4070
Langmuir 1998, 14, 4070-4073
Observation of Adsorbed N,N-Dimethylformamide Molecules on Colloidal ZnS Nanocrystallites. Effect of Coexistent Counteranion on Surface Structure Hiroaki Fujiwara, Kei Murakoshi, Yuji Wada, and Shozo Yanagida* Material and Life Science, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan Received November 24, 1997. In Final Form: May 11, 1998 Adsorption of N,N-dimethylformamide (DMF) molecules on the surface of ZnS nanocrystallites was investigated by in situ Fourier transform IR and NMR spectroscopy in acetonitrile (AN)/methanol mixture solution. The CdO stretching vibration (νCdO) of DMF molecule shifted to a lower wavenumber when adsorbed on ZnS nanocrystallites. Their peak wavenumbers and intensities reflect the strength of interaction in DMF-ZnS surface bonding and a number of adsorbed DMF molecules. 1H NMR measurements of this system also revealed strong interaction of DMF molecules with the surface of ZnS nanocrystallites. The surface interaction of DMF molecules was dependent on the coexistent counteranions in solution. DMF molecules coordinate more effectively to the surface of ZnS nanocrystallites prepared from Zn(ClO4)2‚6H2O than that prepared from Zn(OAc)2‚2H2O. These results agree well with the characteristics of interacting structure of DMF molecule observed by in situ extended X-ray absorption fine structure analysis reported previously (Langmuir 1996, 12, 3598).
Introduction Adsorption of molecules at the solid-liquid interface has been studied for many years.1 Especially, the interaction of solvent with a solid surface is also of considerable interest.2,3 A specific solvent mode due to small, highly polar molecules is known to play a significant role in various important processes such as electron transfer and photoexcitation/relaxation at interfaces,3a because preferentially oriented molecules on the surface modify a local dielectric constant at the interface. Thus, details on the information of solvent molecules on surface are essential to understand characteristics of heterogeneous systems. Earlier, we reported that stable suspension of colloidal CdS (CdS-DMF) and ZnS (ZnS-DMF) nanocrystallites can be prepared from the corresponding metal salts and H2S in DMF without using stabilizers or matrixes. These colloids showed a relatively high catalytic activity for photoreduction of CO2.4 The stability of the nanocrystallites prepared in DMF was found to be higher than those prepared in other solvents such as acetonitrile, methanol, and water. This fact indicates that DMF molecules should work as a stabilizer of nanocrystallites via effective adsorption on the surface of nanocrystallites. (1) (a) Somorjai, G. A. Chemistry in Two Dimensions: Surfaces; Cornell University Press: New York, 1981. (b) Urban, M. W. Vibrational Spectroscopy of Molecules and Macromolecules on surfaces; Wiley: New York, 1993. (2) (a) Anton, A. B.; Avery, N. R.; Toby, B. H.; Weinberg, W. H. J. J. Am. Chem. Soc. 1986, 108, 684. (b) Davis, J. L.; Barteau, M. A. Surf. Sci. 1989, 208, 383. (c) Vannice, M. A.; Erley, W.; Ibach, H. Surf. Sci. 1991, 254, 1. (3) (a) Miller, R. J. D. In Surface Electron-Transfer Process; Miller, R. J. D., McLendon, G. L., Nozik, A. J., Scmickler, W., Willig, F., Eds.; VCH Publishers: New York, 1995; p 95. (b) Villegas, I.; Weaver, M. J. J. Am. Chem. Soc. 1996, 118, 458. (4) (a) Kanemoto, M.; Ishihara, K.; Wada, Y.; Sakata, T.; Mori, H.; Yanagida, S. Chem. Lett. 1992, 835. (b) Kanemoto, M.; Hosokawa, H.; Wada, Y.; Murakoshi, K.; Yanagida, S.; Sakata, T.; Mori, H.; Ishikawa, M.; Kobayashi, H. J. Chem. Soc., Faraday Trans. 1996, 92, 2401. (c) Yanagida, S.; Kanemoto, M.; Wada, Y.; Murakoshi, K.; Sakata, T.; Mori, H. Bull. Chem. Soc. Jpn. 1997, 70, 2063. (d) Fujiwara, H.; Hosokawa, H.; Murakoshi, K.; Wada, Y.; Yanagida, S.; Okada, T.; Kobayashi, H. J. Phys. Chem. B 1997, 101, 8270.
In situ extended X-ray absorption fine structure (EXAFS) analysis of CdS-DMF and ZnS-DMF is one of the powerful tools to determine the microscopic structure of the interface.5,6 For example, in situ Zn-K edge EXAFS analysis of ZnS-DMF clarified the interacting structure of DMF molecules on the surface of ZnS nanocrystallites and their dependence of the coexistent counteranion of the starting zinc salts on the structural parameters of atomic distances and coordination numbers of the Zn-S and Zn-O shell.6 However, EXAFS analysis gives only microscopic structural information of a nanocrystallite surface, rather than the information of chemical species on the surface themselves. IR and NMR spectroscopy have been often used to obtain the information of the structure and orientation of adsorbed species on the metal ions,7 particles,8 and electrode.2,3,9 The use of IR and NMR spectroscopies gives important information on vibrational structure and electronic distribution of the adsorbed molecules. We have now attempted to characterize the adsorption of DMF molecules on the surface of semiconductor nanocrystallites by employing in situ attenuated total (5) Hosokawa, H.; Fujiwara, H.; Murakoshi, K.; Wada, Y.; Yanagida, S.; Satoh, M. J. Phys. Chem. 1996, 100, 6649. (6) Hosokawa, H.; Murakoshi, K.; Wada, Y.; Yanagida, S.; Satoh, M. Langmuir 1996, 12, 3598. (7) (a) Fawcett, W. R.; Liu, G.; Faguy, P. W.; Foss, C. A., Jr.; Motheo, A. J. J. Chem. Soc., Faraday Trans. 1993, 89, 811. (b) Fawcett, W. R.; Liu, G.; Kloss, A. A. J. Chem. Soc., Faraday Trans. 1994, 90, 2697. (c) Xu, T.; Torres, P. D.; Beck, L. W.; Haw, J. F. J. Am. Chem. Soc. 1995, 117, 8027. (8) (a) Sachleben, J. R.; Wooten, E. W.; Emsley, L.; Pines, A.; Colvin, V. L.; Alivisatos, A. P. Chem. Phys. Lett. 1992, 198, 431. (b) Majetich, S. A.; Carter, A. C.; Belot, J.; McCullough, R. D. J. Chem. Phys. 1994, 98, 13705. (c) Nosaka, Y.; Shigeno, H.; Ikeuchi, T. J. Phys. Chem. 1995, 99, 8317. (d) Rajh, T.; Ostafin, A. E.; Micic, O. I.; Tiede, D. M.; Thurnauer, M. C. J. Phys. Chem. 1996, 100, 4538. (e) Ohtani, B.; Yako, T.; Samukawa, Y.; Nishimoto, S.-i.; Kanamura, K. Chem. Lett. 1997, 91. (f) Duffy, N. W.; Dobson, K. D.; Gordon, K. C.; Robinson, B. H.; McQuillan, A. J. Chem. Phys. Lett. 1997, 266, 451. (9) (a) Chadrasekaran, K.; Bockris, J. O’M. Surf. Sci. 1986, 175, 623. (b) Okabayashi, Y.; Hayashi, F.; Terui, Y.; Kitagawa, T. Chem. Pharm. Bull. 1992, 40, 692. (c) Kwon, Y. J.; Son, D. H.; Ahn, S. J.; Kim, M. S.; Kim, K. J. Phys. Chem. 1994, 98, 8481. (d) Son, D. H.; Ahn, S. J.; Lee, Y. J.; Kim, K. J. Phys. Chem. 1994, 98, 8488.
S0743-7463(97)01285-7 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/24/1998
DMF on ZnS Nanocrystallites
Langmuir, Vol. 14, No. 15, 1998 4071
reflectance infrared (ATR-IR) and NMR spectroscopy. The effect of coexistent counteranions on the interaction between DMF molecules and ZnS nanocrystallites surface is also investigated. Experiments Colloidal ZnS nanocrystallites (ZnS-AN) were prepared from acetonitrile (spectral grade, Dojin Chemical Laboratories)/ methanol (spectral grade, Dojin Chemical Laboratories) solution (95:5 v/v mixture) containing 10 mM of Zn(ClO4)2‚6H2O (reagent grade, Mitsuwa Pure Chemicals) or Zn(CH3COO)2‚2H2O (reagent grade, Wako Pure Chemicals) by introducing H2S gas.4-6 A slight amount of methanol (5%) was necessary to dissolve Zn(CH3COO)2‚2H2O. Resulting ZnS nanocrystallites are abbreviated as ZnS-AN(ClO4) and ZnS-AN(OAc), depending on the choice of counteranion. Infrared spectroscopy measurements were performed using a Fourier transform infrared (FT-IR) spectrometer (Parkin Elmar System-2000) and a cylindrical ATR cell with ZnSe rod (SpectraTeck, CIRCLE cell). The crystal rod with the length of 3.25 in. provides the appropriate optical path length for dilute liquid samples. All spectra were measured by averaging 32 scans with a spectral resolution of 4 cm-1. Difference infrared spectra were obtained by reference spectrum from extracting sample spectra. The same acetonitrile solution containing methanol (5%) was used for measurement of a reference spectrum. 1H NMR spectra were recorded on a JNR EX-270 spectrometer (JEOL). All 270 MHz 1H NMR spectra were done in acetonitriled3 (spectral grade, Aldrich)/methanol-d4 (spectral grade, Aldrich) solution (95:5 v/v mixture). Tetramethylsilane (TMS, NMR grade, Wako Pure Chemicals) was used as an external reference for the chemical shift and the signal intensity. The signal intensity was normalized to that of TMS.
Results and Discussion Figure 1a shows difference infrared spectra of the carbonyl group of DMF in acetonitrile/methanol solution containing only 26 mM DMF, Zn(ClO4)2‚6H2O (10 mM), or ZnS-AN(ClO4) (10 mM). The band observed at 1679 cm-1 is assigned to the CdO stretching vibration (νCdO) of DMF molecule. In the Zn(ClO4)2‚6H2O system, the 1679 cm-1 band decreased, as a new band appeared at 1663 cm-1 as a shoulder. The decrease of the 1679 cm-1 band was more apparent in the ZnS-AN(ClO4) system than that in the Zn(ClO4)2‚6H2O system. The spectral feature of the band at 1658 cm-1 and another band at 1712 cm-1 could be clearly seen in the spectra recorded in Figure 1a. The effects of Zn(OAc)2‚2H2O (10 mM) and ZnS-AN(OAc) (10 mM) on the spectral feature of νCdO of DMF molecules were also investigated (Figure 1b). The presence of Zn(OAc)2‚6H2O did not alter the feature of νCdO at 1679 cm-1 significantly. The intensity decreased slightly, but no new band was observed. However, in the case of ZnS-AN(OAc), a new band appeared as a shoulder at 1651 cm-1 with a simultaneous decrease of the intensity at 1679 cm-1. All of the systems containing Zn2+ ions or ZnS nanocrystallites except for the system of Zn(OAc)2‚2H2O showed the evolution of a new band at lower wavenumber compared with that of free DMF molecules in acetonitrile/ methanol solution. It is well-known that the DMF molecule has a polar structure resulting from the negatively charged oxygen atom and the positively charged nitrogen atom,10 which results in the interaction of DMF to the metal cation through the oxygen atom. When DMF molecules interact with Zn2+ ions or surface Zn atoms of ZnS nanocrystallites via its negative oxygen atom, a weakening of the CdO bond is expected because of electron (10) Gutmann, V. The Donor-Acceptor Approach to Molecular Interactions; Plenum: New York, 1978.
Figure 1. Difference IR spectra of DMF molecules in acetonitrile/methanol solution: (a) only DMF (s), DMF in the presence of Zn(ClO4)2 (- -), DMF in the presence of ZnS-AN(ClO4) (‚‚‚); (b) only DMF (s), DMF in the presence of Zn(OAc)2 (- -), DMF in the presence of ZnS-AN(OAc) (‚‚‚). Table 1. Observed Vibrational Frequencies of DMF free DMF
adsorbed DMF
system
freq/cm-1
AbsIR
freq/cm-1 a
AbsIR
none Zn(ClO4)2‚6H2O Zn(OAc)2‚2H2O ZnS-AN(ClO4) ZnS-AN(OAc)
1679 1679 1679 1679 1679
0.0684 0.0509 0.0656 0.0334 0.0635
1663 n.d.b 1658 1651
0.0454 n.d.b 0.0295 0.0149
a The peaks of the bands were determined from second derivations of these infrared spectra of DMF. b Not detected.
donation from oxygen atoms to Zn atom species. As a result, such weakening of the CdO bond is expected to cause a low wavenumber shift of νCdO of DMF. Therefore, the new band at 1650-1660 cm-1 can be assigned to the νCdO of DMF molecules, which are interacting with Zn2+ ions or the surface Zn atoms of ZnS nanocrystallites. EXAFS analysis has indicated that Zn2+ ion in DMF has the six-coordinate octahedral structure as a interaction structure caused by the oxygen atoms of DMF.6 EXAFS analysis has also supported the interaction of oxygen atoms of DMF molecules to the surface of ZnS nanocrystallites (ZnS-DMF) prepared from Zn salts and H2S in DMF.6 The degree of the spectral shifts observed in Figure 1 should depend on the strength of interaction between DMF molecules and Zn atom species in the present system. The peak wavenumber and intensity of the bands in the spectra of Figure 1 are summarized in Table 1. The values of the shifts to lower wavenumber are 16, 21, and 29 cm-1 for the Zn(ClO4)2‚6H2O, the ZnS-AN(ClO4), and the ZnS-AN(OAc), respectively. The larger spectral shift thus indicates stronger interaction of DMF molecules with Zn atom
4072 Langmuir, Vol. 14, No. 15, 1998
species in the ZnS-AN(OAc). In situ EXAFS analysis of ZnS-DMF nanocrystallites has indicated that the bond lengths between the oxygen atom of DMF and Zn species are 2.08, 2.06, and 2.03 Å in the presence of Zn2+ ion, ZnS-DMF(ClO4), and ZnS-DMF(OAc), respectively.6 The shortening of the bond length, which reflects the increment of interaction, is consistent with the order of the interaction between DMF molecules and surface Zn atom species determined by the IR measurements. The intensities of respective bands in the IR measurements reflect the extent of DMF interaction with Zn2+ ions and surface Zn atoms in ZnS nanocrystallites. For example, the intensity of the new band (1658 cm-1) in the ZnS-AN(ClO4) system is larger than that in the ZnS-AN(OAc) system. The coordination number (CN) for the coordinating oxygen atoms to zinc atoms of ZnS-DMF(ClO4) (CN ) 2.1) is greater than that of ZnS-DMF(OAc) (CN ) 1.1), indicating that ZnS-DMF(ClO4) has zinc atoms that efficiently interact with DMF oxygen on the surface.6 We expected this difference of interaction between DMF molecule and ZnS surface to arise from the coexistent counteranion. Preferential adsorption of anion such as OAc- results in the decrease of number of DMF molecules that directly interact with the ZnS surface. However, fewer DMF molecules that interact with ZnS surface still exhibit strong interaction as indicated from the shift of the band to lower wavenumber. It is rather difficult to assign the band observed at higher wavenumber (1712 cm-1) in the ZnS-AN(ClO4) system. Similar behavior was also observed by Weaver et al. in the infrared reflection-absorption spectroscopic (IRAS) measurements of the adsorption of acetone on Pt(111). They observed two new bands at 1642 and 1719 cm-1 due to the adsorption attributed to νCdO of the acetone molecule bound to the metal via the oxygen lone pair in a η1 configuration and to the multilayer, respectively.3b In this case, multilayered acetone on the surface shows the same characteristics as pure liquid acetone because the νCdO of multilayered acetone (1719 cm-1) is similar with that of liquid acetone (1712 cm-1).11 In present case, the νCdO of new band (1712 cm-1) is very close to that of liquid DMF at 1715 cm-1.12 Thus, the signal at 1712 cm-1 in Figure 1a can be attributed to multilayered adsorbed DMF on ZnS surface. We also extended the IR measurements to investigate the C(formyl)-N bonding of DMF molecule. Figure 2 shows difference infrared spectra of DMF in the 1300-1550 cm-1 region. The band at 1390 cm-1 are assigned to the C-N stretching vibration (νC-N) of DMF molecule.13 In the presence of Zn(ClO4)2‚6H2O, the band at 1390 cm-1 shifted to 1388 cm-1 and broadened. In the ZnS-AN(ClO4) system, the band at 1390 cm-1 was decreased, and a new band appears at 1330 cm-1. The low wavenumber shift observed in the ZnS-AN(ClO4) system indicates the presence of the strongly adsorbed DMF molecules. Although the interaction between DMF molecule and the surface of ZnS was stronger in the ZnS-AN(OAc) system as described above, such a noticeable change was not detected because of the lower number of adsorbed DMF molecules. To prove the structural change in the adsorbed DMF molecules in solution, we measured 1H NMR spectroscopy of the systems by using TMS as an external reference. 1H (11) Dellopiane, G.; Overend, J. Spectrochim. Acta 1966, 22, 593. (12) (a) Jao, T. C.; Scott, I.; Steel, D. J. Mol. Spectrosc. 1982, 92, 1. (b) Steel, D.; Quatermain, A. Spectrochim. Acta 1987, 43A, 781. (13) (a) Durgaprasad, G.; Sathyanarayana, D. N.; Patel, C. C. Bull. Chem. Soc. Jpn. 1971, 44, 316. (b) Zhou, X. Z.; Krauser, J. A.; Tate, D. R.; vanBuren, A. S.; Clark, J. A.; Moody, P. R.; Liu, R. J. Phys. Chem. 1996, 100, 16822.
Fujiwara et al.
Figure 2. Difference IR spectra of DMF molecules in acetonitrile/methanol solution: (a) only DMF (s), DMF in the presence of Zn(ClO4)2 (- -), DMF in the presence of ZnS-AN(ClO4) (‚‚‚).
Figure 3. 1H NMR spectra of DMF molecules in acetonitrile/ methanol solution: (a) only DMF, (b) DMF in the presence of Zn(ClO4)2, (c) DMF in the presence of ZnS-AN(ClO4).
NMR signals of DMF in a acetonitrile-d3/methanol-d4 mixture solution (95:5 v/v) were observed at 2.781, 2.899, and 7.928 ppm as sharp signals (Figure 3a). The signals at 2.781 and 2.899 ppm were attributed to cis-methyl protons and trans-methyl protons of DMF, respectively. A downfield shift of these signals was observed in the presence of Zn(ClO4)2‚6H2O (10 mM) as shown in Figure 3b. In the system containing ZnS-AN(ClO4) (10 mM), the downfield shift became remarkable, and the width of the signals became broad compared with those in the presence of Zn(ClO4)2‚6H2O as shown in Figure 3c. Such shifts and shape changes of the signals were almost negligible in the system of Zn(OAc)2‚2H2O and ZnS-AN(OAc), respectively. The effect of Zn(ClO4)2‚6H2O and ZnS-AN(ClO4) was also observed at the signal of 7.928 ppm attributed to the formyl proton of DMF. In the presence of Zn(ClO4)2‚6H2O, this signal shifted to upfield, opposite to that of methyl protons. In the presence of ZnS-AN(ClO4), this signal shifted to downfield and became broad. In the system of Zn(OAc)2‚2H2O and ZnS-AN(OAc), such changes in this signal were negligible as well as those of methyl protons. These results suggest that the shift and broadening of NMR peaks occur by the adsorption of DMF
DMF on ZnS Nanocrystallites
Langmuir, Vol. 14, No. 15, 1998 4073
Table 2. Chemical Shift of DMF in 1H NMR Spectroscopy δ/ppma (∆δb) system
cis-CH3
trans-CH3
-COH
none Zn(ClO4)2‚6H2O Zn(OAc)2‚2H2O ZnS-AN(ClO4) ZnS-AN(OAc)
2.781 2.819 (+ 0.038) 2.783 (+ 0.002) 2.865 (+0.084) 2.784 (+0.003)
2.899 2.948 (+0.049) 2.902 (+0.003) 3.002 (+0.103) 2.903 (+0.004)
7.928 7.907 (-0.021) 7.928 ((0.000) 7.989 (+0.061) 7.929 (+0.001)
a TMS was used as a reference. b Difference in the chemical shift of DMF in the absence of Zn2+ or ZnS nanocrystallites.
molecules on the surface of ZnS nanocrystallites prepared from Zn(ClO4)2‚6H2O. The chemical shifts (δ) and differences of the chemical shifts (∆δ) are summarized in Table 2. In the presence of Zn(ClO4)2‚6H2O, the interaction between the oxygen atom of the DMF molecule and the cationic Zn2+ ion leads to an increment of δ- charge at the oxygen atom. Such electron localization of DMF molecules causes the shifts of formyl proton to an upfield direction. The resulting δ+ charge localized on a nitrogen atom of a DMF molecule also contributes to the downfield shift of the methyl proton signal (Figure 3b). It is well-known that DMF molecules have a resonance structure between the neutral state and the polarized state.14 Our results suggest that the addition of Zn2+ ions induces stabilization of the polarized state. Reynolds et al. reported very similar behavior with the system of Zn(ClO4)‚6H2O by 13C NMR spectroscopy.15 They observed the downfield shift of methyl carbon and the upfield shift of formyl carbon in the system of protonated DMF molecules. These shifts were also explained by electron localization due to the protonation of oxygen atom as the coordination of DMF molecule to Zn2+ in the present system. In the presence of ZnS-AN(ClO4), the downfield shift of methyl protons and formyl proton, and the broadening of the signals support the strong interaction between DMF molecules and ZnS nanocrystallites through (14) (a) Wiberg, K. B.; Laidig, K. E. J. Am. Chem. Soc. 1987, 109, 5935. (b) Schultz, G.; Hargittai, I. J. Phys. Chem. 1995, 99, 11412. (15) McCelelland, R. A.; Reynolds, W. F. J. Chem. Soc., Chem. Commun. 1974, 824.
oxygen atom of DMF molecule. The downfield shifts were caused by donation of electrons from DMF molecules to the surface of ZnS nanocrystallites. Accordingly, all of DMF molecule should have δ+ charge as shown in Figure 3c. In this model, the weakening of CO bonding is proposed as a structural characteristics of adsorbed DMF molecule bound to Zn2+ or ZnS nanocrystallites. These characteristics are also supported by the wavenumber shift of νCdO in IR measurements, indicating a weakening of the CdO bond. The broadening of the signals was attributed to the effective and rigid adsorption of the capping groups, DMF molecules in this case, on the surface of nanocrystallites as reported by Alivisatos et al.8a The largest ∆δ in this system suggests the most number of the adsorbed DMF molecules in this system compared with other systems. Smaller ∆δ values in the system of Zn(OAc)2‚ 2H2O and ZnS-AN(OAc) reflect a lower number of adsorbed DMF molecules. These results are also in good agreement with the observation in the IR measurements. Conclusions The CdO and C-N stretching vibrations of DMF molecule adsorbed on ZnS shifted to smaller wavenumber, thus indicating a strong interaction between DMF molecule and Zn surface atom. This effect is especially seen in the case of ZnS nanocrystallites prepared from Zn(ClO4)2‚6H2O. Their peak wavenumbers and intensities reflect the strength of interaction of DMF molecules with the surface of ZnS nanocrystallites and number of interacting DMF molecules, respectively. The interaction of DMF molecules on ZnS surface also depends on the coexistent counteranion. The results presented here highlight the usefulness of FT-IR and NMR spectroscopy in probing adsorbed molecules on the surface of ZnS nanocrystallites. Acknowledgment. This work was supported in part by Grants-in-Aid for Scientific Research from the Ministry of Education, Science, Sports, and Culture, Japan, and by a Research Fellowship for Young Scientists from the Japan Society for the Promotion of Science (to H.F.). LA9712855
