Influence of annealing temperature on photoelectrochemical water splitting of α-Fe2O3 films prepared by anodic deposition
Introduction
Photoelectrochemical (PEC) water splitting using visible light (sunlight) offers a promising solution to the problem of generating hydrogen and thus energy on a global scale [1], [2], [3], [4], [5], [6]. Therefore, considerable efforts target the development of ideal photoanodes, that is semiconductive materials with an ideal band-gap for visible light absorption and adequate band edge positions for water decomposition. Among various oxide and nitride semiconductors, hematite (α-Fe2O3) has for decades been considered as a strong candidate for a photoelectrochemical water oxidation reaction because it possesses a band gap that permits the absorption of visible light (Eg ≈2.2 V), it is prepared from cheap and abundant elements, and it is stable against photocorrosion. However, the PEC activity of α-Fe2O3 is limited by several factors, including a short hole diffusion length and a poor oxygen evolution reaction kinetics. To overcome these limitations and improve solar conversion efficiencies, intense efforts have been dedicated to (i) the development of α-Fe2O3 nanostructures (to accommodate the short hole diffusion path), (ii) modification of the electronic structure via elemental doping, and (iii) decoration of the surface with oxygen evolution catalysts [3], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. Recently, using a IrO2 decorated nanostructure of hematite produced by atmospheric pressure chemical vapor deposition (APCVD) [10], a water splitting photocurrent of over 3 mA cm−2 has been achieved at an applied potential of 1.23 V (vs. RHE) under AM 1.5 G 100 mW cm−2 simulated sunlight conditions.
Nevertheless, a considerably lower cost alternative to prepare hematite electrodes is electrodeposition. Hence various precipitation based routes combined with suitable post annealing have been established to produce electrodes. Anodic deposition is based on an oxidative precipitation of FeOOH typically formed from a neutral Fe2+ containing electrolyte [13], [20], [21]. Choi and co-workers [13] used this approach to deposit nanocrystalline FeOOH films on conductive FTO glass using a slightly acidic aqueous medium (pH 4.1). The deposited FeOOH film was subsequently annealed in air at 520 °C to obtain α-Fe2O3. The material was investigated under short circuit conditions (300 W xenon arc lamp (Oriel)) and was shown to be photoactive in an iodine electrolyte. In following work [22], [23], the water oxidation photocurrents were studied for such α-Fe2O3 layers modified with various dip coated metal salts which resulted in a maximum photocurrent of 0.3 mA cm−2 at 1.4 V vs. RHE. A possible reason for the comparably modest result may be that α-Fe2O3 electrodes produced on FTO glass usually require a high temperature (>700 °C) annealing step for “activation” [2], [4], [11]. This electrode activation is attributed to Sn atoms diffusing from the FTO substrate into the Fe2O3 (Sn4+ dopants are considered to be substitutionally incorporated at Fe3+ sites) [4], [11].
In this study, we examine the feasibility to use the above described simple anodic deposition approach, and to increase the limited photoelectrochemical response by adequate measures, namely substrate based thermal Sn doping combined with the application of a suitable oxygen evolution catalysts (IrO2, cobalt species).
Section snippets
Preparation of α-Fe2O3 electrodes
Anodic electrodeposition was carried out using an aqueous acidic solution (pH 4.1) containing 0.02 M FeSO4·7H2O (99%, Sigma–Aldrich) according to literature [13]. DI water purified with a Barnstead purification system (resistivity ≥ 18.2 MΩ) was used to prepare all the solutions used in this study. Anodic deposition was carried out at 1.2 V potentiostatically for 3, 8, 15 and 20 min at 70–80 °C using a power supply (Voltcraft VSP 2653). A fluorine-doped tin oxide glass (FTO-15 Ω, Solaronix,
Results and discussion
For screening parameters, the FTO surfaces were coated with different thickness FeOOH layers using the anodic deposition technique described in the experimental section -different layer thicknesses were produced using different anodization times. Fig. 1a shows an example of a top view and a cross-section obtained from an as-deposited layer formed by 8 min anodization. From the SEM images the surface shows that a layer of approx. 200 nm thickness has been formed with a uniform morphology; only
Conclusions
In the present work we investigate the formation of photoactive hematite layers using a low cost alternative to techniques such as atmospheric pressure chemical vapor deposition or atomic layer deposition. For this we use anodic FeOOH layers on FTO, and evaluate adequate annealing techniques to improve the usually low photocurrents obtained from such layers. Most effective is a heat treatment at sufficiently high temperatures to achieve substrate induced Sn doping. The optimal α-Fe2O3 exhibited
Acknowledgements
The authors would like to acknowledge Anja Friedrich, Helga Hildebrand, and Ulrike Marten-Jahns for their assistances in the SEM, XPS, and XRD measurements. We thank DFG and the DFG cluster of excellence “Engineering of Advanced Materials” (EAM) for financial support.
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