Photoelectrochemical and structural characterization of carbon-doped WO3 films prepared via spray pyrolysis

international journal of hydrogen energy 34 (2009) 8476–8484

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Photoelectrochemical and structural characterization of carbon-doped WO3 films prepared via spray pyrolysis Yanping Sun, Carl J. Murphy, Karla R. Reyes-Gil, Enrique A. Reyes-Garcia 1, Jason M. Thornton, Nathan A. Morris, Daniel Raftery* Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN 47907, USA

article info

abstract

Article history:

Carbon-doped tungsten trioxide (WO3) films were produced using a spray-pyrolysis

Received 7 May 2009

methodology, with glucose used as the carbon dopant source. The films were characterized

Received in revised form

by X-ray diffraction, X-ray photoelectron spectroscopy, UV–vis, scanning electron

30 July 2009

microscopy, and solid-state nuclear magnetic resonance. The photoelectrochemical

Accepted 6 August 2009

activity was evaluated under near UV–visible light and visible light only irradiation

Available online 9 September 2009

conditions. The presence of carbonate-type species in the C-doped sample was confirmed

Keywords:

1.6 mA/cm2 in 1 M HCl electrolyte and as high as 2.6 mA/cm2 with the addition of methanol

WO3

as a sacrificial agent. A high contribution (w50%) of the photocurrent density was observed

by XPS and SSNMR. The C-doped WO3 electrodes exhibited photocurrent densities up to

Photocatalysis

from visible light. C-doped WO3 produced approximately 50% enhanced photocurrent

Solar hydrogen production

densities compared with the undoped WO3 electrode synthesized using the same proce-

Water splitting

dures. The photoelectrochemical performance was optimized with respect to several

Anion doping

synthetic parameters, including dopant concentration, calcination temperature and film

NMR

thickness. These results indicate the potential for further development of WO3 photocatalysts by simple wet chemical methods, and provide useful information towards understanding the structure and enhanced photoelectrochemical properties of these materials. ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Tungsten trioxide (WO3) is an n-type semiconductor with an excellent electrochromic, photochromic, and gasochromic properties, and has been extensively used in a variety of applications, including gas and temperature sensing, catalysis, electrochromic windows and displays, and flat panel displays [1–7]. WO3 has been investigated as a photoanode for photoelectrochemical water-splitting systems since the mid1970s [8] due to its stability in acid, good transport properties, its potential to absorb a reasonable fraction of the solar

spectrum, and an ability to generate sizable photcocurrents [9,10]. WO3 photoanodes have also been used as top layer junctions in multi-junction devices designed to split water [11,12]. Because of its relatively large band gap (Eg: 2.6w3.0 eV) [9], WO3 mainly absorbs in the near ultraviolet and blue regions of the solar spectrum. To extend its absorption to the longer wavelengths and therefore increase its photoelectrochemical performance, the band gap of WO3 needs to be reduced. Doping anions into photoactive metal oxides for the purpose of improving photoresponse was proposed in 2001 [13], and in

* Corresponding author. Tel.: þ1 765 494 6070; fax: þ1 765 494 0239. E-mail address: [email protected] (D. Raftery). 1 Present address: Halliburton Energy Services, Duncan, OK, USA. 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.08.015

international journal of hydrogen energy 34 (2009) 8476–8484

particular, anion-doped TiO2 materials have been widely studied in recent years [14–24]. Among different doping anions, nitrogen doping and carbon doping have been proven to be the most promising dopants for TiO2 material. Recently, this approach has been extended to In2O3 [25,26]. Preliminary work on N-doped WO3 shows that, similar to TiO2, nitrogen doping is an effective way to reduce the band gap of WO3 [9,27]. Paluselli et al. reported that N doping narrowed the WO3 band gap from 3.0 eV to 2.2 eV [27], and Cole et al. reported that N doping narrowed the WO3 band gap from 2.5 eV to 1.9 eV [9]. However, the photoelectrochemical performance of N-doped WO3 films has not been promising. Cole et al. reported that the photocurrent density of the N-doped WO3 is only 25% of the undoped WO3 electrode, and they claimed that this poor photocurrent for the nitrogen doped sample is attributed to a degradation of the electron transport properties as the result of a highly defective lattice [9]. To our knowledge, there is no report related to photoelectrochemical evaluation or water splitting based on C-doped WO3. In this study, we describe the effect of C-doping on the photoelectrochemical properties of WO3 thin films prepared by spray pyrolysis. Spray pyrolysis offers an inexpensive and simple method to introduce carbon into the WO3 lattice by merely adding a carbon source (glucose in this case) to the spray precursor solutions. The deposition rate and the thickness of the films can be easily controlled over a wide range by changing the spray parameters, thus eliminating a major drawback of chemical methods such as sol–gel which produces films of limited thickness. In this paper, the structural, optical, morphologic and photoelectrochemical properties of an efficient C-doped WO3 as well as undoped WO3 materials were evaluated and show the benefits of C-doping in these materials that have promise for water splitting applications.

2.

Experimental method

2.1.

Preparation of C-doped and undoped WO3 electrodes

C-doped WO3 films were synthesized by a spray-pyrolysis route from an aqueous precursor containing acetylated peroxotungstic acid electrolyte. The spray precursor was prepared as follows: 8.5 g of tungsten powder (Aldrich) was dissolved in a mixture of 81 mL 30% hydrogen peroxide (Mallinckrodt) and 8 mL deionized water to yield a colorless solution of peroxotungstic acid. Since the reaction is strongly exothermic, an ice bath was employed. The solution was refluxed at 55  C for 24 h after addition of 81 mL glacial acetic acid (Mallinckrodt). The solution was then evaporated at room temperature to obtain a yellow flaky solid of acetylated peroxotungstic acid. 1.84 g of this material was dissolved in 15 mL ethanol (Mallinckrodt) while heating at 50  C. An aqueous glucose (Sigma) solution (4.23 M) was added to the above solution to provide the C dopant source. The aqueous solution was used to make the spray precursor solution since the solubility of glucose in ethanol is very low. The molar concentration of glucose in the precursor solution used to make all the materials in this paper was 0.07 M, except for the concentration dependent study.

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Glass dishes and conductive fluorine-tin oxide (FTO) sheets were used separately as the substrates for the spray-pyrolysis deposition. Thin films prepared on glass dishes were scraped and collected after calcination for powder X-ray diffraction (XRD), UV–vis and solid-state nuclear magnetic resonance (SSNMR) measurements, while films prepared on FTO sheets were used to prepare electrodes for photoelectrochemical analysis. When preparing samples for SSNMR, uniformly 13 C-labeled glucose was used as the carbon dopant. An area of w1.0 cm2 of the FTO surface was exposed to the spray solution. A portion of the FTO was covered with aluminum foil to avoid spray deposition and to prepare an electrical connection. The typical spray-pyrolysis method was carried out by placing the FTO substrates on a hotplate set to 150  C and spraying the solution for 25 min. The resulting films were then calcinated in air at 600  C for 2 h. The undoped WO3 film used as reference electrode was synthesized in exactly the same manner but in the absence of glucose. The electrical contact for all of these films during the photoelectrochemical measurement was made by connecting the uncoated area of FTO with a copper wire using silver epoxy, and the metallic contact was then covered by epoxy resin to isolate it from the HCl electrolyte solution.

2.2.

Characterization

XRD patterns of the prepared powders were measured on a Siemens D500 diffractometer employing Cu Ka radiation (0.1540 nm). Diffraction patterns were recorded in the range of 20 2q  80 with a step width of 0.02 2q s1. UV–vis measurements were made using a Cary Bio300 Varian spectrophotometer in diffuse reflectance mode and employing BaSO4 as a background for the analysis of the powder. The thickness and morphology of the films were characterized using cross-sectional and surface scanning electron microscopy (SEM) images, respectively. X-ray photoelectron spectra (XPS) were collected using a Kratos Axis Ultra X-ray photoelectron spectrometer employing monochromatic Al Ka excitation. Spectral processing was conducted with the CASA XPS software [28], and binding energies were calibrated with respect to the residual C (1s) peak at 284.6 eV. 13 C magic angle spinning (MAS) NMR spectra were acquired using a wide-bore Chemagnetics Varian Infinity 400 MHz spectrometer at 9.4 T and employing a 5 mm HXY T3 probe under magic angle spinning at 8 kHz. Single pulse excitation with decoupling and phase cycling experiments were calibrated with glycine and employed 90 13C pulses of 4 ms in duration. During the experiment, a pulse delay of 6 s was used, 2048 data points were acquired, and 512 scans were averaged. The spectrum was referenced to the isotropic peak of the carbonyl carbon of glycine (176.4 ppm). Photoelectrochemical measurements were carried out in a three-electrode cell equipped with an optical quality quartz window. 1 M HCl was applied as the electrolyte, a Ag/AgCl/KCl (sat.) electrode was utilized as the reference electrode, and a platinum foil was used as the counter electrode. Potentiostatic control was maintained with a Bioanalytical Systems CV-27 potentiostat. A 300 W xenon arc lamp was employed as the light source with an intensity of 0.13 W/cm2 at the

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international journal of hydrogen energy 34 (2009) 8476–8484

3.

Results and discussion

3.1.

XRD

Table 1 – Glucose concentration, calcination temperature, particle size, and photocurrent density under near UV– visible light irradiation for C-doped WO3 samples.

0 0.04 0.07 0.10 0.13 0.07 0.07 0.07

Glucose: 0.13 M

Glucose: 0.10 M

Glucose: 0.07 M Glucose: 0.04 M

Calcination temperature ( C)

Particle size (nm)a

Photocurrent densityb (mA/cm2)

600 600 600 600 600 500 550 650

16 18 20 27 32 32 27 12

1.0 1.5 1.6 1.3 1.1 1.5 1.6 1.3

a Based on (200) XRD peak (2q ¼ 24.4 ). b Measured at 1.8 V.

Glucose: 0 M

20

30

40

50

60

70

80

2θ (º)

b Calcination temp.: 650 ºC

Intensity (a.u.)

C-doped WO3 powders prepared with different glucose concentrations and over a range of calcination temperatures, as outlined in Table 1, were subject to X-ray diffraction measurements. All the annealed C-doped and undoped WO3 samples displayed a monoclinic, polycrystalline WO3 structure with a main peak at 24.4 corresponding to the (200) plane. No additional peaks were detected at the sensitivity limit of the instrument, indicating that C-doping did not change the crystalline structure of the WO3 films. Fig. 1a shows the XRD patterns for the C-doped WO3 with different glucose concentrations in the spray precursor solutions. The average particle size calculated from the Scherrer equation based on the (200) peak was found to be 16 nm for the undoped WO3 sample. At increased precursor glucose concentrations, the average particle size increased from 18 nm using 0.04 M glucose to 32 nm using 0.13 M glucose. Fig. 1b shows the XRD patterns for C-doped WO3 prepared using a 0.07 M precursor glucose concentration and different calcination temperatures. The average particle size calculated from the Scherrer equation based on the (200) peak was found to be 32 nm when calcinated at 500  C, 27 nm at 550  C, 20 nm at 600  C, and 12 nm at 650  C. Therefore, the particle size increased with increasing precursor glucose concentration, but decreased at increased calcination temperatures. In contrast, Cole et al. [9] found that the particle size decreased with increased nitrogen partial pressure used in N-doped WO3 samples prepared by RF magnetron sputtering, and Yang et al. [30] found that the particle size increased with increased calcination temperature in their WO3 samples prepared by a sol–gel technique. Differences in these results might be attributed to the different synthetic method used in our study.

Glucose concentration (M)

a Intensity (a.u.)

electrode surface to simulate the reported total solar irradiance of 0.1366 W/cm2 [29]. A 387 or 400 nm cutoff filter was placed into the path of the xenon arc lamp to remove the UV irradiation, and a water filter was used to remove the IR energy and avoid overheating.

Calcination temp.: 600 ºC Calcination temp.: 550 ºC

Calcination temp.: 500 ºC

20

30

40

50

60

70

80

2θ (º)

Fig. 1 – X-ray diffraction patterns for the (a) C-doped WO3 samples with different precursor glucose concentrations (calcination temperature: 600 8C), and (b) C-doped WO3 samples at different calcination temperatures (glucose concentration: 0.07 M).

3.2.

UV–vis spectroscopy

The UV–vis absorption spectra of the C-doped and undoped WO3 powders are shown in Fig. 2a. Compared to the undoped sample, the C-doped WO3 absorptions are red-shifted in the optical response, and show no separate absorption peaks, which indicates a reduction of the band gap. UV–vis spectra of the C-doped WO3 powders with different glucose spray solution concentrations indicated that the sample synthesized with a 0.07 M glucose concentration exhibits the most visible absorption. Assuming the materials are indirect semiconductors, a plot of the modified Kubelka–Munk function versus the energy of exciting light [2,18] for the undoped and C-doped WO3 powders are shown in Fig. 2b, affording band gap energies of 2.62 and 2.57 eV for the undoped and C-doped WO3 ([glucose] ¼ 0.07 M), respectively. This band gap reduction can be attributed to the doping of C into the WO3 lattice. Compared with our synthesized C-doped WO3 materials, the band gap narrowing of N-doped WO3 prepared by Cole et al. is much larger; they reported band gap reductions up to 0.6 eV for their samples [9]. Different anion doping and synthetic methods appear to narrow the band gap of WO3 by different levels.

international journal of hydrogen energy 34 (2009) 8476–8484

a

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1.0

(A) 0.8

(B)

Abs (a.u.)

(C) 0.6

(D) (E)

0.4

0.2

0.0 350

400

450

500

550

Wavelength (nm)

b

3.5

(A) 3.0

(B)

[F(R)E]1/2

2.5

(C) (D)

2.0 1.5

(E)

1.0 0.5 0.0 1.9

2.3

2.7

3.1

3.5

E (eV)

Fig. 2 – (a) UV–vis diffuse reflectance spectra and (b) transformed diffuse reflectance spectra of the C-doped and undoped WO3 powders with different glucose spray solution concentrations: (A) [glucose] [ 0.07 M, (B) [glucose] [ 0.04 M, (C) [glucose] [ 0.10 M, (D) [glucose] [ 0.13 M, and (E) [glucose] [ 0.00 M.

3.3.

SEM

The surface micrographs of the C-doped and undoped WO3 films (Fig. 3a and b) show cracked morphologies that are likely due to the combustion and pyrolysis of excess glucose and shrinkage of the films during the calcination process. Similar SEM images were observed in C-doped TiO2 and C-doped In2O3 films by spray pyrolysis using glucose as the dopant source [26,30,31]. Cross-sectional analysis (Fig. 3c) shows that the films are approximately 2.5 mm thick for a 1 min spray time and about 50 mm for 25 min of spray time.

3.4.

XPS

Fig. 4 shows XPS high-resolution spectra of C 1s, W 4f, and O 1s core levels for the C-doped and undoped WO3 powders, and Table 2 summarizes corresponding binding energies. In the C 1s spectra, binding energies of 284.6 eV and 285.7 eV were observed for the undoped WO3 sample, while binding energies of 284.6 eV and 285.9 eV were observed for the C-doped sample. These peaks are due to the presence of adventitious elemental carbon which is an unavoidable presence on all airexposed materials. The peak at 289.9 eV was only found in the

Fig. 3 – (a) SEM surface image of C-doped WO3 film, (b) SEM surface image of undoped WO3 film, and (c) cross-sectional image of C-doped WO3 film.

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international journal of hydrogen energy 34 (2009) 8476–8484

2600

Table 2 – Binding energies for C 1s, W 4f, and O 1s core levels in C-doped and undoped WO3 powders.

a 2400

CPS

C 1s (eV)

(A)

2200

Undoped WO3 284.6, 285.7 C-doped WO3 284.6, 285.9, 288.9

2000

W 4f7/2 W 4f5/2 (eV) (eV) 35.8 35.7

38.0 37.8

O 1s (eV) 530.4, 531.1 530.4, 531.2

1800

(B) 1600 1400 297

295

293

291

289

287

285

283

281

279

Binding Energy (eV)

C 1s core level 12000 10000

b (A)

CPS

8000 6000

The binding energies for the W 4f7/2 peak were observed at 35.8 eV for the undoped sample and 35.7 eV for the C-doped sample, while the Wf5/2 peak were observed at 38.0 eV for the undoped sample and 37.8 eV for the C-doped sample. The O 1s spectra show two components. The peak at 530.4 eV for both undoped and C-doped samples is assigned to the O2 1s ion corresponding to WO3, and the peak at 531.1 eV for the undoped sample and at 531.2 eV for the C-doped sample are assigned to hydroxyl groups or water bonded on the film surface [37]. All these binding energies were in agreement with the literature values for WO3 [37,38]. Based on the fact that no obvious binding energy shifts in W 4f or O 1s peaks were observed due to the C doping, we believe that the C is located at interstitial sites.

4000

3.5.

(B)

2000 0 44

42

40

38

36

34

32

30

Binding Energy (eV)

W 4f core level 15000

c CPS

12000

(A)

9000

6000

(B)

3000

0 535

532

529

526

Binding energy (eV)

O 1s core level Fig. 4 – XPS high-resolution spectra for the (a) C 1s, (b) W 4f, and (c) O 1s core levels. The individual spectra correspond to (A) C-doped WO3 powder and (B) synthesized undoped WO3 powder.

C-doped WO3 which corresponds to a carbonate species. This result is in a good agreement with the C-doped TiO2, C-doped In2O3, C-doped SiO2, and C-doped SrTiO3 materials which contain carbonate species [26,32–34]. No C peak is visible around 281 eV, suggesting carbide species were not found in our C-doped sample [35,36]. The carbon content is 1.1%, based on the 288.9 eV peak.

SSNMR

The 13C SSNMR spectrum for the C-doped WO3 sample synthesized using uniformly 13C-labeled glucose as the dopant is displayed in Fig. 5(a). The spectrum shows two high intensity peaks and two low intensity peaks. The high intensity peak near 111 ppm is attributed to carbon in the sample rotor and/or the probe material, and can be ignored in the analysis. The narrow peak at 124 ppm is assigned to a O]C]O structure, which probably results from CO2 gas either adsorbed on the sample surface, or more likely trapped within the micro crystalline material. The low intensity peak observed at 161 ppm is assigned to an (O2) C]O structure, and indicates the presence of a carbonate-type species [26,39,40] which is consistent with the XPS analysis. To the best of our knowledge, the low intensity peak near 189 ppm is likely related to a C]O structure. Additional NMR experiments are needed to identify this peak. The NMR data was collected again 1 year later to examine the same C-doped WO3 sample (Fig. 5(b)). Very similar peak positions and intensities were observed, indicating that the structure of the sample does not change in a year, including the narrow CO2 peak.

3.6.

Photoelectrochemical analysis

The plot of photocurrent density as a function of the applied potential for the C-doped and undoped WO3 electrodes is shown in Fig. 6. The samples were evaluated under near UV–visible light and visible light only irradiation conditions, separately. As shown in Fig. 6, under near UV–visible light irradiation and without any sacrificial reagents, the photocurrent density increased from 1.1 mA/cm2 for the undoped electrode to 1.6 mA/cm2 for the C-doped electrode at 1.8 V (data labeled as Fig. 6(A) and (B)). Both 378 nm and 400 nm cutoff filters were used to measure the photocurrent density under visible light irradiation. As shown in Fig. 6(C)–(F), the

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international journal of hydrogen energy 34 (2009) 8476–8484

Photocurrent Density (mA/cm2)

1.6

340

225

110

-5

a

(A)

1.2

(B) 0.8

0.4

0.0

-120

0.4

0.6

PPM

0.8

1.0

1.2

1.4

1.6

1.8

Applied Potential (V) vs Ag/AgCl

a Photocurrent Density (mA/cm2)

0.8

(C)

b

(D) 0.6

(E) (F)

0.4

0.2

0.0 0.4

340

225

110

-5

-120

PPM

b 13

Fig. 5 – C magic angle spinning SSNMR spectrum of (a) C-doped WO3 synthesized with uniformly 13C-labeled glucose, and (b) the same sample obtained 1 year later.

photocurrent density under visible light irradiation (l > 400 nm) is about 95% of that under near visible light irradiation (l > 378 nm). The 378 nm filter was used for the remaining measurements, but the results obtained with the 400 nm cutoff filter were very similar. As shown in Fig. 6(C) and (F), the photocurrent density is increased from 0.5 mA/cm2 for the undoped electrode to 0.8 mA/cm2 for the C-doped electrode at 1.8 V. This enhanced photoactivity can be attributed to carbon doping. The fact that the visible light photocurrent is a large percentage (approximately 50%) of the total response for both of these C-doped materials is highly encouraging. Fig. 7 shows the photocurrent density of the C-doped WO3 electrode in 1 M HCl electrolyte before and after addition of 20% (v/v) methanol. The addition of methanol as a sacrificial reagent can reduce the e/hþ recombination rate [41], and therefore improves the photoresponse of the C-doped WO3 electrodes. The addition of methanol resulted in an increase in photocurrent density at 1.8 V from 1.6 to 2.6 mA/cm2 under near UV–visible light irradiation, and from 0.8 to 1.2 mA/cm2 under visible light only irradiation. The photoelectrochemical measurements were repeated 6 months later and the photocurrents densities were about 90% of original measured

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Applied Potential (V) vs Ag/AgCl

Fig. 6 – Photocurrent density for the C-doped and undoped WO3 electrodes in 1 M HCl electrolyte. All electrodes were illuminated with 130 mW/cm2 illumination using (a) near UV–visible light and (b) visible light only. (A) C-doped WO3 electrode under near UV–visible light irradiation, (B) undoped WO3 electrode under near UV–visible light irradiation, (C) C-doped WO3 electrode under visible light (l > 378 nm) irradiation, (D) C-doped WO3 electrode under visible light (l > 400 nm) irradiation, (E) undoped WO3 electrode under visible light (l > 378 nm) irradiation, and (F) undoped WO3 electrode under visible light (l > 400 nm) irradiation.

photocurrent densities, indicating that the C-doped WO3 materials are stable over long time periods. The photoresponse was studied as a function of several synthetic variables. The dependence of the C-doped WO3 film photoresponse on the precursor spray glucose concentration is shown in Fig. 8(a). The photocurrent density increased with increasing glucose concentration up to 0.07 M under both near UV–visible light and visible light illumination, and then decreased with further increases in the glucose concentration. This decrease in photoresponse is consistent with UV–visible diffuse reflectance results (Fig. 2). Because the particle size (calculated based on XRD patterns) increases with increasing the glucose concentration, as shown in Table 1, it is clear that the dependence of photocurrent on glucose concentration is not determined entirely by the particle size, but also doping level and other parameters.

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Photocurrent Density (mA/cm2)

3.0

a

Photocurrent Density (mA/cm2)

international journal of hydrogen energy 34 (2009) 8476–8484

(A)

2.5

2.0

(B) 1.5

1.0

0.5

a

1.6

1.2

(A) 0.8

0.4

(B)

0.0 0.00

0.0 0.4

0.6

0.8

1.0

1.2

1.4

1.6

0.05

0.10

0.15

Glucose Concentration (M)

1.8

Photocurrent Density (mA/cm2)

1.4

b

Photocurrent Density (mA/cm2)

Applied Potential (V) vs Ag/AgCl

(A)

1.2 1.0

(B)

0.8 0.6 0.4 0.2

b

1.6

(A)

1.2

0.8

(B) 0.4

0.0 500

550

600

650

Temperature (ºC) 0.0 0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

1.6

Fig. 7 – Photocurrent density for the C-doped WO3 electrode in 1 M HCl electrolyte (A) with addition of methanol and (B) without addition of methanol. The electrode was illuminated with 130 mW/cm2 illumination using (a) near UV–visible light and (b) visible light only (l > 378 nm).

The dependence of the photocurrent density on calcination temperature for the C-doped WO3 electrodes is shown in Fig. 8(b). Four different calcination temperatures were applied to optimize the heating conditions at a given glucose concentration. Under near UV–visible light irradiation, the photocurrent density first increased from 1.1 mA/cm2 at 500  C to 1.6 mA/cm2 at 600  C, and then dropped to 1.3 mA/cm2 at 650  C. This result is different from Yang et al., who reported that a maximum was observed at a calcination temperature of between 475 and 500  C. Because the particle size (calculated based on XRD patterns) decreases with increased calcination temperature, as shown in Table 1, it is clear that the dependence of photocurrent density on calcination temperature is not determined entirely by the crystallite size, which is consistent with the results of Yang et al. [30]. To determine the optimized synthetic conditions for achieving the highest photoactivity, C-doped WO3 electrodes of various thicknesses were prepared by increasing the spray time. Fig. 8(c) shows the photocurrent densities as a function of the spray time used in the electrode preparation. A spray time of 1 min produced a film with a thickness of 2.5 mm, determined by a SEM cross-section image. It was found that the photocurrent density first increased with an increasing

Photocurrent Density (mA/cm2)

Applied Potential (V) vs Ag/AgCl

c (A)

1.2

0.8

(B) 0.4

0.0 0

5

10 15 20 Spray Time (min)

25

30

Fig. 8 – Photocurrent density for the C-doped WO3 electrodes depends on (a) the glucose spray solution concentration (b) the calcination temperature and (c) the spray time: (A) illuminated under near UV–visible light, and (B) illuminated under visible light (l > 378 nm). The photocurrent densities were measured at 1.8 V.

spray time, reaching a maximum at 25 min (w 50 mm thickness), and then decreased with further increases in spray time. Similar results have been reported for many other semiconducting materials, such as TiO2 and CdSe thin films [20,42–45], however, the thickness achieved in this study is larger than in most reports. According to Kavan and Gratzel [44], the photocurrent density depends strongly on the film thickness. When the film thickness is lower than the penetration depth of the light, the film cannot absorb all of the light. However, when the film thickness is

international journal of hydrogen energy 34 (2009) 8476–8484

considerably higher than the penetration depth of the light, the photocurrent density starts to decrease, which can be attributed to the combination of increased resistance and a higher recombination rate of photogenerated carriers due to a reduction of the electric field gradient in a thicker film [46]. It is likely that useful film thicknesses achieved in this study are possible due to the high degree of conductivity of the WO3 material.

4.

Conclusion

C-doped WO3 films have been synthesized by spray pyrolysis using glucose as the dopant source in ethanolic solution followed by calcination. Doping C into the WO3 reduces the band gap as evidenced by a red shift (but no separate peak) in the optical absorption. XPS and SSNMR show the existence of the carbonate species in the C-doped WO3 samples, and this carbonate species is most likely located at interstitial sites based on XPS results. C-doped WO3 electrodes show enhanced photoelectrochemical activity under ultraviolet and visible light irradiation compared with undoped WO3. The observed photocurrent densities are as large as 1.6 mA/cm2 using a 1 M HCl electrolyte and up to 2.6 mA/cm2 when methanol is added as a sacrificial agent. Both experiments show that approximately 50% of the photocurrent density arises from the visible spectral region. The photocurrents are strongly related to the glucose concentration, calcination temperature and the film thickness. For C-doped WO3 thin films, a 0.07 M glucose concentration, 600  C calcination temperature, and 25 min spray time give the highest efficiency. Current efforts are aimed at improving the photoresponse of this material, especially in the visible region.

Acknowledgments Support for this work from the National Science Foundation (CHE-0616748 and DMR-0805096), and the Purdue Research Foundation is gratefully acknowledged. The authors also thank Dr. D. Zemlyanov of the Surface Analysis Laboratory, Birck Nanotechnology Center, Purdue University, for helpful comments on and acquisition of the XPS spectra, and to Professor K. Choi for enabling the diffuse reflectance measurements. D.R. is a member of the Purdue Energy Center.

references

[1] Cheng L, Zhang X, Liu B, Wang H, Li Y, Huang Y, et al. Template synthesis and characterization of WO3/TiO2 composite nanotubes. Nanotechnology 2005;16:1341–5. [2] Yang B, Barnes PRF, Bertram W, Luca V. Strong photoresponse of nanostructured tungsten trioxide films prepared via a sol–gel route. J Mater Chem 2007;17:2722–9. [3] Tamaki J, Okochi Y, Konishi S. Effect of oxide-electrode interface on dilute NO2 sensing properties of WO3 thin film nanosensors. Electrochemistry 2006;74:159–62. [4] Lee DS, Han SD, Huh JS, Lee DD. Nitrogen oxides-sensing characteristics of WO3-based nanocrystalline thick film gas sensor. Sensor Actuat B-Chem 1999;60:57–63.

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[5] Deepa M, Sharma R, Basu A, Agnihotry SA. Effect of oxalic acid dihydrate on optical and electrochemical properties of sol–gel derived amorphous electrochromic WO3 films. Electrochim Acta 2005;50:3545–55. [6] Habazaki H, Hayashi Y, Konno H. Characterization of electrodeposited WO3 films and its application to electrochemical waste water treatment. Electrochim Acta 2002;47:4181–8. [7] Gratzel M. Materials science-ultrafast colour displays. Nature 2001;409:575–6. [8] Hodes G, Cahen D, Manassen J. Tungsten trioxide as a photoanode for a photoelectrochemical cell (PEC). Nature 1976;260:312–3. [9] Cole B, Marsen B, Miller E, Yan Y, To B, Jones K, et al. Evaluation of nitrogen doping of tungsten oxide for photoelectrochemical water splitting. J Phys Chem C 2008; 112:5213–20. [10] Marsen B, Miller EL, Paluselli D, Rocheleau RE. Progress in sputtered tungsten trioxide for photoelectrode applications. Int J Hydrogen Energy 2007;32:3110–5. [11] Bak T, Nowotny J, Rekas M, Sorrell CC. Photo-electrochemical hydrogen generation from water using solar energy. Materials-related aspects. Int J Hydrogen Energy 2002;27: 991–1022. [12] Gratzel M. Photoelectrochemical cells. Nature 2001;414: 338–44. [13] Asahi R, Morikawa T, Ohwaki T, Aoki K, Taga Y. Visible-light photocatalysis in nitrogen-doped titanium oxides. Science 2001;293:269–71. [14] Yuan J, Chen M, Shi J, Shangguan W. Preparations and photocatalytic hydrogen evolution of N-doped TiO2 from urea and titanium tetrachloride. Int J Hydrogen Energy 2006; 31:1326–31. [15] Khan SUM, Al-Shahry M, Ingler WB. Efficient photochemical water splitting by a chemically modified n-TiO2. Science 2002;297:2243–5. [16] Shaban YA, Khan SUM. Visible light active carbon modified n-TiO2 for efficient hydrogen production by photoelectrochemical splitting of water. Int J Hydrogen Energy 2008;33:1118–26. [17] Sreethawong T, Laehsalee S, Chavadej S. Comparative investigation of mesoporous- and non-mesoporousassembled TiO2 nanocrystals for photocatalytic H2 production over N-doped TiO2 under visible light irradiation. Int J Hydrogen Energy 2008;33:5947–57. [18] Sakthivel S, Janczarek M, Kisch H. Visible light activity and photoelectrochemical properties of nitrogen-doped TiO2. J Phys Chem B 2004;108:19384–7. [19] Reddy KM, Baruwati B, Jayalakshmi M, Rao MM, Manorama SV. S-, N- and C-doped titanium dioxide nanoparticles: synthesis, characterization and redox charge transfer study. J Solid State Chem 2005;178:3352–68. [20] Reyes-Gil KR, Reyes-Garcia EA, Raftery D. Photoelectrochemical analysis of anion-doped TiO2 colloidal and powder thin-film electrodes. J Electrochem Soc 2006;153: A1296–301. [21] Wang H, Lewis JP. Second-generation photocatalytic materials: anion-doped TiO2. J Phys: Condens Matter 2006;18: 421–34. [22] Reyes-Garcia EA, Sun Y, Reyes-Gil K, Raftery D. 15N solid state NMR and EPR characterization of N-doped TiO2 photocatalysts. J Phys Chem C 2007;111:2738–48. [23] Reyes-Garcia EA, Sun Y, Raftery D. Solid-state characterization of the nuclear and electronic environments in a boron–fluoride co-doped TiO2 visible-light photocatalyst. J Phys Chem C 2007;111:17146–54. [24] Reyes-Garcia EA, Sun Y, Reyes-Gil K, Raftery D. Solid-state NMR and EPR analysis of carbon-doped titanium dioxide

8484

[25]

[26]

[27]

[28] [29] [30]

[31]

[32]

[33]

[34]

[35]

international journal of hydrogen energy 34 (2009) 8476–8484

photocatalysts (TiO2xCx). Solid State Nucl Magn Reson 2009; 35:74–81. Reyes-Gil KR, Sun Y, Reyes-Garcia E, Raftery D. Characterization of photoactive centers in N-doped In2O3 visible photocatalysts for water oxidation. J Phys Chem C 2009 [accepted]. Sun Y, Murphy CJ, Reyes-Gil KR, Reyes-Garcia EA, Lilly JP, Raftery D. Carbon-doped In2O3 films for photoelectrochemical hydrogen production. Int J Hydrogen Energy 2008;33:5967–74. Paluselli D, Marsen B, Miller EL, Rocheleau RE. Nitrogen doping of reactively sputtered tungsten oxide films. Electrochem Solid State Lett 2005;8:G301–3. Fairley N, Carrick A. The casa cookbook, part I: recipes for XPS data processing. Knutsford: Acolyte Science; 2005. 1. Willson RC. Solar total irradiance observations by active cavity radiometers. Sol Phys 1981;74:217–29. Yang B, Zhang YJ, Drabarek E, Barnes PRF, Luca V. Enhanced photoelectrochemical activity of sol–gel tungsten trioxide films through textural control. Chem of Mater 2007;19:5664–72. Xu C, Khan SUM. Photoresponse of visible light active carbon-modified-n-TiO2 thin films. Electrochem Solid State Lett 2007;10:B56–9. Ren W, Ai Z, Jia F, Zhang L, Fan X, Zou Z. Low temperature preparation and visible light photocatalytic activity of mesoporous carbon-doped crystalline TiO2. Appl Catal B 2007;69:138–44. Altshuler S, Chakk Y, Rozenblat A, Cohen A. Investigation of simultaneous fluorine and carbon incorporation in a silicon oxide dielectric layer grown by PECVD. Microelectron Eng 2005;80:42–5. Ohno T, Tsubota T, Nakamura Y, Sayama K. Preparation of S, C cation-codoped SrTiO3 and its photocatalytic activity under visible light. Appl Catal A 2005;288:74–9. Irie H, Watanabe Y, Hashimoto K. Carbon-doped anatase TiO2 powders as a visible-light sensitive photocatalyst. Chem Lett 2003;32:772–3.

[36] Wang S, Chen T, Rao CK, Wong M. Nanocolumnar titania thin films uniquely incorporated with carbon for visible light photocatalysis. Appl Catal B 2007;76:328–34. [37] Bathe SR, Patil PS. Influence of Nb doping on the electrochromic properties of WO3 films. J Phys D: Appl Phys 2007;40:7423–31. [38] Xia H, Wang Y, Kong F, Wang S, Zhu B, Guo X, et al. Au-doped WO3-based sensor for NO2 detection at low operating temperature. Sensor Actuat B-Chem 2008;134:133–9. [39] Stueber D, Orendt AM, Facelli JC, Parry RW, Grant DM. Carbonates, thiocarbonates, and the corresponding monoalkyl derivatives: III. The 13C chemical shift tensors in potassium carbonate, bicarbonate and related monomethyl derivatives. Solid State Nucl Magn Reson 2002;22:29–49. [40] Duncan TM. 13C chemical shieldings in solids. J Phys Chem Ref Data 1987;16:125–51. [41] Thompson TL, Yates JT. Monitoring hole trapping in photoexcited TiO2(110) using a surface photoreaction. J Phys Chem B 2005;109:18230–6. [42] Fushimi T, Oda A, Ohkita H, Ito S. Fabrication and electrochemical properties of layer-by-layer deposited ultrathin polymer films bearing ferrocene moieties. Thin Solid Films 2005;484:318–23. [43] Kale SS, Pathan HM, Lokhande CD. Thickness dependent photoelectrochemical cells performance of CdSe and HgS thin films. J Mater Sci 2005;40:2635–7. [44] Kavan L, Gratzel M. Highly efficient semiconducting TiO2 photoelectrodes prepared by aerosol pyrolysis. Electrochim Acta 1995;40:643–52. [45] Hodes G, Howell IDJ, Peter LM. Nanocrystalline photoelectrochemical cells – a new concept in photovoltaic cells. J Electrochem Soc 1992;139:3136–40. [46] Khan SUM, Akikusa J. Photoelectrochemical splitting of water at nanocrystalline n-Fe2O3 thin-film electrodes. J Phys Chem B 1999;103:7184–9.