TiO(2) is one of the most studied compounds in materials science. Owing to some outstanding properties it is used for instance in photocatalysis, dye-sensitized solar cells, and biomedical devices. In 1999, first reports showed the feasibility to grow highly ordered arrays of TiO(2) nanotubes by a simple but optimized electrochemical anodization of a titanium metal sheet. This finding stimulated intense research activities that focused on growth, modification, properties, and applications of these one-dimensional nanostructures. This review attempts to cover all these aspects, including underlying principles and key functional features of TiO(2), in a comprehensive way and also indicates potential future directions of the field.

TiO2 nanotubes: synthesis and applications.

Solar driven catalysis on semiconductors to produce clean chemical fuels, such as hydrogen, is widely considered as a promising route to mitigate environmental issues caused by the combustion of fossil fuels and to meet increasing worldwide demands for energy. The major limiting factors affecting the efficiency of solar fuel synthesis include; (i) light absorption, (ii) charge separation and transport and (iii) surface chemical reaction; therefore substantial efforts have been put into solving these problems. In particular, the loading of co-catalysts or secondary semiconductors that can act as either electron or hole acceptors for improved charge separation is a promising strategy, leading to the adaptation of a junction architecture. Research related to semiconductor junction photocatalysts has developed very rapidly and there are a few comprehensive reviews in which the strategy is discussed (A. Kudo and Y. Miseki, Chemical Society Reviews, 2009, 38, 253–278, K. Li, D. Martin, and J. Tang, Chinese Journal of Catalysis, 2011, 32, 879–890, R. Marschall, Advanced Functional Materials, 2014, 24, 2421–2440). This critical review seeks to give an overview of the concept of heterojunction construction and more importantly, the current state-of-the art for the efficient, visible-light driven junction water splitting photo(electro)catalysts reported over the past ten years. For water splitting, these include BiVO4, Fe2O3, Cu2O and C3N4, which have attracted increasing attention. Experimental observations of the proposed charge transfer mechanism across the semiconductor/semiconductor/metal junctions and the resultant activity enhancement are discussed. In parallel, recent successes in the theoretical modelling of semiconductor electronic structures at interfaces and how these explain the functionality of the junction structures is highlighted.

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Visible-light driven heterojunction photocatalysts for water splitting – a critical review

This Review focuses on recent developments in the use of ZnO nanostructures for dye-sensitized solar cell (DSC) applications. It is shown that carefully designed and fabricated nanostructured ZnO films are advantageous for use as a DSC photoelectrode as they offer larger surface areas than bulk film material, direct electron pathways, or effective light-scattering centers, and, when combined with TiO2, produce a core–shell structure that reduces the combination rate. The limitations of ZnO-based DSCs are also discussed and several possible methods are proposed so as to expand the knowledge of ZnO to TiO2, motivating further improvement in the power-conversion efficiency of DSCs.

https://depts.washington.edu/solgel/documents/pub_docs/journal_docs/2009/advanced_material2009.pdf

ZnO Nanostructures for Dye-Sensitized Solar Cells

There is a growing interest in the conversion of water and solar energy into clean and renewable H2 fuels using earth-abundant materials due to the depletion of fossil fuel and its serious environmental impact. This critical review highlights some key factors influencing the efficiency of heterogeneous semiconductors for solar water splitting (i.e. improved charge separation and transfer, promoted optical absorption, optimized band gap position, lowered cost and toxicity, and enhanced stability and water splitting kinetics). Moreover, different engineering strategies, such as band structure engineering, micro/nano engineering, bionic engineering, co-catalyst engineering, surface/interface engineering of heterogeneous semiconductors are summarized and discussed thoroughly. The synergistic effects of the different engineering strategies, especially for the combination of co-catalyst loading and other strategies seem to be more promising for the development of highly efficient photocatalysts. A thorough understanding of electron and hole transfer thermodynamics and kinetics at the fundamental level is also important for elucidating the key efficiency-limiting step and designing highly efficient solar-to-fuel conversion systems. In this review, we provide not only a summary of the recent progress in the different engineering strategies of heterogeneous semiconductors for solar water splitting, but also some potential opportunities for designing and optimizing solar cells, photocatalysts for the reduction of CO2 and pollutant degradation, and electrocatalysts for water splitting.

Engineering heterogeneous semiconductors for solar water splitting

Lithium-sulfur (Li-S) batteries have attracted tremendous interest because of their high theoretical energy density and cost effectiveness. The target of Li-S battery research is to produce batteries with a high useful energy density that at least outperforms state-of-the-art lithium-ion batteries. However, due to an intrinsic gap between fundamental research and practical applications, the outstanding electrochemical results obtained in most Li-S battery studies indeed correspond to low useful energy densities and are not really suitable for practical requirements. The Li-S battery is a complex device and its useful energy density is determined by a number of design parameters, most of which are often ignored, leading to the failure to meet commercial requirements. The purpose of this review is to discuss how to pave the way for reliable Li-S batteries. First, the current research status of Li-S batteries is briefly reviewed based on statistical information obtained from literature. This includes an analysis of how the various parameters influence the useful energy density and a summary of existing problems in the current Li-S battery research. Possible solutions and some concerns regarding the construction of reliable Li-S batteries are comprehensively discussed. Finally, insights are offered on the future directions and prospects in Li-S battery field.

More Reliable Lithium-Sulfur Batteries: Status, Solutions and Prospects.

Novel CdS quantum dots (QDs) sensitized TiO2 nanotube-array photoelectrodes were investigated for their photoelectrochemical (PEC) performance. The highly ordered TiO2 nanotube arrays were synthesized by anodic oxidation and CdS QDs were deposited into the pores of the nanotube arrays by a sequential chemical bath deposition method. It is found that the CdS QDs deposited in the pores of the TiO2 nanotube arrays may significantly increase the liquid junction PEC short circuit photocurrent (from 0.22 to 7.82 mA/cm2) and cell efficiency (up to 4.15%). These results clearly demonstrate that the unique nanotube structure can facilitate the propagation and kinetic separation of photogenerated charges, suggesting potentially important applications of the inorganic QDs sensitized TiO2 nanotube-array films in solar cell applications.

CdS quantum dots sensitized TiO2 nanotube-array photoelectrodes.

TiO 2 nanotube array films, formed by anodic oxidation, have been shown to yield high efficiency of charge generation and collection in photoelectrochemical (PEC) devices. However, the wide band gap (3.2 eV) of TiO 2 limits the efficiency of these devices in the visible region. In this work, four types of presynthesized CdTe quantum dots (QDs) of different sizes were deposited into the TiO 2 nanotubes to serve as the sensitizers, and the performance of the CdTe QD-sensitized TiO 2 nanotube arrays was measured in a PEC solar cell. It is found that, with decreasing particle size, the driving force for electron injection increases while the visible light response decreases. Maximum photocurrent was obtained for the QDs that have an absorption peak at 536 nm. Under AM 1.5 G illuminations, a 6 mA/cm 2 short circuit current density is achieved, which presents a 35 times improvement compared to that based on using a plain TiO 2 nanotube film.

CdTe Quantum Dots-Sensitized TiO2 Nanotube Array Photoelectrodes

A heterojunction CdS/TiO2 photoelectrode is prepared by filling CdS nanoparticles into one-dimensional TiO2 nanotube (NT) array films. The self-assembled TiO2 NTs are fabricated by an anodization method and sensitized with CdS nanoparticles by the close space sublimation technique. The photoactivity of the as-prepared photoelectrodes was measured in a photoelectrochemical solar cell. Under AM 1.5G illumination, a 5.6 mA/cm2 short-circuit current density is achieved using the CdS modified photoelectrode, which represents an enhancement by a factor of 36 in photoactivity compared with that of the plain TiO2 NT array film.

An Efficient Method To Form Heterojunction CdS/TiO2 Photoelectrodes Using Highly Ordered TiO2 Nanotube Array Films

https://pubs.rsc.org/en/content/articlepdf/2014/cc/c4cc01864h

Xianfeng Gao

Papers

CdS quantum dots sensitized TiO2 nanotube-array photoelectrodes.

CdTe Quantum Dots-Sensitized TiO2 Nanotube Array Photoelectrodes

An Efficient Method To Form Heterojunction CdS/TiO2 Photoelectrodes Using Highly Ordered TiO2 Nanotube Array Films

Enhanced photovoltaic performance of perovskite CH3NH3PbI3 solar cells with freestanding TiO2 nanotube array films

Life cycle assessment of lithium sulfur battery for electric vehicles