The Journal of
Physical Chemistry ~
~~
0 Copyright, 1986, b y the American Chemical Society
VOLUME 90, NUMBER 8 APRIL 10, 1986
LETTERS Influence of the Alkali Metal Cation in Solution on the Electroluminescence of a ZnO Electrode during Persulfate Reduction Denis Fichou and Jean Kossanyi* Laboratoire de Photochimie Solaire, E.R. 241 du C.N.R.S.,94320- Thiais, France (Received: July 31, 1985; In Final Form: February 19, 1986)
The electroluminescence intensity of a ZnO electrode in an aqueous electrolyte during the pulsed reduction of S208*-anions is shown to be strongly dependent on the monovalent alkali countercation M' (Li', Na', K', and NH4+)in solution. Besides, during a stationary pulse, the current density and current transients differ greatly from one cation to the other. The experimental results confirm that, whatever the cation is, the key step of the luminescence is hole injection by the S,OS2-anions into the electrode but suggest that the countercations M' are injected inside the electrode surface at specific lattice sites of ZnO where they affect the luminescence and the electrical properties of the semiconductor. In the case of Li' and Na' ions, a correlation has been made with solid-state luminescence of deliberately doped ZnO crystals.
1. Introduction
Radiative recombinations of electrochemically generated electron-hole pairs at a semiconductor/electrolytejunction provide a direct and convenient insight of the complex interfacial charge-transfer processes. It is now well established that electroluminescence (EL) of wide bandgap semiconductors with a valence band edge E,S lying at very positive potentials (Le., >2.0 V/SCE in neutral media) can be m ~ n i t o r e d l -by ~ hole injection during the two-step reduction of persulfate ions S208*-. It has been p r o p o ~ e dthat ~ ? ~the SO4-.radical anion, a short-lived species generated in situ, is the injecting agent due to its strong oxidizing (1) Yamase, T.; Gerischer, H. Ber. Bunsenges. Phys. Chem. 1983,87,349. (2) Fan, F.-R. F.; Leempoel, P.; Bard, A. J. J. Electrochem. SOC.1983, 130, 1866. (3) Carpenter, M. K.; Ellis, A. B. J. Elecrroanal. Chem. 1985, 184, 289. (4) Memming, R. J. Electrochem. Soc. 1969, 116, 785. (5) Pettinger, B.; Schoppel, H.-R.; Gerischer, H . Ber. Bunsenges. Phys. Chem. 1916, 80, 849.
0022-3654/86/2090- 1485$01 .50/0
character (EO > 3.4 V/SCE at pH 7). The mechanism of hole injection into the valence band is supported by (i) the EL onset which is currently found to be slightly negative potential VOnEL to the flatband potential V, and (ii) the band-to-band EL emission observed in the energy gap E region of the spectra. We have reported recentlyI the EL characteristics of polycrystalline pure and Co-doped ZnO electrodes under either cathodic or anodic pulsed excitation; it was shown that the potential dependence upon the EL intensity during the persulfate reduction was complex, displaying two successive emission waves at -2.5 and -3.5 V/SCE, respectively. The most cathodic EL wave has been attributed to a nucleation phenomenom, Le., to a partial reduction of the electrode surface into Zno, the resulting metal layer increasing the current density and consequently the EL intensity. The spectrum of undoped ZnO shows the characteristic UV emission, which is split into two components at 384 (6) (a) Fichou, D.; Kossanyi, J. Chem. Lett. 1985, 1205. (b) Fichou, D.; Kossanyi, J., J. Electrochem. SOC.,in press.
0 1986 American Chemical Society
1486 The Journal of Physical Chemistry, Vol. 90, No. 8, 1986
Letters
and 394 nm, together with a broad self-activated band extending up to 850 nm. Cobalt doping has been shown to decrease the visible emission intensity and to induce a red emission at 705 nm. The typical lifetime of the EL emission is roughly 50-60 f i s all over the spectrum. In all these experiments, the electrolyte was a 0.05 M Na,S,Ox + 0.4 M NaCl aqueous solution, the EL active species being the anion S,0x2-. However, the cathodic polarization of the ZnO electrode, which is necessary to induce the reduction of thib ion and the subsequent emission, can also drive toward the surface of the electrode the positively charged species present in the solution. This would then induce a modification of both the luminescence and the electrical properties by either adsorption or injection of the cations from the solution into the ZnO lattice. The increased interest of the luminescent properties of 11-VI semiconductor^,^ particularly in the case of Zn0,8-10prompts u5 to report our results on the role of the alkali countercation M' (Li', Naf, K', NH4+) of the persulfate salt M2S208,on the EL of polycrystalline ZnO electrodes in aqueous electrolyte. .4n attempt has been made to correlate the experimental results with the electrochemical properties of strong electrolytes and w i t h solid-state luminescence of Li- and Na-doped zinc oxide
2. Experimental Section The polycrystalline ZnO materials are sinters prepared by firing high-purity (Koch-Light 99.99%) zinc oxide powder according to the ceramic technique, as previously described.l' The electrode used in this work has a resistivity of p = 6.2 Qecm and a donor concentration N D of ca. 8 X 10l8 C I T - ~ . Prior to each EL measurement, the electrodes are polished with diamond paste down to 1 .O pm, then etched with 4 N HCI for 15 s and washed with twice-distilled water. The electrolyte is an aqueous solution of the reagent grade salt, 0.05 M M2S208in molar MCl, where M is Na, K, or NH,. Molar Licl in 0.05 molar Na,S2O8 has been used as substitute for Li2S208since it is not commercially available. A detailed description of the experimental EL setup is given elsewhere.6 In short, an electrochemical cell equiped with a flat quartz window and a standard three electrodes configuration, a saturated calomel electrode (SCE) being used as reference, is substituted to the cell holder inside a Perkin-Elmer M P F 44B spectrofluorimeter. For the sake of comparison, each EL spectrum is recorded with a 2-nm spectral resolution and a 1 nm/s scan rate; similarly, the EL luminescence of the EL-potential curves is monitored at 560 nm, with a 20-nm slit width. Electrode potentials are quoted vs. SCE, and measurements are made at room temperature. The EL emission i s obtained by pulsing the electrode (pulsed mode) between 0 V and a negative potential down to -8.0 V, by means of a P.A.R. Model 173 potentiostat monitored by a P.A.R. Model 175 programmer. One-millisecond pulses separated by 5-ms intervals (potential frequency 167 Hz) are used. Simultaneously to each EL intensity measurement, synchronized current and EL transients are displayed on a Tektronix 7834 storage oscilloscope. Cyclic voltammograms are recorded under continuous (sweep mode) polarization from 0 V down to the H 2 0 reduction wave. Argon is bubbled through the solution prior and during the measurements. 3. Results No luminescence is detected in supporting electrolytes (1 M MCl), Le., without the S,Oa2-species, down to a -8-V pulsed potential. This excludes a cation injection into the ZnO surface to be responsible for the EL generating process, as it has been (7) Neumark, G. F.; Catlow, C. R. A . J . Phys. C:Solid Srate Phys. 1984, 17, 6087 and references therein. (8) (a) Schirmer, 0. F.; Zwingel, D. Solid State Commun. 1970. 8, 1559. (b) Zwingel, D. J . Lumin. 1972, 5, 385. (9) Tomzig, E.; Helbig, R. J . Lumin. 1976, 1 4 , 403. (10) Cox, R. T.; Block, D.; Herve, A,; Picard, R.; Santier, C.; Helbig, R. Solid Srare Commun. 1978, 25, 11. ( 1 1 ) Fichou, D.; Pouliquen, J.; Kossanyi, J.; Jakani. M.; Campet. G.; Claverie, J. J . Electroanal. Chem. 1985, 188, 167.
I
'4
;
0
00
-50 POTENTIAL i V vs SCE I
Figure 1. Potential dependence of the EL intensity, monitored at 560 nm, of a ZnO electrode pulsed between 0 V (for 5 ms) and a negative step (for 1 ms) in an aqueous 1 M MCI 0.05 M M2S20,solution containing various alkali cations M'. At each selected potential, the EL intensity is measured 15 s after the pulsed excitation has been switched on, when
+
the emission has been stabilized.
/-
Na*
\
00 WAVELENGTH / nml
Figure 2. Cationic dependence of the EL spectra of a ZnO electrode pulsed between 0 and -3 V in aqueous 1 M MCl + 0.05 M M2S208 solution. The spectral resolution is 2 nm and the scan rate 1 n m j s .
proposed with p-GaP under strong cathodic bias.12 Then, the basic electrochemical mechanism which generates the EL at the interface involves first the two-step reduction of the S,O?- anion and a hole injection into the valence band by the SO4-- radical anion. This is corroborated by the EL potential onset VonEL = -0.6 V, which is slightly negative to the flatband potential V,,, = -0.3 V. Figure 1 shows the potential dependence of the EL intensity for different cations Mf in the solution. It can be seen that, for pulsed potentials ranging from 0 to -8 V, the EL intensity monitored at 560 nm is maximum with Li' and decreases on the sequence Li' > Na+ > K+ > NHJ'. At -4 V, the EL emission with Li' is approximately 2, 3, and 4 times more intense than that obtained with Na', K+, or NH,', respectively. The EL spectra obtained (Figure 2) by pulsing the same ZnO electrode between 0 and -4 V show no important influence of M' on the EL spectral distribution. The typical ZnO luminescence (12) Butler, M. A.; Ginley, D. S. Appl. Phys. Letr. 1980, 36, 845
Letters
The Journal of Physical Chemistry. Vol. 90, No. 8, 1986 1487 '
7
-10
00
FUTENftAL I V v r . SCEI I.
,n:
lli
Figure 3. Typical current transients obtained when the electrode is pulsed between 0 and -4 V. The time scale is I ms/div and the current scale 100 mA/cm'/div (cathodic values are downward): (a) Li+;(b) Na'; (c)
K+; (d) NH4+.
TABLE I EL and Current Intensities of a ZnO Electrode in I M MCI and 0.05 M M&08 Aqueous Solution under B -4.0-V Pulsed Potential (167 Hz)' cations
E L intensity at 560 nm, arb units current density,' mA/cm crystallogr radius. A hydrated radius.'A sp conductance of MCI." %'.cm'.equiv-'
Li+
Nat
Kt
NH,+
100
50 36 28 200 220 270 370 0.68 0.97 1.33 1.43 4.66 3.97 3.63 3.51 115.0 126.5 149.9 149.7
"Characicrisiics of ihc caiton as discussed m the i e x i bAr mearured on a ringlc \ u l i o n a r y pulw 'Taken lrom ref I 3 I n aqueous wluiion ill 25 DC.
i s observed. uith a L V emission at 384 nm (band-to-band recombination) and a shoulder at 394 nm, together with a broad visible band (mmbinationr at surface statcs or bulk dislocations). However. for the 1,). spectrum a band peaking at 560 nm i s superimposed onto the broad structureless visible band observed with the other cations. Thuq. this cniisslon seems 10 be sharacleiistic o f the [.if cation. I n order to correlate the luminescence and electrical properties of the ZnOtelectrolvte interface. the tvmcal transient current responses during a siationary pulse o f 1 ms at -4 V are given in Figure 3 for the four cations studied. As can be seen, the shape of the current transients is strongly dependent on M+ and the intensity of the cathodic current during the pulse decreases in the order NH: > K+ > Na+ > Li+(seeTable I); the anodic transient, which appears after the excitation has been turned off, i s more intense and structured in the same order. Similar oscillograms are obtained whether the persulfate anion i s present or not. In Figure 3, oscillograms a and b show one anodic transient while the other two, c and d, present two oxidation steps. The sharp step which appears in the four eases can correspond to the reoxidation o f the reduced ZnO and/or to the oxidation of water; on the other hand, the second anodic step could be attributed to a change of the cation environment. from inserted in the ZnO lattice to the solvated cation. This part is s t i l l under investigation. Cyclic voltammograms of the ZnO electrode have been carried they are depicted in Figure 4 together out with Li' and NH,;' with the i-Vcurves obtained in the absence of the M2Sz0, salt (dashed lines). The reduction wave at -0.6 V can be attributed unambiguously to the S2082-ion, since it does not appear when the solution contains only M C I . To our knowledge, this is the first clear report of the Sz082-reduction wave on a semiconducting
..
-IO
00
P0TENTt"LIV".
SCE,
Figure 4. Cyclic voltammograms of a n-ZnO electrcdc (I) in I M MCI (dashed line) and (2) in 1 M MCI 0.05 M M,S20s (full line). The simultaneous EL intensity, monitored at 560 nm, under continuous (nonpulsed) polarization has been added (3) by using the same scale (in arbitrary units) for both a and b. (a) M = Lit; (b) M = NH,+.
+
material. The reduction wave at -1.2 V for Lit and at -1.1 V for NH4' i s accompanied by a strong gac evolution together w i t h a grey dewsit on the surface. which can be attributed to the H,O and Zn'"reductions. respectivcl). On the return sweep. the peak at ca. -I V ma) result frum the ZnO reoxidation for the grcy ZnO layer formed at the surface disappears simultaneously. The current densities measured with UHI+ arc approximately twice as intense as the one found uith Li'. Thcse results corroborate the relative values obtaincd on the ascillascope (Figure 3) during a single pulse for the transient currcnt response at -4 V. Figure 4 also shows the E l . obtained under continuous polarization. I t is nuteuorthy that the E L intensity (shown with thcsamexale in Figure 4a.b) i s higher w i t h Li' than with S H 4 * . Furthermore. the E L onset pnential YonkLroughly corresponds to the reduction potential of the persulfate ion.
4. Discussion According to the results given in the table, the E L intensity and the cathodic current resulting from thc persulfate reduction are both modified when the alkaline cation present in the elmtrol)te solution i s changed. But if the E l . intensity decrcaser in the order 1.1. > Nal > K ' > NH4'. the current density varies in the opposite urder. This double cationic dependence can result either from a variation in the conduction properties (mainly ionic activity and
1488
J . Phys. Chem. 1986, 90. 1488-1491
conductivity) of the electrolyte, or from a modification of the ZnO surface by adsorption or injection of cations under the influence of the electric field at the interface. The crystallographic radius of monovalent alkali ions increases from Li' to NH4+, but the corresponding hydrated radius dec r e a s e ~in ' ~the same order (see Table I). This explains the higher conductivity of the electrolyte containing NH4' instead of Li' ions as shown by the variation of the specific equivalent conductance of the M' ions. Such a cation effect on the photocurrent performances and the stability of CdSe electrodes in polysulfide solutions has been reportedI4 already. Nevertheless, this analysis seems insufficient to account for the luminescence enhancement when we change NH4' to Li'. The more intense current observed with NH4' should inject more positive charges in ZnO and, consequently, increase the number of electron-hole recombinations as well as the EL intensity.6 Figure 4 shows that the EL intensity is stronger with Li' than with NH,' even before the Zn2+ reduction potential (-1.2 V) has been reached. Furthermore, the reduction wave of the S208*-ions at -0.6 V is twice as intense with NH4' (14 mA/cm2) as with Li+ (7 mA/cm2), predicting an EL emission more intense for NH4+ than for Li' in the -0.6 to -1.2 V potential range. As this is not the case, it appears necessary to put forward a modification of the electrode itself to account for all the experimental results. Alkali ions, mainly Li' and Na', when introduced inside the lattice of 11-VI semiconductors, are known to increase considerably their resistivity. In ZnO for instance, Li" creates a deep acceptor center which lies about 0.8 eV above the valence band.* On the other hand, the Li and Na' ions induce donor-acceptor pairs in the ZnO lattice and play a determinant role in its UV9 and visible*~1° luminescence. These studies on solid-state ZnO doped with alkali ions are consistent with our observations and we propose that M+ cations from the solution are injected into the ZnO electrode under the effect of the applied potential. Occupancy of preferential sites in the host lattice would modify its luminescence and electrical properties near the surface. The strong accumulation layer built up in ZnO at potentials cathodic to the flat-band potential implies (13) Harned, H. S.; Owen, B. B. In The Physical Chemistry of Elecfrolytic Solufions; 3rd ed.; Rheinhold: New York, 1958; p 537. (14) Licht, S.; Tenne, R.; Flaisher, H.; Manassen, J. J . Electrochem. So?.
1984, 131, 950.
a narrow space charge layer and impedes the cations to penetrate deeply inside the semiconductor. The cations injected during the excitation are then expected to be expelled when the potential is turned off, Le., at the end of each pulse. This is verified by using the electrode successively with various cations without surface pretreatment between each experiment; the same cationic dependence on EL and current is then observed. Besides, due to their large hydration shell, the hydrated cations may be trapped only at superficial sites from where they relax when the excitation is turned off. Since the EL spectral distribution is not strongly influenced by the cation, the radiative centers inside ZnO would be simply activated (or deactivated) by the metal ions M+. Such an EL activation of ZnO by small alkali ions probably reflects a compensation process in the lattice, decreasing the concentration of the nonradiative centers. However, in the case of Li+, the EL band peaking at 560 nm seems to correspond to the so-called "yellow luminescence" of ZnO:Li, which involves a hole trapped next to a Li' acceptor state.la This similarity constitutes an argument to favor a mechanism of countercation injection into the ZnO lattice; nevertheless, it is difficult, on the basis of these only results, to exclude a mechanism involving adsorption of the ions at the surface of ZnO. Then, the NH4' ions would be expected to show easier adsorption properties than Li', as a result of a weaker solvation than K+, Na', and Li'. It seems interesting to verify that a similar cation effect is operating with other 11-VI semiconductors in liquid junction,I5 specially with Li- or Na-doped ZnSe, for which the solid-state luminescence and conductivity have been reported7J6recently.
Conclusion Pulsed EL of the n-ZnO/aqueous electrolyte interface during persulfate reduction, combined with cyclic voltammetry, demonstrates the strong influence of the alkali cation upon the interfacial luminescence and electrical properties of ZnO. This cationic effect seems to result from both a variation of the conductivity of the electrolyte and a modification of the ZnO surface by cation injection in lattice sites or adsorption at the surface. ( 1 5) Further studies in this field are in progress. (16) Neumark, G. F.; Herko, S.P.; McGee, T. F.; Fitzpatrick, B. J. Phys. Reti. Lett. 1984, 53, 604.
Time-Resolved Emission Spectra of Ru(bpy),CI, and cis-Ru(bpy),(CN), at Low Temperature Noboru Kitamura, Haeng-Boo Kim, Yuji Kawanishi, Ritsuko Obata, and Shigeo Tazuke* Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori- k u , Yokohama 227, Japan (Received: October 16, 1985; In Final Form: February 3, 1986)
Time-resolved emission spectroscopy of Ru(bpy)&12 and ci~-Ru(bpy)~(CN)~, where bpy is 2,2'-bipyridine, was conducted in an ethanol-methanol mixture above 80 K. The emission spectra of both complexes shifted to the red with increasing temperatures (>80 K) as well as with the delay time after excitation (110-150 K). The apparent activation energies for the time-dependent red shift of the emission (1 10-130 K) were calculated to be 570 and 1360 cm-' for Ru(bpy),C12 and cis-Ru(bpy)*(CN)*,respectively. It was concluded that charge localization in the excited state was not an intrinsic property of the complex but was induced by solvent relaxation processes.
Introduction The excited state of tris(2,2/-bipyridine)ruthenium(II), R ~ (bpy),", has been widely employed as a potential photocatalyst for chemical conversion of solar energy in fluid media including micellar and polymeric systems and also on electrodes or semi0022-3654/86/2090-1488$01.50/0
conductor partic1es.l The excited-state properties of R ~ ( b p y ) ~ * + , however, are known to be very sensitive to environmental conditions such as solvents and temperature,2 and thus, evaluation of pho(1) Kaiyanasundararn, K. Coord. Chem. Reo. 1982, 46, 159.
0 1986 American Chemical Society
