for ZnIn1.6S4 þ y and ZnIn2.0S4 þ y, their potential of CB are both more negative than that of In2S3, while their potential of VB are both more positive than that of In2S3. In this situation, the photoinduced electrons and holes of ZnIn2S4 can transfer to CB and VB of In2S3, respectively, which can suppress the separation ability of the photoexcited electrons and holes [35]. Thus, the hydrogen production decreases. For ZnIn2.3S4 þ y, ZnIn2.6S4 þ y, ZnIn2.9S4 þ y and ZnIn3.1S4 þ y, their potential of CB and VB are both more positive than that of In2S3. In this situation, the photoinduced electrons of In2S3 can transfer to CB of ZnIn2S4 and the holes of ZnIn2S4 will transfer to VB of In2S3, which can enhance the separation ability of the photoexcited electrons and holes [29,36,37]. Therefor, the hydrogen production can significantly improve. However, there should be an optimal band gap of catalyst. This is because the conduction band potential may be more positive than that of H þ /H2 when the band gap decreases enough, which will not be benifit for the hydrogen production [38]. So, the best photocatalyst can be obtained by keeping the balance between the conduction band potential and the band gap, and the best catalyst is ZnIn2.3S4 þ y.
ECB 0 = X − Ec − 0.5Eg
(1)
EVB 0 = Eg + ECB 0
(2)
3.2. Evaluation of photocatalytic activity 3.2.1. Photocatalytic activity Fig. 5 shows the time profile of photocatalytic H2 evolution over ZnInxS4 þ y (x ¼1.6, 2.0, 2.3, 2.6, 2.9, 3.1) samples under visible light irradiation. The H2 production rate is also listed in Table 1. As shown in Fig. 5, the efficiency of H2 evolution increases rapidly when the value of x increases from 1.6 to 2.3 and then decreases gradually when the value of x is higher than 2.3. The catalyst activity with the increase of the indium content is in the order ZnIn2.3S4 þ y 4ZnIn2.6S4 þ y 4ZnIn2.9S4 þ y 4ZnIn3.1S4 þ y 4ZnIn2.0S4 þ y 4 ZnIn1.6S4 þ y. The average H2 production rate of ZnIn2.3S4 þ y is the maximum and reaches 363 μmol/g h. These results indicate that the H2 production activity of ZnInxS4 þ y is influenced by the indium contents. In order to analyze the reasons for the catalytic activity, the correlations between the activity order and the characteristics of the catalyst were analyzed. For optimum catalyst ZnIn2.3S4 þ y, it has the smallest particle size, the largest surface area and pore volume (as shown in Table 1). For ZnIn2.6S4 þ y, although it has the highest hole–electron separation capability (as shown in Fig. 4), its activity is lower than ZnIn2.3S4 þ y since the crystal size increases, the specific surface area and pore volume decrease. Compared with ZnIn2.0S4 þ y, ZnIn2.9S4 þ y has the same crystal size (as shown in Table 1), ZnIn3.1S4 þ y even has greater crystal size (as shown in Table 1), weaker separation capability of hole–electron pairs (as shown in Fig. 4) and much smaller specific surface area (as shown in Table 1), the activities of ZnIn2.9S4 þ y and ZnIn3.1S4 þ y are higher than that of ZnIn2.0S4 þ y because they have better light absorption
Based on the above analysis, with the indium content increasing, the properties of the catalysts are changed, and thus their catalytic activities are altered correspondingly. Among those characteristics such as catalyst crystal size, light absorption properties, the surface area and pore volume, surface defects, the catalytic activity mainly associates with the crystal size and followed by light absorption properties. The suitable surface defects, large specific surface area and pore volume also have a beneficial effect on the activity. 3.2.2. Photostability Fig. 6 shows the photostability of ZnIn2.3S4 þ y sample in five consecutive runs for total 15 h. As shown in Fig. 6, no significant deactivation is observed during the photocatalytic reaction for at least 15 h, indicating a very good photostability of the catalyst.
4. Conclusions A series ZnInxS4 þ y (x ¼1.6, 2.0, 2.3, 2.6, 2.9, 3.1) photocatalysts were successfully synthesized using the hydrothermal method. With the increase of indium content, the crystal size decreases initially and then increases. The minimum value was 3.6 nm when x¼ 2.3. The morphology of these ZnInxS4 þ y catalysts was affected greatly by the indium contents. The band gap of ZnInxS4 þ y catalysts became small gradually, and its specific surface area and total pore volume of the catalyst increased. Catalytic activity experiments showed that the hydrogen production increased in the
K. Song et al. / Journal of Solid State Chemistry 232 (2015) 138–143
Fig. 6. Stability study of photocatalytic H2 evolution over ZnIn2.3S4 þ y under the visible-light irradiation from an aqueous Na2SO3 and Na2S solution.
beginning and then decreased under visible light with the increase of indium content. The best rate of hydrogen production reached 363 μmol/g h when the value of x was 2.3. The catalytic activity mainly associated with the crystal size and followed by light absorption properties. The suitable surface defects, large specific surface area and pore volume also had beneficial effect on the activity.
Acknowledgments All the authors gratefully acknowledge support from the Special fund for the development of Strategic and new industry in Shenzhen (No. JCYJ20120613114951217 and JCYJ20130329162012793), the Fund for the Research and Development of Science and Technology in Shenzhen (No. CXZZ20130516145955144).
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