A DFT Study On Gas Sensing Properties Of Cuprous Oxide

The increasing needs of the detection of polluting gases make the study of gas sensor one of the hottest topics. P-type semiconductors start to attract the researchers’ attention while most gas sensors are based on n-type semiconductors. Cuprous oxide (Cu2O), a p-type semiconductor, has been observed good sensitivities to a lot of gases. But the sensing mechanism is far from clear. So, herein, we study the sensing properties of cuprous oxide to CO, NO2, NO and C2H5OH based on density functional theory. As the adsorption of oxygen species on semiconductors has been reported and generally accepted, so we focus on the effect of the adsorbed oxygen species. We employ Cu2O (111) surface which is the most thermal stable surface during our studies. We employ the charge variation value to describe the sensing properties. If the value is large enough, then we believe cuprous oxide is sensitive to this specific gas.In the second chapter, we build several adsorption models of carbon monoxide on perfect and oxygen pre-adsorbed Cu2O (111) surface. We first investigate the adsorption properties of CO and oxygen atom on perfect Cu2O (111) surface through the analysis of the optimized adsorption configurations and the density of states. And it turns out that Cuic and3Cu site are the most favorable adsorption site for CO molecule and oxygen atom, respectively. Then the Mulliken population analysis is carried out for every configuration. The most stable adsorption configuration has the largest charge variation of0.414e from CO molecule to the substrate and0.48e from substrate to Oad respectively. Moreover, it is found that the adsorption of CO on the O pre-adsorbed Cu2O(111) surface is strengthened. The adsorbed CO molecule tends to interact with the Oad atom and release the electrons trapped by the oxygen adatom back to the substrate, resulting in a significant charge variation (0.619e). All these findings indicate that Cu2O would be a good candidate for the detection of CO gas. In the third chapter, the adsorption of NO molecule on the perfect and O pre-adsorbed Cu2O (111) surface is studied based on the density functional theory calculation. NO tends to adsorb over Cuic atom with nitrogen atom with adsorption energy of1.296eV. The adsorption of NO molecule causes a charge variation of0.093e from NO molecule to the substrate. But the NO molecule tends to interact with the pre-adsorbed oxygen atom on the oxygen pre-adsorbed Cu2O (111) surface with larger adsorption energies. The charge transfers between the adsorbates and the substrate are significantly increased. This phenomenal is similar to the adsorption of CO on perfect and O pre-adsorbed Cu2O (111) surface.In the fourth chapter, the adsorption of NO2on the perfect and the O pre-adsorbed Cu2O (111) surface has been investigated by density functional theory (DFT) calculation. NO2molecules tend to adsorb over the3Cu sites on the perfect Cu2O (111) surface with a considerable adsorption energy of1.706-1.918eV and capture electrons from the surface (0.231-0.334e). While on the O pre-adsorbed surface, the adsorption is weaker except for near the Oad atom areas. Although the electron transfers are a little lower, they are still sufficient for the detection of NO2. But the electron transfers are negligible (about0.06e) in the most stable configuration where NO2molecule bond to the Oad atom and form the NO3-complexes. Overall, the adsorption of oxygen species will cause performance reduction of the Cu2O based NO2sensor. That means we should try to avoid the adsorption of O2during the production of Cu2O based NO2gas sensor.Taken all together, the adsorption of oxygen has different effect to different gases. It can strength the adsorption of reducing gases like CO and NO and increase the charge transfer. When it comes to oxidizing gases, the charge transfer is even less with the participation of pre-adsorbed oxygen atom. While the pre-adsorption of H shows exactly the opposite effect (The pre-adsorption of H reduces the charge transfer between CO/NO and the surface and increases the charge transfer between NO2and the surface). This is because the adsorption of oxygen atom causes the charge transfer from the surface to the adsorbates and make the surface in a state of lacking electrons. This means the electron accepting ability of the surface is enhanced and the electron donating ability is weakened. So the charge transfers between electron donors like CO and NO molecules and the O pre-adsorbed surface are larger than the perfect surface, while the charge transfers between electrophile (NO2molecule) and the O pre-adsorbed surface are smaller than the perfect surface. The pre-adsorption of H atom causes the charge transfer from the H atom to the surface and make the surface in a electron-rich state. The same principle holds true.In the fifth chapter, the adsorption of ethanol on Cu2O (111) surface is studied firstly. There is a possibility that H-O or O-C bond broke during the adsorption of ethanol molecule as the elongation of the H-O and O-C bond lengths are observed in the stable adsorption configurations. So we further investigate the adsorption and co-adsorption of the fragments on the surface. Calculation results show that perfect surface is sensitive to ethanol. We also find that the broken of the H-O or O-C bond will significantly reduces the charge transfer. This phenomenon is consistent with the performance degradation with the increase of the operation temperature when it is higher than critical temperature.All these results are valuable for the development of not only the cuprous oxide based gas sensors but also other semiconductor gas sensors.