Since 1D nanostructured materials exhibited singular physical and chemical characteristics that differ from those of bulk materials, applications of 1D nanostructured materials have attracted increasing attention. Tungsten oxides have been applied to photocatalysis, photochromic and electrochromic devices as well as gas sensors. Among different known tungsten bronzes, the sodium tungsten bronzes (NaxWO3) have been of great interest due to variation of their electronic and optical properties with composition. The nanostructured sodium-tungsten oxides may potentially be used for these technological applications.
In this work, a simple method for synthesis of NaxWO3 nanorods on tungsten film by using vapor-solid-liquid (VLS) method and sodium in soda lime substrate as the catalyst is presented for the first time. In this regard, the effect of annealing time on growth of the nanorods was studied.
Initially, W thin films with 1µm thickness were deposited on soda lime by DC magnetron sputtering in argon ambient. Then the reaction was carried out in a furnace with a horizontal quartz tube. The heating temperature was 650°C for different times such as 15, 80 and 180min in an N2 environment with a constant flow rate of 400sccm. The synthesized product was characterized and analyzed by SEM, XPS, XRD and UV–visible spectrophotometry. According to SEM observations, after the heating process for 15min, low density nanowires grew on the film with dimension lower than 100nm and 1µm in diameter and length, respectively. By increasing the heating time to 80min, the diameter of the nanowires became about twice but the length of them increased about ten orders of magnitude. Although, after the heat treatment for 180min, the dimensions of the nanowires did not change but they diffused into the substrate. XRD analysis determined that the samples mainly composed of cubic NaxWO3 with strong (001) diffraction peak and this peak increased by increasing the heating time. Based on XPS analysis, most of the surface tungsten on the samples was in W6+ chemical state. Optical transmission and reflection of the samples were also measured and it was indicated the transmission increased while the reflection decreased by increasing the heating time. |