CHEMSUSCHEM FULL PAPERS DOI: 10.1002/cssc.201402525

Nano-Sized Quaternary CuGa2In3S8 as an Efficient Photocatalyst for Solar Hydrogen Production Tarek A. Kandiel,*[a, b] Dalaver H. Anjum,[c] and Kazuhiro Takanabe*[a] The synthesis of quaternary metal sulfide (QMS) nanocrystals is challenging because of the difficulty to control their stoichiometry and phase structure. Herein, quaternary CuGa2In3S8 photocatalysts with a primary particle size of  4 nm are synthesized using a facile hot-injection method by fine-tuning the sulfur source injection temperature and aging time. Characterization of the samples reveals that quaternary CuGa2In3S8 nanocrystals exhibit n-type semiconductor characteristics with a transition band gap of  1.8 eV. Their flatband potential is located at

0.56 V versus the standard hydrogen electrode at pH 6.0 and is shifted cathodically by 0.75 V in solutions with pH values greater than 12.0. Under optimized conditions, the 1.0 wt % Ru-loaded CuGa2In3S8 photocatalyst exhibits a photocatalytic H2 evolution response up to 700 nm and an apparent quantum efficiency of (6.9  0.5) % at 560 nm. These results indicate clearly that QMS nanocrystals have great potential as nanophotocatalysts for solar H2 production.

Introduction As a result of economic development and population growth, increases in the global energy supply are needed, particularly from solar-energy-driven systems. Solar energy can be utilized to generate electricity by employing photovoltaic cells; however, the high costs relative to fossil-fuel-based energy as well as storage and transportation difficulties limit its widespread application. H2 derived from renewable energy sources is considered as the energy carrier of the future. On an industrial level, H2 is produced currently by the steam reforming of methane or naphtha, that is, fossil fuels. As the depletion of fossil fuels is expected over the next few decades, the search for sustainable and green technologies for H2 production is becoming urgent. Photoelectrochemistry and photocatalysis as green technologies for H2 production have attracted great interest since Fujishima and Honda[1] reported that part of the photon energy (under UV illumination) can be utilized to achieve water photolysis using single-crystal rutile TiO2. The water splitting reac-

[a] Dr. T. A. Kandiel, Prof. K. Takanabe Division of Physical Sciences and Engineering KAUST Catalysis Center (KCC) King Abdullah University of Science and Technology (KAUST) Thuwal 23955-6900 (Saudi Arabia) E-mail: [email protected] [b] Dr. T. A. Kandiel Department of Chemistry Faculty of Science, Sohag University Sohag 82524 (Egypt) E-mail: [email protected] [c] Dr. D. H. Anjum Advanced Nanofabrication, Imaging and Characterization CoreLab King Abdullah University of Science and Technology (KAUST) Thuwal 23955-6900 (Saudi Arabia) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201402525.

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

tion is a challenging, energetically uphill reaction that requires a large positive change in Gibbs free energy (DG8 = 237 kJ mol1). Until recently, few visible-light-active photocatalysts have yielded reasonable H2 production from pure water using a single semiconductor material system.[2] Thus, to enhance the H2 production rate, organic or inorganic electron donors such as alcohol and sulfide ions (S2), respectively, are commonly used.[3] These reagents react irreversibly with the photogenerated holes, and the photogenerated electrons promote the H2 evolution reaction. In principle, as an intermediate step, photocatalytic H2 production that employs sacrificial reagents (electron donors) can be converted into a practical application provided that a highly active visible-light photocatalyst is identified. For example, photocatalytic H2 production through the oxidation of H2S and SO2 (a byproduct of crude-oil desulfurization) would be of great economic and environmental interest. Generally, the S content in crude oils ranges from 0.1 to greater than 5 wt %, which results in the production of millions of tons of S per year. Large quantities of H2S are treated by an energy-intensive multireactor Claus process to produce water and S.[4, 5] Photocatalytic H2S splitting operates without external heat input and generates H2 as a useful product. The complete splitting reaction of H2S under ambient conditions is thermodynamically unfavorable (DG8 = 33 kJ mol1), therefore, external energy is required. Theoretically, photons that have an energy greater than 0.17 eV can split H2S to generate H2 and solid S, that is, nearly the entire solar irradiation spectrum can be utilized. Practically, H2S can be dissolved chemically in an alkaline solution (e.g., NaOH) to generate S2 ions, which are then converted. One of the most promising candidates for photocatalytic H2 production by S2 oxidation is metal sulfide photocatalysts. CdS-based photocatalysts have been employed and investigated thoroughly since the 1980s.[6] Bare and modified Zn1xCdxS solid solutions have also been inChemSusChem 2014, 7, 3112 – 3121

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CHEMSUSCHEM FULL PAPERS vestigated, and an apparent quantum efficiency up to 19.8 % at 420 nm has been achieved.[7] Recent advances in H2S photocatalytic splitting by Pt–PdS-decorated CdS nanoparticles have resulted in 90 % quantum efficiency at 420 nm.[8] Although metal sulfides exhibit photocorrosion in water, the presence of S2 (also as a reactant) prevents the dissolution of the semiconductor by Cd2 + formation.[9] Recently, Kudo et al. introduced a new class of quaternary metal sulfide (QMS) photocatalysts.[3b, 10] DFT calculations of the band structure and photophysical analyses revealed that the incorporation of Cu and/or Ag into the photocatalyst led to an increased valence band edge and narrowed band gap.[10a] Through this strategy, CuGa2In3S8 (CGIS) visible-light-driven photocatalysts, which can absorb light up to  700 nm, have been prepared successfully using the solid-state method by heat treatment at a high temperature in an evacuated sealed quartz glass tube.[10b, 11] The photocatalytic activity of a catalyst is affected strongly by its crystal structure, crystallinity, and particle size.[12] Generally, a higher crystallinity is associated with fewer defects and a reduced probability of photogenerated charge-carrier recombination.[12] Unfortunately, to obtain highly crystalline materials, heat treatment at high temperature is required, which generally yields large particles. Although highly crystalline large particles typically exhibit higher activities in the overall water splitting reaction,[13] nanoscale particles are of great interest for solar H2 evolution from sacrificial systems, that is, systems that employ electron donors, such as S2, which are thermodynamically less stringent than the water splitting reaction. The distance over which the photogenerated charge carriers must migrate to reach the surface is smaller for nanocrystals than for microscale particles, which reduces the probability of recombination.[12] This effect depends, however, on the diffusion length of the photogenerated charge carriers that can differ from one material to another. Unlike binary chalcogenides,[14] the synthesis of QMS nanocrystals is challenging because of the difficulty to control their stoichiometry and phase structure.[15] The synthesis of ternary and quaternary CuInxGa1xS2 (CIGS) nanocrystals has been reported recently for 0  x  1,[15a, b] but CuGaxIn5xS8 (CGIS, x = 2) nanocrystals have not yet been synthesized. Herein, we report the synthesis of quaternary CuGa2In3S8 (CGIS) nanocrystals with a particle size of less than 5 nm and a band gap of 1.8 eV. The nanocrystals were fully characterized, and the photocatalytic activities for H2 evolution from aqueous sodium sulfide/sodium sulfite solutions were evaluated under visible-light ( 420 nm) and solar-light irradiation (AM 1.5 G). The photocatalytic activities of the CGIS nanocrystals were also compared with those of CGIS microparticles prepared by the solid-state method, which revealed that the photocatalytic activity of the former was higher than that of the latter.

Results and Discussion The synthesis of Cu-containing QMS nanocrystals is complicated by the facile formation of Cu2S during the hot injection. To overcome this problem, the temperature of the 1-dodecanethiol (1-DDT) injected into the Cu-Ga-In solution must be opti 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemsuschem.org mized. The optimal temperature for 1-DDT injection to avoid Cu2S formation in the current work was 150 8C, in good agreement with previous reports.[15a, 16] Kruszynska et al. observed that during the synthesis of CuInS2, the injection of the S source at temperatures above 200 8C led to the formation of Cu2S as an impurity phase in the product mixture.[17] Han et al. observed that during the synthesis of Cu2S particles from Cu(acac)2 and dodecanethiol as precursors, the precursor solution became clear as the temperature increased to 148–152 8C followed by rapid precipitation of Cu2S particles at 200 8C.[16] These observations suggest that to avoid the formation of Cu2S phases, the optimal temperature for the injection of the S source should be below 200 8C. Therefore, our strategy to avoid the formation of Cu2S was to form the intermediate CuInGa(SR)x complex before the reaction system reached the Cu2S-formation temperature at 200 8C, as reported previously for the synthesis of CuInxGa1xS2 nanocrystals.[15a] The formation of the intermediate complex was apparent from the color change from dark blue to shiny yellow after 1-DDT injection at 150 8C. The reaction was maintained at this temperature for 30 min to ensure the formation of the CuInGa(SR)x intermediate complex. This intermediate likely resulted in the formation of stoichiometric CGIS clusters that could then grow during the subsequent heating and aging at 285 8C.[15a] The results of inductively coupled plasma atomic emission spectroscopy (ICPOES) of the as-prepared CGIS nanocrystals isolated after aging at 285 8C for different durations (2, 4, 6, 12, and 24 h), denoted hereafter as CGIS-2, CGIS-4, CGIS-6, CGIS-12, and CGIS-24, respectively, are shown in Table 1. The results indicated that the

Table 1. Composition (determined by ICP-OES) of CuGa2In3S8 nanocrystals isolated after aging at 285 8C for different reaction times. Nanocrystals CGIS-2 CGIS-4 CGIS-6 CGIS-12 CGIS-24

Ratio Cu/Cu

Ga/Cu

In/Cu

S/Cu

1.0 1.0 1.0 1.0 1.0

1.0 1.9 1.8 1.8 1.9

2.8 2.9 2.9 2.9 3.0

6.6 7.3 7.5 7.8 8.0

CGIS nanocrystals isolated after aging at 285 8C for 4 h or more had a Cu/Ga/In/S atomic ratio of 1:2:3:8 with a relative error of less than 10 % from the theoretical value. Ga deficiency was observed in the sample isolated after aging for 2 h at 285 8C, which indicates that a longer synthesis reaction time is needed to obtain CGIS with the desired stoichiometry. To determine the electronic states of the as-prepared CGIS nanocrystals, their spectra were measured by X-ray photoelectron spectroscopy (XPS), and the data are presented in Figure 1. The Cu 2p core is split into 2p3/2 (binding energy (BE) = 932.6 eV) and 2p1/2 (BE = 952.5 eV) peaks (Figure 1 a), which can be assigned unambiguously to the Cu + state, but is different from the Cu 2p3/2 satellite peak of Cu2 + , which should appear at approximately BE = 942 eV. These Cu + peaks are in good agreement with those observed for ternary and quaterChemSusChem 2014, 7, 3112 – 3121

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Ag1.12Ga2.63In3.7S10 (equivalent to AgGa2In3S8) reference pattern (PDF 01-070-8366). XRD patterns are an effective tool to identify crystalline structures; however, standard XRD patterns of CuGa2In3S8 are not readily available in the literature or in the JCPDS card database. Haeuseler et al.[11] investigated the crystal structures of AgGa2In3S8 and CuGa2In3S8 prepared by quenching samples by using a powder XRD diffractometer. They associated the XRD pattern of CuGa2In3S8 with similar lattice constants of AgGa2In3S8, from which they determined that CuGa2In3S8 has a layered structure with hexagonal crystal packing (Figure S1). Therefore, the quaternary CGIS nanocrystals prepared in this study most likely have the same structure as AgGa2In3S8. The disappearance of the diffraction peak along the c [001] direction might be attributable to excessively small particle sizes in this direction. To confirm this speculation, the as-prepared materials were heat-treated at 800 8C for 8 h in an evacuated quartz tube. Before the heat treatment, the surface-attached ligands were removed by treating the nanocrystals with ammonium sulfide in a formamide and toluene mixture according to a method Figure 1. XPS spectra of CuGa2In3S8 nanocrystals isolated after a 24 h reaction time at reported previously to avoid carbonization of the 285 8C for (a) Cu 2p, (b) Ga 2p, (c) In 4d, and (d) S 2p. ligand.[20] The XRD pattern of the CGIS-24 sample after the heat treatment is shown in Figure 2 b. Diffraction peaks developed along the [001] direction, which connary metal sulfide nanocrystals.[18] Therefore, only Cu + was firm that the crystal structure of CuGa2In3S8 is similar to that of present in the CGIS nanocrystals, and the starting Cu2 + was reduced during the reaction. The Ga 2p3/2 core peak at BE = Ag1.12Ga2.63In3.7S10 (equivalent to AgGa2In3S8) as shown in Fig1118.6 eV is shown in Figure 1 b, and Figure 1 c shows that the ure S1. In 3d core is split into 3d5/2 (BE = 444.6 eV) and 3d3/2 (BE = To investigate the morphology and particle size of the prepared materials, high-resolution transmission electron micros452.2 eV) peaks, which are consistent with a valence of copy (HRTEM) micrographs were obtained (Figure 3). The + 3.[18d, 19] The shoulder peak shown in Figure 1 d located at image confirmed the successful preparation of very small partiBE = 161.4 eV can be assigned to the S 2p BE with a valence of cles in the range of 4 nm as a primary particle size. The higher 2.[19] Thus, the formation of CuGa2In3S8 was somewhat coninterfacial energy of smaller particles generally facilitate Ostfirmed by determining the electronic state obtained by XPS wald ripening, the process in which particles are dissolved and and the composition obtained by ICP-OES. redeposited on the surfaces of larger crystals or sol particles.[21] The XRD patterns of the as-prepared CGIS nanocrystals isoAn increase of the aging time at 285 8C from 2 to 24 h, howevlated after aging at 285 8C for different durations are shown in er, did not lead to severe nanocrystal coarsening (compare the (Figure 2). The synthesized materials exhibit three prominent image of the sample aged for 2 h; Figure S2), which indicates peaks at 2 q = 28.5, 47.6, and 56.68 (Figure 2 a). These peaks are the effectiveness of the surfactant ligands to control the partiin general agreement with the structure for the cle size and prevent further particle growth. Selected area electron diffraction (SAED) pattern analysis (Figure 3 inset) revealed well-resolved (1 0 3) lattice fringes (distance: 0.32 nm) consistent with the XRD pattern of AgGa2In3S8 nanocrystals, which indicates that the crystal structure of the CuGa2In3S8 nanocrystals is indeed similar to that of AgGa2In3S8. The band gap of the as-preFigure 2. (a) XRD patterns of CuGa2In3S8 nanocrystals isolated after aging at 285 8C for different durations and pared CGIS nanocrystals was de(b) XRD patterns of CuGa2In3S8 nanocrystals as-prepared and after heat treatment at 600 and 800 8C for 8 h. The termined by UV/Vis diffuse revertical lines indicate Bragg positions for the layered structure of Ag1.12Ga2.68In3.7S10 (PDF 01-070-8366).  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 3. HRTEM micrographs of CGIS nanocrystals isolated after aging at 285 8C for 24 h.

www.chemsuschem.org beneficial for photocatalysis because they can act as a trapping center for electrons and holes and act as an electron (e) or hole (h + ) pool to promote the spatial separation of the charge carriers.[24] The increase in the defect density, however, can also act as a recombination center. The quaternary nanocrystals have a complex structure as a result of the incorporation of four elements, that is, Cu, Ga, In, and S. The donor states in this system can originate from S vacancies, interstitial Cu ions, or Ga- or In-substituted Cu sites, whereas acceptors include Cu vacancies, interstitial Ga or In ions, and Cu-substituted Ga or In sites. Consequently, the PL mechanism is strongly dependent on the exact stoichiometry and structure of the nanocrystals. The underlying recombination pathways can be identified from the behavior of the PL intensity as the excitation energy is varied. The PL intensity (I) of the near-band-edge PL is generally proportional to Lk, (I / Lk) in which L is the intensity of the excitation light and k is a dimensionless exponent.[25] The value of k is greater than one and less than two for the exciton-like transition, whereas the value is less than one for the

flectance spectroscopy (DRS; Figure 4 a). The onset of the Kubelka–Munk function F(R) shifted to lower wavelengths as the aging time at 285 8C increased from 2 to 4 h and remained nearly constant after 6 h. This shift can be attributed to the deficiency of Ga and/or S in the CGIS nanocrystals isolated after aging for less than 4 h. At longer aging times, CGIS nanocrystals with the desired stoichiometry are obtained as evidenced from the ICP-OES results (Table 1). To estimate the band gap and the transition mode of these nanocrystals, the modified Kubelka–Munk function [F(R)E]n versus the energy of the excitation light Figure 4. (a) Diffuse reflectance spectra of CuGa2In3S8 nanocrystals isolated after different E was plotted, and we assumed n = 0.5 for indirect aging times at 285 8C. (b) Tauc plots of the modified Kubelka–Munk function (F(R)E)n transitions and n = 2 for direct transitions.[22] The in- versus the energy of light absorbed (E). tercept of the linear fit with the tangent to the baseline, corrected for light scattering, yielded a band free-to-bound and donor–acceptor pair transitions. The relagap energy of 1.8 eV for the indirect transition and 2.0 eV for tionship between log(I) and log(L) as a function of light intensithe direct transition. In both transition modes, a linear portion ty for CGIS-24 is shown in Figure 5 b, and I increased linearly was observed (Figure 4 b). Although Tauc plots provide with L. The data points were fitted according to the simple a formal procedure to analyze the absorption data, they do power law (i.e., I / Lk), and the k value was 0.97, which demonnot fulfill the necessary requirements to distinguish between strates that the PL was the result of free-to-bound and donor– the two types of allowed transitions explicitly for the investiacceptor pair transitions. gated CGIS nanocrystals.[23] The band gap was determined The photocatalytic activities of the prepared materials were from the absorption onset as 1.77 eV (Figure 4 a), in good assessed by measuring photocatalytic H2 evolution from Na2S/ agreement with the value (1.8 eV) calculated by assuming the indirect transition. Na2SO3 aqueous solutions. The bare CGIS nanocrystals (without The photoluminescence (PL) spectra of CGIS nanocrystals co-catalyst modification) exhibited low photocatalytic H2 evoluisolated after different intervals are shown in Figure 5 a. The PL tion activity (i.e., 4 mmol h1). Loading CGIS nanocrystals with spectra were dominated by a broad luminescence band, the a small amount of co-catalyst, such as Rh, enhanced the activiintensity of which increased with increasing synthesis reaction ty significantly. The loading of a photocatalyst surface with time up to 4 h, after which the luminescence decreased. As a small amount of noble-metal islands, for example, Rh or Pt, the aging time increased, a shift of the PL band to shorter typically promotes photocatalytic H2 evolution by creating wavelengths was observed, and it is parallel to that of the abelectron sinks that facilitate photogenerated electron–hole pair sorption band edge, which indicates that the shift of the lumiseparation and catalysis of HC radical formation and counescence peak arises from the broadening of the band gap. pling.[10c, e, 26] The results obtained under different conditions, This PL band can be attributed to the presence of donor and that is, aging time, Na2S/Na2SO3 concentrations, and co-cataacceptor states in the CGIS nanocrystals. These states can be lysts, are summarized in Table 2. The CGIS-24 nanocrystals ex 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Table 2. Rate of photocatalytic H2 evolution measured under visible-light illumination (l  420 nm) under different experimental conditions. Photocatalyst

Co-catalyst

[S2]/[SO32] [mol L1]

H2 evolution rate[a] [mmol h1]

CGIS-24 CGIS-2 CGIS-4 CGIS-6 CGIS-12 CGIS-24 CGIS-24 CGIS-24 CGIS-24 CGIS-24 CGIS-24 CGIS-24 CGIS-24 CGIS-24 CGIS-24 CGIS-24 CGIS-24 CGIS-24 CGIS-24 CGIS-24 CGIS-SS CGIS-SS CGIS-SS

none Rh[b] Rh[b] Rh[b] Rh[b] Rh[b] Rh[b] Rh[b] Rh[b] Rh[b] Rh[b] Rh[b] Rh[b] Rh[b] Rh[b] Rh[b] Rh[b] Rh[b] Ru[c] Pt[b] Rh[b] Ru[c] Pt[b]

0.05/0.3 0.05/0.5 0.05/0.5 0.05/0.5 0.05/0.5 0.05/0.5 0/0.5 0.01/0.5 0.03/0.5 0.05/0.5 0.1/0.5 0.2/0.5 0.5/0.5 0.05/0 0.05/0.05 0.05/0.1 0.05/0.3 0.05/0.5 0.05/0.3 0.05/0.3 0.05/0.3 0.05/0.3 0.05/0.3

4 87 98 118 129 157 50 116 160 157 149 85 22 101 131 148 189 157 338 44 108 254 38

[a] The relative errors are less than 10 % judged from repeated measurements. [b] 0.5 wt %. [c] 1.0 wt %.

increased the H2 production rate by a factor of only 1.3. A decrease in efficiency is reported frequently for CdS photocatalysts if photocatalytic H2 production is performed in an aqueous sulfite rather than sulfide solution.[9b] The same behavior was observed for the CGIS nanocrystals studied here; however, the oxidation of SO32 [Eq. (6); E8 = 0.25 V vs. SHE, pH 14) is thermodynamically more favorable than the oxidation of S2 ions [Eq. (4); E8 = 0.52 V vs. the normal hydrogen electrode (NHE), pH 14).[9b] This observation may be attributed to the greater adsorption of S2 onto the photocatalyst surface. The decreased rate of H2 evolution in the case of the solution that contains only S2 compared to that for the mixture of Na2S/ Na2SO3 can be attributed to the formation of disulfide ions, S22, which have a less negative reduction potential (E8 = 0.48 V vs. SHE)[27] than the protons and are able to act as an optical filter to reduce light absorption.[9b] The presence of SO32 prevents the formation of S22 by forming S2O32.[9b] S2 and SO32 act as sacrificial reagents for photocatalytic H2 generation because they are very efficient hole acceptors that enable the effective separation of charge carriers.[10e, 28] The oxidation of S2 and SO32 can either occur by a two-electron transfer process or through a one-electron oxidation (e.g., through the intermediate formation of CSO32), which is thermodynamically less favorable. The reaction mechanism suggested for photocatalytic H2 evolution in the presence of S2/ SO32 mixtures is described by Equations (1)–(7).[9b, 10a] hn

hibited the highest photocatalytic activity among the synthesized samples. It was anticipated that an increase of the aging time at 285 8C would enhance the crystallinity of the materials, but the differences in crystallinity among the prepared samples were difficult to identify because of the small particle size. However, it can be concluded readily from the PL spectra (Figure 5 a) that the CGIS nanocrystals isolated after aging for 24 h at 285 8C exhibited less defects than the other samples and thus resulted in better activity. To investigate the effect of Na2S/Na2SO3 concentrations on the photocatalytic H2 evolution efficiency, the H2 evolution rates were measured by employing different Na2S concentrations at constant Na2SO3 concentration (0.5 mol L1). The rate of H2 evolution increased from 50 mmol h1 in the absence of Na2S to an average value of 150 mmol h1 with Na2S concentrations in the range of 0.03–0.1 mol L1 before it decreased with further increases in the Na2S concentration. The measurement of the H2 evolution rate upon addition of different Na2SO3 concentrations at a constant Na2S concentration (0.05 mol L1) revealed that the amount of evolved H2 increased with the increasing Na2SO3 concentration up to 0.3 mol L1. At higher Na2SO3 concentrations, the amount of evolved H2 decreased. On the basis of these results, it can be concluded that the oxidation of Na2S is preferred over the oxidation of Na2SO3 on the photocatalyst investigated here. For example, an increase in the concentration of Na2S from 0 to 0.05 mol L1 at a constant Na2SO3 concentration led to a threefold-greater H2 production rate, whereas an increase in the Na2SO3 concentration from 0 to 0.05 mol L1 at a constant Na2S concentration (0.05 mol L1)  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

CGIS ƒ! ecb  þ hvb þ

ð1Þ

2 H2 O þ ecb  ! H2 þ 2 OH

ð2Þ

SO3 2 þ 2 OH þ 2 hvb þ ! SO4 2 þ H2 O

ð3Þ

2 S2 þ 2 hvb þ ! S2 2

ð4Þ

SO3 2 þ S2 þ 2 hvb þ ! S2 O3 2

ð5Þ

2 SO3 2 þ 2 hvb þ ! S2 O6 2

ð6Þ

S2 2 þ SO3 2 ! S2 O3 2 þ S2

ð7Þ

For metal sulfides, for example, CdS, the flatband potential (Efb) depends on the concentration of S2 rather than on the pH. S2 can be adsorbed strongly on the CdS surface, which leads to a shift toward a more negative potential according to Equation (8);[9b, 29] however, no data have been presented for quaternary metal sulfides. Thus, to understand the effect of the S2 concentration on the photocatalytic activity and flatband potential of CGIS nanocrystals at the semiconductor– electrolyte junction, Mott–Schottky plots were constructed by employing an impedance spectroscopy technique (measured in the dark) and Equation (9).[30] E fb ½V vs: NHE ¼ 1:160:059 log½S2 

ð8Þ

1 2 ¼ ðEE FB k B T=qÞ C SC 2 e e0 q ND A2

ð9Þ

in which CSC is the capacitance of the space charge layer, e is the dielectric constant of the semiconductor, e0 is the vacuum ChemSusChem 2014, 7, 3112 – 3121

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Figure 5. (a) PL spectra of CuGa2In3S8 nanocrystals isolated after aging at 285 8C for different durations. (b) Plot of log(I) versus log(L). The excitation light intensities were adjusted by employing neutral density filters (Hoya Filters). The excitation wavelength was 490 nm.

Figure 6. Mott–Schottky plots obtained at a frequency of 1 kHz for CGIS film electrodes at different S2 concentrations ranging from (a) 0–0.01 mol L1 and (b) 0.05–1.0 mol L1. All of the measurements were performed in 0.2 mol L1 Na2SO4 aqueous solution. Note: open circles in (b) denote 0 mol L1 S2 concentration at pH 13.5.

permittivity, A is the area of the electrode exposed to the electrolyte, ND is the donor density, E is the applied potential, EFB is the flatband potential, q is the elementary charge, kB is Boltzmann’s constant, and T is the temperature. A plot of CSC2 versus E should thus yield a straight line that intersects the potential axis at EFBkB T/q. The Mott–Schottky plots measured at different S2 concentrations are shown in Figure 6. All of the measurements were obtained in 0.2 mol L1 Na2SO4 solution at a frequency of 1.0 kHz. Although the concentration of S2 changed from 0 to 0.01 mol L1, which consequently changed the solution pH from 6.0 to 11.8, the EFB value remained nearly constant (Figure 6 a). A change in the slope of the Mott– Schottky plot was observed with increasing S2 concentration. To obtain EFB accurately, the non-Faradaic, double-layer region of the potential in the cyclic voltammogram was chosen, which reflects the linear part of the Mott–Schottky plots. At an S2 concentration greater than 0.01 mol L1 (Figure 6 b), EFB was shifted cathodically by 0.75 V and then remained nearly constant regardless of the change in the S2 concentration or the solution pH. To determine if this cathodic shift is caused by S2, the Mott–Schottky plot was also measured in the absence of S2 (pH 13.5, adjusted using NaOH, open circles Figure 6 b and different pH values presented in Figure S3). As shown in Figure 6 b, EFB was also shifted by the same value (i.e.,  0.75 V), which demonstrates that EFB of quaternary CGIS, unlike CdS, does not obey Equation (8). This shift is probably

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caused by the adsorption of S2 and/or OH to the surface at high pH, as reported previously for a CdS single-crystal electrode.[29, 31] The band structure and the standard oxidation potentials for both H + and S2 are shown in Figure 7. Under all conditions, the conduction band and the valence band energetic levels are positioned sufficiently cathodically and anodically, respectively, that they can reduce protons and oxidize sulfide ions, respectively, except at low sulfide ion concentration with a solution pH of approximately 11.8, and the conduction band potential coincides with the proton reduction potential. The increase of the S2 concentration (up to 0.01 mol L1) leads to a simultaneous increase of the solution pH and thus the reduction potential of the protons, whereas the EFB of CGIS remains constant (Figure 6 a), and the driving force for proton reduction is decreased. This might explain why the CGIS nanocrystals exhibit lower activity at lower sulfide ion concentration. The observed increase in the H2 evolution rate if the sulfide ion concentration increases up to 0.1 mol L1 (Table 2) can be thus explained by the greater driving force for proton reduction under these conditions. Although EFB and thus the driving force for proton reduction remain constant at sulfide ion concentrations above 0.1 mol L1, the decrease in the H2 evolution rate at high sulfide ion concentrations can be attributed to either the deficiency of available protons at high pH or a surface disturbance that results from the strong

Figure 7. The band structure of CGIS at different S2 concentrations and standard reduction potentials of redox reagents in the electrolyte solution.

adsorption of S2 ions at the surface, which might induce a recombination surface state. Under optimized Na2S/Na2SO3 concentrations, the effect of different co-catalysts, namely, Pt, Rh, and Ru, was investigated. The optimum loadings were 0.5, 0.5, and 1.0 wt % for Rh, Pt, and Ru, respectively. Typical time courses of photocatalytic H2 evolution over CGIS-24 under visible-light illumination with different co-catalysts are shown in Figure 8. For comparison, the photocatalytic H2 evolution over CuGa2In3S8 particles (prepared by the solid-state method and denoted as CGIS-SS) is also shown. XRD, DRS, and SEM measurements indicated the sucChemSusChem 2014, 7, 3112 – 3121

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Figure 8. Time courses of photocatalytic H2 evolution over CGIS-24 nanocrystals (solid symbols) and CGIS-SS (open symbols) loaded with different co-catalysts from an aqueous Na2S (0.05 mol L1)/Na2SO3 (0.3 mol L1) solution under visible-light illumination (l  420 nm).

cessful preparation of CGIS-SS with a layered structure according to previous reports (Figures S4–S6).[10b, 11] The photocatalytic results showed clearly that the Ru-loaded photocatalysts, either nanocrystals or microparticles, that were prepared by the solid-state method exhibited a superior photocatalytic activity compared to those modified with Pt or Rh co-catalysts. These added metals (or corresponding metal sulfides under reaction conditions) can act as H2 evolution catalysts and/or S oxidation catalysts. If we consider the metallic co-catalysts as H2 evolution sites, the electronic interface between the metal and n-type CGIS must be important in addition to the electrocatalytic function of the co-catalysts. One of the parameters to be considered is the work function of the metals. In this study, a good correlation was observed between the photocatalytic activities and the work function of Pt, Rh, and Ru co-catalysts, that is, 5.64, 4.98, and 4.71, respectively.[32] The high photocatalytic performance with the Ru co-catalyst may be attributed to its low work function, which in turn contributes to the minimized height of the Schottky barrier created at the co-catalyst–semiconductor interface. In addition, the Ru co-catalyst not only enhanced the rate of photocatalysis but also suppressed the deactivation in comparison with the Rh co-catalyst. Some of the Ru species (as metallic or sulfide forms, such as Ru2S3) can also act as a co-catalyst for sulfide ion oxidation, which may result in enhanced charge separation and thus a reduced degree of deactivation. Conversely, the gradual deactivation of CGIS-SS was still observed even if it was decorated with a Ru co-catalyst. It seems that the activity depends strongly on the nature of the photocatalyst and it was not sufficient to prevent the self-oxidation of the CGIS-SS photocatalyst even in the presence of Ru co-catalysts. To prove that the photocatalytic H2 evolution reaction proceeds through band gap excitation, the apparent quantum efficiencies of H2 evolution were measured by using band-pass filters (the photon distributions are presented in Figure S7). The diffuse reflectance spectrum and action spectrum of H2 evolution over 1.0 wt % Ru-loaded CGIS-24 nanocrystals are shown in Figure 9. The onset of the action spectrum was in good agreement with the diffuse reflectance spectrum, which indicates that the photocatalytic H2 evolution indeed proceeds  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 9. Diffuse reflectance spectrum of the 1.0 wt % Ru-loaded CGIS-24 photocatalyst and the action spectrum of H2 evolution from an aqueous Na2S (0.05 mol L1)/Na2SO3 (0.3 mol L1) solution.

through band gap excitation. Moreover, the CIGS-24 nanocrystals loaded with 1.0 wt % Ru exhibited a photocatalytic response toward H2 evolution from an aqueous Na2S/Na2SO3 solution up to 700 nm and exhibited (6.9  0.5) % apparent quantum efficiency at 560 nm. Photocatalytic H2 evolution over 1.0 wt % Ru-loaded CGIS-24 nanocrystals was also investigated under solar-light irradiation by using a solar simulator (Figure 10). The photocatalytic H2

Figure 10. Time courses of photocatalytic H2 evolution over CGIS-24 nanocrystals (open symbol) and CGIS-SS (solid symbol) loaded with 1.0 wt % Ru from an aqueous Na2S (0.05 mol1)/Na2SO3 (0.3 mol L1) solution under solar light illumination (1 sun, AM 1.5 G).

evolution over CGIS-SS under the same conditions was measured for comparison. It is clear from the results shown in Figure 10 that although the initial rate of H2 evolution over CGIS-SS is comparable to that over CGIS-24, the latter exhibited superior activity over a longer period of time. This is also the case in the presence of a higher concentration of CGIS-SS photocatalyst (Figure S8). As mentioned in the Introduction, the distance over which the photogenerated charge carriers must migrate to reach the surface is smaller for nanocrystals than for microscale particles. With adequate co-catalysts, improved photocatalysis may lead to improved stability by enhanced charge separation to avoid self-oxidation. The evaluaChemSusChem 2014, 7, 3112 – 3121

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CHEMSUSCHEM FULL PAPERS tion of photocatalytic H2 evolution induced by a solar simulator as a standard light source is still an ideal method; however, electron donors must be employed. Based on the average rate of H2 evolution over 22 h, 29 LH2 h1 was obtained per m2 of the CGIS nano-photocatalyst suspension exposed to solar irradiation.

Conclusions We have demonstrated a facile hot-injection method for the synthesis of quaternary CuGa2In3S8 nanocrystals with a desired stoichiometry and an average primary particle size of  4 nm by controlling the injection temperature of the S source and the aging time. The synthesized nanocrystals have a transition band gap (  1.8 eV) and can absorb UV and visible light up to 700 nm. Impedance spectroscopy measurements (Mott–Schottky) indicate that the synthesized CuGa2In3S8 nanocrystals are ntype semiconductor and that their flatband potential is located at 0.56 V (versus the standard hydrogen electrode at pH 6.0). The flatband potential is shifted cathodically by 0.75 V in aqueous sulfide solution if the S2 concentration is greater than 0.01 mol L1 (pH  12); however, it remains nearly constant at lower concentrations. The photocatalytic measurements indicated that 1.0 wt % Ru-loaded CuGa2In3S8 nanocrystals isolated after aging for 24 h at 285 8C exhibit superior activity toward H2 evolution from an aqueous Na2S (0.05 mol L1)/Na2SO3 (0.3 mol L1) solution. Under the optimum conditions, an apparent quantum efficiency of (6.9  0.5) % at 560 nm can be achieved. The photocatalytic H2 evolution activity of the CuGa2In3S8 nanocrystals was also investigated under simulated solar irradiation. Based on the average rate of H2 evolution over 22 h, 29 LH2 h1 per m2 was obtained, which evidences that quaternary metal sulfide nanocrystals have great potential as nano-photocatalysts for solar H2 production.

Experimental Section Preparation of CuGa2In3S8 nanocrystals The CuGa2In3S8 nanocrystals were prepared in a fume hood under inert conditions as follows. A mixture of copper(II) acetylacetonate (0.262 g, 1.0 mmol, Sigma–Aldrich,  99.99 % trace metals basis), gallium(III) acetylacetonate (0.734 g, 2.0 mmol, Sigma–Aldrich,  99.99 % trace metals basis), indium(III) acetylacetonate (1.236 g, 3.0 mmol, Sigma–Aldrich,  99.99 % trace metals basis), trioctylphosphine oxide (3.5 mmol, 90 %, Aldrich, technical), and oleylamine (15 mL, 70 %, Aldrich, technical grade) was placed in a fournecked, round-bottomed flask and stirred at RT for 30 min with Ar purging. The solution was then heated to 120 8C and maintained at this temperature for 1 h to remove water. The temperature was then increased to 150 8C, and 1-dodecanethiol (4.0 mL,  16 mmol, Sigma–Aldrich) was injected rapidly into the solution under an Ar atmosphere with continuous stirring. The solution color changed from dark blue to shiny yellow. The temperature of the solution was maintained at 150 8C for 30 min before it was increased gradually to 285 8C over 30 min. After the desired time, that is, 2, 4, 6, 12, or 24 h, the mixture was cooled, and the nanocrystals were isolated by centrifugation and washed thoroughly with ethanol/ hexane (50 % v/v). The particles were then dried in a vacuum oven  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemsuschem.org at 45 8C and denoted as CGIS-2, CGIS-4, CGIS-6, CGIS-12, and CGIS24, respectively.

Preparation of CuGa2In3S8 microparticles CuGa2In3S8 microparticles with layered structures were prepared according to previous reports.[10b, 11] The desired amounts of Cu2S, Ga2S3, and In2S3 (Sigma–Aldrich;  99.99 % trace metals basis 99.99 %) were mixed in a mortar in a glove box. The mixture was sealed in a quartz ampoule tube in vacuo and heat-treated at 850 8C for 8 h.

Characterization of CuGa2In3S8 nanocrystals and microparticles XRD patterns of the CuGa2In3S8 materials were collected by using a Bruker D8 Advance diffractometer (DMAX 2500) operating with a CuKa1,2 energy source at 40 kV and 40 mA. XPS analysis was performed by using an AMICUS/ESCA 3400 KRATOS instrument equipped with Mg anodes at 12 kV and 10 mA. A prominent maximum peak of C 1s at 284.2 eV was taken as the reference to calibrate the XPS spectra. High-resolution transmission electron microscopy (HRTEM) was performed at 300 kV by using an instrument of the type TITAN G2 80–300 ST. SAED patterns were obtained to determine the interplanar d-spacings of the crystalline phases present in the samples. SEM measurements were performed by using a Nova Nano 630 scanning electron microscope from FEI Company using a TLD detector at an accelerating voltage of 5 kV. DRS was performed by using a JASCO (V-670) spectrophotometer equipped with a 60 mm ø integrating sphere. Labsphere USRS-99–010 was employed as a reflectance standard. The reflectance spectra were converted to absorbance mode using the Kubelka–Munk method. PL spectra were measured using a Fluoromax-4 spectrofluorometer (HORIBA Scientific). ICP-OES was accomplished by weighing  15 mg of each nanocrystal sample and digesting it in a mixture of concentrated HCl (3 mL) and concentrated HNO3 (1.0 mL). After dilution to 50 mL, the ICP data were recorded by using a Varian 715-ES instrument.

Flatband potential measurements The flatband potentials of CuGa2In3S8 nanocrystals were measured by impedance spectroscopy using Mott–Schottky plots. CuGa2In3S8 nanocrystals were deposited onto F-doped SnO2 glass (FTO; TCO22–15, Solaronix, ~ 15 ohm/square) using the electrophoretic deposition (EPD) method according to a previous report.[33] For this step, the CuGa2In3S8 powder (50 mg) was suspended in an acetone solution (50 mL) that contained iodine (25 mg). The addition of iodine produced H + by reaction with acetone, which made the particles positively charged. Two slides of the FTO glass were immersed in the solution in parallel at a distance of  2 cm, and 20 V was applied between the electrodes for 5 min by using a DC power supply (PowerPac, Bio-Rad). The coated area was approximately 1  1.25 cm. The coated FTO glass slides were then dried in a vacuum oven at 45 8C and retreated with a solution of CuGa2In3S8 in toluene (10 mg per 10 mL) to enhance the connection between the particles and the electrode and to fill any cracks. Next, toluene (10 mL) was dropped onto the film and allowed to dry. This process was repeated ten times, and the slides were dried in a vacuum oven at 45 8C. The uncoated FTO glass area was covered with nonconductive epoxy except for  0.5 cm left uncoated for the electrical connection with a stainless-steel rod. This contact area and the ChemSusChem 2014, 7, 3112 – 3121

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stainless-steel rod were later covered with Teflon tape to isolate them from the electrolyte solution. The electrochemical setup for the impedance measurements consisted of three electrodes: the working electrode (CuGa2In3S8 nanocrystals film), a Pt wire used as a counter electrode, and a saturated calomel electrode (SCE) as the reference (+ 0.241 V vs. SHE). The experiments were performed in aqueous 0.2 m Na2SO4 solutions that contained different sodium sulfide concentrations. The potential was varied systematically between 0.5 and + 0.5 V, and the frequency range was modulated between 10 and 10 000 Hz by using a 16-channel, research-grade potentiostat system (VMP3) electrochemical interface and an impedance analyzer from Bio-Logic Science Instruments.

[7]

[8]

[9] [10]

Photocatalytic H2 evolution activity measurements The photocatalytic H2 production assessment of the prepared nanomaterials was performed by using a Pyrex top-irradiation reaction vessel connected to a glass closed-gas circulation system. In a typical run, the CuGa2In3S8 powder (0.1 g) was dispersed in Na2S/ Na2SO3 aqueous solution (100 mL) by sonication. The suspension was poured into the photoreactor, and the desired amount of cocatalyst (i.e., Pt, Rh or Ru as Na2PtCl6·6 H2O, Na3RhCl6, or RuCl3·x H2O (Ru 38–40 %) aqueous solutions, respectively) was added. The photoreactor was then sealed and connected to the circulation system. After photoreactor was evacuated and Ar was introduced several times, the photoreactor was irradiated either with visible light by using a 300 W Xe lamp adapted with a cut-off filter (l  420 nm, HOYA, L42) and a cold mirror, CM1, or with a solar simulator (PECL15, Peccell Technologies, Inc.). For the quantum efficiency measurements, the photocatalytic H2 evolution was measured by employing the photocatalyst suspension (25 mL) and the photoreactor was irradiated by using a 300 W Xe arc lamp adapted with a band-pass filter (MAX-303, Asahai Spectra). The photon flux was measured by using a spectroradiometer (EKO, LS-100; Figure S7). The apparent quantum efficiency was calculated as the rate of H2 evolution [mmol s1] multiplied by two and divided by the incident photon flux [mmol s1].

[11] [12] [13] [14] [15]

[16] [17] [18]

[19] [20]

Acknowledgements Funding for this work was provided by Saudi Aramco under contract 6600024505/01. T.A.K. would like to thank the Chemistry Department, Faculty of Science, Sohag University for granting him a leave of absence. Keywords: copper photochemistry

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gallium

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hydrogen

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indium

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Received: June 9, 2014 Revised: July 14, 2014 Published online on September 3, 2014

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