2324
J . Phys. Chem. 1988, 92, 2324-2329
an explanation is not in harmony with the following facts: (1) The present n-Si electrodes coated with Pt islands are stable under illumination at anodic potentials, quite contrary to the naked ones, indicating that the Pt/Si contact is chemically tight and stable. (2) The vacuum-deposited Pt on Si forms a thin metallic alloying (silicide) layer at the interfaces*'5and 4g is determined by the Si/silicide interface. The n-Si electrodes coated with Pt (or Pt silicide) layers of submonoatomic thickness showed only low, constant V, irrespective of c(R/Ox), as mentioned above, indicating that the I#$ at the silicide/Si interface stays constant even if the silicide layer is very thin and in contact with redox electrolyte solutions. (3) The R/n-Si(porous, 0 = 20") and Pt/n-Si(porous, 0 = 9 0 ° ) electrodes show quite different behavior (Figure 7 ) , though they were prepared in the same way except for a change in the tilt angle 0 (Figure 6). It should be emphasized that the V, value of 0.685 V (Figure 2) obtained from one of the Pt-coated and alkali-etched n-Si electrodes is much higher than that (about 0.59 V) reported for an n+-pjunction Si solid solar cell having an antireflection coating and a back-surface-field structure3s and even is higher than that (0.66 1 V) reported for the most elaborate n+-n-pp+-junction solid solar cell using FZ-type silicon that showed a 20.9% con(35) Nunoi, T.; Nishimura, N.; Nammori, T.; Sawai, H.; Suzuki, A. Jpn. J. Appl. Phys. 1980, 19 (Supplement 19-2). 67
version e f f i ~ i e n c y The . ~ ~ short-circuit current and the fill factor for the present type of electrodes will be increased by making textured surfaces and the back-surfacefield structure, for instance. The present result is very encouraging, indicating a novel approach to highly efficient and stable solar cells. In conclusion, the present work has revealed, both experimentally and theoretically, the importance of the microscopic discontinuity of metal overlayers at metal-coated semiconductor electrodes (and powder photocatalysts). Our present theory can be applied to (naked) semiconductor electrodes having localized high-density surface states because they play essentially the same role as deposited metal islands in photoelectrochemical processes. The theory is also applicable to a solid-state-type solar cell having a transparent conductive layer in place of the redox electrolyte solution in PEC cells. Aside from the practical importance, the present work has opened a new insight in the field of the metal-semiconductor j ~ n c t i o n . ' ~ . ' ~
Acknowledgment. We express our thanks to Akashi-Seisakusho Ltd. for taking scanning electron micrographs of Pt-coated n-Si electrodes. This work was partly supported by the Ministry of Education, Science and Culture, Grant-in-Aid for Scientific Research on Priority Areas (No. 62603010). Registry No. Si, 7440-21-3; Pt, 7440-06-4; Fe, 7439-89-6; Fe(CN),, 13408-63-4; Fe(C,O,),,
30948-48-2.
Photoelectrochemlstry in Particulate Systems. 7. Electron-Transfer Reactions of Indium SuHlde Semlconductor Colloids Prashant V. Kamat,* Nada M. Dimitrijeviir,t and Richard W. Fessenden Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received: August 5, 1987, In Final Form: November 1 1 , 1987)
Small semiconductor colloids of In2S3have been prepared in aqueous and nonaqueous media and their absorption properties characterized. A transient photobleaching and formation of S'- and S2H2'radicals have been observed upon laser pulse ( 3 5 5 nm) excitation of these colloids. With the aid of transient absorption spectra, the anodic corrosion process in these semiconductor colloids has been elucidated by using laser flash photolysis and pulse radiolysis techniques. With the use of a zwitterionic viologen compound, the interfacial charge-transfer process at the semiconductor surface has been studied. The quantum yield for the reduction of zwitterionic viologen was 0.07, which is similar to the value obtained with other metal chalcogenide semiconductor colloids. The microenvironment of the stabilizer (Nafion) influenced the charge-transfer process between the semiconductor and the redox relay.
Introduction A variety of approaches has been used in recent years to investigate the charge-transfer processes in colloidal semiconductor systems. Of particular interest is the observation of optical quantization effects in extremely small semiconductor particles.]-' Such a quantum confinement can occur in one, two, or three dimensions. Metal chalcogenides have received considerable attention in recent years because of their photocatalytic activity under visible light irradiation. Bandgap excitation of these semiconductors leads to the trapping of charge carriers that can be characterized by fast kinetic s p e c t r o s c ~ p y The . ~ ~ study of trapped charge carriers in semiconductor systems is important as they can greatly influence the surface corrosion processes of the semiconductor. Indium sulfide (In2&), a member of the II12-V13 group of materials, is known to possess some interesting features because of its defect structure. In2S3,which usually exhibits n-type semiconducting properties, is known to exist in three different forms: 'On leave of absence from the Faculty of Science, Belgrade University, Yugoslavia. 0022-3654/S8/2092-2324$01.50/0
a,which is a defect-centered cubic; p, which is a defect spinel; y, which is a layered structure. The transition temperatures are 420 (a 8) and 740 O C (8 y).*-'O Rehwald and Harbeke* -+
-
~~
~
~~
~~
(1) See,for example: (a) Rosetti, R.; Nakahara, S.; Brus, L. E.J. Chem. Phys. 1983, 79, 1086. (b) Rosetti, R.; Hill, R.; Gibson, J. M.; Brus, L. E. J . Chem. Phys. 1985, 82, 552. (2) See, for example: (a) Williams, F.; Nozik, A. J. Nature (London) 1984, 311, 21. (b) Nozik, A. J.; Williams, F.; Nenadovic, M. T.; Rajh, T.; Micic, 0. I. J . Phys. Chem. 1985.89, 397. (c) Ploog, K.; DBhler, G. H. Adu. Phys. 1983, 32, 285. (d) Petroff, P. M.; Gossard, A. C.; Logan, R. A,; Wiegmann, W. Appl. Phys. Lett. 1982, 41, 633. (e) Sandroff, C. J.; Hwang, D. M.; Chung, W. M. Phys. Rev. B Condens. Matter 1986, 33, 5933. (3) (a) Fojtik, A.; Weller, H.; Koch, U.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 969. (b) Fojtik, A.; Jirkovsky, J. Chem. Phys. Lett. 1987, 137, 226. (4) Henglein, A.; Kumar, A,; Janata, E.; Weller, H. Chem. Phys. Lett. 1986, 132, 133. (5) Baral, S.; Fojtik, A,; Weller, H.; Henglein, A. J . Am. Chem. Sac. 1986, 108, 375. (6) Kamat, P. V.; Dimitrijevic, N. M.; Fessenden, R. W. J . Phys. Chem. 1987, 91, 396. (7) Dimitrijevic, N. M.; Kamat, P. V. J . Phys. Chem. 1987, 91, 2096. (8) Rehwald, W.; Harbeke, G .J. Phys. Chem. Solids 1965, 26, 1309.
0 1988 American Chemical Society
Indium Sulfide Semiconductor Colloids
The Journal of Physical Chemistry, Vol. 92, No. 8, 1988 2325
investigated the electrical conduction mechanism in In2S3single crystals and observed energies of 2.03 eV for direct transition and 1 eV for a very weak indirect transition. From the photoconductivity9 and photoelectrochemical" experiments, a bandgap of 2.0 eV has been estimated. Despite the interesting semiconductor properties, no major effort has been made to employ this class of semiconductors in designing photoelectrochemical systems. A recent study by Becker et a1.I' has highlighted the importance of In2S3semiconductors in photoelectrochemical cells. In order to establish the mechanism of corrosion processes and the role of trapped charge carriers in influencing the electron-transfer reactions, we have undertaken a detailed photochemical study of colloidal In2S3particles. We report here a laser flash photolysis study that elucidates the photochemical behavior of In2S3colloids.
16-
w
y
12-
3 a
-
0 v)
m
WAVELENGTH("
G8-
a
.
GO
200
Experimental Section Materials. In(C104),.8Hz0 (99.9%) was obtained from Johnson Matthey Inc. Nafion ( 5 % petfluorinated ion exchange polymer solution) was obtained from Aldrich. Acetonitrile (Aldrich, Gold Label), methanol (Fisher Scientific, spectroanalyzed), and Millipore water were used for preparing solutions. All other chemicals were analytical reagents. Preparation of In#, Colloids. The method employed was similar to the one employed in the preparation of CdS colloid.6J2 Twenty-five milliliters of In(C104)3(0.1 mM) solution in acetonitrile containing 1% water was taken in a flask. With use of a water pump, the solution was degassed under vacuum with stirring. After cooling the solution to --30 OC, 25 mL of H2Swas injected into the flask. After the solution was gently stirred for 10 min, the solution was degassed and purged with N2 to remove any dissolved H2S. The colloidal suspension was stable for several days when stored in the dark under refrigeration. The same method was employed to prepare the colloidal suspension in methanol. For preparation of In#, colloids in an aqueous medium, 3 mM sodium metaphosphate (Fisher) was used as a stabilizer, and the reaction was carried out at room temperature. All the experiments were performed with the freshly prepared colloidal suspension. Apparatus. Absorption spectra were recorded with a Cary 219 spectrophotometer. Flash photolysis experiments were performed with a 355-nm laser pulse (pulse width 6 ns) from a Quanta-Ray Nd:YAG laser system. A typical experiment consisted of 10-20 replicate shots per sample, and the average signal was processed with a PDP 11/55 computer. The experimental details can be found elsewhere.', Unless otherwise specifically mentioned, all the solutions were purged with argon, and the experiments were performed at 22 "C. Pulse radiolysis experiments were performed with the Notre Dame 7-MeV A R C 0 LP-7 linear accelerator. The operating conditions have been described e1~ewhere.l~Absorbed doses in the pulse radiolysis experiments were in the range of 2-10 Gy/ pulse.
-
Results and Discussion Absorption Characteristics in In#, in Nonaqueous Media. The absorption spectra of colloidal In2S3prepared in acetonitrile and methanol are shown in Figure 1. The colloids prepared at room temperature or at -30 OC are expected to be of the a form as the other crystalline modifications occur at higher temperatures. As reported earlier,8s9,11In#, single crystals exhibit a bandgap corresponding to a direct transition around 2 eV. The colloids (9) Gilles, .IM.; . Hatwell, H.; Offergeld, G.; Van Cakenberghe, J. Phys. Siatus Solidi 1962, 2, K73.
(10) Garlic, G. F. J.; Springford, M.; Checinska, H. Proc. Phys. SOC.1963, No. 82, 16. (11) Becker, R. S.; Zheng, T.; Elton, J.; Saeki, M. Sol. Energy Mater. 1986, 13, 97. (12) Ramdsen, J. J.; Webber, S. E.; Gratzel, M. J . Phys. Chem. 1985, 89,
300
400
500
WAVELENGTH ( n m )
Figure 1. Absorption spectra of 0.05 mM In2S3colloids prepared (a) in acetonitrile and (b) in methanol. Insert: absorption spectra of 0.05 m M In& colloids in acetonitrile recorded before photolysis (-), and after 10 (- - -) and 30 min of photolysis with a xenon lamp. (a
a)
20
340 "m
I6
y
z a m
12
CL
0
2a
oa
04
n "n " 200
400 WAVELENGTH (nm)
300
500
Figure 2. Absorption spectra of 0.05 mM In$, colloidal suspension in acetonitrile (a) 5, (b) 12, (c) 18, (d) 25, ( e ) 40, and (f) 60 min after its preparation. Insert shows the growth in the absorption a t 340 nm.
prepared in acetonitrile exhibited an onset of absorption around 500 nm, which corresponded to a direct transition with a bandgap > 2.5 eV. However, no indirect transition could be observed for these colloids. This behavior was similar to our previous observations with colloidal In,Se3.'5 The formation and growth of In,S, colloidal particles involved a slower process and can be monitored with conventional spectrophotometry. After H,S gas was injected into the reaction vessel, a small portion of the mixture was quickly transferred to a 1-cm cuvette and was sealed with a septum. The absorption spectra recorded at various time intervals are shown in Figure 2. The wavelength range (500-200 nm) was scanned in 2.5 min. The absorption in the UV region confirmed In#, colloid formation immediately after injecting H,S. However, the growth of colloidal particles continued for about an hour, yielding stable particles. The absorption spectrum of these stable colloids exhibited shoulders around 260, 340, and 440 nm. As established earlier with CdS3%l6 and In2Se3l5colloids, the growth of colloidal particles in the present experiments can be explained on the basis of nucleation, combination, and Ostwald ripening processes. The absorption monitored at 340 nm increased continuously (insert in Figure 2), indicating the overall growth of colloidal particles. The small particles formed immediately after the injection of H,S continued to grow until the equilibrium condition was reached. Electron micrographs obtained with Hitachi H600 TEM indicated these fully grown particles as
2740
(13) Das, P. K.; Encinas, M. V.; Small, R. D., Jr.; Scaiano, J. C. J . Am. Chem. SOC.1979, 101, 6965. (14) Patterson, L. K.; Lilie, J. I n f . J . Radiat. Phys. Chem. 1974, 6, 129. -
(15) Dimitrijevic, N . M.; Kamat, P. V. Langmuir 1987, 3 , 1004. (16) Fischer, Ch.-H.; Weller, H.; Fojtik, A,; Lume-Periera, C.; Janata, E.; Henglein, A . Ber. Bunsen-Ges. Phys. Chem. 1986, 90,46-49.
2326 The Journal of Physical Chemistry. Vol. 92, No. 8, 1988
Kamat et al.
s herical with particle diameter ranging between 1000 and 2000
1.When subjected to photolysis (irradiation with a xenon lamp),
InzS3colloids underwent photodegradation as can be seen from the decrease in its absorption (insert in Figure 1 ) . Both anodic and cathodic corrosion processes are expected to occur when the semiconductor colloids are subjected to direct bandgap excitation. Laser flash photolysis experiments have been performed to elucidate the photochemical events that lead to the corrosion of the In$, semiconductor. Laser Pulse Excitation of In2S3Colloids. The transient absorption spectra observed after laser pulse excitation of In2S3 colloidal suspension in acetonitrile are shown in Figure 3. A transient bleaching was observed immediately following the laser pulse with maxima around 420 and e 3 4 0 nm. The position of the bleaching maxima matched well with the shoulders observed in the absorption spectrum, indicating thereby the transient bleaching originated from the depletion of In2S3colloids. The transient bleaching was found to recover quickly with a lifetime of 1-2 ps for the slow process. Because of scattered light from the laser pulse itself, it was not possible to resolve the fast component of bleaching recovery. Similar photobleaching has been observed earlier for CdS6J7and CdSe' colloids. As the transient bleaching recovered, formation of two other transients with absorption maxima at 380 and around 470 nm could be seen. Bandgap excitation of the semiconductor leads to charge separation followed by trapping of the charge carriers in deep and shallow traps lying within the valence- and conduction-band edges (reactions 1-3). The combination of the trapped charge carriers,
-
In2S3
hv
h+ + e-
(111~s~)~ + e(In2S3)n+ h+
-
-
+ h,+
-
e;
(2)
h,+
(3)
2(In2S3),
(4) the resolution of this process. More than 90% of the bleaching recovered to regenerate In&. Trapped holes, if not reacted with the trapped electrons, could lead to anodic corrosion (reaction 5). h,'
-
1
N
0
-0 4
1 ;;, ,
,
300
,
,
,
,
,
400
,
a;;!
,
I:
,
-
,r*,
, ,
600
500
WAVELENGTH ( n m )
Figure 3. Time-resolved transient absorption spectra of 0.1 m M colloidal In& in Ar-saturated acetonitrile, recorded after 355-nm laser pulse excitation: ( 0 )0.15, (0)0.5, (m) 2.5, and (A) 8.0 p.. Inserts a and b show the absorption time profile at 340 and 380 nm.
N
0
(1)
h,+ and e;, to regenerate the semiconductor is a fast process (reaction 4), and light scattering by the colloidal suspension limited e,-
,p:..
0 2
(InzS3),,(In2SzZf)S-
Reaction 5 further leads to the formation of S-, which is identified by its absorption around 470 nm. As confirmed in the pulse radiolysis studies,' the S' radical when formed within the colloidal In2S3particle exhibits absorption at longer wavelengths in the visible region. Photoemission of electrons and their localization on the particle were observed earlier by Henglein and co-workers for CdS particles in aqueous medium.I8 Such a process, which occurred in microsecond-millisecond time scale, resulted in bleaching of CdS absorption. The bleaching observed in the present case is in acetonitrile and in a short time domain (nanosecond) as compared to the experiments of Henglein et al.18 The possibility of a rapid trapping of charge carriers has also been suggested recently for CdS cry~tallites.'~The role of trapped charge carriers in inducing photobleaching can also be understood on the basis of the Burstein-Moss effect.2(tZ2 It has been shown for semiconductors such as InSb and InAs that the absorption edge could be shifted by varying the charge-carrier density of the semiconductor. In a (17) Albery, W. J.; Brown, G.T.; Darwent, J. R.; Saievar-Iranizad, E. J . Chem. Soc., Faraday Trans. 1 1985, 81, 1999. (18) (a) Henglein, A.; Fojtik, A.; Weller, H. Ber. Bunsen-Ges. Phys. Chem. 1987, 91, 441. (b) Alfassi, 2.;Bahnemann, D.; Henglein, A. J . Phys. Chem. 1982, 86,4656. (19) Ramsden, J . J. PFOC.R. SOC.London, A 1987, No. 410, 89. (20) (a) Burstein, E. Phys. Rea 1954, 93, 632. (b) Moss, T. S. Proc. Phys. Soc., London 1954, 67, 775. (21) Pankove, J. I . Optical Processes in Semiconductors: Dover Publications: New York, pp 37-43. ( 2 2 ) (a) Dixon, J. R.; Ellis, J. M. Phys. Reu. 1961, 123, 1560. (b) Palik, E. D.; Mitchell, D. L. Phys. Reu. [Sec.] A 1964. 135, A763.
-04t
14
4 00
300
500
600
WAVELENGTH
(nm) Figure 4. Time-resolved absorption spectra of 0.1 mM colloidal In2S3 in H2S-saturated acetonitrile, recorded after 355-nm laser excitation; ( 0 ) 0.15. (0)0.5, (m) 2.5, and ( A ) 8.0 ~ s .
similar way, one can expect the presence of trapped charge carriers to modify the energy levels of In2S3and shift the absorption edge to higher energies. This shift in absorption would appear as bleaching in the difference absorption spectra. Formation of H2St*-. The transient absorption spectrum recorded in Figure 3 also indicated the formation of the sulfhydril radical HzSz'-. The absorption maximum at 380 nm matched well with the absorption previously ascribed to H2S2'-produced by pulse radiolysis of H2S-saturated solutions (Amx 380, e = 7000 M-' cm-1).23.24Even though the colloidal suspension was purged with Ar, there seems to be strong adsorption of HzS on the particles; the H2Scould exist in the dissociated form HS- and H+ (pK, = 7). The direct oxidation of HS'- by trapped holes at the surface of the semiconductor could lead to the formation of H2S2'(reactions 6 and 7).' The association constant for reaction 7 h:
+ HSHS'
-
HS'
+ HS-
+ (In2S,),,
(6)
H2S2'-
(7)
has been reported to be 2.5 X lo4 M-1.23Since HS- (the dissociated form of H2S on the colloidal surface) acts as a hole scavenger, reaction 6 should be a favorable reaction in retarding anodic corrosion. Further evidence for this scheme (reaction 6) is gathered from the effect of dissolved H I S on the transient absorption spectrum (Figure 4). The colloidal suspension in this experiment was used without purging by argon. The presence (23) Karmann, W.; Meissner, G.; Henglein, A. Z . Naturforsch., B: Anorg. Chem., Org. Chem. 1967, 22, 273. (24) Mills, G.; Schmidt, K. H.; Matheson, M. H.; Meisel, D. J . Phys. Chem. 1987, 91, 1590.
Indium Sulfide Semiconductor Colloids
The Journal of Physical Chemistry, Vol. 92, No. 8, I988 2327
O3I
R
04t
02
0
100
50
0
150
50
TIME
TIME(p5)
100
150
0
(PSI
Figure 5. Absorption time profile of S2H2*and S'- recorded at (a) 380 and (b) 470 nm with band-gap excitation of 0.1 mM In2S3 colloidal suspension in acetonitrile (excitation: 355 nm). -0 2
WAVELENGTH (nm)
Figure 7. Transient absorption spectra of 0.1 mM InzS3 colloidal suspension containing 4 pM ZV in Ar-saturated 14/86 vol/vol ?6 H,O-CH$N, recorded (0)immediately after and (A)7 MS after 355-nm laser pulse excitation.
Figure 6. Transient absorption spectra of 0.1 mM colloidal In2S, in acetonitrile (Ar saturated) in the presence of I- (excitation: 355 nm).
of excess H2S on the surface of colloidal In2S3 enhanced the recovery of photobleaching as reaction 6 occurred more efficiently. However no major effect of dissolved H2S on the decay of S' could be seen. An alternate r o ~ t efor ~ the ~ *formation ~ ~ of H2S;- involves the contribution of So- formed at the surface of the particles (i.e., reaction 8 followed by reaction 7. If this scheme is valid, the S*-
+ H+ * HS'
(8)
formation of H2S2*-should also represent the anodic corrosion process. However as can be seen from Figures 3-5, the formation of H2S2'-was completed within 5-10 M S , and no further growth in the absorption at 380 nm could be seen corresponding to the decay of (Figure 5 ) . While we do not completely rule out the possibility of reaction 8 in the formation of S2H2*-,the experimental evidence suggests reaction 6 as the major route in forming
S2H2'-. Considerable attention has been drawn in recent years to the corrosion processes in single-crystal semiconductor^.^^ It was observed that chalcogenide ions can retard the anodic corrosion by effectively scavenging the surface trapped holes. The experiments described here confirm this process and highlight the importance of the flash photolysis technique in probing the anodic corrosion process. Effect of I- on Transient Formation. Hole scavengers such as I- have a beneficial effect in retarding anodic corrosion of metal chalcogenide semiconductors. For example, naked CdS electrodes employed in photoelectrochemical cells were found to be stable at high concentrations of I-?6 The influence of I- on the transient absorption spectrum of colloidal In2S3is shown in Figure 6 . The I' atom formed at the semiconductor surface (reaction 9) reacts I-
+ h,' I'
-
+ I-
+ (In&),
-
I'
12-
(9) (10)
(25) See, for example: (a) Ellis, A. B.; Kaiser, S. W.; Wrighton, M. S . J. Am. Chem. SOC.1976, 98, 1635, 6418, 6855. (b) Gerischer, H. J . Elecrrounul. Chem. 1977,82, 133. ( c ) Gerischer, H.; Liibke, M.Ber. Bunsen-Ges. Phys. Chem. 1983,87, 120-1 28. (26) Rajeshwar, K.; Kaneko, M.; Yamada, A,; Noufi, R. N . J . Phys. Chem. 1985,89, 806.
quickly with I- to yield 12'- (reaction IO). The absorption maxima at 400 and 700 n m in Figure 6 matched the previously reported absorption spectrum of 12'- (Amx 380 nm, E = 9400 M-'cm-1).27928 If the concentration of I- was kept low (less than 0.8 mM), one could still observe the formation of So-and H2S2'- from the absorbance at 470 and 380 nm, respectively. But at concentrations greater than 2 mM, Iz'- dominated the absorption spectrum. This observation indicated the role of I- in scavenging holes and hence retarding the anodic corrosion process effectively. The suppression of H2S2'-formation was also evident from the onset of bleaching of In2S3colloids, which is seen in the region where H2S2*-usually dominates the absorption. The observed corrosion may also be influenced by the cathodic corrosion process:
e;
-
(1n2S3)n-1(In2S3)-
(1 1)
After the holes are scavenged, excess trapped electrons are left a t the surface and facilitate cathodic corrosion. The scattering of monitoring light at shorter wavelengths limited the study of the corrosion products such as Ino. However, as will be seen later in this article, it is also possible to scavenge these electrons by means of a scavenger such as a viologen compound and beneficially perform electrolysis at these semiconductor microphotoelectrodes. Reduction of Zwitterionic Viologen (ZV). Semiconductor colloids serve as excellent microelectrodes to carry out reduction of various redox systems, and viologen compounds have proved to be excellent probes to study charge transfer at the semiconductor-electrolyte interface.29 The absorption spectrum of the transient obtained upon laser pulse excitation of In2S3colloids in the presence of the zwitterionic viologen (1,l'-bis(su1fopropyl)-4,4'-bipyridine) is shown in Figure 7. The water content in the acetonitrile solution was raised to facilitate the dissolution of ZV. The absorption spectrum recorded immediately after the flash exhibited maxima at 395 and 600 nm, which confirmed the formation of the radical ion ZV'- (reaction 12). eCB-(or e;)
+ ZV
-
ZV'-
(12)
The formation of ZV'- was prompt (insert a in Figure 7), indicating that the interfacial charge-transfer process occurred between In2S3colloid and the adsorbed ZV. The photobleaching of In2S3colloids could still be seen at wavelengths where the Z V absorption was small. This observation supported our previous assignment of the observed photobleaching as due to trapped holes. The absorptions of H2S2'- and S'- could not be resolved as they were buried within the absorption peak of ZV'-. However broadening of the tail region could be seen in the absorption spectrum recorded 8.7 ~s after the laser pulse. (27) Devonshire, R.; Weiss, J. J. J. Phys. Chem. 1968, 72, 3815. (28) Hug, G. L. NBS Res. Dura Ser. 1981, 69, 55. (29) See, for example: (a) Duonghong, D.; Ramsden, J.; Gratzel, M. J. J . Am. Chem. SOC.1984, 56, 1215. (b) Bahnemann, D.; Henglein, A,; Lilie, J . J . Phys. Chem. 1984, 88, 709.
"
2328 The Journal of Physical Chemistry, Vol. 92, No. 8, 1988
Kamat et al.
0
I / [ zv]
, 10'
M-'
Figure 8. Dependence of the inverse of the observed quantum yield of production of the ZV'- on the inverse of ZV concentration (0.1 mM In2Sp colloids in Ar-saturated 10/90 vol/vol % H2GCH3CN).
The transient absorption spectrum recorded 8.7 ws after the laser pulse also exhibited a small decrease in the absorption of ZV'-. Insert a in Figure 7 shows the decay of ZV'- during this period. Such a decay could be attributed to the back electron transfer between ZV'- and the trapped holes at the colloidal semiconductor surface (reaction 13). A major fraction of ZV'h,+
+ ZV'-
-
ZV
(13) survives as it diffuses away from the particle. This process is favored by the fact that particles that are stabilized by Nafion are negatively charged and repel the negatively charged ZV- from the colloidal particle. Hence the negatively charged stabilizer (Nafion) plays two important roles in effecting the electron transfer in these particulate systems: (1) It facilitates the adsorption of the relay (ZV) species on the semiconductor particle. (2) It drives away the reduced product (ZV'-) from the particle surface with electrostatic repulsion. Similar influence of electrostatic interaction on the production of ZV' has been highlighted in negatively charged SiOz and Ti02-SiO, systems.30 On a longer time scale, however, a slower formation of ZV'was seen (insert b in Figure 7). This slower component, as monitored from its absorption a t 600 nm, corresponded to the decay of S'- as observed at 480 nm. Increasing the concentration of ZV'- enhanced the decay of S'-. It is evident that both Soand HzS2'- are capable of reacting with viologen compounds (reactions 14 and 15). Details of the reaction between sulfhydril
s*- + zv H2S2'-
+ ZV
-
-+
ZV*-
+ so
(14)
ZV'-
+ H2S2
(15)
radicals and the viologen compound have been published elsewhere. l9 The dependence of the quantum yield of ZV'- produced with In2S3colloids on the concentration of ZV was studied with the aid of the previously described expression3'
where 4 is the true quantum yield of ZV'- production and K , is the apparent association constant of In2S3colloid and ZV. It was assumed that the prompt formation of ZV'- corresponded to the interfacial charge transfer between In2S3colloid and the adsorbed ZV. The quantum yields ($obJd) at different ZV concentrations were determined from the absorption of ZV'- at 400 ( e = 4.18 X lo4 M-' cm-' ) and 600 nm (1.39 X 104 M-I cm-1)32immediately after the laser pulse excitation. (Anthracene in cyclohexane was used as a reference; the absorption maximum of 3anthracene* is at 422 nm with e = 64700 M-' cm-' andT' = 0.7.)33 The plot of 1/40bsd versus 1/ [ZV], as shown in Figure 8, was a straight line. The values of 4 and K , were 0.07 and 3 X 10' (30) (a) Laane, C.; Willner, I.; Otvos, J. W.; Calvin, M. Proc. Narl. Acad. Sci. U.S.A.1981, 78, 5928. (b) Frank, A. J.; Willner, I.; Zafrir, G . ;Degani, Y. J . A m . Chem. SOC.1987, 109, 3568. (31) (a) Kamat, P. V. J . Photochem. 1985, 28, 513. (b) Kamat, P. V. Langmuir 1985, 1 , 608. (32) Watanae, T.; Honda, K. J . Phys. Chem. 1982, 86, 2617. (33) Amand, B.; Bensasson, R. Chem. Phys. Left. 1975, 34, 44.
-- 0 0 0 4 O
-0005
o
300
o
350
3
400
450
L
500
WAVELENGTH (nm)
Figure 9. (a) Absorption spectrum of 0.2 m M In2S3colloids in aqueous solution. (b) (0)Absorption spectrum of the intermediate obtained 100 ps after the pulse in N20-saturated solution; dose 2.5 Gy/pulse. Solid line: spectrum corrected for the decrease concentration of In2S3(G = 5.4).
M-I, respectively. This value of 4 represents the maximum limit of electron-transfer yield under present experimental conditions. The quantum efficiency of photoelectrochemical reduction of ZV at In2S3colloidal particle is similar to the value obtained for the reduction of ZV and oxazine dyes (4 5 0.1) at CdS and CdSe microelectrode^.^^^ In2S3,which has a similar flat-band potential (-1.1 V)" and a similar interband threshold of optical absorption as those of CdS, can be considered as an important candidate for carrying out microphotoelectrolysis. Efforts are currently being made to enhance the efficiency of photoelectrochemical reduction at the semiconductor electrodes by simultaneous scavenging of holes and electrons with suitable redox systems. In2S3 Colloids in Aqueous Medium. The In2S3colloids prepared in water exhibited a colrsiderable blue shift in their absorption, with a maximum around 295 nm (Figure 9a). Such a hypsochromic shift could be attributed to quantization effects which are observed with small size (Dp < 50 A) semiconductor particles.'-' The appearance of a maximum is usually attributed to an excitonic transition associated with "magic agglomeration number^".^,^ However, recent r e p o r t ~ ~have ~ q questioned ~~ such pronounced absorption peaks in the UV region, as various types of complexes or association of ions in solution can also contribute to the absorption in the UV region. No detectable absorption was observed in blank experiments when indium perchlorate solution was replaced with a solution of sodium perchlorate. Dilution of the colloidal solution led to the decrease of absorption in agreement with Beer's law. Further analysis with X-ray diffraction is being carried out to characterize these semiconductor colloids. Oxidation of the In2S3colloids was performed by the attack of OH' radicals5 that were generated in pulse radiolysis experiments (reactions 17 and 18). The absorption spectrum recorded H20
---
eaq-, H' , OH', H2, H202,H30C,OH-
eaq- + N 2 0
-
OH'
+ OH- + N2
(1 7)
(18)
100 p s after the pulse is shown in Figure 6b. The spectrum shows bleaching with a maximum at -295 nm and a transient absorption at wavelengths greater than 320 nm. The kinetics of formation of the transient absorption was the same at all wavelengths, and (34) Mi66 0.I.; Nenadovic, M. T.; Peterson, M. W.; Nozik, A. J. J . Phys. Chem. 1987, 91, 1295. (35) MiCiC, 0 . I.; Li, 2.; Mills, G.; Sullivan, J. C.; Meisel, D. J . Phys. Chem. 1987, 91, 6221.
J . Phys. Chem. 1988, 92, 2329-2333 the signal was constant for milliseconds. The spectrum corrected for the absorption of In& is also presented in Figure 9b. The transient spectrum exhibits a UV absorption with a tail in the visible region, which may be attributed to intermediates such as So- formed during the anodic corrosion process (reaction 19) at 60H'
-
+ h2S3
60H-
+ 21n3+ + 3S0
(19) the surface of colloidal particles. The study of OH' attack on In& colloids mentioned here is in agreement with the obervations of flash photolysis experiments. For pulse radiolysis experiments the colloidal solutions were bubbled with N 2 0 for 2 h to remove any excess amount of H,S. Therefore, no absorption due to H,S;was observed in the transient spectrum. When the concentration of dissolved H2S (Le., HS-) was increased, a transient absorption with a maximum at 380 nm was observed, which corresponded to H2SZC.In these cases the H,S;- was formed from the reaction of HS- with OH' radicals that competed with reaction 19.23 Further details of the pulse radiolysis experiment that describe the one-electron oxidation of In2S3colloids are published elsewhere.36 (36) Dimitrijevic, N. M.; Kamat, P. V. Radiar. Phys. Chem. 1988, 32, 53.
2329
Conclusion Laser flash photolysis and pulse radiolysis techniques have been demonstrated to be useful for the characterization of trapped charge carriers and the elucidation of corrosion processes in short bandgap semiconductors such as metal sulfides. Trapping of charge carriers led to the bleaching of In,S, colloids, which further led to anodic corrosion. The characterization of transients such as sulfhydril radicals gives useful information in establishing the mechanism of the anodic corrosion process. The photoelectrochemical properties of In& colloids have been demonstrated for the first time by carrying out oxidation and reduction processes under visible light irradiation. Charged stabilizers such as Nafion provide an effective microenvironment for controlling charge transfer between the semiconductor colloid and the redox relay. Acknowledgment. We thank Dr. William E. Ford for providing us the sample of l,l'-bis(sulfopropyl)-4,4'-bipyridine. The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Document No. NDRL-3022 from the Notre Dame Radiation Laboratory. Registry No. ZV'-, 1131 11-40-3; In& 12030-24-9; H2S2'-, 1229310-6; Sa-, 14337-03-2; I-, 20461-54-5.
Distribution of Metal Ions Cation Exchanged onto Porous Vycor Glass and Its Effect on the Emission Quenching of Coadsorbed Ru(bpy),2+ Wei Shi and Harry D. Gafney* Department of Chemistry, City University of New York, Queens College, Flushing, New York 11367 (Received: August 20, 1987; In Final Form: October 28, 1987)
Ru(bpy)?+, Cu2+,Cr3+,and Fe3+cation exchange onto porous Vycor glass. Quenching of Ru(bpy),2+(ads) by the coadsorbed metal ions occurs by static processes and reflects the distribution of the adsorbates within the glass. The distribution of the adsorbed complex is limited to the outer volumes of the glass, principally by a size constraint, whereas the distributions of the smaller metal ions reflect their ionic potentials and kinetic labilities. A model based on a uniform surface coverage predicts the partitioning of the weakly bound, labile Cu2+ in the presence of the coadsorbed complex.
Introduction Controlling the photoredox chemistry of R ~ ( b p y ) , ~by + heterogeneous media has received considerable Particularly with adsorbed reagents, the immediate effect is a significant change in mobility.1° Bard and co-workers, for example, have shown that the translational motion of the complex on clay is negligible during its excited-state lifetime. DellaGuardia and Thomas report an emission polarization ratio of 0.1 1 f 0.02 for (1) Shi, W.; Gafney, H. D. J . Am. Chem. SOC.1987, 109, 1582-1583. (2) Milosavljevic, B. H.; Thomas, J. K. J . Am. Chem. SOC.1986, 108, 25 13-25 17. (3) Fendler, J. J. Phys. Chem. 1985, 89, 2730-2740. (4) Kennelly, T.; Braun, M.; Gafney, H. D. J. Am. Chem. SOC.1985,107, 4431-4440. ( 5 ) Kuczynski, J.; Thomas J. K. J . Phys. Chem. 1985, 89, 2720-2722. (6) Calvin, M. Phofochem. Photobiol. 1983, 37, 349-360. (7) Wilner, I.; Degani, Y . J . Am. Chem. SOC.1983, 105, 6228-6233. (8) Wilner, I.; Otvos, J. W.; Calvin, M. J . Am. Chem. SOC.1981, 103, 3203-3205. (9) Borgarello, E.; Kiwi, J.; Pelizzetti, E.; Visca, M.; Gratzel, M. Nature (London) 1981. 289. 158-159. ' (10) Nijs, H.; Va'n Damme, H.; Bergaya, F.; Habti, A.; Fripiat, J. J. J . Mol. Catal. 1983, 21, 223. (11) Ege, D.; Ghosh, P. K.; White, J. R.; Eqney, J. F.; Bard, A. J. J . A m . Chem. SOC.1985, 107, 5644-5652.
0022-3654/88/2092-2329$01.50/0
Ru(bpy)32+adsorbed onto "dry" cellophane.I2 Similar measurements with the complex adsorbed onto porous Vycor glass (PVG) yield a polarization ratio at room temperature, 0.16 f 0.02, that is within experimental error of that in a 77 K ethanol glass.', Severe reductions in adsorbate mobility convert the electrontransfer reaction from one dependent simply on the amount adsorbed to one dependent on adsorbate distribution, Le., the spatial array of donor and acceptor on the support. Uniform distributions of R ~ ( b p y ) , ~ + ( a d s(ads ) denotes an adsorbed species) have been found on colloidal silicates and cation-exchange resins dispersed in water.l4-I6 The original assumption of a uniform distribution of R ~ ( b p y ) , ~between + the silicate sheets of clay,]' however, is questionable. A recent in(12) DellaGuardia, R. A.; Thomas, J. K. J . Phys. Chem. 1983, 87, 990-998, 3550-3557. (13) Shi, W.; Strekas, T. C.; Gafney, H. D. J . Phys. Chem. 1985, 89, 974-978. (14) Furlong, D. N. Ausf. J . Chem. 1982, 35, 911-917. (15) Thorton, A. T.; Laurence, G. S. J . Chem. SOC.,Chem. Commun. 1978. 408-409. (16) Gaudiello, J. C.; Ghosh, P. K.; Bard, A. J. J . A m . Chem. SOC.1985, 107, 3027-3032. (17) Habti, A,; Keravis, D.; Levitz, P.; Van Damme, H . J . Chem. SOC., Faraday Trans. 1 1984, 80, 67-83.
0 1988 American Chemical Society
