| In order to enhance the photocatalytic activity of Ag3PO4catalysts, highly active Ag3PO4/Fe2O3composite photocatalysts with various weight percentages of Fe2O3have been prepared by ultrasound-assisted precipitation. These photocatalysts were characterized by XRD, XPS, SEM, TEM, UV-vis and PL spectroscopy. The photocatalytic activity and stability of Ag3PO4/Fe2O3composites were evaluated by photocatalytic degradation of methyl orange under visible light irradiation. Ag3PO4thin film was synthesized by an in situ electrochemical deposition method. The impedance analysis of Ag3PO4thin film/electrode system was studied as a function of applied potential under different pH value. The flat-band potential was investigated by Mott-Schottky plot. The role of n-n Ag3PO4-Fe2O3heterojunction was discussed in view of theory of semiconductor physics.(1) The photocatalytic activity analysis of Ag3PO4/Fe2O3composite photocatalysts. Ag3PO4/x%Fe2O3composite photocatalysts have been successfully prepared by ultrasound-assisted precipitation(x=5,10,20,30,40,80and90). The photodegradation of methyl orange in aqueous solution under visible-light irradiation showed that the photocatalytic activity of the Ag3PO4/Fe2O3composites was much higher than that of pure Ag3PO4. The photocatalytic activity of Ag3PO4/Fe2O3improved remarkably with increasing Fe2O3concentration, but at the higher Fe2O3level the photocatalytic activity decreased slightly, suggesting that the optimal Fe2O3concentration in Ag3PO4/Fe2O3composite existed. Ag3PO4/10%Fe2O3composite had the best photocatalytic activity whose rate constant was about6times as large as that of Ag3PO4. Furthermore, Ag3PO4/90%Fe2O3composite showed slightly higher photocatalytic activity than Ag3PO4, but the Ag usage of the former is only about one half of the later, which can reduce the cost and environmental pollution.(2) The stability analysis of Ag3PO4/Fe2O3composite photocatalysts. The recycle experiments were carried out to investigate the reuse of Ag3PO4/Fe2O3composites. The results showed that significant photocatalytic activity losses were observed for all one time used Ag3PO4/Fe2O3composites. The weak peak of silver can be observed in the XRD pattern of one time used Ag3PO4/10%Fe2O3composite, indicating that Ag3PO4particles had been partially reduced into metallic silver. The iron concentration of the methyl orange solution after irradiation for60mins using Ag3PO4/10%Fe2O3as photocatalysts was found to be1.3mg/L, indicating photoinduced dissolution of Fe2O3happened. In sum, for Ag3PO4/Fe2O3composites, photocatalytic activity losses were due to the partially reduction of Ag3PO4into metallic silver and the photocorrosion of Fe2O3. To improve the stability of Ag3PO4/Fe2O3composites, further investigations are needed.(3) The flatband potential measurement of Ag3PO4. Ag3PO4thin film was synthesized directly on F-doped SnO2glass substrate (FTO) using an in situ electrochemical deposition method. The impedance analysis of Ag3PO4thin film/electrode system was studied as a function of applied potential under different pH value. The Nyquist plot was fitted with a plausible equivalent circuit. Based on the determined capacitance vs. bias voltage, namely Mott-Schottky plot, the flat-band potential was investigated. When the pH value is4,7and10, the Vfb is1.259V,1.277V and1.216V (vs. SCE) respectively, namely1.014V,1.032V and0.971V (vs. NHE), which is correspond with the calculated value from the photocurrent vs potential curve.(4) The role of n-n Ag3PO4-Fe2O3heterojunction. When Ag3PO4particles were mixed with Fe2O3particles, at the interfaces between Ag3PO4and Fe2O3particles, an n-n heteroj unction will be formed. Because the flatband potential of Fe2O3is more negative than that of Ag3PO4, the electrons keep on flowing from Fe2O3to Ag3PO4until the Fermi levels become coincident, creating an accumulation of negative charges in the Ag3PO4region and a positive section in the Fe2O3region in the vicinity of the junction. This sets up an internal electrostatic field directed from the Fe2O3region to the Ag3PO4region. Under irradiation, both Ag3PO4and Fe2O3can absorb the band-gap photos and then the electron-hole pairs are generated. Photo-generated holes and electrons separate under the influence of the internal electrostatic field in the n-n heteroj unction region. Holes move to the Ag3PO4region, and electrons to the Fe2O3region. Therefore, the chance of electron-hole recombination is reduced, which leads to high photocatalytic activity of photooxidation of methyl orange. |
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