Article pubs.acs.org/JPCC
Facile Fabrication of Heterostructured Bi2O3−ZnO Photocatalyst and Its Enhanced Photocatalytic Activity Subramanian Balachandran and Meenakshisundram Swaminathan* Department of Chemistry, Annamalai University, Annamalainagar 608 002, India S Supporting Information *
ABSTRACT: The development of coupled semiconductor photocatalysts makes a significant advancement in catalytic functional materials. A new heterostructured Bi2O3−ZnO, synthesized by a simple hydrothermal−thermal decomposition method, exhibited higher photocatalytic activity for the degradation of Acid Black 1 (AB 1) under UV light than pure ZnO, Bi2O3, and commercial Degussa P25. X-ray powder diffraction analysis reveals that the as-synthesized product has the monoclinic lattice phase of Bi2O3 and the hexagonal wurtzite phase of ZnO. HR-SEM images show that Bi2O3−ZnO has an ordered mixture of nanofiber and nanochain structures. This heterostructured Bi2O3−ZnO has increased UV absorption when compared with ZnO. The enhanced photocatalytic activity of Bi2O3−ZnO is attributed to the low recombination rates of photoinduced electron−hole pairs, caused by the vectorial transfer of electrons and holes between ZnO and Bi2O3. Higher efficiency at neutral pH 7 and reusability in the degradation of AB 1 makes Bi2O3−ZnO, a promising candidate for the photocatalytic treatment of dye effluent.
1. INTRODUCTION Photocatalytic degradation of organic pollutants by semiconductor photocatalysts has been the beneficial technology for environmental purification.1−3 Semiconductor photocatalysis affords a potential solution to the problems of energy shortages and environmental pollution. However, photoefficiency of the bare semiconductor catalyst is limited because of the rapid electron−hole recombination. Therefore, it is urgent to develop highly efficient photocatalytic materials for pollutant degradation. The photocatalytic activity of semiconductor oxide depends on its physical and chemical properties. Because the recombination of photoexcited electrons and holes occurs at crystal lattice defects, crystallinity (i.e., the extent of crystallization) is one of the main factors for photocatalytic reaction efficiency. It is also found that photogenerated charge carriers can be effectively separated inside semiconducting composite materials according to different band gap structures of their components.4−6 In the field of photocatalysis, ZnO is believed to be an efficient photocatalytic material alternative to TiO2 because both have similar band gap and photocatalytic mechanism.7,8 The enhanced photocatalytic activity of the nanomaterials was ascribed to the larger surface area, increased oxygen vacancy, and the facilitation of diffusion and mass transportation of the reactant molecules.9,10 The synthesis of zinc oxide nanomaterial has received great attention due to its size-dependent properties and photocatalytic applications.11 Bismuth oxide (Bi2O3) is an important p-type semiconductor with four main crystallographic polymorphs denoted by α-, β-, γ-, and δ-Bi2O3.12 © 2012 American Chemical Society
Bismuth-based oxides appear to be good candidates because they have a band gap in the visible range.13−15 Bi2O3 with a band gap of 2.8 eV accounts for its ability to oxidize water and possibly generate highly reactive species, such as O2−• and OH• radicals, for initiating oxidation reactions.16,17 One of the effective ways to enhance the electron−hole separation is to use coupled semiconductors. In coupled semiconductors, a heterojunction interface between the semiconductors of matching band potentials is constructed. In this way, the electric-field-assisted charge transport from one particle to the other via interfaces is favorable for the electron−hole separation in the coupled materials and for the consequent electron or hole abundance on the surfaces of the two semiconductors.18 Hence in coupled semiconductors, advantages such as improvement of charge separation, increase in the lifetime of the charge carrier, and enhancement of the interfacial charge transfer efficiency to adsorbed substrate can be achieved. Previously there was a report on the synthesis of ZnO film/Bi2O3 microgrid heterojuncion by microsphere lithography technique for the degradation of methyl orange dye.19 In recent years, hydrothermal method has gained much interest due to its operational simplicity and capability of large scale production of material. Because this method does not require rigorous conditions, it can be considered as a low-cost and convenient method for the synthesis of coupled semiReceived: July 11, 2012 Revised: November 17, 2012 Published: November 29, 2012 26306
dx.doi.org/10.1021/jp306874z | J. Phys. Chem. C 2012, 116, 26306−26312
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of Bi2O3−ZnO in reaction tube and irradiated it using four parallel medium pressure mercury lamps emitting 365 nm light. Prior to irradiation, the reaction mixture was continuously aerated by a pump to provide oxygen and for complete mixing. At regular intervals, 3 mL of the sample was taken and centrifuged to remove the catalyst. One mL of centrifugate was diluted to 10 mL and its absorbance at 320 nm was measured to determine the concentrations of AB 1.
conductors. Hence, we attempted to fabricate a heterostructured Bi2O3-ZnO material by hydrothermal−thermal decomposition method and to test its photocatalytic performance on the degradation of acid black 1.
2. EXPERIMENTAL SECTION 2.1. Materials. Analar grade zinc nitrate hexahydrate, oxalic acid dihydrate, bismuth nitrate pentahydrate, nitric acid, sodium hydroxide, and ethanol were obtained from Himedia chemicals and used as such. Acid black 1 (Color Chem, Pondicherry; molecular formula: C22H14N6Na2O9S2; molecular weight: 616.57) and ZnO (Merck chemicals; surface area: 5 m2 g−1; particle size: 4.80 μm) were used as received. A gift sample of TiO2−P25 (80% anatase, 20% rutile with BET surface area 50 m2 g−1 and mean particle size of 30 nm) was supplied by Evonik, Germany. Deionized distilled water was employed throughout experiments. 2.2. Preparation of Bi2O3−ZnO. Bi(NO3)3·5H2O (0.97 g)was dissolved in 10 mL of 1.12 M nitric acid to avoid hydrolyzation of Bi3+ ions and made up to 100 mL after adjusting its pH to 11 by the addition of small amount of 0.2 M NaOH. This Bismuth nitrate solution was mixed with a 100 mL of solution of 11.90 g of Zn (NO3)2·6H2O (0.4 M) in deionized water. Finally, 100 mL of solution of Oxalic acid dihydrate in deionized water (0.6 M) was introduced into the above solution dropwise with stirring and stirring was continued for 2 h to ensure complete precipitation of zinc oxalate and bismuth oxalate. This mixed precipitate was transferred into a Teflon-lined stainless-steel autoclave, sealed, and heated at 120 °C for 6 h. During the hydrothermal process, the pressure was maintained at 18 psi. Mixed precipitate of zinc oxalate dihydrate and bismuth oxalate was filtered, washed with distilled water and ethanol, dried in air at 90 °C for 12 h and calcined at 400 °C for 12 h in a muffle furnace to get 11.7 wt % Bi2O3-ZnO. The pure ZnO, various percentages of Bi2O3-ZnO (6.8, 18.9 wt %), and pure Bi2O3 were prepared using the same method. 2.3. Analytical Methods. X-ray diffraction (XRD) patterns were recorded with a Siemens D5005 diffractometer using Cu Kα (k = 0.151 418 nm) radiation. Maximum peak positions were compared with the standard files to identify the crystalline phase. High-resolution scanning electron microscope (HRSEM) images were taken using FEI Quanta FEG 200 HR-SEM. Energy-dispersive spectra (EDS) analysis was performed on gold-coated samples using a JEOL JSM-5610 SEM equipped with EDS. X-ray photoelectron spectra (XPS) of the catalysts were recorded in an ESCA-3 Mark II spectrometer (VG Scientific, England) using AlKα (1486.6 eV) radiation as the source. The spectra were referenced to the binding energy of C1s (285 eV). The Brunauer−Emmett−Teller (BET) surface area was estimated by nitrogen adsorption−desorption method. The specific surface areas of the samples were determined through nitrogen adsorption at 77 K on the basis of the BET equation using a Micrometrics ASAP 2020 V3.00 H. A PerkinElmer LS55 fluorescence spectrometer was employed to record the photoluminescence (PL) spectra of the oxides at room temperature. Diffuse reflectance spectra (DRS) were recorded using Shimadzu UV-2450. UV absorbance measurements were taken using Hitachi-U-2001 spectrometer. 2.4. Photocatalysis. All experiments were carried out under identical conditions using Heber multilamp photoreactor model HML-MP 88. The detail of this model is described elsewhere.20 We took 50 mL of AB 1 dye solution and 200 mg
3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. X-ray diffractograms of the prepared ZnO and 6.8, 11.7, and 18.9 wt % Bi2O3−ZnO nanocatalysts are shown in Figure 1a−d. The 2θ values of
Figure 1. XRD patterns of (a) prepared ZnO, (b) 6.8 wt % Bi2O3− ZnO, (c) 11.7 wt % Bi2O3−ZnO, and (d) 18.9 wt % Bi2O3−ZnO.
prepared ZnO at 31.77, 34.49, 36.24, 56.60, 62.85, 66.38, 67.94, 69.08, 72.50, and 76.93° in Figure 1a correspond to (100), (002), (101), (110), (103), (220), (112), (201), (004), and (202) diffraction planes of wurtzite ZnO.21 The new peaks observed at 27.9, 30.11, 33.7, 46.6, and 47.1° in Figure 1b−d correspond to (121), (012), (−122), (041), and (−104) diffraction planes of α-Bi2O3 (JCPDS no. 71−2274) in Bi2O3− ZnO nanocatalysts.22 The crystallographic phase of the prepared Bi2O3−ZnO is confirmed as the α-Bi2O3 (monoclinic) form present in hexagonal wurtzite ZnO base material. The relatively high intensity of the (101) peak is indicative of anisotropic growth and implies a preferred orientation of the crystallites. There is no remarkable shift of other diffraction peaks and other crystalline impurities are not observed. Increase in Bi2O3 concentration in the catalyst increases peak intensity of (121), (012), (−122), (041), and (−104) diffraction planes corresponding to α-Bi2O3 in ZnO material. Primary analysis of photodegradation reveals that 11.7 wt % Bi2O3−ZnO is the most efficient in the degradation of AB 1 dye. The HR-SEM images of 11.7 wt % Bi2O3−ZnO at four different magnifications are shown in Figure 2. HR-SEM images show that Bi2O3−ZnO has an ordered mixture of nanofiber and nanochain structures, which are seen at two magnifications of 5 and 2 μm in Figure 2a,b, respectively. Nanofiber structure in Figure 2a is expanded and given as Figure 2c, and the expanded 26307
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Figure 2. HR-SEM images of 11.7 wt % Bi2O3−ZnO at different magnifications: (a) 5 μm, (b) 2 μm, (c) 1 μm, and (d) 500 nm.
Figure 3. XPS analysis of 11.7 wt % Bi2O3−ZnO: (a) survey, (b) O1s, (c) Zn2p, and (d) Bi4f.
nanochain structure of Figure 2b is given in Figure 2d. Nanofiber- and nanochain-like structures are clearly seen in Figure 2c,d. Sizes of the particles are in the range of 40−65 nm, as shown in Figure 2d. The hydrothermal method reported here is a simple process without rigorous conditions, and hence it is a low-cost and convenient method to prepare a heterostructured Bi2O3−ZnO.
EDS is generally accurate up to trace amount of metal present in the surface of base materials. The EDS recorded from the selected area is shown in Figure S1 of the Supporting Information, which reveals the presence of Bi, Zn, and O in the catalyst. To find out the presence of elements and to determine their valence states in Bi2O3−ZnO, we carried out XPS study. The binding energy peaks of Zn, O, and Bi were analyzed. The 26308
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emission is due to surface defects in the ZnO powders, as in the case of ZnO nanowires reported by Wang and Gao.31 Reduction of PL intensity at 416 nm by Bi2O3−ZnO when compared with prepared ZnO indicates the suppression of recombination of the photogenerated electron−hole pair by loaded Bi2O3 on ZnO. This leads to a higher photocatalytic activity. The pore structure of the heterojunction Bi2O3−ZnO composite sample was investigated by nitrogen adsorption− desorption isotherms, and the pore size distribution was calculated by BJH method. The N2 adsorption−desorption isotherms of the synthesized Bi2O3−ZnO shown in Figure 5a
Survey spectrum in Figure 3a shows the presence of Zn, O, and Bi. C1s peaks of the Bi2O3−ZnO are ascribed to adventitious hydrocarbon from XPS instrument itself. In Figure 3b, the O1s profile is asymmetric and can be fitted to two symmetrical peaks α and β locating at 530.2 and 532.2 eV, respectively, indicating two different kinds of O species in the sample. The peaks α and β should be associated with the lattice oxygen (OL) of ZnO and chemisorbed oxygen (OH),23 respectively. Figure 3c presents the XPS spectra of Zn2p, and the peak positions of Zn2p1/2 and Zn2p3/2 are at 1045.1 and 1021.9 eV. Comparing the peak positions to those in the Handbook of Xray Photoelectron Spectroscopy,24,25 we can conclude that: (i) Zn is in the state of Zn2+ and (ii) the whole XPS spectra have a downward shift of 0.5 eV because the standard peak position of Zn2p3/2 is at 1022.4 eV. Two signals ascribed to Bi4f7/2 and Bi4f5/2 at binding energies of 158.6 and 164.1 eV are observed in Figure 3d. Atomic ratio of Bi/O is found to be about 2:3 by the quantification analysis from the data of XPS peaks of bismuth and oxygen. This indicates the presence of Bi2O3, which is consistent with the result of XRD. The optical properties of the heterojunction Bi2O3−ZnO were explored by UV−vis diffuse reflectance and PL spectroscopy. The DRS of ZnO and Bi2O3−ZnO are displayed in Figure 4. Bi2O3−ZnO shows a strong absorption in the
Figure 5. N2 adsorption−desorption isotherm of (a) 11.7 wt % Bi2O3−ZnO and (b) their pore size distribution.
exhibited a hysteresis loop, typical of type II pattern representing the predominant nonporous structure according to the classification of IUPAC.32 A sharp increase in adsorption volume of N2 was observed and located in the P/P0 range of 0.45 to 0.99. This sharp increase can be attributed to the capillary condensation, indicating the good homogeneity of the sample and macropore size because the P/P0 position of the inflection point is related to the pore size.33 Average pore radius of Bi2O3−ZnO, shown by pore size distribution curve in the inset of Figure 5, is 250 Å. The pore size distribution of the Bi2O3−ZnO sample thus confirms the nonporous structure. Surface area measurements, made by the BET method, provide the specific surface area of Bi2O3−ZnO as 28.86 m2 g−1, which is higher than the prepared ZnO (11.52 m2 g−1) and Bi2O3 (14.96 m2 g−1). The single-point adsorption total pore volume of pores
