One-dimensional TiO2 (1D TiO2) nanomaterials with unique structural and functional properties have been extensively used in various fields including photocatalytic degradation of pollutants, photocatalytic CO2 reduction into energy fuels, water splitting, solar cells, supercapacitors and lithium-ion batteries. In the past few decades, 1D TiO2 nanostructured materials with a well-controlled size and morphology have been designed and synthesized. Compared to 0D and 2D nanostructures, more attention has been paid to 1D TiO2 nanostructures due to their high aspect ratio, large specific surface area, and excellent electronic or ionic charge transport properties. In this review, we present the crystal structure of TiO2 and the latest development on the fabrication of 1D TiO2 nanostructured materials. Besides, we will look into some critical engineering strategies that give rise to the excellent properties of 1D TiO2 nanostructures such as improved enlargement of the surface area, light absorption and efficient separation of electrons/holes that benefit their potential applications. Moreover, their corresponding environmental and energy applications are described and discussed. With the fast development of the current economy and technology, more and more effort will be put into endowing TiO2-based materials with advanced functionalities and other promising applications.

A review of one-dimensional TiO2 nanostructured materials for environmental and energy applications

On the reverse indentation size effect and microhardness measurement of solids

The microhardness indentation load/size effect (ISE) on the Knoop microhardness of single crystals of TiO2 and SnO2 has been investigated. Experimental results have been analysed using the classical power law approach and from an effective indentation test load viewpoint. The Hays/Kendall concept of a critical applied test load for the initiation of plastic deformation was considered, but rejected to explain the ISE. A proportional specimen resistance (PSR) model has been proposed that consists of the elastic resistance of the test specimen and frictional effects at the indentor facet/specimen interface during microindentation. The microhardness test load, P, and the resulting indentation size, d, have been found to follow the relationship 
$$P = a_1 d + a_2 d^2 = a_1 d + (P_c /d_0^2 ) d^2$$

 The ISE is a consequence of the indentation-size proportional resistance of the test specimen as described by a
1. a
2 is found to be related to the load-independent indentation hardness. It consists of the critical indentation load, P
c, and the characteristic indentation size, d
o.

The microhardness indentation load/size effect in rutile and cassiterite single crystals

Differential Thermal Analysis

The role of friction between the microhardness indenter and the test specimen is addressed through the analysis of dry (unlubricated) and lubricated tests on iron by Atkinson and Shi. Quantitative evaluation through a proportional specimen resistance model accurately describes the results. It suggests that friction is a major portion of the observed hardness increase at low test loads, the indentation size effect. The ISE is related to the surface-area-to-volume ratio of the indentation, which is inversely related to the indentation dimension.

The frictional component of the indentation size effect in low load microhardness testing

Mechanical characteristics of flux-grown calcium titanate and nickel titanate crystals

Microhardness, fracture mechanism and dielectric behaviour of flux-grown GdFeO3 single crystals

The results of microhardness measurements on flux-grown crystals of (i) single (pure) rare earth aluminates RAlO3 (R = Eu, Gd, Dy, Er) and rare earth orthochromites RCrO3 (R = Y, Gd, Yb), (ii) rare earth aluminates doped with neodymium, erbium, ytterbium and holmium, and (iii) mixed rare earth aluminate crystals of the type (La1−x) Pr(x)AlO3 (x=0, 0.25, 0.75 and 1.00) are presented. The variations in the microhardness value with load are non-linear in all cases. Kick's law fails to explain the observed variations. Instead, they are best explained by the application of the idea of materials resistance pressure in the modified law proposed by Hays and Kendall. The results indicate that the doping does not increase the hardness value of crystals in all cases. The hardness instead depends on the composition of the parent material as well as the dopant entering into the crystal lattice. Mixed rare earth aluminate crystals are shown to be harder than those of single rare earth aluminates.

Microhardness of flux grown pure doped and mixed rare earth aluminates and orthochromites

Fracture mechanics, crack propagation and microhardness studies on flux grown ErAlO3 single crystals

The dependence of loss tangent (tan  δ ) and both real and imaginary parts of the dielectric constant ( ɛ ′ and ɛ ″) on temperature in the range 298–923 K and frequency in the range 10 3 –10 6  Hz for flux grown CdTiO 3 single crystals is reported. The ln  σ ac versus T plots suggest the conduction mechanism to be ionic hopping conduction. From ln  σ ac versus frequency curves, it can be seen that the slope decreases with the rise in temperature, suggesting that the ionic hopping conduction diminishes with the rise in temperature. The activation energy at various fixed frequencies is calculated from the slope of the graph between ln  σ ac versus 1/ T (×10 3  K −1 ). Thermal behaviour of flux grown CdTiO 3 crystals using thermoanalytical techniques including TG, DTA and DTG is discussed. Thermal analysis suggests decomposition of CdTiO 3 in the temperature interval of 1386–1693 K leading to the formation of TiO 2 as the final product. Results obtained on application of TG based models viz. Horowitz–Metzger, Coats–Redfern and Piloyan–Novikova are reported. The results of kinetics of thermal decomposition suggest contracting cylinder model as the one that is relevant to the decomposition of CdTiO 3 . The kinetic parameters viz. the order of reaction, activation energy, frequency factor, and entropy of activation using the above mentioned models are computed.

P. N. Kotru

Papers

Mechanical characteristics of flux-grown calcium titanate and nickel titanate crystals

Microhardness, fracture mechanism and dielectric behaviour of flux-grown GdFeO3 single crystals

Microhardness of flux grown pure doped and mixed rare earth aluminates and orthochromites

Fracture mechanics, crack propagation and microhardness studies on flux grown ErAlO3 single crystals

Dielectric properties, ac conductivity and thermal behaviour of flux grown cadmium titanate crystals