Surface Science Techniques Research Articles

Multimodal spectromicroscopy of monolayer WS2 enabled by ultra-clean van der Waals epitaxy

Van der Waals epitaxy enables the integration of 2D transition metal dichalcogenides with other layered materials to form heterostructures with atomically sharp interfaces. However, the ability to fully utilize and understand these materials using surface science techniques such as angle resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM) requires low defect, large area, epitaxial coverage with ultra-clean interfaces. We have developed a chemical vapor deposition van der Waals epitaxy growth process where the metal and chalcogen sources are separated such that growth times can be extended significantly to yield high coverage while minimizing surface contamination. We demonstrate the growth of high quality 2D WS2 over large areas on graphene. The as-grown vertical heterostructures are exceptionally clean as demonstrated by ARPES, STM and spatially resolved photoluminescence mapping. With these correlated techniques we are able to relate defect density to electronic band structure and, ultimately, optical properties. We find that our synthetic approach provides ultra-clean, low defect density (~1012 cm−2), ~10 μm large WS2 monolayer crystals, with an electronic band structure and valence band effective masses that perfectly match the theoretical prediction for pristine WS2.

Atomic Layer Deposition of ORR Catalysts for Rechargeable Zinc-Air Batteries

The move towards cleaner and renewal energy production, such as solar and wind power, necessitates the development of efficient, safe and cost effective energy storage methods. Zinc-air batteries (ZABs) are prime candidates to meet these requirements and store large amounts of electricity. They use oxygen in the atmosphere as the cathode reactant in the discharge process. Oxygen diffuses into the cell and is reduced to hydroxyl ions in the presence of a catalyst at the air electrode (oxygen reduction reaction or ORR). At the same time, the zinc electrode is oxidized and dissolves in the electrolyte, producing zincate ions. During recharging, zinc metal is reduced from zincate ions and plated at the zinc electrode. Oxygen evolution (OER) occurs at the air electrode. There are challenges in implementing zinc-air batteries for large scale energy storage. The ORR and OER processes at the air electrode are sluggish and electrode stability can be poor because of catalyst delamination and/or substrate corrosion during battery cycling. Effective electrodes for ZABs require a catalyst that is efficient, stable and electrochemically active. Atomic layer deposition (ALD) is a gas phase deposition technique capable of fabricating thin films from a wide variety of materials. ALD utilizes alternating pulses of reactants that each undergo self-limiting reactions on the sample surface, producing uniform and conformal films, with good composition control and sub-nanometer thickness control. These features of ALD are particularly attractive for fabricating ORR catalysts in zinc-air battery electrodes, as they can lead to increased surface areas and three-phase boundary areas, both of which should lead to better performance and cyclability. In this study, ALD is used to produce air electrodes with Mn oxide as the ORR catalyst. The process is optimized and deposits are characterized using microstructural characterization (e.g., electron microscopy, X-ray diffraction, surface science techniques and Raman spectroscopy) and electrochemical characterization (e.g., linear sweep voltammetry, electrochemical impedance spectroscopy and galvanostatic cycling) methods. Successful materials are tested in two different battery configurations, i.e., a two electrode set-up and a three-electrode set-up where the ORR and OER reactions are decoupled. The three-electrode configuration limits the potential window to which each electrode is subjected, greatly improving stability and cyclability.

Voltage dependent STM imaging of inorganic adsorbates

Scanning tunneling microscopy is sensitive to surface adsorbates to a much lower impurity level than most other surface science techniques. Even under the best vacuum and preparation conditions, a very low concentration of depressions of unknown origin is often observed in STM images of the coin metal surfaces. We outline a procedure to identify impurities by apparent height spectroscopy; a technique that can be easily performed by standard scanning tunneling microscopes. Apparent height spectroscopy, performed with a low-temperature scanning tunneling microscope, records the apparent height of an adsorbate with respect to the surface level over an extended voltage range at distinct voltage intervals. The spectra show characteristic features that can be used to identify adsorbates. We exemplify our method for two common impurities on Cu(111), oxygen atoms and carbon monoxide molecules. We reveal three characteristic differences in the apparent height spectroscopy of the two adsorbates: the dark region, the voltage of contrast reversal, and the onset of the lowest unoccupied molecular orbital. Each of these features is characteristic for the specific adsorbate/substrate system; giving three possibilities to identify the two species. The procedure can easily be extended to other impurities.

Laser-Ablated ZnO Nanoparticles and Their Photocatalytic Activity toward Organic Pollutants.

This work aimed to prepare nanostructures of ZnO with various lasers, testing them as photocatalysts, and comparing their morphology and activity in the degradation of organic pollutants in aqueous media. ZnO nanospheres (ns-ZnO) and ZnO nanorods (ms-ZnO) were prepared via the laser ablation of a Zn metal plate in water using nanosecond- and millisecond-pulsed lasers, respectively. The obtained materials were characterized using a set of optical, structural, and surface-science techniques, such as UV-vis spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Under visible-light irradiation, both nanostructures were found to be catalytically active toward the oxidation of methylene blue, which was used as a model compound. The ZnO nanorods fabricated with the millisecond laser showed better photocatalytic performance than their spherically shaped counterparts obtained by means of the nanosecond laser, which could be assigned to a larger number of defects on the ms-ZnO surface.

Recent advances on sputtered films with Cu in ppm concentrations leading to an acceleration of the bacterial inactivation

Abstract This review focuses on the acceleration of the bactericidal and fungicidal effects by uniform, adhesive Cu-based nanocomposites on textile and thin polymer surfaces. The acceleration of the bacterial inactivation kinetics mediated by Cu, TiO2/Cu and ZrO2-TiO2-Cu films in aerobic and anaerobic media is discussed for Gram negative, Gram positive bacteria and fungi. The Cu induced bacterial inactivation kinetics in the dark was observed to occur within the minute range for diverse bacteria. The characterization of the innovative Cu-films microstructure by several surface science techniques is described. This allows the correlation of the surface-reactivity features of the Cu-composite films mediating bacterial inactivation. The redox reactions on the Cu-films during the bacterial inactivation are documented by X-ray-photoelectron spectroscopy (XPS). Attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy was used to monitor the changes in the functional groups leading to bacterial inactivation as a function of the bacterial inactivation time. The dependence of the inactivation kinetics on the Cu-loaded surface roughness is accounted for in qualitative way by atomic force microscopy (AFM). A mechanism for the bacterial inactivation induced by the Cu-nanoparticles by way of reactive oxygen species (ROS) in aerobic and anaerobic processes is discussed. The interfacial charge transfer (IFCT) mechanism under light irradiation between Cu and TiO2 or for the Cu with the binary oxide ZrO2-TiO2 is suggested. The relative potential band-position in the Cu-sputtered semiconductors responsible for the charge transfer is discussed. Evidence was found for the presence in anaerobic media of highly oxidative valence band holes (h+)vb at −0.4 eV generated by Cu2O. The variation in the interface surface potential (Eigenvalues) of the Cu-films during the bacterial inactivation was monitored and allowed to suggest the bacterial mineralization mechanism. Details for the reactions of the Cu-surfaces with Escherichia coli (E. coli) and Methicillin Resistant Staphylococcus Aureus (MRSA) inactivation are described. Evidence for the synergism between Cu and the TiO2 accelerating the bacterial inactivation kinetics is presented in this review.

Resolution along both infrared and visible frequency axes in second-order Fourier-transform vibrational sum-frequency generation spectroscopy

Vibrational sum frequency generation (SFG) spectroscopy is a versatile technique in surface science, which enables the study of physical and chemical properties of interfaces at the molecular scale. This technique reliably measures molecular vibrational resonances. However, utilization of the SFG spectroscopy in its conventional implementation to reveal correlations between vibrational and electronic resonances at interfaces meets substantial practical limitations. The limitations can, in principle, be overcome by using Fourier-transform SFG (FT SFG) spectroscopy and resolving both frequency axes for the second-order nonlinear optical susceptibility. Here, we experimentally demonstrate this approach using two model systems, quartz and a decanoic acid monolayer at the water/air interface. Implementation of the FT SFG spectroscopy requires heterodyne detection and, thus, the technique can provide the phase-resolved response of molecular resonances. To this end, we develop the phasing procedure for this spectroscopy.

Identification of Copper Surface Index by Optical Contrast

AbstractWith the rise of 2D materials, copper (Cu) is revealed as good surface catalyst, especially in the self‐limited growth of graphene. In the regime of surface catalyst, the catalytic activities and functionalities of Cu should be highly dependent on its surface type. Traditional methods to determine the surface index are mainly high‐vacuum based surface science techniques and are typically of low throughput and in small scale. A method to fast detect the surface index of Cu in large scale is still lacking. Here, the authors report an effective optical contrast method to identify the Cu surface index in large area. This method is based on the Cu2O‐thickness dependent color of Cu surface after a mild oxidation in hot air. It is revealed that different Cu surfaces (Cu(111), Cu(100), and Cu(110) as examples) have various oxidation barriers and would exhibit distinct color evolution with heating time. It is also showed that graphene grown on Cu surfaces with varied orientations has totally different growth behaviors. The results would greatly facilitate the high‐throughput determination of Cu surface index and accelerate the large‐scale facet‐dependent catalytic research of Cu, such as in single‐crystal graphene growth.

Molecular Water Adsorption and Reactions on α-Al2O3(0001) and α-Alumina Particles

The adsorption and reaction of water on single crystal α-Al2O3(0001) in ultrahigh vacuum, and α-alumina particles in ambient conditions, were investigated using temperature-programmed desorption (TPD), infrared reflection absorption spectroscopy (IRAS), diffuse reflectance infrared Fourier transform spectra (DRIFTS), and other surface science techniques. For a water coverage of 1 H2O/(surface Al3+) on α-Al2O3(0001), no evidence for the surface hydroxyls expected from dissociative adsorption was observed in the infrared spectra, while the ν2 vibration of molecular water was observed. Electron-stimulated desorption of molecular water at low coverages also indicated molecular or mixed (molecular plus dissociative) adsorption. Analysis of Kr TPD spectra showed that the water films wet the alumina substrate and suggested that the films were initially growing layer-by-layer. In contrast with the single crystal results, DRIFTS of water adsorption on alumina particles indicated the presence of surface hydroxyls t...

Defect Characterization, Imaging, and Control in Wide-Bandgap Semiconductors and Devices

Wide-bandgap semiconductors are now leading the way to new physical phenomena and device applications at nanoscale dimensions. The impact of defects on the electronic properties of these materials increases as their size decreases, motivating new techniques to characterize and begin to control these electronic states. Leading these advances have been the semiconductors ZnO, GaN, and related materials. This paper highlights the importance of native point defects in these semiconductors and describes how a complement of spatially localized surface science and spectroscopy techniques in three dimensions can characterize, image, and begin to control these electronic states at the nanoscale. A combination of characterization techniques including depth-resolved cathodoluminescence spectroscopy, surface photovoltage spectroscopy, and hyperspectral imaging can describe the nature and distribution of defects at interfaces at both bulk and nanoscale surfaces, their metal interfaces, and inside nanostructures themselves. These features as well as temperature and mechanical strain inside wide-bandgap device structures at the nanoscale can be measured even while these devices are operating. These advanced capabilities enable several new directions for describing defects at the nanoscale, showing how they contribute to device degradation, and guiding growth processes to control them.

Structure formation and surface chemistry of ionic liquids on model electrode surfaces-Model studies for the electrode electrolyte interface in Li-ion batteries.

Ionic liquids (ILs) are considered as attractive electrolyte solvents in modern battery concepts such as Li-ion batteries. Here we present a comprehensive review of the results of previous model studies on the interaction of the battery relevant IL 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP]+[TFSI]-) with a series of structurally and chemically well-defined model electrode surfaces, which are increasingly complex and relevant for battery applications [Ag(111), Au(111), Cu(111), pristine and lithiated highly oriented pyrolytic graphite (HOPG), and rutile TiO2(110)]. Combining surface science techniques such as high resolution scanning tunneling microscopy and X-ray photoelectron spectroscopy for characterizing surface structure and chemical composition in deposited (sub-)monolayer adlayers with dispersion corrected density functional theory based calculations, this work aims at a molecular scale understanding of the fundamental processes at the electrode | electrolyte interface, which are crucial for the development of the so-called solid electrolyte interphase (SEI) layer in batteries. Performed under idealized conditions, in an ultrahigh vacuum environment, these model studies provide detailed insights on the structure formation in the adlayer, the substrate-adsorbate and adsorbate-adsorbate interactions responsible for this, and the tendency for chemically induced decomposition of the IL. To mimic the situation in an electrolyte, we also investigated the interaction of adsorbed IL (sub-)monolayers with coadsorbed lithium. Even at 80 K, postdeposited Li is found to react with the IL, leading to decomposition products such as LiF, Li3N, Li2S, LixSOy, and Li2O. In the absence of a [BMP]+[TFSI]- adlayer, it tends to adsorb, dissolve, or intercalate into the substrate (metals, HOPG) or to react with the substrate (TiO2) above a critical temperature, forming LiOx and Ti3+ species in the latter case. Finally, the formation of stable decomposition products was found to sensitively change the equilibrium between surface Li and Li+ intercalated in the bulk, leading to a deintercalation from lithiated HOPG in the presence of an adsorbed IL adlayer at >230 K. Overall, these results provide detailed insights into the surface chemistry at the solid | electrolyte interface and the initial stages of SEI formation at electrode surfaces in the absence of an applied potential, which is essential for the further improvement of future Li-ion batteries.

Application of work function measurements in the study of surface catalyzed reactions on Rh(1 0 0)

The present article aims to show how work function measurements (WF) can be applied in the study of elementary surface reaction steps on metallic single crystal surfaces. The work function itself can in many cases not be interpreted directly, as it lacks direct information on structural and chemical nature of the surface and adsorbates, but it can be a powerful tool when used together with other surface science techniques which provide information on the chemical nature of the adsorbed species. We here, illustrate the usefulness of work function measurements using Rh(100) as our model catalyst. The examples presented include work function measurements during adsorption, surface reaction, and desorption of a variety of molecules relevant for heterogeneous catalysis. Surface coverage of adsorbates, isosteric heat of adsorption, and kinetic parameters for desorption, desorption/decomposition temperatures of surface species, different reaction regimes were determined by WF with the aid of other surface science techniques.

Electrodeposition of Pt-bimetallic Model Systems for Electrocatalysis and Electrochemical Surface Science

In spite of the impressive progress of the fuel cells technology over the last few decades, we still need to address the critical challenge of designing low-cost Pt electrocatalysts with enhanced activity, and improved stability. Exploiting combination of nanoscale Pt structures with another metal, so called Pt-bimetallic nanosystems, has been the most promising route to make significant advances. The electrocatalytic studies on single crystal surfaces combined with the advanced surface science techniques have been essential for our current understanding of Pt reactivity. However, such studies on Pt-bimetallic nanosystems are challenging and not always possible. Utilization of a surface limited redox replacement method (SLRR) in the design of epitaxial Pt films and alloys provides a unique platform to create such model 2D systems. The SLRR approach allows deposition of Pt-nanostructures with the atomic level control of the structure, and coverage which can hardly be matched by any other method. In this article, we give a brief overview of the design of different model Pt-bimetallic systems along with a selection of systematic studies conducted on them that improved our current understanding of the links between the nanostructure and electrocatalytic behavior. Although we provide only an incomplete account of the work undertaken so far, our goal is to motivate the broader electrochemical community to learn more and to inspire new studies and applications.

Effect of CO2 on Surface Film Formation in Magnesium-Aluminum Alloys

Interest in magnesium alloys has increased due to their high strength to weight ratios; however, pure magnesium is highly active and Mg alloys lack corrosion resistance. Alloying additions are employed in the magnesium system to increase corrosion performance, formability and strength. This study focuses on the commercially popular AZ series because of their wide range of applications and ease of accessibility. The AZ series, magnesium-aluminum-zinc alloys, have a complex microstructure comprising second phase particles at grain boundaries formed through solid-state reactions and intermetallic particles formed during solidification processing. A range of alloy compositions have been studied to develop empirical and mechanistic relationships between alloy composition and microstructure; as well as, strength, ductility, and corrosion resistance. Electrochemical, microscopy, and surface science techniques have been used to determine relationships between microstructural composition and corrosion properties of the AZ series. Previous studies have shown discrepancies in the results from atmospheric testing to the accelerated chamber and bulk solution testing.1 Under atmospheric conditions, Mg-alloys corrode at a much slower rate than the bulk solution or chamber test methods.2,3 The lack of available CO2 results in the inability to continuously buffer the bulk solution; therefore, allowing the pH of the bulk solution to reach alkaline conditions similar to the surface/electrolyte interfacial pH. Experiments have been completed to qualitatively measure pH using universal pH indicator and open circuit potential collection to observe any pH change indicative of film formation or corrosion product. Electrochemical data has been collected through electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization to assess the kinetics and any time dependence this protective film may have. Characterization of the surface film was done using in-situ Raman infrared spectroscopy in conjunction with X-ray Photoelectron spectroscopy. Results show that CO2 availability alters the corrosion behavior of the alloy significantly. Surface characterization techniques have revealed the presence of carbonate, CO3 2-, as well as aluminum on the surface. Results indicate a synergistic relationship between Al content and CO2 affecting surface film formation and alloy corrosion rate. Practically, the surface film formation and interaction with the existing atmosphere must be considered in characterizing the protective action of inhibitors and coatings to determine viable methods of protection for this alloy class and other Mg-alloys. This work also sheds light on the inconsistencies between accelerated laboratory testing and atmospheric application conditions. [1] Avedesian, M.M. and H. Baker, ASM Specialty Handbook: Magnesium and Magnesium Alloys, in Magnesium and Magnesium Alloys, M.M. Avedesian and H. Baker, Editors. 1999. [2] Lindström, R., L.G. Johansson, and J.E. Svensson, The Influence of NaCl and CO2 on the Atmospheric Corrosion of Magneisum Alloy AZ91. Materials and Corrosion, 2003(54): p. 7. [3] Lindström, R., J.E. Svensson, and L.G. Johansson, The Influence of Carbon Dioxide on the Atmospheric Corrosion of Some Magnesium Alloys in the Presence of NaCl. Journal of The Electrochemical Society, 2002. 149(4): p. B103.

(Invited) In Situ/Operando Characterization of Electrocatalysts Using Ambient Pressure XPS

What is really happening at an electrochemical interface? This is what I want to reveal. The interface is where the true magic of electrochemistry occurs and yet we know so little about it. The reason for our lack of understanding exists in the challenges that need to be overcome to probe an interface. It’s often buried underneath a gas or liquid phase making it difficult for traditional surface science techniques to be used, while those that can easily work in gas or liquid environments are often probing bulk properties. Many researchers have worked actively to overcome these challenges. Their effort has yielded significant advances to probe the interface under in situ and operando conditions. Ambient pressure X-ray photoelectron spectroscopy (APXPS) is one of the promising techniques developed that is capable of providing this information. This talk will introduce APXPS and how it has been utilized to provide new insight into the solid/gas electrochemical interface for PEM fuel cells. Then I will share the progress our group has made extending this technique into the tender X-ray regime (2 keV to 6 keV) to probe solid/liquid electrochemical interfaces under operando conditions. This will include several studies detailing our investigations to directly probe the electrochemical double layer (EDL), and insight into how platinum and oxide electrodes behave undergoing oxygen evolution reaction. Gaining new insight to guide the design and control of future electrochemical interfaces.

Self-Sterilizing Sputtered Films for Applications in Hospital Facilities.

This review addresses the preparation of antibacterial 2D textile and thin polymer films and 3D surfaces like catheters for applications in hospital and health care facilities. The sputtering of films applying different levels of energy led to the deposition of metal/oxide/composite/films showing differentiated antibacterial kinetics and surface microstructure. The optimization of the film composition in regards to the antibacterial active component was carried out in each case to attain the fastest antibacterial kinetics, since this is essential when designing films avoiding biofilm formation (under light and in the dark). The antimicrobial performance of these sputtered films on Staphylococcus aureus (MRSA) and Escherichia coli (E. coli) were tested. A protecting effect of TiO2 was found for the release of Cu by the TiO2-Cu films compared to films sputtered by Cu only. The Cu-released during bacterial inactivation by TiO2-Cu was observed to be much lower compared to the films sputtered only by Cu. The FeOx-TiO2-PE films induced E. coli inactivation under solar or under visible light with a similar inactivation kinetics, confirming the predominant role of FeOx in these composite films. By up-to-date surface science techniques were used to characterize the surface properties of the sputtered films. A mechanism of bacteria inactivation is suggested for each particular film consistent with the experimental results found and compared with the literature.

Decoupling of graphene from Ni(111) via formation of an interfacial NiO layer

A combination of surface science techniques (STM, XPS, ARPES) and density-functional theory calculations was used to study the decoupling of graphene from Ni(111) by oxygen intercalation. The formation of an antiferromagnetic (AFM) NiO layer at the interface between graphene and the ferromagnetic (FM) Ni is found, where graphene protects the underlying AFM/FM sandwich system. It is found that graphene is fully decoupled in this system and strongly p-doped via charge transfer with a position of the Dirac point of (0.69±0.02) eV above the Fermi level. Our theoretical analysis confirms all experimental findings, addressing also the interface properties between graphene and AFM NiO.

(Invited) Atomic-Scale Surface Science Investigations for Understanding and Producing Electrochemical Interfaces

Key ingredients in solid-state electrochemistry are charge transfer to, and adsorption/reaction of molecules, both heavily influenced by the availability of charge carriers and O vacancies. Surface science techniques, in particular Scanning Tunneling Microscopy (STM) in combination with surface spectroscopy, can help unravel how these processes happen at the atomic scale. The talk will showcase several examples of STM studies of adsorption and reaction of O2 and H2O on titania and selected perovskite materials. It will also exemplify how STM can be used to judiciously steer thin film growth in in Pulsed Laser Deposition (PLD) towards fabricating model electrodes with a desired surface structure, which could help increasing functionality in electrochemical devices.

Electrochemical Promotion of Propene Combustion on Ag Catalytic Coatings

The non-faradaic electrochemical modification of catalytic activity (NEMCA effect) or the electrochemical promotion of catalysis (EPOC) has been investigated thoroughly for more than 120 catalytic reaction systems [1,2]. In electrochemical promotion studies, the conductive catalyst-electrode is in contact with an ionic conductor and the catalyst is electrochemically promoted by applying a current or potential between the catalyst film and a reference electrode. Numerous surface science and electrochemical techniques have shown that EPOC is due to the electrochemically controlled migration of promoting or poisoning ionic species (O2-in case of YSZ) between the ionic conductor and the gas exposed catalytic surface [1]. Propene is one of the major unburnt hydrocarbons containing in diesel cars exhausts. The most effective materials for propene combustion are Platinum-Group Metals. In spite of their high efficiency, these catalysts cannot be considered in the medium-term due to their excessive cost, which makes necessary stages of recovery and recycling [1]. Ag-based catalysts represent a possible alternative. This study reports the electrochemical promotion of the propene combustion on Ag films deposited on 8 mol% Y2O3 stabilized ZrO2 solid electrolyte, an O2-ionic conductor. Nanostructured electrochemical catalysts were prepared by screen-printing and reactive Physical Vapor Deposition (PVD) method. Screen-printing technique is a flexible tool to prepare few µm thick porous films at low cost whereas extremely thin coatings of Ag can be produced by PVD. Thickness and porosity of Ag coatings were modified by changing the deposition parameters (duration and pressure for PVD, nature of the ink and calcination temperature for screen-printing) to optimize the catalytic properties. Catalytic and electrocatalytic tests have been carried out in a quartz reactor [3] which operated under continuous flowing conditions at atmospheric pressure. The catalytic activity was monitored in a temperature range of 100 to 400oC under lean-burn conditions, as encountered in Diesel exhausts. The most active Ag films were also evaluated under closed circuit conditions (± 2V) in order to measure the effect of polarisation between the silver working electrode and an Au reference electrode. Both electrodes were exposed to the same atmosphere in a single chamber configuration. The catalytic activity of samples is depicted in Figure 1. All Ag coatings are effective from around 200°C. The catalytic performances were correlated with the microstructure of the films. Furthermore, the propene combustion was found to be electropromoted on the most active Ag films at low temperature with Faradaic efficiencies larger than 100.

Nanoscopic Ni Interfaced with Oxygen Conductive Supports: Link between Electrochemical and Catalytic Studies

Electrochemical promotion of catalysis (EPOC) or non-faradaic modification of catalytic activity (NEMCA) is a general phenomenon observed at the interface of solid-state electrochemistry and heterogeneous catalysis [1,2]. When the metal or metal oxide catalyst is interfaced with ionically conductive ceramic (solid electrolyte), the application of small electrical stimulus (current or potential) between the catalyst and a counter electrode results in occurrence of two parallel processes: (i) electrochemical reactions at the three phase boundary (tpb) and (ii) spillover/backspillover of ionic species to/from the solid electrolyte to the gas exposed catalyst surface. These ionic species act as promoters for the catalytic reaction and the promoted rate increase is several orders of magnitude higher than one predicted by Faraday’s law. The two most common promoters are Na+ cation from β″-Al2O3 and O2-anion provided from yttiria-stabilized-zirconia (YSZ) electrolyte. The existance of these ionic promoters was confirmed by a variety of surface science and electrochemical techniques [1]. The application of EPOC to nanoscopic, highly dispersed catalysts is of significant interest and represents one of the main challenges for practical application of this phenomenon [2, 3]. The goal of the present work is to investigate EPOC with Pt and Ru nanoparticles (2.5 nm average size) interfaced with YSZ (8% Y2O3-ZrO2) solid electrolyte for ethylene combustion reaction at T= 220 – 375 oC. To this end, the nanoparticles of Pt and Ru were synthesized using polyol method [4] and deposited on YSZ disk. Nanoparticles were subject to detailed electrochemical characterization to find the exchange current (Io) of electrochemical process at tpb and to link it to the apparent faradaic efficiency, |Λ| (eq. 1) and (2) defined in EPOC studies as [1]: |Λ| = Δr/(I/nF) (1) or |Λ| ≈ 2Fro/Io (2) where Δr (mol O/s) is the increase in the catalytic rate divided by the electrochemical rate, ro is the open circuit reaction rate, I/nF (n = 2 for O2-). The process is considered non-Faradaic if |Ʌ|is greater than 1. Figure 1 shows the transient reaction rate response of ethylene combustion over Pt nanoparticles under open circuit (U = 0 V) and under positive potentiostatic polarization (U = 0.5V) at 350 °C. Under closed circuit, the significant and reversible increase in the reaction rate is observed with the rate enhancement ratio r (ratio of the rate under closed circuit over the rate value at the open circuit) reaching 1.79 (i.e. 79 % increase) and the corresponding Faradaic efficiency, Ʌ, value of 23. This indicates that each O2-ion supplied to the Pt catalyst causes on average the catalytic reaction of 23 adsorbed O species originating from the gas phase. The effect of the applied constant potential and current, as well as partial pressure of ethylene, reaction temperature on EPOC effect for both Pt and Ru nanoparticles will be presented. The role of electrified nanoscopic interfaces on the extent of the rate increase will be discussed and rationalized using the exchange current density values [5].

Niobium pentoxide: a promising surface-enhanced Raman scattering active semiconductor substrate

Surface-enhanced Raman scattering technique, as a powerful tool to identify the molecular species, has been severely restricted to the noble metals. The surface-enhanced Raman scattering substrates based on semiconductors would overcome the shortcomings of metal substrates and promote development of surface-enhanced Raman scattering technique in surface science, spectroscopy, and biomedicine studies. However, the detection sensitivity and enhancement effects of semiconductor substrates are suffering from their weak activities. In this work, a semiconductor based on Nb2O5 is reported as a new candidate for highly sensitive surface-enhanced Raman scattering detection of dye molecules. The largest enhancement factor value greater than 107 was observed with the laser excitation at 633 and 780 nm for methylene blue detection. As far as literature review shows, this is in the rank of the highest sensitivity among semiconductor materials; even comparable to the metal nanostructure substrates with “hot spots”. The impressive surface-enhanced Raman scattering activities can be attributed to the chemical enhancement dominated by the photo-induced charge transfer, as well as the electromagnetic enhancement, which have been supported by the density-functional-theory and finite element method calculation results. The chemisorption of dye on Nb2O5 creates a new highest occupied molecular orbital and lowest unoccupied molecular orbital contributed by both fragments in the molecule-Nb2O5 system, which makes the charge transfer more feasible with longer excitation wavelength. In addition, the electromagnetic enhancement mechanism also accounts for two orders of magnitude enhancement in the overall enhancement factor value. This work has revealed Nb2O5 nanoparticles as a new semiconductor surface-enhanced Raman scattering substrate that is able to replace noble metals and shows great potentials applied in the fields of biology related.