Crystallization of VO₂ Via Hydrothermal Synthesis–A Study on Reaction Conditions and Crystal Formation

Vanadium dioxide can exist in a number of stable and metastable polymorphs with different unit cell parameters, including the VO₂ A, B, D, M1, M2, P and R phase. Each of these polymorphs has very specific properties and characteristics that can be used in various applications. The changing requirements make it important to be able to synthesize individual polymorphs at high purity and crystallinity to match their properties with targeted applications. An application area, where VO₂ has garnered increased interest from the scientific community as well as industry, is smart windows which autonomously adapt their solar heat gain to a buildings energy and comfort needs.¹ Here the structural phase transition (STP) between the M1 and R phase of VO₂ is used to trigger a change in solar infrared light transmission at a specific temperature. For this application it is important to selectively synthesize the highly pure and crystalline M1 phase to realize optimized optical properties in thermochromic glass coatings, combining high visible transparency with high solar modulation. Furthermore, precise and selective metal ion doping of the VO₂ crystals, replacing a small number of V ions with metal ions of a different ionic radius, is crucial to generate distortion and defects in the crystal lattice, leading to a reduction of the phase transition temperature to application oriented regions. Finally, the crystallite size is important for transparent applications. Here the size of individual VO₂ crystals within a coating matrix needs to be kept below a certain threshold size to prevent Mie scattering, which induces opacity. Additionally, it has been shown that plasmonic absorption can be triggered within metallic VO₂ (R) nanoparticles in the correct size range, further increasing the solar modulation capacity. All these parameters make it very important to have a prices control over the crystallization behavior of VO₂.<br/>Here we present an in depth study on the crystallization behavior of VO₂ within a hydrothermal reaction.² We investigate various reaction parameters, such as temperature, time, reactants and concentrations and their influence on the crystallization of VO₂ into various polymorphs. Furthermore, we add tungsten ions as dopant to change the phase transition temperature. We show that the addition of W plays a crucial role in the crystallization behavior of VO₂ and investigate the impact of different dopant concentrations on the formation of the VO2 A, B and M polymorphs. Our here developed process excludes the use of hazardous reagents, making it compatible with the concept of green chemistry and industrial production processes. Since the precursor complex also plays a crucial role in the crystallization behavior of VO₂, we present a comparative analysis of the crystallization of the here used vanadyl oxalate precursor with other precursors used in literature, that were obtained from reactions using hazardous reagents. Finally, we present an optimized reaction procedure within a Teflon lined hydrothermal reactor exposing an aqueous solution of vanadyl oxalate in the presence of (NH₄)₆H₂W₁₂O₄₀ for 96 hours to a temperature of 230°C. Using his process, we realized highly crystalline, phase pure W-doped VO₂ (M1) microparticles of uniform size and asterisk shape (ΔH = 28.30 J×g^–1, arm length = 6.7 ± 0.4 μm, arm width = 0.46 ± 0.06 μm). Furthermore, we give an outlook to reduce the particle size during crystal growth, which is essential to realize VO₂ nanoparticles applicable for smart window coatings.<br/>¹C.P.K. Yeung <i>et al. </i><i>Sol. Energy Mater. </i><i>Sol. Cells</i> <b>2021</b>, <i>230</i>, 111256; D. Mann <i>et al. </i><i>IOP Conf. Ser. Earth Environ. Sci.</i> <b>2022</b>, <i>1085</i>, 012060; L. Calvi <i>et al. Sol. Energy Mater. Sol. Cells</i> <b>2023</b>, <i>257</i>, 112350.<br/>²K. Timmers <i>et al. Inorg . </i><i>Chem .</i> <b>2024</b>, <i>63</i>, 5400.