One-dimensional Metal Oxides Mo_x (m = Sn, Zinc) Synthesis And Gas-sensing Properties Of The Composite Material

Tin dioxide （SnO₂） is an environmentally friendly n-type semi-conductor with a wideband gap of3.6eV at room temperature. It possesses high chemical stability andexcellent optical and electrical properties. They have been widely used for variousdevices, especially for gas sensors. Most NO_xgas sensors require high-temperatureoperation, doped noble-metal, trival synthesis process, expensive consumtion, and thedefect of morphology. Therefore, it is meaningful to investigate low-cost and ambientoperation NO_xgas sensors with high sensitivity and fast response. Operation at roomtemperature would be conducive to application. Electrospunning is the excellent methodfor fabricating one-dimensional materials.Firstly, we present a simple method for the fabrication of SnO₂and porous-carboncomposite by directly annealing electrospun composite fibers at inert atmosphere. Atroom temperature, SnO₂and porous-carbon composites were tested to NO_xgas, andthen discuss the mechanism for response.Secondly, tin dioxide nanocrystalline tubes （TONTs） are fabricated via theelectrospinning process, followed by heat treatment to remove PVP and to convert solidcomposite nanofibers into hollow SnO₂nanotubes. The walls of SnO₂nanotubespredominantly consist of5-10nm SnO₂nanoparticles. The high surface-to-volume ratioof the TONTs makes it an excellent gas sensing material. The TONTs show uniquenanocrystalline structure and exhibit excellent sensing properties with not only highsensitivity but also fast response and recovery at room temperature to low-concentrationNO_xdetection. The fast response and high sensitivity of TONTs may be attributed toone-dimensional hollow nanostructures with two adsorbed layers on both the outer andinner surfaces, with nanocrystallite overlapped and connected with four or moreadjacent grains through necks, large amount of sensing activity sites （chemisorbedoxygen: O-, O2-, O22-）, and （101） preferential orientation. Such conditions make it easyfor electrons pass to through the potential barrier of grain junctions. The total energycalculations were performed under density functional theory （DFT） to investigate theadsorption of NO_x（NO and NO2） molecules on the TONTs surface. Then the best a adsorption crystal surface can be found.To further enhance the adsorption capacity of the material for NO_xgas, thenanomaterial need be doped to increase the sensitivity. The different proportion ofCa²⁺-doped SnO₂porous and loosen one-dimensional nanorods were prepared byElectrospinning method, through the adjustment of the viscosity of PVP solution to getthe uniform one-dimensional structure. With the different atomic ratios, the1Dnanostructure which can be synthetized after calcination are consist of SnO₂nanoparticles. The1D nanostructures of none-doped and the2at%Ca²⁺-doped are moreloosen, while1D nanostructures of the other two proportions are compact. When theproportion of2at%Ca²⁺-doped, the SnO₂nanoparticles are smaller, the sensitivity arethe highest among the smaples to NO_xgas and the excellent selectivity to NO_xgas. Theoutstanding performance may be attributed to the position of CaO nanoparticlesbetween the two SnO₂nanoparticles, which can restrain the size of SnO₂nanoparticlesto stabilize at the ideal size. And they don’t destroy the structure of electrontransmission. CaO as the centre of alkalinity adsorption can enhance the adsorptioncapacity of NO_xgas, thun the conductivity will have the enormous changes. The linearrelationship of2at%Ca²⁺-doped SnO₂nanorods is formed by plotting log（S） vs.log[NO_x] with R2=0.99034from97ppm to9.7ppb. Thus, it is suitable for commercialapplication to NO_x sensing systems. Promoting the further application of thenanocomposite fiber and providing valuable theoretical basis in the sensor fields.