hydrothermal growth and characterizations of dandelion-like zno

SEM observations depict that the ZnO product grows in the form of nanorods united together to form 3D dandelion-like nanostructures.. Hence, intensive re-search has been focused on fabri

Hydrothermal growth and characterizations of dandelion-like ZnO

nanostructures

Rohidas B Kalea,⇑, Shih-Yuan Lub,⇑

a

Department of Physics, The Institute of Science, Madam Cama Road, Mumbai 400 032, (M.S.), India

b

Department of Chemical Engineering, National Tsing-Hua University, Hsinchu 30013, Taiwan, ROC

a r t i c l e i n f o

Article history:

Received 16 April 2013

Received in revised form 13 May 2013

Accepted 16 May 2013

Available online 19 June 2013

Keywords:

Chemical synthesis

X-ray diffraction

Scanning electron microscopy

Luminescence

a b s t r a c t

Three dimensional (3D) ZnO nanostructures have been synthesized by using a facile low-cost hydrother-mal method under mild conditions Aqueous alkaline ammonia solution of Zn(CH3COO)2is used to grow 3D ZnO nanostructures The X-ray diffraction (XRD) study reveals the well crystallized hexagonal struc-ture of ZnO SEM observations depict that the ZnO product grows in the form of nanorods united together

to form 3D dandelion-like nanostructures The elemental analysis using EDAX technique confirms the stoichiometry of the ZnO nanorods The product exhibits special optical properties with red-shifts in opti-cal absorption peak (376 nm) as compared with those of conventional ZnO nanorods PL spectra show emission peak (396 nm) at the near band-edge and peak (464 nm) originated from defects states that are produced during the hydrothermal growth TEM and SAED results reveal single crystalline structure

of the synthesized product The reaction and growth mechanisms on the morphological evolution of the ZnO nanostructures are discussed The morphology of ZnO product is investigated by varying the reaction time, temperature, and type of complexing reagent

Ó 2013 Elsevier B.V All rights reserved

1 Introduction

Size, shape, and dimensions strongly affect the physicochemical

properties of nanomaterials Integrated three-dimensional (3D)

platforms of nanostructure semiconductor materials are highly

desirable for advanced nanoscale optoelectronic applications[1]

In the past and present decades, numerous efforts have been taken

to control the size and shape of inorganic nano/micro-crystals

Since these parameters play a vital role in their size tunable

elec-tro-optical properties[2–4] As an example, due to changes in

crys-tallite sizes of nanomaterials, the band edge emission peak of

photoluminescence (PL) and absorption/transmission peak of

UV–visible spectra will be blue or red shifted, known as the

quan-tum size effect[5]

Among the various semiconductor nanomaterials, ZnO has

drawn considerable research attentions, because of its wide energy

band gap (3.37 eV), large exciton binding energy (60 meV) at room

temperature, low lasing threshold, friendliness to the environment,

cheapness, and excellent chemical and thermal stabilities It has a

broad range of high-technology applications, including surface

acoustic wave filters[6], photonic crystals[7], light emitting diodes

[8], photodetectors[9], photodiodes[10], optical modulator

wave-guides[11], varistors[12], gas and piezoelectric sensing[13–15],

and also the potential candidate for the fabrication of several other functional nano-/micro devices[16–19] Recent research has dem-onstrated that the creation of ZnO nanostructures in highly ori-ented and ordered manners is of crucial importance for the development of novel functional devices[20] Hence, intensive re-search has been focused on fabricating ZnO nanostructures and revealing their growth mechanisms along-with structural, mor-phological, optical, and electrical properties

Different chemical, electrochemical, and physical deposition techniques have been used to synthesize ZnO nanorods and nano-wires For example, chemical bath deposition[18], spray pyrolysis

[21], catalytic growth via the vapor–liquid–solid epitaxial (VLSE) mechanisms[22], metal–organic chemical vapor deposition (MOC-VD)[23], pulsed laser deposition[24], templating with anodic alu-mina membranes[25], and epitaxial electrodeposition[26]have been successfully applied in creating highly oriented arrays of anisotropic ZnO NRs

Apart from various other methods, hydrothermal methods have numerous advantages, such as simple, catalyst free growth, low cost, low reaction temperatures, large scale production, well crys-tallized materials with maximum yield, easy to incorporate any type of doping, no need of vacuum or carrier gas, and environmen-tal friendliness Also, the remarkable advantage of this method is to study the influence of numerous organic additives on the size, crystallinity, and morphology of the synthesized products [27] Recently, the synthesis of ZnO nanowires and nanorods by

0925-8388/$ - see front matter Ó 2013 Elsevier B.V All rights reserved.

⇑Corresponding authors Tel.: +886 3 571464; fax: +886 3 5715408.

E-mail addresses: rb_kale@yahoo.co.in (R.B Kale), sylu@nthu.edu.tw (S.-Y Lu).

Contents lists available atSciVerse ScienceDirect

Journal of Alloys and Compounds

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j a l c o m

solvothermal processes has been reported[28–30] However, it is

inevitable to use toxic, dangerous, and expensive solvents such

as amine and methanol in the solvothermal process

In this communication, we report on the controlled synthesis of

dandelion-like ZnO morphologies with nanorods united together

to form 3D nanostructures by using a simple hydrothermal route

Moreover, our product shows enhanced ultraviolet emissions at

room temperature, which may inspire researchers and

technolo-gists to fabricate convenient nano/micro-devices of ZnO materials

with novel unique optical properties

2 Experimental section

2.1 Experimental procedure

All the reagents used in the experiments are in analytical grade (purchased from

Shaowa Chem Co Ltd., Japan) and used without further purification To prepare the

present dandelion-like ZnO nanostructures, 0.05 M of zinc acetate dihydrate

[Zn(CH 3 COO) 2 2H 2 O] is dissolved in 120 mL of deionized water ((Milliq 18.2 MX).

An appropriate amount of ammonium hydroxide NH 4 OH (0.5 M) is dripped under

constant stirring The mixture is enclosed into a Teflon-lined stainless steel

auto-clave The autoclave is kept in an oven maintained at 180 °C for 24 h White

prod-ucts are collected from the bottom of the autoclave and repeatedly washed with

distilled water and absolute ethanol Finally, the product is dried in air at 80–

100 °C for 6 h and used for further characterizations.

2.2 Characterization

The crystallographic structure of the resulting ZnO product is analyzed using an

X-ray diffractometer (XRD, Mac Science, MXP18) with Cu Ka(k = 1.5406 Å)

radia-tion in the 2h range from 20 to 80° The morphology is studied using a scanning

electron microscope (SEM) (JEOL, JSM-5600) equipped with an energy dispersive

X-ray analyzer (EDAX, Oxford Instruments) to detect the elemental proportion A

small amount of collected product is dispersed in ethanol, and small drops of the

sample are deposited on a carbon grid The sample is coated with a thin gold

(Au) layer using an SEM sputter (SPI-Module Sputter Coating Unit, USA) The

ob-tained ZnO product is further characterized with a transmission electron

micro-scope (TEM) and selected area electron diffraction (SAED), using JEOL JEM-2010.

To study the optical properties, optical absorption spectra are recorded in the

wave-length range of 325–800 nm, using a UV–visible spectrometer (Hitachi Model-3300,

Japan) Photoluminescence (PL) study is carried out using a Hitachi F-4500 model

equipped with a xenon lamp as the light source The room-temperature photo

lumi-nescence (PL) spectra were measured using 330 nm as an excitation wavelength An

appropriate amount of ZnO powder is well dispersed in ethanol to study optical

properties.

3 Results and discussion

3.1 Crystal structure

The most stable crystalline structure of ZnO is the hexagonal

(wurtzite) phase that occurs in nature as mineral zincite Its ionic

and polar structure can be described with hexagonal close packing

(HCP) of oxygen and zinc atoms in space group of P63mc with zinc

atoms occupying the tetrahedral sites (point group 3m) The

occu-pancy of four of the eight tetrahedral sites of the HCP array controls

the crystal structure The unit cell contains two formula units and

the typical crystal growth habit exhibits two types of low-index

surfaces: (i) polar surfaces (0 0 0–1) (O-terminated surface) and

(0 0 0 1) (Zn-terminated surface) and (ii) non-polar (0 1–1 0), (1 0–

1 0), etc surfaces; and C6v-symmetric ones parallel to the c-axis

Also, there is no centre of inversion in the wurtzite structure and

therefore an inherent asymmetry is present which allows

aniso-tropic growth along the c-axis[31]

The balance between the kinetic and thermodynamic controls

during the crystal growth determines the final growth habit of a

formed crystal Thermodynamically, crystal growth occurring in

aqueous solutions mainly depends on the characteristics of

me-tal-legend complexes, their solubility’s and ionic products along

with crystal surface energies From the kinetically controlled

reac-tion process, the growth habit of a crystal mainly depends on the

relative growth rate of its various crystal faces Hence, kinetically controlled crystal growth may be different from the thermody-namically predicted crystal growth habit ZnO is an interesting candidate to study the growth habit of crystal under hydrothermal conditions It has been proposed that when the growth rate is slow (thermodynamic control), crystals tend to grow as low aspect ratio polyhedral If kinetically controlled, i.e at higher growth rates, the commonly observed morphologies are elongated crystals along the c-axis and with hexagonal prismatic faces[33–35] Growth mech-anisms and the proposed models were developed and depend on the experimentally observed crystal growth habits The periodic bond chain (PBC) theory [35] is an important model to predict the thermodynamically stable growth habit of a crystal However,

in general, this model fails to explain the usually observed mor-phologies of polar crystals, such as ZnO, for crystal growth under kinetic control and hydrothermal conditions For this reason, other models such as the coordination polyhedron growth (CPG) model have been postulated According to this model, the relative rates

of the ZnO crystal growth in different directions are reported to

be V(0 0 0 1) > V(0 1 1) > V(0 1 1 0) > V(0 0 0 1) that gives the aniso-tropic one-dimensional growth of ZnO crystals [31 and references therein]

3.2 Structural study

Fig 1shows the X-ray diffraction (XRD) pattern of as-synthe-sized ZnO products The diffraction peaks positioned at different 2h values 31.6, 34.26, 36, 47.34, 56.42, 62.74, 66.18, 67.74, 68.88, 72.42, 76.56 and their relative intensities compared with standard intensities can be indexed to pure hexagonal wurtzite phase of zinc oxide (JCPDS No 05-0664) The well resolved sharp XRD peaks with narrow FWHM of the diffraction pattern indicate that the product is well crystallized with excellent crystalline quality and there is no need for further thermal treatments The peaks corresponding to possible impurity phases are not detected

in the XRD pattern, confirming the high purity of the ZnO product The calculated average lattice constants are a = 3.263 Å and

c = 5.230 Å that are in good agreement with the standard values (JCPDS #05-0664)

3.3 Compositional study

In order to determine the elemental stoichiometric ratio, EDAX

is taken at different locations of the samples.Fig 2shows a typical

EDAX pattern of the ZnO product The elemental analysis is carried

out only for Zn and O elements The average atomic percentage of

Zn:O was 49:51, that confirms the stoichiometric ratio (1:1) of the

ZnO product The peaks marked as ‘‘1’’ and ‘‘2’’ are contributed by

the carbon coated grid and Au sputtering, respectively

3.4 Morphological study

The morphology of semiconductor nanomaterials needs to be

optimized for specific applications The morphology has been

shown to affect catalytic and photocatalytic activities and

influ-ence the structural and optoelectronic properties[31–33] Hence,

the morphology of the product is further examined by a scanning

electron microscope.Fig 3a and b shows the SEM images of the

as-synthesized product with different magnifications SEM images

clearly reveal that the product consists of closely packed nanorods

that are united together to form dandelion-like 3D nanostructures

Additional morphological characterizations of the ZnO

nano-structure are carried out using a transmission electron microscope

Fig 4a and b shows TEM images of a 3D ZnO nanostructure and an

individual nanorod TEM pictures reveal that the one end of the

nanorod is wider (100 nm) with gradual decrease in diameter

along its length and finally emerges to form a sharp tip The SAED

pattern is as shown inFig 5a, reveals the single-crystallinity of the

ZnO nanorod.Fig 5b shows a high-resolution TEM (HRTEM) image

of an individual sword-like ZnO nanorod The image clearly reveals

the fringes of the ZnO with a lattice spacing of 0.259 nm,

indicat-ing that the ZnO nanorod is sindicat-ingle crystalline in nature, consistent

with the conclusion of the SAED pattern Furthermore, the

inter-layer distance of 0.259 nm corresponds well with the lattice

spac-ing of the (0 0 2) crystal planes of ZnO, suggestspac-ing growth of the

ZnO nanorods in the (0 0 1) direction

A wide variety of ZnO nanorods[21,34–37]is observed in both

powder and thin film form, but the diameters of the reported

nano-rods are nearly constant throughout their lengths The present

interesting morphology of the ZnO structure consists of long sword

shaped, bundled, closely packed nanorods that may offer unique

optical properties along with gas sensor applications

For nanomaterials, not only length and diameter but also the tip

shape impact their physical properties and performances in

elec-tronic devices In the last decade, researchers are greatly attracted

towards the size effects of 1D nanostructure; however the shape

effects of the 1D nanomaterial remained relatively unexplored

Furthermore, the anisotropic growth of the crystal contributes to

the formation of the nanorods At the first step of the growth, the

initial ZnO crystal seeds nucleate randomly, without any preferred

orientation Since the growth rate along the c-axis of ZnO is the highest, the nuclei with this orientation will grow faster and tend

to dominate Eventually, well-oriented nanorods unite at the initial nucleating sites that form the dandelion-like nanostructure with bundled nanorods

3.5 Optical properties ZnO exhibits a wide band gap at room temperature with a large exciton binding energy, which makes it an intriguing candidate for effective UV emissions However, because of the often dominant presence of structural defects, the UV emission of ZnO nanomate-rials is liable to be quenched and only defect emissions in visible region are detected[38,39] This limits the progress and use of ZnO in optoelectronic and lasing devices Therefore, how to im-prove the crystalline quality of ZnO by synthetic processing and how to realize UV emissions and lasing are still major challenges The 3D ZnO nanostructure obtained in the typical synthesis pro-cess is 3D nanostructure consisting of bundles of nanorods grown with sharp tips This suggests possible interesting optical and electrical properties of this novel 3D structure Also, the optical measurements, such as optical absorption/transmission and pho-toluminescence, are very useful for the determination of the struc-ture, defects, and impurities present in these nanostructures.Fig 6

illustrates the UV–visible absorption and transmission spectra of the 3D ZnO nanostructure It shows a strong and sharp excitonic absorption (maximum)/transmission (minimum) peak centered

at 376 nm, which is slightly red-shifted as compared with that of the bulk ZnO (Egbulk= 3.35 eV, kmax 370 nm) Generally, an exci-tonic absorption/transmission peak appears if the defect density

Fig 3 SEM images of dandelion-like ZnO nanostructures (a) 10 kX and (b) 15 kX.

is considerably low Therefore, from the optical absorption/

trans-mission spectrum, it is evident that the ZnO nanostructures so

formed are of high optical quality To confirm the optical behavior,

the room temperature PL spectrum has been recorded and is shown in Fig 7 The sample exhibits strong UV emissions at

396 nm, followed by much weaker blue-emissions centered at

464 nm The near UV emission of the ZnO originates from the

Fig 4 TEM of (a) dandelion-like ZnO nanostructure and (b) an individual ZnO nanorod.

Fig 5 (a) SAED pattern of ZnO nanorod and (b) HRTEM image of a ZnO nanorod.

Fig 6 UV–visible absorption spectrum of the dandelion-like ZnO nanostructures.

Inset shows the corresponding transmission spectrum.

Fig 7 Room-temperature photoluminescence spectrum of dandelion-like ZnO nanostructures.

recombination of free excitons[40] The weak blue emission peak

centered at 464 nm may be attributed to surface deep traps

emis-sion of ZnO nanorods could originated from transitions involving

Zn interstitial defect states[41] It is worth mentioning, as

com-pared with common ZnO nanorods and nanowires [40,41], the

UV absorption/transmission and PL emission peaks of the present

product exhibit red-shifts from the corresponding bulk values

Pre-vious research has shown that the PL spectrum of ZnO is sensitive

to the product shape, product size, temperature, preparation

meth-od[42], etc Therefore, these particular UV emission behaviors may

be related to the special 3D dandelion-like ZnO nanostructure

assembled by uniform nanorods with narrow tips Such a type of

red-shift was also reported by Ge et al.[43]for 3D ZnO

nanostruc-ture In the present case, a strong and dominant UV emission with

a weak blue emission has been observed which confirms that the

synthesized ZnO nanostructures have good optical properties with

very few zinc interstitials defects

3.6 Reaction and growth mechanisms of 3D ZnO nanostructures

According to previous researchers[37]and our own

experimen-tal evidences, the growth process of ZnO nanostructures generally

takes place via the following reaction mechanisms in the presence

of an excess amount of ammonium hydroxide:

ZnðCH3COOÞ2! Zn2þþ 2ðCH3COOÞ2

NH3þ H2O ! NHþ4þ OH

Zn2þþ 4NH3! ZnðNH3Þ2þ4

ZnðNH3Þ2þ4 þ 4OH! ZnðOHÞ24 þ 4NH3

ZnðOHÞ24 ! ZnO þ H2O þ 2OH

The morphology of ZnO is very sensitive to the preparative

param-eters and ZnO is thus an intriguing candidate to study the growth

habit of hexagonal structure under different experimental

condi-tions To understand the growth mechanism of the 3D ZnO

nano-structures, the ZnO product is also synthesized for a much shorter

reaction time of 6 h.Fig 8shows SEM images of the ZnO product

of this case It clearly reveals the flowerlike morphology that

con-sists of small nanorods surrounded by some thin sheet structure

Fig 9shows a typical EDAX pattern of the ZnO product, with the

inserted box showing the region selected for the analysis, and the

derived elemental composition The analysis shows that the region consists of compounds rich in oxygen, clearly confirming that the surrounded thin sheet structure is of ZnðOHÞ2

4 and converts into ZnO nanorods at prolonged times

In the presence of an excess aqueous ammonia, the quantity of Zn(OH)2 is almost negligible and the abundant quantity of ZnðNH3Þ2þ4 forms in the resultant precursor The species of ZnðNH3Þ24 can be readily decomposed to free Zn2+and form a large quantity of active species of [Zn(OH)4]2 Therefore, there are en-ough growth units of [Zn(OH)4]2to produce ZnO nanorods, that grow from the circumference of the ZnO particles leading to forma-tion of the dandelion-like ZnO nanostructure

Keeping all the experimental conditions invariant but increas-ing the reaction temperature up to 200 °C, the morphology (Fig 10) of the synthesized ZnO product shows an oriented attach-ment growth of microrods that are considerably shortened in length with enlarged diameter The aspect ratio of the constituent rods of this ZnO product is thus much decreased Also the morphol-ogy is observed (Fig 11) for products prepared without disturbing the experimental conditions described in the experimental section but replacing the pH adjusting reagent NH4OH with NaOH It re-veals evident changes in morphology, showing the existence of individual nanorods of diameter 200 nm and of elongated length

in the range of 3lm

Fig 9 EDAX pattern and its relevant compositional analysis.

4 Conclusion

In summary, we have developed a facile hydrothermal method

for the preparation of dandelion-like ZnO nanostructures by using

aqueous ammonia solutions, at relatively low temperatures The

XRD, UV–visible, and PL studies reveal that the as-prepared 3D

ZnO nanostructure possesses hexagonal crystalline structure with

a remarkable optical quality The present synthetic route, free of

templates and surfactants, can be readily adjusted to prepare novel

flowerlike 3D ZnO nanostructure The hydrothermal method might

be extended for the preparation of other oxide semiconductors

The reaction and growth mechanisms clearly reveal that the active

species [Zn(OH)4]2plays an inevitable role to grow the 3D

dande-lion-like morphology The present morphology of ZnO offers new

insight to prepare other oxide nanomaterials of desired

morphol-ogy, which is useful for applications in optoelectronic devices

We also conclude that the reaction time and temperature play an

important role that dramatically changes the morphology of the

synthesized ZnO product

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