Tungsten oxide nanoparticles are oxide particles having high surface area and are capable of showcasing magnetic properties. This happens when tungsten oxide is presented in the form of nanoparticles that are extremely tiny that they are observed with a microscope.
The Nanoparticles field is progressing rapidly day by day and bringing various advancements in each field that it is associated with. A lot of common applications of tungsten oxide nanoparticles are being incorporated in daily human lives and activities which are proving to be highly effective and beneficial for both, industries and mankind.
Recently, there has been a rapid development in nanoparticle research mainly because of the remarkable characteristics of the basic elements which are attained by altering their molecular and atomic characteristics. In many fields, for instance, plastics, coatings, electronics, cosmetics, and biomedicine, etc. nanoparticles have many usages because of these characteristics. In this study, the focus is on tungsten oxide nanoparticles, their applications, and their characteristics.
Tungsten oxide nanoparticles
In nanofluid’s form or in the form of faceted high surface area oxide particles displaying magnetism, tungsten oxide (WO3) nanoparticles or nanopowders are available. Other available forms of these particles are coated, high purity, transparent, and dispersed forms. Tungsten oxide particles of a diameter less than 100 nm make up a fine light yellow powder. Its magnetic characteristics and high surface area is the reason for this material being well-known and famous. This material is usually held as a nanofluid. It provides advantages in a broad range of fields and it has been gaining a lot of attention from different researchers and engineers from different fields.
The yellow powder is the form in which tungsten oxide nanoparticles occur and they have a similar spherical morphology. The molar mass of the tungsten oxide nanoparticles is 231.84 g/mol, and its density is 7.16 g/cm3
Tungsten oxide nanoparticles have the following thermal characteristics. Its boiling point is 1700 C, and its melting point is 1473 C.
Tungsten oxide nanoparticles have many applications. Its main applications are listed below.
- It is used in synthesizing semiconducting and conducting materials
- It is used in mechanochemical applications and optics.
- In different compound materials and ceramics, it is utilized as a pigment and colorant.
- Data storage. It is used for making memory storage devices of high density.
- Semi-conductors: One can use tungsten oxide powders for forming particular composite semiconducting materials.
- Conductors: Due to its electromagnetic characteristics, this material is utilized usually for making conducting nanofluids and conductors.
- Alloys: It can be utilized to make particular forms of tungsten alloys and metal tungsten.
- Optics: As smaller scales, tungsten oxide provides remarkable optical characteristics like other various nanoparticles, making tungsten oxide useful and beneficial in particular displays and imaging applications.
- Mechanochemical: Numerous mechanochemical applications use tungsten oxide’s remarkable characteristics, especially for them to be used in smart windows, solar energy conversion, and associated technologies.
Recent Advances in Tungsten-Oxide-Based Materials and Their Applications
Formation of Tungsten-oxide-based Materials
Here, numerous methods of preparation of tungsten oxide-based materials are discussed along with the advantages and disadvantages of forming nanomaterials for instance MxWO3 and WO3−x. Following are the included methods; solvothermal, hydrothermal, inductively coupled thermal plasma, solid-phase reaction, vapor-phase synthesis, and mechanochemical methods. Due to their easy scalability, solvothermal and hydrothermal methods have been broadly adopted.
Grinding or ball-milling is a mechanochemical technique that is a potential candidate for solvent-free synthesis. Mechanical energies like friction, shearing, or compression, are the mechanical energies that induce chemical transformation that is involved in this method. Na0.88WO3 nanocrystals were made in 2003 by Wang et al. through the grinding of the precursor Na pieces and WO3 powders whose grain has an average 17 nm size.
According to the test of electrical property, Na0.88WO3 displayed properties of a semiconductor, causing material’s lattice distortion due to its electrical conductivity being strongly affected by the high-energy ball-milling. Many benefits are contained by this process, some of which are the process being simple, using cheap raw materials, and the capability of attaining fine particles. Although, the main limitation for the chemical reaction is that it should be monitored for moisture- and air-sensitive substances.
Chemical Vapor Transport
According to chemical vapor transport (CVT) and its principles, a solid’s volatilization in gaseous reactant’s presence (transport agent) deposits the solid of crystalline appearance, anywhere. This method was used in 1997 by Hussain et al. He also let the growth of the crystal of alkali metal tungsten bronzes MxWO3−x, where M can be Cs, Rb, or K).
In order to grow tungsten bronzes’ large crystals, various transport agents like PtCl2, Cl2, Hgl2, HgBr2, and HgCl2 were utilized, however, HgBr2 and HgCl2 turned out to be as efficient and effective as the transport agents themselves. For hexagonal tungsten bronzes, the length of the crystals reached 6 mm. Although, HgBr2 and HgCl2 were used to prepare the tetragonal tungsten bronzes (MxWO3) at 0.1 mm size, where x= 0.25 and there was no or very little transport when x was equal to or greater than 0.35.
With an increase in the concentration of alkali metal, the size of the crystal and transport rate reduced. These were made with and without the addition of transport agents under isothermal conditions, showing almost similar results. Although when utilized as transport agents, an appreciable transport effect is shown in the results. Environmental damage can be caused by the usage of transport agents with high-energy consumption.
A process in which there is a solid reaction taking place between the 2 solids, forming a solid product with no kind of chemical equilibrium. Despite being simple, this technique demands cheap equipment. Although, at high temperatures, its reaction rate is slow. When the solid-phase reaction method was being used, it prepared tungsten bronze K, Na, Rb, and Cs. As a precursor, when metal salts were mixed with WO3NH3 as a precursor, they were used to make cubic tungsten bronze (CTB) Na0.75 WO3 powder and hexagonal tungsten bronze (HTB) Cs0.33WO3, in 2007 in Rb0.33WO3 powder by Takeda and Adachi.
The precursor went for 1 hour at 550°C with H2/Ar or H2/N2. For 1 hour, it was annealed in an N2 atmosphere at 800 °C. At almost 1500 nm, Rb0.33WO3 and Cs0.33WO3 led to broad and strong NIR absorption peaking. Due to its best absorption in the NIR range, HTB phase M0.33WO3 is a little bit more appropriate for solar filter applications. NIR absorption characteristics were demonstrated by Moon et al. in 2013. According to him, due to the modulate optical response by sodium’s quaternary element, better NIR absorption characteristics are possessed by quaternary tungsten bronze as compared to tungsten bronze.
Inductively Coupled Thermal Plasma Method
To decomposite gaseous precursors and evaporate solid precursors, the heat source is mainly thermal plasma. Plasma flame is formed by using reactive gases as significant constituents for synthesizing nano-sized materials. In 2010, Mamak et al. reported thermal plasma synthesis of MxWO3 in which there is a powder mixture that contains (NH4)10(H2W12O42) precursor. In M and W varied ratio, 4H2O and salt of K3C6H5O7, Na2CO3, and HCOOCs, were utilized. Although, Ar was utilized as central gas and H2 of a small amount for providing a lessening environment that is needed for the preparation of MxWO3. The precursor that’s used was a mixture of alkali salts and tungsten with low decomposition temperature.
Processing throughput, raw material cost, and material handling are the benefits of thermal plasma synthesis. One of the remarkable approach for MxWO3’s high throughput production is inductively coupled thermal plasma (ICTP) synthesis, where M is Cs, K, and Na. Low precursor materials were used to make tungsten bronze nanopowders at high purity.
Generally, thermal plasma makes materials, and those materials have a satisfactory tunable composition, high purity, and optical absorption when precursor materials of low cost are being used. Heat shielding filters and coatings are the main applications as a high extinction coefficient is displayed by them in the NIR region with a slight influence on visible color or transparency. For high production at low temperatures, ICTP is a fast reaction method. Its potential is very high and it can be broadly used for research in the near future.
It is a versatile and simple method that is utilized under conditions of high pressure and high temperature for manufacturing inorganic nanomaterials from aqueous solutions. One should adjust precursor concentration, pressure, and temperature, as such parameters should be adjusted according to the nanomaterial’s characteristics. In the hydrothermal process, the most frequently utilized solvent is water. Pressure and temperature highly determine the dielectric constant and water density. A decrease in pressure and increase in temperature makes up for a drop in the dielectric water constant. The rate of reaction faces a significant increase as water’s dielectric constant lessens, therefore facilitating the nucleation growth of the crystals.
Inexpensive instrumentation, good dispersion in solutions, production feasibility, environmental friendliness, and one-step synthetic procedure are some of the many advantages that it provides us. In addition, H2 usage is avoided in this method but it does enhance safety. Although this method encourages high-temperature energy consumption. Hydrothermal methods have reported tungsten bronze-like (NH4)0.33WO3 nanorods, K0.26WO3 nanorods, and Cs0.33WO3.
The important note is that different processing conditions and different reaction schemes are possessed by different nanoparticles according to these preparations. Thus, it is desirable to control the appropriate chemical reactions with suitable circumstances for numerous nanoparticles.
Applications of Tungsten-oxide-based Materials
Tungsten-oxide-based materials MxWO3, WO2.72, and their hybrids have gained a huge amount of attention in numerous fields like gas-sensor, energy-related, photocatalysis, and heat generation applications. All of these applications are interesting and significant too. Following is its discussion in detail.
One of the promising and emerging technology right now is heat generation and it has a significant practical application in the production of PET (polyethylene terephthalate) bottles. IR irradiation heats an extruder PET preform above its Tg (glass transition) point so the extruder PET preform can be blown into the needed shape.
WO3’s small amount is added into the PET for reducing the time of IR irradiation, and therefore it speeds up the productivity. WO2.7’s photothermal conversion characteristics display a huge potential in heat generation. IR irradiation time can be further reduced by such an increase in the temperature in a short span of time with WO2.72, significantly enhancing the manufacturing of the PET bottles. Due to their potential in heat conversion and harvesting solar energy, numerous applications like NIR shielding, water evaporation, and pyro/thermoelectricity are discussed.
Heat can be converted into electrical energy by the means of thermoelectric technology through the Seebeck effect. Pyroelectricity is the least-known characteristics of certain condensed materials and certain solids and it relates to temperature-dependent spontaneous polarization in particular anisotropic solids and indicates some materials’ capability of generating an electric charge when they are being consecutively cooled and heated.
Due to the variations in the temperature, the position of atoms in the structure of the crystal gets a little modified, resulting in effects like polarization change. A voltage is created by the polarization’s change across the material. Therefore, it is capable of being utilized as a thermal-electric convertor. According to Wu et al., one can use PVDF/ WO2.72 for NIR sensing and solar energy harvester applications. There has been a recent increase in the attention towards the hybrids of WO3 in pyro/thermoelectric studies. Although, exploration still needs to be done on MxWO3, WO2.72, and their hybrids.
In various practical applications, there is the demand for a water evaporation process that is driven by solar and uses sunlight as a renewable energy resource. Distillation, desalination, and freshwater production are some of such applications. According to Wang et al., solar heating can trap a selective broad solar spectrum through the strengthening of the air-water interface. Although the transfer of heat lessens from the interfacial to underlying bulk water.
Self-floating is induced by the photothermal layer on the water surface’s top. That photothermal layer is made as a heat barrier, introducing interfacial heating in solar thermal applications. An opportunity is given to the researchers by the multilayered materials for including multifunctionality in a single-component material. WO2.72 photothermal materials have been utilized effectively with polylactic acid (PLA). According to Chala et al., light energy can be converted to thermal energy because of the photoabsorption characteristics of these photothermal materials.
They were made as fiber membranes that have the capability of self-floating and they function at air-water interfaces as heat barriers for light-driven water evaporation. For WO2.72/PLA, a fast increase can be seen in temperature from 19.4-44.7°C and then to 75.3°C for irradiation of over 5 min. Such distinct characteristics of WO2.72 make WO2.72 feasible for commercial applications like sterilization, desalination, and steam generation.
For the application of water oxidation, this material is promising and has the potential. Electrons are required and acquired from the abundant water of the Earth for a water oxidation reaction. The required electrons can be provided by an effective water oxidation catalyst for proton reduction. Although, when it comes to utilizing solar light effectively, the concept of the catalyst design needs light absorption properties in the visible light range. One main thermodynamic criteria should be fulfilled by water oxidation: Semiconductor’s valence band (VB) level should be more positive as compared to the standard redox potential of H2O/O2.
Reduction of CO2
For the generation of holes and electrons, the catalyst (CO2’s photocatalytic reduction) should be excited. Those holes and electrons eventually migrate to the catalyst’s surface. A series of chemical reactions are triggered by the molecules absorbed on the catalyst’s surface which later on synthesizes numerous products like CH3OH, HCHO, HCOOH, and CH4. The process of CO2’s photoreduction is tough. The most complex processes are the production of C-H bonds and the cleavage of C-O bonds. According to Xi, CH4 can be obtained by efficient reduction of carbon dioxide which is enabled by WO2.72.
Lithium-Ion Batteries (LIB)
WO3 is environmentally friendly, less expensive, and has a high theoretical capacity, making it good enough to be utilized as an anode material for LIB. There is one drawback of WO3 though and that is its low electrical conductivity but that has also been enhanced and solved with the WO3−x materials. Development of high-performance anode mesoporous WO3−x was the main focus of Yoon et al. in 2011 through the usage of a hard template that has high electrical conductivity. In comparison with the bulk WO3−x, a high volumetric capacity of 1,500 mAh cm−3 and a high reversible capacity of 748 mAh g−1 were displayed by the developed material.
According to Lee et al. in 2014, enhanced rate performance, stable cycle, and a high reversible capacity of 481 mAh g−1 were possessed by the flexible reduced tungsten oxide–carbon composite nanofiber (WOx-C-NF) films that are utilized in lithium-ion batteries as anode materials than WOx-C-nano and WOx-nanoelectrodes. According to the results of many such studies, WO3−x comes in the list of very promising anode materials for the lithium-ion battery.
There have been recent reports by Li et al., Yue et al., and Zhang et al. about the usage of WO3−x composites for high-performance Li-ion batteries as anode materials. Discharge capacities of 1074 and 2435 mAh g−1 can be delivered by the flower-like WO3/CoWO4/Co nanostructure electrodes according to the reports in 2017 by Liu et al. during the 1st cycle at 200 and 100 mAg-1 current densities, respectively. At 200 mAg-1, a capacity of 331 mAh g−1 whereas at 100 mAg-1, a capacity of 1151 mAh g−1 was possessed by the pristine WO3 electrode with no doping. In comparison with the hybrids of WO3 particles, a remarkable cycling performance is possessed by the hybrids of WO3 (Co composites/ CoWO4WO3 utilized as anode materials).
There have been extensive investigations on H2 fuel. Enhanced durability and catalytic activity are very much in demand for this technology. In this article, there is a partial focus on WO3−x-based electrocatalysts and their usages in the fuel cells. Pd tetrahedron–tungsten oxide nanosheet hybrids (Pd/WO2.72) are synthesized in 2014 by Lu et al. and they offered durability to the fuel cells while enhancing their electrocatalytic activity. In alkaline solutions, high activity along with superior stability is displayed by Pd/WO2.72 hybrids than Pd nanocrystal, for oxygen reduction reaction (ORR).
At 0.90 V, Pd/WO2.72 mass activity is 0.216 A mg −1, way higher as compared to commercial Pd/C, Pd NPs, and Pt/C. A hard template was used in 2010 by Kang et al. to form ordered mesoporous WO3−x. Along with a hard template, mesostructured WO3−x was also used and it was the actual reason for its high conductivity.
A significant tolerance between 0.6-1.3 VNHE is displayed by Pt/mesoporous WO3−x to cycling. It can be utilized as an ORR catalyst support, thereby providing us with long-term stability. For the past few decades, WO3 hybrids have generally been researched broadly for energy-related devices. Recently, a huge amount of attention has been gained by WO2.72 hybrids in energy applications, however, MxWO3 is still not noticed.
In the crystal lattice of WOx, there are oxygen defects, causing the bending of the band and enabling conductivity. Electrons from semiconductor’s surface are absorbed by oxygen when oxygen is in contact with the material for producing negative ions, bending the surface energy band upward, and thus leading to a lessening in the gas sensing material’s surface electron concentration, sensor’s increased resistance, and a reduction in the electrical conductivity. Although, if reducing gas comes in contact with the gas-sensitive material, desorption takes place, which decreases the sensor’s resistance value, lowers the surface energy band, and increases the electrical conductivity and electron concentration.
Tungsten oxide nanoparticles are one of the most applicable materials being used for the welfare of human activities and the modification of industries. The most common uses of these can be found in the area of batteries and electrical productivities. Thorough researches have been carried out in this regard and evidences have been collected about the efficacy that tungsten oxide nanoparticles bring to the industrial upfront.