Article pubs.acs.org/JPCC
Thermally Activated D2 Emission upon Decomposition of Thin Deuterofullerene Films on Au(111) Seyithan Ulas,† Sharali Malik,‡ Artur Böttcher,*,† and Manfred M. Kappes*,†,‡ †
Institute of Physical Chemistry, Karlsruhe Institute of Technology (KIT), Fritz-Haber-Weg 2, 76131 Karlsruhe, Baden-Württemberg, Germany ‡ Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Baden-Württemberg, Germany S Supporting Information *
ABSTRACT: We have studied the formation and thermal properties of thin, deuterofullerene-containing films on Au(111) under ultrahigh vacuum conditions. The films were prepared in situ by exposure of predeposited C60 layers to a flux of atomic deuterium. With increasing deuterium dose, a D + C60 → C60Dx reaction front propagates through the fullerene film toward the gold surface. Heating the resulting deuterofullerene-containing films to >600 K leads to desorption of predominantly C60 and C60Dx. Interestingly, some D2 is also evolved while a significant fraction of the carbon initially deposited is left on the surface as nondesorbable residue. This is in contrast to analogous deuterofullerene-containing films prepared on graphite, which sublime completely but do not measurably evolve D2, suggesting that the gold surface can act as a catalyst for D2 formation. To explore this further, we have systematically studied (i) the thermal properties of C60/Au(111) reference films, (ii) the reaction of C60/Au(111) films with D atoms, and (iii) the heatinginduced degradation of deuterofullerene-containing films on Au(111). In particular, we have recorded temperature-resolved mass spectra of the desorbing species (sublimation maps) as well as performed ultraviolet photoionization spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and scanning tunneling microscopy measurements of the surfaces at various stages of study. We infer that heating deuterofullerene-containing films generates mobile deuterium atoms which can recombine to form molecular deuterium either at the gold surface or on fullerene oligomers in direct contact with it.
1. INTRODUCTION Hydrofullerenes, C60Hx (x < 44) have been discussed as hydrogen storage materials.1,2 At low temperatures, solid C60 physisorbs molecular hydrogen into interstitial octahedral sites, resulting in a relatively low storage capacity of 0.3 wt %.3 This storage level can be increased significantly by hydrogen “chemisorption”, i.e., by hydrogenation of the fullerene cages thus forming hydrofullerenes with strong covalent −C−H bonds. Theoretically, a maximum storage capacity of 7.7 wt % is then possible (corresponding to C60H60). However, such high loadings cannot be achieved experimentally.4 Hydrofullerenes are prepared in bulk quantities by exposing solid C60 to molecular hydrogen in the presence of an appropriate metal catalyst (Ru, Pt, Pd, and others5), whereby hydrogen storage capacity and loading time are primarily governed by the reaction temperature (TS > 523 K) and the H2 partial pressure (p > 20 MPa). Unfortunately, under such comparatively stringent conditions, there are also parallel reactions leading to gradual degradation of the fullerene storage material itself.6 Furthermore, effective hydrogen storage capacities have been shown to depend sensitively on impurities and dopants, fullerene sample prehistory, as well as on the reactor chamber surface.7 The latter seems to play an as yet © 2014 American Chemical Society
poorly understood role particularly in the thermal decomposition of the resulting hydrofullerenes.8 An optimal hydrogen storage medium should exhibit not only high storage capacity but also a low activation energy for dihydrogen release. Unfortunately, deuterofullerene-containing thin films on graphite have been shown to intactly desorb without measurable molecular deuterium evolution upon being heated under vacuum.9 Apparently, the activation energy for C−D bond cleavage is not competitive with that required for desorption of the intact deuterofullerene cage (thermally induced dehydrogenation of C60H2 is expected to be activated by ∼61.4 kcal/mol).10 More recently, we have shown that this situation can be changed by doping preformed deuterofullerene films with (electron-donating) Cs atoms.11 Now, some D2 evolution is also observed during the sublimation process. We attribute this to a series of thermally activated solid-state reactions also involving electron transfer from Cs to C60Dx and thus weakening C−D bonds, which effectively convert Cs doped C60Dx into CsyC60 + D2. Unfortunately, the resulting saturated CsxC60 Received: December 6, 2013 Revised: March 7, 2014 Published: March 10, 2014 6788
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fulleride film does not exhibit any capacity toward further hydrogen storage. Also taking into account the observations of ref 8, it appeared interesting to perform model studies like our experiments with deuterofullerenes on graphite but using a metal substrate instead. Conceivably, electron transfer is possible here without degrading or converting the storage material. As before, we have prepared deuterofullerenes by reaction of C60 with deuterium atoms (using D instead of H for better mass resolution of volatiles), thus dispensing with a D2 chemisorption catalyst to simplify the reaction system.9 Specifically, we have studied the deuteration of C60 films deposited onto Au(111) and have compared the thermal properties of the resulting deuterofullerene-containing films with those on C(0001) (basal plane of a highly oriented pyrolytic graphite (HOPG) substrate). Here we focus on thermal desorption mass spectroscopic analysis of species evolving when such films are heated to 1100 K. Interestingly, for deuterofullerene films on Au(111) we observe some evolution of D2 in addition to dominant C60 and C60Dx emission. We rationalize this in terms of recombination of mobile deuterium atoms either at or in close proximity to the gold surface. Whereas physisorptively bound C60 and C60Dx desorb quantitatively from HOPG, a significant portion of the carbon initially deposited onto Au(111) in the form of C60 cannot be desorbed at all.
procedure allows us to assess effusive C60 deposition fluxes and therefore layer thicknesses to better than ∼10%. Deuteration of C60 films held at constant temperature Ts was carried out by exposing the sample to a dose of atomic deuterium. D atom-containing effusive beams were obtained by dissociating D2 molecules in a microwave discharge (Tectra Gen2). All charged particles produced in the discharge were electrostatically deflected from the beam prior to its hitting the surface (perpendicularly). The flux of atomic deuterium effusing from the source depended only on the partial D2 pressure which was varied in the range of 10−4−10−5 mbar (at constant discharge voltage). Usually D2 plasmas are far from thermal equilibrium; therefore, the kinetic energy distribution of resulting neutral beam constituents (D, D2, ...) are not fully thermal. We did not determine the D atom energy distribution but expect it to have a significant hyperthermal energy component (>0.3 eV9,11). Absolute D flux at the sample surface was independently calibrated by positioning a mass spectrometer such that its ionization filament was located at the same distance from the discharge source as from the surface. All D-exposures were performed at a constant D flux of 5.4 × 1014 cm−2 s−1. The D dose was varied simply by changing the exposition time. Both the effusive C60 and D beams were significantly wider than the Au(111) substrate (circular, 8 mm in diameter). Thus, the surface overlap degree between C60 and D beams was nearly perfect (∼1); this was also checked visually. Deposited film thickness was assumed to be uniform throughout. The thermal properties and sublimation behavior of deuterofullerene-containing films has been investigated by temperature-resolved mass spectroscopy using an Extrel quadruple mass spectrometer and a 70 eV electron impact ionization, either by following a specific ion signal or by rapidly recording mass spectra of desorbing neutral species while heating the sample at a constant rate of 5 K/s. This time−temperature sequence of mass spectra is then plotted as a (T,M)-diagram or sublimation map which allows us to directly identify escaping species as well as their sublimation temperature range.9,11 We generally present sublimation maps taken over the ranges T = 300−1025 K and M = 680−820 amu to cover the emission of the dominantly desorbing species C60 and C60Dx 600 K) differ from those pertaining to equilibrated deuterium gas under steady-state conditions (T = 300 K). C60Dx desorption has been calibrated by relating integrated C60Dx+ signal (from (T,M) maps) to the corresponding signal obtained upon subliming a monolayer of C60 desorbed from HOPG. This approximation ignores possible errors associated with different electron impact ionization cross sections in going from C60 to C60Dx. The density of states in the valence region of deuterofullerene films has been routinely monitored by means of ultraviolet
2. EXPERIMENTAL SECTION The instrumental setup and procedures used have been previously described.9 Briefly, all experiments were performed in an ultrahigh vacuum (UHV) apparatus under a base pressure 1200 K (band C).
initially incident deuterium atoms are trapped and can be subsequently desorbed via C60Dx. Band B shows that another 3.6 × 1013 of the incident deuterium atoms are evolved as LT-D2. Finally, 1.3 × 1014 D atoms are associated with the HT-D2 emission feature, C. Thus, while some D2 emission is observed upon thermal decomposition of the Au(111)-supported film, most of the trapped deuterium is returned to gas phase in the form of C60Dx. Note, that we see no mass spectral evidence for the desorption of smaller hydrocarbons. In particular, CD4 is not observed. In the next sections, we explore how the branching ratio between the three deuterium-evolving reaction pathways, A, B, and C, depends on the initial deuteration conditions applied and what this tells us about the mechanism leading to D2 formation/ desorption from deuterofullerene thin films on Au(111). 3.4. Dependence of C60Dx Sublimation on Deuteration Conditions. Detailed results are shown in Supporting Information (Figures S1−S3). Here we discuss only the most important trends. First, deuteration temperature, TS, was varied over the range 300 K < TS < ∼520 K (limited by C60Dx sublimation onset). As previously reported for C60/HOPG, deuteration of C60 films on Au(111) is thermally activated (see Figure S1 in Supporting Information). In particular, both the mean deuteration degree and the integral C60Dx intensity increase with Ts. Interestingly, whereas the C60 desorption peak β* remains essentially unaffected by increasing TS, peak γ* becomes comparatively weaker. We interpret this as being related to the −C60−C60− C60− polymers created in the C60/Au interface region. Conceivably this material is resistant against (further) deuteration. Next we investigated the dependence on C60 film thickness Δ (see Figures S2 and S3 in Supporting Information and ref 25) while keeping the deuterium dose constant. Sublimation maps were acquired for 7 < Δ < 70 MLE (TS = 425 K, ND =2 × 1017 atoms). Upon increasing film thickness, the average deuterium loading per desorbed C60Dx cage was observed to decrease while the
Figure 4. Upper panel: total numbers of D atoms carried away by sublimating C60Dx molecules and their mean deuteration degree x, both as functions of Δ. Lower panel: numbers of C60, C60Dx, and their sum as functions of increasing film thickness Δ (ND = 2 × 1017, TS = 425 K). Note that the total number of sublimating C60Dx cages is always lower than the number of deposited cages.
films (70 MLE) we observed the highest D → C60Dx conversion yield, i.e., at Ts = 425 K, 32% of incident D-atoms are trapped and may be subsequently desorbed as C60Dx. Overall, these trends reflect transport processes which ensure that some of the D atoms impinging on the outermost C60Dx layers can make their way to pristine or unsaturated C60 cages located underneath, i.e., D → C60Dx conversion is limited by thermally activated diffusion of D atoms through already partially deuterated layers.26 This picture is additionally supported by the previous inference of facile hydrogen migration on C60H36 cage surfaces at elevated temperatures.27 3.5. Dependence of D2 Evolution on Deuteration Conditions. Next we explore the dependence of LT-D2 emission on deuteration conditions (Ts, Δ, and ND). Figure 5 shows the effect of varying the deuteration temperature, Ts, throughout the range 300−520 K for 14 ML thick C60 films (D dose held constant at ND = 2 × 1017 atoms). In all cases, we observe D2 evolution upon subsequently heating the deuterofullerene-containing films. Samples prepared at Ts ∼ 350−425 K 6792
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Figure 5. Upper panel: thermally activated LT-D2 evolution as a function of the deuteration temperature Ts as indicated (ND = 2 × 1017, Δ = 14 ML). Lower panel: total number of LT-D2 molecules evolved, N(D2), together with the corresponding conversion yield, Y(D → D2), both as functions of Ts.
Figure 6. Upper panel: LT-D2 emission as a function of film thickness Δ. (ND = 2 × 1017, Ts = 425 K). Lower panel: total number of LT-D2 molecules emitted, N(D2), together with the low-temperature conversion yield, Y(D → D2), both as functions of the film thickness Δ.
show the largest integral D2 signals partitioned into two welldistinguished desorption peaks, δ and ρ, located at ∼640 K and ∼680 K, respectively. It is also of interest to consider the dependence of LT-D2 evolution on C60 film thickness for films exposed to the same D dose (ND = 2 × 1017 atoms) at Ts = 425 K (Figure 6). The total LT-D2 emission intensity is first observed to increase with film thickness and then decay again after reaching a maximum at Δ ∼ 14 ML. Whereas thin films show resolved δ and ρ desorption features, thicker films exhibit only one broad ρ peak. Interestingly, the temperature at which band ρ is maximal increases with Δ (from 640 to 730 K). Figure 7 shows the dependence of LT-D2 desorption yield on D dose as probed for 14 ML thick C60 films held at Ts = 425 K. D2 evolution starts to be detected for D doses ND > 3 × 1014 atoms, becomes significant at 1015 atoms, and from there on increases linearly with D dose. In contrast, for D doses higher than ∼1016 atoms, the mean conversion yield (Y(D → D2) = 2N(D2)/ND) decreases. The above data allow us to estimate the range of experimental parameters (Ts, Δ, and D dose) for which conversion yields,
Y(D → D2), are maximal. This is seen for Ts* ≅ 360−425 K, Δ* ≅ 14 ML, and N*D ≅ 1016. Under these conditions, 0.1% of incident D atoms are converted to evolved LT-D2 (corresponding to the emission of ∼2.2 × 1013 D2 molecules). However, 2 orders of magnitude more D atoms are carried away by the C60Dx desorbed from these films. 3.6. HT-D2 Evolution. HT-D2 evolution during desorption of a deuterofullerene film as shown in Figure 3 is unexpected. For reference, a room-temperature Au(111) surface was exposed to molecular deuterium, and then thermal desorption from this system was measured. No D2 signal could be detected. Thus, as generally accepted, D2 does not stick on Au(111) at room temperature.28 In contrast, intense HT-D2 desorption is detected upon exposing a room-temperature Au(111) surface to a comparable dose of atomic deuterium. Moreover, this HT-D2 signal scales with D exposure. A surprisingly high level of up to 1% of the D incident onto pristine Au(111) can be stored and evolved in the form of HT-D2 (for details see Figure S4 in Supporting Information). Given that associative desorption of surface-bound atomic hydrogen from Au(111) is known to occur at much lower temperatures, Tdes < 500 K,29 we rationalize this 6793
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Figure 8. Dependence of the conversion probability, Y(D → D2) on deuteration temperature, Ts (Δ = 14 ML, ND = 2 × 1017) as measured in the low- and high-temperature regions, HT-D2 and LT-D2, squares and circles, respectively. Note the counter-correlation.
3.7. Desorption of D2, C60, and C60Dx versus Nondesorbable Residue. Comparing integrated C60Dx and C60 desorption signals to the number of initially deposited C60 cages shows that a large fraction of the deposited carbon load survives heating as a new nondesorbable carbon material (see lower panel in Figure 4), e.g., for Δ = 70 ML the residue is equivalent to nearly 15 ML. Whereas this material cannot be desorbed by heating to 1200 K, the highest temperature accessible to us, it can however be removed by extended Ar+ sputtering and annealing cycles. Note that carbon traces were never observed upon heating pristine Au(111) substrates to 1200 K, i.e., the residue does not originate from surface segregation of carbon impurities stored in bulk gold but is instead a thermal degradation product of the deuterofullerene-containing film. Note that we observed comparable amounts of nondesorbable carbon material after heating C60/Au(111).16 To study their nature, residues were probed by XPS (1253.6 eV; for details see Figure S5 in Supporting Information), and the spectra were compared to various reference materials. In agreement with the literature,30,31 the C1s spectrum of a thick pristine C60 film (panel A of Figure S5) can be well fit by two Gaussian components a1 (285.5 eV) and a2 (284.5 eV). After deuteration, this C1s state signature is modified. Three Gaussian components now provide a better fit: a superposition of slightly reduced (old) a1 and a2 components plus a third strong component a3 (283.7 eV, panel B of Figure S5) are required. The latter reflects sp2 → sp3 conversion as induced by formation of −C−D bonds. Thermal treatment, i.e., sublimation/residue formation, then leads to dramatic modifications of the XP-C1s band profile (panel C of Figure S5). This shows strongly reduced intensity and is now best fit by three new components, b1 (285.7 eV), b2 (284 eV), and b3 (283.2 eV), which do not match the states characteristic for C60 cages, a1, a2, and a3. Instead, both b1 and b2 components match the XPS signature of a clean HOPG surface. We tentatively assign the third, very weak component b3 (283.2 eV) to carbon atoms associated with defects or edges in planar graphene networks. Ex-situ SEM images of the surface taken after heating/desorption reveal a randomly distributed network of flat islands (size distribution centered at ∼40 nm). Conceivably, the strong chemisorptive interaction of C60 with Au(111) mediates thermally induced fragmentation/isomerization of the carbon cages. Interestingly, bulk deuterofullerene
Figure 7. Upper panel: dependence of LT-D2 evolution on ND, the number of incident deuterium atoms (Δ = 14 ML, Ts = 425 K). Lower panel: total number of D2 molecules emitted, N(D2), together with the low-temperature conversion yield, Y(D → D2), both as functions of ND.
high storage level as being due to D penetration into subsurface gold regions upon hyperthermal kinetic energy impact. HT-D2 emission would appear to result from associative recombination of D atoms reemerging from the bulk into the topmost layer upon heating to high temperatures. Note that we also detect a broad, weak D2 desorption feature in the low-temperature range (500−700 K). It grows slightly with D exposure but always remains more than 2 orders of magnitude weaker in intensity than the HT-D2 band. We assign this to submonolayers of chemisorbed D atoms which escape as D2 upon heating. Compared to that of the Au(111) reference, the HT-D2 signal observed upon heating a 14 ML thick deuterofullerene film is ∼4.5 times weaker. This relationship depends on the film thickness and on D dose. Apparently, the intermediate C60Dx/ C60 layer hinders penetration of D atoms into the underlying gold substrate. Figure 8 shows that the HT-D2 yield can also depend on the deuteration temperature Ts. Over the range Ts = 360−425 K, we observe a minimum in HT-D2 yield which counter-correlates with the LT-D2 yield, suggesting that to a significant degree the reaction processes leading to LT- and HT-D2 emission are complementary. 6794
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Figure 9. Morphological and mechanistic model illustrating the molecular composition of deuterofullerene-containing C60 films on Au(111). (a) At a very early deuteration stage; (b) at a very late deuteration stage; (c) during desorption of the C60Dx network; and (d) after desorption of all volatile material (nondesorbable residue). Zone ZA comprises the topmost layers which are primarily comprised of deuterated cages, C60Dx, as well as some D atoms located in interstitial sites (red circles). Sublimation from ZA zone gives rise to band A in (T,M) maps. Zone ZB represents the reaction front or “capping layer” which we propose consists of D-interlinked polymeric chains of fullerene cages, −D−C60−D−C60−D−C60−. NMR measurements on analogous materials support the stabilizing role of interlinking D atoms.37 Thermal decomposition of these chains leads to the C60 desorption peak α* and, associated with this, to a significant release of D2. Zone ZC comprises the immediate C60/Au interface region and is constituted by 2-D (and perhaps 3-D) covalently interlinked fullerene polymers bound strongly to the substrate as well as by individual C60 cages bound even more strongly to atomic defects in the Au surface. The thermal decomposition of these bonding types and materials results in C60 desorption peaks β* and γ* (with some fraction of these reactions also leading to nondesorbable residues).
Figure 9 shows a series of schematics at various stages of the experiment which help in understanding the model. After exposure of a C60 multilayer to a flux of deuterium atoms, the reaction front has propagated into the volume of the film and the topmost layers are constituted by a random network of mainly v.d.Waals-interacting C60Dx cages with some less strongly bound (mobile) D atoms trapped therein. Sublimation starts by removal of these cages (giving rise to the C60Dx desorption band A in Figure 2). As the layer is gradually reduced in thickness while the temperature is raised, lower and lower deuteration degrees are uncovered (and the corresponding cages evolved). This lower deuteration degree also reflects diffusive transport and local trapping and binding of D-atoms during the initial deuteration stage. Underneath the C60Dx- layer, D-stabilized oligomeric chains, −D−C60−D−C60−, constitute the reaction front, which can act as a “capping-layer” for any more volatile material (e.g., pristine C60) underneath. Thermally activated fragmentation of these D-stabilized oligomers takes place in a narrow temperature interval 650−700 K and is associated with the simultaneous sublimation of C60/C60D1/C60D2 (not mass resolvable) as well as bulk C60, all of which can contribute to the broad sublimation peak α*. Below the reaction front and adjacent to the gold
materials as created by high-pressure molecular deuteration of C60 exhibit a dramatic structural transition at 773 K, which has been assigned to fragmentation/collapse of the constituent fullerene cages.32 Similar findings, i.e., formation of pyrolytic graphite, have been reported upon heating a material of average composition C60H18.7 to 900 K.6 In our D + C60/Au(111) system we see clear evidence that strong C60−Au interactions can mediate cage fragmentation followed by nucleation and growth of graphitelike islands. 3.8. Morphological and Mechanistic Model. We propose a simple morphological and mechanistic model to explain the formation and the thermal decomposition of deuterofullerenecontaining films on Au(111). The model requires (i) three types of C60 (bulk-like, polymerized (as a result of electron transfer from the gold substrate) and strongly covalently bound to gold defect sites), (ii) multiple types of C60Dx (even and odd numbered x = 1−50) with x-dependent reactivities and stabilities, and (iii) two generic types of “stored” D (covalently bound D in C60Dx and (thermally activated) mobile D). The nature of the latter is unclear; we speculate that D atoms “dissolved” in the interstitial regions of the C60Dx network and/or D atoms bridging radical centers on adjacent cages may be involved. 6795
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Figure 10. Left panel: (T,M) map taken during sublimation of a 7 ML thick C60 film deuterated at constant sample temperature (Ts = 425 K, ND = 2 × 1017). Right panel: thermally activated LT-D2 emission. The position of peak ρ correlates with α*, and the position of peak δ coincides roughly with the emission of deuterated cages, C60Dx band A.
interface, convoluted with the enhanced reactivity of the less deuterated cages encountered further down. To first order, lighter cages are more reactive simply because they offer a high number of unsaturated carbon atoms. The dependence on D atom dose, while keeping film thickness constant, is consistent. LT-D2 emission sets in only above a threshold of ∼1015 atoms. Its intensity then increases by only ∼50% upon further raising the D atom dose by more than 2 orders of magnitude. Apparently, at low doses essentially every impinging D atom (1 ML ∼ 1014 cages/cm2) binds covalently to C60 cages in the topmost layers. As the dose is increased, some D atoms are also less strongly trapped in the nascent C60Dx network and eventually migrate subsurface. Apparently, D-atom migration is rate-limiting (corresponding to a reduction of the effective sticking coefficient). Such rate limitation is primarily effected by the terminating deuterofullerene layers. It can be seen qualitatively in the temperature dependence of the LT-D2 yield (keeping all other parameters constant). At low deuteration temperatures (Ts ∼ 300 K), the reactive D atoms cannot be transported efficiently through the topmost layers and very few D atoms reach the gold surface (in maximum 1011−1012 cm−2). At the high-temperature extreme (Ts > 520 K), deuterofullerene formation competes with thermal desorption and the reactive D atoms can no longer be stored in the C60Dx network. Formation of the C60Dx network is most effective in the intermediate temperature range (360 K < Ts < 480 K). This is also accompanied by efficient interstitial storage of D atoms.
interface, we propose that 3D domains consisting of [C60]derived covalently bound polymers form during sample heating. This polymerization is a consequence of the strong interaction of chemisorbed C60 cages with the gold surface (and mediated by the associated electron transfer).33 The corresponding polymers start to decompose at ∼770−900 K, leading to C60 evolution as evidenced by peak β* (independent of the deuteration degree), while a nondesorbable carbonaceous residue is formed in parallel. Finally, we propose strong fullerene chemisorption to (additional) surface defects and dislocations as evidenced by peak γ*.34 How does this morphological model translate into a mechanism for LT-D2 evolution? Figure 10 indicates that δ(LT-D2) evolution is associated with the thermal decomposition of C60Dx containing regions, evolving mainly C60D30
