J. Phys. Chem. 1993,97, 8839-8841
8839
Quantitative Analysis of External Magnetic Field Effects on the H2-02 Reaction on the Sn02 . Surface Hisao Ohnishi,* Hirokazu Sasaki, and Masamichi Ippommatsu Fundamental Research Laboratories, Osaka Gas Co., Ltd., 6- 19-9, Torishima, Konohana- ku, Osaka 554, Japan Received: April 28, 1993; In Final Form: June 30, 1993
@
In a steady state, the dependence of an external magnetic field effect on the H2-02 reaction on the SnO2 surface upon field direction, field intensity, temperatures, and hydrogen concentration was measured. Measurements made of the dependence upon the magnetic field direction confirmed that the magnetic field effects resulted not from changes in the mobility of the carrier electrons but from the increased rate of the reaction between hydrogen and surface-adsorbed oxygen. Magnetic field effects were found to be higher in the 573-623 K temperature range than in the 673-773 K range. The dependence of the magnetic field effects upon the field intensity and hydrogen concentration could be described by an equation in which a magnetic field effect term was introduced into the H2-02 reaction term: da/dt = (a(1 kB2)2PG+ b)/a - ccr.
+
Introduction
A2-
r
-in flammable gas - I I I I
The mechanisms of electric conduction and surface reaction on SnO2 have been resolved to elucidate the sensing mechanism of SnO2 gas sensors. In atmospheres of pure air or air containing flammablegases, the electricconductivityof SnO2 was determined by carrier electron density alone, and the mobility was not affected by the concentration of flammable gas in air1
p
= constant
(2)
where u is the electricconductivity,n is the carrier electron density, and p is the mobility. In the atmosphere of air containing H2 at 623-773 K, the carrier electron density is determined by the balance between the rate of the reaction producing electrons (reactions A and B in Figure 1) and the rate of the reaction donsuming electrons (reaction C in Figure 1).2 The balance equation of carrier electrons in SnO2 is described by the following relationship: (3) From eqs 1 and 2 and the kinetic equations for reactions A-C, we can obtain the following relationship: (4)
where PG is H2 concentration, t is the time, and a, b, and c are constants proportional to the kinetic constants of reactions A-C. In the steady state (du/dt = 0), eq 4 can be rewritten as
We have reported on the first findings regarding external magnetic field effects on the H2-02 reaction on an SnO2 surface by the measurements of the electric conductivity of SnO2 thin films.3 We had proposed that the increase in the electrical conductivity of tin oxide thin film resulted from an increase in V,. Moreover, we measured the transient response for the conductivity of SnO2 thin film to fluctuations in the magnetic field inten~ity.~The response had a lag of several seconds; therefore these results suggest that this phenomenon is based on ~~
* To whom correspondence should be addressed. Abstract published in Aduance ACS Abstracts, August 15, 1993.
0022-3654/93/2097-8839$04.00/0
-
(
- - - - - - - - - - - - -' 1 : Surface Adsorption Site, '+ ; 0 2 q:Rate-determinimgStep ) L
Figure 1. Reaction model on the SnOz surface.
the reaction. We were able to describe the transient response properties of external magnetic field effects by applying a simple magnetic field term to the H2-02 reaction term in eq 4. The magnetic field term is expressed by (1 + kB2)2 and multiplied by the kinetic constant for the reaction of H2 and adsorbed 0 2 - on the Sn02 surface: d a-- a(l + kB2)2PG+ b - cu -
dt b where B is the magnetic field strength and k is a magnetic field effect coefficient. Although we have reported that the rate of increase in the conductivity was independent of H2 concentration in the results of ref 3, the quantitative explanation for this was not clear. The study of dependenceon field direction,temperature, and hydrogen concentration provides important information for elucidating the mechanism of the magnetic field effects. In this paper, we analyze the dependence of the magnetic field effects on field direction, field intensity, temperature, and hydrogen concentration in order to analyze detailed quantitative properties of the magnetic field effects. We then analyze the degree of conformity between the experimental results and eq 6. Experimental Section SnO2 thin film was prepared in the form shown in Figure 2 by the reactive R F magnetron sputtering technique. As the sput0 1993 American Chemical Society
Letters
8840 The Journal of Physical Chemistry, Vol. 97, No. 35, 1993 61
1
SnO2 film
Pt electrode \
/
Figure 2. Sample arrangement for conductivity measurement.
TABLE I: Dependence of the Magnetic Field Effect on Field Direction (573 K, 0.359%Hz in Air, B = A5 T) I vs B vs surface I l B I I S (BY, 1x1 I I I B I I S (BY, 0 ) I l B l S (Bz, Iy)
a0
gB
1.2877 1.3134 1.3283
1.3222 1.3486 1.3630
Aclco 2.679 2.682 2.612
tering targets, sintered SnO2 was used for sample I and metallic Sn was used for sample 11. A magnetic field was applied to the sample as shown in Figure 2, and its intensity was varied between -5 and +5 T. Electrical conductivity was measured by the same method as that described in the previous reports3 Results and Discussion Dependence on the Direction of the Magnetic Field. The following potential factors causing changes in electrical conductivity on the application of a magnetic field to tin oxide thin film could be considered: (1) changes in the mobility of the carrier electrons (magnetoresistanceeffect, Hall effect, etc.);(2) changes in the oxygen adsorption rate (Vc);(3) changes in the thermal oxygen desorption rate (VB);(4) changes in the oxygen-hydrogen reaction rate ( VA);(5) changes in the water adsorption. Factors 2,3, and 5 can be ruled out on the basis of the results of the measurements of gas atmosphere dependence discussed previously.3 In our previous rep0rts,39~we had applied the field perpendicularly to the surface of the tin oxide thin film. Moreover, since the electrical conductivity increased with the application of an external magnetic field regardless of the polarity of the field, it was also clearly possible to rule out the magnetoresistance effect and the Hall effect as causes of the phenomenon. In order to ascertain whether or not changes in carrier electron mobility could also be ruled out, we measured the dependence of the phenomenon on the direction of the magnetic field. Table I shows the results for sample I obtained at 573 K in an air atmosphere containing 0.359% H2 by applying +5 or -5 T. Here, I is the direction of the current, B is the direction of the magnetic field, andS is the surfaceof the thin film. Measurements of I IB // S, I // B // S, and I I B I S agreed well on the rate of increase in conductivity upon application of magnetic field. In the presence of By, since the current Ix can induce the Hall effect, the carrier electron tends to rise or sink from the surface, which may affect the reaction. However, because the rate of increase in conductivity did not depend on the polarity of magnetic field and current, the contribution of the Hall effect toward the change in conductivitywas eliminated. These results appear to suggest that the magnetic field effects do not depend in any way on the orientation of the field. Similar results were obtained in air containing 0.05-1.0 vol % H2 by applying +5 or -5 T at 573-773 K. Phenomena that involve changes in the mobility of carrier electrons include not only the magnetoresistance effect and the Hall effect but also, in special cases, the magnetostriction effect or the coupling of conduction electron spin and magnetic field. However, these phenomena are found specificallyin ferromagnetic substances, and the effects should differ between I I B and I //
10
0
20
30
Square of Magnetic field intensity / T2 Figure 3. Dependence of increase rate in conductivity on magnetic field intensity and temperature (sample I). I
B the results of these measurements suggested that the present phenomenon was somethingcompletelydifferent. It was therefore concludedthat “changesin the mobility of carrier electrons”played no part whatsoever in the generation of changes in electrical conductivity, and this seemed in turn to provide further strong support for the theory that the observed changes in electrical conductivity were due to changes in the reaction rate between hydrogen and surface-adsorbed oxygen. Dependence on Magnetic Field Intensity and Temperature. Figure 3 illustrates the dependence of the rate of increase of electrical conductivity of sample I on magnetic field intensity at temperatures between 573 and 773 K. Clearly, the relationship is always expressed by Aa/ao = kB2 (7) at any temperature. Temperature dependence indicated that the effect was higher at 573-623 K than at 673-773 K. The rate of increase in conductivity reached 5.593% with application of 5 T at 623 K. It is known from temperature programmed desorption (TPD) spectroscopy that a considerable amount of surface-adsorbed oxygen desorbed at temperatures up to 673 K.5 It is possible that the magnitude of the magnetic field effectsis related to the amount of surface-adsorbed oxygen species. Dependenceon HydrogenConcentration. In experimentsusing sample I (film made from the tin oxide target), the changes in electrical conductivity induced by the magnetic field were found to be only marginally dependent on hydrogen concentration in a range between 0.10 and 0.68 vol % at 723 K, as shown as squares in Figure 4. However, for sample I1 (film made from metallic tin target), it was found that the magnetic field effect was dependent on the concentration of hydrogen in the atmosphere, as shown as squares in Figure 5. By solving eqs 4 and 6 under the steady-state condition (daldt = 0), we obtain gg
= [(aPG(l go =
+ kB2)2+ b ) / ~ ] ’ / ~
[(UP,
+b)/~]’/~
(9)
and from these
go go The difference between samples I and I1 can be explained as the difference between the a / b of the two different samples. Since a and b were constants proportional to the kinetic constant of
The Journal of Physical Chemistry, Vol. 97, No.35, 1993 8841
Letters 2.5
contrast, the value of b is too large to be ignored and it must be assumed that there was an effect of term b. The relationship between POand AU/UO was expressed well by a curve calculated from eq 10 in Figure 5. Here, k = 6.33 X lW(T-2) and o / b = 8.33 X lo2 were used for the curve fitting. Although the theoretical details of the mechanism for these magnetic field effects have not been clarified yet, the field dependence of the reaction rate, VA 1 kB2, indicates the possibility that the magneticfield driva thesinglet-triplet mixing in radical pairs that can be induced by the Zeeman mechanism under a gradient magnetic field. In the Zeeman mechanism, the singlet-triplet conversion rate is proportional to the square of the magnetic field intensity.61’ The hyperfine mechanism can be excluded because the magnetic field effect was not saturated up to very high fields.
I
. 8
2.0
$18
Q:
+
Concl~ioas
0.0 0
0.2
0.4
0.8
0.6
1.0
1.2
H I concentration / vol.%
Figure4. Dependence of magnetic field effect on hydrogen concentration (sample I, 723 K).
1.4
. 8
1.2
Measurements made of the dependence on the orientation of the magnetic field confirmed that the magnetic field effects resulted not from changes in the mobility of carrier electrons but from the increased velocity of the reaction between surfaceadsorbed oxygen and hydrogen. Magnetic field effects were found to be higher in the 573-623 K temperature range than in the 673-773 K range. We then analyzed thedependence of magnetic field effects on field intensity and hydrogen concentration. These results were described well by the equation
$16
p-
1.0
c.
V
a
0.8
derived from W
Measured value
-
Calculated value
o.2 0.0
I
d 0
0.2
0.4
0.6
0.8
1.0
I
1.2
H I concentration I vol.% Figure 5. Dependence of magnetic field effect on hydrogen concentration (sample 11, 723 K).
reactionsA and B, the ratio of a / b represents the ratio of activity of reaction A to that of reaction B. In sample I, the value of b is extremely small, meaning that, in a range of H2 concentrations between 0.1 and 1.0 vol %, we can safely ignore the term b, and this in turn suggests that the effect is not dependent on gas concentration. The relationship between PO and Au/uo was expressed well by a curve calculated from eq 10 in Figure 4. Here, k = 9.17 X 10-4(T-2) and a / b = 3.38 X 104 were used for the curve fitting. In sample 11, by
in which a magnetic field effect term was introduced into the H&2 reaction term. The dependence on H2 concentration was very different for samples made from targets of tin oxide and metallic tin. These differences could be explained as the difference in the ratio of activity of reaction A to the activity of reaction B in the two samples.
References iad Notes (1) Ippommatsu, M.;Ohnkhi. H.; Smki, H.; Matsumoto, T.J. Appl. Phys. 1991,69,8368. (2) Ohnirhi, H.; Sluaki. H.; Matsumoto. T.; Ipwmmatsu. M.Sensors __ and Actuators B 1993, 14, 677.
(3) Saki, H.; Ohnishi, H.; Ippommatsu, M.J. Phys. Chcm. 1990.94, 428 1. (4) Ohnishi, H.;Smki, H.; Ippommatsu,M.;Marteau, S.J. Phys. Chcm. 1992, %, 312. ( 5 ) Yamazoe, N.;Fuchigami, J.; Kishikawa, M.;Sciyama, T. Sur$ Sci. 1979,86,335. (6) Hayashi, H.; Nagakura, S. Bull. Chcm. Soc. Jpn. 1978, 51,2862. (7) Sakapchi, Y.; Hayashi, H. J. Phys. Chcm. 1984,88, 1437.
