J Mater Sci DOI 10.1007/s10853-015-8901-8
Drastic modification of graphene oxide properties by incorporation of nickel: a simple inorganic chemistry approach Olena Okhay • Rahul Krishna • Alexander Tkach Mathias Kla¨ui • Luis M. Guerra • Joa˜o Ventura • Elby Titus • Jose J.A. Gracio
•
Received: 22 September 2014 / Accepted: 7 February 2015 Ó Springer Science+Business Media New York 2015
Abstract Strong increase in electrical conductivity of graphene oxide (GO) (I & 10-9 A) is found by addition of Ni nanoparticles (NiNPs) preliminarily solved by HCl (Nisol) (I & 10-4 A) or powder (Nipow) obtained from this solution (I & 10-6 A), while simply mixing GO with NiNPs an insulator similar to pure GO is obtained. Thus, Nisol and Nipow can be used to transform GO from insulator to semiconductor. One of the transformation mechanisms is Ni as spillover. At the same time, different kinds of the magnetic response are obtained on GO and reduced GO (rGO) samples with and without Ni. Weak paramagnetic response is detected in pure GO. Stronger paramagnetic behavior is observed for GO and rGO mixed with Nisol or Nipow. Pure rGO sample shows weak ferromagnetism represented by slim but visible hysteresis with remnant magnetization Mr of 0.05 emu/g. GO with NiNPs presents clear hysteresis with Mr of 2.8 emu/g, while sample
O. Okhay (&) R. Krishna E. Titus J. J.A.Gracio Nanotechnology Research Division, Department of Mechanical Engineering, Center for Mechanical Technology and Automation (TEMA), University of Aveiro, 3810-193 Aveiro, Portugal e-mail:
[email protected] A. Tkach M. Kla¨ui Institute of of Physics, Johannes Gutenberg University, Mainz, Staudinger Weg 7, 55128 Mainz, Germany A. Tkach CICECO–Aveiro Institute of Materials, Department of Materials and Ceramic Engineering, University of Aveiro, 3810-193 Aveiro, Portugal L. M. Guerra J. Ventura Institute of Physics of Materials of the University of Porto (IFIMUP), 4169-007 Porto, Portugal
prepared by addition of NiNPs to rGO presents the largest hysteresis with Mr as high as 11.8 emu/g. Thus, the optimal procedure to obtain the magnetic response requested for particular application can be chosen.
Introduction Most of the experimental research on graphene focuses on its electronic properties. As an electronic material, graphene has been receiving much attention due to possible replacement for silicon in the complementary metal-oxide semiconductor technology. Compared to silicon and even III–V semiconductors, graphene has superior mobility and ballistic transport that makes its use attractive in transistors [1]. Graphene as a single atomic layer graphite was first isolated at 2004 [2] and since then many properties previously predicted for this 2D structure have been confirmed experimentally. The most notable feature of graphene is an ability to continuously tune the charge carriers from holes to electrons (gate dependence) [3–5] that can be used for either sensing, transistor or solar cells applications. This effect is most pronounced in the thinnest samples, whereas samples with multiple layers show much weaker gate dependence due to screening of the electric field by the other layers. Transistors fabricated from graphene nanowires have shown impressive on–off ratios [6, 7]. The problem with graphene is that it has no band gap; electrons can flow at any energy, and the corresponding resistivity changes are small. So the major focus of graphene engineers has been to find ways of creating an artificial band gap using such methods as applying electric fields, doping or stretching and squeezing the material.
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In addition to the extraordinary mechanical and electronic transport properties, numerous reports have predicted that ferromagnetic ordering could exist due to various defects in graphene structures, such as vacancy, topological defects or frustration, and hydrogen chemisorption [8, 9], as well as dislocations and impurity atoms [10–12]. Moreover, the unconventional magnetism observed in carbon-based materials has attracted a great deal of interest in metal-free magnetic materials [13, 14]. It was also shown that the energy band gap and magnetic coupling strength can be manipulated by varying the defect concentration and a defective graphene phase has been predicted to be a roomtemperature ferromagnetic semiconductor [13]. Moreover, the most important application in the near future most likely is in the emerging field of spintronics (where a signal is processed using magnetic spin properties rather than electric charge) and it was reported that graphene is a potential material for 2D organic molecular magnet [15]. Another way to control the band gap width can be performed by variation of the oxidation/reduction degree of graphene. Thus, graphene oxide (GO) is known to be an insulator, having large band gap, while reduced GO (rGO) reveals semiconducting properties similar to graphene. In this work, we study the effect of ferromagnetic Ni and procedures of its addition on the electric and magnetic properties of GO and rGO.
characterization, GO and rGO films without and with Ni were obtained by the dropping of corresponding solution on Si substrate. After being deposited on substrates, all samples were stored at room temperature until completely dried. All samples, together with a detailed explanation of the preparation processes, are present in Table 1. Phase purity and crystallinity for some of the samples were analyzed by X-ray diffraction (XRD) technique (Rigaku, Japan, CuKa radiation). The Fourier transform infrared (FTIR) spectra of GO were recorded using a Bruker Tensor 27 FTIR spectrometer mixing the sample with KBr (Aldrich, 99 %, FTIR grade). Morphology of the obtained samples was checked using a Scanning Electron Microscope (SEM) Hitachi S4100. The current–voltage (I–V) cycles were obtained using a Keithley Source Meter 2400 and manual probes with tungsten tips at room temperature. For magnetic measurements, all studied solutions were dropped in capsules and dried at room temperature. Magnetic moment was measured in the magnetic fields up 1000 Oe, using superconducting quantum interference device magnetometer (Quantum Design MPMS XL). Then magnetization was deduced using the sample mass determined as difference between the capsutulated sample and the empty capsule mass.
Results and discussion Structural and microstructural analysis
Experiment XRD study GO samples were obtained from the high-quality GO dispersion in water (1 mg/ml) that was synthesized using modified Hummer’s method on the basis of our previous works [16]. rGO was achieved by addition of hydrazine hydrate (N2H4 9 H2O, 50 %) (HH) to the dispersion in proportion 1:1 and corresponding rGO samples were prepared. For the preparation of GO or rGO with nanoparticles (NPs) of Ni, commercially available Ni nanoparticles (NiNPs) (Quantum SphereTD, USA Co.) were utilized. In each case, the 3 ml of GO dispersion was mixed with 0.5 mg of NiNPs to prepare GO ? NiNPs and after that reduced by 3 ml of HH to obtain r(GO ? NiNPs). For the preparation of two more sets of GO and rGO samples with Ni, next steps were done: i)
ii)
0.5 mg of NiNPs was dissolved in 2 ml of HCl 9 H2O (1:1) to obtain Ni ? HCl-solution (Nisol) and prepare GO ? Nisol and r(GO ? Nisol) samples; Nisol was dried at 75 °C to obtain a powder (Nipow) and prepare GO ? Nipow and r(GO ? Nipow) samples.
Moreover, dispersion of Ni in each solution was obtained using of ultrasonication method (15 min). For electric
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Figure 1 shows the XRD patterns of GO, GO with NiNPs, and rGO samples. A typical peak near 2h = 9.86°, corresponding to a layer–to–layer distance (d-spacing) of *0.896 nm, was observed for the GO. This value was higher than the interlayer spacing of graphite flakes (dspacing = 0.334 nm, 2h = 26.4°), due to the presence of oxygenated functional groups and intercalated water molecules [17]. After the intercalation of NiNPs with GO, additional peaks from Ni were observed in GO ? Ni that were related to the various crystallographic planes of facecentered cubic (fcc) NiNPs [JCPDS card No. 04-0850] and the highest intense diffraction peak at around 2h = 44.4° suggests the crystallinity of NiNPs [18]. A dramatic shift 2h from 9.86° for GO to higher angles *25.56° with decreased d-spacing of *0.349 nm was observed for rGO. This reduction of the d-spacing clearly indicates a better ordering of the two-dimensional structure [19]. FTIR measurement To check the transformation of GO to rGO after reduction of GO by HH, the FTIR spectroscopy was done for these
J Mater Sci Table 1 List of the studied samples, preparation processes and designation here followed Called here
Process of preparation
GO
Powder of GO was mixed with H2O (1 mg/ml)
GO ? NiNPs
0.5 mg of NiNPs were mixed with 3 ml of GO dispersion
GO ? Nisol
(1) 0.5 mg of NiNPs were mixed with 2 ml of HCl 9 H2O (1:1)
GO ? Nipow
(1) 0.5 mg of NiNPs were mixed with 2 ml of HCl 9 H2O (1:1)
(2) 3 ml of GO dispersion were added to Ni ? HCl solution (2) Obtained solution was dried to get a powder (3) Obtained powder was added to 3 ml of GO dispersion rGO
GO dispersion was mixed with N2H4 9 H2O (1:1)
r(GO ? NiNPs)
(1) 0.5 mg of NiNPs were mixed with 3 ml of GO dispersion
r(GO ? Nisol)
(1) 0.5 mg of NiNPs were mixed with 2 ml of HCl 9 H2O (1:1) (2) 3 ml of GO dispersion were added to prepared Ni ? HCl solution
r(GO ? Nipow)
(1) 0.5 mg of NiNPs were mixed with 2 ml of HCl 9 H2O (1:1)
(2) Obtained solution was mixed with 3 ml of N2H4 9 H2O
(3) Obtained solution was mixed with 3 ml of N2H4 9 H2O (2) Obtained solution was dried to get a powder (3) Obtained powder was mixed with already reduced rGO (3 ml of GO dispersion and 3 ml of N2H4 9 H2O) rGO ? NiNPs
(1) 3 ml of GO dispersion were mixed with 3 ml of N2H4 9 H2O (1:1) (2) Obtained rGO dispersion was mixed with NiNPs
samples. Figure 2 shows the FTIR spectra with the clear visible difference between GO and rGO. Modes observed at 1060–1220 cm-1 are C–O stretching vibrations due to the incorporation of the carbonyl groups during the process [20, 21]. The mechanism of exfoliation mainly depends on the peeling of the graphitic structure to pave the way for the entering of oxygen moiety during the oxidation process. The stretching vibrations from C=O can be found at 1720 cm-1 also suggests the carbonyl bonding formation in GO [21].
Fig. 1 XRD pattern of the GO, GO with Ni, and rGO
Fig. 2 FTIR spectra of GO and rGO
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Moreover, the spectrum of GO showed no peaks between 2700 and 3000 cm-1. However, the spectrum of rGO displayed three well-defined fingerprint C–H peaks at around 2853, 2923, and 2970 cm-1. The first two peaks were attributed to symmetric and asymmetric stretching vibration modes of the sp3-CH2 bond. The additional mode of vibration at around 2953 cm-1 is due to asymmetric sp3CH2 stretching vibration that clearly shows the formation of graphene. Similar peak positions have been noticed in earlier work of catalytic-assisted hydrogenation of graphene by a catalytic chemical vapor deposition [22]. So, the spectra indicate that the oxygen-containing functional groups were removed from GO, confirming its chemical reduction. Microscopic analysis of samples Figure 3 represents SEM images of the sample, prepared by dropping the GO, rGO, and r(GO ? NiNPs) solution on Si substrate. Thin continuous layers are evident for GO (Fig. 3a) and rGO (Fig. 3b), while some agglomeration of NiNPs can be observed in the case of GO with NiNPs further reduced by HH (r(GO ? NiNPs)) (Fig. 3c). The continuous layers of GO are confirmed to be quite thin and transparent in transmission electron microscopy (TEM) mode (Fig. 4a), studying the samples prepared on the carbon grid. The agglomeration of NiNPs can be observed in the TEM mode for sample r(GO ? NiNPs)
(Fig. 4b). At the same time, the TEM image in Fig. 4c shows that the agglomeration problem can be overcome by addition of NiNPs to already reduced GO ((rGO) ? NiNPs) that leads to a homogenous distribution with NiNPs being covered by a thin continuous rGO film. Electrical measurements The I–V curves of the GO films without and with Ni are presented in Fig. 5. It is seen from Fig. 5a that the GO film possesses an insulating behavior, revealing high resistance and low current (I & 10-9 A). Electrical conductivity of the insulating GO does not become much higher even after addition of the metallic NiNPs, as presented in Fig. 5b. Figure 5c shows the I–V data of GO ? Nisol, revealing a strong increase of the measured current to 10-4 A. Moreover, some resistive switching-like behavior of the current is detected for this sample at low applied voltage (\5 V). Addition of the Nipow to GO (Fig. 5d) leads also to a higher conductivity than that of pure GO or GO ? NiNPs (Fig. 5a, b, respectively), although not so high as in GO ? Nisol (Fig. 5c). Moreover, analyzed GO ? Nisol and GO ? Nipow samples shows some nonlinear behavior and some set-like and reset-like states can be detected that is very typical for a memristive structure and never detected in insulate GO. At the positive sweeping voltage, the current changes in an counterclockwise direction, suggesting a ‘‘set’’-like
Fig. 3 SEM image of GO (a), rGO (b), and GO mixed with NiNP and further reduced (r(GO ? NiNPs)) (c, d)
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Fig. 4 TEM image of graphene oxide GO (a), GO mixed with NiNPs and further reduced (r(GO ? NiNPs)) (b), and already reduced GO mixed with NiNPs (rGO ? NiNPs) (c)
condition (SLC) or ‘‘on’’-state that resembles the memristive behavior (in which current switches at certain voltage range) [16]. However, the reduction by HH of GO ? NiNPs (Fig. 6b) or GO ? Nisol (Fig. 6c) or GO ? Nipow (Fig. 6d) did not lead to any conductivity increase. Nevertheless,
r(GO ? Nisol) shows higher current values than r(GO ? NiNPs) or r(GO ? Nipow). rGO samples show semiconductor behavior with the current up to *10-4 A (Fig. 6a) that is typical for this material [23]. Let us here discuss mechanisms behind the observed electrical phenomena. Comparing the Fig. 5a, b, it is seen that the addition of NiNPs to the GO does not affect much of its conductivity (despite the metallic nature of Ni with electrical conductivity higher than that of insulating GO). Such behavior can be explained by the formation of an insulating layer between or at the surface of the NiNPs. To change the NiNPs interface, they were previously mixed with HCl 9 H2O and only then the obtained solution (Nisol) was mixed with GO dispersion. HCl was added to NiNPs and suspension was continuously stirred. After the completion of reaction, top part of the product (Ni ? HCl) was precisely collected and bottom part of the solution was discarded to remove the unreacted metallic impurities. After that the top part Ni ? HCl was added to GO solution and stirred. Part of the obtained Ni ? HClsolution was dried and the obtained powder was also mixed with GO dispersion to see the influence of Ni as powder (Nipow) on the electrical conductivity. It is possible to conclude from Fig. 5 that the addition of Nisol or Nipow strongly improves the conductivity of GO, contrarily to the case where NiNPs are used. It well known that nickel chloride is formed during the procedure described above. It is an insulator with the band gap of 4.7 eV, but dissolving in water it can exhibit an ionic conductivity [24]. In the current study, water was used to obtain GO solution. It can be supposed that a good attachment of NiNPs to GO provides the easiness of hydrogenation of GO and facile transformation of radical hydrogen (H) to the carbon skeleton. In this process, NiNPs reacts with hydrochloric acid (aq) and produces the Ni2? and 2e- through electrochemical reaction between Ni metal and acid (Ni ? 2HCl = Ni2? ? 2Cl- ? 2H? ? 2e-). Further, proton and electrons combine together (2H? ? 2e- = H2) and produce the hydrogen gas [25]. In this process, NiNPs were continuously reacted with acid and reduce its own size. Due to this effect it shows very high catalytic activity (smaller size tends to larger surface area and higher surface energy) and enhances spillover of hydrogen on its surfaces and makes the radical hydrogen. It is already reported that the Ni is a very prominent catalyst for hydrogenation of olefinic and benzene double bond and for unsaturated benzenederived carbonyl functionalities also [26–30]. The generated radical H can be supposed to migrate from Ni catalyst surface to the C–C bond through a ‘‘bridge’’ built and to complete the process. Moreover, due to the in-situ generation of hydrogen molecule (through electrochemical reaction
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J Mater Sci Fig. 5 I–V curves of GO (a), GO ? NiNPs (b), GO ? Nisol (c), and GO ? Nipow (d)
Fig. 6 I–V curves of reduced GO (rGO) (a), GO ? NiNPs and further reduced (r(GO ? NiNPs)) (b), GO ? Nisol and further reduced (r(GO ? Nisol)) (c), and GO ? Nipow and further reduced (r(GO ? Nipow)) (d)
between metallic Ni and HCl), it easily spillover on own generator (due to the increase number of defects and grain boundaries on own generator nano Ni surface) and readily transforms to the unsaturated carbon sites. To describe this migration of H, here we are assuming that one part of the Ni particles works as a source and simultaneously another part of the NPs is an activator to dissociate the hydrogen molecule and finally the unsaturated carbon skeleton (–C=O or –C=C) behaves as a receptor. In this heterogeneous catalysis, the source and activator is a metal (Ni) and the receptor is also a semimetal (graphene) and this feature can be explained as diffusion of hydrogen inside the carbon system [31]. Thus,
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during the reaction GO ? (Ni ? HCl) some spillover mechanism was presented and measured electrical conductivity for this system was much higher (Fig. 5c, d) than that for system GO ? NiNPs (Fig. 5b). It is also shoud be stressed here that the influence of HCl can directly imparts the proton (H?), which eventually provides the high current value due to the migration (HCl = H? ? Cl-). Moreover, the negatively charged chloride ion (Cl-) could make stern layer on NiNPs and remain intact on the surface of Ni. In this case, charge fluctuation directly understood as field-dependent migration of H? and e- at counter electrode side
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(Ni ? 2[H–Cl] = NiCl2 ? 2H? ? 2e-). And in the case of Nipow sample, HCl was evaporated together with H2O before the addition to GO and during the drying process some of the protons could be lost and their concentration decreased, leading to smaller increase of the current in GO ? Nipow (Fig. 5d). Finally, it can be supposed that the reaction in GO with Nisol or Nipow is illustrated in Fig. 7. At the same time, addition of Ni ? HCl to rGO does not give possibility to start of such spillover due to GO is already reduced by hydrazine hydrate that is supported by electrical conductivity measurements (Fig. 6c, d). It is well known that most switching materials show filamentary-type switching behavior [32]. Also, it should be stressed here that NiO is one of the most widely studied
materials for unipolar switching-type RRAMs and, as already reported by Park et al., the conducting filaments (CFs) are estimated to be composed of metallic Ni that is phase-separated from the NiO mother matrix [33]. On another hand, diffusion from electrode plays the main role in electrical conductivity mechanism in the case of graphene or GO films. For example, the bipolar switching could be observed in a carbon-based cell employing an Al electrode, but a diamond-like carbon film with W electrode has also been studied as a unipolar switching material [34, 35]. Thus, we can suppose that CFs play a key role in observed resistive switching studied samples.
Magnetic measurements results and discussion
Fig. 7 Schematic view of the chemical process between graphene oxide and Ni during the current experiment
Fig. 8 Magnetization curves of GO (a) and GO with NiNPs (b), Nisol (c), and Nipow (d)
Figure 8 represents the magnetization curves for GO-based samples. Pure GO powder reveals linear behavior with zero remnant magnetization (Fig. 8a). The addition of NiNPs to GO leads to a strong increase of the magnetization from 10-5 emu/g to almost 20 emu/g at the applied magnetic field of 1000 Oe. Moreover, clear hysteresis can be observed for dried solution of GO ? NiNPs (Fig. 8b). However, linear dependence of the magnetization and its small value (10-2 emu/g at 1000 Oe) were detected for GO mixed with Nisol (Fig. 8c) as well as with Nipow (Fig. 8d). Magnetization curves for samples based on rGO are shown in Fig. 9. In contrast to GO, rGO sample has the magnetization as high as 10-2 emu/g at 1000 Oe without addition of any Ni. Moreover, it shows a slim hysteresis loop (Fig. 9a). The shape of the M–H loop and value of the magnetization observed in analyzed rGO are very similar to that reported by Wang et al. [36].
(a)
20
GO
0.00001
-0.00001
(c) 0.01
GO+Ni NPs
0
Magnetization (emu/g)
Magnetization (emu/g)
0.00000
(b)
GO+Nisol
-20
(d) 0.01
0.00
0.00
-0.01
-0.01
-1000
-500
0
500
Magnetic field (Oe)
1000
GO+Ni pow
-1000
-500
0
500
1000
Magnetic field (Oe)
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0.2
rGO
40
Magnetization (emu/g)
(a)
Magnetization (emu/g)
Fig. 9 Magnetization curves of pure GO (a), GO ? NiNPs (b), GO ? Nisol (c), and GO ? Nipow (d) after reduction by hydrazine hydrate
0.0 -0.2
0.004
(c) r(GO+Nisol)
-40
0.4 0.0
-0.004
-0.4
-500
0
500
1000
Magnetic field (Oe)
They prepared graphene by partial reduction of GO with hydrazine and annealed the samples at 400 and 600 °C in an argon atmosphere. The value of magnetization of rGO obtained in the current work without heat treatment is much larger than that for samples annealed at 400 °C and equal to samples annealed at 600 °C reported by Wang et al. [36]. Moreover, it was also reported by Ramakrishna Matte et al. [37] that the graphene samples prepared by thermal exfoliation of graphitic oxide, by conversion of nanodiamond, by arc evaporation of graphite in hydrogen, or by reduction of GO with hydrazine hydrate exhibit roomtemperature magnetic hysteresis. Addition of NiNPs to GO with following reduction by HH leads to significant increase of the magnetization to almost 45 emu/g at 1000 Oe with very large hysteresis (Fig. 9b). However, samples of rGO ? Nisol (rGO ? Nisol, Fig. 9c) or GO ? Nipow (rGO ? Nipow, Fig. 9d) present no hysteresis and very low value of magnetization. Such behavior of the magnetization as a function of magnetic field can be explained by formation of NiCl2, known as a paramagnetic material [38].
Conclusions We observed that the electrical properties of GO and rGO were changed by the addition of Ni. It is found that GO with NiNPs is an insulator similar to pure GO. However, the addition of NiNPs solved by HCl or powder obtained from this solution leads to an increase of the measured current (I) from 10-9 to 10-4 A because of spillover mechanism.
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0
0.000
-1000
(b) r(GO+NiNPs)
(d) r(GO+Nipow)
-1000
-500
0
500
1000
Magnetic field (Oe)
On the other hand, the conductivity of rGO (I & 10-4 A) decreases with the addition of any type of Ni (to 10-5 A for NiNPs and Nisol and to 10-6 A, Nipow). Thus, although the addition of Ni does not lead to increase of conductivity in rGO, Nisol, and Nipow can be used to transform GO from insulator to semiconductor. At the same time, different kinds of the magnetic response are obtained on GO and rGO samples with and without Ni. Weak paramagnetic response is detected in pure GO. Stronger paramagnetic behavior is observed for GO and rGO mixed with Nisol or Nipow. Pure rGO sample shows weak ferromagnetism represented by slim but visible hysteresis with remnant magnetization Mr of 0.05 emu/ g. GO ? NiNPs presents clear hysteresis with Mr of 2.8 emu/g, while sample prepared by addition of NiNPs to GO with subsequent reduction by HH (r(GO ? NiNPs)) presents the largest hysteresis with Mr as high as 11.8 emu/ g. Thus, the optimal procedure to obtain the magnetic response requested for particular application can be chosen: addition of only 0.5 mg of NiNPs to GO and its further reduction by HH leads to increase of Mr in more than 200 times (from 0.05 emu/g for rGO to 11.8 emu/g). Acknowledgements Olena Okhay acknowledges FCT for financial support (SFRH/BD/77704/2011). This work was funded also by the EU’s 7th Framework program IFOX (NMP3-LA-2010 246102), the Graduate School of Excellence MAINZ (GSC 266 Mainz), the German Science Foundation (DFG SPP 1459 Graphene), and the ERC (2007-Stg 208162). Alexander Tkach acknowledges also funds by FEDER through Programa Operacional Factores de Competitividade—COMPETE and national funds through FCT within CICECO Project—FCOMP-01-0124-FEDER-037271 (FCT Ref. PEst-C/CTM/LA0011/2013) and independent researcher grant IF/ 00602/2013.
J Mater Sci
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