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Potentiometric RuO2–Ta2O5 pH sensors fabricated using thick film and LTCC technologies
Author’s Accepted Manuscript Potentiometric RuO2-Ta2O5 pH Sensors fabricated using thick film and LTCC Technologies Libu Manjakkal, Krzysztof Zaraska, Katarina Cvejin, Jan Kulawik, Dorota Szwagierczak www.elsevier.com
PII: DOI: Reference:
S0039-9140(15)30359-3 http://dx.doi.org/10.1016/j.talanta.2015.09.069 TAL16002
To appear in: Talanta Received date: 20 July 2015 Revised date: 24 September 2015 Accepted date: 27 September 2015 Cite this article as: Libu Manjakkal, Krzysztof Zaraska, Katarina Cvejin, Jan Kulawik and Dorota Szwagierczak, Potentiometric RuO2-Ta2O5 pH Sensors fabricated using thick film and LTCC Technologies, Talanta, http://dx.doi.org/10.1016/j.talanta.2015.09.069 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Potentiometric RuO2-Ta2O5 pH Sensors Fabricated Using Thick Film and LTCC Technologies
Libu Manjakkal, Krzysztof Zaraska, Katarina Cvejin, Jan Kulawik, Dorota Szwagierczak* Institute of Electron Technology, Krakow Division, 30-701 Kraków, Zabłocie 39, Poland *Corresponding author: [email protected]
Abstract The paper reports on the preparation, properties and application of potentiometric pH sensors with thick film RuO2-Ta2O5 sensing electrode and Ag/AgCl/KCl reference electrode screen printed on an alumina substrate. Furthermore, it presents fabrication procedure and characterization of a new miniaturized pH sensor on LTCC (low temperature cofired ceramics) substrate, destined for wireless monitoring. The crystal structure, phase and elemental composition, and microstructure of the films were investigated by X-ray diffractometry, Raman spectroscopy, scanning electron microscopy and energy dispersive spectroscopy. Potentiometric characterization was performed in a wide pH range of 2-12 for different storage conditions and pH loops. The advantages of the proposed thick film pH sensors are: (a) low cost and easy fabrication, (b) excellent sensitivity close to the Nernstian response (56 mV/pH) in the wide pH range, (c) fast response, (d) long lifetime, (e) good reproducibility, (f) low hysteresis and drift effects, and (g) low cross-sensitivity towards Li+, Na+ and K+ as interfering ions. The applicability of the sensors for pH measurement of river, tap and distilled water, and some drinks was also tested. KEYWORDS: pH sensor, thick film, LTCC, RuO2-Ta2O5, potentiometric analysis, hysteresis effect 1
1. Introduction Miniaturized, highly sensitive and accurate electrochemical pH sensors which contain both sensitive and reference electrodes in a single platform have become of great importance due to their remarkable predicted application scope in the next generation of the electrochemical sensors. For the fabrication of miniaturized analytical devices, LTCC (low temperature cofired ceramics) technology is offering numerous advantages, including integration of sensing layer and electronic circuits for the wireless sensor part in one module [1, 2]. LTCC technology enables development of the complex three dimensional structure of a device. The electrochemical solid state pH sensors made in microsystem technology are especially useful for the application in wireless sensor networks. Such sensors have been already successfully used in water pollution monitoring and remote sensing applications [3]. Furthermore, solid state pH sensors have been recognized for several decades for bio-medicalchemical applications, food processing, agricultural and industrial applications, etc. [4, 5, 6]. RuO2 is a versatile and attractive solid material for developing electrochemical sensors, biosensors and supercapacitors [3, 7-10]. RuO2 based pH sensors have been found to exhibit outstanding sensing performance over wide pH ranges. Their significant advantages over the traditional glass pH electrode and other metal oxide based pH electrodes include high sensitivity and stability, long lifetime, very fast response, small hysteresis effect, low sensitivity to interferences caused by different ions and very good repeatability [3, 8, 11]. In our previous papers, we reported a thick film potentiometric pH sensor based on RuO2 with a Nernstian response close to the theoretical value [12] and impedance spectroscopic analysis of RuO2 based pH conductimetric sensor [13]. However, since RuO2 is an expensive component, for reducing the cost, as well as to enhance the stability and sensitivity, RuO2 as the pH sensing electrode material can be mixed with other oxides [11, 14, 15]. Recently, there are
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several studies reporting on the development of low cost pH sensors by using binary metal oxides. The advantage of such sensors is low cost, stability, long lifetime and good electrochemical activity. For the sensing electrodes, the RuO2- TiO2 system is the most frequently used one [11, 15, 16]. As a pH sensitive material, Ta2O5 has received significant attention for the fabrication of thin film pH sensors, especially of ISFET (Ion Sensitive Field Effect Transistor) type [17], and EIOS (electrolyte-ion sensitive membrane-oxide-semiconductor) type [18]. In our previous work, we developed a thick film Ta2O5 based pH sensor which exhibited a subNernstian potentiometric response [19]. So far, major studies devoted to RuO2-Ta2O5 system were focused on supercapacitors [20] and DSA-type electrodes (Dimensionally Stable Anodes) [21]. It was found that the introduction of Ta2O5 which is a proton conducting solid electrolyte enhances the rate of proton intercalation at the surface of RuO2 grains [20]. DSA electrodes are prepared in the form of thin film coatings on Ti-substrate and employed as electrochemical devices for oxidation of organic compounds, in chlorine and oxygen production [21, 22]. For this application, good electrochemical properties, high corrosion resistance, and long lifetime of RuO2-Ta2O5 thin films on Ti substrate were reported by Ribeiro et al. [22, 23]. This article presents the studies of thick film potentiometric pH sensors based on RuO2-Ta2O5 binary composition. The nanostructured and porous morphology of the applied thick film RuO2-Ta2O5 electrode provides a high surface to volume ratio and hence large number of adsorption centers on the surface of the sensitive film. This improves sensing performance, contributing to high sensitivity, fast response and low power consumption [24, 25]. The structural properties and composition of the films were characterized by using X-ray diffractometry (XRD), Raman spectroscopy, scanning electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDX). In this work, the use of the binary RuO2-Ta2O5
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composition and screen printing method allowed for fabrication of a thick film pH sensor characterized by low cost, ease of miniaturization and excellent sensing performance. Furthermore, a potentiometric sensor for wireless application was fabricated using LTCC technology and its sensing performance was discussed. The inbuilt reference and sensitive electrodes and the possibility of integration of passive components for the electrical circuit of the wireless system are significant advantages of the LTCC pH sensor. To the best of our knowledge, this article reports for the first time the application of RuO2-Ta2O5 as the sensing material in potentiometric thick film pH sensors.
2. Experimental Two binary RuO2-Ta2O5 mixtures were prepared by ball milling. An active oxide RuO2 (99.9%, Aldrich) was mixed with Ta2O5 (99.8%, Aldrich) in a suitable ratio to obtain RuO2-Ta2O5 (70:30) wt.% and RuO2-Ta2O5 (30:70) wt.% compositions destined for sensitive electrodes of thick film pH sensors. After weighing, the starting materials were wet-ball milled in isopropyl alcohol for 5 h in a planetary ball mill (Fritsch Pulverisette 5, Germany) and dried at 70°C. As a result of ball milling, fine powders were obtained and proper homogeneous mixing of the components was ensured. The prepared RuO2-Ta2O5 based mixtures were used for the preparation of thick film pastes by mixing thoroughly the powders with a suitable amount of ethyl cellulose (binder) and terpineol (solvent) in an agate mortar for 30 min. Thick film potentiometric pH sensors were fabricated by screen printing. The fabrication steps of the potentiometric pH sensors on an alumina substrate (96% Al2O3) were similar to the process that we reported in our previous article [15]. Initially, a silver-palladium (ESL 9695) paste for conducting paths was screen printed on an alumina substrate and sintered at 850°C for 30 min. Then the sensitive RuO2-Ta2O5 layer was printed and fired at
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850°C for 1 h with heating rate of 4°C/min. The schematic representation of the fabricated thick film potentiometric (I) and LTCC based (II) pH sensors is shown in Fig. 1. For the reference electrode (RE) fabrication, sodium hypochlorite solution was used to chloridize a part of the conductive layer and to form AgCl [26]. In order to stabilize AgCl concentration in the reference electrode typically a KCl solution is utilized. However, the thick film containing pure KCl easily dissolves in water. Thus, for increasing the lifetime of RE, a glass-KCl paste was used. The glass based on Bi2O3-SiO2-B2O3-CdO-Li2O3 system was mixed with KCl powder in equal weight proportion and milled for 3 h in isopropyl alcohol using a planetary ball mill. The obtained fine powder was applied for making a paste which was overprinted on AgCl layer and fired at 550°C for 1 h. Finally, an insulative resin was painted on the top of the substrate surface, leaving uncovered the sensing electrode area, points for electrical contact and small opening for hydration port in the glass-KCl layer. The performance of the fabricated thick film reference electrode was cross-checked by using a commercial glass RE (Ag/AgCl/KCl, HYDROMET, Poland). The fabrication procedure of the reference electrode was based on the work reported by Horton et al. [26]. These authors [26] observed perfect functioning of the AgCl layer even after firing of the KCl paste at relatively high temperature of 750°C. In this work, the KCl paste was modified by mixing with a glass of a low temperature melting point. The temperature of 550°C was chosen as optimal for firing of the glass-KCl paste. It was found that after increasing to 600°C the firing temperature of the KCl-glass layer, a minor deterioration of the performance of the reference electrode was observed. A commercial DuPont 951 green tape was used for making LTCC substrates for RuO2-Ta2O5 sensors. The applied LTCC process comprises: cutting of the tape to suitable dimensions, screen printing of conductive and sensitive layers, lamination and co-firing, printing and firing of the reference electrode, painting of the insulative layer and finally
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soldering of leads to provide electrical contact. Fabrication of LTCC pH sensor takes place in six steps illustrated in Fig. 2, described in more detail below: 1.
The green LTCC tape was cut by a laser (Oxford Lasers, UK) into individual sheets with preferred dimensions and shapes.
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Two strips of the conducting layer (Ag, ESL 9916) were screen printed on the top of the green sheet and dried at 120°C for 20 min. Then the sensitive electrode with an area of 3x3 mm2 was printed on the top of the Ag conducting layer and dried. For larger surface area of the sensing electrode, its deformation during cofiring was difficult to avoid.
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The sheet with printed layers and a required number of blank sheets were stacked in a suitable die. The external blank sheet with a proper shape plays the role of electrical insulation which protects conducting paths against the contact with a solution. Isostatic lamination (Pacific Trinetics Corporation, USA) of the stack was carried out under a pressure of 20 MPa at 70°C for 10 min. The next step was cofiring of the ceramic green tapes with the conducting and sensing layers. The proper burnout of the organics was ensured at 450°C and co-firing was carried out at 850°C for 1 h. After co-firing process the sensor became rigid.
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One end of the conducting layer was converted into Ag-AgCl layer. On the top of this AgAgCl layer, glass-KCl layer was overprinted and fired at 550°C for 1 h.
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A polyurethane resin was painted on the top of the glass-KCl layer with a small opening for hydration port. This insulative layer reduces the salt loss from the reference electrode.
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After soldering of electrical leads to the contact pads at the opposite end of the substrate, the sensor was ready for operation. The crystal structure and phase composition studies of the sensitive film were carried
out by X-ray diffraction method (XRD, Philips X’Pert, USA) using Cu-Kα radiation. A Raman microscope (Thermo Fisher DXR, USA) was used for the confirmation of crystal
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structure of the sensitive film. The microscope has a DPSS (Diode Pumped Solid State Laser) laser, with a wavelength 532 nm and maximum power level 10 mW, coupled with 50x magnifying objective. It has a CCD camera as a detector and 900 lines/nm grating. We used 50 µm aperture with estimated spot size 1.1µm. The laser power level of 9 mW was used with an exposure time 30 s. Scanning electron microscopy (FEI Nova Nano, SEM 200, USA) was applied to examine the surface, cross-sectional microstructure and thickness of the sensitive films. X-ray energy dispersive spectroscopy (EDS, EDAX System Genesis Software) method was used for the elemental composition analysis of the sensitive electrode. The ability of the potentiometric RuO2-Ta2O5 electrode to determine pH of test solutions was investigated by measuring the emf (electromotive force) versus the thick film reference electrode. All electrochemical measurements were performed at ambient temperature. The test solutions of pH values 2-12 were utilized for studying the sensing performance of thick film sensors. A commercial calibrated glass pH sensor and a conductivity meter (ELNEIRON-CPC-411) were used to confirm the pH and conductivity value of each test solution. The generated potential difference between thick film sensitive and reference electrodes was measured by using a multimeter (Keithley 2002). The output results were gathered and monitored by a PC which was connected to the multimeter by GPIB interface and analyzed by LabVIEW program.
3. Results and Discussion 3.1. Microstructural Analysis The XRD pattern of RuO2-Ta2O5 (70:30%) thick film sintered at 850°C on an alumina substrate is presented in Fig. 3a. The observed distinct diffraction peaks can be assigned to the rutile structure of RuO2 and to the orthorhombic structure of Ta2O5 [23] and no additional phases were found. The intensities of RuO2 peaks are higher than those of Ta2O5, due to higher amount of RuO2 in the composition. The average crystallite sizes calculated from the 7
major peaks width on the basis of Scherrer equation are in the nanocrystalline range and are almost the same for RuO2 and Ta2O5 (about 10 nm). Small Al2O3 peaks appear also in the pattern due to the effect of an alumina substrate. For RuO2-Ta2O5 film prepared by using the Pechini-Adams method Ribeiro et al. [23] revealed that the intensity of Ta2O5 peak on the XRD pattern was very low in the composition with a high percentage of RuO2 due to amorphous structure of Ta2O5 phase [23]. In this work, both constituent oxides exhibit crystalline nature resulting from another film deposition method and higher sintering temperature. Fig. 3b represents the Raman spectrum of RuO2-Ta2O5 (70:30%) thick film sintered at 850°C. In this spectrum, splitting of the major peaks was found. The peaks are observed in the position of 521, 527, 632, 642, 707 and 711 cm-1. Comparison of the Raman spectrum of RuO2-Ta2O5 film with that of pure RuO2 [27] reveals that the peaks at positions 527, 642 and 711 cm-1 correspond to three major Raman active modes Eg, A1g and B2g of rutile RuO2 with a 14 space group of D4h . Eg is a doubly degenerated mode and A1g a symmetric mode,
respectively [27]. The third mode at 711 cm-1 depends on the Ru-O bond length which is changing due to oxygen atom either moving away or towards the Ru atom. The frequency of this mode is high, and it shows low intensity peak compared to other two modes. The peaks in the Raman spectrum at 521, 632 and 707 cm-1 are attributed to Ta2O5 and are almost similar to those found in our previous study for pure Ta2O5 thick film [19], and reported by other authors for this oxide [28]. The Raman bands corresponding to 450-900 cm-1 are mainly related to the internal stretching vibrations of various Ta-O modes. Furthermore, other peaks for Ta2O5 [19, 28] are not present in the spectrum shown in Fig. 3b, presumably due to the prevailing amount of RuO2 in RuO2-Ta2O5 (70-30%) film. Hence, the obtained Raman spectrum indicates that the film has distinct rutile structure and the obtained Raman active
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modes are similar to those for the well aligned nanocrystalline RuO2 reported by Chen et al. [27]. SEM cross-sectional images of RuO2-Ta2O5 thick film on an alumina substrate before and after treatment in solutions with different pH values are illustrated in Figs. S1 and 4, respectively. The film morphology is almost similar for the freshly prepared samples (Fig. S1) (S=supplementary file) and after reaction with acidic and basic solutions (Fig. 4). The thickness of the screen printed film is nearly 15 µm, as presented in Fig. 4. The EDX analysis performed at the cross-section of the sensor shows the presence of Ru, Ta and oxygen atoms as the constituent elements of the sensing layer. Additional Al and Ag peaks indicate the effect of alumina substrate and conducting layer. Microscopic observations show uniform thicknesses and good cooperation between KCl-glass and AgCl layers. The thickness of the screen printed Ag conducting layer after its partial chloridization to AgCl is nearly 7 µm and that of the glass-KCl layer is 9 µm. Ribeiro et al. [23] obtained by Pechini method RuO2-Ta2O5 thin film without nanoporous structure which showed rough and cracked morphology related to the formation of amorphous Ta2O5 phase surrounding RuO2 crystallites. The RuO2-Ta2O5 thick film prepared in this work is homogeneous, with smooth surface, and nanometric pores and grain sizes ranging from 180 to 520 nm. The observed differences in the crystallinity degree and porosity of the films originate from different deposition methods and much higher temperature of thermal treatment used in this work (850°C) as compared with that applied by Ribeiro et al. [23] (450700°C).
3.2. Potentiometric Analysis of the Sensors The sensitivity of the proposed potentiometric pH sensor at room temperature was validated by dipping the sensor in solutions of different pH values. The tests were conducted by measuring the open circuit potential of the sensor (electromotive force of the cell 9
comprising the sensitive RuO2-Ta2O5 electrode and thick film Ag/AgCl/KCl reference electrode). The measurements were performed initially in acidic solutions and then in alkaline solutions with increasing pH values. Figs. S2 and 5 show the pH dependences of the potential of the sensors with sensing electrodes based on two compositions RuO2-Ta2O5 (30:70%) and RuO2-Ta2O5 (70:30%), respectively. Both plots were found to be linear, although there is a great difference between these two compositions concerning accordance of the experimental results with the theoretical Nernstian response. For RuO2-Ta2O5 (30:70%) sensitive electrode, the slope is 35.3±1 mV/pH in the pH range 2-12 and the standard potential (E0) (intercept of the straight line at pH=0) of the sensor is 594.1±7 mV (Fig. S2). On the other hand, the sensitivity of RuO2-Ta2O5 (70:30%) electrode in the pH range 2-12 is 56.1±0.8 mV/pH and the standard potential is equal to 753.4±5mV (Fig. 5), these values being very close to those derived from the theoretical Nernst relationship. When the metal oxide is exposed to a solution, as suggested by the site-binding theory, negative surface charged groups ( M O ), neutral sites ( M OH ) and positive surface charged groups ( M OH 2 ) are developed [29]. These charged groups at the electrodeelectrolyte interface can create an electrical double layer (edl) structure [29, 30]. H+ or OHions from the solution are attracted to oxygen ions from the metal oxide crystal lattice and to surface cations, respectively, to form surface hydroxyl groups [30, 31]. In acidic solutions, H+ ions are released and the open circuit potential increases with pH changing towards strongly acidic solutions [31]. In basic solutions, the electrode potential decreases with pH rising towards basic values due to the surface metal oxide becoming negatively charged by releasing OH- groups [31]. The formation of metal hydroxyl groups on RuO2 surface was confirmed by the XPS analysis in our previous study [15] concerning RuO2-TiO2 system. The sensitivity obtained for the electrode with a smaller percentage of RuO2 is significantly lower than that for the electrode with a higher amount of RuO2 and the 10
theoretical Nernstian slope factor (59.14 mV/pH). The observed sub-Nernstian response of the RuO2-Ta2O5 (30:70)% electrode may be due to the combination of H+ ion exchange on the surface of the film and charge transfer reaction involving one or two electrons, as suggested for a pH sensor based on pure Ta2O5 [19]. Chen et al. [18] stated that for the EIOS pH sensor with pure Ta2O5 thin film, H+/OH- ions from the aqueous solution react with Ta2O5 surface to form surface hydroxyl groups (Ta-OH) which donate or accept proton from the solution. This reaction is responsible for the Nernstian response. These authors observed a Nernstian pH sensitivity of 56.19 mV/pH in the pH range 1 to 10 [18]. However, in our previous study of Ta2O5 based thick film pH sensor, we observed a sub-Nernstian response [19]. This implies that the preparation method, the sensor construction, surface morphology and crystal structure of the film strongly influence the pH sensing performance of Ta2O5 based pH sensors [17-19]. In addition, the sub-Nernstian response of the RuO2-Ta2O5 (30:70)% electrode may be caused by a too low content of nanostructured RuO2, resulting in lower electrochemically active area of this component [23] which exhibits very good electronic and ionic conductivity. Due to much higher concentration of RuO2 than Ta2O5 in the RuO2-Ta2O5 (70:30)% sensitive layer it can be expected that dissociative adsorption and formation of –OH groups mainly occur on the surface of RuO2 grains and is responsible for the Nernstian response. Pocrifka et al. [11] found that the best sensing performance of the pH sensor based on a binary metal mixture was obtained for the compositions richer in RuO2 than in an inert oxide. When RuO2 surface reacts with an acidic or basic solution, Ru(IV) and Ru(III) oxidation states are observed. This Ru(IV)/Ru(III) reduction potential follows the Nernstian response with the sensitivity of 59 mV/pH at a temperature of 25°C [31]. In the present study, the electromotive force of the sensor with the sensing electrodes with prevailing RuO2 content follows a nearly Nernstian dependence with 56 mV/pH slope at room temperature (Fig. 5). Moreover, the correlation coefficient of this plot being 0.998 reflects a perfect straight line fit and suggests that only one
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electron is involved in the above mentioned redox reaction [32]. The Nernstian response of the metal oxide based pH sensor is related to the processes of adsorption, ion exchange and diffusion of ions from the solution into the porous material structure, accompanied by electron charge transfer. Such mechanism explanation based on the results of potentiometric and impedance spectroscopic studies was proposed in our previous work for pH sensors with the sensing electrodes made of RuO2 commercial pastes [13] and RuO2-TiO2 [15]. A comparison of the sensitivities reported by different authors for pH sensitive electrodes based on RuO2 and RuO2-Ta2O5 binary metal oxide compositions is shown in Table I. The response time of thick film pH sensor with the RuO2-Ta2O5 (70:30%) electrode was determined by measurement of a potential change after dropping of 0.1M HCl or KOH solutions into a test solution. The response time is defined as the time required for a sensor to reach more than 90% of its open circuit potential value in equilibrium [8]. Fig. 6a shows the results of the response time test comprising a cycle of a few abrupt step changes of pH in the range of 2-10. A commercial glass pH sensor was used to confirm the pH value of the solution during the measurement. Compared with the response time 15 s of our previously reported RuO2-TiO2 sensing electrode [15], the response time of RuO2-Ta2O5 based pH sensor is shorter and more dependent on the solution pH. In acidic solutions, the sensor shows a very short response time (less than 8 s), whereas in basic solutions a little longer one (less than 15 s). This indicates that faster response is related to diffusion of H+ ions, dominant in acidic solutions [8]. Moreover, the response time of the metal oxide based pH sensor may also be dependent on the structural properties and morphology of the film [14]. Nanostructured nature and porosity of the fabricated film may improve the response time of the sensor. The response times obtained for our sensor are in close agreement with those reported in literature for other pH sensors (Table I).
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As illustrated in Fig. 6b, the conductivity of the solution at a given pH has very small influence on the sensor potential, especially in the range of 15-2000 µS. However, for the conductivity exceeding 2000 µS there is a drop in potential of 8 mV. The similar dependences were obtained for measurements performed using both the glass and thick film reference electrodes. This confirms the suitability of the fabricated sensor for different applications. Potentiometric investigation shows the emf drift during preliminary measurements. This drift in potential may be due to the influence of the reference electrode [15, 26]. The performance of the fabricated reference electrode after the applied procedure was good, reflected by a small potential difference (7 mV) between the commercial and the fabricated thick film reference electrodes. However, the thick film reference electrode needs a preliminary period (nearly 20 min) to form a conducting channel inside the glass-KCl layer after immersing the electrode in the solution. Similarly, the freshly prepared RuO2-Ta2O5 based sensors show for each pH a drift in potential during initial measurement and their stabilization cannot be attained during a short test period. To avoid this effect, in this work, all meaningful potentiometric measurements data are considered after initial stabilization of the sensor. For checking the stability and drift for every pH, the emf value (Fig. 5) was determined after keeping the sensor near 30 min in each solution. The drift in potential may also be related to the composition, thickness, porosity and homogeneity of the sensitive film [32]. According to Zhuiykov [32], RuO2 based sensing electrode shows a voltage drift during the first month of testing. Presumably, it is due to H+ transport through the sensing layer being governed by H2 trapping at the trap sites existing at grain boundaries or micropores of the nanostructured sensing electrode [32]. After several measurements, the sensor was kept in atmospheric conditions and measured again after 6 and 12 months. Fig. S3 shows the response of the potentiometric sensor after six months. The slope factor of the Nernstian response (56.4±0.7 mV/pH) is
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similar to that determined during the initial period. Furthermore, after one year of measurement the sensor exhibits sensitivity of 54±1 mV/pH. Hence, it was observed that the fabricated thick film pH sensor can be useful for long lifetime applications. We observed that after long term storage there is a decrease in potential value of 30±5 mV for each solution pH. This decrease may result from changes in the composition and morphology of thick film reference electrode. In the initial period of measurement, the potential difference between the glass and thick film reference electrode is ±7 mV, but after storage in distilled water lasting six months this difference grows to 30-50 mV. This may be caused by the salt loss from the glass-KCl layer of RE. Thus, calibration of the sensor is required for long term usage of the fabricated thick film reference electrode. The hysteresis effect (memory effect) of the fabricated thick film pH sensor is illustrated in Figs. 7a, 7b, and 7c for different cycles of pH changes. Hysteresis phenomenon occurs when different output voltages are obtained for a pH electrode measured in the same buffer solution after a series of measurements [15]. Figs. 7a and 7b represent the hysteresis effect for the loops in the acidic (pH 2-4-6-4-2) and the basic (pH 7-9-11-9-7) regions, respectively. The potential deviation was found to be ±3 mV in the acidic region and equal to ±8 mV in the basic region. It was observed that when the pH loop path consists of more steps, as shown in Fig. 7c for pH 3-6-9-12-9-6-3 cycle, the hysteresis width is almost the same as for the shorter loop paths applied in both the acidic and basic regions. Hence, it was found that the hysteresis effect for the fabricated sensor is lower in the acidic pH range than in the basic region and is not dependent on the loop path. The hysteresis effect of the developed sensors was observed after several measurements in different acidic and basic solutions and also for the sensors after long time of storage in atmospheric conditions. Small hysteresis effect (210 mV) was found by Xu et al. [8] for a pH sensor with RuO2/carbon nanotubes electrodes deposited by magnetron sputtering, and by Liao and Chou [33] for RuO2 electrodes obtained
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by rf sputtering. Comparison of the hysteresis width for different RuO2 based pH sensors is shown in Table I. The fabricated thick film RuO2-Ta2O5 pH sensor shows excellent repeatability. Sensors from different lots exhibit almost similar Nernstian slope, with deviations lower than ±1 mV/pH. Selectivity measurements revealed that the presence of Li+, Na+ and K+ ions has no significant effect on the sensitivity of the proposed thick film pH sensors. The influence of these ions on the output emf of the sensor was studied in test solutions containing LiCl, NaCl, KCl salts at a concentration of 0.01 M. These ions have a very small influence on the open circuit potential of the sensor, at a level of ±2 mV. However, in the presence of the interfering cations the slope factor remains the same. The prepared thick film paste based on RuO2-Ta2O5 (70:30 wt.%) was finally utilized as the sensing electrode material for the fabrication of a miniaturized potentiometric pH sensor in LTCC technology (Fig. 2). This sensor is destined for wireless solution monitoring. The potentiometric characteristics of the sensor with the sensitive electrode cofired with LTCC substrate were almost the same as the discussed earlier properties of the thick film sensor screen printed on an alumina substrate. The sensitivity of the LTCC sensor equal to 55.2±0.4 mV/pH (Fig. 8) is close to the theoretical value calculated from the Nernst relationship. The sensing LTCC part of the sensor can be combined with a circuit for the wireless part. For the evaluation of applicability of the fabricated potentiometric RuO2-Ta2O5 sensor in real conditions, it was used for measuring pH of tap water, river water, distilled water and lemon juice. The measured pH values were compared with those obtained by a commercial glass pH electrode. For example, the emf value of the sensor measured after its dipping for 30 min in river water was 310 mV. This value was substituted to the Nernstian relationship
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with the sensitivity of 56 mV/pH and the standard potential of 753 mV, previously determined while testing the sensor in buffer solutions (Fig. 5). The pH value of river water measured using the glass electrode and thick film pH sensor was 8.08 and 7.97, respectively. Similarly for other tested samples (water and juice), almost the same pH values were obtained by means of both sensors. The comparison shows that our thick film pH sensor achieves similar performance as the commercial glass pH sensor, yet with additional advantages of the smaller size, better robustness, lower cost and good applicability for online monitoring.
4. Conclusions In this work, new RuO2-Ta2O5 based thick film pastes were prepared and applied for pH sensing electrodes. Potentiometric pH sensors were fabricated on alumina substrates by thick film technology. Furthermore, LTCC technology was successfully utilized for development of a miniaturized potentiometric pH sensor for wireless solution monitoring. The XRD pattern reveals the binary phase composition of the film with nanometric crystal sizes of RuO2 and Ta2O5. Raman studies confirm the presence of RuO2 and Ta2O5 crystalline phases with prevailing rutile structure for the RuO2-Ta2O5 (70:30%) composition. EDX and SEM investigations show the uniform binary composition, porous and fine-grained microstructure of the sensing electrode. Potentiometric analysis indicates that the sensitivity of RuO2-Ta2O5 (70:30%) film is close to the theoretical Nernstian response in a wide pH range (2-12). The fabricated sensor exhibits very fast response and good repeatability. The drift and hysteresis effects, influence of Li+, Na+ and K+ as interfering ions, and conductivity of solution have no significant impact on the sensing performance of the potentiometric pH sensor. The applicability of the fabricated RuO2-Ta2O5 thick film pH sensor was successfully checked for measurement of river water, tap water, distilled water and lemon juice, and compared with the commercial pH sensor.
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5. Acknowledgment The authors gratefully acknowledged the European Commission in the framework of the FP 7 project SENSEIVER, grant number 289481 for financially supporting the work. The authors are grateful to Elvira Djurdjic, Department of Physics, for Raman experiment at Faculty of Science, University of Novi Sad, Serbia.
References [1] N. Ibanez-Garcia, J. Alonso, C. S. Martinez-Cisneros, F. Valdes, Green-tape ceramics. New technological approach for integrating electronics and fluidics in microsystems, Trac Trend. Anal. Chem. 27 (2008) 24-33. [2] M. R. Gongora-Rubio, Ma. B.A. Fontes, Z. Me. da Rocha, E. M. Richter, L. Angnes, LTCC manifold for heavy metal detection system in biomedical and environmental fluids, Sensor. Actuat. B-Chem. 103 (2004) 468-473. [3] S. Zhuiykov, Solid-state sensors monitoring parameters of water quality for the next generation of wireless sensor networks, Sensor. Actuat. B- Chem. 161 (2012) 1-20. [4] D. O’Hare , K. H. Parker, C. P. Winlove, Metal-metal oxide pH sensors for physiological application, Med. Eng. Phys. 28 (2006) 982–988. [5] Cl. Bohnke, H. Duroy, J.-L. Fourquet, pH sensors with lithium lanthanum titanate sensitive material: applications in food industry, Sensor. Actuat. B-Chem. 89 (2003) 240247. [6] A. Fog, R. P. Buck, Electronic semiconducting oxides as pH sensors, Sensor. Actuat.BChem. 5 (1984) 137-146. [7] S. , E. Kats, D. Marney, Potentiometric sensor using sub-micron Cu2O-doped RuO2 sensing electrode with improved antifouling resistance, Talanta 82 (2010), 502-507. [8] B. Xu, W.-De Zhang, Modification of vertically aligned carbon nanotubes with RuO2 for a solid-state pH sensor, Electrochim. Acta 55 (2010) 2859-2864. [9] W. Wang, S. Guo, I. Lee, K. Ahmed, J. Zhong, Z. Favors, F. Zaera, M. Ozkan, C. S. Ozkan, Hydrous ruthenium oxide nanoparticles anchored to graphene and carbon nanotube hybrid foam for supercapacitors, Sci. Rep. 4 (2014) 1-9.
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[10] J.-C. Chou, R.-T. Chen, Y.-H. Liao, J.-S Chen, M.-S. Huang, H.-T. Chou, Dynamic and wireless sensing measurements of potentiometric glucose biosensor based on graphene and magnetic beads, IEEE Sens. J. 15 (2015) 5718-5725. [11] L. A. Pocrifka, C. Goncalves, P. Grossi, P.C. Colpa, E.C. Pereira, Development of RuO 2– TiO2 (70–30) mol% for pH measurements, Sensor. Actuat. B-Chem. 113 (2006) 10121016. [12] L. Manjakkal, K. Cvejin, J. Kulawik, K. Zaraska, D. Szwagierczak, The effect of resistivity and storage conditions on sensitivity of RuO2 based pH sensors, Key Eng. Mat. 605 (2014) 457-460. [13] L. Manjakkal, E. Djurdjic, K. Cvejin, J. Kulawik, K. Zaraska, D. Szwagierczak, Electrochemical impedance spectroscopic analysis of RuO2 based metal oxide thick film pH sensors, Electrochim. Acta 168 (2015) 246-255. [14] S. Zhuiykov, E. Kats, K. Kalantar-zadeh, M. Breedon, N. Miura, Influence of thickness of sub-micron
Cu2O-doped
RuO2 electrode
on
sensing
performance
of
planar
electrochemical pH sensors, Mater. Lett. 75 (2012) 165-168. [15] L. Manjakkal, K. Cvejin, J. Kulawik, K. Zaraska, D. Szwagierczak, R. P. Socha, Fabrication of thick film sensitive RuO2-TiO2 and Ag/AgCl/KCl reference electrodes and their application for pH measurements, Sensor. Actuat. B-Chem. 204 (2014) 57-67. [16] G. M. da Silva, S. G. Lemos, L. A. Pocrifka, P. D. Marreto, A. V. Rosario, E. C. Pereira, Development of low-cost metal oxide pH electrodes based on the polymeric precursor method, Anal. Chim. Acta 616 (2008) 36-41. [17] D-H. Kwon, B-W. Cho, C-S. Kim, B-K. Sohn, Effects of heat treatment on Ta2O5 sensing membrane for low drift and high sensitivity pH-ISFET, Sensor. Actuat. B-Chem. 34 (1996) 441-445. [18] M. Chen, Y. Jin, X. Qu, Q. Jin, J. Zhao, Electrochemical impedance spectroscopy study of Ta2O5 based EIOS pH sensors in acid environment, Sensor. Actuat. B-Chem. 192 (2014) 399-405. [19] L. Manjakkal, K. Cvejin, B. Bajac, J. Kulawik, K. Zaraska, D. Szwagierczak, Microstructural, impedance spectroscopic and potentiometric analysis of Ta2O5 electrochemical thick film pH sensors, Electroanal. 27 (2015) 770-781. [20] M.J. Lee, J.S. Kim, S. H. Choi, J. J. Lee, S. H. Kim, S. H. Jee, Y.S. Yoon, Characteristics of thin film supercapacitor with ruthenium oxide electrode and Ta2O5+x solid oxide thin film electrolyte, J. Electroceram. 17 (2006) 639-643
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[21] F. Cardarelli, P. Taxil, A. Savall, Ch. Comninellis, G. Manoli, O. Leclerc, Preparation of oxygen evolving electrodes with long service life under extreme conditions, J. Appl. Electrochem. 28 (1998) 245-250. [22] J. Ribeiro, F.L.S. Purgato, k.B. Kokoh, J.-M. Leger, A.R. de Andrade, Application of Ti/RuO2–Ta2O5 electrodes in the electrooxidation of ethanol and derivants: Reactivity versus electrocatalytic efficiency, Electrochim. Acta 53 (2008) 7845-7851. [23] . Ribeiro, M. S. Moats, A. R. De Andrade, Morphological and electrochemical investigation of RuO2-Ta2O5 oxide films prepared by Pechini-Adams method, J. Appl. Electrochem. 38 (2008) 767-775. [24] Z. Dai, L. Xu, G. Duan, T. Li, H. Zhang, Y. Li, Y. Wang, Y. Wang, W. Cai, Fastresponse, sensitive and low-powered chemosensors by fusing nanostructured porous thin film and IDEs-microheater chip, Sci. Rep. 3 (2013) 1669-1676. [25] S.-J. Bao, C.-M. Li, J.-F. Zang, X.-Q. Cui, Y. Qiao, J. Guo, New nanostructured TiO2 for direct electrochemistry and glucose sensor applications, Adv. Funct. Mater. 18 (2008) 591-599. [26] B. E. Horton, S. Schweitzer, A. J. DeRouin, K. G. Ong, A varactor-based, inductively coupled wireless pH Sensor, IEEE Sensor. J. 11 (2011) 1061-1066. [27] Y. M. Chen, A. Korotcov, H. P. Hsu, Y. S. Huang, D. S. Tsai, Raman scattering characterization of well-aligned RuO2 nanocrystals grown on sapphire substrates, New J. Phys. 9 (2007) 130, 1-11. [28] R. P. Devan, W.-D. Ho, S. Y. Wu, Y.-R. Ma, Low-temperature phase transformation and phonon confinement in one-dimensional Ta2O5 nanorods, J. Appl. Crystallogr. 43 (2010) 498-503. [29] D. E. Yates, S. Levine, T. W. Healy, Site-binding model of the electrical double layer at the oxide/water interface, J. Chem. Soc. Farad. T. 1, 170 (1974) 1807-1818. [30] S. Al-Hilli, M. Willander, The pH response and sensing mechanism of n-type ZnO/electrolyte interfaces, Sensors 9 (2009) 7445-7480. [31] P.
Kurzweil,
Precious
metal
oxides
for
electrochemical
energy
converters:
Pseudocapacitance and pH dependence of redox processes. J. Power Sources, 190 (2009) 189-200. [32] S. Zhuiykov, Morphology of Pt-doped nanofabricated RuO2 sensing electrodes and their properties in water quality monitoring sensors, Sensor. Actuat. B-Chem. 136 (2009) 248256.
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[33] Y. Liao, J. Chou, Preparation and characteristics of ruthenium dioxide for pH array sensors with real-time measurement system, Sensor. Actuat. B-Chem. 128 (2008) 603– 612 [34] S. Zhuiykov, Development of ceramic electrochemical sensor based on Bi 2Ru2O7+x -RuO2 sub-micron oxide sensing electrode for water quality monitoring, Ceram. Int. 36 (2010) 2407–2413.
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Figure Captions Fig. 1. Schematic representation of potentiometric RuO2-Ta2O5 sensors: (I) on an alumina substrate (II) on LTCC substrate. Fig. 2. Fabrication steps of LTCC miniaturized pH sensor. Fig. 3. (a) XRD pattern, and (b) Raman spectrum of RuO2-Ta2O5 (70:30%) nanostructured thick film. Fig. 4. SEM cross-sectional image of RuO2-Ta2O5 thick film after treatment in solutions with different pH values Fig. 5. Potential as a function of pH for RuO2-Ta2O5 (70:30%) thick film sensor. Fig. 6. Potentiometric analysis of RuO2-Ta2O5 (70:30%) based thick film pH sensor: (a) dynamic response for different solution pH values and (b) potential difference between the sensitive electrode and glass reference electrode (I) or thick film reference electrode (II) as a function of the solution conductivity at pH=7, after 6 months. Fig. 7. Potential deviations caused by hysteresis effect during different cycles of pH changes for: (a) pH 2-4-6-4-2 loop, (b) pH 7-9-11-9-7 loop, and (c) 3-6-9-12-9-6-3 loop. Fig. 8. Open circuit potential versus pH for LTCC pH sensor with RuO2-Ta2O5 electrode cofired with LTCC substrate.
21
Table I: Methods of fabrication and sensing properties of RuO2 based pH sensors Materials
Method of fabrication
Pt doped RuO2
Screen printing Radio frequency sputtering
RuO2
RuO2-TiO2
Nernstian response (mV/pH) 58 55.64
Potential drift ±1.5mV/ month 0.38 mV/h
Response time (s)
Hysterisis width
Ref.
1-2
-
32
<1 s
4.36 mV for pH loop 7-4-7-10-7, 2.2 mV for pH loop 7-10-7-4-7 -
33
±5 mV in basic solutions, ±3 mV in acidic solutions 6.4 mV for pH loop 7-4-7-10-7, 5.1 mV for pH loop 7-10-7-4-7, 10.2 mV for pH loop 2-8-12-8-2
15
…….
34
±3 mV for pH loop 2-4-6-4-2, ±8 mV for pH loop 7-9-11-9-7
This work
Pechini method Screen printing
56.03
-
-
56.11
3% of potential value
<15
RuO2/MWCNT
Magnetron sputtering
55
2.3 mV/pH
<40
Bi2Ru2O7+xRuO2
Screen printing
58
±1.5 mV/ month
RuO2-Ta2O5
Screen printing
56
....
Response time dependent on solution temperature in acidic solutions >8 s, in basic solutions >15s
RuO2-TiO2
Highlights Binary metal oxide pastes based on RuO2-Ta2O5 were developed. Thick film potentiometric pH sensors were fabricated on alumina and LTCC substrates. Structural and composition studies were performed. Nonlinear characteristics of thick film pH sensors were investigated. LTCC technology was utilized for wireless pH sensor application.
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PII: DOI: Reference:
S0039-9140(15)30359-3 http://dx.doi.org/10.1016/j.talanta.2015.09.069 TAL16002
To appear in: Talanta Received date: 20 July 2015 Revised date: 24 September 2015 Accepted date: 27 September 2015 Cite this article as: Libu Manjakkal, Krzysztof Zaraska, Katarina Cvejin, Jan Kulawik and Dorota Szwagierczak, Potentiometric RuO2-Ta2O5 pH Sensors fabricated using thick film and LTCC Technologies, Talanta, http://dx.doi.org/10.1016/j.talanta.2015.09.069 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Potentiometric RuO2-Ta2O5 pH Sensors Fabricated Using Thick Film and LTCC Technologies
Libu Manjakkal, Krzysztof Zaraska, Katarina Cvejin, Jan Kulawik, Dorota Szwagierczak* Institute of Electron Technology, Krakow Division, 30-701 Kraków, Zabłocie 39, Poland *Corresponding author: [email protected]
Abstract The paper reports on the preparation, properties and application of potentiometric pH sensors with thick film RuO2-Ta2O5 sensing electrode and Ag/AgCl/KCl reference electrode screen printed on an alumina substrate. Furthermore, it presents fabrication procedure and characterization of a new miniaturized pH sensor on LTCC (low temperature cofired ceramics) substrate, destined for wireless monitoring. The crystal structure, phase and elemental composition, and microstructure of the films were investigated by X-ray diffractometry, Raman spectroscopy, scanning electron microscopy and energy dispersive spectroscopy. Potentiometric characterization was performed in a wide pH range of 2-12 for different storage conditions and pH loops. The advantages of the proposed thick film pH sensors are: (a) low cost and easy fabrication, (b) excellent sensitivity close to the Nernstian response (56 mV/pH) in the wide pH range, (c) fast response, (d) long lifetime, (e) good reproducibility, (f) low hysteresis and drift effects, and (g) low cross-sensitivity towards Li+, Na+ and K+ as interfering ions. The applicability of the sensors for pH measurement of river, tap and distilled water, and some drinks was also tested. KEYWORDS: pH sensor, thick film, LTCC, RuO2-Ta2O5, potentiometric analysis, hysteresis effect 1
1. Introduction Miniaturized, highly sensitive and accurate electrochemical pH sensors which contain both sensitive and reference electrodes in a single platform have become of great importance due to their remarkable predicted application scope in the next generation of the electrochemical sensors. For the fabrication of miniaturized analytical devices, LTCC (low temperature cofired ceramics) technology is offering numerous advantages, including integration of sensing layer and electronic circuits for the wireless sensor part in one module [1, 2]. LTCC technology enables development of the complex three dimensional structure of a device. The electrochemical solid state pH sensors made in microsystem technology are especially useful for the application in wireless sensor networks. Such sensors have been already successfully used in water pollution monitoring and remote sensing applications [3]. Furthermore, solid state pH sensors have been recognized for several decades for bio-medicalchemical applications, food processing, agricultural and industrial applications, etc. [4, 5, 6]. RuO2 is a versatile and attractive solid material for developing electrochemical sensors, biosensors and supercapacitors [3, 7-10]. RuO2 based pH sensors have been found to exhibit outstanding sensing performance over wide pH ranges. Their significant advantages over the traditional glass pH electrode and other metal oxide based pH electrodes include high sensitivity and stability, long lifetime, very fast response, small hysteresis effect, low sensitivity to interferences caused by different ions and very good repeatability [3, 8, 11]. In our previous papers, we reported a thick film potentiometric pH sensor based on RuO2 with a Nernstian response close to the theoretical value [12] and impedance spectroscopic analysis of RuO2 based pH conductimetric sensor [13]. However, since RuO2 is an expensive component, for reducing the cost, as well as to enhance the stability and sensitivity, RuO2 as the pH sensing electrode material can be mixed with other oxides [11, 14, 15]. Recently, there are
2
several studies reporting on the development of low cost pH sensors by using binary metal oxides. The advantage of such sensors is low cost, stability, long lifetime and good electrochemical activity. For the sensing electrodes, the RuO2- TiO2 system is the most frequently used one [11, 15, 16]. As a pH sensitive material, Ta2O5 has received significant attention for the fabrication of thin film pH sensors, especially of ISFET (Ion Sensitive Field Effect Transistor) type [17], and EIOS (electrolyte-ion sensitive membrane-oxide-semiconductor) type [18]. In our previous work, we developed a thick film Ta2O5 based pH sensor which exhibited a subNernstian potentiometric response [19]. So far, major studies devoted to RuO2-Ta2O5 system were focused on supercapacitors [20] and DSA-type electrodes (Dimensionally Stable Anodes) [21]. It was found that the introduction of Ta2O5 which is a proton conducting solid electrolyte enhances the rate of proton intercalation at the surface of RuO2 grains [20]. DSA electrodes are prepared in the form of thin film coatings on Ti-substrate and employed as electrochemical devices for oxidation of organic compounds, in chlorine and oxygen production [21, 22]. For this application, good electrochemical properties, high corrosion resistance, and long lifetime of RuO2-Ta2O5 thin films on Ti substrate were reported by Ribeiro et al. [22, 23]. This article presents the studies of thick film potentiometric pH sensors based on RuO2-Ta2O5 binary composition. The nanostructured and porous morphology of the applied thick film RuO2-Ta2O5 electrode provides a high surface to volume ratio and hence large number of adsorption centers on the surface of the sensitive film. This improves sensing performance, contributing to high sensitivity, fast response and low power consumption [24, 25]. The structural properties and composition of the films were characterized by using X-ray diffractometry (XRD), Raman spectroscopy, scanning electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDX). In this work, the use of the binary RuO2-Ta2O5
3
composition and screen printing method allowed for fabrication of a thick film pH sensor characterized by low cost, ease of miniaturization and excellent sensing performance. Furthermore, a potentiometric sensor for wireless application was fabricated using LTCC technology and its sensing performance was discussed. The inbuilt reference and sensitive electrodes and the possibility of integration of passive components for the electrical circuit of the wireless system are significant advantages of the LTCC pH sensor. To the best of our knowledge, this article reports for the first time the application of RuO2-Ta2O5 as the sensing material in potentiometric thick film pH sensors.
2. Experimental Two binary RuO2-Ta2O5 mixtures were prepared by ball milling. An active oxide RuO2 (99.9%, Aldrich) was mixed with Ta2O5 (99.8%, Aldrich) in a suitable ratio to obtain RuO2-Ta2O5 (70:30) wt.% and RuO2-Ta2O5 (30:70) wt.% compositions destined for sensitive electrodes of thick film pH sensors. After weighing, the starting materials were wet-ball milled in isopropyl alcohol for 5 h in a planetary ball mill (Fritsch Pulverisette 5, Germany) and dried at 70°C. As a result of ball milling, fine powders were obtained and proper homogeneous mixing of the components was ensured. The prepared RuO2-Ta2O5 based mixtures were used for the preparation of thick film pastes by mixing thoroughly the powders with a suitable amount of ethyl cellulose (binder) and terpineol (solvent) in an agate mortar for 30 min. Thick film potentiometric pH sensors were fabricated by screen printing. The fabrication steps of the potentiometric pH sensors on an alumina substrate (96% Al2O3) were similar to the process that we reported in our previous article [15]. Initially, a silver-palladium (ESL 9695) paste for conducting paths was screen printed on an alumina substrate and sintered at 850°C for 30 min. Then the sensitive RuO2-Ta2O5 layer was printed and fired at
4
850°C for 1 h with heating rate of 4°C/min. The schematic representation of the fabricated thick film potentiometric (I) and LTCC based (II) pH sensors is shown in Fig. 1. For the reference electrode (RE) fabrication, sodium hypochlorite solution was used to chloridize a part of the conductive layer and to form AgCl [26]. In order to stabilize AgCl concentration in the reference electrode typically a KCl solution is utilized. However, the thick film containing pure KCl easily dissolves in water. Thus, for increasing the lifetime of RE, a glass-KCl paste was used. The glass based on Bi2O3-SiO2-B2O3-CdO-Li2O3 system was mixed with KCl powder in equal weight proportion and milled for 3 h in isopropyl alcohol using a planetary ball mill. The obtained fine powder was applied for making a paste which was overprinted on AgCl layer and fired at 550°C for 1 h. Finally, an insulative resin was painted on the top of the substrate surface, leaving uncovered the sensing electrode area, points for electrical contact and small opening for hydration port in the glass-KCl layer. The performance of the fabricated thick film reference electrode was cross-checked by using a commercial glass RE (Ag/AgCl/KCl, HYDROMET, Poland). The fabrication procedure of the reference electrode was based on the work reported by Horton et al. [26]. These authors [26] observed perfect functioning of the AgCl layer even after firing of the KCl paste at relatively high temperature of 750°C. In this work, the KCl paste was modified by mixing with a glass of a low temperature melting point. The temperature of 550°C was chosen as optimal for firing of the glass-KCl paste. It was found that after increasing to 600°C the firing temperature of the KCl-glass layer, a minor deterioration of the performance of the reference electrode was observed. A commercial DuPont 951 green tape was used for making LTCC substrates for RuO2-Ta2O5 sensors. The applied LTCC process comprises: cutting of the tape to suitable dimensions, screen printing of conductive and sensitive layers, lamination and co-firing, printing and firing of the reference electrode, painting of the insulative layer and finally
5
soldering of leads to provide electrical contact. Fabrication of LTCC pH sensor takes place in six steps illustrated in Fig. 2, described in more detail below: 1.
The green LTCC tape was cut by a laser (Oxford Lasers, UK) into individual sheets with preferred dimensions and shapes.
2.
Two strips of the conducting layer (Ag, ESL 9916) were screen printed on the top of the green sheet and dried at 120°C for 20 min. Then the sensitive electrode with an area of 3x3 mm2 was printed on the top of the Ag conducting layer and dried. For larger surface area of the sensing electrode, its deformation during cofiring was difficult to avoid.
3.
The sheet with printed layers and a required number of blank sheets were stacked in a suitable die. The external blank sheet with a proper shape plays the role of electrical insulation which protects conducting paths against the contact with a solution. Isostatic lamination (Pacific Trinetics Corporation, USA) of the stack was carried out under a pressure of 20 MPa at 70°C for 10 min. The next step was cofiring of the ceramic green tapes with the conducting and sensing layers. The proper burnout of the organics was ensured at 450°C and co-firing was carried out at 850°C for 1 h. After co-firing process the sensor became rigid.
4.
One end of the conducting layer was converted into Ag-AgCl layer. On the top of this AgAgCl layer, glass-KCl layer was overprinted and fired at 550°C for 1 h.
5.
A polyurethane resin was painted on the top of the glass-KCl layer with a small opening for hydration port. This insulative layer reduces the salt loss from the reference electrode.
6.
After soldering of electrical leads to the contact pads at the opposite end of the substrate, the sensor was ready for operation. The crystal structure and phase composition studies of the sensitive film were carried
out by X-ray diffraction method (XRD, Philips X’Pert, USA) using Cu-Kα radiation. A Raman microscope (Thermo Fisher DXR, USA) was used for the confirmation of crystal
6
structure of the sensitive film. The microscope has a DPSS (Diode Pumped Solid State Laser) laser, with a wavelength 532 nm and maximum power level 10 mW, coupled with 50x magnifying objective. It has a CCD camera as a detector and 900 lines/nm grating. We used 50 µm aperture with estimated spot size 1.1µm. The laser power level of 9 mW was used with an exposure time 30 s. Scanning electron microscopy (FEI Nova Nano, SEM 200, USA) was applied to examine the surface, cross-sectional microstructure and thickness of the sensitive films. X-ray energy dispersive spectroscopy (EDS, EDAX System Genesis Software) method was used for the elemental composition analysis of the sensitive electrode. The ability of the potentiometric RuO2-Ta2O5 electrode to determine pH of test solutions was investigated by measuring the emf (electromotive force) versus the thick film reference electrode. All electrochemical measurements were performed at ambient temperature. The test solutions of pH values 2-12 were utilized for studying the sensing performance of thick film sensors. A commercial calibrated glass pH sensor and a conductivity meter (ELNEIRON-CPC-411) were used to confirm the pH and conductivity value of each test solution. The generated potential difference between thick film sensitive and reference electrodes was measured by using a multimeter (Keithley 2002). The output results were gathered and monitored by a PC which was connected to the multimeter by GPIB interface and analyzed by LabVIEW program.
3. Results and Discussion 3.1. Microstructural Analysis The XRD pattern of RuO2-Ta2O5 (70:30%) thick film sintered at 850°C on an alumina substrate is presented in Fig. 3a. The observed distinct diffraction peaks can be assigned to the rutile structure of RuO2 and to the orthorhombic structure of Ta2O5 [23] and no additional phases were found. The intensities of RuO2 peaks are higher than those of Ta2O5, due to higher amount of RuO2 in the composition. The average crystallite sizes calculated from the 7
major peaks width on the basis of Scherrer equation are in the nanocrystalline range and are almost the same for RuO2 and Ta2O5 (about 10 nm). Small Al2O3 peaks appear also in the pattern due to the effect of an alumina substrate. For RuO2-Ta2O5 film prepared by using the Pechini-Adams method Ribeiro et al. [23] revealed that the intensity of Ta2O5 peak on the XRD pattern was very low in the composition with a high percentage of RuO2 due to amorphous structure of Ta2O5 phase [23]. In this work, both constituent oxides exhibit crystalline nature resulting from another film deposition method and higher sintering temperature. Fig. 3b represents the Raman spectrum of RuO2-Ta2O5 (70:30%) thick film sintered at 850°C. In this spectrum, splitting of the major peaks was found. The peaks are observed in the position of 521, 527, 632, 642, 707 and 711 cm-1. Comparison of the Raman spectrum of RuO2-Ta2O5 film with that of pure RuO2 [27] reveals that the peaks at positions 527, 642 and 711 cm-1 correspond to three major Raman active modes Eg, A1g and B2g of rutile RuO2 with a 14 space group of D4h . Eg is a doubly degenerated mode and A1g a symmetric mode,
respectively [27]. The third mode at 711 cm-1 depends on the Ru-O bond length which is changing due to oxygen atom either moving away or towards the Ru atom. The frequency of this mode is high, and it shows low intensity peak compared to other two modes. The peaks in the Raman spectrum at 521, 632 and 707 cm-1 are attributed to Ta2O5 and are almost similar to those found in our previous study for pure Ta2O5 thick film [19], and reported by other authors for this oxide [28]. The Raman bands corresponding to 450-900 cm-1 are mainly related to the internal stretching vibrations of various Ta-O modes. Furthermore, other peaks for Ta2O5 [19, 28] are not present in the spectrum shown in Fig. 3b, presumably due to the prevailing amount of RuO2 in RuO2-Ta2O5 (70-30%) film. Hence, the obtained Raman spectrum indicates that the film has distinct rutile structure and the obtained Raman active
8
modes are similar to those for the well aligned nanocrystalline RuO2 reported by Chen et al. [27]. SEM cross-sectional images of RuO2-Ta2O5 thick film on an alumina substrate before and after treatment in solutions with different pH values are illustrated in Figs. S1 and 4, respectively. The film morphology is almost similar for the freshly prepared samples (Fig. S1) (S=supplementary file) and after reaction with acidic and basic solutions (Fig. 4). The thickness of the screen printed film is nearly 15 µm, as presented in Fig. 4. The EDX analysis performed at the cross-section of the sensor shows the presence of Ru, Ta and oxygen atoms as the constituent elements of the sensing layer. Additional Al and Ag peaks indicate the effect of alumina substrate and conducting layer. Microscopic observations show uniform thicknesses and good cooperation between KCl-glass and AgCl layers. The thickness of the screen printed Ag conducting layer after its partial chloridization to AgCl is nearly 7 µm and that of the glass-KCl layer is 9 µm. Ribeiro et al. [23] obtained by Pechini method RuO2-Ta2O5 thin film without nanoporous structure which showed rough and cracked morphology related to the formation of amorphous Ta2O5 phase surrounding RuO2 crystallites. The RuO2-Ta2O5 thick film prepared in this work is homogeneous, with smooth surface, and nanometric pores and grain sizes ranging from 180 to 520 nm. The observed differences in the crystallinity degree and porosity of the films originate from different deposition methods and much higher temperature of thermal treatment used in this work (850°C) as compared with that applied by Ribeiro et al. [23] (450700°C).
3.2. Potentiometric Analysis of the Sensors The sensitivity of the proposed potentiometric pH sensor at room temperature was validated by dipping the sensor in solutions of different pH values. The tests were conducted by measuring the open circuit potential of the sensor (electromotive force of the cell 9
comprising the sensitive RuO2-Ta2O5 electrode and thick film Ag/AgCl/KCl reference electrode). The measurements were performed initially in acidic solutions and then in alkaline solutions with increasing pH values. Figs. S2 and 5 show the pH dependences of the potential of the sensors with sensing electrodes based on two compositions RuO2-Ta2O5 (30:70%) and RuO2-Ta2O5 (70:30%), respectively. Both plots were found to be linear, although there is a great difference between these two compositions concerning accordance of the experimental results with the theoretical Nernstian response. For RuO2-Ta2O5 (30:70%) sensitive electrode, the slope is 35.3±1 mV/pH in the pH range 2-12 and the standard potential (E0) (intercept of the straight line at pH=0) of the sensor is 594.1±7 mV (Fig. S2). On the other hand, the sensitivity of RuO2-Ta2O5 (70:30%) electrode in the pH range 2-12 is 56.1±0.8 mV/pH and the standard potential is equal to 753.4±5mV (Fig. 5), these values being very close to those derived from the theoretical Nernst relationship. When the metal oxide is exposed to a solution, as suggested by the site-binding theory, negative surface charged groups ( M O ), neutral sites ( M OH ) and positive surface charged groups ( M OH 2 ) are developed [29]. These charged groups at the electrodeelectrolyte interface can create an electrical double layer (edl) structure [29, 30]. H+ or OHions from the solution are attracted to oxygen ions from the metal oxide crystal lattice and to surface cations, respectively, to form surface hydroxyl groups [30, 31]. In acidic solutions, H+ ions are released and the open circuit potential increases with pH changing towards strongly acidic solutions [31]. In basic solutions, the electrode potential decreases with pH rising towards basic values due to the surface metal oxide becoming negatively charged by releasing OH- groups [31]. The formation of metal hydroxyl groups on RuO2 surface was confirmed by the XPS analysis in our previous study [15] concerning RuO2-TiO2 system. The sensitivity obtained for the electrode with a smaller percentage of RuO2 is significantly lower than that for the electrode with a higher amount of RuO2 and the 10
theoretical Nernstian slope factor (59.14 mV/pH). The observed sub-Nernstian response of the RuO2-Ta2O5 (30:70)% electrode may be due to the combination of H+ ion exchange on the surface of the film and charge transfer reaction involving one or two electrons, as suggested for a pH sensor based on pure Ta2O5 [19]. Chen et al. [18] stated that for the EIOS pH sensor with pure Ta2O5 thin film, H+/OH- ions from the aqueous solution react with Ta2O5 surface to form surface hydroxyl groups (Ta-OH) which donate or accept proton from the solution. This reaction is responsible for the Nernstian response. These authors observed a Nernstian pH sensitivity of 56.19 mV/pH in the pH range 1 to 10 [18]. However, in our previous study of Ta2O5 based thick film pH sensor, we observed a sub-Nernstian response [19]. This implies that the preparation method, the sensor construction, surface morphology and crystal structure of the film strongly influence the pH sensing performance of Ta2O5 based pH sensors [17-19]. In addition, the sub-Nernstian response of the RuO2-Ta2O5 (30:70)% electrode may be caused by a too low content of nanostructured RuO2, resulting in lower electrochemically active area of this component [23] which exhibits very good electronic and ionic conductivity. Due to much higher concentration of RuO2 than Ta2O5 in the RuO2-Ta2O5 (70:30)% sensitive layer it can be expected that dissociative adsorption and formation of –OH groups mainly occur on the surface of RuO2 grains and is responsible for the Nernstian response. Pocrifka et al. [11] found that the best sensing performance of the pH sensor based on a binary metal mixture was obtained for the compositions richer in RuO2 than in an inert oxide. When RuO2 surface reacts with an acidic or basic solution, Ru(IV) and Ru(III) oxidation states are observed. This Ru(IV)/Ru(III) reduction potential follows the Nernstian response with the sensitivity of 59 mV/pH at a temperature of 25°C [31]. In the present study, the electromotive force of the sensor with the sensing electrodes with prevailing RuO2 content follows a nearly Nernstian dependence with 56 mV/pH slope at room temperature (Fig. 5). Moreover, the correlation coefficient of this plot being 0.998 reflects a perfect straight line fit and suggests that only one
11
electron is involved in the above mentioned redox reaction [32]. The Nernstian response of the metal oxide based pH sensor is related to the processes of adsorption, ion exchange and diffusion of ions from the solution into the porous material structure, accompanied by electron charge transfer. Such mechanism explanation based on the results of potentiometric and impedance spectroscopic studies was proposed in our previous work for pH sensors with the sensing electrodes made of RuO2 commercial pastes [13] and RuO2-TiO2 [15]. A comparison of the sensitivities reported by different authors for pH sensitive electrodes based on RuO2 and RuO2-Ta2O5 binary metal oxide compositions is shown in Table I. The response time of thick film pH sensor with the RuO2-Ta2O5 (70:30%) electrode was determined by measurement of a potential change after dropping of 0.1M HCl or KOH solutions into a test solution. The response time is defined as the time required for a sensor to reach more than 90% of its open circuit potential value in equilibrium [8]. Fig. 6a shows the results of the response time test comprising a cycle of a few abrupt step changes of pH in the range of 2-10. A commercial glass pH sensor was used to confirm the pH value of the solution during the measurement. Compared with the response time 15 s of our previously reported RuO2-TiO2 sensing electrode [15], the response time of RuO2-Ta2O5 based pH sensor is shorter and more dependent on the solution pH. In acidic solutions, the sensor shows a very short response time (less than 8 s), whereas in basic solutions a little longer one (less than 15 s). This indicates that faster response is related to diffusion of H+ ions, dominant in acidic solutions [8]. Moreover, the response time of the metal oxide based pH sensor may also be dependent on the structural properties and morphology of the film [14]. Nanostructured nature and porosity of the fabricated film may improve the response time of the sensor. The response times obtained for our sensor are in close agreement with those reported in literature for other pH sensors (Table I).
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As illustrated in Fig. 6b, the conductivity of the solution at a given pH has very small influence on the sensor potential, especially in the range of 15-2000 µS. However, for the conductivity exceeding 2000 µS there is a drop in potential of 8 mV. The similar dependences were obtained for measurements performed using both the glass and thick film reference electrodes. This confirms the suitability of the fabricated sensor for different applications. Potentiometric investigation shows the emf drift during preliminary measurements. This drift in potential may be due to the influence of the reference electrode [15, 26]. The performance of the fabricated reference electrode after the applied procedure was good, reflected by a small potential difference (7 mV) between the commercial and the fabricated thick film reference electrodes. However, the thick film reference electrode needs a preliminary period (nearly 20 min) to form a conducting channel inside the glass-KCl layer after immersing the electrode in the solution. Similarly, the freshly prepared RuO2-Ta2O5 based sensors show for each pH a drift in potential during initial measurement and their stabilization cannot be attained during a short test period. To avoid this effect, in this work, all meaningful potentiometric measurements data are considered after initial stabilization of the sensor. For checking the stability and drift for every pH, the emf value (Fig. 5) was determined after keeping the sensor near 30 min in each solution. The drift in potential may also be related to the composition, thickness, porosity and homogeneity of the sensitive film [32]. According to Zhuiykov [32], RuO2 based sensing electrode shows a voltage drift during the first month of testing. Presumably, it is due to H+ transport through the sensing layer being governed by H2 trapping at the trap sites existing at grain boundaries or micropores of the nanostructured sensing electrode [32]. After several measurements, the sensor was kept in atmospheric conditions and measured again after 6 and 12 months. Fig. S3 shows the response of the potentiometric sensor after six months. The slope factor of the Nernstian response (56.4±0.7 mV/pH) is
13
similar to that determined during the initial period. Furthermore, after one year of measurement the sensor exhibits sensitivity of 54±1 mV/pH. Hence, it was observed that the fabricated thick film pH sensor can be useful for long lifetime applications. We observed that after long term storage there is a decrease in potential value of 30±5 mV for each solution pH. This decrease may result from changes in the composition and morphology of thick film reference electrode. In the initial period of measurement, the potential difference between the glass and thick film reference electrode is ±7 mV, but after storage in distilled water lasting six months this difference grows to 30-50 mV. This may be caused by the salt loss from the glass-KCl layer of RE. Thus, calibration of the sensor is required for long term usage of the fabricated thick film reference electrode. The hysteresis effect (memory effect) of the fabricated thick film pH sensor is illustrated in Figs. 7a, 7b, and 7c for different cycles of pH changes. Hysteresis phenomenon occurs when different output voltages are obtained for a pH electrode measured in the same buffer solution after a series of measurements [15]. Figs. 7a and 7b represent the hysteresis effect for the loops in the acidic (pH 2-4-6-4-2) and the basic (pH 7-9-11-9-7) regions, respectively. The potential deviation was found to be ±3 mV in the acidic region and equal to ±8 mV in the basic region. It was observed that when the pH loop path consists of more steps, as shown in Fig. 7c for pH 3-6-9-12-9-6-3 cycle, the hysteresis width is almost the same as for the shorter loop paths applied in both the acidic and basic regions. Hence, it was found that the hysteresis effect for the fabricated sensor is lower in the acidic pH range than in the basic region and is not dependent on the loop path. The hysteresis effect of the developed sensors was observed after several measurements in different acidic and basic solutions and also for the sensors after long time of storage in atmospheric conditions. Small hysteresis effect (210 mV) was found by Xu et al. [8] for a pH sensor with RuO2/carbon nanotubes electrodes deposited by magnetron sputtering, and by Liao and Chou [33] for RuO2 electrodes obtained
14
by rf sputtering. Comparison of the hysteresis width for different RuO2 based pH sensors is shown in Table I. The fabricated thick film RuO2-Ta2O5 pH sensor shows excellent repeatability. Sensors from different lots exhibit almost similar Nernstian slope, with deviations lower than ±1 mV/pH. Selectivity measurements revealed that the presence of Li+, Na+ and K+ ions has no significant effect on the sensitivity of the proposed thick film pH sensors. The influence of these ions on the output emf of the sensor was studied in test solutions containing LiCl, NaCl, KCl salts at a concentration of 0.01 M. These ions have a very small influence on the open circuit potential of the sensor, at a level of ±2 mV. However, in the presence of the interfering cations the slope factor remains the same. The prepared thick film paste based on RuO2-Ta2O5 (70:30 wt.%) was finally utilized as the sensing electrode material for the fabrication of a miniaturized potentiometric pH sensor in LTCC technology (Fig. 2). This sensor is destined for wireless solution monitoring. The potentiometric characteristics of the sensor with the sensitive electrode cofired with LTCC substrate were almost the same as the discussed earlier properties of the thick film sensor screen printed on an alumina substrate. The sensitivity of the LTCC sensor equal to 55.2±0.4 mV/pH (Fig. 8) is close to the theoretical value calculated from the Nernst relationship. The sensing LTCC part of the sensor can be combined with a circuit for the wireless part. For the evaluation of applicability of the fabricated potentiometric RuO2-Ta2O5 sensor in real conditions, it was used for measuring pH of tap water, river water, distilled water and lemon juice. The measured pH values were compared with those obtained by a commercial glass pH electrode. For example, the emf value of the sensor measured after its dipping for 30 min in river water was 310 mV. This value was substituted to the Nernstian relationship
15
with the sensitivity of 56 mV/pH and the standard potential of 753 mV, previously determined while testing the sensor in buffer solutions (Fig. 5). The pH value of river water measured using the glass electrode and thick film pH sensor was 8.08 and 7.97, respectively. Similarly for other tested samples (water and juice), almost the same pH values were obtained by means of both sensors. The comparison shows that our thick film pH sensor achieves similar performance as the commercial glass pH sensor, yet with additional advantages of the smaller size, better robustness, lower cost and good applicability for online monitoring.
4. Conclusions In this work, new RuO2-Ta2O5 based thick film pastes were prepared and applied for pH sensing electrodes. Potentiometric pH sensors were fabricated on alumina substrates by thick film technology. Furthermore, LTCC technology was successfully utilized for development of a miniaturized potentiometric pH sensor for wireless solution monitoring. The XRD pattern reveals the binary phase composition of the film with nanometric crystal sizes of RuO2 and Ta2O5. Raman studies confirm the presence of RuO2 and Ta2O5 crystalline phases with prevailing rutile structure for the RuO2-Ta2O5 (70:30%) composition. EDX and SEM investigations show the uniform binary composition, porous and fine-grained microstructure of the sensing electrode. Potentiometric analysis indicates that the sensitivity of RuO2-Ta2O5 (70:30%) film is close to the theoretical Nernstian response in a wide pH range (2-12). The fabricated sensor exhibits very fast response and good repeatability. The drift and hysteresis effects, influence of Li+, Na+ and K+ as interfering ions, and conductivity of solution have no significant impact on the sensing performance of the potentiometric pH sensor. The applicability of the fabricated RuO2-Ta2O5 thick film pH sensor was successfully checked for measurement of river water, tap water, distilled water and lemon juice, and compared with the commercial pH sensor.
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5. Acknowledgment The authors gratefully acknowledged the European Commission in the framework of the FP 7 project SENSEIVER, grant number 289481 for financially supporting the work. The authors are grateful to Elvira Djurdjic, Department of Physics, for Raman experiment at Faculty of Science, University of Novi Sad, Serbia.
References [1] N. Ibanez-Garcia, J. Alonso, C. S. Martinez-Cisneros, F. Valdes, Green-tape ceramics. New technological approach for integrating electronics and fluidics in microsystems, Trac Trend. Anal. Chem. 27 (2008) 24-33. [2] M. R. Gongora-Rubio, Ma. B.A. Fontes, Z. Me. da Rocha, E. M. Richter, L. Angnes, LTCC manifold for heavy metal detection system in biomedical and environmental fluids, Sensor. Actuat. B-Chem. 103 (2004) 468-473. [3] S. Zhuiykov, Solid-state sensors monitoring parameters of water quality for the next generation of wireless sensor networks, Sensor. Actuat. B- Chem. 161 (2012) 1-20. [4] D. O’Hare , K. H. Parker, C. P. Winlove, Metal-metal oxide pH sensors for physiological application, Med. Eng. Phys. 28 (2006) 982–988. [5] Cl. Bohnke, H. Duroy, J.-L. Fourquet, pH sensors with lithium lanthanum titanate sensitive material: applications in food industry, Sensor. Actuat. B-Chem. 89 (2003) 240247. [6] A. Fog, R. P. Buck, Electronic semiconducting oxides as pH sensors, Sensor. Actuat.BChem. 5 (1984) 137-146. [7] S. , E. Kats, D. Marney, Potentiometric sensor using sub-micron Cu2O-doped RuO2 sensing electrode with improved antifouling resistance, Talanta 82 (2010), 502-507. [8] B. Xu, W.-De Zhang, Modification of vertically aligned carbon nanotubes with RuO2 for a solid-state pH sensor, Electrochim. Acta 55 (2010) 2859-2864. [9] W. Wang, S. Guo, I. Lee, K. Ahmed, J. Zhong, Z. Favors, F. Zaera, M. Ozkan, C. S. Ozkan, Hydrous ruthenium oxide nanoparticles anchored to graphene and carbon nanotube hybrid foam for supercapacitors, Sci. Rep. 4 (2014) 1-9.
17
[10] J.-C. Chou, R.-T. Chen, Y.-H. Liao, J.-S Chen, M.-S. Huang, H.-T. Chou, Dynamic and wireless sensing measurements of potentiometric glucose biosensor based on graphene and magnetic beads, IEEE Sens. J. 15 (2015) 5718-5725. [11] L. A. Pocrifka, C. Goncalves, P. Grossi, P.C. Colpa, E.C. Pereira, Development of RuO 2– TiO2 (70–30) mol% for pH measurements, Sensor. Actuat. B-Chem. 113 (2006) 10121016. [12] L. Manjakkal, K. Cvejin, J. Kulawik, K. Zaraska, D. Szwagierczak, The effect of resistivity and storage conditions on sensitivity of RuO2 based pH sensors, Key Eng. Mat. 605 (2014) 457-460. [13] L. Manjakkal, E. Djurdjic, K. Cvejin, J. Kulawik, K. Zaraska, D. Szwagierczak, Electrochemical impedance spectroscopic analysis of RuO2 based metal oxide thick film pH sensors, Electrochim. Acta 168 (2015) 246-255. [14] S. Zhuiykov, E. Kats, K. Kalantar-zadeh, M. Breedon, N. Miura, Influence of thickness of sub-micron
Cu2O-doped
RuO2 electrode
on
sensing
performance
of
planar
electrochemical pH sensors, Mater. Lett. 75 (2012) 165-168. [15] L. Manjakkal, K. Cvejin, J. Kulawik, K. Zaraska, D. Szwagierczak, R. P. Socha, Fabrication of thick film sensitive RuO2-TiO2 and Ag/AgCl/KCl reference electrodes and their application for pH measurements, Sensor. Actuat. B-Chem. 204 (2014) 57-67. [16] G. M. da Silva, S. G. Lemos, L. A. Pocrifka, P. D. Marreto, A. V. Rosario, E. C. Pereira, Development of low-cost metal oxide pH electrodes based on the polymeric precursor method, Anal. Chim. Acta 616 (2008) 36-41. [17] D-H. Kwon, B-W. Cho, C-S. Kim, B-K. Sohn, Effects of heat treatment on Ta2O5 sensing membrane for low drift and high sensitivity pH-ISFET, Sensor. Actuat. B-Chem. 34 (1996) 441-445. [18] M. Chen, Y. Jin, X. Qu, Q. Jin, J. Zhao, Electrochemical impedance spectroscopy study of Ta2O5 based EIOS pH sensors in acid environment, Sensor. Actuat. B-Chem. 192 (2014) 399-405. [19] L. Manjakkal, K. Cvejin, B. Bajac, J. Kulawik, K. Zaraska, D. Szwagierczak, Microstructural, impedance spectroscopic and potentiometric analysis of Ta2O5 electrochemical thick film pH sensors, Electroanal. 27 (2015) 770-781. [20] M.J. Lee, J.S. Kim, S. H. Choi, J. J. Lee, S. H. Kim, S. H. Jee, Y.S. Yoon, Characteristics of thin film supercapacitor with ruthenium oxide electrode and Ta2O5+x solid oxide thin film electrolyte, J. Electroceram. 17 (2006) 639-643
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[21] F. Cardarelli, P. Taxil, A. Savall, Ch. Comninellis, G. Manoli, O. Leclerc, Preparation of oxygen evolving electrodes with long service life under extreme conditions, J. Appl. Electrochem. 28 (1998) 245-250. [22] J. Ribeiro, F.L.S. Purgato, k.B. Kokoh, J.-M. Leger, A.R. de Andrade, Application of Ti/RuO2–Ta2O5 electrodes in the electrooxidation of ethanol and derivants: Reactivity versus electrocatalytic efficiency, Electrochim. Acta 53 (2008) 7845-7851. [23] . Ribeiro, M. S. Moats, A. R. De Andrade, Morphological and electrochemical investigation of RuO2-Ta2O5 oxide films prepared by Pechini-Adams method, J. Appl. Electrochem. 38 (2008) 767-775. [24] Z. Dai, L. Xu, G. Duan, T. Li, H. Zhang, Y. Li, Y. Wang, Y. Wang, W. Cai, Fastresponse, sensitive and low-powered chemosensors by fusing nanostructured porous thin film and IDEs-microheater chip, Sci. Rep. 3 (2013) 1669-1676. [25] S.-J. Bao, C.-M. Li, J.-F. Zang, X.-Q. Cui, Y. Qiao, J. Guo, New nanostructured TiO2 for direct electrochemistry and glucose sensor applications, Adv. Funct. Mater. 18 (2008) 591-599. [26] B. E. Horton, S. Schweitzer, A. J. DeRouin, K. G. Ong, A varactor-based, inductively coupled wireless pH Sensor, IEEE Sensor. J. 11 (2011) 1061-1066. [27] Y. M. Chen, A. Korotcov, H. P. Hsu, Y. S. Huang, D. S. Tsai, Raman scattering characterization of well-aligned RuO2 nanocrystals grown on sapphire substrates, New J. Phys. 9 (2007) 130, 1-11. [28] R. P. Devan, W.-D. Ho, S. Y. Wu, Y.-R. Ma, Low-temperature phase transformation and phonon confinement in one-dimensional Ta2O5 nanorods, J. Appl. Crystallogr. 43 (2010) 498-503. [29] D. E. Yates, S. Levine, T. W. Healy, Site-binding model of the electrical double layer at the oxide/water interface, J. Chem. Soc. Farad. T. 1, 170 (1974) 1807-1818. [30] S. Al-Hilli, M. Willander, The pH response and sensing mechanism of n-type ZnO/electrolyte interfaces, Sensors 9 (2009) 7445-7480. [31] P.
Kurzweil,
Precious
metal
oxides
for
electrochemical
energy
converters:
Pseudocapacitance and pH dependence of redox processes. J. Power Sources, 190 (2009) 189-200. [32] S. Zhuiykov, Morphology of Pt-doped nanofabricated RuO2 sensing electrodes and their properties in water quality monitoring sensors, Sensor. Actuat. B-Chem. 136 (2009) 248256.
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[33] Y. Liao, J. Chou, Preparation and characteristics of ruthenium dioxide for pH array sensors with real-time measurement system, Sensor. Actuat. B-Chem. 128 (2008) 603– 612 [34] S. Zhuiykov, Development of ceramic electrochemical sensor based on Bi 2Ru2O7+x -RuO2 sub-micron oxide sensing electrode for water quality monitoring, Ceram. Int. 36 (2010) 2407–2413.
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Figure Captions Fig. 1. Schematic representation of potentiometric RuO2-Ta2O5 sensors: (I) on an alumina substrate (II) on LTCC substrate. Fig. 2. Fabrication steps of LTCC miniaturized pH sensor. Fig. 3. (a) XRD pattern, and (b) Raman spectrum of RuO2-Ta2O5 (70:30%) nanostructured thick film. Fig. 4. SEM cross-sectional image of RuO2-Ta2O5 thick film after treatment in solutions with different pH values Fig. 5. Potential as a function of pH for RuO2-Ta2O5 (70:30%) thick film sensor. Fig. 6. Potentiometric analysis of RuO2-Ta2O5 (70:30%) based thick film pH sensor: (a) dynamic response for different solution pH values and (b) potential difference between the sensitive electrode and glass reference electrode (I) or thick film reference electrode (II) as a function of the solution conductivity at pH=7, after 6 months. Fig. 7. Potential deviations caused by hysteresis effect during different cycles of pH changes for: (a) pH 2-4-6-4-2 loop, (b) pH 7-9-11-9-7 loop, and (c) 3-6-9-12-9-6-3 loop. Fig. 8. Open circuit potential versus pH for LTCC pH sensor with RuO2-Ta2O5 electrode cofired with LTCC substrate.
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Table I: Methods of fabrication and sensing properties of RuO2 based pH sensors Materials
Method of fabrication
Pt doped RuO2
Screen printing Radio frequency sputtering
RuO2
RuO2-TiO2
Nernstian response (mV/pH) 58 55.64
Potential drift ±1.5mV/ month 0.38 mV/h
Response time (s)
Hysterisis width
Ref.
1-2
-
32
<1 s
4.36 mV for pH loop 7-4-7-10-7, 2.2 mV for pH loop 7-10-7-4-7 -
33
±5 mV in basic solutions, ±3 mV in acidic solutions 6.4 mV for pH loop 7-4-7-10-7, 5.1 mV for pH loop 7-10-7-4-7, 10.2 mV for pH loop 2-8-12-8-2
15
…….
34
±3 mV for pH loop 2-4-6-4-2, ±8 mV for pH loop 7-9-11-9-7
This work
Pechini method Screen printing
56.03
-
-
56.11
3% of potential value
<15
RuO2/MWCNT
Magnetron sputtering
55
2.3 mV/pH
<40
Bi2Ru2O7+xRuO2
Screen printing
58
±1.5 mV/ month
RuO2-Ta2O5
Screen printing
56
....
Response time dependent on solution temperature in acidic solutions >8 s, in basic solutions >15s
RuO2-TiO2
Highlights Binary metal oxide pastes based on RuO2-Ta2O5 were developed. Thick film potentiometric pH sensors were fabricated on alumina and LTCC substrates. Structural and composition studies were performed. Nonlinear characteristics of thick film pH sensors were investigated. LTCC technology was utilized for wireless pH sensor application.
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