Study of photoelectrochemical water splitting using composite films based on TiO₂ nanoparticles and nitrogen or boron doped hollow carbon spheres as photoanodes

Novel composite films based on TiO₂/hollow carbon spheres were used as photoanodes in photoelectrochemical water splitting studies. CVD grown pristine hollow carbon spheres (HCSs), N-doped HCSs (NHCSs) and B-doped HCSs (BHCSs) were used to prepare composites with TiO₂ nanoparticles. The presence of B and N doping in these carbonaceous materials was confirmed by X-ray photoelectron spectroscopy (XPS). The composite films were characterized by X-ray diffraction, UV–vis spectroscopy and electron microscopy techniques. In the photoelectrochemical studies, the influence of the addition of NHCSs or BHCSs in the TiO₂ matrix was evaluated and compared with a pristine TiO₂ film. The TiO₂/NHCS (0.5 wt%) composite electrode exhibited a 2.4 increase in anodic current density compared to the pure TiO₂ electrode; it also showed an enhanced incident photon to current efficiency (IPCE) and good photoelectrochemical stability over time. The EIS analysis at open circuit conditions showed that the irradiated TiO₂/NHCS electrode exhibited smaller resistance values (R₁ = 43 Ω and R₂ = 3.1 kΩ) than the irradiated TiO₂/BHCS electrode (3.3 kΩ and 10 kΩ). From the experimental results, we propose a photocatalytic mechanism for the NHCS/TiO₂ and BHCS/TiO₂ composites as a photoanode. The presence of p-type BHCSs between the n-type TiO₂ nanoparticles impairs the photocatalytic processes by the formation of a p-n diode with enhanced resistance and recombination. On the other hand, an appreciable enhancement of photocurrent density was achieved for the NHCS/TiO₂ composite films due to a combination of their electronic properties, crack free compact film morphology and enhanced light absorption.