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

The surface science of titanium dioxide

Energy storage is more important today than at any time in human history. Future generations of rechargeable lithium batteries are required to power portable electronic devices (cellphones, laptop computers etc.), store electricity from renewable sources, and as a vital component in new hybrid electric vehicles. To achieve the increase in energy and power density essential to meet the future challenges of energy storage, new materials chemistry, and especially new nanomaterials chemistry, is essential. We must find ways of synthesizing new nanomaterials with new properties or combinations of properties, for use as electrodes and electrolytes in lithium batteries. Herein we review some of the recent scientific advances in nanomaterials, and especially in nanostructured materials, for rechargeable lithium-ion batteries.

Nanomaterials for rechargeable lithium batteries

Photoinduced reactivity of titanium dioxide

Molecular self-assembly is the spontaneous association of molecules under equilibrium conditions into stable, structurally well-defined aggregates joined by noncovalent bonds. Molecular self-assembly is ubiquitous in biological systems and underlies the formation of a wide variety of complex biological structures. Understanding self-assembly and the associated noncovalent interactions that connect complementary interacting molecular surfaces in biological aggregates is a central concern in structural biochemistry. Self-assembly is also emerging as a new strategy in chemical synthesis, with the potential of generating nonbiological structures with dimensions of 1 to 10(2) nanometers (with molecular weights of 10(4) to 10(10) daltons). Structures in the upper part of this range of sizes are presently inaccessible through chemical synthesis, and the ability to prepare them would open a route to structures comparable in size (and perhaps complementary in function) to those that can be prepared by microlithography and other techniques of microfabrication.

Molecular self-assembly and nanochemistry: A chemical strategy for the synthesis of nanostructures

Metal oxide-based resistive-type gas sensors are solid-state devices which are widely used in a number of applications from health and safety to energy efficiency and emission control. Nanomaterials such as nanowires, nanorods, and nanoparticles have dominated the research focus in this field due to their large number of surface sites facilitating surface reactions. Previous studies have shown that incorporating two or more metal oxides to form a heterojunction interface can have drastic effects on gas sensor performance, especially the selectivity. Recently, these effects have been amplified by designing heterojunctions on the nano-scale. These designs have evolved from mixed commercial powders and bi-layer films to finely-tuned core–shell and hierarchical brush-like nanocomposites. This review details the various morphological classes currently available for nanostructured metal-oxide based heterojunctions and then presents the dominant electronic and chemical mechanisms that influence the performance of these materials as resistive-type gas sensors. Mechanisms explored include p–n and n–n potential barrier manipulation, n–p–n response type inversions, spill-over effects, synergistic catalytic behavior, and microstructure enhancement. Tables are presented summarizing these works specifically for SnO2, ZnO, TiO2, In2O3, Fe2O3, MoO3, Co3O4, and CdO-based nanocomposites. Recent developments are highlighted and likely future trends are explored.

Nanoscale metal oxide-based heterojunctions for gas sensing: A review

Recently one dimensional (1-D) nanostructured metal-oxides have attracted much attention because of their potential applications in gas sensors. 1-D nanostructured metal-oxides provide high surface to volume ratio, while maintaining good chemical and thermal stabilities with minimal power consumption and low weight. In recent years, various processing routes have been developed for the synthesis of 1-D nanostructured metal-oxides such as hydrothermal, ultrasonic irradiation, electrospinning, anodization, sol-gel, molten-salt, carbothermal reduction, solid-state chemical reaction, thermal evaporation, vapor-phase transport, aerosol, RF sputtering, molecular beam epitaxy, chemical vapor deposition, gas-phase assisted nanocarving, UV lithography and dry plasma etching. A variety of sensor fabrication processing routes have also been developed. Depending on the materials, morphology and fabrication process the performance of the sensor towards a specific gas shows a varying degree of success. This article reviews and evaluates the performance of 1-D nanostructured metal-oxide gas sensors based on ZnO, SnO2, TiO2, In2O3, WOx, AgVO3, CdO, MoO3, CuO, TeO2 and Fe2O3. Advantages and disadvantages of each sensor are summarized, along with the associated sensing mechanism. Finally, the article concludes with some future directions of research.

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Gas sensors based on one dimensional nanostructured metal-oxides: a review.

Advancement of gas sensor technology over the past few decades has led to significant progress in pollution control and thereby, to environmental protection An excellent example is the control of automobile exhaust emissions, made possible by the use of oxygen gas sensors Since early 1970's there have been sustained studies on oxygen sensors and has led to development of sensors for various applications with varying performance characteristics Solid electrolyte based potentiometric, amperometric and metal oxide based semiconducting resistive type sensors are used for high temperature applications For solution-based pollution monitoring, dissolved oxygen sensors based on Clark electrodes have played a major role More recently, for biological and medical applications, optical oxygen sensors are beginning to have an impact In this review, we focus on both high temperature as well as dissolved oxygen sensors and compare the different methods of oxygen sensing, discuss underlying principles, and outline the designs and specific applications

Oxygen sensors: Materials, methods, designs and applications

Metal oxide resistive-type nano-scale gas sensors have been investigated for their low cost, high sensitivity, and environmentally friendly fabrication. In these sensors, electrical resistance measurements are used to detect the presence of gas. In n-type metal oxides, resistance is increased by coverage of adsorbed oxygen and lowered by removal of adsorbed oxygen through reactions with reducing gasses. The sensitivity and selectivity of these sensors have been improved by incorporation of heterostructures. Heterostructures may improve sensor performance through facilitating catalytic activity, increasing adsorption, and creating a charge carrier depletion layer that produces a larger modulation in resistance. Synergistic effects in these gas sensors describe the improved sensor signal due to these combined effects which act to amplify the reception and transduction of the sensor signal. Receptive mechanisms may be improved by increasing adsorption and reactivity. Transduction mechanisms may be improved by restriction of the major charge conduction channels which helps to maximize resistance modulation. In this review, the synergistic effect achieved by combining these two mechanisms are examined. Fundamental properties of the metal oxide surface are used to provide insight for the large body of experimental evidence available for metal oxide resistive-type gas sensors. This review aims to connect experimental evidence to conceptual mechanistic descriptions by examining adsorption processes, charge transfer, reaction mechanisms, morphology, and ambient gas interactions.

Synergistic effects in gas sensing semiconducting oxide nano-heterostructures: A review

Investigations into the mechanisms governing the behavior of metal oxide gas sensors continue to be of great interest. Oxygen vacancies are a ubiquitous defect in this class of materials and their characteristics can be affected by synthesis, processing and operating parameters. The primary role of oxygen vacancies in modifying sensing performance cited in the gas sensing literature is based on a modulation of the amount of surface adsorbed oxygen or alternatively, the baseline resistance. Unfortunately, this generalized description does not provide a complete representation of the role of oxygen vacancies that would aid in more a fundamental understanding of their role in gas sensing. To this end, an attempt is made to distinguish between the role of surface and bulk oxygen vacancies where emphasis on proper characterization is first highlighted. The influence of surface oxygen vacancies on factors affecting adsorption, such as surface structure, are examined to gain understanding on improved sensing performance. The effect of bulk oxygen vacancy concentration and distribution on sensing are also discussed. Finally, the importance of these concepts within the context of doped and heterostructured gas sensors are then briefly discussed.

Sheikh A. Akbar

Papers

Nanoscale metal oxide-based heterojunctions for gas sensing: A review

Gas sensors based on one dimensional nanostructured metal-oxides: a review.

Oxygen sensors: Materials, methods, designs and applications

Synergistic effects in gas sensing semiconducting oxide nano-heterostructures: A review

Role of Oxygen Vacancies in Nanostructured Metal-Oxide Gas Sensors: A Review