SnO₂, Co₃O₄ And Ni_xCo_{1- Study On Gas Sensing And Piezoelectric Properties Of X}Co₂O₄

The gas sensing properties of Co-Co3O4,Co/Ni-NixCo1-xCo2O4 and SnO2 nanoparticles were improved by exposing the polar {111} crystal faces of Co3O4 and NixCo1-xCo2O4,as well as the surface of Co-Co3O4,Co/Ni-NixCo1-xCo2O4 and SnO2 nanoparticles were activated by hydrogenation.The relationship between the surface structure and sensing properties was studied.The effect of active crystal face and active atom in gas sensing was revealed and the gas sensing mechanism at atomic and molecular level was proposed.In addition,Co3O4 thin films with different[111]orientations were prepared and the photoelectric and piezoelectric properties were studied.For the first time,[111]orientation dependent diode rectification,photovoltaic and piezoelectric power generation properties were observed,Based on the Co3O4[111]polarity structure,the relevant physical mechanism was proposed.The research results have important theoretical significance and potential application value for the development of high sensitivity gas sensing materials and high performance piezoelectric power generation materials.The specific research results are as follows:(1)Sensing reaction mechanism is crucial for enhancing the sensing performance of semiconductor-based sensing materials.Here we show a new strategy to enhancing sensing performance of SnO2 nanocrystals by increasing the density of unsaturated Sn atoms with dangling bonds at the SnO2 surface through hydrogenation.A concept of the surface unsaturated Sn atoms serving as active sites for the sensing reaction is proposed,and the sensing mechanism is described in detail at atomic and molecule level for the first time.Sensing properties of other metal oxide sensors and catalytic activity of other catalysts may be improved by using the hydrogenation strategy.The concept of the surface unsaturated metal atoms serving as active sites may be very useful for understanding the sensing and catalytic reaction mechanisms and designing advanced sensing sensors,catalysts and photoelectronic devices.(2)Co3O4 octahedrons enclosed by {111} facets were synthesized using Cl-ions as a crystal facet control agent.The Co3O4 octahedrons show higher responses than Co3O4 commercial particles to ethanol,methanol,acetone and triethylamine,and the responses are further enhanced by increasing the numbers of 3-coordinated Co atoms(Co3c)with one dangling bond at Co-Co3O4(111)plane by eliminating surface Clions and OH groups.Accordingly,we deem that the Co3c atom at(111)plane acts as an active center for sensing reaction and present an atomic and molecular level sensing mechanism.The Co3c atoms can produce free electrons,absorb O2 molecules and catalyze the gas-sensing reactions.The surface engineering may be used to bolster gas-sensing properties of other metallic oxides.The concept of Co3c atom as an active center will contribute to understand the true nature of the gas sensing and provide guidance on designing highly performance sensing materials.(3)Co3O4 octahedrons were prepared by heating the aqueous solution of AlCl3-CoCl2 with different molar ratios at 450? for 1 h.It was found that the octahedron size decreased from 1.03 ?m to 0.08 ?m and the specific surface increased from 4.56 m2g-1 to 28.96 m2g-1 with the increase of the molar ratio of AlCl3/CoCl2 from 1%to 20%.The area of the active(111)surface increases gradually,and the density of three coordinated Co atoms(Co3c)on the(111)surface increases,which makes the gas sensing properties of the gas sensor increase in turn with respect to NO2 and ethanol.When the molar ratio of AlCl3 to CoCl2 is 15%and 20%,the proportion of octahedron decreases,the proportion of active(111)surface decreases,so the density of Co3c decreases,this leads to the decrease of its response to NO2 and ethanol.(4)NixCo1-xCo2O4 octahedron exposed {111} crystal facets was prepared from cobalt CoC12-NiCl2 aqueous solution at 450? for 1 h.It was found that the response of the hydrogenated NixCo1-xCo2O4 octahedron to acetone and diethylamine gas was significantly improved.The sensitivity was attributed to the removal of hydroxyl groups and chloride ions on the surface.Based on the atomic structure of the active(111)facet of NixCo1-xCo2O4,we consider that the unsaturated Co/Ni atoms with dangling bond on the surface of NixCo1-xCo2O4 octahedron is the active center of the sensing reaction,and a detailed gas sensing mechanism is proposed.The results show that the surface unsaturated metal atomic engineering method should be a general method to improve the sensing sensitivity.(5)?-Co(OH)2-ethanol-PVP colloidal solution was prepared from ?-Co(OH)2 prepared by cobalt chloride-ammonia-glycol hydrothermal system at 130?.Co3O4 thin films were obtained on FTO glass by spin-coating and heat treatment.The[111]orientation of Co3O4 films was controlled through changing the pretreatment temperature.The photoelectric and piezoelectric properties of Co3O4 thin films were studied.The phenomena of diode rectification,photovoltaic and piezoelectric power generation are observed for the first time.The properties of these films increase with the 'enhancement of[111]orientation,which indicates that the rectifying,photovoltaic and piezoelectric properties originate from the polar structure of Co3O4.Based on the polar structure of Co3O4,the physical mechanism of diode rectification,photovoltaic and piezoelectric power generation at the atomic level is proposed.These results show that it is a general rule that the polarity structure has rectifying,photovoltaic and piezoelectric effects,which is expected to change people's understanding of piezoelectric mechanism.