Controlled Synthesis Of Single-Atom Catalysts And Gas Sensing Studies

Noble-metal nanoparticles supported oxides have been widely used in semiconductor gas sensors.Benefiting from their small size and low power consumption,semiconductor gas sensors have been widely used in smart home,air quality monitoring and healthcare.However,due to the scarcity and expensiveness of noble metals,the traditional noble-metal nanoparticles supported materials are difficult to meet the requirements of commercial sensors.Thus,it is vital to develop new generation of gas-sensing materials.At present,the most core solution is how to fully use the noble-metal atoms to activate the target gases and improve the sensing performance,leading into lower metal loading.In this regard,single-atom catalysts with 100%atomic utilization efficiency,high catalytic reactivity and unique selectivity have provoked extensive attentions.However,even in single-atom catalysts,the metal atoms inherently maintain high mobility over the support surface,which lead spontaneous aggregation and the deterioration of catalysts property in the whole catalytic processes.Therefore,the delicate creations of single-atom catalysts with high activity and stability to meet the industrialized needs and combining with advanced manufacturing technology to achieve commercial application are urgently desired but challenging.In this thesis,we designed a series of single-atom catalysts with specific sites to optimize and improve the gas-sensing performance.By designing the special configurations on the supports,the metal monomers can be stabilized in the highly active reaction regions,which greatly improves the intrinsic activity of the single-atom site.In addition,by precisely regulating the type and concentration of defects on the oxides,the site-specific framework can be created,and thus both enhancing the reactivity and stability of catalysts.This method can be applied to a variety of single-atom systems and exhibit the industrial value.Therefore,we further equip the catalysts with advanced processing technology to achieve wireless monitoring,cloud storage and other commercial applications.The main contents are as follows:1.One-dimensional segregated single Au sites on step-rich ZnO ladder for gas-sensing studiesWe reported a layer-stack ZnO materials with abundant unsaturated step defects to stablize highly active Au1 single atoms.Following the role of crystal growth for layer-by-layer mode,the nucleation and growth rate of ZnO precursors can be precisely regulated and controlled.Ladder-like ZnO materials can be synthesized with various sizes and thicknesses.We find that ZnO crystals can preferentially grow into hexagonal nanosheets in the supersaturated states.As the reaction goes on,the new nucleation sites will be formed at the center of the layers.These nucleation sites are more conducive to the growth of the new layer and prevent the lateral extension of initial layer.This process is repeated until the end of crystal growth,leading a ladder-like nanoframework.Moreover,these unsaturated sites in step edges can not only enhance the adsorption of the target gas,but also effectively stabilize the metal monomers.The experiments showed that Au species could orderly linear-arrangement along the steps of ZnO,which greatly enhanced the reactivity of Au single atoms.Compared with ZnO and Au nanoparticles supported ZnO(Au NP-ZnO),Au single-atom supported ZnO(Au1-ZnO)exhibited excellent NO2 gas-sensing response(12.6 to 300 ppb)at a low temperature.Such a high response surpasses that of most reported NO2 sensors.Moreover,even at very low NO2 concentration(10 ppb),the response value was as high as 1.07.2.Nitrogen-assisted unsymmetrical-coordinated single atoms sites for gas-sensing studiesWe designed an unsymmetrical-coordinated single-atom catalysts.By constructing a nitrogen-assisted Sn vacancy(N-Vsn)configuration on the SnO2 supports,a nitrogen-oxygen multi-coordinated environment can be achieved for introduced metal monomers,and thus improving the activity and stability of the catalysts.The SnO2 materials containing only inherent Sn defects are firstly synthesized,and then the unsymmetrical nanopocket center is built by the introduced nitrogen atoms to partially replace the oxygen atoms around the Sn vacancy.According to the positron annihilation lifetime spectra and coincidence Doppler broadening spectra,we find that only the single oxygen atom around the Sn vacancy can be replaced by the invasive nitrogen species.The resultant nanopocket can further stabilize the Pt atoms.Moreover,this site-special single-atom catalysts also exhibit good universality,which can be applied for Cu,Ni metal monomers.Density functional theory calculation shows that the introduction of nitrogen is conducive to(?)e formation of the nitrogen-assisted Vsn pockets,which can provide a more stable configuration for the metal monomers.The resulting Pt-N/SnO2 catalysts are thus able to significantly improve the formaldehyde sensing performance(26.2 at 15 ppm)at a low operating temperature(100℃)and remain stability over 30 days.Therefore,the Pt-N/SnO2 sensors could indeed satisfy the detective requirements of permissible occupational limit for HCHO in urban environment at 15 ppm.Benefiting its superior sensing characteristics and stability,the Pt-N/SnO2 sensors can be combined with wireless Bluetooth system,and thus achieving real-time detection,wireless transmission,cloud-storage and data sharing.Our work sheds light on future way of single-atom catalysts for industrialized application.