Fabrication Of Titanium Dioxide And First-principle Investigation Of Crystal-facet-dependent Sensing Mechanism

Gas sensors are able to detect various kinds of gases,such as environmentally hazardous gases which are harmful to human’s healthy and industrial manufacture.They can also monitor the working situation inside the equipment.The gas sensor must possess good detecting limitations,fast response/recovery time and long stability.Being an important semiconductor metal oxide,the TiO₂-based gas sensor has been found sensitive to a range of harmful gases.Synthesizing TiO₂ with large specific surface area or rationally control the surface orientation of the grain,can greatly improve the sensing properties of the material.Although many researchers have successfully synthesized TiO₂particles with a large portion of specific facet,the microscopic mechanism is still incomplete and many proposed theories only based on experimental observation.In this work,we employed the first-principle calculation software VASP to investigate the adsorption of H₂、CO and H₂S on some low-index facets of two common crystallographic forms of TiO₂ from an atomic level.For those facets which can be easily obtained,we use the experiment in combination with theoretical simulations.But for some hardly obtained facet,we merely use calculation to make predictions.The thoughts used in this work and the explanation given in this study is of great significance for the further fabrication of the high-performance TiO₂-based sensor.The main achievements are concluded as follows:1)By controlling the concentration of H⁺in the solution,we synthesized partly agglomerated flower-like rutile TiO₂ with fundamental units as nano-petals comprised of ultrafine nano-pillars,by a one-pot hydrothermal method.The p H value of the solution modified the rate of hydrolysis of the Ti³⁺,inducing the discrepancy in surface growth,thus finally resulting in rutile TiO₂.In the hydrogen sensing test,the gas sensor fabricated by this flower-like material did not saturate in a wide range of temperatures and delivered excellent performance in response/recover test.The sensing process was simulated,and we found that surface bridging oxygen was easily generated and pre-adsorbed oxygen intended to adhere to the defective site.After the interaction between H₂and surface five-coordinated sites,we discovered obvious charge transfer and change of electronic structures,which were all the reasons for the decrease of resistance after adsorption.The adsorption of Ti_5csites largely contributed to the sensing signals in the test.2)Further,the adsorption of hazardous CO and H₂S on four low-index surfaces of rutile TiO₂ surfaces was investigated using first-principle calculations,in an effort to distinguish the difference of surface reactivity on these surfaces in sensing process.We predicted that rutile TiO₂ possessed an excellent capacity for H₂S detection than CO based on adsorptive structure,the density of states（DOS）analysis and charge density difference（CDD）plots.Particularly,in H₂S adsorption,the dissociative adsorption process occurred on the（001）surface of TiO₂ with largest adsorption energy and noticeable modification in the electronic structure,rendering high sensitivity on this surface.The decomposition of H₂S yielded HS and H species,whose bonding features were also analyzed.It was suggested that the 1s states of H were considerably overlapped with 2p states of surface bridging oxygen,forming OH radicals on（001）surface.The charge density difference plots conformed to the breakage of the H-S bond and the formation of the O-H bond.The reason for different sensing properties on these surfaces was pinpointed,and the significance of rutile（100）facet in H₂S monitoring was underlined.3)In the present study,crystal-facet-dependent gas sensing performance was thoroughly investigated and the sensing mechanism of TiO₂ was elaborated in-depth.Anatase TiO₂ nano-polyhedron with highly reactive（001）facet was successfully synthesized via a one-pot hydrothermal method using fluoride as a facet stabilizer that selectively decreased the surface energy of initially high-energy surface and was utilized for fabrication of carbon monoxide gas sensors,followed by characterization of microstructure,phase-purity,and gas-sensing properties.Chemiresistive properties of（001）-dominated gas sensor exhibited a superior response to CO with a higher response,lower working temperature and faster response/recovery time.Particularly,the first-principle calculation was carried out to expound the sensing mechanism,which showed that CO adsorption on（001）facet was more stable and favorable than that on normally exposed（101）facet,corroborating the reactive nature of（001）facet.Finally,we proposed several reasons for the enhancement of nanomaterial with high energy surfaces.This systematical DFT interpretation of our experiments can be generalized as a method to investigate the response of other adsorption systems,assisting researchers to theoretically expound the sensing mechanism.