Raffaello Mazzaro

A supramolecular strategy to leverage the liquid-phase exfoliation of graphene in the presence of surfactants: unraveling the role of the length of fatty acids.

Achieving the full control over the production as well as processability of high-quality graphene represents a major challenge with potential interest in the field of fabrication of multifunctional devices. The outstanding effort dedicated to tackle this challenge in the last decade revealed that certain organic molecules are capable of leveraging the exfoliation of graphite with different efficiencies. Here, a fundamental understanding on a straightforward supramolecular approach for producing homogenous dispersions of unfunctionalized and non-oxidized graphene nanosheets in four different solvents is attained, namely N-methyl-2-pyrrolidinone, N,N-dimethylformamide, ortho-dichlorobenzene, and 1,2,4-trichlorobenzene. In particular, a comparative study on the liquid-phase exfoliation of graphene in the presence of linear alkanes of different lengths terminated by a carboxylic-acid head group is performed. These molecules act as graphene dispersion-stabilizing agents during the exfoliation process. The efficiency of the exfoliation in terms of concentration of exfoliated graphene is found to be proportional to the length of the employed fatty acid. Importantly, a high percentage of single-layer graphene flakes is revealed by high-resolution transmission electron microscopy and Raman spectroscopy analyses. A simple yet effective thermodynamic model is developed to interpret the chain-length dependence of the exfoliation yield. This approach relying on the synergistic effect of a ad-hoc solvent and molecules to promote the exfoliation of graphene in liquid media represents a promising and modular strategy towards the rational design of improved dispersion-stabilizing agents.

Light-Enhanced Liquid-Phase Exfoliation and Current Photoswitching in Graphene-Azobenzene Composites

Multifunctional materials can be engineered by combining multiple chemical components, each conferring a well-defined function to the ensemble. Graphene is at the centre of an ever-growing research effort due to its combination of unique properties. Here we show that the large conformational change associated with the trans–cis photochemical isomerization of alkyl-substituted azobenzenes can be used to improve the efficiency of liquid-phase exfoliation of graphite, with the photochromic molecules acting as dispersion-stabilizing agents. We also demonstrate reversible photo-modulated current in two-terminal devices based on graphene–azobenzene composites. We assign this tuneable electrical characteristics to the intercalation of the azobenzene between adjacent graphene layers and the resulting increase in the interlayer distance on (photo)switching from the linear trans-form to the bulky cis-form of the photochromes. These findings pave the way to the development of new optically controlled memories for light-assisted programming and high-sensitive photosensors.

Liquid-Phase Exfoliation of Graphite into Single- and Few-Layer Graphene with α-Functionalized Alkanes

Graphene has unique physical and chemical properties, making it appealing for a number of applications in optoelectronics, sensing, photonics, composites, and smart coatings, just to cite a few. These require the development of production processes that are inexpensive and up-scalable. These criteria are met in liquid-phase exfoliation (LPE), a technique that can be enhanced when specific organic molecules are used. Here we report the exfoliation of graphite in N-methyl-2-pyrrolidinone, in the presence of heneicosane linear alkanes terminated with different head groups. These molecules act as stabilizing agents during exfoliation. The efficiency of the exfoliation in terms of the concentration of exfoliated single- and few-layer graphene flakes depends on the functional head group determining the strength of the molecular dimerization through dipole-dipole interactions. A thermodynamic analysis is carried out to interpret the impact of the termination group of the alkyl chain on the exfoliation yield. This combines molecular dynamics and molecular mechanics to rationalize the role of functionalized alkanes in the dispersion and stabilization process, which is ultimately attributed to a synergistic effect of the interactions between the molecules, graphene, and the solvent.

Photoactive Dendrimer for Water Photoreduction: A Scaffold to Combine Sensitizers and Catalysts

We report on the synthesis and characterization of platinum nanoparticles (PtNps) inside the cavities of a PAMAM dendrimer decorated with [Ru(bpy)3](2+) units at the periphery. The phosphorescent ruthenium complexes are used as signaling units of the Pt(2+) complexation in the dendritic architecture and as photosensitizer units in the photocatalytic production of H2 from water. This is the first example of water photoreduction in which the catalyst and the sensitizer are anchored on a dendritic molecular scaffold. This study provides a new outlook in the design of new supramolecular systems and materials for developing artificial photosynthesis.

Photophysical and structural characterisation of in situ formed quantum dots

Conjugated polymer-semiconductor quantum dot (QD) composites are attracting increasing attention due to the complementary properties of the two classes of materials. We report a convenient method for in situ formation of QDs, and explore the conditions required for light emission of nanocomposite blends. In particular we explore the properties of nanocomposites of the blue emitting polymer poly[9,9-bis(3,5-di-tert-butylphenyl)-9H-fluorene] together with cadmium sulphide (CdS) and cadmium selenide (CdSe) precursors. We show the formation of emissive quantum dots of CdSe from thermally decomposed precursor. The dots are formed inside the polymer matrix and have a photoluminescence quantum yield of 7.5%. Our results show the importance of appropriate energy level alignment, and are relevant to the application of organic-inorganic systems in optoelectronic devices.

Synthesis and properties of ZnTe and ZnTe/ZnS core/shell semiconductor nanocrystals

We report the synthesis of spherical ZnTe nanocrystals and the successive coating with a ZnS shell to afford core/shell quantum dots. These nanocrystals can represent alternatives to cadmium-based quantum dots but their preparation and properties are challenging and relatively unexplored. The effect of various synthetic parameters on the reaction outcome was investigated, and the resulting nanocrystals were characterized by TEM, EDX, XPS, and spectroscopic measurements. The optical data indicate that these core/shell quantum dots belong to type I, i.e., both the electron and the hole are confined within the ZnTe core. Both the ZnTe core and ZnTe/ZnS core/shell quantum dot samples absorb in the visible region and are not luminescent. The ZnS shell preserves the optical properties of the core and improves the chemical and photochemical stability of the nanoparticles in air equilibrated solution, whereas they appear to be quite fragile in the solid state. XPS results have evidenced the distinct nature of core and core/shell QDs, confirming the formation of QDs with shells of different thicknesses and their evolution due to oxidation upon air exposure. Anodic photocurrent generation was observed when an ITO electrode functionalized with ZnTe/ZnS nanocrystals was irradiated in the visible region in a photoelectrochemical cell, indicating that the quantum dots perform spectral sensitization of the electron injection into the ITO electrode. Conversely, cathodic photocurrent generation was not observed; hence, the QD-modified electrode performs electrical rectification under a photon energy input.

Poly(3-hexylthiophene) Nanoparticles Containing Thiophene-S,S-dioxide: Tuning of Dimensions, Optical and Redox Properties, and Charge Separation under Illumination

We describe the preparation of poly(3-hexylthiophene-S,S-dioxide) nanoparticles using Rozens reagent, HOF·CH3CN, either on poly(3-hexylthiophene) (P3HT) or on preformed P3HT nanoparticles (P3HT-NPs). In the latter case, core-shell nanoparticles (P3HT@PTDO-NPs) are formed, as confirmed by X-ray photoelectron spectroscopy measurements, indicating the presence of oxygen on the outer shell. The different preparation modalities lead to a fine-tuning of the chemical-physical properties of the nanoparticles. We show that absorption and photoluminescence features, electrochemical properties, size, and stability of colloidal solutions can be finely modulated by controlling the amount of oxygen present. Atomic force microscopy measurements on the nanoparticles obtained by a nanoprecipitation method from preoxidized P3HT (PTDO-NPs) display spherical morphology and dimensions down to 5 nm. Finally, Kelvin probe measurements show that the coexistence of p- and n-type charge carriers in all types of oxygenated nanoparticles makes them capable of generating and separating charge under illumination. Furthermore, in core-shell nanoparticles, the nanosegregation of the two materials, in different regions of the nanoparticles, allows a more efficient charge separation.

Uniform Functionalization of High-Quality Graphene with Platinum Nanoparticles for Electrocatalytic Water Reduction.

Graphene–metal composites have potential as novel catalysts due to their unique electrical properties. Here, we report the synthesis of a composite material comprised of monodispersed platinum nanoparticles on high-quality graphene obtained by using two different exfoliation techniques. The material, prepared via an easy, low-cost and reproducible procedure, was evaluated as an electrocatalyst for the hydrogen evolution reaction. The turnover frequency at zero overpotential (TOF0 in 0.1 m phosphate buffer, pH 6.8) was determined to be approximately 4600 h−1. This remarkably high value is likely due to the optimal dispersion of the platinum nanoparticles on the graphene substrate, which enables the material to be loaded with only very small amounts of the noble metal (i.e., Pt) despite the very highly active surface. This study provides a new outlook on the design of novel materials for the development of robust and scalable water-splitting devices.

Controlling the functional properties of oligothiophene crystalline nano/microfibers via tailoring of the self-assembling molecular precursors

Oligothiophenes are π‐conjugated semiconducting and fluorescent molecules whose self‐assembly properties are widely investigated for application in organic electronics, optoelectronics, biophotonics, and sensing. Here an approach to the preparation of crystalline oligothiophene nano/microfibers is reported based on the use of a “sulfur overrich” quaterthiophene building block, T4S4 , containing in its covalent network all the information needed to promote the directional, π–π stacking‐driven, self‐assembly of Y‐T4S4‐Y oligomers into fibers with hierarchical supramolecular arrangement from nano‐ to microscale. It is shown that when Y varies from unsubstituted thiophene to thiophene substituted with electron‐withdrawing groups, a wide redistribution of the molecular electronic charge takes place without substantially affecting the aggregation modalities of the oligomer. In this way, a structurally comparable series of fibers is obtained having progressively varying optical properties, redox potentials, photoconductivity, and type of prevailing charge carriers (from p‐ to n‐type). With the aid of density functional theory (DFT) calculations, combined with powder X‐ray diffraction data, a model accounting for the growth of the fibers from molecular to nano‐ and microscale is proposed.