RESUMEN
Atomically dispersed catalysts refer to substrate-supported heterogeneous catalysts featuring one or a few active metal atoms that are separated from one another. They represent an important class of materials ranging from single-atom catalysts (SACs) and nanoparticles (NPs). While SACs and NPs have been extensively reported, catalysts featuring a few atoms with well-defined structures are poorly studied. The difficulty in synthesizing such structures has been a critical challenge. Here we report a facile photochemical method that produces catalytic centers consisting of two Ir metal cations, bridged by O and stably bound to a support. Direct evidence unambiguously supporting the dinuclear nature of the catalysts anchored on α-Fe2O3 is obtained by aberration-corrected scanning transmission electron microscopy (AC-STEM). Experimental and computational results further reveal that the threefold hollow binding sites on the OH-terminated surface of α-Fe2O3 anchor the catalysts to provide outstanding stability against detachment or aggregation. The resulting catalysts exhibit high activities toward H2O photooxidation.
RESUMEN
The addition of a co-catalyst onto the surface of a photocathode often greatly enhances the harvested photovoltage of the system. However, the true nature of how the catalyst improves the onset potential remains poorly understood. As a result, how to best utilize effective co-catalysts is still a limiting factor in achieving high performance earth abundant photoelectrochemical hydrogen evolution. Using intensity modulated photocurrent spectroscopy (IMPS), we have probed charge behaviors at the photoelectrode co-catalyst interface. We find that Pt drastically reduces charge recombination at the semiconductor liquid interface (SCLI). Further studies reveal that the onset potentials can be improved either by accelerating the reaction kinetics or reducing the recombination at the SCLI. The knowledge permits us to understand how earth abundant HER catalysts, such as CoP, behave at the SCLI. It is found that CoP is more effective at accelerating the reaction kinetics than reducing recombination.
RESUMEN
CdSe quantum dots (QDs) were attached to single crystal ZnO(0001) and ZnO(1100) substrates using capping groups, 4-mercaptobenzoic acid, 2-mercaptoacetic acid, 3-mercaptopropionic acid, 8-mercaptooctanoic acid, and 11-mercaptoundecanoic acid, as bifunctional linker molecules. The spectral response and photosensitization yields of the adsorbed QDs were studied with photocurrent spectroscopy. Atomic force microscopy (AFM) was used to verify the surface structure of the ZnO crystals and to examine the coverage and arrangement of the QDs on the single crystal surface. The inner-sphere aqueous redox couple Sx(2-)/S(2-), often used as a regenerator for chalcogenide-based QDs, as well as outer-sphere redox couples such as ferrocene, were able to regenerate the photoexcited CdSe QDs and suppress their photocorrosion. Differences in the binding of the QDs to different ZnO crystal faces are also reported.
RESUMEN
Comparison of the UV-vis absorption spectra of CdSe quantum dots (QDs) capped with various mercaptocarboxylic acid capping ligands reveals that only 4-mercaptobenzoic acid (MBzA) capping ligands lower the apparent optical band gap. We propose that the delocalization of the excitons in the CdSe QDs is extended onto the ligands via electronic coupling to the π system of the 4-mercaptobenzoic acid molecules through the Cd-S bond. Furthermore, we demonstrate that the electronic coupling between the QDs and the (MBzA) thiol ligands is influenced by the strength of the Cd-S bond that can be changed by protonating the S atom.
RESUMEN
Operando spectroelectrochemical analysis is used to determine the water oxidation reaction kinetics for hematite photoanodes prepared using four different synthetic procedures. While these photoanodes exhibit very different current/voltage performance, their underlying water oxidation kinetics are found to be almost invariant. Higher temperature thermal annealing was found to correlate with a shift in the photocurrent onset potential toward less positive potentials, assigned to a suppression of both back electron-hole recombination and of charge accumulation in intra-bandgap states, indicating these intra-bandgap states do not contribute directly to water oxidation.
RESUMEN
Photoelectrochemical (PEC) water splitting holds the potential as a direct route for solar energy conversation and storage. The performance of a PEC device is strongly influenced by the electronic properties of the photonanode surface. It has been shown that the synthesis methods can have a profound impact on the electronic properties and PEC performance of various photoelectrode materials such as hematite. Soft X-ray spectroscopic techniques, including O K-edge XAS and Fe L-edge XAS/XES, have been employed to investigate how the synthesis methods impact the electronic structure of resulting hematite materials. It is found that the hematite samples via solution regrowth methods show dramatically increased 3d-4sp band ratios in O K-edge XAS spectra and decreased relative elastic peak intensities in Fe L-edge RIXS spectra compared with samples synthesized via ALD or solution grown. The difference observed in O-K and Fe-L spectra indicated that solution regrowth strategy alters the O 2p-Fe 3d hybridization and hence the electronic structure of the hematite films, which proves to be beneficial for PEC performance of the hematite photoanode. Our findings provided new insights and potentially useful strategies for enhancing the PEC performance of photoanode materials.
RESUMEN
Heterogeneous catalysts with atomically defined active centers hold great promise for high-performance applications. Among them, catalysts featuring active moieties with more than one metal atom are important for chemical reactions that require synergistic effects but are rarer than single atom catalysts (SACs). The difficulty in synthesizing such catalysts has been a key challenge. Recent progress in preparing dinuclear heterogeneous catalysts (DHCs) from homogeneous molecular precursors has provided an effective route to address this challenge. Nevertheless, only side-on bound DHCs, where both metal atoms are affixed to the supporting substrate, have been reported. The competing end-on binding mode, where only one metal atom is attached to the substrate and the other metal atom is dangling, has been missing. Here, we report the first observation that end-on binding is indeed possible for Ir DHCs supported on WO3. Unambiguous evidence supporting the binding mode was obtained by in situ diffuse reflectance infrared Fourier transform spectroscopy and high-angle annular dark-field scanning transmission electron microscopy. Density functional theory calculations provide additional support for the binding mode, as well as insights into how end-on bound DHCs may be beneficial for solar water oxidation reactions. The results have important implications for future studies of highly effective heterogeneous catalysts for complex chemical reactions.
RESUMEN
Photoexcited carrier dynamics in a hematite film with and without amorphous NiFeO x on the surface was investigated using the heterodyne transient grating method. We found that two different electron/hole dynamics took place in the micro- and millisecond time regions and successfully assigned each component to the decay processes of electrons and holes trapped at surface states, respectively. It was also demonstrated that the amorphous NiFeO x coating plays a crucial role in increasing the survival of the holes at the surface trap states, which was caused by the decrease in the surface recombination rate.
RESUMEN
Bismuth vanadate (BiVO4) is a promising material for photoelectrochemical water oxidation. Recently, it has been shown that "photocharging" BiVO4 results in an improved water oxidation performance. However, the understanding of how BiVO4 is being improved has been lacking. Here we study the surface kinetics of BiVO4 using intensity-modulated photocurrent spectroscopy and show that photocharging BiVO4 results in both surface and bulk improvements. This result sheds light on how the surface charge transfer and bulk charge transport of BiVO4 respond to illumination.
RESUMEN
Photoelectrochemical (PEC) water splitting holds the potential to meet the challenges associated with the intermittent nature of sunlight. Catalysts have often been shown to improve the performance of PEC water splitting, but their working mechanisms are not well understood. Using intensity modulated photocurrent spectroscopy (IMPS), we determined the rate constants of water oxidation and recombination at the surface of three different hematite-based photoanodes. It was found that the best performing electrodes, in terms of photocurrent onset potential, exhibited the slowest water oxidation rate constants, which was a surprise. The performance of these photoelectrodes was enabled by the slow surface recombination. When amorphous NiFeO x , a water oxidation catalyst, was present, the rate of surface hole transfer actually slowed down; what was slowed more was the recombination rate at the hematite surface, resulting in better water oxidation performance. As such, NiFeO x primarily serves as a passivation layer rather than a catalytic layer. Together a better understanding of the role of catalytic overlayers for water oxidation has been achieved.
RESUMEN
Photoelectrochemistry (PEC) holds potential as a direct route for solar energy storage. Its performance is governed by how efficiently photoexcited charges are separated and how fast the charges are transferred to the solution, both of which are highly sensitive to the photoelectrode surfaces near the electrolyte. While other aspects of a PEC system, such as the light-absorbing materials and the catalysts that facilitate charge transfer, have been extensively examined in the past, an underwhelming amount of attention has been paid to the energetics at the photoelectrode/electrolyte interface. The lack of understanding of this interface is an important reason why many photoelectrode materials fail to deliver the expected performance in harvesting solar energy in a PEC system. Using hematite (α-Fe2O3) as a material platform, we present in this Perspective how surface modifications can alter the energetics and the resulting consequences on the overall PEC performance. It has been shown that a detailed understanding of the photoelectrode/eletrolyte interfaces can contribute significantly to improving the performance of hematite, which enabled unassisted solar water splitting when combined with an amorphous Si photocathode.