RESUMEN
Monolayer metal oxide coatings on metal oxide supports have the possibility of tuning the surface chemical properties of the coated systems. However, the (meta)stability of these structures makes experimental discovery challenging. A computational approach can help to determine properties that make a coating/substrate system stable and evaluate the stability of a variety of combinations. Herein, we use density functional theory (DFT) to study the stability of monolayer transitional metal oxides over different facets of anatase, brookite, and rutile phase of TiO2. We find that coatings that have a stable polymorph matching that of the support, as well as substrates with higher surface energies, are more likely to form monolayer-coated systems. DFT calculations recommend a number of coating/TiO2 surface facet combinations that may be stable. Despite these predictive observations, we did not find a significant correlation between monolayer stability and a single atomic, surface, or structural property of the coating/support metal/metal oxide and coating oxide monolayer stability. More complex predictive relationships need future study.
RESUMEN
Surface-confined mixed metal oxides can have different chemical properties compared to their host metal oxide support. For this reason, mixed transition metal oxides can offer tunable redox properties. Herein, we use density functional theory to predict the stability of the (0001) surface termination for mixed metal oxides consisting of Fe2O3, Cr2O3 and V2O3. We show that the pure oxide surface stability can predict the surface segregation preference of the surface-confined mixed metal oxides. We focus on substitution of Fe in the V2O3(0001) surface, for which we observe that Fe substitution increases the reducibility of the resulting mixed metal oxide surface. Our results suggest Fe is only stable on the surface under very high temperature and/or low-pressure conditions. Using thermodynamic relationships, we predict the transition points for these surface-confined mixed metal oxides at which exchange between surface/subsurface and subsurface/surface metal atoms occur due to changes in the oxygen chemical potential.
RESUMEN
We developed a ReaxFF reactive force field for NASICON-type Li1+xAlxTi2-x(PO4)3 (LATP) materials, which is a promising solid-electrolyte that may enable all-solid-state lithium-ion batteries. The force field parameters were optimized based on density functional theory (DFT) data, including equations of state and the heats of formation of ternary metal oxides and metal phosphate crystal phases (e.g., LixTiO2, Al2TiO5, LiAlO2, AlPO4, Li3PO4 and LiTi2(PO4)3 (LTP)), and the energy barriers for Li diffusion in TiO2 and LTP via vacancies and interstitial sites. Using ReaxFF, the structural and the energetic features of LATP were described properly across various compositions - Li occupies more preferentially the interstitial site next to Al than next to Ti. Also, as observed in experimental data, the lattice parameters decrease when Ti is partly substituted by Al because of the smaller size of the Al cation. Using this force field, the diffusion mechanism and the ionic conductivity of Li in LTP and LATP were investigated at T = 300-1100 K. Low ionic conductivity (5.9 × 10-5 S cm-1 at 300 K) was obtained in LTP as previously reported. In LATP at x = 0.2, the ionic conductivity was slightly improved (8.4 × 10-5 S cm-1), but it is still below the experimental value, which is on the order of 10-4 to 10-3 S cm-1 at x = 0.3-0.5. At higher x (higher Al composition), LATP has a configurational diversity due to the Al substitution and the concomitant insertion of Li. By performing a hybrid MC/MD simulation for LATP at x = 0.5, a thermodynamically stable LATP configuration was obtained. The ionic conductivity of this LATP configuration was calculated to be 7.4 × 10-4 S cm-1 at 300 K, which is one order of magnitude higher than the ionic conductivity for LTP and LATP at x = 0.2. This value is in good agreement with our experimental value (2.5 × 10-4 S cm-1 at 300 K) and the literature values. The composition-dependent ionic conductivity of LATP was successfully demonstrated using the ReaxFF reactive force field, verifying the applicability of the LATP force field for the understanding of Li diffusion and the design of highly Li ion conductive solid electrolytes. Furthermore, our results also demonstrate the feasibility of the MC/MD method in modeling LATP configuration, and provide compelling evidence for the solid solution sensitivity on ionic conductivity.
RESUMEN
We adapt a graphics processing unit (GPU) to dynamic quantitative second-harmonic generation imaging. We demonstrate the temporal advantage of the GPU-based approach by computing the number of frames analyzed per second from SHG image videos showing varying fiber orientations. In comparison to our previously reported CPU-based approach, our GPU-based image analysis results in â¼10× improvement in computational time. This work can be adapted to other quantitative, nonlinear imaging techniques and provides a significant step toward obtaining quantitative information from fast in vivo biological processes.