Photocatalytic conversion of CO₂ into methanol using graphitic carbon nitride under solar, UV laser and broadband radiations

Photocatalytic conversion of carbon dioxide into methanol using highly efficient g-C₃N₄, in conjunction with three different radiations (solar radiation, broad-band ultraviolet (UV)–visible lamp, and laser beam) is presented. The optical, structural, and morphological properties of the synthesized g-C₃N₄ were studied using advanced analytical techniques like Fourier transform infrared spectroscopy, UV–visible spectrometer, X-ray diffraction, high-resolution transmission electron microscopy, high-angle annular dark field, and X-ray photoelectron spectroscopy. The relative merits of the three sources of radiation in the presence of g-C₃N₄ were studied in terms of key figures of merit of the photocatalytic process, namely, methanol production yield and quantum yield. As expected, after 40 min of irradiation, 355-nm laser (40 mJ/pulse, 10 Hz) with g-C₃N₄ rendered the best methanol production yield (510 μmol g⁻¹ h⁻¹), followed by solar radiation (130 μmol g⁻¹ h⁻¹), and UV broadband lamp. This indicates that the photon flux and the spectral properties of incident light are the key factors for the enhancement of methanol production yield. Although the methanol production yield with 355-nm laser radiation is quite impressive because of the inherent high photon flux and the monochromatic nature of laser, the methanol yield of 130 μmol g⁻¹ h⁻¹ with natural sunlight is quite an important result, as it can be used for the development of large-scale solar fuel generation facilities by harnessing the naturally abundant solar radiation.