| In the last decade, the gas detection and monitoring became greatly valued as the increase of the demand for environment protection, modern life, industry and national security, etc. Gas sensors are applied widely in many areas, such as chemistry, biomedical, food industry, wine-quality monitoring, and breath analysis. Those applications can be classified into two groups which are the detection of single gases (as NOx, NH3,O3, CO, CH4, H2, SO2) and odor discrimination or the monitoring of changes in the ambient. For example, gas sensor for single gas can be used as fire detectors, leakage detectors, controllers of ventilation in cars and planes, alarm devices warning the overcoming of threshold concentration values of hazardous gases in the work places, while sensors for odor discrimination or monitoring changes in the ambient can be applied to detect volatile organic compounds (OVCs) or smells generated from food and household products in food industry and indoor air quality control. Therefore, research on gas sensors with high qualities as well as sensor fabrication are of great importance in both theory and practice.TiO2 is a kind of n-type metal oxide semiconductor with excellent chemical stability and erosion resistance, which is sensitive to several kinds of gases. Previous research has mainly focused on titanium dioxide particles and thin films, but there are very few studies on 1-D titanium dioxide nanobelts. However, the sensing performance of titanium dioxide particles or thin films was found to be limited by such two main drawbacks as low sensitivity and inferior selectivity.1-D titanium dioxide nanomaterials not only keep many specific properties of nanomaterials, but also maintain a much higher carrier transferring rate since the chance for electron loss is decreased because interior carriers transfer along 1-D long axis direction without crossing grain boundaries, so they are much better gas sensor materials candidates than particles and thin films. Titanium dioxide nanobelt is a kind of quasi 1-D nanomaterials with a much bigger size and operable surface area, which also keeps the specifics of nanomaterials. However, the gas sensing property of titanium dioxide nanobelt is limited by the fact that TiO2 nanobelt has a perfect surface structure and a single phase without any defects. Surface modification, such as surface acid corrosion, and assembly of surface heterostructures, is one of the most important ways to further enhance the gas sensing property of TiO2 nanobelt.In the thesis, surface-coarsened TiO2 nanobelts were successfully prepared by acid-assisted hydrothermal method based on the mass production of TiO2 nanobelts through hydrothermal method, and several kinds of surface-coarsened TiO2 nanobelts based heterostructures were also designed and fabricated by photocatalytic reduction method or the liquid phase method. Moreover, the gas sensing properties of surface-coarsened TiO2 nanobelts as well as the heterostructures were also studied in order to provide a theoretical basis for future practical applications of TiO2 based gas sensors.The main research contents of the thesis are as follows:1. TiO2 nanobelts and surface-coarsened TiO2 nanobelts were synthesized via hydrothermal method or acid-assisted hydrothermal method, and three kinds of gas sensors were fabricated based on P25, TiO2 nanobelts and surface-coarsened TiO2 nanobelts, through which the gas sensing properties of those three materials above were studied and analyzed. TiO2 nanobelt prepared by hydrothermal method is a kind of 1-D nanostructure with the length of several or several tens of micrometers, width of more than 100 nm, and thickness of about 50nm. Its surfaces are very smooth and of good crystallinity. The size of surface-coarsened TiO2 nanobelts is decreased in all three dimensions, the smooth surfaces were destroyed and the coarsened surfaces were formed with plenty of defects on. The three types of sensors based on P25, TiO2 nanobelt and surface-coarsened TiO2 nanobelt showed good gas sensing performance.Three sensors all respond to ethanol vapor, acetone vapor, and hydrogen, but not to CO and methane. The response time and recovery time are shorter than 3 s, and the response are all very stable. Among the three sensors, surface-coarsened TiO2 nanobelts based sensor displayed the best performance which means higher sensitivity, lower working temperature and optimal working temperature, while P25 based sensor did the worst. The differences in the properties of three sensors were explained on the base of the microstructure of three materials and the surface morphology of the films on sensors. The sensing mechanism was interpreted with the carrier transferring model or the surface carrier depletion model, and the influence of acid corrosion on nanobelt's gas sensing property was also explained. In brief, surface-coarsened TiO2 nanobelts possess the best gas sensing property in this experiment due to the best surface structures.2. Noble metal (Ag, Au)-TiO2 heterostructures were synthesized on the surface of surface-coarsened TiO2 nanobelts by photocatalytic reduction method, the size of the metal particles was modified by changing the illuminating time, and the gas sensing property of the noble metal-TiO2 heterostructure materials were also studied. The diameter of metal particles (Ag, Au) increases as the extension of the illuminating time. Ag-TiO2 heterostructures have efficiently improved the sensing property of TiO2 nanobelts to ethanol vapor but reduced that to acetone vapor, which means Ag-surface-coarsened TiO2 nanobelt heterostructures are potential to act as ethanol sensing materials. Au-TiO2 heterostructures remarkably enhance the sensing property of TiO2 nanobelts to hydrogen and weaken that to ethanol and acetone vapor, which shows that it is promising to fabricate hydrogen sensing devices based on Au-surface-coarsened TiO2 nanobelt heterostructures after further modification. In addition, the sensitivitis of both heterostructures decrease as the metal particles grow. The chemical interaction mechanism was used to interpret the improvement of gas sensing property of noble metal (Ag, Au)-surface-coarsened TiO2 nanobelts, on the basis of which it was also interpreted about the influence of the metal particle size on the sensing propert. On one hand, metal nanoparticles act as the absorption center of oxygen and accelerate the transfer of electrons from nanobelts to oxygen, which both improve the sensing property of nanobelts;on the other hand, metal nanoparticles favor the absorption of specific reducing gas(es) molecules (e.g. ethanol molecules for Ag) on the surface of nanobelts, and catalyze the reaction betweem such molecules and absorbed oxygen, meanwhile they inhibit the absorption of other kinds of reducing gas molecules and their reaction with absorbed oxygen. The absorption of reducing gas molecules and their reaction with absorbed oxygen will be both weakened as the decrease of specific surface area of metal particles resulting from the increase of the diameter, which finally leads to the reduced gas sensing property.3. Semiconductor (ZnO, CdS)-TiO2 heterostructures were assembled on the surface of surface-coarsened TiO2 nanobelts by liquid phase method, and the gas sensing properties of those heterostructure materials were also studied. ZnO nucleated on the surface of surface-coarsened TiO2 nanobelts and grew up with a petal shape. The particles, which are wide of 30-50 nm and long of 50-80 nm, distribute uniformly on the surface. The ZnO-TiO2 heterostructures enhance the sensitivity and selectivity of surface-coarsened TiO2 nanobelts to hydrogen, and reduce that to ethanol and acetone vapor. CdS particles deposited on the surface of surface-coarsened TiO2 nanobelts with a diameter of 5 nm, and distribute uniformly on the surface. The gas sensing property of CdS-TiO2 heterostructures is similar with that of ZnO-TiO2 heterostructures. Furthermore, CdS-TiO2 heterostructures are more sensitive and selective to hydrogen, which makes it a much better candidate for hydrogen sensor materials. The mechanism of the influence of semiconductor (ZnO, CdS)-TiO2 heterostructures on the sensing property of nanobelts was also discussed. The formation of semiconductor (ZnO, CdS)-TiO2 heterostructures has changed the surface characteristics of the nanobelts, and altered the type and number of active sites on the surface. Those active sites, which favor the absorption of hydrogen molecules and the reaction with absorbed oxygen on the surface, have increased, and those, which are favorable to the absorption of ethanol or acetone molecules and their reactive with absorbed oxygen, have decreased. Therefore, the hydrogen sensing property of such heterostructures is improved. |
PDF Full Text Request