Surfaces — Open Access Journal

In this communication, we report surface state, with Rashba impurities, of a generic topological Kondo insulator (TKI) system by performing a mean-field theoretic (MFT) calculation within the framework of slave-boson protocol. The surface metallicity together with bulk insulation is found to require very strong f-electron localization. The possibility of intra-band as well as inter-band unconventional plasmons exists for the surface state spectrum. The paramountcy of the bulk metallicity, and, in the presence of the Rashba impurities, the TKI surface comprising of ‘helical liquids’ are the important outcomes of the present communication. The access to the gapless Dirac spectrum leads to spin-plasmons with the usual wave vector dependence q^1/2. The Rashba coupling does not impair the Kondo screening and does not affect the quantum critical point (QCP) for the bulk. Full article

An electrochemical sensor, based on a graphene paste electrode (GPE), was modified with a polymerization method, and the electrochemical behavior of catechol (CC) and hydroquinone (HQ) was investigated using electroanalytical methods like cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The effect of CC at the modified electrode was evidenced by the positive shift of the oxidation peak potential of CC at the poly (rosaniline)-modified graphene paste electrode (PRAMGPE) and the nine-fold enhancement of the peak current, as compared to a bare graphene paste electrode (BGPE). The sensitivity of CC investigated by DPV was more sensitive than CV for the analysis of CC. The DPV method showed the two linear ranges of 2.0 × 10⁻⁶–1.0 × 10⁻⁵ M and 1.5 × 10⁻⁵–5 × 10⁻⁵ M. The detection limit and limit of quantification were determined to be 8.2 × 10⁻⁷ and 27.6 × 10⁻⁷ M, respectively. The obtained results were compared successfully with respect to those obtained using the official method. Moreover, this sensor is applied for the selective determination of CC in the presence of HQ. The high sensitivity, good reproducibility, and wide linear range make the modified electrode suitable for the determination of CC in real samples. The practical application of the sensor was demonstrated by determining the concentration of CC in water samples with acceptable recoveries (97.5–98%). Full article

The Surfaces Special Issue entitled “Surface Aspects of Semiconductor Photochemistry” is mainly devoted to the 7th International Conference on Semiconductor Photochemistry (SP7), which was held on 11–14 September 2019 in Milano, Italy, in the beautiful Renaissance “Ca’ Granda” main building of the University Milan [...] Full article

In this review, the evolution of paradigm shifts in CH₄ adsorbent design are discussed. The criteria used as characteristic of paradigms are first reports, systematic findings, and reports of record CH₄ storage or deliverable capacity. Various paradigms were used such as the systematic design of micropore affinity and pore size, functionalization, structure optimization, high throughput in silico screening, advanced material property design which includes flexibility, intrinsic heat management, mesoporosity and ultraporosity, and process condition optimization. Here, the literature is reviewed to elucidate how the approach to CH₄ adsorbent design has progressed and provide strategies that could be implemented in the future. Full article

Surface density of adsorption sites on an adsorbent (including affinity-based sensors) is one of the basic input parameters in modeling of process kinetics in adsorption based devices. Yet, there is no simple expression suitable for fast calculations in current multiscale models. The published experimental data are often application-specific and related to the equilibrium surface density of adsorbate molecules. Based on the known density of adsorbed gas molecules and the surface coverage, both of these in equilibrium, we obtained an equation for the surface density of adsorption sites. We applied our analysis to the case of pristine graphene and thus estimated molecular dynamics of adsorption on it. The monolayer coverage was determined for various pressures and temperatures. The results are verified by comparison with literature data. The results may be applicable to modeling of the surface density of adsorption sites for gas adsorption on other homogeneous crystallographic surfaces. In addition to it, the obtained analytical expressions are suitable for training artificial neural networks determining the surface density of adsorption sites on a graphene surface based on the known binding energy, temperature, mass of adsorbate molecules and their affinity towards graphene. The latter is of interest for multiscale modelling. Full article

Hydrogen is considered as one of the promising clean energy sources for future applications including transportation. Nevertheless, the development of materials for its storage is challenging particularly as a fuel in vehicular transport. In the present study, density functional theory simulations for hydrogen adsorption on the surfaces of pristine, Ru-encapsulated, -doped and -supported C₆₀ are reported. The results show that adsorption on the pristine C₆₀ is exoergic and there is an enhancement in the adsorption upon encapsulation of a single Ru atom. The Ru-doped surface also adsorbs H₂ more strongly than the pristine surface, but its efficacy is slightly less than the Ru-encapsulated surface. The strongest adsorption is calculated for the C₆₀ surface supported with Ru. Full article

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

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

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

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

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 ${}^{\circ }$ 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/ ${}^{\circ }$ C and 23.3 nm/ ${}^{\circ }$ C with resolutions of $2.78\times {10}^{-1}$ ${}^{\circ }$ C and $4.29\times {10}^{-3}$ ${}^{\circ }$ C, respectively, when using ethanol or benzene as the sensing medium. Full article

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

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 (J_sc) as well as open-circuit voltage (V_oc) 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 (R_sh) and better interlayer connections at the electron collector side. Full article

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

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

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

This research dealt with the preparation and characterization of silver oxide (SLO) nanomaterials (NMs) and their composite catalysts (i.e., silver and ruthenium oxide (SLORUO)). The prepared materials were tested for their catalytic performance in carbon monoxide (CO) oxidation. Generally, silver in its pure state is not widely used for CO oxidation due to stability and structural issues. However, the usage of subsurface oxygen and oxygen-induced reconstruction could be effective as an oxidation catalyst at a slightly high temperature. The low-temperature reaction of highly active RuO₂ (RUO) is a well-known phenomenon. Thus, the possibility of using it with SLO to observe the combined catalytic behavior was investigated. The wet chemically prepared SLO and SLORUO NMs exhibited spherical and rods in spherical aggregate-type surface morphology belonging to cubic and rutile crystalline structures, respectively. The NMs and catalysts (i.e., the NMs on γ-Al₂O₃ catalyst support at 0.5 and 1.0 wt.% ranges) showed good thermal stability. The dry and wet CO oxidation using RUO and SLO showed concentration-dependent catalytic activity. The RUO, SLO, and SLORUO composites using 0.5 wt.% showed full CO oxidation at 200, 300, and 225 °C, respectively. The reasons for the observed activity of the catalysts are explained based on the pore characteristics, chemical composition, and dispersion using H₂ temperature-programmed reduction (TPR) behaviors. Full article

Stainless steel is renowned for its wide use as a biomaterial, but its relatively high corrosion rate in physiological environments restricts many of its clinical applications. To overcome the corrosion resistance of stainless steel bio-implants in physiological environments and to improve its osseointegration behavior, we have developed a unique zein/hydroxyapatite (HA) composite coating on a stainless steel substrate by Electrophoretic Deposition (EPD). The EPD parameters were optimized using the Taguchi Design of experiments (DoE) approach. The EPD parameters, such as the concentration of bio-ceramic particles in the polymer solution, applied voltage and deposition time were optimized on stainless steel substrates by applying a mixed design orthogonal Taguchi array. The coatings were characterized by using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and wettability studies. SEM images and EDX results indicated that the zein/HA coating was successfully deposited onto the stainless steel substrates. The wettability and roughness studies elucidated the mildly hydrophilic nature of the zein/HA coatings, which confirmed the suitability of the developed coatings for biomedical applications. Zein/HA coatings improved the corrosion resistance of bare 316L stainless steel. Moreover, zein/HA coatings showed strong adhesion with the 316L SS substrate for biomedical applications. Zein/HA developed dense HA crystals upon immersion in simulated body fluid, which confirmed the bone binding ability of the coatings. Thus the zein/HA coatings presented in this study have a strong potential to be considered for orthopedic applications. Full article

Graphene quantum dots (GOQDs)-MoS_x nanohybrids with different MoS_x stoichiometries (x = 2 and 3) were prepared in order to investigate their chemical stability under hydrogen evolution reaction (HER) conditions. Combined photoemission/electrochemical (XPS/EC) measurements and operando X-ray absorption spectroscopy (XAS) were employed to determine the chemical changes induced on the MoS_x-based materials as a function of the applied potential. This in situ characterization indicates that both MoS₂ and MoS₃ materials are stable under operating conditions, although sulfur terminal sites in the MoS₃ nanoparticles are converted from S-dimer (S₂²⁻) to S-monomer (S²⁻), which constitute the first sites where the hydrogen atoms are adsorbed for their subsequent evolution. In order to complete the characterization of the GOQDs-MoS_x nanohybrids, the composition and particle size were determined by X-ray photoemission spectroscopy (XPS), X-ray diffraction (XRD) and Raman spectroscopy; whereas the HER activity was studied by conventional electrochemical techniques. Full article