Directed synthesis of Mn$_{\mathrm{x}}$Ti$_{\mathrm{1-x}}$O$_{2}$ tunnel structured materials for energy applications

Mn oxides with tunnel structures are crucial in technological applications such as Li batteries, catalysis, fuel cells, electrochemical capacitors, sensors, and groundwater remediation. However, the complexity and poor quality of natural Mn oxide has hindered efforts to understand their fundamental structure-property relationships. To address this issue, we used PLD to make high-quality Mn$_{\mathrm{x}}$Ti$_{\mathrm{1-x}}$O$_{2-\delta}$ single-crystal films. Attempts to synthesize pure $\beta $-MnO$_{2}$ thin films on TiO$_{2}$ substrates resulted in Mn$_{2}$O$_{3}$ dominant films. Results of ab initio thermodynamics to explain film stability as a function of growth conditions and Mn/Ti composition suggest that ``protecting'' the Mn in a TiO$_{2}$ matrix by co-deposition would be beneficial. This approach has met with initial success even though the resultant films have oxygen vacancies. XPS indicates that (110) oriented films are Mn-rich near the surface while (001)-oriented films are Mn-rich near the interface. XRD shows that the films are coherently strained to the substrate which may influence the oxygen non-stoichiometry. Aberration corrected TEM results corroborate the XPS and XRD results and indicate a potential Mn-dependent defect. These films will be discussed along with multilayered (MnO$_{2})_{\mathrm{m}}$-(TiO$_{2})_{\mathrm{n}}$ films. Atomistic modeling shows that alternating cation rows that only contained Ti or Mn (m,n $=$ 1) greatly lower the activation energy for Li diffusion relative to films where Mn and Ti were homogeneously mixed.