Cesium Tungsten Oxide itself is written in two ways one of which is Cesium Tungsten as well. It is a chemical compound found inorganically in nature to form any sort of dense solutions of liquid. 

The compound, Cesium Tungsten Oxide is quite popular among the industries of the entire world as they boost their production rates and demands. The following article comprises the production method, properties both physical and chemical, and applications of the Cesium Tungsten Oxide. The production method is a long and hard process and therefore, takes keen supervision along with the authenticity of the things involved. However, the properties and applications of Cesium Tungsten Oxide are vast and are the building blocks of this entire product.

Introduction

An inorganic chemical compound, cesium tungsten or cesium tungsten, produces an extremely dense liquid in the solution. In the processing of diamonds, the solution is utilized. Most of the rocks float in that solution but diamond sinks in it. Cesium tungsten oxide nanoparticles, which are categorized among the plasmonic nanomaterials, have attracted considerable attention as promising near-infrared (NIR) shielding materials due to their high transmittance of visible light and efficient NIR absorption capability. Cesium tungsten oxide nanoparticles are also advantageous as pigments with favorable heat resistance and a longer lifetime.

Characteristics of nanoscale materials

Nanoscale plasmonic materials exhibit characteristic absorption through surface plasmon excitation. In this case, the maximum absorption band wave is very sensitive to the particle size, local environment, morphology, and coupling of adjacent and close particles. Particularly, cesium-doped tungsten oxide nanoparticles possess considerable visible light transparency and have a broad range of absorption from 780 to 2600 nm with the best wavelength blocking between 800 and 1200 nm perfectly applicable to mini LED NIR shielding purposes. Cesium tungsten oxide nanoparticles are originally crystals with no color and are strongly hygroscopic and the phase transition in them from orthorhombic to the hexagonal system takes place at 536℃.

Production of Cesium Tungsten

When there are no extra stabilizers, the Cs0.33WO3 coarse powder’s bead milling is done in the aqueous solution. As a result, Cs0.33WO3 nanoparticles are produced which are then stabilized in an aqueous solution through the mechanism of electrostatic repulsion because of their electric double layer. The particles’ surface charge influences the electrostatic repulsion strongly. For determining the appropriate solution pH, there is an ongoing investigation on the effect of the pH on the Cs0.33WO3 nanoparticle’s zeta potential. According to the preliminary study, a pH of 1.8 is possessed by the isoelectric point of Cs0.33WO3 nanoparticles. The zeta potential decreases with an increase in the pH and when the zeta potential is up to -35 mV constant, the pH is more than 8. Therefore, at pH 8, the aqueous solution for the Cs0.33WO3 coarse powder’s bead milling was fixed by adding the potassium hydroxide to the deionized water.

Mean diameter of Cesium Tungsten Oxide

Cs0.33WO3 powder’s mean hydrodynamic diameter reduced rapidly in 3 hours from 1310 nm to 50 nm, indicating that the Cs0.33WO3 powder’s size can efficiently be lessened to nanoscale by the bead milling process. The powder was grinded for 1-3 hours. The increase in the time of grinding results in the lessening of hydrodynamic diameters, making the hydrodynamic diameter distribution narrower because the more the grinding time, the more will be the contact between the ground powder and the grinding beads. The photographs were taken of the Cs0.33WO3 powder’s aqueous dispersions before and after 3 hours of grinding for observation.

In the light of observations

According to the observations, before the aqueous dispersion of Cs0.33WO3 powder was starting to grind, it was pretty unstable. In only some minutes, they totally precipitated. Although, when it has been grinding for 3 hours, we could get a homogeneous and stable aqueous dispersion of Cs0.33WO3 nanoparticles with 50 nm as the mean hydrodynamic diameter.

The typical TEM images

There were typical and different TEM images of the Cs0.33WO3 powder before and after grinding. Before grinding, a large particle size was possessed by the Cs0.33WO3 powder. The particles that appear as a result of grinding possess an irregular shape as they were waste that came during the milling process from the collisions with grinding beads. The size of the particle becomes more and more small and uniform as the grinding time increases. The result guaranteed many things along with the fact that the uniform size of the Cs0.33WO3 nanoparticles could be attained by a stirred bead milling process.

The characteristic peaks

The observation was done on the Cs0.33WO3 powder’s characteristic peaks corresponding to the planes of a hexagonal structure before grinding. There is no observable major change in the XRD patterns after grinding, but the characteristic peaks turn broader. There was no change in the crystal structure of Cs0.33WO3 nanoparticles during the bead milling process. It got broader because of the decrease in the size of the particle. In the Cs0.33WO3 nanoparticles, ZrO2 generally acts as a contaminant due to the destruction of grinding beads during the stirred bead milling process. Although, there were no major characteristic peaks for cubic and monoclinic ZrO2 because of the Cs0.33WO3 powder’s lower hardness as compared to the yttrium-stabilized zirconia grinding beads; therefore, neglecting the contamination from the grinding beads.

Image after grinding

After 3 hours of grinding, the typical Cs0.33WO3 nanoparticle’s HRTEM image shows that the 0.375 nm main lattice spacing is related to the (002) planes of hexagonal structure. But, Cu and C elements from the copper grid were not observed whereas the O, W, and Cs elements were observed.

Absorption spectra before and after grinding

For avoiding the precipitation’s occurrence during measurement, 5 wt % of PEG 6000 was added to the samples before and after grinding for 1-2 hours. Before grinding, Cs0.33WO3 powder had no major absorption. According to the studies of Adachi and Takeda, the Cs0.33WO3 nanoparticles displayed a major absorption in the NIR region after grinding due to the polarons or free electrons. The NIR absorption turned more important with the increase in the grinding time and the decrease in the visible absorption. Cs0.33WO3 powder turns out to be efficient as a transparent NIR absorption material when its size is lessened to the nanoscale. The increase in the concentration of the particle can improve the NIR absorption.

Mean hydrodynamic diameters

Before and after grinding for 1-3 hours, the Cs0.33WO3 powder’s mean hydrodynamic diameters were 1,310, 250, 180, and 50 nm.

Variations in the temperature

The solution temperature’s variation with the NIR irradiation time for Cs0.33WO3 nanoparticles’ aqueous dispersions with different concentrations of the particle was obtained after 3 hours of grinding. Obviously, due to photothermal conversion, the increase in the concentration of particles can enhance the temperature increase. After 10 minutes, the temperature of the solution could increase to almost 55 C when the Cs0.33WO3 nanoparticles’ concentration was 0.08 wt %. The increase in temperature was above 30 degrees Celsius. Although, there can be no more enhancement in the temperature even if the Cs0.33WO3 nanoparticles’ concentration rises above 0.08 wt %.

Characteristics of photothermal conversion

Remarkable NIR absorption and characteristic of photothermal conversion were possessed by the Cs0.33WO3 nanoparticles. To kill the cancer cells, the increase in temperature to 55°C was more than enough. Therefore, there should be further investigations on other applications that are due to their remarkable NIR photothermal conversion characteristic (photothermal therapy) including the NIR shielding.

Production of Cesium Tungsten Oxide Particles

With a mean 50 nm hydrodynamic diameter, hexagonal Cs0.33WO3 nanoparticles were successfully made by bead milling in an aqueous solution whose pH was 8. Great stability and NIR photothermal conversion characteristic was possessed by them. There is an improvement in the increase of the NIR photothermal conversion-induced temperature as the concentration of particle increases or the size of the particle decreases. In biomedicine, such a kind of nanomaterial can be used in developing NIR-triggered photothermal conversion materials and can also be utilized in transparent solar heat-shielding filters.

Physical and Chemical Properties of Cesium Tungsten Oxide Nanoparticles

Cesium tungsten oxide nanoparticles (CTONPs) are capable of converting the absorbed photon energy into heat under optical illumination and can consecutively transmit the heat to the ambient media as fast as a few picoseconds. This property of CTONPs in the fast conversion of light to heat has made them promising nano heaters and potential candidates for relevant applications. These optical properties are induced by the electrons hybridization on W 5d and O 2p orbitals in the conduction band, which are basically derived from vacant orbitals and alkali dopants. Particularly as their electronic property, there exists a coordinated linear structural change in their lattice dimensions known to arise from a distortion called the pseudo-Jahn-Teller (PJT) effect caused by Cesium defection decline along with an increase in vacant orbitals.

Absorptive techniques

The strong absorption in the electromagnetic radiation’s NIR coming from the free electrons or pillars in cesium tungsten oxide nanoparticles could be an efficient property in NIR photothermal therapy. Nevertheless, it should be noted that there is a report on their application in photothermal therapy and heating the reaction media using NIR photothermal conversion.

Photothermal conversion

It is a process in which the electromagnetic radiation’s energy is absorbed in a particular wavelength and is transformed directly to heat. The generated heat can have applications in the field such as water evaporation, photothermal therapy, photocatalysis, and electrochemical devices. A therapeutic technique, known as photothermal therapy utilizes photosensitizers for generating heat by absorbing light and also kills the cancer cells. To do so, the NIR region wavelength is used to avoid healthy cells nonspecific heating and have a deeper penetration into tissues to reach unhealthy cells.

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Applications of cesium tungsten oxide

When it comes to manufacturing transparent insulation coating which has the best infrared transparent barrier property, cesium tungsten oxide nanopowder/nanoparticles are utilized. Cesium tungsten oxide nanopowder/nanoparticles are utilized in non-metal surface treatment too.

Heat Generation

In producing polyethylene terephthalate (PET) bottles, one of the emerging promising technology is heat generation. Heat generation has many applications, the production of PET bottles is a very significant application of it. An extruder PET preform is heated by IR irradiation to more than its glass transition (Tg) point in order to be blown into the desired shape. The productivity’s speed can be increased and the time of IR irradiation can be reduced when PET is incorporated with a little amount of WO3. Huge potential in heat generation is shown by WO2.7 due to its characteristics of photothermal conversion. The time of IR irradiation with WO2 can be reduced by an increase in temperature in a very short time, therefore enhancing the manufacturing of the polyethylene terephthalate (PET) bottles. Different applications like NIR shielding, water evaporation, pyro/thermoelectricity are discussed due to their potential in heat conversion and in harvesting solar energy.

Water Evaporation

In various practical applications like distillation, desalination, and freshwater production, there is a usage of a solar-driven water evaporation process as in this process, sunlight is used as a renewable energy resource. (Hua et al., 2017; Shang et al., 2017; Awad et al., 2018; Kim et al., 2018). A selective wide solar spectrum can be trapped by the solar heating which is designed as ‘air-water interface solar heating’ by strengthening the air-water interface (Wang et al., 2017d). Although, from interfacial to the underlying bulk water, there is a minimization in heat transfer. There is a photothermal layer that is used for inducing self-floating on the water surface's top but it is designed deliberately as a heat barrier. In solar thermal applications, the heat barrier introduces interfacial heating (Liu et al., 2015; Lou et al., 2016).

Thermo/Pyroelectricity

Thermoelectric technology has been utilized broadly for converting heat into electrical energy through the Seebeck effect. Certain condensed materials and solids possess a property known as pyroelectricity which is one of their least-known characteristics. Pyroelectricity is a property that is possessed by the materials of a particular class for generating an electric charge when it is consecutively cooled and heated.

Polarization

Polarization change is one of the side effects that is caused when the temperature variations slightly modify the position of atoms in the crystal structure. A voltage is created across the material because of the polarization change. Therefore, it can also be utilized as a thermal-electric converter but the difficult part is obtaining a high-temperature difference in a non-conducted way. Thus, it can't be utilized for converting thermal energy into electric energy. In 2017, according to Wu et al., the WO2.72/PVDF materials are capable of being utilized for NIR sensing and solar energy harvester applications. A huge amount of attention is gained by the hybrids of WO3 in pyro/thermoelectric studies. Although, MxWO3, WO2.72, and their hybrids will be explored in the recent future.

Photocatalysts

There has been a broad range of studies on solar energy's photochemical utilization (for instance, photocatalytic oxidation of alcohols, photocatalytic reduction of CO2, production of hydrogen, and photocatalytic degradation of organic pollutants). (Wang et al., 2017c). MxWO3, WO3−x, and other semiconductor materials have been utilized broadly in various photocatalysis fields, for instance, in remediation of crude oil spills, nitrogen oxide fixation, oxygen production, and photolysis of hydrogen, anti-virus sterilization, wastewater treatment, and air purification. Following are the 5 photochemical usages of solar energy for MxWO3 and WOx materials.

Water Oxidation

The abundant water of Earth provides electrons for the water oxidation reaction. Necessary electrons can be provided by an efficient water oxidation catalyst for proton reduction. Although to utilize the solar light effectively, light absorption characteristics were required by the catalyst design in the visible light range. At least one of the major thermodynamic criteria should be met by water oxidation: as compared to the H2O/O2’s standard redox potential (1.23 Ev), the semiconductor’s valence band (VB) level should be more positive. WO3−x’s VB potential is located at ca. 3.0 eV. According to Wang et al. and Yang et al., it is a capable material for water oxidation application.

Reduction of CO2

One of the catalysts is CO2's photocatalytic reduction, which generates holes and electrons when it is exciting. Those holes and electrons migrate to the catalyst's surface. A series of chemical reactions is triggered when the molecules are absorbed on the catalyst’s surface, therefore manufacturing different products like CH3OH, HCHO, HCOOH, and CH4. Although there is no complication in the photoreduction process, the formation of C-H bonds and the cleavage of C-O bonds are complex processes. According to Xi, CH4 is obtained when WO2.72 allows efficient reduction of CO2. (Xi et al., 2012).

Degradation of Organic Compounds

As compared to the H2/H2O reaction’s reduction potential, the WO3’s conduction band is a little higher along with its valence band which is way higher than the oxidation potential of an H2O/O2 reaction. All of this enables the WO3 photocatalytic oxidation degradation of many of the organic compounds like bacteria contaminants and textile dyes.

In order to treat organic acids-containing wastewater, WO3 is needed, as strong stability is provided by it in an acidic environment, making it suitable to treat such wastewater. Due to the transfer of an electron between mixed-valence states, the oxygen vacancies in WO3−x are useful for the reduction of O2. In addition, due to their strong mechanical characteristics, high melting temperature, high intrinsic density, low price, and multiple oxidation states, WO3−x and WO3 have more effective prospects for energy storage devices. They have been studied broadly in fuel cells, lithium-ion batteries (LIB), and supercapacitor electrodes. Following are the details of it.

Supercapacitors

The investigations on the supercapacitors show that due to the presence of the oxides, WO3 suffers from low electrical conductivity (Liu et al., 2018). Due to multiple oxidation states (W+6 and W+5), the electrical conductivity was improved by the partial reduction of WO3−x. As compared to other tungsten oxides, it has faster performance and high capacity. Mesoporous WO3−x (m-WO3−x) was made by Yoon et al. (2011b) and later it was observed that it has a remarkable capacitance of 639 Fcm−3 and 366 μFcm−2 and it also has a high rate capability, respectively.

Fuel Cells

There have been extensive investigations on the H2 fuel. This technology has a high desire for enhanced durability and catalytic activity. In this article, the focus is on the usage of WO3−x-based electrocatalysts in fuel cells. Pd/WO2.72 (Pd tetrahedron-tungsten oxide nanosheet hybrids) were made by Lu et al. in 2014 and they provided durability for the fuel cells and improved the electrocatalytic activity. Pd/WO2.72 exhibited high activity and superior stability in alkaline solutions for the oxygen reduction reaction as compared to the Pd nanocrystal. At 0.90 V, Pd/WO2.72 mass activity is 0.216 A mg −1, which is way more as compared to the commercial Pd/C, Pd/NPs, and Pt/C. In 2010, Kang et al. used mesostructured WO3−x and a hard template to produced ordered mesoporous WO3−x. Its high conductivity was because of the mesostructured WO3−x that was used in its production.

Gas Sensors

In the crystal lattice of WOx, there are some oxygen defects, which bend the band and allow conductivity. When oxygen is in contact with the material, negative ions are formed as the electrons from the semiconductor’s surface are absorbed by the oxygen. On contact, the surface energy band also faces an upward bent, leading to an increase in the sensor's resistance, a decrease in the electrical conductivity, and a decrease in the gas sensing material's surface electron concentration. Although if the reducing gas makes contact with the gas-sensitive material, there will be an increase in the electrical conductivity and electron concentration, a decrease in the sensor’s resistance value, a lowering of the surface energy band, and desorption will take place.

Conclusion

Cesium tungsten oxide is rich in its applications and due to the vastness of applications, they have been divided into various categories to align them more formally. All of these applications have been proved excellent in their own respective manners and it is due to them that the demand for more production of cesium tungsten oxide has continuously increased as it benefits the industries to a greater extent. 

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References

https://www.frontiersin.org/articles/10.3389/fmats.2019.00049/full

https://link.springer.com/article/10.1186/1556-276X-8-57

https://www.nature.com/articles/srep40928

https://aip.scitation.org/doi/10.1063/1.4831950

https://link.springer.com/article/10.1186/1556-276X-9-294