| Hydrothermal method is widely acceptable for synthesis of monoclinic BiVO4. However, the application of hydrothermal method was limited by its high hydrothermal temperature, long reaction time, poisonous additives or high additive consumption, etc. Besides, the dyes for photodegradation were of low concentration and mostly restricted to Rhodamine B (RB) and methylene blue (MB). To solve these problems efficiently, microwave was introduced into hydrothermal method. But little work has been done on synthesis of BiVO4 by a microwave-assisted hydrothermal route. In this work monoclinic BiVO4 was synthesized by a hydrothermal route assisted with microwave as a heat source and sodium carboxymethyl cellulose (CMC) as a surfactant. Several conditions were tuned to optimize the process of BiVO4 synthesis. To investigate the influence of different synthesis conditions on photocatalysis activity, the obtained samples were characterized by XRD, FESEM, FTIR and UV-vis techniques. CR and ARS were used as target pollutants to evaluate visible-light-driven photocatalytic properties of the BiVO4 samples. Conditions of catalytic reaction under visible light were also optimized by experiments. Furthermore, discussions were carried out on reaction mechanisms of synthesis and degradation as well as recycling capacities and stability of catalyst. Detail conclusions were given as follows:(1) Monoclinic BiVO4 with sizes of 30 nm-4μm and band gaps of 2.35-2.41 eV were prepared under different conditions and their appearances were sheet, T shape, fish-bone like and ellipsoidal. Among them, the best ellipsoidal catalysts of average size 50 nm and 30 nm with band gaps 2.36 eV and 2.35 eV were obtained under conditions of Bi source (Bi:V=1:1) 0.15 mol·L-1, pH 9, microwave heating for 90 min at 120 ℃, calcination for 4 h or 0 h at 400 ℃, separately. Through changes of morphologies, structures and photochemical characteristics of catalysts, their photocatalytic activities were influenced greatly.(2) During the catalytic degradation of dyes over obtained catalysts, the degradation efficiencies raised with increase of catalyst dosage and illumination time. With initial dye concentration rising, degradation efficiencies for CR were continuously decreased, while that for ARS was firstly up and then quickly down. Degradation efficiencies for ARS and CR maintained stable under initial dye solution pH 6-10. The photocatalytic degradation efficiencies for dyes were above 96% under optimal catalytic reaction conditions of 2.5 g·L-1 catalyst dosage, illumination for 3 h, CR concentration of 30 mg·L-1 and ARS concentration of 20 mg·L-1 without adjusting pH values.(3) Theoretical arguments showed that the Ostwald ripening reaction and nucleation-dissolution-recrystallization reaction were carried out in precursor solution during the microwave-hydrothermal process and CMC surfactant worked as a template agent and capping agent; The degradation of dyes might happen by means of dye photosensitization and heterogeneous photocatalysis. During the catalytic process, hydroxide ions and dissolved O2 acted as proper scavengers which suppressed combination of photogenerated carriers, electrons and holes. Removal of sulfonate group might be carried out in dyes degradation processes.(4) During five consecutive recycling runs, the photodegradation efficiencies were kept above 90% and the XRD patterns were nearly the same before and after experiments. It indicated that the obtained catalysts hardly suffered from photocorrosion and possessed favorable recycling characteristics and high stability.Our work revealed that monoclinic BiVO4 were successfully synthesized via a fast mild microwave hydrothermal method assisted with low-concentration CMC. Although of high substrate concentration, the visible-light-driven photocatalytic performance of obtained catalysts remained outstanding. |
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