Rozprawy doktorskie na temat „Graphene-metal nanostructures”

This thesis was dedicated to decorate graphene sheets with metal and metal oxide nanostructures by RF sputtering technique. Two main objectives were focused in this thesis. 1) To decorate graphene sheets uniformly with metal and metal oxide nanostructure without agglomeration. 2) To explore different kinds of application of decorated graphene sheets with metal and metal oxide nanostructures In the first step, we presented the experimental study results about Nb2O5 deposition onto graphite nanoplatelets (GNPs) by the variation of the deposition process parameters. The structural, chemical and electronic properties of the decorated GNPs with Nb2O5 layers were studied. It was found that with deposition of Nb2O5 layers onto GNPs, tensile strain was developed into the planes of the GNPs. The induced tensile strain in and between the planes of GNPs increased with raising the amount of the Nb2O5 concentration. TEM images shows that GNPs decorated with around 5 to10 nm uniform layer of Nb2O5 at 100 W on their surface were successfully fabricated. From the XPS analysis it was confirmed that, by increasing Nb2O5 layer thickness on the GNPs surface with rising RF power values binding energy downshift in C 1s peak suggests a p-type doping of GNPs due to charge transfer at the interface as a consequence of the higher work function difference between the Nb2O5 (4.70 eV) and GNPs (4.33 eV). In the second step, the interface between the graphene sheets and Nb2O5 nanoparticles were studied. It was established that the structural defects were pronounced with increasing amounts of the Nb2O5 concentration. XPS measurement on graphene/Nb2O5 suggests p-type doping of graphene due to charge transfer at the interface as a consequence of the high work function of Nb2O5. The strong p-doping effect was also confirmed by Raman analysis where the positions of the G and 2D peaks of graphene gradually upshifted upon increasing the Nb2O5 concentration. The uniform distribution of decorated Nb2O5 nanoparticles onto graphene was confirmed from TEM analysis. The ferromagnetic behavior was observed for the undecorated graphene and decorated graphene with Nb2O5 nanoparticles. The ferromagnetic behavior of graphene was enhanced with decoration of the Nb2O5 nanoparticles. In the third step, the effect of the Mg concentration on the structural, chemical and morphological properties of the graphene was described. Well dispersed Mg nanoparticles were decorated onto graphene sheets. It was found that from the XRD results, different sizes of the crystalline Mg nanoparticles were obtained onto graphene sheets with variation of the process parameters.. Raman spectra indicated that G and 2D bands of the graphene were shifted to higher wavenumber with deposition of Mg nanoparticles. The well dispersed and small size of Mg nanoparticles in the range of (8-12 nm) onto graphene sheets was decorated by using a high powder vibration frequency. No agglomeration of the sputtered particles was observed with high powder vibration frequency. This observation was confirmed by TEM micrographs. XPS analysis revealed that the decorated Mg nanoparticles onto graphene were oxidized due to exposure to the atmosphere. The well dispersed decorated Mg nanoparticles onto graphene sheets were studied for the hydrogen absorption and desorption at two different temperatures 330 oC and 360 oC at 2 and 8 bars pressure. The hydrogen up taking capacity for the decorated graphene sheets with Mg nanoparticles was 3 wt. % in whole composite. However, the up taking hydrogen storage capacity of the only Mg nanoparticles was 6.6 wt. %. In the last step, the interaction of the graphene sheets with TiO2 nanoparticles was studied. The XRD results indicated that the lattice of the graphene sheets was distorted with increasing amount of the TiO2 concentration. The particle nature of the deposited TiO2 was confirmed by TEM examination and also the TEM analysis shows that TiO2 nanoparticles were uniformly distributed onto graphene sheets. The Raman analysis showed that the G and 2D bands of graphene were shifted to higher wavenumber with increasing TiO2 concentration onto graphene sheet confirming the p doped graphene with TiO2 nanoparticles. The XPS analysis further confirmed the p doping of graphene upon the deposition of the TiO2 nanoparticles. The binding energy downshift the C 1s core level of was observed after charge transfer from graphene to TiO2 nanoparticles due to the larger work function of TiO2 relatively to that of graphene. It was observed that decorated graphene sheets with TiO2 nanoparticles shows reasonably catalytic activity.

This thesis discusses the formation of metal-organic and organic structures grown on surfaces using bottom-up self-assembly techniques. Three systems are investigated primarily using scanning probe microscopy techniques. The growth of metal-organic frameworks (MOFs) on functionalised surfaces is investigated using high resolution atomic force microscopy (AFM). The earliest stages of MOF crystal nucleation are imaged using a layer-by-layer (LBL) growth technique and the ability to track the growth of individual nanocrystallites throughout the LBL process is demonstrated. This LBL method has been suggested as a route to fabricating epitaxially grown, oriented thin-films of MOFs. However, results from these studies indicate that, rather than a uniform crystalline layer, the morphology is that of a preferentially oriented but laterally polycrystalline film and the growth rates of the individual nanocrystallites exceed those expected for a LBL growth mode. This has significant implications for the fabrication of novel devices that incorporate MOFs due to the presence of domain boundaries and defects. Self-assembled monolayers of light-harvesting porphyrin nanorings are investigated with scanning tunnelling microscopy (STM) and AFM. The nanorings are found to form large supramolecular networks in ambient conditions on graphite and boron nitride surfaces. The size and order of these networks is found to be dependent on the number of porphyrin macrocycles that make up each ring. In addition, simulations of isolated nanorings are also performed using Monte Carlo methods to model the distortion previously been observed for isolated nanorings on gold surfaces. These are discussed in the context of spectroscopic measurements which suggest that both size dependent and thermally induced distortion affects the lifetime and delocalisation of excited states in these molecules. Graphene is grown on hexagonal boron nitride surfaces using high-temperature molecular beam epitaxy. Large domains of monolayer graphene are successfully grown and are investigated using AFM and Raman spectroscopy. These domains are found to exhibit hexagonal moiré patterns on the graphene surface which is suggestive of orientational alignment with the underlying boron nitride substrate. Regions with high period and distorted moiré patterns are also observed which suggest that the graphene is under tensile strain which is attributed to the high growth temperatures used. The strain is found to significantly affect the Raman spectrum of graphene and a relationship between the strain and the shifting of Raman spectral peaks is determined. Successful attempts are also made to modify the strain in the graphene monolayer using an AFM tip which is observed to relax when defects are introduced in a controlled manner to the graphene monolayer. These results represent new approaches to the introduction and control of strain in graphene which may be useful for the fabrication of high-performance graphene devices.

Le graphène, une monocouche de graphite, est composé d'atomes de carbone avec une structure en nid d'abeilles. Ses propriétés exceptionnelles ont attiré un intérêt mondial, dont le Prix Nobel de Physique en 2010. Le graphène épitaxié sur métal à rapidement été identifié comme un moyen de production de graphène de haute qualité de taille métrique, et est le sujet d'intenses activités de recherche en sciences de surface pour caractériser ses propriétés. En outre, ces études concernent aussi des systèmes plus complexes avec pour base le graphène, par exemple les réseaux ordonnés de nanoparticules à sa surface. Tout cela a mené à l'étude de la croissance, de la structure et des défauts du graphène épitaxié avec un grande variété de techniques expériementales, tel que la microscopie par effet tunnel, spectroscopie par photo-émission résolue en angle ou encore la microscopie électronique à basse énergie. Ce travail de recherche se concentre sur le graphène obtenu par croissance sur la surface (111) d'un monocristal d'iridium dans des conditions d'ultra vide et étudié avec plusieurs techniques de mesure par diffraction (diffraction de surface des rayons X, diffraction des rayons X en incidence rasante, réflectivité des rayons X et diffraction des électrons à haute énergie en réflexion). Ces expériences ont été faites au synchrotron européen ESRF à Grenoble, en France. La première partie de cette étude a été de déterminer la structure du graphène à l'échelle atomique. Le système montre une tendance à la commensurabilité, mais sa structure précise dépend fortement des conditions de préparation et de la température appliqué au système. En outre, en combinant des techniques de diffraction à haute résolution, une caractérisation précise de la structure, qui fait débat dans la littérature, est dévoilée. Le système étudié présente aussi une surperstructure, typique du graphène épitaxié, nommé moiré pour ses similarités avec l'effet optique du même nom. Celle-ci est utilisée comme gabarit pour faire croître des nanoparticules monodisperses à la surface en réseau auto-organisé. Durant cette étude, trois types de nanoparticules ont été examinés, des particules de platine de deux tailles différentes et des particules composées de platine et de cobalt. Ces systèmes hybrides présentent un fort degré d'organisation, partiellement hérité de la superstructure du moiré. Les nanoparticules forme une interaction forte avec leur support et elles subissent des contraintes de surface causées par leurs petites tailles. Par ailleurs, les nanoparticules de platine-cobalt, dont la croissance est en deux étapes, gardent une structure en couche et non une structure d'alliage métallique
Graphene, a monolayer of graphite, is composed of carbon atoms arranged in a honeycomb lattice. Its exceptional properties have attracted a worldwide interest, including the Novel Prize in Physics in 2010. Epitaxial graphene on a metal was rapidly identified as an efficient method for large-area production of high quality graphene, and also was the matter of intense activities exploiting surface science approaches to address the various properties of graphene and of advanced systems based on graphene, for instance ordered lattice of metal nanoparticles on graphene. This resulted in the study of growth, structure and defects of epitaxial graphene on a wide variety of substrates with various techniques such as scanning tunneling microscopy, angle-resolved photoemission spectroscopy or low-energy electron microscopy. This work focuses on graphene grown on the (111) surface of iridium in ultra-high vacuum conditions and studied with several diffraction techniques (surface X-ray diffraction, grazing incidence X-ray diffraction, X-ray reflectivity, and reflection-high energy electron diffraction). These experiments were performed at the European Synchrotron Radiation Facility in Grenoble, France. The first step in our study was to determine the structure of graphene at the atomic scale. The system was found to have a tendency to commensurability, but that the precise structure depends on temperature and on preparation conditions. Moreover, with the combination of high resolution diffraction techniques, a precise characterization about the debated structure of graphene perpendicular to the surface was unveiled. The system, exhibits a superstructure, typical of epitaxial graphene, called a moiré, as an equivalent of the moiré effect in optics. This is used as a template to grown nanoparticles on top of the system to achieve the self-organisation of monodisperse nanoparticles. In this study, three type of nanoparticles were investigated, two different size of pure platinum ones and bimetallic ones, platinum and cobalt. These hybrid systems show very high degree of order, partly inherited by the superstructure lattice. The nanoparticles were found to strongly bond to their support, experience substantial surface strain related to their small size, and that bimetallic ones grown in a sequential manner retain a chemically layered structure

This research aims to explore the optimization strategies of two-dimensional (2D) nanostructures for high-performance rechargeable batteries. Three effective strategies, including 2D-based phase engineering, component engineering and van der Waals (vdW) heterostructures, were proposed for improving electrochemical properties of 2D nanomaterials. These effective strategies will offer good references for researchers to develop practical next-generation rechargeable batteries using the emerging 2D nanomaterials.

In this thesis, two-dimensional materials such as graphene are tested for their suitability for opto-electronic applications using terahertz time domain spectroscopy (THz-TDS). This ultrafast all-optical technique can probe the response of novel materials to photoexcitation, and yield information about the dynamics of the material systems. Graphene grown by chemical vapour deposition (CVD) is studied using optical-pump THz-probe time domain spectroscopy in a variety of gaseous environments in Chapter 4. The photoconductivity response of graphene grown by CVD is found to vary dramatically depending on which atmospheric gases are present. Adsorption of these gases can open a local bandgap in the material, allowing stimulated emission of THz radiation across the gap. Semiconducting equivalents to graphene, molybdenum disulphide (MoS₂) and tungsten diselenide (WSe₂), grown by CVD, are investigated in Chapter 5. These members of the transition metal dichalcogenide family show sub-picosecond responses to photoexcitation, suggesting promise for use in high-speed THz devices. In Chapter 6, an alternative production route to CVD is studied. Liquid-phase exfoliation offers fast, easy production of few-layer materials. THz spectroscopy reveals that the dynamics of these materials after photoexcitation are remarkably similar to those in CVD-grown materials, offering the potential of cheaper materials for future devices. Finally in Chapter 7, it is shown that carbon nanotubes can be used to make ultrafast THz devices. Unaligned, semiconducting single walled carbon nanotubes can be photoexcited to produce an ultrafast, dynamic THz polariser. The work in this thesis demonstrates the potential for these novel materials in future opto-electronic applications. THz spectroscopy is shown to be an important tool for the characterisation of new materials, providing information that can be used to understand the dynamics of materials, and improve production methods.

The aim of this research is to develop high performance gas sensors with low power consumption and high portability. This was achieved by synthesizing carbon nanomaterials decorated with alkali-metal dopants and metal oxides, and by optimizing ultrathin layer of carbon nanobutes coupled to a new deposition technique. These materials demonstrated excellent sensitivity at room temperature to both nitrogen dioxide and ammonia, down to ppm level, providing a new pathway to realise room temperature gas sensors. Our fabrication methods are highly scalable and do not involve the use of expensive equipment which makes them excellent candidates for mass production.

Water contaminations by many pollutants, especially heavy metals such as Pb(II), Hg(II), Cu(II), Cd(II), and Cr(VI) pose many public health and environmental concerns as reported in the list of hazardous substances compiled by the US Environmental Protection Agency due to their high toxicity, refractory degradation, and ease of entering food chain. Adsorption by chelating resins is proven to be the most effective method for the extraction of metal ions from polluted and wastewater. However, traditional absorbents such as activated carbon, activated alumina, clay, zeolite, etc., show limited adsorption abilities for these heavy metal ions. The major goal of this thesis is to develop efficient and cost-effective adsorbents for the extraction of heavy metals from wastewater. This dissertation will focus on the development of four chemically modified high surface area adsorbents with accessible chelating sites for capturing and retaining toxic metal ions from polluted water. The first adsorbent, Nitrogen Doped Carboxylated Activated Carbon (ND-CAC), is prepared by a polymerization reaction between melamine and formaldehyde to form the melamine formaldehyde resin (MF-R) followed by carbonization at 800 oC under nitrogen atmosphere to form nitrogen doped carbon (ND-C), and finally oxidation to form the ND-CAC adsorbent. The ND-CAC adsorbent shows high adsorption capacities of 750.5, 250.5, 98.2 mg/g for the extraction of Pb(II), Hg(II), and Cr(VI), respectively from aqueous solutions with a high selectivity to Pb(II). The second adsorbent, Melamine Zirconium Phosphate (M-ZrP) is prepared by a precipitation reaction between Melamine Phosphate (MP) and ZrCl4 in an aqueous solution. The M-ZrP adsorbent is used for the removal of Pb(II), Hg(II), and Cd(II) with maximum adsorption capacities of 680.4, 119.0, and 60.0 mg/g, respectively with a high selectivity to Pb(II). The third adsorbent is chemically functionalized metal organic framework (UIO-66-IT) was prepared by post-synthetic modification using the chelating ligand 2-Imino-4-Thioburit. The adsorbent was used to extract Hg(II) and (HPO4)- ions from aqueous solutions and the results revealed exceptionally high adsorption capacities toward mercury and phosphate ions of 700 and 160 mg/g, placing it among the top functionalized MOF known for the high capacity of Hg(II) removal from aqueous solutions. The fourth adsorbent, Melamine Thiourea Partially Reduced Graphene Oxide (MT-PRGO) prepared by the amidation reaction between chemically modified graphene oxide and melamine thiourea, is used for the effective extraction of Hg(II), Co(II) and Cu(II) from polluted water. The MT-PRGO adsorbent shows exceptional selectivity for the extraction of Hg(II) with a capacity of 651 mg/g, placing it among the top of carbon-based materials known for the high capacity of Hg(II) removal from aqueous solutions. Desorption studies demonstrate that the new adsorbents ND-CAC, M-ZrP, UIO-66-IT, and MT-PRGO are easily regenerated with the desorption of the heavy metal ions Hg(II), Pb(II), Cd(II), and Cr(VI) reaching 99 % - 100 % recovery from their maximum sorption capacities using different eluents. Moreover, all prepared adsorbents showed tremendous abilities to clean contaminated water from toxic heavy metals at trace concentrations. That prove the ability of using them at water contamination level when the concentration of heavy metals is very low. The new adsorbents ND-CAC, M-ZrP, UIO-66-IT, and MT-PRGO are proposed as top performing remediation adsorbents for the extraction of the heavy metals Pb(II), Hg(II), Cd(II), Cr(VI), and (HPO4)- from waste and polluted water.

The thesis consists of four parts of which part 1 presents a brief overview of nanomaterials. Parts 2, 3 and 4 contain results of investigations of graphene, nanofilms of noble metal nanoparticles and ZnO nanostructures respectively. Investigations of graphene are described in Part 2 which consists of six chapters. In Chapter 2.1, changes in the electronic structure and properties of graphene induced by molecular charge-transfer have been discussed. Chapter 2.2 deals with the results of a study of the interaction of metal and metal oxide nanoparticles with graphene. Electrical and dielectric properties of graphene-polymer composites are presented in Chapter 2.3. Chapter 2.4 presents photo-thermal effects observed in laser-induced chemical transformations in graphene and other nanocarbons system. Chapter 2.5 describes the mechanical properties of polymer matrix composites reinforced by fewlayer graphene investigated by nano-indentation. The extraordinary synergy found in the mechanical properties of polymer matrix composites reinforced with two nanocarbons of different dimensionalities constitute the subject matter of Chapter 2.6. Investigations of noble metal nanoparticles have been described in Part 3. In Chapter 3.1, ferromagnetism exhibited by nanoparticles of noble metals is discussed in detail while Chapter 3.2 deals with surface-enhanced Raman scattering (SERS) of molecules adsorbed on nanocrystalline Au and Ag films formed at the organic–aqueous interface. Factors affecting laser-excited photoluminescence from ZnO nanostructures are examined in great detail in Part 4.

The thesis consists of four parts of which part 1 presents a brief overview of nanomaterials. Parts 2, 3 and 4 contain results of investigations of graphene, nanofilms of noble metal nanoparticles and ZnO nanostructures respectively. Investigations of graphene are described in Part 2 which consists of six chapters. In Chapter 2.1, changes in the electronic structure and properties of graphene induced by molecular charge-transfer have been discussed. Chapter 2.2 deals with the results of a study of the interaction of metal and metal oxide nanoparticles with graphene. Electrical and dielectric properties of graphene-polymer composites are presented in Chapter 2.3. Chapter 2.4 presents photo-thermal effects observed in laser-induced chemical transformations in graphene and other nanocarbons system. Chapter 2.5 describes the mechanical properties of polymer matrix composites reinforced by fewlayer graphene investigated by nano-indentation. The extraordinary synergy found in the mechanical properties of polymer matrix composites reinforced with two nanocarbons of different dimensionalities constitute the subject matter of Chapter 2.6. Investigations of noble metal nanoparticles have been described in Part 3. In Chapter 3.1, ferromagnetism exhibited by nanoparticles of noble metals is discussed in detail while Chapter 3.2 deals with surface-enhanced Raman scattering (SERS) of molecules adsorbed on nanocrystalline Au and Ag films formed at the organic–aqueous interface. Factors affecting laser-excited photoluminescence from ZnO nanostructures are examined in great detail in Part 4.

University of Technology Sydney. Faculty of Engineering and Information Technology.
Energy storage systems (ESSs) play a critical role in plenty of applications including renewable energy systems, power systems for electric vehicles (EVs) and hybrid electric vehicles (HEVs), and electrical power grids for improving reliability and overall use of the entire system. Currently, there are several types ESSs dominated the energy storage. Each kind of ESSs has their own operation mechanism, energy efficiency, energy density, power density, cycle life, charge and discharge capability, cost efficiency, operating temperature. The common ESS is based on lead acid battery which stores electrical energy in the form of chemical energy. However, if the batteries are overdischarged or kept at a discharged state, its capability will be irreversibly undermined because the sulfate crystals become larger and more difficult to break up during recharge. Since the first NiCd battery was created by Waldemar Jungner in 1899, even though NiCd battery technologies have experienced a series of evolutionary developments, its demerits are obvious including 1) shorter life cycle; 2) memory effect; 3) toxicity of Cd; 4) lower energy density; and 5) limited negative temperature coefficient. Based on the development of NiCd battery technology, nickel metal hydride (NiMH) batteries was proposed by researchers which possess better performance than NiCd batteries in cycle life, energy density and charge&discharge rates. Lithium ion is the preferred chemistry, having a superior specific energy and power density to nickel metal hydride. More lithium per gram stored in the electrodes contributes to higher energy density and power density. In addition to chemical battery system, researchers recently proposed some new sorts of ESSs including flywheel, compressed air energy storage (CAES), superconductive magnetic energy storage (SMES), etc. All of them can provide super energy density and power density. But they are more or less blocked ether in complex mechanical construction or cooling device. Supercapacitor has emerged to be an exciting energy storage device, which is able to provide high specific power, charge and discharge up to million times, have long lifetime and broad range of working temperature. Even though supercapacitor has been widely seen as a promising energy storage candidate to replace the traditional chemical batteries, it also suffer its drawback that the low energy density (the energy stored in per unit of volume and weight), high equivalent series resistance (ESR) and its high cost associated with its performance. Therefore, this PHD thesis project aims to address these drawbacks of supercapacitor by designing different nanotechnologies and fabrication methods to synthesize advance materials with better performance than that of conventional supercapacitor. A Series of designed structures and materials were fabricated by designed methods. All the materials were also investigated by using X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM) observation techniques, Brunauer–Emmett–Teller (BET) surface area measurement and electrochemical testing. A facile and effective hydrothermal treatment that is able to control the condensation speeds of precursors in the solution along the <010>, <100> and <001> directions was designed to fabricate vanadium oxide nanoribbon used for the electrode of supercapacitor. It was achieved by controlling the hydrothermal reaction time and the weight ratio to synthesize the ultralong vanadium oxide nanoribbon with controlled width. It has high specific capacitance of 453 F g⁻¹ at the scam rate of 2 mV s⁻¹ in 2 M NaCl electrolyte, and it still maintained a high capacitance of 201 F g⁻¹ at a higher scan rate of 50 mV s⁻¹, attributing to the easy ion insertion and electronic transport along the a-b plane rather through the layers of the c-axis. Vanadium oxide nanotubes were synthesized by a revised hydrothermal treatment with high-speed stirring. The preparation involved dissolution of V₂O₅ into H₂O₂ and high-speed stirring (10000 r/min) with hexadecylamine. The product was characterized by scanning electron microscopy, transmission electron microscope, X-ray diffraction and thermogravimetric analysis. The electrochemical properties of the materials as electrodes for electrochemical capacitors were evaluated by cyclic voltammetry in a three electrode system consisting of a saturated calomel electrode as reference electrode, platinum as a counter electrode and the active materials as the working electrode. A high capacitance of 148.5 F g⁻¹ was obtained at a scan rate of 2 mV s⁻¹ in 2M KCl. The electrode maintained a high capacitance of 105 F g⁻¹ at a higher scan rate of 50 mV s⁻¹ in 2M KCl electrolyte. 3D mesoporous hybrid NiCo₂O₄@graphene nanoarchitectures were successfully synthesized by a combination of freeze drying and hydrothermal reaction. Field-emission scanning electron microscopy (FESEM) and TEM analyses revealed that NiCo₂O₄@graphene nanostructures consist of a hierarchical mesoporous sheet-on-sheet nanoarchitecture with a high specific surface area of 194 m² g⁻¹. Ultrathin NiCo₂O₄ nanosheets, with a thickness of a few nanometers and mesopores ranging from 2 to 5 nm, were wrapped in graphene nanosheets and formed hybrid nanoarchitectures. When applied as electrode materials in supercapacitors, hybrid NiCo₂O₄@graphene nanosheets exhibited a high capacitance of 778 F g⁻¹ at the current density of 1 A g⁻¹, and an excellent cycling performance extending to 10000 cycles at the high current density of 10 A g⁻¹. We also presented a rational, large-scale and general method, called controllable freeze casting (CFC), to fabricate a high-densely assembled and aligned free-standing NiCo₂O₄@graphene 3D foam by vacuum filtration and air compress pressure assembly method. In the designed method, the amount of water is controllable, therefore controlling the size and the shape of the ice when the material was introduced into freeze drying system, finally achieving controllable pore size and aligned structure. This free-standing foam retains the intrinsic properties of graphene sheet, such as high surface area and high electrical conductivity. In the foam, the graphene sheets build the high conductive skeletons. And the skeletons with high surface areas support the uniform distribution of NiCo₂O₄ nanoparticles on the graphene sheets. By controlling the amount of water in the precursor, it is possible to fabricate 3D NiCo₂O₄@graphene foams with a wide range of thickness and pore size. This dense NiCo₂O₄@graphene material exhibited a high capacitance of 790 F g⁻¹ at a current density of 2 A g⁻¹, and an excellent cycling performance at a high current density of 10 A g⁻¹. The compression test revealed that the 3D NiCo₂O₄@graphene foam exhibited strong mechanical property which is able to support 20,000 times its own weight without structure collapsing. The novel synthesis method of such 3D foam with excellent properties paves the way to explore the application of lamellar materials like graphene in a self-supporting, metal oxide deposition and 3D foam.

Nanoscience dominates almost all areas of science and technology in the 21st century. Nanoparticles are of fundamental interest since they possess unique size dependent properties (optical, electrical, mechanical, chemical, magnetic etc.), which are quite different from the bulk and the atomic state. The research work presented in the thesis is on the preparation, characterization and studies on Ir, Os and graphene oxide-based systems. Interconnected Ir and Os nanochains are prepared under environmentally friendly conditions in aqueous media and subsequently used as substrates for surface enhanced Raman scaterring studies and also as electrocatalysts for oxygen reduction and formaldehyde oxidation. Ir and IrOx nanostructures are prepared using borohydride at different temperatures. The nature of interaction of heme proteins with IrOx is studied using spectroscopic techniques. Electrochemical studies on reduced graphene oxide include sensing of biomolecules with high sensitivity and oxygen reduction reaction (ORR) in aqueous alkaline medium. rGO is also used as support for anchoring Ir nanoparticles and the catalyst is used for the oxidation of benzyl amines to corresponding imines. The thesis is divided in to seven chapters and details are given below. Chapter 1 gives an introduction about the synthetic strategies and properties of metal nanostructures. This is followed by literature survey on Ir, Os and graphene oxide-based systems relevant to the present study. Aim and scope of the present investigation is given at the end. Chapter 2 discusses the experimental procedures and characterization techniques used in the present study. Chapter 3 involves the preparation, characterization and studies on interconnected Ir nanochains. Assemblies of small sized nanoparticles forming network-like structures have attracted enormous interest and different metal nanoassemblies have been reported using different procedures. Ir3+ reduction is kinetically not a very favourable process and hence there are not many attempts to synthesize Ir-based nanostructures. Assemblies of interconnected Ir nanoparticles have been synthesized in the present studies using borohydride as reducing agent and ascorbic acid as capping agent, at high temperatures. Polyfunctional capping molecules such as ascorbic acid and vitamin P play important role for the formation of network- like Ir nanostructures. Optical properties of the networks are probed using UV-Vis spectroscopy and evolution of coupled plasmon of Ir nanochains at 418 nm (figure 1) is observed. The nanochains are used as substrates for SERS studies while the catalytic activity is followed for the reduction of nitroaromatics. Electrocatalytic activity of Ir nanochains is exemplified using oxygen reduction and formaldehyde oxidation. Ir nanochains show better electrocatalytic activities than nanoparticles as shown in figure 2. Figure 1. Time dependent UV-Vis absorption spectra of Ir nanoparticles recorded at various time intervals of (a) 5; (b) 15; (c) 30 and (d) 60 minutes of reduction of Ir3+ using borohydride and the corresponding TEM images. Figure 2. Polarization curves for oxygen reduction on (i) Ir nanochains and (ii) Ir nanoparticles in (A) 0.5 M H2SO4 and (B) 0.1 M KOH at a scan rate of 0.005 V/s. Rotation speed used is 1000 rpm. Chapter 4 discusses the preparation of Ir and IrOx using borohydride. The reaction temperature determines the product. Various physicochemical, microscopic and spectroscopic techniques have been used to understand the evolution of nanostructures. Borohydride reduces Ir3+ at high temperatures to form high surface area foams, while at 25oC, it results in an alkaline environment that helps in the hydrolysis of the Ir precursor to form IrOx nanoparticles. Porous IrOx is formed when Ir foams are annealed at high temperatures. Water oxidation has been demonstrated using IrOx nanoparticles and foams. Biocompatibility of IrOx is used to study the nature of interaction of heme proteins and the formation of bioconjugates using spectroscopic techniques. IrOx forms bioconjugates with substantial changes observed in secondary and tertiary structures of proteins. Chapter 5 explores the synthesis of interconnected ultrafine Os nanoclusters and the nanostructured materials are used as SERS substrates. Os nanochains are prepared under environmentally friendly conditions using polyfunctional molecules like ascorbic acid and vitamin P as both reducing agent and capping agent in aqueous media. Small sized (1-1.5 nm) Os nanoparticles spontaneously self-assemble to form clusters of few tens of nm that in turn self-organize to form branched nanochains of several microns in size. The as-formed nanochains show surface plasmon absorption in the visible region 540 nm which make them active substrates for surface enhanced Raman scattering (SERS) studies. High SERS activity is observed for fluorescent analyte, rhodamine 6G and non-fluorescent analyte, mercaptopyridine, with different laser excitation sources. Efficient energy transfer from fluorescent R6G dye to Os nanochains is observed based on steady state and time resolved fluorescence measurements.Figure 3. (I) Time dependent UV-Vis absorption spectra of Os nanochains recorded at different time intervals of (a) 5; (b) 7; (c) 15; (d) 30 and (e) 60 minutes. Inset shows the TEM images of Os nanochains after 60 minutes of reduction. (II) SERS spectra of 4-MPy adsorbed on Os nanochains from (a) 1 mM; (b) 10 µM and (c) 1 µM solutions using 514 nm laser excitation. Chapter 6 discusses the studies based on reduced graphene oxide. Reduced graphene oxide (rGO) is explored as electrodes for simultaneous determination of dopamine (DA), ascorbic acid (AA) and uric acid (UA) at low concentrations useful in medical diagnostics (figure 4A). It is also used as metal-free electrocatalyst for ORR (figure 4B). The use of rGO as a support for anchoring Ir nanoparticles is probed and subsequently the Ir/rGO is used as catalyst for direct aerobic oxidation of benzyl amine derivatives to corresponding imines. Chapter 7 describes the summary of the work and scope for further studies. Appendix 1 discusses the preparation of different Ir nanostructures using simple galvanic displacement reaction on copper foil while appendix 2 describes the preparation of different sized Ir nanoparticles and their electrocatalytic activity towards oxygen reduction reaction

Nanoscience dominates almost all areas of science and technology in the 21st century. Nanoparticles are of fundamental interest since they possess unique size dependent properties (optical, electrical, mechanical, chemical, magnetic etc.), which are quite different from the bulk and the atomic state. The research work presented in the thesis is on the preparation, characterization and studies on Ir, Os and graphene oxide-based systems. Interconnected Ir and Os nanochains are prepared under environmentally friendly conditions in aqueous media and subsequently used as substrates for surface enhanced Raman scaterring studies and also as electrocatalysts for oxygen reduction and formaldehyde oxidation. Ir and IrOx nanostructures are prepared using borohydride at different temperatures. The nature of interaction of heme proteins with IrOx is studied using spectroscopic techniques. Electrochemical studies on reduced graphene oxide include sensing of biomolecules with high sensitivity and oxygen reduction reaction (ORR) in aqueous alkaline medium. rGO is also used as support for anchoring Ir nanoparticles and the catalyst is used for the oxidation of benzyl amines to corresponding imines. The thesis is divided in to seven chapters and details are given below. Chapter 1 gives an introduction about the synthetic strategies and properties of metal nanostructures. This is followed by literature survey on Ir, Os and graphene oxide-based systems relevant to the present study. Aim and scope of the present investigation is given at the end. Chapter 2 discusses the experimental procedures and characterization techniques used in the present study. Chapter 3 involves the preparation, characterization and studies on interconnected Ir nanochains. Assemblies of small sized nanoparticles forming network-like structures have attracted enormous interest and different metal nanoassemblies have been reported using different procedures. Ir3+ reduction is kinetically not a very favourable process and hence there are not many attempts to synthesize Ir-based nanostructures. Assemblies of interconnected Ir nanoparticles have been synthesized in the present studies using borohydride as reducing agent and ascorbic acid as capping agent, at high temperatures. Polyfunctional capping molecules such as ascorbic acid and vitamin P play important role for the formation of network- like Ir nanostructures. Optical properties of the networks are probed using UV-Vis spectroscopy and evolution of coupled plasmon of Ir nanochains at 418 nm (figure 1) is observed. The nanochains are used as substrates for SERS studies while the catalytic activity is followed for the reduction of nitroaromatics. Electrocatalytic activity of Ir nanochains is exemplified using oxygen reduction and formaldehyde oxidation. Ir nanochains show better electrocatalytic activities than nanoparticles as shown in figure 2. Figure 1. Time dependent UV-Vis absorption spectra of Ir nanoparticles recorded at various time intervals of (a) 5; (b) 15; (c) 30 and (d) 60 minutes of reduction of Ir3+ using borohydride and the corresponding TEM images. Figure 2. Polarization curves for oxygen reduction on (i) Ir nanochains and (ii) Ir nanoparticles in (A) 0.5 M H2SO4 and (B) 0.1 M KOH at a scan rate of 0.005 V/s. Rotation speed used is 1000 rpm. Chapter 4 discusses the preparation of Ir and IrOx using borohydride. The reaction temperature determines the product. Various physicochemical, microscopic and spectroscopic techniques have been used to understand the evolution of nanostructures. Borohydride reduces Ir3+ at high temperatures to form high surface area foams, while at 25oC, it results in an alkaline environment that helps in the hydrolysis of the Ir precursor to form IrOx nanoparticles. Porous IrOx is formed when Ir foams are annealed at high temperatures. Water oxidation has been demonstrated using IrOx nanoparticles and foams. Biocompatibility of IrOx is used to study the nature of interaction of heme proteins and the formation of bioconjugates using spectroscopic techniques. IrOx forms bioconjugates with substantial changes observed in secondary and tertiary structures of proteins. Chapter 5 explores the synthesis of interconnected ultrafine Os nanoclusters and the nanostructured materials are used as SERS substrates. Os nanochains are prepared under environmentally friendly conditions using polyfunctional molecules like ascorbic acid and vitamin P as both reducing agent and capping agent in aqueous media. Small sized (1-1.5 nm) Os nanoparticles spontaneously self-assemble to form clusters of few tens of nm that in turn self-organize to form branched nanochains of several microns in size. The as-formed nanochains show surface plasmon absorption in the visible region 540 nm which make them active substrates for surface enhanced Raman scattering (SERS) studies. High SERS activity is observed for fluorescent analyte, rhodamine 6G and non-fluorescent analyte, mercaptopyridine, with different laser excitation sources. Efficient energy transfer from fluorescent R6G dye to Os nanochains is observed based on steady state and time resolved fluorescence measurements.Figure 3. (I) Time dependent UV-Vis absorption spectra of Os nanochains recorded at different time intervals of (a) 5; (b) 7; (c) 15; (d) 30 and (e) 60 minutes. Inset shows the TEM images of Os nanochains after 60 minutes of reduction. (II) SERS spectra of 4-MPy adsorbed on Os nanochains from (a) 1 mM; (b) 10 µM and (c) 1 µM solutions using 514 nm laser excitation. Chapter 6 discusses the studies based on reduced graphene oxide. Reduced graphene oxide (rGO) is explored as electrodes for simultaneous determination of dopamine (DA), ascorbic acid (AA) and uric acid (UA) at low concentrations useful in medical diagnostics (figure 4A). It is also used as metal-free electrocatalyst for ORR (figure 4B). The use of rGO as a support for anchoring Ir nanoparticles is probed and subsequently the Ir/rGO is used as catalyst for direct aerobic oxidation of benzyl amine derivatives to corresponding imines. Chapter 7 describes the summary of the work and scope for further studies. Appendix 1 discusses the preparation of different Ir nanostructures using simple galvanic displacement reaction on copper foil while appendix 2 describes the preparation of different sized Ir nanoparticles and their electrocatalytic activity towards oxygen reduction reaction

There are many challenges in materializing the applications utilizing inorganic nanoparticles. The primary drawback is the degradation of properties due to aggregation and sintering either due to elevated temperatures or prevailing chemical/electrochemical conditions. In this thesis, various wet chemical synthesis methods are developed to obtain metal nanostructures with enhanced thermal stability. The thesis is organized as below: Chapter 1 presents the problems and challenges in materializing the application of nanomaterials associated with the thermal stability of nanomaterials. A review of the existing techniques to improve the thermal stability and the scope of the thesis are presented. Chapter 2 gives a summary of the various materials synthesized, the method adopted for the synthesis and the characterization techniques used in the material characterization. Chapter 3 presents a general template-less strategy for the synthesis of nanoporous alloy aggregates by controlled aggregation of nanoparticles in the solution phase with excellent control over morphology and composition as illustrated using PdPt and PtRu systems as examples. The Pt-based nanoporous clusters exhibit excellent activity for methanol oxidation with good long term stability and CO tolerance. Chapter 4 presents a detailed study on the thermal stability of spherical mesoporous aggregates consisting of nanoparticles. The thermal stability study leads to a general conclusion that nanoporous structures transform to hollow structures on heating to elevated temperatures before undergoing complete densification. Chapter 5 presents a simple and facile method for the synthesis of single crystalline intermetallic PtBi hollow nanoparticles. A mechanism is proposed for the formation of intermetallic PtBi hollow structures. The intermetallic PtBi hollow structures synthesised show excellent electrocatalytic activity for formic acid oxidation reaction. Chapter 6 presents a robust strategy for obtaining a high dispersion of ultrafine Pt and PtRu nanoparticles on graphene. The method involves the nucleation of a metal precursor phase on graphite oxide surfaces and subsequent reduction with a strong reducing agent. The electrocatalytic activity of the composites is investigated for methanol oxidation reaction. Chapter 7 presents a microwave-assisted synthesis method for selective heterogeneous nucleation of metal nanoparticles on oxide supports leading to the synthesis of high activity catalysts. The catalytic activity of the hybrids synthesized by this method for investigated for H2 combustion. Chapter 8 presents thermal stability studies carried out on nanostructures by in-situ heating in transmission electron microscope. The microstructural changes during the sintering process are observed in real time and the observations lead to the understanding of the mechanism of particle growth and sintering. At the end, the results of the investigations were summarized with conclusions drawn.

There are many challenges in materializing the applications utilizing inorganic nanoparticles. The primary drawback is the degradation of properties due to aggregation and sintering either due to elevated temperatures or prevailing chemical/electrochemical conditions. In this thesis, various wet chemical synthesis methods are developed to obtain metal nanostructures with enhanced thermal stability. The thesis is organized as below: Chapter 1 presents the problems and challenges in materializing the application of nanomaterials associated with the thermal stability of nanomaterials. A review of the existing techniques to improve the thermal stability and the scope of the thesis are presented. Chapter 2 gives a summary of the various materials synthesized, the method adopted for the synthesis and the characterization techniques used in the material characterization. Chapter 3 presents a general template-less strategy for the synthesis of nanoporous alloy aggregates by controlled aggregation of nanoparticles in the solution phase with excellent control over morphology and composition as illustrated using PdPt and PtRu systems as examples. The Pt-based nanoporous clusters exhibit excellent activity for methanol oxidation with good long term stability and CO tolerance. Chapter 4 presents a detailed study on the thermal stability of spherical mesoporous aggregates consisting of nanoparticles. The thermal stability study leads to a general conclusion that nanoporous structures transform to hollow structures on heating to elevated temperatures before undergoing complete densification. Chapter 5 presents a simple and facile method for the synthesis of single crystalline intermetallic PtBi hollow nanoparticles. A mechanism is proposed for the formation of intermetallic PtBi hollow structures. The intermetallic PtBi hollow structures synthesised show excellent electrocatalytic activity for formic acid oxidation reaction. Chapter 6 presents a robust strategy for obtaining a high dispersion of ultrafine Pt and PtRu nanoparticles on graphene. The method involves the nucleation of a metal precursor phase on graphite oxide surfaces and subsequent reduction with a strong reducing agent. The electrocatalytic activity of the composites is investigated for methanol oxidation reaction. Chapter 7 presents a microwave-assisted synthesis method for selective heterogeneous nucleation of metal nanoparticles on oxide supports leading to the synthesis of high activity catalysts. The catalytic activity of the hybrids synthesized by this method for investigated for H2 combustion. Chapter 8 presents thermal stability studies carried out on nanostructures by in-situ heating in transmission electron microscope. The microstructural changes during the sintering process are observed in real time and the observations lead to the understanding of the mechanism of particle growth and sintering. At the end, the results of the investigations were summarized with conclusions drawn.

The thesis is divided into two parts. The first part deals with the research work carried out on the synthesis and chemical modification of nanomaterials whereas the second part describes the preparation and characterisation of polymer films and their use as separation membranes. Part I of the thesis describing the synthetic strategies and chemical manipulation schemes employed on various types of nanomaterials is divided into six chapters. Chapter 1 describes a chemist’s approach towards synthesizing and tuning the properties of different classes of nanomaterials along with a brief account of their potential applications. Chapter 2 of the thesis describes the synthesis and characterization of various metal nanostructures (viz. nanoparticles, nanorods, nanosheets etc.) of nickel, ruthenium, rhodium and iridium using a solvothermal procedure. Chapter 3 deals with the nanoparticles of the novel oxide metal ReO3. ReO3@Au, ReO3@Ag, ReO3@SiO2 and ReO3@TiO2 core-shell nanostructures with ReO3 as the core nanoparticle have been synthesized through a two-step process and characterized. Dependence of the plasmon band of the ReO3 nanoparticles on the interparticle separation has been examined by incorporating the nanoparticles in various polymer matrices and the results compared with those obtained with gold nanoparticles. Chapter 4 presents the dispersion of nanostructures of metal oxides such as TiO2, Fe3O4 and ZnO in solvents of differing polarity (water, DMF and toluene) in the presence of several surfactants. In Chapter 5 of the thesis, fluorous chemical method of separation of metallic and semiconducting single-walled carbon nanotubes is described. This method involves the selective reaction of the diazonium salt of a fluorous aniline with the metallic nanotubes in an aqueous medium and subsequent extraction of the same in a fluorous solvent leaving the semiconducting nanotubes in the aqueous layer. Chapter 6 presents the studies on the interaction of single walled nanotubes and graphene with various halogen molecules (I2, IBr, ICl and Br2) of varying electron affinity probed by employing Raman spectroscopy and electronic absorption spectroscopy. Part II of the thesis describes a general method of fabricating ultrathin free-standing cross-linked polymer films and their subsequent use as separation membranes. A particular class of 1-D nanomaterials namely cadmium hydroxide nanostrands were used in this method throughout, to generate a sacrificial layer upon which the polymer films were generated.

The thesis is divided into two parts. The first part deals with the research work carried out on the synthesis and chemical modification of nanomaterials whereas the second part describes the preparation and characterisation of polymer films and their use as separation membranes. Part I of the thesis describing the synthetic strategies and chemical manipulation schemes employed on various types of nanomaterials is divided into six chapters. Chapter 1 describes a chemist’s approach towards synthesizing and tuning the properties of different classes of nanomaterials along with a brief account of their potential applications. Chapter 2 of the thesis describes the synthesis and characterization of various metal nanostructures (viz. nanoparticles, nanorods, nanosheets etc.) of nickel, ruthenium, rhodium and iridium using a solvothermal procedure. Chapter 3 deals with the nanoparticles of the novel oxide metal ReO3. ReO3@Au, ReO3@Ag, ReO3@SiO2 and ReO3@TiO2 core-shell nanostructures with ReO3 as the core nanoparticle have been synthesized through a two-step process and characterized. Dependence of the plasmon band of the ReO3 nanoparticles on the interparticle separation has been examined by incorporating the nanoparticles in various polymer matrices and the results compared with those obtained with gold nanoparticles. Chapter 4 presents the dispersion of nanostructures of metal oxides such as TiO2, Fe3O4 and ZnO in solvents of differing polarity (water, DMF and toluene) in the presence of several surfactants. In Chapter 5 of the thesis, fluorous chemical method of separation of metallic and semiconducting single-walled carbon nanotubes is described. This method involves the selective reaction of the diazonium salt of a fluorous aniline with the metallic nanotubes in an aqueous medium and subsequent extraction of the same in a fluorous solvent leaving the semiconducting nanotubes in the aqueous layer. Chapter 6 presents the studies on the interaction of single walled nanotubes and graphene with various halogen molecules (I2, IBr, ICl and Br2) of varying electron affinity probed by employing Raman spectroscopy and electronic absorption spectroscopy. Part II of the thesis describes a general method of fabricating ultrathin free-standing cross-linked polymer films and their subsequent use as separation membranes. A particular class of 1-D nanomaterials namely cadmium hydroxide nanostrands were used in this method throughout, to generate a sacrificial layer upon which the polymer films were generated.

Accumulative Roll Bonding (ARB) was used to fabricate Graphene Oxide-reinforced Al-matrix composites, benefiting from a grain refinement effect of the metal-lic matrix. Graphene Oxide reinforcement was suspended in a stabilized aqueous solution and applied, prior to each ARB cycle, through airgun spraying, in order to obtain a ho-mogeneous distribution. Concentrations of 0.5, 2.5 and 10 mg/ml (graphene ox-ide/millipore water) were used. For each concentration, samples produced have under-gone up to 5 rolling cycles. Optical and electron scanning microscopies were used for microstructural char-acterization of the rolled composites which revealed a non-homogenous deformation of the layers across the composite’s thickness. Although not desired but expected, Alumin-ium oxide formation occurred where the reinforcement was applied. Although the presence of graphene-oxide promoted an increase in the micro-hardness, higher values were obtained with its lowest concentration for the same num-ber of ARB cycles. The number of ARB cycles and the direction of the tested sections also influenced the microhardness results since the 5-cycle samples and the rolling di-rection sections for all the samples achieved higher hardness results. Graphene Oxide revealed to be a major contributor to the increase of stiffness during bending of the test-ed samples. The ARB processed samples revealed a decrease on the electrical conductivity when compared with the annealed Aluminium, since the Graphene Oxide and Alumini-um oxide have insulator properties. The contribution of the reinforcement to this de-crease was only noticeable when using higher concentrations, since the sample with less concentration of the applied Graphene Oxide revealed slightly better results than the sample without any reinforcement.

In this thesis, different types of innovative highly performing piezoelectric nanomaterials and nanocomposites have been synthesized and characterized for energy harvesting application. In order to evaluate the piezoelectric properties of the produced materials, a novel approach to quantitatively evaluate the effective piezoelectric coefficient d33, trough Piezoresponse Force Microscopy (PFM), has been developed. PFM is one of the most widely used techniques for the characterization of piezoelectric materials at nanoscale, since it enables the measurement of the piezo-displacement with picometer resolution. PFM is a non-invasive and easy to use test method; it requires only a bottom electrode (no need of a top-electrode deposition over the material under test), thus considerably simplifying the test structure preparation. In particular, in order to have a quantitative information on the d33 a calibration protocol was developed. To get a macroscale characterization of the piezoelectric coefficient, the PFM signal is averaged over different areas of the sample. The proposed method allows to precisely evaluate the piezoelectric coefficient enabling a proper comparison among the different materials analysed. Two different classes of piezoelectric materials have been synthesized and characterized: zinc oxide nanostructures, in particular zinc oxide nanorods (ZnO-NRs) and zinc oxide nanowalls (ZnO-NWs), polyvinylidene fluoride (PVDF) nanocomposites films. The produced piezoelectric materials were fabricated using process which are cost-effective, time-consuming and easy to scale-up. The ZnO nanostuctures were grown by chemical bath deposition (CBD), that guarantees high deposition rate on a wide variety of substrates. PVDF nanocomposite films were produced with a simple solution casting method, without the need of subsequent electrical poling step. To enhance the piezoelectric properties of PVDF films we investigated different PVDF nanocomposite films: PVDF filled with Graphene nanoplatelets (GNPs) or with ZnO-NRs; PVDF filled with different types of hexahydrate metal-salts (HMS); PVDF filled with HMS in combination with nanofillers, like GNPs or ZnO-NRs. We found that the piezoelectric coefficient of the ZnO-NRs is (7.01±0.33) pm/V and (2.63±0.49) pm/V for ZnO-NWs. The higher piezoelectric response of ZnO-NRs is believed to be due to a better crystallinity and a less defectiveness of the ZnO-NRs if compared to the ZnO-NWs, as it has been confirmed by X-ray diffraction (XRD) spectra and by photoluminescence spectroscopy (PL) measurements. The neat PVDF show a d33 limited to 4.65 pm/V; when the nanofillers are added the d33 increases up to 6 pm/V. This value reaches 8.8 pm/V when a specific hexahydrate metal-salts: [Mg(NO3)2∙6H2O] is dispersed in the PVDF polymer matrix. From the comparative analysis of the synthesized materials we found that the sample produced using the dissolution of HMS in PVDF shows the best piezoelectric response (8.8 pm/V) and the most attractive structural and mechanical properties to fabricate a flexible nanogenerators. Therefore, a porous piezoelectric HMS-PVDF nanocomposite film has been used as active material to fabricate flexible nanogenerator. To build such a device, graphene-gold flexible top electrodes were developed. The bilayer electrode structure avoids short circuits between top and bottom electrodes, observed in the absence of graphene interlayer. The nanogenerator was tested using a commercial mini-shaker and operated successfully. The piezoelectric coefficient determined from the electromechanical tests was 9.00 pm/V, which is in good agreement with the one (8.88±3.14) pm/V measured through PFM on the same PVDF film without top electrode. We also measured the piezoelectric coefficient of PVDF using PFM with and without top electrode and both values were found to be in close agreement. This finding suggests that the local characterization using PFM is also a good representation of the global piezoelectric properties of the samples. The progress on advanced piezoelectric materials reported in this work opens new opportunities to fabricate energy harvesters and sensors for wearable and smart clothing applications.