Genban Sun

Hexagonal and cubic Ni nanocrystals grown on graphene: phase-controlled synthesis, characterization and their enhanced microwave absorption properties

Hexagonal close-packed Ni (h-Ni) nanocrystals and face-centered cubic Ni (c-Ni) nanoflowers with uniform size and high dispersion have been successfully assembled on graphene nanosheets (GN) via a facile one-step solution-phase strategy under different reaction conditions, where the reduction process of graphite oxide (GO) sheets into GN was accompanied by the generation of Ni nanocrystals. The reduction of GO by this method is effective, which was confirmed by X-ray diffraction (XRD), Fourier transform infrared (FTIR) and Raman spectroscopy and is comparable to conventional methods. The phase and morphology of nickel can be easily tuned by varying the reaction temperature and solvent. It was shown that the as-formed h-Ni nanocrystals with a diameter as small as 3 nm are grown densely and uniformly on the graphene sheets, and as a result the aggregation of the h-Ni nanocrystals was effectively prevented. In addition, c-Ni nanospheres assembled by c-Ni nanocrystals with a size of 15 nm were also uniformly deposited on the graphene sheets. The investigation of the microwave absorbability reveals that the three Ni/GN nanocomposites exhibit excellent microwave absorbability, which is stronger than the corresponding Ni nanostructures.

Efficient Uranium Capture by Polysulfide/Layered Double Hydroxide Composites

There is a need to develop highly selective and efficient materials for capturing uranium (normally as UO2(2+)) from nuclear waste and from seawater. We demonstrate the promising adsorption performance of S(x)-LDH composites (LDH is Mg/Al layered double hydroxide, [S(x)](2-) is polysulfide with x = 2, 4) for uranyl ions from a variety of aqueous solutions including seawater. We report high removal capacities (q(m) = 330 mg/g), large K(d)(U) values (10(4)-10(6) mL/g at 1-300 ppm U concentration), and high % removals (>95% at 1-100 ppm, or ∼80% for ppb level seawater) for UO2(2+) species. The S(x)-LDHs are exceptionally efficient for selectively and rapidly capturing UO2(2+) both at high (ppm) and trace (ppb) quantities from the U-containing water including seawater. The maximum adsorption coeffcient value K(d)(U) of 3.4 × 10(6) mL/g (using a V/m ratio of 1000 mL/g) observed is among the highest reported for U adsorbents. In the presence of very high concentrations of competitive ions such as Ca(2+)/Na(+), S(x)-LDH exhibits superior selectivity for UO2(2+), over previously reported sorbents. Under low U concentrations, (S4)(2-) coordinates to UO2(2+) forming anionic complexes retaining in the LDH gallery. At high U concentrations, (S4)(2-) binds to UO2(2+) to generate neutral UO2S4 salts outside the gallery, with NO3(-) entering the interlayer to form NO3-LDH. In the presence of high Cl(-) concentration, Cl(-) preferentially replaces [S4](2-) and intercalates into LDH. Detailed comparison of U removal efficiency of S(x)-LDH with various known sorbents is reported. The excellent uranium adsorption ability along with the environmentally safe, low-cost constituents points to the high potential of S(x)-LDH materials for selective uranium capture.

Enhancing the electromagnetic performance of Co through the phase-controlled synthesis of hexagonal and cubic Co nanocrystals grown on graphene.

Cobalt is a promising soft metallic magnetic material used for important applications in the field of absorbing stealth technology, especially for absorbing centimeter waves. However, it frequently presents a weak dielectric property because of its instability, aggregation, and crystallographic form. A method for enhancing the electromagnetic property of metal Co via phase-controlled synthesis of Co nanostructures grown on graphene (GN) networks has been developed. Hexagonal close-packed cobalt (α-Co) nanocrystals and face-centered cubic cobalt (β-Co) nanospheres with uniform size and high dispersion have been successfully assembled on GN nanosheets via a facile one-step solution-phase strategy under different reaction conditions in which the exfoliated graphite oxide (graphene oxide, GO) nanosheets were reduced along with the formation of Co nanocrystals. The as-synthesized Co/GN nanocomposites showed excellent microwave absorbability in comparison with the corresponding Co nanocrystals or GN, especially for the nanocomposites of GN and α-Co nanocrystals (the reflection loss is -47.5 dB at 11.9 GHz), which was probably because of the special electrical properties of the cross-linked GN nanosheets and the perfect electromagnetic match in their microstructure as well as the small particle size of Co nanocrystals. The approach is convenient and effective. Some magnetic metal or alloy materials can also be prepared via this route because of its versatility.

Intercalation of organic sensitisers into layered europium hydroxide and enhanced luminescence property

Two organic sensitisers 4-biphenylcarboxylate (BPC) and terephthalate (TA) were intercalated into the gallery of layered europium hydroxide (LEuH). PL spectra tests indicated that BPC markedly enhanced the red luminescence of Eu(3+) due to efficient energy transfer between BPC and Eu(3+), forming a contrast to intercalated TA and the starting NO(3)(-) anions in the gallery. The energy level matching of the organic guests and Eu(3+) was also discussed to explain the energy transfer from sensitiser to Eu(3+).

Nanocage structure derived from sulfonated β-cyclodextrin intercalated layered double hydroxides and selective adsorption for phenol compounds.

Nanocage structures derived from decasulfonated β-cyclodextrin (SCD) intercalated ZnAl- and MgAl- layered double hydroxides (LDHs) were prepared through calcination-rehydration reactions. The ZnAl- and MgAl-LDH layers revealed different basal spacings (1.51 nm for SCD-ZnAl-LDH and 1.61 nm for SCD-MgAl-LDH) when contacting SCD, while producing similar monolayer and vertical SCD orientations with cavity axis perpendicular to the LDH layer. The structures of the SCD-LDH and carboxymethyl-β-cyclodextrin (CMCD)-LDH intercalates were fully analyzed and compared, and a structural model for the SCD-LDH was proposed. The thermal stability of SCD after intercalation was remarkably enhanced, with decomposition temperature increased by 230 °C. The adsorption property of the SCD-LDH composites for phenol compounds (the effects of adsorption time and phenol concentration on adsorption) was investigated completely. The monolayer arrangement of the interlayer SCD did not affect the adsorption efficiency toward organic compounds, which verified the highly swelling ability of the layered compounds in solvents. Both composites illustrated preferential adsorptive efficiency for 2,3-dimethylphenol (DMP) in comparison with other two phenols of hydroquinone (HQ) and tert-butyl-phenol (TBP), resulting from appropriate hydrophobicity and steric hindrance of DMP. For the two phenols of HQ and TBP, SCD-MgAl-LDH gave better adsorption capacity compared with SCD-ZnAl-LDH. The double-confinement effect due to the combination of the parent LDH host and intercalated secondary host may impose high selectivity for guests. This kind of nanocage structure may have potential applications as adsorbents, synergistic agents, and storage vessels for particular guests.

Intercalation of azamacrocyclic crown ether into layered rare-earth hydroxide (LRH): secondary host-guest reaction and efficient heavy metal removal.

A carboxyethyl substituted azacrown ether (CSAE) derivative was intercalated as a second host into a parent host of layered gadolinium hydroxides (LGdH) by an anion-exchange reaction. The influence of intercalation temperature and starting material ratios of CSAE/LRH on the structures and compositions of CSAE-LRH nanocomposites were investigated. Higher temperature and larger initial CSAE-LGdH weight ratios favor of higher degree of ion exchange at a certain range, while lower temperature gives good morphology for the composites. The adsorptive properties for transition and heavy metal ions were studied using the 20 °C-reacted composite, which showed higher adsorptivity toward transition and heavy metal ions, accompanied by the introduction of nitrate anions. The adsorptive capacity for transition metal ions was in the sequence of Cu(2+) > Zn(2+)∼Ni(2+)∼Co(2+) with a high selectivity to Cu(2+). For the heavy metal ions Ag(+), Hg(2+), Pb(2+), and Cd(2+), the composite showed markedly high selectivity for Ag(+) and Hg(2+). When putting Cu(2+), Ag(+), Hg(2+), Pb(2+), and Cd(2+) together, Ag(+) and Hg(2+) still have higher adsorptive selectivity over Pb(2+) and Cd(2+), and Cu(2+) has also relatively high selectivity but not as high as Ag(+) and Hg(2+). The nanocomposites with a second host in the interlayer are one promising kind of material because of the synergy of the steric effect of the parent host (LRH layer) and the particular characteristics of the secondary host (interlayer crown ether anions).

Direct Synthesis of Unilamellar MgAl-LDH Nanosheets and Stacking in Aqueous Solution

Two-dimensional (2D) materials, such as graphene, inorganic oxides, and hydroxides, are one of the most extensively studied classes of materials due to their unilamellar crystallites or nanosheet structures. In this study, instead of using the universal exfoliation method of the bulky crystal precursor, 2D crystals/nanosheets of MgAl-layered double hydroxides (LDHs) were synthesized in formamide. We propose that the obtained crystals are unilamellar according to the XRD, TEM, and AFM observations. The HRTEM and fast Fourier transform images confirm that the crystal structures are the same as those of the exfoliated MgAl-LDH nanosheets. The directly synthesized sheets can stack into a 3D crystal structure, which is the same as that of typical LDHs except for the disordered orientation of the a-/b- crystal axis of each sheet. This result provides not only a novel approach to the preparation of 2D crystals but also insight into the formation mechanism of LDHs.

Rapid Simultaneous Removal of Toxic Anions [HSeO3]−, [SeO3]2–, and [SeO4]2–, and Metals Hg2+, Cu2+, and Cd2+ by MoS42– Intercalated Layered Double Hydroxide

We demonstrate fast, highly efficient concurrent removal of toxic oxoanions of Se(VI) (SeO42-) and Se(IV) (SeO32-/HSeO3-) and heavy metal ions of Hg2+, Cu2+, and Cd2+ by the MoS42- intercalated Mg/Al layered double hydroxide (MgAl-MoS4-LDH, abbr. MoS4-LDH). Using the MoS4-LDH as a sorbent, we observe that the presence of Hg2+ ions greatly promotes the capture of SeO42-, while the three metal ions (Hg2+, Cu2+, Cd2+) enable a remarkable improvement in the removal of SeO32-/HSeO3-. For the pair Se(VI)+Hg2+, the MoS4-LDH exhibits outstanding removal rates (>99.9%) for both Hg2+ and Se(VI), compared to 81% removal for SeO42- alone. For individual SeO32- (without metal ions), 99.1% Se(IV) removal is achieved, while ≥99.9% removals are reached in the presence of Hg2+, Cu2+, and Cd2+. Simultaneously, the removal rates for these metal ions are also >99.9%, and nearly all concentrations of the elements can be reduced to <10 ppb, a limit acceptable for drinking water. The maximum sorption capacities for individual Se(VI) and Se(IV) are 85 and 294 mg/g, respectively. The 294 mg/g capacity for Se(IV) reaches a record value, placing the MoS4-LDH among the highest-capacity selenite adsorbing materials described to date. More interestingly, the presence of metal ions extremely accelerates the capture of the selenium oxoanions because of the reactions of the metal ions with the interlayer MoS42- anions. The sorptions of Se(VI)+Hg and Se(IV)+M (M = Hg2+, Cu2+, Cd2+) are exceptionally rapid, showing >99.5% removals for Hg2+ within 1 min and ∼99.0% removal for Se(VI) within 30 min, as well as >99.5% removals for pairs Cu2+ and Se(IV) within 10 min, and Cd2+ and Se(IV) within 30 min. During the sorption of SeO32-/HSeO3-, reduction of Se(IV) occurs to Se0 caused by the S2- sites in MoS42-. Sorption kinetics for the oxoanions follows a pseudo-second-order model consistent with chemisorption. The intercalated material of MoS4-LDH is very promising as a highly effective filter for decontamination of water with toxic Se(IV)/Se(VI) oxoanions along with heavy metals such as Hg2+, Cd2+, and Cu2+.

Synthesis, characterization and electromagnetic performance of nanocomposites of graphene with α-LiFeO2 and β-LiFe5O8

Face-centered cubic α-LiFeO2 and spinel β-LiFe5O8 with uniform size and high dispersion have been successfully assembled on 2D graphene sheets via a facile one-pot strategy under different reaction conditions. The reduction of GO by this method is effective and comparable to conventional methods, which was confirmed by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The structure of the products can be easily controlled by changing the solvent and reaction temperature. It was shown that the as-formed β-LiFe5O8 and α-LiFeO2 nanocrystals with a diameter of ca. 5 nm and 7 nm, respectively, were densely and uniformly anchored on the graphene sheets, and as a result, the aggregation of the nanoparticles was effectively prevented. The investigation of the microwave absorbability reveals that the α-LiFeO2–GN and β-LiFe5O8–GN nanocomposites exhibit excellent microwave absorbability, which is stronger than that of the corresponding α-LiFeO2 and β-LiFe5O8 nanostructures, respectively.

Intercalation of coumaric acids into layered rare-earth hydroxides: controllable structure and photoluminescence properties

Organic compounds of ortho-coumaric acid (abbr. o-CMA) and para-coumaric acid (abbr. p-CMA) are intercalated into the layered rare-earth hydroxides (LRHs, R = Eu, Gd) via the ion exchange method. The two organics having the same phenolic hydroxyl and carboxyl groups located at different positions demonstrate variant intercalation structures. The CMA anion guests within the LRH gallery indicate monolayered or bi-layered arrangements depending on their own characteristic features and deprotonation degrees. The combination of o-/p-CMA molecules with LEuH/LGdH layers generates hybrid materials exhibiting versatile luminescence properties. In the solid state, for LEuH layers, the luminescence of layer Eu3+ and interlayer o-CMA is co-quenched; however, for LGdH layers, green emissions (∼520 nm) are observed, both of which are different from the cyan emission (475 nm) of free o-CMA anions. When dispersed in formamide (FM), o-CMA–LEuH composites exhibit weak luminescence, in sharp contrast to o-CMA–LGdH composites displaying green emissions (495–520 nm) with markedly enhanced intensity. The p-CMA–LGdH composites display blue emission (457 nm) quite different from the green emission (499 nm) for free p-CMA in FM, compared with the unobservable emission within LEuH layers. In addition, co-intercalation of the surfactant OS (1-octane sulfonic acid sodium) with o-CMA anions into LEuH produces composites showing desirable blue emission or violet emission, due to the change of the microenvironment of organic guests. The energy transfer between layer Eu3+ and interlayer CMA was proposed to account for the co-quenching or blue shift. This work offers a beneficial approach to fabricate organic–inorganic photofunctional materials with a controllable structure and tunable fluorescence properties.