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Materials Chemistry and Physics 130 (2011) 1169– 1174

Contents lists available at SciVerse ScienceDirect

Materials Chemistry and Physics

j ourna l ho me pag e: www.elsev ier .com/ locate /matchemphys

H-dependant structural and morphology evolution of Ni(OH)2 nanostructuresnd their morphology retention upon thermal annealing to NiO

aqoob Khana, S.K. Durranib,∗, Mazhar Mehmooda, Abdullah Janc, Mazhar Ali Abbasid

National Centre for Nanotechnology, Department of Chemical and Materials Engineering, Pakistan Institute of Engineering and Applied Sciences, P. O. Nilore, Islamabad, 45650,akistanMaterials Division, Pakistan Institute of Nuclear Science and Technology, P. O. Nilore, Islamabad, PakistanCentralized Resource Laboratory, University of Peshawar, Peshawar, PakistanDepartment of Science and Technology, Linkoping University, 60174 Norrkoping, Sweden

r t i c l e i n f o

rticle history:eceived 19 January 2011eceived in revised form 24 August 2011ccepted 25 August 2011

eywords:

a b s t r a c t

Nickel hydroxide nanosheets, nanobelts and nanorods were prepared by hydrothermal treatment of theprecipitates obtained at different pH values. The morphology and crystal structure of the products couldbe controlled simply by adjusting the pH value at precipitation. Interconnected nanosheets of hexagonal�-Ni(OH)2 with thickness around 10–20 nm were formed at pH ∼ 11, whereas nanobelts with typicalwidths around 40–80 nm, and nanorods with diameters around 50–60 nm of phase pure �-Ni(OH)2 con-

anostructuresrecipitationrystal growthnnealing

taining intercalated sulphate ions were obtained in the pH range ∼9.5–8.5. Thermal annealing of thehydroxides at 500 ◦C yielded cubic phase NiO with morphologies similar to their hydroxide precursors.X-ray powder diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spec-troscopy (FTIR), thermogravimetric analysis (TGA), and energy dispersive X-ray (EDX) analysis were usedto characterize the as-prepared products. The role of pH in controlling the phase and morphology of theproducts was discussed.

© 2011 Elsevier B.V. All rights reserved.

. Introduction

Nickel hydroxide [Ni(OH)2] is most widely used as the bat-ery positive electrode of alkaline rechargeable batteries, such asi–Zn, Ni–Fe, Ni–MH, and Ni–H2 storage batteries for its highower density, high proton diffusion coefficient, low toxicity, etc.hey also find use in electrochromic devices [1], fuel cells [2],dsorbents [3], supercapacitors [4,5], and as precursors for NiOanostructures [6,7]. Recently, much research efforts have beenaken to prepare Ni(OH)2 in various morphologies like nanowires8,9], nanorods [10], nanotubes [11], nanoribbons [12], andanoflowers [13] due to the fact that the electrochemical perfor-ance of Ni(OH)2 is strongly influenced by its morphology [14,15].hen synthesized via a precipitation route, control over shape

nd crystal structure of Ni(OH)2 is achieved by controlling differ-nt reaction parameters such as reaction temperature, reactantsoncentration, pH at precipitation, choice of alkali, and the type of

ickel salt used. The pH value at precipitation is a key parameterhat influences both the crystal structure and the morphology ofhe Ni(OH)2 formed. For example, Song et al. [16] observed that

∗ Corresponding author. Tel.: +92 51 2208064; fax: +92 51 9248808.E-mail address: durranisk@gmail.com (S.K. Durrani).

254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2011.08.052

the structural characteristics, such as, degree of crystallinity, lat-tice disorder, crystallite size, and crystal growth orientation of thesynthesized �-Ni(OH)2 and the amount of adsorbed anions in thecrystal were strongly related to the pH value of the precipitationreaction. Ramesh and Kamath [17] studied the effect of precipita-tion pH on the amounts of structural disorders present in �-Ni(OH)2and concluded that pH had a profound effect on the degree of struc-tural disorders in Ni(OH)2. Liang et al. [18] synthesized �-Ni(OH)2nanosheets hydrothermally (200 ◦C, 5 h), using nickel acetate andammonia as the starting material. They observed no change in mor-phology of the nanosheets synthesized at pH ∼ 7.5 and ∼9.6. Usinga home-made nickel functionalized surfactant, Coudun et al. [19]were able to maintain 2D morphology of Ni(OH)2 at 60 ◦C in thepH range of 8–11.5. However, the specific effects of pH on themorphologies of Ni(OH)2 nanostructures still need further inves-tigation, as the previous reports mainly focus on the crystallinestructure of the Ni(OH)2.

In the present study, amorphous nickel hydroxide was formedby employing NiSO4 as the source of Ni2+ ions and NH3 and H2O2as precipitating agents, followed by hydrothermal treatment to

form crystalline Ni(OH)2. Particular attentions has been paid tothe effects of pH on the morphology and crystalline structureof hydrothermally formed Ni(OH)2 nanostructures. NiO nanos-tructures were prepared by thermally decomposing the prepared

1170 Y. Khan et al. / Materials Chemistry and

Fig. 1. XRD patterns of the Ni(OH)2 precipitate (a) and Ni(OH)2 obtained afterhydrothermal treatment at pH ∼ 11 (b) and pH ∼ 10.5 (c).

Fig. 2. SEM images and EDX spectrum of the Ni(OH)

Physics 130 (2011) 1169– 1174

Ni(OH)2 at 500 ◦C. The morphologies of Ni(OH)2 were sustainedafter thermal annealing to NiO.

2. Experimental

Nickel sulphate (NiSO4·6H2O), aqueous ammonia (28%), and hydrogen peroxide(30%) were used as the starting materials. In a typical synthesis, 7 g of NiSO4·6H2Owere dissolved in 100 ml of distilled water to prepare a 0.25 M solution. The pH ofthis solution was adjusted to a desired pH in the range of 11–8.5 by adding aqueousNH3. This solution was stirred for 10 min and then a few drops of H2O2 were addedas anionic precursor. Addition of H2O2 immediately caused the appearance of dirtygreen precipitates in the solution. After stirring for another 30 min, the solution wassealed in a Teflon lined stainless steel autoclave and heated to 200 ◦C for 5 h. Afterthe reaction was complete, the resulting Ni(OH)2 samples were filtered via suctionfiltration and washed thoroughly with deionized water to remove any impurities.The samples were then dried in air at 60 ◦C. The as prepared hydroxide samples wereannealed in air at 500 ◦C for 3 h to obtain NiO nanostructures.

Powder X-ray diffraction (XRD) patterns of the as-prepared samples were col-lected using a Rigaku Geiger flux diffractometer with CuK� radiation. The size andmorphology of the final products were determined by scanning electron microscopy(SEM) (JEOL-JSM-5910). Composition was determined via energy dispersive X-ray

(EDX) spectrometer attached with the SEM. FTIR transmittance spectra of the pow-ders in KBr pellets were recorded in the range of 400–4000 cm−1 using NICOLET6700, thermo electron Co., USA. Thermal properties of the samples were stud-ied from room temperature to 700 ◦C in air, using Mettler Toledo, TG/SDTA 851e

instrument.

2 obtaind at pH ∼ 11 (a–c) and pH ∼ 10.5 (d–f).

try and Physics 130 (2011) 1169– 1174 1171

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Y. Khan et al. / Materials Chemis

. Results

.1. Structure and morphology of Ni(OH)2 prepared at pH ∼ 11nd ∼10.5

The XRD patterns of the Ni(OH)2 samples obtained from theydrothermal treatment of precipitates at pH ∼ 11 and ∼10.5, andf the Ni(OH)2 precipitates before hydrothermal crystallizationre shown in Fig. 1. The broad and featureless diffraction pat-ern (Fig. 1a) of the Ni(OH)2 precipitates before hydrothermalreatment indicates the amorphous nature of initial precipitates.owever good crystalline Ni(OH)2 samples were obtained afterydrothermal treatment at 200 ◦C for 5 h. The positions and inten-ities of the XRD reflections for the sample obtained at pH ∼ 11Fig. 1b) were consistent with the well-crystallized hexagonal �-i(OH)2 phase (JCPDS No. 14-0117). However, for the samplebtained at pH ∼ 10.5, the XRD pattern in Fig. 1c showed a mixedhase, containing peaks from both �-Ni(OH)2 as well from thei(SO4)0.3(OH)1.4 or �-Ni(OH)2 consistent with JCPDS No. 41-1424

peaks labeled with star).The low and high magnification SEM images in Fig. 2(a, b) show

hat the �-Ni(OH)2 sample prepared at pH ∼ 11 consisted of inter-ocked nanosheets, connected together in a network like structure.s indicated by arrows, the nanosheets were thin enough to be

ransparent for the electron beam. The EDX spectrum of �-Ni(OH)2anosheets in Fig. 2c showed Nickel and Oxygen elements only,

ndicating the absence of SO4−2 anions in the nanosheets. For the

ixed phase sample obtained at pH ∼ 10.5, The SEM images inig. 2(d, e) showed predominantly the nanosheets morphologylong with a few nanobelts. We presumed that the nanosheets inhis sample were composed of �-Ni(OH)2 and the nanobelts werehose of the �-Ni(OH)2 type. To verify the assumption, a region fromhe nanobelts was selected for EDX analysis (Fig. 2f) where the ele-

ental composition had sulphur, nickel and oxygen, whereas forhe nanosheets, the element sulphur was not detected. This resultuggests, as will be confirmed below, that the nanobelts belong tohe Ni(SO4)0.3(OH)1.4 phase and the nanosheets were composed of-Ni(OH)2, plus the pH at precipitation controls both the crystallinehase and morphology of Ni(OH)2 during hydrothermal crystalliza-ion.

The FTIR spectra of these two samples in Fig. 3 further supporthe XRD findings. The two samples show similar absorption bandst 3640, 3430, 1630, 510 and 460 cm−1. The weak absorption band

ig. 3. FTIR spectra of the Ni(OH)2 synthesized at pH ∼ 11 (a) and pH ∼ 10.5 (b).

Fig. 4. XRD patterns of the Ni(OH)2 obtained at pH ∼ 9.5 (a) and pH ∼ 8.5 (b).

at 460 cm−1 and a broad band around 510 cm−1 were assigned toNi–O stretching and the in-plane Ni–O–H bending vibrations [17],respectively. The narrow band at 3640 cm−1 indicates the presenceof free O–H groups, characteristic of �-Ni(OH)2. In addition to theseabsorptions, both samples show peaks due to hydrogen bonded OHstretching at 3430 cm−1 and a bending vibration at 1630 cm−1 dueto adsorbed water molecules. The bands observed around 1100 and710 cm−1 for the mixed phase sample prepared at pH ∼ 10.5 weredue to HSO4

− and SO42− vibrations respectively, further confirming

the presence of �-Ni(OH)2 in this sample.

3.2. Structure and morphology of Ni(OH)2 prepared at pH ∼ 9.5and ∼8.5

The XRD patterns of Ni(OH)2 prepared at pH ∼ 9.5 and ∼8.5are shown in Fig. 4. The XRD reflections for both these sampleswere indexed to the Ni(SO4)0.3(OH)1.4 phase, consistent with JCPDScard no. 41-1424. None of the reflections from the �-Ni(OH)2 wereobserved, indicating the stability and phase purity of �-Ni(OH)2

in this pH range. However a lower crystallinity was observedfor the sample prepared at a lower pH, as indicated by its rela-tively week and diffused reflections in Fig. 4b. The composition ofNi(SO4)0.3(OH)1.4 or �-Ni(OH)2 is close to, and isostructural with

Fig. 5. FTIR spectra of the Ni(OH)2 synthesized at pH ∼ 9.5 (a) and pH ∼ 8.5 (b).

1172 Y. Khan et al. / Materials Chemistry and Physics 130 (2011) 1169– 1174

oduct

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Fig. 6. SEM images and EDX analysis of the pr

araotwayite, a mineral with monoclinic unit cell and empirical for-ula Ni0.99Mg0.01(OH)1.43(SO4)0.17(CO3)0.12·0.37H2O. While some

eak positions of the paraotwayite are close to �-Ni(OH)2, theyiffer from each other in their relative intensities. For example,he (2 1 2) reflection in our case is the most intense reflection for-Ni(OH)2 compared to the (0 0 2) reflection of paraotwayite [20].o further discard the presence of paraotwayite, FTIR spectroscopyas employed to determine the species inserted in the interlayer

pace or adsorbed on the surface. In the FTIR spectra shown inig. 5(a, b), the symmetric CO3

−2 stretching mode of paraotwayitet 1435 cm−1 is not observed [21], indicating that the preparedamples were �-Ni(OH)2 containing the intercalated SO4

2− ions.he absorption band at 456 cm−1 was assigned to Ni–O vibrationnd the bands between 600 and 1105 cm−1 were attributed to the

ntercalated SO4

2− anions [16]. The stretching and bending vibra-ional modes of the adsorbed water molecules can also be observedt 3450 and 1630 cm−1 respectively. The sharp band at 3600 cm−1

rises from the free O–H bond stretching vibration.

s obtaind at pH ∼ 9.5 (a–c) and pH ∼ 8.5 (d–f).

Fig. 6 shows the SEM images and EDX analysis of the samplesprepared at pH ∼ 9.5 and ∼8.5. Fig. 6(a, b) reveals that the sampleprepared at pH ∼ 9.5 consist of a large number of nanobelts withtypical widths of 40–80 nm and lengths of several microns, whereasfor the sample prepared at pH ∼ 8.5, shorter nanorods with diame-ters around 50–60 nm along with a few nanobelts can be observed(Fig. 6(d, e)). The EDX analysis of these samples (Fig. 6(c, f)) showsnickel and oxygen, as well as sulphur element which arise from theintercalated SO4

2− anions.

3.3. Morphology retention upon heat treatment in air

The possible use of the synthesized Ni(OH)2 as precursors of NiOnanostructures was investigated by annealing different samples at

500 ◦C for 3 h in air. The TG curves for the phase pure Ni(OH)2 sam-ples are shown in Fig. 7. The weight loss observed up to 280 ◦Cfor all the three samples was due to the removal of physicallyadsorbed water on the surface and intercalated water molecules.

Y. Khan et al. / Materials Chemistry and Physics 130 (2011) 1169– 1174 1173

Fig. 7. TG-curves of (a) �-Ni(OH)2 nanosheets obtained at pH ∼ 11, (b) �-Ni(OH)2

nanobelts prepared at pH ∼ 9.5 and (c) �-Ni(OH)2 nanorods prepared at pH ∼ 8.5.Fig. 8. XRD patterns of NiO obtained after air annealing Ni(OH)2 prepared at (a)pH ∼ 11, (b) pH ∼ 10.5, (c) pH ∼ 9.5 and (d) pH ∼ 8.5.

Fig. 9. SEM images of NiO obtained after air annealing Ni(OH)2 prepared at (a) pH ∼ 11, (b) pH ∼ 10.5, (c) pH ∼ 9.5 and (d) pH ∼ 8.5. A representative EDX spectrum of NiOis shown at (e).

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174 Y. Khan et al. / Materials Chemis

or �-Ni(OH)2, the water molecules are only adsorbed as the smallnterlayer distance exclude the intercalation of such molecules,

hereas �-Ni(OH)2 contains both intercalated and adsorbed waterolecules in its large interlayer spacing. The water content for �-i(OH)2 is therefore larger than �-Ni(OH)2. The second weight lossp to ∼350 ◦C was attributed to the decomposition of Ni(OH)2 toiO by the reaction Ni(OH)2 → NiO + H2O. No further weight lossas observed up to 700 ◦C, indicating that dehydration process

ets completed up to 350 ◦C and the resulting NiO was stable upo 700 ◦C. The small differences in the TG curves (Fig. 7b, c) of-Ni(OH)2 are due to experimental procedure. The XRD patternsf NiO samples obtained after annealing the hydroxide samplesre shown in Fig. 8. The peak positions in the XRD patterns are ingreement with the cubic phase NiO (JCPDS No. 47-1049). However,ompared to the standard pattern, the intensity of (1 1 1) reflectionn all the samples was maximum instead of (2 0 0) reflection. Thisreater intensity of the (1 1 1) reflection suggests an abundance of1 1 1) planes in the NiO samples. The broadening of peaks possi-ly indicates a reduced crystallite size of NiO. The SEM images and

representative EDX pattern of the annealed samples are shownn Fig. 9. From the SEM images it can be observed that the over-ll morphology of the Ni(OH)2 precursors was retained quite welly the NiO products. Rosette like aggregates of NiO thin sheets

n Fig. 9a were obtained from �-Ni(OH)2 nanosheets, while theanosheets prepared at pH ∼ 10.5 aligned along their widths, form-

ng a porous lamellar structure (Fig. 9b). The general morphologyf the nanobelts and nanorods of �-Ni(OH)2 was also preserved inorming NiO, although the widths were slightly reduced and lengthsecreased sharply as compared to their precursors. The EDX anal-sis of the NiO samples showed nickel and oxygen elements, withu peak coming from the gold coating used to make the sampleonducting.

. Discussions

Based on our starting materials, the various chemical processeseading to the formation of Ni(OH)2 can be found in Ref. [17].n this reaction system, Precipitation pH value is the key factor

hich controls both the crystal structure and morphology of thei(OH)2 during hydrothermal processing of the initial precipitates.he influence of pH value on the phase and morphology can bescribed to the concentration of OH−1 ions, which mainly changeshe ionic super saturation, which leads to variation in morphol-gy and crystal structure of the products formed. Both �- and-forms of Ni(OH)2 possess a hexagonal layered structure that dif-

er from each other in the interlayer spacing. The � form has annter layer spacing of c = 4.60 A, without any intercalated speciesn the interlayer space, and the OH groups of adjacent layers areot associative via hydrogen bonding. The interlayer spacing in the-Ni(OH)2 is greater than 7.5 A which depending on the startingaterials can contain anions like sulphates, carbonates, nitrates,

hlorides, and hydrogen bonded water molecules. The initiallyormed Ni(OH)2 precipitates in our case were amorphous, indicat-ng that hydrothermal treatment was necessary for the synthesisf Ni(OH)2 nanocrystals. Under higher hydrothermal pressures andemperatures, the solubility of amorphous Ni(OH)2 increases sohat a highly supersaturated solution was formed. This was fol-owed by the nucleation and growth of Ni(OH)2. During the growthrocess, the phase composition and morphology of the growingnits are controlled by the intrinsic crystal structure and the sur-ounding environments. At higher pH, where the concentration of

H−1 ions is high, the differences in magnitude of electrostatic

epulsions between different crystal planes result in an anisotropicrowth of �-Ni(OH)2. For �-Ni(OH)2, the electrostatic repulsionetween (0 0 1) planes is large as compared to (1 0 0) or (0 1 0)

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Physics 130 (2011) 1169– 1174

planes because the Ni2+ ions bordering the (0 0 1) planes are sep-arated by two layers of OH−1 ions while the Ni2+ ions along the(1 0 0) and (0 1 0) planes are linked by OH−1 ions. Thus to minimizeelectrostatic repulsion, rapid crystal growth occurs along the 1 0 0and 0 1 0 directions, resulting in a sheet like morphology. Also thesheets with thickness direction along c axis are more stable ther-modynamically [8,12]. Thus a proper adjustment of pH under thecurrent experimental conditions could fulfill the anisotropic crys-tal growth requirement which leads to the formation of �-Ni(OH)2nanosheets.

For the formation of �-Ni(OH)2, it has been suggested that SO42−

ions are involved in precipitation [22]. When the concentration offree SO4

2− ions is high (i.e. at relatively lower pH value), the rel-ative affinity of the Ni(OH)2 layers for SO4

2− ions is higher thanOH−1 ions. Later during crystal growth, these SO4

2− ions can playthe role of a capping agent in directing the morphology and crys-tal structure of the �-Ni(OH)2 formed. The steric hindrance arisingfrom the intercalated SO4

2− anions can restrict the growth rate of(0 1 0) and (0 0 1) faces. Thus, a preferential growth along (1 0 0)planes would lead to the formation of �-Ni(OH)2 nanobelts andnanorods.

5. Conclusion

Hydrothermal crystallization of amorphous nickel hydrox-ide was used to prepare different shapes such as, nanosheets,nanobelts, and nanorods of crystalline nickel hydroxides.Nanosheets of pure �-Ni(OH)2 could be obtained at pH ∼ 11,whereas nanobelts and nanorods of pure �-Ni(OH)2 were formedin the pH range 9.5–8.5. A mixed phase Ni(OH)2 sample consistingof nanosheets and nanobelts was obtained at an intermediatepH ∼ 10.5. These results indicate the importance of precipitationpH value in controlling the phase and morphology of Ni(OH)2.Moreover, cubic NiO nanostructures of almost similar morpholo-gies as their hydroxide precursors can be produced by air annealingthe as-prepared Ni(OH)2 nanostructures at 500 ◦C for 3 h.

Acknowledgement

The financial support from the Higher Education Commissionof Pakistan for PhD fellowship of Yaqoob Khan is highly acknowl-edged.

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