The research on this topic is mainly focussed on the synthesis of mono- and polymetallic oxides, sulphides, halogenides and hydroxides through sustainable wet chemistry and colloidal routes with the further aim of optimising the protocols to obtain the desired materials in crystalline form with high yields and purity and in the nanometric size range at the lowest temperature affordable[1, 2]. Additional desired features for the process are short treatment times, use of earth-abundant and safe chemicals and green solvents, as well as ease of implementation and up-scalability
One part of the activity was in the last three years focussed on the optimisation of novel low temperature wet chemistry synthesis protocols combining coprecipitation of oxalates (already used coupled with high temperature processing)[3-5] with hydrothermal processing conditions (see Fig. 1). Through this route the ferrite spinels CoFe2O4, MnFe2O4, NiFe2O4 and ZnFe2O4[6-9] were synthesised. These compounds are of particular interest due to their magnetic properties, as they all display soft ferrimagnetic behaviour (with the exception of CoFe2O4 which is a hard ferrimagnetic material).
The hydrothermal protocol allowed the nanocrystalline ferrites to be achieved at very low temperatures (as low as 75°C). The speed at which the reaction occurs, and the fact that the produced particles have sizes in the 10-50 nm range (see Fig. 2) suggest that both nucleation and growth processes occur very rapidly, with the former being predominant over the latter. Very recentely, we pointed out as formation of the crystalline materials occur very fastly, within 1-3 hours[10,11]
It should moreover be underlined that the developed one-pot route yields, in an easy and reproducible way, high amounts of the desired nanostructured ferrite, without need of time consuming separation and/or purification steps.
This route has also been successfully adapted to prepare doped zinc oxides ZnO:M and zinc sulphides ZnS:M (M=Cu, Mn, Dy, Tb, Mg, Eu, Tb) for applications in the field of photocatalytic degradation of organic compounds, as well as in the application of ZnO nanostructures for the sensing of H2S, showing outstanding performances also as "gas dosimeter". This further application of the protocol afforded nanocrystalline pure samples at temperatures as low as 135°C.
A final current implementation of this synthetic protocol targets the preparation of nanocrystalline manganites (MxMnyOz). Currently three systems have been successfully synthesised at 180°C: the credenerite CuMnO2 and the spinel compounds ZnMn2O4 and ZnMnO3. Though manganites in general are useful materials which find applications in several fields (lithium batteries, ReRAM components, fuel cells, energy storage, catalysis etc.) due to their versatile (magnetic, catalytic etc.) properties, this last species (i.e., ZnMnO3) is particularly interesting, as very few works exist in the literature, where it is addressed as a pure crystalline compound. These compounds are currently under investigation for photocatalytic applications in the field of water oxidation.
Recently, this hydrothermal approach has been used alto to prepare nanocrystalline NaYF4 and ZnO[14,12], as well as Ag2S and quaternary ferrites.[16-17]
Previous experimental activity in this workgroup, not featuring hydrothermal synthesis and focussed instead on nucleation and growth from optimised solutions, concerned the preparation of colloidal zinc oxide and copper sulphide. In the former case, zinc acetylacetonate was reacted with NaOH in different solvents (i.e., water, ethanol, propanediol, glycerol) yielding hexagonal ZnO nanostructures. The effect of synthetic parameters on the final product and on the growth process and resulting morphology, in particular solvent viscosity and dielectric constant, was extensively investigated.
As a further example, CuS nanoparticles were synthesised using carboxylic acid as a solvent (namely, a thioacetic acid/acetic acid mixture was employed) to react copper carboxylate precursors (acetates or propionates). The innovative combined use of thiocarboxylic acid (namely thioacetic acid) and carboxylic acids as solvents in the presence of water provided several advantages: it dispensed with the necessity to use potentially toxic sulphur sources (such as sulphides) and it promoted an enhanced nucleation rate, whilst hindering particle growth (thereby promoting the formation of small particles) due to solvent properties (acidity, viscosity, low dielectric constant). It is furthermore important to note that the choice of thiocarboxylic acid as sulphur sources contributed to the preservation of the aforementioned properties during the reaction since, as the reaction proceeded, the thiocarboxylic acid precursors were converted into carboxylic acid (i.e., solvent) thereby maintaining unaltered the solvent dielectric constant. By using the optimised suspension, CuS thin films were prepared through spray-coating on silica slides yielding monodisperse nanoparticles with an average diameter of 5 nm.
The research on this topic is mainly focussed on exploiting the colloidal method of miniemulsion for the synthesis, in confined space, of transition metals- and lanthanides-doped metal oxides[20-25], sulphides[26-29], halogenides and hydroxides, as well as complex oxides and nanocomposites (see Fig. 3), for potential applications in the field of optical bioimaging and catalysis.
Due to their characteristics (see Fig. 4), miniemulsions (30-500 nm droplet size) are perfectly suited to obtain monodispersed nanoparticles, with a good control on both size and morphology of the final material (nanostructure size: 10-200 nm). Additionally, the confinement induced by the miniemulsions strongly influences the crystallisation processes, for example allowing the obtainment, already at room temperature, of crystalline phases usually produced at high temperatures.
Crystallisation of the target nanostructures is accomplished by mixing two miniemulsion separately containing the precursors(two-miniemulsions approach), or by adding a precipitating agent to the pre-formed miniemulsion containing the metal precursor (diffusion approach). Both approaches allow the confinement of the precipitation within the droplets of the final miniemulsion, thus allowing control on size and morphology of the final material.
By exploiting the precipitation approach, it is also easy to endow the crystalline matrices with functional properties, by incorporating luminescent doping ions (in particular MnII, SmIII, EuIII, TbIII) in a quantitative way. The selected matrices are specifically chosen to maximise biocompatibility, so that the monodispersed, luminescent nanoparticles might be used as nanoprobes in bioimaging applications[21,26,28,30-31].
Alternatively, we have also pioneered the use of tailor-made single-source precursors which, upon UV irradiation, decomposed in the confined space of the droplets to yield a Au-TiO2 nanocomposite.
Part of the activity is also focussed on the surface engineering of these particles in order to enhance redispersibility in physiological media and biocompatibility. For more details see Surface Chemistry and Functionalisation
To better understand the crystallisation process in confined environment, we are currently developing a continuous-flow setup which allows us to follow in situ and in a time-resolved fashion the crystallisation of our target materials within the droplets produced by miniemulsion. Such experiments were performed at synchrotron facilities (Elettra sincrotrone, Swiss Light Source - Paul Scherrer Institute) with complementary techniques (SAXS/WAXS and XAS).
Moreover, our experimental investigations (inorganic synthesis and analytical investigations of crystallisation phenomena within miniemulsion droplets) are backed-up by theoretical modelling in collaboration with Prof. A. Polimeno. In particular, the aim of the collaboration is to rationalise and correlate mesoscopic properties at the fluid dynamic and thermodynamic levels and to understand the key parameters which determine the relative thermodynamic stability of different polymorphs in the confined conditions of miniemulsion droplets.
In order to accomplish a more sustainable and water-based synthesis of functional inorganic materials, we have recently developed a field of research concerning the microfluidic synthesis of inorganic nanocrystalline materials, in collaborations with Proff. M. Maggini and T. Carofiglio. In particular, we focussed on the room-temperature synthesis of very small and monodispersed pure and transition metal and lanthanide-doped zinc sulphide nanoparticles, without the use of any ligand and/or surfactant.[36-37]
Indeed, thanks to the unique conditions that are achieved in a microfluidic reactor, namely i) the highly efficient and fast mixing and ii) the flowing and dynamic nature of the system, a high control on the product's size and size distribution can be achieved.
In Fig. 5 a schematic representation of our microfluidic set-up is reported.
The focus of our research in this field is the study of metal oxides and sulphides surface chemistry in view of their stable functionalisation. This topic is investigated by means of a combined approach based on both theoretical and analytical techniques. Binary and ternary metal oxides and metal sulphides, also doped with transition metal and lanthanide ions, are synthetised by exploiting wet chemistry and colloidal routes, which are a relevant part of our synthetic activity (for further details see Low temperature hydrothermal synthesis of nanocrystalline transition metal oxides and sulphides in an aqueous environment and Inorganic chemistry in a nanoreactor: room temperature miniemulsion approach to crystalline inorganic nanostructures).
The aim of this part of the research activity is the investigation of physical and chemical surface properties of nanostructured zinc sulphide nanostructures, obtained with a simple hydrothermal route and taken as a model of metal sulphides. The synthetic method is explained in Low temperature hydrothermal synthesis of nanocrystalline transition metal oxides and sulphides in an aqueous environment. While many oxide model systems have been thoroughly studied, the surface properties of this material are more difficult to be unraveled due to practical constraints, inter alia, the tendency to oxidation and the limited thermal stability of the crystalline phase. This issue necessarily requires a synergy of different methods. Therefore a combined approach based on experimental characterisation and theoretical modelling is employed.
The crystalline structure is assessed by the X-ray diffraction technique (XRD) performed on powders, which is also employed to confirm the successful obtainment of the targeted compound. Spectroscopic FT-IR and Raman techniques, as well as thermal analysis TGA-DSC are employed to prove the presence and amount of both physi-/chemisorbed species and surface functional groups. The surface composition as well as the electronic valence band structure are assessed through X-ray photoelectron spectroscopy (XPS), using both a conventional instrument (equipped with Al K-α or Mg K-α sources) and synchrotron-based photoemission spectroscopies. In this latter case, the charging effects that afflicts the measurements of semiconducting or low conductive materials are addressed employing special synthetic routes and deposition techniques (i.e. dip-coating) aiming at efficiently coupling the analysed sample with a conducting support. The surface acidity and basicity of zinc sulphide nanoparticles are probed with the controlled adsorption of different small molecules followed by DRIFT spectroscopy (in collaboration with Dr. M.M. Natile).
The theoretical modelling activity is aimed at obtaining a detailed description of the surface structure and reactivity, combining the results from Density Functional Theory (DFT) calculations with information gained from the previously described experimental work. The surface is modeled using asymmetrical slabs, while a plane-wave pseudopotential implementation of the DFT in periodic boundaries conditions is used. We want to focus on the mechanisms of chemisorption of model ligands as this represents a crucial point to explain the observed characteristics of passivation and selectivity of the material.
ZnO and ZnS are II-VI semiconductor materials and their electronic features, such as wide band gap and high exciton binding energies, make them suitable for applications, inter alia, in optoelectronics, photonics and development of optical devices. Moreover, they can be endowed with luminescence, are easily prepared by wet chemistry, environmental friendly and not-cytotoxic, thus disclosing also possible applications in biomedicine, in particular in the field of inorganic optical bioimaging. In this view, their doping with luminescent transition metal or lanthanide ions has also been pursued [27-30].
In order to exploit ZnO and ZnS in bio-oriented applications, i) long-term dispersability in water and physiological environment, as well as ii) their functionalisation with moieties enabling molecular recognition with targeted structures, are required (see Figure 6). To achieve this goal, we have already successfully functionalised zinc oxide with different kind of cathecolic anchors, attached on different structural motifs, in one case adamantane-derivatives (in collaboration with Prof. Wolfgang Maison, Universität Hamburg, Germany) as well as polyoxazolines (in collaboration with Dr. Edmondo M. Benetti from ETH, Zürich).
Neverthless, the most challenging implementation concerns the functionalisation of zinc sulphide, which we are now trying to derivatise with biomimetic dopamine-based polymers (poly-methyloxazolines, see Fig.7), in collaboration with Dr. Edmondo M. Benetti from ETH, Zürich.
Typically, characterisation of naked and functionalised nanostructures involves X-Ray Diffraction (XRD), Selected Area Electron Diffraction (SAED), X-Ray Photoelectron Spectroscopy and ICP-MS for the structural and compositional investigation, Transmission Electron Microscopy and Scanning Electron Microscopy for assessing the morphology features and Dynamic Light Scattering in order to check the stability of functionalised nanostructures in aqueous media. The cytotoxicity behavior of the functionalised nanostructures is investigated in cooperation with Dr. M. Mognato of the Department of Biology, University of Padova.
Disclosing the fine details of the chemo-physical and structural properties of inorganic nanostructures is a necessary requirement also to optimise their synthesis. This is actually a challenging task, and a multi-technique approach is necessary in order to gain a complete picture of the studied systems. We have extensively used a combined spectroscopic and diffraction approach to unravel the microstructural and chemical composition of inorganic nanostructures, also in cooperation with the group of Prof. B. Smarsly, Justus Liebig Universität Gießen[45, 46]
In this framework, the structural features are uncovered by integrating powder X-ray diffraction and microRaman evidence with data collected with synchrotron-based techniques, in particular X-ray absorption spectroscopy. Such an approach has proven, for example, very useful in the investigation of doped materials, since it enables a characterisation of both long- (XRD) and short-range (XAS) order[47, 48]. XPS and XAS have instead proven to be highly complementary for the investigation of chemical environment and oxidation state of the constituting atoms.
A relevant part of the experimental activity is also devoted to unravel i. the mechanisms of hydrolysis and condensation involved in the sol-gel process[49-51] as well as ii. the nucleation, growth and crystal formation of inorganic nanostructures from optimised colloidal suspensions by using different X-ray based spectroscopic and scattering methods, also in time-resolved fashion. This work is prevalently carried out at different European synchrotron radiation facilities (ESRF, Elettra, Soleil, ANKA, DESY, PSI), where the Group members regularly get beamtimes to perform these experiments.
Part of the research activity is also devoted to the study of the surfaces of inorganic materials, in particular by means of X-ray photoelectron spectroscopy, using both conventional and synchrotron-based photoemission instrumentation. The experimental evidences have also been successfully combined with the results from Density Functional Theory (DFT), modelling the surface structure and reactivity. For more details see Surface chemistry and functionalisation.