Structural Control Of Nano-tungsten Trioxide And Its Nonlinear Optical Limiting Performance

The design and effective control of nanomaterials morphology have always been a major challenge for chemical synthesis.Tungsten trioxide（WO₃）semiconductors have been widely used in the field of optoelectronics due to their multiple crystal forms and special electronic structure.However,there has seldom reports concerned on the nonlinear optical limiting performance of nano WO₃,especially the structure-effect relationship between nonlinear optical limiting（OL）performance and morphology,which will prominently promote the application of WO₃ nanostructures in the field of nonlinear optics.In this work,WO₃ nanostructures of different morphologies and dimensions were prepared by changing synthesized parameters（for example,structure-directing agent,pH value and the concentration ratio of precursors）via hydrothermal method.Furthermore,the Au nanoparticles with uniform size were homogeneously deposited on the surface of WO₃ nanowires through the reduction of chloroauric acid with sodium citrate.The morphological,structural and linear optical properties of WO₃ nanostructures together with its composites were systematically characterized by field emission scanning electron microscopy（FESEM）,transmission electron microscopy（TEM）,Raman spectroscopy（Raman）,X-ray diffractometer（XRD）,thermogravimetry analyzer（TG）and UV-visible absorption spectroscopy（UV-Vis）,In addition,the nonlinear OL performance of WO₃ with different morphology and its composites were investigated using open aperture（OA）Z-scan technique and their nonlinear mechanisms were also explored.Based on the above investigation,some valuable conclusions were obtained and listed as following.WO₃ nanostructures of 0-dimensional nanospheres,1-dimensional nanorods and2-dimensional nanoplates were synthesized by changing the concentration ratio of sodium tungstate and hydrochloric acid.Among them,the nanospheres were octahedral WO₃·0.33H₂O while the phases of nanorods and nanoplates were both hexagonal WO₃.All of the three WO₃nanostructures exhibit good nonlinear OL performance and the limiting mechanism is reverse saturable absorption（RSA）result from free carrier absorption（FCA）.Moreover,the dimension of WO₃ nanostructures have significantly influenced on their nonlinear OL performance.WO₃ nanowires（NWs）,square nanopaltes（SNPs）and nanoflower clusters（NFCs）were successfully prepared by altering the pH value and the structure directing agent.The obtained phase structures of NWs and NFCs are both hexagonal structure WO₃ while the NFCs are orthorhombic WO₃·H₂O.The OA Z-scan results demonstrate that all of the three morphologies of WO₃ nanostructures present good nonlinear OL performance.Compared with SNPs and NFCs,NWs show the best nonlinear OL properties;The nonlinear OL activities of NWs,SNPs and NFCs are originate from the synergistic effect of nonlinear absorption and nonlinear scattering.Au nanoparticles with uniform size were homogeneously loaded on WO₃ NWs using using the reduction of chloroauric acid by sodium citrate.No chemical bond occurs between Au nanoparticles and WO₃ NWs,but the introduction of Au nanoparticles narrow the band gap of WO₃ NWs.The nonlinear OL performance of the WO₃ NWs dramatically enhanced after the deposition of Au nanoparticles,attributing to the nonlinear scattering induced by the Au nanoparticles and the charge transfer between Au nanoparticles and WO₃ NWs.Hexagonal WO₃ nanosheets were self-assembled by WO₃ nanorods through two-step hydrothermal method.The regular hexagonal nanosheets were testified to be orthorhombic phase of WO₃·0.33H₂O.Unexpectedly,the self-assembly procedure alters the band gap characteristics of original WO₃ nanostructures,inducing the indirect band gap of WO₃ nanorods transfer to direct and narrow the band gap of hexagonal WO₃ nanosheets.In addition,OA Z-scan and photocatalytic results showed hexagonal sheet-like WO₃ show good nonlinear OL performance and excellent photocatalytic properties.The above results will provide theoretical and experimental foundation for the development of WO₃ with peculiar nanostructures.In addition,it will also have an important promotion for expanding the application of WO₃ nanostructures in the field of nonlinear optics and photocatalysis.