Photoelectrochemical properties of “increasingly dark” TiO2 nanotube arrays

Abstract

Black" TiO2 nanotube arrays (TNTAs) of different darkness are formed by thermal reduction of anodic TiO2 nanotube layers in H2 atmosphere at different temperatures (300-500°C), and their (photo-)electrochemical properties are studied by IPCE spectroscopy, transient photocurrent measurements, IMPS, PEIS, and Mott-Schottky analysis. Hydrogenation of the TNTAs leads to the enhancement of their IPCE (by up to 1.5 times), despite increased recombination losses at anodic potentials. Reduced TNTAs demonstrate a "metallic" behavior by a characteristic frequency response of their impedance and phase angle, reduced charge transfer resistance at anodic bias, increased double layer capacitance, and remarkably increased donor densities (up to 7.25 × 10 23 cm-3). The observed alterations in the properties of TNTAs after hydrogenation are ascribed to the formation of defects and TiHx.

Galvanic Deposition of Pt Nanoparticles on Black TiO2 Nanotubes for Hydrogen Evolving Cathodes

“…Various synthetic routes have been reported to synthesize nanostructured black titania,[ 2 , 4 , 21 ] such as hydrogenation (H 2 , H 2 /Ar, H 2 /N 2 at high/ambient pressure, H 2 plasma),[ 18 , 22 , 23 , 24 , 25 , 26 , 27 ] chemical reduction (by Mg, Zn, Li, Al, NaBH 4 , CaH 2 ),[ 5 , 11 , 13 , 15 , 28 , 29 ] laser ablation/pulsed laser ablation, and electrochemical reduction. [22] Also, microwave irradiation, ultrasonication, [30] and one‐pot gel combustion have been reported in the literature[ 2 , 3 , 21 ] for the production of black titania.…”

Thermal Ramping Rate during Annealing of TiO2 Nanotubes Greatly Affects Performance of Photoanodes

“…However, the crystalline structure can be influenced by intrinsic point/structural defects [ 24 ] that play a crucial role in controlling the physical properties, especially while dealing with nanometer‐sized tubes. [ 25 ] In addition to the nanometer dimension, an ideal geometry is also an important factor for high surface area, long‐term stability, and generation of charge carriers with directional charge transport. Therefore, various methods have been reported for the synthesis of 1D TiO 2 nanotubular geometry.…”

DC sputter deposited TiO2 layers on FTO: towards a maximum photoelectrochemical response of photoanodes

“…In more recent years, 1D or 3D geometries, such as tubes, wires, rods, flakes, etc. of TiO 2 have become extensively investigated [3,[9][10][11][12][13][14][15][16][17][18][19][20][21]. Compared to bulk or compact TiO 2 layers, electrodes made of 3D assemblies of nanostructured TiO 2 are widely considered more effective electrodes as they provide a higher surface area, a higher number of reaction sites, and the nanoscale geometries may allow for orthogonal (beneficial) electron/hole separation [22].…”

“…In view of the electrode geometry, typical IPCE data (acquired under moderate anodic bias and in the UV range) are 5-20% for compact films [9,21,26]. For 3D structures, typical IPCE values are reported as 15-35% for nanotubes [9][10][11]21], 40-65% for nanoflakes [18,27], 15-30% for nanowires [1,22], and 2-8% for compacted nanoparticle photoanodes [9,29,30]. The better IPCE performance for the 3D structures often is ascribed to the higher surface area of the corresponding geometries.…”