Synthesis,characterization And Photoelectrochemical Properties Of Tungsten-Containing Nanostructures

With the reduction of the fossil fuel and the destruction of the environment,using clean resources and solving air or water pollutions become an urgent task.As important semiconductors,tungsten-containing compounds have extensively attracted ever increasing attention in the past few years due to their wide range of optical and electrical applications,such as photocatalysts,electrocatalysts,solar cells,cathode materials in lithium ions rechargeable batteries,sensors and so on.However,to make them a useful material platform,it is necessary to develop feasible synthesis methods to control their morphologies.In this dissertation,several WO3-x nano-/micro-rods,Z-scheme C fiber@WO3-x core-shell composites,WS2 nanoplates and C fiber@WS2 nanoplates core-shell composites were synthesized via simple thermal evaporation methods in a horizontal tube furnace,and the phase composition,microstructure and photoelectrochemical properties were systematically investigated.(1)A series of WO3-x nano-/micro-rods(NMRs)were synthesized through calcining WO3 powders under a reducing atmosphere of S vapor at 950-1150 °C in a vacuum furnace.For comparison,the as-prepared WO3-x NMRs were then annealed at 500 °C for 2 h to obtain their corresponding WO3 nanostructures.The single component WO3-x crystals presented interesting positive impedance humidity sensitivity,and the resistance of WO3-x crystal sensors in response to relative humidity could be tuned from a negative to positive one by increasing the compositional x.It was revealed that the positive humidity impedance was driven by the defect of oxygen vacancy.The decolourization of organic model dyes methylene blue(MB)and rhodamine B(RhB)under visible light over the as-prepared products was investigated,revealing that the oxygen-deficient sample will present better comprehensive performance due to its stronger surface adsorption to the dye molecules,which could be attributed to the oxygen vacancies.(2)A series of novel Z-scheme C fibers@WO3-x(0?x<3)core-shell composites were developed by simply pyrolyzing preoxidized polyacrylonitrile(PAN)fibers impregnated with WO3 nanopowder in absolute ethanol in a vacuum furnace.Such structured composites can present excellent solar-driven photocatalytic performance on the degradation of organic dyes in aqueous solution.When they were applied in photodegrading MB and RhB,greatly improved photocatalytic activities could be observed compared with those over the pure tungsten oxides catalysts without carbon fiber under the same condition.The improved photocatalytic performance can be mainly attributed to the carbon fiber core and the metal connection layer between the tungsten oxides and C core in the composites,which can promptly conduct electrons and reduce the recombination of the photogenerated electron-hole pairs.Resultantly,the developed composites exhibit great potential for environmental remediation by degrading toxic industrial chemicals in waste water under sunlight.(3)A simple approach was proposed to prepare various morphologies of WS2 nanostructures by direct thermal evaporation of WO3 and S powders onto Si substrates sputtered with W film.This method can produce bulk quantities of pure hexagonal,horizontally grown WS2 nanoplates,vertically grown nanoplates,and nanoplate formed flowers simply by tuning the distance between the substrate and source powders.The synthesis mechanism and morphology evolution model were proposed.Moreover,when employed as a thin-film anode material,the Li-ion battery with the as-prepared,vertically grown WS2 nanoplates presented a rechargeable performance between 3 and 0.01 V with a discharge capacity of about 773 mAh/cm3 after recycling for three times,much better than its already-reported counterparts with randomly distributed WS2 nanosheet electrodes,but the battery with horizontally grown WS2 nanoplates could not show any charge-discharge cycling property,which could be attributed to the different structures of WS2 anodes for Li+ ion intercalation or deintercalation.(4)A one-step strategy was established for the large-scale in-situ synthesis of C fibers@WS2 nanoplates core-shell composites simply by heating the PAN fibers impregnated with WO3 nanopowder in absolute ethanol in S vapor.The photocatalytic performances of the as-synthesized composites were evaluated by the degradation of MB,RhB,Cr(VI)and E.coli.Marvelously,such composites presented greatly improved photocatalytic activity and excellent stability under full-spectrum light irradiation.Such strategy will be a general,low-cost and environmentally friendly solution for the synthesis of highly efficient semiconductor photocatalysts for industrial applications.