Synthesis And Gas-sensing Performance Of Hierarchical Nanocomposites Based On Tungsten Oxide Nanoplates And Graphene

This dissertation focused on the design and synthesis of novel hierarchical nanocomposites (HNCs) for gas-sensing applications, based on modifying two-dimensional WO3 and graphene nanoplates using metal oxide and precious metal nanocrystals. The gas-sensing properties of the HNCs obtained were systematically investigated, and the detection of low-concentration toxic gases at low temperatures were achieved to some extent.Firstly, zero-dimensional (OD) / two-dimensional (2D) hierarchical nano-composites, were synthesized via a microwave-assisted method and other wet chemical methods, by anchoring discrete second-phase nanoparticles (NPs) on 2D WO3 nanoplates that were obtained by the intercalation and topochemical transformation approach. The heterogeneous interfaces of the 0D/2D HNCs consisting of various components could modulate their gas-sensing behavior. The effects of components, particle sizes and microstructures of the second-phase nanocrystals on their gas-sensing properties were systematically investigated, and the related mechanisms were also been discussed. The main research contents are summarized as follows:(1) Noble metal-modified WO3 nanoplates hierarchical nanocomposites and their NO-sensing property.Au@WO3 and Ag@W03 nanocomposites were successfully prepared by a wet chemical method and photo-induced reduction, respectively. The Au and Ag NPs immobilized on the surfaces of WO3 nanoplates, resulting in reduced resistance. The amount, particle size and number density of Au NPs influence the NO-sensing performance of the Au@W03 HNCs. Few Au NPs lead to less active sites; while the Au NPs are excess, the increase in the number density of Au NPs weakens the resistance-variation degree of Au@WO3 HNCs because of the possible inter-connection of Au NPs. The lwt.% Au@W03 sample shows the best NO-sensing performance at an operation temperature of～170℃ upon exposure to 0.5-10 ppm of NO gases. The Ag NPs loading nanocomposites showed the similar NO-sensing property, and the 0.5wt.%Ag@WO3 sample has a high selectivity at a large operation temperature range of 25-200℃ to low-concentration NO gases. The morphology of the substrates of WO3 nanocrystals also influenced the NO-sensing performance of the Ag@W03 sensors, and the plate-like WO3 was prior to particle-like WO3. The enhancement in NO-sensing performance of the Au@W03 and Ag@WO3 sensors is attributed to the functional modification of Au and Ag NPs and to the loose aggregates of WO3 nanoplates with a house-of-card structure.(2) Fabrication and enhanced H2S gas-sensing performance of metal oxide@WO3 HNCs with heterogeneous interfaces.① Hierarchical Fe2O3@WO3 heterostructures have been fabricated via a simple microwave-heating process. The α-Fe2O3 NPs with particle sizes of 5-10 nm are uniformly, tightly anchored on the surfaces of WO3 nanoplates. The BET specific surface area of the 5%Fe2O3@WO3 composite is up to 1207 m2g-1,5.9 times higher than that of the WO3 nanoplates. The Fe2O3@WO3 nanocomposites are particularly sensitive and selective toward H2S gas. The 5%Fe2O3@WO3 sensor shows the highest H2S-sensing property at the optimum operating temperature of 150℃, and its sensitivity upon exposure to a 10-ppm-H2S gas is as high as 192,4 times higher than that of the WO3 nanoplates. The improved low-temperature gas-sensing performance ought to be attributed to the synergistic effect of the α-Fe2O3 and WO3 species in chemical compositions and microstructures. The hierarchical 0D/2D nanostructures consisting of 0D NPs on 2D ultrathin nanoplates and the microwave-heating process developed provides a simple and robust strategy to achieve high-performance gas-sensing materials.② The In2O3-modified WO3 nanoplate composites have been synthesized via a microwave-assisted method. The In2O3 NPs uniformly immobilized on the surface of WO3 nanoplates. The In2O3@WO3 nanocomposite is particularly active and selective toward H2S. The In2O3@WO3(In/W=0.8) sensor shows a high response of 143 upon exposure to 10 ppm H2S operating at a low temperature of 150℃. The improved selectively sensitive of In2O3@WO3 sensors to H2S gas at low temperature is mainly own to the synergistic effect of the In2Os and WO3 species.③ CuO@W03 nanocomposites have been synthesized via a hydrothermal method, and the low-temperature (even at room temperature) and selective H2S-sensing property of CuO@WO3 nanocomposites has been achieved. The CuO NPs uniformly immobilized on the surface of WO3 nanoplates and formed p-n heterostructures. The Cu0@W03 nanocomposite is particularly active and selective toward H2S. The 5wt.%CuO@WO3 sensor shows a high response of 830 upon exposure to 10 ppm H2S operating at a low temperature of 100℃, and its sensitivity is 16 times higher than that of the sensor of WO3 nanoplates. The 5wt.%CuO@WO3 sensor has an obvious response to 1 ppm H2S at low temperatures, even at room temperature, and its response time is about 3 min. The improved selective response of CuO@WO3 sensors to H2S gas at low temperatures is mainly own to the synergistic effect of the CuO and WO3 species.(3) The Au/SnO2-modified WO3 nanoplates, ternary Au/SnO2@WO3 HNCs, were synthesized by an in-situ reducing method, showing enhanced low-temperature H2S-sensing performance. Novel Au/SnO2@WO3 composites were successfully synthesized by in-situ reducing HAuCl4 with SnCl2 formed Au/SnO2 NPs on the surfaces of WO3 nanoplates. The Au/SnO2 species immobilized highly enhance the H2S-sensing performance of the WO3-based sensors. The 0.5%Au/SnO2@WO3 sensor shows the best H2S-sensing performance operating at 50℃, and its sensitivity upon exposure to a 10-ppm-H2S gas is as high as 220,28 times higher than that of the WO3 nanoplates. The Au/SnO2@WO3 sensors have highly selective responses to H2S gases among various inorganic gases and organic vapors when the operating temperature is lower than 150℃. The greatly enhanced H2S-sensing performance of the Au/SnO2@WO3 sensors is attributed to the synergistic effect of Au/SnO2 and WO3 species in the process of adsorption, diffusion and reaction.Secondly, the selectivity and non-thermal effect of the microwave assisted heating process can promote heterogeneous nucleation and growth of OD second-phase nanocrystals on the 2D nanoplates. Using this microwave assisted process, it is easy to obtain second-phase nanocrystals with controllable size, uniform distribution, to make 0D/2D HNCs with high specific surface areas. As an expanding application of this process, graphene-based HNCs (i.e., SnO2@rGO) were synthesized as an example, illustrating the applicability of the microwave assisted synthesis method. The main contents are summarized as follows:Hierarchical SnO2@reduced graphene oxide (rGO) nanostructures with ultrahigh surface areas have been successfully synthesized via a simple redox reaction between Sn2+ ions and GO nanosheets under the microwave irradiation. SnO2 NPs with particle sizes of 3-5 nm are uniformly anchored on the surfaces of rGO nanosheets through a heteronucleation and growth process. The SnO2@rGO sample with a hierarchically sesamecake-like microstructure consisting of 92 mass% SnO2 has a superhigh specific surface area of 2110 m2g-1. The sensors derived from the SnO2@rGO nanostructures show highly sensitive and selective to H2S, and it can also be used as electroactive materials to detect sunset yellow molecules. The enhancement in gas-sensing and electroactive performance is mainly attributed to the unique hierarchical microstructure, high surface areas and the synergistic effect of SnO2 and rGO species. This work provides an efficient, robust and cost-effective approach to achieve multicomponent-functional nanocomposites for environmental and energy applications.The research results achieved in this dissertation provide the practical experience and theoretical guidance for understanding the effect of microstructure on the gas-sensing properties of HCNs, as well as for the efficient design and synthesis of high-performance gas-sensing materials.