NANO LETTERS
Stable Cluster Motifs for Nanoscale Chromium Oxide Materials
2004 Vol. 4, No. 2 261-265
Denis E. Bergeron and A. Welford Castleman, Jr.* Departments of Chemistry and Physics, 152 DaVey Laboratory, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802
Naiche O. Jones and Shiv N. Khanna Physics Department, Virginia Commonwealth UniVersity, Richmond, Virginia 23284-2000 Received November 7, 2003; Revised Manuscript Received December 9, 2003
ABSTRACT We show that by varying the formation conditions, two distinct families of stable chromium oxide nanoparticles can be generated, each with unique electronic and magnetic properties. Both families exhibit a strong tendency to scavenge electronic charge, making them potentially valuable as catalysts. What is more interesting is that one family is found to have the magnetic characteristics of tiny bar magnets and may therefore find use in the design of novel magnetic materials.
A major impediment in creating nanoscale materials with useful electronic and magnetic properties has been the identification and production of classes of clusters with structural motifs that simultaneously display the stability requisite for implementation as building blocks and the preservation of electronic and magnetic properties upon growth within this motif. Herein, we demonstrate the existence of a class of oxygen-passivated chromium oxide clusters with specific electronic and magnetic properties. The clusters in the shape of single or multiple cages are stable toward oxidation, making them ideal candidates for the building blocks of novel nanoscale materials. Remarkably, irrespective of the size, the multiple joined cages are ferromagnetic and ideal for exhibiting magnetic shape anisotropy, enabling them to act as nanoscopic bar magnets. In addition, we demonstrate the existence of a completely separate class of stable clusters, which are nonmagnetic. Theoretical and experimental studies of metal clusters over the past decade1-3 have shown that physical, chemical, electronic, and magnetic properties at small sizes can be very different from the bulk. The differences in properties arise not only because of the preponderance of surface sites but also because the geometrical arrangements and electronic energy levels can be very different from the bulk and change with size. Furthermore, in mixed clusters, the compositions can be varied over a wider range than in bulk solids. These diversities allow us to design clusters with desirable traits that could eventually lead to materials with tailored properties. The biggest challenge in using clusters to make new materials, however, is that many clusters coalesce when * Corresponding author. E-mail:
[email protected]. 10.1021/nl034997q CCC: $27.50 Published on Web 01/22/2004
© 2004 American Chemical Society
assembled, thus negating the peculiar characteristics that arise in species of finite dimension. This can be overcome if one can identify clusters that are particularly stable, chemically inert, and easily produced. One way to accomplish this is to carry out a systematic investigation of the reactivity of small clusters and identify the magic species. This communication represents a step in this direction by investigating the properties of fully oxidized, and therefore oxygen-insensitive, chromium-based clusters. Our interest in these clusters stems partly from their magnetic properties. Bulk chromium oxide usually forms4 at stoichiometries corresponding to CrO2 and Cr2O3. CrO2, widely used in magnetic data storage media, has a rutile structure and is metallic and ferromagnetic. This is rather unusual because most metal oxides are nonmetallic and antiferromagnetic. Chromium oxide materials become even more intriguing upon consideration of the other stable bulk oxide, Cr2O3. It has a corundum structure and is antiferromagnetic. In the bulk, then, it is known that the magnetic properties of chromium oxides are highly sensitive to changes in stoichiometric composition. In this communication, we show how this behavior evolves at the reduced size and can be used to design stable species with size- and composition-specific properties. We identify two specific families of CrnOm clusters and demonstrate that the magnetic properties are preserved upon cluster growth within the specific motif. There have been some previous studies on the CrnOm clusters,5-12 most considering only substoichiometric, nonsaturated species. Aubriet and Muller5 have recently generated MnOm (M ) Cr, Mo, and W) clusters through laser ablation of MO3. For CrnOm clusters, their mass spectrum
extends up to clusters containing six Cr atoms. Whereas they do observe large intensities for Cr2O4+, Cr3O6+, and Cr4O10+, their approach favors the formation of highly oxygenated cluster cations. On the theoretical side, Veliah et al.6 carried out density functional calculations on CrnOm clusters containing up to two Cr and three O atoms. Their calculated vibrational frequencies were in good agreement with experiment for these small clusters. Reddy and Khanna7 examined the evolution of magnetic interaction between Cr2 spins as O atoms were successively added to form Cr2Om clusters. They showed that the magnetic coupling oscillates from antiferromagnetic to ferromagnetic to antiferromagnetic. Tono et al.13 have also recently shown that whereas the Cr spins in a Cr2 molecule are antiferromagnetically coupled, the ground state of Cr2O- is a ferromagnet. In contrast to the above, our approach of first generating substoichiometric CrnOm clusters and then reacting them with oxygen shows the formation of a distinct family of saturated oxide clusters. In addition, oxide clusters formed under energetic conditions whereby oxygen is introduced at the cluster source are shown to represent a separate family of saturated clusters. With respect to the formation of clusters via laser ablation of CrO3, our approach allows for more active control of composition and thus control of the magnetic properties. The chromium oxide clusters treated here were generated in the fast-flow tube reaction apparatus described in detail elsewhere.14 In the present experiments, a translating and rotating chromium rod (Research and PVD Materials, 99.7%) is laser ablated in the presence of He carrier gas (8000 sccm). It has been shown previously that chromium’s susceptibility to oxidation can lead to the formation of substoichiometric (oxygen-deficient) oxide clusters even without the “intentional” introduction of oxygen.12 In this study, the carrier gas and cluster species exit the source through a conical nozzle, entering the flow tube that is typically maintained at ∼0.32 Torr; higher pressures can assist in the generation of larger cluster species. The thermalized chromium oxide clusters are then reacted with oxygen introduced through a flow-controlled reactant gas inlet (RGI). Reactants and products are sampled through a 1-mm orifice and analyzed via quadrupole mass spectrometry. The reactions were carried out under oxygen-rich conditions to generate fully oxidized products. We also used another approach to generate oxygenated clusters. In this alternative scheme, the oxygen (∼2.5%) was seeded in He at the cluster source. Unlike the previous case where the substoichiometric clusters are first formed and then oxidized, here atomic oxygen is available in the source plasma, and oxygen participates directly in the initial cluster formation. As we show, such growth conditions favor the formation of the most-oxidized species. Although the source generates both cationic and anionic species and both have been examined, here we will focus mainly on anionic clusters. Figure 1a shows the mass spectrum of the oxidized CrnOmclusters obtained via the reaction of thermalized substoichiometric clusters with 200-sccm O2 introduced at the RGI. One can identify several families. The most prominent peaks at small size (fewer than eight Cr atoms) correspond to the 262
Figure 1. Mass spectra of CrnOm anions formed (a) via reaction of thermalized substoichiometric CrnOm clusters with 200-sccm O2 introduced at the RGI. Peaks highlighted green represent CrnO2n+1, red are CrnO2n+2, and blue are CrnO2n+3. (b) Mass spectra for clusters formed via the introduction of (∼2.5%) O2 seeded in He at the cluster source. Peaks are again color-coded such that purple represents CrnO3n-2, gray is CrnO3n-1, blue is CrnO3n, and yellow is CrnO3n+1.
CrnO2n+2 composition. In addition, there are peaks corresponding to CrnO2n+1 and CrnO2n+3 compositions. Passivated clusters are formed as molecular oxygen interacts with the cluster surface, effectively inserting atomic oxygen across Cr-Cr bonds. CrO2 units, then, are the apparent “building units” in the spectrum shown here. Whereas the mass spectrum does indicate the feasibility of producing different sizes, a clear indication of the stability can come from experiments where one observes the persistence of a given cluster with increasing oxygen content; when the cluster distribution remains unchanged with the introduction of additional oxygen at the RGI, the final product clusters are considered to be completely passivated and therefore promising as viable building blocks for materials. Branching ratios (Figure 2) were extracted from the mass spectra, clearly illustrating the emergence of saturated clusters at the expense of substoichiometric species. Note that the CrnO2n+2 clusters shown by hollow squares are all stable and consistently represent the most abundant species for clusters containing more than two Cr atoms. This, coupled with their large intensity in the mass spectrum in Figure 1a, shows that they are ideal building blocks. The other feature of the mass spectrum in Figure 1 is the conspicuous change in the spectrum from eight to nine Cr atoms. As we will show later, this may be attributable to a transition to bulklike structure.4 Although our primary focus here is the anionic clusters, we have also studied cationic clusters. These spectra also exhibit saturation reaction products corresponding to CrnO2n+2+ Nano Lett., Vol. 4, No. 2, 2004
Figure 2. Branching ratios indicating the evolution of the various cluster species as the CrnOm clusters are exposed to increasing amounts of oxygen at the RGI. The fraction of each peak’s contribution to the integrated intensity of all considered peaks is plotted against the flow rate of oxygen at the RGI.
clusters up until around eight Cr atoms. However, in general, the cationic clusters displayed peaks having one additional oxygen atom with respect to their anionic counterparts. This observation is a consequence of the electronic character of the clusters and will be addressed in a future publication. Figure 1b shows the mass spectrum observed when O2 was seeded in the He carrier gas introduced at the cluster source. Note that under these conditions the formation of saturated products with stoichiometries corresponding to CrnO3n is favored. The mass spectrum does contain clusters with compositions of CrnO3n-2, CrnO3n-1, and a few CrnO3n+1. In most cases, however, there are no clusters with oxygen content higher than 3n, where n is the number of Cr atoms. When clusters formed in this manner are subsequently exposed to oxygen introduced at the RGI, no reaction occurs at all, indicating that these clusters represent a second stable family of completely oxygen-passivated chromium oxide clusters. The experimental spectra broadly exhibit three features. (1) The anion mass spectra and the branching ratios show the emergence of saturated species corresponding to CrnO2n+2. The chemical inertness of these passivated clusters makes them intriguing as potential building blocks for clusterassembled materials. What are their electronic and magnetic properties, and why are they so stable? (2) The mass spectra show a striking change in character from eight to nine Cr atoms and then a resumption of the overall trend. Might this change be related to a transition to bulklike structure? (3) The clusters produced under energetic conditions exhibit peaks at CrnO3n. How do the properties of this family differ from those of CrnO2n+2? To answer some of these questions, we have carried out ab initio electronic structure calculations on small CrnOm clusters, considering several Cr/O stoichiometric ratios. These studies were carried out using a first principles linear Nano Lett., Vol. 4, No. 2, 2004
combination of atomic orbitals molecular orbital approach within a gradient-corrected density functional scheme.15,16 Here, the atomic orbitals are expressed as a linear combination of gaussian functions centered at the atomic positions. The matrix elements of the Hamiltonian are calculated by numerically integrating over a mesh of points. The actual studies were carried out using the NRLMOL set of codes developed by Pederson and Jackson.17 The basis function for Cr had 7s, 5p, and 4d gaussians with a supplementary d gaussian, and that for O had 5s, 4p, and 3d gaussians with a supplementary d gaussian; the reader is referred to the original papers for details.18 Because chromium oxide exhibits several crystallographic arrangements in the bulk, several initial geometrical arrangements were examined. For each initial structure, the geometry was optimized by moving atoms in the direction of forces until the forces reached a threshold value of 10-3 au/bohr. Because we are interested in the magnetic properties, ferromagnetic and possible ferrimagnetic arrangements were examined to find the optimum spin multiplicity. We note that in the overlapping size regime our calculations are in complete agreement with the findings of Veliah et al.6 We begin with the ground-state geometries of CrnO2n+2 clusters. Because the successive members of the series can be analyzed in terms of the addition of CrO2 units, a natural candidate for a structural motif would be a linear chain that grows by the addition of such units. In these structures, each Cr is bound to another by two O atoms, and the end Cr sites have two terminal O atoms not shared by other Cr atoms. Each Cr is then bonded to four O atoms. We ascertained that the ground state of the Cr3O8 anion is indeed such a structure. We found, however, that another class of structures where each Cr is bound to more than two bridging O atoms is more stable. Figure 3 shows the ground-state geometries, binding energies, and spin magnetic moments of neutral and anionic CrnO2n+2 clusters containing three to six Cr atoms. For Cr3O8-, the structure shown in Figure 3 is almost degenerate with a linear chain. We have, however, chosen to show the cage structure because the neutral species prefer this structure. Note that the structures of clusters containing four or more Cr atoms are cages where each Cr shares three or four O atoms with other Cr atoms. Those Cr that share three O atoms with other Cr atoms have an extra terminal O, but those Cr sites that share more than three O atoms generally do not have the terminal oxygen. The cluster that ideally satisfies this rule is Cr4O10, shown in Figure 3, where each Cr has one terminal O and shares three O atoms with others. Note that the structure resembles that of a sputnik. The peculiar stability of this cluster in the current study is also attested to by the observation that it appears as the magic species in the earlier studies where the clusters were generated using laser ablation of CrO3.5 To understand the stability of all of these clusters, one can consider that the O atoms bound to only one Cr form a double bond whereas the Cr-O bonds for O shared by two Cr atoms are single bonds. For Cr4O10, each Cr has one terminal O and shares three O atoms with other Cr sites. The Cr are therefore in a state with a valence of 5. This feature is also seen in larger clusters such as Cr5O12 and Cr6O14. The other interesting 263
Figure 3. Ground-state geometries, atomization energies, and spin multiplicities of anionic and neutral CrnO2n+2 clusters containing three to six Cr atoms. For anionic clusters, the electron affinities are also listed. All of the bond lengths are in angstroms, and the atomization energies are in electronvolts.
aspects of these clusters are their spin multiplicities. Note that all of the clusters have nonzero spin magnetic moments, indicating that they are ferromagnetic. A study of the local magnetic moments shows that the magnetic moments are mostly centered on the Cr sites. Next, it is significant that the anionic spectrum exhibits a marked pattern change in the region going from Cr8Om to Cr9Om. Up until eight Cr atoms, the most prominent peaks in the mass distribution correspond to CrnO2n+1, CrnO2n+2, and CrnO2n+3. For nine Cr atoms, one notices the appearance of a peak corresponding to Cr9O24, a cluster far more oxygenrich than observed in the lower masses. Starting from the bulk structure, it is easy to note that a structure containing 9 Cr atoms, each bonded to the O atoms in the bulk structure, has exactly 9 Cr and 24 O atoms. Starting from an initial configuration that corresponds to a distorted fragment of the bulk, we found that the optimized structure does correspond to the bulk fragment. This structure is shown in Figure 5 and has a spin magnetic moment of 6µB. It is highly stable, with an atomization energy of 155.28 eV. As shown in the Figure, at masses higher than that of this species, the previous pattern is resumed. In the mass region extending beyond that shown here, more breaks in the pattern shown in Figure 1 are observed, consistent with the hypothesis that nanocrystalline shell closures represent a persistent source of interruptions to the morphological motifs. As pointed out before, the introduction of oxygen into the He at the cluster source was found to generate highly 264
Figure 4. Ground-state geometries, atomization energies, and spin multiplicities of neutral CrnO3n clusters containing three to six Cr atoms. For anionic clusters, the electron affinities are also listed. All of the bond lengths are in angstroms, and the atomization energies are in electronvolts.
Figure 5. Ground-state geometry, atomization energy, and spin multiplicity of the neutral Cr9O24 cluster. For the anionic cluster, the electron affinities are also listed. All of the bond lengths are in angstroms, and the atomization energies are in electronvolts.
oxidized species. The most-oxidized species correspond to the CrnO3n composition. A detailed study of the possible geometrical configurations shows that they all have ground states in the form of rings. Here each Cr shares an O with other Cr atoms and has two terminal O atoms. The groundstate structures for the neutral and anionic clusters are shown Nano Lett., Vol. 4, No. 2, 2004
in Figure 4. Assuming that each Cr has a double bond with a terminal O and a single bond with an O shared by two Cr atoms, each Cr has a valence of 6, which is the highest valence exhibited by Cr atoms. Furthermore, no structures with an oxygen-to-chromium ratio higher than 3 were generally observed in the mass spectra of clusters with 3 or more Cr atoms. One can therefore classify these structures as fully coordinated. The saturation of the Cr valence quenches any unpaired spins, and the ground state for each cluster is nonmagnetic. We did examine the possibility of the antiferromagnetic arrangements leading to zero net spin. However, the local magnetic moments at each Cr were zero, showing that the local and overall moments are quenched. Although the mass spectra are measured for charged clusters, making materials using clusters would require assembling neutral species. It is therefore important to ask how the properties, in particular, the magnetic properties, change with charge. Figures 3 and 4 also show the corresponding ground-state geometries, binding energies, and spin multiplicities of the neutral clusters. It is important to note that except for Cr4O10 the neutral clusters are all magnetic. What other electronic features mark these clusters? In addition to magnetism, the clusters have other interesting electronic properties. Figures 3-5 show the atomization energies (AE) of the various charged clusters calculated using the expression AE (CrnOm) ) nE(Cr) + mE(O) - E(CrnOm) where E(Cr) and E(O) are the energies of the atoms. For the anionic clusters, we have also listed the adiabatic electron affinity (EA). Notice that all of the clusters have high EA. Furthermore, the EA increases with increasing oxygen content. For example, the EA in the CrnO2n+2 series is around 3.13 eV, but those of the CrnO3n clusters average around 4.73 eV. Noting that the EAs of halogen atoms range from 3.2 to 3.7 eV, we regard the CrnO3n clusters as highly electronegative. That calculated EAs are higher than those for halogen atoms is consistent with experimental observations. In one of the experiments, the flow tube was contaminated with iodine from other experiments. Remarkably, the contamination was not evident during the study of anionic oxide clusters but was revealed during the study of cationic clusters. No halogen anions survive in the presence of these oxide clusters, but instead cationic halogen species emerge. The highly electronegative clusters could therefore serve as electron scavengers either in pristine form or when collected as materials. Finally, it is interesting to probe whether simple physical principles could be used to understand the stability of the various classes. A careful analysis of the molecular orbitals shows that the bonding is characterized by localized electrons participating in the Cr-O bonds. Furthermore, because of oxygen’s electronegativity, such bonds would be nearly polar. The stability cannot be then reasoned in terms of the collective valence electron shell picture such as the Jellium model.19 A more realistic picture is in terms of the molecular orbitals formed from the Cr d5s1 and the O 2p4 manifolds. Note that a Cr2 bond is weaker than a Cr-O bond, and thus Nano Lett., Vol. 4, No. 2, 2004
it is energetically favorable to have structures with no CrCr bonds. Assuming that O prefers to be in an O2- state, the optimal ratio corresponds to CrnO3n, where the Cr sites share one O atom. Here, all of the Cr electrons are paired with O, resulting in no net unpaired spins and the highest oxidation state of Cr6+ for Cr. These structures are also highly electropositive, consistent with their large EAs. Examining clusters with smaller ratios of O to Cr reduces the oxidation state of Cr. Consequently, Cr sites share more than one O atom, generating cage- or chainlike structures. All of the Cr electrons are unpaired; consequently, the Cr sites have finite spin magnetic moments. To conclude, the present studies show that it is possible to generate two different classes of very stable, chemically inert, and potentially useful CrnOm clusters by controlling the formation conditions. The individual clusters are oxygenpassivated and unreactive and therefore could be ideal building blocks for nanostructured materials. Each series has its own distinct electronic and magnetic properties. Whereas CrnO2n+2 are ferromagnetic, CrnO3n are nonmagnetic; the former may be useful in the design of new nanomagnets. Both families are characterized by high EAs and may make efficient electron scavengers. A unique aspect common to all series is that there are no O-O or Cr-Cr bonds. Because the energy to remove an O atom is smaller in CrnO3n clusters, they may also act as catalysts for CO oxidation. These investigations are in progress and will be reported later. Acknowledgment. We are grateful to the U.S. Department of Energy for financial support (grant DE-FG0202ER46009). References (1) Cluster Assembled Materials; Sattler, K., Ed.; Trans Tech Publications: New York, 1996. (2) Quantum Phenomena in Clusters and Nanostructures; Khanna, S. N., Caslteman, A. W., Jr., Eds.; Springer: New York, 2003. (3) Khanna, S. N.; Jena, P. Phys. ReV. Lett. 1992, 69, 1664. (4) Transition Metal Oxides; Cox, P. A., Ed.; Oxford University Press: New York, 1992. (5) Abriet, F.; Muller, J.-F. J. Phys. Chem. A 2002, 106, 6053. (6) Veliah, S.; Xiang, K.; Pandey, R.; Recio, J. M.; Newsam, J. M. J. Phys. Chem. B 1998, 102, 1126. (7) Reddy, B. V.; Khanna, S. N. Phys. ReV. Lett. 1999, 83, 3170. (8) Wenthold, P. G.; Jonas, K. L.; Lineberger, W. C. J. Chem. Phys. 1997, 106, 9961. (9) Chertihin, G. V.; Bare, W. D.; Andrews, L. J. Chem. Phys. 1997, 107, 2798. (10) Griffin, J. B.; Armentrout, P. B. J. Chem. Phys. 1998, 108, 8062. (11) Nieman, G. C.; et al. High Temp. Sci. 1986, 22, 115. (12) Wang, X.; et al. Appl. Phys. B 2001, 73, 417. (13) Tono, K.; Terasaki, A.; Ohta, T.; Kondow, T. Phys. ReV. Lett. 2003, 90, 133402. (14) Castleman, A. W., Jr.; Weil, K. G.; Sigsworth, S. W.; Leuchtner, R. E.; Keesee, R. G. J. Chem. Phys. 1987, 86, 3829. (15) Kohn, W.; Sham, L. J. Phys. ReV. 1965, 140, A1133. (16) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865. (17) Pederson, M. R.; Jackson, K. A. Phys. ReV. B 1990, 41, 7453. (18) Porezag, D. V.; Pederson, M. R. Phys. ReV. A 1999, 60, 2840. (19) Knight, W. D.; et al. Phys. ReV. Lett. 1984, 52, 2141.
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