Vibrational Spectroscopic Observation of Atomic-Scale Local Surface Sites Using Site-Selective Signal Enhancement

Correlating the orbital overlap area and vibrational frequency shift of an isocyanide moiety adsorbed on Pt and Pd covered Au(111) surfaces

Orbital interactions between adsorbed molecules and the underlying metal surfaces play critical roles in a wide range of surface and interfacial processes. Establishing a correlation between an experimental observable (e.g., vibrational frequency shift of the adsorbed molecule) and the orbital interactions is of vital importance. Herein, theoretical calculations are used to investigate the vibrational frequency shift of phenyl isocyanide molecules as a probe molecule adsorbed on mono- and bi-layer Pt and Pd covered Au(111) surfaces and Pd₂Au₄ and Pt₂Au₄ clusters. By analyzing the density of states (DOS) of the adsorption system, we show that the orbital overlap area of d electronic DOS with a molecular σ or π* orbital, particularly their ratio (R_d-σ/d-π*), can be a meaningful descriptor to explain the frequency shift of the CN moiety. This hypothesis has been verified by simulations for phenyl isocyanide with electron donating NH₂- and withdrawing CF₃- substituent groups, formonitrile and carbon monoxide. Quasi-linear dependence of the frequency shift on R_d-σ/d-π* is observed for both the red and blue shift regions. Our findings build up on previous notions of electronic interactions, which will provide a more quantitative and solid footing to understand and analyze the frequency shift of adsorbed molecules on metal surfaces and the related electronic interactions and catalytic properties.

Steering Metal-Organic Network Structures through Conformations and Configurations on Surfaces

Molecular adsorption conformations and arrangement configurations on surfaces are important structural aspects of surface stereochemistry, but their roles in steering the structures of metal-organic networks (MONs) remain vague and unexplored. In this study, we constructed MONs by the coordination self-assembly of isocyanides on Cu(111) and Ag(111) surfaces and demonstrated that the MON structures can be steered by surface stereochemistry, including the adsorption conformations of the isocyanide molecules and the arrangement configurations of the coordination nodes and subunits. The coordination self-assembly of 1,4-phenylene diisocyanobenzene afforded a honeycomb MON consisting of 3-fold (isocyano)₃-Cu motifs on a Cu(111) surface. In contrast, geometrically different chevron-shaped 1,3-phenylene diisocyanobenzene (m-DICB) failed to generate a MON, which is ascribable to its standing conformation on the Cu(111) surface. However, m-DICB was adsorbed in a flat conformation on a Ag(111) surface, which has a larger lattice constant than a Cu(111) surface, and smoothly underwent coordination self-assembly to form a MON consisting of (isocyano)₃-Ag motifs. Interestingly, only C₃-Ag nodes with heterotactic configurations could grow into larger subunits; those subunits with heterotactic configurations further grew into Sierpiński triangle fractals (up to fourth order), while subunits with homotactic configurations afforded a triangular MON.

Recent advances in plasmon-enhanced Raman spectroscopy for catalytic reactions on bifunctional metallic nanostructures

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.

Probing Single-Atom Catalysts and Catalytic Reaction Processes by Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy

Developing advanced characterization techniques for single-atom catalysts (SACs) is of great significance to identify their structural and catalytic properties. Raman spectroscopy can provide molecular structure information, and thus, the technique is a promising tool for catalysis. However, its application in SACs remains a great challenge because of its low sensitivity. We develop a highly sensitive strategy that achieves the characterization of the structure of SACs and in situ monitoring of the catalytic reaction processes on them by shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) for the first time. Using the strategy, Pd SACs on different supports were identified by Raman spectroscopy and the nucleation process of Pd species from single atoms to nanoparticles was revealed. Moreover, the catalytic reaction processes of the hydrogenation of nitro compounds on Pd SACs were monitored in situ, and molecular insights were obtained to uncover the unique catalytic properties of SACs. This work provides a new spectroscopic tool for the in situ study of SACs, especially at solid-liquid interfaces.

Synthesis of large gold nanoparticles with deformation twinnings by one-step seeded growth with Cu(ii)-mediated Ostwald ripening for determining nitrile and isonitrile groups

In this work, uniform and large gold nanoparticles (Au NPs) including quasi-spherical (QS) Au NPs with average diameters of 70 to 196 nm and trisoctahedral (TOH) Au NPs with average diameters of 140 to 195 nm were successfully synthesized by controlling the concentration of Cu2+ ions and the particle number of 3 nm Au-NP seeds, respectively, using a one-step seeded growth method with Cu2+-mediated Ostwald ripening. It is found that because of the concentration-dependent under-potential deposition of Cu2+ ions (CuUPD), 3 nm Au-NP seeds are firstly changed into Au NPs with a controlled QS- or TOH shape at the initial growth stage, followed by the conformal growth of Au atoms onto the initially formed Au NPs due to Cu2+-mediated Ostwald ripening, in which the extra Au atoms come from the dissolution of in situ Au nuclei by the unavoidable self-nucleation. Moreover, the as-prepared QS Au NPs with a rough surface exhibit a better SERS performance for physically adsorbed probes (crystal violet, CV) than the TOH Au NPs with sharp tips and with a comparable size. Furthermore, the as-prepared QS Au NPs can be used to distinguish nitrile and isonitrile groups by surface-enhanced Raman scattering (SERS) due to the presence of deformation twinnings. Thus, the as-prepared QS Au NPs with a rough surface and deformation twinnings can be further used as templates for the fabrication of bimetallic materials with multi-functionalities.

A quantum chemical study of substituent effects on CN bonds in aryl isocyanide molecules adsorbed on the Pt surface

A periodicity implemented scheme of natural bond orbital (NBO) theory and normal mode analysis has been employed to investigate the tendency of the chemical bond strength of aryl isocyanide molecules with different para-substituted groups adsorbed on the Pt(111) surface. The NC bond order shows a clear correspondence with the NC stretching frequency; both of them exhibit a "volcano-like" profile as a function of the Hammett constant of the para-substituted groups for isolated molecules. When a molecule is adsorbed on the Pt(111) surface, the NC stretching frequency variations are determined by the resultant effect of σ donation and π back-donation between the molecule and the surface. The present comprehensive and systematic computations clarify the electron donating and withdrawing effects of the substituted groups on the interaction between the aryl isocyanide molecule and the transition metal substrate.

Heterogeneous gap-mode nanostructure for surface-enhanced Raman spectroscopic evaluation of charge transfer between noble metal nanoparticles and formaldehyde vapor

Gap-mode nanostructures offer a reliable, scalable and controllable Raman substrate with high signal enhancement, and they are widely used in surface-enhanced Raman spectroscopy. Heterogeneous gap-mode structures composed of different types of nanoparticles with the underlying substrate have been studied only in terms of understanding the electromagnetic field enhancement mechanism up to now, just by focusing on the role of hot spot as enhancing the Raman signal itself. In this study, gold and platinum nanoparticle-based heterogeneous gap-mode structures were fabricated on gold surface, and used to evaluate minute changes in the surface charged state (surface potential) of the nanoparticle interacting with different organic vapors. By monitoring the surface-enhanced Raman signal change of isonitrile probes in the hot spot, it was revealed that gold and platinum nanoparticles show opposite directions of charge transfer over the same formaldehyde treatment. This strategy offers a new way to evaluate the charge transfer phenomenon between organic vapor and nanoparticles, which is especially important in catalytic application, using conventional surface-enhanced Raman spectroscopy.

In Situ Atomic-Level Tracking of Heterogeneous Nucleation in Nanocrystal Growth with an Isocyanide Molecular Probe

We report the use of 2,6-dimethylphenyl isocyanide (2,6-DMPI) as a spectroscopic probe to study the heterogeneous nucleation and deposition of Pd on Ag nanocubes under different conditions by surface-enhanced Raman scattering. As a major advantage, the spectroscopic analysis can be performed in situ and in real time with the nanoparticles still suspended in the reaction solution. The success of this method relies on the distinctive stretching frequencies (ν_NC) of the isocyanide group in 2,6-DMPI when it binds to Ag and Pd atoms through σ donation and π-back-donation, respectively. Significantly, we discovered that ν_NC was sensitive to the arrangement of Pd adatoms on the Ag surface. For example, when the isocyanide group bound to one, two, and three Pd atoms, we would observe the atop, bridge, and hollow configurations, respectively, at different ν_NC frequencies. As such, the ν_NC band could serve as a characteristic reporter for the Pd adatoms being deposited onto different types of facets on Ag nanocubes with atomic-level sensitivity. When 2,6-DMPI molecules were introduced into the reaction solution, we further demonstrated in situ tracking of heterogeneous nucleation and early stage deposition of Pd on Ag nanocubes by monitoring the evolution of ν_NC bands for both Ag and Pd surface atoms as a function of reaction time. This in situ technique opens up the opportunity to investigate the roles played by reaction temperature and the type of Pd(II) precursor in influencing the heterogeneous nucleation and growth of bimetallic nanocrystals. The sensitivity of isocyanide group to Pd atoms helps elucidate some of the details on the reduction, deposition, and diffusion processes involved in heterogeneous nucleation.

Electrical Matching at Metal/Molecule Contacts for Efficient Heterogeneous Charge Transfer

In a metal/molecule hybrid system, unavoidable electrical mismatch exists between metal continuum states and frontier molecular orbitals. This causes energy loss in the electron conduction across the metal/molecule interface. For efficient use of energy in a metal/molecule hybrid system, it is necessary to control interfacial electronic structures. Here we demonstrate that electrical matching between a gold substrate and π-conjugated molecular wires can be obtained by using monatomic foreign metal interlayers, which can change the degree of d-π* back-donation at metal/anchor contacts. This interfacial control leads to energy level alignment between the Fermi level of the metal electrode and conduction molecular orbitals, resulting in resonant electron conduction in the metal/molecule hybrid system. When this method is applied to molecule-modified electrocatalysts, the heterogeneous electrochemical reaction rate is considerably improved with significant suppression of energy loss at the internal electron conduction.

Probing the electronic and catalytic properties of a bimetallic surface with 3 nm resolution

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 3 nm resolution in real space using phenyl isocyanide as a probe molecule (Fig. 1a). We observe a weakened N≡C 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 3 nm spatial resolution we demonstrate here is the result of an enhanced electric field and distinct electronic properties at the step edges.