controlled synthesis of 1d zno nanostructures via hydrothermal

Controlled

synthesis

of

1D

ZnO

nanostructures

via

hydrothermal
process
Di

Liu,

Yanfang

Liu,

Ruilong

Zong,

Xiaojuan

Bai,

Yongfa

Zhu
*
Department

of

Chemistry,

Tsinghua

University,

Beijing

100084,

PR

China
1.

Introduction
ZnO

is

a

potentially

useful

semiconductor

with

a

direct

band-
gap

of

3.37

eV

and

can

be

used

as

photocatalyst

to

destroy

the
organic

pollutants.

In

some

cases,

ZnO

may

exhibit

a

better
efficiency

than

TiO
2
in

photo-catalytic

degradation

for

some

dyes
[1].

Because

the

electrical,

optical

and

some

other

properties

of
nanocrystals

were

governed

by

the

sizes

and

shapes

[2],

much
works

have

been

done

to

fabricate

ZnO

nanostructures

of
controlled

shape

and

size.

Generally,

in

comparison

to

0D
nanostructures,

1D

nanostructures

have

less

grain

boundary,
surface

defect,

dislocation

etc.,

thus

leading

to

more

effective
carrier

transport

[3].

Therefore,

the

methods

to

fabricate

one-
dimensional

(1D)

ZnO

such

as

nanowires

and

nanorods

have
received

the

most

attention,

for

example,

the

high-temperature
physical

evaporation

[4],

the

template-induced

method

[5–7],
hydrothermal

synthesis

[8–11],

reverse

micelle

[12],

biominerali-
zation

method

[13],

precursors

induced

solution

phase

method
[14],

direct

calcination

of

zinc

acetate

[15]

were

used.

Among

all
these

methods,

the

simple

solution

synthesis,

by

thermal
treatment

of

the

reactant

in

different

solvents,

may

be

the

most
simple

and

effective

way

to

prepare

sufficiently

crystallized
materials

at

relatively

low

temperatures

[16–18].

Therefore,

it

is
necessary

to

elucidate

the

relationship

between

reaction

condi-
tions

and

ZnO

morphologies

and

explore

an

effective

and

simple

route

to

control

the

morphology

of

ZnO

for

practical

applications.
This

work

focuses

on

the

synthesis

of

1D

ZnO

with

the
hexagonal

system

wurtzite-type

structure.

A

wide

range

of

ZnO
nanostructures

with

various

shapes

were

obtained

by

adjusting
synthetic

parameters

such

as

precursor

concentration,

reaction
time,

solvent,

reaction

temperature,

annexing

agent,

alkali

source.
The

mechanisms

of

reaction

of

the

formation

of

ZnO

with

various
morphology

and

structure

are

investigated

respectively.
2.

Materials

and

methods
2.1.

Chemicals
All

chemicals

(analytical-grade

reagents)

were

purchased

from
Beijing

Chemicals

Co.

Ltd.

and

used

without

further

purification.
2.2.

The

preparation

of

ZnO

micro/nanostructures
ZnO

nanostructures

were

synthesized

by

using

a

simple
hydrothermal

method.

A

certain

amount

of

zinc

nitrate

hexahy-
drate

(Zn(NO
3
)
2
Á6H
2
O)

and

some

corresponding

amount

of

alkali
were

mixed

with

solvent

composed

of

distilled

water

and

ethanol
(v/v

=

1:2).

It

is

noteworthy

that

the

solvents

for

samples

in

Fig.

3
are

water-methanol

(v/v

=

1:2),

water-alcohol

(v/v

=

1:2),

water-n-
butyl

alcohol

(v/v

=

1:2)

respectively.

The

mixture

was

stirred

for
two

hours.

Then

a

certain

amount

of

annexing

agent

(CTAB/
PEG400)

and

were

transferred

into

the

mixture

under

stirring.
Materials

Research

Bulletin

49

(2014)

665–671
A

R

T

I

C

L

E

I

N

F

O

Article

history:
Received

13

December

2012
Received

in

revised

form

19

August

2013
Accepted

29

September

2013

Available

online

17

October

2013
Keywords:
A.

Nanostructures
A.

Semiconductors
B.

Chemical

synthesis
B.

Crystal

growth
A

B

S

T

R

A

C

T
ZnO

nanostructures

with

various

morphologies

and

sizes

were

successfully

prepared

via

a

hydrothermal
method.

The

influence

of

precursor

concentration,

reaction

time,

solvent,

reaction

temperature,
annexing

agent

and

alkali

source

on

the

morphology

and

structure

was

elucidated

systematically.

The
preferential

growth

of

ZnO

crystal

along

C-axis

direction

was

obvious

when

the

precursor

concentration
was

low.

A

balance

between

growth

and

dissolve

existed

during

the

process

of

forming

ZnO.

The

solvent
of

water-methanol

would

make

for

the

generation

of

products

with

a

big

size

and

regular

morphology.
The

adsorption

of

CTA
+
from

the

additive

of

cetyltrimethyl

ammonium

bromide

(CTAB)

and

hydroxyl
groups

from

polyethylene

glycol

(PEG400)

on

ZnO

would

slow

down

the

preferential

growth

rate

along
C-axis

direction,

leading

to

rod-like

products

with

smaller

aspect

ratios.

When

the

concentration

of
NaOH

was

increased

to

a

certain

degree,

the

growth

rate

of

ZnO

was

slower

than

the

decomposition

rate,
leading

to

appearance

of

irregular

grain-like

products.
ß

2013

Elsevier

Ltd.

All

rights

reserved.
*
Corresponding

author.

Tel.:

+86

10

62787601;

fax:

+86

10

62787601.
E-mail

address:



(Y.

Zhu).
Contents

lists

available

at

ScienceDirect
Materials

Research

Bulletin
jo

u

rn

al

h

om

ep

age:

ww

w.els

evier.c

o

m/lo

c

ate/mat

res

b

u
0025-5408/$

–

see

front

matter

ß

2013

Elsevier

Ltd.

All

rights

reserved.

/>Next,

the

solution

mixture

was

pretreated

under

an

ultrasonic
water

bath

for

30

min

and

then

was

hydrothermally

treated

at

a
given

temperature

for

certain

time

in

a

Teflon-lined

autoclave.
After

the

reaction,

the

white

precipitate

was

collected

and

washed
several

times

with

distilled

water

and

pure

ethanol.

Finally,

ZnO
were

obtained

after

dried

at

80

8C

for

8

h.
A

series

of

condition-dependent

experiments

were

carried

out
to

investigate

the

growth

process

of

ZnO.

Ultrasonic

pretreatment
can

promote

the

generation

of

a

certain

number

of

active

nucleus,
so

all

experiments

in

this

work

adopted

the

ultrasonic

pretreat-
ment

operation

so

as

to

accelerate

reaction

process

[11,19].

The
reaction

parameters

for

each

series

were

summarized

in

Table

1.
2.3.

Characterizations
The

morphologies

and

dimensions

of

the

as-synthesized

ZnO
nanostructures

were

observed

with

transmission

electron

micros-
copy

(TEM)

by

a

JEOL

JEM

2100

electron

microscope

operated

at

an
accelerating

voltage

of

100

kV.
2.4.

Photocatalytic

experiments
The

photocatalytic

performance

of

the

as-prepared

samples
was

evaluated

by

photocatalytic

degradation

of

methylene

blue
(MB)

under

UV

light

irradiation.

The

samples

(25

mg)

were
dispersed

in

the

50

ml

MB

aqueous

solution

(3

Â

10

À5
M).

The
mixed

suspension

was

magnetically

stirred

for

0.5

h

in

the

dark

to
reach

an

adsorption–desorption

equilibrium.

Under

ambient
conditions

and

stirring,

the

mixed

suspensions

were

exposed

to
UV

irradiation

produced

by

a

300

W

high

pressure

Hg

lamp

with
the

main

wave

crest

at

365

nm.

At

certain

time

intervals,

2

ml

of
the

mixed

suspension

were

extracted

and

centrifugated

to

remove
the

photocatalyst.

The

degradation

process

was

monitored

by
measuring

the

absorption

of

MB

in

filtrate

at

664

nm

using

UV–vis
absorption

spectrometer.
3.

Results

and

discussion
ZnO

nanostructures

with

various

sizes

and

morphologies

were
obtained

with

different

precursor

concentration,

reaction

time,
solvent,

reaction

temperature,

annexing

agent,

alkali

source

etc.
The

detailed

effects

of

these

conditions

are

shown

in

the

following
paragraphs.

Determining

these

effects

will

be

helpful

to

further
extend

the

scope

of

the

hydrothermal

synthesis

method

employed
in

this

work.
3.1.

Effects

of

precursor

concentration
Fig.

1

shows

the

TEM

images

of

the

ZnO

prepared

with

different
precursor

concentrations.

When

the

precursor

concentration

was
high

(>0.0167

M),

the

product

had

a

flake-like

structure

(Fig.

1a
and

b).

Long

spindle-like

products

formed

when

the

precursor
concentration

decreased

below

0.0067

M

(Fig.

1c

and

d).

The
diameters

were

in

the

range

of

180–370

nm

and

140–300

nm,

and
the

aspect

ratios

were

in

the

range

of

6–13

and

12–15

for

the
products

prepared

at

0.0067

M

and

0.001

M,

respectively.

It

is
considered

that

the

crystal

prefers

lateral

growth

when

the

zinc
source

is

abundant,

leading

to

the

appearance

of

flake-like

structure.

The

formation

of

the

spindle-like

structure

can

be
attributed

to

the

reactant

shortage

and

subsequent

limited

growth
during

the

late

period

of

the

reaction.

Because

the

preferential
growth

of

ZnO

crystal

is

along

C-axis

direction,

the

(0

0

0

1)

crystal
plane

which

has

a

rapid

growth

rate

would

be

covered

gradually,
forming

a

cuspate

shape

structure.

With

the

concentration

further
reduced,

diameter

of

rods

decreased

and

the

aspect

ratio

increased.
The

above

results

show

that

when

the

precursor

concentration

is
low,

the

preferential

growth

of

ZnO

crystal

along

C-axis

direction

is
obvious.

At

this

time,

because

of

the

low

concentration

of
reactants,

the

reaction

rate

is

slow

and

the

distinction

of

the
probability

that

growth

units

combine

to

each

crystal

plane

is

also
significant.

Then

the

crystal

grain

presents

a

polar

growth

trend,
leading

to

1D

morphology.

Our

proposition

is

consistent

with

the
previous

conclusion

[20].

At

the

same

time,

due

to

the

strict
monomer

diffusion,

the

crystal

growth

rate

is

restricted,

thus

the
product

size

is

much

smaller.
3.2.

Effects

of

reaction

time
TEM

images

of

ZnO

obtained

with

different

reaction

time

are
shown

in

Fig.

2.

There

exist

some

short

columnar

products

at

the
initial

stage

of

the

reaction

(1

h).

Their

average

diameters

are

in

the
range

of

180–300

nm.

It’s

important

to

note

that

some

hexagon
thin

slices

formed

within

1

h.

An

hour

later,

the

hexagon

thin

slices
began

to

deform,

the

short

columnar

structure

coarsened

and
lengthened

at

the

same

time.

The

aspect

ratio

increased

a

little
since

it

grew

faster

in

the

length

direction

than

in

the

diameter.

The

hexagon

thin

slice

seemed

to

dissolve

away

after

5

h.

Then

the
hexagon

slices

gradually

disappeared

when

the

reaction

time

is
prolonged

and

the

diameters

of

nanorods

increased

as

the

reaction
time

increased

from

5

h

to

16

h.

Based

on

Fig.

2,

it

is

concluded

that

there

exists

a

balance

of

growth-dissolve

during

the

formation
process

of

ZnO.

It

is

considered

that

the

crystal

prefers

lateral
growth

at

the

initial

stage

of

the

reaction

due

to

the

abundant

zinc
source,

leading

to

the

appearance

of

the

complete

hexagon

section
slice.

As

reaction

time

increases,

the

hexagon

thin

slices

gradually
dissolve

and

then

disappear,

the

short

columnar

structure

further
coarsens,

whereas

the

aspect

ratio

slowly

increases.
3.3.

Effects

of

solvent
All

products

shown

in

Fig.

3

present

flake-like

shape

when

the
adding

amount

of

additive

(PEG400)

was

set

at

5

ml

and

the
precursor

concentration

was

0.067

M.

ZnO

prepared

in

the

solvent
of

water-methanol

has

a

relatively

regular

morphology

which

is
close

to

rectangle

shape

with

the

shorter

length

of

about

500

nm.
However,

some

small

irregular

pieces

with

average

length

of
several

dozens

of

nanometers

and

smaller

length–width

ratios
were

obtained

when

the

solvent

changed.

These

observations
demonstrate

that

solvent

plays

an

important

role

in

the
morphology

of

product.

This

may

be

caused

by

the

different
Table

1
The

experimental

conditions

of

ZnO

nanostructure

preparation.
Designation

n(Zn
2+
):n(HMT)

C(Zn
2+
)

Solvent

(V
H2O
:V
ethanol
)

Reaction

temperature

(8C)

Reaction

time

(h)
Samples-Fig1

1:1

1:2

100

24
Samples-Fig2

1:1

0.0067

M

1:2

100
Samples-Fig3

1:1

0.067

M

150

24
Samples-Fig4

1:1

0.0067

M

1:2

16
Samples-Fig5/6

1:1

0.0067

M

1:2

100

16
Samples-Fig7

0.050

M

1:2

150

16
D.

Liu

et

al.

/

Materials

Research

Bulletin

49

(2014)

665–671
666
Fig.

1.

TEM

images

of

ZnO

with

different

precursor

concentrations:

(a)

0.067

M;

(b)

0.0167

M;

(c)

0.0067

M

and

(d)

0.001

M.
Fig.

2.

TEM

images

of

ZnO

samples

with

different

reaction

time:

(a)

and

(b)

1

h;

(c)

and

(d)

2

h;

(e)

5

h;

(f)

8

h;

(g)

12

h

and

(h)

16

h.
D.

Liu

et

al.

/

Materials

Research

Bulletin

49

(2014)

665–671

667
properties

of

each

solvent

such

as

polarity

of

solvent,

saturated
vapor

pressure,

adhesion

coefficient,

dissolving

capacity

to

salt

or
metal

ions,

solvability

and

so

forth

[18].

Firstly,

the

solvation

of
PEG400

is

obvious

when

water-methanol

is

used

as

solvent
because

of

its

strong

polarity.

When

the

PEG400

molecules

are
surrounded

by

solvent

molecules,

their

adsorption

on

the

surface
of

ZnO

grain

will

decrease.

In

addition,

the

solute

is

difficult

to
disperse,

thus

large

sheet-like

products

with

complete

morphol-
ogies

are

easy

to

form

in

relatively

concentrated

solute.

Secondly,
methanol

has

a

low

boiling

point,

and

the

reaction

temperature

in

this

experiment

has

reached

its

boiling

point.

As

a

result,

the
collision

and

coalescence

of

nucleuses

are

significant

in

the
violently

boiling

solution,

leading

to

product

with

large

size.

On

the
contrary,

ethanol

and

n-butyl

alcohol

both

have

lower

polarity

and
higher

boiling

points

compared

to

methanol,

thus

at

the

initial
stage

of

nucleation

when

the

temperature

is

below

the

boiling
point,

the

collision

and

coalescence

of

nucleuses

and

the

solvation
of

PEG400

are

not

strong

and

the

dispersion

of

solute

is

good,

thus
well-scattered

small

pieces

were

obtained.
3.4.

Effects

of

reaction

temperature
In

Fig.

4,

the

aspect

ratio

of

ZnO

obtained

at

different

reaction
temperature

has

little

change.

However,

the

aspect

ratio

of

product
decreases

and

the

quantity

of

long

spindle-like

product

decreases
at

lower

temperature,

which

indicates

that

the

growth

rate

along
C-axis

direction

weakens.

It

is

also

can

be

observed

that

when

the
reaction

temperature

is

reduced,

the

hydrolysis

velocity

(that

is

the
rate

of

releasing

OH
À
)

of

hexamethylenetetramine

(HMT)

slows
down,

the

property

of

crystal

and

growth

integrity

of

product

both
weaken.
3.5.

Effects

of

annexing

agent
Scheme

1

is

a

schematic

procedure

to

prepare

ZnO

nanos-
tructures

in

the

presence

of

annexing

agent.

As

shown

in

Scheme

1,
CTA
+

and

hydroxyl

groups

coming

from

the

additive

of

CTAB

and
PEG400

respectively

can

adsorb

on

the

surface

of

ZnO

crystal

grain
through

the

electrostatic

attraction

effect

with

the

growth

units

or
zinc

ion

in

solution.

However,

based

on

the

results

reported

before,
the

growth

rate

of

each

crystal

plane

of

ZnO

is

different

[21].

The
more

active

the

crystal

plane

is,

the

more

growth

units

or

zinc

ions
it

would

grasp.

As

a

result,

the

clapping

role

acted

by

CTAB

or
PEG400

molecules

realizes

the

regulation

of

crystal

growth.
Figs.

5

and

6

shows

TEM

micrographs

of

the

ZnO

crystals

grown
in

the

aqueous

solutions

with

certain

amount

of

additive.

As

shown
in

Fig.

5,

all

products

prepared

with

or

without

the

additive

of

CTAB
exhibit

rod-like

structure.

It’s

worth

noting

that

the

aspect

ratio

of
the

product

prepared

free

of

additive

is

about

6–13.

However,

the
aspect

ratio

decreases

to

2–5

after

adding

some

amount

of

CTAB.

At
the

same

time,

when

the

adding

amount

is

more

than

0.2

mmol,
the

diameters

of

the

products

increased

to

1

m
m.

CTAB

is

a

cationic
surfactant

which

can

be

completely

hydrolyzed

in

water

or
ethanol.

The

cation

generated

from

hydrolysis

has

a

tetragonal
structure

with

a

hydrophobic

long

chain

tail,

therefore

there

is

an
ion

pair

of

the

growth

elementary

Zn(OH)
4
2À
and

CTA
+
.

Thus

it

is
Fig.

4.

TEM

images

of

ZnO

prepared

at

different

reaction

temperature:

(a)

and

(b)

100

8C;

(c)

and

(d)

80

8C.
Fig.

3.

TEM

images

of

ZnO

prepared

in

different

solvent:

(a)

water-methanol;

(b)

water-alcohol;

(c)

water-n-butyl

alcohol.
D.

Liu

et

al.

/

Materials

Research

Bulletin

49

(2014)

665–671
668
considered

that

the

surfactant

molecules,

which

can

be

adsorbed
on

the

surface

of

zinc

oxide

during

the

crystallization

process,

have
the

following

two

roles:

structural

directing

agent

and

protectant
for

preventing

product

from

gathering.

CTAB

may

also

act

as

the
transmission

medium

or

the

modifying

agent

for

the

small

crystal
nucleus

in

the

initial

solution.

The

surfactant

molecules

adsorbed
on

the

surface

of

crystal

nucleus

can

be

seen

as

links

for

small
crystal

nucleus

gathering

into

crystal

nucleus

cluster

[22].

As

can
be

seen

from

Fig.

5(b–e),

CTAB

plays

a

role

of

limiting

domain,

and
the

CTA
+
is

much

easier

to

adsorb

on

the

active

(0

0

0

1)

crystal
plane

due

to

the

electrostatic

interaction

as

shown

in

Scheme

1(a).
Thus

the

preferential

growth

rate

along

C-axis

direction

is
weakened

to

some

extent,

leading

to

relatively

uniform

growth
rates

of

the

crystal

planes.
Fig.

6

shows

that

the

diameters

of

products

are

in

the

range

of
150–800

nm

when

the

adding

amount

of

PEG400

changes

from
Scheme

1.

Schematic

illustration

of

the

proposed

formation

mechanisms

of

ZnO.
Fig.

5.

TEM

images

ZnO

prepared

with

different

adding

volume

of

CTAB:

(a)

0;

(b)

0.02

mmol;

(c)

0.1

mmol;

(d)

0.2

mmol

and

(e)

0.4

mmol.
D.

Liu

et

al.

/

Materials

Research

Bulletin

49

(2014)

665–671

669
0

ml

to

2

ml.

However,

when

the

adding

amount

is

increased

to
5

ml,

the

diameter

of

the

product

increases

to

several

microns

and
the

aspect

ratio

decreases

obviously.

It

is

also

found

that

with

the
adding

amount

of

PEG400

increased,

long

spindle-like

structure
gradually

reduces,

quite

to

the

opposite,

long

columnar

structure
increases.

As

an

organic

polymer

with

a

long

non-polar

carbon
chain,

PEG

is

often

used

as

surfactant

to

control

the

growth

of
nanocrystals.

The

atom

O

in

the

entire

long

PEG400

molecule

has
coordination

abilities

with

metal

ions,

thus

it

has

a

relatively
strong

electrostatic

attraction

to

Zn
2+

.

Then

the

adsorption

of
PEG400

on

the

surface

of

ZnO

crystal

grain

would

make

the

activity
of

ZnO

particles

greatly

reduced

[9].

The

corresponding

illustration
can

be

seen

in

Scheme

1(b).

It

can

be

seen

in

general

that

PEG400
has

a

similar

function

as

CTAB

which

is

different

only

in

the

ways

of
adsorption

because

of

their

different

molecular

structures.
Solvation

effect

caused

by

pure

water

will

weaken

the

adsorption
of

PEG400

molecules

on

the

surface

of

ZnO

crystal

grain.

Thus,
water-ethanol

(V:V

=

1:2)

was

selected

as

solvent

to

avoid

the
solvation

effect.

3.6.

Effects

of

alkali

source
TEM

micrographs

of

ZnO

prepared

with

different

concentration
of

NaOH

are

shown

in

Fig.

7.With

the

molar

ratio

of

Zn
2+
to

NaOH
Fig.

6.

TEM

images

of

ZnO

with

different

adding

volume

of

PEG400:

(a)

0;

(b)

0.2

ml;

(c)

1

ml;

(d)

2

ml

and

(e)

5

ml.
Fig.

7.

TEM

images

of

ZnO

prepared

with

different

concentration

of

NaOH:

(a)

0.25

M

(Zn
2+
:NaOH

=

1:5);

(b)

0.4

M

(Zn
2+

:NaOH

=

1:8)

and

(c)

0.6

M

(Zn
2+
:NaOH

=

1:12).
D.

Liu

et

al.

/

Materials

Research

Bulletin

49

(2014)

665–671
670
increased

from

1:5

to

1:12,

the

morphology

of

product

turns
gradually

from

rod-like

to

grain-like

shape.

Based

on

the

crystal
growth

characteristics

of

ZnO,

the

product

presents

column-like
morphology.

When

the

pH

rises,

the

supersaturation

of

solution
which

can

affect

the

size

of

critical

nucleus

is

bigger,

thus

making
the

size

of

crystal

nucleus

much

smaller

at

the

stage

of

nucleation
and

leading

to

the

formation

of

products

with

small

size.

ZnO

is

a
kind

of

amphoteric

oxide,

so

when

the

concentration

of

NaOH

is
high,

the

growth

rate

of

ZnO

may

be

smaller

than

the

decomposi-
tion

rate

[23].

Thus

the

products

prepared

with

high

concentration
of

NaOH

present

irregular

grain-like

structure

due

to

the

larger
decomposition

rate

of

(0

0

0

1)

crystal

plane.

The

growth

of

crystal
experiences

a

process

of

dissolution-crystallization.

3.7.

Photocatalytic

activity
It

is

well

known

that

ZnO

has

been

used

as

a

semiconductor
photocatalyst

for

the

degradation

of

pollutants.

The

photocatalytic
activity

of

samples

in

Fig.

1

in

the

degradation

of

well-known
organic

dye

MB

was

presented

in

Fig.

8.

The

photocatalytic
properties

of

samples

in

Fig.

1,

from

Fig.

1a

to

d,

show

a

trend

which
is

decreased

at

first

and

then

increased.

From

the

comparison
between

Fig.

1a

and

b,

it

can

be

seen

that

the

former

has

a

better
crystalline

performance

which

also

means

that

it

would

have

less
bulk

defects.

The

bulk

defects

are

usually

seen

as

recombination
centers

of

photo-generated

electrons

and

holes.

Thus,

Fig.

1b

is
supposed

to

have

a

higher

probability

of

the

recombination

of

the
photo-generated

electron/hole

pairs

in

comparison

to

sample

a,
resulting

in

a

lower

photocatalytic

activity.

Fig.

1c

and

d

with
smaller

crystal

sizes

and

larger

specific

surface

area

were

obtained
by

further

reduce

the

concentration

of

reactants.

The

large

surface
area

supplies

more

active

sites

to

adsorb

MB,

and

then

facilitates
the

diffusion

and

mass

transportation

of

MB

molecules

and
hydroxyl

radicals

during

the

photochemical

reaction.

Moreover,
the

structures

of

nanoscale

favor

the

movement

or

transfer

of
electrons

and

holes

generated

inside

the

crystal

to

the

surface

[24],
which

also

helps

to

enhance

the

photocatalytic

activity

of

Fig.

1c
and

d

to

some

degree.

The

photocatalytic

activity

of

ZnO

is

also
strongly

dependent

on

the

surface

orientation

of

the

nanocrystals
which

could

result

in

orientation-dependent

charge-transfer
processes

[25,26].

Thus,

as

for

the

hexagonal

ZnO

nanorods
(Fig.

1c

and

d),

their

regular

surface

orientation

with

larger
emergences

of

{1

0

1

0}

and

{0

0

0

1}

surfaces

would

be

propitious

to

the

separation

of

the

photo-induced

electrons

and

holes,

leading
to

relatively

higher

photocatalytic

efficiencies.

However,

in
comparison

to

Fig.

1c

and

d

has

a

smaller

size

and

a

larger

specific
surface

area

which

result

in

a

higher

photocatalytic

performance.
4.

Conclusions
Summarizing,

ZnO

nanostructures

with

various

well-defined

morphologies,

such

as

columnar-,

long

spindle-,

flake-

and

grain-
like

samples

have

been

successfully

synthesized

by

adjusting
synthetic

parameters.

A

possible

mechanism

of

the

growth

process
of

the

ZnO

nanostructures

under

different

conditions

has

been
elucidated

systematically.

The

controllable

fabrication

also

pro-
vides

a

chance

for

studying

the

morphology-dependent

properties
of

ZnO

micro/nanocrystals.

Besides,

a

growth

mechanism

of

the
influence

of

surfactants

on

the

growth

of

crystal

was

discussed

in
detail.
Acknowledgments
This

work

was

partly

supported

by

the

Chinese

National

Science
Foundation

(20925725,

21373121)

and

National

Basic

Research
Program

of

China

(2013CB632403)

and

National

High

Technology
Research

and

Development

Program

of

China

(2012AA062701)
and

Special

Project

on

Innovative

Method

from

the

Ministry

of
Science

and

Technology

of

China

(2009IM030500).
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8.

Photocatalytic

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of

MB

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the

presence

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Sample-Fig.

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(C)

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/

Materials

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Bulletin

49

(2014)

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671