A simple green approach to synthesis of sub-100 nm carbon spheres as template for TiO₂ hollow nanospheres with enhanced photocatalytic activities

Carbon spheres (CSs) have attracted great attention given their wide applications in bio-diagnostics, photonic band-gap crystals and drug delivery, etc. The morphology and size of CSs greatly affect their performances and applications. Herein, we report a green and catalyst-free hydrothermal carbonization (HTC) method to synthesize CSs with glucose as carbon precursor. The diameter of CSs can be tuned within a wide range from 450 to 40 nm by controlling the glucose concentration, reaction time and temperature. Using the obtained CSs as template, hollow TiO₂ nanospheres (HTNSs) with controllable diameters are prepared via a sol-gel method. As photocatalysts for hydrogen generation, the photoactivity of the HTNSs shows strong dependence upon size, and is much higher than that of solid TiO₂. With particle size decreasing, the photoactivity of the obtained HTNSs gradually increases. Without any co-catalyst, the highest photocatalytic hydrogen generation activity is obtained with HTNSs of 40 nm in diameter, which exceeds that of solid TiO₂ and commercial P25 by 64 times and 3 times, respectively.