A facile room temperature synthesis of Zn O nanoflower thin films grown at a solid–liquid interface.

A facile room temperature synthesis of ZnO nanoflower thin filmsgrown at a solid–liquid interface

Aarti H. Jadhav • Sagar H. Patil •

Shivaram D. Sathaye • Kashinath R. Patil

Received: 22 February 2014 / Accepted: 7 May 2014 / Published online: 24 May 2014

� Springer Science+Business Media New York 2014

Abstract Hierarchical ZnO films consisting of nano-

flower particulates are successfully grown by a solid–liquid

interface reaction technique at room temperature without

additives like surfactants, capping agent, or complexing

agent. The structural, morphological, and photocatalytic

properties of these films are studied using scanning elec-

tron microscopy (SEM), energy dispersive X-ray analysis

(EDX), transmission electron microscopy (TEM), X-ray

photoelectron spectroscopy (XPS), X-ray diffraction

(XRD), and UV–Vis spectroscopy. The nucleation, growth

processes and hence the resulting morphology of the end

product can be regulated by changing the concentration of

LiOH and the time of reaction. SEM throws light on the

chronology of the flower formation by studying the inter-

mediate morphology. Electron microscopy results indi-

cated that these ZnO nanostructures self-assembled to

produce flower-like nanostructures. The highest photocat-

alytic efficiency was observed for the films prepared at the

concentration of LiOH 0.5 mg/mL in ethanol at 24 h. On

the basis of the results, a plausible growth mechanism for

the formation of flower-like ZnO nanostructures is

discussed.

Introduction

Zinc oxide (ZnO) is a promising semiconductor having a

direct band gap 3.37 eV with a large excitation binding

energy of about 60 meV [1] and unique acoustic [2], cat-

alytic [3] and electronic [4] properties. Due to these

properties, ZnO is used in various applications such as

room temperature UV lasers [5], ultraviolet light-emitting

diodes [6], field-effect transistors [7], solar cells [8], and

optoelectronics [9]. ZnO is biocompatible [10] and there-

fore it is considered for cell labeling applications [11]. It is

an important material in ‘‘green chemistry,’’ taking into

account its usefulness for photocatalytic degradation of

hazardous organic dyes [12]. ZnO is considered a fore-

runner as an effective photocatalyst along with TiO2 due to

their comparable properties, namely, biocompatibility,

electro affinity, higher electron mobility, and similar

energy level structure in a band diagram. ZnO is sometimes

claimed as more efficient than TiO2 [13] in photodegra-

dation of some organic compounds [14, 15]. The photo-

catalytic activity of ZnO is strongly dependent on the

morphology of crystallites, exposing suitable plane/s to

substrates in the reaction. Concomitantly, higher surface

area of the catalyst facilitates the higher efficiency of the

catalyst. Incidentally, both the properties are dependent on

the process of preparation of catalyst.

The development of controlled synthesis with desired

ZnO morphology is indispensable for exploring the true

potential of ZnO as a photocatalyst. So far, the synthesis of

nanostructured ZnO with different morphologies such as

nanoparticles [16], nanowires [17], nanobelt [18], nano-

tubes [19], flowers-like microstructure [20], nanotetrapods

[21] have been reported for the application of photocata-

lytic activity and also the variety of method dependant

morphologies [22–24] as reported by Salavati-Niasari et al.

Electronic supplementary material The online version of thisarticle (doi:10.1007/s10853-014-8313-1) contains supplementarymaterial, which is available to authorized users.

A. H. Jadhav � S. H. Patil � K. R. Patil (&)

Center for Materials Characterization, National Chemical

Laboratory, Pune 411008, India

e-mail: [email protected]

S. D. Sathaye

759/83 Deccan Gymkhana, Pune 411004, India

123

J Mater Sci (2014) 49:5945–5954

DOI 10.1007/s10853-014-8313-1

Consequently, various synthesis approaches have been

studied to tune the size, shape and hence the properties of

ZnO. It is reported that the precursor of Zn2? affect the

size, shape, and morphology of resulting ZnO particles.

These observations are well exhibited by the work of

Salavati-Niasari et al. who have used various Zn2? pre-

cursors for ZnO nanoparticles formation, mainly through

thermal decomposition [25, 26]. When bulk ZnO is being

used as a photocatalyst in heterogeneous reaction, the

recovery of the photocatalyst is likely to be a lengthy

process made up of the steps like filtration and then acti-

vation. To avoid these problems, many researchers pre-

ferred thin films of ZnO, well adhered to the substrates, as a

catalyst which has additional advantage of requirement of

smaller amount of the catalyst. Therefore, various methods

have been reported such as hydrothermal synthesis [27],

thermal evaporation [28], molecular beam epitaxy [29],

vapor deposition [30], and metal–organic chemical vapor

deposition [31] for the preparation of ZnO thin films hav-

ing suitable structure and morphology for catalytic appli-

cation. Many of these methods consist of complex

procedures, use of sophisticated and expensive instrumen-

tal systems, high temperature, low or high pressure toxic

source materials, and higher energy inputs, etc. On the

other hand, the solution chemical approaches can allow the

growth of ZnO crystals in thin film form at much lower

temperatures (\200 �C) without using any sophisticated

instrumentation. There are also different methods like

solvothermal [32], sol–gel [33], chemical bath deposition

(CBD) [34], and template-based self-assembly processes

[35]. Chemical methods normally use capping agent

capable of stabilizing a particular crystal facets and also to

tune the size and shape of ZnO nanostructures. However,

these capping agents may become detrimental to the

catalytic activity of the ZnO requiring the removal of

templates/capping agents, through tedious operational

procedures.

Besides these, there are some specific examples of ZnO

nanostructure synthesis; for example, Zhang et al. have

reported a site-specific sequential nucleation and growth

route to synthesize oriented ZnO nanostructures [36]. Du

et al. have reported the synthesis of ZnO nanoflowers in

water in the presence of cetyltrimethylammonium bromide

(CTAB) as structure guiding agent [37]. Liu et al. syn-

thesized ZnO nanoflowers by alkaline hydrolysis of zinc

nitrate using ammonium hydroxide [38]. The morphology

closer to that of a flower has also been prepared by Masuda

et al. [39] using site-selective deposition technique. Du

et al. also synthesized flower-like ZnO on the surface of

multiwalled carbon nanotubes [40]. Recently, ethanol-

based precipitation method [11] was found to be attractive

for growing ZnO nanoparticles by precipitation of Zn2?

with LiOH in ethanol wherein the size can be readily tuned

via adjustment of the pH of solution. ZnO nanoparticles

were subsequently stabilized by encapsulating those by

silica to form ZnO@silica core–shell nanostructures.

Obviously, the developed material is not expected to be

considered suitable for catalytic applications as photoactive

material namely, ZnO, as it may not have sufficient

exposure to substrate. All these references appropriately

reveal the desire to explore a simple and facile process for

nanoparticulate ZnO formation for catalytic application.

However, it was thought that the basic chemistry of this

method [11] may be useful by the clever application to

grow thin films of ZnO without using any capping agent.

Among various applications of ZnO nanostructures, the

photocatalytic degradation of hazardous organic dyes and

chemicals are very important considering the present day

problem of environment protection. Rhodamine B is a

commonly discharged material from papers, plastics, tex-

tiles, and rubber industries and cause severe water pollution

and hence effectively disturbs the ecosystems [41].

Therefore, it is important to develop an effective method to

degrade the Rhodamine B in water. Photocatalytic degra-

dation can become green technology for industrial down-

flow water treatment. ZnO nanomaterial is proved as one of

the effective photocatalysts for the said purpose in earlier

studies [42, 43].

In continuation of our efforts to enhance the applications

of novel solid–liquid interface reaction technique (SLIRT)

[44, 45] to synthesize nanostructures, herein, we report for

the first time an environmentally benign approach to

achieve directly the thin films of ZnO at room temperature

having flower-like morphology. Further, we show that

these films are efficient for the photocatalytic degradation

of Rhodamine B, a common dye pollutant in water. The

flower-like ZnO nanostructures may enhance the photo-

catalytic performance due to its high surface-to-volume

ratio and stability against aggregation [46]. In the present

communication, we report the results of the studies of

structural, morphological, and optical properties of ZnO

films formed at solid–liquid interface as a function of

concentrations of precursor LiOH, time of reaction, etc. A

plausible mechanism of formation is also discussed.

Experimental section

Materials

Zinc acetate dihydrate (Zn (CH3COO)2�2H2O) and lithium

hydroxide (LiOH) were purchased from Sigma-Aldrich.

Rhodium B and absolute ethanol was purchased from

Merck. For the synthesis, all the chemicals were used as

received, without further purification. The water used

throughout all experiments was double distilled deionized

5946 J Mater Sci (2014) 49:5945–5954

123

(DI) water. The substrates used are glass slides, quartz

plate, and silicon wafers. Glass and quartz substrates were

cleaned by pretreating with freshly prepared piranha solu-

tion (H2SO4/H2O2 = 70:30, v/v) at 70 �C for 15 min,

thoroughly rinsed with deionized water, and dried by in

nitrogen flow.

Preparation of ZnO thin film

ZnO thin films were synthesized by facile SLIRT. Initially,

thin film of zinc acetate dihydrate on a glass substrate was

formed by modified spin-coating method using equipment

and procedures reported in the earlier references [44, 45].

Typically, a film of 0.05 M zinc acetate dihydrate was

deposited on the substrate (2 9 2 cm) by the modified

spin-coating method using ethanol as a solvent. The pro-

cedure was repeated five times to obtain sufficient thick-

ness of the film. The films were dried at room temperature

in vacuum desiccators. The solutions of LiOH with various

concentrations (1.8, 0.7, 0.5, and 0.25 mg/mL) in absolute

ethanol were prepared. Zinc acetate dihydrate-coated sub-

strates (glass/silicon wafer) were immersed in an alcoholic

solution of LiOH for varying lengths of time (15 min–

24 h). Zinc acetate dihydrate film reacted with alcoholic

solution of LiOH and gets converted into ZnO. The sub-

strates in each case were removed after the reaction from

LiOH solution and rinsed with ethanol three times to

remove untreated zinc acetate and excess LiOH. The

conversion process was studied as a function of time. It was

noted that the changes occurring in the film reach com-

pletion within 24 h.

The resulting ZnO samples are referred, hereafter as A,

B, C, and D, respectively. In other words, A refers to a film

formed by five times coated zinc acetate dihydrate on glass

substrate and subsequently immersed in 1.8 mg/mL LiOH

in ethanol for 1 day and so on. The film thus formed on the

substrate are dried and used for further physicochemical

characterization.

Characterization

The as-synthesized products were investigated for their

morphological, structural, and optical properties. The

morphologies were examined by environmental scanning

electron microscopy (ESEM, FEI Quanta 200 3D dual

Beam E-SEM) attached with energy dispersive spectros-

copy (EDS) and transmission electron microscopy (TEM,

FEI Tecnai F-20) at an acceleration voltage of 200 kV.

High-resolution transmission electron microscopy

(HRTEM, FEI Tecnai F-30) was used at an acceleration

voltage of 300 kV. For TEM and HRTEM analysis, sam-

ples were prepared on carbon-coated copper grids by

scraping ZnO film from glass substrate and dispersed in

ethanol. A drop of dispersed solution was put on grid and

then dried at room temperature. The electron diffraction

facility was employed for assessment of the structure and

the phases present, during the morphological character-

ization of the film. The absorption in the UV–Vis region

was studied using the films deposited on the quartz sub-

strate, using JASCO dual beam spectrophotometer (JASCO

V-570) operated at a resolution of 1 nm. FTIR spectra of

as-prepared films were recorded on a Perkin Elmer 1090

spectrometer. The films were also characterized by means

of X-ray diffraction (XRD) using PAN analytical D8

Model with copper radiation (Ka of k = 1.54 A). The

surface characterization of the film was also done by XPS

analysis using (ESCA-3000, VG Scientific Ltd, UK) with a

base pressure of better than 1.0 9 10-9 Pa. Mg Ka radia-

tion (1253.6 eV) was used as an X-ray source and operated

at 150 W. The products were also utilized as a photocat-

alyst for the decomposition of Rhodamine B.

Photodegradation of Rhodamine B

The photocatalytic activity of as-synthesized ZnO film was

evaluated by monitoring the photocatalytic decomposition

of Rhodamine B. 0.2 ppm of Rhodamine B aqueous solu-

tion was used to study the catalytic activity of film. Before

the sample was exposed to UV–Vis radiation by a 250 W

mercury lamp to study photocatalytic degradation, sample

was immersed in 10 mL of 0.2 ppm Rhodamine B dye

solution in petri-dish and then kept in dark for 30 min to

obtain adsorption equilibrium. The concentration of Rho-

damine B in solution after reaction at various time intervals

was calculated by measuring the UV–Vis absorption at

wavelength 553 nm using UV–Vis spectrometer.

To estimate the photostability of catalyst, three cycles of

photodegradation experiments were carried out. The

decomposition of the Rhodamine B was observed by

measuring the absorbance at regular time intervals after

each cycle.

Results and discussion

The product from the reported synthesis procedure is

examined by using XRD. Figure 1 exhibits the typical

XRD pattern of (a) zinc acetate drop casted from ethanolic

solution, (b) modified spin coated from ethanolic solution,

and (c) subsequently dipped in LiOH (0.5 mg/mL of eth-

anol) on glass substrates. The drop casted sample shows a

sharp peak at 2h = 6.1� which is assigned to (100) plane of

monoclinic zinc monoacetate (JCPDS 14-0902). The

observed peaks at higher 2h can be assigned to higher order

reflections of the plane (100). The spin-coated sample

shows diffraction peaks at 2h = 11.3 and 19.7�,

J Mater Sci (2014) 49:5945–5954 5947

123

respectively, which can be assigned to tetragonal beta zinc

acetate (JCPDS 21-1467). Thus, XRD of a drop casted film

from a solution of zinc acetate dihydrates in absolute

alcohol can be attributed to the formation zinc monoacetate

film [47].

The inference of this observation is that absolute alcohol

reacts with loosely bound water of crystallization, con-

verting zinc dihydrate into zinc monoacetate through the

reaction

ðCH3COOÞ2Zn2H2O! ½ðCH3COOÞZn�þ þ ½CH3COO��þ 2Hþ þ 2OH�

When the film of zinc acetate is dipped in LiOH

solution in absolute alcohol, the film initially forms

Zn(OH)2 on the surface. However, Zn(OH)2 spontane-

ously decomposes to ZnO and H2O. H2O is absorbed by

hygroscopic absolute alcohol. In fact, hygroscopic abso-

lute alcohol is proposed to be a driving force for the

reaction. This is how, at no juncture of film formation,

the presence of OH is prominently observed in the film

during IR or XPS study.

It would be proper to differentiate between the reported

mechanisms of ZnO formation from zinc acetate dihydrate

precursor and what is being proposed presently. The

reaction media in the reported literature are different and

perhaps that is the reason that end product ZnO is formed

via Zn(OH)2. As a consequence, higher temperatures are

reported for ZnO formation [48, 49]. In the present case,

the end product is formed at room temperature. Obviously,

the mechanism is different than the one reported in the

literature.

The samples processed in LiOH alcoholic solution show

an XRD peak at 31.7� and which can be indexed as (100)

plane of hexagonal wurtzite ZnO (JCPDS No. 36-1451).

Morphological evolution

Further, the evolution of the morphology of ZnO film was

studied by characterizing it by SEM as a function of LiOH

concentration. The results are shown in Fig. 2. The inset of

Fig. 2 shows enlarged SEM image of single flower-like

ZnO nanostructures. The SEM observations indicate that

the products are composed of flower-like nanostructures for

samples A, B, C, and D. The size of the flower is in the

range 1–5 lm. The concentration of LiOH has been found

to be critical parameter for tuning the morphology of ZnO

nanostructures in the film grown in each case. It can be

seen that the flowers in sample C have identical shape and

size, much larger when compared to A, B, and D samples.

The effect of time of reaction on the morphology of the

product is also studied by analyzing SEM images of rep-

resentative sample C which are shown in Fig. 3. The SEM

observation reveals that the products obtained after 15 min

contains ZnO rods with a diameter of about 60 nm

(Fig. 3a). The inset is HRTEM taken on the petals shows a

lattice spacing of about 0.26 nm which corresponding to

the interspacing of (002) planes of ZnO crystal lattice.

When the reaction time is increased to 1 h, ZnO rods grow

vertically as well as laterally. It is clearly seen that rods are

not exactly vertical. Therefore, when those grow laterally,

the portions near their bases come closer to each other

while top tips project outside. Ultimately, rods’ bases get

fused forming a single entity with a bulging mass at the

base and projecting ‘‘antennas’’ above (Fig. 3b). With

increasing time of reaction, the lateral growth at the base is

restricted while the ‘‘antennas’’ grow non-uniformly to get

the shape of the petal of a flower. This is how the total

structure looks like flowers with projecting petals. With

prolonging the growth time to 6 h, the lengths of ZnO

petals (height of 2D petals) are increased to 550 nm, as

shown by Fig. 3c. When the growth time is increased to

15 h, the overall dimensions of ZnO flowers are increased

to 500 nm to 1 lm (Fig. 3d). When the growth time is

prolonged to 24 h, the width of the flowers is about 1–5 lm

(Fig. 3e), respectively. Thus, dimensions of the ZnO

flowers can also be controlled by tuning the reaction time.

The composition of as-synthesized ZnO flowers is

checked by EDS. It is clear from the EDS analysis that the

synthesized product are made up of Zn and O only. Further

quantitative analysis shows the mean atomic ratio Zn/O is

about 1.0 and therefore, ZnO structure seems to be nearly

stoichiometric.

Detailed structural information about this flower-like

ZnO was obtained from TEM and HRTEM studies. Figure 4

shows a selected TEM image and the corresponding

SAED pattern of flower-like ZnO nanostructures for sample

C. From Fig. 4a, it is clear that the ZnO flowers are made

by the collection of various triangular-shaped petals.

Fig. 1 XRD pattern of a ethanolic zinc acetate drop casted on glass,

b zinc acetate solution spin coated on glass, and c ZnO. Inset Zinc

acetate dihydrate powder

5948 J Mater Sci (2014) 49:5945–5954

123

Fig. 2 SEM images of ZnO nanostructures after 24 h a 1.8 mg/mL, b 0.7 mg/mL, c 0.5 mg/mL, and d 0.2 mg/mL. Inset is a magnified image

Fig. 3 SEM image of the ZnO for sample C at different time a 15 min. Inset is HRTEM image b 1 h, c 6 h, d 15 h, and e 24 h

J Mater Sci (2014) 49:5945–5954 5949

123

The petals have sharp tips and wider base, connected with

each other in a peculiar fashion that they make flower-shape

morphologies. The size of the flower is around 1–5 lm

which matches well with SEM images. Close examination of

the nanoflowers reveals that the flowers are formed from

distinct petals. The corresponding SAED pattern (Fig. 4c)

indicates the crystalline nature of these flower-like struc-

tures. In addition, the flowers have a lattice spacing of about

0.28 nm (Fig. 4b), which corresponds to the distance

between the (100) planes in the ZnO crystal lattice (JCPDS

36-1451).

The purity and chemical composition of the ZnO film

examined under XPS is depicted in Fig. S1 (supporting

information). The full survey spectrum shows mainly Zn,

O, and C species and no impurities could be determined on

the surface of the ZnO thin film. Moreover, there is no peak

at about 55.5 eV corresponds to Li species, confirms that

film is free from Li content. Two peaks at binding energies

1021.7 and 1044.8 eV with a separation by 23.1 eV are

attributed to characteristic peaks of Zn2p3/2 and Zn2p1/2,

respectively. The peak of Zn2p3/2 appears at 1021.7 eV,

confirming that the Zn2? linked to an oxygen atom. The

scan of the O1s spectrum is shown in Fig. S1, exhibiting a

peak at 531.7 eV which is attributed to oxidized Zn atom in

the film.

The quality and chemical composition of the as-synthe-

sized flower-shaped ZnO nanostructures are examined by

FTIR spectroscopy. Figure 5 shows the FTIR spectrum of

the precursor and assembly of ZnO nanoflowers. Figure 5a

shows the FTIR spectrum of the precursor. The strong peaks

at 1550 and 1420 cm-1 represent the asymmetric stretching

vibration of C=O mode of zinc acetate. However, it was

proposed by Gonzalez [47] during the reaction of zinc

acetate dihydrate in alcohol leads zinc monoacetate for-

mation which is consistent with the observation of XRD in

chemical reaction considered in present work. The peak at

2925, 2853, and 1045 cm-1 belong to –CH3 and –CH2–

vibrations. IR spectra of the end product ZnO nanoflowers

show that absorption peak of –CO and organic groups are

negligibly small. The peaks have totally vanished in the

range of 1500–800 cm-1, indicating complete decomposi-

tion of the precursor and formation of ZnO. The sharp and

strong peaks appear at 417 and 495 cm-1 for Zn–O vibra-

tion confirm formation of ZnO [50].

Fig. 4 Typical a low

magnification, b high-

resolution, and c selected area

electron diffraction pattern of

as-synthesized ZnO

nanostructure

500 1000 1500 2000 2500 3000 3500 4000

C-HC=O O-HZn-O

Inte

nsity

(a.

u.)

(b)

(a)

Wavenumber (cm-1)

Fig. 5 FTIR spectra of a the precursor and b ZnO for sample C

5950 J Mater Sci (2014) 49:5945–5954

123

Figure S2 shows UV–Visible absorption spectra of

the as grown ZnO thin film for A, B, C, and D

samples (supporting information).The absorption peak

at 378 nm is attributed to ZnO absorption [51].

To demonstrate the potential application of the end

product ZnO film, photocatalytic decomposition of

Rhodamine B, is studied as a function of time of

reaction under UV exposure. Absorption at wave length

553 nm is used as a measure of concentration of

Rhodamine B. Figure S3 (supporting information)

shows a typical time-dependent UV–Vis spectrum of

Rhodamine B dye in the presence of ZnO film. It clearly

shows that with increasing the UV-irradiation time, the

absorption intensity of Rhodamine B decreases con-

firming the degradation of Rhodamine B. The catalytic

effect of ZnO film is exhibited in Fig. 6a. It shows that

in the presence of catalyst, *80 % Rhodamine B is

degraded within 50 min under UV-light illumination.

The plot of remaining dye concentration (A/A0 where

A is the dye concentration at time t and A0 is the initial

concentration) versus time is shown in Fig. 6b for all

samples.

The significantly high dye degradation over the sur-

face of as-synthesized flower-shaped ZnO nanostructures

is explained by relatively larger surface area available

for reaction due to flower morphology of the catalyst

film along with efficient charge separation of electron–

hole pair under the UV-light illumination [46, 51]. In

general, the generation of electron–hole pairs by ZnO

under illuminations reacts with water which acts as a

strong oxidizing agent to break the large organic mole-

cule into less harmful small organic molecules [52].

Figure 6c shows the % dye degradation for three cycles

for all samples which indicate that the film catalyst has

good stability.

The product of present method confirms that the

efficiency of flower-like morphology of ZnO is useful as

a catalyst for the degradation reaction of polluting ionic

dye.

Mechanism of ZnO film formation

The mechanism of film formation can be summarized in

Scheme 1.

The specific points to be noted are: (1) although, we

start with Zn precursor as zinc acetate dihydrate in

absolute alcohol, spin-coated film show the formation of

beta zinc acetate as explained in XRD results. Further,

we show that when this film is dipped in alcoholic LiOH

solution, the top layer gets converted to ZnO. The con-

sequence of the top protective layer formation is that the

reaction of converting zinc acetate to ZnO becomes

Fig. 6 Typical plot of a %

degradation versus irradiation

time, b relative absorption (A/

A0) versus irradiation time, and

c reproducible of catalytic

activity of samples during three

consecutive reactions for

sample A, B, C, and D

J Mater Sci (2014) 49:5945–5954 5951

123

diffusion controlled and hence slow which makes the end

product formation time dependent, facilitating observed

nanostructures formed in the present process. OH- ions

need to diffuse in through this protective layer at the

interface while bulky acetate ions need to diffuse out to

further the reaction to the product ZnO.

Thus, the reaction at the solid–liquid interface forms a

thin layer of ZnO which can act as a nucleation center or a

‘‘seed’’ layer for the growth of the film. As the diffusion of

OH- ions take place through the top layer of ZnO, reaction

leads to the end product. However, there are some subtle

points which have serious consequences on the final

morphology of the films. It is known that growth rate of

ZnO along (002) plane is higher than along (100) plane

observed in the present case. It is proposed that initially,

the growth along (002) plane does occur as observed in

SEM after 15 min of reaction. In fact, to the best of our

knowledge, this is the first time observation where film is

being seeded from the top. Also, it is known that the (002)

planes are made up of all Zn or all O atoms and has a polar

nature; while vertical prismatic faces of (100) planes,

making wurtzite structure is nonpolar and has two coor-

dinated zinc. Zinc on prismatic face is prone to bound to

ligand, such as acetate in the present case [53]. Thus,

initially, the growth along (100) plane would be controlled.

As the reaction proceed with time, OH- ions diffuse in, to

react with zinc acetate, an intermediate compound like

Zn(OH)2 acetate is likely to be formed. Such metal

hydroxide salts having a tendency to form layered struc-

tures are well reported in the literature [54]. Therefore,

during the growth stage, the layers would originate from

(100) planes, get clustered with nearby similar plane and

continue to form a petal after due course of reaction.

Subsequently, zinc hydroxide decomposes to ZnO and

concomitantly acetate ion diffuse out in alcoholic LiOH

solution. Also, it is known that fast growing faces disap-

pear and slow growing faces survive in the ultimate pic-

ture. Thus, the observation of only (100) plane in XRD in

fully grown flower can be rationalized. It may be noted

that while the increment along the original rod length is 10

times, the width grows few hundred fold and therefore

XRD pattern dominantly shows reflection of (100) plane.

Also, the presence of fringes corresponding to both (100)

and (002) plane in HRTEM is well understood. As dis-

cussed above, since the growth process is diffusion con-

trolled, initially only petals are observed to be formed

which ultimately grow into flower morphology.

Conclusion

In conclusion, we have successfully synthesized at room

temperature a well-crystalline ZnO film with flower-shaped

morphology by a facile SLIRT without any additives. The

as-synthesized ZnO flowers are characterized in detail in

terms of their morphological and structural properties by

various analytical techniques such as ESEM, TEM, XRD,

and FTIR. We have analyzed the flower-like morphological

evolution ZnO particles. The as-synthesized flower-shaped

ZnO nanostructures are utilized as efficient photocatalyst

for the photocatalytic degradation of Rhodamine B which

exhibits sufficiently high degradation *80 % within

50 min. A suitable mechanism is proposed to explain the

formation of ZnO films by SLIRT having a flower-like

nanostructure morphology.

Acknowledgements We thanks to A. B. Gaikwad, R. S. Gholap,

Ketan, and Shravani for helping in SEM and TEM characterization.

Virendra, Ashwini, Babasaheb, and Prashant Gaikwad for their help

and moral support. One of the author (Sagar Patil) wish to express his

gratitude to CSIR and UGC, New Delhi for their support in this work.

Scheme 1 Schematic

presentation of formation

process of the nanostructure

assembly

5952 J Mater Sci (2014) 49:5945–5954

123

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