First Principles Studies On Adsorption Property Of Potassium Sodium Tantalate Niobate (001) Surfaces

Paraelectric K1yNayTa1xNbxO3(KNTN) crystal has come to prominencebecause of their excellent electro-optic properties, and has been a promisingmaterial for electro-optic modulators and electroholography applications. For allthese applications, the surface structure and the associated surface electronic andphysics properties are of primary importance. Molecules adsorption would affect thestructure, electrical and optical properties of material surface, and influence theperformance of the devices based on this material. First-principles method iseffective for calculation of geometry structure, electronic structure and opticalproperties of crystals and surfaces in materials. Therefore, the adsorption of waterand carbon dioxide molecules on KNTN surface terminations were studied byfirst-principles calculations.Firstly, clean KNTN (001) surface models were established, suggested byprevious studies. To obtain a stable structure, the geometry optimization task wasdone by performing an iterative process in which the coordinates of the atoms areadjusted so that the total energy of the structure is minimized. After geometricoptimization, the surface energies, band structure and density of states of KNTNsurfaces were evaluated and the results with KNTN bulk and KTN surfaces werecompared. The calculated results show good agreement with the availableexperimental and theoretical data.Secondly, We present a density functional theory study of water adsorption onKNTN (001) surface terminations.The energetically favorable configurations forwater monomer adsorption is at the K-Na bridge site on the K(Na)O-termination andthe Nb top site on the Nb(Ta)O2-termination. The bond formation between watermolecule and KNTN surface were investigated by analysis of difference electrondensity and partial density of states. With the coverage increasing, the valence bandmaximum of KNTN surface shifts towards lower energy obviously, resulting in bandgap widening. The absorption coefficient and refractive index spectrum of KNTNsurface show obvious blueshift with coverage. Upon illumination by light with awavelength greater than350nm, the refractive index decreases with increasingwater coverage.Finally, the adsorption of CO2on KNTN (001) surface terminations wasstudied. The preferred adsorption sites are found to be the Na-O bridge site on theK(Na)O-termination and O top site on the Nb(Ta)O2-termination. In the mostfavorable configurations, the CO2molecule is bent and the C O bonds are elongated, indicating the CO2activation. After adsorption, the band gap of the surface isenlarged, relevant to the electrons density redistribution. In the coverage rangebetween0and2ML, the valence band maximum of KNTN surface shifts towardslower energy obviously, and the absorption coefficient and refractive index spectrumexhibit a blueshift with increasing coverage. With wavelengths greater than350nm,the refractive index of KNTN surface decreases with the coverage increasing, andthe variation is significantly greater than the case of water adsorption. The resultsillustrate that the electronic structure and optical properties of KNTN surface issensitive with CO2adsorption, and provide fundamental insight into the CO2adsorption mechanism on KNTN surface and potential application on gas sensing.