J. Phys. Chem. 1992, 96, 1819-1826
nothing to do with zeolite dilution) causes the change of the slope. It is known that as (corresponding to the pure zeolite) decreases as temperature increases.I5J6 The decrease of 6- depends not only on the increase of temperature but also on the degree of dilution. This temperature influence increases with the degree of dilution. To see how dilution can change the 6- dependence on temperature, we consider the simplest case of the powder sample. From eq 7 one can obtain the relative change of 6with temperature:
Again
where AE is the energy difference between the xenon inside and outside the zeolite. AE is negative. Therefore
-
dM = -d = M{ d(100- W) d W
l > O
(W+
(13)
c)2
In agreement with our experimental results, the higher the dilution, the more 6- decreases as T rises. Conclusion Adsorbed xenon on zeolite can diffuse rather rapidly (on the N M R time scale used) from one crystallite to another. This diffusion process can be affected by either dilution or compression of the zeolite bed which in turn influences the xenon NMR be-
1819
havior. At low dilution, 6- does not change much with dilution and compression (Figures 1 and 2). Therefore, for routine application of the xenon NMR technique to a pure zeolite or sample containing less than 50% of nonporous impurity the result should be acceptable. At higher dilution,, 6 decreases rapidly with dilution. This tells us that special attention must be paid to the application of the technique to industrial catalysts which sometimes contain more than 50% of binder. For a mixture, the increase of line width with xenon pressure can be a good indication of the presence of a second phase in the sample. The xenon diffusion process depends also on the structure of the zeolite in which xenon is adsorbed. The less open the structure or the higher the surface barrier for xenon diffusion, the smaller the influence of the intercrystallite diffusion on xenon NMR. X e X e interactions depend only upon the xenon density inside the zeolite and not on the state of xenon outside the zeolite crystallite (xenon gas or xenon adsorbed on binder). Therefore, the xenon chemical shift can always be expressed by the formula'
6 = bref + 6p-0 + 6E + 6M + 6xe-xc (14) As the adsorption behavior of the zeolite is not affected by dilution or compression and the concentration, tinter, is low for a mixture of zeolite and nonporous binder, the detected signal intensity is proportional to the zeolite content in the mixture. We have checked this pointIg and it can also be proved by the result presented in ref 7. However, when the binder is also porous, a low-temperature experiment is most helpful. Finally, the form of the xenon NMR signal depends markedly on the homogeneity of mixtures. This can be used as a tool for controlling the homogeneity of industrial samples. Registry No. Xe, 7440-63-3. (19) The xenon spin-lattice relaxation time T I depends on the conditions of compression and dilution of the zeolite. The higher the degree of dilution, the longer T , . In the case of sample NaY [lO]+NaA, a pulse repetition time of 5 s was used for intensity measurements.
Investigation of the Electrode Processes Causing Characteristic Emission from Rare Earth Doped Anodic Ta,O, Films Eric A. Meulenkamp,* John J. Kelly, and George Blasse Debye Research Institute, University of Utrecht, P.O. Box 80.000, 3508 T A Utrecht, The Netherlands (Received: June 21, 1991)
Rare earth doped anodic Ta2O5 layers can show characteristic light emission. In the case of Eu3+photoluminescence, excitation is restricted to common charge transfer and 4f shell transitions. No electroluminescence can be observed. This is different for the case of Tb3+. Experiments involving irradiation with suprabandgap light and hole injection by H202 show that charge carriers can recombine on terbium ions. The time and potential dependence of Tb3+electro- and photoluminescence are governed by the solid-state and defect properties of the electrode. Hence, Tb3+emission can serve as a valuable probe in the study of the cathodic behavior of Ta205. Several experiments indicate that electroluminescenceoriginates, at least partially, from the surface.
1. Introduction
Efforts aimed at characterization of processes occurring at electrified interfaces with the help of surface-sensitive techniques have increased considerably over the past decade. The use of photoluminescence (PL) and electroluminescence (EL) is unique to the study of the semiconductor/electrolyte interface. Holes created in the valence band (VB)of an n-type semiconductor can recombine radiatively with conduction band (CB)electrons. This is shown schematically in Figure 1. In PL, electrons are excited to the CB by using suprabandgap irradiation. EL can occur if a strong oxidizing agent is present in the electrolyte. Such agents are reduced by means of VB electrons, a process which corresponds 0022-365419212096-18 19$03.00/0
to hole injection. The emission spectrum and the potential dependence of the luminescence may give in situ information on features such as the reduction mechanism and the surface recombination In recent years the study of the electroluminescence of compound semiconductors has been directed toward the elucidation of the spatial origin of the electron-hole recombination. In the (1) Decker, F.;Pettinger, B.; Gerischer, H. J. Electrochem. Soc. 1983,130, 1335. (2) Smandek, B.; Chmiel, G.; Gerischer, H. Ber. Bumenges. Phys. Chem. 1989, 93, 1094; Erratum 1989, 93, 1579.
0 1992 American Chemical Society
1820 The Journal of Physical Chemistry, Vol. 96, No. 4, 1992 SEMICONDUCTOR
CONDUCTION BAND
ELECTROLYTE
E
I i" I 1 VALENCE BAND Figure 1. Mechanism of electro- and photoluminescence in an n-type semiconductor. The solid lines represent radiative processes, dashed lines nonradiative processes. The numbers refer to (1) hole injection by an oxidizing agent in solution; (2) creation of a hole by irradiation with suprabandgap light; (3) nonradiative recombination through surface states; (4) radiative recombination via various energy levels.
late 1970s and the early 1980s much experimental work was interpreted in terms of surface EL since extra bands appeared in the EL spectrum that were absent in photol~minescence.~-~ Smandek and Gerischer, on the other hand, have shown recently6 that many discrepancies in the EL and PL of a typical group III/V semiconductor like GaP can be explained on the basis of the concentration profile of the injected minority carriers. They narrowed down the many claims of surface EL to just one, viz., the 650-nm emission during the reduction of S2OS2-at Ti02.' T i 0 2 is a wide bandgap semiconductor which belongs to the class of the valve metal oxides. These have been studied as solar energy materials, as chemical sensors, and for application in electrochromic displays. The use of Ta205as a dielectric is widespread. Such oxides are very stable, both chemically and under high cathodic polarization, which makes them suitable for the study of EL. Dopant ions can be incorporated. Recombination on these ions can result in characteristic luminescence if their emission energy is smaller than the bandgap. An example is the work by Bard et al. on ZnS:Mn2+.8 Haapakka et al. have described electroluminescenceof thin films of valve metal oxides in the presence of such strong oxidizing agents as H202and S2OS2-.The thin oxide layers were prepared by anodization. They have used this EL for the quantitative determination of heavy metal ions and organic compounds in aqueous solution?JO Two particularly interesting articles describe the EL of rare earth doped oxide f i l m ~ . ~ ' Rare J ~ earth ions are especially attractive as dopants because of their spectral properties. The characteristic emission is due to transitions within the 4f ~he1l.l~Decay time, relative intensity, and crystal field splitting (3) Pettinger, B.; Schoppel, H. R.; Gerischer, H. Ber. Bunsenges. Phys. Chem. 1976,80, 849. (4) Nakato, Y.; Tsumura, A.; Tsubomura, H. Chem. Phys. Lett. 1982,85, 3x7.. _.
(5) 2402. (6) (7) (8) 1033. (9) 259.
Nakato, Y.; Tsumura, A.; Tsubomura, H. J . Phys. Chem. 1983, 87, Smandek, B.; Gerischer, H. Electrochim. Acta 1985, 30, 1101. Smandek, B.; Gerischer, H. Electrochim. Acta 1989, 34, 1411. Ouyang, J.; Fan, F.-R. F.; Bard, A. J. J. Electrochem. Soc. 1989, 136, Haapakka. K.; Kankare, J.; Kulmala, S. Anal. Chim. Acta 1985, 171,
(10) Haapakka, K.; Kankare, J.; Puhakka, 0.Anal. Chim. Acta 1988,207, 195. (1 1) Haapakka, K.; Kulmala, S. Anal. Chim. Acta 1988, 208, 69. (12) Haapakka, K.; Kankare, J.; Kulmala, S.Anal. Chim. Acta 1988,209, 165.
Meulenkamp et al.
1
1
1
+ time
1
1
(s)
Figure 2. Anodization procedures. The anodizing conditions are described in the text. (a) Symmetric double step (SDS) pulse form; in luminescence measurements, EL occurs during shaded cathodic pulse. (b) Triangular potential sweep (dc).
may provide information on the environment of the ions, for example, the site symmetry, the mechanism of charge compensation, and the number of attached water We report here on some solid-state and photo- and electroluminescence properties of rare earth containing anodized tantalum oxide films. It is shown how an investigation of the luminescence can give insight into such aspects as the nature of the absorption bands and the energy levels of the dopant ion. The spatial origin of the EL is also studied. The possible benefits of time-resolved measurements of the EL of this system to studies of the cathodic behavior of valve metal oxides are indicated. 2. Experimental Section Tantalum rod (4 mm diameter) of 99.9+% purity was purchased from Highways International (The Netherlands) and cut into disks, 1 mm thick. The metal was mounted on copper using silver epoxy and embedded in epoxy min. Before each anodization experiment, the electrodes were polished down to 1 pm using diamond spray and subsequently etched in a solution of 40% HF, 65% H N 0 3 , and 98% H2S04 1/1/2.5 by volume. Measurements were performed in a three-electrode configuration using rotating disk electrodes, a large Pt counter electrode, and a saturated calomel electrode (SCE) as reference. All potentials are given with respect to SCE. High voltage (>lo V) anodization experiments were carried out in a two-electrode configuration by using a scan generator and an Oltronix A2.5K1OHR power supply as amplifier. For impedance measurements a Solartron 1250 frequency response analyzer was used in combination with a Solartron 1286 electrochemical interface. The pulse generator and the potentiostat were homebuilt. Steady-state EL was measured on Perkin-Elmer MPF-44B and SPEX DM3000F Fluorolog spectrofluorometers. In the case of EL, the RDE was placed vertically in the sample chamber of the spectrofluorometer. Time-resolved EL was measured by using a single photon counting system described in detail elsewhere." The monochromator was set in the mirror mode. PL and photocurrent experiments, with the electrodes mounted horizontally, were performed in the SPEX Fluorolog spectrofluorometer. Far-UV excitation measurements were performed on a PerkinElmer MPF-3L spectrofluorometer equipped with a Hamamatsu 200 W deuterium lamp. The excitation spectra are corrected for lamp intensity. Diffuse reflection spectra were measured on a Perkin-Elmer Lambda 7 UV/vis spectrophotometer. (13) (a) Blasse, G. In Handbook on the Physics and Chemistry of Rare Earths; Gschneidner, K. A. Jr., Eyring, L.,Eds.;North-Holland Amsterdam, 1979; Vol. 3, Non-metallic compounds I; Chapter 34. (b) Peacock, R. D. Struct. Bonding 1975, 22, 83. (14) Henderson, B.; Imbusch, G. F. Optical Spectroscopy of Inorganic Solids; Clarendon Press: Oxford, U.K., 1989; Chapter 8. (15) See e.g.: Porter, L.C.; Wright, J. C. J . Chem. Phys. 1982, 77,2322. (16) Horrocks, W. Dew. Jr.; Sudnick, D. R. Acc. Chem. Res. 1981, 14, 384. (17) Donker, H.; van Schaik, W.; Smit, W. M. A.; Blasse, G. Chem. Phys. Lett. 1989, 158, 509.
Rare Earth Doped Anodic Ta20, Films
The Journal of Physical Chemistry, Vol. 96, No. 4, 1992 1821 rl
All chemicals were of analytical grade. Rare earth oxides (99.99%, Highways International) were dissolved in perchloric acid which was then neutralized. D 2 0 (99.9 atom 7%D) was from Sigma. Doubly distilled deionized water was used for the experiments. All solutions were purged with nitrogen. The measurements were carried out a t room temperature. 3. Results and Discussion 3.1. Preparation and Characterizationof tbe Oxide Layer. The anodizing solution contained 1.0 M NaCIO, and had a pH of 3.0. Doped layers were prepared with a rare earth ion concentration of 1.0 X lo4 M in the anodizing solution. Two anodization procedures were used; see Figure 2. The first consists of the symmetric double step (SDS) pulse potential, which was devised by Haapakka et al." The amplitude of the SDS potential was 8 V and its frequency 104 Hz. The anodization was carried out for 60 min. These values have been adopted from Haapakka et al. The rotation rate was lo00 rpm. The metal oxide forms during the anodic pulse. Under cathodic polarization vigorous hydrogen evolution occurs. It has been suggested that field-assisted diffusion of rare earth ions takes place a t negative potentials." The intervening zero voltage pulse is used to stabilize the electrode. The second procedure involves a triangular potential sweep (dc layer), which has frequently been used to grow well-defined oxide films of known t h i c k n e s ~ . ' ~ JThe ~ sweep rate was 100 mV/s. Dc anodization was performed with a stationary electrode. The final potential was 10 V. The thickness of the SDS layer has been reported" to be larger than that expected on the basis of the anodization ratio, which is approximately 1.8 nm/V.l8J9 Auger depth profiling measurements indicated thicknesses of more than 40 nm, but no details were given of the calibration procedure in determining the relation between sputter time and sputter rate. Double anodization experiments can give information on the thickness of the SDS films. An electric field exceeding the anodizing field is required to cause further growth of oxide films on valve metals. A steep increase in anodic current during a dc anodization of an SDS layer is expected at a potential that corresponds to the thickness of that layer. Indeed, we observed such an increase at 14.0 f 1 V (SCE), which implies a thickness of 26 f 2 nm, if we take into account a zero field thickness of 1 n m . I 9 The dc anodizing current density was 0.40 mA cm-2, yielding a roughness factor r of 1.1, in good agreement with other a u t h ~ r s . ' ~ J ~ The capacity C was potential-independent in the blocking region. Therefore, the simple capacitor formula can be applied to estimate the thickness d C = coe,r/d (1) Here, eo and e, denote the vacuum permittivity and the relative dielectric constant, respectively. Almost no dispersion was observed up to frequencies of 10 kHz. On using a value for the relative dielectric constant of 25,I8J9we obtain a thickness of 22 f 2 nm. Both estimates are in reasonable agreement in view of the rather large uncertainties in the literature values of notably e, and of the anodization ratio. On the basis of the SDS anodic pulse potential a thickness of only about 15 nm would be expected. The larger thickness is probably due to a large transient electric field when the anodic pulse is applied. Low-voltage anodic oxide films are usually amorphous and probably have a glasslike structure.l8 The bandgap of such amorphous semiconducting oxides can be estimated from photocurrent measurements. We have to remark that the term bandgap is not fully justified. We use it here instead of the mobility gap. The photocurrent spectrum of the SDS electrode is shown in Figure 4b. No linear correlation between (iphhv)" and hv was found for various values of n. It is clear, however, that the bandgap is about 4.2 eV, which is not an uncommon value.zo-21This value can also be inferred from a plot of In (iph) (18) Young, L. Anodic Oxide Films; Academic Press: London, 1961. (19) Macagno, V.; Schultze, J. W. J. Electroanal. Chem. 1984,180, 157. (20) Metikos-Hukovic,M.; Ceraj-Ceric,M. Thin Solid Films 1986, 145, 39.
1
I
450
f
550
650
wavelength ( nm ) Figure 3. Normalized EL (---) and PL (-) emission spectra of the Tb3+-dopedTazOS. EL was excited by an SDS pulse. PL was excited by 260-nm radiation; the resolution was 2 nm. The assignment of the peaks is indicated (see also energy level scheme, Figure 7). Inset: background Ta20SEL emission; resolution 20 nm.
-
I
'
l
I
200
'
(
240
1
'
280
l
'
l
320
4 wavelength (
nm )
h
200
240
-+
wavelength ( nm )
280
320
h
id
300
400
500
wavelength ( nm ) Figure 4. Various excitation spectra of doped and undoped Ta205. Broad bands measured on the MPF-3L; resolution 20 nm. Tb3+and Eu3+4 f 4 f excitation lines measured on the SPEX Fluorolog;resolution 4 and 7 nm, respectively. (a) Normalized broad PL excitation bands (X,= 545 and 612 nm in the case of Tb3+and Ed+, respectively) and diffuse reflection spectrum: (-) Tb3+PL; (---) Eu3+ PL; reflection. (b) Photocurrent spectra measured at 5 V: (+) Tb3+-doped;( 0 )Eu3+-doped;(0) undoped. (c) PL excitation (&,,, = 545 and 612 nm in the case of Tb3+ and Eu3+,respectively): (-) Tb3+, (---) E d + . The sharp lines correspond to transitions within the 4f shell.39 The scale on the intensity axis is the same as in Figure 4a. (--e)
vs hv, i.e., a plot of the so-called Urbach tail,22which yields a straight line between 3.7 and 4.2 eV. This reflects the density (21) Haapakka, K. E.;Kankare, J. J.; LindstrBm, R.; Virtanen, J. Anal. Insrrum. 1984-1985, 13 (3&4), 241.
Meulenkamp et al.
1822 The Journal of Physical Chemistry, Vol. 96, No. 4, 1992
of states within the mobility gap of the amorphous oxide. The bandgap of the dc film seems somewhat wider. Indeed, values up to 4.6 eV have been r e p ~ r t e d . ~ ~Hence, - ~ ~ the electronic structure of the film depends critically on the anodizing conditions, which is also clear from the spread in the literature data. The quenching of the photocurrent in the far-UV region will be discussed below. It is well-known that valve metal oxides show rectifying current-potential characteristics.l* Although the bandgap is rather wide, they effectively behave as n-type semiconductors. No substantial anodic current was observed at moderate anodic potentials with the present Ta205films. H2evolution starts at about -1.2 V for both SDS and dc anodized tantalum. The flatband potential is -0.8 V.'9*2' In contrast to what has been reported by other workers,l' we were able to induce cathodic terbium emission in dc layers. The emission spectrum and the cathodic potential dependence of the EL were identical with those of SDS oxide films. EL of the dc oxide proved to be less stable, however, and its intensity was approximately a factor of 5 lower than that of SDS anodized specimens. This is probably due to a lower concentration of incorporated rare earth ions. According to AES measurements by Haapakka et al." the concentration of Tb3+in the SDS layer is about 0.3 atom % at the surface. It rapidly falls off inside the oxide. The total content is approximately 100 times larger than in the dc layer; the surface concentration differs by a factor of 10.'' It is clear from the above discussion that the main features of the electrical and optical properties of anodic Ta20, are essentially independent of the anodization procedure. Hence, we can rule out any contribution of the complex sequence of SDS and dc anodization (see below). On the basis of the above results, further experiments were performed with SDS anodized tantalum only. 3.2. Luminescence. Kinetics. All EL measurements were performed under optimized conditions in 1.O M NaC104, 1.O X M H202,and pH 3.0 unless otherwise stated. The electrode rotation rate was 1000 rpm. An SDS potential pulse (see Figure 2) with an amplitude of 3 V and frequency 62.5 Hz was used to excite EL. Spectral EL measurements were performed by integrating the total emission intensity. The intensity of the EL is greatly enhanced and stabilized by repeatedly dc polarizing the SDS layers a t an anodic potential of 10 V. Both the anodic leakage current and the cathodic hydrogen evolution current are greatly diminished. Cracks and flaws, which are likely to occur in these oxide films, are sealed. It is also probable that deep traps, due to, for example, hydrogen species, which have been incorporated during the anodization, are removed. Such traps can act as nonradiative recombination centers.26 The EL emission spectrum of a Tb3+doped oxide is shown as a dashed line in Figure 3. The PL and EL intensity are low compared to the emission intensity of efficient phosphors. The well-defined peaks correspond to Tb3+ 5D4emission. No ,D3 emission was observed. The amorphous nature of the anodic oxide is reflected in the inhomogeneous broadening of the emission peaks, which smears out the crystal field components. The difference between the EL and PL spectra is discussed in the section on the spatial origin of the luminescence. In EL experiments some background emission could be observed (Figure 3, inset). This emission constitutes about 0.5% of the total emission intensity. It consists of a single broad band, with an onset a t about 3.7 eV near the bandgap. This emission must be due to intrinsic subbandgap recombination, as it is also evident in undoped oxide layers. We can also describe this EL as localized emission from tantalate groups.27 It proved impossible to measure the sub(22) Stimming, U. Electrochim. Acta 1986, 31, 415. (23) CICchet, P.;Martin, J. R.; Ollies, R.; Vallouy, C. C. R. Acud. Sci. 1976. -. -282C. - - - ,887. ~(24) Apker, L.; Taft, E. A. Phys. Rev. 1952, 88, 58. (25) Schultze, J. W.; Elfenthal, L. J . Elecrroonal. Chem. 1986, 204, 153. (26) Yamase, T.; Gerischer, H. Eer. Eunsenges. Phys. Chem. 1983, 87, 349.
bandgap emission reliably in PL. As its intensity is very low, interference from the light source and scattering may have masked its presence. The background emission provides clear evidence that the oxidizing agent is able to inject holes into the valence band. Subsequent recombination of these holes with conduction band electrons results in luminescence. We observed EL only with the two-electron systems 02,H202, and S2OS2-. The reduction mechanism of these agents is known to proceed in two steps:28 H202 e- (CB) 'OH OH(2a) 'OH OH- h+ (VB) (2b) The oxidizing agent itself is not able to inject holes. The radical intermediate, on the other hand, has a very positive redox potential, which Memming estimated as 1+2.65 V.28 The trivalent terbium ion can act as a hole trap, owing to its tendency to become tetravalent. The highly oxidizing Tb4+ ion is subsequently reduced by electron capture. Luminescence is observed if this proceeds via the excited state of the trivalent species: Tb3+ + h+ (VB) Tb4+ (3a) Tb4+ e- (CB) (Tb3+)* (3b)
+
- +
+
+
- +
(Tb3+)* Tb3+ hu (3c) The net result is the recombination of electrons and holes on the terbium ion. A combination of (2) and (3) accounts for the occurrence of characteristic Tb" EL. Such an oxidation/reduction cycle has also been used8to explain the characteristic Mn2+ (3d5) EL in ZnS:Mn2+. Further support comes from EL measurements of Haapakka and Kankare on AZO3,doped with various rare earth ions.I2 They showed that only those ions exhibiting redox reactions within reasonable potential limits luminesce. The recombination mechanism (3a) - (3c) is also commonly used to account for the Occurrence of thermoluminescence and suprabandgap excited photoluminescence of rare earth doped X-ray storage phosphors.29 The intensity I of the Tb3+ emission depends on the H202 concentration ( I (H202)n,with n = 0.8-0.9). The small deviation from a first-order process is probably due to an adsorption equilibrium prior to the first reduction step. The integrated intensity depends linearly on the square root of the rotation frequency. Therefore, convective diffusion of bulk hydrogen peroxide seems the rate-determining step. Such a diffusion-limited reduction has also been observed for SZOs2-a t Ta205.30 Eu3+EL could not be observed either in Ta205or in A1203in the presence of H202,02,or S202-. We were able to induce Eu3+ EL on N20,in the presence of nitrate ions as previously described by Haapakka et al.I2 In the present Ta205system, however, no characteristic emission was observed upon addition of NO 270 nm; (2) absorption at X < 270 nm; (3) Eu3+
excitation bands. results in a quenching of the photocurrent. If we take a to be 4 X lo5cm-I,l8 the penetration depth is 25 nm, Le., about the film thickness. In this way, we are able to account for a significant decrease of the photocurrent at shorter wavelengths. In order to explain the complete absence of photocurrent, which is observed here, an unrealistically high value of a (2 X lo6an-')is required. We must therefore assume an additional quenching mechanism. Electrons and holes created by irradiation with 240-nm light, have a certain kinetic energy since the bandgap is only about 4.2 eV. Before thermalization these charge carriers are so mobile that the recombination rate is enhanced, leading to a further decrease of the photocurrent. The photocurrent spectra show that electrons and holes are created by irradiation around 300 nm. The Tb3+PL excitation intensity, however, is very low in this region. This is again due to the low absorption strength in this wavelength region as the excitation intensity is the product of the PL quantum efficiency and the absorption strength. Therefore, the photocurrent has its maximum in the long-wavelength region of Figure 4 because the absorption strength is low. The PL excitation band has its maximum in the short-wavelength region; this is not only due to the high absorption strength, but also to two other effects. Firstly, there is the additional recombination mechanism considered above. Secondly, the PL intensity a t 240 nm is enhanced by the small penetration depth, since m a t Tb3+ ions are incorporated near the surface. The complete energy level scheme of the doped layers is shown in Figure 5 . The importance of the defect chemistry and trapping is paramount in determining the time dependence of the rare earth PL intensity. The Tb3+luminescence intensity in the case of excitation at 260 nm is close to zero when the electrode is illuminated for the first time. A rapid increase is observed upon illumination for periods of up to about 20 min. Then the increase slows down and finally a limiting value is reached. The slope of the intensity-time curve depends strongly on the incident wavelength: light of wavelength longer than 300 nm does not alter the intensity significantly; below 300 nm, the slope increases as the wavelength is decreased. The emission intensity upon excitation in the 4f shell at 378 nm is zero before illuminating with srprabandgap radiation and reaches a finite value afterwards. If the electrode is ke$ in the dark for some time, it takes a few minutes of illumination to regain the previous emission intensity. The size of the decrease in the dark does not exceed the intensity gain in the slow increase regime. The rapid increase is thus irreversible. The photocurrent behaves in the opposite manner. Upon illuminating the sample for the first time, a rapid decrease is followed by a slower one. After the radiation has been switched off, the photocurrent is a fraction higher on recommencing illumination. The intensity of the Eu3+ PL is only slightly dependent on illumination time. A small decrease is observed. Darkness restores the original value. Clearly, the electronic structure of the oxide is irreversibly changed upon initial illumination. Deep trapping centers which
-
Meulenkamp et al.
1824 The Journal of Physical Chemistry, Vol. 96, No. 4, 1992 pulse on
pulse on
C.T.
u
pulse off
j
4t8 - 4f75d'
U
time ( minutes )
Figure 6. Time dependence of the EL intensity (Ae,,, = 545 nm) of an undoped oxide layer upon addition of Tb3+to the electrolyte. The arrow indicates the addition of Tb3+. The dashed line represents the estimated EL intensity assuming that pulsing is not interrupted.
compete with terbium for electrons and holes are filled by capturing these charge carriers. Similar behavior has been observed with X-ray storage phosphors. The glow c w e of such compounds, after irradiation with X-rays, can drastically alter after the first heating cycle. This is usually ascribed to the removal of deep trap levels which have been introduced during the firing process.33 The absence of any Tb3+ 4f excitation intensity points clearly to the absence of Tb3+ in the unirradiated samples, since the quantum efficiency of this excitation is close to 1." The terbium must then be. present in the tetravalent state, which is known to show no lumine~cence.~~ Illumination induces the capture of electrons and concomitant reduction to the trivalent state. The slow changes upon continuing illumination or darkness point to the existence of a thermal exchange between Tb3+ and Tb4+. Indeed, a lowering of the excitation intensity at 378 nm is observed after a period of darkness. Tb4+shows an intense absorption band in the near-UV region.34 For the reason already outlined above, no contribution of this absorption can be seen in the reflection spectrum. The relatively small changes in the E 3 + PL intensity are in good agreement with the above described energy level scheme, which excludes Eu3+ as a trapping center. Spatial Origin of EL. The excitation mechanism of the EL of the rare earth doped Ta205system seems analogous to that of the doped A1203system, which has been studied by Haapakka et a1.l2 In both, an oxidizing agent is required to obtain cathodic EL which is characteristic of the dopant ions. EL of Tb3+is found only in the case of H202,02,or S203-reduction. Terbium acts as a hole trap as described above. The reduction of nitrate ions gives rise to characteristic Eu3+ and Sm3+ emission at A1203. These rare earth ions are relatively easily reduced to the divalent state. The unique combination of oxidizing agent and luminescent ion in the case of A1203points to a direct interaction between both, which can only take place at the solid/electrolyte interface or in the electrolyte. The second possibility can be rejected, the Tb3+ ion is strongly bound since the emission intensity is constant over several weeks of experimentation. Direct evidence for the Occurrence of radiative surface recombination comes from the experiment shown in Figure 6 . An undoped Ta20S layer is SDS polarized in the presence of H202. Only the very weak background emission is observed. At t = 10 min, M Tb3+ is added to the electrolyte. The increase in emission intensity is due to characteristic Tb3+ EL. The time dependence of the emission intensity must reflect the concentration of adsorbed Tb3+ ions; it is likely that no incorporation of ions or oxide growth takes place during the cathodic and anodic pulses, respectively, because the pulse amplitude is considerably smaller than that used during the anodization. The effect of switching (33) Schipper, W. J.; Blasse, G . To be published. (34) Hoefdraad, H.E. J . Inorg. Nucl. Chem. 1975, 37, 1917.
Eu3+
Figure 7. Energy level scheme of the Eu" (4F)and Tb3+(4f8) ions. Nonradiative (---) and main radiative (-) decay pathways are indi-
cated. off the SDS pulse and keeping the electrode a t open circuit potential for some time is also shown in Figure 6 . When the SDS potential is reapplied, the intensity rises rapidly to a value which corresponds to the extrapolated intensity, expected if pulsing were not interrupted. This again indicates that adsorbed ions can show luminescence. Additional information showing that EL of the doped films originates, at least in part, from the surface is provided by an investigation of the spectrum and of the decay time of the Tb3+ emission. The interaction between the 4f electrons results in a number of well-defined energy levels, which are shown in Figure 7. The 4f electrons are screened from the surroundings of the ion by outer lying s and p electrons. The position of the various levels is thus rather insensitive to the nature of the ligands. The levels are denoted by the term symbol =+'LJ, where S stands for the total spin quantum number, L represents the total orbit quantum number, and J is the total quantum number. The crystal field splitting of the levels may give direct information on the site symmetry of the luminescent ion. No such information can be obtained here because of the large inhomogeneous broadening. The relative intensity of the transitions is also governed by the site symmetry. This originates from the J selection rule which for the Tb3+ ion can be summarized as follows:13 for forced electric dipole transitions IAJI I 6
(4a)
for magnetic dipole transitions IAJ( I 1
(4b)
Transitions with AJ = 0, f2, are called hyper~ensitive,'~ because their intensity is extremely sensitive to the environment. If the Tb3+ ion is placed in a site of inversion symmetry, the 5D4-7F6 emission intensity is zero. In all other cases it is nonzero. The 5D4-7F5emission intensity is much less dependent on the environment for two reasons: firstly, the electric dipole (ED) contribution is not hypersensitive; secondly, the magnetic dipole (MD) contribution is, to a good approximation, independent of the site symmetry. We can use the intensity ratio of the 5D4-7F6 and 5D4-7F5 transitions to compare the EL and PL spectra (Figure 3). We have done this by normalizing on the basis of the SD4-7Fstransition intensity. The relative intensity of the ED transitions SD4-7F6and 5D4-7F4is clearly higher in the EL case, pointing to a larger deviation from inversion symmetry. In PL, all Tb3+ions in the sample contribute to the emission signal, which is necessarily an
The Journal of Physical Chemistry, Vol. 96, No. 4, 1992 1825
Rare Earth Doped Anodic Taz05 Films n
20 time ( m s )
10
0
+
30
Figure 8. Time-resolved electroluminescence, resolution 100 c(s. SDS frquency is 31.25 Hz (shown schematically) in the case of terbium EL and 62.5 Hz in the case of background EL. At r = 0 the potential is pulsed cathodically. (-) Tb3+emission in H20(---) Tb3+emission in D20,(-.-) background emission. Note the logarithmic scale.
average of all possible site symmetries, including bulk and surfacebound ions. The higher ED transition intensity in EL is thus indicative of a larger contribution of surface-bound ions. We must add here that the PL spectrum is independent of excitation wavelength. As EL is measured during cathodic polarization, we checked the potential dependence of EL and PL simultaneously in a potential scan experiment. Both emission spectra did not change at potentials anodic of -2.3 V. The EL spectrum in this potential region was identical with that measured during SDS polarization. Other experiments showed that the rate of change in the EL spectrum depends on the cathodic potential and the cathodic pulse time. This effect, and the change of EL and PL spectral properties cathodic of -2.3 V during a potential scan, will be discussed elsewhere.3s More information on the environment of the luminescent ions can be obtained from the decay time. The lifetime of the 5D4 excited state is determined by nonradiative and radiative processes. The radiative decay rate is relatively insensitive to the nature of the ligands, as compared to the nonradiative rate. Hence, we can regard fr8d-I as a constant, where Trad, the radiative decay time, is of the order of several milliseconds. The nonradiative process involves the dissipation of heat by emission of phonons. The rate of multiphonon emission is determined by the energy of the available vibrations. We can roughly distinguish two cases: if more than 5 phonons are required to span the energy gap between the excited level and the next lower level, radiative emission dominates; otherwise the quantum efficiency is low. We can also describe this by stating that the nonradiative decay time 7,"rad in the latter case is much smaller than Tra& The gap between the 5D4 and the 'Fo level of the Tb3+ion is about 15 OOO an-'.It is obvious that the most energetic phonons will determine the nonradiative decay rate. The frequency of the OH stretching vibration is -3200 cm-I. The nonradiative decay is thus very fast in water. By replacing HzO by D20, Horrocks has indeed shownI6 that the decay rate of the 5D4excited state is a linear function of the number of attached HzO molecules: n(HzO) = 4.2(f1(H20)- T-I(D~O))
Here T is expressed in milliseconds. Because of the isotope effect the OD vibration frequency is -2250 cm-', which is too small to play a significant role in the quenching of the Tb3+luminescence. We can use this isotope effect to gain more insight into the environment of the electroluminescent ions. The emission intensity during the SDS potential pulse is shown in Figure 8 on a logarithmic scale. The intensity grows during cathodic polarization and decays during the subsequent zero potential and anodic pulses. Both growth and decay are in principle a complex function of recombination kinetics, possible changes in the oxide properties ~
~~~
~
~~~
(3s) Meulenkamp, E. A.; Blasse, G.;Kelly, J. J. To be published.
and the decay time of the 5D4excited state. We can deduce the influence of the nonradiative decay rate by comparing the intensity-time profile of the background emission (dotted line) and the Tb3+emission in H,O (drawn line). Since the rise of both is nearly identical, some other process must be involved here. The most likely explanation is that a change in the oxide properties takes place, because the rate of increase depends strongly on the cathodic potential. A similar effect has been reported by Bard et Since the decay of the background emission and the Tb3+ emission is rather nonexponential, we use to describe the decay a phenomenological decay time constant Tphen, which is defined as the time required to reduce the emission intensity to 36.8%of its original value. The luminescent decay time of a typical tantalate emission is a few microseconds at room temperature.3' Therefore, the decay of the background emission (Tphcn = 100 ps) must reflect the recombination kinetics. The decay time of the Tb3+emission in H20, which is about 800 ps, can only correspond to the 5D4excited level lifetime. This is confirmed by the fact that the potential of the pulse directly following the cathodic pulse does not affect T significantly. Any significant contribution of recombination g e t i c s would be expected to depend strongly on that potential. Further support coma from measurement8 in heavy water, which are also shown in Figure 7 (dashed line). The change in decay time to about 1.4 ms can only be attributed to a slower nonradiative decay of the 5D4 excited level. The number of attached water molecules is 2.5 f 0.5 using Horrocks' formula (eq 5). It is likely that even more water molecules are attached to the electroluminescent ions. The reason for this is the following. A few percent of hydrogen is undoubtedly present in the heavy water electrolyte since 1 M NaC104.Hz0 has been added. Furthermore, DzO exchanges rapidly. Terbium ions which are coordinated by light water will have a major contribution to the decay at short times since the nonradiative decay rate of these ions is high. The phenomenological decay time in heavy water is therefore likely to be underestimated. It would be better to compare the decay in HzO and DzOjust before the cathodic pulse, but in that time regime recombination kinetics interfere. The EL of water-coordinated terbium ions is a strong indication for recombination of electrons and holes on surface-bound ions. It is possible, however, that incorporation of hydrogen in the oxide16~z3~36 causes the formation of hydroxyl groups in the bulk. As quenching of the EL during the cathodic pulse occurs only on a longer time scale, Le., t 1 20 ms,35we do not expect that hydrogen indiffusion affects the electronic properties during the cathodic pulse time. Furthermore, quenching of Eu3+and Tb3+ PL under excitation in the 4f shell did not occur upon cathodic polarization up to -3 V. As the energy gap between the excited and lower levels is much smaller for Eu3+than for Tb3+(see Figure 7), a pronounced effect on the PL intensity would be expected if OH groups were formed in the solid. This quenching by water can also be responsible for the absence of EL from Pr3+, which has about the same redox properties as Tb3+.39Because the energy gap between the excited and the next lower level of Pr3+ is only 6800 cm-I, even three OD stretching vibrations are sufficient to cause nonradiative decay. 4. Conclusions
We have shown that several features of the EL and PL of the rare earth doped tantalum oxide films are governed by the solid-state properties of the electrode. These have been explored by the usual means; the role of the dopant ion also provides information. The absence of characteristicEu3+emission in EL could ~
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~
(36) Fan, F.-R. F.; Leempoel, P.; Bard, A. J. J . Electrochem. Soc. 1983, 130, 1866. (37) Blasse, G.;Bril, A. J. Lumin. 1970, 3, 109. (38) Novichkov, V. Yu.; Orlov, V. V. Elekrrokhimiya 1988, 24, 691. (39) Carnall, W. T. In Handbook on the Physics and Chemistry of Rare Earrhs; Gschneidner, K. A. Jr., Eyring, L., Eds.; North-Holland: Amsterdam, 1979; Vol. 3, Non-metallic compounds I; Chapter 24.
J. Phys. Chem. 1992, 96, 1826-1835
1826
be explained on the basis of the energy level scheme. It was deduced that Eu3+ cannot act as a recombination center. The time and potential dependence of the Tb3+PL and EL reflect the defect properties and the charge carrier recombination kinetics of the Ta205film. Experiments involving the Tb3+ SD4decay time in H 2 0 and D2Q and the EL of adsorbed ions show that the characteristic EL of the terbium-doped tantalum oxide layer
originates at least partially from the surface. Acknowledgment. The work described here was supported by the Netherlands Foundation for Chemical Research (SON), with financial aid from the Netherlands Organization for Scientific Research (NWO). Registry No. Ta205, 1314-61-0; Tb3+,22541-20-4; H202,7722-84-1.
Kinetics of CO Adsorption on Ni(ll0) Studied in Real Time by Ion-Stimulated Desorption of Neutral Molecules J. H. Campbell, Department of Chemistry, Stanford University, Stanford, California 94305
J. J. Vajo,+ and C. H. Becker* Molecular Physics Laboratory, SRI International, Menlo Park, California 94025 (Received: June 24, 1991; In Final Form: September 27, 1991)
The adsorption of CO on Ni( 110) has been studied in situ using a low-current pulsed Ar+ beam, followed by laser ionization of the desorbed neutral CO species and reflecting time-of-flight mass spectrometry of the resulting CO+ photoions. Since the fractional surface coverage of CO is monitored in real time, adsorption kinetics are conveniently studied even at temperatures where adsorption and desorption occur concurrently and equilibrium coverages are below saturation. We have measured the adsorption kinetics with a constant CO pressure of 3 X IO-* Torr for temperatures between 196 and 429 K. At 196 K the probability of adsorption of CO is independent of CO coverage, ec0, until Oc0 -0.75 of saturation coverage; this behavior clearly suggests precursor-mediated adsorption kinetics. The initial adsorption probability at low CO coverages is constant for surface temperatures between 196 and 429 K. The implications of this observation on the energy barrier separating the precursor and the chemically adsorbed states are discussed within the context of several kinetic adsorption models. In addition, for temperatures between 357 and 403 K, where the equilibrium CO coverage is 0.7-0.3 monolayer (ML), the adsorption kinetics are unusual. Specifically, the adsorption probability under these conditions is constant up to nearly 95% of the equilibrium coverage. No single set of kinetic parameters, whether coverage independent or linearly dependent on coverage, can explain the CO adsorption data over the entire temperature range studied.
1. Introduction The adsorption of CQ on the group 10 (formerly group VIIIA) metals has been well studied because of its relevance to commercially important catalytic processes.'+ Typical experimental observations for CQ adsorption on the group 10 metals are that the adsorption or sticking probability, s, remains constant up to high CO coverages and that s is often close to one.'O This type of adsorption behavior is generally attributed to the existence of a weakly bound mobile state on the surface which acts as an intermediate or precursor state to chemisorption.1° The concept of a mobile precursor state originated with Langmuir,11*12who noted that the adsorption probability of cesium on tungsten did not scale with the number of open sites on the surface but instead remained constant up to nearly saturation ~ o v e r a g e . ' ~ - 'Two ~ types of precursors may be defined: an "intrinsic" precursor which exists over an empty site on the surface and an "extrinsic" precursor which exists over a filled site.1° The precursor molecule need not be the same molecule as that found in the final adsorption state. For example, the precursor to the dissociative chemisorption of CO on Ni is the molecularly adsorbed state, which will not dissociate until a surface temperature of T, 600 K is reached." Due to the stability of chemisorbed molecules, their isolation and detection are fairly straightf0rward;'O thus, many dissociatively chemisorbed systems with precursor-mediated kinetics have been ob~erved.~*-~~ Direct observations of precursors to molecular chemisorption, however, are more difficult because these precursors are considered to be in a physisorbed state, which are more weakly bound than
-
'Current address: Hughes Research Laboratories, Malibu, CA 90265.
0022-3654/92/2096-1826$03.00/0
molecular precursors to dissociative chemisorption. Typical values for physisorption well depths are -2-3 kcal/mol and are almost (1) Vannice, M. A. In Catalysis Science and Technology; Anderson, J. K., Boudart, M., Eds.; Springer-Verlag: Berlin, 1982; Vol. 3. (2) Ertl, G. In Catalysis Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: Berlin, 1983; Vol. 4. (3) Christmann, K.; Schober, 0.;Ertl, G. J . Chem. Phys. 1974,60,4719. (4) Campuzano, J. C.; Dus, R.; Greenler, R. G. Surf. Sci. 1981, 102, 172. (5) Klier, K.; Zettlemoyer, A. C.; Leidheiser, Jr., A. J. Chem. Phys. 1970, 52, 589. (6) Pfniir, H.;Menzel, D. J. Chem. Phys. 1983, 79, 2400. (7) Thiel, P. A,; Williams, E. D.; Yates, Jr., J. T.; Weinberg, W. H. Surf. Sci. 1979, 84, 54. (8) (a) Madden, H. H.;Kiippers, J.; Ertl, G. J . Chem. Phys. 1973, 58, 3401. (b) Taylor, T. N.; Estrup, P. J. J . Vac. Sci. Technol. 1973, 10, 26. (9) Banhofer, J.; Hock, M.; Kiippers, J. Surf. Sci. 1987, 191, 395. (IO) Weinberg, W. H. In Kinerics of Interface Reactions; Grunze, M., Kreuzer, H.J., Eds.; Springer-Verlag: Berlin, 1987; p 94. ( I 1) Langmuir, I. Chem. Rev. 1929, 6, 451. (12) Taylor, J. B.; Langmuir, I. Phys. Reu. 1933, 44, 423. (1 3) Kisliuk, P. J. Phys. Chem. Solids 1957, 3, 95. (14) Kisliuk, P. J . Phys. Chem. Solids 1958, 5 , 78. ( I 5 ) Becker, J. A. In Structure and Properties at Solid Surfaces; Gomer, R., Smith, C., Eds.; University of Chicago: Chicago, 1953; p 459. (16) Ehrlich, G. J . Phys. Chem. 1955, 59, 473. (17) (a) Steinriick, H. P.; D'Evelyn, M. P.; Madix, R. J. Surf. Sci. 1986, 172, L561. (b) DEvelyn, M. P.; Madix, R. J. Surf. Sci. Rep. 1984, 3, 413. (18) Gland, J. L. Surf. Sci. 1980, 93, 487. (19) B o c k , C.; DeGroot, C. P. M.; Biloen, P. Surf. Sri. 1981, 104, 300. (20) Hsu,Y.-P.; Jacobi, K.; Rotermund, H. H. Surf. Sci. 1982, 117, 581. (21) Shayegan, M.; Cavallo, J. M.; Glover 111, R. E.; Park, R. L. Phys. Rev. Lett. 1984, 53, 1578. (22) Poelsema, B.; Verheij, L. K.; Comsa, G . Surj. Sci. 1985, 152/153, 486.
0 1992 American Chemical Society
