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The coupled effects of pH and ionic strength impact a variety of geochemical processes in the subsurface. In this study, we investigate the interactions of H+ and major ions at the surface-solution interface of silica porous media under advection-dominated flow-through conditions. A series of 21 column experiments were performed by systematically injecting solutions of different pH and ionic strengths. Three types of porous media (i.e., two natural sands and quartz beads) were considered in order to explore differences in surface/solution interactions among quartz materials. Multiple lines of evidence were used to characterize the geochemical processes taking place during the flow-through experiments: (i) breakthrough curves of pH and major ions were measured at the column outlets; (ii) the natural sand surfaces were characterized by chemical extractions and scanning electron microscopy and the quartz structure was analyzed by XRD; (iii) reactive transport modeling was performed to quantitatively interpret the experimental results. We observed strong reactivity of the quartz surface characterized by significant release of protons when interacting with the major ions. The results also show significant differences in the quantity of protons emitted from the surfaces of the three silica porous media, as well as in the shape of the pH breakthrough curves. Reactive transport modeling was based on a multicomponent ionic transport formulation and on surface complexation description of the solid/solution interactions. The surface complexation model included the individual contributions of quartz and Fe/Al oxides present in the sand coatings with the component additive approach. For each medium, a single set of surface complexation parameters capable of reproducing the experimental dataset (i.e., 7 columns) was calibrated by parallelizing the simulations of the flow-through experiments. This approach allowed us to capture the pH and ionic species behavior within a large range of ionic strengths (i.e., [0–100] mM). The SCMs quantitatively show that H+ is released from quartz upon adsorption of Na+ and that the protonation of the oxides surfaces retards the pH front. Differences in acidity behavior between the silica surfaces seem to be primarily controlled by differences in surface topology, crystal structure of quartz and/or various presence of aluminosilicates. This study demonstrates that the reactivity and diversity of quartz surfaces in silica porous media and the complex interplay with oxide surfaces in the coatings control the transport of aqueous charged species in flow-through systems.

Cellulose nanocrystals (CNCs) are promising bio-derived nanomaterials for the bottom-up fabrication of biomedical constructs. In this report, dicarboxylic acid-functionalized CNC (DCC) was functionalized with arginylglycylaspartic acid (RGD) tripeptide as a motif for improved cell adhesion and targeting. The product (DCC-RGD) self-assembled into a more elongated nanofibrillar structure through lateral and end-to-end association. When added into poly(ethylene imine) (PEI)/pDNA polyplex solution, nanocelluloses interacted electrostatically with positively charged polyplexes without affecting their integrity. The constructs were tested for their potentials as non-viral transfection reagents. Cell viability and transfection efficiency of fibroblast NIH3T3 cells were monitored as a function of CNC concentration where, in general, viability increased as the CNC concentration increased, and transfection efficiency could be optimized. Using wild-type MDCK and αV-knockout MDCK cells, the construct was able to provide targeted uptake of polyplexes. The findings have potential applications, for example, cell-selective in vitro or ex vivo transfection of autologous mesenchymal stem cells for cell therapy, or bottom-up design of future innovative biomaterials.

The synergistic combination of different nanomaterials for improved performance in environmental applications such as the removal of aqueous micropollutants has attracted increasing interest in recent years. This study demonstrates a novel sequential adsorption–aggregation concept that harnesses tubular halloysite nanotubes (HNTs) and flexible cellulose nanofibrils (CNFs) for the removal of a small, anionic dye molecule, chrome azurol S, from water. Hollow HNTs were first allowed to interact with the aqueous dye solution, after which the dye-loaded colloidal nanotubes were aggregated and separated from the water phase with cationized CNFs. The combination of 25 mg CNFs with 1 g HNTs at pH 7 resulted in efficient removal of dye (80%) and turbidity (~100%) and the removal of dye was further promoted in more acidic conditions (within the pH range of 6–8.5) because of the attractive electrostatic interactions. Cationic CNFs not only enabled the separation of dye-loaded clay particles from the water phase through a rapid aggregation but also participated in dye removal through adsorption (~20%). In comparison with nano-sized HNTs, the dye removal performance of micro-sized and chemically similar kaolin was poor (43%). Given the good availability of both HNTs and CNFs and the low consumption of the more expensive component (i.e., CNFs) in the process, the concept is straightforward, readily applicable, environmentally benign, and potentially cost-effective.

In the present work, the degradation of size-selected Pt nanoclusters is studied under electrochemical conditions. This model catalyst mimics carbon supported Pt nanoclusters and nanoparticles typically employed in proton exchange membrane fuel cells (PEMFCs). Insight into the early stage of degradation is given by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and confirmed by transmission electron microscopy (TEM). In contrast to common assumptions, it is demonstrated that even extremely small Pt clusters exhibit a remarkable stability under electrochemical conditions. Such nanoclusters are then relevant to develop electrodes for energy applications. Furthermore, applying mixed cluster samples of Pt22 and Pt68, no preferential dissolution of Pt22 by Ostwald ripening - usually held responsible to be the main mechanism for activity loss in Pt fuel cell catalysts - is observed. In light of the findings reported, developing highly-dispersed subnanometer Pt clusters as catalyst for PEMFCs is a realistic approach provided the operation conditions are suitably adjusted. Furthermore, mitigation strategies to improve the stability of few-atoms catalyst under electrochemical operation will benefit from addressing cluster-support interactions.

This review article discusses the current and future possibilities for the application of in situ transmission electron microscopy to reveal synthesis pathways and functional mechanisms in complex and nanoscale materials. The findings of a group of scientists, representing academia, government labs and private sector entities (predominantly commercial vendors) during a workshop, held at the Center for Nanoscale Science and Technology-National Institute of Science and Technology (CNST-NIST), are discussed. We provide a comprehensive review of the scientific needs and future instrument and technique developments required to meet them. Published by Elsevier B.V.

International audience; Catalyst selection is very crucial for controlled growth of single-walled carbon nanotubes (SWNTs). Here we introduce a well-designed Fesingle bondTisingle bondO solid solution for SWNT growth with a high preference to large chiral angles. The Fesingle bondTisingle bondO catalyst was prepared by combining Ti layer deposition onto premade Fe nanoparticles with subsequent high-temperature air calcination, which favours the formation of a homogeneous Fesingle bondTisingle bondO solid solution. Using CO as the carbon feedstock, chemical vapour deposition growth of SWNTs at 800 °C was demonstrated on the Fesingle bondTisingle bondO catalyst. Nanobeam electron diffraction characterization on a number of individual SWNTs revealed that more than 94% of SWNTs have chiral angles larger than 15°. In situ environmental transmission electron microscopy study was carried out to reveal the catalyst dynamics upon reduction. Our results identify that the phase segregation through reducing Fesingle bondTisingle bondO catalyst leads to the formation of TiOx-supported small Fe nanoparticles for SWNT growth. The strong metal-support interactions induced by partial reduction of TiOx support promote the wettability of Fe nanoparticle, accounting for the preferential growth of large-chiral-angle SWNTs. This work opens a new avenue for chiral angle selective growth of SWNTs.

The stability of Ni-MoS2/ZrO2 toward water, potassium, and chlorine containing compounds during hydrodeoxygenation (HDO) of a mixture of phenol and 1-octanol was investigated in a high pressure gas and liquid continuous flow fixed bed setup at 280 °C and 100 bar. To maintain the stability of the catalyst, sufficient co-feeding of a sulfur source was necessary to avoid oxidation of the sulfide phase by oxygen replacement of the edge sulfur atoms in the MoS2 structure. However, the addition of sulfur to the feed gas resulted in the formation of sulfur containing compounds, mainly thiols, in the oil product if the residence time was too low. At a weight hourly space velocity (WHSV) of 4.9 h−1 the sulfur content in the liquid product was 980 ppm by weight, but this could be decreased to 5 ppm at a WHSV of 1.4 h−1. A high co-feed of sulfur was needed when water was present in the feed and the H2O/H2S molar ratio should be below ca. 10 to maintain a decent stability of the catalyst. Chlorine containing compounds caused a reversible deactivation of the catalyst when co-fed to the reactor, where the catalytic activity could be completely regained when removing it from the feed. Commonly, chlorine, H2O, and H2S all inhibited the activity of the catalyst by competing for the active sites, with chlorine being by far the strongest inhibitor and H2S and H2O of roughly the same strength. Dissimilar, potassium was a severe poison and irreversibly deactivated the catalyst to 2 slabs.

Micro-supercapacitors (MSCs) hold great promise as highly competitive miniaturized power sources satisfying the increased demand in microelectronics; however, simultaneously achieving high areal and volumetric capacitances is still a great challenge. Here we demonstrated the designed construction of binder-free, electrically conductive, nanoporous activated graphene (AG) compact films for high-performance MSCs. The binder-free AG films are fabricated by alternating deposition of electrochemically exfoliated graphene (EG) and nanoporous AG with a high specific surface area of 2920 m2/g, and then dry transferring onto the target substrates with a high-pressure mechanical densification process. Remarkably, the resulting compressed AG films showed uniform morphology in lateral dimensions, high conductivity (60 S/cm), nanoporous feature ( 99.6% after 10,000 cycles). Our results suggested that AG-MSCs are competitive for prospective applications of miniaturized energy storage devices.

The reduction of a metal oxide is often a critical preparation step for activating catalytic behaviour. This study addresses the reduction process of NiO in pure form and in a composite of NiO/yttria-stabilized zirconia (YSZ) in hydrogen relevant for solid oxide electrochemical cells by comparing results from environmental transmission electron microscopy (ETEM) with thermogravimetric analysis (TGA). The temperature dependent reduction profiles obtained from TGA confirm an inhibitive effect from YSZ on the NiO reduction. The ETEM images show the growth of Ni in decaying NiO and reveal the nanoscale morphological changes such as pore formation in NiO above 280°C and densification and collapse of the pore structures above 400°C. The accelerated Ni front in NiO illustrates the auto catalysis of the reaction. A rapid temperature ramping from room temperature to 780°C in hydrogen in 1 second resulted in immediate morphological changes at the nanoscale from dense NiO to dense Ni. The analysis suggests that the inhibitive effect of YSZ on the NiO reduction reaction is not due to a direct local interaction between YSZ and NiO, but instead due to gas and/or mass transport limitations.© 2014 Elsevier B.V. All rights reserved.

In this work, we present a detailed study of the formation of supported intermetallic Ni–Ga catalysts for CO2 hydrogenation to methanol. The bimetallic phase is formed during a temperature-programmed reduction of the metal nitrates. By utilizing a combination of characterization techniques, in particular in situ and ex situ X-ray diffraction, in situ X-ray absorption spectroscopy, transmission electron microscopy combined with electron energy loss spectroscopy and X-ray fluorescence, we have studied the formation of intermetallic Ni–Ga catalysts of two compositions: NiGa and Ni5Ga3. These methods demonstrate that the catalysts with the desired intermetallic phase and composition are formed upon reduction in hydrogen and enable us to propose a mechanism of the Ni–Ga nanoparticles formation. By studying the effect of calcination prior to catalyst reduction, we show that the reactivity depends on particle size, which suggests that the reaction is structure sensitive.

Molybdenum carbide supported on active carbon, carbon nanotubes, and titanium dioxide, and promoted by K 2 CO 3 , has been prepared and tested for methanol and higher alcohol synthesis from syngas. At optimal conditions, the activity and selectivity to alcohols (methanol and higher alcohols) over supported molybdenum carbide are significantly higher compared to the bulk carbide. The CO conversion reaches a maximum, when about 20 wt% Mo 2 C is loaded on active carbon. The selectivity to higher alcohols increases with increasing Mo 2 C loading on active carbon and reaches a maximum over bulk molybdenum carbide, while the selectivity to methanol follows the opposite trend. The effect of Mo 2 C loading on the alcohol selectivity at a fixed K/Mo molar ratio of 0.14 could be related to the amount of K 2 CO 3 actually on the active Mo 2 C phase and the size, structure and composition of the supported carbide clusters. Unpromoted, active carbon supported Mo 2 C exhibits a high activity for CO conversion with hydrocarbons as the dominant products. The K 2 CO 3 promoter plays an essential role in directing the selectivity to alcohols rather than to hydrocarbons. The optimum selectivity toward higher alcohols and total alcohols is obtained at a K/Mo molar ratio of 0.21 over the active carbon supported Mo 2 C (20 wt%).

Carbon particle structures containing only a few graphene layers have been collected from a Bunsen (propane) diffusion flame in the low particle concentration pale yellow luminous regions close to the soot inception. These particles were sampled directly on transmission electron microscopy grids for structural and elemental analysis. They were found to be several hundreds of nanometers in size. Such large structures are not easily explained from gas-phase kinetic models, yet the sheets occurred relatively frequent in the images. Some pictures also showed interesting polygonal few-layered graphitic structures perpendicular to the graphene planes.

Environmental transmission electron microscopy is used in combination with density functional theory calculations to study the redox stability of a nickel/yttria-stabilized zirconia solid oxide fuel cell anode. The results reveal that the transfer of oxygen from NiO to yttria-stabilized zirconia triggers the reduction reaction. During Ni reoxidation, the creation of a porous structure, due to mass transport, accounts for the redox instability of the Ni-based anode. Both the expansion of NiO during a redox cycle and the presence of stress in the yttria-stabilized zirconia grains are observed directly. Besides providing an understanding of the Ni-YSZ anode redox degradation, the observations are used to propose an alternative anode design for improved redox tolerance. © 2010 Acta Materialia Inc.

The development of a ternary AlxIn1-xP shell grown around GaAs nanowires epitaxially grown in the [(1) over bar (1) over bar (1) over bar] direction has been studied. Morphology and composition of the shell have been studied using cross-sectional transmission electron microscopy (TEM). The side facets of the shell are found to develop dominant {1 (1) over bar 0} macro facets with small (approx. 5 nm) {1 1 (2) over bar} facets independent of the GaAs core side facets. Phase segregation is observed as AIP developing from the {1 1 (2) over bar} facets, while Al0.5In0.5P is found in the rest of the ternary shell. (C) 2010 Elsevier B.V. All rights reserved.

We report the growth and characterization of GaSb nanowires grown by MOVPE. The structural properties of the nanowires are investigated by the means of transmission electron microscopy, X-ray diffraction and single nanowire photoluminescence. The measurements confirm a high material quality in the GaSb nanowires. Also, a back-gated nanowire transistor structure is used to extract values for the polarity and resistivity of the GaSb. Finally, a simple kinetic model is presented to explain the non-linear time dependence of the GaSb nanowire growth. (C) 2008 Elsevier B.V. All rights reserved.

A series of triclinic Fe1−xAlxVO4 phases with 0

We report Au-assisted growth of GaAs/GaSb nanowire heterostructures on GaAs(1 1 1)B-substrates by metal-organic vapor phase epitaxy. The growth is studied at various precursor molar fractions and temperatures, in order to optimize the growth conditions for the GaSb nanowire segment. In contrast to most other III-V nanowire systems, the GaSb nanowire growth is Group V-limited under most conditions. We found that depending on the TMSb molar fraction, the seed particle is either supersaturated AuGa or AuGa2 during GaSb growth. The high Ga content in the particle gives a characteristic diameter increase between the GaAs and GaSb segment. From TEM and XEDS measurements we conclude that the GaSb nanowire growth occurs along either the AuGa-GaSb or AuGa2-GaSb pseudo-binaries of the Au-Ga-Sb ternary phase diagram. Finally, the GaSb nanowires exhibit untapered radial growth on the {1 (1) over bar 0} side facets. (C) 2008 Elsevier B.V. All rights reserved. (Less)

Substitution of W for the Mo in M2-type Mo-V-Te-oxide was investigated. XRD, FTIR, XANES, and HRTEM verified that the substitution is possible in the whole range up to complete replacement of Mo by W. Catalytic measurements for propene ammoxidation show that both the specific activity and the selectivity to acrylonitrile pass through a maximum with increase of the W-content of the catalyst. XRD analysis indicates that the catalytic results, in part, can be explained by a disordered distribution of Mo, W, and V among the octahedral sites in M2 giving better catalytic performance than a more ordered arrangement does. Moreover, SEM and HRTEM imaging show that with increasing W-content, the M2 crystals change from μm-sized elongated crystals to spherical crystals of nm-size, influencing the type of surfaces being exposed and consequently the catalytic properties. Among the catalysts investigated, a catalyst with the composition (Mo0.3W0.7)1.9V1.0Te1.1O10 is the most active and selective one, giving 80% selectivity to acrylonitrile.

The aim of the Letter is to elucidate the nature of metal-support interaction in the 2 wt% Rh/Al 2 O 3 catalyst obtained by annealing Rh–O–Al xerogel at 1113 K in air. XPS, HRTEM, and XRD results reveal that during the Rh–O–Al xerogel annealing in air, rhodium incorporates into forming alumina, which results mostly in Rh 4+ /δ-Al 2 O 3 solid solution formation. However, in the course of the catalyst reduction at 773 with H 2 and at 823 K with CH 4 the Rh 4+ /δ-Al 2 O 3 solid solution transforms into Rh–Al alloy. The islands of rhodium form on the surface of the Rh–Al alloy nanocrystallites if the reduction is slow enough.

Binary molybdenum oxides are active catalysts for propene oxidation to acrolein if they are structurally different from orthorhombic MoO 3 (o-Mo). The periodical structure of the as-synthesized precatalyst must be different from o-Mo to create the activity. Additional features typical for efficient catalysts are a bulk nanostructuring and a complex surface termination that are detectable in TEM examinations only. After transition into defective o-Mo the samples exhibit much reduced activities. The implications of these findings on the functional analysis of complex selective oxidation catalysts are discussed.

A set of Mo, V, and W mixed -oxide samples with additives P and Cs showing high performance in select ive partial oxidation of propene is studied by means of elemental mapping and crystal structure determination using scanning and transmission electron microscopy, energy-dispersive X-ray spectroscopy (EDS), and electron energy loss spectroscopy (EELS) as supplement to XRD measurements which are insufficient for phase determination. The set is divided into two subsets of samples denoted A and B, where subset A samples show the highest performance in select ive partial oxidation of propene. These high-performance catalysts are all predominated by structures of the Mo5O14 type in contrast to a negligible amount of the orthorhombic MoO3, which is known to favor total oxidation.

Photoluminescence and photoluminescence excitation spectroscopy on Ga0.47In0.53As multi quantum wells confined either by homogeneous ternary Al0.48In0.52As barriers or by Ga0.47In0.53As/Al0.48In0.52As short-period superlattice (SPS) barriers show that the confinement by SPS barriers improves the edge luminescence significantly. The spectral width of the free-exciton absorption and the low-temperature emission peak as well as the Stokes shift between emission and excitation spectra are reduced as compared to samples clad by homogeneous ternary Al0.48In0.52As barriers. The dominant low-temperature emission line in the SPS-clad Ga0.47In0.53As multi quantum wells is assigned to intrinsic excitonic recombination. The small Stokes shift of the excitonic luminescence line can be explained by intrinsic relaxation processes caused by monolayer fluctuation of the well width and by statistical fluctuation of the composition of the ternary Ga0.47In0.53As well material. (IAF)