Portrait of Professor Anna Corrias

Professor Anna Corrias

Professor of Chemistry
Director of Research

About

Professor Anna Corrias graduated in Chemistry at Cagliari University (Italy) and went on to be awarded her PhD there. At the same university she was first Lecturer and then Associate Professor of Physical Chemistry.

Anna joined the School of Physical Sciences in March 2013 as a Reader in Chemistry and was promoted to Professor in 2018. She is currently working on a British Council funded New Ceria Based Materials for Challenges in Energy and Environment research project.

Research interests

Professor Anna Corrias’s main research expertise is in the field of aerogels, the lightest materials ever made, which find applications as thermal insulators and catalyst supports.

Currently, she is working on using silica aerogels to stabilise cerium oxide, also called ceria, in form nanocubes. Ceria possess an extraordinary reactivity, especially when it is at the nanoscale and when it is grown in the form of nanocubes. Due to their extreme low density and high porosity, silica aerogels play the role of acting as an effective catalyst support and as a medium that keeps the nanocubes apart even when the samples are submitted to high temperature, to fully exploit ceria reactivity in processes of great interest for the environment and energy, spanning energy conversion, energy storage and environmental protection and remediation.

The structural and morphological characterisation of these materials is done using a multi-technique approach involving thermal analysis, physisorption, transmission electron microscopy (TEM), X-ray diffraction, and X-ray absorption spectroscopy.

Teaching

Anna teaches across a range of chemistry modules, covering content that includes chemical reactions, thermodynamics and kinetics, and topics in inorganic synthetic chemistry. 

Publications

Showing 50 of 113 total publications in the Kent Academic Repository. View all publications.

Article

  • Loche, D., Morgan, L., Casu, A., Mountjoy, G., O’Regan, C., Corrias, A. and Falqui, A. (2019). Determining the maximum lanthanum incorporation in the fluorite structure of La-doped ceria nanocubes for enhanced redox ability. RSC Advances [Online] 9:6745-6751. Available at: https://doi.org/10.1039/c8ra09766f.
    Ceria nanocubes have been doped with increasing amounts of lanthanum to enhance their redox ability. X-ray diffraction and transmission electron microscopy techniques provide a consistent picture indicating that there is an upper limit to the lanthanum that can be incorporated in the fluorite structure of ceria nanocubes, which is close to 7.5 mol% La. This limited loading is nevertheless able to produce a significant enhancement of the ceria redox ability as evidenced by the use of X-ray absorption spectroscopy to determine the Ce3+/Ce4+ ratio in samples submitted to a degassing treatment at room temperature.
  • Morgan, L., Molinari, M., Corrias, A. and Sayle, D. (2018). Protecting Ceria Nanocatalysts - The Role of Sacrificial Barriers. ACS Applied Materials & Interfaces [Online] 10:32510-32515. Available at: https://doi.org/10.1021/acsami.8b08674.
    Forces acting on a functional nanomaterial during operation can cause plastic deformation and extinguish desirable catalytic activities. Here, we show that sacrificial materials, introduced into the catalytic composite device, can absorb some of the imposed stress, and protect the structural integrity and hence activity of the functional component. Specifcally, we use molecular dynamics (MD) to simulate uniaxial stress on a ceria (CeO2) nanocube, an important functional material with respect to oxidative catalysis, such as the conversion of CO to CO2. We predict that the nanocube, protected by a `soft' BaO or `hard' MgO sacrifcial barrier, is able to withstand 40.1 GPa or 26.5 GPa respectively before plastic deformation destroys the structure irre-
    versibly; the sacrificial materials, BaO and MgO, capture 71% and 54% of the stress respectively. In comparison, the unprotected nanoceria catalyst deforms plastically at only 2.5 GPa. Furthermore, modelling reveals the deformation mechanisms and the importance of microstructural features, insights that are difficult to measure experimentally.
  • Caddeo, F., Loche, D., Casula, M. and Corrias, A. (2018). Evidence of a cubic iron sub-lattice in t-CuFe2O4 demonstrated by X-ray Absorption Fine Structure. Scientific Reports [Online] 8. Available at: http://dx.doi.org/10.1038/s41598-017-19045-8.
    Copper ferrite, belonging to the wide and technologically relevant class of spinel ferrites, was grown in the form of t-CuFe2O4 nanocrystals within a porous matrix of silica in the form of either an aerogel or a xerogel, and com-pared to a bulk sample. Extended X-ray absorption fine structure (EXAFS) spectroscopy revealed the presence of two different sub-lattices within the crystal structure of t-CuFe2O4, one tetragonal and one cubic, defined by the Cu2+ and Fe3+ ions respectively. Our investigation provides evidence that the Jahn-Teller distortion, which occurs on the Cu2+ ions located in octahedral sites, does not affect the coordination geometry of the Fe3+ ions, regardless of their location in octahedral or tetrahedral sites.
  • Wang, J., Ivaturi, S., Wang, S., Corrias, A. and Boi, F. (2017). Observation of enhanced magnetic transition in Pbnm SmFeO3. Journal of Applied Physics [Online] 122:174103. Available at: https://doi.org/10.1063/1.5000959.
    Rare-earth orthoferrite RFeO3 materials have recently attracted great attention for their intriguing technological potential. Among these materials, SmFeO3 holds great promise, not only for its excellent physical properties (fast magnetic switching, spin reorientation, and magnetization reversal), but also for its potential ferroelectric properties, which have been recently under debate. Here, we focus our attention on the T-dependent Zero Field Cooled and Field Cooled magnetization properties of micrometer scale crystals of SmFeO3 obtained by annealing methods. We report the observation of an enhanced magnetic transition at a temperature of approximately 139?K. From literature bulk susceptibility measurements, it has been suggested that below the temperature of about 140?K, Sm3+ moments begin to order antiparallel to the Fe-moments due to antiferromagnetic f-d exchange interactions. We attribute the observed transition to compensation effects induced by the appearance of long range ordering in Sm3+ spins. The magnetic-nature of the observed transition is confirmed by additional temperature dependent XRD analysis, which did not show structural changes in the samples in the same temperature range (from 298?K to 100?K). Due to residual small fractions of ferromagnetic ?-Fe from the sample-growth, possible interactions between the magnetic moment of ?-Fe and the SmFeO3 crystals at the compensation temperature cannot be excluded and could be at the origin of the enhanced magnetic signal reported in this work
  • Boi, F., He, Y., Wen, J., Wang, S., Yan, K., Zhang, J., Medranda, D., Borowiec, J. and Corrias, A. (2017). Cl-Assisted Large Scale Synthesis of Cm-Scale Buckypapers of Fe3C-Filled Carbon Nanotubes with Pseudo-Capacitor Properties: The Key Role of SBA-16 Catalyst Support as Synthesis Promoter. Materials [Online] 10. Available at: http://dx.doi.org/10.3390/ma10101216.
    We show a novel chemical vapour deposition (CVD) approach, in which the large-scale fabrication of ferromagnetically-filled cm-scale buckypapers is achieved through the deposition of a mesoporous supported catalyst (SBA-16) on a silicon substrate. We demonstrate that SBA-16 has the crucial role of promoting the growth of carbon nanotubes (CNTs) on a horizontal plane with random orientation rather than in a vertical direction, therefore allowing a facile fabrication of cm-scale CNTs buckypapers free from the onion-crust by-product observed on the buckypaper-surface in previous reports. The morphology and composition of the obtained CNTs-buckypapers are analyzed in detail by scanning electron microscopy (SEM), Energy Dispersive X-ray (EDX), transmission electron microscopy (TEM), high resolution TEM (HRTEM), and thermogravimetric analysis (TGA), while structural analysis is performed by Rietveld Refinement of XRD data. The room temperature magnetic properties of the produced buckypapers are also investigated and reveal the presence of a high coercivity of 650 Oe. Additionally, the electrochemical performances of these buckypapers are demonstrated and reveal a behavior that is compatible with that of a pseudo-capacitor (resistive-capacitor) with better performances than those presented in other previously studied layered-buckypapers of Fe-filled CNTs, obtained by pyrolysis of dichlorobenzene-ferrocene mixtures. These measurements indicate that these materials show promise for applications in energy storage systems as flexible electrodes.
  • Carta, D., Montini, T., Casula, M., Monai, M., Bullita, S., Fornasiero, P. and Corrias, A. (2017). Water Gas Shift Reaction over Pt-CeO2 Nanoparticles Confined within Mesoporous SBA-16. Journal of Materials Chemistry A [Online] 5:20024-20034. Available at: http://dx.doi.org/10.1039/C7TA03640J.
    Novel nanocomposite catalysts for single step Water Gas Shift Reaction (WGSR) were prepared by deposition-precipitation and impregnation of Pt-CeO2 nanophases onto an ordered mesoporous silica support featuring a cubic arrangement of mesopores (SBA-16 type). The highly interconnected porosity of the SBA-16 developing in three-dimension (3D) provides a scaffold which is easily accessible to reactants and products by diffusion. The textural and morphological properties of the final catalyst were affected by the procedure utilized for dispersion of the nanophases onto SBA-16. Catalysts prepared by deposition-precipitation present highly dispersed nanocrystalline CeO2 on the surface of SBA-16 and retain high surface area, high thermal stability and high Pt accessibility. Catalysts prepared by impregnation show improved Pt-CeO2 interaction but a more significant decrease of surface area compared to pure SBA-16, due to the confinement of the CeO2 crystallites within the mesoporous matrix. As a result, catalysts prepared by deposition-precipitation are effective for WGSR under working conditions in the high temperature range (around 300-350 °C), whereas catalysts prepared by impregnation are suitable for the process operative at low temperature (LT-WGSR). Our results point out that catalyst preparation procedures can be used to optimise the performance of heterogenous catalysts, by controlling the CeO2 crystallites size and optimizing Pt-CeO2 contact by embedding. Improved thermal and chemical stability was achieved using a mesoporous scaffold.
  • Loche, D., Marras, C., Carta, D., CASULA, M., Mountjoy, G. and Corrias, A. (2017). Cation Distribution and Vacancies in Nickel Cobaltite. Physical Chemistry Chemical Physics [Online] 19:16775-16784. Available at: http://dx.doi.org/10.1039/C7CP02260C.
    Samples of nickel cobaltite, a mixed oxide occurring in the spinel structure which is currently extensively investigated because of its prospective application as ferromagnetic, electrocatalytic, and cost-effective energy storage material were prepared in the form of nanocrystals stabilized in a highly porous silica aerogel and as unsupported nanoparticles. Nickel cobaltite nanocrystals with average size 4 nm are successfully grown for the first time into the silica aerogel provided that a controlled oxidation of the metal precursor phases is carried out, consisting in a reduction under H2 flow followed by mild oxidation in air. The investigation of the average oxidation state of the cations and of their distribution between the sites within the spinel structure, which is commonly described assuming the Ni cations are only located in the octahedral sites, has been carried out by X-ray Absorption Spectroscopy providing evidence for the first time that the unsupported nickel cobaltite sample has a Ni:Co molar ratio higher than the nominal ratio of 1:2 and a larger than expected average overall oxidation state of the cobalt and nickel cations. This is achieved retaining the spinel structure, which accommodates vacancies to counterbalance the variation in oxidation state.
  • Loche, D., Casula, M., Corrias, A., Marras, C., Gozzi, D. and Latini, A. (2016). Catalytic Chemical Vapour Deposition on MFe2O4–SiO2 (M = Co, Mn, Ni) Nanocomposite Aerogel Catalysts for the Production of Multi Walled Carbon Nanotubes. Journal of Nanoscience and Nanotechnology [Online] 16:7750-7757. Available at: http://doi.org/10.1166/jnn.2016.11596.
    Highly porous MFe2O4-SiO2 (M = Co, Mn, Ni) nanocomposite aerogels were tested for the first time as catalysts for multi walled carbon nanotubes production by Catalytic Chemical Vapour Deposition. Structural and textural characterization points out that the catalysts are made out of nanocomposites where nanocrystalline ferrite phases with controlled composition and size in the range 9-13 nm are finely dispersed into the highly porous silica support. Remarkably, CoFe2O4-SiO2 and MnFe2O4-SiO2 catalysts have shown poor catalytic activity, whereas NiFe2O4-SiO2 catalyst has given rise to good quality nanotubes with high yields at deposition temperatures in the range 500-650 °C. The different catalytic behaviour of the mixed ferrite-silica aerogels as a function of the bivalent metal (M = Co, Mn, Ni) can been ascribed to how easily the nanophase can be reduced. In particular, it was demonstrated that the most active NiFe2O4 nanoparticles undergo an in-situ reduction process during the Catalytic Chemical Vapour Deposition reaction, forming a NiFe alloy phase that is responsible for promoting the multi walled carbon nanotubes growth.
  • Caddeo, F., Corrias, A. and Sayle, D. (2016). Tuning the Properties of Nanoceria by Applying Force: Stress Induced Ostwald Ripening. Journal of Physical Chemistry C [Online] 120:14337-14344. Available at: http://doi.org/10.1021/acs.jpcc.6b02279.
    The ionic conductivity and activity of a material, such as ceria, is central to its exploitation in applications such as fuel cells or catalysis. Grain Boundaries (GB) influence profoundly the ionic conductivity and activity of a material. Accordingly, the ability to control GB concentrations is pivotal to these applications. Here, we show that Oswald ripening can be induced by applying uniaxial force. In particular, grain-boundaries are purged to facilitate single nanocrystals from polycrystalline precursors. Our simulations thus predict mechanisms for strain-tunable properties.
  • Marras, C., Loche, D., Carta, D., Casula, M., Schirru, M., Cutrufello, M. and Corrias, A. (2016). Copper-based catalysts supported on highly porous silica for the Water Gas Shift reaction. ChemPlusChem [Online]. Available at: http://doi.org/10.1002/cplu.201500395.
    Cu-based nanoparticles, supported on either a silica aerogel or cubic mesostructured silicas obtained using two different synthetic protocols, were used as catalysts for the Water Gas Shift reaction. The obtained nanocomposites were thoroughly characterized before and after catalysis using N2 adsorption-desorption measurements at -196 °C, transmission electron microscopy, wide-angle and low-angle X-ray diffraction. It was found that the samples before catalysis contain nanoparticles of copper oxides – either CuO or Cu2O – while the formation of metallic copper nanoparticles, constituting the active catalytic phase, is observed either using a pretreatment in reducing atmosphere or directly during the catalytic reaction due to the presence of CO. A key role in determining the catalytic performances of the samples is played by the ability of the different matrices in promoting a high dispersion of Cu metal nanoparticles. The best catalytic performances are obtained with the aerogel sample, which also exhibits constant CO conversion values at constant temperature and quite a reproducible behaviour after subsequent catalytic runs. On the other hand, in the catalysts based on cubic mesostructured silica the detrimental effects related to sintering of Cu nanoparticles are avoided only on the silica support which is able to produce a reasonable dispersion of the copper nanophase.
  • Sestu, M., Carta, D., Casula, M., Corrias, A. and Navarra, G. (2015). Novel interpretation of the mean structure of feroxyhyte. Journal of Solid State Chemistry [Online] 225:256-260. Available at: http://dx.doi.org/10.1016/j.jssc.2015.01.003.
    The structure of the iron oxyhydroxide called feroxyhyte (?-FeOOH), which shows an elusive X-ray powder diffraction pattern, has been represented so far using models describing a mean structure based on the crystalline network of the iron (III) oxide hematite (?-Fe2O3). In this paper, a novel description of the mean structure of feroxyhyte is presented, which is based on the structure of the thermodynamically stable iron oxyhydroxide goethite. Starting from different local arrangements present in the goethite network, a mean structural model is determined which shows an X-ray powder diffraction pattern almost coincident with previous studies. This outcome enables to integrate the structure of feroxyhyte among those of other well characterized iron oxyhydroxides.
  • Marras, C., Loche, D., Corrias, A., Konya, Z. and Casula, M. (2015). Bimetallic Fe/Mo–SiO2 aerogel catalysts for catalytic carbon vapour deposition production of carbon nanotubes. Journal of Sol-Gel Science and Technology [Online] 73:379-388. Available at: http://dx.doi.org/10.1007/s10971-014-3544-9.
    Abstract Highly porous nanocomposite aerogels based on bimetallic Fe and Mo nanoparticles with a variable Fe:Mo weight ratio ranging from 5:1; 3:1; and 0.7:1 dispersed on amorphous silica were obtained. N2 physisorption, X-ray diffraction, and transmission electron microscopy indicate that the Fe/Mo–SiO2 nanocomposite aerogels as obtained by
    a co-gelation sol–gel route followed by supercritical drying and reduction treatment under H2 exhibit Fe and Mo nanocrystals with size in the range 4–10 and 15 nm, respectively, supported on highly porous silica. The catalytic performance of the Fe/Mo–SiO2 aerogels for the synthesis of multi wall carbon nanotubes (MWCNT) by catalytic chemical vapour deposition (CCVD) was evaluated in terms of amount and quality of the produced CNTs as assessed by gravimetric results, thermal analysis, and TEM. The effect of catalyst composition and CCVD temperature was investigated, pointing out that high reaction temperatures (800 ?C) favor the formation of MWCNTs with high quality in elevated yield, the highest C uptake value being[400 %. Catalyst composition and CCVD temperature were also found to affect the homogeneity of CNT morphology, the best MWCNT quality (with outer diameter 23–25 nm) being achieved at 800 ?C with the catalyst having the largest Mo content.
  • Corrias, A., Conca, E., Cibin, G., Mountjoy, G., Gianolio, D., De Donato, F., Manna, L. and Casula, M. (2015). Insights into the Structure of Dot@Rod and Dot@Octapod CdSe@CdS Heterostructures. Journal of Physical Chemistry C [Online] 119:16338-16348. Available at: http://www.dx.doi.org/10.1021/acs.jpcc.5b04593.
    CdSe@CdS dot@rods with diameter around 6 nm and length of either
    20, 27, or 30 nm and dot@octapods with pod diameters of ?15 nm and lengths of ?50
    nm were investigated by X-ray absorption spectroscopy. These heterostructures are
    prepared by seed-mediated routes, where the structure, composition, and morphology of
    the CdSe nanocrystals used as a seed play key roles in directing the growth of the second
    semiconducting domain. The local structural environment of all the elements in the
    CdSe@CdS heterostructures was investigated at the Cd, S, and Se K-edges by taking
    advantage of the selectivity of X-ray absorption spectroscopy, and was compared to pure
    reference compounds. We found that the structural features of dot@rods are
    independent of the size of the rods. These structures can be described as made of a
    CdSe dot and a CdS rod, both in the wurtzite phase with a high crystallinity of both the
    core and the rod. This result supports the effectiveness of high temperature colloidal
    synthesis in promoting the formation of core@shell nanocrystals with very low
    defectivity. On the other hand, data on the CdSe@CdS with octapod morphology suggest the occurrence of a core composed of
    a CdSe cubic sphalerite phase with eight pods made of CdS wurtzite phase. Our findings are compared to current models
    proposed for the design of functional heterostructures with controlled nanoarchitecture.
  • Bullita, S., Casu, A., Casula, M., Concas, G., Congiu, F., Corrias, A., Falqui, A., Loche, D. and Marras, C. (2014). ZnFe2O4 nanoparticles dispersed in a highly porous silica aerogel matrix: A magnetic study. Physical Chemistry Chemical Physics [Online] 16:4843-4852. Available at: http://dx.doi.org/10.1039/c3cp54291b.
    We report the detailed structural characterization and magnetic investigation of nanocrystalline zinc ferrite nanoparticles supported on a silica aerogel porous matrix which differ in size (in the range 4-11 nm) and the inversion degree (from 0.4 to 0.2) as compared to bulk zinc ferrite which has a normal spinel structure. The samples were investigated by zero-field-cooling- field-cooling, thermo-remnant DC magnetization measurements, AC magnetization investigation and Mössbauer spectroscopy. The nanocomposites are superparamagnetic at room temperature; the temperature of the superparamagnetic transition in the samples decreases with the particle size and therefore it is mainly determined by the inversion degree rather than by the particle size, which would give an opposite effect on the blocking temperature. The contribution of particle interaction to the magnetic behavior of the nanocomposites decreases significantly in the sample with the largest particle size. The values of the anisotropy constant give evidence that the anisotropy constant decreases upon increasing the particle size of the samples. All these results clearly indicate that, even when dispersed with low concentration in a non-magnetic and highly porous and insulating matrix, the zinc ferrite nanoparticles show a magnetic behavior similar to that displayed when they are unsupported or dispersed in a similar but denser matrix, and with higher loading. The effective anisotropy measured for our samples appears to be systematically higher than that measured for supported zinc ferrite nanoparticles of similar size, indicating that this effect probably occurs as a consequence of the high inversion degree. © 2014 the Owner Societies.
  • Carta, D., Boi, F., Corrias, A., Bullita, S., KónyaZ. and Casula, M. (2014). Iron/cobalt-SBA-16 cubic mesoporous composites as catalysts for the production of multi-walled carbon nanotubes. Journal of Porous Materials [Online] 21:1123-1131. Available at: http://dx.doi.org/10.1007/s10934-014-9862-z.
    A series of catalysts containing iron and cobalt nanoparticles supported on a highly ordered mesoporous cubic Im3m silica (SBA-16) were prepared by wet impregnation and used for the production of multi-walled carbon nanotubes (MWCNTs) by catalytic chemical vapor deposition (CCVD) of acetylene. The catalysts were characterized by low- and wide-angle X-ray diffraction, N2 physisorption analysis at 77 K and transmission electron microscopy to study the influence of different metal loading and impregnation time on the CCVD process. Quality and morphology of the MWCNTs was assessed by transmission and scanning electron microscopy, whereas thermal analysis was used to estimate the amount of CNTs produced. It was found that the nanocomposites are catalytically active with particular reference to samples with relatively high metal loading, and are stable under the conditions adopted for the CNT production by the CCVD process.
  • Conca, E., Aresti, M., Saba, M., Casula, M., Quochi, F., Mula, G., Loche, D., Kim, M., Manna, L., Corrias, A., Mura, A. and Bongiovanni, G. (2014). Charge separation in Pt-decorated CdSe@CdS octapod nanocrystals. Nanoscale [Online] 6:2238-2243. Available at: http://dx.doi.org/10.1039/c3nr05567a.
    We synthesize colloidal CdSe@CdS octapod nanocrystals decorated with Pt domains, resulting in a metal-semiconductor heterostructure. We devise a protocol to control the growth of Pt on the CdS surface, realizing both a selective tipping and a non-selective coverage. Ultrafast optical spectroscopy, particularly femtosecond transient absorption, is employed to correlate the dynamics of optical excitations with the nanocrystal morphology. We find two regimes for capture of photoexcited electrons by Pt domains: a slow capture after energy relaxation in the semiconductor, occurring in tipped nanocrystals and resulting in large spatial separation of charges, and an ultrafast capture of hot electrons occurring in nanocrystals covered in Pt, where charge separation happens faster than energy relaxation and Auger recombination. Besides the relevance for fundamental materials science and control at the nanoscale, our nanocrystals may be employed in solar photocatalysis. © 2014 The Royal Society of Chemistry.
  • Carta, D., Casula, M., Bullita, S., Falqui, A., Casu, A., Carbonaro, C. and Corrias, A. (2014). Direct sol-gel synthesis of doped cubic mesoporous SBA-16 monoliths. Microporous and Mesoporous Materials [Online] 194:157-166. Available at: http://dx.doi.org/10.1016/j.micromeso.2014.03.032.
    Porous monoliths of a 3D cubic cage mesostructured silica (SBA-16 type) and of composites containing Co (5% wt.) and FeCo (5% wt.) nanophases dispersed in the SBA-16 type matrix were prepared following a templated-gelation method. Optically clear disk-shaped monoliths with diameter around 10 cm were obtained whose absorption spectra are tunable based on the composition. Low-angle X-ray diffraction, N2 physisorption and transmission electron microscopy show that the ordered mesoporous structure and high surface area are preserved after calcination at 500 °C and after reduction at 800 °C, indicating a very high thermal stability. X-ray absorption spectroscopy at the Fe and Co K-edges allowed us to identify the intermediate products before and after reduction. In particular, the formation of a nanophase of layered cobalt silicate hydroxide was determined in the Co containing sample calcined at 500 °C. This compound is easily reducible giving rise to a superparamagnetic nanocomposite containing pure fcc Co nanoparticles dispersed within the 3D mesoporous silica structure. Extensive characterization points out that the proposed procedure is promising in the preparation of functional nanocomposites with adjustable magnetic and optical properties.
  • Carta, D., Loche, D., Casula, M., Oláh, N., Olasz, D. and Corrias, A. (2014). Nickel-based nanocrystals dispersed on SBA-16 gels: Synthesis and structural characterization. Journal of Non-Crystalline Solids [Online] 401:134-138. Available at: http://dx.doi.org/10.1016/j.jnoncrysol.2014.01.010.
    Porous monoliths of nanocomposites containing Ni (5 wt.%) and FeNi (5 wt.%) nanoparticles dispersed on an SBA-16 type matrix were prepared following a templated-gelation method based on the sol-gel process. The nanocomposites were characterized by energy dispersive X-ray spectroscopy, N2 physisorption at 77 K, X-ray diffraction and transmission and scanning electron microscopy. In particular, N2 physisorption and transmission electron microscopy analysis show that the ordered mesoporous structure and the high surface area of all the samples are preserved after calcination in air at 500 °C and also after reduction in H2 flux at 800 °C, indicating a very high thermal stability of the samples. As a result of the effective dispersion of the nanophase within the porous texture, nanocomposites containing Ni nanocrystals with an average size of 6 nm homogeneously dispersed within the pores of the amorphous silica matrix were obtained. © 2014 Elsevier B.V.
  • Carta, D., Marras, C., Loche, D., Mountjoy, G., Ahmed, S. and Corrias, A. (2013). An X-ray absorption spectroscopy study of the inversion degree in zinc ferrite nanocrystals dispersed on a highly porous silica aerogel matrix. Journal of Chemical Physics [Online] 138:054702-054702. Available at: http://dx.doi.org/10.1063/1.4789479.
    The structural properties of zinc ferrite nanoparticles with spinel structure dispersed in a highly porous SiO2 aerogel matrix were compared with a bulk zinc ferrite sample. In particular, the details of the cation distribution between the octahedral (B) and tetrahedral (A) sites of the spinel structure were determined using X-ray absorption spectroscopy. The analysis of both the X-ray absorption near edge structure and the extended X-ray absorption fine structure indicates that the degree of inversion of the zinc ferrite spinel structures varies with particle size. In particular, in the bulk microcrystalline sample, Zn2+ ions are at the tetrahedral sites and trivalent Fe3+ ions occupy octahedral sites (normal spinel). When particle size decreases, Zn2+ ions are transferred to octahedral sites and the degree of inversion is found to increase as the nanoparticle size decreases. This is the first time that a variation of the degree of inversion with particle size is observed in ferrite nanoparticles grown within an aerogel matrix. © 2013 American Institute of Physics.
  • Latini, A., Cavallo, C., Aldibaja, F., Gozzi, D., Carta, D., Corrias, A., Lazzarini, L. and Salviati, G. (2013). Efficiency improvement of DSSC photoanode by scandium doping of mesoporous titania beads. Journal of Physical Chemistry C [Online] 117:25276-25289. Available at: http://dx.doi.org/10.1021/jp409813c.
    Solid solutions of scandium in anatase as semiconductor material for DSSC photoanodes were prepared by the controlled hydrolysis of titanium(IV) isopropoxide and scandium(III) isopropoxide in hydroalcoholic medium. The final powder was constituted by mesoporous anatase beads doped with Sc. A superstructure characterizes the beads, which are spherical at the microscopic level (?650 nm) and rice-grain-shaped at the nanoscopic level (?20 nm). The BJH pore size distribution and BET surface area of the powder beads were found depending from the Sc content ranging the peak of the former between 7 and 25 nm and between 65 and 128 m2 g–1 the latter. Data obtained by XRD and EXAFS confirm that we are dealing with real solid solutions with ScTi substitution defects. The electronic properties of the synthesized semiconductor material as a function of Sc doping were investigated by the measure of the flat band potential, band gap, and deep levels. In the range 0.0–1.0 at. % of Sc, the flat band energy changes from ?4.15 to ?4.07 eV, whereas the band gap height increases by 0.03 eV. The presence of Sc modifies heavily the cathodoluminescence spectrum of anatase at the lowest concentration too. Several DSSCs with photoanodes at different Sc doping were tested both under solar simulator and in the dark. The maximum efficiency of 9.6% was found at 0.2 at. % of Sc in anatase that is 6.7% higher with respect to the DSSCs with pure anatase.
  • Boi, F., Mountjoy, G., Luklinska, Z., Spillane, L., Karlsson, L., Wilson, R., Corrias, A. and Baxendale, M. (2013). The origin of long-period lattice spacings observed in iron-carbide nanowires encapsulated by multiwall carbon nanotubes. Microscopy and Microanalysis [Online] 19:1298-1302. Available at: http://dx.doi.org/10.1017/S1431927613001918.
    Structures comprising single-crystal, iron-carbon-based nanowires encapsulated by multiwall carbon nanotubes self-organize on inert substrates exposed to the products of ferrocene pyrolysis at high temperature. The most commonly observed encapsulated phases are Fe3C, α-Fe, and γ-Fe. The observation of anomalously long-period lattice spacings in these nanowires has caused confusion since reflections from lattice spacings of �0.4 nm are kinematically forbidden for Fe3C, most of the rarely observed, less stable carbides, α-Fe, and γ-Fe. Through high-resolution electron microscopy, selective area electron diffraction, and electron energy loss spectroscopy we demonstrate that the observed long-period lattice spacings of 0.49, 0.66, and 0.44 nm correspond to reflections from the (100), (010), and (001) planes of orthorhombic Fe3C (space group Pnma). Observation of these forbidden reflections results from dynamic scattering of the incident beam as first observed in bulk Fe3C crystals. With small amounts of beam tilt these reflections can have significant intensities for crystals containing glide planes such as Fe3C with space groups Pnma or Pbmn. © Microscopy Society of America 2013.
  • Carta, D., Corrias, A., Falqui, A., Brescia, R., Fantechi, E., Pineider, F. and Sangregorio, C. (2013). EDS, HRTEM/STEM, and X-ray absorption spectroscopy studies of co-substituted maghemite nanoparticles. Journal of Physical Chemistry C [Online] 117:9496-9506. Available at: http://dx.doi.org/10.1021/jp401706c.
    A detailed study of the composition and structure of Co-doped maghemite nanoparticles with systematically varying composition has been carried out by transmission electron microscopy (TEM) techniques, such as high-resolution TEM, scanning TEM, and energy-dispersive X-ray spectrometry, and by X-ray absorption spectroscopy at the Fe and Co K-edges, analyzing both the extended X-ray absorption fine structure and the X-ray absorption near-edge structure regions. The latter techniques, in particular, allow us to determine the degree of inversion of divalent and trivalent metal ions among the octahedral and tetrahedral sites in the spinel structure of the nanoparticles and give detailed information on atomic distances. The samples consist of single-crystal nanoparticles with a composition corresponding to the Fe/Co ratio used in the synthesis. The degree of inversion is quite similar for all samples and close to the value found in a pure cobalt ferrite bulk sample. © 2013 American Chemical Society.
  • Carta, D., Bullita, S., Falqui, A., Casula, M., Corrias, A. and KónyaZ. (2013). Carbon nanotubes synthesis over FeCo-based catalysts supported on SBA-16. Nanopages [Online] 8:1-8. Available at: http://dx.doi.org/10.1556/Nano.2013.00002.
    A series of Fe/Co based nanocomposites where the matrix is mesoporous ordered cubic Im3m silica (SBA-16 type) characterized by a three dimensional cage-like structure of pores were obtained by two different approaches: impregnation and gelation. X-ray diffraction and transmission electron microscopy analysis show that after metal loading, calcination at 500 C and reduction in H2 fl ux at 800 C the nanocomposites retain the well-ordered structure of the matrix with cubic symmetry of pores. All nanocomposites prepared were tested for the production of carbon nanotubes by catalytic chemical vapour deposition. Transmission electron microscopy points out that good quality multi-walled carbon nanotubes are obtained.
  • Carta, D., Bullita, S., Casula, M., Casu, A., Falqui, A. and Corrias, A. (2013). Cubic mesoporous silica (SBA-16) prepared using butanol as the Co-surfactant: A general matrix for the preparation of FeCo-SiO2 nanocomposites. ChemPlusChem [Online] 78:364-374. Available at: http://dx.doi.org/10.1002/cplu.201200283.
    A mesoporous ordered cubic Im3m silica (SBA-16) characterized by a three dimensional cage-like structure of pores was used as a host matrix for the preparation of a series of FeCo-SiO2 nanocomposites with different alloy loading and composition by the wet impregnation method. The mesoporous structure of the SBA-16-type support, prepared according to a versatile sol-gel templated synthetic method, which makes use of n-butanol as a co-surfactant, is stable during the treatments necessary to obtain the final nanocomposites, as pointed out by low-angle X-Ray diffraction, transmission electron microscopy, and N2 physisorption at 77 K. Wide-angle X-ray diffraction shows that upon reduction at 800 °C, FeCo nanocrystals (6-7 nm) with the typical bcc structure are formed and energy-dispersive X-ray spectroscopy analysis, performed by scanning transmission electron microscopy on one of the samples, shows that the Fe/Co atomic ratio in the alloy nanoparticles is very close to the expected value of two. Electron tomography was used for the first time to gain evidence on the highly interconnected mesoporous structure of SBA-16 and the arrangement of the nanoparticles within the matrix. It was found that spherical alloy nanocrystals with narrow size distribution are homogeneously distributed throughout the mesoporous matrix and that the resulting FeCo-SiO2 nanocomposite material displays superparamagnetic behavior with high strength dipolar interactions, as expected for particles with a large magnetic moment.
  • Fantechi, E., Campo, G., Carta, D., Corrias, A., De Julián Fernández, C., Gatteschi, D., Innocenti, C., Pineider, F., Rugi, F. and Sangregorio, C. (2012). Exploring the effect of co doping in fine maghemite nanoparticles. Journal of Physical Chemistry C [Online] 116:8261-8270. Available at: http://dx.doi.org/10.1021/jp300806j.
    We present a study of the structural, magnetic, and magneto-optical properties of a series of Co-substituted ferrite nanoparticles (NPs) prepared by thermal decomposition of metallo-organic precursors in high boiling solvents. The structural characterization, carried out by using several techniques (transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and magnetic circular dichroism measurements), showed all the samples are high crystalline, 5-6 nm spherical NPs with the cubic spinel structure typical of ferrites. The evolution of lattice parameters with cobalt content suggests that the material is Co-substituted maghemite, also confirmed by XAS and magneto optical (MO) characterizations. The investigation of the magnetic and magneto-optical properties displays peculiar trends with the cobalt content, the main features being the large increase of the saturation magnetization and the anomalous dependence of magnetic anisotropy which reaches its maximum values for intermediate compositions. The large tuneability of this material makes it possible to implement the performances of devices used in biomedical and sensing applications.
  • Loche, D., Casula, M., Corrias, A., Marras, S. and Moggi, P. (2012). Bimetallic FeCo nanocrystals supported on highly porous silica aerogels as fischer-tropsch catalysts. Catalysis Letters [Online] 142:1061-1066. Available at: http://dx.doi.org/10.1007/s10562-012-0877-2.
    In this work, nanocomposites constituted of FeCo alloy nanoparticles dispersed on a highly porous silica aerogel have been designed as catalysts for low temperature Fischer-Tropsch synthesis. The catalysts were characterized by XRD, TEM, N 2 physisorption and SEM analysis. A high catalytic activity with CO conversions up to 95 % has been obtained, with enhanced selectivity for the C 2-C 4 hydrocarbons.
  • Carta, D., Navarra, G., Falqui, A., KónyaZ. and Corrias, A. (2012). Structural characterization of FeCo alloy nanoparticles embedded in SBA-16 and their catalytic application for carbon nanotubes production. RSC Advances [Online] 2:7886-7893. Available at: http://dx.doi.org/10.1039/C2RA01147F.
    The formation of FeCo alloy nanoparticles embedded in a highly ordered 3D cubic mesoporous silica matrix (SBA-16) was thoroughly studied using several techniques. In particular, the selectivity of Extended X-ray absorption fine structure and X-ray absorption near-edge structure spectroscopy at both the Fe and Co K-edges allowed us to determine that before reduction treatment Fe and Co are present in a poorly crystalline environment, while after reduction treatment FeCo nanoparticles with the typical bcc structure are formed. FeCo alloy nanoparticles are used in several applications: biomedical (magnetic carriers for drug delivery and cell separation), magnetic (data storage) and catalytic. In this work, FeCo nanoparticles formed in situ in the SBA-16 matrix were used for the production of carbon nanotubes by catalytic chemical vapour deposition. Transmission electron microscopy indicates that good quality multi-walled carbon nanotubes are obtained.
  • Carta, D., Mountjoy, G., Apps, R. and Corrias, A. (2012). Effect of the support on the formation of FeCo alloy nanoparticles in an SBA-16 mesoporous silica matrix: An X-ray absorption spectroscopy study. Journal of Physical Chemistry C [Online] 116:12353-12365. Available at: http://dx.doi.org/10.1021/jp302927t.
    A series of nanocomposites consisting of FeCo alloy nanoparticles supported on a three-dimensional cubic mesoporous silica matrix (SBA-16) were prepared by wet impregnation of the matrix with a solution of Fe and Co nitrates. FeCo alloy nanoparticles were obtained by heat treatment at 800 °C in reducing atmosphere of the impregnated SBA-16 previously calcined at 500 °C. Three different SBA-16 types were used as a support of the nanophase. The influence of the matrix on the absorption of Fe and Co ions was investigated using X-ray diffraction and X-ray absorption spectroscopy. In particular, extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) techniques at the Fe and Co K-edges were used in order to identify the intermediate products before reduction and to study the formation of the bcc FeCo alloy, which cannot be assessed unambiguously using X-ray diffraction. An important influence of the matrix has been observed in the phases formed before reduction, in the size of nanoparticles, and in the oxidation of the FeCo alloy nanoparticles.
  • Mountjoy, G., Loche, D., Wang, P., Sader, K. and Corrias, A. (2011). Scanning transmission electron microscopy study of the evolution of needle-like nanostructures in CoFe2O4 and nife 2O4 silica nanocomposite aerogels. Journal of Physical Chemistry C [Online] 115:5358-5365. Available at: https://doi.org/10.1021/jp110472d.
    Magnetic nanocomposite materials consisting of 10 wt % CoFe 2O4 or NiFe2O4 nanoparticles in a silica aerogel matrix have been synthesized by the sol?gel method. A 100-kV aberration-corrected scanning transmission electron microscope (STEM) has been used to study these materials, and bright field and high angle annular dark field images show that after heat treatment at both 450 and 900 °C, they contain needle-like nanostructures Ì?1 nm in width and 10 nm in length. High resolution STEM images show that the needle-like nanostructures have a layered internal structure with typical interlayer spacings of 0.33 ± 0.02 nm. Electron energy loss spectroscopy using a 0.13-nm diameter probe gives information on the composition of these nanostructures. The results presented here for samples heat treated at 450 °C are consistent with needle-like nanostructures arising from Co and Ni silicate hydroxides which are separate from the also present Fe-containing phase of ferrihydrite nanoparticles. Samples heat treated at 900 °C have previously been shown to contain round ferrite nanoparticles ?8 nm in diameter. The results presented here are consistent with the needle-like nanostructures being transformed into ferrite-like phases after heat treatment at 900 °C, and the needle-like nanostructures are often found attached to round ferrite nanoparticles. © 2011 American Chemical Society.
  • Carta, D., Corrias, A. and Navarra, G. (2011). An X-ray absorption spectroscopy study of FeCo alloy nanoparticles embedded in ordered cubic mesoporous silica (SBA-16). Journal of Non-Crystalline Solids [Online] 357:2611-2614. Available at: http://dx.doi.org/10.1016/j.jnoncrysol.2011.02.053.
    Nanocomposites containing FeCo alloy nanoparticles dispersed in a highly ordered cubic mesoporous silica (SBA-16) matrix were prepared using two different synthetic methods, co-precipitation and impregnation. Extended X-ray Absorption Spectroscopy (EXAFS) technique at both Fe and Co K-edges was used to investigate the structure of FeCo nanoparticles and the presence of additional disordered oxide phases. EXAFS technique gives evidence of differences in the oxidation degree of the FeCo nanoparticles depending on the synthetic method used.
  • Casula, M., Corrias, A., Arosio, P., Lascialfari, A., Sen, T., Floris, P. and Bruce, I. (2011). Design of water-based ferrofluids as contrast agents for magnetic resonance imaging. Journal of Colloid and Interface Science [Online] 357:50-55. Available at: http://dx.doi.org/10.1016/j.jcis.2011.01.088.
    We report the synthesis, characterization and relaxometric study of ferrofluids based on iron oxide, with potential for use as magnetic resonance imaging (MRI) contrast agents (CAs). The effect of different cost-effective, water-based surface modification approaches which can be easily scaled-up for the large scale synthesis of the ferrofluids has been investigated. Surface modification was achieved by silanization, and/or coating with non-toxic commercial dispersants (a lauric polysorbate and a block copolymer with pigment affinic groups, namely Tween 20 and Disperbyk 190) which were added after or during iron oxide nanoparticle synthesis. It was observed that all the materials synthesized functioned as negative contrast agents at physiological temperature and at frequencies covered by clinical imagers. The relaxometric properties of the magnetic nanoparticles were significantly improved after surface coating with stabilizers compared to the original iron oxide nanoparticles, with particular reference to the silica-coated magnetic nanoparticles. The results indicate that the optimization of the preparation of colloidal magnetic ferrofluids by surface modification is effective in the design of novel contrast agents for MRI by enabling better or more effective interaction between the coated iron oxide nanoparticles and protons present in their aqueous environment.
  • Casula, M., Concas, G., Congiu, F., Corrias, A., Loche, D., Marras, C. and Spano, G. (2011). Characterization of stoichiometric nanocrystalline spinel ferrites dispersed on porous silica aerogel. Journal of Nanoscience and Nanotechnology [Online] 11:10136-10141. Available at: http://dx.doi.org/10.1166/jnn.2011.4975.
    Stoichiometric magnetic nanosized ferrites MFe 2O 4 (M = Mn, Co, Ni) were prepared in form of nearly spherical nanocrystals supported on a highly porous silica aerogel matrix, by a sol-gel procedure. X-ray diffraction and transmission electron microscopy indicate that these materials are made out of non-agglomerated ferrite nanocrystals having size in the 5-10 nm range. Investigation by Mössbauer Spectroscopy was used to gain insights on the superparamagnetic relaxation and on the inversion degree. Magnetic ordering at room temperature varies from superparamagnetic in the NiFe 2O 4 sample, highly blocked (?70%) in the MnFe 2O 4 sample and nearly fully blocked in the CoFe 2O 4 sample. A fitting procedure of the Mössbauer data has been used in order to resolve the spectrum into the tetrahedral and octahedral components; in this way, an inversion degree of 0.68 (very close to bulk values) was obtained for 6 nm silica-supported CoFe 2O 4 nanocrystals.
  • Cutrufello, M., Rombi, E., Ferino, I., Loche, D., Corrias, A. and Casula, M. (2011). Ni-based xero- and aerogels as catalysts for nitroxidation processes. Journal of Sol-Gel Science and Technology [Online] 60:324-332. Available at: http://dx.doi.org/10.1007/s10971-011-2460-5.
    Porous nanocomposites made out of nickel dispersed on silica or alumina matrices were prepared as prospective catalysts for the nitroxidation of hydrocarbons in the form of aerogel or xerogel by adopting either a supercritical or a conventional gel drying procedure. The structural and textural features of the materials were investigated by X-ray diffraction, transmission electron microscopy and N 2 physisorption and combined to the acid/base and reducibility data as deduced by adsorption microcalorimetry and temperature programmed reduction (TPR) profiles. The alumina-based samples are made out of nanocrystalline nickel aluminate and are mesoporous, although the aerogel has larger pore volumes and surface area than the xerogel. On the other hand, in the silica-based samples nickel oxide nanocrystals are dispersed on amorphous silica, the size of the nanocrystals being around 5 nm in the microporous xerogel and 14 nm in the mainly mesoporous aerogel. TPR data point out that the aluminabased samples have similar reducibility, whereas significant differences were observed in the silica-supported composites, the NiO-SiO 2 aerogel exhibiting improved reducibility at low temperature. The NO-catalyst interaction was monitored by temperature programmed NO reaction coupled to mass spectrometry and preliminary tests on the use of the NiO-SiO 2 xerogel and aerogel nanocomposites for the catalytic nitroxidation of 1-methylnaphthalene to 1-naphthonitrile were obtained in a fixedbed continuous-flow reactor. The data indicate that the aerogel exhibits larger selectivity than the corresponding xerogel, pointing out the importance of tuning the sol-gel parameters in the design of porous composite materials for catalytic applications.
  • Carta, D., Casula, M., Bullita, S., Falqui, A. and Corrias, A. (2011). Iron-cobalt nanocrystalline alloy supported on a cubic mesostructured silica matrix: FeCo/SBA-16 porous nanocomposites. Journal of Nanoparticle Research [Online] 13:3489-3501. Available at: http://dx.doi.org/10.1007/s11051-011-0270-x.
    A series of novel nanocomposites constituted of FeCo nanoparticles dispersed in an ordered cubic Im3m mesoporous silica matrix (SBA-16) have been successfully synthesized using the wet impregnation method. SBA-16, prepared using the non-ionic Pluronic 127 triblock copolymer as a structure-directing agent, is an excellent support for catalytic nanoparticles because of its peculiar three-dimensional cage-like structure, high surface area, thick walls, and high thermal stability. Low-angle X-ray diffraction, N2 physisorption, and transmission electron microscopy analyses show that after metal loading, calcination at 500 °C, and reduction in H2 flux at 800 °C, the nanocomposites retain the well-ordered structure of the matrix with cubic symmetry of pores. FeCo alloy nanoparticles with spherical shape and narrow size distribution (4-8 nm) are homogeneoulsy distributed throughout the matrix and they seem in a large extent to be allocated inside the pores.
  • Carta, D., Casula, M., Corrias, A., Falqui, A., Dombovári, Ã, Gálos, A. and KónyaZ. (2011). One-step preparation of feco nanoparticles in a SBA-16 matrix as catalysts for carbon nanotubes growth. Journal of Nanoscience and Nanotechnology [Online] 11:6735-6746. Available at: http://dx.doi.org/10.1166/jnn.2011.4230.
    Nanocomposites containing FeCo alloy nanoparticles dispersed in a highly ordered 3D cubic Im3m mesoporous silica (SBA-16) matrix were prepared by a novel, single-step templated-assisted sol-gel technique. Two different approaches were used in the synthesis of nanocomposites; a pure SBA-16 sample was also prepared for comparison. Low-angle X-ray diffraction, transmission electron microscopy and N 2 physisorption at 77 K show that after metal loading, calcination at 500 °C and reduction in H 2 flux at 800 °C the nanocomposites retain the cubic mesoporous structure with pore size not very different from the pure matrix. X-ray absorption fine structure (EXAFS) analysis at Fe and Co K-edges demonstrates that the FeConanoparticles have the typical bcc structure. The final nanocomposites were tested as catalysts for the production of carbon nanotubes by catalytic chemical vapour deposition and high-resolution TEM shows that good quality multi-walled carbon nanotubes are obtained.
  • Carta, D., Corrias, A. and Navarra, G. (2011). A total X-ray scattering study of MnFe2O4 nanoparticles dispersed in a silica aerogel matrix. Journal of Non-Crystalline Solids [Online] 357:2600-2603. Available at: http://dx.doi.org/10.1016/j.jnoncrysol.2011.01.013.
    Structural information on a MnFe2O4-SiO2 nanocomposite aerogel and on the pure silica aerogel matrix were obtained by total X-ray scattering experiments. The total pair distribution function of the silica aerogel is in agreement with literature data on melt-quenched silica. The total pair distribution function of the nanocomposite contains the contribution of all the pair correlations of the atomic species making the interpretation more difficult. The difference curve obtained by subtracting the total pair distribution function of the matrix from that of the nanocomposite, allows to selectively study the structural environment of the nanoparticles.
  • Teddy, J., Falqui, A., Corrias, A., Carta, D., Lecante, P., Gerber, I. and Serp, P. (2011). Influence of particles alloying on the performances of Pt-Ru/CNT catalysts for selective hydrogenation. Journal of Catalysis [Online] 278:59-70. Available at: http://dx.doi.org/10.1016/j.jcat.2010.11.016.
    The influence of PtRu bimetallic particle size and composition on cinnamaldehyde selective hydrogenation has been investigated for the first time using well-defined catalysts based on carbon nanotubes support. Very high selectivity towards cinnamyl alcohol together with high activity have been obtained provided a high temperature treatment of the catalyst is performed. HRTEM, WAXS and EXAFS analyses permit us to conclude that the remarkable influence of this high temperature treatment on both activity and selectivity arises from different phenomena. First, a particle size and a structural effect have been evidence that permits to increase the selectivity. WAXS and EXAFS point the formation of alloyed PtRu nanoparticles. Second, the heat treatment allows the removal of oxygenated groups from CNT surface. This may increase the cinnamaldehyde adsorption capacity and decrease the activation barrier for diffusion of substrate and product on the CNT surface, thus contributing to an increase in the activity.
  • Falqui, A., Loche, D., Casula, M., Corrias, A., Gozzi, D. and Latini, A. (2011). Synthesis and characterization of multiwalled carbon Nanotube/FeCo nanocomposites. Journal of Nanoscience and Nanotechnology [Online] 11:2215-2225. Available at: http://dx.doi.org/10.1166/jnn.2011.3725.
    Multiwalled carbon nanotube/FeCo nanocomposites were produced by Catalytic Chemical Vapour Deposition using highly porous FeCo - SiO 2 aerogels with different loadings and dimensions of FeCo nanoparticles as catalysts. Multiwalled carbon nanotubes with average number of walls depending on the size of the catalyst nanoparticles w ere obtained. Inside the nanotubes spherical or elliptical FeCo nanoparticles are retained, and the magnetic properties of the resulting nanocomposites were characterized in detail.
  • Vanyorek, L., Loche, D., Katona, H., Casula, M., Corrias, A., KónyaZ., Kukovecz, A. and Kiricsi, I. (2011). Optimization of the catalytic chemical vapor deposition synthesis of multiwall carbon nanotubes on FeCo(Ni)/SiO2 aerogel catalysts by statistical design of experiments. Journal of Physical Chemistry C [Online] 115:5894-5902. Available at: http://dx.doi.org/10.1021/jp111860x.
    We report on optimizing the catalytic chemical vapor deposition synthesis of multiwall carbon nanotubes (MWCNTs) from ethene over supported transition metal SiO2 nanocomposite aerogels using the statistical design of experiments (DOE) approach. DOE allowed us to test 19 different catalysts in a total of 49 reactions instead of testing 27 catalysts in 729 runs as required by a three-level full factorial design. Both catalyst-related and process-related variables were optimized; in particular varied parameters were Fe + Co loading, Fe/Co ratio, Ni loading, C2H4 flow rate, temperature, and duration of the reaction. The results of the optimization indicate that a good catalyst should contain a high overall loading (10 wt %) of iron and cobalt in similar amount, should be free of nickel and should be operated at a relatively low temperature (650-700 °C) at high carbon source space velocity for optimum performance. The uniqueness of this work is that we demonstrated that catalyst-related and process-related variables can be optimized simultaneously in the DOE of MWCNT synthesis.
  • Falqui, A., Corrias, A., Wang, P., Snoeck, E. and Mountjoy, G. (2010). A transmission electron microscopy study of CoFe2O4 ferrite nanoparticles in silica aerogel matrix using HREM and STEM imaging and EDX spectroscopy and EELS. Microscopy and Microanalysis [Online] 16:200-209. Available at: http://dx.doi.org/10.1017/S1431927610000061.
    Magnetic nanocomposite materials consisting of 5 and 10 wt% CoFe 2O4 nanoparticles in a silica aerogel matrix have been synthesized by the sol-gel method. For the CoFe2O4-10wt% sample, bright-field scanning transmission electron microscopy (BF STEM) and high-resolution transmission electron microscopy (HREM) images showed distinct, rounded CoFe2O4 nanoparticles, with typical diameters of roughly 8 nm. For the CoFe2O4-5wt% sample, BF STEM images and energy dispersive X-ray (EDX) measurements showed CoFe2O4 nanoparticles with diameters of roughly 3 ± 1 nm. EDX measurements indicate that all nanoparticles consist of stoichiometric CoFe2O4, and electron energy-loss spectroscopy measurements from lines crossing nanoparticles in the CoFe2O4-10wt% sample show a uniform composition within nanoparticles, with a precision of at best than ± 0.5 nm in analysis position. BF STEM images obtained for the CoFe2O 4-10wt% sample showed many "needle-like" nanostructures that typically have a length of ? 10 nm and a width of ? 1 nm, and frequently appear to be attached to nanoparticles. These needle-like nanostructures are observed to contain layers with interlayer spacing 0.33 ± 0.1 nm, which could be consistent with Co silicate hydroxide, a known precursor phase in these nanocomposite materials.
  • Carta, D., Casula, M., Floris, P., Falqui, A., Mountjoy, G., Boni, A., Sangregorio, C. and Corrias, A. (2010). Synthesis and microstructure of manganese ferrite colloidal nanocrystals. Physical Chemistry Chemical Physics [Online] 12:5074-5083. Available at: http://dx.doi.org/10.1039/b922646j.
    The atomic level structure of a series of monodisperse single crystalline nanoparticles with a magnetic core of manganese ferrite was studied using X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) techniques at both the Fe and Mn K-edges, and conventional and high resolution transmission electron microscopy (TEM and HRTEM). In particular, insights on the non-stoichiometry and on the inversion degree of manganese ferrite nanocrystals of different size were obtained by the use of complementary structural and spectroscopic characterization techniques. The inversion degree of the ferrite nanocrystals, i.e. the cation distribution between the octahedral and tetrahedral sites in the spinel structure, was found to be much higher (around 0.6) than the literature values reported for bulk stoichiometric manganese ferrite (around 0.2). The high inversion degree of the nanoparticles is ascribed to the partial oxidation of Mn2+ to Mn3+ which was evidenced by XANES, leading to non-stoichiometric manganese ferrite.
  • Loche, D., Casula, M., Falqui, A., Marras, S. and Corrias, A. (2010). Preparation of Mn, Ni, Co ferrite highly porous silica nanocomposite aerogels by an urea-assisted sol-gel procedure. Journal of Nanoscience and Nanotechnology [Online] 10:1008-1016. Available at: http://dx.doi.org/10.1166/jnn.2010.1907.
    The preparation of highly porous MnFe 2O 4-SiO 2 and NiFe 2O 4-SiO 2 nanocomposite aerogels with high purity and homogeneity was successfully achieved by a sol-gel procedure involving urea- assisted co-gelation of the precursor phases firstly applied for the synthesis of CoFe 2O 4-SiO 2. This method allows fast gelation, giving rise to aerogels with 97% porosity. The structural, morphological and textural characterization as a function of thermal treatments was carried out by a multitechnique approach confirming that, as in the case of CoFe 2O 4-SiO 2, the formation of single nanocrystals of manganese ferrite and nickel ferrite with spinel structure occurs after heating at 750 °C and is complete at 900 °C when the high porosity typical of aerogels is still retained. Thermogravimetric analysis (TG), differential thermal analysis (DTA), N 2-physisorption at 77 K, powder X-ray diffraction (XRD), and transmission electron microscopy (TEM) indicate that the compositional homogeneity, crystallite size, thermal stability, and porosity are controlled by the sol-gel parameters of the preparation.
  • Corrias, A., Mountjoy, G., Loche, D., Puntes, V., Falqui, A., Zanella, M., Parak, W. and Casula, M. (2009). Identifying Spinel Phases in Nearly Monodisperse Iron Oxide Colloidal Nanocrystal. Journal of Physical Chemistry C [Online] 113:18667-18675. Available at: http://dx.doi.org/10.1021/jp9047677.
    Nearly monodisperse iron oxide colloidal nanocrystals prepared by nonhydrolytic high-temperature solution method were obtained with two different sizes and degrees of oxidation. The characterization of the structural features of the nanocrystals was performed by a multitechnique approach including transmission electron microscopy, X-ray diffraction and X-ray absorption spectroscopy, energy filtered electron microscopy imaging, and SQUID magnetometry. The different techniques provided complementary information on the local oxidation state of iron in the iron oxide nanoparticles, the stability of the phases, the exact crystal structure, and the compositional homogeneity. X-ray diffraction, transmission electron microscopy, and extended X-ray absorption spectroscopy show that the addition of oxidizer to the iron precursor gives rise to monodisperse polycrystalline nanoparticles made out of FeO plus a spinel phase. X-ray absorption near-edge structure, which is very sensitive to the oxidation state and local environment of iron in the different iron oxides, was used to distinguish among isostructural spinel phases of iron (II,III) oxide (magnetite) and iron(III) oxide (maghemite). Single-crystalline spinel nanoparticles are obtained upon sequential oxidation: in smaller nanoparticles a mixture of mainly Fe3O4 and ?-Fe2O3 is present, whereas the larger nanoparticles are made out of ?-Fe2O3, as also supported by SQUID magnetization measurements. The importance of a multitechnique approach for the elucidation of the compositional and structural details in addition to geometrical parameters in the characterization of nanocrystalline iron oxides is pointed out.
  • Carta, D., Casula, M., Corrias, A., Falqui, A., Navarra, G. and Pinna, G. (2009). Structural and magnetic characterization of synthetic ferrihydrite nanoparticles. Materials Chemistry and Physics [Online] 113:349-355. Available at: http://dx.doi.org/10.1016/j.matchemphys.2008.07.122.
    Ferrihydrite is a generic term for various poorly ordered Fe(III) oxyhydroxides which are naturally occurring as nanocrystals and are believed to constitute the ferric core of ferritine, the main iron storage protein in biological systems. Unlike other iron oxides, the exact structure and composition of ferrihydrite is still a matter of debate. In this work, we have prepared and characterized the two main forms of ferrihydrite referred to as 2-lines and 6-lines, on the basis of the number of reflections observed in the (X-ray) diffraction pattern. Thermal and textural properties have been studied; structural characterization has been performed by X-ray diffraction, transmission electron microscopy and X-ray absorption spectroscopy (EXAFS and XANES). The structure of the two forms results to be quite similar. The study of the magnetic properties indicates that the small differences between the 2-lines and 6-lines ferrihydrite samples are mainly caused by the different weight of the magnetic spins located on the particle surface, related to the different nanoparticles mean size.
  • Carta, D., Casula, M., Corrias, A., Falqui, A., Loche, D., Mountjoy, G. and Wang, P. (2009). Structural and magnetic characterization of Co and Ni silicate hydroxides in bulk and in nanostructures within silica aerogels. Chemistry of Materials [Online] 21:945-953. Available at: http://dx.doi.org/10.1021/cm8029714.
    X-ray diffraction, transmission electron microscopy, X-ray absorption spectroscopy, and magnetic measurements were used to investigate highly porous silica aerogels containing iron and cobalt or iron and nickel, calcined at 450 °C, which are intermediates in the formation of CoFe2O 4-SiO2 and NiFe2O4-SiO2 aerogel nanocomposites. Aerogels without iron and pure Co and Ni silicate hydroxides were also studied. The aerogel samples containing iron and cobalt or iron and nickel present two separate phases, one containing iron in the form of ferrihydrite and the other either Co or Ni silicate hydroxides. The silicate hydroxides are present in the aerogels in form of nanoparticles with a one-dimensional appearance in electron microscopy images, which are therefore anisotropic nanostructures, such as nanoneedles or nanosheets. The study of the magnetic properties indicates that the coercivity is greatly modified by size and shape of the particles going from pure Co or Ni silicate hydroxides to the aerogel samples containing only nickel or only cobalt. Finally, in the samples also containing iron, the total magnetic behavior is influenced both by the presence of ferrihydrite and by the possible diffusion of iron in the nanostructures of cobalt or nickel silicate hydroxide.
  • Carta, D., Casula, M., Falqui, A., Loche, D., Mountjoy, G., Sangregorio, C. and Corrias, A. (2009). A Structural and Magnetic Investigation of the Inversion Degree in Ferrite Nanocrystals MFe2O4(M = Mn, Co, Ni). Journal of Physical Chemistry C [Online] 113:8606-8615. Available at: http://dx.doi.org/10.1021/jp901077c.
    The structural and magnetic properties of nanocrystalline manganese, cobalt, and nickel spinel ferrites dispersed in a highly porous SiO2 aerogel matrix were studied. X-ray diffraction and high-resolution transmission electron microscopy indicate that single crystalline ferrite nanoparticles are well dispersed in the amorphous matrix. The cation distribution between the octahedral and tetrahedral sites of the spinel structure was investigated by X-ray absorption spectroscopy. The analysis of both the X-ray absorption near edge structure and the extended X-ray absorption fine structure indicates that the degree of inversion of the spinel structure increases in the series Mn, Co, and Ni spinel, in accordance with the values commonly found in the corresponding bulk spinels. In particular, fitting of the EXAFS data indicates that the degree of inversion in nanosized ferrites is 0.20 for MnFe2O4, 0.68 for CoFe2O4, and 1.00 for NiFe2O4. Magnetic characterization further supports these findings.
  • Falqui, A., Corrias, A., Gass, M. and Mountjoy, G. (2009). A transmission electron microscopy study of fe-Co alloy nanoparticles in silica aerogel matrix using HREM, EDX, and EELS. Microscopy and Microanalysis [Online] 15:114-124. Available at: http://dx.doi.org/10.1017/S1431927609090114.
    Magnetic nanocomposite materials consisting of 5.5 wt% Fe-Co alloy nanoparticles in a silica aerogel matrix, with compositions FexCo1x of x = 0.50 and 0.67, have been synthesized by the sol-gel method. The high-resolution transmission electron microscopy images show nanoparticles consisting of single crystal grains of body-centered cubic Fe-Co alloy, with typical crystal grain diameters of approximately 4 and 7 nm for Fe0.5Co0.5 and Fe0.67Co0.33 samples, respectively. The energy dispersive X-ray (EDX) spectra summed over areas of the samples gave compositions FexCo1x with x = 0.48 0.06 and 0.68 0.05. The EDX spectra obtained with the 1.5 nm probe positioned at the centers of 20 nanoparticles gave slightly lower concentrations of Fe, with means of x = 0.43 0.01 and x = 0.64 0.02, respectively. The Fe0.5Co0.5 sample was studied using electron energy loss spectroscopy (EELS), and EELS spectra summed over whole nanoparticles gave x = 0.47 0.06. The EELS spectra from analysis profiles of nanoparticles show a distribution of Fe and Co that is homogeneous, i.e., x = 0.5, within a precision of at best 0.05 in x and 0.4 nm in position. The present microscopy results have not shown the presence of a thin layer of iron oxide, but this might be at the limit of detectability of the methods.
  • Carta, D., Casula, M., Mountjoy, G. and Corrias, A. (2008). Formation and cation distribution in supported manganese ferrite nanoparticles: An X-ray absorption study. Physical Chemistry Chemical Physics [Online] 10:3108-3117. Available at: http://dx.doi.org/10.1039/b800359a.
    Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) techniques at both Fe and Mn K-edges were used to investigate the formation of MnFe2O4 nanoparticles embedded in a silica aerogel matrix as a function of calcination temperature (at 450, 750 and 900°C). Up to 450°C, two separated highly-disordered phases of iron and manganese are present. With increasing the temperature (to 750 and 900°C), the structure of aerogel nanoparticles becomes progressively similar to that of the spinel structure MnFe2O4 (jacobsite). Quantitative determination of cations distribution in the spinel structure shows that aerogels calcined at 750 and 900°C have a degree of inversion i = 0.20. A pure jacobsite sample synthesised by co-precipitation and used as a reference compound shows a much higher degree of inversion (i = 0.70). The different distribution of iron and manganese cations in the octahedral and tetrahedral sites in pure jacobsite and in the aerogels can be ascribed to partial oxidation of Mn2+ to Mn3+ in pure jacobsite, confirmed by XANES analysis, probably due to the synthesis conditions.

Book section

  • Corrias, A. and Casula, M. (2011). Aerogels Containing Metal, Alloy and Oxide Nanoparticles Embedded into Dielectric Matrices. In: Aegerter, M., Leventis, N. and Koebel, M. eds. Aerogels Handbook. Springer, pp. 335-363. Available at: https://doi.org/10.1007/978-1-4419-7589-8_16.
    Aerogels are regarded as ideal candidates for the design of functional nanocomposites based on supported metal or metal oxide nanoparticles. The large specific surface area together with the open pore structure enables aerogels to effectively host finely dispersed nanoparticles up to the desired loading and to provide nanoparticle accessibility as required to supply their specific functionalities. The incorporation of nanoparticles as a way to increase the possibility of the use of aerogels as innovative functional materials and the challenges in the controlled preparation of nanocomposite aerogels is reviewed in this chapter.

Conference or workshop item

  • Corrias, A., Loche, D. and Casula, M. (2011). Magnetic nanocomposite aerogels. In: MRS Fall Meeting. pp. 31-39. Available at: http://www.scopus.com/inward/record.url?eid=2-s2.0-84860213468&partnerID=40&md5=9f732de7d2947d5f0f5e045018f1c972.
    Aerogels are regarded as ideal candidates for the design of functional nanocomposites containing supported metal or metal oxide nanoparticles. The large specific surface area together with the open pore structure enables aerogels to effectively host finely dispersed nanoparticles up to the desired loading, to provide nanoparticle accessibility and/or to prevent nanoparticle agglomeration, as required to supply their specific functionalities. The preparation of highly porous nanocomposite aerogels containing magnetic metal, alloy or metal oxide nanoparticles dispersed into amorphous silica, with high purity and homogeneity, was successfully achieved by a novel sol-gel procedure involving urea-assisted co-gelation of the precursor phases. This method allows fast gelation, giving rise to aerogels with 97% porosity, and it is very versatile allowing to vary composition, loading and average size of the nanoparticles. The characterization of the morphological and structural features of the nanocomposite aerogels is carried out using different techniques, such as X-ray diffraction, Transmission Electron Microscopy and X-ray Absorption Spectroscopy. The characterization of the magnetic properties is carried out by SQUID magnetometry. © 2011 Materials Research Society.
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