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Effect of ZnO doping on morphology and electrochemical properties of sub-micron RuO2 sensing electrode of DO sensor
Materials Letters 65 (2011) 991–994
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Effect of ZnO doping on morphology and electrochemical properties of sub-micron RuO2 sensing electrode of DO sensor Serge Zhuiykov a,⁎, Vladimir Plashnitsa b, Norio Miura c a b c
CSIRO, Materials Science and Engineering Division, 37 Graham Road, Highett, VIC. 3190, Australia Research and Education Centre of Carbon Resources, Kyushu University, Kasuga-shi, Fukuoka, 816-8580, Japan KASTEC, Kyushu University, Kasuga-shi, Fukuoka, 816-8580, Japan
a r t i c l e
i n f o
Article history: Received 10 December 2010 Accepted 29 December 2010 Available online 3 January 2011 Keywords: ZnO RuO2 sensing electrode DO sensor Nano-materials
a b s t r a c t Thick-film 20 mol% ZnO-doped RuO2 sensing electrodes (SEs) were fabricated by screen-printing technique on the platinised alumina substrate of the planar electrochemical dissolved oxygen (DO) sensor. The effect of ZnO doping on morphology, electrochemical properties and sensing characteristics of the sensor was investigated. It was found that ZnO doping has not only improved the SE structure, but has also enhanced 3− , Ca2+, PO3− selectivity of the DO sensor. Selectivity testing exhibited that the presence of Cl−, Li+, SO2− 4 , NO 4 , Mg2+, Na+ and K+ with a concentration range of 10−7 to 10−1 mol/L in the solution had practically no effect on the sensor's emf. The reason in enhancement of the sensor characteristics could be related to the establishment of the better structured SE as more advanced crystallization is achieved for the doped RuO2-SE. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Nanostructured RuO2 is an electronically conductive semiconductor oxide with rutile structure. Due to its high catalytic activity it has been considered as one of the promising materials for a variety of applications such as super-capacitors, bio-sensors and potentiometric pH and DO sensors [1,2]. Doping of RuO2 by other nanostructured noble metals or nano-oxides has often being very effective method of improvement not only the structure of RuO2-SE, but also its electrochemical properties [3–5]. Moreover, high surface-to-volume ratio of nano-particles and their associated high surface activities can vary the kinetics of redox reactions in water for the doped-SEs. Therefore, from the embedded chemical sensor's development point of view, doped thin- and thick-film RuO2-SEs possess the electrode structure with increased adsorption capability toward DO (pH) sensing. However, some noble metals, such as Pt, Au and Rh, are too expensive to be used on an industrial scale. Consequently, research of nano-oxide-doped RuO2-SE has a significant practical value. Our previous attempts in the development of Cu2O-doped RuO2-SE have shown that it is possible to improve both the sensor's selectivity and its antifouling resistance [6,7]. Since ZnO is also a well-known oxide with reported antifouling capabilities [8], it is essential to investigate its influence on the morphology and electrochemical properties of RuO2-SE. To the best of our knowledge, no study has been dedicated so far to the analysis of such influence relevant to the electrochemical water quality sensors.
⁎ Corresponding author. Tel.: + 61 3 9252 6236; fax: +61 3 9252 6246. E-mail address: [email protected] (S. Zhuiykov). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.12.060
Thus, this research represents the first study towards the better understanding of the influence of ZnO doping on the morphology and electrochemical properties of sub-micron RuO2-SE of the potentiometric DO sensor. 2. Experimental 20 mol% ZnO-doped RuO2-SEs were fabricated from the ZnO and RuO2 nano-particles of high-purity analytical grade which had a particle size range of ca. 50 and 360 nm, respectively. In brief, Pt current conductors of ca. 5 μm thickness were applied onto each alumina sensor substrate and sintered at 1000 °C for 1 h in air prior to RuO2 and ZnO nano-particles deposition. This was followed by the screen-printing of thick-film SEs, which were obtained by mixing ZnO and RuO2 nano-powders with organic α-terpineol (C10H18O, 99.9%) suspension as binder [6]. All fabricated sensors attached with 20 mol% ZnO-doped RuO2-SEs were subsequently sintered at 800 °C. The surface morphology of the ZnO-doped RuO2-SE was characterized by a JEOL JSM-6340F field emission scanning electron microscope (FE-SEM). X-ray photoelectron spectroscopy (XPS) measurements (XPS; AXIS-165, Shimidzu/Kratos, Japan) were performed using a monochromatic aluminium X-ray source (1386.6 eV) operating at 15 kV and 7 mA under ultra-high vacuum (10−5 Pa). The carbon peak at 284.6 eV was used as a reference to estimate the electrical charge effect. X-ray diffraction (XRD) analyses were carried out using a Bruker D8 Advance X-Ray Diffractometer with CuKα (with wavelengths Kα1λ = 1.5406 Å, Kα2λ = 1.544439 Å and a Kα2 ratio of 0.5) radiation operating at 40 kV, 40 mA and monochromatised with a graphite monochromator. Cyclic voltammetry (CV) was performed at a scan rate of 100 mV s−1 in a KH2PO4–Na2HPO4 solution to observe
992
S. Zhuiykov et al. / Materials Letters 65 (2011) 991–994
the electrochemical characteristics of ZnO-doped RuO2-SE. The electrochemical measurements of complex impedance were performed using AUTOLAB analyser, PGSTAT, The Netherlands. Impedance spectra were collected in the frequency range of 1 Hz to 1 MHz at amplitude 5 mV at different pHs. Dissolved salt solutions including KCl, KBr, Li2SO4, Na2SO4, Mg(NO3)2, Ca(NO3)2 and Na2HPO4 were used to determine the electrochemical characteristics of the SEs including their cross-sensitivity, selectivity limits of detection and working concentration span. Concentrations of 10−7 to 10−2 mol/L in aqueous solutions of these salts were prepared from 10−1 mol/L stock solutions. A separated external Ag/AgCl, Cl− reference electrode was used for potentiometric measurements. 3. Results and discussion Fig. 1 depicts both SEM images of ZnO-doped RuO2-SE and XRD patterns of this SE deposited on the alumina sensor substrate. As shown in Fig. 1(A), the structure of ZnO-doped RuO2-SE consists of the grains, which were homogeneously distributed in the relatively dense SE. Few pores were also developed. It can be observed that the particles appear to be made up of a combination of tetragonal, cubic and rhombohedral structures sized between 500 and 900 nm, as
presented in Fig. 1(B). The XRD spectra of the ZnO-doped RuO2-SE presented in Fig. 1(C) showing the RuO2 diffraction pattern is consistent with a tetragonal structure with a symmetry or space group P42 and lattice parameters 4.499 × 4.499 × 3.107 Å b90 × 90 × 90N. The hexagonal, P63 phase of ZnO was similarly identified with lattice parameters 3.25 × 3.25 × 5.207 Å b90 × 90 × 120N. In both cases, identification was assisted by the high narrow diffraction peaks, suggesting a high degree of crystallinity of the developed structure. A number of rhombohedral alumina peaks with lattice parameters 4.759 × 4.759 × 12.993 Å b90 × 90 × 120 N are also noted, however, their intensity was somewhat low. XPS analysis of 20 mol% ZnO-doped RuO2-SE is presented in Fig. 2, with (A) showing the characteristic binding energy shape of the core level spectra for Zn 2p3/2, (B) the characteristic binding energy shape of the Ru 3d shell electrons and (C) spectrum of the O 1s spectral region. Due to the fact that the XPS data for Ru 3d encompassed the C 1s peak for adventitious carbon at 284.6 eV, all spectra illustrated in Fig. 2, have been recalibrated by the carbon peak. The survey XPS spectrum of the Zn 2p3/2 spectral region for ZnO revealed the presence of only zinc and oxide without any obvious contaminant species. In the Zn 2p core level XPS spectrum, the peak corresponding to the Zn 2p3/2, is observed at around 1015 eV. The Ru 3d core level spectrum
Fig. 1. SEM images of 20 mol% ZnO-doped RuO2-SE illustrating surface morphology (A), magnified view of the main grains at nano-scale (B) and XRD patterns of SE deposited on Al2O3 sensor substrate (C).
S. Zhuiykov et al. / Materials Letters 65 (2011) 991–994
6.00E-05
31200 31000
993
A
RuIII
RuIV
ZnO-RuO2
Zn 2p3/2
4.00E-05
30800 2.00E-05
Current (A)
30600
Counts /s
30400 30200
0.00E+00
-2.00E-05
30000 29800
-4.00E-05 29600 -6.00E-05 -1.5
29400
-1
-0.5
0
0.5
1
1.5
Voltage (V)
29200 29000 1020 1019 1018 1017 1016 1015 1014 1013 1012 1011 1010
Fig. 3. Cyclic voltammetry at scan rate of 100 mV/s for 20 mol% ZnO-doped RuO2-SE in KH2PO4–Na2HPO4 solution at a temperature of 25 °C.
Binding energy / eV 8500
6000
previously published value of 4.1 eV for un-doped RuO2 [9] and 2.9 eV for the Cu2O-doped RuO2 [6]. The O 1s region, as shown in Fig. 2(C), is characterized by two bands: the metal-oxygen (M–O) at 533.1 eV from the metal oxide and at 529.8 eV, which is attributed to hydroxide ions (OH−) and water (H2O), adsorbed to the RuO2 surface. This is consistent with the partial oxidation of Ru(IV) to Ru(III) in the SE discussed elsewhere [7,10]. Considering the changes that the Ru/RuO2 sustained within the ZnO-doped SE, it may be reasonable to suggest that the reaction involved the transfer of two electrons in the following way [10]:
5500
2RuO2 þ 2Hag þ 2e ↔ Ru2 O3 þ H2 O:
5000
Typical CV for ZnO-doped RuO2-SE in KH2PO4–Na2HPO4 solution at a temperature of 25 °C is presented in Fig. 3. The curve had a monotonic behaviour and did not show any plateau typical of a phase change. Two cathodic and two anodic peaks were observed within the measuring potential range. On the positive potential range only surface reactions associated with the redox pairs RuIII↔RuIV and corresponding H+ adsorption occur [1]. The Nyquist plot in a buffer solution (pH 7.0) for the 20 mol% ZnOdoped RuO2-SE is presented in Fig. 4. As expected, the influence of
8000
Ru 3d3/2
B
Ru 3d5/2
7500
Counts / s
7000 6500
þ
4500 4000 292
290
288
286
284
282
Binding energy / eV 17000
O2-
O 1s
C
15000
2000
14000
1800
13000
1600 1400
12000
-Z" / Ohm
Counts / s
16000
−
11000
H2
O/OH-
10000
1200
Rgb
Rbulk
1000 800
9000 600 8000 540
538
536
534
532
530
528
526
Binding energy / eV Fig. 2. XPS core level photoelectron peaks for Zn 2p (A), Ru 3d (B) and O 1s (C).
was characterized by the pair of narrow peaks corresponding to the 5/2 and 3/2 spin–orbit components at 286.8 and 285.8 eV, respectively. The observed 1.0 eV separation in this case is different to the
ZnO-RuO2-SE
400
pH = 7.0
200
T = 25°C
0 2000
4000
6000
8000
10000
12000
14000
Z' / Ohm Fig. 4. The Nyquist plot for 20 mol % ZnO–RuO2-SE in buffer solution at a temperature of 25 °C.
994
S. Zhuiykov et al. / Materials Letters 65 (2011) 991–994
Sensor Response Delta EMP (mV)
10
sensitivity to KBr. Only one Br− ion exhibited cross-sensitivity to 20 mol% ZnO-doped RuO2-SE in the high concentration range of 10−2 to 10−1 mol/L, whereas at low concentration range of 10−7 to 10−2 mol/L no effect on the sensor's emf is observed. One possible explanation of this cross-sensitivity is the likelihood that bromide is enhanced in concentration at the surface of SE at the heterogeneous oxidation [12]. This cross-sensitivity must be taken into consideration when the industrial sensor prototype will be manufacturing.
0
-10
-20
-30
Li2SO4 Na2SO4 KBr KCl Mg(NO3)2 Na2HPO4 Ca(NO3)2
4. Conclusions
-40
-50
-60 0.0000001 0.000001 0.00001
0.0001
0.001
0.01
0.1
1
Concentration (Mol/l) Fig. 5. Selectivity of the DO sensor based on sub-micron 20 mol% ZnO-doped RuO2-SE at a temperature range of 15–30 °C.
the grain boundary resistance (Rgb) dominated the changes. The bulk contribution (Rbulk) to the SE conductivity is seen to be minimal, whereas the major contributing factor appears to be processes occurring on the grain boundaries of SE. Noteworthy that the presented semi-arcs for the developed ZnO-doped RuO2-SE are very different to the Nyquist plot recently presented for the pH sensor based on nanostructured RuO2 attached to the carbon nano-tubes-SE [2]. The anomalies can be explained as a result of both RuO2-SE porosity and the microstructural changes taking place in the relatively dense ZnO–RuO2-SE. Partial transfer Ru(IV) to the Ru(III) occurred mostly on the grain boundaries of SE [11] has increased the capacitance of Rgb. However, the presence of the “inner” active surfaces owing the porosity of the developed ZnO–RuO2-SE structures has also contributed to the increase of Rbulk capacitance. The investigation of interfering effects of various dissolved ions on the sensor measuring potential in aqueous solutions has been performed using the method of fixed interference [6]. Fig. 5 depicts Δemf of the DO sensor based on 20 mol% ZnO-doped RuO2-SE vs various ion concentrations at a temperature range of 15–30 °C. The results obtained indicate that the DO sensor based on ZnO–RuO2-SE is insensitive to the most of common ions, with exception of the
Influence of ZnO doping on the morphology and electrochemical properties of the RuO2-SE has been investigated throughout this research. It was found that ZnO doping can not only improve the RuO2 structure, but also capable to enhance the selectivity of DO sensor based on ZnO-doped RuO2-SE. The reason in enhancement of the sensor characteristics could be related to the establishment of the better structured SE as more advanced crystallization is achieved for the doped RuO2-SE. It is therefore evident that for the planar design of the thick-film solid-state DO sensor, the doping of the RuO2-SE is also an important parameter that must be considered in order to optimise the sensor's performance. Acknowledgement The work was supported by the CSIRO Sensors and Sensor Networks Transformational Capability Platform and CSIRO Materials Science and Engineering Division — strategic co-investment SIP5 project “Advanced Sensors”. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
Zhuiykov S. Electrochem Commun 2008;10:839–43. Xu B, Zhang WD. Electrochim Acta 2010;55:2859–62. Zhuiykov S, Marney D, Kats E, Int. J. Appl. Ceram. Techn. in press. Macounova K, Makarova M, Jirkovsky J, Frans J, Kritl P. Electrochim Acta 2008;53: 6126–34. Zhuiykov S. Sens Actuators B 2009;136:248–56. Zhuiykov S, Kats E, Marney D. Talanta 2010;10:502–7. Zhuiykov S, Kats E, Marney D, Kalantar-zadeh K. Prog Org Coat 2011;70:67–73. Sorensen PA, Kiil S, Dam-Johansen K, Weinell CE. J Coat Technol Res 2009;6: 135–76. Rochefort D, Dabo P, Guay D, Sherwood PMA. Electrochim Acta 2003;48:4245–52. Zhuiykov S. Ionics 2009;15:693–701. Kurzweil P. J Power Sources 2010;190:189–95. George IJ, Abbott JPD. Nature: Chem 2010;2:713–7.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Effect of ZnO doping on morphology and electrochemical properties of sub-micron RuO2 sensing electrode of DO sensor Serge Zhuiykov a,⁎, Vladimir Plashnitsa b, Norio Miura c a b c
CSIRO, Materials Science and Engineering Division, 37 Graham Road, Highett, VIC. 3190, Australia Research and Education Centre of Carbon Resources, Kyushu University, Kasuga-shi, Fukuoka, 816-8580, Japan KASTEC, Kyushu University, Kasuga-shi, Fukuoka, 816-8580, Japan
a r t i c l e
i n f o
Article history: Received 10 December 2010 Accepted 29 December 2010 Available online 3 January 2011 Keywords: ZnO RuO2 sensing electrode DO sensor Nano-materials
a b s t r a c t Thick-film 20 mol% ZnO-doped RuO2 sensing electrodes (SEs) were fabricated by screen-printing technique on the platinised alumina substrate of the planar electrochemical dissolved oxygen (DO) sensor. The effect of ZnO doping on morphology, electrochemical properties and sensing characteristics of the sensor was investigated. It was found that ZnO doping has not only improved the SE structure, but has also enhanced 3− , Ca2+, PO3− selectivity of the DO sensor. Selectivity testing exhibited that the presence of Cl−, Li+, SO2− 4 , NO 4 , Mg2+, Na+ and K+ with a concentration range of 10−7 to 10−1 mol/L in the solution had practically no effect on the sensor's emf. The reason in enhancement of the sensor characteristics could be related to the establishment of the better structured SE as more advanced crystallization is achieved for the doped RuO2-SE. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Nanostructured RuO2 is an electronically conductive semiconductor oxide with rutile structure. Due to its high catalytic activity it has been considered as one of the promising materials for a variety of applications such as super-capacitors, bio-sensors and potentiometric pH and DO sensors [1,2]. Doping of RuO2 by other nanostructured noble metals or nano-oxides has often being very effective method of improvement not only the structure of RuO2-SE, but also its electrochemical properties [3–5]. Moreover, high surface-to-volume ratio of nano-particles and their associated high surface activities can vary the kinetics of redox reactions in water for the doped-SEs. Therefore, from the embedded chemical sensor's development point of view, doped thin- and thick-film RuO2-SEs possess the electrode structure with increased adsorption capability toward DO (pH) sensing. However, some noble metals, such as Pt, Au and Rh, are too expensive to be used on an industrial scale. Consequently, research of nano-oxide-doped RuO2-SE has a significant practical value. Our previous attempts in the development of Cu2O-doped RuO2-SE have shown that it is possible to improve both the sensor's selectivity and its antifouling resistance [6,7]. Since ZnO is also a well-known oxide with reported antifouling capabilities [8], it is essential to investigate its influence on the morphology and electrochemical properties of RuO2-SE. To the best of our knowledge, no study has been dedicated so far to the analysis of such influence relevant to the electrochemical water quality sensors.
⁎ Corresponding author. Tel.: + 61 3 9252 6236; fax: +61 3 9252 6246. E-mail address: [email protected] (S. Zhuiykov). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.12.060
Thus, this research represents the first study towards the better understanding of the influence of ZnO doping on the morphology and electrochemical properties of sub-micron RuO2-SE of the potentiometric DO sensor. 2. Experimental 20 mol% ZnO-doped RuO2-SEs were fabricated from the ZnO and RuO2 nano-particles of high-purity analytical grade which had a particle size range of ca. 50 and 360 nm, respectively. In brief, Pt current conductors of ca. 5 μm thickness were applied onto each alumina sensor substrate and sintered at 1000 °C for 1 h in air prior to RuO2 and ZnO nano-particles deposition. This was followed by the screen-printing of thick-film SEs, which were obtained by mixing ZnO and RuO2 nano-powders with organic α-terpineol (C10H18O, 99.9%) suspension as binder [6]. All fabricated sensors attached with 20 mol% ZnO-doped RuO2-SEs were subsequently sintered at 800 °C. The surface morphology of the ZnO-doped RuO2-SE was characterized by a JEOL JSM-6340F field emission scanning electron microscope (FE-SEM). X-ray photoelectron spectroscopy (XPS) measurements (XPS; AXIS-165, Shimidzu/Kratos, Japan) were performed using a monochromatic aluminium X-ray source (1386.6 eV) operating at 15 kV and 7 mA under ultra-high vacuum (10−5 Pa). The carbon peak at 284.6 eV was used as a reference to estimate the electrical charge effect. X-ray diffraction (XRD) analyses were carried out using a Bruker D8 Advance X-Ray Diffractometer with CuKα (with wavelengths Kα1λ = 1.5406 Å, Kα2λ = 1.544439 Å and a Kα2 ratio of 0.5) radiation operating at 40 kV, 40 mA and monochromatised with a graphite monochromator. Cyclic voltammetry (CV) was performed at a scan rate of 100 mV s−1 in a KH2PO4–Na2HPO4 solution to observe
992
S. Zhuiykov et al. / Materials Letters 65 (2011) 991–994
the electrochemical characteristics of ZnO-doped RuO2-SE. The electrochemical measurements of complex impedance were performed using AUTOLAB analyser, PGSTAT, The Netherlands. Impedance spectra were collected in the frequency range of 1 Hz to 1 MHz at amplitude 5 mV at different pHs. Dissolved salt solutions including KCl, KBr, Li2SO4, Na2SO4, Mg(NO3)2, Ca(NO3)2 and Na2HPO4 were used to determine the electrochemical characteristics of the SEs including their cross-sensitivity, selectivity limits of detection and working concentration span. Concentrations of 10−7 to 10−2 mol/L in aqueous solutions of these salts were prepared from 10−1 mol/L stock solutions. A separated external Ag/AgCl, Cl− reference electrode was used for potentiometric measurements. 3. Results and discussion Fig. 1 depicts both SEM images of ZnO-doped RuO2-SE and XRD patterns of this SE deposited on the alumina sensor substrate. As shown in Fig. 1(A), the structure of ZnO-doped RuO2-SE consists of the grains, which were homogeneously distributed in the relatively dense SE. Few pores were also developed. It can be observed that the particles appear to be made up of a combination of tetragonal, cubic and rhombohedral structures sized between 500 and 900 nm, as
presented in Fig. 1(B). The XRD spectra of the ZnO-doped RuO2-SE presented in Fig. 1(C) showing the RuO2 diffraction pattern is consistent with a tetragonal structure with a symmetry or space group P42 and lattice parameters 4.499 × 4.499 × 3.107 Å b90 × 90 × 90N. The hexagonal, P63 phase of ZnO was similarly identified with lattice parameters 3.25 × 3.25 × 5.207 Å b90 × 90 × 120N. In both cases, identification was assisted by the high narrow diffraction peaks, suggesting a high degree of crystallinity of the developed structure. A number of rhombohedral alumina peaks with lattice parameters 4.759 × 4.759 × 12.993 Å b90 × 90 × 120 N are also noted, however, their intensity was somewhat low. XPS analysis of 20 mol% ZnO-doped RuO2-SE is presented in Fig. 2, with (A) showing the characteristic binding energy shape of the core level spectra for Zn 2p3/2, (B) the characteristic binding energy shape of the Ru 3d shell electrons and (C) spectrum of the O 1s spectral region. Due to the fact that the XPS data for Ru 3d encompassed the C 1s peak for adventitious carbon at 284.6 eV, all spectra illustrated in Fig. 2, have been recalibrated by the carbon peak. The survey XPS spectrum of the Zn 2p3/2 spectral region for ZnO revealed the presence of only zinc and oxide without any obvious contaminant species. In the Zn 2p core level XPS spectrum, the peak corresponding to the Zn 2p3/2, is observed at around 1015 eV. The Ru 3d core level spectrum
Fig. 1. SEM images of 20 mol% ZnO-doped RuO2-SE illustrating surface morphology (A), magnified view of the main grains at nano-scale (B) and XRD patterns of SE deposited on Al2O3 sensor substrate (C).
S. Zhuiykov et al. / Materials Letters 65 (2011) 991–994
6.00E-05
31200 31000
993
A
RuIII
RuIV
ZnO-RuO2
Zn 2p3/2
4.00E-05
30800 2.00E-05
Current (A)
30600
Counts /s
30400 30200
0.00E+00
-2.00E-05
30000 29800
-4.00E-05 29600 -6.00E-05 -1.5
29400
-1
-0.5
0
0.5
1
1.5
Voltage (V)
29200 29000 1020 1019 1018 1017 1016 1015 1014 1013 1012 1011 1010
Fig. 3. Cyclic voltammetry at scan rate of 100 mV/s for 20 mol% ZnO-doped RuO2-SE in KH2PO4–Na2HPO4 solution at a temperature of 25 °C.
Binding energy / eV 8500
6000
previously published value of 4.1 eV for un-doped RuO2 [9] and 2.9 eV for the Cu2O-doped RuO2 [6]. The O 1s region, as shown in Fig. 2(C), is characterized by two bands: the metal-oxygen (M–O) at 533.1 eV from the metal oxide and at 529.8 eV, which is attributed to hydroxide ions (OH−) and water (H2O), adsorbed to the RuO2 surface. This is consistent with the partial oxidation of Ru(IV) to Ru(III) in the SE discussed elsewhere [7,10]. Considering the changes that the Ru/RuO2 sustained within the ZnO-doped SE, it may be reasonable to suggest that the reaction involved the transfer of two electrons in the following way [10]:
5500
2RuO2 þ 2Hag þ 2e ↔ Ru2 O3 þ H2 O:
5000
Typical CV for ZnO-doped RuO2-SE in KH2PO4–Na2HPO4 solution at a temperature of 25 °C is presented in Fig. 3. The curve had a monotonic behaviour and did not show any plateau typical of a phase change. Two cathodic and two anodic peaks were observed within the measuring potential range. On the positive potential range only surface reactions associated with the redox pairs RuIII↔RuIV and corresponding H+ adsorption occur [1]. The Nyquist plot in a buffer solution (pH 7.0) for the 20 mol% ZnOdoped RuO2-SE is presented in Fig. 4. As expected, the influence of
8000
Ru 3d3/2
B
Ru 3d5/2
7500
Counts / s
7000 6500
þ
4500 4000 292
290
288
286
284
282
Binding energy / eV 17000
O2-
O 1s
C
15000
2000
14000
1800
13000
1600 1400
12000
-Z" / Ohm
Counts / s
16000
−
11000
H2
O/OH-
10000
1200
Rgb
Rbulk
1000 800
9000 600 8000 540
538
536
534
532
530
528
526
Binding energy / eV Fig. 2. XPS core level photoelectron peaks for Zn 2p (A), Ru 3d (B) and O 1s (C).
was characterized by the pair of narrow peaks corresponding to the 5/2 and 3/2 spin–orbit components at 286.8 and 285.8 eV, respectively. The observed 1.0 eV separation in this case is different to the
ZnO-RuO2-SE
400
pH = 7.0
200
T = 25°C
0 2000
4000
6000
8000
10000
12000
14000
Z' / Ohm Fig. 4. The Nyquist plot for 20 mol % ZnO–RuO2-SE in buffer solution at a temperature of 25 °C.
994
S. Zhuiykov et al. / Materials Letters 65 (2011) 991–994
Sensor Response Delta EMP (mV)
10
sensitivity to KBr. Only one Br− ion exhibited cross-sensitivity to 20 mol% ZnO-doped RuO2-SE in the high concentration range of 10−2 to 10−1 mol/L, whereas at low concentration range of 10−7 to 10−2 mol/L no effect on the sensor's emf is observed. One possible explanation of this cross-sensitivity is the likelihood that bromide is enhanced in concentration at the surface of SE at the heterogeneous oxidation [12]. This cross-sensitivity must be taken into consideration when the industrial sensor prototype will be manufacturing.
0
-10
-20
-30
Li2SO4 Na2SO4 KBr KCl Mg(NO3)2 Na2HPO4 Ca(NO3)2
4. Conclusions
-40
-50
-60 0.0000001 0.000001 0.00001
0.0001
0.001
0.01
0.1
1
Concentration (Mol/l) Fig. 5. Selectivity of the DO sensor based on sub-micron 20 mol% ZnO-doped RuO2-SE at a temperature range of 15–30 °C.
the grain boundary resistance (Rgb) dominated the changes. The bulk contribution (Rbulk) to the SE conductivity is seen to be minimal, whereas the major contributing factor appears to be processes occurring on the grain boundaries of SE. Noteworthy that the presented semi-arcs for the developed ZnO-doped RuO2-SE are very different to the Nyquist plot recently presented for the pH sensor based on nanostructured RuO2 attached to the carbon nano-tubes-SE [2]. The anomalies can be explained as a result of both RuO2-SE porosity and the microstructural changes taking place in the relatively dense ZnO–RuO2-SE. Partial transfer Ru(IV) to the Ru(III) occurred mostly on the grain boundaries of SE [11] has increased the capacitance of Rgb. However, the presence of the “inner” active surfaces owing the porosity of the developed ZnO–RuO2-SE structures has also contributed to the increase of Rbulk capacitance. The investigation of interfering effects of various dissolved ions on the sensor measuring potential in aqueous solutions has been performed using the method of fixed interference [6]. Fig. 5 depicts Δemf of the DO sensor based on 20 mol% ZnO-doped RuO2-SE vs various ion concentrations at a temperature range of 15–30 °C. The results obtained indicate that the DO sensor based on ZnO–RuO2-SE is insensitive to the most of common ions, with exception of the
Influence of ZnO doping on the morphology and electrochemical properties of the RuO2-SE has been investigated throughout this research. It was found that ZnO doping can not only improve the RuO2 structure, but also capable to enhance the selectivity of DO sensor based on ZnO-doped RuO2-SE. The reason in enhancement of the sensor characteristics could be related to the establishment of the better structured SE as more advanced crystallization is achieved for the doped RuO2-SE. It is therefore evident that for the planar design of the thick-film solid-state DO sensor, the doping of the RuO2-SE is also an important parameter that must be considered in order to optimise the sensor's performance. Acknowledgement The work was supported by the CSIRO Sensors and Sensor Networks Transformational Capability Platform and CSIRO Materials Science and Engineering Division — strategic co-investment SIP5 project “Advanced Sensors”. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
Zhuiykov S. Electrochem Commun 2008;10:839–43. Xu B, Zhang WD. Electrochim Acta 2010;55:2859–62. Zhuiykov S, Marney D, Kats E, Int. J. Appl. Ceram. Techn. in press. Macounova K, Makarova M, Jirkovsky J, Frans J, Kritl P. Electrochim Acta 2008;53: 6126–34. Zhuiykov S. Sens Actuators B 2009;136:248–56. Zhuiykov S, Kats E, Marney D. Talanta 2010;10:502–7. Zhuiykov S, Kats E, Marney D, Kalantar-zadeh K. Prog Org Coat 2011;70:67–73. Sorensen PA, Kiil S, Dam-Johansen K, Weinell CE. J Coat Technol Res 2009;6: 135–76. Rochefort D, Dabo P, Guay D, Sherwood PMA. Electrochim Acta 2003;48:4245–52. Zhuiykov S. Ionics 2009;15:693–701. Kurzweil P. J Power Sources 2010;190:189–95. George IJ, Abbott JPD. Nature: Chem 2010;2:713–7.