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.

A Wireless Magnetoelastic Biosensor for the Selective Detection of Low Density Lipoprotein (LDL) Particles

Electrochemical oxidase biosensors have been widely applied in healthcare, environmental measurements and the biomedical field. However, the low and fluctuant oxygen levels in solution and the high anodic detection potentially restrict the assay accuracy. To address these problems, in this work, we constructed a three-phase interface enzyme electrode by sequentially immobilizing H2O2 electrocatalysts and an oxidase layer on a superhydrophobic laser-induced graphene (LIG) array substrate. The LIG-based enzyme electrode possesses a solid–liquid–air three-phase interface where constant and sufficient oxygen can be supplied from the air phase to the enzymatic reaction zone, which enhances and stabilizes the oxidase kinetics. We discovered that the enzymatic reaction rate is 21.2-fold improved over that of a solid–liquid diphase system where oxygen is supplied from the liquid phase, leading to a 60-times wider linear detection range. Moreover, the three-phase enzyme electrode can employ a cathodic measuring principle for oxidase catalytic product H2O2 detection, which could minimize interferences arising from oxidizable molecules in biofluids and increase the detection selectivity. This work provides a simple and promising approach to the design and construction of high-performance bioassay systems.

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Laser-Induced Graphene Arrays-Based Three-Phase Interface Enzyme Electrode for Reliable Bioassays

Correction for ‘Robust electrochemical metal oxide deposition using an electrode with a superhydrophobic surface’ by Jun Zhang, et al., Nanoscale, 2017, 9, 87–90.

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Correction: Robust electrochemical metal oxide deposition using an electrode with a superhydrophobic surface

Coupling the oxidase enzymatic and H2O2 cathodic reactions is an ideal approach for accurate bioassays development because it naturally avoids most endogenous/exogenous electroactive interferents in biofluids. However, the accompanying oxygen cathodic reaction at the surface of commonly used stable catalysts compromises the bioassay accuracy, thus limiting practical application of this detection principle. Here, highly active oxygen‐insensitive electrocatalysts for H2O2 reduction by modifying noble metals (Pt, Au) with an ultra‐thin tin oxide layer that can selectively block the diffusion of oxygen from the electrolyte to the noble metal surface, are reported. This empowers the electrocatalysts with enhanced resistance to current interference from oxygen reduction reaction, thus enabling highly selective sensing of sarcosine, a model analyte (a biomarker for prostate cancer), in the presence of various electroactive interferences. This work highlights the importance of H2O2 cathodic reactions in accurate biomarker detection and their advanced applications in life science, including developing reliable biomarker assays.

Oxygen‐Insensitive Peroxide Reduction Catalysts for Reliable Electrochemical Bioassays

Abnormal hydrogen peroxide (H2O2) levels in the cellular environment are closely related to cell dysfunction and serious diseases. Thus, selective and sensitive H2O2 detections are urgently needed for clinical diagnosis and therapy. Herein, via surface defect engineering, an oxygen‐tolerant electrocatalyst based on tin oxide for selective H2O2 reduction and detection with exceptional stability and activity is designed and developed. When introduced at an appropriate level (≈5.3%), surface oxygen vacancies help lower the charge transfer resistance for enhancing the H2O2 reduction reaction, while maintain the weak oxygen (O2) adsorption, which enables a constant H2O2 reduction (sensing) response in the electrolyte at variable oxygen levels. Moreover, the tin oxide‐based assay system exhibits outstanding stability over a wide pH range of 4–9, as well as selectivity in the presence of interferent endogenous and exogenous electroactive species, which is suitable for trace H2O2 monitoring secreted from NB4 cells, a model cancer cell. The oxygen vacancy‐mediated tin oxide achieves the highest stability as well as high selectivity compared to reported electrochemical probes for specific H2O2 detection in biological environments, with the potential for biological and biomedical applications.

Xinjian Feng

Papers

A Wireless Magnetoelastic Biosensor for the Selective Detection of Low Density Lipoprotein (LDL) Particles

Laser-Induced Graphene Arrays-Based Three-Phase Interface Enzyme Electrode for Reliable Bioassays

Correction: Robust electrochemical metal oxide deposition using an electrode with a superhydrophobic surface

Oxygen‐Insensitive Peroxide Reduction Catalysts for Reliable Electrochemical Bioassays

Oxygen Vacancy‐Mediated Catalysts toward Selective H2O2 Reduction in Cellular Environment