Nanostructured Zn-Fe2O3 thin film modified by Fe-TiO2 for photoelectrochemical generation of hydrogen
Introduction
Photoelectrochemical (PEC) splitting of water into hydrogen and oxygen attracted researchers all over the world since the generation of hydrogen using TiO2 as a photoanode by Fujishima and Honda in 1972 [1]. The search has continued for ideal PEC water splitting material having the characteristics of efficient sun light absorption, proper band edge energetic, high quantum efficiency, practical durability and low cost [2], [3]. A wide range of oxide semiconductors like TiO2, α-Fe2O3, ZnO, BaTiO3 and WO3 etc. has been thoroughly investigated in PEC cell for the generation of hydrogen to get reasonable efficiency at low cost [4], [5], [6], [7], [8], [9]. However, no single semiconducting material has yet been found to satisfy all the above said requirements simultaneously. Compared to others, hematite (α-Fe2O3) is considered to be an attractive PEC material, having desired property of narrow band gap (approximately 2.2 eV), which in principle allows utilization of a larger fraction of the solar spectrum, low cost, electrochemical stability and low toxicity [9], [10], [11], [12]. But, the water splitting efficiency for α-Fe2O3 is reported to be much lower than the theoretical maximum efficiency of 12.9% [13]. This is mainly due to the rapid electron-hole recombination in the semiconductor resulting in short diffusion lengths of charge carriers [14], slow surface reaction kinetics [5] and falloff in the absorption cross-section of the material for wavelengths approaching the band gap value. Additionally, the conduction band edge of hematite is slightly below that of the H+/H2 redox potential; due to which an external electrical bias is needed to generate hydrogen [15]. To overcome these problems and to improve performance of hematite nanostructuring techniques have been suggested and studied by several groups [5], [9], [10], [13]. However the reported value of the photocurrent density is still much below to meet the challenge.
Recently, composite semiconductor thin films of different band gap energies have gained considerable interest on account of its modified optical and charge transportation properties [16]. It is well accepted that the wide band gap semiconductors generate a high photovoltage but exhibit low photocurrent whereas, smaller band gap semiconductors can utilize a larger fraction of the incident photons but generate lower photovoltage [17]. Therefore, a device having multiple band gap energy layers can cover broad range of solar spectrum. It is expected that combining best of α-Fe2O3 and TiO2 in one photoelectrode, may provide a better and efficient PEC system for generation of hydrogen. With this idea, this paper presents the PEC study on nanostructured Zn doped α-Fe2O3 (Zn-Fe2O3) thin film, modified by introducing a layer of Fe doped TiO2 (Fe-TiO2) below the hematite thin film. Zn-Fe2O3 and Fe-TiO2 thin films were chosen because of their good optical and photoelectrochemical properties than pristine α-Fe2O3 and TiO2 thin films [18], [19]. Prepared photoelectrodes were also characterized for their structural, electrical and optical properties to assess the mechanism by which this concept influences the photoelectrode performance.
Section snippets
Experimental
All chemicals used in this study were analytical grade reagents; Fe(NO3)3·9H2O (99.9%, Aldrich), Zn(NO3)2·6H2O (99.9%, Aldrich), titanium tetra isopropoxide (TTIP, 97% Pure) and diethanolamine were used to prepare the precursor solution for Zn-Fe2O3 and Fe-TiO2 respectively.
X-ray diffraction
Fig. 1 represents XRD patterns of nanostructured Zn-Fe2O3, Fe-TiO2 and modified thin films. The Peaks obtained at 2θ = 24.02, 33.22, 42.9, 49.54, 56.0 and 57.8° are due to reflection from the planes (012), (104), (202), (024), (211) and (018) of hematite, respectively, indicating the existence of hematite phase with rhombohedral structure. The peaks observed at 2θ = 25.3 and 48.0° are due to reflection from the planes (101) and (200), respectively of the anatase phase of TiO2 with the
Conclusion
The present study showed that the Zn-Fe2O3 thin film modified with underlying Fe-TiO2 is a better photoelectrode in PEC cell for splitting of water to generate hydrogen using solar energy, as compared to single material photoelectrode. The maximum solar to hydrogen conversion efficiency of 0.47% was exhibited by modified Zn-Fe2O3 photoelectrode with overall thickness 0.89 μm. The improved photoresponse of this photoelectrode may be attributed to the efficient separation of photogenerated charge
Acknowledgments
The authors gratefully acknowledge the partial financial support obtained for this work by the Department of Science & Technology, New Delhi under the project no: SR/S2/CMP-47/2005 and by the University Grant Commision, New Delhi under the project no: 37-128/2009.
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