J. Phys. Chem. 1982, 86, 2617-2619
2617
Measurement of the Extinction Coefficient of the Methyl Viologen Cation Radical and the Efficlency of Its Formatlon by Semiconductor Photocatalysis Tadashi Watanabe' and Kenichi Honda Department of Synthetic Chemisby, Faculty of Engineering, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan (Received: Ju/y 29, 1981; In Final Form: February 17, 1982)
With a thin-layer spectroelectrochemicaltechnique the molar extinction coefficient (e) of the methyl viologen cation radical (MV+.) has been measured to obtain, at the red peak, 13700,13800,13800, and 13900 M-' cm-', in H20,CH,OH, C2H50H,and CH3CN,respectively. These values are 15-25% higher than those usually reported or cited. The present data are used to assess the quantum efficiency (6)of photocatalytic MV+.production. The 6 value well exceeds unity for some semiconductor-solvent combinations.
Introduction Methyl viologen cation radical (MV+.),electrogenerated from the colorless dication (MVz+)at a standard redox potential of -0.45 V vs. NHE,'q2 is a strong reductant capable of reducing the proton to yield molecular hydrogen in neutral to acidic media. In view of this, many workers have been recently directed to photocatalytic generation of MV+. as a possible means of light-to-chemical energy c o n v e r s i ~ n . ~MV+. - ~ can also reduce molecular oxygen, and this process is considered to be a key reaction in the herbicidic action of methyl viologen.1J*'2 Further, an apparatus for detecting trace amounts of oxygen in gases has been devised13 based on this reaction. To discuss quantitatively the efficiency and kinetics of these important processes, we must know an accurate value for the extinction coefficient (e) of MV2+. However, significant scatter is seen in the literature regarding the e value of MV+.. For instance, the tmax in the red region (605-610 nm) ranges from 86003 to 13300 M-' cm-l,lo*llwith the most frequently cited value being 11000-12000 M-' cm-l 4-1
The parent dication ( M V ) possesses a relatively strong absorption peak around 260 nm, for which a precise extinction coefficient can readily be determined. Hence, if one follows the spectral evolution over the whole UVvisible wavelength range on reduction of MV2+to MV+., the t value for the latter could be determined accurately. The thin-layer spectroelectrochemical technique2J4-16is (1)C. L. Bird and A. T. Kuhn, Chem. SOC.Reu., 10,49 (1981). (2)E. Steckhan and T. Kuwana, Ber. Bunsenges. Phys. Chem., 78,253 (1974). (3)A. I. Krasna, Photochem. Photobiol., 31,75 (1980). (4)J. R. Darwent, J. Chem. SOC.,Chem. Commun., 805 (1980). ( 5 ) R. J. Crutchley and A. B. P. Lever, J.Am. Chem. SOC.,102,7129 (1980). (6)T. Tanno, D.WBhrle, M. Kaneko, and A. Yamada, Ber. Bunsenges. Phvs. Chem.. 84.1032 (1980). 17) D. Meiee1,'W. A. Mulac, and M. S. Matheson, J. Phys. Chem., 85, 179 (1981). (8)I. Okura and N. Kim-Thuan, J.Chem. SOC.,Faraday Tram. 1,77, 1411 (1981). (9)A. Harriman, G.Porter, and M.-C. Richoux, J . Chem. SOC.,Faraday Tram. 2,77,833 (1981). (10)J. A. Farrington, M. Ebert, E. J. Land, and K. Fletcher, Biochim. Biophys. Acta, 314 372 (1973). (11)R. N. F. Thorneley, Biochim. Biophys. Acta, 333, 487 (1974). (12)J. A. Farrington, M. Ebert, and E. J. Land, J. Chem. SOC.,Faraday Tram. 1 , 74,665 (1978). (13)P.B. Sweetser, Anal. Chem., 39,979 (1967). (14)R. W. Murray, W. R. Heineman, and G. W. ODom, A d . Chem., 39,1666 (1967). (15)W. R. Heineman, Anal. Chem., 50,390A (1978). 0022-3654/82/2086-2617$01.25/0
a potential method for such a purpose. In the present work we employed this technique covering a wide spectral range (220-740 nm) under rigorous deoxygenation, in order to find out which of the reported e value is more reliable. Further, the e values thus determined are used to assess the quantum efficiency (4) for photocatalytic MV+. production on semiconductor powders. Cases where @ exceeds 1.0 are demonstrated for the first time.
Experimental Section Reagent grade MV2+dichloride (Sigma) was recrystallized from methanol and dried overnight at 60 "C in vacuo. Elemental analysis of a well-dried sample gave C, 55.96%; H, 5.42%; and N, 10.8770, in good agreement with those expected for Cl2H1,N2Cl2: C, 56.05%; H, 5.49%; and N, 10.89%. On this basis it was found that commercially obtained methyl viologen contained 3.5-4.0 HzO per molecule of methyl viologen. Figure 1shows the construction of an air-tight thin-layer spectroelectrochemical cell. The working electrode is platinum gauze (1X 4 cm, 80 mesh, transmittance ca. 55%) and is sandwiched by a pair of quartz plates with a 160-pm thick Teflon spacer. An Ag layer, vacuum-deposited onto the inner face of a quartz plate so as to surround the working electrode, is used as a pseudo-reference electrode. The thin-layer cell (TLC) is assembled only by screw tightening and can be dismounted and washed to permit repeated use. A platinum wire served as an auxiliary electrode. First the all-quartz cell container is set with the side arm directing downward into which a 4-mL MV2+ aqueous (0.1 M KCl), CH30H (0.1 M LiCl), CzH60H(0.1 M LiCl), or CH&N (0.1 M NaC10,) solution is injected. After at least three freeze-pump-thaw cycles at ca. torr, the stopcocks are closed and the cell is set in the normal position (as in Figure 1)and put into the sample compartment of a Shimadzu Model MPS-5000 spectrophotometer. The spectral evolution was measured during electrolysis of the thin-layer solution with a Model HA-101 Hokuto Denko potentiostat. Photocatalytic MV+. formation was measured as follows. MV2+solutions (25 mL, 1 X 10-4-2 X M) were prepared with distilled water (pH 6) and reagent grade CH30H, C2H,0H, and CH,CN in a Pyrex cell having a 3-cm diameter flat window. Suprapure powders of ZnO (99.99%, below 200 mesh), TiOz (99.99%, below 300 mesh), or WO, (99.9%, below 200 mesh), supplied from Furuuchi Chemicals, was then added. Neither reduction nor pla(16)T.Watanabe and K. Honda, J. Am. Chem. SOC.,102,370(1980).
0 1982 American Chemical Society
2818
Watanabe and Honda
The Journal of Physical Chemistry, Vol. 86, No. 14, 1982 WE (Pt (MESH)
160.3 TEFLON
QUARTZ
_____
4
___ ___
TEFLON BAR
EeFoore Electrolysis
-0.4--0.55
jMV2+ 1
V
(MVt)
- 0 . 7 5 ~ - 0 . 9V
[MYo)
MASK
4
Figwe 2. Spectra of the electrolyte solution in the TLC at different electrode potentials: [MV2+]o= 2 mM in C,H,OH; [LiCI] = 0.1 M.
LIGHT 1/ Fwe I. ~onstructionof ttie thwayer di.n c (top), and its assembly
in the air-tight container (bottom): WE, Pt gauze working electrode; CE, Pt wire auxiliary electrode; RE, Ag (pseudo) reference electrode.
TABLE I: Peak Wavelengths and Molar Extinction Coefficients for MVZ and MV'. in the Four Solvents Employed +
solvent
(hm,,/nm) Cmax/(M-l cm-') MV2+ MV'.
H20
(257) 20700
t
200
CH30H
!286420)0
t
200
t
200
t
200
C2H50H CH3CN
!2s63odo '2"064odO
(396) 42100 (397) 42700 (398) 41100 (397) 41800
t
800
t
800
t
800
t
800
(606) 13700 (609) 13800 t (611) 13800 t (607) 13900 t
300 300 300 300
tinization treatments1' were applied to the semiconductor powders. Under magnetic stirring and nitrogen bubbling, the suspension was illuminated by a monochromatic light (390 nm) from a 500-W xenon arc lamp (Ushio Electric). The photon flux reaching the cell had been determined by ferrioxalate actinometry. After an appropriate period of illumination, the absorbance change at ca. 610 nm was measured by introducing the supernatant solution to a 1 x 1 x 5 cm optical cell via a side arm. In the calculation of the quantum efficiency (4) for MV+- formation, no corrections were made for the light scattered by the suspended semiconductor powder. Hence the reported 4 values should be regarded as a lowest estimate. Results and Discussion Determination of the Extinction Coefficient of MV+.. In the first place we attempted to determine the extinction coefficient of the parent dication MV2+, which has an absorption peak around 260 nm in the four solvents used. So that MV2+solutions of accurate concentrations could (17)B.Kraeutler and A. J. Bard, J.Am. Chem. Soc., 100,4317(1978).
be prepared, care was taken, as in elemental analysis, to keep this rather hygroscopic crystal from being hydrated just before dissolving it in the solvents. Measurements of the t value for M V + were repeated at least twice in each solvent, and the results (Table I) were consistent to within f0.5%. Given the exact t values for MV2+,one can determine the t values for MV+. by a simple procedure. Introducing a MV2+electrolyte solution into the TLC, one records the absorbance at ca. 260 nm before electrolysis and the absorbance at any wavelength after the complete reduction MV2+ MV., by checking the reversibility and completion of this transformation in the absorption spectra. The t values for MV+- are thus calculated to the same accuracy as the e values for MV2+. One can even start the thin-layer measurement with a M V + solution of unknown concentration. Such simplicity comes from the use of quartz along the optical path. Otherwise one is led to measure only the absorbance increase in the visible range upon reduction and, in order to calculate the e values for MV+., coulometric titration has to be used. In coulometry, however, one must ensure strict uniformity and accuracy of the effective internal spacing of the TLC. Moreover, the residual current can be a source of error, particularly at the lower concentrations of electrolyzed species. These problems do not arise in the experimental procedure adopted here. Figure 2 shows a typical example of the spectral change on reduction of W + to MV+.in C2H60H. With negative polarization the spectrum of MV' (broken curve) changes into the solid curve, which becomes constant beyond -0.4 V vs. Ag. The wavelength for a small peak at ca. 250 nm in the MV+. spectrum is clearly different from that in the original MV2+spectrum, and we can conclude that the reduction process is complete. In the course of this transformation, the relative concentrations of MV2+and MV+. were determined at a given electrode potential and were inserted in the Nernst equation, and a slope of 60 f 1 mV per tenfold change in the concentration ratio was found, in accordance with the one-electron process. On further negative polarization the MV+. spectrum changes to that shown by the dash-dot curve, again with a Nernstian slope of ca. 60 mV, corresponding to the reduction MV+. MVO. All these spectral changes were practically reversible with respect to the electrode potential. In aqueous solution, use of a M V + concentration higher than ca. lo4 M led to deformation of the MV+. spectrum, reflecting dimer formation.1*2Hence in this case the initial MV2+ concentration was kept below M. In other solvents such complexity did not arise even at much higher
-
-
Extinction Coefficient of the Methyl Viologen Cation Radical
1.51
L ZnO 0 2 g / 2 5 m l EtOH
00
X 1390 nm
Irradiation Time
2
/
man
Flgure 3. Orowth of the MV+. absorbance in a photocatalytic system: incident photon flux, 1.64 X loie s-’.
concentrations (>1mM), and the pattern of the spectral evolution was essentially the same as in Figure 2. The values of tmarfor MV+. thus determined are summarized in Table I. The t- of MV+. in the red region is 13700-13900 M-l cm-l, depending little on the nature of solvent, and is ca. 15-25% higher than those usually rep ~ r t e d . ~Three - ~ major error sources could be envisaged in the measurements reported here: sample purity, absorbance reading in general, and ensurance of reduction completion. We estimate the magnitudes of these uncertainties to be ca. 0.5, 1, and 1%, respectively. A quadrature sum gives 1.5% as a total uncertainty. Thus, as a modest estimate, we could say the values of ,E of MV+. are accurate to within 2%. Several factors might have influenced the accuracy of previously reported t values of MV+.. One of these is certainly the water content in the MV2+sample. While the stoichiometric number of H 2 0 per MV2+molecule is 2.0,” this could, as we found, readily increase to ca. 4 during contact with moist air. A thin-layer coulometric method might be influenced by the presence of residual current and by the accuracy of the internal spacing of a TLC (see above), while a radiation chemical method7J2 might be influenced by the uncertainty in the G values and t value for the solvated electron. Absorption measurement on chemical reduction of MV2+ can be a good direct method only if the completion of reduction as well as the purity of the sample is rigorously checked. The cause for obtaining smaller values o f t by such a method (e.g., reduction with dithionite18) is immediately evident if one compares the solid curve in Figure 2 with the “MV+. spectrum” reported in ref 18. A rough examination reveals that the latter spectrum corresponds to a combination of 25% MV2+ 75% MV’., and hence the reported 6605 should be modified to ca.14O00 M-’ cm-’, in fair agreement with the present result. Quantum Efficiencyfor Photocatalytic MV+. Formation. Using the t values given in Table I, we determined the quantum efficiency (4) of photocatalytic MV+. production for several semiconductor-solvent combinations.
+
(18)E. M. Kosower and J. L. Cotter, J. Am. Chem. SOC.,86, 5524 (1964).
The Journal of Physical Chemistty, Vol. 86, No. 14, 7982 2619
As an example, the growth of the MV+. absorbance in the course of illumination of a ZnO (0.2 g)-C2H50H (25 mL) system is illustrated in Figure 3. On the basis of the incident photon flux (1.64 X 10l6s-l) the lowest quantum efficiency at the initial stage ( $ O ) is calculated to be 1.44. When the amount of the ZnO powder was 0.1 and 0.4 g/25 mL, the value of $o was 1.35 and 1.55, respectively. With Ti02 (0.2-0.3 g/25 mL) as a photocatalyst, we obtained = 1.1-1.3. For a given photocatalyst the value of $o depended on the solvent. Thus, with 0.2 g of ZnO, 4O was 0.015,0.44,0.47, and 0.76, in H20, CH3CN,H20/C2H50H (3,2:1),and CH30H, respectively. The relatively high value of in CH3CN, which is known to have high resistance against o~idation,’~ might have arisen from some impurities contained in this solvent. In the case of W03 having a low-lying conduction band level,20 was 0.0 in all the solvents employed. A remarkable difference in the @O values in aqueous and alcoholic solutions may reflect the difference in the rates of photocatalytic oxidation of these solvents. Product analyses have not been conducted, but O2 or H202(in HzO) and HCHO and CH3CH0 (in CH30H and C2H,0H) are possible candidates. A close examination of oxidation products is beyond the scope of the present work. well exceeded unity in The fact that the value of C2H50Hwith ZnO and Ti02as photocatalysts suggests the occurrence of a “current-doubling effect”,21whereby a one-electron oxidized C2H50Hmolecule injects another electron to the conduction band of the semiconductor. In this picture, the one-electron oxidized form (X) of ethanol must have sufficiently strong reducing power. Hence, as an alternative explanation, the attack of X on M V + in the solution phase could be envisaged. For a final distinction between these two mechanisms, further investigation on the lifetime of X and on the rate constant for the reaction of X with MV2+will be required. Since in the present study the Ti02 powder was used as received, the reduction and/or platinization pretreatments of the powder” may not be a prerequisite for a one-electron photocatalytic reduction such as the MV2+ MV+. conversion. In fact, no further improvement in the value was noted after reduction and platinization treatment of the Ti02powder. This reaction has recently been shown to proceed efficiently also on illuminated GaAs electrodes.22 Surface platinization of Ti02 plays an essential role in promoting photocatalytic reactions such as carboxylic acid decompositiona and amino acid synthesis,% where multielectron pathways are possibly involved in the cathodic processes. Acknowledgment. The authors are grateful to M. Shinohara for his participation in the photocatalytic measurements.
-
(19)S.N.Frank and A. J. Bard, J. Am. Chem. SOC.,97,7427(1976). (20)T.Inoue, A. Fujishima, S. Konishi, and K. Honda, Nature (London),277,637 (1979). (21)W . P.Gomes. T. Freund. and S. R. Morrison. J. Electrochem. So, 115, 818 (1968): (22) F.-R. F. Fan, B. Reichmann, and A. J. Bard, J.Am. Chem. Soc., 102,1488 (1980). (23)B. Kraeutler and A. J. Bard, J.Am. Chem. SOC.,100,5985(1978). (24)H.Reiche and A. J. Bard, J. Am. Chem. SOC.,101,3127 (1979).
