Enhancement of H2-sensing Properties of F-doped SnO2 Sensor by Surface Modification with SiO2

Effects of surface chemical modification with sodium silicate on the gas-sensing properties of F-doped SnO2 gas sensor designed and fabricated employing micro-electro mechanical system (MEMS) technology were investigated. Gas sensing properties of the sensor were checked against combustible gases like H2, CO, CH4 and C3H8 at a heater voltage of 0.7 V. The H2 sensitivity of the surface modified F-doped SnO2 micro sensor markedly increased and reached S = 175 which was found to be about 40 times more than that of unmodified sensor (S = ∼ 4.2). The increase in the sensitivity is discussed in terms of increased resistivity and reduced permeation of gaseous oxygen into the underlying sensing layer due to the surface modification of the sensor. The present micro-hydrogen sensor with enhanced sensitivity due to SiO2 incorporation is a low energy consuming portable sensor module that can be mass-produced using MEMS technology at low cost.


Introduction
The demand for a hydrogen sensor with high sensitivity, fast regeneration, and an even faster response time is gaining momentum as efforts to develop a hydrogen economy continue to grow [1][2]. Large numbers of companies and organizations such as NASA and DOE, that use large quantities of hydrogen and oversee the development of the technology, have outlined a detailed performance criterion for an acceptable hydrogen sensor.
Tin oxide is the most used n-type semiconductor in gas sensing device because of its capabilities to detect combustible gases like CH 4 , C 2 H 5 OH, CO, H 2 and so on [3][4][5][6][7]. This SnO2 based sensor is generally operated at 250°C-500°C in air. SnO 2 adsorbs oxygen from the atmosphere leading to an electron depleted zone and the reducing gases react with the adsorbed oxygen increasing thereby the electronic concentration in the material resulting in decrease of the electrical resistance. The electrical resistance of SnO 2 based sensors is controlled by a potential barrier mechanism. Due to the consumption of negatively charged oxygen adsorbates by inflammable gases, decrease in resistance is resulted because the potential barrier height decreases due to back transfer of electrons trapped by oxygen adsorbates into the outer regions of the SnO 2 particles [8][9]. Recently, room temperature operating SnO 2 based sensors were also reported. Shukla et al have reported about the hydrogen discriminating nanocrystalline doped tin oxide room temperature MEMS sensor [10] and investigated effect of air pressure, ultra violet radiation exposure as well as inverse catalytic effect at lower operating temperatures on the sensing characteristics of the sensors [11][12]. Wei et al have reported about a SnO 2 gas sensor doped with carbon nanotubes operating room temperature [13].
Previously, we have reported about synthesis of nano-crystalline F-doped SnO 2 and its application to micro gas sensor [14]. This micro gas sensor has shown higher sensitivity and better selectivity for the hydrogen gas in comparison to commercially available SnO 2 . However, the sensitivity to 100ppm hydrogen gas at best heater voltage has been found to be as low as 2.8. The low sensitivity stems from the low resistivity of the F-doped SnO 2 ( ~ 1.8 Ω•cm). There are various techniques to modify the sensing properties of the gas sensors. One critical approach is to modify the metal oxide surface by using catalyst layer [15] or gas filter layer [16 -18]. Wada and Egashira have reported that electrical resistance in air of SnO 2 sensors increased straightly with the amount of the incorporated SiO 2 . A significant increase in sensitivity to hydrogen has also been observed with the repeated incorporation of SiO 2 on the SnO 2 surface [19].
The aim of the present study is to evaluate the effect of surface modification of F-doped SnO 2 sensor on its hydrogen sensing sensitivity. The present micro-hydrogen sensor with enhanced sensitivity due to SiO 2 incorporation is a low energy consuming portable sensor module that can be mass-produced using MEMS technology at low cost.

Preparation of sample
The F-doped SnO 2 was synthesized by firing of fluoro(2-methylbutan-2-oxy)di(pentan-2,4dionato)tin(II) at 550 o C for 30min. The sol-gel precursor fluoro(2-methylbutan-2-oxy)di(pentan-2,4dionato)tin(II) was prepared by the procedure described elsewhere [20]. Figure 1 shows the surface chemical modification procedure with sodium silicate. Sodium silicate was used to incorporate SiO 2 for the surface modification of F-doped SnO 2 . In the mixture solution of 0.4M boric acid and 0.4M KCl , 0.4M NaOH solution was added to attain a particular value of pH (between 9 ~ 10). To this 50 ml mixture solution, 3g F-doped SnO 2 was dispersed with constant stirring. Then 10% sodium silicate solution was added drop-wise to the buffer solution of constant pH value. The solution was further stirred for an hour after the addition of sodium silicate. The solid sample was recovered by decantation and repeatedly washed with doubly distilled water to remove the electrolytes. Finally it was washed with methanol. The sample was dried at 60°C and then heated at 600°C for 1 hour in the furnace.

Characterization of the modified F-doped SnO 2
The morphology of the surface modified SnO 2 was investigated by a scanning electron microscope (SEM, Philips XL-30) and an energy dispersive X-ray spectrometer (EDAX, PV99). Crystallinity and phase homogeneity of the samples were observed by powder X-ray diffraction using Philips PW 1280 diffractometer.

Fabrication of Sensor
Surface modified F-doped SnO 2 micro-sensor with SiO 2 as well as unmodified F-doped SnO 2 micro sensor was fabricated on the silicon-based substrate with Pt electrode and heater. The fabrication of the silicon-based chip is described elsewhere [14]. Nano-crystalline F-doped SnO 2 micro-sensor was fabricated on the silicon-based substrate with Pt electrode and heater. The fabrication of the siliconbased chip is made in the following steps; (a) thermal oxidation of the surface for insulating layer of SiO 2 formation, (b) pre-patterning for the cavity by wet etching with HF, (c) Cr deposition followed by Au deposition by thermal evaporation in a chamber, (d) Photo-resistive film formation by hexamethyldisilazane (HMDS) coating with spin coater (TOK, TELR N101 PM), (e) patterning for the heater and the sensing electrode (Pt electrode), (f) removing of photo-resistive film, (g) wet etching using HF for cavity making and the (h) drop coating as a micro-bead with the mixed solution of nano-

Temperature of sensor
Changes in heater resistance were monitored when a linearly increasing current was applied to the heaters. The resistance was converted to sensor temperature according to the well-known equation [21].
Where R T1 is the resistance at the initial temperature, R T2 is the resistance at the final temperature, α is the temperature coefficient (+0.00377/ o C), T 1 is the initial temperature, T 2 is the final temperature.

Gas sensing test
Gas sensing properties were evaluated in a gas chamber. The experimental setup for the gas sensitivity measurements was the same as those used in our previous report [14]. The sensing activity was tested by keeping the test chamber at room temperature. In the beginning, pure nitrogen gas was passed through the test chamber to remove any residual gas or water vapors then fresh air was passed and maintained till constant sensor resistance was obtained as R a . The test gas was then admitted along with air and R g was measured, till a constant value of R g was obtained. Sample gases containing CO, CH 4 , C 3 H 8 , H 2 were mixed by manual gas blender and led to the measuring chamber. (size: 250 mm × 250 mm × 160 mm). In the previous work, maximum hydrogen sensitivity of the microsensor was at 0.7 V [14]. Hence heater voltage of each sensor was maintained at the voltage of 0.7 V. The electrical resistances of each sensor element in air (R a ) and in a gas (R g ) were measured to evaluate gas sensitivity. Gas sensitivity was defined as the ratio (S= R a /R g ) of electrical resistance of sensor in air (R a ) to electrical resistance of sensor in a particular sample gas (R g ).

Results and discussion
Microstructure of the sensor material Figure 2 shows the SEM photographs of the F-doped SnO 2 and F-doped SnO 2 modified with SiO 2 . It was observed that F-doped SnO 2 consisted of nanoparticle in the range of ~ 15 nm to ~ 30 nm, indicating an almost uniform size distribution (Figure 2a). After surface chemical treatment with sodium silicate, network slightly aggregated and particle size increased to ~ 45 nm (Figure 2b), indicating deposition of SiO 2 on the surface of F-doped SnO 2 . Figure 3 shows the SEM and EDAX results of the F-doped SnO 2 modified with SiO 2 . As shown in Figure 3a   Surface modified F-doped SnO 2 as well as unmodified F-doped SnO 2 were also characterized by XRD technique (Figure 4). The F-doped SnO 2 had a cassiterite structure without any impurity (JCPDS 41-1445). The nanocrystalline nature of the F-doped SnO 2 and F-doped SnO 2 modified with SiO 2 was confirmed by the broad peaks in the XRD pattern. Decreased intensity of the surface modified F-doped SnO 2 can be explained by the scattering of the X-ray by SiO 2 thin layer on F-doped SnO 2 .

Gas sensing property
Hydrogen has a much wider range of flammability in air 4% to 75% by volume and the lowest limit of hydrogen concentration in air to explosion is 4.65% [22]. These characteristics would tend to indicate that flammability is a greater risk for hydrogen than for other fuels. In many accidental situations the lower flammable limit (LFL) is more important. The LFL for hydrogen is similar to that of methane, about twice that of propane, and four times that of gasoline. So, the development of a sensor with high performance should be to detect from ppm level to a few percentage of the gas in air. In this investigation, the detection of low concentration of hydrogen (ppm level) was kept as a target. Concentrations ranging from 100 to 600 ppm of H 2 , CH 4 , C 3 H 8 , and CO were injected in the test chamber and the sensing signals were recorded. The sensor was operated at 0.7 V. This heater voltage was selected because the selectivity and sensitivity for the hydrogen was the best at this voltage [14]. The calculated heater temperature at this voltage was 320 o C as shown in Figure 5.
Sensitivities of the micro-sensor made with nano-crystalline F-doped SnO 2 as well as with SiO 2 modified F-doped SnO 2 against various reducing gases were recorded. As shown in the Figure 6a, the surface modified F-doped SnO 2 showed very high sensitivity for H 2 , on introduction of 600ppm hydrogen gas (S = 175). On the other hand, the sensitivities to C 3 H 8 , CO, and CH 4 were relatively low(S = 1-3), as seen in Figure 6a. It is obvious that incorporation of SiO 2 is very much effective for the enhancement of the sensitivity of sensor to the hydrogen. But the sensitivity to C 3 H 8 , CO, and CH 4 increased slightly. This may be due to the difference in molecular size between H 2 and these gases. Small H 2 molecules can go through the aggregated SiO 2 particles to react with negatively charged oxygen adsorbates on the surface of the sensor, while large C 3 H 8 , CO, and CH 4 molecules are prevented from reaching the surface of F-doped SnO 2 by SiO 2 particles. Similar behavior in the gassensing properties of the sensor after surface modification was observed and explained on the basis of difference in molecular size and decreased oxidation activities by Wada and Egashira [19].

Comparison of the sensors made with surface modified F-doped SnO 2 and unmodified F-doped SnO 2
F-doped SnO 2 has shown higher sensitivity to hydrogen gas in comparison to the commercially available SnO 2 sensor [14]. The higher sensitivity and selectivity of the nano-crystalline F-doped SnO 2 for hydrogen gas may be due to the greater adsorption of hydrogen molecules on the favorable sites at fluorine atoms of the materials or the increased n-type property of SnO 2 by fluorine doping. Higher sensitivity shown by nano-crystalline F-doped SnO 2 is also because of its higher surface area. Similar results have been obtained on SnO 2 nano-crystalline H 2 gas sensors for their sensitivities by Seal and Shukla [23] and Gong et al. [24]. However, the sensitivity to 600ppm hydrogen gas at best heater voltage was ~ 4.2 ( Figure 6b). The H 2 sensitivity of the surface modified F-doped SnO 2 micro sensor markedly increased and reached S = 175 on introduction of 600ppm hydrogen (Figure 6a). Although the sensitivity of the present device to hydrogen is not very high but still the value of sensitivity of this surface modified F-doped SnO 2 sensor to hydrogen, is about 40 times more than that of unmodified sensor. This might be explained by the increased resistivity of the F-doped SnO 2 due to the surface modification with SiO 2 . It has been observed in our previous report that F-doped SnO 2 had good selectivity to H 2 but its sensitivity was low because of the reduced resistivity [14]. Wada and Egashira have reported that electrical resistance of the SnO 2 sensor in air increased straightly with the amount of the incorporated SiO 2 resulting in the increase of H 2 sensitivity [19]. So, in this case also, incorporated SiO 2 on the surface of F-doped SnO 2 is responsible for increase in electrical resistance of the sensor, resulting in the remarkable increase of its sensitivity to hydrogen. As SiO 2 particles can reduce permeation of gaseous oxygen into the underlying sensing layer and hence to enhance the gas sensitivity due to the suppressed oxygen readsorption. Also, it was found that the hydrogen oxidation activity of SnO 2 was promoted by the incorporation of the SiO 2 component [18]. Anyway, it should be noted that the surface chemical modification with SiO 2 resulted in a remarkable increase in the hydrogen sensitivity of the sensor. The present micro-hydrogen sensor with enhanced sensitivity due to SiO 2 incorporation is a low energy consuming portable sensor module that can be mass-produced using MEMS technology at low cost.

Response and recovery times of the sensor
Response and recovery times are the important parameters of gas sensor. The time taken for the sensor to attain 80% of the maximum change in resistance upon exposure to the gas is the response time. The time taken by the sensor to get back 80% to the original resistance is the recovery time. Figure 7 shows variation in logarithmic resistance of the SiO 2 coated F-doped SnO 2 sensor upon exposure to and removal of 600ppm H 2 at 320 o C. The response time calculated for the sensor was T 80 =58sec. In contrast, the recovery time was very slow and prolonged at 320 o C as seen in Figure 7. Similar behavior of response and recovery time of the surface modified SnO 2 sensors with SiO 2 has been reported by Wada and Egashira [19].

Conclusions
Hydrogen sensitivity of F-doped SnO 2 sensor could be improved by surface chemical modification with sodium silicate treatment. SEM and EDAX observations revealed the existence of fine SiO 2 particles on the surface of the sensor. The maximum hydrogen sensitivity of the sensor was at a heater voltage of 0.7 V. The H 2 sensitivity of the surface modified F-doped SnO 2 micro sensor markedly increased and reached S = 175 which was found to be about 40 times more than that of unmodified sensor (S = ~ 4.2). The increase in the sensitivity is discussed in terms of increased resistivity and reduced permeation of gaseous oxygen into the underlying sensing layer due to the surface modification of the F-doped SnO 2 with SiO 2 . The present micro-hydrogen sensor with enhanced sensitivity due to SiO 2 incorporation is a low energy consuming portable sensor module that can be mass-produced using MEMS technology at low cost.