The effect of surfactant on the structure and properties of ZnO films prepared by electrodeposition

The effect of surfactant on the structure and properties of ZnO films prepared by electrodeposition

Materials Science and Engineering B 177 (2012) 1678–1681 Contents lists available at SciVerse ScienceDirect Materials Science and Engineering B jour...

1MB Sizes 3 Downloads 53 Views

Materials Science and Engineering B 177 (2012) 1678–1681

Contents lists available at SciVerse ScienceDirect

Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb

Short communication

The effect of surfactant on the structure and properties of ZnO films prepared by electrodeposition Xiujuan Qin a,b , Guangjie Shao a,b,∗ , Lin Zhao a a b

Hebei Key Laboratory of Applied Chemistry, College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, China State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China

a r t i c l e

i n f o

Article history: Received 14 January 2012 Received in revised form 6 June 2012 Accepted 13 August 2012 Available online 28 August 2012 Keywords: Surfactant Hydrogen evolution reaction Electrodeposition pH ZnO film

a b s t r a c t The effect of surfactant on the formation, structure and properties of ZnO films synthesized by electrodeposition was studied in this work. It was carried out in an aqueous Zn(NO3 )2 solution containing surfactant op-10 using cathodic galvanostatic method. The results showed that the additive surfactant effectively inhibited hydrogen evolution reaction on cathode surface, maintained stability of the solution pH and improved deposition rate of the films to two times. Grown ZnO films with uniform grain and smooth surface were observed by using atomic force microscopy. Optical characterizations indicated that average optical transmittance of such films was more than 80% in the visible wavelength range, and its optical band gap was near 3.21 eV. © 2012 Elsevier B.V. All rights reserved.

1. Introduction In recent years, ZnO films have been object of quickly growing attention because of its high electrochemical stability, abundance in nature and absence of toxicity. They are widely used in photodiodes, chemical sensors, surface acoustic wave devices, piezoelectric transducers, light emitting devices, catalysis and solar cells. ZnO thin films have been prepared by a wide variety of techniques such as sputtering [1], spray pyrolysis [2], pulsed laser deposition [3], chemical vapor deposition [4], electrodeposition [5–13], sol–gel method [14] and so on. In particular, the electrodeposition method has advantages over other processes owing to its simple equipment, low cost, low temperature (lower than 100 ◦ C) and the possibility of making large area thin films. Furthermore, morphology and thickness of the films can be easily controlled by adjusting deposition parameters, and high deposition rate being especially suited to solar cells. So far, electrolyte used in electrodeposition ZnO films is divided into aqueous solution and non-aqueous solution. It is expensive that non-aqueous solution is used as electrolyte. However, the crystal grain of ZnO electrodeposited from aqueous solution is

∗ Corresponding author at: Hebei Key Laboratory of Applied Chemistry, College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, China. E-mail address: [email protected] (G. Shao). 0921-5107/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2012.08.012

micron-sized and it has poor quality due to hydroxyl ion adsorption on the surface of the films [10]. There are a lot of papers on the application of surfactants for ZnO electrodeposition, research emphasis is mostly focused on controlling the surface morphology of ZnO crystals using different surfactants, such as sodium dodecyl sulfate (SDS), hexamethylenetetramine (HMT), cetyltrimethylammonium bromide (CTAB), and ethylene-diamine (EDA) [7,9,11–13]. Whereas there are few studies on the effect of surfactant on the quality of ZnO films electrodeposited from aqueous solution. In this paper, ZnO films with good properties were prepared by using electrodeposition method in aqueous solutions containing surfactant op-10. Op-10 is short for octylphenol-ethoxylate, and the structural formula of it is C8 H17 C6 H4 O(CH2 CH2 O)10 H. It is a kind of non-ionic surfactant, whose state is a colorless and transparent thick liquid. The pH of the surfactant op-10 solution is about 6. The effect of surfactant on the quality of ZnO films was investigated. 2. Materials and methods A homemade three-electrode cell was used to electrodeposit ZnO films with ITO (tin-doped In2 O3 coated glass, supplied by QinhuangdaoYao-hua Co. Ltd.) glass substrate, 25 mm × 15 mm × 3 mm, sheet resistance of about 30 /, as the working electrode, platinum as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. The electrolyte consisted of 0.07 M (mol/L) Zn(NO3 )2 and some surfactant op-10 with an adjusted initial pH 5.5. Analytical grade reagents

X. Qin et al. / Materials Science and Engineering B 177 (2012) 1678–1681

1679

1500

1.75

1.25

Scan height (Å)

2

Current density (mA/cm )

1.50

b a

1.00 0.75

1000

a b 500

0.50 0.25 0 0.0

0.00 -0.7

-0.8 -0.9 Applied potential(V vs.SCE)

-1.0

0.5

-1.1

1.0

1.5

2.0

Scan-length (mm)

Fig. 1. Linear sweep voltammogram of ZnO deposition from Zn(NO3 )2 aqueous solutions: (a) with 0.5% op-10 and (b) without op-10.

Fig. 2. Step pattern of ZnO films deposited: (a) with 0.5% op-10 and (b) without op-10.

Concomitant reaction: were used. Cathodic galvanostatic deposition mode was used with a current density of 0.27 mA cm−2 . The temperature was held at 65 ◦ C in water bath and the electrolyte was stirred continuously using a magnetic stirrer. Electrodes were held vertically with a distance of 2 cm. Deposition time was maintained at about 20 min. Prior to the film deposition, the ITO substrates were cleaned with acetone, ethanol and distilled water and were then dried. The asprepared samples were annealed in air at the temperature of 200 ◦ C for 30 min. The linear sweep voltammogram of prepared ZnO films were performed on a LK2005A Microcomputer-based Electrochemical System. X-ray diffraction (XRD) measurements were performed by a D/Max-2500/PC diffractometer with Cu K␣ radiation. The surface morphology of the films was studied by a MultiMode 8 atomic force microscope (AFM). Optical properties studies were carried out employing an UV-2550 ultraviolet–visible spectrum and FL-1039 fluorescence spectrum using an excitation wavelength of 325 nm. The films thickness was measured by a XP-2TM surface Talysurf. 3. Results and discussion 3.1. Content of surfactant Structure and properties of ZnO films were affected by the surfactant concentration in aqueous solutions according to the report in the literature [7]. ZnO films were electrodeposited from Zn(NO3 )2 aqueous solution containing different surfactant op-10 content, whose volume ratio is 0%, 0.25%, 0.5% and 0.75%, respectively. The quality of ZnO films was evaluated. Results showed that the surfactant content of 0.5% is suitable in this work. 3.2. Linear sweep voltammogram of ZnO films The linear sweep voltammogram of electrodeposition ZnO films is shown in Fig. 1. The sweep was scanned cathodically at 1 mV/s. The cathodic polarization curve labeled as “a”, sample prepared from Zn(NO3 )2 aqueous solution containing op-10 of 0.5%, significantly moves to negative potential compared with curve labeled as “b”, sample prepared without op-10. This difference reaches to 97 mV when current density is about 0.27 mA cm−2 . The cathodic electrodeposition of ZnO thin films from nitrate solution is thought to proceed via following [10]. Cathodic reaction: NO3 − + H2 O + 2e = NO2 − + 2OH−

(1)

2H+ + 2e = H2

(produces 2OH− ions)

(2)

Zn2+ + 2OH− = Zn(OH)2 = ZnO + H2 O

(3)

Anodic reaction: 2H2 O = O2 + 4H+ + 4e

(4)

Cathodic current consists of the process of Eqs. (1) and (2) and competes with one another. In Fig. 1, the curve labeled as “b” is not smooth, obvious bubbles on cathode surface were observed during electrodeposition when potential is between −0.7 V and −0.92 V (vs. SCE), it may result from hydrogen evolution reaction on cathode. The curve labeled as “a” turns to smoothness in the same sweep range, no obvious bubbles on cathode surface were observed during electrodeposition. This implies that adsorption of surfactant molecules on the cathode surface inhibits hydrogen evolution reaction, resulting in the increase of cathodic polarization. When electrodeposition of ZnO films was carried out in aqueous solution without surfactant, H2 produced on the cathode surface promoted OH− ions away from the electrode surface due to its stir action. Thereby, the deposition rate of ZnO films (Eq. (3)) becomes slow, and grain size of films becomes coarseness because of lower supersaturation ratio on electrode interface. Based on the relationship between supersaturation ratio and the nucleation rate [15]:



N˙ = Zc exp



2N 16ka3 E¯ s3 Vm a

3kv2 2 (RT )3 (ln S)2



(5)

where N˙ is the nucleation rate, S is the supersaturation ratio of Zn(OH)2 , E¯ s is the average surface energy,  is the number of ions in a solute molecule, Zc is the frequency factor of 1025 , ka and kv are the area shape factor and volume shape factor of crystal, respectively. The higher the supersaturation ratio is, the faster nucleation rate becomes according to Eq. (5), which benefits to obtain small crystal grain. By contrast, the deposition rate of ZnO films in the additive surfactant solution is faster and grain size of films is fine under higher supersaturation ratio on electrode interface because of restraining of hydrogen evolution reaction on cathode surface. Electrodeposited ZnO films thickness was measured, as shown in Fig. 2. A good agreement has been found between analysis above and the test result. Electrodeposited ZnO films thickness is about 110 nm (with surfactant op-10, curve labeled as “a”) and 50 nm (without surfactant op-10, curve labeled as “b”), respectively. The solution pH was about 4.6 after electrodeposition. That is, variation of the solution pH with op-10 is from 5.5 to 4.6. However,

1680

X. Qin et al. / Materials Science and Engineering B 177 (2012) 1678–1681

101 100

Intensity

002

110

102 a

b

30

35

40

45

50

55

60

2 (deg)

Fig. 3. XRD pattern of ZnO films: (a) with 0.5% op-10 and (b) without op-10.

pattern labeled as “a” (deposited films with op-10). This indicates that deposited ZnO films structure was not affected by adding surfactant. Moreover, relative intensity (I0 0 2 /I1 0 1 ) of the pattern labeled as “a” is weakened compared with the pattern labeled as “b”. During electrodeposition, films crystallization would be affected by the kinetics of atomic arrangements. It is reported that the (0 0 2) orientation of ZnO has the lowest surface energy among all orientations [6]. Therefore, at a relative low deposition rate (corresponding to simple aqueous solution), adatoms on the substrate surface would have enough time to move to look for the lowest energy sites before these adatoms are covered by the next layer of atoms. However, a relative high deposition rate (corresponding to aqueous solution containing surfactant op-10) would make the adatoms have no time to arrange their sites, and hence the films exhibit random orientations. The test result of XRD corresponds with discussion in Section 3.2. The strain () and grain size are calculated using Eq. (6) [16] and Sheere expressions based on the data obtained from XRD analysis.  = −453.6 × 109

the solution pH without op-10 dropped quickly after electrodeposition, it changed from initial 5.5 to final 3.8, variation of 1.7 units. The pH is one of the most important factors which influence electrodeposition ZnO films quality. In galvanostatic deposition mode, OH– number produced by cathodic electrode process (Eq. (1) and (2)) is the same as H+ number produced by anodic electrode process (Eq. (4)). The formation of ZnO (Eq. (3)) made OH− concentration in solution decrease. As a result, the solution pH fell after electrolysis. But decrease speed of the pH in additive surfactant solution is inhibited. Hydrogen evolution reaction on cathode surface (Eq. (2)) easily occurs in aqueous solution without surfactant. This can reduce the reaction speed of Eq. (1). Thus, NO2 − concentration in additive-free surfactant solution is lower than that in additive surfactant solution. The decrease of NO2 − concentration in solution accelerates decomposition of weak acid HNO2 (NO2 − + H+  HNO2 ), resulting in the increase of H+ concentration. So, decrease speed of the pH in additive-free surfactant solution is faster.

c − c  0

c0

(6)

where c is the lattice constant of the sample and c0 is the lattice constant of the standard ZnO sample. They are −7.84 × 108 Pa, 54.5 nm (without surfactant op-10) and −3.05 × 108 Pa, 38.8 nm (with surfactant op-10), respectively. Obviously, the strain and grain size of sample are reduced after adding surfactant. 3.4. Films surface morphology AFM technique was used to investigate the surface morphology of deposited ZnO thin films. Sample electrodeposited in aqueous solution containing surfactant op-10 emerges compact, uniform as shown in Fig. 4a. Appearance exhibits spherical particle. In Fig. 4b, the morphology of the films electrodeposited in aqueous solution without op-10 presents visible difference from Fig. 4a. Particle boundaries appear blurry, with clusters formed by agglomeration. Roughness values of ZnO films with and without surfactant op-10 are 29.8 nm and 46.5 nm, respectively.

3.3. Film structure 3.5. Optical properties of ZnO films Fig. 3 illustrates the XRD patterns of ZnO thin films deposition from Zn(NO3 )2 aqueous solutions with and without surfactant op-10. The prepared films are polycrystalline with a hexagonal wurtzite structure. No noticeable additive peak is observed in the

Fig. 5 shows the optical transmittance of ZnO films electrodeposited with and without surfactant op-10. Optical transmittance average values (>80%) of ZnO samples are obtained in the visible

Fig. 4. AFM images of ZnO films: (a) with 0.5% op-10 and (b) without op-10.

X. Qin et al. / Materials Science and Engineering B 177 (2012) 1678–1681

100

competition of two factors. Namely, increase of the band gap resulted from quantum size effect competes with decrease of the band gap resulted from surface barrier [17]. In this study, the effect of surface barrier is far more than that of quantum size effect for ZnO sample prepared in aqueous solution with op-10 from the data mentioned above. Thereby, optical band gap of the sample narrows, optical absorption edge is red shift. The room temperature photoluminescence (PL) spectra of ZnO films electrodeposited are shown in Fig. 6. ZnO films exhibit the strong intrinsical UV emission at about 390 nm. Comparing the relative intensity of the exciton emission to the DLE (Iexc /IDLE ) from defects is a way to evaluate the quality of ZnO films. From Fig. 6, for the sample prepared in aqueous solution with op-10 labeled as “a”, the high intensity ratio of Iexc /IDLE reveals that the crystal quality of ZnO films is improved.

90

b

(Absorption coefficient ) 2

Transmittance/(%)

80 70

a

60 50

b a

40 30 20

2.0

2.5

3.0

3.5

4.0

Photon evergy(eV)

350

400

450

500 550 Wavelength( nm)

600

650

1681

700

4. Conclusions

Fig. 5. The curves of transmittance and (˛h)2 –(h) of ZnO films: (a) with 0.5% op-10 and (b) without op-10.

a

Intensity

b

In Zn(NO3 )2 aqueous solution containing surfactant op-10, ZnO films were prepared by cathodic galvanostatic method on the ITO glass substrate. Surfactant added in Zn(NO3 )2 aqueous solution plays an important role in inhibiting hydrogen evolution reaction and holding the solution pH. Hence, the quality of deposited ZnO films was improved. After adding surfactant op-10, deposited films with high deposition rate and narrow band gap is especially suited to solar cells. Obtained ZnO layer exhibits a smooth, uniform, compact, transparent film with smaller strain for an optimized surfactant ratio 0.5%. Acknowledgements We are grateful for the financial support from the Natural Science Foundation in Hebei Province, China (No. B2012203070). References

350

400

450

500

550

600

Wavelength(nm) Fig. 6. Room temperature PL spectra of ZnO thin films on ITO glass produced by electrochemical deposition: (a) with 0.5% op-10 and (b) without op-10.

wavelength range. Whereas, optical absorption edge of ZnO sample electrodeposited in aqueous solution containing surfactant op-10 is slightly red shift. Absorption coefficient satisfies Eq. (7) for a direct band gap material [8]. The band gap (Eg) is obtained by extrapolation of the plot of (˛h)2 vs. (h) and is found to be 3.21 eV, 3.27 eV for the ZnO films electrodeposited with and without surfactant op-10, as shown inset in Fig. 5. 2

(˛hv) = A(hv − Eg)

(7)

Variation of the band gap (Eg) for semiconductor material is affected by many factors, for example: grain size, doping, surface barrier, etc. The effect of surface barrier on the band gap becomes considerable distinctness for film material under nanometer size. Increase or decrease of the band gap depends on the result of

[1] P. Prepelita, R. Medianu, B. Sbarcea, Applied Surface Science 256 (2010) 1807–1811. [2] S.M. Rozati, S. Akesteh, Materials Characterization 58 (2007) 319–322. [3] K. Wang, Z.B. Ding, S.D. Yao, H. Zhang, S.L. Tan, F. Xiong, P.X. Zhang, Materials Research Bulletin 43 (2008) 3327–3331. [4] T. Terasako, T. Yamanaka, S. Yura, M. Yagi, S. Shirakata, Thin Solid Films 519 (2010) 1546–1551. [5] M.A. McLachlan, H. Rahman, B. Illy, D.W. McComb, M.P. Ryan, Materials Chemistry and Physics 129 (2011) 343–348. [6] L.J. Luo, G. Lv, B.H. Li, X.Y. Hu, L. Jin, J.B. Wang, Y.W. Tang, Thin Solid Films 518 (2010) 5146–5151. [7] H. Usui, Electrochimica Acta 56 (2011) 3934–3940. [8] M. Fahoume, O. Maghfoul, M. Aggour, B. Hartiti, F. Chraıbi, A. Ennaoui, Solar Energy Materials & Solar Cells 90 (2006) 1437–1444. [9] F. Xu, Y. Lu, Y. Xie, Y. Liu, Materials and Design 30 (2009) 1704–1711. [10] L.Sh. Zhang, Zh.G. Chen, Y.W. Tang, Zh.J. Jia, Thin Solid Films 492 (2005) 24–29. [11] X. Gan, X. Gao, J. Qiu, X. Li, Applied Surface Sciences 254 (2008) 3839–3844. [12] T. Pauporte, J. Rathousky, Microporous and Mesoporous Material 117 (2009) 380–386. [13] M. Sofos, J. Goldberger, D.A. Stone, J.E. Allen, Q. Ma, D.J. Herman, W.W. Tsai, L.J. Lauhon, S.I. Stupp, Nature Materials 8 (2009) 68–74. [14] H. Oh, J. Krantz, I. Litzov, T. Stubhan, L. Pinna, C.J. Brabec, Solar Energy Materials & Solar Cells 95 (2011) 2194–2199. [15] J. Nyvlt, Industrial Crystallization from Solutions, Buttereworths, London, 1971. [16] T.L. Won, H.L. Chang, Thin Solid Films 353 (1999) 12–15. [17] Z.G. Ji, Physics of Semiconductor, Zhejiang University Press, Hangzhou, 2005.