The Influence of W-Doping on The Crystallization of VO2(M) within Hydrothermal Synthesis

30%-40% of a buildings operational energy consumption is due to heat loss or unwanted solar heat gain through windows [1]. To reduce the energy demand for heating and cooling simultaneously, a window has to adapt to changing requirements, which can be achieved via so called “smart windows”. Vanadium dioxide can be used as active material in smart windows to realize adaptive solar heat regulating properties. In its monoclinic form, the crystal displays thermochromic properties, changing electrical and optical properties based on its structural phase transition from monoclinic VO2 (M) to rutile VO2 (R) at 68 °C. For application purposes, the switching temperature can be lowered by introducing defects in the crystal structure via a dopant, like tungsten [2]. VO2 (M) can be synthesized from V2O5 using a reducing agent under hydrothermal conditions at mild temperatures [3]. However, due to the many existing non-thermochromic crystal phases of VO2, as well as the potential to be reduced to V(III) or oxidized to V(V), it is not trivial to synthesize highly pure and crystalline VO2 (M). The difficulty to realize mixing in hydrothermal batch reactors, placed in an oven, often leads to inhomogeneous products. Here we present a method to synthesize VO2(M) from V2O5 and oxalic acid, as reducing agent, via hydrothermal reaction. We introduce a new method to enable mixing of the reaction solution within a hydrothermal batch reactor, increasing homogeneity of the resulting crystalline VO2 (M). Furthermore, we investigate the influence of the W dopant on the crystallization of VO2 (M). We show that under established conditions VO2 tends to crystallize in the (A) and (B) phases, whereas with introduction of W a preferential crystallization in the (M) phase can be achieved. By optimizing the reaction conditions, we obtain highly crystalline and pure VO2 (M) particles at a lowered switching temperature.