Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications

Energy harvested directly from sunlight offers a desirable approach toward fulfilling, with minimal environmental impact, the need for clean energy. Solar energy is a decentralized and inexhaustible natural resource, with the magnitude of the available solar power striking the earth’s surface at any one instant equal to 130 million 500 MW power plants.1 However, several important goals need to be met to fully utilize solar energy for the global energy demand. First, the means for solar energy conversion, storage, and distribution should be environmentally benign, i.e. protecting ecosystems instead of steadily weakening them. The next important goal is to provide a stable, constant energy flux. Due to the daily and seasonal variability in renewable energy sources such as sunlight, energy harvested from the sun needs to be efficiently converted into chemical fuel that can be stored, transported, and used upon demand. The biggest challenge is whether or not these goals can be met in a costeffective way on the terawatt scale.2

http://pubs.acs.org/doi/pdf/10.1021/cr1002326

Solar Water Splitting Cells

TiO 2 photocatalysis is widely used in a variety of applications and products in the environmental and energy fields, including self-cleaning surfaces, air and water purification systems, sterilization, hydrogen evolution, and photoelectrochemical conversion. The development of new materials, however, is strongly required to provide enhanced performances with respect to the photocatalytic properties and to find new uses for TiO 2 photocatalysis. In this review, recent developments in the area of TiO 2 photocatalysis research, in terms of new materials from a structural design perspective, have been summarized. The dimensionality associated with the structure of a TiO 2 material can affect its properties and functions, including its photocatalytic performance, and also more specifically its surface area, adsorption, reflectance, adhesion, and carrier transportation properties. We provide a brief introduction to the current situation in TiO 2 photocatalysis, and describe structurally controlled TiO 2 photocatalysts which can be classified into zero-, one-, two-, and three-dimensional structures. Furthermore, novel applications of TiO 2 surfaces for the fabrication of wettability patterns and for printing are discussed.

TiO2 photocatalysis: Design and applications

We report the first demonstration of hydrogen treatment as a simple and effective strategy to fundamentally improve the performance of TiO2 nanowires for photoelectrochemical (PEC) water splitting. Hydrogen-treated rutile TiO2 (H:TiO2) nanowires were prepared by annealing the pristine TiO2 nanowires in hydrogen atmosphere at various temperatures in a range of 200–550 °C. In comparison to pristine TiO2 nanowires, H:TiO2 samples show substantially enhanced photocurrent in the entire potential window. More importantly, H:TiO2 samples have exceptionally low photocurrent saturation potentials of −0.6 V vs Ag/AgCl (0.4 V vs RHE), indicating very efficient charge separation and transportation. The optimized H:TiO2 nanowire sample yields a photocurrent density of ∼1.97 mA/cm2 at −0.6 V vs Ag/AgCl, in 1 M NaOH solution under the illumination of simulated solar light (100 mW/cm2 from 150 W xenon lamp coupled with an AM 1.5G filter). This photocurrent density corresponds to a solar-to-hydrogen (STH) efficiency of ∼1...

Hydrogen-Treated TiO2 Nanowire Arrays for Photoelectrochemical Water Splitting

Rising atmospheric levels of carbon dioxide and the depletion of fossil fuel reserves raise serious concerns about the ensuing effects on the global climate and future energy supply. Utilizing the abundant solar energy to convert CO2 into fuels such as methane or methanol could address both problems simultaneously as well as provide a convenient means of energy storage. In this Review, current approaches for the heterogeneous photocatalytic reduction of CO2 on TiO2 and other metal oxide, oxynitride, sulfide, and phosphide semiconductors are presented. Research in this field is focused primarily on the development of novel nanostructured photocatalytic materials and on the investigation of the mechanism of the process, from light absorption through charge separation and transport to CO2 reduction pathways. The measures used to quantify the efficiency of the process are also discussed in detail.

Photocatalytic reduction of CO2 on TiO2 and other semiconductors.

High-yield three-dimensional (3D) flower-like nanostructures self-assembled from 1 D ZnO nanorods and nanotubes are experimentally demonstrated. The Zn and O terminated crystal planes of ZnO nanorods results in positively and negatively charged top (001) and bottom (00-1) surfaces, respectively. The nanorods self-assembled into 3D nanostructures via the electrostatic interaction between the crystal planes with opposite charges. Moreover, on the basis of the different stability of polar and nonpolar planes in wurtzite-type ZnO, the nanorods based 3D nanostructures transformed into nanotubes based ones spontaneously. This provides a new approach to prepare multi-dimensional materials without the necessity to employ any external intervention.

High-yield self-assembly of flower-like ZnO nanostructures.

Developing nanostructured electrocatalysts, with low overpotential and high activity, has fundamental and technical importance in many fields. A new electrocatalyst for H2O2 oxidation based on strongly coupled rhodium nanocrystal/reduced graphene oxide (Rh/RGO) hybrid nanomaterials is described. The Rh/RGO nanohybrids markedly reduce the oxidation potential to about + 0.1 V, which is 0.5 V lower than that of a traditional platinum electrode. The nanohybrids have a strong synergetic effect and are catalytically more active than single-component Rh and RGO and state-of-the-art commercial Rh/C catalysts.

Strongly Coupled Rhodium/Graphene Hybrids for H2O2 Oxidation with Ultra-Low Potential and Enhanced Activity

Photoelectrodes based on TiO2 decorated with Au nanoparticles (NPs) (Au-TiO2) have attracted great attention for visible-light-driven photoelectrochemical (PEC) water splitting applications. Herein, we report a highly efficient visible-light-responsive photoanode composed of Au NPs decorated well-separated TiO2 nanowire (NW) arrays with a length up to over 9 μm. The photocurrent was found to be about 16 times higher than that of bare TiO2 NWs under visible-light illumination. Moreover, the photocurrent increased linearly with NW length, suggesting good charge dynamic properties and efficient utilization of the decorated Au NPs of the photoanode. These results indicate that the long and well-separated NW arrays have great potential for future development of PEC devices. 

Long and Well-Separated TiO2 Nanowire Arrays Decorated with Au Nanoparticles for Visible-Light-Driven Photoelectrochemical Water Splitting

The invention discloses a super-hydrophobic solid-liquid-gas three-phase coexistence bio-enzyme sensor and a preparation method thereof. The super-hydrophobic solid-liquid-gas three-phase coexistence bio-enzyme sensor comprises a substrate with surface having super-hydrophobic performance, a catalysis material having hydrogen peroxide catalysis function, and biological enzyme capable of being reacted with a substance to be measured to generate hydrogen peroxide from down to up. By forming a solid-liquid-gas three-phase coexistence state on the surface of the super-hydrophobic material, sufficient oxygen can be provided for supplying the enzymatic reaction, so that the sensor has the characteristics of wide detection scope, low detection limit, fast response speed, accurate detection result, stable performance, easy preparation and low cost.

Super-hydrophobic solid-liquid-gas three-phase coexistence bio-enzyme sensor and preparation method thereof

Rational interfacial microenvironment design and modulation are crucial to photocatalysis performance. Herein, we report a photocatalytic system with a liquid (water)–liquid (organic liquid)–solid (L–L–S) triphase reaction interface microenvironment and demonstrate its utility in semiconductor-mediated photodegradation. The triphase system was fabricated using TiO2 nanowire arrays as the solid phase and polydimethylsiloxane as the inert organic liquid phase. The presence of the polydimethylsiloxane layer was confirmed via cryotransmission electron microscopy analysis, and the presence of the L–L–S interface was demonstrated using three-dimensional (3D) laser scanning confocal microscopy. Based on this triphase system, the concentrations of reactant O2 and organic molecules could be both significantly improved at the interface microenvironment, leading to over 30 times higher reaction kinetics than that of a conventional liquid–solid (L–S) diphase system. Moreover, the L–L–S system exhibited excellent stability, and the design showed universal applicability. These findings highlight the importance of interface architecture design and reveal an effective path for the development of efficient catalysis systems. 

Xinjian Feng

Papers

High-yield self-assembly of flower-like ZnO nanostructures.

Strongly Coupled Rhodium/Graphene Hybrids for H2O2 Oxidation with Ultra-Low Potential and Enhanced Activity

Long and Well-Separated TiO2 Nanowire Arrays Decorated with Au Nanoparticles for Visible-Light-Driven Photoelectrochemical Water Splitting

Super-hydrophobic solid-liquid-gas three-phase coexistence bio-enzyme sensor and preparation method thereof

Liquid–Liquid–Solid Triphase Interface Microenvironment Mediates Efficient Photocatalysis