Photoelectrochemical Behavior of Bi2S3 Nanoclusters and

(2) Kamat, P. V. InNanoparticles and Nanostructural Films; Fendler,. J., Ed.; Wiley-VCH: New York, 1998; p 207. (3) Kamat, P. V. Chemtech 1995, June, ...
0 downloads 0 Views 113KB Size
Download PDF
3236

Langmuir 1998, 14, 3236-3241

Photoelectrochemical Behavior of Bi2S3 Nanoclusters and Nanostructured Thin Films Raul Suarez,† P. K. Nair,† and Prashant V. Kamat*,‡ Departamento de Materiales Solares, Centro de Investigacion en Energia, Universidad Nacional Autonoma de Mexico Temixco, Morelos 62580, Mexico, and Notre Dame Radiation Laboratory, Notre Dame, Indiana 46556 Received February 10, 1998. In Final Form: April 2, 1998 Quantized semiconductor nanoclusters of Bi2S3 are prepared in acetonitrile by reacting BiI3 with H2S or Na2S. Both preparation methods yield stable colloids with particle diameters of e5 nm. Excitation with a 355-nm laser pulse results in transient bleaching in the 400-500-nm region. This process is followed by the formation of S-surf with a difference absorption maximum around 540 nm. This we attribute to the chemical changes associated with the hole-trapping process. A composite thin film electrode comprised of SnO2/Bi2S3 nanocrystallites has been prepared by sequential deposition of SnO2 and Bi2S3 films onto an optically transparent electrode, and its photoelectrochemical behavior has been studied. The thin film is photoactive in the visible and near-IR and exhibits an incident photon to photocurrent efficiency (IPCE) of ∼15% at 400 nm.

Introduction Significant efforts have been made in recent years to develop photoelectrochemically active semiconductor nanoparticles and thin nanostructured semiconductor films.1-5 In the case of size-quantized semiconductor particles it has been possible to observe the transformation from solidstate electronic properties to molecular properties (See, for example, refs 6-10). These nanoparticles can be cast into thin semiconductor films of highly porous morphology. They can be further modified with sensitizing dyes or short band gap semiconductor nanocrystallites. Recently, we reported the fabrication and photoelectrochemical properties of thin film electrodes obtained from nanometer-sized colloidal SnO2,11 TiO2,12 WO3,13 and ZnO.14 Composite semiconductor systems such as TiO2-CdS, ZnO-CdS, and TiO2-CdSe have been successfully em* Address correspondence to this author. E-mail: [email protected] or http//www.nd.edu/∼pkamat. Telephone: 219-631-5411. Fax: 219-631-8068. † Universidad Nacional Autonoma de Mexico Temixco. ‡ Notre Dame Radiation Laboratory. (1) Hagfeldt, A.; Graetzel, M. Chem. Rev. 1995, 95, 49. (2) Kamat, P. V. In Nanoparticles and Nanostructural Films; Fendler, J., Ed.; Wiley-VCH: New York, 1998; p 207. (3) Kamat, P. V. Chemtech 1995, June, 22. (4) Gorer, S.; Hodes, G. In Semiconductor NanoclusterssPhysical, Chemical and Catalytic Aspects; Kamat, P. V., Meisel, D., Eds.; Elsevier Science: Amsterdam, 1997; p 297. (5) Graetzel, M. In Semiconductor NanoclusterssPhysical, Chemical and Catalytic Aspects; Kamat, P. V., Meisel, D., Eds.; Elsevier Science: Amsterdam, 1997; p 353. (6) Brus, L. E. J. Chem. Phys. 1984, 80, 4403. (7) Weller, H.; Koch, U.; Gutierrez, M.; Henglein, A. Ber. BunsenGes. Phys. Chem. 1984, 88, 649. (8) Rossetti, R.; Hull, R.; Gibson, J. M.; Brus, L. E. J. Chem. Phys. 1985, 83, 1406. (9) Nozik, A. J.; Williams, F.; Nenadovic, M. T.; Rajh, T.; Micic, O. I. J. Phys. Chem. 1985, 89, 397. (10) Ekimov, A. I.; Efros, A. L.; Shubina, T. V.; Skvortsov, A. P. J. Lumin. 1990, 46, 97. (11) Bedja, I.; Hotchandani, S.; Kamat, P. V. J. Phys. Chem. 1994, 98, 4133. (12) Vinodgopal, K.; Hotchandani, S.; Kamat, P. V. J. Phys. Chem. 1993, 97, 9040. (13) Hotchandani, S.; Bedja, I.; Fessenden, R. W.; Kamat, P. V. Langmuir 1994, 10, 17. (14) Hotchandani, S.; Kamat, P. V. J. Electrochem. Soc. 1992, 139, 1630.

ployed as thin film electrodes in photoelectrochemical cells.15-19 A series of metal sulfides, CdS, PbS, Ag2S, Sb2S3, and Bi2S3, were employed by Weller and co-workers20 to sensitize metal oxide thin films and demonstrate the role of relative energy levels of semiconductor systems in controlling the interparticle charge transfer. The coupling of two semiconductor systems with favorable energetics not only provides a convenient means to extend the photoresponse of the large band gap semiconductor but also improves the charge rectification properties of the composite film.18,21,22 The choice of semiconductors such as Bi2S3 (Eg ) 1.7 eV) as a light-harvesting substrate is ideal, since its absorption reasonably overlaps with the visible and nearIR part of the solar spectrum. Chemical deposition of thin Bi2S3 films and their photoelectrochemical properties have been reported earlier.23-26 Photoelectrochemical studies have now been performed by coupling nanostructured SnO2 films with Bi2S3 nanocrystallites. Since SnO2 is a good electron acceptor (ECB ) 0.0 V vs NHE at pH ) 1), we expect it to facilitate charge rectification within the composite film. The photoelectrochemical behaviors of Bi2S3 nanoclusters and thin films are presented here. Experimental Section Materials. Optically transparent electrodes (OTE) were cut from an indium tin oxide-coated glass plate (1.3-mm thick, 20 (15) Gerischer, H.; Luebke, M. J. Electroanal. Chem. Interfacial Electrochem. 1986, 204, 225. (16) Vogel, R.; Pohl, K.; Weller, H. Chem. Phys. Lett. 1990, 174, 241. (17) Hotchandani, S.; Kamat, P. V. J. Phys. Chem. 1992, 96, 6834. (18) Liu, D.; Kamat, P. V. J. Phys. Chem. 1993, 97, 10769. (19) Nasr, C.; Kamat, P. V.; Hotchandani, S. J. Electroanal. Chem. 1997, 420, 201. (20) Vogel, R.; Hoyer, P.; Weller, H. J. Phys. Chem. 1994, 98, 3183. (21) Vinodgopal, K.; Kamat, P. V. Environ. Sci. Technol. 1995, 29, 841. (22) Kamat, P. V. In Semiconductor NanoclusterssPhysical, Chemical and Catalytic Aspects.; Kamat, P. V., Meisel, D., Eds.; Elsevier Science: Amsterdam, 1997; p 237. (23) Peter, L. M. J. Electroanal. Chem. 1979, 98, 49. (24) Bhattacharya, R. N.; Pramanik, P. J. Electrochem. Soc. 1982, 129, 332. (25) Biswas, S.; Mandal, A.; Mukherjee, D.; Pramanik, P. J. Electrochem. Soc. 1986, 133, 49. (26) Nair, M. T. S.; Alvarez-Garcia, G.; Estrada-Gasca, C. A.; Nair, P. K. J. Electrochem. Soc. 1993, 140, 212.

S0743-7463(98)00166-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/13/1998

Bi2S3 Nanoclusters and Nanostructured Thin Films Ω/square) obtained from Donnelley Corporation, Holland, MI. SnO2 colloidal suspension (15%) was obtained from Alfa Chemicals and used without further purification. BiI3 and Na2S were obtained from Aldrich Chemical Co. All other chemicals were analytical reagents of the highest available purity. Preparation of Bi2S3 Colloids. (a) H2S Method. A BiI3 solution in acetonitrile (0.1-0.5 mM; 100 mL) was placed in a flask which has a provision for degassing. Nafion solution (0.1 mL; 5%, Aldrich) was added, and the solution was degassed for about 5 min. The solution was cooled to 273 K, and 25 mL of H2S was injected into the flask. After the solution was left standing in the cold bath for about 15 min, the solution was degassed to remove excess H2S. (b) Na2S Method. The procedure was similar to the chemical precipitation methodology described above using Na2S solution as the reactant instead of H2S. An equimolar Na2S solution was added to an acetonitrile solution containing e0.5 mM Bi2I3 and 0.05% Nafion. The solution quickly turned lightly brown, thereby indicating the formation of Bi2S3. The solution was degassed with N2. For transmission electron microscopic examination of colloids, a drop of the colloid sample was applied to a carbon-coated copper grid. Particle sizes were determined from the photographs taken at a magnification of 150000× using a Hitachi H600 transmission electron microscope. The pictures were further magnified by photographic enlargement. Absorption spectra were recorded with Milton Roy 3000 diode array spectrophotometer. Preparation of Nanostructured Thin Film Electrodes. (a) OTE/SnO2. The procedure of casting transparent thin films of SnO2 on an OTE electrode has been reported previously.11 A small aliquot (usually 0.3-0.5 mL) of SnO2 colloidal suspension (1%) was applied to the conducting surface of 0.8 × 5 cm2 OTE and was dried in air on a warm plate. The SnO2 colloid-coated OTE was then annealed for 1 h at 673 K. The thickness of the film as determined from the gravimetric analysis was about 0.5 µm. This electrode will be referred to as the OTE/SnO2 electrode. (b) OTE/SnO2/Bi2S3. Thin films of Bi2S3 were deposited on OTE/SnO2 electrodes using the chemical precipitation method described earlier.3,17 The OTE/SnO2 electrode was successively dipped in solutions of Na2S and Bi2S3. The procedure was repeated for about five times. Before it was dipped in each of the solutions, the electrode was washed with water. The film color slowly changed to brown as the Bi2S3 nanocrystallites grew on the SnO2 surface. Picosecond Laser Flash Photolysis. Picosecond laser flash photolysis experiments were performed with 355-nm laser pulses from a mode-locked, Q-switched continuum YG-501 DP Nd:YAG laser system (output 2-3 mJ/pulse, pulse width ∼18 ps). The white continuum picosecond probe pulse was generated by passing the fundamental output through a D2O/H2O solution. The details of the experimental setup and its operation are described elsewhere.27,28 Electrochemical and Photoelectrochemical Measurements. These measurements were carried out in a thin layer cell consisting of a 2- or 5-mm path length quartz cuvette with two side arms attached for inserting reference and counter (Pt gauze) electrodes.29 A Princeton Applied Research (PAR) Model 173 potentiostat and Model 175 Universal Programmer or a BAS model 100 Electrochemical Analyzer was used in electrochemical measurements. Photocurrent and photovoltage were measured with a Keithley Model 617 programmable electrometer. The source of excitation was a 250-W xenon lamp, and a Bausch and Lomb high-intensity grating monochromator and a 400-nm cutoff were introduced in the path of the excitation beam for selecting the excitation wavelengths. The light intensity was measured with a precalibrated Si diode from UDT Sensors. Inc., (Hawthorne, CA). In all these experiments, unless and otherwise stated, the active area of the electrode was 1 cm2 and the illumination of the electrodes has been carried out through the front face, i.e., from the electrolyte side. (27) Ebbesen, T. W. Rev. Sci. Instrum. 1988, 59, 1307. (28) Kamat, P. V.; Ebbesen, T. W.; Dimitrijevic, N. M.; Nozik, A. J. Chem. Phys. Lett. 1989, 157, 384. (29) Kamat, P. V.; Bedja, I.; Hotchandani, S.; Patterson, L. K. J. Phys. Chem. 1996, 100, 4900.

Langmuir, Vol. 14, No. 12, 1998 3237

Figure 1. Absorption spectra of Bi2S3 colloidal suspension in acetonitrile: (a) H2S method; (b) Na2S method.

Figure 2. Transmission electron micrograph of Bi2S3 nanoclusters, initially recorded with a magnification of 150000×.

Results and Discussion Properties of Bi2S3 Nanoclusters in Acetonitrile. The colloidal Bi2S3 suspension prepared by controlled addition of Na2S (equimolar) solution to BiI3 solution in acetonitrile or exposure of the latter to H2S produced a transparent yellow-brown colloidal suspension. By lowering the reaction temperature to 283 K and by adding Nafion solution, it is possible to achieve stable colloids in the concentration range (1-5) × 10-4 M. The absorption spectra of the Bi2S3 colloidal suspensions prepared by these two methods are shown in Figure 1. The onset of absorption for these colloids is around 500 nm, which is significantly blue shifted compared to the bulk band gap of Bi2S3 (Eg ) 1.7 eV). The size quantization effects are likely to play an important role in shifting the absorption onset of these ultrasmall nanoclusters. The transmission electron micrograph of Bi2S3 nanoclusters synthesized by the Na2S method is shown in Figure 2. The Bi2S3 particles in these samples exhibit clustering of nearly spherical particles of diameter in the range e5 nm. Because of the wide dispersity of particle size, it was difficult to establish a direct correlation between the absorption onset and particle diameter. Picosecond Dynamics of Bi2S3 Nanoclusters. Timeresolved transient absorption spectra recorded after the 355-nm laser pulse excitation of Bi2S3 colloids are shown

3238 Langmuir, Vol. 14, No. 12, 1998

Figure 3. Transient absorption spectra of 0.5 mM Bi2S3 colloidal suspensions prepared by the (a) H2S method and (b) Na2S method. The difference absorption spectrum was recorded 50 ps after a 355-nm laser pulse (pulse width 18 ps) excitation.

in Figure 3. The spectra recorded immediately after the laser pulse excitation show an intense bleaching with a maximum around 480 nm. This bleaching observed near the band edge of Bi2S3 essentially represents a blue shift in its absorption following photoinduced charge separation. The transient bleaching is dominant in the case of colloids prepared from Na2S. This is in accordance with the prominent absorption band observed for these colloids at 480 nm. The nonlinear optical property of metal chalcogenide nanoclusters has been the topic of many recent investigations.30-38 In these studies, explanations for the observed transient bleaching were provided on the basis of various photophysical processes, viz., the dynamic Burstein effect, charge trapping, and Stark effects. In the present experiments excitation of Bi2S3 colloids with a 355-nm laser pulse results in charge separation followed by charge trapping. The intense electric field created as a result of photoinduced charge separation is likely to cause a blue shift in the band edge of Bi2S3. Similar transient bleaching has been observed for other metal chalcogenide colloids. This effect, which is usually explained on the basis of the Stark effect, has also been observed by depositing excess charge on CdS colloids or by applying an electric field to CdSe film.37,38 Photoinduced processes in Bi2S3 colloids were further probed by recording time-resolved transient absorption spectra (Figure 4). Following the 355-nm laser pulse excitation of Bi2S3 colloids, we observe two distinct processes, as indicated by the transient bleaching at short times and a broad absorption in the red-IR region at longer times. The absorption time profiles monitored at 500 and 540 nm are shown in Figure 5A and B, respectively. The bleaching trace at 500 nm initially shows a growth, but with increasing time, the bleaching recovers. The 500nm bleaching trace was analyzed with a biexponential (30) Haase, M.; Weller, H.; Henglein, A. J. Phys. Chem. 1988, 92, 4706. (31) Henglein, A.; Kumar, A.; Janata, E.; Weller, H. Chem. Phys. Lett. 1986, 132, 133. (32) Banyai, L.; Koch, S. W. Phys. Rev. Lett. 1986, 57, 2722. (33) Liu, C.; Bard, A. J. J. Phys. Chem. 1989, 93, 3232. (34) Kamat, P. V.; Dimitrijevic, N. M.; Nozik, A. J. J. Phys. Chem. 1989, 93, 2873. (35) Hilinski, E. F.; Lucas, P. A.; Wang, Y. J. Chem. Phys. 1988, 89, 3435. (36) Wang, Y.; Suna, A.; McHugh, J.; Hilinski, E. F.; Lucas, P. A.; Johnson, R. D. J. Chem. Phys. 1990, 92, 6927. (37) Colvin, V. L.; Alivisatos, A. P. J. Chem. Phys. 1992, 97, 730. (38) Luangdilok, C.; Lawless, D.; Meisel, D. In Fine Particles Science and Technology; Pelizzatti, E., Ed.; Kluwer Academic Publishers: Boston, 1996; p 457.

Suarez et al.

Figure 4. Time-resolved transient absorption spectra recorded following a 355-nm laser pulse excitation of 0.5 mM Bi2S3 in acetonitrile prepared by the Na2S method.

Figure 5. Absorption-time profiles monitored at (A) 500 and (B) 540 nm. The experimental conditions were the same as those in Figure 4. The 500-nm trace was fitted with a biexponential kinetic fit (solid trace) with lifetimes of 55 and 480 ps, respectively. A pseudo-first-order kinetic fit of the 540nm trace gave the lifetime 3.3 ( 0.05 ns.

kinetic fit with lifetimes of 55 ps (growth) and 480 ps (recovery), respectively (Figure 5A). In the longer time scale, however, we observe a growth in the red region. A pseudo-first-order kinetic fit of the 540-nm trace (Figure 5B) indicated a lifetime of 3.3 ( 0.05 ns for this growth component. Upon excitation of Bi2S3 colloids, the charge separation is followed by the charge recombination and chargetrapping processes (eqs 1-3)

Bi2S3 + hν f Bi2S3(eCB + hVB)

(1)

Bi2S3(eCB + hVB) f Bi2S3 + hν

(2)

Bi2S3(eCB + hVB) f Bi2S3 (et + ht)

(3)

It has been shown earlier28,31 that one of the primary photochemical events associated with hole trapping in

Bi2S3 Nanoclusters and Nanostructured Thin Films

Langmuir, Vol. 14, No. 12, 1998 3239

Figure 6. Absorption spectra of nanostructured thin films of (a) SnO2 and (b) SnO2/Bi2S3 cast on optically transparent electrodes (OTEs). The films were first cast with SnO2 colloids followed by the chemical precipitation of Bi2S3 on the conducting glass plates (inset).

metal sulfides is the formation of sulfide radicals at the semiconductor surface (Ssurf•-). For CdS colloids, this process has been characterized from the absorption band in the region of 480 nm. In addition, surface-adsorbed sulfide ions are also capable of scavenging photogenerated holes to generate sulfide radicals at the surface. In the present experiments, we attribute the broad absorption observed in Figure 4 at longer delay times to a similar process with Ssurf•- formation at the colloid surface.

Bi2S3(ht) f Bi2S2(Ssurf•-)

(4)

The rate constant for the formation of Ssurf•- as determined from the pseudo-first-order growth of the 540nm absorption was 3 × 108 s-1. This value is similar to that for Ssurf•- formation in CdS colloids.28 Formation of Ssurf•- has important consequences of inducing anodic corrosion in metal sulfide colloids. In the present case most of the Ssurf•- generated at the surface underwent slow recombination with trapped electrons. Absorption Spectra of Nanostructured Thin Films of SnO2 and SnO2/Bi2S3. To investigate the photoelectrochemical properties of Bi2S3 nanoclusters, we deposited thin films of Bi2S3 nanoclusters by the chemical deposition method. The principle of casting such metal sulfide films on porous SnO2 films has been discussed elsewhere.3,39 Conducting glass electrodes were first cast with a thin film of SnO2 nanocrystallites by the procedure reported earlier. The use of nanocrystalline SnO2 films as the initial layer has two advantages. (i) The SnO2 films cast from small colloidal particles (particle diameter 10-15 nm) possess a highly porous morphology, which facilitates adsorption of Bi3+ and S2- ions from solution to produce nanoparticles of Bi2S3, and (ii) the SnO2 film itself is photoelectrochemically active and can accept electrons from the adjacent semiconductor layer of Bi2S3 nanoparticles. Thus, deposition of a Bi2S3 particulate layer enables us to extend the photoresponse of the SnO2 film into the visible and near-IR. A similar strategy of building composite films has been shown to be useful for developing electrodes for photoelectrochemical cells.15-20 The absorption spectra of OTE/SnO2 and OTE/SnO2/ Bi2S3 are compared in Figure 6. While the SnO2 film exhibits absorption at wavelengths less than 350 nm, the Bi2S3 film shows absorption extending up to 850 nm. The infrared absorption onset which corresponds to a band (39) Hotchandani, S.; Kamat, P. V. Chem. Phys. Lett. 1992, 191, 320.

Figure 7. (A, top) Photocurrent-time characteristics of (a) OTE/SnO2 and (b) OTE/SnO2/Bi2S3 electrodes during illumination with monochromatic light of 425 nm (Iinc ) 1 mW/cm2). Electrolyte: 0.1 M Na2S and 1 M KCl in water, pH 12. The area of electrode exposure was ∼1 cm2. (B, bottom) Electrode response to monochromatic light (425 nm, 1 mW/cm2) irradiation. The open-circuit photovoltage Voc of the OTE/SnO2/Bi2S3 electrode was monitored during an ON-OFF cycle of illumination. Electrolyte: 0.1 M Na2S and 1 M KCl in water, pH 12.

gap of ∼1.45 eV is similar to those of the films deposited in a chemical bath.24,25 These bulk semiconductor properties of the Bi2S3 nanocrystallites grown on nanoporous SnO2 films indicate that they are not size-quantized; instead they collectively exhibit bulk semiconductor properties similar to those of the polycrystalline semiconductor thin films. Photoelectrochemical Characteristics of a Coupled OTE/SnO2/CdSe Electrode. The OTE/SnO2/ Bi2S3 electrodes when irradiated with visible light in a photoelectrochemical cell responded with the generation of anodic photocurrent. The direction of the photocurrent confirmed the electron flow from excited Bi2S3 into SnO2 nanocrystallites. The performance of the OTE/SnO2/Bi2S3 electrode was further evaluated by carrying out a series of photoelectrochemical measurements: (a) Generation of Photocurrent and Photovoltage. The photocurrent response of OTE/SnO2 and OTE/SnO2/Bi2S3 electrodes to 425-nm illumination is shown in Figure 7A. The photocurrent generation consisted of two steps. The major fraction of this photocurrent appeared promptly following the excitation with 425-nm light, which was then followed by a slower growth. This pattern of photocurrent was reproducible for several ON-OFF cycles of illumination. Single-component semiconductor films of metal oxides usually show only one fast photocurrent component following the band gap excitation. We attribute the initial fast rise of the photocurrent observed in Figure 7A to the initial charge separation in individual semiconductor layers. This is followed by electron transfer from Bi2S3

3240 Langmuir, Vol. 14, No. 12, 1998

Figure 8. i-V characteristics of the OTE/SnO2/Bi2S3 electrode (RE, Ag/AgCl; CE, Pt; electrolyte, 0.1 M Na2S and 1 M KCl in water, pH 12; scan rate, 5 mV/s).

to SnO2 nanocrystallites and electrochemical equilibration. The difference in conduction band energies between these two semiconductors favors such an interparticle electron transfer. Such a charge rectification is evident from the secondary component in Figure 7A, which consists of a slow rise in the photocurrent. Although photoinduced electron transfer between the two particles is an ultrafast process, we notice a relatively longer time needed for electrochemical equilibration. Under these low-current operating conditions, the photocurrent observed with Bi2S3 was quite stable. As shown earlier,18,40 the composite films consisting of two different semiconductor systems promote charge rectification and improve the photocurrent stability. Since the electron-hole recombination is significantly decreased in the composite semiconductor system, we expect a larger and steadier photocurrent than that for the singlecomponent films. Unfortunately, we could not selectively check the performance of the Bi2S3 film, since it was not possible to extend the present chemical precipitation method to cast nanostructured films of Bi2S3 directly on a conducting glass electrode. The photovoltage response of the OTE/SnO2/Bi2S3 electrode is shown in Figure 7B. The electrode response under open-circuit circuit conditions was rather slow, as the photovoltage continued to grow for several minutes following the excitation. Again, this is indicative of the longer time needed for the equilibration of the composite electrode. At 425-nm excitation, charge separation occurs only at the Bi2S3 layer and not at the SnO2 one. The electrons that are transferred from Bi2S3 into SnO2 nanocrystallites are likely to occupy shallow and deep traps. The slow growth in the photovoltage observed in Figure 7B is likely to be influenced by such trap-filling phenomena. Effects of charge trapping on the performance of nanostructured thin film-based photoelectrochemical cells are discussed elsewhere.41 (b) i-V Characteristics. The i-V characteristics of the OTE/SnO2/Bi2S3 electrode in an aqueous medium are shown in Figure 8. At bias potentials more negative than -0.8 V vs Ag/AgCl one observes cathodic current under both dark and illumination conditions. This effect is mainly due to the fact that electrons accumulate within these particles at potentials more negative than the flat band potential of Bi2S3. Upon illumination with visible light (λ > 400 nm), the OTE/TiO2 electrode exhibits anodic photocurrent at potentials greater than -0.9 V vs Ag/ AgCl. The photocurrent increases with increasing anodic bias and attains saturation. These i-V characteristics (40) Vinodgopal, K.; Bedja, I.; Kamat, P. V. Chem. Mater. 1996, 8, 2180. (41) Schwarzburg, K.; Willig, F. Appl. Phys. Lett. 1991, 58, 2520.

Suarez et al.

Figure 9. Photocurrent action spectra of different electrodes, (a) OTE/SnO2 (electrolyte, 0.02 M NaOH); (b) OTE/SnO2/Bi2S3 (electrolyte, 0.1 M Na2S and 1 M KCl in water, pH 12). IPCE % was determined from eq 5.

indicate that the Bi2S3 film typically possesses an n-type semiconducting property. At potentials greater than +0.2 V vs Ag/AgCl, we see a steep rise in the current caused by the anodic corrosion of Bi2S3 film. As indicated earlier, the n-type semiconducting behavior is based on preferential hole injection into the electrolyte.11,12,18,42-44 The application of an anodic bias to an OTE/TiO2 electrode provides the necessary energy gradient to drive away the photogenerated holes and electrons in different directions and thus minimizes charge recombination. On the other hand, at zero-current-potential (-0.75 to -0.9 V) all the photogenerated charge carriers recombine. (c) Incident Photon-to-Current Conversion Efficiency (IPCE). Short-circuit photocurrents were measured at various excitation wavelengths, and IPCE was determined from the following expression,

IPCE (%) )

Isc (A/cm2) 2

Iinc (W/cm )

×

1240 × 100 λ (nm)

(5)

where Iinc is the light incident on the electrode. The electrolyte consisted of 0.1 M Na2S and 1 M KCl. The action spectra, representing IPCE vs wavelength, of the OTE/SnO2 and coupled OTE/SnO2/Bi2S3 electrodes are shown in Figure 9. The OTE/SnO2 electrode responds only to UV excitation below 350 nm, as it predominantly absorbs in the UV region, while OTE/SnO2/Bi2S3 shows a photoresponse extending up to 700 nm. Since Bi2S3 is a small band gap semiconductor (Eg ) 1.7 eV), its absorption in the nanostructured film extends up to 900 nm. Thus, we are able to observe the photoresponse of the OTE/SnO2/Bi2S3 electrode beyond the visible region. Unfortunately, we were not able to assess the IR response and the photocurrent onset in present studies. (Because of monochromator grating limitations, we could not probe the reponse of the electrode accurately beyond 700 nm.) As shown earlier, a better charge separation in the SnO2/ Bi2S3 coupled film is likely to improve the charge separation and hence the photocurrent generation. Interestingly, we observe a flat IPCE value in the visible region which increases significantly at wavelengths below 400 nm. The maximum IPCE at 425 nm is about 15%. It should be (42) Hodes, G.; Howell, I. D. J.; Peter, L. M. J. Electrochem. Soc. 1992, 139, 3136. (43) Hagfeldt, A.; Lindquist, S. E.; Graetzel, M. Sol. Energy Mater. Sol. Cells 1994, 32, 245. (44) Sodergren, S.; Hagfeldt, A.; Olsson, J.; Lindquist, S.-E. J. Phys. Chem. 1994, 98, 5552.

Bi2S3 Nanoclusters and Nanostructured Thin Films

noted that not all the incident light is absorbed by Bi2S3 at this excitation wavelength. If we consider only the absorbed fraction of the incident light, we expect a lightharvesting efficiency in the range 50-60%. The mechanism of charge separation in nanostructured thin films has been discussed in earlier studies.3,42 The kinetics of charge transfer at the semiconductor/electrolyte interface is expected to control the charge separation in semiconductor nanocrystallites and hence the photocurrent generation. Usually, one of the photogenerated charge carriers in the nanocrystallites is quickly captured by the redox couple in the electrolyte while the other diffuses through the film to the back contact OTE to generate photocurrent. Thus, the quick capture of electron or hole by the electrolyte is essential to obtain a photocurrent. The observation of an anodic photocurrent in SnO2 and SnO2/Bi2S3 electrodes suggests that the holes are preferentially scavenged by the electrolyte Na2S, while electrons diffuse toward OTE and flow through the external circuit. Conclusions Bi2S3 semiconductor nanoclusters prepared in acetonitrile exhibit size quantization effects. Upon excitation

Langmuir, Vol. 14, No. 12, 1998 3241

with a 355-nm laser pulse, these colloids undergo intense bleaching in the 450-500-nm region. Chemical changes associated with hole trapping can be monitored from the absorption of S-surf, which exhibits a broad absorption around 540 nm. Photoelectrochemically active composite films of SnO2 and Bi2S3 nanocrystallites cast on a conducting glass electrode are capable of generating photocurrent under visible light excitation. Such electrodes are useful to develop efficient photoelectrochemical cells for the conversion of solar energy into electricity. Our preliminary experiments with Bi2S3 films, though promising, suffer from low-energy conversion efficiency. Optimization of redox couple, electrolyte, and film thickness is therefore essential for improving the performance of Bi2S3-based photoelectrochemical cells. Acknowledgment. We would like to thank our colleagues C. Nasr and B. Shanghavi for their assistance and useful discussions. The work described herein was supported by the Office of Basic Energy Sciences of the U.S. Department of Energy (P.V.K.). This is Contribution No. NDRL 4048 from Notre Dame Radiation Laboratory. LA9801662