Adsorbed Water Layer Research Articles

Diamond surface conductivity: Properties, devices, and sensors

Abstract

Electrolyte layering at the calcite(104)-water interface indicated by Rb(+)- and Se(VI) K-edge resonant interface diffraction.

Calcite-water interface reactions are of major importance in various environmental settings as well as in industrial applications. Here we present resonant interface diffraction results on the calcite(104)-aqueous solution interface, measured in solutions containing either 10 mmol L(-1) RbCl or 0.5 mmol L(-1) Se(VI). Results indicate that Rb(+) ions enter the surface adsorbed water layers and adsorb at the calcite(104)-water interface in an inner-sphere fashion. A detailed analysis based on specular and off-specular resonant interface diffraction data reveals three distinct Rb(+) adsorption species: one 1.2 Å above the surface, the second associated with surface adsorbed water molecules 3.2 Å above the surface, and the third adsorbed in an outer-sphere fashion 5.6 Å above the surface. A peak in resonant amplitude between L = 1.5 and L = 3.0 is interpreted as signal from a layered electrolyte structure. The presence of a layered electrolyte structure seems to be confirmed by data measured in the presence of Se(VI).

Analysis of Factors in the Nanoscale Physical and Electrical Characterization of High-K Materials by Conductive Atomic Force Microscope

The conductive atomic force microscope (CAFM) has been shown to be a powerful tool to study the physical and electrical properties of high-k materials on nanoscale. However, such accurate measurements could be altered by external factors. In this work, the main factors affecting CAFM measurements including environmental conditions, tip coating materials and the tip-sample bias are evaluated by analyzing the topographic and current maps performed on HfO2 stacks in different conditions. It is found that environmental conditions have notable effects on the CAFM measurements due to the water meniscus formed by adsorbed water layers on the tip and sample surface in air. And the lateral resolution of the technique can be improved remarkably by measuring in a high vacuum environment. The mechanical and electrical properties of tip coating materials are found to be related to the stability of tip conductivity and the real voltage drop in the stack, hence have a key influence on the current measurement. Moreover, the tip-sample interaction can be altered by the electrostatic force induced by the applied bias, leading to variations of the topographic image of the surface with the bias voltage. Our results indicate that the impact of these factors must be taken into account when performing CAFM measurements under different conditions or comparing the results obtained in different experiments.

Effects of O-deficiency on the interaction between rutile and Arg: A density functional theory study

Density functional theory (DFT) was used to investigate the adsorption of arginine (Arg) on three types of rutile (110) surface [R(110)], i.e., a pristine R(110), an R(110) with two kinds of oxygen deficiency, and an R(110) with an adsorbed water layer. The most stable adsorption configuration on pristine R(110) was identified when the aliphatic straight chain of Arg was parallel to the surface. The hindering effect of in-plane oxygen deficiency is larger than that of bridging oxygen deficiency on the interactions between Arg and R(110). The water layer hinders the Arg adsorption on R(110). These results deepened our understanding of the interfacial interactions between Arg and R(110), and would guide the design and development of tailored biomaterials at the electronic level.

Water surface coverage effects on reactivity of plasma oxidized Ti films

The behavior of the adsorbed water on the surface of thin sputter deposited Ti films maintained at room temperature was investigated in dependence on the thickness of the resulting adsorbed water layer, controllably injecting water vapor into plasma. The surface morphology and microstructure were used to characterize the surfaces of plasma treated titanium films. Presented experimental results showed that titanium films immersed in water vapor plasma at pressure of 10–100Pa promoted the photocatalytic activity of overall water splitting. The surfaces of plasma oxidized titanium covered by an adsorbed hydroxyl-rich island structure water layer and activated by plasma radiation became highly chemically reactive. As water vapor pressure increased up to 300–500Pa, the formed water multilayer diminished the water oxidation and, consequently, water splitting efficiency decreased. Analysis of the experimental results gave important insights into the role an adsorbed water layer on surface of titanium exposed to water vapor plasma on its chemical activity and plasma activated electrochemical processes, and elucidated the surface reactions that could lead to the split of water molecules.

Enhanced sensitivity and contrast with bimodal atomic force microscopy with small and ultra-small amplitudes in ambient conditions

Here, we introduce bimodal atomic force microscopy operated with sub-nm and ultra-small, i.e., sub-angstrom, first and second mode amplitudes in ambient conditions. We show how the tip can be made to oscillate in the proximity of the surface and in perpetual contact with the adsorbed water layers while the second mode amplitude and phase provide enhanced contrast and sensitivity. Nonlinear and nonmonotonic behavior of the experimental observables is discussed theoretically with a view to high resolution, enhanced contrast, and minimally invasive mapping. Fractions of meV of energy dissipation are shown to provide contrast above the noise level.

Closure to “Residual Shear Strength Measured by Laboratory Tests and Mobilized in Landslides” by Gholamreza Mesri and Nejan Huvaj-Sarihan

The authors present both laboratory and in situ data that support that the available residual friction angle ðfr9Þs on a presheared sliding plane depends on effective stress and not on aging. This discussion presents an alternate reason for why ðfr9Þs depends on effective stress. It involves the adsorbed water layer (AWL) in clays and may also help explain the aging behavior. The discusser recently published a paper (Schmertmann 2012), based on extensive experimental results, which presents new concepts for the Mohr-Coulomb (M-C) components of mobilized shear resistance in clay. Two of these components, denoted Ic and Ia, appear to relate to the behavior of the AWL; a viscous, plastic, Ic component that does not depend on the effective stress, and therefore, behaves as a M-C cohesion; and a second viscous, plastic, Ia component that depends linearly on the effective stress, and therefore, behaves as a M-C friction with an angle denoted fa9. Fig. 15 in Schmertmann (2012), presented herein as Fig. 1, shows, versus the plasticity index, a range of 7–19 for fr9 surrounding a range of 8– 14 for fa9. These fr9 come from a mixture of secant and tangent values. The authors’ Fig. 1 shows a measured laboratory and mobilized field secant ðfr9Þs range of 7–21 from approximately 75 data points in 16 references. The similarity in fr9 ranges in these two figures, and how they surround fa9 may provide more evidence of the importance of fa9 and AWL behavior in fr9. The authors do not include a residual cohesion in their paper. If residual strength behavior results from the AWL behavior, then the residual secant friction angle ðfr9Þs used by the authors should include both the Ic cohesion and the Ia friction. Including both components gives Eq. (1)

Comparison of nonaqueous hydrophilic interaction chromatography with aqueous normal‐phase chromatography on hydrosilated silica‐based stationary phases

We investigated the retention behavior of phenolic acids in nonaqueous normal-phase (NP) LC with buffered methanol/acetonitrile mobile phases on hydrosilated silica-based stationary phases. The silica hydride, Diamond hydride, Bidentate C18, and Cholesterol columns showed a higher retention of phenolic acids in the nonaqueous mobile phases than in aqueous NP mobile phases. There are some selectivity differences between the aqueous and nonaqueous mobile phases, but generally the resolution and selectivity are better in the aqueous systems. The retention of the phenolic acids tested decreased with increasing concentration of methanol in the mobile phase, up to 20% v/v methanol. At increased temperatures, the retention factors and peak widths decrease in both NP modes, showing linear ln k versus 1/T plots, due to a single retention mechanism over the temperature range from 25°C up to the column stability limit, however, the best separations are achieved at low temperatures. The enthalpic and entropic contributions to the retention were determined, and the differences between the aqueous and nonaqueous modes are possibly due to the adsorbed water layer.

Monitoring surface ion mobility on aluminum oxide: Effect of chemical pretreatments

Despite of its crucial role in many electrochemical processes, a clear surface chemistry-correlated understanding of interfacial ion mobility is missing. This study presents an improved application of a previously introduced method making use of the scanning Kelvin probe (SKP) technique on aluminum oxide surfaces. The local work function changes, induced by the displacement of the mobile surface ions in an electric field applied across the surface, are monitored as a function of surface chemistry modified by different surface pretreatments and ambient atmosphere. Complementary quartz crystal microbalance (QCM) and surface current measurements reveal how the modified surface chemistry affects the thickness of the adsorbed water layer and the surface conductivity, respectively. Besides the usefulness of the SKP technique for mobility studies, we demonstrate a selective control of the superficial ion mobility by engineering the surface fixed charges.

Probing the temperature dependence of proton transfer to charged platinum electrodes by reactive molecular dynamics trajectory studies

We have performed reactive trajectory calculations of proton discharge on charged platinum surfaces as a function of temperature and charge. A recently developed 9-state empirical valence bond model has been employed. The temperature dependence follows an Arrhenius law with activation energies in the range of 0.1eV. The activation energy for the discharge reaction decreases significantly with increasing driving force as modeled by an increasingly negative surface charge on the electrode. The analysis shows that the average orientation of molecules in the adsorbed water layer reacts to the approaching proton. Within increasing temperature, configurations become more prevalent which facilitate fast proton transfer by Grotthuss style proton hops from the second to the first layer. This effect becomes more pronounced near more negatively charged surfaces and leads to the computed reduction of the activation energy.

Molecular Simulations of Water Adsorbed on Mesoporous Silica Thin Films

Grand canonical Monte Carlo and canonical ensemble molecular dynamics simulations were used to investigate water adsorption properties in mesoporous silica thin films. The effect of pore radius on the adsorption properties was assessed using two models of mesoporous silica thin films having different pore radius and film thickness (1.38 and 5.66 nm in Model 1, respectively, and 1.81 and 7.30 nm in Model 2, respectively). In the simulations, a water adsorption layer or water menisci were formed in a mesopore accompanying the growth or shrinkage of stable adsorption layers on the upper and lower surfaces. The thickness of a water adsorption layer immediately before the onset of capillary condensation and the curvature radius of a water meniscus prior to capillary evaporation were identified. In the initial stage of layer adsorption, the surface state and radius of the pore affected the coordination structure of water around silanol groups. In the pore-filling state, the hydrogen bond network of water in the pore was similar but not the same as that in the bulk liquid; the difference was because the pore wall affected the position of water molecules. This difference may confer unique dynamic properties upon the water in mesoporous silica.

On the Transition from Bulk to Ordered Form of Water: A Theoretical Model to Calculate Adhesion Force Due to Capillary and van der Waals Interaction

The adhesion force due to capillary interaction between two hydrophilic surfaces is strongly dependent on the partial pressure of water and is often calculated using the Kelvin equation. The validity of the Kelvin equation is questionable at low relative humidity (RH) of water, like in high vacuum and dry nitrogen environments, where water is only present as layers of several molecules thick at the surfaces. A model from ordered to bulk form of water has been developed using the Brunauer, Emmett, and Teller adsorption model. The results show that the adhesion force calculated using the Young–Laplace and Kelvin equations at low (5–30 %) RH is underestimated. The total adhesion force shows changes when the RH is changed from 0 to 100 %. In dry conditions, at RH below 10 %, the total adhesion force is contributed by the van der Waals interaction due to solid–solid contact. The total adhesion force then increases and remains constant being equal to the superposition of van der Waals interaction due to solid–solid contact and van der Waals interaction due to adsorbed water layers on the surfaces. The total adhesion force further increases slowly with the increase in RH incorporating capillary forces and then decreases at very high RH due to screening of van der Waals forces. This change in adhesion force occurs from solid–solid interaction to ordered form of water at low RH and from ordered form to bulk form of water at high RH along with the screening effect of van der Waals interaction. The results have been compared with the experiments and it has been seen that at small length scales, the model is in agreement with the existing experimental data. However, at large length scales roughness of the surfaces should be taken into account.

Nanowear behaviour of monocrystalline silicon against SiO2 tip in water

By using an atomic force microscopy, the wear tests of Si(100) surface against SiO2 microsphere were performed both in water and in air. During the tests, the contact pressure was varied between 0.78GPa and 1.1GPa, the displacement amplitude was set as 250nm and the number of wear cycles was 200. As a comparison, the wear tests of Si(100) surface against diamond tip were also performed in water and air. The experimental results indicated that water environment played an important role in the wear behaviour of Si/SiO2 pair. Under a contact pressure of 1.1GPa, the wear depth of silicon in water was about 0.25nm, which was less than 10% of that in air. Since the wear depths of silicon in water were similar at various sliding velocities (0.04–5μm/s), the hydrodynamic lubrication was not the main reason causing the slight wear of silicon in water. Further analysis indicated that the distinct wear behaviour of Si/SiO2 pair in air and in water could be mainly explained as the different intensity of chemical reactions in two environments for the pair of Si/SiO2. In air, the SiOSi bridges could easily form between SiO2 microsphere and silicon substrate and induced serious tribochemical wear of silicon. When the Si/SiO2 pair was immersed in water, because of the different structure of the adsorbed water layer on silicon surface and the effect of the electrical double layer, the tribochemical reaction was largely resisted and the wear of silicon was slight. Even though, since the contact pressure was too low to induce the mechanical wear of silicon, the limited wear of silicon in water was still attributed to the tribochemical reaction. The results may help us understand the nanowear mechanism of silicon and optimize the tribological design of dynamic MEMS working in water environment.

Anomalous stability of graphene containing defects covered by a water layer

Defects are inevitably present in graphene and can alter its properties and thus its applications. Interestingly, we find that commonly observed Stone-Wales and double vacancy defects do not affect graphene's hydrophilic and hydrophobic properties and that an adsorbed single water layer does not noticeably affect the defect-containing graphene's electronic properties. Our findings are based on calculations using a density functional tight-binding theory. Specifically, we observe negligible alteration in the interaction strength (less than 0.1 kcal mol(-1)) between a single water layer and graphene upon the incorporation of the various types of defects, which indicates that graphene has relatively stable hydrophilic and hydrophobic properties. The presence of a single water layer causes only negligible changes in the energy gap and a small charge transfer to the aqueous layer (less than 0.1 e). The results indicate that the electronic properties of graphene are determined mainly by its own structural characteristics and are not considerably affected by the adsorbed water layer. Further electronic structure analysis reveals that the two commonly observed defects do not change the sp(2) hybridization characteristics of the C atoms of graphene even in the water environment. Our results are significant for graphene studies and applications in areas such as life sciences and materials science where hydrophilic and hydrophobic properties and electronic properties are important.

Rapid, conformal gas-phase formation of silica (SiO2) nanotubes from water condensates

An innovative atomic layer deposition (ALD) concept, with which nanostructures of water condensates with high aspect ratio at equilibrium in cylindrical nanopores can be transformed uniformly into silica (SiO2) at near room temperature and ambient pressure, has been demonstrated for the first time. As a challenging model system, we first prove the conversion of cylindrical water condensates in porous alumina membranes to silica nanotubes (NTs) by introducing SiCl4 as a metal reactant without involving any catalytic reaction. Surprisingly, the water NTs reproducibly transformed into silica NTs, where the wall thickness of the silica NTs deposited per cycle was found to be limited by the amount of condensed water, and it was on the orders of ten nanometers per cycle (i.e., over 50 times faster than that of conventional ALD). More remarkably, the reactions only took place for 10-20 minutes or less without vacuum-related equipment. The thickness of initially adsorbed water layers in cylindrical nanopores was indirectly estimated from the thickness of formed SiO2 layers. With systematic experimental designs, we tackle the classical Kelvin equation in the nanosized pores, and the role of van der Waals forces in the nanoscale wetting phenomena, which is a long-standing issue lacking experimental insight. Moreover, we show that the present strategy is likely generalized to other oxide systems such as TiO2. Our approach opens up a new avenue for ultra-simple preparation of porous oxides and allows for the room temperature formation of dielectric layers toward organic electronic and photovoltaic applications.

Investigation of humidity-dependent nanotribology behaviors of Si(1 0 0)/SiO2 pair moving from stick to slip

With an atomic force microscopy, the humidity-dependent nanotribology behaviors of Si(100) against SiO2 microsphere were investigated while the relative movement translated from stick to slip. The relative humidity RH of air exhibits a strong effect on the motion behavior of Si(100)/SiO2 pair. With the increase in RH, relative movement of Si(100)/SiO2 pair is easier to keep into stick state, namely, the relative slip becomes more difficult to occur in a higher humidity range. The adhesion Fa will increase with the increase in RH in the given humidity range. In the low RH range (<20%) where the adsorbed water layer forms the ‘solid-like’ structure, due to the absence of water meniscus, Fa increases very slowly. However, in relative higher RH range (>20%), Fa increases very sharply once ‘liquid-like’ adsorbed water layer forms, because it increases the capillary force. The initial friction forces Ft of Si(100)/SiO2 pair also increase with the increase in RH in the given humidity range. However, different from Fa, it increases sharply in the low RH range (<30%) and slightly in the higher RH range (>30%). During the cyclic friction process, under the higher RH, relative stable tangential force is easier to be observed at higher displacement amplitude, here, the relative movement usually keeps into stick state. With the increase in RH, the surface damage of Si(100) transforms from mechanical deformation (forming hillock) to tribochemical wear (material removal). The tribochemical wear is sensitive to the absorbed water film with ‘solid-like’ structure, here, the wear volume increases drastically in this RH range (<20%); further increase of wear is small in higher relative humidity regime where the ‘liquid-like’ water layer is formed. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis verifies the water molecules participate into tribochemical reaction to rupture the SiSi and SiO network bonds on Si(100) substrate.

Spatial distribution of adsorbed water layers at the TiO2 rutile and anatase interfaces

The spatial distribution functions of adsorbed water layers at hydrated rutile-(110) and anatase-(101) surfaces have been studied at 300K, using equilibrium classical molecular dynamics. It was found that there is evidence of some hydrogen-bonding with water molecules outside the adsorbed layer for anatase-(101), but less for rutile-(110) – on average, about 1.6 per water molecule in anatase-(101), in contrast to 0.9 for rutile-(110). This is due to adsorbed water molecules being confined more firmly in the area between bridging oxygen atoms at the rutile-110 surface, while the more open-like nature of anatase-101 allows for greater contact with molecules outside the first layer.

Structuring of Interfacial Water on Silica Surface in Cyclohexane Studied by Surface Forces Measurement and Sum Frequency Generation Vibrational Spectroscopy

We investigated interfacial water, formed by adsorption or phase separation (prewetting transition), on a silica surface in water-cyclohexane binary liquids using a combination of colloidal probe atomic force microscopy (AFM) and sum frequency generation (SFG) vibrational spectroscopy. At 33 ± 9 ppm water, the long-range attraction extending to 19.4 ± 2.9 nm appeared, which was caused by the contact of water layers formed on silica surfaces. The attraction range increased with increasing water concentration and reached 97 ± 17 nm at the saturation concentration of water in cyclohexane (C*), indicating that the thickness of the water layer formed on silica was ca. 50 nm. The interfacial energy between the water adsorption layer and bulk solution (γ = 79.3 ± 2.0 mN/m) was estimated from the pull-off force, and was significantly larger than the value for the bulk water/cyclohexane interface (γ = 50.1 mN/m). SFG spectroscopy demonstrated that the interfacial water formed an icelike structure at C*. These results indicated that the interfacial water molecules formed an icelike ordered structure induced by the hydrogen bonding with surface silanol groups, resulting in the free OH groups being more exposed to the bulk solution. On the other hand, the water adsorption layer induced by phase separation at water concentrations above C* was found to be less ordered and its structure at the adsorption layer/bulk interface was almost the same as that of bulk water, although its thickness was almost the same as that formed at C*. To our knowledge, this is the first report of the observation of liquid adsorption layers formed by chemical interaction up to saturation and by the wetting transition above saturation, and their differences in the structure and properties at the molecular level.

Viscosity and Wetting Property of Water Confined in Extended Nanospace Simultaneously Measured from Highly-Pressurized Meniscus Motion.

Understanding fluid and interfacial properties in extended nanospace (10-1000 nm) is important for recent advances of nanofluidics. We studied properties of water confined in fused-silica nanochannels of 50-1500 nm sizes with two types of cross-section: (1) square channel of nanoscale width and depth, and (2) plate channel of microscale width and nanoscale depth. Viscosity and wetting property were simultaneously measured from capillary filling controlled by megapascal external pressure. The viscosity increased in extended nanospace, while the wetting property was almost constant. Especially, water in the square nanochannels had much higher viscosity than the plate channel, which can be explained considering loosely coupled water molecules by hydrogen bond on the surface within 24 nm. This study suggests specificity of fluids two-dimensionally confined in extended nanoscale, in which the liquid is highly viscous by the specific water phase, while the wetting dynamics is governed by the well-known adsorbed water layer of several-molecules thickness.

Humidity Effects on Friction and Wear Between Dissimilar Metals

The effect of water vapor on the friction and wear between copper and 440C stainless steel was studied using a ball-on-flat tribometer, polarization-modulation reflection–absorption infrared spectroscopy, and Auger electron spectroscopy. The wear behavior changed drastically as the relative humidity (RH) varied in inert gas (nitrogen or argon). In a dry environment, a small degree of abrasive wear of soft copper was observed. In the RH range of 10–70 %, catastrophic adhesive wear of the soft copper surface was dominant. A high RH (>80 %) environment exhibited wear of the hard 440C stainless steel surface and the steel wear debris was deposited onto the copper. The adsorption isotherm measurements for copper and stainless steel revealed that water adsorption increases quickly between zero and 10 % RH and then the adsorption proceeds more slowly as RH increases further. The adsorbed water layer thickness increases rapidly again as saturation is approached. It seems that the thin layer of adsorbed water under 70 % RH facilitates the adhesive wear through passivation of grain boundaries or acceleration of crack propagations, but the thick water layer formed over 80 % RH acts as an electrolyte medium allowing galvanic corrosion to commence.