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Effect of UV irradiation on magnetic behavior of reduced graphene oxide decorated with nickel nanostructure
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Effect of UV irradiation on magnetic behavior of reduced graphene oxide decorated with nickel nanostructure ⁎
Geeta Sharmaa, , S.W. Gosavib a b
Mechanical Metallurgy Division, Bhabha Atomic Research Centre, Mumbai, India Dept. of Physics, Savitribai Phule Pune University, Pune, India
A R T I C L E I N F O
A BS T RAC T
Keywords: Graphene Composite Magnetism UV-irradiation
Reduced graphene oxide (RGO) decorated with nickel has been synthesized via an in-situ reduction of graphene oxide (GO) and nickel nitrate using NaOH and hydrazine. The starting materials Ni (NO3)2 and GO were taken in two different ratios and the products formed were designated as RGNi2 and RGNi1. The formation of the composite was confirmed by the appearance of X-ray diffraction peaks at 44.5°, 51.9°, 76.5° corresponding to Ni and at 24.8°and 43.2° for RGO. The RGNi2 was irradiated with UV light (λ=254 nm) for different durations (2, 6, 12, 24 and 48 h). Intensity ratio of d and g-bands (Id/Ig) of Raman spectra increases from 1.18 to 1.47 over the duration of irradiation period (2–48 h). The magnetization measurements using the vibrating sample magnetometer (VSM) of these samples reveal their ferromagnetic behavior. The calculated saturation magnetization (MS) value of Ni, RGNi1and RGNi2 is 47.86, 30.56 and 8.25 emu/g respectively and the corresponding coercivity (HC) value is found to be 181, 227 and 296 Oe. The MS of RGNi2 is found to increase to 10.65 emu/g after 48 h of irradiation. This enhancement in the MS(~23%) with irradiation may be due to defect formation by the UV light.
1. Introduction Graphene, a two dimensional array of sp2-hybridized carbon atoms has been a topic of interest for more than past six decade since Wallace calculated its electronic structure [1]. However, recently the interest has renewed due to the discovery of new physics [2]. Graphene holds a lot of potential in basic research as well as technological applications due to the fact that, it possesses large surface area, exceptional electrical, mechanical, thermal, optical and magnetic properties. Defects in graphene such as vacancies, disorders, non-hexagonal rings and doped impurities affect various physical, chemical and magnetic properties [3]. The scattering centers are created by these defects, which affect the carrier mobility and electrical conductivity of graphene. Apart from causing deteriorating effects, these defects give rise to some fascinating phenomenon in graphene such as Kondo effect in which local magnetic moments at the defects interact with the conducting electrons at low temperature [4,5]. The defects in graphene have been extensively studied theoretically and experimentally [6]. In most of the experiments high-energy electron beams, alpha radiation, ion bombardment and photo irradiation is generally used [4,5,7,8]. In comparison to all these irradiation sources, photo irradiation is least employed. Photo irradiation such as UV irradiation has a great
⁎
potential for inducing defects in graphene by creating electron hole pairs [9]. Magnetic nanoparticles are of great interest owing to their large number of applications in magnetic fluids [10], data storage [11], biotechnology, biomedicine [12], catalysis [13,14], magnetic resonance imaging [15,16]. Recently, RGO sheets decorated by magnetic nanoparticles were used as a strong absorbent in waste water purification and magneto-photothermal therapy of cancer [17,18] Dispersion of magnetic nanoparticles on reduced graphene oxide sheets carves an efficient route for catalytic, magnetic, adsorbing and electrode material [22,23]. Composites of magnetic nanoparticles such as Fe3O4, ZnFe2O4, CoFe2O4, Co with graphene have been recently reported [24–30]. Ni nanoparticles are also one of the important magnetic materials having applications as catalyst, filler of nano inks and magnetic fluids [31–34]. The effect of UV irradiation on the various properties of graphene-based materials is very important subject which has recently attracted the attention of researchers [19–21]. To the best of our knowledge effect of photo-irradiation on the reduced graphene oxide and nickel nanoparticle (NP) composite is rarely studied. In the present study, the RGO-Ni NP composites were synthesized and photo-irradiated with the UV-light (λ=254 nm) for different time durations. Their structural and magnetic properties were
Corresponding author. E-mail address: [email protected] (G. Sharma).
http://dx.doi.org/10.1016/j.ceramint.2016.12.137 Received 13 November 2016; Received in revised form 25 December 2016; Accepted 29 December 2016 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Sharma, G., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2016.12.137
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studied by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), Transmission electron microscopy (TEM), Raman spectroscopy and vibrating sample magnetometer (VSM). The aim of the present study is to investigate photo-induced modification in the structure of reduced graphene oxide-Ni nanocomposite. Systematic study on effect of increasing UV dose on the magnetic properties of the composites is reported. 2. Experimental 2.1. Preparation of reduced graphene oxide All the chemicals used for the synthesis were of analytical reagent grade and were purchased from Sigma Aldrich. At first the graphene oxide was synthesized after oxidizing graphite with KMnO4 in the presence of sulphuric acid using well-reported Hummer's method [35]. Oxidation helps in exfoliating the sheets by increasing the interlayer spacing. The solid product was separated by centrifugation, after washing repeatedly with acetone, and dried in ambient at 65 °C for 12 h. To synthesize quantum sheets of RGO, we started with reducing the graphene oxide (GO) by hydrazine [36]. The hydrazine has been found to be the best reducing agent in producing very thin graphene like sheets [37]. The larger particles of graphite, which did not exfoliate, were removed by centrifuging (in de-ionized water) at 4500 rpm for 10 min, and the product so formed was suitable for further studies. The obtained product was again diluted with de-ionized water and sonicated in an ultrasonic bath for 30 min. Hydrazine was added and sonication continued for a further 210 min. The total synthesis time was 4 h and the sample is designated as reduced graphene oxide (RGO). The lightweight black products floated on the solution and were washed thoroughly with distilled water and alcohol and dried under vacuum at 370 °C for 2 h.
Fig. 1. The XRD patterns of (top panel) Ni, RGNi1, RGNi2; (bottom panel) RGO.
length 254 nm or 365 nm was used to expose the samples at room temperature. In the present study, samples were exposed only to 254 nm UV radiations. One minute of exposure by 254 nm radiations from this lamp at a distance of 3 in. from the sample corresponds to an irradiation of 55.8 mJ/cm2. 3. Results and discussion 3.1. X-ray diffraction and field emission scanning electron microscopy (FESEM) Fig. 1 shows X-ray diffraction (XRD) patterns of Ni NPs, Ni NPsRGO composites (RGNi1 & RGNi2) and RGO. The reduction of graphene oxide leads to the formation of highly reduced graphene which is indicated by appearance of peak at 24.4° and a weak peak at 43.2° [36]. Both RGNi1and RGNi2 composites show peaks at 2θ=44.5°, 51.9°, 76.5° corresponding to (111), (200) and (222) planes of fcc Ni (JCPDS 01−1260) respectively and a weak signature of reduced graphene oxide is also visible at 24.8° which differs slightly (0.4°) from RGO peak may be due to difference in the degree of reduction in both the compounds and the fact that layers of reduced graphene oxide in the composites are more closely spaced. No peak corresponding to the NiO was detected. The average crystallite size (D) calculated using Debye Scherer formula, for the most intense peak (at around 2θ=44.5°) is listed in Table 1. The average particle size of Ni in the composite is found to be less than 20 nm. Further insight into the structure and surface morphology of the samples was obtained by using field emission scanning electron microscope. FESEM images of as prepared a) Ni NPs, b) RGNi1, c) RGNi2 and d) RGO are shown in Fig. 2. The Fig. 2a shows irregular morphology of Ni nanoparticles with needle like projections on them. As expected high agglomeration among the nanoparticles is visible due to reduced surface energy and the dipolar attraction between magnetic NPs [39,40]. In RGNi1 and RGNi2 sample the Ni NPs are seen decorated on reduced graphene oxide sheet. Arrows in the images
2.2. Synthesis of Ni nanoparticles on RGO nanosheets The typical procedure for the synthesis of RGO-Ni nanocomposite is as follows: 35.0 mg of GO sheets, 70 mg (5 mmol) of Ni (NO3)2· 6H2O, were dispersed in 50 mL of ethylene glycol (EG) with ultra sonication for 1 h (weight % ratio of Ni (NO3)2·6H2O: GO=2:1). Next, we added 25 mL of hydrazine hydrate and 1.0 M NaOH solution in sequence. The solution was heated with rapid stirring at a temperature of 60 °C; nickel-RGO nanocomposite was formed after about 1 h. This sample was designated as RGNi1. Using the same procedure, another sample having weight % ratio of Ni (NO3)2·6H2O: GO=1:1 was prepared and this sample was designated as RGNi2. These samples were preserved and used for further experimentation. Wu et al. have explained the formation of Ni nanoparticles with ethylene glycol and NaOH. These authors have reported that no particles were formed in ethylene glycol without adding sufficient amount of hydrazine at 25– 60 °C, which reveals the particles, were reduced by hydrazine instead of ethylene glycol. Also addition of NaOH solution favors formation of pure Ni nanoparticles in ethylene glycol. Instead of ethylene glycol water was also tried as the possible solvent, but the resulting product was impure [38]. 2.3. Characterization The crystalline phase of as-synthesized samples was characterized using powder X-Ray diffraction (XRD, Bruker D8 Advance diffractometer) with CuKα radiation (λ=1.5406 Å). Surface morphology and particle size of the composite was studied using FESEM (Hitachi, S4800 operating at 30 kV and 10 microA). Raman spectra were recorded at room temperature using a DXR Raman microscope (ThermoFisher Scientific) with 514.5 nm excitation source from an Ar+ laser. The magnetic measurements were carried out with VSM, Lake Shore7037. A switchable UVGL 58 lamp emitting UV radiations of either wave-
Table 1 FWHM, lattice parameter, (2Ɵ) position of (111) plane and particle size (nm) of Ni, RGNi1, RGNi2.
2
Sample
FWHM
Lattice parameter (d)Å
Position (2Ɵ)
Particle size (nm)
Ni RGNi1 RGNi2
0.50 0.68 0.77
2.033 2.033 2.033
44.52 44.51 44.52
17 12 11
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Fig. 2. FESEM images of a) Ni nanoparticles, b) RGNi1, c) RGNi2 d) RGO.
graphene Ni composite. In perfect graphite, the G band is highly intense (in-plane bond stretching motion of C sp2 pairs), whereas the D band is generally active in the presence of high disorder in the reduced graphene oxide. The 2D is observed at 2680 cm−1, 2685 cm−1, 2683 cm−1 for RGO, RGNi2, RGNi1 respectively. The d-band to g-band intensity ratio (Id/Ig) increases from 1.1 in RGO to 1.3 in the composites (RGNi1 and RGNi2) suggesting that disorder increases marginally due to the presence of Ni crystallites on reduced graphene oxide [41,42]. Raman spectroscopy is also used to investigate the single and multilayer characteristics of graphene and graphene oxide. It is well established that the G and 2D bands of single layer graphene sheets are usually located at ~1585 cm−1 and ~2679 cm−1 while for multilayer graphene (2–6 layers), the positions of G and 2D bands shifts to lower and higher wavenumbers 6 and 19 cm−1 [59,60]. In the present work, in RGO sample the position of G band (1594 cm−1) and 2D band (2679 cm−1) and asymmetric shape of 2D band indicates the presence of multilayers of reduced graphene oxide. Also in case of composites RGNi2 and RGNi1 the G band shifts towards lower wavenumber side (1584 cm−1) and 2D band shifts to higher wavenumbers 2683 cm−1 and 2685 cm−1 respectively indicating multilayers of reduced graphene oxide in them. The RGNi2 sample was photo-irradiated with UV for different time periods (2 h, 6 h, 12 h, 24 h and 48 h) under ambient conditions. Fig. 5 shows the Raman spectra of RGNi2 sample after UV irradiation for different time periods. The position of D, G, 2D bands and Id/Ig after different time durations are listed in Table 2. The D band shifts to lower wavenumber region with increasing irradiation time. The peak at 500 cm−1 is due to Ni and with increasing irradiation time no signature of NiO (expected at 1556 cm−1) is seen in any of the spectra which could have been formed due to photoirradiation with UV light [51]. To further investigate the formation of NiO on the surface of Ni nanoparticles due to photo irradiation by UV light, the FTIR spectra of RGNi2 irradiated for 48 h was recorded (Fig. 6). The peak due to Ni at 1067 cm−1 is seen, which are in good agreement with the results reported earlier and no peaks due to NiO are observed [44]. Hence no formation of NiO on the surface of Ni nanoparticles is observed. The intensity ratios of D band (Id) and G band (Ig) were calculated for different doses of UV to estimate the amount of defects in the system. It is seen from the Fig. 7, Id/Ig increases from 1.18 to 1.47 over the duration of irradiation period (2–48 h). This considerable increase
points towards Ni nanoparticles. Ni is homogenously distributed on reduced graphene oxide sheet and the particles are seen attached to each other owing to their magnetic behavior. The needle like projections present on the nickel NPs are absent in case of Ni- RGO composite. Fig. 2(d) is the image of RGO. Thin reduced graphene oxide sheet is clearly visible in the figure. Morphological structures of the samples were further observed by TEM analysis. Fig. 3(i) shows TEM images along with their corresponding SAED patterns a) RGNi2 b) Ni c) RGO. Fig. 3i(b) is the image of Ni NP. Nearly spherical shaped particles having size less that 10 nm is seen in the image. Fig. 3i(a) is the image of RGNi2 composite. The Nickel nanoparticles are seen uniformly distributed over the ultra thin RGO sheet. The size of the Ni nanoparticles seen is much larger than the pure Ni particles may be due attraction among nickel nanoparticles. The SAED pattern of RGNi2 shows the six fold pattern arising due to graphene in addition to ring pattern from Ni particles, indicating the formation of graphene–Ni composite. Fig. 3i(c) (left) shows the images of RGO. Folded thin layers of graphene resembling crumpled silk veil waves with approximate size of few hundred square nanometers are clearly visible in the images. These sheets are transparent and very stable under electron beam. SAED pattern of HRG shows the six fold ring pattern. Further, the chemical composition of the RGNi2 and Ni nanoparticles was characterized by EDXA analysis. As can be seen in Fig. 3(ii)a, carbon can be detected in the spectrum as the major component and in Fig. 3(ii)b Ni is having the maximum percentage. 3.2. Raman spectroscopy Raman spectroscopy is a powerful non-destructive technique mainly to study graphitic materials such as graphene and also for examining the ordered and disordered crystal structures. Fig. 4 shows Raman spectra of RGNi2, RGNi1 and RGO. The D band is observed at 1325 cm−1 in all the three materials i.e. RGO, RGNi2 and RGNi1. The G band in RGO is observed at 1594 cm−1 and it shifts to lower wavenumber at 1587 cm−1 in RGNi2 and RGNi1. The similar shift in Raman spectra of the composite was also observed by Li et.al. in the Ni @ reduced graphene oxide nanostructures [49]. The G band is characteristic of sp2 hybridized carbon; the broad and intense D band is due to the disorder induced in the sp2 hybridized carbon lattice. Presence of Ni NPs on RGO affects the vibrations of sp2 hybridized carbon atoms leading to shift in the position of G band in RGO and 3
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Fig. 3. (i): TEM images and SAED patterns of a) RGNi2 b) Ni c) RGO. (ii): EDAX of a) RGNi2 b) Ni.
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Fig. 4. Raman spectra of RGO, RGNi2and RGNi1.
Fig. 6. FTIR spectra of RGNi2 irradiated for 48 h.
Fig. 5. Raman spectra of RGNi2 sample after UV irradiation for different time periods (a) 2 h; b) 4 h; c) 6 h; d) 24 h; e) 48 h).
Fig. 7. Id/Ig ratio with increasing irradiation time.
dissociation and recombination of C–C bonds, leading to formation of structural defects in graphene.
Table 2 Variation in D, G, 2D and Id/Ig band position of RGNi2 after irradiation for different time periods. Irradiation Time (h)
Position of D band (cm−1)
Position of G band (cm−1)
Position of 2D band (cm−1)
Id/Ig
2 6 12 24 48
1343 1343 1344 1330 1325
1568 1575 1569 1583 1587
2688 2690 2690 2698 2699
1.18 1.19 1.26 1.44 1.47
3.3. Magnetic characterization The magnetic characterization of the samples at room temperature was performed by vibrating sample magnetometer. Magnetic feature of graphene are generally very weak. It is reported earlier that the weak ferromagnetism in graphene may arise from unpaired spins from topological and bonding defects. The localized unpaired spins could be induced by the edge states [45] as well as defects [46] and stacking faults [47] between graphene sheets. Though bulk graphene shows poor magnetic behavior but significant magnetism could be achieved in it by functionalizing with magnetic nanoparticles [48,49]. The ferromagnetic behavior of Ni; RGNi1 and RGNi2 was confirmed by measuring the hysteresis loop (M vs H) at room temperature. Fig. 8 shows magnetization curves for a) Ni; b) RGNi1; c) RGNi2. The presence of a nonzero coercivity (Hc=296 Oe for RGNi2 and Hc=227 Oe for RGNi1) and a non zero remanence (Mr =3.64 emu/g for RGNi2 and Mr=9.57 emu/g) demonstrate the ferromagnetic behavior of the functionalized Ni NPs-graphene composites. The Ms, Mr, Hc values of Ni, RGNi1 and RGNi2 are listed in the Table 3. The saturation magnetization (Ms) of nickel is found to be 47.86 emu/g, which is much lower than that for the reported bulk Ni metal (58.57 emu/g) at room
in the Id/Ig can be assigned to the appearance of the carbonaceous defects in the reduced graphene oxide sheets. The UV irradiation is reported to degrade the carbon content of graphene structure [43,59,61]. The defect formation by UV irradiation is triggered by the optical transition from π band to π* band. In the visible region the absorption by graphene follows a linear energy momentum dispersion relationship [54,55] contrary to UV region, where dispersion relation is no longer linear and absorption increases. The maximum absorption is at a photon energy of 4.62 eV, which arises from optical transition at the M point [56]. Hence the transition from π to π* band is responsible for UV light absorption by graphene. The states excited via π–π* transitions would cause weakening of C–C bonds and give rise to 5
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increase in Hc value of RGO-Ni composites. 3.3.1. Effect of irradiation The studies on light-induced defects are of great interest. The RGNi2 sample was exposed to UV radiations for 48 h. Fig. 9 shows the M versus H plot of RGNi2 sample after irradiation for 48 h. It is observed that after irradiation the saturation magnetization, Ms increases. and it is found to be 10.65 emu/g after 48 h of irradiation. The coercivity also changes and is 300 Oe. With increased dose of UV irradiation some light induced defects are formed. The defect formation by UV irradiation is triggered by the optical transition from π band to π* band. In the visible region the absorption by graphene follows a linear energy momentum dispersion relationship [54,55] contrary to UV region, where dispersion relation is no longer linear and absorption increases. The maximum absorption is at a photon energy of 4.62 eV, which arises from optical transition at the M point [56]. Hence the transition from π to π* band is responsible for UV light absorption by graphene. The states excited via π–π* transitions would cause weakening of C–C bonds and give rise to dissociation and recombination of C–C bonds, leading to formation of structural defects in graphene. The reduced graphene oxide may contain grain boundaries surrounded by edges. These graphene edges become more reactive than the ordered sp2 lattice. The defective site may initiate defect formation by UV light and defects are extended by UV light [57]. Moreover such defects have local magnetic moment and may give rise to flat bands and eventually to the development of magnetic ordering [58]. The UV radiations are unable to cause ionization in Ni since these radiations possess a maximum energy of 4.8 eV, while the threshold energy required for ionizing nickel is 5.1 eV. Hence there are very less chances of UV radiation ionizing nickel. Hence the defects created in reduced graphene oxide are responsible for enhanced ferromagnetism. It is suggested that defect engineering could be a useful tool for enhancing the ferromagnetic properties.
Fig. 8. Magnetic hysteresis loops of a) Nickel; b) RGNi1 c) RGNi2 at 300 K. The inset is magnified hysteresis loop.
Table 3 Summarized magnetic properties of Ni nanoparticles and composite measured at 300 K. Sample
Ms (emu/g)
Mr (emu/g)
Mr/Ms
Hc (Oe)
Ni RGNi1 RGNi2
47.86 30.56 8.25
8.96 9.57 3.64
0.187 0.313 0.44
181 227 296
4. Conclusions In summary, we have observed the room-temperature ferromagnetism of reduced graphene oxide Ni composites prepared via in-situ reduction of graphene oxide and nickel salt (nickel nitrate). We have also observed enhanced ferromagnetism with UV irradiation. Post UV irradiation led to ~23% increase in ferromagnetism (22.9% increase in Ms value and 22.4% rise in Mr value). The possible origin of the enhanced ferromagnetism may come from the long-range coupling of spin units existing as defects in graphene sheets, which are generated in irradiation process. The room-temperature ferromagnetism, combined with solution-processing capability, should have wide-reaching consequences in material science, and these collective properties could make graphene-based materials a competent choice for many important device applications, including spintronic, magnetoresistance, magnetic memory devices, and so on.
Fig. 9. Magnetic hysteresis loops of RGNi2 after 48 h of irradiation at 300 K. The inset is magnified hysteresis loop.
temperature [50]. With decreasing particle surface energies such as domain wall energies become progressively more effective compared with volume energies [52]. It is clear from the Table 3 that with increasing graphene percentage in the composite, saturation magnetization (Ms) is decreasing and coercivity (Hc) is increasing. There exists significant charge transfer as well as magnetic interaction between graphene and magnetic nanoparticles. It is reported earlier that in graphene metal composites there is an effective charge transfer between graphene and adsorbed nanoparticles [53]. In case of nickel nanoparticles the charge transfer occurs from nickel to graphene. The outer most d orbital of nickel is partially occupied. Hence the transfer of charge from nickel to graphene will lead to reduction in unpaired electrons, leading to decrease in Ms value in reduced graphene oxide - Ni composite. The charge transfer interaction is responsible for decrease in Ms and
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Effect of UV irradiation on magnetic behavior of reduced graphene oxide decorated with nickel nanostructure ⁎
Geeta Sharmaa, , S.W. Gosavib a b
Mechanical Metallurgy Division, Bhabha Atomic Research Centre, Mumbai, India Dept. of Physics, Savitribai Phule Pune University, Pune, India
A R T I C L E I N F O
A BS T RAC T
Keywords: Graphene Composite Magnetism UV-irradiation
Reduced graphene oxide (RGO) decorated with nickel has been synthesized via an in-situ reduction of graphene oxide (GO) and nickel nitrate using NaOH and hydrazine. The starting materials Ni (NO3)2 and GO were taken in two different ratios and the products formed were designated as RGNi2 and RGNi1. The formation of the composite was confirmed by the appearance of X-ray diffraction peaks at 44.5°, 51.9°, 76.5° corresponding to Ni and at 24.8°and 43.2° for RGO. The RGNi2 was irradiated with UV light (λ=254 nm) for different durations (2, 6, 12, 24 and 48 h). Intensity ratio of d and g-bands (Id/Ig) of Raman spectra increases from 1.18 to 1.47 over the duration of irradiation period (2–48 h). The magnetization measurements using the vibrating sample magnetometer (VSM) of these samples reveal their ferromagnetic behavior. The calculated saturation magnetization (MS) value of Ni, RGNi1and RGNi2 is 47.86, 30.56 and 8.25 emu/g respectively and the corresponding coercivity (HC) value is found to be 181, 227 and 296 Oe. The MS of RGNi2 is found to increase to 10.65 emu/g after 48 h of irradiation. This enhancement in the MS(~23%) with irradiation may be due to defect formation by the UV light.
1. Introduction Graphene, a two dimensional array of sp2-hybridized carbon atoms has been a topic of interest for more than past six decade since Wallace calculated its electronic structure [1]. However, recently the interest has renewed due to the discovery of new physics [2]. Graphene holds a lot of potential in basic research as well as technological applications due to the fact that, it possesses large surface area, exceptional electrical, mechanical, thermal, optical and magnetic properties. Defects in graphene such as vacancies, disorders, non-hexagonal rings and doped impurities affect various physical, chemical and magnetic properties [3]. The scattering centers are created by these defects, which affect the carrier mobility and electrical conductivity of graphene. Apart from causing deteriorating effects, these defects give rise to some fascinating phenomenon in graphene such as Kondo effect in which local magnetic moments at the defects interact with the conducting electrons at low temperature [4,5]. The defects in graphene have been extensively studied theoretically and experimentally [6]. In most of the experiments high-energy electron beams, alpha radiation, ion bombardment and photo irradiation is generally used [4,5,7,8]. In comparison to all these irradiation sources, photo irradiation is least employed. Photo irradiation such as UV irradiation has a great
⁎
potential for inducing defects in graphene by creating electron hole pairs [9]. Magnetic nanoparticles are of great interest owing to their large number of applications in magnetic fluids [10], data storage [11], biotechnology, biomedicine [12], catalysis [13,14], magnetic resonance imaging [15,16]. Recently, RGO sheets decorated by magnetic nanoparticles were used as a strong absorbent in waste water purification and magneto-photothermal therapy of cancer [17,18] Dispersion of magnetic nanoparticles on reduced graphene oxide sheets carves an efficient route for catalytic, magnetic, adsorbing and electrode material [22,23]. Composites of magnetic nanoparticles such as Fe3O4, ZnFe2O4, CoFe2O4, Co with graphene have been recently reported [24–30]. Ni nanoparticles are also one of the important magnetic materials having applications as catalyst, filler of nano inks and magnetic fluids [31–34]. The effect of UV irradiation on the various properties of graphene-based materials is very important subject which has recently attracted the attention of researchers [19–21]. To the best of our knowledge effect of photo-irradiation on the reduced graphene oxide and nickel nanoparticle (NP) composite is rarely studied. In the present study, the RGO-Ni NP composites were synthesized and photo-irradiated with the UV-light (λ=254 nm) for different time durations. Their structural and magnetic properties were
Corresponding author. E-mail address: [email protected] (G. Sharma).
http://dx.doi.org/10.1016/j.ceramint.2016.12.137 Received 13 November 2016; Received in revised form 25 December 2016; Accepted 29 December 2016 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Sharma, G., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2016.12.137
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studied by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), Transmission electron microscopy (TEM), Raman spectroscopy and vibrating sample magnetometer (VSM). The aim of the present study is to investigate photo-induced modification in the structure of reduced graphene oxide-Ni nanocomposite. Systematic study on effect of increasing UV dose on the magnetic properties of the composites is reported. 2. Experimental 2.1. Preparation of reduced graphene oxide All the chemicals used for the synthesis were of analytical reagent grade and were purchased from Sigma Aldrich. At first the graphene oxide was synthesized after oxidizing graphite with KMnO4 in the presence of sulphuric acid using well-reported Hummer's method [35]. Oxidation helps in exfoliating the sheets by increasing the interlayer spacing. The solid product was separated by centrifugation, after washing repeatedly with acetone, and dried in ambient at 65 °C for 12 h. To synthesize quantum sheets of RGO, we started with reducing the graphene oxide (GO) by hydrazine [36]. The hydrazine has been found to be the best reducing agent in producing very thin graphene like sheets [37]. The larger particles of graphite, which did not exfoliate, were removed by centrifuging (in de-ionized water) at 4500 rpm for 10 min, and the product so formed was suitable for further studies. The obtained product was again diluted with de-ionized water and sonicated in an ultrasonic bath for 30 min. Hydrazine was added and sonication continued for a further 210 min. The total synthesis time was 4 h and the sample is designated as reduced graphene oxide (RGO). The lightweight black products floated on the solution and were washed thoroughly with distilled water and alcohol and dried under vacuum at 370 °C for 2 h.
Fig. 1. The XRD patterns of (top panel) Ni, RGNi1, RGNi2; (bottom panel) RGO.
length 254 nm or 365 nm was used to expose the samples at room temperature. In the present study, samples were exposed only to 254 nm UV radiations. One minute of exposure by 254 nm radiations from this lamp at a distance of 3 in. from the sample corresponds to an irradiation of 55.8 mJ/cm2. 3. Results and discussion 3.1. X-ray diffraction and field emission scanning electron microscopy (FESEM) Fig. 1 shows X-ray diffraction (XRD) patterns of Ni NPs, Ni NPsRGO composites (RGNi1 & RGNi2) and RGO. The reduction of graphene oxide leads to the formation of highly reduced graphene which is indicated by appearance of peak at 24.4° and a weak peak at 43.2° [36]. Both RGNi1and RGNi2 composites show peaks at 2θ=44.5°, 51.9°, 76.5° corresponding to (111), (200) and (222) planes of fcc Ni (JCPDS 01−1260) respectively and a weak signature of reduced graphene oxide is also visible at 24.8° which differs slightly (0.4°) from RGO peak may be due to difference in the degree of reduction in both the compounds and the fact that layers of reduced graphene oxide in the composites are more closely spaced. No peak corresponding to the NiO was detected. The average crystallite size (D) calculated using Debye Scherer formula, for the most intense peak (at around 2θ=44.5°) is listed in Table 1. The average particle size of Ni in the composite is found to be less than 20 nm. Further insight into the structure and surface morphology of the samples was obtained by using field emission scanning electron microscope. FESEM images of as prepared a) Ni NPs, b) RGNi1, c) RGNi2 and d) RGO are shown in Fig. 2. The Fig. 2a shows irregular morphology of Ni nanoparticles with needle like projections on them. As expected high agglomeration among the nanoparticles is visible due to reduced surface energy and the dipolar attraction between magnetic NPs [39,40]. In RGNi1 and RGNi2 sample the Ni NPs are seen decorated on reduced graphene oxide sheet. Arrows in the images
2.2. Synthesis of Ni nanoparticles on RGO nanosheets The typical procedure for the synthesis of RGO-Ni nanocomposite is as follows: 35.0 mg of GO sheets, 70 mg (5 mmol) of Ni (NO3)2· 6H2O, were dispersed in 50 mL of ethylene glycol (EG) with ultra sonication for 1 h (weight % ratio of Ni (NO3)2·6H2O: GO=2:1). Next, we added 25 mL of hydrazine hydrate and 1.0 M NaOH solution in sequence. The solution was heated with rapid stirring at a temperature of 60 °C; nickel-RGO nanocomposite was formed after about 1 h. This sample was designated as RGNi1. Using the same procedure, another sample having weight % ratio of Ni (NO3)2·6H2O: GO=1:1 was prepared and this sample was designated as RGNi2. These samples were preserved and used for further experimentation. Wu et al. have explained the formation of Ni nanoparticles with ethylene glycol and NaOH. These authors have reported that no particles were formed in ethylene glycol without adding sufficient amount of hydrazine at 25– 60 °C, which reveals the particles, were reduced by hydrazine instead of ethylene glycol. Also addition of NaOH solution favors formation of pure Ni nanoparticles in ethylene glycol. Instead of ethylene glycol water was also tried as the possible solvent, but the resulting product was impure [38]. 2.3. Characterization The crystalline phase of as-synthesized samples was characterized using powder X-Ray diffraction (XRD, Bruker D8 Advance diffractometer) with CuKα radiation (λ=1.5406 Å). Surface morphology and particle size of the composite was studied using FESEM (Hitachi, S4800 operating at 30 kV and 10 microA). Raman spectra were recorded at room temperature using a DXR Raman microscope (ThermoFisher Scientific) with 514.5 nm excitation source from an Ar+ laser. The magnetic measurements were carried out with VSM, Lake Shore7037. A switchable UVGL 58 lamp emitting UV radiations of either wave-
Table 1 FWHM, lattice parameter, (2Ɵ) position of (111) plane and particle size (nm) of Ni, RGNi1, RGNi2.
2
Sample
FWHM
Lattice parameter (d)Å
Position (2Ɵ)
Particle size (nm)
Ni RGNi1 RGNi2
0.50 0.68 0.77
2.033 2.033 2.033
44.52 44.51 44.52
17 12 11
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Fig. 2. FESEM images of a) Ni nanoparticles, b) RGNi1, c) RGNi2 d) RGO.
graphene Ni composite. In perfect graphite, the G band is highly intense (in-plane bond stretching motion of C sp2 pairs), whereas the D band is generally active in the presence of high disorder in the reduced graphene oxide. The 2D is observed at 2680 cm−1, 2685 cm−1, 2683 cm−1 for RGO, RGNi2, RGNi1 respectively. The d-band to g-band intensity ratio (Id/Ig) increases from 1.1 in RGO to 1.3 in the composites (RGNi1 and RGNi2) suggesting that disorder increases marginally due to the presence of Ni crystallites on reduced graphene oxide [41,42]. Raman spectroscopy is also used to investigate the single and multilayer characteristics of graphene and graphene oxide. It is well established that the G and 2D bands of single layer graphene sheets are usually located at ~1585 cm−1 and ~2679 cm−1 while for multilayer graphene (2–6 layers), the positions of G and 2D bands shifts to lower and higher wavenumbers 6 and 19 cm−1 [59,60]. In the present work, in RGO sample the position of G band (1594 cm−1) and 2D band (2679 cm−1) and asymmetric shape of 2D band indicates the presence of multilayers of reduced graphene oxide. Also in case of composites RGNi2 and RGNi1 the G band shifts towards lower wavenumber side (1584 cm−1) and 2D band shifts to higher wavenumbers 2683 cm−1 and 2685 cm−1 respectively indicating multilayers of reduced graphene oxide in them. The RGNi2 sample was photo-irradiated with UV for different time periods (2 h, 6 h, 12 h, 24 h and 48 h) under ambient conditions. Fig. 5 shows the Raman spectra of RGNi2 sample after UV irradiation for different time periods. The position of D, G, 2D bands and Id/Ig after different time durations are listed in Table 2. The D band shifts to lower wavenumber region with increasing irradiation time. The peak at 500 cm−1 is due to Ni and with increasing irradiation time no signature of NiO (expected at 1556 cm−1) is seen in any of the spectra which could have been formed due to photoirradiation with UV light [51]. To further investigate the formation of NiO on the surface of Ni nanoparticles due to photo irradiation by UV light, the FTIR spectra of RGNi2 irradiated for 48 h was recorded (Fig. 6). The peak due to Ni at 1067 cm−1 is seen, which are in good agreement with the results reported earlier and no peaks due to NiO are observed [44]. Hence no formation of NiO on the surface of Ni nanoparticles is observed. The intensity ratios of D band (Id) and G band (Ig) were calculated for different doses of UV to estimate the amount of defects in the system. It is seen from the Fig. 7, Id/Ig increases from 1.18 to 1.47 over the duration of irradiation period (2–48 h). This considerable increase
points towards Ni nanoparticles. Ni is homogenously distributed on reduced graphene oxide sheet and the particles are seen attached to each other owing to their magnetic behavior. The needle like projections present on the nickel NPs are absent in case of Ni- RGO composite. Fig. 2(d) is the image of RGO. Thin reduced graphene oxide sheet is clearly visible in the figure. Morphological structures of the samples were further observed by TEM analysis. Fig. 3(i) shows TEM images along with their corresponding SAED patterns a) RGNi2 b) Ni c) RGO. Fig. 3i(b) is the image of Ni NP. Nearly spherical shaped particles having size less that 10 nm is seen in the image. Fig. 3i(a) is the image of RGNi2 composite. The Nickel nanoparticles are seen uniformly distributed over the ultra thin RGO sheet. The size of the Ni nanoparticles seen is much larger than the pure Ni particles may be due attraction among nickel nanoparticles. The SAED pattern of RGNi2 shows the six fold pattern arising due to graphene in addition to ring pattern from Ni particles, indicating the formation of graphene–Ni composite. Fig. 3i(c) (left) shows the images of RGO. Folded thin layers of graphene resembling crumpled silk veil waves with approximate size of few hundred square nanometers are clearly visible in the images. These sheets are transparent and very stable under electron beam. SAED pattern of HRG shows the six fold ring pattern. Further, the chemical composition of the RGNi2 and Ni nanoparticles was characterized by EDXA analysis. As can be seen in Fig. 3(ii)a, carbon can be detected in the spectrum as the major component and in Fig. 3(ii)b Ni is having the maximum percentage. 3.2. Raman spectroscopy Raman spectroscopy is a powerful non-destructive technique mainly to study graphitic materials such as graphene and also for examining the ordered and disordered crystal structures. Fig. 4 shows Raman spectra of RGNi2, RGNi1 and RGO. The D band is observed at 1325 cm−1 in all the three materials i.e. RGO, RGNi2 and RGNi1. The G band in RGO is observed at 1594 cm−1 and it shifts to lower wavenumber at 1587 cm−1 in RGNi2 and RGNi1. The similar shift in Raman spectra of the composite was also observed by Li et.al. in the Ni @ reduced graphene oxide nanostructures [49]. The G band is characteristic of sp2 hybridized carbon; the broad and intense D band is due to the disorder induced in the sp2 hybridized carbon lattice. Presence of Ni NPs on RGO affects the vibrations of sp2 hybridized carbon atoms leading to shift in the position of G band in RGO and 3
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Fig. 3. (i): TEM images and SAED patterns of a) RGNi2 b) Ni c) RGO. (ii): EDAX of a) RGNi2 b) Ni.
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Fig. 4. Raman spectra of RGO, RGNi2and RGNi1.
Fig. 6. FTIR spectra of RGNi2 irradiated for 48 h.
Fig. 5. Raman spectra of RGNi2 sample after UV irradiation for different time periods (a) 2 h; b) 4 h; c) 6 h; d) 24 h; e) 48 h).
Fig. 7. Id/Ig ratio with increasing irradiation time.
dissociation and recombination of C–C bonds, leading to formation of structural defects in graphene.
Table 2 Variation in D, G, 2D and Id/Ig band position of RGNi2 after irradiation for different time periods. Irradiation Time (h)
Position of D band (cm−1)
Position of G band (cm−1)
Position of 2D band (cm−1)
Id/Ig
2 6 12 24 48
1343 1343 1344 1330 1325
1568 1575 1569 1583 1587
2688 2690 2690 2698 2699
1.18 1.19 1.26 1.44 1.47
3.3. Magnetic characterization The magnetic characterization of the samples at room temperature was performed by vibrating sample magnetometer. Magnetic feature of graphene are generally very weak. It is reported earlier that the weak ferromagnetism in graphene may arise from unpaired spins from topological and bonding defects. The localized unpaired spins could be induced by the edge states [45] as well as defects [46] and stacking faults [47] between graphene sheets. Though bulk graphene shows poor magnetic behavior but significant magnetism could be achieved in it by functionalizing with magnetic nanoparticles [48,49]. The ferromagnetic behavior of Ni; RGNi1 and RGNi2 was confirmed by measuring the hysteresis loop (M vs H) at room temperature. Fig. 8 shows magnetization curves for a) Ni; b) RGNi1; c) RGNi2. The presence of a nonzero coercivity (Hc=296 Oe for RGNi2 and Hc=227 Oe for RGNi1) and a non zero remanence (Mr =3.64 emu/g for RGNi2 and Mr=9.57 emu/g) demonstrate the ferromagnetic behavior of the functionalized Ni NPs-graphene composites. The Ms, Mr, Hc values of Ni, RGNi1 and RGNi2 are listed in the Table 3. The saturation magnetization (Ms) of nickel is found to be 47.86 emu/g, which is much lower than that for the reported bulk Ni metal (58.57 emu/g) at room
in the Id/Ig can be assigned to the appearance of the carbonaceous defects in the reduced graphene oxide sheets. The UV irradiation is reported to degrade the carbon content of graphene structure [43,59,61]. The defect formation by UV irradiation is triggered by the optical transition from π band to π* band. In the visible region the absorption by graphene follows a linear energy momentum dispersion relationship [54,55] contrary to UV region, where dispersion relation is no longer linear and absorption increases. The maximum absorption is at a photon energy of 4.62 eV, which arises from optical transition at the M point [56]. Hence the transition from π to π* band is responsible for UV light absorption by graphene. The states excited via π–π* transitions would cause weakening of C–C bonds and give rise to 5
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increase in Hc value of RGO-Ni composites. 3.3.1. Effect of irradiation The studies on light-induced defects are of great interest. The RGNi2 sample was exposed to UV radiations for 48 h. Fig. 9 shows the M versus H plot of RGNi2 sample after irradiation for 48 h. It is observed that after irradiation the saturation magnetization, Ms increases. and it is found to be 10.65 emu/g after 48 h of irradiation. The coercivity also changes and is 300 Oe. With increased dose of UV irradiation some light induced defects are formed. The defect formation by UV irradiation is triggered by the optical transition from π band to π* band. In the visible region the absorption by graphene follows a linear energy momentum dispersion relationship [54,55] contrary to UV region, where dispersion relation is no longer linear and absorption increases. The maximum absorption is at a photon energy of 4.62 eV, which arises from optical transition at the M point [56]. Hence the transition from π to π* band is responsible for UV light absorption by graphene. The states excited via π–π* transitions would cause weakening of C–C bonds and give rise to dissociation and recombination of C–C bonds, leading to formation of structural defects in graphene. The reduced graphene oxide may contain grain boundaries surrounded by edges. These graphene edges become more reactive than the ordered sp2 lattice. The defective site may initiate defect formation by UV light and defects are extended by UV light [57]. Moreover such defects have local magnetic moment and may give rise to flat bands and eventually to the development of magnetic ordering [58]. The UV radiations are unable to cause ionization in Ni since these radiations possess a maximum energy of 4.8 eV, while the threshold energy required for ionizing nickel is 5.1 eV. Hence there are very less chances of UV radiation ionizing nickel. Hence the defects created in reduced graphene oxide are responsible for enhanced ferromagnetism. It is suggested that defect engineering could be a useful tool for enhancing the ferromagnetic properties.
Fig. 8. Magnetic hysteresis loops of a) Nickel; b) RGNi1 c) RGNi2 at 300 K. The inset is magnified hysteresis loop.
Table 3 Summarized magnetic properties of Ni nanoparticles and composite measured at 300 K. Sample
Ms (emu/g)
Mr (emu/g)
Mr/Ms
Hc (Oe)
Ni RGNi1 RGNi2
47.86 30.56 8.25
8.96 9.57 3.64
0.187 0.313 0.44
181 227 296
4. Conclusions In summary, we have observed the room-temperature ferromagnetism of reduced graphene oxide Ni composites prepared via in-situ reduction of graphene oxide and nickel salt (nickel nitrate). We have also observed enhanced ferromagnetism with UV irradiation. Post UV irradiation led to ~23% increase in ferromagnetism (22.9% increase in Ms value and 22.4% rise in Mr value). The possible origin of the enhanced ferromagnetism may come from the long-range coupling of spin units existing as defects in graphene sheets, which are generated in irradiation process. The room-temperature ferromagnetism, combined with solution-processing capability, should have wide-reaching consequences in material science, and these collective properties could make graphene-based materials a competent choice for many important device applications, including spintronic, magnetoresistance, magnetic memory devices, and so on.
Fig. 9. Magnetic hysteresis loops of RGNi2 after 48 h of irradiation at 300 K. The inset is magnified hysteresis loop.
temperature [50]. With decreasing particle surface energies such as domain wall energies become progressively more effective compared with volume energies [52]. It is clear from the Table 3 that with increasing graphene percentage in the composite, saturation magnetization (Ms) is decreasing and coercivity (Hc) is increasing. There exists significant charge transfer as well as magnetic interaction between graphene and magnetic nanoparticles. It is reported earlier that in graphene metal composites there is an effective charge transfer between graphene and adsorbed nanoparticles [53]. In case of nickel nanoparticles the charge transfer occurs from nickel to graphene. The outer most d orbital of nickel is partially occupied. Hence the transfer of charge from nickel to graphene will lead to reduction in unpaired electrons, leading to decrease in Ms value in reduced graphene oxide - Ni composite. The charge transfer interaction is responsible for decrease in Ms and
Acknowledgements Authors are thankful to Dr. Pankaj Poddar, Staff Scientist, National Chemical Laboratory, Pune, for his valuable suggestions and discussions. Geeta Sharma would like to thank UGC-DSK PDF (Grant No. 237) and Bhabha Atomic Research Centre for financial support. References [1] P.R. Wallace, The band theory of graphite, Phys. Rev. 71 (1947) 622. [2] A.K. Geim, K.S. Novoselov, The rise of graphene, Nat. Mater. 6 (2007) 183–191. [3] W. Wei, X. Qu, Extraordinary physical properties of functionalized graphene, Small 8 (2012) 2138–2151. [4] J. Chen, W.G. Cullen, C. Jang, M.S. Fuhrer, E.D. Williams, Defect scattering in graphene, Phys. Rev. Lett. 102 (2009) 236805. [5] J. Chen, L. Li, W.G. Cullen, E.D. Williams, M.S. Fuhrer, Tunable Kondo effect in
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