ORIGINAL PAPER

Microwave assisted hydrothermal synthesis of zinchydroxystannate films on glass substrates

Mohammad Abbas Mahmood • Joydeep Dutta

Received: 27 January 2012 / Accepted: 13 March 2012 / Published online: 27 March 2012

� Springer Science+Business Media, LLC 2012

Abstract Zinc stannate (ZnSnO3, Zn2SnO4) and its

precursor, i.e. zinc hydroxystannate (ZnSn(OH)6), have

emerged as technological nanomaterials for different

applications. Herein, we report synthesis of polycrystalline

zinc hydroxystannate (ZHS) film on glass substrate through

facile and efficient microwave assisted hydrothermal

growth. The method comprises of three steps; deposition of

ZnO seed films on glass substrates through spray pyrolysis,

growth of ZnO nanorod arrays over the seeded substrates

through microwave assisted hydrothermal method and

transformation of the as-synthesized ZnO nanorod arrays

into the ZHS films through microwave treatment in aque-

ous precursor solution of SnCl4 and NaOH. The films were

characterized by energy dispersive X-ray spectroscopy,

X-ray diffraction and scanning electron microscopy

(SEM). The films contain two crystalline phases namely

ZnO with [002] as preferred growth direction and

ZnSn(OH)6 preferably grown along [200] vector. The

obtained ZHS films consist of crystals of exclusively cubic

structure with sizes up to several microns. Microwave

irradiation time, NaOH/SnCl4 molar ratio, concentration of

Sn4? ions, and the applied power are the four parameters

which influence the size, aerial density and growth rate of

ZHS microblocks.

Keywords Microwave � Hydrothermal � Nanorod �Array � Zinc hydroxystannate � Film

1 Introduction

Synthesis of nanostructured zinc stannate (ZnSnO3,

Zn2SnO4), a ternary oxide semiconductor, has been the

focus of an increasing interest for the last couple of years.

Its good optoelectronic properties [1–3], render the mate-

rial a suitable candidate for a variety of applications, like,

Photocatalysis [4–6], gas sensing [7–9], humidity sensing

[10, 11] and photovoltaic devices [12–14]. Zinc hydroxy-

stannate (ZnSn(OH)6) along with other organometallic

stabilizers, improve thermal stability of chlorine containing

polymers, particularly poly vinyl chloride (PVC) [15]. Zinc

hydroxystannate (ZHS) acts as precursor for zinc stannate

(ZTO) growth which through dehydration, turns into the

stannate [6, 16, 17]. ZHS has been found as an excellent

inorganic synergist in certain polymeric materials which

contain halogenated fire retardants and smoke suppressant

[18]. ZHS together with anhydrous Al2O3 are highly

effective in suppressing flame and smoke evolution and is

known to reduce emission of carbon monoxide during

burning. The combined action of ZHS and Al2O3 was seen

to result in the formation of thermally stable char, a non-

combustible specie, at the expense of combustible products

[19]. On the basis of limiting oxygen index (LOI) mea-

surements, it was found that the addition of a specific

amount of ZTO or ZHS significantly improved the fire

retarding characteristic of solid state epoxy resins without

affecting their optical properties [20].

M. A. Mahmood � J. Dutta (&)

Center of Excellence in Nanotechnology, Asian Institute of

Technology, Klong Luang, Pathumthani 12120, Thailand

e-mail: joy@ait.asia

M. A. Mahmood

Department of Basic Sciences and Islamiat, University of

Engineering and Technology, Peshawar, Pakistan

J. Dutta

Chair in Nanotechnology, Water Research Center,

Sultan Qaboos University, PO Box 17,

Al-Khoud 123, Sultante of Oman

e-mail: dutta@squ.edu.om

123

J Sol-Gel Sci Technol (2012) 62:495–504

DOI 10.1007/s10971-012-2754-2

Zinc hydroxystannate mostly reported to have been

synthesized through hydrothermal routes, which because of

its convenience, environmental friendliness, and cost

effectiveness is most suited for industrial applications. The

technique through variation of the precursor contents and/

or other synthesis parameters gives better control over the

crystallinity, size and morphology of the nanostructured

products [21]. Wrobel et al. [22] synthesized films of single

crystal ZHS microcubes on tin metal substrates which was

assumed to play a key role in aerial density and growth rate

of film by releasing Sn4? ions to the precursor and also by

providing proper nucleation sites for crystal growth. Using

similar technique, Qin et al. [14] reported the formation

of perovskite cubic structured nanocubes of ZHS on

a-{Cu,Sn} copper foils. Through the dissolution-precipitation

hydrothermal route, Zhang et al. [23] successfully synthe-

sized cubic crystals ZHS on glass substrates. For this

purpose, the substrates were pre-coated with a layer of ZnO

which acted as source of Zn2? ions during etching at higher

pH. The aqueous precursor solution contained SnCl4 and

NaOH as a source of Sn4? and OH- ions, respectively,

while synthesis was carried out at 120 �C in an autoclave.

Aerial density and size of the product crystals were con-

trolled by adjusting the concentration of Sn4? ions and

NaOH/SnCl4 molar ratios in the precursor [23].

Microwave irradiation, wherein energy is imparted

through the interaction of high frequency electromagnetic

waves to water molecules, has been found to heat up an

aqueous system uniformly at several times higher rates

than the conventional heating processes. Microwave

assisted hydrothermal synthesis technique has been suc-

cessfully applied for the growth of ZnO nanorod array on

solid substrates [24, 25]. Not only for the synthesis of

binary oxide, rather microwave heating has also been

successfully applied for the synthesis of several ternary

oxides, like, niobates (LiNbO3, NaNbO3, KNbO3) and

titanates (BaTiO3, PbTiO3) [26], and tungstates (PbWO3,

BaWO3) [27, 28]. To our knowledge, application of

microwave irradiation for the synthesis of zinc stannate

(ZnSnO3, Zn2SnO4) or ZHS (ZnSn(OH)6) has not been

reported so far.

Herein we report a facile low temperature hydrothermal

route for synthesis of ZHS films on glass substrates using

microwave heating technique. The complete fabrication

process comprised of three main steps including deposition

of polycrystalline ZnO seeding films on glass substrates,

microwave assisted hydrothermal growth of ZnO nanorod

arrays on the as-seeded substrates followed by transfor-

mation of the as-synthesized nanorod array into ZHS film

under microwave irradiation in an aqueous precursor

solution of SnCl4 and NaOH. The films were characterized

by energy dispersive X-ray (EDX) spectroscopy, X-ray

diffraction pattern, and scanning electron microscopy

(SEM). The ZHS film consisted of cubic crystals of size up

to a few microns. The plausible growth mechanism of the

microblocks is discussed here and results of experiments on

effect of different synthesis parameters like irradiation

time, NaOH/SnCl4 molar ratio, Sn4? concentration and

irradiation power are presented.

2 Materials and synthesis

Substrates used in these experiments were microscope

glass slides which were cleaned in an ultrasonic bath for

10 min successively in soap water, acetone and deionized

(DI-water) and then dried in an electric oven at 100 �C.

Deposition of ZnO polycrystalline seeding films was car-

ried out through a spray pyrolysis method. Precursor

solution was prepared by dissolving a measured amount of

zinc acetate dihydrate (Zn(COOCH3)2�2H2O, Merck) in

DI-water through sonication for 2 min. The substrates were

arranged on an alumina plate (4.500 9 4.500) placed on a hot

electric plate (400 �C). Using commercial spray gun, 20 ml

of the 20 mM precursor solution was sprayed over the hot

substrates at 1 bar air pressure. Solution flow rate and

distance of the gun from the substrates were adjusted to

*10 ll/s and 25–30 cm, respectively. After finishing the

spray process, substrates were left to cool down at room

temperature.

Growth of ZnO nanorod arrays on the as-prepared

substrates was carried out through microwave assisted

hydrothermal route using commercial microwave oven

(SHARP-R276, 2.45 GHz). The substrates were mounted

on supports, at both the ends, in a petri dish with the seeded

part facing downward. 500 ml solutions 60 mM of zinc

nitrate hexahydrate (Zn(NO3)2�6H2O, QReCTM) and hexa-

methylenetetramine (C6H12N4, Aldrich) were prepared

separately in DI-water by sonication for 10 min. 100 ml

from each of the two stock solutions were poured into the

petri dish to obtain equimolar concentration (30 mM) of

Zn2? and OH- ions in the precursor solution. The petri

dish was transferred into the microwave oven which was

run at low power mode (80 W) for a time interval of

50 min. After the microwave irradiation, the system was

left to cool down for 10 min and precursor solution was

replenished prior to further growth process being under-

taken [29]. The same procedure was repeated five times. At

the end of the growth process, the films were rinsed with

DI-water so that the loosely attached particles could be

removed. All the films were dried in an electric oven in air

at 100 �C for 4–5 h prior to further studies.

Transformation of the ZnO nanorod films into the ZHS

films were also carried out in the microwave oven. Typi-

cally, two stock solutions A (200 mM) and B (2 mM) of

496 J Sol-Gel Sci Technol (2012) 62:495–504

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sodium hydroxide (NaOH, Merck) and tin chloride penta-

hydrate (SnCl4�5H2O, ACROS ORGANICS) were pre-

pared in DI-water and sonicated for 5 min. 100 ml of stock

B was taken in glass container and X ml of stock A was

pipetted into it so that the precursor mixture had the

required molar ratio (MR) of NaOH/SnCl4. The substrate

carrying ZnO nanorod films were then immersed into the

mixture. The glass container was closed with a lid and

transferred into the microwave oven which was operated at

specific power mode (80, 240, 400, 560 W) for a certain

interval of time. At the end, the substrates were rinsed with

DI-water and dried in electric oven in air at 100 �C for

several hours. All the chemicals used were analytical grade

and were not purified further.

The films were characterized by scanning electron

microscope (SEM-JSM-6301F) fitted with an energy dis-

persion spectroscopy (EDS) attachment working at 20 kV

and XRD equipment (XRD-JEOL-JDX 3530) operated at

40 kV and 40 mA using Cu-Ka radiation (k = 1.54056 A).

3 Results and discussions

Low magnification scanning electron micrographs (SEM)

of ZnO nanorods show a homogeneous growth (Fig. 1a)

with an aerial density of *50 rods/lm2 and average rod

thickness of *100 nm (Fig. 1a, inset). In Fig. 1b the cross

sectional view of the film shows that the arrays consisted of

nanorods of narrow size distribution growing vertically

from the substrates. Nearly 2.8 lm long rods could be

grown by applying 80 W power in the microwave oven

within 5 h of reaction (Fig. 1b) that is 4–5 times faster than

what has been observed for conventional hydrothermal

growths at low temperatures [30]. Observations regarding

uniformity, density and alignment of the nanorod arrays

established effectiveness of the spray pyrolysis technique

for the deposition of polycrystalline ZnO seeding film for

the controlled growth of such arrays. Compared to other

seeding techniques, spray pyrolysis has also been found to

be advantageous as it is fast and simple.

Fig. 1 SEM images of; a top and b cross sectional view of ZnO nanorod array synthesized through microwave assisted hydrothermal process

under 80 W irradiation for 5 h using 30 mM precursor; inset of a is the high magnification SEM image of the top view of the array

Fig. 2 a, b, SEM images of top and cross sectional views, respectively, of ZHS microcube film formed over pre-synthesized ZnO nanorod array

under 80 W microwave irradiation for 1.75 h in a precursor solution having NaOH/SnCl4 molar ratio of 10/1

J Sol-Gel Sci Technol (2012) 62:495–504 497

123

Immersing these ZnO nanorod coated substrates into an

aqueous precursor solution (Sn4?-100 ml-2 mM, NaOH-

10 ml-200 mM) and subjecting to continued thermal

treatment with 80 W microwave power for about 1 h or

more, growth of an uniform polycrystalline film could be

observed. SEM images of the top and cross sectional views

of a typical fabricated film (reacted for 1.75 h) are shown

in the Fig. 2a, b, respectively. The low magnification SEM

image of the top view (Fig. 2a), shows that the substrate

was completely covered with cubes wherein no ZnO

nanorods could be observed. Higher magnification image

(inset of Fig. 2a) reveals that the film consisted of cubic as

Fig. 3 EDS spectrum of ZHS

cube formed over ZnO nanorod

film due to thermal treatment

under 80 W microwave

irradiation for 2 h in a 2 mM

precursor solution with NaOH/

SnCl4 molar ratio of 10/1

Fig. 4 X-ray diffraction (XRD) pattern of the powder obtained by scratching ZHS film synthesized over ZnO nanorod array with 80 W

microwave irradiation for 2 h, using precursor solution with NaOH/SnCl4 molar ratio of 10/1

498 J Sol-Gel Sci Technol (2012) 62:495–504

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well as rectangular blocks of sizes prevailingly in the range

of 1–2 lm. The SEM image of the cross sectional view of

the film given in Fig. 2b shows that these blocks were not

merely laying on top of the array rather they were extended

down to the bottom by engulfing the nanorods in stacks. A

typical EDS spectrum taken from the surface of a single

block (Fig. 3) shows the presence of zinc (Zn) and tin (Sn)

along with the predominant amount of oxygen (O). Com-

parable sizes of Zn and Sn peaks indicated that the

microblocks could be zinchydroxystannate (ZnSn(OH)6),

as the material contains the same molar concentrations of

zinc and tin [23]. The gold (Au) peaks in the spectrum are

due to the gold film sputtered on top of the film during

preparation of samples for SEM microscopy.

X-ray diffraction (XRD) pattern of the film given in

Fig. 4, shows a mixed phase of hexagonal ZnO crystals and

cubic crystals of ZnSn(OH)6 when compared with standard

data files JCPDS PDF#89-7102 and PDF#20-1455,

respectively. Relative intensities of peaks show the per-

pendicular growth of hexagonal ZnO nanorods along the

c-axis [002] while the ZHS blocks grow preferentially

along the (200) planes. The presence of mixed phases of

ZnO and ZHS could also be observed in the SEM image of

the cross sectional view of the film given in Fig. 2b.

Perceived mechanism for the formation of ZHS micro-

blocks involves dissolution-precipitation processes as pro-

posed by Zhang et al. [23], wherein Sn4? and OH- present

in the precursor solution react with Zn2? ions supplied by

the ZnO nanorods. The mixture precursor for synthesis of

ZHS in all the experiments was found with pH ranging

between 10 and 13. The large surface area renders ZnO

nanorod array to high rate dissolution and release of Zn2?

ions, particularly at elevated pH [31]. In one of the reports

from our group, we have dwelt on the induction of high

concentration of defects through the microwave assisted

fast crystallization and growth of ZnO nanorods [32].

Furthermore, the post growth annealing was reported to

mitigate defects in the hydrothermally synthesized ZnO

nanorods [33]. The fast hydrothermal growth under

microwave irradiation and lack of post synthesis annealing

would leave the nanorods with high concentration of sur-

face defects and would in turn increase its surface energy,

leading to a higher etching/dissolution and the facile

release of Zn2? ions. High concentration of OH- in the

Fig. 5 a, b, c, d Low magnification SEM images of top views of ZHS

films synthesized over pre-synthesized ZnO nanorod arrays with heat

treatment for 0.5, 1.0, 2.0, and 3.0 h, respectively, under 80 W

microwave irradiation in solution having NaOH/SnCl4 molar ration of

10/1; insets of a and d are higher magnification top views, while that

of c is that of the cross sectional view

J Sol-Gel Sci Technol (2012) 62:495–504 499

123

precursor solution leads to the formation Sn(OH)6- ions

which by reacting with Zn2? cations form ZnSn(OH)6 [6].

Crystallization of ZnSn(OH)6 starts through heterogeneous

nucleation at points on the surface of ZnO nanorod where

Sn(OH)6- ions are held at Zn2? cites. The plausible

chemical reactions in the process have been summarized in

Eqs. (1–3).

SnCl4 þ 4NaOH ! Sn(OH)4 þ 4NaCl ð1Þ

SnðOHÞ4 þ 2ðOHÞ� $ SnðOHÞ2�6 ð2Þ

Zn2þ þ SnðOHÞ2�6 ! ZnSnðOHÞ6 # ð3Þ

Figure 5 shows SEM images of the transformed ZnO

nanorod arrays treated for different period of time in

precursor solution (Sn4?-150 ml-2 mM, NaOH-15 ml-

200 mM), under 80 W microwave irradiation. A 30 min

microwave irradiation leads to the formation of cubic

blocks with size distribution in 0.9–1.25 lm range

scattered all over the nanorod array (Fig. 5a). Higher

magnification SEM image of the array, given in the inset of

Fig. 5, shows that the blocks grow in random directions

due to which they intersect one another while some blocks

grow at some depth within the nanorod array. SEM image

of the array after 1 h continuous irradiation of microwaves,

given in Fig. 5b, shows the presence of cubes with nearly

the same concentration and similar distribution on the

nanorod arrays. However, the average size of the cubes

increased to *2.5 lm. SEM image of sample prepared

with 2 h microwave irradiation shows the formation of a

dense layer of closely packed microblocks of different

shapes and size distribution within the range of 2–4.5 lm

(Fig. 5c). The cross sectional view of film (inset of Fig. 5c)

shows the maximum transformation of ZnO nanorod array

into ZHS film. The microwave treatment beyond 2 h

results in thermal decomposition of ZHS blocks (Fig. 5d)

and the material gelatinizes the top surface of the nanorod

array (inset of Fig. 5d). This decomposition is presumably

caused by the dehydration of ZnSn(OH)6 to form ZnSnO3

[17] (Eq. 4) which could not crystallize at the prevailing

temperature and, therefore, settle at the top of nanorod

array. The presence of 6–8 l cubes after a 3 h microwave

treatment (Fig. 5d) may be due to some ZnSn(OH)6

building blocks which survive dehydration and adsorb/

crystallize at a nearby ZHS crystal.

Fig. 6 a, b, c, d, Low magnification SEM images of ZHS film

formed over pre-synthesized ZnO nanorod array under 80 W

microwave irradiation for 2 h in precursor solutions having NaOH/

SnCl4 molar ratios of 5/1, 20/1, 30/1, and 50/1, respectively; insetsshow the magnified cross sectional views of the corresponding films

500 J Sol-Gel Sci Technol (2012) 62:495–504

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2ZnSnðOHÞ6 ! 2ZnSnO3 þ 6H2O ð4ÞFigure 6 includes SEM images of four samples prepared

with NaOH/SnCl4 molar ratios (MR) of 5/1, 20/1, 30/1, and

50/1, under microwave treatment for 2 h. MR of 5/1 did

not produce any ZHS cubic crystals (Fig. 6a) the reason

obviously is the composition of ZHS i.e. ZnSn(OH)6,

which suggests that the concentration of OH- ions should

be at least six times that of Sn4? ions. The treatment

resulted in the deposition of *0.5 l thick layer on top of

the nanorod array (inset of Fig. 6a). MR of 20/1, on other

hand produced 1.3–4 lm cubic blocks, randomly scattered

on top of ZnO nanorod array (Fig. 6b). Upon increasing the

NaOH/SnCl4 MR to 30/1, aerial density of the ZHS cubic

crystals were found to increase and smaller crystallites of

*0.5–1.25 lm were formed (Fig. 6c). Higher alkaline

solution (MR of 50/1) did not allow the formation of any

ZHS cubes (Fig. 6d). Insets of Fig. 6b–d show that the

under lying nanorod arrays are free from the deposition of

continuous layer like the one observed in the inset of

Fig. 6a. From the image analysis, it could be concluded

that OH-/Sn4? molar ratio less than 6/1 would not produce

ample concentration of ZnSn(OH)6 to crystallize. In case

higher OH-/Sn4? molar ratios, i. e. 30/1 and 50/1

(pH = 12.75 and 12.89), the predominantly high concen-

tration of free OH- ions are expected to dissolve the newly

created crystallite at faster rate than its growth. At molar

ratio of 20/1, some crystals might have managed to attain a

critical size after which they grow at a faster rate by taking

up all the growth units where as nucleation at new sites

were restricted. In such a scenario, molar ratio of 10/1 was

found to be optimum one for unrestricted nucleation and

growth of ZnSn(OH)6 crystals as shown in Fig. 2.

To investigate the effect of molar concentration of Sn4?

on the formation of ZHS microblocks, four samples were

prepared with different Sn4? concentrations, i. e. 0.5, 1.0,

3.0, and 5.0 mM. Rest of the parameters such as precursor

volume, NaOH/SnCl4 molar ratio, and irradiation time and

power were kept constant as 100 ml, 10/1, 105 min and

80 W respectively. Results of the experiment depicted

through SEM images of the as-prepared samples (Fig. 7)

reveal that molar concentration of Sn4? ion in the precursor

solution has a profound effect on size and surface density

of ZHS microblocks. The 0.5 mM concentration of Sn4?

ions produced ZHS cubes with size ranging between 0.8

Fig. 7 a, b, c, d, Low magnification SEM images of ZHS film

formed over pre-synthesized ZnO nanorod arrays under 80 W

microwave irradiation for 105 min using precursor solutions having

the NaOH/SnCl4 molar ratio of 10/1 but Sn4? concentration of 0.5,

1.0, 3.0, and 5.0, respectively; insets show the cross sectional views of

the corresponding films

J Sol-Gel Sci Technol (2012) 62:495–504 501

123

and 2.0 lm and surface density of *900 cubes per mm2

(Fig. 7a). In 1.0 mM concentration of Sn4? ion (Fig. 7b),

size distribution of the cubes shifted into 1.8–4.4 lm while

surface density increased to *3,980 cubes per mm2. Using

3.0 mM Sn4? concentration, the ZnO nanorod array was

almost completely transformed into a dense layer of ZHS

cubes (Fig. 7c). It could be seen that the cubes had size

distribution in the range 0.5–1.8 lm and surface density of

*7,407,40 cubes per mm2. Upon increasing the Sn4?

concentration to 5.0 mM, the cube size distribution was

found to shift to 3.5–6.5 lm (Fig. 7d); however, surface

density decreased to 82,407 per mm2. Insets in all sections

of Fig. 7 show the cross sectional views of the corre-

sponding films. Results of our experiments indicate that

2–3 mM concentration of Sn4? ions lead to the optimum

formation of highly dense film of ZHS cubic crystals. It has

been seen that crystallization starts with nucleation

(homogeneous/heterogeneous) at some specific sites;

while, an extremely small crystal (nucleus) is thermody-

namically unstable and dissolve instantly if ample number

of growth units are not present around which adsorb onto

the nucleus and stabilize it by growing it quickly up to a

stable size [34]. A low precursor concentration, therefore,

could not produce a significant number of ZHS crystals. In

case of higher precursor concentration, i. e. 5 mM, a pretty

big number of Sn(OH)62- anions might have not found their

complements (i.e. Zn2? cations) and have precipitated after

being converted into Sn(OH)4 and/or SnO and settled on

top of the nanorod array (Fig. 7d). These synthesis condi-

tions would then have restricted creation of new nuclei,

allowing the existing crystals to take up the growth units

and expand vigorously.

All the experiments discussed so far were carried out

with microwave power of 80 W. To investigate the effect

of applied microwave power on the formation of ZHS

microblock film, several samples were prepared in a precursor

solution (Sn4?-200 ml-2 mM, OH--20 ml-200 mM) with

240 W, 400 W and 560 W microwave power for different

time intervals. SEM images of the as-synthesized films are

given in Fig. 8, which shows that under 240 W microwave

irradiation for 30 min very less number of ZHS micro-

blocks are formed, with maximum size \2 lm (Fig. 8a).

Upon increasing the irradiation time to 40 min (Fig. 8b),

density as well as size range (2–6 lm) of microblocks

increased, while, majority of them were partially embedded

into the nanorod array. By extending the irradiation time to

Fig. 8 a, b, c, SEM images of ZHS films formed over pre-

synthesized ZnO nanorod arrays under 240 W microwave irradiation

for 30, 40, and 50 min, respectively; d, e, f, SEM images of ZHS

films formed over pre-synthesized ZnO nanorod arrays under 400 W

microwave irradiation for 10, 15, and 30 min, respectively; precursor

used in all the experiments was (Sn4?-200 ml-2 mM, OH--20 ml-

200 mM)

502 J Sol-Gel Sci Technol (2012) 62:495–504

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50 min, density of the microblock was further increased by

several times (Fig. 8c) with majority of the blocks \4 lm

sizes. In the image (Fig. 8c), it could be observed that there

was a significant number of smaller ZHS blocks (\1 lm)

adsorbed at the surface of the bigger blocks, showing that

the conditions, at this stage, were favorable for the nucle-

ation process. Given in the Fig. 8d, e, f, are the low mag-

nification images of ZHS films synthesized with 400 W

microwave irradiation for a period of 10, 15, and 30 min,

respectively. It could be seen that the treatment resulted in

gelatinization of the nanorod arrays with an amorphous film

whose thickness increased with irradiation time; however,

no signs of crystallization were observed. The same was the

case with 560 W microwave irradiation (SEM images of the

as-prepared samples are not shown here).

4 Conclusion

Microwave heating has been successfully applied for the

fast synthesis of ZnO nanorod arrays on glass and their

transformation into polycrystalline films of zinchydroxy-

stannate (ZnSn(OH)6). Maximum transformation was

obtained by adjusting the four synthesis parameters i.e.,

irradiation time, NaOH/SnCl4 molar ratio and concentra-

tion of Sn4? ions in the precursor solution, and microwave

power as 1.75–2 h, 10/1, 2–3 mM, and 80 W, respectively.

The method being simple, cost effective, and fast turns out

to be significantly effective for practical/industrial appli-

cation of ZHS coating on solid substrates.

Acknowledgments The authors would like to acknowledge partial

financial supports from the National Nanotechnology Center,

belonging to the National Science and Technology Development

Agency (NSTDA), Ministry of Science and Technology (MOST),

Thailand and the Center of Excellence in Nanotechnology at the

Asian Institute of Technology, Thailand and NWFP University of

Engineering and Technology, Peshawar, Pakistan.

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