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Solid State Sciences 13 (2011) 438e443

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Solid State Sciences

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Facile synthesis of hollow and porous nickel microspheres by low temperaturemolecular self-assembly

Qingwei Zhu a, Yihe Zhang a,c,*, Jiajun Wang b, Fengshan Zhou a, Paul K. Chu c

aNational Laboratory of Mineral Materials, School of Materials Science and Technology, State Key Laboratory of Geological Processes & Mineral Resources,China University of Geosciences, Beijing 100083, ChinabDepartment of Powder Metallurgy and Special Materials, General Research Institute of Non-Ferrous Metal, Beijing 100088, ChinacDepartment of Physics & Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China

a r t i c l e i n f o

Article history:Received 2 September 2010Received in revised form26 November 2010Accepted 1 December 2010Available online 9 December 2010

Keywords:SynthesisHollow nickel microsphereSelf-assemblyMagnetic property

* Corresponding author. National Laboratory of MMaterials Science and Technology, State Key LaboratoMineral Resources, China University of Geosciences, Bþ86 10 82323433.

E-mail address: zyh@cugb.edu.cn (Y. Zhang).

1293-2558/$ e see front matter � 2010 Elsevier Masdoi:10.1016/j.solidstatesciences.2010.12.008

a b s t r a c t

Hollow and porous nickel microspheres with potential applications as catalysts, magnetic materials, andadsorbing materials were synthesized by self-assembly in an aqueous medium in which hydrazinehydrate acted as the reducing agent. The crystal structure, phase, particle size and magnetic performanceof the hollow nickel microspheres were characterized and the effects of the template, pH value,temperature, and reaction time were evaluated. Uniform hollow nickel spheres with porous structurescould be obtained at 75 �C and pH of 9e10 with a heating time of 30 min. The average diameter of thespheres was 1.4 mm and mean thickness of the sphere wall was 120 nm. Polyvinyl pyrrolidone (PK40)played an important role in the formation of the spherical template which controlled the formation ofhollow spheres. Magnetic measurements indicated that the hollow nickel spheres are soft-magneticmaterials with much lower coercivity and much higher saturation magnetization compared to the nickelcrystals with irregular shapes.

� 2010 Elsevier Masson SAS. All rights reserved.

1. Introduction

Different morphology and structure of materials usually lead todifferent physical and chemical properties and various potentialapplications. Hollow inorganic particles have been used widely aslight fillers, catalysts, adsorbents, capsules, photonic crystals,chemical sensors, and superhydrophilic coatings due to their loweffective density, high specific surface area, and stability [1e10]. Infact, hollow spherical particles of MoO2, PtRuPd, TiO2, CuS, Co2O3,SnO2, Eu2O3, ZnO, ZrO2, etc. have attracted scientific interest anddifferent hollow particles have been synthesized by using differentmethods and templates [11e19].

Being an important ferromagnetic material, anisotropic nickelparticles often exhibit interesting magnetic properties. Hencenickel particle with various morphology have potential applica-tions in magnetic materials, and adsorbing materials. Some groupshave synthesized nickel with different morphologies such as

ineral Materials, School ofry of Geological Processes &eijing 100083, China. Tel./fax:

son SAS. All rights reserved.

nanoparticles, nanodots, nanowires, nanorods, nanocones andnanofibres by a variety of methods like hydrothermal reduction,electrodeposition and template-based methods [20e23]. However,there have only been scattered reports on the preparation of hollownickel spheres. Hu et al. [24] synthesized hollow nickel sphereswith controllable shell thicknesses using silica particles as thetemplate and the Au surface as the seed. Deng et al. [25] producedsub-micrometer-sized hollow nickel spheres by auto-catalyzing theredox reaction around a sacrificial colloidal particle surface withsodium hypophosphite as the reducing agent. Jiang et al. [26]obtained hollow nickel spheres by ultrasonic electroless platingand Chen et al. [27]mademono-dispersed hollow SiO2/Ni magneticspheres by using radiofrequency magnetron sputtering and thesol-gel method with polymethyl methacrylate (PMMA) spheres asthe template. And these aforementioned approaches are quitecomplex and time consuming, and the synthesized spheres aretypically not uniform. Hence, great attention has been paid tosynthesis of uniform spherical nickel particles with hollow struc-ture using a simply method, expecting an enhancement in themagnetic and catalytic properties.

In this work, a simple novel approach is introduced to synthe-size uniform hollow and porous nickel microspheres with a diam-eter of w1.4 mm at a low temperature in a relatively short time.Since this method is simple, controllable and environmentally

mailto:zyh@cugb.edu.cnwww.sciencedirect.com/science/journal/12932558http://www.elsevier.com/locate/sssciehttp://dx.doi.org/10.1016/j.solidstatesciences.2010.12.008http://dx.doi.org/10.1016/j.solidstatesciences.2010.12.008http://dx.doi.org/10.1016/j.solidstatesciences.2010.12.008

Fig. 1. (a) XRD pattern and (b) FT-IR spectrum of the as-synthesized hollow nickelmicrospheres.

Q. Zhu et al. / Solid State Sciences 13 (2011) 438e443 439

friendly, it can be used for commercial applications. The effectfactors during the process of synthesis of hollow nickel micro-sphere were analyzed. Subsequently, the phase, morphology andmagnetic property of hollow nickel microsphere were investigated.

2. Experimental details

2.1. Synthesis of hollow nickel spheres

The analytical reagents used in the experiments included nickelsulfate, hydrazine hydrate, anhydrous ethanol, n-butanol, ascorbicacid and anhydrous sodium carbonate. The bio-chemical reagentsused here were gelatin and mixed fatty acid. Polyvinyl pyrrolidone(average molecular weight¼ 40,000e55,000, PK40) was synthe-sized in the laboratory using radical polymerization with ammo-nium peroxydisulfate as initiator at 80 �C and separated by dialysismembrane for 48 h. And deionized water was used throughoutour experiments.

To prepare the hollow nickel spheres, a mixture containing3.0 mol L�1 hydrazine hydrate and 1.60 mol L�1 sodium carbonatesolutions, referred to as solution A, was preheated to w70 �C.Solution B containing 20.00 mL of 0.75 mol L�1 nickel sulfatesolution, an appropriate amount of 5% (M/V) PK40 solution, 1 mL of5% (M/V) gelatin, and 1 mL of 1% (M/V) ethanol solutionwith mixedfatty acids were also preheated to about 70 �C. That these twosystems were preheated to w70 �C is to control the reactiontemperature in the following process well. On the other hand, it isto avoid the volatilization of hydrazine hydrate as far as possible.Solution B was added into solution A under rigorous magneticstirring. Anhydrous ethanol was added drop by drop to defoamduring the reaction. The obtained precipitate was washed by 1% (M/V) aqueous ascorbic acid and then n-butanol several times so as toremove the templates, followed by filtration through a Buchnerfunnel connected to a vacuum pump. The final product was dried ina vacuum dryer at 80 �C for 1 h.

2.2. Characterization

Scanning electron microscopy (SEM) was conducted on a Hita-chi S-4300 field emission scanning electron microscope at anacceleration voltage of 1 5 kV. All the samples were coated witha thin layer of gold by sputtering before microscopic observation.Transmission Electron Microscopy (TEM) measurements wereperformed on an H-8100 transmission electron microscope at anacceleration voltage of 150 kV. Powder X-ray Diffraction (XRD) wasperformed on the nickel powders on a Rigaku D/Max-2000 X-raydiffractometer with Cu-Ka radiation (2 kV rotating anode andl¼ 0.15406 nm) at 45 kV and 300 mA. A scanning rate of 0.06 s�1was applied to record patterns in the 2q range of 10e110�. Fouriertransform infrared (FT-IR) spectra of the samples were obtainedwith a PerkinElmer Spectrum 100 FT-IR spectrophotometer in the4000e400 cm�1 range. The samples were prepared into KBrpellets. The magnetic measurement of the as-prepared productswas carried out in a BHV-50HTI Vibrating Sample Magnetometer(VSM, Riken, Japan). Magnetization curves were recorded at roomtemperature by first saturating the sample in a field of 15,000 Oe.

3. Results and discussion

3.1. XRD of nickel powders

The XRD data in Fig. 1a are used to determine the phase of thenickel particles produced bymolecular self-assembly. There are fivecharacteristic peaks with 2q values of 44.4�, 51.7�, 76.3�, 92.8�, and98.3�, which correspond to the (1 1 1), (2 0 0), (2 2 0), (3 1 1), and

(2 2 2) planes of crystalline nickel, respectively. Other characteristicpeaks caused by impurities of nickel oxide or hydroxide, are notobserved indicating that the synthesized nickel powders do notcontain detectable impurities. The pattern can be indexed todemonstrate the sole existence of metallic nickel powders withthe face-center cubic (fcc) structure (JCPDS card NO.04-0850). Alsothe broad peaks emblematizing amorphous components are notfound in the curve. Our results show that the synthesized nickelpowders have high purity.

3.2. FT-IR of hollow nickel spheres

The FT-IR spectrum of the sample (Fig. 1b) shows bands at 3468and 1637 cm�1. The strong and broad absorption peak around3468 cm�1 is caused by stretching vibration of hydroxyl group; themedium strong peak at 1637 cm�1 results from bending vibrationof eOH group in water molecules. It also further demonstrates thatno organic impurity but water remains on this sample, i.e. the te-mplates were removed entirely in the successive aqueous ascorbicacid and n-butanol washing steps done after the synthesis of thismaterial. Water molecules probably came from the air by means ofabsorbing in the surface of nickel particles. From the FT-IR spec-trum, we cannot found the bands at nearby 455 cm�1 and 526 cm�1

which are associated with the bending vibration of NieO or

Q. Zhu et al. / Solid State Sciences 13 (2011) 438e443440

stretching vibration of NieOeH [28]. It can be inferred that thereexists no amorphous nickel oxide and nickel hydroxide. This resultfurther confirms the homemade hollow nickel microspheres arehighly pure.

3.3. SEM and TEM of hollow nickel spheres

The SEM micrographs in Fig. 2 show uniform spheres with anaverage diameter of w1.4 mm. Some spheres exhibit slight agg-lomeration by forming chains to the surface probably caused by themagneto-static energy of ferromagnetic particles [29]. The porouscage structures are clearly demonstrated in Fig. 2b and the poremay be formed when nickel particles adsorb onto the templateswhich are removed through these minute apertures by washingwith aqueous ascorbic acid and n-butanol. The porous structureyields a high specific surface area which is important to applica-tions to catalysis and adsorption. The TEM image in Fig. 3 disclosesthe spherical outline of the nickel sphere and confirms the hollowstructure within the sphere with a diameter of w1.4 mm and a wallthickness of about 120 nm.

3.4. Effects of the reaction conditions

The pH value is a significant factor affecting the reductionefficacy of hydrazine hydrate which is used as the reducing agentbecause its product is N2, which is not a pollutant and can act asa protective agent preventing the metal particles from

Fig. 2. (a) SEM image of the hollow nickel microspheres synthesized at 75 �C for30 min with PK40 as the template and (b) high resolution image of (a).

oxidization. According to the standard reductioneoxidationpotential, hydrazine hydrates have different electrode potentials atdifferent pH values. Under alkaline conditions, the electrode reac-tion is:

N2H4 þ 4OH�/N2 þ 4H2Oþ 4e� Eq ¼ �1:15 V (1)In comparison, under acidic conditions, the electrode reaction is:

N2Hþ5/N2 þ 5Hþ þ 4e� Eq ¼ �0:23 V (2)

The degree of reduction of hydrazine hydrate is thus higherunder alkaline conditions. Therefore, it was important to adjust thepH value during the reductioneoxidation reaction of hydrazinehydrate. Here, aqueous sodium carbonate (1.6 mol L�1) is used toadjust the pH value. When the pH value is less than 9, no matterhow long themixture is heated up to 80 �C, only purple precipitatesor a mixture of purple and grey precipitates are obtained. It isbecause the reduction reaction is not completed and the productsare not pure nickel spheres. Fig. 4 depicts the XRD patterns of theproducts which are composed of complex components. When thepH value is between 9 and 10, blackegrey nickel metal powders canbe obtained.

Temperature is another important factor affecting the reactionrate. Sometimes, the reactions are impeded if the wrong temper-ature is adopted. To determine the appropriate temperature, solu-tions A and B with a pH value of 9e10 are not preheated. Thereaction temperature is then varied from room temperature to90 �C. At room temperature, nothing is observed to adsorb onto themagnetic stirring rod, indicating that no magnetic nickel powdersare created. When the temperature is raised to 66e70 �C, a quitesmall amount of grey nickel powders are obtained suggesting theonset of the reaction. The apparent reaction does not commenceuntil it is heated to 75 �C. However, if the temperature is too high,the reaction occurs violently and a large number of bubbles andblackegrey precipitates appear in a very short time. A suitable

Fig. 3. TEM image of the as-synthesized hollow nickel microspheres.

Fig. 4. XRD pattern of as-synthesized samples when the pH value was less than 9.

Fig. 6. SEM image of the hollow nickel microspheres synthesized without addingPK40.

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temperature promotes collisions among molecules increasing thereactivity and accelerating hydrolysis of sodium carbonate tomaintain a favorable pH value in the solution.

To evaluate the effects of the reaction time, 10 mL of 1.6 mol L�1

sodium carbonate solution are added to obtain the optimal pHvalue. The blackegrey products begin to appear after approxi-mately 25 min when heated to w75 �C. A violent reaction thentakes place subsequently. Large quantities of blackegrey metalpowders are found on the magnetic stirring rod in a short time.This is possibly because of auto-catalysis of nickel particles result-ing in the accelerated reaction [25]. The nickel ions can be re-duced completely after an additional 5-min reaction. Increasing thereaction time cannot increase the amount of the products butrather yield particles that tend to grow and coalesce (Fig. 5).

In self-assembling of nanomaterials, one of the importantfactors is the self-assembly-induced template. When the particlesgrow on the template, it may be affected by liquid interfacialtension, capillary force, and different hydrophilic or hydrophobicforces. Moreover, selective adsorption of ions and molecules ontodifferent surfaces also affects the particle shape. Any of thesefactors can become the key controlling parameter for the

Fig. 5. SEM image of the hollow nickel microspheres synthesized when the reactiontime is prolonged to 35 min showing agglomeration of nickel particles.

resulting morphologies [30]. On account of their amphiphilicnature, surfactant molecules often spontaneously self -assembleinto microscopic structures such as spheres, cylinders, vesicles, andmembranes in the aqueous solutions depending on outside factors[31]. Hence, to synthesize inorganic particles, these microscopicstructures are often used to act as the soft templates to controlthe resulting morphologies.

In the case of PK40, due to its amphiphilic character (highlypolar lactam group and nonpolar alkyl backbone affording hydro-philicity and hydrophobic property, respectively), it is reasonable tothink that PK40 could be dissolved in water to a certain concen-tration to form micelles with spherical shape to minimize thesurface energies [32]. In this study, 0.4 mL of 5% PK40, 1 mL of 1%mixed fatty acid and 1 mL of 5% gelatin were added to the nickelsulfate solution, PK40 with a concentration more than the criticalmicelle concentration (CMC) may form spherical vesiculate struc-ture and the nonpolar alkyl backbones point toward the interior ofthe micelle and the polar lactam group outward the water [32e35].

The polar lactam group outward the water interacts with nickelelectrostatically subsequently inducing nucleation, crystallization,and growth of nickel particles and giving rise to the final mor-phology and size. In our experiments, hollow nickel spheres areproduced. To demonstrate the impact of PK40 on the morphologyof the nickel particles, a controlled experiment in the absence ofPK40 was conducted while other conditions remained unchanged.SEM image in Fig. 6 reveals that although the spherical nickelparticles can be produced without PK40, the shape of the productsis irregular. But in contrast, when 0.4 mL of 5% PK40 was added,uniform spherical particles were formed as shown in Fig. 2. Itproves that PK40 with spherical self-assembly structure had exer-ted a key influence on the formation of the nickel hollow spheres.The presence of gelatin and mixed fatty acid ensured the as-synthesized spherical particles maintain dispersive, stable andhomogeneous in aqueous solution [36].

3.5. Magnetic property of samples

It is well known that the magnetic property of material isclosely related to its size, morphology, crystallinity and composi-tion. Also the presence of shape anisotropy plays an important rolein enhancing the magnetic properties [37]. Fig. 7 depicts the plotof magnetization versus magnetic field for hollow nickel micro-sphere and irregular nickel particle (as shown in Fig. 6) recorded atroom temperature. Their magnetic parameters are summarized in

Fig. 7. Magnetization curves of (a) hollow nickel microsphere and (b) irregular nickelparticle at roomtemperature.The insetsare theenlargementsof thecentrepartof thecurve.

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Table 1. It can be seen that both samples show hysteresis loop,revealing the ferromagnetic behavior. The coercivity (Hc), satura-tion magnetization (Ms) and remnant magnetization (Mr) of hollownickel microsphere are 168.7 Oe, 58.6 emu g�1 and 14.3 emu g�1,respectively. For irregular nickel particle, the Hc, Ms and Mr are213.0 Oe, 15.9 emu g�1 and 5.1 emu g�1, respectively. The magneticproperty of hollow nickel microsphere shows enhancementcomparing to that of irregular nickel particles. The saturationmagnetization of hollow nickel microsphere is very close to thatof bulk nickel (55 emu g�1) [38]. Both the samples exhibit higherHc values than bulk nickel (100 Oe) [39], and the Hc value of theparticle with hollow spherical structure is much lower comparedto that of irregular nanoparticle. This may be result of shapeanisotropy.

Generally, it would be more helpful for non-retentive materialsto absorb electromagnetic wave if they havemuch higher saturatedmagnetization and much lower coercivity though they are likely tobe affected by other factors. Therefore, hollow nickel microsphereshould show excellent microwave absorbing performance due to

Table 1The magnetic properties of the samples.

Samples Hc (Oe) Ms (emu g�1) Mr (emu g�1)

Hollow microspheres 168.7 58.6 14.3Irregular particles 213.0 15.9 5.1

its high-saturated magnetization and low coercivity resulted fromits special structure.

4. Conclusion

Uniform, hollow and porous nickel microspheres are synthe-sized in the aqueous phase via self-assembly. The average diameterand wall thickness of the synthesized nickel microspheres areapproximately 1.4 mm and 120 nm, respectively. The factors af-fecting the synthesis efficacy include the pH value, temperature,reaction time, and PK40. When the pH value is less than 9,a mixture instead of pure nickel is produced and uniform nickelparticles are formed only at pH between 9 and 10. PK40, a macro-molecule surfactant, plays a critical role in controlling themorphology and size of the hollow nickel microspheres. The reac-tion temperature also has a large influence on the generation ofpure nickel. The ideal temperature is about 75 �C and the optimalreaction time is 30 min. Magnetic measurements indicated that thehollow nickel spheres are soft-magneticmaterials withmuch lowercoercivity and much higher saturation magnetization compared tothe nickel crystals with irregular shapes. And their porous struc-tures, high-saturatedmagnetization and low coercivity ensure theirpotential applications as magnetic adsorbing materials.

Acknowledgements

This study was supported by the open foundation of NationalLaboratory of Mineral Materials of China University of Geosciences(Grant No. 08A006), the Key Project of Chinese Ministry of Educa-tion (No: 107023), Special fund of Co-construction of BeijingEducation Committee, and City University of Hong Kong StrategicResearch Grant (SRG) No. 7008009.

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Facile synthesis of hollow and porous nickel microspheres by low temperature molecular self-assemblyIntroductionExperimental detailsSynthesis of hollow nickel spheresCharacterization

Results and discussionXRD of nickel powdersFT-IR of hollow nickel spheresSEM and TEM of hollow nickel spheresEffects of the reaction conditionsMagnetic property of samples

ConclusionAcknowledgementsReferences