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We monitored the conversion of aragonite to calcite in water by comparing single and mixed polymorph suspensions. We demonstrate that the enhanced aragonite-to-calcite conversion in mixed polymorph suspensions is dramatically inhibited by adding polyphosphate (sodium hexametaphosphate). 13C and 31P solid-state magic angle spinning (MAS) NMR and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra allow us to follow quantitatively these effects as imparted by the dissolution-recrystallization processes. 31P{13C} and 13C{31P} rotational echo double resonance (REDOR)NMR experiments reveal coprecipitated phosphate that is embedded only within the surfaces of both polymorphs during the initial dissolution and recrystallization processes, causing passivation that arrests phase conversion.

This study illustrates pervasive challenges in studying wetting dynamics, including dynamic contact angles, on irregularly roughened surfaces. We demonstrate that asymmetric water droplet shapes occur more than 50% of the time during static and dynamic contact angle measurements on sandblasted Zn-plated stainless steel with a polymeric overcoat. The pinning that causes the asymmetric drop shape distortion on horizontal surfaces also influences the sliding behavior on inclined surfaces. These effects lead to a poor correlation between the measured dynamic contact angles and the observed sliding angles (critical tilt angles). Our work emphasizes that large variations in the values of these dynamic wetting parameters are inherent to the heterogeneity of the surface roughness, and thus they limit the usefulness of standard dynamic wetting criteria. These findings have implications for academic and industrial research focused on making coated materials that have consistent wettability properties throughout their usage life cycle.

We demonstrate that mixtures of graphite and lab-oxidized graphenic carbon materials can be separated into three individual components (graphite, graphene/graphite oxide (GO) and oxidative debris (OD)) by a series of aqueous treatments. Our results show that a key part of this separation procedure involves a water treatment and sonication near neutral pH in order to separate GO from OD. We show that the relative proportions of OD and GO – independent of any humic substances – can affect the ability of oxidized graphite to be suspended in water, which can influence the efficiency of the separation procedures we describe. We compare and contrast our protocol with others that are widely used for cleaning archaeological charcoal prior to radiocarbon dating. Our protocol has potential applications for tailored cleaning procedures for graphenic carbon materials, including the possibility of separating GO from both OD and graphite, for radiocarbon dating purposes.

Impedance spectroscopy data show that polycrystalline ZnO films can show either increases or decreases in their effective resistances after UV exposure, depending on the frequency of the applied AC excitation. Simple equivalent circuit models, based on resistance (R) and capacitance (C) in parallel, are sufficient to confirm the observed experimental trends. Simulated data demonstrate that that arbitrary R and C values will not produce the sign change, but that typical resistance and capacitance characteristics for photoconductive semiconductors like ZnO can cause the sign change. These results suggest that it could be desirable to manipulate the R and C values of photodetector materials to either control - or eliminate - such frequency-dependent UV responses.

We show that mildly alkaline electrolytes can be used to produce Zn coatings that improve the water repellent properties of stainless steel. Optimal Zn deposits were prepared under potentiostatic conditions from electrolytes that contained ZnCl2, NH4Cl, and a surfactant (polyethyleneimine). After deposition, the Zn electrodeposit was capped with stearic acid to prevent oxidation and to provide a lower surface energy. The capped electrodeposits display an impressive degree of water repellency, including extremely poor water droplet adhesion. We discuss the range of deposition parameters (electrolyte composition, pH, and applied potential) that produce the best water-repellent electrodeposits.

We demonstrate a proof-of-principle method to separate particles of two CaCO3polymorphs, calcite and aragonite, based on surface charge density differences that affect electrophoretic mobility values. Calcite and aragonite standards show significant differences in their electrophoretic mobility distributions in polyphosphate-containing suspensions. Phosphate additives, which are commonly used to reduce particle aggregation, have a serendipitous added benefit of stabilizing CaCO3against dissolution. However, the mobility differences among archaeologically relevant samples, such as lime plasters and chalk, are not consistently different enough to make this a reliable separation strategy. Nevertheless, this study is important because it takes a new and fundamentally different approach to non-destructive separation of archaeological materials.

We show that two different sources of calcite (geogenic flowstones and anthropogenic lime plaster) have complex, yet distinctive, structural disorder signatures. It is effective to identify these differences by pairing a rapid, fieldwork-compatible technique (Fourier transform infrared (FTIR) spectroscopy) with a robust laboratory-based technique (X-ray diffraction (XRD) peak width analyses). We demonstrate that crystalline domain size, microstrain fluctuations, and lattice strain each affect the FTIR spectra of calcite. To focus on each variable separately, XRD data and FTIR absorption spectra are compared among calcite samples formed by different processes. Small crystalline domain sizes cause changes to FTIR peak intensity ratios (grinding curves). However, larger microstrain fluctuations or larger lattice strain also produce similar changes. Thus, inferring structural differences from calcite FTIR spectra alone is not advisable. Instead, we advocate using FTIR grinding curves in conjunction with analyses of angle-dependent XRD peak widths using the Williamson–Hall relation. Thus, combining these two analysis techniques is more powerful and informative than using either one alone. These findings are relevant for heritage science, including archaeology.

Comparing different synthesis methods to investigate ambient degradation differences among ZnO crystals with different crystal habits, we examined the effects of particle size, surface area, shape (surface polarity), and zeta (surface) potential. Neither surface polarity nor surface area, on their own, can account for the differences in the surface carbonation among differently synthesized ZnO samples. Our results demonstrate that surface dissolution and carbonation tendencies must be considered together, in the context of surface polarity, to explain different propensities toward degradation in ZnO powders.

Ambient moisture can dramatically promote the response of ZnO to ethanol vapor, a hydrophilic gas. By comparing sensor responses in a broad range of humidities, we show that there is a consistent enhancement in ethanol adsorption on ZnO when physisorbed water, detected by capacitance measurements, is present. The time constants related to the capacitive signal recovery during desorption are consistent with the formation of C2H5OH-(H2O)n clusters that have a different desorption rate than water alone. These room temperature results indicate that surface water mediates the dynamic adsorption/re-evaporation equilibrium of solvated ethanol molecules. Thus, attention to interactions between the target gas molecules and their environment is important for understanding the mechanisms behind selective gas sensing.

We show that unintentional hydrogen doping of ZnO during the electrodeposition process can impact the material’s carrier concentration as significantly as others have reported for intentional extrinsic doping. Mott−Schottky analyses on the natively n-type electrodeposits show a decrease in the carrier concentrations from 1021to 1018cm−3with increasing overpotential. A strong link exists between larger optical band gaps (determined from diffuse reflectance spectroscopy) and higher carrier concentrations, which suggests that hydrogen-based doping underlies the n-type conductivity (Moss−Burstein effect). We propose that kinetic defects introduced during growth at larger overpotentials compete with hydrogen doping, thereby leading to lower net carrier concentrations. This has important implications for using the deposition potential to tune other electrodeposit properties such as the growth rate and morphology.

Electrochemically produced ZnO/metal rectifying (Schottky) junctions can exhibit consistent barrier heights and high rectifying ratios when prepared using optimized electrolyte pH (6.5) and applied voltage (=-1.1 V vs Ag/AgCl) conditions. An increase in soft breakdown for more acidic deposition electrolytes (pH 4) correlates with a diminished preferred orientation in the resulting ZnO electrodeposit. Forward-biased junctions exposed to increased relative humidities show increased current as a result of protonic conduction from water hydrolysis at the ZnO/air interface. At moderate to high relative humidities (50-85% RH), hydrophobic coatings improve the quality of the rectifying response by changing the wetting properties of the ZnO surface. Our findings suggest that electrodeposition, in conjunction with postdeposition surface coatings, can offer improved functionality for electron transport materials in wet or humid environments.

CeAuSn, Ce3Rh4Sn13, and Ce3lr4Sn13were prepared by reaction of the elements in an arc-melting furnace and subsequent annealing at 970 K for two weeks. The three stannides were investigated by X-ray powder and single crystal techniques. CeAuSn crystallizes with the NdPtSb type, space group P63mc: a = 472.7(2), c = 771.6(3) pm, wR2 = 0.0230,208 F2values, 11 variable parameters, and BASF = 0.40(2). The gold and tin atoms form a pronounced two-dimensional [AuSn] polyanion which consists of slightly puckered Au3Sn3hexagons. ,19Sn Mössbauer data at 78 K show one signal at an isomer shift of δ = 1.90(7) mm/s subjected to unresolved quadrupole splitting of ΔEQ= 0.55(2) mm/s. Ce3Rh4Sn13and Ce3lr4Sn13adopt the cubic Yb3Rh4Sn13type structure, space group Pm3n: a = 970.51(3) pm, wR2 = 0.0721, 267 F2values (Ce3Rh4Sn13) and a = 972.29(6) pm, wR2 = 0.0850, 267 F2values (Ce3lr4Sn13) with 14 variable parameters for each refinement. Striking structural motifs in Ce3Rh4Sn13are condensed distorted trigonal [RhSn6] prisms with Rh-Sn distances of 266 pm. The polyhe­dral network leaves two different cages which are occupied by cerium (6c position) and tin (2a position) atoms. The Sn2 atoms show occupancy parameters of only 92% (Ce3Rh4Sn13) and 76% (Ce3Ir4Sn13) and an extremely large displacement parameter indicating a rattling of these atoms within the icosahedral Sn12cages. Magnetic susceptibility measurements of Ce3Rh4Sn13show paramagnetic behavior down to 2 K with an experimental magnetic moment of 2.45(2) μB/Ce. No magnetic ordering is observed. Magnetization measurements show a moment of 0.78(2) μB/Ce at 2 K and 5.5 T. Resistivity data reveal only a very weak tem­perature dependence. The two crystallographically different tin sites are resolved in the 119Sn Mössbauer spectrum which shows a signal at δ = 2.12(1) mm/s subject to quadrupole splitting of 1.54(1) mm/s, superimposed by a singlet at δ = 2.47(1) mm/s. The Seebeck coefficient of Ce3Rh4Sn13is within a few μ V/K of zero over the temperature range of 10 - 300 K.

Impedance spectroscopy data show that polycrystalline ZnO films can show either increases or decreases in their effective resistances after UV exposure, depending on the frequency of the applied AC excitation. Simple equivalent circuit models, based on resistance (R) and capacitance (C) in parallel, are sufficient to confirm the observed experimental trends. Simulated data demonstrate that that arbitrary Rand Cvalues will not produce the sign change, but that typical resistance and capacitance characteristics for photoconductive semiconductors like ZnO can cause the sign change. These results suggest that it could be desirable to manipulate the Rand Cvalues of photodetector materials to either control – or eliminate – such frequency-dependent UV responses.

In this work, nanosecond laser machining is used to fabricate hydrophobic 17-4 PH stainless steel surfaces with microscale and submicron structures. Four surface structures were designed, with microscale channels and pillars (100 μm pitch size) of uniform heights (100 μm) or alternating heights (between 100 μm and 50 μm). During fabrication, the high-power laser beams also created submicron features on top of the microscale ones, leading to hierarchical, multiscale surface structures. Detailed wettability analysis was conducted on the fabricated samples. Measured static contact angles of water on these surfaces are over 130 deg without any coating, compared to ∼70 deg on the original steel surface before laser machining. Slightly lower contact angle hysteresis was also observed on the laser machined surfaces. Overall, these results agree with a simple Cassie–Baxter model for wetting that assumes only fractional surface area contact between the droplet and the surface. This work demonstrates that steel surfaces machined with relatively inexpensive nanosecond laser can achieve excellent hydrophobicity even with simple microstructural designs.

We show that mildly alkaline electrolytes can be used to produce Zn coatings that improve the water repellent properties of stainless steel. Optimal Zn deposits were prepared under potentiostatic conditions from electrolytes that contained ZnCl2, NH4Cl, and a surfactant (polyethyleneimine). After deposition, the Zn electrodeposit was capped with stearic acid to prevent oxidation and to provide a lower surface energy. The capped electrodeposits display an impressive degree of water repellency, including extremely poor water droplet adhesion. We discuss the range of deposition parameters (electrolyte composition, pH, and applied potential) that produce the best water-repellent electrodeposits.

Comparing different synthesis methods to investigate ambient degradation differences among ZnO crystals with different crystal habits, we examined the effects of particle size, surface area, shape (surface polarity), and zeta (surface) potential. Neither surface polarity nor surface area, on their own, can account for the differences in the surface carbonation among differently synthesized ZnO samples. Our results demonstrate that surface dissolution and carbonation tendencies must be considered together, in the context of surface polarity, to explain different propensities toward degradation in ZnO powders.

Ambient moisture can dramatically promote the response of ZnO to ethanol vapor, a hydrophilic gas. By comparing sensor responses in a broad range of humidities, we show that there is a consistent enhancement in ethanol adsorption on ZnO when physisorbed water, detected by capacitance measurements, is present. The time constants related to the capacitive signal recovery during desorption are consistent with the formation of C2H5OH-(H2O)nclusters that have a different desorption rate than water alone. These room temperature results indicate that surface water mediates the dynamic adsorption/re-evaporation equilibrium of solvated ethanol molecules. Thus, attention to interactions between the target gas molecules and their environment is important for understanding the mechanisms behind selective gas sensing.

We demonstrate that ZnO/metal junctions, produced by a commonly used electrochemical oxidation procedure, are prone to lateral (two-dimensional) heterogeneities. These heterogeneities are not apparent in bulk structural measurements (such as X-ray diffraction data), but are evident in the electrodeposit’s electrical (current–voltage) and optical (luminescence) properties. The spatial variations in the ZnO films are related to incomplete oxidation during the final stage of their multi-step electrochemical formation process. Support for this explanation comes from a surprisingly simple equivalent circuit that accurately models the current-voltage response as a combination of resistive (Ohmic) and rectifying (Schottky) junction contacts at the ZnO/substrate interface.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.