Single-crystalline sodium tungsten bronze (Na-WO₃) nanorods with typical diameters of 10−200 nm and lengths of several micrometers were prepared via hydrothermal synthesis. The as-prepared Na-WO₃ nanorods crystallized in a hexagonal structure (space group P6/mmm) with unit cell parameters a=7.3166(8) Å and c= 3.8990(8) Å and elongated along the ⟨001⟩direction.

Chemical analyses indicated a stoichiometry of Na_0.18WO_3.09·0.5H₂O, revealing the existence of tunnel Na⁺ ions and water molecules in the structure, as confirmed also by the vibrational spectroscopic study. The as-prepared Na-WO₃ nanorods exhibited a direct-allowed electronic transition with band-gap energy of about 2.5 eV, which allows a visible-light-driven photochromism related to photogenerated carriers and a proton−electron double injection process. The proposed photochromism was discussed in detail by means of Fourier transform infrared spectroscopy. The involved local structural evolutions such as water decomposition and ion intercalation during the photochromic process were identified.

One of the most effective actions for reduction of energy loss through the building envelope is to optimize the thermal performance, area and localization of the transparent components in the façade in order to obtain minimal heat losses and optimal solar gains.

When considering the thermal performance of these transparent components, one should consider, not only heat loss (or gains) caused by thermal transmission, but also the beneficial effects of incident solar radiation and hence reduced demand for heating and artificial lighting.

This study presents calculations for a range of windows as part of a building where the coupled effects of incident solar radiation and thermal transmission heat losses are accounted for in terms of a net energy balance for the various solutions. Effects of varying thermal transmittance values (U-values) are studied in connection with solar heat gain coefficients.

Three different rating methods have been proposed and applied to assess the energy performance of several window configurations. It has been found that various rating methods give different energy saving potentials in terms of absolute figures. Furthermore, it has been found that windows, even with existing technology, might outperform an opaque wall in terms of heating and cooling demands.