facile synthesis of zno micro-nanostructures with controllable morphology and

Applied

Surface

Science

261 (2012) 759–

763
Contents

lists

available

at

SciVerse

ScienceDirect
Applied

Surface

Science
j

our

nal

ho

me

p

age:

www.elsevier.com/loc

ate/apsusc
Facile

synthesis

of

ZnO

micro-nanostructures

with

controllable

morphology

and
their

applications

in

dye-sensitized

solar

cells
Yi

Zhou
a,∗
,

Dang

Li
a
,

Xiangchao

Zhang
b
,

Jianlin

Chen

a
,

Shiying

Zhang
b
a
Department

of

Chemistry

and

Biological

Engineering,

Changsha

University

of

Science

and

Technology,

Changsha

410114,

China
b
Department

of

Science

and

Technology,

Changsha

University,

Changsha

410003,

China
a

r

t

i

c

l

e

i

n

f

o
Article

history:
Received

11

May

2012
Received

in

revised

form

31

July

2012
Accepted

31

July

2012
Available online 31 August 2012
Keywords:
ZnO
Micro-nanostructures
Urchin
Dye

sensitization

solar

cell

Photoelectric

properties
a

b

s

t

r

a

c

t
Different

morphologies

of

ZnO

micro-nanostructures

were

successfully

prepared

by

hydrothermal
method

at

relatively

mild

conditions

using

ammonia

to

adjust

the

pH

of

the

reaction

system.

The

samples
were

characterized

by

X-ray

powder

diffraction,

scanning

electron

microscopy,

optical

reflectance

spec-
tra,

and

photocurrent–voltage

curve.

The

results

demonstrated

that

the

morphologies

of

ZnO

changed
from

“wire”

to

“flower”,

“urchin”

and

“wire”

with

increase

in

the

pH

of

the

reaction

system

due

to

the
increased

concentration

of

ammonia.

The

diffused

reflectance

spectra

illustrated

that

the

reflectance
of

denser

urchin-like

ZnO

was

low

at

18%

in

the

visible

region.

When

the

as-synthesized

ZnO

micro-
nanostructures

were

used

as

the

anode

of

the

dye

sensitization

solar

cell,

the

denser

urchin-like

ZnO
exhibited

the

best

photoelectric

properties.

The

short

circuit

current

(J
sc
),

open

circuit

voltage

(V
oc
),

and
conversion

efficiency

(Á)

were

6.50

mA/cm
2
,

0.682

V,

and

1.92%,

respectively.
© 2012 Elsevier B.V. All rights reserved.
1.

Introduction
As

an

important

low-cost

semiconductor

functional

mate-
rial

with

large

band

gap

(3.37

eV)

and

large

excitation

binding
energy

(60

meV)

[1],

zinc

oxide

(ZnO)

is

recognized

as

one

of
the

most

promising

materials

for

optoelectronic

applications.
Micro-nanostructured

ZnO

has

drawn

considerable

attention

due
to

its

unique

electrical,

mechanical,

and

optical

properties,

in
addition

to

its

applications

in

numerous

fields,

such

as

solar
cells

[2],

gas

sensors

[3,4],

piezoelectric

materials

[5],

pho-
tonic

crystals

[6],

and

optoelectric

devices

[7].

Previous

studies
demonstrated

that

the

properties

of

ZnO

are

closely

related
to

the

size

and

shape

of

the

structures.

For

example,

tetra-
pod

ZnO

nanostructures

exhibit

strong

UV

emission

[8]

and

needle-like

ZnO

arrays

exhibit

strong

blue

light

emission

[9].
Thus,

studying

the

morphology

of

micro-nanostructured

ZnO

is
important.
ZnO

micro-nanostructures

have

been

synthesized

with

various
methods,

such

as

chemical

vapour

deposition

[10],

template-based
method

[11],

laser

ablation

[12],

spray

pyrolysis

technique

[13],
hydrothermal

method

[14],

and

electrodeposition

method

[15].
Researchers

have

prepared

micro-nanostructured

ZnO

with

dif-
ferent

morphologies

using

these

different

methods.

For

example,
Polsongkram

et

al.

[14]

prepared

ZnO

nanorods

by

hydrothermal
∗
Corresponding

author.

Tel.:

+86

731

85258328;

fax:

+86

731

85258328.
E-mail

addresses:

,



(Y.

Zhou).
method.

Chen

et

al.

[16]

synthesized

ZnO

nanotubes

by

a
sonochemical

method

at

low

temperature.

Liu

and

Zeng

[17]
fabricated

ZnO

dandelions

by

a

modified

Kirkendall

process.
Jana

et

al.

[18]

prepared

water

lily-type

ZnO

flowers

by

a
simple

solution

method,

and

Elias

et

al.

[19]

prepared

hollow
urchin-like

ZnO

thin

films

by

electrochemical

deposition.

How-
ever,

majority

of

these

studies

are

limited

to

the

research

of
one

kind

of

morphology.

Few

studies

have

reported

on

ZnO
micro-nanostructures

with

different

morphologies

using

a

single
method.
In

the

present

study,

several

types

of

ZnO

micro-nanostructures
with

different

morphologies

and

photoelectric

properties

were
prepared

by

a

simple

hydrothermal

method

at

relatively

mild
conditions.

The

concentration

of

ammonia,

which

can

adjust
the

pH

of

the

reaction

system,

was

controlled.

Subsequently,
the

influence

of

pH

on

the

photoelectric

properties

of

the
ZnO

micro-nanostructures

was

investigated

by

studying

the
photocurrent–voltage

(I–V)

characteristics

of

the

dye-sensitized
solar

cell

(DSSC).
2.

Experimental
2.1.

Materials
All

chemicals

were

of

analytical

reagent

grade

and

used

with-
out

further

purification.

All

aqueous

solutions

were

prepared

using
double

distilled

water.
0169-4332/$

–

see

front

matter ©

2012 Elsevier B.V. All rights reserved.
/>760 Y.

Zhou

et

al.

/

Applied

Surface

Science

261 (2012) 759–

763
2.2.

Preparation

of

ZnO

film

ZnO

micro-nanostructures

were

synthesized

using

a

hydro-
thermal

method.

The

procedure

was

as

follows:

1.18

g

zinc

nitrate
hexahydrate

(Zn(NO
3
)
2
·6H
2
O)

and

0.56

g

hexamethylenetetramine
(C
6
H
12
N
4
)

were

added

to

40

mL

double

distilled

water

under

strong
magnetic

stirring

at

room

temperature

to

obtain

a

transparent

and
homogeneous

solution.

NH
3
(25%)

was

then

dropped

into

the

solu-
tion

at

60

drops/min

to

change

the

pH

from

7

to

11.

The

solution
was

kept

at

room

temperature

for

0.5

h

under

vigorous

stirring

to
obtain

the

precursor.
The

ZnO

micro-nanostructures

used

in

this

work

were

grown

on
FTO-coated

glass

substrates.

First,

the

FTO-coated

glass

substrates
were

successively

cleaned

in

an

ultrasonic

bath

with

acetone,
ethanol,

and

double

distilled

water

for

15

min

to

remove

dust

and
prevent

surface

contamination.

The

FTO-coated

glass

substrates
were

then

dipped

in

0.5

mol/L

ZnCl
2
aqueous

solution

for

5

min
at

room

temperature.

Finally,

the

substrates

were

pulled

upward
by

a

hoist

with

constant

speed,

and

then

dried

in

the

air

to

obtain
functionalized

FTO-coated

glass

substrates

[20].
The

precursor

solution

and

the

functionalized

substrates

were
transferred

to

a

Teflon-sealed

autoclave.

Then

the

reaction

was

kept
at

90
◦
C

for

9

h

to

synthesize

the

ZnO

micro-nanostructures.

After
deposition,

the

samples

were

cleaned

several

times

with

double
distilled

water

and

then

dried

in

the

air.
2.3.

Construction

of

dye-sensitized

solar

cell
The

construction

of

DSSC

has

been

reported

in

a

previous
research

[21].
2.4.

Characterization
The

samples

were

characterized

using

scanning

electron
microscopy

(SEM,

JEOL

JSM-6700F)

and

X-ray

diffraction

patterns
were

recorded

on

an

X-ray

diffraction

system

(SIEMENS

D5000).
The

diffused

reflectance

spectra

were

measured

by

an

IPCE

tester
(Solar

Cell

Scan

100,

Beijing

Zhuo

Li

Han

Guang).

The

I–V

charac-
teristics

were

measured

using

a

computer-controlled

digital

source
meter

(Keithley,

Model

2400)

under

the

illumination

of

a

Newport
solar

simulator

(AM

1.5,

100

mW/cm
2
).
3.

Results

and

discussion

3.1.

Morphology

and

structural

analyses
Fig.

1

depicts

the

SEM

images

of

micro-nanostructured

ZnO
grown

under

different

pH,

namely,

7,

8,

9,

10

and

11.

The

growth
temperature

and

time

were

90

◦
C

and

9

h,

respectively.

As

shown

in
Fig.

1,

the

morphology

of

the

as-grown

micro-nanostructured

ZnO
was

closely

related

to

the

pH

of

the

precursor

solution.
Fig.

1a

indicates

that

ZnO

nanowires

formed

on

the

substrate
when

the

applied

pH

was

7.

The

dense

ZnO

nanowires

with
hexagonal

structure

were

vertically

well-aligned

and

uniformly
distributed

on

the

substrate.

The

average

diameters

of

the

ZnO
nanowires

were

approximately

30–50

nm;

the

length–diameter
ratios

were

approximately

6–10.

The

sample

prepared

with

pH

=

8
resulted

in

the

formation

of

flower-like

ZnO

whose

petals

were
approximately

500–700

nm

in

length

and

300–400

nm

in

width
(Fig.

1b).

When

the

pH

was

increased

to

9,

urchin-like

ZnO

were
formed.

Fig.

1c

reveals

that

the

urchin-like

ZnO

was

comprised
of

nanorods,

which

had

similar

centers

and

were

approximately
5–6

␮m

in

length

and

300–500

nm

in

width.

Notably,

urchin-like
ZnO

also

formed

when

the

applied

pH

was

controlled

at

10

(Fig.

1d).
However,

this

urchin-like

ZnO

was

comprised

of

needle-like

ZnO
nanowires,

and

the

sizes

and

amounts

of

ZnO

nanowires

were

also
different

from

those

in

Fig.

1c.

When

the

applied

pH

was

increased
to

11,

the

urchin-like

morphologies

disappeared

and

changed

to
ZnO

nanowires

with

poor

orientations.

The

average

diameters

of
these

ZnO

nanowires

were

approximately

50–80

nm,

and

their
average

lengths

were

approximately

500–600

nm.
Fig.

1f

and

g

is

the

lower

magnification

SEM

images

of

Fig.

1c

and
d,

respectively.

The

urchin

structures

were

lined

by

a

single

layer

on
the

substrate,

and

all

the

ZnO

nanowires

were

relatively

homoge-
neous.

Fig.

1h

reveals

the

side

view

images

of

micro-nanostructured
ZnO

at

pH

=

10.

As

shown

in

Fig.

1h,

the

film

thicknesses

were

about
2

␮m.
Urchin-like

ZnO

micro-nanostructures

were

formed

on

FTO-
coated

glass

substrates

by

a

hydrothermal

method.

The

formation
process

can

be

expressed

as

follows

[22]:
(CH)
6
N
4
+

6H
2
O

→

6HCHO

+

NH
3
(1)
NH
3
+

H
2
O

→

NH
4
+
+

OH
−
(2)
Zn
2+
+

NH
3

→

Zn(NH
3
)
4
2+
(3)
Zn
2+
+

4OH
−
→

Zn(OH)
4
2−
(4)
Zn(NH
3
)
4
2+
+

2OH
−
→

ZnO

+

4NH
3
+

H
2
O

(5)
Zn(OH)
4
2−
→

ZnO

+

H
2
O

+

2OH

−
(6)
Based

on

the

growth

habits

of

ZnO

crystals

in

aqueous

solu-
tions,

urchin-like

ZnO

micro-nanostructures

can

be

obtained

only
when

the

pH

of

the

bulk

solution

are

controlled

at

certain

values.
When

the

pH

is

low,

the

concentrations

of

OH
−
and

NH
3
in

the
precursor

solution

are

correspondingly

low,

leading

to

the

small
amount

of

Zn(OH)
4
2−
and

Zn(NH
3
)
4
2+
,

which

are

insufficient

to
form

the

nuclei.

Therefore,

ZnO

nanowires

form

on

the

substrate
when

the

applied

pH

is

controlled

at

7

(Fig.

1a).

The

amount

of
Zn(OH)
4
2−
and

Zn(NH
3
)
4
2+

increases

with

the

pH.

When

the

pH
is

above

8,

Zn(OH)
4
2−
and

Zn(NH
3
)
4
2+
will

gather

and

decompose
to

ZnO

nuclei

at

the

beginning

of

the

reaction.

The

growth

units
of

Zn(OH)
4
2−
and

Zn(NH
3
)
4
2+
are

then

adsorbed

on

the

nuclei
due

to

intermolecular

absorption

forces,

such

as

van

der

Waals
interactions,

and

finally

grow

to

nanowires

in

all

directions

to

form

three-dimensional

urchin-like

ZnO.

The

SEM

micrographs

in
Fig.

1

reveal

that

the

morphologies

of

three-dimensional

ZnO

are
different

under

different

conditions.

As

the

pH

increases,

the

mor-
phologies

of

ZnO

change

from

“flower”

(Fig.

1b),

to

sparse

“sea
urchin”

(Fig.

1c)

and

denser

“sea

urchin”

(Fig.

1d).

The

pH

has
an

important

function

during

the

formation

of

the

ZnO

micro-
nanostructures.

This

can

be

explained

as

follows.

Ammonia

can
easily

separate

from

the

solution

when

the

pH

is

high,

which

results
in

an

increase

in

the

air

pressure

of

the

Teflon-sealed

autoclave.
This

will

influence

the

growth

of

ZnO

micro-nanostructures,

lead-
ing

to

change

in

the

morphology

of

ZnO.

The

pH

of

the

reaction
solution

increases

with

the

addition

of

ammonia.

At

the

same

time,

the

growth

units

are

more

likely

to

come

in

contact

with

the

nuclei.
This

phenomenon

can

be

propitious

for

the

formation

of

Zn(OH)
4
2−
and

Zn(NH
3
)
4
2+
,

finally

leading

to

an

increase

in

the

amount

of
nanowires

on

the

nuclei.

However,

when

the

pH

increases

to

a
certain

degree,

the

air

pressure

in

the

Teflon-sealed

autoclave

will
increase

to

a

greater

degree;

thus,

Zn(OH)
4
2−
and

Zn(NH
3
)
4
2+
are
unable

to

gather

to

form

the

initial

ZnO

nuclei.

Therefore,

the

pre-
cursor

solution

is

generated

for

the

single

and

independent

ZnO
nanowires,

hindering

them

from

forming

flower-

or

urchin-like
ZnO.

The

concentration

of

ammonia,

the

pH,

and

the

air

pressure
of

the

Teflon-sealed

autoclave

has

an

important

function

in

the
above

transformation

processes

of

ZnO

morphologies.

However,
the

specific

reaction

mechanism

requires

further

research.
Y.

Zhou

et

al.

/

Applied

Surface

Science

261 (2012) 759–

763 761
Fig.

1.

SEM

images

of

the

micro-nanostructured

ZnO

under

different

pH:

(a)

7;

(b)

8;

(c

and

f)

9;

(d

and

g)

10;

and

(e)

11.

Lower

magnification

SEM

images

(f)

and

(g).

Side
view

images

of

micro-nanostructured

ZnO

at

pH

=

10

(h).
3.2.

XRD

patterns
Fig.

2

shows

the

XRD

spectra

of

the

micro-nanostructured

ZnO
under

different

pH.

All

diffraction

peaks

can

be

indexed

to

a

hexag-
onal

wurtzite

phase

of

ZnO,

in

agreement

with

the

standard

card
(JCPDS

78-2486).

No

characteristic

peaks

of

any

impurities,

except
polycrystalline

SnO
2
(from

the

FTO

substrate),

were

detected

in
the

pattern,

confirming

that

the

obtained

products

are

pure

ZnO.
The

characteristic

peaks

were

high

in

intensity

and

narrow,

which
indicated

that

ZnO

micro-nanostructure

had

high

crystallinity.

The
intensities

of

the

diffraction

peaks

of

micro-nanostructured

ZnO
were

obviously

different,

indicating

that

the

pH

had

effect

on

the
crystallinity

of

grown

ZnO

micro-nanostructure.
3.3.

Optical

reflection

spectra

analyses
Fig.

3

shows

the

optical

reflection

spectra

of

the

micro-
nanostructured

ZnO

under

different

pH.

Fig.

3

indicates

that
the

light

scattering

of

the

ZnO

micro-nanostructure

is

closely
related

to

its

morphology.

The

surface

areas

of

denser

urchin-
like

structure,

sparse

urchin-like

structure,

vertically

well-aligned
nanowire

structure,

disorderly

nanowire

structure

and

flower-like
structure

were

350,

290,

223,

147,

96

m
2
/g,

respectively.

And

due
to

the

different

surface

areas

of

the

different

morphologies,

the
order

of

intensity

of

the

ZnO

micro-nanostructure

light

scatter-
ing

is

as

follows:

denser

urchin-like

structure

<

sparse

urchin-like
structure

<

vertically

well-aligned

nanowire

structure

<

disorderly
nanowire

structure

<

flower-like

structure.

Among

these

ZnO
micro-nanostructures,

the

reflectance

of

the

denser

urchin-like
ZnO

is

the

lowest

at

approximately

18%

in

the

visible

region.
The

reflectance

of

the

flower

structure

is

the

highest.

This

could
attribute

to

the

size

[23]

and

the

morphology

[24]

of

ZnO,

which
play

important

roles

for

controlling

the

light

scattering.

In

addition,
the

graph

indicates

that

the

entire

ultraviolet

absorption

spectrum
edge

is

approximately

380

nm,

which

is

in

agreement

with

the
direct

wide

band

gap

(3.37

eV)

of

ZnO.
3.4.

Application

of

ZnO

micro-nanostructure

in

DSSC
Fig.

4

compares

the

I–V

characteristics

of

the

DSSC

based

on

ZnO
with

different

micro-nanostructures.

The

corresponding

values
are

summarized

in

Table

1,

which

demonstrates

photoelectro-

chemical

characteristics,

such

as

current

density

at

short

circuit
(J
sc
),

voltage

at

open

circuit

(V

oc
),

fill

factor

(FF),

and

efficiency
762 Y.

Zhou

et

al.

/

Applied

Surface

Science

261 (2012) 759–

763
Fig.

2.

XRD

spectra

of

the

micro-nanostructured

ZnO

under

different

pH:

(a)

7;

(b)
11;

(c)

8;

(d)

9;

and

(e)

10.
Fig.

3.

Optical

reflection

spectra

of

the

micro-nanostructured

ZnO

under

different
pH:

(a)

8;

(b)

11;

(c)

7;

(d)

9;

and

(e)

10.
Fig.

4.

Photocurrent–voltage

characteristics

of

ZnO

with

different

micro-
nanostructure-based

DSSCs.

(a)

Denser

urchin-like

ZnO;

(b)

sparse

urchin-like

ZnO;
(c)

orderly

ZnO

nanowire;

(d)

flower-like

ZnO;

and

(e)

disorderly

ZnO

nanowire.
Table

1
Photovoltaic

parameters

of

micro-nanostructured

ZnO

with

different

micro-
nanostructures.
Film

type

J
sc
(mA/cm
2
)

V
oc
(V)

FF

(%)

Á

(%)
Orderly

ZnO

nanowire

4.48

0.598

36.7

0.98
Disorderly

ZnO

nanowire

3.13

0.526

27.3

0.45
Flower-like

ZnO

3.93

0.537

33.2

0.70
Sparse

urchin-like

ZnO

4.82

0.654

38.5

1.21
Denser

urchin-like

ZnO

6.50

0.682

43.4

1.92
of

power

conversion

(Á)

in

different

samples.

Table

1

indicates
that

the

photoelectrochemical

characteristics

of

the

DSSC-based
denser

urchin-like

ZnO

are

high,

reaching

a

maximum

value

of

1.92%.

Notably,

the

denser

urchin-like

structure

is

beneficial

for

the
transfer

of

electrolytes,

and

this

specific

structure

can

increase

the
production

of

carriers

and

photoelectric

activity

due

to

its

larger
surface

area

and

increased

activity

centers

for

absorbing

dye.

In
the

urchin-like

structure,

the

needle-like

ZnO

nanowires

have

the
same

center

extended

to

the

surroundings.

This

special

structure
can

effectively

improve

the

efficiency

of

electron

transmission,
decreases

the

transmission

path

of

charge

in

the

electrode

materi-
als

and

the

recombination

of

carriers.

As

a

result,

the

photoelectric
activity

of

ZnO

increases.
Table

1

further

shows

that

the

ZnO

nanowires

were

formed

on
the

substrate

when

the

applied

pH

was

controlled

at

7

and

11,
but

the

photoelectric

parameters

of

these

two

different

nanowires
varied

enormously.

Due

to

their

excellent

orientation,

the

ZnO
nanowires

prepared

at

pH

=

7

can

provide

an

effective

transmis-
sion

path

for

electrons,

reduce

the

recombination

of

carriers,

and
increase

the

photoelectric

activity

of

ZnO.
4.

Conclusions
Several

kinds

of

ZnO

micro-nanostructures

with

different

mor-
phologies

and

photoelectric

properties

have

been

prepared

by

a
simple

hydrothermal

method

at

relatively

mild

conditions.

The
pH

is

essential

in

the

growth

of

ZnO,

and

results

in

the

mor-
phology

of

ZnO

changing

from

“wire”

to

“flower”,

“urchin”

and
“wire”

with

the

addition

of

different

amounts

of

ammonia.

Due
to

the

different

surface

areas

of

the

various

morphologies,

the
order

of

intensity

of

the

ZnO

micro-nanostructure

light

scatter-

ing

is

as

follows:

denser

urchin-like

structure

<

sparse

urchin-like
Y.

Zhou

et

al.

/

Applied

Surface

Science

261 (2012) 759–

763 763
structure

<

vertically

well-aligned

nanowire

structure

<

disorderly
nanowire

structure

<

flower-like

structure.

Among

the

foregoing,
the

reflectivity

of

the

denser

urchin-like

structure

was

the

lowest
at

18%.

When

the

obtained

ZnO

micro-nanostructures

were

used

as
the

anode

of

the

DSSC,

the

photoelectrochemical

characteristics

of
the

DSSC

based

on

ZnO

with

different

micro-nanostructures

vary.
The

denser

urchin-like

ZnO

micro-nanostructures

display

excellent
photoelectric

properties

due

to

their

larger

surface

area,

increased
activity

centers,

and

more

effective

transmission

paths.
Acknowledgments
This

work

was

supported

by

the

National

Natural

Science
Foundation

of

China

(grant

no.

21171027).

The

authors

are

also
grateful

to

the

aid

provided

by

the

Science

and

Technology

Inno-
vative

Research

Team

in

Higher

Educational

Institutions

of

Hunan
Province.
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