N. D. Lenenko

Distortion of MoS2 host layers in intercalation compounds with various guest species: correlation with charge transfer

Abstract The MoS 2 host layers in various intercalation compounds freshly prepared from single-layer dispersions are substantially distorted compared to those in parent crystalline 2 H -MoS 2 . The coordination number for the Mo–Mo sphere ( d ∼3.2 A) obtained from Mo K-edge EXAFS data can be used as a convenient parameter to estimate qualitatively the degree of distortion, which is shown to correlate fairly with the residual negative charge kept by the host layers.

Solvent effects on the formation and absorption spectra of nanodisperse molybdenum disulfide

The optical absorption spectra of molybdenum disulfide prepared by monolayer dispersion of a crystalline precursor in a liquid medium were studied. On the whole, the three-dimensional structure of the crystalline analogue was reconstructed in disperse MoS2 dried to a powder state. A substantial fraction of the material transformed into a nanosized state during dispersion and was stabilized in the form of nanoparticles in various solvents. The stabilizing action of the solvents studied decreased in the series acetonitrile, ethanol > chloroform > water. The data on the optical absorption of MoS2 suspensions in various solvents were evidence of a broad particle-size distribution. Along with nanoscale particles, the samples contained larger particles responsible for long-wave absorption (>600 nm). The size of nanoparticles was estimated using an empirical correlation between the position of maximum A (the long-wave maximum of the first allowed absorption band) in the absorption spectrum and the size of particles (2R). This correlation was obtained using the literature data on monodisperse particles and mixtures of particles. The number of particles showed a tendency to increase as the concentration of disperse MoS2 in acetonitrile decreased.

Modification of molybdenum disulfide (2H-MoS2) and synthesis of its intercalation compounds

We have shown that powdered molybdenum disulfide prepared as a result of the exfoliation (singlelayer dispersion) of a crystalline precursor (2H-MoS2) and subsequent precipitation in an acid medium, is capable of forming intercalation compounds in reacting with salts of copper and tetramethylammonium, as well as diethylamine and triethylamine. We consider factors influencing the composition and type of the alternation of guest and host layers in the resulting layered compounds. Relationships between charge transfer to MoS2 layers and the extraordinary reactivity of its modified species are discussed.

Thermal transformations of nanocomposite material consisting of molybdenum disulfide and nickel hydroxide layers

Abstract The structure of Ni(OH)2/MoS2 nanocomposite material obtained via MoS2 single-layer dispersion and the products of its annealing in the 100–400°C temperature range were studied using X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS). The transformation of the initial MoS2 superlattice structure with Mo–Mo bonding to the usual structure with unique Mo–Mo distance was found to occur upon annealing. The decomposition of two-dimensional Ni(OH)2-like species in the interlayer space of MoS2 at 400°C leads to complete substitution of O by S in the Ni local environment.

Mössbauer evidence for fast electron hopping between Fe(II) and Fe(III) in iron hydroxide/molybdenum disulfide lamellar nanocomposite

Abstract Nanocomposite compounds with iron hydroxide and molybdenum disulfide alternating layers have been studied using 57Fe Mossbauer spectroscopy. Evidence is given of the presence of two oxidation states for iron (Fe(II)/Fe(III)≈1.9). In the temperature range 60–160 K thermal evolution of the absorption areas obeys Debye’s model, with the lattice temperature ΘM equal to 275 and 320 K for Fe(II) and Fe(III), respectively. At higher temperature (T≥180 K), a new iron state is observed, which can be related to fast electron hopping between certain heterovalent iron ions. Tin-doping enhances this process which can already be detected at 140 K.

Synthesis of layered metal dichalcogenides with organic cations by electrochemical reduction in suspensions

Intercalated group IVB-VIB metal disulfides and diselenides MX2 (M = Ti, Zr, Nb, Mo) can be obtained by electrochemical reduction of the corresponding MX2 in the presence of tetraalkylammonium salt. An original approach was used, which allowed effective reduction of these insoluble substances in the form of powders suspended in an organic solvent using the standard electrolysis technique in solutions in a one-space electrolyzer with a soluble anode. Using electron transfer mediators in the process afforded intercalated compounds with yields of 60–100%.

The role of interface in photo processes in photoconductive heterophase composites based on monolayer dispersions of molybdenum disulfide

Two-layer heterostructures with a high yield of charged current carriers were obtained and investigated. The heterostructures comprised monolayer dispersions of MoS2 in polyvinyl alcohol as the lower layer and the upper p-, n-, or bipolar transport layer (TL). It was found that dark conductivity of the two-layer heterostructures increased considerably compared with the conductivity of reference samples based on TL but containing no MoS2. After excitation from the TL side, maximum photoelectric sensitivity was obtained where TL long-wave absorption decreased and there was a high concentration of TL excited states at the interface that interacted with the surface of MoS2 particles. This interaction quenched the luminescence of TL transport centers and led to the photogeneration of both positively and negatively charged current carriers at the interface. Luminescence quenching could be of 50–60% at high MoS2 contents in the lower layer (90–100 wt %). This was evidence that the contact area of MoS2 particles with the polymeric transport layer exceeded the geometric interface dimensions and the interface had a relief surface (was fairly thick). It was found that the donor-acceptor properties of both MoS2 and the polymeric TL were very important for the photogeneration of charge carriers at the interface. When the MoS2 content in the lower layer was maximum, the effectiveness of the accumulation of minority carriers increased because of the formation of an alternative transport network associated with MoS2 particles. The conclusion was drawn that the photogeneration of charges at the interface resulted from the phototransfer of electrons between MoS2 particles and TL excited states.

Preparation and photoelectric properties of organoinorganic polymer nanocomposites on the basis of ultradispersed molybdenum disulfide particles

Photoconducting nanocomposites were prepared by incorporating nanoparticles obtained by monolayer dispersion of crystalline molybdenum disulfide into various polymer matrices (nonconducting and p-or n-type conducting). The shift of the absorption spectrum caused by changes in the concentration of MoS2 nanoparticles demonstrated that such particles exhibit a quantum-size effect, the characteristics of which were found to be determined by the efficiency of their stabilization in each of the polymer matrices. Studying the photoelectric sensitivity of films of n-type-conduction composites on the basis of polymethylphenylsiloxane modified with 2,4,7-trinitrofluorenone (an electron acceptor) units demonstrated that molybdenum disulfide nanoparticles possess acceptor properties (characterized by an electron affinity energy of Ea > 2 eV, in agreement with the observation that composite films based on poly(N-epoxypropylcarbazole (a p-type-conduction matrix) doped with p-diethylaminobenzaldehyde-N,N-diphenylhydrazone (an effective donor), exhibit an enhancement in the proper photosensitivity of the p-type-conduction matrix in its absorption band (λ 600 nm (formed at a MoS2 concentration of > 2.0 wt %), a weakening of the acceptor properties was observed, a feature responsible for decreases in the photosensitivity and the quantum yield of the photogeneration of holes and for the capture of holes in traps. A mechanism of the photogeneration of charge carriers with the participation of the phototransfer of electrons leading to the formation of electron-hole pairs was discussed.

Preparing films of nanocrystalline molybdenum disulfide and examining their surface structure

A method for preparing MoS2 nanocrystalline films at room temperature in which the single-layer dispersions of MoS2 crystals are used and the films are formed by depositing a suspension of the obtained MoS2 nanoparticles onto a substrate has been developed. The peculiarities of the structure and morphology of the new films have been studied by diffractometry and atomic force microscopy. Data on the sizes of MoS2 nanocrystallites and the character of their distribution on the surface of films of various thicknesses have been obtained. A mechanism of the formation of the nanocrystalline structure of the films is proposed.

Characteristics of electrodeposition of composite coatings based on nanosized molybdenum disulfide and copper

Voltammetry was used to study the charge state of molybdenum disulfide nanoparticles in single-layer aqueous dispersions obtained by hydration of LiMoS2. MoS2 nanoparticles were shown to have a negative charge. Nanodispersed MoS2 can be used to deposit Cu-MoS2 coatings. The influence of some deposition parameters (electrolyte composition, current mode) on the coating composition was determined.