Physical and photoelectrochemical studies for hydrogen photo-evolution over the spinel ZnCr 2 O 4 S. Boumaza a,b , A. Bouguelia c , R. Bouarab a , M. Trari c, * a Laboratoire de Chimie du Gaz Naturel, Faculte ´ de Chimie, USTHB, BP32 El-Alia, Algiers, Algeria b Centre de Recherche Scientifique et Technique en Analyses Physico-Chimiques (C.R.A.P.C), BP248 Algiers RP, 16004, Algeria c Laboratoire de Stockage et de Valorisation des Energies Renouvelables, Faculte ´ de Chimie, USTHB, BP32 El-Alia, Algiers, Algeria article info Article history: Received 28 May 2008 Received in revised form 18 November 2008 Accepted 19 November 2008 Available online 6 January 2009 Keywords: Hydrogen Photoelectrochemical Spinel ZnCr 2 O 4 abstract The present work deals with the photoelectrochemical hydrogen production over the spinel ZnCr 2 O 4 . The photoactivity is dependent on the synthesis conditions and the oxide has been prepared by nitrate way in order to produce homogeneous powder with large active surface. The transport properties indicate p-type conductivity with activation energy of 0.21 eV. A corrosion potential of 0.404 V SCE and an exchange current density of 50 mA cm 2 have been determined from the semi logarithm plot. The photocurrent onset potential, assimilated to the flat band potential, was found to be 0.39 V SCE . ZnCr 2 O 4 /S 2 O 3 2 is a self driven system where absorption of light promotes electrons into the conduction band with a potential (1 V) sufficient to reduce water into hydrogen. The activity shows a tendency toward saturation whose deceleration is the result of the competitive reductions of end products namely S 2 O 6 2 and S 2 O 4 2 with water. A comparative study with CuCr 2 O 4 is reported. ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. 1. Introduction Hydrogen is an important energy source and promising in the future [1,2]. It is clean and ideal for the long term storage and its production from water over semiconductor (SC) materials remains a challenging target [3–8]. Most oxides investigated to date are stable against corrosion but have wide forbidden band (E g ) and would therefore require external bias assistance; this limits seriously the choice of SC that can be used in photoelectrochemical (PEC) devices [9,10]. In addition, the difference jE CB  E H2O=H2 j, is constant and both the redox level and the conduction band (CB) vary almost by the same quantity namely 0.06 V/pH resulting in large over-voltages. For oxides with partially filled d-levels, the flat band potential (V fb ) and the band gap (E g ) are two contradictory parameters which obey the empirical relation: V fb (SCE) ¼ 2.70  E g [11]. The above handicaps can be partially overcome by using materials whose electronic bands do not derive from O 2 :2p orbital [12]. The spinels AM 2 O 4 , where M denotes a transition metal, emerge as an interesting family for the PEC conversion [13]. More recently, they begin to draw attention in the field of the photocatalysis [14] and optoelectronic applications [15]. p-type electrode material should be chemically stable and must combine small band gap energy (E g ) and a positive flat band (V fb ). The electronic bands are pH independent and we have exploited this property in PEC conversion [16]. The photoactivity is intrinsically determined by the solid proper- ties and is enhanced when the catalyst is elaborated by chemical route [17] or sol gel technique [18,19] in order to increase the surface-to-volume ratio. ZnCr 2 O 4 is a narrow * Corresponding author. Tel.: þ213 21 24 79 50; fax: þ213 21 24 73 11. E-mail addresses: mtrari@caramail.com, labosver@gmail.com (M. Trari). Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he 0360-3199/$ – see front matter ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.11.059 international journal of hydrogen energy 34 (2009) 4963–4967