Enhancing the solar water splitting activity of TiO₂ nanotube-array photoanode by surface coating with La-doped SrTiO₃

Abstract

Photoelectrochemical water splitting has been considered one of the most promising alternatives for the substitution of fossil fuels with a clean and renewable fuel, such as H₂. Two of the most studied photocatalysts for such application are titanium dioxide and strontium titanate due to their favourable photoactivity, robustness and availability; although specific limitations have hindered their large-scale implementations. In this investigation, we produced photoanodes comprising heterojunctions of TiO₂ nanotubes and lanthanum-doped SrTiO₃ for the photoelectrochemical split of water, under simulated solar irradiation. A thin layer of SrTiO₃ formed on the surface of TiO₂ nanotubes through a hydrothermal route was effective in promoting the electron-hole separation at the interface, leading to a photocurrent density enhancement from 58.9 to 109.4 μA cm⁻² (at 1.23 V vs RHE). The charge transfer resistance at the solid/liquid interface was substantially reduced, according to electrochemical impedance spectroscopy measurements. Further photoactivity increase was achieved by doping the SrTiO₃ overlayer with La³⁺. This is the first time the application of such La:SrTiO₃/TiO₂ nanocomposite in sustainable energy production is reported. The doping with 10 mol% of La³⁺ into the Sr²⁺ sites enhanced the photoelectrochemical response to 144.2 μA cm⁻² (1.23 V vs RHE) due to the suppression of deep trapping states related to formed SrO species.

Introduction

Globally, hydrogen is widely employed in ammonia synthesis, oil refining and chemical production. More recently, hydrogen has been used in fuel cell vehicles with high autonomy and zero carbon emission. However, hydrogen production is based on the reforming of methane, which is a fossil fuel. Harnessing and storing solar energy in hydrogen molecules through the artificial photosynthesis has been arising as a promising method to supply the global clean energy needs [1]. TiO₂ and SrTiO₃ are the most studied semiconductor materials for overall water splitting, howbeit, these materials present problems related to the charge carrier recombination and low charge carrier mobility [[2], [3], [4], [5], [6]].

Nanostructuring of semiconductor materials, heterojunction formation and doping hold the key to improve the efficiency of artificial photosynthesis to split water into H₂ and O₂. In particular, one-dimensional nanostructure, such as nanotubes, is a promising geometry to improve the charge mobility by allowing a preferential pathway to charge carrier mobility [7,8]. Template-assisted, hydrothermal and anodization methods are the most common chemical routes used to produce semiconductor nanotubes [[9], [10], [11]]. In special, the anodization method is one the most prominent approach to produce semiconducting nanotubes for photoelectrochemical water splitting [8,[11], [12], [13], [14]].

Despite the simplicity of the anodization method to produce 1D geometry, the chemical composition of the semiconductor oxide nanotubes depends exclusively on the composition of the metallic substrate. Thus, the combination of the anodization method with another methodology can produce heterojunction keeping the 1-D geometry. Liu et al. produced a heterojunction between TiO₂ and Fe₂TiO₅ using electrodeposition as a second-step [13]. Also, Zhang et al. produced a heterojunction between TiO₂ and SrTiO₃ using hydrothermal treatment. As a matter of fact, anodization and hydrothermal synthesis have enormous potential to produce heterojunction with 1-D nanostructures [[15], [16], [17], [18]]. Herein, we synthesized a La³⁺ doped SrTiO₂/TiO₂ nanotubes heterojunction by anodization and hydrothermal treatment. Furthermore, we determined the presence of defects resulting from the employed synthesis method and how to mitigate them by La³⁺ doping.