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

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 <i>T</i> = 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

Impact of Pacific and Atlantic sea surface temperatures on interannual and decadal variations of GRACE land water storage in tropical South America

We analyze 10 years of Gravity Recovery and Climate Experiment (GRACE) terrestrial water storage anomalies (TWSAs) over tropical South America along with seven climate indices linked to equatorial Pacific and tropical Atlantic oceans sea surface temperatures (SSTs) using a multichannel singular spectrum analysis and lagged cross correlations. We focus on the interannual, nonlinear modes of covariability between TWSAs and SSTs. By comparing the relative distributions of the leading modes, we identify teleconnections between TWSAs, Pacific and Atlantic SSTs at different time periods. Thus, the northern and northeastern regions of tropical South America are mainly influenced by Pacific SSTs, while the central and western Amazon regions are more influenced by Atlantic SSTs. The former regions are more sensitive to central Pacific SSTs than to eastern Pacific SSTs. A quasi-biennial mode explains the largest part (27%) of the residual, interannual cross covariance and is found both in the El Niño–Southern Oscillation and in the Atlantic meridional mode. A trend-like mode explains the second largest part (24%) of the residual cross covariance and may be caused by the following: (1) the decadal variability in the North Pacific climate, as expressed by the negative trend in the Pacific decadal oscillation and by increased water storage in northern and northeastern South America, (2) the melting of Andean glaciers in Peru and Bolivia due to man-induced increase in land surface temperatures, and (3) the land use/cover changes after deforestation leading to increased runoff and groundwater recharge, expressed by increased water storage in southern Amazon regions