Platinum carbonyl clusters decomposition on defective graphene surface

Surface Science 691 (2020) 121499

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Platinum carbonyl clusters decomposition on defective graphene surface a,b

c

d,e

T

a

Mattia Gaboardi , Roberta Tatti , Giovanni Bertoni , Giacomo Magnani , ⁎ Roberto Della Pergolaf, Lucrezia Aversac, Roberto Verucchic, Daniele Pontirolia, , Mauro Riccòa a

Department of Mathematical, Physical and Computer Sciences, University of Parma, I-43124 Parma, Italy Elettra Sincrotrone Trieste SCpA, Area Science Park, S.S. 14km 163.5, 34012 Basovizza, Italy IMEM-CNR, Institute of Materials for Electronics and Magnetism, Trento site c/o Fondazione Bruno Kessler, Via alla Cascata 56/C, Povo, I-38123 Povo (Tn), Italy d CNR – Istituto Nanoscienze, Via Campi 213/A, I-41125 Modena, Italy e IMEM-CNR, Institute of Materials for Electronics and Magnetism, Parco Area delle Scienze 37/A, I-43124 Parma, Italy f Department of Earth and Environmental Sciences, University of Milano-Bicocca, I-20126 Milano, Italy b c

A R T I C LE I N FO

A B S T R A C T

Keywords: Graphene Catalysis Platinum nanoparticles, metal carbonyl clusters

Having single atoms or small clusters docked onto a single layer graphene represents a charming feature for energy-storage and catalysis. Unfortunately, the large cohesion energy of transition metals often prevents the isolation of nanoscopic clusters, which invariably tend to aggregate. The decoration of defective graphene layers with single Pt atoms and sub-nanometric clusters is herein achieved by exploiting metal carbonyl clusters, as precursor, and investigated by means of transmission electron microscopy and X-ray photoemission spectroscopy. Unexpectedly, the process of aggregation of Pt into larger clusters is inhibited onto the surface of defective graphene, where the Pt-clusters are found to fragment even into single metal atoms.

1. Introduction The availability of chemically and physically produced bulk graphene is of prominent relevance in applications requiring large surface area materials or components [1,2]. Above all, catalysis and energy storage are widely explored research areas of potential use, in which graphene has shown better performances as compared to other porous materials, especially when decorated with small metallic clusters, or even single atoms [3–5]. However, transition metals possess large cohesive energy and they easily aggregate in the form of nanoparticles (NPs). The adoption of suitable synthetic methods and an efficient anchoring of the nanoparticles just after their formation can however contrast this tendency. In 2009, Singh et al. demonstrated that the graphene surface is theoretically able to support the hydrogen spillover process when decorated by Pt, Pd, or Ni clusters [3]. However, the high cohesive energies of transition metals often reduce the control of nanoparticle sizes during the synthesis, although it was shown [6] that, unlike other supports, graphene promotes the formation of Pt sub-nanometric clusters showing an improved catalytic performance [7] . The graphene-related material produced by the chemical/thermal exfoliation of graphite oxide is furthermore inevitably rich in structural defects, which also can help in the metal nanoparticle docking. In the case of platinum, it has been calculated that the presence of mono-, di-

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and tri-carbon atom vacancies on the graphene layer, promotes the binding between graphene and the metal (either single atoms, or small clusters) with three-times higher stabilization energy compared to a perfect layer [8–10]. This has partly been demonstrated in atom/ion bombardment (Pt, Co, and In) experiments [9]. In particular, the stabilization of a Pt adatoms onto graphene is calculated to be promoted on a di-vacancy, while a Pt cluster gains the same binding energy (BE) only when its nuclearity (i.e. the number of Pt atoms constituting the cluster) reaches the value of six (about 180 kcal/mol). Even five- or seven-member carbon rings are considered to increase the stabilization of Pt adatoms [10], while grain boundaries behave as nucleation centres for Pt decoration [11]. Moreover, the presence of hydroxyl or carboxyl groups on graphene oxide can supply basic groups, able to stabilize Pt ions by coordination bonds. It consequently appears that defects in graphene can increase the probability of finding Pt adatoms on the carbon layer, rather than nanoparticles. Generally, it can be inferred that, starting from high nuclearity clusters or NPs, the chance to obtain small clusters, or even Pt adatoms, is significantly reduced by the fact that the cohesive force of Pt atoms predominates over their individual stabilization on the carbon surface. Several experimental studies report about Pt NPs on graphene [12–19], whilst many theoretical predictions have been made so far [3,20]. Nitrogen defects prevent the agglomeration of platinum, even

Corresponding author. E-mail address: [email protected] (D. Pontiroli).

https://doi.org/10.1016/j.susc.2019.121499 Received 9 July 2019; Received in revised form 4 September 2019; Accepted 10 September 2019 Available online 12 September 2019 0039-6028/ © 2019 Elsevier B.V. All rights reserved.

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333 K for one day. Batches of about 600 mg of GO were then placed on the bottom of a quartz vial with 2 dm3 inner volume and connected to a turbo-molecular plus rotary vacuum pump system. The TEGO was obtained, as described in ref. [29], by suddenly exposing the GO to 1423 K in dynamic high vacuum (<10−5 mbar), in order to exfoliate it into graphene layers. The exfoliated material was held at this temperature for 30 min before undergoing a fast cooling in air to room temperature. The vial was then disconnected and opened in Ar atmosphere (<1 ppm of O2 and H2O), in order to prevent the saturation of dangling bonds formed during the detachment of oxygen-containing groups during the exfoliation. From here on, TEGO was always handled under inert atmosphere, or vacuum, and never exposed to air. Acetonitrile (CH3CN, 99.8%, Sigma-Aldrich) was anhydrified and degassed before use. Its use was preferred, with respect to the commonly used dimethylformamide (DMF) or n-methyl-2-pyrrolidone (NMP), because of its lower boiling point and good effectiveness in suspending TEGO. This facilitates the evaporation at room tem-perature, avoiding the thermally-induced decomposition of Pt carbonyl clusters. Platinum carbonyl cluster (TBA+)4[Pt19(CO)22]4− (TBA = tetra-n-butylammonium), whose synthesis is described elsewhere [30,31], was chosen because of the low nuclearity and the high solubility in CH3CN. The relatively high thermal stability of [Pt19(CO)22]4− allows preserving its integrity in acetonitrile up to the boiling temperature of the solvent [31]. Assuming TEGO as entirely composed of carbon, we synthesized the Pt decorated TEGO (Pt-TEGO) according to the atomic ratio (Pt:C) 1:100. This ratio was chosen as representative of metal decorated graphene for electrochemical and catalytic applications (see also investigations of Ni-TEGO in ref. [29]). About 200 mg of TEGO and 1 mol% of (TBA)4[Pt19(CO)22]4− were suspended/dissolved separately in acetonitrile (10 mL each) by continuous magnetic stirring inside Schlenk glassware. The mixtures were then combined at room temperature and kept under stirring overnight. The solvent was removed by slow evaporation at room temperature. In order to remove the carbonyl groups from Pt clusters, Pt-TEGO was heated up to 573 K in dynamic high vacuum with a fast ramp (20 K/min) and held at this temperature for 20 min before cooling. The rapid heating promotes the molecular decomposition, facilitating the nucleation process and inhibiting the growth of nuclei.

promoting the dispersion of adatoms on graphene layers [21], although the method of synthesis (by atomic layer deposition) limits the upscaling of this procedure [22]. A high-yield production of nanostructured Pt on graphene, demanded for several applications, requires new synthetic routes, following alternative approaches to standard techniques NPs (e.g. thermal decomposition of metal-organic precursors or reduction of H2PtCl6). A possible breakthrough can be expected by starting from small, already nanostructured, platinum clusters, such as the family of Pt carbonyl cluster salts, i.e. (L+)n[Ptx(CO)y]n− (where x = 4–38; y = 5–44; n = 2–4; and L+: counter ions) [23,24]. These metal-organic compounds are characterised by weak lattice energies and they are available in a wide range of structures and nuclearities. Such molecules can be further stabilized by different ligands to facilitate solubility in common solvents. Imaoka et al. recently carried out an electrochemical study on small-nuclearity Pt clusters, discovering that Pt19 shows the best catalytic activity for oxygen-reduction reactions [25]. Interestingly, these clusters display a different coordination with respect to bulk face-centered cubic (fcc) Pt and they are characterized by different BEs. Moreover, the carbonyl cage, responsible for the chemical stability of the cluster, can be easily removed by mild heating, leaving behind a naked metallic core [26]. Usually, this process brings to the formation of large metal clusters or metal droplets formed by the coalescence of many carbonyl clusters cores. However, the use of highly defective fewlayers graphene, such as the one obtained from thermal exfoliation of graphite oxide, is hereby shown to 1- promote the fragmentation of the precursor clusters into Pt atoms and 2- hinder their aggregation, this probably due to the large number of active sites. These vacancies can anchor the clusters and avoid their diffusion onto the surface (pinning the clusters rather than allowing them to migrate and aggregate). With this conceptual approach, we present here a simple method for the preparation of a new functional material with potential application in catalysis, based on the graphene layer decoration with Pt starting from thermally exfoliated graphite oxide (TEGO), with active defects. This was obtained by operating under strictly controlled atmosphere after the graphite oxide (GO) exfoliation and by using carbonyl metal clusters as molecular metal precursor. During the decoration process, unlike all the other cases reported so far, a fragmentation of the original cluster, rather than and aggregation, is observed.

2.2. Transmission electron microscopy 2. Materials and methods For transmission electron microscopy (TEM) analysis, a small amount of Pt-TEGO was dispersed in isopropanol and a droplet was deposited onto holey carbon coated copper grids. The morphology and structure were determined by high resolution transmission electron microscopy (HRTEM) and by high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM). The latter was used to reveal the possible different species of the particles, due to the high contrast (proportional to Zα, with α∼2, Z being the atomic number of the element in the sample) of this technique. The observations were carried out on a JEOL JEM-2200FS microscope equipped with a Schottky gun working at 200 kV (point resolution 0.19 nm), an incolumn energy filter (Ω-type), a CCD high-resolution camera, STEM detectors, and an energy dispersive spectroscopy (EDS) detector. X-ray photoelectron spectroscopy (XPS) investigation was undertaken on the (TBA+)4[Pt19(CO)22]4− compound, TEGO and Pt-TEGO, using powder samples that were first dispersed in isopropanol. The Pt cluster was deposited onto a cleaned Si surface and directly measured after the solvent evaporation, while the other powders were post-collected on a polyethylene foil. The reliability of this procedure has been investigated previously, and it did not introduce any significant contribution or artefacts [32].

2.1. Materials RW-A grade graphite (SGL Carbon, average size 66 µm) was oxidized following the Brodie method [27]. We adopted this approach as: 1- it avoids the use of potassium permanganate (used in the more common Hummers’ method), which can easily pollute the obtained graphite oxide and graphene with manganese NPs; 2- unlike the Hummer method, the Brodie method promotes the in plane functionalization rather than the edge functionalization [28], so that the GO obtained is less hydrophilic, but more suitable for thermal exfoliation. The TEGO obtained is thus richer in single/few layers graphene than in other cases. Specifically, the graphite (5 g) was mixed with sodium chlorate powder (40 g) in a 1:8 wt ratio and cooled with an ice bath in a fumehood. By keeping the mixture under continuous stirring, 50 ml of concentrated HNO3 was slowly added, with negligible change in temperature. The suspension was heated at 333 K for 8 h with a slow thermal ramp (20 K/h) and then cooled to RT (30 K/h). After continuous stirring for 1 day, the suspension appeared dark green. The product was then diluted in type-1 purified water and filtered through a coarse filter paper. The product was then suspended in a 3 M solution of HCl 37%, filtered, and carefully washed on the filter until the pH of the liquid phase increased to 7. Finally, the product was dried in an oven at 333 K overnight. The as obtained GO was washed and dried in air at

2.3. X-ray photoemission spectroscopy X-ray photoemission spectroscopy (XPS) was carried out ex situ in 2

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Fig. 1. Morphology of the Pt-TEGO film. (a) HAADF image, showing the small Pt particles uniformly dispersed on the graphene substrate. (b) Corresponding HRTEM image from few Pt NPs. Some primal atomic planes are formed, with a distance ∼0.23 nm, as expected from (111) Pt0 planes (see inset). (c, d) Particle diameter histograms from the as-synthesized (c) and 573 K treated sample (d), respectively (numbers inside parenthesis denote associated uncertainties). (e, f) High magnification HAADF images from the as-synthetized (e) and the 573 K treated sample (f) showing in both cases the presence of ultrasmall particles (as the ones pointed by the arrows), compatible with Pt single atoms.

d = 1.4(0.2) nm in the as-synthesized sample (Fig. 1(c)). After thermal treatment of Pt-TEGO at 573 K the average size slightly increases to 1.5(0.4) nm (Fig. 1(e)). It is worth pointing out that the size of a single Pt19 cluster is about 0.8 nm in the longest dimension (see Fig. 2) and 0.45 nm in the shortest one (extent of an inner Pt pentagon). The HAADF images even illustrate single Pt atoms lying on TEGO. These appear as small dots in the regions between the particles (e.g. see magnified region in Fig. 1 (e, f)), with apparent diameter below 0.3 nm, mainly resulting from the finite size of the convergent beam (around 0.2 nm). It is worthwhile mentioning that the Pt atoms dynamically spread on the TEGO film during imaging, attaching to and detaching from the bigger particles continuously (see movie in SI), energetically excited by the electron beam. Such diffusion does not alter significantly the dimension of the particles bigger than ∼1 nm, to possibly affect the size distributions reported in Fig. 1.

an ultra-high vacuum (UHV) chamber, using a non-monochromatized Mg-Kα photon source (1253.6 eV), while the photoelectrons were analyzed by a VSW HA 100 electron energy analyser, leading to a total energy resolution of 0.86 eV. The BEs of all the core levels were referred to the Au 4f7/2 core level signal at 84.00 eV. The peak lineshape was deconvoluted using a Voigt profile, after subtracting the background by means of a Shirley function. The typical uncertainty for the peak energy position is ± 0.05 eV, whereas for the full width at half maximum (FWHM) and for the area evaluation is about ± 5%. 3. Experimental results 3.1. Platinum distribution on TEGO TEM and Raman characterization of our as-synthesized TEGO can be found in ref. [29]. TEGO consists of single and few-layer graphene (1–3 layers) composed by very thin flat sp2 domains (average diameter ∼12 nm) surrounded by a thicker defective network (sp2/sp3 carbon). After the decoration by Pt clusters, the Pt-TEGO appears as shown in Fig. 1. HAADF-STEM images reveal a uniform decoration of the graphene layers by Pt NPs (Fig. 1(a)). By inspection of the HRTEM images (as the one in Fig. 1(b)), and by their fast Fourier transforms, some particles present rudimental planes at 0.23 nm as expected for (111) atomic planes in fcc platinum. The diameter of these aggregates has been measured from STEM images by considering the Feret's diameter. Histograms of NP diameters adapt to a log-normal distribution. Leastsquare minimization of histograms resulted in an average size of

3.2. Composition and bonding XPS analysis reveals the presence of different chemical species (see Fig. SI1 and Tables SI1, SI2, and SI3 in the supporting information), leading to atomic percentages of C, O, Pt, and N elements as reported in Table 1. The solid state structure of the anion [Pt19(CO)22]4− cluster is displayed in Fig. 2. Detailed XPS C1s, O1s core level spectra of TEGO, Pt-TEGO and (TBA+)4[Pt19(CO)22]4− cluster are shown in Fig. 3 (a, b). In TEGO, the dominant carbon fraction is sp2-hybridized (C=C, 284.20 eV, 74.6% of the whole C1s area), with the presence of sp3 CeC groups (285.33 eV, 16.4%), probably due to amorphous grain-boundary 3

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C1s peaks are related to the Pt cluster, one associated to the CO groups placed as a bridge between two Pt atoms (285.68 eV, 13.0%), and the other one belonging to the CO directly bound to single Pt atoms and placed at the edge of the cluster (286.68 eV, 15.6%) [34]. The corresponding features in the O1s core level are located at 531.63 (43.6%) and 532.80 eV (51.8%), respectively (Fig. 3(b)). Both in C1s and in O1s, peaks at higher BE (288.78 eV and 535.81 eV) are shake-up structures, related to energy loss processes of photoelectrons, typical of organic molecules. Thus, the C1s and O1s analysis well reflect the structure of the metal-carbonyl complex counterbalanced by 4 TBA units, also from the quantitative point of view, as shown in SI1, SI2. The Pt 4f core level, characterized by a spin orbit doublet (4f5/2 and 4f7/2), reveals the presence of three different valence states Pt0, PtII and PtIV (Fig. 3(c)), in agreement with previous studies [35]. About 19.9% of the total platinum is present in the zero valence state, Pt0 (Pt 4f7/2 peak at 71.15 eV), 69.6% is attributed to PtII (71.87 eV), and 10.5% belongs to PtIV (73.85 eV). The Pt° feature can be attributed to the cluster core (2 atoms), with a possible contribution from the two Pt atoms experiencing only a single bond with a CO group, but all other links with Pt atoms. PtII and PtIV species can be ascribed to electron-deficient Pt atoms involved in d-π* back-donation toward CO molecules [36–38]. The 4/15 theoretical ratio of metal to oxidized platinum species is in good agreement with Pt0/(PtII+PtIV) value experimentally found, being 0.28. TEGO decoration by Pt19(CO)22 is successful, as suggested by the presence of platinum (Table 1), with a Pt:C ratio close to 1:100 value, used for their synthesis. The C1s core level is characterized by five components related to pristine TEGO (Fig. 3(a)). Other two peaks at 284.80 (7.5% of the total C signal) and 287.22 eV (7.4%) can be ascribed to a residue of acetonitrile used for the preparation of this material [39], as evidenced by the presence of nitrogen (Figure SI1 in SI). The decoration by platinum does not seem to affect the carbon chemistry of TEGO, with the ratio between sp2 and sp3 carbon species at 284.25 and 285.38 eV apparently unchanged (64.3% and 14.1%, respectively). The O1s core level is characterized by two main peaks at 532.27 eV and 533.50 eV, related to the C=O/COOH and CeO species (Fig. 3(b)), in agreement with pristine TEGO results. It is worthwhile to mention that the peculiar features of C=O groups observed in C1s and O1s core levels for Pt19(CO)22 cluster are not present in the as synthesized Pt-TEGO, and similarly happens for TBA+ features. This suggests that clusters readily react with TEGO during the synthesis at room temperature, possibly releasing most of the C=O molecules. A third component is present in the O1s core level at 533.30 eV (7.4% of the total oxygen) whose origin is discussed hereafter. The Pt 4f core level in Pt-TEGO shows three doublets with Pt 4f7/2 peaks located at 71.18, 72.06 and 73.66 eV. Although some BE differences are present with respect to Pt19(CO)22 results, these peaks can also be related to Pt0, PtII and PtIV metal valence states [38,40,41]. Moreover, the intensity of PtIV (36.0%) is increased with respect to the Pt19(CO)22 molecule, while PtII component decrease to 45.6% and Pt0 is almost unchanged (Fig. 3(c), Table SI3). The increase of PtIV weight and the higher BE (+0.43 eV) for PtII species once more confirm the modification of the original metallorganic complex structure, with formation of new Pt-O bonds. Indeed, the O1s component at 533.30 eV can be identified as the oxygen counterpart of these chemical species, which were not found in the genuine complex. Being the graphene structure almost unchanged, these Pt-O links are due to reaction of Pt particles with C=O/COOH and CeO groups on graphene. It has been suggested that Pt-O bonds are nucleation sites for Pt nanoparticles on graphene [18,40], in fair agreement with STEM results where aggregates larger than single Pt19 clusters have been found. The thermal treatment at 573 K removes any trace of nitrogen (see Table 1 and Figure SI1), as evidenced also by the absence of acetonitrile peaks in C1s lineshape (Fig. 3A, Table SI1). The five components associated to the as-prepared graphene are still present and, once more, strictly similar to the pristine graphene. Same results are obtained by

Fig. 2. Pt19(CO)22 cluster (Pt, blue; C, gray; O, red). Table 1 Quantitative XPS analysis of samples.

TEGO Pt-TEGO as synth. Pt-TEGO 573 K (TBA)4[Pt19(CO)22]

%C

%O

% Pt

%N

96.8 86.9 93.7 61.2

3.2 5.9 5.5 15.2

0 0.7 0.8 14.8

0 6.5 0 8.8

regions as already described by TEM analysis [29]. Peak at 290.03 (4.0%) is a fingerprint of aromatic domains, being related to a π-π* electron promotion process. Components at 287.66 and 286.48 eV are related to carbonyl groups C=O (1.9%) and presence of hydroxyl groups CeO (3.1%), respectively. Indeed, as previously observed, the high temperature exfoliation of GO leaves about 3% of oxygen (see Table 1) [29,32], ascribed to the two aforementioned oxidized carbon C1s components and to the corresponding two components in the O1s core level (Fig. 3b) at 532.21 eV (C=O/COOH, 30.9%) and 533.42 eV (CeO, 69.1%). Atomic percentages of (TBA+)4[Pt19(CO)22]4− for C, O, Pt and N reported in Table 1 are in agreement with theoretical values of 65.6%, 16.8%, 14.5% and 3.1%. The C1s lineshape of the organic cation is characterized by several features (see Fig. 3(a)). Two components at 284.83 and 285.93 eV can be associated to the CeC bonds in the butyl chains (53.7%) and to the CeN bonds (13.5%) of the TBA [33]. Two 4

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Fig. 3. C1s (a), O1s (b), and Pt 4f (c) lineshape analysis of as-prepared TEGO, Pt-TEGO, Pt-TEGO after thermal treatment at 573 K, and (TBA+)4[Pt19(CO)22]4− cluster. The background has been subtracted and the intensity normalized for each spectrum.

responsible for the Pt atoms stabilization. Although speculative, due to the difficulties in measuring atomic Pt on a non-flat bulk graphene, this hypothesis is supported by theoretical studies, showing a stabilization effect of Pt (and other metals) on vacancy type defects [42], which were proved to be the dominant type of defects in TEGO [29,43,44]}. It has been previously reported that the interaction between Pt NPs and graphene is mediated by PtII and PtIV species grafted on the edge and on the basal plane of the graphene, possibly bound to oxygen groups [18,40]. This is in contrast to what happens to Pt clusters supported on an electronic insulator, such as MgO or γ-Al2O3, where the metallic core maintains its structure, even after decarbonylation at 393 K under vacuum [45,46]. Conversely, in the case of TiO2 supported Pt-catalysts, it was found that CO adsorption induces a significant increase in the initial size of Pt NPs, even at room temperature. This reductive agglomeration is preceded by the oxidative disruption of smaller clusters at low CO pressure, following the Ostwald ripening mechanism (static coalescence) [47]. This is apparently facilitated by the electron transfer from TiO2 to the supported metal, resulting in the reduction of Ptn+ ions to Pt0 atoms, the latter undergoing agglomeration [48]. During this process, single Pt adatoms are temporary detached from clusters, forming mobile carbonyl species (e.g. Pt(CO)2), thus clarifying that the substrate plays an important role, allowing the diffusion toward larger agglomerates. The case of epitaxial graphene has been investigated by Gerber et al. in 2013, showing the marked tendency of Pt clusters to sinter upon CO exposure, through a Smoluchowski ripening mechanism [49]. This implies that clusters, rather than Pt adatoms, constitute the mobile species, leading to their dynamic coalescence. In the present case, TEGO is far from the ‘perfect’ single layer graphene, having a high amount of topological defects (e.g. atom vacancies, Stone-Wales defects [50], oxygen containing groups, and sp3 carbons) and border regions (i.e. armchair or zig-zag edges) that can

analysis of O1s components related to C=O/COOH and CeO. The O1s component at 533.30 eV, previously attributed Pt-O bonds, is still present but with lower intensity (3.5%). This suggests a significant reduction of Pt species after thermal treatment, as also confirmed by the Pt 4f lineshape analysis, which shows a lower intensity of the PtII (Pt 4f7/2 at 72.16 eV, 30.9%) and PtIV (73.92 eV, 16.5%) components and a strong increase of the metal species Pt0 (71.10 eV, 52.6%), which becomes dominant. 4. Discussion Although the synthesis has been carried out well below the cluster decomposition temperature [31], both TEM and XPS clearly show that the simple contact of Pt19(CO)22 with TEGO, even at RT, inevitably leads to the disruption of the carbonyl cage and to the structural fragmentation of the cluster. When Pt19(CO)22 clusters are fragmented on TEGO, they release most of the carbonyl groups, a fraction of which can bind to dangling bonds on graphene (which has never been exposed to air) and gives rise to the observed increased oxidation of TEGO. The PtTEGO sample does not show the peculiar XPS features of the (TBA+)4[Pt19(CO)22]4− cluster, i.e. the components related to the TBA and C=O groups are absent both in the C1s and O1s core levels, thus demonstrating the carbonyl cluster fragmentation. This structural modification underpins the variation of the chemical environment, reflecting an increase in the intensity of the PtIV species in the Pt 4f core level. The lack of cluster aggregation, commonly observed upon decomposition of carbonyl clusters, is also confirmed by the analysis of the relative abundance of Pt oxidized species, which changes after TEGO decoration. By using degassed anhydrous solvents, and avoiding air-exposure, defects on TEGO can be kept active. The presence of these paramagnetic centers and non-hydrogenated defects could be 5

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Acknowledgements

capture Pt clusters, thus inhibiting the sintering process. A single graphene layer grown on a Pt(111) surface can be easily separated from the substrate by weakening the interlayer interactions through the intercalation of CO, which is promoted to bind to the metal [51]. This would suggest an increased stability of Pt NPs when these interact with a perfect graphene (small clusters present a different structure and mechanism from larger NPs or bulk). Nevertheless, the carbonyl cage of Pt19(CO)22 is broken by the interaction with TEGO, leading to cluster “melting” on the surface. TEGO itself might promote the Pt19(CO)22 fragmentation, thanks to the high dissociative efficiency of single atomvacancies (dangling bonds), as observed in the case of H2[43]. Topological defects can act as trapping centres for both Pt adatoms and CO molecules, forming stable species that persist even after thermal treatment at 573 K. Small clusters may recombine during the synthesis, undergoing Smoluchowski ripening and therefore coalescing in larger NPs (i.e. ∼1.5 nm particles in Fig. 1). The thermal treatment is expected to increase the mobility of these clusters, thus increasing the number of larger particles. The efficiency of TEGO in capturing Pt atoms by forming stable bonds is demonstrated by the fact that, even after thermal treatment at 573 K, their presence is still confirmed by TEM analysis (see Fig. 1). XPS analysis shows that the oxygen concentration of Pt-TEGO does not change significantly after the thermal treatment. It is a strong indication of stable bonds between the carbonyl groups in graphene and the Pt particles, responsible for the observation of platinum oxidized species (PtII and PtIV) [18]. Indeed, the Pt 4f core level in Pt-TEGO is characterized by three different oxidative states (Pt0, PtII, and PtIV), persisting even after heating at 573 K. The zerooxidation state may be ascribed to Pt atoms belonging to the core of metallic particles. Single Pt adatoms onto a graphene layer should exhibit oxidation states higher than zero, as their orbitals should be hybridized with the underlying π-structure or by oxygen-electronic states belonging to functional groups attached on TEGO. Thus, the presence of PtII and PtIV species can be attributed to these species while their significant reduction after thermal treatment supports the coalescence process, leading to the formation of slightly larger clusters. The Pt atoms on the surface of the metal particles, forming a bridge with the graphene layer, might exhibit the Pt(IV) oxidative state. Thermal treatment at 573 K promotes the clusters coalescence, rising the fraction of Pt0 inner cores species, as confirmed by the increased intensity of the Pt° component in the Pt 4f core level, and consequently of the Pt0/ (PtII+PtIV) ratio of the XPS contributions with respect to the not treated Pt-TEGO and to the (TBA+)4[Pt19(CO)22]4− cluster.

This work has been financially supported by the Marie Curie IRSESEU Project MagNonMag number 295180 and by the Cariplo foundation (Project number 2013-0592, “Carbon based nanostructures for innovative hydrogen storage systems”). The authors would like to acknowledge Prof. Alessandro Ceriotti, Neeraj Sharma and James C. Pramudita for fruitful discussions. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.susc.2019.121499. References [1] A. Züttel, A. Borgschulte, L. Schlapbach, Hydrogen as a Future Energy Carrier, WILEY-VCH, 2011. [2] J. Zhu, D. Yang, Z. Yin, Q. Yan, H. Zhang, Graphene and graphene-based materials for energy storage applications, Small 10 (2014) 3480–3498. [3] A.K. Singh, M.A. Ribas, B.I. Yakobson, H-Spillover through the catalyst saturation: an Ab Initio Thermodynamics study, ACS Nano 3 (2009) 1657–1662. [4] E. Quesnel, F. Roux, F. Emieux, P. Faucherand, E. Kymakis, G. Volonakis, et al., Graphene-based technologies for energy applications, challenges and perspectives, 2D Mater. 2 (2015) 030204. [5] F. Giglio, D. Pontiroli, M. Gaboardi, M. Aramini, C. Cavallari, M. Brunelli, et al., Li12C60: a lithium clusters intercalated fulleride, Chem. Phys. Lett. 609 (2014) 155–160. [6] R. Siburian, J. Nakamura, Formation process of Pt subnano-clusters on graphene nanosheets, J. Phys. Chem. 116 (2012) 22947–22953. [7] R. Siburian, T. Kondo, J. Nakamura, Size control to a sub-nanometer scale in platinum catalysts on graphene, J. Phys. Chem. C 117 (2013) 3635–3645. [8] T. Yumura, T. Awano, H. Kobayashi, T. Yamabe, Platinum clusters on vacancy-type defects of nanometer-sized graphene patches, Molecules 17 (2012) 7941–7960. [9] H. Wang, Q. Wang, Y. Cheng, K. Li, Y. Yao, Q. Zhang, et al., Doping monolayer graphene with single atom substitutions, Nano Lett. 12 (2012) 141–144. [10] Y. Okamoto, Density-functional calculations of icosahedral M13 (M=Pt and Au) clusters on graphene sheets and flakes, Chem. Phys. Lett. 420 (2006) 382–386. [11] K. Kim, H.-B.-R. Lee, R.W. Johnson, J.T. Tanskanen, N. Liu, M.-.G. Kim, et al., Selective metal deposition at graphene line defects by atomic layer deposition, Nat. Commun. 5 (2014) 4781. [12] P. Xu, L. Dong, M. Neek-Amal, M.L. Ackerman, J. Yu, S.D. Barber, et al., SelfOrganized platinum nanoparticles on freestanding graphene, ACS Nano 8 (2014) 2697–2703. [13] S. Guo, D. Wen, Y. Zhai, S. Dong, E. Wang, Platinum nanoparticle ensemble-ongraphene hybrid nanosheet: one-Pot, rapid synthesis, and used as new electrode material for electrochemical sensing, ACS Nano 4 (2010) 3959–3968. [14] S. Mukherjee, B. Ramalingam, S. Gangopadhyay, Hydrogen spillover at sub-2 nm Pt nanoparticles by electrochemical hydrogen loading, J. Mater. Chem. A 2 (2014) 3954–3960. [15] B. Seger, P.V. Kamat, Electrocatalytically active graphene-platinum nanocomposites. Role of 2-D carbon support in PEM fuel cells, J. Phys. Chem. C 113 (2009) 7990–7995. [16] N. Shang, P. Papakonstantinou, P. Wang, S.R.P. Silva, Platinum integrated graphene for methanol fuel cells, J. Phys. Chem. C 114 (2010) 15837–15841. [17] Y. Si, E.T. Samulski, Exfoliated graphene separated by platinum nanoparticles, Chem. Mater. 20 (2008) 6792–6797. [18] M.-.Y. Yen, C.-.C. Teng, M.-.C. Hsiao, P.-.I. Liu, W.-.P. Chuang, C.-C.M. Ma, et al., Platinum nanoparticles/graphene composite catalyst as a novel composite counter electrode for high performance dye-sensitized solar cells, J. Mater. Chem. 21 (2011) 12880–12888. [19] M.M. Devi, S.R. Sahu, P. Mukherjee, P. Sen, K. Biswas, Graphene: a self-reducing template for synthesis of graphene–nanoparticles hybrids, RSC Adv. 5 (2015) 62284–62289. [20] G. Kim, S.-.H. Jhi, Carbon monoxide-tolerant platinum nanoparticle catalysts on defect-engineered graphene, ACS Nano 5 (2011) 805–810. [21] S. Stambula, N. Gauquelin, M. Bugnet, S. Gorantla, S. Turner, S. Sun, et al., Chemical structure of nitrogen-doped graphene with single platinum atoms and atomic clusters as a platform for the PEMFC electrode, J. Phys. Chem. C 118 (2014) 3890–3900. [22] S. Sun, G. Zhang, N. Gauquelin, N. Chen, J. Zhou, S. Yang, et al., Single-atom catalysis using Pt/Graphene achieved through atomic layer deposition, Sci. Rep. 3 (2013) 1775. [23] I. Ciabatti, C. Femoni, M.C. Iapalucci, G. Longoni, S. Zacchini, Platinum carbonyl clusters chemistry: four decades of challenging nanoscience, J. Cluster Sci. 25 (2014) 115–146. [24] J.C. Calabrese, L.F. Dahl, P. Chini, G. Longoni, S. Martinengo, Synthesis and structural characterization of platinum carbonyl cluster dianions bis,tris,tetrakis, or pentakis(tri-.mu.2-carbonyl-tricarbonyltriplatinum)(2-). new series of inorganic oligomers, J. Am. Chem. Soc. 96 (1974) 2614–2616. [25] T. Imaoka, H. Kitazawa, W.-.J. Chun, K. Yamamoto, Finding the most catalytically

5. Conclusions We succeeded in decorating thermally exfoliated graphite oxide with Pt starting from the carbonyl metal cluster (TBA+)4[Pt19(CO)22]4−. Surprisingly, molecular decomposition is triggered by the interaction of the carbonyl clusters with TEGO already at room temperature and it results in the fragmentation of clusters and the formation of a narrow-size distribution of nanoparticles (average size around 1.4 nm). Moreover, TEM investigations highlight the presence of single Pt adatoms functionalizing the graphene surface. Thermal treatment of Pt-TEGO in dynamic vacuum at 573 K does not significantly alter the morphology of the system and the expected coalescence of the Pt clusters into larger Pt drops is not observed. However, the fractions of Pt0, PtII, and PtIV species, as detected by XPS analysis, change. The increase in fraction of Pt0 has been ascribed to the Smoluchowski ripening of small clusters. Single Pt adatoms attached on graphene are present either in the IV or II oxidation state, depending on their position. The structural rearrangement of particles following the thermal treatment is demonstrated by the reduction of PtIV oxidized species.

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