Efficient photocatalytic H2 production using visible ...

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Feb 10, 2014 - ... of Civil and Environmental Engineering, New Jersey Institute of Technology, 323 Martin Luther King Blvd, .... (Spectral Evolution, SR-1100).
INTERNATIONAL JOURNAL OF ENERGY RESEARCH Int. J. Energy Res. 2014; 38:1513–1521 Published online 10 February 2014 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/er.3157

Efficient photocatalytic H2 production using visible-light irradiation and (CuAg)xIn2xZn2(1  2x)S2 photocatalysts with tunable band gaps Guangshan Zhang1,*,†, Wen Zhang2,‡,§, Daisuke Minakata3, Peng Wang1,

Yongsheng Chen4 and John Crittenden4,5

1 State Key Laboratory of Urban Water Resource and Environment, School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China 2 Department of Civil and Environmental Engineering, New Jersey Institute of Technology, Newark, NJ 07102, USA 3 Department of Civil and Environmental Engineering, Michigan Technological University, Houghton, MI 49931, USA 4 School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA 5 Brook Byers Institute for Sustainable Systems, Georgia Institute of Technology, Atlanta, Georgia 30332, USA

SUMMARY The band structures of semiconductor photocatalysts fundamentally determine the photocatalytic activity and the H2 production from the visible-light-driven water-splitting reaction. We synthesize a suite of multicomponent sulfide photocatalysts, (CuAg) xIn2xZn2(1  2x)S2 (0 ≤ x ≤ 0.5), with tunable band gaps and small crystallite sizes to produce H2 using visible-light irradiation. The band gap of the photocatalysts decreases from 3.47 eV to 1.51 eV with the increasing x value. The (CuAg)0.15In0.3Zn1.4S2 (x = 0.15) photocatalyst yielded the highest photocatalytic activity for H2 production owing to the broad visible-light absorption range and suitable conduction band potential. Under the optimized reaction conditions, the highest H2 production rate is 230 μmol m2 h1 with a visible-light irradiation of 2.7 × 105 einstein cm2 s1, and the quantum yield reaches 12.8% at 420 ± 5 nm within 24 h. Furthermore, the photocatalytic H2 production is shown to strongly depend on their band structures, which vary with the elemental ratios and could be analyzed by the Nernst relation. Copyright © 2014 John Wiley & Sons, Ltd. KEY WORDS (CuAg)xIn2xZn2(1  2x)S2; H2 production; photocatalyst; band gap; water splitting Correspondence * Guangshan Zhang, State Key Laboratory of Urban Water Resource and Environment, School of Municipal and Environmental Engineering, Harbin Institute of Technology, 73 Huanghe Road, Nangang District, Harbin 150090, China. † E-mail: [email protected] ‡ Wen Zhang, Department of Civil and Environmental Engineering, New Jersey Institute of Technology, 323 Martin Luther King Blvd, Newark, NJ 07102, United States. § E-mail: [email protected] Received 24 October 2013; Revised 25 December 2013; Accepted 29 December 2013

1. INTRODUCTION Visible-light photocatalytic H2 production from water splitting has elicited considerable attention because of the increasing demand for renewable, clean, and sustainable energy [1–4]. Numerous semiconductors have been used as active photocatalysts for the water-splitting reaction, including TiO2 [5,6], Ta2O5 [7,8], ZnGa2O4 [9], CuInS2 [10,11], and (Ga1  xZnx)(N1  xOx) (0 < x < 1) [12,13], but most of them is for the ultraviolet range of wavelength. To capture the visible-light energy from solar irradiation, which constitutes 46% of the solar energy [14,15], development of new visible-light-driven photocatalysts is critical. Copyright © 2014 John Wiley & Sons, Ltd.

There are several important considerations for the development of visible-light-driven photocatalysts including (i) band gap energy; (ii) band gap position; and (iii) stability. First, water splitting into H2 and O2 requires endothermic free energy of reaction (+237 kJ/mol). As a result, photocatalysts must absorb radiant light with photon energies of greater than 1.23 eV, which is the minimum required band gap energy. Given the wavelength of visible light (400–780 nm), the band gap energy of the photocatalytic materials must be less than 3.0 eV to effectively absorb the visible light for the photoexcitation [16,17]. Furthermore, to achieve overall water splitting, the lowest unoccupied molecular orbital at the conduction band 1513

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H2 production using visible-light-driven water-splitting reaction

(CB) must be located at a more negative potential than the reduction potential of H+ to H2. In contrast, the highest occupied molecular orbital at the valence band (VB) must be positioned more positively than the oxidation potential of H2O to O2. Finally, photocatalysts must be stable in water to avoid photocorrosion [18,19] and releases of metal ions into water [20]. There have been some visible-light-driven photocatalysts developed that exhibit features described above. ZnS-based photocatalysts have small band gaps (