In this work, we propose a solution-based carbon precursor coating and subsequent carbonization strategy to form a thin protective carbon layer on unstable semiconductor nanostructures as a solution to the commonly occurring photocorrosion problem of many semiconductors. A proof-of-concept is provided by using glucose as the carbon precursor to form a protective carbon coating onto cuprous oxide (Cu₂O) nanowire arrays which were synthesized from copper mesh. The carbon-layer-protected Cu₂O nanowire arrays exhibited remarkably improved photostability as well as considerably enhanced photocurrent density. The Cu₂O nanowire arrays coated with a carbon layer of 20 nm thickness were found to give an optimal water splitting performance, producing a photocurrent density of −3.95 mA cm^–2 and an optimal photocathode efficiency of 0.56% under illumination of AM 1.5G (100 mW cm^–2). This is the highest value ever reported for a Cu₂O-based electrode coated with a metal/co-catalyst-free protective layer. The photostability, measured as the percentage of the photocurrent density at the end of 20 min measurement period relative to that at the beginning of the measurement, improved from 12.6% on the bare, nonprotected Cu₂O nanowire arrays to 80.7% on the continuous carbon coating protected ones, more than a 6-fold increase. We believe that the facile strategy presented in this work is a general approach that can address the stability issue of many nonstable photoelectrodes and thus has the potential to make a meaningful contribution in the general field of energy conversion.

Hetero-element doping (e.g., N, F, C) of TiO₂ is inevitably accompanied by significantly increased structural defects due to the dopants' nature being foreign impurities. Very recently, in situ self-doping with homo-species (e.g., Ti³⁺) has been emerging as a rational solution to enhance TiO₂ photoactivity within both UV and visible light regions. Herein we demonstrate that conventional electrochemical reduction is indeed a facile and effective strategy to induce in situ self-doping of Ti³⁺ into TiO₂ and the self-doped TiO₂ photoelectrodes showed remarkably improved and very stable water splitting performance. In this study, hierarchical TiO₂ nanotube arrays (TiO₂ NTs) were chosen as TiO₂ substrates and then electrochemically reduced under varying conditions to produce Ti³⁺ self-doped TiO₂ NTs (ECR-TiO₂ NTs). The optimized saturation photocurrent density and photoconversion efficiency on the ECR-TiO₂ NTs under simulated AM 1.5G illumination were identified to be 2.8 mA cm⁻² at 1.23 V vs. RHE and 1.27% respectively, which are the highest values ever reported for TiO₂ based photoelectrodes. The electrochemical impedance spectra measurement confirms that the electrochemical induced Ti³⁺ self-doping improved the electrical conductivity of the ECR-TiO₂ NTs. The versatility and effectiveness of the electrochemical reduction method for Ti³⁺ self-doping in P25 based TiO₂ was also examined and confirmed.