RESUMO
We study the electrocatalytic oxygen evolution reaction using in situ X-ray absorption spectroscopy (XAS) to track the dynamics of the valence state and the covalence of the metal ions of LaFeO3 and LaFeO3/LaNiO3 thin films. The active materials are 8 unit cells grown epitaxially on 100 nm conductive La0.67Sr0.33MnO3 layers using pulsed laser deposition (PLD). The perovskite layers are supported on monolayer Ca2Nb3O10 nanosheet-buffered 100 nm SiNx membranes. The in situ Fe and Ni K-edges XAS spectra were measured from the backside of the SiNx membrane using fluorescence yield detection under electrocatalytic reaction conditions. The XAS spectra show significant spectral changes, which indicate that (1) the metal (co)valencies increase, and (2) the number of 3d electrons remains constant with applied potential. We find that the whole 8 unit cells react to the potential changes, including the buried LaNiO3 film.
RESUMO
Carrier dynamics affects photocatalytic systems, but direct and real-time observations in an element-specific and energy-level-specific manner are challenging. In this study, we demonstrate that the dynamics of photo-generated holes in metal oxides can be directly probed by using femtosecond X-ray absorption spectroscopy at an X-ray free-electron laser. We identify the energy level and life time of holes with a long life time (230 pico-seconds) in nano-crystal materials. We also observe that trapped holes show an energy distribution in the bandgap region with a formation time of 0.3 pico-seconds and a decay time of 8.0 pico-seconds at room temperature. We corroborate the dynamics of the electrons by using X-ray absorption spectroscopy at the metal L-edges in a consistent explanation with that of the holes.
RESUMO
We study the particle size distribution and phase changes of the anatase TiO2 nanopowder samples when they are subject to the plasma treatments of three different kinds of gases as nitrogen (N2), oxygen (O2), and argon (Ar). The plasma gas pressures vary as 0.1, 0.3, and 0.6 Torr. We demonstrate that the plasma treatments have an effect neither on the phase structure nor on the mean nanocrystalline size. The phase and size invariances of the samples are attributed to their nanoscale thermodynamic aspects. We find out that elevating the gas pressure in some cases creates fine-size amorphous nanoparticles with a narrow distribution. Our findings authenticate that plasma treatment affects the amorphous phase with etching particles down to a mean value of â¼3 nm. The small-angle X-ray scattering (SAXS) technique was utilized to obtain the size distribution of the nanoparticles, and the wide-angle X-ray scattering (WAXS) technique was used to probe the phase and size changes of the crystalline structure.
RESUMO
Perovskite solar cells (PSCs) based on organic-inorganic metal halide perovskites are a recent ground-breaking advancement in attaining power conversion efficiencies exceeding 21%. However, the toxicity of lead in these PSCs could be a deterrent for large-scale development due to the environmental concerns. The methylammonium tin triiodide (CH3NH3SnI3) perovskite has been successfully employed in lead-free PSCs as an alternative to CH3NH3PbI3 perovskite. The PSCs have mostly been realized with a highly expensive spiro-OMeTAD hole-transporting material (HTM). Herein, copper thiocyanate (CuSCN) was implemented as a HTM instead of the highly expensive spiro-OMeTAD counterpart. The results show that CuSCN is a promising HTM for the lead-free CH3NH3SnI3-based PSCs. We investigated and optimized the parameters of the lead-free CH3NH3SnI3-based PSCs with the CuSCN HTM. The simulated device shows a power conversion efficiency exceeding 26% under AM 1.5G illumination and an absorption onset up to 1080 nm. The reported CH3NH3SnI3-based PSCs provide a viable path to the realization of environmentally benign, low-cost, and high-efficiency PSCs.
