Mechanism of luminescence quenching of CdS single crystal by redox species: electric-field effect or electron-transfer quenching?

Volume 182,number 2

CHEMICALPHYSICS LETTERS

26 July 1991

Mechanism of luminescence quenching of CdS single crystal by redox species: electric-field effect or electron-transfer quenching? M. Hiramoto a, K. Hashimoto a Chemical Process Engineering, Far&y b Deparlmenf

ofSyntheflc

’ Deparfment

ofElectromc

Chemistry, Chemistry,

b and T. Sakata c OfEngineering, Faculty

Osaka University,

OfEngineering,

Tokyo Instirute

Yamadaoka

The University

Suta,

of Tokyo, Hongo,

of Technology at Nagatsuia,

Midori-ku,

Osaka 565, Japan Bunkyo-ky, Yokohama

Tokyo 113, Japan 227, Japan

Received 22 January 1991;in final form 10 April 1991

Photoluminescenceof a single-crystallineCdS electrode was measured in solutions ofvarious redox speciesunder potentiostatic conditions. Both the green and red emission of CdS crystal were quenched efiiciently by the addition of various electron acceptors. Quenching efficiency depends both on the electrode potential and the kind of electron acceptors. Since the flat-band potential is

hardly affected by the addition of electron acceptors, it is concluded that the luminescence quenching by these electron acceptors is not due to an electric-fieldeffect but due to the interfacial electron transfer.

1. Introduction In order to elucidate

the electronic

structure

of a

colloidal semiconductor

and its interfacial

process,

the photoluminescence

of semiconductors

such as

CdS [l-6], ZnS [7], TiO, [S], etc. [9] has been intensively investigated and many examples of the luminescence quenching by various redox species have been reported. Recently, we reported the photoluminescence (PL) quenching of CdS and ZnS particles and discussed the quenching mechanism based on electron transfer (ET) between electrons in the conduction band of semiconductors and electron acceptors on the surface [ IO,1 I]. Some workers, however, claim that the PL quenching can be explained only by an electric-field effect [ 5,121. So, in order to study the interfacial ET by means of PL quenching, it is important to separate the quenching due to the change of electric field caused by the addition of redox species from real ET quenching, although there are some approaches [ 13,141 based both on the electric field and on the surface recombination. In this Letter, we report the results of photoluminescence quenching of a single-crystalline CdS electrode under potentiostatic conditions by various 0009-2614/91/$

03.50

0

redox species and discuss the role of the electric field and the interfacial electron transfer at the interface on the photoluminescence quenching.

2. Experimental The photoluminescence of a single-crystalline CdS (Teikokutsushin Co. Ltd.; parallel to c axis) electrode was measured in an aqueous solution of Na$O, (0.1 M ) at various electrode potentials. Before measurements, the single crystal was etched with a concentrated hydrochloric acid for 15 s. The electrode potential was controlled by the potentiostat (Hokuto Denko Ltd., HA-501 ). Effects of the redox species on the photoluminescence were measured by adding the redox species into the electrolyte solution. The luminescence spectra were measured by using a Spex Fluorolog 2 spectrometer. For capacitance measurements of the CdS electrode in various solutions, a phase-sensitive detector at 1 kHz was used. The flatband potentials were determined from MottSchottky plots.

1991 - Elsevier Science Publishers B.V. (North-Holland)

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3. Results and discussion

Fig. 1 shows the luminescence spectra of a singlecrystalline CdS electrode in the absence of an electron acceptor (a) and in the presence of Fe3+ (b). Two emission peaks were observed for a single-crystalline CdS. One is the so-called “red emission” which is observed with a peak at 730 nm. This emission is essentially the same one observed for CdS/PVG (porous Vycor glass) [ lo]. The other is the so-called “green emission” with a peak at 5 10 nm. The intensity of these emissions depends strongly on the electrode potentials as reported by Uchihara et al. [ 121. The effect of the electrode potential on the red-emission intensity in the absence of redox species is shown in fig. 2 (curve (a)). When the electrode potential becomes negative, the intensity of the red emission increased remarkably, keeping the spectral shape the same. The green emission also showed similar dependence as shown in fig. 2 (curve (e ) ). The potential dependence of luminescence intensity of a semiconductor electrode was already reported by Karas et al. [ 15 ] for single-crystalline CdS, by Ferrer and Salvador [ 5 ] for polycrystalline CdS, and Nakato et al. [ 91. It is well known that the electric field in the space-charge layer at semiconductorelectrolyte interface affects the charge separation,

a

;

b I

WAVELENGTH Fig. 1. Photoluminescence

800

7co

600

/

nm

spectra of single-crystalline CdS elec-

trode whose electrode potential is fixed at -0.7 V versus saturated calomel electrode (SCE): (a) without electron acceptor; (b) with 0.01 M Fe’+. Excitation wavelength is 453 nm. Measurements were carried out in an aqueous solution of 0.1 M Na,S04 at room temperature.

140

-1.0 U versus

-0.5 .CXE/V

Fig. 2. Electrode potential versus red-emission intensity at 730 nm of a single-crystalline CdS in the solution without electron acceptors (a) and with MV*+ (5x10-’ M) (b), benzoquinone (10-j M) (c), Fe’+ (10-l M) (d). Curve (e) is for the greenemission intensity without electron acceptors.

since the photoexcited electrons and holes in the space-charge layer move oppositely in the electric field. Because of this charge separation, the luminescence due to recombination between electrons and holes (regardless of being free or trapped) decreases its intensity in the space-charge region. Thus, in the presence of an electric field, the space-charge layer is a kind of “dead layer” of the luminescence. On the other hand, the thickness ofthe photoabsorption layer is determined by the absorption coefficient and is independent of the electrode potential. Since the thickness of the space-charge layer, i.e. “the dead layer of the luminescence”, increases by increasing the anodic polarization in the case of an n-type semiconductor, the luminescence intensity is expected to decrease by increasing the anodic polarization. The remarkable decrease of the luminescence with increase of the anodic polarization (curves (a) and (e) in fig. 2) is explained reasonably by this electric-field effect. Ferrer and Salvador [5] reported that the photoluminescence of the polycrystalline CdS electrode was quenched by anodic polarization of the electrode potential and explained the quenching of electron acceptors by this electric-field effect. Uchihara et al. [ 121 also reported the photoluminescence quenching of the single-crystalline CdS by anodic

Volume 182,number 2

polarization and explained the photoluminescence quenching by electron acceptors based on the electric-field effect. On the other hand, in the previous work [ lo], we discussed the luminescence quenching of CdS particles on porous Vycor glass (PVG) based on the electron-transfer quenching. In order to clarify whether electron-transfer quenching takes place or not, the effects of various redox species were examined by using this single-crystalline electrode. In fig. 2, the quenching effect of various electron acceptors against the intensity of the red emission at various electrode potentials is also shown. Interestingly, the addition of electron acceptors such as benzoquinone, methyl viologen, Fe3+, was found to case a drastic quenching of the red emission at any electrode potential between - 1.05 and -0.6 V as shown in fig. 2. On the other hand, most electron donors, such as sulfite and lactic acid, do not cause such luminescence quenching. This behavior is the same as observed for the red emission of CdS particles deposited on PVG [lo]. Nearly the same quenching behavior was observed for the green emission. In fig. 1 (curve (b)), the quenching effect of Fe3+ on the luminescence spectrum is shown as an example. Fig. 3 shows the electrode potential dependence of 1,/l,. Here, lo and IA represent the luminescence intensity in the absence and in the presence of the electron acceptor. The fact that Z,/l, is smaller than I .O means

-$

0.5 ,

0.2

O-II_

-1.0

that quenching both of the green (G) and red (R) emissions occurs by adding the electron acceptors even under the constant electrode potential. However, if the addition of an electron acceptor causes a large shift of the flat-band potential (CT,) of CdS in the negative direction, the band bending at the space-charge layer would be increased [ 161. Then, the decrease of luminescence intensity would also be explained by the increase of the electric field in the space-charge region. In order to clarify whether the quenching is caused by the electric-field effect or electron transfer, the flat-band potentials of the single-crystalline CdS in the solutions of various redox acceptors were measured. The result is shown in table 1. As seen in table I, the addition of electron acceptors hardly shifts U,. This result means that the magnitude of the band bending of the CdS electrode is not affected by these electron acceptors as far as the electrode potential is fixed. Therefore, it is concluded reasonably that the luminescence quenching in figs. 2 and 3 is not due to the electric-field effect but due to interfacial electron transfer. Thus, the quenching effect caused by the interfacial electron transfer is distinguished from that caused by the electric-field effect. When the photoluminescence is measured in an aqueous solution containing a large amount of electron donor (NarSO, ( 1 M)), the quenching effect of methyl viologen ( 10m3 M) is not observed. This is consistent with the result of Uchihara et al. [ 121. The same aqueous solution containing both Na2S03 and methyl viologen shows a new absorption band Table 1 Shift of the flat-band potential (9) of CdS single crystal from that without electron acceptors by addition of electron acceptors. Measurementswere carried out in an aqueous solutionof NaZS04 (0.1 M) in the dark. Sample was polished and etched with concentrated HCI before the measurements. In the absence of electron acceptors, cih is -0.73 V versus a saturated calomel electrode (SCE) a)

c

-1.2

26 July 199I

CHEMICALPHYSICSLETTERS

-0.8

-0.6

-0.4

U vemu~SCE/V Fig. 3. 1,/f, versus electrode potential of the single-crystalline CdS. Methyl viologen (5x IO-’ M) and Fe’+ ( 10s2 M) were used as acceptors. 1, and 19denote the photoluminescenceintensity in the solution with and without electron acceptor, respectively. I,/[, for green emission (G) and that for red emission (R) are shown.

Electron acceptor

Ua,shift (V)

benzoquinone ( I 0W3M ) methyl viologen (5 x 10-j M) Fe’+ (IO-*M)

-0.04 -0.02 f0.04

a1Our Vu,value measured in SO:- (0.1 M) solution is more positive compared to that measured in the SO:- (1 M) solution reported in ref. [ 121.The negative shift of U, by the adsorption of SO:-

ion was reported by Minoura and Tsuiki

[171. 141

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CHEMICAL PHYSICS LETTERS

(450-320 nm), presumably due to the formation of the charge-transfer complex between MV2’ and SO:-. The above result suggests that MV2+ cannot act as an electron acceptor in a concentrated solution of SO:-, This explains well the result observed by Uchihara et al. [ 121. Moreover, the photoluminescence of CdS single crystal, which was already quenched by methyl viologen, did not recover by the addition of Na,SO, (1 M). This suggests an irreversible strong adsorption of methyl viologen on the CdS surface. The present results for the CdS single crystal clearly show an important role of interfacial ET on the photoluminescence quenching. In the case of CdS/PVG, because of the small particle size of CdS (about 10 nm) [ lo], the band bending of each particle seems to be very small, especially, under illumination. Thus, we may conclude that the quenching observed in our CdS/PVG is mostly due to the interfacial ET, even If some small part of the quenching behavior is attributed to the electric-field effect. As show in fig. 3, 1,/Z, increases gradually with increasing anodic polarization. This is explained as follows: As shown in fig. 2, the decreasing rate of I,, 1smuch faster than that ofl,. This means that in the presence of electron acceptors, the electron-transfer effect exceeds the electric-field-induced electron-hole separation. As a result, the ratio of IA to I,, i.e. IA/ I,, appears to increase with increasing anodic polarization. Z,/Zo of the red emission is always larger than that of the green emission for any acceptor. This means that the green emission is quenched more easily than the red emission. The green emission is observed at the wavelength just near the absorption edge of CdS (edge emission), and this emission arises from the transition between very shallow traps for the electron and hole [ 181. Because of these shallow traps, electrons must be easily excited thermally to the conduction band and transferred easily to an electron acceptor. This may be the reason why the green emission is quenched more efficiently than the red emission. The rise of the red and green emission is very rapid, following the excitation pulse of 50 ps [ 191. These results indicate that the trapping rate of carriers by the luminescence centers is faster than 50 PS. The interfacial ET rate seems comparable with or faster than this value. 142

26 July 199 I

4. Conclusion The interfacial electron-transfer process from the conduction band of CdS to the adsorbed electron acceptor can be observed by measuring the photoluminescence quenching of single-crystalline CdS electrode under potentiostatic conditions. Though the emission centers are located mainly in the bulk of CdS, the luminescence quenching relates with the interfacial ET. The quenching, due to the electric-field effect, is concluded to be minor under potentiostatic conditions, or in the case of very small CdS particles such as CdS/PVG [ lo].

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[IS] B.R. Karas. H.H. Streckert, R. Schreiner and A.B. Ellis, J. Am, Chem. Sot. 103 ( I98 1) 1648. [ 161 T. Uchihara, M. Matsumura, J. Ono and H. Tsubomura, J. Phys. Chem. 94 ( 1990) 415. [ 171 H. Minoura and M. Tsuiki, Electrochim. Acta 23 ( 1978) 1377.

[ 181 F.J. Bryant and C.J. Radford, Phys. Stat. Sol. 12 (a) ( 1972) 73.

[ 191 M. Hiramoto, K. Hashimoto and T. Sakata, unpublished results.