J . Phys. Chem. 1989, 93, 2607-2609 band. Figure 5 shows the reaction of NADH with oxygenated samples of dehydrated Co-Y and ethylenediamine-Co-Y (prepared by heat treatment at 235 "C) for Na+- and Cs+-exchanged zeolites. In the case of CoNa-Y, the 0, generated is mobile and can oxidize NADH. However, in the presence of ethylenediamine all of the cobalt ions have migrated back into the supercages to form complexes and the diffusible 02- formed by reaction of O2 with Co(I1) in the internal sites no longer occurs. Also, the superoxo and peroxo complexes of Co(I1)-ethylenediamine are trapped inside the zeolite cages and do not react with NADH. In the case of CoCs-Y, the oxygenated ethylenediamine sample exhibits a diminished initial reactivity toward NADH, indicating that only a fraction of the Co2+ ions in site SI or SI' migrate back into the supercages, in contrast to the Na+-exchanged samples. A plausible explanation could be that the Co2+migrating into the supercage from internal sites needs to be replaced by other cations at these sites, and the Cs+, because of its size (radius 1.67 A) cannot enter these sites. The partial formation of Co-ethylenediamine complex is taking place in the CoCs-Y due to the presence of Na+ ions in these samples, since all of the Na+ cannot be exchanged with Cs+.
2607
Conclusions In summary, this study has shown the following: 02-can be generated by oxidation of Co(I1) in CoM-Y ( M = alkali-metal cations) at room temperature. Dehydration of the zeolite is necessary for this reaction to occur. We have proposed that Co2+ ions in the internal sites (site SI, SI') undergo this reaction. Oxygenation of dehydrated CoM-Y-ethylenediamine samples leads to the formation of superoxide bound to cobalt, as well as pperoxo and pperoxo-w-hydroxo complexes. The superoxide ions generated by oxidation of CoM-Y are mobile and will oxidize NADH at the periphery of the zeolite. Upon formation of cobalt-ethylenediamine complexes, this reaction does not occur on Na-Y and only to a limited extent on Cs-Y. The lack of 02-formation is due to the migration of the Co2+ from the internal sites back into the supercages. Acknowledgment. We acknowledge the National Science Foundation (CHE 8510614) for supporting this work. Registry No. NADH, 58-68-4; 02, 7782-44-7; 02-, 11062-77-4; ethylenediamine, 107-15-3.
Photochemistry of ZnO in Heptane: Detection by Oxygen Uptake and Spin Trapping Ciping Chen,+Richard P. Veregin,* John R. Harbour, Michael L. Hair, Sandra L. Issler, and John Tromp Xerox Research Centre of Canada, 2660 Speakman Drive, Mississauga. Ontario L5K 2 L 1 ~Canada (Received: August 1 , 1988)
The utilization of a Clark O2electrode to monitor O2consumption in low-dielectric,aliphatic solvents has been successfully realized. Thus, the consumption of oxygen in ZnO dispersions in heptane could be readily monitored upon photoexcitation of the ZnO. Addition of an amine as electron donor enhanced this O2uptake with the rate of O2uptake being directly proportional to the concentration of added amine. ESR results from use of the technique of spin trapping revealed that a nitrogen-centered radical was photogenerated in the presence of amines. Electrochemical oxidation of the amine in the presence of the spin trap provided evidence that the amine cation radical is the species being trapped. The mechanism of this reaction is discussed.
Introduction There has been a considerable amount of research directed at the events which occur during illumination of aqueous pigment dispersions.'-Is Solvents with intermediate dielectric constants have also been utilized for both pigment dispersions and photoelectrochemical However, very little attention has been paid to dispersing solvents with low dielectric constants such as heptane.20,2' Imaging systems have been devised which are based upon a photoinduced charge exchange of pigments dispersed in an aliphatic solvent such as Sohio. These PAPE (photoactive pigment electrophotography) systems have been previously described.22 In addition, photooxidation of aliphatic solvents has potential for solar energy utilization. It is therefore of interest to study the reactions of pigments under illumination and to determine the efficiency of the process(es) in low-dielectricmedia. To continuously monitor such events, we have extended the application of an O2electrode to organic solvents. It will be shown that the Clark electrode normally used for monitoring O2concentration in aqueous medium can also be effectively employed for aliphatic solvents. In the present study, ZnO particles were dispersed in both heptane and Sohio, and the oxygen uptake was measured as a function of illumination. This photochemistry was also monitored by electron spin resonance spectroscopy using the technique of spin trap-
'
Present address: Institute of Photographic Chemistry, Academia Sinica, Beijing, Peoples Republic of China.
0022-3654/89/2093-2607$01.50/0
ping.23,24 This study therefore provides insight into the photoreactions of ZnO in low-dielectric media.
(1) Rubin, T. R.; Calvert, J. G.; Rankin, G. T.; MacNevin, W. J . A m . Chem. SOC.1953, 75, 2850. (2) Markham, M. C.; Laidler, K. J. J . Phys. Chem. 1953, 57, 363. (3) Freund, T.: Gomes, W. P. Catal. Rev. 1969, 3. 1. (4) Gerischer, H. J . Electroanal. Chem. 1975, 58, 263. (5) Ellis, A. B.; Kaiser, S. W.; Bolts, J. M.; Wrighton, M. S. J . Am. Chem. SOC.1976, 98, 1635. (6) Murray, R. W. Acc. Chem. Res. 1980, 13, 135. (7) Heller, A. Acc. Chem. Res. 1981, 14, 154. (8) Bard, A. J. J . Photochem. 1979, 10, 59. (9) Gratzel, M. Acc. Chem. Res. 1981, 14, 376. (10) Fox, M. A. Acc. Chem. Res. 1983, 16, 314. (11) Harbour, J. R.; Hair, M. L. Photochem. Photobiol. 1978, 28, 721. (12) Harbour, J. R.; Hair, M. L. J . Phys. Chem. 1979,83, 652. (13) Harbour, J. R.; Tromp, J.; Hair, M. L. J . Am. Chem. SOC.1980,102, 1874. (14) Harbour, J. R.; Wolkow, R.; Hair, M. L. J . Phys. Chem. 1981, 85, 4026.
(15) Darwent, J. R.; Porter, G. J . Chem. Soc., Chem. Commun. 1981,145. (16) Fox, M. A,; Lindig, B.; Chen, C. C. J . A m . Chem. SOC.1982, 104, 5828. (17) Kohl, P. A,; Bard, A. J. J . A m . Chem. SOC.1977, 99, 7531. (18) Fujihira, M.; Satoh, Y . ;Osa, T. J . Electroanal. Chem. Interfacial Electrochem. 1981, 126, 211. (19) Lin, M. S.; Hung, N.; Wrighton, M. S. J . Electroanal. Chem. Interfacial Electrochem. 1982, 135, 121. ( 2 0 ) Giannotti, C.; Le Greneur, S.; Watts, 0. Tetrahedron Lett. 1983.24, 5071.
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 6, 1989
2608
100
Chen et al.
I LIGHT ON
z 0
ot
I
0
5
10
15
20
25
TIME (minutes)
Figure 1. O2uptake as a function of time of irradiation for ZnO particles dispersed in Sohio. The apparent increase in O2 concentration at the onset of illumination is due to heating of the probe and is reversible.
Experimental Section Oxygen uptake was measured by using a YSI Model 53 oxygen monitor. Illumination was achieved through a Quartzline 300-W projection lamp that was filtered to remove the UV light. The quantum efficiency was determined with a 380-nm narrowbandpass filter as previously described.I2 The Clark electrode was calibrated by comparing the solvent before and after purging with N,, with use of the known concentration of 0, in these solvents.25 ZnO (20 mg/5 mL) was dispersed in heptane (BDH) or Sohio 3440 mineral spirits (a mixture of fully-saturated branched alkanes from Sohio Chemical Co.) by ultrasonic irradiation with both solvents yielding similar results. For experiments with an electron donor, hexylamine (99+%, Aldrich), diethylamine (98%, Aldrich), triethylamine (99+%, Aldrich), or propylamine (99+%, Aldrich) was added. The ZnO was from Fischer. The 5-mL dispersion was added to the cell, and the electrode lowered into place to complete the closed cell. The interface of the heptane/water was maintained by the semipermeable membrane that is used in this system. The system functioned normally in response to argon and 0, purging, demonstrating that 0, equilibration between the two phases was occurring. Electron spin resonance (ESR) spectra were recorded by using a Varian E109E spectrometer system described previously.I2 Illumination of the ZnO dispersion in situ was achieved with an Oriel Model 6107, universal arc lamp source with a 150-W xenon lamp and a UV cutoff filter. The spin trap was 5,5-dimethyl-lpyrroline 1-oxide (DMP0)26 from Aldrich and was used as received. The electrochemical oxidation of hexylamine was done in situ in an electrochemical ESR flat cell (Wilmad) with Pt electrodes. The oxidation was done in acetonitrile (Aldrich 99%, anhydrous) with 0.1 M tetrabutylammonium perchlorate (Kodak). The amine concentration was 0.4 M, and the DMPO concentration was 0.2 M. All solutions and the ESR cell were well purged with N2. Results 0, Upfake. When a dispersion of ZnO in heptane or Sohio was illuminated with light from the 300-W Quartzline projection lamp, oxygen was consumed as shown in Figure 1. The control experiment of illumination in the absence of ZnO showed no 0, uptake. By varying the wavelength of excitation with filters, it was demonstrated that only light that can promote bandgap transitions (A C ABG, where XBG = 380 nm) was effective in (21) Filimonov, V . N. Elem. Fotoprofsessy Mol., Akad. Nauk S S S R
1960, 346.
Weigl, J. W. Angew. Chem., I n t . Ed. Engl. 1977, 16, 374. Janzen, E. G.Arc. Chem. Res. 1971, 4 , 31. Harbour, J. R.; Hair, M. L. Adu. Colloid Interjace Sci. 1986, 24, 103. Murov, S . L. Handbook of Photochemistry; Marcel Dekker: New York, 1973; p 89. (26) Janzen, E. G.; Liu, J . I.-P. J . Magn. Reson. 1973, 9, 510.
(22) (23) (24) (25)
Figure 2. ESR spectrum of the 02-adduct of DMPO generated in a partially N2-purged heptane dispersion of ZnO: modulation amplitude, 0.5 G; microwave power, 10 mW. 1-20~4
Figure 3. ESR spectrum generated upon irradiation of a ZnO/heptane dispersion containing hexylamine (20 pL/mL) and DMPO (10 pL/mL): modulation amplitude, 0.4 G; microwave power, 10 mW. TABLE I: ESR Hvoerfine Couolings for DMPO Adducts
hyperfine couplingb electron donor' heptylamine hexylamine propylamine diethylamine hexylamine (electrochemical)
AN
ABH
AaN
13.64 13.90 13.68 14.13 14.25
15.68 16.00 16.06 16.07 15.83
1.68 1.70 1.68 1.91 1.78
All adducts were produced photochemically with ZnO in heptane except hexylamine (electrochemical), which was produced electrochemically in acetonitrile. bAll hyperfine couplings are in gauss. AN, ABH,and AaN are the couplings to the nitrogen and 8-proton of the DMPO and the coupling to the amine nitrogen, respectively.
bringing about 0, consumption. In the donor-free case the heptane dispersion was centrifuged (after illumination), and the resultant supernatant analyzed with UV-vis spectroscopy. A peak was observed at 270 nm along with a shoulder at 210 nm. These observations are consistent with previous findings20-21 that demonstrated that heptanone was generated upon photolysis of TiO, or ZnO in the presence of heptane and 0,. The quantum efficiency of oxygen uptake, 9, was found to be -0.05 (5%) for a light intensity of -5 X 1015 photons/s at a wavelength of 380 nm. Addition of either n-heptylamine or triethylamine (TEA) resulted in an enhanced photochemical oxygen uptake with a linear dependence on amine concentration. However, as the amine concentration exceeded 0.6 mL of amine/5 mL of solution, the rate leveled off. Since % is 0.05 for the additive-free case, these results indicate that the 9 in the presence of the donor amine can reach -0.2. This is still well below the value of 1.0 achieved for H 7 0 2production from ZnO in aqueous media in the presence of a donor.
The Journal of Physical Chemistry, Vol. 93, No. 6,1989 2609
Photochemistry of ZnO in Heptane
Spin Trapping. When ZnO in heptane was illuminated in the presence of DMPO, an ESR spectrum of the Oz- adduct was observed (AN = 12.9 G, ABH= 6.3 G, and AYH= 1.6 G) as shown in Figure 2 . This is consistent with the one-electron reduction of O2 to generate the 02- r a d i ~ a l . ~ ' In the presence of diethyl-, propyl-, hexyl-, or heptylamine, a nitrogen-centered radical adduct was generated with splittings shown in Table I. It is important to note that these spectra were initially quite broad (due to the presence of 0,) but within minutes gave well-resolved spectra (Figure 3). This is the result of photoinduced 0, consumption and is consistent with the O2uptake results. The N-centered radical adduct is present in such large quantities relative to the 0, adduct that the ESR spectrum reveals only the N-centered radical adduct. With TEA present in the ZnO/heptane dispersion an ESR spectrum of the 0,- adduct is again observed with no evidence of the nitrogen-centered radical. This spectrum, recorded during illumination, reveals that the Oz- adduct itself is unstable during excitation of ZnO. In addition the lines in the ESR spectrum are much sharper relative to those of the additive-free case; reflecting a more efficient removal of 0, in the presence of TEA relative to the additive-free case. The fact that no nitrogen-centered radical adduct is observed with TEA is believed to be due to the fact that TEA is a tertiary amine. This will be discussed after the identity of the N-centered radical is established. When hexylamine was electrochemically oxidized in acetonitrile/O.l M TBAP in the presence of DMPO, a nitrogen-centered radical adduct was again observed. The ESR spectrum of the electrochemically produced adduct is identical with that produced photochemically with ZnO (see Figure 3), as shown in Table I. The slight difference in the ESR hyperfine couplings between the two experiments is due to the change to acetonitrile as the solvent for the electrochemistry. The one-electron electrochemical oxidation can be identified with the production of the hexylamine radical cation as shown by eq 1. On the basis of these results, :NH2(CH2)&H3 -++NH2(CH2)5CH3+ e-
(1)
the DMPO spin adducts observed in the electrochemical and photochemical experiments can be identified as 1-4. For TEA, where R , = Rz = R3 = -CHzCH3, it appears the nitrogen-centered radical is too sterically hindered with three alkyl substituents to form the corresponding adduct with DMPO. The oxygen-uptake experiments do suggest strongly that the nitrogen-centered radical is produced from TEA, although it is not trapped.
Discussion The absorption of light by ZnO is known to generate elec(27) Harbour, J.
R.;Hair, M . L. J . Phys. Chem. 1978, 82, 1397.
l : R l = H , R2=R3=CHzCH3 2: R i =R2= H , R ~ = - ( C H ~ ) Z C H ~
3: R ~ = R Z ' H , R ~ ' - ( C H ~ ) ~ C H ~
2: Ri'R2=H,
R3=-(CH2)6CH3
tron/hole pairs that can undergo redox reactions at the solid-liquid interface in competition with recombination. The fact that oxygen uptake is enhanced by the addition of amines supports the contention that the uptake of 0, results from reduction of 0,. That is, the amine reacts with the hole, thus reducing the recombination loss and providing the conduction band electrons with a longer lifetime, which enhances interfacial electron transfer. The spin-trapping experiments are consistent with this mechanism. In the absence of an amine donor, the 0,- adduct is observed presumably due to the transfer of an electron in the conduction band to adsorbed Oz to generate OF, which is subsequently trapped. When diethyl-, n-propyl-, n-hexyl-, or nheptylamine was present as a donor, an intense nitrogen-centered radical adduct signal was obtained. Electrochemical studies suggest that this adduct is due to the trapping of the amine cation radical by the DMPO. Hence, the amine reacts with the hole in the valence band to oxidize the amine to its cation radical. In the case of TEA no amine radical was trapped, presumably due to the three ethyl groups, which inhibits reaction with the DMPO via steric repulsion. 0, uptake revealed that TEA was as effective as heptylamine in increasing 0, uptake. The ultimate product of this reaction in the absence of amine appears to be an oxygen-containing species such as heptanone.20s2' The @ for 0, uptake is surprisingly high at 0.05 compared to 0.14 for ZnO-photomediated production of H 2 0 2in aqueous media. It is important to note that the 0, concentration in solvents such as heptane are 10 times higher than in H 2 0 . This may increase the rate at which 0, is consumed in nonaqueous solvents. These experiments reveal that a Clark electrode is an effective probe for monitoring O2uptake in low-dielectric solvents. In this case, the consumption of 0, was monitored upon illumination of a ZnO dispersion, but the technique could be applied to other pigments as well.
-
Registry No. 1, 118476-70-3; 2, 118476-71-4; 3, 118476-72-5; TEA, 121-44-8; DMPO, 3317-61-1; 02,7782-44-7; ZnO, 1314-13-2; (CH3CH2)2NH, 109-89-7; CH3(CH2)2NH2, 107-10-8; CH3(CH2)5NH*, 11 126-2; CHj(CH2)6NH2, 11 1-68-2; CH3(CH2)5CH3, 142-82-5.
