Adsorption of ethanol on V2O5 (010) surface for gas-sensing applications: Ab initio investigation
Introduction
Vandium pentoxide(V2O5), as a very important n-type semiconductor material, has been extensively investigated due to its unique properties and potential for use in catalysts, lithium-ion batteries, gas sensors and thermochromic devices [1], [2], [3], [4], [5], [6]. In particular, V2O5 has an interesting layered structure, which permits a wide variety of other molecules or cations to be embedded between the layers. With water or other molecules intercalated, the large layer spacing permits the gas molecules to enter and approach the active position easily. This makes layered V2O5 become a more attractive material for high sensitive detection of ethanol gas due to its high active surface area for gas adsorption [7], [8], [9], [10], [11]. Nanostructures of single crystalline V2O5, which can be synthesized via thermal evaporation [6], hydrothermal method [8], [9], sol–gel process [10], and solvothermal method [11], have been found experimentally to possess high gas-sensing properties to ethanol vapor.
Generally, the sensing properties of semiconductor oxide-based gas sensors are evaluated by monitoring the changes of electrical conductivity resulting from adsorption of gas [12]. Thus, a systematic investigation of the gas molecules adsorption on oxide surface is important since it can brings a deep understanding of the gas-sensing mechanism as well as theoretical hints for development of high sensitive gas-sensing materials. In this point, ab initio calculation is well suitable for providing detailed and reliable information on structural and electronic processes on gas-adsorbed solid surfaces [13]. Based on ab initio calculations, studies on different adsorption systems such as NOx-ZnO [14], CO2-PuO2 [15], NOx-SnO2 [16], ethanol-Si [17], [18], ethanol-LaFeO3 [19] have been preformed. As for the case of V2O5, X. Yin et al. ever studied the adsorption of NH3 and H2O on V2O5 (010) surface via ab initio calculation for catalytic applications [20], [21]. For the studies of V2O5-based ethanol gas sensor, however, relatively rich experimental works at present show the lack of the correlative theoretical studies. Up to date, theoretical studies about the adsorption of ethanol on V2O5 are still scarce. At present, the details occurring during adsorption, such as the geometry of the adsorbed ethanol molecule, the binding mechanism, the dominant atom sites for adsorption and so on, is still vague. To gain clear insights into the surface-controlled sensing mechanism of V2O5, the microscopic adsorption process of ethanol molecule as well as the correlation with the conductivity variation of oxide has to be clarified theoretically.
In this work, we present ab initio investigation of ethanol adsorption on V2O5 (010) surface. The gas-surface interaction, adsorption energy, density of states and atomic charge transfer are thoroughly calculated and discussed. Particular attention is paid to understanding the modification of electronic structures and the quantitative feature of charge transfer due to the ethanol molecule adsorption, with aim to provide the theoretical understanding for ethanol sensing process of crystalline V2O5. Such theoretical works are highly important to in-depth understand the gas-sensing mechanism and underpin the experimental studies of V2O5-based ethanol gas sensors. On the other hand, the detailed study about adsorption mechanisms of ethanol on crystalline V2O5 surfaces also is one of the most important topics of modern material and surface science.
Section snippets
Modeling system
The crystal lattice of vanadium pentoxide is of an orthorhombic symmetry with a space group D2h-Pmmn and unit cell parameters given by a = 11.51 Å, b = 3.56 Å, c = 4.37 Å [22], [23]. In this structure, each vanadium atom is connected to five oxygen atoms to create pyramids sharing their vertices and corners in building a double chain. The chains are connected along the edges to form layers that are then stacked to form the bulk structure along c-axis [24]. The (010) face, which is the thermodynamically
Results and discussion
To validate the computational setup, we have preliminarily calculated the structural properties of the free ethanol molecule. Since periodic boundary conditions were used in this work, the ethanol molecule was placed in a cubic supercell with side dimension of R = 10 Å to eliminate the interactions between periodic images. Fig. 2 shows the structure of the isolated molecule. The H of hydroxyl, methylol and methyl are denoted by HOH, HCH2 and HCH3, respectively. The calculations to different bond
Conclusions
An ab initio calculation is employed to investigate the ethanol adsorption on V2O5 (010) surface. It is found that molecular adsorption of ethanol can spontaneously take place at all the considered surface sites of the two pentacoordinated V atoms and the five structurally surface O atoms. This indicates that there are high density of active sites available on V2O5 (010) surface for ethanol adsorption. In other words, V2O5 (010) surface possesses high adsorption activity to ethanol gas, which
Acknowledgement
This work was financially supported by the National Natural Science Foundation of China (Nos. 61274074, 61271070, 61574100).
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