Decorating (C60) n+, n = 1-3, with CO2 at low temperatures: Sterically enhanced physisorption.

Multiple attachment of CO2 to the monomer, dimer and trimer cations of C60 has been observed in the mass spectra of He nanodroplets sequentially doped with C60 and CO2 and exposed to electron ionization at 50 eV. Remarkable anomalies were seen in the ion yield for CO2 coverage for (C60)2+(CO2)8 and (C60)3+(CO2)1,2. These provide insight into the influence of steric properties on the nature of physisorption. The enhanced stabilities of (C60)2+(CO2)8 and (C60)3+(CO2)1,2 are attributed to physisorption inside the "groove" of the dimer and the two "dimples" in the trimer cations of C60. Molecular dynamics simulations provide a qualitative assessment of the observed physisorption and a useful visualization of structural aspects.

Introduction

Physisorption is a general phenomenon which is observed in any solid–fluid or solid–gas systems. It is characterized by weak bonding caused by the interaction of the induced or permanent dipole moment of the adsorbate with its own image charge in a polarizable solid. In the bound state both the geometry and the electronic structure of the atom or molecule is barely perturbed and multilayer coverage is possible. Due to the low binding energy of the adsorbates this process becomes particularly relevant at low temperatures. In the interstellar medium for example, in which temperatures are as low as 10 K, physisorption of atoms and molecules onto dust grains, including polycyclic hydrocarbon molecules, is considered to be a significant step toward molecular synthesis [1].

Isolated fullerene molecules offer a curved carbonaceous surface for the physisorption of atoms and molecules. In gas phase studies these molecules are heated, typically to temperatures >700 K, to vaporize them and they are therefore vibrationally highly excited and this prevents the adsorption of weakly bound atoms or molecules. Nevertheless, the decoration of hot fullerenes has been achieved with metal atoms in a vapor condensation source [2]. The group of Martin has reported magic numbers due to electronic and geometric [2] shell closures for metal decorated C60 and C70 [3]. In our laboratory we have very recently observed physisorption to fullerene cations at very low temperatures (370 mK). In studies of the physisorption of helium and hydrogen, C60+ and C70+ are formed by electron ionization of He nanodroplets doped with fullerenes [4] or sequentially doped with fullerenes and hydrogen [5]. In both cases a shell closure for 32 adsorbates is observed in the case of the C60 monomer cation.

Steric effects appear in the physisorption onto multimers of fullerenes. We have seen this clearly in a very recent investigation of the adsorption of methane to small cationic multimers of C60 [6] that combines experiment and theory. In the case of dimer and trimer cations, “groove” and “dimple” sites respectively, have been assigned as favorable attachment sites. Enhanced stabilities are observed for methane adsorbed at the groove in the dimer and the two dimples in the trimer cations. The situation is different for polar molecules. In the case of water [7] or ammonia [8] as adsorbates on C60+, these adsorbates are formed preferentially as clusters adjacent to the C60 unit due to the strong interaction between these adsorbates.

Here we study the interaction of CO2 with fullerene cations embedded in superfluid He nanodroplets. Nagano et al. have shown previously that a large amount of CO2 can be absorbed in solid C60 under a supercritical CO2 treatment [9]. CO2 infrared absorptions suggest a strong interaction between CO2 and C60. The physisorption of CO2 on well crystallized samples of C60 has been studied by Monte Carlo simulations [10]. The gas was seen to be adsorbed in the voids of the C60 structure in a solid state with densities slightly larger than the CO2 bulk solid. The interaction of CO2 with a single C60 molecule has been investigated by density functional theory [11]. The GGA/PBE functionals and the DNP basis set led to a binding energy of 0.037 eV.

We focus here on an investigation of the formation of mixed C60–CO2 cation aggregates and the experimental search for distinct anomalies in the ion yield for CO2 coverage of (C60)+, n = 1–3, with a view toward elucidating the influence of steric properties on the nature of physisorption. The search is augmented by molecular dynamics (MD) simulations that provide a qualitative assessment of the observed physisorption and a useful visualization.

Experimental

The experimental setup has been described in detail before [12]. In brief, helium (purity 99.999%) is cooled to 9.5 K by a closed cycle two-stage cryocooler (SRDK-415D-F50H, Sumitomo Heavy Industries Ltd.). Helium nanodroplets are formed by helium expansion at a stagnation pressure of 2.3 MPa through a 5 μm nozzle into vacuum. Under these conditions the estimated average number of helium atoms per droplet is in the order of 5 × 105. The droplets are superfluid with a temperature of ∼0.37 K [13]. After formation, the helium droplet beam passes a 0.8 mm conical skimmer to avoid shock waves and enters a differentially pumped pickup chamber. The pickup chamber is again divided into two differentially pumped regions. A small amount of C60 (SES research, purity 99.95%) is vaporized into the first region from a heated crucible. In the second region CO2 (Messer; purity 99.9995%) is introduced from an external reservoir and fed into the chamber with a flow controller. Stable and efficient pickup conditions are achieved at a constant temperature of 330 °C for C60 and 1 mPa for CO2. After the pickup process the He beam enters the ionization chamber and is crossed with an electron beam of 50 eV. The cations formed from electron ionization are guided by a weak electrostatic field toward the entrance of a time-of-flight mass spectrometer. The commercial orthogonal reflectron time-of-flight mass spectrometer (Tofwerk) separates the masses and achieves a mass resolution R ∼ 5000 FWHM (in V-mode). The ions are finally detected by a multichannel plate operated in single counting mode.

Computational section

MD-simulations were performed to provide structural details of the observed states of physisorption. We used the OPLS force field [14,15] in vacuum with a distributed charge of +1e on the fullerenes. The charges on the CO2 were chosen as 0.379e on C and −0.1895e on O according to standard Mulliken populations (B3LYP/aug-cc-pVTZ). The procedure was first to heat the system up to 20 K in steps of 2 K and then cool it down again followed by a conjugate gradient optimization. The calculations were done in Hyperchem 7 [16]. They were visualized in Chemcraft 1.6 [17]. The results are still qualitative as more elaborate simulations to assess various properties of CO2-fullerene clusters are still lacking. The determination of the number of molecules in the grooves, dimples and first adsorption shells, the adsorption energies and the mutual orientation of CO2 molecules at different adsorption sites could be accomplished in the manner described in Ref. [6].

Results and discussion

Mass spectra

Fig. 1 shows a partial mass spectrum (on a semi-logarithmic scale) obtained for the electron ionization of helium droplets doped with C60 and CO2. The highest ion yields are due to complexes of pure (C60)+ which are indicated in the graph and can easily be resolved. The monomer and multimers of (C60)+ all show attachment of at least 30 CO2 molecules leading to clusters which are denoted as (C60)+(CO2). A distinct change in the ion yield intensity for m = 32 could not be observed in the present experiment. Therefore CO2 behaves differently from the other non-polar molecules studied so far, which might be founded in the high quadrupole moment of CO2. The ion abundance of (C60)+(CO2), n = 1–3, is seen in Fig. 1 as a function of the number of the adsorbed CO2 molecules. Since 720 is not a multiple of 44 there is no overlap of the ion peaks of these three series in the mass spectrum and the ion series can be tracked until the signal-to-noise ratio gets too low. The trends in ion abundance are smooth with only a few exceptions. The dimer shows a stable configuration for eight attached CO2 molecules while the first two CO2 adducts of the trimer exhibit a higher stability. Stability features could not be assessed unambiguously for clusters (C60)+(CO2) with n > 3.

Fig. 1

Mass spectrum obtained by electron ionization (50 eV, 88 μA) of He nanodroplets sequentially doped with C60 and CO2. The conditions of the source were T0 = 9.5 K, P0 = 2.3 MPa; C60 was vaporized at 330 °C. At P(CO2) = 10−3 Pa multiple attachment of CO2 is evident for the C60 monomer (red dots), dimer (blue squares) and trimer (magenta triangles) cations. Note the stability anomalies for (C60)2+(CO2)8 and (C60)3+(CO2)1,2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

Other minor features are embedded in the mass spectrum in Fig. 1; these are exemplified by the spectrum shown in Fig. 2. All important mass peaks have been checked for their isotope pattern, identified and labeled. The highest ion yield in Fig. 2 at 736Th is identified as C60O+. Among the remaining ions, aside from the strong C60+CO2 peak, clusters are seen that contain oxidized C60+ as well as oxygen and also water impurities present in the background gas in the apparatus and efficiently picked up by the helium nanodroplets upon collision. The pure (CO2)17+ cluster also was identified. Quantitative analysis of the isotope enriched mass peaks of C60O+ reveals a significant contribution of C60+H2O (16.5%) and some C60OH+ (3.5%).

Fig. 2

Section of the mass spectrum from Fig. 1. All important mass peaks have been checked for their isotope pattern, identified and labeled. Aside from the main C60+CO2 peak, clusters are seen that contain oxidized C60 as well as oxygen and water impurities. The pure (CO2)17+ cluster also was observed.

The origin of C60O+, C60OH+ and C60+O2 (or perhaps C60O2+ in which two O atoms are chemically bonded to C60+) that feature in the mass spectrum shown in Fig. 2 is intriguing. The ionization process of doped helium nanodroplets is initiated by the formation of a He+. Its charge migrates through the droplet via resonant charge transfer without nuclear motion, on a time scale of 10 fs [18]. To ionize the dopant the charge has to migrate toward the dopant, which is directed by the attraction of the charge to the induced or permanent dipole moment of the dopant [19,20]. Charge transfer between isolated He+ and C60 would result in an electronically excited C60+* with an excitation energy of 17.0 eV. This energy results from the neutralization energy of He+ (24.6 eV) and the ionization energy of C60 (7.6 eV). The interaction of the excited C60+* with CO2 and H2O could lead directly to the formation of C60O+ and C60OH+, respectively, according to reactions (1) and (2).Further oxidation analogous to reaction (1) might then lead to C60O2+. Charge transfer between isolated He+ and the CO2 dopant can be expected to lead to the products CO+ (79%), CO2+ (11%), O+ (9%) and O2+ (1%) reported for the analogous gas-phase reaction at room temperature [21], but perhaps not in the same proportions. Production of CO2+ and the further charge transfer from CO+ to CO2 [21] readily lead to the formation of clusters of CO2+. A more rigorous discussion on cluster formation of pure CO2 in helium nanodroplets will be topic of a separate study.

Physisorption

The stability anomalies for (C60)2+(CO2)8 and (C60)3+(CO2)1,2 apparent in Fig. 1 are reminiscent of those reported in a very recent combined experimental and theoretical investigation of the adsorption of methane to small cationic multimers of C60 [6]. In the case of dimer and trimer cations, “groove” and “dimple” sites respectively, have been assigned as favorable attachment sites for physisorption. Enhanced stabilities were observed for methane adsorbed at the groove in the dimer and the two dimples in the trimer cations.

In order to model CO2 molecules in the groove and dimple sites we optimized 8 carbon dioxide molecules in the groove for the dimer (Fig. 3a) and two in the dimples for the trimer (Fig. 3b). Our simulations show the interesting feature that 8 CO2 molecules in the dimer-groove are stable only at low temperatures ∼<60 K. At higher temperatures one or two of them are displaced from the groove. In case of 6 remaining molecules this leads to a highly symmetric ring, similar to what was found for methane in the groove of the C60 dimer [22]. We aim to determine the transition temperature more accurately in extended simulations. Since the interaction between CO2 molecules (64 meV for the CO2 dimer [23]) is larger than the one between methane (22 meV for the CH4 dimer [24]) the mutual orientation of the CO2 molecules is much more strongly influenced by its neighboring adsorbents. The charge distribution on CO2 favors a slipped configuration for the C60 dimer. This could lead to a windmill or fence-like structure (compare Fig. 3a). This is also different from the H2 adsorption on fullerenes, where H2 lies flat against the surface [25].

Fig. 3

Occupation of the groove sites on a cationic fullerene dimer by 8 CO2 molecules in a fence-like manner (a) and of the dimple sites (one CO2 molecule each) on a trimer (b).

Conclusions

At least 30 molecules of CO2 are attached to C60 monomer, dimer and trimer cations in He nanodroplets at 9.5 K, sequentially doped with C60 and CO2 and exposed to electron ionization at 50 eV. Remarkable anomalies occur in the ion yield for CO2 coverage for (C60)2+(CO2)8 and (C60)3+(CO2)1,2. Computations show that these can be attributed to the influence of steric properties on the nature of physisorption. The enhanced stabilities of (C60)2+(CO2)8 and (C60)3+(CO2)1,2 result from physisorption to the “groove” in the dimer and the two “dimples” in the trimer cations of C60 in a manner similar to that we recently have observed for methane physisorption under similar operating conditions and analogous to the adsorption of CO2 in the voids of the C60 structure in a solid state as studied by Monte Carlo simulations [10].