Clarification of Photoelectrochemical Oxygen Evolution Sites in TiO₂ Nanotube Array Electrodes by PbO₂ Deposition Method

Abstract

TiO2 nanotube (TNT) photoanodes exhibit activity and stability for the oxygen evolution reaction (OER) by photoelectrochemical water oxidation. However, the location of the OER site by the photogenerated holes has not been clarified for the TNT photoanodes, unlike well-studied macrocrystalline photocatalysts. In this study, we performed reactions of TNT photoanodes in a 0.1 M Pb(NO3)2 under UV irradiation. The photoelectrochemically deposited PbO2 particles were observed through scanning electron microscopy in the backscattered electron mode. We found that β-PbO2 was deposited on the nanotubes with photocurrent decay and that the reaction site was located on the upper part (~1 µm) of the TNT array with ~3 µm length. The photocurrent decay implies the selective deposition of PbO2 on the catalytic site for water oxidation. The PbO2 nanoparticles were deposited on the inner and outer surfaces of the tube walls. This result is consistent with the mechanism of charge separation at the space charge layers formed on both surfaces of the walls. We also confirmed that the OER site changes depending on the wavelength of the incident light due to the change in the light penetration depth.

Porous Transport Photoelectrodes Fabricated on Felt Substrates and Applications to Polymer Electrolyte Photoelectrochemistry

“…Tsampas et al [17,18] and Amano et al, [7,9,19] separately reported the preparation of TiO 2 nanotube arrays (TNTAs) on 3D porous Ti-felt substrate by electrochemical anodization and subsequent annealing in air/O 2 , as depicted in Figure 3A. Tsampas et al conducted anodization at 30 V using the electrolyte (ethylene glycol containing 0.3 wt % ammonium fluoride and 2 vol % water) pre-aged with a dummy sample for 10 h to minimize cracks and the detachment of TNTA.…”

“…[5,6] Several porous photoanodes (ntype TiO 2 , WO 3 , BiVO 4 , Fe 2 O 3 , and SrTiO 3 ) and photocathodes (ptype CuInS 2 and Cu 2 O) have been prepared on porous felt substrates composed of carbon and titanium microfibers. [4][5][6][7][8][9][10][11][12][13] Carbon microfibers could be oxidatively decomposed during thermal treatments and photoanodic reactions. In contrast, titanium microfibers have better tolerance under oxidative conditions compared with carbon microfibers.…”

“…[21] A comparison of the 3D Ti felt and 2D Ti plate revealed that the porous TNTAs/Ti felt exhibited a higher IPCE than the flat TNTAs/Ti plate in both neutral and acidic solutions (nanotube length: 3 μm). [9] The reaction sites in the TNTAs were visualized by scanning electrochemical cell microscopy. [19] PEC-deposited PbO 2 particles revealed that reaction sites were located on the inner and outer surfaces of the tube walls, and depended on the wavelength, which affects the penetration depth of the light.…”

“…[19] PEC-deposited PbO 2 particles revealed that reaction sites were located on the inner and outer surfaces of the tube walls, and depended on the wavelength, which affects the penetration depth of the light. [9] Gas-phase PEC-performance data for TiO 2 /Ti-felt photoanodes were reported at RH < 100 %. The surface of the photoanode needs to be coated with a perfluorosulfonic acid (PFSA) ionomer for vapor-fed PEC water-splitting applications in the absence of liquid condensation.…”

Photoelectrochemical C–H activation of methane to methyl radical at room temperature

“…Ti felt was used as the conductive substrate for the gasdiffusion photoanodes. The WO 3 electrode was prepared by dip coating with an aqueous solution of (NH 4 ) 6 H 2 W 12 O 40 and polyethylene glycol; the electrode was then calcined at 923 K for 2 h. 7,28,40 The TiO 2 electrode was prepared by the anodization of Ti felt in ethylene glycol with 0.25 wt% NH 4 F and 10 vol% H 2 O at 50 V for 3 h. 29,41,42 The anodized Ti felt was calcined at 823 K for 1 h to crystallize into anatase TiO 2 . The photoanodes were modified using a Nafion ionomer dispersion (Sigma-Aldrich).…”