Reduced graphene oxide and Ag wrapped TiO2 photocatalyst for enhanced visible light photocatalysis

A well-organised reduced graphene oxide (RGO) and silver (Ag) wrapped TiO2 nano-hybrid was successfully achieved through a facile and easy route. The inherent characteristics of the synthesized RGO-Ag/TiO2 were revealed through crystalline phase, morphology, chemical composition, Raman scattering, UV-visible absorption, and photoluminescence analyses. The adopted synthesis route significantly controlled the uniform formation of silver nanoparticles and contributed for the absorption of light in the visible spectrum through localized surface plasmon resonance effects. The wrapped RGO nanosheets triggered the electron mobility and promoted visible light shift towards red spectrum. The accomplishment of synergised effect of RGO and Ag well degraded Bisphenol A under visible light irradiation with a removal efficiency of 61.9%.

Reduced graphene oxide wrapped Cu2O supported on C3N4: An efficient visible light responsive semiconductor photocatalyst

Herein, Cu 2O spheres were prepared and encapsulated with reduced graphene oxide (rGO). The Cu 2O–rGO–C3N4 composite covered the whole solar spectrum with significant absorption intensity. rGO wrapped Cu 2O loading caused a red shift in the absorption with respect to considering the absorption of bare C3N4. The photoluminescence study confirms that rGO exploited as an electron transport layer at the interface of Cu 2O and C3N4 heterojunction. Utmost, ∼2 fold synergistic effect was achieved with Cu 2O–rGO–C3N4 for the photocatalytic reduction of 4-nitrophenol to 4-aminophenol in comparison with Cu 2O–rGO and C3N4. The Cu 2O–rGO–C3N4 photocatalyst was reused for four times without loss in its activity.

Effect of graphene substrate on the SERS Spectra of Aromatic bifunctional molecules on metal nanoparticles

The design of molecular sensors plays a very important role within nanotechnology and especially in the development of different devices for biomedical applications. Biosensors can be classified according to various criteria such as the type of interaction established between the recognition element and the analyte or the type of signal detection from the analyte (transduction). When Raman spectroscopy is used as an optical transduction technique the variations in the Raman signal due to the physical or chemical interaction between the analyte and the recognition element has to be detected. Therefore any significant improvement in the amplification of the optical sensor signal represents a breakthrough in the design of molecular sensors. In this sense, Surface-Enhanced Raman Spectroscopy (SERS) involves an enormous enhancement of the Raman signal from a molecule in the vicinity of a metal surface. The main objective of this work is to evaluate the effect of a monolayer of graphene oxide (GO) on the distribution of metal nanoparticles (NPs) and on the global SERS enhancement of paminothiophenol (pATP) and 4-mercaptobenzoic acid (4MBA) adsorbed on this substrate. These aromatic bifunctional molecules are able to interact to metal NPs and also they offer the possibility to link with biomolecules. Additionally by decorating Au or Ag NPs on graphene sheets, a coupled EM effect caused by the aggregation of the NPs and strong electronic interactions between Au or Ag NPs and the graphene sheets are considered to be responsible for the significantly enhanced Raman signal of the analytes [1-2]. Since there are increasing needs for methods to conduct reproducible and sensitive Raman measurements, Grapheneenhanced Raman Scattering (GERS) is emerging as an important method [3].

Direct Observation of Tetragonal Distortion in Epitaxial Structures through Secondary Peak Split in a Synchrotron Radiation Renninger Scan

This paper reports a direct observation of an interesting split of the (022)(022) four-beam secondary peak into two (022) and (022) three-beam peaks, in a synchrotron radiation Renninger scan (phi-scan), as an evidence of the layer tetragonal distortion in two InGaP/GaAs (001) epitaxial structures with different thicknesses. The thickness, composition, (a perpendicular to) perpendicular lattice parameter, and (01) in-plane lattice parameter of the two epitaxial ternary layers were obtained from rocking curves (omega-scan) as well as from the simulation of the (022)(022) split, and then, it allowed for the determination of the perpendicular and parallel (in-plane) strains. Furthermore, (022)(022) omega:phi mappings were measured in order to exhibit the multiple diffraction condition of this four-beam case with their split measurement.

Wetting of Graphene

Graphene, a remarkable 2D material, has attracted immense attention for its unique physical properties that make it ideal for a myriad of applications from electronics to biology. Fundamental to many such applications is the interaction of graphene with water, necessitating an understanding of wetting of graphene. Here, molecular dynamics simulations have been employed to understand two fundamental issues of water drop wetting on graphene: (a) the dynamics of graphene wetting and (b) wetting of graphene nanostructures. The first problem unravels that the wetting dynamics of nanodrops on graphene are exactly the same as on standard, non-2D (or non-layered) solids – this is an extremely important finding given the significant difference in the wetting statics of graphene with respect to standard solids stemming from graphene’s wetting translucency effect. This same effect, as shown in the second problem, interplays with roughness introduced by nanostructures to trigger graphene superhydrophobicity following a hitherto unknown route.

Graphene and Semicondutor or Metallic Nanoparticles for Energy Conversion

The purpose of the present PhD thesis is to investigate the properties of innovative nano- materials with respect to the conversion of renewable energies to electrical and chemical energy. The materials have been synthesized and characterized by means of a wide spectrum of morphological, compositional and photophysical techniques, in order to get an insight into the correlation between the properties of each material and the activity towards different energy conversion applications. Two main topics are addressed: in the first part of the thesis the light harvesting in pyrene functionalized silicon nanocrystals has been discussed, suggesting an original approach to suc- cessfully increase the absorption properties of these nanocrystals. The interaction of these nanocrystals was then studied, in order to give a deeper insight on the charge and energy extraction, preparing the way to implement SiNCs as active material in optoelectronic devices and photovoltaic cells. In addition to this, the luminescence of SiNCs has been exploited to increase the efficiency of conventional photovoltaic cells by means of two innovative architectures. Specifically, SiNCs has been used as luminescent downshifting layer in dye sensitized solar cells, and they were shown to be very promising light emitters in luminescent solar concentrators. The second part of the thesis was concerned on the production of hydrogen by platinum nanoparticles coupled to either electro-active or photo-active materials. Within this context, the electrocatalytic activity of platinum nanoparticles supported on exfoliated graphene has been studied, preparing an high-efficiency catalyst and disclosing the role of the exfoliation technique towards the catalytic activity. Furthermore, platinum nanoparticles have been synthesized within photoactive dendrimers, providing the first proof of concept of a dendrimer-based photocatalytic system for the hydrogen production where both sensitizer and catalyst are anchored to a single scaffold.

Synthesis of porous graphene/TiO2 by use of recycled graphite

Graphene-based nanomaterials are a kind of new technological materials with high interest for physicists, chemists and materials scientists. Graphene is a two-dimensional (2-D) sheet of carbon atoms in a hexagonal configuration with atoms bonded by sp2 bonds. These bonds and this electron configuration provides the extraordinary properties of graphene, such as very large surface area, a tunable band gap, high mechanical strength and high elasticity and thermal conductivity [1]. Graphene has also been investigated for preparation of composites with various semiconductors like TiO2, ZnO, CdS aiming at enhanced photocatalytic activity for their use for photochemical reaction as water splitting or CO2 to methanol conversion [2-3]. In this communication, the synthesis of porous graphene@TiO2 obtained from a powder graphite recycled, supplied by ECOPIBA, is presented. This graphite was exfoliated, using a nonionic surfactant (Triton X-100) and sonication. Titanium(IV) isopropoxide was used as TiO2 source. After removing the surfactant with a solution HCl/n-propanol, a porous solid is obtained with a specific area of 358 m2g-1. The solid was characterized by XRD, FTIR, XPS, EDX and TEM. Figure 1 shows the graphene 2D layer bonded with nanoparticles of TiO2. When a water suspension of this material is exposed with UV-vis radiation, water splitting reaction is carried out and H2/O2 bubbles are observed (Figure 2)

Graphene NanoPlatelets Reinforced Tantalum Carbide consolidated by Spark Plasma Sintering

Hypersonic aerospace vehicles are severely limited by the lack of adequate high temperature materials that can withstand the harsh hypersonic environment. Tantalum carbide (TaC), with a melting point of 3880°C, is an ultrahigh temperature ceramic (UHTC) with potential applications such as scramjet engines, leading edges, and zero erosion nozzles. However, consolidation of TaC to a dense structure and its low fracture toughness are major challenges that make it currently unviable for hypersonic applications. In this study, Graphene NanoPlatelets (GNP) reinforced TaC composites are synthesized by spark plasma sintering (SPS) at extreme conditions of 1850˚C and 80-100 MPa. The addition of GNP improves densification and enhances fracture toughness of TaC by up to ~100% through mechanisms such as GNP bending, sliding, pull-out, grain wrapping, crack bridging, and crack deflection. Also, TaC-GNP composites display improved oxidation behavior over TaC when exposed to a high temperature plasma flow exceeding 2500 ˚C.

Thermally induced all-optical inverter and dynamic hysteresis loops in graphene oxide dispersions

We experimentally study the temporal dynamics of amplitude-modulated laser beams propagating through a water dispersion of graphene oxide sheets in a fiber-to-fiber U-bench. Nonlinear refraction induced in the sample by thermal effects leads to both phase reversing of the transmitted signals and dynamic hysteresis in the input- output power curves. A theoretical model including beam propagation and thermal lensing dynamics reproduces the experimental findings. © 2015 Optical Society of America.

Manipulating the structural and electronic properties of epitaxial NaNbO3 films via strain and stoichiometry

Due to their intriguing dielectric, pyroelectric, elasto-electric, or opto-electric properties, oxide ferroelectrics are vital candidates for the fabrication of most electronics. However, these extraordinary properties exist mainly in the temperature regime around the ferroelectric phase transition, which is usually several hundreds of K away from room temperature. Therefore, the manipulation of oxide ferroelectrics, especially moving the ferroelectric transition towards room temperature, is of great interest for application and also basic research. In this thesis, we demonstrate this using examples of NaNbO3 films. We show that the transition temperature of these films can be modified via plastic strain caused by epitaxial film growth on a structurally mismatched substrate, and this strain can be fixed by controlling the stoichiometry. The structural and electronic properties of Na1+xNbO3+δ thin films are carefully examined by among others XRD (e.g. RSM) and TEM and cryoelectronic measurements. Especially the electronic features are carefully analyzed via specially developed interdigitated electrodes in combination with integrated temperature sensor and heater. The electronic data are interpreted using existing as well as novel theories and models, they are proved to be closely correlated to the structural characteristics. The major results are: -Na1+xNbO3+δ thin films can be grown epitaxially on (110)NdGaO3 with a thickness up to 140 nm (thicker films have not been studied). Plastic relaxation of the compressive strain sets in when the thickness of the film exceeds approximately 10 – 15 nm. Films with excess Na are mainly composed of NaNbO3 with minor contribution of Na3NbO4. The latter phase seems to form nanoprecipitates that are homogeneously distributed in the NaNbO3 film which helps to stabilize the film and reduce the relaxation of the strain. -For the nominally stoichiometric films, the compressive strain leads to a broad and frequency-dispersive phase transition at lower temperature (125 – 147 K). This could be either a new transition or a shift in temperature of a known transition. Considering the broadness and frequency dispersion of the transition, this is actually a transition from the dielectric state at high temperature to a relaxor-type ferroelectric state at low temperature. The latter is based on the formation of polar nano-regions (PNRs). Using the electric field dependence of the freezing temperature, allows a direct estimation of the volume (70 to 270 nm3) and diameter (5.2 to 8 nm, spherical approximation) of the PNRs. The values confirm with literature values which were measured by other technologies. -In case of the off-stoichiometric samples, we observe again the classical ferroelectric behavior. However, the thermally hysteretic phase transition which is observed around 620 – 660 K for unstrained material is shifted to room temperature due to the compressive strain. Beside to the temperature shift, the temperature dependence of the permittivity is nearly identical for strained and unstrained materials. -The last but not least, in all cases, a significant anisotropy in the electronic and structural properties is observed which arises automatically from the anisotropic strain caused by the orthorhombic structure of the substrate. However, this anisotropy cannot be explained by the classical model which tries to fit an orthorhombic film onto an orthorhombic substrate. A novel “square lattice” model in which the films adapt a “square” shaped lattice in the plane of the film during the epitaxial growth at elevated temperature (~1000 K) nicely explains the experimental results. In this thesis we sketch a way to manipulate the ferroelectricity of NaNbO3 films via strain and stoichiometry. The results indicate that compressive strain which is generated by the epitaxial growth of the film on mismatched substrate is able to reduce the ferroelectric transition temperature or induce a phase transition at low temperature. Moreover, by adding Na in the NaNbO3 film a secondary phase Na3NbO4 is formed which seems to stabilize the main phase NaNbO3 and the strain and, thus, is able to engineer the ferroelectric behavior from the expected classical ferroelectric for perfect stoichiometry to relaxor-type ferroelectric for slightly off-stoichiometry, back to classical ferroelectric for larger off-stoichiometry. Both strain and stoichiometry are proven as perfect methods to optimize the ferroelectric properties of oxide films.

Development and characterization of innovative graphene-based composites

Over the last decade, graphene and related materials (GRM) have drawn significant interest and resources for their development into the next generation of composite materials. This is because these nanoparticles have the ability to operate as reinforcing additives capable of imparting considerable mechanical property increases while also embedding multi-functional advantages on the host matrix. Because graphene and 2D materials are still in their early stages, the relative maturity of different types of composite systems varies. As a result, certain nanocomposite systems are currently commercially accessible, while others are not yet sufficiently developed to enter the market. A substantial emphasis has been placed on developing thermoplastic and thermosetting materials that combine a variety of mechanical and functional qualities. These include higher strength and stiffness, increased thermal and electrical conductivity, improved barrier properties, fire retardancy, and others, with the ultimate goal of providing multifunctionality to already employed composites. The work presented in this thesis investigates the use and benefits that GRM could bring to composites for a variety of applications, with the goal of realizing multifunctional components with improved properties that leads to lightweight and, as a result, energy and cost savings and pollution reduction in the environment. In particular, we worked on the following topics: • Benchmarking of commercial GRM-based master batches; • GRM-coatings for water uptake reduction; • GRM as thermo-electrical anti-icing /de-icing system; • GRM for Out of Oven curing of composites.

A glassful of graphene: graphene-based materials for drinking water remediation

Modern world suffers from an intense water crisis. Emerging contaminants represent one of the most concerning elements of this issue. Substances, molecules, ions, and microorganisms take part in this vast and variegated class of pollutants, which main characteristic is to be highly resistant to traditional water purification technologies. An intense international research effort is being carried out in order to find new and innovative solutions to this problem, and graphene-based materials are one of the most promising options. Graphene oxide (GO) is a nanostructured material where domains populated by oxygenated groups alternate with interconnected areas of sp2 hybridized carbon atoms, on the surface of a one-atom thick nanosheets. GO can adsorb a great number of molecules and ions on its surface, thanks to the variety of different interactions that it can express, such as hydrogen bonding, p-p stacking, and electrostatic and hydrophobic interaction. These characteristics, added to the high superficial area, make it an optimal material for the development of innovative materials for drinking water remediation. The main concern in the use of GO in this field is to avoid secondary contaminations (i.e. GO itself must not become a pollutant). This issue can be faced through the immobilization of GO onto polymeric substrates, thus developing composite materials. The use of micro/ultrafiltration polymeric hollow fibers as substrates allows the design of adsorptive membranes, meaning devices that can perform filtration and adsorption simultaneously. In this thesis, two strategies for the development of adsorptive membranes were investigated: a core-shell strategy, where hollow fibers are coated with GO, and a coextrusion strategy, where GO is embedded in the polymeric matrix of the fibers. The so-obtained devices were exploited for both fundamental studies (i.e. molecular and ionic behaviour in between GO nanosheets) and real applications (the coextruded material is now at TRL 9).

Graphene-based bioelectronic interfaces and devices and interpretative models targeting glial cells

Bioelectronic interfaces have significantly advanced in recent years, offering potential treatments for vision impairments, spinal cord injuries, and neurodegenerative diseases. However, the classical neurocentric vision drives the technological development toward neurons. Emerging evidence highlights the critical role of glial cells in the nervous system. Among them, astrocytes significantly influence neuronal networks throughout life and are implicated in several neuropathological states. Although they are incapable to fire action potentials, astrocytes communicate through diverse calcium (Ca2+) signalling pathways, crucial for cognitive functions and brain blood flow regulation. Current bioelectronic devices are primarily designed to interface neurons and are unsuitable for studying astrocytes. Graphene, with its unique electrical, mechanical and biocompatibility properties, has emerged as a promising neural interface material. However, its use as electrode interface to modulate astrocyte functionality remains unexplored. The aim of this PhD work was to exploit Graphene-oxide (GO) and reduced GO (rGO)-coated electrodes to control Ca2+ signalling in astrocytes by electrical stimulation. We discovered that distinct Ca2+dynamics in astrocytes can be evoked, in vitro and in brain slices, depending on the conductive/insulating properties of rGO/GO electrodes. Stimulation by rGO electrodes induces intracellular Ca2+ response with sharp peaks of oscillations (“P-type”), exclusively due to Ca2+ release from intracellular stores. Conversely, astrocytes stimulated by GO electrodes show slower and sustained Ca2+ response (“S-type”), largely mediated by external Ca2+ influx through specific ion channels. Astrocytes respond faster than neurons and activate distinct G-Protein Coupled Receptor intracellular signalling pathways. We propose a resistive/insulating model, hypothesizing that the different conductivity of the substrate influences the electric field at the cell/electrolyte or cell/material interfaces, favouring, respectively, the Ca2+ release from intracellular stores or the extracellular Ca2+ influx. This research provides a simple tool to selectively control distinct Ca2+ signals in brain astrocytes in neuroscience and bioelectronic medicine.