Nanostructured Zn-Fe2O3 thin film modified by Fe-TiO2 for photoelectrochemical generation of hydrogen

https://doi.org/10.1016/j.ijhydene.2010.07.016Get rights and content

Abstract

Nanostructured semiconductor thin films of Zn-Fe2O3 modified with underlying layer of Fe-TiO2 have been synthesized and studied as photoelectrode in photoelectrochemical (PEC) cell for generation of hydrogen through water splitting. The Zn-Fe2O3 thin film photoelectrodes were designed for best performance by tailoring thickness of the Fe-TiO2 film. A maximum photocurrent density of 748 μA/cm2 at 0.95 V/SCE and solar to hydrogen conversion efficiency of 0.47% was observed for 0.89 μm thick modified photoelectrode in 1 M NaOH as electrolyte and under 1.5 AM solar simulator. To analyse the PEC results the films were characterized for various physical and semiconducting properties using XRD, SEM, EDX and UV–Visible spectrophotometer. Zn-Fe2O3 thin films modified with Fe-TiO2 exhibited improved visible light absorption. A noticeable change in surface morphology of the modified Zn-Fe2O3 film was observed as compared to the pristine Zn-Fe2O3 film. Flatband potential values calculated from Mott–Schottky curves also supported the PEC response.

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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