Surface Restructuring Research Articles

Surface Reconstruction and Reactivity of Platinum–Iron Oxide Nanoparticles

The electrochemical stability of platinum–iron oxide nanoparticles has been studied using X-ray absorption spectroscopy. Samples were monitored for changes in their surface structure and elemental composition after electrochemical cycling in acidic media, which were then correlated with their electronic properties and a thorough analysis of their electrocatalytic activities. In addition to the observation of significant surface restructuring, greater Fe content following electrochemical treatment was found to be associated with higher half-wave potentials and specific current densities (up to 0.965 ± 0.001 VRHE and 1.32 ± 0.03 × 10–5 mA cm–2, respectively). This work highlights the potential to improve the electrocatalytic activity of platinum–iron oxide nanoparticles by controlling the Fe content and platinum–iron bonding, and demonstrates the critical importance of characterizing these nanocatalysts both before and after use.

Nanocatalysis II: In Situ Surface Probes of Nano-Catalysts and Correlative Structure–Reactivity Studies

Model nano-catalysts with monodisperse particle sizes and architectures are essential for a fundamental understanding of surface property dynamics during catalytic reactions. Surface tools and techniques, when conducted under catalytically relevant temperature and pressure conditions, render possible measurements of dynamic surface properties such as oxidation state, composition, coordination, and bonding. Near edge X-ray absorption fine structure (NEXAFS) spectroscopy with purposely built in situ reaction cells and ambient pressure X-ray photoelectron spectroscopy (APXPS) provide (near) surface sensitive and chemical specific information on the oxidation states of metal and oxide (co-)catalysts as well as adsorbent functional elements such C, O and N under reactive gas atmospheres and even liquid environments. Likewise, sum frequency generation (SFG) vibrational spectroscopy with in situ reaction cells helps uncover the bonding geometry and configuration of the topmost surface again under conditions pertinent to catalysis. Furthermore, the local dynamics in the nanoscale and on the single particle level are revealed by environmental transmission electron microscopy (ETEM) and the spectro-microscopy techniques equipped within. A correlative approach, where an array of these in situ tools and techniques were conducted in parallel with catalytic measurements, was employed to gain molecular insight into some of the modern scientific challenges in heterogeneous catalysis. Several case examples of this correlative approach are presented here. The CO oxidation reaction over hybrid nano-catalysts of Pt nanoparticles (NPs) with various mesoporous metal oxides such as Co3O4, MnO2 and CeO2 was explored in relation to bifunctional catalysis and interfacial charge transfer chemistry by using in situ NEXAFS spectroscopy. Likewise, bimetallic CoPt and PtSn nanoparticle catalysts supported on silica were investigated by using a combination of in situ NEXAFS spectroscopy and APXPS. Next, CO2 hydrogenation was carried out over bimetallic CoPt/SiO2 and Co/TiO2 hybrid nano-catalysts. In this case, in situ NEXAFS spectroscopy, APXPS, and ETEM indicated severe, yet reversible, surface restructuring that involved hydrogen atom spillover. Finally, ~2 nm Pt NPs were investigated using in situ SFG to study hydrogenation and hydrogenative isomerization reactions. Specifically, SFG indicated that the hydrogenation of furfural and crotonaldehyde proceed by interfacial hydrogen atom spillover from TiO2, while the hydrogenative isomerization of methylcyclopentane (MCP) proceeds by spillover and surface diffusion of cyclohexene over mesoporous zeolites. These studies unequivocally indicated the presence of a particular reaction channel that involved one way flow of charged (i.e. electrons or protons) or neutral species (i.e. reactants) at a broadly defined interface between metals and oxides. In addition to these case studies, experimental approaches employing capillary flow micro-reactors are discussed in relation toward the goal of short time resolutions that could help isolate such charged or neutral intermediates in the future.

Homogeneous Palladium Nanoparticles Surface Hosts Catalyzed Reduction of the Chromophoric Azo (-N=N-) Group of Dye, Acid Orange 7 by Borohydride in Alkaline Media

In alkaline media, well-characterized gelatin-stabilized palladium (GPd) nanoparticles catalyze the reduction of the azo group containing pollutant dye, Acid Orange 7 (AO7) by sodium borohydride (NaBH4) to 1-amino-2-napthol and sulfanilic acid. Kinetic observations and detailed FTIR studies suggests that the reaction follows Langmuir–Hinshelwood kinetic model, where during the reaction both AO7 and borohydride are adsorbed on the GPd surface. Plots of lnko versus ln[AO7] or ln[NaBH4] show that the order of reaction with respect to AO7 and NaBH4 remains almost same over different molar ratios of [NaBH4]/[AO7]. The catalyzed reaction shows an initial induction period (t0) due to a surface-restructuring process of GPd nanoparticles, and (1/t0) can be defined as the rate of surface restructuring. The activation energy of the catalyzed reaction and energy of the surface-restructuring process of GPd are estimated as 22 ± 3 and 25 ± 7 kJ M−1, respectively.

In situ high-pressure high-temperature scanning tunneling microscopy of a Co(0 0 0 1) Fischer–Tropsch model catalyst

The Fischer–Tropsch synthesis of hydrocarbons from carbon monoxide and hydrogen over cobalt catalysts is expected to become the key method for the future production of liquid fuels. Despite decades of research on the reaction mechanism, the state of the surface of the operating catalyst is still uncertain. Using in situ high-temperature high-pressure scanning tunneling microscopy, we have investigated the Fischer–Tropsch reaction over a Co(0001) single crystal model catalyst in the methanation limit. Atomically resolved images show that the surface does not transform into an oxide or carbide, but remains metallic under reaction conditions. The data are consistent with a mobile layer of reversibly adsorbed particles on the surface. The surface morphology under reaction conditions is unchanged from the surface in ultra-high vacuum. Widespread assumptions about a surface restructuring that seemed to explain the activity of cobalt-based Fischer–Tropsch catalysts are not confirmed.

Nanotexturing and Wettability Ageing of Polypropylene Surfaces Modified by Oxygen Capacitively Coupled Radio Frequency Plasma

Polypropylene (PP) was treated by an oxygen capacitively coupled radio frequency plasma (CCP) under a radio frequency (RF) power of 200 W for exposure time of 1, 5, and 10 min. The ageing process of the plasma-treated PP was studied at an ageing temperature of 90 °C during an ageing time up to 25 h. The formation of the nanotextures with different geometry and aspect ratio and the grafting of large number of oxygen-containing groups were achieved on as-treated PP surfaces under the oxygen CCP treatment for the increased exposure time. The hydrophilicity on the as-treated PP surfaces with the stable nanotextures was rapidly depressed during the ageing process at 90 °C due to the restructuring of chemical composition. The surface restructuring rate was dependent on the aspect ratio and the oxygen-containing groups on the nanotextured PP with increasing exposure time. The hydrophobic over-recovery to high hydrophobicity and superhydrophobicity were observed on the post-aged surfaces with the stable nanofibrils from as-treated hydrophilic surfaces. The superhydrophobicity with the low water adhesion was achieved on the post-aged surfaces preserving the nanofibrils with high aspect ratio and large distance due to the decrease of the oxygen-containing groups after the surface restructuring.

Role of oxygen in Cu(1 1 0) surface restructuring in the vicinity of step edges

Surface steps are typically assumed as a source of adatoms for oxygen-chemisorption induced surface reconstruction, but few microscopic observations have been made in the vicinity of steps on reconstructing surfaces. Using in situ scanning tunneling microscopy, we provide direct evidence that surface steps are the source of Cu adatoms for the Cu(110)(2×1)-O restructuring. Using density functional theory, we show that the role of oxygen is to stabilize Cu adatoms detached from step edges via the barrier-less formation of CuO dimers on terraces. Incorporating this atomic process of capturing Cu adatoms into kinetic Monte Carlo simulations reproduces the experimentally observed (2×1)-O reconstruction.

Restructuring of an Ir(210) electrode surface by potential cycling.

This study addresses the electrochemical surface faceting and restructuring of Ir(210) single crystal electrodes. Cyclic voltammetry measurements and in situ scanning tunnelling microscopy are used to probe structural changes and variations in the electrochemical behaviour after potential cycling of Ir(210) in 0.1 M H2SO4. Faceted structures are obtained electrochemically as a function of time by cycling at a scanrate of 1 V·s−1 between −0.28 and 0.70 V vs SCE, i.e., between the onset of hydrogen evolution and the surface oxidation regime. The electrochemical behaviour in sulfuric acid solution is compared with that of thermally faceted Ir(210), which shows a sharp characteristic voltammetric peak for (311) facets. Structures similar to thermally-induced faceted Ir(210) are obtained electrochemically, which typically correspond to polyoriented facets at nano-pyramids. These structures grow anisotropically in a preferred direction and reach a height of about 5 nm after 4 h of cycling. The structural changes are reflected in variations of the electrocatalytic activity towards carbon monoxide adlayer oxidation.

Increased silver activity as a result of controllable reaction-driven reconstruction for high-index facets

Metal nanoparticles (NPs) have been studied extensively as catalysts due to their high activity. However, the application of metallic nanostructures in catalytic reactions is hampered by their uncontrollable performance due to their reconstruction in reactive environments. Here, controllable restructuring of silver NPs in the reactive environment has been achieved to increase their activity. Surface restructuring of the as-synthesized silver NPs was monitored in situ by UV–vis spectra. Kinetic analysis, transmission electron microscopy (TEM), and molecular simulation were conducted to investigate the mechanism of this process. Results indicate that this reaction-driven restructuring leads to the formation of reactive high-index facets on silver NPs. In the reduction of 4-nitrophenol by sodium borohydride, silver NPs with extremely high catalytic activity (surface-area-normalized rate constant of 13.22s−1m−2L) were obtained via accurately controlled surface restructuring of silver NPs obtained by UV irradiation.

Probing Surface Structure Quality of ZnO Nanorods by Second Harmonic Generation

The surface quality of ZnO nanorod after postannealing was inspected by second harmonic generation (SHG) with the assistance of photoluminescence (PL) spectra, and the X-ray photoelectron (XPS) spectroscopy. The SHG is sensitive to the quality of the surface structure, which involves the elimination of surface defects and restructuring. Oxygen (O)-deficient defects and surface defects were generated during the growth of ZnO nanorods. The PL and XPS analyses indicate that the surface defects on ZnO nanorods were reduced at annealing temperatures above 600 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C. The bulk crystal structure was repaired for higher activation temperatures (> 700 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C) concluded by X-ray diffraction. Results from SHG revealed the relationship between surface restructuring of ZnO nanorods and annealing temperature.

Surface restructuring of lignite by bio-char of Cuminum cyminum – Exploring the prospects in defluoridation followed by fuel applications

Recently, there has been an interest in the areas of developing new carbon materials for fluoride removal applications. The development of new carbon materials is of recent choice which involves the synthesis of hybrid carbon from various sources. In this context, the present contribution is made to focus on the study the restructured surface of lignite using a bio-material called Cuminum cyminum. The restructured lignite (RSL) was synthesized with an improved carbon content of 13% and its BET surface area was found to be 3.12 times greater than lignite (L). The amorphous nature of lignite and RSL was quite explicable from XRD studies. SEM studies exhibited a fibrous and finer surface of lignite which was well restructured into a semi-melt (5μm) surface for RSL. Defluoridation potential of Restructured Lignite (15.8mgg−1) was greater than the lignite (13.8mgg−1) at pH 7.93±0.03. Kinetic and isotherm parameters derived from various models helped in comprehending the nature and dynamics of fluoride sorption. Both the normal and the restructured lignite were found to be consistent with its fluoride uptake of 57% and 60% respectively even after fifth cycle of regeneration. High heating values of 22.01MJkg−1 and 26.90MJkg−1 respectively for lignite and RSL deemed their additional application as fuel materials.

Capturing Surface Processes

A modified surface x-ray diffraction geometry allows dynamic restructuring of surfaces to be studied. [Also see Report by Gustafson et al. ]

Illuminating surface atoms in nanoclusters by differential X-ray absorption spectroscopy.

We use differential extended X-ray absorption fine structure (Δ-EXAFS) to monitor the Ar-induced surface restructuring of silica-supported Pd nanoclusters (1 nm diameter) at 77 K. Δ-EXAFS analysis shows 9 ± 2 nearest-neighbor Pd-Pd bonds expand by 0.104 ± 0.005 Å as a result of Ar adsorption. Atomistic molecular dynamics simulations provide evidence for a model in which Ar drives restructuring of under-coordinated Pd atoms, leading to an increased Pd-Pd bond length of surface Pd atoms with no change in overall nearest-neighbor Pd-Pd coordination number. Based on observations from the atomistic simulations, it is likely that under-coordinated atoms are trapped in metastable states at 77 K and Ar provides the kinetic energy needed to overcome the barrier for surface restructuring. Together, experiment and theory highlight the ability of Δ-EXAFS to probe surface atoms of Pd nanoclusters.

Interfacial molecular restructuring of plasticized polymers in water.

Upon water contact, phthalate-plasticized poly(vinyl chloride) (PVC) surfaces are highly unstable because the plasticizer molecules are not covalently bound to the polymer network. As a result, it is difficult to predict how the surface polymer chains and plasticizers may interact with water without directly probing the plastic/water interface in situ. We successfully studied the molecular surface restructuring of 10 wt% and 25 wt% bis 2-ethylhexyl phthalate (DEHP)-plasticized and pure PVC films (deposited on solid substrates) in situ due to water contact using sum frequency generation (SFG) vibrational spectroscopy. SFG spectral signals from both the top and the bottom of the plastic film were obtained simultaneously, so a thin-film model spectral analysis was applied to separately identify the molecular changes of plastics at the surface and the plastic/substrate interface in water. It was found that in water both the structures of the plastic surface and the buried plastic/substrate interface changed. After removing the samples from the water and exposing them to air again, the surface structures did not completely recover. Further SFG experiments confirmed that small amounts of DEHP were transferred into the water. The leached DEHP molecules could reorder and permanently transfer to new surfaces through water contact. Our studies indicate that small amounts of phthalates can transfer from surface to surface through water contact in an overall scope of minutes. This study yields vital new information on the molecular surface structures of DEHP plasticized PVC in water, and the transfer behaviors and environmental fate of plasticizers in polymers.

Spatiotemporal catalytic dynamics within single nanocatalysts revealed by single-molecule microscopy

This review discusses the latest advances in using single-molecule microscopy of fluorogenic reactions to examine and understand the spatiotemporal catalytic behaviors of single metal nanoparticles of various shapes including pseudospheres, nanorods, and nanoplates. Real-time single-turnover kinetics reveal size-, catalysis-, and metal-dependent temporal activity fluctuations of single pseudospherical nanoparticles (<20 nm in diameter). These temporal catalytic dynamics can be related to nanoparticles' dynamic surface restructuring whose timescales and energetics can be quantified. Single-molecule super-resolution catalysis imaging further enables the direct quantification of catalytic activities at different surface sites (i.e., ends vs. sides, or corner, edge vs. facet regions) on single pseudo 1-D and 2-D nanocrystals, and uncovers linear and radial activity gradients within the same surface facets. These spatial activity patterns within single nanocrystals can be attributed to the inhomogeneous distributions of low-coordination surface sites, including corner, edge, and defect sites, among which the distribution of defect sites is correlated with the nanocrystals' morphology and growth mechanisms. A brief discussion is given on the extension of the single-molecule imaging approach to catalysis that does not involve fluorescent molecules.

A near ambient pressure XPS study of Au oxidation

The surface of a gold foil under ozone oxidation was examined by near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and scanning electron microscopy (SEM). Our in situ observations show that a surface oxide phase is formed during the exposure to ozone; however this phase decomposes under vacuum and even in the presence of ozone at temperatures higher than 300 °C. Assuming that an oxide overlayer completely covers the Au surface, the thickness of the oxide phase was estimated to be between 0.29 and 0.58 nm by energy-dependent XPS depth profiling. The surface oxidation led to structural modifications of the gold surface. These morphological changes do not disappear even under vacuum. In the Au 4f spectra, an additional component at low binding energy (83.3 eV), which appears during/after O3 treatment, is assigned to the presence of low-coordinated atoms which appear on the Au surface as a result of surface restructuring under oxidation. Ex situ SEM images demonstrate that only the region of the sample that was exposed to O3 shows the presence of ridges on the Au surface.

Formation of nanoclusters and nanopillars in nonequilibrium surface growth for catalysis applications: growth by diffusional transport of matter in solution synthesis

Growth of nanostructures and nanopillars is considered in a model of deposition of atoms diffusing from solution to the forming surface structure. Surface restructuring is accounted for, yielding morphologies of interest in catalysis. Kinetic Monte Carlo approach is utilized to explore the emergence of face-centered cubic-symmetry surface features in Pt-type metal nanostructures. Results exemplify evaluation of the fraction of the active sites with desirable properties for catalysis, suggesting optimal growth regimes.

Fundamental issues on practical Fischer–Tropsch catalysts: How surface science can help

The present article highlights the contribution of surface science and molecular modeling to the understanding of Fischer–Tropsch catalysis, in particular related to carbon-induced Co Fischer–Tropsch catalyst deactivation. The role of atomic and graphitic carbon in surface restructuring is discussed. Both forms of surface carbon stabilize surface roughness, while molecular CO promotes mobility of Co surface atoms. In a proposed chain growth mechanism on Co(0001) chain elongation proceeds via alkylidyne+CH. The resulting acetylenic species is hydrogenated to alkylidyne, the route to further growth. (Cyclo-)polymerization of acetylenic species produces (aromatic) forms of polymeric surface carbon, a slow side reaction.

In Situ Probing the Surface Restructuring of Antibiofouling Amphiphilic Polybetaines in Water.

Antibiofouling materials have a wide range of applications in biomedical devices and marine coatings. Due to the amphiphilic nature of proteins and organisms, amphiphilic materials have been designed to resist their unspecific adsorption. Surface restructuring behavior of amphiphilic materials in water is believed to play a key role in the antibiofouling mechanisms. In this work, the surface structures of several amphiphilic polybetaine coatings in water have been probed in situ using sum frequency generation (SFG) vibrational spectroscopy. These are novel polybetaines constructed from functionalized polynorbornenes. The polybetaines with oligo(ethylene glycol) (OEG), octyl (C8), or fluorinated (F13) side chains exhibit different surface restructuring behaviors upon contacting water due to their different surface hydrophobicity. The OEG and C8 chains were present and ordered at the water interface, while the F13 chain withdrew from water. The hydrophilic betaine group extended into the water and formed hydrogen bonds with water molecules. The surface restructuring of these materials detected using SFG can be well correlated to their antibiofouling performance, providing an understanding of their antibiofouling mechanisms.

Synthesis of cobalt nanoparticles on Si (100) by swift heavy ion irradiation

We report the growth and characterization of uniform-sized nanoparticles of cobalt on n-type silicon (100) substrates by swift heavy ion (SHI) irradiation. The Co thin films of 25-nm thicknesses were grown by e-beam evaporation and irradiated with two different types of ions, 45-MeV Li3+ and 100-MeV O7+ ions with fluences ranging from 1 × 1011 to 1 × 1013 ions/cm2. SHI irradiation, with the beam rastered over the area of the film, resulted in the restructuring of the film into a dense array of Co nanostructures. Surface topography studied by atomic force microscopy revealed narrowed size distributions, with particle sizes ranging from 20 to 50 nm, formed through a self-organized process. Ion fluence-dependent changes in crystallinity of the Co nanostructures were determined by glancing angle X-ray diffraction. Rutherford backscattering spectroscopy analysis showed the absence of beam-induced mixing in this system. Surface restructuring and beam-induced crystallization are the dominant effects, with the nanoparticle size and density being dependent on the ion fluence. Results are analyzed in the context of molecular dynamics calculations of electron-lattice energy transfer.

Methanation on mass-selected Ru nanoparticles on a planar SiO2 model support: The importance of under-coordinated sites

Mass-selected Ru nanoparticles were deposited onto planar SiO2 support and their capability for the methanation reaction investigated. The catalytic activity for the methanation reaction at 100mbar under hydrogen rich conditions (1:99 CO/H2 ratio) was measured as a function of particle size. We found that the Turnover Frequency increased with nanoparticle diameter in the range 4–10nm.As the TOF decreases over reaction repetition, we investigated the possible reasons and ruled out carbon deposition as a cause for activity loss. As no particle sintering was observed, we invoked surface restructuring as a possible cause.