In Situ Coordination-Induced Langmuir Film Formation of Water

Sep 3, 2003 - In Situ Coordination-Induced Langmuir Film Formation of Water-Soluble 2,5-Dimercapto-1,3,4-thiadiazole at the Air/Water Interface and th...
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In Situ Coordination-Induced Langmuir Film Formation of Water-Soluble 2,5-Dimercapto-1,3,4-thiadiazole at the Air/Water Interface and the Growth of Metal Sulfide Nanostructures in Their Templated Langmuir-Schaefer Films Haofei Gong, Meifang Yin, and Minghua Liu* CAS Key Laboratory of Colloid and Interface Science, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China Received January 21, 2003. In Final Form: May 26, 2003 Water-soluble bismuthiol(I) (2,5-dimercapto-1,3,4-thiadiazole, DMTD) was found to form stable Langmuir films on the aqueous subphase containing concentrated metal ions such as Cd(II), Cu(II), and Ag(I), although the compound had no long alkyl chain. It was confirmed that the Langmuir film formation was due to the in situ interfacial coordination between DMTD and the metal ions. The in situ coordinated Langmuir films could be transferred onto solid substrates by the horizontal lifting method as Langmuir-Schaefer (LS) films. The transferred LS films were characterized by a series of methods such as UV-visible spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and atomic force microscopy (AFM). The transferred LS films from the DMTD on metal ion subphases showed ordered layer structures with the layer distances of 0.68, 0.69, and 0.55 nm for the films transferred from the Cu(II), Cd(II), and Ag(I) subphases, respectively. The metal ion-coordinated DMTD ultrathin films could be used as templates to grow metal sulfides in aqueous Na2S solution. Pyramid morphologies of metal sulfides were obtained in the film. A possible growth mechanism was discussed. It was suggested that the alkyl-chainfree and water-soluble characteristics of DMTD play an important role in forming the large-sized, wellorganized, and pyramid-shaped metal sulfide thin films.

Introduction The Langmuir film at the air/water interface is one of the most important organized molecular films and can serve as a basis for understanding a series of molecular assemblies.1,2 Recently, many efforts have been devoted to the investigations on the physical and chemical processes in monolayers such as morphological changes,3-5 chemical reactions,6,7 and molecular recognitions.8-10 * Corresponding author. E-mail: [email protected]. Telephone: +86-10-6256-9563. Fax: +86-10-6256-9564. (1) Gaines, G. L., Jr. Insoluble monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966. (2) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: New York, 1991. (3) Francis, R.; Skolnik, A. M.; Carino, S. R.; Logan, J. L.; Underhill, R. S.; Angot, S.; Taton, D.; Gnanou, Y.; Duran, R. S. Macromolecules 2002, 35, 6483. (4) Gillgren, H.; Stenstam, A.; Ardhammar, M.; Norden, B.; Sparr, E.; Ulvenlund, S. Langmuir 2002, 18, 462. (5) Seoane, R.; Minones, J.; Conde, O.; Minones, J., Jr.; Casas, M.; Iribarnegaray, E. J. Phys. Chem. B. 2000, 104, 7735. (b) Buzin, A. I.; Godovsky, Yu. K.; Makarova, N. N.; Fang, J.; Wang, X.; Knobler, C. M. J. Phys. Chem. B. 1999, 103, 11372. (c) Rodriguez Patino, J. M.; Sanchez, C. C.; Rodriguez Nino, M. R. Langmuir 1999, 15, 2484. (d) Huo, Q.; Russell, K. C.; Leblanc, R. M. Langmuir 1998, 14, 2174. (6) Wadia, Y.; Tobias, D. J.; Stafford, R.; Finlayson-Pitts, B. J. Langmuir 2000, 16, 9321. (7) Lucia, L. A.; Wyrozebski, K.; Chen, L.; Geiger, C.; Whitten, D. G. Langmuir 1998, 14, 3663. (8) Ariga, K.; Terasaka, Y.; Sakai, D.; Tsuji, H.; Kikuchi, J. I. J. Am. Chem. Soc. 2000, 122, 7835. (9) Li, C.; Huang, J.; Liang, Y. Langmuir 2001, 17, 2228. (b) Li, C.; Huang, J.; Liang, Y. Langmuir 2000, 16, 7701. (10) Ariga, K.; Kamino, A.; Cha, X.; Kunitake, T. Langmuir 1999, 15, 3875. (b) Bohanon, T. M.; Caruso, P.-L.; Denzinger, S.; Fink, R.; Mobius, D.; Paulus, W.; Preece, J. A.; Ringsdorf, H.; Schollmeyer, D. Langmuir 1999, 15, 174. (c) Marchi-Artzner, V.; Artzner, F.; Karthaus, O.; Shimomura, M.; Ariga, K.; Kunitake, T.; Lehn, J.-M. Langmuir 1998, 14, 5164. (d) Ariga, K.; Kunitake, T. Acc. Chem. Res. 1998, 31, 371.

Among these, interfacial reactions, particularly, the in situ coordination reaction in the monolayers, have attracted much attention because the ultrathin films of coordination complexes have particular optical,11-14 electrical,15,16 magnetic,17-19 and catalytic20,21 properties. The in situ interfacial coordination in the monolayer gives many possibilities to the fabrication of functional metal complex-based molecular assemblies. First, by taking advantage of in situ coordination, monolayers of some small ligand molecules that could not form stable monolayers alone at the air/water interface can be formed, which greatly enriched the choice of monolayer-forming materials, and many functional metal complexes can be fabricated as ultrathin films.22 Moreover, the stability and rigidity of the monolayers can be greatly enhanced after (11) Schick, G. A.; Schreiman, I. C.; Wagner, R. W.; Lindsy, J. S.; Bocian, D. F. J. Am. Chem. Soc. 1988, 111, 1344. (12) Zhang, R. J.; Yang, K. Z. Langmuir 1997, 13, 7141. (13) Taniguchi, M.; Ueno, N.; Okamoto, K.; Karthaus, O.; Shimomura, M.; Yanagishi, A. Langmuir 1999, 15, 7700. (14) Yam, V. W.-W.; Yang, Y.; Yang, H.-P.; Cheung, K.-K. Organometallics 1999, 18, 5252. (15) Nakamura, T.; Kojima, K.; Matsumoto, M.; Tachibana, H.; Tanaka, M.; Kawabata, Y. Chem. Lett. 1989, 367. (16) Xiao, Y.; Yao, Z.; Jin, D. Thin Solid Films 1993, 223, 173. (17) Poertz, M.; Dacal, F.; Segmuller, A. Phys. Rev. Lett. 1978, 10, 246. (18) Byrd, H.; Pike, J. K.; Talham, D. R. J. Am. Chem. Soc. 1994, 116, 7903. (19) Aiai, M.; Ramos, J.; Mingotaud, C.; Amiell, J.; Delhaes, P.; Jaiswal, A.; Singh, R. A.; Singh, B.; Singh, B. P. Chem. Mater. 1998, 10, 728. (20) Maassen, E.; Tieke, B.; Jordan, G.; Rammensee, W. Langmuir 1996, 12, 5595. (21) To¨llner, K.; Popovitz-Biro, R.; Lahav, M.; Milstein, D. Science 1997, 278, 2100. (22) Liu, M.; Kira, A.; Nakahara, H. Langmuir 1997, 13, 4807. (b) Liu, M.; Cai, J. Langmuir 2000, 16, 2899. (c) Cai, J.; Liu, M.; Dong, C.; Li, J.; Tang, J.; Jiang, L. Colloids Surf., A 2000, 175, 165.

10.1021/la034098h CCC: $25.00 © 2003 American Chemical Society Published on Web 09/03/2003

Metal Sulfide Growth in Langmuir-Schaefer Films

interfacial coordination compared with their pure monolayers.23 Second, the in situ coordination reactions make it possible to fabricate the thin films of coordination polymers.24 Most of the coordination polymers are insoluble in common organic solvents, which makes it difficult to fabricate their ultrathin films and further investigate their properties. However, sometimes it could be easy to fabricate the ultrathin solid films of coordination polymers directly by using in situ coordination.24 Third, the properties and functions of the monolayers can be regulated upon using different metal ions in the subphase, which may optimize their applications subsequently. Considering the versatile choices in the metal ions and the ligands, it is expected that the in situ coordination method will be effective in fabricating a series of functional ultrathin films of metal complexes. Previously, we have investigated the interfacial coordination-induced monolayer formation of a serious of benzimidazole derivatives with short or no alkyl chains.22,24 Some new properties and functions were found.23-25 In addition, by using the in situ coordinated ultrathin films as templates, the growth of silver halide nanoparticles was studied, where various novel morphologies of silver halides were observed in the film.26 Bismuthiol(I) is an important reagent to detect metal ions as a result of its excellent reactivity with metal ions.27 In this paper, we used bismuthiol(I), probably the smallest film-forming compound, to investigate its in situ coordination with various metal ions and studied the properties of thus formed ultrathin films. The in situ coordination-induced film formation was studied by the π-A isotherms, and the characteristics of the transferred Langmuir-Schaefer (LS) films were investigated by UV-visible spectra, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and atomic force microscopy (AFM). The transferred LS films were used as templates to grow metal sulfide nanoparticles in the film. Pyramid-shaped Ag2S, Cu2S, and CdS were obtained in the film upon immersion of the film in an aqueous Na2S solution. It is noted that during the formation of metal sulfide nanoparticles, bismuthiol(I) in the film was removed simultaneously as a result of its water solubility. The formation and growth of the nanoparticles in the templated films were investigated using UV-vis spectra, field emission scanning electron microscopy (FE-SEM) and energy-dispersive X-ray (EDS) analysis. A possible growth process was discussed that includes the dissolving of the template bismuthiol(I) and the directed aggregation of the metal sulfide nanoparticles within the film. Experimental Section Materials. Bismuthiol(I) (2,5-dimercapto-1,3,4-thiadiazole, DMTD) was purchased from Tianjin Chemical Reagent Company and recrystallized from a methanol solution before use. Metal nitrates: Cu(NO3)2, AgNO3, Cd(NO3)2, Ni(NO3)2, Zn(NO3)2, and Na2S were all analytical reagents and used without further purification. Chloroform and methanol were distilled before use. Millipore water (18 MΩ cm) was used in all the cases. Procedure. Measurements of surface pressure-area (π-A) isotherms and film depositions were carried out on a KSV mini trough (Helsinki, Finland). A Langmuir film of DMTD was formed by spreading its chloroform solution (5 × 10-4 mol L-1, 1% methanol was added to help the dissolution of DMTD in (23) Liu, M. H.; Kira, A.; Nakahara, H. Langmuir 1997, 13, 779. (24) Gong, H.; Liu, M. Langmuir 2001, 17, 6228. (25) Cai, J.; Liu, M.; Yu, G.; Liu, Y. J. Mater. Chem. 2001, 11, 1924. (26) Gong, H.; Liu, M. Chem. Mater. 2002, 14, 4933. (27) Huang, L.; Tang, F.; Hu, B.; Shen, J.; Yu, T.; Meng, Q. J. Phys. Chem. B 2001, 105, 7984 and references therein.

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Figure 1. Surface pressure-area isotherms of DMTD spreading on aqueous subphases containing metal ions. (a) AgNO3. (b) Cu(NO3)2. (c) Cd(NO3)2. (d) Zn(NO3)2. (e) Ni(NO3)2. (f) The previous five ions mixed. The concentrations of the subphases were 0.1 M, except for AgNO3 (0.01 M). chloroform) onto the aqueous subphases containing various metal ions. The concentration of all these metal salts was 0.1 M, except for AgNO3, which was 0.01 M.28 After the evaporation of the solvent and the coordination reaction was equilibrated for 40 min, the π-A isotherms of the spreading films were recorded at 20 °C by compressing the Langmuir film at a barrier speed of 10 mm/min. The film was transferred onto solid substrates by a horizontal lifting method and rinsed with water thoroughly during each transfer to wash out the possible adsorbed metal salts. The surface pressure was kept at 10 mN/m in all the cases. The transferred metal ion-coordinated thin films were immersed in an aqueous 0.05 M Na2S solution for the growth of metal sulfides at room temperature. For scanning electron microscopy (SEM) observation, three layers of the Langmuir film were transferred on a silicon wafer and immersed in a 0.05 M Na2S solution. The films were rinsed with water thoroughly before SEM observations. Instruments. To characterize the LS films, 20 or 40 layers were transferred onto quartz, silicon wafer, and CaF2 substrates for measurements of UV-vis spectra, XRD patterns, XPS spectra, and FT-IR spectra. One layer of the in situ coordinated films was transferred to freshly cleaved mica for AFM measurements. XRD patterns were obtained using a Hitachi Natural D/Max-RB X-ray diffractometer (Japan) with Cu KR radiation (λ ) 0.154 nm). UV-vis spectra were measured with a JASCO UV-530 spectrophotometer. XPS spectra were measured with a VG Scientific ESCALAB 220-IXL spectrometer using Al KR as the excitation source (1486.6 eV). AFM measurements were performed using a tapping mode atomic force microscope (Nanoscope IIIa, Digital Instruments). FT-IR spectra were recorded using a JASCO FTIR 660 spectrometer. FE-SEM was performed on JSM-6301F with an attachment of an EDS spectrometer.

Results and Discussion In Situ Coordination-Induced Langmuir Film Formation: Surface Pressure-Area Isotherms. As a water-soluble small molecule, DMTD showed no surface pressure when it was spread on a plain water surface. However, surface pressure-area (π-A) isotherms were measurable when DMTD was spread on aqueous solutions containing metal salts. A water-insoluble Langmuir film could be formed through the in situ coordination of DMTD with metal ions. Figure 1 shows the π-A isotherms of spreading films of DMTD on the subphase containing various metal ions. Compared with DMTD on a plain water surface, all kinds of metal ions in the subphase can induce the formation of insoluble films on the surface, indicating that the in situ coordination reaction took place. The onset areas of the spreading film on metal ion-containing subphases were (28) For the silver ion, the high concentration may cause the obvious reduction in the natural conditions. So, a less-concentrated silver ion was used.

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Table 1. XPS Analysis of the LS Films Transferred from Metal Ion Subphases subphase

Mn+ a (eV)

S(2p) (eV)

content N(1s) of metal (eV) ionb

Cu2+ 932.8 (2p3/2), 167.8 [S2p)] 399.7 952.2 (2p1/2) 165.3 [S2p1/2)], 164.1[S2p3/2)] 163.4 [S(2p1/2)], 162.3 [S(2p3/2)] Cd2+ 405.3 (3d5/2) 165.3 [S(2p1/2)], 164.1 [S(2p3/2)] 399.5 412.1 (3d3/2) 163.3 [S(2p1/2)], 162.0 [S(2p3/2)] Ag+ 368.2 (3d5/2) 165.1 [S(2p1/2)], 164.1 [S(2p3/2)] 399.6 374.4 (3d3/2) 163.5 [S(2p1/2)], 162.2 [S(2p3/2)]

0.30 0.35 3.51

a Take the C(1s) ) 284.6 eV as a standard. b Estimated from the atomic ratio of metal ion to nitrogen.

Figure 3. XPS spectra of the S(2p) and Cu(2p) regions of the transferred LS film on the Cu(NO3)2 subphase. Figure 2. UV-vis absorption spectra of the LS films transferred from the spreading films on the subphases containing various metal ions as well as the methanol solution of DMTD.

much smaller compared with the molecular area estimated from the Corey-Pauling-Koltun model. This suggested that a multilayer ultrathin Langmuir film or threedimensional structure was formed at the surface of the metal ions.29 In addition, the onset areas are dependent on the metal ions used. It was found that the onset areas decrease in the order of Ag+ > Cu2+ > Cd2+ > Zn2+ > Ni2+. The isotherm of the DMTD on the subphase containing all the above ions showed an average onset area between the Cu2+ and Cd2+, indicating that all of the metal ions worked to induce the Langmuir film formation. On the subphases of Cu(NO3)2 and AgNO3, the isotherms were much expanded, and the surface pressure increases steadily even at a very small molecular area, indicating the polymeric nature of the coordinated films.30 Considering the larger onset molecular area, we will focus our discussions on the effect of the Cu(II), Cd(II), and Ag(I) ions in the following. LS Films. To verify the above in situ coordinationinduced thin film formation, the Langmuir films on various metal ion subphases were transferred onto solid substrates using a LS method, and the transferred films (LS films) were characterized by UV-vis spectroscopy, FT-IR spectroscopy, XPS, XRD, and AFM. UV-Vis Spectra. Figure 2 shows the UV-vis spectra of 20-layer transferred LS films on quartz substrates as well as the spectrum of DMTD in a dilute methanol solution. DMTD in the methanol solution shows an absorption peak at 332 nm and a shoulder at 256 nm, which is characteristic of DMTD.27 Different from its solution, the transferred LS film from the aqueous Ag+, Cu2+, and Cd2+ subphases showed peaks at 335, 340, and 329 nm, respectively. The different absorption band of (29) Dutta, A. K.; Vanoppen, P.; Jeuris, K.; Grim, P. C. M.; Pevenage, D.; Salesse, C.; De Schryver, F. C. Langmuir 1999, 15, 607. (30) Sigwart, C.; Kroneck, P.; Hemmerich, P. Helv. Chim. Acta 1970, 53, 177.

Figure 4. XRD patterns of the transferred LS films at various metal ion subphases.

the transferred LS films from solution indicates the coordination of DMTD with metal ions in the subphase. In addition, different absorption bands are observed depending on the metal ions. XPS Spectra. To ensure the metal ion coordination at the air/water interface and evaluate the ion contents in the transferred films, XPS was performed on the 40-layer LS films on a silicon wafer transferred from Cu2+, Cd2+, and Ag+ subphases. Table 1 lists the binding energies of the metal ions, S(2p), N(1s), and the calculated contents of metal ions. It is confirmed that, in any case, metal ions were incorporated into the films. Careful analysis of the S(2p) band revealed different binding energies. In the cases of Cd(II) and Ag(I), two kinds of sulfur were found with the binding energy of S(2p3/2) at about 164 and 162 eV, corresponding to the unbound sulfur and the bound thiol, respectively.31 The N(1s) peak showed a binding energy at 399.5 eV, which is slightly higher than the natural nitrogen in the heteroatom form,32 indicating that nitrogen in DMTD is also taken into the coordination. In addition, no N(1s) peak was observed at around 407 eV, indicating that no nitrate anion was incorporated into the film. The fact that no nitrate ions were incorporated into the film indicated that the metal ions were not adsorbed into the films but coordinated with DMTD.

Metal Sulfide Growth in Langmuir-Schaefer Films

Figure 5. FT-IR spectra of the transferred LS films at various metal ion subphases and the cast film of DMTD on CaF2 substrates.

In the case of the Cu(II)-coordinated film, two peaks at 932.8 eV [Cu(2p3/2)] and 952.2 [Cu(2p1/2)] were found as shown in Figure 3, clearly indicating that Cu is presented as a +1 oxidation state in the film.27,33 Careful analysis of the S(2p) band reveals three binding energies for the copper-ion-coordinated film. A small ratio of sulfur with the binding energy of 167.8 eV was found besides the two kinds of sulfur at lower binding energies discussed previously. This can be attributed to the high valent state of sulfur that is oxidized by Cu(II),34 which further verified that the Cu ion in the film was in the +1 oxidation state. It is reported that Cu(II) can oxidize the vacant thiol to disulfides.33,35 In our case, a higher valence of sulfur was found in the film upon coordination with Cu(II) at the air/water interface. On the other hand, quantitative analysis of the metal ions and the DMTD revealed that the ratios of Cu(I) or Cd(II) ions to DMTD were about 0.3, while for Ag(I), the value was 3.5. The smaller content of Cu(I) or Cd(II) in the film indicates the insufficient coordination reaction at the air/water interface. It is much different in the case of the silver(I) ion; the atomic ratio of Ag to DMTD is

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much higher than 1. The silver(I) ion easily forms a linear complex with DMTD, so the theoretical ratio of the silver ion to the ligand molecule is 1:1 in the film.22 The high content of silver in the film may arise from the reduction of the silver ion at the interface.24 In addition, because both the sulfur atoms in the heteroatomic ring and those in the thiol group have strong affinities to silver, the reduced silver clusters are supposed to be adsorbed around the sulfur, which makes the silver content in the film much higher than the theoretical value. XRD Pattern. XRD is a powerful technique to evaluate the layer structure of a thin film. Sophisticated LangmuirBlodgett films from long-alkyl-chain derivatives have been proven to be well-ordered. To study the layer structure of the LS films transferred from the in situ coordinated Langmuir films of DMTD with metal ions, the XRD patterns of transferred LS films (40 layers, 10 mN/m) were investigated as shown in Figure 4. Three diffraction peaks were observed for the Cd2+-coordinated DMTD LS film with 2θ values at 12.86, 25.71, and 38.90°. According to the Bragg equation (2d sin θ ) nλ), these three peaks could be ascribed to the 001, 002, and 003 diffraction indexes, respectively, corresponding to a layer distance of 0.69 nm. For the film transferred from the Cu2+ subphase, the observed diffraction peaks have almost the same 2θ value as the Cd2+-coordinated film, and the long d spacing was estimated to be 0.68 nm. These XRD patterns indicated that ordered layer structures are formed for Cu(I)- and Cd(II)-coordinated LS films. The XRD pattern of the LS film transferred from the Ag+ subphase showed three diffraction peaks with 2θ values at 2.62, 16.04, and 32.33°. The latter two peaks can be ascribed to the 001 and 002 diffractions of the transferred film, which correspond to a layer distance of 0.55 nm. This value is reasonable in considering the similarity between the structure of Ag(I)-DMTD and Cd(II)-DMTD. The much stronger 002 peak than 001 peak in the XRD of the Ag(I)-DTMD film indicates that electron density is very high in the 002 layer. This is just in accordance with the previously described XPS results, where a higher silver content was detected in the film and these silver clusters were supposed to be adsorbed on the sulfur atoms. The additional diffraction peak at 2.62°, corresponding to a layer distance of 3.34 nm, could be regarded as a

Figure 6. AFM images of the transferred LS films at various metal ion subphases. (a) Cu(NO3)2. (b) Cd(NO3)2. (c) AgNO3.

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Table 2. Infrared Spectral Assignment of the 40-Layer Transferred Films in Comparison with the Cast Film of DMTD on CaF2 Substratesa cast film 2474m 1502s 1450s 1263s 1049s 939m 916w

Cu(NO3)2

Cd(NO3)2

AgNO3

1456m 1357s 1070s

1473m 1356s 1074s 1053s

1458m 1359s 1057s

assignment27 υ(SsH) thioamide bands Ib thioamide bands Ib thioamide bands IIc thioamide bands IIc υ(CdS) + υ(CsN) υ(CdS) υ(CdS)

a υ ) stretch; δ ) deformation; s ) symmetric; as ) asymmetric. Due to δ(NsH) (major contribution) + δ(CdN) (minor contribution). c Due to δ(NsH) (minor contribution) + δ(CdN) (major contribution). b

superlattice structure because this distance is just exactly 6 times the d spacing of 0.55 nm. FT-IR Spectra. To further investigate the coordination structure of DMTD on the metal ion surface, FT-IR spectra were performed on the 40-layer LS films as well as the cast film of DMTD on CaF2 substrates as shown in Figure 5. The assignments of the vibration bands are listed in Table 2. Compared with the cast film of DMTD, the LS film transferred from the metal ion subphases showed no absorption band at the position of 2474 cm-1, which is characteristic of S-H vibration, suggesting the formation of a metal-sulfur bond in the film. The vibration bands at 1049, 916, and 939 cm-1, which mainly contributed from stretching modes of CdS and CsN of DMTD, disappeared completely in all the films. It is also noticed that there are strong absorption bands at about 1070 cm-1 in all the cases, which could be attributed to the CdN stretching vibration band of the coordinated DMTD in the film. In addition, the vibration bands at 1502 and 1450 cm-1 in the cast film, which contributed mainly to the deformation vibration of N-H, have a low-frequency shift in all the cases, suggesting the coordination of N to the metal ions.36 These FT-IR spectral changes clearly indicated the coordination of the metal ions with DMTD. AFM. AFM was proven to be a powerful method to detect the domain and morphology of the ultrathin films. Up to now, AFM has been widely studied in those of long-alkylchain substituted Langmuir-Blodgett films,37-39 selfassembly films,40,41 and layer-by-layer assembly films,42,43 but few studies have been performed on the thin films of alkyl-chain-free molecules, especially to those of coordi(31) Brower, T. L.; Garno, J. C.; Ulman, A.; Liu, G.; Yan, C.; Go¨lzha¨user, A.; Grunze, M. Langmuir 2002, 18, 6207. (32) Patsch, M.; Thieme, P. Angew. Chem., Int. Ed. Engl. 1971, 10, 569. (33) Brust, M.; Blass, P. M.; Bard, A. J. Langmuir 1997, 13, 5602. (34) Chanunpanich, N.; Ulman, A.; Malagon, A.; Strzhemechny, Y. M.; Schwarz, S. A.; Janke, A.; Kratzmueller, T.; Braun, H. G. Langmuir 2000, 16, 3557. (35) Corsim, A.; Fernaudo, Q.; Freiser, H. Talanta 1964, 11, 63. (36) Nakamoto, K. In Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed.; John Wiley & Sons: NewYork, 1986. (37) Gaffo, L.; Constantino, C. J. L.; Moreira, W. C.; Aroca, R. F.; Oliveira, O. N., Jr. Langmuir 2002, 18, 3561. (38) Oishi, Y.; Umeda, T.; Kuramori, M.; Suehiro, K. Langmuir 2002, 18, 945. (39) Morita, S.-i.; Iriyama, K.; Ozaki, Y. J. Phys. Chem. B. 2000, 104, 1183. (40) Liu, D.; Zhang, H.; Grim, P. C. M.; De Feyter, S.; Wiesler, U.-M.; Berresheim, A. J.; Mullen, K.; De Schryver, F. C. Langmuir 2002, 18, 2385. (41) Han, S. W.; Ha, T. H.; Kim, C. H.; Kim, K. Langmuir 1998, 14, 6113. (42) Lee, S. H.; Balasubramanian, S.; Kim, D. Y.; Viswanathan, N. K.; Bian, S.; Kumar, J.; Tripathy, S. K. Macromolecules 2000, 33, 6534. (43) Locklin, J.; Youk, J. H.; Xia, C.; Park, M. K.; Fan, X.; Advincula, R. C. Langmuir 2002, 18, 877.

Figure 7. UV-vis absorption spectral change of the film upon immersion in an aqueous 0.05 M solution of Na2S transferred from the subphase of (a) Cu(NO3)2, (b) Cd(NO3)2, and (c) AgNO3. The insets show the change of the UV-vis absorbance at a certain wavelength as a function of the immersion time.

nated polymeric thin films. To observe the morphology of the interfacial coordinated polymers, one layer of the LS films were transferred on the freshly cleaved mica from the subphases of Cu(NO3)2, Cd(NO3)2, and AgNO3 for AFM observations as shown in Figure 6. For silver(I)- and copper(I)-ion-coordinated films, obvious weblike morphologies were observed. The roughnesses in the two films are both at about 1.7 nm. Some aggregates with a higher vertical distance were observed in the case of the copperion-coordinated film, which suggested that multilayer films or three-dimensional structures are formed in the film. These results are in agreement with the previously mentioned surface pressure-area isotherms. In the case of the Cd(II)-coordinated film, dotted domains are ob-

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Figure 8. FE-SEM images of the films after immersion in an aqueous 0.05 M Na2S solution for (a) 30 min, (b) 1 h, and (c) 24 h transferred from the subphase of Cu(NO3)2. (d and e) After 24 h of immersion in a Na2S solution for the film transferred from subphase of (d) Cd(NO3)2 and (e) AgNO3.

served, showing the similar multilayer nature of the transferred film, although the dotted nanoparticles are not connected. Growth of the Metal Sulfides in the LS Films. As discussed before, although DMTD was a water-soluble molecule, it can form water-insoluble thin films at the surface of metal ions through the in situ coordination at interfaces. Moreover, the transferred LS films have wellordered layer structures. In our previous work,26 we have found that this kind of metal-ion-coordinated ultrathin film can be used as templates for the growth and aggregation of inorganic nanoparticles. Similarly, by immersing the metal-ion-coordinated LS films of DMTD into an aqueous solution of Na2S, the nanoparticles of metal sulfides with interesting morphologies were observed. UV-Vis Spectral Study. The formation of metal sulfide can be monitored by the UV-vis spectra as shown in

Figure 7. All three of the spectra of the Cu+-, Cd2+-, and Ag+-coordinated films show the absorption band in the range of 300-320 nm, which is the absorption of metalion-coordinated DMTD in the Na2S solution. The position of the absorption band did not change dramatically, but the absorbance intensity changed significantly with increasing time in all three of the cases, as shown in the insets of each figure. They all have fast intensity increase processes at the initial stage of immersion followed by a sharp intensity decrease. After that, the absorbance intensities have a steady increase for Cu+- and Cd2+coordinated films. For the Ag+-coordinated film, the absorbance intensity has another decrease process after increasing. As we have discussed before, DMTD was a water-soluble molecule especially in the basic conditions, so DMTD can dissolve in Na2S solution when it was unbound from the coordination polymer with the formation of metal sulfides. The increase of the intensity in the initial

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Scheme 1. Possible Growth Mechanism of Metal Sulfides in the Metal-Ion-Coordinated DMTD Ultrathin Films

stage can be ascribed to the formation of metal sulfides in the film, and the subsequent decrease could be attributed to the dissolution of DMTD into the solution. The slow intensity increase process can be attributed to the growth and aggregation of the metal sulfides in the film. For the Ag+-coordinated film, the absorbance intensity decreased steadily at the final immersion stage after an intensity increase process, which maybe was caused by the further dissolving of DMTD in the film. SEM Study. To further reveal the metal sulfide growth and aggregation process in the film, FE-SEM was performed on the silicon wafer of three-layer transferred thin films after immersion in the Na2S solution for different times. Figure 8 shows the SEM images of the metal sulfides grown in the concentrated Na2S solution. In the case of the Cu(I)-coordinated thin film, netlike morphologies were seen in the initial stage of immersion as shown in Figure 8a, indicating the formation of copper sulfide in the film. Some light square areas were seen in the net in the film when immersing for 1 h as shown in Figure 8b, which indicates the aggregate of the copper sulfide in the film. Finally, a pyramid morphology was formed in the film when immersing for 24 h. In the cases of the Cd(II) and Ag(I) ions, similar SEM pictures were found, and in any case, pyramid morphology of the metal sulfides was formed in the final stage of immersion. EDS analysis revealed that the observed morphologies were composed of metal sulfides. Quantitative analysis revealed that the pyramid morphologies were composed of Cu2S, CdS, and Ag2S, which was in accordance with the oxidation state of cupric ion discussed previously. Moreover, no N element was found in the film, which strongly verified that DMTD was dissolved in the solution during the formation of the metal sulfides. The possible formation process of the metal sulfides in the DMTD LS film is shown in Scheme 1. When the metalion-coordinated LS films were immersed into the Na2S solution, as a result of being free of an alkyl chain, hydrated S2- can easily penetrate into the film and form the metal sulfide. As a result of the water-soluble nature of DMTD, the DMTD was dissolved in the solution during the formation of the metal sulfides. The formed metal sulfide

Gong et al.

nanoparticles were directly aggregated in the film to form a pyramid shape. It should be further noted than the alkylchain-free and water-soluble characteristics of DMTD play important roles in the formation of the well-defined shaped and large-sized metal sulfides in the film. Some groups44-46 reported the formation of a pure metal sulfide thin film including the template dissolving process in organic solvents after exposure of the film in gases or immersion in a S2- solution, which could not form well-shaped, directed aggregation materials in the film. The watersoluble nature of the template DMTD facilitated the directed aggregation of the metal sulfides and made it possible to form large-sized, well-ordered, and regularshaped pure metal sulfide ultrathin films. So far, there are many reports on the formation of globular nanoparticles of metal sulfides but less on the formation of such pyramid-structured metal sulfides. Our method provides a simple way to fabricate and assemble such kinds of pyramid metal sulfides. Conclusions Water-soluble DMTD was found to form insoluble metalion-coordinated Langmuir films on the subphase containing concentrated metal salts through an in situ coordination at the air/water interface. Such in situ coordinationinduced Langmuir films can be transferred to a solid substrate by the horizontal lifting method, forming LS films with a good layer structure. A series of characterization methods such as UV-vis spectra, XPS, XRD, and FT-IR spectra have confirmed the coordination of DMTD with metal ions. AFM measurements on the one-layer transferred films revealed that the ultrathin Langmuir film was composed of aggregated nanoparticles of the complexes. The metal-ion-coordinated DMTD films could be used as templates to grow metal sulfides. Pyramidshaped metal sulfides were grown in the films by immersing them in a concentrated Na2S solution. It was revealed that the alkyl-chain-free and water-soluble nature of the DMTD template play important roles in forming the large-sized, well-organized, and regularshaped metal sulfide thin films. Acknowledgment. This work was supported by the Major State Basic Research Development Program 973 (No. G2000078103), Outstanding Youth Fund (No. 20025312), National Natural Science Foundation (No. 29992590-3), and the Fund of the Chinese Academy of Sciences. LA034098H (44) Facci, P.; Diaspro, A.; Rolandi, R. Thin Solid Films 1998, 327329, 532. (45) Elliot, D. J.; Furlong, D. N.; Grieser, F. Colloids Surf., A 1998, 141, 9. (46) Moriguchi, I.; Nii, H.; Hanai, K.; Nagaoka, H.; Teraoka, Y.; Kagawa, S. Colloids Surf., A 1995, 103, 173.