Langmuir 1996, 12, 5739-5741
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Notes Sonochromic Effect in WO3 Colloidal Suspensions Prashant V. Kamat*,† Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 K. Vinodgopal*,‡ Department of Chemistry, Indiana University Northwest, Gary, Indiana 46408 Received June 6, 1996
Introduction WO3 is a large band gap semiconductor with a variety of applications in electrochromic and photochromic sensors.1-5 We have recently shown that the WO3 colloids under the influence of an electrochemical bias or UV irradiation exhibit reversible blue color.3 A similar effect has also been observed with other metal oxide semiconductor colloids such as TiO26 and SnO2.7 Trapping of electrons and the intercalation of cations were considered to be the primary reasons for observing the blue color in these colloids. In recent years there has been a burst of activities in investigating sonolytic reactions.8-14 The usefulness of this technique in synthesizing colloidal semiconductors15 and metals9 and dissolution of MnO2 colloids16 has also been demonstrated. The primary reactive species generated in a sonolytic reaction are •H and •OH radicals. Several recent studies have focused on the aspect of understanding the chemical reactivity of these radicals in a sonolytic reaction.11,13,17-19 We have now employed semiconductor colloids to investigate the radical reactions in sonolytic processes. In this study we present our * To whom correspondence should be addressed. † E-mail:
[email protected]. http://www.nd.edu:80/∼pkamat. ‡ E-mail:
[email protected]. (1) Giraudeau, A.; Fan, F. R.; Bard, A. J. J. Am. Chem. Soc. 1980, 102, 5137. (2) Lampert, C. M. Sol. Energy Mater. 1984, 11, 1. (3) Bedja, I.; Hotchandani, S.; Kamat, P. V. J. Phys. Chem. 1993, 97, 11064. (4) Bedja, I.; Hotchandani, S.; Carpentier, R.; Vinodgopal, K.; Kamat, P. V. Thin Solid Films 1994, 247, 195. (5) Nenadovic, M. T.; Rajh, T.; Micic, O. I.; Nozik, A. J. J. Phys. Chem. 1984, 88, 5827. (6) Hagfeldt, A.; Vlachopoulos, N.; Graetzel, M. J. Electrochem. Soc. 1994, 141, L82. (7) Bedja, I.; Hotchandani, S.; Kamat, P. V. J. Phys. Chem. 1994, 98, 4133. (8) Suslick, K. S. Science 1990, 247, 1439. (9) Suslick, K. S. MRS Bull. 1988, 29. (10) Suslick, K. S. In Ultrasound, its chemical, physical and biological effects; Suslick, K. S., Ed.; VCH: New York, 1988; pp 138. (11) Henglein, A.; Gutierrez, M. J. Phys. Chem. 1990, 94, 5169. (12) Hart, E. J.; Fischer, C. H.; Henglein, A. Radiat. Phys. Chem. 1990, 36, 511. (13) Hart, E. J.; Henglein, A. J. Phys. Chem. 1985, 89, 4342. (14) Colarusso, P.; Serpone, N. Res. Chem. Intermed. 1996, 22, 61. (15) Hobson, R. A.; Mulvaney, P.; Grieser, F. J. Chem. Soc., Chem. Commun. 1994, 823. (16) Sostaric, J. Z.; Mulvaney, P.; Grieser, F. J. Chem. Soc., Faraday Trans. 1995, 91, 2843. (17) Gutierrez, M.; Henglein, A.; Dohrmann, J. K. J. Phys. Chem. 1987, 91, 6687. (18) Gutierrez, M.; Henglein, A. J. Phys. Chem. 1990, 94, 3625. (19) Henglein, A.; Gutierrez, M. Int. J. Radiat. Biol. Relat. Stud. Phys., Chem. Med. 1986, 50, 527.
S0743-7463(96)00560-4 CCC: $12.00
Figure 1. Design of the sonolysis cell.
preliminary results from the reaction of WO3 colloids with sonolytically generated H atoms. To the best of our knowledge this is the first such attempt to demonstrate a sonochromic effect and quantify the H atom induced reactions in colloidal semiconductor suspensions. Experimental Section Materials. Sodium tungstate, oxalic acid, and Acid Orange 7 were obtained from Aldrich. Acid Orange 7 was purified by column chromatography. All other chemicals were analytical reagents of highest available purity. Preparation of WO3 Colloids. A transparent colloidal suspension of WO3 (0.1 M) was prepared in water by the method described earlier.3,5 Concentrated HCl was added dropwise to a solution of 0.1 M sodium tungstate until a white precipitate settled down. Tungstic acid (WO3‚2H2O) precipitate was dispersed in water by adding oxalic acid (0.2 M) at elevated temperatures. The diameter of these particles as measured from transmission electron microscopy is in the range 50-100 Å. Absorption spectra were recorded with a Perkin Elmer 3840 diode array spectrophotometer. Steady-state γ-radiolysis was carried out at 296 K with a 60Co source. The dose rate as determined by Fricke dosimetry was 88 Gy/min. Sonolysis Experiments. The sonolysis experiments were carried out with a 640 kHz sonolysis setup of Ultrasonic Energy Systems (Panama City, FL). A specially designed glass vessel (500 mL capacity) was attached to the transducer with silicon rubber. The vessel had two ports to purge gas during the sonolysis and to extract sample during sonolysis. A spectrophotometer cell was fitted to one of the ports with the aid of air-tight joints so that the sample could be transferred into the cell without exposure to air. The design of the cell assembly is shown in Figure 1.
Results and Discussion The colloidal suspension of WO3 is colorless with absorption below 400 nm. Sonolysis of WO3 colloidal suspension was carried out in the vessel described in Figure 1, and the samples were extracted into the spectrophotometer cell at various time intervals. As the sonolysis progressed, the sample turned blue, with the color deepening with time. The absorption spectra of © 1996 American Chemical Society
5740 Langmuir, Vol. 12, No. 23, 1996
Notes
other chemical species present in the medium. It has been shown that the reactivity of these radicals is strongly dependent on the frequency of the sonolysis.20,21 At high frequency, such as the one employed in the present experiments, the collapse of the bubble occurs quickly. Thus, the probability of ejection of H• and •OH radicals before they undergo recombination is significantly enhanced at higher frequency. In the present experiments •OH radicals quickly react with oxalic acid present in the solution while H• atoms interact with the WO3 colloids (reactions 2 and 3). The initial attachment of H• to WO3 colloids is followed by trapping of electrons at W6+ sites
Figure 2. Absorption spectra of 0.08 M WO3 colloids in water recorded at various time intervals following 640 kHz sonolysis. The absorption spectra were recorded at the following time intervals: (a) 0 min; (b) 30 min; (c) 50 min; (d) 70 min; and (e) 120 min. Inset shows the growth of absorbance at 750 nm.
Figure 3. Absorption spectra of 0.08 M WO3 colloids in water recorded following γ-radiolysis at the following time intervals: (a) 0 min; (b) 5 min; (c) 10 min; and (d) 15 min.
samples at different duration of sonolysis are shown in Figure 2. All the absorption spectra exhibited broad absorption in the red-IR region. Similar broad absorption has also been observed earlier in UV-photolysis and radiolysis of WO3 colloidal suspensions. This was attributed to the trapping of electrons in the surface defects followed by the intercalation of cations such as H+ or Na+. During the first 30 min the changes in the absorption are very small. It is possible that the surface adsorbed oxygen acts as a scavenger for the trapped electrons in the early part of the sonolysis. However, the growth of the red-IR absorption in the later part of the sonolysis is quite linear, suggesting a quantitative increase in the production of trapped electrons. Acoustic cavitation is the single most step that influences the sonochemical process. The nonlinear acoustic process that is controlled by the nucleation, growth, and implosive collapse of bubbles produces enormous local temperatures (10 000 K) and pressure (up to 10 000 atm). Under these extreme conditions the water molecule is cleaved to form H• and •OH radical species. )))
H2O 98 H• + •OH
(1)
Transient radicals would then recombine or react with
(HCOOH)2 + •OH f CO2 + H2O
(2)
H• + WO3 f H(WO3) f H+ + (WO3(e))
(3)
The electron trapping followed by the intercalation of cations such as Na+ or H+ results in the blue color. The reactivity of sonolytically produced H• atoms toward yielding reduction products is not unusual. For example, Henglein and his co-workers17 have demonstrated the reduction of Br2, I2, MnO4-, AuCl4-, and Ag+ in the sonolysis of aqueous solutions. The ability to undergo electron transfer with surface-modified semiconductor colloids has also been demonstrated recently.22 In order to confirm the reductive role of H• atoms in inducing blue coloration, we independently carried out γ-radiolysis experiments with WO3 colloids containing oxalic acid (pH 1.6). γ-Radiolysis of aqueous solution generates radicals such as H•, •OH, and eaq. By scavenging the •OH radicals with oxalic acid and by maintaining an acidic medium (pH 1.6 in the present example), one can selectively react WO3 colloids with H• atoms. The absorption spectra recorded at different time intervals after γ-radiolysis are shown in Figure 3. These spectra exhibit broad absorption in the red and IR, and the magnitude of the absorption increases with increasing time of radiolysis. The similarity of the absorption bands in Figures 2 and 3 confirms the participation of H• atoms in inducing the blue coloration (reaction 3). If indeed the observed blue coloration is the result of an electron-trapping process, we should be able to scavenge these electrons with suitable acceptors. Upon exposure of the sonolyzed WO3 sample to air, the blue color quickly disappeared (Figure 4A). The blue coloration can be achieved again following the sonolysis of the deaerated sample. This shows that the sonochromic effect (colorless-blue) coloration observed in WO3 colloids is a reversible phenomenon and can be useful in detecting the production of H• atoms in sonolysis experiments. It has been shown in our earlier studies that oxygen and azo dyes are good electron acceptors which can scavenge trapped electrons from the semiconductor colloids such as TiO2 and WO3.23,24
WO3(e) + O2 f WO3 + O2•-
(4)
WO3(e) + AO7 f WO3 + AO7•- f products (5) Figure 4B shows the effect of an azo dye, Acid Orange 7 (AO7), on the absorption characteristics of the sonolyzed (20) Petrier, C.; Jeunet, A.; Luche, J.-L.; Reverdy, G. J. Am. Chem. Soc. 1992, 114, 3148. (21) Petrier, C.; Lamy, M. F.; Francony, A.; Benahcene, A.; David, B.; Renaudin, V.; Gondrexon, N. J. Phys. Chem. 1994, 98, 10514. (22) Meisel, D. Private communication, 1996. (23) Kamat, P. V. Prog. React. Kinet. 1994, 19, 277.
Notes
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sonolyzed for 2 h. The sample was transferred to an airtight cuvette (sample cell). The absorption spectra were recorded with respect to an equal amount of WO3 suspension (without sonolysis) in the reference cuvette. Spectrum a in Figure 4B shows the difference absorption spectrum of the sonolyzed sample. A known amount of concentrated (degassed) AO7 solution was then injected into both sample and reference cuvettes. The difference absorption spectrum shows the decreasing absorption at wavelengths greater than 550 nm and increased bleaching at 470 nm as the concentration of AO7 is increased. The AO7 dye has a strong absorption band in the visible with an absorption maximum at 470 nm. The increased bleaching of the absorption and decrease in the red-IR absorption band in Figure 4B with increasing AO7 concentration suggest that the dye is consumed during its reaction with trapped electrons. Since AO7 quantitatively scavenges electrons from the sonolyzed sample, it should be possible to determine the concentration of electron accumulation in WO3 colloids. A decrease of about 1 absorption unit corresponds to the reduction of 0.16 µmol of AO7 in a 4 mL sample. We are further continuing our efforts to quantify the production of H• atoms in a sonolytic process.
Figure 4. (A) Effect of oxygen on the absorption properties of a sonolyzed WO3 suspension (0.08 M): (a) deaerated suspension after 60 min sonolysis; (b) after exposing the same solution to air. (B) Effect of an electron scavenger, acid orange 7, on the sonolyzed WO3 suspension (deaerated). Spectrum a was recorded after the sonolysis of a 0.08 M WO3 colloidal suspension. The difference absorption spectra (b-d) were recorded after addition of AO7 solution (deaerated) to the sonolyzed WO3 suspension. The concentrations of AO7 were (a) 0, (b) 10, (c) 19.9, and (d) 39.7 µM. The corresponding concentration of AO7 was taken as reference.
WO3 sample. First the WO3 colloidal suspension was
Acknowledgment. We would like to thank Mr. Ian Ducanson for the elegant design of the sonolysis cell employed in this study. We would also like to thank Prof. Atilla Tuncay at Indiana University Northwest for assistance with our initial experiments as well as helpful discussions. K.V. acknowledges the support of Indiana University Northwest through a Grant-in-Aid. P.V.K. acknowledges the support of the Office of Basic Energy Sciences of the U.S. Department of Energy. This is Contribution No. NDRL-3943 from the Notre Dame Radiation Laboratory. LA9605605
(24) Vinodgopal, K.; Bedja, I.; Hotchandani, S.; Kamat, P. V. Langmuir 1994, 10, 1767.
