The Oxygen Vacancy in Crystal Phases of WO₃

Abstract

The oxygen vacancy in WO(3) has previously been implicated in the electrochromism mechanism in this material. Previous theoretical calculations on the oxygen vacancy in WO(3) have not considered the full range of crystal structures adopted by the material. Here we report studies of the oxygen vacancy in seven crystal phases. The use of a very accurate tungsten plane-wave pseudopotential means that a byproduct of this study is a more detailed and complete picture of undefected WO(3) than previously available. Electronic structures of the crystal phases in both undefected and defected systems have been calculated and are discussed. The band gap in WO(3) is dependent upon bonding-antibonding interactions, these being dependent upon overlap in each direction. The effect of an oxygen vacancy is dependent upon the availability of both Op and Wd electrons, this being different for the various phases. A variety of behavior is predicted, which may be explained in terms of O2p-W5d mixing, including the formation of long W-W dimer bonds. It is found that the nature of a polaron in this material is dependent upon both the crystal structure and distribution of oxygen vacancies.

Structural and optical properties of visible active photocatalytic WO3 thin films prepared by reactive dc magnetron sputtering

“…47 Under these sputter conditions, significant amounts of oxygen vacancies may also be created, which give rise to polarons (see below). 34 In nano-WO 3 thin films, both grain size and substrate interactions are suggested to change the relative thermodynamic stability of WO 3 phases. 37 A transition to a compressed phase can be obtained either by using high external pressure or by accumulating internal compressive strain due to polarons.…”

“…29,30 For small values of substoichiometry, x, crystals of WO 3Àx are reported to undergo a structural phase transition from the room temperature d phase to the low temperature e phase at T ; 250 K. 31 The d phase shows metallic behavior, and the e phase has insulating properties for the electron/hole carriers. 32 Reported values for the electronic band gap of nano-WO 3 vary considerably from E g 5 2.6 to 3.25 eV, 33,34 and this is probably due to a combination of one or several of the effects discussed above. Further, the small size of nano-WO 3 crystallites will enhance the surface energy of the system.…”

Defect calculations in semiconductors through a dielectric-dependent hybrid DFT functional: The case of oxygen vacancies in metal oxides

“…77 However, the microscopic origin of this effect, which is intimately connected with the typical blue coloration of WO 3−x films, 78,79 has been longly debated, and several models have been put forward for rationalizing experimental observations. 77,80,81 On the theory side, a few first-principle investigations have been performed on O-deficient RT monoclinic WO 3 , [82][83][84] but among them only one 85 employed state-of-the-art methods yielding a correct description of the bulk electronic structure. The latter study of Wang et al, 85 revealed a delicate interplay between the concentration of O vacancies and the metallic or insulating nature of WO 3−x .…”

Synthesis and characterization of tungstite (WO3·H2O) nanoleaves and nanoribbons

“…The most common methods to synthesize tungstite/tungsten oxide are electrochemical [8], solution-based colloidal [9], bioligation [10][11], chemical vapor deposition [12][13], and hydrothermal (HT) [14][15]. A simple heat-treatment of tungstite (WO 3 .H 2 O) allows for the phase transformation to tungsten oxide (WO 3 ), an important class of n-type semiconductors with a tunable band gap of 2.5-2.8 eV [16]. Moreover, its high chemical stability, low production costs and non-toxicity have recently generated significant interests for a wide variety of applications in microelectronics and optoelectronics [17][18], super-hydrophilic thin films [15], dye-sensitized solar cells [19], colloidal quantum dot LEDs [20], photocatalysis [21] and photoelectrocatalysis [22], water splitting photocatalyst as main catalyst [23][24][25][26][27][28][29][30][31][32][33][34].…”