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Applied Surface Science 258 (2012) 5038– 5045

Contents lists available at SciVerse ScienceDirect

Applied Surface Science

jou rn al h om epa g e: www.elsev ier .com/ locate /apsusc

hotoelectrochemical properties and photocatalytic activity of nitrogen-dopedanoporous WO3 photoelectrodes under visible light

uyang Liua, Ya Lib, Wenzhang Lia,∗, Song Hana, Canjun Liua

Key Laboratory of Resources Chemistry of Nonferrous Metals (Ministry of Education), School of Chemistry and Chemical Engineering, Central South University, Changsha 410083,hinaCollege of Chemistry, Xiangtan University, Xiangtan 411105, China

r t i c l e i n f o

rticle history:eceived 24 November 2011ccepted 13 January 2012vailable online 21 January 2012

eywords:ungsten oxideanoporous photoelectrodeitrogen-dopedhotocatalytic activity

a b s t r a c t

In the present work, nitrogen-doped tungsten oxide (WO3) nanoporous photoelectrode was studied byphotoelectrochemical and photocatalytic methods in order to evaluate the photoactivity and the pos-sibility of its application in solar photocatalysis. WO3 nanoporous photoelectrodes were prepared byanodization of tungsten foil in NH4F/(NH4)2SO4 electrolytes, followed by annealing in NH3/N2 to incor-porate N as a dopant. The crystal structure, composition and morphology of pure and nitrogen doped WO3

were compared using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electronmicroscopy (SEM). The results indicate that nitrogen can be doped successfully into WO3 nanoporousphotoelectrodes by controlling annealing temperature. Incident photon to current efficiency measure-ments carried out on PEC cell with N-doped WO3 nanoporous photoelectrodes as anodes demonstrate a

significant increase of photoresponse in the visible region compared to undoped WO3 nanoporous pho-toelectrodes prepared at similar conditions. In particular, the photocatalytic and photoelectrocatalyticactivity under visible light irradiation for newly synthesized N-doped WO3 nanoporous photoelectrodeswere investigated by degradation of methyl orange. The photoelectrocatalytic activity of N-doped WO3

nanoporous photoelectrodes was 1.8-fold enhancement compared with pure WO3 nanoporous photo-electrodes.

. Introduction

As a promising material, tungsten oxide (WO3) has received great deal of attention due to its no-photo-corrosion, stabilityn acid, and potential to absorb a reasonable fraction of the solarpectrum [1–4]. In the past two decades, WO3 has been exten-ively studied for their fundamental science and a wide varietyf application in photovoltaics [5–7], photocatalysis [8–10], andater splitting [11–14]. However, due to the relatively large band

ap (2.6–3.0 eV), WO3 mainly absorbs in the near ultraviolet andlue region of the solar spectrum. To overcome this limitation ando enlarge its absorption range, many methods have been investi-ated, including noble metal deposition [15–19], transition metaloping [20,21], and dye sensitization [7,22]. It has been foundhat WO3 doped with some elements, such as carbon, sulfur anditrogen would enhance photoresponse under visible irradiation

23–25]. Comparing the effect of above anion dopants through den-ity of states calculations, nitrogen (N) doping has been identified

∗ Corresponding author. Tel.: +86 731 8887 7364; fax: +86 731 8887 9616.E-mail address: liwenzhang@csu.edu.cn (W. Li).

169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved.oi:10.1016/j.apsusc.2012.01.080

© 2012 Elsevier B.V. All rights reserved.

as the most promising species because its p states mix with O 2pstates [26], resulting in a reduction of the band gap.

To the best of our knowledge, there are two synthetic routesabout the modification of WO3 by nitrogen, including sputteringmethods [24,27,28] or calcinations in the presence of ammonia[23,29]. Paluselli et al. [27] reported that N-doped WO3 with aband gap of 2.2 eV can be prepared by RF reactive sputtering, andCole et al. [28] reported N doping narrowed WO3 band gap from2.5 eV to 1.9 eV in the same technique. In those synthesis routes,N2 was used as the nitrogen sources for the preparation of N-doped WO3. The doping of nitrogen has reduced the energy bandgap efficiently. However, the photoelectrochemical performanceof N-doped WO3 films prepared by RF reactive sputtering is nopromising. Recent studies by Schmuki and co-workers [23] showedthat the photocurrent of N-doped WO3 films by NH3 treatment issignificantly enhanced in the visible light region. In their studies,the N-doped WO3 films were synthesized by a two step annealingmethod, including annealed at 450 ◦C for 1 h in ambient air step andtreated at 300 ◦C in NH3 for 4 h step in sequence.

It is feasible that doping impurities into WO3 powder canimprove photocatalytic activity under visible light [20,21,30]. How-ever, the separation problems of WO3 powder hold back itsutilization in the application. Therefore, many kinds of WO3 films

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sbmNuWoeNn

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Y. Liu et al. / Applied Surfac

n various substrates were used to substitute WO3 nanopow-ers as photocatalyst [8,31,32]. Fortunately, the recently reportedossibility to fabricate WO3 nanoporous films provides a uniquepportunity for the photocatalysis application [1,4,33–36]. In ordero obtain such nanostructure oxide layers, a new approach waseveloped that is based on anodization of W in diluted F−-ontaining electrolytes. In our previous report [37], we investigatedhe visible light photoelectrochemical properties of the orderedanoporous WO3 layers prepared in neutral NaF/Na2SO4 stronglectrolytes by voltage stepping anodization. These nanostruc-ured oxide layers have shown excellent corrosion resistance andigher photocurrent conversion efficiency as compared with com-act anodic WO3 layers grown under comparable electrochemicalonditions.

In the present work, we describe the effect of nitrogen doping ofelf-organized anodic nanoporous WO3 photoelectrodes obtainedy one step annealing in a NH3/N2 gas mixture. The structural,orphologic and photoelectrochemical properties of an efficient-doped WO3 as well as undoped WO3 photoelectrodes were eval-ated. The photoelectrocatalytic activity of N-doped nanoporousO3 photoelectrodes was also evaluated by degradation of methyl

range under visible light irradiation. To the best of our knowl-dge, this is the first report on the visible photocatalytic activity of-doped nanoporous WO3 photoelectrodes. This work may offerew insights on the photoactivity enhancement.

. Experimental

.1. Preparation of WO3 nanoporous photoelectrodes

To produce anodic WO3 nanoporous photoelectrodes, we usedungsten foils (99.95% purity, Alfa Aesar) with thicknesses of.05 mm as substrates. The tungsten foils were cleaned in four5 min steps in acetone, isopropanol, methanol and finally deion-

zed (DI) water and then dried in a nitrogen stream. The anodizationxperiments were carried out in a two-electrode cell consisting of

tungsten working electrode and a Pt counter electrode at roomemperature (25 ± 1 ◦C). The tungsten samples were anodized at0 V in 1 M (NH4)2SO4 + 0.5 wt% NH4F for 30 min. The cell voltageas applied by a single step from the open-circuit potential to a

iven value and was supplied by a direct current voltage sourceDH1719A-5, Dahua, China). The current between the working andounter electrodes was measured by a digital multimeter (34401A,gilent) interfaced to a personal computer. After the electrochemi-al treatment, the samples were immediately rinsed with DI waternd then dried in a N2 stream. All solutions were prepared fromeagent-grade chemicals and DI water.

.2. Preparation of N-doped WO3 nanoporous photoelectrodes

N-doped WO3 nanoporous photoelectrodes were preparedy annealing anodic oxide layers in a NH3/N2 gas mixture120 mL min−1). The anodic oxide layers were also annealed in pure2 at 450 ◦C as reference. Accordingly, modulating the gaseousmmonia–nitrogen ratio was to detect its effect on N-doped WO3anoporous photoelectrodes.

.3. Microstructure and chemical analysis

A field-emission scanning electron microscope (FESEM, Sirion

00, FEI, Holland) was employed for the structural and morpho-

ogical characterization of the as-prepared photoelectrodes. Therystalline structure of the samples was measured by X-ray diffrac-ion (XRD, D/Max2250, Rigaku, Japan). The chemical compositions

ce 258 (2012) 5038– 5045 5039

of the samples were identified by X-ray photoelectron spectroscopy(XPS, XSAM800, Kratos, Britain).

2.4. Photoelectrochemical measurement

In order to investigate the photoresponse of the N-doped WO3nanoporous photoelectrodes, a standard three-electrode electro-chemical cell with a flat quartz window to allow illuminationwas used for the photoelectrochemical and electrochemical mea-surements, which were performed in 0.5 M H2SO4 (pH 0) by anelectrochemical workstation (Zennium, Zahner, Germany). A Ptcounter electrode and an Ag|AgCl|satd. KCl reference electrodealong with the working electrode completed the cell setup. Thepotentials were swept linearly at a scan rate of 10 mV/s, and allpotentials in the photocurrent data below were quoted with respectto this reference electrode. A 500 W Xe lamp (CHF-XM35, TrusttechCo. Ltd., Beijing) served as the visible light source with an intensityof 100 mW/cm2. A 400 nm cutoff filter was placed into the pathof the Xe lamp to remove the UV irradiation. Photo-action spec-tra were recorded by using an apparatus comprising a Xe lampsource (150 W, Oriel), a monochromator with bandwidth of 10 nm.A focusing lens was equipped with monochromator to enhance theincident light power on the photoelectrode. For the IPCE calcula-tion, the absolute intensity of the incident light was measured bya model BS2281 Si detector and the photocurrent was measured at1.2 V vs Ag/AgCl (pH 0).

2.5. Evaluation of photocatalytic activity

For the photocatalytic (PC) and photoelectrocatalytic (PEC)degradation experiments, methyl orange (MO) was chosen as a tar-get compound with an initial concentration of 20 mg/L. A 500 Wxenon lamp with a cutoff filter (� ≥ 400 nm) was employed asthe visible light source. The distance between the photoreactionand lamp was 10 cm and the visible light intensity was about100 mW/cm2. The photoreacted solution was analyzed by record-ing variations of the absorption band maximum (508 nm) in theUV–vis spectra of MO using an UV–vis spectrophotometer (UV2100, Shimadzu). The PEC of MO under visible light irradiation wascarried out in a quartz reactor with samples serving as the photo-anode, platinum foil serving as the cathode, and an Ag|AgCl|satd.KCl electrode serving as the reference electrode. The bias poten-tial applied on the photo-anode was 0.8 V (vs Ag/AgCl). All theexperiments were performed with magnetic stirring at 25 ◦C andcontinually bubbled with N2.

3. Results and discussion

3.1. XRD

The crystal structure significantly influenced the photocatalyticactivity of WO3 photoelectrodes. Due to crystal defect state, i.e. theunbonded oxygen, which might become the recombination centerof photoelectron and hole, amorphous WO3 showed lower photo-catalytic activity [38,39]. Fig. 1 shows the XRD patterns of undopedWO3 and modified WO3 annealed at different temperature. TheWO3 films annealed at 450 ◦C in N2 show sharp diffraction peaksat 23.3◦, 23.8◦ and 24.6◦, which suggest monoclinic WO3 phaseaccording to JCPDS data (JCPDS 43-1035). Similarly, the samplesannealed in NH3/N2 form a monoclinic structure at 300 and 450 ◦C.At 550 ◦C, the intensities of WO3 peaks obviously reduce and two

weak diffraction peaks in the regions 37–43◦ which corresponds toW2N appears. Since the XRD indicates the presence of W2N phase,it is also determined that the oxygen sites are occupied by nitro-gen atoms [40]. The higher temperature anneal at 650 ◦C induces

5040 Y. Liu et al. / Applied Surface Science 258 (2012) 5038– 5045

Fd

asap

3

eNniiodwbtow[ftTgas

3

ahotpmamAtopst

ig. 1. The X-ray diffraction patterns of nanoporous WO3 electrodes annealed atifferent temperature.

stronger intensity of W2N with a further decrease in the inten-ity of the WO3 phase. It is reported that the formation of W2Nnd the loss of crystallinity in the WO3 induce deactivation of thehotoresponse [23,41].

.2. XPS

The XPS spectra of undoped WO3 and N-doped WO3 photo-lectrodes annealed at 450 ◦C are shown in Fig. 2a. The surfaces of-doped WO3 photoelectrode are composed of tungsten, oxygen,itrogen and carbon contaminants. No nitrogen signal was detected

n the undoped WO3 sample whist weak nitrogen signals appearedn N-doped WO3 samples. Fig. 2b shows the enlarge N 1s peaksf the undoped WO3 and N-doped WO3 samples. In the case of N-oped WO3, there is a board peak observed from 392 to 402 eV. Twoell-defined peaks can be distinguished, which indicated the N 1s

inding energies were 399.75 and 395.59 eV, respectively. Usuallyhe peak at 396 eV corresponds to the bonding state of N W and thene at 400 eV can be assigned to N that is surface bond with N or O,hich is in agreement with the findings reported by Ghicov et al.

42]. Considering that no W2N was observed at this temperaturerom the XRD pattern, it is suggested the fact that the substitu-ion of O in WO3 by N element and formation of W O N band.herefore, one can conclude from the XPS results that the nitro-en was not only successfully implanted in the structure but it waslso present in a chemically bonded state, indicating that nitrogen isuccessfully doped in the WO3 lattice annealed in NH3/N2 at 450 ◦C.

.3. SEM

Fig. 3 shows the morphology of the samples over the rangenneal temperature. To illustrate the evolution, the sample beforeeat treatment is also included here. Fig. 3a is top-view SEM imagef WO3 film formed by anodization of W in the (NH4)2SO4 elec-rolyte with NH4F. A highly ordered porous oxide structure isresent after anodization, where a typical pore size of approxi-ately 70 nm is observed. These samples were then exposed to

heat treatment in NH3/N2 at different temperature. The sampleaintained its morphology for anneal in NH3/N2 gas up to 300 ◦C.

further increase in temperature to 450 ◦C causes an alteration inhe morphology, with WO3 nanorods growing randomly on surface

f the porous films. However, it is quite clear, that at a anneal tem-erature of 550 ◦C, a change in the morphology. The ordered poroustructure is deformed and consists of massive particles. It appearshat a compact layer forms with increasing in anneal temperature.

Fig. 2. (a) XPS spectrums of undoped and N-doped nanoporous WO3 electrodescalcined at 450 ◦C, and (b) XPS high-resolution spectra of N 1s peak for undoped andN-doped WO3 electrodes.

The sample sintering in N2 at 450 ◦C provided layer with morphol-ogy similar to the NH3/N2 case, but with large feature size of theparticles, as shown in Fig. 3e.

3.4. Photoelectrochemical analysis

Little is known about the effect of chemical state of the N dopanton the electronic properties of WO3 nanoporous photoelectrodes.In order to understand the intrinsic electronic of properties N-doped WO3 nanoporous photoelectrodes in electrolyte solution, weperformed electrochemical impedance measurement in the dark todetermine the capacitance of layers. Based on the Mott–Schottkyequation, the space charge layer capacitance (Csc/F cm−2) of a semi-conductor determined from measured impedance (C = −1/ωZ′′) isrelated to the applied potential (U/V):

1

C2sc

= 2εε0qND

(U − Ufb − kT

q

)

where ε is the relative permittivity of the semiconductor (50 forWO3) [43], ε0 is the vacuum permittivity (8.85 × 10−14 F cm−2), q isthe electronic charge (1.60 × 10−19 C), ND/cm−3 is the donor carrier

density (for an n-type semiconductor), Ufb/V is the flat band poten-tial, k is the Boltzmann constant (1.38 × 10−23 J K−1), and T/K is theKelvin temperature, respectively. Assuming that the capacitance ofthe Helmholtz layer can be neglected, the Mott–Schottky plots of

Y. Liu et al. / Applied Surface Science 258 (2012) 5038– 5045 5041

Fig. 4. Mott–Schottky plots of undoped and N-doped nanoporous WO3 photo-electrodes. The capacitance was measured at 1 kHz of a frequency and 10 mV ofamplitude potential.

the WO3 layers annealed in N2 and N2/NH3 at the AC frequencyof 1 kHz in a 0.5 M H2SO4 aqueous electrolyte are shown in Fig. 4.The positive slopes indicate that both of the samples are n-typesemiconductors. The Ufb values were provided from the x-interceptand the donor density ND was estimated from the slope of the plot(C−2

sc vs potential) using the relationship slope = 2/εε0qND. Thedonor concentrations were calculated to be 2.58 and 6.84 × 10−22

for undoped and N-doped samples, respectively. The N-doped WO3nanoporous photoelectrodes exhibited higher donor concentrationcompared to the undoped sample due to N incorporation and/ornative defects such as W interstitials and O vacancies. Moreover, itis found that the flat band potentials were 0.25 and 0.31 V for theN-doped and undoped WO3, respectively. The Ufb of the N-dopedWO3 obviously shifts negatively in contrast to that of the undopedWO3, indicating a modified band structure by doping. Usually, themore negative the flat band potential, the better is the ability ofsemiconductor film to facilitate the charge separation [44].

In order to address the quantitative correlation betweenN-doping and light absorption of the nanoporous WO3 photoelec-trodes, the photocurrent response of the samples as a function ofwavelength of incident light was measured at a potential of 1.2 Vvs Ag/AgCl in a 0.5 M H2SO4 solution. Photoaction spectra for theundoped and N-doped WO3 photoelectrodes are compared in Fig. 5,where the incident photon to electron conversion efficiency (IPCE)is plotted vs wavelength

IPCE (%) =(

1240 jpP�

)× 100

where jp is the photocurrent density (mA/cm2), P is the incidentphoton flux density at the photoelectrode location (mW/cm2) and� is wavelength (nm). With an onset of IPCE at 450 nm, the undopedsamples have strong photoresponse in the near-UV region but littlephotoresponse in >450 nm ascribed to intrinsic band gap energyof WO3. In contrast, the samples at 450 ◦C in NH3/N2 (N-doping)

show substantial photo-activity in the visible light region from400 to 600 nm in addition to strong photoresponse in the near-UV. These results clearly confirm that nitrogen can be successfullydoped in WO3 by annealed in NH3/N2, resulting in considerable

Fig. 3. SEM images of the (a) as-anodized WO3 electrode and annealed in NH3/N2

(v/v = 1:2) at (b) 300 ◦C, (c) 450 ◦C, (d) 550 ◦C, and in N2 at (e) 450 ◦C, respectively.

5042 Y. Liu et al. / Applied Surface Science 258 (2012) 5038– 5045

Ft

va6

cppsasts∼tcitsma

Fp

ig. 5. Photoaction spectra (IPCE vs wavelength) of the nanoporous WO3 photoelec-rodes, recorded in 0.5 M H2SO4 at 1.2 V vs Ag/AgCl.

isible response. Additionally, more interesting is that the samplennealed at 550 ◦C in NH3/N2 also extended its optical response to00 nm, although the IPCE in UV region significantly decreased.

To further quantify the PEC performance, systematic electro-hemical measurements were carried out to evaluate the PECroperties of photoanodes fabricated from the different WO3 sam-les in a 0.5 M H2SO4 solution (pH 0). Fig. 6 shows a set of linerweep voltammagrams recorded on these WO3 layers in the darknd with illumination of 100 mW/cm2 (AM 1.5). Dark scan linerweep voltammagrams from 0 to 1.6 V indicate a weak current inhe range of 10−8 A/cm2. Upon illumination with a visible lightource, all samples exhibit pronounced photocurrent starting at+0.4 V. In comparison to undoped WO3 annealed at 450 ◦C in N2,

he N-doped samples annealed at 450 ◦C in NH3/N2 show a signifi-ant enhancement in photoresponse with the photocurrent densityncreasing from 2.8 to 3.3 mA/cm2. There is no saturation of pho-

ocurrent observed at positive potential, indicating efficient chargeeparation in nanoporous layers upon illumination [45]. Further-ore, it was demonstrated that the photocurrent density is strongly

ffected by the temperature during anneal in NH3/N2 from the

ig. 6. Liner sweep voltammagrams, collected at a scan rate of 10 mV/s at appliedotentials from 0 to +1.6 V from the nanoporous WO3 photoelectrodes.

Fig. 7. The photocurrent density of WO3 nanoporous photoelectrodes annealed inNH3/N2 atmosphere with the different ratio at 450 ◦C, recorded in 0.5 M H2SO4 at1.2 V vs Ag/AgCl.

temperature-dependence liner sweep voltammagrams collectedfrom the N-doped WO3 nanoporous photoelectrodes. The samplesannealed at 300, 450, 550 ◦C showed 1.5, 2.3, 3.3 mA/cm2 at 1.6 V (vsAg/AgCl) on the front side illumination, respectively. The maximumphotocurrent was obtained with the sample annealed at 450 ◦C andphotocurrent decreased with further increase of annealed temper-ature above 550 ◦C. This tendency may be attributed to the crystalphase conversion and the structure of WO3 layers. Recent studiesby Schmuki and co-workers [23] suggested that the formation ofW2N and the loss of crystallinity in the WO3 induce deactivation ofthe photoresponse.

To determine the optimized synthetic conditions for achievingthe highest photoelectrochemical performance, the WO3 photo-electrodes were annealed in NH3/N2 atmosphere with the differentratio. Effect of gaseous NH3 concentration on photocurrent densitycan be seen from Fig. 7. The photocurrent increased with increas-ing the NH3/N2 ratio to 1/2. Further increase in the NH3/N2 ratioresulted in a decrease in the photocurrent density. One possiblereason is that higher concentration of NH3 would increase theefficiency of nitrogen doping on WO3 layers. Increasing the nitro-gen concentration will lower the quantum yields. In other words,the nitrogen doping sites could also serve as recombination sites,which will decrease photocurrent density [46,47]. So far, when theNH3–N2 ratio was 1/2 (v/v), the optimum photoelectrochemicalperformance could be obtained.

3.5. PC and PEC degradation of MO under visible light irradiation

According to the results of the photocurrent action measure-ment, the N-doped samples are expected to show the differentphotocatalytic properties from those of pure WO3. Thus, the photo-catalytic activities of the samples were measured with visible light(� ≥ 400 nm) degradation of MO as model reaction. The photocat-alytic (PC) degradation of MO follows pseudo-first-order kinetics.The kinetic constant k can be calculated from the linear transform[48]ln

(C0Ct

)= kt where C0 is the initial concentration of MO and

Ct is the concentration of MO at time t. The MO concentration

does not change for every measurement using various samplesunder dark conditions without light illumination. Illumination inthe absence of WO3 photoelectrodes does not result in the pho-tocatalytic decolorization of MO. Therefore, the presence of both

Y. Liu et al. / Applied Surface Science 258 (2012) 5038– 5045 5043

Fig. 8. (a) Degradation rates of MO and (b) kinetic rate constants of variousn

icdrn

WstAiWtoAatatN5fiT

OH(surf) + h → OH (3)

e − + O → •O − (4)

anoporous WO3 photoelectrodes under visible light irradiation.

llumination and WO3 photoelectrodes is necessary for the effi-ient degradation. These results also suggest that the degradationecolorization of MO aqueous solution is caused by photocatalyticeactions on WO3 photoelectrodes surface under the visible illumi-ation.

Fig. 8 shows the comparison of photocatalytic activity of theO3 photoelectrodes annealed at different temperatures. It can be

een that the calcination temperatures have a great influence onhe photocatalytic activity of the WO3 layers annealed in NH3/N2.t 300 ◦C, the sample shows a relative low photocatalytic activ-

ty due to the weak crystallization of WO3 in the samples [38].ith increasing the calcination temperature to 450 ◦C, the pho-

ocatalytic activity obviously increases owing to the enhancementf WO3 crystallization and the decrease of defects in the samples.t 450 ◦C, the N-doped photoelectrode shows the highest photocat-lytic activity and its k reaches 0.0459 min−1. The k was determinedo be 0.0352 min−1 for the undoped sample. The photocatalyticctivity of the sample annealed in NH3/N2 at 450 ◦C exceeds that ofhe sample in N2 by a factor of 1.3, which could be ascribed to the

doping. With further increasing the calcination temperature to50 ◦C, the photocatalytic activity decreases. This is ascribed to theollowing two causes. The first cause is due to the WO3 changing

nto W2N, which has lower photocatalytic activity than WO3 [23].he second cause is due to destroy of architectures and nanoporous

Fig. 9. Degradation rates of MO by various processes on undoped and N-dopednanoporous WO3 photoelectrodes under visible light irradiation.

structures of WO3 photoelectrodes, resulting in the decrease ofphotocatalytic activity [49].

The electrochemical (EC), PC and PEC process of MO in aqueoussolution under visible light irradiation are performed on undopedand N-doped WO3 photoelectrodes. The experimental results aredisplayed in Fig. 9. The corresponding kinetic constants and regres-sion coefficients of MO degradation are listed in Table 1. It is obviousthat the EC process of MO is much slower than PC and PEC process,suggesting that the EC degradation of MO could not rapidly reactwithout visible light irradiation. As a photocatalyst, WO3 results inthe effective photocatalytic degradation of MO. This is attributedto the more effective separation of photogenerated electron–holepairs and the higher internal surface area of the special nanoporousstructure [8]. When the bias potential of 0.8 V is applied to undopedWO3 photoelectrode, PEC degradation rate of MO is approximately2.2 times of PC degradation rate. It is found that PC and PEC degra-dation rate of MO on N-doped WO3 electrode are 1.3 and 1.6times of that on pure WO3 layer. Obviously, N-doped WO3 showsthe highest decomposition rage in the PEC process, suggestingthat the applied bias potential inhibits the recombination of thephotogenerated electron–hole pairs effectively and lengthens thephotogenerated carriers life [50]. Especially, PC degradation rate ofMO on N-doped WO3 electrode is almost equal to the PEC degrada-tion rage of MO on undoped photoelectrode. With an appropriateamount of nitrogen on WO3, an efficient charge separation ofthe photogenerated electron–hole pairs can be accomplished [51].These results are in accordance with that of photocurrent spectra.

The process of visible light photocatalytic oxidation of MO isdescribed in Fig. 9. First, holes in the N impurity level and elec-trons in conduction band were generated (Eq. (1)) under visiblelight irradiation [52,53].

WO3 N + hv → h+ + eCB− (1)

In the photocatalytic process, the hole and electrons will reactwith molecule O2 and OH− on the catalyst surface to form •O2

−

superoxide anion radicals and •OH radical [54], respectively (Eqs.(2)–(4)). The •O2

− radicals then interact with H2O absorbed to pro-duce more •OH radical (Eq. (5)).

H2O ↔ OH(surf)− + H+ (2)

− + •

CB 2 2(surf)

•O2(surf)− + H2O → •OH + OH− (5)

5044 Y. Liu et al. / Applied Surface Science 258 (2012) 5038– 5045

Table 1PEC kinetic rate constants and regression coefficient of undoped and N-doped nanoporous WO3 photoelectrodes under visible light irradiation.

Process Undoped WO3 N-doped WO3

Kinetic constant k(min−1)

Correlationcoefficient, R2

Kinetic constant k(min−1)

Correlationcoefficient, R2

PC 0.0352 0.983

EC 0.0124 0.994

PEC 0.0553 0.989

Fsv

•

oiiFcwfhp(

W

H

e

•

tNalp

4

pis

[[

[

[

[[

[

[

[

[[[[

ig. 10. Schematic diagram of the interface charge-carrier transfer of photocataly-is and photoelectrocatalysis for N-doped nanoporous WO3 photoelectrodes underisible irradiation.

Finally, these •OH radicals react with MO mineralize it (Eq. (6)).

OH + MO → CO2 + H2O (6)

For the photocatalytic reaction, the lower recombination ratef photogenerated electrons and holes of WO3 photoelectrodenduces the significant enhancement of the photocatalytic activ-ty. A feasible mechanism of the photoelectrocatalysis is shown inig. 10. The photogenerated electrons are forced to transfer to theounter electrode by applying an anodic bias potential to the WO3orking electrode and then reduce the oxygen absorbed on the sur-

ace of counter electrode to form •O2−, while the photogeneratedoles in the WO3 photoelectrode react with H2O to form •OH. Thehotoelectrocatalytic reaction is summarized in the following Eqs.7)–(10):

Photoanode:

O3 N + h� → h+ + eCB− (7)

2O + h+ → •OH + H+ (8)

Cathode (counter electrode):

CB− + O2 → •O2(surf)

− (9)

O2(surf)− + H2O → •OH + OH− (10)

A positive potential applied on WO3 photoelectrode can inhibithe recombination of eCB

− and h+. While the working electrode is-doped WO3 photoelectrode, N doping extended the visible lightbsorption edge and the electrons were excited form the N impurityevel to the conduction band, resulting in the enhancement of thehotoelectrocatalytic activity.

. Conclusions

In summary, we have synthesized N-doped WO3 nanoporoushotoelectrode by one step annealing the anodic oxide layers

n a NH3/N2 gas mixture. The results of XRD and XPS demon-trated that the nitrogen element was indeed incorporated into

[

[

0.0473 0.9920.0156 0.9950.0903 0.991

the crystal lattice of WO3. The microstructure and morphology ofWO3 nanoporous photoelectrodes were significantly affected bythe temperature during the NH3/N2 treatment. When the calcina-tion temperature was as high as 550 ◦C, the WO3 phase graduallytransformed to W2N and the order porous structure is deformed.IPCE studies of the N-doped WO3 nanoporous photoelectrodesshow a significant enhancement in conversion efficiency in thevisible light. The photocatalytic activity of as-prepared N-dopedWO3 nanoporous photoelectrodes was determined by degradationof MO under visible light irradiation, and compared to the undopedsample. The PC and PEC degradation rate of the N-doped WO3 pho-toelectrode annealed at 450 ◦C in a NH3/N2 (v/v = 1/2) gas mixtureare 1.3 and 1.6 times of that on undoped photoelectrode, respec-tively.

Acknowledgment

This study was supported by the National High TechnologyResearch and Development Program of China (863 Program, No.2011AA050528) and the National Nature Science Foundation ofChina (nos. 51072232 and 21171175).

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