Electrodeposited zinc oxide thin films: Nucleation and

ARTICLE IN PRESS

Solar Energy Materials & Solar Cells 91 (2007) 864–870 www.elsevier.com/locate/solmat

Electrodeposited zinc oxide thin films: Nucleation and growth mechanism A.I. Inamdar, S.H. Mujawar, S.B. Sadale, A.C. Sonavane, M.B. Shelar, P.S. Shinde, P.S. Patil Thin Film Materials Laboratory, Department of Physics, Shivaji University, Kolhapur 416 004, India Received 7 September 2006; received in revised form 28 November 2006; accepted 31 January 2007 Available online 13 March 2007

Abstract The nucleation and growth mechanism of the electrodeposited zinc oxide thin films on fluorine-doped tin oxide (FTO) coated (10–20 O/cm2) glass substrates from acetate solution, without and with ex situ oxygen bubbling, has been studied by cyclic voltammetry (CV), chronoamperometry (CA) and scanning electron microscopy (SEM) techniques. Ethylene diamine tetra acetic acid (EDTA) was used as a complexing agent. The cyclic voltammograms exhibit crossover, a characteristic of nucleation process on FTO-coated conducting glass substrates for all the baths bubbled with oxygen. The current transients were analyzed by fitting chronoamperometric data into the Scharifker–Hills nucleation model. The plausible nucleation and growth mechanism is proposed. For mother bath and lower oxygen bubbling time, the nucleation and growth mechanism follows 3D progressive nucleation and growth, which became instantaneous in case of baths for higher oxygen bubbling time. The SEM study showed that the films become compact when the oxygen bubbling time was increased. The thin films were further characterized by X-ray diffraction technique for structural studies and the ZnO film formation was confirmed. With the increase in oxygen bubbling time, the shift in band gap energies from 3.2 to 3.3 eV is observed. r 2007 Elsevier B.V. All rights reserved. Keywords: Zinc oxide thin films; Electrodeposition; Cyclic voltammetry; Chronoamperometry; Scanning electron microscopy (SEM); X-ray diffraction

1. Introduction Zinc oxide (ZnO) is a direct wide band gap (3.3 eV) semiconductor with large exciton binding energy of 60 meV at room temperature. It has various applications in optical, electrical and acoustic devices like UV lasing [1–3], optoelectronics [4], photovoltaics, photoconductive sensors and piezoelectric transducers [5]. In photovoltaics, they have been used as transparent conducting electrodes in solar cells based on copper indium diselenide [6] or amorphous silicon [7] absorbers. The ultraviolet lasing of bulk ZnO material has been demonstrated at cryogenic temperature many years ago [8]; potential applications of ZnO nanostructures include UV lasing [9,10], electrooptical switch [4] and hydrogen storage [11] devices. Several studies have been devoted to the electrodeposition of ZnO thin films directly from oxygen-dissolved Corresponding author. Tel.: +91 231 2690571; fax: +91 231 2691533.

E-mail address: [email protected] (P.S. Patil). 0927-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2007.01.018

precursor, as for instance for cathodic deposition using nitride ions as the oxygen precursor [12]. It has been discovered that good quality zinc oxide thin films can be deposited simply using dissolved oxygen as the precursor [13,14] according to overall reaction. Zn2þ þ 0:5O2 þ 2e ! ZnO:

(1)

Various other techniques have also been used to deposit ZnO thin films such as sputtering [6], metal-organic chemical vapour deposition [15], molecular beam epitaxy and atomic layer epitaxy [16]. The effect of polyehoxylated additives and organic additives on zinc electrocrystallization and on their kinetic parameters was studied by Trejo and Alvarez et al. [17,18]. Peulon and Lincot [13,14] described the cathodic electrodeposition of ZnO thin films from oxygenated aqueous zinc salt solution. The effect of oxygen concentration on morphology and on deposition parameters was studied by Pauporte and Lincot [19]. The nucleation and growth mechanism of electrosynthesized

ARTICLE IN PRESS A.I. Inamdar et al. / Solar Energy Materials & Solar Cells 91 (2007) 864–870

zinc oxide thin films from ex-situ oxygen bubbled zinc acetate bath has been sparsely studied. Accordingly, the present work has been carried out with an aim to understand nucleation, growth mechanism and the effect of ex-situ oxygen bubbling time on morphology of zinc oxide thin films [13,14].

standard hydrogen electrode). The working and counter electrodes were placed parallel to each other separated by a distance of 1 cm. Thin films were obtained by potentiostatic electrolysis at 508 mV (vs. SCE) for 30 min in zinc acetate bath. The films prepared from baths bubbled with oxygen for different times viz. 30, 60 and 90 min are denoted as Z1, Z2 and Z3, respectively, and those produced from baths without oxygen are denoted as Z0. All the experiments were performed at room temperature and in a quiescent solution. The surface morphology of the samples was observed using a JEOL-JSM-6360 scanning electron microscope (SEM). X-ray diffraction (XRD) spectra of the films were recorded on an X-ray diffractometer (Philips PW-1710)

2. Experimental Zinc oxide thin films were synthesized via an aqueous zinc acetate bath containing 50 mM solution concentration. 0.1 M KCl was used as a supporting electrolyte. The zinc acetate solution was complexed at pH 10.5 with ethylene diamine tetra acetic acid (EDTA) to restrict the mobility of H+ ions. Oxygen was bubbled through the bath for 30, 60 and 90 min. Electrolytic baths thus prepared were utilized for the deposition of ZnO thin films. Electrochemical measurements were performed by using EG & G make Versastat-II (PAR 362) model controlled by personal computer in three electrode cell configuration with electrochemistry software M270. Fluorine-doped tin oxide (FTO) coated conducting glass substrate of 3 cm2 area was used as a working electrode, a platinum wire as a counter electrode and saturated calomel electrode (SCE) as a reference electrode (SCE, E0 ¼ 244 mV vs SHE, the

a

Sample

Anodic peak current (mA/cm2)

Cathodic peak current (mA/cm2)

Cathodic peak potential (mV)

Z0 Z1 Z2 Z3

3.88 32.94 57.54 57.54

80 100 120 130

758 752 742 694

-20 -40

4 3.5 3 2.5 2 1.5 1 0.5 0 -300 -100 100

100 Ea Current density (μA/cm2)

0 Current density (μA/cm2)

Table 1 Parameters obtained from cyclic voltamograms of zinc oxide thin film deposited from 50 mM zinc acetate solution

b 20

300

-60 -80

Ea

-100 -120 -140 -2000 -1500 -1000 -500 0 Potential (mV) Vs SCE

0 -100 -200 -300 -400 -500 -2000 -1500 -1000 -500 0 Potential (mV) Vs (SCE)

500

c

500

d 100

100

0

0

Current density (μA/cm2)


865

-100 -200

Cross Over

-300 -400 -500 -600 -2000 -1500 -1000 -500 0 Potential (mV) Vs (SCE)

500

-100 -200 -300 -400 -500 -600 -2000 -1500 -1000

-500

0

500

Potential (mV) Vs (SCE)

Fig. 1. Cyclic voltammograms recorded at the scan rate of 20 mV/s for all the samples: (a) Z0, (b) Z1, (c) Z2 and (d) Z3.


866

with Cu Ka radiation of 1.5418 A˚ wavelength. The transmission spectra in the 350–850 nm were recorded using Hitachi model 330 spectrophotometer. 60

OBT60 OBT50

OBT40

40 OBT30 30 OBT20 20

OBT10

10 MB 0 0

20

40 60 80 100 Potential (mV) vs(SCE)

120

140

Fig. 2. An enlarge view of the anodic peak recorded for the intermediate OBTs.

a

Fig. 1a–d shows cyclic voltammetry (CV) spectra obtained with zinc acetate solution without oxygen bubbling, referred hereafter as mother bath (MB), and with ex-situ oxygen bubbling times viz. 30, 60 and 90 min (OBT30, OBT60, OBT90), respectively, at a scan rate of 20 mV/s. A well-defined cathodic peak (Ec) is discernable at 758 mV (vs SCE) with peak current of 80 mA/cm2, during forward cathodic scan (Fig. 1a). The peak is followed by a sharp rise in cathodic current, nearing hydrogen evolution potential. During reverse scan, current decreases rapidly up to 1250 mV (vs SCE), followed by a monotonic decrement up to 400 mV (vs SCE) and leveled off with a weak anodic peak with peak current of 3.88 mA/ cm2 (shown in the inset). The CV spectra, recorded for all oxygen-bubbled baths (Fig. 1b–d), remained almost similar, except three added features: (i) the crossover between anodic and cathodic scans becomes prominent, (ii) anodic shift in the cathodic peak potential is observed and (iii) an evolution of an anodic peak during reverse scan. The prominent crossover during cathodic scan can be related to the onset of nucleation process [20,21]. The anodic shift of cathodic peak is due to faster kinetics resulting in the increment of nuclei density and nucleation

b 10 Region-II

-40

Cint

-90 -140 -190



10

Cf Region-I

-40

Cint

-90 -140 -190

Cf

-240 -290

-240 -500

Ci 0

500 1000 Time t (sec)

1500

-340 -500

2000

c

Ci 0


500 1000 Time t (sec)

1500

2000

d 10 -40 -90 -140 -190

Cf

-240 -290 -340 -390 -500

Cint

-30

Cint Current density (μA/cm2)


50

OBT90

3. Results and discussion

-80 -130 -180

Cf

-230 -280 -330 -380

Ci 0

500 1000 Time t (sec)

1500

2000

-430 -500

Ci 0

500 1000 Time t (sec)

1500

2000

Fig. 3. Current vs transit time obtained at deposition potential of 1.3 V (vs SCE) for the samples: (a) Z0, (b) Z1, (c) Z2 and (d) Z3.


rate. The improvement in the kinetics is the net effect of rise in oxygen concentration upon oxygen bubbling which causes decrease in pH (10.5 for MB to 7.5 for OBT90) and increase in ionic conductivity (20.9 S/cm for MB to 203 S/cm for OBT90).This improvement in kinetics with decrease in pH is well known [22]. Similar observations have been reported by Pauporte and Lincot [19]. The anodic peak is evolved due to oxidation of ‘‘Zn’’, which is reduced cathodically. This observation reveals that the above processes occur prominently after oxygen bubbling in the MB. However, this nucleation signature is not revealed by the MB, it might be very slow or undetectable within the studied time domain or limited by the current resolution. The oxidation peak current density at around 80 mV (vs SCE) increases from 3.88 mA/cm2 (for MB) to 57.54 mA/

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cm2 (for OBT60) and remains constant for further increase in oxygen bubbling time (OBT90) (Table 1). Fig. 2 shows an enlarged view of the oxidation peak recorded for intermediate OBT intervals viz. 10, 20, 40 and 50 min. Anodic peak current density increases by regular magnitude up to 60 min beyond which it saturates up to OBT90. Fig. 3 shows curves for current vs transit time obtained for MB, OBT30, OBT60 and OBT90 (a, b, c and d), respectively. After an initial surge, current decays abruptly as cations in the close vicinity of the electrode are reduced and a transient signal is observed. The current Ci (shown in Fig. 3) depends on a number of initially accessible cationic species in the close vicinity of the cathode, which discharge immediately after electrode is polarized. This leads to the birth of the nucleation centers:

Table 2 Parameters obtained from chronoamperometric graphs Sample

Oxygen bubbling time (min)

Ci (mA/cm2)

Cint (mA/cm2)

Cf (mA/cm2)

Ci–Cint (mA/ cm2)

CfCint (mA/ cm2)

Nucleation and growth process

Z0 Z1 Z2 Z3

0 30 60 90

230 320 370 413

90 110 100 95

130 120 100 95

140 210 270 318

40 10 0 0

Progressive Progressive Progressive Instantaneous

a

c 1.0

: experimental : instantaneous : progressive


0.8 i2/imax2

i2/imax2

0.8

1.0

0.6 0.4

0.6 0.4 0.2

0.2

0.0 0.0 0

1

2

3

4 5 t/tmax

6

7

1

2

3

4

5

6

7

8

9

10

9

10

t/tmax

b

d 1.0

1.2



1.0 i2/imax2

0.8 i2/imax2

0

8

0.6 0.4

0.8 0.6 0.4 0.2

0.2

0.0

0.0 0

1

2

3

4

5 6 t/tmax

7

8

9

10

0

1

2

3

4

5 6 t/tmax

7

8

Fig. 4. Nondimensional i2/i2max vs t/t2max plots of data for zinc oxide electrodeposition onto FTO-coated conducting glass substrates for the samples: (a) Z0, (b) Z1, (c) Z2 and (d) Z3.


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either large number of small nuclei or small number of large nuclei. This introduces temporal depletion regime near cathode, leading to the decrement in the current; Cint. As fresh cationic species diffuse, current starts increasing gradually and reach Cf. Hence, it is reasonable that the difference, CfCint, is proportional to the number of cationic species deposited onto the electrode during the second event. This current is used to judge whether the nuclei formed in the first stage grow progressively or instantly. The difference, CfCint, illustrates the nature of the process in the studied time domain. Based on this discussion and values given in Table 2, the nucleation and growth mechanism is proposed. From the current-transit time (Fig. 3a) recorded for the MB, the current after initial surge abruptly decreases, attains minimum (region I) and then increases steadily, in the studied time domain (region II). The nuclei formed in region I grow during region II. The growth process is slow, which leads to cluster formation. The progressive growth of the critical nuclei leads to cluster formation due to process like adsorption and coalescence. In case of oxygenbubbled baths (Fig. 3b–d), the current after initial surge decreases relatively slow without exhibiting a sharp minimum as observed in case of MB. Thus, a negligible rise in current from Cint to Cf was noticed. However, the

initial surge in current, Ci, increases with oxygen bubbling time (see Table 2). This is due to increase in rate of nucleation leading to faster kinetics. Scharifker and Hills [23] model is further used to describe the nucleation process during initial few seconds using chronoamperometric techniques. Progressive nucleation corresponds to slow growth of nuclei on a less number of active sites, all activated at same time. Instantaneous nucleation corresponds to fast growth of nuclei on many active sites, all activated during the course of electroreduction [24]. Hence, the transients have been further analyzed in reduced form in terms of the maximum current imax and the time at which the maximum current is observed, tmax [23]. This method of obtaining information about kinetics of the nucleation process has been used extensively for wide variety of systems [25,26]. The expressions for instantaneous nucleation and progressive nucleation with 3D growth are given by following equations, respectively, i2 i2max i2

(203)

(104)

Intensity (Arb. unit)

(110)

(103)

i2max

Z3

Z2

*

*

Z1 Z0 45

55

65 75 2θ (degrees)

85

95

Fig. 5. XRD patterns recorded for the samples deposited from 50 mM aqueous zinc acetate solution.

¼ 1:9542

2 ht i t max , 1 exp 1:2564 tmax t

( " # )2 ht i t 2 max ¼ 1:2254 , 1 exp 2:3367 tmax t

(2)

(3)

where imax and tmax are the current and time coordinates of the peaks, respectively. Eqs. (3) and (4) provide a convenient criterion to these two extreme cases of nucleation kinetics [27], during initial time period. The fitting of the experimental curves with theoretical curves for MB and OBTs are shown in Fig. 4a–d. For the MB, the experimental curve is very similar to that of progressive in the entire studied time domain. This corroborates our conclusion that the growth process in case of MB is of progressive nature. For oxygen bubbled baths (OBT30 and OBT60), the experimental curve follows progressive nature as well (Fig. 4b and c). Similarly, progressive nucleation and growth mechanism prevails for the other intermediate OBTs. However, for OBT90 experimental curve follows instantaneous nature (Fig. 4d). The general reaction route of electrocrystallization of zinc oxide thin films from acetate bath is as follows [28,29].

Table 3 Comparison of observed d-values, obtained from XRD data, with standard d-values from JCPDS data Sr. no.

1 2 3 4 5 6

Observed d-values

Standard d-values

Possible reflections (h k l)

MB (Z0)

OBT30 (Z1)

OBT60 (Z2)

OBT90 (Z3)

ZnO

1.636 1.571 1.502 1.326 1.187

1.636

1.636

1.636

1.632

Zn(OH)2

1.571 1.499

1.500

1.499

1.493

1.188

1.188

1.186 1.085

1.187 1.090

1.322

(1 1 0) (0 0 3) (1 0 3) (1 1 2) (1 0 4) (2 0 3)


The dissociation of zinc acetate salt (after dissolving in deionized water) is as follows ZnðCH3 COOÞ2 2H2 O þ 2H2 O 3Zn2þ þ 2CH3 COOH þ 4OH þ xH2 :

(4)

Reaction mechanism for the MB is 2Zn2þ þ 2CH3 COOH þ 4OH þ xH2 þ 4e ! ZnðOHÞ2 þ ZnO þ H2 O þ xH2 þ 2CH3 COOH:

(5)

The reaction mechanism in case of oxygen-bubbled baths is Zn2þ þ 2CH3 COOH þ 4OH þ H2 O þ ðx 1ÞH2 þ 2e ! ZnO þ 2CH3 COOH þ 4H2 O:

(6)

The films were further investigated by XRD and SEM for structural and morphological studies. The samples

(αhυ)2×1010 (eV/cm2)

2.35 1.85 MB

1.35

OBT30 OBT60

0.85

OBT90 0.35 -0.15 1.5

1.8

2.0

2.3

2.5 2.8 hθ (eV)

3.0

3.3

3.5

Fig. 6. (ahu)2 vs. (hu) plot for the films deposited at various oxygen bubbling times.

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synthesized from MB, OBT30, OBT60 and OBT90 are denoted by Z0, Z1, Z2 and Z3, respectively. Fig. 5 shows the XRD spectra of Z0, Z1, Z2 and Z3 samples. The angle of diffraction, 2y, was varied over 10–1001. The observed XRD spectra were compared with standard JCPDS data files (number 75–1533, 36–1451 and 79–0208 for ZnO, 72–2032 for zinc hydroxide and 77–0447 for FTO-coated conducting glass substrate (not shown in figure)). These observed d values are in good agreement with standard d values (Table 3). For Z0, two Zn (OH)2 phases along (0 0 3) and (2 0 1) planes (denoted by star) are observed in addition to three ZnO peaks along (1 1 0), (1 0 3) and (1 0 4) planes. This indicates that the Z0 sample is a composite of ZnO and Zn (OH)2 phases. The Zn (OH)2 phase disappears when bath is bubbled with oxygen for 30 min and above, and ZnO formation is clearly observed. Increase in the peak intensity is due to improvement in crystallinity with OBT. The above results were supported by band gap determination for all the films. The optical absorption spectra were recorded in the wavelength range over 350–850 nm. The direct band gap is confirmed from (ahu)2 vs. (hu) variation as shown in Fig. 6. The band gap energy varies between 3.2 and 3.3 eV, which is in agreement with the documented room temperature values [30,31]. Fig. 7a–d shows the surface morphology of the zinc oxide deposits formed on FTO-coated conducting glass substrates from MB, OBT30, OBT60 and OBT90, respectively. For Z0, loosely packed clusters are observed. A well-defined grain of about 250 nm was obtained for Z1. Relatively dense morphology of irregular grains (larger grain size 750 nm) is revealed by the Z2 films. In case of Z3, the grains are diffused with each other to form compact layer. From this, it is concluded that different

Fig. 7. Scanning electron micrographs recorded for the samples: (a) Z0, (b) Z1, (c) Z2 and (d) Z3.

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morphologies can be obtained by employing corresponding deposition conditions. 4. Conclusions ZnO thin films were successfully electrodeposited onto the FTO-coated conducting glass substrates at room temperature electrochemical deposition from an aqueous zinc acetate solution. From the fitting of experimental data to theoretical Scharifker–Hills model, it is concluded that the progressive nucleation and growth mechanism prevails for the thin films deposited from MB, OBT30 and OBT60. For OBT90 experimental curve follows instantaneous nature. The films deposited from zinc acetate bath are of composite phases of zinc hydroxide and zinc oxide, whereas zinc oxide thin films were obtained from oxygen bubbled baths. The band gap energies determined for the films deposited from MB and oxygen-bubbled baths echo above findings. Different morphologies can be obtained by employing the corresponding deposition conditions. Acknowledgements The authors wish to acknowledge the UGC, New Delhi for the financial support through the UGC-DRS IInd phase programme (2004–2009) and DST through FIST programme (2002–2007). References [1] Z.K. Tang, G.K.L. wang, P. Yu, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Appl. Phys. Lett. 72 (1998) 3270. [2] Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897. [3] J.H. Choy, E.S. Jang, J.H. Chung, D.J. Jang, Y.W. Kim, Adv. Mater. 15 (2003) 1911. [4] H. Kind, H. Yan, M. Law, B. Messer, P. Yang, Adv. Mater. 14 (2002) 158.

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