Investigation On The Structure Control And Room-temperature Ammonia Sensing Properties Of Cu₂O Nanocrystals And Their Composites

Ammonia gas is an important chemical raw material,but it can cause air pollution and harm human health.In this sense,real-time monitoring for ammonia concentration is urgently needed.However,ammonia is flammable and violent,leading to a lot of difficulties in its detection.Therefore,it is necessary to develop a rapid and accurate method for detection of ammonia concentration at room temperature.Among many methods to detect ammonia,the gas sensor based on P-type semiconductor metal oxide is one of ideal choices.As a typical and cheap P-type semiconductor,Cu₂O nanocrystals exhibited certain room-temperature gas sensing performance,but its baseline resistance at room temperature is large,the response value to ammonia is low,and the response recovery speed is slow.To solve these problems,Cu₂O was taken as the research object in this work,theoretical calculation method was employed to selects its optimal exposed crystal surface for ammonia sensors,and its micro/nano structure was adjusted,and the internal relationship between its micro/nano structure and its room-temperature ammonia sensing performance was revealed.In this sense,Cu₂O based sensor with better room-temperature ammonia sensing performance was fabricated.The main research contents are presented as follows.（1）Based on first-principles density functional theory,surface models of exposed crystal surfaces corresponding to three common morphologies of Cu₂O（concave octahedron,cube and dodecahedron）were established.As a high-index facet,{511}crystal facets showed significantly more active sites than{100}and{110}crystal facets.In addition,the{511}crystal surface is the highest active surface for adsorption of NH₃ molecules,and the adsorption effect on NH₃ molecules is obviously stronger than that of the interference gases,indicating that the{511}crystal facets of Cu₂O,namely,the concave octahedron,exhibited better selectivity for NH₃ gas.（2）According to the above theoretical calculation results,nano-Cu₂O concave octahedron with{511}exposed crystal facets,Cu₂O cube with{100}exposed crystal facets and Cu₂O dodecahedron with{110}exposed crystal facets were prepared successfully by using a facile hydrothermal method and regulating the p H value in the reaction process.Their ammonia sensing properties at room temperature were also investigated in detail.The results showed that the response value of Cu₂O concave octahedron to 100 ppm ammonia gas was 3.84 times and 1.41 times higher than that of dodecahedron and cube,respectively,which are consistent with the above theoretical calculation results.（3）The above nano-Cu₂O concave octahedral based sensor showed very large baseline resistance（～GΩ）,which makes it difficult to test the gas sensing performance at room temperature.In this sense,carbon nanotubes（CNTs）with good electrical conductivity were added to the precursor of Cu₂O to form Cu₂O concave octahedron/CNTs hybrids.The baseline resistance of the composite based sensor was significantly reduced and decreased with the increase of CNTs content,and the ammonia response at room temperature reached the maximum when the CNTs content was 5%.This was mainly because that the heterogeneous interface of Cu₂O/CNTs in the hybrid can facilitate the charge transfer,leading to a reduction in the baseline resistance and the improvement of response.（4）Although the ammonia sensing performance of the above nano-Cu₂O concave octahedral/CNTs hybrids at room temperature was improved,there was still a large room response to improve the response.In this sense,Ag nanocrystals were loaded on the surface of the hybrids by light reduction reaction to form Cu₂O/CNTs/Ag ternary composite.Based on the overflow effect of noble metals,the response to ammonia gas at room temperature was significantly enhanced,especially the response value was increased by 1.73 times.Moreover,the sensor also exhibited short response time,good selectivity and stability.Therefore,this result can promote the practical application of room-temperature ammonia gas sensors.