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Nanocristalline TiO2 obtained by a facile and environment-friendly sonochemical method was subjected to thermal treatment in the temperature range of 400-900 °C in order to produce variable anatase-rutile phases ratio. The relationship between the optical bandgap and the electrochemical behavior was studied. All the stages of phase transformation of the as-prepared sample such as: nucleation, growth and coarsening were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). It was found that phase transformation mechanism stems from the redistribution of energy in the system and a critical particle size. On the other hand, the samples were characterized by UV-vis spectroscopy for the bandgap studies. The optical band gap of as-prepared sample increases to 3.31 eV with respect to 3.20 eV for bulk-anatase. This expansion could be attributed to quantum size effect. The i-E characteristics of samples with variable anatase-rutile ratio were obtained using cyclic voltammetry technique in a 0.5 M H2SO4 solution at room temperature. The foremost charge magnitude was obtained when anatase had a critical size of 17 nm. Analyzing both particle size for anatase and rutile, we observed that when rutile is the dominating phase and its size difference larger in 35% than anatase, the current reaches its minimum values. Based on electrochemical results, the optimal particle size and content phases control are important in order to obtain an increase in the electrochemical performance in the Hydrogen Evolution Reaction (HER) zone

Supersaturated solid solutions of Cu-44.5at.%Ni-22.5at.%Fe and Cu-37at.%Ni-6at.%Cr alloy were produced by ball milling of a pure chemical elemental mixture for 1080 ks. Two fcc supersaturated solid solutions with a grain size of about 20 and 50 nm, respectively, were obtained after milling. These alloys were subsequently aged at temperatures between 800 and 1003 K for different times. The aging promoted the phase decomposition of the supersaturated solid solution into a mixture of Cu-rich and Ni- phases in both the aged MA alloy powders. The growth kinetics of the modulation wavelength was determined from the X-ray diffraction results and followed the Lifshitz-Slyozov- Wagner theory for a diffusion-controlled coarsening in the mechanically-alloyed Cu-Ni-Fe alloy after aging. However, the sidebands intensity seems to be very low and overlapped with the peaks corresponding to the Cu-rich phase in the aged mechanically-alloyed Cu-Ni-Cr alloy. The growth kinetics of composition modulation wavelength for the aged MA Cu-Ni-Fe alloy was faster at 803 and 898 K than that for the same alloy composition obtained by a conventional processing and then aged at the same temperatures.

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