Impact of valence-band-edge lifting on the doping-free bandgap tunability of 1D Magnéli-phase W18O49 nanorods

•1D W18O49 nanorods are synthesized on ITO/glass substrates at various temperatures.•1D W18O49 nanorods possess monoclinic crystals with mixtures of W5+ and W6+ cations.•The bandgap of 1D W18O49 nanorods is reduced with elevated synthesis temperature.•The reduction in the bandgap relates to the increase in the amount of W5+ cations.