Effects of Boron Doping on Photocatalytic Activity and Microstructure of

May 13, 2006 - Boron-doped TiO2 nanoparticles were prepared by the sol-gel method and characterized by XRD, TEM,. XPS, FT-IR, and UV-vis spectroscopy...
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APPLIED CHEMISTRY Effects of Boron Doping on Photocatalytic Activity and Microstructure of Titanium Dioxide Nanoparticles Daimei Chen, Dong Yang, Qun Wang, and Zhongyi Jiang* Key Laboratory for Green Chemical Technology, School of Chemical Engineering and Technology, Tianjin UniVersity, Tianjin 300072, People’s Republic of China

Boron-doped TiO2 nanoparticles were prepared by the sol-gel method and characterized by XRD, TEM, XPS, FT-IR, and UV-vis spectroscopy. XRD results showed that the doping of boron ions could efficiently inhibit the grain growth and facilitate the anatase-to-rutile transformation prior to the formation of diboron trioxide phase. FT-IR and XPS results revealed that the doped boron was present as the form of B3+ in B-doped TiO2 samples, forming a possible chemical environment like Ti-O-B. The lattice parameters at different boron contents and calcination temperatures indicated that B3+ was likely to weave into the interstitial TiO2 structure. The photocatalytic activity of the B-doped TiO2 nanoparticles was evaluated by the photoregeneration of reduced nicotinamide adenine dinucleotide (NADH). All B-doped TiO2 nanoparticles calcined at 500 °C showed higher photocatalytic activity than pure TiO2 sample in the photocatalytic reaction of NADH regeneration under UV light irradiation. When the molar ratio of B to Ti was 5%, the TiO2 nanoparticles could photocatalytically reproduce 94% NADH. Introduction In the past several decades, much effort has been devoted to semiconductor photocatalysis because it is an efficient method for the chemical utilization of solar energy.1,2 Titanium dioxide is the most promising photocatalyst because of its high efficiency, low cost, chemical inertness, and long-term stability against photocorrosion and chemical corrosion, and it has been used in various fields, such as solar cells, photocatalytic splitting of water for green-energy hydrogen production, selective synthesis of organic compounds, air purification, removal of organic and inorganic pollutants, and photokilling of pathogenic organisms.3-9 However, undesired recombination of photoexcited carriers and a high band gap (Eg ) 3.2 eV) seriously limited the practical application of TiO2. Doping with other elements, as a simple and feasible method, has been widely used for TiO2 modification to improve its photocatalytic activity or to extend its light absorption into the visible region. An initial approach is doping of TiO2 with transition metal elements such as Cr, Fe, or Ni,10-18 which can substitute for the Ti site in the crystal structure of TiO2. However, the photocatalytic activity of metal-doped TiO2 is impaired by the thermal instability and an increase in carrierrecombination facilities. Recently, a new approach to improve the photocatalytic efficiency by doping with nonmetal elements has been introduced. TiO2 doped with nonmetal elements, such as F,19-21 I,22 and P,23 and codoped with Cl and Br24 showed higher photocatalytic activity under UV irradiation. Also, TiO2 doped with N,25-27 C,28,29 and S,30-32 and codoped with N and F,33,34 showed high photocatalytic activity under visible light owing to band gap narrowing. Among the nonmental elements, nitrogen has been extensively studied in the past few years. For * To whom correspondence should be addressed. Tel.: +86-2227892143. Fax: +86-22-27892143. E-mail: [email protected].

the different synthetic routes of nitrogen-doped TiO2, the interpretation of visible light response is proposed differently, from the presence of NOx35 or NHx36 to the substitutional nitrogen doping. However, the detailed mechanism of other nonmetal dopants influencing the optical absorption and photocatalytic activity of TiO2 is poorly understood and deserves to be further explored. Recently, TiO2 nanomaterials coupled with diboron trioxide due to their possible capability of photocatalytically splitting water, narrowing the band gap of TiO2 and reducing the recombination of photoinduced electrons and holes, have received a lot of attention.37-42 For example, Zhao et al.37 reported that doping with boron and Ni2O3 in TiO2 resulted in the improvement of TiO2 in both spectral response and photocatalytic efficiency. Moon and co-workers38,39 found that Pt-loaded Ti/B binary oxide exhibited special photocatalytic activity for the stoichiometric decomposition of pure water in suspension systems, in which the boron oxide played a significant role in the suppression of recombination of envolved H2 and O2 of the reaction system. The study of Grey et al.40 suggested that boron incorporation into rutile TiO2 led to partial reduction of Ti4+ to Ti3+, which could improve the photocatalytic activity of rutile TiO2 since Ti3+ sites could act as the photogenerated electron traps and thus facilitate the charge separation. To the best of our knowledge, however, the effort for TiO2 doped with only boron element has been little until now. In the present work, B-doped TiO2 nanoparticles with different atomic ratios of B to Ti (from 1 to 20%) were prepared by a sol-gel method, and their intrinsic characteristics were investigated using X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), and ultraviolet-visible (UV-vis) spectroscopy. The effects of boron

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doping on the intrinsic characteristics of TiO2 were investigated in detail. The photocatalytic activity of B-doped TiO2 nanoparticles was evaluated by the photoregeneration of reduced nicotinamide adenine dinucleotide (NADH), and the photocatalytic mechanism is discussed. Experimental Section Chemicals. Nicotinamide adenine dinucleotide (NAD) and reduced nicotinamide adenine dinucleotide (NADH) were reagent grade supplied from Sigma and were used as received. Pentamethylcyopentadienylrhodium(III) chloride (Cp*RhCl2) was purchased from Aldrich and was used to synthesize [Cp*Rh(bpy)Cl]Cl according to literature methods,43 which can readily hydrolyze to be [Cp*Rh(bpy)(H2O)]2+ in water. All other reagents were of analytical grade, and the water used was deionized. Preparation of Photocatalysts. Titanium tetra-n-butyl oxide (TTB) and H3BO3 were used as TiO2 and boron precursor, respectively. Boron-doped TiO2 nanoparticles were prepared by the following procedure: 0.02 mol of TTB was added dropwise to 50 mL of the H2O-H3BO3 mixed solution under vigorous stirring at room temperature with different atomic ratios of B to Ti; the atomic ratio of B to Ti is hereafter designated as RB. Subsequently, the resulting solution was stirred in a closed beaker at room temperature for 12 h to further hydrolyze TTB and obtain monodisperse TiO2 nanoparticles. The sol solution obtained was dried at 100 °C for 8 h in air to vaporize water and alcohol to obtain an xerogel, and then calcined at high temperature in air for 1 h to make TiO2 crystallize. A series of B-doped TiO2 nanoparticles were prepared by changing RB and the calcination temperature. The B-doped nanoparticles were denoted as TiO2-X-Y, where “X” and “Y” represent RB and the calcination temperature in degrees Celsius, respectively. For comparison, undoped TiO2 was also prepared with the same procedure without H3BO3. Characterization. The particle morphology was observed with a JEM-100CX II transmission electron microscope (TEM). The Brunauer-Emmett-Teller (BET) surface areas (SBET) of the powder samples were determined by nitrogen adsorptiondesorption isotherm measurements at 77 K on a CHEMBET3000 nitrogen adsorption apparatus. The X-ray powder diffraction (XRD) pattern was obtained with a Philips X’Pertpro diffractometer using Co KR radiation with an accelerating voltage of 40 kV and current of 40 mA, which was used to determine the crystallite size and identity of TiO2 samples. The surface properties of TiO2 samples were characterized by X-ray photoelectron spectroscopy (XPS) in a Perkin-Elmer PHI 1600 ESCA system with a monochromatic Mg KR source and a charge neutralizer. The FT-IR spectra were measured using a Nicolet spectrometer. The concentration of the samples mixed with KBr was kept around 0.25-0.3%. The diffuse reflectance spectra of TiO2 samples in the 200-1100 nm range were recorded using a Perkin-Elmer Lambda35 UV-vis spectrophotometer. Measurement of Photocatalytic Activity. The photocatalytic activities of pure and B-doped TiO2 nanoparticles were measured by the photocatalytic regeneration of NADH in N2 atmosphere at room temperature. The photocatalytic regeneration of NADH by pure and B-doped TiO2 nanoparticles was carried out in a 100 mL water-cooled cylindrical quartz reactor. The weight of catalyst powder used in each experiment was kept at 80 mg. Prior to experimentation, the solution of NAD+ and [Cp*Rh(bpy)(H2O)]2+ was allowed to reach adsorption equilibrium with the photocatalyst in darkness. The initial concentrations of

Figure 1. XRD patterns of TiO2-RB-500 with different RB values: (a) 0; (b) 1; (c) 3; (d) 5; (e) 10; (f) 20.

NAD+ and [Cp*Rh(bpy)(H2O)]2+ at the adsorption equilibrium were 0.2 and 0.4 mM, respectively. The pH of reactive solution was kept at 6.0 using 100 mM phosphate buffer solution. The experiment started once the 250 W high-pressure Hg lamp (λ < 400 nm) above the reactor was turned on. At a defined time interval, the concentration of NADH produced in the photocatalytic reaction was analyzed using the UV-vis spectrophotometer (Hitachi U-2800) at 340 nm. Results and Discussion Crystal Structure. XRD was carried out to investigate the changes of TiO2 phase structure after boron doping and calcination treatment. Figure 1 shows the effect of RB on the crystal structure of TiO2 nanoparticles calcined at 500 °C for 1 h. The anatase phase is the main crystal form of TiO2 in the undoped TiO2 powders, containing a small amount of brookite. With the increase of RB value, the intensity of brookite peak (B) changes little, indicating that boron ion cannot suppress the crystallization of brookite or catalyze the brookite to anatase transformation like F-.19 Moreover, with increasing RB, the anatase peaks (A) gradually become wider, which means that the size of B-doped TiO2 nanoparticles is reduced correspondingly. According to the line width analysis of the anatase (101) diffraction peak based on the Scherrer formula, the average crystallite sizes of all TiO2 samples are estimated to be about 15.1, 11.4, 11.0, 9.2, 8.7, and 9.3 nm, as RB increases from 0 to 20. Accordingly, the BET surface areas of TiO2-RB-500 samples changed from 95.4, 99.3, 101.4, 108.9, and 119.6 m2/g to 110.4 m2/g. These results confirm that boron doping can efficiently inhibit the crystal size and increase the surface area of TiO2. When RB is 20, the B2O3 phase appears, probably due to the fact that the doped boron ions segregate from the internal of anatase structure and form a layer of diboron trioxide phase on the surface of TiO2 nanoparticles. Figure 2 shows the effect of calcination temperature on the phase structure of B-doped TiO2 samples with RB ) 10. It is found that, with the calcination temperature increasing from 400 to 800 °C, the intensity and the width of the diffraction peaks of anatase become higher and narrower, respectively, suggesting that the crystallinity of anatase becomes much improved. When the calcination temperature is over 600 °C, the diboron trioxide crystal appears. Considering the fact that the B2O3 phase emerged at 500 °C in the B-doped TiO2 sample when RB was 20 and did not occur at 500-800 °C when RB was no more than 10, the separation of boron from the TiO2 crystal is related to not only the calcination temperature but also the RB value. It is inferred that the doped boron ions may be present in the interstitial site of anatase, which can balance the residual charge

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sites. On the other hand, since the transformation involves an overall contraction or shrinking of the oxygen structure and the cooperative movement of the ions, the removal of oxygen ions, which presumably reduces the strain energy that must be overcome before structural rearrangement, accelerates the transformation. Since the ion radius of B3+ (0.023 nm) is smaller than that of Ti4+ (0.064 nm), boron oxides existing at the surface or grain boundary or in the matrix of titanium dioxide can easily incorporate into the framework of titanium and thus promote an increase in surface defects (oxygen vacancies). In addition, the formation of diboron trioxide phase is also a key factor for the anatase-to-rutile transformation. When the temperature and boron amount are high enough to form the diboron trioxide phase, such as RB ) 10 and 700 °C, the doped boron ions segregate from the internal of anatase structure and form a layer of diboron trioxide phase on the surface of TiO2 nanoparticles. This can suppress diffusion between anatase particles in direct contact and limit their ability to act as surface nucleation sites for rutile. To the best of our knowledge, it is the first report that nonmetal elements can affect the phase transformation of TiO2. To further investigate the effect of boron doping on the crystal structure of TiO2, we calculated the lattice parameters of TiO2 sample listed in Table 2 as the calcination temperature and RB changed. All the lattice parameters were obtained by using Bragg’s law (2d sin θ ) λ) and a formula for a tetragonal system: 1/d2 ) (h2 + k2)/a2 + l2/c2. It is clearly seen from Table 2 that the lattice parameters of all TiO2 samples remain almost unchanged along the a-axis with the change of RB and calcination temperature. The c-axis parameter increases as RB changes from 0 to 5, indicating that boron ions may enter the interstitial site of TiO2 anatase crystal structure and lead to the swell of unit cell volume. Given that the radius of B3+ (0.023 nm) is much smaller than that of Ti4+ (0.064 nm), it is difficult for B3+ to replace the Ti4+ site. Thus, it is estimated that most boron ions are doped in the interstitial of TiO2 matrix. With the sequential increase of RB (from 5 to 20), however, the c-axis parameter decreases. It is inferred that a part of boron ions separates from the TiO2 lattice to form diboron trioxide phase at this time, which retards the crystal growth and results in the decrease of unit cell volume. At the calcination temperature of 500 °C, since the diboron trioxide phase formed at RB ) 20, the maximum c-axis occurs at RB ) 10. Figure 3 shows the typical TEM images of B-doped TiO2 samples with RB ) 20 calcined at different temperatures for 1 h. A clear relationship between the size of B-doped TiO2 and calcination temperature is observed. As the calcination temperature increases from 500 to 800 °C, the microcrystal size of

Figure 2. XRD patterns of B-doped TiO2 samples with RB ) 10 and calcined at (a) 400, (b) 500, (c) 600, (d) 700, and (e) 800 °C for 1 h.

of the TiO2 nanoparticles, rather than at the substituted site of oxygen ion.37 This can reduce the surface energy of the nanoparticles and hinder the growth of anatase grains. When the concentration of doped boron is beyond the solubility limit of the boron in the TiO2 anatase structure due to the increase of calcination temperature or RB, the boron ions expelled from the anatase structure would form nanoclusters on the surface of TiO2 nanoparticles and grow slowly until clusters merged into a thin film of diboron trioxide. Meanwhile, it can also be observed from Figure 2 that the phase transformation of anatase to rutile occurs at 700 °C, and the proportion of rutile phase obviously becomes greater as the calcination temperature increases from 700 to 800 °C. The ratio between anatase and rutile extracted from XRD spectra, which is often used to quantify the anatase-to-rutile transformation, is calculated with the empirical relationship44

( )

IR IR R(T) ) 0.679 + 0.312 IR + IA IR + IA

2

where R(T) is the percentage content of rutile at each temperature, IA is the intensity for the main anatase reflection, and IR is the intensity for the main rutile reflection. Table 1 shows the percentage content of rutile and anatase in the TiO2-RB-700 and TiO2-RB-800 samples. From Table 1 it can be seen that, with the increase of RB from 1 to 5, the rutile content increases from 1.5% to 4.7% at 700 °C and from 23.4 to 56.9% at 800 °C. However, the rutile content decreases when RB is greater than 5. This means that there is an inflection point at RB ) 5, before which the anatase-to-rutile transformation rate is accelerated and after which the transformation process is retarded. It has been reported6 that an increase of surface defects will enhance the rutile transformation ratio, as these defects can act as nucleation

Table 1. Effect of Calcination Temperature and RB on Phase Structure of TiO2 700 °C 800 °C

RB ) 0

RB ) 1

RB ) 3

RB ) 5

RB ) 10

RB ) 20

A: R: A: R:

A: R: A: R:

A: R: A: R:

A: R: A: R:

A: R: A: R:

A: R: A: R:

98.5 1.50 76.6 23.4

98.4 1.60 75.8 24.2

97.2 2.80 62.2 37.8

95.3 4.70 43.1 56.9

97.6 2.40 81.7 18.3

99.0 1.00 96.9 3.08

Table 2. Effect of Calcination Temperature and RB on Lattice Parameters of TiO2 RB ) 0 500 °C 600 °C 700 °C 800 °C

a: c: a: c: a: c: a: c:

3.7995 9.5958 3.7975 9.5826 3.7954 9.5651 3.7934 9.5326

RB ) 1 a: c: a: c: a: c: a: c:

3.7918 9.6082 3.7986 9.5914 3.7970 9.5870 3.7937 9.5558

RB ) 3 a: c: a: c: a: c: a: c:

3.7934 9.6263 3.7940 9.6091 3.7885 9.6047 3.7933 9.5624

RB ) 5 a: c: a: c: a: c: a: c:

3.7904 9.6946 3.7954 9.6358 3.7967 9.6047 3.7953 9.5830

RB ) 10 a: c: a: c: a: c: a: c:

3.7910 9.7220 3.7921 9.6348 3.7984 9.5914 3.7934 9.5630

RB ) 20 a: c: a: c: a: c: a: c:

3.7951 9.4958 3.7932 9.5738 3.7943 9.5738 3.7956 9.5347

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Figure 4. XPS spectrum of TiO2-10-500.

Figure 5. B 1s high-resolution XPS spectra of TiO2-RB-500 with different RB values: (a) 3; (b) 10; (c) 20.

Figure 6. O 1s high-resolution XPS spectra of (A) TiO2-0-500 and (B) TiO2-20-500.

Figure 3. TEM photographs of B-doped TiO2 samples with RB ) 20 calcined at (a) 500, (b) 600, (c) 700, and (d) 800 °C for 1 h.

B-doped TiO2 gradually increases because of the fusion of TiO2 nanoparticles, in good agreement with XRD results. At the same time, the BET surface areas decrease from 110.4, 60.1, and 24.7 m2/g to 14.9 m2/g. However, the phase separation in B-doped TiO2 is not observed, indicating that the B-doped TiO2 nanopartilcles possess a homogeneous structure. XPS Studies. Figure 4 shows the X-ray photoelectron spectroscopic (XPS) survey spectrum of TiO2-10-500. XPS peaks show that the B-doped TiO2 powders contain only Ti, O, B, and C elements and the binding energies of Ti 2p, O 1s, B 1s, and C 1s are 458.7, 531, 193, and 284 eV, respectively. The C element can be ascribed to the residual carbon from

precursor solution and the adventitious hydrocarbon from XPS instrument itself. The XPS spectra of other samples are similar. Figure 5 shows the high-resolution XPS spectra of the B 1s region on the surfaces of TiO2-3-500, TiO2-10-500, and TiO220-500. It is observed that the B 1s region contains one peak only, and the binding energy increases from 191.1 to 192.2 eV with the increase of RB from 3 to 20, suggesting the increase of B-O bonds on the surface of B-doped TiO2. Taking into account the standard binding energy of B 1s in B2O3 or H3BO3 (193.0 eV, B-O bond) and in TiB2 (187.5 eV, B-Ti bond), this result displays that the boron atom is probably incorporated with TiO2 to some extent, and the chemical environment surrounding boron is similar to that in pure B2O3. Figure 6 shows the high-resolution XPS spectra of the O 1s region taken on the surface of undoped (Figure 6A) and B-doped (Figure 6B) TiO2 calcined at 500 °C for 1 h. The O 1s region of the calcined undoped TiO2 is composed of two peaks at 529.8 and 531.4 eV, corresponding to Ti-O and the hydroxyl group, respectively. However, the broad O 1s region of the calcined B-doped TiO2 can be fitted by three peaks, which are Ti-O in TiO2,

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Figure 7. FT-IR spectra of TiO2-RB-500 with different RB values: (a) 0; (b) 1; (c) 3; (d) 5; (e) 10; (f) 20.

Figure 8. UV-visible absorption spectra of TiO2-RB-500.

B-O bond, and hydroxyl groups, respectively, further confirming the presence of B-O bond. FT-IR Spectra. Figure 7 shows the FT-IR spectra of pure and B-doped TiO2 samples calcined at 500 °C for 1 h. It is believed that the broad peaks at 3400 and 1650 cm-1 correspond to the surface-adsorbed water and hydroxyl groups. As RB increases, the change of band shape is observed in two peaks in Figure 7. When RB is up to 20, a distinct peak around 1200 cm-1 can be observed, which belongs to the B-O bond in the B2O3 crystal. However, this peak cannot be found on the undoped and low-content B-doped TiO2 samples, in good accordance with the results of XRD. Another peak around 1400 cm-1 is present in the IR spectra of B-doped TiO2 but absent for the calcined pure TiO2, and its intensity increases with the increase of boron content. Sagawa et al. reported that the tricoordinated boron framework in B-ZSM-5 gave an IR spectrum at 1385 cm-1,45 and Jung et al.41 thought that the peak at 1393 cm-1 is attributed to the Ti-O-B bond in the B2O3SiO2/TiO2 ternary mixed oxide prepared by a sol-gel method. Therefore, the peak around 1400 cm-1 may be attributed to TiO-B bonds in this work. UV-Vis Absorption Spectra. Figure 8 illustrates the UVvis absorption spectra of undoped and five B-doped TiO2 samples calcined at 500 °C for 1 h; the small chart in the upper right corner of Figure 8 is the enlarged figure at 360-410 nm. It can be clearly observed that the absorption edge position of all B-doped TiO2 moved toward shorter wavelength, which is called a blue shift, meaning the band gap of TiO2 increases after boron doping. Given that the crystal size of TiO2 samples becomes smaller as the boron amount increases, the band gap widening should be attributed to the decrease of TiO2 crystal size. When the boron amount is greater than 5%, the size of TiO2 crystal is less than 10 nm, indicating that the band structure of titanium in B-doped TiO2 becomes quantized. Larger band gap energy of B-doped TiO2 can result in larger thermodynamic driving force and faster charge carrier transfer rates in the normal

Figure 9. NADH regeneration photocatalyzed by TiO2-RB-500, together with [Cp*Rh(bpy)(H2O)]2+ as the electron mediator and H2O as the electron donor.

Marcus region than its bulk phase counterparts. Moreover, the boron doping can also lead to the formation of partial Ti3+, which limits the recombination rate of charge carriers.40 It can be seen from Figure 8 that the absorbance of TiO2 at 250-320 nm increases with the increase of doped boron amount, which can result in the improvement of the photocatalytic activity in the UV region. Photoregeneration of NADH. To examine the photocatalytic efficiency of the B-doped TiO2 nanomaterials, the photoregeneration of NADH has been introduced. NADH46 is one of the most important cofactors in the biosynthesis involving many oxidoreductases, and a number of strategies including enzymatic catalysis,47-49 whole-cell conversion,50-52 and chemical method53-60 have been devised for the regeneration of NADH in order to reduce the cost of products. In this work, the regeneration of NADH catalyzed by B-doped TiO2 nanoparticles under UV light irradiation has been studied, together with [Cp*Rh(bpy)(H2O)]2+ and H2O as the electron mediator and electron donor, respectively. Figure 9 shows the changes of NADH concentration with reaction time for undoped TiO2 and B-doped samples calcined at 500 °C for 1 h. All B-doped TiO2 samples exhibit improved photoactivity compared with pure TiO2, which may be interpreted in terms of the quantization effect and the enhanced absorption in the UV range. The size quantization resulting in an increase of the band gap energy is expected to be helpful for the photoactivity because the photoexcited electrons are confined in the conduction band and their lifetime is elongated. The enhancement of absorbance in the UV region increases the number of photogenerated electrons and holes to participate in the photocatalytic reaction, which can enhance the photocatalytic activity of TiO2. Furthermore, H2O acts as an electron donor in our photocatalytic system; therefore, the rate of the water ionization may be a key factor for the NADH regeneration.61 After excitation by UV light irradiation, the dispersed TiO2 particles in solution can be regarded as nanosized minicells, which induces H2O to ionize into H+ and OH-. Because of the increase of band gap of B-doped TiO2, the rate of ionizaton water increases. It is thus estimated that B-doped TiO2 can improve the electron-supplying capacity of H2O, consequently improving the rate of NADH regeneration. When RB is no more than 5, the photoactivity of B-doped TiO2 nanoparticles increases slightly with the increase of RB. The conversion of NADH is maximal and up to 94%, when RB ) 5. When RB is greater than 5, the photoactivity of TiO2-10500 decreases at first and then increases again. One possible reason is that boron doping converts some Ti4+ to Ti3+ by charge compensation. 39 The existence of a certain amount of Ti3+ on the surface of TiO2 nanoparticles could enhance the photoac-

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tivity because it can act as an active site to assist the adsorption of reactant and trap the photogenerated electron to reduce the recombination of photoexcited electrons and holes. On the other hand, it also impairs the photoactivity because it can serve as a recombination center for photoexcited electron and hole pairs. When RB is no more than 5, the Ti3+ state seems to be formed on the surface or the sublayer of anatase, which can facilitate the separation of photoexcited electron and hole pairs and slow their recombination. When RB is greater than 5, more Ti3+O-B bonds are produced in the sublayer or bulk of anatase matrix, the recombination centers of photogenerated carriers are formed, and their recombination accelerates. Therefore, the photocatalytic activity of TiO2-10-500 is lower than that of TiO25-500. Compared with sample TiO2-10-500, the photocatalytic activity of TiO2-20-500 has a significant improvement, which is only lower than that of TiO2-5-500. This result can be attributed to the formation of a new phase of diboron trioxide. It is well-known that binary oxide catalysts often exhibit higher catalytic activity than what can be directly predicted from the properties of their components. The interface between the two phases may act as a rapid separation site for the photogenerated electrons and holes due to the difference in the energy level of their conduction bands and valence bands. In addition, Moon et al.36,37 found that the Pt-loaded Ti/B binary oxide exhibited special photocatalytic activity for the stoichiometric decomposition of pure water. Therefore, it is estimated that when RB is up to 20 the titanium-boron binary oxides form, which enhances the rate of water decomposition, consequently, and improves the rate of NADH regeneration. Conclusion In summary, a simple sol-gel method was developed for the preparation of B-doped anatase TiO2 nanoparticles by hydrolysis of titanium tetra-n-butyl oxide in H3BO3 aqueous solution. The doping of boron could efficiently inhibit the grain growth and facilitate the anatase-to-rutile transformation before the formation of diboron trioxide phase. It was inferred that the doping boron was present in the form of B3+ in B-doped TiO2 samples and was likely to weave into the interstitial TiO2 structure, whose chemical environment may be the state of Ti-O-B. When the temperature and boron amount were high enough to form the diboron trioxide phase, the doped boron ions segregated from the internal of the anatase structure and formed a layer of diboron trioxide phase on the surface of TiO2 nanoparticles, which could suppress diffusion between anatase particles in direct contact and limit their ability to act as surface nucleation sites for rutile. All B-doped TiO2 nanomaterials calcined at 500 °C showed increased photocatalytic activity over that of pure TiO2 sample in the photocatalytic reaction of NADH regeneration under UV light irradiation, due to the quantization effect and the intense absorption in the UV range. When the molar ratio of B to Ti was 5%, the B-doped TiO2 nanoparticles could photocatalytically regenerate 94% of NADH. Acknowledgment This work is supported by the Program for the Cross-Century Talent Raising Program of Ministry of Education of China and Changjiang Scholars and Innovative Research Teams in University (PCSIRT). We thank Professor Zhong Shunhe and Professor He Fei for their kind assistance in diffuse reflectance spectra and XPS measurements, respectively. Literature Cited (1) Mills, A.; Hunte, S. L. J. Photochem. Photobiol., A: Chem. 1997, 108, 1.

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ReceiVed for reView January 19, 2006 ReVised manuscript receiVed April 5, 2006 Accepted April 19, 2006 IE0600902