ARTICLE pubs.acs.org/JPCC
ZnO/BiOI Heterostructures: Photoinduced Charge-Transfer Property and Enhanced Visible-Light Photocatalytic Activity Jing Jiang, Xi Zhang, Peibei Sun, 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 ABSTRACT: In this study, ZnO/BiOI heterostructures were synthesized by a facile chemical bath method at low temperature. Control of the morphology and constituents of the ZnO/ BiOI heterostructures was realized by simply tuning the Bi/Zn molar ratios. The resulting ZnO/BiOI heterostructures exhibited high photocatalytic activity in the degradation of methyl orange under visible-light irradiation. The high photocatalytic activity of the ZnO/BiOI heterostructures was first attributed to their high surface area. Surface photovoltage spectroscopy and transient photovoltage measurements revealed that the photoinduced charge-transfer property of p-type BiOI could be improved greatly by coupling with n-type ZnO. The heterojunction at the interface between the BiOI and ZnO could efficiently reduce the recombination of photoinduced electronhole pairs to increase the lifetime of charge carriers by 15 times and thus enhance the photocatalytic activity of the ZnO/BiOI heterostructures, in addition to the high surface area. This study reveals that the heterostructure construction between two different semiconductors plays a very important role in determining the dynamic properties of their photogenerated charge carriers and their photocatalytic properties.
’ INTRODUCTION Semiconductor photocatalysis has attracted great interest because it provides a promising pathway for solving energy supply and environmental pollution problems. To date, various kinds of semiconductor materials, including metal oxides,1 sulfides,2 nitrides,3 and their mixed solid solutions,4,5 have been exploited as photocatalysts responsive to both the UV and visible light ranges. However, their applications are usually restricted by photocorrosion, short lifetimes of photogenerated electronhole pairs, and limited visible-light responses.6 It is, therefore, of considerable significance to develop an efficient strategy toward producing catalysts with high photoactivity and high stability for practical applications. Although narrow-band-gap semiconductors are able to capture visible light in the solar spectrum, their photogenerated electronhole pairs suffer from fast recombination. Thus, tailoring semiconductor properties by fabricating designed structures is indispensable to improving the overall charge-transfer efficiency. For instance, reducing doping defects can suppress the recombination of photogenerated charge carriers, resulting in an improved photocatalytic activity.7 In addition, combining a semiconductor with a metal or coupling two different semiconductors to form a heterostructure is another way to promote the separation of photogenerated charge carriers and thus increase their lifetime.8,9 Heterostructure construction between two different semiconductors has been extensively exploited in many fields such as photocatalysis and solar energy conversion to enhance the performance of photovoltaic devices,10,11 because heterojunctions dominate some behaviors of photogenerated charges, such as the direction of transportation, the distance for separation, and the recombination rate.12,13 Furthermore, the internal electric field built at a r 2011 American Chemical Society
heterojunction interface can greatly decrease the photogenerated charge-carrier recombination and increase the charge-carrier lifetimes, thus enhancing the photocatalytic activity. Therefore, heterostructure construction is not only a feasible approach for developing highly active photocatalysts response to visible light, but also a rational route to studying the relationship between photogenerated charge-carrier transfer and photocatalytic properties. Bismuth oxyhalides (BiOX, X = Cl, Br, and I) are well-known layered compounds that have a crystal structure of [Bi2O2]2+ layers interleaved by slabs comprising halide atoms. Owing to strong intralayer bonding and weak interlayer van der Waals interactions, these unique layered structures usually exhibit fascinating properties (e.g., anisotropic structural, electrical, optical, and mechanical properties) and have promising applications in cosmetics, pigments, catalysis, and photoelectrochemical devices.14 In particular, bismuth oxyhalides are interesting as they offer the possibility to manipulate the electronic structure by choosing different halide atoms in the crystal structure, which is highly desirable from the viewpoint of the photocatalytic applications.15 Among these compounds, BiOI, with a band gap of 1.8 eV, has the strongest absorption in the visible light range and exhibits the best visible-light photocatalytic activity. As a typical p-type semiconductor, BiOI can serve as an efficient visible-light photosensitizer for n-type TiO2 with a large band gap to greatly enhance its photocatalytic efficiency.16
Received: June 23, 2011 Revised: September 7, 2011 Published: September 19, 2011 20555
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The Journal of Physical Chemistry C ZnO is also an n-type semiconductor with a band-gap energy (about 3.5 eV) similar to that of TiO2. Some studies have reported that ZnO exhibited better efficiency than TiO2 in the photocatalytic degradation of organic pollutants and photoelectric conversion.1719 However, to the best of our knowledge, there is no report on the design and fabrication of coupled ZnO/ BiOI heterostructures and their photocatalytic performances. Herein, we report a low-temperature method to synthesize ZnO/ BiOI heterostructures in one step. By simply tuning the Bi/Zn molar ratios, the morphology and constituents of the ZnO/BiOI heterostructures can be controlled. The photoinduced chargetransfer properties of the ZnO/BiOI heterostructures were systematically investigated by surface photovoltage spectroscopy (SPS) and transient photovoltage (TPV) techniques. The relationship between the photogenerated charge-carrier-transfer behavior and the photocatalytic activity in ZnO/BiOI heterostructure systems is also discussed in detail.
’ EXPERIMENTAL SECTION Chemicals. Bi(NO3)3 3 5H2O, Zn(NO3)2 3 6H2O, KI, and NaOH were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and used without further purification. All chemicals used in this study were of commercially available analytical grade. Preparation of the ZnO/BiOI Heterostructures. ZnO/BiOI heterostructures were synthesized by a facile chemical bath method at low temperature. Typically, Zn(NO3)2 3 6H2O was added to 40 mL of deionized water containing a stoichiometric amount of NaOH (Zn/OH molar ratio of 1:2) with constant stirring, and white precipitates formed. Then, a stoichiometric amount of KI was added, and the mixture was stirred for 30 min at room temperature. Subsequently, different stoichiometric amounts of Bi(NO3)3 3 5H2O were added to the resultant mixed solution. The Bi/Zn molar ratios were kept to 0, 1:1, 1:2, 1:3, and 1:4 (denoted as ZnO, ZB-1, ZB-2, ZB-3, and ZB-4), respectively. After being stirred for another 30 min at room temperature, the resulting mixtures were heated at 80 °C for 2 h in a water bath. Finally, the precipitates were collected, washed thoroughly with deionized water and ethanol, and dried at 50 °C in air. For comparison, pure BiOI was prepared by the same procedure without the addition of Zn(NO3)2 3 6H2O and NaOH. Characterization. Powder X-ray diffraction (XRD) measurements were recorded on a Rigaku D/MAX-RB diffractometer with monochromatized Cu Kα radiation (λ = 0.15418 nm). The morphology was observed with scanning electron microscopy (SEM, JEOL 6700-F) and transmission electron microscopy (TEM, JEOL JSM-2010). The samples for TEM were prepared by dispersing the final powders in ethanol with ultrasonic irradiation and then dropping the dispersion on carboncopper grids. Furthermore, the obtained powders deposited on a copper grid were observed by high-resolution TEM (HRTEM). A nitrogen adsorption system (Micrometritics ASAP2010) was employed to record the adsorptiondesorption isotherms at the liquid-nitrogen temperature of 77 K. A Varian Cary 100 Scan UVvisible system equipped with a Labsphere diffuse reflectance accessory was used to obtain the reflectance spectra of the catalysts over the range of 200800 nm. Laboratory sphere USRS-99-010 was employed as the reflectance standard. Surface electronic states were analyzed by X-ray photoelectron spectroscopy (XPS, Perkin-Elmer PHI 5000C, Al KR). All binding energies were
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Figure 1. XRD patterns of (a) pure BiOI; ZnO/BiOI heterostructures (b) ZB-1, (c) ZB-2, (d) ZB-3, and (e) ZB-4; and (f) pure ZnO.
calibrated using the contaminant carbon (C 1s = 284.6 eV) as the reference. Photocatalytic Activity Tests. The visible-light-driven photocatalytic activities of ZnO/BiOI heterostructures were evaluated in the degradation of methyl orange (MO) at ambient temperature using a 500-W halogentungsten lamp with a 420-nm cutoff filter as the light source. Typically, 0.1 g of photocatalyst was added to 100 mL of 10 mg L1 MO aqueous solution in a container. The solution was continuously stirred in the dark for 1 h to ensure the establishment of an adsorptiondesorption equilibrium between the photocatalyst and the MO before irradiation. During the degradation, the MO solution with photocatalyst was continuously stirred with a dynamoelectric stirrer, and the concentration of MO was monitored by colorimetry with a Hitachi U-3310 UVvis spectrometer. The photonic efficiency (ξ) was calculated according to the equations20,21 I0 ¼
Iλ NA hc
ξ ð%Þ ¼
ð1Þ
kc0 V 100 I0 A
ð2Þ
where I0 is the photon flux (einstein s1 cm2), I is the light intensity (J s1 cm2), NA is Avogadro’s number (mol1), h is Planck’s constant (J s), c is the speed of light (m s1), k is the rate constant (s1), A is the illuminated area (cm2), c0 is the initial MO concentration (mol L1), λ is the average illumination wavelength of the visible light (m), and V is the volume of MO aqueous solution (L). Photoelectrochemical Measurements. For the fabrication of the photoanode, as-prepared samples were obtained by mixing 1 mL of ethanol and 20 mg of as-prepared powder homogeneously. The as-prepared samples were spread on an indium tin oxide (ITO) conducting glass and allowed to dry under ambient conditions. The photocurrents were measured with an electrochemical analyzer (CHI660D, CHI Shanghai, Inc.) in a standard three-electrode system with the as-prepared sample as the working electrode, a Pt foil as the counter electrode, and Ag/AgCl (saturated KCl) as the reference electrode. A 500-W Xe arc lamp with a 420-nm cutoff filter was utilized as the light source. A 0.5 M Na2SO4 aqueous solution was used as the electrolyte. Surface Photovoltage Spectroscopy (SPS) and Transient Photovoltage (TPV) Measurements. SPS was recorded on a 20556
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Figure 2. XPS spectra of the ZnO/BiOI heterostructure ZB-1: (a) survey, (b) Zn 2p, (c) Bi 4f, (d) O 1s, and (e) I 3d.
self-made instrument.22 Monochromatic light was obtained by passing light from a 500-W xenon lamp (CHF XQ500W, Global xenon lamp) through a double-prism monochromator (Hilger and Watts, D 300). The slit width of the entrance and exit was 1 mm. A lock-in amplifier (SR830-DSP, Stanford Research Systems), synchronized with a light chopper (SR540) was employed to amplify the photovoltage signal. The range of modulating frequency was from 20 to 70 Hz. The spectral resolution was 1 nm. The raw SPS data were normalized using an illuminometer (Zolix UOM-1S). The samples were studied without further treatment during the SPS measurements. Transient PV measurements were also carried out on a self-made instrument.23 The sample was excited with a laser radiation pulse (wavelength of 532 nm and pulse width of 5 ns) 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).
’ RESULTS AND DISCUSSION Figure 1 shows X-ray diffraction (XRD) patterns of pure BiOI, pure ZnO, and ZnO/BiOI heterostructures. All of the diffraction peaks shown in spectra a and f of Figure 1 can be indexed to the tetragonal phase of BiOI (JCPDS card 73-2062) and the hexagonal phase of ZnO (JCPDS card 36-1451), respectively. The diffraction peaks of both BiOI and ZnO were sharp and intense, indicating their highly crystalline nature. No impurity peaks were observed, confirming the high purity of the two products. Two sets of XRD peaks of tetragonal BiOI and hexagonal ZnO can be clearly observed in Figure 1be. 20557
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Figure 3. SEM images of (a) pure BiOI; ZnO/BiOI heterostructures (b) ZB-1, (c) ZB-2, (d) ZB-3, and (e) ZB-4; and (f) pure ZnO.
Meanwhile, the diffraction peaks corresponding to ZnO and the diffraction peak intensity ratio (IZnO,(100)/IBiOI,(012)) between the two components decrease gradually with increasing Bi/Zn molar ratio. Notably, the diffraction peaks assigned to BiOI in the patterns of ZnO/BiOI heterostructures became broader and weaker, suggesting that the presence of ZnO could inhibit the crystal growth of BiOI. The chemical composition and surface chemical states of the ZnO/BiOI heterostructures were further investigated by X-ray photoelectron spectroscopy (XPS). The peak positions in all of the XPS spectra were calibrated with C 1s at 284.60 eV. The typical survey XPS spectrum of the ZnO/BiOI heterostructure (ZB-1) indicates that the product consists of Zn, Bi, O and I elements (Figure 2a). Two symmetric peaks at 1022.1 and 1045.2 eV in the high-resolution XPS spectrum of Zn 2p are assigned to Zn 2p3/2 and Zn 2p1/2, indicating the existence of Zn2+ in the ZnO/BiOI heterostructure (Figure 2b).24 Two strong peaks at 159.5 and 164.8 eV shown in Figure 2c are attributed to Bi 4f7/2 and Bi 4f5/2, respectively, which are characteristic of Bi3+ in BiOI. The satellite peaks with a distance of 1.8 eV between Bi 4f7/2 and Bi 4f5/2 main peaks are consistent with the reported values.16,25 The O 1s peaks for the ZnO/BiOI heterostructure (Figure 2d) can be deconvoluted into three peaks at 529.5, 530.6, and 532.2 eV, which correspond to the BiO bonds in [Bi2O2] slabs of the BiOI layered structure,26 the ZnO bonds of ZnO,27 and the OH bonds of the surface-adsorbed water,28 respectively. As for the high-resolution I 3d XPS spectrum (Figure 2e), two peaks at 618.9 and 630.5 eV are associated with I 3d3/2 and I 3d5/2, respectively, in good agreement with those in BiOI.16 All of these results further confirm the coexistence of ZnO and BiOI in the ZnO/BiOI heterostructure. Figure 3 shows SEM images of pure BiOI, pure ZnO, and the ZnO/BiOI heterostructures. Figure 3a reveals that the BiOI sample consisted of large numbers of irregular plates with smooth surfaces. These plates were 0.21.0 μm in width and 100200 nm in thickness. The SEM images in Figure 3be suggest that the morphology of ZnO/BiOI heterostructures depends highly on the content of BiOI in the heterostructures.
The ZB-1 sample (Figure 3b) exhibited a platelike structure similar to that of pure BiOI, but its plates became much thinner. For ZB-2 with a Bi/Zn molar ratio of 1:2, the width and thickness of the plates were markedly shrunk to 250 and 20 nm, respectively. As the content of BiOI decreased, the morphology of the ZnO/BiOI heterostructures changed further. As shown in images d and e of Figure 3, ZB-3 and ZB-4 were composed of some irregularly shaped aggregates rather than plates, indicating that ZnO aggregates were well wrapped on the surface of the BiOI. The pure ZnO sample had a flower-shaped superstructure constructed of plenty of aggregated small particles (Figure 3f). These SEM observations reveal that the coexistence of BiOI and ZnO significantly affects the morphologies and crystal growth habits of both components. Detailed structural information about the ZnO/BiOI heterostructures was further investigated by TEM and HRTEM. Figure 4a shows typical TEM images of the representative ZnO/BiOI heterostructure (ZB-1) with a plate-shape morphology, consistent with the SEM observations. A magnified TEM image (Figure 4b) reveals that some nanoparticles with sizes of about 7 nm are anchored on the plates. The interaction between the nanoparticles and the plates was so strong that ultrasonication during the sample preparation procedure for TEM analysis could not peel off these nanoparticles. The HRTEM image reveals the highly crystalline nature of ZB-1 (Figure 4c). The HRTEM images in Figure 4d,e were taken from the yellow and blue square regions, respectively, indicated in Figure 4c. In Figure 4d, the spacing of the adjacent lattice planes is ca. 0.28 nm, consistent with the interplanar spacings of the (100) plane of hexagonal ZnO. The clear lattice fringes with an interplanar lattice spacing of 0.27 nm in Figure 4e correspond to the (111) plane of tetragonal BiOI. The fast Fourier transform (FFT) patterns of the yellow and blue square regions in Figure 4c further confirm the coexistence of hexagonal ZnO and tetragonal BiOI (Figure 4f,g), respectively. UVvis diffuse reflectance spectra of BiOI, ZnO, and ZnO/ BiOI heterostructures are shown in Figure 5. Pure ZnO has almost no absorbance in the visible light range, with an 20558
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Figure 4. (a,b) TEM images, (ce) HRTEM images, and (f,g) FFT patterns of the ZnO/BiOI heterostructure ZB-1.
Figure 5. (a) UVvis diffuse reflectance spectra and (b) corresponding colors of pure BiOI, pure ZnO, and ZnO/BiOI heterostructures. Plots of (αhν)1/2 versus energy (hν) for the band-gap energies of (c) BiOI and (d) ZnO.
absorbance edge of about 400 nm, whereas pure BiOI has strong absorbance in the visible light range, with an absorbance edge around 660 nm (Figure 5a). The color of ZnO is white, whereas BiOI is deep red. In contrast, all of the ZnO/BiOI
heterostructures have prominent absorbance in the visible light range, and their edges range from 620 to 660 nm, which is in accordance with the color changing from orange to red (Figure 5b). As a crystalline semiconductor, optical absorption near the band 20559
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Table 1. Textural and Photocatalytic Properties of ZnO/ BiOI Heterostructures k0 a
MO ABET 2 1
VBJH
2
degradation k (10 , (103, g
3 1
samples (m g ) (cm g ) in 4 h (%)
a
Figure 6. Nitrogen adsorptiondesorption isotherms of pure BiOI, pure ZnO, and ZnO/BiOI heterostructures.
edge follows the formula αhν = A(hν Eg)n/2, where α, ν, Eg, and A are the absorption coefficient, light frequency, band-gap energy, and a constant, respectively.29 In addition, n is a constant that depends on the characteristics of the transition in the semiconductor, namely, direct transition (n = 1) or indirect transition (n = 4). For BiOI and ZnO semiconductor, the value of n is 4 for the indirect transition.30 Therefore, the band-gap energy (Eg value) of the products can be estimated from a plot (αhν)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 products (Figure 5c,d). The band-gap energy of BiOI is about 1.81 eV, whereas that of ZnO is 3.04 eV, which is smaller than the value reported in the literature because of quantum confinement effects in nanosized ZnO.31,32 Figure 6 shows the nitrogen adsorptiondesorption isotherms of ZnO, BiOI, and ZnO/BiOI heterostructures. All of the samples display type-IV isotherms with a distinct hysteresis loop in the range of 0.51.0P/P0, indicating the presence of mesoporous structure.33 The formation of mesoporous structures can be ascribed to the aggregation of the primary crystallites of the samples. From the isotherms, the BET surface areas (ABET) of the samples were calculated and are summarized in Table 1. ZnO and BiOI had low BET surface areas of 8.5 and 4.6 m2 g1, respectively. Interestingly, the ZnO/BiOI heterostructures exhibited much higher BET surface areas than pure ZnO and BiOI. The significant increase in BET surface area should be ascribed to the aforementioned crystal growth inhibition effect and/or the loose agglomeration between the two components. The trend in the BET surface areas of the ZnO/BiOI heterostructures was found to be different from that of BiOI/TiO2 heterostructures.16 In that case, the surface areas of the BiOI/ TiO2 heterostructures were significantly lower than that of pure TiO2, and an increase in the BiOI content obviously resulted in a decrease of the surface areas of the BiOI/TiO2 heterostructures. MO is one type of organic dye that is often used as a model pollutant to study the catalytic performance of photocatalysts. In this study, the photocatalytic activity of ZnO/BiOI heterostructures was evaluated in the degradation of MO under visible-light irradiation (λ > 420 nm). Figure 7a shows the time profiles of MO photodegradation over BiOI, ZnO, and ZnO/BiOI heterostructures. Pure ZnO had no visible-light photocatalytic activity, whereas pure BiOI exhibited a weak photocatalytic activity that could degrade MO by only 7% after 4 h of visible-light irradiation.
ZnO
8.5
0.05
2
ZB-1
40.2
0.28
78
ZB-2
41.0
0.36
30
ZB-3
35.8
0.35
ZB-4
32.9
0.29
BiOI
4.6
0.03
h1)
h1 m2) ξ (%)
0.33 36.9
0.39
0.0002
9.17
0.0258
8.89
2.17
0.0062
23
6.72
1.88
0.0047
21
6.36
1.93
0.0044
7
1.81
3.89
0.0013
k' values are k values normalized by the surface areas.
Obviously, the photocatalytic efficiency of the pure BiOI nanoplates synthesized in this study was much lower than that of the solvothermally synthesized hierarchical BiOI nanoplate microspheres reported in our previous study.15 This phenomenon reflects the fact that the photocatalytic activity of BiOI is highly dependent on the shape, size, and aggregation of the nanoplates. It is interesting to note that the formation of the ZnO/BiOI heterostructures greatly enhanced the photocatalytic activity. After 4 h of visible-light irradiation, the photodegradation efficiencies of MO were about 78%, 30%, 23%, and 21% for ZB-1, ZB-2, ZB-3, and ZB-4, respectively. The ZB-1 sample exhibited the highest photocatalytic activity. Figure 7b reveals that the photocatalytic degradation of MO on different catalysts fits pseudo-first-order kinetics, ln(C/C0) = kt, where C is the concentration of the MO at time t, C0 is the initial concentration of the MO solution, and the slope k is the apparent reaction rate constant. Table 1 summarizes the determined k values and photonic efficiency for different catalysts. The degradation constant of ZB-1 (0.369 h1) was found to be about 112 times that of ZnO (0.0033 h1) and 20 times that of BiOI (0.0181 h1). The photonic efficiencies of the ZnO/BiOI heterostructures increased with increasing Bi/Zn ratio. The highest photonic efficiency was observed for the Bi/Zn ratio of 1:1 (ZB-1, 0.0258%), which is 129 times that of ZnO (0.0002%) and 19.8 times that of BiOI (0.0013%). We also evaluated the stability and reusability of the ZnO/BiOI heterostructure (ZB-1). As shown in Figure 8, no marked change in the photocatalytic activity was observed after three cycles of photodegradation, suggesting the good stability of the ZnO/BiOI heterostructures. The enhanced photocatalytic activity of the ZnO/BiOI heterostructure can be associated with the morphology evaluation and the heterojunction effect at the BiOI and ZnO interface. The aforementioned structure analysis demonstrated that the presence of ZnO could inhibit the crystal growth of BiOI, leading to thinner BiOI nanoplates in the heterostructures. These thinner nanoplates could provide a shorter pathway for the migration of photogenerated charger carriers, whereas the heterojunction effect at the BiOI and ZnO interface could reduce the recombination probability of photogenerated electrons and holes through an internal electric field and thus lead to a highly photocatalytic efficiency. The reason that excess ZnO content decreased the photocatalytic efficiency of the ZnO/BiOI heterostructures can be explained by the following aspects: First, higher ZnO content means lower BiOI content in the ZnO/BiOI heterostructures, resulting in fewer photocatalytically active centers for the photocatalytic process. Second, a large amount of ZnO on the surface of BiOI in the ZnO/BiOI heterostructures 20560
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Figure 7. (a) Photocatalytic activities of pure BiOI, pure ZnO, and ZnO/BiOI heterostructures in the degradation of MO under visible-light irradiation. (b) Pseudo-first-order kinetics curves of MO degradation over different catalysts.
Figure 8. Recycling test on the ZnO/BiOI heterostructure ZB-1 for the degradation of MO under visible-light irradiation.
Figure 9. Schematic illustration of ZnO/BiOI heterostructures with different Bi/Zn molar ratios.
could block the visible-light capture of BiOI, as illustrated by Figure 9. To investigate the effect of surface area on of the photcatalytic activity, we normalized the apparent reaction rate constants by surface area (Table 1). After comparing the apparent reaction rate constants of MO before and after normalizing by surface area, we found that the order of normalized rates was different from the original order of rates. The normalized MO degradation rates of ZB-2, ZB-3, and ZB-4 became lower than that of pure BiOI. Thus, high surface area is an important factor in improving
Figure 10. Photocurrent responses of pure BiOI and ZnO/BiOI heterostructure ZB-1 in 0.5 M Na2SO4 aqueous solutions under visible-light irradiation (λ > 420 nm) at 0.5 V vs Ag/AgCl.
photocatalytic efficiency. More strikingly, ZB-1 still exhibited the highest normalized rate constant, indicating that the heterojunction effect should be another crucial factor in enhancing the photocatalytic activity of ZnO/BiOI in addition to high surface area. Generally, semiconductor photocatalysis involves the generation of electrons in the conduction band and holes in the valence band within a semiconductor upon light irradiation at energies equal to or greater than the band gap of the semiconductor. Subsequently, the utilization of photoexcited charge carriers to initiate redox reactions with suitable substrates on the semiconductor surface. To further understand the heterojunction effect on the photocatalytic activity enhancement of ZnO/BiOI heterostructures, we carefully studied the photoinduced chargetransfer properties of the ZnO/BiOI heterostructures. The photoelectrochemical cell (PEC) responses of BiOI and the ZnO/BiOI heterostructures (ZB-1) were first recorded under visible-light irradiation (λ > 420 nm). Figure 10 shows the currentvoltage curves for BiOI and ZB-1 under several on/ off visible-light irradiation cycles. Both samples are prompt in generating photocurrent with a reproducible response to on/off cycles, demonstrating the effective charge transfer and successful 20561
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Figure 11. Surface photovoltage (SPV) spectra of pure BiOI, pure ZnO, and ZnO/BiOI heterostructure ZB-1.
electron collection for the samples within the PEC. In comparison with pure BiOI, BZ-1 exhibited an increased current density, demonstrating that the photoinduced electrons and holes in BiOI prefer to separate and further transfer to the ZnO because of the pn junction between them. As a result, the recombination of the photogenerated electronhole pairs is greatly reduced by the internal electrostatic field in the junction region. Surface photovoltage (SPV) spectroscopy was implemented to investigate the behavior of photogenerated charges in ZnO/ BiOI heterostructures. The signal of surface photovoltage (SPV) is attributed to the change in surface potential barriers before and after light illumination. The SPV amplitude as a function of the incident wavelength reveals the light-responsive wavelength range and the separation extent of the photogenerated charges in the semiconductor materials. Figure 11 shows the SPV spectra of BiOI, ZnO, and the ZnO/BiOI heterostructure ZB-1. The samples had a distinct SPV response. The similarity between the UVvis optical absorption and SPV spectra confirms that photon absorption indeed induces charge generation and separation. A quite weak SPV response in the UV light range was observed for pure ZnO, whereas an apparent SPV response ranging from 300 to 700 nm was observed for pure BiOI. Interestingly, the observed SPV response intensity was significantly strengthened in the ZnO/BiOI heterostructure (ZB-1), indicating that the improved separation of photoinduced electronhole pairs was achieved by the coupling of ZnO and BiOI. The transient photovoltage (TPV) spectroscopy technique was used to further investigate the dynamic properties of photoinduced charge carriers in ZnO/BiOI heterostructures. It can provide direct information about the charge dynamics, including the generation, separation, and recombination of photoinduced charge carriers.13,3436 Figure 12 shows the TPV spectra of ZnO, BiOI, and the ZnO/BiOI heterostructure ZB-1. Pure ZnO did not respond upon illumination with a 532-nm laser pulse. Pure BiOI displayed a positive photovoltage transient signal with a retardation of about 108 s compared to the laser pulse and an abrupt rise at 2 107 s upon irradiation with a 532-nm laser pulse. In general, the positive TPV response means that the positive charges accumulate at the top electrode under irradiation. 12,37,38 BiOI has a unique layered structure (inset of Figure 12), which is characterized by [Bi2O2] slabs interleaved by double slabs of halogen atoms. This induces the presence of
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Figure 12. Transient photovoltage spectroscopy (TPS) of pure BiOI, pure ZnO, and ZnO/BiOI heterostructure ZB-1. The inset shows the crystal structure of BiOI.
internal electric fields between [Bi2O2] slabs and halogen anionic slabs in BiOI.26 As a result, the photogenerated holes could move to the surface of BiOI induced by the self-built electric field. Thus, a positive signal of the TPV response at 2 107 s for BiOI is the effect of positive charge drift under the self-built electric field at the surface region. Moreover, the drift velocity of the holes is much lower than that of the electrons, so the enrichment of positive charge in the BiOI surface could be detected. Afterward, charge concentration on the BiOI surface rapidly decreased as a result of recombination, so that the TPV response is sharply weakened. The short lifetime suggests a low separation efficiency of the photogenerated charges in pure BiOI. Unlike the case for pure BiOI, the TPV curves of ZB-1 clearly displayed two response processes with response times of 107105 and 105103 s. More significantly, the lifetime of the TPV response for the ZnO/BiOI heterostructure became much longer than that of BiOI. These TPV features of ZnO/BiOI can be attributed to the heterojunction formed at the interface between BiOI and ZnO. The potentials of the conduction-band (CB) and valenceband (VB) edges of BiOI and ZnO were estimated by Mulliken electronegativity theory EVB ¼ X Ee þ 0:5Eg
ð3Þ
where Ee is the energy of free electrons on the hydrogen scale (ca. 4.5 eV) and X is the absolute electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms. The CB can be obtained by ECB = EVB Eg. The X values for BiOI and ZnO were determined to be 5.99 and 5.95 eV, respectively. Therefore, the EVB values of BiOI and ZnO were calculated to be 2.40 and 2.97 eV, respectively, and the ECB values of BiOI and ZnO were estimated to be 0.59 and 0.07 eV, respectively. The energy band structure diagram of BiOI and ZnO is thus schematically illustrated in Figure 13. BiOI is a p-type semiconductor whose Fermi level is located close to the valence band, whereas ZnO is a typical n-type semiconductor whose Fermi energy level lies close to conduction band.3942 When BiOI is in contact with ZnO to form a pn junction, the electrons diffuse from n-ZnO into p-BiOI, resulting in an accumulation of negative charges in the p-BiOI region near the junction; holes diffuse from the p-BiOI region to the n-ZnO region, creating a positive section in the n-ZnO region in the 20562
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Figure 13. Diagram of the (a) band energy of ZnO and BiOI before contact and (b) formation of a pn junction and the proposed charge separation process of ZnO/BiOI heterostructures under visible-light irradiation.
vicinity of the junction. When the Fermi levels of BiOI and ZnO reach equilibration, an internal electric field directed from n-ZnO to p-BiOI is simultaneously built to stop the charge diffusion from n-ZnO into p-BiOI. Meanwhile, the energy bands of ZnO shift downward along with the Fermi lever, whereas the energy bands of BiOI shift upward in this process. Under visible-light irradiation, electronhole pairs would be photogenerated within BiOI. The aforementioned difference in the drift velocity between holes and electrons would result in the positive signals shown in Figure 12. It is known that the plate thickness of BiOI in the ZnO/BiOI heterostructure is much thinner than that of pure BiOI (Figure 3), providing a shorter pathway and more opportunities for the photoholes to reach the surface of the plates. The observed increase in positive TPV response at 2 107 s is therefore reasonable. Subsequently, the photogenerated electrons would migrate from BiOI to the CB of ZnO. However, the VB potential of BiOI is higher than that of ZnO, so holes can only stay on the surface of BiOI. Furthermore, the internal electric field can promote the separation of the electronhole pairs, causing the electrons to move across the interface to the region of ZnO and the holes to transfer to the surface. Because the diffusion coefficients for excess electrons and holes are different, this leads to the formation of the diffusion PV.12,43 Thus, the positive TPV response gradually increased and reached about 1.15 mV in 1.5 105 s. After that, the delay for the TPV peak can be ascribed to the time retardation of the maximum separation in ZB-1. The complete recombination time of ZB-1 was about 0.03 s, much longer than that of pure BiOI (0.002 s). This suggests that the heterojunctions at the interface between BiOI and ZnO can greatly reduce the recombination of electronhole pairs and extend the lifetime of charge carriers by 15 times, which is another factor in the superior photocatalytic activity of ZnO/ BiOI heterostructures in addition to high surface area.
’ CONCLUSIONS In summary, we have synthesized ZnO/BiOI heterostructures through a facile one-pot chemical bath method at low temperature. The morphology, constituents, and optical properties of ZnO/BiOI heterostructures can be rationally controlled through simply tuning the Bi/Zn molar ratios. The as-obtained ZnO/ BiOI heterostructures exhibited high visible-light-driven photocatalytic activity in the degradation of MO in aqueous solution. The surface photovoltage and transient photovoltage measurements demonstrated that the photoinduced charge-transfer properties of the BiOI were greatly improved by coupling with
ZnO to form the heterostructures. The heterojunction at the interface between p-type BiOI and n-type ZnO can efficiently reduce the recombination of electronhole pairs and greatly extend the lifetime of charge carriers, which accounts for the enhancement in photocatalytic activity in addition to high surface area. This finding indicates that the heterojunction effect created between two different semiconductors is of importance in determining the dynamic properties of photogenerated charge transfer and the related photocatalytic properties.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Phone/Fax: +86-27-6786-7535.
’ ACKNOWLEDGMENT This work was supported by the National Basic Research Program of China (973 Program) (Grant 2007CB613301), National Science Foundation of China (Grants 21073069, 91023010, and 21177048), 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), and Program for Changjiang Scholars and Innovative Research Team in University (Grant IRT0953). ’ REFERENCES (1) Xie, H.; Li, Y. Z.; Jin, S. F.; Han, J. J.; Zhao, X. J. J. Phys. Chem. C 2010, 114, 9706. (2) Tabata, M.; Maeda, K.; Ishihara, T.; Minegishi, T.; Takata, T.; Domen, K. J. Phys. Chem. C 2010, 114, 11215. (3) Jung, H. S.; Hong, Y. J.; Li, Y. R.; Cho, J. H.; Kim, Y. J.; Yi, G. C. ACS Nano 2008, 2, 637. (4) Huang, J. H.; Cui, Y. J.; Wang, X. C. Environ. Sci. Technol. 2010, 44, 3500. (5) Zou, Z. G.; Ye, J. H.; Sayama, K.; Arakawa, H. Nature 2001, 414, 625. (6) Kamat, P. V. Chem. Rev. 1993, 93, 267. (7) Nowotny, M. K.; Sheppard, L. R.; Bak, T.; Nowotny, J. J. Phys. Chem. C 2008, 112, 5275. (8) Kim, H. G.; Borse, P. H.; Jang, J. S.; Jeong, E. D.; Jung, O. S.; Suhd, Y. J.; Lee, J. S. Chem. Commun. 2009, 5889. (9) Zheng, Y. H.; Zheng, L. R.; Zhan, Y. Y.; Lin, X. Y.; Zheng, Q.; Wei, K. M. Inorg. Chem. 2007, 46, 6980. (10) Chen, X. B.; Shen, S. H.; Guo, L. J.; Mao, S. S. Chem. Rev. 2010, 110, 6503. (11) Kamat, P. V. J. Phys. Chem. C 2007, 111, 2834. 20563
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