Nanostructured Metal Oxide Semiconductuors: Synthesis, Gas Sensing And Photocatalytic Properties

Semiconductors have great practical value in catalysis, sensing, optical devices，photoelectric conversion and other realms because of their superior physical andchemical properties. With the development of nanotechnology, the traditional bulksemiconductor materials cannot meet the demands of modern industry. Nanostructuredsemiconductors possess smaller particles, larger specific surface area and more surfaceactive sites than those of traditional bulk materials, so nanostructured materials offerendless possibilities for the optimization of material properties, even in solvingenvironmental and energy problems. Especially, design and synthesis of function-orientednanomaterials has been a focal point of research.With the development of society and economy, mankind faces increasingly energycrisis and environmental pollution. People need constantly to create new ways of solvingthese problems. The detection of toxic, hazardous, flammable, explosive gas in theenvironment becomes a critical issue, which can be realized following the emergence ofgas sensors. Semiconductor nanomaterials have superior sensing properties and arewidely employed in gas sensors. And the metal oxide semiconductor nanomaterials havebeen widely used in gas sensor owing to their low cost, easy preparation and controllablesize. Phenols, a kind of common chemical raw material, are potentially toxic,carcinogenic and teratogenic, and phenol is the typical example of them. PhotocatalysisPhotocatalytic degradation of phenol has emerged as a promising technique to deal withthe problem of environmental pollution because it has some advantages of efficiency,stability and no second pollution. In order to resolve the issue fundamentally, we mustfind out alternative clean energy to reduce the dependence on non renewable fossil fuelsand the solar energy is the best choice in the field of clean energy. Catalytic splitting ofwater into hydrogen and oxygen is long regarded as a promising method to store solarenergy in the form of chemical bonds, not least because of hydrogen’s attraction as agreen and renewable fuel. Photocatalyst is the most critical part for the highly efficientphotocatalytic reaction. Semiconductor materials as one of the most widely studied photocatalysts, their researches mainly foucus on improving the photocatalytic activity ofthem.The thesis mainly focuses on the preparation of nanostructured metal oxidesemiconductor by optimizing the synthesis condition and explores the relationship ofnanostructure and properties in depth. And the achievements made here will offervaluable data for further designing mew semiconductor based materials.Firstly, In2O3-SnO2composite nanofibers are synthesized via an electrospinningmethod and then La0.7Sr0.3FeO3coated In2O3-SnO2composites (LISc) which haveresponse to TMA gas witnh humidity resistant ability are obtained.In addition we test theinfluence of La0.7Sr0.3FeO3nanoparticle to the sensing property of ISc, the result showthat loading of La0.7Sr0.3FeO3particles can improve the sensitivity to TMA greatly. Froma practical perspective, we also investigated the influence of humidity to the LISccomposite sensors. The test shows that LISc composite sensors still response sensitivilyto TMA with a concentration of1ppm when the humidity reaches85%, and the responsevalue is about6. Long-term stability of the sensors was recorded for60days, slightvariation in response was observed, proving good stability and repeatability of the sensor.High efficiency, highsensitivity and good stability are the characteristics of TMA sensors,and that may ascribe to the unique heterojunction structure andporosity. And it will makeit possible for researchers to further explore their practical applications in sensors.Secondly, without any additives, hierarchical In2O3materials composed ofnanoparticles or porous hollow nanoplates are prepared by a simple thermal treatment of“pre-synthesized” glycerolate precursors. They are denoted as S-In2O3and F-In2O3,respectively. After thermal treatment of the precursor (heat treatment in air atmosphere at350°C at2h) we get the corresponding In2O3nanospheres consists of nanoparticles witha size of about30nm and hierarchical flower-like hollow In2O3materials consists ofnanosheets. It is clear that the formation of hierarchical flower-like hollow In2O3materials is an Oswald ripening process-the same phase dissolution-recrystallizationprocess by time dependent experiment. Two In2O3nanomaterials of different structures were studied in gas sensing performance test. The optimized response temperature areboth245°C, and F-In2O3performance better than S-In2O3under the same condition. Thatmay be attributed to their novel hierarchical structure and larger specific surface area. Theselectivity of hierarchical flower-like hollow F-In2O3was also investigated, whenexposed to ethanol, acetone, methanol, benzene, CO, H2, ammonia with the sameconcentration of100ppm, F-In2O3responsed most sensitive to ethanol, and the resonsevalue is about55in less than10s, which is much better than that of S-In2O3.Thirdly, hierarchical flower-like titanium glycerolate precursors are prepared by atemplate-free solvothermal method with tetra-isopropyl titanateastitanium source,glycerol as main solvent. The morphology of precursors was modulated by changing theconcentration of isopropyl alcohol titanium and the amount of the mixed solvent in thereaction system. We find that the concentration of isopropyl alcohol titanium and theamount of the mixed solvent paly an important role in the morphology of the precursors.And what’s more, the morphologies of the precursors were maintained after calcination.The photocatalytic degradation of phenol was also investigated for the materials calcinedat different temperatures. From the phonel degradation test, we may take550oC as theoptimimal temperature. At this tempreture, under the irradiation of UV light, phenol canbe degradated89%in40minutes. In addition, we tested the photocatalytic performancesof TiO2with different and morphologies but the same treatment tempreture (550oC), andfound that nanosheets assembled flower-like TiO2gives the best performance, which iscomparable to that of P25. However, the obtained TiO2is easier to separate than othernano-sized materials owing to its hierarchical structure and large size in micron scale.Fourthly, we have successfully synthesized acrocoite PbCrO4nanomaterial, made ofnanorods at room temperature. The results show that: the absorption band of the obtainedcrocoite PbCrO4is at about550nm, the calculated the band gap of PbCrO4is about2.27eV, meaning it has strong absorption ability of visible light and can effectively utilizevisible light. We tested the photocatalytic oxygen production performance of theacrocoite PbCrO4nanomaterial under visiable light (>400nm) irradiation with silver nitrate as sacrificial agent. Actualy O2was detected. We also studied the influence of heattreatment to the microstructure and photocatalytic properties. It is found that with theincrease in calcination temperature, the intensity of the XRD peak decreases, probablybecause in the calcination process PbCrO4decomposd partially, which leads to thedecrease of photocatalytic activity.