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Langmuir 1996, 12, 3598-3603
Extended X-ray Absorption Fine Structure Analysis of ZnS Nanocrystallites in N,N-Dimethylformamide. An Effect of Counteranions on the Microscopic Structure of a Solvated Surface Hiroji Hosokawa,† Kei Murakoshi,† Yuji Wada,† Shozo Yanagida,*,† and Mitsunobu Satoh‡ Graduate School of Engineering, Osaka University, Suita, Osaka 565, Japan, and Laboratory for Coordination Chemistry and Catalysis Research, Kogakuin University, Hachioji, Tokyo 192, Japan Received February 16, 1996. In Final Form: May 6, 1996X In-situ Zn K-edge extended X-ray absorption fine structure (EXAFS) analysis of colloidal ZnS nanocrystallites (ZnS-DMF, mean diameter 3 nm, hexagonal) prepared by reacting zinc perchlorate (ZnSDMF-ClO4), zinc sulfate (ZnS-DMF-SO4), or zinc acetate (ZnS-DMF-Ac), with H2S in N,Ndimethylformamide (DMF) was performed to clarify the microscopic surface structure of the nanocrystallites. EXAFS analysis reveals that the zinc atoms on the surface of ZnS nanocrystallites in DMF are solvated by the oxygen atoms of DMF molecules. The structural parameters, such as atomic distances and coordination numbers of the Zn-S and Zn-O shell, depend on the coexistent counteranions of the starting zinc salts. The coordination numbers for the Zn-S shell of ZnS-DMF-SO4 and -Ac are greater than that of ZnS-DMF-ClO4, indicating that ZnS-DMF-ClO4 has zinc atoms which are most efficiently solvated by DMF oxygens on the surface, while ZnS-DMF-SO4 and -Ac have a S-rich surface in DMF. The S-rich surface of ZnS-DMF-SO4 and -Ac well explains the spectral behavior, i.e., the red shift of the absorption onsets by the addition of Zn2+ ions to the respective ZnS-DMF solutions.
Introduction Quantum-confined semiconductor nanocrystallites have attracted considerable attention due to their unique properties, which are not present in bulk materials.1 They exhibit size-dependent properties (size quantization effects), e.g., a blue shift of absorption onset, a change of electrochemical potential of band edge, and an enhancement of photocatalytic activities, with decreasing crystallite size. Furthermore, the surface of nanocrystallites also plays a crucial role in determining the properties, because nanocrystallites are characterized by large surfaceto-volume ratios. The optical and photocatalytic properties of semiconductor nanocrystallites depend upon the surface structure of nanocrystallites induced by surface modification (passivation) with coexistent anions,2-7 organic molecules,8-23 and metal cations.24-30 A key problem for improvement in the properties is an understanding of †
Osaka University. Kogakuin University. X Abstract published in Advance ACS Abstracts, July 1, 1996. ‡
(1) For reviews see (a) Henglein, A. Top. Curr. Chem. 1988, 143, 113. (b) Henglein, A. Chem. Rev. 1989, 89, 1861. (c) Brus, L. E. J. Phys. Chem. 1986, 90, 2555. (d) Steigerwald, M. L.; Brus, L. E. Acc. Chem. Res. 1990, 23, 183. (e) Brus, L. E. Appl. Phys. A 1991, 53, 465. (f) Wang, Y.; Herron, N. J. Phys. Chem. 1991, 95, 525. (g) Weller, H. Angew. Chem., Int. Ed. Engl. 1993, 32, 41. (h) Kamat, P. V. Chem. Rev. 1993, 93, 267. (2) (a) Ramsden, J. J.; Gra¨tzel, M. J. Chem. Soc., Faraday Trans. 1 1984, 80, 919. (b) Ramsden, J. J.; Webber, S. E.; Gra¨tzel, M. J. Phys. Chem. 1985, 89, 2740. (3) Rossetti, R.; Hull, R.; Gibson, J. M.; Brus, L. E. J. Chem. Phys. 1985, 82, 552. (4) Bahnemann, D. W.; Kormann, C.; Hoffmann, M. R. J. Phys. Chem. 1987, 91, 3789. (5) Amadelli, R.; Maldotti, A.; Bartocci, C.; Carassiti, V. J. Phys. Chem. 1989, 93, 6448. (6) Misawa, K.; Yao, H.; Hayashi, T.; Kobayashi, T. J. Chem. Phys. 1991, 94, 4131. (7) Torimoto, T.; Sakata, T.; Mori, H.; Yoneyama, H. J. Phys. Chem. 1994, 98, 3036. (8) (a) Dannhauser, T.; O’Neil, M.; Johansson, K.; Whitten, D.; McLendon, G. J. Phys. Chem. 1986, 90, 6074. (b) O’Neil, M.; Marohn, J.; McLendon, G. J. Phys. Chem. 1990, 94, 4356.
S0743-7463(96)00143-6 CCC: $12.00
characteristics of the microscopic surface structure of semiconductor nanocrystallites. Surface characterization of semiconductor nanocrystallites has been performed by using various spectroscopies, such as NMR,9,12,13b,13c,31-35 XPS,36-39 AES,26,37 EDAX,40 ESCA,27 and EXAFS.41,42 However, no report on in-situ observation of the microscopic surface structure of colloidal nanocrystallites in solution has been published. (9) Steigerwald, M. L.; Alivisatos, A. P.; Gibson, J. M.; Harris, T. D.; Kortan, A. R.; Muller, A. J.; Thayer, A. M.; Duncan, T. M.; Douglass, D. C.; Brus, L. E. J. Am. Chem. Soc. 1988, 110, 3046. (10) Kamat, P. V.; Dimitrijevic, N. H. J. Phys. Chem. 1989, 93, 4259. (11) Fischer, C. H.; Henglein, A. J. Phys. Chem. 1989, 93, 5578. (12) Herron, N.; Wang, Y.; Eckert, H. J. Am. Chem. Soc. 1990, 112, 1322. (13) (a) Yanagida, S.; Enokida, T.; Shindo, A.; Shiragami, T.; Ogata, T.; Fukumi, T.; Sakaguchi, T.; Mori, H.; Sakata, T. Chem. Lett. 1990, 1773. (b) Ogata, T.; Hosokawa, H.; Oshiro, T.; Wada, Y.; Sakata, T.; Mori, H.; Yanagida, S. Chem. Lett. 1992, 1665. (c) Yanagida, S.; Ogata, T.; Shindo, A.; Hosokawa, H.; Mori, H.; Sakata, T.; Wada, Y. Bull. Chem. Soc. Jpn. 1995, 68, 752. (14) Wang, Y.; Suna, A.; McHugh, J.; Hilinski, E. F.; Lucas, P. A. Johnson, R. D. J. Chem. Phys. 1990, 92, 6927. (15) (a) Chandler, R. R.; Coffer, J. L.; Atherton, S. J.; Snowden, P. T. J. Phys. Chem. 1992, 96, 2713. (b) Bigham, S. R.; Coffer, J. L. J. Phys. Chem. 1992, 96, 10581. (c) Coffer, J. L.; Chandler, R. R.; Gutsche, C. D.; Alam, I.; Pinizzotto, R. F.; Yang, H. J. Phys. Chem. 1993, 97, 696. (d) Chandler, R. R.; Coffer, J. L. J. Phys. Chem. 1993, 97, 9767. (16) (a) Eychmu¨ller, A.; Ha¨sselbarth, A.; Katsikas, L.; Weller, H. Ber. Bunsenges. Phys. Chem. 1991, 95, 79. (b) Resch, U.; Eychmu¨ller, A.; Haase, M.; Weller, H. Langmuir 1992, 8, 2215. (c) Vossmeyer, T.; Katsikas, L.; Giersig, M.; Popovic, I. G.; Diesner, K.; Chemseddine, A.; Eychmu¨ller, A.; Weller, H. J. Phys. Chem. 1994, 98, 7665. (17) Bawendi, M. G.; Carroll, P. J.; Wilson, W. L.; Brus, L. E. J. Chem. Phys. 1992, 96, 946. (18) (a) Torimoto, T.; Uchida, H.; Sakata, T.; Mori, H.; Yoneyama, H. J. Am. Chem. Soc. 1993, 115, 1874. (b) Torimoto, T.; Maeda, K.; Sakata, T.; Mori, H.; Yoneyama, H. J. Phys. Chem. 1994, 98, 13658. (c) Inoue, H.; Ichiroku, N.; Torimoto, T.; Sakata, T.; Mori, H.; Yoneyama, H. Langmuir 1994, 10, 4517. (19) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (20) Willner, I.; Eichen, Y.; Frank, A. J.; Fox, M. A. J. Phys. Chem. 1993, 97, 7264. (21) Majetich, S. A.; Carter, A. C. J. Phys. Chem. 1993, 97, 8727. (22) Rajh, T.; Micic, O. I.; Nozik, A. J. J. Phys. Chem. 1993, 97, 11999. (23) Nirmal, M.; Murray, C. B.; Bawendi, M. G. Phys. Rev. B 1994, 50, 2293.
© 1996 American Chemical Society
EXAFS Analysis of ZnS Nanocrystallites
Recently, we obtained in-situ information regarding the microscopic surface structure of CdS nanocrystallites by means of in-situ Cd K-edge EXAFS measurements for colloidal CdS nanocrystallites in N,N-dimethylformamide (DMF) solution for the first time.43 The EXAFS analysis revealed that CdS nanocrystallites were stabilized by solvation of the oxygen atoms of DMF to the cadmium atoms on the surface of CdS in DMF and that the microscopic surface structure of the solvated CdS nanocrystallites was changed by the surface modification with thiol molecule or with S2- ion. The characteristics of the surface structure determined by EXAFS analysis were correlated with the optical properties of CdS nanocrystallites in DMF. The in-situ EXAFS analysis of semiconductor nanocrystallites in solution has been demonstrated as a useful technique to obtain in-situ information regarding the microscopic structure of semiconductor nanocrystallites in solution. As well as the colloidal CdS nanocrystallites in DMF, we recently found that colloidal ZnS nanocrystallite solution prepared from Zn2+ and H2S in DMF (ZnS-DMF) was stable without coagulation and had unique properties due to quantum confinement, e.g., a blue shift of the absorption onset from the band-gap in bulk ZnS, and (24) (a) Gutie´rrez, M.; Henglein, A. Ber. Bunsenges. Phys. Chem. 1983, 87, 474. (b) Henglein, A.; Gutie´rrez, M. Ber. Bunsenges. Phys. Chem. 1983, 87, 852. (c) Henglein, A.; Gutie´rrez, M.; Fischer, C. H. Ber. Bunsenges. Phys. Chem. 1984, 88, 170. (d) Weller, H.; Koch, U.; Gutie´rrez, M.; Henglein, A. Ber. Bunsenges. Phys. Chem. 1984, 88, 649. (e) Spanhel, L; Weller, H.; Fojtik, A.; Henglein, A. Ber. Bunsenges. Phys. Chem. 1987, 91, 88. (f) Ha¨sselbarth, A.; Eychmu¨ller, A.; Eichberger, R.; Giersig, M.; Mews, A.; Weller, H. J. Phys. Chem. 1993, 97, 5333. (25) Rabani, J. J. Phys. Chem. 1989, 93, 7707. (26) Kortan, A. R.; Hull, R.; Opila, R. L.; Bawendi, M. G.; Steigerwald, M. L.; Alivisatos, A. P.; Carrol, P. J.; Brus, L. E. J. Am. Chem. Soc. 1990, 112, 1327. (27) Dunstan, D. E.; Hagfeldt, A.; Almgren, M.; Siegbahn, H. O. G.; Mukhtah, E. J. Phys. Chem. 1990, 94, 6797. (28) Hoener, C.; Allan, K. A.; Bard, A. J.; Campion, A.; Fox, M. A.; Mallouk, T. E.; Webber, S. E.; White, J. M. J. Phys. Chem. 1992, 96, 3812. (29) (a) Shiragami, T.; Ankyu, H.; Fukami, S.; Pac. C.; Yanagida, S.; Mori, H.; Fujita, H. J. Chem. Soc., Faraday Trans. 1992, 88, 1055. (b) Shiragami, T.; Fukami, S.; Pac. C.; Yanagida, S. J. Chem. Soc., Faraday Trans. 1993, 89, 1857. (c) Shiragami, T.; Fukami, S.; Wada, Y.; Yanagida, S. J. Phys. Chem. 1993, 97, 12882. (d) Kanemoto, M.; Nomura, M.; Wada, Y.; Yanagida, S. Chem. Lett. 1993, 1687. (30) (a) Zhou, H. S.; Honma, I.; Komiyama, H.; Haus, J. W. J. Phys. Chem. 1993, 97, 895. (b) Honma, I.; Sano, T.; Komiyama, H. J. Phys. Chem. 1993, 97, 6692. (31) Thayer, A. M.; Steigerwald, M. L.; Duncan, T. M.; Douglass, D. C. Phys. Rev. Lett. 1988, 60, 2673. (32) Sachleben, J. R.; Wooten, E. W.; Emsley, L.; Pines, A.; Colvin, V. L.; Alivisatos, A. P. Chem. Phys. Lett. 1992, 198, 43. (33) Becerra, L. R.; Murray, C. B.; Griffin, R. G.; Bawendi, M. G. J. Chem. Phys. 1994, 100, 3297. (34) Majetich, S. A.; Carter, A. C.; Belot, J.; McCullough, R. D. J. Phys. Chem. 1994, 98, 13705. (35) Bowers, C. R.; Pietrass, T.; Barash, E.; Pines, A.; Grubbs, R. K.; Alivisatos, A. P. J. Phys. Chem. 1994, 98, 9400. (36) Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc. 1992, 114, 5221. (37) Hoener, C.; Allan, K. A.; Bard, A. J.; Campion, A.; Fox, M. A.; Mallouk, T. E.; Webber, S. E.; White, J. M. J. Phys. Chem. 1992, 96, 3812. (38) Mahamuni, S.; Khosravi, A. A.; Kundu, M.; Kshirsagar, A.; Bedekar, A.; Avasare, D. B.; Singh, P.; Kulkarni, S. K. J. Appl. Phys. 1993, 73, 5237. (39) Katari, J. E. B.; Colvin, V. L.; Alivisatos, A. P. J. Phys. Chem. 1994, 98, 4109. (40) Swayambunathan, V.; Hayes, D.; Schmidt, K. H.; Liao, Y. X.; Meisel, D. J. Am. Chem. Soc. 1990, 112, 3831. (41) (a) Herron, N.; Wang, Y.; Eddy, M. M.; Stucky, G. D.; Cox, D. E.; Moller, K.; Bein, T. J. Am. Chem. Soc. 1989, 111, 530. (b) Moller, K.; Eddy, M. M.; Stucky, G. D.; Herron, N.; Bein, T. J. Am. Chem. Soc. 1989, 111, 2564. (42) (a) Marcus, M. A.; Flood, W.; Steigerwald, M.; Brus, L. Bawendi, M. J. Phys. Chem. 1991, 95, 1572. (b) Marcus, M. A.; Brus, L. E.; Murray, C.; Bawendi, M. G.; Prasad, A.; Alivisatos, A. P. Nanostruct. Mater. 1992, 1, 323. (43) Hosokawa, H.; Fujiwara, H.; Murakoshi, K.; Wada, Y.; Yanagida, S.; Satoh, M. J. Phys. Chem. 1996, 100, 6649.
Langmuir, Vol. 12, No. 15, 1996 3599
relatively high photocatalytic activities for reduction of CO2 under UV-light irradiation.44 In a preliminary experiment, spectral behavior of ZnS-DMF depended upon the coexistent counteranion of zinc salts used for the preparation of ZnS-DMF. Addition of excess Zn2+ ions to ZnS-DMF induced red shifts of the absorption onsets of ZnS-DMF prepared from zinc acetate or zinc sulfate, in contrast to no effect of the addition of Zn2+ ions on the absorption spectrum of ZnS-DMF prepared from zinc perchlorate. Such spectral behavior may reflect differences in the microscopic surface structure of ZnS nanocrystallites in DMF, which depends upon the counteranions from the starting zinc salts. With these in view, in-situ Zn K-edge EXAFS measurements for ZnS-DMF were carried out to observe DMFcoordinated surfaces of ZnS nanocrystallites and to investigate characteristics of the surface structure of ZnS nanocrystallites prepared from various zinc salts in DMF. In this paper, we deal with EXAFS analysis of DMFcoordinated surfaces of ZnS nanocrystallites whose microscopic structures depend upon the choice of zinc salts as a source of Zn2+. Experimental Section Materials. Zinc perchlorate hexahydrate (reagent grade, Mitsuwa Chemicals), zinc sulfate heptahydrate (reagent grade, Wako Pure Chemicals), zinc acetate dihydrate (reagent grade, Wako Pure Chemicals), N,N-dimethylformamide (DMF) (spectral grade, Dojin Chemical Laboratories), and hydrogen sulfide (reagent grade, Sumitomo Pure Chemicals) were used as received. Preparation and Characterization. Colloidal ZnS nanocrystallites in DMF (ZnS-DMF) were prepared as previously described:44 H2S gas was introduced into an argon-purged DMF solution (5 mL) of a zinc salt (10 mM) with vigorous stirring on an ice bath in a Pyrex tube (16 mm in diameter). The resulting colorless solution of ZnS-DMF was purged with nitrogen gas for 1.5 h to remove unreacted H2S. The ZnS-DMF prepared from Zn(ClO4)2‚6H2O, ZnSO4‚7H2O, and Zn(CH3COO)2‚2H2O are abbreviated to ZnS-DMF-ClO4, ZnS-DMF-SO4, and ZnSDMF-Ac, respectively. High-resolution transmission electron microscopy (TEM) images and electron diffraction patterns were obtained on a Hitachi H-9000, operating at 300 kV. TEM samples were prepared by placing a drop of colloidal ZnS solution on a copper grid covered with amorphous carbon. The TEM image of ZnS-DMF (Figure 1) shows that nearly spherical nanoparticles had groups of fringes without stacking faults. The size distribution was relatively narrow (2-5 nm in diameter) with a mean diameter of 3 nm. ZnS-DMF was confirmed to have hexagonal crystalline structure as well as bulk ZnS by electron diffraction measurement. Apparent difference in the size and crystal structure between ZnS-DMF prepared from a different salt, such as zinc perchlorate, zinc sulfate, and zinc acetate, were not observed in TEM images. UV-vis absorption spectra were measured on a Hitachi U-3300 spectrophotometer using a quartz cell with 1 mm optical path length. EXAFS Measurement and Analysis. The concentration of zinc ions in solution for in-situ EXAFS measurements was 10 mM, which gave a transparent solution without any coagulation. A polyethylene bag was used as a cell for EXAFS measurements. A few milliliters of sample solution was transferred to the cell just before use. For powder samples, pellets of Zn compounds (ca. 1 wt %) mixed with polyethylene as a binder were used. Bulk ZnS powder (99.99%, Aldrich) and an aqueous solution of Zn(ClO4)2‚6H2O were used as standard samples to determine reference parameters for the first shell of Zn-S and Zn-O, respectively. Zn K-edge (9660 eV) EXAFS measurements were performed on the BL-6B at the Photon Factory of the National Laboratory for High Energy Physics. Synchrotron radiation for the EXAFS (44) Kanemoto, M.; Hosokawa, H.; Wada, Y.; Murakoshi, K.; Yanagida, S.; Sakata, T.; Mori, H.; Ishikawa, M.; Kobayashi, H. J. Chem. Soc., Faraday Trans., in press.
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Hosokawa et al.
Figure 1. TEM image and the size distribution of ZnS-DMFAc. The magnification is 5 625 000. The figure has been reduced to fit in the column. measurements was operated at a ring energy of 2.5 GeV with a maximum stored current of 345 mA, and was monochromatized by a set of two Si(111) crystals. Data were collected at room temperature in the fluorescence mode. A short ionization chamber (17 cm long) filled with nitrogen gas was used to monitor the intensity of the incident beam. The fluorescence intensity was measured using a fluorescent ion chamber detector45 filled with argon gas. A copper filter (3 mm thick) was used to reduce elastically and inelastically scattered X-rays.46 The X-ray energy was scanned with an integration time of 2 s in the region from 9160 to 9760 eV (225 data points), with 4 s in the region from 9760 to 10 160 eV (161 data points), and with 10 s in the region from 10 160 to 10 500 eV (247 data points). The energy calibration was carried out by measuring Cu K-edge (8980 eV) EXAFS spectra of copper foil (6 mm thick). The EXAFS data were analyzed according to a previously described standard procedure.43,47-49 The background spectra were separately measured with each pure solvent or polyethylene without Zn compounds, and then were subtracted from the sample spectra. The EXAFS spectra were extracted with normalization of the edge step, interpolation onto a photoelectron momentum vector, and subtraction of the post edge background. The resulting EXAFS oscillations were multiplied by k3 and Fourier filtered with Hamming windows. The Fourier-filtered data were analyzed by nonlinear least-squares curve-fitting techniques. Further details of the analysis procedures have been presented elsewhere.43
Results and Discussion Optical Properties of ZnS-DMF. Parts a-c of Figure 2 show the addition effects of excess Zn2+ ions into the ZnS-DMF solution on the absorption spectra of ZnSDMF prepared from a zinc salt with one of the different counteranions, ClO4-, SO42-, or CH3COO-, respectively. The absorption spectra of ZnS-DMF were never changed (45) Lytle, F. W.; Greegor, R. B.; Sandstrom, D. R.; Marques, E. C.; Wong, J.; Spiro, C. L.; Huffman, G. P.; Huggins, F. E. Nucl. Instrum. Methods 1984, 226, 542. (46) Wong, J. Nucl. Instrum. Methods 1984, 224, 303. (47) Teo, B. K. EXAFS: Basic Principles and Data Analysis; SpringerVerlag: Berlin, 1986. (48) Koningsberger, D. C., Prins, R., Eds. X-ray Absorption: Principles, Applications, Techniques of EXAFS, SEXAFS, and XANES; Wiley: New York, 1988. (49) Sakane, H.; Miyanaga, T.; Watanabe, I.; Matsubayashi, N.; Ikeda, S.; Yokoyama, Y. Jpn. J. Appl. Phys. 1993, 32, 4641.
Figure 2. Absorption spectra (solid line) of ZnS-DMF prepared from various zinc salts: (a) zinc perchlorate, (b) zinc sulfate, and (c) zinc acetate ([ZnS] ) 10 mM, 1 mm path-length cell), and those with excess 10 mM DMF solution of Zn2+ ions (equi. mol) (dashed line).
under the experimental conditions, proving the stability of ZnS-DMF. The absorbances at the absorption peak wavelength of ZnS-DMF prepared from respective zinc salts are the same. This agreement indicates that the amounts of ZnS nanocrystallites formed by the present preparation method should not depend on the choice of the zinc salts. The absorption onsets of ZnS-DMF (305 nm)50 were at shorter wavelengths than 330 nm which is the band-gap in bulk ZnS. The shift of band gap to the higher energy could be explained as due to the quantum confinement of ZnS nanocrystallites with a diameter of 3 nm, as estimated on the basis of the effective mass approximation.51,52 There was a crucial difference in the addition effect of excess Zn2+ ions on the absorption spectra of ZnS-DMF. While the addition of Zn2+ ions did not affect the absorption spectrum of ZnS-DMF-ClO4 (Figure 2a), the addition of Zn2+ ions to ZnS-DMF-SO4 and -Ac induced red shifts (50) The shifts of the absorption onsets depending on the zinc salts are estimated to correspond to the differences in diameter by less than 2 Å on the basis of the effective mass approximation.51,52 We could not observe the difference of the size of ZnS nanocrystallites by TEM measurements under the present conditions. (51) (a) Brus, L. E. J. Chem. Phys. 1984, 80, 4403. (b) Rossetti, R.; Hull, R.; Gibson, J. M.; Brus, L. E. J. Chem. Phys. 1985, 82, 552. (52) Lippen, P. E.; Lannoo, M. Phys. Rev. B 1989, 39, 10935.
EXAFS Analysis of ZnS Nanocrystallites
Langmuir, Vol. 12, No. 15, 1996 3601 Table 1. Curve-Fitting Results for Fourier-Filtered k3χ(k) Zn K-Edge EXAFS of Zn Compoundsa sample
shell
∆r/Åb
bulk ZnSg,h Zn(ClO4)2 in H2Og,h Zn(ClO4)2 in DMFg ZnS-DMF-ClO4m
Zn-S Zn-O Zn-O Zn-S Zn-O Zn-S Zn-O Zn-S Zn-O
1.44-2.32 1.19-2.07 1.19-2.07 1.19-2.32
ZnS-DMF-SO4m ZnS-DMF-Acm
r/Åc
2.342i 2.08k 2.08 2.343 2.06 1.19-2.32 2.333 2.06 1.19-2.32 2.346 2.03
CNd
σ2/Å2 e
R/% f
4j 6l 5.7 3.1 2.1 3.9 1.4 3.8 1.1
0.0058 0.0074 0.0076 0.0073 0.0073 0.0077 0.0135 0.0058 0.0100
6.0 9.7 10.1 5.6 5.6 6.5
a Curve-fitting was performed over a k range of 3.7-12.7 Å-1. The window for the inverse Fourier transform. Hamming window ) 0.04 Å. c Atomic distance. d Coordination number. e The square of the Debye-Waller factor. f Quality of the fit, defined as {∑(k3χobs - k3χcald)2/∑(k3χobs)2}1/2. g One-shell fit. h Standard sample. i Fixed parameter. See ref 56. j The reduction factor is 0.879. k Fixed parameter. See ref 53. l The reduction factor is 0.977. m Two-shell fit. Standard deviations of one-shell fit: r, 0.003 Å; CN, 0.3; σ2, 0.0008 Å2 for the Zn-S shell; r, 0.01 Å; CN, 0.9; σ2, 0.0013 Å2 for the Zn-O shell. Standard deviations of two-shell fit: r, 0.009 Å; CN, 0.7; σ2, 0.0029 Å2 for the Zn-S shell; r, 0.02 Å; CN, 1.0; σ2, 0.0060 Å2 for the Zn-O shell. b
Figure 3. Phase-uncorrected Fourier transforms of k3χ(k): (a) bulk ZnS crystallites; (b) an aqueous solution of zinc perchlorate; (c) a DMF solution of zinc perchlorate; (d) ZnS-DMF-ClO4; (e) ZnS-DMF-SO4; (f) ZnS-DMF-Ac. The k-space data ranges used in the transforms were (a) 3.60-13.40 Å-1, (b) 3.20-12.85 Å-1, (c) 3.25-12.95 Å-1, (d) 3.45-13.15 Å-1, (e) 3.55-13.05 Å-1, and (f) 3.65-13.00 Å-1.
of the absorption onsets by 6 nm (Figure 2b) and 10 nm (Figure 2c), respectively. On the basis of the effective mass approximation,51,52 these red shifts reflect an increase in the radius of ZnS nanocrystallites by ca. 2 Å. Compared the increase in the radius by 2 Å with the Zn-S bond length (2.342 Å), the addition of Zn2+ ions should lead to the monolayer growth of ZnS nanocrystallites by the reaction of the added Zn2+ ions with sulfur-rich sites on the surface of ZnS nanocrystallites. The difference in the spectral behavior may reflect the amounts of ionic sulfur sites on the surface of ZnS nanocrystallites in DMF, which depends on the choice of the counteranions. Thus, ZnSDMF-SO4 and -Ac, where the shifts of the absorption onsets with excess Zn2+ ions were observed, may have an S-rich surface. On the other hand, ZnS-DMF-ClO4 should not have excess S2- ions on the surface of ZnS nanocrystallites, since no addition effect of Zn2+ ions was observed. In contrast to the addition effect of Zn2+, the addition of excess H2S did not alter the absorption spectrum of each ZnS-DMF. This result indicates that the amounts of ionic zinc sites on the surface of ZnS-DMF and those of free Zn2+ ions in ZnS-DMF must be extremely small. In-Situ EXAFS Observation of the DMF-Coordinated Surface of ZnS-DMF. Parts a and b of Figure 3 show phase-uncorrected Fourier transforms of k3weighted EXAFS for standard samples (bulk ZnS powder and an aqueous solution of zinc perchlorate). The peak at 1.95 Å in the Fourier transform (Figure 3a) of EXAFS for bulk ZnS powder can be assigned to a four-coordination of sulfur atoms in the ZnS crystal lattice. For an aqueous solution of zinc perchlorate, the observed peak at 1.66 Å in the Fourier transform (Figure 3b) was assigned to a six-coordination of oxygen atoms of water molecules hydrated to Zn2+ ions in an aqueous perchlorate solution, since solvated Zn2+ ions in water are known to have a six-coordinate octahedral structure.53-55 Calculated Fourier transforms using the reported bond lengths of Zn-S
(2.342 Å)56 and Zn-O (2.08 Å)53 fitted well with experimental Fourier-filtered k3χ(k) of bulk ZnS powder and an aqueous zinc perchlorate solution, respectively (Table 1). Reference parameters were derived from these fitting procedures.57 Figure 3c shows phase-uncorrected Fourier transforms of k3-weighted EXAFS for DMF solutions of zinc perchlorate. It is well-known that the DMF molecule has a polarized structure resulting from the negatively charged oxygen atom and the positively charged nitrogen atom,58 which results in the solvation of DMF to metal cations through the oxygen atom.54,59,60 The main peak, therefore, could be assigned to a coordination by the DMF oxygens. Acceptable fitting for Fourier-filtered k3χ(k) of a DMF solution of zinc perchlorate was obtained with a one-shell fit of Zn-O (Table 1). The derived atomic distance (2.08 Å) and coordination number (5.8) indicate that Zn2+ ion in DMF-containing ClO4- should have the six-coordinate octahedral structure. This solvation structure coincides with that in a DMF solution containing BF4- 60 and with that of an aqueous zinc perchlorate solution. Figure 3d shows phase-uncorrected Fourier transform of k3-weighted EXAFS for ZnS-DMF-ClO4. Note that the main peak at 1.92 Å in the transform of ZnS-DMFClO4 is broader than that in bulk ZnS crystallites (Figure 3a). Especially, the FT magnitude around 1.7 Å for ZnSDMF-ClO4 was greater than that for bulk ZnS. This greater transform may be attributed to superposition of a different coordination shell around the zinc atoms. Since (53) Ohtaki, H.; Yamaguchi, T.; Maeda, M. Bull. Chem. Soc. Jpn. 1976, 49, 701. (54) Marcus, Y. Chem. Rev. 1988, 88, 1475. (55) Ohtaki, H.; Radnai, T. Chem. Rev. 1993, 93, 1157. (56) Wyckoff, R. W. G. Crystal Structures, 2nd ed.; Wiley & Sons: New York, 1963; Vol. 1. (57) The extracted reference parameters were as follows; Zn-S shell: ∆E0, 13.37 eV; ηj(k), 0.791 Å-2; Sj(k), 0.879; Cj , 1.551 rad; Zn-O shell: ∆E0, 11.48 eV; ηj(k), 1.072 Å-2; Sj(k), 0.977; Cj , 1.675 rad. The parametrized function used to fit the data is the following:43,47-49
χ(k) )
∑{N S (k)F (k)/kr } exp(-2σ k ) × 2 2
2
j j
j
j
j
exp(-2rjηj(k)/k) sin(2krj + φ(k) - k3Cj)
k ) 2π{2me(E - E0)1/2}/h
(58) Gutmann, V. The Donor-Acceptor Approach to Molecular Interactions; Plenum: New York, 1978. (59) Ozutsumi, K.; Takamuku, T.; Ishiguro, S.; Ohtaki, H. Bull. Chem. Soc. Jpn. 1989, 62, 1875. (60) Ozutsumi, K.; Koide, M.; Suzuki, H.; Ishiguro, S. J. Phys. Chem. 1993, 97, 500.
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the oxygen atom of DMF molecule can coordinate to Zn2+ ions in DMF as described above, the different coordination may be due to the oxygen atoms of DMF molecules solvating to zinc atoms on the surface of ZnS nanocrystallites. Two-shell fitting for Fourier-filtered k3χ(k) of ZnS-DMF-ClO4 was performed to quantitatively interpret the coordination in the system. As shown in Table 1, the best fitting was achieved with a two-shell fit of Zn-S and Zn-O. In contrast, a one-shell fit of Zn-S gave no acceptable fitting results. Thus, these curve-fitting results indicate that zinc atoms on the surface of ZnS nanocrystallites should be coordinated not only by the sulfur atoms in ZnS nanocrystallites, but also by the oxygen atoms of solvating DMF molecules. The bond length and the value of the square of the Debye-Waller factor for the Zn-S shell in ZnS-DMFClO4 (r, 2.343 Å; σ2, 0.0073 Å2) were comparable to those in bulk ZnS powder (r, 2.342 Å; σ2, 0.0058 Å2). These results indicate that the lattice structure of ZnS nanocrystallites in DMF should be comparable to that of bulk ZnS crystallites. This agreement in the lattice structure between ZnS-DMF-ClO4 and bulk ZnS coincides with the TEM observation, as described in the Experimental Section. The derived coordination numbers of the Zn-S and Zn-O shell reflect the contribution of both the interior and surface of the ZnS nanocrystallites. In order to extract the information regarding the surface structure of the system, the contribution of surface zinc atoms should be estimated. Since ZnS-DMF was confirmed to consist of hexagonal nanocrystallites with a mean diameter of 3 nm, it is reasonable to assume that the ZnS nanocrystallites have 35-95 hexagonal units, taking into account two arrangements of the units; i.e. when the number of step atoms is maximized or when the number of terrace atoms is maximized.43 These two models for ZnS-DMF lead to the estimation that the respective coordination numbers of the zinc atom should be 3.0-3.5 with sulfur atoms and 1.0-0.5 with oxygen atoms, by assuming that all bonds of the surface zinc atoms which are not fully coordinated by sulfur atoms are saturated by DMF oxygen. Note that the coordination number of the Zn-S shell from the EXAFS analysis for ZnS-DMF-ClO4 (3.1) is in the range of the estimated value (3.0-3.5), which supports the validity of the models to extract the surface contribution from the EXAFS results. The coordination number of the Zn-O shell determined from the EXAFS (2.1) is significantly greater than that (1.0-0.5) estimated from the above models. Taking into account the absence of free Zn2+ ions in the system as described in the above section, this large coordination number of the Zn-O shell indicates that more than one DMF molecule should coordinate to each zinc atom on the ZnS nanocrystallite surface. This surface structure, which was efficiently solvated by DMF oxygens, should be a characteristic of ZnS-DMF-ClO4. Effects of the Counteranion on the Surface Structure of ZnS-DMF. Parts e and f of Figure 3 show phaseuncorrected Fourier transforms of k3-weighted EXAFS for ZnS-DMF-SO4 and -Ac. Note that the transforms of ZnS-DMF-SO4 and -Ac were quite different from that of ZnS-DMF-ClO4. The magnitudes around 1.7 Å in the transforms of ZnS-DMF-SO4 and -Ac were smaller than that in ZnS-DMF-ClO4, suggesting the less contribution of the scattering by oxygen atoms of DMF. Excellent fits were obtained (Table 1) with a two-shell fit of Zn-S and Zn-O, but not with a one-shell fit of Zn-S, showing that DMF molecules solvating to the ZnS nanocrystallite surface also exist in the systems of ZnS-DMF-SO4 and
Hosokawa et al.
-Ac, although the contributions of oxygen atoms seem to be much smaller than that in ZnS-DMF-ClO4. The bond lengths and the Debye-Waller factors for the Zn-S shell were not affected by the choice of the counteranions as well as the crystal structure of ZnS nanocrystallites. In contrast, the coordination numbers of the Zn-S and Zn-O shell and the bond length of the Zn-O shell depended on the counteranion. Note that the Zn-O bond length in ZnS-DMF-Ac (2.03 Å) was shorter than those in ZnS-DMF-ClO4 and -SO4 (2.06 Å). Taking into account the fact that the Zn-O bond length become shorter with decreasing the Zn-O coordination number,61 the shortest bond length in ZnS-DMF-Ac should reflect the smallest coordination number of DMF oxygens in the system (1.1). In addition, the coordination numbers of the Zn-S shell in ZnS-DMF-SO4 (3.9) and -Ac (3.8) were significantly greater than that in ZnS-DMF-ClO4 (3.1). The coordination numbers of the Zn-O shell in ZnS-DMF-SO4 (1.4) and -Ac (1.1) were smaller than that in ZnS-DMF-ClO4 (2.1). These differences in the coordination numbers reflect the difference in the surface structure of ZnS nanocrystallites, responsible for the choice of the starting Zn salts. The larger coordination numbers of the Zn-S shell and the smaller coordination numbers of the Zn-O shell suggest that the surface zinc atoms should be terminated by more sulfur atoms and be solvated by less DMF molecules. The characteristics of the surface structure of ZnS-DMF determined by the EXAFS analysis are in agreement with those evaluated from the absorption behavior, i.e., ZnS-DMF-SO4 and -Ac should have an S-rich surface while ZnS-DMF-ClO4 should not have an S-rich surface. It has been proposed that the octahedrally solvated structure of Zn2+ ion in water depends on the coexistent counteranion due to the interaction of the anion with Zn2+ ion.54,55 The octahedral structure is disordered with the stronger interaction between Zn2+ ion and the coexistent anion in the order of ClO4- < SO42- < CH3COO-.66 Taking into account the anion effect on the solvation structure of Zn2+ ion, the anion-dependent surface structure of ZnSDMF may be explained as due to the different interaction of the anions with zinc atoms on the surface of ZnSDMF. Since ClO4- should not strongly interact with Zn2+ ion, which is experimentally supported by the EXAFS analyses of aqueous and DMF solutions of zinc perchlorate (the six-coordinate octahedral structure of Zn2+ ion), the surface zinc atoms of ZnS-DMF-ClO4 should be efficiently solvated by DMF oxygens without interference by ClO4-, resulting in the greater coordination number of (61) The Zn-O bond length for a four-coordinate tetrahedral structure, e.g., [Zn(hmpa)4]2+ in hexamethylphosphoric triamide (HMPA) (1.93 Å)62 and hexagonal ZnO crystallites (1.98 Å),63 has been established to be shorter than that for a five-coordinate structure, e.g., monoaquobisacetylacetonatozinc (2.02 Å)64 and ZnMoO4 (2.025 Å),65 and also that for a six-coordinate octahedral structure, e.g., [Zn(H2O)6]2+ in water (2.08 Å)53 and [Zn(dmf)6]2+ in DMF (2.08 Å).60 (62) Ozutsumi, K.; Abe, Y.; Takahashi, R.; Ishiguro, S. J. Phys. Chem. 1994, 98, 9894. (63) Abrahams, S. C.; Bernstein, J. L. Acta Crystallogr., Sect. B 1969, 25, 1233. (64) Montgomery, H.; Lingafelter, E. C. Acta Crystallogr. 1963, 16, 748. (65) Abrahams, S. C. J. Chem. Phys. 1967, 46, 2052. (66) Although Zn2+ ion in an aqueous perchlorate solution has the six-coordinate octahedral structure with six H2O molecules,53 it was proposed that a part of the octahedral structures of Zn2+ ions consists of five H2O molecules and one sulfate group in an aqueous sulfate solution.67 In the case of an aqueous zinc acetate solution, the solvation structure was proposed to have five-coordination of H2O molecules and monodentate acetate anion to Zn2+ ion.68 (67) Radnai, T.; Pa´linka´s, G.; Caminiti, R. Z. Naturforsch. 1982, 37a, 1247. (68) Caminiti, R.; Cucca, P.; Monduzzi, M.; Saba, G.; Crisponi, G. J. Chem. Phys. 1984, 81, 543.
EXAFS Analysis of ZnS Nanocrystallites
the Zn-O shell (2.1) and the smaller one of the Zn-S shell (3.1). On the other hand, SO42- and CH3COO- may interfere with the solvation of DMF to the ZnS surface due to the interaction of the anions, SO42- and CH3COO-, with zinc atoms on the surface.69 The resulting surface zinc atoms which should not be fully solvated by DMF oxygens may be bound to dissolved S2- ions to form Zn-S bonding70 on the ZnS surface, leading to the greater coordination number of the Zn-S shell, i.e., a S-rich surface. Conclusion In-situ Zn K-edge EXAFS measurements of ZnS nanocrystallites in DMF (ZnS-DMF) prepared from zinc perchlorate (ZnS-DMF-ClO4), zinc sulfate (ZnS-DMFSO4), or zinc acetate (ZnS-DMF-Ac), differentiate the microscopic surface structure of ZnS nanocrystallites in DMF. The zinc atoms on the surface of ZnS-DMF-ClO4 were well solvated by oxygen atoms of DMF molecules in DMF, but ZnS-DMF-SO4 and -Ac have different surface structures influenced by the coexistent counteranions, SO42- and CH3COO-. The larger coordination numbers (69) In-situ Zn K-edge EXAFS measurements of DMF solutions of zinc sulfate and zinc acetate were carried out. However, no acceptable fits were obtained for Fourier-filtered k3χ(k) of DMF solutions of zinc sulfate and zinc acetate. These results may be explained as due to disorder of the octahedral solvation structure of Zn2+ ions in DMF by the interaction between Zn2+ ions and the coexistent anions, SO42- and CH3COO-. (70) The solubility of ZnS in solution (1.43 × 10-7 wt% in water at 20 °C) is much lower than that of these zinc salts (zinc sulfate, 34.98 wt % in water at 20 °C; zinc acetate, 29.38 wt % in water at 15 °C).71 (71) Stephan, H.; Stephan, T. Solubilities of Inorganic and Organic Compounds; Pergamon Press: London, 1963; Vol. 1.
Langmuir, Vol. 12, No. 15, 1996 3603
of the Zn-S shell and the smaller ones of the Zn-O shell were obtained in the EXAFS spectra for ZnS-DMF-SO4 and -Ac, indicating the S-rich surface in ZnS-DMFSO4 and -Ac, when compared with those for ZnS-DMFClO4. The S-rich surface of ZnS-DMF-SO4 and -Ac was also supported by the spectral behavior, i.e., the red shift of the absorption onsets by the addition of Zn2+ ions to the respective ZnS-DMF solutions. The effects of the coexistent anions on the microscopic surface structure of semiconductor nanocrystallites were first confirmed by in-situ EXAFS measurements. Physical and photochemical properties of a semiconductor nanocrystallite would be influenced by the surface structure depending on the kinds of the coexistent anions. Acknowledgment. We gratefully acknowledge the technical assistance of Drs. M. Nomura and A. Koyama at the National Laboratory for High Energy Physics for the EXAFS measurements. We gratefully appreciate the TEM measurements of Professor H. Mori and Dr. T. Sakata of the Research Center for Ultra-high Voltage Electron Microscopy at Osaka University. This work was supported in part by Grants-in-Aid for Scientific Research from the Ministry of Education, Science, Sports, and Culture, Japan (Nos. 06403023, 07242248, and 07740462), and by a Research Fellowship for Young Scientists from the Japan Society for the Promotion of Science (to H.H.). Supporting Information Available: Fourier-filtered EXAFS spectra of bulk ZnS, an aqueous solution of zinc perchlorate, a DMF solution of zinc perchlorate, ZnS-DMFClO4, ZnS-DMF-SO4, and ZnS-DMF-Ac (6 pages). Ordering information can be found on any current masthead page. LA960143S
