Platinum Metals Rev., 2003, 47, (3), 98
Nucleation and Growth of Platinum Clusters in Solution and on Biopolymers
- By Lucio Colombi Ciacchi
- Theory of Condensed Matter Group, Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 OHE, U.K
- Michael Mertig
- Wolfgang Pompe
- Institut für Werkstoffwissenschaft und Max Bergmann Zentrum für Biomaterialien, Technische Universität Dresden, D-01069 Dresden, Germany
- Sergio Meriani
- Dipartimento di Ingegneria dei Materiali e Chimica Applicata, Università di Trieste, 1-34100 Trieste, Italy
- De Vita Alessandro Email: E-mail: email@example.com
- INFM DEMOCRITOS National Simulation Center, Trieste, Italy
The molecular mechanisms of platinum cluster nucleation and growth in solution and on biopolymers have been investigated by means of first-principles molecular dynamics. In contrast with the classical picture where clusters nucleate by aggregation of metallic Pt(0) atoms, it was found that Pt-Pt bonds can form between dissolved Pt(II) complexes after only a single reduction step. Furthermore, small clusters were observed to grow by addition of unreduced [PtCl2(H2O)2] complexes, in agreement with an autocatalytic growth mechanism. Moreover, Pt(II) ions covalently bound to biopolymers were found to act as preferential nucleation sites for the formation of clusters. This is a consequence of the presence of heterocyclic donor ligands which both enhance the electron affinity of the metal nuclei and induce the formation of metal-metal bonds that are stronger than those obtained in solution. In fact, in metallisation experiments a clean and purely heterogeneous metallisation of single DNA molecules leading to thin and uniform Pt cluster chains extending over several microns was obtained.
The mechanisms through which metal clusters and colloids form in solution have received a great deal of attention in recent years (1– 6). This is due to the importance of achieving accurate control over the processes of cluster nucleation and growth in order to produce clusters of uniform size (7–8) and shape (9). Monodisperse colloidal suspensions present optimal catalytic properties(10), and can be employed for the production of self-assembled particle arrays with peculiar electronic and optical properties (11–13). Moreover,the controlled, selectively heterogeneous metal growth on biomolecular templates allows the fab-rication of nanosized metal structures in a perfectlyclean surrounding medium (14), which is an important step toward the bottom-up fabrication of nanodevices with metallic functionality. To improve the control on all phases of the cluster formation process, a detailed understanding of the elementary steps of cluster nucleation and growth is desirable. However, very little is known at an atomic level about the agglomeration reactions which take place in solution between reduced metal complexes and eventually lead to the formation of metallic clusters.
In this review we report the results of our studies dealing with the molecular mechanisms of the nucleation and growth of platinum nanoparticles after the reduction of a dissolved platinum salt (1,2,14). The [PtCL4]2- ion is chosen as a representative and widely studied example of a square-planar complex capable of producing cluster suspensions upon reduction. Particular attention is paid todefining the minimum reducing conditions necessary to initiate the nucleation of metal clusters and to promoting cluster growth.
We address this problem by means of molecular dynamics techniques taken to the quantum accuracy level (15). Namely, the forces acting on the atoms are computed from first principles, solving the many-body electronic problem within the spin-polarised Density Functional Theory (16).The atoms are then moved according to the classical equation of motion integrated with standard algorithms (17). Both the minimisation of the electronic states and the dynamics of the atoms are performed using the Car-Parrinello (CP) method(18), the gradient-corrected exchange-correlationpotential PW91 (19), and separable, norm-conserving atomic pseudopotentials (20). Given the metallic character of small noble metal clusters(21, 22), our CP simulations are performed with the algorithm proposed in References 23 and 24,which is especially suited to treat metallic systems.
Details of the computational techniques and of the simulation parameters can be found in References 1, 2 and 14. All calculations were carried out on the massively parallel computer platforms of the Center for High Performance Computing at the Technical University of Dresden using the Lautrec code (25). This is a highly optimised parallel code for performing CP molecular dynamics simulations and structure optimisations, originallydeveloped from a serial version by A. De Vita and A. Canning in R. Car’s group at EPFL, Lausanne.
Nucleation of Pt Clusters in Solution
Formation of Pt Dimers
Our investigation begins with a simulation of the reduction of two [PtCl2(H2)O)2] complexes(Figure 1) surrounded by water molecules randomly placed in a periodically repeated cubic cell of edge length 12.0 Å. These complexes are the hydrolysis products of [PtCL4]2– ions which actively take part in the process of metal cluster formation(3). First the system is annealed for about 0.5 ps in a constant temperature first-principles molecular dynamics (FPMD) simulation at 300 K. A second simulation at 300 K is started after one additional electron is added to the annealed system (Figure1(a)). The unpaired electron localises immediatelyon one of the two complexes, causing the detachment of the two water ligands from the central Ptatom and the formation of a linear [PtCl2]– complex (Figure 1(b)). After that, unexpectedly, a Pt-Ptbond forms between the linear Pt(I) complex and the unreduced Pt(II) complex (Figure 1(c)). The formed dimer remains stable until the end of the simulation, which is stopped after ~ 3.5 ps of simulated time. After quenching the atomic motion,the Pt-Pt distance is 2.87 Å and the calculated bond energy of the obtained dimer is 1.5 eV.
A third FPMD simulation at 300 K is started from this point after adding a second reducing electron to the annealed configuration of the obtained Pt(I)-Pt(II) dimer. In this case a chlorine ligand is immediately lost, and the structure of the dimer gradually changes so that after ~ 1.5 ps the Pt-Pt bond is in the same plane as the other bonds that the Pt atoms have with their ligands. The Pt-Pt equilibrium distance is now 2.6 Å, which is a typical value for Pt(I) dimers (26). Indeed, the final geometry (Figure 1(d)) very closely resembles that of the [Pt2Cl4(CO)2]2– ion, which was synthesisedvia the reduction of K2PtCl4 with CO in concentrated HCl solution (27, 28).
Formation of Pt Trimers
Interestingly, both the Pt(I)-Pt(II) and the Pt(I)-Pt(I) dimers obtained in the previous simulations are able to react further with unreduced Pt(II) complexes. This is observed in two FPMD simulations where a [PtCl2(H2O)2] complex approachesthe dimers from the side of the Pt(I) unit, which isexpected to be a reactive site for addition reactions(29). In both cases, a further Pt(II)-Pt(I) bond is formed. Bond distances for the trimers are 2.9 Å for Pt(II)-Pt(I)-Pt(II) and 2.8 Å for Pt(II)-Pt(I)-Pt(I). The same reactions were observed to occur in simulation cells filled with water molecules to simulate the solution environment (Figure 2).
We note that the formation of these oxidised complexes (dimers and trimers) is an intermediate step towards the growth of bigger clusters. In particular, during reduction processes at relatively high concentration of Pt(II) complexes and in the presence of mild reducing agents – typical conditions for the formation of monodisperse colloidal suspensions (3) – Pt(II) complexes are likely to react with partially reduced complexes or dimers before reduction. The reduction (possibly involving dechlorination, as shown in Figure 1(d)), is expected to occur via electron transfer from the reducing agent to the formed nucleus, which has higher electron affinity than isolated complexes due to orbital delocalisation on more than one metal atom site. We expect that the growth to bigger particles requires neither the reduction of isolated complexes before addition to the growing nuclei, nor the complete reduction of the nuclei to the zerovalent state. This issue is investigated in a series of FPMD simulations in the next section.
Growth of Pt Clusters via Addition of Pt(II) Complexes
The simulations presented in the previous section revealed that unreduced [PtCl2(H2O)2]complexes easily form Pt-Pt bonds with open-shell platinum complexes and dimers with average oxidation states intermediate between zero and two. This leads to the hypothesis that analogous reactions may occur on the surface of bigger clusters(not necessarily reduced to the zerovalent state) and be an important process for the overall growth mechanism. Therefore, we have performed direct simulations of the addition reactions of[PtCl2(H2O)2] complexes to growing clusters that are in various oxidation states.
In particular, we consider both a metallic, fully reduced Pt12 cluster and a neutral Pt12CL4 cluster, where the Pt atoms have, on average, a formal oxidation state of +(4/12). The relaxed cluster structures are shown in Figure 3 together with the electronic states that have energy above the Fermi level. It is note worthy that these empty orbitals protrude widely out of the cluster surface, and are thus expected to be highly active acceptor sites in addition reactions. In particular, donation into these orbitals can be expected to occur from the filled dz2orbital of square-planar Pt(II) complexes.
Indeed, the dynamical simulation of the reaction between a [PtCl2(H2O)2] complex and a Pt12Cl4 cluster (Figure 4) begins with the adsorption of the Pt(II) complex on the cluster surface(Figure 4(b)). Unexpectedly, the reaction proceeds with the dissociation of the Pt(II) complex soon after the adsorption. While a water ligand detaches from the Pt(II) atom and remains isolated until the end of the simulation, both chlorine ligands adsorb on the cluster surface (Figure 4(c)). The Pt atom originally belonging to the Pt(H) complex becomes completely incorporated into the skeleton of the cluster and is indistinguishable from the other Pt atoms at the end of the simulation (Figure4(d)). The obtained cluster presents a structure consisting of three stacked planes of Pt atoms arranged in a triangular lattice. This geometry ensures a high mean coordination number of the Pt atoms within the clusters and thus a high cohesive energy (21). The same reaction was also observed to occur for the naked Pt12 cluster, both in gas-phase simulations and in a simulation cell filled with water molecules to model the solution environment Moreover, we observed that a further Pt(II) complex is able to react with the(hydrated) Pt13Cl6 cluster obtained in the simula-tion shown in Figure 4 (2).
A Novel Mechanism of Cluster Formation
The results of our FPMD simulations strongly suggest that the formation of platinum clusters in solution after reduction of a dissolved platinum salt can be initiated by formation of a Pt(I)-Pt(II) dimer immediately after the first reduction step. This is in contrast with the classical nucleation picture where clusters are supposed to form through aggregation of fully reduced Pt(0) atoms only when they are present in solution in sufficient concentration. We propose that, after the very first step, the formation of clusters does not progress through novel reduction of isolated complexes, but via addition of Pt(II) complexes to already formed nuclei. This mechanism is expected to take place, in particular, under conditions of high concentration of Pt(II) complexes and/ or if mild reducing agents are used. We note that under different conditions the full reduction to the zerovalent state prior to cluster nucleation cannot be a priori excluded on the basis of our results.
Interestingly, in our FPMD simulations further steps of cluster growth were in fact found to proceed via the addition of unreduced complexes to a growing, partially oxidised nucleus. The reduction to the metallic state may then happen independently of the addition of complexes and is expected to involve the whole cluster, the electron affinity of which is appreciably bigger than that of isolated complexes. On the other hand, the electron affinity of the cluster is further increased by the addition of Pt(II) complexes, which have the effect of increasing both the cluster and the mean oxidation state of the Pt atoms. In turn,the reduction of the cluster (with possible desorp-tion of Cl- atoms, as shown in Figure 1(d)) favours the further addition of unreduced complexes on the cluster surface. Thus, the processes both of reduction and Pt addition become easier and easier with increasing particle size; that is, our model naturally accounts for an autocatalytic cluster growth. This is fully consistent with the auto-accelerating kinetics of metal particle formation observed experimentally during chemical reduction of metal salts (6).
Nucleation of Pt Clusters on Biopolymer Templates
According to the mechanism suggested above, each Pt(II) complex that reacts with a reducing electron can be thought of as a ‘critical nucleus’ for the growth of metallic platinum clusters. Thus, the kinetics of cluster formation can be influenced by selectively acting on the early steps of the nucleation, and in particular on the very first Pt-Pt bond formation reaction. In the following, we show how this property can be exploited to achieve aselectively heterogeneous metal cluster growth on biological templates, such as DNA and proteins.
Pt(II) complexes are known to react with DNA via covalent bonding between the metal ion and nitrogen atoms of the DNA bases (30, 31). The N7 atoms in guanine and adenine bases are very favourable binding sites. Moreover, similar reactions are known to occur with protein amino acids, and in particular with the N3 site of histidine (32).
In a series of FPMD simulations we now investigate how the heterogeneous formation of Ptdimers occurs between free [PtCL2H2O)2] complexes and Pt complexes bound to DNA bases(adenine and guanine) or histidine amino adds,after a single reduction step. We consider two cases:
reduction of ions bound to the biomolecular template followed by reaction of the bound Pt(I)ions with free Pt(II) complexes, and
reduction in solution followed by reaction of [PtCl2]- molecules with unreduced Pt(II) ions covalently bound to heterocyclic ligands.
The calculated HOMO-LUMO (highest occupied molecular orbital-lowest unoccupied molecular orbital) energy gaps of Pt(II) complexes bound to an adenine, a guanine, and two stacked guanines are 2.23, 2.25, and 1.13 eV, respectively. For comparison, the free [PtCl2(H2O)2] complexpresents a higher energy gap of 2.34 eV. Thesevalues indicate that the reduction of Pt(II)-bio-polymer complexes should be favoured over the reduction of free complexes in solution. The higher electron affinity of the Pt(II).biopolymer complexes can be associated with the presence of delocalised orbital states on the heterocyclic ligands (such as guanine, adenine and histidine),which can ‘accommodate’ the additional electronin a more favourable way. Moreover, as we explainbelow, the presence of heterocyclic ligands has theeffect of stabilising the Pt-Pt bonds formed during the investigated dimerisation reactions. Indeed, in all the simulated heterogeneous dimerisation reactions (14, 33), we observe that immediately after the Pt-Pt bond formation, a water ligand detaches from the Pt(II) complex. Loss of this water ligand considerably strengthens the Pt-Pt bond (Figure 5).
The obtained Pt-Pt equilibrium distances are ~ 2.6 Å in all cases and the bond energies are between 1.8 and 2.5 eV, depending on the hydrolysis states of the formed dimers (14, 33). Therefore, all heterogeneously-formed Pt dimers appear to be more stable than the corresponding homogeneously-formed dimers. The reason for the increased dimer stability lies in the observed water substitution step occurring during the dimer formation reaction which is only possible in the presence of heterocyclic ligands with a strong donor character, such as purine bases or histidine amino acids. In fact, the charge density accumulation on the Pt atoms, induced by the presence of the heterocyclic ligands, leads to a highly repulsive antibonding interaction between the Pt(II) atomand one of its water ligands (Figure 5(b, c)). The water ligand eventually detaches causing a strengthening of the Pt-Pt bond. This in turn has the effect of increasing the electron affinity of the dimer (14), and thus favours further steps of reduction and complex addition to the growing nucleus. These results suggest that, going through the autocatalytic growth process described in the previous sections, the first-formed heterogeneous nuclei may quickly develop into larger particles, consuming the metal complex feedstock present in the solution and thus suppressing homogeneous particle formation.
Selectively Heterogeneous Cluster Growth on Biopolymers
To substantiate the results of the simulations reported above we performed metallisation experiments in which an aged solution of K2PtCl4 was reduced in the presence of DNA molecules (14). To form Pt(II)-DNA adducts in sufficient concentration, we incubated a 1 mM aged solution of the Pt(II) metal salt with λ-DNA for about 24 hours,keeping the ratio of Pt(II):base at 65:1. This large Pt(II):base ratio is used both to bind as many complexes as possible to the DNA bases (primarily, but not exclusively, to guanine and adenine), and to create a sufficient reservoir of metal atoms for the formation of metallic clusters after the reduction of the whole solution. Indeed, after 24 hours of activation about 3 to 5% of the total complexes are expected to be bound to the DNA. As shown in the previous section, these Pt(II)-DNA complexes are expected to be reduced and to form a nucleus more easily than free complexes in solution. Therefore, the growth of metallic particles (proceeding through steps of addition of Pt(II)complexes and reduction of the whole cluster) is expected to occur preferentially starting from the complexes bound to the bases.
After the ‘activation’ step, dimethylaminoborane is added to the solution in excess with respect to the Pt(II) concentration, to induce metal particle formation. TEM imaging of the metallisation products reveals that the chemical reduction of the Pt salt in the presence of activated DNA indeed occurs through a purely heterogeneous reaction channel, and that continuous chains of Pt clusters of size ~ 5 nm aligned along the entire DNA molecule are obtained (Figure 6).
Remarkably, the homogeneous nucleation channel is completely suppressed, as proven by the absence of homogeneously nucleated particles in the background of the sample. An atomic force microscope analysis of the metallisation products revealed that a purely heterogeneous nucleation of clusters is obtained only after long activation times(that is, when a large number of Pt(II) complexes are bound to the DNA bases). When reduction is started immediately after the addition of Pt(II) complexes to a DNA solution, besides a small number of heterogeneously formed clusters, bigger aggregates of homogeneously nucleated particles are found on the sample (14). Moreover,the increasingly heterogeneous reaction channel is linked to an acceleration of the overall metallisation kinetics, with increasing activation times (14).
These results confirm that heterocyclic organicligands (such as DNA bases) actively promote the heterogeneous nucleation of metal clusters according to the mechanism predicted by theoretical simulations reported in the previous sections. The active effect of the DNA template in promoting the formation of metallic particles enables, in particular, the fabrication of exceptionally thin and regular necklaces of platinum clusters, as shown in Figure 6.
Finally, in very recent work (34), palladium clusters were grown on the surface of cytoskeletal proteins (microtubules) by a two-step procedure similar to the one we used to metallise DNA. It was demonstrated that, after incubation of the microtubules with palladiumions and subsequent reduction, palladium clusters grow on the microtubule surface forming particle arrays which reflectthe helical symmetry of the underlying assembly of tubulin monomers. By looking at the correlation between the amino acid positions in tubulin monomers and the distribution of palladium clusters on a microtubule, the authors suggest that the selective arrangement of clusters along the biotemplate is induced by histidine amino addslocated on the outer microtubule surface (34). On the basis of our results, we believe that the metalclusters are in fact grown in situ, selectively at the histidine sites to which Pd(II) ions were previously bound after the incubation of the microtubuleswith a Pd salt solution.
We studied the molecular mechanisms of the nudeation and growth of platinum dusters after the reduction of K2PtCl4, both free in solution and supported on biopolymers. The obtained results are explained by an autocatalytic mechanism of metal cluster formation, where Pt clusters nucleate and grow via the addition of unreduced Pt(II) complexes to partially-reduced Pt complexes or to small clusters.
We note that the observed reactions are fully in agreement with a surface-growth cluster mechanism which was proposed to account for a shape-controlled cluster fabrication (9), based on the competition between the effect of surface stabilisation by capping agents and the addition of new complexes to reactive surface sites.Interestingly, the addition reactions take place with a considerable rearrangement of the cluster structure and involve a redistribution of the ligands on the cluster surface. This suggests that, in general, low-energy structure rearrangements and ligand fluxionality should be taken into account for achieving a controlled growth of noble metal clusters in reduction baths.
Moreover, we found that Pt(II) complexes covalently bound to heterocyclic donor ligands, such as purine DNA bases or histidine amino acids, can act as preferential nucleation sites for the formation of metal particles. In particular, we were able to grow Pt clusters on DNA molecules by a selectively heterogeneous metallisation protocol that avoids any spurious formation of metal clusters in the solution while remaining very simple, and thus is suitable for industrial processes. This approach could open the way to a wide range of applications in nanotechnology, based on the selective metallisation of nanostructured biotemplates formed, for example, by exploiting the properties of self-assembly and molecular recognition of biopolymers (35). The possibility of selectively metallising DNA molecules previously inserted into a lithographically-fabricated circuit structure appears to be particularly interesting for nanoelectronics (36–39). Finally, thin, uniform metal films or arrays of metal particles could be grown onto insulating substrates functionalised with a monolayer of organic molecules terminated with imidazole or similar groups, by electroless in situ metallisation. In this case it should be possible to structure the organic layer previously (for instancevia imprinting or analogous techniques (40)) to achieve a complex metallic pattern after selective metallisation.
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We are grateful to R. Seidel (TU Dresden) for the fruitful collaboration and cowork. L. C. C. acknowledges support from the EPSRC. M. M. and W. P. acknowledge the Bundesministerium für Bildung und Forschung and the Deutsche Forschungsgemeinschaft for financial support. All computational resources were provided by the Center for High Performance Computing at TU Dresden.
Lucio Colombi Ciacchi is a Postdoctoral Research Associate at the Cavendish Laboratory of the University of Cambridge, U.K. Hismain research field is the first-principles modelling of clusters,surfaces and biological systems.
Michael Mertig is the Head of the BioNanotechnology andStructure formation (BNS) group at the Max Bergmann Center ofBiomaterials, Dresden, Germany. His main research interest is theinvestigation of biomimetic processes and their application in anengineering context.
Michael Mertig is the Head of the BioNanotechnology andStructure formation (BNS) group at the Max Bergmann Center ofBiomaterials, Dresden, Germany. His main research interest is theinvestigation of biomimetic processes and their application in anengineering context.
Wolfgang Pompe is the Professor of Materials Science and Nanotechnology at the Technische Universität Dresden, Germany.His main research fields are the mechanical behaviour of ceramicsand thin films, functional ceramics, formation of nanostructures byusing biological templates, and hard tissue engineering.
Sergio Meriani is a Professor of Materials Science and Technology at DIMCA, University of Trieste, Italy. His main research interestsare in solid state chemistry and in flame retardant materials.
Alessandro De Vita is Lecturer of Materials Science at DIMCA,University of Trieste, Italy. His main research interest is the development and application of novel techniques for electronic-structure-based atomistic modelling of materials. These techniques are specifically designed to elucidate the microscopic mechanisms underlying the properties of materials, particularly intechnological applications such as the development of new materials and devices of nanometre size scale.