| The safety issue and effective utilization of hydrogen gas(H2)are of great importance to future environment and society as H2 is a green,clean and renewable source.Particularly,investigation of H2 sensor which is capable of working in ambient condition with considerable sensitivity and fast response/recovery is significantly important to the production,storage,transportation,and usage of H2 energy.Semiconductor with special surface structure processes high reactive sites including edge,kink and hanging bonds,and it is extensively applied in lithium batteries,solar cells,gas sensors,etc.In recent years,H2 sensor based on titania(TiO2)has achieved great progress,however,lowering down the working temperature,improving the sensitivity and reducing the response and recoverytime simultaneously have always been the ultimate goal for the TiO2 based hydrogen sensor.In this work,H2 sensor based on TiO2 nanoarray films was fabricated and surface of the oriented TiO2 thin film was modified,and the working mechanism for H2 sensing of the modified thin films was also investigated.The main research results can be summarized as follows:1.Enhancement of H2 sensitivity of TiO2 array thin film hydrogen sensor by ultraviolet(UV)irradiation.In this work,UV irradiation was adopted to help improving the sensitivity of the already exist room temperature working(002)-oriented rutile TiO2 nano array thin film based H2 sensor.XPS was used to analyze the surface chemical state change of the thin film before and after UV irradiation.The result suggested that UV irradiation is benefit to elimination of surface contamination and to the increase of oxygen vacancy,which were considered as the reason for the enhancement of the sensitivity of H2 sensor in various H2 concentration.2.H2 sensing performance of the sensors based on platinum(Pt)nanoparticles functionized(002)oriented rutile TiO2.Pt nanoparticles with various amounts were deposited on(002)surface of the oriented rutile TiO2 by adjusting direct current magnetron sputtering time(10 s,15 s,20 s,30 s),and the H2 sensing performance and mechanism has been revealed.An unusual inversion response pattern of Pt nanoparticles functionized(002)oriented rutile TiO2 based sensor was observed with the increasing of H2 concentration and amount of Pt NPs:The sensitivity of sensor Pt with 10 s Pt sputtering time was positive and changed to negative when Pt sputtering time increased(15 s,20 s,30 s);the sensitivity of the Pt modified TiO2 sensor was negtive in low H2 concentration(1-20 ppm)and inversed to positive in higher concentration(>40 ppm).The results could be attributed that a competitive effect exists between Pt NPs and the(002)surface of TiO2 nanorod array film for H2 adsorbing.That is,H2 molecules were preferentially adsorbed on Pt nanoparticles until the Pt sites reached satuation,then H atoms dissociated and diffused to(002)surface of rutile TiO2.3.The enhancement of hydrogen sensing performance of the(002)oriented rutile TiO2 by co-exposing the(101)facets.Rutile TiO2 thin film with(101)and(002)facets exposing was controllably synthesized through adjusting the ethanol content of the hydrothermal solvent.The crystalline structure,morphologies and H2 sensing performance of the samples varied with the ratio change of ethanol in the hydrothermal precursor.By increasing the ethanol content,the(002)orientation growth was enhanced and the(101)orientation growth was restrained,size of the nanorods composed of the thin film declined and the density of the film increased.All of the prepared TiO2 nanorod array film based sensors performed very well at room temperature and those with both(101)and(002)facets exposing have faster response,as well as good repeatability and stability than those with only(002)facets.Modelling results based on density functional theory(DFT)simulations have been adopted to reveal the surface interaction of H2 and the TiO2 surface.The results suggested that H2 intended to be adsorbed and dissociated on the(002)surface and(101)surface,with the energy barrier lower on the(002)surface.The dissociated H atoms inclined to diffuse into sublayers of TiO2 lattice and then to escape from the(110)surface.Thin films with smaller density,in which more(110)surface is exposed,offered more space for H2 regeneration,thus have shorter response and recovery time as well as higher sensitivity.4.The fabrication and H2 sensing performance of(004)oriented anatase TiO2 array thin films.(004)oriented anatase TiO2 array thin films have been synthesized by hydrothermal method on florin doped tin oxides(FTO)substrate with TBOT as Ti source and SO42-as additive.A pre-deposited seed layer was introduced and a post annealing treatment process is carried out to improve the crystal quality of the thin films.The experimental results showed that,the sensor had a detection limit as low as 1 ppm with a remarkable sensitivity of 17%and fast response and recovery time less than 30 s and 70 s respectively to 1-2000 ppm H2 at ambient condition,which was much better than that of the oriented rutile TiO2 thin film and other reported data in the literature.DFT indicated that the low dissociation barrier for H2 on the(004)surface was responsible for the high sensitivity and fast response at room temperature. |
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