Building a Better Window onto Nanoparticulate Catalyst Films

Conventional optical spectroscopies like UV/Vis or infrared absorption are useful but intrinsically bulk sensitive. Using these techniques to interrogate catalyst/liquid interfaces operando thus requires distinguishing the signal of a relatively small number of molecules at the interface from that of the much larger number of molecules in the adjoining bulk phase(s). Even-order nonlinear optical techniques like sum or difference frequency generation spectroscopy are intrinsically surface sensitive by their symmetry selection rules and thus in principle overcome this hurdle. While application of SFG/DFG to optically flat, model catalyst, surfaces is relatively well established, application of these techniques to characterize nanoparticulate films (more important in application) is substantially more challenging due to multiple scattering and the a lack of phase resolution. We have recently developed (see here ) a technique that quantitatively corrects for multiple scattering and offers phase resolution for such systems, by depositing the nanoparticulate film on a crystalline quartz substrate. As we demonstrate in the manuscript, this approach appears to offer the possibility to exploit the full power of nonlinear optical techniques in the operando characterization of the particulate catalyst/liquid interface. 

Identifying Surface States on Oxide Surfaces During OER using Photocurrent Kinetics 

Identifying earth-abundant, active, selective, stable catalysts for the oxygen evolution reaction (OER) is a grand challenge along a path to a sustainable future. In pursuit of this goal much work has explored the potential applicability of various transition metal oxides. Understanding the OER mechanism on such materials, and thus understanding the physical basis for empirical differences in activity/stability, has proven surprisingly difficult. Part of the difficulty arises because in a potential range similar to that of the OER the metal oxide surface may further oxidize or (de)protonate. Thus gaining insight into the OER mechanism requires distinguishing these, possible, co-existing surface species and their relevance for the liquid phase oxidation chemistry operando. In a recent manuscript (see here) we have demonstrated a chopped photocurrent approach that overcomes this difficulty for the OER on the photoanode material hematite (\alpha-Fe2O3). By collecting wavelength dependent photocurrent kinetics, following shut off of illumination, we show that the oxide surface is characterized by two distinct surface OH populations operando*: monodentate and bidentate coordinated by underlying Fe Atoms. The later is the site active for the OER while the former regulates surface charge. In part II of this story, currently in preparation, we interrogate these surface sites using nonlinear optical surface phonon spectroscopy.  

Our group is excited to share our latest research, currently under review at Angewandte Chemie International Edition, which introduces a novel spectroelectrochemical strategy to probe the local electric fields governing the Oxygen Evolution Reaction (OER). By combining vibrational sum frequency generation (SFG) with the vibrational Stark effect (VSE) at an IrOx/Nb-SrTiO3 model anode, we utilized a Ti-O dangling bond as a built-in reporter to map how interfacial fields vary with potential and pH. Our findings reveal that a proton-rich Helmholtz layer strengthens the field in acidic conditions, while phosphate adsorption hinders kinetics at neutral pH, offering a conceptually simple but powerful tool for the rational design of catalysts and electrolytes. This work is supported by the DFG, German Research Foundation, project number 388390466 TRR 247, as well as GteX Japan and MEXT. The open access version of the paper can be found here: 10.26434/chemrxiv-2025-k2pd5

Exciting New Insights into Monolayer Transition Metal Dichalcogenides and Gold Interaction

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Congratulation on Tao’s first PRB paper (Phys. Rev. B 107, 155433)! In this work, we employed azimuthal-dependent sum frequency generation (SFG) spectroscopy to analyze the interaction between monolayer MoS2, a TMDC, and gold surfaces. This was achieved by comparing the second-order nonlinear optical (NLO) response of MoS2 on both Si/SiO2 and gold, with a focus on how the patterns and intensities change with the azimuthal angle and under different polarizations. The study revealed that the interaction between MoS2 and gold significantly alters both the magnitude and symmetry of the NLO response, in contrast to its behavior on Si/SiO2. This finding aligns with prior theoretical studies, emphasizing a strong interaction between monolayer MoS2 and the gold substrate. This research offers a contact-free method to probe substrate-induced changes in the electronic structure of TMDC monolayers. Its implications are far-reaching, particularly in the development of optoelectronic devices, and it opens new avenues for exploring the potential of TMDCs in technological applications.

Innovative Study Unveils New Optical Responses in 2D Semiconductor-Metal Junctions

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In this second piece of work (under review in PRB, preprint is available here 
arXiv:2310.19657), we seek to employ the different symmetries of the responses from metal and TMDC to isolate the optical responses of a MoS2 monolayer in a MoS2/Au junction. The SFG spectra exhibited a linear lineshape without any A or B exciton features. This characteristic was attributed to the strong dielectric screening and substrate-induced doping at the junction. The linear lineshape of the spectra interestingly reflects the constant density of states (DOS) at the band edge of the 2D semiconductor. We were able to extrapolate from their findings to determine the onset of a direct quasiparticle bandgap of approximately 1.65 ± 0.20 eV, indicating a substantial renormalization of the bandgap. This study not only enhances our understanding of the optical responses of 2D semiconductors under extreme screening conditions but also serves as a vital reference for the advancement of 2D semiconductor-based photocatalytic applications.