Visible light driven (VLD) reduced TiO2-x nanocatalysts designed by inorganic and organic reducing agent-mediated solvothermal methods for electrocatalytic and photocatalytic applications

This work presents a comparative study on the structural, optical and electrochemical characteristics of visible light driven (VLD) reduced titanium dioxide (TiO2-x ) nanocatalysts synthesized via inorganic and organic synthetic routes. X-ray diffraction (XRD) patterns, Raman spectra and X-ray absorption fine structure (XAFS) analyses reflected anatase phase titania. Whereas, the quantitative EXAFS fit and XANES analysis revealed structural distortion due to the presence of oxygen and titanium vacancies with low valent Ti states in anatase lattices of certain nanocatalysts, which subsequently leads to better electrochemical and photocatalytic activities. Moreover, owing to the large surface area and mesoporous structures, the Mg-TiO2-x nanocatalysts exhibited enhanced water adsorption and ultimately increased overall water splitting with an OER overpotential equal to 420 mV vs. RHE at a current density of 10 mA cm-2 (Tafel slope = 62 mV dec-1), extended visible light absorbance, decreased photoluminescence (PL) intensity and increased carrier lifetime in comparison with commercial titania.

Mechanistic study of visible light driven photocatalytic degradation of clofibric acid using Fe-based metal organic frameworks (MOFs)

Although a series of past studies proved the potential usage of Fe-based metal-organic frameworks (MOFs) as photocatalysts, there remains a knowledge gap of the photocatalytic mechanism stemming from the challenge to separate the simultaneous sorption and photocatalytic degradation. Thus, this article aimed to suggest a novel approach by desorbing target molecules during photocatalysis to excavate the underlying mechanisms of sorption and photocatalytic degradation. In this study, two Fe-based MOFs, MIL-101(Fe) and MIL-101(Fe)-NH2, were selected to remove clofibric acid under visible light irradiation. Prior to photocatalysis, sorption mechanism was uncovered based on the sorption kinetic, isotherm, thermodynamic interpretation, and of its dependence on solution pH. The results inferred that the primary sorption mechanism was through the π-π interaction between the benzene ring of clofibric acid and the organic ligand of Fe-based MOFs. Based on these results, photocatalytic mechanism could be independently or jointly assessed during the photocatalytic degradation of clofibric acid. Subsequently, the application of the Tauc method and XPS spectra revealed that the bandgap structure of Fe-based MOFs had the potential to oxidize clofibric acid by producing ROS through the electron excitation upon visible-light illumination. On top of that, the amine functionalization of Fe-based MOF altered the structural moiety that led to an additional strong acid-base interaction with clofibric acid but a decrease in the bandgap limiting the ROS production during photocatalytic activity.

Modulating Co-catalyst/Facet Junction for Enhanced Photoelectrochemical Water Splitting

Rational construction of electric field via assembling appropriate co-catalysts on anisotropic facets is of great significance for improving the photogenerated charge separation efficiency. However, this strategy usually gives rise to Fermi-level pinning which is not contributive to the charge separation but deleterious to the photoelectrochemical performance through consuming the measurable photovoltage. Herein, we demonstrate that manganese dioxide electrodeposited on the (111) facet of titanium dioxide nanorods could tremendously boost the catalytic activity of pristine photoanode via a stronger interface electric field and less photovoltage decay compared with the counterpart grown on the (110) facet. A photocurrent density of 1.65 mA·cm^-2 at 1.23 V (vs reversible hydrogen electrode), nearly the theoretical maximum of titanium dioxide, is achieved by the optimum photoanode with an extremely high separation efficiency of 95.15%. This study offers more in-depth insights into the design of carrier separation strategy through loading co-catalysts on different substrate surfaces for more efficient solar energy conversion.

The formation and detection techniques of oxygen vacancies in titanium oxide-based nanostructures

TiO2 and other titanium oxide-based nanomaterials have drawn immense attention from researchers in different scientific domains due to their fascinating multifunctional properties, relative abundance, environmental friendliness, and bio-compatibility. However, the physical and chemical properties of titanium oxide-based nanomaterials are found to be explicitly dependent on the presence of various crystal defects. Oxygen vacancies are the most common among them and have always been the subject of both theoretical and experimental research as they play a crucial role in tuning the inherent properties of titanium oxides. This review highlights different strategies for effectively introducing oxygen vacancies in titanium oxide-based nanomaterials, as well as a discussion on the positions of oxygen vacancies inside the TiO2 band gap based on theoretical calculations. Additionally, a detailed review of different experimental techniques that are extensively used for identifying oxygen vacancies in TiO2 nanostructures is also presented.

Room-temperature single-photon emitters in titanium dioxide optical defects

Fluorescence properties of crystallographic point defects within different morphologies of titanium dioxide were investigated. For the first time, room-temperature single-photon emission in titanium dioxide optical defects was discovered in thin films and commercial nanoparticles. Three-level defects were identified because the g(2) correlation data featured prominent shoulders around the antibunching dip. Stable and blinking photodynamics were observed for the single-photon emitters. These results reveal a new room-temperature single-photon source within a wide bandgap semiconductor.

On the relationship between rutile/anatase ratio and the nature of defect states in sub-100 nm TiO2 nanostructures: experimental insights

Black TiO₂ is being widely investigated due to its superior optical activity and potential applications in photocatalytic hydrogen generation. Herein, the limitations of the hydrogenation process of TiO₂ nanostructures are unraveled by exploiting the fundamental tradeoffs affecting the overall efficiency of the water splitting process. To control the nature and concentration of defect states, different reduction rates are applied to sub-100 nm TiO₂ nanotubes, chosen primarily for their superiority over their long counterparts. X-Ray Photoelectron Spectroscopy disclosed changes in the stoichiometry of TiO₂ with the reduction rate. UV-vis and Raman spectra showed that high reduction rates promote the formation of the rutile phase in TiO₂, which is inactive towards water splitting. Furthermore, electrochemical analysis revealed that such high rates induce a higher concentration of localized electronic defect states that hinder the water splitting performance. Finally, incident photon-to-current conversion efficiency (IPCE) highlighted the optimum reduction rate that attains a relatively lower defect concentration as well as lower rutile content, thereby achieving the highest conversion efficiency.

Polymer-Mediated Self-Assembly of TiO2@Cu2O Core-Shell Nanowire Array for Highly Efficient Photoelectrochemical Water Oxidation

Phototoelectrochemical (PEC) water splitting represents a highly promising strategy to convert solar energy to chemical energy in the form of hydrogen, but its performance is severely limited by the water oxidation reaction. We conformally grew an ultrathin and continuous coating of Cu2O on TiO2 nanowire array (NWA) to form a truly core-shell TiO2@Cu2O NWA via a new facile, economical, and scalable polymer-mediated self-assembly approach, in which the polymer serves as a stabilizer, reductant, and linker simultaneously. This heteronanostructure was subsequently directly used as a photoanode for PEC water splitting, showing a photocurrent density of 4.66 mA cm(-2) at 1.23 V vs RHE in 0.5 M Na2SO4 solution and a maximum photoconversion efficiency of 0.71%, both of which are the highest reported for TiO2-based photoanodes measured under the same conditions (neutral conditions and without any sacrificial agent). The superior PEC performance of the TiO2@Cu2O NWA toward water oxidation is primarily due to much enhanced visible light collection and charge separation for high charge carrier density as well as greatly facilitated charge transfer and transport. This work not only offers a novel TiO2@Cu2O core-shell NWA photoanode for highly efficient PEC water oxidation and investigate its enhancement mechanism but also provides scientific insights into the mechanism of the polymer-mediated self-assembly, which can be further extended to fabricate various other core-shell nanoarchitectures for broad applications.

Single-step electrospun TiO2–Au hybrid electrodes for high selectivity photoelectrocatalytic glutathione bioanalysis

One-step electrospun Au nanoparticle decorated TiO₂ nanofiber membrane served as effective photoanode for highly selective glutathione analysis with a photoelectrocatalytic oxidation process.

Rutile nanotubes by electrochemical anodization

We present a facile method to synthesize rutile titanium dioxide nanotubes (R-TiNT), directly in powder form through rapid breakdown electrochemical anodization by modifying the post anodization processing and annealing temperature.

Electrochemical properties of novel titania nanostructures

In this study, the supercapacitive properties of six new TiO2 nanostructures-including nanodishes, three-layer nanosheets, ancient Chinese coins, single-layer nanosheets, hollow nanocubes, and commercial rutile TiO2 are investigated mainly by cyclic voltammetry, chronopotentiometry, and electrochemical impedance spectroscopy. The results show that among them, the TiO2 nanodishes have the highest discharging capacitance at 1792 mFg(-1), which is 6.4 and 1.5 times higher than that of TiO2 single-layer nanosheets and commercial rutile TiO2, respectively. We found that the electrochemical properties of the TiO2 samples are predominated primarily by the high-energy facets exposed, instead of by the Brunauer-Emmett-Teller area. An important and previously unknown finding of our work is that the electrochemical properties of electrode materials can be improved by controlling the high-energy facets.