Electrochemistry Communications 23 (2012) 59–62

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Electrochemistry Communications

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Thermal air oxidation of Fe: rapid hematite nanowire growth andphotoelectrochemical water splitting performance

Sabina Grigorescu a,b, Chong-Yong Lee a, Kiyoung Lee a, Sergiu Albu a, Indhumati Paramasivam a,Ioana Demetrescu b, Patrik Schmuki a,⁎a Department of Materials Science and Engineering, University of Erlangen-Nuremberg, Martensstrasse 7, D-91058 Erlangen, Germanyb University Politehnica of Bucharest, Faculty of Applied Chemistry and Materials Science, Polizu no 17, 011061 Bucharest, Romania

⁎ Corresponding author.E-mail address: schmuki@ww.uni-erlangen.de (P. Sc

1388-2481/$ – see front matter. Crown Copyright © 20doi:10.1016/j.elecom.2012.06.038

a b s t r a c t

a r t i c l e i n f o

Article history:Received 16 May 2012Received in revised form 8 June 2012Accepted 29 June 2012Available online 5 July 2012

Keywords:Iron oxideThermal annealingWater splittingPhotoanode

Different iron oxide structures were formed by annealing of iron foils in air at temperatures between 500 °Cto 800 °C. Depending on temperature, a significant variation in the hematite/magnetite ratio and a stronglytemperature dependent morphology is obtained. While over a wide range of conditions more or less compactFe3O4/Fe2O3 layers are obtained, at 600 °C rapid growth (several micrometer per hour) of highly crystallinehematite nanowires can be observed. Visible light photocurrent measurements in 1 M NaOH under AM 1.5100 mW/cm2 conditions show that photocurrent density and the onset potential for water oxidation stronglyshifted in the cathodic direction for the nanowire morphology. The results indicate that a simple air oxidationof iron can provide a rapid path to form hematite nanowires. Obtained layers are considerably active asphotoanodes for solar water splitting.

Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.

1. Introduction

Ever since Fujishima and Honda [1] reported in 1972, a TiO2

photoelectrode approach to produce hydrogen from water, ways andmeans to use sunlight as a primary energy source for hydrogen fuel gen-eration has been a highly investigated topic in science and technology. Ingeneral, to achieve an optimized sunlight response, materials must havea suitably low band gap and, to be able to split water, have suitableband-edge positions relative to the red-ox potential of water. Therefore,large efforts are directed toward band-gap engineering of “ideal” semi-conductors that allow high efficiency and direct (open circuit) watersplitting [2]. Nevertheless due to cost, less optimized but abundant ma-terials such as hematite, with Eg. ~2.1 eV (~12 % solar conversion effi-ciency) have regained strong research interests to be used inphotoelectrochemical configurations [3–11]. The main reason for thisinterest is the expectation that some material inherent drawbacks(such as low carrier mobilities (10−2 to 10−1 cm2 V−1 s−1) and shorthole diffusion lengths (2 to 4 nm) that in practice lead to very lowlight harvesting efficiencies, and a large overpotential for photoassistedwater oxidation, can be overcome by designing optimized hematitenanostructures as photo anodes [3]. In particular, the formation of one‐dimensional nanostructured metal oxide architectures, for examplenanotubes and nanowires, are highly promising, not only as they mayenable directional charge transport but also because small tube walls

hmuki).

12 Published by Elsevier B.V. All rig

or wire diameters (~some nm) could accommodate the short hole diffu-sion length.

The most widely used synthesis procedures for Fe2O3 structures in-volve PVD, CVD and electrochemical precipitation techniques. Neverthe-less, thermal oxidation in various oxidizing atmospheres is amost simple,cheap and direct procedure to form hematite [12–21]. For some thermalannealing conditions, mostly by long‐term annealing [17] and in highlyspecific atmospheres [18–21], the formation of so‐called whiskers (ornanowires) has been reported. In the present work, we investigate thedirect thermal oxidation of iron foils in air at different temperatures.We find that a simple short‐term (1 h) annealing of these foils can leadto the formation of well‐defined crystalline hematite nanowires. Suchlayers can showa considerable photoelectrochemicalwater splitting per-formance under visible light.

2. Experimental

For the preparation of the photoanodes we used iron foils (99.99%purity, Alfa Aesar) that were degreased by sonicating in acetone andethanol for several minutes, followed by rinsing with distilled waterand drying in a nitrogen stream. The samples then were thermallyannealed in a tube furnace (Heraeus TYP R0K 6.5/60) for differenttimes under open air at 500 °C, 600 °C, 700 °C and 800 °C, respective-ly. For this, the samples were placed in the ceramic boat and insertedin the furnace at room temperature. The temperature was ramped upwith a heating rate of 15 °C min−1, kept at the designated tempera-ture for 1 h, and finally removed from the furnace (static air flow, rel-ative humidity of 65%).

hts reserved.

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A field-emission scanning electron microscope (FE-SEM, S4800,Hitachi) equipped with an energy dispersive X-ray (EDX) analyzerwas used to investigate the morphology and composition of the sam-ples. A transmission electron microscope and SAD pattern (Philips CM30 T/STEM) was used to evaluate morphology and crystal orientationof the formed nanowire. The structures of the samples were identifiedby a X-ray diffractometer (X'pert-MPD PW3040, Phillips) using CuKαradiation.

All photoelectrochemical water splitting measurements wereperformed in a three‐electrode configuration using an Ag/AgCl(3 M KCl) electrode as a reference electrode, a counter electrodeof platinum foil and the thermally annealed iron oxide electrodeas a photoanode. The measurements were performed in 1.0 MNaOH and the potential value was converted to a RHE scale usingthe relationship ERHE=EAg/AgCl+0.0591×pH+0.1976 V. The sam-ples were illuminated under AM 1.5 solar simulator where the lightintensity was calibrated to be 100 mWcm−2 using a calibrated

Fig. 1. SEM top and side views of the iron oxide grown at 50

silicon detector. Photocurrent transients as a function of the ap-plied potential were recorded by chopped light irradiation (20 sin the dark and 20 s in the light).

3. Results

Fig. 1 shows examples of the oxidemorphologies formed on the ironfoils after annealing at different temperatures for 1 h. Fig. 2a providesthe corresponding XRD patterns. At 500 °C (Fig. 1a) a homogeneouscompact layer with a flake-like top morphology with typical featuresizes around~0.6 μm is formed. The XRD investigations shown inFig. 2a indicate that the samplewas crystallized after the heat treatmentat 500 °C, mainly to magnetite and some hematite. This is in line withbroad literature that reports generally a three-layer structure for heattreatments of Fe in air or O2 with an inner FeO, then a Fe3O4 layer, andan outer hematite (Fe2O3) layer [16,22]. This structure is formed for allannealing temperatures—i.e. from the cross sections (Fig. 1(b, d, f, h))

0 °C (a, b), 600 °C (c, d), 700 °C (e, f), and 800 °C (g, h).

Fig. 2. (a) XRD spectra of iron oxide obtained at different temperatures measured under X-ray incident angle of 1° (H=hematite; M=magnetite). (b) XRD spectra of the sampleannealed at 800 °C taken under different incident angles of X-rays. (c) EDX data obtained from sample annealed at 800 °C with a representative HR-SEM showing multilayeredcrystalline structure of formed iron oxides. (d) TEM of a single iron oxide nanowire grown at 600 °C, inset shows the corresponding selected area diffraction pattern.

Fig. 3. Current–voltage curves of the iron oxide layers formed at different temperaturesperformed in 1.0 M KOH under chopped light irradiation, with a scan rate of 2 mVs−1.

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a strong increase in the layer thickness (growth rate) is observed for700° and 800 °C. Here layer thicknesses of ~30 and ~70 μm areobtained. For these layers XRD at different angles (Fig. 2b) shows hema-tite to be located in the outermost part in line with SEM cross-sectionalimages (Fig. 2c). EDX analysis performed in this cross-sectional config-uration confirms the Fe2O3 layer at 700 and 800 °C to be 0.2–5 μmthick, with underneath 1–5 μm Fe3O4 layer (Fig. 2c). The quantitativerest consists of a layer of FeO.

The top morphology for elevated temperatures changes from theplatelet structure at 500 °C to a coral-like structure at 700 and 800 °C.The most striking feature, however, is observed at 600 °C. As illustratedin Fig. 1c,d, at this temperature a mixed hematite–magnetite layer iscovered with extended whiskers (or nanowires) that protrude severalmicrometers from the surface. In contrast to otherwork [16] the growthrate is extremely rapid and occurs under normal air condition. i.e. 2‐μm‐

long nanowireswere formedwithin 1 h as shown in the cross section ofthe sample in Fig. 1d. XRD patterns reveal that for this sample the peaksassociated to hematite become more intense in comparison with theamount of magnetite. TEM investigations in Fig. 2d show that theoxide wires are of straight shape and of a diameter of approximately20 nm. It should also be noted that there is a small variation in diame-ters typically within 10 nm. The selected area diffraction (SAD) patternin the inset of Fig. 2d shows for the wires a highly crystalline hematitestructure with (110) and (012) orientation.

At 700 °C and 800 °C (Fig. 1), neither flakes nor a wiremorphologycan be observed. As mentioned, a typical high‐temperature three‐layer morphology and a top coral morphology are formed.

To evaluate the water splitting performance at the nanowire layerwe recorded current–voltage curves in 1 M NaOH solution usingintermittant AM 1.5 100 mW/cm2 illumination and compared the re-sults with the other investigated annealing conditions. The results arecompiled in Fig. 3. At 500 °C, the onset potential for water oxidation

comparatively is much higher than for samples prepared at highertemperatures. Among all samples, clearly the nanowire‐coated struc-ture formed at 600 °C shows the highest photocurrent magnitude. Aphotocurrent density of 1.0 mA cm−2 at 1.4 V vs. RHE is obtained. Incomparison with literature [3], the value is within the highest reportedfor the undoped, or cocatalyst‐free hematite samples. The samples pre-pared under identical conditions showed a good reproduciblity with astandard deviation in the photocurrent of ±10 %.

Overall the water splitting result may well indicate the signifi-cance of achieving suitable surface morphology and crsytalline struc-tures by a simple thermal annealing procedures. From cross‐sectionalSEM/EDX combined with XRD data, we can conclude that under all

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conditions a top hematite layer is present (in line with the literature)and that this layer is the key for the photoactivity, where Fe3O4 andFeO layers mainly served as a back-contact. At 600 °C, the nanowiresmay provide the typical 1-D structural benefits that are a short minor-ity carrier diffusion path and directional majority transport (the latteris particularly important in a photoelectrochemical arrangement).From a number of additional measurements we noted that the purityand the surface condition of the substrate can also play a role. Overall,this work shows that an appropriate heat treatment at Fe can createrapid nanowire growth, enhances the magnitude of the photocurrentresponse but also can significantly shift the onset potental for thewater oxidation reaction when comparing the samples heat treatedat 500 °C and 600 °C.

4. Conclusions

In summary, thermal annealing conditions of an iron foil were—asexpected—found to significantly affect the surface morphology andcrystal structure of iron oxide. We show, however, that a remarkablefeature is the rapid growth of highly crystalline hematite nanowiresthat can be observed when annealing the iron foils at 600 °C in air.These conditions lead to the highest photocurrent and the lowestonset potential for photoelectrochemical water splitting. Therefore,the present work represents a valuable platform for further optimiza-tion of thermal treatments of iron substrates toward a maximizedwater splitting performance.

Acknowledgment

The authors would like to thank DFG and the DFG cluster of excel-lence (EAM). We thank Robert Hahn for valuable advice.

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