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Understanding the factors that govern the deposition and

morphology of thin films of ZnO from aqueous solution{

Kuveshni Govender, David S. Boyle, Peter B. Kenway and Paul O’Brien*

The Manchester Materials Science Centre and Department of Chemistry, University of

Manchester, Oxford Road, Manchester, UK M13 9PL. E-mail: paul.obrien@man.ac.uk;

Fax: 0161-2754616; Tel: 0161-2754652

Received 31st March 2004, Accepted 19th May 2004

First published as an Advance Article on the web 15th June 2004

The influence of the choice of complexing ligand, zinc counter-ion, pH, ionic strength, supersaturation, deposition

time and substrate on the nature of ZnO films grown from chemical baths (CBD) are discussed. There are

significant differences between CBD and similar routes such as hydrothermal methods for ZnO films. Modelling of

speciation and experimental results suggest that acicular ZnO morphologies are best obtained by limiting the

concentration of one of either Zn21 or OH2 in the presence of a large excess of the other. The presence of a prior

ZnO layer can facilitate nucleation at lower levels of supersaturation and enable size tailoring of ZnO columns.

The point at which the substrate is introduced into the bath is crucial and can lead to a significant difference in

both the width of the rods and optical transparency of the films. HR-TEM has yielded important structural

information and a growth mechanism for single crystalline ZnO rods by CBD is described for the first time.

1. Introduction

The deposition from aqueous solution or CBD (chemical bathdeposition) of metal oxide thin films involves controlled pre-cipitation on a substrate via hydrolysis and/or condensationreactions of metal ions and/or complexes from aqueous solu-tion.1 In CBD, the crystal morphology is strongly influenced byexperimental conditions including chemical speciation in thesolution (ligand, pH, metal counter-ion, ionic strength are allimportant), the level of supersaturation, the temperature andthe nature of the substrate. Control of the size, shape andorientation of ZnO crystallites on the substrates is a pre-requisite for creation of high surface area materials for use inmany types of devices including photovoltaic and optoelec-tronic devices.2 The fundamental optical physics of ZnOnanostructures as a function of crystallite dimensionality haveonly recently been experimentally verified.3 In this paper, wehave modelled the initial composition of deposition baths anddeveloped strategies for morphological control during CBD ofZnO, leading to reproducible routes to polycrystalline andsingle crystal array thin films. The mechanisms of growth ofthin films of ZnO rod arrays have been studied using HR-TEMwith FIB sample preparation techniques.

In order to understand the important physiochemical processesinvolved in the CBD of ZnO, it is useful to summarise its solutionchemistry. In an aqueous solution, metal cations Mz1 are solvatedby water giving rise to aquo-ions, typically [M(OH2)n]

z1. TheM–OH2 bond is polarised which facilitatesdeprotonation of thecoordinated water. In dilute solutions, a range of monomericspecies exist such as ([M(OH2)n2p(OH)p]

(z2p)1 and other hydroxyspecies [M(OH)n] (it is often customary to omit the water);

ultimately oxoanions are formed. In order to form the poly-nuclear species, which subsequently develop into metal oxideparticles, reactions involving condensation reactions must occur.Two important processes have been recognized, olation is theformation of an ‘‘ol’’ bridge by reaction of a hydroxo- and aquo-species as follows:

M–OH 1 M–OH2 A M–OH–M 1 H2O (1)

Oxolation leads to an ‘‘oxo’’ bridge by the dehydration ofhydroxo-species:

M2–(OH)2 A M–O–M 1 H2O (2)

Zinc hydroxide is amphoteric and complexation by OH2

can lead to soluble species such as ‘‘[Zn(OH)3]2’’ and‘‘[Zn(OH)4]22’’ and hence ‘‘zinc hydroxide’’ is more solublein basic solution than a simple consideration of the solubilityproduct (y10216) would suggest.4

In the CBD of ZnO, ligands are employed to keep the freezinc ion concentration low. Raising the bath temperature pro-motes some dissociation of the zinc complex, leading tocontrolled supersaturation of the free metal ion. Zinc is a labilemetal ion in aqueous solution and equilibria within stirredsolutions are generally attained quickly. Thermodynamic model-ling of solutions representing the initial states of deposition bathsis a useful aid to understanding these systems.5,6

Morphology of ZnO crystallites

The thermodynamically stable phase for ZnO is zincite withthe wurtzite (hcp) structure although two cubic phases, ametastable sphaleritic (zincblende)7 and high pressure rocksalttype8 have been reported. The elongated hexagonal zinc oxidecrystal has both polar and non-polar faces. The former areeither Zn-terminated (the (0001) face) or O-terminated (the(0001) face). The latter include (1120) and (1010) faces. Themorphology of a particular crystal is determined by the slowestgrowing faces. Polar faces with surface dipoles are thermody-namically less stable than non-polar faces, often undergorearrangement to minimise their surface energy and also tend togrow more rapidly. Therefore it is necessary to distinguish between

{ Electronic supplementary information (ESI) available: Fig. A,B: filmthickness profiles for Zn–en and Zn–TEA systems; Fig. C: SEM imagesof ZnO films on ZnO template layers from HMT baths; Fig. D: XRDpatterns of ZnO microcolumns grown on ZnO template layers onTO(F) glass, and on Au/TO(F) glass; Fig. E: SEM images showingeffect of increasing ionic strength on ZnO film growth; Fig. F: SEMimages and grain size distributions of ZnO films from HMT baths;Fig. G: micrographs of ZnO thin film samples (Methods 1 and 2);Table A: thermodynamic data; Table B: rod dimensions for differentdeposition times. See http://www.rsc.org/suppdata/jm/b4/b404784b/D

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equilibrium and kinetic growth morphologies. The former corres-ponds to a minimum in the surface free energy. However, ther-modynamic models that consider morphological development as afunction of internal structure are poor at describing the solutiongrowth of ZnO at moderate temperatures.9

The morphology of films or powders needs to be optimisedfor each application. The morphology has been shown to affectcatalytic and photocatalytic activity.10 In ZnO powders, thenon-polar faces typically account for y80% of the total surfaceand the (1010) faces can determine the adsorption of com-plexes.11 A wide variety of ZnO crystallite morphologies areobserved for both precipitates and thin films includingcolumnar grains,12–14 rods,2,15 stars5 and spherical habits.2,16

Despite numerous studies, there is little understanding of themechanisms and factors that govern the observed morphology.Two growth mechanisms are usually discussed for the growthof single crystals: growth-dissolution-recrystallization pheno-mena or nanoparticle aggregation.17

The morphology of ZnO is greatly complicated by its pro-pensity to twinning. Twinned ZnO crystals have been investi-gated and shown to have a fourling structure, in which three‘‘legs’’ are related to the fourth (spine) by twinning. The spinesare related by twinning along the (1122) planes with a slightdistortion at the twinned faces, such that two angles betweenpairs of spines are 97.6u and four are 116.1u.18,19 In the secondphase of fourling crystal growth, three sheets lying in the threeplanes determined by the leg and the spine grow out from thespine. Many deviations from this idealised structure can occur.Fourlings of ZnO have been reported during forced hydrolysis ofzinc nitrate using HMT (hexamethylenetetramine),20 en (ethyl-enediamine)21,22 and NH4OH23 using hydrothermal methodsand by the decomposition of zinc hydroxide carbonate.24 Underhydrothermal conditions, bipyramidal twinning occurs via the(0001) plane whereas dumbbell-like examples are formed via the(0001) plane.25 The twinned relations of the crystallites werefound to be influenced by additives in the reaction medium.

Solution growth of ZnO rod array films

There is interest in highly structured metal oxide thin films ashigh surface area substrates for solar cells (e.g. in the Gratzel andeta configurations).26,27 One problem can be a need to annealas-deposited films in order to effect more complete crystal-lization. A low-temperature synthetic procedure that requires nopost-deposition annealing step is potentially very useful.28 Fewsystematic studies have been reported although recently Imai hasshown that using different substrates with identical baths yieldsdifferent morphologies.29 We have investigated solution-basedapproaches with a defined range of experimental conditions forgrowing single crystal arrays of ZnO by CBD.2

The literature describing ZnO precipitates is widely dis-persed. The formation of high-aspect ratio ZnO crystallites hasattracted interest.30 Earlier studies of ZnO single crystal rodshave not led to a universally accepted mechanism for the crystalgrowth. There is some evidence from ageing experiments thatthe formation of ‘‘Zn(OH)2‘‘ is a prerequisite for the controlledgrowth of needle-like ZnO crystallites.31 This observation isconsistent with a dissolution–reprecipitation mechanism inwhich Zn(OH)2 acts as a reservoir of zinc. The Zn21 concen-tration is held below the level where undesirable secondarynucleation processes, which can lead to twinning, occur.

The use of hexamethylenetetramine (HMT) is reportedwidely in the growth of acicular ZnO precipitates and thinfilms. An early report in the patent literature describes heatingaqueous solutions of zinc chloride (ZnCl2) and an organiccompound that form a base (such as HMT) or an acid (such asethylene chlorohydrin) by hydrolysis.32 The control of ZnOcrystallite morphology using HMT has been described byFujita et al.,33 where precipitates derived from zinc nitrate wereglobular or rod-shaped whereas those from zinc chloride were

both globular and acicular. Formation of ZnO in acidifiedbaths containing ZnCl2 and HMT was suggested via tworoutes; direct formation or via a Zn5Cl2(OH)8 intermediate.34

The globular form was predominant at high concentrations ofHMT. Andres-Verges and co-workers also used aqueous bathscontaining zinc nitrate and HMT to form ‘‘rod-like’’ pre-cipitates.20 Individual microcrystals were formed within anarrow concentration range of zinc salt and HMT dependingon the salt used, whereas aggregates were formed when thesolution pH was not lowered. A decrease in the pH of thesolution resulted in the formation of individual needles orprismatic microcrystals. These workers first suggested the ideaof orientated attachment to explain the acicular growth of ZnOcrystallites from solution. Spherical particles formed in solu-tion tended to aggregate along their polarized anisotropicc-axes to yield primary ‘‘rod-like’’ zinc oxide crystallites. Similarobservations have been made more recently for quasi-sphericalZnO nanoparticles formed in basic methanolic solutions.35

The deposition of thin films of rod-like ZnO arrays onfluorine doped tin oxide (TO(F)) glass substrates has beenreported by Vayssieres, using zinc nitrate–HMT solutionssimilar to the work of Andres-Verges’ but in closed vessels.15

The success of the method depends on separating homo-geneous and heterogeneous nucleation by controlling the inter-facial tension. We have grown ZnO rods on ZnO templatelayers on TO(F)-glass using carboxylate precursors in openbaths2 and demonstrated room temperature lasing from sucharrays.36 There is an important distinction to be made betweenaqueous solution deposition of ZnO (thin films or precipitates)from open and closed baths; the latter conditions may appro-ach hydrothermal growth. In general, the solubility of ZnO isgreater under hydrothermal conditions and growth is typicallyslower. Imai has also demonstrated the importance of substrateand shown that upright ZnO rods could be grown only oncrystalline ZnO templates.37

HMT is commonly used in solution routes to acicular ZnO,however it is not essential and its role is often misunderstood.The primary role of HMT in aqueous deposition baths is to actas a pH buffer. The kinetics of decomposition of HMT havebeen investigated.38 The conversion of HMT to formaldehyde(and ammonia) was found to be pH dependent in buffers ofconstant ionic strength, with the reaction half-life decreasingfrom 13.8 h at pH 5.8 to 1.6 h at pH 2.0 In a typical CBDexperiment, HMT provides a slow controlled supply of OH2

and is a poor ligand for zinc. Significant formation of theZn(OH)2 phase cannot be discounted, therefore a growthmechanism based on dissolution–reprecipitation of preliminaryhydroxy-zinc clusters and zinc oxide is possible in heateddeposition baths containing HMT.31

2. Results and discussion

Forced hydrolysis of zinc carboxylates

The forced hydrolysis described by Matijevic is the simplestprecipitative process for metal oxides and involves ageingof aqueous metal salt solutions at elevated temperatures (80–100 uC).23 The pH decreases during the course of the reactiondue to the deprotonation of the hydrated cation.

½M(H2O)x�nz DA

DH ½M(H2O)x{1(OH)�(n{1)z

??M(OH)n or MOy

(3)

Studies of the speciation of Zn21 in chemical baths allowdetermination of the initial degree of supersaturation withinthe system. Speciation calculations were carried out for bathscontaining only zinc acetate in order to determine the point atwhich the solution becomes supersaturated with respect to theZn(OH)2 (Fig. 1a). At 25 uC this critical value of pH is 6.68, ingeneral agreement with experimental results.

2 5 7 6 J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 5 7 5 – 2 5 9 1

Preliminary experiments involved attempts to depositZnO films using CBD methods (open baths) on TO(F) glassby forced hydrolysis of zinc salts. Due to the volatility of zinccarboxylates as compared to nitrate, chloride and sulfate salts,deposition of material only occurred for the former. Zincacetate and zinc formate solutions (0.025 mol dm23) containingthe TO(F) substrates were heated at 90 uC for 2 hours.

There are some unique characteristics of ZnO–carboxylatesystems, for example, Li et al. have demonstrated that forma-tion of ZnO from basic solutions of zinc acetate by forcedhydrolysis is only possible under conditions which allow therelease of volatile by-products and is hence facilitated by use ofopen baths or the periodic release of pressure in hydrothermalsystems.9 Carboxylates are often selectively adsorbed on thesurface sites on ZnO. Surface defects, on nominally Zn-freeO-terminated faces, in the form of steps with {1010} orientatedfaces that expose zinc cations, have been evidenced byexposure to formic acid with subsequent formation ofZn-bound formate species.39 There is indirect evidence fromwork involving etching of ZnO single crystals that indicatescarboxylates inhibit the rates of dissolution one hundred foldand tend to aid ageing and growth of crystallites.40,41 Dis-solution processes are controlled by Zn–O hydrolysis reactionsat Zn21 kink sites and catalysed by both H1 and ligands forzinc.

Precipitates were visible in the deposition bath with zincacetate, but not zinc formate. For the former, the homogeneousprocess was dominant and precipitation occurred almostimmediately upon heating of the solution. The films obtainedwere powdery and non-adherent and scanning electron micro-scopy of the films revealed poor surface coverage with rod-like particles, often with dumbbell morphology, randomlyorientated on the substrate with little evidence for perpendi-cular growth (Fig. 1b). The particle size distribution was largeand there is also evidence of twinning.

Development of crystal morphology

The production of a supersaturated (SS) solution is a pre-requisite for crystallization and the level of SS is important indetermining the final crystal morphology, as exemplified byCBD ZnO and shown in Fig. 2. We can broadly identify threemain growth regions, defined by the level of supersaturation.. for relatively low levels of supersaturation in the region

below SS*, heterogeneous nucleation dominates. Crystal facesgrow via the outward displacement of a growth spiral originat-ing from screw dislocations (the so-called BCF mechanism)42 inthe central region of the face. Well-formed polyhedral crystalsare generally produced under these conditions.. in the intermediate supersaturation range SS* v SS v

SS**, growth occurs by a mixture of slow spiral growth and2-dimensional nucleation of clusters on to crystal faces fromsolution (polynuclear processes). In this region, 2D growthgenerally predominates. Since the probability of occurrence of2D nucleation is far greater near to edges and corners of faces,growth steps will advance from them to yield a crenellated faceand often ‘‘hollowed’’ rod morphology.. for high supersaturation SS w SS**, homogeneous nuclea-

tion is more important. The crystal faces become rough andcontinuous linear growth occurs. In this region, ZnO crystalsadopt a dendritic to spherulitic morphology and withincreasing supersaturation, growth kinetics are controlled bybulk diffusion as the solution becomes more viscous.

The acicular rod morphologies, typically observed for CBD-ZnO films using baths containing HMT and a zinc salt atmoderately acidic pH 5,2 can be explained in terms of akinetically-controlled reaction involving relatively high con-centrations of one (i.e. Zn21) over the other (i.e. OH2 ) com-ponent. Similar arguments apply to growth of columnar ZnOfilms using basic baths and ethylenediamine ligand, where thehigh–low concentration relationship is reversed.

Influence of ligand on crystallite morphology

For cations that are not easily hydrolysed, such as Zn21,precipitation of the metal hydroxide can be promoted byraising the pH with a base. Lewis bases, such as ethylenedia-mine (en) and triethanolamine (TEA), can also form complexeswith the metal. The complexes can also act as a reservoir formetal, by buffering the free concentration to below the pre-cipitation point.

In a ZnO CBD system where both ZnO and ‘‘Zn(OH)2’’ maybe formed, the less stable phase in contact with the media will ingeneral precipitate first. Under conditions such that thehydroxide precipitates first, changing the temperature or pHcan result in the formation of the oxide. The oxide can therebyform via a dissolution/reprecipitation process or by a phase

Fig. 1 a. Speciation diagram illustrating the hydrolysis of zinc acetate(0.025 mol dm23) at 25 uC. Zn(OH)2 (s), represented by AH-2(s),is predicted to precipitate at pH 6.68. A ~ Zn21; B ~ CH3COO2.b. SEM image of ZnO precipitates on TO(F) glass formed from forcedhydrolysis of zinc acetate (0.025 mol dm23) at 90 uC.

Fig. 2 Relationship between supersaturation in deposition baths, rateof crystallite growth and morphology.

J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 5 7 5 – 2 5 9 1 2 5 7 7

transformation. A lattice rearrangement, however, is morecomplicated and depends on the degree of rearrangement anddissolution of hydroxide to provide Zn21. The uniformity ofthe initial hydroxide particles may be key in controllinguniformity of the oxide particles.

There are several reports in the literature on the effect ofligands in the deposition bath on the morphology ofZnO.20,21,30 The hydrolysis of the cation can be controlled bya slow release of hydroxide into the metal salt solution. Atelevated temperatures both HMT and urea undergo slowchemical decomposition to generate OH2 ions.

Deposition of ZnO films on TO(F) glass using en as complexingagent

We have previously demonstrated that good quality ZnO thinfilms may be deposited from baths containing zinc acetate(0.018 mol dm23) and en (0.042 mol dm23, at a final pH of 11).5

Further experiments were conducted using similar bathconditions but with different zinc salts. Films were formedfrom baths containing en and either zinc acetate, formate,sulfate or chloride. No films could be deposited from bathscontaining zinc nitrate. Speciation calculations were performedfor baths containing zinc acetate and en (at 25 uC), thedistribution plots indicated that baths are supersaturated withrespect to the hydroxide by pH ~ 6.92 (Fig. 3a and ESI:{ TableA for binding constants). Small decreases in stability constantvalues at higher temperatures did not produce significantchanges to speciation diagrams. It was found that good qualityfilms were produced at pH 11 (at 70 uC, 2 h), such baths aresupersaturated and the concentration of free zinc is very low,orders of magnitude lower than [OH2].

All ZnO films were of the zincite phase (JCPDS 36-1451) asdetermined by XRD (Fig. 3b). The crystallinity was slightlyimproved upon annealing in air at 400 uC. In general, films werethin (y1 mm) and peaks originating from the TO(F) substratewere evident in the XRD. Infrared spectra (ATR-FTIR) were

recorded to determine whether material such as zinc hydroxide(crystalline or amorphous) was present. No peaks indicative ofzinc hydroxide were observed.

Scanning electron micrographs of ZnO films grown frombaths containing en and zinc (as the acetate, formate, chlorideor sulfate) provided useful information and, with the exceptionof zinc sulfate, films comprised twinned crystallites that formedas starlike particles with needle-like spines (Fig. 4a: i, ii, iii).McBride has postulated that these star-like morphologies growby a modified LaMer mechanism.31 Stars are formed fromtwinning along the (1122) plane of the hexagonal lattice. Cross-sectional SEM indicated that the film thickness in all cases wasof the order y1 mm. Films formed from sulfate-containingbaths were less crystalline and appeared to be a mixture ofsmall flowerlike particles and larger flat platelets (Fig. 4a: iv).More detailed examination of films deposited from baths con-taining en and zinc acetate appeared to show that the indivi-dual arms of the starlike ZnO crystallites possessed somehelical structure (Fig. 4b). Similar morphology has beenreported for ZnO nanorods grown on nanocrystalline ZnOtemplates, using baths containing HMT and zinc nitratemodified by addition of sodium citrate.43 It has been suggestedthat the presence of citrate generates the helical morphology.Moreover, the structure closely resembles that observed fornacreous calcium carbonate in red abalone (gastropod Haliotisrufescens) and thus may offer insights into biomineralisationprocesses. It appears that in the presence of acetate and en,ZnO may also be deposited with similar helical structures.Growth kinetics were monitored using the QCM technique,typically an induction period was followed by a growth and ashort termination phase. The QCM technique is not directly

Fig. 3 a. Speciation diagram for Zn–en system at 25 uC. Dashedline represents zinc hydroxide precipitation point. A ~ [Zn21] ~0.006 mol dm23 and B ~ [en] ~ 0.042 mol dm23, [CH3COO2] ~0.012 mol dm23. AH-x represents the soluble zinc-hydroxy species.b. X-Ray diffractograms obtained for as-deposited ZnO films fromi. en-baths (70 uC). ii. TEA-baths and iii. HMT-baths on TO(F) glass.* Reflections assigned to SnO2–glass substrate (JCPDS 41-1445).

Fig. 4 a. SEM images (top) of ZnO films from en-baths (70 uC, pH 11)containing: i. zinc acetate; ii. zinc chloride; iii. zinc formate and iv. zincsulfate. [Zn] ~ 0.018 mol dm23, [en] ~ 0.042 mol dm23. b. SEM image(below) of ZnO film deposited from zinc acetate–en bath showing indivi-dual crystallite arms possessing helical structures as described elsewhere.43

2 5 7 8 J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 5 7 5 – 2 5 9 1

comparable to growth on TO(F)-glass substrates as the micro-balance uses an Au-coated crystal. However useful com-parisons can be made for different baths, assuming all otherfactors (temperature, stirring rate, etc.) are equal. For theen-system (ESI:{ Figure A) two distinct linear growth phasesfollowing the induction period could be identified; a short andrapid growth period over y250 s (rate ~ 0.12 nm s21) and aslower growth phase from y1500–3000 s (rate ~ 0.01 nm s21).

Deposition of ZnO films on TO(F) glass using TEA ascomplexing agent

In order to investigate the sensitivity of particle morphologiesto changing bath chemistry, our existing literature preparationwas followed with minor modifications (pH 12 vs. pH 11 andTEA : metal ratio 5 : 1 vs. 4 : 1 and a similar metal ion con-centration). Experiments were conducted at 70 uC with TEA(0.072 mol dm23) using different zinc salts (0.018 mol dm23) onTO(F) glass substrates as for the Zn–en experiments. The finalpH of the bath was 11. In initial experiments films were onlyobtained from baths containing zinc nitrate.

Speciation calculations were carried out on baths containingzinc nitrate and TEA. Supersaturation with respect to thehydroxide occurred at pH 6.44 similar to the en system (Fig. 5a).Moreover, the best quality films were produced at pH 11, wherethe bath is again supersaturated and the concentration of the freezinc is orders of magnitude lower than [OH2], similar to theen-system. However in contrast to the en-system, in which thezinc is mainly in the form of Zn–en complexes at pH 11, for TEAthe zinc speciation is defined by zinc-hydroxy complexes.

Comparison of micrographs of thin films from the presentstudy (Fig. 5b) with those obtained for ZnO powders,21 undersimilar bath chemistry, leads to the following conclusions.Firstly, the particle morphologies are virtually identical in bothcases. However, there is a substrate effect, as the widths ofparticles comprising films are narrower than those formed insolution (Fig. 5b: i and iii). The morphology of the films can bedescribed as spherical aggregates.

For both ‘‘as-deposited’’ and annealed films, the formationof crystalline zincite was evident from XRD (Fig. 3b: ii).However, unlike films deposited from en-containing baths, nochanges in the diffraction pattern were observed upon annealing

in air. Again no evidence for zinc hydroxide in the films wasobtained from IR spectroscopy. Results obtained from QCMmeasurements were different from those with en-containingbaths (ESI:{ Figure B). The induction period (0–500 s)was characterised by oscillations, followed sequentially by arelatively linear period of rapid growth (y500–1500 s; averagerate ~ 0.09 nm s21) and then slower deposition (y1500–2500 s;average rate ~ 0.02 nm s21). In comparison to the previouslydescribed en-system, in the TEA system the transition from the‘‘fast’’ to ‘‘slow’’ growth was relatively abrupt.

Deposition of ZnO films on TO(F) glass using HMT

Further studies of the effect of the chelating ligand on ZnOparticle morphology were carried out with HMT, whichdecomposes slowly in heated aqueous solutions to yieldammonia and formaldehyde. The concentration of HMT hasbeen reported to influence the rate of ZnO formation.34

Speciation calculations for the HMT system indicate that thepH at which the bath becomes supersaturated at 25 uC withrespect to the hydroxide is y6.8 (Fig. 6a). All other factorsbeing equal (i.e. ligand and metal concentration), at values ofpH w 6, supersaturation in terms of ‘‘zinc hydroxide’’ willbe greater for HMT than en. Thin film formation requiresproduction of a supersaturated system hence we would expectZnO film formation to occur at lower pH values for HMT thanen. HMT decomposes under CBD conditions, which com-promises any simple thermodynamic speciation calculations.However, under typical experimental conditions, ZnO thinfilms will be deposited from supersaturated bath solutionscontaining relatively high concentrations of free zinc and low

Fig. 5 a. Speciation diagram of Zn–TEA system (above) at 25 uC withA ~ [Zn21] ~ 0.018 mol dm23 and B ~ [TEA] ~ 0.072 mol dm23.Dashed line represents zinc hydroxide precipitation point. b. (below)i. Top view and ii. cross-section SEM images of annealed ZnO filmsfrom TEA-baths (70 uC, pH 11). [Zn] ~ 0.018 mol dm23; [TEA] ~0.072 mol dm23. iii. Precipitates obtained from solution.

Fig. 6 a. Speciation diagram (top) of Zn–HMT system at 25 uC.Dashed line represents ‘‘zinc hydroxide’’ precipitation point. A ~[Zn21] ~ 0.0083 mol dm23 and B ~ [CH3COO2] ~ 0.016 mol dm23

and C ~ [HMT] ~ 0.025 mol dm23. b. SEM images (below) ofZnO films deposited from HMT-baths (0.1 mol dm23 Zn(NO3)2 and0.1 mol dm23 HMT) in stoppered flasks in oven: i. pH ~ 6.8, TO(F)glass, precipitation initiated at 50 uC; ii. pH ~ 6.8, ZnO templatelayers, precipitation initiated at 50 uC; iii. pH ~ 5.0, TO(F) glass,precipitation initiated at 95 uC; iv. pH ~ 5.0, ZnO template layers,precipitation initiated at 95 uC.

J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 5 7 5 – 2 5 9 1 2 5 7 9

concentrations of hydroxide. In essence this represents areversal of the situation encountered with the en-system.

Several workers have reported the homogeneous precipita-tion of ZnO in HMT baths but Andres-Verges and co-workersdemonstrated that a decrease in solution pH resulted in theformation of individual needles or prismatic microcrystals frombaths containing zinc nitrate and HMT.20 We have reasonedthat the presence of a pre-existing template layer (deliberate orotherwise) would promote heterogeneous growth of ZnO films.Moreover, as precipitation occurred rapidly in baths in whichno pH adjustment was made, by using both ZnO templates andlower solution pH, good quality films can be deposited. SEMimages were recorded for films deposited on ZnO templatelayers at an unadjusted pH of 6.8 (Fig. 6b: ii) and the lower pHof 5 (Fig. 6b: iv), using bath conditions described above. It isclearly seen that films deposited at both pH values are dense,importantly however, those films deposited at lower values ofpH have well defined faces. The phenomenon has beenrationalised in terms of slower growth kinetics.20

The above observations have led to a new two-step approachfor the solution growth of highly orientated ZnO microcolumnarrays. The procedure involves deposition of ZnO templatelayers of the desired morphology and subsequent overgrowthof ZnO microcolumns on the templates.2 Similar strategieshave been employed subsequently by others.44–46

Acicular morphologies encountered for solution grown ZnOare generally assumed to result from one of two mechanismsbased on dissolution–recrystallization phenomena or alter-natively nanoparticle-oriented aggregation. Results obtained byAndres-Verges et al. are more consistent with the latter process.However, the processes by which the particles couple at a certainsize are not clear. The spherical particle size was found to becritical; these fuse to form embryonic rod-like microcrystals. Itwas postulated that the oriented coupling through the particlec-axis may be related to the net polarisation that occurs in thisdirection in ZnO microcrystals. The particles subsequentlydevelop by a ripening mechanism. In this latter step, the particlesincrease in length but not in width until they reach the requireddimensions. The formation of the well-developed faces is the finalstep that takes place under slower kinetic conditions.20 Recentlythe group of Weller have reported similar observations for ZnOnanoparticle assemblies.35

The overall pattern of results indicates the sensitivity of themorphology of generated ZnO particles to the nature of thecomplexing ligand. The presence of en in the baths typicallyleads to star-like morphologies, those with TEA produce spheri-cal aggregates and HMT leads to formation of rods/needles.

Influence of counter-ion on growth of ZnO crystallites

All other factors being equal, changing the counter-ion for zincoften yields different crystallite morphology. Changes inmorphology may derive from the effects of species that actas promoters or inhibitors for nucleation and growth processes.For example, by impeding growth on one or more faces, thecrystal develops only in certain directions and hence the finalmorphology is altered. The process is often termed ‘‘crystalgrowth inhibition’’. For example, studies of the interaction ofdiblock copolymers with ZnO crystal faces have shown thatthese substances influence both morphology and particle sizedistribution of growing crystallites by adsorbing onto specificfaces and retarding growth perpendicular to these faces.47 Anadditional factor associated with the counter-ion of the zincsalt is volatility of the component or derived species (especiallyfor use of zinc carboxylates).

Adsorption of ions from solution to substrate may arise simplyas a consequence of simple electrostatic attraction of species onthe charged surface of ZnO (as a consequence of the solution pHand amphoteric surface hydroxyl groups). However specificadsorption effects may occur and lead to concentration

dependent effects. With increasing concentration of a givenspecifically adsorbed ion (SAI), the point of zero charge (PZC)may shift and in addition, the sign of the surface charge as givenby the zeta (f)-potential may reverse. The first effect occursbecause the SAI may interfere with the adsorption of potential-determining ions (i.e. H1 and OH2 for ZnO in contact withaqueous solution). The second effect arises from adsorption ofthe SAI into the inner layer of the solid–solution interface, whichin turn can reverse the charge of the outer diffuse double-layer.

In the present study, the influence of zinc salt counter-ion oncrystallite morphology has been investigated. As rod-like ZnOmorphologies were of particular interest in the present study,initial experiments were conducted using TO(F) glass substratesand baths containing HMT and zinc nitrate, chloride, per-chlorate, acetate or sulfate. Growth morphology is highlysensitive to the level of supersaturation. Speciation calculationsindicate that there are small, but significant, differences in levelsof ‘‘zinc hydroxide’’ supersaturation and ZnO precipitationpoints for baths containing different counter-ions. In terms ofthermodynamic stability of complexes, the calculations suggestthat precipitation should follow the order sulfate, nitrate ychloridey formate and acetate. Empirical observations followedthe order sulfate, perchlorate, chloride, nitrate, formate andacetate anion. The induction period for baths containing acetatewas significantly longer than for baths containing the other salts.Baths containing sulfate ion did not produce films on TO(F) glassin this study. Images obtained by SEM of films deposited usingzinc chloride (Fig. 7b), perchlorate (Fig. 7c) and nitrate (Fig. 7d)on TO(F) glass indicated that films were not dense and theacicular crystallites poorly orientated. Although films depositedusing zinc acetate were dense the rod-like crystallites were notwell orientated (Fig. 7a). The particle dimensions also variedsubstantially for each counter-ion used.

In subsequent experiments, ZnO template layers were used assubstrates (prepared using TEA baths) deposited on TO(F)coated glass. It was observed that good quality films, comprisingdiscrete upright ZnO rods, on template layers, could only beformed when substrates were immersed just before or as visibleprecipitation occurred. For comparison attempts were also madeto deposit films using zinc formate. Scanning electron micro-graphs (ESI:{ Figure C) revealed that films deposited on ZnOtemplates were well orientated. The dimensions of crystallitesvaried significantly for each precursor used. In agreement withthe earlier results, no films were obtained using zinc sulfate.

From the results obtained in the present study, it is clearthat use of different counter-ions in baths containing growingcrystallites has a noticeable effect on final film morphology.Differences in kinetics (growth rates, adsorption of counter-ions on growing crystal faces, etc.) are the likely origin. The

Fig. 7 SEM images of ZnO films on TO(F) glass from HMT baths(0.025 mol dm23 zinc salt and 0.025 mol dm23 HMT, pH 5, 90 uC ,1 hour); A. Zn(CH3COO)2; B. ZnCl2; C. Zn(ClO4)2; D. Zn(NO3)2.

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effect is more pronounced in precipitates removed from thebaths used to deposit thin films and characterised by SEM.Material obtained from baths containing acetate (Fig. 8a: i),formate (Fig. 8a: ii) and chloride baths (Fig. 8a: iii) were mainlyrod-like whilst crystallites obtained from baths containingnitrate (Fig. 8a: iv) and perchlorate (Fig. 8a: v) anions wereacicular. Those using sulfate were composed of flat hexagonalplatelets (Fig. 8a: vi). Twinning was observed in ZnO particlesobtained from perchlorate baths.

A comparison of the dimensions of the crystalline pre-cipitates from the bath with crystals comprising films (ESI:{Figure C) gives an indication of the effect of the template layer.The ZnO precipitates obtained from acetate and formate bathsdiffer significantly in their typical dimensions. By contrast, the

crystallites comprising films do not exhibit significant differencein width. The platelets arising from sulfate baths (no films couldbe deposited) are flat and arey30 mm wide, much larger than anyof the particles formed from other systems. The most likelyexplanation for the formation of these platelets is considerablecrystal growth inhibition of the polar c faces by sorption of thesulfate anions.48,49 Films obtained from chloride baths werehomogeneous and again show the effect of the template layer onmorphology. The powders, however, are composed of bothagglomerated rods and much larger flakes. Nitrate- andperchlorate-containing baths produced less homogeneous films.

The driving force behind the directional and ordered aggre-gation has been studied by Ocana et al., who observed similargrowth patterns using TEM for the production of ellipsoidala-Fe2O3 particles with phosphate anions.50 Matijevic et al. alsostudied the effect of anionic surfactants on the size and shape ofZnO particles. They found that the addition of a fluorinatedpolyether carboxylate to a system, which typically producedintertwined ZnO crystallites, results in the formation of separ-ated rod-like particles.30

The clear differences in CBD of the ZnO crystallite mor-phology as a function of counter-ion are interesting, as theycontrast with previous results obtained by our group usingchemically similar systems and hydrothermal conditions.21 Inthe first instance, it is clear that the behaviour of differentcounter-ions cannot be adequately described as simple adsorp-tion of different ions on the ZnO surfaces. As described earlier, itis known that carboxylate species are often selectively adsorbedon a variety of surface sites on ZnO. Studies in the vapour phasehave shown that formate species adsorb on the ZnO (0001) polar(Zn terminated) and ZnO (1010) non-polar faces while thepristine (0001) surface was unreactive.51 The difference in reac-tivity was accounted for by the lack of acid–base pairs on theoxygen polar face. Interaction of formate with the (0001) face canoccur at defect sites, i.e. at exposed Zn21 at steps or oxygenvacancies. Three structures have been proposed for the formateanion sorbing onto metal oxide surfaces, viz. bridging, bidentateand unidentate.52 Formate was found to coordinate to the metalin a bidentate fashion on the (1010) face and in a monodentatemanner on the (0001) face or (0001) face at defect sites.

TEM images and SAED of crystallites removed from ZnOfilms deposited from HMT/acetate-containing baths ontotemplate layers are shown in Fig. 8b: i and ii. Precipitatesobtained from these baths were similarly characterised (Fig. 8b:iii and iv). Although the sample preparation resulted in somecontamination of ZnO rods by organic material, it was clearthat many of the rods were composed of discrete subunits,orientated and connected along the c-axis of the growingcrystallites. The subunits are of the order y1 mm in length, forboth ZnO rods formed in solution and as thin films. Themechanism of growth of ZnO rods is clearly complex and mayinvolve a number of processes. More detailed studies wereconducted using TEM and focussed ion beam (FIB) samplepreparation techniques and are discussed later.

Influence of substrate

The effect of the substrate on the nature of the ZnO films formedwas studied. Deposition experiments were performed usingidentical bath conditions (0.025 mol dm23 Zn(CH3COO)2 and0.025 mol dm23 HMT, pH 5, 90 uC, 1 h) but differentsubstrates: gold-coated TO(F) glass, ZnO template layers onTO(F) glass, TO(F) glass and single crystalline (0001) sapphire.

Micrographs for the first three are shown in Fig. 9a(identical magnification). There are marked differences inaverage diameter of the crystallites deposited on the threesubstrates. By contrast, the lengths of rods (as-determined bycross-sectional SEM) were similar, in the range 2–2.5 mm. Thefilms deposited on both Au-coated TO(F) substrates and the ZnOtemplate layers were well-aligned. For the microcolumns grown

Fig. 8 a. SEM images (top) of ZnO precipitates obtained from HMTbaths (0.025 mol dm23 zinc salt and 0.025 mol dm23 HMT, pH 5, 90 uC,1 hour); i. Zn(CH3COO)2; ii. Zn(HCOO)2; iii. ZnCl2; iv. Zn(NO3)2;v. Zn(ClO4)2; vi. ZnSO4. b. TEM images and SAED (below) of ZnOprecipitates obtained from zinc acetate–HMT baths.

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directly on Au/TO(F) glass there are no signals arising frompolycrystalline ZnO template layers (ESI:{ Figure D). Crystallitesize distributions were determined from micrographs of ZnOnanocolumns deposited on Au-TO(F) glass (Fig. 9b) in order toquantify the polydispersity and homogeneity of films. Using animage analysis routine,53 average (mean) radii values of 266 nm(standard deviation 54 nm) were determined.

Growth of ZnO films on single crystal (0001) sapphire fromHMT-containing baths was attempted. Preliminary experi-ments using similar material from co-workers appeared toindicate that unusual 60u rotational twinned structures couldbe deposited (Fig. 10a). Previously a great deal of researchhas focussed on ZnO films grown on (0001) sapphiresubstrates.54,55 Other workers have demonstrated epitaxial

relationships between (1120) ZnO and (0112) Al2O3 and (0001)ZnO and (0111) Al2O3.56 Deposition experiments were alsoconducted on substrates that were used to produce the filmsshown in Fig. 10a, after rinsing in nitric acid to remove depositsand then using the standard cleaning procedure. Arrays of ZnOrods (rather than the previously observed rotational twins)were found to grow in troughs or scratches, formed presumablythrough the cleaning process (Fig. 10b).

Influence of solution pH

In the present study, the influence of bath pH on as-depositedfilms of ZnO, grown on ZnO template layers, using the zincacetate/HMT system was investigated. As for zinc nitrate/HMTsystems, homogeneous precipitation occurred more rapidly inzinc acetate/HMT systems where no pH adjustment was made ascompared to those where the pH was lowered. The SEMmicrographs (Fig. 11) reveal that films from such baths (pH ~6.8) comprised crystallites of different sizes with poorlydeveloped crystal faces. In subsequent experiments, baths wereadjusted to pH ~ 5 using HCl or acetic acid. In these cases, filmswere composed of rods with well-defined faces and in addition,the individual particles had a smaller average diameter than thoseobtained from the former. The non-chloride containing bathsalso showed the slowest homogeneous precipitation, i.e.possessed the longest induction time before reaction occurred.

Effect of changing ionic strength

For sparingly soluble metal compounds, such as ZnO or Zn(OH)2

in the present study, increasing the ionic strength tends to increasethe solubility of the solid phase because the activity of the ionsdecreases. In order to investigate the effects of ionic strength onthe nature of ZnO films, experiments were conducted using the Znacetate/HMT system on ZnO template layers.

This work involved the variation of the ionic strength in theZn–HMT system using KCl as a background electrolyte. Theionic strength of the system was varied in the range [KCl] ~0.025–0.4 mol dm23. The precise ionic strength of the bathcould not be determined as solution pH was adjusted bydropwise addition of glacial acetic acid. However, useful trendscould be identified. It was observed that as [KCl] increased,

Fig. 9 a. SEM images (top) of ZnO films from HMT baths (0.025 mol dm23

Zn(CH3COO)2 and 0.025 mol dm23 HMT, pH 5, 90 uC , 1 hour) oni. Au-coated TO(F) glass; ii. ZnO template layers on TO(F) glass;iii. TO(F) glass. b. Crystallite size distribution (below) determined fromSEM images of ZnO nanocolumns deposited on Au-TO(F) glass. Radiiof ZnO microcolumns reach terminal values within few minutes ofgrowth, leading to development of a stable size range that could befitted to a normal Gaussian distribution (400 data sets, R ~ 0.984).

Fig. 10 SEM images of ZnO films deposited on single crystal sapphire(0001) from baths containing zinc acetate (0.025 mol dm23) and HMT(0.025 mol dm23) at 90 uC. a. Starlike crystallites obtained, some 60urotational twins are observed; b. rod arrays grown in troughs oncleaned substrate previously used to deposit films shown in a.

Fig. 11 SEM images of ZnO films from HMT baths (0.025 mol dm23

Zn(CH3COO)2 and 0.025 mol dm23 HMT) deposited on ZnO templatelayers at A. pH 6.8; B. pH 5 (adjusted with HCl); C. pH 5 (adjusted withCH3COOH).

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homogeneous precipitation occurred more rapidly. It wasapparent that no ZnO rods were deposited from baths with[KCl] w 0.2 mol dm23. The effect (if any) of zinc-chlorocomplex formation, which would tend to increase stability toprecipitation with increasing [KCl], appears to be insignificant.

As shown in Fig. 12, SEM images reveal that within therange [KCl] ~ 0.025–0.15 mol dm23 the width of individualcrystallites was decreased from those formed from bathscontaining no KCl. The observation could be rationalised interms of specific ion adsorption e.g. on kink sites on the a-facesof the ZnO crystal, which could indirectly promote growth inthe c direction. However, no measurements were done todetermine if particles also decreased in length. Ionic strengtheffects have been demonstrated for solution growth of calcitecrystals, at low ionic strength, aggregation and alignmentphenomena are present, no such mechanisms operate at higherionic strength.57 For the range [KCl] w 0.15 mol dm23 parti-cles changed morphology from rods to discs. Similar resultswere obtained using zinc formate rather than acetate as aprecursor (see ESI:{ Figure E).

The influence of ionic strength on ZnO film growth is clearlycomplex. From CBD experiments employing zinc acetate orformate and HMT, it appears that there is a limiting ionic strengthwhereby film growth is possible, beyond which only homogeneousprecipitates may be formed. Changing ionic strength also hasmorphological effects, as evidenced from decreasing ZnO rodcrystallite width with increasing ionic strength.

Effect of varying growth times

Andres-Verges et al. found that for ZnO grown under hydro-thermal conditions, an increase in bath temperature forsolutions containing ZnCl2 and HMT resulted in a change inmorphology from rods to needle morphology.20 In thesestudies, the length and final shape of the rod were shown to bevery dependent on the growth time. It is also reported thatageing time also has a major influence on final crystalmorphology, ZnO habit changing from flaky aggregates toflower-like morphology with increasing growth time.58

The development of ZnO rod array films (using zinc acetate)on ZnO template layers as a function of deposition time wasinvestigated. Films were removed from baths after differentperiods of time and characterised by SEM. The micrographs(Fig. 13) show that crystallites possess an average diameter ofy300 nm after 2 minutes and y1 mm after 1 h. The cross-sectional SEM images allow the length of the rods to bedetermined. However it was only possible to determine theaverage rod length for those grown for times greater than10 min. At shorter growth times, it was difficult to distinguishthe template layer from the ZnO rod growth (ESI:{ Table B).Evidence for oriented attachment processes was obtained forhomogeneous precipitates lying on films (Fig. 13D).

Summary of preceding survey of factors that influence CBD of ZnO

In general, the most likely crystal morphology for ZnO underkinetically controlled growth conditions and at low super-saturation is as acicular rods, whereas under thermodynamiccontrol, prismatic structures are expected.17 Hence by limitingthe supply of one reactant employing a large excess of theother, anisotropic growth is facilitated along the polar c-axis ofthe zincite structure. Several factors exert a profound effect onthe morphology of ZnO thin films grown by CBD and there areimportant differences between CBD and the chemically similarhydrothermal routes to ZnO powders or films. Under hydro-thermal conditions, Trindade et al. have reported that neither

Fig. 12 SEM images showing the effect of ionic strength on ZnO films(on template layers) obtained from HMT baths (0.025 mol dm23

Zn(CH3COO)2 and 0.025 mol dm23 HMT, pH 5, 1 h). The ionic strengthwas varied using different concentrations of KCl: A. 0 mol dm23;B. 0.025 mol dm23; C. 0.05 mol dm23; D. 0.075 mol dm23; E. 0.1 mol dm23;F. 0.125 mol dm23; G. 0.3 mol dm23; H. 0.4 mol dm23.

Fig. 13 SEM images of ZnO films from HMT baths (0.025 mol dm23

Zn(CH3COO)2 and 0.025 mol dm23 HMT) deposited on ZnO templatelayers deposited for: A. 2 min; B. 5 min; C. 10 min; D. 20 min; E. 30 min;F. 1 h. For D, a large dumbbell precipitate formed homogeneously in thebath appears to indicate formation by orientated attachment.

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solution pH, choice of counter ion and ionic strength, bathtemperature or reaction time exert any significant influence onthe characteristics of resultant ZnO powders. The choice ofligand and nature of hydrothermal treatment were found to beimportant,21 results that are clearly different from thoseobtained for CBD films.

The results obtained in the present study suggest that nuclea-tion and growth promotion/inhibition are significant and varyconsiderably with small changes in bath chemistry. For example,only large homogeneous precipitates can be obtained fromdeposition baths in the presence of sulfate, which appears tocompletely impede nucleation and growth of ZnO thin films, onboth TO(F) glass and ZnO template layers. Moreover, themorphology of these precipitates (flat hexagonal y30 mm wideplatelets) is very different to those obtained using other zinc salts(i.e. acetate, formate, nitrate, chloride and perchlorate). Flattenedcrystallites were also observed in experiments involving increasingionic strength (using KCl) and HMT/zinc acetate systems.

In the aqueous CBD of ZnO, the influence of counter-ion oncrystallite morphology may be through weak electrostaticforces (e.g. specific adsorption of ions resulting in change inp.z.c. or f-potential, double layer compression, etc.) or moresignificant physi- or chemisorption behaviour (e.g. at attractivekink sites on crystal faces). However, increasing ionic strengthalso tends to increase the solubility of the solid phase beingdeposited but decrease the values of thermodynamic stabilityconstants of the metal–ligand complexes in deposition baths.These facts highlight the complexity of CBD systems andillustrate the difficulty in unambiguously assigning directcausal relationships between the many interrelated parameters(e.g. crystal morphology and choice of counter-ion).

Dependence of film morphology on point of introduction ofsubstrate in deposition bath

Empirically it has been found that the size of the crystallitescomprising films depends largely on when the ZnO templatesubstrate is immersed in the bath. If the substrate is immersed inthe bath from the point at which the pH of the solution is adjustedto 5 at ambient temperature, the nanocrystalline templatedissolves as the bath is heated to 90 uC and no film is deposited.If the substrate coated with a nanocrystalline ZnO template isinserted in the bath about 15 min after the desired temperature of90 uC is reached (TA), the bath produces no homogeneousprecipitation during the 2 h reaction period and a dense white filmis obtained. Images obtained by SEM reveal that the particles arey 250 nm wide and y1.2 mm long (Fig. 14C,D). However, if thesubstrate is placed in the heated deposition bath at the point ofonset of visible turbidity (ca. 30 min after reaching 90 uC) (TB), atransparent film is obtained and some homogeneous precipita-tion is present in the bath (Fig. 14A,B). The ZnO rods comprising

such films are y90 nm wide and y600 nm long. The aspect ratioof the latter (y6.7), grown from time TB, is clearly better than theformer (y4.8) grown from time TA.

Use of nanocrystalline template layers for overgrowth of ZnOfilms by forced hydrolysis of zinc carboxylates and using differentchelating ligands

The pattern of results in the preceding sections indicated thatthe simplest and most promising synthetic route towardsgrowth of ZnO nanorod array films would involve depositionon to nanocrystalline ZnO template layers. Preliminaryexperiments involved the forced hydrolysis of zinc acetateand zinc formate using nanocrystalline ZnO templates on glasssubstrates. It was very apparent that very low levels ofhomogeneous precipitation occurred using baths containingzinc acetate, moreover no visible homogeneous precipitationcould be detected using zinc formate solutions. Transparentfilms were obtained with both precursors. Subsequent experi-ments involved the use of these template layers for theovergrowth of ZnO films deposited using different chelatingligands viz. en, TEA and HMT. Reactions were similar to thosereported earlier for each ligand on TO(F) glass substrates. Thetransparent films were characterised by XRD and SEM.

X-Ray diffractograms obtained for nanocrystalline ZnO tem-plates and overlying films are shown in Fig. 15. For films pro-duced by forced hydrolysis of zinc acetate, using en and HMT,there is strong enhancement of the (002) peak. For films depositedfrom baths containing TEA ligand the effect is not as pronounced.

As indicated earlier, attempts to deposit ZnO films by forcedhydrolysis of zinc carboxylates with no added ligand wereunsuccessful using TO(F) glass substrates. However, SEMimages provide evidence that films are produced using identicalbaths (0.025 mol dm23 zinc acetate or zinc formate) onnanocrystalline ZnO templates (Fig. 16A,B and C,D respec-tively). The films were homogeneous and composed of rod-likearray structures (ca. 140 nm wide and 650 nm long) when zincacetate was used (Fig. 16B), in contrast to the more nodularparticles (ca. 140 nm wide and 300 nm long) obtained when zincformate was used (Fig. 16D).

Films obtained from baths using en as a ligand for zinc werecomposed of fused columnar crystals (ca. 110 nm wide and300 nm long), with well-defined hexagonal end facets (c-faces).Hence the presence of a template layer appears to direct growthtowards acicular, rather than starlike, morphologies. Thecrystallite formed homogeneously in the bath and laying on topof the film possesses a star-like morphology (Fig. 16E, inset)and resembles those comprising films deposited over TO(F)glass (Fig. 4a). Films deposited using the TEA ligand have a

Fig. 14 SEM images of ZnO films deposited on nanocrystalline ZnOtemplates introduced in baths from time: A,B TB and C,D TA.

Fig. 15 XRD patterns of ZnO films from: a. HMT-system ([ZnAc] ~0.025 mol dm23, [HMT] ~ 0.025 mol dm23, pH 5, 90 uC, 2 h); b. en-system ([ZnAc] ~ 0.018 mol dm23, [en] ~ 0.042 mol dm23, pH 11,70 uC, 2 h); c. forced hydrolysis of zinc acetate (0.025 mol dm23, 90 uC, 2 h);d. TEA-system ([Zn(NO3)2] ~ 0.018 mol dm23, [TEA] ~ 0.042 mol dm23,pH 11, 70 uC, 2 h) deposited on nanocrystalline ZnO templates.Nanocrystalline ZnO template shown in e. ZnAc ~ zinc acetate.

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more globular appearance and are less defined than others inthese investigations.

Based on the success of deposition baths containing HMTand using microcrystalline ZnO template layers, further workwas conducted on the influence of the nanocrystalline templatelayer on the subsequent growth of films, using HMT as theligand. The results were unambiguous, micrographs obtainedfrom films provide clear evidence that all other factors beingequal, the width of ZnO rods grown on nanocrystalline(Fig. 17D, E and F) rather than microcrystalline (Fig. 17A, Band C) template layers is far smaller.

Influence of choice of counter-ion on growth of ZnO rods onnanocrystalline ZnO template layers

The influence of the zinc counter-ion on the ZnO rod dimen-sions was investigated using similar strategies to thoseemployed for microcrystalline template layers. Micrographs

obtained from films (Fig. 18) suggest that films grown frombaths containing zinc nitrate and HMT possess crystallites withthe largest aspect ratio (ca. 10).

Growth of ZnO rods on nanocrystalline ZnO templates frombaths containing the products of decomposition of HMT(formaldehyde and ammonia)

The amine HMT decomposes slowly in heated aqueous solu-tions to yield ammonia and formaldehyde as initial reactionproducts. Attempts were made to grow ZnO rods on nano-crystalline ZnO templates directly from aqueous baths (90 uC)containing zinc acetate, ammonia and formaldehyde. Theconcentrations of the latter components were chosen assumingstoichiometric decomposition of HMT. Reactions were carriedout at the initial solution pH of 6.9 and at pH 5, which was thepH employed earlier in this study for the successful CBD ofordered arrays of ZnO rods. No precipitation occurred from

Fig. 16 SEM images of ZnO films deposited on nanocrystalline ZnO templates from: A,B. forced hydrolysis of zinc acetate; C,D. forced hydrolysisof zinc formate; E,F. en-system; G,H. TEA-system.

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baths held at pH 5 in which dissolution of the nanocrystallineZnO template layer occurred. Experiments were also conductedwithout formaldehyde i.e. only zinc acetate and ammonia pre-sent. Speciation calculations were performed in order to assessthe initial degree of supersaturation and identify the importantcomponents in deposition baths (Fig. 19a). Baths containingeither ammonia or ammonia and formaldehyde were found besupersaturated with respect to zinc hydroxide at pH 6.8. Theseresults are in general agreement with experimental observations.

Similar thermodynamic calculations were performed for bathscontaining different concentrations of HMT. Speciation calcula-tions indicate that with an increase in HMT concentrations, there

are small differences in levels of ‘‘zinc hydroxide’’ super-saturation and precipitation points (Fig. 19b). The hydroxideprecipitation points for the baths containing Zn:HMT in theratios, 2:1, 1:1, 1:2, 1:4, 1:8 occur at pH values of 6.8, 6.8, 6.9.7.04 and 7.16 respectively. The calculated pH, at which the bathis supersaturated with respect to the hydroxide containingZn:HMT in a 1:1 ratio, is exactly the same as the bath con-taining the stoichiometric amount of ammonia and formalde-hyde (or ammonia alone). However, these small differences insupersaturation at a given pH do appear to be associated withmorphological changes in films, as SEM images for the filmsdeposited using different HMT concentrations show that as theHMT concentration increases, the width of the rods decreases.The aspect ratio for the rods, however, does not change

Fig. 17 A. CBD-ZnO microcrystalline templates on TO(F) glass comprising nodular grains (deposited from TEA-systems); B,C. ZnO microcolumnsgrown on CBD-ZnO templates from HMT-system ([Zn] ~ 0.025 mol dm23, [HMT] ~ 0.025 mol dm23, pH 5, 90 uC); D. nanocrystalline ZnO templateson glass microscope slides (prepared by sol gel (SG) method); E,F. ZnO nanocolumns grown on SG-ZnO templates from HMT-system.

Fig. 18 SEM images of ZnO films on nanocrystalline ZnO templatesfrom HMT baths (0.025 mol dm23 zinc salt and 0.025 mol dm23 HMT,pH 5, 90 uC, 2 hours); A,B. Zn(CH3COO)2; C,D. Zn(HCOO)2;E,F. ZnCl2; G,H Zn(NO3)2.

Fig. 19 a. Speciation diagram of Zn–NH3–HCHO system at 25 uC withA ~ [Zn21] ~ 0.0083 mol dm23, B ~ [CH3COO2] ~ 0.016 mol dm23,C ~ [NH3] ~ 0.0083 mol dm23 and [HCHO] ~ 0.016 mol dm23. Dashedline represents zinc hydroxide precipitation point. b. Speciation diagramof Zn–HMT system at 25 uC showing [Zn21]free and zinc hydroxideprecipitation points for increasing [HMT].

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significantly i.e. narrower rods tend to reach smaller terminallengths.

For films deposited from aqueous baths (90 uC) at initialpH ~ 6.9, containing zinc acetate, ammonia and formaldehydeon nanocrystalline ZnO template layers, XRD and SEMmeasurements were recorded. X-Ray diffraction patterns werevery similar to those obtained for ZnO rod films deposited frombaths containing HMT and zinc acetate on ZnO template layersand exhibited enhancement of the (002) reflection. In agreementwith previous results, no evidence for incorporation of hydroxidewas obtained from ATR-FTIR analysis. SEM micrographs werealso very similar to those obtained for analogous HMT systems,as films comprised ordered rod arrays of perpendicularlyorientated ZnO rods on nanocrystalline template layers(Fig. 20A,B). The dimensions of the rods were y85 nm inwidth and y600 nm long, which were smaller than thoseobtained for films deposited from HMT/zinc acetate systems andgrown on nanocrystalline ZnO templates (Fig. 18A). For bathscontaining only zinc acetate and ammonia, the films comprisedrods with globular caps (Fig. 20D) in contrast to the rods formedwhen formaldehyde was present (Fig. 20A,B).

Effect of growth time

The XRD patterns for the films, deposited for different times,are shown in Fig. 21. All patterns are consistent with zincitephase. As the growth time increases, the (002) reflectionincreases in relative intensity, consistent with formation of ZnOrod crystallites. The (100) and (101) reflections remain broadduring deposition in contrast to the (002) peak, which becomesnoticeably sharper with increasing deposition time. Thisobservation is consistent with the growth of ZnO rodscrystallites, along the c-axis.

Scanning electron micrographs were recorded and grain sizedistributions determined for films deposited for different growthperiods (ESI:{ Figure F). Grain size profiles were unimodal,generally asymmetric and typical for abnormal grain growthwhere grain size distributions are directed by different substrate–grain relationships. The average grain diameter increased linearlyfromy82 nm after 5 min toy136 nm after 2 h, corresponding toan average growth rate ofy0.23 nm min21. From cross sectionalSEM images it was clear that growing ZnO rods tend to broadenslightly with deposition time, hence the definition of ‘‘averagegrain size’’ is not precise and a direct comparison is only sensiblefor rods of similar length.

In attempts to improve the aspect ratio of ZnO rods, as-deposited ZnO films were immersed into fresh baths. Anadditional motivation for this work was to gain further insightsinto the actual growth mechanism of ZnO single crystals. Inpreliminary experiments, a direct comparison was made of filmsprepared using baths (HMT–zinc acetate system) identical tothose employed previously but using three different reactionsequences. In the first group, ZnO rods were deposited onnanocrystalline ZnO templates for 2 h (Method 1). In the secondgroup, ZnO rods were deposited on nanocrystalline ZnO for 2 hand subsequently transferred into fresh heated baths, at the pointwhere visible turbidity became apparent, for a period of 1 h(Method 2). The final experiments followed the second procedureand then films were removed and placed in another fresh bath atthe critical point for an additional 1 h (Method 3).

Films were characterised by SEM and it was apparent thatalthough the length of the ZnO rods increases upon exposure tofresh CBD solution, the width of rods also increases and henceno improvement in aspect ratio is observed (Table 1 and Fig. 22).In addition, crystallites comprising the films become very denselypacked, which is not desirable for applications that require post-deposition processing such as infiltration of dye molecules ontothe rods. No conclusive evidence for film growth via orientatedattachment processes could be gained. The resolution of theSEM is insufficient for detailed interpretation of growth ofnanostructured films. However, it was clear that in common withthe microcrystalline systems described earlier (see Fig. 13D),growth of homogeneous precipitates may occur throughorientated attachment processes (Fig. 22G).

Attempts were made to gain more detailed structural informa-tion, by use of HR-TEM and focussed ion beam (FIB) samplepreparation techniques. Samples were prepared from ZnO thinfilms deposited by Method 1 (2 h deposition) and Method 2 (2 1

1 h deposition) as described previously (ESI:{ Figure G). The FIBtechnique allows thin (y50 nm) specimens for HR-TEM to be

Fig. 20 SEM images of ZnO rod arrays deposited on nanocrystallineZnO templates layers from baths containing: A,B. zinc acetate(0.025 mol dm23), HCHO (0.016 mol dm23) and NH3 (0.0083 mol dm23)(pH 6.9, 90 uC, 2 h) and C,D. zinc acetate (0.025 mol dm23) and NH3

(0.0083 mol dm23) (pH 6.9, 90 uC, 2 h).

Fig. 21 XRD patterns of ZnO films deposited on ZnO nanocrystallinetemplate layers from HMT baths (0.025 mol dm23 Zn(CH3COO)2

and 0.025 mol dm23 HMT, pH 5, 90 uC) for different periods of time.a. 2 min. b. 5 min. c. 10 min. d. 20 min. e. 30 min. f. 1 h. g. 2 h.

Table 1 Rod dimensions and aspect ratio of the films deposited forthe various times

time diameter/nm length/nm aspect ratio

2 min y77 ¡ 4 a —5 min y83 ¡ 6 a —10 min y85 ¡ 10 a —20 min y102 ¡ 9 220 ¡ 10 2.130 min y108 ¡ 11 530 ¡ 25 4.91 h y122 ¡ 10 1000 ¡ 50 8.22 h y137 ¡ 11 1500 ¡ 75 10.92 h 1 1 h y320 ¡ 11 2000 ¡ 100 6.32 h 1 1 h 1 1 h y560 ¡ 12 2500 ¡ 125 4.5a The presence of the template layer compromised accurate lengthmeasurements.

J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 5 7 5 – 2 5 9 1 2 5 8 7

obtained with minimal damage in preparation. The integrity ofthe original structure is maintained and each interface is clearlyrepresented (Fig. 23). Moreover, the single crystalline nature ofindividual ZnO rods is clear from SAED.

The crystalline rods of ZnO formed comprise oriented nano-crystallites, in which the lattice planes of individual particles areperfectly aligned to form a single crystalline entity (Fig. 24). It isalso evident from TEM that the outer layers of ZnO rods (ca.

y5 nm) are composed of randomly orientated nanoparticles.The atomic structure is clearly resolved in Fig. 25. Nucleationwill involve conditions of low to moderate supersaturation in

conjunction with an active interface. Under typical dynamicbath conditions, it is more likely that these external aggregatesof nanoparticles will be subject to dissolution–reprecipitationprocesses. The predominance of one process over the other willdetermine the velocity of growth in a given direction. Sorptionof ions or anisotropic nanoparticles with favourable orienta-tion approaching the polar faces of wurtzite ZnO (in contrastto the non-polar faces), is expected to be effective and prohibitdesorption and rearrangement of growth units.

Evidence for orientated attachment of large ZnO crystallitesto existing ZnO rods grown on templates has been obtained,for both films grown by Method 1 (2 h deposition on ZnOtemplates) and Method 2 (2 h deposition followed by 1 hgrowth in fresh baths). The pattern of results obtained from theTEM studies appears to show that the process is not assignificant as the mechanism involving ZnO nanocrystalaggregation. An example of the phenomena for films grownby Method 1 is shown in Fig. 26, where the rod–rod interface ofa composite unit is shown. Although visually there appears to

Fig. 22 SEM images of ZnO rod arrays deposited on nanocrystallineZnO template layers for: A,B. 2 h (Method 1); C,D. 2 h 1 1 h(Method 2) and E,F,G. 2 h 1 1 h 1 1 h (Method 3).

Fig. 23 HR-TEM of cross-section of ZnO thin film sample prepared byMethod 1 (mag. 627.5k) and SAED recorded for single crystalline ZnOrod along the (100) zone axis. For reference the first series of reflectionsare indexed. Double diffraction phenomena occurs leading to thepresence of forbidden reflections (e.g. (001)) and are denoted by X.

Fig. 24 HR-TEM of single ZnO rod (mag. 310k) grown on nanocrystalline ZnO template layer (Method 1). a. The outer porous layer is composedof randomly orientated nanoparticles. b. Area indicated in a is shown in greater detail. The extended lattice structure, comprising individual ZnOnanocrystals, is clearly evident.

2 5 8 8 J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 5 7 5 – 2 5 9 1

be considerable misalignment of the rods, at higher magnifica-tion (Fig. 26B) the lattice configuration between the twoseparate rods is evidently good. Similar structures were foundfor ZnO rod arrays grown by Method 2. Hence it appears thatunder the appropriate conditions, ZnO rods can develop by acombination of different growth processes.

The primary aim of the HR-TEM investigation was to betterunderstand the growth mechanism of ZnO rods on nano-crystalline templates. If we assume that deposition occurs fromsolutions of low to intermediate levels of supersaturation,growth would be expected to proceed via dislocation-basedgrowth spiral or 2D-nucleation based mechanisms. For allsamples in the present study (both in preliminary work and thelater samples) no evidence for the presence of significantconcentrations of defects such as dislocations or stacking faults

in ZnO rods could be obtained, from HR-TEM or SAED. Thepattern of results obtained in this study for the deposition ofZnO rods on nanocrystalline ZnO template layers do notsupport a mechanism based exclusively on a growth spiral.However, our observations do not preclude the possibility thatsuch a mechanism may operate in early stages of growth.

A likely sequence of events would be 2D-nucleation on thetemplate layers, via the orientated aggregation of polar nanopar-ticles and the subsequent development of single crystalline materialby dissolution-reprecipitation phenomena. As supersaturation inbaths decreases due to consumption of reagents, 2D-nucleationand growth processes on surface terraces decrease rapidly insignificance. Crystal faces develop a smoother appearance throughaging via dissolution-recrystallisation processes, whereas adsorp-tion and incorporation of growth units is limited by diminishingconcentrations of growth sites such as kinks on crystal faces.

3. Conclusions

The influence of reaction conditions including ligand, counter-ion, pH, ionic strength, supersaturation, deposition time andsubstrate on the nature of ZnO films grown by CBD has beenexamined and discussed, the most important of which aresummarised in Fig. 27.

Use of different ligands leads to deposition of ZnO films withdifferent morphologies on TO(F) glass, e.g. ethylenediamineusually gave rise to starlike crystals, TEA produced nodulesand HMT usually produced rods. Similarly, using identicalligands but different zinc salts often gives rise to differentmorphologies, with sulfate usually producing the most drama-tic change. Speciation studies and empirical evidence suggestthat growth of acicular ZnO morphologies by CBD are bestobtained by limiting the concentration of one reactant (i.e.either Zn21 or OH2) in the presence of an excess of the secondcomponent, promoting the kinetically controlled form of thefinal crystal. The presence of an existing ZnO substrate caninfluence the morphology of subsequent ZnO growth. Astriking example is that ZnO films could not be deposited on

Fig. 25 HR FEG-TEM image of ZnO rod showing lattice structure.

Fig. 26 (Left) HR-TEM of ZnO rods grown by Method 1 showing orientated attachment of two smaller rods. (Right) Inset is shown at highermagnification to demonstrate the good lattice alignment of the two ZnO single crystals.

J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 5 7 5 – 2 5 9 1 2 5 8 9

TO(F) glass by the forced hydrolysis of zinc carboxylates, butrod-like arrays of ZnO were produced from chemicallyequivalent baths on nanocrystalline ZnO templates. Templatesof ZnO can direct the deposited film e.g. in the en system, whichtypically produced starlike crystallites, columnar structureswere deposited. High surface area arrays of upright ZnOcrystallites on nanocrystalline ZnO templates have beendeposited using baths containing HMT. The point at whichthe substrate is introduced into the bath is crucial and leads to asignificant difference in the width of the rods and opticaltransparency of the films. Thinner rods are produced when thesubstrate is immersed in the bath at the point of visibleturbidity i.e. at the onset of precipitation. Zinc nitrate pre-cursors tended to lead to formation of rod arrays onnanocrystalline templates that possessed the highest aspectratios. It was also found that deposition baths containing zincacetate and the decomposition products of HMT (initial pH ~6.9) produced similar rods arrays of ZnO to the standard HMTsystem (initial pH ~ 5). It was found that increasing HMTleads to slightly thinner rods, however, there was no change inthe aspect ratio as crystallites were shorter in length.

Attempts have been made to improve the aspect ratio of ZnOrods, by growing ZnO rods using fresh solutions on preformedZnO rods arrays. The procedure was unsuccessful. The finalZnO films comprised dense rod arrays that were increased inlength and in width with some fused columnar structures.

HR-TEM has yielded important structural information andan insight into the growth of ZnO rods by CBD. The mostlikely sequence of events involves 2D-nucleation on ZnOtemplate layers, growth via orientated aggregation of polarnanoparticles and subsequent development of single crystallinematerial by dissolution–reprecipitation.

Experimental

Film growth

Films of ZnO were grown on substrates immersed in aqueousdeposition solutions using forced hydrolysis and CBD

methods. Substrates (microscope slides, TO(F) glass, micro-or nanocrystalline ZnO templates, Au-coated TO(F) glass,single crystal sapphire (0001)) were cleaned by a standardprocedure. A Mettler Toledo MA 235 pH/ion analyser andInLab 413 electrode were used to record solution pH.Modelling of solutions was performed using SPECIES(AcadSoft Ltd.). The program calculates and displays specia-tion curves as a function of pH. The input parameters forSPECIES are the stability constants for the homogeneousequilibria being modelled and their stoichiometric coefficients.Film thickness measurements were performed using the QCMtechnique, using a Maxtek PM-700 Series Plating Monitor andprobe employing a quartz crystal oscillator (5 MHz, AT-cut,gold crystal, unpolished). The QCM apparatus was controlledand experimental data recorded by use of a PC.

Forced hydrolysis was performed using solutions of zincacetate (0.025 mol dm23) or zinc formate (0.025 mol dm23) onTO(F) glass. Final pH values for the zinc acetate and formatebaths were 6.9 and 6.4 respectively. Solutions were heated at95 uC for a period of two hours. For experiments involvingnanocrystalline ZnO template layers on glass microscope slides,substrates were immersed in solutions at ambient temperatureprior to heating of baths using the temperature-controlledwaterbath.

Deposition of ZnO thin films by CBD methods using ethyl-enediamine (en) as ligand was achieved using aqueous solutionsof zinc acetate (0.018 mol dm23) and en (0.042 mol dm23),adjusted to a pH of 11 with aqueous sodium hydroxide(5 mol dm23). Films were grown at the desired temperature(50–80 uC) for 1 hour. For TEA systems, solutions contain-ing zinc nitrate (0.018 mol dm23) and triethanolamine(0.072 mol dm23), adjusted to a pH of 11 with aqueoussodium hydroxide (5 mol dm23), were used. Films were grownover a range of temperatures (50–80 uC) for 1 hour. Unlessotherwise stated, experiments employing HMT were conductedusing bath solutions containing zinc acetate (0.025 mol dm23)and HMT (0.025 mol dm23), adjusted to pH 5 with aqueousacetic acid (5 mol dm23), for deposition times of 1 h.

Fig. 27 Schematic overview of main results obtained in this study.

2 5 9 0 J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 5 7 5 – 2 5 9 1

Microcrystalline template layers were deposited using theTEA method described above. Nanocrystalline templates weregrown by sol–gel methods,59 involving hydrolysis and con-densation of an alcoholic Zn-precursor under basic conditions,in order to produce a homogeneous sol for subsequent dip-coating on glass microscope slides. A suspension of zinc acetatedihydrate (17.8 g, 0.08 mol) in n-propanol (120 cm3) was heatedunder reflux conditions (130 uC, 20 min). The mixture wasallowed to cool to ambient temperature before rapid additionof tetramethylammonium hydroxide (TMAH; 25% in MeOH,36 cm3) to yield a transparent nanoparticulate ZnO coating sol(y0.5 mol dm23), which could be concentrated further byremoval of solvent using a rotary evaporator. Films wereprepared by dip-coating substrates in the sol and sintering in afurnace at 400 uC for 2 min.

Characterisation studies

X-Ray diffraction studies were performed using secondarygraphite monochromated Cu Ka radiation (40 kV) on either aPhilips X’Pert Materials Diffractometer (APD) or Bruker AXSD8 diffractometer. Measurements on the former were takenusing a glancing angle incidence detector at an angle of 3u, for2h values over 10–95u in steps of 0.04u with a count time of 2 s.For the latter, scans were done over 2h values of 5u–90u and astep size 0.01u or 0.02u. Scanning electron microscopy (SEM)and energy dispersive X-ray analysis (EDAX) on carbon-coated films was performed using a Philips Excel 30 FEG SEMinstrument or Philips 525 SEM instrument with an EDAXDX4 EDS unit. Samples were carbon-coated (using anEdwards Coating System E306A) for EDAX or otherwisegold coated (Edwards Sputter Coater S150B). The focused ionbeam (FIB) thinning technique was used to prepare cross-sectional ZnO thin film samples for characterisation by highresolution transmission electron microscope (HR-TEM). Thiswork was conducted at FEI Bristol using a FIB 200 instrument(Ga1 beam). In order to minimise ion beam induced artefacts(i.e. amorphous layer and Ga implantation), a Pt buffer layerwas deposited on the area of interest on the specimen by ion-beam assisted CVD, prior to thinning. TEM electron micro-scopy was accomplished using either a Philips CM200 (200 kV)microscope or a FEI Tecnai F30 FEGTEM (300 kV) instru-ment. TEM specimens were mounted on a carbon-coatedcopper TEM grid. Electronic absorption spectra were obtainedusing a Helios Beta Thermospectronic spectrophotometer.Infrared spectra were recorded using a Specac single reflectanceATR instrument (4000–400 cm21, resolution 4 cm21).

Acknowledgements

POB, DSB and KG thank the EPSRC and Royal Society(United Kingdom) and NRF (South Africa) for financialsupport. The authors thank Dr Chengee Jiao from FEICompany for FIB-TEM sample preparation.

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