ELECTRON TRANSFER AT SEMICONDUCTOR SURFACES
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Reactions Involving Electron Transfer at Semiconductor Surfaces. 111. Dissociation of Methyl Iodide over Zinc Oxide1 by Joseph Cunningham* and A. L. Penny Department of Chemistry, University College, Cork, Ireland
(Received August 9, 1971)
Publication costs borne completely by The Journal of Physical Chemistry
Gaseous methyl iodide has been observed to undergo dissociation when contacted with semiconducting zinc oxide at 20' in the absence of any illumination. Additional dissociation of methyl iodide was observed at the CHJ-ZnO interface under illumination by ultraviolet light of wavelengths longer than 290 nm. The gas-phase product of the dissociation in the presence and absence of illumination was methane. At room temperature in the dark, adsorption obeyed a Langmuir type isotherm and monolayer coverage was closely approached at all methyl iodide pressures >5 x 10+ Torr. Conductivity and esr studies of zinc oxide layers showed initial electron localization by adsorbed methyl iodide but also revealed a secondary process which slowly returned electrons to the conduction band. Parallel studies on kinetics of methane formation at CDJ-ZnO interfaces revealed marked differences between ZnO surfaces preactivated at 350 or 500'. Samples baked in vacuo a t 350' had lower dark conductivity and CD8H represented >50% of methanes produced in the initial 30 min dark reaction, thus implying hydrogen transfer reactions with residual surface hydroxyls. Kinetics of initial methane production, or of decrease in conductivity, were consistent with electron localization being rate determining at these hydroxylated surfaces, followed by fast reaction with surface hydroxyls. Samples baked in vacuo to 500' prior to CDaI adsorption had higher conductivity and CDI was the major methane product. Kinetics analysis indicated that electron localization was fast at these dehydroxylated surfaces and that reaction of CD31-(ads) with CDaI(ads) was rate determining. The isotopic composition of additional methane produced when CDJ-ZnO interfaces were illuminated with photons inside the ZnO band edge, resembled the composition of methanes formed in dark reaction a t the same surface. The apparent initial quantum efficiency of the photoassisted methane formation was ca. 3 x 10-6, but kinetics of this process, like kinetics of the slow secondary dark reaction, were complex and the mechanisms are not yet fully understood.
Introduction Investigations carried out by the authorsa on the transfer and sharing of electrons between n-type semiconducting oxides and nitrous oxide adsorbed on their surfaces at room temperature revealed dissociation of the nitrous oxide to a very limited extent in the dark and additional dissociation under uv illumination. In explanation of these phenomena it was proposed that nitrous oxide, a well known electron-attaching agent in liquida or gaseous s y ~ t e m s ,decomposed ~,~ by dissociative electron attachment when contracted with the surfaces of n-type zinc oxide and ferric oxide. This paper presents the results of the continuation of these investigations employing methyl iodide as an adsorbate and zinc oxide as an adsorbent, Methyl iodide has been widely used as an electron attaching additive in g a s e o ~ sliquidJ8 , ~ ~ ~ or heterogeneous system^.^,^^ It is generally considered that electron attachment in the gas phase is accompanied by dissociation according t o CHaI(g)
+ e(g)
-
CHJ-(g)
-+
CHdg)
+ 1-k)
(la)
Mass spectrometric studies showed that electrons at thermal energies were sufficient t o effect dissociation via (la), but that an electron energy threshold of 2 eV
was required t o effect the analogous process with N,O(g). Since nitrous oxide was already shown t o undergo dissociative electron attachment at dark and uv-illuminated zinc oxide surfaces,2 the reported lower threshold for dissociative electron attachment t o methyl iodide in the gas phase7 made it appear probable that methyl iodide would undergo the analogous process (lb) on zinc oxide, i.e. CH31(ads)
+ e(Zn0) + CH31-(ads)
4CH8(ads)
+ I-(ads)
(lb)
(1) This work is supported in part by the U. S. Air Force Office of Scientific Research through the European Office of Aerospace Research, OAR, United States Air Force, under Contract AF 61(052)67C-0044. (2) . , (a) , , J. Cunnineham. J. J. Kellv. and A. L. Pennv. J . Phus. Chem.. 74, 1992 (1970);( b ) J. Cunningham, J. J. Kelly, knd A. t.Penny; ibid., 75, 617 (1971). (3) G. Scholes and M.Simic, Nature (London), 202, 895 (1964). (4) A. V. Phelps and R. E. Voshall, J. Chem. Phys., 49, 3246 (1968). (5) J. M.Warman, iVature (London), 213, 381 (1967). (6) G. Jacobs and A. Henglen, 2.Naturforsch., 19a,906 (1964). (7) V. H. Dibeler and R . M. Reese, J . Res. Nat. Bur. Stand., 54, 127 (1955). (8) P.R. Geissler and J. E . Willard, J . Amer. Chem. SOC.,84, 4627 (1962). (9) R . F. Claridge and J. E. Willard, ibid., 87, 4992 (1965). (10) N. H.Sagert, J. A. Reid, and R. W. Robinson, Can. J . Chem., 48(1), 17 (1970). The Journal of Physical Chemistry, Vol. 76, No. 17, 1979
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At dark CHJ-ZnO interfaces, process l b is only probable when the symbol e(Zn0) in (lb) corresponds to electrons with equilibrium Boltzmann distribution between conduction band and shallow donor levels. The reduced energy of electrons in the valence band or of electrons trapped at deep electron traps (depth >> h!!'), renders involvement of such electrons in (lb) energetically improbable at the dark interface. Illuminating a CHJ-ZnO interface with photons of energies greater than the band gap of ZnO, 3.2 eV (photons of X 300 nmll allows these additional photo-induced possibilities for charge transfer to be explored at such wavelengths without significant direct excitation of methyl iodide. Results reported very recently of infrared studies on high surface area zinc oxides established that extensive hydroxylation of the surface persists after preactivation a t 350" but that above this "critical" temperature rapid loss of surface hydroxyls occurs.12 The possible role of surface hydroxyls in bringing about reaction at dark or illuminated methyl iodide-Zn0 interfaces may therefore be examined in the present study by comparison of results obtained over zinc oxide samples preactivated a t 350" with results over surfaces preactivated at higher temperatures. Mass spectrometric analysis of product(s) from CD31-ZnO interfaces is employed in this study to distinguish clearly between products arising via hydrogen-transfer reactions with surface hydroxyls (e.g., CD3H) and those arising via electron attachment (e.g., CDq or CZD,).
Experimental Section Materials. Details of the origin, surface area, and electron concentration of the zinc oxide have been given previously, together with a description of the activation treatment of the zinc oxide.2 One important practical difference between the K;2O-ZnO and CH31-ZnO systems was that exposure of the zinc oxide to methyl iodide poisoned the activity of the oxide, which could not be regenerated by heating under vacuum. Zinc oxide samples were therefore always discarded after conducting a measurement with methyl iodide. Details of the reaction systems used in measurements of gas adsorption, gas decomposition, and oxide conductivity have been given previously2"together with a description of the irradiation system.2b I n this present study, however, a 100-W Hanau medium-pressure mercury lamp The Journal of Physical Chemistry, Vol. 76, No. 17, 1079
JOSEPH CUNNINGHAM AND A. L. PENNY was used instead of a 125-W Hanovia. The esr spect r a were recorded with a Decca XI spectrometer fitted with a TElo2 mode rectangular cavity. A simple homodyne detection system was employed and the instrument was operated mainly in a "fixed frequency" mode at 9270.450 MHz and field modulation of 100 kHz. Xanganese-doped magnesium oxide was used as a reference for g-value measurements. A finely powdered sample of the doped MgO in a sealed capillary tube was affixed to the outer wall of the esr tube. Methyl iodide, BDH reagent grade, and methyl& iodide (>95% in deuterium), supplied by Prochem Ltd., were degassed under vacuum by the freeze-pumpthaw method and distilled through a 6411. column of phosphorus pentoxide. Throughout a 6-month storage period the methyl iodide showed no detectable coloration. Methyl chloride, supplied by the Matheson Co., was purified by fractional distillation. Purity of all gases was monitored by mass spectrometric analysis. Mass spectra were recorded on a quadrupole mass spectrometer having an m/e range 4-50. The instrument was unable to resolve below m/e 4.
Experimental Results A. Dark Reaction. (i) Adsorption Measurements. At room temperature the introduction of methyl iodide to a zinc oxide sample previously activated at 500" resulted in a rapid adsorption of methyl iodide. The adsorption isotherm measured at room temperature is shown in Figure 1. The adsorption data fitted a Langmuir isotherm with monolayer coverage corresponding to V , = 3.9 X 10-1 cm3 of methyl iodide. A significant practical consequence of these adsorption measurements was that monolayer coverage appeared to exist for any equilibrium pressure of methyl iodide above 5X Torr. (ii) Analysis. Since mass spectrometric analysis of the product gas shovcred it to be >99% methane, which was noncondensable at liquid nitrogen tempera-
PresrureCg /Torr
Figure 1. Adsorption of methyl iodide on 0.9-g ZnO sample activated at 500"; adsorption measured at 20".
(11) J. G. Calvert and J. N. Pitts, "Photochemistry," Wiley, Kew York, N. Y . , 1966. (12) K. Atherton, G. Newbold, and J. A. Hockey, Discussions Furuduy SOC.,52, 33 (1971).
ELECTRON TRANSFER AT SEMICONDUCTOR SURFACES ture, reaction a t the CHJ-ZnO interface could be followed by measuring the pressure of the noncondensable gas with a Pirani gauge after various contact times. No ethane was observed a t any stage of reaction. A trace of ethylene was observed but only during the degassing of the zinc oxide at elevated temperatures following a reaction. In order to determine the source of the fourth hydrogen in the methane product, m e t h y l d iodide was used in the reaction. Analysis showed the methane to
Table I: Effect of Activation Temperature on CDr :CDaH Ratio Act. temp,
OC
CD,: CDsH
350 430 450 480 490
1:3 2:3 2:3 1:l 1:1
430 followed by 5 X l o + Torr of Dz for 5 hr
3:l
(iii) Influence of Activation Temperature. The ratio CD4:CD3H in the initial stages of the dark reaction depended on the activation temperature. Table I shows the increase in CD4: CD3H with activation temperature. This suggested that the surface hydrogen content of the zinc oxide decreased with increasing activation. Indirect evidence that this surface hydrogen was probably in the form of OH came from the observation that water was one of the species desorbed during activation procedures. It has been suggested elsewhere12 that dehydroxylation at high activation temperatures proceeds via loss of HzO from vicinal pairs of surface hydroxyls. (iv) Decomposition Kinetics. Typical results showing the increase in the volume of methane with contact time are shown in Figure 2. Although no outstanding differences are visually apparent in the shapes of the kinetic curves for different activation temperatures, it was found that while kinetic data for lower activation temperatures (350-400") fitted Elovich kinetic plots, data for higher activation temperatures (480-500") did not. (v) T h e Effect of Methyl Iodide Pressure. For zinc oxide activated at 490" the reaction rate after the first 30 min contact was independent of pressure over a to 50 Torr. Higher pressures showed range of 4 X a slight increase in the rate of reaction in its initial stages, although no difference was detected between
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Timclmin
these initial rates a t 50 and 5 Torr. On the basis of these results it appeared that two processes were contributing to dissociation, an initial fast process with some slight dependence on pressure and a slower secondary process which was essentially pressure independent. (vi) E f e c t of Deuterium. Zinc oxide, activated at 430", produced an initial CD4:CDgH ratio of 2:3. When 5 X Torr of Dz was contacted with the zinc oxide a t room temperature for 5 hr following activation at 430", a subsequent reaction of the zinc oxide with methyl iodide in the presence of the Dzproduced a CD4: CDaH ratio of 7:2. This showed participation of preadsorbed hydrogen in the formation of product methanes, since Dz showed no influence when a DzCDaI mixture was introduced to an activated oxide surface. (vii) Competition between Methyl Iodide and Methyl Chloride. Introduction of a 1 : l mixture of methyl chloride and methyl-dr iodide to a zinc oxide sample activated at 490" produced a ratio of methanes resulting from CDII (i.e,, CD3 CD3H) to methanes resulting from CH3C1 (i.e., CH3D CH4) of approximately 4 : l . When the CHaCl was introduced to the zinc oxide some seconds prior to admitting CDJ, the above ratio became 2 :5, showing that both gases were effective in reacting with surface sites in the fast initial process. (viii) E f e c t of Methyl Iodide o n the Nitrous Oxide Decomposition Capacity of Z i n c Oxide. Previous work had shown that 6 g of oxide when activated had a cm3 of nitrous oxide. capacity to decompose 9.6 X To a 6-g sample of zinc oxide activated at 490" was introduced 2.3 X lov2 cm3 of methyl iodide. All was cm3 of nitrous oxide was adsorbed. When 1.4 X then introduced, 3.7 X cm3 decomposed. This amount of preadsorbed methyl iodide thus reduced the nitrous oxide "decomposition capacity" of zinc oxide to 4001, of its value after activation at 490". ( i x ) Esr Measurements. The esr spectrum of a zinc oxide sample activated at 500" showed an intense singlet at g = 1.96 and a weak signal at g = 2.003. This was consistent with signals reported by other
+
+
The Journal of Phgsical Chemistry, Val. 76,No. 17, 1978
2356
JOSEPH CUNNINGHAM AND A. L. PENNY its rate had decayed to an approximately constant low value before attempting to measure photoassisted for-
30
60
T i m / min
x)hi
Figure 3. Variation with time of the corrected intensity of g = 1.96 esr signal following introduction of CHJ.
mation of product. Illuminating the dark-equilibrated interface with wavelengths longer than 290 nm caused an increase in the rate of the dissociation of methyl iodide. Use of methyl-d3 iodide showed that the product gas, which was methane, had a CDd:CD3H ratio equal to or greater than that of the preceding dark reaction (see Table 11). Table 11: Mass Analyses of Methane Product temp, 350°Time, CD4/ min CDsH
-Act.
Conditions
Dark reaction
Illumination on
temp, 490'CD4/ min CDsH
-Act.
Time,
5 15 30
1/3 2/3 2/3
10 20 30
3/1 3/1
60 90
2/3 2/3
60 100
4/1 4/1
1/1
CDa : CDaH : CHsD : CHI
Figure 4. Change in the conductivity of activated ZnO film with time on introduction of (a) 3 X 10-8 Torr of CHaI and (b) 5 X 10-1 Torr of CHaI.
worker^.^^^^^ The cavity Q , measured under the same conditions as the spectrum, was decreased by a factor of 0.2 relative to the unloaded cavity. Introduction of 5 Torr of methyl iodide caused the apparent intensity of the signal at g = 1.96 t o decrease significantly but this was accompanied by an increase in the cavity Q. The intensities of the g = 1.96 signal (11.96) were normalized with respect to a constant Q. Figure 3 shows the behavior of 11.g6/& with time. The corrected intensity of the g = 2.003 signal was not affected by the methyl iodide. (x) Conductivity Measurements, Methyl iodide altered the conductivity of an activated zinc oxide film in the manner shown in Figure 4,where i was the current transported by the film when a 1-V potential difference was applied across it. Note the initial drop and the subsequent rise toward its initial value. When the conductivity had returned to its original activated value, an increase in the methyl iodide pressure had no effect on it but the magnitude of the initial decrease in the current did depend on methyl iodide pressure. B. Photoreaction. (i) Analysis. Because of the relatively large dark reaction occurring initially when methyl iodide was introduced to an activated zinc oxide sample, such dark reaction was allowed to proceed until The Journal of Physical Chemistry, Vol. 76, No. 17, 197g
Equimolar mixture CDaI and CHaCl, 45 min dark reaction 90 minphotoreaction
18 47
32 33
3 24
10 25
CHaCl introduced before CDaI, 45 min dark reaction 24 hr photoreaction 30 min photoreaction 120 min photoreaction
2 1 2 3
7 3 7 8
4
19 20 35 35
6
15 15
(ii) Kinetics. The appearance of methane from the photoinduced dissociation of methyl iodide is shown in Figure 5 . The rapid initial rate decays progressively over a long period. The reaction was not followed for longer than 20 hr at which time the reaction rate was comparable to the dark reaction rate. At methyl 5, and 50 Torr the voliodide pressures of 4 X umes of photoproduced methane product with time appeared identical so that, in this pressure range the photoassisted reaction mas pressure independent. (iii) Quantum Eficiency. The average quantum efficiency for all wavelengths longer than 290 nm emitted by a 100-W medium-pressure mercury lamp was 3 X This value was based on the volume of methane from the first 60 min of the photoreaction and on the intensity of the radiation at the zinc oxide surface measured by a calibrated uv meter.2b (iv) Effect of Methyl Chloride. An equimolar mixture of CH3C1and CD31was introduced to an activated (13) T. Kwan, Symposium on Electronic Phenomena in Chemisorption and Catalysis on Semiconductors, Moscow 1968, Walter De Gruyter & Co., Berlin, 1968, p 184. (14) (a) P. J. Kokes, J . Phys. Chem., 66, 99 (1962); (b) M. Setaka, K. ,M.Sancier, and T. Kwan, J. Cutul., 16, 44 (1970).
ELECTRON TRANSFER AT SENICONDUCTOR SURFACES
Time I min
Figure 5 . Photoproduced methane at ZnO surface under ( a ) 4 x 10-2 Torr of CHBI,(b) 5 Torr C&I, and (e) 50 Torr of CH31. Arrow indicates start of illumination.
zinc oxide sample. Analyses of the dark reaction and subsequent photoreaction are shown in Table I1 and show that there is little difference in the ratio of the methanes from CD3I and CH3Cl for the dark and photoreactions. (v) Conductivity Measurements. As described in section A(x), the introduction of methyl iodide to an activated zinc oxide film in the dark influenced the current transported by the film (see Figure 4). Illumination of the film when the current was at its minimum resulted in a photocurrent (Figure 6). On removal of the illumination the photocurrent decayed slowly and did not reach its starting value within 60 min. Illumination of the film when the current had returned to its activated value caused a slow, almost linear increase in current (Figure 6) which was greater, however, than the photocurrent at the minimum. Removal of the illumination caused a very slow photocurrent decay. Evidence that methyl iodide was decreasing the photoconductivity came from the observation that an increased photocurrent was always observed when the methyl iodide mas condensed at liquid nitrogen temperature.
Discussion Dark Reaction. Initial Interaction (Contact T i m e s
