Fabrication And Gas Sensing Performance Of Biomorphic In Oxides With Hierarchical Porous Structures

With the rapid industrialization and the great increase of population, indiscriminateemission of toxic and harmful gases and flammable and explosive gases not only causesserious air pollution to the ecological environment, but also pose a great threat to theindustries and our everyday lives. In this case, the exploration of gas sensing materialsbecomes one of the urgent problems that must be solved for the implementation of thestrategy of sustainable development. Current researches show that hierarchical structures,especially the hierarchical porous structures, are excellent structural candidates to endowmetal oxides based gas sensing materials with high sensitivity and response rate, whichmeans that hierarchical structures are one of the powerful solutions for the gas detectionand monitoring. However, due to the insufficiency of current synthetic processes andtechnologies, the building of hierarchical structures is always confined by somebottlenecks, such as hard to be synthesized, simplifying of structures, limited types, hardto be modified, and so on, which greatly speed down the exploration of gas sensingmaterials with high sensitivity and response rate. Fortunately, nature provides usblue-prints for the design of elaborate hierarchical structures after thousands of millionsof years’ evolution. Firstly, nature creates astonishing varieties of amazing hierarchicalstructures with multi-morphology in multi-scales and multi-dimensions. This provides usa giant treasure of structural templates. Secondly, the synthetic processes of these naturalstructures are effective, facile, low-cost, green and morphology-tunable, and thus couldbreak a new way towards the artificial synthesis of hierarchical materials. Thirdly, naturalelaborate hierarchical structures are always correlated to specific optical, electrical,magnetic, sonic, thermal, mechanical and other functionalities. Thesestructures-coupling-functionalities provide us new models for the design of advancedfunctional materials. Fourthly, natural structures are composed of lots of biomolecules,which offer abundant and well-dispersed active sites for wet-chemical synthesis andchemical modifications of final products.In view of these points upwards, we explored the fabrication procedures and gas sensing application of open hierarchical porous tin oxides (SnO2), hierarchical porousSnO2with loose and thin scaffolds, and open hierarchical porous SnO2with loose andthin scaffolds in the perspectives of structural bio-templates, self-assembly bio-templatesand functional bio-templates, respectively. Then we analyzed the influences ofhierarchical structures and compositions optimization on as-fabricated metal oxides,which could provide us new insights and models for the exploration of gas sensingmaterials. The main contents and results are as follows:1. Inspired by the open hierarchical porous structures of butterfly wings, weexplored their application in gas sensing SnO2materials, which could provide us newmodels for the structural design of gas sensing materials.Templated from butterfly wings, we successfully developed a one-step solutiondeposition process combined with thermal treatments to fabricate open hierarchicalporous and hollow SnO2materials. By changing the concentration of precursory solutionand deposition time, the wall thickness could be tuned from30to110nm withoutchanging the crystal size, the distribution of pore size and the surface area of thematerials. Butterfly-wings-morphic SnO2could work at a relative low workingtemperature (170C), and showed the6times high gas response of the blank sample andless than the half response/recovery times of the blank sample to50ppm ethanol. Thesuperior sensing performances were ascribed to the specific structures. The openstructures provided convenient entrance for gas molecules, and the hierarchical structuresfacilitated the gas transport in the inner. The research on structure-performancerelationship showed that the increase of wall thickness prevented the gas diffusion on thehierarchical scaffolds and thus induced the degradation of gas sensing performances,which included the decrease of gas response and the increase of optimal workingtemperature. This change phenomenon directs a new way towards the structuralexploration for high performance gas sensing materials, which is to build hierarchicalstructures with loose and thin-walled scaffolds facilitating gas transport.2. For the requirement of loose and thin-walled scaffolds, we explored the directutilization of biological membrane-like self-assembly system to fabricate threedimensional (3D) hierarchical porous structures possessing loose and thin-walledscaffolds, inspired by the thin thickness and strong self-assembly ability of biologicalmembranes. This can break a new way towards the synthesis of hierarchical gas sensingstructures.Pollen coats were utilized as biological self-assembly systems, in which SnO2 precursory ions were guided to self-assemble to3D hierarchical porous structuresconstructed by interconnected membranes. The self-assembly process was similar to theformation process of biological membranes: hydrophobic lipids self-assembled to theinner of the membranes, and hydrophilic proteins adsorbing SnO2precursory ionsself-assembled to the external of the membranes. The membranes thickness of the finalproducts was only tens of nanometers, loosely packed up by nanoparticles of about8.0nm. The loosening and porosity of the scaffolds could be tuned by the calcinationtemperature. They were optimal at the calcination temperature of about700C, anddegraded seriously when the temperature elevated. With the degradation of scaffolds’loosening and porosity, the gas responses decreased while the response time and theworking temperatures increased. The performances variations should be ascribed to themodulation role of the loosening and porosity on the gas diffusion efficiency on thescaffolds. This work provides us some new insights on the design of hierarchical gassensing materials.3. To combine the advantages of open hierarchical porous structures and loose andporous scaffolds, we proposed to mimic the structures of pollen grains to fabricate3Dopen hierarchical porous structures possessing loose and porous scaffolds in theperspective of functional biological templates. Such structures could act as models forhigh and controllable gas sensor.To make full use of the strong absorbing ability of surface biomolecules (proteins, etc)to precursory ions, we developed a two-step soakage process followed by thermaltreatment to replicate the functional hierarchical porous structures templated from pollengrains. Biomolecules acted as mesotemplates and prevented the crystal growth andparticles accumulation, making sure of the building of loose and porous scaffolds andallowing them to be tuned by the volume ration of ethanol and water in the treatingsolution. With the hierarchical structures of pollen grains, the gas response and responserate of SnO2materials were improved. The improvements varied with the gas species,concentrations and working temperatures. To reducing gases, the gas responses ofpollen-grain-morphic SnO2was about1.5~3.4times high of that of the blank samples,while to oxidizing gases of NO2and Cl2the gas responses was about4~6.5times high.The different influences suggested that the oxygen species absorbed on SnO2partilces isinsufficient. This gives us the indication on the further improvement of hierarchical gassensing materials. In addition, the loosening and porosity of the scaffolds could tune thegas response and the variation rate (n) of gas response to gas concentration. This should be due to the cooperative effects of surface area, surface coarseness, particles packageand crystal size.4. Aroused by the performance insufficiency and application confinement of puregas sensing metal oxide, we optimized the pollen-grains-morphic SnO2in terms ofcompositions. This work built up a good basis for the compositional optimization ofhierarchical gas sensing materials and the exploration of high sensitive and fast responsegas sensing materials.On the basis of the synthesis of pollen-grains-morphic SnO2, we added the soakageprocess in PdCl2solution to realize the controllable surface modification ofpollen-grains-morphic SnO2(Pd/Sn atoms molar ratios range in0~3.82%). Thewell-dispersed PdO largely influenced the gas sensing performances: i) Decreased theoptimal working temperature. The decreases were dependent on gas species, PdOcontents and the microstructures; ii) Improved the gas response. The improvement to H2was the best and increased with the increase of PdO contents; iii) Decreased theresponse-recovery times. The decrease was larger at lower working temperature, and wasnot affected by PdO contents. The influence of PdO on gas sensing performance wasrealized by the different cooperative interactions between microstructures and surfaceperformances. When the Pd/Sn <2.89%, PdO existed as tiny particles and prevented thecrystal growth and particles accumulation. The spillover and back-spillover effect zonesof PdO could not overlap. The absorbing ability of oxygen species and O/Sn atoms molarratio changed a little, and thus the gas response enhancement was in the middle-level;When the Pd/Sn≥2.89%, PdO particles accumulated and the roles on crystal growth andSnO2particles accumulation decreased. The spillover and back-spillover effect zones ofPdO could overlap entirely or in some extent. The absorbing ability of oxygen speciesand O/Sn atoms molar ratio increased sharply, and thus the gas response enhancementwas the best.The research provides new direction for the synthesis of hierarchical gas sensingmaterials and brings in new insights for the structural design and compositionaloptimization of good gas sensing materials. It is greatly meaningful for the exploration offunctional materials in multi-scale and multi-dimension and structure-enhancedmaterials.