Understanding Surfactant Stabilization of MoS2 Nanosheets in Aqueous Dispersions from Zeta Potential Measurements and Molecular Dynamics Simulations

The sonication-assisted exfoliation of MoS2 in aqueous media in the presence of ionic surfactants to give stable dispersions is an attractive procedure for obtaining single or few-layered nanosheets, as it is easily scalable and does not involve toxic or high boiling solvents. Here, we have investigated the origin of the stability of aqueous dispersions of MoS2 nanosheets obtained by sonication in the presence of the cationic surfactant cetyltrimethylammonium bromide (CTAB) by zeta potential measurements at different ionic strengths and molecular dynamics (MD) simulations. Our measurements show that the dispersions are stabilized by electrostatic repulsive interactions between the delaminated MoS2 nanosheets, which acquire a positive charge because of the adsorption of the cationic surfactant. MD simulations were performed to understand the interaction between MoS2 nanosheets and the CTAB surfactant chains in the dispersion and the structure and arrangement of the adsorbed surfactant chains. Our simulations are able to reproduce the experimentally measured variation of the zeta potential with ionic strength. In addition, the relative contribution and role of different intermolecular interactions between various components of the dispersion was estimated by simulating the potential of mean force (PMF) between two surfactant-adsorbed MoS2 sheets. On the basis of experiment and simulations, we are able to establish that the stability of aqueous dispersions of MoS2 in the presence of an ionic surfactant can be understood based on classical models of charged interfaces

Cyclodextrin-functionalized Fe3O4@TiO2: reusable, magnetic nanoparticles for photocatalytic degradation of endocrine-disrupting chemicals in water supplies

Water-dispersible, photocatalytic Fe3O4@TiO2 core shell magnetic nanoparticles have been prepared by anchoring cyclodextrin cavities to the TiO2 shell, and their ability to capture and photocatalytically destroy endocrine-disrupting chemicals, bisphenol A and dibutyl phthalate, present in water, has been demonstrated. The functionalized nanoparticles can be magnetically separated from the dispersion after photocatalysis and hence reused. Each component of the cyclodextrin-functionalized Fe3O4@TiO2 core shell nanoparticle has a crucial role in its functioning. The tethered cyclodextrins are responsible for the aqueous dispersibility of the nanoparticles and their hydrophobic cavities for the capture of the organic pollutants that may be present in water samples. The amorphous TiO2 shell is the photocatalyst for the degradation and mineralization of the organics, bisphenol A and dibutyl phthalate, under UV illumination, and the magnetism associated with the 9 nm crystalline Fe3O4 core allows for the magnetic separation from the dispersion once photocatalytic degradation is complete. An attractive feature of these ``capture and destroy'' nanomaterials is that they may be completely removed from the dispersion and reused with little or no loss of catalytic activity

Trap-State Dynamics in Visible-Light-Emitting ZnO:MgO Nanocrystals

Oleate-capped ZnO:MgO nanocrystals have been synthesized that are soluble in nonpolar solvents and which emit strongly in the visible region (450−600 nm) on excitation by UV radiation. The visible emission involves recombination of trap states of the nanocrystalline ZnO core and has a higher quantum yield than the band gap UV exciton emission. The spectrally resolved dynamics of the trap states have been investigated by time-resolved emission spectroscopy. The time-evolution of the photoluminescence spectra show that there are, in fact, two features in the visible emission whose relative importance and efficiencies vary with time. These features originate from recombination involving trapped electrons and holes, respectively, and with efficiencies that depend on the occupancy of the trap density of states

Covalently Linked, Water-Dispersible, Cyciodextrin: Reduced-Graphene Oxide Sheets

Reduced-graphene oxide (rGO) sheets have been functionalized by covalently linking beta-cyclodextrin (beta CD) cavities to the sheets via an amide linkage. The functionalized beta-CD:rGO sheets, in contrast to rGO, are dispersible over a wide range of pH values (2-13). Zeta potential measurements indicate that there is more than one factor responsible for the dispersibility. We show here that planar aromatic molecules adsorbed on the rGO sheet as well as nonplanar molecules included in the tethered beta-CD cavities have their fluorescence effectively quenched by the beta-CD:rGO sheets. The beta-CD:rGO sheets combine the hydrophobicity associated with rGO along with the hydrophobicity of the cyclodextrin cavities in a single water-dispersible material

Accommodating Unwelcome Guests in Inorganic Layered Hosts: Inclusion of Chloranil in a Layered Double Hydroxide

The host-guest chemistry of most inorganic layered solids is limited to ion-exchange reactions. The guest species are either cations or anions to compensate for the charge deficit, either positive or negative, of the inorganic layers. Here, we outline a strategy to include neutral molecules like ortho- and para-chloranil, that are known to be good acceptors in donor-acceptor or charge-transfer complexes, within the galleries of a layered solid. We have succeeded in including neutral ortho- and para-chloranil molecules within the galleries of an Mg-Al layered double hydroxide (LDH) by using charge-transfer interactions with preintercalated p-aminobenzoate ions as the driving force. The p-aminobenzoate ions are introduced in the Mg-Al LDH via ion exchange. The intercalated LDH can adsorb ortho- and para-chloranil from chloroform solutions by forming charge-transfer complexes with the p-aminobenzoate anions present in the galleries. We use X-ray diffraction, spectroscopy, and molecular dynamics simulations to establish the nature of interactions and arrangement of the charge-transfer complex within the galleries of the layered double hydroxide

Spectrally Resolved Resonance Energy Transfer from ZnO:MgO Nanocrystals

Resonance energy transfer (RET) from the visible emission of core−shell ZnO:MgO nanocrystals to Nile Red chromophores, following band gap excitation in the UV, has been investigated for four different nanocrystal sizes. With use of steady state and time-resolved fluorescence spectroscopic measurements the wavelength dependent RET efficiencies have been determined. The RET process in ZnO:MgO nanocrystals occurs from emissions involving trap state recombination. There are two such processes with different RET efficiencies for the same particle size. This is shown to be a consequence of the fact that the recombination processes giving rise to the two emissions are located at different distances from the center of the particle so that the donor−acceptor distances for the two are different, even for the same particle size

Magnetism, exchange and crystal field parameters in the orbitally unquenched Ising antiferromagnet FePS3

FePS3 is a layered antiferromagnet (T N=123 K) with a marked Ising anisotropy in magnetic properties. The anisotropy arises from the combined effect of the trigonal distortion from octahedral symmetry and spin-orbit coupling on the orbitally degenerate5 T 2g ground state of the Fe2+ ion. The anisotropic paramagnetic susceptibilities are interpreted in terms of the zero field Hamiltonian, ?=?i [?(L iz 2 ?2)+|?|L i .S i ]?? ij J ij S i .S j . The crystal field trigonal distortion parameter ?, the spin-orbit coupling ? and the isotropic Heisenberg exchange,J ij, were evaluated from an analysis of the high temperature paramagnetic susceptibility data using the Correlated Effective Field (CEF) theory for many-body magnetism developed by Lines. Good agreement with experiment were obtained for ?/k=215.5 K; ?/k=166.5 K;J nn k=27.7 K; andJ nnn k=?2.3 K. Using these values of the crystal field and exchange parameters the CEF predicts aT N=122 K for FePS3, which is remarkably close to the observed value of theT N. The accuracy of the CEF approximation was also ascertained by comparing the calculated susceptibilities in the CEF with the experimental susceptibility for the isotropic Heisenberg layered antiferromagnet MnPS3, for which the high temperature series expansion susceptibility is available

Glass, Gel, and Liquid Crystals: Arrested States of Graphene Oxide Aqueous Dispersions

Colloidal systems with competing interactions are known to exhibit a range of dynamically arrested states because of the systems' inability to reach its underlying equilibrium state due to intrinsic frustration. Graphene oxide (GO) aqueous dispersions constitute a class of 2D-anisotropic colloids with competing interactions long-range electrostatic repulsion, originating from ionized groups located on the rim of the sheets, and weak dispersive attractive interactions originating from the unoxidized graphitic domains. We show here that aqueous dispersions of GO exhibit a range of arrested states, encompassing fluid, glass, and gels that coexist with liquid-crystalline order with increasing volume fraction. These states can be accessed by varying the relative magnitudes of the repulsive and attractive forces. This can be realized by changing the ionic strength of the medium. We observe at low salt concentrations, where long-range electrostatic repulsion dominates, the formation of a repulsive Wigner glass, while at high salt concentrations, when attractive forces dominate, the formation of gels exhibits a nematic to columnar liquid-crystalline transition. The present work highlights how the chemical structure of GO hydrophilic ionizable groups and hydrophobic graphitic domains coexisting on a single sheet gives rise to a rich and complex array of arrested states

Magnetism and exchange in the layered antiferromagnet NiPS3NiPS_3

The anisotropic susceptibility of the layered antifemomagnet $NiPS_3(T_N = 155 K)$ has been measured between 45 K and 650 K. The system may be described by the effective spin Hamiltonian $H = DS^2_{iz}-\sum_{ij} J_{ij}S_i . S_j$, with the quadratic single-ion anisotropy terms introducing anisotropy in an otherwise isotropic situation. The exchange I and single-ion anisotropy parameter D were determined from an analysis of the anistropic susceptibility data for two different models: (i) the Oguchi model, in which a pair of spins chosen at random is treated exactly while its interactions with the rest of the crystal are approximated by a mean field and (ii) the correlated effective field (CEF) approximation developed by Lines, which reduces the many-body problem to a single-particle, non-interacting ensemble form, by the introduction of static temperature-dependent correlation parameter, which are evduated by forcing consistency with the fluctuation-dissipation theorem. It is found that the CEF approximation is superior to the Ognchi model in describing the susceptibility of $NIPS_3$. The exchange and crystal field parameters for the CEF approximation are J / k = -58.0 K; D / k = 16.1 K; g_l_l = 2.05 and $g_\perp =2.13$

Dilution of a layered antiferromagnet: Magnetism in MnxZn1−xPS3Mn_xZn_{1-x}PS_3

The magnetic properties of the layered antiferromagnet $MnPS_3$ have been studied as a function of dilution with zinc. The magnetic susceptibility of $Mn_xZn_{1-x}PS_3$can be accounted for reasonably well by a randomly diluted antiferromagnetic honeycomb lattice. For compositions above the percolation threshold the suceptibility was calculated using the high-temperature series expansion (HTSE) for a dilute magnetic lattice. For compositions below the percolation threshold uncompensated spins of the finite Mn clusters give rise to an additional Curie correction to the HTSE which is maximum near the $p_c$