CrystEngComm
PAPER
Cite this: CrystEngComm,2014,16,
1525
Received 22nd August 2013,
Accepted 21st October 2013
DOI: 10.1039/c3ce41670d
www.rsc.org/crystengcomm
Li-doped ZnO nanorods with single-crystal
quality –non-classical crystallization and
self-assembly into mesoporous materials
Carlos Lizandara-Pueyo,
a
Stefan Dilger,
a
Markus R. Wagner,
bc
Melanie Gerigk,
a
Axel Hoffmann
b
and Sebastian Polarz*
a
The benefits and promise of nanoscale dimensions for the properties of (ceramic) semiconductors are
widely known. 1-D nanostructures in particular have proven to be of extraordinary relevance due to their
applicability in future electronic and optoelectronic devices. Key to successful technological implementation of
semiconductor nanostructures is the control of their electronic properties via doping. Despite its tremendous
importance, precise chemical doping of defined nano-objects has been addressed rarely so far. Frequent
problems are the creation of secondary defects and related undesired property changes by incorporation of
hetero-elements, and the difficulty in ensuring a uniform and precise positioning of the dopant in the nano-
crystal lattice. Here, we present the synthesis of Li-doped zinc oxide nanorods, which possess excellent
(single-crystal) quality. The method is based on a novel non-classical crystallization mechanism, comprising
an unusually oriented disassembly step. Afterwards, the nanorods are incorporated into mesoporous layers
using colloidal self-assembly. Proof-of-principle gas sensing measurements with these novel materials
demonstrate the beneficial role of Li-doping, indicating not only better conductivity but also the occurrence
of catalytic effects.
1. Introduction
The exploitation of semiconductors is of utmost importance
to existing and forthcoming technologies. Some have even
argued that in analogy to the ‘Copper Age’following the
‘Stone Age’, we are currently living in the Semiconductor
Age.
1
For sure, silicon is the semiconductor which is used
the most in a technological context (e.g. microelectronics).
However, there are various applications for which silicon is
not suitable due to either its relatively narrow band-gap or its
indirect band-structure. Therefore, there is large interest in
wide, direct gap semiconductors like III/V compounds, for
instance gallium nitride (GaN),
2–4
or II/VI compounds such
as zinc oxide (ZnO).
5–7
Furthermore, for silicon, it is well
documented that the pure compound itself is only of very
limited use. Functional devices like photovoltaic cells
require p- and n-doped Si. Interestingly, the exploitation of
doping to control the properties of ceramic semiconductor
nanostructures still stands at the beginning.
8–11
Most nano-
materials presented in the literature consist of the pure
semiconductor, or unintentional doping with impurities
has occurred.
Zinc oxide, one of the most important binary semi-
conductors in current research,
12,13
is interesting because
of its multifunctional character with applications in various
fields such as optoelectronics (e.g. photovoltaics, transparent
electronics),
14
cosmetics (UV protection), sensor technology
(gas sensors, bio sensors) or catalysis (methanol synthesis).
6,15–17
One-dimensional ZnO nanostructures have also attracted sig-
nificant attention.
14
Despite much recent research, important
challenges remain.
18
ZnO is intrinsically n-doped due to its ten-
dency to form native point defects and due to the presence of
residual shallow donors such as interstitial hydrogen or of Al,
Ga, or In on Zn lattice sites.
19–21
Major efforts have been under-
taken to achieve p-doping of ZnO via partial substitution of
zinc by lithium or,
10,11
alternatively, of oxygen by nitrogen.
22–24
The desired p-doping has often been found to be compensated
by unintentional n-doping resulting from dopants being
located on interstitial sites in the lattice.
25,26
The development
of reliable methods for the preparation of Li–ZnO colloidal
particles, ultimately of nanorods, with precise lattice site
control of the dopants, would thus represent a major advance.
Particular importance in this context is attributed to routes
leading to colloidal ZnO nanorods. A common way to prepare
ZnO is to start from an aqueous solution of a Zn
2+
salt (e.g. zinc
acetate), followed by the precipitation of zinc–oxo–hydroxo
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a
University of Konstanz, Universitaetsstr. 10, 78464 Konstanz, Germany.
Tel: +49 7531 884415
b
Technical University of Berlin, Hardenbergstr. 36, 10623 Berlin, Germany
c
Catalan Institute of Nanotechnology (ICN), Campus UAB, 08193 Bellaterra
(Barcelona), Spain
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species at elevated pH and elimination of water resulting in
ZnO.
27,28
Guo and Xu et al. and independently Weller et al.
prepared single-crystalline ZnO nanorods in 2002.
29,30
Water is
a highly polar, protic solvent, and it permits Ostwald-ripening
as an equilibrating process due to the sufficient solubility of
the ions Zn
2+
and OH
-
in the continuous phase. Therefore, it is
tempting to prepare ZnO nanostructures in organic, non-
coordinating solvents. There are only a few reports about the
preparation of ZnO in aprotic, organic solvents, which is
mainly due to the lack of precursors which are soluble enough
in these media. Chaudret et al. have published a series of
highly interesting papers about the reaction of dialkylzinc
compounds to zinc oxide colloids in cyclohexane.
31–36
For
instance, the authors described the successful synthesis of
defined nanorods and nanodiscs.
31
Experimental section
The zinc oxide precursor was prepared as described in previous
papers.
37–39
All other chemicals were obtained from chemical
suppliers and were purified and/or dried prior to use.
Preparation of nanorods
Water (1.5 ml), cyclohexane (35 ml) and a polyglyceryl-
containing emulsifier like polyglyceryl-3 polyricinoleate (1 ml)
are mixed. For the synthesis of Li-containing materials, Li-salts
(lithium acetate, lithium stearate) are added to the aqueous
phase. An emulsion is prepared by applying ultrasound for
15 min using a Bandelin-Sonopuls TT13/FZ ultrasonication
device. The emulsion is heated to 70 °C. [MeZnOiPr]
4
is
dissolved in dry cyclohexane (15 ml) and added drop-wise at
0.63 ml min
-1
to the emulsion using a syringe pump from KD
Scientific (model KS200). During the addition, ultrasonication
is continued for 300 min. The final colloids are isolated by
removing the solvent under vacuum.
Characterization methods
PXRD data were acquired using a Bruker D8 Advance.
Conventional TEM measurements were performed using a
Zeiss Libra 120, while high resolution TEM measurements
were performed using a Jeol JEM2200FS. Solid-state NMR
spectra were recorded using a Bruker DRX 400 spectrometer.
N
2
physisorption measurements were recorded using a Micro-
meritics TriStar. The Raman spectra were recorded using a
LabRAM HR800 spectrometer (HORIBA Jobin Yvon) with the
532 nm laser line of a frequency doubled Nd:YAG laser. The
laser was focused on the sample using an Olympus MplanN
100×objective lens (NA = 0.90). The spectra were collected in
backscattering geometry with a spectral resolution better
than 0.3 cm
-1
. The spectrally dispersed Raman signal was
detected using a Peltier-cooled CCD camera. The laser power on
the sample was tuned to 1 mW. Micro-photoluminescence (μPL)
spectra were measured using the fourth harmonic (266 nm) of
a pulsed Nd:YAG laser (Coherent Antares 76s) with a repetition
rate of 76 MHz and a pulse length of 50 ps as excitation source.
The laser was focused through a UV enhanced objective lens
(NA = 0.65) on the samples which were mounted on a helium
flow cryostat and cooled to a temperature of T=4K.The
excitation power was kept below 100 μW to avoid local
heating or high excitation effects. The luminescence was
spectrally dispersed and recorded using a 1 m spectrometer
(SPEX) with a UV enhanced CCD (Princeton Instruments)
achieving a spectral resolution of 0.01 nm. Time resolved
photoluminescence (TRPL) measurements were performed
by single photon counting using a micro channel plate with
a bialkali cathode as detector (Hamamatsu R3809 U-52). The
emitted light was dispersed using a subtractive double mono-
chromator (McPherson) at a spectral resolution of 0.1 nm. EPR
measurements were performed using an X-band Miniscope
spectrometer (MS200, Magnettech GmbH).
Gas sensor preparation
8 mg of the colloid samples were dispersed in 4 mL of
ethanol (abs.) by ultrasonication. The dispersions were drop
coated on 3 mm ×3 mm sensor substrates from
“Umweltsensortechnik”. Ten drops of the dispersion were
subsequently dropped onto the substrate and allowed to dry.
The organic, colloidal stabilizers were removed in air at
475 °C during 5 hours. The overall gas stream through the
setup was kept constant at 0.5 L min
−1
(80% N
2
, 20% O
2
).
The ethanol concentration (5 ×10
3
ppm) was adjusted using an
OWL-stone OVG-4 setup. The sensors were heated by varying
the heater voltage (10 Ωheating resistor built into the sensor
substrate). Temperatures at the sensor were measured with a
type K thermocouple, and the electric current through the
sensor was measured using a Keithley 2401 source meter at
a constant measuring voltage of 2 V.
Results and discussion
Li–ZnO nanorods via non-classical crystallization
Based on our experience with the use of alkylzinc alkoxides
[MeZnOR]
4
(R = organic group) as ZnO precursors, and
regarding the preparation of colloidal ZnO nanoparticles with
characteristic shape anisotropy,
37,40,41
the following experi-
ment was designed (see also the experimental section): a
water-in-oil emulsion is prepared, and [MeZnOR]
4
is added to
the oil-phase by way of a syringe pump, which allows control
of the precursor concentration at all times. The precursor is
hydrolyzed in the vicinity of the oil–water interface, and ZnO
formation is initiated. Li
+
ions can be introduced by dissolv-
ing lithium species in the aqueous phase of the emulsion.
A unique mechanism for the formation of the final parti-
cles is revealed by transmission electron microscopy (TEM)
images of samples taken from the reaction mixture at differ-
ent times. At t= 5 min, a great number of spherical particles
have formed (Fig. 1). These particles are amorphous, as
shown by the absence of any pattern in electron diffraction
(ED). Apparently, the water droplets have gelated. One also
sees some larger particles composed of several smaller ones
(Fig. 1c). After t= 15 min, the TEM data show that the
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intermediary amorphous phase starts to crystallize (Fig. 1e).
Surprisingly, the results of this crystallization process are
bundles of highly oriented nanorods, a so-called super-
crystal.
41
In agreement with this, a large degree of orientation
is apparent not only from the TEM images but also from the
ED pattern, which is characterized by spots rather than
rings. Finally, after t= 100 min, the entire sample consists
of crystalline nanorods with lengths of 100–200 nm and
widths of ≈15 nm.
The described crystallization mechanism is highly
unusual. Unlike classical crystallization, it does not involve
nucleation followed by growth via attachment of single ions.
The process just described is much closer to many systems
reported in biomineralization,
42
where complex shapes
are generated via crystallization of an intermediary and
often amorphous phase inside confining compartments like
cellular vesicles.
The polydispersity of the size-distribution proved to be
changeable by adapting a method known as focusing.
43
Feed-
ing the precursor in a continuous way allowed an adjustment
of the aspect ratio. The more and the longer the precursor is
added, the longer the particles become (Fig. 2). To find out
about the main growth direction, powder X-ray diffraction
(PXRD) was recorded and is shown in Fig. 2e.
The observed pattern is characteristic of ZnO in the
wurtzite modification. PXRD indicates that the particles are
elongated along the c-direction. It is apparent that the
signal corresponding to elongation of the particles in the
c-direction, the [002] diffraction at 2θ= 34.4°, is much
narrower and becomes more intense as the particles get
more elongated. Particle extensions can be obtained via the
evaluation of the widths of the [100] signal (for the a,b
direction) and the [002] signal (for the c-direction) using
the Scherrer equation.
44,45
It can be concluded that the
extension of the growth phase affects mainly the crystallo-
graphic c-direction D
c
=55→120 nm, while D
a,b
=15nm
remains almost constant.
The non-classical crystallization pathway is the key to the
successful incorporation of Li so as to replace Zn in the ZnO
lattice, because Li ions dissolved in the water droplet of the
emulsion become entrapped during the solidification
(Fig. 1b,d,f). The characterization of the Li-containing mate-
rial is a demanding task. This is because Li, as a very light
element, cannot be detected using energy-dispersive X-ray
spectroscopy (EDX). Furthermore, the ionic radius of Li
+
in
tetrahedral coordination (59 pm) is so close to the radius of
Zn
2+
in wurtzite (60 pm) that a differentiation using
Fig. 1 Schematic representation of the formation mechanism (left side)
and time-dependent TEM and ED data (right side). Indication of one water
droplet containing lithium ions plotted in green (a); the gelated, spherical
particles after addition of the ZnO precursor (b) and the corresponding
TEM data recorded after t= 5 min (c); crystallization of a bundle of
rod-like nanocrystals (d, e; t= 15 min); detachment of the particles into
isolated nanorod colloids (f, g). An excerpt of the ZnO lattice with Li
replacing tetrahedral zinc sites is also shown.
Fig. 2 TEM (a–d) and PXRD data (e) of samples prepared for continuous
addition of ZnO precursor (a; black graph ↔a; red graph ↔b;
blue graph ↔c; orange graph ↔d).
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crystallographic methods is not possible either. Therefore,
the formation of the desired Zn
1-x
Li
x
O materials (LZO) was
investigated using a combination of more sophisticated,
independent analytical methods (Fig. 3). The composition,
x= 0.103, was determined from elemental analysis using ion-
coupled plasma mass spectrometry (ICP-MS) together with
NMR-data.
7
Li solid-state NMR spectroscopy, a sensitive tool
for investigating the local environment of the lithium ions,
shows two species at δ= 1.83 and 1.04 ppm. The latter
species originates from residual, non-incorporated Li-salt
as shown by reference measurements (Fig. 3a). The residual
Li
+
could be separated by purifying the samples using a
Li-specific crown ether (12-crown-4).
It can be seen that only one signal remains, which is char-
acteristic of Li
+
on Zn
2+
lattice sites (Fig. 3a). Vibrational
modes and strain in the Li-doped nanorods were studied
using micro Raman spectroscopy and compared to a pure
ZnO nanorod sample with an identical aspect ratio as refer-
ence (Fig. 3b). Both samples show the typical ZnO lattice
modes with narrow line widths of only 1.5 cm
-1
in case of the
E
2
(low), which is comparable to excellent ZnO single crys-
tals.
46
No broadening of modes is observed in the Li-doped
samples. This indicates that the structural quality of the
nanorods is not reduced by the Li
+
incorporation. The strain-
sensitive E
2
(high), observed at 439.4 cm
-1
in both samples, is
characteristic of a small compressive strain.
47,48
The identical
position of this mode in the Li doped and undoped nanorods
proves that no additional lattice strain is induced by the Li
doping. In addition to the ZnO modes, the Li doped samples
show Raman modes at 95.6, 127.2, 156.8, 193.6, 271.4 and
1091.0 cm
-1
with exceptional intensities. These modes appear
exclusively in Li doped ZnO and thus provide clear evidence
for the incorporation of Li into the ZnO nanorods.
48,49
In addition, optical emission spectra were recorded
(Fig. 3c). The low temperature photoluminescence (PL) spec-
trum is dominated by a bound exciton luminescence (DX) at
3.3567 eV which coincides with the reported peak position of
the I
9
line in ZnO.
19
In the emission spectra of the LZO nano-
rods, an additional emission band with a maximum at 3.31 eV
is observed, which corresponds to a free electron to acceptor
transition (e,A).
50
Recombination dynamics of the DX transi-
tions, studied by time-resolved photoluminescence (TRPL)
measurements in both samples, show moreover that an exciton
lifetime of 1.4 ns in the undoped sample is exceptionally long –
i.e. comparable to that of high quality ZnO single crystals and
shortens to 0.2 ns in the LZO nanorods, possibly due to non-
radiative energy transfer to deeper Li-related electronic states.
51
Furthermore, a new signal at a g-factor of 2.014 appears in
electron paramagnetic resonance (EPR) spectroscopy measure-
ments of LZO. This signal is characteristic of heteroatoms
occupying Zn
2+
lattice sites.
25,52,53
All of the mentioned analytical data lead to the conclusion
that the nanorods prepared by our method possess excellent
single-crystal-like quality. The high crystallinity could be con-
firmed by HRTEM measurements shown in Fig. 4a, b. ED dif-
fraction (not shown) confirms that the long axis of the single-
crystalline nanoparticles is equal to the crystallographic
c-direction. The value of the lattice constant c=2×d
002
is
5.18 (±0.013) Å, which is the same value as that of single-
crystalline ZnO. There is practically no difference from nano-
rods consisting of pure ZnO (Fig. 4c).
Assembly of novel, mesoporous materials
LZO nanorods represent promising nanoparticle building
blocks for advanced functional devices. Here, we introduce a
unique method in which the formation of such devices is
driven mainly by self-organization. From TEM images shown
in Fig. 2, it is apparent that the nanorods tend to align parallel
to each other at high concentrations. Drying of a dispersion
with higher concentrations of the nanorods leads to the forma-
tion of mesoscopic structures, which have been investigated
using scanning electron microscopy (SEM) as shown in Fig. 5b.
The removal of organic compounds located at the ZnO inter-
faces for stabilization via calcination leads to a novel porous
material and at the same time to a network of electrically
connected nanorods (see Fig. 5c, d). The porous nature and the
reminiscent orientation of the nanorods are clearly apparent
from TEM. Formation of a mesoporous material with an
average pore size of 8.5 nm is proven by its N
2
physisorption
isotherm and pore-size distribution function, as shown in
Fig. 3 Spectroscopic investigation of LZO nanorods (≡red) in comparison
to ZO nanorods (≡blue): (a)
7
Li solid-state NMR spectra of LZO rods
(prior ≅dotted line, after purification ≅solid line) and lithium stearate
(≡grey line) as reference. (b) Raman spectra taken at room temperature.
(c) PL spectra taken at T= 5 K. Both spectra are normalized to neutral
donor bound exciton luminescence at 3.3567 eV (I
9
). (d) TRPL of I
9
(D
0
X).
(e) X-band EPR spectra.
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Fig. 5c. To the best of our knowledge, this is the first time that
formation of a porous material of this kind has been achieved.
In order to demonstrate the benefits of this special
material's architecture with regard to its functionality, e.g. for
chemical gas-sensing,
54
the porous solid described above is
brought between an interdigitating array of electrodes (Fig. 5d).
Working at constant voltage and constant reactant concentra-
tion, the current was measured as a function of the temperature
at the sensor. Direct comparison of LZO to an analogous mate-
rial composed of pure ZnO (Fig. 6a) shows that the current for
LZO is always higher than that for ZnO, a direct consequence of
Fig. 4 HRTEM micrographs of Li-containing ZnO nanorods (a, b) in
comparison to pure ZnO nanorods (c).
Fig. 5 Oriented attachment (b; SEM data and schematic image) of
single LZO nanorods (a). The porous nanorod material obtained after
calcination (c; TEM data, nitrogen physisorption data and pore-size
distribution function and schematic image). Device architecture for
testing the materials in chemical sensing (d).
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the lower resistance due to the successful doping described
above.
A direct advantage of the higher overall conductivity is
that the sensor device affords reliable results at comparably
low temperatures. As the usual temperature for the operation
of conventional gas sensors is 300–400 °C, data of very poor
quality are obtained with pure zinc oxide material at a much
lower temperature of 150 °C. Even at this low temperature
however, the LZO sensor produces a clear signal as soon as
the reactant gas is turned on. However, also the response of
the sensor is of importance (respectively the measured cur-
rents prior to and after exposure to the analyte).
The current is a direct measure of the conversion of the
reactant (R ≅ethanol) into the products (P ≅carbon dioxide,
water)
55
at the surfaces. Thus, the temperature dependency of
the current amplification (Fig. 6b) can be evaluated using the
Arrhenius formalism. Indeed, there is a difference in current
amplification. It can be seen that the LZO material is superior
to the materials prepared in an analogous way, except for nano-
rods composed of pure ZnO. Fitting the data to the Arrhenius
law affords virtual activation energies for the surface processes
(E
a,LZO
=49.8kJmol
-1
,E
a,ZO
= 61.31 kJ mol
-1
). The lowering of
activation energy is indicative of a catalytic effect caused by
Li-incorporation. The latter assumption is supported by the
difference in temporal behavior of the two materials. Whereas
the sensing signal of pure ZnO decreases more or less slowly
for continuing exposure to the analyte, LZO is characterized by
a constant or even increasing sensor response.
Conclusions
Our results open new vistas toward a controlled doping of
nanoparticles, and in particular of those with anisotropic
shape, by use of a non-classical crystallization methodology
related to biomineralization. It could be revealed that aniso-
tropic ZnO particles form from an amorphous precursor state
followed by an unusual disassembly step. Li
+
could be incorpo-
rated without losing the perfect crystallinity of the rod-like
nanoparticles. It was proven by several methods that Li
+
occupies the Zn
2+
position in the ZnO lattice, resulting in
higher electric conductivity than conventional ZnO. The nano-
rods could be reassembled into a parallel array. This array was
converted into a unique mesoporous material, which could be
used for the fabrication of functional devices. By facilitating
chemical gas-sensing measurements, our results provide a
unique perspective with regard to testing the catalytic activities
of nanostructured materials.
Acknowledgements
We gratefully acknowledge the Carl-Zeiss Foundation for
funding (REFINE research initiative). We thank Prof. A. Seubert
(Marburg) for performing the ICP-MS measurements.
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