Microwave-assisted self-doping of TiO₂ photonic crystals for efficient photoelectrochemical water splitting

In this article, we report that the combination of microwave heating and ethylene glycol, a mild reducing agent, can induce Ti³⁺ self-doping in TiO₂. A hierarchical TiO₂ nanotube array with the top layer serving as TiO₂ photonic crystals (TiO₂ NTPCs) was selected as the base photoelectrode. The self-doped TiO₂ NTPCs demonstrated a 10-fold increase in visible-light photocurrent density compared to the nondoped one, and the optimized saturation photocurrent density under simulated AM 1.5G illumination was identified to be 2.5 mA cm^-2 at 1.23 V versus reversible hydrogen electrode, which is comparable to the highest values ever reported for TiO₂-based photoelectrodes. The significant enhancement of photoelectrochemical performance can be ascribed to the rational coupling of morphological and electronic features of the self-doped TiO ₂ NTPCs: (1) the periodically morphological structure of the photonic crystal layer traps broadband visible light, (2) the electronic interband state induced from self-doping of Ti³⁺ can be excited in the visible-light region, and (3) the captured light by the photonic crystal layer is absorbed by the self-doped interbands.