Highly-stable aqueous dispersion of (NH4)(x)WO3 crystallites (ac. 6-12 nm in diameter and 20-100 nm in length) with decent infrared shielding ability was synthesized at only 100 degrees C. The effects of reducing agent (N2H4 center dot 2HCl) dosage on the phase, morphology, zeta potential and composite of products have been systematically addressed. It was shown that the addition of N2H4 center dot 2HCl in precursor solution introduced obvious oxygen deficiency into (NH4)(x)WO3 crystallites, resulting in a further improved NIR shielding ability. However, high dosages of N2H4 center dot 2HCl were found to depress the zeta potentials and accelerate the growth of (NH4)(x)WO3 crystallites. A mechanism, which was reasonably supported by confirmatory experiments, was extracted to explain the result. To show potential application of our product, (NH4)(x)WO3-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. (C) 2014 Elsevier B.V. All rights reserved.