Effect of Structural Parameters of TiO2 Nanotube Arrays upon Their Photocatalytic/Photoelectrocatalytic Performance

The effect of structural parameters of TiO₂ nanotube arrays (TNAs) upon their photocatalytic/photoelectro‐catalytic performance is studied by comparing the morphological characteristics and physicochemical properties with different tube lengths prepared from three kinds of electrolytes. The results show that the UV‐Vis absorption edge of TNAs red‐shifted with the increment of tube length and the short TNAs possess higher bandgap energy. The variation tendency of electrochemical window of TNAs is DMSO (5.5 V)>Cit (3.2 V)>HF (1.8 V). The long TNAs possess higher photocatalytic (PC) reactivity suggesting the surface roughness factor is the main determinant of PC efficiency, although, there is obvious recombination effects for the long TNAs. Evidenced by the positive correlation between tube length and photoelectrocatalytic (PEC) efficiency for TNAs from the same electrolyte, the enhancement of the tube length could lead to better PEC reactivity, but when the tube length is over a certain value, the PEC degradation rate no longer increases but decreases. The long TNAs with large surface roughness factor prepared from Cit and DMSO electrolytes exhibit comparative or even lower PEC performance compared with the short TNAs prepared from HF electrolyte, indicating that the PEC performance of TNAs was dominated by charge separation and photoelectron transfer properties rather than surface roughness coefficient and the tube length.     

Oscillatory forces of nanoparticle suspensions confined between rough surfaces modified with polyelectrolytes via the layer-by-layer technique

This paper addresses the systematic study of surface roughness effects on the internal structuring of silica nanoparticle suspensions under confinement. The confining surfaces are modified by physisorption of layers of oppositely charged polyelectrolytes with the so-called layer-by-layer technique. The layer-by-layer technique modifies the surface roughness without changing the surface potential of a multilayer with the same outermost layer, by increasing the number of constituent layers and ionic strength of the polyelectrolyte solutions and by selecting an appropriate pair of polyelectrolytes. The oscillatory forces of nanoparticle suspensions with a particle diameter of 26 nm are measured by a colloidal-probe atomic force microscope (CP-AFM). The characteristic lengths of the oscillatory force, i.e., wavelength, which indicates interparticle distance, and decay length, or particle correlation length, are not affected by the surface roughness. The corresponding reduction in the oscillatory amplitude and the shift in the phase correlate with an increase in surface roughness. Increasing surface roughness further induces a disappearance of the oscillations, and both confining surfaces contribute to the effect of surface roughness on the force reduction. In order to show an oscillatory force, the particles have to show positional correlation over a reasonably long range perpendicular to the surface, and the correlation function should be the same over a larger lateral area. This requires that both the particles and the surfaces have a high degree of order or symmetry; otherwise, the oscillation does not occur. A roughness of a few nanometers on a single surface, which corresponds to about 10% of the nanoparticle diameter, is sufficient to eliminate the oscillatory force.     

Self-affine roughness influence on the friction coefficient for rubbers onto solid surfaces

In this paper we investigate the influence of self-affine roughness on the friction coefficient mu(f) of a rubber body under incomplete contact onto a solid surface. The roughness is characterized by the rms amplitude w, the correlation length xi, and the roughness exponent H. It is shown that with increasing surface roughening at short and/or long length scales (decreasing H and/or increasing ratio w/xi, respectively), the maximum of the friction coefficient mu(f) shifts to lower sliding velocities. The latter occurs only for conditions of incomplete contact for small contact length scales lambda (xi).     

The influence of substrate surface roughness on microstrip transmission line loss using conventional analyses and length‐scale fractal analysis

Two components of conductor topography can impact conductor loss for signals in the GHz frequency range: conductor–ceramic interface roughness and conductor edge angle. This study is an experimental investigation of the influence of these conductor topographies on conductor loss in microstrip circuits produced by thick‐film technology. The aluminum nitride ceramic substrates have different surface roughnesses due to different surface finish processes. The substrate surfaces were characterized using conventional and length‐scale fractal analysis. The conductor–ceramic interface was measured with a contact profilometer. The conductor edge angle and conductor edge profile were measured optically. It was found that there is a direct correlation between conductor loss and conductor edge angle, whereas there is an inverse correlation between loss and substrate roughness or relative length of the conductor–ceramic interface. This is the opposite result to the conventional expectation of surface roughness effects on conductor loss. There is also a negative correlation between conductor edge angle and surface roughness or relative length. The loss behavior can be explained by the interaction of the conductor paste with the surfaces during processing. The paste tends to spread more on the smoother surfaces, and thus creates an elongated edge of diminishing cross‐section and a small edge angle. This leads to greater conductor loss. Copyright © 2009 John Wiley & Sons, Ltd.     

Droplet morphologies on particles with macroscopic surface roughness

The equilibrium configuration of liquid droplets on the surface of macroscopically rough solid particles was determined by numerical simulations using the volume-of-fluid (VOF) method. The fractional surface coverage of the particle as a function of the droplet size, equilibrium contact angle, and the particle surface roughness amplitude and correlation length has been systematically investigated. Droplet size and contact angle were found to generally have a stronger effect on surface coverage than particle surface roughness. Because of droplet coalescence, a relatively large variation in surface coverage was observed for any given total liquid volume, particularly for larger values of the equilibrium contact angle.     

Optical properties of polymer films for transparent insulation

Commercially available, highly transparent polymer films for transparent insulation applications were investigated systematically as to their relevant optical properties in the solar and infrared wavelength range. The photometric characterisation in the solar range and the calculation of non-spectral, solar optical film properties using models for scattering-absorbing media have shown, that the solar extinction is dominated by scattering occuring mainly at the surface. For various amorphous and semi-crystalline films the root-mean-square surface roughness correlated well with the solar optical thickness. Regarding high infrared absorptance in the wavelength range of about 10 μm the carbon-oxygen single bond is highly effective for commercial materials with maximum service temperatures of about 100°C. For 50 μm thick films of different polymer types with carbon-oxygen single bonds in the molecular structure a good correlation between the concentration of the functional corbon-oxygen group and the nonspectral, infrared optical thickness was found.     

Characterization of synthetic hematite (α-Fe2O3) nanoparticles using a multi-technique approach

The aim of this work was to investigate the surface structure of aqueous hematite dispersions characterized by a large variability of morphology and particle size combining structural investigations obtained from Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM) techniques with in vitro particle size distributions and zeta potential measurements from Dynamic Light Scattering (DLS) technique, and we achieved a self-consistent and detailed characterization of hematite particles whose sizes and morphologies could be correlated to the synthesis conditions (type of added anion, Al substitution and pH). Surface AFM characterization provided an accurate analysis of particle microstructure and also indicated that the growth of microcrystals followed different surface roughness. DLS, AFM, and TEM techniques furnished complementary information on the average particle dimensions, whose variation could be attributed to the morphological difference of hematites, ranging from platy to regular or irregular hexagonal or ellipsoidal shape. Finally, a correlation between the average particle dimensions and the measured zeta potential was also been found in aqueous dilute suspensions characterized by neither pH nor-ionic-strength-control, for which a drop of zeta potential from positive to negative values was detected for hematite particle dimensions larger than a threshold size of ~150 nm.     

Surface structure of amorphous PMMA from SPM: auto‐correlation function and fractal analysis

The surface topography of amorphous poly(methyl methacrylate) has been investigated by scanning probe microscopy technique and analysed using an auto‐correlation function approach. Spatial correlations in the arrangement of sub‐macromolecular fragments on the surface are studied depending on preparation conditions (whether the surface was free, i.e. in contact with air, or confined with the silicon substrate). The correlation length of the roughness distribution on the surface of amorphous polymers corresponds to the internal dimensions of unperturbed polymer chains (the radius of gyration). The scaling behaviour of the roughness is analysed with the Hurst approach, and found to be different for the free surface and for the confined one. As a result of annealing at a temperature above the glass transition temperature of the bulk polymer, both surfaces converge to similar fractal dimensionalities and correlation lengths.     

Characterization of Plasma Polymerized Hexamethyldisiloxane Films Prepared by Arc Discharge

Herein, we present a simple method for fabricating plasma polymerized hexamethyldisiloxane films (pp-HMDSO) possessing superhydrophobic characteristics via arc discharge. The pp-HMDSO films were deposited on a soda–lime–silica float glass using HMDSO monomer vapor as a precursor. A detailed surface characterization was performed using scanning electron microscopy and atomic force microscopy. The growth process of the pp-HMDSO films was investigated as a function of deposition time from 30 to 300 s. The non-wetting characteristics of the pp-HMDSO films were evaluated by means of contact angle (CA) measurements and correlated with the morphological characteristics, as obtained from microscopy measurements. The deposited films were found to be nano-structured and exhibited dual-scale roughness with the static CA values close to 170°. Fourier transform infrared spectroscopy analysis was carried out to investigate chemical and functional properties of these films. Methyl groups were identified spectroscopically to be present within the pp-HMDSO films and were proposed to result in the low surface energy of material. The synergy between the dual-scale roughness and low surface energy resulted in the superhydrophobic characteristics of the pp-HMDSO films. A possible mechanism for the pp-HMDSO film formation is proposed.     

Characterizing the molecular order of phosphonic acid self-assembled monolayers on indium tin oxide surfaces

Self-assembled monolayers (SAMs) of alkanephosphonic acids with chain lengths between 8 and 18 carbon units were formed on thin films of indium tin oxide (ITO) sputter-deposited on silicon substrates with 400 nm thermally grown SiO(2). The silicon substrates, while not intended for use in near-IR or visible optics applications, do provide smooth surfaces that permit systematic engineering of grain size and surface roughness as a function of the sputter pressure. Argon sputter pressures from 4 to 20 mTorr show systematic changes in surface morphology ranging from smooth, micrometer-sized grain structures to <50 nm grains with 3× higher surface roughness. Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy experiments are conducted for alkanephosphonic acids deposited on these wide range of ITO surfaces to evaluate the effects of these morphological features on monolayer ordering. Results indicate that long-chain SAMs are more highly ordered, and have a smaller tilt angle, than short-chain SAMs. Surprisingly, the 1-octadecyl phosphonic acids maintain their order as the lateral grain dimensions of the ITO surface shrink to ～50 nm. It is only when the ITO surface roughness becomes greater than the SAM chain length (～15 ?) that SAMs are observed to become relatively disordered.     

Orientation and order in thin films of a combined liquid crystalline polymer

The order in thin films of a combined liquid crystalline polymer is studied by X-ray reflection. Films of thicknesses of less than 200 nm on float glass are investigated as a function of temperature. The polymer with mesogenic groups in the main and side-chains exhibits smectic and cholesteric mesophases. Measurements in the smectic phases show a Bragg peak and smectic layers are oriented parallel to the substrate. The sample is thus macroscopically ordered by the influence of substrate and free surface. The film surface is very smooth after spincoating; surface roughness is typically 0.8 nm. First annealing of samples leads to a significant roughening of the free surface; roughness increases to 2.1 nm. Order as a function of film thickness depends on the interaction of the polymer with the substrate and free surface. These interactions give rise to a typical correlation length of perturbations in smectic ordering.     

The Role of inclusion size in toughening of epoxy resins by spherical micelles

We report our finding of an optimal length scale for toughening of epoxies using spherical micelles formed by block copolymers. The amphiphilic diblock copolymer poly(hexylene oxide)‐poly(ethylene oxide) (PHO‐PEO) with 30 wt % PEO self‐assembled to form spherical micelles in a bisphenol A epoxy resin with a phenol novolac hardener. We systematically increased the size of the spherical micelles from 20–30 nm to 0.5–10 μm by swelling their PHO core using PHO homopolymer. Although all the blends were tougher than the unmodified epoxy, the largest enhancement of fracture resistance was measured in blends containing 0.1–1 μm spherical inclusions. This enhanced toughness was correlated with plastic deformation by shear banding in tensile test and greater roughness of the fracture surface. Smaller micelles neither induced plastic deformation nor contributed to surface roughness significantly whereas larger micelles acted as local defects resulting in early failure. These findings provide a framework in assessing the toughening effects of blended block copolymers on epoxy resins. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1125–1129, 2009     

Surface Corrosion Studies on High-Purity Quartz Vessels for Digestive Sample Preparation

 High purity quartz is the most common material for pressure vessels of high pressure digestion systems. Such systems are in worldwide use especially for matrix decomposition and mineralization procedures with either conventional heating or heating in a microwave field. Usually, the quartz vessels are used for hundreds of decompositions and corrosive attack is often already visible with the bare eye. On the other hand it is well known that adsorption as well as leaching phenomena increase with rising surface roughness of a material. This is especially critical in trace and ultratrace work. For the latter such interactions with the vessel wall might even become the limiting factor in ultratrace determinations at the ng/g-level and below. Therefore, systematic morphological and compositional investigations by high resolution scanning electron microscopy were performed on the inner surfaces of decomposition vessels which were subjected to a rising number of digestions under pressure in a Multiwave digestion apparatus. Milk powder was digested with nitric acid for these investigations. Rising corrosive attack was observable but did not lead to severely damaged surfaces. The latter were found on a quartz pressure vessel for an HPA-digestion apparatus which was in intensive use over a long time. Recrystallization was also observable in this case. In addition to investigations by scanning electron microscopy, FT-NIR-Raman spectroscopy was also applied for the surface characterization of the various quartz vessels. The onset of recrystallization is detected much earlier with this method than by morphological changes in the scanning electron microscope (SEM). It appears that the combination of surface morphological and topochemical investigations in the SEM with vibrational spectroscopy is ideal for an optimal characterization of corroded quartz surfaces. Received January 15, 2001. Revision April 14, 2001.     

Influence of the substrate type on the microstructural,optical and electrical properties of sol–gel ITO films

Indium tin oxide (ITO) is recognized as the best transparent and conductive material [transparent conducting oxide (TCO)] until now and its properties are dependent on the preparation method. In the present work ITO films with In:Sn atomic ratio 9:1 were prepared by a sol–gel route on different substrates (microscope glass slides, microscope glass covered with one layer of SiO₂ and Si wafers) for TCO applications. The multilayer ITO films were obtained by successive deposition by the dip-coating method and the films were characterized from the structural, morphological, optical, and electrical points of view using X-ray diffraction, scanning electron microscopy, atomic force microscopy, spectroscopic ellipsometry and by Hall effect measurements, respectively. The results showed that the thickness, optical constants and carrier numbers depend strongly on the type of substrate, number of deposited layers and sol concentration. The optical properties of ITO films are closely related to their electrical properties. The enhancement of the conductivity was possible with the increase of crystallite size (which occurred after thermal treatment) and with the reduction of surface roughness.     

Fabrication and characterization of plasma processed surfaces with tuned wettability

Engineered surfaces with controlled hydrophilic/ hydrophobic character have been fabricated by tailoring the substrate topography and chemistry. In this method, the substrate to be treated was first coated by a photoresist, which was then surface-roughened using SF6 plasma etching. The resulting rough texture was then transferred to the underlying silicon surface by over-etching of the photoresist. At this point, the topographically modified surface was modified chemically by controlled deposition of a thin polymer layer using plasma processing. In this way, both the surface texture and the surface chemistry could be varied independently, producing surfaces with variable wetting character, including super-hydrophilicity and super-hydrophobicity, depending on the choice of plasma polymer deposited. Chemical characterization demonstrates a correlation between the surface chemistry and the wettability of the samples after etching. The surface elementary composition contained more C-F groups as the measured contact angle increased, indicating that the change of wettability is due to both the roughness and the surface energy of the deposited photoresist. In the case of materials deposited on the plasma-treated rough surfaces, the strengthening of the wetting character is only due to the created surface roughness, as XPS analyses showed no significant chemical difference as compared to the flat polymer.     

Investigation of the effect of dual-size coatings on the hydrophobicity of cotton surface

A series of silica nanoparticles with two different length scales were introduced onto the cotton substrates to study the correlation between the surface structure and observed hydrophobicity. SiO₂ nanoparticles of 7, 12, 20, and 40 nm in size were individually functionalized using 3-aminopropyl triethoxysilane or 3-glycidoxypropyltrimethoxysilane. Amino functionalized silica nanoparticles were durably attached to the cotton surface that was previously treated with epichlorohydrin. By depositing an additional layer of epoxy modified silica nanoparticles, a dual-size hierarchical coating was obtained. It was found that the order of deposition of particles to develop dual-size coatings determines the surface roughness, hydrophobicity and the amount of silica loaded on the cotton substrate. Deposition of the bigger nanoparticles on top of smaller ones resulted in rougher surfaces, higher hydrophobicity and higher amount of silica loading onto the cotton surface. A strong correlation between the size ratio of deposited nanoparticle combinations and the amount of silica loading was observed. It was found that there is also a direct relationship between the surface roughness and the hydrophobicity of the samples generated. Based upon these correlations, it is now possible to tune surface roughness and subsequent wettability by controlling the sizes of the dual-type nanoparticle layers.     

Plasma pretreatment of polycarbonate substrates for indium zinc oxide film deposition

The influence of plasma treatment of polycarbonate (PC) substrates on the morphological, electrical, and adhesion properties of deposited amorphous transparent indium zinc oxide (IZO) by direct current magnetron sputtering was investigated by analyzing atomic force microscopy, contact angles, Hall, and nano‐scratch measurements. The surfaces of PC substrates were performed by plasma treatment at various processing times in Ar/O₂ mix atmosphere. The atomic force microscopy images indicated that the microstructure of the substrates considerably influenced the surface morphology of deposited IZO films, and the least surface roughness of IZO was obtained after 5‐s plasma treatment. The IZO film deposited on PC with 5‐s plasma treatment presented an improved electrical conductivity and thermal stability after annealing at 120 °C in air, whereas the significant decrease in carrier concentration and increase in resistivity with extending plasma treatment time were observed, which was attributed to the elevated oxygen adsorption during annealing for a loosely packed structure. Moreover, the adhesion properties of IZO films with PC substrates decreased after 30‐s plasma treatment because of the significant difference on the surface polarity between the PC and thin films and the increased roughness caused by plasma etching. Copyright © 2016 John Wiley & Sons, Ltd.     

High surface water interaction in superhydrophobic nanostructured silicon surfaces: convergence between nanoscopic and macroscopic scale phenomena

In the present work, we investigate wetting phenomena on freshly prepared nanostructured porous silicon (nPS) with tunable properties. Surface roughness and porosity of nPS can be tailored by controlling fabrication current density in the range 40-120 mA/cm(2). The length scale of the characteristic surface structures that compose nPS allows the application of thermodynamic wettability approaches. The high interaction energy between water and surface is determined by measuring water contact angle (WCA) hysteresis, which reveals Wenzel wetting regime. Moreover, the morphological analysis of the surfaces by atomic force microscopy allows predicting WCA from a semiempiric model adapted to this material.     

Protein interaction with a Pluronic-modified poly(lactic acid) Langmuir monolayer

Interaction of bovine serum albumin (BSA) with poly(lactic acid) (PLA) layers mixed with poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers (Pluronic) at air/solution interfaces was studied by the Langmuir balance technique. Wettability of the mixed PLA-Pluronic system was characterized in the form of a transferred one-layer Langmuir-Blodgett film, and considerable hydrophilization was obtained for all of the Pluronics (6400, 6800, 10500, and 12700) applied here. The density of PEO chains in the monolayer and hence the coverage of PLA was controlled by the composition and the compression of the mixed monolayers. Tensiometric investigations revealed that a significant reduction of BSA adsorption/penetration was achieved by applying the Pluronic 6800 and 12700 with long PEO blocks for hydrophilization of PLA. Interaction of BSA with the modified PLA monolayer depended on the density and length of the PEO chains. The surface morphological characteristics of the films determined by atomic force microscopy were in good correlation with the results of BSA interaction. The average roughness of the polymer LB layer was high due to BSA penetration into the PLA film, while smooth surfaces with small roughness were obtained when the PLA layer was modified by Pluronic 6800.     

Determination of colloidal gold nanoparticle surface areas, concentrations, and sizes through quantitative ligand adsorption

Determination of the true surface areas, concentrations, and particle sizes of gold nanoparticles (AuNPs) is a challenging issue due to the nanoparticle morphological irregularity, surface roughness, and size distributions. A ligand adsorption-based technique for determining AuNP surface areas in solution is reported. Using a water-soluble, stable, and highly UV–vis active organothiol, 2-mercaptobenzimidazole (MBI), as the probe ligand, we demonstrated that the amount of ligand adsorbed is proportional to the AuNP surface area. The equivalent spherical AuNP sizes and concentrations were determined by combining the MBI adsorption measurement with Au³⁺ quantification of aqua regia-digested AuNPs. The experimental results from the MBI adsorption method for a series of commercial colloidal AuNPs with nominal diameters of 10, 30, 50, and 90 nm were compared with those determined using dynamic light scattering, transmission electron microscopy, and localized surface plasmonic resonance methods. The ligand adsorption-based technique is highly reproducible and simple to implement. It only requires a UV–vis spectrophotometer for characterization of in-house-prepared AuNPs.