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Controlled Growth of Palladium Nanoparticles on Graphene Nanoplatelets via Scalable Atmospheric Pressure Atomic Layer Deposition Hao Van Bui, Fabio Grillo, Ryan Helmer, Aristeidis Goulas, and J. Ruud van Ommen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b02285 • Publication Date (Web): 07 Apr 2016 Downloaded from http://pubs.acs.org on April 8, 2016

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The Journal of Physical Chemistry

Controlled Growth of Palladium Nanoparticles on Graphene Nanoplatelets via Scalable Atmospheric Pressure Atomic Layer Deposition Hao Van Bui,∗,†,¶ Fabio Grillo,†,¶ Ryan Helmer,† Aristeidis Goulas,‡ and J. Ruud van Ommen† †Department of Chemical Engineering, Delft University of Technology, 2628 BL, Delft, The Netherlands ‡Delft IMP B.V., 2628 BL, Delft, The Netherlands ¶Contributed equally to this work E-mail: [email protected] Phone: +31 (0) 15 278 2635

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Abstract We demonstrate the deposition of crystalline palladium nanoparticles on graphene nanoplatelets (Pd/graphene) via atmospheric pressure atomic layer deposition (ALD) carried out in a fluidized bed reactor. The nucleation and growth of Pd nanoparticles on the inert graphene surface was enabled by applying an ozone pretreatment step, without significantly affecting the graphene crystalline structure. Uniform nucleation on both basal planes and edges of the graphene was obtained. The Pd loading and dispersion as well as the average particle size could be controlled by varying the number of ALD cycles. By analyzing the evolution of the particle size distribution and NPs spatial density we obtained insights into the nucleation and growth of Pd ALD on graphene. Furthermore, by shortening the pretreatment time, selective growth of Pd nanoparticles on the edges of the graphene was achieved. The Pd/graphene fabricated with our method showed significantly lower level of impurities compared to the Pd/graphene synthesized by wet chemistry routes. Our approach provides a 100% solvent-free, controllable and scalable process for producing the bulk quantities of Pd/graphene required for practical applications in, for example, catalysis.

Introduction Graphene-supported nanoparticles (NPs) are emerging as a novel class of high-performance catalysts 1–4 . The use of graphene for supporting NPs, and catalytic materials in general, boasts several advantages. For instance, the high chemical, thermal and mechanical stability of graphene makes it an ideal support for durable catalysts. Its high surface area coupled with the high surface-to-volume ratio of NPs enables optimal active site dispersion. Moreover, the high electron mobility of graphene can facilitate the electron transfer during chemical reactions and thus enhance the catalytic activity of the supported NPs 1,2 . Hence, the coupling of NPs and graphene opens up new avenues for meeting the incessant demand for high performance catalysts. For this reason, a great deal of research has been devoted to 2


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the fabrication of graphene-supported NPs of several noble and base metals (e.g. Pt, Pd, Au, Fe, Co, Ni), as well as metal oxides (e.g. TiO2 , WO3 , ZnO, SnO2 , Co3 O4 ), for various applications such as electrocatalysis, photocatalysis, water treatment and splitting, and energy generation and storage 1–8 . In particular, catalysts based on graphene-supported palladium NPs (Pd/graphene) have been shown to outperform conventional Pd-based catalysts for many catalytic reactions such as Suzuki-Miyayra coupling 9–11 , hydrogenation of carboxylic acids 12 , oxygen reduction 13 , and various oxidation reactions of aromatic alcohols 14,15 , formic acid 16 , carbon monoxide 17 and methanol 18 . A number of methods have already been developed for the synthesis of Pd/graphene. However, most of these methods rely on wet chemistry and thus the use of various Pd-precursors in the presence of solvents and surfactants 9,10,14–16,19–30 . Such methods, despite their simplicity and inexpensiveness, have several shortcomings that limit the practical application of catalysts based on Pd/graphene. For instance, one cannot easily control the metal dispersion as solution-based techniques are highly sensitive to experimental parameters such as the metal precursor concentration, type of solvent, presence of a dispersing and/or reducing agent, and especially deposition time and temperature 4 . In addition, wet chemistry syntheses inherently result in Pd/graphene with high level of impurities arising from residual solvent and reaction by-products. This can significantly affect the lifetime and durability of the catalysts. Therefore, a solvent-free and scalable method for the synthesis of Pd/graphene, capable of depositing NPs in a controlled fashion, is highly desirable. Atomic layer deposition (ALD) is a solvent-free gas-phase technique that has been developed for the deposition of conformal thin films and NPs on various substrates. By relying on sequential self-limiting surface reactions, ALD boasts a thickness control down to the sub-nanometer level 31 . Such unprecedented precision has seen ALD flourish in the semiconductor industry, where fabrication techniques capable of accessing the nanoscale are vital to the fabrication of ever-shrinking electronic devices 31,32 . Unlike many other gas-phase techniques, ALD is not a line-of-sight technique as the ALD surface-reactions can take place on 3



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any exposed surface that has suitable active sites 33 . For this reason, ALD lends itself to the deposition of material on substrates with various geometries, and, in particular, powders. ALD of metals and metal oxides on powders is proving to be an attractive route for the fabrication of catalysts with tailored properties 34–43 . In fact, ALD enables the deposition of NPs, clusters 7,34,44–46 and even single atoms 47,48 . ALD of Pd has been developed for depositing Pd thin films on flat substrates using conventional ALD reactors with chemistries based on either a reducing or an oxidizing step. 49–55 . As discussed in a recent review by Hämäläinen et al. 56 , the choice of the ALD chemistry and experimental conditions (e.g., temperature) significantly influences the product morphology and composition (i.e., metallic Pd or PdO). For instance, metallic Pd can be deposited via ALD by using Pd(hfac)2 and H2 at temperatures in the range of 80-130◦C. However, if formaline is used instead of H2 as the reducing agent, one needs to operate at higher temperatures (i.e., 200 ◦C) in order to deposit metallic Pd. Alternatively, metallic Pd can be deposited on oxygen terminated surfaces via ALD by using Pd(thd)2 and an oxidizer such as O2 , as long as one either carries out the deposition at temperatures above 200 ◦C or adds a reducing step with H2 after each oxidation step 56 . The deposition of Pd NPs via ALD on nanopowders and active carbon has been demonstrated by using viscous flow and fluidized bed reactors (FBRs) 35,38–40 . Yan et al. 48 recently reported the deposition of single-atom Pd on graphene via ALD in a viscous flow reactor. The single-atom Pd/graphene thus fabricated showed unprecedented catalytic performance for the selective hydrogenation of 1, 3-butadiene. Nevertheless, in this work a solution-based method was used to preoxidize the graphene nanosheets in order to activate the relatively inert graphene surface. In addition, viscous flow reactors, despite being a convenient means for coating powders at the lab scale, are not suitable for the production of the bulk quantities of products required for most practical applications 33,36,40,57 . FBRs on the other hand, are inherently scalable, especially for atmospheric pressure operations 36,41,58 . Furthermore, ALD in FBRs on high-surface-area substrates has been shown to be a forgiving process with 4


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regard to the precursor utilization 36,59,60 . The objective of this work is to demonstrate how ALD carried out at atmospheric pressure in FBRs can be used to deposit crystalline metallic Pd NPs on graphene nanoplatelets in a scalable and controlled fashion. We show that by using a gas-phase ozone pretreatment step we could activate the graphene surface towards ALD of Pd based on oxidative chemistry. In doing so, we could achieve uniform nucleation of Pd NPs over the graphene surface, and their controlled growth by varying the number of ALD cycles. The Pd/graphene obtained with our method exhibits a significantly lower content of impurities compared to the Pd/graphene synthesized by the wet chemistry routes reported in the literature. Moreover, by tuning the pretreatment conditions we could achieve selective growth of Pd NPs on the edges of graphene nanoplatelets. Finally, by analyzing the evolution of the particle size distribution (PSD) and NPs spatial density with the number of cycles we obtained further insights into the nucleation and growth of Pd ALD on graphene.

Experimental The experiments were carried out in a custom-built fluidized bed ALD reactor operating at atmospheric pressure already described in a previous work 61 . The system consisted of a glass column (26 mm in internal diameter and 500 mm in height) placed on top of a single motor Paja PTL 40/40-24 vertical vibration table to assist the fluidization. Graphene nanoplatelets (6-8nm thick and 12 µm wide, and a surface area of about 150 m2 g−1 ) and the noble metal precursor, palladium 2,2,6,6-tetramethyl-3,5-heptanedione (Pd(thd)2 ), were obtained from Strem Chemicals and used as received. Synthetic air (20 wt% oxygen) was used as the oxidizing medium. The precursor, contained in a stainless steel bubbler, was heated and maintained at 120 ◦C, whereas the steel tubes connecting the bubbler and the reactor were maintained at 130 ◦C to avoid the condensation of the precursor before reaching the reactor. The reactor was heated by an infrared lamp placed parallel to the column with

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feedback control to maintain a constant temperature of 220 ◦C. In each experiment, 1.5 g of graphene powder was used. Prior to each experiment, the powders were dried at 120 ◦C for 1 hour. An optimized gas flow of 0.6 l min−1 was introduced through a distributor plate at the bottom of the reactor column in order to fluidize the powders. In order to initialize the growth of Pd on graphene, an ozone treatment step at 150 ◦C was applied. This was performed by flowing synthetic air (0.6 l min−1 ) through a OAS Topzone ozone generator. The ozone-enriched air thus obtained had an ozone content of about 1.5 wt%. The ALD process consisted of sequential exposures of the powders to the Pd precursor (5 min) and synthetic air (10 min), separated by a purging step (10 min) using nitrogen as an inert gas (N2 , 99.999 vol%). As-synthesized Pd/graphene powder were suspended in ethanol and transferred to regular transmission electron microscopy (TEM) grids (3.05 mm in diameter). High resolution TEM (HRTEM) micrographs were taken using a FEI Titan G2 60-300 transmission electron microscope operated at 300 kV. TEM images were taken at several locations on the grids using a JEOL JEM1400 transmission electron microscope operating at a voltage of 120 kV and a current density of 50 pA cm−2 . The images were then analyzed by using the software ImageJ to obtain the PSDs. More than 4000 particles were analyzed for each PSD. The Pd/graphene was transferred onto a Si wafer coated with 300 nm of SiO2 . This was to eliminate the influence of the substrate (Si) signal in the XRD patterns of the powders. XRD patterns were obtained by a PANalytical X-pert Pro diffractometer with Cu Kα radiation, secondary flat crystal monochromator and X’celerator RTMS Detector system. The angle of interest 2θ was measured from 10◦ to 90◦ with fine steps of 0.001◦ . A Renishaw inVia Raman microscope equipped with a 514 nm excitation laser was used for Raman spectral study of the Pd/graphene powders, which were immobilized on monocrystalline Si wafers. The Raman peak of Si substrate at 520 cm−1 was used as the reference for the Pd/graphene peak position calibration in each measurement. The Raman spectra were measured in air with a 20 mW power. The integration time for all Raman spectra was 6


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100 ms. The scanned range was 500-3500 cm−1 with intervals of 1.5 cm−1 . A Mettler Toledo TGA/SDTA 851e thermogravimetric analyzer was used for studying the thermal behavior of the synthesized powders. About 3 mg of Pd/graphene was used for each TGA measurement. The TGA curves were recorded while ramping up the powders from 25 ◦C to 800 ◦C with a ramping rate of 5 ◦C min−1 in a synthetic air flow of 100 ml min−1 . Inductively coupled plasma optical emission spectrometry (ICP-OES) was used as the main method to estimate the Pd loading. To assess the reliability of such method for Pd/graphene, we also carried out Instrumental Neutron Activation Analysis (INAA) for several experimental cases obtaining comparable loadings. For each experimental case, the elemental analysis was carried out on material sampled from at least two different locations of the powder bed (top and bottom). For ICP-OES, approximately 25 mg of the sample were destructed in a solution containing 4.5 ml HCl (30%), 1.5 ml HNO3 (65%) and 1 ml HF (40%) using a microwave (Multiwave PRO). The destruction time in the microwave was 180 min at maximum power. After destruction the samples were diluted with 50 ml with Milli-Q water and then analyzed with a PerkinElmer Optima 4300 DV optical emission spectrometer.

Results and discussion The ALD process used in this work is based on the cyclic alternation of the self-terminating chemisorption of Pd(thd)2 and the reactivation of the substrate surface by exposure to an oxidizing agent such as synthetic air. The oxidizing step serves two purposes, namely, the removal of the organic precursor ligands and the restoration of an oxygen-terminated surface, without which the Pd(thd)2 chemisorption cannot proceed 47 . It follows that the absence of active oxygen on bare graphene surfaces is liable to prevent the nucleation of Pd deposition. In fact, 12 cycles of Pd ALD on untreated graphene only resulted in sporadic nucleation, as shown in Figure 1a. The extent of the deposition is consistent with the low defect density of the as-received graphene confirmed by Raman spectroscopy as discussed

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Figure 1: TEM micrographs showing Pd NPs deposited on untreated graphene (a) and O3 -treated graphene (60 min) (b) for 12 ALD cycles.

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Figure 2: TEM micrograph showing selective growth of Pd NPs at the graphene edges after 12 ALD cycles on O3 -treated graphene (30 min) and corresponding PSD.

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Figure 3: TEM micrographs showing Pd NPs deposited on O3 -treated graphene (60 min) for 3 (a), 6 (b) and 12 (c) ALD cycles.

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12 cycles 12 cycles - fit

Figure 4: Particle size distribution of Pd NPs deposited for 3, 6 and 12 cycles on ozonetreated graphene (60 min), and corresponding fitted Weibull distributions.

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later. The initial lack of active sites on graphene surfaces can be addressed with surface pretreatments. Depending on the ALD chemistry of choice, and thus the nature of the required active sites, the use of gas-phase pretreatments such as exposure to NO2 , trimethylaluminum 62,63 and ozone 64 , has been shown to enable ALD growth. In this work, given the role of surface oxygen in the chemisorption of Pd(thd)2 , we employed an ozone treatment prior to the deposition. Ozone molecules can adsorb on graphene surfaces and further react to form oxygen-containing groups such as epoxy and carbonyl groups, thus providing reactive sites for the inception of ALD surface reactions 65–67 . Despite introducing defects into the graphene structure, the ozone treatment did not significantly affect the crystalline structure, as confirmed by the XRD patterns and Raman spectra discussed later. However, the ozone exposure considerably enhanced the nucleation of Pd NPs. The extent of the surface functionalization depends on several parameters such as temperature, partial pressure of ozone, exposure time and the graphene quality. In particular, graphene nanoplatelets consist of two-dimensional, mostly defect-free, basal planes bounded by inherently defective edges. Hence, the edges are expected to readily react with ozone, whereas the functionalization of the basal planes is likely to follow a slower kinetics 68,69 . In virtue of this, by using either short or long exposures, we obtained selective nucleation at the graphene edges or uniform nucleation on both basal planes and edges, respectively. Figure 2 shows a representative TEM micrograph of graphene pretreated with ozone at 150 ◦C for 30 min and then decorated with Pd NPs for 12 ALD cycles at 220 ◦C. The TEM micrographs show that the Pd NPs were mainly deposited on the graphene edges. In particular, the average particle size in this case is about 1.5 nm (Figure 2), which is considerably lower than the 3.9 nm estimated for the NPs deposited for the same number of cycles on the graphene that was pretreated for 60 min (Figure 1b). These results suggest that by tuning the pretreatment conditions (e.g. the exposure time) one can manipulate the nucleation and growth of Pd ALD on graphene, and, in particular, achieve selective growth at the edges. Such growth behavior might prove to be useful in applications such as sensing 70 . 12


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Ozone exposures of 60 min at 150 ◦C prior to Pd ALD resulted in Pd NPs that are uniformly dispersed on both the basal planes and edges of the graphene nanoplatelets. The metal loading and particle size could be varied by changing the number of cycles. Figure 3a, Figure 3b and Figure 3c show TEM micrographs of the Pd NPs deposited on graphene at 220 ◦C, after a long ozone exposure (60 min), for 3, 6 and 12 cycles, respectively. According to ICP-OES the Pd loadings were 0.7%, 1.3% and 3.7% in weight for 3, 6 and 12 cycles, respectively. To obtain further insights into the nucleation and growth of Pd NPs, we extracted the PSD from several TEM micrographs by estimating the projected area and thus the equivalent diameter of more than 4000 particles per sample via image analysis. The histograms thus obtained are plotted in Figure 4. The best fit for all the histograms was given by Weibull distributions. The average particle diameters are 1.5 nm, 2.4 nm and 3.9 nm for 3, 6 and 12 cycles, respectively. Increasing the number of cycles not only resulted in an increase in the average particle diameter but also in wider and right-skewed PSDs. To elucidate the mechanism underlying the broadening and increased right-skewness of the PSD with increasing number of cycles, we estimated the average spatial density of NPs. Since Pd is a face-centered cubic metal, Pd atoms are likely to arrange on graphene surfaces in either cubo-octahedral or icosahedral NPs 71,72 , which is consistent with the presence of the (111), (200) and (220) facets detected by XRD as shown in Figure 5 (see also Figure 6). Hence, by geometric construction, the total number of atoms Ntot in a Pd NP is given by 71–73 :

Ntot =

1 10n3 − 15n2 + 11n − 3 , n ≥ 1 3

(1)

whereas the number of surface atoms Nsurf is: Nsurf = 10n2 − 20n + 12 , n > 1

(2)

where n is an integer number that expresses the number of atomic layers that construct the NP. For example n equals to 1 corresponds to a single atom or central site, whereas n equals 13



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to 2 corresponds to a central site fully coordinated by 12 surface atoms and thus forming a NP of 13 atoms. The diameter of such NP linearly depends on n according to the formula 71 : √ d = (2n − 1)a/ 2

(3)

where a is the lattice constant. Therefore, given the number of cycles i and the corresponding normalized PSD fi (d) and Pd weight fraction wi , we estimated the NPs spatial density si , by imposing the following mass balance: Z

dmax

fi (x)Ntot (x)dx =

si dmin

wi (NA /Ar ) (1 − wi )S

(4)

where NA is the Avogadro constant, Ar the relative atomic mass of Pd and S the surface area per unit weight of graphene nanoplatelets. The left-hand side of Eq. 4 is the average number of Pd atoms per unit of area estimated with the PSD, whereas the right-hand side is the same value estimated with the Pd loading. In addition, we also estimated the Pd dispersion Di , or in other words the fraction of Pd surface atoms (Nsurf /Ntot ) by using the following expression: 5 Di = 6

Z

dmax

fi (x)Nsurf (x)/Ntot (x)dx

(5)

dmin

where 5/6 is a correction factor accounting for the fact that a certain fraction of the surface atoms might not be available for adsorption due to interactions with the substrate 38,74 .

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Table 1: Change in Pd loading, normalized NPs spatial density si /s3 , dispersion Di , average particle size dave i , and standard deviation σ and skewness factor γ1 with the number of ALD cycles.

ALD cycles

Pd loading wt%

3 6 12

0.7 1.3 3.7

si /s3 -

Di %

dave i nm

σ nm

γ1 -

1.0 0.4 0.3

61 49 37

1.5 2.4 3.9

0.6 1.3 2.0

≈0 0.4 0.8

The values of si , normalized with respect to s3 , and Di are tabulated in Table 1 together with the Pd loading, average NP size dave i , and σ and γ1 of the PSD. According to our calculations, the number of NPs decreases significantly with the number of cycles, in agreement with a qualitative analysis of the TEM micrographs (see Figure 1, Figure S1, Figure S2 and Figure S3). By increasing the number of cycles from 3 to 12, the number of NPs decreases by 70%. This suggests that nucleation of new NPs takes place only in the very first cycles. Hence, we speculate that, given the negligible deposition observed on untreated graphene, the maximum number of NPs is formed in the first cycle when the chemisorption of Pd(thd)2 depletes all the active sites created by the ozone treatment. Thus, after the first cycle, the graphene surface will recover its inactivity towards Pd(thd)2 and ALD growth will proceed on preexisting nuclei. The negative net nucleation rate, or in other words the disappearance of NPs with the number of cycles is ascribable to sintering via particle coalescence and/or Ostwald ripening. Ostwald ripening has been speculated to play a major role in noble metals ALD when oxygen is used as the second precursor, as noble metals form volatile oxides which facilitate the transport of atoms between NPs 75 . In our case, ripening would take place mostly during the oxidation step as the carbonaceous layer formed upon the chemisorption of Pd(thd)2 hinders the transport of atoms during the first half-cycle 76 . An increase in number of cycles translates in longer ripening periods and thus in small NPs 15



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feeding larger ones until their disappearance, which is not compensated by nucleation of new NPs. Nevertheless, the appearance of long tails in the PSD with increasing number of cycles suggests that dynamic and/or static coalescence rather than Ostwald ripening is the dominant sintering mechanism under the conditions considered here 76–78 (see Figure 3 and Figure 4). In light of these results, we speculate that by optimizing the ozone pretreament one can vary the initial Pd NPs density, and thereby obtain the desired particle particle size at a given Pd loading by varying the number of cycles, provided that the deposition conditions (e.g. oxygen partial pressure, temperature, exposure times) are tuned so as to suppress sintering while retaining ALD growth. The optimization of the deposition conditions aimed at decoupling the increase of the metal loading from the broadening of the PSD with the number of cycles will be the subject of follow-up studies. Figure 5 shows the XRD patterns obtained for the untreated graphene (Figure 5a), the ozone-treated graphene (Figure 5b), the graphene decorated with Pd nanoparticles for 6 cycles (Figure 5c) and 12 cycles (Figure 5d). The pattern in Figure 5a shows the characteristic peaks of multilayer graphene 79 . The characteristic peak of graphene oxide (i.e., expected at 14.6◦ 80 ) was not observed. This remained unchanged after the ozone surface treatment step (Figure 5b) and the deposition of Pd (Figure 5c and Figure 5d), suggesting that the graphene was not significantly oxidized by the ozone treatment and subsequent Pd deposition. The peaks at 40.1◦ , 46.5◦ , 68.2◦ and 82.3◦ in Figure 5c and Figure 5d are consistent with the presence of the (111), (200), (220) and (311) planes, respectively, of the face-centered cubic crystal structure of Pd 79,81 . The crystallinity of the Pd NPs was also confirmed by HRTEM as shown in Figure 6. No XRD peaks consistent with the presence of PdO phase, expected at 34◦ and 54.4◦ 82 , were observed. The absence of significant amounts of PdO in either crystalline or amorphous form is also supported by the absence of the PdO fingerprint in the Raman spectra (Figure 7, expected at ca. 636 cm−1 ) 17,82 . The results indicate that the Pd NPs are crystalline and metallic. Nevertheless, given the ALD chemistry employed, the Pd NPs are bound to be oxygen terminated. This is in good agreement with the results 16


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a.

Untreated graphene

b.

Ozone-treated graphene

10 20 30 40 50 60 70 80 90 10 20 30 40 50 60 70 80 90 d.

(311)

(220)

(311)

(220)

(200)

(111)

12 cycles

(200)

6 cycles

c. (111)

Intensity (arb. units)

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10 20 30 40 50 60 70 80 90 10 20 30 40 50 60 70 80 90 o

o

2 theta ( )

2 theta ( )

Figure 5: XRD patterns of untreated graphene (a), O3 -treated graphene (b), and Pd NPs deposited on O3 -treated graphene for 6 (c) and 12 (d) ALD.

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a.

b.

c.

d.

Figure 6: HRTEM micrographs of Pd NPs deposited on O3 -treated graphene (60 min) after 12 cycles showing the lattice spacing of the NPs facets.

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reported by Rikkinen et al. 53 , where a similar chemical process was employed to synthesize Pd NPs on a porous carbon support.


Raman spectroscopy is a highly sensitive and versatile technique to characterize graphene-

a.

b.

G

G

D

D

1000 1200 1400 1600 1800 2000 1000 1200 1400 1600 1800 2000 Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


c.

d.

G

G

D D

D'

D'

1000 1200 1400 1600 1800 2000 1000 1200 1400 1600 1800 2000 -1

-1

Raman shift (cm )

Raman shift (cm )

Figure 7: Raman spectra obtained for the untreated graphene (a), O3 -treated graphene (60 min) (b) and Pd NPs deposited on O3 -treated graphene (60 min) for 6 (c) and 12 (d) ALD cycles. based materials as it enables the determination of the number and orientation of graphene layers, disorder and defect density in graphene as well as other physical properties 83 . Figure 7 shows the Raman spectra of the untreated graphene (Figure 7a), the graphene treated in ozone at 150 ◦C for 1 hour (Figure 7b), and the graphene with Pd deposited for 6 cycles (Figure 7c) and 12 cycles (Figure 7d) in the spectral range of interest (i.e., 1000-2000 cm−1 ). The main Raman spectroscopic features of graphene are represented by the D, G, and 2D peaks 83,84 . The G peak at around 1580 cm−1 represents the E2g vibrational mode at the 19



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Γ-point caused by the stretching of the C=C bond in graphitic materials. The 2D peak at around 2700 cm−1 (see Figure S6 in the Supporting Information), also known as G’, can be used to determine the number of layers of graphene. For all the spectra, the peak intensity ratio IG /I2D is larger than unity (Figure S6), which is indicative of multilayer graphene 84 . The D peak at 1357 cm−1 is due to the breathing modes of the sp2 rings, which require a defect for its activation. This peak therefore represents disorder and defects of graphene 84 . For the untreated graphene, the intensity of the D peak was very low, indicating the low defect density of the graphene. The higher intensity of the D peak in the Raman spectrum of the ozone treated graphene (Figure 7b) suggests that defects were introduced by the treatment. We speculate that such defects play a crucial role in activating the nucleation of Pd ALD on graphene, as discussed in the previous section. The broader D peak of the ozone-treated graphene compared to that of the Pd/graphene could be ascribed to the change in the number and nature of the functional groups present on the graphene surface after ALD 68 . The increasing in the ratio ID /IG and the appearance of the D’ peak with increasing number of ALD cycles are caused by the increase of Pd loading 10 . No Raman peak of PdO (at ca. 636-640 cm−1 ) 17,82 was found. Hence, the Raman spectra provide further evidence of the metallic state of the ALD-grown Pd NPs. The quality of the synthesis in terms of impurities was assessed by means of thermogravimetric analysis (TGA). The results are shown in Figure 8. The TGA curve of the untreated graphene exhibits a gradual weight loss (up to about 15%) in the temperature range of 300500 ◦C. This could arise from the desorption of oxygen-containing functional groups (e.g., -OH, -COOH) 15 . The sporadic deposition of Pd NPs on untreated graphene (Figure 1a) suggests that these groups are not suitable for the nucleation of Pd ALD. A qualitatively similar trend is observed for the ozone-treated graphene that undergoes a lower weight loss (i.e.,

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