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Surfaces, Volume 3, Issue 3 (September 2020) – 10 articles

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Open AccessArticle
Stochastic Analysis of Electron Transfer and Mass Transport in Confined Solid/Liquid Interfaces
Surfaces 2020, 3(3), 392-407; https://doi.org/10.3390/surfaces3030029 - 08 Aug 2020
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Abstract
Molecular-level understanding of electrified solid/liquid interfaces has recently been enabled thanks to the development of novel in situ/operando spectroscopic tools. Among those, ambient pressure photoelectron spectroscopy performed in the tender/hard X-ray region and coupled with the “dip and pull” method makes it possible [...] Read more.
Molecular-level understanding of electrified solid/liquid interfaces has recently been enabled thanks to the development of novel in situ/operando spectroscopic tools. Among those, ambient pressure photoelectron spectroscopy performed in the tender/hard X-ray region and coupled with the “dip and pull” method makes it possible to simultaneously interrogate the chemical composition of the interface and built-in electrical potentials. On the other hand, only thin liquid films (on the order of tens of nanometers at most) can be investigated, since the photo-emitted electrons must travel through the electrolyte layer to reach the photoelectron analyzer. Due to the challenging control and stability of nm-thick liquid films, a detailed experimental electrochemical investigation of such thin electrolyte layers is still lacking. This work therefore aims at characterizing the electrochemical behavior of solid/liquid interfaces when confined in nanometer-sized regions using a stochastic simulation approach. The investigation was performed by modeling (i) the electron transfer between a solid surface and a one-electron redox couple and (ii) its diffusion in solution. Our findings show that the well-known thin-layer voltammetry theory elaborated by Hubbard can be successfully applied to describe the voltammetric behavior of such nanometer-sized interfaces. We also provide an estimation of the current densities developed in these confined interfaces, resulting in values on the order of few hundreds of nA·cm−2. We believe that our results can contribute to the comprehension of the physical/chemical properties of nano-interfaces, thereby aiding to a better understanding of the capabilities and limitations of the “dip and pull” method. Full article
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Open AccessArticle
Molecular Dynamics Simulation Study of the Mechanical Properties of Nanocrystalline Body-Centered Cubic Iron
Surfaces 2020, 3(3), 381-391; https://doi.org/10.3390/surfaces3030028 - 04 Aug 2020
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Abstract
In the present work, the mechanical properties of nanocrystalline body-centered cubic (BCC) iron with an average grain size of 10 Å were investigated using molecular dynamics (MD) simulations. The structure has one layer of crystal grains, which means such a model could represent [...] Read more.
In the present work, the mechanical properties of nanocrystalline body-centered cubic (BCC) iron with an average grain size of 10 Å were investigated using molecular dynamics (MD) simulations. The structure has one layer of crystal grains, which means such a model could represent a structure with directional crystallization. A series of uniaxial tensile tests with different strain rates and temperatures was performed until the full rupture of the model. Moreover, tensile tests of the models with a void at the center and shear tests were carried out. In the tensile test simulations, peak stress and average values of flow stress increase with strain rate. However, the strain rate does not affect the elasticity modulus. Due to the presence of void, stress concentrations in structure have been observed, which leads to dislocation pile-up and grain boundary slips at lower strains. Furthermore, the model with the void reaches lower values of peak stresses as well as stress overshoot compared to the no void model. The study results provide a better understanding of the mechanical response of nanocrystalline BCC iron under various loadings. Full article
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Open AccessFeature PaperArticle
Response of Photoluminescence of H-Terminated and Hydrosilylated Porous Si Powders to Rinsing and Temperature
Surfaces 2020, 3(3), 366-380; https://doi.org/10.3390/surfaces3030027 - 03 Aug 2020
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Abstract
The photoluminescence (PL) response of porous Si has potential applications in a number of sensor and bioimaging techniques. However, many questions still remain regarding how to stabilize and enhance the PL signal, as well as how PL responds to environmental factors. Regenerative electroless [...] Read more.
The photoluminescence (PL) response of porous Si has potential applications in a number of sensor and bioimaging techniques. However, many questions still remain regarding how to stabilize and enhance the PL signal, as well as how PL responds to environmental factors. Regenerative electroless etching (ReEtching) was used to produce photoluminescent porous Si directly from Si powder. As etched, the material was H-terminated. The intensity and peak wavelength were greatly affected by the rinsing protocol employed. The highest intensity and bluest PL were obtained when dilute HCl(aq) rinsing was followed by pentane wetting and vacuum oven drying. Roughly half of the hydrogen coverage was replaced with –RCOOH groups by thermal hydrosilylation. Hydrosilylated porous Si exhibited greater stability in aqueous solutions than H-terminated porous Si. Pickling of hydrosilylated porous Si in phosphate buffer was used to increase the PL intensity without significantly shifting the PL wavelength. PL intensity, wavelength and peak shape responded linearly with temperature change in a manner that was specific to the surface termination, which could facilitate the use of these parameters in a differential sensor scheme that exploits the inherent inhomogeneities of porous Si PL response. Full article
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Open AccessArticle
Montecarlo Simulation and HAXPES Analysis of Organosilane Segregation in Titania Xerogel Films; Towards a Generic Surface Chemofunctionalization Process
Surfaces 2020, 3(3), 352-365; https://doi.org/10.3390/surfaces3030026 - 28 Jul 2020
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Abstract
The formation of xerogels implies a sequence of hydrolysis and condensation reactions, which are intricate to analyze in heteromolecular sols. We analyze by probabilistic Montecarlo methods the development of hybrid organosilane–titania xerogels and illustrate how partial charges of the reacting molecules can help [...] Read more.
The formation of xerogels implies a sequence of hydrolysis and condensation reactions, which are intricate to analyze in heteromolecular sols. We analyze by probabilistic Montecarlo methods the development of hybrid organosilane–titania xerogels and illustrate how partial charges of the reacting molecules can help estimating relative probabilities for the condensation of the molecules. Since the condensation rate of Ti alkoxides is much higher than the corresponding rate of Si alkoxides (especially if bearing a non-hydrolizable group), by imposing a fast condensation process in agreement with low pH kinetics, the process leads to a surface segregation of the organosilane. The simulation results are compared with results of characterization of thin condensates of two different organosilanes within a titanium–isopropoxide matrix. Non-destructive in-depth profiles were obtained by hard x-ray photoelectron spectroscopy, which can resolve through estimation of Si and specific moieties of the organosilane molecules the progress of the condensation. These results are relevant for the generalization of chemo-functionalization processes by kinetic demixing of organosilanes, which have myriad applications in biomedicine and biotechnology. Full article
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Open AccessArticle
Surface Plasmon Resonance-Based Temperature Sensor with Outer Surface Metal Coating on Multi-Core Photonic Crystal Fibre
Surfaces 2020, 3(3), 337-351; https://doi.org/10.3390/surfaces3030025 - 20 Jul 2020
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Abstract
We report an innovative design of a multi-core photonic crystal fibre-based surface plasmon resonance temperature sensor using ethanol and benzene as temperature-sensitive materials with a segmented outer-surface metal coating scheme. A stable sensing performance for a detection range of 10–80 C was [...] Read more.
We report an innovative design of a multi-core photonic crystal fibre-based surface plasmon resonance temperature sensor using ethanol and benzene as temperature-sensitive materials with a segmented outer-surface metal coating scheme. A stable sensing performance for a detection range of 10–80 C was found while using ethanol as the temperature-sensitive material; while using benzene both blue and red frequency shifts were observed. The maximum temperature sensitivities obtained from this proposed temperature sensor were 360 pm/ C and 23.3 nm/ C with resolutions of 2.78 × 10 1 C and 4.29 × 10 3 C, respectively, when using ethanol or benzene as the sensing medium. Full article
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Open AccessArticle
In Situ Monitoring of Growth of Vertically Stacked h-BN/Graphene Heterostructures on Ni Substrates and Their Interface Interaction
Surfaces 2020, 3(3), 328-336; https://doi.org/10.3390/surfaces3030024 - 14 Jul 2020
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Abstract
Vertically stacked hexagonal boron nitride (h-BN)/graphene heterostructures present potential applications in electronic, photonic, and mechanical devices, and their interface interaction is one of the critical factors that affect the performances. In this work, the vertical h-BN/graphene heterostructures with high coverage are synthesized by [...] Read more.
Vertically stacked hexagonal boron nitride (h-BN)/graphene heterostructures present potential applications in electronic, photonic, and mechanical devices, and their interface interaction is one of the critical factors that affect the performances. In this work, the vertical h-BN/graphene heterostructures with high coverage are synthesized by chemical vapor deposition (CVD) of h-BN on Ni substrates followed by segregation growth of graphene at the h-BN/Ni interfaces, which are monitored by in situ surface microscopy and surface spectroscopy. We find that h-BN overlayers can be decoupled from Ni substrates by the graphene interlayers. Furthermore, the h-BN domain boundaries exhibit a confinement effect on the graphene interlayer growth and the lower graphene domains are limited within the upper h-BN domains. This work provides new insights into the formation mechanism and interface interaction of the vertical heterostructures. Full article
(This article belongs to the Special Issue Surface Science and Catalysis of Graphene-Related 2D Materials)
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Open AccessArticle
Stability of Non-Flexible vs. Flexible Inverted Bulk-Heterojunction Organic Solar Cells with ZnO as Electron Transport Layer Prepared by a Sol-Gel Spin Coating Method
Surfaces 2020, 3(3), 319-327; https://doi.org/10.3390/surfaces3030023 - 09 Jul 2020
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Abstract
In this research, inverted bulk heterojunction organic solar cells (BHJ OSC) with poly(3-hexylthiophene-2,5-diyl): (6,6)-phenyl C61 butyric acid methyl (P3HT:PCBM) as the active layer were fabricated by a sol-gel spin coating method using flexible PET and non-flexible glass as substrates. The power conversion efficiency [...] Read more.
In this research, inverted bulk heterojunction organic solar cells (BHJ OSC) with poly(3-hexylthiophene-2,5-diyl): (6,6)-phenyl C61 butyric acid methyl (P3HT:PCBM) as the active layer were fabricated by a sol-gel spin coating method using flexible PET and non-flexible glass as substrates. The power conversion efficiency (PCE) and the stability of the cells were investigated. According to the results, the non-flexible device showed higher short circuit current (Jsc) as well as open-circuit voltage (Voc) as compared to the flexible one so that 2.52% and 0.67% PCE for non-flexible and flexible cells were obtained, respectively. From the stability point of view, the non-flexible device maintained 51% of its initial efficiency after six weeks in a dark atmosphere, while it was about 19% for the flexible cell after four weeks. The most important reason for the higher PCE with the higher stability in the non-flexible cell can be attributed to its higher shunt resistance (Rsh) and better interlayer connections at the electron collector side. Full article
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Open AccessArticle
ZnO Surface Doping to Enhance the Photocatalytic Activity of Lithium Titanate/TiO2 for Methylene Blue Photodegradation under Visible Light Irradiation
Surfaces 2020, 3(3), 301-318; https://doi.org/10.3390/surfaces3030022 - 07 Jul 2020
Viewed by 324
Abstract
Wastewater contaminated with dyes produced by textile industries is a major problem due to inadequate treatment prior to release into the environment. In this paper, the ability of ZnO to enhance the interfacial photocatalytic activity of lithium titanate/TiO2 (LTO/TiO2) for [...] Read more.
Wastewater contaminated with dyes produced by textile industries is a major problem due to inadequate treatment prior to release into the environment. In this paper, the ability of ZnO to enhance the interfacial photocatalytic activity of lithium titanate/TiO2 (LTO/TiO2) for the photodegradation of methylene blue (MB) under visible light irradiation (4.38 mW/cm2) was assessed. The ZnO-doped lithium titanate/TiO2 (ZnO/LTO/TiO2) was synthesized using a combination of hydrothermal and wetness impregnation methods. The high-resolution transmission electron microscope (HRTEM) and X-ray Diffraction (XRD) analyses indicate that the ZnO/LTO/TiO2 contain several phases (ZnO, LTO, and TiO2). The adsorption capacity of LTO/TiO2 (70%) was determined to be higher compared to its photocatalytic activity (25%), which is attributed to the strong interaction between the Li and surface oxygen atoms with the MB dye molecules. The introduction of ZnO improved the photocatalytic ability of LTO/TiO2 by 45% and extended the life span of ZnO/LTO/TiO2. The ZnO/LTO/TiO2 can be reused without a significant loss up to four cycles, whereas LTO/TiO2 had reduced adsorption after the second cycle by 30%. The ZnO increased the surface defects and restrained the photo-induced electrons (e) from recombining with the photo-induced holes (h+). Scavenging tests indicated that the hydroxyl radicals played a major role in the photodegradation of MB, which is followed by electrons and holes. Full article
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Open AccessArticle
Hydrogen in Aluminium-Coated Steels Exposed to Synthetic Seawater
Surfaces 2020, 3(3), 282-300; https://doi.org/10.3390/surfaces3030021 - 04 Jul 2020
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Abstract
Thermally sprayed aluminium (TSA) coatings provide protection to offshore steel structures without the use of external cathodic protection (CP) systems. These coatings provide sacrificial protection in the same way as a galvanic anode, and thus hydrogen embrittlement (HE) becomes a major concern with [...] Read more.
Thermally sprayed aluminium (TSA) coatings provide protection to offshore steel structures without the use of external cathodic protection (CP) systems. These coatings provide sacrificial protection in the same way as a galvanic anode, and thus hydrogen embrittlement (HE) becomes a major concern with the use of high strength steels. The effect of TSA on the HE of steel seems to remain largely unknown. Further, the location of hydrogen in TSA-coated steel has not been explored. To address the above knowledge gap, API 5L X80 and AISI 4137 steel coupons, with and without TSA, were prepared and the amount of hydrogen present in these steels when cathodically polarised to −1.1 V (Ag/AgCl) for 30 days in synthetic seawater was determined. One set of TSA-coated specimens was left at open circuit potential (OCP). The study indicates that the amount of hydrogen present in TSA-coated steel is ~100 times more than the amount found in uncoated steel, and that the hydrogen seems to be largely localised in the TSA layer. Full article
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Open AccessArticle
Relationship between Surface Properties and Fiber Network Parameters of Eucalyptus Kraft Pulps and Their Absorption Capacity
Surfaces 2020, 3(3), 265-281; https://doi.org/10.3390/surfaces3030020 - 30 Jun 2020
Viewed by 289
Abstract
Water absorption capacity is a key characteristic of cellulosic pulps used for different commodities. This property is influenced by the affinity of the pulp fiber surface with water, chemical composition of the pulp, morphology, and organization of fibers in the network. In this [...] Read more.
Water absorption capacity is a key characteristic of cellulosic pulps used for different commodities. This property is influenced by the affinity of the pulp fiber surface with water, chemical composition of the pulp, morphology, and organization of fibers in the network. In this study, surface properties of six industrial Eucalyptus bleached kraft pulps (fluff pulps) dry-defiberized in a Hammermill, which were obtained by wood pulping and pulp bleaching under different production conditions, were studied while employing dynamic water vapor sorption and contact angles measurements. The absorption properties of air-laid pulp pads were analyzed following the absorbency testing procedure and the relationship between these properties and pulp’s chemical composition and fiber network structure were assessed by multivariate analysis. The results showed that the accessibility of the fiber surface is related to the reduction of the contact angles, but, at the same time, to the longer absorption time and less absorption capacity of the fiber network. Therefore, the absorption properties of the pulps are not necessarily directly related to their surface properties. Indeed, absorptivity is related to the surface chemical composition, fiber morphology, and fiber network structure. Thus, surface carboxylic groups promote total water uptake, resulting in better absorption capacity. Greater fiber coarseness and deformations (curl and kink) provide a less wettable surface, but a more porous network with higher specific volume, resulting in more absorbent air-laid formulations. Full article
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