Eosin Y-sensitized graphitic carbon nitride fabricated by heating urea for visible light photocatalytic hydrogen evolution: the effect of the pyrolysis temperature of urea

Abstract

Graphitic carbon nitride (g-C₃N₄) was prepared by pyrolysis of urea at different temperatures (450–650 °C), and characterized by thermogravimetric and differential thermal analysis (TG-DTA), elemental analysis (C/H/N), X-ray diffraction (XRD), UV-vis diffuse reflectance spectra (DRS), Brunauer–Emmett–Teller (BET) analysis, Fourier transform-infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) spectra. The samples prepared at low temperatures (450 and 500 °C) are a mixture of g-C₃N₄ and impurities, whereas the samples prepared at high temperatures (550, 600 and 650 °C) should be g-C₃N₄ (polymeric carbon nitride). The polymerization degree of g-C₃N₄ for the prepared samples increases to a maximum at 600 °C with increasing pyrolysis temperature and then decreases, whereas the defect concentration changes conversely, that is, g-C₃N₄ prepared at 600 °C has the lowest defect concentration. Using Eosin Y (EY) and the prepared sample as the sensitizer and the matrix, respectively, the photocatalytic activity for hydrogen evolution from aqueous triethanolamine solution was investigated. The g-C₃N₄ prepared at 600 °C exhibits the highest sensitization activity. Under optimum conditions (1.25 × 10⁻⁵ mol L⁻¹ EY and 7.0 wt% Pt), the maximal apparent quantum yield of EY-sensitized g-C₃N₄ prepared at 600 °C for hydrogen evolution is 18.8%. The highest activity can be attributed to the pure composition, the higher dye adsorption amount and the lowest defect concentration.