Enhanced Photocatalytic Activity of Phosphorus-Doped Graphitic Carbon Nitride Under Visible Light Irradiation

Abstract

INTRODUCTION In the current age, the development and utilization of environment-friendly energy resources have become the major concerns because of the depletion of fossil fuel and the severe global environmental pollution.(1) Recently, a metal-free photocatalyst graphitic carbon nitride (g-C3N4) has attracted intensive interest for its promising applications in splitting water to produce H2, decomposition of organic pollutants, and organic synthesis under visible light.(2) The g-C3N4 attractive intensive attention since Wang et al. have reported its photocatalytic ability to produce hydrogen from a methanol aqueous solution under visible-light irradiation. It has been expected that photocatalysts should exhibit the merits of stability, earth-abundance, nontoxity, and visibl-light absorbance. However, the speed of electrons-hole recombination in the g-C3N4is very fast, therefore photocatalytic activity is very low. It is known that metal/nonmetal co-doped photocatalyst enhances photocatalytic activity. And, Nonmetal doping can increase the visible light absorption, modify the carrier mobility of catalysts and modulate the electronic structure of catalysts.(3) And by morphology control, exotic optoelectronic properties and high specific surface areas have been obtained. In this study, phosphorus was chosen as dopant into g-C3N4. And, the valence band position of g-C3N4 shifted negatively via phosphorus doping. Then phosphorus-doped g-C3N4 (p/g-C3N4) was further calcinated at 500 °C for 10 h,. Finally, we obtained T-P/g-C3N4photocatalyst. This modification provided materials with enhanced optical absorption, high surface area, and accelerated charge carriers transfer rate as well as the increased photocatalytic activities for RhB. The photocatalyst was characterized by XRD, BET measurements, PL, UV-DRS, XPS, SEM, TEM and its photocatalytic activities were evaluated by degradation of RhB under visible light irradiation. This work proposed a simple and low-cost method to synthesize the phosphorus doping catalyst, which had great potentials for application in treatment of environmental pollution. EXPERIMENTAL Preparation of photocatalysts P/g-C3N4 photocatalysts were prepared by thermal polymerization of melamine and NH4H2PO4 in atmosphere. The preparation of P/g-C3N4 photocatalysts was as follow: 5.0 g of melamine and (0.1, 0.2, 0,3, 0,4 g of)NH4H2PO4 was put in a Muffle Furnace and heated to 550 °C for 4 h to complete the reaction. The as-prepared P/g-C3N4 was further calcinated at 500 °C for 10 h, leading to T-P/g-C3N4. Photodegradation of RhB For evaluation of photocatalytic activity, we examined photodegradation of RhB. 30 mg of g-C3N4was added into 5 ppm RhB (30 mL). Then, the solution was stirred until adsorption-desorption equilibrium using magnetic stirrer. A Xe lamp (420≦λ≦800 nm) was applied as light source. The irradiation time was 1 h. After irradiation, we measured degradation percentage by using UV-visible spectrophotometer. RESULTS AND DISCUSSION Photodegradation of RhB The pure g-C3N4 and T-P/g-C3N4 decomposed 40 % and 100 % of RhB under visible light after 60 min, respectively. T-P/g-C3N4 with 0.2 g of NH4H2PO4had highest photocatalytic activity for the degradation of RhB under visible light irradiation. CHARACTERIZATIONS XRD BET measurements PL UV-DRS XPS SEM TEM UV-vis spectrophotometer CONCLUSIONS The g-C3N4 with phosphorus-doping and additional thermal process showed high photocatalytic activity for degradation of RhB under visible light irradiation. Photodegradation rate of RhB over T-P/g-C3N4 was about 3.7 times greater than that of the pristine one. The enhanced photocatalytic activity of T-P/g-C3N4in RhB degradation could be attributed to several reasons; (Ⅰ) The greatly increased surface area, (Ⅱ) The improved photogenerated charge carrier transportation and separation, (Ⅲ) The negatively shifted valance band. REFERENCES (1) A. Yujiao Zhong, B. Zhiqiang Wang, C. Jianyong Feng, D. Shicheng Yan, E. Haitao Zhang, F. Zhaosheng Li, G. Zhigang Zou, Improvement in photocatalytic H2 evolution over g-C3N4prepared from protonated melamine, Applied Surface Science 295 (2014) 253– 259 (2) A. Hongjian Yan, B. Haoxin Yang, TiO2–g-C3N4 composite materials for photocatalytic H2evolution under visible light irradiation, Journal of Alloys and Compounds 509 (2011) L26–L29 (3) A. Mo Zhang, B. Xiaojuan Bai, C. Di Liu, D. Jun Wang, E. Yongfa Zhu Enhanced catalytic activity of potassium-doped graphitic carbon nitride induced by lower valence position Applied Catalysis B: Environmental 164 (2015) 77–81