Preparation And Properties Of Tungsten Oxide Nanomaterials

Tungsten oxide(WO3-x)is a widely used transition metal oxide.As energy consumption continues to increase,the development of energy devices is also in urgent.Therefore,the development of the technology of exploiting green energy and renewable energy related applications(for example,for photocatalysis,solar cells,and photothermal conversion)has become a hot focus for the researchers.Although the effects on the efficiency of these energy conversion devices have various factors,the main factor is the material structure and nature.Many energy related materials,including metals,metal chalcogenides,oxides,carbides,nitrides and phosphides,have attracted the attention of a large number of researchers.Among them,tungsten oxide(WO3-x)has received a considerable attention because of its multifarious merits:rich earth content,highly adjustable composition,high chemical stability at low pH and excellent conductivity.Tungsten oxide has a potential value in the field of photocatalysis,which is determined by the nature of the material itself.Tungsten oxide has a suitable band gap(Eg = 2.6-2.8 eV)which absorbs about 12%of the solar spectrum.Through the introduction of oxygen vacancies can increase the absorbance of more solar spectrum.The electron transport rate(12 cm2 V-1 s-1)of tungsten oxide is faster than that of TiO2(0.3 cm2 V-1 s-1),revealing its potential application in the field of photocatalysis.In addition,it is also an excellent electrode material,the application of electrochromic devices and supercapacitors is mainly due to the good charge storage/transport performance.Non-stoichiometric tungsten oxide is also well known because the lattice can withstand considerable oxygen vacancies.In addition,many oxygen vacancies containing tungsten oxide such as WO2.9,WO2.83,WO2.72 are air-stable.Oxygen vacancies act as a shallow donator,can improve the conductivity and free carriers density,and enhance the surface of the adsorption species(such as CO2,H2 and NO2),making them promising photocatalysts and gas sensor materials.At present,WO3-x attract a wide range of interests,and they have strong absorption in the near infrared(NIR 780-1100 nm)and have good photothermal conversion effect.Because the material has a prerequisite for photothermal therapy,WO3-x is expected to be a new material for the treatment of cancer by photothermal therapy.In this paper,the regulation of the synthesis of tungsten oxide crystal,preparation of reduced cubic phase WO3-x through direct reduction method,the design and synthesis of special crystal channels of tungsten oxide crystals and their applications in the field of photothermal conversion are elaborated as following points:1.The W18O49 nanowires were prepared by hydrolysis of tungsten hexachloride in n-butanol system by solvothermal method.The oxidized state W18O49 and the reduced state W18O49 were prepared by adding sodium nitrate and sodium borohydride as redox agents in the system,respectively,to control the redox atmosphere of the reaction system.The XRD results show that both the oxidized state W18O49 and the reduced state W18O49 have the same crystal structure as W18O49 prepared without any additives.The only difference is that they have different proportions of tungsten elements with different valence states.The reduced W18O49 contains more W5+,which leads to its stronger absorbance in the NIR region,and it also exhibits better photothermal conversion effect and photothermal stability.The photothermal conversion efficiency of reduced W18O49 at 808 nm is 59.6%.2.Sodium tungstate dihydrate solution was acidified with nitric acid at room temperature to give hydrated tungstic acid,which was treated as an intermediate and subjected to heat treatment under different atmospheres to obtain tungsten oxide with different crystal structures.In the air and nitrogen atmosphere,monoclinic W03 is obtained after heat treatment.In reducing atmosphere under the same heat treatment,the cubic phase WO3-x is obtained instead.Cubic crystal structure of tungsten oxide is not common to be observed because of thermodynamic stability.And we introduce oxygen vacancies into the lattice of tungsten oxides to stabilize the cubic crystal structure.The photothermal conversion performance of the cubic phase WO3-x was tested and the electrocatalytic hydrogen production was performed.It was found that its photothermal conversion efficiency is 48.7%at 808 nm and has a good photothermal stability.In 0.5 M H2SO4 solution(pH=0),the overpotential of hydrogen production is-294 mV,which has an obvious catalytic activity for hydrogen evolution reaction compared with that of monoclinic phase WO3.3.By using the reaction system of tungsten hexachloride and n-butanol,hexagonal(NH4)xWO3 was obtained by adding urea which is decomposed at 200 ? to introduce ammonium ions that stabilize the hexagonal crystal channel.We also studied the effect of reaction parameters(the amounts of urea addition and reaction time)on the product.The results show that the pH value of the reaction system increases with the increase of the urea amount,and other productions appear with the yield decreasing.With the extension of the reaction time,the crystallinity of the product is improved and the growth of the one-dimensional morphology is favorable.Finally,we investigated the photothermal conversion performance of(NH4)xWO3 which is obtained under the relative optimized conditions.It was found that the photothermal conversion efficiency of 2 mg/mL(NH4)xWO3 at 808 nm is 69.3%,after five laser irradiation cycles,(NH4)xWO3 still maintains a considerable temperature increment,indicating its good thermal stability.