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Applied Surface Science 256 (2010) 4397–4401

Contents lists available at ScienceDirect

Applied Surface Science

journa l homepage: www.e lsev ier .com/ locate /apsusc

hotoelectrochemical property and photocatalytic activity of N-doped TiO2

anotube arrays

ingjing Xua,∗, Yanhui Aob,∗, Mindong Chena, Degang Fuc

College of Environmental Science and Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, ChinaKey Laboratory of Integrated Regulation and Resource Development on Shallow Lakes, Ministry of Education,ollege of Environmental Science and Engineering, Hohai University, Nanjing 210092, ChinaState Key Laboratory of Bioelectronics, Southeast University, Nanjing 210096, China

r t i c l e i n f o

rticle history:eceived 15 July 2009eceived in revised form 11 February 2010ccepted 11 February 2010

a b s t r a c t

N-doped TiO2 nanotube arrays (NTN) were prepared by anodization and dip-calcination method.Hydrazine hydrate was used as nitrogen source. The surface morphology of samples was characterizedby SEM. It showed that the mean size of inner diameter was 65 nm and wall thickness was 15 nm for NTN.The ordered TiO2 nanotube arrays on Ti substrate can sustain the impact of doping process and post-heat

vailable online 18 February 2010

eywords:hotoelectrochemical-doped

treatment. The atomic ratio of N/Ti was 8/25, which was calculated by EDX. Photoelectrochemical prop-erty of NTN was examined by anodic photocurrent response. Results indicated the photocurrent of NTNwas nearly twice as that of non-doped TiO2 nanotube arrays (TN). Photocatalytic activity of NTN wasinvestigated by degrading dye X-3B under visible light. As a result, 99% of X-3B was decomposed by NTNin 105 min, while that of TN was 59%.

iO2 nanotube arrays

isible lighthotocatalysis

. Introduction

There is a much current interest in the development of TiO2eterogeneous photocatalysis as it was approved to be a highlyfficient catalyst to deal with environmental remediation, due tots high photo-stability, low-cost, and environmental friendly fea-ure [1]. The photocatalytic activity of TiO2 strongly depends onuch as morphology, crystalline structure, size, and the preparedethods. Nanotubular semiconductor structures are particularly of

nterest because of their unusual electronic transport and mechani-al strength characteristics [2]. Thus, applications of TiO2 nanotuberrays including gas sensor [3], photoelectrochemical [4], bio-lectrocatalysis [5], and so on. There are many papers related toiO2 nanotube arrays and its applications [6–14]. To date, vari-us methods such as anodization [2,5], template method [15–17],emplate-assisted sol–gel method [18], have been developed torepare TiO2 nanotube arrays structure. Among these methods,he anodization of titanium sheet has proven to be a classical andeasible approach because of its outstanding advantages.

Owing to its wide band gap (3.2 eV for anatase), anatase TiO2an only be excited under ultraviolet irradiation. However, thisection occupies only less than 5% of the solar irradiance at thearth’s surface. For the sake of efficient use of sunlight, enlarge-

∗ Corresponding authors.E-mail addresses: xujj@seu.edu.cn (J. Xu), andyao@hhu.edu.cn (Y. Ao).

169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.apsusc.2010.02.037

© 2010 Elsevier B.V. All rights reserved.

ment the absorption band border of TiO2 may then appear as anappealing challenge for developing the future generation of pho-tocatalysts. Many groups had focus on developing synthesis routesto modified TiO2 with visible light response by metal doping, andcoupling with other semiconductors of band gap narrower thanthat of TiO2. Recently, studies report that substitutional dopingof non-metallic elements, like N, C, F [19–23] seem to be a verypromising approach for the production of TiO2 operating at visiblewavelength. According to Asahi et al. [19], doping TiO2 with nitro-gen leads to a narrowing in the bang gap due to the mixing of pstates of nitrogen with O 2p states.

At present, there are very few papers related on TiO2 nanotubearrays with visible light response. In this paper, TiO2 nanotubearrays were prepared by anodization method. And then, hydrazinewhich is an active chemical and can be burned in air at a rela-tively low temperature with a strong heat release was chosen asnitrogen source. The N-doped TiO2 nanotube arrays were preparedby dip-calcinations method. The as-prepared novel materials canrespond under visible light, and excited excellent photoelectro-chemical property and photocatalytic activity.

2. Experimental

2.1. Materials

Titanium sheet, purity >99.5%, is of 0.20 mm thickness. The Ptfoil, purity >99.99%, is of 0.05 mm thickness. Chemical reagents are

4398 J. Xu et al. / Applied Surface Science 256 (2010) 4397–4401

Fo

aCaawd

2

w(asatpHo

2

whid3adda

experiment. The detailed process was shown in early article [25].

ig. 1. (a) Schematic diagram of photoelectrochemical reaction system, (b) structuref dye reactive brilliant X-3B.

ll analytical reagent grade quality and were purchased from J&Khina Chemical Ltd. Titanium sheets were polished mechanicallynd washed with abluent, and further deionized water, alcohol, andcetone in turn by ultrasonic wash. At last, titanium sheets wereashed in mixture of HF (3.5 M) and HNO3 (6.5 M), and rinsed ineionized water for 10 s to obtain a fresh smooth surface.

.2. Preparation of TiO2 nanotube arrays

The self-organized and well-aligned TiO2 nanotube arraysere fabricated by anodization process. A rectangle Ti sheet

12 mm × 8 mm) was used as an anode and Pt foil (15 mm × 10 mm)s a cathode in the anodic oxidation experimental set-up (ashown in Fig. 1). The distance between Ti and Pt foil was fixedt 2.0 cm. A direct current power supply is utilized for the con-rol of experimental voltage in the electrochemical process. Byotentiostatic anodization at 20 V in electrolyte HF (0.1 M) and2SO4 (1.0 M) for 60 min, noncrystal TiO2 nanotube arrays werebtained.

.3. Preparation of N-doped TiO2 nanotube arrays

Totally, nitrogen-doped TiO2 nanotube arrays (denoted as NTN)ere prepared by treating amorphous TiO2 nanotube arrays withydrazine hydrate. Firstly, amorphous TiO2 nanotube arrays were

mmersed hydrazine hydrate (80%) for 6 h. And then, they wereried at 110 ◦C for 4 h in air. A post-calcination process (450 ◦C,h) was followed to crystallize TiO2 nanotube from amorphous to

natase phase. The noncrystal TiO2 nanotube arrays, which wereirectly calcined at 450 ◦C for 3 h without doping with N, wereenoted as TN. The colors of both TN and NTN were cyaneous withlittle difference.

Fig. 2. Spectrum of visible light source (MVL-210).

2.4. Characterization

The morphologies of nanotubes were characterized with ascanning electron microscopy (SEM, Quanta200, FEI, USA). Thecomposition of nitrogen was measured by an energy dispersiveX-ray analyzer (EDX, EDAX, USA) attached to the SEM.

2.5. Experimental set-up

The photoelectrochemical property was measured with an elec-trochemical workstation (CHI660A, CH Instruments Co.). TN wasused for comparison. A three-electrode configuration was used inexperiments (as seen in Fig. 1(a)). An external 250 W halogen lampof artificial solar light was utilized as light source. As a photoan-ode, NTN (or TN) was close to solar light to receive light irradiation.Pt plate was used as cathode. The distance of two electrolytes wasof 20 mm. A saturated calomel electrode (SCE) was used as a refer-ence electrode. X-3B (20 mg/L, the structure was shown in Fig. 1(b))+0.01 M Na2SO4 was used as electrolyte.

The photocatalytic activity of NTN (or TN) was tested in pho-tocatalytic degradation of Reactive Brilliant Red, X-3B in aqueoussolution. Visible light (MVL-210, the spectrum is shown in Fig. 2)with vertical irradiation was used as light source. Two pieces ofNTN (or TN) were dipped into 5 ml of X-3B solution with an initialconcentration of 20 mg l−1. Prior to photoreaction, air was pumpedinto reactor in the dark for 30 min to reach adsorption–desorptionequilibrium. Then, with continuous pump, the reaction was irra-diated by artificial solar light from the top vertically. During thephotoreaction, samples were collected at a time interval of every15 min for analysis.

3. Results and discussions

3.1. SEM and EDX analysis

The formation of TiO2 nanotube arrays on the surface of titaniumdepends on both reaction of electrochemical etching and chemi-cal dissolution [24]. The surface morphology of NTN and TN werecharacterized by SEM, and shown in Fig. 3. For both samples, NTNand TN, the highly ordered TiO2 nanotube arrays have been wellfabricated on Ti substrate. It shows that the ordered structure cansustain the impact of doping process and post-heat treatment. Themean size of inner diameter is about 65 nm and its wall thicknessis about 15 nm.

The nitrogen content of NTN was determined by EDX (seeFig. 3(d)). The detection limit for N is estimated to be 2 at.% in our

Numerous dots were selected for determining the doping elementcontent, and then average value was calculated. For sample NTN,the atomic ratio of N/Ti is 8/25. While for the TN sample, therewas no nitrogen detected (see Fig. 3(c)). The results showed that

J. Xu et al. / Applied Surface Science 256 (2010) 4397–4401 4399

NTN, a

nm

Nmo(s

E

Fig. 3. SEM images of (a) TN and (b)

itrogen doping titania nanotube array can be obtained by thisethod.Fig. 4 compares the UV–visible diffuse reflectance spectrum of

TN and TN. The results indicated that doping nitrogen by thisethod can give rise to a clear red-shift in the optical response

f titania nanotube array. Furthermore, higher visible absorbance

400–550 nm) was observed for NTN. The absorption edge of theample is determined by the following equation [26]:

g = 1239.8�

(1)

Fig. 4. DRS spectra of TN and NTN.

nd EDX analysis (c) TN and (d) NTN.

where Eg is the band gap (eV) of the sample, � (nm) is the wave-length of the onset of the spectrum. From the data of UV–vis diffusereflectance spectrum, the energy band gaps of TN and NTN can becalculated to be 3.26 and 3.04 eV, respectively. From the resultswe can predict that NTN would show high photocatalytic activityunder visible light irradiation.

3.2. Photoelectrochemical property

The photocurrent response measurement was carried out undervisible light pulsed irradiation to investigate the photo-inducedcharges separation efficiency of un-doped and N-doped samples.The results are shown in Fig. 5. The working electrode potentialswere located at 0 V to simulate the same working condition as pho-tocatalysis reaction system. We can see that photocurrent of NTN(0.182 mA/cm2), is much higher than undoped sample NT, whichis 0.092 mA/cm2 (Fig. 5). Higher photocurrent means more photo-induced electrons can transfer from NTN to counter electrode viaexternal circuit efficiently, illuminated by visible light. The photo-catalytic activity of titania greatly depended on the electron–holetransfer ability. Thus, it can be forecasted that the NTN would showhigher photocatalytic activity than TN.

3.3. Photocatalytic activity

The photocatalytic activity of NTN and TN was studied by degra-dation of X-3B solution, and the results are shown in Fig. 6(a). Directphotolysis ability was also studied. Only less than 1% of X-3B wasdecomposed. By effect of TiO2 catalysts and visible light, the photo-catalytic degradation percent of X-3B was 59.3% for TN and 99.0% for

4400 J. Xu et al. / Applied Surface Scien

Nsaooe

l

Fa

Fig. 5. Photocurrent response of NTN and TN.

TN, respectively. The apparent rate constant (kapp) has been cho-en as the basic kinetic parameter for the different photocatalysts,s it enables one to determine a photocatalytic activity independentf the previous adsorption period in the dark and the concentration

f X-3B remaining in the solution. The apparent first order kineticquation:

n(

C0

C

)= kapp × t (2)

ig. 6. Kinetic of X-3B degradation for different samples (a), variations in ln(C0/C)s a function of irradiation time and linear fits of samples (b).

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ce 256 (2010) 4397–4401

is used to fit experimental data. The variations in ln(C0/C) as afunction of irradiation time are given in Fig. 6(b). It can be seenthat NTN shows higher visible light photo-activity than TN. Theenhanced photocatalytic ability could be ascribed to the N dop-ing. As in our early articles [21], during N doping, the 2p orbital ofthe doped N atom are significantly interacting with that of O 2p,leading to a charge transfer between a dopant and a conduction orvalence band. As a result, giving rise to the red-shift in the bandgap transition. Therefore, catalyst NTN could be excited under visi-ble light, and shows high photocatalytic activity. The results are ingood agreement with the DRS investigation.

4. Conclusions

In summary, N-doped TiO2 nanotube arrays were prepared bytwo-step method. Firstly, TiO2 nanotube arrays through anodiza-tion method. And then, hydrazine was used as nitrogen source, toform N-doped TiO2 nanotube arrays by dip-calcinations method.The as-prepared novel materials can respond under visible light.They exhibit excellent photoelectrochemical property and photo-catalytic activity under visible light, compared with non-dopedTiO2 nanotube arrays. Due to that N doping could decrease the bandgap and enlarge response range to long wavelength.

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