Synthesis Of Fast Response Tungsten Oxide-based Photochromic Hydrogel Composites

This paper reports the synthesis of tungsten oxide-molybdenum oxide composite using a simple,cost-effective and environmentally friendly method using deep eutectic solvent,DES(ethaline)at low temperatures.The XRD revealed that the composites were made up of agglomerated particles in the amorphous state at crystal sizes ranging from ?56.0 nm to ?68.0 nm,depending on the hydrothermal temperatures,while it was found that the crystal sizes reduced with increasing temperature.The SEM and TEM showed that the as-prepared composite consists of aggregated,cuboctahedra-like crystal particles with nanopores at 60 °C,which form the building block.Furthermore,the UV-Vis measurement indicated an intense response to UV radiation in the range of 200-460 nm,with a maximum absorption at ? 460 nm and a threshold value at ?540 nm and a wide band gap energy of 3.87-3.95 e V.In addition,the as-prepared composite exhibited color change from cream-yellow to blue within 5 seconds irradiation exposure and reversibility in90 minutes,with remarkable cyclability.In this study,we report the utilization of different water contents(0-40 vol.%)as cosolvent to ethaline,ChCl:EG(deep eutectic solvent)at the mole ratio of 1:2 to prepare binary oxide of tungsten oxide-molybdenum oxide nanocomposite in an ambient environment,followed by characterization using XRD,FTIR,SEM,TEM,TGA,XPS and UV-Vis spectroscopy.The asprepared composite composed of aggregated particles at low temperature of 60 °C,in the crystallite sizes ranging from ?48.40-71.97 nm,with water cosolvent acting as bad solvent.Further characterizations revealed that the atomic percentage ratios of tungsten and molybdenum components increase with decrease in carbon content as the water content increases,while the morphology changes from cubic to prismatic polyhedral-like structures at low temperature,in addition to retaining ?73% of its original weight above 700 °C.The UV-Vis analysis revealed broad absorption at 239-374 nm with a threshold value of 527 nm and can attain maximum photochromic response in less 60 s on exposure to UV-visible light irradiation.Tungsten oxide-based composites are important smart materials for wider usage in energy efficient windows rewritable inks and sensors.However,the problems of agglomeration have made it challenging to be employed as transparent,photo-reversible materials especially in functional hydrogels.We herein report the preparation of water-soluble tungsten oxidemolybdenum oxide nanocomposite in polymeric monomers to form high transparent hydrogel.The as-prepared hydrogel was characterized with XPS,FTIR,TGA,SEM,and UV-Vis-NIR,while the mechanical properties were also analyzed.The SEM images showed several microscopic particles with approximately same sizes,well-dispersed in the hydrogel and without the appearance of any cluster phenomenon.The as-prepared hydrogel revealed good mechanical properties as it could sustain up to a deformation ratio(?)of 86.3% at a stress of 60.3 MPa.Moreover,the hydrogel exhibited high transparency with percentage transmittance(T%)of ?83%,as well as highly remarkable photochromic response,with absorption at ?490 nm while additional absorption peaks at ?630 nm and ?730 nm appeared as the UV light exposure timing increases.Furthermore,coloration change was observed in 5 s,with the coloration exhibiting blue-black on180 s exposure to UV light,while reverse fading was achieved in 6 h in ambient air without external stimulus.