Synthesis of Ta3N5 Nanotube Arrays Modified with Electrocatalysts for Photoelectrochemical Water Oxidation

Tantalum nitride (Ta3N5) is a promising material for photoelectrochemical (PEC) water oxidation with a narrow band gap (2.1 eV) that can effectively utilize visible light in the solar spectrum. Ta3N5 nanotube (NT) arrays were synthesized on a Ta foil by electrochemical anodization followed by an ammonia treatment at 800 °C. The photocurrent of nanostructured Ta3N5 was over 3 times higher than that of a dense regular Ta3N5 film in 0.1 M Na2SO4 aqueous solution at pH 11. Several electrocatalysts (IrO2 nanoparticles (NPs), Co3O4 NPs, cobalt phosphate, and Pt NPs) were used to modify Ta3N5 NTs for PEC water oxidation. The photocurrent of Ta3N5 NTs modified with IrO2 and Co3O4 was ca. four times higher than that of unmodified NTs. Cobalt phosphate also showed a positive improvement for PEC water oxidation on Ta3N5 NTs, whereas Pt was ineffective. Scanning electrochemical microscopy was used to measure the faradaic efficiency of the Ta3N5 photoanodes for water oxidation, which can reach as high as 88% for a Co3O4−Ta3N5 NTs photoanode, but is less than 15% at best, for Ta3N5 without the electrocatalyst. The results indicate that cobalt oxide and cobalt phosphate are promising candidates as electrocatalysts on Ta3N5 for water oxidation because Co is an earth-abundant material. ■ INTRODUCTION We report the photoelectrochemical (PEC) performance of Ta3N5 nanotube (NT) arrays modified with several electrocatalysts for water oxidation. IrO2 nanoparticles (NPs), Co3O4 NPs, cobalt phosphate, and Pt NPs were prepared as electrocatalysts of Ta3N5 NTs for water oxidation. The generation of renewable clean energy is one of the most profound challenges of the 21st century. Water splitting driven by solar energy into hydrogen and oxygen is a promising process for achieving renewable clean energy. Various semiconductors have been developed since Fujishima and Honda first suggested water splitting with TiO2 under UV illumination in 1972, and TiO2 is one of the most studied semiconductors. However, TiO2 has a large band gap (3.0 eV for rutile and 3.2 eV for anatase) and can only perform efficiently under UV irradiation. Stable photocatalysts responsive to visible light are still few in number. Recently, transitional metal (oxy)nitrides have attracted considerable attention as a new type of visible light-driven photocatalyst. Among the metal (oxy)nitrides, tantalum nitride (Ta3N5) shows great promise with a narrow band gap (2.1 eV) that can utilize up to 600 nm visible light. In addition, its conduction band edge and valence band edge positions are suitable for water splitting. Ta3N5 powders and NPs have been developed for water oxidation in the presence of sacrificial reagents. A 10% quantum efficiency for Ta3N5 under visible light irradiation (420 nm < λ < 600 nm) was obtained in a 0.01 M AgNO3 aqueous solution. 16 Most recently, Yokoyama et al. have prepared a Ta3N5 thin film using a reactive sputtering technique. The anodic photocurrent of the film after NH3 treatment increased by ca. 10 times relative to the untreated electrode at both 0.0 and 0.5 V versus Ag/AgCl in an aqueous solution containing Fe(CN)6 /Fe(CN)6 4‐ as a redox couple. In addition to physicochemical properties, the PEC performance of a material also depends on the molecular-scale architecture and the nature of the active sites (cocatalyst). A NT array architecture can provide large surface area and sufficient lengths to effectively capture incident irradiation and improve the separation of photogenerated charge carriers. Highly ordered metal oxide NTs on Ti, Fe, Ta, Ti−Ru alloy, and Ti−Fe alloy have been successfully fabricated by the anodization method as a photocatalyst for solar energy applications. Feng et al. have prepared highly oriented Ta3N5 NT films by electrochemical anodization of a Ta foil followed by nitridation. The incident photon-to-current conversion efficiency (IPCE) at a wavelength 450 nm for 240 nm long Ta3N5 NTs reached 5.3% in 1 M KOH solution with Received: May 4, 2012 Revised: June 1, 2012 Published: June 1, 2012 Article