research article characterization and growth mechanism of...

Research ArticleCharacterization and Growth Mechanism of Nickel NanowiresResulting from Reduction of Nickel Formate in Polyol Medium

Olga A Logutenko Alexander I Titkov Alexander M VorobrsquoyovYriy M Yukhin and Nikolay Z Lyakhov

Institute of Solid State Chemistry and Mechanochemistry Siberian Branch of the Russian Academy of SciencesKutateladze 18 Novosibirsk 630128 Russia

Correspondence should be addressed to Olga A Logutenko ologutenkosolidnscru

Received 29 August 2016 Revised 2 November 2016 Accepted 13 November 2016

Academic Editor Jean M Greneche

Copyright copy 2016 Olga A Logutenko et al This is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work is properlycited

Nickel linear nanostructures were synthesized by reduction of nickel formate with hydrazine hydrate in ethylene glycol medium inthe absence of any surfactants or capping agents for direction of the particles growth The effect of the synthesis conditions suchas temperature reduction time type of polyol and nickel formate concentration on the reduction products was studied The sizeand morphology of the nickel nanowires were characterized by X-ray diffraction scanning and transmission electron microscopyIt was shown that the nickel nanocrystallites were wire-shaped with a face-center-cubic phase Ethylene glycol was found to play acrucial role in the formation of the nickel nanowires The possible growth processes of the wire-shaped particles taking place at 110and 130∘C are discussed It was shown that under certain synthesis conditions nickel nanowires grow on the surface of the crystalsof the solid intermediate of nickel with hydrazine hydrate

1 Introduction

In recent years transition metal nanoparticles (Fe Co andNi) have received much attention owing to their wide appli-cations in catalysis optical electronic and magnetic devices[1 2] There are many various techniques that are usedfor making metal nanoparticles with different morphologiessuch as spherical nanoparticles [3] triangles [4] nanorods[5] chains [2] and nanowires [6ndash11] Anisotropic magneticnanomaterials are known to exhibit unique magnetic prop-erties due to which one-dimensional (1D) nanostructuredmaterials have great potential for use in optical electronicand magnetic devices [12 13] As to 1D metallic nickel itis increasingly required for use in gas sensors magnetic re-cording devices catalysts and drug deliveries [14ndash16] Themagnetic properties of nickel nanoparticles allow their appli-cation in cell separation and manipulation Thus nickelnanowires functionalized with antibodies can be used forefficient cell separation [17] with little or no effects on cellsand no disruption to the cellular growth cycle [18]

To date a range of 1D structures have been synthe-sized for various transition metals including nickel To

prepare 1Dnanostructures such as nanorods nanofibers andnanowires the template assisted method spatially restrictingthe particle growth [19 20] or the magnetic-field assistedgrowth route in which magnetic particles align under amagnetic field forming linear chains [21ndash24] is often usedin practice Thus polycrystalline nickel nanowires of around10 microns in length and of about 200 nm in diameter wereobtained using magnetic induction [25] However thesemethods which have drawbacks are being quite complicatedwhich is caused by multistep preparation of the matrix forsynthesis and the subsequent purification of the final productfrom residues and contaminants Furthermore they do notallow production of a large amount of nanomaterials

The chemical reduction route for synthesis of nanopar-ticles is the most widely used because it is relatively simpleallows control of the size and the size distribution of the parti-cles and is more promising in terms of industrial implemen-tation due to its relative simplicity and low cost In this case torestrict radial growth of nanowires various surfactants werenoted to be suitable instead of the conventional templatesThe surfactants acting as capping agents can change the ratio

Hindawi Publishing CorporationJournal of NanomaterialsVolume 2016 Article ID 9058686 9 pageshttpdxdoiorg10115520169058686

2 Journal of Nanomaterials

of the growth rates of different crystallographic planes ofthe particles and result in the formation of one-dimensionalnanostructures Generally PVP is used as the surfactantwhich can direct the growth of wire-like structures andprevent them from aggregation [2 11] Apart from that theformation of nanorods was shown to be promoted by a highhexadecylamine content in the reaction medium [26] Liuet al [27] found that the anion surfactant sodium dodecylbenzenesulfonate played a key role in the growth of Ninanobelts

However some experimental results indicated that thecapping material is not always essential for the growth ofthe Ni nanowires Thus Krishnadas et al [7] synthesizednickel nanowires via the reduction of nickel chloride inethylene glycol using hydrazine hydrate as the reducing agentwithout the assistance of surfactants templates and externalmagnetic field They showed that an increase in the nickelconcentration resulted in an increase in both the diameter(from 60 to 130 nm) and the length (from sim150 to sim600 120583m)of the nanowires They also found that temperature playsan important role in determining the dimensions and mor-phology of the nanowires while the surfactant and magneticfield have negligible roles in their formation Howeverdespite the authors providing some details into the growthof anisotropic nickel nanostructures the exact mechanismof their formation including the role of the solvent in thisprocess remained unclear Also the effect of the nature of thesolvent on the formation of the nanowires was not consideredby the authors

In this paper we present the results of a study of thereduction of nickel formate with hydrazine hydrate usingpolyol (ethylene glycol diethylene glycol 12-propylene gly-col and 15-pentanediol) as a medium and provide a fairlysimple method for preparation of nickel nanowires havingthe potential for high-volume production No morphology-controlling media such as external magnetic forces tem-plate materials and surfactants were needed to promotethe one-dimensional growth in this system Nickel formatewas chosen as a precursor because the possibility of theuse of short chain carboxylates for stabilization of nickelnanoparticles has not been studied yet Furthermore unlikelong chain carboxylates nickel formate is quite soluble inethylene glycol which allows carrying out the reduction ofthe precursor in the solution Also washing the final product(powder metal) from impurities is much easier in this caseThe effect of the synthesis conditions (temperature reductiontime nickel formate concentration and the nature of thepolyol) on the nanowires growth process has been studiedand a tentative mechanism of their formation is discussed

2 Experimental

Nickel (II) formate dihydrate (Ni(HCOO)2sdot2H2O) and ethy-lene glycol (HO(CH2)2OH) of 998 purity grade ge995pure 12-propylene glycol (CH3CH(OH)CH2OH) ge970pure 15-pentanediol (HO(CH2)5OH) 99 pure diethyleneglycol ((HOCH2CH2)2O) 95 pure ethanol and 80 solu-tion of hydrazine hydrate supplied by Sigma Aldrich were

used in the experiments without further purification Solu-tions were prepared using distilled water

The reduction of nickel formatewas carried out as followsAn appropriate amount of nickel formate was dissolved ina polyol under stirring by heating in an oil bath to thedesired temperature Then a concentrated solution (80)of hydrazine hydrate was added to the reaction mixtureunder continuousmechanical stirring at a nickel to hydrazinehydrate molar ratio of 1 18 After the reaction was completethe mixture was air-cooled and the supernate was decantedThe resulting nickel powder was washed several times withethanol and distilled water to remove the impurities and thenit was dried in air at room temperature The particles sizeand their morphology were investigated by means of X-raydiffraction (XRD) analysis and electron microscopy

XRD patterns were recorded on a D8 Advance powderX-ray diffractometer equippedwith a one-dimensional Lynx-Eye detector and a K120573 filter using bu K120572 radiation Thecrystallite size and lattice parameters were estimated by theRietveld method [28] using software for the profile andstructural analysis Topas 42 (Bruker AXS Germany) Thebroadening of the patterns due to the crystallite size wasmodeled by the ldquoDouble-Voigtrdquo function Analysis of thesamples by transmission electron microscopy (TEM) wasperformed using a JEM 2010 electron microscope (JEOLJapan) operating at 200 kVandhaving a resolution of 014 nmStudy of the samples by scanning electronmicroscopy (SEM)was performed using a Hitachi 3400N scanning electronmicroscope (Hitachi Ltd Japan)

The carbon hydrogen and nitrogen contents in thesynthesized samples were determined by the modified Pregl-Dumasmethod ending in gravimetric analysis using a PerkinElmer 2400 Elemental Analyzer The nickel content in thesolution was determined by complexometric titration in thepresence of the indicator murexide

3 Results and Discussion

Ethylene glycol (EG) can act as a reducing agent as well as thereaction medium in many systems However in the case ofnickel its reduction tometal requires the process to be carriedout in the presence of alkaline agents and at the boiling pointof ethylene glycol (sim198∘C) [29] which makes the controlof size and morphology very difficult To reduce nickel atlower temperatures additional reducing agents need to beintroduced into the system In our work we used hydrazinehydrate for that To accelerate the reduction reaction ratethe solution should be alkaline enough as the reductionof nickel ions and generation of nanoparticles require thesynthesis to be carried out at pH values of gt9 In the caseof nickel formate the reduction reaction proceeds withoutadding alkaline reagents to the system which excludes theformation of nickel hydroxide as an intermediate and avoidscontamination of the final product by them

It should be noted that ethylene glycol was also foundto play the role of a protective layer on the particle surfacespreventing them from agglomerating which might occurvia interactions between OH groups and nickel atoms [3031] That is why we did not introduce additional stabilizing

Journal of Nanomaterials 3

(a) (b)

Figure 1 SEM images of the samples obtained by reduction of nickel formate with hydrazine hydrate at temperatures of 100∘C (a) and 110∘C(b)

reagents into the reaction medium Lond et al found [32]that when dissolving a nickel chloride in ethylene glycol acomplex of the composition NiCl2(EG)3is formed

31 Effect of the Reduction Temperature and Time on theProperties of the Nickel Nanoparticles In order to determinethe optimum temperature required for the reduction ofnickel formate with hydrazine hydrate in ethylene glycoland to understand the influence of temperature a seriesof experiments was carried out at reaction temperatures of80 100 110 120 130 and 150∘C and at a nickel formate toethylene glycol weight ratio of 1 100 It was shown that thereaction rate increased as the temperature increased Thusat temperatures below 110∘C the reduction does not occureven over a time interval of 4 h After adding hydrazinehydrate to the solution of nickel formate in ethylene glycolat 100∘C a light violet-pink precipitate is formed As followsfrom the X-ray diffraction (XRD) and scanning electronmicroscopy (SEM) analysis the precipitate obtained underthese conditions does not contain metallic nickel and hasquite highly elongated crystal morphology The crystals areuniform in size and have a length of about 7ndash9 micronsand a width of about 300 to 600 nm (Figure 1(a)) At 110∘Caccording to the XRD data the incomplete reduction ofnickel formate occurs (Figure 2 curve 2) As seen from thefigure the product obtained is a mixture of metallic nickel(Braggrsquos reflections at 2Θ value of 445∘ 5198∘ and 764∘) andsome nickel compound precipitated first from the solutionThis is also confirmed by the electron microscopy data(Figure 1(b)) As seen from the electron microscopy imagesthe final product contains large crystals of the unreactednickel compound of sim15120583m in width and sim8120583m in lengthand small (100minus150 nm in size) spherical nickel nanoparticlesnucleated directly on the crystals from which the nanowiresare growing

At 120∘C the reduction reaction proceeds slowly resultingin the formation of a mixture containing mostly metallicnickel with the impurity of the initial complex of nickelformate with hydrazine hydrate At a temperature of 130∘Cwhen adding hydrazine hydrate to the hot solution of nickelformate in ethylene glycol the reactionmixture changes from

10 20 30 40 50 60 70

1

2

3

80 90

Inte

nsity

(arb

uni

ts)

degrees)2120579 (

(111) N

i

(200) N

i

(222) N

i

Figure 2 XRD patterns of the samples obtained by reductionof nickel formate with hydrazine hydrate at different reductiontemperatures ∘C 100 (1) 110 (2) and 130 (3)

a green colour to deep violet-blue but remains transparentdue to the formation of some soluble nickel complex withhydrazine hydrate For a while no precipitate formation tookplace under these conditions However 1-2 minutes later thesolution becomes turbid and black and solid product appearsindicating the formation of metallic nickel Figure 2 (curve3) shows the XRD patterns of the precipitate obtained at130∘C As seen only the diffraction peaks at 2120579 values of445∘ 5198∘ and 764∘ attributed to the main characteristicpeaks of the (111) (200) and (222) crystal planes of face-centered cubic (fcc) nickel (JCPDS file number 04-0850) areobserved There are no other crystalline phases includingNiO in the diffraction patterns which indicate that the puremetallic nickel could be obtained under these conditionsThe lack of oxidation of the nickel nanoparticles prepared inethylene glycol via the reduction by hydrazine hydrate hasbeen discussed inmany publications for example [6 33] andwas attributed to the formation of nitrogen gas during thereaction processwhich could prevent the nickel nanoparticlesfrom oxidation However even though the characteristic


20nm 50nm

Figure 3 TEM images of nickel nanowires synthesized by reduction of nickel formate with hydrazine hydrate at 130∘C

peaks assigned to the formation of nickel oxide were notdetected in the XRD patterns (Figure 2 curve 3) analysis ofthe nickel nanoparticles obtained by transmission electronmicroscopy (Figure 3) revealed the presence of a very smalllayer of amorphous nickel oxide on the nanowires surfaceThe thickness of the layer does not exceed 1-2 nm and itdoes not increase over time The morphology of the samplesinvestigated by TEM and SEM showed (Figures 3 4(a) and4(b)) that the obtained particles are wire-like and nanocrys-talline Their average size is about 100 nm in diameter andtheir length ranges from 2 to 4 microns According to XRDdata the crystallite size of the resulting nickel particles was13 nm It should be noted that no individual spherical nickelparticles are observed in the SEM image and the wires have aquite uniform shape

An increase in the temperature up to 140 and 150∘C resultsin an increase in the rate of the nickel reduction in thefirst place but does not change very much the dimensionsand morphology of the resultant nanoparticles The resultsof studies of the obtained powders by SEM showed that theparticles have a wire-like shape and their diameter rangesfrom 150 to 200 nm while their length is in the range of 7ndash10 120583m It is also seen that the wires are less homogeneous inthe resultant powders According to XRD data the productsobtained are pure metallic nickel with a crystallite size of9 nm Thus at temperatures below 130∘C the reductionprocess proceeds very slowly or it does not occur at all Anincrease in temperature above 130∘C results in an increaseof both the diameter and the length of the nanowires butthe increase in length is more noticeable than the increase indiameter Apart from that an increase in temperature resultsin a large variation in length so the temperature of (130plusmn5)∘Cseems optimal for the synthesis of nickel nanowires

In order to investigate the effect of reaction time nickelnanoparticles were synthesized at a reduction time of 1minand 1 hour SEM images of the samples obtained at differentreaction times showed (Figures 4(c) and 4(d)) that at theinitial stage of reaction in the first 1min nickel nanowiresare already properly formed which indicates that processof the nanowire formation is rapid The resulting particleshave a size of about 150 nm in width and 6ndash10 120583m inlength A further increase in the reaction time results inlittle change in the dimensions of the nanowires but makesthem less uniform (Figure 4(d)) As seen from the figure

some nanowires become thicker due to the aggregationprocess reaching approximately 300 nm in diameter whiletheir length remains almost the same According to the XRDdata an increase in reduction time results in an increase inthe crystallite size from 9 to 12 nm

32 Effect of Nickel Formate Concentration on the Morphologyof the Nickel Nanoparticles The effect of the nickel concen-tration in the initial solution on the size and morphology ofthe resulting nickel particles has been investigated at 130∘Cwith fixed nickel to hydrazine hydrate molar ratio of 1 18and reaction time of 3min The concentrations of nickel inethylene glycol varied from 0005molL to 027molL InFigures 4(e) and 4(f) SEM micrographs of nickel nanowiresprepared at nickel formate to ethylene glycol weight ratios of1 20 (027molLNi) and 1 400 (00135molLNi) are shownAs seen from this figure a decrease in the nickel formateconcentration results in a decrease in the dimensions of thenanowires At the same time the distribution of the lengthof the nanowires becomes much broader with predominantshorter chains as the initial nickel concentration in the solu-tion is decreased Thus average diameters of the nanowiresare approximately 300 150 100 and 50 nm at nickel toethylene glycol ratios of 1 20 1 100 1 400 and 1 1000respectivelyTheir lengths almost do not change remaining inthe range of 2-3 120583mwhen changing the nickel to ethylene gly-col ratio from 1 20 to 1 1000 but the distribution of the lengthof the nickel nanowires tends to be wider with a decrease innickel concentration Therefore the concentration of nickelin the reaction system should be kept in the 005ndash02molLrange to retain the wire-like morphology of the synthesizednickel nanoparticles

33 Mechanism of Nanowire Formation The mechanism of1D nanostructures growth is widely discussed in the liter-ature [7 27 34 35] but it has not yet been fully clarifiednot least because many factors affect the growth processSo the proposed mechanisms for nanowire formation aremostly qualitative and all authors who tried to preciselydescribe the growth process of the 1D nickel nanostructuresconcluded that it is a problem In the case of our systemthe possible reactions resulting in the formation of nickelnanowires can be presented as follows Initially when nickelformate is dissolved in ethylene glycol the formation of the


(a) 119879 = 130∘C Ni EG = 1 100 and 120591 = 3min (b) 119879 = 140∘C Ni EG = 1 100 and 120591 = 3min

(c) 120591 = 1min 130∘C and Ni EG = 1 100 (d) 120591 = 1 h 130∘C and Ni EG = 1 100

(e) Ni EG = 1 20 130∘C and 120591 = 3min (f) Ni EG = 1 400 130∘C and 120591 = 3min

Figure 4 SEM images of nickel nanowires synthesized by reduction of nickel formate with hydrazine hydrate at different temperatures (ab) reaction times (c d) and nickel concentrations (e f)

nickel formate species solvated by ethylene glycol moleculesoccurs (1) Then when adding hydrazine hydrate to thesolution ethylene glycol ligands are exchanged for N2H4ligands resulting in the formation of a more stable complexbetween nickel formate and hydrazine hydrate as describedby (2) This complex is precipitated from the solution attemperatures below 130∘C as a violet-pink compound whileat temperatures above 130∘C it is soluble The change inthe solution colour from green to deep violet-blue confirmsits formation As known from the literature [36] hydrazinehydrate can play a role in both a bridging bidentate ligandtowards the metal and the reduction agent for the metal Thecomplex composition was shown to depend on the amountof hydrazine hydrate in the system and on the acidity ofthe reaction medium According to published information

[36ndash38] several complexes can be present in the solutioncontaining nickel chloride and hydrazine hydrate such asNi(N2H4)2Cl2 Ni(N2H4)3Cl2 and Ni(N2H4)6Cl2 which actas precursors for the production of nickel nanoparticles

Ni (HCOO)2 sdot 2H2O + 119899HOCH2CH2OH

997888rarr [Ni (HOCH2CH2OH)119899] (HCOO)2 + 2H2O(1)

[Ni (HOCH2CH2OH)119899] (HCOO)2 + 119898N2H4

997888rarr [Ni (N2H4)119898] (HCOO)2 + 119899HOCH2CH2OH(2)

As the solution is heated for longer the free (excess)hydrazine reduces dissolved nickel species followed by thenucleation and growth of the nickel particles as chain-like


(a)

(a)

(b)

100nm

(b)

(c)

(c)

(d)

100nm

(d)

Figure 5 SEM (a c) and TEM (b d) images of the nanoparticles obtained by reduction of Ni(N2H4)2(CHOO)2 in ethylene glycol withhydrazine hydrate after different reaction time periods at 130∘C (a b) 1min and (c d) 2min Ni EG = 1 100

nanostructures via homogeneous nucleation (3)The reactionproceeds very quickly resulting in a very rapid change incolour from gray to black

4 [Ni (N2H4)119898]2++ 4119899N2H4

997888rarr 4Ni + 4 (119898 + 119899)NH3 + 2 (119898 + 119899)N2

+ (119898 + 119899)H2 + 2 (119898 + 119899)H+

(3)

Since at temperatures below 130∘C nickel nanowiresgrow from seeds initially formed on the crystals precipitatedfrom the solution we isolated the intermediate obtained bythe addition of hydrazine hydrate to the solution of nickelformate in ethylene glycol at 100∘C first studying itsmorphol-ogy and composition and then the process of its reductionin ethylene glycol with hydrazine hydrate As mentionedabove at temperatures below 130∘C (100ndash110∘C) as soonas hydrazine hydrate was added to the solution of nickelformate the solution immediately became turbid and alight violet-pink precipitate was formed The precipitate soobtained was separated from the solution by centrifugationand washed with ethanol several times Then it was dried inair and sent for elemental composition analysis Analysis ofthe samples revealed the presence of nickel nitrogen carbonhydrogen and oxygen Based on the weight percentages of270 262 45 115 and 305 obtained for Ni N H C and Orespectively the empirical formula of the complex obtainedwas found to be Ni(N2H4)2(CHOO)2 The calculated weight

percentages for Ni N H C and O are 276 263 47 113 301respectively It should be noted that the same compositionwas found for the compounds precipitated from ethyleneglycol at Ni to ethylene glycol ratio of 1 20 and from thesolution of propylene glycol at Ni to propylene glycol ratioof 1 100 As seen from the electron microscopy images thefinal product contains large crystals of the unreacted nickelcompound of sim15 120583m in width and sim8 120583m in length andsmall (100minus150 nm in size) spherical nickel nanoparticlesnucleated directly on the crystals from which the nanowiresare growing

The synthesized complex Ni(N2H4)2(CHOO)2 was fur-ther used as a starting material (precursor) for synthesis ofnickel nanoparticles via the reduction route In this syntheticprocedure a weighed amount of the precursor was mixedwith ethylene glycol at Ni(N2H4)2(CHOO)2 to ethylene gly-col weight ratio of 1 100 and reduced by hydrazine hydrate at130∘C Electron microscopy images of the samples taken outfrom the reaction mixture at different reaction time periodsare given in Figure 5 A SEM image of the initial compoundNi(N2H4)2(CHOO)2 prior to starting the reduction processis given in Figure 1(a) As seen the compound is present ashighly elongated crystals of about 400 to 800 nm in widthand about 4microns in length Figure 5 shows SEM and TEMimages of the products obtained from a reduction time of 1and 2min In the figure the early growth of nanowires canbe seen It is also seen that small spherical particles of 50ndash150 nm in size are formed on the initial crystal surface while


anisotropic nanostructures are growing away from it ThesesmallNi nanocrystalsmay act as nucleation sites allowing thenewly reduced nickel atoms from the solution to be absorbedthereon which results in the formation of the larger wire-likeshaped particles So the wires are likely formed via adatomaddition and direct growth on the nickel seeds Five minutesafter the reaction started the precursor has been completelyreduced resulting in the formation of nickel nanowires withan average size of about 100 nm in diameter and 2ndash4 micronsin length (Figures 4(a) and 4(b)) According to the XRDdata the product obtained was a pure metallic nickel with acrystallite size of 15 nm

There could be a few reasons why such a shape is realizedduring the reduction of nickel formate by hydrazine hydratein ethylene glycol The first reason could be an inherent self-generated magnetic field which nickel particles possess anddue to which they can aggregate along the magnetic forcelines forming the 1D nanostructures According to informa-tion in the literature [39ndash41] nanoparticles with magneticproperties can spontaneously align into chain structuresHuelser et al [42] using model calculations showed thatformation of the chains of iron nanoparticles up to 300 120583min length can be explained by a magnetic interaction betweenthe ferromagnetic particles in the reaction medium Compu-tational studies of purely metallic (Fe Co and Ni) magneticnanoparticle chains [43] have also confirmed that So thestrong magnetic dipole interactions between nickel particlescan make them align and could be the reason for the forma-tion of the magnetic wire-like structures

On the other hand the solvent used for nanowirespreparation is known to have a significant effect on theirmor-phology Some solvents which are able to interact with themetal atoms can promote the growth of anisotropic structureswhereas others suppress completely such growth The polyolmedium we used in the synthesis may well be responsiblefor the formation of the wire-like structures acting as acapping agent thus affecting the nucleation and growth ratesof the nickel crystals As found by Gong et al [44] themorphologies of the Ni nanocrystallites are closely related tothe types of the solvents and only the solvents with doublehydroxyl groups could be used for the preparation of the Nifibres So it is reasonable to suppose that ethylene glycol isable to influence the growth of anisotropic nanostructures viabinding to certain Ni nanocrystal facets and that can result inthe formation of wire-like structures It should be noted thatnickel nanowires are formed in the ethylene glycol solutionbut they do not form under the same reaction conditionswhen changing the type of polyol Thus it was shown thatnickel nanoparticles resulting from the reduction of nickelformate in diethylene glycol 12-propylene glycol and 15-pentanediol do not possess a wire-like shape (Figure 6) Asseen from the TEM micrograph given in Figure 6 nickelnanoparticles prepared in 12-propylene glycol are mostlyspherical in shape and have smaller sizes less than 50 nmand this morphology did not change even when varying thereaction conditions (temperature and precursor concentra-tion) So the polyol type affects both the shape and size ofthe resultant nanoparticles The reason may be that all thesesolvents being more viscous than ethylene glycol affect the

100nm

Figure 6 TEM image of nickel nanoparticles synthesized byreduction of nickel formate in 12-propylene glycol with hydrazinehydrate at 130∘C

diffusion and growth processes slowing them and inhibitingnanowire formation As reported by Tao et al [45] fast nucle-ation and fast growth promote nanowires formation so theethylene glycol reaction medium seems to play an importantrole in the formation of nickel anisotropic nanostructuresAlso it should be noted that polyols ability to tune particle sizeand shape is certainly related to their complexing strength (ortheir ability to coordinate the Ni(II) ions) which is not thesame for different polyols and decreases when passing from ashort chain polyol to a long chain polyol [46] Therefore themore stable intermediate formed in the Ni-EG systemmay befavourable to the nanowires formation but elucidation of thisissue requires further investigation

4 Conclusions

Nickel nanowires have been prepared in two simple waysby reduction of both nickel formate and nickel hydrazineformateNi(N2H4)2(CHOO)2 with hydrazine hydrate in ethy-lene glycol at 130∘C without the use of other componentssuch as additional capping agents The dependence of thestructural characteristics of nickel nanoparticles on the syn-thesis conditions showed that an increase in the reaction timeresults in little change in the dimensions of the nanowires Anincrease in the temperature results in an increase in the rateof the nickel reduction but does not change very much thedimensions andmorphology of the resultant nanoparticles Adecrease in the nickel formate concentration in the solutionresults in a decrease in the diameters of the nanowires Thusat nickel to ethylene glycol ratios of 1 20 1 100 1 400and 1 1000 the average diameters of the nanowires are300 150 100 and 50 nm respectively while their lengthsalmost do not change remaining in the range of 2-3 120583mFinal morphology of the nickel nanoparticles was shown todepend on the choice of polyol used in the reaction Themechanism of the nanowire formation has been discussedand it has been shown that when using Ni(N2H4)2(CHOO)2as the precursor Ni nanowires grow from spherical seedsinitially formed on the crystal surface via heterogeneousnucleation When using nickel formate which is soluble inethylene glycol one could assume that Ni nanowires growfrom spherical seeds initially formed in the solution via


homogeneous nucleation Although it might be possible thatdue to the high temperature which leads to higher reactionrates large crystallites do not have time to be formed so thenanowires growth takes place on very small solid particleswhich appear in the solution just before the nickel reductionThe as-synthesized nickel nanowires are expected to showenhanced ferromagnetic properties and could be suited foruse in catalysis

Competing Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

This work was supported by the Russian Science FoundationResearch Project no 15-13-00113

References

[1] J HuMOuyang P Yang andCM Lieber ldquoControlled growthand electrical properties of heterojunctions of carbon nan-otubes and silicon nanowiresrdquoNature vol 399 no 6731 pp 48ndash51 1999

[2] C-M Liu L Guo R-MWang Y Deng H-B Xu and S YangldquoMagnetic nanochains of metal formed by assembly of smallnanoparticlesrdquo Chemical Communications no 23 pp 2726ndash2727 2004

[3] W Xu K Y Liew H Liu T Huang C Sun and Y ZhaoldquoMicrowave-assisted synthesis of nickel nanoparticlesrdquoMateri-als Letters vol 62 no 17-18 pp 2571ndash2573 2008

[4] J S Bradley B Tesche W Busser M Maase and M T ReetzldquoSurface spectroscopic study of the stabilization mechanism forshape- selectively synthesized nanostructured transition metalcolloidsrdquo Journal of the American Chemical Society vol 122 no19 pp 4631ndash4636 2000

[5] FMa J Ma J Huang and J Li ldquoThe shape dependence of mag-netic andmicrowave properties for Ni nanoparticlesrdquo Journal ofMagnetism andMagnetic Materials vol 324 no 2 pp 205ndash2092012

[6] J Wang L Y Zhang P Liu et al ldquoPreparation and growthmechanism of nickel nanowires under applied magnetic fieldrdquoNano-Micro Letters vol 2 no 2 pp 134ndash138 2010

[7] K R Krishnadas P R Sajanlal and T Pradeep ldquoPristineand hybrid nickel nanowires template- magnetic field- andsurfactant-free wet chemical synthesis and raman studiesrdquoJournal of Physical Chemistry C vol 115 no 11 pp 4483ndash44902011

[8] A Cortes G Riveras J L Palma et al ldquoSingle-Crystal growthof nickel nanowires influence of deposition conditions onstructural andmagnetic propertiesrdquo Journal of Nanoscience andNanotechnology vol 9 no 3 pp 1992ndash2000 2009

[9] Z R Smith R L Smith and S D Collins ldquoMechanism ofnanowire formation in metal assisted chemical etchingrdquo Elec-trochimica Acta vol 92 pp 139ndash147 2013

[10] J Zhang W Xiang Y Liu M Hu and K Zhao ldquoSynthesis ofhigh-aspect-ratio nickel nanowires by dropping methodrdquo Na-noscale Research Letters vol 11 no 1 pp 118ndash122 2016

[11] Z Xia andWWen ldquoSynthesis of nickel nanowires with tunablecharacteristicsrdquo Nanomaterials vol 6 no 1 article no 19 2016

[12] W Zhou K Zheng L He et al ldquoNiNi3C core-shell nanochainsand its magnetic properties one-step synthesis at low tempera-turerdquo Nano Letters vol 8 no 4 pp 1147ndash1152 2008

[13] H Wu R Zhang X Liu D Lin and W Pan ldquoElectrospinningof Fe Co and Ni nanofibers synthesis assembly and magneticpropertiesrdquoChemistry ofMaterials vol 19 no 14 pp 3506ndash35112007

[14] M JamalMHasan AMathewson andKM Razeeb ldquoDispos-able sensor based on enzyme-free Ni nanowire array electrodeto detect glutamaterdquo Biosensors and Bioelectronics vol 40 no 1pp 213ndash218 2013

[15] L He Z-M Liao H-C Wu et al ldquoMemory and thresholdresistance switching in NiNiO core-shell nanowiresrdquo NanoLetters vol 11 no 11 pp 4601ndash4606 2011

[16] S Senapati S K Srivastava S B Singh and H N MishraldquoMagnetic NiAg core-shell nanostructure from prickly Ninanowire precursor and its catalytic and antibacterial activityrdquoJournal of Materials Chemistry vol 22 no 14 pp 6899ndash69062012

[17] N Gao H Wang and E-H Yang ldquoAn experimental study onferromagnetic nickel nanowires functionalized with antibodiesfor cell separationrdquo Nanotechnology vol 21 no 10 2010

[18] F Byrne A Prina-Mello A Whelan et al ldquoHigh content anal-ysis of the biocompatibility of nickel nanowiresrdquo Journal ofMagnetism and Magnetic Materials vol 321 no 10 pp 1341ndash1345 2009

[19] S Wen and J Szpunar ldquoDirect electrodeposition of highlyordered magnetic nickel nanowires on silicon waferrdquo Micro ampNano Letters vol 1 no 2 pp 89ndash93 2006

[20] A K Bentley M Farhoud A B Ellis G C Lisensky A-ML Nickel and W C Crone ldquoTemplate synthesis and magneticmanipulation of nickel nanowiresrdquo Journal of Chemical Educa-tion vol 82 no 5 pp 765ndash768 2005

[21] P Liu Z Li B Zhao B Yadian and Y Zhang ldquoTemplate-free synthesis of nickel nanowires by magnetic fieldrdquoMaterialsLetters vol 63 no 20 pp 1650ndash1652 2009

[22] HWang X LiM Li K Xie and L Liao ldquoPreparation of NiCucomposite nanowiresrdquo Beilstein Journal of Nanotechnology vol6 no 1 pp 1268ndash1271 2015

[23] M Li K N Xie J W Ye Y ZWu and L Li ldquoFacile synthesis ofnickel nanowires under magnetic fieldrdquo Materials Science andTechnology vol 30 no 6 pp 712ndash714 2014

[24] Y Soumare A Dakhlaoui-Omrani F Schoenstein S MerconeG Viau and N Jouini ldquoNickel nanofibers and nanowireselaboration by reduction in polyol medium assisted by externalmagnetic fieldrdquo Solid State Communications vol 151 no 4 pp284ndash288 2011

[25] H Niu Q Chen M Ning Y Jia and X Wang ldquoSynthesis andone-dimensional self-assembly of acicular nickel nanocrystal-lites undermagnetic fieldsrdquo Journal of Physical Chemistry B vol108 no 13 pp 3996ndash3999 2004

[26] N Cordente M Respaud F Senocq M-J Casanove C Am-iens and B Chaudret ldquoSynthesis and magnetic properties ofnickel nanorodsrdquo Nano Letters vol 1 no 10 pp 565ndash568 2001

[27] Z P Liu S Li Y Yang S Peng Z K Hu and Y T QianldquoComplex-surfactant-assisted hydrothermal route to ferromag-netic nickel nanobeltsrdquo Advanced Materials vol 15 no 22 pp1946ndash1948 2003


[28] H M Rietveld ldquoA profile refinement method for nuclear andmagnetic structuresrdquo Journal of Applied Crystallography vol 2pp 65ndash71 1969

[29] F Fievet J P Lagier B Blin B Beaudoin and M FiglarzldquoHomogeneous and heterogeneous nucleations in the polyolprocess for the preparation of micron and submicron size metalparticlesrdquo Solid State Ionics vol 32-33 no 1 pp 198ndash205 1989

[30] D Ai and S Kang ldquoSynthesis of Ni nanopowders using an EHAsystemrdquo Materials Transactions vol 47 no 8 pp 2056ndash20592006

[31] GViau F Fievet-Vincent andF Fievet ldquoNucleation and growthof bimetallicCoNi andFeNimonodisperse particles prepared inpolyolsrdquo Solid State Ionics vol 84 no 3-4 pp 259ndash270 1996

[32] P B Lond P S Salmon and D C Champeney ldquoStructureof Ni2+ solutions in ethylene glycol by neutron diffraction anobserved hydrogen bond between the solvent ligands in the firstand second cation coordination shellsrdquo Journal of the AmericanChemical Society vol 113 no 17 pp 6420ndash6425 1991

[33] S-H Wu and D-H Chen ldquoSynthesis and characterization ofnickel nanoparticles by hydrazine reduction in ethylene glycolrdquoJournal of Colloid and Interface Science vol 259 no 2 pp 282ndash286 2003

[34] Y-H Choi Y-S Chae J-H Lee Y-W Kwon and Y-SKim ldquoMechanism of metal nanowire formation via the polyolprocessrdquo Electronic Materials Letters vol 11 no 5 pp 735ndash7402015

[35] C Gong J Zhang X Zhang et al ldquoStrategy for ultrafine nifibers and investigation of the electromagnetic characteristicsrdquoThe Journal of Physical Chemistry C vol 114 no 22 pp 10101ndash10107 2010

[36] D Nicholls and R Swindells ldquoHydrazine complexes ofnickel(II) chloriderdquo Journal of Inorganic and Nuclear Chemistryvol 30 no 8 pp 2211ndash2217 1968

[37] L Guo C Liu R Wang H Xu Z Wu and S Yang ldquoLarge-scale synthesis of uniform nanotubes of a nickel complex bya solution chemical routerdquo Journal of the American ChemicalSociety vol 126 no 14 pp 4530ndash4531 2004

[38] J W Park E H Chae S H Kim et al ldquoPreparation of fine Nipowders from nickel hydrazine complexrdquo Materials Chemistryand Physics vol 97 no 2-3 pp 371ndash378 2006

[39] M P Pileni ldquoNanocrystal self-assemblies fabrication andcollective propertiesrdquo Journal of Physical Chemistry B vol 105no 17 pp 3358ndash3371 2001

[40] M Grzelczak J Vermant E M Furst and L M Liz-MarzanldquoDirected self-assembly of nanoparticlesrdquoACS Nano vol 4 no7 pp 3591ndash3605 2010

[41] H Kitching M J Shiers A J Kenyon and I P Parkin ldquoSelf-assembly ofmetallic nanoparticles into one dimensional arraysrdquoJournal of Materials Chemistry A vol 1 no 24 pp 6985ndash69992013

[42] T P Huelser HWiggers P Ifeacho O Dmitrieva G Dumpichand A Lorke ldquoMorphology structure and electrical propertiesof ironnanochainsrdquoNanotechnology vol 17 no 13 pp 3111ndash31152006

[43] A Hucht S Buschmann and P Entel ldquoMolecular dynamicssimulations of the dipolar-induced formation of magneticnanochains and nanoringsrdquo EPL (Europhysics Letters) vol 77no 5 Article ID 57003 2007

[44] C-H Gong C Q Du Y Zhang Z-S Wu and Z-J ZhangldquoMorphology-controlled synthesis of nickel nanostructuresunder magnetic fieldsrdquo Chinese Journal of Inorganic Chemistryvol 25 no 9 pp 1569ndash1574 2009

[45] A R Tao S Habas and P Yang ldquoShape control of colloidalmetal nanocrystalsrdquo Small vol 4 no 3 pp 310ndash325 2008

[46] K J Carroll J U Reveles M D Shultz S N Khanna and EE Carpenter ldquoPreparation of elemental Cu and Ni nanopar-ticles by the polyol method an experimental and theoreticalapproachrdquo Journal of Physical Chemistry C vol 115 no 6 pp2656ndash2664 2011

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ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CorrosionInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

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Biomaterials


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Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

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Nano

materials


Journal ofNanomaterials


of the growth rates of different crystallographic planes ofthe particles and result in the formation of one-dimensionalnanostructures Generally PVP is used as the surfactantwhich can direct the growth of wire-like structures andprevent them from aggregation [2 11] Apart from that theformation of nanorods was shown to be promoted by a highhexadecylamine content in the reaction medium [26] Liuet al [27] found that the anion surfactant sodium dodecylbenzenesulfonate played a key role in the growth of Ninanobelts

However some experimental results indicated that thecapping material is not always essential for the growth ofthe Ni nanowires Thus Krishnadas et al [7] synthesizednickel nanowires via the reduction of nickel chloride inethylene glycol using hydrazine hydrate as the reducing agentwithout the assistance of surfactants templates and externalmagnetic field They showed that an increase in the nickelconcentration resulted in an increase in both the diameter(from 60 to 130 nm) and the length (from sim150 to sim600 120583m)of the nanowires They also found that temperature playsan important role in determining the dimensions and mor-phology of the nanowires while the surfactant and magneticfield have negligible roles in their formation Howeverdespite the authors providing some details into the growthof anisotropic nickel nanostructures the exact mechanismof their formation including the role of the solvent in thisprocess remained unclear Also the effect of the nature of thesolvent on the formation of the nanowires was not consideredby the authors

In this paper we present the results of a study of thereduction of nickel formate with hydrazine hydrate usingpolyol (ethylene glycol diethylene glycol 12-propylene gly-col and 15-pentanediol) as a medium and provide a fairlysimple method for preparation of nickel nanowires havingthe potential for high-volume production No morphology-controlling media such as external magnetic forces tem-plate materials and surfactants were needed to promotethe one-dimensional growth in this system Nickel formatewas chosen as a precursor because the possibility of theuse of short chain carboxylates for stabilization of nickelnanoparticles has not been studied yet Furthermore unlikelong chain carboxylates nickel formate is quite soluble inethylene glycol which allows carrying out the reduction ofthe precursor in the solution Also washing the final product(powder metal) from impurities is much easier in this caseThe effect of the synthesis conditions (temperature reductiontime nickel formate concentration and the nature of thepolyol) on the nanowires growth process has been studiedand a tentative mechanism of their formation is discussed

2 Experimental

Nickel (II) formate dihydrate (Ni(HCOO)2sdot2H2O) and ethy-lene glycol (HO(CH2)2OH) of 998 purity grade ge995pure 12-propylene glycol (CH3CH(OH)CH2OH) ge970pure 15-pentanediol (HO(CH2)5OH) 99 pure diethyleneglycol ((HOCH2CH2)2O) 95 pure ethanol and 80 solu-tion of hydrazine hydrate supplied by Sigma Aldrich were

used in the experiments without further purification Solu-tions were prepared using distilled water

The reduction of nickel formatewas carried out as followsAn appropriate amount of nickel formate was dissolved ina polyol under stirring by heating in an oil bath to thedesired temperature Then a concentrated solution (80)of hydrazine hydrate was added to the reaction mixtureunder continuousmechanical stirring at a nickel to hydrazinehydrate molar ratio of 1 18 After the reaction was completethe mixture was air-cooled and the supernate was decantedThe resulting nickel powder was washed several times withethanol and distilled water to remove the impurities and thenit was dried in air at room temperature The particles sizeand their morphology were investigated by means of X-raydiffraction (XRD) analysis and electron microscopy

XRD patterns were recorded on a D8 Advance powderX-ray diffractometer equippedwith a one-dimensional Lynx-Eye detector and a K120573 filter using bu K120572 radiation Thecrystallite size and lattice parameters were estimated by theRietveld method [28] using software for the profile andstructural analysis Topas 42 (Bruker AXS Germany) Thebroadening of the patterns due to the crystallite size wasmodeled by the ldquoDouble-Voigtrdquo function Analysis of thesamples by transmission electron microscopy (TEM) wasperformed using a JEM 2010 electron microscope (JEOLJapan) operating at 200 kVandhaving a resolution of 014 nmStudy of the samples by scanning electronmicroscopy (SEM)was performed using a Hitachi 3400N scanning electronmicroscope (Hitachi Ltd Japan)

The carbon hydrogen and nitrogen contents in thesynthesized samples were determined by the modified Pregl-Dumasmethod ending in gravimetric analysis using a PerkinElmer 2400 Elemental Analyzer The nickel content in thesolution was determined by complexometric titration in thepresence of the indicator murexide

3 Results and Discussion

Ethylene glycol (EG) can act as a reducing agent as well as thereaction medium in many systems However in the case ofnickel its reduction tometal requires the process to be carriedout in the presence of alkaline agents and at the boiling pointof ethylene glycol (sim198∘C) [29] which makes the controlof size and morphology very difficult To reduce nickel atlower temperatures additional reducing agents need to beintroduced into the system In our work we used hydrazinehydrate for that To accelerate the reduction reaction ratethe solution should be alkaline enough as the reductionof nickel ions and generation of nanoparticles require thesynthesis to be carried out at pH values of gt9 In the caseof nickel formate the reduction reaction proceeds withoutadding alkaline reagents to the system which excludes theformation of nickel hydroxide as an intermediate and avoidscontamination of the final product by them

It should be noted that ethylene glycol was also foundto play the role of a protective layer on the particle surfacespreventing them from agglomerating which might occurvia interactions between OH groups and nickel atoms [3031] That is why we did not introduce additional stabilizing


(a) (b)





10 20 30 40 50 60 70

1

2

3

80 90

Inte

nsity

(arb

uni

ts)

degrees)2120579 (

(111) N

i

(200) N

i

(222) N

i




20nm 50nm





















(a)

(a)

(b)

100nm

(b)

(c)

(c)

(d)

100nm

(d)



4 [Ni (N2H4)119898]2++ 4119899N2H4

997888rarr 4Ni + 4 (119898 + 119899)NH3 + 2 (119898 + 119899)N2

+ (119898 + 119899)H2 + 2 (119898 + 119899)H+

(3)








100nm



4 Conclusions




Competing Interests


Acknowledgments


References























































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




(a) (b)





10 20 30 40 50 60 70

1

2

3

80 90

Inte

nsity

(arb

uni

ts)

degrees)2120579 (

(111) N

i

(200) N

i

(222) N

i




20nm 50nm





















(a)

(a)

(b)

100nm

(b)

(c)

(c)

(d)

100nm

(d)



4 [Ni (N2H4)119898]2++ 4119899N2H4

997888rarr 4Ni + 4 (119898 + 119899)NH3 + 2 (119898 + 119899)N2

+ (119898 + 119899)H2 + 2 (119898 + 119899)H+

(3)








100nm



4 Conclusions




Competing Interests


Acknowledgments


References























































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




20nm 50nm





















(a)

(a)

(b)

100nm

(b)

(c)

(c)

(d)

100nm

(d)



4 [Ni (N2H4)119898]2++ 4119899N2H4

997888rarr 4Ni + 4 (119898 + 119899)NH3 + 2 (119898 + 119899)N2

+ (119898 + 119899)H2 + 2 (119898 + 119899)H+

(3)








100nm



4 Conclusions




Competing Interests


Acknowledgments


References























































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials
















(a)

(a)

(b)

100nm

(b)

(c)

(c)

(d)

100nm

(d)



4 [Ni (N2H4)119898]2++ 4119899N2H4

997888rarr 4Ni + 4 (119898 + 119899)NH3 + 2 (119898 + 119899)N2

+ (119898 + 119899)H2 + 2 (119898 + 119899)H+

(3)








100nm



4 Conclusions




Competing Interests


Acknowledgments


References























































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials







100nm



4 Conclusions




Competing Interests


Acknowledgments


References























































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials





Competing Interests


Acknowledgments


References























































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials






























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials



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