Hydrothermal Systhesis And Gas Sensing Properties Of ZnO Semiconducting Metal Oxide Materials

Zinc oxide （ZnO）, a direct wide band gap （3.37eV） semiconductor with a largeexcitation binding energy （60meV）, is one of the most important n-type semiconductormaterials. For gas-sensing applications, it has attracted great attention for a long timedue to their advantageous features, such as high sensitivity to various oxidizing andreducing gases, low cost, simplicity in fabrication, and miniaturized as well. Up to now,nano-and microcrystalline ZnO with various size and morphologies have been reported.In particular, the promising material platforms for the applications as a gas sensor restwith those hierarchical, porous and hollow structures with a dimension of micro ornanostructures composed of as many low dimensional nano-building blocks as possible.On the other hand, doping ZnO with various elements has been suspected to enablemodulation of surface charge states of ZnO, modifying significantly its functionality. Inthis paper, we report a facile and tunable synthesis of ZnO with various hierarchical,porous and hollow structures via simple hydrothermal process and we also dopped ZnOwith rare-earth metals. Furthermore, the crystal structure, morphologies, growthmechanism are elaborated, and their ethanol sensing properties on the sensitivity,response-recovery time, stability, and selectivity are investigated too. The main resultsand significance are as follows.Using hydrothermal method and reagents of sodium citrate and polyethyleneglycol （PEG）, we synthesize the ZnO nano-crystals and find that they exhibithierarchical flower-like architectures assembled by nanosheets. Further comparativestudies demonstrate that the sodium citrate assists a uniform growth of the nanosheets,and the PEG provides nucleation sites for the assembling of the nanosheets, both ofwhich play a critical role in producing such unique flower-like architectures.Consequently, this sample is found to show excellent sensing performances to the targetethanol owing to its largest amount of petals, intervals, and pores. The gas sensors havebeen fabricated based on the above sensitive ZnO materials, the responses of the sensoris about22.6, response and recovery time is6s and15s to50ppm ethanol at250℃.ZnO nanocrystals with various morphologies are synthesized via a simple andfacile HMT assisted hydrothermal process. Here, HMT is introduced as a structuredirecting and assembling agent to controllable synthesis of ZnO architectures and theconcentration of HMT play a critical role in producing such different ZnO morphologies. We find that the sandglass-like ZnO exhibit excellent sensing performances to the targetethanol gas owing to its largest amount of Zn-terminated ZnO （0001） polar surfaces,which can easily adsorb more oxygen species on the surface and therefore improve thegas-sensing performances of ZnO. The sandglasslike ZnO architecture has a highresponse （72.5）, fast response and recovery time （3s,6s）, excellent selectivity and circlestability to50ppm ethanol at350℃, which might be promising for applications idetectin ethanol.We have fabricated successfully novel hollow and porous ZnO nanospheres via thesimple template-free hydrothermal technique in organic solution. In addition to formingthe spherical configuration, the MEA also plays a pivotal role in making thenanospheres hollow and porous. Such hierarchical nanospheres possess a large specificsurface area and can be functionalized with Y for advanced chemical gas-sensingapplication. Y doping can increase the concentration of O vacancy and hence absorbmore oxygen on the ZnO surface, which as a result increases the concentration of O.Gas-sensing performance to the HCHO is found to be enhanced in the doped samplewith the Y concentration of4%. However, gas response is saturated to a maximumvalue of65.7when the Y concentration reaches4%. Further increase in the Y-dopingconcentration results in an adverse effect, i.e., lowers the gas response. The Y-dopednanospheres have a lower optimal tem-perature （300C） than the Y-free ones （350C）.The response–recovery characteristics of4%Y doped ZnO spheres is4s and6s,excellent selectivity and circle stability to50ppm HCHO.We have successfully fabricated the ultrathin ZnO nanosheets with a thickness of10–13nm via the simple yet efficient hydrothermal approach followed by calcination.We find that the generated sheet-like [Zn4（CO₃）（OH）6] precursor is heavilyagglomerated, yet transformed to ultrathin ZnO nanosheets in a dispersive fashion aftercalcination process. Further gas-sensing measurements reveal that the sensor made ofultrathin ZnO nanosheets show sensitivity as high as62.9to the ethanol of50ppm atthe optimal temperature as low as300℃. We fabricate successfully theZn4（OH）6CO₃·H2O （ZHC） microspheres which consist of the nanoflakes by using asimple hydrothermal process, and demonstrate that the ZHC precursor can betransformed utterly to ZnO porous microspheres after calcination. We find that the ureaadditive promotes the formation of nanoflakes and that the MEA enables shrinking ofthe nanoflakes and easier assembling of nanoflakes to spheres. Further gas-sensingmeasurements reveal that the sensor made of porous ZnO microspheres exhibits the highest gas response （87.5） and the shortest response and recovery time （7and9s,respectively） to the ethanol gas of50ppm at an optimal temperature of350°C.We have successfully fabricated ZnO/SnO₂microspheres with an average size of1–2mm via a two-step solution method for the first time, which exhibit a uniquecore–shell architecture with a uniform dispersion of SnO₂nanoparticles on surfaces ofspherical ZnO cores. The gas-sensing measurements reveal that the sensor made of thecore–shell structural ZnO/SnO₂micro-spheres shows sensitivity as high as52.7toethanol gas of50ppm at an optimal temperature as low as250℃, along with theresponse and recovery time as short as3and5s. We have prepared the unique hollowZnSnO₃nanocages through the simple hydrothermal method and investigate theirgas-sensing properties. The sensor made of hollow ZnSnO₃nanocages have a highspecific surface area and exhibitexcellent sensor response, selectivedetection, shortresponse and recovery time, and good repeatability and stability to the HCHOgas.