Determining the Location and Role of Al in Al-Modified TiO2 Nanoparticles Using Low-Temperature Heat Capacity, Electron Energy-Loss Spectroscopy, and X-ray Diffraction

Abstract

The location and function of dopants in metal oxide nanoparticles have been poorly characterized for many systems. We have performed heat capacity measurements, electron energy-loss spectroscopy (EELS), and X-ray diffraction (XRD) on 10 TiO2 nanoparticle samples that have different amounts of Al dopant to determine the location and function of the Al3+ cations. From the heat capacity data, lattice vacancies are observed to increase significantly with the addition of the Al dopant, suggesting Al3+ cations enter the TiO2 lattice and create vacancies due to the charge difference between Al3+ and Ti4+. The presence of gapped terms in fits of the low-temperature heat capacity data also suggests that small regions of short-range order are created within the TiO2 lattice. Entropies at T = 298.15 K were determined from the heat capacity data and show effects related to the entropy of mixing, suggesting that a solid solution of Al/TiO2 is formed. EELS data confirm that Al enters the TiO2 lattice but also indicates that the short-range structure around the Al atoms shifts from a TiO2-like environment toward an Al2O3-like environment as the dopant concentration increases. XRD data suggest that the long-range order of the particles decreases as the dopant concentration increases but retains a basic TiO2-like structure. This is the first investigation to use heat capacity data in this manner to determine the location of the dopant.