J. phy~.Chem. lB02, 86, 3581-3585
The thermal dynamics of this complicated system have to be studied again before a proper mechanism of isomerization can be developed. We have made many attempts to obtain the P E spectrum of the cis dimer using a variety of methods, including heating the dimer within 2 cm of the ionization point at minimal temperatures, and adiabatically expanding CH3N0 through a small nozzle. In both instances, CH2=NOH or CH3N0 alone are formed, and no evidence for the cis dimer was obtained. The HAM/3 calculations serve as a basis for the assignments of the PE spectra and are presented in Tables I-IV. They will not be d m s e d in any detail here, except to say that those for CH,=NOH (Table 11) are in agreement with the previous results,20*28 especially concerning the relative ordering of the first two closely spaced states. Those for trans-(CH,NO), (Table 111) closely fit our experimental spectrum (Figure 2) and Bergmann et al.’s experimental IP’s.l9 They also indicate that the earlier spectrum of Egdell et al.18 is actually that of the trans dimer. CH3N0 presents an interesting assignment problem (Table I), since earlier calculations on HNO involving perturbation corrections to Koopman’s theorem indicated that the theorem did not hold in this case.23 This mediction was justified by our own PCKT calculation on CH3N0. The low-lying unoccupied level (3a”) in the SCF results caused a switching of 2a” with 9a’ and la” with 8a‘ orbitals after corrections are aDDlied. HAMI3 also predicted this ordering. The PCKT calculated IP’s are all within 0.5 eV of the experimental values. A plot of predicted IPS against the values (where T is the truncation criterion) shows that this discrepancy should (28) M. A. Robb and I. G. Ceizmadia, J. Chem. Phys., 50,1819(1969).
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decrease as more determinants are considered. An exception to this trend is exhibited by the small Pt band at 15.8 eV. Thus if the 8a’, 7a/, and la” ionizations are present in the band at 16.9 eV we might assign the 15.8eV band to a shakeup satellite peak. This, however, is tentative and will have to be confirmed by a further extensive theoretical study.
Summary It has been demonstrated that a combination of in-situ P E and quadrupole mass spectroscopy, combined with semiempirical HAM/3 calculations, can be successfully used to evaluate the ionization processes in a complicated gas-phase system, in the case, the CH,NO, cis- and t r a n ~ - ( C H ~ N 0and ) ~ , CH2=NOH mixture. This clarifies several ambiguities noted in earlier work. We note that while this work was in its concluding stages, the authors of ref 18 have repeated their earlier work and now showm that the spectrum of CH3N0 is as described in this present manuscript. In addition, their configuration interaction calculations confirm the breakdown of Koopmans’ theorem for this molecule. This work complements our present study where we have shown the interrelationship between the two dimer species, monomeric CH3N0 and CH,=NOH. Acknowledgment. We gratefully acknowledge the financial support of the Natural Sciences and Research council of sot., (29)N. P. Emeeting, TrarLs.2, J. ,6, Pfab, 844 (1980). J. C. Green, and J. Romelt, J. Chem.
(30)I. N.Levine, J. Chem. Phys., 38, 2326 (1963). (31)M. van Meerssche and G. Germain, Bull. Soc. Chim.Belg., 68,244 (1969). (32)G. Germain, P. Piret, and M. van Meerseche, Acta Crystallogr., 16, 109 (1963).
New Preparation Method for Doped Polycrystalline T102 and Nb205and Their Photoelectrochemical Properties Yawmlchl Matsumoto,’ Tatekl Shlmlzu, Arata Toyoda, and El-lchl Sato w r t m e n t of Indusblel Chemisby, FacW of Englneerlng. UtsmtWa lJnivm?nV. Ishclcho 2753, utsunomlya, Japan *
(ReceM: November 13, 1981; I n Final Form: M r e h 31, 1982)
Doped polycrystalline Ti02 and Nb205have been prepared by a new method and examined as photoanodes. Polycrystalline oxide films on metal substrates which are thick and porous are prepared by anodic oxidation accompanied by sparking in various electrolytes. It was found that cations in the electrolyte are doped into the oxide films during anodic oxidation, which has been proved by SIMS. Therefore, this is a new easy method of preparing doped polycrystalline TiOz and Nb20s. The TiOz film prepared in NazSOl solution at 100 V is a good photoanode, showing a relatively high photocurrent under visible-light illumination. The doping is carried out by addition of the doping ions in saturated Na2S04solution. The Cr-, Ir- and Rh-doped Ti02films exhibit a high visible photocurrent but a low W photocurrent. The Cr-doped Nb205films also exhibit similar spectral responses. An increase in the UV photoresponse and a decrease in the visible photoresponse by reduction are observed. The relationship in which the band-gap photocurrents decrease with increasing visible photocurrents is obtained for the doped oxide films. This suggests, with the reduction effects, that the impurity and the interstitial metal d bands or states bringing about the visible response also act as recombination centers.
Introduction TiO, and m206 are suitable materials for photoanodes in the photoelectrolysis of water, especially in terms of stability in aqueous solutions. Polycrystalline oxide has the advantages of being easier to prepare and low in cost, and some forms of this material have photoelectrochemical 0022-3654/82/208&3581$0 1.2510
properties similar to those of a single crystal. Various preparation methods of polycrystalline TiOz have been thermal dereported: thermal oxidation of the (1)Fujishima, A.; Kohayakawa, K.; Honda, K. J. Electrochem. SOC. 19711,122,1487.
0 1982 Amrlcan Chemical Society
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composition of metal salts,"12 anodic oxidation of the metal,1J4-16chemical vapor decomposition,"J8 arc-plasma-sprayed raw rutile powder,lSplasma jet sprayingmand sintering of r ~ t i l e . ' On ~ ~ the ~ ~ other ~ hand, polycrystalline Nb205can be prepared by thermal oxidation22*23 and anodic oxidation of the metal.24 Recently, it has been reported that the response of polycrystalline Ti02 to the solar spectrum is improved by In these cases, the doping with various elements.'~~JOJ~-~~ doping has been done by thermal decompitions of metal salts on Ti substrates. In this paper, a new preparation method for doped polycrystalline Ti02and NbZO5 will be demonstrated, and the photoelectrochemical properties of these materials will be presented. These polycrystalline oxide films have been prepared by anodic oxidation accompanied by sparking in various electrolytes, which is similar to that reported by Clechet et al.16 It was discovered in this investigation that cations in the electrolyte are doped into the oxide layer during anodic oxidation accompanied by sparking.
Experimental Section The titanium plate (99.6'70,1.5 X 5.0 X 0.1 cm, Kobe Steel Ltd.) and the niobium plate (99.8%,1.5 X 5.0 X 0.05 cm, Shinku Yakin Ltd.) were polished with emery papers and then washed with a cleanser. The sample was cleaned by an ultrasonic technique, followed by washing with acetone. Anodic oxidation was done in various electrolytes (200 mL), using a full-wave rectifier circuit system with a platinum counterelectrode (5.0 X 0.5 mm in diameter). The distance between the working metal plate and the counter platinum wire was 4.0 cm. The composition of the prepared oxide films was determined by X-ray analysis.
TABLE 1: TiO, Films and Their Photoelectrochemical Properties photoelectrochemical propertiesb
additives
reduction methoda
OPh, mA/ cmz
VPh, uA/ cml
OP,V
none E
3.0 6.0 6.2 2.8 1.9 3.9 5.6 5.6 6.7 3.1 3.0 1.6 4.7 6.1 7.0 0.1 8.6
6.4 4.8 0.1 42.0 53.5 1.5 21.8 30.5 0.2 23.0 46.0 47.0 0.1 3.8 0.3 5.6 0.0
-0.90 -0.94 -0.97 -0.82 -0.80 -0.91 -0.91 -0.99 -0.99 -0.90 -0.90 -0.93 -1.02 -0.88 -0.95 -0.73 -1.00
none none none 5x M Cr04,1 x lo-) M Cr04,1 x lo-) M CrO4,5 x 10-4 M 1r4+ 7.5 x 10-4 M 1r4+ 5 x 10-4 M 1r4+ 1x M Rh3+ 2.5 x M Rh3+ 5x M Rh3+ 5x M Rh3+ thermally oxidized TiO, (800 'C, in air) single-crystal rutile (Nd = 1.4 x ioi9 cm-3)"
TZ E
E T, E E T*
E E E
T, T, T,
E
T,
a E and T denote electrochemical reduction for 1 min and thermal reduction in H, for 1 h, respectively. T, and T, denote thermal reductions a t 700 and 800 "C, respectively. OPh, VPh, and OP denote overall photocurrent, visible photocurrent, and onset potential, respectively. The electrode surface ((001) plane) was etched in H,SO,-(NH,),SO, solution a t 240 "C for 30 min. Nd was determined from a Mott-Schottky plot.
TABLE 11: Nb,O, Films and Their Photoelectrochemical ProDertieP photoelectrochemical properties
(2)Mavroides, J. G.; Tcherov, D. I.; Kafalas, J. A,; Kolesar, D. F. Mater. Res. Bull. 1975,10, 1023. (3)Houlihan, J. F.;Madacsi, D. P. Mater. Res. Bull. 1976,11,1191. (4)Tamura, H.; Yoneyama, H.; Iwakura, C.; Murai, T. Bull. Chem. SOC.Jpn. 1977,50,753. (5)Haneman, D.;Holmes, P. Sol. Energy Mater. 1979,1, 233. (6)Laser, D.;Gottesfeld, S. J. Electrochem. SOC.1979,126, 475. (7)Mataumoto, Y.; Kurimoto, J.; Amagasaki, Y.; Sato, E. J. Electrochem. SOC.1980,127,2148. (8) Sprijnken, H. R.; Schumacher, R.; Schindler, R. N. Ber. Bunaenges. Phys. Chem. 1980,84,1040. (9)Stalder, C.; Augustymki,J. J. Electrochem. Soc. 1979,126,2007. (IO) Monnier, A.; Augustynski, J. J. Electrochem. SOC.1980,127,1576. (11) Yoneyama, H.; Sasaki, H.; Tamura, H. Denki Kagaku 1981,49, 222. (12)Mataumoto, Y.;Kurimoto, J.; Shimizu, T.; Sato, E. J. Electrochem. SOC.1981,128,1040. (13)Matsumoto, Y.; Shimizu, T.; Sato, E. Electrochim. Acta 1982,27, 419. (14)Keeny, J.;Weinstein, D. H.; Haas, G.M. Nature (London) 1976, 253,719. (15)Blondeau, G.; Froelicher, M.; Froment, M.; Hugot-Le Goff, A.; Zerbino, J. J. Electrochem. SOC.1979,126,1592. (16)Clechet, P.;Martelet, C.; Martin, J. R.; Olier, R. Electrochim. Acta 1979,24,457. (17)Hardee, K. L.; Bard, A. J. J. Electrochem. SOC.1976,122,739. (18) Hardee, K. L.; Bard, A. J. J. Electrochem. SOC.1977, 124,216. (19)Wang, R,;Henager, C. H. J. Electrochem. SOC.1979, 126, 56. (20)Gisler, W.; Lensi, P. L. J. Appl. Electrochem. 1976,6, 9. (21)Gautron, J.; Lemasson, P.; Marucco, J. F. Faraday Discuss. Chem. SOC.1980,70,81. (22)Reichman, B.; Bard, A. J. J. Electrochem. SOC.1980,127, 241. (23)Takehara, Z.;Yokoyama, H.; Yoshizawa, S., presented at the Electrochemical Society of Japan Meeting, Tokyo, Oct 1-2,1980,Paper D209. (24)Heusler, K. E.; Yun, K. S. Electrochim. Acta 1977,22,977.
additives none none none
M Cr,O,,10-1 M Cr,O,'10-l M CrO4,10-l M Cr,O,z7.5 X 10-,M Ir4+
reduction OPh, VPh, method mA/cmz pA/cmZ none E
Tl
E E E
0.05 0.57 0.07 2.26 7.10 1.81 1.50 0.81
0.023 0.191 0.230 0.024 0.025 0.006 0.050 0.159
a Each symbol is as shown for Table
OP, V -1.00 -1.06 -1.10 -0.85 -1.26 -1.23 -1.20 -1.12
I.
In-depth analysis of the oxide layers for the doped cations was conducted by means of SIMS (Hitachi Ltd.). These ions were produced by bombarding the sample with at 10 kV as the primary ion. The accelerated Ar+ or 02+ sample current was 0.4 pA/beam, and the beam diameter was 0.4-0.7 mm. The electrolyte was preelectrolyzed 1 M KOH in the photoelectrochemical measurements. A Hg/HgO electrode was used as the reference electrode, and electrode potentials cited in this paper are in reference to this electrode. The counterelectrode was a platinum plate. A quartz H-type cell was used as the electrolytic cell. The polarization curves were determined by using a potentiostat (Hokuto Denko L a . , HA-201) and a function generator (Nichia Keiki Ltd.,) and recorded by an X-Y recorder (Yokogawa Electric Works Ltd., Type-3036). Monochromatic light of various wavelengths was obtained with a prism monochrometer. A 500-W xenon lamp was used as the light source. The photocurrents with the monochromatic light were transformed into quantum efficiencies
The Journal of Physical Cbmistty, Vol. 86, No. 18, 1982 3583
Doped Polycrystalllne Ti02 and Nb2O5
I
300
1
400 Wavelength
I
I
300
400 Wavelength
500 (nm)
600
Flgure 1. Spectral dependences of quantum efflciencles of TO,: (a) prepared In 10" M Cr0:solution and electrochemically reduced, (b) in 2.5 X lo4 M Rh3+ solution and electrochemically reduced, (c) In 7.5 X lo4 M Ifl' solution and electrochemicaUy reduced, (d) in SOkRkn containing no additive and electrochemlcally reduced, (e) In M Cr04'- solution and thermally reduced at 800 OC In H,.
500 ( nm 1
600
Flgue 2. Spectral dependences of quantum etficlcies of Nb205: (a) prepared In 0.1 M Cr20:- solutlon and electrochemically reduced, (b) In solutlon containing no addlthre and electrochemically reduced.
using a thermopile (Epply Lab., Inc.). Results Tables I and I1 show some of the photoelectrochemical properties of Ti02 (rutile) and Nb205films, respectively. Anodic photocurrents in these tables were measured at 0 V, and the photocurrents under the illumination of visible light (> 440 nm) were measured by using a cutoff filter of y44 (Koshin Kogaku). Oxide films were difficult to form in acidic and alkaline solutions. When the oxidation was carried out over a cell voltage of about 80 V, local breakdowns accompanied by sparking occurred on the oxide film at the electrode surface. Such oxide layers were porous with many regular pores as observed with SEM. All of the electrodes in these tables were prepared at a cell voltage of 100 V for 16 min in saturated Na2S04solution, and the formed oxide films were 5-10 pm in thickness. it was found that the oxide films were doped with the added metal ions as well as Na+ from the Na2S04in the electrolyte, based on the SIMS measurement. The concentrations of the doped cations were high in the surface layer of the film and low in the bulk. Thus, the cations in the electrolyte are directly doped by the anodic oxidation accompanied by sparking. For the purpose of increasing the conductivity of the electrode, the reduction was carried out by electrochemical reduction or thermal reduction. The former is done by cathodic electrolysis in a saturated Na#04 solution without additives in 1 min at a cell voltage of 30 V with the rectifier circuit system stated above, and the latter is done at 700 or 800 OC in Hz for 1h. It is found in Tables I and I1 that the increases in the visible light photocurrents and the decreases in the overall photocurrents are brought about by the Cr, Ir, and Rh dopings for TiOz and by Cr doping for Nb205. This tendency is more remarkable with the electrode prepared at higher concentrations of the additives, which will result in higher concentration of the doping metal in the formed oxide. However, unknown islands are formed in the oxide films by anodic oxidation at higher concentrations of the additives than those listed in these tables, leading to the decreases in the overall and visible photocurrents, probably because of the disturbance caused by light passing into the Ti02 or Nb205surface. In Table I, the photoelectrochemical properties of the thermally oxidized Ti02 and single-crystal rutile are also listed for comparison. It is clear that the visible photocurrent is much higher for the doped electrodes than for the above electrodes. The reduction procedures, especially
Light intensity
( a.u.1
Flgure 3. Photocurrent as a function of light lntenslty of 500 nm. All samples are electrochemically reduced: (a) TIO, prepared In 10" M Cr0-: solution, (b) T102 In 7.5 X lo4 Ir4+solution, (c) TiO, In 2.5 X M Rh3+, (d) T10, In solution containing no addlthre, (e) Nb205in lo-' M Cr,O?- solution.
thermal reduction, cause an increase in the overall photocurrent and a decrease in the visible photocurrent in all of the electrodes listed in this table. The degree of reduction will be higher for thermal reduction than for electrochemical reduction, as shown in the results for thermally oxidized Ti02in which the overall photocurrent is much higher for the thermally reduced sample than for the electrochemically reduced sample. The same phenomenon is also observed for the Nb205electrodes, as shown in Table 11. The spectral dependences of the quantum efficiencies of the Ti02and Nb2O5 oxide films prepared in this study are shown in Figures 1 and 2, respectively. All of the Ti02 electrodes doped with Cr,Ir, and Rh show almost the same spectral dependences. The decrease in the overall photocurrent stated above caused by the doping is based on the UV photoresponse. Similar spectral dependences were originally found by Maruska et al. for single-crystal rutile doped with Cr and Mn.25926 The spectral dependence of the thermally reduced sample of Cr-doped Ti02 is also shown in Figure 1. An increase in the UV response and a decrease in the visible response are observed. The doped Nb205also shows spectral dependences similar to those of TiOz. Figure 3 shows the visible photocurrents as a function of the light intensity of the 500-nm monochromatic light. The photocurrents linearly increase with the light intensity for all of the electrodes. The same linear relationships are also obtained for other monochromatic light (300-600 nm). ~
~~
(26) Ghosh, A. K.; Maruska, H.P. J . Electrochem. SOC.1977, 124, 1516. (26) Maruska, H.P.; Ghosh, A. K. Sol. Energy Mater. 1979, I , 237.
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The Journal of phvslcal Chemlstty, Vol. 86, No. 18, 1982
A
0'
O'
100 500 Wavelength ( nm)
600
I
Flgure 4. Absorption spectra of TiO, films: (a) prepared In lo3 M M Cr0,2- solution and thermally reduced at 800 OC in H,, (b) in GO,* solutkn and electrochemically reduced, (c) in 2.5 X le M Rh3+ solution and electrochemically reduced, (d) in solution containing no additive and electrochemlcally reduced.
The same linear relationship was reported by Butler et aLn for nondoped single-crystal Ti02 and SrTiOs. They postulate that a two-step excitation process is responsible for this phenomenon. However, it is more likely that the linear relationship is based on a one-step excitation process and that the slope reflects the degree of the recombination process.28 Figure 4 shows the optical absorption spectra of the doped TiOz films, which were observed with an integral sphere spectrophotometer (Type 323, Hitachi Ltd.). The absorption spectra are almost consistent with the spectral dependences of the photocurrenta in Figure 1 for the electrochemically reduced samples but not for the thermally reduced sample. The absorption spectra for the latter sample, i.e., the strongly reduced sample, do not indicate an excitation process for hole creation contributing to the electrochemical process but only a simple absorption process. The visible absorption of this sample is probably mainly based on the excitation process from the defect levels concerned with oxygen vacancies below the Fermi level to the conduction band, which will bring about no visible photocurrent. Thus, it seems that oxygen vacancies do not affect the photoelectrochemicalproperties. Similar spectra were also obtained for Nb205films.
Discussion The oxide films prepared by anodic oxidation accompanied by sparking (breakdown) in NazS04solution are thick and porous and are useful as photoanodes. Moreover, it is noteworthy that the cations in the electrolyte are easily doped into the oxide layer. This is a new method for doping into a polycrystalline oxide. Strangely, S ion from SO4%ion in the electrolyte was not detected by SIMS. The cations in the electrolyte may not be electrochemically doped into the oxide layer but may be thermally doped. The cation adsorbed on the electrode surface will diffuse into the oxide lattice with oxide growth caused by the generated temperature of the electrode surface under the sparking voltage. In fact, the temperature of the electrolyte (200 mL) rose to 60-70 "C after anodic oxidation for 16 min at 100 V. However, the detail of the doping mechanism in this case is not clear. Oxide films prepared in this study will have many recombination states near the Fermi level. The increase in (27) Butler, M. A.; Abramovich, M.; Decker, F.; Juliano, J. F. J. Electrochem. SOC.1981,128, 200. (28) Morrison, S. R. 'Electrochemistry at Semiconductor and Oxidized Metal Electrodes";Plenum Press: New York, 1980.
Ob
20
60
40
Visible photocurrent ( pA/cm2 )
Figure 5. Relationship between overall photocurrent and visible photocurrent for TiO, film. Circle and open symbols represent solution containing no additive; circle and closed symbols represent Be2+ solutkn; cirde and haff-closed symbds represent solution; triangle and open symbols represent P+solution; triangle and closed symbols represent Be2+ and CrO,,- solution; triangle and half-closed symbols represent Rh3+ solution.
w:-
the UV photocurrent due to the reduction will be assigned to the decrease in the recombination centers, which will be the structural defects near the conduction-band energy. These defect levels will be filled with electrons with the Fermi level rising because of the reduction; the filled levels are no longer the recombination states for electrons. A similar phenomenon was also observed for thermally oxidized Ti02,13 This phenomenon accompanies a decrease in the visible response. The visible photocurrents observed on oxide films without additives will also be due to the structural defects, i.e., interestitial metal (Ti or Nb) ion, but not doped Na ion, because the energy levels of the latter ion are very different from those of Ti ion, Nb ion, and oxygen ion. We claimed in previous papers7J2that the visible photoresponse is assigned to the d orbitals of the interstitial cations situated near the K* conduction band, which seems to form the impurity band (d band). A similar conclusion is also drawn for the doped samp1es?J2 Thus, it will be concluded that the recombination states are consistent with the impurity and interestitial metal d bands or states bringing about the visible response in the energy position. Moreover, it will follow from the following relationship between the UV photocurrent and the visible photocurrent that both states will be the same. Figure 5 shows the relationship between the overall photocurrent and the visible photocurrent for the various Ti02 electrodes prepared in this investigation (at 100 V in K 8 0 4and N a 8 0 4 solutions with Be, Cr, Rh, and Ir ions and with no additive). The electrodes in this figure are limited to those having a thick oxide layer (>5 pm) which was controlled by anodic oxidation time (>1 min), because the magnitude of the photocurrent is influenced by the thickness of the oxide layer. A thin oxide film produces only a small photocurrent because of the thin space charge layer which is limited by its thickness. The preparation of the doped electrodes was carried out in electrolyte containing less than M additive ions so that disturbing islands are not formed on the electrode surface. Be doping brings about a slight increase in the UV response. All electrodes were electrochemically reduced at 30 V for 1 min. It is found in Figure 5 that the overall photocurrent, Le., UV photocurrent, decreases with increasing visible photocurrent. From this relationship, we conclude that the doped ions (Cr, rh, and Ir ions) and structural defects
J. phys. Chem. lB82, 86, 3585-3591 impurity or interstitial d-band or states
Band model of TO, and Nb206 prepared in this study. The recombination process and the vlslble photoredponse are illustrated. Figure 6.
(probably interstitial Ti ions) bringing about the visible photocurrent will simultaneously produce the recombination states. That is, a natural conclusion based on the reduction effects is that the impurity and interstitial Ti d bands or states bringing about the visible response also act as recombination centers in the elecwdes used in this study. The band model is illustrated i e Figure 6. However, we do not preclude the existence of impurity or defect levels near the 7r valence band; some of these levels may
3585
also possibly contribute to the visible response and the recombination process. The energy position of the impurity band or states agrees with that proposed by Mirlin et al.,29but not with those proposed by Ghosh et al.25and Gautron et al.21The disappearence of the visible photoresponse on strong reduction2lPBwould confirm the existence of the impurity or defect levels near the conduction band proposed by us and Mirlin et aLa These levels will be filled with ele?trons as the Fermi level rises because of the strong reduction, and, therefore, visible photoexcitation will not occur. Especially, Mirlin et al.29observed the disappearance of visible absorption on thermal reduction for Cr-doped single-crystal Ti02,although this disappearance was not observed in the present study, as Figure 4 shows.
Acknowledgment. We thank Professor H. Tamura, Dr. H. Yoneyama, and Dr. 0. Ikeda, at Osaka University, and Mr. Y, Okajima, of Hitachi Research Laboratory, for SIMS measurement. (29) Mirlin, D. N.; Reshina, I. I.; Sochana, L. S. Sou. Phys.-Solid State Engl. Transl. 1979,11, 1995.
Quenching of Singlet and Triplet Excited States of Aromatic Molecules by Europium Ions N. Sabbatlnl,’la M. T. Inddll,lb M. T. GandoH1,la and V. BaIzanlla~c IsHMO ChknlcO “0.Ciamician” dell’UnlversP, Bobgna, Italy: Centro di Studio sulk Fotochimlca e ReattivlrS degii Stati Eccitati dei CompaPtl W coordlnazkns del C M ,Utllv6fsltd di F m m , Italy; and IsHtuto di Fotochimlca e Radiazionl d’Aita Energia del CNR. Bologna, Italy ( R W : Febnvry 24, 1982: I n Flnai F m : M y 13, 1982)
The rate constants of the quenching processes of the lowest singlet excited state of naphthalene, pyrene, anthracene, 9,10-diphenylanthracene,tetracene, coronene, rubrene, and protoporphyrin IX dimethyl ester, and of the lowest triplet excited state of anthracene, tetracene, and coronene by Eu3+in acetonitrile solution have been measured. Singlet excited s t a t a are quenched with practically diffusion-controlled rates, the main quenching products being radical ions and triplets. Triplet excited states are quenched with very low rate constants (
