Electrochemistry Communications 17 (2012) 10–13

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Electrochemistry Communications

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Photoelectrochemical anodization for the preparation of a thick tungsten oxide film

Sunmi Kim, Jinsub Choi ⁎Department of Chemical Engineering, Inha University, Incheon, Republic of Korea 402–751

⁎ Corresponding author. Tel.: +82 32 860 7476; fax:E-mail address: jinsub@inha.ac.kr (J. Choi).

1388-2481/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.elecom.2012.01.006

a b s t r a c t

a r t i c l e i n f o

Article history:Received 28 September 2011Received in revised form 3 January 2012Accepted 6 January 2012Available online 12 January 2012

Keywords:AnodizationTungsten oxidePhotoelectrochemistry

Nanoporous WO3 with a thickness of about 2.6 μm was achieved by photoelectrochemical anodization; thisthickness was around twice the thickness possible with normal (photo-absent) anodic film. The thickness in-crease is ascribed to the much formation of a protective crystalline oxide on the surface and acceleration ofdissolution at the tip by holes generated during illumination. The nanoporous WO3 film prepared by photo-electrochemical anodization shows photocurrents of 1.85 mA/cm2 in 0.33 M H3PO4 under AM 1.5 illumina-tion, a value nearly four times higher than that for film prepared with normal anodization.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Tungsten (VI) oxide (WO3) has been intensively studied in a widerange of technological applications such as electrochromic devices[1,2], rechargeable Li-ion batteries [3], and so on due to its uniquephysical and chemical properties. Especially, combining a good chem-ical stability against photocorrosion in acidic aqueous solutions with amoderate bandgap, tungsten oxide is an attractive and promisingcandidate electrode for photoelectrochemical applications [4–6].

Several techniques, including CVD (chemical vapor deposition)[7], evaporation [8,9], sol–gel coating [10,11] and anodization[12–15] have been studied as ways of depositing nanostructuredWO3 films. Among them, anodization is one good candidate methodbecause it can provide uniform layers on large-area substrates atvery low cost. For example, nanoporousWO3 thin films were fabricat-ed by galvanostatic anodization in oxalic acid [15]. Schmuki et al. pre-pared self-organized nanoporous WO3 films with an average porediameter of 70–90 nm by anodization in a electrolyte containing neu-tral F− [16]. Since resultant WO3 is an n-type semiconductive materi-al [17,18], the final morphology, oxide structure and thickness will begreatly influenced by external photo-energy during anodization. Upto now, the photoelectrochemical anodization (PA) has been mainlyapplied to semiconductive materials such as Si, InP, GaAs and so onfor the production of nano(or micro) porous channels [19–22].

In this work, we demonstrate that PA of a tungsten metal can pro-duce thicker tungsten oxide film, which can be used as electrode witha better photocatalytic activity, compared to that prepared by normalanodization (NA). The mechanism for the increase in thickness due toPA is described in detail.

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2. Experimental

AW foil with a thickness of 0.3 mm (99.95%, Goodfellow, UK) wasused as the starting substrate for the fabrication of nanoporous WO3.The anodization was carried out at a constant potential of 50 V for dif-ferent durations at 95 °C with or without exposure to a light source (a150 W Xe lamp (LS-150-Xe, ABET TECHNOLOGIES)). An anhydrouselectrolyte of 10 wt.% K2HPO4/glycerol was used as the electrolyte[23]. Prior to anodization, the electrolyte was heated at 180 °C for3 h to reduce the water content; solution was subsequently cooledto 95 °C. The anodization setup consisted of a conventional two-electrode configuration with a platinum mesh as the counter elec-trode and the degreased tungsten foil as the working electrode,using a power supply (SourceMeter 2400, Keithley) interfaced witha computer.

A field emission scanning electron microscope (FE-SEM, 4300S,Hitachi, Japan) and a high resolution transmission electron micro-scope (HR-TEM, JEM-2100F, JEOL, Japan) were employed for thestructural and morphological characterization. The samples for thecross-sectional images of TEM analysis were prepared with a focusedion beam (FIB, XVision200DB) [24]. The crystallinity of the sampleswas characterized by X-ray diffraction (XRD, Rigaku D/max-RB,Japan) using graphite monochromatized Cu Ka radiation(l=0.1542 nm).

For the characterization of photoactivity, the anodized sampleswere annealed at 400 °C for 5 h and subsequently, they were charac-terized in 0.33 M H3PO4 using a potentiostat (AUTOLAB). The three-electrode setup consisted of a platinum counter electrode, WO3

films as the working electrode and an Ag/AgCl 3 M KCl reference elec-trode. After 60 s equilibration at open circuit potential, sweep poten-tials in the range of 0.2 V to 3.5 V (vs. Ag/AgCl 3 M KCl) were appliedwith illumination generated by the AM 1.5 (global) solar simulator(300 W, 100 mW/cm2, XES-301S, SAN-El ELECTRIC, Japan).

Fig. 1. (a) Thickness variations of anodic WO3 at different anodizing times. A and B indicate the film prepared by normal anodization (NA) and photoelectrochemical anodization(PA), respectively. Cross-sectional TEM images of nanoporous WO3 prepared for 20 h by (b) NA and (c) PA. (A: outer layer, B: middle layer, C: inner layer). The growth direction isindicated by the arrow. ‘m’ of (b) and (c) indicates the length of mouth of pores. Enlarged views of A, B, and C (or B+C) layers are shown on the bottom, respectively.

11S. Kim, J. Choi / Electrochemistry Communications 17 (2012) 10–13

(a) (b)

(c)

Fig. 2. (a) Current-time transient during the fabrication of anodic WO3. The insets show the magnified spectra. A and A′ indicate the time at minimum current density during NA andPA, respectively. B and B′ indicate the time at maximum current density during NA and PA, respectively. (b) XRD patterns of WO3 films prepared by NA and PA for different times.Black and gray lines are XRD results for NA and PA, respectively. Black circle, diamond and star are W (110), WO3 (110), andWO3 (020), respectively. (c) photocurrent performanceof WO3 films prepared for 20 h by (A) NA and (B) PA, respectively.

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3. Results and discussion

The anodic formation of tungsten oxide in non-aqueous electro-lyte can be described to following equation.

Wþ 3OH−→WO3 þ 3Hþ þ 3e−

where OH− can be provided from rest H2O.Fig. 1(a) provides the thickness of the porous layer as a function of

anodizing time. In the case of NA, the average thickness of the anodicWO3 film increases linearly with a proportionality constant of0.05 μm/h as a function of anodization time (A of Fig. 1(a)). However,B of Fig. 1(a) shows that the thickness of the samples prepared by PAfollows a “sigmoidal pattern”. The total thickness of the films pre-pared by PA is much higher than that prepared by NA. Interestingly,the thickness of NA films is slightly greater than that of PA films at ini-tial periods (e.g., 1 h and 3 h), whereas thicker films are formed by PAafter 4 h. Nearly two times higher thickness is achieved at 20 h by PA.Fig. 1(b) and (c) provide cross-sectional TEM images of the samplesprepared by NA and PA for 20 h, respectively. These layers can be di-vided to three different regions according to its morphology: nano-porous structure (outer layer, A), intermediate-nanoporousstructure consisting of wall pores with a size of a few hundred nano-meters perpendicular to the growth direction (middle layer, B) andcolumnar structure (inner layer, C). In Fig. 1(b), the thickness ofeach layer is 0.5, 0.5 and 0.4 μm. In comparison with (B) and (C)layer, the outer layer (A), which is formed first, dissolves much

more during anodization due to its longer exposure to the electrolyte.In the middle layer (B), relatively larger wall pores perpendicular tothe growth direction is observed, indicating that not only the tips ofthe pores but also their walls are dissolved during the growth. Thisis probably due to the fact that the oxide formed in the middle layerhas insufficient chemical stability due to a longer exposure to theelectrolyte. In the inner layer (c), the columnar layer is formed sincethere is less chemical dissolution due to diffusion problems of chan-nels and shorter exposure time.

On the other hand, the WO3 layer produced via PA (Fig. 1(c)) con-sists of a top layer of 0.3 μm and a middle and inner layer of 1.7 μm;these layers have unclear boundary, showing evidence of larger co-lumnar layer with relatively smaller wall pores, compared to that pre-pared NA. We suppose that PA helps formation of the crystallinephase of the outer layer in the film. Since crystalline oxide has greaterchemical stability that does the amorphous phase, the pore nucle-ation by etching crystalline oxide is relatively slow [25]. This leadsto less growth rate of porous oxide at the initial stage. This is invery good agreement with the observation that initial thickness ofPA films is less than that of NA films (Fig. 1(a)). In fact, much greaterdissolution in NA is observed in the outer layer (A) of TEM images.Note that larger mouth of pores is observed in the outer layer (A) ofFig. 1(b) compared to that of Fig. 1(c) (see the denotation of “m”).

Since the crystalline outer layer does not dissolvewell during PA, theapplied current can be used to form the columnar film underneath theouter layer, resulting in thicker total oxide thickness. However, duringthe long anodization, the formed columnar film can be chemically

13S. Kim, J. Choi / Electrochemistry Communications 17 (2012) 10–13

dissolved, still resulting in the formation of wall pores. In addition, theholes generated from the illuminationwill probably promote the disso-lution of oxide at the tips, as happened in electrochemical n-typemacroporous silicon formation with illumination [20–22].

Fig. 2(a) provides a comparison of current density–time curves dur-ing the anodization of tungsten with and without illumination. Notethat the decrease in the initial current density down to a minimumvalue (A and A′, in the inset of Fig. 2(a)) is ascribed to the formationof barrier-type oxide on the whole surface and the successive increasein current density up to the maximum value (B and B′, in the inset ofFig. 2(a)) is due to the formation of pores. The current density decreasedslightly after the maximum current density due to the stoppage ofgrowth of over-nucleated pores [26]. From the time of marked B andB' in Fig. 2(a), we can assume that pore nucleation in PA is delayed;this result accords with the results shown in Fig. 1. Overall, a highertotal current density is observed during PA compared to that in NA, im-plying that the dissolution rate at the tip is greater in the case of PA,compared to that for NA.

Fig. 2(b) shows XRD patterns of theWO3 layer prepared for 1 h, 6 hand 20 h by NA and PA. For NA samples, a peak from metallic tungstensubstrate and a peakwith veryweak intensity at 2θ=16.8°, which is as-cribed to crystalline WO3 (110), are detected (see 1 h and 6 h ofFig. 2(b)). Whereas, more strong peak at 2θ=16.8° is detected for thePAWO3 film and the peak intensity increases from 1 h to 6 h. These dif-ferent intensities at 2θ=16.8° indirectly reflect that the degree of for-mation of crystalline WO3 (110), which probably works as aprotective layer during anodization, is strongly influenced by photo-energy.

For films grown for 20 h, both NA and PA films have a broad dif-fraction peak at 2θ=23.5°, which is attributed to the WO3 (020),and the WO3 (110) peak disappears, meaning that the initial oxidewith (110) phase is finally dissolved during long anodization and sub-sequently new crystalline structure with a phase of (020) are formed.

In a photoelectrochemical cell, the photocurrent of a photoanodeindirectly indicates the photocatalytic activity [27]. Fig. 2(c) showsthe practical photocurrent spectra vs. applied voltage curves. Thephotocurrent density dramatically increases as the bias potential in-creases. For the PA WO3 films with a thickness of 2.6 μm preparedfor 20 h, the photocurrent density at 1.5 V (vs. Ag/AgCl 3 M KCl)was measured at 1.85 mA/cm2 (B of Fig. 2(c)), which is almost fourtimes higher than that for the NA films with a thickness of 1.33 μm(0.5 mA/cm2, A of Fig. 2(c)) prepared for 20 h.

4. Conclusions

The effects of PA on the formation of anodic nanoporous WO3

were studied in detail. We found that the thickness of WO3 can be ex-tended to more than 2.6 μm, which is double the thickness that can beobtained by NA. The mechanism for the production of PA film with agreater thickness was in part attributed to higher degree of formation

of crystalline outer oxide layers and was in part ascribed to the accel-eration of dissolution at the tip due to holes generated byillumination.

PA films show very high photoelectrochemical activity due to theirthickness. We expect that PA WO3 photoanodes with crystalline sur-face and thicker films can be integrated into water splitting devices.

Acknowledgement

This work was supported by the Regional Innovation Center forEnvironmental Technology of Thermal Plasma (ETTP) at Inha Univer-sity designated by MKE (2011).

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