Surface Science Techniques Research Articles

Historical Perspectives on Electroplating during the Past 100 Years

The development of electrodeposition practice and the underlying science and engineering methods that emerged during the past century will be traced. Beginning in the late 19th century, many large-scale electrolytic technologies became feasible owing to the invention of the electric generator. These included electrowinning, electrorefining, and electrodeposition, among others. Their early development and commercial use took place before the recognition of many fundamental scientific and engineering principles. As a result, these industries came to be characterized by slow evolutionary change based on past experience and intuitive insight.In 1913, a symposium on electrodeposition was arguably the first to apply systematic academic effort to the art of the plater, and thus promote cooperation between science and technology. The success of these activities led, in 1922, to formation of the Electrodeposition Division. For several subsequent decades, the growth of electrodeposition technology took place while electrochemists developed experimental tools (e.g. polarography), data (e.g., thermodynamic) and theories (e.g. non-ideal electrolytic solutions).By the 1950s there were an enormous number of electrodeposition applications, but the sense was emerging that progress based on empirical experimentation was rapidly coming to a close, and that further significant advances could be made only when the fundamentals of the plating processes are more completely understood. During the 1960s, the invention of new materials revolutionized the electrodeposition industries. In addition, the digital computer came into use for obtaining the current distribution in simple geometries. In addition, refined experimental research methods were developed, iincluding the potentiostatic power supply, rotating disk, “model” experimental systems, and various electroanalytical and surface-science techniques, In the 1970s, the field of electrodeposition technology saw significant new demands arising from changed availability of energy, feedstock, and capital as well as increased attention to waste treatment. These events shattered the empirical traditions of the past, and triggered new interest in ‘modern’ electrodeposition science and engineering built o a foundation of thermodynamics, kinetics, transport phenomena, and current distribution aspects. In the 1980s, the magnetic thin-film storage head, energized the entire microelectronics field of electrodeposition technology. Also, studies with single crystal electrodes and with surface scanning microscopies provided spectacular new capabilities for investigations at time scales, molecular specificity, and spatial resolution that were orders of magnitude superior to those of only a decade earlier.By the 1990s, important advances were made in understanding phenomena associated with defects, additives, solvent effects, nanoscale phenomena, surface films, mechanisms of lattice formation, among others. In addition, mathematical modeling of electrodeposition systems moved down-scale to include both continuum and non-continuum phenomena. During the 2000s, the shift from aluminum to electrodeposited copper for on-chip interconnections represented one of the most important change in materials since the beginning of the semiconductor industry. In the 2010s, the mathematical tools used to explore electrochemical systems expanded beyond the traditional continuum methods to include kinetic Monte Carlo, molecular dynamics, and quantum chemistry.In conclusion, throughout the history of electrodeposition science and technology, several high-level trends may be recognized: Advances often came from outside the electrodeposition field. It is important to read the literature widely, and with enough informed judgment to recognize analogies between seemingly different situations;Many electrodeposition systems have been improved over the course of many years. The literature contains a gold mine of applications worth further study. It is important to recognize when new science or engineering materials and methods can provide fresh insights to improving old, but very important, applications;Over the past century, there have been periods when significant gaps existed between scientific understanding of electrochemical phenomena, and our ability to incorporate it into engineering practice. It is important to identify problems worth solving and to release impedements to introduction of new ideas.Today, the ability to use numerical simulations to achieve precise quantitative understanding at new levels of magnitude, sophistication, and completeness offers a significant challenge. It is therefore important to develop re-usable electrochemical engineering methods, and to align tight integration of discovery science, application design, research prototyping and manufacturing collaboration.

Detection of Lead and Environmental Parameters with Sulfur Film Modification of Screen Print Electrode

Environmental electrochemical sensors are an exciting field with a high volume of possibilities. Screen print electrodes have a tremendous advantage for environmental applications as they are 1) capable of performing laboratory and field measurements 2) allow a variety of surface science techniques and provide 3) an excellent outreach medium for students to learn about a variety of engineering and scientific disciplines. The use of redox-active (faradaic or “duck curve”) and non-redox (non-faradaic or capacitance) elements were used to detect lead and bicarbonate respectively using cyclic voltammetry. Aims of this project include the performance assessment of a screen print electrode with sulfur films treatments to detect these environmental compounds. This research education and outreach project summarizes efforts made to conduct and disseminate environmental electrochemistry to students in the New Brunswick area and Rutgers University. Remote and in-person learning labs were conducted to teach students fundamentals of electrochemistry and empower an underserved population to pursue science and engineering disciplines.

(Invited) Electrocatalyst Development for Efficient CO2 Electrolysis

This talk will discuss recent efforts at the National Energy Technology Laboratory (NETL) to develop efficient electrocatalysts that convert CO2 and water into C1 products, such as carbon monoxide (CO) and formic acid (HCOOH). The first example combines advanced surface science techniques, computational modeling, and electrochemical evaluation to track the size-dependent CO2 reduction vs. HER selectivity of silver nanocatalysts with sub-nanometer resolution. Understanding size-dependent catalytic properties is important to maximize the utilization of costly, state-of-the-art catalyst materials. Experimental efforts synthesized and evaluated Ag nanocatalysts with controlled diameters between 2-6 nm, and we identified ~4 nm Ag particles as the optimum balance of activity, selectivity, and catalyst utilization. Computational modeling techniques, including DFT, transition state theory, and microkinetic modeling identified key catalytically active surface sites that were responsible for HER vs. CO2 reduction, determined the respective active sites’ population as a function of particle size, and then predicted HER vs. CO2 reduction rates and selectivity for Ag nanocatalyst with diameters between 1-10 nm. Our computational results predicted ~4 nm particle provided the highest combination of product selectivity, catalyst utilization, and catalytic activity based on a total metal basis, which was in near perfect agreement with experiential results. These experimental and computational results should guide future catalyst design efforts by placing a lower bound on the size of practical Ag catalysts. A second example will highlight efforts to develop 3D structured and chemically-doped SnO2 catalysts for the conversion of CO2 into formate and formic acid. We coupled novel synthetic techniques with advanced synchrotron characterization, computational modeling, and evaluation MEA-style electrolyzer devices. In the case of 3D structured SnO2 catalysts we found CO2 conversion activity was substantially improved by increasing the electrocatalytically active surface area, while doping SnO2 catalysts with heteroatoms created new active sites that lowered the CO2 conversion overpotential and boosted performance.

A green approach for the preparation of ZnO@C nanocomposite using agave americana plant extract with enhanced photodegradation

The present study demonstrates the crucial role of agave americana extract in enhancing the optical properties of zinc oxide (ZnO) through thermal treatment method. Various analytical and surface science techniques have been used to identify the morphology, crystalline structure, chemical composition, and optical properties, including scanning electron microscopy, x-ray diffraction, high resolution transmission electron microscopy (HRTEM), x-ray spectroscopy (EDS) and UV–visible spectroscopy techniques. The physical studies revealed the transformation of ZnO nanorods into nanosheets upon addition of an optimized amount of agave americana extract, which induced large amount of amorphous carbon deposited onto ZnO nanostructures as confirmed by HRTEM analysis. The use of increasing amount of americana extract has significantly reduced the average crystallite size of ZnO nanostructures. The resultant hybrid system of C@ZnO has produced a significant effect on the ultraviolet light-assisted photodegradation of malachite green (MG) dye. The photocatalyst dose was fixed at 10 mg for each study whereas the amount of agave americana extract and MG dye concentration are varied. The functionality of hybrid system was greatly enhanced when the amount of agave americana extract increased while dye concentration kept at lower level. Ultimately, almost 100% degradation efficiency was achieved via the prepared hybrid material, revealing combined contribution from synergy, stabilization of ZnO due to excess of carbon together with the high charge separation rate. The obtained results suggest that the driving role of agave americana extract for surface modification of photocatalyst can be considered for other nanostructured photocatalysts.

Adsorption, thermal conversion, and catalytic hydrogenation of acrolein on Cu surfaces

The adsorption, thermal chemistry, and catalytic hydrogenation of acrolein on copper model surfaces was characterized by a combination of surface-science techniques, namely, reflection–absorption infrared spectroscopy (RAIRS) and temperature program desorption (TPD), quantum mechanics (DFT) calculations, and catalytic kinetic measurements using a so-called “high-pressure cell”. Adsorption of acrolein on the Cu surface was found to involve both the carbonyl oxygen atom and the CC bond. Thermal activation leads to early dissociation and dehydrogenation to produce acetylene and carbon monoxide below 160 K, and later to the production of ketene (220 K) and propene (260 K). Under vacuum hydrogenation with H2 is not possible, but upon predosing of the surface with atomic H all the possible hydrogenation products, namely, propanal, 1-propanol, and allyl alcohol, were detected in TPD experiments, at 162, 190, and 210 K, respectively. Finally, catalytic hydrogenation under atmospheric pressures is slow and only produces propanal, at turnover frequencies below 0.2 s−1.

Structure and Properties of Ultrathin SiOx Films on Cu(111).

The metal-oxide interface plays a crucial role in catalysis, and it has attracted increasing interest in recent years. Cu/SiO2, as a common copper-based catalyst, has been widely used in industrial catalysis. However, it is still a challenge to clarify the structures of the interface of Cu-SiOx and the effect on catalytic performance. Herein, we prepared ultrathin SiOx films by evaporating Si onto a Cu(111) surface followed by annealing in an O2 atmosphere, which were characterized by various surface science techniques. The results showed that a SiOx film could grow nearly layer-by-layer on the Cu(111) surface in the present condition. Both X-ray photoelectron spectroscopy (XPS) and high-resolution electron energy loss spectroscopy (HREELS) results confirmed the presence of Cu-O-Si and Si-O-Si species. Thermal stability experiments illustrated that the well-ordered silica films were stable under annealing in vacuum. The feature of CO adsorption suggested a CO-Cuδ+ species induced from the Cuδ+-O-Si. Low-energy ion scattering spectroscopy (LEIS) and XPS results demonstrated that some Cu2O appeared on the surface when the 1 ML SiOx/Cu(111) was exposed in O2 at 353 K.

Chemical and Morphological Changes of Graphite Electrodes at Low Cathodic Potentials and Its Relevance to Li-Ion Batteries

Lithium-ion (Li-ion) batteries have become ubiquitous to modern-day electronics and electric vehicles. A detailed understanding regarding the reversible and irreversible nature of Li-ion electrode processes is a prerequisite to rationally delineate the factors affecting device performance and clarify degradation mechanisms. This includes, for instance, the role of electrode cracking and solid solid-electrolyte-interphase (SEI) formation which can contribute toward irreversible capacity loss. In terms of experimental approaches, aside from the high surface area electrodes used in practical systems, one powerful approach is to incorporate the study of well-defined electrodes and surface science techniques.1 Herein, we employ highly oriented pyrolytic graphite (HOPG) as a well-defined model anode in Li-ion batteries and reveal the chemical and morphological changes at low cathodic potentials prior to the potentials of bulk SEI formation.2 To achieve this, we employ the use of X-ray photoelectron spectroscopy (XPS) and ultrahigh vacuum scanning tunneling microscopy (UHV-STM). We utilize a so-called UHV-EC setup to provide a snapshot-like analysis of the electrode surface without the involvement of electrode washing or exposure to air.Upon cathodic polarization at ca. 1.75V (Figure 1), STM imaging shows the onset and occurrence of graphite exfoliation which can originate from the intercalation of solvated Li+ ions. Meanwhile, XPS shows a minimal amount of residual electrolyte along with the observation of LiF indicating the decomposition of the Li salt (LiPF6). The future extensions of our methodology and additional details will be provided at the presentation. Figure 1. (a) Cathodic linear sweep voltammogram in 1.1 M LiPF6 EC/DMC. Following polarization at ca. 1.75 V (b) STM imaging, and (c) XPS F 1s and Li 1s spectra.

Two-Dimensional Ultrathin Silica Films.

Two-dimensional (2D) ultrathin silica films have the potential to reach technological importance in electronics and catalysis. Several well-defined 2D-silica structures have been synthesized so far. The silica bilayer represents a 2D material with SiO2 stoichiometry. It consists of precisely two layers of tetrahedral [SiO4] building blocks, corner connected via oxygen bridges, thus forming a self-saturated silicon dioxide sheet with a thickness of ∼0.5 nm. Inspired by recent successful preparations and characterizations of these 2D-silica model systems, scientists now can forge novel concepts for realistic systems, particularly by atomic-scale studies with the most powerful and advanced surface science techniques and density functional theory calculations. This Review provides a solid introduction to these recent developments, breakthroughs, and implications on ultrathin 2D-silica films, including their atomic/electronic structures, chemical modifications, atom/molecule adsorptions, and catalytic reactivity properties, which can help to stimulate further investigations and understandings of these fundamentally important 2D materials.

The micro-fabrication and performance analysis of non-magnetic heating chip for miniaturized SERF atomic magnetometer

The elevated temperature field is one of the fundamental conditions for the spin-exchange relaxation-free (SERF) regime, which can be accomplished by electric heating chip without introducing stray field. Therefore, a non-magnetic heating chip composed of double identical ring-structure layers with a silicon oxide insulating layer between them is carefully designed and fabricated with the material of nichrome alloy on a borosilicate glass substrate by Micro Electro-Mechanical Systems (MEMS) technology. What’s more, the influence of annealing treatment process on the microstructure of the heating chip is characterized and explicated utilizing modern surface science technique, including four-probe meter, X-ray diffraction (XRD) and atomic force microscope (AFM). Then, the integrated miniature single-beam SERF atomic magnetometer is developed based on the microfabricated heating chip. The temperature stability of heating chip is tested with the value less than ± 10 mK at 120 °C by the proportional integral derivative (PID) control program, and the magnetic current coefficient is determined of only 0.506 nT/mA by the spin resonance response of the SERF atomic magnetometer near the zero field. Finally, the developed SERF atomic magnetometer achieves a high measuring sensitivity of 70 fT/Hz1/2. These experimental results provide a new avenue for the wafer-level microfabrication of the high-performance non-magnetic heating chip. More importantly, the presented work paves the way towards chip-scale atomic magnetometers with high sensitivity and mm-scale spatial resolution in the applications of magnetic source imaging and magnetic anomaly detection.

A high-resolution X-ray photoelectron spectroscopy study on the adsorption and reaction of ethylene on Rh(1 1 1)

The adsorption and thermal evolution of ethylene on Rh(111) were investigated by state-of-the-art surface science techniques. Synchrotron radiation-based high-resolution X-ray photoelectron spectroscopy (XPS) and temperature-programmed XPS allowed for the examination of the adsorption process and the reaction pathway of ethylene on Rh(111). At 140 K, ethylene adsorbs molecularly in a symmetrical geometry. The reaction intermediates formed upon heating were identified by the vibrational fine structure of XPS. At approximately 200 K, ethylidyne emerges, as expected from literature. Above 350 K, we observe the evolution of methylidyne. Simultaneously, decomposition to carbides sets in, which is completed at ∼ 460 K.

An insight of CO 2 hydrogenation to methanol synthesis: Thermodynamics, catalysts, operating parameters, and reaction mechanism

Catalytic hydrogenation of CO2 to methanol is an exciting avenue to curb the rising CO2 emissions and generate renewable energy or value-added products. Methanol synthesis via the thermal catalysis route gets increasing emphasis due to its fast kinetics and flexible combination of active components. In the last decade, many studies on CO2 hydrogenation to methanol have been reported with different kinds of catalysts that have been synthesized and characterized using state-of-the-art surface science tools and techniques. In situ analysis techniques as well as theoretical (eg, density functional theory, Monte Carlo simulations, and Micro-Kinetics modeling) studies have been performed to understand the insights of morphology changes, the interaction of active sites, and the formation of intermediate species under the reaction conditions. In the present review, the advancements on CO2 to methanol via hydrogenation route have been presented taking into consideration different perspectives spanning across thermodynamic aspects, the influence of reaction temperature, pressure, feed composition, space-velocity, and morphologically tuned novel catalyst on CO2 conversion and methanol selectivity. Among the reported catalysts, the Al2O3-supported Cu-Zn catalyst showed better performance with 25% CO2 conversion and 73% methanol selectivity at 170°C and 50 bar pressure. The CeO2-supported Pd-Zn catalyst showed 14% CO2 conversion and 97% methanol selectivity at 220°C under 20 bar pressure. Also, CeO2-nanorods supported Cu-Ni catalyst showed good performance at 260°C and 30 bars, with around 18% CO2 conversion and 73% methanol selectivity. Additionally, the mechanistic insights of the process are emphasized with necessary figures and diagrams. The rate-determining steps for each mechanism are also highlighted with chemical structures for further clarity. This review also summarizes potential catalysts and their optimum operating conditions to achieve maximum CO2 conversion and methanol selectivity. We believe, the review is unique and not found in any article which addressed the above aspects altogether in a compact form.

Using Surface Science Techniques to Investigate the Interaction of Acetonitrile with Dust Grain Analogue Surfaces

Surface science methodologies, such as reflection-absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD), are ideally suited to studying the interaction of molecules with model astrophysical surfaces. Here we describe the use of RAIRS and TPD to investigate the adsorption, interactions and thermal processing of acetonitrile and water containing model ices grown under astrophysical conditions on a graphitic dust grain analogue surface. Experiments show that acetonitrile physisorbs on the graphitic surface at all exposures. At the lowest coverages, repulsions between the molecules lead to a decreasing desorption energy with increasing coverage. Analysis of TPD data gives monolayer desorption energies ranging from 28.8–39.2 kJ mol−1 and an average multilayer desorption energy of 43.8 kJ mol−1. When acetonitrile is adsorbed in the presence of water ice, the desorption energy of monolayer acetonitrile shows evidence of desorption with a wide range of energies. An estimate of the desorption energy of acetonitrile from crystalline ice (CI) shows that it is increased to ~37 kJ mol−1 at the lowest exposures of acetonitrile. Amorphous water ice also traps acetonitrile on the graphite surface past its natural desorption temperature, leading to volcano and co-desorption. RAIRS data show that the C≡N vibration shifts, indicative of an interaction between the acetonitrile and the water ice surface.

Unveiling the Mechanisms Ruling the Efficient Hydrogen Evolution Reaction with Mitrofanovite Pt3Te4.

By means of electrocatalytic tests, surface-science techniques and density functional theory, we unveil the physicochemical mechanisms ruling the electrocatalytic activity of recently discovered mitrofanovite (Pt3Te4) mineral. Mitrofanovite represents a very promising electrocatalyst candidate for energy-related applications, with a reduction of costs by 47% compared to pure Pt and superior robustness to CO poisoning. We show that Pt3Te4 is a weak topological metal with the invariant, exhibiting electrical conductivity (∼4 × 106 S/m) comparable with pure Pt. In hydrogen evolution reaction (HER), the electrode based on bulk Pt3Te4 shows a very small overpotential of 46 mV at 10 mA cm–2 and a Tafel slope of 36–49 mV dec–1 associated with the Volmer–Heyrovsky mechanism. The outstanding ambient stability of Pt3Te4 also provides durability of the electrode and long-term stability of its efficient catalytic performances.

Structural and Chemical Transformations of Zinc Oxide Ultrathin Films on Pd(111) Surfaces.

Structural and chemical transformations of ultrathin oxide films on transition metals lie at the heart of many complex phenomena in heterogeneous catalysis, such as the strong metal-support interaction (SMSI). However, there is limited atomic-scale understanding of these transformations, especially for irreducible oxides such as ZnO. Here, by combining density functional theory calculations and surface science techniques, including scanning tunneling microscopy, X-ray photoelectron spectroscopy, high-resolution electron energy loss spectroscopy, and low-energy electron diffraction, we investigated the interfacial interaction of well-defined ultrathin ZnOxHy films on Pd(111) under varying gas-phase conditions [ultrahigh vacuum (UHV), 5 × 10-7 mbar of O2, and a D2/O2 mixture] to shed light on the SMSI effect of irreducible oxides. Sequential treatment of submonolayer zinc oxide films in a D2/O2 mixture (1:4) at 550 K evoked reversible structural transformations from a bilayer to a monolayer and further to a Pd-Zn near-surface alloy, demonstrating that zinc oxide, as an irreducible oxide, can spread on metal surfaces and show an SMSI-like behavior in the presence of hydrogen. A mixed canonical-grand canonical phase diagram was developed to bridge the gap between UHV conditions and true SMSI environments, revealing that, in addition to surface alloy formation, certain ZnOxHy films with stoichiometries that do not exist in bulk are stabilized by Pd in the presence of hydrogen. Based on the combined theoretical and experimental observations, we propose that SMSI metal nanoparticle encapsulation for irreducible oxide supports such as ZnO involves both surface (hydroxy)oxide and surface alloy formation, depending on the environmental conditions.

(Invited) Probing the Electronic Structure of Oxide Electrocatalysts and the Formation of Reaction Intermediates

Perovskite oxides (ABO3) are highly active for the oxygen evolution reaction (OER). Activity is observed to correlate with changes in bulk electronic structure parameters, such as metal-oxygen covalency and transition metal oxidation state. We employ spectroscopic approaches to consider such descriptors at the surface of perovskite oxides in situ, as well as their implications on the formation of surface adsorbates proposed to act as reaction intermediates during the OER.Many surface science techniques, such as X-ray photoelectron spectroscopy (XPS), collect information from inherently surface-sensitive low-energy processes, requiring operation in ultrahigh vacuum. This constraint is lifted for ambient pressure XPS, which can probe the surface in equilibrium with the gas phase at pressures up to ~a few Torr, or with thin liquid layers using a higher incident photon energy. This presentation will discuss the insights obtained with this technique regarding the electronic structure of oxide electrocatalysts in an oxidizing or humid environment, as well as the reaction intermediates of relevance to electrocatalysis.1 We will then extend the technique to probe electrocatalysts in operando,2 driving current through a thin layer of liquid electrolyte and employing a tender X-ray source.

(Invited) Role of the Electrolyte on Li Cation Electrodeposition and Intercalation

Cation transport through the electrolyte to the anode is one most important steps to understand desolvation and diffusion through the electrolyte phase To help understanding this step, there is a sustained development of sophisticated in situ surface science and electrochemical techniques, and first-principles based computational methods. In this work, we analyze the structure and dynamics of state-of-the-art electrolytes particularly at interfaces of typical intercalation anodes. We use classical reactive molecular dynamics and ab initio molecular dynamics to characterize the interfacial systems, and constrained molecular dynamics to investigate the role of highly structured solvation structures on cation deposition and intercalation.

(Invited) Surfactant Mediated Electrochemical Deposition: Principles and Extensions

Historically speaking, and to the present day, the commercial success of thin films has much to do with the ability of surfactants to yield smooth bright surfaces. The microelectronics industry has provided significant motivation for the development of a more fundamental understanding the role electrolyte additives in electrochemical deposition, one obvious example being the innovations related to superconformal growth for 3-D Cu interconnects. This talk will provide an overview of the combined use of electroanalytical and surface science techniques to gain insights into surfactant mediated growth processes. Going further still, the knowledge garnered from investigations, that were originally focused on understanding Cu electrodeposition for interconnects, have been, and continue to be, extended to other metals and chemical systems with important applications ranging from microsystems to batteries to electrocatalysis.

An In Situ and Real-Time Plasmonic Approach of Seed/Adhesion Layers: Chromium Buffer Effect at the Zinc/Alumina Interface

The effect of additives on metal/oxide interfaces is explored in situ and in real time on evaporated films by a combination of surface science techniques, among which a very flexible optical method shows a unique ability to scrutinize the growth and wetting properties of supported clusters that involve several elements. The study focuses on Cr at the Zn/α-Al2O3(0001) interface at 300 K. A particular interest of the present interface is that Zn does not stick at all on bare alumina. The sticking and morphology of both Cr and Zn films during their growth are analyzed from sub-monolayer to multilayer thicknesses. After an initial oxidation reaction with residual OH groups, shown to be detrimental to Zn adhesion, Cr growth proceeds through the formation of high aspect ratio particles that percolate around an average thickness of 10 Å. With regard to Zn growth on a Cr deposit, two very distinct stages can be distinguished. In the sub-monolayer thickness range, Cr forms a seed layer that drastically increases the Zn sticking coefficient from 0 to nearly 1 due to a diffusion length of physisorbed Zn adatoms before desorption larger than Cr island separation; Zn clusters are anchored on the Cr seeds that they encapsulate, but their wetting behavior is dictated by the interaction with alumina. In a second stage, as soon as the Cr film percolates, it forms an adhesion layer on which Zn grows in a nearly two-dimensional mode. In all cases, Cr films are stable upon annealing. On Cr-covered alumina, the Zn desorption energy is enhanced as compared to bare surfaces, which, in line with atomistic simulations, is assigned to the formation of more favorable Cr–Al2O3 and Cr–Zn than Zn–Al2O3 bonds. Generally speaking, the ability demonstrated herein of small amounts of additives to dramatically increase the adhesion of films is of great practical interest. It shows that noncontinuous and partially oxidized films of additives, closer to realistic cases of application, can strongly enhance the sticking of films. Also, anchoring a functional film by discrete predeposited seeds can keep its properties intact.

Ni-modified Fe3O4(001) surface as a simple model system for understanding the oxygen evolution reaction

Electrochemical water splitting is an environmentally friendly technology to store renewable energy in the form of chemical fuels. Among the earth-abundant first-row transition metal-based catalysts, mixed Ni-Fe oxides have shown promising performance for effective and low-cost catalysis of the oxygen evolution reaction (OER) in alkaline media, but the synergistic roles of Fe and Ni cations in the OER mechanism remain unclear. In this work, we report how addition of Ni changes the reactivity of a model iron oxide catalyst, based on Ni deposited on and incorporated in a magnetite Fe3O4(001) single crystal, using a combination of surface science techniques in ultra-high vacuum such as low energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), low-energy ion scattering (LEIS), and scanning tunneling microscopy (STM), as well as atomic force microscopy (AFM) in air, and electrochemical methods such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in alkaline media. A significant improvement in the OER activity is observed when the top surface presents an iron fraction among the cations in the range of 20-40%, which is in good agreement with what has been observed for powder catalysts. Furthermore, a decrease in the OER overpotential is observed following surface aging in electrolyte for three days. At higher Ni load, AFM shows the growth of a new phase attributed to an (oxy)-hydroxide phase which, according to CV measurements, does not seem to correlate with the surface activity towards OER. EIS suggests that the OER precursor species observed on the clean and Ni-modified surfaces are similar and Fe-centered, but form at lower overpotentials when the surface Fe:Ni ratio is optimized. We propose that the well-defined Fe3O4(001) surface can serve as a model system for understanding the OER mechanism and establishing the structure-reactivity relation on mixed Fe-Ni oxides.

Near-field probing of dielectric screening by hexagonal boron nitride in graphene integrated on silicon photonics

Hexagonal boron nitride (hBN) is one of the most suitable 2D materials for supporting graphene in electronic devices, and it plays a fundamental role in screening out the effect of charge impurities in graphene in contrast to inhomogeneous supports such as silicon dioxide (SiO2). Although many interesting surface science techniques such as scanning tunneling microscopy (STM) revealed dielectric screening by hBN and emergent physical phenomena were observed, STM is only appropriate for graphene electronics. In this paper, we demonstrate the dielectric screening by hBN in graphene integrated on a silicon photonic waveguide from the perspective of a near-field scanning optical microscopy (NSOM) and Raman spectroscopy. We found shifts in the Raman spectra and about three times lower slope decrease in the measured electric near-field amplitude for graphene on hBN relative to that for graphene on SiO2. Based on finite-difference time-domain simulations, we confirm lower electric field slope and scattering rate in graphene on hBN, which implies dielectric screening, in agreement with the NSOM signal. Graphene on hBN integrated on silicon photonics can pave the way for high-performance hybrid graphene photonics.