Electrochemical exfoliation of graphite in H2SO4, Li2SO4 and NaClO4 solutions monitored in-situ by Raman microscopy and spectroscopy

The electrochemical exfoliation of graphite is one of the cheapest and most tunable industrial techniques to produce graphene nanosheets with tunable degree of oxidation and solubility. Anodic oxidation allows high-yield production of electrochemically exfoliated graphene oxide (EGO) in either acids or salt solutions, with the key role played by ions electrochemically driven in between the graphene sheets. This chemical intercalation is followed by a mesoscale mechanical exfoliation process, which is key for the high yield of the process, but which is still poorly understood. In this work, we use Raman spectroscopy to simultaneously monitor the intercalation and oxidation processes taking place on the surface of highly ordered pyrolytic graphite (HOPG) during electrochemical exfoliation. The mechanism of EGO formation in either acidic (0.5 M H2SO4) or neutral (0.5 M Li2SO4) electrolytes through blistering and cracking steps is discussed and described. This process is compared also to non-destructive intercalation of graphite in an organic electrolyte (1 M NaClO4 in acetonitrile). The results obtained show how high exfoliation yield and low defectivity shall be achieved by the combination of efficient, non-destructive intercalation followed by chemical decomposition of the intercalants and gas production

Pyrolytic Formation of TiO2/Carbon Nanocomposite from Kraft Lignin: Characterization and Photoactivities

This article reports on the formation of pyrolytic carbon/TiO2 nanocomposite (p-C/TiO2) by pyrolysis of a mixture of the P25 TiO2 and kraft lignin at 600 °C. The result was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, UV-visible spectroscopy, electron paramagnetic resonance spectrometry (EPR), thermogravimetry (TGA) and SEM microscopy. Its photocatalytic activity was ascertained using three classes of chemical probes, namely (i) degradation of methylene blue (MB) and rhodamine-B (RhB) dyes in UV light-irradiated aqueous suspensions, (ii) depletion of phenol and (iii) degradation of antibiotics. The p-C/TiO2 nanocomposite is a strong phisisorbent of both MB and RhB nearly twofold with respect to neat TiO2. Although it is nearly twofold more photoactive toward the degradation of MB (0.091 min−1 versus 0.047 min−1), it is not with regard to RhB degradation (0.064 min−1 versus 0.060 min−1). For the degradation of phenol in aqueous media (pH 3), pristine TiO2 was far more effective than p-C/TiO2 for oxygenated suspensions (17.6 × 10−3 mM min−1 versus 4.3 × 10−3 mM min−1). Under an argon atmosphere, the kinetics were otherwise identical. The activity of the material was tested also for a real application in the degradation of a fluoroquinolone antibiotic such as enrofloxacin (ENR) in tap water. It is evident that the photoactivity of a semiconductor photocatalyst is not a constant, but it does depend on the nature of the substrate used and on the experimental conditions. It is also argued that the use of dyes to assess photocatalytic activities when suspensions are subjected to visible light irradiation is to be discouraged as the dyes act as electron transfer photosensitizers and or can undergo photodegradation from their excited states

CVD-graphene/graphene flakes dual-films as advanced DSSC counter electrodes

The use of graphene-based electrodes is burgeoning in a wide range of applications, including solar cells, light emitting diodes, touch screens, field-effect transistors, photodetectors, sensors and energy storage systems. The success of such electrodes strongly depends on the implementation of effective production and processing methods for graphene. In this work, we take advantage of two different graphene production methods to design an advanced, conductive oxide- and platinum-free, graphene-based counter electrode for dye-sensitized solar cells (DSSCs). In particular, we exploit the combination of a graphene film, produced by chemical vapor deposition (CVD) (CVD-graphene), with few-layer graphene (FLG) flakes, produced by liquid phase exfoliation. The CVD-graphene is used as charge collector, while the FLG flakes, deposited atop by spray coating, act as catalyst for the reduction of the electrolyte redox couple (i.e., I3-/I-- and Co+2/+3). The as-produced counter electrodes are tested in both I3-/I-- and Co+2/+3-based semitransparent DSSCs, showing power conversion efficiencies of 2.1% and 5.09%, respectively, under 1 SUN illumination. At 0.1 SUN, Co+2/+3-based DSSCs achieve a power conversion efficiency as high as 6.87%. Our results demonstrate that the electrical, optical, chemical and catalytic properties of graphene-based dual films, designed by combining CVD-graphene and FLG flakes, are effective alternatives to FTO/Pt counter electrodes for DSSCs for both outdoor and indoor applications

Optical and transport experiments in nano-structures with semiconductor superlattices

I presented the recent results on the RRS, PLE, and PL in ordered and intentionally disordered semiconductor SL’s in the presence of an external magnetic field. From the RRS and PLE spectra, we calculated and compared the behaviours of the energy dispersions of the dephasing times, relating them to the ordered and disordered nature of the SL’s. We also studied the behaviour of the emission energy and linewidth with the magnetic field, comparing the results with the available results obtained on QW systems. As regards the dependence of the PL linewidth on the field, we found distinct difference in the evolution of the PL width between the ordered and disordered SL’s. While in the ordered SL the PL linewidth increases monotonously with the field, in the disordered SL the evolution is not monotonous and this behaviour can be correlated to the interplay between localization length and exciton extension in the z direction along which the intentional disorder is present

Influenceof Ion Diffusionon the Lithium−Oxygen Electrochemical Process and Battery Application Using Carbon Nanotubes−Graphene Substrate

Lithium-oxygen (Li-O2) battery is nowadays among the most appealing next-generation energy storage systems in view of a high theoretical capacity and the use of transition-metal-free cathodes. Nevertheless, the practical application of this battery is still hindered by limited understanding of the relationships between cell components and performances. In this work, we investigate a Li-O2 battery by originally screening different gas-diffusion layers (GDLs) characterized by low specific surface area (<40 m2 g-1) with relatively large pores (absence of micropores), graphitic character, and the presence of a fraction of hydrophobic PTFE polymer on their surface (<20 wt.%). The electrochemical characterization of Li-O2 cells using bare GDLs as the support indicates that the oxygen reduction reaction (ORR) occurs at potentials below 2.8 V vs. Li+/Li, while the oxygen evolution reaction (OER) takes place at potentials higher than 3.6 V vs. Li+/Li. Furthermore, the relatively high impedance of the Li-O2 cells at the pristine state remarkably decreases upon electrochemical activation achieved by voltammetry. The Li-O2 cells deliver high reversible capacities ranging from ~6 mAh cm-2 to ~8 mAh cm-2 (referred to the geometric area of the GDLs). The Li-O2 battery performances are rationalized by the investigation of a practical Li+ diffusion coefficient (D) within the cell configuration adopted herein. The study reveals that D is higher during ORR than during OER, with values depending on the characteristics of the GDL and on the cell state of charge (SOC). Overall, D values range from ~10-10 to ~10-8 cm2 s-1 during the ORR, and ~10-17 to ~10-11 cm2 s-1 during the OER. The most performing GDL is used as substrate for the deposition of few-layer graphene (FLG) and multiwalled carbon nanotubes (MWCNTs) to improve the reaction kinetics, leading to a Li-O2 cell operating with a maximum specific capacity of 1250 mAh g-1 (1 mAh cm-2) at current density of 0.33 mA cm-2. XPS on electrode tested in our Li-O2 cell setup suggests the formation of a stable solid electrolyte interphase (SEI) at the surface which extends the cycle life

Origin of the blue fluorescence in Dominican amber

We report on optical absorption, fluorescence, and time-resolved fluorescence measurements in Dominican ambers. The "blue" variety reveals an intense fluorescence emission in the visible wavelength region, between 430 and 530 nm, with spectral features typical of aromatic hydrocarbons. On the contrary, the "red" and "yellow" varieties have a much weaker and featureless emission. The data for blue amber, including fluorescence lifetime, allow the identification of perylene as the compound responsible for its distinctive showy fluorescence

Optical properties of Cd(1-x)Mn(x)Te/Cd(1-y)Mn(y)Te superlattices with high difference of Mn concentration between wells and barriers

We studied the optical properties of Cd(1–x)Mn(x)Te/Cd(1–y)Mn(y)Te semiconductor superlattices with medium (x=0.3,y=0.01) and high (x=0.8,y=0) difference of the Mn concentration between wells and barriers, by means of photoluminescence and photoreflectance spectroscopy. Photoluminescence allows us to study the emission due to the fundamental heavy (H) hole 11H interminiband excitonic transition and evidences the emission characteristics. Photoreflectance reveals several heavy and light (L) holes interminiband excitonic transitions, up to the 33H. The experiments are in good agreement with the theory with an envelope-function approximation approach, taking into account strain effects due to lattice mismatch between wells and barriers. The combined use of photoluminescence and photoreflectance gives a complete information on the electronic configuration of these Cd(1–x)Mn(x)Te/Cd(1–y)Mn(y)Te superlattices that can find specific applications in spintronic devices