Catalysis Today 199 (2013) 22–26 Contents lists available at SciVerse ScienceDirect Catalysis Today j ourna l ho me p ag e: www.elsevier.com/lo cate/cattod Photocatalytic H 2 generation from spinels ZnFe 2 O 4 , ZnFeGaO 4 and ZnGa 2 O 4 Xiaoxiang Xu ∗ , Abul K. Azad, John T.S. Irvine School of Chemistry, University of St Andrews, Fife KY16 9ST, UK a r t i c l e i n f o Article history: Received 19 November 2011 Received in revised form 20 February 2012 Accepted 11 March 2012 Available online 8 April 2012 Keywords: Photocatalyst H2 production Spinels a b s t r a c t The three spinels ZnFe 2 O 4 , ZnFeGaO 4 and ZnGa 2 O 4 were synthesised by conventional solid state reaction method. ZnFeGaO 4 has unit cell parameter between the parent spinels and shows appreciable absorption in visible light range with a band gap ∼1.9 eV. Photocatalytic experiments show that ZnFeGaO 4 has a reasonable hydrogen production rate (∼9 mol h -1 /g catalyst) under visible light irradiation ( ≥ 420 nm) but exhibits an improved performance (hydrogen production rate ∼971 mol h -1 /g catalyst) compared with parent ZnFe 2 O 4 compound (hydrogen production rate ∼861 mol h -1 /g catalyst) under full range irradiation ( ≥ 250 nm). The photo-electrodes prepared by electrophoretic deposition of spinel powders onto the fluorine doped tin oxide glass showed a clear impedance response under irradiation. Introducing Ga into the ZnFe 2 O 4 structure seems to enhance the light absorbance in the UV region and modify the electronic structure which accounts for the improved photocatalytic activity. © 2012 Elsevier B.V. All rights reserved. 1. Introduction The increasing global demand for energy has been providing scientists a strong incentive to seek alternative energy supplies for the existing fossil-fuels based energy economy [1,2]. Our cur- rent energy consumption each year is likely to deplete fossil fuel reserves in the near future, needlessly to say the severe environ- mental impact of fossil fuel usage such as CO 2 emission that is associated with global warming. Thereby, a clean and sustainable energy supply is highly desired. Producing hydrogen from water using solar energy is very attractive not only because hydrogen is a carbon free energy vector, whose oxidation yields only water but also because solar energy is inexhaustible in nature and is dis- tributed fairly all over earth on a daily basis [3,4]. Solar hydrogen production from water has been achieved by photocatalysts such as TiO 2 for decades [5–11]. However, their efficiency is generally poor, mainly due to their large band gaps that allow absorbance of only UV portion of the solar spectrum [12]. Developing a photocat- alyst that can absorb visible light is the major challenge. ZnFe 2 O 4 has been quite attractive because of the small band gap (∼1.9 eV) [13] and proper band edges (flat band potential ∼-0.5 eV vs. SCE) [14] that meet the thermodynamic requirements of water splitting [15]. However, its photocatalytic activity for hydrogen generation is generally poor due to its narrow conduction and valence bands that favour charge recombination [14,16–18]. A recent strategy is ∗ Corresponding author at: Room 219, School of Chemistry, University of St Andrews, North Haugh, St Andrews, Fife KY16 9ST, UK. Tel.: +44 133 446 3680; fax: +44 133 446 3808. E-mail address: xx26@st-andrews.ac.uk (X. Xu). to combine ZnFe 2 O 4 with other wide band gap semiconductors to form hetero-junctions so that photo-generated charges can be physically separated [19–21]. However, intimate contact between individual particles is required otherwise charge recombination cannot be avoided. An alternative strategy is to introduce extrin- sic elements into ZnFe 2 O 4 so that the electronic structure can be modified [22]. Here, we choose Ga as the modifier as ZnGa 2 O 4 has a highly dispersed conduction band which is favourable for charge migration [23–26]. More importantly, both compounds have the same crystal structure with comparable unit cell parameters there- fore small disturbance to the crystal structure is expected. 2. Materials and methods 2.1. Material synthesis All samples were synthesised by conventional solid state reac- tion methods. Appropriate amounts of ZnO (Aldrich 99.9%), Fe 2 O 3 (Aldrich 99.9%) and Ga 2 O 3 (Aldrich 99+%) were mixed accord- ing to the stoichiometry. These powders were firstly pre-dried at 400–500 ◦ C prior to weighing for the removal of the adsorbed water and gases. The mixtures were then planetary ball milled for 1 h in a zirconia container with zirconia balls (13 mm in diameter). The typ- ical rotation speed is 200 rpm. Acetone was used to ensure thorough mixing and the acetone to powder ratio is controlled as 4 mL ace- tone to 1 g powder. The finely mixed powders were then self-dried in air and uniaxially pressed into pellets under a pressure of 50 MPa. The resulting pellets were then loaded into alumina crucibles and calcinined at 1000 ◦ C for 24 h in a muffle furnace. Intermediate grindings, pressing and re-calcining at 1000 ◦ C was needed in order to eliminate secondary phases and were repeated twice. The final 0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2012.03.013