Complexity of Intercalation in MXenes: Destabilization of Urea by Two-Dimensional Titanium Carbide.

MXenes are a new class of two-dimensional materials with properties that make them important for applications that include batteries, capacitive energy storage, and electrocatalysis. These materials can be exfoliated and delaminated to create high surface areas with interlayers accessibility. Intercalation is known to be possible, and it is critical for many applications including electrochemical energy storage, water purification, and sensing. However, little is known about the nature of the intercalant and bonding interactions between the intercalant within the MXene. We have investigated urea interaction within a titanium carbide based MXene using inelastic neutron scattering (INS) to probe the state of intercalated species. By comparison with reference materials, we find that under intercalation conditions urea decomposes readily, leading to intercalation of ammonium cations observable by INS and evolving carbon dioxide detected by infrared spectroscopy. Reactive molecular dynamics calculations were conducted to provide atomistic insights about reaction pathways and their energetics. These results have implications for understanding intercalation in active layered materials.

Ultrasensitive Detection of Cu2+ Using a Microcantilever Sensor Modified with L-Cysteine Self-Assembled Monolayer.

A microcantilever was modified with a self-assembled monolayer (SAM) of L-cysteine for the sensitively and selectively response to Cu(II) ions in aqueous solution. The microcantilever undergoes bending due to sorption of Cu(II) ions. The interaction of Cu(II) ions with the L-cysteine on the cantilever is diffusion controlled and does not follow a simple Langmuir adsorption model. A concentration of 10-10 M Cu(II) was detected in a fluid cell using this technology. Other cations, such as Ni2+, Zn2+, Pb2+, Cd2+, Ca2+, K+, and Na+, did not respond with a significant deflection, indicating that this L-cysteine-modified cantilever responded selectively and sensitively to Cu(II).

A high performance hybrid battery based on aluminum anode and LiFePO4 cathode.

A novel hybrid battery utilizing an aluminum anode, a LiFePO4 cathode and an acidic ionic liquid electrolyte based on 1-ethyl-3-methylimidazolium chloride (EMImCl) and aluminum trichloride (AlCl3) (EMImCl-AlCl3, 1-1.1 in molar ratio) with or without LiAlCl4 is proposed. The hybrid ion battery delivers an initial high capacity of 160 mA h g(-1) at a current rate of C/5. It also shows good rate capability and cycling performance.

Quantitative electrochemical measurements using in situ ec-S/TEM devices.

Insight into dynamic electrochemical processes can be obtained with in situ electrochemical-scanning/transmission electron microscopy (ec-S/TEM), a technique that utilizes microfluidic electrochemical cells to characterize electrochemical processes with S/TEM imaging, diffraction, or spectroscopy. The microfluidic electrochemical cell is composed of microfabricated devices with glassy carbon and platinum microband electrodes in a three-electrode cell configuration. To establish the validity of this method for quantitative in situ electrochemistry research, cyclic voltammetry (CV), choronoamperometry (CA), and electrochemical impedance spectroscopy (EIS) were performed using a standard one electron transfer redox couple [Fe(CN)6]3-/4--based electrolyte. Established relationships of the electrode geometry and microfluidic conditions were fitted with CV and chronoamperometic measurements of analyte diffusion coefficients and were found to agree with well-accepted values that are on the order of 10-5 cm2/s. Influence of the electron beam on electrochemical measurements was found to be negligible during CV scans where the current profile varied only within a few nA with the electron beam on and off, which is well within the hysteresis between multiple CV scans. The combination of experimental results provides a validation that quantitative electrochemistry experiments can be performed with these small-scale microfluidic electrochemical cells provided that accurate geometrical electrode configurations, diffusion boundary layers, and microfluidic conditions are accounted for.

Ionic liquid derived carbons as highly efficient oxygen reduction catalysts: first elucidation of pore size distribution dependent kinetics.

Metal-free N-doped carbons with controllable pore texture were derived from carbonization of ionic liquid and served as catalysts for oxygen reduction reaction (ORR) with an activity comparable to that of Pt/C. The investigation shows that both the ORR activity and kinetics are strongly correlated with the pore size distribution.

Dynamics of phenanthrenequinone on carbon nano-onion surfaces probed by quasielastic neutron scattering.

We used quasielastic neutron scattering (QENS) to study the dynamics of phenanthrenequinone (PQ) on the surface of onion-like carbon (OLC), or so-called carbon onions, as a function of surface coverage and temperature. For both the high- and low-coverage samples, we observed two diffusion processes; a faster process and nearly an order of magnitude slower process. On the high-coverage surface, the slow diffusion process is of long-range translational character, whereas the fast diffusion process is spatially localized on the length scale of ∼4.7 Å. On the low-coverage surface, both diffusion processes are spatially localized; on the same length scale of ∼4.7 Å for the fast diffusion and a somewhat larger length scale for the slow diffusion. Arrhenius temperature dependence is observed except for the long-range diffusion on the high-coverage surface. We attribute the fast diffusion process to the generic localized in-cage dynamics of PQ molecules, and the slow diffusion process to the long-range translational dynamics of PQ molecules, which, depending on the coverage, may be either spatially restricted or long-range. On the low-coverage surface, uniform surface coverage is not attained, and the PQ molecules experience the effect of spatial constraints on their long-range translational dynamics. Unexpectedly, the dynamics of PQ molecules on OLC as a function of temperature and surface coverage bears qualitative resemblance to the dynamics of water molecules on oxide surfaces, including practically temperature-independent residence times for the low-coverage surface. The dynamics features that we observed may be universal across different classes of surface adsorbates.

Treatment of perchlorate-contaminated groundwater using highly selective, regenerable ion-exchange technologies.

Treatment of perchlorate-contaminated water using highly selective, regenerable ion-exchange and perchlorate-destruction technologies was demonstrated at a field site in California. Four treatment and four regeneration cycles were carried out, and no significant deterioration of resin performance was noted in 2 years. The bifunctional resin (Purolite A-530E) treated about 37,000 empty bed volumes (BVs) of groundwater before a significant breakthrough of perchlorate occurred at an average flow rate of 150 gpm (or 1 BV/min) and a feed perchlorate concentration of about 860 microg/L. Sorbed perchlorate (approximately 20 kg) was quantitatively recovered by eluting with as little as 1 BV of the FeCl3-HCl regenerant solution. The eluted ClO4- was highly concentrated in the third quarter of the first BV of the regenerant solution with a concentration up to 100,000 mg/L. This concentrated effluent greatly facilitated subsequent perchlorate destruction or recovery by precipitation as KClO4 salts. High perchlorate destruction efficiency (92-97%) was observed by reduction with FeCl2 in a thermoreactor, which enabled recycling of the FeCl3-HCl regenerant solution, thereby minimizing the need to dispose of secondary wastes containing ClO4-. This study demonstrates that a combination of novel selective, regenerable ion-exchange and perchlorate-destruction and/or recovery technologies could potentially lead to enhanced treatment efficiency and minimized secondary waste production.

Optically directed molecular transport and 3D isoelectric positioning of amphoteric biomolecules.

We demonstrate the formation of charged molecular packets and their transport within optically created electrical force-field traps in a pH-buffered electrolyte. We call this process photoelectrophoretic localization and transport (PELT). The electrolyte is in contact with a photoconductive semiconductor electrode and a counterelectrode that are connected through an external circuit. A light beam directed to coordinates on the photoconductive electrode surface produces a photocurrent within the circuit and electrolyte. Within the electrolyte, the photocurrent creates localized force-field traps centered at the illuminated coordinates. Charged molecules, including polypeptides and proteins, electrophoretically accumulate into the traps and subsequently can be transported in the electrolyte by moving the traps over the photoconductive electrode in response to movement of the light beam. The molecules in a single trap can be divided into aliquots, and the aliquots can be directed along multiple routes simultaneously by using multiple light beams. This photoelectrophoretic transport of charged molecules by PELT resembles the electrostatic transport of electrons within force-field wells of solid-state charge-coupled devices. The molecules, however, travel in a liquid electrolyte rather than a solid. Furthermore, we have used PELT to position amphoteric biomolecules in three dimensions. A 3D pH gradient was created in an electrolyte medium by controlling the illumination position on a photoconductive anode where protons were generated electrolytically. Photoelectrophoretic transport of amphoteric molecules through the pH gradient resulted in accumulation of the molecules at their apparent 3D isoelectric coordinates in the medium.

Perchlorate isotope forensics.

Perchlorate has been detected recently in a variety of soils, waters, plants, and food products at levels that may be detrimental to human health. These discoveries have generated considerable interest in perchlorate source identification. In this study, comprehensive stable isotope analyses (37Cl/35Cl and 18O/17O/16O) of perchlorate from known synthetic and natural sources reveal systematic differences in isotopic characteristics that are related to the formation mechanisms. In addition, isotopic analyses of perchlorate extracted from groundwater and surface water demonstrate the feasibility of identifying perchlorate sources in contaminated environments on the basis of this technique. Both natural and synthetic sources of perchlorate have been identified in water samples from some perchlorate occurrences in the United States by the isotopic method.

Sorption and desorption of perchlorate and U(VI) by strong-base anion-exchange resins.

This study investigated the sorption affinity and capacity of six strong-base anion-exchange (SBA) resins for both uranium [U(VI)] and perchlorate (ClO4-) in simulated groundwater containing varying concentrations of sulfate (SO4(2-)). Additionally, desorption of U(VI) from spent resins was studied to separate U(VI) from resins with sorbed ClO4- for waste segregation and minimization. Results indicate that all SBA resins investigated in this study strongly sorb U(VI). The gel-type polyacrylic resin (Purolite A850) showed the highest sorption affinity and capacityfor U(VI) butwasthe least effective in sorbing ClO4-. The presence of SO4(2-) had little impact on the sorption of U(VI) but significantly affected the sorption of ClO4-, particularly on monofunctional SBA resins. A dilute acid wash was found to be effective in desorbing U(VI) but ineffective in desorbing ClO4- from bifunctional resins (Purolite A530E and WBR109). A single wash removed approximately 75% of sorbed U(VI) but only approximately 0.1% of sorbed ClO4- from the bifunctional resins. On the other hand, only 21.4% of sorbed U(VI) but approximately 34% of sorbed ClO4- was desorbed from the Purolite A850 resin. This study concludes that bifunctional resins could be used effectively to treatwater contaminated with ClO4- and traces of U(VI), and dilute acid washes could minimize hazardous wastes by separating sorbed U(VI) from ClO4- prior to the regeneration of the spent resin loaded with ClO4-.

Photon-driven nanomechanical cyclic motion.

Microcantilevers modified by a monolayer of azobenzene molecules bend up and down periodically, switched by a 365 nm UV light, as a result of the conversion of the two configurations of azobenzene molecules in the monolayer.

Detection of femtomolar concentrations of HF Using an SiO(2) microcantilever.

Femtomolar concentrations of hydrogen fluoride, a decomposition component of nerve agents, were detected using a SiO(2) microcantilever. The microcantilever underwent bending due to the reaction of HF with SiO(2). The microcantilever deflection increased as the concentration of HF increased. Other acids, such as HCl, had no effect on the deflection of the cantilever. The mechanism of reaction-induced bending and the correlation of microcantilever deflection with the HF concentration are discussed. The deflection in response to HF of a commercially available silicon cantilever was also studied, and its response was compared with that of the SiO(2) cantilever. Much less bending amplitude and sensitivity were observed for the silicon cantilever.

Detection of CrO4(2-) using a hydrogel swelling microcantilever sensor.

Hydrogels containing various mounts of tetraalkylammonium salts were used to modify microcantilevers for measurements of the concentration of CrO4(2-) in aqueous solutions. These microcantilevers undergo bending deflection upon exposure to solutions containing various CrO4(2-) concentrations as a result of swelling or shrinking of the hydrogels. The microcantilever deflection as a function of the concentration of CrO4(2-) ions is nearly linear in most concentration ranges. It was found that a concentration of 10(-11) M CrO4(2-) can be detected using this technology in a fluid cell. Other ions, such as Br-, HPO4(2-), and NO3-, have minimal effect on the deflection of this cantilever. The anions SO4(2-) and CO3(2-) could interfere with the CrO4(2-) detection, but only at high concentrations (> 10(-5) M). Such hydrogel-coated microcantilevers could potentially be used to prepare microcantilever-based chemical and biological sensors when molecular recognition agents are immobilized in the hydrogel.

Complete degradation of perchlorate in ferric chloride and hydrochloric acid under controlled temperature and pressure.

Despite favorable thermodynamics, the reduction of perchlorate (ClO4-) is kinetically limited in aqueous media because of its high activation energy. In this paper, a new methodology has been presented for degrading ClO4- in an FeCl3-HCl solution at an elevated temperature (< 200 degrees C) and/or pressure (approximately 20 atm). Results indicate that the rate constant for the pseudo-first-order reaction between ClO4- and ferrous Fe(II) (in excess) increased nearly 3 orders of magnitude when the temperature was increased from 110 to 195 degrees C, and a complete reduction of ClO4- occurred in < 1 h at 195 degrees C in the FeCl3-HCl solution. The activation energy of the reaction was calculated to be about 120 kJ/mol. Additionally, a flow-through reactor was constructed based on the batch kinetic measurements, and a nearly complete degradation of ClO4- was observed under continuous-flow mode. Because the FeCl3-HCl solution has been successfully used in regenerating selective anion-exchange resins sorbed with ClO4- during water treatment, this new methodology offers a cost-effective means to degrade ClO4- while not altering the chemical properties of the FeCl3-HCl regenerant solution so it can be reused, eliminating the production of secondary wastes.

Detection of Hg2+ using microcantilever sensors.

Trace amounts of Hg2+ are detected by using a microcantilever coated with gold. The microcantilever undergoes bending due to accumulation of Hg2+ on the gold surface. It is found that a concentration of 10(-11) M Hg2+ can be detected using this technology. Other cations, such as K+, Na+, Pb2+, Zn2+, Ni2, Cd2+, Cu2+, and Ca2+ have little or no effect on the deflection of the cantilever. The selectivity of the Hg2+ sensor could be improved by coating the gold surface of microcantilever with a self-assembled monolayer of a long-chain thiol compound.