Materials Research Bulletin 46 (2011) 1276–1282

Solution precursor plasma deposition of nanostructured ZnO coatings

Raghavender Tummala, Ramesh K. Guduru *, Pravansu S. Mohanty

Department of Mechanical Engineering, University of Michigan – Dearborn, MI 48128, USA

A R T I C L E I N F O

Article history:

Received 7 October 2010

Received in revised form 17 March 2011

Accepted 29 March 2011

Available online 5 April 2011

Keywords:

A. Electronic materials

A. Nanostructures

B. Plasma deposition

B. Electrical properties

D. Optical properties

A B S T R A C T

Zinc oxide (ZnO) is a wide band gap semiconducting material that has various applications including

optical, electronic, biomedical and corrosion protection. It is usually synthesized via processing routes,

such as vapor deposition techniques, sol–gel, spray pyrolysis and thermal spray of pre-synthesized ZnO

powders. Cheaper and faster synthesis techniques are of technological importance due to increased

demand in alternative energy applications. Here, we report synthesis of nanostructured ZnO coatings

directly from a solution precursor in a single step using plasma spray technique. Nanostructured ZnO

coatings were deposited from the solution precursor prepared using zinc acetate and water/isopropanol.

An axial liquid atomizer was employed in a DC plasma spray torch to create fine droplets of precursor for

faster thermal treatment in the plasma plume to form ZnO. Microstructures of coatings revealed

ultrafine particulate agglomerates. X-ray diffraction confirmed polycrystalline nature and hexagonal

Wurtzite crystal structure of the coatings. Transmission electron microscopy studies showed fine grains

in the range of 10–40 nm. Observed optical transmittance (�65–80%) and reflectivity (�65–70%) in the

visible spectrum, and electrical resistivity (48.5–50.1 mV cm) of ZnO coatings are attributed to ultrafine

particulate morphology of the coatings.

� 2011 Elsevier Ltd. All rights reserved.

Contents lists available at ScienceDirect

Materials Research Bulletin

jo u rn al h om ep age: ww w.els evier .c o m/lo c ate /mat res b u

1. Introduction

Zinc oxide (ZnO) is a well studied metal oxide that hasapplications in different fields including optoelectronics, piezo-electrics, biomedical, sensors and pharmaceuticals [1,2]. It is awide bandgap semiconducting material with high excitationenergy, which makes it very useful for many engineeringapplications, such as high breakdown voltages, ability to sustainlarge electric fields and stable optical transmission [3,4]. It is a goodsubstitute for indium tin oxide as conductive electrodes ofamorphous silicon solar cells because of high chemical andmechanical stability in hydrogen plasma as well as high opticaltransparency in the visible and near – infrared region [5]. ZnOcoatings along with metallic substrates have considerable applica-tions as back end reflectors in thin film solar cells to enhance theefficiency [6]. It is also a promising candidate for anti-reflectioncoatings, transparent conductive oxide electrodes, liquid displays,heat mirrors, and surface acoustic wave devices, etc. [7].

Usually the structure and properties of ZnO coatings, such ascrystallite orientation, grain size, resistivity, carrier mobility oroptical transparency are influenced by the fabrication techniqueemployed [8]. There are different techniques that have been in useto develop ZnO films or coatings, for example atomic layer

* Corresponding author. Tel.: +1 313 593 4927; fax: +1 313 593 3851.

E-mail address: rkguduru@umich.edu (R.K. Guduru).

0025-5408/$ – see front matter � 2011 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2011.03.028

deposition (ALD) [9,10], chemical vapor deposition (CVD) [11,12],physical vapor deposition (PVD) [13,14], spray pyrolysis [15,16]and sol–gel technique [17,18]. Some of these techniques are eitherquite expensive or time consuming processes. Spray pyrolysis isrelatively inexpensive and easier compared to the remaining, but itrequires heating of substrates to high temperatures. Also, as thenumber of layers increases, substrate temperature needs to beincreased in order to achieve right decomposition of the precursorin top layers of the coatings. Finally, less control over themicrostructures and properties makes spray pyrolysis an unat-tractive process for real time applications. Hence there is arequirement for an inexpensive and fast deposition technique(s)for developing ZnO coatings in a single step process with goodcontrollability. Thermal spray techniques are usually known fortheir mass production rates with high commercial interest.However, synthesis of ZnO coatings via thermal spray techniquesas reported by Tului et al. [19] from pre-synthesized ZnO powdersis not very efficient and economical because it does not provide aclose control in tailoring the microstructures of the coatings.Added to that, it puts a limitation on size of the pre-synthesizedpowder particles that could be used in the spray process. On theother hand, production of zinc oxide coatings directly from asolution precursor in a single step makes solution precursorplasma spray technique very attractive for enabling good controlover the microstructures as well as high throughput. In thistechnique, the solution precursor is injected through hightemperature plasma plume where it thermally decomposes into

ZnO solution precurso r

Peristaltic pump/Dampener

Plasma Gun Inje ctor (Atomi zation int o high temperature plume)

Deposit ion on th e subs trat e (Titan ium /Glas s)

Fig. 2. Flow chart for plasma spray process using solution precursor.

R. Tummala et al. / Materials Research Bulletin 46 (2011) 1276–1282 1277

solid nanoclusters of the desired compound that is deposited onthe substrate. Also, this process does not require heating ofsubstrates.

In the present research work, solution precursor plasma sprayprocess was employed, for the first time, to develop nanostruc-tured ZnO coatings. The ZnO was sprayed on different substrateswith a solution precursor that was developed using zinc acetate.Then the coatings were examined for microstructures, electricaland optical properties and their characteristics are compared withthe properties of ZnO films developed by other techniques.

2. Experimental procedure

2.1. Plasma spray setup

Fig. 1 shows the schematic for the DC plasma gun (100HE,Progressive Technologies Inc, Grand Rapids, MI, USA) used forspraying ZnO coatings from a solution precursor. The power of thisgun can be varied between 35 kW and 100 kW. An axial liquidatomizer was developed for this gun by Mohanty et al. [20,21] tofeed the solution precursors of various materials. Fig. 2 shows aflow chart for the solution precursor plasma spray process used inthe present work. The solution precursor is fed with help of aperistaltic pump through a polyurethane tube. The precursor thenflows through an injector that holds an atomizer to atomize thesolution into fine droplets in order to facilitate and accelerate hightemperature chemical reactions. Atomized liquid droplets of theprecursor undergo chemical reactions in the plume and turn intomolten/semimolten particles of desired compound while exitingthe plume, which are deposited on the substrate to develop acoating. Usually compressed air or Argon is employed foratomizing the solution precursor at a pressure of 140–310 kPa.To avoid back pressure from the gun, check valves are connected tothe feed ports. More details on the solution precursor plasma sprayprocess can be found elsewhere [20–23].

2.2. Solution precursor and substrates

Solution precursor was prepared using zinc acetate dihydrate(Alfa Aesar, ACS 98.0–101.0%), de-ionized water and isopropanol(91% pure, Walgreens pharmacy). Initially, 0.4 mol of zinc acetatedihydrate was added to 250 ml of de-ionized water and stirredthoroughly using a magnetic stirrer to form a 1.6 molar solution.Following this, 150 ml of isopropanol was added. Thus formedclear solution was used in the plasma spray process. Titanium(surface roughness �0.44 mm), steel (surface roughness�0.61 mm) and glass substrates (surface roughness �0.01 mm)were chosen for depositing the ZnO coatings. Titanium (Ti) and

Fig. 1. Schematic for plasma spray setup (100 HE Plasma Gun) for solution precursor

spray.

steel substrates (6 cm � 5 cm � 2.5 cm), and Corning glass sub-strates (7.5 cm � 5 cm � 0.25 cm, TEDPELLA) were cleaned withacetone following distilled water before depositing ZnO. Thicknessof the coatings was varied on glass substrates by varying thenumber of spray cycles of plasma deposition to study the variationin optical transmission and absorbance properties of nanostruc-tured ZnO as a function of thickness. The ZnO coatings on steelsubstrates were used to measure the reflectance for back reflectorapplications in solar cells.

2.3. Plasma spray process and parameters

Table 1 shows the optimized conditions used for spraying ZnOcoatings on Ti, steel and glass substrates. During plasma sprayprocess, the glass substrates were cooled from rear side usingcompressed air to avoid cracking due to thermal stresses. However,Ti and steel substrates did not require cooling as they wereconducting the heat away to substrate holder while spraying theZnO coatings.

Table 1Solution precursor plasma spray process parameters.

Parameter Value

Power (kW) 65

Voltage (V) 265

Current (A) 235

Primary gas (m3/h) (Argon) 5.66

Secondary gas (m3/h) (nitrogen, hydrogen) 2.83, 3.16

Standoff distance (mm) 105

Feed rate (ml/min) (solution precursor) 20

Atomizing gas (kPa) (compressed air) 172.36

Cooling gas (kPa) (compressed air) 344.73

Number of passes Ti substrate – 75 passes

Steel substrate – 75 passes

Glass substrate – 10,

20 and 30 passes

R. Tummala et al. / Materials Research Bulletin 46 (2011) 1276–12821278

2.4. Characterization of ZnO coatings

Phase formation of all the ZnO coatings was determined from X-ray diffraction (XRD) studies conducted using a Rigaku Miniflex X-ray diffraction machine with a Cu Ka radiation (l = 1.5402 A).Surface morphology of the coatings was studied using Hitachi2600-N Scanning Electron Microscope (SEM). Microstructures ofthe coatings were investigated by transmission electron micro-scope (TEM). The samples for TEM studies were prepared followingdrop-cast method from scraped powders of ZnO coatings. First, theZnO coatings were scraped with a stainless steel knife to obtainpowder particles of the coating. These powders were suspended inmethanol and sonicated for 5 min to attain a uniform dispersion.Droplets of sonicated solution were then drop-casted onto a holeycarbon coated copper grid, and subsequently heated to 80 8C toevaporate methanol. These samples were then examined in JEOL2010F TEM at different magnifications in conventional and darkfield imaging modes as well as diffraction mode. Electricalresistivity of ZnO coatings was measured using a Keithley(Voltmeter: 2182A; Source meter: 6220) four point probe analyzer.Electrical measurements were done on different locations of thecoatings. Optical properties were studied in terms of percentagetransmission and absorbance on ZnO coated glass substrates usinga Varian Cary 5000 spectrophotometer. Variation in absorbanceand transmittance were measured as a function of number ofplasma spray cycles, i.e. ZnO coating thickness. Reflectivitymeasurements were also done on ZnO coated glass and steelsubstrates using the above spectrophotometer.

3. Results and discussion

3.1. X-ray diffraction studies

Fig. 3 shows the XRD spectra obtained for plasma sprayed ZnOcoatings as well as commercial ZnO powder. The XRD patterns ofZnO coatings on Titanium and steel substrates show Ti and steelpeaks also, respectively. These studies confirm polycrystallinenature and hexagonal Wurtzite crystal structure (a = 0.3296 nm,c = 0.52065 nm) of the ZnO coatings on all the substrates.

3.2. Morphology and microstructure

Adherence tests conducted using cellophane (sticky) tapeshowed poor bonding of ZnO coatings on the glass substrates;

30 40 50 60 70

Angle (2θ)

Inte

nsit

y (

A.U

.)

Commer cial Zn O Powder

ZnO coating on Ti

ZnO coating on Glas s

ZnO coating on Stee l

( 1

0 0

)

( 0

0 2

)

( 1

0 1

)

∆

( 1

0 2

)

( 1

1 0

)

( 1

0 3

)

( 2

0 0

)

( 1

1 2

)(

2 0

1 )

∆

## #

# : Ti Sub strate

∆ : Steel Subs trate

#

Fig. 3. XRD pattern for commercial ZnO powder and solution precursor sprayed ZnO

coatings.

on the other hand, only very top layers of ZnO particulates werepeeled off in case of Ti and steel substrates. Poor adherence of thecoatings on glass substrates could possibly be because of smoothsurface of the glass substrates.

Fig. 4 shows the surface morphology of solution precursorplasma sprayed ZnO coatings at low (70�) and high (5000�)magnifications on Ti substrate. Similar morphologies of thecoatings were observed on all the three substrates. The ZnOcoatings in Fig. 4(a) show rough particulate morphology. Similarfeatures were observed for ZnO coatings developed via spraypyrolysis [24], combustion CVD[12] and electrochemical deposi-tion techniques [25]. On the other hand, the ZnO coatingsdeveloped via Metal Organic CVD [26], atmospheric pressure mistCVD [27], and e-beam evaporation techniques [28] showedsmoother morphologies compared to the above. Vapor depositedfilms are usually expected to exhibit smooth morphologiesbecause of atom by atom deposition mechanism. However,similarities between the solution precursor plasma spray processand spray pyrolysis could be expected as both the processes makeuse of liquid precursors. Droplets of solution precursor undergochemical reactions upon impacting a hot substrate in the spraypyrolysis, whereas atomized droplets of the solution precursorundergo accelerated chemical reactions in the high temperatureplasma plume to form semi-molten particles before impacting onthe substrate. Following reaction is expected to occur during thethermal decomposition of solution precursor in the plasma plume:

ZnðCH3COOÞ2 þ H2O �!heat energyZnO þ 2CH3COOH (1)

Fig. 4. SEM images of plasma sprayed ZnO coating on Ti susbtrate (a) at low

magnification (70�) and (b) at high magnification (5000�).

Fig. 5. TEM images of ZnO (a) dark field TEM image of particulate agglomerates, (c) HREM image of ZnO grains in an agglomerate (c) Diffraction pattern for ZnO.

0

0.1

0.2

0.3

0.4

0 10 20 30 40 50

Grain size (nm)

Nu

mb

er

fracti

on

Average grain size ~ 23.7 nm

Fig. 6. Grain size distribution for ZnO coating.

R. Tummala et al. / Materials Research Bulletin 46 (2011) 1276–1282 1279

The solution precursor plasma spray process is advantageousover spray pyrolysis because of rapid production of the coatings aswell as no requirement for preheating of the substrates. Forexample, it could be very useful for spraying coatings on lowmelting or polymer substrates. In addition, plasma spray processcan yield fine structures by tailoring the extent of atomization ofthe solution precursor. Fig. 4(b) shows agglomerates of nanopar-ticulates of ZnO. Formation of agglomerates could be attributed tohigh temperatures and rapid quenching in the plasma plume. Theparticles that form during thermal decomposition of the atomizedliquid droplets could possibly fuse together into agglomeratesbefore exiting the plume.

Microstructures of the agglomerates and ultrafine particulateswere examined in TEM. Fig. 5 shows the TEM images of agglomeratedZnO particulates with very fine grain size. It is evident from the TEMimages that each agglomerate has several equiaxed grains with fewtens of nanometers in size. Grain size distribution for ZnO coatings isshown in Fig. 6. The grain size measurements were done using severalbright field and dark field images with line intercept method. A totalof 272 individual grains were considered for the Gaussian distribu-tion shown in Fig. 6. The grain size varied between 10 and 40 nm andaverage grain size was calculated to be approximately 23.7 nm.Similar grain sizes were reported in the ZnO coatings developed viaALD, spray pyrolysis, thermal evaporation and CVD techniques also[29–32]. The diffraction pattern, shown in Fig. 5(c), confirmed thecrystal structure of ZnO. High resolution image shown in Fig. 7indicates an interplanar spacing of 2.60 A between two consecutive(0 0 2) planes along the C axis of hexagonal Wurtzite structure.

Similar to the CVD and PVD techniques, the grain size ofsolution precursor plasma sprayed coatings can also be controlledin situ either by heating or cooling the substrates, prior to the

deposition or during the deposition process, respectively. Also, theamount of porosity can be tailored by plasma spray parameteroptimization. Highly porous nanostructures shown in Fig. 4(a)could be very useful for sensing applications because of their largesurface area contributed by ultrafine particulate boundaries, roughmorphology and intermittent porosity between the particulatesand agglomerates.

3.3. Electrical measurements

Fig. 8 shows the graph between current (I) and voltage (V) forZnO coatings sprayed on Ti and steel substrates. Electrical

4

Fig. 7. HREM lattice image of ZnO.

R. Tummala et al. / Materials Research Bulletin 46 (2011) 1276–12821280

measurements were done by detecting the voltage while varyingthe current across the terminals of the four point probe analyzer.Slope of the I–V graph indicates resistance of the coating betweentwo terminals that measured the voltage. However, based on theprocedures provided by Keithley Instruments [33] for four-pointprobe analyzer, following formulae were employed to determinethe bulk resistivity of ZnO coating on conducting substrates.

Sheet resistance ¼ 4:53 � slope of I�V graph (2)

Resistivity ¼ Thickness of the coating � Sheet Resistance (3)

The resistivity of ZnO was calculated to be 48.5 � 4.65 � 10�3

and 50.1 � 3.13 � 10�3 V cm from the above formulae for a coatingthickness of 23.6 � 2.27 mm on Ti and 23.55 � 1.47 mm on steelsubstrates, respectively; which is lower than the resistivity of ZnOcoatings developed via sol–gel method (99 � 10�3 to 225 �10�3 V cm) [34,35] and spray pyrolysis techniques (100 � 10�3 to315 � 10�3 V cm) [36,37]. However, it is comparable or higherthan the resistivity of ZnO coatings developed by e-beamevaporation (58 � 10�3 V cm) [28] and CVD coatings (�1 � 10�3

to 18 � 10�3 V cm) [38], respectively. The room temperatureconductivity for ZnO coatings (thickness >0.2 mm) at low voltages

0

0.001

0.002

0.003

0.004

0.005

0.00120.0010.00080.00060.00040.00020

Current (A)

Vo

ltag

e (

V)

Res istivity on Ti: 48 .5 ± 4.65 mΩ - cm

Resistivity on Stee l: 50 .1 ± 3.13 mΩ - cm

Fig. 8. Electrical measurements for ZnO coatings on Ti and steel substrates.

(<0.1 V) was determined to be ohmic in nature according to Riad et al.[39], and it was proved to change to space charge limited conductionprocess at high voltages (>0.1 V). In the current studies, measure-ments were done below 0.01 V, therefore the I–V data shown in Fig. 8should fall under ohmic conductivity of ZnO. However, conductivityof ZnO is expected to get influenced by various microstructuralparameters. The intrinsic defects in ZnO are usually notorious for theelectrical activity. Grain boundaries act as charge–carrier traps,leading to band bending and potential barriers all around the grains[39]. With decreasing the grain size, number of gain boundaries aswell as the total amount of grain boundary area will increase. Fay et al.[32] showed an inverse relation between the grain size and resistivityfor ZnO coatings developed by low pressure CVD. In addition, porescan also act as scattering sites for charge carriers. Therefore, weattribute the grain boundaries, ultrafine particulate boundaries andpores as possible root causes for higher resistivity in solutionprecursor plasma sprayed ZnO coatings compared to the vapordeposited coatings, which are usually free of porosity and particulateboundaries.

3.4. Optical properties

Absorption spectra for different thicknesses of ZnO layerssprayed on Corning glass substrates are shown in Fig. 9.Absorbance (A) was observed to increase with increasing thenumber of spray cycles, i.e., with increasing the thickness of ZnOcoatings. Similarly, transmittance values, shown in the inset ofFig. 9, have decreased with increasing the thickness of ZnOcoatings. The transmittance of ZnO coatings for 10 passes isapproximately 65–80% in the visible region, which is comparableor slightly lower than the transmittance of ZnO coatings developedby electrochemical deposition (75–80%) [25], sol–gel (80–90%) [5],CVD (80–85%) [8], pulsed laser deposition (80–90%) [40], and spraypyrolysis techniques (60–90%) [41]. Usually, low absorbance alongwith high transmittance of ZnO in the visible spectrum is due to theoxygen vacancies and interstitial Zinc atoms which act as donorimpurities. These impurities may be ionized by low energyphotons. However, electron excitation from valence band toconduction band occurs only when the photon energy is equivalentto or higher than the band gap of ZnO [42]. Therefore, increasedabsorbance below 400 nm could be correlated to the band gap ofZnO. Using absorbance data, a graph between hn and (ahn)2 isplotted, see Fig. 10, to determine the band gap of ZnO coatings.Here, h is the Planck’s constant (6.626 � 10�34 J s), n is thefrequency of photons (n = speed of light/wavelength of the

0

1

2

3

350 400 45 0 50 0 55 0 600 65 0 70 0

Waveleng th (nm)

Ab

so

rba

nc

e (

A.U

.)

30 passe s

20 passe s

10 passe s

0102030405060708090

100

350 42 0 49 0 56 0 63 0 70 0Wavelength (nm)

%T

10 pass es

20 pass es

30 pass es

Fig. 9. Absorbance for ZnO coatings with different number of plasma spray passes.

Inset shows variation of transmittance with respect to the number of spray passes.

0

0.5

1

1.5

2

2.5

3

1.3 1.6 1.9 2.2 2.5 2.8 3.1 3.4 3.7 4

hν(eV)

( αhν)

2 . 10

6 [(ev

/cm

)2 ]

Fig. 10. Optical band gap for ZnO coating.

R. Tummala et al. / Materials Research Bulletin 46 (2011) 1276–1282 1281

photon), and a is the absorption coefficient (a = A/L, L is thethickness of ZnO film). Fig. 10 shows a band gap of 3.24 eV(corresponds to �370 nm) for ZnO coating on a 10 passes glasssample. The band gap value is comparable with the literature [8]and also it was observed to be independent of ZnO coatingthickness. However, decrease in transmittance with increasing thecoating thickness could be attributed to rough morphology as wellas increased number of defects, such as large number of grainboundaries, porosity and particulate boundaries that can causeinternal scattering of the light within the coating.

Light scattering is usually a required phenomenon in backreflectors in solar cells. ZnO is quite often employed as a backreflector along with Ag or Al or steel coatings or substrates in thebottom of the solar cells to reflect the light back into the active areafor improving the light absorption and there by the efficiency.Fig. 11 shows the reflectance measurements done on ZnO coatedsteel and glass substrates. These measurements indicate areflectance of 65–70% in the visible spectrum for back reflectormade of ZnO/steel, which is slightly lower than the reflectancereported for Ag/ZnO (70–90%) back reflectors developed by Yanet al. [6] using sputtering process. The reason for lower reflectancecould be because of internal scattering of light within the coatingdue to porosity and particulate boundaries. However, theseproperties could be tailored by controlling the solution precursorplasma spray process as well as texturing of the coatings. Poor

0

20

40

60

80

100

350 400 45 0 500 550 60 0 65 0 700

Wavelength (nm)

% R

efl

ecta

nce

ZnO on steel substrate - 75 passes

ZnO on corning glass - 10 passe s

Fig. 11. Reflectivity measurements for ZnO coatings.

reflectance on ZnO coated glass must be because of transmission oflight through the glass substrate.

4. Conclusions

ZnO coatings with reasonably good electrical and opticalproperties were successfully deposited using an inexpensivesolution precursor plasma spray process that has the capabilityto produce coatings with high throughput as well as tailor themicrostructures. The microstructural parameters, electrical andoptical properties are comparable with the ZnO coatings developedby other techniques. Higher resistivity and lower transmittancecompared to the CVD films is attributed to ultrafine particulateboundaries, grain boundaries and porosity. However, highlyporous nanostructured ZnO with large surface area could be veryuseful in sensor as well as solar applications.

Acknowledgements

The authors would like to thank Nagaswetha Pentyala for fourpoint probe electrical measurements.

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