Materials Letters 63 (2009) 2442–2444

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Materials Letters

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Preparation of B-doped titania hollow sphere and its photocatalytic activity undervisible light

Jingjing Xu a,⁎, Yanhui Ao b,⁎, Mindong Chen a

a College of Environmental Science and Engineering, Nanjing University of Information Science and Technology, Nanjing 210044, Chinab College of Environmental Science and Engineering, Hohai University, Nanjing 210092, China

⁎ Corresponding authors.E-mail addresses: xujj@seu.edu.cn (J. Xu), andyao@s

0167-577X/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.matlet.2009.08.031

a b s t r a c t

a r t i c l e i n f o

Article history:Received 20 July 2009Accepted 13 August 2009Available online 23 August 2009

Keywords:Composite materialsNanomaterialsSemiconductors

In this study, B-doped titania hollow spheres were prepared using hydrothermally prepared carbon spheresas template. The photocatalytic activity of as-prepared hollow titania spheres was determined bydegradation of Reactive Brilliant Red dye X-3B (C.I. reactive red 2) under visible light irradiation, and wascompared to commercial P25 titania. It was revealed that the photocatalytic activity of the hollow titaniaspheres enhanced a lot. The apparent rate constant of the hollow titania spheres was almost 22 times as thatof P25 titania. A photocurrent–time spectrum was also applied to investigate the efficiency of electrontransfer in the process of photocatalysis reactions by different samples.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

In the past several decades, a promising method as an alternativeto conventional methods for wastewater treatment is represented bytitania photocatalysis due to its high efficiency, low cost, and long-term stability against photo-corrosion [1]. However, its technologicalapplication seems limited by several factors, among which the mostrestrictive one is the need of using an ultraviolet (UV), wavelength(λ)<387 nm, as excitation source due to its wide band-gap (3.2 eV foranatase) [2]. Doping with non-metallic elements has received muchattention for extending of its light absorption into visible region [3–7].Asahi et al. [3] reported the preparation of nitrogen-doped TiO2 withvisible light sensitivity. Hattori et al. [5] found a significantenhancement on photocatalytic activity of titania by doping with F−

ions. More recently, TiO2 doped with boron has received a lot ofattention [8–12]. They found that the absorption edge was red-shiftedand photocatalytic efficiency was enhanced.

On the other hand, fabrication of titania hollow microspheres hasrecently attracted enormous attention because of their low density,high surface area, good surface permeability as well as large light-harvesting efficiencies [13]. However, the preparation of hollow titaniawith visible responsive activity has not been investigated until to now.Therefore, we prepared B-doped titania hollow spheres in the presentwork by a simple method. Furthermore, we investigated the photo-catalytic activity of the as-prepared B-doped titania hollow spheresunder visible light irradiation.

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2. Materials and methods

2.1. Preparation of carbon spheres

6 g of glucose was firstly dissolved in 40 ml of water. The solutionwas then sealed in a 50 ml Teflon-lined autoclave and maintained at170 °C for 5 h. The products were then washed by alcohol and waterfor five cycles, respectively. The carbon spheres were then dried at80 °C for 2 h under vacuum.

2.2. Preparation of B-doped titania

B-doped titania was prepared as follows: 50 ml Ti (OBu)4 dilutedwith 16 ml (i-PrOH) was dropwise added into NaBH4 aqueoussolution, whose acidity was adjusted with HNO3 (pH=2). After Ti(OBu)4 was hydrolyzed completely, the solution was transferred intoa stainless steel Teflon-lined autoclave andmaintained at 55 °C for 4 hwith pressure of 0.5 MPa. Then, the B-doped titania sol was obtainedafter the remove of PrOH and n-butyl alcohol from the solution in arotatory evaporator under vacuum.

2.3. Preparation of B-doped titania hollow spheres (BTH)

0.1 g carbon spheres were added into 50 ml B-doped titania solprepared by abovementionedmethod. The suspensionwas stirred for12 h. Then it was centrifuged, washed by water for 3 times. Thus, B-doped titania coated carbon spheres were obtained. In order toproduce B-doped titania hollow spheres of anatase, the titania–carboncomposite particles were calcined at 500 °C for 3 h in the air. For

Fig. 1. The diffuse reflectance UV–vis spectra of BTH and TH, XRD patterns of BTH(inset).

Fig. 3. XPS spectrum of BTH and high-resolution spectrum of the B1s (inset).

2443J. Xu et al. / Materials Letters 63 (2009) 2442–2444

comparison, titania hollow spheres (TH) were prepared by the sameway using pure titania sol.

2.4. Photocatalytic and photoelectrochemical activity

0.01 g of samples was dispersed into 20 ml of an X-3B aqueoussolution (25 mgl−1) and then irradiated with a 250W halogen lamp(Instrumental Corporationof BeijingNormalUniversity,with a lightfiltercutting the light below 400 nm) under continuous stirring. Before theirradiation, the suspension was maintained in the dark for 1 h to reachcomplete adsorption–desorption equilibrium. The concentration of X-3Bwas determined from the absorbance at the wavelength of 535 nm.

The photoelectrochemical activity of the different samples sus-pended in X-3B aqueous solution was also investigated. The measure-ments were carried out by using a standard three-electrode systemequippedwith a quartzwindow, a saturated calomel reference electrode(SCE) and a platinum plate counter-electrode (CE) placed in the samecell, and optically transparent In2O3–SnO2 oxide conductive glass sheetwas selected as working electrode. The electrolyte was 1 gl−1 ofdifferent photocatalysts and 50 mgl−1 of X-3B aqueous mixture. Theworking electrode was illuminated from the front side with the abovementioned halogen lamp. The photocurrent–time (I–t) measurementwas recorded on an electrochemical workstation.

Fig. 2. TEM images of carbon spheres (inset) and BTH.

3. Results and discussion

3.1. Characterization of BTH

Fig. 1 compares theUV–visible diffuse reflectance spectra of BTH andTH. The results indicated that doped boron by this method can give riseto a clear red-shift in the optical response of the titania hollow spheres.Furthermore, high visible absorbance (400–550 nm) was observed forB-doped samples. The XRD pattern of BTH is shown in the inset of Fig. 1.As shown in thefigure, the particles had formed anatase phase since thecharacteristic diffraction peaks of anatase (major peaks: 25.4°, 38.0°,48.0°, 54.7°, 63.1°) are evident in the sample. No other peaks such asB2O3 appeared. Itmay be due to the reason that the content of Bwas toolittle to be detected.

The TEM image of carbon spheres is shown in the inset of Fig. 2. It canbe seen from thefigure that the diameter of as-prepared carbon spheresranges from 250 to 300 nm. Fig. 2 shows TEM images of BTH. The strongcontrast between thedark edges and bright centers indicates thehollowstructure of titania spheres. It can be seen that the hollow spheres havean average diameter of 250–300 nm and the shell thickness is about30 nm.

X-ray photoelectron spectroscopy analysis (as shown in Fig. 3)indicates that the peak contains Ti, O, C and B atoms. Part of C atomsmaycome from the residual carbon of precursor solution. The other C atomsprobably come from carbon spheres during the calcinations. The high-

Fig. 4. Photocurrent response in different system under visible light impulse irradiation.

Fig. 5. Kinetics of X-3B degradation with the irradiation time and Linear transformln(C0/C)= f(t) of the kinetic curves of X-3B degradation (inset).

2444 J. Xu et al. / Materials Letters 63 (2009) 2442–2444

resolution XPS spectrum of the B1s is shown in the inset of Fig. 3. It canbe seen that the binding energy of B1s is centered at 191.3 eV. Takinginto account the standard binding energy of B1s in B2O3 (193.0 eV, B–Obond) and in TiB2 (187.5 eV, B–Ti bond), it may be deduced that boronatoms are probably incorporated into TiO2 to some extent. Most of theboronatomsexist in thephase of B2O3, and there are still a small amountof boron forming B–Ti bonds.

3.2. Photoelectrochemical activity

To study the photo induced charges separation efficiency of the as-prepared BTH, the photocurrent response experiments have beencarried out under visible light pulsed irradiation and the results areshown in Fig. 4. Results show that the photocurrent of BTH (0.80 μA) ismuch higher than that of TH (0.32 μA) and P25 (0.12 μA). Higherphotocurrent means photo induced electrons have transferred moreeffectively from titania to counter electrode via external circuit. Thephotocatalytic activity of titania greatly depended on the efficiency ofelectron–hole pair excitation, electron–hole transfer, and electron–hole charge pairs separation. Therefore, it can be forecasted that BTHwould show higher photocatalytic activity than TH and P25.

3.3. Photocatalytic activity

Degradation experiments of X-3B were studied under visible lightand results are shown in Fig. 5. The blank experiment without catalystswas also investigated. And the value can be neglected with about 2% of

conversion after 2h illumination. For the absolute degradation ratio ofX-3B, the trend is BTH>TH>P25. The apparent first order kineticequation ln(C0/C)=kappt is used to fit experimental data, where kappis the apparent rate constant, C is the solution-phase concentration ofX-3B, and C0 is the initial concentration at t=0 [14]. The variations in ln(C0/C) as a function of irradiation time are given in the inset of Fig. 4.The obtained kapp are 7.15⁎10−4, 0.00316 and 0.0159 min−1 for P25,TH and BTH, respectively. From the results, we can see that BTH ex-hibited a strong photocatalytic activity for decomposition of X-3B undervisible light irradiation.

The different photocatalytic activity of different samples can beattributed to the following factors. It is well known that the doping ofB elements in titania plays an important role in the visible-lightphotocatalytic activity of titania. It can be seen from the UV–vis diffusereflectance spectrumof BTH that it results in an increase in absorption inthe visible light region and a red-shift in the absorption edge. The band-gap narrowing of titania by B-doping leads to enhanced photocatalyticactivity of thehollow titania sphere under visible light. Furthermore, thehollow structure of BTH and TH would show high energy conversionefficiency [15,16].

4. Summary

A green and simple method was reported to prepare B-dopedtitania hollow spheres. The photocatalytic activity of as-preparedhollow titania spheres was determined by degradation of X-3B undervisible light irradiation, and compared to commercial P25 titania. Theapparent rate constant of the hollow titania spheres was almost 22times as that of P25 titania. Photoelectrochemical results illustratethat B-doped titania hollow spheres behave enhanced photocatalyticactivity for enhancing interfacial electron transfer in the heteroge-neous reaction system.

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