Growth of ultra-long sodium tungsten oxide and tungsten oxide nanowires: Effects of impurity and residue deposition

•Ultra-long nanowires of sodium tungsten oxide and tungsten oxide were synthesized with lengths up to several hundred microns.•The nanowires were found to be triclinic Na5W14O44 with monoclinic WO3.•Rectangular microplates grown with the nanowires were identified to be triclinic Na2W4O13.•The composition of the nanowires was controlled by the low concentration of sodium impurity in the tungsten source at ppm levels.•Enhanced growth of nanowires with the presence of residue deposition was explained by the enhancement of local vapor pressure.

Ultra-long nanowires of sodium tungsten oxide and tungsten oxide were synthesized by simply heating tungsten source under a low oxygen pressure environment. The nanowires have diameters of ~40 to 500 nm and lengths from tens to several hundred microns. The majority of the nanowires were found to be triclinic Na5W14O44 with small amount of monoclinic WO3. Triclinic Na2W4O13 microplates with a rectangular shape grown together with the nanowires were also identified. The formation of ultra-long nanowires is explained by the vapor–solid (VS) growth mechanism. Effects of impurity and residue deposition have been thoroughly investigated. With a low concentration even smaller than 10 ppm (parts per million), the sodium impurity in the tungsten source could result in the formation of sodium tungsten phases. The growth of nanowires could be enhanced with the presence of residue deposition and the enhancement was attributed to the production of local vapor pressure from the residue deposition.