RESEARCH PAPER

Synthesis and characterization of cadmium hydroxidenanowires by arc discharge method in de-ionized water

Volkan Eskizeybek • Okan Demir •

Ahmet Avci • Manish Chhowalla

Received: 5 August 2010 / Accepted: 16 May 2011 / Published online: 29 May 2011

� Springer Science+Business Media B.V. 2011

Abstract In this study, Cd(OH)2 nanowires have

been synthesized by using arc discharge method in

de-ionized water. The morphology and properties of

the Cd(OH)2 nanowires were characterized by X-ray

diffraction analysis (XRD), scanning electron micros-

copy, transmission electron microscopy (TEM), and

UV–Vis spectroscopy. TEM observations revealed

that Cd(OH)2 nanowires were abundant morphology

in synthesized nanostructures, and the diameter of the

Cd(OH)2 nanowires ranges from 5 to 40 nm with

several micrometers of length. In addition, the width

of nanowires is not uniform and varies throughout the

nanowire. XRD analysis revealed that the Cd(OH)2

nanowires grow along [001] direction. Furthermore,

hexagonal- and irregular-shaped Cd(OH)2 nanoplates

were synthesized during arc discharge. It was

obtained that required arc current is 50 A for the

effective and large scale production of Cd(OH)2

nanowires. Furthermore, the optical properties of the

nanowires have been characterized by UV–Vis

spectra. By the means of the optical studies, the

direct band gap of Cd(OH)2 nanowires was found to

be 4.0 eV with strong quantum size effect. It is also

shown that a simple and cheap method which does

not require relatively expensive vacuum and laser

equipment stipulates an economical alternative for

the synthesis of Cd(OH)2 nanowires.

Keywords Arc discharge � Cd(OH)2 � Nanowires �Characterization

Introduction

Owing to the unique size and shape-dependent

physical properties, one-dimensional (1D) nanostruc-

tures such as nanotubes, nanowires, and nanobelts

have received increasing interest in the field of

nanoscience. While most of the study has been

focused on single-crystal 1D nanostructures, studies

on nanoparticles assemblies into 1D nanostructures

have been scarcely reported due to the difficulties

associated with their preparations (Peng et al. 2000;

Iijima 1991; Pan et al. 2001; Han et al. 1997; Xia

et al. 2003). Remarkable advances have been made in

the synthesis and characterization of 1D to fabricate

complex-nanoscaled electronic or opto-electronic

devices (Miao et al. 2006). 1D semiconductor

nanostructures have been attracting extensive

research interest in recent years because of their

novel properties and potential applications. Further-

more in recent years, metal hydroxide nanostructures

V. Eskizeybek (&) � O. Demir � A. Avci

Mechanical Engineering Department, Selcuk University,

42075 Selcuklu, Konya, TURKEY

e-mail: veskizeybek@selcuk.edu.tr

M. Chhowalla

Department of Materials Science and Engineering,

Rutgers University, Piscataway, NJ 08854, USA

123

J Nanopart Res (2011) 13:4673–4680

DOI 10.1007/s11051-011-0430-z

such as Ni(OH)2, Cu(OH)2, Mg(OH)2, and Cd(OH)2

have been synthesized as potential templates or

precursors for the corresponding oxide nanostructures

(Dai et al. 1995; Heath and LeGoues 1993; Ren et al.

1998). Cadmium hydroxide Cd(OH)2 is a potential

candidate for application as a cathode material in

batteries owing to its high stability (Singh 1998;

Motupally et al. 1998). Furthermore, Cd(OH)2 is an

important material that can be used as a precursor of

CdO films or powders. Cd(OH)2 can be converted

into cadmium oxide through dehydration or into other

functional materials such as CdS and CdSe by

reactions with appropriate elements or compounds

(Ristic et al. 2004; Zhang et al. 2005; Kondo Ri

Okhimura and Sakai 1971). Cd(OH)2 is a wide band

gap (3.2 eV) semiconductor with a wide range of

possible applications including solar cells, phototran-

sistors, photodiodes, transparent electrodes, gas sen-

sors, etc. These applications of Cd(OH)2 are based on

its specific optical and electrical properties (Ghoshal

et al. 2007; Zhou et al. 2008; Li et al. 2006;

Santamaria et al. 2009; Mane and Han 2005). For

example, Cd(OH)2 thin films show a high transpar-

ency in visible region of solar spectrum, as well as

high ohmic conductivity. Within the past few years,

efforts have been made for the fabrication of

semiconducting oxide nanowires (Peng et al. 2002;

Kong et al. 2001; Liang et al. 2001; Yang and Lieber

1996; Zhu et al. 1999), with less attention has been

focused on Cd(OH)2. Numerous techniques have

been proposed to synthesize nano-sized Cd(OH)2

with promising control of properties (Ristic et al.

2004; Mane and Han 2005; Luo et al. 2005). Arc

discharge in liquid environment is a simple and cheap

method for large-scale synthesis of nanoparticles

(Wang et al. 2004). This method needs only a dc

power supply and an open vessel full of de-ionized

water, liquid nitrogen, or aqueous solution. It does

not require any vacuum media, a furnace, a heat

exchange, and reacted gases such as argon, nitrogen,

helium, and hydrogen compared with any other well-

known methods. In this study, we report using a

simple method for the synthesis of large scale

Cd(OH)2 nanowires by using arc discharge method

in de-ionized water. The idea is to induce the

generation of OH- ions through the reduction of

H2O and the production of nanoparticles in a liquid

environment in which it secures the isolation of

produced nanostructures from air atmosphere.

Experimental

All the chemicals were of analytical grade and used

as received without further purification. Cd rod

(99.99%) was purchased from Alfa Aesar. De-ionized

water was used throughout.

Synthesis of Cd(OH)2 nanowires

The arc discharge apparatus used for the synthesis of

Cd(OH)2 nanowires in this study is shown schemat-

ically in Fig. 1. The apparatus consists of Cd

electrodes. A 5 l isolated glass beaker was filled with

3 l de-ionized water. A high purity Cd uniform rod,

which has 10-mm diameter and 13.3 g weight, was

used as the cathode and has a flat surface to keep

uniform arcing during experiment. Another Cd con-

ical-shaped rod, which has 6-mm diameter at the end

and 45 mm in length and 33.5 g weight, was used as

the anode.

The arc discharge was initiated in the de-ionized

water by touching the anode to the cathode, and then,

the gap between the electrodes was controlled by

hand at about 1 mm to maintain stable arc discharge.

The arc current was supplied by a direct current (dc)

welding power supply. The applied arc current was

decided as 50 A, after several experiments as an

optimum current, to produce large scale and effective

nanoparticles. Furthermore, welding of the cadmium

anode and cathode electrodes to each other occurred

during arc discharge at high currents more than 50 A

and resulted with interruption of the experiment.

During arc discharge, the temperature between elec-

trodes can increase more than 5,000�K (Startsev et al.

2000; Levchenko et al. 2010). A dense black smoke

was readily observed around the arc plasma in a few

seconds and dispersed in the de-ionized water

Fig. 1 Schematic view of the arc discharge apparatus

4674 J Nanopart Res (2011) 13:4673–4680

123

rapidly. The arc discharge was continued for 3 min,

and the discharge voltage was measured between 28

and 34 V by the multi-meter when the arc was stable.

Surprisingly, it was observed that a natural segrega-

tion of the constituents takes place after discharging.

The glass beaker was kept at room temperature to

complete settling of synthesized particles at the

bottom for 24 h. The settled products were collected

carefully by pouring the products into a 500 mL

beaker after decantation of almost all the suspension.

The collected products were washed with de-ionized

water and absolute ethanol to remove the ions

possibly in the final product for five times, dried at

40 �C under vacuum for 24 h.

Characterization

Powder X-ray diffraction (XRD), transmission elec-

tron microscopy (TEM), high resolution TEM

(HRTEM), and scanning electron microscopy

(SEM) were applied to characterize the crystal

structure, size, shape, composition, and structure of

as-prepared samples. XRD was carried out by means

of Shimadzu XRD-6000 X-ray diffractometer using

Cu Ka (k = 0.15418 nm) radiation at 40 kV and

30 mA ranging from 2 to 70o at a scanning rate of 2o/

min. SEM images of the synthesized products were

carried out using a JEOL/JSM-6335F-EDS SEM. The

samples for TEM prepared by some part of the

particles were dispersed in ethanol, and a drop of the

solution was placed on a carbon-coated copper grid.

The solution was allowed to evaporate before imag-

ing. The TEM and HRTEM images were performed

using JEOL 2100 HRTEM at 300 kV. The optical

absorption spectrum of nanoparticles was recorded by

a UV–Vis spectrophotometer (Ocean Optics

HR4000) in the solution form by suspended small

amount of powder in aqueous medium.

Results and discussion

The production rate of arc discharge method under

de-ionized water was determined by measuring the

weight of the product and electrodes after drying to

remove any excess water. It was calculated that

22 wt% of the anode (7.36 g) was consumed during

arc discharge in 4 min. On the other hand, the weight

of the cathode was increased 4.9 g after reaction

because of the adhesion of consumed Cd?2 ions on

the cathode in bulk form. Furthermore, 11.5 wt% of

consumption was dissolved in de-ionized water and

decanted after settlement of the product. As a result,

27.6 wt% of consumption formed by Cd(OH)2 nano-

structures such as nanowires and hexagonal- and

irregular-shaped nanoparticles was obtained. The

highest production rate of the synthesized particles

was found 40.62 g/h when anode has 6-mm diameter

and 45-mm length, and the current of the arc was

50 A. Nevertheless, the production yield of cadmium

hydroxide nanoparticles synthesized by cadmium–

cadmium electrodes became lower with increasing

the arc current because of the decreasing of the arc

stability.

The XRD analysis were performed to obtain the

structural and composition of the Cd(OH)2 nanowires

which were produced by arc discharge method in de-

ionized water. Figure 2 shows XRD patterns of the

product. All the diffraction peaks can be well indexed

to hexagonal Cd(OH)2 with calculated lattice con-

stants of a = 0.3494 nm and c = 0.4708 nm, which

are in good agreement with the literature (JCPDS

Card No. 31-0228). The relatively stronger (001) and

(002) diffraction peaks suggest that the products grow

along [001] and [002] directions. In addition, all the

peaks are related to Cd(OH)2 diffraction peaks which

indicate that the high purity products were synthe-

sized. Sharp peaks of the XRD patterns indicate that

the fabricated Cd(OH)2 nanostructures possesses

good crystallinity. The average crystallite sizes were

calculated from XRD patterns using Scherrer’s

Fig. 2 XRD pattern of Cd(OH)2 nanowires synthesized by arc

discharge in de-ionized water

J Nanopart Res (2011) 13:4673–4680 4675

123

equation (D = 0.9k/Bcosh, where D = crystalline

diameters, k = X-ray wavelength, typically as

1.541 A, B = half width of diffraction peak mea-

sured in radians and h = Bragg angle). The average

crystallite size obtained was about 18 nm.

Figure 3 show typical SEM images of Cd(OH)2

nanowires. The mostly entangled (Fig. 3a) rarely

uniform oriented (Fig. 3b) Cd(OH)2 nanowires were

observed with the diameter around 25 nm and the

ultra long length can easily come up to several

micrometers. The analysis of the samples reveals that

very well-packed Cd(OH)2 nanowires were synthe-

sized in a large scale and high purity.

The TEM images of Cd(OH)2 nanowires fabricated

from the arc discharge in de-ionized water are shown

in Fig. 4a, b which exhibit in morphology of the

nanowires with the diameters in the range 5–40 nm.

High resolution TEM image of the nanowire in

Fig. 4c shows that some of the nanowires were

covered with variable sizes and shaped Cd(OH)2

nanoparticles along the surface of the nanowires.

Similar formations were reported by Fan (2009) for

CdO nanowires, and nanotubes resided along the

exterior surface with non-ordered polyhedron-shaped

nanoparticles. Another type formation of nanowires

was given by Ye et al. (2007) that the Cd(OH)2

nanowires were assembled from small nanoparticles

by rotation of one crystal relative to the other.

HRTEM image (Fig. 4d) reveals that lots of nanopar-

ticles coalesced one another and formed Cd(OH)2

nanowires with a specifically oriented self assembly.

In addition to the nanoparticles-covered nanowires,

the smooth surface nanowires were also synthesized

from the arc discharge method as shown in Fig. 4e.

It is observed that the diameters of the nanowires

vary throughout the nanowires. Smoothness of the

nanowires is important for efficiency of the nanode-

vices since it provides the alignment formation to the

devices. On the other hand, high surface area in the

nanomaterials increases the properties of the

nanostructures.

The HRTEM images of two different particles

settled on the outer surface of a Cd(OH)2 nanowire

are shown in Fig. 5a, b. In both images, lattice spaces

are 0.260 nm apart between adjacent lattice planes.

The obtained result is in agreement with the value of

the (101) lattice planes of hexagonal Cd(OH)2

nanoparticles.

Besides the nanowires, the hexagonal-shaped and

irregular-shaped Cd(OH)2 nanoplates were also

observed among the nanowires as shown in Fig. 6a.

Similar hexagonal nanoplates with different sizes can

be seen in the literature (Miao et al. 2006; Chen and

Gao 2006; Shi et al. 2006). The lengths of the

hexagonal edges of the plates ranges 50–250 nm. The

TEM image (Fig. 6a) reveals that a couple of

adherent nanoplates are perpendicular to Cu grid.

The HRTEM image of a perfect hexagonal nanoplate

with a side length about 120 nm is shown in Fig. 6b.

The angles of all the corners were measured as 120o.

Figure 6c shows the selected area electron diffraction

(SAED) pattern of the hexagonal plate that is the

single crystal nature.

The formation mechanism of Cd(OH)2 nanowires

has to be taken into account in two steps: (1)

nucleation and growth of Cd(OH)2 nanoparticles; (2)

self-assembly by oriented attachment. The nucleation

and growth of Cd(OH)2 nanoparticles can be

explained on the basis of buffer action of cadmium

ions. This process is often referred to as ‘‘olation’’

(Ichinose et al. 2004) and can be represented as

follows:

Fig. 3 SEM image of

Cd(OH)2 nanowires.

a Mostly entangled

Cd(OH)2 nanowires.

b Rarely uniform oriented

Cd(OH)2 nanowires

4676 J Nanopart Res (2011) 13:4673–4680

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Fig. 4 a TEM image of Cd(OH)2 nanowires, b HRTEM

image of an individual Cd(OH)2 nanowire covered by Cd(OH)2

nanoparticles along the surface of the nanowire, c HRTEM

image of an individual Cd(OH)2 nanowire assembled from

small nanoparticles by rotation of one crystal relative to the

other, d HRTEM image of an individual Cd(OH)2 nanowire

with varying diameter throughout the nanowire

Fig. 5 HRTEM (a, b) images of different Cd(OH)2 nanoparticles which settled on the outer surface of Cd(OH)2 nanowire with the

same lattice spaces

J Nanopart Res (2011) 13:4673–4680 4677

123

n Cd H2Oð Þp½ �2þþmOH�

$ Cdn OHð Þm H2Oð Þnp�m

h i 2n�mð Þþ

! Cdn OHð Þ2n

� �sð Þ þ ðnp� mÞHþ

The de-ionized environment in which the synthesis of

nanoparticles realized during arc discharge acts as a

H2O source, and this advantage simplifies the

production by avoiding to use complex chemicals

which contains Cd and H2O.

The assembly of the nanoparticles should be a key

for the formation of the nanowires. The coalesce of

nanoparticles to form nanowires due to the reduction

of the total surface energy through elimination of the

higher surface energy of the lattice faces by the

aggregation is referred as ‘‘oriented attachment’’

(Ichinose et al. 2004). This growth mechanism

involves the irreversible and specifically oriented

self-assembly of primary nanocrystals and results in

the formation of the nanostructures. This formation

mechanism depends on the synthesize temperature

and Ostwald ripening that forms larger particles takes

place at high temperatures (Claudia et al. 2002; Pen

2004; Cui and Zheng 2003; Zhang et al. 2003; Huang

et al. 2003).

The UV–Vis absorbtion spectroscopy is one of the

most widely used methods to examine the optical

properties of nanomaterials. The samples were well

dispersed in de-ionized water by sonication for

inspection of optical properties of the Cd(OH)2

nanostructures at room temperature. Figure 7 shows

the variation of optical absorbance with incident

photon wavelength of Cd(OH)2 nanowires from 200

to 800 nm. The absorption spectrum exhibits a peak

at about 220 nm with considerable absorption ultra-

violet region.

The nature of the optical band gap can be

determined using the fundamental absorption, which

corresponds to the electron excitation from valence

band to conduction band. Direct absorption band gaps

of the Cd(OH)2 nanostructures can be obtained by

conforming the absorption data to the following

equation (Pankove 1971); ahv = B(hv-Eg)n,where ais the absorption coefficient, hv is the photon energy,

Eg is the optical band gap of the material, B is the

material constant, and n is either 2 for direct transition

or � for an indirect transition. Therefore, the optical

band gap Eg of the Cd(OH)2 nanostructures for the

absorption edge can be determined by extrapolating

Fig. 6 a TEM image of Cd(OH)2 nanoplates synthesized with Cd(OH)2 nanowires during arc, b TEM image of an individual

hexagonal-shaped Cd(OH)2 nanoplate, c SAED pattern of hexagonal shaped Cd(OH)2 nanoplate

800600400200

Inte

nsity

(a.

u.)

Wavelength (nm)

1 2 3 4 5

(αhv

)2

h ν (eV)

Fig. 7 Wavelength versus absorbance plot of UV spectra for

Cd(OH)2 nanowires. Inset shows the variation of (ahv)2 versus

hv for determination of band gap energy

4678 J Nanopart Res (2011) 13:4673–4680

123

the straight portion of the curve (ahv)2 versus hv when

a = 0. The inset of Fig. 7 shows the curves of the

(ahv)2 versus hv for Cd(OH)2 nanostructure. From the

figure, direct band gap was calculated to be 4.0 eV

which is bigger than the reported value for Cd(OH)2

thin films (Eg = 2.75 eV) but smaller than Cd(OH)2

nanobelts (Eg = 4.45 eV) (Mane and Han 2005;

Zhang et al. 2008; Qu et al. 2010; Singh et al. 2009).

The band gap energy of the as-synthesized Cd(OH)2

nanostructures is higher than the corresponding bulk

material. The band gap increasing is a direct conse-

quence of the size reduction. In this diameter range for

Cd(OH)2 nanowires, quantum confinement effect

occurs when electron (hole) energy can increase (Liao

et al. 2001).

Conclusion

In summary, Cd(OH)2 nanowires were synthesized in

large scale by arc discharge method in de-ionized

water. XRD analysis that introduced the purity of the

synthesized Cd(OH)2 nanowires is high due to the

detection of only hexagonal Cd(OH)2 peaks. Since

the apparatus and its operation are simple, this

technique can be practical option for the large scale

synthesis of cadmium hydroxide nanowires with high

purity. Cd(OH)2 nanowires are not smooth and their

width is varying along the nanowire. Also Cd(OH)2

nanowires consist of coalesce of hexagonal Cd(OH)2

nanoparticles which have a few nanometers diameters

by self-assembly. Besides the nanowires, the hexag-

onal-shaped and irregular-shaped Cd(OH)2 nano-

plates were also observed among the nanowires.

Optical absorption analysis indicate Cd(OH)2 nano-

wires have significant absorption in the visible

spectral range. Furthermore, the direct band gap of

Cd(OH)2 nanowires is found to be as 4.0 eV which is

comparably larger than corresponding bulk form.

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