Silvia Gross

Hybrid Materials Based on the Embedding of Organically Modified Transition Metal Oxoclusters or Polyoxometalates into Polymers for Functional Applications: A Review

The covalent incorporation of inorganic building blocks into a polymer matrix to obtain stable and robust materials is a widely used concept in the field of organic-inorganic hybrid materials, and encompasses the use of different inorganic systems including (but not limited to) nanoparticles, mono- and polynuclear metal complexes and clusters, polyhedral oligomeric silsesquioxanes (POSS), polyoxometalates (POM), layered inorganic systems, inorganic fibers, and whiskers. In this paper, we will review the use of two particular kinds of structurally well-defined inorganic building blocks, namely transition metals oxoclusters (TMO) and polyoxometalates (POM), to obtain hybrid materials with enhanced functional (e.g., optical, dielectric, magnetic, catalytic) properties.

Green and low temperature synthesis of nanocrystalline transition metal ferrites by simple wet chemistry routes

Crystalline and nanostructured cobalt (CoFe2O4), nickel (NiFe2O4), zinc (ZnFe2O4) and manganese (MnFe2O4) spinel ferrites are synthesized with high yields, crystallinity and purity through an easy, quick, reproducible and low-temperature hydrothermal assisted route starting from an aqueous suspension of coprecipitated metal oxalates. The use of water as a reaction medium is a further advantage of the chosen protocol. Additionally, the zinc spinel is also prepared through an alternative route combining coprecipitation of oxalates from an aqueous solution with thermal decomposition under reflux conditions. The nanocrystalline powders are obtained as a pure crystalline phase already at the extremely low temperature of 75 °C and no further thermal treatment is needed. The structure and microstructure of the prepared materials is investigated by means of X-ray powder diffraction (XRPD), while X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analyses are used to gain information about the surface and bulk composition of the samples, respectively, confirming the expected stoichiometry. To investigate the effect of the synthesis protocol on the morphology of the obtained ferrites, transmission electron microscopy (TEM) observations are performed on selected samples. The magnetic properties of the cobalt and manganese spinels are also investigated using a superconducting quantum device magnetometer (SQUID) revealing hard and soft ferrimagnetic behavior, respectively.

Zirconium and hafnium oxoclusters as molecular building blocks for highly dispersed ZrO2 or HfO2 nanoparticles in silica thin films

A novel synthetic route for the preparation of ZrO2 or HfO2 nanoparticles homogeneously dispersed in SiO2 thin films was developed. This route is based on the copolymerisation of organically modified crystalline oxozirconium or oxohafnium clusters (M4O2(OMc)12, M = Zr, Hf; OMc = OC(O)–C(CH3)CH2) with (methacryloxymethyl)triethoxysilane (MAMTES, CH2C(CH3)C(O)O–CH2Si(OCH2CH3)3). These crystalline clusters, which are the precursors for the corresponding metal oxides (MO2), were prepared via the sol–gel route by reaction of zirconium or hafnium butoxide (M(OBu)4) with methacrylic acid. The copolymerisation of the clusters with the methacrylate-functionalised siloxane was photoinitaited by Irgacure 184 and allowed the anchoring of the cluster to the forming silica network. The solution was cast into films by dip-coating and UV cured (10 min, 125 W) to promote the copolymerisation of the methacrylate groups of the cluster with those of the silane. Transparent and homogeneous films 200–450 nm thick were obtained after calcination at 800 °C in air. This route allowed the production of a very homogeneous dispersion of the MO2 precursors inside the silica matrix. The surface and in-depth composition of the thin films was investigated through IR, X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS). SIMS and XPS depth profiles evidenced a very homogenous distribution of both zirconium or hafnium throughout the silica films and sharp film–substrate interfaces. The surface morphology of the coatings was investigated through atomic force microscopy (AFM), which showed smooth, homogeneous and crack-free surfaces. Through X-ray diffraction (XRD) the crystallisation of hafnium and zirconium oxides was revealed, while the presence of isolated crystalline nanoparticles having a diameter of 5–10 nm was evidenced by transmission electron microscopy (TEM). A pull-off test indicated a very good adhesion of the films to the substrate.

Mesoporous tin-doped indium oxide thin films: effect of mesostructure on electrical conductivity

Abstract We present a versatile method for the preparation of mesoporous tin-doped indium oxide (ITO) thin films via dip-coating. Two poly(isobutylene)-b-poly(ethyleneoxide) (PIB-PEO) copolymers of significantly different molecular weight (denoted as PIB-PEO 3000 and PIB-PEO 20000) are used as templates and are compared with non-templated films to clarify the effect of the template size on the crystallization and, thus, on the electrochemical properties of mesoporous ITO films. Transparent, mesoporous, conductive coatings are obtained after annealing at 500 °C; these coatings have a specific resistance of 0.5 Ω cm at a thickness of about 100 nm. Electrical conductivity is improved by one order of magnitude by annealing under a reducing atmosphere. The two types of PIB-PEO block copolymers create mesopores with in-plane diameters of 20–25 and 35–45 nm, the latter also possessing correspondingly thicker pore walls. Impedance measurements reveal that the conductivity is significantly higher for films prepared with the template generating larger mesopores. Because of the same size of the primary nanoparticles, the enhanced conductivity is attributed to a higher conduction path cross section. Prussian blue was deposited electrochemically within the films, thus confirming the accessibility of their pores and their functionality as electrode material.

Dielectric investigation of inorganic–organic hybrid film based on zirconium oxocluster-crosslinked PMMA

Abstract Through photocopolymerisation of the cluster Zr4O2(OMc)12 bearing 12 polymerisable groups with methylmethacrylate in a monomer:cluster molar ratio of 50, a novel inorganic–organic hybrid thin film was obtained. The prepared thin film, having a thickness of about 70 μm, was characterised through dielectric spectroscopy. The dielectric spectra of the film were measured at different temperatures (20–73.4 °C) and in the frequency range 20 Hz–1 MHz. The α and β relaxations typical of polyalkylmethacrylate polymers were detected and the electric response mechanisms were investigated by analysing the dielectric spectra in terms of the Davidson–Cole phenomenological equation. Finally, a dielectric constant of 1.93 at 25 °C and 1 kHz was determined. This value classifies this system as a promising dielectric material for the development of electronic devices, such as polymer based field effect transistors.

Chemical and physical routes for composite materials synthesis: Ag and Ag2S nanoparticles in silica glass by sol–gel and ion implantation techniques

Two composite systems, “Ag” and “Ag–S” nanoparticles in silica films, were approached by using two different synthesis routes, namely sol–gel and ion implantation. Silica composites containing embedded nanosized silver- and silver sulfide-crystallites were obtained by the sol–gel process. The formation of silver nanograins was also observed in Ag-implanted silica samples, while sequential implantation (first Ag then S) led to the formation of core–shell Ag–Ag2S nanoclusters. The systems were then characterised using different analytical tools, i.e. X-ray photoelectron spectroscopy (XPS), X-ray-excited Auger electron spectroscopy (XE-AES), X-ray diffraction (XRD), secondary-ion mass spectrometry (SIMS), Rutherford backscattering spectrometry (RBS) and transmission electron microscopy (TEM). These advanced microscopic and X-ray analytical methods were combined to gain complementary information concerning the composition and microstructure of the investigated composite systems. In addition, the characterisation of both systems by means of several investigation techniques provided a valuable insight into the potential features offered by sol–gel and ion implantation and enabled a fruitful comparison between these preparative routes. The influence of the different synthesis parameters on the final features of the composites is analysed and discussed.

Oxocluster-reinforced organic–inorganic hybrid materials: effect of transition metal oxoclusters on structural and functional properties

The focus of this Highlight is on the structural and functional properties which organically modified transition oxoclusters can provide, once embedded into a polymer matrix, to the resulting hybrid materials. Some selected case studies are discussed to highlight the role of these polynuclear inorganic building blocks in determining appealing material properties.

ω-Mercapto-functionalized hafnium- and zirconium-oxoclusters as nanosized building blocks for inorganic–organic hybrid materials: synthesis, characterization and photothiol-ene polymerization

Two isostructural mercapto-functionalized zirconium- and hafnium-oxoclusters [M12(μ3-O)8(μ3-OH)8(MP)24·n(MPA), MPA = HS–(CH2)2–C(O)OH; MP = HS–(CH2)2–C(O)O−; M= Zr, Hf; n = 4 for Zr, n = 5 for Hf] were prepared by reacting 3-mercaptopropionic acid with zirconium and hafnium butoxide, respectively, in an alkoxide : acid 1 : 7 molar ratio. The two oxoclusters Zr12 and Hf12, which are composed of two Zr6 and Hf6 sub-units bridged by four carboxylate ligands, were thoroughly characterized by single crystal X-ray diffraction, infrared spectroscopy (FT-IR), 1H and 13C nuclear magnetic resonance (NMR) spectroscopy, thermogravimetry (TGA), differential thermoanalysis (DTA) and extended X-ray absorption spectroscopy (EXAFS). The zirconium oxocluster was then embedded in a polymethacrylic matrix by a photothiol-ene polymerization reaction by using different cluster : monomer molar ratios. EXAFS measurements performed on the polymethacrylic-based hybrid materials showed that the structure of the oxocluster is retained even after being embedded in the polymer matrix. Solid-state 13C NMR spectroscopy as well as FT-IR spectroscopy demonstrated that the inorganic–organic hybrid materials are characterized by an almost complete polymerization of methacrylic acid, in which the oxoclusters are embedded in and covalently bound to the polymer matrix.

Photocatalytic performances of mesoporous TiO2 films doped with gold clusters

We report on the single-pot fabrication of ordered mesoporous crystallized titania films doped with gold. Au is incorporated in TiO2 films by adding to the coating solution precursors such as AuCl3 or monodisperse Au113+ nanoclusters, and Au nanoparticles are formed by calcination. A systematic study is performed to correlate structure, Au doping and photocatalytic activity of such films. Two-dimensional small angle X-ray scattering (2D-SAXS), transmission electronic microscopy (TEM), scanning electronic microscopy (SEM) and porosimetry–ellipsometry show that the films retain their mesoporous order even for doping levels as high as 1% Au : Ti atomic ratio. Wide angle X-ray scattering (WAXS), and cyclovoltammetry (CV) show that Au113+ nanoclusters promote the formation of the TiO2 (B) phase in competition with the anatase phase. AuCl3 stabilizes instead only the anatase phase. The highest photocatalytic activity is exhibited by films where Au113+ is employed as a precursor, which we attribute to the combination of the mixed anatase/TiO2 (B) phase, of Au nanoparticle doping and of a well-ordered mesoporous TiO2 matrix.

Sol-Gel and CVD Co3O4 Thin Films Characterized by XPS

The present investigation is focused on x-ray photoelectron spectroscopy (XPS) and x-ray excited Auger electron spectroscopy (XE-AES) analysis of the main core levels (O 1s, Co 2p, and Co LVV) of nanocrystalline Co3O4 coatings. The samples were obtained by CVD and sol-gel routes. Co(dpm)2 (Hdpm = 2,2-6,6-tetramethyl-3,5-heptanedione) was chosen as CVD precursor thanks to its appreciable volatility, the absence of direct Co–C bonds and the presence of thermally labile Co–O moieties. The preparation of the sol-gel films was accomplished starting from methanolic solutions of Co(OCOCH3)2⋅4H2O due to the clean conversion of cobalt acetate into cobalt oxides. The obtained Co3O4 films were bluish-brown, homogenous, crack-free, and adhered well to the substrates. The microstructural analyses revealed the formation of single-phase nanostructured layers with average crystallite dimensions ranging between 15 and 26 nm.