Developing efficient catalytic systems for the hydrogen oxidation and evolution reactions (HOR/HER) is essential in the world's transition to renewable energy. There is a growing recognition that the ...HOR/HER activity depends on properties of the electrochemical interface, rather than just the composition and structure of the catalyst. Herein, we demonstrate that specifically adsorbed organic additives (theophylline derivatives) could enhance the intrinsic HOR/HER activity in base on polycrystalline Pt by up to a factor of 3 via introducing weakly hydrogen‐bonded water, as confirmed by in situ surface enhanced infrared and Raman spectroscopies. Optimal HOR/HER activity is achieved on a 7‐n‐butyltheophylline decorated Pt surface, which sufficiently disrupts the hydrogen bonding network in the double layer without depleting the interfacial water. This work demonstrates the promise of electrochemical interfacial engineering as a strategy to boost electrocatalytic performance.
Specifically adsorbed organic additives are found to enhance the intrinsic kinetics of hydrogen evolution and oxidation reactions in alkaline solution on Pt surfaces by reorganizing the interfacial water structure. In situ vibrational spectroscopies present compelling evidence that weakly hydrogen‐bonded water has a substantial impact on the hydrogen electrocatalytic process.
Surface and interfaces play key roles in heterogeneous catalysis, electrochemistry and photo(electro)chemistry. Tip-enhanced Raman spectroscopy (TERS) combines plasmon-enhanced Raman spectroscopy ...with scanning probe microscopy to simultaneously provide a chemical fingerprint and morphological information for the sample at the nanometer spatial resolution. It is an ideal tool for achieving an in-depth understanding of the surface and interfacial processes, so that the relationship between structure and chemical performance can be established. We begin with the background of surfaces and interfaces and TERS, followed by a detailed discussion on some issues in experimental TERS, including tip preparation and TERS instrument configuration. We then focus on the progress of TERS for studying the surfaces and interfaces under different conditions, from ambient, to UHV, solid-liquid and electrochemical environments, followed by a brief introduction to the current understanding of the unprecedented high spatial resolution and surface selection rules. We conclude by discussing the future challenges for TERS practical applications in surfaces and interfaces.
TERS offers the high spatial resolution to establish structure-function correlation for surfaces and interfaces.
Tip‐enhanced Raman spectroscopy can provide molecular fingerprint information with ultrahigh spatial resolution, but the tip will be easily contaminated, thus leading to artifacts. It also remains a ...great challenge to establish tip‐enhanced fluorescence because of the quenching resulting from the proximity of the metal tip. Herein, we report shell‐isolated tip‐enhanced Raman and fluorescence spectroscopies by employing ultrathin shell‐isolated tips fabricated by atomic layer deposition. Such shell‐isolated tips not only show outstanding electromagnetic field enhancement in TERS but also exclude interference by contaminants, thus greatly promoting applications in solution. Tip‐enhanced fluorescence has also been achieved using these shell‐isolated tips, with enhancement factors of up to 1.7×103, consistent with theoretical simulations. Furthermore, tip‐enhanced Raman and fluorescence signals are acquired simultaneously, and their relative intensities can be manipulated by changing the shell thickness. This work opens a new avenue for ultrahigh resolution surface analysis using plasmon‐enhanced spectroscopies.
Combined tip‐enhanced Raman and fluorescence spectroscopy using shell‐isolate tips is presented. With this method, clear and strong Raman and fluorescence signals can be obtained, and their relative intensities can be tuned by changing the shell thickness.
Resolving atomic site‐specific electronic properties and correlated substrate–molecule interactions is challenging in real space. Now, mapping of sub‐10 nm sized Pt nanoislands on a Au(111) surface ...was achieved by tip‐enhanced Raman spectroscopy, using the distinct Raman fingerprints of adsorbed 4‐chlorophenyl isocyanide molecules. A spatial resolution better than 2.5 nm allows the electronic properties of the terrace, step edge, kink, and corner sites with varying coordination environments to be resolved in real space in one Pt nanoisland. Calculations suggest that low‐coordinate atomic sites have a higher d‐band electronic profile and thus stronger metal–molecule interactions, leading to the observed blue‐shift of Raman frequency of the N≡C bond of adsorbed molecules. An experimental and theoretical study on Pt(111) and mono‐ and bi‐atomic layer Pt nanoislands on a Au(111) surface reveals the bimetallic effect that weakens with the increasing number of deposited Pt adlayer.
With a spatial resolution better than 2.5 nm, tip‐enhanced Raman spectroscopic imaging reveals unprecedented site‐specific electronic properties of different atomic sites with varying coordination numbers within a single, sub‐10 nm sized Pt nanoisland, using the distinct Raman fingerprints of adsorbed molecules. The work pushes forward the atomic‐ and molecular‐level characterization of heterogeneous catalysts.
An atomic- and molecular-level understanding of heterogeneous catalysis is required to characterize the nature of active sites and improve the rational design of catalysts. Achieving this level of ...characterization requires techniques that can correlate catalytic performances to specific surface structures, so as to avoid averaging effects. Tip-enhanced Raman spectroscopy combines scanning probe microscopy with plasmon-enhanced Raman scattering and provides simultaneous topographical and chemical information at the nano/atomic scale from ambient to ultrahigh-vacuum and electrochemical environments. Therefore, it has been used to monitor catalytic reactions and is proposed to correlate the local structure and function of heterogeneous catalysts. Bimetallic catalysts, such as Pd-Au, show superior performance in various catalytic reactions, but it has remained challenging to correlate structure and reactivity because of their structural complexity. Here, we show that TERS can chemically and spatially probe the site-specific chemical (electronic and catalytic) and physical (plasmonic) properties of an atomically well-defined Pd(sub-monolayer)/Au(111) bimetallic model catalyst at 3nm resolution in real space using phenyl isocyanide as a probe molecule (Fig. 1a). We observe a weakened NC bond and enhanced reactivity of phenyl isocyanide adsorbed at the Pd step edge compared with that at the Pd terrace. Density functional theory corroborates these observations by revealing a higher d-band electronic profile for the low-coordinated Pd step edge atoms. The 3nm spatial resolution we demonstrate here is the result of an enhanced electric field and distinct electronic properties at the step edges.
Metallic nanostructures exhibit superior catalytic performance for diverse chemical reactions and the in-depth understanding of reaction mechanisms requires versatile characterization methods. ...Plasmon-enhanced Raman spectroscopy (PERS), including surface-enhanced Raman spectroscopy (SERS), shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS), and tip-enhanced Raman spectroscopy (TERS), appears as a powerful technique to characterize the Raman fingerprint information of surface species with high chemical sensitivity and spatial resolution. To expand the range of catalytic reactions studied by PERS, catalytically active metals are integrated with plasmonic metals to produce bifunctional metallic nanostructures. In this minireview, we discuss the recent advances in PERS techniques to probe the chemical reactions catalysed by bifunctional metallic nanostructures. First, we introduce different architectures of these dual-functionality nanostructures. We then highlight the recent works using PERS to investigate important catalytic reactions as well as the electronic and catalytic properties of these nanostructures. Finally, we provide some perspectives for future PERS studies in this field.
This minireview provides a comprehensive discussion on recent advances in plasmon-enhanced Raman spectroscopy for catalytic reactions on bifunctional metallic nanostructures.
Active oxygen species (AOS) play key roles in many important catalytic reactions relevant to clean energy and environment. However, it remains challenging to characterize the active sites for ...producing AOS and to image the surface properties of AOS, especially on multicomponent metallic catalyst surfaces. Herein, we utilize tip-enhanced Raman spectroscopy (TERS) to probe the local generation and diffusion of OH radicals on a Pd/Au(111) bimetallic catalyst surface. The reactive OH radicals can be catalytically generated from hydrogen peroxide (H2O2) at the metal surface, which then oxidizes the surface adsorbed thiolate, a reactant that is used as the TERS probe. By TERS imaging of the spatial distribution of unreacted thiolate molecules, we demonstrate that the Pd surface is active for generation of OH radicals and the Pd step edge shows much higher activity than the Pd terrace, whereas the Au surface is inactive. Furthermore, we find that the locally generated OH radicals at the Pd step edge could diffuse to both the Au and the Pd surface sites to induce oxidative reactions, with a diffusion length estimated to be about 5.4 nm. Our TERS imaging with few-nanometer spatial resolution not only unravels the active sites but also characterizes in real space the diffusion behavior of OH radicals. The results are highly valuable to understand AOS-triggered catalytic reactions. The strategy of using reactants with large Raman cross sections as TERS probes may broaden the application of TERS for studying catalysis with reactive small molecules.
In the field of surface plasmon-mediated photocatalysis, the coupling reactions of p-aminothiophenol (PATP) and p-nitrothiophenol (PNTP) to produce p,p′-dimercaptoazobenzene (DMAB) are the most ...widely investigated systems. However, a clear understanding of the structure–function relationship is still required. Here, we used tip-enhanced Raman spectroscopy (TERS) to study the coupling reactions of PATP and PNTP on well-defined Ag(111) and Au(111) surfaces using 632.8 and 532 nm lasers. On Au(111), the oxidative coupling of PATP can proceed under irradiation by a 632.8 nm laser, and the reductive coupling of PNTP can only occur under irradiation by a 532 nm laser. Neither wavelength of laser light can induce the coupling reactions of these two molecules on Ag(111). Density functional theory (DFT) was used to calculate the stable adsorption configurations of PATP and PNTP on Ag(111) and Au(111). Both the adsorption configurations of the two molecules on the surfaces and laser energies were, experimentally and theoretically, found to determine whether the coupling reactions can occur on different substrates. These results may help the rational design of photocatalysts with enhanced reactivity.
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