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Understanding the structure and dynamic process of water at the solid–liquid interface is an extremely important topic in surface science, energy science and catalysis1–3. As model catalysts, atomically flat single-crystal electrodes exhibit well-defined surface and electric field properties, and therefore may be used to elucidate the relationship between structure and electrocatalytic activity at the atomic level4,5. Hence, studying interfacial water behaviour on single-crystal surfaces provides a framework for understanding electrocatalysis6,7. However, interfacial water is notoriously difficult to probe owing to interference from bulk water and the complexity of interfacial environments8. Here, we use electrochemical, in situ Raman spectroscopic and computational techniques to investigate the interfacial water on atomically flat Pd single-crystal surfaces. Direct spectral evidence reveals that interfacial water consists of hydrogen-bonded and hydrated Na+ ion water. At hydrogen evolution reaction (HER) potentials, dynamic changes in the structure of interfacial water were observed from a random distribution to an ordered structure due to bias potential and Na+ ion cooperation. Structurally ordered interfacial water facilitated high-efficiency electron transfer across the interface, resulting in higher HER rates. The electrolytes and electrode surface effects on interfacial water were also probed and found to affect water structure. Therefore, through local cation tuning strategies, we anticipate that these results may be generalized to enable ordered interfacial water to improve electrocatalytic reaction rates. Interfacial water consists of hydrogen-bonded water and Na·H2O, its structure changes at hydrogen evolution reaction (HER) potentials, and when structurally ordered it aids interfacial electron transfer, resulting in higher HER rates.

The analysis of plasmonic photoelectrochemical reactions on nanostructured noble metal electrodes is an attractive field, but little is known about the detailed competition between the photochemica...

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.

The fundamental structure-biofunction relationship of calcium phosphates (CaPs) remains unclear despite their clinical successes as important biomaterials. Herein, a series of CaP coatings with gradual change of topography and crystallinity is constructed by electrochemical deposition, and the roles of the two basic physicochemical properties are scrutinized for further understanding the mechanism behind the superior bioactivities of octacalcium phosphate (OCP). We observe a distinct modulation on cell proliferation on the prepared CaP coatings for different cells. The magnitude of the modulation seems to depend on the cellular size, and the effect is attributed mainly to the microstructure of the coatings. On the other hand, the crystallinity manifests its significance for the osteogenic property of the OCP coatings. Further transmission electron microscopy analysis and density functional theory calculations reveal a surface rich in HPO42- for the high-crystalline OCP nanocrystals. The results highlight that the nanocrystal surface properties of the OCP coatings, including the periodic structure and the HPO42- composition, may play significant roles surpassing the ion release effect in determining its osteogenic property, probably via surface spatial and/or chemical recognitions. The present findings shed light on the fundamental understanding of the structure-biofunction relationship for CaP biomaterials.

Atomically dispersed M-N-C (M refers to transition metals) materials represent the most promising catalyst alternatives to the precious metal Pt for the electrochemical reduction of oxygen (ORR), yet the genuine active sites in M-N-C remain elusive. Here, we develop a two-step approach to fabricate Cu-N-C single-atom catalysts with a uniform and well-defined Cu2+-N4 structure that exhibits comparable activity and superior durability in comparison to Pt/C. By combining operando X-ray absorption spectroscopy with theoretical calculations, we unambiguously identify the dynamic evolution of Cu-N4 to Cu-N3 and further to HO-Cu-N2 under ORR working conditions, which concurrently occurs with reduction of Cu2+ to Cu+ and is driven by the applied potential. The increase in the Cu+/Cu2+ ratio with the reduced potential indicates that the low-coordinated Cu+-N3 is the real active site, which is further supported by DFT calculations showing the lower free energy in each elemental step of the ORR on Cu+-N3 than on Cu2+-N4. These findings provide a new understanding of the dynamic electrochemistry on M-N-C catalysts and may guide the design of more efficient low-cost catalysts.

RuO2 is one of the most promising catalysts for oxygen evolution reaction (OER), a key reaction in water electrolysis. However, the fundamental insights about the structural evolution of RuO2 under different electrochemical environments remain unclear. Herein, the molecular reaction pathways of the electroreduction of RuOx in alkaline and neutral conditions are identified using in-situ electrochemical surface-enhanced Raman spectroscopy. Au@RuO2 core–shell nanostructures are fabricated to amplify the Raman signals of species adsorbed on RuOx surfaces. Using such core–shell nanostructures, direct spectroscopic evidences of OH intermediate during the RuO2 electroreduction process are obtained, which is further confirmed by D2O isotopic substitution experiments. A molecular pathway that RuO2 is reduced to Ru through a two-step reduction process has been proposed. This work provides insightful information on the structural evolution of RuOx with electrochemical environments.

All-solid-state Z-scheme photocatalysts for overall water splitting to evolve H2 is a promising strategy for efficient conversion of solar energy. However, most of these strategies require redox mediators. Herein, a direct Z-scheme photoelectrocatalytic electrode based on a WO3-x nanowire-bridged TiO2 nanorod array heterojunction is constructed for overall water splitting, producing H2. The as-prepared WO3-x/TiO2 nanorod array heterojunction shows photoelectrochemical (PEC) overall water splitting activity evolving both H2 and O2 under UV–vis light irradiation. An optimum PEC activity was achieved over a 1.67-WO3-x/TiO2 photoelectrode yielding maximum H2 and O2 evolution rates roughly 11 times higher than that of pure TiO2 nanorods without any sacrificial agent or redox mediator. The role of oxygen vacancy in WO3-x in affecting the H2 production rate was also comprehensively studied. The superior PEC activity of the WO3-x/TiO2 electrode for overall water splitting can be ascribed to an efficient Z-scheme charge transfer pathway between the WO3-x nanowires and TiO2 nanorods, the presence of oxygen vacancies in WO3-x, and a bias potential applied on the photoelectrode, resulting in effective spatial charge separation. This study provides a novel strategy for developing highly efficient PECs for overall water splitting.

International audience; A simple and readily applicable voltammetric approach is described to characterize and measure thesite-hopping surface diffusion of underpotential-deposited (UPD) metal adatoms at nanoelectrodes. UPD refers tothe deposition of atoms on foreign metal supports at potentials lower than those predicted by Nernst law for a bulkdeposition. Despite its importance in several fields of catalysis, advanced nanofabrication or even atomicnanoengineering by atomic layer epitaxy, diffusion of UPD adatoms is difficult to observe at micro- andmacroelectrodes. In fact, at electrodes of usual dimensions, UPD adatoms surface diffusion is masked by otherelectrochemical phenomena of greater relative amplitudes. Conversely, at nanowires electrodes sealed in glass,only an extremely small fraction of the surface of the wires is exposed to the electrolyte solution and is rapidlyloaded. This allows the spillage of UPD adatoms onto the much larger area of the nanowire rod which is immuneto Faradaic reactions due to its isolation from the electrochemical solution by the glass casing. Therefore,surprisingly for a UPD process, voltammetric peaks currents and integral desorption charges primarily reflect thesediffusional surface processes so that the integral charges vary linearly with the inverse of the square root of thescan rate. This is easily observable and measurable with usual bench-level electrochemical instrumentation. In thiswork, by using nanoelectrodes with diameters between 90 and 260 nm, we were able to establish the majorinvolvement of this site-hopping surface diffusion of UPD Pb adatoms on polycrystalline gold (Au) and characterizeit quantitatively. The equivalent surface diffusion coefficient of UPD Pb adatoms was determined to be ~4.4 × 10-11 cm2 s-1 at room temperature, corresponding to a Gibbs free energy activation barrier of ca. 18.72 kJ mol-1 (i.e.,0.19 eV per Pb adatom) for the reaction of inter-sites exchange of Pb adatoms on a polycrystalline Au surface.

Electrical and mechanical properties of gold‐ para ‐benzenedimethanethiol (BDMT)‐gold molecular junctions with different binding configurations have been investigated using density functional theory (DFT) combined with a non‐equilibrium Green’s function (NEGF) approach. We first determined the most stable structure in total electronic energy by geometry optimization of the molecular junction. We then studied how different stretching processes affect the conductance and the stretching forces. Particularly two stretching modes can bring about different conductance behavior. When only a single‐end molecular contact in interfacial configurations at the top and bridge sites is stretched, the molecular conductance first decreases exponentially. This is followed by a flat platform, and finally produced an abrupt conductance drop from the fracture of interfacial chemical bond Au‐Au. In the junction with a double‐end stretching mode, the fracture of Au‐S bond occurs either between the sulfur‐bonded gold atom and the underlying Au surfaces, or at either of the two Au‐S bonds in the double‐end stretching mode at the top‐pyramidal site. The latter interfacial structure consists of a four‐Au‐atom pyramidal structure adsorbed on gold surface, resulting in a sharp conductance increase just before complete fracture. This is closely associated with resonance tunneling of beta spin electrons in the critical breaking state of interfacial Au‐S bonds in gold‐BDMT‐gold molecular junction.

Layered P2–Na2/3Ni1/3Mn2/3O2 is a promising cathode material. It exhibits a high capacity and suitable operating voltage and undergoes a phase transition from P2 to O2 during charge/discharge. Researchers have used Ti substitution to improve the cathode, yet the chemical principles that underpin elemental substitution and functional improvement remain unclear. To clarify these principles, we used in situ Raman spectroscopy to monitor chemical changes in P2–Na2/3Ni1/3Mn1/3Ti1/3O2 and P2–Na2/3Ni1/3Mn2/3O2 during charge/discharge. Based on the change in the A1g and Eg peaks during charge/discharge, we concluded that Ti substitution compressed the transition metal layer and expanded the planar oxygen layer in the unit cell. Titanium stabilized the P2 phase structure, which improved the cycling stability of P2–NaNMT. Our results provide clear theoretical support for future research on modifying electrodes by elemental substitution.

Elucidating hydrogen oxidation reaction (HOR) mechanisms in alkaline conditions is vital for understanding and improving the efficiency of anion-exchange-membrane fuel cells. However, uncertainty remains around the alkaline HOR mechanism owing to a lack of direct in situ evidence of intermediates. In this study, in situ electrochemical surface-enhanced Raman spectroscopy (SERS) and DFT were used to study HOR processes on PtNi alloy and Pt surfaces, respectively. Spectroscopic evidence indicates that adsorbed hydroxy species (OHad ) were directly involved in HOR processes in alkaline conditions on the PtNi alloy surface. However, OHad species were not observed on the Pt surface during the HOR. We show that Ni doping promoted hydroxy adsorption on the platinum-alloy catalytic surface, improving the HOR activity. DFT calculations also suggest that the free energy was decreased by hydroxy adsorption. Consequently, tuning OH adsorption by designing bifunctional catalysts is an efficient method for promoting HOR activity.

Electrochemical fabrication of functional three-dimensional micro/nanostructures (3D-MNSs) at the wafer scale is important in the semiconductor industry, but it involves complex interfacial reactio...

The electrical double layer (EDL) is the extremely important interfacial region involved in many electrochemical reactions, and it is the subject of significant study in electrochemistry and surface science. However, the direct measurement of interfacial electric fields in the EDL is challenging. In this work, both electrochemical resonant Raman spectroscopy and theoretical calculations were used to study electric field distributions in the EDL of an atomically flat single-crystal Au(111) electrode with self-assembled monolayer molecular films. This was achieved using a series of redox-active molecules containing the 4,4'-bipyridinium moiety as a Raman marker that were located at different precisely controlled distances away from the electrode surface. It was found that the electric field and the dipole moment of the probe molecule both directly affected its Raman signal intensity, which in turn could be used to map the electric field distribution at the interface. Also, by variation of the electrolyte anion concentration, the Raman intensity was found to decrease when the electric field strength increased. Moreover, the distance between adjacent Raman markers was ∼2.1 Å. Thus, angstrom-level spatial resolution in the mapping of electric field distributions at the electrode-electrolyte interface was realized. These results directly evidence the EDL structure, bridging the gap between the theoretical and experimental understandings of the interface.

Solid/liquid interfacial structure occupies great importance in chemistry, biology, and materials. In this paper, by combining EC-SERS study and DFT calculation, we reveal the adsorption and dimerization of sulfite (SO32-) at a gold electrode/water solution interface, and establish an adsorption displacement strategy to suppress the dimerization of sulfite. At the gold electrode/sodium sulfite solution interface, at least two layers of SO32- anions are adsorbed on the electrode surface. As the applied potential shifts negatively, the adsorption strength of the first SO32- layer is weakened gradually and then is dimerized with the second orientated SO32- layer to form S2O52-, and S2O52- is further reduced to S2O32-. After hydroxyethylene disphosphonic acid (HEDP) is introduced to the gold electrode/sodium sulfite solution interface, the second oriented SO32- layer is replaced by a HEDP coadsorption layer. This results in the first layer of SO32- being desorbed directly without any structural transformation or chemical reaction as the potential shifts negatively. The suppression of sulfite dimerization by HEDP is more clear at the gold electrode/gold sulfite solution interface owing to the electroreduction of gold ions. Furthermore, the electrochemical studies and electrodeposition experiments show that as the sulfite dimerization reaction is suppressed, the electroreduction of gold ions is accelerated, and the deposited gold coating is bright and dense with finer grains.

Here, we report tuning the size of contact area through contact force to study the interfacial reactions at the metal/semiconductor/electrolyte (MSE) three-phase interface of electrochemical nanoim...

With the constantly increasing demands of high energy density and long life span energy storage devices, people are looking for high-performance and safe anode materials as alternatives to replace graphite in lithium-ion batteries. A facile coprecipitation method has been developed to synthesize Ag/ZnMn2O4 (Ag/ZMO) hybrids with various weight ratios of Ag as anode materials in lithium-ion batteries. Electrochemical measurements indicate that the addition of Ag nanoparticles not only improves the conductivity of electrode material but also enhances the initial coulombic efficiency and the cycle stability of electrode materials. After 100 cycles at 100 mA g−1, the optimum Ag/ZMO hybrid can still maintain an ultra-high average capacity of 1300 mAh g−1. It is anticipated that our work may open up a new avenue to rapidly and massively produce various anode materials decorated with Ag nanoparticles for lithium-ion batteries.

Investigating the chemical nature of the adsorbed intermediate species on well-defined Cu single crystal substrates is crucial in understanding many electrocatalytic reactions. Herein, we systemati...

Coordination behavior between theophylline (THP) molecule and Au(III) ion as well as electrochemical reduction of THP-Au(III) complexing ion were studied in detail. UV–vis absorption spectroscopy clarifies that Au(III) is coordinated with THP in a 1:2 M ratio. Raman spectroscopy indicates that it is N in THP chemically bonding to Au(III), and density functional theory (DFT) together with time-dependent density functional theory (TD-DFT) further reveal that the specific N atom is N9 in the imidazole ring (N7H form). Based on the invented formula of cyanide-free gold electroplating with THP as a complexant, the electrode behavior of THP-Au(III) ion on the gold electrode was studied. The results show that the stability constant Kf of THP-Au(III) ion is 2.7 × 1035, and the reduction of THP-Au(III) ion is in irreversible with diffusion control.

Solid/liquid interfaces are ubiquitous in nature and knowledge of their atomic-level structure is essential in elucidating many phenomena in chemistry, physics, materials science and Earth science1. In electrochemistry, in particular, the detailed structure of interfacial water, such as the orientation and hydrogen-bonding network in electric double layers under bias potentials, has a significant impact on the electrochemical performances of electrode materials2–4. To elucidate the structures of electric double layers at electrochemical interfaces, we combine in situ Raman spectroscopy and ab initio molecular dynamics and distinguish two structural transitions of interfacial water at electrified Au single-crystal electrode surfaces. Towards negative potentials, the interfacial water molecules evolve from structurally ‘parallel’ to ‘one-H-down’ and then to ‘two-H-down’. Concurrently, the number of hydrogen bonds in the interfacial water also undergoes two transitions. Our findings shed light on the fundamental understanding of electric double layers and electrochemical processes at the interfaces. Interfacial water structures in electric double layers under bias potentials can impact the electrochemical performance of electrodes. Two structural transitions of interfacial water at electrified Au single-crystal electrode surfaces have now been identified.

We propose programed potential modulation strategies to balance the ion intercalation/deintercalation, surface tailoring and bubbling dispersion processes in the electrochemical exfoliation of graphite, resulting in high-quality graphene with high crystallinity, low oxidation degree, uniform size distribution and few layers.

Hydrogen fuel generation using solar light via photoelectrochemical (PEC) methods can help meet growing global energy demands and decrease environmental pollution. The key to efficient PEC hydrogen production is the synthesis of solar light driven photoelectrodes with efficient charge carrier separation. Here, we designed and prepared a ternary Bi2MoO6/Pd/TiO2 photoelectrode composed of Bi2MoO6 nanosheets, Pd nanoparticles (NPs), and TiO2 nanotube arrays (NTAs) on a Ti substrate using electrochemical methods. This novel photoelectrode had good visible light absorbance and significantly improved PEC hydrogen production rates (∼5- and >15-times higher under UV–vis and visible light irradiation, respectively, compared with TiO2NTAs). The interfacial charge transfer mechanism of Bi2MoO6/Pd/TiO2 NTAs was comprehensively studied by comparing its PEC and photoelectrocatalytic performance with that of other TiO2 NTAs (i.e., Pd/TiO2 NTAs, Bi2MoO6/TiO2 NTAs, and Pd/Bi2MoO6/TiO2 NTAs). For Bi2MoO6/Pd/TiO2 NTAs, Pd N...

Developing an understanding of structure–activity relationships and reaction mechanisms of catalytic processes is critical to the successful design of highly efficient catalysts. As a fundamental reaction in fuel cells, elucidation of the oxygen reduction reaction (ORR) mechanism at Pt(hkl) surfaces has remained a significant challenge for researchers. Here, we employ in situ electrochemical surface-enhanced Raman spectroscopy (SERS) and density functional theory (DFT) calculation techniques to examine the ORR process at Pt(hkl) surfaces. Direct spectroscopic evidence for ORR intermediates indicates that, under acidic conditions, the pathway of ORR at Pt(111) occurs through the formation of HO2*, whereas at Pt(110) and Pt(100) it occurs via the generation of OH*. However, we propose that the pathway of the ORR under alkaline conditions at Pt(hkl) surfaces mainly occurs through the formation of O2−. Notably, these results demonstrate that the SERS technique offers an effective and reliable way for real-time investigation of catalytic processes at atomically flat surfaces not normally amenable to study with Raman spectroscopy. This work was supported by the NSFC (21522508, 21427813, 21521004, 21533006, 21621091 and 21775127), “111” Project (B17027), Natural Science Foundation of Guangdong Province (2016A030308012), the Fundamental Research Funds for the Central Universities (20720180037), and the Thousand Youth Talents Plan of China. Support from MINECO and Generalitat Valenciana (Spain), through projects CTQ2016–76221-P (AEI/FEDER, UE) and PROMETEOII/2014/013, respectively, is greatly acknowledged. V.B.-M. acknowledges MINECO for the award of a pre-doctoral grant (BES-2014–068176, project CTQ2013–44803-P).

The adsorption and electrooxidation of CO molecules at well-defined Pt(hkl) single-crystal electrode surfaces is a key step towards addressing catalyst poisoning mechanisms in fuel cells. Herein, we employed in situ electrochemical shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) coupled with theoretical calculation to investigate CO electrooxidation on Pt(hkl) surfaces in acidic solution. We obtained the Raman signal of top- and bridge-site adsorbed CO* molecules on Pt(111) and Pt(100). In contrast, on Pt(110) surfaces only top-site adsorbed CO* was detected during the entire electrooxidation process. Direct spectroscopic evidence for OH* and COOH* species forming on Pt(100) and Pt(111) surfaces was afforded and confirmed subsequently via isotope substitution experiments and DFT calculations. In summary, the formation and adsorption of OH* and COOH* species plays a vital role in expediting the electrooxidation process, which relates with the pre-oxidation peak of CO electrooxidation. This work deepens knowledge of the CO electrooxidation process and provides new perspectives for the design of anti-poisoning and highly effective catalysts.

Electrochemical electron transfer (ET) of transition metal complexes or redox metalloproteins can be catalyzed by more than an order of magnitude by molecular scale metallic nanoparticles (NPs), often rationalized by concentration enhancement of the redox molecules in the interfacial region, but collective electronic AuNP array effects have also been forwarded. Using DFT combined with molecular electrochemical ET theory we explore here whether a single molecular scale Au nanocluster (AuC) between a Au (111) surface and the molecular redox probe ferrocene/ferricinium (Fc/Fc+) can trigger an ET rate increase. Computational challenges limit us to Au n Cs (n up to 147), which are smaller than most electrocatalytic AuCs studied experimentally. AuC-coating thiols are addressed both as adsorption of two S atoms at the structural Au55 bridge sites and as superexchange of variable-size AuCs via a single six-carbon alkanethiyl bridge. Our results are guiding, but enable comparing many AuC surface details (apex, ridge, face, direct vs superexchange ET) with a planar Au(111) surface. The rate-determining electronic transmission coefficients for ET between Fc/Fc+ and AuC are highly sensitive to subtle AuC electronic features. The transmission coefficients mostly compete poorly with direct Fc/Fc+ ET at the Au(111) surface, but Fc/Fc+ 100 face-bound on Au79 and Au147 and ridge bound on Au19 leads to a 2- or 3-fold rate enhancement, in different distance ranges. Single AuCs can thus indeed cause rate enhancement of simple electrochemical ET, but additional, possibly collective AuNC effects, as well as larger clusters and more complete coating layers, also need to be considered.

Knowledge of atomistic structures at solid/liquid interfaces is essential to elucidate interfacial processes in chemistry, physics, and materials sciences. The (√3 × √7) structure associated with a pair of sharp reversible current spikes in the cyclic voltammogram on a Au(111) electrode in sulfuric acid solution represents one of the most classical ordered structures at electrode/electrolyte interfaces. Although more than 10 adsorption configurations have been proposed in the past four decades, the atomistic structure remains ambiguous and is consequently an open problem in electrochemistry and surface science. Herein, by combining high-resolution electrochemical scanning tuning microscopy, electrochemical infrared and Raman spectroscopies, and, in particular, the newly developed quantitative computational method for electrochemical infrared and Raman spectra, we unambiguously reveal that the adstructure is Au(111)(√3 × √7)-(SO4···w2) with a sulfate anion (SO4*) and two structured water molecules (w2*) in a unit cell, and the crisscrossed [w···SO4···w]n and [w···w···]n hydrogen-bonding network comprises the symmetric adstructure. We further elucidate that the electrostatic potential energy dictates the proton affinity of sulfate anions, leading to the potential-tuned structural transformations. Our work enlightens the structural details of the inner Helmholtz plane and thus advances our fundamental understanding of the processes at electrochemical interfaces. The authors acknowledge funding by the National Natural Science Foundation of China (91950121, 21727807, 21403179, 21872115, and 21533006).

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.

In situ monitoring of electrocatalytic processes at solid-liquid interfaces is essential for the fundamental understanding of reaction mechanisms, yet quite challenging. Herein, Pt-on-Au nanocatalysts with a Au-core Pt-satellite superstructure have been fabricated. In such Pt-on-Au nanocatalysts, the Au cores can greatly amplify the Raman signals of the species adsorbed on Pt, allowing the in situ surface-enhanced Raman spectroscopy (SERS) study of the electrocatalytic reactions on Pt. Using the combination of an electrochemical method and in situ SERS, size effects of Pt on the catalytic performance of the core-satellite nanocomposites towards CO and methanol electrooxidation are revealed. It is found that such Pt-on-Au nanocomposites show improved activity and long-term stability for the electrooxidation of CO and methanol with a decrease in the Pt size. This work demonstrates an effective strategy to achieve the in situ monitoring of electrocatalytic processes and to simultaneously boost their catalytic performance towards electrooxidation.

Understanding the reaction mechanism for the catalytic process is essential to the rational design and synthesis of highly efficient catalysts. MoS2 has been reported to be an efficient catalyst toward the electrochemical hydrogen evolution reaction (HER), but it still lacks direct experimental evidence to reveal the mechanism for MoS2-catalyzed electrochemical HER process at the atomic level. In this work, we develop a wet-chemical synthetic method to prepare the single-layer MoS2-coated polyhedral Ag core-shell heterostructure (Ag@MoS2) with tunable sizes as efficient catalysts for the electrochemical HER. The Ag@MoS2 core-shell heterostructures are used as ideal platforms for the real-time surface-enhanced Raman spectroscopy (SERS) study owing to the strong electromagnetic field generated in the plasmonic Ag core. The in situ SERS results provide solid Raman spectroscopic evidence proving the S-H bonding formation on the MoS2 surface during the HER process, suggesting that the S atom of MoS2 is the catalytic active site for the electrochemical HER. It paves the way on the design and synthesis of heterostructures for exploring their catalytic mechanism at atomic level based on the in situ SERS measurement. Ministry of Education (MOE) Accepted version This research was financially supported by MOE under AcRF Tier 1 (Project No. 2017-T1-002-119) and AcRF Tier 2 (Project No. MOE2017-T2-1-162; MOE2016-T2-2-103) in Singapore, NTU’s Start-Up Grant (Project No. M4081296.070.500000), and NSFC (21775127 and 21522508). H.Z. acknowledges the financial sup-port from ITC via the Hong Kong Branch of National Precious Metals Material Engineering Research Center (NPMM), and the grants (Project No. 9380100, 9610478 and 1886921) in the City University of Hong Kong.

Electrocatalytic CO2 reduction to CO emerges as a potential route of utilizing emitted CO2 . Metal-N-C hybrid structures have shown unique activities, however, the active centers and reaction mechanisms remain unclear because of the ambiguity in true atomic structures for the prepared catalysts. Herein, combining density-functional theory calculations and experimental studies, the reaction mechanisms for well-defined metal-N4 sites were explored using metal phthalocyanines as model catalysts. The theoretical calculations reveal that cobalt phthalocyanine exhibits the optimum activity for CO2 reduction to CO because of the moderate *CO binding energy at the Co site, which accommodates the *COOH formation and the *CO desorption. It is further confirmed by experimental studies, where cobalt phthalocyanine delivers the best performance, with a maximal CO Faradaic efficiency reaching 99 %, and maintains stable performance for over 60 hours.

The increase in natural gas reserves makes methane a significant hydrocarbon feedstock. However, the direct catalytic conversion of methane into liquid fuels and useful chemicals remains a great challenge, and many studies have been devoted to this field in the past decades. Electrocatalysis is considered as an important alternative approach for the direct conversion of methane into value-added chemicals, although many other innovative methods have been developed. This review highlights recent advances in electrocatalytic conversion of methane to ethylene and methanol, two important chemicals. The electrocatalytic systems efficient for methane conversions are summarized with an emphasis on catalysts and electrolytes. The effects of reaction conditions such as the temperature and the acid-base property of the reaction medium are also discussed.

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.

Currently, lithium-ion batteries play a key role in energy storage; however, their applications are limited by their low energy density. Here, we design a facile method to prepare mesoporous ZnMn2O4 microspheres with ultrahigh rate performance and ultralong cycling properties by finely tuning the solution viscosity during synthesis. When the current density is raised to 2 A·g−1, the discharge capacity is maintained at 879 mA·h·g−1 after 500 cycles. The electrochemical properties of mesoporous ZnMn2O4 microspheres are better than that for most reported ZnMn2O4. To understand the electrochemical processes on the mesoporous ZnMn2O4 microspheres, in situ Raman spectroscopy is used to investigate the electrode surface. The results show that mesoporous ZnMn2O4 microspheres have a great potential as an alternative to commercial carbon anode materials.

Sodium-ion batteries are currently being considered as a promising concept for large-scale energy storage. This is partly due to the low cost and abundancy of sodium and the possibility to use cheaper parts on cell level such as current collector and electrolyte. As an important electrode in a safe rechargeable sodium-ion battery, the electrochemical activity on the anode involves a series of complex electrochemical processes. Therefore, new strategies for material design are required to promote each pivotal step in the optimization of battery performance. Herein, we present a multiscale design of a mesoporous MoS 2 foam with a coating of graphene layers as a highly efficient anode for sodium-ion batteries. The resulting composite material delivers impressive electrochemical performance in a half-cell versus metallic sodium with a high specific capacity for sodium ions and a stable performance during cycling for more than 1000 cycles. The strategy introduced in this study opens new opportunities, not only for the development of MoS2 composite anode materials through a multiscale design to maintain high capacity and stability, but also for the development of other sodium-ion battery anode materials.

Ionic liquids (ILs) are novel type of electrolytes and have found various applications in electrochemistry. Understanding their electrochemical stability has been one of fundamental and important subject of researches, which would promote further application of ionic liquids in electrochemistry. Electrochemical surface enhanced Raman spectroscopy (EC-SERS) can provide abundant finger print information of surface-bound species for elucidating reaction mechanism, while normal Raman (NR) spectroscopy of thin layer region near electrode surface can provide more information about products without interference of signals from surface adsorption. Here we report combined EC-SERS and NR studies with DFT calculations on the cathodic process of 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide (EMITFSI) on Ag electrodes, aiming to identify reaction products and to reveal reaction mechanisms. It has been found that during the reduction of EMI cation, N-heterocyclic carbenes (NHCs) as an intermediate product were formed first and then underwent dimerization to form double-bond dimer. Single-bond dimer were also generated through single-electron radicals whose lifetime was too short to be detected directly though. In addition, dealkylation may happen, leading to formation of methylimidazole and ethylimidazole. This work not only deepens our insight into the cathodic process of EMI + , but also provides a guideline for electrochemical Raman study to be employed for tracing and understanding the process of electrode reaction.

Two-dimensional (2D) materials such as graphene and hexagonal boron nitride (h-BN) can be used as robust and flexible encapsulation overlayers, which effectively protect metal cores but allow reactions to occur between inner cores and outer shells. Here, we demonstrate this concept by showing that Pt@h-BN core–shell nanocatalysts present enhanced performances in H2/O2 fuel cells. Electrochemical (EC) tests combined with operando EC-Raman characterizations were performed to monitor the reaction process and its intermediates, which confirm that Pt-catalyzed electrocatalytic processes happen under few-layer h-BN covers. The confinement effect of the h-BN shells prevents Pt nanoparticles from aggregating and helps to alleviate the CO poisoning problem. Accordingly, embedding nanocatalysts within ultrathin 2D material shells can be regarded as an effective route to design high-performance electrocatalysts.

Eutectic Gallium-Indium (EGaIn) is a promising technique to fabricate self-assembled monolayers (SAMs)-based molecular devices. However, several abnormal signals had been reported in previous measurements on the as-fabricated devices, like the offset of zero-bias current and others. To date, the limited study has been taken to explore the origin of the offset, which prevents the understanding of electrical transport through large-area molecular tunneling junctions, and hinders the development of high-performance devices. To explore the offset of zero-bias current, we built a new EGaIn automatic instrument platform, which allowed a systematic study on the electrical behavior of SAMs of n-alkanethiolate (n = 14, 16, 18). The measurements showed that the ambient water affected charge transport through as-fabricated molecular junctions. With the presence of water, a primary cell was established at the interface between SAMs and EGaIn/GaOx, which generated an internal potential and caused the offset of zero-bias current when the geometry contact area of the molecular junction is smaller than 600 μm2.

The concentration distribution of hydroxyl radicals ( OH) has a crucial influence on planarization accuracy and efficiency in a photoinduced confined chemical etching system. To elucidate the concentration distribution of OH near the titanium dioxide (TiO2) photoanode, we proposed two in situ strategies with micrometer-scale spatial resolution: the substrate-generation/tip-collection mode of scanning electrochemical microscopy (SECM) and the deposition–etching–stripping method, by which the influence of the scavenger (i.e., glycine) and the apparent thicknesses of confined etching layers were estimated. The developed SECM methodologies provide powerful analytical tools for the screening of photoinduced chemical etching systems as well as further research on confining and etching mechanisms.

Strain effect has been utilized to tune the catalytic properties of metal nanoparticles. However, in-depth understanding of the strain effect on CO2 electroreduction over the core-shell structure catalyst is still unclear. In this work, we report a prominent strain-dependent activity/selectivity in the electroreduction of CO2 over Pd-based core-shell nanoparticles. Tensile and compressive strain are generated in Au@Pd and AuCu@Pd nanoparticles, respectively, due to the different lattice spacing of Au > Pd > AuCu. In CO2 electroreduction, Au@Pd exhibits a maximum Faradaic efficiency for CO production of 89.6% at −0.9 V versus reversible hydrogen electrode, which is 1.2 and 1.7 times of Pd and AuCu@Pd, respectively. Moreover, when using Au@Pd with different thicknesses of Pd shell as electrocatalysts, we also find that the decreased tensile strain with the increasing thickness of Pd shell results in inferior catalytic activity. This may be because tensile strain on Pd-based electrocatalysts enhanced the CO2 adsorption, thus promoting the catalytic performance. This work offers a pathway with the regulation of lattice strain to rationally design effective electrocatalysts towards CO2 electroreduction.

In-situ electrochemical infrared (EC-IR) and Raman (EC-Raman) spectroscopies are powerful tools for characterizing electrochemical interfacial structures. However, the electrochemical interfaces even for model systems such as single-crystal electrode/electrolyte interfaces are usually very complexly related to the applied potential, adsorbates, electrolyte ions and solvent. As a result, the observed EC-IR and -Raman spectra including the potential-dependent vibrational frequencies and spectral intensities are difficult to be unambiguously interpreted for over four decades. By combining vibrational spectra simulations with electrochemical interface models, we developed a computational method capable of quantitatively simulating EC-IR and EC-Raman spectra observed in the classic metal single-crystal electrochemical systems. In this work, we introduce in detail the theoretical derivation and the optimization of computational parameters for the method. The selections of applied potential, vibrational model, as well as electrode surface configuration are further elaborated for the exact simulation of electrochemical infrared and Raman spectra of the ordered CO adstructures at the Pt(1 1 1) electrode/0.1 M perchloric acid solution interface. The developed method can be expanded to simulate the spectral features in different specific adsorption systems for understanding more complicated materials and interface structures.

Electrochemical surface-enhanced Raman spectroscopy (EC-SERS) can provide the fingerprint information at the molecular level during electrochemical processes and the laser used in the EC-SERS process can simultaneously induce the plasmon-mediated chemical reaction (PMCR). In this work, EC-SERS has been used to understand the plasmonic effects on the selective dimerization reduction route of para-halonitrobenzene on the AgNPs@Ag electrode. Para-iodonitrobenzene (pINB) was first chosen as the probe molecule for photoelectrochemical reductions and investigated in acidic and neutral electrolyte solutions. The density functional theory (DFT) calculations confirmed that the electrochemical reduction of pINB produces 2-amino-5-iodophenol in HClO4 and p,p’-diiodoazobenzene (DIAB) in neutral electrolyte. However, when the PMCR is introduced in the electroreduction of pINB, the route of reduction path can be altered to the reduction product DIAB. Thus DIAB was generated firstly both in acidic and neutral conditions by PMCR and then reduced to p-iodoaniline. Finally, all the para chloro- and bromo- nitrobenzene can generate the p,p’-dihalo-azobenzene dimer by the PMCR under acidic conditions. Therefore, the plasmonic photoelectrochemical dimerization and reduction reactions can give us an idea to introduce the light on the selectivity of chemical reactions on plasmonic noble electrodes of nanostructures to alter the reduction pathway. This is practically significant for obtaining highly selective reduction products.