Materials Science and Engineering B 177 (2012) 1678–1681
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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 , spray pyrolysis , pulsed laser deposition , chemical vapor deposition , electrodeposition [5–13], sol–gel method  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 . 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
Scan height (Å)
Current density (mA/cm )
a b 500
0.50 0.25 0 0.0
-0.8 -0.9 Applied potential(V vs.SCE)
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 . 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 . Cathodic reaction: NO3 − + H2 O + 2e = NO2 − + 2OH−
2H+ + 2e = H2
(produces 2OH− ions)
Zn2+ + 2OH− = Zn(OH)2 = ZnO + H2 O
Anodic reaction: 2H2 O = O2 + 4H+ + 4e
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 :
N˙ = Zc exp
2N 16ka3 E¯ s3 Vm a
3kv2 2 (RT )3 (ln S)2
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,
X. Qin et al. / Materials Science and Engineering B 177 (2012) 1678–1681
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 . 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)  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
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
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 . 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.
(Absorption coefficient ) 2
40 30 20
500 550 Wavelength( nm)
Fig. 5. The curves of transmittance and (˛h)2 –(h) of ZnO ﬁlms: (a) with 0.5% op-10 and (b) without op-10.
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
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 . 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)
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
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