In the present review we try to give a comprehensive and most up to date view to the field, with an emphasis on the currently most investigated anodic TiO2 nanotube arrays. We will first give an overview of different synthesis approaches to produce TiO2 nanotubes and TiO2 nanotube arrays, and then deal with physical and chemical properties of TiO2 nanotubes and techniques to modify them. Finally, we will provide an overview of the most explored and prospective applications of nanotubular TiO2.

One-dimensional titanium dioxide nanomaterials: nanotubes.

Low-cost and resourceful transition metal phosphides (TMPs) have gradually received wide acceptance in the energy industry through exhibiting comparable catalytic activity and long-term stability to traditional catalysts (e.g., Pt/C, LiCoO2, LiFePO4, etc.). With the emergence of the research hotspot of TMPs, probing their mechanism of catalytic energy conversion and storage inspired by the superb structure of metal-phosphorus chelate is of great significance. To this end, recent developments in TMPs with various crystal structures and morphologies have attracted much attention. The design of TMPs ranging from the choice of different transition metals to phosphorus sources has been intensively explored. This research has indicated that multidimensional morphologies of TMPs prominently enrich the patterns of charge storage and electron transportation, and ultra-nanoscaled TMPs obtained by multiple tools and techniques might challenge the threshold of electrocatalytic reactions. Here, recent developments in synthetic strategies of TMPs from different precursors are classified. The underlying mechanism between the structural and crystallographic characteristics and the tuned properties of TMPs in energy applications is also presented. Additionally, the key trends in structure and morphology characterization of TMPs are highlighted. Future perspectives on the challenges and opportunities facing TMPs catalysts are thereby proposed.

Earth-Rich Transition Metal Phosphide for Energy Conversion and Storage

The humidity dependence of the gas-sensing characteristics in SnO2-based sensors, one of the greatest obstacles in gas-sensor applications, is reduced to a negligible level by NiO doping. In a dry atmosphere, undoped hierarchical SnO2 nanostructures prepared by the self-assembly of crystalline nanosheets show a high CO response and a rapid response speed. However, the gas response, response/recovery speeds, and resistance in air are deteriorated or changed significantly in a humid atmosphere. When hierarchical SnO2 nanostructures are doped with 0.64–1.27 wt% NiO, all of the gas-sensing characteristics remain similar, even after changing the atmosphere from a dry to wet one. According to diffuse-reflectance Fourier transform IR measurements, it is found that the most of the water-driven species are predominantly absorbed not by the SnO2 but by the NiO, and thus the electrochemical interaction between the humidity and the SnO2 sensor surface is totally blocked. NiO-doped hierarchical SnO2 sensors exhibit an exceptionally fast response speed (1.6 s), a fast recovery speed (2.8 s) and a superior gas response (Ra/Rg = 2.8 at 50 ppm CO (Ra: resistance in air, Rg: resistance in gas)) even in a 25% r.h. atmosphere. The doping of hierarchical SnO2 nanostructures with NiO is a very-promising approach to reduce the dependence of the gas-sensing characteristics on humidity without sacrificing the high gas response, the ultrafast response and the ultrafast recovery.

The Role of NiO Doping in Reducing the Impact of Humidity on the Performance of SnO2-Based Gas Sensors: Synthesis Strategies, and Phenomenological and Spectroscopic Studies

Advances and new directions in gas-sensing devices

On the enhanced corrosion resistance of a selective laser melted austenitic stainless steel

The electrochemical dissolution of MnS inclusions and pit initiation processes on type 303 stainless steel were investigated by an electrochemical microcell technique. In 1 M Na 2 SO 4 , the electrochemical dissolution of MnS inclusions started at 0.1-0.3 V vs Ag/AgCl(3.33 M KCl), which was followed by the initiation of a hemispherical and smooth wall pit at MnS/matrix boundary. It was suggested that the dissolution products of MnS inclusions induced the formation of a salt film, which caused localized electropolishing. In 0.1 M NaCl, the onset potential of electrochemical dissolution of MnS inclusions was 0.3-0.4 V and the stable pit growth occurred at a potential of around 0.5 V. Field-emission scanning electron microscopy observations revealed that many metastable pits with a diameter of approximately 1 μm were formed at MnS/matrix boundary on the specimen, of which the polarization measurement was stopped immediately after stable pit initiation. From a morphological point of view, the metastable pits were found to be very similar to flat-walled etch pits. The dissolution products of MnS inclusions and chloride ions would attack the metal surface newly exposed by dissolution of the inclusions. It is proposed that the chemistry of dissolved species from MnS inclusions have a substantial influence on pit initiation at MnS inclusions.

https://iopscience.iop.org/article/10.1149/1.2745639/pdf

Microelectrochemical Measurements of Dissolution of MnS Inclusions and Morphological Observation of Metastable and Stable Pitting on Stainless Steel

Pit Initiation Mechanism at MnS Inclusions in Stainless Steel: Synergistic Effect of Elemental Sulfur and Chloride Ions

A Microelectrochemical System for In Situ High-Resolution Optical Microscopy: Morphological Characteristics of Pitting at MnS Inclusion in Stainless Steel

TiO 2 nanotube films were formed on pure Ti, Ti-Pt, and Ti-Pd thin films by anodization in a mixture of glycerol and water (1:1 volume ratio) containing 0.5 wt % NH 4 F and were applied as a resistance-type hydrogen gas sensor. A transmission electron microscopy analysis of the TiO 2 nanotube films revealed that the nanosized particles of Pt and Pd were dispersed in the wall of the nanotube. These dispersed Pt or Pd particles effectively improved the performance of the hydrogen gas sensor perhaps due to the acceleration of hydrogen chemisorption on the wall of the nanotube. Pt- and Pd-added TiO 2 nanotube sensors showed a two-order decrease in resistance upon exposure to 1000 ppm H 2 at 290°C and had little or no response to 1000 ppm CH 4 , 1000 ppm CO, and 1 ppm SO 2 .

https://iopscience.iop.org/article/10.1149/1.3374643/pdf

Izumi Muto

Papers

Microelectrochemical Measurements of Dissolution of MnS Inclusions and Morphological Observation of Metastable and Stable Pitting on Stainless Steel

Pit Initiation Mechanism at MnS Inclusions in Stainless Steel: Synergistic Effect of Elemental Sulfur and Chloride Ions

A Microelectrochemical System for In Situ High-Resolution Optical Microscopy: Morphological Characteristics of Pitting at MnS Inclusion in Stainless Steel

Hydrogen Gas Sensor Using Pt- and Pd-Added Anodic TiO2 Nanotube Films

Fabrication of nanoporous copper by dealloying amorphous binary Ti–Cu alloys in hydrofluoric acid solutions