No abstract

An aqueous deposition process for V(6)O(13) films is developed whereby the vanadium oxidation state is continuously controlled throughout the entire process. In the precursor stage, a controlled wet chemical reduction of the vanadium(V) source with oxalic acid is achieved and monitored by (51)Vanadium Nuclear Magnetic Resonance ((51)V-NMR) and Ultraviolet-Visible (UV-Vis) spectroscopy. The resulting vanadium(IV) species in the aqueous solution are identified as mononuclear citrato-oxovanadate(IV) complexes by Electron Paramagnetic Resonance (EPR) and Fourier Transform Infra-Red (FTIR) spectroscopy. This precursor is successfully employed for the deposition of uniform, thin films. The optimal deposition and annealing conditions for the formation of crystalline V(6)O(13), including the control of the vanadium oxidation state, are determined through an elaborate study of processing temperature and O(2) partial pressure. To ensure a sub 100 nm adjustable film thickness, a non-oxidative intermediate thermal treatment is carried out at the end of each deposition cycle, allowing maximal precursor decomposition while still avoiding V(IV) oxidation. The resulting surface hydrophilicity, indispensable for the homogeneous deposition of the next layer, is explained by an increased surface roughness and the increased availability of surface vanadyl groups. Crystalline V(6)O(13) with a preferential (002) orientation is obtained after a post deposition annealing in a 0.1% O(2) ambient for thin films with a thickness of 20 nm.

No abstract

Deposition of functional materials on nonplanar surfaces remains a challenge for various applications, including three-dimensional (3D) all-solid-state Li-ion batteries. In this Letter we present a new process to deposit functional oxide materials on high aspect ratio microstructures without the use of vacuum-based deposition methods. Using ultrasonic spray deposition in combination with metal citrate chemistry, we were able to deposit high-quality coatings on Si microcylinders with an aspect ratio of 10. These results were achieved by controlling the precursor chemistry, wetting properties, gel mobility, and precursor decomposition. The versatility of the process was shown by depositing titanium oxide (TiO2), lithium lanthanum titanate (Li0.35La0.55TiO3), and tungsten oxide (WO3) on Si microcylinders of 50 μm length with an intercylinder distance of 5 μm. Finally, a proof of the 3D battery concept was achieved by coating of TiN/Si microcylinders with WO3 using a minimized thermal budget to preserve the (oxidative) TiN current collector. This led to an almost 3-fold electrode capacity enhancement per footprint area, due to the high surface-to-bulk ratio of the 3D coating. Therefore, these results represent a breakthrough in the field of solution-processing of nonplanar microstructures. In addition, the flexibility, low cost, and high scale-up potential of this approach are very promising for various applications requiring coated 3D microstructures.