J. Phys. Chem. 1991,95, 779-783 larities between the spectra of the cathodes and Li2FeS2at 4.2 K, there is not a complete correspondence. In particular, the spectrum of Li2FeS2shows an intense absorption at +4.8 mm s-l and a low-intensity absorption of +1.7 mm s-' which do not appear in the cathode spectrum. A comparison of the spectra of Li3Fe2S4with those of the cathodes again shows some similarities but some differences. Of particular note here is the result of an experiment in which a cell was discharged at 55 OC. On the assumption that discharge at a higher temperature would lead to a closer approach to thermodynamic equilibrium during discharge, it was surmised that Li2FeS2would emerge as a more dominant intermediate at 55 However, the room-temperature M6ssbauer spectrum following discharge at 55 OC, while somewhat less complicated than that of cathodes discharged at room temperature, clearly indicated the presence of Li3Fe2S,. Certainly the absorptions at -3.8, -3.0, -0.8, +2.2, and +5.7 mm 6' observed in the Li3FezS4 spectrum are also observed in the 55 OC discharged cathode spectrum (Figure 6c). In Figure 7 a comparison is drawn between the 57FeM k b a u e r spectrum at 4.2 K of a half-discharged cell and a simulated spectrum which contains Li3Fe2S4,Li2FeS2,and FeS2 as components. Again, while the agreement is not exact, the two are very similar. The above results suggest that, in the discharge of the Li/FeS2 cell at or near to room temperature, a mixture of discrete reduction products is produced in the cathode. This mixture includes (i) Li3Fe2S4,(ii) a species that is similar, but not identical, to Li2FeS2 which may possibly be Li233Fe,,6,S2, and finally (iii) small particles of iron. FeS2
--
Lil,5FeS2
Lil.SFeSz Li2FeS2
Li2FeS2
(7) (8)
+ FeO
(9)
Li2S
Not surprisingly, the reduction is not a simple stepwise process. Thus,at halfdischarge significant quantities of both Li3FeS4and a product similar to Li2FeS2appear to be present at the same time, together with unreduced Fa2. It appears that discharge does not ~
(13)
Fong, R. M.Sc. Thesis.Simon Fraser,
1988 (unpublished).
779
occur at thermodynamic equilibrium and that kinetic factors are important. This is emphasized in the LiJLiAsF6-propylene carbonate)FeS2cell discussed earlier, where iron particles were the only macroscopically observed product at any point in the discharge. In that case, the reduction to iron appears to occur rapidly without the buildup of any measurable quantities of intermediate. It is not clear why the change in electrolyte produces such a significant change in the reduction path. The fact that the spectrum of Li2FeS2does not coincide exactly with that of the reduction product in the cell is perhaps not too surprising. Given the layered structure of Li2FeS2and the passibility of varying the Li and Fe occupancies of the tetrahedral holes in layer 11, or indeed the lithium occupancies of the octahedral holes in layer I, a spectrum of iron sites could be present which are not precisely the same as those in stoichiometric Li2FeS2. Chemical Reduction of FeS2 with n-BuLi. The s7Fe Mijssbauer spectra of samples of FeS2 chemically reduced with n-BuLi in hexane at room temperature were very similar to those noted above for the Li(LiC104-propylene carbonatelFeSzcell experiments, as shown in Figure 8. Indeed, there is remarkably good agreement in the spectrum of FeSz reacted with 2.4 f 0.1 equiv of lithium and a cell cathode discharge to 2.5 equiv (compare Figure 8b with Figure 2b). In addition, spectra were recorded at room temperature on chemically lithiated samples of FeS2in a weak external magnetic field. The presence of a reduction product in a 1.5 equiv of lithium sample could be discerned in the presence of an external field, although in the absence of such a field the parent FeS2 spect?m looked little changed (Figure 8a) at room temperature. This is evidence that the reduction product is formed as very small particles which are superparamagneticat room temperature. On further reduction superparamagnetic particles of iron are formed which in an external field exhibit the characteristic internal magnetic field of iron.
Acknowledgment. We thank Dr. R. R. Haering for suggesting this work, Dr. Ray Batchelor for his assistance in computational aspects, and Dr. Tom Birchall of the Department of Chemistry, McMaster University, Hamilton, Ontario, for supplying us with a copy of the MBssbauer fitting routine GMFP. We are grateful to the Natural Sciences and Engineering Research Council of Canada for support of this work.
Photon Emission via Surface State at the Gold/Acetonitrlle Solution Interface Kobei Uosaki,* Kei Murakoshi, and Hideaki Kits Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060, Japan (Received: April 6, 1990; In Final Form: July 10, 1990)
The emission of light caused by an electron-transfer reaction at a gold electrode in acetonitrile solution containing one of three redox species (benzophenone, tram-stilbene,and benzonitrile) with different redox potentials was studied. The high-energy threshold of the spectrum decreases linearly as the potential of the gold electrode becomes more negative. The peak position with respect to the high-energy threshold of the spectrum varies with electrode potential and is not affected by the redox potential of the electron injection species at the same electrode potential. The emission efficiency also depends on the potential. From these results, we proposed that the emission is due to a charge-transfer reaction inverse photoemission (CTRIP)process that takes place via a surface state.
Introduction Infomation on electronic of electrodes, e,g., metals and semiconductors, is very important to understanding how elect m h e m i a l reactions at electde/electrolyte interfaces. Recently, several techniques to obtain this information either in situ or ex situ have been developed. Ultraviolet photoemission
spectroscopy (UPS)and inverse photoemission spectroscopy (IPS) are useful in determining the distribution of the occupied and unoccupied states, respectively, under ultrahigh vacuum (UHV) conditions.' Unfortunately, however, there are not many tech(1) Smith, N. V.;Woodruff,D.P.Prog. Surf. Sci. 1986. 21, 295.
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780 The Journal of Physical Chemistry, Vol. 95, No. 2, 1991
niques available to obtain this kind of information about electrodes in solution. Electroreflectance (ER) spectroscopy, which measures the optical transition from occupied state to unoccupied state, is often applied to the study the electronic states of metals2 and semiconductors3 in electrolyte solution in situ. McIntyre and Sass reported that a photon is emitted from a gold electrode, induced by electron injection from an electron donor in solution to the e l e ~ t r o d e . ~They suggested that the emission is due to radiative relaxation of excited states, which are created by the electron injection, to empty electronic bulk states in gold electrode, Le., inverse photoemission process. By analyzing the emission spectrum, one should be able to obtain the information of unoccupied states. Since the electron injection process in this case is actually a charge-transfer reaction, they proposed to call this phenomenon charge-transfer inverse photoemission spectroscopy (CTRIPS).S They pointed out that the emission of a photon can be induced not only by electron injection but also by hole injection to the electrode from an electron a~ceptor.~,’In the latter, filled states can be studied. Bard and Ouyang applied this technique to platinum/acetonitrile By employing various electron or hole injection species with different redox potentials, they found that light is emitted only when either electrons are injected by donors that have the redox potential more negative than a critical value or holes are injected by acceptors that have the redox potential more positive than a critical value. From these results, they concluded that light is emitted in CTRIPS by the relaxation of excited states to Schcckley-type surface state. CTRIPS seems to be very useful to probe both unoccupied and occupied electronic states of electrodes in solution, although the quantitative interpretation of the observed spectrum is not easy. One complication arises from the fact that injected electrons or holes have an energy distribution that reflects the energy distribution of a donor or an acceptor in solution, although the injected electrons have single energy in the case of IPS measurement under UHV. Thus, the CTRIP spectrum contains the information of energy distribution not only of the electrode but also of a donor or an acceptor in solution. In the present work, we have investigated the light emission process at a gold electrode in acetonitrile solution containing one of three redox species (benzophenone, trans-stilbene, benzonitrile). From the potential dependence of the spectrum of emitted light, we found that the emission is by a CTRIP process, mainly due to the relaxation of injected electrons to surface states. The present results are compared with the results of ER spectroscopy.
Experimental Section Materials. Spectral grade acetonitrile (Dojin Chemical Laboratories Co.) was used as solvent and was dried with P20, by continuous stirring for 24 h. The solvent was further purified by repeating the vacuum transfer and stirring with P20sfor 3 h three times.’O Reagent-grade tetrabutylammonium tetrafluoroborate ( (C4H&NBF4) (Aldrich Chemical Co.), benzophenone, and trans-stilbene (Wako Pure Chemicals Co. Ltd.) were recrystallized three times from ethyl acetate, hexane, and methanol, respectively, and dried under reduced pressure at 90-100 OC for 24 h. Reagent-grade benzonitrile (Wako Pure Chemicals Co. Ltd.) was dried with MgS04, distilled from P20s under reduced pressure, and stored over 4A-1/16 molecular sieves that were activated by heating in vacuum at 250 O C . I l (2)
(3)
Boeck, W.;Kolb, D. M. Surf. Sci. 1982, 118, 613. Seraphin, B. 0. J . Appl. Phys. 1966, 37, 721. McIntyre, R.;Sass, J. K. Phys. Rev.Lett. 1986, 56, 651.
(4) (5) Mclntyre, R.; Sass, J. K. J . Electroanal. Chem. 1985, 196, 199. (6) Mclntyre, R.;Roe,D. K.; Sass, J. K.; Storck, W .J. Electroanal. Chem. 1987, 228, 293. (7) McIntyre, R.; Roe,D. K.;Sass, J. K.; Gerischer, H. Electrochemical Surface Science; Soriap, M. P.. Ed.; ACS Symposium Series 378; American Chemical Society: Washington, DC, 1988; pp 233-244. (8) Ouyang, J.; Bard, A. J. J . Phys. Chem. 1987, 91, 4058. (9) Ouyang, J.; Bard, A. J. J . Phys. Chem. 1988, 92, 5201.
(IO) Gerischer, H.:McIntyre, R.; Scherson, D.; Stork, W . J . Phys. Chem. 1987, 91, 1930. ( I 1) Perrin, D. D.; Armarego, W.L. F.;Perrin, D. R. Purification of Luborarory Chemicals, 2nd ed.; Pergamon: New York, I98 1.
2L k D B.
Figure 1. Vertical sectional view of the spectroelectrochemical cell: A, quartz window; B, window holder; C, cell body; D, solution inlet channel; E, reference electrode channel; F, counter electrode; G, working electrode.
Gold electrodes were prepared by vacuum deposition using a gold wire (99.99%) as a source, on clean glass or cleaved mica in a 10-S-Torr vacuum. The temperature of the substrate during deposition was room temperature or 300 OC and controlled by a hot-plate controller (Chino Co., DB-01-3). The deposition rate (0.1 nm/s) and the thickness of the film were measured with a quartz crystal thickness monitor (ULVAC Co., CRTM-1000). Orientation and smoothness of the films were studied by X-ray diffraction measurements and scanning tunneling microscopy (STM, Digital Instruments Co., NanoScope I), respectively. Apparatus. A spectroelectrochemical cell used in this experiment is shown in Figure 1. The cylindrical cell is made of Teflon with a UV quartz (Fujihara Factory Co.) optical window. The gold working electrode (WE, area = 0.28 cm2) and a platinized platinum mesh counter electrode (CE) were held firmly in the cell so that they are in parallel position. The separation between the two electrodes was kept at ca. 8 mm by a Teflon spacer. The Luggin capillary was positioned near the working electrode by means of a 0.5-mm-diameter channel in the cell. A Ag/O.Ol M AgNO, reference electrode (RE) was used, and all potentials reported herein are referred to this reference. Electrolyte solutions were prepared in a vacuum line under I O-, Torr and transferred through Teflon tube to the cell by positive N2 (99.999%) pressure. The cell has a channel to purge N2 around the Teflon spacer that separates the WE and CE. The WE potential was controlled by using a potentiostat (Nikko Keisoku, NPGS-301s). An external potential was provided by either a programmable function generator (Toho Technical Research Co., FG-02) or a home-built pulse-generating system controlled by a personal computer (NEC Co., PC-8801 mk 11).12 The light intensity was monitored with a photomultiplier tube (PMT; Hamamatsu Photonics Co. Ltd., R636) that has a fairly flat response over the region 300-800 nm according to the supplier. For the transient measurement, the PMT response of unfiltered emitted light was amplified by a fast response amplifier ( N F Electronics, LI-75A, 1 MHz, gain = 100). In the emission spectrum measurements the optical window of the cell was placed in front of an entrance slit of a monochromator (Ritsu Oyo Kagaku Co. Ltd., MC-ZONP,/= 5.9), and a photon counter (NF Electronics, LI-574) was employed. The PMT response was averaged four times at each wavelength. The photon-counting duration was 100 ms and synchronized to potential modulation. Since the emission intensity was weak, the slit was wide open ( 5 mm), and therefore spectral resolution was low (20 nm). The personal computer was used to collect the PMT response (output of either the fast response amplifier or the photon counter) and current via a 12-bit analog-to-digital converter (ADC) and to control the monochromator via a P I 0 interface. All experiments were carried out at room temperature. ~~
(12) Uosaki, K.; Kita, H. J . Am. Chem. SOC.1986, 93, 6521.
Photon Emission at Gold/Acetonitrile Interface
The Journal of Physical Chemisrry, Vol. 95, No. 2, 1991 781
f
Figure 2. Time course of potential (top panel), emission intensity (middle panel), and current (bottom panel); 50 mM benzophenone in acctonitrile/0.2 M (C4H9)4NBF,.
Results It was confirmed by cyclic voltammetric measurements that all the aromatic compounds used in the present study showed reversible redox behavior at a gold electrode in rigorously purified acetonitrile solution. Symmetric reduction and oxidation waves with small peak separations were observed a t all cases. No electrochemical reaction other than the redox of the species was observed in the voltammogram within the potential region investigated (-2.6 to 0.7 V). Redox potentials ( V ) of benzophenone, tram-stilbene, and benzonitrile were determined as the average of the cathodic and anodic peak potentials and are -2.15, -2.6, and -2.7 V, respectively. In emission measurements, the electrode potential was stepped first to a negative potential where the anion radical of the added species is formed and then to various positive potentials (Uf = -0.5 to 0.7 V) where the anion radical is oxidized. Figure 2 shows a typical time course of light intensity (middle panel) and current (bottom panel) at the gold electrode in acetonitrile solution containing 50 mM benzophenone and 0.2 M (C4H9)4NBF4when the potential pulse was applied as shown in the top panel. In this particular case, the potential was stepped from -0.4 to -2.4 then to 0.8 V. The pulse width was 51.2 ms. During the cathodic pulse at -2.4 V, reduction current due to the generation of benzophenone anion radical flowed but no emission was detected. When the potential was stepped to 0.8 V, anodic current due to the oxidation of anion radical flowed, and light was ~bserved.’~Light intensity was very sensitive to the condition of the electrode surface and impurities in the solution. If too negative a potential was applied, decomposition of solvent and solute took place and organic polymeric compound was formed on the electrode surface,14 leading to a noticeable decrease of emission intensity and current while double-potential steps were repeated. Thus, negative potential limit a t CTRIP spectrum measurement was restricted to 0.1 V more positive than the redox potential of each species. Light intensity was constant even after more than 100 cycles of double-potential step as far as the above condition was maintained. The pulse width was set at either 102.4 or 512 ms for the spectrum measurement. When the emission was weak, the longer pulse width was used to generate much more radical anion and more strong emission. Figure 3 shows the emission spectra obtained at various Uf in solution containing benzonitrile as a redox species. From these spectra, it is clear that the high-energy threshold of each spectrum (Eth)increases as Ufbecomes more positive. The peak energy of each spectrum (E,) also increases as Ufbecomes more positive. The width and shape of spectrum also have a Ufdependence. Similar tendencies were observed in the emission spectra of gold in acetonitrile solution containing rranr-stilbene or benzophenone as a redox species. If purification of the solute and solvent was (1 3) A photograph taken with long exposure showed that the emission is
only from the gold and not from the counter electrode. (14) Ouyang, J.; Bard, A. J. J . Electroanal. Chcm. 1987, 222, 331.
u -0.5V
2.0 3.0 4.0 Photon Energy / eV Figure 3. Emission spectra of gold electrode. The potential was stepped from -2.6 V to various electrode potentials Uf;50 mM benzonitrile in acetonitrile/0.2 M (C4H9),NBF,.
-0.5
0 0.5 P o t e n t i a l / V vs. A g / A g +
Figure 4. Emission efficiency-potential relation. The conditions are as in Figure 3. not appropriate or a too negative potential was applied, the spectrum shape and emission intensity were not reproducible and a spectrum with no Ufdependence was observed. The emission intensity was also a function of Ufand increased as Ufbecame more positive. Figure 4 shows the emission efficiency, a, in benzonitrile solution as a function of Up The efficiency is defined as the integrated emission intensity over the integrated oxidation current for a given duration. The efficiency increases exponentially with Uf in the negative potential region but saturates around 6 X IO4 counts/mC at relatively positive Uf. Three different gold electrodes were used, i.e., gold thin films deposited on glass at room temperature, on glass at 300 OC, and on mica at 300 “C. X-ray diffraction studies proved that the gold (1 1 1) face dominates over these film surfaces. STM measurements showed that the temperature of substrate influenced the surface morphology. The surface of the film deposited at 300 OC had relatively large flat terraces of lateral dimensions of 60-100 nm. On the other hand, the films deposited at room temperature had rolling hills and valleys with sizes of -20 nm.15-16The shape of the emission spectrum and the emission intensity were not strongly affected by the sample preparation conditions, although the effect of sample preparation conditions on the crystallinity (15) Putnam, A.; Blackford, B. L.; Jericho, M.H.;Watanak, M.0.Surf. Sci. 1989, 217, 276. (16) Chidsey, C. E. D.; Loiacono, D. N.; Sleator, T.; Nakahara, S.Surf. Sci. 1988, 200, 45.
Uosaki et al.
782 The Journal of Physical Chemistry, Vol. 95, No. 2, 1991
1
-0.5
Photon Energy
- El,)
/ eV
Figure 5. Emission spectrum of gold at various electrode potentials, taking photon energy with respect to Einjas abscissa: a, 0.7 V; b, 0.5 V; c, 0.2 V; d, 0 V: e, -0.2 V; f, - 0 . 5 V.
and roughness is quite significant. Discussion
There are various possibilities for the origin of light emission at metal/electrolyte interfaces. One possibility is electrogenerated chemiluminescence (ECL).” Highly exergonic electron-transfer reactions often yield electronically excited, rather than groundstate, chemical species in solution. If the decay of an excited species to its ground state is radiative, the electron-transfer reaction may be accompanied by light emission. The ECL spectrum should be unique for a given system and should have no electrode potential dependence. The observed spectra in the present study, however, depended on Up This fact excludes the possibility that the origin of the emission observed in the present study is due to ECL. An additional reason for eliminating ECL is that the excited-state energy of molecules used do not correlate to the emitted photon energy observed here? As mentioned before, potential-independent spectra were also observed a t certain cases even in the present study. These spectra originated from an ECL process induced by the decomposition or polymerization of the chemical species a t the gold/solution interface. A second possibility for the emission is the CTRIP process. As pointed out in previous reports,e9 Eth of a CTRIP spectrum corresponds to maximum radiative energy due to the decay from the electron injected state to the Fermi level and is, therefore, determined by the difference between the redox potentials of the chemical species and the electrode potentials. The energy of injected electron with respect to the Fermi level (Ein,)may be represented by the difference between LIO and Uf (Einj]. = -(V - Uf)). Figure 3 shows that Eth decreases linearly as Uf becomes more negative, Le., the energy difference between Einjand Ethis almost constant at each spectrum (04.1 eV). The linear relations between Ein.and Eth were found in all cases investigated in the present study. These results support that the emission is due to CTRIP process. If the relaxation is solely due to the bulk unoccupied states, not only (Eth- Einj)but also the shape of the spectrum should be independent on U p The difference between Einjand E,, however, becomes larger when U,becomes more positive as shown in Figure 3. The effect of Uf on the shape of the emission spectra is more clearly seen in Figure 5 , where the emission spectra are redrawn by taking the photon energy with respect to Einjas the abscissa. If there is no potential dependence, obviously all spectra should be superimposed on each other in this plot. It is clear, however, that both the shape and the values of (Ein,- E,) have the electrode potential dependence. The values of (Einj- EP)obtained for the three redox species are plotted against Uf in Figure 6. The data of not only the present study but also of the results by McIntyre et aLSv7are quoted.l* There is a linear relation between (Einj- EP)and Uf with the positive (17) Faulkner, L. R. In Methods in Enzymology; DeLuca, M. A., Ed.; Academic: New York, 1978; Vol. 57, pp 454-526. (18) The values of (Eioj EP)were calculated from V,, V ,and E, of Mclntyre et al. in the same manner as in this study. Note the redox potentials of benzophenone (CP = -2.0 V)l and trans-stilbene (V = -2.5 V)’ quoted by Mclntyre et al. were different from our values by 0.15 and 0.1 V, respectively.
-
1
1
1
1
1
,
1
1
1
1
1
1
1
0 0.5 Potential / V vs. Ag/Ag’
Figure 6. Relation between (Einj- EP)and electrode potential: (0) bcnzonitrile, present results; (A) trans-stilbene, present results; (A) trans-stilbene, results of McIntyre et al.;’ (0)benzophenone, present results; (m) benzophenone, results of McIntyre et aLs
slope of 0.5 eV/V, although there is some deviation from the linear line at relatively positive potentials. The deviation from the linear line seems to be more significant for benzophenone. It must be also noted that there is no clear effect of electron injecting species on this relation. If the emission spectrum reflects only the bulk band distribution of the metal as previously suggested by McIntyre et ale,’ the observed spectra at various electrode potentials should have no potential dependence and (Eq - E ) should be constant, contrary to the results shown in Figure 6. t h e results presented here suggest that the distribution of the unoccupied electronic states of gold, which is responsible for the CTRIP process, shifts to the high-energy direction against the Fermi energy, EF,when Uf becomes more positive. This shift can be explained if the states are located at the surface, i.e., surface states. In CTRIPS measurement, the mean free path of the injected electron is so short that one can expect that the spectrum reflects the distribution of the energy states closer to the metal surface rather than to the b ~ l k . ~ -Surface ~ q ~ ~ states of metals were observed under UHV condition by Although the situation in an electrochemical environment is quite different, the existence of intrinsic surface states of metals in solution was also suggested from the result of ER s p e c t r o ~ c o p y .The ~ ~ ~peak ~ ~ position of ER spectra shifts to a higher energy when the electrode potential becomes more positive. In the case of Au( loo), the peak shift is 0.4 eV/V.23 This result was explained as follows. Since surface states are localized a t the metal electrolyte interface, the energy position of the states is affected by the field within the electric double layer.% When the electrode potential becomes positive, the energy position shifts to higher energy with respect to EF. Although a quantitative comparison between the results of ER spectroscopy and the present results is impossible at this stage, the fact that the potential dependences of the peak position of the CTRIPS and ER spectra are similar in both cases supports that the surface states are also involved in the CTRIP process. The fact that (Einj - Ep) at a given potential is not affected by electron injecting species indicates that the states involved are intrinsic. To understand the potential dependence of emission efficiency shown in Figure 4, we need more investigation, but we may speculate as follows. McIntyre et al. showed that the emission intensity was affected by the density of bulk band states to which the charge, Le., electron or hole, was injected. They found that light emission intensity by hole injection is much stronger than that by electron i n j e ~ t i o n .They ~ considered that the stronger emission at hole injection is due t o the fact that the density of d-band states is higher at the energy region where a hole is injected than at the energy region where an electron is injected. The potential dependence of the efficiency was explained by this consideration. The density of bulk states of gold is much higher (19) Himpel, F. J.; Fauster, Th.; Straub, D. J . Lumin. 1984,31/32,920. (20) Goldmann, A.; Dose, V.; Borstel, G. Phys. Reo. 1985, B32, 1971. (21) Woodruff, D. P.;Royer, W. A.; Smith, N . V. Phys. Reo. 1986,834, 164. (22) Kolb, D.M.; Franke, C. Appl. Phys. 1989,A49, 379. (23) Liu, S.H.; Hinnen, C.; Huong, C. N . V.; Tacconi. N . R. D.; Ho, K. M . J . Electroanal. Chem. 1984, 176, 325. (24) Kolb, D.M.; Boeck, W.; Ho, K. M.; Liu, S. H. Phys. Reo. Leo. 1981. 47, 1921.
J. Phys. Chem. 1991.95, 783-787 at an energy region higher than 3 eV above EFthan at the region lower than 3 eV above In the present system, Einjis linearly changed by changing Uf, and at U,= 0.3 eV, Einjis about 3 eV with respect to EF. Thus, one expects lower efficiency if Uf is more negative than 0.3 V as experimentally observed (Figure 4). Light emission from metals stimulated by electron injection was also reported at the tunneling junctionu and during the scanning tunneling microscopy (STM) measurement in a v a c ~ u m . ~ ~In. ~ * these systems light emission have been considered to be caused by the radiative decay of a localized surface plasmon polariton mode that was excited by tunneling electron and surface roughness. The contribution of this process to the CTRIP process is not clear (25) Marel, D. V. D.; Sawatzky, G. A.; Zellcr, R.; Hillebrecht, F. U.; Fuggle, J. C. Solid Stare Commun. 1984, 50, 47. (26) Dawson, P.; Walmslcy, D. G.; Quinn, H. A.; Ferguson, A. J. L. Phys. Rev. 1984,830, 3164. (27) Gimzewski, J. K.; Sass, J. K.; Schlitter, R. R.; Schott, J. Europhys. Lrrr. 1989. 8. 435. (28) Cwmbs. J. M.;Gimzcwski, J. R. R. J. Microsc. 1988, 152, 325.
M.;Reihl, B.; Sass, J. K.; Schlittler,
783
at the present time. To elaborate the process of the light emission from the metal/electrolyte interface, further investigation about the characteristics of injected electron at the metal/electrolyte interfacez9is necessary. In summary, we studied the emission of light from a gold electrode in acetonitrile solution containing one of three redox species, i.e., benzophenone, trans-stilbene, and benzonitrile, and proposed that the emission is due to the CTRIP process taking place via surface states.
Acknowledgment. Prof. Y . Hariya is acknowledged for his arrangement of X-ray diffraction measurement. Thanks are due to Mr. M. Kohiyama and Mr. T. Kiya of Hokkaido University for manufacturing the cell and solution transfer system, respectively. This work was partially supported by the International Scientific Research Program (Joint Research 0104405) and a Grant in Aid for Scientific Research (02453001) of the Ministry of Education, Science and Culture, Japan. (29) Sass, J. K.; Gimzewski, J. K. J. Electroanal. Chem. 1988, 251,241.
Luminescence of [E~C2.2.1]~+ and [Cec2.2.1l8+ Cryptates Adsorbed on Oxlde Surfaces M. F. Hazenkamp,* C. Blasse, Debye Research Institute, University of Utrecht, P.O. Box 80000, 3508 TA Utrecht, The Netherlands
and N. Sabbatini Dipartimento di Chimica “G. Ciamician” dell’Universitb, 40126 Bologna, Italy (Received: July 12, 1990)
The luminescence properties of the (E~C2.2.11~’and (CeC2.2.1I3+cryptates adsorbed on Sios and Ti02 surfaces are reported. There are considerable changes in the excitation and emission spectra of the luminescence of the Eu3+cryptate compared to those of the complex in aqueous solution. This suggests that HzOmolecules in the first coordination sphere of the Eu3+ ion are replaced by S i 4 entities. From the long decay time of this complex on silica it follows that there are no H 2 0 molecules coordinating the Eu3+ion anymore. This suggests that the cryptate molecule has a specific, hemispherical, conformation on this surface. Evidence is given that the [EuC2.2.1I3+complex decomposes on a Ti02 surface. Since the excitation and emission spectra of [CeC2.2.1I3+on silica are very similar to those of the complex in aqueous solution and of those of uncomplexed Ce3+ ions on silica, no conclusions about the differences in the coordination of the Ce3+ion in these surroundings can be drawn based on our measurements.
Introduction
It is known that several diazapolyoxabicyclic ligands (cryptates) form complexes with metal ions that are stable in aqueous solution and in the solid state.’ The luminescence properties of several lanthanide ions encapsulated in these cryptates have been reported over the past few years.” One of the first studies was on the Eu3+ ion in the 2.2.1 cryptand (see Figure 1) by Sabbatini et al.’ This study revealed that the luminescence efficiency of the Eu3+ ion increases considerably compared to that of the ion in aqueous solution. This is due to the shielding of the Eu’+ ion by the cryptate cage from the surrounding H 2 0 molecules which induce an efficient nonradiative decay. Moreover, the cryptate gives rise to a low-lying charge-transfer transition, which accounts for a much higher absorptivity compared to the noncomplexed Eu3+ ion. (1) Lchn, J . 4 . Acc. Chem. Res. 1978, 11, 49. ( 2 ) Sabbatini, N.; Perathoner, S.;Balzani, V.; Alpha, B.; Lehn, J . 4 . Supramolecular Photochemistry; Balzani, V., Ed.;Reidel: Dordrecht, 1987; p 187. ( 3 ) Sabbatini, N.; Dellonte, S.;Ciano, M.; Bonaui, A.; Balzani, V. Chem. Phys. Lett. 1984, 107, 212. ( 4 ) Sabbatini, N.; Dellonte, S.;Blasse, G. Chem. Phys. Lrtr. 1986, 129, 541. (5) Blase, G.; Dirksen, G. J.; Sabbatini, N.; Perathoner, S.Inorg. Chim. Acta 1987. 133. 167. ( 6 ) Blasse. G.; Buys, M.;Sabbatini. N. Chem. Phys. L r r r . 1986,124,538.
0022-3654/91/2095-0783$02.50/0
Recently, there is much interest in modifying the photophysical and photochemical properties of such cage complexes. This can be achieved by pairing the complex with a perturbing ion7 or by interaction with a host such as solid interfacesa9or zeolites.1° In this paper the photophysical properties of [EuC2.2.1I3+ and [CeC2.2.1I3+adsorbed on SiO, and Ti02surfaces are reported.
Experimental Section The 2.2.1 cryptates with Eu3+ and Ce3+ were prepared as described in ref 3. The cryptates are adsorbed from aqueous solutions of the complexes. The concentrations were 3 X lW3 M and the pH was about 6. The solid oxide supports which were used were porous vycor glass (Corning 7930) and TiOZ(Degussa P25). Porous vycor glass (PVG) is a porous material consisting of 96% Si02and 4% BzO3. The BET surface area is about 150 m2/g, and the average pore size is 40 A.” Disk-sha pieces of the glass were pretreated as described previously.’IrdThe cryptates ( 7 ) Sabbatini, N.; Perathoner, S.;Lattanzi, G.; Dellonte, S.;Balzani, V. J . Phys. Chem. 1987. 91, 6136. (8) Shi, W.; Wolfgang, S.;Strekas, T. C.; Gafney. H. D. J . Phys. Chem. 1985, 89. 974. ( 9 ) Blasse, G.; Dirksen, G. J.; van der Voort, D.; Perathoner. S.;Lehn, J.-M.; Alpha, B. Chem. Phys. k i t . 1988, 146. 347. (IO) Incavo, J. A.; Dutta. P. K. J. Phys. Chem. 1990, 94, 3075. ( 1 1) Corning technical information.
0 1991 American Chemical Society
