Preparation of Ni-doped carbon nanospheres with different surface chemistry and controlled pore structure

In classic carbon supports is very difficult to control pore size, pore size distribution, and surface chemical properties at the same time. In this work microporous carbons derived from furfuryl alcohol are used as support to prepare Ni-doped carbon materials. The N₂ flow rate used during the carbonisation process of the precursor influences on the size of the nanospheres obtained but not in their textural properties. Microporous carbon nanospheres have been synthesised with a narrow pore size distribution centred in 5.5 Å. The surface chemistry of these materials can be easily modified by different treatments without detriment of the pore structure of the doped carbon nanospheres.     

Synthesis and characterization of a hexagonal mesoporous silica with enhanced thermal and hydrothermal stabilities

A synthetic route was developed for a novel hexagonal mesoporous silica that has remarkably wide channel diameters and thick walls. The procedure involved the acid-catalyzed hydrolysis of tetraethylorthosilicate in a water/ethanol/isopropoanol solvent mixture while employing 1-hexadecylamine as a templating agent and mesitylene as an auxiliary agent. After removal of the template by either extraction with ethanolic hydrochloric acid or by calcination at 550 °C, the resulting mesoporous materials had surface areas of 1283 and 1211 m²/g. The channel diameters were found to be 47.2-51.1 Å, while the wall thicknesses were 20.9-21.1 Å. X-ray powder diffraction demonstrated that the novel mesoporous silica belonged to the MCM-41 structural family. Notably, they displayed higher thermal and hydrothermal stabilities, and have higher surface areas than conventionally prepared MCM-41 silica. The thickest channel walls (21.1 Å) can withstand calcination to nearly 850 °C with minimal structural damage. The calcined sample was more resistant to hydrothermal treatment in boiling water than was the solvent-extracted product but both materials showed minimal change after 25 h of hydrothermal treatment.     

Characterization of nanoporous carbons by combining CO2 and H2 sorption data with the Monte Carlo simulations

The Monte Carlo method in its grand ensemble variant (GCMC) is used in combination with experimental data in order to characterize microporous carbons and obtain the optimal pore size distribution (PSD). In particular, the method is applied in the case of AX-21 carbon. Adsorption isotherms of CO₂ (253 and 298 K) and H₂ (77 K) up to 20 bar have been measured, while the computed isotherms resulted from the GCMC simulations for several pore widths up to 3.0 nm. For the case of H₂ at 77 K quantum corrections were introduced with the application of the Feynman-Hibbs (FH) effective potential. The adsorption isotherms were used either individually or in a combined manner in order to deduce PSDs and their reliability was examined by the ability to predict the experimental adsorption isotherms. The combined approach was found to be capable of reproducing more accurately all the available experimental isotherms.     

Controlling the micropore size of activated carbons for the treatment of fuels and combustion gases

Exclusively microporous activated carbons have been prepared from cork by physical and chemical activation under different conditions. The results show that it is possible to control the pore size of the activated carbons and to obtain materials with narrow micropore size (≥0.69 nm) and high micropore volume (≤0.64 cm³ g⁻¹) equal to or better than the best activated carbon fibres. Higher micropore volumes are generally obtained by chemical activation at higher temperature using dry or potassium hydroxide impregnation. On the other hand, wet or carbonate impregnation, as well as high temperature, or physical activation with CO₂ or H₂O under appropriate conditions, favours low mean pore widths.     

Synthesis of lanthanum nickelate perovskite nanotubes by using a template-inorganic precursor

A new route to obtain metal oxide nanotubes is presented: an inorganic coordination complex precursor containing the metal ions and impregnated into alumina membrane templates yield hollow tubular nanostructures of LaNiO₃ by calcination at 600 °C as characterized by powder X-ray diffraction (XRD). Scanning electron microscopy (SEM) shows that the resulting nanotubes have 200 nm in diameter in good agreement with the template pore. Transmission electron microscopy (TEM) and dark field transmission electron microscopy (DF-TEM) show that the nanotubes with 10-20 nm walls and internal separations are composed of 3-5 nm crystals.     

Structural and superconducting properties of ion beam sputtered Nb thin films and Nb/Cu bilayers

We present the results of a study of structural and superconducting properties of polycrystalline Nb thin films (200 Å, 300 Å, 400 Å, 700 Å and 1000 Å) and Nb/Cu bilayers (300 Å/300 Å and 400 Å/300 Å) prepared on Si substrates by ion beam sputtering at room temperature. The thicknesses, roughnesses at the surfaces and interfaces were determined by X-ray reflectivity whereas the grain sizes were determined from grazing incidence X-ray diffraction and transmission electron microscopic studies. The superconducting transition temperature (T_C) of Nb thin films are smaller than T_C of bulk Nb. The Nb-200 Å sample does not show T_C down to 2.3 K. The average size of the grains varies from 42 Å for Nb-200 Å sample to 69 Å for Nb-1000 Å sample. Our results show that the T_C in these polycrystalline films is not only limited by its thickness but also by the size of the grains. The Nb films deposited in situ on the Cu layer (Nb/Cu) show a marginal increase in average sizes of the grains as compare to their respective values in Nb films of same thicknesses. As a result a marginal increase in T_C of these films is also observed. The maximum decrease in T_C due to oxygen intake during deposition should be about 0.5 K from its bulk value (9.28 K). We have attributed the large decrease in T_C in our case on the basis of decrease in the Debye temperature and density of states at the Fermi level for Nb thin films as compared to their respective values for bulk Nb.     

Evolution of pores in mesoporous silica films: Porogen loading effect

Mesoporous silica films were synthesized via a sol-gel process under an acidic condition. Various amounts of triblock copolymer F38 were loaded to precursor sols as the pore generator. The evolution of the pores generated by porogen decomposition was investigated as a function of F38 loading by positron annihilation gamma-ray energy spectroscopy and positron annihilation lifetime spectroscopy based on slow positron beams. The threshold of pore percolation is found to be around 10 wt% of F38 loading by positron annihilation gamma-ray energy spectroscopy. Positron annihilation lifetime spectroscopy in the films show that the pore size increases from 1 nm to 3 nm with increasing F38 loading from 5 wt% to 30 wt%.     

On the short-range order of the SiOx (0 ≤ x ≤ 2) surface

Fine (oscillating) structure (FS) in the elastically scattered electron spectra (ESES) [O. Bondarchuk, S. Goysa, I. Koval, P. Melnik, M. Nakhodkin, Surf. Sci. 258 (1991) 239; O. Bondarchuk, S. Goysa, I. Koval, P. Melnik, M. Nakhodkin, Surf. Rev. Lett. 4 (1997) 965] was used to investigate surface structure of the SiO_x (0 ≤ x ≤ 2). SiO_x surface with different stoichiometry was prepared by implantation of 500 eV oxygen ions into a silicon wafer. Fourier transformation of the FS ESES contains one peak at 2.32 Å for Si, two peaks at 1.62 Å and 2.65 Å for a-SiO₂ and three peaks centered at 1.6-1.7 Å, 2.1-2.2 Å and 2.65-3.04 Å for SiO_x. Peaks at 1.62 Å and 2.65 Å are assigned to Si-O and O-O nearest distances correspondently. Ratio of the area under the peak at 2.65 Å to the area under the peak at 1.62 Å turned out to be not constant but grows linearly with the composition parameter x. The latter is considered to prove validity of the Random Bond Model to describe short-range order on the surface of non-stoichiometric silicon oxide.     

A metal-ion-assisted assembly approach to synthesize disulfide-bridged periodical mesoporous organosilicas with high sulfide contents and efficient adsorption

Well-ordered two-dimensional (2D) hexagonal periodic mesoporous organosilicas (PMOs) with a high content of disulfide groups have been prepared by a simple metal-ions-assisted amphiphilic surfactant templating process under a strong acidic condition. Long-chain organic bridge silane, bis(triethoxysilylpropyl)disulfide (BTSPDS) was used as a precursor which can be co-condensed with tetraethoxysilane (TEOS) to assemble with the triblock copolymer Pluronic P123 template and to construct the mesostructured organic-inorganic frameworks. The content of disulfide functional groups is up to 20% (BTSPDS molar content in the initial silane mixture) incorporated into the framework. The obtained ordered mesoporous DS-PMO materials have relatively high BET surface area (∼580 m²/g), large uniform pore size (up to 6.3 nm) and thick pore walls (thickness up to 7.1 nm), because of the long-chain disulfide bridges. The metal ions such as Zn²⁺ formed the four-coordination complex with two sulfides of BTSPDS and ethylene oxide moieties of P123 template, which could enhance the interaction between the “soft” long disulfide groups and P123 template, thus improving the mesostructural regularity correspondingly. The disulfide-bridged PMO materials exhibit excellent hydrothermal stability in boiling water for 5 days, probably due to the thick pore walls. SEM images show that after the hydrothermal treatment, the morphology of the ordered disulfide-bridged PMO materials is retained, as that of the wheat-like SBA-15. Excellent adsorption efficiency (∼716 mg/g) for Hg²⁺ ions is observed, suggesting a potential application in removal of heavy metal ions in wastewater.     

Ordering temperature of L10-type FePt films reduced by CuO addition

Effects of addition of CuO layers in L1₀-type FePt thin films are investigated. The ordering temperature of L1₀-type FePt films can be reduced by CuO addition. The coercivities of 0.78 and 0.82 T are achieved in [Pt(10 Å)/Fe(14 Å)/CuO(2 Å)]₁₀ film annealed at 550 °C for 20 min and [Pt(10 Å)/Fe(15 Å)/CuO(3 Å)]₁₀ film annealed at 600 °C for 20 min, respectively, and these values are compared to the coercivity of 0.8 T in [Pt(10 Å)/Fe(13 Å)]₁₀ film annealed at 650 °C. The thickness of Fe and CuO layers strongly influences the ordering temperature of L1₀-type FePt and the magnetic properties of the films. The addition of CuO not only brings microstructure and surface morphology changes of FePt film, but also lowers the ordering temperature.     

The adsorption geometry of sulphur on Ir{1 0 0}: A quantitative LEED study

A quantitative low energy electron diffraction (LEED) analysis has been performed for the p(2 × 2)-S and c(2 × 2)-S surface structures formed by exposing the (1 × 1) phase of Ir{1 0 0} to H₂S at 750 K. S is found to adsorb on the fourfold hollow sites in both structures leading to Pendry R-factor values of 0.17 for the p(2 × 2)-S and 0.16 for the c(2 × 2)-S structures. The distances between S and the nearest and next-nearest Ir atoms were found to be similar in both structures: 2.36 ± 0.01 Å and 3.33 ± 0.01 Å, respectively. The buckling in the second substrate layer is consistent with other structural studies for S adsorption on fcc{1 0 0} transition metal surfaces: 0.09 Å for p(2 × 2)-S and 0.02 Å for c(2 × 2)-S structures. The (1 × 5) reconstruction, which is the most stable phase for clean Ir{1 0 0}, is completely lifted and a c(2 × 2)-S overlayer is formed after exposure to H₂S at 300 K followed by annealing to 520 K. CO temperature-programmed desorption (TPD) experiments indicate that the major factor in the poisoning of Ir by S is site blocking.     

Multiple antiferromagnet/ferromagnet interfaces as a probe of grain-size-dependent exchange bias in polycrystalline Co/Fe50Mn50

We have used ferromagnet/antiferromagnet/ferromagnet trilayers and ferromagnet/antiferromagnet multilayers to probe the grain size dependence of exchange bias in polycrystalline Co/Fe₅₀Mn₅₀. X-ray diffraction and transmission electron microscopy show that the Fe₅₀Mn₅₀ (FeMn) grain size increases with increasing FeMn thickness in the Co (30 Å)/FeMn system. Hence, in Co(30 Å)/FeMn(t_AF Å)/Co(30 Å) trilayers the two Co layers sample different FeMn grain sizes at the two antiferromagnet/ferromagnet interfaces. For FeMn thicknesses above 100 Å, where simple bilayers have a thickness-independent exchange bias, we are therefore able to deduce the influence of FeMn grain size on the exchange bias and coercivity (and their temperature dependence) simply by measuring trilayer and multilayer samples with varying FeMn thicknesses. This can be done while maintaining the (1 1 1) orientation, and with little variation in interface roughness. Increasing the average grain size from 90 to 135 Å results in a fourfold decrease in exchange bias, following an inverse grain size dependence. We interpret the results as being due to a decrease in uncompensated spin density with increasing antiferromagnet grain size, further evidence for the importance of defect-generated uncompensated spins.     

Acquisition of vector hysteresis loops from micro-arrays of nano-magnets

A modified scanning Kerr microscope has been used as a static Kerr magnetometer to acquire in-plane vector hysteresis loops from square Si/Ta(50 Å)/Co₈₀Fe₂₀(40 Å)/Ni₈₈Fe₁₂(108 Å)/Ta(100 Å) elements with size ranging from 123 nm to 10 μm. The nanoscale elements were arranged in square arrays of 4 μm size. The laser beam was focused to a sub-micron spot, while polarization changes were recorded with an optical bridge detector containing a beam-splitting polarizer and two quadrant photodiodes. The coercive field exhibited a non-monotonic increase from 11 Oe in the 10 μm element to 170 Oe in the 123 nm elements. Loops acquired with the field applied parallel to the easy and hard in-plane uniaxial anisotropy axes were observed to become more similar in shape as the element size decreased.     

Effects of carbonization temperature on microstructure and electrochemical performances of phenolic resin-based carbon spheres

Activated phenol resin-based carbon spheres (APCS) electrodes with high double layer capacitance and good rate capability were prepared from phenol resin-based spheres (PS) at different carbonization temperatures prior to KOH activation. The carbonization temperature has a marked effect on both the pore structure and the electrochemical performances of the APCS in 6 M KOH electrolyte. APCS carbonized at 600 °C results in higher specific surface area and larger pore size, and hence higher capacitance and better rate capability. The specific capacitance of the APCS in 6 M KOH aqueous solution can be as high as 282 F g⁻¹. It remains 252 F g⁻¹ as the current density increases to 1000 mA g⁻¹.     

The structure and optical properties of AlN nanocrystals prepared by Schlenk techniques at atmospheric pressure and low temperature

AlN nanocrystals were prepared in organic solvent at atmospheric pressure and low temperature by the Schlenk technique. Both hexagonal and cubic AlN nanocrystals were obtained. The hexagonal nano-AlN powder possessed a wurtzite structure with a=3.124 Å, c=5.024 Å, the average grain size was about 2 nm. The lattice constant of the cubic nano-AlN was a=9.171 Å, the average grain size was about 4 nm. The structural and optical properties of the obtained AlN were analyzed. The emission related to deep-level defects was investigated by using temperature-dependent photoluminescence.     

Adsorption in cylindrical pores: Mixed lattice-site/off-site Monte Carlo simulations in pores with heterogeneous wall structure

We present results of grand canonical Monte Carlo simulations of adsorption in cylindrical pores with rough surface modeled by lattice-site approach. Each site is characterized by two parameters: structural and energetic, which locally modify the structure and energy properties of the surface. There are three types of sites, randomly distributed over the wall: attractive, neutral and repulsive with respect to the smooth pore model. The results presented here show how this model affects the mechanism of adsorption and how it changes the forms of adsorption isotherm. We compare our numerical results with the experimental data of adsorption of a simple fluid (CH₄, T = 77 K) in cylindrical silica pore of diameter d = 4 nm (MCM-41 material).     

Investigation of ferromagnetic coupling between Co layers in a Co/Pt multilayer with perpendicular anisotropy

The coercivity of a Co/Pt multilayer with out-of-plane anisotropy can be lowered greatly if it is grown onto an ultrathin NiO underlayer . By making use of this characteristic, a series of samples glass/NiO(10 Å)/[Co(4 Å)/Pt(5 Å)]₃/Pt(x Å)/[Co(4 Å)/Pt(5 Å)]₃ with different Pt spacer thickness have been prepared to determine the ferromagnetic (FM) coupling between Co layers across the Pt layer. The measurements of major and minor hysteresis loops have shown that the FM coupling between the top and bottom Co/Pt multilayers decreases monotonically with the Pt layer thickness and disappears above the Pt layer thickness of 40 Å. This thickness of 40 Å is much larger than that in the literature. In addition to the FM coupling between the top and bottom Co/Pt multilayers across the Pt spacer, there exists a weak biquadratic coupling, which induces the broad transition of the bottom Co/Pt multilayer.     

Surface and adsorptive properties of carbons prepared from biomass

A number of activated carbons were prepared from a locally available by-product, corncobs, under currently established activation schemes. Obtained carbons were characterized by N₂ adsorption at 77 K and the isotherms were analyzed by BET and α_s methods. Steam-activation at 900 °C produced a microporous carbon having the highest S^α of 788 m² g⁻¹, whereas activation with air at 350 °C produced a carbon of S^α = 321 m²/g and possess wider pores. KOH impregnation with char in ratio 1:1 (w/w) and impregnated in the same ratio with the raw material prior to pyrolysis at 700 °C for 1 h, gave CK700, K700 respectively. An additional sample was obtained by oxidizing part of K700 with conc. HNO₃. All three KOH carbons show pore structures much close to char itself which may be due to potassium salt left in pores and is not easily leached with repeated water washings. In addition, KOH is more effective on the precursor itself than on its char of already developed porosity. FT-IR spectra show an increase in oxygen functionalties on the carbon surface as a result of activation process and the bands become stronger in the spectra of the acid-treated sample. The oxidized carbon sample showed relatively higher uptake of Pb²⁺ and MB and its surface chemistry plays the key role in their adsorption, while sharp decrease was observed in the uptake of phenol and mono-nitrophenols from aqueous solutions. An SEM study showed that air activation produce obvious voids reflecting its erosive effect on the external carbon surface.     

Polymer-templated mesoporous carbons synthesized in the presence of nickel nanoparticles, nickel oxide nanoparticles, and nickel nitrate

Mesoporous carbon composites, containing nickel and nickel oxide nanoparticles, were obtained by soft-templating method. Samples were synthesized under acidic conditions using resorcinol and formaldehyde as carbon precursors, poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock co-polymer Lutrol F127 as a soft template and nickel and nickel oxide nanoparticles, and nickel nitrate as metal precursors. In addition, a one set of samples was obtained by impregnation of mesoporous carbons with a nickel nitrate solution followed by further annealing at 400 °C. Wide angle X-ray powder diffraction along with thermogravimetric analysis proved the presence of nickel nanoparticles in the final composites obtained using nickel and nickel oxide nanoparticles, and Ni(NO₃)₂ solution. Whereas, the impregnation of carbons with a nickel nitrate solution followed by annealing at 400 °C resulted in needle-like nickel oxide nanoparticles present inside the composites’ pores. Low-temperature (−196 °C) nitrogen physisorption, X-ray powder diffraction, and thermogravimetric analysis confirmed good adsorption and structural properties of the synthesized nickel-carbon composites, in particular, the samples possessed high surface areas (>600 m²/g), large total pore volumes (>0.50 cm³/g), and maxima of pore size distribution functions at circa 7 nm. It was found that the composites were partially graphitized during carbonization process at 850 °C. The samples are stable in an air environment below temperature of 500 °C. All these features make the synthesized nickel-carbon composites attractive materials for adsorption, catalysis, energy storage, and environmental applications.