Full Length ArticleEnhanced photocatalytic activity of Ag3PO4 for oxygen evolution and Methylene blue degeneration: Effect of calcination temperature
Graphical abstract
Introduction
Semiconductor-based photocatalysts have been widely explored for water splitting and pollutant removal because of the use of renewable solar energy. The conventional TiO2 photocatalyst possesses high activity and good stability, and thus meet the requirements of water splitting and pollutant removal [1], [2], [3]. However, due to the large band gaps (3.05 eV for rutile and 3.15 eV for anatase), only ca. 5% solar irradiation could be utilized. This has largely limited the practical application of TiO2. Compared with TiO2, visible-light-driven photocatalysts, which could utilize up to ca. 45% solar energy, have been received great attention. Many photocatalysts, such as modified-TiO2 [3], multimetal oxides [4], sulfides [5], oxynitrides [6] and heterojunctions [7], [8], were widely developed. Recently, Ye reported silver orthophosphate (Ag3PO4) possesses a high visible-light photocatalystic activity [9], [10]. Especially, the quantum yield is up to 90% for O2 evolution from water at wavelength >420 nm, which is significantly higher than these of other previously reported semiconductors [9]. Followed by, great efforts have been made to further enhance the photocatalytic activity and stability of Ag3PO4, such as fabricating single crystals [11], morphology-control [12], [13], [14], size-control [15], and heterocoupling [16], [17], [18], etc. On other hand, Ag3PO4 suffers from photo and thermal stability issues in practical applications, because the interstitial Ag+ is easily to be reduced to Ag by the photo-generated electron in the conduction band of Ag3PO4 and the Ag3PO4 itself is tend to be decomposed into Ag or Ag2O under the humid environment. To address this issue, much effort in recent years has been focused on the exploration of the novel Ag3PO4 catalysts and the deactivation mechanism of Ag3PO4 to improve its photocatalytic stability [19], [20], [21].
The high temperature calcination, which could induce the changes of physicochemical properties, is an alternative strategy to enhance the photocatalytic activities of a semiconductor [22], [23]. Noda reported Ta2O5 crystallized by high temperature calcination showed a higher photocatalytic activity for the overall water splitting than Ta2O5 with an amorphous structure [24]. Yu found the photocatalytic activity of anatase TiO2 gradually increased with the increasing calcination temperature. The enhanced photocatalytic efficiency was ascribed to the improved crystallization [25]. Sato also experimentally verified that the high temperature calcination could promote the release of lattice oxygen, which played an important role for enhancing the photocatalytic activity of TiO2 [26]. Accordingly, the high temperature calcination is a potential approach to create synergistic effects for enhancing the activity of photocatalysts, including Ag3PO4. However, until now there are few tentative investigations on the influences of calcination temperature on the photocatalytic activity of Ag3PO4.
Here, high active Ag3PO4 photocatalysts were prepared via a facile high temperature calcination of as-prepared Ag3PO4 from coprecipitation process. The Ag3PO4 photocatalysts were characterized by using X-ray diffraction (XRD), ultraviolet–visible diffuse reflection spectroscopy (UV–vis DRS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), specific surface area (BET), X-ray photoelectron spectroscopy (XPS) and electrochemical measurements. Ag3PO4 remained cubic phase without impurity after being calcined at moderate temperatures. The photocatalytic activities of Ag3PO4 were evaluated by photocatalytic oxygen evolution and methylene blue (MB) degradation, respectively. Based upon the analysis of XRD, UV–vis DRS, BET, XPS and electrochemical tests, the change of the physicochemical properties of Ag3PO4 was discussed and the possible mechanism for the enhanced photocatalytic activity was rationalized.
Section snippets
Materials
Silver nitrate (AgNO3, 99%) was furnished by Sigma–Aldrich. Sodium dihydrogen phosphate (NaH2PO4, 99.5%) and Methylene blue (C16H18ClN3S·3H2O, MB, 98.5%) were purchased from Tianjin Chemical Reagent Company. The high purity water (resistivity >18 MΩ cm) was used throughout the experiments. Fluorine doped tin oxide conductive glass (FTO) was purchased from Nippon Sheet Glass Company 5 (Japan) and was ultrasonic cleaned with acetone, ethanol and high purity water for 20 min in sequence before use.
Characterizations of Ag3PO4 photocatalysts
Fig. 1a shows powder XRD patterns of Ag3PO4 photocatalysts calcined at 200, 300, 400 and 500 °C for 1 h. It is obvious all samples display characteristic diffraction peaks (2θ) at 21.1°, 29.9°, 33.5°, 36.8°, 42.7°, 48.0°, 52.9°, 55.2°, 57.5°, 61.8°, 66.0°, 70.1°, 72.1°, and 74.0°. All diffraction peaks are in accord with cubic Ag3PO4 (JCPDS No.06-0505). No other peaks could be seen, indicating the pure phase of Ag3PO4 photocatalyst was obtained. It is notable that the peak intensity became
Conclusion
A facile high temperature calcination strategy has been developed to prepare high-crystallinity Ag3PO4 without introducing impurity. The high temperature can induce the improvement of crystallinity, the increase of oxygen vacancies, and the decrease of specific surface area. The improved crystallinity would facilitate the separation and transfer of charges, thus promoting the photocatalytic activities. The increased oxygen vacancies might act the active sits for O2 evolution and MB
Acknowledgments
This work was supported by the People's Government of Henan Province No. 15A150008, and the National Natural Science Foundation of China (NSFC, No. 51502078).
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