Single-Atom-Based Catalysts for Photocatalytic Water Splitting on TiO2 Nanostructures

H2 generation from photocatalytic water splitting is one of the most promising approaches to producing cost-effective and sustainable fuel. Nanostructured TiO2 is a highly stable and efficient semiconductor photocatalyst for this purpose. The main drawback of TiO2 as a photocatalyst is the sluggish charge transfer on the surface of TiO2 that can be tackled to a great extent by the use of platinum group materials (PGM) as co-catalysts. However, the scarcity and high cost of the PGMs is one of the issues that prevent the widespread use of TiO2/PGM systems for photocatalytic H2 generation. Single-atom catalysts which are currently the frontline in the catalysis field can be a favorable path to overcome the scarcity and further advance the use of noble metals. More importantly, single-atom (SA) catalysts simultaneously have the advantage of homogenous and heterogeneous catalysts. This mini-review specifically focuses on the single atom decoration of TiO2 nanostructures for photocatalytic water splitting. The latest progress in fabrication, characterization, and application of single-atoms in photocatalytic H2 generation on TiO2 is reviewed

Photoelectrochemical H2 generation from suboxide TiO2 nanotubes: Visible light absorption versus conductivity

In the present work we report on the key factors dictating the photo-electrochemical (PEC) performance of suboxide titania (TiOx) nanotubes. TiOx nanotubes were produced by a systematic variation of reduction heat treatments of TiO2 in Ar/H2. The properties of the TiOx tubes were investigated by electron paramagnetic resonance (EPR), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), solid-state conductivity, reflectivity measurements, photocurrent spectroscopy, and photoelectrochemical hydrogen evolution. In line with earlier literature, these suboxide tubes show a drastically improved photoelectrochemical water splitting performance compared to non-reduced anatase TiO2 tubes. In this work we show that the key improvement in water splitting performance is due to the strongly improved conductivity of TiOx semimetalic tubes, reaching 13.5 KΩ/tube compared to 70 MΩ (for non-reduced anatase), and is not due to the enhanced visible light absorbance

Self-organized, free-standing TiO2 nanotube membranes: Effect of surface electrokinetic properties on flow-through membranes

In the present work we investigate the effect of the surface electrokinetic properties and presence of background ions on the flow of a marker dye through TiO2 nanotube membranes. We believe the results to be of high significance not only for filtration but also for the design of microphotoreactor application based on photoactive TiO2 nanotubes membrane. First, both-side open, high aspect ratio TiO2 nanotube membranes were obtained by fast anodization of Ti to self-aligned TiO2 nanotube layers, followed by a lift-off process. Then we investigated the permeation through the TiO2 nanotube membranes by diffusion of acid orange 7 (AO7) and extracted the dye diffusion rates. The effects of pH, ionic concentration, and size of ions were investigated, and the results were compared with theoretical modeling of the surface charge of TiO2 and the neighbouring electric double layer as a function of different species of ions; the modeling confirms the experimental data. We observed a remarkable influence of the background ion species, as well as of ion concentrations and pH in the feed solution on the diffusion rate of AO7. The results of modeling are well in line with the observed influence of TiO2 nanotube inner surface charge and effective size (hydrodynamic radius) of the ions in the background solution. It is also observed that the absolute permeate flux and the membrane’s permeability strongly depend on the electric and wetting conditions of the membrane surface. Keywords: TiO2 nanotube, Membrane, Electrochemical anodizatio

Depth elemental characterization of 1D self-aligned TiO2 nanotubes using calibrated Radio Frequency Glow Discharge Optical Emission Spectroscopy (GDOES)

In this work we study the depth composition of anodic TiO2 nanotube layers. We use elemental depth profiling with Glow Discharge Optical Emission Spectroscopy and calibrate the results of this technique with X-ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDS). We establish optimized sputtering conditions for nanotubular structures using the pulsed RF mode, which causes minimized structural damage during the depth profiling of the nanotubular structures. This allows to obtain calibrated sputter rates that account for the nanotubular “porous” morphology. Most importantly, sputter-artifact free compositional profiles of these high aspect ratio 3D structures are obtained, as well as, in combination with SEM, elegant depth sectional imaging

Hematite photoanode with complex nanoarchitecture providing tunable gradient doping and remarkable low onset potential for advanced PEC water splitting

Over the past years, αFe2O3 (hematite) has reemerged as a promising photoanode material in photoelectrochemical (PEC) water splitting. In spite of considerable success in obtaining relatively high solar conversion efficiency, the main drawbacks hindering practical application at hematite are related to an intrinsically hampered charge transport and a sluggish kinetics of the oxygen evolution reaction on the photoelectrode surface. In the present work, we report a strategy on how to synergistically address both these critical limitations. Our approach is based on three key features that are applied simultaneously, specifically i) a careful nanostrcuturing of hematite photoanode in the form of nanorods, ii) doping of hematite by Sn4+ ions by a controlled gradient, and iii) surface decoration of hematite by a new class of double hydroxide layered (LDH) OER cocatalysts based on ZnCo LDH. All three interconnected forms of functionalization result in an extraordinary cathodic shift of the photocurrent onset potential by more than 300 mV and a PEC performance that reaches a photocurrent density of 2.00 mA/cm2 at 1.50 VRHE

Long-Living Holes in Grey Anatase TiO2 Enable Noble-Metal-Free and Sacrificial-Agent-Free Water Splitting

Titanium dioxide has been the benchmark semiconductor in photocatalysis for more than 40 years. Full water splitting, that is, decomposing water into H2 and O2 in stoichiometric amounts and with an acceptable activity, still remains a challenge, even when TiO2-based photocatalysts are used in combination with noble-metal co-catalysts. The bottleneck of anatase-type TiO2 remains the water oxidation, that is, the hole transfer reaction from pristine anatase to the aqueous environment. In this work, we report that “grey” (defect engineered) anatase can provide a drastically enhanced lifetime of photogenerated holes, which, in turn, enables an efficient oxidation reaction of water to peroxide via a two-electron pathway. As a result, a Ni@grey anatase TiO2 catalyst can be constructed with an impressive performance in terms of photocatalytic splitting of neutral water into H2 and a stoichiometric amount of H2O2 without the need of any noble metals or sacrificial agents. The finding of long hole lifetimes in grey anatase opens up a wide spectrum of further photocatalytic applications of this material. © 2020 The Authors. Published by Wiley-VCH Gmb

Inducing a nanotwinned grain structure within the TiO2 nanotubes provides enhanced electron transport and DSSC efficiencies > 10 %

Titania is one of the key materials used in 1, 2 and 3D nanostructures as electron transport media in energy conversion devices. In the present work we show that the electronic properties of TiO2 nanotubes can be drastically improved by inducing a nanotwinned grain structure in the nanotube wall. This structure can be exclusively induced for “single-walled” nanotubes with a high temperature treatment in pure oxygen atmospheres. Nanotubes with a twinned grain structure within the tube wall show a strongly enhanced conductivity and photogenerated charge transport compared to classic nanotubes. We exemplify this remarkable improvement in the electronic properties by using nanotwinned TiO2 nanotubes in dye-sensitized solar cells where a significant increase in efficiency of up to 10.2% is achieved