Electronic and Band Structure Tuning of Ternary Semiconductor

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J. Phys. Chem. C 2010, 114, 18198–18206

Electronic and Band Structure Tuning of Ternary Semiconductor Photocatalysts by Self Doping: The Case of BiOI Xi Zhang and Lizhi Zhang* Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal UniVersity, Wuhan 430079, People’s Republic of China ReceiVed: June 3, 2010; ReVised Manuscript ReceiVed: September 10, 2010

Foreign nonmetal or metal element doping has been widely used to tailor the electronic and band structures of wide band gap binary oxide semiconductor photocatalysts, extending their absorption edges into the visible light range for better utilization of solar light. Besides doping with foreign elements, self-doping can also tune the electronic and band structures of semiconductor photocatalysts but only limited to binary metal oxides, such as oxygen-deficient TiOx (x < 2). In this study, we demonstrate that self-doping is able to tune the electronic and band structures of ternary semiconductor photocatalysts and thus significantly enhance their photocatalytic activities by utilizing BiOI as the example. Density functional theory calculations revealed that iodine self-doping could effectively tune the electronic structures of BiOI. Motivated by the calculations, iodine self-doped bismuth oxyiodide photocatalysts were synthesized with a soft chemical method to illustrate this band structure tailoring approach. Experimental results confirmed that self-doping could change the electronic structures to intrinsically improve the optical absorption property and charge transfer ability, thus enhancing the photocatalytic activity of ternary semiconductors. Meanwhile, the intense absorption with a steep absorption edge of self-doped ternary semiconductors is different from that of foreign elements doped TiO2 with discrete bands, confirming this is a novel electronic and band structure tuning method. This successful band structure tailoring example of ternary semiconductors suggests the self-doping strategy could be general to develop novel visible light driven ternary photocatalysts with enhanced performances. 1. Introduction Titanium dioxide (TiO2) has received much attention due to its potential photocatalytic applications in wastewater treatment and air purification.1,2 However, owing to its wide band gap (Eg ) 3.2 eV), TiO2 is only responsive to ultraviolet light (UV) instead of more abundant visible light. Therefore, it is indispensable to develop efficient photocatalysts to extend the absorption edge into visible light range. One of the efforts is modification of TiO2, including nonmetal/metal doping, surface functionalization, coupling with other semiconductors, etc.3-11 In the field of semiconductor photocatalysis, foreign nonmetal or metal element doping has been widely used to tailor the electronic and band structures of wide band gap binary oxide semiconductors such as TiO2 and ZnO. As a rule, the photocatalytic activity is closely related to the positions of the conduction band (CB) and valence band (VB) in photocatalyst. In typical metal oxides, the bottoms of the conduction bands, which consist mainly of empty transition-metal d orbitals, are located at a potential slightly more negative than 0 V (vs NHE) at pH 0 and the tops of the valence bands, consisting of O2p orbital, are more positive than 3 V.12 This caused large gaps between the band edges, such as ZnO (3.4 eV),13 In2O3 (3.7 eV),14 and Ga2O3 (4.8 eV),15 which are too large to harvest visible light. When other elements having an atomic orbital with a potential energy higher than the O2p orbital are introduced into the metal oxide, new hybrid valence bands can be formed instead of the pure O2p orbital, which results in increasing the valence-band potential and thus producing a photocatalyst with a band gap suitable for visible light absorption. * To whom correspondence should be addressed. Phone/Fax: +86-276786 7535. E-mail:[email protected].

Besides doping with foreign elements, self-doping has been used to tune the physicochemical properties of semiconductors. For example, self-doping can also tune the electronic and band structures of binary metal oxides, such as oxygen-deficient TiOx (x < 2).16 The self-doping of TiO2 enables its effective absorption in the visible light range. In order to gain insight about the enhanced absorption properties in the visible range, Justicia et al.17,18 performed a series of first-principles calculations to assess the influence of oxygen vacancies on the electronic states of TiOx (x < 2). It is concluded that the oxygen vacancies concentration increases the gap states to form a miniband below the conduction band. The electrons in these defect states are able to transfer and contribute to the conduction by absorbing a visible photon. These carriers can give rise to intermediate species able to oxidize organic compounds (i.e., air oxygen acts as an electron acceptor by forming highly reactive particles such as superoxide ions), thus enhancing the photocatalytic performance of TiOx (x < 2). Recently, as a novel ternary oxide semiconductor, bismuth oxyhalides (BiOX, X ) Cl, Br, and I) have drawn much attention for their potential application in photocatalysis. From the density functional theory (DFT) calculation on the electronic structure of BiOX,19,20 both O2p and Xnp (n ) 3, 4, and 5 for X ) Cl, Br, and I, respectively) states dominate the valence bands, whereas Bi6p states contribute the most to the conduction bands. With the growing X atomic number, the density peak of the localized Xnp states in the valence band shifts toward the valence-band top, which produced a narrowing band gap from BiOCl to BiOI. Obviously, the X atom has an important effect on the band gap structure of BiOX. In our previous work,21 we reported a general one-pot nonaqueous synthesis of BiOX (X

10.1021/jp105118m  2010 American Chemical Society Published on Web 10/01/2010

Tuning of Ternary Semiconductor Photocatalysts ) Cl, Br, I) nanoplate microspheres. These BiOX microspheres exhibited attractive photocatalytic activity on the degradation of organic pollutant. Among them, BiOI possessed the best photocatalytic activity under visible light irradiation. Therefore, it is of great importance to improve the photocatalytic activity of this new visible light photocatalyst BiOI either by coupling or by doping for potential applications. Herein, we demonstrate theoretically and experimentally that self-doping is also able to tune the electronic and band structures of ternary semiconductor photocatalysts and thus significantly enhance their photocatalytic activities by utilizing BiOI as the example. We first used density functional theory (DFT) calculations to investigate the effects of iodine self-doping on the electronic and band structure of BiOI. Motivated by density functional theory (DFT) calculation results, iodine self-doped bismuth oxyiodide photocatalysts were synthesized with a soft chemical method to illustrate this band structure tailoring approach. After systematical characterization, we discuss the effects of iodine self-doping on the band structure, optical properties, and enhanced photocatalytic activities of BiOI. 2. Experimental Section 2.1. Theoretical Calculations. The band structures and density of state (DOS) calculations of BiOI and iodine self-doped BiOI were investigated via the plane-wave-pseudopotential approach based on density functional theory (DFT). All structures were optimized. Then their electronic structures were calculated within the generalized gradient approximation (GGA-PBE), as implemented in the highly efficient Cambridge serial total energy package (CASTEP) code.22 The k-point meshes for Brillouin zone sampling were constructed using the Monkhorst-Pack scheme. A plane wave cutoff energy of 380 eV was used. 2.2. Sample Preparation. All reagents were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and used as received without further purification. In a typical synthesis, Bi(NO3)3 · 5H2O was added into an ethylene glycol (EG) solution containing KI with different I/Bi molar ratios of 1, 1.5, 2, 2.5, and 3. The mixture was stirred for 0.5 h at room temperature in air and then poured into a 40 mL Teflon-lined stainless autoclave until 80% of the volume was filled. The autoclave was allowed to be heated at 160 °C for 12 h and then air cooled to room temperature. The resulting precipitates were collected and washed with ethanol and deionized water thoroughly and dried at 50 °C in air. Correspondingly, the obtained powders were labeled as BiOIx (x ) 1, 1.5, 2, 2.5, and 3) for a I/Bi molar ratio of 1, 1.5, 2, 2.5, and 3, respectively. 2.3. Characterization. X-ray powder diffraction (XRD) measurements were performed on a Rigaku Ultima III X-ray diffractometer with Cu KR radiation (λ ) 1.5418 Å). The morphology was determined by scanning electron microscopy (SEM, JEOL 6700-F). The samples for transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were prepared by dispersing the final powders in ethanol, and then the dispersion was dropped on carbon-copper grids, which was observed by a high-resolution transmission electron microscope (JEOL JSM-2010). UV-vis diffuse reflectance spectra (DRS) were obtained using an UV-vis spectrometer (Shimadzu UV-2550) and converted from reflection to absorbance by the Kubelka-Munk method.23 Brunauer-Emmett-Teller (BET) specific surface area was measured by nitrogen adsorption at 77 K using a Micromeritics Tristar-3000 surface area and porosity analyzer. Prior to analysis, 0.2 g of sample was degassed at 150 °C for 3 h in a flow of N2. The total surface area was determined by curve fitting the BET

J. Phys. Chem. C, Vol. 114, No. 42, 2010 18199 equation to adsorption isotherms of N2 at 77 K in the region of 0.05-0.99 relative pressure. The I:Bi molar ratios of the obtained self-doped photocatalysts were detected by IRIS (INTREPID 2) inductively coupled plasma atomic emission spectrometry (ICP-AES). 2.4. Photocatalytic Activity Test. Photocatalytic Degradation of Methyl Orange (MO). The optical system used for the photocatalytic reaction consisted of a 500 W halogen-tungsten lamp and a 420 nm cutoff filter, which was placed under the reaction cell to ensure irradiation with visible light only. All experiments were conducted at room temperature in air. In each experiment, 0.1 g of photocatalysts was added into MO solution (100 mL, 10 mg/L) in a reaction cell with a Pyrex jacket. Prior to irradiation, the suspensions were stirred in the dark for 1 h to reach adsorption/desorption equilibrium. At irradiation time intervals of 1 h, 4 mL of the suspensions were collected and then centrifuged to remove the photocatalyst particles. The obtained solutions were analyzed by a Hitachi U-3310 UV-visible spectrophotometer, and the absorbance at 464 nm was monitored. Photocatalytic RemoWal of NO. The NO removal with obtained powders was performed in a continuous flow reactor at ambient temperature.24 The NO gas was acquired from a compressed gas cylinder at a concentration of 48 ppm of NO (N2 balance, BOC gas) with traceable National Institute of Standards and Technology (NIST) standard. The initial concentration of NO was diluted to about 400 ppb by the air stream supplied using a zero air generator (Thermo Environmental Inc. model 111). The desired humidity level of the NO flow was controlled at 70% (2100 ppmv) by passing the zero air streams through a humidification chamber. The gas streams were premixed completely by a gas blender, and the flow rate was controlled at 4 L/min by a mass flow controller. After the adsorption-desorption equilibrium among water vapor, gases, and photocatalysts was achieved, the lamp was turned on. The concentration of NO was continuously measured by a chemiluminescence NO analyzer (Thermo Environmental Instruments Inc. model 42c), which monitors NO, NO2, and NOx (NOx represents NO + NO2) with a sampling rate of 0.7 L/min. The reaction of NO with air was ignorable when performing a control experiment with or without light in the absence of photocatalyst. 2.5. Surface Photovoltage Spectroscopy (SPS) and Transient Photovoltage (TPV) Measurements. The SPS instrument was made according to a previous report.25 Monochromatic light was obtained by passing light from a 500 W xenon lamp (CHF XQ500W, Global xenon lamp power) through a double-prism monochromator (Hilger and Watts, D 300). The slit width of the entrance and exit is 1 mm. A lock-in amplifier (SR830DSP), synchronized with a light chopper (SR540), was employed to amplify the photovoltage signal. The range of modulating frequency is from 20 to 70 Hz. The spectral resolution is 1 nm. The raw SPS data were normalized using the illuminometer (Zolix UOM-1S). Transient PV measurement was also carried out on a self-made instrument. The sample was excited with a second-harmonic 532 nm laser radiation pulse with 5 ns pulse width from a third-harmonic Nd:YAG laser (Polaris II, New Wave Research, Inc.). The intensity of the pulse was regulated with a neutral gray filter and determined by an EM500 single-channel Joule meter (Molectron, Inc.). The PV transient signal was registered by a 500 MHz digital phosphor oscilloscope (TDS 5054, Tektronix).

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Figure 1. Calculated band structures and total density of states (DOS) of (a, b) BiOI and (c, d) BiOI1+1/3.

3. Results and Discussion 3.1. Electronic and Band Structure Calculation of BiOI and Iodine Self-Doped BiOI. The electronic and band structures of photocatalysts play a crucial role in determining the photoexcitation of charge carriers. Using the CASTEP program package, the band structures of BiOI and iodine self-doped BiOI were calculated by the plane-wave density function theory (DFT). Herein, we first used plane-wave density function theory (DFT) to check how the iodine self-doping influences the electronic and band structures of BiOI by constructing a (3 × 1 × 1) supercell of iodine self-doped BiOI (BiOI1+1/3) based on the BiOI tetragonal structure (Figure S1, Supporting Information). The calculation results revealed that BiOI is an indirect gap semiconductor from Figure 1a. Surprisingly, the clear direct transition was observed for BiOI1+1/3 from Figure 1b. Obviously, the optical absorption property of BiOI was changed from indirect to direct nature after being iodine self-doped. Referring to the total DOS as shown in Figure 1c and 1d, it can be concluded that the top of the VB in BiOI is mainly composed of Bi6s, O2p, and I5p orbits, the bottom of the CB is Bi6s and Bi6p orbits, while BiOI1+1/3 has the same band composition. However, compared with BiOI, the CB bottom of BiOI1+1/3 is depressed after I5p orbits doping, which could narrow the band gap for a more broad absorption region. Meanwhile, the composition of s orbital in the hybridized CB increased obviously. Because the photogenerated carriers in the s orbital have a high mobility due to the dispersive characteristic of the s orbital, it favors the separation of photoinduced electron/hole pairs.26

3.2. Characterization of BiOIx Powders. XRD Analysis. Figure 2 shows the XRD patterns of the as-prepared powders with different I/Bi molar ratios. It showed that all the powders were well crystallized and could be indexed to a pure hexagonal structure for BiOI (JCPDS file No. 73-2062). Moreover, it illustrates that iodine doping does not result in new crystal orientations or changes in preferential orientations. The peak intensity of BiOIx increased with the increase of x value, as observed from Figure 2a. This indicates the relatively higher crystallinity and bigger crystalline size of BiOIx (x > 1) compared to BiOI. This phenomenon is different from that in the case of foreign element doping, which often results in a crystal size decrease of doped photocatalysts by providing dissimilar boundaries. The crystallite size of BiOIx was calculated from the full width at half-maximum (fwhm) of the most intense diffraction peak (012) by using Scherrer’s equation. The average crystalline sizes are summarized in Table 1, which were about 10.8, 12.0, 18.1, 21.7, and 27.1 nm for the samples of BiOIx (x ) 1, 1.5, 2, 2.5 and 3, respectively). However, a careful comparison of the (012) diffraction peaks in the range of 2θ ) 28-31° (Figure 2b) shows that the peak position of BiOI with different iodine contents shifts slightly toward a higher 2θ value. The same trends are also observed on other diffraction peaks. According to Bragg’s law, dhkl ) λ/(2 sin 2θ), where dhkl is the distance between the crystal planes of (hkl), λ is the X-ray wavelength, and θ is the diffraction angle of the crystal plane (hkl). The observed shift of diffraction peaks toward higher 2θ values could not be attributed to substitution of O2- with I-, because the radius of I- (0.216 nm) is larger than that of O2- (0.140 nm) and therefore the substitution of

Tuning of Ternary Semiconductor Photocatalysts

Figure 2. (a) XRD patterns of as-prepared BiOIx (x ) 1, 1.5, 2, 2.5, and 3) powders. (b) Diffraction peak positions of the (012) plane in the range of 2θ ) 28-31°.

smaller O2- with larger I- should increase lattice parameters (dhkl value), which corresponds to the shift of diffraction peaks toward lower 2θ values. We think this unexpected lattice shrinkage may be attributed to the appearance of bismuth vacancy to reach new charge balance after the substitution of oxygen with iodine.27 XPS Spectrum. The XPS investigation of BiOI and BiOI1.5 powders was performed for evaluation of their composition. The peak positions of different atoms were determined by internally referencing the adventitious carbon at a binding energy of 284.6 eV. Figure 3a is a typical XPS survey spectrum of different atoms. From Figure 3a, both of them were composed of four elements of Bi, O, I, and a trace amount of carbon. The peaks at binding energies of 163.8, 529.4, and 619.1 eV can be assigned to Bi4f5/2, O1s, and I3d5/2 in pure BiOI, respectively. To clearly reveal the chemical states of the I species in BiOIx (x ) 1, and 1.5), a careful comparison of I3d spectra is shown in Figure 3b. In BiOI, the peaks at binding energies of around 630.1 (I3d3/2) and 618.6 eV (I3d5/2) could be ascribed to pure BiOI. As for BiOI1.5, the peaks at binding energies of around 630.0 (I3d3/2) and 618.5 eV (I3d5/2) could be fitted with two sets of peaks, 629.8 and 630.6 eV and 618.3 and 619.1 eV, respectively. The first set of peaks at 629.8 and 619.1 eV could be ascribed to I3d3/2 and I3d5/2, which is similar with that in pure

J. Phys. Chem. C, Vol. 114, No. 42, 2010 18201 BiOI. Another set of peaks at 630.6 and 618.3 eV might be assigned to I- from the excess KI.28 However, there was no potassium species detected by the XPS analysis (Figure S2a, Supporting Information). We also checked the changes in the valence-band structure induced by iodine doping. Valence-band XPS was used for the observation of extra electronic states above the valence band of BiOI (Figure S2b, Supporting Information). The valence band of undoped BiOI is dominated by Bi6p and I5p states overlapping with O2p states. Although a very similar peak configuration was observed for BiOIx, its valence band was changed after the iodine self-doping. This change may be interpreted as an addition of diffusive electronic states and thus a shift in the top of VB to lower binding energy, representing a narrowing of the band gap. Moreover, these additional diffusive electronic states above the VB of BiOI1.5 suggest an increase of the density of states just above the VB edge, which is consistent with the DFT calculations. This confirms the effective tuning of the iodine self-doping on the band structures of BiOI. SEM Images. Figure 4 shows the SEM images of BiOIx powders. It is found that undoped BiOI is composed mostly of sphere-like superstructures (Figure 4a). The diameters of these spheres were in the range of 1-4 µm. Obviously, iodine doping did not significantly affect the morphology of BiOI. All BiOIx samples were of microsphere superstructures with similar sizes (Figure 4b-e). Higher magnification SEM images (Figure 4f) reveal that these microspheres were composed of nanoplates about 500 nm in length. These nanoplates aligned radically to form hierarchical microspheres because of the direction role of ethylene glycol (EG).21 TEM and HRTEM Images. Electron microscopy characterization of undoped BiOI photocatalyst was presented in our previous study.21 Figure 5 displays the TEM and HRTEM images of the as-synthesized self-doped photocatalyst BiOI1.5. As shown in Figure 5a, BiOI1.5 consisted of solid structured spheres. Irregular nanoplates can be observed on the sphere surface, which confirms SEM observation (Figure 4f). Highresolution transmission electron microscopy (HRTEM) was further utilized to investigate the structural information of BiOI1.5 (Figure 5b). The HRTEM image of BiOI1.5 revealed that the material was highly crystallized, as evidenced by well-defined lattice fringes. A clear lattice image was observed with d spaces of about 0.278 nm, which is consistent with the d spacing of the [110] reflection (0.275 nm). Moreover, the HRTEM image reveals that amorphous shells are covered on the crystallized BiOI nanoplates to form core-shell structures. These amorphous shells might be the excess iodine species adsorbed on the surface of BiOI nanoplates. On the basis of XRD, XPS, and HRTEM results, we conclude that the self-doped iodine species in BiOI1.5 might be incorporated into the BiOI lattice and adsorbed on the surface of BiOI. We also analyzed the I:Bi ratios of BiOIx by using ICP-AES (Table 2). It is found that the final I:Bi ratio (1.044) of undoped BiOI is close to 1, while the final I:Bi ratios of doped BiOIx were less than the original ones added in the solution. With x value increasing, the deviation of final I:Bi ratios of doped BiOIx from the original ones became larger. However, all these final I:Bi ratios of doped BiOIx (x ) 1.5, 2, 2.5, and 3) are significantly higher than that of undoped BiOI, confirming their self-doping character. 3.3. Optical Absorption Properties of BiOIx. It is interesting to note that the color of the products is strongly dependent on their composition (Figure 6a). With a higher iodine content in the product, its color gradually deepens from orange, to

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TABLE 1: Summary of Lattice Parameters, Crystal Size, Textural Properties, Photocatalytic Activities, and Band Gap of BiOIx Powders lattice parameters [Å] x in BiOIx 1 1.5 2 2.5 3

a)b 3.9834 3.9720 3.9650 3.9781 3.9848

c 9.1696 9.1923 9.1751 9.1182 9.1321

crystal sizea [nm] 10.8 ((0. 2) 12.0 ((0. 2) 18.1 ((0. 1) 21.7 ((0.2) 27.1 ((0.1)

ABET [m2 g-1] 28.6 ((0. 1) 29.7 ((0. 1) 19.5 ((0. 2) 12.8 ((0. 1) 9.4 ((0. 1)

k b [h-1] -3

5.5 × 10 11.1 × 10-3 4.3 × 10-3 2.2 × 10-3 1.5 × 10-3

k′ c [g · h-1m-2] -4

1.92 × 10 3.74 × 10-4 2.21 × 10-4 1.72 × 10-4 1.60 × 10-4

Egd [eV] 2.04 ((0. 01) 1.96 ((0. 01) 1.93 ((0. 01) 1.91 ((0. 01) 1.88 ((0. 01)

a The crystallite size of BiOIx was calculated from the full width at half-maximum (fwhm) of the most intense diffraction peak (012) using Scherrer’s equation. b The reaction kinetics of MO degradation were analyzed with the pseudo-first-order model as expressed by ln(C0/C) ) kt. c The k′ values were k values normalized with the surface areas. d Band gap energy was estimated from the main absorption edges of the UV-vis diffuse reflectance spectrum.

Figure 3. (a) XPS survey spectrum and (b) high-resolution I3d spectra of BiOIx.

carmine red, and last to deep red, which implies that the intrinsic optical properties have changed. UV-vis absorption measurements were carried out to further investigate the optical properties of these powders. The UV-vis diffuse reflectance spectra (DRS) of the samples are shown in Figure 6b. It was found that the absorption edges of pure and iodine self-doped BiOI were quite different. BiOI has an absorption edge up to about 600 nm, while the absorption edge of iodine self-doped BiOIx sample shifts toward the visible light region. This indicates the band gap was reduced after iodine doping. Besides, Figure 6b also reveals that the absorption spectrum of BiOI in the visible region is not very steep and has a tail in the edge,

indicating that its absorption relevant to the band gap is due to the indirect transition. However, for iodine self-doped BiOIx, intense absorption bands with a steep edge in the visible light region were observed, meaning that their visible light absorption bands are induced by the direct transition instead.29 The band gap energy of a semiconductor could be calculated by the formul: Rhν ) A (hν - Eg)n/2, where R, ν, Eg, and A are the absorption coefficient, the light frequency, the band gap, and a constant, respectively. Among them, n depends on the characteristics of the transition in a semiconductor, i.e., direct transition (n ) 1) or indirect transition (n ) 4). For BiOI, the value of n is 4 for the indirect transition.20 Therefore, the band gap energy (Eg value) of the resulting samples can be estimated from a plot (Rhν)1/2 versus photon energy (hν). The intercept of the tangent to the X axis would give a good approximation of the band gap energy of the samples (Figure 6c). The band gap of BiOI is evaluated to be 1.92 eV, close to that reported in our previous work. It is found that the band gaps of BiOIx powders ranged from 1.92 to 1.84 eV and decreased with the x value increased from 1 to 3 (Table 1). 3.4. Photocatalytic Activities of BiOIx Powders. Photocatalytic Degradation of MO. Figure 7a exhibits the photocatalytic activities of BiOIx powders on the degradation of MO under visible light irradiation (λ > 420 nm). MO is a kind of chemically stable and persistent dye pollutant, and its photolysis upon visible light irradiation is negligible under our experimental conditions. The concentration of methyl orange did not change after 1 h adsorption at intervals of 0.5 h by a UV-vis spectrophotometer, confirming that MO adsorption on photocatalysts reached equilibrium in 1 h. From Figure 7a, the adsorption of MO on BiOIx powders is different before irradiation. The adsorption decreases with the increase of x value, which is probably attributed to the different BET surface areas of BiOIx (Table 1). It is easily acceptable that self-doped BiOIx photocatalysts exhibited different photocatalytic performances on MO degradation on the basis of the above characterization. After 4 h of visible light irradiation, the MO degradation over undoped BiOI is about 48% and 74% for BiOI1.5. When the x value increased to 2, about 29% of MO could be degraded on the photocatalyst. Further increasing the x value to 2.5 and 3, the degradation efficiencies were 10% and 6%, respectively. Therefore, BiOI1.5 exhibited better photocatalytic activity among all the BiOIx (x ) 1, 1.5, 2, 2.5, and 3). To evaluate the role of I- on the photocatalytic activity of BiOIx, we compared the photocatalytic performances of BiOI1.5 and pure BiOI with adding equally excess KI (BiOI + KI) in the solution on the degradation of MO under the same condition (Figure 7b). It is found that BiOI + KI showed similar photocatalytic performance with BiOI, which was much lower

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Figure 4. SEM images of BiOIx powders: x ) (a) 1, (b) 1.5, (c) 2, (d) 2.5, and (e) 3. (f) High-resolution image of the surface of the sphere (BiOIx, x ) 3).

than that of BiOI1.5. This suggests that excess I- in the solution has no positive effect on the photocatalytic activity of BiOI. In other words, the photocatalytic activity enhancement of BiOI1.5 is attributed to the iodine doping, not excess iodine adsorbed on the surface as observed on the HRTEM image (Figure 5b). Photocatalytic RemoWal of NO. We further evaluated the photocatalytic performance of BiOIx (x ) 1, 1.5, 2, 2.5, and 3) to remove pollutants in the gas phase. Figure 7c shows the relative variations of the NO removal rate in the presence of BiOIx at a single pass flow through the reactor under a humidity level of 2100 ppmv. As shown in Figure 7c, the removal efficiency reached the highest value of 28 and 32% for BiOI and BiOI1.5 respectively, after only 30 min under visible light irradiation. The BiOIx (x ) 2 and 2.5) showed lower removal efficiencies of 20% and 24%, respectively, after being irradiated for 30 min. In the case of the highest doping dose, BiOI3 exhibited the lowest removal efficiency of 12%. Therefore,

BiOI1.5 also exhibited the best photocatalytic performance among all these BiOIx samples for pollutant removal in the gas phase. The NO removal efficiency of BiOI1.5 was much higher than the famous TiO2 photocatlayst (Degussa P25), which could only remove 4% NO in 30 min under visible light irradiation. The unexpected photocatalytic activity change of BiOI2 in Figure 7c may be related to photocatalytic activity loss during gasphase photocatalysis, which was observed previously.24,30,31 The origin for this kind of photocatalytic activity loss is unknown to us at present. To check the stability of self-doped BiOI photocatalyst, we examined the phase structures of BiOI1.5 after the photocatalytic degradation of MO and the chemical states of Bi, O, and I in BiOI1.5 before and after visible light irradiation (Figure S3, Supporting Information). There were no changes in phase structure or the chemical states of iodine self-doped BiOI powders after visible light photocatalysis, suggesting self-doped

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Figure 5. (a) TEM and (b) HRTEM images of BiOIx (x ) 1.5) powders. High-resolution image of the surface of the sphere (square parts) is shown in the inset of a.

TABLE 2: Bi:I Ratios of BiOIx (x ) 1, 1.5, 2, 2.5, and 3) Analyzed by Using ICP x in BiOIx from ICP

1

1.5

2

2.5

3

1.044 1.276 1.261 1.320 1.431 ((0. 001) ((0. 001) ((0. 002) ((0. 002) ((0. 002)

BiOI is photochemically stable for pollutant removal under visible light irradiation. 3.5. Surface Photovoltage Spectroscopy and Transient Photovoltage Measurements. The SPV method relies on analyzing illumination-induced changes in the surface voltage. The signal of photovoltage is attributed to the changes of surface potential barriers before and after illumination. Thus, it can provide a rapid and straightforward investigation of the carrier separation and transfer behavior. We used surface photovoltage spectroscopy (SPS) and transient photovoltage (TPV) measurements to detect charge transfer in BiOIx (x ) 1 and 1.5). As seen in Figure 8a, each sample has a distinct SPV response. The SPV response of pure BiOI appears in the visible light region at about 500 nm, while BiOI1.5 has an obvious absorption edge at about 620 nm. The similarity between the UV-vis optical absorption and SPV spectra is a clear indication that photon absorption indeed induces charge generation and separation. The SPS response observed on pure BiOI can be attributed to the electron transition from the valence band to the conduction band of BiOI. Differently, the intensity of the SPS signal on BiOI1.5 increased. This indicates that the generation of electron-hole pairs can be improved in BiOI1.5 because of its narrower band gap. Transient photovoltage measurements were further used to study the difference of charge transfer between BiOI and BiOI1.5. For BiOI (Figure 8b), innumerable charges could accumulate on the surface after being excited. This would bring about a rise of photovoltage response (the first apex at 3 × 10-7 s) after the laser was switched off. After the first quick photovoltage transient, charge concentration in the BiOI surface is decreasing due to the recombination. Because the drift velocity of the holes is much slower than that of the electrons, the enrichment of charge in the BiOI surface could be still detected, corresponding to the positive signals as seen in Figure 7b. However, the interaction of the electron with internal surface states may strongly influence electron diffusion, so the transport of charge carrier through this process is slow, resulting in the second

recombination process corresponding to the apex at 1 × 10-4 s. For BiOI1.5, it has a similar photovoltage response with that of BiOI, suggesting that BiOIx (x ) 1 and 1.5) has a similar charge transfer process. However, BiOI1.5 has more strong signals, indicating that it can be more easily excited than BiOI and the photoinduced e/h pairs can be more easily transformed in the surface of BiOI1.5 than in BiOI. This further confirms that the self-doping could speed up the charge transfer on BiOI1.5. 3.6. Effects of Self-Doping on the Electron and Band Structure and Photocatalytic Activity. It is known that photocatalytic processes are based on photoinduced electron-hole (e/h) pairs. Therefore, the generation and separation of photoinduced e/h pairs, which are related to the electronic and band structures of photocatalysts, are the key factors to influence a photocatalytic reaction. DFT calculation suggests that iodine self-doping of BiOI leads in general to an increase of the density of states just above the BiOI valence-band edge, which is confirmed by valence-band XPS analysis. This special electronic structure makes the band gap narrower and the VB flatter, which favors the mobility of photoinduced holes and thus is beneficial to photocatalytic oxidation of organic pollutants.26,32,33 Moreover, the iodine self-doping changes the optical absorption of BiOI from indirect to direct, which is more helpful for visible light excitation. This is confirmed by the DRS experimental results. Furthermore, the band structures of doped BiOI were more beneficial for the separation of e/h pairs, which was proved experimentally by surface photovoltage measurements. Meanwhile, iodine doping in BiOI could increase the carrier transfer of photoinduced electron-hole pairs, as revealed by surface photovoltage spectroscopy and transient photovoltage measurements. Therefore, we demonstrated that self-doping is able to tune the electronic and band structures of ternary semiconductor photocatalyst BiOI and thus significantly enhance its photocatalytic activity. Besides the band structure, there exist many other factors to influence the photocatalytic activity of semiconductors, including surface area, crystallinity, optical absorption, and so on. When x > 1.5, the surface areas of BiOIx photocatalysts decreased, resulting in their photocatalytic activity drops partially. Meanwhile, like foreign-elements doping for TiO2, there is an optimal amount of iodine doping. Excess iodine doping might cause too many crystal defects and/or surface-adsorbed iodine ions

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Figure 6. (a) Gradual color changes of the as-prepared BiOIx powders. (b) UV-vis diffuse reflectance spectra (DRS) of BiOIx powders. (c) Plots of the (Rhν)1/2 vs photon energy (hν) for BiOIx powders.

on BiOIx photocatalysts, which would also decrease the photocatalytic activities of BiOIx for x > 1.5. In order to rule out the effect of different surface areas of the self-doped photocatalysts on their photcatalytic activities, we normalized the photocatalytic degradation rates by the surface areas (Table 1) and found that the order for the normalized rates was the same as that of the original rates, confirming that iodine self-doping could enhance the photocatalytic activity of BiOI. This self-doping strategy could also change the optical properties of other ternary semiconductor photocatalysts, such as BiOCl, BiOBr, and BiVO4, and enhance their photocatalytic activities (Figures S4-6 and Table S1, Supporting Information). Meanwhile, there are also optimal dopant contents for selfdoping in other ternary semiconductor photocatalysts. The selfdoped photocatalysts exhibit the best photocatalytic activity only in the case of the optimal self-doping content. Although the intrinsic origin for the existence of optimal self-doping content is unclear at present, we do believe self-doping is a general and convenient method to tune the electronic and band structures

Figure 7. Photocatalytic activities of BiOIx powders under visible light irradiation: (a) degradation of MO, (b) effects of surface-adsorbed iodine species on the photocatalytic activity of BiOI1.5, and (c) removal of NO.

of other ternary photocatalysts and thus open up new perspectives on development of highly efficient ternary semiconductor photocatalysts. Moreover, compared with the methods (metalorganic chemical vapor deposition,18 Ar ion bombardment,34 or oxygen desorption induced by electronic excitations,35 etc.) to fabricate self-doped TiO2, the soft chemical approach to synthesize self-doped ternary semiconductor photocatalysts in this study is much more convenient and easily scaled up.

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Zhang and Zhang 0352), and Program for Changjiang Scholars and Innovative Research Team in University (grant IRT0953). Prof. Zhigang Zou and Prof. Zhaosheng Li at Nanjing University are highly appreciated for their help with theory calculations. We also thank Prof. Dejun Wang and Prof. Tengfeng Xie in Jinlin University for the SPS and TPV measurements. Supporting Information Available: Crystal structures of BiOI and BiOI1.5, valence-band XPS spectra of BiOI and BiOI1.5, photostability test results of BiOI1.5, UV-vis diffuse reflectance spectra, photocatalytic activities, and the composition of BiOClx, BiOBrx, and BiVxO4. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 8. Surface photovoltage measurements of BiOI and BiOIx (x ) 1.5): (a) surface photovoltage spectroscopy (SPS) and (b) transient photovoltage (TPV) measurements.

4. Conclusions In summary, we demonstrated that self-doping is able to tune the electronic and band structures of ternary semiconductor photocatalysts and thus significantly enhance their photocatalytic activities by utilizing BiOI as the example. Density functional theory calculations revealed that iodine self-doping could effectively tune the electronic and band structures of BiOI, which is confirmed by DRS and valence-band XPS results experimentally. The BiOIx (x ) 1, 1.5, 2, 2.5, and 3) hierarchical microspheres were synthesized through a simple soft-chemical method. Among them, BiOI1.5 exhibited the best photocatalytic activity both on the photocatalytic degradation of methyl orange and on removal of NO under visible light irradiation. When x > 1.5, the photocatalytic activities of BiOIx deceased, suggesting there is an optimal amount of iodine self-doping. This study suggests that this self-doping strategy and its facile preparation method are very attractive for the development of novel visible light driven photocatalysts with enhanced performance. Acknowledgment. This work was supported by the National Basic Research Program of China (973 Program) (grant 2007CB613301), National Science Foundation of China (grants 20777026, 21073069, and 91023010), Program for Innovation Team of Hubei Province (grant 2009CDA048), Self-Determine Research Funds of CCNU from the Colleges’ Basic Research and Operation of MOE (grant CCNU09C01009), Program for New Century Excellent Talents in University (grant NCET-07-

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