Thermally Triggered Dedeuteration of Cesium-doped

J. Phys. Chem. C 2008, 112, 13789–13796

13789

Thermally Triggered Dedeuteration of Cesium-doped Deuterofullerene Films Daniel Lo¨ffler, Patrick Weis, Artur Bo¨ttcher,* and Manfred M. Kappes* Institut fu¨r Physikalische Chemie, UniVersita¨t Karlsruhe, 76131 Karlsruhe, Germany ReceiVed: March 7, 2008; ReVised Manuscript ReceiVed: May 16, 2008

We have probed the electronic structure and thermal stability of Cs-doped deuterofullerene thin films, Csx(C60Dn), as generated under ultrahigh vacuum conditions, using ultraviolet photoionization (UP) spectroscopy and thermal desorption spectroscopy. Mass spectra taken during sublimation indicate that deuterofullerene (C60Dn) and cesium fulleride regions (CsxC60) can coexist at elevated temperatures. The yield of Cs fulleride phases obtained by heating increases with the amount of Cs atoms initially deposited/intercalated into the deuterofullerene thin films. At highest initial Cs doses, a CsxC60 phase with doping degree of 4 e x e 6 is eventually generated. UP spectra of the as-prepared films taken before heating suggest that C60-D-d-Cs+d complexes are initially formed. Thermally activated conversion of these complexes into Cs fullerides can be followed by recording UP features characteristic for (increasing) C60-LUMO state occupation. The transition from cesium intercalated C60Dn into CsxC60 sets in at a temperature of 610 K and is completed by 670 K. This narrow transition temperature range is also characterized by synchronous desorption of Cs and D2. We conclude that Cs intercalation can significantly weaken >C-D bond strengths relative to C60Dn, thus facilitating thermally activated dedeuteration. 1. Introduction Assuming that each carbon binds one hydrogen atom, the nominal hydrogen storage capacity of fullerene C60 is 7.7 wt % and thus is of potential practical interest should it become possible to reversibly add/remove dihydrogen to/from the carbon cages under moderate conditions.1,2 Unfortunately, at least three issues (beyond the present high price of the storage medium itself) have so far hindered the use of fullerenes in hydrogen storage: (1) C60 does not by itself significantly react with molecular hydrogen, even at elevated temperatures and pressures. This is perhaps surprising, given that the first step in the corresponding gas-phase reaction sequence, C60 + H2 f C60H2, has been calculated to be endoergic by ca. -0.5 eV3 (and subsequent H2 chemisorption steps are also thought to be endoergic up to high hydrogen contents).4 Various supported transition metal cluster catalysts have recently been applied to reducing the activation energy for dihydrogen addition. For example, C60 in toluene solution has been hydrogenated using a supported nickel catalyst, with significant reaction yields obtainable under comparatively mild conditions.5 (2) As the hydrogen-to-carbon ratio approaches 1, increased cage strain leads to competing (and irreversible) cage fragmentation reactions.6 Consequently, the various bulk-scale methods so far applied to the preparation of hydrofullerene derivatives of C60 (Birch reduction,7 hydrogen transfer reaction with metal hydrides,8 high pressure hydrogenation at elevated temperatures,9,10 and exposure to hydrogen plasma11,12) do not presently achieve hydrogen contents higher than ∼5 wt %. (3) Molecular hydrogen can be regained only by heating the hydrofullerenes up to rather high sample temperatures, T*(H2) > 800 K. Furthermore, although some release of H2 has been reported under these conditions, its overall yield and the nature of the resulting solid material are unknown. It seems unlikely * Corresponding authors e-mail: [email protected], [email protected].

that this quite stringent procedure leads to only pristine cages without other significant side reactions.13 Note that palladiumcatalyzed dehydrogenation of C60H36 in 1,2-orthodichlorobenzene solution has recently been demonstrated under more moderate conditions. However, the reaction stops at C60H18, leaving a significant fraction of the “stored” hydrogen still covalently attached to the fullerene cage.14 Thermal release of dihydrogen from hydrofullerenes would obviously be facilitated by a reduction in the corresponding C-H bond strengths and associated activation barriers. Conceivably, this can be achieved by doping of the pristine or hydrogenated fullerene solids with electron donating metal atoms. An analogous effect has recently been demonstrated in the case of Li- and Na-doped AlH3 solids.15 The doped phases exhibit up to 50 K lower H2 desorption temperatures. Alkalimetal-induced weakening of the C-H bonds in hydrofullerenes might also reduce the strain in the hydrogenated cages, thus allowing higher reversible hydrogen loadings. Moreover, alkali metals are capable of cleaving the H-H bond and might therefore also act as catalysts for dissociative chemisorption of dihydrogen and attachment of H atoms to the surrounding fullerene cages.16 Alkali-doped hydrofullerenes would also constitute a novel ternary system, X-H-Y, in which the overall properties (crystal structure, thermal stability, conductivity, phase transitions, etc.) are expected to depend on the balance between two competing interactions, X-H and H-Y,. One prominent example of such a materials class is MHXO4 (M ) Rb, Cs; X ) S, Se).17,18 A thermally driven phase-transition in CsHSO4 (around 414 K) raises its proton conductivity by 4 orders of magnitude. Another unique property resulting from the fact that hydrogen, as the key constituent of the ternary system, is involved in two competing types of bonds is the superconductivity found in hydrogenated alkali-metal-graphite intercalation compounds (AM-GICs).19 Here, a clear dependence of the superconductivity on hydrogen content has been established. Moreover, among known AM-GICs, the two stable structures C8M and C12nM,

10.1021/jp802034j CCC: $40.75  2008 American Chemical Society Published on Web 08/12/2008

13790 J. Phys. Chem. C, Vol. 112, No. 35, 2008 (n > 2, M ) K, Rb, and Cs) exhibit catalytic activities toward both dihydrogen dissociation and exchange of the atomic products.20 The relative strength of binding of hydrogen atoms to their various neighbors also appears important for catalytically active systems consisting of alkali metal and hydrogen atoms coadsorbed onto transition metal surfaces.21 Correspondingly, there is a strong dependence of the dissociative H2 chemisorption rate on Cs coverage for such surfaces. To our knowledge, alkali-metal-doped hydrofullerene solids have not yet been systematically studied. In contrast, alkalimetal fullerides (refs 22 and 23 and references therein) have been well-characterized in solid state, whereas significantly less is known about the hydrofullerenes themselves.24,25 Several hydrofullerene derivatives (C60Hx) have been prepared, isolated, and characterized.26 Of these, C60H2, C60H18, and C60H36 (airand light-sensitive solids at room temperature) are best characterized.27,8,25 The highest hydrogen contents so far obtained in purified bulk samples correspond to a mixture of C60H42 and C60H44. Very recently we have established an ultrahigh vacuum (UHV)-based procedure which allows for the efficient deuteration/hydrogenation of solid C60 films by exposing them to a flux of thermal energy D-atoms. Specifically, two welldistinguishable types of deuterated solids can be formed, depending on the deuterium dose used: (1) highly deuterated films consisting of predominantly C60D36 (with a low admixture of C60D44),28-30 and (2) a capping layer constituting mainly of C60Dn cages with n < 18. The two materials exhibit quite different thermal stabilities and can each be used as a welldefined model system for studying metal doping of deuterofullerenes. In this contribution we have studied the incorporation of alkali metal atoms into preformed, highly deuterated C60 films deposited on HOPG. On the basis of our recent study of the thermal properties and sublimation behavior of CsxC60 thin films.29 Cs was chosen as the specific dopant. We concentrate on doping induced modifications to the electronic structure of the valence band as well as thermal stability of the resulting CsxC60Dn materials. For this, we have applied ultraviolet photoionization spectroscopy (UPS) (21.2 eV) and thermal desorption spectroscopy (TDS). The initial Cs doping appears to involve the formation of partially ionic C60-D-Cs complexes. We observe that cesium intercalated deuterofullerenes can lead to the formation of Cs fullerides, CsxC60, by heating. UP spectra demonstrate that C60Dn f CsxC60 conversion can be taken to completion via thermally induced decomposition of C60-D-Cs complexes as also manifested by synchronous thermal desorption of Cs and D2. 2. Experimental Section C60 thin films were prepared under UHV conditions by low energy deposition of mass-selected C60+ ions (“ion softlanding”).31 This technique provides low-contaminant fullerene films (mass-selected ion beam purity was >99.99% under the deposition conditions used) and enables precise control of the layer thickness simply by measuring the neutralization current during deposition. Details of the soft-landing apparatus have been described in ref. 26 Briefly, C60+ was generated by electron-impact ionization (60 eV electron energy) of C60 vapor (Alfa Aesar Company, 99.5% purity). Resulting cations were collimated into an ion beam, electrostatically separated from residual neutrals, and then directed by a system of electrostatic lenses through a quadrupole mass selector (Extrel) toward an HOPG surface (SPI Corporation, SPI-II quality, sample size of

Lo¨ffler et al. 7 × 7 mm). All experiments described here were performed on freshly cleaved HOPG samples. Deposition of mass selected C60+ was carried out at a kinetic energy of 6 eV as realized by applying a retarding potential to the conductive target surface. This impact energy choice was motivated by our foregoing experiments with smaller, more labile fullerene cages, which indicated that fragmentation-free deposition can be achieved with high yields under these conditions. Typical C60+ ion fluxes were between 10 and 20 nA. Prior to C60+ deposition, the HOPG sample was flashed several times up to 1100 K in order to remove -OH and -C-H terminations of step edges. The deposited C60 load was determined by integrating the incident ion current as measured with a picoamperemeter (Keithley). Thick C60 films of up to 200 MLE (nominally) were usually generated at room temperature (where 1 MLE ) 1014 cm-2 means roughly the molecular packing density in crystalline C60). UPS, TDS, and AFM data obtained for C60 films generated by soft-landing ion deposition in this study were indistinguishable from literature results for C60 films as generated by Knudsencell-based deposition (evaporation of neutral C60) onto room temperature HOPG. Deposited C60 films were kept at room temperature during subsequent UHV transfer to a reaction chamber and then exposed to a constant, calibrated flux of neutral deuterium atoms. We used a microwave discharge setup (Tectra) operated under a partial pressure of 4 × 10-5 mbar D2 (Roth, purity >99%) as a stable source of deuterium atoms. All deuteration experiments were carried out at the same atomic flux of 2.7 × 1014 atoms/ cm2 s. The atomic flux was calibrated by means of quadrupole mass spectrometer (IQ+ Smart, VG) mounted collinearly in front of the deuterium source with the entrance at the same position in space as was used for the C60 sample surface when exposing it to deuterium atoms. Note that the mass spectral signal at 2 amu was observed to increase significantly when turning on the plasma discharge, while maintaining the chamber at constant preselected D2 pressure. This signal increase was repeatedly quantified at various background D2 pressures. Using this procedure, we estimate the overall accuracy of D flux determinations to be (8%. To exclude deuterium ions from the impinging flux, the plasma source was equipped with an electrostatic deflector. Optimum deflection potentials were found by minimizing the ionic current measured at the sample (typically to C-D derived peak exhibits a step-like change (Figure 4b, right upper panel). All of these discontinuous changes occur within the same temperature range, thus indicating a common origin. The thermal desorption spectra shown in Figure 4c provide further insight. The left panel shows Cs- and D2-TD measurements. The right panel shows the corresponding C60D36 signal (see also Figure 2). In each case, measurements were performed on an as-prepared highly doped DF (II) film. Furthermore, a C60-TD spectrum is shown, which was obtained after completing the T-dependent UP measurements discussed in the previous section (i.e., after first heating to 775 K and then cooling back to room temperature). Whereas only very weak emission of C60D36 is observed, Cs, D2, and C60 all exhibit very strong sublimation peaks. Importantly, Cs- and D2-TD signals appear in the same temperature interval and both are peaked at 610 K. The essentially synchronous appearance of the occupied C60LUMO, between 610 and 670 K, implies that thermally induced removal of Cs and D2 is associated with the formation of Cs fullerides. Note that the Cs-TD curve exhibits a second much less intense γ peak centered at 860 K. Its appearance coincides with the C60-TD peak and indicates the decomposition of the CsxC60 phase (right lower panel in Figure 4c). Thus, the 610 K emission of Cs and D2 result from decomposition of C60-D-Cs complexes, whereas peak γ corresponds to decomposition of the resulting CsxC60 phase at higher temperatures. The temperature-dependent evolution of the valence band found for the weakly doped DF film (I) is not shown but is quite analogous to the behavior found for the highly doped film (II). In particular, a sudden occupation of the C60-LUMO state is observed within the same temperature interval (610-670 K). Interestingly, the C60-LUMO state peaks at the same binding energy of 0.55 eV as was the case for DF film (II), indicating

that the same Cs4-5C60 phase is generated independent of amount of Cs initially incorporated. The reasons for this remain unclear. We note however, that decomposition of >C-D-Cs complexes generates local concentrations of Cs and D atoms, which can principally be involved in three (competing) reactions: (i) associative recombination of D to generate D2, (ii) formation of CsxC60, and (iii) desorption of Cs atoms. Presumably, the kinetic balance between these reaction steps (two of which are quite exoergic) is responsible for the resulting Cs4-5C60 phase. 3.3. DF Conversion Model. Above we have inferred that Cs-induced DF conversion to Cs4-5C60 goes by way of intermediate >C-D-Cs complexes. We speculate that these complexes are likely formed via short-range electron transfer from interstitial Cs-6s into empty deuterofullerene cage orbitals having primarily >C-D bond character. >C-D-Cs complexes begin to decompose around 610 K. Decomposition is associated with desorption of Cs and D2 as wellas the appearance of the occupied C60-LUMO state in UP spectra. Note that the ionic solid CsH thermally decomposes at 440 K as indicated by monitoring H2 emission.43 Note further that hydrogenation of the ternary CsC8 graphite intercalation compound leads to complexes consisting of H anions interacting with Cs cations and partly charged graphene sheets.44 The H2 thermal desorption spectra obtained for this material exhibits a pronounced peak at 573 K, attributed to the decomposition of Cs-H bonds. Both observations support our contention that decomposition of intermediate >C-D-Cs bonds is responsible for Cs fulleride formation. In closing, we briefly consider possible molecular mechanistic steps responsible for D2 formation. In the temperature range within which DF conversion takes place (610 - 670 K), one would expect high mobility of any D and Cs atoms released into the carbon cage network (low diffusion barriers), thus also ensuring correspondingly high D recombination rates, D + D f D2. Formation of D2 via concerted decomposition of neighboring >C-D-Cs · · · Cs-D-C< moieties also appears conceivable, leading to the additional release of two Cs atoms. As mentioned in the introduction, C60 cages are quite inert toward D2. In contrast, the released Cs can be expected to react

Dedeuteration of Cesium-doped Deuterofullerene

J. Phys. Chem. C, Vol. 112, No. 35, 2008 13795 (b) CsD-mediated deuterium abstraction followed by recombination: C60-D-Cs f C60 + [CsD] f C60 + [Cs] + [D]. (c) Concerted decomposition of adjacent >C-D-Cs complexes: 2[C60-D-Cs] f 2CsC60 + [D2]. Note that these three schematic processes correspond to the simplest possible explanations for the observed phenomenology and as such must be regarded as very speculative. In the future, more-detailed in situ spectroscopic studies are required in order to understand the underlying solid state reaction mechanism at a molecular level. Summary Deuterofullerenes can be converted into cesium fullerides (CsxC60) by depositing Cs atoms onto C60Dn films under UHV conditions and subsequent heating. The reaction proceeds to completion in two steps: (1) Cs deposited at room temperature forms ionic bonds with deuterium atoms terminating C60Dn cages (thus concurrently weakening the C-D interaction); and (2) heating these intermediate complexes to beyond 610 K leads to their decomposition. This is associated with the evolution of D2 and (excess) Cs. At the same time we observe occupation of the C60-LUMO state, as a marker for the formation of Cs fullerides. The detailed molecular level mechanism responsible for these observations is presently unclear. In future work it will be of interest to confirm and quantify Cs-induced >C-D bond weakening in deuterofullerenes by means of appropriate quantum chemical calculations. From a hydrogen storage point of view, a study of the temperaturedependent reactivity of the accessible cesium fulleride phases with both atomic and molecular hydrogen is called for. This is presently being pursued in our laboratory. Acknowledgment. This work was supported by the Deutsche Forschungsgemeinschaft (DFG) as administered by the Karlsruhe Cluster of Excellence on Functional Nanostructures (CFN). References and Notes

Figure 4. (a) UP spectra of the valence band regions of highly cesiumdoped DF films (II) taken after annealing to various temperatures. The thin line represents the UP spectrum of the undoped DF film. The deposition of 18 × 1016 Cs atoms onto the DF film has been performed at 475 K. (b) The left panel shows the corresponding thermally induced evolution of the C60-LUMO band. The right panel shows the thermally induced shift of the binding energy of the >C-D feature (upper plot) as well as the intensity of the C60-LUMO peak vs sample temperature (lower plot). (c) Cs-, D2-, C60D36-, and C60-TD spectra taken for a highly doped DF (II) film (see text for details).

with ensuing dedeuterated cages to generate cesium fullerides. Note, that the highly reactive D atoms could also undergo back reaction with exposed C60 in competition with associative recombination to D2. However, due to enhanced volatility of the product and perhaps also due to the large difference between the related formation enthalpies, D2 evolution apparently prevails over cage deuteration (∼4.5 eV per D + D event45 and ∼2.8 eV per C-D bond3). On this basis, three elementary reaction routes appear plausible as sources for D2: (a) One-step deuterium atom abstraction followed by recombination to generate D2: C60-D-Cs f CsC60 + [D].

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