Hydrothermal growth of ZnO nanorods: The role of KCl in controlling
rod morphology
Jonathan M. Downing
a
, Mary P. Ryan
b
, Martyn A. McLachlan
a,
⁎
a
Department of Materials & Centre for Plastic Electronics, Imperial College London, London SW7 2AZ, UK
b
Department of Materials & London Centre for Nanotechnology, Imperial College London, London SW7 2AZ, UK
abstractarticle info
Article history:
Received 17 August 2012
Received in revised form 19 April 2013
Accepted 22 April 2013
Available online 9 May 2013
Keywords:
Zinc oxide
Hydrothermal
Nanorod
Ionic additive
Alignment
Self-assembly
The role of potassium chloride (KCl) in controlling ZnO nanorod morphology of large area thin films prepared by
hydrothermal growth has been extensively investigated. The influence of KCl and growth time on the orienta-
tion, morphology and microstructure of the nanorod arrays has been studied with systematic changes in the
length, width, density and termination of the nanorods observed. Such changes are attributed to stabilization
of the high-energy (002) nanorod surface by the KCl. At low KCl concentrations (b 100 mM) c-axis growth

,highoptical
transparency and a large exciton binding energy at room temperature
(60 meV) [11]. Further enhancement of many of these physical proper-
ties may be possible through intrinsic and extrinsic doping.
In many nanostructures the large surface-to-volume ratio, prefer-
ential growth of polar/non-polar surfaces and anisotropic charge trans-
port have been explored as means of improving device performance
[7,12,13]. For e xampl e, Z nO nanorods hav e bee n used to form pho tovol-
taic devices in which the power conversion efficiency was improved
by almost an order of magnitude compared with comparable devices
fabricated with planar ZnO layers [7,14]. Furthermore, improvements
in the turn-on field values of electroluminescent devices by a factor of
eight have been reported through control of nanorod morphology
[15]. Future improvements in emerging devices are likely if methods
for the reproducible fabrication of nanorod arrays can be achieved.
Using high temperature, vacuum techniques such as Pulsed Laser
Deposition [16], Vapor Liquid Solid Growth [17] and Chemical Vapor
Deposition [18] high quality nanorod arrays have been fabricated. In
comparison solution processing routes are more attractive owing to
their low cost, low deposition temperatures and compatibility with
large area deposition. However, morphological control and batch-to-
batch reproducibility continue to be problematic with such processes.
To date, the most widely reported solution processes for nanorod
growth are electrochemical deposition [19,20] and hydrothermal
growth [21]. The latter is particularly well suited to large area deposi-
tion, requires little capital investment of equipment and can be
achieved on electrically conducting and insulating substrates, includ-
ing flexible polymeric substrates [21].
For the growth of nanorod arrays, the hydrothermal technique
is outlined in detail by Vayssieres et al. [22], where nanorod growth

À
→NO
À
2
þ 2OH
À
ð2Þ
C
6
H
12
N
4
þ 10H
2
O⇌6CH
2
O þ 4NH
þ
4
þ 4OH
À
ð3Þ
Zn
2þ
þ 2OH
À
→ZnðOHÞ
2
→ZnO þ H

with the effect to produce stacked plate like crystals [33].
We have recently reported on t he incorporation of polyethy leneimine
(PEI) and potassium chloride (KCl) in the hydrothermal growth of ZnO
and t heir influence on nanorod m orpholog y and vertical o rientation
[34].Herewereportasignificant advance of this preliminary work in
which exceptional control o f nanorod morphology and vertical alignment
are achi eved. The role o f KCl in co ntrolling nanorod morphology and its
influence on the grow th process i s described in deta il. The n anorod arrays
here are proposed as highly suitable and tunable active layers in emerg-
ing optoelectron ic devices, for example homojunctions in hybrid solar
cells and as electron injecting layers in organic light emitting diodes.
2. Experimental details
2.1. ZnO nanorod preparation
A two-stage deposition process for nanorod growth was adapted
from existing literature [21,24];
Seed lay er d eposition A 0.75 M solution o f z inc acetate dihydrate (Zn (O
2
CCH
3
)
2
.2(H
2
O)) and 2-am inoethanol (H
2
N(CH
2
)
2
OH) in 2-methoxyethano l (HO(CH

) were mixed in
a closed vessel immersed in a controlled tempera
ture water bath 95°C±1°C. The substrate
supported seed layers were suspended directly into
the hydrothermal solution. The additives, 0–500 mM
KCl and 10 mM PEI (H(NHCH
2
CH
2
)
n
NH
2
)were
added to the solution imme diate ly pr ior to seed
layer immersio n. Followin g rod depo sition the films
were rinsed thoroughly with deionized water and
allowed to dry at 95 °C.
The morphologies of the films were characterized using a LEO 1525
field emission scanning electron microscope. Surface images were
obtained on the as-prepared films whilst cross-sectional images were
obtained after the scratching with a diamond scribe (imaging typically
2–10 kV). Image analysis was carried out from the micrographs by
counting (areal density) and with use of ImageJ software to collate
rod length and width data. The crystal structures of the films were ana-
lyzed using Panalytical X'Pert MPD diffractometer equipped with an
Accelerator detector, operated at 40 kV/40 mA (Cu K
α
source, theta-
theta configu ratio n). X-ray diffraction (XRD) patterns were corrected

(1040 ± 180 nm), at higher KCl concentrations a gradual reduction
in nanorod length is observed (Fig. 2d-f). At KCl concentrations
(b 200 mM) the nanorods are terminated with sharp points, above
this concentration the rods are terminated by flat surfaces and show
clear hexagonal faceting.
Fig. 1. Showing measured average nanorod length plotted against growth time for
hydrothermal baths containing equimolar (25 mM) Zn(NO
3
)
2
/HMT in addition to
10 mM PEI and 100 mM KCl.
19J.M. Downing et al. / Thin Solid Films 539 (2013) 18–22
To further quantify the influence of KCl on nanorod growth detailed
analysis of films grown under each set of growth conditions was carried
out using SEM images. The calculated averages are accompanied by the
standard error of the mean, it should be noted that such errors are small
due to the large measured sample size. A minimum of two samples at
each KCl concentration were analyzed at a number of locations on
each substrate. In summary, and consistent with the trends shown in
Fig. 2, nanorod length increases on addition of KCl, reaching a maximum
at 50 mM (1040 nm (13 nm)); above this concentration nanorod
length is gradually reduced. The measured rod diameter increases
across the concentration series to a maximum of 90 nm (2 nm) at
500 mM KCl. The areal density is reduced on addition of small amounts
of KCl but increases with increasing KCl concentration. Fig. 3 shows the
processed results obtained from the image analysis for the calculated
deposition volume, and the measured rod length, diameter and areal
density as KCl concentration is varied.
Films grown over the entire KCl concentration range for both

2
-
,Cl
-
,
OH
-
. Typically these are between − 285 and − 430 kJ mol
−1
[37],indi-
cating that these species will be heavily solvated during nanorod
growth hence reactions between these species in solution are unlikely.
Furthermore all ionic species are in low concentrations, the maximum
([KCl] 500 mM), is significantly lower than the solubility limit at
100 °C (7.6 M).
Analysis of the SEM micrographs of short (40 min) growth time
samples yields some information about nanorod nucleation and early-
stage growth behavior. In our experiments similar areal densities
were measured without KCl and across the whole KCl concentration
series, on average 149 μm
−2
indicating that the nucleation density is
independent of KCl concentration.
Increasing the KCl concentration results in a marked reduction in
the standard deviation of rod length, showing improved uniformity
as KCl concentration is increased (Table 1).
In the presence of KCl the observed nanorod growth behavior can
be explained by considering nanorod surface termination (Fig. 2i-j).
In order to minimize the area of high energy (002) surfaces nanorod
growth proceeds primarily along the b002> direction with slow growth

interaction is highlighted (all SEM scale markers 200 nm). TEM images highlighting the change in nanorod tip termination at i) 10 mM and j) 300 mM KCl.
20 J.M. Downing et al. / Thin Solid Films 539 (2013) 18–22
The longest rods are formed at 50 mM KCl but the greatest volume
of ZnO is deposited at 100 mM KCl—owing to the marked increase in
lateral growth. Control of rod termination may be advantageous in
some device applications e.g. photovoltaics, where a smaller (002)
surface may reduce the polar barrier for electron transfer [13].
4.2. Physical nanorod interaction
As the growth time is extended from 40 to 120 min there is a dis-
tinct reduction in areal density of nanorods (cf. Fig. 3d), indicating
that many of the nucleated nanorods observed early in the linear
growth regime do not continue to grow. We propose that this phe-
nomenon results from the physical interaction occurring when adja-
cent nanorods grow at angles whereby their growth paths intersect,
Fig. 2c, resulting in the termination of some nanorods as growth
time increases. This process is supported by the observed increase
in (002) diffraction intensity at longer growth times (cf. 40 vs.
120 min, Fig. 4b). Very recent work shows the validation of this phe-
nomenon by application of the geometrical selection model [39],
where three distinct growth regimes are outlined, namely isolated,
competitive and aligned.
The variation in nanorod alignment between different KCl concen-
trations is ascribed to the differences in growth rate between lateral
and vertical directions. In regimes where c-axis growth is hindered
(>100 mM KCl), nanorod diameters are increased; hence rod-to-rod
interactions occur at an earlier growth stage. Under these conditions
(>100 mM KCl), misaligned nanorods are more likely to interact early
in the growth process and terminate, i.e. growth in the competitive re-
gime is reduced, resulting in the highly orientated films at lower time
periods. The XRD data support this hypothesis; diffraction intensity

b) length, c) diameter and d) areal density of ZnO nanorods grown for 120 min with
varying concentrations of KCl (error bars show standard error).
21J.M. Downing et al. / Thin Solid Films 539 (2013) 18–22
5. Conclusions
A reproducible method for preparing tailored ZnO nanorods from
aqueous solution has been developed through incorporation of KCl into
the hydrothermal growth bath. KCl acts as a growth modifier through
stabilization of the polar (002) nanorod surfaces. The range of KCl con-
centrations inve stigat ed (0–500 mM) spans growth modes where par-
tial adsorption results in the formation of high aspect ratio nanorods
and complete adsorption results in the formation of near-coalesced
rods. In comparison to electrochemical growth, where 60 mM KCl is
reported to be sufficient to stabilize the (002) surface and change the
morphology from rods to platelets [40], platele t d e po si tio n has not
been reported by the hydrothermal method, furthermore in our work
only rod-like structures were prepared. At high KCl concentrations
(>100 mM), the growth of short er and wider Z nO nanorods is o bserved,
consistent with reduced b 002> growth due to stabilizat ion of the (0 02)
surfaces by KCl.
The incorporation of simple ionic additive into the hydrothermal
growth bath provides a convenient method affording control of the
nanorod dimensions, areal density and surface termination. This rep-
resents a significant step in the controlled and reproducible low-cost
solution processing of tailored nanostructures and should facilitate
the uptake of these structures into relevant device architectures.
Acknowledgments
The authors acknowledge useful discussions withDr.JosephFranklin,
Imperial College. JMD is supported by the EPSRC, EP/J016039/1, and in
part by the Energy Futures Lab Imperial College Lo ndon. M AM is grateful
for the support of a Royal Academy of Engineering/EPSRC Research

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