| With the development of science and technology and the improvement of people's living standards,people have put forward higher requirements on living and working environment.In the past two decades,the development of small-scale gas sensing devices for toxic gases has been extensively studied for manufacturing and monitoring processes.In many industries,gas has become increasingly important as a raw material and for this reason,it is important to develop high-sensitivity gas detectors that should allow for the continuous and quantitative monitoring of environmentally specific gas concentration.Metal oxide semiconductor(MOS)sensors have drawn much attention due to their high sensitivity,long-term stability,excellent durability,low production cost and low power consumption.MOS gas sensors are widely used in the following fields:(1)flammable gas detection;(2)environmental monitoring and safety.It is revealed that the gas sensing mechanism of metal oxide gas sensors can help to design and construct novel gas sensing materials with excellent properties.Although the basic mechanism of gas reactions remains controversial,grabbing electrons and energy band bending for the adsorbed molecules making the conductivity change is the most basic reason.The research system in this paper is the perovskite materials and the SnO2-based semiconductor gas sensing materials.Perovskite materials and SnO2-based semiconductor gas sensing materials have always been the hot research topics both at home and abroad.In this paper,the LaFeO3,LaCrO3 and SnO2-based powder materials were prepared and the gas-sensing properties of these materials under different atmospheres of CO2,acetone,CO,H2 were tested.The adsorption between the gas molecules and the surface atoms was calculated to reveal the gas sensing mechanism.Through analyzing of atomic structure and electronic properties,the simulation results were used to explain the experimental phenomena and the gas sensing mechanism.The main research results of this paper:1.We investigated the CO2 gas sensing property of LaCrO3 both from experiment and the first principle calculations.LaCrO3 nanocrystalline powders were prepared by sol-gel method and subsequently annealed at 700?,900? for 4h,respectively.At room temperature,LaCrO3 is orthorhombic(Pnma)structure.LaCrO3 undergoes crystallographic transitions to rhombohedral(hexagonal)phase(R 3 c)first at around 550K and then to the cubic(Pm3m)phase at around 1300K.At temperatures above 550K,the optimum operating temperature is 360? and the resistance of the LaCrO3 sensor decreases when exposed to CO2,which is different from other p-type semiconductors.When exposed to 300000 ppm CO2 at 360?,the response of LaCrO3 annealed at 700? and 900? are 1.046 and 1.034,respectively.The first principle calculation results show that CO2 molecule chemically binds with the surface O atom of LaCrO3(012)through C atom,and two O atoms of CO2 molecule bind with the adjacent surface Cr atoms.During the interaction process,CO2 molecule captures electrons from LaCrO3 surface,which is consistent with experimental results.2.We employ the density functional theory to study the sensing properties of CO2 on SnO2(110)surface and the influence of pre-adsorbed oxygen.Our calculation results indicate that CO2 molecule can not be adsorbed onto stoichiometric SnO2(110)surface or SnO2(110)surface pre-adsorbed by O2-and O-in dry air.When CO2 is introduced to SnO2 surface in wet air,CO2 reacts with O of pre-adsorbed OH-,bringing about the formation of carbonates containing(C03)2-and the dissociation/movement of surface OH-group,accompanying the releasing of electron from CO2 to SnO2 surface.The appropriate pre-adsorption of OH-on SnO2(110)surface promotes the sensing response of CO2.At high relative humidity,the active adsorption sites at the surface of SnO2 are mainly occupied by hydroxyl OH groups,and there are few adsorption sites on SnO2 surface for CO2,which leads to the decrease of the CO2 sensing response.3.We investigated the the acetone sensing properties and mechanism of SnO2 thick-films both from experiment and the DFT calculations.SnO2 thick film based on nanocrystalline co-precipitation powders annealed at 600? could sensitively detect acetone vapor.At the optimal operating temperature of 180?,the responses are 3.33,3.94,5.04 and 7.27 for SnO2 sensor with 1,3,5,and 10 ppm acetone,respectively.The DFT calculations results show that the acetone molecule can molecularly adsorb on the 5-fold-coordinated Sn and the vacancy(VO)site with O-down,accompanying with electron transfer from.acetone to SnO2(110)surface.The acetone molecule acted as an donor in these modes which can be used to explain the reason why the resistance of SnO2 or n-type metal oxide decreased after acetone molecule was introduced into the system.4.We investigated the acetone gas sensing property of LaFeO3 both from experiment and the first principle calculations.LaFeO3 nanocrystalline powders were prepared by sol-gel method and annealed at 600?,700?,800?,900? and 1000?for 4h,respectively.LaFeO3 annealed at 800? can be used to detect low concentration of acetone sensitively.For 0.5,1,5 and 10 ppm acetone,the responses of LaFeO3 are 2.068,3.245,5.195 and 7.925,respectively.For 0.5 ppm acetone,the response time and the recovery time of LaFeO3 thick film are 62s and 107s,respectively.Calculated results show that acetone releases electrons to LaFeO3(010)surface pre-adsorbed with oxygen species(O-or O2-).Acetone molecule reacts with oxygen species mainly through adsorbing on oxygen species(O-)or replacing the weakly pre-adsorbed oxygen species(O2).During the reaction process,there was oxygen molecule formed.The above two processes may play important roles in acetone sensing for LaFeO3.We did not find the formations of CO2 and H2O from the DFT calculation results,which maybe happens at higher temperatures.We also find that acetone molecule can adsorb on Fe site,meanwhile some electrons transfer to LaFeO3 surface.5.We investigate the H2-sensing mechanism of SnO2(110)surfaces with the first principle calculations to understand the H2-sensing behaviors and sensing mechanism of SnO2 surfaces with different reduction degrees.From the calculation results we found that oxygen concentration in the ambient atmosphere greatly affects the H2-sensing mechanism of SnO2.At considerable high oxygen concentrations atmosphere,H2 interacts with oxygen species pre-adsorbed onto SnO2(110)surface,leading to electron release back to the semiconductor SnO2.When interacting with O2-,H2 molecule dissociates with one H atom to form hydroxyl adsorbed onto Sn site and another H atom adsorbed onto the oxygen atom of pre-adsorbed O2-;when interacting with the O-,H2O molecule is formed in the process.At very low oxygen concentration,structural reconstruction is induced by the interaction between H2 and SnO2 sub-reduced surface with removed two fold-coordinated bridging oxygen rows,accompanying electron transfer from H2 to surface without H2O formation.The above calculated results are consistent with the experimental observation.6.We investigated the CO gas sensing property of Pd doped SnO2 both from experiment and the first principle calculations.Pd doped SnO2 nano-particles were synthesized using co-precipitation method and annealed at 600? for 5h.For Pd doping and temperature effect,the resistances of the samples first decrease and then increase.Pd doping in SnO2 can improve its CO sensing response.At temperatures from 20? to 80?,the responses of Pd:SnO2 sensors decrease at first,undergo a minimum,and finally increase to the maximum response value at 80?.At 80?,the response of 2.0 wt.%Pd doped SnO2 sensor for 400 ppm CO is 171.9.At low temperature range,CO response decreases with increasing of operating temperature,which was caused by physical adsorption.At higher operating temperature,the response of Pd doped SnO2 for CO increases at first,undergoes a maximum at 260?,and finally drops.The 1.5 wt.%PdO doping SnO2 was verified to be significantly sensitive.At 260?,the response of 1.5 wt.%PdO doping SnO2 is 6.59 for 400 ppm CO.For 200 ppm CO,the response time and recovery time were about 43s and 10s,respectively.From the calculation results,we know that CO molecule can grab O from the pre-adsorbed oxygen on Pd4 cluster or from the PdO cluster on SnO2 surface and formed CO2. |
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