Surface Relaxation Research Articles

Correlating Surface Chemistry to Surface Relaxivity via TD-NMR Studies of Polymer Particle Suspensions.

This study elucidates the impact of surface chemistry on solvent spin relaxation rates via time-domain nuclear magnetic resonance (TD-NMR). Suspensions of polymer particles of known surface chemistry were prepared in water and n-decane. Trends in solvent transverse relaxation rates demonstrated that surface polar functional groups induce stronger interactions with water with the opposite effect for n-decane. NMR surface relaxivities (ρ2) calculated for the solid-fluid pairs ranged from 0.4 to 8.0 μm s-1 and 0.3 to 5.4 μm s-1 for water and n-decane, respectively. The values of ρ2 for water displayed an inverse relationship to contact angle measurements on surfaces of similar composition, supporting the correlation of the TD-NMR output with polymer wettability. Surface composition, i.e., H/C ratios and heteroatom content, mainly contributed to the observed surface relaxivities compared to polymer % crystallinity and mean particle sizes via multiple linear regression. Ultimately, these findings emphasize the significance of surface chemistry in TD-NMR measurements and provide a quantitative foundation for future research involving TD-NMR investigations of wetted surface area and fluid-surface interactions. A comprehensive understanding of the factors influencing solvent relaxation in porous media can aid the optimization of industrial processes and the design of materials with enhanced performance.

Resonance and relaxation effects governing the sorption of O2 and CO2 in deionized water under dynamic conditions at 21 ± 1 °C

The behavior of water samples (deionized, deaerated and washed with He) was investigated in dynamics by physicochemical and electrochemical methods. The susceptibility of water samples to sorption of O2 and CO2 from air depends on their pretreatment. Sorption of O2 and CO2 accompanied by the desorption of dissolved helium is a multivariable nonlinear process, presumably, based on resonance and relaxation phenomena within the surface layer. Sorption of O2 and CO2 in water is considered as an interaction between gas molecules and ensembles of H2O molecules, which is assumed to be based on resonance effects. Sorption of gases leads to a, decrease in the surface tension at the water/gas interface. The effect is most pronounced in case of He-washed samples. The decrease in the surface energy upon CO2 absorption and the increase in the relaxation properties of the water surface are probably factors that facilitate sorption of gases, including O2. A fraction of sorbed O2 are contained in nanobubbles formed via water surface relaxation. A change in the viscosity of water samples during the sorption of gases may serve as a relaxation factor favorably affecting the formation and stability of nanobubbles in the surface layer and in bulk water. Irradiation of water with a laser light at λ = 633 nm can generate nano-cavities in the laser beam. In case of very low dissolved O2 concentrations, nanobubbles have no noticeable effect on the oxygen reduction reaction. Water samples washed with helium demonstrate a higher physical and chemical activity compared to other water samples, the activity being manifested in sorption properties. Electrochemical properties of water samples - deionized, deaerated and washed with He - are somewhat different. This is primarily due to the different content of dissolved O2 in the samples. Nonempirical simulations helped in clarifying the experimental data. The key factor that predetermines stability and/or collapse of nanobubbles is the organization of the hydrogen-bond network of water that strongly depends on the presence of water ions and foreign particles. In the case of both deionized water and water washed with He, HCO3-anion cannot participate in stabilization of nanobubbles. On the other hand, OH– anions can stabilize nanobubbles.

Development of a Single-Sided Magnetic Resonance Surface Scanner: Towards Non-Destructive Quantification of Moisture in Slaked Lime Plaster for Maintenance and Remediation of Heritage Architecture

Single-sided time-domain proton magnetic resonance (MR) surface scanners are useful for non-destructive measurements of moisture. A mobile single-sided MR sensor unit employing two concentric ring magnets was constructed for the in situ quantification of moisture in slaked lime plaster used in the outer walls and roofs of heritage architecture. This sensor unit allows for non-destructive measurements of water proton relaxation 1.5 to 13.5 mm beneath the surfaces of walls and roofs. The following laboratory experiments on water-saturated porous slaked lime plaster samples were performed. (i) The porosity (water volume fraction) was measured in approximately eight minutes with a root mean square error of 1.9 vol%. (ii) The fundamental MR-related property (i.e. proton surface relaxivity) needed for the estimation of the pore size distribution was also measured successfully. (iii) The pore volume expansion due to frost damage was successfully detected as a significant change in the transverse relaxation time distribution. These results demonstrate that the portable MR surface scanner is a promising non-destructive testing tool for the maintenance and remediation of heritage architecture made of plaster.

Fast-nondestructive measurement of axial slice water content and water distribution with NMR SFG-MSCPMG sequence

Measuring hydraulic conductivity by instantaneous profile method needs to detect the pore water content of soil at different positions, but most of the detecting methods will disturb the soil sample. In the study, the axial slice scanning of soil samples was carried out by using static filed gradient multi-slice Carr-Purcell-Meiboom-Gill (SFG-MSCPMG) sequence of nuclear magnetic resonance (NMR) technology, which can nondestructively determine water content and distribution at different positions of soil column. In this experiment, three clays were used to illustrate the relationship between NMR test results and oven drying results of water content. The results show that there is a good linear relationship between the NMR signal and water content. Combining mercury intrusion porosimetry (MIP) data, the transverse surface relaxivity parameters (ρ2) of clays are obtained. With ρ2 = 41.90 μm/s the T2 distribution curve of pore water in the ordinary clay can be converted into the pore water distribution curve, which is in good agreement with that obtained with MIP. Nevertheless, it is found that the T2 distribution curve of pore water in expansive clay obtained with NMR technique is more suitable for analyzing the average pore water distribution because of the fast exchange of pore water between the interlayers, intra-aggregates and inter-aggregates in expansive clay.

Structures, Energies, and Electronic Properties of Low‐Index Surfaces of γ″‐Ni3Nb: A First‐Principles Calculations

First‐principles calculations are carried out to study the surface structure, energies, and electronic properties of Ni3Nb(100), Ni3Nb(001), and Ni3Nb(110) based on the density functional theory (DFT). The surface relaxation results reveal that the relaxations are mainly localized in the first and second atomic layer, and Ni3Nb(110)‐Ni experiences the largest surface relaxation (–16.95%), whereas Ni3Nb(001)‐NiNb undergoes smallest relaxations. The surface energies of nonstoichiometric surfaces present a linear relationship with the chemical potential of Ni (ΔμNi), while those of stoichiometric surface are independent of ΔμNi. Furthermore, Ni3Nb(001)–Ni and Ni3Nb(001)–NiNb are the most stable surfaces owing to their having the lowest surface energy in a wide range of ΔμNi, while the nonstoichiometric Ni3Nb(110)–Ni and Ni3Nb(110)–NiNb surfaces with the largest surface energies are the most unstable surfaces. The electronic structures of nonstoichiometric surfaces are different from that of the bulk Ni3Nb, whereas the effect of surface relaxation on the electronic properties of the stoichiometric surface is weak.

Wettability and fluid characterization in shale based on T1/T2 variations in solvent extraction experiments

Rock wettability and pore fluid properties are critical parameters for evaluating shale reservoirs, enhancing oil recovery, sequestering waste, and storing energy. Multiple petrophysical and geochemical experiments are routinely used to characterize these properties of shales. Two-dimensional nuclear magnetic resonance (NMR)-based shale evaluation methods usually rely on T1/T2 difference in various fluids. However, T1/T2 value is inadequate for petrophysical characterization due to the similar T1/T2 value of different hydrogen types in shales. A brand-new characteristic parameter, T1/T2 variation during a solvent extraction experiment, is proposed for the characterization of shale petrophysical properties. The theoretical ranges of T1/T2 variation for different hydrogen components were determined from the type and content changes of hydrogen during extraction based on surface relaxation model. Joint experiments of multi-step solvent extraction and NMR T1-T2 were designed to validate the theoretical results. Both theoretical and experimental results show that the heterogeneity of shale is responsible for the different hydrogen components with a similar T1/T2 value. The results of joint experiment (1) show that the T1/T2 variation ranges for different hydrogen components are different, and (2) indicate that T1/T2 variations can be used to quantitatively evaluate the hydrogen type and saturation. The experimentally determined ranges and patterns of T1/T2 variation are consistent with the theoretical results. The quantitative relationship between T1/T2 variation and saturation, mobility, or wettability was further determined based on the theoretical variation of correlation time. Finally, a new interpretation scheme was established for the quantitative evaluation of wettability and fluid properties based on T1/T2 variations.

Experimental and simulation study on the estimation of surface relaxivity of clay minerals

Nuclear Magnetic Resonance (NMR) is an important tool for the evaluation of pore structure and petrophysical properties of reservoir rocks. Pore Surface relaxivity (ρ2), which controls the relaxation decay due to the interaction between the spins and the pore surface, is an important parameter that impacts the interpretation of NMR for petrophysical properties. Sandstone rocks have complex minerology and consist of different clay minerals. Earlier efforts have reported the surface relaxivity of different rock types such as sandstone or carbonate, but less attention was given to the estimation of individual clay minerals surface relaxivity. The surface complexity and variable mineralogy (especially paramagnetic and ferromagnetic minerals) of clays can both impact the surface relaxivity and relaxation times in clays and porous media containing clays. This study, using for the first time both simulation and experimental approaches, aims to systematically investigate the surface relaxivity of clays characterized by variable mineralogy and iron content. A glass beads sample (iron free) and 5 different pure clay minerals samples (ranked by increasing iron content: Kaolinite, Smectite, Illite, Chlorite, and Nontronite) were prepared with similar average grain sizes (500 μm). The determination of surface relaxivity requires prior measurement of pore surface-to-volume ratio (S/V) using an independent method in addition to obtaining T2 relaxation time. Two methods, among the most widely used, were used here for S/V determination including BET N2 gas adsorption and 3D micro CT imaging. The 3D images were also used to conduct NMR simulations to reproduce the experimental data. The estimated surface relaxivity values from both methods were compared and correlated to the iron content of the different clays. The surface relaxivity values obtained based on BET-derived S/V showed reasonable values (29–343 μm/s) that have a strong positive relationship with iron content of the clays. The documented equation relating surface relaxivity and iron content can be potentially utilized to predict surface relaxivity based on the compositional analysis of clays. On the other hand, digital images did not capture the surface complexity of clays thus significantly underestimated S/V which consequently resulted in overestimated surface relaxivity (898–11000 μm/s) that were physically non-feasible. Supporting the same argument, the simulated T2 relaxation distribution showed significant mismatch with the experimental data for the clay samples while an excellent match was observed for the glass beads sample. This can be attributed to the smooth surface of the glass beads, unlike the complex clays. The reported surface relaxivity values (based on BET-derived S/V) for the individual clay minerals can be used to perform NMR simulation and interpretation studies conducted on sandstone or shale formations. Careful attention should be given when performing NMR simulation on micro-CT digital images of samples containing clay due to the inadequacy of such images in capturing the complex surface and microporous nature of clays. The outcomes from this study can improve the interpretation of NMR T2 data and the determination of NMR-derived pore size distribution and petrophysical properties. The reported surface relaxivity values for clay can also inform the NMR simulation.

Pore size distribution measurement with magnetic resonance [formula omitted] distributions outside the fast diffusion regime

Based on Brownstein-Tarr (BT) theory, a new method is proposed to determine the pore size distribution which is an important petrophysical property in studying fluid storage and fluid transport in reservoir rocks. For reservoir rocks with large pores, or high surface relaxivity, direct conversion from the magnetic resonance (MR) T2 distribution with the fast diffusion assumption may be unreliable. In this study, the ground mode lifetime T20 distribution was proposed to be utilized to estimate the pore size distribution. The T20 lifetime of large pores within a porous system can be determined directly through experiment. However, the T20 lifetimes of smaller pores may overlap with the first nonground mode T21 lifetimes of the larger pores, which collectively contribute to the MR signal with a shorter lifetime. To accurately determine the T20 lifetime of smaller pores, it is essential to remove the contribution to the distribution of the T21 lifetimes of the larger pores. The T21 distribution can be both experimentally observed and calculated through the proposed method. The proposed pore size distribution method is verified by calculation in the present work. Water-saturated glass bead packs were employed for initial experiments. The calculated pore size, and its distribution, match the geometric pore size. The proposed method was applied to determine the pore size distribution of a Berea sandstone. The results are in good agreement with SEM measurements.

Relaxation of the Distorted Lattice of 4H-SiC (0001) Surface by Post-Oxidation Annealing

Thermal oxidation of 4H-SiC to grow native-oxide SiO2 is always followed by the generation of crystal defects and lattice distortion. We studied the relaxation of this distorted lattice on thermally-oxidized 4H-SiC surface by performing annealing process with several conditions. The surface distortion could be relaxed partially by annealing under argon, nitrogen monoxide, and H2O gases, confirmed by in-plane X-ray diffractometer. This surface relaxation is possibly induced by the release of oxygen-related defects, as confirmed by thermal desorption analysis. The surface distortion caused by thermal oxidation is due to the existence of oxygen in 4H-SiC lattice, while the relaxation is caused by the migration of the oxygen-related defect structure, and emitted from 4H-SiC surface region as CO molecule.

Reversible and Irreversible Layer Edge Relaxation in Laser‐Radiation‐Hardened 2D Organic–Inorganic Perovskite Crystals

The layer edge states or low energy state (LES) in 2D hybrid organic–inorganic perovskites demonstrate a prolonged carrier lifetime for better performance of optoelectronic devices. However, the fundamental understanding of LES in 2D perovskites is still inconclusive. Herein, a photoluminescence (PL) study of LES in 2D Ruddlesden–Popper perovskites is presented with n = 2 and n = 3 from their cleaved cross sections that are more stable than the natural edge. The PL measurements clearly observe reversible, and irreversible surface relaxations (case I and case II) in three laser intensity ranges, further supported by a PL excitation cycle from low to high laser intensity, and vice versa. The PL wavelength of LES is tunable with laser intensity and blueshifts with increasing laser intensity during irreversible surface relaxation process (case I). Fluorescence lifetime imaging (FLIM) shows that the LES has a longer lifetime than the band‐edge emission in the sample without a photodegradation, while the BE lifetime becomes relatively longer in the area with a photodegradation. The presented laser tunable LES and the related irreversible relaxation process provide a new insight that can help improve the photostability in 2D perovskites and understand roles of LESs in optoelectronic device performance.

Pore structure and fractal dimension analysis of ancient city wall bricks in China

Comprehensive quantitative characterization of the pore structures of ancient city wall bricks is important for predicting the durability of historical masonry buildings. However, in most previous studies, only one or two methods were used to analyse the pore structures of fired clay bricks. In this study, mercury intrusion porosimetry (MIP), low-temperature nitrogen adsorption (LTNA), and low-field nuclear magnetic resonance (LF-NMR) were used to characterize the pore structure and fractal dimension of nine different ancient city wall bricks. The results show that the surface relaxivity of ancient bricks measured via the Hahn spin echo (HSE) in LF-NMR ranged from 1.55 to 1.75 μm/s. Furthermore, the porosity of the blue bricks measured via LF-NMR were much lower than that obtained via the gravimetric method owing to a higher number of paramagnetic iron mineral phases. LTNA was not suitable for describing pore size distribution of ancient bricks as the pore diameters were mainly concentrated in the range of 100–1000 nm. The pore structures of ancient bricks in dried and water-saturated states were different, the pore size of the white bricks obtained via LF-NMR was smaller than the MIP-derived pore-throat size by a factor of approximately 2.

Direct Measurement of Pore Size and Surface Relaxivity with Magnetic Resonance at Variable Temperature

Direct Measurement of Pore Size and Surface Relaxivity with Magnetic Resonance at Variable Temperature

Surface relaxivity estimation of coals using the cutting grain packing method for coalbed methane reservoirs

Low-field nuclear magnetic resonance (NMR) has been widely used to express the pore-size distributions of conventional and unconventional reservoirs. Based on the equation 1T2=ρFs1r, the determination of the surface relaxivity ρ can help to quantitatively convert the transverse relaxation time T2 to pore radius (r). The existing calculation of ρ using NMR requires supplementary and expensive destructive or non-destructive methods, and consolidated cylinders are required as samples to be well tested. However, the operation of multiple drilling projects and the production of many drilling cuttings are barriers to making rapid measurement of the ρ value of the unconsolidated coal cuttings challenging. In this study, an NMR grain packing method is proposed using different coal grains. The surface-to-volume ratio was transformed into the ratio of the derivative of an arbitrary pore volume to its volume, and the governing NMR equation was established by considering only the NMR porosity and T2 distribution. Sphere and cubic-like models were also constructed. The results showed that the coal with different grain sizes had different relaxation responses, and the different assemblies with the same wgrain/wgrain,c also exhibited some relaxation differences, which might be affected by the diversity in grain packing patterns and grain morphology, resulting in the variation in porosity. In addition, the water film distribution on the grain surface and the soluble minerals in the coal contribute to the variation in ρ values by influencing the relaxation properties of the voids between grains and the pores in the grains. The T2 spectrum indicated that the macropores contributed the most to the porosity, owing to the loose grain packing, and the larger grains might not strictly obey the fast diffusion limit. For the sphere-like model, the ρ values of the Ji15 and Ji17 coals were in the range of 19.47–44.04 μm/s and 36.50–46.66 μm/s, respectively; whereas for the cubic-like model, the corresponding ρ values of these two coals ranged from 24.16 μm/s to 54.64 μm/s and from 45.29 μm/s to 57.90 μm/s, respectively. Finally, different methods of ρ calculation are compared, and its potential field application is discussed.

Analysis Method of Full-Scale Pore Distribution Based on MICP, CT Scanning, NMR, and Cast Thin Section Imaging—A Case Study of Paleogene Sandstone in Xihu Sag, East China Sea Basin

Using different experimental methods, the pore radius ranges vary greatly, and most scholars use a single experiment to study pore structure, which is rarely consistent with reality. Moreover, the numerical models used in different experiments vary and cannot be directly compared. This article uniformly revised all experimental data into a cylinder model. Quantitative analysis of the full-scale pore distribution is established by mercury withdrawal–CT data, and semi-quantitative distribution is obtained by mercury–NMR–cast thin section imaging. In this paper, we introduce the tortuosity index (τ) to convert the CT ball-and-stick model into a cylinder model, and the pore shape factor (η) of the cast is used to convert the plane model into the cylinder model; the mercury withdrawal data is applied to void the influence of narrow throat cavities, and the NMR pore radius distribution is obtained using the mercury-T2 calibration method. Studies have shown that the thickness of bound water is 0.35~0.4 μm, so the pores with different radius ranges were controlled by different mechanisms in the NMR tests, with pores < 0.35~0.4 μm completely controlled by surface relaxation, including strong bound water and weak bound water; pores in the 0.4~4 μm reange were controlled by surface relaxation; and pores > 10 μm were completely controlled by free relaxation. The surface relaxivity rate of fine sandstone was 18~20 μm/s. The tortuosity index τ was generally 1~7; the larger the value, the more irregular the pores. The pore shape factor η was generally 0.2~0.5; the smaller the value, the more irregular the pores. Mercury withdrawal–CT scan data can quantitatively determine the pore radius distribution curve. The coefficient of the logarithm is positive considering porosity, and the constant is negative considering porosity. Permeability controls the maximum pore radius, with a max pore radius > 100 μm and a permeability > 1 mD. Mercury withdrawal–NMR–cast thin section imaging data can semi-quantitatively establish a pore radius distribution histogram. The histogram represents quasi-normal, stepped, and unimodal data. When 60 μm is the inflection point, if a large proportion of pores measure > 60 μm, good reservoir quality is indicated. If a large proportion of pores measures < 60 μm, the permeability is generally <0.5 mD.

Tailored Lewis Acid Sites for High-Temperature Supported Single-Molecule Magnetism.

Generating or even retaining slow magnetic relaxation in surface immobilized single-molecule magnets (SMMs) from promising molecular precursors remains a great challenge. Illustrative examples are organolanthanide compounds that show promising SMM properties in molecular systems, though surface immobilization generally diminishes their magnetic performance. Here, we show how tailored Lewis acidic Al(III) sites on a silica surface enable generation of a material with SMM characteristics via chemisorption of (Cpttt)2DyCl ((Cpttt)- = 1,2,4-tri(tert-butyl)-cyclopentadienide). Detailed studies of this system and its diamagnetic Y analogue indicate that the interaction of the metal chloride with surface Al sites results in a change of the coordination sphere around the metal center inducing for the dysprosium-containing material slow magnetic relaxation up to 51 K with hysteresis up to 8 K and an effective energy barrier (Ueff) of 449 cm-1, the highest reported thus far for a supported SMM.

NMR-Mapped Distributions of Dielectric Dispersion

Combinations of NMR and dielectric measurements frequently address challenging saturation and wettability determinations in conventional reservoirs. When pore structure effects are addressed, the nuclear magnetic resonance (NMR) characteristics are interpreted based on the evaluations of surface relaxivity, and the dielectric structural response is attributed to the “texture” of the rock matrix. Both pore structure descriptors can be improved if the molecular motions and charge mobility common to the measurements are considered. Similar to the dipolar relaxation equivalence of NMR and dielectric correlation time measurements in the Bloembergen, Purcell, and Pound (BPP) model, we develop a relaxation time correlation assuming representative Maxwell-Wagner relaxations. Dielectric dispersion curves for the carbonate matrix and vug pore components demonstrated by Myers are quantified using a dielectric relaxation time (DRT) model. The modeled pore system fractions are spectrally mapped to the NMR T1 or T2 distributions based on enhanced Debye shielding distances correlated with the conductivity. The characterized NMR distributions are validated with micro-CT pore-size determinations and diffusion correlations. The mapped distributions provide petrophysical insight into the frequently used Archie exponent combination (mn) associated with conductivity tortuosity and additional wettability screening criteria.

Effect of layer structure of AlN interlayer on the strain in GaN layers during metal-organic vapor phase epitaxy on Si substrates

It is highly challenging to grow high-quality gallium nitride (GaN) layers on silicon (Si) substrates due to the intrinsic mismatching of their structural and thermal properties. Aluminum nitride (AlN) interlayers have been used to induce a compressive strain to GaN layers during growth, which compensates for the tensile strain in these layers on Si substrates during cooling. In this study, we investigated the effect of the growth temperature and layer structure of the AlN interlayer to understand the relationship between surface flatness and relaxation ratio of the AlN interlayer and the compressive strain in the overlying GaN layer. Low-temperature (LT) growth enhanced lattice relaxation of the AlN interlayer, whereas the AlN surface was atomically flat at high temperature (HT). We also examined a two-step growth to combine the advantages of LT- and HT-AlN. This approach resulted in a surface with multiple flat regions separated by grooves, which had the largest compressive strain in the overlying GaN layer at the early stages of growth. At later stages, the strain was the largest on the HT-AlN interlayer. In both cases, the experimentally measured compressive strain exceeded simulated predictions. Finally, possible solutions for inducing a larger compressive strain in the GaN layer using interlayers were discussed.

On the Stability of Pickering and Classical Nanoemulsions: Theory and Experiments

Emulsification is a crucial technique for mixing immiscible liquids into droplets in various industries, such as food, cosmetics, biomedicine, agrochemistry, and petrochemistry. Quantitative analysis of the stability is pivotal before the utilization of these emulsions. Differences in X-ray attenuation for emulsion components and surface relaxation of the droplets may contribute to X-ray CT imaging and low-field NMR spectroscopy as viable techniques to quantify emulsion stability. In this study, Pickering (stabilized solely by nanoparticles) and Classical (stabilized solely by low molecular weight polymers) nanoemulsions were prepared with a high-energy method. NMR and X-ray CT were employed to constantly monitor the two types of nanoemulsions until phase separation. The creaming rates calculated from NMR match well with the results obtained from X-ray CT. Furthermore, we show that Stokes' law coupled with the classical Lifshitz-Slyozov-Wagner theory underestimates the creaming rate of the nanoemulsions compared to the experimental results from NMR and X-ray CT imaging. A new theory is proposed by fully incorporating the effects of Pickering nanoparticles, hydrocarbon types, volume fraction, size distribution, and flocculation on the droplet coarsening. The theoretical results agree well with the experimentally measured creaming rates. It reveals that the attachment of nanoparticles onto a droplet surface decreases the mass transfer for hydrocarbon molecules to move from the bulk aqueous phase into other droplets, thus slowing the Ostwald ripening. Therefore, Pickering nanoemulsions show a better stability behavior compared to Classical nanoemulsions. The impacts of hydrocarbon and emulsification energy on the stability of nanoemulsions are reported. These findings demonstrate that the stability of the nanoemulsions can be manipulated and optimized for a specific application, setting the stage for subsequent investigations of these nanodroplets.

Analysis of Adsorbed Polyphosphate Changes on Milled Titanium Dioxide, Using Low-Field Relaxation NMR and Photoelectron Spectroscopy.

In this study, changes in the adsorbed amount and surface structure of sodium hexametaphosphate (SHMP) were investigated for aluminum-doped TiO2 pigment undergoing milling. Relaxation NMR was utilized as a potential at-line technique to monitor the effect of milling on surface area and surface chemistry, while XPS was used primarily to consider the dispersant structure. Results showed that considerable amounts of weakly adsorbed SHMP could be removed with washing, and the level of dispersant removal increased with time, highlighting destructive effects of sustained high-energy milling. Nonetheless, there were no significant chemical changes to the dispersant, although increases to the bridging oxygen (BO) peak full width at half-maximum (FWHM) suggested some chemical degradation was occurring with excess milling. Relaxation NMR revealed a number of important features. Results with unmilled material indicated that dispersant adsorption could be tracked with pseudo-isotherms using the relative enhancement rate (Rsp), where the Rsp decreased with dispersant coverage, owing to partial blocking of the quadrupolar surface aluminum. Milled samples were also tracked, with very accurate calibrations of surface area possible from either T1 or T2 relaxation data for systems without dispersant. Behavior was considerably more complicated with SHMP, as there appeared to be an interplay between the dispersant surface coverage and relaxation enhancement from the surface aluminum. Nevertheless, findings highlight that relaxation NMR could be used as a real-time technique to monitor the extent of milling processes, so long as appropriate industrial calibrations can be achieved.

Zn Vacancies as Hydrogen Trap Sites in Polar Surfaces: A New Stabilization Mechanism for the ZnO(0001)-(2×2) Surface Reconstruction

The (2×2) reconstruction of the ZnO(0001) surface has been investigated by X-ray photoelectron diffraction (XPD). Comparing the XPD measurements with multiple-scattering simulations, the single Zn vacancy per (2×2) surface unit cell model is confirmed, and structures with O adatoms are ruled out. The analysis indicates an outward relaxation of the topmost Zn layer, in contrast to the usually reported results by density-functional theory (DFT) calculations. On the basis of DFT, we describe a new stabilizing mechanism of the polar ZnO surface through surface reconstruction where the Zn vacancies are occupied by three hydrogens atoms. The DFT surface relaxation of the proposed model is in excellent agreement with the XPD findings. Our DFT simulations also strongly indicate that the migration of hydrogens atoms to the surface, coming from the bulk, may influence the desorption of the surface Zn atom to create the vacancy.