Facile Method for Fabricating Boron-Doped TiO2 Nanotube Array with

Apr 24, 2008 - rent density of the boron-doped sample is greater than that of .... oxidation of rose bengal in aqueous solution using a Ti/TiO2 mesh e...
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Ind. Eng. Chem. Res. 2008, 47, 3804–3808

Facile Method for Fabricating Boron-Doped TiO2 Nanotube Array with Enhanced Photoelectrocatalytic Properties Jingyuan Li, Na Lu, Xie Quan,* Shuo Chen, and Huimin Zhao Key Laboratory of Industrial Ecology and EnVironmental Engineering (Ministry of Education), School of EnVironmental and Biological Science and Technology, Dalian UniVersity of Technology, Dalian 116024, People’s Republic of China

Highly ordered boron-doped TiO2 nanotube arrays were fabricated via a facile electrodeposition method. X-ray photoelectron spectroscopy (XPS) analysis revealed incorporated B atoms in the lattice of a TiO2 nanotube array. The X-ray diffraction (XRD) spectrum indicated improved crystallinity of boron-doped TiO2 nanotube arrays, relative to undoped TiO2 nanotube arrays. A shift of the absorption edge toward the visible region and a new absorption shoulder (380–510 nm) of boron-doped TiO2 nanotube arrays were observed via diffuse reflectance spectroscopy (DRS). In photoelectrochemical measurements, under either ultraviolet (UV) or visiblelight irradiation, the photocurrent conversion efficiency was enhanced because of boron doping. The photoelectrocatalysis of phenol under simulated solar irradiation was performed using boron-doped or undoped TiO2 nanotube arrays, and the kinetic constant of a boron-doped TiO2 nanotube array photoelectrode was increased by ca. 28%, compared to that of an undoped TiO2 nanotube array photoelectrode. 1. Introduction TiO2 electrodes have been extensively investigated for their superior semiconducting material properties,1,2 and among the TiO2 photoelectrodes with various morphologies and architectures, the anodic TiO2 nanotube array exhibits more promising photochemical and photocatalytic properties, because of its nanotube-array architecture, which enhances the electron percolation pathway for vectorial charge transfer, promotes ion diffusion in the semiconductor/electrolyte interface,2–4 and restrains photogenerated electron–hole pairs from recombination.5 Recently, TiO2 nanotube arrays have been investigated in photoelectrochemical and photoelectrocatalytic regions, such as solar cells,4,6 photoelectrocatalytic water splitting,7 and the degradation of organic pollutants.8 We found that the mineralization efficiency of pentachlorophenol, using TiO2 nanotube array photoelectrodes, was much higher than that of TiO2 nanoparticle film photoelectrodes under ultraviolet (UV) light, and the photoelectrocatalytic activity was greater than the photocatalytic activity, because of charge separation that was caused by an external bias supply.8 However, the utilization efficiency of solar light for TiO2 nanotube arrays is still poor, because TiO2 nanotube arrays with a wide band gap are photochemically excited only by UV light. Several studies have shown that doping TiO2 with nonmetals, such as nitrogen,9,10 carbon,11,12 or boron,13 would increase its visible-light photoresponse. In our previous work, boron-doped TiO2 nanotube arrays were fabricated for the first time via a vapor method, and it was found that boron doping in TiO2 nanotube arrays contributed to the increased visible photoresponse and the enhanced photoelectrocatalytic activity.14 However, the vapor method for boron doping should involve a gaseous boric source, which is not as safe and convenient as using a nitrogen source. Ion implantation is another main strategy for doping nonmetals into TiO2 nanotube arrays, whereas this process may cause morphology disintegration and amorphization, which requires secondary treatment to compensate.10 Therefore, the doping methods still need innovation and development. * To whom correspondence should be addressed. Tel.: +86-41184706140. Fax: +86-411-84706263. E-mail: [email protected].

Electrodeposition is a common technique that can be used to achieve a uniform deposition of elements on a conducting substrate. However, to our knowledge, until now, it has not been used as a doping method to incorporate impurity atoms in the lattice of a nanostructured film. In consideration of the controllability of the electrodeposition technique and the unique nanostructure of the TiO2 nanotube array, which allows the electrolyte to permeate throughout the internal and external nanotube layer,7 we extrapolate that boron-doped TiO2 nanotube arrays can be achieved using the electrodeposition technique. In this paper, we have presented a facile electrodeposition method for doping TiO2 nanotube arrays with elemental boron, and we have investigated the effects of boron doping on the properties of the TiO2 nanotube array, including crystal structure, optical properties, and photoelectrocatalytic activity. 2. Experimental Section 2.1. Fabrication of Boron-Doped TiO2 Nanotube Arrays. Titanium sheets (0.5 mm thick, 25 mm width × 40 mm length, 99% purity) were sonicated in acetone for 3 min and in deionized (DI) water for 20 min, and then they were chemically etched in a mixture of acids (HF:HNO3:H2O ) 1:4:5, by volume) for 30 s, followed by rinsing with DI water and, finally, drying in a nitrogen stream for 30 s at 25 °C. The electrolyte was 1 M (NH4)2SO4 + 0.5 wt% NH4F,15 and then the pH value was adjusted to 5.0 using 0.3 M H2SO4, to prevent the anodized titanium sheet from forming a precipitate that is produced during the anodization process. The anodization was conducted with increasing potential (from 0 V to 20 V, then holding at 20 V for 2 h), using a direct current (dc) power (Beijing Dahua, PRC) in a two-electrode electrochemical setup that consisted of the pretreated titanium sheet anode and a platinum sheet cathode. After anodization, the sample was immediately rinsed with DI water. A potentiostat (model DJS-292) and a three-electrode cell with an anodized titanium sheet as the working electrode, a saturated calomel electrode (SCE) as the reference electrode, and a platinum sheet as the counter electrode were used in the electrodeposition process. The efficient volume of the electro-

10.1021/ie0712028 CCC: $40.75  2008 American Chemical Society Published on Web 04/24/2008

Ind. Eng. Chem. Res., Vol. 47, No. 11, 2008 3805

Figure 1. Schematic diagram of photoelectrochemical reactor. Legend: 1, light source; 2, quartz cell; 3, anode; 4, cathode; 5, reference electrode; 6, stirring device; 7, stirring bar; 8, CHI electrochemical analyzer; and 9, computer.

chemical cell is 80 mL. The electrodeposition processes were performed using current densities of 10 µA/cm2 for 27 min in the 0.1 M H3BO3 electrolyte, and an anodized titanium sheet that was not subjected to electrodeposition was prepared for comparison. Afterward, the resulting samples were rinsed with DI water, dried in air, and then annealed at 500 °C for 2 h with heating and cooling rate of 2 °C/min. 2.2. Characterization. The morphology of the boron-doped TiO2 nanotube array was characterized by environmental scanning electron microscopy (ESEM) (model Quanta 200 FEG). The crystal structures of the boron-doped and undoped TiO2 nanotube arrays were investigated using X-ray diffractometry (XRD, Shimadzu, model LabX XRD-6000) equipment with Cu KR radiation. The chemical composition of the borondoped and undoped TiO2 nanotube array was examined using X-ray photoelectron spectroscopy (XPS, model Escalab 250) equipment with Al KR monochromatic radiation (1486.6 eV). The optical absorption properties of the two samples were investigated using diffuse reflectance spectroscopy (DRS), using an ultraviolet-visible light (UV–vis) spectrophotometer (Jasco, model UV-550). 2.3. Photocurrent Measurements. A schematic diagram of the experimental setup for the photocurrent measurements is shown in Figure 1. The photocurrent measurements, using 1 M KOH as the electrolyte, were conducted in a standard threeelectrode cell with boron-doped or undoped TiO2 nanotube arrays as the photoanode, a platinum sheet as the cathode, and SCE as the reference electrode, which were connected to a CHI electrochemical analyzer (CH Instruments, model 650B, Shanghai Chenhua). A 350-W high-pressure mercury lamp (Beijing Huiyixin), which provided an illumination intensity of 0.75 mW/ cm2 was used as the UV light source; a high-pressure xenon short arc lamp (CHF-XM35-500W, Beijing Changtuo) with an illumination intensity of 80 mW/cm2 provided full-spectrum illumination; a filter was added to this high-pressure xenon short arc lamp to allow visible light (λ > 400 nm) to pass through with an illumination intensity of 37 mW/cm2. Radiometers were used to measure the intensity of the incident UV light (model UV-A, Photoelectric Instrument Factory, Beijing Normal University) and the visible light (model FZ-A, Photoelectric Instrument Factory, Beijing Normal University). 2.4. Direct Photolysis and Photoelectrocatalysis. The experimental setup for the photoelectrocatalysis (PEC) of phenol was the same as that described for photocurrent measurements, as shown in Figure 1. A constant potential of 0.4 V vs SCE was applied using the CHI electrochemical analyzer during the PEC processes. The high-pressure xenon short arc lamp served as the light source, with an illumination intensity of 80 mW/ cm2 at the position of the photoanode. Eighty milliliters of an aqueous solution with an initial phenol concentration of 20 mg/ L, with 0.01 M Na2SO4 as the supporting electrolyte, was added

Figure 2. ESEM top view image and cross-section (inset picture) of a borondoped TiO2 nanotube array.

Figure 3. B 1s XPS spectra of (a) a boron-doped TiO2 nanotube array and (b) an undoped TiO2 nanotube array.

into a quartz cell (50 mm length × 40 mm width × 60 mm height). All the experiments were conducted with stirring at room temperature. Direct photolysis (DP) of the phenol, without any photocatalyst, was conducted under the same conditions as those for PEC. The concentration of phenol was determined using a high-performance liquid chromatography (HPLC) system (Separations Module 2695, Waters) that was equipped with a photodiode array detector (Module 2996, Waters). The mobile phase consisted of methanol and water (in a 2:1 ratio), and the detection wavelength was 280 nm. 3. Results and Discussion As shown in Figure 2, the nanotubes of boron-doped TiO2 nanotube array are open at the top, with an average diameter of ca. 100 nm, which is the same as that for an undoped anodized TiO2 nanotube array,14,15 and there is no distinct deposit on the film surface. The cross-sectional image of the boron-doped sample (Figure 2) provide direct evidence that the self-organized and ordered nanotube arrays are vertical to the substrate, with a nanotube layer thickness of ca. 1.0 µm. Figure 3 shows the B 1s XPS spectrum of (a) the borondoped TiO2 nanotube array and (b) the undoped TiO2 nanotube array. Figure 3a shows that a B 1s peak at ca. 191.4 eV is observed for the boron-doped sample, whereas the XPS spectra contain no distinct peak at the same binding energy for the

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Figure 4. XRD spectra of boron-doped and undoped TiO2 nanotube arrays after annealing at 500 °C.

undoped sample in Figure 3b. According to the standard B 1s binding energies in TiB2 (187.5 eV, Ti-B) and B2O3 (193.0 eV, B-O),16 the B 1s binding energy of 191.4 eV might be assigned to the mix state Ti-O-B of TiB2 and B2O3, which is consistent with the published value of the B 1s signal for borondoped TiO2 in XPS analysis.13,14,16 Ti3+ cations should be produced by the reduction of Ti4+ cations, because of charge compensation.17 However, there is no discernible difference in the Ti 2p peaks between boron-doped and undoped samples (figure not shown). This phenomenon might be attributed to the subtle amount of Ti3+ cations that are produced by boron incorporation, relative to the large amount of Ti4+ cations. Accordingly, the results of XPS analysis indicate that the B atoms can be incorporated into an ordered TiO2 nanotube array. However, the XPS measure sensitivity for boron is poor, and the amount of boron dopant is low, relative to TiO2; therefore, the B 1s peak is weak and, to some extent, can be influenced by instrument noise. The XRD patterns of boron-doped and undoped TiO2 nanotube arrays are compared in Figure 4. The crystal structures of these two samples both correspond predominantly to anatasephase TiO2 at 2θ ) 25.3° after annealing at 500 °C, which implies that the boron-doped and undoped samples mainly consist of anatase-phase TiO2. The (101) anatase peak intensity of the boron-doped sample is larger than that of undoped sample, which demonstrates that the crystallinity of TiO2 could be improved by boron doping. Correspondingly, the grain boundaries and amorphous regions that can serve as charge-carrier recombination centers are reduced.7 Figure 5 shows the optical absorption coefficient (R) of borondoped and undoped TiO2 nanotube arrays, as a function of the wavelength (λ). The increased optical absorption coefficient in the main absorption region at 270–320 nm was observed for the boron-doped sample. Furthermore, a red shift of optical absorption edge and a new optical absorption shoulder at 380–510 nm are obtained for the boron-doped sample. In terms of the reasons for the red shift of element-doped TiO2, two types of debatable theories exist: one is that “bandgap narrowing” leads to a red shift by mixing the impurity state with valence-band states, such as a mixture of N 2p or C 2p with O 2p states;9,12 the other one is that the “introduction of donor or acceptor levels” in the band gap results in a visible response.18–21 Zhao et al.13 reported that the B 2p states mixing with the O 2p states in boron-doped TiO2 lead to the bandgap narrowing. However, in this paper, the absorption shoulder in the visible-light region with a steady absorption coefficient from 380-420 nm cannot be interpreted appropriately using this “bandgap narrowing” mechanism, because a narrowed interband

Figure 5. Difuse reflectance spectroscopy (DRS) spectra of (;) borondoped TiO2 nanotube arrays and (---) undoped TiO2 nanotube arrays.

transition should exhibit an abrupt decrease in the absorption coefficient in the absorption-edge region, rather than such an absorption shoulder. Therefore, the red shift and the new absorption shoulder in the visible range are likely due to the excitation of electrons from the impurity energy levels, located above the valence-band edge (O 2p states), provided by the substituted B atoms, to the conduction band edge (Ti 3d states). It is, to some extent, consistent with the cases of nitrogen- and carbon-doped TiO2.18–21 It is well-known that the interband electron transition is accompanied by relaxation and recombination in TiO2 semiconducting material, and consequently, only partial photons absorbed by TiO2 can contribute to the generation of a photocurrent. Therefore, it is necessary to take into account the photocurrent–voltage characteristics for further investigation of the photoelectrochemical properties of boron-doped TiO2 nanotube arrays. The net photocurrent density was calculated by subtracting the dark current density from the measured photocurrent density, and the dark currents were negligible in all cases. As shown in Figure 6a, the saturated photocurrent density of a boron-doped TiO2 nanotube array photoelectrode generated under UV light at 1.0 V vs SCE is ca. 20% higher than that of an undoped TiO2 nanotube array photoelectrode, which is consistent with that of the boron-doped sample fabricated via the vapor method in our previous work.14 The superior photocurrent conversion efficiency may be contributed to improved optical absorption capability and increased crystallinity caused by boron doping. Similarly, there is a noticeable increase in photocurrent density under visible light for the boron-doped TiO2 nanotube array photoelectrode, compared to the undoped TiO2 nanotube array photoelectrode, as shown in Figure 6b. The photocurrent density of the boron-doped sample at 1.0 V vs SCE is ca. 18% higher than that of the undoped sample. The absorption shoulder in the visible region and the red shift of the absorption edge may have important roles in the enhanced visible-light photocurrent conversion efficiency of the borondoped sample. However, the photocurrent density of the borondoped sample in visible light is still far from that in UV light, despite the presence of a new absorption shoulder. It may result from harvest losses of charge carriers that are due to high recombination in these impurity energy levels under visiblelight excitation.18,19 According to the superior photocurrent conversion efficiency of the boron-doped TiO2 nanotube array photoelectrode under UV and visible illumination, it is reasonable that the photocurrent density of the boron-doped sample is greater than that of the undoped sample under full spectrum illumination (see Figure

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Figure 6. Photocurrent density versus voltage for boron-doped and undoped TiO2 nanotube arrays under (a) UV light, (b) visible light (λ > 400 nm), and (c) full spectrum illumination.

nanotube array as a photocatalyst. Furthermore, the kinetic constant of the boron-doped sample is increased by ca. 28%, as compared to that of the undoped sample, which indicates that enhanced photoelectrocatalytic activity was obtained by boron doping via this electrodeposition method. The reasons may be as follows: boron-doped TiO2 nanotube array photoelectrodes of superior photoelectrochemical properties are able to afford more photogenerated electrons and holes to decompose phenol; on the other hand, the surface defects that are induced by the incorporated B atoms are able to serve as catalytic centers, which could enhance the photoelectrocatalytic activity of TiO2.24 4. Conclusions Figure 7. Concentration variation of phenol in direct photolysis (DP) and photoelectrocatalysis (PEC) degradation. Table 1. Kinetic Constants and Regression Coefficients of DP and PEC Degradations of Phenol under Full Spectrum Illumination

DP PEC PEC

sample

kinetic constant, k (h-1)

R2

undoped TiO2 nanotube array B-doped TiO2 nanotube array

0.252 0.431 0.554

0.975 0.995 0.998

6c). The photoconversion efficiencies of light energy to chemical energy without applied bias were calculated according to related expressions in refs 3 and 18, and values of ca. 0.4% and 0.6% are obtained for the undoped and boron-doped TiO2 nanotube array photoelectrodes, respectively. To investigate the applied potential of boron-doped TiO2 nanotube arrays for use as photocatalysts, DP and PEC tests were performed. As seen from Figure 7, the resulting concentration-reaction time curves show noticeable differences in the removal efficiencies among these tests. Clearly, 35% of the phenol was removed via DP degradation after 2 h. In contrast, via PEC degradation for 2 h, 56% and 66% of the phenol was removed with undoped and boron-doped samples, respectively. In terms of kinetics study, the DP and PEC degradations of phenol under these experimental conditions follow a pseudofirst-order kinetics equation, in accord with previous studies that DP or PEC degradations of organic contaminants of low initial concentration fit the pseudo-first-order kinetics equation.22,23 The kinetic constants (k) are listed in Table 1. The kinetic constants of the DP and PEC degradations were increased from 0.252 h-1 to 0.431 h-1, which indicated that the overall degradation rate was significantly increased by the addition of the TiO2

Boron-doped TiO2 nanotube arrays were obtained via an electrodeposition method with a current density of 10 µA/cm2 for 27 min. Compared to undoped TiO2 nanotube array photoelectrodes, the photocurrent conversion efficiencies of the boron-doped sample were enhanced under ultraviolet (UV) light, visible light, and a full solar spectrum, respectively. The tests of photoelectrocatalysis (PEC) degradation of phenol under full spectrum illumination demonstrated a higher catalytic activity for the PEC process than that of the direct photolysis (DP) process, and boron doping could further accelerate the PEC reaction rate. The effects of boron doping on ordered TiO2 nanotube array photoelectrodes may result from improved crystallinity and impurity energy levels that are located in the TiO2 band gap. More work is still needed to further understand the mechanism of this doping method, and to develop this electrodeposition method for doping with other elements, or with multiple elements, to increase the utilization efficiency of solar light for TiO2 nanotube arrays. Acknowledgment This work was supported jointly by the National Science Foundation of Distinguished Young Scholars of China (under Project No. 20525723) and National Nature Science Foundation China (under Project No. 20407005). Literature Cited (1) Fujishima, A.; Rao, T. N.; Tryk, D. A. Titanium dioxide photocatalysis. J. Photochem. Photobiol. C 2000, 1, 1. (2) Mor, G. K.; Varghese, O. K.; Paulose, M.; Shankar, K.; Grimes, C. A. A review on highly ordered, vertically oriented TiO2 nanotube arrays: fabrication, material properties, and solar energy applications. Sol. Energy Mater. Sol. Cells 2006, 90, 2011. (3) Frank, A. J.; Kopidakis, N.; Lagemaat, van, de. Electrons in nanostructured TiO2 solar cells: transport, recombination and photovoltaic properties. J. Coord. Chem. ReV. 2004, 248, 1165.

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(15) Macak, J. M.; Tsuchiya, H.; Schmuki, P. High-aspect-ratio TiO2 nanotubes by anodization of titanium. Angew. Chem., Int. Ed. 2005, 44, 2100. (16) Chen, D. M.; Yang, D.; Wang, Q.; Jiang, Z. Y. Effects of boron doping on photocatalytic activity and microstructure of titanium dioxide nanoparticles. Ind. Eng. Chem. Res. 2006, 45, 4110. (17) Grey, I. E.; Li, C.; MacRae, C. M.; Bursill, L. A. Boron incorporation into rutile phase equilibria and structure considerations. J. Solid State Chem. 1996, 127, 240. (18) Torres, G. R.; Lindgren, T.; Lu, J.; Granqvist, C. G.; Lindquist, S. E. Photoelectrochemical study of nitrogen-doped titanium dioxide for water oxidation. J. Phys. Chem. B 2004, 108, 5995. (19) Neumann, B.; Bogdanoff, P.; Tributsch, H.; Sakthivel, S.; Kisch, H. Electrochemical mass spectroscopic and surface photovoltage studies of catalytic water photooxidation by undoped and carbon-doped titania. J. Phys. Chem. B 2005, 109, 16579. (20) Irie, H.; Watanabe, Y.; Hashimoto, K. Nitrogen-concentration dependence on photocatalytic activity of TiO2-xNx powders. J. Phys. Chem. B 2003, 107, 5483. (21) Lindgren, T.; Mwabora, J. M.; Avendano, E.; Jonsson, J.; Hoel, A.; Granqvist, C. G.; Lindquist, S. E. Photoelectrochemical and optical properties of nitrogen doped titanium dioxide films prepared by reactive DC magnetron sputtering. J. Phys. Chem. B 2003, 107, 5709. (22) Ollis, D. F.; Hsiao, C. Y.; Budiman, L.; Lee, C. L. Heterogeneous photoassisted catalysis: conversions of perchloroethylene, dichloroethane, chloroacetic acids, and chlorobenzenes. J. Catal. 1984, 88, 89. (23) Li, X. Z.; Liu, H. L.; Yue, P. T.; Sun, Y. P. Photoelectrocatalytic oxidation of rose bengal in aqueous solution using a Ti/TiO2 mesh electrode. EnViron. Sci. Technol. 2000, 34, 4401. (24) Jung, K. Y.; Park, S. B.; Ihm, S. K. Local structure and photocatalytic activity of B2O3-SiO2/TiO2 ternary mixed oxides prepared by solgel method. Appl. Catal., B 2004, 51, 239.

ReceiVed for reView September 05, 2007 ReVised manuscript receiVed February 26, 2008 Accepted March 01, 2008 IE0712028