Temperature-independent TiO2-ZrO2 oxygen lambda sensor 1
A. Lari, 1A. Khodadadi, 2Y. Mortazavi Catalysis and Reaction Engineering Research Laboratory, School of Chemical Engineering, University of Tehran 2 Nanoelectronics Centre of Excellence, University of Tehran Tehran, Iran Email: [email protected]
cubic fluorite or tetragonal structure is of considerable importance in material technology . It is a high temperature refractory with resistance to thermal shock and undergoes transformation toughening. Zirconia also exhibits high oxygen ion conductivity. Among various metal oxide catalysts, the combination of titania-zirconia has attracted much attention in recent years. These mixed oxides have been extensively used as catalysts and catalyst supports for a wide variety of reactions . In addition to catalytic application, these mixed oxides have also been employed for various other purposes such as photoconductive thin films, gas sensors, in fuel cells and ceramic technology . TiO2 is unique for its photocatalytic and strong metal support interaction properties and the ZrO2 is a well known solid acid catalyst. More over, the composite materials often exhibit enhanced mechanical and thermal properties than the two participating components . In this study, we have investigated the sensing properties of Ti-Zr as a solid reference for the YSZ sensor in order to detecting O2 in the gas phase without a reference. One of the obstacles to commercialization of the sensors without solid reference is usually their temperature-dependent response. It is shown that the EMF of the sensor developed in this study exhibits a temperature-independent behavior to O2.
Abstract—The electromotive force (EMF) of a new type of YSZ-based planar oxygen sensor exposed to simulated automobile exhaust gas, with no air reference, was measured under different conditions. A sensor with two parallel Pt electrodes, one coated with Ti0.75Zr0.25O2 (Ti-Zr) as a solidstate reference, was fabricated. The performance of this sensor was measured at different temperatures and λs, i.e. the actual air/fuel ratio to that of stoichiometric value. The transition in EMF occurs around λ=1. Also the amplitude of the transition between rich (λ=0.8) and lean (λ=1.2) measured at a temperature range of 350-650oC remains almost constant, indicating a temperature-independent performance.
Transportation is one of the most important sources of air pollution, especially in urban areas. Therefore, strict regulations on pollution control are being enforced worldwide. Oxygen sensors in automotive applications are used to measure the air fuel (A/F) ratio of engine exhaust gases and to control the optimum A/F ratio in order to have an efficient treatment of the exhaust gases by catalytic converters. Conventional oxygen sensors, used in automobiles, should be exposed to air in one side as a reference which requires thorough sealing of the exhaust gas and the air reference. This requirement leads to a relatively large sensor with long response time. One of the main approaches for elimination of air reference is to use solid state reference . Various oxides with ionic or mixed ionic and electronic conductivity may in principle be used to design oxygen sensors operating at high temperatures . Titanium oxide usually has high specific surface area with photocatalytic property which makes it suitable for gas sensing application. In reaction involving titania, oxygen vacancies are one of the major advantages of such material . There is a great interest in materials with different combinations to achieve a considerable degree of performances for gas detection. It is clear that dopant incorporation has a major influence on the transport behavior of oxygen ion conduction. ZrO2, stabilized in the
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YSZ powder was prepared by a co-precipitation method. The starting compounds are: zirconyl nitrate, ZrO(NO3)2.6H2O, yttrium nitrate, and ammonium hydroxide. Nitrates are dissolved in deionized water and precipitated with ammonium hydroxide. After filtration, washing and drying, the precipitate is calcined in air at 600°C for 3 h. The powder is then ball milled for 12 h. Dense pellets of YSZ is formed by pressing the powder followed by sintering at 1560°C for 12 h. Ti-Zr catalyst is prepared by a combustion synthesis method. Titanium tetrachloride and zirconyl nitrate are used as titanium and zirconium precursors respectively. Sorbitol, C6H14O6 is used as the fuel. The binary oxide of Ti-Zr is formed according to
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The schematic of the sensor fabricated is shown in Fig. 1. The experimental setup used for measuring sensitivity of the sensors is shown in Fig. 2. The response of the sensors are measured in the presence of the gas mixtures with various air / (CO + C3H8) molar ratios corresponding to the automotive exhaust gases from different air/fuel ratios or λs. C3H8 is a model compound for unburned hydrocarbons in the exhaust gas. The gas mixtures include 6.0 mol% CO, 0.2 mol% C3H8 (in Argon) and air proportional to λ, i.e. 0.8 and 1.2, chosen to stand for rich and lean conditions in auto engines.
Figure 1. Structure of the sensor.
RESULTS AND DISCUTIONS
the following reactions:
Figure 3 presents the powder X-ray diffraction (XRD) spectrum of Ti-Zr binary mixture prepared by the combustion method when sintered at 800oC. The peaks designated with arrows are ZrO2 peaks and the rest are those of titania. The spectra of TiO2 in the figure had the characteristic peaks of the anatase phase of TiO2 and cubictype ZrO2. For this sample, the crystallite size is calculated to be 31 nm. The BET area of the sample is 23 m2/g.
0.75TiCl4 + 0.25 ZrO(NO3)2. 6 H2O + 0.096 C6H14O6 → Ti0.75Zr0.25O2 + 0.25 N2 + 0.576 CO2 + 0.673 H2O + 3 HCl Ti-Zr powder synthesized as such was calcined in air at 600oC for 3 h. Crystalline phases of the samples were determined by X-ray diffraction (Philips X’pert), and phase crystallite sizes were calculated from the broadening of XRD peaks, using Scherrer’ equation . Specific surface area of Ti-Zr powders was determined by nitrogen adsorption using BET method with CHEMBET 3000 equipment.
TiO2 may exist in three crystal phases: rutile, anatase, and rookit. At high temperatures anatase is metastable, and easily transformed into rutile which leads to the formation of a binary mixture of both phases. As reported by Liang et al. , the phase transformation of anatase to rutile for TiO2 could be prevented to some extent by introducing a small amount of dopants. It is well known that anatase posses a higher photocatalytic activity as compared to rutile, and thus better gas-sensing properties .
To fabricate the final form of the sensor following steps are performed: 1. Platinum paste is deposited on both sides of the YSZ pellet.
Figure 4 shows the EMF transition between λ=0.8 and λ=1.2 at different temperatures. It is interesting to note that the amplitude of the changes in EMF remains almost constant for the temperature range examined, i.e. 350-650°C, indicating a temperature-independent oxygen sensor.
2. Platinum wires are connected onto both sides of the pellet. 3. The pellet is then sintered at 900°C for 30 minutes. 4. The Ti-Zr powder is mixed with α-terpineol as a binder and then screen printed onto one side of the solid electrolyte pellet. The pellet is dried then at 160°C for 5 h followed by sintering at 800°C for 4h.
In t e n s it y ( a .u .)
According to Wagner's equation , the voltage of a solid electrolyte (SE) galvanic cell for the Nernst's equilibrium
Figure 3. XRD pattern of Ti0.75Zr0.25O2.
Figure 2. Schematic diagram of the experimental setup.
performed by Luerburn et al.  have shown that in vicinity of TPB, the response was quick with large amplitude, whereas at the points far from TPB, the response time was longer and the amplitude was quite small. For the electrochemical measurement, they used the microfabricated Pt electrode on the top side of the YSZ pellet as working electrode and a counter and a reference electrode- both prepared with Pt paste. They employed photoelectron emission electron microscopy, PEEM, to study the process at the three-phase boundary. They observed a bright region after a few seconds, which spread on the electrolyte, starting from the three-phase boundary which finally covered the complete electrolyte surface. They attributed this observation to the oxidation properties of zirconium oxide.
Figure 3. EMF response of the sensor to transition between λ=0.8 and 1.2 at different temperatures. Total gas flow = 130.0 sccm.
When TiO2 is reduced, defects are formed in the form of Ti+3. The Ti+3 have one negative charge compared to the normal lattice. These negative defects are balanced by oxide
cell is calculated from:
vO.. . Diffusion of both oxygen and titanium
cations are detected between 100oC to 400oC but without a significant change in the stoichiometry of the surface. Above 400oC, only titanium cations diffusion from the surface to the bulk is detected and the ratio of oxygen to reduced titanium of surface increases. Therefore, when negative charges increase in bulk, Ti+3 diffuse from bulk to surface and then oxygen of surface is released .
f O//2 RT =− ln / Z i ξF f O2
Where F, Zi and ξ are respectively, the faraday number, the charge number of the mobile ionic charge carrier i, and the number of atoms associated in the standard state. The superscripts ' and " denote the positions of the electrolyte surfaces which are identical with reference and the measuring electrodes, respectively, of the galvanic cell.
In the present paper, we placed platinum paste underneath the Ti-Zr catalyst layer. This combination results in the influence of Pt on the adsorption of carbon monoxide, propane, and oxygen  which in turn leads to the improved activity of the platinum layer. This enhancement in the activity may be attributed to the enhanced O2 dissociation rate at the Ti-Zr/Pt interface. Another effect of Pt paste is probably to increase the redox properties of TiO2, resulting in reducing the amount of O2- produced from Pt paste in another side of the sensor. Oxygen seems to spill over from the Pt surface to the Ti-Zr catalyst decreasing the catalyst oxidation state during the λ=0.8 and spill back to the Pt surface during the λ=1.2.
This type of Wagner's equation (Nernst's equation) is applied when the electrolyte has an infinitely extended width of the ionic domain. If the chemical potential of the mobile ion oxygen is fixed but different on both sides of the solid electrolyte (YSZ), an electric field is produced within the electrolyte, the magnitude of which is related to the difference in chemical potential (or, equivalently in ideal situation, the oxygen partial pressure). If the performance of the sensor tested depends on temperature, equation used for the sensor is different from Nernst's equation due to the presence of electrons and holes which affects conduction of the YSZ electrolyte.
Ti0.75Zr 0.25O2 170 150
In general, the most active reaction area is around the triple phase boundary, TPB, of electrode, electrolyte, and gas phase. This doesn't mean that all surface reaction, mass transport and charge transfer reaction must take place at the narrow area around TPB. If the surface diffusion is fast compared with the surface reaction, the adsorptiondesorption reaction site may expand onto the electrode surface. If the bulk diffusion inside the electrode or the diffusion along the electrode/electrolyte boundary is fast, the oxygen transport path can extend into the electrolyte/electrode boundary . The experiments
Figure 5. EMF changes versus λ.
Figure 6. Conversion versus temperature.
Figure 5, presents the change in EMF when the sensor exposed to a step change between two simulated gases representing rich and lean gases having λ=0.8 and λ=1.2 respectively. As is evident the low-high transition occurs around λ=1 which is similar to the behavior of conventional lambda sensors. In fact, the main transition occurred in the stoichiometric point which means that TiO2 tend to release its lattice oxygens around this point.
Figure 6 present the catalytic performance of Ti-Zr for oxidation of CO and propane at different temperatures. As can be seen the conversions are complete at 500 to 600oC. The presence of vacancies and redox properties of TiO2 facilitate oxidation reaction. It was reported that CO conversion didn't complete up to 700oC over plain TiO2 . It seems that the presence of ZrO2 lead to an increase in the surface area as compared to TiO2 at a given firing temperature , the inhibition of rutile formation , the rise in surface acidity  or the creation of active defects on the TiO2 surface, have been proposed as possible causes for this improvement.
CONCLUSION TiO2-based sensor show a low-high transition response, when exposed to exhaust gas composition changes from rich to lean condition. Doping the TiO2 with ZrO2 enhances the ionic conductivity and conversion of carbon monoxide and propane. The response of the sensor is independent of the temperature changes. As a result, performance of the sensor follows the Nernst's equation. The transition take place in λ=1 (point of stoichiometric A/F) which is indicative of complete conversion of the components of the exhaust gas. REFERENCE 
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