Inorganic semiconductor nanostructures are ideal systems for exploring a large number of novel phenomena at the nanoscale and investigating the size and dimensionality dependence of their properties for potential applications. The use of such nanostructures with tailored geometries as building blocks is also expected to play crucial roles in future nanodevices. Since the discovery of carbon nanotubes much attention has been paid to exploring the usage of inorganic semiconductor nanostructures as field-emitters due to their low work functions, high aspect ratios and mechanical stabilities, and high electrical and thermal conductivities. This article provides a comprehensive review of the state-of-the-art research activities in the field. It mainly focuses on the most widely studied inorganic nanostructures, such as ZnO, ZnS, Si, WO3, AlN, SiC, and their field-emission properties. We begin with a survey of inorganic semiconductor nanostructures and the field-emission principle, and then discuss the recent progresses on several kinds of important nanostructures and their field-emission characteristics in detail and overview some additional inorganic semiconducting nanomaterials in short. Finally, we conclude this review with some perspectives and outlook on the future developments in this area.

Inorganic semiconductor nanostructures and their field-emission applications

Hydrogen production from water splitting by photo/photoelectron-catalytic process is a promising route to solve both fossil fuel depletion and environmental pollution at the same time. Titanium dioxide (TiO2) nanotubes have attracted much interest due to their large specific surface area and highly ordered structure, which has led to promising potential applications in photocatalytic degradation, photoreduction of CO2, water splitting, supercapacitors, dye-sensitized solar cells, lithium-ion batteries and biomedical devices. Nanotubes can be fabricated via facile hydrothermal method, solvothermal method, template technique and electrochemical anodic oxidation. In this report, we provide a comprehensive review on recent progress of the synthesis and modification of TiO2 nanotubes to be used for photo/photoelectro-catalytic water splitting. The future development of TiO2 nanotubes is also discussed.

One‐dimensional TiO2 Nanotube Photocatalysts for Solar Water Splitting

The generation of fuels and chemicals from sunlight offers a promising approach for the storage of solar energy. Natural photosynthesis is the best-known example for converting solar energy into chemical energy. In the light-dependent reactions of photosynthesis, the photoinduced electron transport chain from water (H2O) to reduced nicotinamide adenine dinucleotide phosphate (NADPH) is a kind of Z-scheme energetics diagram, which involves two photoexcitation steps and a downhill electron flow from photosystem II (PSII) to photosystem I (PSI) (Figure 1). The Z-scheme pathway is advantageous in realizing efficient charge separation and producing active species with strong reducibility for subsequent reduction of CO2 to hydrocarbons. In practice, artificial photosynthesis is an alternative approach to generate hydrogen energy using sunlight to split Photoelectrocatalytic water splitting offers a promising approach to convert sunlight into sustainable hydrogen energy. A thorough understanding of the relationships between the properties and functions of photoelectrocatalytic materials plays a crucial role in the design and fabrication of efficient photoelectrochemical systems for water splitting. This review presents the advances in the development of efficient photoelectrocatalytic materials. First, the fundamentals involved in the photoelectrocatalytic water splitting are elaborated. Then, the critical properties of photoelectrocatalytic materials are classified and discussed according to the associated photoelectrochemical processes, including light absorption, charge separation, charge transportation, and photoelectrocatalytic reactions. The importance of heterointerfaces in photoelectrodes is also mentioned in conjunction with the illustration of some functional interlayer materials. Finally, some strategies that can be employed in material screening and optimization for the construction of highly efficient photoelectrochemical devices for water splitting are also discussed. Water Splitting

Photoelectrocatalytic Materials for Solar Water Splitting

Resistive-type hydrogen gas sensor based on TiO2: A review

Spontaneous-emission lifetime of Wannier excitons in mesoscopic semiconductor quantum disks

Black Ni-doped TiO2 photoanodes for high-efficiency photoelectrochemical water-splitting

Doping of titania nanotubes is one of the efficient way to obtain improved physical and chemical properties. Through electrochemical anodization and annealing treatment, Ni-doped TiO2 nanotube arrays were fabricated and their hydrogen sensing performance was investigated. The nanotube sensor demonstrated a good sensitivity for wide-range detection of both dilute and high-concentration hydrogen atmospheres ranging from 50 ppm to 2% H2. A temperature-dependent sensing from 25°C to 200°C was also found. Based on the experimental measurements and first-principles calculations, the electronic structure and hydrogen sensing properties of the Ni-doped TiO2 with an anatase structure were also investigated. It reveals that Ni substitution of the Ti sites could induce significant inversion of the conductivity type and effective reduction of the bandgap of anatase oxide. The calculations also reveal that the resistance change for Ni-doped anatase TiO2 with/without hydrogen absorption was closely related to the bandgap especially the Ni-induced impurity level.

https://link.springer.com/content/pdf/10.1186/1556-276X-9-118.pdf

Ni-doped TiO2 nanotubes for wide-range hydrogen sensing

Surface modification and ions doping are the efficient ways to enhance photocatalytic activity of titanium oxide. The cobalt-phosphate/Ni-doped TiO2 nanotubes composite photoanodes were fabricated via electrodeposition method and its photocatalytic properties were investigated. The cobalt-phosphate/Ni-doped TiO2 composite photoanodes were characterized by scanning electron microscope, Raman spectrum, X-ray photoelectron spectroscopy and diffuse reflectance absorption spectra. The photocatalytic activity of the cobalt-phosphate/Ni-doped TiO2 composite photoanodes exhibited a remarkable improvement. The experimental results indicated that the onset potential of cobalt-phosphate/Ni-doped TiO2 composite photoanodes shifted to more negative position compared to that of the undoped TiO2. The photocatalytic performance of the cobalt-phosphate/Ni-doped TiO2 composite photoanodes is attractive for further investigation of solar water oxidation.

Cobalt-phosphate/Ni-doped TiO2 nanotubes composite photoanodes for solar water oxidation

This work reports the facile synthesis of reduced N/Ni-doped TiO2 nanotubes photoanodes and their photocatalytic activity application. The obtained photoanodes were characterized by scanning electron microscope (SEM), Raman spectrum (Raman), X-ray photoelectron spectroscopy (XPS) and diffuse reflectance absorption spectra. The narrowed band gap of TiO2 due to the doping of N and Ni elements could enhance the light absorption effectively. The NaBH4 reduction process resulted in the formation of oxygen vacancies in photoanodes. The electrochemical characterization revealed that photo-induced carriers were more efficient charge separation and transportation in reduced N/Ni-doped TiO2 nanotubes photoanodes. The highest photocurrent density obtained from reduced N/Ni-doped TiO2 nanotubes photoanodes was 2.52 mA cm−2 at 0 V vs. Ag/AgCl in 1 M KOH solution, which was about five times as high as that obtained from undoped TiO2.

Reduced N/Ni-doped TiO2 nanotubes photoanodes for photoelectrochemical water splitting

Surface modification with oxide nanostructures is one of the efficient ways to improve physical or biomedical properties of shape memory alloys. This work reports a fabrication of highly ordered Ti-Ni-O nanotube arrays on Ti-Ni alloy substrates through pulse anodization in glycerol-based electrolytes. The effects of anodization parameters and the annealing process on the microstructures and surface morphology of Ti-Ni-O were studied using scanning electron microscope and Raman spectroscopy. The electrolyte type greatly affected the formation of nanotube arrays. A formation of anatase phase was found with the Ti-Ni-O nanotube arrays annealed at 450 °C. The oxide nanotubes could be crystallized to rutile phase after annealing treatment at 650 °C. The Ti-Ni-O nanotube arrays demonstrated an excellent thermal stability by keeping their nanotubular structures up to 650 °C.

https://www.mdpi.com/1996-1944/7/4/3262/pdf

Qiang Liu

Papers

Black Ni-doped TiO2 photoanodes for high-efficiency photoelectrochemical water-splitting

Ni-doped TiO2 nanotubes for wide-range hydrogen sensing

Cobalt-phosphate/Ni-doped TiO2 nanotubes composite photoanodes for solar water oxidation

Reduced N/Ni-doped TiO2 nanotubes photoanodes for photoelectrochemical water splitting

Anodic Fabrication of Ti-Ni-O Nanotube Arrays on Shape Memory Alloy.