Heteroepitaxial Growth of Narrow Band Gap Carbon-Rich Carbon Nitride Using In Situ Polymerization to Empower Sunlight-Driven Photoelectrochemical Water Splitting

We describe an <i>in situ</i>-polymerized conformal thin layer coating of narrow band gap carbon-rich carbon nitride (NBG-CRCN) on titania nanorod arrays to design a binary semiconductor heterojunction photocatalyst. The <i>in situ</i> polymerization creates a strong interaction between the TiO₂ nanorod substrate and the carbon nitride film, which prevents leaching of CRCN in liquid electrolytes. A unique aspect of our work is developing an easy and inexpensive technique for the heteroepitaxial growth of mechanically and photochemically stable carbon nitride thin films with intimate contact at the CN:TNR heterojunction interface. This method aids in overcoming one of the main problems with carbon nitride (CN), namely, the inability to produce an evenly distributed CN coating on a substrate. The synthesized NBG-CRCN@TNR extends the visible light absorption to 700 nm (E_g = 1.7 eV) and red-shifts the photoluminescence (PL) emission peak to 580 nm. The peak shifts and broadening in the Raman spectra of the NBG-CRCN@TNR hybrid compared to those in TNR confirm an unusually strong interaction between TiO₂ and NBG-CRCN. An easy and inexpensive technique to heteroepitaxially grow CRCN (002) on rutile TiO₂ (110) is confirmed by advanced characterization. High-resolution transmission electron microscopy (HRTEM), selected-area electron diffraction (SAED), and grazing-incidence wide-angle X-ray scattering (GIWAXS) suggest the heteroepitaxial growth of (002) CRCN on rutile TiO₂ (110). Under AM1.5G solar illumination, the NBG-CRCN@TNR hybrid shows superior performance in photoelectrochemical water splitting, generating a photocurrent density as high as 4.3 mA cm^-2 in 1 M KOH under 0.6 V external bias, rising to 8.4 mA cm^-2 in the presence of a hole scavenger (methanol). An impressive hydrogen evolution rate of 26.51 μmol h^-1 with 88.12% Faradaic efficiency is recorded. Establishing a high-quality interface between g-C₃N₄ and titania permits effective charge carrier separation, leading to enhanced photocatalytic activity.