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Metal clusters and nanoparticles assembled in zeolites: an example of stable materials with controllable particle size
Materials Science and Engineering C 19 Ž2002. 327–331 www.elsevier.comrlocatermsec
Metal clusters and nanoparticles assembled in zeolites: an example of stable materials with controllable particle size V.S. Gurin a,) , V.P. Petranovskii b, N.E. Bogdanchikova b a
Physico-Chemical Research Institute, Belarusian State UniÕersity, Leningradskaja Str., 14, Minsk, 220080, Belarus b CCMC-UNAM, Ensenada, B.C., 22800, Mexico
Abstract An ion-exchangeable zeolite Žmordenite. is used to control the formation of nanoparticles and clusters within the solid matrix by the hydrogen reduction of metal ions ŽAgq, Cu2q, and Ni 2q .. SiO 2rAl 2 O 3 molar ratio in mordenite appears to be an efficient tool to manage the reducibility of the metal ions. Few-atomic silver clusters in line with the larger silver nanoparticles were observed with DRS for the reduced Agq-exchanged mordenites. Cu2q-exchanged ones produce the copper nanoparticles with different optical appearance, and Ni 2q-exchanged mordenites are reduced up to complicated species with no explicit assignment of metal particles under the conditions studied. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Zeolite; Clusters; Metal nanoparticles
1. Introduction In spite of intensive research in the field of metal nanoparticles and clusters in the last decades, the problem of a proper size-control and materials fabrication with particles of the desired properties exists, since each type of materials requires a development of new approaches to the synthesis of clusters and nanoparticles. Nanostructured materials containing small metal particles and clusters have been fabricated by different techniques: ion-implantation w1–3x, sol–gel synthesis w4–6x, thermal w7x and photochemically stimulated processes w8x, plasma-polymerization w9x, etc.; however, in most examples a matrix appears as a passive support keeping the nanoparticles within a solid phase, and chemical processes with solid supports have only a secondary importance by the participation of superficial groups providing more metal-support interaction. An intradendrimer exchanged and reduced materials are an exception in the sense of matrix activity w10x, similar to zeolites; however, this is an organic matrix instable under thermal treatment. Zeolites possess variable ionic properties, and the chemistry of metal ions with zeolite matrix proposes a number of interactions both with
)
Corresponding author. Fax: q7-375-172-264696. E-mail address: [email protected] ŽV.S. Gurin..
simple cation substitution, built-up framework positions, and they are able to be incorporated into the cavities of molecular size w11x. A specific of metal–zeolite interactions provides wide routes for the control of metal species produced in zeolites w12,13x. A great variety of natural and synthetic zeolites w11x is very different with respect to transition metal chemistry in spite of the common basis construction of alumosilicate frameworks. The amount of substituted silicon atoms Ži.e. SiO 2rAl 2 O 3 molar ratio— MR. is one of the important parameters managing acidity, ion-exchangeability, and metal ion coordination. Else, a more prominent feature of zeolites, in contrast with another matrices with non-regular structure, is the occurrence of regular intracrystalline cavities, the size of which is determined by the type of zeolite lattice rather than the composition and MR. These cavities can incorporate and stabilize both homonuclear and heteronuclear clusters when the size of the cavities falls into correspondence with the clusters. Thus, one can use acidity monitoring for some chemical processes in the solid phase of zeolites like that in liquid solutions in order to control the products of H-dependent chemical processes. As an example of such processes, we concern here the reduction by hydrogen ŽM nqq H 2 ™ M ny1 q 2Hq . which results in partial or complete reduction of metal ions pre-incorporated within a zeolite. We study the dependence of reduction efficiency of the metal ions on the acidity of zeolite matrix variable
0928-4931r02r$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 8 - 4 9 3 1 Ž 0 1 . 0 0 4 1 1 - 8
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V.S. Gurin et al.r Materials Science and Engineering C 19 (2002) 327–331
in the wide range for mordenite Ž10 F MR F 206.. This variation of MR of mordenite retains invariably the size of ˚ cross section. and the channels Želliptic, with 6.5 = 7.0 A can provide the stabilization of mono-sized clusters. We study the reduction of Ag, Cu, and Ni ions and demonstrate the formation of the different size particles and clusters depending on the metal ion type, MR and temperature of reduction. The produced species are of practical interest as active catalysts with regulation of catalytic activity due to different reduced species stabilized within the matrix.
2. Experimental The mordenite samples in protonated forms were supplied by TOSOH ŽJapan.. They had MR from 10 through 206. An ion-exchange with Agq-, Cu2q and Ni 2q was performed in the corresponding aqueous solutions of nitrates with large excess of the metal ions. The suspensions of mordenite powders were filtered, a solid product was washed and dried at the ambient conditions. Metal concentration in the final samples was in the range 0.5–2 wt.% Žaccording to the data of energy dispersive spectroscopic analysis.. A subsequent heating in hydrogen flow Žat temperatures 293–773 K. resulted in the change of optical properties monitored by diffuse reflectance spectroscopy ŽDRS. recorded on a Varian Cary-300 spectrometer with Kubelka–Munk processing.
3. Experimental results and discussion Ag-mordenite samples appear to be easily reducible under different temperatures Žbeginning from 100 8C., and the temperature interval providing the maximum intensity of peaks of reduced silver is 200–300 8C. This process is illustrated for the series of Ag-mordenite samples reduced at 200 8C ŽFig. 1a.. Intensity of these maxima and their shape depend strongly on the MR value, but their position of the short-wavelength part is almost the same for the different mordenites: the principal peaks are at 280–285 and 318–323 nm. In contrast, in the range of wavelengths 370–450 nm the broad absorption band appears to be dependent from MR remarkably. The most pronounced maxima in the UV range are inherent to the medium values of MR, while the long-wavelength part develops more for the lowest and highest MR Ž15 and 206.. These UV maxima were assigned recently to the silver clusters, Ag 8 , according to their observation in solutions w14x, the mass-spectroscopic detection w15x and EXAFS refinement w16x. The broad absorption band with l ) 370 nm is easily interpreted as the plasmon resonance of silver nanoparticles Žwith sizes in the range 1–5 nm. often observable in a number of system with reduced ultrafine silver w17x. The
Fig. 1. DRS spectra of a series of Ža. Ag-, Žb. Cu-, Žc. Ni-exchanged mordenites reduced in hydrogen at 200 8C ŽAg., 450 8C ŽCu., 250 8C ŽNi. with various SiO 2 rAl 2 O 3 molar ratio: Ag, Cu— Ž1–15; 2–20; 3–30; 4–206. and Ni— Ž1–15; 2–20; 3–128; 4–206..
UV peaks of the silver reduced products are described much scarcely, however, species of lower nuclearity were also detected in zeolites w13,18,19x, and numerous fewatomic silver species in solution were pointed out to be of the lower stability w20x. The clusters formed in mordenite Žand some other zeolites with similar size of cavities— erionite, beta, LTL w21x., in contrast, discover little variations under storage during months and years. The formation of the clusters and nanoparticles can proceed through the aggregation of the reduced silver
V.S. Gurin et al.r Materials Science and Engineering C 19 (2002) 327–331
atoms stopped by the cavities andror prolonged in the extra-crystalline region where no geometric obstacles occur for further growth, and diffusion of the reducible Agq determines a maximum size of the particles formed. Mordenite with variable MR serves in these processes as ‘medium’ kinetically controlling the process due to the change of acidity and silver ion concentration. The size of Ag 8 formed can correspond approximately to the cavity dimension Žsee below.. Silver reduced in a less appropriate medium produces aggregates of more size, which depends on the samples with different MR, mainly, on silver ion concentration. Thus, the acidity of the matrix Žgoverned by MR. is the tool optimizing products of the silver reduction. The optimum temperature of Cu2q reduction is higher than for Agq reduction, and the corresponding DRS spectra are shown in Fig. 1b. Under lower temperatures, the sample with MR s 206 is reduced, but another samples reveal no specific features of the reduced copper Žnot shown., and a large amount of initial Cu2q leaves as the long-wave broad band Žsemi-band due to cut of the spectra at l ) 850 nm not available for the device used.. This band is also noticeably developed even for the reduced Cu-mordenites at MR s 15, 20, 30 ŽFig. 1b.. An assignment of this band to the d-d transition of Cu2q-ions in pseudooctahedral position is not complicated, and such appearance is usual for solids with CuŽII. w22x. The principal absorption band in the reduced Cumordenites peaks at 580–600 nm and appears under the highest MR Žfrom the samples under study., MR s 206, but does not present at MR s 15, while it begins to be seen
at MR s 20, and more develops at MR s 31 ŽFig. 1b.. In the cases of low reduction degree ŽMR s 15, 20., the broad long-wave absorption band Ž l ) 600 nm., related to Cu2q, occurs and the common rising of the spectra is seen in the range of l - 400 nm. This absorption band at 580–600 nm can be associated with the plasmon resonance in copper nanoparticles w2,6,17,20x, and we simulated them with the Mie theory ŽSection 4.. As well as in the case of Cu2q-exchanged mordenites, the samples with Ni 2q reveal a noticeable amount of this ionic form, the band at 390–400 nm and the weak doublet, l ; 650 and 720 nm, which is typical for Ni 2q ions, e.g., in ligand octahedral environment, NiŽH 2 O. 62q w22x ŽFig. 1c.. The reduction temperature providing a remarkable effect of MR for Ni-mordenite samples is 250 8C ŽFig. 1c.. If the ‘grey’ background essentially rising the spectral curves Že.g. curve 4. means an appearance of the reduced metal in a form without any pronounced band unlike copper and silver in this range, one can assume that the mordenites with maximum MR value Žs 206. again provide the most efficient reduction with production of metal nanoparticles. The medium values of MR have less reducibility, the bands remain Žcurves 2 and 3. with different intensity, and the band at l ; 390 nm is shifted from sample 3 to sample 2. This can reflect the variation of local Ni 2q environment under different MR Žsample 2 contains more aluminum than sample 3.. The reduced Ni-mordenite samples with different MR display the band peaked at l ; 290 nm which scarcely can be associated with a reduced Ni-containing species since there are no
Table 1 Results of MOLCAO calculations of the selected Ag 8 clusters Initial geometry
Charge
329
Binding energy per
Ag–Ag interatomic
atom, BErn, eV
˚ distances, A
First allowed transitions, l, nm Žin the range l ) 200 nm.
Oh
y 0 q
2.34 2.30 2.35
2.88 2.86 2.89
330; 319; 222; 208 – 319; 306; 221; 207
Td
y 0 q
2.34 2.24 2.30
2.91; 3.92 2.90; 3.19 2.97; 3.32
319; 221; 219 – 319; 314; 211; 206
D4h
y 0 q
2.18 1.81 2.35
2.88; 3.35 2.65; 3.25 2.80; 2.98
461; 242; 237; 235; 234; 227 474; 239 311; 222; 205
C 2v
y 0 q
2.34 2.26 2.34
2.79; 2.91; 2.92 2.85; 2.89; 2.90 2.78; 2.95; 2.95
296; 266 – 323; 299; 209; 203
330
V.S. Gurin et al.r Materials Science and Engineering C 19 (2002) 327–331
changes in its intensity depending on MR ŽFig. 1c.. This band is typical for charge transfer complexes of Ni 2q w22x, and its appearance through different samples means incomplete reducibility of NiŽII. prevented by complexation in zeolite, remnant water within zeolite crystals, formation of silicates and alumosilicates. Thus, unlike Ag and Cu-containing mordenites, in the case of Ni-mordenites we obtained no unambiguous evidence on the formation of any nanoparticles or clusters. The specific plasmon resonance peak of Ni particles would be located at l ; 230 nm w23x.
4. Simulation of clusters and nanoparticles formed in mordenites In order to simulate small silver clusters stabilization, which was supposed within the mordenite cavities, a series of selected geometric structures was calculated at ab initio level by restricted and unrestricted self-consisted field Hartree–Fock methods with the molecular orbital-linear combination of atomic orbitals ŽMOLCAO. approach ŽTable 1. using HONDO7 code w24x. Basis sets were used with the effective core potentials w25x counting 19 valence electrons for Ag atoms. The data obtained for the cluster structures under study ŽTable 1. allow us to propose isomers Ag 8 probable for stabilization within the mordenite cavities. The neutral cubic Ag 8 cluster is slightly more stable than the C 2v isomers, but positively and negatively charged clusters have almost the same value of the binding energy. The charged O h structures are distorted to C 1 –C 2v symmetry. This means a possible coexistence of different isomers at ambient temperature. Electronic transitions estimated for this family reveal that the neutral ones have very high energies of the first allowed transitions, while the transitions for the charged clusters are more reasonable and can match experimental observations: Agq 8 of one type of geometry with C 2v symmetry has the first transitions corresponding to l s 323 and 299 nm and fit the best agreement with the above-observed experimental peaks of DRS data. Thus, we suggest this cluster as a candidate for species stabilized in the mordenite cavities. This cluster possesses also rather short Ag–Ag interatomic distance, and, consequently, it can easily enter a cavity without large distortion. The recent EXAFS refinement w16x evidences the structure for silver clusters Ag 8 stabilized in erionite Žthe type of zeolite with cavities of circular cross-section and near dimensions. similar to that prescribed here. Within the framework of the present paper, we could not detect any unambiguous features assigned to small copper clusters, and we simulate the principal band in copper optical spectra for a model of spherical particles in a medium with the dielectric constant ´o . The optical constants of Cu were used from Ref. w26x, and their size-dependence was accounted only for the imaginary part
by the limitation of the mean free path length of electrons w27x. The calculated absorption spectra ŽFig. 2. display the remarkable effect of particle size and medium dielectric constant. Within the ranges under study, they affect mainly the shape of the absorption feature evolving from the shoulder in the case of low ´o and less particle size to the pronounced peak for higher ´ o and larger particles. On the basis of these conclusions, we can associate the changes in experimental spectra of the reduced Cu-mordenites with the variation of the size of copper particles and their position in mordenite. The particles of this size range Ž) 1 nm. could not fit the intracrystalline cavities unlike the above Ag 8 . They can be located on microcrystal surface Žthis version corresponds to low ´ o . or inside the mordenite in the mesopores and cleaved areas Žmore ´o .. Then, copper nanoparticles produced in mordenites with the low MR are of less size than those formed under the high MR. The higher acidity under the lower MR inhibits the reduction reaction, and particles of less size may be formed in spite of more copper amount exchanged under the more number of substituted Al in such mordenites. Thus, MR value in mordenite appears as the efficient tool of copper reduction.
Fig. 2. Calculated absorption spectra of Cu nanoparticles in the medium with different ´ o for a series of particle radii.
V.S. Gurin et al.r Materials Science and Engineering C 19 (2002) 327–331
5. Conclusions Ag-, Cu-, and Ni-ion exchanged zeolites Žon the example of mordenite. were studied for the production of reduced species of the metals with emphasis on clusters and nanoparticles with pronounced optical appearance. The MR value of the mordenite allows us to control the properties of the reduction products. Ag-mordenite system results in both mono-sized clusters Žassigned as 3D polyhewith C 2v symmetry according to quantum dron Agq 8 chemical calculations. and nanometer-range particles. Cumordenites show only the nanoparticles with different shapes of the plasmon resonance absorption band, which was simulated in dependence on particle size and the medium dielectric properties. The effect of the zeolite properties upon the reduction reaction of nickel ions was recorded also, however, without a direct detection of any metal nanoparticles in optical data. Acknowledgements The authors acknowledge funding for this research by CONACYT, Mexico, through grants 32118-E, 31366-U and 1 E120.2403. References w1x R. Bertoncello, F. Trivillin, E. Cattaruzza, P. Mazzoldi, G.W. Arnold, G. Battaglin, M. Catalano, J. Appl. Phys. 77 Ž1995. 1294. w2x R.H. Magruder III, R.F. Haglund Jr., L. Yang, J.E. Wittig, R.A. Zuhr, J. Appl. Phys. 76 Ž1994. 708. w3x A.L. Stepanov, D.E. Hole, P.D. Townsend, J. Non-Cryst. Solids 260 Ž1999. 65.
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w4x G. De, A. Licciulli, C. Massaro, L. Tapfer, M. Catalano, G. Battaglin, C. Meneghini, P. Mazzoldi, J. Non-Cryst. Solids 194 Ž1996. 225. w5x T. Li, J. Moon, A.A. Morrone, J.J. Mecholsky, D.R. Talham, J.H. Adair, Langmuir 15 Ž1999. 4328. w6x J.F. Perez-Robles, F.J. Garcia-Rodriguez, J.M. Yanez-Limon, F.J. Espinoza-Beltran, Y.V. Vorobiev, J. Gonzalez-Hernandez, J. Phys. Chem. Solids 60 Ž1999. 1729. w7x W. Kleemann, Z. Phys. 215 Ž1968. 113. w8x U. Kreibig, Appl. Phys. 10 Ž1976. 255. w9x A. Heilmann, M. Quinten, J. Werner, Eur. Phys. J. B 3 Ž1998. 455. w10x M. Zhao, R.M. Crooks, Chem. Mater. 11 Ž1999. 3379. w11x D.W. Breck, Zeolite Molecular Sieves, Structure, Chemistry and Use, Wiley-Interscience Publ., New York, 1974. w12x B.C. Gates, Chem. Rev. 95 Ž1995. 511. w13x T. Sun, K. Seff, Chem. Rev. 94 Ž1994. 858. w14x B.G. Ershov, E. Janata, A. Henglein, J. Phys. Chem. 97 Ž1993. 339. w15x S. Fedrigo, W. Harbich, J. Buttet, Phys. Rev. B 47 Ž1993. 10706. w16x J.S. Ogden, N.E. Bogdanchikova, J.M. Corker, V.P. Petranovskii, Eur. Phys. J. D 9 Ž1999. 608. w17x U. Kreibig, M. Vollmer, Optical Properties of Metal Clusters, Springer-Verlag, Berlin, 1995. w18x G.A. Ozin, F. Hugues, S.M. Mattar, D.F. McIntosh, J. Phys. Chem. 87 Ž1983. 94, 3445. w19x R. Seitert, R. Rytz, G. Calzaferri, J. Phys. Chem. A 104 Ž2000. 7473. w20x A. Henglein, Chem. Rev. 89 Ž1989. 1861; A. Henglein, Ber. Bunsenges. Phys. Chem. 101 Ž1997. 1562. w21x V.S. Gurin, N.E. Bogdanchikova, V.P. Petranovskii, Mater. Sci. Eng., C Ž2001. in press. w22x A.B.P. Lever, Inorganic Electronic Spectroscopy, 2nd ed., Elsevier, Amsterdam; New York, 1984. w23x B.G. Ershov, Russ. Chem. Bull. 49 Ž2000. 1715. w24x M. Dupuis, J.D. Watts, H.O. Villar, G.J.B. Hurst, HONDO: Ab Initio HFMO Calculations ŽVersion 7.0., QCPE Program No. 544, IBM Sci. Eng. Comp., Kingston, NY, 1987. w25x P.J. Hay, W.R. Wadt, J. Chem. Phys. 82 Ž1985. 299. w26x P.B. Johnson, R.W. Christy, Phys. Rev. B 6 Ž1972. 4370. w27x C.F. Bohren, D.R. Huffman, Absorption and Scattering of Light by Small Particles, Wiley-Interscience Publ., New York, 1983.
Metal clusters and nanoparticles assembled in zeolites: an example of stable materials with controllable particle size V.S. Gurin a,) , V.P. Petranovskii b, N.E. Bogdanchikova b a
Physico-Chemical Research Institute, Belarusian State UniÕersity, Leningradskaja Str., 14, Minsk, 220080, Belarus b CCMC-UNAM, Ensenada, B.C., 22800, Mexico
Abstract An ion-exchangeable zeolite Žmordenite. is used to control the formation of nanoparticles and clusters within the solid matrix by the hydrogen reduction of metal ions ŽAgq, Cu2q, and Ni 2q .. SiO 2rAl 2 O 3 molar ratio in mordenite appears to be an efficient tool to manage the reducibility of the metal ions. Few-atomic silver clusters in line with the larger silver nanoparticles were observed with DRS for the reduced Agq-exchanged mordenites. Cu2q-exchanged ones produce the copper nanoparticles with different optical appearance, and Ni 2q-exchanged mordenites are reduced up to complicated species with no explicit assignment of metal particles under the conditions studied. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Zeolite; Clusters; Metal nanoparticles
1. Introduction In spite of intensive research in the field of metal nanoparticles and clusters in the last decades, the problem of a proper size-control and materials fabrication with particles of the desired properties exists, since each type of materials requires a development of new approaches to the synthesis of clusters and nanoparticles. Nanostructured materials containing small metal particles and clusters have been fabricated by different techniques: ion-implantation w1–3x, sol–gel synthesis w4–6x, thermal w7x and photochemically stimulated processes w8x, plasma-polymerization w9x, etc.; however, in most examples a matrix appears as a passive support keeping the nanoparticles within a solid phase, and chemical processes with solid supports have only a secondary importance by the participation of superficial groups providing more metal-support interaction. An intradendrimer exchanged and reduced materials are an exception in the sense of matrix activity w10x, similar to zeolites; however, this is an organic matrix instable under thermal treatment. Zeolites possess variable ionic properties, and the chemistry of metal ions with zeolite matrix proposes a number of interactions both with
)
Corresponding author. Fax: q7-375-172-264696. E-mail address: [email protected] ŽV.S. Gurin..
simple cation substitution, built-up framework positions, and they are able to be incorporated into the cavities of molecular size w11x. A specific of metal–zeolite interactions provides wide routes for the control of metal species produced in zeolites w12,13x. A great variety of natural and synthetic zeolites w11x is very different with respect to transition metal chemistry in spite of the common basis construction of alumosilicate frameworks. The amount of substituted silicon atoms Ži.e. SiO 2rAl 2 O 3 molar ratio— MR. is one of the important parameters managing acidity, ion-exchangeability, and metal ion coordination. Else, a more prominent feature of zeolites, in contrast with another matrices with non-regular structure, is the occurrence of regular intracrystalline cavities, the size of which is determined by the type of zeolite lattice rather than the composition and MR. These cavities can incorporate and stabilize both homonuclear and heteronuclear clusters when the size of the cavities falls into correspondence with the clusters. Thus, one can use acidity monitoring for some chemical processes in the solid phase of zeolites like that in liquid solutions in order to control the products of H-dependent chemical processes. As an example of such processes, we concern here the reduction by hydrogen ŽM nqq H 2 ™ M ny1 q 2Hq . which results in partial or complete reduction of metal ions pre-incorporated within a zeolite. We study the dependence of reduction efficiency of the metal ions on the acidity of zeolite matrix variable
0928-4931r02r$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 8 - 4 9 3 1 Ž 0 1 . 0 0 4 1 1 - 8
328
V.S. Gurin et al.r Materials Science and Engineering C 19 (2002) 327–331
in the wide range for mordenite Ž10 F MR F 206.. This variation of MR of mordenite retains invariably the size of ˚ cross section. and the channels Želliptic, with 6.5 = 7.0 A can provide the stabilization of mono-sized clusters. We study the reduction of Ag, Cu, and Ni ions and demonstrate the formation of the different size particles and clusters depending on the metal ion type, MR and temperature of reduction. The produced species are of practical interest as active catalysts with regulation of catalytic activity due to different reduced species stabilized within the matrix.
2. Experimental The mordenite samples in protonated forms were supplied by TOSOH ŽJapan.. They had MR from 10 through 206. An ion-exchange with Agq-, Cu2q and Ni 2q was performed in the corresponding aqueous solutions of nitrates with large excess of the metal ions. The suspensions of mordenite powders were filtered, a solid product was washed and dried at the ambient conditions. Metal concentration in the final samples was in the range 0.5–2 wt.% Žaccording to the data of energy dispersive spectroscopic analysis.. A subsequent heating in hydrogen flow Žat temperatures 293–773 K. resulted in the change of optical properties monitored by diffuse reflectance spectroscopy ŽDRS. recorded on a Varian Cary-300 spectrometer with Kubelka–Munk processing.
3. Experimental results and discussion Ag-mordenite samples appear to be easily reducible under different temperatures Žbeginning from 100 8C., and the temperature interval providing the maximum intensity of peaks of reduced silver is 200–300 8C. This process is illustrated for the series of Ag-mordenite samples reduced at 200 8C ŽFig. 1a.. Intensity of these maxima and their shape depend strongly on the MR value, but their position of the short-wavelength part is almost the same for the different mordenites: the principal peaks are at 280–285 and 318–323 nm. In contrast, in the range of wavelengths 370–450 nm the broad absorption band appears to be dependent from MR remarkably. The most pronounced maxima in the UV range are inherent to the medium values of MR, while the long-wavelength part develops more for the lowest and highest MR Ž15 and 206.. These UV maxima were assigned recently to the silver clusters, Ag 8 , according to their observation in solutions w14x, the mass-spectroscopic detection w15x and EXAFS refinement w16x. The broad absorption band with l ) 370 nm is easily interpreted as the plasmon resonance of silver nanoparticles Žwith sizes in the range 1–5 nm. often observable in a number of system with reduced ultrafine silver w17x. The
Fig. 1. DRS spectra of a series of Ža. Ag-, Žb. Cu-, Žc. Ni-exchanged mordenites reduced in hydrogen at 200 8C ŽAg., 450 8C ŽCu., 250 8C ŽNi. with various SiO 2 rAl 2 O 3 molar ratio: Ag, Cu— Ž1–15; 2–20; 3–30; 4–206. and Ni— Ž1–15; 2–20; 3–128; 4–206..
UV peaks of the silver reduced products are described much scarcely, however, species of lower nuclearity were also detected in zeolites w13,18,19x, and numerous fewatomic silver species in solution were pointed out to be of the lower stability w20x. The clusters formed in mordenite Žand some other zeolites with similar size of cavities— erionite, beta, LTL w21x., in contrast, discover little variations under storage during months and years. The formation of the clusters and nanoparticles can proceed through the aggregation of the reduced silver
V.S. Gurin et al.r Materials Science and Engineering C 19 (2002) 327–331
atoms stopped by the cavities andror prolonged in the extra-crystalline region where no geometric obstacles occur for further growth, and diffusion of the reducible Agq determines a maximum size of the particles formed. Mordenite with variable MR serves in these processes as ‘medium’ kinetically controlling the process due to the change of acidity and silver ion concentration. The size of Ag 8 formed can correspond approximately to the cavity dimension Žsee below.. Silver reduced in a less appropriate medium produces aggregates of more size, which depends on the samples with different MR, mainly, on silver ion concentration. Thus, the acidity of the matrix Žgoverned by MR. is the tool optimizing products of the silver reduction. The optimum temperature of Cu2q reduction is higher than for Agq reduction, and the corresponding DRS spectra are shown in Fig. 1b. Under lower temperatures, the sample with MR s 206 is reduced, but another samples reveal no specific features of the reduced copper Žnot shown., and a large amount of initial Cu2q leaves as the long-wave broad band Žsemi-band due to cut of the spectra at l ) 850 nm not available for the device used.. This band is also noticeably developed even for the reduced Cu-mordenites at MR s 15, 20, 30 ŽFig. 1b.. An assignment of this band to the d-d transition of Cu2q-ions in pseudooctahedral position is not complicated, and such appearance is usual for solids with CuŽII. w22x. The principal absorption band in the reduced Cumordenites peaks at 580–600 nm and appears under the highest MR Žfrom the samples under study., MR s 206, but does not present at MR s 15, while it begins to be seen
at MR s 20, and more develops at MR s 31 ŽFig. 1b.. In the cases of low reduction degree ŽMR s 15, 20., the broad long-wave absorption band Ž l ) 600 nm., related to Cu2q, occurs and the common rising of the spectra is seen in the range of l - 400 nm. This absorption band at 580–600 nm can be associated with the plasmon resonance in copper nanoparticles w2,6,17,20x, and we simulated them with the Mie theory ŽSection 4.. As well as in the case of Cu2q-exchanged mordenites, the samples with Ni 2q reveal a noticeable amount of this ionic form, the band at 390–400 nm and the weak doublet, l ; 650 and 720 nm, which is typical for Ni 2q ions, e.g., in ligand octahedral environment, NiŽH 2 O. 62q w22x ŽFig. 1c.. The reduction temperature providing a remarkable effect of MR for Ni-mordenite samples is 250 8C ŽFig. 1c.. If the ‘grey’ background essentially rising the spectral curves Že.g. curve 4. means an appearance of the reduced metal in a form without any pronounced band unlike copper and silver in this range, one can assume that the mordenites with maximum MR value Žs 206. again provide the most efficient reduction with production of metal nanoparticles. The medium values of MR have less reducibility, the bands remain Žcurves 2 and 3. with different intensity, and the band at l ; 390 nm is shifted from sample 3 to sample 2. This can reflect the variation of local Ni 2q environment under different MR Žsample 2 contains more aluminum than sample 3.. The reduced Ni-mordenite samples with different MR display the band peaked at l ; 290 nm which scarcely can be associated with a reduced Ni-containing species since there are no
Table 1 Results of MOLCAO calculations of the selected Ag 8 clusters Initial geometry
Charge
329
Binding energy per
Ag–Ag interatomic
atom, BErn, eV
˚ distances, A
First allowed transitions, l, nm Žin the range l ) 200 nm.
Oh
y 0 q
2.34 2.30 2.35
2.88 2.86 2.89
330; 319; 222; 208 – 319; 306; 221; 207
Td
y 0 q
2.34 2.24 2.30
2.91; 3.92 2.90; 3.19 2.97; 3.32
319; 221; 219 – 319; 314; 211; 206
D4h
y 0 q
2.18 1.81 2.35
2.88; 3.35 2.65; 3.25 2.80; 2.98
461; 242; 237; 235; 234; 227 474; 239 311; 222; 205
C 2v
y 0 q
2.34 2.26 2.34
2.79; 2.91; 2.92 2.85; 2.89; 2.90 2.78; 2.95; 2.95
296; 266 – 323; 299; 209; 203
330
V.S. Gurin et al.r Materials Science and Engineering C 19 (2002) 327–331
changes in its intensity depending on MR ŽFig. 1c.. This band is typical for charge transfer complexes of Ni 2q w22x, and its appearance through different samples means incomplete reducibility of NiŽII. prevented by complexation in zeolite, remnant water within zeolite crystals, formation of silicates and alumosilicates. Thus, unlike Ag and Cu-containing mordenites, in the case of Ni-mordenites we obtained no unambiguous evidence on the formation of any nanoparticles or clusters. The specific plasmon resonance peak of Ni particles would be located at l ; 230 nm w23x.
4. Simulation of clusters and nanoparticles formed in mordenites In order to simulate small silver clusters stabilization, which was supposed within the mordenite cavities, a series of selected geometric structures was calculated at ab initio level by restricted and unrestricted self-consisted field Hartree–Fock methods with the molecular orbital-linear combination of atomic orbitals ŽMOLCAO. approach ŽTable 1. using HONDO7 code w24x. Basis sets were used with the effective core potentials w25x counting 19 valence electrons for Ag atoms. The data obtained for the cluster structures under study ŽTable 1. allow us to propose isomers Ag 8 probable for stabilization within the mordenite cavities. The neutral cubic Ag 8 cluster is slightly more stable than the C 2v isomers, but positively and negatively charged clusters have almost the same value of the binding energy. The charged O h structures are distorted to C 1 –C 2v symmetry. This means a possible coexistence of different isomers at ambient temperature. Electronic transitions estimated for this family reveal that the neutral ones have very high energies of the first allowed transitions, while the transitions for the charged clusters are more reasonable and can match experimental observations: Agq 8 of one type of geometry with C 2v symmetry has the first transitions corresponding to l s 323 and 299 nm and fit the best agreement with the above-observed experimental peaks of DRS data. Thus, we suggest this cluster as a candidate for species stabilized in the mordenite cavities. This cluster possesses also rather short Ag–Ag interatomic distance, and, consequently, it can easily enter a cavity without large distortion. The recent EXAFS refinement w16x evidences the structure for silver clusters Ag 8 stabilized in erionite Žthe type of zeolite with cavities of circular cross-section and near dimensions. similar to that prescribed here. Within the framework of the present paper, we could not detect any unambiguous features assigned to small copper clusters, and we simulate the principal band in copper optical spectra for a model of spherical particles in a medium with the dielectric constant ´o . The optical constants of Cu were used from Ref. w26x, and their size-dependence was accounted only for the imaginary part
by the limitation of the mean free path length of electrons w27x. The calculated absorption spectra ŽFig. 2. display the remarkable effect of particle size and medium dielectric constant. Within the ranges under study, they affect mainly the shape of the absorption feature evolving from the shoulder in the case of low ´o and less particle size to the pronounced peak for higher ´ o and larger particles. On the basis of these conclusions, we can associate the changes in experimental spectra of the reduced Cu-mordenites with the variation of the size of copper particles and their position in mordenite. The particles of this size range Ž) 1 nm. could not fit the intracrystalline cavities unlike the above Ag 8 . They can be located on microcrystal surface Žthis version corresponds to low ´ o . or inside the mordenite in the mesopores and cleaved areas Žmore ´o .. Then, copper nanoparticles produced in mordenites with the low MR are of less size than those formed under the high MR. The higher acidity under the lower MR inhibits the reduction reaction, and particles of less size may be formed in spite of more copper amount exchanged under the more number of substituted Al in such mordenites. Thus, MR value in mordenite appears as the efficient tool of copper reduction.
Fig. 2. Calculated absorption spectra of Cu nanoparticles in the medium with different ´ o for a series of particle radii.
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5. Conclusions Ag-, Cu-, and Ni-ion exchanged zeolites Žon the example of mordenite. were studied for the production of reduced species of the metals with emphasis on clusters and nanoparticles with pronounced optical appearance. The MR value of the mordenite allows us to control the properties of the reduction products. Ag-mordenite system results in both mono-sized clusters Žassigned as 3D polyhewith C 2v symmetry according to quantum dron Agq 8 chemical calculations. and nanometer-range particles. Cumordenites show only the nanoparticles with different shapes of the plasmon resonance absorption band, which was simulated in dependence on particle size and the medium dielectric properties. The effect of the zeolite properties upon the reduction reaction of nickel ions was recorded also, however, without a direct detection of any metal nanoparticles in optical data. Acknowledgements The authors acknowledge funding for this research by CONACYT, Mexico, through grants 32118-E, 31366-U and 1 E120.2403. References w1x R. Bertoncello, F. Trivillin, E. Cattaruzza, P. Mazzoldi, G.W. Arnold, G. Battaglin, M. Catalano, J. Appl. Phys. 77 Ž1995. 1294. w2x R.H. Magruder III, R.F. Haglund Jr., L. Yang, J.E. Wittig, R.A. Zuhr, J. Appl. Phys. 76 Ž1994. 708. w3x A.L. Stepanov, D.E. Hole, P.D. Townsend, J. Non-Cryst. Solids 260 Ž1999. 65.
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