Synthesis of tellurium dioxide nanoparticles by spray pyrolysis

Tellurite glasses (amorphous TeO₂ based materials) have two useful optical properties, a high retractive index and high optical nonlinearity, that make them attractive for a range of applications. Unlike other metal oxide nanoparticles that are prepared in large quantities by vapor-phase processes, TeO₂ nanoparticles have not been prepared by such methods. Here, we describe the vapor-phase synthesis of TeO₂ nanoparticles from an aqueous solution of telluric acid, Te(OH)₆, that is dispersed into fine droplets by an atomizer. These droplets are carried by nitrogen through a furnace, where they evaporate and decompose, resulting in nucleation of TeO₂ nanoparticles. The resulting nanoparticles were characterized by TEM, XPS, FTIR, and XRD. Particle size distributions measured online using a scanning mobility particle spectrometer demonstrated that a furnace temperature above 700°C was required for full evaporation of the precursor droplets. Thermogravimetric analysis shows that the Te(OH)₆ converts to TeO₂ without evaporating when heated at 5-100°C/min. This contrasts sharply with the complete evaporation observed in the spray pyrolysis reactor, where heating rates exceed 50 000°C/min. The effects of operating parameters on nanoparticle composition, production rate, and size were also studied. This laboratory scale process is capable of producing up to 80 mg/h amorphous TeO₂ nanoparticles with primary particle diameters from 10 to 40 nm. Particles this small exhibit negligible scattering of visible light and have potential applications in the linear and nonlinear refractive index engineering of polymer nanocomposites and as host matrices for rare-earth ions that can act as IR to visible upconverters. The method presented here allows their synthesis in macroscopic quantities from an inexpensive and environmentally friendly precursor.