New strategy for IR regulation could be effective as radiative heat barrier

The following article is published by Department of Science and Technology (DST), Govt. of India.

https://dst.gov.in/new-strategy-ir-regulation-could-be-effective-radiative-heat-barrier

This article was also highlighted by The Hindu on 20 December 2024.

https://www.thehindu.com/news/cities/bangalore/cens-scientists-develop-new-strategy-for-infrared-regulation/article69008984.ece

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A novel strategy to control and regulate infrared (IR) radiation using 2-dimensional nanosheets of a chemical called hexagonal boron nitride have potential for deployment as radiative heat barrier, thermal camouflage and in thermal management applications.

Infrared (IR) radiation with wavelength ranging from 780 nm to 1000 nm, present in solar radiation, sustains life on Earth. However, excess exposure to IR radiation, which constitutes about 50% of Sun’s radiation, can be undesirable from the viewpoints of both discomforting ambient temperature and possible health disorders. Effective control and management of IR radiation has attracted much attention for reducing the ever-increasing energy demands, such as, air-conditioning.

Scientists from the Centre for Nano and Soft Matter Sciences (CeNS), Bengaluru, an autonomous institute of Department of Science and Technology have come up with a novel strategy to control and regulate IR radiation. The method employs solution-processed 2-dimensional nanoflakes of hexagonal boron nitride (h-BN) developed by Dr. HSSR Matte and his student Priyabrata Sahoo, and preferentially assembling them in a polymer network liquid crystal.

Top panel: Schematic diagram showing (a) PNLC and (b) PNLC-BN architecture with polymer fibres (maroon) and the h-BN flakes (cyan) in the midst of liquid crystal molecules (yellow). Bottom panel: Thermal imaging and quantification of thermal regulation on exposure to hot surface with captured IR camera images. The image is much cooler for PNLC-BN device than PNLC device alone.

"The new concept could be a paradigm shift in this area as every single constituent of this multicomponent material is IR transparent, but their careful combination under specific conditions and preferential placement brings about a device that is extremely effective in IR regulation," explained Dr. Krishna Prasad and Dr. Shankar Rao who jointly supervised this research,.

This feature is demonstrated in several ways including scattering measurements, thermal imaging, passive radiative cooling, and reflectivity features, and is well supported by numerical simulations carried out by Dr. Sikdar at IIT Guwahati.

The system also has much higher mechanical strength as confirmed by nanoindentation studies. Gayathri Pishorady, a student and primary researcher in this work, foresees the strategy to offer a more generic way of developing IR regulators.

The work is published in a recent issue of Materials Horizons (Mater. Horiz., 11, 554 (2024); and a patent application has been filed.

Publication link : DOI: https://doi.org/10.1039/D3MH01467C

New technique for efficient fabrication of liquid crystal display devices with lowered cost

An essential requirement of liquid crystal devices (LCDs) is the unidirectional planar alignment of the constituent liquid crystals (LC) over large areas. Although the conventional polymer rubbing method yields quality LC alignment, it possesses unavoidable and undesirable drawbacks such as production of electrostatic charges and dust particles that interfere with display operation, and even cause damage to the electronic components of the display. While electrostatic charges increase the failure rate, dust creates defects which seriously compromises with the performance of the device. Other problems include multistep process for coating and the necessity for high-temperature curing. This has led to a surge in demand to replace this rubbing method with new non-contact techniques.

Figure: (a) Schematic showing the electrophoretic deposition (EPD) of h-BN nanoflakes over transparent conducting (TC) substrates and the FESEM image of the substrate, (b) The obtained birefringence when the polarizer is parallel and perpendicular/at 45o to the LC orientation, (c) Twisted Nematic (TN) configured LC device in the voltage ON and voltage OFF states showing high contrast ratio, and (d) Comparison of the alignment area obtained using CVD technique with the present research work.

The latest among these techniques is to employ 2D nanomaterials e.g. graphene, hexagonal boron nitride (h-BN), transition metal dichalcogenides, and so on as alignment layers. However, all these have a built-in lacuna owing to the deployment of the chemical vapour deposition (CVD) method as this technique necessitates high deposition temperature, necessity for precursors and by-products that are often hazardous or toxic. Besides, when the CVD method is used, unidirectional LC alignment is observed over only small regions.

A team of scientists from the Centre for Nano and Soft Matter Sciences (CeNS), Bengaluru, an autonomous institute of the Department of Science and Technology (DST) conceptualized and implemented a novel way of employing a 2D materials to overcome the drawbacks of current methods. Using h-BN nanoflakes as the specific material, the group comprising of Gayathri Pisharody, Priyabrata Sahoo, Dr. D.S. Shankar Rao, Dr. HSSR Matte and Dr. S. Krishna Prasad employed a procedure called solution-processed deposition technique and found it to be effective in getting the LC alignment over a much larger area. They also found the resultant crystals to be quite robust with no evidence of decay in LC orientation over several months.

The work published in Advanced Materials Interfaces formed the basis for an Indian patent application. Lead researcher Dr. Prasad added that the method demonstrating a non-conventional and contact-free route for getting unidirectional alignment of LC, is also much simpler, scalable, flexible for adaptation, and cost-effective.

Publications link: DOI: 10.1002/admi.202200486

Article published and highlighted by PIB India, EE Times India, The Graphene Council, and The Hindu Businessline.

Solution Processing of Topochemically Converted Layered WO3 for Multifunctional Applications

Transition metal oxides (TMOs) attracts huge interest because of their tuneable optoelectronic properties, high carrier mobility, stability against harsh environments, etc. Among these, tungsten oxide (WO3) is one of the most interesting metal oxide, which has been studied over the past few decades in several applications like sensors, electrochromic devices, supercapacitors, optoelectronic devices, water splitting, etc. WO3 has been utilized in several nanomaterials form like nanoparticles, nanorods, nanoflowers, nanoplates, nanosheets, etc. to enhance the performance in several applications. WO3 nanosheets which are having 2D morphology got special attention and are utilized in various applications.

Usually, nanosheets of any material are synthesized from their bulk counterpart using different types of exfoliation mechanisms, either through a top-down approach or a bottom-up approach. One of the common top-down mechanisms is liquid-phase exfoliation (LPE). However, the problem of WO3 lies in the non-layered structure. To have it layered, the WO3 structure must contain water molecules (WO3. nH2O). That is the reason there is no report in the literature on anhydrous layered WO3. Therefore no report on LPE of this kind of WO3 to obtain nanosheets.

However, recently we crossed this barrier by synthesizing layered WO3 using topochemical conversion route from commercially available tungsten disulfide (WS2), and importantly, converted WO3 is shown to have anhydrous behavior. Recently we have reported this discovery in Chemistry-A European Journal published by Wiley (https://doi.org/10.1002/chem.202100751).

Chemistry-A European Journal, 2021, Accepted manuscript

WS2 is heated at 500C for 3 h in ambient conditions in a furnace and as a result, WS2 gets converted to WO3 retaining its layered morphology. This process is commonly known as topochemical conversion. For the general reader, the definition of this process can be read as:

"Here, for topochemical synthesis, it can be broadly defined as adding, extracting or substituting elements to or from precursors in an environment of liquid or gas for synthesis of new materials which retain the structure or, at least, morphology of precursors." Xiao et al, Chem. Soc. Rev., 2018, 47, 8744--8765

Further, converted bulk layer WO3 is subjected to LPE to obtain WO3 nanosheets. Before that, the solvent selection is the most important step to achieve successful exfoliation. Because all solvents are not equally efficient to exfoliate and stabilize the material in the dispersion. The surface tension and Hansen solubility parameter (HSP) of solvent should be close to that of solute in order to have successful exfoliation. Therefore twenty-two solvents have been tried and among them, isopropyl alcohol-water (1:1) co-solvent system is found to be the best.

In this paper, detailed studies on LPE of converted WO3 have been carried out. Finally, obtained dispersions in co-solvent system are used to fabricate thin films which are utilized in supercapacitor and photoelectrochemical water oxidation. This processing technique shows its potential which can also be extended in other applications. The whole synthesis-LPE-thin film fabrication process can be summarized by the following schematic.

Schematic illustrations of the whole process starting from synthesis to fabrication of thin films.

The terms shown in the schematic are taken from the original paper. Interested readers can go through the original publication through this link. If you wish to have a copy of this paper, please contact me.

Have a good day!