A comparative evaluation of the photocatalytic and optical properties of nanoparticulate ZnO synthesised by mechanochemical processing

In this study, mechanochemical processing has been used to manufacture a nanoparticulate powder of ZnO with a controlled particle size and minimal hard agglomeration. The suitability of this ZnO powder for use as either a photocatalyst or an optically transparent UV-filter was evaluated by comparing its optical and photocatalytic properties with those of three commercially available powders that were synthesised by chemical precipitation and flame pyrolysis. The ZnO powder synthesised by mechanochemical processing was found to exhibit high optical transparency and low photocatalytic activity per unit of surface area, which indicates that it is suitable for use in optically transparent UV-filters.

Nanopowders synthesized by mechanochemical processing

The activation of chemical reactions by milling reactants in a ball mill is presented here as a novel, low cost method for the synthesis of wide range of nanopowders with mean particle sizes as small as 4 nm. The factors controlling such mechanochemical reactions are discussed with respect to their influence on particle size, size distribution, and dispersion.

Nanocrystalline zirconia powders synthesised by mechanochemical processing

Mechanochemical processing of zirconium and yttrium chloride precursors with lithium hydroxide has been used to synthesise ultrafine powders of yttria-stabilised zirconia. The precursors reacted during milling to form a composite consisting of nanocrystalline oxide grains embedded within a matrix of lithium chloride. The ultrafine powder was recovered subsequently by removing the lithium chloride through washing with deionised water and methanol. The powders were characterised using X-ray diffraction (XRD), transmission electron microscopy (TEM), and BET gas adsorption. The sintering behaviour of cold pressed pellets was examined by dilatometry.

Synthesis of ultrafine ceria powders by mechanochemical processing

The synthesis of ultrafine cerium dioxide (CeO₂) powders via mechanochemical reaction and subsequent calcination was studied. Anhydrous CeCl₃ and NaOH powders, along with NaCl diluent, were mechanically milled. A solid-state displacement reaction—CeCl₃+ 3NaOH → Ce(OH)₃+ 3NaCl—was induced during milling in a steady-state manner. Calcination of the as-milled powder in air at 500°C resulted in the formation of CeO₂ nanoparticles in the NaCl matrix. A simple washing process to remove the NaCl yielded CeO₂ particles ∼10 nm in size. The particle size was controlled in the range of ∼10–500 nm by changing the calcination temperature.

Mechanochemical synthesis of nanoparticulate ZnO–ZnWO4 powders and their photocatalytic activity

In this study, mechanochemical reaction systems with H₂WO₄ as a precursor were investigated for the synthesis of nanoparticulate powders of WO₃, ZnWO₄, and dual-phase (ZnWO₄)_x(ZnO)_1–x. The objective was to establish whether mechanochemical processing can be used to manufacture high activity photocatalysts in the ZnO–WO₃ system. Milling and heat treatment of H₂WO₄ + 12NaCl was found to result in the formation of irregularly shaped platelets of a sodium tungstate rather than nanoparticles of WO₃. Powders of single-phase ZnWO₄ and dual-phase (ZnWO₄)_x(ZnO)_1–x were successfully synthesised by incorporating H₂WO₄ into the ZnCl₂ + Na₂CO₃ + 4NaCl reactant mixture. The photocatalytic activity of these powders was evaluated using the spin-trapping technique with electron paramagnetic resonance spectroscopy. It was found that the photocatalytic activity decreased with the ZnWO₄ content. This decrease in activity was attributed to the larger average particle size of the ZnWO₄ component compared to the ZnO, which reduced the surface area available for interfacial transfer of the photogenerated charge carriers.

Mechanochemical synthesis and characterization of GaN nanocrystals

A solid-state displacement reaction of Ga₂O₃ with Mg₃N₂ has been used to synthesize GaN nanocrystals by mechanochemical processing. X-ray diffraction, transmission electron microscopy (TEM) and selected area electron diffraction (SAED) measurements indicated that the nanocrystals had a hexagonal structure and sizes ranging from 4 to 20thinspnm. Optical absorption and transmission measurement showed the bandgap of the nanocrystals was consistent with that of bulk GaN samples (3.43thinspeV). This study  demonstrates that mechanochemical processing has significant potential for the synthesis of GaN nanocrystals in a simple and efficient way.

Recent developments in mechanochemical nanoparticle synthesis

The results of recent investigations of the mechanochemical synthesis of CaC0₃, Cr₂0₃ and Nb₂0₅ nanopowders are reported. With all three materials studied, it is shown that the volume fraction of the matrix phase is crucial to the formation of separate, unagglomerated particles. With Cr₂0₃ and Nb₂0₅, amorphous particles were formed by mechanochemical reaction and low temperature heat treatment was required for crystallization. It is shown that, as a 'bottom up' process, mechanochemical processing enables the building up nanoparticles through solid-state chemical reaction in a microscopically uniform environment, leading to the formation of nanoparticles with narrow size distributions.

Mechanochemical synthesis of nanocrystalline SnO2-ZnO photocatalysts

Mechanochemical processing of anhydrous chloride precursors with Na₂CO₃ has been investigated as a means of manufacturing nanocrystalline SnO₂ doped ZnO photocatalysts. High-energy milling and heat-treatment of a 0.1SnCl₂+0.9ZnCl₂+Na₂CO₃+4NaCl reactant mixture was found to result in the formation of a composite powder consisting of oxide grains embedded within a matrix of NaCl. Subsequent washing with deionized water resulted in removal of the NaCl matrix phase and partial hydration of the oxide reaction product with the consequent formation of ZnSn(OH)₆. The extent of this hydration reaction was found to decrease in a linear fashion with the temperature of the post-milling heat-treatment over the range of 400–700 °C. For a heat-treatment temperature of 700 °C, the SnO₂ doped ZnO powder was found to exhibit significantly higher photocatalytic activity than either single-phase SnO₂ or ZnO powders that were synthesized using similar processing conditions. The heightened photocatalytic activity of the SnO₂ doped ZnO was attributed to its higher specific surface area and the enhanced charge separation arising from the coupling of ZnO with SnO₂.

Mechanochemical synthesis of nanoparticles

The results of recent investigation of the mechanochemical synthesis of inorganic nanoparticles are reviewed. It was demonstrated that, by selecting suitable chemical reaction paths, stoichiometry of starting materials and milling conditions, mechanochemical processing can be used to synthesise a wide range of nanocrystalline particles dispersed within a soluble salt matrix. Selective removal of the matrix phase by washing the resulting powder with appropriate solvents can yield nanoparticles of the desired phase. This technique has been shown to have advantages over other methods of producing nanoparticles in terms of low cost, small particle sizes, low agglomeration, narrow size distributions and uniformity of crystal structure and morphology.

UV-shielding ceramic nanoparticles synthesised by mechanochemcial processing

ZnO, TiO₂ and CeO₂ are known as UV-shielding ceramic materials that have advantages over organic UV absorbers for their photo-stability and non-hazardous nature to human bodies. However, they normally cause low transparency in the visible-light range due to light scattering by large particles, which is undesirable for many transparent UV-blocking applications in cosmetic and plastic industries. Light-scattering efficiency of particles can be drastically reduced by decreasing the particle sizes down below 100 nm. This paper reviews recent investigation on the synthesis of ZnO and CeO₂ nanoparticles by mechanochemical processing. The resulting particles had a significantly low degree of agglomeration, having mean particle sizes of ~ 25 nm and ~ 10 nm, respectively. The aqueous suspensions of the nanoparticles showed strong absorption in the UV-light range and high transmittance in the visible-light range. Mechanochemical processing offers the possibility of industrial-scale production of transparent UV-shielding ceramic particles for many applications.

Mechanochemical synthesis of ceramic powders with perovskite structure

The mechanical activation is one of the most effective method for obtaining highly disperse system due to mechanical action stress fields form in solids during milling procedure. This effect results in changes of free energy, leading to release of heat, formation of a new surface, formation of different crystal lattice defects and initiation of solid-state chemical reaction. The accumulated deformation energy determines irreversible changes of crystal structure and consequently microstructure resulting in the change of their properties. Mechanochemical processing route has been developed recently for the production of intermetallic and alloy compounds. The intrinsic advantage of this process is that the solid-state reaction is activated due to mechanical energy instead of the temperature. It was shown that the chemical reactivity of starting materials could be improved significantly after mechanochemical activation and, subsequently, the calcination temperature was reduced. Besides, it was apparent that the mechanochemical treatment could enhance the reactivity of constituent oxides; however, the sintering process could not be avoided to develop the desired ceramics. A novel mechanochemical technique for synthesis of fine-grained perovskite structured powders has shown that it is possible to form perovskite at room temperature. The effect of milling on the formation of perovskite structure of barium titanate (BT), lead titanate (PT), PZT, PZN, magnesium niobate (PMN) and LM ceramic materials was analyzed. The dielectric properties of sintered ceramics are comparable with those prepared by other methods in the literature. (C) 2003 Elsevier B.V. B.V. All rights reserved.

Influence of heavy rare earth ions substitution on microstructure and magnetism of nanocrystalline magnetite

In this work we report results on the influence of heavy rare earth ions substitution on microstructure and magnetism of nanocrystalline magnetite. A series of Fe(2.85)RE(0.15)O(4) (RE = Gd, Dy, Ho, Tm and Yb) samples have been prepared by high energy ball milling. Structure/microstructure investigations of two selected samples Fe(2.85)Gd(0.15)O(4) and Fe(2.85)Tm(0.15)O(4), represent an extension of the previously published results on Fe(3)O(4)/gamma-Fe(2)O(3), Fe(2.85)Y(0.15)O(4) and Fe(2.55)In(0.45)O(4) [Z. Cvejic, S. Rakic, A. Kremenovic, B. Antic, C. Jovalekic. Ph. Colomban, Sol. State Sciences 8 (2006) 908], while magnetic characterization has been done for all the samples. Crystallite/particle size and strain determined by X-ray diffractometry and Transmission electron microscopy (TEM) confirmed the nanostructured nature of the mechanosynthesized materials. X-ray powder diffraction was used to analyze anisotropic line broadening effects through the Rietveld method. The size anisotropy was found to be small while strain anisotropy was large, indicating nonuniform distribution of deffects in the presence of Gd and Tm in the crystal structure. Superparamagnetic(SPM) behavior at room temperature was observed for all samples studied. The Y-substituted Fe(3)O(4) had the largest He and the lowest M(S). We discuss the changes in magnetic properties in relation to their magnetic anisotropy and microstructure. High field irreversibility (H>20kOe) in ZFC/FC magnetization versus temperature indicates the existence of high magnetocrystalline and/or strain induced anisotropy. (C) 2008 Elsevier B.V. All rights reserved.

Effect of particle size on the photocatalytic activity of nanoparticulate zinc oxide

In this study, a three-stage process consisting of mechanical milling, heat treatment, and washing has been used to manufacture nanoparticulate ZnO powders with a controlled particle size and minimal agglomeration. By varying the temperature of the post-milling heat treatment, it was possible to control the average particle size over the range of 28–57 nm. The photocatalytic activity of these powders was characterized by measuring the hydroxyl radical concentration as a function of irradiation time using the spin-trapping technique with electron paramagnetic resonance spectroscopy. It was found that there exists an optimum particle size of approximately 33 nm for which the photocatalytic activity is maximized. The existence of this optimal particle size is attributable to an increase in the charge carrier recombination rate, which counteracts the increased activity arising from the higher specific surface area for a sufficiently small particle size.