Metal oxides of group 4 (titania, zirconia, and hafnia) form an interesting class of nanomaterials. Titania nanocrystals have applications in medicine, photocatalysis, and photovoltaics. Zirconia nanocrystals are components for (in)-organic composites. Hafnia nanocrystals have found use as resistive switching elements, as scintillators, as computed tomography contrast agents, and as sensitizers in radiation therapy. All three oxides (but zirconia and hafnia especially) have high thermal and chemical stability making them attractive for a wide range of applications from nanoelectronics to medicine. Highly monodisperse and colloidally stable oxide nanocrystals can be obtained via nonaqueous surfactant-assisted syntheses. A particularly successful strategy is the reaction of metal alkoxide with metal halide in tri-n-octylphosphine oxide (TOPO). Anatase titania is thus produced at 300 • C, while tetragonal zirconia is formed at 340 • C, and monoclinic hafnia is formed at 340-360 • C. The surface of the obtained nanocrystals is covered by a mixture of protonated TOPO, dioctyl phosphinate, and dioctyl pyrophosphonate. Control over the final properties of the nanocrystals is highly sought after: shape, size, doping, and crystal structure. However, so far, achieving such synthetic control remains a challenge. For instance, the ZrO 2 nanocrystal size can currently only be tuned between 3 and 5.5 nm. Nanocrystal heterostructures, such as core/shells or alloys, have attracted increasing interest. For example, mixed phase TiO 2 /ZrO 2 nanoparticles were recently shown to display better electrocatalytic activity towards hydrogen evolution than the constituent metal oxides on their own, making them interesting materials for hydrogen fuel cells. Alloyed Hf x Zr 1-x O 2 nanocrystals have also been synthesized in TOPO by varying the amount of zir-conium and hafnium precursors, yet the composition of the nanocrystals was always more zirconium-rich than the composition of the precursor mixture. ZrTiO 4 and HfTiO 4 remain inaccessible. 17 From these reports on group 4 metal oxide alloys, it has been proposed that the reactivity of the metals can be ranked as Ti >> Zr > Hf, but direct evidence is still lacking. While core-shell structures have been extensively explored for other nanocrystals, the design of core/shell heterostructures of group 4 metal oxides has lagged behind. Recently, we reported epitaxial ZrO 2 /HfO 2 core/shell nanocrystals using nonaqueous surfactant-assisted synthesis. Shelling europium-doped zirconia nanocrystals with pure zirconia, significantly increased the photoluminescence lifetime of europium. While exciting, issues with independent nucleation of shelling material remain a problem as the factors affecting nucleation or shell growth are poorly understood. To rationally develop heterostructures by design, a better understanding of the reaction kinetics and the reaction mechanism is required. Indeed, the reaction rate has a direct impact on the type of heterostructures that can be produced. In 1996, Arnal and coworkers 32 studied the reaction between TiCl 4 and Ti(O i Pr) 4 at 100 • C which forms an amorphous gel (there was no surfactant added). Isopropyl chloride was found to be the only coproduct of the reaction (Equation ).