Corrosion of sodium tungsten bronze nanoparticles in high-humidity environments

Abstract

Sodium tungsten bronzes (NaxWO3) are reduced metal oxides with applications in nanoparticle plasmonics. Despite decades of research, little is known about the conditions under which they can corrode. Here, NaxWO3 nanoparticles were exposed to high-humidity air for 71 days to induce corrosion. The samples were studied ex-situ by using X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The low-Na sample (x = 0.4) corroded slowly into a single, insulating, hydrated Na–W–O–N phase over 10 weeks. The high-Na sample (x = 0.7) fully corroded after only 4 weeks, decomposing into at least 3 corrosion products, including the phase observed in the low-Na sample. Despite the complex corrosion process, the corrosion rate of both samples was reasonably well-described by a first-order rate equation, allowing for straightforward estimation of the corrosion kinetics. Although the corrosion phases could not be unambiguously determined, the EDS and XRD data suggests that the phases are hydrated, sodiated tungstates which appear to contain N. These findings help clarify the conditions under which the tungsten bronzes can corrode and are important for designing applications which use tungsten bronze nanoparticles.