Colloidal inorganic nanocrystals stand out as an important class of advanced nanomaterials owing to the flexibility with which their physical-chemical properties can be controlled through size, shape, and compositional engineering in the synthesis stage and the versatility with which they can be implemented into technological applications in fields as diverse as optoelectronics, energy conversion/production, catalysis, and biomedicine. The use of microwave irradiation as a non-classical energy source has become increasingly popular in the preparation of nanocrystals (which generally involves complex and time-consuming processing of molecular precursors in the presence of solvents, ligands and/or surfactants at elevated temperatures). Similar to its now widespread use in organic chemistry, the efficiency of "microwave flash heating" in dramatically reducing overall processing times is one of the main advantages associated with this technique. This Review illustrates microwave-assisted methods that have been developed to synthesize colloidal inorganic nanocrystals and critically evaluates the specific roles that microwave irradiation may play in the formation of these nanomaterials.

Microwave-assisted synthesis of colloidal inorganic nanocrystals.

Microwave-assisted preparation of inorganic nanostructures in liquid phase.

Gas sensors based on a chemiresistive metal oxide semiconductor are widely used including nitrogen dioxide (NO 2 ) at a moderate temperature. In this work efforts are taken to fabricate NO 2 gas sensor using thin films of tungsten oxide (WO 3 ) grown directly on to a soda-lime glass substrate without assistance of any seed layer by a simple and a facile hydrothermal technique. As per our knowledge, the deposition of nanostructured WO 3 thin films without assistance of any seed layer on the glass substrate was rarely reported. The WO 3 thin film samples were synthesized at various deposition times ranging from 3 h to 7 h and were characterized by X-ray diffraction, Raman spectroscopy, field emission scanning electron microscopy, transmission electron microscopy, UV–vis spectroscopy and Brunauer-Emmett-Teller techniques. The surface morphological and structural characterization showed the two dimensional (2D) nanoplate-like structure of as synthesized WO 3 thin films with plate thickness ranging from 90 to 150 nm and had an orthorhombic structure, respectively. Moreover, the 2D nanoplates of WO 3 exhibited a gas response ∼10 for 5 ppm for toxic NO 2 gas at relatively low operating temperature. The new synthesis route and sensing behavior of as synthesized WO 3 nanoplates revealed a promising candidate for the fabrication of the cost effective gas sensors.

Sensitive and selective NO2 gas sensor based on WO3 nanoplates

The morphology-controlled synthesis and near-infrared (NIR) absorption properties of W18O49 were systematically investigated for the application of innovative energy-saving windows. Various morphologies of W18O49, such as nanorods, nanofibers, nanograins, nanoassembles, nanoplates, and nanoparticles, with various sizes were successfully synthesized by solvothermal reactions using organic alcohols as reaction media and WCl6, W(EtO)6, and WO3 solids as the tungsten source. W18O49 nanorods of less than 50 nm in length showed the best optical performance as an effective solar filter, which realized high transmittance in the visible region as well as excellent shielding properties of NIR light. Meanwhile, the W18O49 nanorods also exhibited strong absorption of NIR light and instantaneous conversion of the absorbed photoenergy to the local heat.

Morphology-controlled synthesis of W18O49 nanostructures and their near-infrared absorption properties.

Nanoparticles probably constitute the largest class of nanomaterials. Nanoparticles of several inorganic materials have been prepared by employing a variety of synthetic strategies. Besides synthesizing nanoparticles, there has been considerable effort to selectively prepare nanoparticles of different shapes. In view of the great interest in inorganic nanoparticles evinced in the last few years, we have prepared this perspective on the present status of the synthesis of inorganic nanoparticles. This article includes a brief discussion of methods followed by reports on the synthesis of nanoparticles of various classes of inorganic materials such as metals, alloys, oxides chalcogenides and pnictides. A brief section on core-shell nanoparticles is also included.

Recent progress in the synthesis of inorganic nanoparticles

WO3 nanoparticles in the 5−30 nm range have been prepared by solvothermal treatment of tungsten chloride in benzyl alcohol up to 210 °C followed by annealing in air. The solvothermal treatment has been performed by resistive or microwave heating and leads to agglomerates of tungsten oxide nanoparticles covered by organic residues. As determined by thermogravimetric measurements in air, the organic residues have been removed at 350 °C for samples prepared by microwave heating and at least at 420 °C for those prepared by resistive heating. When resistive heating is used, the specific surfaces are in the 20−90 m2/g range corresponding to particles of 10−30 nm, but smaller sizes, 140 m2/g of specific surface and 6 nm in diameter, can been obtained by fast microwave heating.

M. Comet

Papers

WO3 Nanoparticles in the 5−30 nm Range by Solvothermal Synthesis under Microwave or Resistive Heating