UV−visible diffuse reflectance spectroscopy was used to probe the electronic structure and domain size of tungsten oxide species in crystalline isopolytungstates, monoclinic WO3, and dispersed WOx species on ZrO2 surfaces. UV−visible absorption edge analysis, CO2 chemisorption, and Raman spectroscopic results show that three distinct regions of WOx coverage on ZrO2 supports appear with increasing WOx surface density:  submonolayer region (0−4 W nm-2), polytungstate growth region (4−8 W nm-2), and polytungstate/crystalline WO3 coexistence region (>8 W nm-2). The structure and catalytic activity of WOx species on ZrO2 is controlled only by WOx surface density (W nm-2), irrespective of the WOx concentration, oxidation temperature, and ZrO2 surface area used to obtain a particular density. The submonolayer region is characterized by distorted octahedral WOx species that are well dispersed on the ZrO2 surface. These species show a constant absorption edge energy, they are difficult to reduce, and contain few a...

/pdf/structure-and-electronic-properties-of-solid-acids-based-on-2ikoo481kf.pdf

Structure and Electronic Properties of Solid Acids Based on Tungsten Oxide Nanostructures

The properties and applications of molybdenum oxides are reviewed in depth. Molybdenum is found in various oxide stoichiometries, which have been employed for different high-value research and commercial applications. The great chemical and physical characteristics of molybdenum oxides make them versatile and highly tunable for incorporation in optical, electronic, catalytic, bio, and energy systems. Variations in the oxidation states allow manipulation of the crystal structure, morphology, oxygen vacancies, and dopants, to control and engineer electronic states. Despite this overwhelming functionality and potential, a definitive resource on molybdenum oxide is still unavailable. The aim here is to provide such a resource, while presenting an insightful outlook into future prospective applications for molybdenum oxides.

Molybdenum Oxides – From Fundamentals to Functionality

In the quest to discover the properties of planar semiconductors, two-dimensional molybdenum trioxide and dichalcogenides have recently attracted a large amount of interest. This family, which includes molybdenum trioxide (MoO 3 ), disulphide (MoS 2 ), diselenide (MoSe 2 ) and ditelluride (MoTe 2 ), possesses many unique properties that make its compounds appealing for a wide range of applications. These properties can be thickness dependent and may be manipulated via a large number of physical and chemical processes. In this Feature Article, a comprehensive review is delivered of the fundamental properties, synthesis techniques and applications of layered and planar MoO 3 , MoS 2 , MoSe 2 , and MoTe 2 along with their future prospects.

Two-Dimensional Molybdenum Trioxide and Dichalcogenides

One-dimensional nanostructures of orthorhombic molybdenum trioxide (α-MoO3) have been synthesized in the forms of ribbons or rods via acidification under hydrothermal conditions at 140−200 °C The reaction path has been revealed with our kinetic investigations, which shows the following sequence:  (i) from the starting compound (NH4)6Mo7O24·4H2O to (ii) formation of intermediate compound ((NH4)2O)00866·MoO3·0231H2O, and then to (iii) final α-MoO3 nanoribbons or nanorods in 100% phase purity The optimal growth temperature is in the range of 170−180 °C under the current experimental settings At higher reaction temperatures, this transformation can be accelerated, but with poorer crystal morphological homogeneity It has been found that the dimensions of these rectangular nanorods are about 50 nm in thickness, 150−300 nm (mean value at 200 nm) in width, and a few tens of micrometers in length The crystal morphology can be further altered with inorganic salts such as NaNO3, KNO3, Mg(NO3)2, and Al(NO3)3 

E.M. McCarron

Papers

Synthesis and properties of thin film polymorphs of molybdenum trioxide