Potentiometric CO2 Sensor Using Li+ Ion Conducting Li3PO4 Thin Film Electrolyte.

Li+ ion conducting Li3PO4 thin film electrolytes with thickness 300nm, 650nm and 1.2μm were deposited on Al2O3 substrate at room temperature by thermal evaporation method. Reference and sensing electrodes were printed on Au interfaces by conventional screen printing technique. The overall dimension of the sensor was 3 × 3 mm and of electrodes were 1 × 1.5 mm each. The fabricated solid state potentiometric CO2 sensors of type: CO2, O2, Au, Li2TiO3-TiO2| Li3PO4 |Li2CO3, Au, CO2, O2 have been investigated for CO2 sensing properties. The electromotive force (emf) and Δemf/dec values of the sensors are dependent on the thickness of the electrolyte film. 1.2μm thickness deposited sensor has shown good sensing behavior than the sensors with less thickness. The Δemf values of the sensor are linearly increased up to 460°C operating temperature and became stable above 460°C. Between 460-500°C temperatures region the sensor has reached an equilibrium state and the experimentally obtained Δemf values are about 80% of the theoretically calculated values. A Nernst's slope of -61mV/decade has been obtained between 250 to 5000 ppm of CO2 concentration at 500°C temperature. The sensor is suitable for ease of mass production in view of its miniaturization and cost effectiveness after some further improvement.


Introduction
The alarming increase of industrial pollutants and combustible gas exhausts in the global environment have stimulated the development of selective, reproducible, long life and affordable CO 2 sensors in order to control the green house effects. Moreover, the use of CO 2 sensors in the fields of agriculture, automobile, air conditioning, chemical processing, and concrete industries have been increasing. Electrochemical sensors, which use the effect of the concentration on the equilibrium of the redox reactions occurring at the electrode-electrolyte interface in an electrochemical cell, are considered to be the most promising ones for the low level detection and monitoring of environmental pollutants such as NO x , SO x , HC and CO 2 . According to Weppner, the potentiometric sensors in which the sensing electrode reaction converts the target gas to the mobile ion or the immobile ion or neither of them of the solid electrolyte are called as type I, II, and III potentiometric sensors [1]. Among the type III sensors, potentiometric CO 2 sensor combining a solid electrolyte with an auxiliary phase of carbonate is eminently suited for practical applications [2]. Li + ion conducting electrolyte [3] and LIPON [4] combined with a Li 2 CO 3 auxiliary phase have been tried in view of their promising ability to use even in humid conditions due to the less reactivity with water [5].
A typical potentiometric sensor is made up of three components such as an ionic conductor, a sensing or auxiliary phase and a reference electrode material. Most of the potentiometric sensors are being fabricated by bulk and stack type. Many researchers have been tried to use thick film technology in electrolyte fabrication [6,7]. Some researchers have also tried to make electrolyte, and sensing materials using thin film process [8,9]. However, some of the qualitative factors of these sensors are still need to be improved further.
In this experiment, we have adopted both thin and thick film processes for the development of a miniaturized, long life and cost effective CO 2 sensor from a commercial point of view. The properties of the thin film electrolyte were studied with the variation of thickness, and operating temperature. The thin film electrolyte of Li 3 PO 4 for potentiometric sensor may be the first attempt and result.

Experimental
Planar type CO 2 sensors were fabricated by using thick film process for sensing and reference materials and thin film process for solid electrolytes. Li 3 PO 4 (Aldrich, 99.9%) thin films of 300 nm, 650 nm and 1.2 µm thickness were deposited on Al 2 O 3 substrate by thermal evaporation. During deposition, the applied chamber pressure was 10 -6 Torr and power was 10 A. The as-deposited thin films were sintered at 700 o C for 2 hr in air. Two Au electrodes were attached to the electrolyte thin films each at 2 mm apart and sintered at 700 o C for 1 hr to make good contact with the electrolyte. Li 2 TiO 3 (Aldrich, 99.99%) and TiO 2 (Aldrich, 99.9%) were mixed with an organic binder and grinded in a three roll mixer. The paste was screen printed of about 10 µm thicknesses on Au electrode as reference material and sintered at 700 o C for 1 hr. In the similar way, Li 2 CO 3 (Alfa Aesar 99.99%) paste was prepared and screen printed as sensing material and sintered at 600 o C for 1 hr. The schematic diagram of the fabricated sensor is shown in Fig.1. The overall dimensions of the fabricated sensor was 3mm x 3mm. Li 3 PO 4 electrolyte was deposited over the whole area, though, the dimensions of the screen printed sensing and reference materials were 1mm x 1.5 mm. The surface morphology and the cross section of the Li 3 PO 4 thin films were observed by FE-SEM (Hitachi S-4700). The electromotive force, emf, of the sensor was measured by a two probe HP34401A multimeter of high impedance (above 10GΩ) connected to a computer through HP 34812A Benchlink software for data acquisition.

Results and discussion
In this work, the studied electrochemical sensor structure was  (3) and (4) Li 2 CO 3 ↔ 2Li + + CO 2 + 1/2O 2 + 2e (Sensor) 2Li + + TiO 2 + 1/2O 2 + 2e ↔ Li 2 TiO 3 (Reference) And the overall reaction for the open cell can be expressed as, Thus, the oxygen partial pressure or chemical potential of the oxygen is same at both the electrodes and cancels in the overall cell reaction causing oxygen independency and hence the sensor depends only on the CO 2 partial pressure.
The SEM micrographs of the surface morphology of Li 3 PO 4 electrolyte thin films are shown in   Fig. 2(a). This suggests that a minimum thickness of electrolyte is needed to obtain a good emf value.  Table.1. The emf values were increased with increasing electrolyte thickness and operating temperatures.  This can be attributed to the effect of low kinetic energies produced between each electrode and the electrolyte and reaching a non-equilibrium state [4]. It is found that in this temperature region the response and recovery of the sensor is good for all the investigated CO 2 partial pressures.  exhibits good response but some irreversible recovery. Fig. 5 (b) shows the ∆emf/dec as a function of CO 2 concentration. A Nernst's slope of -61mV/decade has been obtained between 250 to 5000 ppm of CO 2 concentration at 500 o C temperature. It was observed for three months a satisfactory performance of the proposed sensor. However, sensing characteristic of the sensor can be improved further if the thickness and microstructure are optimized by the deposition process and sintering temperatures. The investigations will be reported elsewhere as soon as possible.

Conclusions
Li 3 PO 4 thin films with thickness 300nm, 650nm and 1.2µm were deposited on Al 2 O 3 substrate as Li + ion conducting electrolytes. Planar type CO 2 potentiometric sensors were fabricated using Li 2 CO 3 as an auxiliary phase and a mixed phase of Li 2 TiO 3 and TiO 2 as the reference electrode. The sensor with 1.2µm thickness electrolyte and sintered at 700 o C has shown good response and recovery characteristics with an ∆emf/dec value of -61mV/dec. It is observed that below 460 o C operating temperature the ∆emf values are deviated from the theoretical values and above 460 o C operating temperatures they are approaching 80% of the theoretically calculated values all over the CO 2 concentrations. The emf of the sensor was increased with increasing electrolyte thin film thickness. We suggest a possibility to make a miniaturized potentiometric CO 2 sensor using Li + ion conducting Li 3 PO 4 thin film electrolyte.