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Langmuir 1997, 13, 708-713
Electron Spin Resonance Study of Radicals Produced by Photoirradiation on Quantized and Bulk ZnS Particles Yasuhiro Nakaoka and Yoshio Nosaka* Department of Chemistry, Nagaoka University of Technology, Nagaoka, Niigata 940-21, Japan Received February 20, 1996. In Final Form: November 13, 1996X Quantized ZnS particles were prepared and isolated from aqueous solution with a capping agent, thioglycerol. The absorption peak of the particles was located at 259-260 nm, and the diameter was estimated to be 1.5-1.6 nm from X-ray diffractograms. Their excitation energy agrees well with that calculated with the finite depth potential well model. ESR spectra were measured for the quantized and bulk ZnS particles at 77 K under photoirradiation. For the quantized ZnS, only surface hole radicals were observed, while several kinds of radicals consisting of trapped electron, inner, and surface holes were observed for the bulk ZnS. A high activity of photocatalytic reactions for quantized semiconductor particles may arise from a large number of radicals photoproduced at the surface.
Introduction Semiconductor photocatalysis has become of special interest in view of chemical storage of solar energy, photodetoxification of waste water, synthesis of organic compounds, and so on.1a Since ZnS has a relatively large band-gap energy of 3.66 eV,2 conduction band electrons and valence band holes possess strong redox potential. Useful photocatalytic reactions have been searched and reported on bulk ZnS powder3-5 and quantized (Q-) ZnS particles.6-10 In quantized semiconductor particles whose size is smaller than the exiton, the energy level of conduction band electrons becomes more negative and that of valence band holes becomes more positive. As a result, the properties of quantized semiconductor particles differ from those of bulk particles.1b-j For these reasons, numerous investigations have been reported for the preparative methodology11,12 and electronic13 and photocatalytic properties6-10 of Q-ZnS particles. From this point X Abstract published in Advance ACS Abstracts, January 15, 1997.
(1) (a) PhotocatalysissFundamentals and Applications; Serpone, N., Pelizzetti, E., Eds.; Willey: New York, 1989. (b) Henglein, A. Chem. Rev. 1989, 89, 1861. (c) Weller, H. Angew. Chem., Int. Ed. Engl. 1993, 32, 41. (d) Brus, L. E. J. Chem. Phys. 1984, 80, 4403. (e) Steigerwald, M. L.; Brus, L. E. Acc. Chem. Res. 1990, 23, 183. (f) Hagfeldt, A.; Gra¨tzel, M. Chem. Rev. 1995, 95, 49. (g) Wang, Y.; Herron, N. J. Phys. Chem. 1991, 95, 525. (h) Kamat, P. V. Chem. Rev. 1993, 93, 267. (i) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226. (j) Bawendi, M. G. NATO ASI Ser., Ser. B 1995, 340, 339. (2) Fan, F.-R. F.; Leempoel, P.; Bard, A. J. J. Electrochem. Soc. 1983, 130, 1866. (3) (a) Kisch, H.; Ferna´ndez, A.; Millini, R. Chem. Ber. 1986, 119, 3473. (b) Zeug, N.; Bu¨cheler, J.; Kisch, H. J. Am. Chem. Soc. 1985, 107, 1459. (c) Kisch, H.; Bu¨cheker, J. Bull. Chem. Soc. Jpn. 1990, 63, 2378. (d) Kisch, H.; Twardzik, G. Chem. Ber. 1991, 124, 1161. (4) (a) Anpo, M.; Matsumoto, A.; Kodama, S. J. Chem. Soc., Chem. Commun. 1987, 1038. (b) Kodama, S.; Matsumoto, A.; Kubokawa, Y.; Anpo, M. Bull. Chem. Soc. Jpn. 1986, 59, 3765. (5) (a) Yanagida, S.; Mizumoto, K.; Pac, C. J. Am. Chem. Soc. 1986, 108, 647. (b) Reber, J.-F.; Meier, K. J. Phys. Chem. 1984, 88, 5903. (c) Mihaylov, B. V.; Hendrix, J. L. J. Photochem. Photobiol., A: Chem. 1993, 72, 173. (d) Ranjit, K. T.; Krishnamoorthy, R.; Viswanathan, B. J. Photochem. Photobiol., A: Chem. 1994, 81, 55. (e) Harada, H.; Ueda, T.; Sakata, T. J. Phys. Chem. 1989, 93, 1542. (6) (a) Henglein, A.; Gutie´rrez, M. Ber. Bunsen-Ges. Phys. Chem. 1983, 87, 852. (b) Henglein, A.; Gutie´rrez, M.; Fischer, Ch.-H. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 170. (c) Weller, H.; Koch, U.; Gutie´rrez, M.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 649. (7) (a) Inoue, H.; Torimoto, T.; Sakata, T.; Mori, H.; Yoneyama, H. Chem. Lett. 1990, 1483. (b) Inoue, H.; Ichiroku, N.; Torimoto, T.; Sakata, T.; Mori, H.; Yoneyama, H. Langmuir 1994, 10, 4517. (c) Kuwabata, S.; Nishida, K.; Tsuda, R.; Inoue, H.; Yoneyama, H. J. Electrochem. Soc. 1994, 141, 1498. (8) Dunstan, D. E.; Hagfeldt, A.; Almgren, M.; Siegbahn, H. O. G.; Mukhtar, E. J. Phys. Chem. 1990, 94, 6797.
S0743-7463(96)00155-2 CCC: $14.00
of view, it is interesting to compare the photocatalytic reactivity of Q-ZnS with that of bulk ZnS. The improvement of the photocatalytic activity of ZnS with decreasing particle size was reported in the literature.7a,9a,b In the initial stage of the semiconductor photocatalysis, electrons and holes are formed in the conduction and valence band, respectively. Subsequently, they may be trapped on the semiconductor particles and form paramagnetic species. Through these paramagnetic intermediates, photocatalytic reactions certainly proceed.1a As a direct means of detecting the photoproduced surface radicals, it is known that electron spin resonance (ESR) spectroscopy is one of the most sensitive and effective tools. Although ESR measurements for bulk4,5a,14-16 and Q-ZnS9c have been reported, no extensive comparison with ESR spectra between irradiated Q-ZnS particles and bulk ZnS powder has been reported. In this study, we isolated Q-ZnS particles as a water soluble powder by the surface modification method with thioglycerol. Their particle diameters were estimated (9) (a) Kanemoto, M.; Shiragami, T.; Pac, C.; Yanagida, S. J. Phys. Chem. 1992, 96, 3521. (b) Yanagida, S.; Ishimaru, Y.; Miyake, Y.; Shiragami, T.; Pac, C.; Hashimoto, K.; Sakata, T. J. Phys. Chem. 1989, 93, 2576. (c) Yanagida, Y.; Kawakami, H.; Midori, Y.; Kizumoto, H.; Pac, C.; Wada, Y. Bull. Chem. Soc. Jpn. 1995, 68, 1811. (d) Yanagida, S.; Yoshiya, T.; Shiragami, T.; Pac, C. J. Phys. Chem. 1990, 94, 3104. (e) Kanemoto, M.; Shiragami, T.; Pac, C.; Yanagida, S. Chem. Lett. 1990, 931. (f) Yanagida, S.; Azuma, T.; Kawakami, H.; Kizumoto, H.; Sakurai, H. J. Chem. Soc., Chem. Commun. 1984, 21. (g) Yanagida, S.; Kawakami, H.; Hashimoto, K.; Sakata, T.; Pac, C.; Sakurai, H. Chem. Lett. 1984, 1449. (h) Yanagida, Y.; Azuma, T.; Sakurai, H. Chem. Lett. 1982, 1069. (i) Yanagida, Y.; Kizumoto, H.; Ishimaru, Y.; Pac, C.; Sakurai, H. Chem. Lett. 1985, 141. (10) (a) Chen, L.; Zhu, X.; Wang, F.; Gu, W. J. Photochem. Photobiol., A: Chem. 1993, 73, 217. (b) Chen, L.; Gu, W.; Zhu, X.; Wang, F.; Song, Y.; Hu, J. J. Photochem. Photobiol., A: Chem. 1993, 74, 85. (c) Hayes, D.; Grieser, F.; Furlong, D. N. J. Chem. Soc., Faraday Trans. 1990, 86, 3637. (11) (a) Heywood, B. R.; Fendler, J. H.; Mann, S. J. Colloid Interface Sci. 1990, 138, 295. (b) Zhao, X. K.; Fendler, J. H. J. Phys. Chem. 1991, 95, 3716. (c) Baral, S.; Zhao, X. K.; Rolandi, R.; Fendler, J. H. J. Phys. Chem. 1987, 91, 2701. (12) (a) Zhang, Y.; Raman, N.; Bailey, J. K.; Brinker, C. J.; Crooks, R. M. J. Phys. Chem. 1992, 96, 9098. (b) Mahamuni, S.; Khosravi, A. A.; Kundu, M.; Kshirsagar, A.; Bedekar, A.; Avasare, D. B.; Kulkarni, S. K. J. Appl. Phys. 1993, 73, 5237. (c) Kaito, C.; Saito, Y. J. Cryst. Growth 1990, 99, 743. (d) Williams, R.; Labib, M. J. Colloid Interface Sci. 1985, 106, 251. (13) (a) Brus, L. E. J. Lumin. 1984, 31-32, 381. (b) Rossetti, R.; Hull, R.; Gibson, J. M.; Brus, L. E. J. Chem. Phys. 1985, 82, 552. (c) Chestnoy, N.; Hull, R.; Brus, L. E. J. Chem. Phys. 1986, 85, 2237. (14) Schneider, J.; Ra¨uber, A. Solid State Commun. 1967, 5, 779. (15) (a) Shono, Y. J. Phys. Soc. Jpn. 1979, 47, 590. (b) Shono, Y. J. Phys. Soc. Jpn. 1981, 50, 2344. (16) (a) Arizumi, T.; Mozutani, T.; Shimakawa, K. Jpn. J. Appl. Phys. 1969, 8, 1411. (b) Lee, K. M.; O’Donnell, K. P.; Watkins, G. D. Solid State Commun. 1982, 41, 881.
© 1997 American Chemical Society
Photoirradiation on Quantized and Bulk ZnS Particles
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from X-ray diffraction analyses, and the excitation wavelength was compared with the reported values. The reported excitation wavelengths were evaluated by using the finite depth potential well model.17 For both bulk ZnS and Q-ZnS powders, paramagnetic species photoproduced on their surfaces were observed by ESR. The analysis of the ESR spectra, similar to the previous report for CdS powder,18 will give evidence for the origin of the difference between bulk and Q-ZnS particles in photocatalysis. Experimental Section Materials. Zn(ClO4)2‚6H2O (Soekawa Chemicals), ZnS, NaOH, NaI, 2-propanol, diethyl ether, methanol (Nacalai Tesque, GR), 1-thioglycerol (GSH) (Nacalai Tesque, EP), and H2S diluted to 5% by He (Sumitomo Seika) were used as received. Preparation of Q-ZnS. Q-ZnS was prepared basically according to the procedure reported by Weller et al. for CdS.19 Two kinds of Q-ZnS (a and b) were prepared. For Q-ZnS-a, 1.75 g (4.70 mmol) of Zn(ClO4)2 and 1 mL (12.0 mmol) of GSH were dissolved in 250 mL of water and the pH of the solution was adjusted to about 11. H2S (1.13 mmol) was added to this solution. The solution was dialyzed against water and then concentrated and mixed with 2-propanol to make a precipitate. The precipitate obtained was washed and dried to serve as a powder of Q-ZnS-a. The preparation procedure for Q-ZnS-b differs from that for Q-ZnS-a in the following points: twice the amount of H2S was added to the Zn2+ solution containing GSH, and the solution was heated for 1 h at 100 °C and cooled to room temperature. Both Q-ZnS-a and Q-ZnS-b particles are completely dispersed in water, and clear solutions were obtained. Method. UV-vis absorption spectra of Q-ZnS in aqueous solution were measured with a Hitachi U-3210 spectrophotometer. At some steps in the preparation procedures, absorption spectra of the solution were also measured. X-ray diffraction (XRD) analyses were carried out for the commercially obtained bulk ZnS powder and the synthesized Q-ZnS powders with a Rigaku RAD-IIIA X-ray diffractometer. Observation with a transmission electron microscope (TEM) was performed with a JEOL JEM-200CT microscope. ESR spectra were measured for the Q-ZnS and bulk ZnS powders placed in quartz sample tubes at 77 K with a JEOL ES-RE2X ESR spectrometer under photoirradiation with a superhigh-pressure mercury lamp (Ushio, USH-500D) through a band pass filter (Toshiba, UV-D36C for bulk ZnS and UVD33S for Q-ZnS; which transmit 340 ( 50 nm and 270 ( 40 nm, respectively). The sample tubes were evacuated to about 5 × 10-3 Torr and sealed. In the case of testing surface radicals, the powder was immersed in a 0.15 M methanol solution of NaI, a hole scavenger, and an ESR spectrum was measured on the frozen solution at 77 K. In some cases, methyl viologen (electron acceptor), Na2S, and a ZnCl2 aqueous solution were impregnated to the powder and dried in vacuum. All obtained spectra were read in a personal computer (EPSON, PC-486GR Super) with an image scanner (EPSON, GT-6500) and converted into a g value scale and then simulated by using a simple home-made simulation program.
Results and Discussion XRD Analyses and TEM Observations. When the isolated Q-ZnS powder was dispersed in water, the solution showed no light scattering, and the absorption spectrum was measured. The peaks in the spectra were located at 259 and 260 nm for Q-ZnS-a and Q-ZnS-b, respectively. The UV-vis spectrum almost remained the same shape when it was compared with that as prepared in water, indicating that Q-ZnS was isolated as a stable powder. In order to determine particle diameter, the powders were analyzed by XRD and transmission electron mi(17) Nosaka, Y. J. Phys. Chem. 1991, 95, 5054. (18) Nakaoka, Y.; Nosaka, Y. J. Phys. Chem. 1995, 99, 9893. (19) Vossmeyer, T.; Katsikas, L.; Giersig, M.; Popovic, I. G.; Diesner, K.; Chemseddine, A.; Eychmu¨ller, A.; Weller, H. J. Phys. Chem. 1994, 98, 7665.
Figure 1. X-ray diffractograms for bulk ZnS powder (a) and Q-ZnS-b powder (b), and diffraction pattern of the zinc-blende crystal structure of ZnS (c).
croscopy. X-ray diffractograms of the bulk ZnS and the Q-ZnS powders are shown in Figure 1a and b, respectively. For bulk ZnS, the diffractogram clearly shows the zincblende structure. For Q-ZnS, although the diffractogram is broad, peak positions likely correspond with those of the zinc-blende structure. On the preparation of ZnS in basic aqueous solution, hydroxide tends to be formed in place of sulfide. In the present experiment, however, zinc hydroxide was not formed, because the X-ray diffractogram of Zn(OH)2 is quite different from that of Q-ZnS and no peaks were obtained. The average diameters of the crystallites are calculated from the half width of the peak with Scherrer’s equation20 to be 1.5 ( 0.2, 1.6 ( 0.2, and 10.3 ( 0.4 nm for Q-ZnS-a, Q-ZnS-b, and bulk ZnS, respectively. TEM observations for bulk ZnS gave a diameter of about 3 µm, indicating that the bulk ZnS particles were constructed from aggregation of the 10 nm crystallites. It was difficult to obtain a clear atomic image of Q-ZnS because of the limited resolution of the TEM used in the present study. We proposed formulas 1-3 to calculate the excitation energy (Eex) of quantized semiconductor particles on the basis of the finite depth potential well model17 and successfully applied them to Q-CdS particles.21
Eex ) Eg + Ve + Vh + Ec
(1)
Vi ) V0(a + b/((V0mi*/me)1/2R + c)2), i ) e, h (2) Ec ) -1.8e2/R
(3)
where Eg is the band gap energy of the bulk semiconductor, Ve and Vh are the energy shifts of the conduction band and valence band calculated from eq 2, Ec is the Coulomb interaction energy between the electron and the hole, V0 (20) Handbook of X-Rays; Kaelble, E. F., Ed.; Mcgraw-Hill: New York, 1967. (21) Nosaka, Y.; Shigeno, H.; Ikeuchi, T. J. Phys. Chem. 1995, 99, 8317.
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Nakaoka and Nosaka Table 1. Summary for the Observed and Reported Size and Absorption Peak Wavelength of the Q-ZnS Particles diameter/nm
absorption peak wavelength/nm
ref
1.5 ( 0.2a 1.6 ( 0.2a 2-3b 1.7b 3b 3-5 4 ( 0.5b 4b 2-3,a 2-5b 0.7a 1.5a 2.0b 2.0b 2.1b 1.9b 2.5b 2.1b
259 260 293 280 302 282 308 288 300-304 233 276 270 268 279 279 282 278
this work this work 6 6c 6c 7a and c 7b 8 9d 12b 12b 13a 13b 13b 13b 13b 13c
a
Figure 2. Relationship between the diameter of the Q-ZnS particles and the peak wavelength of absorption spectra of Q-ZnS. Experimental data were obtained from this work (O), Henglein et al. (9),6 Yoneyama et al. (b),7 Dunstan et al. (]),8 Yanagida et al. (0),9d Mahamuni et al. (4),12b and Brus et al. (2).13 The solid and broken curves are calculated on the basis of the finite depth quantum well model.17 See text for the parameters.
is the depth of the potential well, me* and mh* are the effective masses of the electron and the hole, R is the radius of the particle, is the dielectric constant of the semiconductor, and a, b, and c are the parameters determined from mi*. Figure 2 shows the relationship between the diameter of the particles and the peak wavelength of the absorption spectra of Q-ZnS. The two curves in the figure were calculated from these equations with the effective masses of me*/me ) 0.42 and mh*/me ) 0.61 for the broken line24 and me*/me ) 0.25 and mh*/me ) 0.59 for the solid line25 and with Eg ) 3.66 eV,2 V0 ) 2.60 eV,22 and r ) (/0) ) 8.3.23 The diameters of the present Q-ZnS particles and reported ones are summarized in Table 1 and also plotted in Figure 2 against the peak wavelength of each absorption spectrum. Our experimental results (O) lie on one of the curves, where the effective masses reported by Kane25 were used. In this case, Eex (in eV) is represented by the following equation as a function of the particle radius R (in nm).
Eex ) 3.66 +
1.396 0.681 0.312 + R (R + 0.500)2 (R + 0.262)2 (4)
As reported by Henglein et al.,6a Yoneyama et al.,7b and Yanagida et al.,9d in many cases the size distributions of Q-ZnS were estimated to be about 1 nm. Therefore, taking (22) Calculated from the potential of the conduction band edge of -2.07 V vs SCE at pH 7 reported in ref 9c. (23) Landolt-Bo¨rnstein. Numerical Data and Functional Relationships in Science and Technology; Springer-Verlag: New York, 1982; Vol. III, p 17b. (24) Lippens, P. E.; Lannoo, M. Phys. Rev. B 1989, 39, 10935. (25) Kane, E. O. Phys. Rev. B 1978, 18, 6849.
Determined by XRD. b Determined by TEM.
the size distribution into account, it may be concluded that all the reported data in Figure 2 agree with our calculation except for that reported by Yoneyama et al.7b and Dunstan et al.8 Their data for relatively large particles lie in a shorter wavelength region than our calculation. It can be explained as a relatively wide size distribution appearing at large average diameter. Since the absorption of the partition of smaller particles overlaps with that of larger particles, the absorption peaks tend to shift to a shorter wavelength. ESR Measurements for Bulk ZnS Powder. Figure 3 shows ESR spectra of the bulk ZnS powder. A small weak signal was observed without irradiation (spectrum 3a). The g values determined by simulation for this signal were g1 ) 2.0035, g2 ) 2.027, and g3 ) 2.051 (spectrum 3b; line widths, W1 ) 0.4 mT, W2 ) 1.8 mT, and W3 ) 3.1 mT). We labeled this signal A. The origin of this signal is unknown at present. Under irradiation, many anisotropic signals appeared in the range g ) 2.00-2.06 (spectrum 3c). Since the shape is complicated, this spectrum may involve several kinds of radicals. These radicals are formed stationary because the increase in the signal intensity was saturated within a few minutes of the irradiation. In order to distinguish these signals, the microwave saturation properties of the signal intensity at several g values were measured. Figure 4 shows the plot of signal height as a function of the square root of applied microwave power. The increase of the signal heights at g ) 2.005, 2.040, and 2.058 were saturated at the same microwave power of about 10 mW. On the other hand, the signal at g ) 1.994 was saturated at a relatively low microwave power of 0.05 mW, and that at g ) 2.023 was saturated at 0.5 mW. We labeled these signals B and C, respectively. These observations suggest that at least three different kinds of radicals were produced in the irradiated bulk ZnS powder. In order to know the nature of these radicals, some chemical reagents were adsorbed on the powder. In the presence of hole scavengers, photoproduced holes on the ZnS surface are expected to disappear by oxidizing them. The increase of the signal of trapped electrons may associate with the decrease of holes, and consequently changes in the intensity of the ESR signal would be observed. Then, in a frozen methanol solution of NaI, ESR measurements were carried out for the bulk ZnS powder under irradiation. As shown in spectrum 3d, signals at each g value maintained their positions but changed their intensities. The signals at g ) 2.00-2.05 decreased in the presence of the hole scavenger. On the
Photoirradiation on Quantized and Bulk ZnS Particles
Figure 3. ESR spectra for bulk ZnS powder obtained before irradiation (a), with its simulation (b), during irradiation (c), and in the presence of methanol and iodide ion (d), and the subtraction of spectrum d from spectrum c (e). Stars represent the signals of the Mn2+ marker. These spectra were recorded at 77 K under the microwave power of 1 mW.
other hand, the signal at g ) 2.05-2.06, which will be identified later, remained the same intensity. We labeled this signal D. The subtraction of the spectrum in the presence of hole scavenger (spectrum 3d) from that of naked ZnS (spectrum 3c) gives spectrum 3e, which will be labeled E later. Signal D, which was not affected by hole scavenger, was canceled out by the subtraction, though no scaling was made in the signal intensities. The fact that the component of spectrum 3e disappeared with hole scavenger indicates an oxidation ability of this radical. ESR signals of sulfur radicals were reported to appear at g ) 2.002-2.0244a and g1 ) 2.043, g2 ) 2.031, and g3 ) 2.0034b for the ground and unground ZnS powders, respectively. The signal which disappeared with hole scavenger (spectrum 3e) is located in this region. Then the signal could be assigned to the holes trapped on sulfur atoms at the particle surface. Since both I- and methanol are good electron donors, they must participate in the hole scavenging. In this experiment, however, the ESR signals of I• and the oxidation products of methanol were not observed. In the case of I- oxidation, the signal of I• could not be observed in the magnetic field region of this experiment, because the signal becomes too broad in frozen solution. Iodine has the nuclear spin quantum number 5 /2, and the g values and hyperfine coupling constants have large anisotropy (gx ) 2.079, gy ) 1.987, and gz ) 1.944; Ax ) 32.6 mT, Ay ) 38.8 mT, and Az ) 68.6 mT).26 Similar ambiguity in the appearance of the signals of the electron donor has been reported for the oxidation of PVA, methanol, and I- on the irradiated TiO2 colloid.27 (26) Bailey, C. E. J. Chem. Phys. 1973, 59, 1599. (27) Howe, R. F.; Gra¨tzel, M. J. Phys. Chem. 1985, 89, 4495.
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Figure 4. Saturation properties of the ESR signals for the irradiated bulk ZnS powder at g ) 1.994 (2), 2.005 (]), 2.023 (9), 2.040 (4), and 2.058 (0).
From the ESR spectrum measured at lower microwave power, we found that signal B is the g⊥ component accompanying g| at 2.000. In the presence of I-, the intensity of signal B increased. On the other hand, this signal disappeared in the presence of an electron acceptor, methyl viologen. These observations suggest that the radicals giving signal B originate from the photoinduced electrons at the particle surface. F center (electron trapped in sulfur vacancy) usually considered as the radicals produced by the electron trapping on the particle surface. The g value of the F center in ZnO was reported to be g| ) 1.9948 and g⊥ ) 1.9963.28 Although the reported g values of the F center in ZnS crystals are 2.0027152.0034,14 it may be possible that the signal shifts to g ) 1.994 when the sulfur vacancy is located at the surface. A similar shift for the trapped electron was reported for TiO2 particles.29 Another possibility is to assign this signal to Zn+. The g value reported for Zn+ is g ) 1.96 and isotropic.30 However, taking into account the possible shift of the g value for the trapped electron, this assignment may not be disregarded. Signal C remained in the presence of Na2S, although it disappeared in the presence of ZnCl2. The existence of an Sn type radical on ZnS was reported,4b and the g value of the S4 radical obtained in the deep blue solution of NaS4 was reported to be 2.024.31 From these facts, we attributed this signal to the Sn type radical on the particle surface. Signal D was unchanged in the presence of I-. The g values of this radical correspond to those of the holes inside ZnS single crystals (g| ) 2.0030 and g⊥ ) 2.0530 and 2.0550), which are localized on the sulfur atoms neighboring the zinc vacancy.15 Thus, the ESR signal D, which (28) Wong, N.-B.; Taarit, Y. B.; Lunsford, J. H. J. Chem. Phys. 1974, 60, 2148. (29) Rajh, T.; Ostafin, A. E.; Micic, O. I.; Tiede, D. M.; Thurnauer, M. C. J. Phys. Chem. 1996, 100, 4538. (30) Anpo, M.; Kubokawa, Y. J. Phys. Chem. 1984, 88, 5556. (31) Giggenbach, W. J. Inorg. Nucl. Chem. 1968, 30, 3189.
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Table 2. Summary for the Experimental Results and ESR Parameters of Radicals for Bulk ZnS Powder and Synthesized Q-ZnS Powders samples bulk ZnS
Q-ZnS-a/Q-ZnS-b a
diameter/nm 3000b
1.5a/1.6a ( 0.2
g value
label
attribution and remarks
g1 ) 2.0035, g2 ) 2.027, g3 ) 2.051 g| ) 2.005, g⊥ ) 2.040 g1 ) 2.005, g2 ) 2.036, g3 ) 2.066 g| ) 2.005, g⊥ ) 2.058
A E
unknown, observed before irradiation holes trapped on sulfur atoms at the particle surface
D
g| ) 2.000, g⊥ ) 1.994
B
g ) 2.023 g| ) 2.005, g⊥ ) 2.040 g1 ) 2.005, g2 ) 2.036, g3 ) 2.066
C E
holes trapped on sulfur atoms next to the zinc vacancy inside the particle F center (electron trapped at sulfur vacancy on the particle surface) Sn type radical on the particle surface holes trapped on sulfur atoms at the particle surface
From XRD Analyses. b From TEM observation.
Figure 5. ESR spectra for Q-ZnS-b powder obtained before irradiation (a), during irradiation (b), and with their simulations (c and d), and the sum of spectra c and d (e).
was not affected by the contact with I-, is attributable to the holes produced in the inner part of the particles. Although Yanagida et al.5a have assigned the signal at g ) 2.054 to the trapped holes, they did not mention the location of the sulfur radicals or the ability of oxidation. ESR Measurements for Q-ZnS Powder. ESR spectra obtained with Q-ZnS-b are shown in Figure 5. For Q-ZnS-a, almost the same spectrum was obtained. As shown in spectrum 5a, very little signal was recorded without irradiation. Under irradiation, several signals in the range g ) 2.00-2.06 were observed, as shown in spectrum 5b. Four sharp signals in the spectrum at around g ) 2.00 likely arise from GSH decomposed by irradiation. Then, this signal is disregarded in the following discussion. The broad spectrum could be simulated by two fundamental spectra. The best fit (spectrum 5e) was given with the combination on the two signals having g| ) 2.005 and g⊥ ) 2.040 (spectrum 5c; line width, W| ) 1.2 mT and W⊥ ) 2.9 mT) and g1 ) 2.005, g2 ) 2.036, and g3 ) 2.066 (spectrum 5d; line width, W1 ) 0.9 mT, W2 ) 3.1 mT, and W3 ) 5.3 mT). The feature
Figure 6. Computer simulation of the ESR spectrum for the irradiated bulk ZnS powder. Summation of the spectra obtained for bulk ZnS before irradiation (a) and for the irradiated Q-ZnS-b powder (b) and the additional spectrum with g| ) 2.005 and g⊥ ) 2.058 (c) gives spectrum d, which fits the experimental one (Figure 3c).
of the signal corresponds to spectrum 3e in bulk ZnS, and we labeled these signals E. Since the spectrum of Q-ZnS (spectrum 5b) seems to be included in that of the bulk ZnS (spectrum 3c), we simulated the latter by taking the former. The simulated spectrum (Figure 6d) consists of the spectra of bulk ZnS before irradiation (signal A, spectrum 6a) and Q-ZnS (signal E, spectrum 6b), and another spectrum having g| ) 2.005 and g⊥ ) 2.058 (spectrum 6c; line width, W| ) 1.1 mT and W⊥ ) 2.1 mT), which corresponds to signal D. The simulated spectrum is almost the same in shape as that of irradiated bulk ZnS (spectrum 3c). Thus, the radicals formed on Q-ZnS (spectrum 5b) appeared to also be produced in the irradiated bulk ZnS (spectrum 3c). Since the spectrum of Q-ZnS-b (spectrum 5b) corresponds to the component (spectrum 3e) which disappeared with Iin the spectrum of bulk ZnS, the spectrum observed with Q-ZnS is attributable to that of the holes trapped on a sulfur atom at the surface.
Photoirradiation on Quantized and Bulk ZnS Particles
ESR measurements for bulk ZnS and Q-ZnS powders are compared in Table 2. Conclusion. We observed the first step of ZnS photocatalysis and found that the valence band holes were trapped both at the surface and inside the particles for the bulk materials. On the other hand, for quantized small particles, only surface radicals of holes were observed. The observation is consistent with most Zn-S bonds of Q-ZnS being located at or near the surface. The holes trapped at the inner part of the particle have no ability to oxidize the surface molecules and become probably inner trap sites to take the role of a recombination center.
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Therefore, the high activity of Q-ZnS is responsible for the radicals formed at the surface, which participate in a particular photocatalytic reaction. Acknowledgment. This study was supported partly by a Grant-in-Aid on Priority-Area-Research (No. 05237105, 07228223) from the Japanese Ministry of Science, Education, Sports and Culture. We thank Prof. N. Fujii for many stimulating discussions and Prof. Y. Inoue for the use of the ESR facility. LA960155D
