synthesis and characterization of β-co(oh) 2 , cuo and zno...

Synthesis and Characterization of �-Co(OH)2, CuO and ZnO Nanostructures bySolvothermal Method without Any Additive

Robabeh Mehdizadeh,* Lotf Ali Saghatforoush and Soheila SanatiDepartment of Chemistry, Payam Noor University, 19395-4697, Tehran, I.R. of Iran

(Received: Aug. 2, 2012; Accepted: Oct. 11, 2012; Published Online: Dec. 20, 2012; DOI: 10.1002/jccs.201200419)

�-Co(OH)2, CuO and ZnO nanostructures with plate-like, particle-like and flower-like morphologieswere prepared through the use of simple solvothermal method using of melt salt and 1,10-phenanthrolineas complexing agent and sodium hydroxide. �-Co(OH)2 consisted of a plate-like structure, and the nano-plates size was about 29 nm. The structure was comprised of regular sheets which were assembled to-gether. Furthermore, the as-obtained �-Co(OH)2 nanoplates can be easily converted into Co3O4 nano-plates by calcining in air at 500 °C for 2 h. The results indicate that ZnO powder is of hexagonal wurtzitestructure and well crystallized with high purity. CuO powder is pure monoclinic-structured crystalline.The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), andFourier transform infrared (FT-IR) spectra. Possible formation mechanism of the nanostructures is pro-posed.

Keywords: Nanostructures; Cobalt hydroxide; ZnO; Solvothermal; CuO.

INTRODUCTION

Inorganic nanostructures (NSs) with well-defined sizes

and complex morphologies have attracted considerable re-

search efforts because of their size, morphology, and nano-

structure dependent properties.1 In particular, hierarchical

NSs of transition metal oxides and hydroxides are a hot

topic due to their attractive structures, large surface-to-vol-

ume ratio, and accessible active interfaces. Among the

transition metal hydroxides, cobalt hydroxides [Co(OH)2]

have versatile applications in catalysis, as electrochemical

supercapacitors, in magnetic recording,2–4 and diverse

other fields. For instance, Co(OH)2 is an important elec-

trode material2,5 and can be used as an effective additive to

improve the electrochemical properties of nickel hydroxide

electrodes.6 Moreover, Co(OH)2 has been investigated as a

precursor for the preparation of cobalt oxide nanomaterials

by a thermal conversion.7,8 Co(OH)2 is polymorphic and

crystallizes in to layered structures with two forms, �- and

�-Co(OH)2.9,10 The hydrotalcite-like �-phase is metastable

and easily transforms into the stable brucite-like �-phase in

strongly alkaline media. The properties of Co(OH)2 materi-

als are closely associated with their microstructures. E.g.

their electrochemical capacitance is significantly influ-

enced by surface area and morphology because double

layer and pseudo capacitances are both interfacial phenom-

ena and pores allow a rapid transfer of electrolytes.11,12

Therefore, much research is devoted to the control of their

microstructure. Divers synthetic methods, such as solution

precipitation,13 precursor conversions,14 and electrode po-

sition,1 have been employed to prepare various NSs of co-

balt hydroxides. Co(OH)2 NSs with different morphologies

including rod,15 needle,16 sheet,17 belt,8 and butterfly-like

shapes15 have been obtained. There are already reports on

three dimensional, hierarchical structures of Co(OH)2, such

as sisal, dandelion, and rose-like shapes,18 and flower-like

hollow core-shell structures.19 Hydrothermal synthesis is

widely employed to prepare various nano-sized inorganic

materials.20 The hydrothermal technique provides a versa-

tile route to control grain size, particle morphology, micro-

structure, and phase composition via adjusting parameters

such as temperature, process duration, and pH value of the

solution.21 The reaction media also play a significant role.22

Compared to the conventional hydrothermal/solvothermal

process, in which usually only a single solvent is used,

mixed solvents allow yet more control via adjusting the

type and ratio of the solvents.23 Synthesis of Co(OH)2 with

abundant morphologies via a facile route still remains a

challenge. Various morphologies of Co(OH)2 with mixed

solvents with different ratios of water to ethanol as media,

are obtained hydrothermally by the aid of dimethylgly-

oxime (dmgH).24 A facile room temperature solution-phase

approach for large-scale synthesis of �-Co(OH)2 nano-

plates reported.25 The synthesis of highly uniform, close to

spherical, coral-like �-Co(OH)2 NSs through a facile, etha-

J. Chin. Chem. Soc. 2013, 60, 339-344 © 2013 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 339

JOURNAL OF THE CHINESE

CHEMICAL SOCIETYArticle

* Corresponding author. Tel: +00984612349868; Fax: +00984612349566; E-mail: [email protected]

nol assisted hydrothermal process reported.26

Many studies have shown that the size, shape, and

properties of ZnO nanocrystals depend strongly on the

preparation method and conditions.27 The synthesis, char-

acterization and application of various ZnO nanostructures

including the belts/ ribbons,28 rings,29 tetrapods,30 combs,31

sheets32 and complex structures33 are presently the subject

of intense research. Different synthesis methods have been

devised, including sol–gel technique, microemulsion syn-

thesis, mechanochemical processing, spray pyrolysis and

drying, thermal decomposition of organic precursor, RF

plasma synthesis, supercritical-water processing, self-as-

sembling, hydrothermal processing, vapor transport pro-

cess, sonochemical or microwave-assisted synthesis, direct

precipitation.34 Many methods have been developed to pre-

pare and synthesize of CuO particles with various mor-

phologies. Li et al, synthesized nano-dendrite like CuO via

hydrothermal route.35 CuO nano-shuttle was also prepared

under surfactant assisted conditions using the other same

method.36 CuO nano-rods and nano-ribbons were synthe-

sized by wet chemical methods. In addition, the nanofibers

of CuO were prepared by thermal oxidation on Cu substrate

through importing the polycarbonate membrane template

for initial deposition of Cu nuclei.37 The above mentioned

Methods cannot be departed from complex chemical reac-

tions or processes. Thermal oxidation may assist the pro-

duction of catalysts, semiconductor devices or functional

oxide films under controlled conditions.38 A direct and sim-

ple thermal oxidation method was employed to synthesize

CuO nano-wires and nano-rods. Using this convenient

route, with no catalyst and template assisted, many re-

search teams prepared CuO nano-wires successfully by ox-

idizing copper foils under different conditions such as dif-

ferent annealing temperatures, time or atmosphere.39-42

Metal complexes built from metal ions and polydentate or-

ganic ligands have been grown rapidly in recent years ow-

ing to their potential applications.43 So far, however, the

studies on the syntheses of nano- or microscaled structures

with metal complexes as precursors have been less re-

ported.

In this study, we report the synthesis of plate-like

�-Co(OH)2, ZnO and CuO nanostructures through a facile

solvothermal process. This reliably reproducible method

uses only MCl2 (Co, Zn, Cu), NaOH and 1,10-phenanthro-

line as reactants without templates or further auxiliary re-

agents.

EXPERIMENTAL

Nanostructures synthesis

In a typical experiment, 3 mmol MCl2 (CoCl2.6H2O, ZnCl2,

CuCl2.2H2O) dissolved in 3 mL of distilled water. In the other

beaker, 6 mmol of 1,10-phenanthroline was dissolved in 2 mL of

distilled water and 5 mL of warm ethanol. Then this solution was

added in to the solution of melt chloride under magnetic stirring.

10 mL of NaOH aqueous solution (2M) was added in to the solu-

tion. This alkaline solution was transferred into a Teflon-lined au-

toclave with about 80% capacity. The autoclave was then sealed

and maintained at 160 °C for 24 h. After cooling to room tempera-

ture, the resulting product was centrifuged, washed with distilled

water and absolute ethanol for several times for remove impuri-

ties. Finally, the resulting products was dried at 50 °C. Co3O4

nanoplates were obtained when the as-synthesized Co(OH)2 sam-

ple was directly calcined at 500 °C for 2 h in a muffle furnace.

Materials and physical measurements

All chemical reagents in this experiment were of analytical

grade and used without further purification. Fourier transform in-

frared (FT-IR) spectra were recorded using KBr disks on a

Shimadzu FT-IR model Prestige 21 spectrometer. The morpholo-

gies of products were observed with scanning electron micros-

copy (SEM, Philips XL-30). X-ray powder diffraction (XRD)

measurements were performed using a Philips diffractometer

manufactured by X’pert with monochromatized CuKa radiation.

RESULTS AND DISCUSSION

The morphology, structure and size of the samples are

investigated by Scanning Electron Microscopy (SEM).

Figure 1 shows the SEM micrograph of cobalt hydroxide

nanostructures. The SEM micrograph in Figure 1 shows

340 www.jccs.wiley-vch.de © 2013 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim J. Chin. Chem. Soc. 2013, 60, 339-344

Article Mehdizadeh et al.

Fig. 1. SEM image of Co(OH)2 nanoplates.

that the cobalt hydroxide consisted of a plate-like structure,

and the size was in the range between 1 and 2 �m. The

structure was comprised of densely regular sheets which

were assembled together.

Figure 2 shows the SEM micrograph of ZnO nano-

structures. As shown in Fig. 2 the morphology of the ZnO

sample is a flower-like nanomaterial with an average size

of 2 �m in diameter. Nanoflowers size is of 50 nm. In the

case of CuO sample, the CuO crystallites are nanoparticles

with an average size of 53 nm which show in Fig. 3 which

in agreement is by XRD results.

Fig. 4 provides XRD patterns of nanostructures. The

diffraction peaks in the XRD pattern (Fig. 4a) can be

readily indexed to crystalline bulk �-Co(OH)2. The lattice

constants (a = 3.180 Å, c = 4.655 Å) calculated from this

XRD pattern correspond well to the values given in the

standard card (JCPDS 30-0443), which is indexed to the

hexagonal phase of brucite-like �-Co(OH)2.25 Compared

with the standard pattern, the intensity of the (0 0 1) peak is

unusually stronger than others, implying the preferential

orientation of (0 0 1) on the surface. No impurity peaks are

found, suggesting a high purity of the as-synthesized �-

Co(OH)2. The pattern of cobalt hydroxide shows a good

degree of crystallinity. All peaks for sample ZnO (Fig. 4b)

accord with the JCPDS (No. 36-1451) data for ZnO with

hexagonal wurtzite structure. The diffraction peaks in the

XRD pattern can be readily indexed to crystalline bulk

ZnO. The lattice constants (a = 3.24 Å, c = 5.19 Å) calcu-

J. Chin. Chem. Soc. 2013, 60, 339-344 © 2013 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.jccs.wiley-vch.de 341


Synthesis and Characterization of �-Co(OH)2, CuO and ZnO CHEMICAL SOCIETY

Fig. 2. SEM image of ZnO nanoflowers.

Fig. 3. SEM image of CuO nanoparticles.

Fig. 4. XRD pattern of nanostructures, a) Co(OH)2, b)ZnO, c) CuO.

lated from this XRD pattern correspond well to the values

given in the standard card. No impurity peaks are found,

suggesting a high purity of the as-synthesized ZnO. While

the peaks for sample CuO (Fig. 4c) can be readily ascribed

to the monoclinic CuO form. The observed indexed peaks

in this XRD pattern are fully matched with the correspond-

ing pure monoclinic-structured crystalline CuO (a = 4.68

Å, b = 3.42 Å, c = 5.12 Å, JCPDS card No. 05-0661). Esti-

mated from the Scherrer formula, D = 0.891�/�cos�,

where, D is the average grain size, � is the X-ray wave-

length (0.15405 nm), and � and � are the diffraction angle

and full-width at half maximum of an observed peak, re-

spectively. The average size of the ZnO nanoflowers and

CuO nanoparticles is calculated were about 37 nm for ZnO

by using the strongest peak (1 0 1) at 2� = 36.20, 32 nm for

CuO by using the strongest peak (0 0 2) at 2� = 35.50 and

25.2 nm for Co(OH)2 by using the strongest peak (0 0 1) at

2� = 19.20.

The nanostructures composition can be confirmed via

FT-IR spectroscopy as depicted in Fig. 5a,b,c. A narrow

band (Fig. 5a) is located at 3630 cm-1, which corresponds to

the � O–H stretching of the OH groups in the brucite-like

structure. A broad band at about 3447 cm-1 is characteristic

of the stretching vibration of interlayer water molecules

and of hydroxyl groups hydrogen-bonded to H2O.26 The

bands at 1655 and 1457 cm-1 corresponds to the bending

mode of water molecules. The peak in the region of 486

cm-1 can be assigned to Co–O stretching vibration.26 In the

FT–IR spectrum of the nanostructures (Fig. 5b,c), a strong

band around 445 and 522 cm-1 was observed, which is re-

lated to the Zn–O and Cu–O stretching vibration, respec-

tively. The broad peaks at ca. 3438 and 3425 cm-1 in (Fig.

5b,c) are due to adsorbed water on the external surface of

the samples during handling to record the spectra.

M(phen)2 complex is formed firstly by the reaction of

M2+ with phen during the precursor preparation, and it then

releases M2+ slowly during solvothermal processing. For

Co2+, Co(OH)2 precipitate is produced gradually from the

reaction of Co2+ with OH-. The plate-like nanostructures of

Co(OH)2 are expected to be promoted by the guide of

Co(phen)2 complex. The process of decomposition was a

controlling step, because complex didn’t decompose as

quickly as other Co-containing inorganic salts, which may

provide enough time and opportunity for the growth of

Co(OH)2 nanomaterial. Thus, �-Co(OH)2 nuclei slowly

grew along reaction, resulting in the plate-like structure of

the sample.

For Zn2+ and Cu2+, under the condition of heavy alka-

line solution, [Zn(OH)4]2- or [Cu(OH)4]

2- ions were first

formed. Then [Zn(OH)4]2- or [Cu(OH)4]

2- ions were dehy-

drated under hydrothermal conditions and they in situ gen-

erated a bit of ZnO or CuO nuclei which acted as the seeds

for the growth of melt oxide. The surface of ZnO or CuO

nuclei is either positively charged or negatively charged. In

either case the surface will selectively adsorb ions of oppo-

site charges (OH� or Cu2+) on it, and the new surface cov-

ered with ions will in turn adsorb ions with opposite charges

to cover the surface next.20,21 In the heavy alkaline syn-

thetic system, more OH� may neutralize positive charges



Fig. 5. FT-IR spectra of nanostructures a) Co(OH)2, b)ZnO, c) CuO.

on the surface of melt oxide, preventing them from possible

crystallite aggregation. Thus, ZnO or CuO nuclei slowly

grew and resulting in the flower-like structure of the sam-

ple ZnO and nanoparticle structure for CuO.44

The as-prepared �-Co(OH)2 nanoplates acting as a

precursor is converted into cobalt oxide through dehydra-

tion in our experiment. This reaction can be schematized as

follows:

�-Co(OH)2dehydration� �� Co3O4 + H2O

The product of the thermal decomposition from the

Co(OH)2 precursor was studied by FT-IR, XRD and SEM.

The FT-IR spectra of the Co3O4 show at Fig. 6a. The FT-IR

spectra of the Co3O4 show absorption peak at 3430 cm-1 are

assigned to the stretching vibration of hydroxyl group.

Peaks around 663 and 567 cm-1 are ascribed to the Co–O

stretching mode.

Figure 6b shows the XRD patterns of the dehydration

product. The diffraction peaks were indexed to the phase of

crystalline cubic structured cobalt oxide with the lattice

constant a = 8.08 which are consistent with the values in the

standard card (JCPDS card No. 42-1467). No peaks from

impurities are observed in this pattern. Figure 7 shows the

SEM micrograph of Co3O4 after calcination, which is com-

posed of nanoplates. Size of nanoplates was in about 35 nm

in diameter.

CONCLUSIONS

In summary, �-Co(OH)2 nanoplates, ZnO nanoflow-

ers and CuO nanoparticles were successfully synthesized

in one step via a template and surfactant free solvothermal

route. �-Co(OH)2 consisted of a plate-like structure, and

the nanoplates size was about 29 nm. Furthermore, the as-

obtained �-Co(OH)2 nanoplates can be easily converted

into Co3O4 nanoplates by calcining in air. The results indi-

cate that ZnO powder is of hexagonal wurtzite structure

and well crystallized with high purity. CuO powder is pure

monoclinic-structured crystalline. Chemical composition,

morphology, and size of the nanostructures have been sys-

tematically characterized using XRD, SEM and FTIR.

Thermal decomposition was employed to produce Co3O4

nanoplatelets of Co(OH)2 precursor. The present method is

simple and low-cost which makes it feasible for scale-up

production nanostructures.

ACKNOWLEDGEMENTS

We are grateful to Payame Noor University for finan-

cial support of this work.

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J. Chin. Chem. Soc. 2013, 60, 339-344 © 2013 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.jccs.wiley-vch.de 343


Synthesis and Characterization of �-Co(OH)2, CuO and ZnO CHEMICAL SOCIETY

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synthesis and characterization of β-co(oh) 2 , cuo and zno...

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