Electrochemical Tip-Enhanced Raman Spectroscopy: An In Situ Nanospectroscopy for Electrochemistry.

Revealing the intrinsic relationships between the structure, properties, and performance of the electrochemical interface is a long-term goal in the electrochemistry and surface science communities because it could facilitate the rational design of electrochemical devices. Achieving this goal requires in situ characterization techniques that provide rich chemical information and high spatial resolution. Electrochemical tip-enhanced Raman spectroscopy (EC-TERS), which provides molecular fingerprint information with nanometer-scale spatial resolution, is a promising technique for achieving this goal. Since the first demonstration of this technique in 2015, EC-TERS has been developed for characterizing various electrochemical processes at the nanoscale and molecular level. Here, we review the development of EC-TERS over the past 5 years. We discuss progress in addressing the technical challenges, including optimizing the EC-TERS setup and solving tip-related issues, and provide experimental guidelines. We also survey the important applications of EC-TERS for probing molecular protonation, molecular adsorption, electrochemical reactions, and photoelectrochemical reactions. Finally, we discuss the opportunities and challenges in the future development of this young technique.

Electrochemical Storage of Atomic Hydrogen on Single Layer Graphene.

If hydrogen can be stored and carried safely at a high density, hydrogen-fuel cells offer effective solutions for vehicles. The stable chemisorption of atomic hydrogen on single layer graphene (SLG) seems a perfect solution in this regard, with a theoretical maximum storage capacity of 7.7 wt %. However, generating hydrogenated graphene from H2 requires extreme temperatures and pressures. Alternatively, hydrogen adatoms can easily be produced under mild conditions by the electroreduction of protons in solid/liquid systems. Graphene is electrochemically inert for this reaction, but H-chemisorption on SLG can be carried out under mild conditions via a novel Pt-electrocatalyzed "spillover-surface diffusion-chemisorption" mechanism, as we demonstrate using dynamic electrochemistry and isotopic Raman spectroscopy. The apparent surface diffusion coefficient (∼10-5 cm2 s-1), capacity (∼6.6 wt %, ∼85.7% surface coverage), and stability of hydrogen adatoms on SLG at room temperature and atmospheric pressure are significant, and they are perfectly suited for applications involving stored hydrogen atoms on graphene.

Photosynergetic Electrochemical Synthesis of Graphene Oxide.

Here we propose a strategy of radical oxidation reaction for the high-efficiency production of graphene oxide (GO). GO plays important roles in the sustainable development of energy and the environment, taking advantages of oxygen-containing functional groups for good dispersibility and assembly. Compared with Hummers' method, electrochemical exfoliation of graphite is considered facile and green, although the oxidation is fairly low. To synthesize GO with better crystallinity and higher oxidation degree, we present a photosynergetic electrochemical method. By using oxalate anions as the intercalation ions and co-reactant, the interfacial concentration of hydroxyl radicals generated during electrochemical exfoliation was promoted, and the oxidation degree was comparable with that of GO prepared by Hummers' method. In addition, the crystallinity was improved with fewer layers and larger size. Moreover, the aniline coassembled GO membrane was selectively permeable to water molecules by the hydrogen-bond interaction, but it was impermeable to Na+, K+, and Mg2+, due to the electrostatic interactions. Thus, it has a prospective application to water desalination and purification. This work opens a novel approach to the direct functionalization of graphene during the electroexfoliation processes and to the subsequent assembly of the functionalized graphene.

Tip-enhanced Raman spectroscopy for nanoscale probing of dynamic chemical systems.

Dynamics are fundamental to all aspects of chemistry and play a central role in the mechanism and product distribution of a chemical reaction. All dynamic processes are influenced by the local environment, so it is of fundamental and practical value to understand the structure of the environment and the dynamics with nanoscale resolution. Most techniques for measuring dynamic processes have microscopic spatial resolution and can only measure the average behavior of a large ensemble of sites within their sampling volumes. Tip-enhanced Raman spectroscopy (TERS) is a powerful tool for overcoming this limitation due to its combination of high chemical specificity and spatial resolution that is on the nanometer scale. Adapting it for the study of dynamic systems remains a work in progress, but the increasing sophistication of TERS is making such studies more routine, and there are now growing efforts to use TERS to examine more complex processes. This Perspective aims to promote development in this area of research by highlighting recent progress in using TERS to understand reacting and dynamic systems, ranging from simple model reactions to complex processes with practical applications. We discuss the unique challenges and opportunities that TERS presents for future studies.

Direct Nanomachining on Semiconductor Wafer By Scanning Electrochemical Microscopy.

Scanning electrochemical microscopy (SECM) is one of the most important instrumental methods of modern electrochemistry due to its high spatial and temporal resolution. We introduced SECM into nanomachining by feeding the electrochemical modulations of the tip electrode back to the positioning system, and we demonstrated that SECM is a versatile nanomachining technique on semiconductor wafers using electrochemically induced chemical etching. The removal profile was correlated to the applied tip current when the tip was held stationary and when it was moving slowly (<20 µm s-1 ), and it followed Faraday's law. Both regular and irregular nanopatterns were translated into a spatially distributed current by the homemade digitally controlled SECM instrument. The desired nanopatterns were "sculpted" directly on a semiconductor wafer by SECM direct-writing mode. The machining accuracy was controlled to the sub-micrometer and even nanometer scales. This advance is expected to play an important role in electrochemical nanomachining for 3D micro/nanostructures in the semiconductor industry.

Illuminating nanostructured gold electrode: surface plasmons or electron ejection?

Recently, the photoelectric effect in metals in electrolyte environments has aroused the interest of researchers. However, direct evidence for surface plasmons-enhanced electrochemical reactions involving classic outer-sphere reactions of reversible redox couples is seldom reported. We used a surface plasmons-active gold-mushroom-array as a working electrode and observed enhanced faradaic current from ferrocenemethanol following illumination with a xenon lamp. The photoelectric current behaved differently in the presence and absence of oxygen in the solution. The preliminary results are discussed with consideration of the possible mechanisms of electron transfer, although they are very complex, due to the lack of direct evidence. Some consideration was also taken of the research on photoelectrochemical reactions on metallic electrodes.

Molecular mechanism of charge inversion revealed by polar orientation of interfacial water molecules: A heterodyne-detected vibrational sum frequency generation study.

"Charge inversion" is a phenomenon in which multivalent counterions overcompensate for interfacial charges and invert the sign of the net charge near a surface. This phenomenon is believed to be relevant to biologically important processes such as DNA condensation, and hence it has attracted much attention. We investigated the polar orientation of interfacial water molecules at two different negatively charged interfaces in the absence and presence of La3+ using heterodyne-detected vibrational sum frequency generation spectroscopy, which can directly determine the up/down orientation of interfacial molecules. It was found that the orientations of water molecules at a bio-relevant phospholipid interface change from the hydrogen-up to the hydrogen-down with the addition of 10 µM La3+. This change of water orientation indicates that the net charge at the phospholipid interface is inverted by adsorption of La3+ to the phosphate headgroup. By contrast, at an alkylsulfate interface, the majority of the interfacial water molecules remain hydrogen-up orientated even in the presence of 25 mM La3+, indicating that the sulfate headgroup is still solvated by up-oriented water. The observed headgroup specificity suggests that charge inversion at the phospholipid interface originates primarily from the chemical interaction between the phosphate and La3+ ion.

Local environment inside a novel aromatic micelle investigated by steady-state and femtosecond fluorescence spectroscopy of an encapsulated solvatochromic probe.

The local environment within a recently developed anthracene-shelled micelle (ASM), which is a micelle-like nanocapsule composed of anthracene-embedded amphiphiles, was investigated by steady-state and time-resolved spectroscopy of an encapsulated solvatochromic fluorescent probe molecule, coumarin 153 (C153). The absorption maximum of encapsulated C153 (452 nm) is more red-shifted than that of free C153 in water, indicating a highly polar environment inside the micelle. Despite this, the fluorescence Stokes shift of encapsulated C153 (∼3700 cm-1) is substantially smaller than that of free C153 in water. Femtosecond time-resolved broadband fluorescence measurements further showed that the dynamic Stokes shift is completed within 1 ps, revealing that the reorganization of the micelle interior following photoexcitation of the C153 probe is characterized by a sub-picosecond, limited-amplitude response. The femtosecond fluorescence anisotropy data showed that the orientational diffusion of the host-guest complex is slower (860 ps) than that of the empty micelle (510 ps), suggesting that the micelle structure is flexible enough to expand when the guest molecule is accommodated and that the micelle rotates with the encapsulated guest molecule. This softness of the micelles further allows some of them to simultaneously encapsulate two C153 molecules, as evidenced by the appearance of blue-shifted, H-dimer-like absorption and fluorescence bands. Based on these steady-state and femtosecond time-resolved spectroscopic data, we discuss the electronic state of C153 and micelle structure as well as the host-guest interaction in this novel flexible synthetic nanocapsule.

Preparation of Highly Fluorescent Host-Guest Complexes with Tunable Color upon Encapsulation.

Unlike previous coordinative host-guest systems, highly emissive host-guest complexes (up to Φ(F) = 0.5) were successfully prepared upon encapsulation of various fluorescent dyes (e.g., BODIPY and coumarin derivatives) by a Pt(II)-linked coordination capsule in water. Picosecond time-resolved spectroscopy elucidates the photophysical behaviors of the obtained complexes. Notably, the emission color of the fluorescent guest within the capsule can be readily modulated upon pairwise encapsulation with planar aromatic molecules.

Nonlinear optical pulse suppression via ultrafast photoinduced electron transfer in an aggregated perylene diimide/oligothiophene molecular triad.

A donor-acceptor-donor triad material in which two quinquethiophene moieties are attached via nonconjugated, flexible bridges to the 1,7-positions (80% isomer) and 1,6-positions (20% isomer) of a perylene diimide (PDI-5T) has been synthesized, and its nonlinear suppression of nanosecond laser pulses in the 680-750 nm range has been studied. The kinetics of the photoinduced charge separation processes have been characterized using femtosecond transient pump-probe spectroscopy. Excitation of either the quinquethiophene donor or perylene diimide acceptor leads to ultrafast (<700 fs) photoinduced charge separation, yielding quinquethiophene and perylene diimide radical ions that are strongly absorbing in the red-near-IR region. Despite the short lifetime (52 ps) of the charge-separated state, reasonably strong nonlinear suppression of nanosecond pulses, with figures-of-merit up to 14, has been realized with 4 mM solutions of PDI-5T. Although the radical ion absorption (RIA) is much stronger at 750 nm than that at 680 or 700 nm, the best optical suppression figures-of-merit were observed at 680 and 700 nm. Comparison of the optical parameters at these wavelengths suggests that the stronger ground-state absorption, due to aggregates of PDI-5T, is responsible for the enhanced figure-of-merit at the shorter wavelength.

Photoinduced electron transfer and nonlinear absorption in poly(carbazole-alt-2,7-fluorene)s bearing perylene diimides as pendant acceptors.

This paper reports the synthesis, photophysical behavior, and use in nanosecond optical-pulse suppression of a poly(2,7-carbazole-alt-2,7-fluorene) and a poly(3,6-carbazole-alt-2,7-fluorene) in which the carbazole N-positions are linked by an alkyl chain to one of the nitrogen atoms of a perylene-3,4,9,10-tetracarboxylic diimide (PDI) acceptor. It was found that the PDI pendants on the polymer side chain aggregated even in dilute solution, which extended the onset of PDI absorption into the near-infrared (NIR). Transient-absorption spectra of these polymers provide evidence for efficient electron transfer following either donor or acceptor photoexcitation to form long-lived charge-separated species, which exhibit strong absorption in the NIR. The spectral overlap between the transient species and the long-wavelength absorption edge of the aggregated PDI leads to reverse saturable absorption at 680 nm that can be used for optical-pulse suppression. Additionally, at high input energies, two-photon absorption mechanisms may also contribute to the suppression. PDI-grafted polymers exhibit enhanced optical-pulse suppression compared with blends of model materials composed of unfunctionalized poly(carbazole-alt-2,7-fluorene)s and PDI small molecules.

Ultrafast dynamics of an azobenzene-containing molecular shuttle based on a rotaxane.

An ultrafast spectroscopic study was carried out for a photoisomerizable, rotaxane-based molecular shuttle, in which photoisomerization of the azobenzene moiety of the thread-like guest drives a shuttling motion of a cyclodextrin host. Femtosecond upconversion and time-resolved absorption measurements revealed distinct S1 dynamics with time constants of 1.2 and 17 ps. Both time constants are smaller when the cyclodextrin host is absent, implying that, within the S1 state, there are mutiple barriers to the isomerization and subsequent shuttling, due to steric interference from the cyclodextrin.

Accelerated interfacial proton transfer for promoting electrocatalytic activity.

Interfacial pH is critical to electrocatalytic reactions involving proton-coupled electron transfer (PCET) processes, and maintaining an optimal interfacial pH at the electrochemical interface is required to achieve high activity. However, the interfacial pH varies inevitably during the electrochemical reaction owing to slow proton transfer at the interfacial layer, even in buffer solutions. It is therefore necessary to find an effective and general way to promote proton transfer for regulating the interfacial pH. In this study, we propose that promoting proton transfer at the interfacial layer can be used to regulate the interfacial pH in order to enhance electrocatalytic activity. By adsorbing a bifunctional 4-mercaptopyridine (4MPy) molecule onto the catalyst surface via its thiol group, the pyridyl group can be tethered on the electrochemical interface. The pyridyl group acts as both a good proton acceptor and donor for promoting proton transfer at the interfacial layer. Furthermore, the pK a of 4MPy can be modulated with the applied potentials to accommodate the large variation of interfacial pH under different current densities. By in situ electrochemical surface-enhanced Raman spectroscopy (in situ EC-SERS), we quantitatively demonstrate that proton transfer at the interfacial layer of the Pt catalyst coated with 4MPy (Pt@4MPy) remains ideally thermoneutral during the H+ releasing electrocatalytic oxidation reaction of formic acid (FAOR) at high current densities. Thus, the interfacial pH is controlled effectively. In this way, the FAOR apparent current measured from Pt@4MPy is twice that measured from a pristine Pt catalyst. This work establishes a general strategy for regulating interfacial pH to enhance the electrocatalytic activities.

Probing nanoscale spatial distribution of plasmonically excited hot carriers.

Surface plasmons (SPs) of metals enable the tight focusing and strong absorption of light to realize an efficient utilization of photons at nanoscale. In particular, the SP-generated hot carriers have emerged as a promising way to efficiently drive photochemical and photoelectric processes under moderate conditions. In situ measuring of the transport process and spatial distribution of hot carriers in real space is crucial to efficiently capture the hot carriers. Here, we use electrochemical tip-enhanced Raman spectroscopy (EC-TERS) to in situ monitor an SP-driven decarboxylation and resolve the spatial distribution of hot carriers with a nanometer spatial resolution. The transport distance of about 20 nm for the reactive hot carriers is obtained from the TERS imaging result. The hot carriers with a higher energy have a shorter transport distance. These conclusions can be guides for the design and arrangement of reactants and devices to efficiently make use of plasmonic hot carriers.

Optimizing the interfacial electron transfer capability of single layer graphene by thermal annealing.

The interfacial electron transfer capability of Si/SiO2 wafer supported single layer graphene is optimized by thermal annealing in an inert gas environment, which facilitates its applications in both electrochemical and electronic devices.

Photoelectric effect accelerated electrochemical corrosion and nanoimprint processes on gallium arsenide wafers.

Here we report photoelectric-effect-enhanced interfacial charge transfer reactions. The electrochemical corrosion rate of n-type gallium arsenide (n-GaAs) induced by the contact potential at platinum (Pt) and GaAs boundaries can be accelerated by the photoelectric effect of n-GaAs. When a GaAs wafer is illuminated with a xenon light source, the electrons in the valence band of GaAs will be excited to the conduction band and then move to the Pt boundaries due to the different work functions of the two materials. This results in an enhanced contact electric field as well as an enlarged Pt/GaAs contact potential. Consequently, in the presence of electrolyte solution, the polarizations of both the Pt/solution interface and the GaAs/solution interface at the Pt/GaAs/solution 3-phase boundary are enhanced. If the accumulated electrons on the Pt side are removed by electron acceptors in the solution, anodic corrosion of GaAs will be accelerated strictly along the Pt/GaAs/solution 3-phase boundary. This photo-enhanced electrochemical phenomenon can increase the corrosion rate of GaAs and accelerate the process of electrochemical nanoimprint lithography (ECNL) on GaAs. The method opens an innovative, highly efficient, low-cost nanoimprint technique performed directly on semiconductors, and it has prospective applications in the semiconductor industry.

Role of Adsorption Orientation in Surface Plasmon-Driven Coupling Reactions Studied by Tip-Enhanced Raman Spectroscopy.

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.

Chaperone-like chiral cages for catalyzing enantio-selective supramolecular polymerization.

Cage catalysis has emerged as an important approach for mimicking enzymatic reactions by increasing the reaction rate and/or product selectivity of various types of covalent reactions. Here, we extend the catalytic application of cage compounds to the field of non-covalent molecular assembly. Acid-stable chiral imine cages are found to catalyze the supramolecular polymerization of porphyrins with an accelerated assembling rate and increased product enantioselectivity. Because the imine cages have a stronger interaction with porphyrin monomers and a weaker interaction with porphyrin assemblies, they can fully automatically detach from the assembled products without being consumed during the catalytic process. We reveal the kinetics of the auto-detachment of cages and the chirality growth of the assemblies using spectroscopic characterization studies. We find that the passivation groups attached to the cages are important for maintaining the structural stability of the cages during catalyzed assembly, and that the steric geometries of the cages can profoundly affect the efficiency of chiral regulation. This strategy demonstrates a new type of catalytic application of cage compounds in the field of molecular assembly, and paves the way to controlling supramolecular polymerization through a catalytic pathway.

Electrogenerated chemiluminescence of a spirobifluorene-linked bisanthracene: a possible simultaneous, two-electron transfer.

We report the electrogenerated chemiluminescence (ECL) of 2,2'-bis(10-phenylanthracen-9-yl)-9,9'-spirobifluorene (spiro-FPA), a dichromophoric molecule composed of two phenylanthracenes linked by a spirobifluorene moiety (PA-X-PA). The results are compared to those for 9,10-diphenylanthracene (DPA), a related molecule with a single chromophore. Cyclic voltammetry (CV) of spiro-FPA shows two reversible, closely spaced, one-electron transfers on both reduction and oxidation, occurring at E(o)(1,red) = -2.02, E(o)(2,red) = -2.07 V vs SCE and E(o)(1,ox) = 1.14, E(o)(2,ox) = 1.20 V vs SCE. The potentials for each pair are close enough to appear as a single peak in CV, indicating that the spirobifluorene moiety interrupts conjugation between the redox centers. The potentials observed are similar to those of DPA, which shows E(o)(red) = -2.06 V vs SCE and E(o)(ox) = 1.15 V vs SCE. The absorbance spectrum of spiro-FPA shows lambda(max,abs) = 377 nm, with 377 = 25,700 M(-1) s(-1), while DPA exhibited lambda(max,abs) = 374 nm, with 374 = 13,800 M(-1) s(-1), demonstrating that spiro-FPA has twice the available chromophores as DPA. Photoluminescence (PL) data for spiro-FPA shows lambda(max,PL) = 434 nm, with Phi(PL) = 0.74, while DPA fluoresces at 420 nm with Phi(PL) = 0.91; thus, there is greater solvent or structural relaxation in the spiro-FPA excited state, which may account for the greater internal conversion. Unlike DPA, the ECL spectrum of spiro-FPA exhibits long-wavelength emission not observed in the PL. We attribute this emission to excimers formed during annihilation ECL. Steric hindrance prevents DPA from forming excimers, even in ECL, but spiro-FPA annihilation can occur between pairs of di-ions (PA(*-)-X-PA(*-) and PA(*+)-X-PA(*+)), which are electrostatically more strongly attracted to one another than the mono-ions. This greater electrostatic attraction may be sufficient to overcome the steric hindrance to excimer formation. Lowering the electrolyte concentration decreases the electrostatic shielding of the ions from one another; thus, the increase in longer wavelength ECL accompanying a decrease in electrolyte concentration supports the role of the di-ions in excimer formation. Additionally, simulations show, consistent with experiment, a more rapid decrease in excimer concentration than in excited monomer concentration as a function of time after each potential pulse. This is probably due to the greater number of scavenging reactions available for di-ions. The simulations are confirmed experimentally when lower potential pulsing frequencies yield lower relative excimer emission. Since an excited state created by one-electron transfer between two di-ions should be rapidly quenched via electron transfer by the other PA moiety, the existence of excimers suggests the possibility of simultaneous, two-electron transfer to generate the excimer.

Synthesis and linear and nonlinear absorption properties of dendronised ruthenium(II) phthalocyanine and naphthalocyanine.

Ruthenium phthalocyanines and naphthalocyanines with axial dendronised pyridine ligands show high solubility in a variety of solvents, and exhibit solid-state absorption spectra that are comparable to those obtained in dilute solution, making them interesting candidates for optical limiting in the visible.