The effect of 1-N-alkyl chain of ionic liquids [Cnmim] +Br- (n = 2, 4, 6, 8) on the aspect ratio of ZnO nanorods: Syntheses, morphology, forming mechanism, photoluminescence and recyclable photocatalytic activity

Article abstract of DOI:10.1039/c1jm12095f

Using a pure ionic liquid-assisted solid-state-like synthesis method under 80 °C, high quality zinc oxide (ZnO) nanorods in 1-alkyl-3-methylimidazolium and bromide anions ionic liquids [C_nmim]⁺Br^- (n = 2, 4, 6, 8) were rea

Full text of DOI:10.1039/c1jm12095f

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links <
C
Journal of
Materials Chemistry
Cite this: J. Mater. Chem., 2011, 21, 15732
www.rsc.org/materials
PAPER
+
ꢀ
The effect of 1-N-alkyl chain of ionic liquids [C mim] Br (n ¼ 2, 4, 6, 8) on
n
the aspect ratio of ZnO nanorods: syntheses, morphology, forming
mechanism, photoluminescence and recyclable photocatalytic activity
a
a
b
a
b
c
Li Wang,* Li-Xian Chang, Lian-Qiang Wei, Shen-Zhi Xu, Ming-Hua Zeng* and Shi-Lie Pan
Received 12th May 2011, Accepted 28th July 2011
DOI: 10.1039/c1jm12095f
ꢁ
Using a pure ionic liquid-assisted solid-state-like synthesis method under 80 C, high quality zinc oxide
+
ꢀ
(
ZnO) nanorods in 1-alkyl-3-methylimidazolium and bromide anions ionic liquids [C
n
mim] Br (n ¼ 2,
4
, 6, 8) were realized. The size effect of the N-alkyl chains of ionic liquids hindered the further growth of
ZnO nanorods and the aspect ratio (length/diameter, L/D) of ZnO nanorods (from 40 : 1, 20 : 1, 10 : 1,
to 5 : 1) were realized by rationally tuning the length of the organic cation groups. Long octyl chains
ꢀ
ꢀ
(
9.93 A) resulted in short ZnO nanorods (with L/D ¼ 5 : 1), otherwise, short ethyl chains (2.53 A)
resulted in long ZnO nanorods (with L/D ¼ 40 : 1). The growth mechanism of the above ZnO nanorods
was mainly due to electrostatic interactions between cations of the ionic liquids and the ZnO crystal
nucleus, causing the preferential growth on the [0001] directions of the hexagonal ZnO. The as-obtained
short ZnO nanorods (with L/D ¼ 5 : 1) exhibited defect-related green-yellow emission under UV
excitation, and acted as effective recyclable photocatalysts during six photodegradation cycles of the
organic dye molecule Rhodamine B, suggesting promising potential for environmental applications.
This systematic study highlighted that the structures and morphology of ZnO nanomaterials could be
controllable via specifically varying the molecular structure of 1-alkyl-3-methylimidazolium cation-
based ionic liquids, and then with the tuned photocatalytic properties and optical and electronic
functions of them.
1
. Introduction
obtained materials is still poorly understood to date, and more
experiments are urgently needed to address these limitations.
Actually, ionic liquids based on dialkylimidazolium cations in
Recently, ionic liquids as a new solvent system have developed to
a focal point of interest due to their intrinsic advantages, such as
negligible vapor pressures, wide liquidus ranges, tunable solu-
bility for both organic and inorganic molecules, and synthetic
the synthesis of inorganic nanomaterials have many advan-
14,15
tages.
First, this kind of ionic liquid has a low interface
tension although polar (e.g. for 1-butyl-3-methylimidazolium
bromide, the g ¼ 38 mNm against air). Very small particles can
be generated since low interface tensions result in high nucleation
rates. Second, they are air- and moisture-stable, have high
thermal stability, are non-volatile, and weakly coordinating.
Third and the most importantly, the physical and chemical
properties of the ionic liquids can be easily varied over a wide
range by the selection of position-1, -2 and -3 alkylimidazolium
cations, which can be used to rationally design the structures, the
morphology and properties of the resulting nanomaterials. These
characters make alkylimidazolium cations ionic liquids an ideal
green reaction medium to be used in establishing the relationship
between molecular structure of ionic liquids and the morphol-
ogies of the resulting inorganic materials.
1–3
flexibility. In contrast on the great progress in exploring their
applications in organic reactions, transition catalysis, and elec-
4–6
trochemistry, etc., up to now, only limited examples have been
7
utilized in the area of the synthesis of inorganic nanomaterials
8
9
10
(
2 3 4 2 3
such as TiO , Fe S , a-Fe O , etc.). However, although a few
studies have attempted to explore how ionic liquids and their
mixed solvent systems influence the nucleation and growth of
inorganic nanomaterials by varying the cations or anions of the
11–13
ionic liquids,
the correlation of the modulated effect of pure
ionic liquids and the structure–function relationship of the as-
a
College of Chemistry and Chemical Engineering, Xinjiang Normal
University, Urumqi, 830054, P R China. E-mail: wangli9805@yahoo.
com.cn
b
On the other hand, the design and rational synthesis of inor-
ganic nanomaterials with controlled structure characteristics has
drawn great attention due to their strongly determined physical
properties for optical, electrical, magnetic, and catalytic appli-
Key Laboratory for the Chemistry and Molecular Engineering of
Medicinal Resources (Ministry of Education), School of Chemistry &
Chemical Engineering of GuangXi Normal University, Guilin, 541004,
P. R. China. E-mail: zmh@mailbox.gxnu.edu.cn
c
Xinjiang Technical Institute of Physics & Chemistry, Chinese Academy of
Sciences, 40-1 South Beijing Road, Urumqi, 830011, China
16–18
cations.
Zinc oxide is a direct wide-band-gap semiconductor
1
5732 | J. Mater. Chem., 2011, 21, 15732–15740
This journal is ª The Royal Society of Chemistry 2011

View Article Online
and is suitable in a wide range of optical and electronic appli-
19
cations. In particular, their potential use in photocatalysis (e.g.,
photolysis of water to generate hydrogen, photodecomposition
of organics) has aroused great interest, as there have been several
examples of ZnO displaying more impressive photocatalytic
20,21
activity than the widely studied titanium dioxide.
Photo-
catalytic properties of ZnO are heavily influenced by surface
area, morphology, primary and secondary crystallite size distri-
bution, nature and concentration of defects and impurities. Thus,
enormous efforts have been made to explore expedient synthetic
ways to ZnO with different structure, morphology and tailor-
made hierarchical organization. However, to the best of our
knowledge, examples of varying the types of cations of ionic
liquids based on dialkylimidazolium cations to probe the rela-
tionships between the structure of ionic liquids and the function
Scheme 1 Illustration for the variety of the 1-N-alkyl chain length of
+
ꢀ
ionic liquids [C
n
mim] Br (n ¼ 2, 4, 6, 8).
acetate was added with thorough mixing. The ethyl acetate was
decanted followed by the addition of fresh ethyl acetate and this
step was repeated twice. After the third decanting of ethyl
acetate, any remaining ethyl acetate was removed by a Rotava-
por. Before use, the ionic liquids were further vacuum-dried
several times at a temperature above their melting points. The
14,22,23
of ZnO nanomaterials are especially very limited.
In this contribution, we report the controlled-assembly of ZnO
nanorods with different aspect ratio via a very simple, one-step,
ꢀ
+
pure [C
n
mim] Br (n ¼ 2, 4, 6, 8) ionic liquid assisted solid-state
route under low temperature, through varying the size of dia-
lkylimidazolium cations; furthermore, we describe the forming
mechanism, photoluminescence and recyclable photocatalytic
activity by as-obtained short ZnO nanorods.
1
H-NMR results of other ionic liquids agreed with the
24
literature.
Preparation of the samples. As shown in Scheme 2, in a typical
procedure of sample 1, 4.0 mmol Zn(CH COO) $2H O were
ground for about 20 min in an agate mortar, followed by the
2
. Experimental section
3
2
2
2
.1 Reagents and characterization
+
ꢀ
addition of 1 mL ionic liquid [C mim] Br and 24.0 mmol NaOH
2
All the reagents are of analytical grade and were purchased
without further purification. X-Ray powder diffraction (XRD)
measurements were recorded on a D/Max-2500 X-ray diffrac-
tometer using Cu-Ka radiation (l ¼ 0.154 nm). The X-ray
photoelectron spectra (XPS) were collected on a Kratos Axis
Ultra DLD multi-technique X-ray photoelectron spectroscope,
using a monochromated Al-Ka X-ray source. The transmission
electron microscopy (TEM) images and high-resolution trans-
mission electron microscopy (HRTEM) images and selected area
electron diffraction (SAED) patterns were taken on an FEI
TECNAI G2 20 S-TWIN microscope. The scanning electron
microscopy images (SEM) were taken on a Hitachi S-3500N
SEM, JEOL-6700F FSEM, and LEO1430VP scanning electron
microscope. Energy-disperse X-ray analyses (EDX) were per-
formed on an EDX G2T20136-5 energy-disperse X-ray instru-
ment. UV-vis absorption was monitored with a U-3310
spectrophotometer. The room-temperature photoluminescence
spectra were recorded on a SPEX FL 212 spectrometer with
a Xenon lamp. FTIR spectroscopy measurements were recorded
with a Bruker Optics Instrument type Tensor 27 spectropho-
tometer. The surface area at the mesoscale was measured using
a Quantachrome NOVA 4200e analyzer and the surface area was
calculated from the linear part of the BET plot.
fine powders. The mixture was ground for 30 min and heated at
ꢁ
80 C for 48 h to ensure the completeness of reaction. Then the
mixture was washed with alcohol and water, and dried in the air.
Yield: 88%.
A procedure similar to that employed for sample 1, except that
+
+
mim] Br was replaced by [C
ꢀ
ꢀ
ionic liquid [C
2
n
mim] Br (n ¼ 4, 6,
8). As for the synthesis of sample 2, yield: ca. 86%, the ionic
+
ꢀ
liquid is [C
ionic liquid is [C
ca. 82%, ionic liquid is [C
4
mim] Br ; the synthesis of sample 3, yield: ca. 83%,
+
ꢀ
mim] Br ; and the synthesis of sample 4, yield:
6
+
mim] Br . Other synthetic conditions
ꢀ
8
were identical with the synthesis of sample 1.
2.3 Photocatalytic activity experiments
For photocatalytic activity measurement, 40 mg of ZnO nano-
ꢀ5
ꢀ1
rods was added to 100 mL of a 10 mol L Rhodamine B
aqueous solution and then magnetically stirred in the dark for 15
min, which allowed it to reach adsorption equilibrium and
uniform dispersity. The solution was then exposed to UV irra-
diation from a 125-W high-pressure Hg lamp with the strongest
emission at 365 nm at room temperature. After a given irradia-
tion time, about 5 mL of the mixture was withdrawn, and the
catalysts were separated from the suspensions by centrifugation.
The degradation process was monitored through a wavelength
scan using a U-3310 spectrophotometer.
2
.2 Preparations
Preparation of ionic liquids [C mim] Br (n ¼ 2, 4, 6, 8) .
+
ꢀ
24
For the examination of the ZnO nanostructures as recyclable
photocatalysts, ZnO in the centrifugal tube and beaker was
collected and combined together, then was subsequently
immersed into fresh Rhodamine B aqueous solution at the same
concentrations for another cycle of examination. The above
procedure was performed for 6 cycles to evaluate the suitability
n
(
Scheme 1) Equal molar amounts of freshly distilled alkyl
bromide and 1-methylimidazole were added to a round-
bottomed flask equipped with a reflux condenser for 24–72 h at
ꢁ
5
0–80 C with stirring until two phases formed. The top phase,
containing unreacted staring materials, was decanted and ethyl
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 15732–15740 | 15733

View Article Online
Scheme 2 The process of ionic liquid-assisted low-temperature solid-state approach for sample 1.
of the ZnO nanostructures for use in the photodecomposition of
organic dye molecules.
morphology of them. Furthermore, this method needs low
ꢁ
temperature, only 80 C, and a small amount of ionic liquids, but
has a high yield of up to ca. 82–88%. This method thus has the
distinct advantages of simple high efficiency, low energy
consumption, convenient operation, and lack of pollution.
3
. Results and discussions
3
.1 Preparation method
3
.2 Structure and composition determination
As is well known, the synthesis of ZnO with different morphol-
ogies using or in the presence of ionic liquids mainly includes
three kinds. (1) Using a mixture of water and ionic liquids as
solvents. As such, Zhu and Wang synthesized flowerlike and
needlelike ZnO structures by heating an aqueous solution of Zn
Fig. 1 shows the corresponding XRD patterns of the samples 1–
4. All the XRD patterns could be indexed as the hexagonal
3
wurtzite ZnO (space group P6 mc, JCPDS Card No. 36-1451).
No characteristic peaks of any impurities are detected in the
patterns, demonstrating that all of the samples have high phase
purity. It is also found that the patterns of (0002) facet from
samples 1–4 increase gradually, indicating there is exposure of
more polar (0002) faces.
(
NO
additive.
morphological ZnO structures by microwave heating in
3
)
2
and NaOH in a microwave oven, with [Bmim][BF
4
] as an
25
Analogously, Cao et al. synthesized various
+
ꢀ 23
a mixture of water and ionic liquids such as [Bmim] Cl , and
ꢀ 26
+
Bmim] BF
ꢀ 23
+
OHmim] Cl ,
+
OHmim] BF
ꢀ 27
[
4
,
[C
2
[C
2
4
.
(2)
Fig. 2 shows the typical XPS spectra taken from Zn and O
regions of sample 1. The binding energies obtained in the XPS
analysis are corrected for specimen charging by referencing the
C1s to 285.2 eV. A strong peak at about 1021.9 eV can be
attributed to Zn2p3/2 (Fig. 2a). The peak profile of O1s is asym-
metric; the peak at about 530.2 eV is indexed to the O (ꢀ2) in the
ZnO, whereas the weaker shoulder peak at about 532.2 eV is due
to chemisorbed oxygen caused by surface hydroxyl (Fig. 2b). No
obvious peaks for other elements or impurities were observed.
The atomic composition of Zn and O (excluding OH) was
calculated by using the integrated peak area and sensitivity
factors, and the atomic ratio of Zn : O is 1 : 1.08, very close to the
composition of ZnO.
Ionic-liquid precursors method. The character to this synthesis
methodology was the use of metal-containing ionic liquids or
alkaline ionic liquids that acted both as solvents and reactants in
the synthesis process. For example, Dai and co-workers recently
employed Zn(L) (NTf ) -type ionic liquids, which can serve as
4
2 2
both solvents and metal-oxide precursors, for manufacturing
22
nanostructured zinc oxide under the ionothermal condition.
Taubert and colleagues reported heating mixture of zinc acetate
and
tetrabutylammonium
hydroxide
(TBAH,
28,29
N
(
C
4
H
9
)
4
OH$30H O) to obtain hollow ZnO mesocrystals.
2
The
character to this synthesis methodology was the use of metal-
containing ionic liquids or alkaline ionic liquids as the zinc
source or alkali source that acted both as solvents and as reac-
tants in the synthesis process. (3) Using neat ionic liquids as
solvents. Xie and co-workers synthesized wurtzite ZnO hexag-
ꢁ
onal micro-pyramids at 286 C, using a mixture of oleic acid and
ethylenediamine as solvent. They regard this mixture as one kind
ꢀ
+ 30
of ionic liquid (R–COOH + R–NH
2
¼ R–COO + R–NH
3
).
However, to make clear how the self-assembly process in ionic
liquids differs from those in conventional solvents and how ionic
liquids interact with inorganic or organic species, reactants, and
products, it is necessary to build up a pure ionic liquids solvents
system in the syntheses inorganic materials.
In this paper, we merely depend on the simple combination of
Zn(CH COO) $2H O and NaOH solid power in a pure ionic
3
2
2
liquids solvents system and ordinary low-temperature grinding,
rather than the traditional tedious procedures and high reaction
temperatures. The synthesis in a pure ionic liquids solvents
system offers a good opportunity to make clear how varying the
organic groups in ionic liquids and their effect influence the
nucleation and growth of ZnO nanomaterials; and the final
Fig. 1 XRD patterns of (a) sample 1; (b) sample 2; (c) sample 3; (d)
sample 4.
1
5734 | J. Mater. Chem., 2011, 21, 15732–15740
This journal is ª The Royal Society of Chemistry 2011

View Article Online
Fig. 2 XPS spectra of the sample 1 (a) Zn2p3/2 region. (b) O1s region.
Fig. 4 (a) TEM images of sample 2; (b) TEM images of sample 3. The
insets show the size distribution histogram.
3
.3 Morphologies of samples prepared in ionic liquid
+
n
ꢀ
[
C
mim] Br (n ¼ 2, 4, 6, 8)
Morphologies of ZnO prepared in different ionic liquids under
similar reaction conditions (the reaction temperature kept as
ꢀ
ꢁ
2+
0 C and the the molar ratio of [OH ]/[Zn ] as 6 : 1), in which
8
the reactions proceed rather completely, have been paralleled.
ꢀ
+
When [C mim] Br is employed, the product 1 is comprised of
2
long nanorods with a length of 1500–2500 nm and average
diameter of 50–100 nm (Fig. 3a). The EDX image recorded from
the same region also confirms that the components of the
products are only Zn and O with a ratio of 1.02 : 1 (inset in
Fig. 3a). The Cu signal is attributed to the copper meshes while
the C signal is attributed to the membrane for HRTEM. The
HRTEM image of the edge of the single nanorod (Fig. 3b)
marked by a square in Fig. 3a shows that the interplanar spacing
of the lattice stripes is 0.261 nm and 0.162 nm, respectively,
Fig. 5 (a) TEM images of sample 4 (the insets show the size distribution
histogram); (b) HRTEM images of single ZnO nanorod.
+
increased to octyl ([C mim] Br ), the product is comprised of
ꢀ
8
ꢁ
corresponding to the distance of (0001) planes and (1120), indi-
short nanorods with an average length of 200 nm and diameter of
cating that the ZnO nanorod exhibits a preferred growth orien-
tation along the [0001] crystal axis. The electron diffraction
pattern recorded on the region marked by the square along [0001]
axis, confirmed that it is a single crystal.
4
0–60 nm (Fig. 5, sample 4).
The morphologic variety with different ionic liquids is further
supported by scanning electron microscopy (SEM) observations
Fig. 6). Fig. 6a shows long ZnO nanorods with an average
length of 1500–2500 nm and average diameter of 50–100 nm
(
+
ꢀ
When the 1-N-alkyl chain of ionic liquids [C
+
n
mim] Br is
4
mim] Br ), the length of the as-
ꢀ
+
ꢀ
increased from ethyl to butyl ([C
prepared in ionic liquid [C
2
mim] Br . Fig. 6b shows the SEM
obtained ZnO nanorods slightly decreased to 1000–1500 nm with
image of the shorter ZnO nanorods with an average length of
the average diameter still being 50–100 nm (Fig. 4a, sample 2);
1
000–1500 nm and average diameter of 50–100 nm prepared in
+
+
when the 1-N-alkyl chain of ionic liquids [C mim] Br is further
ꢀ
ꢀ
n
4
ionic liquid [C mim] Br . When the 1-N-alkyl chain of ionic
+
liquids [C mim] Br was increased to hexyl, short ZnO nanorods
+
increased to hexyl ([C mim] Br ), the length of the as-obtained
ꢀ
ꢀ
6
n
ZnO nanorods rapidly decreased to 500–1000 nm with the
average diameter still being 50–100 nm (Fig. 4b, sample 3); when
+
ꢀ
n
the 1-N-alkyl chain of ionic liquids [C mim] Br was finally
Fig. 3 (a) TEM images of sample 1 and the EDX image from the region
marked by rectangle (inset); (b) HRTEM images of single ZnO nanorod
region marked by the square in part (a), and the fast Fourier transform
electron diffraction images (inset).
Fig. 6 SEM images of (a) sample 1; (b) sample 2; (c) sample 3; (d)
sample 4.
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 15732–15740 | 15735

View Article Online
Table 1 Experimental conditions and morphology of the samples 1–4
ꢁ
ꢀ
2+
[OH ]/[Zn ] (molar atio)
Sample
Reaction T/ C
Morphology
Diameter/nm
Length/nm
Ionic liquid
+
mim] Br
ꢀ
ꢀ
ꢀ
ꢀ
1
2
3
4
80
80
80
80
6
6
6
6
Nanorods
Nanorods
Nanorods
Nanorods
50–100
50–100
50–100
40–60
1500–2500
1000–1500
500–1000
200
[C
[C
[C
[C
2
4
6
8
+
mim] Br
+
mim] Br
+
mim] Br
2
+
ꢀ
2ꢀ
4
with an average length of 500–1000 nm and average diameter of
0–100 nm were obtained (Fig. 6c). Fig. 6d shows that the
product is comprised of short nanorods with an average length of
Zn + 4OH / [Zn(OH) ] ;
5
2
00 nm and an average diameter of 50 nm prepared in ionic
ꢀ
2ꢀ
+
mim] Br . This experimental result indicates that the
2
[Zn(OH)4] / ZnO + 2H O
liquid [C
8
aspect ratio of 1D ZnO nanostructures decreases as the length of
the N-alkyl chain on the imidazolium cation increases. The
detailed synthetic conditions and morphology of the samples 1–4
were listed in Table 1.
After nucleation, the particles grow by diffusion of reactant units
to the surface of the particles. Low interface tensions of ionic
liquids result in high nucleation rates, and thus very small
particles can be generated which undergo weak Ostwald
ripening.
It is notable that blank experiments without any solvents and
parallel experiments in water, methanol, and ethanol as solvents
have also been done. No ZnO nanorods, only irregular particles
with different diameters, were obtained which confirmed that
ionic liquids play an important role in the formation of ZnO
nanorods with different aspect ratios.
(
ii) Zn-teminated (0001) polar faces adopting preferential
growth. Hexagonal ZnO crystals are described as a number of
alternating planes composed of four-fold tetrahedrally-coordi-
2ꢀ
2+
nated O and Zn ions stacked alternatively along the c-axis.
The oppositely-charged ions produce positively-charged Zn
ꢁ
0001) and negtively-charged O (0001) surfaces, resulting in
(
3
.4 Formation mechanism
a normal dipole moment and spontaneous polarization along the
Good understanding of the formation mechanism of inorganic
materials in ionic liquids is critically necessary for future inves-
tigations into this new class of solvents in inorganic applications.
The forming mechanism of ZnO nanorods in ionic liquids is
related to a number of important processes that should be dis-
cussed. On the basis of the following discussion, the growth
process of ZnO nanorods can be illustrated in Scheme 3.
c-axis, as well as a divergence in surface energy. Besides the polar
ꢁ
surfaces, ZnO has other typical non-polar facets: (1010), (1010),
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
(1100), (1100), (0110), and (0110) facets. The polar and non-
polar surfaces make up the ZnO hexagonal prism, as shown in
the schematic model (the upper right corner of Scheme 3).
During the crystal growth, the polar surfaces usually appear as
growing surfaces because of their high surface energy during the
crystal growth. To tune the growth behavior of ZnO nano-
structures, the surface energy of the polar planes can be reduced
by compensating the surface charge with ionic liquids. Due to the
(
i) Very small particles can be generated in ionic liquids.
ꢀ
2+
When the molar ratio of Zn to OH is higher, the initial reac-
tion mixture may be represented by the reaction equations:
Scheme 3 Illustration for the growth process of ZnO nanorods
5736 | J. Mater. Chem., 2011, 21, 15732–15740 This journal is ª The Royal Society of Chemistry 2011
1

View Article Online
+
ꢀ
2
Fig. 7 FTIR spectra of (a) pure [C mim] Br and (b) ZnO nanorods/
+
mim] Br .
ꢀ
[
C
2
Fig. 8 The average aspect ratio of the ZnO nanostructures as function of
+
ꢀ
n
the number of the n of ionic liquid [C mim] Br in the reaction system.
electrostatic interaction between the H–C(2) of the imidazole
2ꢀ
2
ꢀ
ꢁ
ring of ionic liquids and O -terminated (0001) planes and O
2+
ꢁ
and Zn co-terminated non-polar facets (1010), (1010), (1100),
ꢁ
ꢁ
(iii) Correlations between molecular structures of ionic
liquids and aspect ratio of ZnO nanorods. Scheme 3 shows the
illustration for the variation of the 1-N-alkyl chain length of ionic
ꢁ
1100), (0110), and (0110) facets, the surface energies of these
ꢁ
ꢁ
(
planes decrease greatly, resulting in a relatively slow growth rate
+
ꢀ
2+
liquids [C
n
mim] Br (n ¼ 2, 4, 6, 8) and Fig. 8 shows the corre-
and thereby exposing the Zn -teminated (0001) polar faces for
preferential growth.
lation between 1-N-alkyl chain of ionic liquids and the aspect
ꢀ
ratio of ZnO nanorods: long octyl chains (9.93 A) chains resulted
The statement about the forming mechanism of ZnO nanorods
in ionic liquids is supported by FTIR (Fig. 7, Table 2). The bands
in short ZnO nanorods (with L/D ¼ 5 : 1), otherwise, short ethyl
ꢀ
ꢀ1
+
ꢀ
chains (2.53 A) resulted in long ZnO nanorods (with L/D ¼
at 3151 cm in pure [C
2
mim] Br (Fig. 7a) are assigned to the
4
0 : 1).
H–C(4) and H–C(5) of the imidazole ring stretching vibration.
ꢀ
ꢀ
1
+
The bands at 3089 cm in pure [C mim] Br are assigned to the
A possible reason is that during the formation of ZnO nano-
+
2
ꢀ
8
rods in long N-alkyl chain ionic liquids, such as [C mim] Br ,
H–C(2) of the imidazole ring stretching vibration. The bands
ꢀ1
weak interactions also exist between the H–C(2) of the imidazole
ꢁ ꢁ
around 2989 cm are assigned to the C–H bonds of alkyl chains
ꢀ1
ring of ionic liquids and (0001) planes and non-polar (1010),
ꢁ ꢁ ꢁ ꢁ ꢁ
1010), (1100), (1100), (0110), and (0110) facets, which makes the
attached with C(1) and C(3). The bands at 1631 cm are
(
assigned to the C]C stretching vibration, while the bands at
ꢀ1
2+
ꢀ1
preferential growth of Zn -teminated (0001) polar faces realized.
However, in the process of further growth of long ZnO nano-
rods, the long alkyl chains of ionic liquids tied the top and side
faces of ZnO nanorods due to the weak interaction between the
1
573 cm and 1178 cm are due to the imidazole ring skeleton
stretching vibration and inplane deformation vibration respec-
+
3
1
ꢀ
tively. Compared with the pure [C
2
mim] Br , several significant
changes are observed in the mixture of ZnO nanorods and
ꢀ
2
ꢀ
ꢁ
ionic liquids and O -terminated (0001) planes and O and Zn
2ꢀ
2+
+
2
[C mim] Br (Fig. 7b). The relative intensity of the H–C(2) of the
ꢁ
co-terminated non-polar facets (1010), (1010), (1100), (1100),
ꢁ
ꢁ
ꢁ
ꢀ1
imidazole ring stretching vibration band at 3089 cm is weak-
ꢁ
0110), hindered the further growth of ZnO nanorods, and thus
(
ened. Besides, the band of the C]C stretching vibration at 1631
ꢀ1
relatively short and thin ZnO nanorods were obtained.
cm almost disappears. The above changes of bands demon-
strate that the formation of ZnO has an effect on the electron
cloud density of the imidazole ring. Based on the analysis of
FTIR spectra, we can conclude that there are interactions
3
.5 Energy band structure and optical properties
between ionic liquids and ZnO, and the interactions focus on the
+
mim] may
ZnO is a direct band gap semiconductor, according to Tauc
H–C(2) of the imidazole ring of ionic liquids. The [C
2
2
equation: (ahn) ¼ A (hn ꢀ E
coefficient is related to the excitation energy (Eexc ¼ hn) by (ahn)
const (hn ꢀ E ), where E is the band gap energy. To obtain the
g
), and therefore its absorption
act as a template to provide effective sites for the nucleation and
growth of ZnO nanorods.
2
g
g
+
mim] Br and ZnO/[C
ꢀ
+
ꢀ
Table 2 Frequencies of FTIR absorption bands for the pure [C
2
2
mim] Br
ꢀ
1
Frequencies of absorption band/cm
+
mim] Br
ꢀ
+
ꢀ
Pure [C
2
2
ZnO nanorods/[C mim] Br
Assignments
3
3
2
1
1
151
089
989
631
3152
3085
2988
H–C(4) and H–C(5) of imidazole ring stretching vibration
H–C(2) of imidazole ring stretching vibration
C–H bonds of alkyl chains attached with C(1) and C(3)
C]C stretching vibration
Imidazole ring skeleton stretching vibration and inplane deformation vibration
573, 1178
1571, 1176
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 15732–15740 | 15737

View Article Online
passive and reducing agents may be helpful for the formation of
oxygen vacancy defects under low temperature conditions. Thus
the green-yellow emission broad band around 560 nm in this case
may be the result from radiative recombination of a photo-
generated hole with an electron occupying the oxygen vacancy.
And the smaller particle size may result in larger the oxygen
vacancy content, the higher probability of exciton occurrence,
and the stronger green-yellow emissions.
3
.6 Photocatalytic activities and mechanism
Fig. 9 (a) Room-temperature UV-vis absorption spectra of ZnO
2
samples 1 and 4; (b) the corresponding plot of (ahn) versus photon energy
To understand the correlation between the morphology and
photocatalytic property of ZnO nanostructures, Rhodamine B is
used as a probe molecule to investigate the photocatalytic
performance. Fig. 11(A) shows the percent of remaining
concentration of Rhodamine B with respect to time in the pres-
ence of samples 1 and 4 monitored at the maximum absorbance
of 554nm. Fig. 11(B) shows photos of the photodegradation of
Rhodamine B aqueous solutions in the presence of sample 4. It
was found that the degradation efficiency of short ZnO nanorods
hn.
2
absorption onset, (ahn) was plotted versus energy hn. Extrapo-
lation of the linear part until its intersection with the hn axis gives
g
the values of E . The room temperature UV-vis absorption
spectra of ZnO samples with typical morphologies are shown in
2
Fig. 9, and Fig. 9(b) shows the corresponding plot of (ahn)
versus photon energy hn. The band gaps of sample 1 (long
nanorods) and 4 (short nanorods) are almost the same, at 3.22
and 3.29 eV, respectively.
(
sample 1) was higher than that of long ZnO nanorods (sample
). We also plot the change in the concentration of RB without
4
catalyst and without UV irradiation, which indicates that the
catalyst and light are essential for photocatalytic degradation.
It is generally accepted that, when a photon with energy of hv
matches or exceeds the band gap energy of the semiconductor, an
electron is exited from the valence band to the conduction band,
leaving a hole behind. The photoelectron can be easily trapped
Room temperature photoluminescent spectra of the ZnO
nanorods with different morphologies excited at 325 nm is shown
in Fig. 10. A weak peak around 390–440 nm and a broad and
intense green-yellow emission around 560 nm are observed in the
products, respectively, which could be attributed to the near
band-edge emission of the intrinsic wide band gap of ZnO,
by electronic acceptors like adsorbed O , and further produce
2
14
resulting from the recombination of excitonic centers. So far,
the origin of the visible emission is highly controversial. Taking
the green-yellow emission, for example, a number of hypotheses
have been proposed, such as a singly ionized oxygen vacancy, an
oxygen antisite, and a zinc vacancy, etc. Recently, Zheng et al.
proposed that reducing agents may be helpful for the formation
of oxygen vacancy defects by theoretical and experimental
analysis, and the higher the annealing temperature that is
adopted, the more surface oxygen vacancy defects should be
eliminated and the more interstitial oxygen defects be generated,
in respect of the competition between oxygen vacancy defects
ꢀ
a superoxide radical anion (O _ ), whereas the photo-induced
2
ꢀ
holes can be trapped by electronic donors, such as OH , to
produce OH_ radicals. Hydroxyl radicals, due to their high
ꢀ
oxidizing potential (2.80 V), together with O
2
_ , react with the
33
majority of the organic substrates, till complete mineralization.
32
and interstitial oxygen defects. In this case, the ZnO nano-
structures are synthesized in an environment of passive and
reducing agents of ionic liquid under a low temperature envi-
ꢁ
ronment (80 C), without post-annealing. The presence of
Fig. 11 (A) The percent of remaining concentration of Rhodamine B
with respect to time in the presence of samples 1 and 4 monitored at the
maximum absorbance of 554 nm. (B) Photos of photodegradation of
Rhodamine B aqueous solutions by sample 4.
Fig. 10 Room-temperature photoluminescent spectra of ZnO samples 1
and 4.
1
5738 | J. Mater. Chem., 2011, 21, 15732–15740
This journal is ª The Royal Society of Chemistry 2011

View Article Online
Several stages may give a significant contribution to the
different photocatalytic activity between samples 1 and 4, several
important factors are discussed.
of superficial inorganic species and organic adsorbates on the
ZnO nanorods at the beginning. After the first cycle, the inor-
ganic species were desorbed from the surface and the organic
adsorbates degraded, resulting in the accelerating degradation
(
i) Surface areas may be of the primary relevance. Sample 1 is
2
comprised of long nanorods with a BET surface area of 23.50 m
34
rate in the following second and third degradation cycles.
ꢀ
1
g
while sample 4 is comprised of short nanorods with a BET
ꢀ1
However, after the third cycle, a slight decrease in the photode-
composition rate was observed due to a possible loss of some
nanorods during the recycling process.
2
surface area of 32.10 m g . The higher the surface areas that the
ꢀ
catalyst possesses, the more reactants (e.g., O
2
, OH , and organic
molecules) are efficiently adsorbed on the surface, thus leading to
higher photocatalytic activity.
4. Conclusions
(
ii) Oxygen vacancies are of importance. Well-defined ZnO
short nanorods (sample 4) with more surface oxygen vacancy
defects working as electron acceptors can trap the photo-
generated electrons temporarily to reduce the surface recombi-
nation of photogenerated electrons and holes, thus benefitting
the efficient separation of electron–hole pairs to minimize the
waste of photo-energy.
In summary, one-dimensional ZnO nanostructures with vari-
able-aspect-ratios are successfully prepared through a pure ionic
liquid-assisted solid-state method under low-temperature. Three
fascinating features are described in the present work. (i) This
ꢁ
method needs low temperature, only 80 C, and a small amount
of ionic liquids, but has a high yield of final products of up to 80–
90%. Merely depending on the simple combination of common
compounds and ordinary low-temperature grinding, rather than
the traditional tedious procedures and high reaction tempera-
tures, this method thus has the distinct advantages of high effi-
ciency, low energy consumption, convenient operation, and lack
of pollution, having the potential to be made generally appli-
cable. (ii) Rationally tuning the 1-N-alkyl chain length of the
(
iii) Highly reactive (0001) faces have a significant contri-
bution to photocatalysis. There is exposure of a greater
proportion of polar (001) faces in sample 4, which can be verified
by XRD images. The (0001) facets of the hexagonal ZnO short
nanorods have particular and specific catalytic activities in
ꢀ
photocatalytic decomposition reactions. For OH ions could
preferentially adsorb onto Zn-terminated (0001) face of ZnO
because of static electronic interaction. This would lead to
a greater rate of production of OH_radicals, and hence favors the
degradation of the dye during the photocatalysis.
+
ꢀ
cations of the ionic liquids [C mim] Br (n ¼ 2, 4, 6, 8) to probe
n
the structure–function relationships, establish the correlations
between molecular structures of the adopted ionic liquids
ꢀ
+
[C mim] Br and the morphology-dependent photochemical
With the successful exhibition of short ZnO nanorods (with
L/D ¼ 5 : 1) as effective photocatalysts, their use as recyclable
photocatalysts was further studied. Fig. 12(a) shows the UV-vis
absorption spectra of Rhodamine B as a function of irradiation
time in the presence of sample 4 over 6 cycles. Fig. 12(b) shows
the tests of absorbance changes as a function of irradiation time
in the presence of sample 4 over 6 cycles for Rhodamine B
monitored at the maximum absorbance of 554 nm. We can see
that almost no decrease but a slight wave in the photodecom-
position rate was observed during 6 cycles. Interestingly, the
degradation rate for the as-prepared short nanorods was rela-
tively small during the first cycle. The abnormal performance of
the ZnO nanorods in the first cycle could be due to the presence
n
properties of the resulting ZnO nanostructures. The alkyl chain
+
length of the ionic liquids [C mim] Br was found to be critical to
ꢀ
n
the L/D of the final 1D ZnO nanostructures. Long alkyl chains
resulted in short ZnO nanorods and short alkyl chains resulted in
long ZnO nanorods. (iii) Most importantly, the as-obtained ZnO
nanostructures exhibit defect-related green-yellow emission
under UV excitation, which is beneficial for the photocatalytic
decomposition of organic dye molecules Rhodamine B, working
as effective recyclable photocatalysts, and even exhibiting
excellent photocatalytic activity during six cycles, suggesting
promising potential for environmental applications. We hope,
this new and convenient approach can pave the way for the
Fig. 12 (a) UV-vis absorption spectra of Rhodamine B as a function of irradiation time in the presence of sample 4 over 6 cycles; (b) tests of absorbance
changes as a function of irradiation time over 6 cycles for Rhodamine B monitored at the maximum absorbance of 554 nm.
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 15732–15740 | 15739

View Article Online
dedicated synthesis of other metal oxide nanomaterials and will,
in turn, motivate the developments of ionic liquids.
14 L. Wang, L. X. Chang, B. Zhao, Z. Y. Yuan, G. S. Shao and
W. J. Zheng, Inorg. Chem., 2008, 47, 1443.
1
5 L. Wang, S. Xu, L. X. Chang, Z. Su, M. H. Zeng, L. N. Wang and
Y. N. Huang, J. Solid State Chem., 2011, 184, 720.
Acknowledgements
16 T. Ming, Wei. Feng, Q. Tang, F. Wang, L. D. Sun, J. F. Wang and
C. H. Yan, J. Am. Chem. Soc., 2009, 131, 16350.
17 S. Hu and X. Wang, J. Am. Chem. Soc., 2010, 132, 9573.
This work is supported by the National Natural Science Foun-
dation of China (Grant No. 21065012, 91022015), the Natural
Science Foundation of Xinjiang Uygur Autonomous Region of
China (Grant No. 200821166), and the Science and Technology
Program in support of Xinjiang Uygur Autonomous Region of
China (Grant No. 201091244), the opening funds of Xinjiang
Laboratory of Phase Transitions and Microstructures in
Condensed Matters, Yili Normal University, as well as the
Program for New Century Excellent Talents in University of the
Ministry of Education China (NCET-07-217).
1
8 D. Kuang, S. Ito, B. Wenger, C. Klein, J. Moser, R. Humphry-Baker,
S. M. Zakeeruddin and M. Gr €a tzel, J. Am. Chem. Soc., 2006, 128,
4
146.
19 X. H. Liu, M. Afzaal, K. Ramasamy, P. O’Brien and J. Akhtar, J.
Am. Chem. Soc., 2009, 131, 15106.
2
0 S. Sakthivel, B. Neppolian, M. V. Shankar, B. Arabindoo,
M. Palanichamy and V. Murugesan, Sol. Energy Mater. Sol. Cells,
2
003, 77, 65.
21 Y. Z. Li, X. Zhou, X. L. Hu, X. J. Zhao and P. F. Fang, J. Phys.
Chem. C, 2009, 113, 16188.
2
2
2
2 H. G. Zhu, J. F. Huang, Z. W. Pan and S. Dai, Chem. Mater., 2006,
8, 4473.
3 J. Wang, J. M. Cao, B. Q. Fang, P. Lu, S. G. Deng and H. Y. Wang,
Mater. Lett., 2005, 59, 1405.
4 J. G. Huddleston, A. E. Visser, W. M. Reichert, H. D. Willauer,
G. A. Broker and R. D. Rogers, Green Chem., 2001, 3, 156.
1
References
1
J. Dupont, G. S. Fonseca, A. P. Umpierre, P. F. P. Fichtner and
S. R. Teixeira, J. Am. Chem. Soc., 2002, 124, 4228.
25 Y. J. Zhu and W. W. Wang, Inorg. Chem. Commun., 2004, 7, 1003.
26 J. M. Cao, J. Wang, B. Q. Fang, X. Chang, M. B. Zheng and
H. Y. Wang, Chem. Lett., 2004, 33, 1332.
27 X. M. Hou, F. Zhou, Y. B. Sun and W. M. Liu, Mater. Lett., 2007, 61,
1789.
28 Z. H. Li, A. Shkilnyy and A. Taubert, Cryst. Growth Des., 2008, 8,
4526.
29 Z. H. Li, A. Geßner, J. P. Richters, J. Kalden, T. Voss, K u€ bel and
A. Taubert, Adv. Mater., 2008, 20, 1279.
2
3
4
H. Itoh, K. Naka and Y. Chujo, J. Am. Chem. Soc., 2004, 126, 3026.
Y. Zhou and M. Antonietti, J. Am. Chem. Soc., 2003, 125, 14960.
J. Dupont, R. F. de Souza and P. A. Z. Suarez, Chem. Rev., 2002, 102,
3667.
Kumar, Chem. Rev., 2001, 101, 1.
T. Welton, Chem. Rev., 1999, 99, 2071.
Z. Ma, J. H. Yu and S. Dai, Adv. Mater., 2010, 22, 261.
W. J. Zheng, X. D. Liu, Z. Y. Yan and L. J. Zhu, ACS Nano, 2009, 3,
5
6
7
8
115.
J. M. Ma, L. Chang, J. B. Lian, Z. Huang, X. C. Duan, X. D. Liu,
P. Peng, T. Kim, Z. F. Liu and W. J. Zheng, Chem. Commun.,
2010, 46, 5006.
0 J. B. Lian, X. C. Duan, J. M. Ma, P. Peng, T. Kim and W. J. Zheng,
30 X. Zhou, Z. X. Xie, Z. Y. Jiang, Q. Kuang, S. H. Zhang, T. Xu,
R. B. Huang and L. S. Zheng, Chem. Commun., 2005, 5572.
31 S. A. Katsyuba, P. J. Dyson, E. E. Vandyukova, A. V. Chernova and
A. Vidis, Helv. Chim. Acta, 2004, 87, 2556.
32 Y. H. Zheng, C. Q. Chen, Y. Y. Zhan, X. Y. Lin, Q. Zheng,
K. M. Wei, J. F. Zhu and Y. J. Zhu, Inorg. Chem., 2007, 46, 6675.
33 A. Testino, I. R. Bellobono, V. Buscaglia, C. Canevali, M. D’Arienzo,
S. Polizzi, R. Scotti and F. Morazzoni, J. Am. Chem. Soc., 2007, 129,
3564.
9
1
1
1
1
ACS Nano, 2009, 3, 3749.
1 H. G. Zhu, J. F. Huang, Z. W. Pan and S. Dai, Chem. Mater., 2006,
18, 4473.
2 S. Lee, Y. S. Chi, J. K. Lee, I. S. Choi, C. E. Song, S. K. Namgoong
and S. Lee, J. Am. Chem. Soc., 2004, 126, 480.
3 G. Trewyn, C. M. Whitman and V. S. Lin, Nano Lett., 2004, 4, 2139.
34 X. G. Han, Q. Kuang, M. S. Jin, Z. X. Xie and L. S. Zheng, J. Am.
Chem. Soc., 2009, 131, 3152.
1
5740 | J. Mater. Chem., 2011, 21, 15732–15740
This journal is ª The Royal Society of Chemistry 2011