The processes occurring during the early stages of the formation of crystalline solids are not well understood thus preventing the rational synthesis of new solids. The investigation of the ...structure-forming processes is an enormous challenge for both analytical and theoretical methods because very small particles or aggregates with different chemical composition and different sizes must be probed, both before and during nucleation. Furthermore, these precursors are present in a complex and dynamic equilibrium. This Review gives a survey of the in-situ methods available for the study of the early stages of crystallization of solids and how they can help in the synthesis of metastable polymorphs, of transient intermediates, and/or precursors displaying new or improved properties. Examples of actual research demonstrate the necessity and potentials but also the limitations of in-situ monitoring of the formation of crystalline solids.
Semimetal-functionalised polyoxovanadates Monakhov, Kirill Yu; Bensch, Wolfgang; Kögerler, Paul
Chemical Society reviews,
12/2015, Letnik:
44, Številka:
23
Journal Article
Recenzirano
Odprti dostop
Polyoxovanadates (POVs), known for their wide applicability and relevance in chemical, physical and biological sciences, are a subclass of polyoxometalates and usually self-assemble in aqueous-phase, ...pH-controlled condensation reactions. Archetypical POVs such as the robust V
IV
18
O
42
12−
polyoxoanion can be structurally, electronically and magnetically altered by heavier group 14 and 15 elements to afford Si-, Ge-, As- or Sb-decorated POV structures (heteroPOVs). These main-group semimetals introduce specific chemically engineered functionalities which cause the generally hydrophilic heteroPOV compounds to exhibit interesting reactivity towards organic molecules, late transition metal and lanthanoid ions. The fully-oxidised (V
V
), mixed-valent (V
V
/V
IV
and V
IV
/V
III
), "fully-reduced" (V
IV
) and "highly-reduced" (V
III
) heteroPOVs possess a number of intriguing properties, ranging from catalytic to molecular magnet characteristics. Herein, we review key developments in the synthetic and structural chemistry as well as the reactivity of POVs functionalised with Si-, Ge-, As- or Sb-based heterogroups.
Recent synthetic advances have greatly expanded the class of polyoxovanadate cluster structures that are in part substituted or augmented by semimetal (Si, Ge, As, Sb) groups, in turn enabling subsequent functionalization steps and resulting in novel materials properties.
We present a systematic study on the magnetotransport properties of HfTe2 single crystals grown by different synthetic protocols. Both chemical vapor transport (CVT) as well as the self-flux method ...were applied. Depending on the synthetic procedure the crystal quality is reflected by the residual resistivity ratio (RRR). The best CVT grown crystal shows a RRR of 262, while the crystal with the highest quality obtained with the Te self-flux method exhibits a value of 404. The superiority of the self-flux method can be traced back to its ability to reduce the amount of Zr as main contaminant more effectively compared to chemical vapor transport. The large RRR value is reflected in the magnetoresistance (MR) effect which reaches more than 9400%, outperforming the data published for HfTe2. The benefit of the self-flux approach was tested for WTe2 and a RRR of 2525 was reached significantly surpassing the data reported in literature. Crystals of both high and low RRR were compared with respect to the magnetotransport properties, i.e., transverse magnetoresistance and the Hall effect. The major factor determining the maximum value of the MR is the carrier mobility which is severely affected by the preparation conditions, while the carrier balance remains virtually unaffected.
Simple test reactions as ethene hydrogenation, 2-butene cis–trans isomerization and H2/D2 scrambling were shown to be catalyzed by MoS2 and WS2 in surface states which did not chemisorb oxygen and ...were, according to XPS analysis, saturated by sulfide species. This is a clear experimental disproof of classical concepts that require coordinative unsaturation for catalytic reactions to occur on such surfaces. It supports new concepts developed on model catalysts and by theoretical calculations so far, which have been in need of confirmation from real catalysis.
Three new arsenato-polyoxovanadates with the composition M(en)32V6As8O26 (M = Co2+ (I), Zn2+ (II), and Cd2+ (III)) were synthesized under solvothermal conditions in high yields, thus significantly ...enhancing the knowledge of As-rich polyoxovanadate cluster chemistry. The compounds are isostructural and feature the very rare V6IVAs8IIIO264− cluster anion. The cluster shell is constructed by interconnection of two trimeric {V3O11} groups consisting of three edge-sharing VO5 polyhedra and four As2O5 units, which are formed by two corner-sharing AsO3 pyramids. While the As2O5 group is a common structural feature in arsenato-polyoxovanadates, the {V3O11} unit is only observed in V-rich high-nuclear heteroatom-containing polyoxovanadates {V14E8} (E = As, Sb, Ge). The complexes adopt the Λ (δδδ) conformation, which is the most stable arrangement. Interestingly, the unit cell parameters do not scale with the volume of the M(en)32+ complexes, assuming a constant volume of the anion. Only a very detailed Hirshfeld surface analysis revealed that the van der Waals volume of the {V6As8O26} moiety is the smallest for the Cd-containing compound, while the volumes of the anions in the other two compounds are very similar. Therefore, the observed trends of the lattice parameters can be explained on the basis of these findings. Furthermore, intermolecular interactions include As⋯H contacts in addition to O⋯H and H⋯H interactions. The electronic spectrum of I contains d–d transitions of the vanadyl group and of the Co2+ cation. As expected only the d–d transitions of the VO2+ unit occur for II and III.
The hydrothermal transformation of the heteroatom polyoxovanadate compound {Ni(en)3}3V15Sb6O42(H2O)·≈15H2O (en = ethylenediamine) in water into {Ni(en)2}2V14Sb8O42·5.5H2O was investigated by in situ ...XRD experiments at different temperatures. First, the precursor undergoes very fast amorphization or dissolution, and an induction period is observed that depends on the reaction temperature. Crystal growth of the product is completed within 3 h, while higher temperatures lead to accelerated reaction progress. Evaluation of the kinetics showed that heterogeneous nucleation is the rate‐limiting step of the reaction. The activation energies for nucleation and crystal growth are very low compared to data reported in the literature for several other chemical systems.
In‐situ X‐ray diffraction experiments on the transformation of a compound containing {V15Sb6O42} clusters into a layered compound consisting of {V14Sb8O42} clusters demonstrate a two‐step mechanism proceeding by amorphization/dissolution and Avrami nucleation. Activation energies for nucleation and crystal growth are remarkably low.
Reaction of Zn(ClO4)2·6H2O with cyclam (cyclam = 1,4,8,11-tetraazacyclotetradecane, C10H24N4) and Na3SbS4 in an acetonitrile/water mixture led to the formation of crystals of the title compound, ...Zn3(SbS4)(C10H24N4)3(SbS4)·2CH3CN·2H2O or (Zn-cyclam)3(SbS4)2(H2O)2(acetonitrile)2. The set-up of the crystal structure is similar to that of (Zn-cyclam)3(SbS4)2.8H2O reported recently Danker et al. (2021). Dalton Trans. 50, 18107–18117. The crystal structure of the title compound consists of three crystallographically independent ZnII cations (each disordered around centers of inversion), three centrosymmetric cyclam ligands, one SbS43– anion, one water and one acetonitrile molecule occupying general positions. The acetonitrile molecule is equally disordered over two sets of sites. Each Zn2+ cation is bound to four nitrogen atoms of a cyclam ligand and one sulfur atom of the SbS43– anion within a distorted square-pyramidal coordination. The cation disorder of the Zn(cyclam)2+ complexes is discussed in detail and is also observed in other compounds, where identical ligands are located above and below the Zn(cyclam)2+ plane. In the title compound, the building units are arranged in layers parallel to the bc plane forming pores in which the acetonitrile solvate molecules are located. Intermolecular C—H...S hydrogen bonding links these units to the SbS43– anions. Between the layers, additional water solvate molecules are present that act as acceptor and donor groups for intermolecular N—H...O and O—H...S hydrogen bonding.
Liquid phase exfoliation (LPE) has been used for the successful fabrication of nanosheets from a large number of van der Waals materials. While this allows to study fundamental changes of material ...properties’ associated with reduced dimensions, it also changes the chemistry of many materials due to a significant increase of the effective surface area, often accompanied with enhanced reactivity and accelerated oxidation. To prevent material decomposition, LPE and processing in inert atmosphere have been developed, which enables the preparation of pristine nanomaterials, and to systematically study compositional changes over time for different storage conditions. Here, we demonstrate the inert exfoliation of the oxidation-sensitive van der Waals crystal, CrTe3. The pristine nanomaterial was purified and size-selected by centrifugation, nanosheet dimensions in the fractions quantified by atomic force microscopy and studied by Raman, X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDX) and photo spectroscopic measurements. We find a dependence of the relative intensities of the CrTe3 Raman modes on the propagation direction of the incident light, which prevents a correlation of the Raman spectral profile to the nanosheet dimensions. XPS and EDX reveal that the contribution of surface oxides to the spectra is reduced after exfoliation compared to the bulk material. Further, the decomposition mechanism of the nanosheets was studied by time-dependent extinction measurements after water titration experiments to initially dry solvents, which suggest that water plays a significant role in the material decomposition.
Reaction of Co(ClO
4
)
2
·6H
2
O with cyclam (cyclam = 1,4,8,11-tetraazacyclotetradecane) and Na
3
SbS
4
·9H
2
O (Schlippesches salt) in a mixture of acetonitrile and water leads to the formation of ...crystals of the title compound with the composition {Co
3
(SbS
4
)
2
(C
10
H
24
N
4
)
3
·2CH
3
CN·2H
2
O}
n
or {(Co-cyclam)
3
(SbS
4
)
2
·2(acetonitrile)·2H
2
O}
n
. The crystal structure of the title compound consists of three crystallographically independent Co-cyclam
2+
cations, which are located on centers of inversion, one SbS
4
3−
anion, one water and one acetonitrile molecule that occupy general positions. The acetonitrile molecule is disordered over two orientations and was refined using a split model. The Co
II
cations are coordinated by four N atoms of the cyclam ligand and two
trans
-S atoms of the tetrathioantimonate anion within slightly distorted octahedra. The unique SbS
4
3−
anion is coordinated to all three crystallographically independent Co
II
cations and this unit, with its symmetry-related counterparts, forms rings composed of six Co-cyclam cations and six tetrathioantimonate anions that are further condensed into layers. These layers are perfectly stacked onto each other so that channels are formed in which acetontrile solvate molecules that are hydrogen bonded to the anions are embedded. The water solvate molecules are located between the layers and are connected to the cyclam ligands and the SbS
4
3−
anions
via
intermolecular N—H...O and O—H...S hydrogen bonding.
Transition metal inserted NbS2 (TxNbS2) compounds receive great attention due to their intriguing and diverse magnetic and electric transport properties. Typically, these compounds are prepared by ...high-temperature synthesis from the elements, which is time and energy-consuming and yields highly crystalline products. So far, no route for preparing these compounds from precursors by thermal decomposition has been reported. Herein, we report the synthesis of a dithiocarbamate of niobium Nb2S4(CS2NH2)4 as a precursor for the synthesis of NbS2 by this preparative strategy. Furthermore, we demonstrate that a co-decomposition with dithiocarbamates of transition metals (here, Co and Pd) is a viable route for the synthesis of TxNbS2-type compounds. This is a promising route for the exploration of these compounds’ properties in the form of, e.g., nanocrystalline or thin film samples.
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