Monica Distaso

Facile synthesis and post-processing of eco-friendly, highly conductive copper zinc tin sulphide nanoparticles

Cu2ZnSnS4 (CZTS) nanoparticles have shown promising properties to be used as an energy harvesting material. They are usually synthesised under inert atmosphere or vacuum, whereas the subsequent step of film formation is carried out under an atmosphere of sulphur and/or Sn in order to avoid the decomposition of CZTS nanoparticles into binary and ternary species as well as the formation of the corresponding oxides. In the present paper we show that both the synthesis of CZTS nanoparticles and the film formation from the corresponding suspension can be considerably simplified. Namely, the synthesis can be carried out without controlling the atmosphere, whereas during the film annealing a nitrogen atmosphere is sufficient to avoid the depletion of the CZTS kesterite phase. Furthermore, an integrated approach including in-depth Raman analysis is developed in order to deal with the challenges associated with the characterization of CZTS nanoparticles in comparison to bulk systems. The formation of competitive compounds during the synthesis such as binary and ternary sulphides as well as metal oxides nanoparticles is discussed in detail. Finally, the as-produced films have ten times higher conductivity than the state of the art.

Influence of the Counterion on the Synthesis of ZnO Mesocrystals under Solvothermal Conditions

Polymers and coordinating solvents have been shown to serve as templating agents to assist the precipitation of ZnO nanoparticles and address their morphology. In this work we show for the first time that a difference in the coordination strength between the polymer (poly-N-vinylpyrrolidone (PVP)) and the two Zn(II) precursor salts (nitrate and acetate) is able to promote or suppress the formation of mesocrystalline structures and even more importantly to tune their three-dimensional organization. On the basis of FTIR and (13)C NMR spectroscopic studies, we propose that not only the polymer (PVP) but also the solvent (DMF) play a key role as directing agents.

Gallium-rich Pd–Ga phases as supported liquid metal catalysts

A strategy to develop improved catalysts is to create systems that merge the advantages of heterogeneous and molecular catalysis. One such system involves supported liquid-phase catalysts, which feature a molecularly defined, catalytically active liquid film/droplet layer adsorbed on a porous solid support. In the past decade, this concept has also been extended to supported ionic liquid-phase catalysts. Here we develop this idea further and describe supported catalytically active liquid metal solutions (SCALMS). We report a liquid mixture of gallium and palladium deposited on porous glass that forms an active catalyst for alkane dehydrogenation that is resistant to coke formation and is thus highly stable. X-ray diffraction and X-ray photoelectron spectroscopy, supported by theoretical calculations, confirm the liquid state of the catalytic phase under the reaction conditions. Unlike traditional heterogeneous catalysts, the supported liquid metal reported here is highly dynamic and catalysis does not proceed at the surface of the metal nanoparticles, but presumably at homogeneously distributed metal atoms at the surface of a liquid metallic phase.

A comprehensive study on the mechanism behind formation and depletion of Cu2ZnSnS4 (CZTS) phases

High efficiency kesterite based solar cells have vigorously raised the research interests in this material. The challenge lies in understanding the formation and co-existence of more than 10 possible by-products during and after the synthesis of Cu2ZnSnS4 (CZTS) and their various different structural and electronic defects. The present contribution shows an in-depth study on the stages of formation and depletion of nanoparticulate CZTS. Employing a hot injection synthesis method, we give direct experimental evidence of the co-existence of cubic, tetragonal and defected CZTS structures and different by-products as a function of time and temperature. SEM, (HR)TEM, XRD, EDX, ICP-OES, Raman spectroscopy and UV-Vis-NIR spectroscopy have been used in order to better evaluate and interpret data for crystal structures and compositions. The obtained understanding on the formation of different phases suggests 250 °C as the most favourable synthesis temperature. Based on our study, general strategies can be developed for controlling the amount of formed phases, the by-products and the defects in kesterite and other similar multicomponent nanoparticles as well as in bulk systems.

Early Stages of Oriented Attachment: Formation of Twin ZnO Nanorods under Microwave Irradiation

Nonclassical crystallization represents a unique tool to fabricate ordered superstructures. The underlying oriented attachment mechanism (OAM) implies the spontaneous assembly of building blocks with common crystallographic orientations. The driving force is the reduction of the surface free energy by eliminating the high-energy facets, but the detailed mechanisms are still not clear. In the framework of our interest for the synthesis of complex nanostructures, we have recently reported that ZnO mesocrystals can be synthesized under solvothermal conditions in the presence of poly-N-vinyl-pyrrolidone (PVP), and that the polymer matrix plays a key role as directing agent during the synthesis. ZnO in wurtzite phase is constructed by a number of alternating planes stacked along c axis direction giving a Zn (0001) and O (000 1) terminated faces. ZnO is able to form twin structures, that is, crystals in which facets with the same polarity are coupled to form a single crystal with various morphologies. According to the state of the art, the twinning process can be promoted by inorganic or polymeric species adsorbed onto the coupling interfaces. Bioinspired synthesis provide an additional tool for studying the nature of the interfaces and forces involved during oriented attachment. Herein, we showed that in water and under microwave (MW) irradiation, ZnO nanorods with a characteristic seam cut perpendicular to the elongation direction form (Figure 1). Scanning electron microscopy (SEM) analysis of the as-obtained particles showed that the sample consists of rods with hexagonal cross-section with a length between 320 nm and 2.5 mm and a width of 71–367 nm with an aspect-ratio (AR) distribution between two and ten (Figure 1). Transmission electron microscopy (TEM) analysis and selected-area electron diffraction (SAED) of the sample isolated after 16 min showed that the interface between two ZnO rod parts is single crystalline and that the growth took place through c axis ([0001] direction; Figure 2a and corresponding inset). XRD evidenced that the final particles crystallized in wurtzite phase (Figure 2b), as was confirmed by SAED analysis (Figure 2a). Kinetic measurements have been carried out with the aim to highlight the formation mechanism through the isolation of intermediate species. Thus, the reaction was repeated and stopped after 45 seconds, one, and two minutes, when the temperature inside the reaction mixture was 45, 58, and 153 8C, respectively. After 45 seconds, no precipitate formed, [a] Dr. M. Distaso, Prof. W. Peukert Institute of Particle Technology FAU Erlangen N rnberg, Cauerstrasse 4 91058 Erlangen (Germany) Fax: (+49)913-185-29401 E-mail : M.Distaso@lfg.uni-erlangen.de [b] Dr. M. Mackovic, Prof. E. Spiecker Centre for Nanoanalysis and Electron Microscopy (CENEM) Department of Materials Science and Engineering FAU Erlangen N rnberg Cauerstrasse 6, 91058 Erlangen (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201201646. Figure 1. a) SEM image of ZnO nanorods after 16 min of irradiation under MW; b) AR distribution of particles, in which q0= [D(AR) in interval between ARi and ARi+1]/[total amount of rods interval width (ARi+1 ARi)].

ZnO superstructures via oriented aggregation initiated in a block copolymer melt

A fast and simple one pot synthesis of ZnO nano- and microparticles initiated and driven by an amino block copolymer O,O′-bis(2-aminopropyl)polypropylene glycol-block-polyethylene glycol-block-polypropylene glycol (Jeffamine®) is reported. The specific building mechanism of ZnO mesocrystals is investigated in detail using electron microscopy and diffraction methods. Mesocrystals with a complex superstructure are formed as a result of a consecutive and oriented multiple stage aggregation process: first a 0D → 1D aggregation process is observed, then a 1D → 3D aggregation process occurs in which secondary particles form cones and multiple cone symmetries. Dots, rods, cones, and multiple cones have been isolated within a time resolved study which clearly supports the growth model. To control the morphology of the product particles, the influence of relevant synthesis parameters including stirring and sonication of the intermediate were investigated. Extensive surface characterization of the resulting mesocrystals is presented using infrared and photoluminescence spectroscopies as well as thermogravimetric analysis. Even after multiple washing steps, the particles exhibit a Jeffamine® coated surface that allows for easy dispersion in both polar and nonpolar solvents. The obtained mesocrystals efficiently scatter in the whole range of visible light.

Shedding light on the effective fluorophore structure of high fluorescence quantum yield carbon nanodots

Carbon nanodots (CD) have great potential for imaging and sensing applications, due to their unique luminescence properties in combination with their low toxicity. Although CDs are currently the subject of intensive research activity, the exact structure of their effective fluorophore species has to be clarified because it can depend on the starting materials and reaction conditions. Most importantly, the limited photostability of some CDs presents an impediment to the much-needed breakthroughs required in the field. In this work, the photobleaching of CDs synthesized hydrothermally from citric acid and urea under various conditions is studied in detail. By visualizing changes in the absorption and fluorescence properties of CDs over time upon exposure to UV-light by means of in situ spectroscopic probes, we demonstrate a trade-off between high initial quantum yield and photostability. We found that the CDs consist of low-molecular weight fluorophores bound to π-conjugated domains (carbon core). In the case of CDs synthesized at higher reaction temperatures and/or for longer reaction times, lower initial quantum yields and higher photostability originate from partial conversion of the low-molecular weight fluorophores to π-conjugated domains during synthesis. Thus, the remaining surface bound fluorophores are bound to larger π-conjugated domains, which, in turn, enable dissipation of absorbed UV-energy and protection against photochemical damage. Reference experiments were performed with citrazinic acid – a pyridone-based structure – using ultrafast time-resolved spectroscopy, steady-state spectroscopy, and theoretical DFT simulations. We conclude that pyridone-like structures are most likely responsible for both the high quantum yields and the photobleaching of CDs.

Formation and Dissolution of Twin ZnO Nanostructures Promoted by Water and Control over Their Emitting Properties

By using ZnO as a model system, the formation of twinned nanostructures has been investigated under microwave irradiation, exploiting experimental conditions ranging from purely solvothermal when N,N-dimethylformamide was used, to purely hydrothermal when water was the solvent. A progressive increase in size, elongation and roughness of the surface was observed with increasing water content in the solvent mixture. Particular attention was paid to the reactivity of the ZnO surfaces towards dissolution. Our results show that the formation of twinned nanorods is a dynamic process and that the coupling interphase itself is highly reactive. Consequently, the twinned rods undergo a number of complex dissolution processes that are responsible for the appearance of a wide distribution of defects either on the surface or inside the structure. Poly(N-vinyl pyrrolidone) influences the photoluminescent properties of the as-synthesised materials and allows control of the ratio of the intensity of the UV and visible emission.

The effects of post-processing on the surface and the optical properties of copper indium sulfide quantum dots

In the current contribution we report on investigations regarding the surface of CuInS2 quantum dots and on different strategies to control the amount of surface ligands in a post-processing step. In particular, the reactivity of the organic components, that is, 1-dodecanthiol and 1-octadecene as ligand and solvent, respectively, during nanocrystal formation was studied. A new method to remove residuals from the reaction mixture and to detach excess organics from the surface of the nanocrystals is reported. Our new method, which is based on the utilization of acids, is compared with standard purification procedures by means of thermogravimetric analysis (TGA) with particular focus on its efficiency to remove organics. As a complement, the surface chemistry is analyzed by nuclear magnetic resonance spectroscopy (NMR) to shed light on the nature of the organic components still present after purification. Further analysis of the product by inductively coupled plasma optical emission spectroscopy (ICP-OES) is performed to verify the influence of the new purification method on surface composition and properties. Moreover, steady state and time resolved spectroscopies give insights into excitonic behavior as well as recombination processes. Finally, the new method is optimized for the purification of CuInS2-ZnS nanocrystals, which show enhanced optical properties.

Selective Aerobic Oxidation of 5‐Hydroxymethylfurfural to 2,5‐Diformylfuran or 2‐Formyl‐5‐furancarboxylic Acid in Water by using MgO⋅CeO2 Mixed Oxides as Catalysts

Mixed oxides based on MgO⋅CeO2 were used as efficient catalysts in the aerobic oxidation of 5-hydroxymethylfurfural (5-HMF) to afford, with very high selectivity, either 2,5-diformylfuran (DFF, 99 %) or 2-formyl-5-furancarboxylic acid (FFCA, 90 %), depending on the reaction conditions. 5-Hydroxymethyl-2-furancarboxylic acid (HMFCA, 57-90 %) was formed only at low concentration of 5-HMF (<0.03 m) or in presence of external bases. The conversion of 5-HMF ranged from a few percent to 99 %, according to the reaction conditions. The oxidation was performed in water, with O2 as oxidant, without any additives. The surface characterization of the catalysts gave important information about their acid-base properties, which drive the selectivity of the reaction towards DFF. FFCA was formed from DFF at longer reaction times. Catalysts were studied by XPS and XRD before and after catalytic runs to identify the reason why they undergo reversible deactivation. XRD showed that MgO is hydrated to Mg(OH)2 , which, even if not leached out, changes the basic properties of the catalyst that becomes less active after some time. Calcination of the recovered catalyst allows recovery of its initial activity. The catalyst is thus recoverable (>99 %) and reusable. The use of mixed oxides allows tuning of the basicity of the catalysts, avoiding the need for external bases for efficient and selective conversion of 5-HMF and waste formation, resulting in an environmentally friendly, sustainable process.