Thin Solid Films 519 (2011) 2415–2420

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Thin Solid Films

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Transparent nanoporous tin-oxide film electrode fabricated by anodization

Akira Yamaguchi a,b,⁎, Teruhiko Iimura c, Kazuhiro Hotta c, Norio Teramae c,⁎a College of Science, Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki 310-8512, Japanb Frontier Research Center for Applied Atomic Sciences, Ibaraki University, Tokai, Ibaraki 319-1106, Japanc Department of Chemistry, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan

⁎ Corresponding authors. Tel./fax: +81 29 228 8389. YCollege of Science, Ibaraki University, 2-1-1 Bunkyo, Mi

E-mail addresses: yakira@mx.ibaraki.ac.jp (A. Yamag(N. Teramae).

0040-6090/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.tsf.2010.11.049

a b s t r a c t

a r t i c l e i n f o

Article history:Received 7 May 2010Received in revised form 12 October 2010Accepted 30 November 2010Available online 8 December 2010

Keywords:Nanoporous filmThin film electrodeTin oxideAnodization

A transparent nanoporous tin oxide film electrode was fabricated by anodizing a tin film on a fluorine-dopedtin oxide (FTO) film electrode. The resulting anodized nanoporous tin oxide (ANPTO) film has columnar-typepore channels with around 50 nm in diameter and is optically transparent. Electrochemical measurementswith Fe(CN)63− as a redox probe clearly revealed that the ANPTO film could be used for a working electrodewith a large internal surface area. Moreover, it was found that ANPTO film had a wider anodic potentialwindow (N ca. 2.0 V) than conventional metal oxide electrodes, such as FTO and indium tin oxide filmelectrodes (N ca. 1.3 V). Thewide anodic potential window improves applicability of a transparent metal oxideelectrode for various electrochemical oxidation reactions, which are often interfered by oxygen evolution inwater. These results conclude that the ANPTO film can be used as an advanced transparent nanoporous filmelectrode.

amaguchi is to be contacted atto, Ibaraki 310-8512, Japan.uchi), teramae@m.tohoku.ac.jp

l rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Optically transparent film electrodes have been widely used forvarious optoelectronic and electro-optic devices, such as solar cells,photosensors, gas sensors, flat panel displays, photocatalysts, andbatteries. The efficiency of such devices could be highly improved ifporous properties are imparted to the transparent film electrodes dueto their high surface area. Tin oxide (SnO2) is one of the representativematerials used for transparent electrodes and nanostructurization ofsuch electrodes has been extensively studied by using varioustemplate- and templateless-fabrication methods in order to developa class of nanoporous tin oxide (NPTO) films [1–6]. The template-assisted sol–gel method has been usually utilized to fabricate an NPTOfilm with hierarchical and/or well-ordered pore structure [1–5],however the pore structure has often suffered from collapse ordistortion during the heat treatment to induce crystallization. Instead,anodization of a metal film can be considered as a simple template-less-fabrication method [6–9]; a nanoporous tin oxide film can beformed by anodizing a tin plate in an acidic solution [6]. Although aporous anodized metal oxide film often lacks uniformity in porediameter and structure, the anodization method has advantages inefficient and large-scale fabrication of a nanoporous metal oxide filmwith large internal surface area [6]. In addition, the pore structure isstable during the heat treatment to induce crystallization of metal

oxide framework after anodization. Thus, the anodization method canbe regarded as a powerful technique for the development of an NPTOfilm electrode. Until now, the anodization method has beenextensively applied to fabricate porous alumina [7], titania [8], andsilicon [9]. However, only one paper has been reported on anodizationof a pure tin plate until now, and availability of NPTO film prepared bythe anodization method has not been clarified [6].

In addition to the fabrication method, a key issue on a usage of theNPTO film is to confirm its characteristics as an electrode forelectrochemical applications. Usual tin-oxide-based film electrodescontain dopants, such as antimony and fluorine, to get high electricalconductivity. However, doping of atoms into the tin oxide frameworkis difficult during or after anodizing tin films. Accordingly, it isessential to assure both high electrical conductivity and the efficientelectrochemical activity at the inner pore surface of the NPTO film. Forthe development of a transparent NPTO film electrode by theanodization method, both the fabrication procedure and electro-chemical characterization should be evaluated.

In the present study, an NPTO layer was fabricated on a fluorine-doped tin oxide (FTO) film electrode by the anodization method andelectrode performances of the NPTO film were examined. Hereafter,the NPTO layer formed by anodization is called as an anodized-NPTO(ANPTO) layer. The ANPTO layer is optically transparent and hasdensely packedmeso- andmacro-pores. The ANPTO layerwasmade ofpure tin oxide without dopants. Nevertheless, the electrochemicalexperiments clearly showed that the ANPTO layer had enoughelectrical conductivity for electrochemical applications and thatredox reactions were able to take place at the inner pore surface ofthe ANPTO layer. Moreover, it was found that the ANPTO layer had

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wider anodic potential window (N ca. 2.0 V) than conventional metaloxide electrodes, such as FTO (N ca. 1.3 V) and indium tin oxide (ITO)film electrodes. The wide anodic potential window improves applica-bility of a transparent metal oxide electrode for various electrochem-ical oxidation reactions, which are often interfered by oxygenevolution in water. These results conclude that the ANPTO layer canbe used as an advanced transparent nanoporous film electrode.

2. Experimental details

2.1. Materials

The FTO film electrode (25 mm×25 mm, FTO thickness=ca.1200 nm) was purchased from SPD Laboratory, Inc. (Hamamatsu,Japan). The ITO film electrode (75 mm×25 mm, ITO thickness=ca.250 nm) was purchased from Sigma-Aldrich Japan K.K. (Tokyo,Japan). Tin wire (99.9%) for the thermal deposition of the tin filmwas purchased from Nilaco Co. (Tokyo, Japan). Oxalic acid waspurchased from Nacalai Tesque, Inc. (Kyoto, Japan). Tris(2,2′-bipyridyl)dichlororuthenium (Ru(bpy)3Cl2) was purchased fromSigma-Aldrich Japan K.K. (Tokyo, Japan). All other chemicals werepurchased from Wako Pure Chemical Industries Ltd. (Osaka, Japan).Ultrapure water, obtained using a Milli-Q™ (Millipore) system, wasused for all experiments.

2.2. Fabrication of ANPTO layer on FTO film

The ANPTO layer was fabricated by anodizing a tin film deposited ona fluorine-doped tin oxide (FTO) film. The FTO film electrode wassonicated in acetone, methanol, and then Milli-Q water for 5 min each.After drying under a nitrogen gas stream, the FTO film electrode washeated in an oven at 500 °C for 30 min to remove organic adsorbates.Thin tin layer (thickness=ca. 15 nm) was deposited on the FTO filmelectrode using a Ulvac model VPC-1100 vacuum deposition system,and then the thin tin layerwas oxidized in an oven at 500 °C for 3 h. Theresulting thin tin oxide (TO) layer was island-like as shown in Fig. 1(b).

Fig. 1. SEM top views of (a) the FTO film, (b) the TO island layer on the FTO film, (c) the tin laanodizing the tin layer.

Finally, an additional tin layer was deposited on the FTO film electrodethrough the TO island layer (Fig. 1(c)). Anodization of the tin layer wascarried out at in a 0.04 M oxalic acid solution at 5 °C. In the anodizationprocedure, the potential of 4.0 V was applied to the tin layer for 15 s atfirst, and then the potential of 6.0 V was applied until the tin layer wascompletely anodized (Fig. 1(d)). After anodization, the resultingANPTO/FTO multilayer film was annealed at 500 °C for 3 h (Fig. 2).

2.3. Measurements

Scanning electron microscopy (SEM) measurements were per-formed on a Field-Emission SEM (Hitachi, S-4300). Optical transmis-sion spectra of the sample substrates were recorded on a UV/Visspectrophotometer (Jasco, V570). The incident angle of light was 90°for all transmission spectral measurements. The XPS measurementsweremadewith SSX-100 (SSI). Amonochromatized Al KαX-ray beamwas used as an excitation source and X-ray photoelectron microscopy(XPS) data were acquired at a photoelectron takeoff angle of 35° withrespect to the normal to the sample surface. The vacuum pressure ofthe sample chamber was 4.5×10−7 Pa. All XPS binding energies werecalibrated using the binding energy of C1s (284.6 eV). The resistancevalues of the FTO and ANPTO/FTO multilayer film electrodes weremeasured by the four-point probe method (Mitsubishi ChemicalAnalytech Co. Ltd., Loresta-GP MCP-T610) under ambient conditions(25 °C). Electrochemical measurements were performed using apotentiostat (CH Instrument, model 1030A). A platinum wire andAg/AgCl electrode were used for the counter and reference electrodes,respectively. All electrochemical measurements were performed atroom temperature (25 °C).

3. Results and discussion

3.1. Characterization of ANPTO/FTO multilayer film

Fig. 2 shows typical scanning electron microscopy (SEM) imagesand schematic illustration of the ANPTO/FTO multilayer film. There

yer deposited on the FTO film through the TO island layer, and (d) the ANPTO layer after

Fig. 2. SEM images: (a) top view of the ANPTO surface with mesopores, (b) perspective view of the cleaved face of the ANPTO layer, (c) cross-sectional view of the ANPTO/FTOmultilayer film on a glass substrate. Scale bars correspond to 500 nm. (d) Schematic illustration of the ANPTO/FTO multilayer film.

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are densely packedmesopores with around 15 nm in pore diameter atthe top surface of the ANPTO layer (Fig. 2(a)). On the other hand,columnar-type pore channels with around 50 nm in diameter arerecognized inside the ANPTO layer (Fig. 2(b) and (c)). These SEMimages suggest that the ANPTO layer is formed by anodization asfollows: mesopores are initially grown from the top surface of the tinlayer and they are progressively merged with each other to formcolumnar-type pore channels as schematically shown in Fig. 2(d). Thethickness of the ANPTO layer after annealing was estimated by SEMimages and the thickness is about 3.5 times larger than that of theoriginal tin layer (Fig. 3). X-ray photoelectron spectrum of the ANPTOlayer in a binding energy range of 0 to 1100 eV indicates that theANPTO layer is composed of tin oxide without any detectablecontaminants from other elements (Fig. 4). The peak positions ofSn3d5/2, 3d3/2, and O1s are 486.5 eV, 494.9 eV, and 530.3 eV,respectively, which are almost the same as those observed for a tin

Fig. 3. Relationship between the initial thickness of the tin layer before anodization andthe thickness of the ANPTO layer.

oxide powder in the literature (486.2±0.2 eV, 494.6±0.3 eV, and530.1±0.2 eV) [10].

It should be noted that the direct vacuum deposition of a thick tinlayer on the FTO film resulted in an accidental exfoliation of the tinand/or ANPTO layer during anodization. The presence of the TO islandlayer on the FTO film ensures the strong adhesion of the additionallydeposited tin layer to the FTO surface and prevents the ANPTO layerfrom exfoliation during anodization. In the anodization of the tin layeron FTO film electrode with TO island layer, the potential applied to thetin layer was changed from 4.0 V to 6.0 V to prevent accidentalexfoliation and dissolution of the tin and/or ANPTO layer duringanodization. When the constant potential of 6.0 V was applied to thetin layer, the exfoliation of the tin and/or ANPTO layer was sometimeshappened at the beginning of the potential application. On the otherhand, when the anodizing of the tin layer was carried out at constantpotential of 4.0 V, the exfoliation was not happened and the tin layercould be anodized. However, anodizing at constant potential of 4.0 V

Fig. 4. XPS spectrum of the ANPTO layer.

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required long time until the tin layer was completely anodized, andthe long time anodization resulted in undesirable dissolution of theanodized tin oxide layer in the oxalic acid solution. Accordingly, twostep potential application (4.0 V to 6.0 V) described in experimentalsection was developed for successive fabrication of ANPTO/FTOmultilayer film electrode.

The ANPTO layer is optically transparent as shown in Fig. 5. The lighttransmittance of the ANPTO/FTO multilayer film is lower than that ofthe FTO film. This lower transmittance is ascribed to the roughness ofthe tin layer deposited on the FTO film as shown in Fig. 1(c). Depositionof a smooth tin layer would be able to improve the light transmittanceof the ANPTO/FTO multilayer film. Resistance values measured by theconventional four-point probe method were 1.1Ω and 1.3Ω for thebare FTO film electrode and the ANPTO/FTO multilayer film electrodewith 1200 nm in ANPTO thickness, respectively.

3.2. Redox reaction occurred inside the ANPTO layer

Electrochemical experiments using Fe(CN)63− as a redox probewere performed to confirm that the redox reaction occurred at theinner pore surface of the ANPTO layer. Fig. 6(a) shows cyclicvoltammograms (CVs) of the ANPTO/FTO multilayer film and theFTO film electrodes in 0.1 M KCl aqueous solution (pH 5.4) containing0.2 mM Fe(CN)63−, and a redox couple centered at 0.22 V is recognizedfor both electrodes. Since the peak currents of the redox couple areproportional to the square root of the scan rate, the redox reaction issuggested to be diffusion-controlled. On the other hand, an additionalredox couple appears at 0.54 V only for the ANPTO/FTO multilayerfilm electrode in an acidic acetate buffer solution (pH 2.7), as shown inFig. 6(b). Hereafter, the diffusion-controlled redox reaction (0.22 V)and the additional redox reaction (0.54 V) are denoted as redox I andredox II, respectively. The redox II appears only for an acidic solutioncontaining K3Fe(CN)6 and the peak current decreases with increasingthe pH value (Fig. 6(b) inset). The peak potential separation (ΔEp) ofthe redox II is small (3 mV for a potential scan rate of 200 mV s−1) asshown in Fig. 6(b) and the peak current is linearly proportional to thescan rate, which is expected for the surface-confined redox process.Accordingly, the redox II can be ascribed to an adsorbed species on theelectrode surface (discuss later).

Fig. 6(c) shows the plots of the cathodic peak current of the redoxII against the concentration of Fe(CN)63− in an acidic solution (pH 2.7)using two ANPTO/FTO film electrodes having different thicknesses(750 nm and 1200 nm) of the ANPTO layer. The cathodic peak currentincreases as the concentration of Fe(CN)63− increases for both

Fig. 5. Light transmission spectra of (a) FTO film electrode and (b, c) ANPTO/FTOmultilayer film electrodes: thicknesses of the ANPTO layer are (b) 750 nm and(c) 1200 nm. Inset shows photographs of the FTO film electrode and the ANPTO/FTOmultilayer film electrode (ANPTO thickness=1200 nm).

Fig. 6. CVs of FTO film (broken lines) and ANPTO/FTO multilayer film (solid lines)electrodes observed in (a) 0.1 M KCl aqueous solution (pH 5.4) containing 0.2 mM K3Fe(CN)6 and (b) in 0.1 M KCl and 0.1 M acetate buffer aqueous solution (pH 2.7)containing 0.2 mM K3Fe(CN)6. Inset shows plots of the oxidation peak current at 0.54 Vagainst the solution pH value. Potential scan rate for (a) and (b) is 200 mV s−1. (c) Plotsof the reduction peak current at 0.54 V against the concentration of Fe(CN)63−: theANPTO layer thicknesses are 750 nm (open circle) and 1200 nm (closed circle). SEMimages of the ANPTO/FTO multilayer films are also shown.

electrodes, indicating that the adsorbed species for the redox II isconcerned with Fe(CN)63−. It should be emphasized that the increasein the cathodic peak current is remarkable for the electrode with athick ANPTO layer (1200 nm), indicating that the amount of adsorbed

Fig. 8. CVs of the FTO film electrode (broken line) and ANPTO/FTO multilayer filmelectrode (solid line) observed in 0.1 M KCl aqueous solution (pH 5.4) containing0.2 mM Ru(bpy)3Cl2. Scan rate is 100 mV s−1.

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species is larger for the ANPTO/FTO film electrode with a thick ANPTOlayer (1200 nm) than the one with a thin ANPTO layer (750 nm).From this result, it is evident that the redox II takes places mainly atthe inner pore surface of the ANPTO layer and that the ANPTO layerhas enough electrical conductivity for using it as a working electrode.Therefore, we can conclude that the ANPTO layer on the FTO layer canwork as an electrode with high surface area due to the large internalsurface area in the porous ANPTO layer.

We discuss the redox II reaction occurred inside the pore channelsof the ANPTO layer. As shown in Fig. 6(b) inset, peak current of theredox II increases as the pH value decreases, indicating that theadsorbed species for the redox II reaction are formed in acidicsolutions. In an acidic solution with K3Fe(CN)6, it is well known thatthe interaction of FeII/III(CN)64−/3− with FeII/III, which is produced bydecomposition of FeII/III(CN)64−/3−, results in the formation of ahexacyanoferrate (mostly water-insoluble Prussian blue (PB)) layer[11–13]. Hence, the possible explanation for the redox II might beredox reaction of PB analogues adsorbed at the inner pore surface ofthe ANPTO layer. To clarify the chemical species concerned with theredox II, further investigation is required and it would be helpful touse several spectroscopic techniques, such as XPS, X-ray diffraction,and Mössbauer spectroscopies [13]. Although the details of the redoxII cannot be fully clarified, the appearance of the redox II peaksupports the usefulness of the ANPTO layer for an electrode.

3.3. Electrochemical potential window

The electrode performance of the ANPTO/FTO multilayer filmelectrode was further examined in a 0.1 M KCl aqueous solution. Asshown in Fig. 7, significant increase in the anodic and cathodiccurrents is recognized above 1.3 V and below−0.3 V, respectively, forthe FTO film electrode. Similar CV is obtained for the ITO filmelectrode. The increase in the anodic current can be ascribed tooxygen evolution, and the increase in the cathodic current, tohydrogen evolution and/or reduction of the electrode surface to themetallic state [14]. On the other hand, the ANPTO/FTO multilayer filmelectrode exhibits increases in the anodic and cathodic currents above2.0 V and below −0.1 V, respectively. These results indicate that theelectrochemical potential window of the present ANPTO/FTO multi-layer film electrode is shifted toward a positive potential region andbecomes to be wide compared to usual transparent metal oxide filmelectrodes such as FTO and ITO film electrodes.

The CV measurement using Ru(bpy)32+ as a redox probe clearlyshowed the advantage of the ANPTO/FTO multilayer film electrodewith thewide anodic potentialwindow. Fig. 8 shows CVs of Ru(bpy)32+

obtained for the ANPTO/FTOmultilayerfilm electrode and the FTOfilm

Fig. 7. CVs of ITO film, FTO film, and ANPTO/FTO multilayer film electrodes in 0.1 M KClaqueous solution (pH 5.4). Scan rate is 5 mV s−1.

electrode. For the FTO film electrode, the oxidation peak of Ru(bpy)32+

is completely overlapped with the current of the oxidation evolution.In contrast, the use of the ANPTO/FTO multilayer film as a workingelectrode allows observation of a clear oxidation peak of Ru(bpy)32+

with less influence of the oxidation evolution. These results confirmthe availability of the ANPTO/FTO multilayer film electrode for theelectrochemical reaction at a high anodic potential.

In the present study, the mechanism for the wide electrochemicalpotential window found for the ANPTO/FTO multilayer film electrodeis not clarified. Having in mind recent carbon electrodes (boron-doped diamond electrode [15,16] and electron cyclotron resonancenanocarbon electrode [17]) with wide electrochemical potentialwindow, one considerable reason for the wide electrochemicalwindow of the ANPTO/FTO multilayer film electrode might be highoverpotentials for the chemical species due to the inertness of theinner pore surface for adsorption. Additionally, specific solvent andsolute properties, such as high viscosity [18,19], slow diffusivity[20,21], clustering [22], and proton dissociation behavior [23], insideinorganic nanopores would be also related to the electrochemicalpotential window for the ANPTO/FTO multilayer film electrode. Themechanism for the wide electrochemical potential window is animportant subject to be clarified for further development of theANPTO/FTO multilayer film electrode.

4. Conclusion

Anodization of a tin film deposited on an FTO film electrode wasapplied to fabrication of an anodized nanoporous tin oxide (ANPTO)layer. The resulting ANPTO/FTO multilayer film was opticallytransparent and had columnar-type pore channels. The electrochem-ical measurements clarified two important features for the use ofANPTO/FTO multilayer film as an electrode. One is that the redoxreaction takes place at the inner surface of the ANPTO layer. The otheris that the ANPTO/FTO multilayer film has wider anodic potentialwindow compared to usual transparent metal oxide film electrodessuch as FTO and ITO film electrodes. These results conclude theavailability of the ANPTO/FTO multilayer film as a transparent filmelectrode with large internal surface area and wide anodic potentialwindow. The ANPTO/FTOmultilayer film electrode would be used as atransparent film electrode for various photoelectrochemical andelectrocatalytic systems.

Acknowledgements

This work was supported in part by Grants-in-Aid for ScientificResearch (No. 21685009) from the Ministry of Education, Culture,

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Sports, Science and Technology, Japan. We acknowledge Dr. ShigeoSato and Mr. Hirokazu Hirai, Institute for Materials Research(IMRAM), Tohoku University, for the XPS measurements.

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