pH-controlled assembly of three-dimensional tungsten oxide hierarchical nanostructures for catalytic oxidation of cyclohexene to adipic acid

Abstract

Tungsten oxide has a wide range of applications in chromic devices and catalysis. Effective control over the morphology of tungsten oxide remains a major challenge. Here, we present a facile pH-controlled strategy for synthesizing three-dimensional tungsten oxide architectures with different morphologies without using any templates or surfactants. These hierarchical architectures with rod-like, disk-like and spherelike morphologies are assembled from one-dimensional tungsten oxide nanowires/nanorods. The influence of the single parameter of the pH value on the morphologies and crystal structures of tungsten oxide is systematically studied. The results indicate that different polytungstate anions obtained at various pH values give rise to different crystal nuclei by a dehydration process. These crystal nuclei grow into similar phases (crystal structures) but distinct morphologies via hydrothermal treatment. Here, the similarity of the crystal structures of h-WO 3 and o-WO 3 •0.33H 2 O plays a key role in the structural transformation. Further experiments show that these three-dimensional tungsten oxide architectures display promising applications as catalysts in green chemistry for the oxidation of cyclohexene to adipic acid using an environmentally friendly oxidant (hydrogen peroxide).

Biomimetic synthesis of nanostructured WO 3 · H 2 O particles and subsequent thermal conversion to WO 3

“…[28–32]. The ions can be precipitated as hydrous tungsten oxides (WO 3 · x H 2 O) [33,34], tungstate (H 2 WO 4 , H 4 WO 5 , etc.) [35,36] crystals or WO 3 .…”

Nano- and micro-structural control of WO3 photoelectrode films through aqueous synthesis of WO3·H2O and (NH4)0.33WO3 precursors

“…Nanostructures (such as nanoparticles, nanowires, and nanorods) provide a larger surface area, that is, a larger number of active sites for photoelectrochemical reactions, which results in a more effective photo-energy conversion. 2,[6][7][8] In addition, submicron-and micron-scale porous structures are reported to work as light-scattering layers, where the path length of the incident light in the lm materials increases, which leads to more effective light absorption. [9][10][11][12][13] Thus, the fabrication techniques of nano-and micro-structures are widely investigated for the practical application of WO 3 materials.…”

“…Tungsten species are reported to exist as monomeric tungstate ions (WO , and so forth) in an aqueous media, [19][20][21][22][23] and such tungstate ions can deposit as various types of hydrous tungsten oxides (WO 3 $xH 2 O) and tungstates (H 2 WO 4 , H 4 WO 5 and so forth). 6,8,24,25 Previously, we have reported the preparation of nanostructured WO 3 particle materials, where WO 3 $H 2 O layered platy particles were rst obtained as precursors through an aqueous solution process and then thermally converted to monoclinic WO 3 . 26 In this case, the macroscopic layered structures of the precursors remained even aer the thermal conversion to WO 3 .…”

Metal oxide nanostructures for sensor applications

“…WO3 can present different crystallographic phases, such as cubic, hexagonal, monoclinic, triclinic, tetragonal and orthorhombic[260][261][262][263][264]. The cubic WO3 is hardly obtained experimentally, but oxygen vacancies in the WO3 lattice are capable of increasing the cell symmetry from monoclinic via tetragonal to cubic phase[265].The most studied WO3 phases are the monoclinic (P21/n), orthorhombic (Pbcn) and hexagonal (P6/mmm).…”

“…The cubic WO3 is hardly obtained experimentally, but oxygen vacancies in the WO3 lattice are capable of increasing the cell symmetry from monoclinic via tetragonal to cubic phase[265].The most studied WO3 phases are the monoclinic (P21/n), orthorhombic (Pbcn) and hexagonal (P6/mmm). The monoclinic phase lattice parameter was reported to be a= 0.7301 nm, b= 0.7539 nm, and c= 0.7689 nm[266] and consists of WO6 octahedral connected by corner sharing of oxygen atoms, connected in the a-, b-and c-directions; the orthorhombic phase with lattice parameters of a= 0.7333 nm, b= 0.7573 nm, and c= 0.7740 nm[267] is based on two octahedral, WO5(H2O) (with two oxygen atoms forming W=O and W-OH2 bonds) and WO6 with W-O bonds sharing the same bond length; the hexagonal phase with cell parameters a= 0.7298 nm and c= 0.7798 nm[268] consists of WO6 octahedral structures sharing equatorial oxygen atoms, forming trigonal and hexagonal tunnels[261,264]. Both monoclinic and hexagonal phases consist of WO6 octahedral, but with different arrangements.Figure 13showsSEM images of the different morphologies and WO3 crystallographic phases.…”