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Photocatalytic activity of graphitic carbon nitride (g-C3N4) was significantly improved by an alkaline hydrothermal treatment. The specific surface area of g-C3N4 obtained by heating melamine at 550 °C was only 7.7 m2 g−1, which was too small for it to be utilized as a catalyst for air purification. By the hydrothermal treatment with NaOH solution at 90–150 °C, the surface area was increased up to 65 m2 g−1, and the oxidation rate of nitrogen oxide (NO) under visible light (380 < λ < 480 nm) was increased by 8.6 times. XRD, ESR, elemental analysis and electron microscopy showed that unstable domains of not-well-ordered carbon nitride were removed by hydrolysis to form a mesoporous structure with a higher surface area. Deactivation of g-C3N4 was not observed during the experimental period, although a small part of carbon nitride was decomposed by self-oxidation.
Sano et al. (Tue,) studied this question.