Hydrophobic Coatings with Charge Permeability via Plasma Deposition of Long-Chain Perfluorocarbons.

Hydrophobization of nanotextured catalyst materials is a promising route to enhance the yield of N2 and CO2 conversion into green fuels. However, these applications require a hydrophobic coating to not only promote air trapping but also allow charge transfer at the electrode-electrolyte interface. In this work, nano thin films with thicknesses as low as 7 nm were deposited from the plasma phase of perfluorohexene, perfluorodecene, and perfluorooctane (PFO) precursors using a mild vacuum and gentle powers. Atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) characterization reveal that the resulting films are conformal and hydrophobic thanks to a good retention of CF2 and CF3 moieties. The PFO films exhibited the highest water contact angle and achieved superhydrophobic states when deposited on top of re-entrant nano features, an indication of successful air trapping. Electrochemical studies further demonstrated that the plasma-deposited PFO films allow charge transfer but could only sustain repeated cyclic voltammetry cycles without losing their hydrophobicity when deposited under optimal conditions. This result indicates that plasma deposition could become a viable route for the hydrophobization of electrocatalysts required to enhance the yield of poorly soluble gas reduction reactions.

Probing Molecular Mechanisms during the Oscillatory Adsorption of Propyl Chain Functionalized Organosilane Films with Sum Frequency Generation Spectroscopy.

The selectivity rules of sum frequency generation spectroscopy were exploited to determine propyl chain order during the time-dependent oscillatory adsorption of propyltrimethoxysilane (PTMS) and Langmuir-type growth of propyldimethylmethoxysilane (PDMMS). During the early stages of film growth, molecular packing density determines the extent of propyl chain defects within both films with high surface coverage resulting in a film with fewer defects. Following this, an ordered monolayer-like film stabilizes on the Al2O3 substrate for both silanes. Although this result is intuitive for the Langmuir-type growth of PDMMS, the stabilization of molecular ordering despite the continuing oscillation in PTMS surface coverage indicates the presence of a stable monolayer, while it is the oligomerized PTMS dendrimers which continue to desorb and readsorb to the substrate. We also reveal for the first time, the formation of a physisorbed bilayer during the self-assembly process of PTMS. The presence of this ordered, physisorbed bilayer on top of the covalently bound PTMS film plays a key role in the process of the molecular self-assembly mechanism and is proposed to enable further condensation of the covalently bound film.

Oxidative Dissolution of Sulfide Minerals in Single and Mixed Sulfide Systems under Simulated Acid and Metalliferous Drainage Conditions.

Chalcopyrite, galena, and sphalerite commonly coexist with pyrite in sulfidic waste rocks. The aim of this work was to investigate their impact, potentially by galvanic interaction, on pyrite oxidation and acid generation rates under simulated acid and metalliferous drainage conditions. Kinetic leach column experiments using single-minerals and pyrite with one or two of the other sulfide minerals were carried out at realistic sulfide contents (total sulfide <5.2 wt % for mixed sulfide experiments), mimicking sulfidic waste rock conditions. Chalcopyrite was found to be most effective in limiting pyrite oxidation and acid generation with 77-95% reduction in pyrite oxidation over 72 weeks, delaying decrease in leachate pH. Sphalerite had the least impact with reduction of pyrite dissolution by 26% over 72 weeks, likely because of the large band gap and poor conductivity of sphalerite. Galena had a smaller impact than chalcopyrite on pyrite oxidation, despite their similar band gaps, possibly because of the greater extent of oxidation and the significantly reduced surface areas of galena (area reductions of >47% for galena vs <1.5% for chalcopyrite) over 72 weeks. The results are directly relevant to mine waste storage and confirm that the galvanic interaction plays a role in controlling acid generation in multisulfide waste even at low sulfide contents (several wt %) with small probabilities (≤0.23%) of direct contact between sulfide minerals in mixed sulfide experiments.

Scanning Auger Microscopy Studies of Silane Films Grown on Plasma-Modified HOPG Surfaces.

The growth of silane films on plasma oxidized highly oriented pyrolytic graphite (HOPG) surfaces has been studied using wet chemical deposition of propyltrimethoxysilane (PTMS) and propyldimethylmethoxysilane (PDMMS). Scanning Auger microscopy (SAM) and X-ray photoelectron spectroscopy (XPS) were used to investigate the chemical composition and morphology of the silane films. The effects of several deposition parameters were examined, including the necessity of oxidation of the HOPG surface, addition of water with the silane, and rinsing before curing. The optimal conditions needed to create a complete uniform film differ for the two silanes due to differences in their structures. Both silanes require an oxidized HOPG surface for a film to grow, the addition of water with PTMS results in a thicker film, while the addition of water with PDMMS decreases the film growth. Rinsing of both samples before curing removes physisorbed species, leaving only the covalently bonded film on the surface.

The Role of Physisorption and Chemisorption in the Oscillatory Adsorption of Organosilanes on Aluminium Oxide.

The effect of physisorbed and chemisorbed species on the time-dependent self-assembly mechanism of organosilane films has been investigated on aluminium oxide using X-ray Photoelectron Spectroscopy. The role of physisorbed species was determined through their removal using a simple rinsing procedure while monitoring film substrate coverage. Removing physisorbed species from Propyldimethylmethoxysilane films, shown to follow a Langmuir-type adsorption profile, reduces the substrate coverage initially but quickly results in coverages equivalent to films that did not undergo a rinsing procedure. This indicates that all Propyldimethylmethoxysilane molecules are covalently bound to the substrate following 15 s of film growth. Removing physisorbed species from films, which have been shown to follow an oscillatory adsorption profile, Propyltrimethoxysilane and Propylmethyldimethoxysilane, reveal the persistence of these oscillations despite a reduction in silane substrate coverage. These results not only confirm the presence of two thermodynamically favourable phases in the condensation equilibrium reaction as physisorbed and chemisorbed species, but also indicate that the desorption of species during film growth involves both states of chemical binding.

The effect of different pyrolysis temperatures on the speciation and availability in soil of P in biochar produced from the solid fraction of manure.

Biochar application to agricultural land has been proposed as a means for improving phosphorus (P) availability in soil. The purpose of the current study was to understand how pyrolysis temperature affects P speciation in biochar and how this affects availability of P in the amended soil. Biochar was produced at different temperatures from digestate solids. The primary species of P in digestate solids were simple calcium phosphates. However, a high co-occurrence of magnesium (Mg) and P, indicated that struvite or other magnesium phosphates may also be important species. At low temperatures, pyrolysis had little effect on P speciation; however, as the temperature increased above 600 °C, the P gradually became more thermodynamically stable in species such as apatite. At very high temperatures above 1000 °C, there were indications of reduced forms of P. Biochar production decreased the immediate availability of P in comparison with the original digestate solids. However, for biochar produced at low temperatures, availability quickly increased to the same levels as in the digestate solids. For biochar produced at higher temperatures, availability remained depressed for much longer. The low availability of P in the biochar produced at high temperatures can probably be explained by the formation of less soluble P species in the biochar. In contrast, the transient decrease of availability of the P in the biochar produced at low temperatures can be explained by mechanisms, such as sorption on biochar, which gradually decreases because of oxidation of the biochar surfaces or changes in pH around the biochar particles.

Classification of time-of-flight secondary ion mass spectrometry spectra from complex Cu-Fe sulphides by principal component analysis and artificial neural networks.

Artificial neural network (ANN) and a hybrid principal component analysis-artificial neural network (PCA-ANN) classifiers have been successfully implemented for classification of static time-of-flight secondary ion mass spectrometry (ToF-SIMS) mass spectra collected from complex Cu-Fe sulphides (chalcopyrite, bornite, chalcocite and pyrite) at different flotation conditions. ANNs are very good pattern classifiers because of: their ability to learn and generalise patterns that are not linearly separable; their fault and noise tolerance capability; and high parallelism. In the first approach, fragments from the whole ToF-SIMS spectrum were used as input to the ANN, the model yielded high overall correct classification rates of 100% for feed samples, 88% for conditioned feed samples and 91% for Eh modified samples. In the second approach, the hybrid pattern classifier PCA-ANN was integrated. PCA is a very effective multivariate data analysis tool applied to enhance species features and reduce data dimensionality. Principal component (PC) scores which accounted for 95% of the raw spectral data variance, were used as input to the ANN, the model yielded high overall correct classification rates of 88% for conditioned feed samples and 95% for Eh modified samples.

Chemometric and multivariate statistical analysis of time-of-flight secondary ion mass spectrometry spectra from complex Cu-Fe sulfides.

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) spectra of mineral samples are complex, comprised of large mass ranges and many peaks. Consequently, characterization and classification analysis of these systems is challenging. In this study, different chemometric and statistical data evaluation methods, based on monolayer sensitive TOF-SIMS data, have been tested for the characterization and classification of copper-iron sulfide minerals (chalcopyrite, chalcocite, bornite, and pyrite) at different flotation pulp conditions (feed, conditioned feed, and Eh modified). The complex mass spectral data sets were analyzed using the following chemometric and statistical techniques: principal component analysis (PCA); principal component-discriminant functional analysis (PC-DFA); soft independent modeling of class analogy (SIMCA); and k-Nearest Neighbor (k-NN) classification. PCA was found to be an important first step in multivariate analysis, providing insight into both the relative grouping of samples and the elemental/molecular basis for those groupings. For samples exposed to oxidative conditions (at Eh ~430 mV), each technique (PCA, PC-DFA, SIMCA, and k-NN) was found to produce excellent classification. For samples at reductive conditions (at Eh ~ -200 mV SHE), k-NN and SIMCA produced the most accurate classification. Phase identification of particles that contain the same elements but a different crystal structure in a mixed multimetal mineral system has been achieved.

Synchrotron FTIR microscopy of Langmuir-Blodgett monolayers and polyelectrolyte multilayers at the solid-solid interface.

Synchrotron FTIR microscopy has been used to probe the structure of model boundary lubricant layers confined at the solid-solid interface. The combination of high brightness of the IR source and a novel contact geometry that uses a hemispherical internal reflection element as the means for light delivery has enabled the detection of <2.5 nm thin monolayer lubricant layers in the solid-solid contact, in addition to allowing for spectral acquisition from specific regions of the contact. Spectra of hydration water from within a confined polyelectrolyte multilayer film have also been acquired, highlighting the altered hydrogen bonding environment within the polymer layer.

The role of mineral surface chemistry in modified dextrin adsorption.

The adsorption of two modified dextrins (phenyl succinate dextrin--PS Dextrin; styrene oxide dextrin--SO Dextrin) on four different mineral surfaces has been studied using X-ray photoelectron spectroscopy (XPS), in situ atomic force microscopy (AFM) imaging, and captive bubble contact angle measurements. The four surfaces include highly orientated pyrolytic graphite (HOPG), freshly cleaved synthetic sphalerite (ZnS), and two surfaces produced through surface reactions of sphalerite: one oxidized in alkaline solution (pH 9, 1 h immersion); and one subjected to metal ion exchange between copper and zinc (i.e. copper activation: exposed to 1×10(-3) M CuSO(4) solution for 1 h). XPS measurements indicate that the different sphalerite surfaces contain varying amounts of sulfur, zinc, oxygen, and copper, producing substrates for polymer adsorption with a range of possible binding sites. AFM imaging has shown that the two polymers adsorb to a similar extent on HOPG, and that the two polymers display very different propensities for adsorption on the three sphalerite surface types, with freshly cleaved sphalerite encouraging the least adsorption, and copper activated and oxidized sphalerite encouraging significantly more adsorption. Contact angle measurements of the four surfaces indicate that synthetic sphalerite has a low contact angle upon fracture, and that oxidation on the timescale of one hour substantially alters the hydrophobicity. HOPG and copper-activated sphalerite were the most hydrophobic, as expected due to the carbon and di/poly-sulfide rich surfaces of the two samples, respectively. SO Dextrin is seen to have a significant impact on the wettability of HOPG and the surface reacted sphalerite samples, highlighting the difficulty in selectively separating sphalerite from carbonaceous unwanted minerals in flotation. PS Dextrin has the least effect on the hydrophobicity of the reacted sphalerite surfaces, whilst still significantly increasing the wettability of graphite, and thus has more potential for use as a polymer depressant in this separation.