Radiation Physics and Chemistry 89 (2013) 5156Contents lists available at SciVerse ScienceDirectRadiation Physics and Chemistry0969-80http://d

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castanoahlamjajournal homepage: www.elsevier.com/locate/radphyschemSynthesis of nickel nanoparticles on multi-walled carbon nanotubesby gamma irradiation

Vivek M. Rao a, Carlos H. Castano b,n, Jessika Rojas b, Ahlam J. Abdulghani c

a Department of Chemical and Biological Engineering, Missouri University of Science and Technology, 400 W. 11th St. Rolla, MO 65409, USAb Department of Mining and Nuclear Engineering, Missouri University of Science and Technology, 301 W. 14th St. Rolla, MO 65409, USAc Department of Chemistry, College of Science, University of Baghdad, Jaderiya, Baghdad, IraqH I G H L I G H T S Nickel nanoparticles were deposited on multi-walled carbon nanotubes by gamma irradiation.

The effect of dose on size distribution of the nanoparticles was evaluated. Between 30 and 60 kGy, as dose increases the average size of the nanoparticles decrease from 16 to 12 nm.a r t i c l e i n f o

Article history:Received 7 September 2012Accepted 2 April 2013Available online 17 April 2013

Keywords:NickelNanoparticlesCarbon nanotubesGamma-rayRadiation induced chemistry6X/$ - see front matter Published by Elsevierx.doi.org/10.1016/j.radphyschem.2013.04.006

esponding author. Tel.: +1 573 341 6766; fax:ail addresses: vmrgy5@mst.edu (V.M. Rao),c@mst.edu (C.H. Castano), jvr45d@mst.edu (J.meel@scbaghdad.edu.iq (A.J. Abdulghani).a b s t r a c t

Multi-walled carbon nanotubes (MWCNTs) were used as a substrate for nickel nanoparticles production.Nickel nanoparticles of average sizes between 9 and 16 nm were synthesized by gamma irradiation ofaqueous solutions containing nickel sulfate or nickel chloride as precursors. MWCNTs were acid treatedand poly vinyl pyrrolidone (PVP, m44,000) or sodium dodecyl sulfate (SDS) were used as stabilizers.Isopropanol was used as a scavenger of hydroxyl radicals, and deionized (DI) water as a solvent. Gammairradiation was carried out at room temperature and ambient pressure in a 60Co gamma source at dosesof 30, 40, 50, and 60 kGy. The nickel nanoparticles were characterized by transmission electronmicroscopy (TEM), scanning transmission electron microscopy (STEM), and X-ray photoelectron spectro-scopy (XPS). By controlling the dose and stabilizer's concentrations, nanoparticles with different sizeswere obtained. Poly vinyl pyrrolidone (PVP) was found to be more efficient at preventing coalescence ofNi seeds than sodium dodecyl sulfate (SDS).

Published by Elsevier Ltd.1. Introduction

Outstanding physical, chemical and electro-mechanical proper-ties have brought carbon nanotubes (CNTs) to the forefront ofmicro and nanotechnology ever since their discovery (Iijima, 1991).Their high thermal conductivity in fluid suspensions has uses inenhancing industrial heat transfer efficiency (Kamali and Binesh,2010; Ding et al., 2006). CNTs have also been investigated for theirmechanical properties due to their low mass and high tensilestrength (tenfold that of stainless steel). The current energy crisisand concerns with our environment have boosted our interest inrenewable and eco-friendly sources of energy. Hydrogen has beenconsidered a reliable and eco-friendly energy carrier for bothportable and stationary applications.Ltd.

+1 573 341 6309.

Rojas),Hydrogen, however, needs to be safely and reversibly stored tobe useful as an energy carrier. Carbon nanotubes are being studiedas hydrogen storage devices (Lipson et al., 2008). The DOE targetestimated for commercialization for hydrogen storage systems is6.5 wt%. Adsorption of hydrogen by varying coverage on CNTs andflip-in mechanisms for hydrogen atoms have been proposed toincrease hydrogen trapping capacities of CNTs (Park et al., 2005;Lee et al., 2001). For pristine single walled CNTs, hydrogen storagecapacities have not exceeded 1 wt% due to weak CH bonding atatmospheric pressure and 295 K. Hydrogen storage capacities havebeen reported up to 6 wt% at cryogenic conditions (12 bar, 77 K),but these conditions are not practical for commercialization(Lawrence and Xu, 2004; Poirier et al., 2004; Pradhan et al.,2002). Hydrogen uptake has been proven to be positively influ-enced by the MWCNT diameter (Hao et al., 2003) and the presenceof metal catalysts (Lipson et al., 2008).

Hydrogen is known to form hydrides readily with manytransition and noble metals (Fukai and Sugimoto, 1985). Choosingthe right metal is expected to improve the hydrogen storage

www.elsevier.com/locate/radphyschemwww.elsevier.com/locate/radphyschemhttp://dx.doi.org/10.1016/j.radphyschem.2013.04.006http://dx.doi.org/10.1016/j.radphyschem.2013.04.006http://dx.doi.org/10.1016/j.radphyschem.2013.04.006http://crossmark.dyndns.org/dialog/?doi=10.1016/j.radphyschem.2013.04.006&domain=pdfhttp://crossmark.dyndns.org/dialog/?doi=10.1016/j.radphyschem.2013.04.006&domain=pdfhttp://crossmark.dyndns.org/dialog/?doi=10.1016/j.radphyschem.2013.04.006&domain=pdfmailto:vmrgy5@mst.edumailto:castanoc@mst.edumailto:jvr45d@mst.edumailto:ahlamjameel@scbaghdad.edu.iqhttp://dx.doi.org/10.1016/j.radphyschem.2013.04.006

V.M. Rao et al. / Radiation Physics and Chemistry 89 (2013) 515652capacity of a nanomaterial. Reported electrochemical loadings ofhydrogen on CNTs encapsulated in a Pd matrix, resulted in 812 wt%hydrogen absorption. The mass of the substrate used (Pd), however,countered the increased hydrogen storage capacity (Lipson et al.,2008). Decoration of CNTs with titaniumwas predicted to store 8 wt% hydrogen (Yildirim and Ciraci 2005) but experimental work hasreported much lower values (Brieno-Enriquez et al., 2009; Ratheret al., 2009). Platinum nanoparticles dispersed on CNTs are pre-dicted to cross the DOE requirements at extremely high pressures(Park and Lee, 2010). Nickel has long been known for its hydrogencatalytic properties. Nickel nanoparticles have been coated on CNTsby thermal evaporation (Bittencourt et al., 2007), and electrolessdeposition (use of a redox reaction to deposit metal on a substratewithout the passage of an electric current) (Chen et al., 2010).Impregnating CNTs with varying amounts of nickel have shown upto 2.8 wt% hydrogen storage capacity (Kim et al., 2005). Nickel hasalso been studied for surface or subsurface hydride formations byelectrochemical methods, which differs from conventional three-dimensional bulk hydride formations (Baranowski, 1999).

Our objective is to produce a nanocomposite structure(MWCNT- Ni matrix) that can enhance the hydrogen storagecapacities of pristine nanotubes by utilizing the nickel hydrideformation to form a link between the CNT walls and hydrogenatoms (similar to Lipson et al., 2008). This work presents thesynthesis of nickel nanoparticles adsorbed on MWCNTS using a60Co gamma source in aqueous solutions containing MWCNT, anickel precursor salt, a stabilizer, and isopropanol as a hydroxylscavenger. The resulting nanostructures were characterized withSTEM, TEM, and XPS.Fig. 1. FTIR spectra of as obtained MWCNTs before (a) and after (b) reflux withnitric acid for 2 h at 80 1C.2. Experimental

2.1. Chemicals

The chemical nickel precursors used in this work were nickelchloride hexahydrate (NiCl2.6H2O) or nickel sulfate hexahydrate(NiSO4 6H2O). MWCNTs with 320 nm outside diameter, 13 nminside diameter, 0.110 m long were used as substrate fordeposition of Ni nanoparticles. PVP with m44,000 and SDS wereused as stabilizers, and isopropanol (99%) as a scavenger ofhydroxyl radicals. All chemicals were purchased from Alfa-Aesar.Purification and functionalization of MWCNTs were done bysurface oxidation reaction using concentrated nitric acid (15.6 M,65% v/v) from Acros Organics. In a typical experiment, a suspen-sion of MWCNTs in HNO3 (1 mg/ml) was ultrasonicated for 30 minto eliminate agglomeration of CNTs. The mixture was then heatedin a round bottom flask fitted with a reflux condenser at 80 1C for2 h in a water bath with continuous stirring. The resulting solutionwas cooled to room temperature, and the removal of acid layerwas done by successive dilution with DI water followed bycentrifugation until the pH of washing water was approximately5. After decantation of the supernatant solution, the MWCNTswere dried overnight at 80 1C.

2.2. Preparation of Ni/MWCNT nanomaterial

Small scale batches for the synthesis of Ni nanoparticles/MWCNTs by gamma irradiation were prepared as follows: Solventmixture of DI water and isopropanol in a volume ratio of (2:1)respectively containing a nickel salt (5103 M), and three differ-ent concentrations of each surfactant (SDS: 0.05 M, 0.07 M, 0.1 Mand PVP: 0.1 mM, 0.3 mM, 0.5 mM were distributed in into small(1 ml) glass vials. To each 1 ml was added 1 mg MWCNTs. Theresulting mixtures were ultrasonicated for 60 min to enhancecomplete dispersion of reactants within the aqueous solutionand were then purged with a stream of argon for 1 h to evacuatethe oxygen. Each vial with a designated concentration of SDS andPVP was then irradiated by a 60Co gamma source at doses of 30,40, 50, and 60 kGy, respectively. Blank samples were also preparedin parallel to test the formation Ni nanoparticles in identicalconditions but in absence of gamma irradiation.

2.3. Apparatus

FTIR spectra of MWCNTs before and after treatment with nitricacid were recorded on a Nicolet NE X US470 FTIR spectrophot-ometer in the region 4004000 cm1 using KBr pellets. Themorphology of Ni nanoparticles deposited on MWCNTs wascharacterized and their sizes and distribution were studied byusing a Helios Nanolab 600 FIB scanning transmission electronmicroscope (STEM) and transmission electron microscope (TEM-Technai F20). Scanning transmission electron microscope (STEM)mode was also used to obtain the energy dispersion X-ray spectra ofNi nanoparticles. Binding energies and detection of nanostructuresNi/MWCNTs were studied by X-ray photoelectron spectroscopy(XPS) using a KRATOS AXIS 1 65-X-ray photoelectron spectrometer.Degassed samples were irradiated in a Gamma Cell 220 Excell 60Coirradiator (MSD NORDION 447 Ontario Canada), dose rate 10 KGy/hwith a cylindrical irradiation chamber of 152 mm diameter and203 mm height.3. Results and discussion

3.1. FTIR spectra

The FTIR spectra of as-prepared and oxidized MWCNTs arecompared in Fig. 1. The immediate distinction between these twospectra is the difference in the peak intensities. It is recognized thattreatment with nitric acid increased intensities of the main absorptionpeaks exhibited by the two samples at 1062, 1406, 1656, 2938 and3468 cm1 corresponding to stretching vibrations of COH, CO (andOH in-plane deformation of sorbed water), CO, CH and OH groups(Stobinski et al., 2010). This indicates the removal of catalytic metallicimpurities from the binding sites of MWCNTs by surface oxidation. Thespectrum of the acid treated MWCNTS exhibited additional absorptionpeaks appeared at 1250, 1530, 1765, 2355, and 2531 cm1 which refersto further functionalization of MWCNTS by chemical oxidation withnitric acid (Stobinski et al., 2010). The peak observed at 1250 cm1 maybe assigned to OH deformation and CO stretching combination insurface phenols and aromatic carboxylic acid (Stobinski et al., 2010),while that at 1530 cm1 is assigned to stretching modes of aromaticCC bonds associated with the backbone of treated nanotubes

V.M. Rao et al. / Radiation Physics and Chemistry 89 (2013) 5156 53(Stobinski et al., 2010; Rojas and Castano, 2012). The bands observed at1765 and 2530 cm1 were attributed to stretching modes of additionalcarboxylic groups while the peak at 2355 cm1 can be associated withthe OH stretching vibration of strongly hydrogen-bonded COOHgroups (Stobinski et al., 2010).

3.2. Preliminary synthesis and characterization of Ni/MWCNTsnanocomposites

Radiolysis of water with gamma radiation leads to the forma-tion of reactive species such as hydrated electron (e aq.), hydroxylradical (OH), and hydrogen radical(H) H3O+, H2O2, and H2. TheFig. 2. STEM and TEM micrographs of nickel nanoparticles adsorbed on MWCNTs with (a60Co gamma source.

Fig. 3. STEM micrographs of the same nickel samples of (a) SDS (0.07 mM) and (b) PVPnanoparticles on MWCNTs in the former, while the Ni nanoparticles still remaining on

Fig. 4. STEM micrographs of nickel chloride samples irradiated with (a) 40 kGy (b) 50 kGhigher population of Ni nanoparticles was adsorbed on MWCNTs. (c) also shows a layenormally accepted values of product yields are well known(Parajon et al., 2008; Appleby and Schwarz, 1969). These speciesplay an important role in the reduction of metal atoms in this case,from Ni+2 to Ni0. The hydrated electrons (e aq.) and hydrogenradicals (H) are strong reducing agents and can reduce Ni2+ ionsinto neutral Ni0 atoms in a two step process similar to otherdivalent metal atoms (Belloni et al., 1998). On the flipside, hydro-xyl radicals (OH) tend to strongly oxidize Ni0 back to Ni+2 ions.To prevent this oxidation, isopropanol is introduced into thereaction medium as a scavenger for hydroxyl radicals (Belloniet al., 1998; Michealis and Henglein, 1992). The OH radicals canreact with isopropanol molecules during and after irradiation to) 0.07 mM SDS and (b) 0.5 mM PVP samples irradiated with a dose of 40 kGy from a

(0.5 mM ) surfactants 30 days after 40 kGy gamma irradiation showing no nickelthe MWCNTs in the latter.

y and (c) 60 kGy absorbed dose using PVP (0.5 mM) as a stabilizer. At higher doses ar of PVP well dispersed over the nanoparticles and MWCNTs.

Fig. 5. Particle size distribution and nickel nanoparticles populations in nickel chloride and PVP (0.5 mM) samples irradiated with (a) 30 kGy (b) 40 kGy (c) 50 kGy(d) 60 kGy gamma radiation doses.

Table 1Average particle sizes and variation with intensity of radiation.

Intensity of radiation(kGy)

Average particle size(nm)

Standard deviation(nm)

30 16.61 73.540 16.04 71.750 13.65 72.160 11.97 72.4

V.M. Rao et al. / Radiation Physics and Chemistry 89 (2013) 515654form reducing isopropanol radicals which react with metal ions toform long lived clusters zero-valent metal atoms M0 (Belloni,2006; Rojas and Castano, 2012). Nickel is an example of non-noble metals whose monomers and oligomers are quite fragile tocorrosion by the solvent. Thus the presence of OH scavengers iscrucial for the formation of long-lived clusters (Belloni et al., 1998).Once formed, nano-sized nickel colloids tend to agglomerate easilyinto large particles and settle out of solution. In addition to van derWaals attractive forces, nickel colloids possess additional magneticattraction (Chou and Huang, 2001). The magnetic Ni nanocrystalshave very large surface areas, and also have very high surfaceenergy. As a result, they will be very reactive with oxygen andwater, forming nickel oxides (Yan et al., 2009). In order to preventagglomeration, surfactants or dispersants such as anionic surfac-tant sodium dodecyl sulfate (SDS) or a polymerpoly vinylpyrrolidone (PVP) were used as stabilizers. Fig. 2(a) and (b) showthe electron micrographs of Ni/MWCNTs nanocompositessynthesized by gamma irradiation at 40 kGy using SDS and PVP,respectively. Repeated STEM analysis after 30 days for SDS treatedsamples showed no presence of Ni nanoparticles on MWCNTs incontrast to what was observed with the samples stabilized by PVP.This indicates that SDS had weak interactions with the Ninanoparticles leading to their oxidation and agglomeration whilePVP had stronger interaction with both the metal nanoparticlesand functionalized MWCNTs compared to SDS. Fig. 3(a) shows asettlement of SDS on the outer walls of the MWCNTs, confirmed byEDS spectra of the outer wall. PVP has N and O atoms in itsheterocyclic five membered ring moiety which can associate withthe nickel atom on the surface of the crystal while the long chainsof PVP stretch out around, causing a steric hindrance effect thatdepresses the collision between nickel crystals and thus preventthe growing of particles effectively and decreased their surfaceenergy (Hussain and Haque 2010). By their high dispersion power,the strongly adsorbed PVP molecules were capable of protecting Ninanoparticles from agglomeration and from oxidation for 30 days,as is shown in Fig. 3(b). According to these preliminary results, PVPwas selected for further experiments.

Attempts to deposit nickel nanoparticles on MWCNTs byadsorption using nickel sulfate as a metal precursor was lesssuccessful compared with nickel chloride. This may be attributedto the lower solubility of nickel sulfate in aqueous solution (44.4 g/100 ml) which makes the synthesized reactive Ni nanoparticlesbecome more susceptible to oxidation forming nickel ions andnickel oxide, or to agglomeration before being adsorbed on CNTs.Furthermore it was reported that reducing agents such as hydro-gen required for synthesis of nickel nanoparticles from nickel

Fig. 7. XPS analysis of Ni/MWCNT nanocomposite (a) Ni peak (b) C peak.

V.M. Rao et al. / Radiation Physics and Chemistry 89 (2013) 5156 55sulfate were twice that required for nickel chloride (Joseph et al.,2011) as is shown in the following equations

NiCl2+H2 (g)-Ni+2HCl (g)

NiSO4+2H2 (g)-Ni+SO2 (g)+2H2O

Nickel chloride was accordingly chosen for further experimen-tation and analysis.

3.3. Synthesis of Ni/MWCNTs by gamma irradiation at differentdoses using nickel chloride as a precursor and PVP as a stabilizer

Based on the preliminary characterization, the experimentswere repeated with doses of ionizing radiation at 30, 40, 50, and60 kGy using nickel chloride hexahydrate as the source of nickelions and PVP as a stabilizer. The samples were prepared andirradiated as before, and the results of STEM analysis are shown inFig. 4.

All three irradiated nickel chloride samples showed highpopulation of nickel nanoparticles with average sizes of 913 nmadsorbed on MWCNTs which indicates that the functionalizedMWCNTs provided active sites for adsorption of nickel atoms/clusters. At higher doses, higher yields and more attachment ofnanoparticles to the MWCNTs took place. Fig. 5 shows the particlesize distributions of Ni nanoparticles supported by MWCNTs ateach dose of gamma radiation. No marked reduction in theaverage particle size was observed for samples irradiated at30 kGy compared with those irradiated at 40 kGy, while therewas a 50% reduction in standard deviation, followed by a moresignificant reduction in average particle sizes for samples irra-diated at doses of 50 and 60 kGy, respectively. Table 1 lists theaverage particle sizes and their corresponding standard deviationsobtained at each dose. The variation of average particle size withabsorbed dose is shown in Fig. 6 evidencing a decrease in the sizeof nickel nanoparticles on MWCNTs with increasing dose.

Varying the PVP concentration (o1 mM) showed insignificanteffects. Since nickel nanoparticles were adsorbed on MWCNTseven at 30 kGy of gamma irradiation, the concentrations of PVPemployed proved sufficient to retain the nanoparticles in their Ni0

state. These results indicate that PVP serves as a good linkerFig. 6. The variation of average particle size (nm) with absorbed gammadoses (kGy).between the Ni nanoparticles and functional groups present onthe oxidized CNT surface. This linking may take place throughinteraction of N and O atoms of PVP ring with the formers andthrough weak intermolecular interactions (such as stacking,hydrophobic or electrostatic attractions) with latters (Georgakilaset al., 2007). The absence of Ni nanoparticles on some MWCNTs inthe STEM images is attributed to a lack of active sites on theirsurfaces.

Fig. 7(a) and (b) show the XPS analysis of the Ni-CNT nano-composite. Fig. 7(a) shows a relatively low intensity Ni 2p3/2 peakat 852.6 eV compared to the NiO peak at 853.8 eV. Since the NiOpeak is sharper, it is suggested that the reduction of Ni+2 to Ni0 isaccompanied by the possible bonding between nickel and oxygen.Fig. 7(b) shows that carbon has peaks at 284.6 V (1s) and 285.3 eV(carbonyl CO). The carbonyl CO peak arises from the functionalizedsites of MWCNTs. In the absence of data for NiC peak, we believethe adsorption of nickel on MWCNTs is through a NiOMWCNTbond. The NiO bond is weaker as compared to the CO bond,indicated by the higher free energy of the former.4. Conclusions

Nickel nanoparticles with average sizes of 913 nm weresuccessfully synthesized and deposited on carbon nanotubesinduced by gamma radiation at doses 30, 40, 50, and 60 KGy usingnickel chloride as a metal precursor, PVP as a stabilizer andisopropanol as hydroxyl radical scavenger and DI water as asolvent. Increasing the dose of gamma radiation caused a reduc-tion in average particle size. Lower concentrations of nickel salt are

V.M. Rao et al. / Radiation Physics and Chemistry 89 (2013) 515656desirable in order to prevent agglomeration and to ensurecomplete adsorption of nickel atoms on MWCNTs. Preliminarycharacterization proved PVP as a more suitable stabilizer for nickelnanoparticles than SDS. Varying the PVP concentration (o1 mM)yields insignificant effects. The physical obstruction provided byPVP restricts both: oxidation of nickel nanoparticles before beingadsorbed on MWCNTs and desorption of nickel atoms fromMWCNTs and subsequent agglomeration of nickel nanoparticles.Functionalization of MWCNTs by nitric acid provided active sitesfor adsorption of Ni0 nanoparticles. The reaction conditions suchas the type of stabilizer used (surfactant, polymer) functionaliza-tion of carbon nanotubes, metal precursor salt, secondary alcoholand their concentrations as well as absorbed dosage are allimportant factors in dictating the stability of synthesized nano-composites (Rojas and Castano, 2012). Optimum conditions forproducing higher yields of smaller Ni nanoparticles (o10 nm),will require to take all these factors into consideration.Acknowledgements

This work was partially supported by NRC Grant NRC-38-10-966512. The authors would like to specially thank Dr. StoyanToshkov for providing the irradiation services at the Universityof Illinois at Urbana Champaign and Dr. Kai Song, Sr. ResearchScientist, Missouri University of Science & Technology for technicalassistance with electron microscopy.

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Synthesis of nickel nanoparticles on multi-walled carbon nanotubes by gamma irradiationIntroductionExperimentalChemicalsPreparation of Ni/MWCNT nanomaterialApparatus

Results and discussionFTIR spectraPreliminary synthesis and characterization of Ni/MWCNTs nanocompositesSynthesis of Ni/MWCNTs by gamma irradiation at different doses using nickel chloride as a precursor and PVP as a stabilizer

ConclusionsAcknowledgementsReferences