Controllable Synthesis And Gas-sensing Performance Of SnO₂ Non-sperical Hollow Micro/Nanostructures

Metal oxide semiconductors with hierarchical hollow structures have drawn enormous attention for drug delivery,nanoreactors,catalysts,chemical sensors,lithium-ion batteries,energy conversion and storage owing to their characteristics of high surface area,surface-activated shell,low tap density,and potential for loading capacity.Altouhg a variety of hollow nanostructures have been successfully prepared using several basic strategies,including microemulsion method,template-engaged method and Ostwald ripening,the direct fabrication of non-spherical hierarchical hollow structures with anisotropic texture still remains a huge challenge,because the paucity of specific non-spherical templates and less uniform coating of high-curvature shells.Therein,a series of non-spherical hierarchical hollow SnO₂ nanoparticles,were systematically designed and synthesized through the self-template methods as high-performance gas sensor.Moreover,considering the formation processes and the possible growth mechanisms,we studied the internal correlation of non-spherical hierarchical hollow morphology/local microstructure and the high sensing performances.The main results are as follows:（1）The unique flowerlike SnO₂ hollow nanosheets with multilayer walls inherited the morphology of flowerlike ZnSn（OH）₆ have been successfully synthesized by a facile three-step route based on flowerlike ZnSn（OH）₆ solid nanosheets.Combining the formation process and mechanism of flowerlike SnO₂ hollow nanosheets with multilayer walls,it revealed that its morphology is an internal hollow structure with porous and passable multilayered walls and the shells are composed of SnO₂ particles with a diameter of about 5 nm.In addition,gas sensors based on flowerlike SnO₂particles with a multilayer hollow nanosheet structure were constructed.It is found that flowerlike SnO₂-based gas sensors show a good selectivity to acetone at the working temperature of 300°C,while it exhibits high sensitivity of 4.3（10 ppm）with the fast response time（0.9 s）and recovery time（5.8 s）.This optimal performance for acetone can be ascribed to the unique micro-nano porous structure that preventing the disordered agglomeration of SnO₂ subunits,and the porous structure and the exposed active sites on the surface of the multilayer SnO₂ facilitate the rapid adsorption and desorption of gas.（2）On the basis of ZnSn（OH）₆ single-shelled hollow cubes as self-template through the anneal and acid etching process,hierarchical SnO₂ triple-shelled hollow cubes with0 D nanoparticle-assembled shells has been successfully prepared.All the shells are composed of primary SnO₂nanoparticles as building blocks with a size of about 5 nm.Combining the formation process and mechanism of SnO₂ triple-shelled hollow cages,it confirmed that the triple-shelled structure is addressed in terms of the forced release of Zn₂SnO₄ and the Ostwald ripening of Sn⁴⁺ions-controlled dissolution recrystallization process.Then,gas sensors based on SnO₂ triple-shelled hollow cages show selectivity to toluene at the optimal working temperature of 250°C.Furthermore,the gas sensor based on the triple-shelled hollow cages of SnO₂ presents a significantly high sensitivity of 38.7（20 ppm）,fast response time（0.76 s）and recovery time（6.1 s）for the targeted toluene.This indicates that the synergy of the non-spherical hierarchical hollow structure and up to three layeres of highly sensitive shells are conducive to the development of higher performance gas-sensitive materials.（3）The novel SnO₂ hollow nano-cubes with ultra-fine rodlike building blocks have been successfully prepared via a facile self-template hydrothermal route.The size of the edge length of as-prepared SnO₂ cubes is distributed in a narrow range of 400–800nm.The typical thickness of these nanorod-assembled shells is estimated to be approximately 130 nm,and the rodlike primary building blocks hold single crystal structure and uniform ultrathin diameter of about 5 nm.The formation process and growth mechanism of SnO₂ hollow nanostructures have also dicussed through the hydrothermal time-dependent growth process.Furthermore,gas sensor based on ultrathin nanorod-assembled SnO₂ hollow cubes was investigated.It is found that SnO₂nanostructures shows selectivity to n-butanol at the temperature of 310°C with a high sensitivity of 75.7（100 ppm）and low detection limit（0.2 ppm）.The results eventually demonstrate that the combination of non-spherical contact surface and high-activity ultra-fine nanorods in the hierarchical structure will extremely improve the sensing performance of gas sensors.