hydrothermal growth of zno nanorods the role of kcl in controlling

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 te

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Hydrothermal growth of ZnO nanorods: The role of KCl in controlling

rod morphology

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

a b s t r a c t

a r t i c l e i n f o

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 thinﬁlms 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 (b100 mM) c-axis growth i.e perpendicular to the polar surface, dominates, leading to nanorods with increased length over the control sample (0 mM KCl) At higher concentrations (>100 mM) stabilization of the high-energy surface by KCl occurs and planar (002) facets are observed accompanied by increased lateral (100) growth, at the highest KCl concen-trations near coalesced (002) terminated rods are observed Additionally we correlate the KCl concentration with the uniformity of the nanorod arrays; a decrease in polydispersity with increased KCl concentration is observed The vertical alignment of nanorod arrays was studied using X-ray diffraction, it was found that this parameter increases as growth time and KCl concentration are increased We propose that the increase in vertical alignment

is a result of nanorod–nanorod interactions during the early stages of growth

1 Introduction

There has been a signiﬁcant volume of published work relating to

the growth of ZnO nanostructures over the past decade The unique

electronic and optical properties of this material combined with the

library of 3-dimensional structures prepared, including; dendrites[1],

tetrapods[2], helixes[3], in addition to lower-dimensional structures

such as particles[4], nanorods[5]and platelets[6]have fuelled

contin-ued research Of the many available structures ZnO nanorods have been

proposed for a wide range of applications, including; photovoltaics[7],

light emitting diodes[8], piezoelectric generators[9]and chemical

sen-sors[10] In addition to structural variations, ZnO continues to attract

interest owing to its outstanding electronic properties, including wide

and direct band gap ~3.3 eV, reported carrier concentrations of up to

2 × 1016cm−3, electron mobilities up to 205 cm2Vs−1, high optical

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 example, ZnO nanorods have been used to form photovol-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-onﬁeld 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-ingﬂexible polymeric substrates[21]

For the growth of nanorod arrays, the hydrothermal technique

is outlined in detail by Vayssieres et al.[22], where nanorod growth

is promoted when a water-soluble zinc precursor, often zinc nitrate (Eqs.(1)–(2)), is mixed with a suitable amine At elevated tempera-tures (b100 °C) thermal decomposition of the amine (Eq.(3)) results

⁎ Corresponding author at: Department of Materials & Centre for Plastic Electronics,

Imperial College London, Royal School of Mines, South Kensington, London SW7 2AZ,

UK Tel.: +44 20 7594 9692.

E-mail address: martyn.mclachlan@imperial.ac.uk (M.A McLachlan).

Contents lists available atSciVerse ScienceDirect

Thin Solid Films

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / t s f

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in a pH increase of the solution, producing thermodynamically

unsta-ble zinc hydroxide, which spontaneously precipitates as ZnO (Eq.(4))

NO3þ H2Oþ 2e→NO2 þ 2OH ð2Þ

C6H12N4þ 10H2O⇌6CH2Oþ 4NHþ4þ 4OH ð3Þ

Zn2þþ 2OH→ZnðOHÞ2→ZnO þ H2O ð4Þ

The formation of nanorods during hydrothermal growth occurs due

to the differing surface energies of the polar (002) and non-polar (100)

surfaces in wurtzite ZnO Minimization of the high-energy polar surface

results in increasedb002> growth and the subsequent formation of

nanorods[23]

The hydrothermal process is well-studied and the inﬂuence of

many growth parameters on nanorod morphology have been reported,

e.g reducing [Zn2+](aq)reduces rod diameter[21], increasing

tempera-ture increases growth rate and improves crystallinity[24] Alternative

Zn precursors and amines have been used to alter rod morphology

[25,26]and seed layers have been used to control areal density[26,27]

and vertical alignment[28,29]

Whilst the addition of chemical additives has been well studied as a

means of altering rod morphology i.e the incorporation of surfactants to

minimizeb100> growth through adsorption on the non-polar surfaces

[30,31] There are few reports of the role of ionic additives in solution;

in alkali deposition baths, metal sulfates are reported to control rod

aspect ratios[15], while increasing Cd2+/Zn2+ratio in solution results

in the formation of bipyramidal structures [32] In reactions using

hexamethylene triamine (HMT), citrate ions adsorb to (002) surface

with the effect to produce stacked plate like crystals[33]

We have recently reported on the incorporation of polyethyleneimine

(PEI) and potassium chloride (KCl) in the hydrothermal growth of ZnO

and their inﬂuence on nanorod morphology and vertical orientation

[34] Here we report a signiﬁcant advance of this preliminary work in

which exceptional control of nanorod morphology and vertical alignment

are achieved The role of KCl in controlling nanorod morphology and its

inﬂuence on the growth process is described in detail The nanorod arrays

here are proposed as highly suitable and tunable active layers in

emerg-ing optoelectronic 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 layer deposition A 0.75 M solution of zinc acetate dihydrate (Zn(O2

CCH3)2.2(H2O)) and 2-aminoethanol (H2N(CH2)2

OH) in 2-methoxyethanol (HO(CH2)2OCH3))[35]

was deposited on to tin-doped indium oxide

coat-ed glass substrates (PsiOteC UK Ltd, 12–16 Ω/sq)

Denseﬁlms were created by loading at 500 rpm before spinning at 2000 rpm for 30 s; three-coats were applied with substrate heating (300 °C for

10 min) between coats Aﬁnal 60 min anneal at

450 °C was carried out, producing continuous ﬁlms of ~120 nm thickness

Nanorod growth 25 mM solutions of HMT (NH2(CH2)6NH(CH2)6

NH2) and zinc nitrate ((Zn(NO3)2) 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(NHCH2CH2)nNH2) were added to the solution immediately prior to seed layer immersion Following rod deposition theﬁlms were rinsed thoroughly with deionized water and allowed to dry at 95 °C

The morphologies of thefilms were characterized using a LEO 1525 field emission scanning electron microscope Surface images were obtained on the as-preparedfilms 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 thefilms were ana-lyzed using Panalytical X'Pert MPD diffractometer equipped with an Accelerator detector, operated at 40 kV/40 mA (Cu Kαsource, theta-theta configuration) X-ray diffraction (XRD) patterns were corrected for Kα2emission and adjusted for background in X'Pert Highscore Plus, integrated peak intensity was found by peakfitting in X'Pert DataViewer

3 Results 3.1 Role of KCl:film structure and rod morphology Nanorod growth was carried out using equimolar concentrations (25 mM) of Zn(NO3)2and HMT and 10 mM PEI, whilst the KCl con-centration was systematically varied (0– 500 mM) The influence of growth time on rod length, Fig 1, shows a linear increase in rod length for growth times of 25 - 120 min Further extensions of the growth time do not lead to a significant increase in average rod length owing to consumption of the reactants Linear growth is a result of the steady decomposition of HMT in solution, which acts as a buffer

to provide hydroxide ions that are subsequently consumed (Eq.(4))

to produce ZnO[36]

In the present study growth times of 40 and 120 min at each KCl concentration were investigated in order to study growth early and late in the linear growth regime.Fig 2shows cross-sectional and sur-face SEM images of ZnO nanorod structures grown for 120 min (40 min, not shown) over the 0–500 mM KCl concentration range Here KCl is seen to be inﬂuencing nanorod length, vertical alignment, aspect ratio and tip termination The introduction of KCl (10 mM) re-sults in a signiﬁcant increase in rod length (860 ± 250 nm) compared with growth in the absence of KCl (370 ± 130 nm) Increasing the con-centration of KCl to 50 mM results in a further increase in rod length (1040 ± 180 nm), at higher KCl concentrations a gradual reduction

in nanorod length is observed (Fig 2d-f) At KCl concentrations (b200 mM) the nanorods are terminated with sharp points, above this concentration the rods are terminated byﬂat 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

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To further quantify the inﬂuence of KCl on nanorod growth detailed

analysis ofﬁlms 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 3shows 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

growth times were characterized using XRD, the diffraction patterns

are shown inFig 4a, the integrated intensity of the (002) diffraction

peak (~34.4 °2θ) has been used to quantify rod alignment Diffraction

intensity for a given peak is linearly dependent on the quantity (sample

volume) and orientation of nanorods on the substrate, i.e for a given

volume (002) diffraction intensity is increased if nanorods are aligned

To deconvolute the orientation and volume effects the integrated

(002) peak areas are corrected for the sample volume Finally to

quan-tify rod alignment, data were normalized to the (002) peak intensity of

the 0 mM KCl sample, providing a direct comparison of KCl addition on

vertical alignment,Fig 4b Samples grown for 40 min show little

varia-tion in vertical alignment as the KCl concentravaria-tion is varied In contrast

those grown for 120 min all show improvements in alignment that

reaches a maximum at around 400 mM KCl

4 Discussion

A simple method for controlling ZnO nanorod morphology through

addition of KCl to the hydrothermal growth bath is presented Our

pro-posed mechanisms to explain the observed changes in morphology as a

function of KCl concentration are outlined below

4.1 Solution chemistry

It isﬁrst necessary to consider the free energy of hydration for each

of the ionic species in the hydrothermal bath i.e NH+, K+, NO-, NO-, 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 149μm−2indicating 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 theb002> direction with slow growth

in the b100> directions Fig 2 shows that at KCl concentrations

≤100 mM the nanorods are terminated by (101) faceted points (con-sistent with ref.[23]), and as the KCl concentration increases a planar (002) surface is stabilized We propose that the observed increase in nanorod length at 10 mM KCl i.e rapidb002> growth, is attributed to the partial stabilization of a small (002) surface at the tip of the growing rod and not lateral (100) adsorption as previously speculated[25] The proposed structure and morphology of the growing rods are shown schematically inFig 5 In the absence of KCl, nanorods are terminated

by sharp points and the (002) surface is fully minimized (Fig 5a) Addi-tion of small amounts of KCl, results in the partial stabilizaAddi-tion of the (002) surfaceFig 5b, which promotes rapidb002> growth and explains the observed increase in nanorod length and accompanying reduction

in diameter Further increases in KCl concentration completely stabilize the (002) surface resulting in a promotion of lateral growth resulting in planar faceted rods, consistent with previous reports[34] Under alka-line conditions (pH 11) where it has been proposed that changes in nanorod aspect ratio are attributed to cation adsorption to the lateral (100) surfaces[15], our hydrothermal method growth occurs at a mea-sured pH of 5.5[38], under these conditions the non-polar (100) surface

is thought to remain neutral, with any charging of the (100) surface being negligible in comparison to the effect of additive stabilization

to the (002) growth surface, growth rates on the relative planes are shown schematically inFig 5

Fig 2 Cross-section (upper) and surface (lower) SEM images showing ZnO nanorods grown from hydrothermal baths containing equimolar (25 mM) Zn(NO 3 ) 2 /HMT and 10 mM PEI The modiﬁcation of nanorod morphology with the addition of a) 0, b) 10, c) 50, d) 100, e) 200, f) 500 mM KCl for a growth time of 120 min is shown, c) nanorod–nanorod 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.

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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 orientatedﬁlms at lower time periods The XRD data support this hypothesis; diffraction intensity per unit volume is at a maximum at 400 mM KCl for 120-min growth, showing increased alignment despite the reduced nanorod length (315 nm) of theseﬁlms

Table 1 Measured nanorod length and calculated statistical information for samples grown over the KCl concentration range 0 – 500 mM.

KCl concentration (mM)

Length (nm) Standard deviation (nm)

Sample size

Standard deviation/Length

Fig 4 XRD patterns and calculated diffraction data for ZnO nanorods ﬁlms grown with varying KCl concentrations, a) volume corrected and normalized ZnO (002) peaks for growth times of 40 (left) and 120 min (right), b) integrated (002) peak intensity corrected for volume and normalized to 0 mM KCl sample where values >1 indicate increased nanorod vertical alignment.

Fig 3 Morphological data calculated from SEM micrographs, showing a) volume,

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).

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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 investigated (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], platelet deposition 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 shorter and wider ZnO nanorods is observed,

consistent with reducedb002> growth due to stabilization of the (002)

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 with Dr Joseph Franklin,

Imperial College JMD is supported by the EPSRC, EP/J016039/1, and in

part by the Energy Futures Lab Imperial College London MAM is grateful for the support of a Royal Academy of Engineering/EPSRC Research Fellowship that supported him during this work

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Fig 5 Schematic illustrations showing the modiﬁcation of nanorod morphology with

KCl addition, a) 0 mM, b) b100 mM, c) >>100 mM.

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