Computational study of NO 2 sensing on the WO 3 (001) surface is presented. Our ab initio calculations reveal a two-step process of NO 2 detection on the WO 3 surface. In a first step the NO 2 molecule is dissociated at an oxygen vacancy site, but a NO molecule remains adsorbed. In a second step NO is re-oxidized into NO 2 by O 2 of the surrounding air leading to the resistance increase which is experimentally observed. We also calculate the adsorption energy of NO on stoichiometric and non-stoichiometric WO 3 surfaces and propose a method for the NO detection.
A detailed study of the formation of neutral oxygen vacancies in monoclinic tungsten oxide (WO 3 ) is performed within the framework of the self-consistent first-principles SIESTA method. This work reveals that the neutral oxygen vacancies are anisotropic with a strong correlation to the structural anisotropy of the WO 3 monoclinic room temperature phase. We show that most of the structural relaxation around the vacancies occurs along a single straight line of W-O-W bonds and that the lowest energy corresponds to the formation of vacancies along the [001] crystallographic direction, where long and short W-O bonds alternate. Moreover, vacancies lead to the partial filling of the conduction band, in which 5d electronic states of the neighbouring W atoms dominate. In term of Mulliken population, the initial charge carried by the removed oxygen atom is almost recovered on the O and W atoms closest to the vacancy.
The gas response of tungsten trioxide (WO 3) based sensors strongly depends on the surface properties. Reconstructed surfaces and oxygen point defects at the surface of the monoclinic WO 3 are studied using a self-consistent scheme based on first-principle. The oxygen vacancy is found to be the predominant defect independently of the oxygen partial pressure. Indeed, under rich oxygen atmosphere the formation enthalpies are found to be 1.45 eV in LDA (1.28 eV in GGA) for the oxygen vacancy instead of 2.70 eV (2.42 eV) for the oxygen adatom. When the oxygen partial pressure is lowered, the oxygen vacancy formation enthalpy decreases and becomes exothermic under very O-poor condition (-1.65 eV in LDA and-1.36 eV in GGA). On the other hand, the formation enthalpy of an oxygen adatom rises. Finally, the oxygen vacancy formation acts as a n-doping by introducing negative charge carriers at the bottom of the conduction band. All these results can be very helpful in order to explain the electrical resistivity measurements.
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