Interface Controlled Synthesis Of SnO₂ Hetero-nanocrystals With Enhanced NO₂ Sensing Performance

Nitrogen dioxide is one of the main gaseous pollutant in atmosphere.It is of great importance to human health and operation safety to achieve ppb-level detection of NO₂ at low temperature.Chemical resistive metal oxide semiconductor nanomaterials are wildly used to detect various flammable and hazardous gases because of their high chemical stability,low cost and simplicity associated with practical applications.The gas sensing properties,such as sensitivity,response time,recover time,operating temperature and selectivity are largely depended on the morphology,crystal facet,and interface modifications with other elements.Tin oxide octahedron nanocrystals,with the only exposed highly active{221}facet,were chosen as the research material.Sulfur doping and noble metal decoration were implemented to control the surface of SnO₂.The influence on morphology and NO₂ sensing performance were investigated as follows:1.SnO₂ octahedron nanocrystals with well-defined surface were hydrothermally prepared.The crystal size ranged from 300 nm to 400 nm.The as-prepared SnO₂ showed high sensitivity and excellent selectivity to ppb-level NO₂ at 100℃.2.Sulfur was doped into SnO₂ at 400℃,450℃and 500℃via chemical vapor deposition.The content of sulfur varied with increasing temperature.The morphology and structure remained the same after doping.XPS result showed that the doped sulfur was mainly in anion species.Sulfur doping increased the oxygen vacancy and active oxygen species,which were responsible for the enhancement of NO₂ sensing properties.The sample doped under 450℃exhibited the best sensing performance.Compared with the pristine SnO₂,the response to 125 ppb NO₂ under 100℃increased from 3.7 to 6.1,while the response and recover time were shortend from 160 s and 46 s to 59 s and 26 s,respectively.3.Au,Ag,Pt,Pd were decorated on the surface of SnO₂ nanocrystals via a photochemical route.Mercaptoethanol were responsible for the high dispersity and loading content of noble metal.TEM and XPS results showed that 2⁸ nm Au nanoclusters were loaded on the surface of SnO₂ in metallic form,while Pd was doped into SnO₂ lattice in oxidized form.The doping of Ag and Pt showed both oxidized and reduced state.Compared with the pristine SnO₂,the sensitivity of Au/SnO₂ to NO₂ nearly doubled in the concentration range from 125 ppb to 1 ppm.Pd doping also enhanced the response but was less than that of Au.Pt and Ag didn’t show much enhancement or even deteriorate the sensitivity.The difference in sensing properties was routed from the chemical state and loading content of noble metals.Moreover,the distinct catalytic effect to NO₂ may also lead to the variation in sensitivity.