Sn/Sb Segregation Effect at Polycrystalline Hematite Interfaces

The effect of atomic structure modification in hematite has become a key factor for the rational design of the highly efficient system. Herein, this study describes the segregation effects of Sn and Sb-dopants on both solid-solid (grain boundaries) and solid-liquid interfaces (surfaces) of polycrystalline hematite. From an experimental point of view, the proper sintering process allows freezing of a state of electronic defects, in which the electrical properties of hematite are controlled by the grain boundary and Sn or Sb segregation. Impedance spectroscopy and dc conductivity measurements show that current flows through preferential pathways associated with Sn or Sb segregation that occurs at the grain boundary, leading to a decrease in grain-boundary resistance. By employing the ab initio approach, the potential barrier reduction was observed on polycrystalline interfaces due to the dopant, which causes an increase in the inter-grain electron transport. Concomitantly, the dopants' segregation on hematite surfaces results in a decrease in the oxygen vacancy formation energy. Such vacancies lead to the experimentally observed rise of the flat band potential. The comprehension of the electronic effects of dopants on both types of interfaces has important implications to explain the experimental results, also enabling the control and design of interfaces for different higher-efficiency applications.<br/><br/>The authors acknowledge financial support from FAPESP (Project Nos. CEPID-13/07296-2 and 17/02317-2). F.L.S. acknowledges the support from the FAPESP (Grant No. 17/11986-5) and Shell and the strategic importance of the support given by ANP (Brazil's National Oil, Natural Gas, and Biofuels Agency) through the R&D levy regulation. The authors also acknowledge the Laboratório Nacional de Computação Científica (Project No. SCAFMat2) for computer time.