Hydrothermal Synthesis Of Tin Oxide Nanostructures And Their Gas Sensing Properties

As an important n-type semiconductor with a wide band gap（Eg=3.6 ev, at 300 K）, SnO₂ has wide applications in many fields such as gas sensors, photocatalyst, solar cells and transparent electrode etc. Sensor is the most important one among the applications. The fabricated SnO₂-based sensor possesses a high response, excellent response–recovery characteristic and high stability. The characteristics promote SnO₂ to be the most potential gas-sensing material. Sensing properties of the gas sensor strongly depend on the structures and morphologies. How to prepare the controlled sizes and morphologies of nano tin oxide has become an important research direction.In this paper, by adjusting the synthetic conditions, adding NaOH/surfactants to the system, sintering process, SnO₂ nanomaterials of different morphologies have been prepared via a simple and low cost hydrothermal method for the propose of improving the gas sensing performances. The synthetic morphologies including hollow structure, hierarchical porous SnO₂ structures which have large specific surface areas are all beneficial to improvement of the sensing properties. Microstructures and micro morphologies of the materials are characterized by characterization testing means such as XRD, SEM and TEM, HRTEM and SAED, discussed the growth mechanism and studied their gas sensing. The results and significance in the thesis are as follows.（1） Rod-like SnO₂ nanostructures consisted of 1D nanorods: we present a facile hydrothermal method to synthesis dense and porous rod-like SnO₂ nanostructures with SnCl₂·2H₂O as reactant and utilizing of CTAB or PEG. The reactants concentration and surfactants have a strong effect on the formation process and morphology of the SnO₂. The sensor based on the porous nanorods exhibited higher response to ethanol gas compared with that of the sensor using the dense nanorods. The highest gas response of the porous nanorods to ethanol is estimated to be 54 with optimal operating temperature of 350 oC. This is due to the porous SnO₂ nanorods surface have large amount of petals and porosity, provide larger number of gas diffusion pathways along the SnO₂ surfaces, which benefit for the gas diffusion.（2） Hierarchical SnO₂ flower-like architectures assembled with 1D nanorods or nanocones: Firstly, a facile hydrothermal route to synthesize SnO₂ flower-like architectures assembled by nanorods align radically form center under mild conditions is investigated. It reveals the as-prepared products how to change from sphere to flower. The presence of PEG-6000 controls the process of crystal growth and a key factor of achieving SnO₂ flower-like architectures with uniform morphologies and well dispersed. In the optimum working temperature is 313 °C, gas response is S=35 and the response and recovery times for the sensor are evaluated to be 7-8 s and 9-10 s. The excellent gas-sensing performances of them demonstrate their potential application as gas sensors. Then, nanocone-assembled 3D flower-like SnO₂ is synthesized via a hydrothermal process. The maximum diameter of an individual nanocone is comparable to twice Debye length of bulk SnO₂, majority carriers heavily depleted due to the ionization of adsorbed oxygen, which is believed to be beneficial for gas sensing. The gas sensing performance of the 1D-3D SnO₂ based sensor is investigated towards 100 ppm H₂ at 50 oC, the response is improved almost 3.5 times（S= 9.5） in few seconds under UV irradiation. the reasons behind which may be attributed to not only the novel 1D-3D configuration, providing vast reaction sites and sufficient diffusion spaces, but also the rearrangement of surface Sn-O bonding from Sn-O（ad） to Sn-O（hv） which makes it possible to detect H₂ at low temperature effectively.（3） Hierarchical SnO₂ flower-like architectures assembled with 2D nanosheets: Firstly, porous flower-like SnO₂ architectures self-assembled by aggregative nanosheets have been synthesized via one-step hydrothermal method. This superior performance can be attributed to the unique 2D sheet-like structure. It shortens the gas diffusion distance and provides highly accessible open channels and active surfaces for the detected gas. Then, a novel snowflake-like SnO₂ hierarchical architecture has been synthesized via a facile hydrothermal method and followed by calcination. The obtained SnO₂ architecture is composed of numerous nanoflakes. Moreover, the snowflake-shape nanoflowers have a porous feature on petals that could further increase the active surface area of the materials, which facilitates gas diffusion and mass transportation in sensing materials. The results of gas sensing tests verify that the snowflake-like SnO₂ architecture exhibit excellent selectivity, high response and fast response-recovery capability.