Bulk and interfacial engineering of Ta3N5 nanotube arrays by Sn(iv) doping, proper passivation and co-catalysts for efficient solar water oxidation

Abstract

Nowadays, photoelectrochemical (PEC) water oxidation is evaluated as an encouraging technology for the production of renewable H₂ fuels. The design or foundation of efficient photoelectrodes is regarded as one of critical keys to realize highly efficient devices for H₂ production. Due to their high light absorption, visible absorbing materials have been widely investigated to achieve high PEC performance. Recently, tantalum nitride (Ta₃N₅) with a band gap of ∼2.1 eV has been actively investigated because it can capture the entire visible light. In the present study, an n-type Ta₃N₅ nanotube (NT) film in a fluorinated-based electrolyte was developed by a two-time electrochemical anodization technique using a Ta foil. To improve the electronic conductivity and retard chemical dissolution, tin cations (Sn⁴⁺) were incorporated into the amorphous TaO_x NT, followed by a high-temperature treatment in a N₂/NH₃ gas mixture environment to obtain enhanced crystallinity and quality of films. Inductively coupled plasma-reactive ion etching (ICP-RIE) was performed to remove the blocking layer made on the top surface region after the electrochemical anodization reaction probably resulting from reaction remains and some oxide contaminants. The Sn-doped Ta₃N₅ NT photoelectrodes showed an improved photocurrent density (J_SC) of ∼1.49 mA cm⁻² at 1.23 V vs. RHE (reversible hydrogen electrode, briefly V_RHE), which was comparatively higher (∼0.84 mA cm⁻²) than that of a bare Ta₃N₅ NT film, which could be ascribed to the enhanced charge transfer/separation yield as well as the suppressed defect state. When transition metal sites were substituted by Sn doping, the electrical conductivity improved, promoting bulk charge separation and transfer with enhanced water reduction ability and a lower onset potential of Ta₃N₅ NTs. Moreover, to obtain a stable Sn-doped Ta₃N₅ NT photoelectrode, thin gallium nitride (GaN) was passivated onto a Ta₃N₅ NT film as a hole storage layer to prevent a quick electron–hole recombination process. As an alternative, anchoring a layer of Co(OH)_x co-catalyst on Ta₃N₅ NT substantially enhanced the photoelectrochemical (PEC) performance, showing a J_sc value of ∼4.58 mA cm⁻² at 1.23 V_RHE under solar light irradiation with more sustainable stability.