Probing the Structure and Orientation of Carboxylic Acid-Terminated Self-Assembled Monolayers.

Self-assembled monolayers (SAMs), such as alkanethiols (AT), are widely used as functional coatings or interfaces between different materials. There is an assumption that the arrangement and alignment of the hydrocarbon chains in films made from carboxyl-terminated alkanethiols are similar to those made from alkanethiols. Here, the structure of the outermost layer and near-surface region of SAMs formed from carboxyl-terminated alkanethiols of various lengths has been analyzed. The chemical composition of the samples was measured using X-ray photoelectron spectroscopy (XPS) and angle-resolved XPS (AR-XPS), allowing the film thickness. Metastable induced photoelectron spectroscopy (MIES) as a surface analytical tool sensitive only for the outermost layer in conjunction with density functional theory (DFT) calculations provided insights into the composition of the topmost layer, showing that it consists mainly of the backbone of the SAM-forming molecules. Through combining AR-XPS concentration depth profiles and the measurement of the composition of the outermost layer, it can be shown that SAMs tend to favor a gauche orientation, enabling interactions between the functional groups.

The interaction of size-selected Ru3 clusters with TiO2: depth-profiling of encapsulated clusters.

Ru is a metal of interest in catalysis. Monodisperse Ru3 clusters as catalytic sites are relevant for the development of catalysts because clusters use significantly lower amounts of precious materials for forming active sites due to the small size of the cluster. However, retaining the mono-dispersity of the cluster size after deposition is a challenge because surface energy could drive both agglomeration and encapsulation of the clusters. In the present work Ru3 clusters are deposited by chemical vapor deposition (CVD) of Ru3(CO)12 and cluster source depositions of bare Ru3 onto radio frequency sputter-deposited TiO2 (RF-TiO2) substrates, TiO2(100), and SiO2. When supported on RF-TiO2, bare Ru3 is encapsulated by a layer of titania substrate material during deposition with a cluster source. Ligated Ru3(CO)12 is also encapsulated by a layer of titania when deposited onto sputter-treated RF-TiO2, but only through heat treatment which is required to remove most of the ligands. The titania overlayer thickness was determined to be 1-2 monolayers for Ru3(CO)12 clusters on RF-TiO2, which is thin enough for catalytic or photocatalytic reactions to potentially occur even without clusters being part of the very outermost layer. The implication for catalysis of the encapsulation of Ru3 into the RF-TiO2 is discussed. Temperature-dependent X-ray photoelectron spectroscopy (XPS), angle-resolved XPS, and temperature-dependent low energy ion scattering (TD-LEIS) are used to probe how the cluster-surface interaction changes due to heat treatment and scanning transmission electron microscopy (STEM) was used to image the depth of the surface from side-on.

Specific Ion Effects at the Vapor-Formamide Interface: A Reverse Hofmeister Series in Ion Concentration Depth Profiles.

Employing neutral impact collision ion scattering spectroscopy (NICISS), we have directly measured the concentration depth profiles (CDPs) of various monovalent ions at the vapor-formamide interface. NICISS provides CDPs of individual ions by measuring the energy loss of neutral helium atoms backscattered from the solution interface. CDPs at the vapor-formamide interface of Cl-, Br-, I-, Na+, K+, and Cs+ are measured and compared to elucidate the interfacial specific ion trends. We report a reverse Hofmeister series in the presence of inorganic ions (anion and cation) at the vapor-formamide interface relative to the water-vapor interface, and the CDPs are found to be independent of the counterion for most ions studied. Thus, ions at the surface of formamide follow a "Hofmeister paradigm" where the counterion does not impact the ion series. These specific ion trends are complemented with surface tension and X-ray absorption near-edge structure (XANES) measurements on formamide electrolyte solutions.

Understanding specific ion effects and the Hofmeister series.

Specific ion effects (SIE), encompassing the Hofmeister Series, have been known for more than 130 years since Hofmeister and Lewith's foundational work. SIEs are ubiquitous and are observed across the medical, biological, chemical and industrial sciences. Nevertheless, no general predictive theory has yet been able to explain ion specificity across these fields; it remains impossible to predict when, how, and to what magnitude, a SIE will be observed. In part, this is due to the complexity of real systems in which ions, counterions, solvents and cosolutes all play varying roles, which give rise to anomalies and reversals in anticipated SIEs. Herein we review the historical explanations for SIE in water and the key ion properties that have been attributed to them. Systems where the Hofmeister series is perturbed or reversed are explored, as is the behaviour of ions at the liquid-vapour interface. We discuss SIEs in mixed electrolytes, nonaqueous solvents, and in highly concentrated electrolyte solutions - exciting frontiers in this field with particular relevance to biological and electrochemical applications. We conclude the perspective by summarising the challenges and opportunities facing this SIE research that highlight potential pathways towards a general predictive theory of SIE.

Modification of the Surface Composition of PTB7-Th: ITIC Blend Using an Additive.

We investigated the effect of adding p-anisaldehyde (AA) solvent to the ink containing poly[[2,60-4,8-di(5-ethylhexylthienyl)benzo[1,2-b:3,3-b]dithiophene][3-fluoro-2[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]](PTB7-Th) and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:20,30-d0]-s-indaceno[1,2-b:5,6-b0]-dithiophene(ITIC) on the morphology of the active layer. The present study focuses on determining the effect of the additive on the compositions at the surface of the PTB7-Th: ITIC composite and its morphology, forming one side of the interface of the blend with the MoOX electrode, and the influence of the structural change on the performance of devices. Studies of device performance show that the addition of the additive AA leads to an improvement in device performance. Upon the addition of AA, the concentration of PTB7-Th at the surface of the bulk heterojunction (BHJ) increases, causing an increase in surface roughness of the surface of the BHJ. This finding contributes to an understanding of the interaction between the donor material and high work function electrode/interface material. The implications for the interface are discussed.

Cyclic Copper Uptake and Release from Natural Seawater-A Fully Sustainable Antifouling Technique to Prevent Marine Growth.

Unwanted growth of fouling organisms on underwater surfaces is an omnipresent challenge for the marine industry, costing billions of dollars every year in the transportation sector alone. Copper, the most widely used biocide in antifouling paints, is at the brink of a total ban in being used in antifouling coatings, as it has become an existential threat to nontargeted species due to anthropogenic copper inputs into protected waters. In the current study, using a porous and cross-linked poly(ethylene imine) structure under marine and fouling environments, available copper from natural seawater was absorbed and electrochemically released back as a potent biocide at 1.3 V vs Ag|AgCl, reducing marine growth by 94% compared to the control electrode (coupon) at 0 V. The coating can also function as an electrochemical copper sensor enabling real-time monitoring of the electrochemical uptake and release of copper ions from natural seawater. This allows tailoring of the electrochemical program to the changing marine environments, i.e., when the vessels move from high-copper-contaminated waters to coastal regions with low concentrations of copper.

Influence of TiO2 surface defects on the adsorption of N719 dye molecules.

Surface defects influence the dye adsorption on TiO2 used as a substrate in dye-sensitized solar cells (DSSCs). In this study, we have used different Ar+ sputtering doses to create a controlled density of defects on a TiO2 surface exposed to different pre-heating temperatures in order to analyse the influence of defects on the N719 dye adsorption. TiO2 was pre-treated using two different treatments. The first treatment involved heating to 200 °C with subsequent sputtering at different doses. The second treatment included heating only, but at four different temperatures starting at 200 °C. After the pre-treatments, the TiO2 samples were immersed into an N719 dye solution for 24 hours at room temperature to dye the TiO2 substrates. The amount of Ti3+ surface defects introduced by the different pre-treatments and their influence on dye adsorption onto the TiO2 surface were examined by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and metastable induced electron spectroscopy (MIES). Neutral impact collision ion scattering spectroscopy (NICISS) was used to determine the coverage of the TiO2 surface by adsorbed dye molecules. It was found that Ti3+ surface defects were formed by Ar+ sputtering but not by pre-treatment through heating alone. MIES analysis of the outer-most layer and density of states calculations show that the thiocyanate ligand of the N719 dye becomes directed away from the TiO2 surface. Both XPS and NICISS results indicate that the amount of adsorbed N719 dye decreases with increasing density of Ti3+ surface defects. Thus, the generation of surface defects reduces the ability of the TiO2 surface to adsorb the dye molecules. Heating alone as pre-treatment of the TiO2 substrates instead increases the dye adsorption, without causing detectable defects on the TiO2 surface.

Cr2O3 layer inhibits agglomeration of phosphine-protected Au9 clusters on TiO2 films.

The properties of semiconductor surfaces can be modified by the deposition of metal clusters consisting of a few atoms. The properties of metal clusters and of cluster-modified surfaces depend on the number of atoms forming the clusters. Deposition of clusters with a monodisperse size distribution thus allows tailoring of the surface properties for technical applications. However, it is a challenge to retain the size of the clusters after their deposition due to the tendency of the clusters to agglomerate. The agglomeration can be inhibited by covering the metal cluster modified surface with a thin metal oxide overlayer. In the present work, phosphine-protected Au clusters, Au9(PPh3)8(NO3)3, were deposited onto RF-sputter deposited TiO2 films and subsequently covered with a Cr2O3 film only a few monolayers thick. The samples were then heated to 200 °C to remove the phosphine ligands, which is a lower temperature than that required to remove thiolate ligands from Au clusters. It was found that the Cr2O3 covering layer inhibited cluster agglomeration at an Au cluster coverage of 0.6% of a monolayer. When no protecting Cr2O3 layer was present, the clusters were found to agglomerate to a large degree on the TiO2 surface.

Creation of High-Performance Heterogeneous Photocatalysts by Controlling Ligand Desorption and Particle Size of Gold Nanocluster.

Recently, the creation of new heterogeneous catalysts using the unique electronic/geometric structures of small metal nanoclusters (NCs) has received considerable attention. However, to achieve this, it is extremely important to establish methods to remove the ligands from ligand-protected metal NCs while preventing the aggregation of metal NCs. In this study, the ligand-desorption process during calcination was followed for metal-oxide-supported 2-phenylethanethiolate-protected gold (Au) 25-atom metal NCs using five experimental techniques. The results clearly demonstrate that the ligand-desorption process consists of ligand dissociation on the surface of the metal NCs, adsorption of the generated compounds on the support and desorption of the compounds from the support, and the temperatures at which these processes occurred were elucidated. Based on the obtained knowledge, we established a method to form a metal-oxide layer on the surface of Au NCs while preventing their aggregation, thereby succeeding in creating a water-splitting photocatalyst with high activity and stability.

Competing Segregation of Br- and Cl- to a Surface Coated with a Cationic Surfactant: Direct Measurements of Ion and Solvent Depth Profiles.

Ion-surface scattering experiments can be used to measure elemental depth profiles on the angstrom scale in complex liquid mixtures. We employ NICISS (neutral impact collision ion scattering spectroscopy) to measure depth profiles of dissolved ions and solvent in liquid glycerol containing the cationic surfactant tetrahexylammonium bromide (THA+/Br-) at 0.013 M and mixtures of NaBr + NaCl at 0.4 M total concentration. The experiments reveal that Br- outcompetes Cl- in its attraction to surface THA+, and that THA+ segregates more extensively when more Br- ions are present. Intriguingly, the depths spanned by THA+, Br-, and Cl- ions generally increase with Br- bulk concentration, expanding from ∼10 to ∼25 Šfor both Br- and Cl- depth profiles. This broadening likely occurs because of an increasing pileup of THA+ ions in a multilayer region that spreads the halide ions over a wider depth. The experiments indicate that cationic surfactants enhance Br- and Cl- concentrations in the surface region far beyond their bulk-phase values, making solutions coated with these surfactants potentially more reactive toward gases that can oxidize the halide ions.

Investigating the Structure of Self-Assembled Monolayers Related to Biological Cell Membranes.

Tethered bilayer lipid membranes are solid supported lipid membranes, where the inner leaflet is covalently linked to the solid supported substrate through anchorlipids. These anchorlipids form a self-assembled monolayer, which serves as the basis of the membrane and also provides submembrane space. The molecular structure and composition of this monolayer has thus significant influence on the membrane structural and functional properties. The density of the self-assembled monolayer can be tailored by adding small molecules to the monolayer. Here, the structure of fully tethered and sparsely tethered monolayers, where the anchorlipid has been diluted with a small surface-active thiol, has been analyzed using neutral impact collision ion scattering spectroscopy, X-ray photoelectron spectroscopy, and metastable induced electron spectroscopy. Combination of these three techniques allowed description of the self-assembly process in detail. The monolayers have been characterized in terms of layer thickness and orientation of the lipids.

The effect of TiCl4 treatment on the performance of dye-sensitized solar cells.

Titanium dioxide (TiO2) is used to form the anode in dye sensitized solar cells (DSSCs) and is thus one of the crucial components of this type of solar cell. A large range of treatments of the titania anode before deposition of the dye have been applied to improve the performance of DSSCs among which the TiCl4 treatment is the most frequently used treatment. The present study investigates how the TiCl4 treatment of the TiO2 changes the anode. For this purpose, different sample sets, nontreated, post-treated, and pre-post treated cells, were fabricated and analyzed with electron and ion scattering spectroscopy to investigate how they change the titania surface and consequently influence the cell performance. After the TiCl4 treatment, the short circuit current density (Jsc), open-circuit voltage (Voc), and efficiency significantly improved. An upward shift in the conduction band of the semiconductor as an effect of TiCl4 treatment was measured using inverse photoemission spectroscopy and UV-photoelectron spectroscopy. An increase in dye adsorption due to the treatment was found through neutral impact collision ion scattering spectroscopy measurement. The change in the energy level alignment of the titania and the adsorbed dye and the increase in dye loading explain the threefold increase of power conversion of the cells.

Versatile PbS Quantum Dot Ligand Exchange Systems in the Presence of Pb-Thiolates.

A robust solution phase ligand exchange system for lead sulfide (PbS) quantum dots (QDs) in the presence of Pb-thiolate ligands is presented that can better preserve the excitonic absorption and emission features as compared to the conventional ligands. The photoluminescence after ligand exchange of PbS QDs with Pb-thiolate ligand is preserved up to 78% of the original oleate capped PbS QDs.

Atomically resolved structure of ligand-protected Au9 clusters on TiO2 nanosheets using aberration-corrected STEM.

Triphenylphosphine ligand-protected Au9 clusters deposited onto titania nanosheets show three different atomic configurations as observed by scanning transmission electron microscopy. The configurations observed are a 3-dimensional structure, corresponding to the previously proposed Au9 core of the clusters, and two pseudo-2-dimensional (pseudo-2D) structures, newly found by this work. With the help of density functional theory (DFT) calculations, the observed pseudo-2D structures are attributed to the low energy, de-ligated structures formed through interaction with the substrate. The combination of scanning transmission electron microscopy with DFT calculations thus allows identifying whether or not the deposited Au9 clusters have been de-ligated in the deposition process.

Change of surface structure upon foam film formation.

The charge distribution and coverage with surfactant molecules at foam film surfaces plays an important role in determining foam film structure and stability. This work uses the concentration depth profiling technique neutral impact collision ion scattering spectroscopy to experimentally observe the charge distribution in a foam film for the first time. The charge distribution at the surface of a foam film and the surface of the corresponding bulk liquid were measured for a cationic surfactant solution and the surface excess as well as the electric potential were determined. Describing the internal pressure of foam films by using the electrochemical potential is introduced as a new concept. The foam film can be seen to have a more negative surface charge compared to the bulk liquid surface due to re-arranging of the surfactant molecules. It is discussed how the change in surface excess and electric potential change the electrochemical potential and the stability of the foam film.

Planar silver nanowire, carbon nanotube and PEDOT:PSS nanocomposite transparent electrodes.

Highly conductive, transparent and flexible planar electrodes were fabricated using interwoven silver nanowires and single-walled carbon nanotubes (AgNW:SWCNT) in a PEDOT:PSS matrix via an epoxy transfer method from a silicon template. The planar electrodes achieved a sheet resistance of 6.6 ± 0.0 Ω/□ and an average transmission of 86% between 400 and 800 nm. A high figure of merit of 367 Ω-1 is reported for the electrodes, which is much higher than that measured for indium tin oxide and reported for other AgNW composites. The AgNW:SWCNT:PEDOT:PSS electrode was used to fabricate low temperature (annealing free) devices demonstrating their potential to function with a range of organic semiconducting polymer:fullerene bulk heterojunction blend systems.

Identification of the vibrational modes in the far-infrared spectra of ruthenium carbonyl clusters and the effect of gold substitution.

High-quality far-IR absorption spectra for a series of ligated atomically precise clusters containing Ru3, Ru4, and AuRu3 metal cores have been observed using synchrotron radiation, the latter two for the first time. The experimental spectra are compared with predicted IR spectra obtained following complete geometric optimization of the full cluster, including all ligands, using DFT. We find strong correlations between the experimental and predicted transitions for the low-frequency, low-intensity metal core vibrations as well as the higher frequency and intensity metal-ligand vibrations. The metal core vibrational bands appear at 150 cm(-1) for Ru3(CO)12, and 153 and 170 cm(-1) for H4Ru4(CO)12, while for the bimetallic Ru3(µ-AuPPh3)(µ-Cl)(CO)10 cluster these are shifted to 177 and 299 cm(-1) as a result of significant restructuring of the metal core and changes in chemical composition. The computationally predicted IR spectra also reveal the expected atomic motions giving rise to the intense peaks of metal-ligand vibrations at ca. 590 cm(-1) for Ru3, 580 cm(-1) for Ru4, and 560 cm(-1) for AuRu3. The obtained correlations allow an unambiguous identification of the key vibrational modes in the experimental far-IR spectra of these clusters for the first time.

On the correlation between dye coverage and photoelectrochemical performance in dye-sensitized solar cells.

Concentration depth profiles of the ruthenium based dyes Z907 and N719 adsorbed onto titania are measured directly and used for determining the adsorption isotherm of the dyes. Dye layers formed by both grow in islands on the titania which do not cover the entire titania surface even at the maximum coverage. Impedance spectroscopy in conjunction with the adsorption isotherms shows that recombination losses mainly appear between the dye and the electrolyte solution. The short circuit current and the efficiency increase linearly with the dye coverage. The open circuit voltage slightly increases with increasing dye coverage which is interpreted as most likely to be a consequence of the higher charge in the particles upon higher dye loading on the TiO2 surface.

Phosphine-stabilised Au9 clusters interacting with titania and silica surfaces: the first evidence for the density of states signature of the support-immobilised cluster.

Chemically made, atomically precise phosphine-stabilized clusters Au9(PPh3)8(NO3)3 were deposited on titania and silica from solutions at various concentrations and the samples heated under vacuum to remove the ligands. Metastable induced electron spectroscopy was used to determine the density of states at the surface, and X-ray photoelectron spectroscopy for analysing the composition of the surface. It was found for the Au9 cluster deposited on titania that the ligands react with the titania substrate. Based on analysis using the singular value decomposition algorithm, the series of MIE spectra can be described as a linear combination of 3 base spectra that are assigned to the spectra of the substrate, the phosphine ligands on the substrate, and the Au clusters anchored to titania after removal of the ligands. On silica, the Au clusters show significant agglomeration after heat treatment and no interaction of the ligands with the substrate can be identified.