Effective improvement of the photocatalytic efficiency of g-C^sub 3^N^sub 4^ by a green hydrothermal treatment method

An environmental-friendly, low cost hydrothermal treatment method was attempted to improve the photocatalytic efficiency of g-C3N4. The hydrothermal treatment of g-C3N4 was implemented in deionized water at 190 °C for 8 h. The characterization of the hydrothermally treated g-C3N4 (HT-g-C3N4) by X-ray diffraction, transmission electron microscopy, N2adsorption-desorption isotherms, UV-vis diffuse reflection spectra, photocurrent response, electrochemical impedance spectroscopy and Zeta potential measurement revealed that: (i) HT-g-C3N4 still had graphite-like structure; (ii) HT-g-C3N4 had much smaller particle size and exceptionally larger specific surface area than g-C3N4; (iii) the absorption edge of HT-g-C3N4 was blue-shifted, compared with that of g-C3N4; (iv) HT-g-C3N4 had far larger photocurrent and far lower interfacial charge transfer resistance than g-C3N4; (v) the surface charge of HT-g-C3N4 was positive, whereas that of g-C3N4 was negative. Furthermore, the photocatalytic tests showed that HT-g-C3N4 had notably higher photocatalytic activity than g-C3N4 in the reduction of aqueous Cr(VI) under visible light (λ>420 nm) irradiation. The reasons for the improved photocatalytic efficiency of HT-g-C3N4 were proposed.