Effect of Different Pretreatments on Indium-Tin Oxide Electrodes
Moonjeong Choi, Kyungmin Jo, and Haesik Yang*
Department of Chemistry and Chemistry Institute of Functional Materials, Pusan National University, Busan 609-735, Korea
*E-mail: hyang@pusan.ac.kr
Received October 25, 2012, Accepted November 12, 2012
The effect of pretreatment on indium-tin oxide (ITO) electrodes has been rarely studied, although that on metal and carbon electrodes has been enormously done. The electrochemical and surface properties of ITO electrodes are investigated after 6 different pretreatments. The electrochemical behaviors for oxygen reduction, Ru(NH3)63+
reduction, Fe(CN)63− reduction, and p-hydroquinone oxidation are compared, and the surface roughness, hydrophilicity, and surface chemical composition are also compared. Oxygen reduction, Fe(CN)63− reduction, and p-hydroquinone oxidation are highly affected by the type of the pretreatment, whereas Ru(NH3)63+ reduction is almost independent of it. Interestingly, oxygen reduction is significantly suppressed by the treatment in an HCl solution. The changes in surface roughness and composition are not high after each pretreatment, but the change in contact angle is substantial in some pretreatments.
Key Words : Indium-tin oxide, Pretreatment, Electrochemical property
Introduction
Pretreatment of electrodes is commonly employed to clean electrode surface and to change their electrocatalytic activities and surface roughness.1-9 In chemical sensors and biosensors, pretreatment plays a crucial role in improving the reproduci- bility of electrochemical data. Pretreatment methods of metal electrodes such as gold and platinum electrodes have been widely studied. The methods include mechanical polishing,1 thermal treatment,2,3 electrochemical treatment,4-6 and Piranha treatment.6,7 However, the effect of pretreatment methods on indium-tin oxide (ITO) electrodes has been rare- ly investigated, although many pretreatment methods have been employed to clean ITO electrodes.10-20 Recently, the authors studied the effect of thermal treatment on the electro- catalytic activities and surface roughness of ITO electrodes:
the electrocatalytic activities and surface roughness of ITO electrodes increase with increasing the incubation temper- ature.10
The electrocatalytic activities of ITO electrodes are much lower than those of common metal and carbon electrodes.
This characteristic of ITO electrodes allows us to obtain low and reproducible background currents.21 In some cases, it is required to obtain higher or lower electrocatalytic activities of ITO electrodes, depending on their application. Higher electrocatalytic activities can be obtained by modifying ITO electrodes with more electrocatalytic materials such as gold nanoparticles,22 carbon nanotubes,23 and graphenes.24 How- ever, it would be better to obtain higher electrocatalytic activities via simple pretreatment of ITO electrodes. When electrochemical measurements are carried out at potentials below 0 V vs Ag/AgCl, the background currents caused by oxygen reduction reaction are problematic. Even in the case of ITO electrodes, oxygen reduction reaction is not negli- gible in negative potential regions. In order to minimize
oxygen reduction reaction, it is required to obtain rather lower electrocatalytic activities.
To clean ITO electrodes, the pretreatment methods that are based on dipping or ultrasonication (i) in an HCl solution, (ii) in a mixed solution of H2O2, NH4OH, and water, (iii) in a methanolic solution of K2CO3, or (iv) in a Piranha solution have been commonly employed.11-20 However, the electro- catalytic activities, surface roughness, hydrophilicity, and surface composition of ITO electrodes obtained after these pretreatments have never been intensively investigated and compared. In this study, the authors have studied the effect of pretreatment methods on the electrochemical properties, surface roughness, contact angle, and surface composition of ITO electrodes. After 6 different pretreatments, the electro- chemical properties toward oxygen reduction, Ru(NH3)63+
reduction, Fe(CN)63− reduction, and p-hydroquinone oxida- tion were compared, and atomic force microscopy (AFM) images, contact angles, and X-ray photoelectron spectro- scopy (XPS) spectra were also compared.
Experimental
All chemicals were purchased from Sigma-Aldrich and used without further purification. Phosphate-buffered saline (PBS) solutions consisted of 0.01 M phosphate, 0.138 M NaCl, and 0.0027 M KCl (pH 7.4).
ITO-coated glasses were obtained from Samsung Corning (Daegu, Korea). The pretreatment methods of ITO electrodes are as follows.
The treatment 1: no treatment.
The treatment 2: dipping in 1 M HCl for 10 min.11 The treatment 3: dipping in a solution of 1:1:5 (v/v) H2O2 (30%)/NH4OH (30%)/ water for 1 h at 70 °C.12-16
The treatment 4: the treatment 2 followed by the treat- ment 3.17
422 Bull. Korean Chem. Soc. 2013, Vol. 34, No. 2 Moonjeong Choi et al.
The treatment 5: ultrasonication in a methanolic solution of 0.5 M K2CO3 for 30 min.18
The treatment 6: dipping in a solution of 1:1:5 (v/v) H2O2 (30%)/NH4OH (30%)/water for 1.5 h at 70 °C after sequen- tial ultrasonication in trichloroethylene, ethanol, and water for 15 min.19
After each pretreatment, the electrodes were washed with water and dried with nitrogen.
The electrochemical experiments were performed using a CHI 405A instrument (CH instruments, Austin, TX, USA) and an electrochemical cell consisting of an ITO working electrode, a Pt counter electrode, and an Ag/AgCl (3 M NaCl) reference electrode at room temperature. The geo- metric area of the ITO electrode exposed to solution was ca.
0.28 cm2. AFM images were obtained using an XE-100 (Park System, Korea), operated in non-contact mode.
Results and Discussion
Four electrochemical reactions [oxygen reduction, Ru(NH3)63+
reduction, Fe(CN)63- reduction, and p-hydroquinone oxida- tion] have been tested to check the dependence of the electrochemical behaviors of ITO electrodes on the pre- treatment method (Figure 1). Cyclic voltammograms obtain- ed in a PBS solution (pH 7.4) without adding electroactive species are shown in Figure 1(a). Because dissolved oxygen
exists in solution, oxygen reduction reaction can occur in negative potential regions. At an untreated ITO electrode, the currents during the cathodic scan gradually negatively increased below −0.4 V. The increased currents are due to oxygen reduction reaction. The currents were significantly increased after the treatment 3 and 5, whereas the currents were negligible up to −0.6 V after the treatment 2. In Figure 2(a), the currents measured at −0.5 V during the anodic scan are compared. The current for the treatment 4 was the highest, whereas the current for the treatment 2 was the lowest. All results indicate that oxygen reduction reaction at ITO electrodes is highly affected by the pretreatment method and that it can be significantly suppressed by the treatment 2 (the treatment in an HCl solution).
Interestingly, capacitive currents above −0.1 V in four voltammograms of Figure 1(a) were very similar irrespective of the pretreatment methods. In general, the capacitive currents of Au, Pt, and carbon electrodes depend on the pretreatment method, which makes it difficult to obtain reproducible capacitive background currents.1,25 However, the capacitive background currents of ITO electrodes were quite reproducible in the pretreatment 1, 2, and 4. The capacitance values that were calculated from anodic and cathodic currents at 0.3 V in cyclic voltammograms (at a scan rate of 20 mV/s) are shown in Table 1. Moreover, the faradaic anodic currents were negligible in positive potential
Figure 1. Cyclic voltammograms obtained (at a scan rate of 50 mV/s) in PBS (pH 7.4) containing (a) no electroactive species, (b) 1.0 mM Ru(NH3)63+, (c) Fe(CN)63−, and (d) p-hydroquinone at ITO electrodes after the different pretreatments.
regions in a PBS solution, and the capacitive background currents were very flat (data not shown).
Ru(NH3)63+ undergoes a fast outer-sphere electron-transfer reaction.26 Therefore, electrochemical behaviors of Ru(NH3)63+
depend much less on the type and electrocatalytic activity of an electrode than other electroactive species.10,26 In Figure 1(b), four cyclic voltammograms for Ru(NH3)63+ were very similar irrespective of the pretreatment method as expected:
four anodic peak potentials were located almost in the same position. The difference between the anodic peak potential and the cathodic peak potential (ΔEp) was very similar in all pretreatment cases (Figure 3(a)), indicating that the electron- transfer rate was similar irrespective of the pretreatment
method.
It is known that the electron-transfer rate of Fe(CN)63−
reduction highly depends on the surface state as well as the electrocatalytic activity of an electrode.10,26,27 Electrostatic repulsion between Fe(CN)63− and negatively charged surface can significantly increase ΔEp, whereas electrostatic attrac- tion between Fe(CN)63− and positively charged surface can decrease ΔEp. If the electrocatalytic activity of an electrode is high, ΔEp can be low, and vice versa. In Figure 1(c), voltammetric behavior for Fe(CN)63− reduction varied with the pretreatment method. The ΔEp value obtained after each pretreatment is shown in Figure 3(b). The treatment 2 showed the highest ΔEp, whereas the treatment 3 showed the lowest ΔEp. It is not clear whether the low ΔEp is due to surface charge or electrocatalytic activity. However, the Figure 2. (a) Cathodic currents at −0.5 V and (b) anodic currents at
0.8 V in cyclic voltammograms obtained (at a scan rate of 50 mV/
s) in PBS (pH 7.4) containing (a) no electroactive species and (b) 1.0 mM p-hydroquinone at ITO electrodes after the different pretreatments.
Table 1. Contact angle, root-mean-square (RMS) roughness, and peak-area ratio of indium to tin in XPS spectra that were obtained after the different pretreatments
Treatment 1 2 3 4 5 6
Capacitance (µF/cm2) 10.6 ± 0.2 10.1 ± 0.2 11.7 ± 1.1 10.2 ± 0.5 16.7 ± 2.1 13.0 ± 0.8
Contact angle (°) 70 75 66 60 < 10 57
RMS (nm) 1.8 1.1 1.8 1.0 1.3 1.4
In:Sn 8.7:1 9.3:1 8.6:1 8.9:1 9.1:1 8.9:1
Figure 3. ΔEp in cyclic voltammograms obtained (at a scan rate of 50 mV/s) in PBS (pH 7.4) containing (a) 1.0 mM Ru(NH3)63+ and (b) Fe(CN)63− at ITO electrodes after the different pretreatments.
424 Bull. Korean Chem. Soc. 2013, Vol. 34, No. 2 Moonjeong Choi et al.
results clearly showed that the voltammetric behavior of Fe(CN)63− is significantly influenced by the pretreatment methods.
Electrochemical behaviors of p-hydroquinone oxidation highly depend on the thermal treatment of ITO electrodes.10 In Figure 1(d), the treatment 1 (for an untreated ITO elec- trode) allowed high anodic currents, whereas other pretreat- ments caused low anodic currents. This trend was not well in accord with that in oxygen reduction reaction. The reason for this is that the electrocatalytic activity depends on the type of electrochemical reaction as well as the pretreatment method.
The pretreatment can change surface properties such as the roughness and hydrophilicity of ITO electrodes. To investigate their dependence on the pretreatment method, the AFM images and contact-angle data were obtained and com- pared after the pretreatments. Four AFM images in Figure 4 show that the change in surface roughness was not high after each treatment. The variation in the root-mean-square (RMS) roughness in Table 1 was not significant. However, contact angle changed substantially after the treatment 5. The elec- trode surface became very hydrophilic after the treatment 5 (the treatment in a methanolic solution of K2CO3).
The variation in surface roughness and contact angle may result from the change in surface chemical composition. To check this possibility, XPS spectra were obtained (Figure 5).
The positions of two In 3d peaks and two Sn 3d peaks in Figure 5(a) correspond to the oxidation state of In3+ and Sn4+, respectively, indicating the presence of In2O3 and SnO2.28,29 There was no significant decrease in the area of C 1s peak after the pretreatments (Figure 5(b)), meaning that the inevitable carbon-based surface contamination was not significantly decreased after the pretreatments. The asym- metric shape of O 1s peaks in Figure 5(b) is due to the
Figure 4. AFM images of ITO electrodes obtained after the different pretreatments: (a) the treatment 1; (b) the treatment 2; (c) the treatment 3; and the treatment 5.
Figure 5. XPS spectra of ITO electrodes obtained after the different pretreatments: (a) narrow scan of In 3d and Sn 3d; (b) narrow scan of C 1s and O 1s. The numbers in figure represent the treatment numbers.
presence of both indium oxide and tin oxide.28,29 To check the change in the surface composition of indium and tin after the pretreatments, the peak-area ratios of indium to tin in XPS spectra (Figure 5(a)) were compared in Table 1. The ratio did not noticeably change after the pretreatments, indicating that the surface chemical composition did not change significantly.
Conclusion
Electrochemical properties of ITO electrodes were signifi- cantly influenced by the pretreatment method. The treatment in an HCl solution significantly decreased the electro- catalytic activity toward oxygen reduction reaction. The changes in the surface roughness and the surface chemical composition were not high after each treatment, but the change in contact angle was substantial in the treatment in a methanolic solution of K2CO3. This study clearly shows that the pretreatment can substantially change the electrochemical and surface properties of ITO electrodes.
Acknowledgments. This work was supported for two years by Pusan National University Research Grant.
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