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.

Superhydrophilic membranes have drawn much attention owing to their outstanding anti-fouling performance and ultrahigh permeation flux for wastewater treatment and oil–water separation. Since most widely used polymer membranes have high intrinsic hydrophobicity, a universal approach for superhydrophilic modification is highly required. Yet, how to simply transform highly hydrophobic membranes into superhydrophilic ones is still a challenge. Herein, we develop a one-step and general strategy to achieve the hydrophobic-to-superhydrophilic transformation of commercial membranes on the basis of catechol chemistry, i.e., co-deposition of tannic acid (TA) and 3-aminopropyltriethoxysilane (APTES) in aqueous solution. Owing to the distinct adhesion properties of TA and the reaction between the oxidative product of TA and the hydrolysis product of APTES, hydrophilic and hierarchical layer-colloidal nanospheres can be in situ assembled on various highly hydrophobic membranes including polyvinylidene fluoride (PVDF), polypropylene (PP), polytetrafluoroethylene (PTFE), copper mesh, stainless steel wire, and nylon mesh. The resulting superhydrophilic membrane can realize high-efficiency separation of various oil-in-water emulsions.