Photoanodes based on TiO₂and α-Fe₂O₃for solar water splitting – superior role of 1D nanoarchitectures and of combined heterostructures

Abstract

Solar driven photoelectrochemical water splitting (PEC-WS) using semiconductor photoelectrodes represents a promising approach for a sustainable and environmentally friendly production of renewable energy vectors and fuel sources, such as dihydrogen (H). In this context, titanium dioxide (TiO) and iron oxide (hematite, α-FeO) are among the most investigated candidates as photoanode materials, mainly owing to their resistance to photocorrosion, non-toxicity, natural abundance, and low production cost. Major drawbacks are, however, an inherently low electrical conductivity and a limited hole diffusion length that significantly affect the performance of TiO and α-FeO in PEC devices. To this regard, one-dimensional (1D) nanostructuring is typically applied as it provides several superior features such as a significant enlargement of the material surface area, extended contact between the semiconductor and the electrolyte and, most remarkably, preferential electrical transport that overall suppress charge carrier recombination and improve TiO and α-FeO photoelectrocatalytic properties. The present review describes various synthetic methods and modifying concepts of 1D-photoanodes (nanotubes, nanorods, nanofibers, nanowires) based on titania, hematite, and on α-FeO/TiO heterostructures, for PEC applications. Various routes towards modification and enhancement of PEC activity of 1D photoanodes are discussed including doping, decoration with co-catalysts and heterojunction engineering. Finally, the challenges related to the optimization of charge transfer kinetics in both oxides are highlighted.

Perovskite solar cell for photocatalytic water splitting with a TiO2/Co-doped hematite electron transport bilayer

“…Hematite (a-Fe 2 O 3 ) based photoanodes are ideal for such applications, due to their stability, abundance and low cost. 1,2 In spite of these encouraging properties, progress towards the manufacture of useful water splitting devices has been limited. The free energy change required to split one molecule of H 2 O to H 2 and 1/2O 2 under standard conditions is 237.2 kJ mol À1 while the cell voltages are in the order of 1.8-2.0 V. 3,4 This potential requirement could be attained from a semiconductor photoanode with appropriate valence and conduction bands illuminated by visible light.…”

Effect of Ionic Compounds of Different Valences on the Stability of Titanium Oxide Colloids

“…Recent reviews indicate a widespread contemporary interest in this compound and its derivatives [1][2][3][4]. For instance, the high refractive index makes it excellent pigment in white paints [5], where primary particles are homogeneously dispersed in the liquid medium.…”

TiO2 Nanotubes on Transparent Substrates: Control of Film Microstructure and Photoelectrochemical Water Splitting Performance

“…For thin films, various TiO 2 one-dimensional (1D) nanostructures involving self-organized nanotubes, highly-ordered nanorod arrays, and nanowires, have attracted much attention due to the combination of highly functional features and controllable nanoscale geometry with the possibility of adjusting length, diameter, and spacing [13,[15][16][17]. In particular, 1D TiO 2 nanotubes (TNT) show better PEC performance than thin compact layers due to higher surface area, favorable charge transfer along the nanotube y-axis perpendicular to charge collecting bottom layer, and enhanced light harvesting efficiency [18].…”