Preparation of Pt-skin PtRhNi Nanoframes Decorated with Small SnO2

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Cite This: ACS Appl. Mater. Interfaces 2019, 11, 22352−22363

Preparation of Pt-skin PtRhNi Nanoframes Decorated with Small SnO2 Nanoparticles as an Efficient Catalyst for Ethanol Oxidation Reaction Grzegorz Gruzeł,*,† Przemysław Piekarz,† Mirosława Pawlyta,‡ Mikołaj Donten,§,∥ and Magdalena Parlinska-Wojtan† †

Institute of Nuclear Physics Polish Academy of Sciences, PL-31342 Krakow, Poland Institute of Engineering Materials and Biomaterials, Silesian University of Technology 44-100 Gliwice, Poland § Faculty of Chemistry, University of Warsaw, 02-093 Warsaw, Poland ∥ Faculty of Chemistry, Biological and Chemical Research Centre, 02-089 Warsaw, Poland

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S Supporting Information *

ABSTRACT: Pt-based nanoframes are one of the most promising catalysts for ethanol oxidation reaction in direct ethanol fuel cells. It is important to understand the mechanisms responsible for creating these hollow nanoframe-based catalysts. Herein, for the first time, Pt-skin PtRhNi rhombic dodecahedral nanoframes were decorated with small SnO2 nanoparticles and were used as an efficient catalyst for the ethanol oxidation reaction. Moreover, by combining the ex situ scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy observations at various stages of synthesis, along with density functional theory calculations, it was possible to track the synthesis route of solid rhombic dodecahedral PtRhNi nanoparticles, which are the precursors of PtRhNi nanoframes. After the chemical etching of the Ni core from solid PtRhNi nanoparticles, the obtained nanoframes were decorated with SnO2 nanoparticles. The resulting SnO2@PtRhNi heteroaggregates were deposited on highsurface-area carbon and electrochemically tested, showing a 6-fold higher mass activity and 10-fold higher specific activity toward ethanol oxidation reaction than commercially available Pt catalysts. KEYWORDS: nanoframes, synthesis, Pt-skin, ethanol oxidation reaction, catalysts

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INTRODUCTION Direct ethanol fuel cells (DEFCs) seem to be a promising source of electric energy for mobile devices or even vehicles. Unfortunately, there are some difficulties associated with anode catalysts used for ethanol oxidation reaction (EOR). First, due to the high cost of platinum, which is used as a catalyst, the cost of DEFC production is very high.1 Second, the currently used catalysts have low selectivity for the complete oxidation of ethanol to CO2, which results in low efficiency of conversion of chemical energy to electrical energy.2 Third, due to the low selectivity of the ethanol oxidation reaction, byproducts, like acetic acid and acetaldehyde, are formed. These compounds can cause poisoning of the anode catalyst, which reduces its catalytic activity.3 Currently, nanocatalysts composed of PtRh/SnO2 nanoparticles (NPs) are considered to be one of the most efficient catalysts for ethanol oxidation reaction.4 The crucial role in these catalysts is played by platinum, adsorbing on its surface ethanol, which undergoes dehydrogenation.4−6 Furthermore, these catalysts have high selectivity toward CO2 due to the presence of rhodium, which favors the cleavage of the C−C © 2019 American Chemical Society

bond in the ethanol molecules. The role of SnO2 is the adsorption and dissociation of H2O on its surface to provide OH groups to oxidize CO and possibly to reduce the PtOH and RhOH formation (making Pt and Rh available to react with ethanol). The adsorption and dissociation of H2O on SnO2 have been verified in a number of studies.7,8 Most of the currently used catalysts for ethanol oxidation are in the form of spherical nanoparticles, lacking specific crystallographic facets,2,9,10 whereas there are studies showing that the oxidation of ethanol occurs more or less efficiently depending on the arrangement of atoms on the surface of nanoparticles.11,12 Several research groups synthesized catalytic nanoparticles for EOR, which were characterized by precisely defined shapes and showed greater activity than spherical nanoparticles.11,13,14 An interesting variation of nanoparticles with a well-defined polyhedral shape is the so-called hollow nanoparticles. The most studied examples of hollow nanoReceived: March 15, 2019 Accepted: June 4, 2019 Published: June 4, 2019 22352

DOI: 10.1021/acsami.9b04690 ACS Appl. Mater. Interfaces 2019, 11, 22352−22363

ACS Applied Materials & Interfaces

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particles are nanoboxes,15,16 nanocages,17,18 and nanoframes.19,20 Among those, nanoframes have the most open structure, when compared with other types of hollow nanostructures of similar sizes. It is possible to obtain nanoframes with different shapes; recently, triangular,21 cubic,22 octahedral,19 and decahedral23 nanoframes have been prepared using various methods. It is also possible to obtain nanoframes with different chemical compositions, for example, PtNi,24 PdRh,25 or PtCu.26,27 Nanoframes have many advantages; among others, they provide many edges and corners, which are catalytically active due to the low coordination number of atoms on these edges and corners. Another advantage of the nanoframes is their high-surfacearea-to-volume ratio due to the removal of the nonfunctional atoms from the interior of nanoparticles. This operation not only improves the efficiency in catalytic reactions but also minimizes the usage of precious metals such as platinum, rhodium, and gold. One of the methods to obtain nanoframes is the synthesis of template solid nanopolyhedral nanoparticles, which are later etched to form nanoframes. The etching process is usually easy to perform; however, synthesis of the template solid nanoparticles could be challenging. Therefore, it is crucial to investigate the synthesis path of the nanoparticles, which helps obtain solid polyhedral nanoparticles in a controlled way and, as a consequence, allows one to obtain nanoframes. As discussed in the previous paragraph, there are many scientific papers in which EOR catalysts have been discussed. However, among these studies, there are only a few reports dealing with the combination of two approaches, that is, simultaneous consideration of the optimal chemical composition and well-defined shape of the catalysts. Only by combining these two features will it be possible to synthesize a new generation of efficient and more active catalysts. Moreover, as shown by the Adzic group,5 the ternary PtRh/ SnO2 catalyst increases the selectivity of ethanol oxidation to CO2. The physical contact between the metal and the oxide particles is crucial due to the fact that the metal−oxide interface is the most catalytic active site. Therefore, in the present study, the syntheses of solid rhombic dodecahedral PtRhNi nanoparticles with PtRhNi edges and a Ni core were performed, which subsequently were transformed into PtRhNi nanoframes by chemical etching of the Ni core. Next, the obtained nanoframes were decorated by small SnO2 NPs to enhance their catalytic performance toward EOR. The resultant nanocatalysts were characterized by transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) to confirm their nanoframe/oxide structure, and their EOR activity was also verified. The crucial step in the preparation of the SnO2@ PtRhNi nanocatalysts is the synthesis of the solid parental PtRhNi nanopolyhedra; therefore, a control experiment was performed, in which synthesis intermediates were examined by TEM and EDS. Also, density functional theory (DFT) calculations were performed to understand the formation of phase-segregated polyhedral nanoparticles. It was expected that the decoration of frames with oxide NPs should lead to a much higher electrochemical activity as the number of contact sites between the metal frame and the oxide NPs increased compared with the spherical ternary PtRh/SnO2 catalysts. Moreover, due to the presence of Rh, the C−C bond splitting should occur directly without passing through intermediate products such as acetic acid and acetaldehyde.

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EXPERIMENTAL SECTION

Reagents. Nickel(II) nitrate hexahydrate, chloroplatinic acid hexahydrate, oleylamine, tin(IV) chloride pentahydrate, and 5 wt % Nafion were purchased from Sigma Aldrich. Acetic acid was purchased from Merck Milipore. Rhodium(III) chloride hydrate was purchased from Acros Organics. Citric acid monohydrate, isopropanol, and ethanol were purchased from POCH. Vulcan XC72R was purchased from Cabot. All reagents were used as received without further purification. Synthesis of PtRhNi Polyhedral and Preparation of PtRhNi Nanoframes. The PtRhNi polyhedra were synthesized using a modified protocol previously reported by Chen et al.24 In a typical synthesis, 50 mg of H2PtCl6·6H2O, 10 mg of RhCl3·H2O, and 43 mg of Ni(NO3)2·6H2O were dissolved in 1 mL of deionized water and then added into 25 mL of oleylamine in a three-necked flask at 160 °C in an argon atmosphere. Then, the solution was heated to 270 °C until it turned into a black slurry. The temperature was maintained for an additional 3 min; after this time, the solution was cooled down to room temperature. The obtained nanoparticles were collected by centrifugation (3000 rpm) and redispersed in 2 mL of chloroform. The nanoparticle solution was then transferred to a two-necked flask with 20 mL of acetic acid. The resulting solution, after being sonicated for 30 min, was kept at 100 °C for 5 h under stirring in an air atmosphere. The yielded PtRhNi nanoframes were collected by centrifugation (3000 rpm) and washed three times with a hexane− ethanol mixture. Synthesis of SnO2 Nanoparticles. Sol−gel microwave-assisted synthesis of SnO2 was performed based on the protocol proposed by Zhu et al.28 First, 10 mL of 0.1 M SnCl4·5H2O and 10 mL of 0.1 M citric acid solutions were separately prepared. Subsequently, the tin chloride and citric acid solutions were mixed thoroughly and sonicated for 30 min. Afterward, the obtained 20 mL of solution was placed in a microwave oven with a maximum power of 280 W and heated for 12.5 min. The final product was washed with ethanol. Decorating PtRhNi Nanoframes with SnO2 Nanoparticles. First, the pH of the solutions containing PtRhNi nanoframes obtained by chemical etching, as well as of the solution with SnO 2 nanoparticles, was adjusted to 4.5 and separately sonicated for 1 h to disperse the nanoparticles in ethanol. After this time, 10 mL of the SnO2 NPs suspension (approximately 13 mg of SnO2 NPs) was added dropwise to the PtRhNi nanoframes. The obtained suspension of SnO2@PtRhNi heteroaggregates was sonicated for another 1 h and washed three times with ethanol. Characterization Methods. The morphology of the synthesized nanoparticles was examined by scanning transmission electron microscopy (STEM) using the high-angle annular dark-field detector (HAADF) in conventional and high-resolution modes. Selected area electron diffraction (SAED) patterns were taken in the TEM mode. All of these measurements were performed on an aberration-corrected FEI Titan electron microscope operating at 300 kV equipped with a FEG cathode. Energy-dispersive X-ray spectroscopy (EDS) was used to analyze the chemical composition of the synthesized nanoparticles. These measurements were performed on a FEI Talos F200 instrument operating a FEG cathode at 200 kV and equipped with the Super-X in-column EDS detector. The X-ray diffraction (XRD) studies were carried out using the X’Pert PRO (PANalytical) diffractometer with Cu Kα (1.5404 Å) radiation, a graphite monochromator, and a strip detector (X’Celerator). To preclude any extra diffraction lines, the samples were placed onto a “zerobackground” silicon plate. The experiments were performed at room temperature. The XRD patterns were vector normalized, and the baseline correction was applied using Origin software. All reference crystallographic data, such as lattice parameter and values of 2θ, were taken from JCPDS files: JCPDS 87-0646 (Pt), JCPDS 05-0685 (Rh), JCPDS 04-0850 (Ni), and JCPDS 88-0287 (SnO2). Inductively coupled plasma optical emission spectrometry (ICP-OES) measurements were performed on a PerkinElmer Plasma 40 instrument to determine global composition of nanoparticles, whereas inductively coupled plasma mass spectrometry (ICP-MS) measurements were 22353


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temperature. The platinum electrochemically active surface area (ECSA) was calculated based on hydrogen desorption peaks on CV curves measured with a 20 mV s−1 scan rate; next, the ECSAs were converted into specific surface area (SSA) by dividing the ECSA by the mass of platinum in the respective catalysts. The EOR measurements were conducted at a 10 mV s−1 scan rate. The EOR results were normalized by SSA.

carried out on PerkinElmer NexION 300D to prepare catalysts with 20% Pt loading. To perform the controlled deposition of SnO2 nanoparticles on the nanoframes, ζ-potential measurements were conducted. The ζ-potential values as a function of the solution pH were determined for the ethanol suspension containing PtRhNi nanoframes, as well as for the SnO2 nanoparticle solution, by the microelectrophoretic method using a Zetasizer Nano Series from Malvern Instruments. The Smoluchowski model was used in the ζpotential measurements. Each value was obtained as an average of three subsequent runs of the instrument with at least 20 measurements. All experiments were performed in ethanol at 25 °C. Theoretical Calculations. Density functional theory (DFT) calculations were performed using the Vienna ab initio simulation package,29 applying the generalized gradient approximation.30 In the calculations, a Monkhorst−Pack31 mesh of 2 × 1 × 2 k-points and plane wave energy cutoff of 300 eV were used. Spin polarization with the ferromagnetic ordering was included in the calculation. The starting cell consisted of 90 Ni atoms forming six layers with (110) surfaces and vacuum spaces of 14 Å. This model was subjected to relaxation of the lattice parameters and atomic positions. Next, the 30 Ni atoms were substituted by 22 Pt and 8 Rh atoms with different positions. The final atomic ratio of Pt/Rh/Ni was 22:8:60, which is in good agreement with the experimental atomic ratio of the obtained PtRhNi nanopolyhedra. The following models were considered (Figure S1):

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RESULTS AND DISCUSSION PtRhNi solid nanoparticles were synthesized using the method adapted from Chen’s work.24 Based on the three different projections of STEM images (Figure 1), the shape of the

(1) Random alloy model, where Pt, Rh, and Ni atoms were randomly mixed in the cell. (2) Cluster model, where platinum and rhodium formed separated clusters. (3) PtRh surface model, where the two top layers of the cell consisted only of Pt and Rh atoms. (4) PtRh subsurface model, where the third and fourth layers of the cell consisted only of Pt and Rh atoms. (5) PtRhNi surface model, where Pt, Rh, and Ni atoms formed the first, second, fifth, and sixth layers, whereas the third and fourth layers consisted only of Ni atoms. (6) PtRhNi subsurface model, where Pt, Rh, and Ni atoms formed the second, third, fourth, and fifth layers, whereas the first and sixth layers consisted only of Ni atoms. To study the diffusion of Pt and Rh atoms in the PtRhNi rhombic dodecahedral nanoparticle, the total energies of the model having Pt and Rh atoms in different positions were calculated. The visualization of the above models, as well as schematic nanoparticle models, was performed in VESTA version 3.3.9 software.32 The differences in total energies of cells containing 90 atoms were compared; the lowest calculated energy was set at 0 eV as a reference. Catalyst Preparation and Electrochemical Characterization. For loading of the nanoparticles on Vulcan XC-72R carbon support, the carbon powder was added to ethanol and sonicated for 1 h. Next, the PtRhNi nanoframes or PtRhNi nanoframes decorated with SnO2 were added to the carbon suspension and stirred for 12 h. After this time, the nanoparticles/C (20 wt % Pt on carbon support) were collected by centrifugation (3000 rpm) and then calcined at 200 °C in air for 14 h to remove the organic surfactants and obtain Pt-skin segregation at the edges of the nanoparticles. Then, 4 mg of dried carbon powder was suspended in a solution containing 3 mL of ultrapure H2O, 1 mL of isopropanol, and 20 μL of 5 wt % Nafion and was sonicated for 40 min. Next, 10 μL of the so-prepared catalyst ink was pipetted onto a polished glassy carbon disc and dried at room temperature. The cyclic voltammetry (CV) and ethanol oxidation reaction (EOR) measurements were performed using a Bio-logic SP200 potentiostat in a three-electrode electrochemical cell. As a counter and a reference electrode, platinum wire and a saturated Ag/ AgCl electrode were used, respectively. All of the potential values were converted to the reversible hydrogen electrode (RHE) scale. The CV curves were recorded in a previously Ar bubbled 0.1 M HClO4 electrolyte, whereas EOR measurements were conducted in an electrolyte containing 0.1 M HClO4 and 0.5 M ethanol at room

Figure 1. Schematic illustration of PtRhNi solid nanoparticles after synthesis, PtRhNi nanoframes after etching, and PtRhNi nanoframes after SnO2 deposition. Below each model, projections from three different positions and corresponding HAADF STEM images of the obtained samples are presented. The models, which are not in scale, are based on literature reports and experimental results obtained by the authors.

obtained nanoparticles could be identified as rhombic dodecahedral, which is enclosed by 12 {110} planes.33 As it can be seen on the overview image (Figure 2a), most of the nanoparticles have a rhombic dodecahedral shape like in previous reports,24,33 which is possible due to using oleylamine as a capping agent.34 Although there are also nanoparticles with a more elongated shape, they constitute only 15% of all of the nanoparticles in the sample. This is probably caused by the presence of rhodium, which has a high surface energy of the {110} surfaces.35 Thus, to reduce the surface energy, the nanoparticles adopt an elongated shape in one or two directions. Still, generally the nanoparticles possess rhombic dodecahedral shapes with the PtRhNi phase segregated on the edges and the Ni phase in the core of the nanoparticles. By using the HAADF STEM detector, due to the difference between the Z-numbers of platinum, rhodium, and nickel, it can be observed that the elements with higher Z-numbers, in this case platinum and rhodium, are as expected segregated on the edges of nanopolyhedra, whereas nickel, having a lower Znumber, is located mainly in the core of the nanoparticles. The average size of the solid PtRhNi nanopolyhedra, measured based on STEM images, is 17.7 ± 2.6 nm (Figure S2a). The PtRhNi solid nanoparticles are crystalline, which is confirmed by both HRSTEM images (Figure 2b,c) and SAED patterns (Figure S3a). The lattice distance, being equal to 0.21 nm measured on the HRSTEM image (Figure 2c), is smaller than that of pure face-centered cubic (fcc) Pt(111) planes and of pure fcc Rh(111) planes; however, it is bigger than the one of pure fcc Ni(111), thus indicating the formation of a PtRhNi alloy on the edges of the nanoparticles. Also, SAED patterns of the as22354


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Figure 2. (a) STEM HAADF overview image of PtRhNi solid nanoparticles; (b) high-resolution scanning transmission electron microscopy (HRSTEM) HAADF image of a single PtRhNi solid nanoparticle; and (c) magnification of the area marked in (b).

4a), small (about 5 nm) polyhedral nanoparticles are formed with no phase segregationPt, Rh, and Ni are mixed within the entire nanoparticle. As the temperature increases to 225 °C (Figure 4b), the nanoparticles grow and polyhedral nanoparticles transform into branched structures with branches composed mainly of platinum and rhodium. The formation of branches is possible due to the diffusion of these elements from the core of nanoparticles to the outside37 and due to constant deposition of freshly reduced Pt and Rh atoms originating from precursors. During further increase of temperature to 240 °C (Figure 4c), the nanoparticles change their shape from branched to rhombic dodecahedral. This is due to the nickel atoms’ deposition on nanoparticles and filling of the space between the PtRh dendrites to minimize the surface energy, which results in obtaining a polyhedral shape close to rhombic dodecahedral. HAADF STEM images show that the edges are slightly brighter than the center of the nanoparticles, which indicates the beginning of PtRh phase-segregation on the edges, although the EDS map shows that Pt, Rh, and Ni are still uniformly mixed on this stage of growth. At 255 °C (Figure 4d), rhombic dodecahedral nanoparticles still keep growing; however, the presence of PtRh-rich edges is confirmed by both HAADF STEM images and EDS maps, which suggests that diffusion of Pt and Rh from the core to edges occurs. Interestingly, at this temperature, not all rhodium diffuses into the edges, but it is also in the core of the nanoparticles, which leads to the conclusion that rhodium diffuses slower than platinum. After increasing the temperature to 270 °C (Figure 4e), both platinum and rhodium are only at the edges and not in the core, which stems from the HAADF STEM images and EDS maps. Interestingly, platinum diffusion from the core of the nanoparticles to the edges occurs despite the fact that platinum has a higher surface energy for the {110} facets than nickel (2.82 vs 2.37 J m−2, respectively).38 This phenomenon was investigated by other groups37,39,40 and was explained by the reduction of the internal strain caused by a bigger atomic radius of Pt than Ni (1.39 and 1.24 Å, respectively).41 However, in the studied nanoparticles, there are additionally rhodium atoms, which also diffuse from the core to the edges. This can be explained by a similar mechanism as for platinum, although rhodium has the highest surface energy among the three elements (2.90 J m−2),38 and it also has a smaller atomic radius than platinum but still bigger than nickel (1.34 Å).41 Generally, in multimetallic systems, the element with lower surface energy and bigger atomic radius tends to diffuse to the surface.42 Rhodium has a higher surface energy than nickel, but due to its bigger atomic radius, it is

synthesized nanopolyhedra (Figure S3a) show diffraction rings placed between those corresponding to (111), (200), (220), (311), and (222) planes of pure Pt, Rh, and Ni. This observation is supported by XRD measurements (Figure S4), in which diffraction peaks from PtRhNi nanopolyhedra are located between diffraction peaks from (111), (200), and (220) planes of pure fcc Pt, Rh, and Ni metals. However, diffraction peaks originating from PtRhNi nanopolyhedra are slightly asymmetric, indicating the occurrence of phase segregation in the nanoalloy.36 Indeed, EDS mapping (Figure 3a) confirms the previous observations that edges of

Figure 3. (a) HAADF STEM image of single PtRhNi nanopolyhedra with the corresponding elemental maps for Pt, Rh, Ni, and their overlapped image; (b) HAADF STEM image with the corresponding EDS linescan through the nanoparticle.

nanopolyhedra are composed of platinum, rhodium, and nickel, whereas the core of the nanoparticles consists mainly of nickel. The EDS linescan (Figure 3b) also reveals that the signal from nickel is strong in the entire nanoparticles, whereas signals from platinum and rhodium are concentrated on their edges. Based on EDS measurements, the atomic ratio of Pt/ Rh/Ni in the solid nanoparticles is 30:9:61, which is in good agreement with ICP-OES results (28:9:62) and roughly corresponds to Pt3Rh1Ni6. To investigate the formation mechanism of PtRhNi nanopolyhedra, control experiments were performed, in which during synthesis, nanoparticles were sampled at different temperatures (from 210 to 270 °C with 15 °C steps). HAADF STEM images and EDS maps revealed that at 210 °C (Figure 22355


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Figure 4. STEM HAADF images and EDS maps of intermediate products obtained during PtRhNi solid nanopolyhedra synthesis at (a) 210 °C, (b) 225 °C, (c) 240 °C, (d) 255 °C, and (e) 270 °C.

the fact that it is reduced later than Pt and Rh. Interestingly, the platinum content in the nanoparticles during synthesis is initially high; next, it decreases and then it again increases. This can be explained as follows: the rhodium precursor has a relatively low concentration in the reaction solution and it was all used to generate nanoparticle seeds. On the other hand, platinum precursors have a higher concentration than rhodium; therefore, generation of the nanoparticle seeds does not use all of the Pt atoms, and they are still floating in the reaction solution. These Pt atoms, during the growth of nanoparticles, are deposited on the nanopolyhedra edges, which explains the increase of the platinum content measured by EDS. To better understand the process of platinum and rhodium surface segregation occurring during the synthesis of PtRhNi nanopolyhedra, DFT calculations were performed (Figure S6). Six different models of PtRhNi(110) surface were analyzed. According to theoretical simulations, the most energetically favorable is the system in which Pt, Rh, and Ni atoms formed surface layers, whereas Ni atoms formed the core of the system. In contrast, the least favorable was the system in which

pushed to the surface. However, because Rh has a higher surface energy than Pt and a slightly smaller atomic radius, it diffuses slower than Pt. It is worth noticing that during the synthesis, except Pt and Rh diffusion, constant deposition of Pt, Rh, and Ni atoms from the precursors also occurs. This causes a size increase of the synthesized nanopolyhedra. The results of EDS chemical analysis (Figure S5) performed during the synthesis are also interesting. Based on EDS measurements, it can be seen that initially (at 210 °C) the nanoparticles are composed mostly of Pt and Rh. This can be explained by the fact that rhodium and platinum have a high reduction potential (0.758 V vs standard hydrogen electrode (SHE) and 0.755 V vs SHE, respectively), whereas nickel has a low reduction potential (−0.257 V vs SHE).43 It is generally known that compounds with higher reduction potential are reduced faster;44 therefore, in this case, platinum and rhodium are reduced before nickel, which explains the excess of these two elements in the initial nanoparticles. Due to the relatively low concentration of rhodium in the reaction solution, its atomic ratio decreases during synthesis. On the other hand, nickel content in the nanoparticles increases during synthesis due to 22356


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Figure 5. (a) STEM HAADF overview image of the etched PtRhNi nanoframes; (b) HRSTEM HAADF image of a single etched PtRhNi nanoframe; and (c) magnification of the area marked in (b).

which corresponds to Rh3Ni2. Based on these results, it can be concluded that the presence of platinum is necessary for the synthesis of PtRhNi rhombic dodecahedra nanoparticles. This can be caused by the low reduction potential of nickel (−0.257 V vs SHE), making it difficult to reduce. Platinum facilitates the reduction of nickel and formation of PtRhNi nanoparticles; it is well known that the reduction of 3d transition metals, such as nickel, is enhanced by the presence of noble metal seeds.46,47 Another control experiment was performed to verify the influence of changing the temperature increase rate during the synthesis. After increasing the rate of temperature increment from 3 to 8 °C min−1, mostly overgrown PtRhNi nanoparticles were obtained (Figure S8a,b); however, some rhombic dodecahedral nanoparticles were still visible in the sample. It is worth noticing that overgrown nanoparticles are much larger than rhombic dodecahedral NPs (45 vs 18 nm average size). The EDS elemental maps (Figure S8c) show that the synthesized nanoparticles are indeed composed of platinum, rhodium, and nickel; nevertheless, no phase segregation occurs, and all elements are uniformly distributed, forming a nanoalloy. The next step was to etch nickel from the solid PtRhNi rhombic dodecahedral nanoparticles to obtain PtRhNi nanoframes. Based on the three different projections of STEM images (Figure 1), it could be unambiguously stated that after etching, the nanoframes became hollow but preserved their rhombic dodecahedral shape. The average size of the PtRhNi nanoframes, measured on the basis of the STEM images, is 16.9 ± 2.1 nm (Figure S2b), which is smaller than their parental counterparts due to the loss of nickel and some platinum and rhodium. The PtRhNi nanoframes are crystalline, which is confirmed by both HRSTEM images (Figure 5b,c) and SAED patterns (Figure S3b); however, locally some of the edges lose their atomic ordering after etching and are rather amorphous (Figure S9). Furthermore, the edges are not smooth like they were before etching. The lattice distance of 0.21 nm measured on the HRSTEM image (Figure 5c) is the same as in the case of solid nanopolyhedra, which suggests that nickel was not removed from the edges of nanoframes but only from the interior of the nanoparticles. SAED patterns (Figure S3b) confirm this observation. The diffraction rings are similar to those observed in PtRhNi nanopolyhedra; however, they are slightly smaller due to the higher Pt and Rh contents in the sample. Analogously, XRD patterns for PtRhNi nanoframes (Figure S4a) contain similar diffraction peaks to PtRhNi solid nanopolyhedra, but they are shifted toward lower angles due to

Pt and Rh atoms were concentrated in the center of the system, with nickel on the surfaces. All calculated energies are summarized in Table S1. The DFT calculations confirmed that despite the high surface energies of platinum and rhodium, the total energy of the PtRhNi system with Pt, Rh, and Ni segregation on the surface and Ni in the subsurface (model “PtRhNi surface”) is the lowest compared with that of the other tested systems. The highest total energy was found in the system in which Pt and Rh atoms are inside the cell, whereas nickel atoms form the surfaces (model “PtRh subsurface”, ΔE = 8.68 eV). This atom arrangement causes lattice strain and, as a consequence, has a higher total energy. However, this model can be considered as an initial state of the growth of the nanoparticles, where Pt and Rh did not start to diffuse yet. Next, Pt and Rh diffuse outward, forming a structure presented on the model “PtRhNi subsurface”, which has a lower total energy (ΔE = 1.62 eV) due to the mixing of the Pt, Rh, and Ni atoms. Finally, platinum, rhodium, and some nickel atoms are arranged on the surface, whereas the rest of the nickel atoms are in the subsurface (model PtRhNi surface). This system has the lowest total energy (ΔE = 0 eV) due to the reduction of the internal strain caused by the differences in atomic radii. Based on the DFT results, it can also be concluded that the most energetically favorable process is the mixing of Pt and Rh with Ni on the surface. The system with only Pt and Rh on the surface and nickel in the subsurfaces (model “PtRh surface”) has a significantly higher total energy than that in the model PtRhNi surface (ΔE = 4.29 eV). The possible explanation of this phenomenon is the lattice mismatch among Pt, Rh, and Ni (3.92, 3.80, and 3.52 Å, respectively),38 which hinders the deposition and growth of PtRh layers on the Ni layers due to the increase of strain energy.45 Moreover, based on the “clusters model”, it can be seen that the separation of the Pt and Rh clusters on the surface increased the total energy (ΔE = 6.09 eV), which can indicate that it is energetically favorable to mix platinum and rhodium, whereas formation of separated Pt and Rh phases is less likely. To verify the role of platinum in the synthesis of rhombic dodecahedral nanopolyhedra, a control experiment was conducted, in which the synthesis was performed in the absence of the Pt precursor. As a result, small (∼10 nm) crystalline RhNi nanoparticles were obtained (Figure S7a,b). As it can be seen on the EDS elemental maps (Figure S7c), rhodium and nickel are uniformly distributed in the obtained nanoparticles; nevertheless, in some cases, it is visible that either nickel or rhodium dominates in the sample. The atomic ratio of Rh/Ni, based on the EDS measurements, is 62:38, 22357


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these two types of NPs. It is generally accepted that nanoparticles with opposite ζ-potentials will be attracted to each other.49 The ζ-potential measurements were performed in a pH range between 3.5 and 13 (Figure S11). The PtRhNi nanoframes and SnO2 NPs have opposite ζ-potentials in two ranges of pH: below 5 and over 11. In the present study, the connection of the SnO2 NPs and nanoframes was conducted at pH 4.5, where ζ-potential for the PtRhNi nanoframes was 14 mV, whereas for the SnO2 NPs the ζ-potential was −5 mV. The SAED patterns (Figure S3d) of SnO2@PtRhNi show no differences compared with diffraction patterns of the PtRhNi nanoframes; only diffraction rings corresponding to the PtRhNi phase are visible. However, on the XRD patterns of PtRhNi/SnO2 (Figure S4), it can be seen that a small, additional diffraction peak appears, which can be indexed as (211) and (112) planes of tetragonal SnO2. This poor detection of SnO2 NPs by diffraction methods is probably caused by the small size of the oxide nanoparticles. Moreover, in electron diffraction, only very few SnO2 NPs undergo diffraction when selected with a selected area aperture, whereas in XRD a much larger sample volume, containing thus more SnO2 NPs, undergoes diffraction. This would explain why a small peak from SnO2 is visible only in XRD. On the overview HAADF STEM images of the SnO2@PtRhNi sample (Figure 7a,b), the PtRhNi nanoframes are visible; however, the SnO2

nickel removal. From both SAED and XRD, it can be seen that the nickel content in the sample was decreased. Indeed, EDS mapping (Figure 6a) confirms the previous observation that

Figure 6. (a) HAADF STEM image of single PtRhNi nanoframes with corresponding elemental maps for Pt, Rh, Ni, and their overlap; and (b) HAADF STEM image with (c) corresponding EDS linescan through one nanoframe.

nickel was removed from the core of the nanoparticles, but remained at the edges of the nanoframes, forming a nanoalloy with platinum and rhodium. The EDS linescan (Figure 6b,c) also reveals that the signals from platinum, rhodium, and nickel are present at the edges, whereas a void is visible between the edges. The atomic ratio of Pt/Rh/Ni in nanoframes is 51:17:32, based on EDS measurement, which is in good agreement with ICP-OES results (48:19:33) and roughly corresponds to Pt3Rh1Ni2. This confirms our previous observation that nickel is present on the edges of the nanoframes. This is caused by the fact that nickel is more stable when it is alloyed with platinum.48 Nevertheless, due to the fact that some of the PtRhNi edges of nanoframes are rough and slightly amorphous, it can be deduced that the acetic acid treatment partially affects the PtRhNi alloy at the edges of the nanoframes. The next stage was to synthesize very small SnO2 NPs. Based on the HAADF STEM images (Figure S10a), it can be seen that the obtained nanoparticles are spherical with an average diameter of 2.6 ± 0.4 nm (Figure S2c). The HAADF HRSTEM image (Figure S10b) confirms the crystallinity of the nanoparticles; the measured lattice distances are 0.27 and 0.34 nm, corresponding to (101) and (110) planes of tetragonal SnO2, respectively. The crystallinity of SnO2 nanoparticles is also confirmed by the SAED patterns (Figure S3c). The presented set of diffraction rings can be indexed as (110), (101), (200), (111), (210), (211), and (220) planes of tetragonal SnO2. Moreover, the crystallinity of SnO2 is confirmed by XRD (Figure S4); the diffraction peaks presented on the diffractogram can be indexed as (200), (211), and (112) planes of tetragonal SnO2. The measured diffraction peaks are broad, which indicates that the SnO2 NPs are very small, as confirmed by the STEM observations. The PtRhNi nanoframes decorated with small SnO2 NPs (SnO2@PtRhNi) were obtained by mixing of these two types of nanostructures. The controlled deposition of the SnO2 NPs on the nanoframes was possible due to the electrostatic interactions resulting from the difference in ζ-potentials of

Figure 7. (a) HAADF STEM image of the PtRhNi nanoframes decorated with SnO2 NPs; (b) magnified HAADF STEM image of the area presented in (a); (c) HAADF STEM and (d) bright-field HRSTEM images of a single PtRhNi nanoframe decorated with SnO2 NPs with measured lattice distances. The SnO2 NPs are marked with yellow ellipses; (e) schematic illustration of a nanoframe decorated with SnO2 NPs shown in (c) and (d).

NPs are not observable. Only high-resolution HAADF STEM (Figures 7c and S12) and bright-field (Figure 7d) STEM images allow one to visualize that indeed the SnO2 NPs are deposited on the PtRhNi nanoframes. The lattice distances of 0.18 and 0.21 nm measured from the HRSTEM bright-field images could be indexed as the (200) and (111) planes, respectively, of the fcc PtRhNi alloy. On the other hand, the measured lattice distances of 0.24, 0.26, 0.32, and 0.33 nm 22358


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Figure 8. (a) HAADF STEM image of a single SnO2@PtRhNi nanostructure, and (b) corresponding overlapped EDS maps for platinum, rhodium, nickel, and tin.

This migration of Pt atoms to the surface influenced by the calcination process causes the formation of the so-called Ptskin, which has been reported before by many groups investigating platinum segregation in Pt-bimetallic systems such as PtNi,24 PtCo,50 PtSn,51 and others. In this case, Pt segregation occurs in the presence of nickel and rhodium, which can be explained by the differences in atomic radii and surface energies, similar to that in the case of Pt diffusion during the synthesis of solid PtRhNi NPs. Usually, Pt-skin surfaces consist of up to 3 Pt monolayers (ML),24,52 which corresponds to about 0.5 nm thickness. Herein, the measured Pt-skin surface is between 0.6 and 0.8 nm, which corresponds to 4 and 5 Pt ML. This can be explained by Ostwald ripening occurring during oxygen calcination, which leads to obtaining thicker Pt-skin surfaces.50 The HAADF STEM images of SnO2@PtRhNi NPs after deposition on carbon (Figure S13) show that SnO2 NPs are still deposited on the nanoframes, and the connection between them was not torn apart. To verify that, EDS tilt series were performed (Figure S14), which show that the signal from tin was present on different sides of the nanoframes. Unfortunately, during exposure to the electron beam, the nanoframes were damaged, which is visible on the images. The obtained catalysts were characterized electrochemically toward oxidation of ethanol. First, based on the CV measurements (Figure S15), the platinum specific surface areas of the tested catalysts were calculated based on ECSA measurement and mass of Pt in each catalyst. The largest surface of platinum per milligram (SSA) has the PtRhNi nanoframes (368 cm2 mg−1), compared with commercial Pt/C (297 cm2 mg−1), solid PtRhNi nanopolyhedra (230 cm2 mg−1), and SnO2@PtRhNi catalysts (180 cm2 mg−1) (Table S2). Interestingly, the SnO2@PtRhNi catalysts have lower SSA and ECSA values than the PtRhNi nanoframe catalysts. This could be explained by the fact that SnO2 deposited on nanoframes decreases the surface of platinum and consequently the platinum SSA. Subsequently, the EOR catalytic performance was investigated (Figures 10a−e and S16). The lowest onset potential among the three tested catalysts has SnO2@PtRhNi (close to 0.34 V vs RHE), in comparison with PtRhNi nanoframes and the commercial Pt catalysts, which have 0.42 V vs RHE and 0.56 V vs RHE, respectively. Moreover, the SnO2@PtRhNi catalyst has the highest current density per milligram of Pt (90 mA cm−2 mgPt−1) compared with both PtRhNi nanoframes (26 mA cm−2 mgPt−1) and commercial Pt catalysts (4 mA cm−2 mgPt−1). Basic electrochemical tests showed that the SnO2@ PtRhNi catalyst exhibits better performance during ethanol

could be indexed as the (200), (101), and (110) planes of tetragonal SnO2, respectively. This strongly suggests that the SnO2 NPs are deposited on the PtRhNi nanoframe edges. EDS mapping (Figure 8) reveals that indeed the SnO2 NPs cover the nanoframe edges; the signal from the tin is located on the nanoframe area. The atomic ratio of Pt/Rh/Ni/Sn estimated based on EDS measurements of the nanoframes is 35:11:21:33, which is different from ICP-OES results (19:6:11:64). However, this can be attributed to the fact that EDS is a local technique, whereas ICP-OES provides global results and SnO2 nanoparticles are not only deposited on the surfaces of nanoframes but also occur alone in the sample. After deposition of PtRhNi and SnO2@PtRhNi on Vulcan carbon, the samples were calcined at 200 °C to remove the surfactants and perform surface restructuring. The HAADF HRSTEM images (Figure 9b) show that after calcination, the

Figure 9. HAADF HRSTEM images of PtRhNi nanoframes before (a) and after calcination (b) with the EDS linescan through the edges of the PtRhNi nanoframes before (c) and after calcination (d); the Ptskin layer was marked with gray rectangles. Note that the EDS linescans do not correspond to the HAADF images.

nanoframes are highly crystalline and have much smoother edges than the nanoframes before calcination (Figure 9a). The EDS linescan (Figure 9c) also reveals that before the calcination, platinum, rhodium, and nickel are homogeneously distributed on the nanoframe edges. However, after calcination, platinum tends to segregate on the surfaces of the nanoframes, forming an approximately 0.6 nm-thick Pt layer (Figure 9d), which corresponds to about three Pt monolayers. 22359


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Figure 10. First EOR forward scan of (a) Pt Tanaka, (b) PtRhNi solid, (c) PtRhNi nanoframes, (d) PtRhNi nanoframes decorated with SnO2 NPs, and (e) comparison of all catalysts recorded in 0.1 M HClO4 + 0.5 M C2H5OH solution at a scan rate of 10 mV s−1. Insets present the magnified area to determine the onset potential; (f) chronoamperometry curves for all tested catalysts recorded at a potential of 0.65 V vs RHE for 1.5 h (5400 s).

platinum, consequently decreasing the Pt area at which ethanol can be adsorbed and oxidized. Nevertheless, this presence of tin oxides on the surfaces of the nanoframes assures the highest catalytic performance. According to Kowal et al.,5 SnO2 strongly adsorbs water on its surface, instead of on the Pt

oxidation reaction than both the PtRhNi nanoframes and the commercial Pt/C catalyst. This might be astonishing, as the calculated specific surface area of the SnO2@PtRhNi catalyst is the lowest among the three tested samples. This might be due to the presence of SnO2 NPs partially covering the surface of 22360


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ACS Applied Materials & Interfaces and Rh surfaces, making them available for ethanol adsorption. Moreover, SnO2 provides OH species, which help in oxidation of CO to CO2.4 The decrease in the specific surface area of SnO2@PtRhNi by half, compared with PtRhNi nanoframes, indicates that there is a high coverage of the PtRhNi nanoframes by small SnO2 NPs, which provides a large number of nanoframe/oxide interfaces. This leads to shortening of the distances between active sites of PtRhNi nanoframes and SnO2 NPs. Based on the previous study,53 it can be assumed that this vicinity of these two types of nanoparticles enhances the EOR activity of these catalysts. Also, the presence of rhodium in the nanocatalysts affects the EOR. Both nanoframe catalysts (PtRhNi and SnO2@PtRhNi) exhibit higher catalytic activities than the commercial Pt/C catalyst due to the fact that rhodium facilitates C−C bond splitting.54,55 Furthermore, besides the chemical composition of the tested catalysts, the morphology of the nanoframes also plays a crucial role in the ethanol oxidation reaction. First, the rhombic dodecahedral shape provides 14 corners and 24 edges with low-coordinated atoms, which are catalytically more active than the face atoms. Second, removing of the nonfunctional interior results in a hollow structure with three-dimensional accessibility for adsorption of ethanol molecules. Third, the nanoframe-based catalysts generally have a higher surface-area-to-volume ratio than solid nanoparticles, which provides the improved efficiency of utilizing precious metals like Pt and Rh.24 The EOR catalytic performance and catalyst stability of all tested nanocatalysts were further examined by potentiostatic chronoamperometry experiments at a potential of 0.65 V vs RHE (Figure 10f). It can be seen that after 1.5 h of EOR, the highest current densities per mg of Pt exhibit the SnO2@ PtRhNi catalysts.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Grzegorz Gruzeł: 0000-0003-0714-3699 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Institute of Engineering Materials and Biomaterials of the Silesian University of Technology for the use of the Titan FEI TEM instrument. The study also was carried out with the Talos F200 FEI TEM and ICP-MS equipment purchased by Biological and Chemical Research Centre, University of Warsaw, established within the project cofinanced by the European Union from the European Regional Development Fund under the Operational Program Innovative Economy, 2007−2013. Financial support from the Polish National Science Centre (NCN), grant UMO-2014/ 13/B/ST5/04497, is acknowledged. Many thanks for the partial financial support by Pik-Instruments. The authors would like to thank A. Kowal, Ph.D., for inspiring with the idea of hollow nanoparticle synthesis and for fruitful discussions. The authors thank Dr. A. Budziak from IFJ PAN for acquiring the XRD diffractograms, Dr A. Ruszczyńska from Biological and Chemical Research Centre, University of Warsaw, for conducting the ICP-MS measurements, and Dr Anna PajorŚwierzy for ζ-potential measurements. P.P. acknowledges the financial support from the Polish National Science Centre (NCN) under project No. UMO-2017/25/B/ST3/02586.

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CONCLUSIONS In summary, the synthesis of PtRhNi rhombic dodecahedral nanoparticles and etching them to PtRhNi nanoframes were performed. The obtained nanoframes were decorated with small SnO2 NPs and applied as catalysts in ethanol oxidation reaction. During the synthesis of nanopolyhedra, by using STEM and EDS techniques, it was possible to track the mechanism of the growth process of the PtRhNi NPs. These observations revealed that in time, with increasing temperature, the Pt and Rh atoms diffuse from the core of the nanoparticles to the edges and corners, which was also confirmed by DFT calculations. These findings could help in the design and synthesis of other shape-controlled PtRh-containing nanoparticles. By etching away the Ni core from the solid PtRhNi nanopolyhedra and depositing the SnO2 NPs on the PtRhNi nanoframes, SnO2@PtRhNi nanocatalysts were obtained with a Pt-skin layer on the frame surfaces formed after calcination. This system exhibits superior catalytic activity compared with PtRhNi nanoframes without the addition of SnO2 and with commercially used Pt/C catalysts. These results show that due to the synergistic effect among Pt, Rh, Ni, and Sn, as well as due to shape control of the nanoframe structure, we designed highly promising EOR catalysts.

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Additional TEM data, SAED, and XRD diffractograms, models used in calculations, CV and ζ-potential data, comparison of mass and specific activity (PDF)

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b04690. 22361


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Preparation of Pt-skin PtRhNi Nanoframes Decorated with Small SnO2

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