We report here a simple hydrothermal synthesis of 100-200 nm flakes of tochilinite (Fe
1−
x
S)·
n
(Mg,Fe)(OH)
2
constructed by interchanging atomic sulfide and hydroxide sheets as a representative of ...a new platform of multifunctional two-dimensional materials. The reliable formation of tochilinites was ensured by an excess of sodium sulfide, with the assembly of the metal sulfide and hydroxide sheets driven by their opposite electric charges. X-ray photoelectron spectroscopy found that the hydroxide layers involved Fe
3+
cations from 10 to 40% of total iron tuned by addition of Al and Li entering the layers; the Fe
1−
x
S sheets comprised comparable amounts of high-spin Fe
3+
and Fe
2+
centers, and minor S-S bonding. The room-temperature Mössbauer spectra fitted with several doublets (chemical shift of 0.35-0.4 mm s
−1
and varying quadrupole splitting) transformed to three six-line patterns (hyperfine fields of ∼290, 350 and 480 kOe) due to magnetic ordering at 4.2 K, albeit the paramagnetic behavior observed in SQUID experiments. A series of UV-vis absorption maxima were explained in terms of both the high-index all-dielectric Mie resonance, in line with the permittivity measurement data, and the ligand-metal charge transfer resembling that in Fe-S clusters in proteins. Prospective properties and applications of the materials are discussed.
We report the reliable synthesis of 2D iron sulfide-magnesium hydroxide nanoflakes. The sulfide and hydroxide sheets assemble
via
opposite electric charges. Comparable amounts of high-spin Fe
3+
and Fe
2+
centers occur in the sulfide layers.
Geological activity on icy planets and planetoids includes cryovolcanism. Until recently, most research on terrestrial permafrost has been engineering-oriented, and many related phenomena have ...received too little attention. Although fast processes in the Earth's cryosphere were known before, they have never been attributed to cryovolcanism. The discovery of a couple of tens of meters wide crater in the Yamal Peninsula aroused numerous hypotheses of its origin, including a meteorite impact or migration of deep gas as a result of global warming. However, the origin of the Yamal crater can be explained in terms of cryospheric processes. Thus, the Yamal crater appears to result from collapse of a large pingo, which formed within a thaw lake when it shoaled and dried out allowing a large talik (that is layer or body of unfrozen ground in a permafrost area) below it to freeze back. The pingo collapsed under cryogenic hydrostatic pressure built up in the closed system of the freezing talik. This happened before the freezing completed, when a core of wet ground remained unfrozen and stored a huge amount of carbon dioxide dissolved in pore water. This eventually reached gas-phase saturation, and the resulting overpressure came to exceed the lithospheric confining stress and the strength of the overlying ice. As the pingo exploded, the demarcation of the crater followed the cylindrical shape of the remnant talik core.
A new approach is being considered for obtaining microfibrillated cellulose with a low degree of polymerization by sulfuric acid hydrolysis with simultaneous ultrasonic treatment under mild ...conditions (temperature 25 °C, 80% power control). Samples of initial cellulose, MCC, and MFC were characterized by FTIR, XRF, SEM, DLS, and TGA. It was found that a high yield of MFC (86.4 wt.%) and a low SP (94) are observed during hydrolysis with ultrasonic treatment for 90 min. It was shown that the resulting microfibrillated cellulose retains the structure of cellulose I and has an IC of 0.74. It was found that MFC particles are a network of fibrils with an average size of 91.2 nm. ζ-potential of an aqueous suspension of MFC equal to -23.3 mV indicates its high stability. It is noted that MFC has high thermal stability, the maximum decomposition temperature is 333.9 °C. Simultaneous hydrolysis process with ultrasonic treatment to isolate MFC from cellulose obtained by oxidative delignification of spruce wood allows to reduce the number of stages, reduce energy costs, and expand the scope.
We introduce here a multifunctional material composed of alternating atomic sulfide sheets close to the composition of CuFeS
2
and Mg-based hydroxide ones (valleriite), which are assembled due to ...their electric charges of opposite sign. Valleriite particles 50-200 nm in lateral size and 10-20 nm in thickness were synthesized
via
a simple hydrothermal pathway using various concentrations of precursors and dopants, and examined with XRD, TEM, EDS, X-ray photoelectron spectroscopy, reflection electron energy loss spectroscopy (REELS), Mössbauer, Raman and UV-vis-NIR spectroscopies, and magnetization, dynamic light scattering, and zeta potential measurements. The electronic, magnetic and optical characteristics are found to be critically dependent on the charge (electron density) at the narrow-gap sulfide layers containing Cu
+
and Fe
3+
cations, and can be tuned
via
the composition of the hydroxide part. Particularly, substitution of Mg
2+
with Al
3+
increases the negative charge of the hydroxide layers and reduces the content of Fe
3+
-OH centers (10-45% of total iron); the effects of Cr and Co dopants entering both layers are more complicated. Mössbauer doublets of paramagnetic Fe
3+
detected at room temperature transform into several Zeeman sextets at 4.2 K; the hyperfine fields up to 500 kOe and complex magnetic behavior, but not pure paramagnetism or antiferromagnetism, were observed for valleriites with the higher positive charge of the sulfide sheets, probably due to the depopulation of the minority-spin 3d states of S-bonded Fe
3+
ions. Aqueous colloids of valleriite show optical absorption at 500-750 nm, which, along with the peaks at the same energies in REELS, may arise due to quasi-static dielectric resonance involving the vacant Fe 3d band and being dependent on the composition of both layers too. These and other findings call attention to valleriites as a new rich family of 2D materials for a variety of potential applications.
We propose a simple hydrothermal synthesis of nanoflakes composed of alternating sulfide and hydroxide quasi-monolayers and their aqueous colloids, as a prospective family of novel multifunctional 2D materials.
Display omitted
•Surface analysis of citrate-stabilized AgNPs immobilized from a dense sol was performed.•Capping species are largely products of citrate decomposition.•Species adsorbed on AgNPs ...having various sizes are different.•Ligands bound to surface Ag via one or two carboxylate and alcohol groups.•No surface ketone group was found.
Citrate is an important stabilizing, reducing, and complexing reagent in the wet chemical synthesis of nanoparticles of silver and other metals, however, the exact nature of adsorbates, and its mechanism of action are still uncertain. Here, we applied X-ray photoelectron spectroscopy, soft X-ray absorption near-edge spectroscopy, and other techniques in order to determine the surface composition and to specify the citrate-related species at Ag nanoparticles immobilized from the dense hydrosol prepared using room-temperature reduction of aqueous Ag+ ions with ferrous ions and citrate as stabilizer (Carey Lea method). It was found that, contrary to the common view, the species adsorbed on the Ag nanoparticles are, in large part, products of citrate decomposition comprising an alcohol group and one or two carboxylate bound to the surface Ag, and minor unbound carboxylate group; these may also be mixtures of citrate with lower molecular weight anions. No ketone groups were specified, and very minor surface Ag(I) and Fe (mainly, ferric oxyhydroxides) species were detected. Moreover, the adsorbates were different at AgNPs having various size and shape. The relation between the capping and the particle growth, colloidal stability of the high-concentration sol and properties of AgNPs is briefly considered.
We report on the fabrication and studies of Ø100 μm half-disk lasers with an active region based on InGaAs/GaAs quantum dots providing very high modal gain. Such resonators support whispering gallery ...modes propagating at the cavity periphery. The microlasers show directional light outcoupling: continuous-wave output power emitted from the flat side reaches 17 mW, which is about 7 times greater than the power emitted from the back semicircular side. Single-mode lasing in a wide range of the injection currents is observed. P-side down bonding of the devices onto Si-board allowed increasing the maximum optical power to more than 30 mW and the lasing was observed up to 93°C. The 3 dB modulation bandwidth of 4.6 GHz was measured likely being limited by RC-parasites.
The development of inexpensive, easy-to-produce, and easy-to-use analytical tools for detection of harmful and toxic substances is a relevant research problem with direct applications in ...environmental monitoring and protection. In this work, we propose a novel composite material based on alumina nanofibers and detonation nanodiamonds for detection of phenol in aqueous medium. The composite material was obtained by mixing an aqueous suspension of alumina nanofibers with a diameter of 10–15 nm and a length of several microns and a hydrosol of nanodiamonds with an average cluster size of 70 nm. The mechanisms underlying the interaction of these nanomaterials are clarified and the physicochemical properties of the composite are investigated. The SEM and TEM studies show that the obtained composite has a network structure, in which clusters of nanodiamonds (10–20 nm in diameter) are distributed over the surface of nanofibers. Coupling of nanomaterials occurs due to opposite signs of their zeta potentials, which results in electrostatic attraction and subsequent chemical bonding as indicated by the X-ray photoelectron spectroscopy and simultaneous thermal analysis. The bonding apparently occurs between functional groups (mainly carboxyl) on the surface of nanodiamonds and amphoteric hydroxyl groups on the surface of alumina nanofibers. The proposed composite allows an easy-to-perform colorimetric analysis for qualitative and quantitative determination of phenol in aqueous samples with linear response over a wide range of concentrations (0.5–106 μM). Multiple tests have shown that the composite is reusable and retains its catalytic function for at least 1 year during storage at room temperature.
We recently synthesized prospective new materials composed of alternating quasi-atomic sheets of brucite-type hydroxide (Mg, Fe)(OH)2 and CuFe1–x S2 sulfide (valleriites). Herein, their thermal ...behavior important for many potential applications has been studied in inert (Ar) and oxidative (20% O2) atmospheres using thermogravimetry (TG) and differential scanning calorimetry (DSC) analyses and characterization with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX). In the Ar media, the processes are determined by the dehydroxylation of the hydroxide layers forming MgO, with the temperature of the major endothermic maximum of the mass loss at 413 °C. Sulfide sheets start to degrade below 500 °C and melt at nearly 800 °C, with bornite, chalcopyrite, and troilite specified as the final products. In the oxidative atmosphere, the exothermic reactions with the mass increase peaked at 345 and 495 °C, corresponding to the partial and major oxidations of Cu–Fe sulfide layers. Sulfur oxides captured in magnesium hydroxide layers to form MgSO4 compromised the layer integrity and promoted the oxidation of the sulfide entities. The final products also contained minor MgO, Cu2MgO3, Fe3O4, and MgFe2O4 phases. Samples doped with Al, which decreases the content of Fe in hydroxide layers, show notably impeded decay of valleriite in argon but facilitated the oxidation of Cu–Fe sulfides, while the impact of Li (it slightly increases the number of the Fe–OH sites) was less expressed. The mutual stabilization of the two-dimensional (2D) hydroxide and sulfide layers upon heating in an inert atmosphere but not in oxygen as compared with bulk brucite and chalcopyrite was suggested to explain high thermal resistance across the stacked incommensurate sheets, which slows down the endothermic reactions and accelerates the exothermic oxidation; the high number of Fe atoms in the hydroxide sheets are expected to promote the phonon exchange and heat transfer between the layers.
Although nanoparticles of heavy metal xanthates and their hydrosols can play important roles in froth flotation, environmental issues, analytics, and manufacturing of metal sulfide nanocomposites, ...they have received little attention. We studied colloidal solutions and immobilized particles prepared via interaction of aqueous lead nitrate with alkyl xanthates applying UV–vis absorption spectroscopy, dynamic light scattering, zeta potential measurement, thermogravimetry analysis, Fourier transform infrared spectroscopy, Raman scattering, X-ray photoelectron spectroscopy, atomic force microscopy, and transmission electron microscopy. The hydrodynamic diameter of colloidal particles of Pb(SSCOR)2 decreased from 500 to 50 nm with an increase in the alkyl radical length and the initial xanthate to lead ratio (X/Pb); the zeta potential magnitude varied similarly, although it remained negative. The effect of pH in the range of 4.5–11 was minor, but the colloids produced using excess of Pb2+ in alkaline media were close to PbX and decomposed much easier than PbX2. The uptake of lead xanthates on supports was generally low because of negative charges of the colloids; however, 50–100 nm thick PbX2 films were deposited on PbS and SiO2 from the media of X/Pb < 2 and pH < 9 because of preadsorption of Pb2+, while nanorods formed on highly oriented pyrolytic graphite.
Gold and silver sulfoselenides are of interest as materials with high ionic conductivity and promising magnetoresistive, thermoelectric, optical, and other physico-chemical properties, which are ...strongly dependent on composition and structure. Here, we applied X-ray photoelectron spectroscopy and Ag L3 X-ray absorption near-edge structure (XANES) to study the electronic structures of low-temperature compounds and solid solutions Ag2SxSe1–x (0 < x < 1), AgAuS, and Ag3AuSxSe2–x (x = 0, 1, 2). Upon substitution of Se with S, a steady increase in the positive charge at Ag(I) sites and only minor changes in the local charge at chalcogen atoms were found from the photoelectron Ag 3d, S 2p, Se 3d, and Ag M4,5VV Auger spectra. The intensity of the Ag L3-edge peak, which is known to correlate with hole counts in the Ag 4d shell having a formal d10 configuration, was enhanced by 20–25% from Ag2Se to Ag2S and from Ag3AuSe2 to Ag3AuS2. The effect of gold is more pronounced, and the number of Ag d holes and the negative charge of S and Se notably decreased for Au-containing compounds; in particular, the Ag L3-edge peak is about 35% lower for AgAuS relative to Ag2S. At the same time, the Au 4f binding energy and, therefore, charge at Au(I) sites increase with increasing S content due to the transfer of electron density from Au to Ag atoms. It was concluded that the effects mainly originate from shortening of the metal–chalcogen and especially the AuAg interatomic distances in substances having similar coordination geometry.
•XPS and Ag L3 XANES spectra from Ag2SxSe1-x, AgAuS, and Ag3AuSxSe2-x were compared.•Positive charge at Ag(I) increased with S content, the charges of ligands were steady.•Presence of Au effectively decreased the local charges both at Ag and chalcogen atoms.•Coordination of Ag to S increased while Au reduced count of holes in Ag 4d shell.•The effects are determined mainly by metal–ligand and AuAg interatomic distances.
PDF
