Complementary Surface-sensitive Techniques Research Articles

Electrochemical dissolution of PtRu/C: Effect of potential, fuels, and temperature

Over the last decade, in situ or online inductively coupled plasma mass spectrometry (ICP-MS) has been established as a powerful tool for time- and potential-resolved assessment of electrocatalyst dissolution stability. On the other hand, much more accessible for practical realization ex situ ICP-MS studies in three-electrode cells are not commonly carried out. Still, they can offer valuable insights into catalyst degradation during prolonged accelerated stress tests, thus complementing the online measurements. This work presents an example of how both techniques can be used jointly to study the effect of potential, fuels (isopropanol and glycerol), and temperature on PtRu/C dissolution. PtRu/C is chosen for its potential use as an electrocatalyst in fuel cells and electrosynthesis. Ru is a significantly less noble metal, so it is anticipated to dissolve preferentially, resulting in a loss of the electrocatalyst's functionality. Elucidating more light into dissolution behavior and the environment's role can help develop mitigating strategies to minimize such degradation. Our study reveals that potential has a significant effect on the dissolution of both metals. On the contrary, fuels only affect Ru dissolution, with trends varying depending on the fuel. Temperature again affects the stability of both metals, increasing Pt dissolution with temperature. In the case of Ru, however, in certain conditions, metal dissolution decreases at an elevated temperature. Two additional complementary surface-sensitive techniques, i.e., Cu underpotential deposition and transmission electron microscopy, were used to investigate how Pt and Ru dissolution change their surface concentration. The finding opens the possibility of PtRu/C lifetime prolonging at applications where the potential reaches up to 1.0 VRHE.

Self-Terminated Surface Reconstruction of Lanthanum Nickelates Promotes Alkaline Oxygen Evolution

Oxygen evolution reaction (OER) itself is expected to have a strong impact on the state of the surface of the electrocatalyst, yet the actual structure of the surface under operating conditions is rarely considered while formulating structure-activity correlations in electrocatalysis. In this study, changes in the surface structure of Ruddlesden-Popper-type lanthanum nickelate, La2NiO4+δ, and the perovskite LaNiO3 are probed and contrasted using a series of complementary surface sensitive techniques, including XPS, HRTEM and O K-edge XAS. We found noticeable surface changes to occur on La2NiO4+δ after exposure its exposure to the alkaline electrolyte that is accompanied by the emergence of a strong Nin+/n+1 redox feature and the coincides with a 45-fold enhancement of the OER activity (at η = 370 mV) in 0.1 M KOH, as compared to the pristine La2NiO4+δ phase. We observe that the in situ formation of an amorphous nickel (oxy)hydroxide layer on the surface increases the electrochemically-active surface area of La2NiO4+δ and imparts a high stability of the catalyst under OER conditions (the formation of the amorphous layer self-terminates at ca. 3 nm). In addition, the transition from octahedral Ni sites (O h ) to square-planar Ni sites (D 4h ) in the reconstructed surface was observed using O K-edge X-ray absorption spectroscopy. Further, we found no evidence for any surface transformations in the perovskite-based catalyst LaNiO3 that also did not demonstrate any improvement in its activity over cycling. This work highlights the importance of a detailed surface analysis of the catalysts under operation to build robust structure-activity correlations for the metal oxides under electrochemical reaction conditions. Figure 1

Stability of supported hybrid lipid bilayers on chemically and topographically-modified surfaces

Hybrid lipid bilayers are a particular case of supported lipid bilayers with the two monolayer leaflets composed by different types of molecules. These nanostructures can be produced in a well-controlled array fashion and are suitable for the study of biomembrane-related phenomena via electrochemical or plasmonic sensing. Understanding how the underlying solid surface affects the supported membrane formation and organization is necessary for the potential use of these hybrid platforms in applications for which surfaces are not flat and topographically complex. Here we assess the role of lipid phase, substrate surface energy and topography on the formation and stability of hybrid supported membranes from vesicle precursors using complementary surface-sensitive techniques, namely quartz crystal microbalance with dissipation and atomic force microscopy. The stability of hybrid bilayers against thermal and osmotic changes is evaluated and compared to standard supported lipid bilayers formed onto hydrophilic SiO2. Force spectroscopy measurements reveal an overall weaker lateral organization of hybrid membranes as a result of the underlying self-assembled monolayer being not optimally organized. Hybrid bilayers display a decoupled behavior between the two leaflets when vertically compressed at constant speed. On microcontact printed Au surfaces, hybrid bilayers were formed over printed patches, while surprisingly, supported lipid bilayers were observed on non-patterned Au regions suggesting a non-trivial self-assembled monolayer reorganization when in aqueous environment.

Copper adatoms mediated adsorption of benzotriazole on a gold substrate

The adsorption of organic molecules on metal surfaces is often mediated by metal adatoms, however their effect on the geometry of the resulting supramolecular structure can be difficult to determine. Herein, the role played by reactive copper atoms in the adsorption chemistry of benzotriazole (BTAH), an organic corrosion inhibitor for copper well-known for forming adatom mediated supramolecular structures, on an inert substrate, Au(111), has been investigated in an ultra-high vacuum environment using a combination of complementary surface sensitive techniques and density functional theory calculations. Pseudo-ordered and hydrogen bonded flat-lying assemblies convert into ordered -[CuBTA]n- flat-lying organometallic species of well-defined stoichiometry upon addition of copper atoms from the gas phase onto an Au(111) surface, which has been pre-dosed with BTAH. The formation and characterisation of these organometallic species are discussed in the light of the experimental results and computational modelling. The combination of complementary experimental and computational methods is crucial in order to obtain a thorough characterisation of such a complex system. The final supramolecular structure shows previously unseen BTA phases, which are favoured by the adsorption distribution of copper adatoms on Au(111), and are stable up to 500 K.

Physicochemical Properties of a Bi-aromatic Heterocyclic-Azo/BSA Hybrid System at the Air-Water Interface.

The interaction of a heterocyclic azo compound with itself and with bovine serum albumin (BSA) is realized by probing the structural modifications in Langmuir (L) monolayers and Langmuir–Blodgett (LB) films. It was found from the pressure–area/molecule isotherms that the elastic, thermodynamic, and hysteretic properties of the pure azo L monolayer were strongly altered due to the variation of temperature and pH of subphase water. In addition to that, the modification of such properties of the azo L monolayer due to mixing with BSA was also studied. The incorporation of BSA within the azo molecular assembly reduced the elasticity of that assembly. Such reduction of in-plane elasticity of the pure azo monolayer can also be achieved by reducing the temperature and pH of subphase water without adding BSA. A reduction in area per molecule of the azo assembly at the air–water interface associated with the conformational change from horizontal to vertical orientation facilitating π–π interaction was observed with increase in temperature and pH of the subphase. Such parameters also affected the interactions between azo and BSA molecules within the azo/BSA binary system. The structures of pure azo and binary films can be determined after they are transferred to hydrophilic and hydrophobic Si surfaces using the LB technique. Their out-of-plane and in-plane structures, as extracted from two complementary surface sensitive techniques, X-ray reflectivity and atomic force microscopy, were found to be strongly dependent on mixing with BSA, subphase pH, temperature, and substrate nature.

Unusual Behavior of the Bipolar Molecule 25-Hydroxycholesterol at the Air/Water Interface-Langmuir Monolayer Approach Complemented with Theoretical Calculations.

In this study, 25-hydroxycholesterol (25-OH), a biamphiphilic compound with a wide range of biological activities, has been investigated at the air/water interface. We were interested in how two hydroxyl groups attached at distal positions of the 25-OH molecule (namely, at C(3) in the sterane system and at C(25) in the side chain) influence its surface behavior. Apart from traditional Langmuir monolayers, other complementary surface-sensitive techniques, such as electric surface potential measurements, Brewster angle microscopy (BAM, enabling texture visualization and film thickness measurements), and polarization modulation-infrared reflection-absorption spectroscopy (PM-IRRAS), were applied. Experimental data have been interpreted with the aid of theoretical study. Our results show that 25-OH molecules in the monomolecular layer are anchored to the water surface alternatively with C(3) or C(25) hydroxyl groups. Theoretical calculations revealed that the populations of these alternative orientations were not equal and molecules anchored with C(3) hydroxyl groups were found to be in excess. As a consequence of such an arrangement, surface films of 25-OH are of lower stability as compared to cholesterol (considered as a non-oxidized analogue of 25-OH). Moreover, it was found that, upon compression, the transition from mono- to bilayer occurred. The molecular mechanism and interactions stabilizing bilayer structure were proposed. The explanation of the observed unusual surface behavior of 25-OH may contribute to an understanding of differences in biological activity between chain- and ring-oxidized sterols.

A biophysical study of the interactions between the antimicrobial peptide indolicidin and lipid model systems

The naturally occurring peptide indolicidin from bovine neutrophils exhibits strong biological activity against a broad spectrum of microorganisms. This is believed to arise from selective interactions with the negatively charged cytoplasmic lipid membrane found in bacteria. We have investigated the peptide interaction with supported lipid model membranes using a combination of complementary surface sensitive techniques: neutron reflectometry (NR), atomic force microscopy (AFM), and quartz crystal microbalance with dissipation monitoring (QCM-D). The data are compared with small-angle X-ray scattering (SAXS) results obtained with lipid vesicle/peptide solutions. The peptide membrane interaction is shown to be significantly concentration dependent. At low concentrations, the peptide inserts at the outer leaflet in the interface between the headgroup and tail core. Insertion of the peptide results in a slight decrease in the lipid packing order of the bilayer, although not sufficient to cause membrane thinning. By increasing the indolicidin concentration well above the physiologically relevant conditions, a deeper penetration of the peptide into the bilayer and subsequent lipid removal take place, resulting in a slight membrane thinning. The results suggest that indolicidin induces lipid removal and that mixed indolicidin-lipid patches form on top of the supported lipid bilayers. Based on the work presented using model membranes, indolicidin seems to act through the interfacial activity model rather than through the formation of stable pores.

Chitosan-fatty acid interaction mediated growth of Langmuir monolayer and Langmuir-Blodgett films

The interaction of chitosan with bio-membranes, which plays important role in deciding its use in biological applications, is realized by investigating the interaction of chitosan with stearic acid (fatty acid) in Langmuir monolayers (at air–water interface) and Langmuir-Blodgett (LB) films (after transferring it onto solid substrate). It is found from the pressure-area isotherms that the chitosan insertion causes an expansion of chitosan-fatty acid hybrid monolayers, which reduces the elasticity and make the film heterogeneous. It is likely that at low surface pressure chitosan is situated at the interface, interacting with stearic acid molecules via electrostatic and hydrophobic interactions whereas at high pressure chitosan mainly located at subsurface beneath stearic acid molecules. In the latter case the interaction is predominantly electrostatic yielding very small contribution to the surface pressure. The reduction of temperature of the subphase water allows more number of chitosan molecules to reach surface to increase the pressure/interaction. On the other hand, although pure chitosan is found difficult to relocate on the substrate from air-water interface due to its hydrophilic-like nature, it alongside stearic acid (amphiphilic molecules) can be transferred onto substrate using LB technique as evident from infrared spectra. Their out-of-plane and in-plane structures, as extracted from two complementary surface sensitive techniques- X-ray reflectivity and atomic force microscopy, are found strongly dependent on the chitosan mole fraction and the deposition pressure. These analysis of the film-structure will essentially allow one to model the system better and provide better insight into the interaction.

Lipidation Effect on Surface Adsorption and Associated Fibrillation of the Model Protein Insulin.

Lipidation of proteins is used in the pharmaceutical field to increase the therapeutic efficacy of proteins. In this study, we investigate the effect of a 14-carbon fatty acid modification on the adsorption behavior of human insulin to a hydrophobic solid surface and the subsequent fibrillation development under highly acidic conditions and elevated temperature by comparing to the fibrillation of human insulin. At these stressed conditions, the lipid modification accelerates the rate of fibrillation in bulk solution. With the use of several complementary surface-sensitive techniques, including quartz crystal microbalance with dissipation monitoring (QCM-D), atomic force microscopy (AFM), and neutron reflectivity (NR), we show that there are two levels of structurally different protein organization at a hydrophobic surface for both human insulin and the lipidated analogue: a dense protein layer formed within minutes on the surface and a diffuse outer layer of fibrillar structures which took hours to form. The two layers may only be weakly connected, and proteins from both layers are able to desorb from the surface. The lipid modification increases the protein surface coverage and the thickness of both layer organizations. Upon lipidation not only the fibrillation extent but also the morphology of the fibrillar structures changes from fibril clusters on the surface to a more homogeneous network of fibrils covering the entire hydrophobic surface.

Understanding the formation of supported lipid bilayers via vesicle fusion-A case that exemplifies the need for the complementary method approach (Review).

In this review, the authors discuss the challenges of studying supported lipid bilayers (SLBs) deposited by vesicle fusion in terms of (1) evaluating SLB formation and quality using quartz crystal microbalance with dissipation and (2) analyzing the composition and asymmetry of SLBs composed by lipid mixtures using complementary surface sensitive techniques. An overview of the literature is presented and the inconsistencies on this topic are discussed with the objective to expand beyond simple lipid compositions and set the basis for forming and analyzing SLBs of complex natural lipid extracts formed via the vesicle fusion method. The authors conclude by providing some guidelines to successfully form SLBs of complex lipid mixtures including natural extracts.

Formation and Characterization of Supported Lipid Bilayers Composed of Hydrogenated and Deuterated Escherichia coli Lipids.

Supported lipid bilayers are widely used for sensing and deciphering biomolecular interactions with model cell membranes. In this paper, we present a method to form supported lipid bilayers from total lipid extracts of Escherichia coli by vesicle fusion. We show the validity of this method for different types of extracts including those from deuterated biomass using a combination of complementary surface sensitive techniques; quartz crystal microbalance, neutron reflection and atomic force microscopy. We find that the head group composition of the deuterated and the hydrogenated lipid extracts is similar (approximately 75% phosphatidylethanolamine, 13% phosphatidylglycerol and 12% cardiolipin) and that both samples can be used to reconstitute high-coverage supported lipid bilayers with a total thickness of 41 ± 3 Å, common for fluid membranes. The formation of supported lipid bilayers composed of natural extracts of Escherichia coli allow for following biomolecular interactions, thus advancing the field towards bacterial-specific membrane biomimics.

Interaction of the Phage Endolysin PlyC with Model Membranes

Endolysins are bacteriophage-encoded peptidoglycan hydrolases expressed during the late stages of a phage replication cycle that function to lyse the bacterial cell wall, thus enabling progeny phage release. When exogenously added, these enzymes lyse the peptidoglycan of Gram-positive pathogens, resulting in osmotic lysis and cell death. One particular endolysin, PlyC, has a distinctive ability to translocate eukaryotic membranes and retain killing activity in the intracellular environment, making it a particularly interesting target molecule for the development of novel therapeutic approaches. However, the protein-membrane interactions and the cell penetration processes remain unknown. Here, we investigate molecular-scale aspects of such membrane interactions using sparsely-tethered lipid bilayer membranes (stBLMs), a robust planar biomimetic lipid membrane model.Applying complementary surface-sensitive techniques such as surface plasmon resonance, electrochemical impedance and neutron reflectometry, we demonstrate the first steps towards a mechanistic understanding of how the PlyC binding domain, PlyCB, initiates membrane translocation. Our data reveals that while the interaction of PlyCB with purely zwitterionic membranes is negligible, the protein strongly interacts with anionic membranes that contain phosphatidylserine (PS) above a well-defined concentration threshold. In contrast, PlyCB affinity for other anionic lipids tested is low, suggesting specificity for PS rather than non-specific ionic interactions. Furthermore, the PlyCB point mutant R66E that lacks the ability to translocate membranes has likewise lost affinity for PS. With Neutron reflection we identified two distinct PlyCB membrane-association modes. Depending on PS membrane concentration, PlyCB is either peripherally associated or membrane-spanning. Because the outer leaflet of eukaryotic membranes is largely zwitterionic, our findings imply that PlyC induces and/or recognizes PS exposure during cellular uptake. In addition, these results show how lipid membrane surface charge density and composition play a critical role for PlyC internalization.

Direct impact of nonequilibrium aggregates on the structure and morphology of Pdadmac/SDS layers at the air/water interface.

We discuss different nonequilibrium mechanisms by which bulk aggregates directly modify, and can even control, the interfacial structure and morphology of an oppositely charged polyelectrolyte/surfactant (P/S) mixture. Samples are categorized at the air/water interface with respect to the dynamic changes in the bulk phase behavior, the bulk composition, and the sample history using complementary surface-sensitive techniques. First, we show that bulk aggregates can spontaneously interact with the adsorption layer and are retained in it and that this process occurs most readily for positively charged aggregates with an expanded structure. In this case, key nonequilibrium issues of aggregate dissociation and spreading of surface-active material at the interface have a marked influence on the macroscopic interfacial properties. In a second distinct mechanism, aggregates inherently become trapped at the interface during its creation and lateral flocculation occurs. This irreversible process is most pronounced for aggregates with the lowest charge. A third mechanism involves the deposition of aggregates at interfaces due to their transport under gravity. The specificity of this process at an interface depends on its location and is mediated by density effects in the bulk. The prevalence of each mechanism critically depends on a number of different factors, which are outlined systematically here for the first time. This study highlights the sheer complexity by which aggregates can directly impact the interfacial properties of a P/S mixture. Our findings offer scope for understanding seemingly mysterious irreproducible effects which can compromise the performance of formulations in wide-ranging applications from foams to emulsions and lubricants.

Formation of supported lipid bilayers by vesicle fusion: effect of deposition temperature.

We have investigated the effect of deposition temperature on supported lipid bilayer formation via vesicle fusion. By using several complementary surface-sensitive techniques, we demonstrate that despite contradicting literature on the subject, high-quality bilayers can be formed below the main phase-transition temperature of the lipid. We have carefully studied the formation mechanism of supported DPPC bilayers below and above the lipid melting temperature (Tm) by quartz crystal microbalance and atomic force microscopy under continuous flow conditions. We also measured the structure of lipid bilayers formed below or above Tm by neutron reflection and investigated the effect of subsequent cooling to below the Tm. Our results clearly show that a continuous supported bilayer can be formed with high surface coverage below the lipid Tm. We also demonstrate that the high dissipation responses observed during the deposition process by QCM-D correspond to vesicles absorbed on top of a continuous bilayer and not to a surface-supported vesicular layer as previously reported.

From Benzenetrithiolate Self-Assembly to Copper Sulfide Adlayers on Cu(111): Temperature-Induced Irreversible and Reversible Phase Transitions

Self-assembly and thermally activated surface chemistry of 1,3,5-benzenetrithiol (BTT) on Cu(111) are studied under ultrahigh vacuum (UHV) conditions by different complementary surface sensitive techniques. Low-energy electron diffraction (LEED) patterns acquired at room temperature and during subsequent heating reveal irreversible phase transitions between in total four different long-range-ordered phases termed α-phase to δ-phase. X-ray photoelectron spectroscopy (XPS) of the different phases facilitates the identification of major chemical changes for the first phase transition from α- to β-phase, whereas in the succeeding phase transitions, no significant chemical shifts are observed anymore. The structural characterization of each phase is carried out by high-resolution scanning tunneling microscopy (STM), and adsorption geometries of the phenyl rings are derived from C 1s near-edge X-ray absorption fine structure (NEXAFS). The combination of the results from this array of experimental techniques leads to a consistent picture of the various phases and underlying processes. Upon room-temperature deposition, BTT fully deprotonates and planar-adsorbed molecules self-assemble into an ordered monolayer. With a temperature onset of 300 K, the carbon–sulfur bonds start dissociating. Sulfur forms a copper sulfide superstructure, whereas the organic remainders form disordered structures. Further heating converts an initial metastable and rarely observed (√3 × √3)R ± 30° copper sulfide superstructure into the more stable and well-known (√7 × √7)R ± 19.1° polymorph.

Towards microstructured optical fibre sensors: surface analysis of silanised lead silicate glass

While protocols to surface functionalise silica glass platforms are well-established, the surface coating of other glass types has received limited attention. Here we fully characterise the surface attachment of a fluoroionophore on extruded lead silicate glass slides and demonstrate these slides as a model for investigating the surface chemistry in a microstructured optical fibre (MOF). This model system allows the utilization of multiple, complementary surface-sensitive techniques that cannot be used within the internal surface of the fibre structure. By characterising the fluoroionophore attachment, we observe that the fluorescence intensity from fluorescence imaging, the atomic nitrogen percentage measured by X-ray photoelectron spectroscopy (XPS), the carbonyl bond component (287.5 eV) in the XPS high resolution carbon spectrum, and Principal Component Analysis (PCA) of the time-of-flight secondary ion mass spectrometry (ToF-SIMS) data can be used to provide relative quantification of the concentration of an attached fluoroionophore. We also show the first use of ToF-SIMS imaging and depth profiling of the Pb content within a glass substrate to provide information on the coverage provided by the coating and the relative thickness of an organic coating. Combined together, these techniques provide a comprehensive picture of the coated glass surface that facilitates fibre sensor development.

Nitrogen-rich plasma polymers: Comparison of films deposited in atmospheric- and low-pressure plasmas

Low- and atmospheric-pressure plasma co-polymerisations of binary gas mixtures of C 2H 4 and NH 3 or N 2, respectively, were investigated for depositing N-rich plasma polymer coatings for biomedical applications. Deposition kinetics and relevant surface characteristics of resulting plasma polymerised ethylene enriched with nitrogen, “PPE:N”, coatings were investigated as a function of the gas mixture ratio, X = NH 3(N 2)/C 2H 4. Physico-chemical properties of the coatings were determined using a combination of complementary surface-sensitive techniques: for example, total nitrogen concentrations, [N], up to 40 at.% were measured by X-ray Photoelectron Spectroscopy, XPS, while those of primary amines, [–NH 2], were determined by performing chemical derivatisation followed by XPS analyses. PPE:N films were further characterised by UV–VIS Ellipsometry, Near Edge X-ray Absorption Fine Structure Spectroscopy, IR Spectroscopy and by Contact Angle Goniometry measurements. The stability of the coatings was tested, both in terms of water solubility and of “ageing” in ambient atmosphere. For the latter, selected samples were stored in the laboratory for different durations, in order to assess possible changes in their chemical structures.

Formation of 1D-Nanowires and 2D Nanophases in Heteroepitaxy of Sbon High Index Si(5 5 12) Surface

The surface topography of high index Si(5 5 12) presents a single-domain planar reconstruction that is composed of (225) and (337) regions with nanoscale widths and row like trenches and provides an unique template for the growth of nanostructures. In this study, Sb has been adsorbed on this Silicon surface to form various superstructural phases by steering the kinetic parameters and post growth annealing of the surface. Different pathways adopted have shown the formation of stabilized ordered superstructural phases corresponding to various coverages and substrate temperatures and are probed in-situ by complementary surface sensitive techniques such as Low Energy Electron Diffraction (LEED), Auger Electron Spectroscopy (AES), and Electron Energy Loss Spectroscopy (EELS). The growth of Sb at 300 degrees C substrate temperature and annealing the formed system to different temperatures upto 820 degrees C leads to the formation of low dimensional phases having anisotropicity along the (110) direction like atomic wires. The results also demonstrate the pathways in tailoring 1 D and 2D nanostructure formation, on this technologically and scientifically important interface.

Playing Tetris at the nanoscale

The ability to fabricate arrays of functional nanostructures with controlled size, shape and stability, precisely positioned on a substrate of choice, is an ongoing challenge in nanotechnology [1,2]. In this context, the bottom-up approach is emerging as a promising method for the growth and processing of nanostructured materials. This approach is based on the concepts of (organic) self-assembly [3] or (inorganic) selforganization [4] of nanostructures on a suitable substrate [5]. Surface nanotemplates [6], i.e. substrates that are artificially or naturally patterned at the nanoscale, are frequently used to control self-assembly or selforganization processes. Such templates are often able to provide surface cues that may guide the formation of ordered structures, including e.g. tissue regeneration [7]. The 6H-SiC(0001) nanomesh is a particularly interesting template in that it expresses a periodic array of nanopores whose size and spacing can be tailored in the 2–3 nm range [8]. Chen et al. had previously identified this SiC nanomesh as a chemically inert nanoscale template which directs the nucleation and controls the growth of Co clusters during metal vapor deposition, so that a 2D array of Co nanoparticles with relatively uniform spacing and a very narrow size distribution results [8,9]. On page 176 of a recent issue, Chen et al. [10] report the first thorough structural characterization of this 6H-SiC(0001) nanomesh template using complementary surface sensitive techniques. Other recent examples of templates include long-range (e.g. mesoscale) reconstructions [11,12] and step bunching [13], host–guest inclusions formed by metal–organic coordinated networks [14,15] and other types of ‘nanomeshes’ [16]. Despite the widespread use of patterned substrates, the structure and properties of such templates are often poorly understood. Silicon carbide (SiC) is a wide band-gap ( 2.35 eV) semiconductor which has been extensively investigated both for its fundamental interest and for possible applications. This extraordinary IV–IV compound

Hydrophobic recovery of VUV/NH3 modified polyolefin surfaces: Comparison with plasma treatments in nitrogen

Film samples of two very pure polyolefins (low density polyethylene, LDPE and biaxially oriented polypropylene, BOPP) were surface-modified by two different methods, namely vacuum ultraviolet (VUV) irradiation with a Kr resonant lamp in low-pressure NH3 gas, and atmospheric pressure glow discharge (APGD) plasma treatment in pure N2 gas. Samples were then stored in air and the time-dependence of surface properties (the surface energy and chemical composition) was monitored using several complementary surface-sensitive techniques: contact angle goniometry (CAG), X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). We show that the main mechanism responsible for hydrophobic recovery is the motion of polymer chains and chain segments, which governs an apparent “loss” of functional groups, within the first monolayers of the surface (∼1nm). Finally, comparing BOPP samples modified by both techniques, we show that aging can be reduced by crosslinking near the surface, as illustrated by depth-sensing nano-indentation measurements.