Enhanced Photocatalytic Activity and Mechanism of La3+- Doped BiPO4

Abstract

Ladoped BiPO4 photocatalysts were synthesized by a hydrothermal method. The morphologies, structures, components, and light absorption properties of the photocatalysts were evaluated using XRD, FESEM, EDX, BET, XPS, and DRS techniques. La-doping could improve the photocatalytic activity of BiPO4 for degrading methylene blue under ultraviolet irradiation, The apparent reaction rate of La-BiPO4 was 0.31min, which was 3.4 times higher than that of pure BiPO4. The outstanding photocatalytic activity could be mainly attributed to the higher valence band position and more effective electron-hole pair separation, ensured by Mott-Schottky and EIS test. Moreover, radical scavenger experiments confirmed that holes constituted the active species. Introduction Photocatalysis has attracted tremendous attention because of its great potential in renewable energy and environmental protection [1][2]. Among the numerous semiconductor photocatalysts, the novel BiPO4 has proven to be a suitable alternative to TiO2[3], and is frequently investigated owing to its low cost and significant photocatalytic oxidative ability in the decomposition of organic dyes and other pollutants [4][5]. Nevertheless, widespread applications of BiPO4 have been hindered by its large grain size and weak response to visible light [3][6]. Therefore, it is necessary to identify effective approaches to enhance the photocatalytic activity of BiPO4. In addition, systemic studies regarding the mechanism and pathway of photogenerated electron-hole pairs (e/h) under irradiation should be conducted in order to design efficient doped photocatalysts, which will promote practical applications in areas such as environmental protection. A significant amount of work has been carried out in an effort to enhance the photocatalytic activity of BiPO4, such as improving preparation methods [6], modificating with noble metals [7], and fabricating heterojunctions [8]. It was also reported that doping with F and Ag could enhance the photocatalytic activity of BiPO4 [9][10]. The effects of lanthanide doping on the photocatalytic activity of BiPO4 have not been reported, to the best of our knowledge. Other research has shown that lanthanide ions were good dopants for TiO2 and BiVO4 [11][12], owing to their 4f electron configuration. Among lanthanide elements, La is attractive because it is cheap and can reduce the recombination rate of e/h effectively. Therefore, doping with La may be an effective method to enhance the photocatalytic activity of BiPO4. In this study, lanthanum ion-doped BiPO4 spherical particles were fabricated by a simple hydrothermal process. The crystal structure, optical properties, and photo-electrochemical performance of La-BiPO4 catalysts were measured. Their photocatalytic activities were evaluated in the decomposition of methylene blue (MB) under ultraviolet (UV) light. Furthermore, the possible mechanism behind the improved photocatalytic properties of La-BiPO4 was investigated on the basis of its energy band structure and the measurement of reactive species. 2nd International Conference on Machinery, Materials Engineering, Chemical Engineering and Biotechnology (MMECEB 2015) © 2016. The authors Published by Atlantis Press 617 Experimental Synthesis of La-BiPO4 photocatalysts All analytically pure reagents were provided by Sinopharm Chemical Reagent Company. Briefly, appropriate amounts of Bi(NO3)3•5H2O and La(NO3)3·6H2O were added to 29 mL of a nitric acid solution (26.4 mL distilled water + 2.6 mL HNO3), into which 1 mL of tri-butyl-phosphate (TBP) was added drop-wise. The mixed solution was subsequently stirred for 0.5 h. The pH was maintained at less than 1. The resulting suspension was transferred into a Teflon-lined stainless steel autoclave and maintained at 200°C for 3 h. The final product was filtered and washed 3 times with distilled water and ethanol, and then dried at 80°C for 12 h. La-BiPO4 catalysts with 2% La contents (mol fraction) were synthesized using the aforementioned method. Characterization of photocatalysts The obtained catalysts were examined by X-ray diffraction (XRD) (Dmax-2500, Rigaku, Japan) with Cu-Kα radiation (λ = 0.15406nm) at 40 kV and 200 mA. Field emission scanning electron microscopy (FE-SEM) images and EDX spectra were collected using a scanning electron microscope (Supera 55, Zeiss, German) with a scanning voltage of 10.00 kV. The elemental compositions and valence states of the samples were analyzed using X-ray photoelectron spectroscopy (XPS) (ESCALAB 210, VG, UK), using a non-monochromatic Mg Kα X-ray source (300 W). The Brunauer-Emmett-Teller (BET) specific surface area was determined by a surface area analyzer (ASAP 2020 HD88, Micromeritics, USA). Diffuse reflectance spectra (DRS) were measured using a UV-vis spectrophotometer (UV-4100, Hitachi, Japan) equipped with an integrating sphere attachment. Evaluation of photocatalytic activity The photocatalytic activities were investigated in the degradation of MB under UV irradiation. A 20 W germicidal lamp (λ = 254 nm) was used as irradiation source. The average light intensity of the lamp was 18.2 uw/cm. Before irradiation, a MB solution (250 mL, 2×10 mol/L) containing 0.125g catalysts was stirred for 30 min in the dark in order to establish the adsorption-desorption equilibrium between MB and the catalysts. Aliquots (5 mL) of the reaction solution were sampled in specified time intervals and were centrifuged (10000 rpm, 10 min) to remove the catalysts particles. The absorbance of MB (the maximum absorption peak occurs at 664 nm) was recorded using a UV-Vis spectrophotometer (TU-1810D, Beijing Purkinje General Instrument Co., Ltd.). The radical and hole trapping experiments were similar to the above-mentioned photodegradation experiment. The scavengers, tert-butyl alcohol (t-BuOH) and ethylenediaminetetraacetic acid disodium salt (EDTA-2Na), were added to the reaction solution prior to the addition of the photocatalysts. Photoelectrochemical measurements Photoelectrochemical measurements were carried out using an electrochemical system (Priceton Parstat-4000, USA). A La-BiPO4 film was deposited on ITO conducting glass (La-BiPO4@ITO). UV irradiation was executed using a 20 W ultraviolet germicidal lamp (Foshan electric lighting co., Ltd.). A standard three electrode cell with a La-BiPO4@ITO film as the working electrode, standard Ag/AgCl electrode as the reference electrode, and platinum wire as the counter electrode was used in the photoelectrochemical measurements. Na2SO4 (0.1 mol/L) was used as the electrolyte solution.