Surface-Polarity-Induced Spatial Charge Separation Boosts Photocatalytic Overall Water Splitting on GaN Nanorod Arrays

Photocatalytic overall water splitting has been recognized as a promising approach to convert solar energy into hydrogen. However, most of the photocatalysts suffer from low efficiencies mainly because of poor charge separation. Herein, taking a model semiconductor gallium nitride (GaN) as an example, we uncovered that photogenerated electrons and holes can be spatially separated to the nonpolar and polar surfaces of GaN nanorod arrays, which is presumably ascribed to the different surface band bending induced by the surface polarity. The photogenerated charge separation efficiency of GaN can be enhanced significantly from about 8 % to more than 80 % via co-exposing polar and nonpolar surfaces. Furthermore, spatially assembling reduction and oxidation cocatalysts on the nonpolar and polar surfaces remarkably boosts photocatalytic overall water splitting, with the quantum efficiency increased from 0.9 % for the film photocatalyst to 6.9 % for the nanorod arrays photocatalyst.     

Surface‐Polarity‐Induced Spatial Charge Separation Boosts Photocatalytic Overall Water Splitting on GaN Nanorod Arrays

Photocatalytic overall water splitting has been recognized as a promising approach to convert solar energy into hydrogen. However, most of the photocatalysts suffer from low efficiencies mainly because of poor charge separation. Herein, taking a model semiconductor gallium nitride (GaN) as an example, we uncovered that photogenerated electrons and holes can be spatially separated to the nonpolar and polar surfaces of GaN nanorod arrays, which is presumably ascribed to the different surface band bending induced by the surface polarity. The photogenerated charge separation efficiency of GaN can be enhanced significantly from about 8 % to more than 80 % via co‐exposing polar and nonpolar surfaces. Furthermore, spatially assembling reduction and oxidation cocatalysts on the nonpolar and polar surfaces remarkably boosts photocatalytic overall water splitting, with the quantum efficiency increased from 0.9 % for the film photocatalyst to 6.9 % for the nanorod arrays photocatalyst.     

Surface restructuring of polymers

Time-dependent contact angle measurements are employed to follow the dynamics of surface modifications of various polymeric surfaces of different hydrophilicities. The equilibration of a hydrophilic polymer in a strong polar environment (such as water) induces an increase in the polarity of the surface; the subsequent exposure of the restructured solid to a nonpolar environment decreases the polarity of the surface. The dynamics of these processes depends on the history of the specimen. Various phenomena, such as surface restructuring by the reorientation of the buried polar or nonpolar moieties, water penetration into the polymer, and the reorganization of water in the neighborhood of the surface, are suggested to be responsible for the time evolution of the dynamic contact angles.     

Delivering octadecylphosphonic acid self-assembled monolayers on a Si wafer and other oxide surfaces

We describe a simple experimental approach for delivering self-assembled monolayers (SAMs) of octadecylphosphonic acid (OPA) on many oxide surfaces using a nonpolar medium with a dielectric constant around 4 (e.g., trichloroethylene). This approach readily results in the formation of full-coverage OPA SAMs on a wide variety of oxide surfaces including cleaved mica, Si wafer, quartz, and aluminum. Especially, the availability of delivering full-coverage OPA SAM on a Si wafer is unique, as no OPA SAMs at all could be formed on a Si wafer when using a polar OPA solution. The reason a nonpolar solvent is superior lies in the very fact that the hydrophilic OPA headgroup tends to escape from the nonpolar solution and is thus enriched at the medium-air interface. It is these OPA headgroups seeking a hydrophilic surface that make possible the well-controlled OPA monolayer on an oxide surface.     

Role of hydration force in the self-assembly of collagens and amyloid steric zipper filaments

In protein self-assembly, types of surfaces determine the force between them. Yet the extent to which the surrounding water contributes to this force remains as a fundamental question. Here we study three self-assembling filament systems that respectively have hydrated (collagen), dry nonpolar, and dry polar (amyloid) interfaces. Using molecular dynamics simulations, we calculate and compare local hydration maps and hydration forces. We find that the primary hydration shells are formed all over the surface, regardless of the types of the underlying amino acids. The weakly oscillating hydration force arises from coalescence and depletion of hydration shells as two filaments approach, whereas local water diffusion, orientation, or hydrogen-bonding events have no direct effect. Hydration forces between hydrated, polar, and nonpolar interfaces differ in the amplitude and phase of the oscillation relative to the equilibrium surface separation. Therefore, water-mediated interactions between these protein surfaces, ranging in character from "hydrophobic" to "hydrophilic", have a common molecular origin based on the robustly formed hydration shells, which is likely applicable to a broad range of biomolecular assemblies whose interfacial geometry is similar in length scale to those of the present study.     

Mixed alkanethiol monolayers on gold surfaces: Wetting and stability studies

Studies of wetting and stability of mixed monolayers containing hydrophobie and hydrophilic components are discussed. We are reporting the observation of an apparent concentration-driven transition in the cosine of the contact angles of liquids on mixed monolayers. It is suggested that this phenomenon is due to a possible (true or rounded) surface phase transition, resulting in the formation of a prewetting water layer. This formation is triggered by variations in the quenched distribution of random surface fields. The variation of the surface free-energy, both polar and dispersive parts, has been determined as a function of surface OH-concentration. The surface free-energy of the 100% OH surface is close to that found for water, as might be expected for a surface coated with several monolayers of water. Zisman plots obtained for several of the surfaces using polar and nonpolar liquids give γ_c values which follow the observed dispersive contribution to the total surface free energy, and thus do not present a good approximation to the surface free energy (i.e., γ_c < γ_sv).Contact angle variation was studied on self-assembled alkanethiol monolayers containing mixtures of OH and CH₃ groups at their air-monolayer interface. It was found that these high free energy organic surfaces yielded contact angles which were not stable over long periods of time. The extent of the variation was found to be related to the surface free energy (%OH). The effect of different storage environments and temperature on the changing contact angles are discussed. We propose that monolayer surfaces containing high concentrations of OH groups on mobile organic chains are not stable. Such monolayer surfaces may stabilize over time, depending on the chain length, by surface reorganization and the adsorption of contaminants.     

Using ferroelectric poling to change adsorption on oxide surfaces

Adsorption has been invoked to explain many phenomena in ferroelectric materials including the unanticipated stability of ultrathin ferroelectric films; however, the intrinsic surface properties of ferroelectric oxides have been largely unexplored. Therefore, the effect of ferroelectric poling on the adsorption/desorption of two polar molecules, acetic acid and 2-propanol, and one nonpolar molecule, dodecane, on LiNbO3(0001) was compared. The two polar molecules were found to adsorb significantly more strongly on the positive surface. Temperature-programmed desorption (TPD) data yielded desorption pre-exponentials of the two polar molecules more than 11 orders of magnitude lower than expected. Ferroelectric materials are also intrinsically pyroelectric, and it is shown that the unusually low desorption pre-exponentials can be explained by temperature dependent heats of adsorption that result from changes in the surface dipole as the samples are heated. This conclusion was supported by dodecane adsorption/desorption, which was independent of polarity with normal desorption pre-exponentials. The differences between the polar and nonpolar molecules indicate that interactions between polar molecules and ferroelectric surfaces are dominated by electrostatics. It is shown that adsorption energy differences between positive and negative surfaces are large enough to switch the polarity of ferroelectric thin films.     

Interfacial Adhesion between Fatty Acid Collectors and Hydrophilic Surfaces: Implications for Low-Rank Coal Flotation

Fatty acids, which are enriched in vegetable oil, have attracted much attention in low-rank coal flotation because of their unique chemical structure. In this study, density functional theory calculations, molecular dynamics simulations, and atomic force microscopy were employed to investigate the adsorption structure and forces between collectors and hydrophilic surfaces. The results show that fatty acids can be easily adsorbed onto surfaces through hydrogen bonds, and can cover the oxygen sites. The existence of hydration film on hydrophilic surfaces prevented nonpolar molecules from being able to adsorb, while polar fatty acids could adsorb and expel water molecules. The adhesion force between the RCOOH-terminated probe and the surface appeared in the retraction process, which differed significantly from that of the RCH₃-terminated probe, indicating that polar fatty acids are more suitable as flotation collectors for low-rank coal than nonpolar hydrocarbon oil. The simulation and AFM test revealed the mechanisms of polar fatty acids, and can provide guidance for low-rank coal flotation applications.     

A new approach to the design of surface treatments for reinforcing agents

A new approach for design of improved interfacial bonding between carbon fibers and polymeric matrices is described. In this method the affinity of surface-treated carbon fibers to various polymer matrices is simulated by analyzing the adsorption characteristics of model compounds, typical of the polymer units, onto very high surface area carbon fibers. The affinity can be evaluated by the use of Thermogravimetric Analysis (TGA) to measure adsorption–desorption characteristics. Since high surface area, activated carbon fibers (ACF) can be chemically treated to obtain acidic, basic, nonpolar or highly polar surfaces, it should be possible to assess the interactions of chemically modified surfaces with model compounds such as methanol (MeOH), acetone, dimethylformamide (DMF), dimethylether, tetrahydrofuran (THF) and water. Some preliminary results are presented, indicating that surface treatments for carbon fibers other than oxidation may lead to enhanced bonding with polymeric matrices.     

Adsorption and structural characteristics of the surface of modified nanodiamond powders

The adsorption of benzene and water vapors on the surface of detonation nanodiamond powders that differed in particle size, degree of their strong aggregation, and chemical state of their surface was studied. Specific features of the processes of adsorption of polar and nonpolar molecules on chemically modified surfaces of nanodiamonds are analyzed.     

Chemical force spectroscopy in heterogeneous systems: intermolecular interactions involving epoxy polymer,mixed monolayers,and polar solvents

We used chemical force microscopy (CFM) to study adhesive forces between surfaces of epoxy resin and self-assembled monolayers (SAMs) capable of hydrogen bonding to different extents. The influence of the liquid medium in which the experiments were carried out was also examined systematically. The molecular character of the tip, polymer, and liquid all influenced the adhesion. Complementary macroscopic contact angle measurements were used to assist in the quantitative interpretation of the CFM data. A direct correlation between surface free energy and adhesion forces was observed in mixed alcohol-water solvents. An increase in surface energy from 2 to 50 mJ/m(2) resulted in an increase in adhesion from 4-8 nN to 150-300 nN for tips with radii of 50-150 nm. The interfacial surface energy for identical nonpolar surface groups of SAMs was found not to exceed 2 mJ/m(2). An analysis of adhesion data suggests that the solvent was fully excluded from the zone of contact between functional groups on the tip and sample. With a nonpolar SAM, the force of adhesion increased monotonically in mixed solvents of higher water content; whereas, with a polar SAM (one having a hydrogen bonding component), higher water content led to decreased adhesion. The intermolecular force components theory was used for the interpretation of adhesion force measurements in polar solvents. Competition between hydrogen bonding within the solvent and hydrogen bonding of surface groups and the solvent was shown to provide the main contribution to adhesion forces. We demonstrate how the trends in the magnitude of the adhesion forces for chemically heterogeneous systems (solvents and surfaces) measured with CFM can be quantitatively rationalized using the surface tension components approach. For epoxy polymer, inelastic deformations also contributed heavily to measured adhesion forces.     

Adhesive properties of gamma-irradiated polyketone

The effect of polyketone (PK) γ-irradiation dose on the energy characteristics of the PK surface, the specific free surface energy, and the interfacial energy at the boundary between polar (water) and nonpolar (octane) liquids has been studied for the first time. The most significant changes in the energy characteristics of the surface occur at doses up to 100 kGy, and improvement in PK adhesion to the polar and nonpolar phases is observed at doses above 75 and 200 kGy, respectively.     

Wetting study of imidazolium ionic liquids

In this work, we present a systematic contact angles study of a series of 1-alkyl, 3-methyl-imidazolium ionic liquids (ILs) on well-defined polar and nonpolar monolayer surfaces supported on Si wafers. The advancing and receding contact angles of ILs were used to determine the surface energy of the monolayer surfaces using Neumann's equation-of-state and Zisman's critical surface tension approaches. In parallel, the contact angles of conventional probe fluids (molecular liquids) including water, formamide, methylene iodide, ethylene glycol, and hexadecane were determined on the same surfaces. The results obtained showed a great deal of similarity in wetting behavior of ionic vs molecular probe fluids: the contact angles of both sets of liquids followed the same patterns in accord with the surface tension of the fluid. A good agreement was found between the surface energy determined by different sets of liquids.     

Determining the conformation of an adsorbed Br-PEG-peptide by long period X-ray standing wave fluorescence

Long-period X-ray standing wave fluorescence (XSW) and X-ray reflectivity techniques are employed to probe the conformation of a Br-poly(ethylene glycol) (PEG)-peptide adsorbate at the hydrated interface of a polystyrene substrate. The Br atom on this Br-PEG-peptide construct serves as a marker atom allowing determination by XSW of its position and distribution with respect to the adsorption surface with angstrom resolution. Adsorption occurs on native or ion-beam-modified polystyrene films that are spin-coated onto a Si substrate and display either nonpolar or polar surfaces, respectively. A compact, oriented monolayer of Br-PEG-peptide can be formed with the peptide end adsorbed onto the polar surface and the PEG end terminating with the Br tag extending into the aqueous phase. The 108-141 A distance of the Br atom from the polystyrene surface in this oriented monolayer is similar to the estimated approximately 150 A length of the extended Br-PEG-peptide. This Br-polystyrene distance depends on adsorption time and surface properties prior to adsorption. Incomplete multilayers form on the polar surface after sufficient adsorption time elapses. By contrast, adsorption onto the nonpolar surface is submonolayer, patchy, and highly disordered with an isotropic Br distribution. Overall, this combination of X-ray surface scattering techniques with a novel sample preparation strategy has several advantages as a real space probe of adsorbed or covalently bound biomolecules at the liquid-solid interface.     

An effective way to stabilize colloidal particles dispersed in polar and nonpolar media

This article offers a new approach to building up self-adjustable invertible polymer coatings at solid surfaces. The approach is based on a two-step process. In the first step, the surface of dispersed TiO2 has been functionalized with the aid of toluene diisocyanate (TDI) as a coupling agent. In the second step, the chains of amphiphilic oligoester have been covalently grafted to the titanium dioxide surface functionalized with isocyanate groups. It is shown that the titania modified in this way can form stable suspensions in both polar (water) and nonpolar (toluene) media. Multiple redispersion cycles show the ability of the modified titanium dioxide particles, after their removal from one type of dispersion and consequent drying, to be redispersed in dispersing media strongly differing by polarity from the previous.     

Properties of the CdSe(0001), (0001), and (1120) single crystal surfaces: Relaxation, reconstruction, and adatom and admolecule adsorption

Details of the chemical mechanism underlying the growth of colloidal semiconductor nanocrystals remain poorly understood. To provide insight into the subject, we have preformed a comprehensive study of the polar (0001) and (0001) and nonpolar (1120) wurtzite CdSe surfaces that are exposed during crystal growth using first-principles density functional theory (DFT-GGA) calculations. Stabilization of these surfaces by relaxation and reconstruction was considered. Two particular reconstructions of the polar surfaces were examined: vacancy formation on a 2 x 2 unit cell and addition of Se and Cd atoms on the (0001) and (0001) surfaces, respectively. Calculation results indicate that the (1120) is the most stable surface when compared to the two polar surfaces. Furthermore, reconstructions of the (0001) surface are energetically favored when compared to reconstructions of the (0001) facet. Adsorption of Cd and Se atoms and the CdSe molecule on the three relaxed surfaces and two reconstructed (0001) surfaces were also investigated. Several binding sites were considered to determine the most stable binding geometries and energetics. Atomic species preferentially bind in either 2-fold or 3-fold sites, while the CdSe molecule binds parallel to the surface on all of the considered surfaces. Vibrational frequencies of the adspecies were calculated for the most stable binding configurations and were included in the zero point energy correction. Diffusion barriers for the atomic and molecular species were estimated where possible to be between 0.2 and 0.4 eV on the three relaxed surfaces. Thermochemistry of the CdSe molecule binding and dissociation was also investigated. On all considered surfaces, dissociation is preferred to desorption with dissociation only exothermic on the (0001) surface. Comparison of the three relaxed and two reconstructed surfaces indicates that CdSe molecule binding and dissociation is thermodynamically favored on the (0001) surface. This implies that under a reaction-controlled regime, the rate of homoepitaxy would be faster on the (0001) Se terminated surface than on the (0001) and (1120) surfaces, making the (0001) surface of a nanocrystal the primary direction of growth.     

Antihydrophobic cosolvent effects in organic displacement reactions

[formula: see text] Rates of reactions in water can be modified by the presence of antihydrophobic cosolvents such as ethanol and DMSO, which lower the energies of nonpolar surfaces. The rate effects reflect changes both in the solvation of nonpolar surfaces and also in the solvation of polar groups. The effects have been sorted out for some displacement reactions, revealing the geometry of an interesting branching reaction whose two paths show different antihydrophobic effects.     

Effects of trace water on charging of silica particles dispersed in a nonpolar medium

This paper presents an investigation of the effects of trace water on the charging of silica (SiO(2)) particles dispersed in a nonpolar medium. There are a growing number of applications that seek to use electrostatic effects in apolar media to control particle movement and aggregation stability in such systems. One factor that is often overlooked in the preparation of nonpolar colloidal dispersions is the amount of water that is introduced to the system by hygroscopic particles and surfactants. The amount and location of this water can have significant effects on the electrical properties of these systems. For nonpolar surfactant solutions it has been shown that water can affect the conductivity, and it has been speculated that this is due to swelling of the polar cores of inverse micelles, increasing the fraction of them that are charged. Some studies have suggested that particle surface charging may also be sensitive to water content, but a clear mechanism for the process has not been fully developed. The situation with particles is further complicated by the fact that it is often unclear whether the water resides on the particle surfaces or in the polar cores of inverse micelles. The current work explores not only the effect of water content on reverse micelle and particle charging but seeks to differentiate between water bound to the particles and water located in the micelles. This is accomplished by measuring the solution conductivity and the electrophoretic mobility of silicon dioxide particles dispersed in solutions of Isopar-L and OLOA 11000. The water content is determined for both the dispersion and the supernatant after centrifuging the particles out. It is found that at equilibrium the majority of the water in the system adsorbs to the surface of the hygroscopic silica particles. In addition, the effect of water on particle electrophoretic mobility is found to be dependent on surfactant concentration. At small OLOA concentrations, additional water results in an increase in particle mobility due to increased particle charging. However, at large OLOA concentrations, additional water leads to a decrease in particle mobility, presumably as a result of increased electrostatic screening or neutralization. Thus, particle charging and electrophoretic mobility in an apolar surfactant solution are found to be highly sensitive to both the total water content in the system and to its concentration relative to the amount of surfactant present.     

Adsorption of organosilanes at a Zn-terminated ZnO (0001) surface: molecular dynamics study

Four different organosilanes (octyltrihydroxysilane, butyltrihydroxysilane, aminopropyltrihydroxysilane, and thiolpropyltrihydroxysilane) adsorbed at a reconstructed Zn-terminated polar ZnO (0001) surface are studied via constant temperature (298 K) molecular dynamics simulations. Both single adsorbed silane molecules as well as adsorbed silane layers are modeled, and the energy, distance, orientation, and alignment of these adsorbates are analyzed. The adsorbed silane molecules exhibit behavior depending on the chemical nature of their tail (nonpolar or polar) as well as on the silane concentration at the solid surface (single adsorption or silane layer). In contrast to the O-terminated ZnO surface studied previously, now adsorption can only occur at the vacancies of this reconstructed crystal surface, thus leading to an arched structure of the liquid phase near the crystal surface. Nevertheless, both nonpolar and polar single adsorbed silanes show a similar orientation and alignment at the surface (orthogonal in the former, parallel in the latter case) as for the O-terminated ZnO surface, although the interaction energy with the surface is considerably increased for nonpolar silanes while it is nearly unaffected for the polar ones. For adsorbed silanes within silane layers, the difference to single adsorbed silanes depends on the polarity of the tail: nonpolar silanes again show an orthogonal alignment, while polar silanes exhibit two different orientations at the solid surface-a head and a tail down configuration. This leads to two completely different but nevertheless stable orientations of these silanes at the Zn-terminated ZnO surface.     

一氧化碳在原子簇Zn_4O_4上吸附的从头算研究

以原子簇Zn_4O_4模型,用量子化学的从头算方法与补偿方法（counterpoise ）相结合研究了一氧化碳在氧化锌极化表面Zn-ZnO(0001)和O-ZnO(000-1)及在非极 化（10-10）表面的吸附态和吸附键能。研究表明,无论锌离子以何种方式出现在 晶体表面,锌离子都是较强的活化吸附中心,CO的碳原子向内的吸附键最强。这一 结论与宏观实验测试的结果相一致,尽管宏观实验测试的结果在上述不同晶体表在 相近的CO脱附热。这种不同氧化锌晶体表面有相近的CO脱附热的现象是由于晶体表 面存在固有的晶体缺陷-表面层阶梯造成的。补偿方法,主要用于计算不同活化吸 附点的吸附质与吸附剂的弱作用。以Zn_4O_4为模型,以氧离子为活化吸附位,对 CO在氧化锌表面的计算结果表明,当CO分子垂直于晶体表面显排斥作用,当CO分子 平行于晶体表面仅有弱的吸附键。