Reduction Of Surface Energy Research Articles

Formation and Characterization of Femtosecond-Laser-Induced Subcluster Segregated Nanoalloys

We report the first synthesis of subcluster segregated nanoalloys formed through the joining of immiscible metallic nanoparticles (NPs) using femtosecond (fs) laser irradiation. Immiscible alloy components consisting of Ag and Ni, and Ag and Fe, all in the form of NPs, were first deposited on a carbon film in vacuum by fs laser ablation from the parent metals. These samples of randomly distributed NPs were then irradiated with multiple fs laser pulses at a fluence of 1.5 mJ/cm2. Transmission electron microscopy (TEM) observations indicate that Ag and Ni as well as Ag and Fe NPs were successfully joined under these conditions. Energy dispersive X-ray (EDX) results show that no mixing layer exists at the interface. The nanostructure in the interface reveals the assumption of a specific angle between two matching planes on either side of the interface. Calculation of the lattice mismatch indicates that the system adjusts to this angle so as to reduce surface energy. Structural ledges were also formed at the ...

Energetics of the formation of Cu–Ag core–shell nanoparticles

This work presents molecular dynamics and Monte Carlo simulations aimed at developing an understanding of the formation of core–shell Cu–Ag nanoparticles. The effects of surface and interfacial energies were considered and used to form a phenomenological model that calculates the energy gained upon the formation of a core–shell structure from two previously distinct, non-interacting nanoparticles. In the majority of cases, the core–shell structure was found to be energetically favored. Specifically, the difference in energy as a function of the radii of the individual Cu and Ag particles was examined, with the assumption that a core–shell structure forms. In general, it was found that the energetic gain from forming such a structure increased with increasing size of the initial Ag particle. This result was interpreted as a result of the reduction in surface energy. For two separate particles, both Cu and Ag contribute to the surface energy; however, for a core–shell structure, the only contribution to the surface energy is from the Ag shell and the Cu contribution is changed to a Cu–Ag interfacial energy, which is always smaller.

Nanoparticles for molecular imaging.

Imaging techniques have been instrumental in the visualization of fundamental biological processes, identification and diagnosis of diseased states and the development of structure-function relationships at the cellular, tissue and anatomical levels. Together with the advancements made in imaging techniques, complementary chemical compounds, also known as imaging probes or contrast agents, are developed to improve the visibility of the image by enhancing sensitivity, and for the identification and quantitation of specific molecular species or structures. Extensive studies have been conducted to explore the use of inorganic nanoparticles which exhibit magnetic and optical properties unique to the nano regime so as to enhance the signals sensitivity for magnetic resonance and fluorescent imaging. These physical properties are tailored by controlling the size, shape and surface properties of nanoparticles. In addition, surface modification of nanoparticles is often required to improve its stability, compatibility and functionality. Surfactants, surface-active agents, have been used to engineer the surface characteristics of nanoparticles to improved particle stability and functionality. Surfactants enhance nanoparticle stability through the reduction of surface energy, and by acting as a barrier to agglomeration through either steric hindrance or repulsive electrostatic forces. Coupling of nanoparticles with biomolecules such as antibodies or tumor targeting peptides are enabled by the presence of functional groups (e.g., carboxyl or amine groups) on surfactants. This paper provides an overview of the chemistry underlying the synthesis and surface modification of nanomaterials together with a discussion on how the physical properties (e.g., magnetic, absorption and luminescent) can be controlled. The applications of these nanoparticles for magnetic resonance, fluorescent and photoacoustic imaging techniques that do not rely on ionizing radiation are also covered in this review.

A facile and green method to hydrophobize films of cellulose nanofibrils and silica by laccase-mediated coupling of nonpolar colloidal particles.

Hydrophobic particles based on dodecyl 3,4,5-trihydroxybenzoate (LG) were coupled onto the surface of cellulose nanofibrils (CNFs) and silica by treatment with a multicomponent colloidal system (MCS) derived from the laccase-mediated reaction of LG in the presence of a sulfonated lignin (SL). Surface modification upon treatment with MCS was monitored in situ and in real time by quartz crystal microgravimetry. The colloidal stability of MCS and its components in water was followed by measuring space- and time-resolved light transmission and back scattering. The sulfonated lignin increased dispersion stability and reduced the characteristic MCS particle size [from ≈4 to ≈80 nm, according to AFM and dynamic light scattering (DLS)]. It also facilitated the surface enzymatic reaction that led to adsorption and coupling of MCS onto CNFs and silica surfaces. The combined effect of reduced surface energy and surface roughness by MCS treatment produced an increase in water contact angle on CNFs and silica of about 90 and 80°, respectively. Surface pretreatment with chitosan further increased the extent of MCS adsorption on the surfaces. This method represents a sustainable alternative to traditional approaches for cellulose hydrophobization and a step forward in implementing green routes for surface modification.

Crystalline Submicrometer Spherical Particles with High Refractive Index for Optical Applications

Nanoparticles are generally aggregated in liquid to reduce surface energy, and therefore surface modification of the nanoparticles is important to obtain well dispersed nanoparticles in liquid. Thus the aggregation of nanoparticles should be avoided for practical use of nanoparticles as much as possible.Recently we found that B4C submicrometer spherical particles by irradiating low-fluence laser onto the B nanoparticles dispersed in organic solvent through the high-temperature carbonization reaction. Furthermore, we also found that similar submicrometer spherical particles were produced for various kinds of materials, such as metals (Au, Ag, etc.), oxides (ZnO, TiO2, etc.) and semiconductors (Si, GaP, etc.) using nanoparticles as raw materials and by unfocused laser beam irradiation.In these cases, raw nanoparticles are presumably not vaporized nor ablated but melted by nanosecond laser irradiation to form liquid droplets, resulting in spherical particle formation by quenching. Thus, this process “laser melting in liquid” is evidently different from the conventional technique, “laser ablation in liquid” for nanoparticle fabrication using high-fluence laser irradiation in liquid.More importantly, the particle size increased from nano-size to sub-micrometer size by laser melting in liquid. This is because aggregated nanoparticles melt together by optical absorption and subsequent temperature increase over melting temperature to form larger spherical particles. Thus the aggregation of nanoparticles plays an important role in submicrometer spherical particle formation.Here we present our recent results on the laser melting in liquid, such as the effect of processing parameters on the products. Especially we discuss the effect of raw particle size by dynamic light scattering and zeta potential on the product morphology and generated particle size. Physicochemical calculation of temperature estimation based on the particle heating-melting-quenching model by laser irradiation is compared with the experimental results. Optical and medical applications of submicrometer spheres is also demonstrated.

Effect of atmospheric pressure plasma pre-treatment and aging conditions on the surface of thermoplastics

The application of atmospheric pressure plasma is a popular pre-treatment method in the field of joining technology. Its purpose is the improvement of adhesion between different materials, e.g., for adhesive bonding, painting, or injection molding. Especially, while bonding parts with lower surface energies, efficient surface activation can be achieved via this treatment method. The surface is specifically modified in its chemical, physical, and topographical properties. However, after surface treatment, it is possible that these new surfaces progressively change by aging and compensation processes. Knowing the time-variant surface state after pre-treatment is extremely important for interpreting the entire process chain. Additionally, the effect of the pre-treatment on other properties—such as the bonding strength of an adhesive bond—must be known. The aging behavior of an atmospheric pressure plasma-treated plastic surface has been demonstrated in several studies based on the surface properties (chemical composition, surface energy, etc.). Whether these property changes also significantly affect the bond strength of structural adhesive joints is to be clarified in the present analysis. In this study, the aging behavior of three typical thermoplastics (polyethylene, polypropylene, and polystyrene) is examined after a treatment with an atmospheric plasma jet. The aging is taken into account by two factors: ambient temperature and time. Kinematic parameters (distance and velocity) of the pre-treatment are integrated in the experimental design to include the effects of the pre-treatment method in the investigation. The dependence of these influencing effects on bond strength and surface energy is analyzed with the help of central composite experimental designs. The results show a change in surface properties over time and ambient temperature. An increase of temperature and time leads to a reduction of surface energy. Furthermore, the kinematic factors of the plasma treatment determine aging. However, a mathematical correlation between bond strength and aging parameters could not be detected in this study. The bond strength is time-invariant but significantly dependent on the parameters distance and speed. Additionally, a dimensional analysis was conducted in order to describe the dependency of surface energy and load factor on the investigated parameters. Therefore, several dimensionless parameters were developed. Regarding the load factor, it was possible to describe the ongoing processes based on the correlations discovered by dimensional analysis. This allows the prediction of the load factor based on process and storage parameters. A relationship between the investigated Π-numbers could not be detected for the surface energy, which means a functional relationship to describe the surface energy could not be found.

Preparation and Characterization of Superhydrophobic Modification of Polyvinylidene Fluoride Membrane by Dip-Coating

The superhydrophobicity polyvinylidene fluoride (PVDF) membranes were modified via reducing surface energy by dip-coating perfluoroalkyl methacrylic copolymer (Zonyl 8740) onto the membranes prepared on mat glass. The chemical component of the unmodified and modified PVDF membranes surface was investigated by ATR-FTIR. Morphology and hydrophobicity of the unmodified and modified PVDF membranes were examined by scanning electronic microscopy and water contact angle, respectively. The effects of concentration of Zonyl 8740, coating time, conditions of heat treatment on hydrophobic capability of PVDF membranes were investigated. The results showed that the water contact angle increased from 141˚ to 151˚ by the dip-coating modification, therefore getting superhydrophobic PVDF membrane. Moreover, the porosity and the morphology of modified PVDF membrane were unchanged by the dip-coating modification. This results suggested that the hydrophobicty stability of the modified PVDF membrane was also good.

Structure Analysis of Al-Modified TiO2 Nanocatalyst Supports

The structures of Al-modified anatase TiO2 supports were analyzed using traditional analysis techniques of X-ray diffraction, transmission electron microscopy, and N2 sorption as well as X-ray absorption near-edge spectroscopy, 27Al magic-angle spinning NMR, and low-temperature heat capacity. The combined results from these characterizations reveal that the Al dopant is incorporated either on the surface or in vacancies depending on the details of the synthetic method used. Alumina incorporated on the surface stabilizes TiO2 by lowering the surface energy of anatase and stabilizing planes with high surface energy that would otherwise join to achieve a reduced surface energy. On the other hand, Al incorporated in TiO2 vacancies stabilizes the structure through increasing lattice strain and limiting mass transport necessary for grain growth. The 22 mol % Al-modified TiO2 with Al on the surface had average crystallite diameter of 6 nm, surface area of 184 m2 g–1, pore volume of 0.34 cm3 g–1, and pore diamete...

Multi-functional SiO2 moth-eye pattern for photovoltaic applications

A moth-eye pattern made of silica (SiO2) nanoparticles mixed with MIBK (methyl isobutyl ketone) was fabricated on a glass substrate using a direct printing method. The patterned glass was annealed to improve hardness and a heptdecafluoro-1,1,2,2-tetrahydrodecyl trichloro-silane based self-assembled monolayer (HDFS-SAM) was applied to the moth-eye pattern to reduce surface energy which improves hydrophobic properties. Compared to flat glass, the moth-eye patterned glass with a HDFS-SAM coating showed reduced reflectance, strong hardness, and high efficiency. As a result, the JSC of photovoltaic module was increased up to 1.9755mA/cm2 and efficiency was improved by 4.66% by the increased current density (JSC) of photovoltaic system, and the external quantum efficiency was enhanced over the entire visible spectrum.

Why Can Organic Liquids Move Easily on Smooth Alkyl-Terminated Surfaces?

The dynamic dewettability of a smooth alkyl-terminated sol-gel hybrid film surface against 17 probe liquids (polar and nonpolar, with high and low surface tensions) was systematically investigated using contact angle (CA) hysteresis and substrate tilt angle (TA) measurements, in terms of their physicochemical properties such as surface tension, molecular weight/volume, dielectric constant, density, and viscosity. We found that the dynamic dewettability of the hybrid film markedly depended not on the surface tensions but on the dielectric constants of the probe liquids, displaying lower resistance to liquid drop movement with decreasing dielectric constant (ε < 30). Interfacial analysis using the sum-frequency generation (SFG) technique confirmed that the conformation of surface-tethered alkyl chains was markedly altered before and after contact with the different types of probe liquids. When probe liquids with low dielectric constants were in contact with our surface, CH3 groups were preferentially exposed at the solid/liquid interface, leading to a reduction in surface energy. Because of such local changes in surface energy at the three-phase contact line of the probe liquid, the contact line can move continuously from low-surface-energy (solid/liquid) areas to surrounding high-surface-energy (solid/air) areas without pinning. Consequently, the organic probe liquids with low dielectric constants can move easily and roll off when tilted only slightly, independent of the magnitude of CAs, without relying on conventional surface roughening and perfluorination.

Producing photoactive, transparent and hydrophobic SiO2-crystalline TiO2 nanocomposites at ambient conditions with application as self-cleaning coatings

Nowadays, the enhancement of atmospheric pollution is dramatically increasing the presence of soiling on buildings in every city of the world. Thus, the development of photocatalysts as self-cleaning coatings is a promising challenge. The first object of this work was to develop a simple synthesis route for obtaining SiO2-crystalline TiO2 nanocomposites at ambient temperature. Thus, it meets the requirements to produce photoactive coatings on buildings and, additionally, it can be used in other applications requiring low temperature. The second objective was to gain insights into the structure of these materials and to establish the relationship between their structure and their performance as photocatalysts.The synthesis process involves mixing titanium and silicon alkoxides in the presence of oxalic acid. An organic silica oligomer is also added to reduce surface energy and consequently, to give hydrophobic properties to the material. These products were applied as a coating on marble specimens in order to investigate their effectiveness. The results obtained highlight that oxalic acid acts a chelating agent of the titanium precursor, giving rise to a homogeneous TiO2–SiO2 material. In addition and most importantly, oxalic acid produces photoactive anatase crystals at ambient temperature. From our investigation on the structure of these materials, we conclude that the formation of independent domains of TiO2 inside of SiO2 matrix, and the anatase crystal size are key factors for improving the photoactivity of the coatings. We also conclude that the oxalic acid concentration plays a crucial role in the formation of this optimum structure.

Interatomic Coulombic interactions as the driving force for oriented attachment

Growth of nano- and mesocrystals in both laboratory and natural environments can proceed via the oriented attachment (OA) pathway. However, the driving force for OA is controversial; surface energy reduction, van der Waals interaction, and/or dipole–dipole interactions have been proposed. Here, we analyzed the interaction energy of two approaching nanoparticles by comparing the magnitudes of Coulombic interactions from molecular energetic calculations, van der Waals interactions from the Hamaker formulation, and surface charge repulsion from the DLVO theory. The analyses were conducted for three materials: SiO2, ZnS and ZnO. Results show that in vacuum, or when two nanoparticles are in close proximity in an electrolyte solution, the strong intrinsic interatomic Coulombic interactions between two nanoparticles provide the primary physical driving force for OA. However, when two particles are far apart in a solution, the interatomic Coulombic interactions are screened and van der Waals interactions become the physical driving force (that superimposes onto the random force from the Brownian motion). In both vacuum and a solution, the energy change that occurs following an OA event (i.e., the thermodynamic driving force) comes largely from the interatomic Coulombic interactions arising from both the surface atoms (accounting for the surface energy reduction) and the atoms in interior of the nanoparticles. This energy change is crystallographic orientation-dependent. The findings of this study indicate the range of conditions under which interatomic Coulombic interactions provide the primary driving force for crystal growth by OA and highlight the effects of aqueous solution and ionic strength on the energetics of the process.

Effect of organic materials used in the synthesis on the emission from CdSe quantum dots

Quantum-dot nanocrystals have particular optical properties due to the quantum confinement effect and the surface effect. This study focuses on the effect of surface conditions on the emission from quantum dots. The quantum dots prepared with 1-hexadecylamine (HDA) in the synthesis show strong emission while the quantum dots prepared without HDA show weak emission, as well as emission from surface energy traps. The comparison of the X-ray patterns of these two sets of quantum dots reveals that HDA forms a layer on the surface of quantum dot during the synthesis. This surface passivation with a layer of HDA reduces surface energy traps, therefore the emission from surface trap levels is suppressed in the quantum dots synthesized with HDA.

Characterization of α-tocopherol as interacting agent in polyvinyl alcohol–starch blends

In this study, the interactions of α-tocopherol (α-TOH) in PVOH–starch blends were investigated. α-TOH is an interacting agent possesses a unique molecule of polar chroman “head” and non-polar phytyl “tail” which can improve surface interaction of PVOH and starch. It showed favorable results when blending PVOH–starch with α-TOH, where the highest tensile strengths were achieved at 60wt.% PVOH–starch blend for 1 phr α-TOH and 50wt.% for 3 phr α-TOH, respectively. This due to the formation of miscible PVOH–starch as resulted by the compatibilizing effect of α-TOH. Moreover, the enthalpy of melting (ΔHm) of 60wt.% PVOH–starch and 50wt.% PVOH–starch added with 1 and 3 phr α-TOH respectively were higher than ΔHm of the neat PVOH–starch blends. The thermogravimetry analysis also showed that α-TOH can be used as thermal stabilizer to reduce weight losses at elevated temperature. The surface morphologies of the compatible blends formed large portion of continuous phase where the starch granules interacted well with α-TOH by acting as compatilizer to reduce surface energy of starch for embedment into PVOH matrix.

In-situ atomic-scale observation of irradiation-induced void formation

The formation of voids in an irradiated material significantly degrades its physical and mechanical properties. Void nucleation and growth involve discrete atomic-scale processes that, unfortunately, are not yet well understood due to the lack of direct experimental examination. Here we report an in-situ atomic-scale observation of the nucleation and growth of voids in hexagonal close-packed magnesium under electron irradiation. The voids are found to first grow into a plate-like shape, followed by a gradual transition to a nearly equiaxial geometry. Using atomistic simulations, we show that the initial growth in length is controlled by slow nucleation kinetics of vacancy layers on basal facets and anisotropic vacancy diffusivity. The subsequent thickness growth is driven by thermodynamics to reduce surface energy. These experiments represent unprecedented resolution and characterization of void nucleation and growth under irradiation, and might help with understanding the irradiation damage of other hexagonal close-packed materials.

Passivation of High-Surface-Energy Sites of Milled Ibuprofen Crystals via Dry Coating for Reduced Cohesion and Improved Flowability

Ibuprofen micronization with dry coating is investigated to examine its influence on passivation/stabilization of high-surface-energy sites and reduced cohesion. A fluid energy mill was used to micronize ibuprofen particles down to 5-28 μm with or without simultaneous nanosilica coating. Powder flow property and dispersibility were characterized using FT4 powder tester and Rodos/Helos laser diffraction particle sizer. Surface energy was characterized using a next generation inverse gas chromatography instrument. Uncoated micronized ibuprofen showed an increased Lifshitz-van der Waals (LW) dispersion component of surface energy with increasing milling intensity. In contrast, dry-coated milled powders showed a significant reduction in the LW component, whereas physical mixture of uncoated micronized ibuprofen and silica exhibited no reduction in surface energy, indicating that dry coating is necessary for the passivation of high-energy sites of ibuprofen created during micronization. Surface energy of pure micronized ibuprofen was highly heterogeneous, whereas dry-coated ibuprofen had greatly reduced heterogeneity. Micronization with dry coating also improved flowability and bulk density as compared with pure active pharmaceutical ingredient micronization without coating, or just blending with silica. Overall, dry coating leads to decreased cohesion and improved flowability because of reduced LW dispersive component of surface energy and creating nanoscale surface roughness.

Dispersion of solvent-wet carbon nanotubes for electrical CNT/polydimethylsiloxane composite

Highly entangled carbon nanotube (CNT) aggregates contain lots of empty spaces formed by entangled nanotubes wherein a viscous fluid has difficultly to diffuse through these spaces without strong driving force. It is necessary to fill the free volume with the matrix before the dispersion of CNTs. This study is focused on the investigation of the solvent wetting effect of CNTs on their dispersion. The heterogeneously distributed narrow and long tunnels, like porous media in entangled clumps, are wetted with a solvent before dispersion. It results in the improved electrical performance of CNT/PDMS composite since the solvent-wet CNTs have reduced surface energy and may improve the dispersion of CNTs under flow. The dispersion of the solvent-wet CNTs may reduce the entangled CNT aggregates since they have sufficient time to wet and the external energy transferred to the composite during dispersion is used only to disperse the CNT aggregate. Also, if the solvent with a low evaporation temperature is chosen, it does not remain in the composite product at all. The solvent-wetting method is suggested as a novel dispersion method having a high potential for industrial applications, from storage of CNTs to dispersion.

Thermal Stability of Gold Nanorods for High-Temperature Plasmonic Sensing

There are many potential sensing applications for Au nanorods due to a tunable localized surface plasmon resonance (LSPR) frequency that changes with aspect ratio. However, their application at high temperatures is limited due to a shape change that can take place well below the melting point of bulk Au, driven by a reduction in surface energy. A method of stabilizing Au nanorods is provided here by encapsulating them with a 15 nm capping layer of yttria stabilized zirconia (YSZ). After annealing rods with nominal dimensions of 100 × 44 nm to a temperature of 600 °C, small reductions in length were observed, but the rods remained stable for all subsequent sensing tests at 500 °C, which amounted to 80 h. It was shown with a separate sample that the rod geometry can be preserved even up to 800 °C over a 12 h annealing period, although a significant shortening of the rod length occurred, leaving a void space in the YSZ. The sensing response of both the transverse and the longitudinal LSPR peaks was monitored...

Twin-Free GaAs Nanosheets by Selective Area Growth: Implications for Defect-Free Nanostructures

Highly perfect, twin-free GaAs nanosheets grown on (111)B surfaces by selective area growth (SAG) are demonstrated. In contrast to GaAs nanowires grown by (SAG) in which rotational twins and stacking faults are almost universally observed, twin formation is either suppressed or eliminated within properly oriented nanosheets are grown under a range of growth conditions. A morphology transition in the nanosheets due to twinning results in surface energy reduction, which may also explain the high twin-defect density that occurs within some III–V semiconductor nanostructures, such as GaAs nanowires. Calculations suggest that the surface energy is significantly reduced by the formation of {111}-plane bounded tetrahedra after the morphology transition of nanowire structures. By contrast, owing to the formation of two vertical {11[overline]0} planes which comprise the majority of the total surface energy of nanosheet structures, the energy reduction effect due to the morphology transition is not as dramatic as that for nanowire structures. Furthermore, the surface energy reduction effect is mitigated in longer nanosheets which, in turn, suppresses twinning.

Hydrothermal Growth of Tailored SnO2 Nanocrystals

We studied the growth of SnO2 nanocrystals with a tailored structure via surface capping assisted hydrothermal approach with tetramethyl ammonium hydroxide (N(CH3)4OH; TMAH). KOH and NaOH were also used instead of TMAH for comparison. The nanocrystals with a size ranging from 3.2 ± 0.9 to 74 ± 20 nm were grown at 150 °C for 24 h depending on the pH. N(CH3)4+ capped the surface of SnO2 and improved the dispersion of the nanocrystals in basic aqueous solutions. The capping provided nanocrystals finer than those grown with KOH and NaOH because of suppressed Ostwald ripening via a reduction in surface energy. High-resolution transmission electron microscopy revealed that the nanocrystals grown in strong basic solutions with TMAH had cubic morphology terminated by the (001) and (110) faces. This strongly suggests that N(CH3)4+ preferentially caps the (001) face with the highest surface energy and decreases its surface energy to be comparable to that of the (110) face with the lowest surface energy. Anisotropic...