Gold Nanorods for Improving Near-Infrared Attenuation in SnO₂:F Thin Films

Glass industry looks for climate adaptative windows, the so-called “smart windows”, to promote energy saving buildings and houses. Tropical countries present high temperatures along all the year and blocking infrared windows are convenient to promote low air conditioning consuming and save energy. In this work we propose a coating for glass, which consists in gold nanorods deposited on SnO₂:F thin film in order to attenuate the near- and mid-infrared radiation coming from the sun and hot objects from outdoor. SnO₂:F is a well-known low emissivity material which is transparent in the visible and reflect mid-infrared radiation. On the other hand, metallic nanoparticles with suitable sizes and geometries can attenuate near infrared radiation. This is the case of gold nanorods.<br/>SnO₂:F thin films of around 536 nm thickness were deposited by spray pyrolysis technique at 475 °C from SnCl₂.2H₂O precursor and NH₄F with ratio of F/Sn=0.15 on microscope slices. Colloidal gold nanorods of spectral response from 700 nm to 800 nm were prepared by seedless approach using HAuCl₄, CTAB, NaBH₄, AgNO₃, C₆H₈O₆ and HCl at 37% to pH control. The colloidal gold nanorods were centrifuged and redispersed in PVP in order to deposit the nanoparticles on SnO₂:F thin film by spin-coating.<br/>The samples were optically characterized by transmittance and specular reflectance in a UV-1800 Shimadzu UV-Visible (190-1100 nm) and a Fourier Transform IRAffinity-1S Shimadzu (350-7800 cm^-1). Numerical simulations (Boundary Element Method) were performed in order to solve Maxwell equations and support the experimental results.<br/>SnO₂:F thin film thickness and dielectric function were calculated by fitting the UV-visible transmittance with Reffit software. In FTIR measurements, the SnO₂:F thin film present an increase in reflectance from wavelength of 2000 nm, reaching a reflectance of 80% at wavelengths larger than 5000 nm, which proves a high capacity to reflect mid-infrared radiation. On the other hand, in the near-infrared (700-1100 nm), the SnO₂:F thin film present low reflectante (below 12%) and a transmittance of 80%. By using the gold nanorods the near-infrared transmittance of the samples was reduced without compromising the visible spectral region. Figures of merit were used to quantify the improvement in blocking infrared radiation with respect to glass.