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. Author manuscript; available in PMC: 2020 Nov 3.
Published in final edited form as: Microsc Microanal. 2019 Jan 30;25(1):80–91. doi: 10.1017/S1431927618016185

Unique Structural Characteristics of Catalytic Palladium/Gold Nanoparticles on Graphene

Kavita Meduri 1, Candice Stauffer 2, Graham O’Brien Johnson 3, Paolo Longo 4, Paul G Tratnyek 3, Jun Jiao 1,2,*
PMCID: PMC7608640  NIHMSID: NIHMS1639551  PMID: 30698125

Abstract

Adding Au to Pd nanoparticles (NPs) can impart high catalytic activity with respect to hydrogenation of a wide range of substances. These materials are often synthesized by reducing metallic precursors; hence, sonochemical and solvothermal processes are commonly used to anchor these bimetals onto thin supports, including graphene. Although similar NPs have been studied reasonably well, a clear understanding of structural characteristics relative to their synthesis parameters is lacking, due to limitations in characterization techniques, which may prevent optimization of this very promising catalyst. In this report, a strategic approach has been used to identify this structural and material synthesis correlation, starting with controlled sample preparation and followed by detailed characterization. This includes advanced scanning transmission electron microscopy and electron energy loss spectroscopy; the latter using a state-of-the-art instrumentation to map the distribution of Pd and Au, and to identify chemical state of the Pd NPs, which has not been previously reported. Results show that catalytic bimetal NP clusters were made of small zero-valent Pd NPs aggregating to form a shell around an Au core. Not only can the described characterization approach be applied to similar material systems, but the results can guide the optimization of the synthesis procedures.

Keywords: bimetal core–shell nanoparticles, direct detection, EDS, EELS, electron microscopy, gold and graphene, palladium, SEM, TEM

Introduction

Bimetallic nanoparticles (NPs) often have remarkable properties due to the interaction between the surfaces of the two metals. For this reason, they are often used in various science and industry fields. Of these, palladium (Pd)/gold (Au) bimetallic NPs are of interest due to their high miscibility, demonstrated in their phase diagrams (Mizukoshi et al., 2000). Particularly, Pd/Au bimetals exhibit high catalytic activity for various reactions (Ferrando et al., 2008), including hydrogenation of hex-2-yne to cis-hex-2-ene (Schmid et al., 1996), acetylene cyclotrimerization yielding benzene (Lee et al., 1995), synthesis of hydrogen peroxide from H2 and O2, and the oxidation of alcohols to aldehydes with O2 (Edwards et al., 2008), and for the hydrodehalogenation of volatile organic compounds (VOCs) like trichloroethylene (Nutt et al., 2005; Chaplin et al., 2012; Li et al., 2012; Meduri et al., 2016, 2017, 2018). Pd/Au bimetals are used to catalyze the aforementioned reactions because Pd has strong catalytic capabilities for hydrogenation, and Au is an excellent promotor of Pd’s catalytic behavior. Together, Pd/Au NPs are very potent as an effective catalyst for hydrogenation reactions.

Pd/Au-based nanomaterials are frequently synthesized as needed, as they have varying uses depending on the sizes and shapes. The most common methods for synthesizing Pd/Au-based nanomaterials involve reducing metal precursors with different reducing agents, such as dialdehyde nanocellulose (Zhang et al., 2018), a combination of trisodium citrate, tannic acid, and potassium carbonate (Qian et al., 2014), and sodium borohydride (Huang et al., 2015).

In this report, the bimetallic Pd/Au NPs were made using a combination of sonochemical and solvothermal processes. While a form of these processes have been used in the past (Nagata et al., 1996; Mizukoshi et al., 2000; Lim et al., 2010; Shi et al., 2013), to the authors’ knowledge, a detailed characterization of the NPs formed by the integration of sonochemical process followed by the solvothermal process has not been reported. These processes were chosen as they do not need an outside reducing agent, and so the materials can be characterized without external and unnecessary material interference. The synthesis process is environmentally friendly and does not use harsh chemicals, surfactants, or stabilizers. Additionally, the precursors for making the NPs were chosen to have minimal impact on the environment while still being able to form effective catalysts. As reported previously, this catalyst is very effective at removing chlorinated VOCs (Meduri et al., 2016, 2017, 2018). The process is simple, and since no other chemicals are involved in the synthesis, the chance of impurities affecting the catalyst activity is reduced.

Sonochemical processing, or sonication, uses ultrasound waves that pass through water to cause acoustic cavitation (formation, growth, and violent collapse of small bubbles) (Riesz et al., 1985). This cavitation leads to the formation of hydrogen and hydroxyl radicals, which can diffuse into the bulk solution and cause further reactions (Tauber et al., 1999; Caruso et al., 2002). When metallic precursors are sonicated, these radicals reduce the metal ion into zero-valent metal particles. In fact, the sonochemistry of specific metal compounds such as Fe(CO)5, and Mo(CO)6, were explored in detail and found to produce unusual nanostructured inorganic materials via a reduction process (Bang & Suslick, 2010). In the past, Pd NPs have been made using a complicated sonication setup (Nagata et al., 1996; Mizukoshi et al., 2000; Akita et al., 2008); however, the NPs made in this report were made using a simple bath sonicator.

The next step is the solvothermal process, which allows these reactions to continue undisturbed by external factors until equilibrium is reached. Solvothermal process usually involves heating the solution (usually between 100 and 1,000°C) (Gersten, 2004) so that the stainless steel container creates an autogeneous pressure that propels the reaction (Demazeau, 2010). Here, as the reaction is initiated in the sonochemical process, the stainless-steel jacket creates a closed environment where the reactions can take place undisturbed. At this stage, once the metal nuclei have been formed by sonication, these nuclei grow into NPs during the solvothermal process. The advantage of the design of the solovothermal process following the sonochemical process is that the reactions can take place at room temperature, and do not need other external parameters to propagate the reaction.

Generally, in most Pd/Au bimetallic materials, Pd is found to cover Au (Ferrando et al., 2008; Li et al., 2012; Huang et al., 2015; Meduri et al., 2017), indicating there is some attraction between these metals. That is, the Au and Pd bimetallic particles have been found to have a core–shell structure. In the past, similar compositions of such particles have been studied with a variety of techniques, including X-ray diffraction (XRD), extended X-ray adsorption fine structure, and X-ray adsorption near edge structure (Lee et al., 1995), atomic absorption spectroscopy and X-ray photo-electron spectroscopy (Edwards et al., 2005; Enache et al., 2006). Some theoretical simulations were also carried out (Liu et al., 2005; Ji et al., 2014). A number of studies used conventional electron microscopy techniques such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM) with energy-dispersive X-ray spectroscopy (EDS) to visualize the particle size and shape (Edwards et al., 2005; Akita et al., 2008).

While these studies contributed significant knowledge of the bimetal NPs, the characterization of catalytic bimetal NPs hybridized on graphene flakes presented in this report not only reveals the visual images of the structure formed by the metallic NPs, but also the size, crystallinity, composition, and chemical state, using the combination of different modes of electron microscopy such as high-resolution TEM (HRTEM), scanning transmission electron microscopy (STEM), and supporting spectroscopy techniques such as EDS and electron energy loss spectroscopy (EELS). This is the first time that the direct detection EELS has been used to identify the chemical phase of the Pd NPs. Traditionally, the EELS acquisition introduces noise from multiple sources, including from electronic read-out of the CCD and the statistics of photon generation/collection. The new design for direct detection of the Gatan K2 camera system consists of a passivation/electronics layer, epilayer, and substrate which lead to an increase in the signal-to-noise ratio and a reliability of analysis (Hart et al., 2017). By using direct detection EELS in this study, the oxidation state of the Pd NPs has been identified to be Pd0. This finding is significant because it confirms the Pd0 catalyzes the TCE degradation through traditional hydrodehalogenation, in which dichloroethene isomers (1,1-DCE,trans-1,2-DCE, cis-1,2-DCE) and vinyl chloride are brief intermediate products (Li et al., 2012). On the other hand, if the Pd NPs had a different oxidation state, such as Pd+2 instead of Pd0, then different degradation routes can be expected, many of which include the formation of stable, unfavorable products that further pollute the environment (Chaplin et al., 2012). In the past, the high noise and low resolution of standard EELS detection prevented this technique from being used for such analyses. The new design K2 detector allows for an improved EELS analysis can be carried out with detailed investigation on oxidation state of the NPs.

It is expected that the new structural phenomena in relation to their chemical states revealed by this study will help further understand the correlation of the synthesis parameters and the bimetal nanostructures, as well as their catalyst behaviors, ultimately leading to the optimization of the design and performance of novel catalysts.

Materials and Methods

Synthesis of Materials

Two organometallic precursors—palladium (II) acetate (reagent grade, 98%, Sigma-Aldrich, St. Louis, Missouri) and tetrachloroauric (III) acid trihydrate (Acros Organics, Pittsburgh, Pennsylvania)—were used to fabricate the Pd/Au-based NPs through a sonochemical process. These NPs hybridize onto graphene support in the following solvothermal process. Here, the term “hybridize” refers to the interaction between the Pd/Au NPs as described in past publications (Ji et al., 2014). While we have shown it is possible to extend this method to making Pd/Au NPs on other carbon supports, such as activated carbon (Meduri et al., 2018), to understand the interaction of the atomic structures of the bimetal particles in relation to the material synthesis process, only graphene was used as the substrate to anchor the bimetal NPs in this study.

Liquid exfoliated graphene was made by measuring 25 mg of expandable graphite (ACS Material, Pasadena, California) and mixing with 50 mL of n-methyl-2-pyrrolidone. This solution was sonicated using a probe sonicator (Sonics Vibra-cell VCX 130, Newtown, Connecticut) for 60 min, forming a sustainable gray suspension. The solid, exfoliated graphene sheets were collected by centrifugation at 14,800 rpm for 15 min. The graphene in the above-mentioned process is not subjected to any heat treatment, surface modification, or oxidation processes.

To hybridize bimetal NPs on exfoliated graphene, the following method was used. A 1:1 mixture of Pd:Au was made by first sonicating 9 mg of Au precursor in acetone for 15 min and then adding it to a solution of 3 mg of Pd (5 wt% loading on 27 mg of graphene support) precursor. In most experiments, ACS grade acetone was used as a solvent, unless otherwise specified. This solution was sonicated for 60 min. Graphene was then added, and the solution was transferred to a Teflon liner in a stainless-steel autoclave for the solvothermal process. In this process, the solution was allowed to react at room temperature (22–25°C) for 24 h, except for certain experiments where the synthesis time has been specified. The catalyst samples were then filtered, washed, and air dried.

Characterization Instruments

A Zeiss Sigma FEG VP SEM with EDS and an FEI Tecnai F20 HRTEM equipped with STEM and EDS capabilities operated at an accelerating voltage of 200 kV were used for the imaging and chemical analysis of the samples. EELS data acquisition was carried out using a JEOL F200 STEM microscope equipped with a Gatan GIF Quantum IS that features a direct detection K2 camera which is capable of acquiring EELS data in electron counting mode, as well as low-dose energy filtered and TEM images at a frame rate up to 1,600 images in in situ mode (Hart et al., 2017). For SEM analysis, samples were loaded onto a standard aluminum stub with a double-sided carbon and single-sided copper tape. For TEM analysis, samples were drop-cast onto 300 mesh copper grids with lacey carbon coatings (Ted Pella Inc., # 01895, Redding, California).

Results and Discussion

All specimens examined in this manuscript have been made using a combination of the sonochemical and solvothermal process, as described in the “Materials and methods” section. That is, the samples were made with a molar ratio of 2:1 Pd:Au with acetone as solvent for a total of 24 h. Any variations in the synthesis process have been specified in the particular section.

Structure of the Nanoparticles and Configuration of the Bimetals

In order to make the Pd catalyst more effective, it must be in good contact with the Au (Nutt et al., 2005; Chaplin et al., 2012; Li et al., 2012; Meduri et al., 2016, 2017, 2018). In addition, supports offer several advantages, such as uniform NP distribution, control over aggregation, and ease in handling and recovery (Meduri et al., 2018). Therefore, for this report, grapheme-supported Pd/Au NPs were synthesized. The combination of a sonochemical and solvothermal processes to synthesize the samples allows certain control over process parameters and design of materials. Here, the Au precursor solution is sonicated first, followed by the sonication of the Pd and Au precursor solutions together. This design is to ensure the formation of a stable Au core surrounded by Pd. Finally, the addition of the graphene allows the NPs to deposit and hybridize with the carbon surface. Here, hybridization of the NPs with the support occurs as described in past publications (Ji et al., 2014).

Following this process, several samples were synthesized, then examined via SEM and TEM for a broad characterization of shape, size, and structure. Some of these results are shown in Figure 1. For SEM images (Figs. 1a and 2b), the same area was observed using both the secondary electron (SE) detector and the backscattered electron (BSE) detector. An area with high NP density was specifically chosen for this analysis. In Figure 1a, the SE image shows the surface structure of the NP clusters, suggesting a rough morphology due to agglomeration of smaller NPs. As this detector collects signals from the surface, generated by SEs due to a lower interaction volume, the outer topographical structure of the NPs becomes visible. In Figure 1b, the backscattered image is formed by the BSEs from deeper in the structure, due to the higher interaction volume, and reveals more Z-contrast but less surface information. In this image, the brighter core is more apparent, suggesting a heavier material is surrounded by a less-dense material. These two images show that the Au core is inside and surrounded by a Pd shell, therefore suggesting the PdShellAuCore structure. This structure is further explained in Figure 1c, which is a schematic representation of this PdShellAuCore structure, showing smaller Pd NPs agglomerate together to form an aggregate shell around an Au core.

Fig. 1.

Fig. 1.

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a: SEM images with SE detector showing surface of NPs and b: BSE detector showing contrast of metals making up the NPs. Arrows indicate regions of interest. c: Schematic representation of PdShellAuCore structure. d: TEM image of Pd/Au NPs on graphene with two types of structures highlighted in boxes. e: HRTEM image of Structure 2 revealing small NPs dispersed evenly through graphene, EDS spectrum of this in (f) indicates strong Pd peaks but no Au, indicating the NPs are Pd on carbon support; Cu is from the grid.

Fig. 2.

Fig. 2.

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TEM analysis of Structure 1. a: shows a cluster with a less dense material surrounding a denser core. b,c: HRTEM images of Pd and Au, respectively, showing crystallinity. d: STEM dark-field image of this cluster confirms two different materials with core-aggregated shell configuration. The arrow points toward an area where the aggregate Pd shell does not uniformly cover the Au core. EDS maps indicate core is Au (e), and aggregate shell is Pd (f) with overlay (g) confirming PdShellAuCore structure.

Next, TEM characterization was carried out. Seen in Figure 1d, a low magnification image shows some clusters of NPs on a graphene sheet, along with some unsupported clusters around the graphene. These clusters, classified here as Structure 1, make-up a majority of the morphology of the sample. While the supported clusters are responsible for catalyzing the removal of TCE, the unsupported NP clusters do not play a role, as they are removed completely upon washing and cleaning of the catalyst after synthesis but before testing of the sample. However, here, these structures are useful for impartial characterization. Along with Structure 1, a secondary structure, classified here as Structure 2, was observed on the graphene support. Structure 2 is further analyzed in Figure 1e, a STEM dark field image of the graphene, where the bright spots are tiny NPs of 1–2 nm dispersed over the support. An EDS spectrum of this area was taken, as shown in Figure 1f, indicating these NPs are made of Pd. The source of the Cu signal in this spectrum is from the TEM lacey carbon copper grid. Additionally, the peak identifying Au at 2.12 keV is missing, as indicated in the spectrum. That is, Structure 2 is only made of Pd and has no Au present. These tiny Pd NPs were found uniformly dispersed throughout the graphene support. This phenomenon indicates that Pd has some affinity for the carbon support.

Examples of Structure 1, the bimetallic NP cluster, were studied in detail. Both supported and unsupported clusters were studied, and no difference was observed between these; hence, results from both supported and unsupported clusters have been presented here. Moreover, it is believed that the presence of Structure 2, consisting of small Pd NPs spread through the surface of graphene, prevents some of the clusters from hybridizing with the support. Figure 2a is an image of an unsupported cluster indicating a core–shell configuration. The image contrast suggests that the NPs of darker contrast act as the core while the NPs of lighter contrast seem to form the shell surrounding the darker NPs. In fact, this structural configuration does not appear to be a traditional smooth-core–shell structure. Rather, both the denser and less-dense materials appear to be made up of smaller particles that have aggregated. The difference in density suggests a PdShellAuCore configuration. The smaller, individual Pd NP that form the shell, seen in Figure 2b, have lattice fringes, suggesting they are single crystalline in nature. The same is true for an exposed core, seen in Figure 2c. The cluster in Figure 2a was further analyzed in dark-field STEM mode, seen in Figure 2d. As can be seen, the image contrast in Figure 2a is reversed compared with that in Figure 2d. Both images confirm that the core is made of a denser material, which is different from the lighter material that forms the shell. The EDS elemental mapping of Pd and Au is shown in Figures 2e and 2f, respectively. The overlay structure of these maps is seen in Figure 2g, revealing a PdShellAuCore structure. The individual Pd NPs which agglomerate together to form the shell were found to have individual sizes in the range of 2–5 nm and the shell itself can measure up to 30 nm. The Au core appears to be NPs in the sizes ranging from 30 to 50 nm that agglomerate to form up to 500 nm size centers. Thus, the overall sizes of the clusters can reach micron scales. Detailed examination over several samples revealed that the Pd shell covering the Au is not very uniform, and on rare occasions, some portions of the Au cores remain exposed. One example of this can be seen in Figure 2d, with the arrow pointing to a location where the aggregate Pd shell does not uniformly cover the Au core.

As seen here, the clusters have a complex configuration; however, the TEM and STEM images allow us to characterize the materials with higher accuracy, while also revealing information about crystallinity. Additionally, the EDS allows the identification of the elements, isolating where one metal is present and the other may be absent. While the material and structural configuration demonstrated here are consistent with those reported by others (Akita et al., 2008; Bang & Suslick, 2010), the sizes of the clusters presented here are relatively larger due to the differences of the preparation procedures. Thus, it is clear that Structure 1 consists of aggregate Pd NPs of 2–5 nm forming a shell around an Au core made of 30–50 nm NPs, and that the NPs appear to be single crystalline. There does not appear to be a regular shape to either of the metallic NPs. Moreover, there are interactions that take place among the Pd, Au, and carbon support.

Chemical State of the Metal NPs

It is important to identify the oxidation state of the metal NPs, specifically of the Pd NPs which are the primary catalysts that promote the various chemical reactions. In a previous study, it has been shown that the role of Au is to promote the catalytic behavior of Pd when used as a catalyst for reactions such as hydrohalogenation (Meduri et al., 2018). If the Pd is zero-valent, then a particular reaction mechanism and kinetics can be expected. If, however, the Pd is oxidized, other mechanisms can be expected, leading to the formation of unfavorable products (Chaplin et al., 2012).

To understand their chemical states, the graphene-supported Pd/Au NPs were characterized using techniques such as XRD and UV-Vis spectroscopy and found to be consistent with previous findings (Nagata et al., 1996; Mizukoshi et al., 1997; Okitsu et al., 1997), indicating that both metals have an oxidation state of zero. These spectroscopy techniques provide information about the overall oxidation state of the metals but are limited for the oxidation states of the individual NPs. Characterization of a sample using EELS addresses this limitation, as it is capable of generating spatially resolved, high-resolution spectral maps for the data collected. Specifically, the new design of the K2 detector allows the acquisition of EELS spectra, as well as images in counting mode, where each single electron is counted. Because the K2 detector is capable of counting electrons at a speed of 400 spectra per second (Hart et al., 2017), it is ideally for characterizing the catalyst NPs.

Here, EELS analysis using the K2 detector was performed on the same samples analyzed in Figures 1 and 2. These results are presented in Figure 3. Note that Figure 3a is the STEM image, which has been analyzed to reveal elemental Pd and Au maps, as seen in Figures 3b and 3c. These maps have been overlapped in Figure 3d to confirm the PdShellAuCore structure. The EELS spectrum collected from this structure is shown in Figure 3e. The top spectrum was taken from the sample, showing the Pd L edges, specifically, L3 (left) and L2 (right). The lower two spectra are of Pd L from standards of a metallic Pd (oxidation state 0) and palladium oxide (PdO) (chemical state of Pd is 2+). The spectrum from the sample most closely resembles the spectrum from the Pd metallic standard; therefore, the chemical state of Pd was characterized in a zero state. Thus, the catalytic activity of this material is attributed to Pd0.

Fig. 3.

Fig. 3.

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STEM/EELS map and analysis of the bimetallic cluster. a: STEM image with EELS maps of (b) Pd L, (c) Au M, and (d) overlaid maps showing a PdShellAuCore structure. e: EELS spectrum from the sample compared with a metallic Pd standard and a Pd2+ standard. Upon comparison, the Pd spectrum from the sample was found to resemble the metallic standard.

The irregular core–shell structure, seen in Figure 3 of the Pd/Au NPs, suggests that the Au is encapsulated by the Pd shell during the NP formations. This prevents the Au core from being exposed to, or reacting with, the external environment that may cause any redox. Furthermore, Au is a noble metal and usually unreactive, and the outer shell Pd is generally more reactive than Au. Since Pd has retained its zero-valent nature during the core–shell particle formation, this suggests Au NPs have an oxidation state of zero as well.

Interaction Between Pd, Au, and Graphene Support

To study the interactions between the metals and their support, samples of individual metal NPs on graphene were made. The procedure to make the samples was the same; that is, the precursors were mixed with acetone and sonicated, except that only one metallic precursor [either palladium (II) acetate or tetrachloroauric (III) acid trihydrate] was used per sample. The TEM images from these samples are shown in Figure 4. Images of Figures 4a and 4b are of Pd on graphene, and Figures 4c and 4d are of Au on graphene. Figure 4a is a relatively low magnification image of Pd NPs on graphene, showing the Pd NPs uniformly dispersed over the graphene surface. The arrangement of Pd NPs on the graphene is very similar to Structure 2 (composed of tiny Pd NPs spread over the graphene) seen in Figure 1e; except the NPs in this sample are larger. Based on these observations, it is believed that in the absence of Au, only Structure 2 is formed, and these NPs grow instead of forming clusters. Figure 4b is a high magnification image in which the lattice structures of the NPs are visible, indicating the NPs are single crystalline.

Fig. 4.

Fig. 4.

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TEM images of a: Pd NPs on graphene at a low magnification and b: HRTEM image showing crystallinity of NPs. c: Low magnification of Au NPs showing sparsely scattered NPs over support and d: HRTEM image showing crystallinity of these NPs.

For the sample with Au only, similar observations were made. Figure 4c is a relatively low magnification image of Au on graphene. Here, a sparse dispersion of the NPs is observed. In fact, for these images, the concentration of the Au precursor was increased by a factor of about 13, from 0.23 to 3 mg/mL. When the original concentration was used, although a color change was detected in the forming solution (indicating a reaction had occurred), no NPs were observed on the graphene. In addition, despite a high concentration of precursor used, very few small, crystalline Au NPs in the range of 5–10 nm were formed on the graphene, as seen in Figure 4c. The HRTEM image in Figure 4d shows these NPs are also crystalline. The lack of NPs despite a change in color of the solvent suggests Au has a weak interaction with graphene, as very few NPs accumulate on the graphene. This is in agreement with a previous report (Ji et al., 2014) that used DFT simulations, in which Au was shown to have a much weaker interaction with graphene than that of Pd.

In both samples, no large clusters were observed, indicating that clusters form only when both metals are present, with Pd on the outside and Au on the inside. This suggests that the formation of the Pd/Au cluster in our experiments is due to attractive interactions between Pd and Au atoms, which cause them to spontaneously move toward each other, as described previously (Ji et al., 2014). In addition, these experiments demonstrate that Pd has a great affinity for graphene, which explains why the Pd/Au NP clusters cling to graphene.

While Pd has a great affinity for both graphene and Au, Au has little affinity for the support. Hence, when both metals and support are present, the Pd clusters around the Au, forming Structure 1, and individually forms on the graphene support, forming Structure 2. This affinity to graphene also causes some of the clusters to form on the support; however, the abundance of Structure 2 might limit the formation of clusters on the surface of graphene, leading to the formation of both supported and unsupported clusters. Interesting, all clusters were found to have an aggregate PdShellAuCore configuration, but not the reverse. This is because the PdShellAuCore configuration was found to have higher surface energy stability over the AushellPdcore structure (Ferrando et al., 2008).

Effect of Synthesis Time on Growth of Bimetallic NP Clusters

To further study the interaction between the Pd/Au NPs, the synthesis time was changed to observe the formation and growth of the bimetallic clusters. The solvothermal process is mostly responsible for the growth of the NPs, and so this time was varied to understand how the NPs form and grow. Samples were observed at intervals of 1 h during synthesis, up to 24 h. Here, the sample that was allowed to react for just 1 h has been evaluated, as it reveals significant findings, seen in Figures 5 and 6.

Fig. 5.

Fig. 5.

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TEM image of a specimen with synthesis time controlled to 1 h, allowing the observation of structure phenomena of an incomplete reaction. a: Low magnification image showing different structures present on the same sample; areas in boxes were examined further. b: Cluster strongly resembling Structure 1, c: dark-field STEM image of this cluster confirming Au–Pd core–shell. Structure 3 is examined in Figure 6.

Fig. 6.

Fig. 6.

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Images showing Structure 3. a: TEM image and corresponding dark-field STEM image in inset b: c: This is a closer look at the cluster. EDS maps of this cluster showing d: Au and e: Pd.

In the relatively low magnification TEM image (Fig. 5a), two types of clusters were observed. The cluster to the left has a similar configuration to Structure 1, and further analysis shown in Figures 5b and 5c confirmed this. However, the cluster to the right, termed here as Structure 3, appears to have a different arrangement. This structure has been analyzed in Figures 6a–6e. Figures 5b and 5c are bright-field TEM and dark-field STEM images, respectively, revealing the same core–shell structure previously observed. However, the Au core is more apparently made of smaller Au NPs congregated together. In the 24 h sample, these cores appear to mesh together more as the smaller NPs lose their distinct outlines.

Similarly, Structure 3, a partly formed Pd/Au NPs cluster, is shown in Figures 6a–6e. Figure 6a is a bright-field TEM image of Structure 3 cluster, while Figure 6b is a dark-field STEM image of the same area rearranged slightly due to prolonged exposure to the beam. A closer look at an area in this cluster is seen in Figure 6c. This structure appears to be “unfinished”. That is, the structure appears to be made of an agglomeration of NPs, but the clear core–shell configuration is absent. The EDS maps in Figures 6d and 6e of Au and Pd, respectively, suggest that the agglomerate is a mix of both metal NPs. The small size of the NPs proves challenging for EM techniques and requires advanced analysis, which is on-going. However, based on the analyses presented by past publications (Ferrando et al., 2008), it is assumed the NPs are of individual metals that are mixed together, which later grow into the core–shell configuration of Structure 1. The dark-field STEM images in Figures 5c and 6c show a clear difference in the morphology of the two structures. It is important to note that Structure 3 is only present at lower synthesis times, suggesting that over time, Structure 3 becomes Structure 1. This means that sufficient time is needed for the reaction to be completed and for the final structures to form. Based on this finding, we believe that the NPs are formed by first coming together to form unfinished clusters. As the NPs grow over time, a thick layer of Pd NPs surround clusters of Au NPs.

Effect of Solvent on the Formation of NPs

Another important parameter that affects the formation of the cluster is the solvent used for the synthesis. When choosing solvents, an important factor is the dissolution of the metallic precursors in the solvent. That is, the precursor needs to ionize in the solution, and this metal ion gets reduced during sonication. Therefore, a solvent that promotes dissolution is highly preferable. In this study, acetone, DI water, and ethanol were tested as potential solvents for their effect on the formation of clusters.

Acetone has been used as a solvent to make carbon-supported Pd/Au NPs structures in the past (Meduri et al., 2016), as well as for all the results present thus far. These samples were compared against samples made using DI water and ethanol as solvents. In the case of DI water as a solvent, when the precursor was initially added to the solvent, the dissolution was found to be very poor, which affected all subsequent cluster formation steps. Reproducibility was poor for these samples and so the results have not been included here.

Ethanol has several advantages as a solvent for the reaction. It is a polar solvent that easily dissolves the precursors. In their review, Bang & Suslick (2010) found that particle size was dependent upon alcohol concentration and alkyl chain length. This is due to the fact the alcohols are absorbed on the samples made using ethanol as a solvent have been characterized in Figure 7. As seen in the TEM Figure 7a, clusters similar to Structure 1 are present throughout the sample, indicating the presence of Pd/Au NPs. Both supported and unsupported clusters were observed, and found to have the same characteristics. Here, an unsupported cluster has been characterized for clarity. Seen in Figure 7a, the contrast between the metals is not as apparent as in Structure 1, which is a similar structure made with acetone. The dark-field STEM image in Figure 7b with an area enlarged in Figure 7e at a higher magnification provides some insight. The images indicate that the thickness of the shell has been truncated, and that while this cluster has a core–shell design, the shell layer is so thin that it becomes hard to distinguish in the TEM image. EDS maps in Figure 7 of (c) Pd and (d) Au confirm the composition of the structure. In Figures 7f and 7g, the extracted EDS spectrums from the center and edge of the cluster can be seen. The overall intensity is quite low due to the small size of the NPs and cluster area analyzed. In Figure 7f from the center of the cluster, both Au and Pd peaks are observed with the Au intensity greater than Pd. In Figure 7g, the Au peak is absent and there is a marginal increase in the Pd peak (increased from eight to ten counts). The Cu peaks are from the grid and the carbon is from the graphene support. The difference in the intensities of the Au and Pd peaks in these EDS spectrums suggest an aggregate-PdShell AuCore structure for this cluster as well.

Fig. 7.

Fig. 7.

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a: TEM of a cluster of NPs made using ethanol as the solvent. b: Dark-field STEM imaging of a cluster in the same cluster. A part of the cluster marked with the square was further examined in (e). EDS Maps (d) and (e) are of Pd and Au, respectively. e: A part of the cluster from (b) shows a less dense material covering a denser material. Extracted EDS spectrums from (f) the center and (g) edge show the Au and Pd peaks in the center and only Pd peaks at the edge, suggesting an Au core surrounded by a uniform Pd shell.

Effect of Sonication on Formation of NPs

The mechanism for the formation of noble metal NPs with chemical state 0 has been explored in the past by Bang & Suslick (2010). This mechanism involves the sonolysis of water; specifically, sonochemically generated H·radicals act as reductants to form zero-valent Pd/Au NPs (Bang & Suslick, 2010). In this mechanism, the AuCl4−/Au0 and Pd2+/Pd0 reactions occur when precursors dissolve in the solvent and are reduced to form Au and Pd NPs. However, the reduction of Pd and Au does not occur simultaneously, but rather sequentially. The difference in redox potential of AuCl4−/Au0 (1.002 V) and Pd2+/Pd0 (0.915 V) is assumed to be partly responsible for the PdShellAuCore configuration (Bang & Suslick, 2010). Au NPs form first, and Pd NPs begin to form at a slower speed. In the section dealing with the effects of solvent, a similar observation and analysis was made, in line with the mechanism proposed by Bang & Suslick.

In further examination of this mechanism, the following experiment was carried out. Both the Pd and Au precursor solutions were sonicated for the same amount of time, together. In the original synthesis method, Au precursor solution is given an extra 15 min of sonication before adding the Pd precursor, giving Au a total of 75 min of sonication, while the Pd is only sonicated for 60 min. If the redox potential was indeed driving the formation of the NPs, then Au would have a small head start. However, since the difference in redox potentials of the metals is so low, it is more likely that the samples would end up competing with each other in the formation of NPs.

The results of this experiment are shown in Figure 8. For simplicity, these samples were made without the graphene support. Figures 8a–8c were taken at different locations of the same sample, showing a nonuniform formation of the core–shell structures. As expected, both Pd and Au were forming almost at the same time, with Au having a small lead, leading to clusters of varied shapes and sizes. Certain areas appear to have a shallow layer of Pd NPs covering Au cores, while in other locations, Pd NPs form a thick, dense layer around the Au core.

Fig. 8.

Fig. 8.

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TEM of a sample in which both the Pd and Au precursors were sonicated and allowed to react for the same amount of time, producing nonuniform bimetallic clusters. Images (a–c) were taken on different locations of the same sample. Arrows point to areas revealing the smaller Au cores deep inside the thicker Pd aggregate shells.

It can be inferred that in the original synthesis, the additional 15 min of sonication for the Au allows for the formation of a stable core that the Pd NPs can surround. That is, these results prove that while the difference in redox potentials of the reactions is not solely responsible for the core-structure, it definitely plays a role in this formation.

Mechanism of Formation of NPs and Structure

Based on the findings outlined in this report, it is believed that the mechanism of material formation occurs as follows. First, the metallic precursors are added to a solvent, preferably acetone, and then sonicated. During this process, the organic end breaks off the precursors and the sonolysis of water assists in the reduction of the metal ions to their native oxidation states. If Au is given a head start, the stable cores are formed first, followed by a thick, aggregate shell of Pd NPs. Different solvents can have an effect on the size of the aggregate shell and other properties of the material.

In the course of the solvothermal process, because of the attractive forces between Pd and Au, the tiny metal NPs simply agglomerate together. Once the graphene is added, the NPs hybridize with the carbon surface and grow in size. Due to Pd’s affinity to graphene, two types of structures are formed—Structure 1 comprises of clusters of Pd and Au in an aggregate-PdShell AuCore configuration, while Structure 2 is smaller Pd NPs dispersed throughout the graphene. In fact, it is speculated that as Structure 2 is spread through the graphene support, it sometimes prevents Structure 1—the clusters—from properly hybridizing with the graphene. This leads to Structure 1 being either supported or unsupported. When used against a contaminant, all unsupported clusters of Structure 2 are removed; however, here they were useful for unobstructed analysis.

Summary

A systematic characterization of graphene-supported Pd/Au bimetal materials has been carried out using electron microscopy and spectroscopy techniques. Pd/Au NPs on graphene are very effective catalysts for the removal of VOCs from groundwater. Here, the materials are made using an environmentally friendly process, by a combination of sonochemical and solvothermal process. In the past, we have shown these materials were found to be effective at removing TCE (Meduri et al., 2018). Pd/Au NPs on carbon supports are of interest as catalysts for several important reactions, and their structure, configuration, shape, crystallinity, and chemical state have been studied.

Here, a range of different electron microscopy techniques such as SEM, TEM, EDS, and EELS have been used to investigate the various features of the carbon-supported bimetallic material and better understand its formation. The ability to visualize the formation and growth of the material at every stage and under different conditions allowed for a conclusive understanding of how the catalyst is formed and behaves. The state-of-the-art Gatan K2 Summit EELS detector attached to a TEM was used to identify the chemical state of Pd as zero-valent. The lower noise and higher sensitivity of the K2 Summit detector, compared with more traditional EELS systems, opens new avenues for research, and the acquisition of the high-energy edges where the signal is very low. This material has been characterized with good certainty and the methods outlined here are applicable to similar material systems and configurations with high reliability.

Acknowledgments.

This study is supported in part by NSF awards No. 1507707, No. 1508115, and No. 1560383, Oregon Nanoscience and Microtechnologies Institute (ONAMI) and NIH Build EXITO program. Microstructural and compositional analysis was performed at the Center for Electron Microscopy and Nanofabrication (CEMN), Portland State University, Portland, Oregon. EELS analysis was carried out at Analytical Projects R&D Gatan, Gatan Inc., Pleasanton, California.

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