Elastic Strain Effects On The Photocatalytic And Electrocatalytic Activities Of Nanofilms

Elastic Strain Engineering（ESE）is a promising interdisciplinary research subject on material science,physics and chemistry,which is aimed at imposing the elastic strain to the functional material in order to modify its physical and chemical activities.The previous reported work mainly concentrated upon elastic-strain effects on the core-shell nanoparticles and epitaxial nanofilms.For other non-epitaxial functional thin films,however,strain-engineered effects could not be fully achieved due to the relative small elastic strain that imposed to the films.As a result,in-depth understanding of ESE is limited.In this dissertation,Fe Ni Co Ti alloy,NiTi（Nb）shape memory alloy and E-beam lithography processed Si O2 wafer with nano-pillars distribution were selected as the substrate to impose the relative larger elastic strain to the TiO₂,WO₃ and Fe₂O₃-NiFe₂O₄ semiconducting nanofilms,Pt nanofilm and MoS₂ monolayer.Differential scanning calorimetry（DSC）and thermal dilatation measurement were used to study the phase transformation behavior of the substrate.Scanning electron microscopy（SEM）and atomic force microscopy（AFM）were used to characterize the surface morphology of the films.Grazing-incident X-ray diffraction（GIXRD）and Raman spectroscopy were utilized to measure the elastic strain in the film.For photocatalysis,bandgap of semiconducting film was obtained from the UV-vis diffuse reflectance spectrum,photocatalytic degradation of methyl orange（MO）and photoelectrocatalytic water oxidation were performed to investigate the elastic-strain effects on the photocatalysis.For electrocatalysis,electrochemical analytical techniques including linear sweep voltammetry（LSV）and scanning electrochemical microscopy（SECM）together with multiphysics modeling and simulation were used for clarifying the elastic strain effects on the oxygen reduction reaction（ORR）activity of Pt nanofilm and hydrogen evolution reaction（HER）of MoS₂ monolayer.The main conclusions obtained here are as follows:（1）The semiconducting film comprised of α-Fe2O3 and Ni Fe2O4 was tensile-strained biaxially with an average value of 0.46% by the means of surface relief of Fe Ni Co Ti alloy substrate,which contributes to a 30% increase in the degradation rate of methyl orange and 2 3 times larger photocurrent of the strained film.（2）Anatase TiO₂ nanofilm was tensilely strained by utilizing the two-way shape memory effect（TWSME）of NiTiNb substrate,and 0.4% tensile strain in the film induces a 60 me V narrowed bandgap and 1.4 times larger photocurrent of the water oxidation reaction of TiO₂.（3）Rutile TiO₂ thin film was biaxially tensilely strained by in-situ mechanical bending,and 0.5% tensile strain results in a narrowed bandgap with ?Eg of 70 me V and 1.46 times larger photocurrent of the water oxidation reaction than that of the strain-free TiO₂.（4）0.52% tensile strain and 1.10% compressive strain were successfully imposed to the 10 nm Pt nanofilm,and compressive strain induces a 39% enhancement of the kinetic rate constant of the ORR,while tensile strain has the opposite effect.Based on the same imposing strain strategy,0.76% tensile strain and 1.93% compressive strain were successfully imposed to the 5 nm Pt nanofilm,and compressive strain induces a 52% enhancement of the kinetic rate constant and a 27 m V positive shift of the half-wave potential of the ORR compared to that of the pristine Pt nanofilm,while tensile strain has the opposite effect.（5）MoS₂ monolayer was 2% tensilely strained by utilizing the nano-cone arrays that were patterned on the Si O2 substrate with Au coating,and tensile strain improves a kinetic rate constant of the HER by 4.3 times and shifts the half-wave potential positively by 0.1 V.