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846 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1998 0935-9648/98/1108-0846 $ 17.50+.50/0 Adv. Mater. 1998, 10, No. 11

Synthesis of Layered Platinum-Based MaterialsThrough Thermal Decomposition ofSelf-Assembled Metal Carbonyl±SurfactantPhases**

By Timothy Bell and David M. Antonelli*

The templating of inorganic materials onto self-as-sembled surfactant phases is an area of much current inter-est.[1±4] By coprecipitation of surfactants with inorganic sol-gel precursors a wide variety of structures can be obtainedin which the condensed inorganic material assumes theshape of the liquid-crystalline surfactant phase. In eachcase the appropriate interaction between the surfactanthead group and the inorganic phase is necessary to directthe templating interaction. This can be achieved throughcharge interactions,[5] hydrogen bonding,[6] or ligand inter-actions[7] in which there is a discrete chemical bond be-tween the surfactant head group and a metal center in theinorganic phase. Through the selection of the appropriatesurfactant and conditions, layered, cubic, and hexagonalstructures can be obtained. One of the most well-known ex-amples of this is the templating of silica onto self-assembledtrimethylammonium surfactant micelles to form hexago-nally packed mesoporous silicate MCM-41.[1,8] While re-moval of the surfactant from layered phases causes collapseof the structure, cubic and hexagonal structures often retaintheir shape after the removal of the organic matter. Thesematerials can also be deposited as films for use in nano-scale sensors and size-selective membranes.[9] Control overinternal porosity and particle morphology can be achievedby varying chain length and synthesis conditions. More re-cent work has extended this approach to synthesize meso-porous niobium,[7a,b] tantalum,[7c] titanium,[7d] zirco-nium,[7e,10] hafnium,[11] and manganese oxides.[12] However,very little has been done in terms of extending research ef-forts to the synthesis of liquid-crystal templated materialsbased on these non-oxide substances.

Due to the importance of zero-valent metals and metalalloys in catalysis,[13] the extension of this self-assembly ap-proach to template metals or metal precursors onto self-assembled surfactant phases is of great interest. A recentreport described the synthesis of hexagonally packed nano-structured platinum by coprecipitation of platinum saltswith trialkylammonium surfactants.[14] In this approach areducing agent such as Fe, Mn, or Zn was required to re-

duce the PtII salt to metallic platinum. The use of milderreducing agents led to loss of structure. From this theauthors concluded that it was important to reduce the ma-terial very quickly to ensure that the diminished charge in-teraction between the neutral zero-valent platinum phaseand the cationic surfactant phase did not lead to loss ofstructural integrity. Although the surfactant could beleached out with retention of structure as determined bytransmission electron microscopy (TEM), the material dis-played a broad diffraction pattern, indicating either smalldomains of hexagonal order or large amorphous regions inthe sample. The surface area of this material was not re-ported.

Because of the great flexibility of composition, charge,and size and the relative ease of thermal decomposition tothe pure metallic state, ionic metal carbonyl clusters areideal building blocks for nanostructured metals. While thereduction of metal cation±containing self-assembled surfac-tant structures leads to loss of charge and possible loss ofstructure, owing to the diminished force of the ionic tem-plating interaction, loss of CO from metal carbonyl clustersis a neutral process in which the charge should be retainedin the metallic phase. The coupling of CO loss and metal±metal bond formation with self-assembly of surfactants canbe seen as being closely analogous to the hydrolysis andcondensation of silicates in the presence of organic tem-plating agents (Scheme 1). In the case of silicate condensa-tion from alkoxide precursors silicon±oxygen bridges areformed with concomitant loss of neutral H2O. In the caseof the condensation of metal carbonyls, metal±metal bondsare formed with accompanying loss of neutral CO.

Because platinum is highly active in a wide variety of cat-alytic processes[15] and its carbonyl complexes are espe-cially prone to CO loss with large cluster formation, wechose to focus on the self-assembly of platinum carbonylanions with cationic surfactants. In previous work, solutionsof sodium hexachloroplatinate in methanol were treatedwith sodium hydroxide in the presence of carbon monoxideto give the anions [Pt3(CO)6]n

2± (n = 2±6) in high yield.[16]

The value of n, and hence the charge-to-metal and metal-to-surfactant ratio of the cluster, can be tuned by control-ling the amount of hydroxide present in the initial synthesismixture. Size and charge are crucial features in determiningthe phase of metal oxide (lamellar, cubic, or hexagonal asobserved in the MS-41 family of molecular sieves) formedfrom the templating of metal oxide clusters with surfac-tants.[2a] This suggests that it may be possible to tune thephase of the platinum carbonyl±cetyltrimethylammoniummesostructure by altering the charge-to-metal ratio of theplatinum cluster employed by controlling the amount ofhydroxide in the synthesis mixture.

Solutions of Na2[(Pt3(CO)6)n] (n = 3±5) in methanolwere treated with an excess of cetyltrimethylammoniumchloride and concentrated under a flow of CO until precipi-tation of the metal carbonyl salt occurred. Degassed waterwas then added until precipitation was complete. The solu-

±

[*] Prof. D. M. Antonelli,[+] Dr. T. Bell[+]

School of Chemistry, Physics, and Environmental SciencesUniversity of SussexFalmer, Brighton, East Sussex, BN1 9QJ (UK)

[+] Current address: Department of Chemistry, University of Windsor,Windsor, Ontario, N9B 4P4, Canada.

[**] The authors are grateful to Johnson Matthey and The Royal Societyfor financial support and Julian Thorpe for his TEM work. GeoffreyOzin is thanked for helpful discussions.

tion was filtered and the resulting solids dried undervacuum to yield the red, blue±green, and yellow±greensalts [C16H33N(CH3)3]2[(Pt3(CO)6)3], [C16H33N(CH3)3]2-[(Pt3(CO)6)4], and [C16H33N(CH3)3]2[(Pt3(CO)6)5], re-spectively. While [C16H33N(CH3)3]2[(Pt3(CO)6)3] and[C16H33N(CH3)3]2[(Pt3(CO)6)4] are air sensitive, [C16H33-N(CH3)3]2[(Pt3(CO)6)5] is stable in air for several hours.The infrared spectra (IR) and elemental analyses of thesematerials were consistent with the formulation of these ma-terials as cetyltrimethylammonium salts of each of the cor-responding anions.[16] In each case the CO bands were onlyslightly shifted from the values originally reported for thecorresponding bis(triphenylphosphoranylidene) ammo-nium (PPN) salts.[17]

Figure 1a shows the powder X-ray diffraction (XRD)pattern obtained for [C16H33N(CH3)3]2[(Pt3(CO)6)5] usingCu Ka radiation (1.54 �, clearly demonstrating the layerednature of the material with peaks at 19.6 �, 9.8 �, and6.4 �. This is consistent with interdigitation of the tails of

surfactant molecules on adjacent clusters as opposed to alamellar phase such as MCM-50 mesostructured silicate,which has two surfactant molecules per layer packed tail totail. Unlike MCM-50 and related layered metal oxidesthere are higher order peaks in the region above 2y = 15�.This indicates that the structural integrity of the metal clus-ter has been retained and that the individual metal clusters,rather than being randomly ordered from layer to layer,display long-range order in the walls of the material. At-tempts to grow crystals of this material large enough for asingle-crystal X-ray structure led only to the formation oflarge plates unsuitable for X-ray analysis. Interestingly,the powder patterns obtained for [C16H33N(CH3)3]2-[(Pt3(CO)6)3] and [C16H33N(CH3)3]2[(Pt3(CO)6)4] are

virtually identical to that obtained for [C16H33N(CH3)3]2-[(Pt3(CO)6)5]. This indicates that changing the charge-to-metal-to-surfactant ratio alters only the interlayer packingdensity of the surfactants but not interlayer spacing or thephase of the material as anticipated. This implies that thelong axis of the triangular cluster blocks lie parallel to theplane of the interdigitated surfactant lamellae and so thespacing will not vary as the cluster length grows. In pre-vious studies[18] it was found that decreasing the surfactant-to-metal ratio in a wide variety of metal oxide phases fa-vored formation of cubic and hexagonal phases overlayered phases.

In an effort to obtain a hexagonal phase related toMCM-41 and the material isolated by Attard et al.,[14] theplatinum carbonyl dianions [(Pt3(CO)6)n]2± (n = 3±5) wereprecipitated out with one rather than two equivalents ofsurfactant. This, however, had no effect on the phase preci-pitated. Instead, the yield of the final product was reducedby approximately fifty percent, consistent with incompleteprecipitation of the salt caused by the change in stoichiom-etry. Increasing the concentration of the solution and/orprecipitation directly from cold methanol without the addi-tion of water had no effect on the isolated phase. This maybe because the combination of a singly charged surfactantwith a doubly charged one results in effectively a double-tailed surfactant system, which invariably favors lamellarand interdigitated phases over hexagonal phases.

TEM samples of [C16H33N(CH3)3]2[(Pt3(CO)6)5] wereprepared by suspension of the salt in hexanes followed bysonication and mounting on carbon-coated grids. The TEMimage of this material obtained at 100 kV (Fig. 2a) showslamellar regions with an interlayer spacings of approxi-mately 20 �, consistent with the XRD data. Lattice spac-ings in the walls were not visible under the conditions em-ployed and gaps between individual clusters were not visi-ble. Amorphous regions were not observed, indicating thatthe majority of the sample displays the layered structure asexpected.

In an effort to eliminate CO from these materials, to linktogether the individual clusters in the layers of this materialand obtain a purely metallic surfactant phase, metal car-bide, or oxide carbide surfactant phase, the material[C16H33N(CH3)3]2[(Pt3(CO)6)5] was heated at 70 �C underdynamic vacuum while monitoring the CO region of the IRspectrum. This synthetic strategy is outlined in Scheme 2.After two days at this temperature all CO bands had van-ished from the spectrum. The XRD pattern of this newgray material (Fig. 1b) shows that the layered structure ofthe material was retained with only a slight shrinkage ofthe interlayer spacing with a d(100) at 19.1 � and new

Adv. Mater. 1998, 10, No. 11 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1998 0935-9648/98/1108-0847 $ 17.50+.50/0 847

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Fig. 1. XRD patterns of: a) cetyltrimethylammonium salt of [Pt3(CO)6]52±;

b) sample from (a) after heat treatment at 70 �C at 10±3 torr for two days;c) sample from (b) after further heat treatment at 120 �C at 10±3 torr for twodays, showing a broad hump typical of poorly ordered hexagonal phases.

Scheme 1. Formation of metals by thermalor photolytic decomposition of metal car-bonyls is analogous to the sol-gel process inthat building of oxide bridges by hydrolysisand condensation of alkoxides is compar-able to loss of CO with metal±metal bondformation in metal carbonyl clusters.

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848 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1998 0935-9648/98/1108-0848 $ 17.50+.50/0 Adv. Mater. 1998, 10, No. 11

peaks in the range 11� < 2y < 16�. The fact that the higherangle peaks were retained is consistent with the materialretaining higher order in the walls after loss of CO. Thesepeaks do not index to any known phase of platinum metal.

The TEM micrograph of this material, shown in Fig-ure 2b, clearly demonstrates the layered nature of this ma-terial. The lack of regularly spaced gaps in the platinumlayers suggests that the individual platinum carbonyl clus-ters have condensed together, however, these gaps may betoo small to observe under the conditions of the experi-ment. Further evidence for the interconnectivity of the

clusters in this material lies in the fact that it is insoluble inpolar and non-polar solvents whereas the parent carbonylphase dissolves in warm methanol.

The elemental analysis of this material (14.41 % C,2.28 % H, 0.58 % N) is consistent with loss of CO and re-tention of the surfactant. The percentage of platinum asmeasured by inductively coupled plasma (ICP) was 82.13.This is consistent with the formula Pt15C38H84N2 (13.05 %C, 2.42 % H, 0.86 % N, 83.67 % Pt), although the slightlyhigher level of carbon observed suggests that there is resi-dual carbon monoxide or carbide left in the platinum layer,which may be necessary to stabilize the structure. Decom-position of metal carbonyls to metal carbides and carbideoxides is a well-documented process.

The 13C MAS NMR (68 MHz, cross polarization) spec-trum of this material recorded from ±200 ppm to 500 ppmdisplayed resonances in the 14 to 32 ppm range, which canbe assigned to the surfactant carbons. Samples of this mate-rial prepared by enriching a solution of Na2[(Pt3(CO)6)5]with 13CO followed by precipitation with cetyltrimethylam-monium chloride and heat treatment at 70 �C for two daysshowed a series of low-intensity resonances from 203±221 ppm in the 13C MAS NMR spectrum. This is consistentwith small amounts of either residual carbon monoxide orcarbide, however carbide resonances are usually broad andappear much further downfield. The presence of oxide inthe material was ruled out by elemental analysis and thelack of an oxygen peak in the energy dispersive X-ray spec-trum (EDS) of the material. Further heating of this materi-al at 120 �C for two days leads to a new black material withan XRD pattern with a broad hump centered around d =36 �. This pattern closely resembles that displayed bymesoporous materials in an early stage of formation or de-composition with randomly ordered tubes.[8,19] Transforma-tions from a layered to a hexagonal phase have been ob-served before,[20] however TEM images of this sample showa completely amorphous structure. The C, H, and N ele-mental analysis of this material was virtually identical tothat of the material heated at 70 �C for two days, indicatingthat there was no further loss of CO or surfactant, but onlya reordering of the structure into an ill-defined mesostruc-tured phase. Further heating of this material at 150 �C ledto loss of all order as determined by XRD.

In summary, the precipitation of platinum carbonyl an-ions of the type [(Pt3(CO)6)n]2± (n = 3±5) with cetyltri-methylammonium chloride has led to layered platinum car-

Fig. 2. a) Low-resolution TEM image at 100 kV of cetyltrimethylammoniumsalt of [Pt3(CO)6]5

2±, showing layered regions with ca. 20 � spacings.b) Sample from (a) after heat treatment at 70 �C at 10±3 torr for two days.The higher magnification image shows that the lamellae are indeed continu-ous, consistent with condensation between neighboring platinum carbonylclusters.

Scheme 2. Thermal treatment of layeredplatinum carbonyl phases results in loss ofCO and formation of a new platinum-basedlayered material. This new strategy showspromise as a new method of preparingnanostructured metals and metal alloys.

bonyl phases, which lose CO under thermal conditions toform layered platinum-based phases. Prolonged heating atthis temperature results in a complete loss of structure asdetermined by XRD. These results suggest that this newapproach of employing metal carbonyl clusters as buildingblocks for mesostructured metals is promising and maylead to isolation of novel metallic nanostructures with hex-agonal, cubic, and layered structures.

Received: December 19, 1997Final version: March 16, 1998

±[1] a) C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartulli, J. S. Beck,

Nature 1992, 359, 710. b) J. S. Beck, J. C. Vartuli, W. J. Roth, M. E.Leonowicz, C. T. Kresge, K. D. Schmitt, C. T.-W. Chu, D. H. Olson,E. W. Shepard, S. B. McCullen, J. B. Higgins, J. L. Schlenker, J. Am.Chem. Soc. 1992, 114, 10 834.

[2] D. M. Antonelli, J. Y. Ying, Curr. Opin. Colloid Interface Sci. 1996, 1,523.

[3] P. Behrens, Angew. Chem. Int. Ed. Engl. 1996, 35, 515.[4] D. Walsh, S. Mann, Nature 1995, 377, 320.[5] a) Q. Huo, D. I. Margolese, U. Ciesla, D. G. Demuth, P. Feng, T. E.

Gier, P. Sieger, A. Firouzi, B. F. Chmelka, F. Schuth, G. D. Stucky,Chem. Mater. 1994, 6, 1176. b) A. Firouzi, D. Kumar, L. M. Bull, T. Be-sier, P. Sieger, Q. Huo, S. A. Walker, J. A. Zasadzinski, C. Glinka, J.Nicol, D. Margolese, G. D. Stucky, B. F. Chmelka, Science 1995, 267,1138.

[6] P. T. Tanev, M. Chibwe, T. J. Pinnavaia, Nature 1994, 368, 321.[7] a) D. M. Antonelli, J. Y. Ying, Angew. Chem. Int. Ed. Engl. 1996, 35,

426. b) D. M. Antonelli, A. Nakahira, J. Y. Ying, Inorg. Chem. 1996,35, 3126. c) D. M. Antonelli, J. Y. Ying, Chem. Mater. 1996, 8, 874. d)D. M. Antonelli, J. Y. Ying, Angew. Chem. Int. Ed. Engl. 1995, 34,2014. e) M. S. Wong, D. M. Antonelli, J. Y. Ying, Nanostruct. Mater.1997, 9, 165.

[8] C.-Y. Chen, S. L. Burkette, H.-X. Li, M. E. Davis, Microporous Mater.1993, 2, 27.

[9] H. Yang, N. Coombs, I. Sokolov, G. A. Ozin, Nature 1996, 381, 589.[10] U. Ciesla, S. Schacht, G. D. Stucky, K. Unger, F. Schuth, Angew. Chem.

Int. Ed. Engl. 1996, 35, 541.[11] P. Liu, J. Liu, A. Sayari, Chem. Commun. 1997, 577.[12] Z. R. Tian, J. Y. Wang, N. G. Duan, V. V. Krishnan, S. L. Suib, Science

1997, 276, 926.[13] a) G. Suess-Fink, G. Meister, Adv. Organomet. Chem. 1993, 35, 41. b)

Metal Clusters in Catalysis (Eds: B. C. Gates, L. Guczi, H. Knoezin-ger), Elsevier, Amsterdam 1986. c) E. L. Muetterties, M. J. Krause,Angew. Chem. Int. Ed. Engl. 1983, 22, 135.

[14] G. S. Attard, C. G. Göltner, J. M. Corker, S. Henke, R. H. Templer,Angew. Chem. Int. Ed. Engl. 1997, 36, 1315.

[15] G. A. Somorjai, An Introduction to Surface Chemistry and Catalysis,Wiley, New York 1994.

[16] a) P. Chini, G. Longoni, V. G. Albano, Adv. Organomet. Chem. 1976,14, 285. b) G. Longoni, P. Chini, J. Am. Chem. Soc. 1976, 98, 7225.

[17] For n = 5: nCO (nujol) 2054, 1887, 1871, 1840, 1832 cm±1. Elementalanalysis calc. for C68H84N2O30Pt15: C, 18.83; H, 1.95; N, 0.65. Found: C,17.81; H, 1.62; N, 0.41.

[18] J. C. Vartuli, K. D. Schmitt, C. T. Kresge, W. J. Roth, M. E. Leonowicz,S. B. McCullen, S. D. Hellring, J. S. Beck, J. L. Schlenker, D. H. Olson,E. W. Sheppard, Chem. Mater. 1994, 6, 2317.

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Ultrathin Diblock Copolymer/TitaniumLaminatesÐA Tool for Nanolithography

By Joachim P. Spatz, Peter Eibeck, Stefan Möûmer,Martin Möller,* Thomas Herzog, and Paul Ziemann

Significant progress has been achieved in various litho-graphic methods based on X-ray, electron, or ion beamtechniques, enabling the preparation of structures smallerthan 100 nm.[1] There is, however, an obvious need forcomplementary approaches that allow easy and reliablepatterning in the nanometer range.[2±5] Based on molecu-lar concepts from organic[6] and macromolecular chemis-try[7±11] efforts are being made to develop biomimeticªbottom upº approaches where molecules assemble intowell-defined functional nanostructures.[12] Ultimately thiscan be expected to lead to an entirely new nanotechnologybased on molecular wires and switches. Here we report anintermediate approach using a self-assembled diblock co-polymer mask in combination with a conventional ion etch-ing technique for the preparation of mesoscopic periodicªpointº patterns in an inorganic substrate.

The microdomain pattern of a thin diblock copolymerfilm can be exploited to etch a 3D surface pattern into anunderlying substrate.[13,14] The usability and versatility ofsuch an etching process depend strongly on the aspect ra-tios that can be achieved, i.e., the maximum length-to-depth ratio of the etched pits. So far, the diblock copolymerapproach has allowed the etching of structures with an as-pect ratio of 1±2.[13,14]

We have studied the formation of ultrathin films of apolystyrene-block-poly(2-vinylpyridine) diblock copolymeron mica, i.e., a highly polar ionic substrate.[15,16] Applied ina CHCl3 solution the 2-vinylpyridine block is strongly ad-sorbed and forms a wetting layer of only about 1 nm thick-ness. Due to the unfavorable interaction and elastic contri-butions to the free energy, the polystyrene blocks do notcover the layer of adsorbed poly(2-vinylpyridine) butrather dewet to yield defined and regularly arranged clus-ters as depicted in Scheme 1.

Scheme 1.

Adv. Mater. 1998, 10, No. 11 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1998 0935-9648/98/1108-0849 $ 17.50+.50/0 849

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_______________________±

[*] Prof. M. Möller, Dr. J. P. Spatz, P. Eibeck, S. MöûmerOrganische Chemie III±Makromolekulare ChemieUniversität UlmD-89081 Ulm (Germany)

T. Herzog, Prof. P. ZiemannAbteilung Festkörperphysik, Universität UlmD-89081 Ulm (Germany)