Crystal Plane-Dependent Gas-Sensing Properties of Cu2o Particles Nanostructures: Experimental and Theoretical Studies

Abstract

Introduction Recently, the study on selective exposure of high-energy facets at the surface of metal-oxide semiconductors nanomaterials (MOSs) have been of great concern. Usually, different facets of a single crystal exhibit distinctive chemical and physical properties, for example, gas-sensing ability, chemical reactivity, and adsorption capacity. In principle, gas-sensing by the MOSs is based on the oxidation–reduction reaction of the detected gases occurring on the semiconductor surface, which leads to an abrupt change in conductance of the sensor. For this reason, the gas-sensing ability of the MOSs is in theory very sensitive to the crystal facets of the sensing materials the applications of MOSs with high-energy facets have been concentrated on the surface-reacted processes. Kaneti et al. [1] and Wang et al. [2] studied on the relation between the sensitivity of metal oxide gas sensor and the nature of the crystal surface exposed to the gas species. They demonstrated that semiconductor metal oxidation with high energy surface has the highest gas sensitivity.But by the method we synthesized, the six Cu2O nanocrystals with different energy facets were achieved by simply adjusting the added PVP. At the optimal working temperature, Cube Cu2O nanocrystal with (100) facet exhibited the best sensitivity to n-propanol gases. The reason for possibility is that the Cu2O nanocrystal with high-energy facets also adsorbed a large number of organic molecules. Method The chemiresistive gas sensor is mainly composed of four parts: 1. A 4 mm long ceramic tube (internal diameter: 0.8 mm, external diameter: 1.2 mm), where a pair of gold electrodes with four Pt wires have been installed originally. 2. Sensing material layer (thickness: ~ 42 um) on the surface of ceramic tube. Taking the material based sensor as an example here. Mix the obtained powder with deionized water to form a slurry and coat the slurry uniformly on the whole surface of the ceramic tube. Then put the well-coated tube in the muffle furnace heating at 100 °C for 2 h in air. 3. A Ni-Cr alloy coil heater, it was inserted through the well-coated and sintered ceramic tube, controlling the working temperature via the flowing current. 4. A hexagon base, which supported the well-coated and sintered ceramic tube through welding. After ageing, the as-prepared sensors were tested through the static test system at a laboratory conditions. Results and Conclusions In this work, A variety of Cu2O architectures with an interesting morphological evolution (from cube, cubooctahedrons, truncated octahedral to octahedral) were realized by simply adjusting the added PVP. The test results demonstrated that Octahedral Cu2O particles exposing the (111) facets exhibit a surprisingly competitive sensing activity to n-propanol gases at the optimal working temperature compared with truncated octahedral Cu2O with (100) and (111) facets. But we found the sensing activity of the cube Cu2O with (100) facet exhibit much better than the octahedral Cu2O particles with (111) facet. The phenomenon could be explained from the PVP organic macromolecular. It can be concluded that PVP had acted as a capping agent and that preferential adsorption occurred on the (111) planes of the Cu2O crystals with large percentage of unsaturated copper atoms. Hence, the adsorbed PVP will also reduce the surface reactivity of the octahedral Cu2O particles with (111) facet. Our findings may provide the mechanism to explore a simple, efficient and promising sensing/catalysis activity materials in gas sensors.