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Synthesis and Enhanced Visible-Light Photoelectrocatalytic Activity of pn Junction BiOI/TiO2 Nanotube Arrays Gaopeng Dai,† Jiaguo Yu,*,† and Gang Liu*,‡ †
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, P. R. China ‡ National Center for Nanoscience and Technology, Beijing 100190, P. R. China ABSTRACT: pn junction BiOI/TiO2 nanotube arrays (NTs) were prepared by coating BiOI on the tube wall of the selforganized TiO2 NTs using a novel impregnatinghydroxylation method. The as-prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and UVvis absorption spectroscopy. The photoelectrocatalytic (PEC) activity toward degradation of methyl orange (MO) aqueous solutions under visible-light irradiation (λ > 420 nm) by pn junction BiOI/TiO2 NTs was investigated. The results from the current study demonstrate that the as-prepared BiOI/TiO2 junction NTs display much greater PEC activity than the respective pure BiOI and TiO2 counterparts due to the synergetic effects of several factors including the strong visible absorption, the formation of pn junction, and the impact of external electrostatic field. The proposed mechanism responsible for the enhanced performance was further confirmed by the transient photocurrent response experiments.
1. INTRODUCTION Over the past decades, titania has received much attention due to its enormous potential applications in photocatalysis,1 sensing,2 biomedicine,3 and photovoltaics.4 A number of methods have been reported for fabricating TiO2 NTs including solgel,5 template-assisted,6 hydrothermal,7 seed growth,8 and anodization.9 Among current synthetic methodology, anodization has shown outstanding advantages in preparing highly ordered TiO2 NTs in that the dimension of TiO2 NTs can be precisely controlled.912 Indeed, uniform TiO2 NTs with various pore sizes (22110 nm), lengths (2001 000 000 nm), and wall thicknesses (734 nm) can be obtained by simply tailoring electrochemical conditions based on anodization. Compared to the immobilized TiO2 nanoparticle films, the perpendicularly aligned and highly ordered TiO2 NT architecture provides an effective electron percolation pathway.13,14 In this regard, highly ordered nanotube arrays could significantly improve the lifetime of photogenerated charge carriers14,15 and benefit the photocatalytic efficiency. From the application point of view, highly ordered TiO2 NTs display excellent photocatalytic properties, far exceeding that of traditional TiO2 films.1619 For example, nanotube arrays are an ideal architecture for water photoelectrolysis, and TiO2 NTs can promote water photocleavage with high photoconversion efficiency under UV radiation.20 In solar-cell applications,2124 perpendicularly oriented onedimensional TiO2 nanotubes can be integrated with hole-transporting materials to make digitated devices, in which photoninduced electrons and holes can be separated and transported r 2011 American Chemical Society
much more quickly. On the other hand, the photoelectric conversion efficiency of dye-sensitized TiO2 NT solar cells can be up to 6.9%.24 In line with the above remarkable properties, TiO2 NTs show great promises in heterogeneous photocatalysis. Nevertheless, the intrinsic band gap of TiO2 (3.2 eV for anatase and 3.0 eV for rutile) limits its absorption in the ultraviolet part of the solar spectrum. Moreover, the rapid recombination of photoinduced electrons and holes greatly lowers the quantum efficiency.2527 Therefore, it is necessary to develop effective solutions to improve the charge separation efficiency and enhance visible-light photoactivity. Forming a heterojunction between TiO2 NTs and a narrow-gap semiconductor, such as CdS,28,29 CdSe,30,31 CdTe,32,33 and Cu2O,34 it was demonstrated to be an efficient route. Under visible-light irradiation, the electrons excited from the narrow bandgap semiconductors can be transferred to TiO2, assisting the charge separation and improving the visible-light photocatalytic activity dramatically.35 In particular, BiOI is an attractive p-type semiconductor36 with a narrow band gap of 1.94 eV.37 To date, several methods have been reported for the preparation of BiOI micro-/nanostructures. For example, Lei et al.38a reported template-free synthesis of BiOI hierarchical structures by a solution route at room temperature and their enhanced visible-light photocatalytic and electrochemical hydrogen storage properties. Received: January 25, 2011 Revised: March 11, 2011 Published: March 25, 2011 7339
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The Journal of Physical Chemistry C Xiao et al.38b described facile synthesis of nanostructured BiOI microspheres with high visible-light-induced photocatalytic activity at low temperature using ethanolwater mixed solvent as reaction media. Xia et al.38c reported self-assembly and enhanced photocatalytic properties of BiOI hollow microspheres via a reactable ionic liquid using an EG-assisted solvothermal method. Recently, it was reported that the coupled BiOI/TiO2 heterojunction exhibited enhanced visible-light photocatalytic activity for the degradation of MO.38d However, it is well-known that powdered samples are difficult to be separated and recycled from the reaction system. In contrast, BiOI/TiO2 NTs can be readily separated from the slurry system after photocatalytic reaction and reused.39 Also, the photoelectrochemical performances of BiOI/TiO2 NTs under visible-light irradiation (λ > 420 nm) have not yet been reported in detail. In this work, BiOI was for the first time coated on the tube wall of the TiO2 NTs by a simple impregnatinghydroxylation method using BiI3 as the precursor. The as-prepared pn junction BiOI/TiO2 NTs show high visible-light photoelectrocatalytic (PEC) activity toward the degradation of MO in water with great stability. To the best of our knowledge, this is the first report on the preparation of pn junction BiOI/TiO2 NTs with enhanced visible-light PEC activity.
2. EXPERIMENTAL SECTION 2.1. Sample Preparation. The self-organized TiO2 NTs were grown by anodization of Ti foils in an ethylene glycol solution containing 0.25% (in mass) NH4F and 1% (in volume) H2O, similar to the method described by Prakasam et al.40 Prior to anodization, Ti foils (0.25 mm in thickness, 99% purity) were, respectively, cleansed by sonicating in acetone, isopropanol, methanol, and distilled water. Finally, Ti foils were dried in air. Anodization was performed in a two-electrode configuration connected to a DC power supply with titanium foils as the working electrode and platinum foils as the counter electrode under a constant 40 V anodic potential for 1 h at room temperature. Upon anodic oxidation, the samples were rinsed with distilled water and dried in an oven at 80 °C for 1 h. The resulting amorphous TiO2 NTs were annealed at 450 °C for 3 h in air to improve the stoichiometry and crystallization. BiOI was coated onto the TiO2 NTs by an impregnatinghydroxylation method using BiI3 as the precursor. In a typical synthesis, 0.94 g of BiI3 was added into 20 mL of absolute ethanol solutions containing 2 drops of 35% HCl. The TiO2 NTs were first immersed in BiI3 solution for 30 min followed by a slight rinse with ethanol and then dried in an oven at 80 °C for 1 h to ensure that BiI3 was adhered to the tube wall uniformly. Subsequently, the TiO2 NTs were immersed in distilled water, and BiOI was deposited on the nanotube wall by hydroxylation of BiI3. For comparison, pure BiOI films were also coated on Ti foils following the same impregnatinghydroxylation method. The weight of BiOI coated on the TiO2 NTs was the same as that on Ti foils. 2.2. Characterization. The morphology was characterized using an S-4800 field emission scanning electron microscope (FESEM, Hitachi, Japan) at an accelerating voltage of 10 KV. Transmission electron microscopy (TEM) analyses were conducted with a JEM-2100F electron microscope (JEOL, Japan) operating at 200 kV. X-ray diffraction (XRD) patterns, obtained on a D/MAX-RB X-ray diffractometer (Rigaku, Japan) using Cu KR radiation at a scan rate (2θ) of 0.05° s1, were used to
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characterize the sample crystalline phases and crystallite size. The accelerating voltage and the applied current were 40 kV and 80 mA, respectively. The average crystallite sizes were determined according to the Scherrer equation using the full-width half-maximum data after correcting the instrumental broadening. X-ray photoelectron spectroscopy (XPS) measurements were performed on a VG ESCALAB MKII XPS system with an Mg KR source and a charge neutralizer. All the binding energies were referenced to the adventitious C1s peak at 284.8 eV. UVvisible diffuse reflectance spectra were obtained on a UVvisible spectrophotometer (UV-2550, Shimadzu, Japan). Fine BaSO4 powders were used as a reflectance standard. 2.3. Photoelectrocatalytic Evaluation. The photoelectrocatalytic activity of the samples was evaluated by photoelectrocatalytic decolorization of MO aqueous solution at ambient temperature. MO is a well-known chemically stable containingnitrogen dye pollutant.41 The photoelectrochemical degradation reactions were tested by a CHI electrochemical analyzer (CHI660C Instruments) in a three-electrode system in which the BiOI/TiO2 NTs acted as the photoanode, Pt wire as the cathode, and a Ag/AgCl electrode as the reference electrode. Bias potentials applied on the photoanode were 0.5 V. 2 2 cm2 BiOI/TiO2 NTs were placed in 15 mL of MO aqueous solution with a concentration of 4 105 M in a rectangle cell (25 W 25 L 50 H mm). The solution was allowed to reach an adsorptiondesorption equilibrium among the photocatalyst, MO, and water before visible-light irradiation. A 300 W xenon lamp with a 400 nm cutoff filter as a light source was positioned ca. 10 cm away from the reaction cell. The integrated visible-light intensity was measured to be ca. 25 mW cm2 by a visible-light radiometer (model: FZ-A, China). Upon light irradiation at every 20 min, the reaction solution was sampled to determine the concentration change of MO using an UVvisible spectrophotometer (UV-2550). As for the MO aqueous solution with low concentration, its photoelectrocatalytic decolorization is a pseudofirst-order reaction, and its kinetics may be expressed as ln(C0/C) = kt, where k is the apparent rate constant and C0 and C are the initial and reaction concentrations of aqueous MO, respectively.42 The visible-light photoelectrocatalytic activity of other samples was also measured under the same conditions. For comparison, the photocatalytic experiment was performed by using the same system without applying an external potential. An electrochemical oxidation experiment was performed at the same bias without visible-light illumination. 2.4. Photoelectrochemical Measurements. Photocurrents were measured by an electrochemical analyzer (CHI660C Instruments) in a standard three-electrode system with the asprepared samples as the working electrodes with an active area of ca. 0.5 cm2, a Pt wire as the counter electrode, and Ag/AgCl (saturates KCl) as a reference electrode. A 300 W Xe arc lamp equipped with an ultraviolet cutoff filter (λ > 400 nm) was utilized as the visible-light source. The integrated visible-light intensity measured with a visible-light radiometer (FZ-A) was 25 mW/cm2. A 1 M Na2SO4 aqueous solution was used as the electrolyte.
3. RESULTS AND DISCUSSION 3.1. Characterization. The morphologies of the TiO2 NTs were characterized using SEM. Figure 1a displays a typical SEM image of the TiO2 NTs with a regularly arranged pore structure. The TiO2 NTs display an average outer pore diameter of ca. 7340
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Table 1. Phase Structures, Average Crystallite Size, and Visible-Light PEC Activity of Various Samples samples
a
Figure 1. SEM (a,b) and TEM (c,d) images of self-organized TiO2 NTs (a,c) and BiOI/TiO2 NTs (b,d). (a) Top and side (inset) view SEM images of the unmodified TiO2 NTs. (b) Top SEM image of BiOI/TiO2 NTs. (c) TEM images of the unmodified TiO2 NTs. (d) TEM images of BiOI/TiO2 NTs. (e) Corresponding EDX pattern of BiOI/TiO2 NTs.
Figure 2. XRD patterns of TiO2 NTs (a), BiOI film (b), and BiOI/ TiO2 NTs (c).
100 nm and wall thickness of ca. 20 nm. The inset in Figure 1a is a cross-section of the NTs, displaying that the NTs are composed of well-aligned nanotubes of about 2.1 μm in length. It is apparent that the space between individual nanotubes is wellresolved. Figure 1b shows that for BiOI/TiO2 NTs the surface of the nanotubes is smooth; the wall thickness of nanotubes increases by ca. 8 nm; and the interspaces between nanotubes disappear. These results indicate that a thin layer of BiOI was coated onto both the inner and outer surface of TiO2 NTs. The
phasea
crystallite size (nm)
activity
TiO2 NTs
A
21
very low
BiOI film BiOI/TiO2 NTs
B AþB
25.2 A:20.6, B:8.2
middle high
A and B denote anatase and BiOI, respectively.
SEM results were consistent with TEM results. Figure 1c is a TEM image of the unmodified NT sample. The outer and inner pore diameter is about 100 and 60 nm, respectively, and the tube wall thickness is about 20 nm. After depositing BiOI, the outer and inner pore diameter was about 105 and 50 nm (Figure 1d), respectively, suggesting that a thin BiOI layer was indeed coated on the inner and outer walls. Furthermore, the color of the tube became black, implying that a thin layer of BiOI film was coated on the tube wall due to the higher electron density of the Bi element than the Ti element. The composition of the BiOI/TiO2 NTs was determined by energy-dispersive X-ray spectroscopy (EDX) experiments. The EDX spectrum (Figure 1e) confirms that the BiOI/TiO2 NTs consist of Ti, O, Bi, and I and BiOI coating deposits onto TiO2 NTs. XRD was used to identify the phase structure of the samples. Figure 2a shows the XRD pattern for TiO2 NTs. Quantitative analysis of this pattern shows that all peaks can be indexed to the TiO2 anatase phase (JCPDS file No: 21-1272, a = 3.7852 Å, c = 9.5139 Å, space group: I41/amd (141)) and the Ti metal phase (JCPDS file No: 44-1294, a = 2.951 Å, c = 4.683 Å, space group: P63/mmc (194)), respectively. The peaks of the anatase phase are from the TiO2 NTs and those of the metal Ti phase from Ti substrate. The XRD pattern for BiOI films coated on Ti foils (Figure 2b) shows that all the diffraction peaks are in good agreement with those of tetragonal structure of BiOI (JCPDS file No: 10-0445, a = 3.994 Å, c = 9.149 Å, space group: P4/nmm (1129)) and Ti, respectively. After TiO2 NTs are coated with BiOI, anatase, BiOI, and Ti metal phase all appear (see Figure 2c), and no other diffraction peaks can be observed, suggesting that no Bi and I related impurities exist in the BiOI/ TiO2 NTs. Table 1 lists the average crystalline sizes of all samples. It can be seen that the crystalline size of BiOI in TiO2 NTs was about 8.2 nm. To further investigate the surface chemical compositions and oxidation states of BiOI/TiO2 NTs, XPS studies were conducted, and the spectra are illustrated in Figure 3. It can be seen from the XPS survey spectrum (Figure 3a) that the BiOI/TiO2 NTs contain not only Ti and O elements but also some C, Bi, and I elements. The XPS peak for C 1s (284.8 eV) is ascribed to the adventitious hydrocarbon from the XPS instrument. A typical high-resolution XPS spectrum of Bi 4f is shown in Figure 3b. Two peaks at 159.0 and 164.3 eV are assigned to Bi 4f7/2 and Bi 4f5/2, respectively, which is characteristic of Bi3þ in BiOI.43 Figure 3c shows the high-resolution XPS spectrum of the O 1s region, which can be fitted into three peaks: the main peak at 530.3 eV is attributed to the TiO of TiO2, the peak at 532.3 eV is assigned to the hydroxyl group,4446 and the peak at 529.8 eV can be ascribed to the BiO bonds in [Bi2O2] slabs of BiOI layered structure.38,47 As to the high-resolution spectra of the I 3d (Figure 3d), two peaks at 630.4 and 618.8 eV are attributed to I 3d3/2 and I 3d5/2, respectively, which can be ascribed to I in pure BiOI.38d The XPS results further confirm the coexistence of 7341
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Figure 3. XPS survey spectrum of BiOI/TiO2 NTs (a) and the corresponding high-resolution XPS spectra of Bi 4f (b), O 1s (c), and I 3d (d).
Figure 4. UVvisible diffuse reflectance spectra of unmodified TiO2 NTs (a), BiOI (b), and BiOI/TiO2 NTs (c).
BiOI and TiO2 in BiOI/TiO2 NTs and no existence of other impurities associated with Bi or I, in good agreement with the XRD studies. Figure 4 shows the UVvis diffuse reflectance spectra of the unmodified TiO2 NTs, BiOI/TiO2 NTs, and BiOI film. The spectrum obtained from the pure TiO2 NT film (Figure 4a) shows that TiO2 NTs primarily absorb the ultraviolet light with a wavelength below 400 nm, which was ascribed to the intrinsic band gap absorption of TiO2. The absorption of pure TiO2 NTs in the visible region can be assigned to scattering of light caused by pores or cracks in the nanotube arrays.42,48 In contrast, BiOI film and BiOI/TiO2 NT film show strong absorptions in the wavelength ranging from 400 to 650 nm (Figure 4b, 4c), which
Figure 5. EO (a), PC (b), and PEC (c) activity of BiOI/TiO2 NTs for the degradation of MO.
can be assigned to the intrinsic band gap absorption of BiOI. This property would facilitate the absorption of the solar energy in the visible region. 3.2. Photoelectrocatalytic (PEC) Activity. The PEC and photocatalytic (PC) activities of the samples were evaluated by degradation of MO aqueous solution under visible-light irradiation, and the electrochemical oxidation (EO) activities of the samples was also evaluated by degradation of MO aqueous solution in the absence of light irradiation. Figure 5 shows a comparison of EO, PC, and PEC activity of BiOI/TiO2 NTs for the degradation of MO. It is apparent that the PEC process is most efficient to degrade MO. The complete removal of MO (92%) was observed after 160 min, while only 57% of MO removal was obtained in the PC process with the same illumination time. The MO 7342
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anatase TiO2 is an n-type semiconductor with a large band gap (3.2 eV).50 The valence band edge positions of BiOI and TiO2 were estimated in this study according to the concepts of electronegativity.51,52 The conduction band (CB) and valence band (VB) potentials of the two semiconductors at the point of zero charge can be calculated by the following equation EVB ¼ X Ee þ 0:5Eg
Figure 6. (A) Comparison of visible-light PEC activity of TiO2 NTs (a), BiOI film (b), and BiOI/TiO2 NTs (c) for the PEC decomposition of MO in water. (B) Cycling degradation curve for BiOI/TiO2 NTs.
removal with EO was insignificant, suggesting that electrochemical oxidation did not occur significantly in this process. Figure 6A shows the comparison of visible-light PEC activity for different samples. The unmodified anatase TiO2 NTs show negligible PEC activity. In contrast, BiOI and BiOI/TiO2 NT films exhibit greater visible-light PEC activity due to the narrow band gap of BiOI. After 160 min of PEC reaction, 50% and 92% of MO was degraded by BiOI film and BiOI/TiO2 NTs, respectively. The coupling of BiOI with TiO2 NTs shows the higher visible-light PEC activity. Provided that the PEC reaction follows a pseudofirst-order reaction, the rate constant of MO decomposition over BiOI/TiO2 NTs is estimated to be about 0.015, exceeding that of BiOI film (0.005) by a factor of 3. It is also found that colorless p-chlorophenol is also quickly decomposed by BiOi/TiO2 NTs under visible-light irradiation (not shown here). According to the above results, the enhanced PEC activity of BiOI/TiO2 NTs can be attributed to the synergetic effects of strong visible-light absorption, pn junction structure,38d and the applied external electrostatic field. It is well-known that the photodegradation process is based on electronhole pairs generated by band gap excitation. The photoinduced electrons and holes could migrate to the surface to react with the adsorbed reactants. However, the photogenerated electrons and holes can recombine easily in the volume or on the surface of the semiconductor particles.49 Therefore, the PEC efficiency can be greatly influenced by the separation rate of the photoinduced electronhole pairs. First, pn junction plays an important role in the effective separation of light-induced electrons and holes. BiOI is a p-type semiconductor36 with a narrow band gap,37 while
ð1Þ
where X is the absolute electronegativity of the semiconductor, which is defined as the geometric mean of the absolute electronegativity of the constituent atoms; Ee is the energy of free electrons on the hydrogen scale (ca. 4.5 eV); EVB is the VB edge potential; and Eg is the band gap of the semiconductor. The conduction band (CB) position can be deduced by ECB = EVB Eg. The X values for TiO2 and BiOI are ca. 5.90 and 5.94 eV. On the basis of the above equations, the top of the VB and the bottom of the CB of TiO2 are calculated to be 3.00 and 0.20 eV, respectively. Accordingly, the VB and CB of BiOI are estimated to be 2.41 and 0.47 eV, respectively. Figure 7A shows the energy band schematic diagram for pBiOI and n-TiO2. Before contact, the conduction band edge of BiOI is lower than that of TiO2, and the Fermi level of BiOI is also lower than that of TiO2.38d After contact, the Fermi level of BiOI is moved up, while the Fermi level of TiO2 is moved down until an equilibrium state is formed (Figure 7B).53 Meanwhile, consistent with the rising up and/or descending of the Fermi level, the whole energy band of BiOI is raised up while that of TiO2 is descended, and as a result, the conduction band edge of BiOI is higher than that of TiO2. At the equilibrium, the inner electric field is formed, thus the p-type BiOI region has the negative charge while n-type TiO2 has the positive charge. Under visible-light illumination, BiOI could be easily excited and induced the generation of photoelectrons and holes. According to the schematic diagram in Figure 7, the excited electrons on the conduction band of the p-type BiOI transfer to that of n-type TiO2, and simultaneous holes remain in the p-BiOI valence band. The migration of photogenerated carriers can be promoted by the internal field. Thus, the photogenerated electronhole pairs will be separated effectively by the pn junction formed in the pBiOI/n-TiO2 interface, and the recombination of electronhole pairs can be reduced. The separated electrons and holes are then free to initiate reactions with the reactants adsorbed on the photocatalyst surfaces with enhanced photocatalytic activity. Furthermore, the applied potential can also greatly promote separation of photogenerated electrons and holes in BiOI/TiO2 NTs and reduce their combination, thus enhancing the photocatalytic activity. According to the above results and discussion, it is apparent that the synergetic effects of the pn junction formed between BiOI and TiO2, and the applied external electrostatic field was responsible for the highly efficient separation of photogenerated electrons and holes. Figure 8 shows the schematic diagram of charge transfer processes at the BiOI/TiO2 NT photoelectrode for photoelectrocatalytic degradation of MO under visible-light irradiation. Due to the formation of a pn junction, the conduction band position of TiO2 is more positive than that of BiOI. Thus, the excited electrons on the conduction band of the p-type BiOI can transfer to the conduction band of n-type TiO2. Consequently, under the external electrostatic field, the electrons traveled along the TiO2 NTs and passed through the interface between TiO2 and Ti to the external circuit, leaving the 7343
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Figure 7. Schematic diagrams for (A) energy bands of p-BiOI and n-TiO2 before contact and (B) the formation of a pn junction and its energy band diagram at equilibrium and transfer of photoinduced electrons from p-BiOI to n-TiO2 under visible-light irradiation.
Figure 8. Schematic diagram of (A) BiOI/TiO2 NTs and (B) the charge transfer process at BiOI/TiO2 NTs photoelectrode for photoelectrocatalytic degradation of MO.
Figure 9. Comparison of transient photocurrent response of the TiO2 NTs (a), BiOI films (b), and BiOI/TiO2 NTs (c) in 1 M Na2SO4 aqueous solutions under visible-light irradiation at 0.5 V vs Ag/AgCl.
photogenerated holes in the valence band of BiOI. In this case, the photogenerated electronhole pairs were separated effectively. Finally, the separated electrons and holes reacted with the reactants absorbed on the electrode surface and subsequently enhanced the photoelectrocatalytic activity.
The stability of highly efficient BiOI/TiO2 NTs photocatalyst was further evaluated by the recycle experiments in the PEC degradation of MO under visible-light irradiation. The results for the recycling BiOI/TiO2 NTs are shown in Figure 6B. It is clear that PEC efficiency does not exhibit significant loss after five recycles, indicating that the BiOI/TiO2 NTs display high stability and do not suffer from photocorrosion during the PEC degradation of MO. 3.3. Transient Photocurrent Response. To further investigate the above proposed photocatalytic mechanism, the transient photocurrent responses of the BiOI films, TiO2, and BiOI/TiO2 nanotube arrays were measured under intermittent visible-light irradiation, respectively. Figure 9 shows a comparison of photocurrent responses of different samples. It can be seen that the photocurrent rapidly decreases to zero as long as the light was turned off, and the photocurrent remained a constant value when the light was on. The reducibility of these results is very good. For all samples, an anodic photocurrent spike, which decayed rapidly followed by a steady current, appears at the initial time of irradiation. When the light is switched off, a cathodic spike can also be observed. The initial current is due to the separation of electronhole pairs at the semiconductor/electrolyte interface: holes are trapped or captured by reduced species in the electrolyte, while the electrons are transported to the back contact substrate via the walls of TiO2 nanotubes.39,54,55 The decay of the photocurrent indicates that a fraction of the holes reaching the semiconductor surface, instead of capturing electrons from the electrolyte, either recombine with electrons from the conduction band and/or accumulate at the surface. After recombination of the excessive holes with electrons, the generation and transfer of electronhole pairs reach an equilibration and form a constant current. When the light is interrupted, the holes accumulated in the surface state still continue to recombine, and a cathodic spike is observed. Further studies show that BiOI/TiO2 NTs exhibit the highest photocurrent density. This can be attributed to the combined effects of several factors: First, the photocurrent is mainly determined by the efficiency of photogenerated hole transfer at the semiconductor/electrolyte and the electron diffusion to the 7344
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The Journal of Physical Chemistry C back contact.5659 Second, the pn junction can reduce the recombination of photogenerated electrons and holes by the internal electrostatic field in the junction region. Finally, an external electrostatic field can further enhance the transfer and separation of photogenerated electrons and holes in BiOI/TiO2 NTs. Therefore, it is not surprising that the combined effects of the internal and external electric fields result in the highest photocurrent density.60
4. CONCLUSIONS Pn junction BiOI/TiO2 NTs are successfully prepared by coating BiOI on the tube wall of the self-organized TiO2 NTs using the impregnatinghydroxylation method. A thin uniform BiOI layer was observed onto both the inner and outer surfaces of TiO2 NTs. The BiOI/TiO2 NT sample showed strong absorptions in the wavelength range from 400 to 650 nm due to the intrinsic band gap absorption of BiOI and scattering of light caused by pores or cracks in the nanotube arrays. The BiOI/TiO2 NTs also display much higher photoelectrocatalytic activity for the photocatalytic decolorization of MO aqueous solution at ambient temperature and greater photocurrent response than the respective pure BiOI and TiO2 counterparts. This is due to the combined effects of two factors, including the pn junction reducing the recombination of photogenerated electrons and holes by the internal electrostatic field in the junction region and an applied external electrostatic field enhancing the transfer and separation of photogenerated electrons and holes in BiOI/TiO2 NTs. After five recycles of the MO photodegradation, the BiOI/ TiO2 NT retained its activity, proving the high stability of the BiOI/TiO2 NTs. This study provides some new insight into the design and fabrication of advanced photocatalytic materials with pn heterojunction structures with enhanced photoelectrocatalytic activity. The BiOI/TiO2 NTs could be promising for practical applications in that they can be more readily recycled and reused than conventional powder photocatalysts. Due to their large specific surface area, high pore volume, unique morphology, and high photocatalytic activity, the novel photocatalysts show potential applications in sensors, solar cells, catalysis, separation technology, biomedical engineering, and nanotechnology. ’ AUTHOR INFORMATION Corresponding Author
*Tel.: 0086-27-87871029. Fax: 0086-27-87879468. E-mail:
[email protected]. Tel.: 0086-10-82545613. Fax: 008610-62656765. E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was partially supported by the National Natural Science Foundation of China (20877061 and 51072154) and the Natural Science Foundation of Hubei Province (2010CDA078). This work was also financially supported by the National Basic Research Program of China (2007CB613302). ’ REFERENCES (1) (a) Liu, S. W.; Yu, J. G.; Jaroniec, M. J. Am. Chem. Soc. 2010, 132, 11914. (b) Yu, J. G.; Wang, W. G.; Cheng, B.; Su, B. L. J. Phys. Chem. C 2009, 113, 6743. (c) Yu, J. G.; Qi, L. F.; Jaroniec, M. J. Phys. Chem. C 2010, 114, 13118.
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