Use of metal tungsten bronze electrodes in chemical analysis

Mechanism and Kinetics of the Phase Formation and Dissolution of NaxWO3 on a Pt Electrode in a Na2WO4-WO3 Melt

A comprehensive study concerning the phase formation mechanism and growth/dissolution kinetics of sodium tungsten bronze crystals during the electrolysis of a 0.8Na2WO4-0.2WO3 melt was carried out. The regularities of deposit formation on a Pt(111) working electrode were investigated experimentally using cyclic voltammetry, chronoamperometry, scanning electron microscopy, and X-ray diffraction analysis. Models have been developed to calculate the current response during the formation, growth and dissolution of a two-phase deposit consisting of NaxWO3 and metallic tungsten or two oxide tungsten bronzes with different sodium content. These models consider mass transfer to the electrode and nuclei; chemical and electrochemical reactions with the participation of polytungstate ions, Na+, Na0, and O2-; as well as the ohmic drop effect. The approach was proposed to describe the dissolution of an NaxWO3 crystal with a nonuniform sodium distribution. The fitting of cyclic voltammograms was performed using the Levenberg-Marquardt algorithm. The NaxWO3 formation/growth/dissolution mechanism was determined. Concentration profiles and diffusion coefficients of [WnO3n]-, reaction rate constants, number density of nuclei, and time dependencies of crystal size were calculated. The proposed approaches and models can be used in other systems for the cyclic voltammogram analysis and study of the mechanism and kinetics of electrode processes complicated by phase formation; parallel and sequential electrochemical and chemical reactions; as well as the formation of a deposit characterized by a nonuniform phase and/or chemical composition.

Cation Defect Mediated Phase Transition in Potassium Tungsten Bronze

Applying in situ transmission electron microscopy, the phase instability in potassium tungsten bronze (K_xWO₃, 0.18 < x < 0.57) induced by heating was investigated. The atomistic phase transition pathway of monoclinic K_0.20WO₃ → hexagonal K_mWO₃ (0.18 < m < 0.20) → cubic WO₃ induced by cationic defects (K and W vacancies) was directly revealed. Unexpectedly, a K⁺-rich tetragonal K_nWO₃ (0.40 < n < 0.57) phase would nucleate as well, which may result from the blockage of K⁺ diffusion at the grain boundaries. Our results point out the critical role of the cationic defects in mediating the crystal structures in K_xWO₃, which provide reference to rational structural design for extensive high-temperature applications.

Ordered and twinned structure in hexagonal-based potassium tungsten bronze nanosheets

Non-stoichiometric hexagonal-based potassium tungsten bronze (KxWO3) nanosheets were synthesized by oxidizing tungsten foil in potassium hydroxide. The tungsten bronze nanosheets exhibited an ordered monoclinic superstructure as revealed by X-ray diffraction patterns. Further detailed structural investigation by employing electron microscopy techniques showed the coexistence of 120° rotation twinning variants in the superstructure phase, which may result from the rotation symmetry reduction induced by the ordered arrangements of K vacancies during crystal growth.

Self-assembly of KxWO3 nanowires into nanosheets by an oriented attachment mechanism

The KxWO3 nanosheets consisting of superfine nanowires were successfully synthesized in ambient air. The detailed electron microscopy and X-ray diffraction investigations imply that the nanosheets were obtained by self-assembly of the ordered nanowires with exposed {0110}H facets. The sheet morphology is closely related with the growth conditions including temperature and time, etc. A possible mechanism based on the oriented attachment of neighboring nanowires for the formation of nanosheets is proposed. Our results shed light on the interfacial characteristics of self-assembled KxWO3 nanowires and can serve as guidance to the future design of relevant two-dimensional structures for various electrical and optical applications.

A Simple Hydrothermal Method for the Large-Scale Synthesis of Single-Crystal Potassium Tungsten Bronze Nanowires

The large-scale synthesis of single-crystal K(x)WO(3) tungsten bronze nanowires has been successfully realized by a hydrothermal method under mild conditions. Uniform K(0.33)WO(3) nanowires with diameters of 5-25 nm and lengths of up to several micrometers are obtained. It is found that the morphology and crystallographic forms of the final products are strongly dependent on the sulfate and citric acid, which may act as structure-directing and soft-reducing agent, respectively. Some other influential factors on the growth of tungsten bronze nanowires, such as temperature and reaction time, are also discussed. It is worth noting that other alkali metal tungsten bronzes such as (NH(4))(x)WO(3), Rb(x)WO(3), and Cs(x)WO(3) could also be selectively synthesized by a similar route. Thus, this novel and efficient method could provide a potential mild route to selectively synthesize various tungsten bronze on-dimensional nanomaterials.