Efficient Visible-Light-Driven Photocatalytic Degradation with Bi2O3

Res. , 2014, 53 (32), pp 12575–12586. DOI: 10.1021/ie501850m. Publication Date (Web): July 20, 2014. Copyright © 2014 American Chemical Society. *E...
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Efficient Visible-Light-Driven Photocatalytic Degradation with Bi2O3 Coupling Silica Doped TiO2 Juan Yang,* Xiaohan Wang, Jun Dai, and Jiantong Li Department of Physics and Chemistry, Henan Polytechnic University, Jiaozuo, 454003, People’s Republic of China S Supporting Information *

ABSTRACT: A new TiO2-based visible light photocatalyst (Bi2O3/Si−TiO2) was synthesized by both Bi2O3 coupling and Si doping via a two-step method. The structural, morphological, light absorption, and photocatalytic properties of as-prepared samples were studied using various spectroscopic and analytical techniques. The results showed that Bi2O3/Si−TiO2 catalysts held an anatase phase and possessed high thermal stability. The doped Si was woven into the lattice of TiO2, and its content had a significant effect on the surface area and the crystal size of Bi2O3/Si−TiO2. The introduced Bi species mainly existed as oxides on the surface of TiO2 particles, and the Bi2O3 photosensitization extended the light absorption into the visible region. Bi2O3 coupling also favored the separation and transfer of photoinduced charge carriers to inhibit their recombination and Si doping enlarged the surface area of photocatalysts. Compared to bare TiO2, Bi2O3/TiO2, and Si−TiO2, Bi2O3/Si−TiO2 samples showed better activities for the degradation of methyl orange (MO) and bisphenol A (BPA) under visible light irradiation (λ > 420 nm). The highest activity was observed for 1.0% Bi2O3/15% Si−TiO2 calcined at 500 °C. The superior performance was ascribed to the high surface area, the ability to absorb visible light, and the efficient charge separation associated with the synergetic effects of appropriate amounts of Si and Bi in the prepared samples. The adsorbed hydroxyl radicals (•OH) were also found to be the most reactive species in the photocatalytic degradation.

1. INTRODUCTION Faced with significant environmental problems, extensive research on the elimination of hazardous chemical compounds has been developed in the past several decades. Heterogeneous photocatalysis, as an emerging advanced oxidizing technology, has consistently drawn much attention worldwide since it can provide a simple way to use light to perform chemical transformations.1 In most cases, photocatalytic degradation is conducted over anatase TiO2 owing to its strong oxidizing power, cost effectiveness, and long-term stability against photocorrosion and chemical corrosion.2,3 However, the large band gap (3.2 eV) of anatase makes it active only under UV light with wavelength less than 387.5 nm, limiting its practical environmental application by using solar light with more than 43% of visible light. With this view, doping of TiO2 has been undertaken by a number of researchers. Metal ion doping into the TiO2 lattice4−7 causes the thermal instability of the prepared TiO2 and the metal centers can act as electron traps, which lowers the photocatalytic efficiency. Nonmetal ion doping has shown great potential in introducing bathochromism, which leads to enhanced photoactivity in the visible light region. Among these doped TiO2’s, N, S, and C doped TiO2’s have been regarded to be excellent photocatalysts in the past decades. 8−11 Nonetheless, it has been found that the substituted nonmetal atoms could also serve as recombination sites of photogenerated charges, reducing the quantum yield of the photocatalytic reactions.12 Therefore, it is highly desirable to develop a new modification or preparation method that can enhance the photocatalytic activity of TiO2. To tackle these drawbacks over TiO2 photocatalysis, hybrid heterostructures between TiO2 and other semiconductors with proper band potentials have been explored. The aim has been © 2014 American Chemical Society

to extend the response range to wider wavelengths and to promote greater separation of photoinduced charge carriers. For instance, ZnO/TiO2 composites displayed enhanced photocatalytic activity for the degradation of dye pollutants rhodamine 6G compared to bare TiO2 or ZnO under UV light illumination.13 Zhao and co-workers have reported improved photocatalytic activity on the removal of toxic organic pollutants by using Ni2O3/TiO2−xBx under visible light irradiation.14 Low content of SnO2 coupled N-doped TiO2 possessed higher photocatalytic properties on the decomposition of rhodamine B than bare TiO2 and single doped TiO2 under visible light irradiation.15 Bismuth oxide (Bi2O3), as a promising semiconductor to be coupled with TiO2, is an attractive material for the photooxidation of pollutants because of its direct band gap of 2.8 eV.16,17 Bi2O3/TiO2 composites or Bi-doped TiO2 are more effective than bare TiO2 in the case of photodegradation of organic pollutants under irradiation. Li et al. reported the enhanced photocatalytic activity of Bi2O3/TiO2 for the degradation of 4-chlorophenol under visible light irradiation, due to the photosensitization effect of coupled Bi2O3.18 According to Zhang et al., the doped Bi ions substituted some of the lattice Ti atoms determined using Xray photoelectron spectroscopy. The resulting Bi impurity level aided in the separation of electron−hole pairs and inhibited their recombination.19 At the same time, Singh and co-workers pointed out that Bi12TiO20 phase was produced when 15% TiO2 was doped into Bi2O3. The produced Bi12TiO20 catalyst Received: Revised: Accepted: Published: 12575

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and stirred for 30 min to form an ethanol−nitric acid−water solution. The TBOT−C2H5OH solution was added dropwise to the latter solution under vigorous stirring for 1 h until a translucent solution was obtained. The resulting solution was then transferred into a 100 mL Teflon-lined steel autoclave and heat treated at 140 °C for 6 h. A yellow deposit was obtained after the autoclave was cooled to room temperature, washed with deionized water and ethanol, and dried at 80 °C overnight. The obtained material was ground into powder and calcined at various temperatures to obtain the final product. The asprepared sample was denoted as mBi2O3/nSi−TiO2 (X), where m and n represented the Bi2O3 and Si molar contents, and X was the calcination temperature. m changed from 0.2, 0.5, 1.0, to 2.0%; n changed from 5.0, 10.0, 15, to 30%, respectively. Bare TiO2, Si doped TiO2, and Bi2O3 coupling TiO2 (Bi2O3/ TiO2) were prepared via a similar procedure. 2.3. Characterization. The X-ray diffraction (XRD) measurements were performed on a D8 Advance diffractometer (Bruker) with Cu Kα radiation (λ = 0.154 05 nm) in the range 10−80° (2θ). The crystallite size was estimated by the Scherrer equation on the full width at half-maximum (fwhm) of the anatase(101) peak. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were obtained with a FEI Tecnai G2 highresolution transmission electron microscope operating at 200 kV. The distribution of Ti and Bi concentrations in Bi2O3/Si− TiO2 was analyzed by energy-dispersive X-ray spectroscopy (EDX; HAADF, FEI Tecnai G2 F20). Nitrogen adsorption and desorption measurements were performed at 77 K with a Micromeritics ASAP 2020 system. The samples were degassed at 473 K before measurements. Ultraviolet−visible diffusereflectance spectra (UV−vis DRS) were obtained on a UV−vis spectrophotometer (UV-2550, Shimadzu, Japan) using BaSO4 as reflectance standard. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo Scientific ECALAB 250xi system with Mg Kα source. All the binding energies were calibrated by the C 1s peak at 284.8 eV of the surface adventitious carbon. Fourier transformation infrared (FT-IR) spectra were acquired in the range 400−4000 cm−1 with a NICOLET 750 FT-IR spectrometer using KBr and sample mixture pellets. Raman spectra were collected by a Renishaw inVia Reflex Raman spectrometer with 514 nm excitation light in the range 100−1400 cm−1. Photoluminescence (PL) spectroscopy was measured at room temperature using a fluorescence spectrophotometer (PerkinElmer LS55) with an excitation wavelength of 325 nm. Electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV) experiments were carried out in a standard threeelectrode cell containing 0.5 mol/L Na2SO4 aqueous solution with a platinum foil and a saturated calomel electrode as the counter electrode and reference electrode, respectively, on a CHI 760D workstation (Shanghai, China). The working electrode was prepared as described in recent reports.28 2.4. Photocatalytic Activity Measurement. The photocatalytic performance of as-prepared catalyst was evaluated by using MO and BPA as model pollutants in aqueous suspension. The light source for photocatalytic reaction was a 300 W xenon lamp (Changtuo Instrumental Corp. of Beijing). All the radiation with wavelengths shorter than 420 nm was removed by a glass filter. The temperature of the reaction system was controlled with a portable air conditioner. The initial concentration of MO and BPA in a 100 mL self-designed quartz photochemical reactor was fixed at 10 mg/L. Prior to

exhibited the increasing photoactivity of degradation of methyl orange (MO) under UV illumination, compared to bare TiO2 or Bi2O3.20 However, the photocatalytic decomposition of pollutants using Bi12TiO20 under visible light illumination was not referred to in that study though the formation of Bi12TiO20 phase resulted in the band gap narrowing (2.55 eV). Another vital method, to enhance the photocatalytic activity of TiO2, is to control the microstructure of TiO2 particles, including the crystalline phase, crystallinity, crystallite size, and specific surface area.21 The crystal phase and crystallinity affect the surface properties of TiO2, which strongly influence the recombination rate of photogenerated charge carriers. The crystallite size and surface area contribute to the efficient light absorption and reactant adsorption. The most photoactive form, anatase TiO2, may transfer to rutile in the calcination process, accompanied by a remarkable decrease of surface area and photocatalytic activity. Silica species or SiO2 was introduced into TiO2 lattice, leading to superior thermal stability toward incorporated nonmetal elements (N, C, P, S, F, etc.). Thereby, the anatase phase was preserved and the surface area was enlarged, as well as the crystallite size was decreased even at high calcination temperatures.22−27 The silicon doped TiO2 can be applied to various processes over a wide range of operating conditions frequently encountered during environmental remediation and fine chemical production. However, the Si-doped TiO2 and SiO2−TiO2 mixed oxides cannot be activated with visible light illumination. According to the above analysis, it could be expected that TiO2 modified by Bi2O3 coupling and Si doping might be a promising photocatalyst with high surface area, superior thermal stability, and excellent visible light performance simultaneously. Herein, we prepared Bi2O3 coupling Si−TiO2 (Bi2O3/Si− TiO2) through a facile solvothermal method. To the best of our knowledge, Bi2O3 coupled Si−TiO2 has not been previously reported. The as-prepared samples show better photocatalytic activity toward the degradation of methyl orange (MO) and bisphenol A (BPA) than Si−TiO2 and Bi2O3/TiO2 under visible light irradiation. The promoting effects of Bi and Si species on the photocatalytic activity of TiO2 were investigated. On the basis of the experimental results, the possible degradation mechanism and synergistic effects of Bi2O3 coupling and Si doping on the enhanced photocatalytic activity were also proposed.

2. EXPERIMENTAL SECTION 2.1. Materials. Tetrabutyl titanate (TBOT), bismuth nitrate pentahydrate (Bi(NO 3 ) 3 ·5H 2 O), tetraethyl orthosilicate (TEOS), and bisphenol A (BPA) were obtained from National Medicines Corporation Ltd., China. Nitric acid (HNO3), methyl orange (MO), and anhydrous ethanol (C2H5OH) were purchased from Shanghai Chemical Reagent Limited Co. All of the chemicals were of analytical grade and used as received without further purification. Double distilled water was used throughout in the experiments. 2.2. Preparation of Photocatalysts. The Bi2O3/Si−TiO2 samples were prepared following a simple modified solvothermal method, using TBOT, Bi(NO3)3·5H2O, and TEOS as the precursor materials for Ti, Bi, and Si, respectively. In a typical synthesis, 20 mL of TBOT was dissolved in 20 mL of ethanol to produce the TBOT−C2H5OH solution. Meanwhile, 0.582 g of Bi(NO3)3·5H2O was dissolved in 10 mL of 3 mol/L HNO3, and then another 40 mL of C2H5OH was added. A 1.8 mL volume of TEOS was introduced dropwise into the solution 12576

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photoreaction, the suspension was magnetically stirred in the dark for 30 min to establish an adsorption/desorption equilibrium between the pollutants and the surface of the photocatalysts. At given irradiation time intervals, 3 mL of suspension was collected and subsequently centrifuged to remove the catalyst particles. The extent of MO and BPA decomposition was determined by measuring the absorbance value at 463 and 278 nm respectively using a Cary100 UV/vis spectrophotometer. The concentrations of H 2 O2 were determined by the spectrophotometric DPD (N,N-diethyl-pphenylenediamine) method.29 Total organic carbon (TOC) assays were carried out on a Tekmar Dohrmann Apollo 9000 TOC analyzer.

3. RESULTS AND DISCUSSION 3.1. XRD Analysis. Figure 1A,B shows the XRD patterns of bare TiO2 and Bi2O3/Si−TiO2 with different amounts of Bi and Si dopants. As indicated in Figure 1A,B, the XRD patterns of Bi2O3/Si−TiO2 samples almost coincided with that of pure TiO2. All the XRD patterns exhibited the presence of anatase phase TiO2 (JCPDS 21-1272) without any other TiO2 phases or impurity phases induced by the dopants. Besides, the (101) peak intensities did not obviously decrease with the increase in Bi contents, indicating that the introduced Bi species affected little the crystallinity of obtained materials (Figure 1A). The diffraction angles also did not shift, suggesting the Bi species could not incorporate into TiO2 lattice, but might present as a separate phase on the surface of Bi2O3/Si−TiO2. This could be attributed to the bigger size of Bi3+ (103 pm) than that of Ti4+ (61 pm), which inhibited the replacement of Ti by Bi in the TiO2 crystal lattice.18 No obvious diffractional peaks indicative of Bi2O3 phase were observed in the XRD spectra of Bi2O3/Si− TiO2 sample (Figure 1A). Additionally, pure Bi2O3 sample was prepared and analyzed by XRD. The comparisons in Figure S1 (see Supporting Information) indicated that there was no overlap for the XRD peaks of single Bi2O3 and Bi2O3/Si−TiO2 sample. The results imply the coupled Bi2O3 was amorphous and highly dispersed on the surface of TiO2. Based on the XRD data, the average crystallite sizes (D) of as-prepared samples were calculated using the well-known Scherrer equation, and the results are displayed in Table 1. It can be seen from Table 1 that the D values changed to some extent when the Bi2O3 content varied from 0.25 to 2.0 mol % under the Si content of 15 mol %. For the mBi2O3/15% Si−TiO2 samples, when 0.25 mol % Bi2O3 was coupled, the D value was 12.3 nm. About 2.0 mol % Bi2O3 was introduced, and the D value changed to 8.3 nm. It might be attributed to that the presence of Bi2O3 would hinder the aggregation of particles. Similar results have been reported by Ku et al., in which the compounding of NiO and ZnO can inhibit the growth of TiO2 particles and therefore increase the surface area of catalysts.30,31 However, the (101) peak intensity decreased significantly and the peak shifted to a higher angle after Si doping, indicating the crystalline size and crystallinity decreased and the doped Si entered the lattice of TiO2 (see Figure 1B and inset). The D values of samples changed remarkably with the incorporation of Si species. For the 1.0% Bi2O3/nSi−TiO2 sample, when 5.0 mol % Si was doped, the D value changed to 13.3 nm, compared with 23.2 nm for bare TiO2. Doping more Si (such as 30 mol %), the D value of sample was 5.5 nm. This demonstrated that the doped Si played a more important role in the crystallite size of TiO2 than the introduced Bi species.

Figure 1. (A) XRD patterns of (a) bare TiO2, (b) 15% Si−TiO2, (c) 0.25% Bi2O3/15% Si−TiO2, (d) 0.5% Bi2O3/15% Si−TiO2, (e) 1.0% Bi2O3/15% Si−TiO2, and (f) 2.0% Bi2O3/15% Si−TiO2 calcined at 500 °C. (B) XRD patterns of (a) bare TiO2, (b) 1.0% Bi2O3/TiO2, (c) 1.0% Bi2O3/5% Si−TiO2, (d) 1.0% Bi2O3/10% Si−TiO2, (e) 1.0% Bi2O3/15% Si−TiO2, and (f) 1.0% Bi2O3/30% Si−TiO2 calcined at 500 °C. (C) XRD patterns of 1.0% Bi2O3/15% Si−TiO2 calcined at different temperatures: (a) 400, (b) 500, (c) 600, (d) 700, and (e) 800 °C.

For the 1.0% Bi2O3/15% Si−TiO2 sample, the intensity of the (101) peak increased with the calcination temperature, depicting the increase in the crystallinity (Figure 1C). The peaks of rutile presented when 1.0% Bi2O3/15% Si−TiO2 was calcined at 800 °C and therefore the as-prepared sample possessed high thermal stability. The crystal size of 1.0% Bi2O3/ 15% Si−TiO2 increased from 4.8 to 33.8 nm when the 12577

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Table 1. Physical Properties of the Prepared Photocatalysts

a

sample name

particle sizea (nm)

SBETb (m2·g−1)

pore sizeb (nm)

TiO2(500) 15% Si−TiO2(500) 0.25% Bi2O3/15% Si−TiO2(500) 0.5% Bi2O3/15% Si−TiO2(500) 1.0% Bi2O3/15% Si−TiO2(500) 2.0% Bi2O3/15% Si−TiO2(500) 1.0% Bi2O3/TiO2(500) 1.0% Bi2O3/5% Si−TiO2(500) 1.0% Bi2O3/10% Si−TiO2(500) 1.0% Bi2O3/30% Si−TiO2(500) 1.0% Bi2O3/15% Si−TiO2(400) 1.0% Bi2O3/15% Si−TiO2(600) 1.0% Bi2O3/15% Si−TiO2(700) 1.0% Bi2O3/15% Si−TiO2(800)

23.2 14.8 12.3 10.9 9.2 8.3 17.6 13.3 11.0 5.5 4.8 8.9 18.2 33.8

35.6 89.8 106.5 138.6 153.2 166.4 64.5 105.2 142.1 293.1 291.7 167.3 74.2 23.6

11.4 9.8 9.4 9.1 8.7 8.5 10.8 9.9 9.0 7.5 7.9 8.4 11.0 12.1

Calculated from XRD. bObtained from N2 adsorption−desorption measurements.

Figure 2. TEM images of (a) 1.0% Bi2O3/TiO2 and (b) 1.0% Bi2O3/15% Si−TiO2 and HRTEM patterns of (c) 1.0% Bi2O3/TiO2 and (d) 1.0% Bi2O3/15% Si−TiO2.

calcination temperature changed from 400 to 800 °C, while it hardly changed from 400 to 600 °C. This indicated that crystallite size of Bi2O3/Si−TiO2 sample hardly grew when the calcination temperature was below 600 °C and it began growing when the calcination temperature was above 600 °C. 3.2. Morphological, BET, and Raman Analysis. Figure 2a,b shows TEM images of 1.0% Bi2O3/TiO2 and 1.0% Bi2O3/ 15% Si−TiO2 samples calcined at 500 °C. TEM graphs exhibited in Figure 2a,b show that the prepared samples consisted of spherical TiO2 nanoparticles partly agglomerated to another. The particle size decreased from 1.0% Bi2O3/TiO2 to 1.0% Bi2O3/15% Si−TiO2. The results are in good

agreement with those obtained from XRD analysis (Table 1). The HRTEM images in Figure 2c,d confirm that 1.0% Bi2O3/ TiO2 and 1.0% Bi2O3/15% Si−TiO2 samples calcined at 500 °C were indeed comprised of anatase phase with the lattice fringe spacing of 0.352 nm corresponding to the (101) crystallographic plane of anatase TiO2, which is also consistent with the XRD analysis. No obvious Bi2O3 particles were observed in TEM images of Bi2O3/TiO2 or Bi2O3/Si−TiO2 (Figure 2), implying the extremely small size and high dispersion for Bi2O3, which coincides with the XRD results. To further investigate the distribution of Bi species in the as-prepared samples, an EDX 12578

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Figure 3. Scanning transmission electron microscopy (STEM) and EDX spectroscopy images of 1.0% Bi2O3/15% Si−TiO2. (a) STEM image; EDX energy maps of (b) Ti, (c) Bi, and (d) Si corresponding to the enlarged image of (a).

area. Coupled Bi2O3 also resulted in the decrease of particle sizes and the increase of surface area to some extent, which might be attributed to hindering the aggregation of TiO2 by Bi2O3. Because of the different effecting mechanisms, the doped Si species had more influence on the surface area and particle size of the as-synthesized nanoparticles than the coupled Bi species. Raman spectroscopy is competent enough to interpret the structural complexity of catalyst because the peaks are clearly separated from each other in frequency for a specific material. Raman spectra of bare TiO2 and Bi2O3/Si−TiO2 calcined at 500 °C are presented in Figure S3 (Supporting Information). The major peaks at 144, 399, 519, and 639 cm−1 can be attributed to the characteristic bands of anatase phase TiO2.32 The results verified the existence of anatase phase TiO2, as demonstrated by XRD and HRTEM analysis. It can be seen from Figure S3A,B in the Supporting Information that the intensity of the characteristic peaks decreased gradually with the increase of Bi and Si contents. Moreover, it could be evidently seen that the intensities of the highest peak at 144 cm−1 were weaker and peak widths were broadened compared to bare TiO2 after Bi2O3 coupling and Si doping. This demonstrated that the crystallinity became poor, resulting in the reduction of particle size. The decrease in particle size caused the increase in specific surface areas of the samples and enhanced photocatalytic activity. 3.3. FT-IR and XPS Analyses. FT-IR spectra of pure TiO2 and Bi2O3/Si−TiO2 samples with different Si contents are shown in Figure 4. FT-IR spectra of all the samples exhibited similar peaks at 3435 and 1632 cm−1, which are assigned to the stretching and bending vibrations of the surface-bound

mapping of the elements was carried out for 1.0% Bi2O3/15% Si−TiO2. Parts b, c, and d of Figure 3 are EDX energy maps of Ti, Bi, and Si elements, respectively, corresponding to the enlarged image of Figure 3a. Additionally, the SEM image, EDX mapping, and element mapping of 1.0% Bi2O3/15% Si−TiO2 are also presented in Figure S2 (Supporting Information). The EDX and element mapping results (shown in Figure 3 and the Supporting Information, Figure S2) demonstrated that the introduced Bi and Si species existed in the samples and were homogeneously distributed in the TiO2 particles. Since heterogeneous photocatalysis is generally influenced by the surface area and pore structure of catalysts, the effects of introduced Bi and Si species on the pore structures and BET surface areas of as-synthesized samples were investigated on the basis of nitrogen adsorption and desorption measurements. Values of the BET surface area (SBET) and pore size of various samples are also presented in Table 1. It can be seen from Table 1 that the SBET of Bi2O3/Si−TiO2 increased with the increase of Bi contents and Si contents, whereas the difference lay in the increasing rate of SBET. From Table 1, it could be seen that SBET increased and the pore size decreased obviously, even when a small amount of Si was introduced into TiO2. For the sample 1.0% Bi2O3/5% Si−TiO2, there were the significant increase of surface area from 64.5 to 105.2 m2·g−1 and the decrease of pore size from 10.8 to 9.9 nm. With further increase of the Si contents, the surface area increased and pore size decreased markedly. When the Si content was 30 mol %, the highest surface area reached 293.1 m2·g−1 and the smallest pore size was 7.5 nm. The doped Si entered the lattice of TiO2, which hampered the crystal growth of TiO2 and then led to the decrement of particle size and increment of specific surface 12579

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peak at ca. 1078 cm−1 could be attributed to the asymmetric stretching vibration of Si−O−Si and the vibration intensity increased with the increase in Si contents. The vibration at about 954 cm−1 changed from a weak shoulder peak to a distinguishable single peak with increasing Si contents. This trend indicated that vibration at 954 cm−1 maybe resulted from the Ti−O−Si linkages.25,34 The peaks for the samples below 800 cm−1 were mainly ascribed to the stretching vibrations of Ti−O bonds. Furthermore, it was observed that, with the increase of Si contents in the Bi2O3/Si−TiO2 samples, the amounts of surface adsorbed water and hydroxyl groups also increased (Figure 4). These findings are important for the photocatalytic performance of catalyst since the surface adsorbed water and hydroxyl groups can react with photogenerated holes to produce hydroxyl radicals, which contributed to alleviating charge carrier recombination and improving the photocatalytic performance of doped TiO2. XPS analysis was performed to investigate the surface chemical states of as-synthesized samples. Figure 5 displays the XPS analysis results for bare TiO2, 1.0% Bi2O3/TiO2, and 1.0% Bi2O3/15% Si−TiO2 calcined at 500 °C. High-resolution XPS spectra of Ti 2p (Figure 5A) show the symmetrical peaks around 458.5 and 464.3 eV, which are attributed to Ti 2p3/2 and Ti 2p1/2, respectively. These results revealed that all the samples

Figure 4. FT-IR spectra of 1.0% Bi2O3/Si−TiO2 as a function of Si content calcined at 500 °C: (a) bare TiO2, (b) 1.0% Bi2O3/5% Si− TiO2, (c) 1.0% Bi2O3/10% Si−TiO2, (d) 1.0% Bi2O3/15% Si−TiO2, and (e) 1.0% Bi2O3/30% Si−TiO2, respectively.

hydroxyl groups and surface adsorbed water molecules.33 Besides the above-mentioned main bands, for Bi2O3/Si−TiO2 two new bands at 1078 and 954 cm−1 appeared. The broad

Figure 5. (A) Ti 2p XPS spectra of (a) bare TiO2, (b) 1.0% Bi2O3/TiO2, and (c) 1.0% Bi2O3/15% Si−TiO2 calcined at 500 °C. (B) O 1s XPS spectra of (a) bare TiO2, (b) 1.0% Bi2O3/TiO2, and (c) 1.0% Bi2O3/15% Si−TiO2. (C) Bi 4f XPS spectra of (b) 1.0% Bi2O3/TiO2 and (c) 1.0% Bi2O3/15% Si−TiO2. (D) Si 2p XPS spectra of (c) 1.0% Bi2O3/15% Si−TiO2. 12580

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were mainly comprised of Ti4+. Furthermore, the 2p3/2 peaks of bare TiO2 and 1.0% Bi2O3/TiO2 were both at 458.5 eV, but the peak of Ti 2p3/2 shifted upward to 458.7 eV after Si doping, indicating Bi did not enter the crystal lattice of TiO2 while silicon did. The substitution of Ti by Si atoms in the near surface region of TiO2 resulted in the octahedral coordination of Si in the titania matrix and formed the Ti−O−Si bonds. Due to the larger electronegativity of Si (1.8) than that of Ti (1.5), the positive charge on Ti species would increase because of the decrease in the electron density around Ti species, which resulted from the greater electronegativity of Si via O acting on Ti.35,36 The O 1s spectra in Figure 5B show the band asymmetrical peaks which were deconvoluted into two different peaks centered at 529.6 and 531.5 eV for bare TiO2 and 1.0% Bi2O3/ TiO2 sample. This was an indication that there were two kinds of oxygen species, which were assigned to bulk oxygen (Ti−O) and surface hydroxyl species (Ti−OH), respectively. For 1.0% Bi2O3/15% Si−TiO2 sample, two new peaks appeared after the curve fitting. The peaks at 530.7 and 532.6 eV may be attributed to the oxygen in Ti−O−Si and Si−O−Si linkages, respectively.23,25 In the XPS spectrum of Si 2p (Figure 5D) for 1.0% Bi2O3/15% Si−TiO2, Si 2p spectra can be divided into two peaks at 101.9 and 103.0 eV, which may be attributed to Ti−O−Si and Si−O−Si bonds, confirming that Si was doped into the lattice of TiO2. Meantime, Si mainly existed in Si−O− Si linkages, which was shown from the greater intensity of Si− O−Si than that of Ti−O−Si. In the high-resolution Bi 4f spectra presented in Figure 5C, two peaks at 158.6 and 163.9 eV corresponded to the signals from the two doublets of Bi 4f5/2 and Bi 4f7/2 in trivalent oxidation state37 for 1.0% Bi2O3/ TiO2 and 1.0% Bi2O3/15% Si−TiO2. In addition, from the XPS data, the measured Bi content of 2.8 mol % was higher than that of the calculated value (2.0 mol % Bi for 1.0% Bi2O3/Si− TiO2) in the preparation process. This implied that the introduced Bi species existed mainly on the surface of photocatalyst particles. 3.4. UV-DRS Spectra. Diffraction reflectance spectroscopy was used to measure the absorbing capacities of the as-prepared samples to visible light, and the experimental results are shown in Figure 6. As indicated in Figure 6A, for bare TiO2, the strong absorption below ca. 380 nm is usually attributed to charge transfer from the valence band (O 2p states) to the conduction band (Ti 3d states).38 The absorption of Bi2O3/Si−TiO2 photocatalysts was enhanced more and more with increasing content of coupled Bi2O3 in the range 400−550 nm. The band gaps could be calculated by using αhν = A(hν − Eg) n,18 where Eg, h, ν, n, and A are the band gap, Planck’s constant, frequency of light, number characterizing transition (for TiO2, n = 1/2), and a constant, respectively. Plotting (αhν)1/2 vs hν based on the spectral response in Figure 6A gave the extrapolated intercept corresponding to the Eg value. As shown in the inset of Figure 6A, the band gap energies of bare TiO2, 0.25% Bi2O3/ 15% Si−TiO2, and 2.0% Bi2O3/15% Si−TiO2 were 3.15, 2.90, and 2.75 eV, respectively. There was the largest enhancement in visible light absorption for the 2.0% Bi2O3/15% Si−TiO2 sample. On the contrary, to coupling Bi2O3, the absorbance of Bi2O3/Si−TiO2 samples slightly shifted blue with the increase of Si content (see Figure 6B). The band gap energies of 1.0% Bi2O3/10% Si−TiO2, 1.0% Bi2O3/15% Si−TiO2, and 1.0% Bi2O3/30% Si−TiO2 were estimated to be 2.78, 2.83, and 2.91 eV, respectively. It was thought that doped Si broadened the band gap of TiO2, resulting in the blue shift of the light

Figure 6. UV−vis DRS spectra of bare TiO2 and Bi2O3/Si−TiO2 calcined at 500 °C. (A) Effect of Bi2O3 coupling: (a) TiO2, (b) 0.25% Bi2O3/15% Si−TiO2, (c) 0.5% Bi2O3/15% Si−TiO2, (d) 1.0% Bi2O3/ 15% Si−TiO2, and (e) 2.0% Bi2O3/15% Si−TiO2. (B) Effect of doped Si content: (a) TiO2, (b) 1.0% Bi2O3/5% Si−TiO2, (c) 1.0% Bi2O3/ 10% Si−TiO2, (d) 1.0% Bi2O3/15% Si−TiO2, and (e) 1.0% Bi2O3/ 30% Si−TiO2. Insets present the corresponding plots of transformed Kubelka−Munk vs energy of light for various samples.

absorption.24 Additionally, the slight blue shift could be assigned to the quantum size effect of semiconductors resulting from Si doping. As a result, the coupling of Bi2O3 would contribute to absorb visible light, and hence enhance the photocatalytic activity under visible light irradiation. 3.5. Photocatalytic Activity of the As-Prepared Samples. The photocatalytic activity of as-synthesized samples was evaluated by the degradation of MO under visible light irradiation. Figure 7A shows the normalized concentration of MO (C/C0) against the reaction time for photocatalyst coupled different contents of Bi2O3. It can be clearly seen that Bi2O3/ Si−TiO2 samples had higher photocatalytic efficiency for MO degradation compared to undoped TiO2, Bi2O3/TiO2, and Si singly doped TiO2 under the stated conditions. The results obtained from Figure 7A demonstrated that the photodegradation activities improved with the increase of Bi2O3 content, reaching the highest photocatalytic activity at Bi2O3 content of 1.0 mol %, and then decreased. To have a better understanding of the reaction kinetics of MO degradation, the experimental data in Figure 7A were fitted by a pseudo-firstorder model. Figure 7B indicates the first-order kinetics data for 12581

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Coupling of Si−TiO2 with Bi2O3 resulted in an abrupt enhancement of photocatalytic activity. Bi2O3/Si−TiO2 possessed excellent visible light activities, owing to the red shift in the absorption wavelength range depicted by UV−vis DRS analysis (see Figure 6). Bi2O3, being an inorganic photosensitizer, has a band gap of 2.8 eV. Under visible light irradiation, photoelectrons (e−) and holes (h+) were generated on the surface of Bi2O3. Furthermore, due to the higher potential of the valence band of Bi2O3 (3.13 eV) than that of TiO2 (2.91 eV), the photogenerated holes were transferred from the valence band of Bi2O3 into that of TiO2 in the Bi2O3/ Si−TiO2 catalysts (Scheme 1). The transfer of photogenerated Scheme 1. Proposed Degradation Mechanism of MO and BPA by Bi2O3/Si−TiO2 under Visible Light Irradiation (λ > 420 nm)

carriers was feasible thermodynamically, which alleviated the recombination of e−/h+ pairs and contributed to the high photocatalytic activity. On the other hand, low photocatalytic activity has been observed for higher Bi2O3 content (>1.0 mol %). This may be due to the fact that coupled Bi2O3 mainly existed on the surface of Bi2O3/Si−TiO2 catalysts and high content of Bi2O3 may block the surface active sites, hence lowering the photocatalytic activity. The interface charge separation efficiency of photogenerated electrons and holes is a crucial factor for the photocatalytic activity, which can be examined by the EIS Nyquist plots. Figure 8A shows the EIS Nyquist plots of TiO2 and Bi2O3/Si− TiO2 under dark and visible light irradiation. The arc radius of the EIS Nyquist plot of Bi2O3/Si−TiO2 was smaller than that of TiO2 in dark and visible light irradiation. Because the arc radius of EIS spectra reflects the interface layer resistance occurring at the surface of electrode, more effective separation of charge carriers and faster interfacial charge transfer occurred on the Bi2O3/Si−TiO2 photocatalyst.39 Additionally, to investigate the hole transfer from Bi2O3 to TiO2, the LSV curves of TiO2 and Bi2O3/Si−TiO2 electrodes were also determined, and the results are presented in Figure 8B. As the applied potential swept from −0.0 to +1.0 V, the photocurrent of Bi2O3/Si− TiO2 electrode was always larger than that of TiO2 electrode. The enhancement of photocurrent also shows the increase of the photoinduced carrier transport rate and the improvement of photogenerated electron−hole pair separation. Furthermore, the enhanced separation of charge carriers in Bi2O3/Si−TiO2 was also supported by photoluminescence (PL) measurements (Supporting Information, Figure S4). These results clearly reveal that the heterojunctions formed between coupled Bi2O3 and Si−TiO2 can effectively separate photogenerated electron− hole pairs and enhance the photocatalytic activity under visible light irradiation.

Figure 7. (A) Degradation rates, (B) first-order kinetics data, and (C) values of the rate constant k of the photodegradation of MO over bare TiO2 and Bi2O3/Si−TiO2 coupled different contents of Bi2O3 calcined at 500 °C: (a) bare TiO2, (b) 15% Si−TiO2, (c) 0.25% Bi2O3/15% Si−TiO2, (d) 0.5% Bi2O3/15% Si−TiO2, (e) 1.0% Bi2O3/15% Si− TiO2, and (f) 2.0% Bi2O3/15% Si−TiO2.

the photodegradation of MO using different catalysts. All fitting curves of the irradiation time against ln(C0/C) were nearly linear. As shown in Figure 7C, the rate constant (k) of Bi2O3/ Si−TiO2 was obviously higher than that of bare TiO2, Bi2O3/ TiO2, or Si−TiO2, respectively. The highest rate constant was also obtained at Bi2O3 content of 1.0 mol %. Therefore, 1.0% Bi2O3 was inferred as the optimal amount of Bi species in Bi2O3/Si−TiO2 catalysts. 12582

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Figure 8. (A) Nyquist plots and (B) linear scan voltammograms of bare TiO2 and Bi2O3/Si−TiO2 nanocomposite photoelectrodes under dark and visible light irradiation.

Figure 9A displays the influence of Si contents on the visible photocatalytic activity of Bi2O3/Si−TiO2 for MO degradation. It was found that the degradation of MO over Bi2O3/Si−TiO2 with different Si contents also obeyed pseudo-first-order kinetics. The corresponding kinetics data and rate constant (k) are presented in Figure 9B,C. It can be clearly seen that the photocatalytic activity of Bi2O3/Si−TiO2 was enhanced rapidly with the increase of Si content. The highest degradation rate of MO was obtained at a Si content of 15%, and then decreased. With the addition of Si, the thermal stability of Bi2O3/Si−TiO2 was enhanced and anatase phase was preserved during the heattreating process (Figure 1C). The surface area of photocatalysts was also evidently enlarged (Table 1), which contributes to adsorbing more reactant molecules. Meanwhile, the Si doping resulted in the decrease of crystallite size (Table 1), which might increase the transfer rate of photogenerated e−/h+ pairs. In addition, according to the results of FT-IR spectroscopy, Si doping could increase the amount of surface hydroxyl groups, which benefited dispersing TiO2 into water and resulted in the enhancement of photocatalytic activity. Therefore, the photocatalytic efficiency of Bi2O3/Si−TiO2 might be better than that of bare TiO2 and single Bi2O3/TiO2. However, if excessive Si was doped into TiO2, the produced Si−O−Ti would act as an insulator and inhibit the transfer of photoinduced e−/h+ pairs. On the other hand, excessive SiO2 prevented TiO2 from

Figure 9. (A) Degradation rates, (B) first-order kinetics data, and (C) values of the rate constant k of the photodegradation of MO over bare TiO2 and Bi2O3/Si−TiO2 with different amounts of Si dopants: (a) bare TiO2, (b) 1.0% Bi2O3/TiO2, (c) 1.0% Bi2O3/5% Si−TiO2, (d) 1.0% Bi2O3/10% Si−TiO2, (e) 1.0% Bi2O3/15% Si−TiO2, and (f) 1.0% Bi2O3/30% Si−TiO2.

contacting with reactant molecules and reduced the visible light photocatalytic performance. In view of the fact that MO is active to visible light, its photodegradation may be caused by a dye-sensitized path which does not require the band gap excitation of a photocatalyst. To further identify the visible light activity of Bi2O3/Si−TiO2 nanocomposites, we also tested the degrada12583

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3.6. Visible Photocatalysis Mechanism of Bi2O3/Si− TiO2. In order to elucidate the degradation mechanism involved in our present system, 1.0% Bi2O3/15% Si−TiO2 was evaluated for degradation of BPA under visible light irradiation in the presence of various radical scavengers. As shown in Figure 11, the degradation of BPA was remarkably

tion of BPA that has no absorption in the visible light region (Supporting Information, Figures S5 and S6). The photodegradation rate of BPA and the corresponding rate constant increased with the increment of coupled Bi2O3 and doped Si, reaching the highest photocatalytic activity at Bi2O3 content of 1.0% and Si content of 15%. The effects of the contents of coupled Bi2O3 and doped Si on the photocatalytic degradation of BPA were analogous to those of MO. Moreover, the above results suggest that the degradation process of MO and BPA should be primarily ascribed to the photocatalytic oxidation, not dye sensitization. Additionally, to investigate the mineralization of organic pollutants in the photocatalytic oxidation, the TOC removal efficiency was operated and the results are shown in Figure 10.

Figure 11. Degradation kinetics of BPA in 1.0% Bi2O3/15% Si−TiO2 suspension under visible irradiation with (a) no scavenger added, (b) NaHCO3, (c) KH2PO4, (d) tert-butyl alcohol, (e) methanol, and (f) 0.5 mM p-benzoquinone. Scavenger concentration: 0.1 M.

depressed by HCO3− and H2PO4−, while it was slightly depressed by methanol and tert-butyl alcohol in 1.0% Bi2O3/ 15% Si−TiO2 suspension. Both HCO3− and H2PO4− had high adsorption on the surface of the catalyst.40 The adsorbed HCO3− reacted with h+ or adsorbed •OH on the catalyst, leading to lower activity. Since •OH scavengers methanol and tert-butyl alcohol hardly adsorbed on the catalyst in aqueous systems, they predominantly scavenged the free •OH radical in solution. Meanwhile, with the addition of O2•− scavenger pbenzoquinone, the degradation of BPA decreased to some extent, suggesting that O2•− was not the main active species for BPA degradation or no O2•− was generated in the present system. These results indicated that the surface adsorbed •OH was the main reactive oxygen species in the reaction. Moreover, Zhao and co-workers reported that many dye pollutants could be degraded over TiO2 on the basis of a self-photosensitization process via the reaction between dye cationic radicals (dye•+) and O2•−.41 However, based on the above-mentioned efficient degradation of BPA (Supporting Information, Figures S5 and S6) and the controlled experiments with various radical scavengers, the photocatalytic process was the predominant process in the system of Bi2O3/Si−TiO2 with visible light, whereas the self-photosensitization process could be ignored. Based on the UV−vis DRS, XPS, EIS, and degradation of BPA over 1.0% Bi2O3/15% Si−TiO2 under different radical scavengers, a degradation mechanism was proposed and is illustrated in Scheme 1. Bi2O3 coupling resulted in the visible light response of as-prepared Bi2O3/Si−TiO2. Because the band gap of Bi2O3 is 2.8 eV, it can be excited by light with wavelength less than 445 nm.42,43 However, the photocatalytic activity of pure Bi2O3 is very low owing to the high electron− hole recombination rate. The valence band edge of Bi2O3 is lower than that of TiO2;44 the Bi2O3/TiO2 heterojunctions formed in the composite will promote the photogenerated holes in Bi2O3 to be transferred to the upper lying valence

Figure 10. Total organic carbon (TOC) removals for 10 mg/L (A) MO and (B) BPA by 15% Si−TiO2, 1.0% Bi2O3/TiO2, and 1.0% Bi2O3/15% Si−TiO2, respectively.

Figure 10A depicts the TOC removal after 2 h of visible irradiation with the optimal photocatalyst of 1.0% Bi2O3/15% Si−TiO2(500), compared to single doped 15% Si−TiO2(500) and 1.0% Bi2O3/TiO2(500). This exhibited that MO was not just decolorized but also mineralized as 56% of TOC was removed from MO aqueous solution by the optimal photocatalyst, which was higher than that by 15% Si−TiO2 (9%) and 1.0% Bi2O3/TiO2 (21%). Seen from Figure 10B, TOC removal from 10 mg/L BPA solution after 3 h of visible light illumination had results similar to those of MO degradation. These results clearly indicated that 1.0% Bi2O3/15% Si− TiO2(500) was a promising catalyst, which can effectively enhance the photomineralization efficiency of pollutants under visible light irradiation. 12584

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TiO2 photocatalysts, and H2O2 formation determined by DPD spectrometric method. This material is available free of charge via the Internet at http://pubs.acs.org.

bands of TiO2 as shown in Scheme 1. The process is thermodynamically feasible. The recombination rate of photoinduced electron−hole pairs was reduced, and many more holes were captured to induce photocatalytic reactions. With regard to the photogenerated electrons in Bi2O3, it was not allowed in thermodynamics for the one-electron reduction of O2 on Bi2O3, owing to the higher potential of the conduction band (+0.33 eV vs NHE) than the redox potential of O2/O2•− (−0.16 eV vs NHE).45 In the present photocatalytic system, H2O2 was detected by the DPD spectrometric method. As shown in Figure S7 (see Supporting Information), a considerable amount of H2O2 was generated over Bi2O3/Si− TiO2 and Bi2O3/TiO2 samples, while almost no H2O2 was detected over bare TiO2. These results suggest that H2O2 can be formed via two-electron reduction of O2 on the surface of coupled Bi2O3 under visible light irradiation. On the other hand, the Si doping enlarged the surface area, which availed the adsorption of reactant molecules. The small crystallite size might increase the generation rate of electron−hole pairs. Furthermore, the doped Si could generate a positive charge difference and the impurity cation (Si) acted as a Lewis site,27 which can generate more hydroxyl groups to balance the positive charge. Therefore, more holes could be quickly scavenged by the hydroxyl groups to produce •OH. The adsorbed •OH eventually degraded BPA and MO into nontoxic compounds such as carbon dioxide, water, and inorganic substrate. Both the hole transferring and scavenging prevented the electrons and holes from recombination, which remarkably promoted the efficiency of the photocatalytic degradation.

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Corresponding Author

*E-mail: [email protected]. Tel.: (86)391-3987818. Fax: (86)391-3987811. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the National Nature Science Foundation of China (21307027, 51074067) and the Foundation of Henan Educational Committee (2010B150009).

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REFERENCES

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4. CONCLUSION Bi2O3/Si−TiO2 photocatalysts were prepared through a simple two-step method and evaluated in terms of performance by the degradation of MO and BPA aqueous solution under visible light irradiation. The as-prepared Bi2O3/Si−TiO2 samples showed significant increase in photoactivity toward the degradation of MO and BPA. To find out the factors behind the enhanced activity of Bi2O3/Si−TiO2, we examined the samples by various techniques. XRD results presented the anatase phase and high thermal stability of the prepared samples. The surface area of TiO2 increased significantly with increasing Si contents, and the doped Si species had more influence on the surface areas and particle sizes of the assynthesized samples than the coupled Bi species. The FT-IR and XPS of the samples revealed the lattice doping of Si species and the presence of more surface hydroxyls owing to Si doping. Bi2O3/Si−TiO2 showed the visible light response and reduced electron−hole recombination due to the photosensitizing effect of Bi2O3. The reduction of O2 by photogenerated electrons on the coupled Bi2O3 was a two-electron transfer process. The enhanced photocatalytic activity for degradation of MO and BPA was attributed to the cooperative effects of both Bi2O3 coupling and Si doping. The photocatalytic decomposition of BPA tended to be achieved via the action of adsorbed hydroxyl radicals.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed information concerning the XRD spectra of Bi2O3, SEM image, EDX and element mapping of 1.0% Bi2O3/15% Si−TiO2, Raman spectra of bare TiO2 and Bi2O3/Si−TiO2, photoluminescence spectra of bare TiO2 and Bi2O3/Si−TiO2, kinetic analysis of BPA degradation with different Bi2O3/Si− 12585

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