11516 | J. Mater. Chem. C, 2015, 3, 11516--11523 This journal is©The Royal Society of Chemistry 2015

Cite this: J.Mater. Chem. C, 2015,3, 11516

Synthesis and characterization of reducedgraphene oxide/spiky nickel nanocomposite fornanoelectronic applications†

Maryam Salimian,a Maxim Ivanov,b Francis Leonard Deepak,c Dmitri Y. Petrovykh,c

Igor Bdikin,a Marta Ferro,d Andrei Kholkin,be Elby Titusa and Gil Goncalves*a

The surface modification of graphene oxide (GO) sheets with Ni nanoparticles has been a subject of intense

research in order to develop new preeminent materials with increased performance for different application

areas. In this work, we develop a new hydrothermal one-step method for the simple and controllable synthesis

of reduced GO/nickel (GO/Ni) nanocomposites. Different reaction parameters have been investigated in order

to control the synthetic process: reaction temperature, concentration of the nickel precursor and reducing

agent. It was observed that the critical parameter for effective control of nickel particle size, morphology,

crystalline structure and distribution at the GO surface during the reaction process was the concentration of

hydrazine. The results obtained showed that control of hydrazine concentration allows obtaining crystalline

metallic Ni nanoparticles, from spherical to spiky morphologies. For nanocomposites with spiky Ni nano-

particle, the reaction time allows controlling the growth of the nanothorn. The electrical properties of

reduced graphene nickel nanocomposites containing spiky nickel particles showed a large resistive

switching, which is essentially due to the switchable diode effect that can be used as a built-in part of

graphene-based embedded electronics.

1. IntroductionGraphene, a two dimensional carbon material with honeycombstructure, has received a lot of attention due to its remarkableproperties since its discovery in 2004.1 The unique structureallows achieving extraordinary properties, such as high thermaland electrical conductivity. These properties have been furtherexplored to overcome the forthcoming thermal problem inelectronic circuits and low specific power density in lithium-ion batteries.2–4 As has been reported, graphene is the strongestmaterial that has ever been examined.5 Besides these extraordinaryproperties that arise from the nature of graphene, new synergeticproperties can be achieved by manipulating or decorating itssurface with different kinds of biological molecules and materials,

macromolecules or nanoparticles. The incorporation of nano-particles has been one interesting approach to increase theapplication range of graphene-based materials.

Graphene nanocomposites can be obtained by methodo-logies that consist of the direct growth of nanoparticles on thegraphene surface or processes that allow the self-assembly ofnanoparticles at the graphene surface.6 It was observed that thedegree of oxidation of graphene7 and heteroatom doping ofgraphene8 have a strong influence on the structure of final nano-composites. Functionalization of graphene with metal and metaloxide nanoparticles allows the development of new nano-composite materials for diverse applications in fields such ascatalysis, electronics, biology, magnetisms, and optoelectronics.

In order to develop new graphene nanocomposites withmagnetic properties, the most used strategy involves the assemblyof magnetic nanoparticles, among which the nanoparticles basedon iron,9 nickel10 and cobalt11 are the most relevant. Differentsynthetic routes have been developed in order to create graphene/nickel hybrids in a reproducible and controlled manner. Chenet al. reported the preparation of Ni–graphene hybrids usingNaHB4 as a reductant and NaOH as an alkaline medium undermicrowave irradiation.12 Choi et al. prepared a nanostructuredNi/graphene hybrid for electrochemical hydrogen storage,using GO in ethylene glycol and nickel(II) nitrate hexahydrate(Ni(NO3)2!6H2O) as a precursor while reducing the reassembled

a TEMA-NRD, Mechanical Engineering Department, University of Aveiro,3810-193 Aveiro, Portugal. E-mail: ggoncalves@ua.pt

b CICECO-Materials Institute of Aveiro & Department of Physics,University of Aveiro, 3810-193 Aveiro, Portugal

c International Iberian Nanotechnology Laboratory (INL), Av. Mestre Jose Veiga,4715-330 Braga, Portugal

d Department of Material and Ceramic Engineering, University of Aveiro, 3810-193,Aveiro, Portugal

e Institute of Natural Sciences, Ural Federal University, 620000 Ekaterinburg, Russia† Electronic supplementary information (ESI) available: FT-IR characterizationdata and HAADF-STEM data with corresponding elemental information. See DOI:10.1039/c5tc02619a

Received 22nd August 2015,Accepted 8th October 2015

DOI: 10.1039/c5tc02619a

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hybrid under a mixed gas of H2/N2 (5 : 95).13 Gotoh et al.produced graphene sheets decorated with metal or metal oxidenanoparticles, including nickel nanoparticles, using GO, NH3

solution and the [Ni(NH3)6]Cl2 precursor followed by calcinationof the nickel complex, at temperatures ranging from roomtemperature to 673 K.14 Bhowmik et al. showed a new multistepmethod for the synthesis of reduced GO/Ni nanocompositesbased on the previous synthesis of Ni(OH)2/GO nanocomposites,followed by thermal treatment in air at 380 1C to promote theconversion of NPs into NiO and under H2 for the final conversionto metallic nickel.15 Recent studies showed the use of alkalihydroxides, such as NaOH or KOH, for previous pH-adjustedreaction medium for the synthesis of nickel nanoparticles.16–18

It was also observed that the complexation of Ni with ureafollowed by reduction with hydrazine allows achieving reducedGO/Ni nanocomposites.19 However, one of the most commonlyused reducing agents for the synthesis of nickel nanoparticles onthe surface of GO nanosheets is hydrazine hydrate. Wang et al.reported a simple microwave-assisted method for the synthesis ofNi nanospheres in ethylene glycol solution for the development ofglucose sensors.20 Graphene/nickel nanocomposites were alsoprepared via a hydrothermal process through hydrazine reductionof nickel precursors on the surface of GO nanosheets.21,22 Ji et al.observed that the concentration of nickel ions has an importantinfluence on the morphology of the nanocomposites.21

In this work, we developed a new approach for the synthesis ofGO/nickel nanocomposites by a one-step facile, cheap and environ-mentally friendly hydrothermal route. We investigated the influ-ence of several experimental parameters, such as time, temperatureand reducing agent concentration (hydrazine), on the morphology,crystalline phase and distribution of nickel nanoparticles on theGO surface. The results revealed that the size, shape and distribu-tion of nickel particles on the GO surface can be simply tailored byadjusting the hydrazine hydrate concentration in the reactionmedium. For nanocomposites with spiky nanoparticles, it wasobserved that the reaction time can control the growth of thenanothorn. It was also observed that hydrazine hydrate promotesthe reduction of GO during the synthesis process of the nano-composites. Electronic characterization showed that spiky nickelparticles implanted in the rGO matrix enhance conductivity, withnonlinearity observed in current–voltage dependence if the outputelectrodes are attached. Via the equilibrium energy band diagrams,we confirmed that all the experimental structure components(Ni particles, rGO, NiO and Pt-tip) are exactly in place, so NiO playsa role of the gate insulator. In general this structure works like agraphene based transistor switch or a switchable diode embeddedin a graphene-based matrix, both of these could be very useful forgraphene-based embedded nanoelectronic applications.

2. Experimental section2.1 Synthesis of graphene oxide

GO was prepared by chemical exfoliation of graphite (graphitepowder, o45 mm, Z99.99%, Sigma-Aldrich) following the modifiedHummers method.16 Basically, it consists of the cleavage of the

interacting carbon planes in graphite into individual nanosheetsunder strong acidic (H2SO4) and oxidizing (KMnO4) conditions.The resultant suspension was extensively washed with distilledwater by filtration and the resulting GO was freeze-dried in orderto avoid agglomeration of the particles.

2.2 Synthesis of reduced graphene oxide/nickelnanocomposites

Graphene oxide (10 mg) was dispersed in 10 ml of deionizedwater using an ultrasonic bath for 3 hours. A solution ofNi(NO3)2 (0.07 mol l"1) was prepared by dissolving 27 mg ofNi(NO3)2 in 2 ml of deionized water. The two solutions weremixed together and stirred for two hours, and further sonicatedfor one hour. After sonication the desired quantity of hydrazinehydrate was added to the solution and stirred for one hour.Then the solution was transferred to a 25 ml Teflon autoclaveand kept in a furnace for different periods of time at 100 1C.The final nanocomposites were washed with deionized waterand freeze-dried. The hydrazine concentration and reactiontime used in this work are summarized in Table 1, with therespective effects on nickel nanoparticle shape, size and crystal-line structure.

3. CharacterizationPowder X-ray diffraction data were collected using a SiemensD500 diffractometer with secondary monochromator CuKaradiation in the 5–851 range with steps of 0.051, the time forcollecting X-rays being 50 s for each measuring point at 30 mAand 40 kV. Morphological studies were performed using anultra-high resolution analytical scanning electron microscopeHR-FESEM Hitachi SU-70 and a Quanta 650 FEG ESEM (FEI).Transmission Electron Microscopy (TEM), High ResolutionTransmission Electron Microscopy (HRTEM), Selected AreaElectron Diffraction (SAED) and Scanning/Transmission Elec-tron Microscopy (STEM) analysis were performed using a TitanChemiSTEM 80–200 kV probe Cs corrected microscope. Energydispersive X-ray analysis (EDS) spectra and elemental mapswere acquired on a Super-X EDS system. X-ray PhotoemissionSpectroscopy (XPS) analysis was carried out on an ESCALAB 250Xi (Thermo Scientific) system equipped with a monochromaticAl Ka X-ray source and a sample-charge neutralization system.Fourier transform infrared (FTIR) spectroscopy was carried outusing a FTIR spectrometer with the ATR accessory using a

Table 1 GO/Ni nanocomposites prepared under different experimentalconditions. A description of the crystalline structure, particle size and shapeof Ni nanoparticles achieved depending on the experimental conditions

SamplesN2H4(mol l"1)

Reactiontime (h)

Crystallinephase (XRD)

Particlesize (nm)

Particleshape

G/Ni1 0.08 22 Ni(OH)2 — —G/Ni2 0.17 22 Ni metallic/Ni(OH)2 145 SphericalG/Ni3 0.83 5 Ni metallic 300 SpikyG/Ni4 0.83 11 Ni metallic 300 SpikyG/Ni5 0.83 22 Ni metallic 300 SpikyG/Ni6 4.1 22 Ni metallic 900 Spherical

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Perkin-Elmer Spectrum BX at room temperature. Samples weremixed with KBr and were then prepared in the shape of platesfor Scanning Probe Microscopy (SPM) measurements. A com-mercial SPM (Ntegra Aura, NT-MDT, Russia) was used in AtomicForce Microscopy (AFM) mode, Conductive AFM (C-AFM) alsoknown as Spreading Resistance (SR) mode, Kelvin Probe (KPFM)mode and Piezoresponse Force Microscopy (PFM) mode in orderto perform morphological, conductivity and supposititiouspiezoeffect analyses of the sample. A conductive Si cantilevercoated with Pt (CSG30/Pt, NT-MDT, Russia) with a spring con-stant of 0.6 N m"1 and a resonance frequency of B48 kHz wereused for topography and spreading resistance investigation.

4. Results and discussion4.1 Influence of reaction parameters on the G/Ninanocomposite structure

The synthesis of G/Ni nanocomposites was performed by thechemical interaction of the nickel ions and the GO surface. Inthe past it has been reported that the metallic ions can bereduced by the oxygen functional groups at the GO surface thatcan act as nucleation sites for the nucleation and growth ofmetallic nanoparticles.16 Besides, our experimental results showedthat nickel nanoparticle growth and crystalline phase can becontrolled at the GO surface by changing the concentration ofhydrazine in the reaction medium.

X-ray diffraction (XRD) patterns of G/Ni nanocomposites areshown in Fig. 1. The corresponding peak of GO at 10.501 shiftedto 23.41 and broadened according to the effective reduction toRGO13 during the synthesis of G/Ni nanocomposites. Thediffraction profile of the G/Ni1 nanocomposite prepared withthe lower concentration of N2H4 (0.08 mol l"1) showed thepreferential formation of Ni(OH)2, with the presence of peaks at19.381, 33.321, 38.781, 52.131, 59.381, 62.981, 70.231, and 73.071which are attributed to (001), (100), (101), (102), (110), (111),(200), (103), and (201) planes, respectively.23 The G/Ni2 nanocom-posite prepared with a high concentration of N2H4 (0.17 mol l"1) inits reaction medium showed the presence of diffraction peaks

corresponding to the metallic nickel phase at 44.71, 52.01 and76.51, which can be attributed to (111), (200) and (220) planes ofnickel in a face-centered cubic structure.24 We also observed thepresence of minor peaks that can be attributed to a secondaryphase of residual Ni(OH)2.

The increase in concentration of N2H4 to 0.83 and 4.1 mol l"1

for samples G/Ni5 and G/Ni6, respectively, showed only thepresence of diffraction peaks of metallic nickel without anyevidence of the Ni(OH)2 phase or other impurities. These resultsclearly demonstrated that the crystalline phase obtained in nickelgraphene nanocomposites through the hydrothermal reaction ofNi(NO3)2 is dependent on N2H4 concentration. However, it wasalso observed that the reaction time does not have a preponderanteffect on the crystalline structure of nickel particles. XRD showedthat for different reaction times from 5 to 11 and to 22 h (samplesG/Ni3, G/Ni4 and G/Ni5, respectively,) the crystalline phaseobtained in nanocomposite materials is pure metallic nickel.

In fact it was observed that the concentration of N2H4 andthe reaction time are important experimental parameters forcontrolling the size and shape of nickel nanoparticles in thestudied G/Ni nanocomposites. SEM images of G/Ni nano-composites (Fig. 2 and 3) showed that the nickel nanoparticleshave different particle morphologies and they are really integratedand well dispersed on the surface of GO sheets. Fig. 2 shows thatthe nanocomposites prepared with a lower concentration of N2H4

do not promote the formation of nickel nanoparticles at the GOsurface (G/Ni1). The increased concentration of N2H4 (G/Ni2)allows the formation of spherical nickel particles with the averagesize of 145 nm and is distributed homogeneously on the surfaceof rGO. A further increase of the N2H4 concentration induceddrastic changes to the morphology and the size of nickel nano-particles. At this stage, spiky nickel particles with the average sizeof 300 nm (G/Ni5) were obtained homogenously at the rGO surface.For the nanocomposites prepared with the highest concentrationof N2H4 (G/Ni6), we observed the presence of agglomerated nickelparticles on the surface of rGO with an average size of 900 nm.These results indicated the crucial role of N2H4 concentration incontrolling the size, morphology and distribution of nickel particlesin G/Ni nanocomposites.

Fig. 1 XRD profile of G/Ni nanocomposites synthesized with differentN2H4 concentrations (G/Ni1, G/Ni2, G/Ni5, G/Ni6, and G/Ni7) and differentreaction times (G/Ni3, G/Ni4, and G/Ni5).

Fig. 2 SEM images of G/Ni nanocomposites synthesized with differentN2H4 concentrations.

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Previous studies showed the effect of reaction medium pHon the formation of nickel particles as well.25 NaOH has beentypically used as a basic precursor for controlling the reactionmedium pH during the synthesis of nickel particles.21 It is wellknown that the basic medium adjusted by NaOH can changethe reaction effects of hydrazine. Ni et al. showed that themorphology of nickel particles can be adjusted in a certain rangeof base concentration for hydrazine reduction reactions.26 Infact, it was already observed that other reaction parameters, suchas temperature and magnetic field, also play an important role incontrolling the morphology of nickel nanoparticles during theirreduction by N2H4.17

According to our FTIR study of nanocomposites (see Fig. S1in ESI†) N2H4 not only affects the particle morphology but alsoinduces the reduction of GO by elimination of oxygen func-tional groups during the hydrothermal process. FTIR spectra ofG/Ni nanocomposites showed a clear reduction of the bandscorresponding to the oxygen functional groups, revealing onlythe presence of few residual bands of oxygen functional groups.27

As mentioned above, oxygen functional groups at the GO surfacecan act as electrophilic agents for the nucleation of nickel ionsand the growth of nanoparticles. However, it was observed thatthe increase of the N2H4 concentration in solution contributes tothe particle shape formation by the possible coordination of Niions though nitrogen donor groups of N2H4, which acts asa strong reducing agent to form nickel particles. The nano-composites exhibited different particle morphologies dependingon the N2H4 concentration: spherical, spiky and big agglomerates(Fig. 2). The growth mechanism of spiky nickel particles can beexplained according to Mathew et al.28 There are two main stepsin the formation of flower-like structures: first, formation of acore, then, as the reduction reaction continues, the newlyformed particles can be adsorbed onto the surface of existingparticles acting as seeds to form nanothorns on the surface ofthe nickel core. It was observed that at higher temperature thespeed of crystal growth is decreased due to a pronouncedmovement of nickel ions in solution. Therefore, a longerreaction time is needed to form the flower-like morphology.29

Although we found that the reaction time does not affect thecrystallinity of the nickel particles, it has an important effect onthe structure of spiky nickel nanoparticles in G/Ni nanocompo-sites. SEM images of samples G/Ni3, G/Ni4 and G/Ni5 showed

the growth of anisotropic nickel particles on the surface of rGOsheets (Fig. 3).

These images present the dependence of nanothorn size onthe reaction time. After 5 hours of hydrothermal treatment,nickel particles showed some anisotropic deformation on theirsurface indicating the beginning of the formation of the nano-thorns. As the reaction time increased to 11 and 22 hours, thesize of the nanothorns also increased.

4.2 rGO nanocomposites with spiky nickel nanoparticles

GO sheets homogenously modified with spiky nickel nano-particles were further explored in this work due to theirpeculiar structure that can confer novel interesting propertiesto the composite materials. TEM analysis of individual spikynickel particles at the surface of the RGO sheet (Fig. 4) showedthat its peculiar structure is composed of two different regions,a core (1) surrounded by nanothorns (2). Statistical analysis ofnickel nanoparticles showed that the size of nanothorns variesfrom 40 nanometers to 100 nanometers and the core size isabout 200 nm. The selected area diffraction pattern (SAED) ofthe regions (1 and 2 in Fig. 4) proved the presence of a thin layerof nickel oxide.

HRTEM images of a single nanothorn, including a magnifiedclose-up of a selected area, are shown in Fig. 5. The lattice planeswith the spacing of 2.089 Å, which correspond to the (200) latticeplanes (NiO SG: Fm3m Cubic 00-047-1049ICDD Database), con-firmed the presence of nickel oxide layer structure on the nano-thorn. The surface oxidation of nickel nanoparticles was notdetected by XRD measurements, which indicates that they are avery small fraction compared with the metallic phase.

The presence of an oxidized surface has been further con-firmed by X-Ray Photoelectron Spectroscopy (XPS) measure-ments carried out for samples G/Ni3, G/Ni4 and G/Ni5 (Fig. 6).30

For all three samples, the C 1s spectra (data not shown) havethe asymmetric lineshape characteristic of graphitic materials,as would be expected for rGO with hydrazine.30 The Ni 2pspectra of these materials are qualitatively similar to thosepreviously reported for NiO on rGO.31 The binding energy(BE) of the strongest Ni 2p3/2 component is above 855 eV for

Fig. 3 SEM images of G/Ni3, G/Ni4 and G/Ni5 nanocomposites at threedifferent reaction times 5, 11 and 22 hours, respectively, showing the increasein the growth of spiky nickel nanoparticles. A schematic representation ofmorphology changes of nickel nanoparticles with reaction time.

Fig. 4 TEM image of one single spiky nickel particle at the surface of theRGO sheet. Diffraction pattern (SAED) of the core (1) and a single nano-thorn (2) with the respective crystalline planes of nickel oxide.

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all the samples in Fig. 6. While BE 4 855 eV is occasionallyreported for NiO samples, in the NIST XPS Database,32 themajority of BE values are below 855 eV for NiO, so based on theBE values it is more likely to assign the material in the oxidizedsurface layer of these samples as Ni(OH)2. The small shouldersobserved at BE of ca. 853 eV indicate the presence of metallic Niwithin the sampling depth of XPS (which is ca. 5 nm at this BE).

For materials with complex nanostructured morphology, anunambiguous interpretation of surface layer composition isdifficult, however, the data in Fig. 6 are consistent with a thin(o5 nm at least in some areas) oxidized surface layer covering ametallic Ni particle. Given the nanoscale size and spiky mor-phology of the nanothorns, the XPS data indicate that nano-thorns are not primarily composed of metallic Ni, as they wouldrepresent a large fraction of the overall nanoparticle volumesampled by XPS. Finally, compared to G/Ni3 and G/Ni4 samples,only a minimal amount of metallic Ni is detected in the G/Ni5sample, suggesting that longer reaction times increase not onlythe nanothorn size but also the degree of surface oxidation.

Fig. 7 shows the high-angle annular dark-field scanningtransmission electron microscopy (HAADF-STEM) image of theG/Ni5 nanocomposite and the corresponding elemental maps ofnickel (green) and oxygen (red). These data further proved the

oxidation of nickel particles on the surface, which is in agree-ment with the SAED and TEM data. Accordingly, the local EDXspectra of a single nickel particle confirmed the presence of avery thin oxidation layer covering the core and the nanothorns(see ESI,† and discussion therein).

4.3 SPM measurements of G/Ni nanocomposites

SPM measurements performed to simultaneously acquire topo-graphy and spreading resistance images showed a clear mani-festation of the presence of Ni particles in the rGO matrix(Fig. 8).

The topography analysis confirmed that the sample surfaceis sufficiently smooth with the RMS roughness less than 1 nm(Fig. 8(a)) and did not influence the measured current distribu-tion. SR images represent a real mapping of the sample con-ductivity (Fig. 8(b)). Moreover, this distribution demonstrateda good correlation with SEM images (Fig. 8(c)) with the Niparticles dispersed in the rGO matrix within the equal appor-tionment statistics. By means of a comparison of both SPM andSEM methods we could mark the Ni shell (Fig. 8(d)) and itsresponse on SR and SEM scans, respectively.

Kelvin Probe Force Microscopy (KPFM) mode showed adistribution of electric potential on the sample surface (Fig. 9(a)),where the Ni particles exhibit higher potential as comparedto the rGO matrix, and Ni particle cores and spiky shells couldbe distinguished, as they have a maximum potential due tothe electric field concentrated on the point of shells (insert toFig. 9(a)).

This resulted in white dots with a maximum KPFM signalaround the Ni particles. The contact SR mode reveals theconductive regions that are related to the Ni particle core andshell conglomerates, as confirmed by KPFM results (Fig. 9(b)).The current–voltage dependence has been then studied in twosteps based on the most conductive (green circle) and least

Fig. 5 HRTEM images of a single nanothorn (on the left) and its selectedarea with high resolution (on the right) showing the lattice structure of NiO.

Fig. 6 Ni 2p XPS spectra of samples G/Ni3, G/Ni4, and G/Ni5.

Fig. 7 High-angle annular dark-field scanning transmission electronmicroscopy (HAADF-STEM) image of the G/Ni5 nanocomposite and thecorresponding elemental maps of nickel (Ni) and oxygen (O).

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conductive ( yellow circle) points, which are related to Niparticles and rGO, respectively. The I–V behavior, showed anonlinear and highly reproducible current hysteresis behavior,indicating a large resistive switching, which can be described asthe switchable diode effect (Fig. 9(c)).33 The measurementswere performed with 20 cycles by sweeping the bias voltage ofthe cantilever tip from "5 to 5 V and back to "5 V, repeatedly.Moreover, the forward and backward curves showed an obviousdiode-like rectifying I–V characteristic, indicating a forward passfor backward pass diode behavior and a reverse diode behaviorfor a forward sweep. It can be seen that during the measuringcycle the diode polarity can be switched at around 2 V.

In order to exclude the supposititious ferroelectric polariza-tion effect, the relationship between the current hysteresis andthe PFM response has been determined by increasing the voltagesweep range step by step.34 During all the experiments the PFMresponse was absent and signal fluctuations were at the noiselevel. The current–voltage behavior of the rGO point also shows

nonlinearity similar to the Schottky effect but does not have anyhysteresis behavior (Fig. 9(d)). This could be due to the effect ofNi particles on current–voltage behavior of the sample thatmainly leads to the switchable diode effect.35

Based on the analysis of our results, the switchable diodebehavior in the sample can be explained qualitatively by the Niparticle modulation of Schottky-like barriers at both bottomand top (cantilever’s tip) electrodes. The ideal Schottky barrierat a metal–semiconductor interface is determined by the differ-ence in the metal work function and the semiconductor electronaffinity. The work function of rGO is taken as 4.4 eV,36 the workfunction of Ni is 4.6 eV37 and the work function of Pt is about5.3 eV.33 The NiO layers should also be taken into accountbecause oxygen contributes significantly to the electronic stateof Ni particle shells, as confirmed by XPS and HAADF-STEMmethods. The NiO band gap energy is 3.6 eV and the electronaffinity is 5.3 eV.38 When rGO, Ni and Pt are joined together in adiode-like structure during the C-AFM experiment, it is obviousthat the electrons move faster from Pt to rGO, due to the higherwork function of Pt than that of rGO, leaving behind positivecharges in NiO. Then current depletion regions are formed bythe differences in charge carrier velocity through the bottom andtop electrodes (cantilever’s tip), respectively, and the built-incurrent–voltage switchable diode effect (Fig. 10).

The influence of NiO is taken into account because of itsconsiderable role as a buffer layer. The effect was consideredin the scope of equilibrium energy band diagrams. The equili-brium energy band diagrams of rGO/NiO/Ni/Pt heterostructuresare shown in Fig. 10. In this structure, the height and width ofthe contact barrier between graphene, Ni and Pt are defined asdifferences in the work function "ej. In the NiO contact areathe widths of the contact barrier are correspondingly definedas eDNiO " ej. Compared with the contact barrier betweenrGO, Pt and Ni the energy band bending at the NiO interface(Fw, CNiO o ej) is much smaller due to the large work functionof NiO. In addition, the ultrathin p-type NiO layer reduces thecontact barrier width at the NiO/Ni interface (DNiO, "ej + Ni).Therefore, with the presence of the NiO buffer layer, electronscan pass through the barrier more easily.35

Fig. 8 Images of the rGO matrix with Ni particles in (a) SPM microscopymode (b) spreading resistance SPM mode, (c) SEM mode and (d) a singlespiky nickel particle.

Fig. 9 SPM images of the rGO matrix with Ni particles in (a) Kelvin ProbeForce Microscopy mode and (b) spreading resistance mode. Current–voltage dependence on the (c) most conductive (Ni particle) and (d) lessconductive (rGO matrix) areas. Fig. 10 The equilibrium energy band diagrams of rGO/Ni/Pt structure.

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5. ConclusionIn summary, we showed a new one-step hydrothermal approachfor the controlled synthesis of rGO/Ni nanocomposites usinghydrazine as a reducing agent. We observed that N2H4!H2Oconcentration is a key experimental parameter to control thesize, morphology, distribution and crystalline structure ofnickel particles at the GO surface. Hydrazine has a major rolein this reaction not only because it allows control of thenucleation and growth of Ni metallic nanoparticles throughthe reduction of nickel ions, but also by reducing the surface ofGO. The results obtained for the synthetic process showed thatthe increase of N2H4!H2O concentration in the reaction mediumcorresponds to an increase of nickel particle sizes, varying from145 to 900 nm, and also affects the nickel particle morphologies,from spherical to spiky and finally to big agglomerates. For thepreparation of all nanocomposites we did not use any alkalinemedia, despite previous reports on such nanoparticle growth,indicating that the pH value can be adjusted in solution using anappropriate amount of N2H4!H2O. For nanocomposites with spikynanoparticles (rGO/spiky Ni) it was observed that the increase ofthe reaction time promotes the growth of nickel nanothorns. Theresults also indicate that spiky nickel particles are composed of acore/shell structure: a metallic Ni core and a few nm thin outerlayer of NiO. Furthermore, it is observed that the thickness of theNiO outer layer increases with the increase of the reaction time.

By means of spreading resistance SPM mode we have shownthat spiky nickel particles implemented in the rGO matrixenhance conductivity with nonlinearity in current–voltage depen-dence if the output electrodes are attached. Via the equilibriumenergy band we confirmed that all the experimental structurecomponents (Ni particles, rGO, NiO and Pt-tip) are exactly inplace, even NiO plays a role of gate insulator. In general thisstructure works like a graphene-based transistor switch orembedded in a graphene based matrix switchable diode bothof which could be very useful for graphene based embeddednanoelectronics applications.

AcknowledgementsMaryam Salimian and Gil Gonçalves thank the Fundaçao para aCiencia e Tecnologia (FCT) for the PhD (SFRH/BD/98337/2013)and PostDoc (SFRH/BDP/84419/2012) grants, respectively. MaximIvanov acknowledges FCT for his postdoctoral grant FCT UID/CTM/50011/2013. This work was developed in the scope of theproject CICECO-Aveiro Institute of Materials (Ref. FCT UID/CTM/50011/2013), financed by national funds through the FCT/MECand when applicable co-financed by FEDER under the PT2020Partnership Agreement.

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Electronic Supplementary Material

Synthesis and characterization of reduced graphene oxide /spiky nickel nanocomposites for nanoelectronic applications

Maryam Salimiana, Maxim Ivanovb, Francis Leonard Deepakc, Dmitri Petrovykhc, Igor Bdikina,b ,Marta Ferrod, Andre Kholkinb, Elby Titusa and Gil Goncalvesa*

aTEMA-NRD, Mechanical Engineering Department, University of Aveiro, 3810-193 Aveiro, Portugal

bCICECO-Materials Institute of Aveiro & Department of Physics , University of Aveiro, 3810-193 Aveiro, Portugal

cINL-International Iberian Nanotechnology Laboratory Av. Mestre Jose Veiga s/n 4715-330 Braga, Portugal

dDepartment of Material and Ceramic Engineering, University of Aveiro 3810-193,Aveiro, Portugal

Figure S1 shows FT-IR spectra of GO, G/Ni3, G/Ni4 and G/Ni5 nanocomposites. GO profile

pattern represents a complete oxidation of graphite. The broad band at high frequency (2800-

3600) cm-1 and also a band at 2360 cm-1 related to vibration of OH group. Adsorption bands in

1722 cm-1, 1620 cm-1 and 1044 cm-1 confirmed the vibration of C=O (in COOH), C=C and C-O

groups respectively. In RGO/Ni FTIR spectrum there are two weak peaks at 1558 cm-1 and 1176

cm-1 .The former is related to C-O vibration band and the later one is related to Graphene sheets

vibration. The rest of oxygen functional groups are not exist anymore according to the reduction

of graphene oxide [1].

Electronic Supplementary Material (ESI) for Journal of Materials Chemistry C.This journal is © The Royal Society of Chemistry 2015

High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) of

G/Ni5 nanocomposite with corresponding elemental information in the core of a nickel particle

and in a single nanothorn are shown in figures S2 and S3 respectively. This information

indicated that the core and the nanothorn of nickel particle composed of about 99% metallic

nickel and around 1 % oxygen. This confirmed a very thin oxidation layer covering the nickel

particles both around the cores and the nanothorns.

Figure S1.FT-IR spectra of GO, G/Ni3, G/Ni4 and G/Ni5 nanocomposites.

References1. Ji, Z.Y., et al., Reduced graphene oxide supported FePt alloy nanoparticles with high

electrocatalytic performance for methanol oxidation. New Journal of Chemistry, 2012. 36(9): p. 1774-1780.

Figure S2. High-angle annular dark-field scanning transmission electron microscopy of G/Ni5 nanocomposite and corresponding elemental information at selected area of nickel core.

Figure S3. High-angle annular dark-field scanning transmission electron microscopy of G/Ni5 nanocomposite and corresponding elemental information at selected area of one single nanothorn.