EXAFS Analysis of Mn-Alloyed TiO₂ Coatings Grown by Atomic Layer Deposition: Probing the Crystallization Behavior

<b>Introduction</b><br/>TiO₂ is a nontoxic, wide bandgap semiconductor with many excellent physical properties such as high chemical stability in acids and bases and good optical transparency to visible light, making it an excellent candidate for photoelectrocatalysis. Atomic Layer Deposition (ALD) enables synthesis of conformal coatings on various substrates by its layer-by-layer, surface-growth mechanism. However, controlling the crystal structure, which can modulate properties such as the band gap and band edge positions, remains a challenge as TiO₂ is often amorphous as deposited but can be annealed into multiple polymorphs such as anatase, rutile, and brookite. Despite progress in understanding the reaction mechanisms and intermediates associated with the deposition of binary materials via ALD, there are still many questions surrounding the structure-processing relationship of ternary materials. For example, the role that dopants or alloying agents play in determining key properties such as oxidation state and crystal structure remain open questions.<br/><b>Methods:</b><br/>The X-Ray Absorption Spectroscopy (XAS) and X-ray Absorption Near Edge Structure (XANES) measurements were conducted at the Inner Shell Spectroscopy beamline of the National Synchrotron Light Source II (NSLS-II) at Brookhaven National Lab. The data was collected at room temperature with the energy calibrated using a Ti foil. The Athena software package was used to calibrate the energy and normalize the data using the recommended parameters. XAS was collected in fluorescence mode with μ(E) = I_f/I₀ where I_f is the fluorescence intensity and I₀ the incident intensity. Each spectrum is flattened so that the overall spectrum has an asymptotic behavior approaching 1.<br/><b>Results:</b><br/>The primary pre-edge feature in the TiO₂ sample, centered at 4968 eV is due to the oxygen vacancies and resultant undercoordinated Ti. The absorption maximum of that pre-edge feature is unchanged after the incorporation of Mn. However, there is a narrowing of the peak in TiMnO_x samples relative to the TiO₂ and an enhancement of the shoulder peak that appears at 4972 eV. The separation between the apex of the first and second pre-edge feature corresponds to the crystal field splitting energy of the Ti 3d states. This indicates that the incorporation of Mn, while not having an effect in the oxidation state of the Ti, does perturb the local steric coordination environment. Quantitative analysis of this feature was not possible due to the background variation between the different samples and the low absolute intensity of the peak.<br/>Analysis of the Mn K-edge reveals that the oxidation state of Mn in the as-grown TiMnOx is independent of Mn concentration. One explanation for this fractional oxidation state could be the incorporation of Mn into multiple different sites. The pre-edge feature is comparable to experimental and computational Mn₃O₄ spectra. The position of the white line can be explained either by the electrostatic model or as continuum resonances. In the electrostatic model, as the oxidation state increases, the electrons are more tightly bound to the nucleus, requiring higher energy to be excited. The continuum model considers both the excited atom and the surrounding ones, and as the absorber-scatter distance gets shorter, the energy of the continuum state increases with 1/r². As higher oxidation states result in shorter bond lengths, the energy increases as oxidation state increases. In addition, the intensity of the white line intensity is proportional to the filling of 3d orbitals. As the 3d orbitals gets more occupied, the intensity of the white line goes down due to the reduction in possible population states. As the position of the white line remains constant between the as grown TiMnOx samples, the increase in sharpness of the K-edge in the 16:1 sample relative to the 2:1 sample could be explained by the combination of a slightly increased oxidation state and longer Mn-O bond.