Transparent (NH4)xWO3 colloidal dispersion and solar control foils: Low temperature synthesis, oxygen deficiency regulation and NIR shielding ability

• ~10×40 nm (NH4)xWO3 nanorods have been hydrothermally prepared at only 100 °C.
• 7×7 cm2 flexible (NH4)xWO3-PVP foils were casted onto PET substrates.
• These foils showed a 67.6% visible transmittance and a 25.5% infrared transmittances.
• Effects of zeta potentials on product morphologies were addressed.

Highly-stable aqueous dispersion of (NH4)xWO3 crystallites (ac. 6–12 nm in diameter and 20–100 nm in length) with decent infrared shielding ability was synthesized at only 100 °C. The effects of reducing agent (N2H4∙2HCl) dosage on the phase, morphology, zeta potential and composite of products have been systematically addressed. It was shown that the addition of N2H4∙2HCl in precursor solution introduced obvious oxygen deficiency into (NH4)xWO3 crystallites, resulting in a further improved NIR shielding ability. However, high dosages of N2H4∙2HCl were found to depress the zeta potentials and accelerate the growth of (NH4)xWO3 crystallites. A mechanism, which was reasonably supported by confirmatory experiments, was extracted to explain the result. To show potential application of our product, (NH4)xWO3-based flexible solar control foils were prepared by directly casting the dispersion onto polyethylene terephthalate (PET) substrates. The foils can selectively cut-off near infrared light (NIR) by 65.8% (transmittance depressed from 91.3% of a bare PET to 25.5% of the foils), while maintaining a high visible transmittance of 67.6%, showing promise as energy-efficient filter.

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