Insights into the synthesis and characterization of thiosemicarbazide and urea-based novel isotype II heterojunctions and their competing physicochemical properties in photochemical hydrogen production

Abstract

Optimizing surface area, light absorption, and band-gap energy of graphitic carbon nitride (g-CN) is crucial for developing efficient hydrogen production photocatalysts. Here, a mass-ratio controlled mix-and-match protocol with precursors, urea and thiosemicarbazide (U and T), is introduced for in situ synthesis of g-CN isotype II heterojunctions (UT-CNs). The specific surface area (SSA), band-gap energy, and product yield of heterostructures were highly mass-ratio-dependent; however, there was no clear systematic trend between the mass ratios and the physicochemical properties. Notably, the heterojunction, synthesized with a low U-ratio (UT3-CN), exhibited a similar to 52 % increase in SSA and similar to 44 % increase in pore volume compared to bulk T-CN, while light absorption and band-gap energy remained largely unchanged. Despite a 10 % increase in yield compared to bulk U-CN, UT3-CN's SSA decreased by similar to 40-50 % at higher U-ratios, and light absorption and yields improved. UT3-CN, with a band gap of similar to 2.88 eV, showed three times higher photocatalytic activity for H-2 evolution than T-CN (similar to 2.85 eV) and UT-CN with the narrowest band gap (similar to 2.59 eV), attributed to high SSA and pore volume, well-ordered structural domains, reduced interlayer spacing, and enhanced charge separation with minimal recombination. The emergence of a sharp X-ray diffraction peak at ca. 2 theta = 43.5 degrees in the heterojunctions suggested that structural ordering was influenced by high ammonia gas concentrations and dithiourazole intermediates during thermal decomposition. These findings highlighted the need for mechanistic studies on T decomposition, and this versatile approach offers a promising strategy for fine-tuning photocatalytic properties using a library of g-CN precursors.