Direct Observation of Rh and La Dopant Positions in SrTiO₃ Nanoparticles with Atomic-Scale Electron Microscopy

Doped SrTiO₃ nanoparticles constitute a leading materials platform for solar hydrogen production. One of the most ubiquitous and efficient SrTiO₃ materials utilizes Rh and La as dopants, designed to improve both visible-light absorption and charge separation ability. Typically, dopant positions in the SrTiO₃ lattice are assigned using ionic size and charge or ensemble-averaged techniques such as power X-ray diffraction or Raman spectroscopy. Direct observations of dopant locations in the SrTiO₃ lattice have not been previously reported. Here, we apply atomic-scale scanning transmission electron microscopy (STEM) coupled with image processing to directly elucidate dopant locations in 2% Rh-doped SrTiO₃ (Rh:SrTiO₃) and 2% La, 2% Rh codoped SrTiO₃ (La,Rh:SrTiO₃). Ensemble measurements were first performed to quantify aggregate properties across many particles of each type. Measurements of H₂ using in-line mass spectrometry suggest that the materials have different quantum yields for photocatalytic H₂ evolution, and Raman spectroscopy suggest that the concentration of dopants in the B-site (Ti site) differs. Simultaneous atomic-scale STEM, energy-dispersive X-ray spectroscopy (EDS), and electron energy loss spectroscopy (EELS) were performed on individual nanoparticles to directly determine dopant locations. Results indicate that in Rh:SrTiO₃, Rh occupies both the A-site and the B-site, which is unexpected because ionic radii argument suggests Rh will occupy only the B-site. In La,Rh:SrTiO₃, La primarily occupies the A-site and Rh primarily occupies the B-site. Precise knowledge of dopant positions is used to inform density functional theory (DFT) simulations for each doped lattice's electronic structure. These combined results suggest that La codoping can hinder photocatalytic H₂ evolution activity when Rh dopants exist in B-site recombination centers. The methods presented here demonstrate the effectiveness of correlating ensemble measurements, atomic-scale STEM imaging, and DFT simulations to establish structure-performance relationships for doped SrTiO₃ nanoparticles.