A series of Bi2O3/ZnO nanorods photocatalyst with different Bi contents were succeffully synthesized through a simple one-pot hydrothermal method. The as-prepared samples were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, UV-vis spectrum, Brunauer-Emmett-Teller. The photocatalytic activity of the as-prepared samples was evaluated by photocatalytic decolorization of Indigo Carmine(IC) aqueous solution at ambient temperature under visible light. The catalytic mechanism was also discussed.
Keywords: ZnO nanorods, Bi2O3, visible light, photocatalysts
In the past decades, semiconductor photocatalysts has attracted extensive attention due to its wide potential application in environmental protection procedures such as air purification, water disinfection, hazardous waste remediation, and water purification. Among various semiconductor photocatalysts, TiO2 and ZnO have been recognized as the excellent materials for photacatalytic process due to their high photosensitivity, nontoxic nature, and easy to get. Although TiO2 is universally considered as the most important photocatalyst, ZnO is also a suitable alternative to TiO2 due to their similar bandgap energy (3.2eV) and its lower cost. Moreover, the growing number of publications devoted to ZnO is more than TiO2, this clearly shows an increase in interest by the scientific community in ZnO considered. However, the photocatalytic activity of ZnO should be further enhanced from the viewpoint of practical use and commerce. Therefore, various methods have been developed to reduce the e-/h+ recombination rate of ZnO in the photocatalytic processes.
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As a wide band gap oxide semiconductor, ZnO need to be excited by an ultraviolet light which is less than 5% of the solar irradiance at the Earth's surface. So, it is highly desirable to develop a photocatalyst that can use visible light in high efficiency under sunlight irradiation. Researchers have been interested in the modification of electronic and optical properties of this semiconductor for its efficient use in water and air purification under visible light irradiation. There are three ways to synthesize the visible light ZnO photocatalyst: (1) doping ZnO with transition metals such as Fe,V, Cr, Mn, Co, Ag, Ni; (2) doping nitrogen into ZnO; (3) coupling of ZnO with a small band-gap semiconductor which extends light absorption into the visible region.
Bismuth oxide, Bi2O3, due to its high dielectric permittivity, refractive index, marked photoconductivity and photoluminescence, has attracted great attention. As a photocatalyst, Bi2O3 is a p-type semiconcuctor with valence and conduction band edges of +3.13V and +0.33V (vs.NHE), respectively. Under visible light irradiation, the photogenered electron and hole is able to oxidize water.
In the present paper, we report the synthesis of Bi2O3/ZnO composite nanorods by one-pot hydrothermal method at first time. The as-prepared samples were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, UV-vis spectrum, Brunauer-Emmett-Teller. The photocatalytic activity of the as-prepared samples was evaluated by photocatalytic decolorization of IC aqueous solution at ambient temperature under visible light. The catalytic mechanism was also discussed.
2. Experimental Section
Zinc acetate (ZnAC2·2H2O), Bismuth nitrate pentahydrate (BiN3O9·5H2O), ammonia water (NH4OH), sodium hydroxide (NaOH), sulphuric acid (H2SO4) were used in the experiments. All the chemicals employed were of analytical grade and applied without further purification (purchased from Shanghai Chemical Industrial Company). Distilled water was used throughout for all experiments.
2.2 Synthesis of nanosized ZnO powders and N-doped TiO2
ZnO was prepared by hydrothermal method. In a typical experiment, ZnAC2·2H2O was dissolved in 120mL distilled water to form solution with certain concentration, and then adjusted the pH of the solution to 10 with NH4OH to form white colloidal semigel under stirring for 30min. Then the reaction solution was transferred into a 200mL Teflon-lined stainless steel autoclave, followed by hydrothermal treatment of the mixture at 120â„ƒ for 18h. After hydrothermal reaction, the white precipitates were centrifuged, and the wash with distilled water and absolute alcohol six times. The washed precipitates were dried in a vacuum oven at 60â„ƒ for 8h and finally were calcined in air at 500 â„ƒ for 2h with the temperature rate of 5â„ƒ/min.
For bismuth-modified ZnO, various mol% (0.5, 1, 5and 10) of Bismuth nitrate pentahydrate were added to the zinc acetate solution under stirring, other steps were the same as mentioned above.
Nitrogen-doped titania (N-doped TiO2) catalyst with tetrabutyl titanate as a titanium precursor was prepared via the sol-gel method at room temperature. The procedure was as follows: 17 mL tetrabutyl titanate and 40 mL absolute ethyl alcohol were mixed as solution a, subsequently solution a was added dropwise under vigorous stirring into the solution b that contained 40 mL absolute ethyl alcohol, 10 mL glacial acetic acid, and 5 mL double distilled water to form transparent colloidal suspension c. Subsequently aqua ammonia with N/Ti proportion of 8 mol% was added into the resulting transparent colloidal suspension under vigorous stirring condition and kept stirring for 1 h. Finally, the xerogel was formed after being aged for 2 days. The xerogel was grounded into powder which was calcined at 500°C for 2 h. Finally, above powder was grounded in agate mortar and screened by shaker to obtain N-doped TiO2 powders.
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The crystalline phase of ZnO and bismuth modified ZnO were analyzed by X-ray diffractometer (D/MAX-RB, Rigaku Corporation, Japan) with Cu Kα radiation (λ = 1.54056Å). The patterns were collected at 295 K with a step-scan procedure in the range of 2 θ = 1 0 − 100°. The step interval was 0.02° and the time per step was 1 s. The accelerating voltage and applied current were 40 kV and 40 mA, respectively. The chemical nature of surface was analyzed by X-ray photoelectron spectroscopy (XPS, PHI 5000 versaProbe, UIVAC-PHI Corporation, Japan).The chemical composition of the compound was determined by scanning electron microscope-X-ray energy dispersion spectrum (SEM-EDS, LEO 1530VP, LEO Corporation, Germany), UV-visible diffuse reflectance spectrum of the photocatalysts were measured with a Shimadzu UV-2550 UV-Visible spectrometer, and BaSO4 was used as the reference material. The surface areas of the photocatalyst were determined by the Brunauer-Emmett-Teller (BET) method (MS-21, Quantachrome Instruments Corporation, USA) with N2 adsorption at liquid nitrogen temperature. All the samples were degassed at 180â„ƒ prior to nitrogen adsorption measurements.
2.4 photocatalytic experiment
In the photocatalytic experiment, indigo carmine was used as a probe molecule to evaluate the photocatalytic reactivity of the as synthesized pure ZnO and Bi-doped ZnO nanorods. The experiments were carried out as follows: 40mg of the samples was dispersed in 50mL of 0.03 mM indigo carmine solution in a quartz test tube. Prior to illumination, the suspensions were magnetically stirred in the dark for some times to ensure the establishment of absorption equilibrium of indigo carmine on the sample surfaces. Subsequently, the suspension was irradiated under a 500W Xe lamp with cutoff filter (λ>420nm), which was positioned about 6cm away from the quartz test tube. UV-vis adsorption spectra (Shimazu, UV2450) were recorded at different time intervals to monitor the process.
3. Results and discussion
XRD patterns of ZnO and Bi-doped ZnO are shown in Fig.1 respectively. A series of characteristic peaks at 31.61is observed, and they are in accordance with the standard ZnO JCPDS pattern (International Centre for Diffraction Data, JCPDS 36-1451), showing the main structure of the samples is the wurtzite structure of ZnO. The particle size was calculated using the Scherrer's formula . The average crystallite size D for ZnO was determined as nm. Along with the increase of the content of Bi, peak intensity first larger and then smaller, it indicates that the dispersion of a little of Bi can improve the crystallinity of ZnO. But with more Bi disperses the crystallinity of ZnO decreases. This is consistent with the SEM.
To be sure about the composition of the product, we performed XPS analysis. Fig. 2 and Fig. 3 show the full survey and high resolution XPS data for the 5%Bi doped ZnO nanoparticles, respectively. From the XPS spectrum, the binding energies of O 1s, Zn 2p1/2, and Zn 2p3/2 for the as prepared ZnO are 529.1, 1045.0, 1021.8 eV, respectively (Fig.3). In addition, a weak C 1s peak at 283.8 eV is observed due to ambient air contamination (Fig. 4(a)). To determine the chemical state, we used Auger parameter of Zn (using Zn 2p3/2 peak (Fig. 4(b))) and Auger peak of Zn (at 497.8 eV) which is equal to 2008.5 eV. This value confirms formation of ZnO at the surface of samples. It is in consistant with the XRD analysis above. The O 1s peak (Fig.4(c)) is located at 529.8 eV, indicating typical metal oxides (centered at 528.1-531.0 eV). High resolution XPS analysis in the range of Bi binding energies depicted in Fig. 4(d). Bi 4f5/2 and Bi 4f7/2 peaks were observed at 163.1 eV and 157.8 eV, respectively, which are in agreement with literature values. This result confirms the presence of Bi2O3 in the surface of ZnO nanoparticles.
Fig.4a-e shows the SEM images of pure ZnO and Bi doped ZnO nanocomposite samples with various morphologies prepared by the hydrothermal method, using diffrernt molar ratios of Bi3+/Zn2+. When we use ZnAC2·2H2O and NH4OH without BiN3O9·5H2O, short ZnO nanorods are observed in the products. Addition of BiN3O9·5H2O in the molar ratio of 0.5/100 in the reaction system, the products obtained are made up of longer and more rodlike photocatalysts. The mean diameter and length of 0.5%Bi were determined at about 10um and 60um. With the ratio of the Bi3+/Zn2+ arises, the crystallinity of ZnO decreases. It shows that the doped Bi impact the formation of rodlike ZnO.
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3.4 UV-vis spectroscopic analysis
The absorption of UV-vis light is an important factor in evolution of of photocatalyst property. Fig.5a shows the comparison of UV-vis absorption spectra of pure ZnO and Bi doped ZnO nanocomposites at room temperature. It is obvious that Bi doped ZnO samples absorb more light in the range of 400-500 nm compared to undoped ZnO, indicating our method is effective to visible light range.
Assuming the Bi doped ZnO to be indirect semiconductor, as is ZnO, a plot of the (ahv)1/2 versus the energy of absorbed light affords the band gap energy as shown in Fig.5b. The band gaps optically obtained in such a way were approximately shown in Table 1. This result obviously reveals the band gap of the Bi doped ZnO is smaller than that of the pure ZnO. It is apparent that the diffuse reflectance spectra (DRS) of all the doped ZnO samples have extended a red shift and increased absorbance in the visible range with the increasing doping content.
4 Photocatalytic activity and mechanism
4.1 Photocatalytic performance of Bi doped ZnO nanocatalysts
IC was adopted as a representative organic pollutant to evaluate the photocatalytic performance of the as-synthesized samples. N-TiO2 was used as a reference to qualitatively understand the photocatalytic activity of the as-synthesized samples. The photocatalytic activities of the as-synthesized samples and N-TiO2 are shown in Fig. n. the degradation efficiency is defined as C/C0, where C0 and C are the initial concentration of IC after the equilibrium adsorption and the reaction concentration, respectively. As seen in Fig. n, the order of the photocatalytic activity of the as-synthesized samples is 5% Bi-ZnO>N-TiO2>10% Bi-ZnO>1% Bi-ZnO>0.5% Bi-ZnO>ZnO. It is obvious that the photocatalytic activities of the Bi-ZnO photocatalysts are significantly higher than that of pure ZnO, the photocatalytic activities of the 5% Bi-ZnO photocatalyst is better than N-TiO2, thus indicating that the as prepared Bi-ZnO photocatalysts have a positive effect on enhancing the photocatalytic activity. With the increasing of Bi content, the photocatalytic activity becomes higher. Among the samples, the 5% Bi-ZnO is found to be the most active.
4.2 Relationship between structure and photocatalytic performance of Bi doped ZnO nanocatalysts
The proposed photocatalytic mechanism and charge transfer in the as-synthesized Bi-doped ZnO photocatalysts during the photocatalytic process is shown in Scheme 1. As the band gap of Bi2O3 is 2.85 eV, it can be excited by light with wavelength less than 435 nm. However, the photocatalytic activity of pure Bi2O3 is very low because of the high electron-hole recombination rate in Bi2O3. The valence band of Bi2O3 is lower than that of ZnO, the Bi2O3-ZnO heterojunctions formed in the composite will promote the photo generated holes in bismuth oxide to be transferred to the upper lying valence bands of ZnO as shown in Scheme 1. In addition, molecular oxygen O2 can be served as the scavenger of the electrons to yield the superoxide radical anion O2- and hydrogen peroxide H2O2. This process is thermodynamically feasible. Recombination rate of photo-induced electron-hole pairs was reduced and much more holes were captured to induce photocatalytic reaction. And factor that suppresses the electron-hole recombination will therefore enhance the photocatalytic activity. As the recombination rate of photo-induced electron-hole pairs was reduced and much more holes were captured to induce photocatalytic reaction. Thus, the photocatalytic activity of Bi2O3-ZnO composite enhanced as compared to the ZnO, N-TiO2.