Effects of carbon dopants concentration on $TiO_2$ nanotube arrays for photoelectrochemical water splitting

Abstract

There are various approaches to replace existing coal fuel. One of famous studies is using solar energy for generating hydrogen fuel through water splitting due to abundancy and high intensity of solar light without contaminating environment. However, water splitting is a non-spontaneous reaction to occur under solar energy in the standard condition. Therefore, lots of studies on photoelectrochemical (PEC) water splitting with semiconductor or metal/semiconductor composite are suggested. $TiO_2$ is one of materials, which can absorb solar light and generate electron and hole for water splitting reaction with proper energy states and stability in a solution. However, $TiO_2$ have efficiency problems of absorbing only ultra-violet due to large band gap and fast recombination of photogenerated electron and hole. In this study, $TiO_2$ nanotube arrays are used to enlarge their active surface area and to elongate charge carriers' diffusion length that suppress charge recombination for higher efficiency. Also, dopant (or defect) levels are introduced into $TiO_2$ band gap to improve photocatalytic activity by a carbon doping. Dopant levels lie above the valence band of $TiO_2$, and help absorbing visible light and form oxygen vacancies. Oxygen vacancies can suppress the recombination of photo-generated electron and hole. Thus carbon dopants can give extra available charge carriers to the system. The carbon dopant concentrations were varied to observe quantitative effects of dopants. Additionally to characterizations of samples, current density and IPCE were tested with three electrode cell under simulated solar light. Through the study, it is examined that concentrations affect dopant level formation and sufficient amount is needed for photocatalytic activity improvement. This is well coincided with several theoretical studies which state that carbon dopants concentration is an important factor for improvement of $TiO_2$ efficiency for PEC water splitting.