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

Article abstract of DOI:10.1039/c5tc02619a

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 nanoparticle, 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.

Full text of DOI:10.1039/c5tc02619a

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Synthesis and characterization of reduced
graphene oxide/spiky nickel nanocomposite for
Cite this:DOI: 10.1039/c5tc02619a
nanoelectronic 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.
Received 22nd August 2015,
Accepted 8th October 2015
DOI: 10.1039/c5tc02619a
www.rsc.org/MaterialsC
macromolecules or nanoparticles. The incorporation of nano-
particles has been one interesting approach to increase the
1. Introduction
Graphene, a two dimensional carbon material with honeycomb
structure, has received a lot of attention due to its remarkable
properties since its discovery in 2004.1 The unique structure
allows achieving extraordinary properties, such as high thermal
and electrical conductivity. These properties have been further
explored to overcome the forthcoming thermal problem in
electronic circuits and low specific power density in lithium-
ion batteries.2–4 As has been reported, graphene is the strongest
material that has ever been examined.5 Besides these extraordinary
properties that arise from the nature of graphene, new synergetic
properties can be achieved by manipulating or decorating its
surface with different kinds of biological molecules and materials,
application range of graphene-based materials.
Graphene nanocomposites can be obtained by methodo-
logies that consist of the direct growth of nanoparticles on the
graphene surface or processes that allow the self-assembly of
nanoparticles at the graphene surface.6 It was observed that the
degree of oxidation of graphene7 and heteroatom doping of
graphene8 have a strong influence on the structure of final nano-
composites. Functionalization of graphene with metal and metal
oxide nanoparticles allows the development of new nano-
composite materials for diverse applications in fields such as
catalysis, electronics, biology, magnetisms, and optoelectronics.
In order to develop new graphene nanocomposites with
magnetic properties, the most used strategy involves the assembly
of magnetic nanoparticles, among which the nanoparticles based
on iron,9 nickel10 and cobalt11 are the most relevant. Different
synthetic routes have been developed in order to create graphene/
nickel hybrids in a reproducible and controlled manner. Chen
et al. reported the preparation of Ni–graphene hybrids using
NaHB4 as a reductant and NaOH as an alkaline medium under
microwave irradiation.12 Choi et al. prepared a nanostructured
Ni/graphene hybrid for electrochemical hydrogen storage,
using GO in ethylene glycol and nickel(II) nitrate hexahydrate
(Ni(NO3)2Á6H2O) 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 characterization
data and HAADF-STEM data with corresponding elemental information. See DOI:
10.1039/c5tc02619a
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hybrid under a mixed gas of H2/N2 (5 : 95).13 Gotoh et al. interacting carbon planes in graphite into individual nanosheets
produced graphene sheets decorated with metal or metal oxide under strong acidic (H2SO4) and oxidizing (KMnO4) conditions.
nanoparticles, including nickel nanoparticles, using GO, NH3 The resultant suspension was extensively washed with distilled
solution and the [Ni(NH3)6]Cl2 precursor followed by calcination water by filtration and the resulting GO was freeze-dried in order
of the nickel complex, at temperatures ranging from room to avoid agglomeration of the particles.
temperature to 673 K.14 Bhowmik et al. showed a new multistep
2.2 Synthesis of reduced graphene oxide/nickel
nanocomposites
method for the synthesis of reduced GO/Ni nanocomposites
based on the previous synthesis of Ni(OH)2/GO nanocomposites,
followed by thermal treatment in air at 380 1C to promote the
conversion of NPs into NiO and under H2 for the final conversion
to metallic nickel.15 Recent studies showed the use of alkali
hydroxides, such as NaOH or KOH, for previous pH-adjusted
reaction medium for the synthesis of nickel nanoparticles.16–18
It was also observed that the complexation of Ni with urea
followed by reduction with hydrazine allows achieving reduced
GO/Ni nanocomposites.19 However, one of the most commonly
used reducing agents for the synthesis of nickel nanoparticles on
the surface of GO nanosheets is hydrazine hydrate. Wang et al.
reported a simple microwave-assisted method for the synthesis of
Ni nanospheres in ethylene glycol solution for the development of
glucose sensors.20 Graphene/nickel nanocomposites were also
prepared via a hydrothermal process through hydrazine reduction
of nickel precursors on the surface of GO nanosheets.21,22 Ji et al.
observed that the concentration of nickel ions has an important
influence on the morphology of the nanocomposites.21
Graphene oxide (10 mg) was dispersed in 10 ml of deionized
water using an ultrasonic bath for 3 hours. A solution of
Ni(NO3)2 (0.07 mol lÀ1) was prepared by dissolving 27 mg of
Ni(NO3)2 in 2 ml of deionized water. The two solutions were
mixed together and stirred for two hours, and further sonicated
for one hour. After sonication the desired quantity of hydrazine
hydrate was added to the solution and stirred for one hour.
Then the solution was transferred to a 25 ml Teflon autoclave
and kept in a furnace for different periods of time at 100 1C.
The final nanocomposites were washed with deionized water
and freeze-dried. The hydrazine concentration and reaction
time used in this work are summarized in Table 1, with the
respective effects on nickel nanoparticle shape, size and crystal-
line structure.
3. Characterization
In this work, we developed a new approach for the synthesis of
GO/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, temperature
and reducing agent concentration (hydrazine), on the morphology,
crystalline phase and distribution of nickel nanoparticles on the
GO surface. The results revealed that the size, shape and distribu-
tion of nickel particles on the GO surface can be simply tailored by
adjusting the hydrazine hydrate concentration in the reaction
medium. For nanocomposites with spiky nanoparticles, it was
observed that the reaction time can control the growth of the
nanothorn. It was also observed that hydrazine hydrate promotes
the reduction of GO during the synthesis process of the nano-
composites. Electronic characterization showed that spiky nickel
particles implanted in the rGO matrix enhance conductivity, with
nonlinearity observed in current–voltage dependence if the output
electrodes 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 plays
a role of the gate insulator. In general this structure works like a
graphene based transistor switch or a switchable diode embedded
in a graphene-based matrix, both of these could be very useful for
graphene-based embedded nanoelectronic applications.
Powder X-ray diffraction data were collected using a Siemens
D500 diffractometer with secondary monochromator CuKa
radiation in the 5–851 range with steps of 0.051, the time for
collecting X-rays being 50 s for each measuring point at 30 mA
and 40 kV. Morphological studies were performed using an
ultra-high resolution analytical scanning electron microscope
HR-FESEM Hitachi SU-70 and a Quanta 650 FEG ESEM (FEI).
Transmission Electron Microscopy (TEM), High Resolution
Transmission Electron Microscopy (HRTEM), Selected Area
Electron Diffraction (SAED) and Scanning/Transmission Elec-
tron Microscopy (STEM) analysis were performed using a Titan
ChemiSTEM 80–200 kV probe Cs corrected microscope. Energy
dispersive X-ray analysis (EDS) spectra and elemental maps
were acquired on a Super-X EDS system. X-ray Photoemission
Spectroscopy (XPS) analysis was carried out on an ESCALAB 250
Xi (Thermo Scientific) system equipped with a monochromatic
Al Ka X-ray source and a sample-charge neutralization system.
Fourier transform infrared (FTIR) spectroscopy was carried out
using a FTIR spectrometer with the ATR accessory using a
Table 1 GO/Ni nanocomposites prepared under different experimental
conditions. A description of the crystalline structure, particle size and shape
of Ni nanoparticles achieved depending on the experimental conditions
N2H4
Reaction Crystalline
Particle Particle
size (nm) shape
Samples (mol lÀ1) time (h) phase (XRD)
2. Experimental section
2.1 Synthesis of graphene oxide
G/Ni1
G/Ni2
G/Ni3
G/Ni4
G/Ni5
0.08
0.17
0.83
0.83
0.83
4.1
22
22
5
11
22
22
Ni(OH)2
Ni metallic/Ni(OH)2 145
Ni metallic
Ni metallic
Ni metallic
Ni metallic
—
—
Spherical
Spiky
Spiky
Spiky
Spherical
300
300
300
900
GO was prepared by chemical exfoliation of graphite (graphite
powder, o45 mm, Z99.99%, Sigma-Aldrich) following the modified
Hummers method.16 Basically, it consists of the cleavage of the G/Ni6
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Perkin-Elmer Spectrum BX at room temperature. Samples were corresponding to the metallic nickel phase at 44.71, 52.01 and
mixed with KBr and were then prepared in the shape of plates 76.51, which can be attributed to (111), (200) and (220) planes of
for Scanning Probe Microscopy (SPM) measurements. A com- nickel in a face-centered cubic structure.24 We also observed the
mercial SPM (Ntegra Aura, NT-MDT, Russia) was used in Atomic presence of minor peaks that can be attributed to a secondary
Force Microscopy (AFM) mode, Conductive AFM (C-AFM) also phase of residual Ni(OH)2.
known as Spreading Resistance (SR) mode, Kelvin Probe (KPFM)
The increase in concentration of N2H4 to 0.83 and 4.1 mol lÀ1
mode and Piezoresponse Force Microscopy (PFM) mode in order for samples G/Ni5 and G/Ni6, respectively, showed only the
to perform morphological, conductivity and supposititious presence of diffraction peaks of metallic nickel without any
piezoeffect analyses of the sample. A conductive Si cantilever evidence of the Ni(OH)2 phase or other impurities. These results
coated with Pt (CSG30/Pt, NT-MDT, Russia) with a spring con- clearly demonstrated that the crystalline phase obtained in nickel
stant of 0.6 N mÀ1 and a resonance frequency of B48 kHz were graphene nanocomposites through the hydrothermal reaction of
used for topography and spreading resistance investigation.
Ni(NO3)2 is dependent on N2H4 concentration. However, it was
also observed that the reaction time does not have a preponderant
effect on the crystalline structure of nickel particles. XRD showed
that for different reaction times from 5 to 11 and to 22 h (samples
G/Ni3, G/Ni4 and G/Ni5, respectively,) the crystalline phase
obtained in nanocomposite materials is pure metallic nickel.
In fact it was observed that the concentration of N2H4 and
the reaction time are important experimental parameters for
controlling the size and shape of nickel nanoparticles in the
studied G/Ni nanocomposites. SEM images of G/Ni nano-
composites (Fig. 2 and 3) showed that the nickel nanoparticles
have different particle morphologies and they are really integrated
and well dispersed on the surface of GO sheets. Fig. 2 shows that
the nanocomposites prepared with a lower concentration of N2H4
do not promote the formation of nickel nanoparticles at the GO
surface (G/Ni1). The increased concentration of N2H4 (G/Ni2)
allows the formation of spherical nickel particles with the average
size of 145 nm and is distributed homogeneously on the surface
of rGO. A further increase of the N2H4 concentration induced
drastic changes to the morphology and the size of nickel nano-
particles. At this stage, spiky nickel particles with the average size
of 300 nm (G/Ni5) were obtained homogenously at the rGO surface.
For the nanocomposites prepared with the highest concentration
of N2H4 (G/Ni6), we observed the presence of agglomerated nickel
particles on the surface of rGO with an average size of 900 nm.
These results indicated the crucial role of N2H4 concentration in
controlling the size, morphology and distribution of nickel particles
in G/Ni nanocomposites.
4. Results and discussion
4.1 Influence of reaction parameters on the G/Ni
nanocomposite structure
The synthesis of G/Ni nanocomposites was performed by the
chemical interaction of the nickel ions and the GO surface. In
the past it has been reported that the metallic ions can be
reduced by the oxygen functional groups at the GO surface that
can act as nucleation sites for the nucleation and growth of
metallic nanoparticles.16 Besides, our experimental results showed
that nickel nanoparticle growth and crystalline phase can be
controlled at the GO surface by changing the concentration of
hydrazine in the reaction medium.
X-ray diffraction (XRD) patterns of G/Ni nanocomposites are
shown in Fig. 1. The corresponding peak of GO at 10.501 shifted
to 23.41 and broadened according to the effective reduction to
RGO13 during the synthesis of G/Ni nanocomposites. The
diffraction profile of the G/Ni1 nanocomposite prepared with
the lower concentration of N2H4 (0.08 mol lÀ1) showed the
preferential formation of Ni(OH)2, with the presence of peaks at
19.381, 33.321, 38.781, 52.131, 59.381, 62.981, 70.231, and 73.071
which 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 lÀ1) in
its reaction medium showed the presence of diffraction peaks
Fig. 1 XRD profile of G/Ni nanocomposites synthesized with different
N2H4 concentrations (G/Ni1, G/Ni2, G/Ni5, G/Ni6, and G/Ni7) and different
reaction times (G/Ni3, G/Ni4, and G/Ni5).
Fig. 2 SEM images of G/Ni nanocomposites synthesized with different
N2H4 concentrations.
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the growth of anisotropic nickel particles on the surface of rGO
sheets (Fig. 3).
These images present the dependence of nanothorn size on
the reaction time. After 5 hours of hydrothermal treatment,
nickel particles showed some anisotropic deformation on their
surface indicating the beginning of the formation of the nano-
thorns. As the reaction time increased to 11 and 22 hours, the
size of the nanothorns also increased.
4.2 rGO nanocomposites with spiky nickel nanoparticles
Fig. 3 SEM images of G/Ni3, G/Ni4 and G/Ni5 nanocomposites at three
different reaction times 5, 11 and 22 hours, respectively, showing the increase
in the growth of spiky nickel nanoparticles. A schematic representation of
morphology changes of nickel nanoparticles with reaction time.
GO sheets homogenously modified with spiky nickel nano-
particles were further explored in this work due to their
peculiar structure that can confer novel interesting properties
to the composite materials. TEM analysis of individual spiky
nickel particles at the surface of the RGO sheet (Fig. 4) showed
that its peculiar structure is composed of two different regions,
a core (1) surrounded by nanothorns (2). Statistical analysis of
nickel nanoparticles showed that the size of nanothorns varies
from 40 nanometers to 100 nanometers and the core size is
about 200 nm. The selected area diffraction pattern (SAED) of
the regions (1 and 2 in Fig. 4) proved the presence of a thin layer
of nickel oxide.
HRTEM images of a single nanothorn, including a magnified
close-up of a selected area, are shown in Fig. 5. The lattice planes
with the spacing of 2.089 Å, which correspond to the (200) lattice
planes (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 not
detected by XRD measurements, which indicates that they are a
very 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) have
the asymmetric lineshape characteristic of graphitic materials,
as would be expected for rGO with hydrazine.30 The Ni 2p
spectra of these materials are qualitatively similar to those
previously reported for NiO on rGO.31 The binding energy
(BE) of the strongest Ni 2p3/2 component is above 855 eV for
Previous studies showed the effect of reaction medium pH
on the formation of nickel particles as well.25 NaOH has been
typically used as a basic precursor for controlling the reaction
medium pH during the synthesis of nickel particles.21 It is well
known that the basic medium adjusted by NaOH can change
the reaction effects of hydrazine. Ni et al. showed that the
morphology of nickel particles can be adjusted in a certain range
of base concentration for hydrazine reduction reactions.26 In
fact, it was already observed that other reaction parameters, such
as temperature and magnetic field, also play an important role in
controlling the morphology of nickel nanoparticles during their
reduction by N2H4.17
According to our FTIR study of nanocomposites (see Fig. S1
in ESI†) N2H4 not only affects the particle morphology but also
induces the reduction of GO by elimination of oxygen func-
tional groups during the hydrothermal process. FTIR spectra of
G/Ni nanocomposites showed a clear reduction of the bands
corresponding to the oxygen functional groups, revealing only
the presence of few residual bands of oxygen functional groups.27
As mentioned above, oxygen functional groups at the GO surface
can act as electrophilic agents for the nucleation of nickel ions
and the growth of nanoparticles. However, it was observed that
the increase of the N2H4 concentration in solution contributes to
the particle shape formation by the possible coordination of Ni
ions though nitrogen donor groups of N2H4, which acts as
a strong reducing agent to form nickel particles. The nano-
composites exhibited different particle morphologies depending
on the N2H4 concentration: spherical, spiky and big agglomerates
(Fig. 2). The growth mechanism of spiky nickel particles can be
explained according to Mathew et al.28 There are two main steps
in the formation of flower-like structures: first, formation of a
core, then, as the reduction reaction continues, the newly
formed particles can be adsorbed onto the surface of existing
particles acting as seeds to form nanothorns on the surface of
the nickel core. It was observed that at higher temperature the
speed of crystal growth is decreased due to a pronounced
movement of nickel ions in solution. Therefore, a longer
reaction time is needed to form the flower-like morphology.29
Although we found that the reaction time does not affect the
crystallinity of the nickel particles, it has an important effect on
the structure of spiky nickel nanoparticles in G/Ni nanocompo-
sites. SEM images of samples G/Ni3, G/Ni4 and G/Ni5 showed
Fig. 4 TEM image of one single spiky nickel particle at the surface of the
RGO 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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Fig. 5 HRTEM images of a single nanothorn (on the left) and its selected
area with high resolution (on the right) showing the lattice structure of NiO.
all the samples in Fig. 6. While BE 4 855 eV is occasionally
reported for NiO samples, in the NIST XPS Database,32 the
majority of BE values are below 855 eV for NiO, so based on the
BE values it is more likely to assign the material in the oxidized
surface layer of these samples as Ni(OH)2. The small shoulders
observed at BE of ca. 853 eV indicate the presence of metallic Ni
within the sampling depth of XPS (which is ca. 5 nm at this BE).
For materials with complex nanostructured morphology, an
unambiguous interpretation of surface layer composition is
difficult, however, the data in Fig. 6 are consistent with a thin
(o5 nm at least in some areas) oxidized surface layer covering a
metallic 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 would
represent a large fraction of the overall nanoparticle volume
sampled by XPS. Finally, compared to G/Ni3 and G/Ni4 samples,
only a minimal amount of metallic Ni is detected in the G/Ni5
sample, suggesting that longer reaction times increase not only
the nanothorn size but also the degree of surface oxidation.
Fig. 7 shows the high-angle annular dark-field scanning
transmission electron microscopy (HAADF-STEM) image of the
G/Ni5 nanocomposite and the corresponding elemental maps of
nickel (green) and oxygen (red). These data further proved the
Fig. 7 High-angle annular dark-field scanning transmission electron
microscopy (HAADF-STEM) image of the G/Ni5 nanocomposite and the
corresponding elemental maps of nickel (Ni) and oxygen (O).
oxidation of nickel particles on the surface, which is in agree-
ment with the SAED and TEM data. Accordingly, the local EDX
spectra of a single nickel particle confirmed the presence of a
very 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 surface
is 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 demonstrated
a good correlation with SEM images (Fig. 8(c)) with the Ni
particles dispersed in the rGO matrix within the equal appor-
tionment statistics. By means of a comparison of both SPM and
SEM methods we could mark the Ni shell (Fig. 8(d)) and its
response on SR and SEM scans, respectively.
Kelvin Probe Force Microscopy (KPFM) mode showed a
distribution of electric potential on the sample surface (Fig. 9(a)),
where the Ni particles exhibit higher potential as compared
to the rGO matrix, and Ni particle cores and spiky shells could
be distinguished, as they have a maximum potential due to
the electric field concentrated on the point of shells (insert to
Fig. 9(a)).
This resulted in white dots with a maximum KPFM signal
around the Ni particles. The contact SR mode reveals the
conductive regions that are related to the Ni particle core and
shell conglomerates, as confirmed by KPFM results (Fig. 9(b)).
The current–voltage dependence has been then studied in two
steps based on the most conductive (green circle) and least
Fig. 6 Ni 2p XPS spectra of samples G/Ni3, G/Ni4, and G/Ni5.
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nonlinearity similar to the Schottky effect but does not have any
hysteresis behavior (Fig. 9(d)). This could be due to the effect of
Ni particles on current–voltage behavior of the sample that
mainly leads to the switchable diode effect.35
Based on the analysis of our results, the switchable diode
behavior in the sample can be explained qualitatively by the Ni
particle modulation of Schottky-like barriers at both bottom
and top (cantilever’s tip) electrodes. The ideal Schottky barrier
at a metal–semiconductor interface is determined by the differ-
ence in the metal work function and the semiconductor electron
affinity. The work function of rGO is taken as 4.4 eV,36 the work
function of Ni is 4.6 eV37 and the work function of Pt is about
5.3 eV.33 The NiO layers should also be taken into account
because oxygen contributes significantly to the electronic state
of Ni particle shells, as confirmed by XPS and HAADF-STEM
methods. The NiO band gap energy is 3.6 eV and the electron
affinity is 5.3 eV.38 When rGO, Ni and Pt are joined together in a
diode-like structure during the C-AFM experiment, it is obvious
that the electrons move faster from Pt to rGO, due to the higher
work function of Pt than that of rGO, leaving behind positive
charges in NiO. Then current depletion regions are formed by
the differences in charge carrier velocity through the bottom and
top electrodes (cantilever’s tip), respectively, and the built-in
current–voltage switchable diode effect (Fig. 10).
The influence of NiO is taken into account because of its
considerable role as a buffer layer. The effect was considered
in the scope of equilibrium energy band diagrams. The equili-
brium energy band diagrams of rGO/NiO/Ni/Pt heterostructures
are shown in Fig. 10. In this structure, the height and width of
the contact barrier between graphene, Ni and Pt are defined as
differences in the work function Àej. In the NiO contact area
the widths of the contact barrier are correspondingly defined
as eDNiO À ej. Compared with the contact barrier between
rGO, Pt and Ni the energy band bending at the NiO interface
(Fw, CNiO o ej) is much smaller due to the large work function
of NiO. In addition, the ultrathin p-type NiO layer reduces the
contact barrier width at the NiO/Ni interface (DNiO, Àej + Ni).
Therefore, with the presence of the NiO buffer layer, electrons
can pass through the barrier more easily.35
Fig. 8 Images of the rGO matrix with Ni particles in (a) SPM microscopy
mode (b) spreading resistance SPM mode, (c) SEM mode and (d) a single
spiky nickel particle.
conductive ( yellow circle) points, which are related to Ni
particles and rGO, respectively. The I–V behavior, showed a
nonlinear and highly reproducible current hysteresis behavior,
indicating a large resistive switching, which can be described as
the switchable diode effect (Fig. 9(c)).33 The measurements
were performed with 20 cycles by sweeping the bias voltage of
the cantilever tip from À5 to 5 V and back to À5 V, repeatedly.
Moreover, the forward and backward curves showed an obvious
diode-like rectifying I–V characteristic, indicating a forward pass
for backward pass diode behavior and a reverse diode behavior
for a forward sweep. It can be seen that during the measuring
cycle 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 and
the PFM response has been determined by increasing the voltage
sweep range step by step.34 During all the experiments the PFM
response was absent and signal fluctuations were at the noise
level. The current–voltage behavior of the rGO point also shows
Fig. 9 SPM images of the rGO matrix with Ni particles in (a) Kelvin Probe
Force Microscopy mode and (b) spreading resistance mode. Current–
voltage dependence on the (c) most conductive (Ni particle) and (d) less
conductive (rGO matrix) areas.
Fig. 10 The equilibrium energy band diagrams of rGO/Ni/Pt structure.
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5. Conclusion
In summary, we showed a new one-step hydrothermal approach
for the controlled synthesis of rGO/Ni nanocomposites using
hydrazine as a reducing agent. We observed that N2H4ÁH2O
concentration is a key experimental parameter to control the
size, morphology, distribution and crystalline structure of
nickel particles at the GO surface. Hydrazine has a major role
in this reaction not only because it allows control of the
nucleation and growth of Ni metallic nanoparticles through
the reduction of nickel ions, but also by reducing the surface of
GO. The results obtained for the synthetic process showed that
the increase of N2H4ÁH2O concentration in the reaction medium
corresponds to an increase of nickel particle sizes, varying from
145 to 900 nm, and also affects the nickel particle morphologies,
from spherical to spiky and finally to big agglomerates. For the
preparation of all nanocomposites we did not use any alkaline
media, despite previous reports on such nanoparticle growth,
indicating that the pH value can be adjusted in solution using an
appropriate amount of N2H4ÁH2O. For nanocomposites with spiky
nanoparticles (rGO/spiky Ni) it was observed that the increase of
the reaction time promotes the growth of nickel nanothorns. The
results also indicate that spiky nickel particles are composed of a
core/shell structure: a metallic Ni core and a few nm thin outer
layer of NiO. Furthermore, it is observed that the thickness of the
NiO outer layer increases with the increase of the reaction time.
By means of spreading resistance SPM mode we have shown
that spiky nickel particles implemented in the rGO matrix
enhance conductivity with nonlinearity in current–voltage depen-
dence if the output electrodes are attached. Via the equilibrium
energy band we confirmed that all the experimental structure
components (Ni particles, rGO, NiO and Pt-tip) are exactly in
place, even NiO plays a role of gate insulator. In general this
structure works like a graphene-based transistor switch or
embedded in a graphene based matrix switchable diode both
of which could be very useful for graphene based embedded
nanoelectronics applications.
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Acknowledgements
˜
Maryam Salimian and Gil Gonçalves thank the Fundaçao para a
ˆ
Ciencia e Tecnologia (FCT) for the PhD (SFRH/BD/98337/2013)
and PostDoc (SFRH/BDP/84419/2012) grants, respectively. Maxim
Ivanov acknowledges FCT for his postdoctoral grant FCT UID/
CTM/50011/2013. This work was developed in the scope of the
project CICECO-Aveiro Institute of Materials (Ref. FCT UID/CTM/
50011/2013), financed by national funds through the FCT/MEC
and when applicable co-financed by FEDER under the PT2020
Partnership Agreement.
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