Facile Strategy To Prepare Rh Nanosheet-Supported PtRh

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A Facile Strategy to Prepare Rh Nanosheet-Supported PtRh Nanoparticles with Synergistically Enhanced Catalysis in Oxidation Shuangfei Cai, Chao Lian, Haohong Duan, Wei Xiao, Qiusen Han, Cui Qi, Chen Wang, and Rong Yang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03889 • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 12, 2019

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Chemistry of Materials

A Facile Strategy to Prepare Rh Nanosheet-Supported PtRh Nanoparticles with Synergistically Enhanced Catalysis in Oxidation Shuangfei Cai†, Chao Lian‡, Haohong Duan§, Wei Xiao†, Qiusen Han†, Cui Qi†, Chen Wang*,† and Rong Yang*,† †

CAS Key Lab for Biomedical Effects of Nanomaterials and Nanosafety, Center of Materials Science and Optoelectronics Engineering, CAS center for Excellence in Nanoscience, National Center for Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing 100049, China ‡

School of Science, Beijing Jiaotong University, Beijing 100044, China

§

Chemistry Research Laboratory, Department of Chemistry, University of Oxford, 12 Mansfield Road, Oxford OX1 3TA, U.K. ABSTRACT: Here we report the one-pot synthesis of metal-based zero-dimensional (0D)/two-dimensional (2D) nanocomposites via a modified seed-mediated growth method. In this strategy, two metal precursors (H2PtCl6·6H2O and Rh(acac)3) were mixed together with HCHO as a reductant, followed by the solvothermal treatment, producing homogeneous PtRh bimetallic nanoparticles (NPs) supported on ultrathin Rh nanosheets (NSs) with trapezoidal shape and crystalline/amorphous hetero-phase characteristics. Intriguingly, successive processes including generation of PtRh NPs, formation of Rh NSs and immobilization of PtRh NPs on Rh NSs were involved in the synthesis, creating a novel 0D/2D PtRh/Rh nanostructure. Several experimental parameters (reductant, precursor, surfactant, solvent and temperature) were crucial to the formation of unique structure, during which the preferentially-formed Pt atoms/clusters as seeds not only accelerated reduction of Rh precursors to form PtRh NPs, but directed formation of Rh NSs around PtRh NPs. Owing to both structural and electronic effects, PtRh/Rh exhibited synergistically enhanced activity in the aerobic oxidation. Mechanistic study further revealed the reaction obeyed the Michaelis-Menten theory, and PtRh/Rh-catalysis originated from both generation of important reactive oxygen species and rapid electron transfer. The results provide a facile yet effective avenue to fabricate a new type of Pt-based heterogeneous catalysts. 1. INTRODUCTION Supported metallic nanostructures have attracted considerable research interest in catalysts, sensors, nanoelectronics, and energy storage/conversion.1-3 Among them, numerous studies have demonstrated that the supported Pt-based nanocatalysts with well-defined shape and controlled size displayed outstanding catalytic performance and stability, owing to metal-support interaction.4-14 As representative examples, 1

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single-layer Ni(OH)2 NS-supported ultrathin Pt nanowires9 and monolayer MoS2 NS-supported Pt NPs10 have exhibited better electrocatalytic activity towards hydrogen evolution reaction (HER) than the commercial Pt/C catalysts in alkaline and acidic solutions, respectively. The enhanced activity and stability could be attributed to easy charge transport and efficient coupling between Pt and NSs, as a result of their unique morphological and structural features. However, conventional synthetic protocols to access supported Pt-based nanostructures are commonly based on a two-step procedure: (1) pre-preparation and/or surface functionalization of supports and (2) in-situ growth of Pt-containing composites. These procedures are time-consuming, laborious, and costly. Thus, developing a one-pot approach that can simultaneously produce support matrices and immobilize metals is fascinating. Recently, ultrathin metallic compounds representing a new research frontier in 2D nanomaterials have aroused enormous enthusiasm in numerous fields.15,16 With many unusual physiochemical attributes such as large specific area, high aspect ratio, unsaturated surface coordination and anisotropic structure, these layered materials provide promising support substrates to anchor other metals. To date, a variety of 0D/2D nanostructures like Ru/Pd NSs and nanoribbons,17,18 (Pt, Au)/Pd NSs,19 and (Ag, Pt, Pd, PdAg, PtAg, PdPtAg)/Au nanocomposites with diverse shapes (square sheets, rhombic nanoplate and nanoribbons),20-22 have been successfully constructed via various synthetic strategies based on organic ligand-assisted growth, seeded growth and crystal phase transformation. The incorporation of other metals into the nanostructures not only enhance the intrinsic properties of materials but bring new functions, thus holding great promise in broad applications in catalysis, surface enhanced Raman scattering (SERS), sensing, photothermal therapy, bioimaging and solar cells. However, controllable synthesis of novel heterostructures based on metallic NSs still remains a grand challenge. Meanwhile, the catalytic oxidations, with molecular oxygen (O2) as an oxidant, are of paramount significance in nature. Oxidases can activate O2 to catalyze oxidation of substrates, but several intrinsic drawbacks limit their large-scale applications.23 Intriguingly, some inorganic nanomaterials behave as oxidases in oxidations, with advantages of adjustable activity and high stability.24-27 In the open air, they catalyze the oxidation of several chromogenic substrates such as 3,3',5,5'-tetramethylbenzidine (TMB), producing colored products easily observed by naked eyes. With oxidase mimic-based colorimetric assay, numerous biologically relevant samples (e. g., proteins, cancer cells, metal ions and small molecules) could be conveniently detected with simplicity, rapidness and low-cost, thus particularly attractive for point-of-care diagnosis. Until now, exploration of efficient nanomaterial-based oxidase-mimics still remains an active topic. Herein, we report a modified see-mediated growth strategy, which allows the one-pot synthesis of PtRh NPs supported on Rh NSs in trapezoidal shape (Figure 1). As seeds, Pt atoms/clusters were preferentially-formed in the synthetic system, followed by the in situ reduction of Rh precursor to produce both PtRh NPs and Rh NSs, creating novel 0D/2D structured PtRh/Rh nanocomposites. Moreover, PtRh/Rh displayed high capability for O2 activation and rapid electron transfer, resulting in enhanced activity in the aerobic oxidation of TMB. The work 2


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presents a useful advance on the development of nanostructured catalytic materials.

Figure 1. Illustration of one-pot synthesis of PtRh/Rh for catalytic oxidation. 2. RESULTS AND DISCUSSION 2.1 Synthesis and Characterization of PtRh/Rh. In a typical synthesis, PtRh/Rh nanocomposites were fabricated by reducing metal precursors (H2PtCl6·6H2O and Rh(acac)3) with HCHO in benzylalcohol, in the presence of polyvinylpyrrolidone (PVP) as a surfactant (see details in Experimental section). As a representative example, the morphology of products obtained using a Pt/Rh feed atomic ratio of 0.218 was determined by transmission electron microscopy (TEM; Figure 2A and Figure S1), scanning electron microscopy (SEM; Figure S2) and the high-angle, annular dark-field scanning transmission electron microscopy (HAADF-STEM; Figure 2B), which revealed a large number of NPs were supported on the ultrathin NSs, with an average size of about 3.3 nm. The NSs have several hundred nanometers in length and trapezoidal shape. The ultrathin nature of NS products was also evidenced by the atomic force microscopy (AFM) analysis. The thickness of one single NS was below 3.5 nm (Figure S3). Moreover, Rh, Pt and overlapped element maps (Figures 2C-2E) showed homogeneous distribution of metals. Signals of both two elements could also be easily detected by the energy dispersive X-ray (EDX) analysis (Figure 2F), with a Pt/Rh atomic ratio of 0.219 very close to the feed one. X-ray diffraction (XRD) was used to characterize the crystal structure of products. Considering both NSs and particles in products, the crystal structure of pure Pt NPs and Rh NSs was also studied for comparison. As shown in Figure 2G, the XRD data of Pt NPs perfectly matched the standard peaks of face-center cubic (fcc) Pt (JCPDS no. 01-087-0646). However, the XRD pattern of pure Rh NSs showed only one weak peak at about 40.8o, which could be assigned to the (111) plane of fcc Rh (JCPDS no. 01-088-2334). Similar phenomenon was also 3



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found with other Rh NSs, due to the ultrathin thickness.28-30 For the nanocomposites, diffraction peaks appear at about 40.4°, 46.7°, 68.5° and 82.7°, which just positioned between the standard peaks of (111), (200), (220) and (311) planes of fcc Pt and Rh. These results indicate the particles in products may be PtRh nanoalloys. Besides, surface analysis of products was performed by X-ray photoelectron spectroscopy (XPS). Peaks located at about 311.83 eV and 307.14 eV (Figure 2H) could be ascribed to Rh 3d3/2 and Rh 3d5/2, respectively.31,32 Meanwhile, peaks representing Pt 4f5/2 and Pt 4f7/2 were near to 73.35 eV and 70.49 eV (Figure 2I), respectively.33,34 The results suggest metallic Rh and Pt as predominant species in products.

Figure 2. (A) TEM image, (B) HAADF-STEM image, elemental maps for (C) Rh and (D) Pt, (E) overlapped elemental map, (F) EDX spectrum, (G) XRD pattern, high-resolution peak-fitting XPS spectra of (H) Rh 3d and (I) Pt 4f regions of products. Scale bar: 100 nm.

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The possible alloy nanostructure of particles was also studied by the high-resolution TEM (HRTEM) technique. A typical HRTEM image of a region near the edge is displayed in Figure 3A, which reveals crystalline/amorphous hetero-phase characteristics of NS. The crystalline domains, marked by the dashed pink curves, show the lattice fringes with interplanar spacing of about 0.222 nm, which is between the fcc Pt (0.225 nm) and fcc Rh (0.221 nm). Thus, these crystalline domains could be composed of alloys. Another HRTEM image of typical region (Figure 3B), which was randomly selected at the center of NS, also reveals its crystalline/amorphous heterostructures, with similar lattice fringes and interplanar spacing of crystalline domains (Figure 3C). Considering subtle difference in the interplanar spacing of fcc Pt and Rh, more detailed characterizations, for accurate discrimination of alloy particles, are needed.

Figure 3. HRTEM images of regions (A) near the edge (B) and close to center as well as (C) the enlarged region indicated in Figure 3B of a randomly selected NS, HAADF-STEM image (D) and corresponding element maps of (E) Pt and (F) Rh of the selected region marked by the blue box near the edge of a single NS shown in Figure 3D. The typical circles marked with yellow-dotted lines shown in Figure 3B, show particles with good crystalinity. Owing to ultrathin nature of NSs, the sample was highly sensitive to the electron beam irradiation. To further verify the alloy structure of particles, high-resolution element maps of Pt and Rh of a typical region near the edge of NS, which is indicated in its STEM image (Figure 3D), were carefully collected. One can see a large number of small bright spots distributed on the selected NS. From Figures 3E and 3F, these spots are composed of both Pt and Rh elements with homogeneous distribution. The map of Rh is slightly larger than that of Pt in particles, owing to existence of Rh NS 5



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substrate. The EDX line scans across a single particle, which was located at the edge of NS (Figure S4A), also revealed the existence of both metal elements (Figure S4B). These results verify the alloy nanostructure from observations by XRD and HRTEM. However, owing to existence of Rh NS substrate, it is challenging to accurately measure the composition of supported particles. To avoid interference of NS, a few freestanding particles with similar size and morphology, which were distributed in the different regions around the NS, were chosen (Figure S4C). From EDX spot scan analysis (Figure S4D), the average Pt/Rh atomic ratio was approximately 1 : 2. Overall, Pt1Rh2/Rh nanocomposites were successfully prepared. 2.2 Formation mechanism of PtRh/Rh. The most interesting feature of present synthesis lies in the formation of 0D/2D nanostructure. Metals like Pt,35,36 Pd,37 Ir,38 Fe39 and Ni,40 could be alloyed with Rh by Galvanic replacement, pyrolysis and co-reduction. However, Pt and Rh were partially alloyed under present conditions. The phase segregation of metals, therefore, provides the opportunity to construct the supported nanostructure. In the following control experiments, the use of H2PtCl6·6H2O alone gave spherical Pt NPs with average diameter of 2.7 nm, while Rh NSs could be obtained using Rh(acac)3 alone (Figure S5). The formation of PtRh alloys in the nanocomposites suggests possible interaction between Pt and Rh atoms/clusters. In principle, since standard reduction potential (E) for PtCl62-/Pt0 (1.48 V)41 is much more positive than that of Rh3+/Rh0 (0.76 V)42 pair in the same coordination environment, Pt4+ ions with high E could be reduced first. To investigate possible role of Pt atoms/clusters in PtRh/Rh formation, yield of Rh in the reaction intermediates, which were collected during the synthesis of Pt1Rh2/Rh and pure Rh NSs at the same time (1, 10, 30 and 120 min), was compared. After purification, Rh concentration of intermediates was determined by ICP-OES. From Figure S6A, at the early reaction stage (1 min), the sample was rich in Pt element during PtRh/Rh synthesis, with yield of 67.4%, calculated based on its theoretical mass from Pt precursors. Meanwhile, the yield of Rh, during 1 to 30 min of time, was increased from 25.6 % to 66.2 %. Contrarily, the intermediates for pure Rh NSs gave lower yield of Rh, from 6.7 % to 34.8 % (Figure S6B). The enhancement of Rh yield for PtRh/Rh synthesis was attributed to the preferentially-formed Pt atoms/clusters, which could accelerate the reduction of Rh precursors. Thus, growth of Rh NSs in PtRh/Rh was catalyzed by Pt atoms/clusters. To further illustrate the role of Pt seeds in Rh NS growth, a two-step procedure was employed, in which Pt seeds were premade with 1 min and Rh(acac)3 was subsequently introduced to the same synthetic system for reduction. As shown in Figure S7, similar shape of NSs was obtained with good particle distribution. Thus, formation of Rh NSs could be closely related to the preferentially-formed Pt seeds. To identify more details over the growth process of Pt1Rh2/Rh, several key factors including reductant, precursor, surfactant, solvent and temperature were studied systematically. In a control experiment without HCHO or using other reducing agents like benzaldehyde, aggregates formed as products with low yield (Figure S8). Thus, HCHO was indispensable for the formation of 0D/2D nanostructured Pt1Rh2/Rh. 6


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Previous studies revealed that CO, either generated from HCHO decomposition or additional introduction to synthetic system, played crucial roles in the 2D anisotropic growth of Rh NSs.28,43 A peak at 2091 cm-1 in the Fourier transform infrared (FT-IR) spectrum of Pt/Rh, related to linearly adsorbed CO based on literatures,28,44,45 was found (Figure S9). Under confinement effect of CO, deposition of Rh carbonyl clusters could be inhibited by the adsorbed CO molecules on the basal (111) faces of Rh, leading to continuous growth of NSs along the [111] direction. Thus, HCHO not only acted as a reductant, but served as a shape controller for Rh NS formation. Another essential parameter for PtRh/Rh synthesis is the precursor. Experimentally, by adjusting feed ratio of Pt/Rh from 0.218 to 0.763, both morphology and composition of products were controllable (Figure S10 and Table S1). However, by increasing Pt precursor concentration, there was no obvious shift of diffraction peak of Pt (111) plane in the XRD patterns of products (Figure S11), indicating that composition of PtRh NPs remained essentially unchanged. Although 0D/2D structure could also be created by changing H2PtCl6·6H2O to other Pt precursors like Pt(acac)2, irregularly shaped NSs were obtained, with smaller size (Figure S12A). This may be related to the lower reduction potential, i. e., Pt2+/Pt0 (1.18 V)42 vs. PtCl62-/Pt0 (1.48 V), affecting reduction kinetics of PtRh/Rh. Meanwhile, replacing Rh(acac)3 by RhCl3 produced spherical NPs (Figure S12B), suggesting that ligands in Rh precursor could also be important for nucleation, growth and assembly of Rh atoms/clusters to NSs. Besides, the role of PVP as a stabilizer for morphology of products was studied. By adjusting PVP concentration from 8.1 to 32.3 g L-1, no obvious morphology change of products was observed (Figures S13A and S13B). However, by changing the molecular mass of PVP, the obtained products were not well-shaped (Figures S13C) or slightly aggregated (Figure S13D). Meanwhile, by replacing PVP with other stabilizers like polyethylene glycol (PEG) or chitosan, which bears –OH or/and –NH2 groups, products were not trapezoidal in shape (Figures S13E and S13F). The results suggest the C=O groups in PVP are essential to support trapezoidal NSs. Additionally, the solvent might also affect the reaction kinetics in synthesis. To study whether benzylalcohol as a solvent/co-reductant is indispensable for Pt/Rh formation, the reductions were conducted in the absence of benzylalcohol or in other common solvents, including N-methyl pyrrolidone (NMP), ethylene glycol (EG) and diethylene glycol (DEG). Without benzylalcohol, irregularly shaped and broken NSs with small sizes were obtained, with a lot of freestanding crystalline nanostructures (Figure S14A). In this case, it was likely that the precursors were vigorously diffused and reduced too quickly, owing to the aqueous phase and high temperature. By displacing benzylalcohol with the selected solvents, trapezoidal NSs were also formed. For NMP, crystalline nanostructures were well distributed on Rh NSs (Figure S14B). However, by using EG or DEG, the crystalline domains were mainly located at edges of Rh NSs (Figures S14C and S14D), which could be related to the stronger reducibility of EG and higher viscosity of DEG. The reduction rate of precursors might be faster in EG, while the diffusion rate of precursors might be slower in DEG, which greatly affected nucleation and growth of metal clusters as well as assembly of NSs. Besides, some of freestanding NPs might also be randomly adsorbed on Rh NSs 7



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during the synthesis or post-treatment of products, leading to poor distribution of crystalline nanostructures on Rh NSs. Thus, for the controlled synthesis of PtRh/Rh, a solvent (e. g., benzylalcohol or NMP) with lower reducibility and viscosity, which is closely associated with reaction kinetics, is required. Additionally, yield of PtRh/Rh was largely affected by temperature. A temperature above 180 oC was necessary. To get a better understanding of formation process, the time-dependent experiment was conducted and the reaction process was monitored by TEM. For comparison, the morphological evolution of pure Rh NSs along with time was first investigated. At the begining (1 min), main products were the folded aggregates, together with a few small NSs (Figure 4A). Increasing reaction time to 10 min, several larger NSs were obtained with irregular shape, accompanied by lots of adsorbed aggregates. After 30 min of reaction, the trapezoidal shape of NSs became clear, among which many small NSs could be found. These results suggest the aggregates could evolve into NSs. As the reaction proceeded, the small NSs gradually became larger, with several hundred nanometers in length. These intriguing findings demonstrate nucleation and growth process of Rh NSs. Figure 4B shows the morphological evolution of Pt1Rh2/Rh within the same time. During the early stage of reaction (1 min), a large amount of smaller freestanding NPs were observed, with a few larger aggregates. Moreover, some of NPs were closer to each other and supported on small-size irregular NSs. When the reaction was extended to 10 min, although a large amount of small freestanding NPs still existed, the NSs became larger and their trapezoidal profiles were formed. Notably, besides NSs, obvious aggregates were not be observed, totally different from the case for Rh NS synthesis. As reaction continued to 30 min, the freestanding NPs evidently decreased, with the appearance of lots of supported NSs. When the reaction was futher increased to 2 h, more and more supported NSs with good NP distribution were formed as final products. On a basis of above results, we hypothesized that the reactions during 10 min were essential for PtRh/Rh synthesis, among which Pt seeds accelerated adsorption and reduction rate of Rh precursor, leading to fast generation of alloy particles and subsequent formation of NSs.

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Figure 4. (A, B) TEM images of intermediates collected at different time (1, 10, 30 and 120 min, from left to right) for Rh NSs and Pt1Rh2/Rh synthesis as well as (C) possible formation mechanism of PtRh/Rh. Based on above results and dissusions, following mechanism was proposed (Figure 4C). Initially, plenty of Pt atoms/clusters were rapidly formed in the synthetic system. With large surface area and high surface energy, the unsaturated surface Pt atoms tend to contact with Rh precursors to reach the adsorption equilibrium. After that, with the assistance of Pt atoms/clusters, the adsorbed Rh precursors were quickly reduced to Rh atoms/clusters at high temperature. Subsequently, these highly reactive Rh atoms/clusters could colloid with active sites of Pt atoms/clusters, followed by diffusion from the surface into the crystal lattice of Pt, producing crystalline PtRh NPs. These bimetallic NPs further provided possible active sites, facilitating adsorption and reduction of residual Rh precursors. As reaction continuously proceeded, the newly yielded Rh atoms/clusters could be surrounded PtRh NPs and gradually assembled to Rh NSs along the Rh [111] direction, owing to confinement effect of CO from HCHO decomposition. Since Rh precursors could be spontaneously adsorbed on PtRh NPs along the different directions, the assembled Rh NSs have amorphous structure, while PtRh NPs may be not only onto, embedded, but also coated with Rh NSs. Here, we describe the PtRh/Rh formation as a multi-step process of preferential generation of Pt atoms/clusters, in-situ reduction of Rh precursors to form PtRh alloys and self-assembly of Rh atoms/clusters to Rh NSs. Notably, contrary to metallic NSs used as heterogeneous neucleation sites for growth of other metals in previous studies, the preferentially-formed Pt atoms/clusters in this study acted as seeds, which not only 9



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promoted reduction of Rh precursors to form partial crystalline PtRh NPs, but directed the formation of amorphous Rh NSs around PtRh NPs. Therefore, our protocol doesn’t involve multiple processes of pre-preparation, surface modification of supports and dispersion of metallic NPs, and thus labor-saving and cost-effective. 2.3 Catalytic activity of PtRh/Rh. Considering rich active sites of PtRh NPs and a large surface area of Rh NSs, the 0D/2D structured PtRh/Rh could be potentially beneficial for O2 activation, and thus enhance catalysis for oxidations. To test this hypothesis, the oxidation of TMB in open air, which affords a blue product (oxidized TMB, oxTMB) with a maximum absorption at 652 nm,46 was chosen (Figure 5A). The control reaction without catalyst showed no obvious color change (Figure 5B). Compared with pure Rh NSs and Pt NPs, Pt1Rh2/Rh as a model catalyst displayed better activity in oxidation, producing a deeper blue color, with an approximate 5- and 3-fold enhancement in the absorbance at 652 nm (Figure 5C), respectively. Moreover, by using Pt NPs together with Rh NSs, the absorbance of products (curve e, Figure 5C) was just equivalent to the case of simple combination of these metals, suggesting each of metal components catalyzed the oxidation independently. Thus, Pt1Rh2/Rh displayed synergistically enhanced activity in TMB oxidation. The oxidation rate was further compared by steady-state kinetic analysis under same conditions. As revealed by typical kinetic curves (Figure 5D) and corresponding linear Lineweaver-Burk plots (Figure 5E), the colorimetric reactions followed the Michaelis-Menten theory. Pt0.219/Rh demonstrated the highest catalytic efficiency, with a value of maximal velocity (Vmax) of 0.188 μM s-1, larger than that of Rh NSs (0.00192 μM s-1) and Pt NPs (0.129 μM s-1). Compared with other nanomaterial-based oxidase-mimics, PtRh/Rh had higher rate to those of Pt NPs (ca. 0.1 μM s-1),47 CeO2 NPs (0.069 μM s-1)48 and PdPt/Au alloy nanorods (ca. 0.01 μM s-1),26 showing superior ability of activating O2.

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Figure 5. (A) Reaction scheme for TMB oxidation, (B) color change of solutions at room temperature: (a) TMB, (b) Pt1Rh2/Rh (28 μM), (c) Rh (23 μM) + TMB, (d) Pt (5 μM) + TMB, (e) Pt (5 μM) + Rh (23 μM) + TMB and (f) Pt1Rh2/Rh (28 μM) + TMB, (C) corresponding UV-Vis spectra as well as kinetic analysis using (D) Michaelis-Menten model and (E) Lineweaver-Burk model for TMB oxidation at room temperature. The concentration of catalyst was based on metals, calculated from ICP-OES. The enhanced activity of Pt1Rh2/Rh is probably attributed to two factors, i. e., structural and electronic effect. The formation of amorphous/crystalline nanostructures, assembled from Rh clusters around PtRh NPs, could not only provide a large surface area but create more active sites. The electronic effect, which derived from the d-band center shift of Pt, could also play an important role in the oxidation. As evidenced by XPS in Figures 3F and 3G, compared with metallic Rh 3d5/2 (307.04 eV) in pure Rh NSs (Figure S15A) and Pt 4f7/2 (70.57 eV) in pure Pt NPs (Figure S15B), the Rh 3d5/2 and Pt 4f7/2 core-level peaks in PtRh/Rh positively and negatively shifted for about 0.10 eV, respectively. The BE shift reveals electron interaction between Rh and Pt atoms, in which electron transfer from Rh to Pt. Upon filling Pt d-band, the binding strength of Pt and adsorbates could be weakened,49 which facilitated adsorption and activation of O2, leading to higher catalytic activity in TMB oxidation. The coverage density of metallic NPs in nanocomposites has significant influence on their catalytic performance. The coverage density of PtRh NPs in nanocomposites depends on the initial concentration of precursors. In the range of Pt/Rh feed ratio of 0.218 to 0.763, with increasing Pt4+ concentration, distribution of PtRh NPs on Rh NSs became more and more dense, while shape of NSs kept trapezoidal. On the other 11



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hand, by raising Pt/Rh feed ratio, the catalytic activity gradually increased but decreased when the ratio exceeded 0.545 (Figure S16). The higher Pt/Rh atomic ratios were unfavorable for catalysis, likely due to the block of active sites of both PtRh NPs and Rh NSs. The optimized catalyst gave a Vmax value of 0.205 μM s-1, calculated by fitting the initial reaction rates to the Michaelis-Menten equation (Figure S17). These results suggest the catalytic activity of PtRh/Rh was feed composition-dependent. 2.4 Mechanism investigation. To gain an in-depth understanding of PtRh/Rh-catalytic mechanism, several key reaction parameters were systematically studied. Similar to previous studies, the activity of Pt1Rh2/Rh exhibited pH- and temperature-dependence for TMB oxidation, among which the highest activity was achieved at pH 4 and 40 oC (Figures 6A-6B). Notably, for higher temperatures (especially above 50 oC), the products could be degraded under acidic conditions, owing to disproportionation of TMB cation radicals.50 Accordingly, the oxidation of ascorbic acid (AA), a typical non-enzyme substrate, was chosen for comparison. As shown in Figure S18, without catalyst, oxidation of AA by O2 was slow. However, in the presence of Pt1Rh2/Rh, conversion of AA continuously increased with temperature. By using 0.14 μM of catalyst at 70 oC, about 95 % of AA was oxidized. These results demonstrate high stability of catalyst at higher temperatures. Therefore, the temperature dependence in the catalytic oxidations is substrate-limited.

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Figure 6. Effect of (A) pH, (B) temperature, (C) Pt1Rh2/Rh concentration (based on total metals) and (D) atmosphere on TMB oxidation, (E) ESR spectra of DMPO-radical adducts for Pt1Rh2/Rh-DMSO system and (F) UV-Vis spectra for CytC oxidation by Pt1Rh2/Rh. Relative activity was calculated based on the maximum absorbance recorded at 652 nm, and relative velocity was relative to the maximum initial velocity. It is unsurprising that the activity showed catalyst concentration-dependence, in which the higher Pt1Rh2/Rh concentration, the faster the initial oxidation rate, which could be observed by comparing the early data (Figure 6C). On the other hand, after saturation of Pt1Rh2/Rh with N2, the initial oxidation rate quickly decreased (Figure 6D), since the active sites of catalyst could be occupied by N2 molecules. Thus, both catalyst and dissolved O2 played important roles in the oxidation. To further investigate the role of O2 as an electron acceptor, possible reactive oxygen species (ROS) like O2•– radicals were studied by the electron spin resonance (ESR) technique, using 5,5-dimethyl-1-pryrroline N-oxide (DMPO) as a spin trap. 13



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Considering relatively longer lifetime of O2•– in aprotic media,51 DMSO was selected for ESR experiments. Two types of peroxy radicals, i. e., O2•– and HO2• (the protonated form of O2•–) were identified, wherein the observed signal was matched well with the simulated one (Figure 6E). The O2•– radicals, as main oxygen species in present system, generated via single-electron reduction of oxygen (O2 + e- = O2•–). Meanwhile, the formation of HO2• might be asscociated with water in the DMPO aqueous solution, since O2•– is always in equilibrium with its conjugate acid (O2•– + H+ = HO2•).52 Considering TMB oxidation in buffer, both O2•– and HO2• radicals were important reactive oxygen intermediates in the oxidation. On the other hand, the possible ability of accepting electrons of Pt1Rh2/Rh during catalytic process was tested with cytochrome C (CytC), an active reactant in the electron transfer process.53 To exclude the influence of O2, the catalyst, substrate and buffer were sufficiently pre-saturated with N2. As shown in Figure 6F, after incubation with Pt1Rh2/Rh, typical absorption peaks at 520 and 550 nm of substrate completely disappeared, accompanied by the appearance of a new peak at 530 nm. The results suggest that cytochrome C, as an electron donor, was oxidized by Pt1Rh2/Rh. Thus, besides the ability of ROS generation, Pt/Rh-catalysis originates from electron transfer between catalyst and substrate. 2.5 Stability of PtRh/Rh. To test the thermal stability of catalyst for TMB oxidation, Pt1Rh2/Rh was pre-incubated in a water bath at higher temperatures (50 and 70 oC). As shown in Figure S19, after incubation for 6 h, the activity of Pt1Rh2/Rh in TMB oxidation at 40 oC remained above 95 %. Moreover, when Pt1Rh2/Rh dispersion was stored at room temperature for 3 months, no obvious decrease in the activity was observed. Thus, PtRh/Rh possessed high thermal and long-term stability. Besides, Pt1Rh2/Rh could be reused for TMB oxidation. Figure 7A shows the TEM image of the recycled catalyst after 2rd run for reaction. Compared to fresh catalyst, the size, alloy structure and distribution of NPs as well as trapezoidal shape of NSs for the recycled one did not undergo appreciable change (Figures 7B-7D). With excellent activity, stability and good recyclability, PtRh/Rh could be potentially severed as an oxidase-mimic for practical applications.

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Figure 7. (A) TEM image, (B) HRTEM image, (C) enlarged HRTEM image of a typical region indicated in Figure 7B, (D) STEM image, element maps of (E) Pt and (F) Rh of the recycled catalyst after 2rd run for TMB oxidation. 3. CONCLUSIONS In summary, novel 0D/2D PtRh/Rh nanocomposites were successfully fabricated, using the one-pot strategy via Pt seed-mediated growth. Different from previous studies on metallic NSs as heterogeneous neucleation sites for growth of other metals, Pt atoms/clusters were preferentially-formed as seeds in this work, which not only accelerated reduction of the second metal precursor to form bimetallic NPs, but directed formation of metallic support materials around NPs. Owing to a synergistic effect of PtRh NPs and Rh NSs, PtRh/Rh displayed enhanced activity in oxidation. The catalysis followed a Michaelis-Menten mechanism, with ROS generation and rapid electron transfer. These findings would be helpful for the studies in nanocatalysis. Most importantly, the novel synthetic strategy opens the door to design and prepare supported Pt nanostructures for various potential applications. 4. EXPERIMENTAL SECTION 4.1 Materials. Rh(acac)3 (99.99 %) was provided by Alfa Aesar. H2PtCl6·6H2O (99.9 %) was purchased from STREM Chemicals, Inc. PVP (K30) was obtained from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Other reagents were used as received without further purification. The water used throughout all experiments was purified by a Milli-Q system (18 MΩ·cm). 4.2 Characterization. TEM images were obtained using a transmission electron microscope (FEI Tecnai G2 F20 U-TWIN) operating at an accelerating voltage of 200 kV. SEM images were collected on a Hitachi S-4800 ﬁeld-emission scanning electron microscope. The sample surface was sprayed with Au to make them conductive. The 15



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thickness of sample was analyzed by Asylum Research AFM Cypher S. The crystalline structure was determined using a X-ray powder diffractometer (XRD, Bruker D8 focus) with Cu Kα radiation (λ = 1.5406 Å) and a high-resolution transmission electron microscope (HRTEM, JEM-2010F). The composition was measured by the inductively coupled plasma optical emission spectroscopy (ICP-OES, Thermo Scientific iCAP 6300) and EDX analysis. Partial EDX data were collected on a JEOL JEM 2100F transmission electron microscope. The surface state was analyzed by a X-ray photoelectron spectrometer (Thermo Fisher ESCALAB 250Xi). The binding energy (BE) of the XPS was calibrated with respect to the pure bulk Au 4f7/2 (BE = 84.0 eV) and Cu 2p3/2 (BE = 932.7 eV) lines. The BE was referenced to the Fermi level (Ef) calibrated by using pure bulk Ni as Ef = 0 eV. FT-IR spectra were recorded on PerkinElmer Spectrum One FT-IR spectrometer in the transmission mode using CaF2. 4.3 Preparation of Rh NSs. Rh NSs were synthesized based on the previous work30 with slight modification. Typically, Rh(acac)3 (8.0 mg) and PVP (120 mg) were dissolved in benzylalcohol (3 mL) and formaldehyde (3 mL). The mixture was stirred vigorously for 10 min and then transferred to a Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained at 180 oC for 2 h and then cooled to room temperature. The resulting black product was precipitated with acetone, separated and washed with a mixture of acetone and ethanol. To study the morphological evolution of Rh NSs, reactions were performed for different time (1, 10 and 30 min), and the obtained products were purified following a similar procedure described above. 4.4 Preparation of Pt NPs. Briefly, H2PtCl6·6H2O (21.75 mM, 0.2 mL) and PVP (120 mg) were dissolved in benzylalcohol (3 mL) and formaldehyde (3 mL). The mixture was stirred vigorously for 10 min and then transferred to a Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained at 180 oC for 2 h and then cooled to room temperature. The resulting black product was precipitated with acetone, separated and washed with a mixture of acetone and ethanol. 4.5 Preparation of PtRh/Rh. PtRh/Rh was synthesized via the one-pot or two-step synthetic route, on a basis of a conventional solution-phase reduction method. One-pot synthesis. PtRh/Rh was synthesized using a similar procedure for preparing Pt NPs described above except that Rh(acac)3 was additionally introduced. Typically, Rh(acac)3 (8.0 mg) and PVP (120 mg) were dissolved in benzylalcohol (3 mL) and formaldehyde (3 mL). Then, the desired quantity (0.1 to 0.8 mL) of H2PtCl6·6H2O (21.75 mM) was added to above solution. The mixture was stirred vigorously for 10 min and then transferred to a Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained at 180 oC for 2 h and then cooled to room temperature. The resulting black product was precipitated with acetone, separated and washed with a mixture of acetone and ethanol. To study the morphological evolution, reactions were performed for different time (1, 3, 10 and 30 min), following a similar procedure described above except that a fixed quantity (0.2 mL) of H2PtCl6·6H2O (21.75 mM) was used. Two-step synthesis. For comparison, Pt seeds were pre-made and Rh(acac)3 was 16


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subsequently introduced in the same synthetic system. Typically, after PVP (100 mg) was dissolved in benzylalcohol (3 mL) and formaldehyde (3 mL), H2PtCl6·6H2O (21.75 mM, 0.2 mL) was added. The resulting mixture was stirred vigorously for 10 min and then transferred to a Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained at 180 oC for 1 min and then cooled to room temperature. After addition of Rh(acac)3 (8.0 mg), the autoclave was sealed again and maintained at 180 o C for 2 h and then cooled to room temperature. Finally, the products were purified following a similar procedure described above. 4.6 Procedure for TMB oxidation. Typically, the catalytic reaction was carried out in acetate buffer (100 mM) containing 0.5 mM of TMB. After incubation with catalyst for 20 min, absorbance of reaction solution at 652 nm was determined immediately. 4.7 Kinetics study. The steady-state kinetic assays were performed by changing TMB concentrations (0.005 to 0.5 mM). The absorbance values monitored at 652 nm (A) were back-converted to TMB concentration derived oxidation products using the Beer–Lambert Law, A = ε • b • C, where the molar absorption coefficient ε was 39000 M-1 cm-1,54 the vitric cuvettes of path length b was 1 cm. Kinetic parameters were determined via Michaelis-Menten equation V0 = Vmax × Cs / (Km + Cs), where V0 represents the initial velocity obtained by fitting the early data to a linear equation, and Vmax refers to the maximal velocity; Cs is the concentration of substrate and Km is the Michaelis constant. The data were also fitted to the Lineweaver-Burk model 1 / V0 = 1 / Vmax + (Km / Vmax) (1 / Cs). 4.8 Measurement of ROS. Possible reactive oxygen intermediates were identified by ESR spectroscopy. Briefly, the dried powder of Pt1Rh2/Rh (ca. 1 mg) was dispersed in 500 μL of DMSO with ultrasonication treatment, followed by addition of DMPO (100 mM in water) as a spin trap at 298 K. 4.9 CytC electron-transfer experiment. Briefly, 45 μL of deoxygenated red-CytC (4 mM) and 170 μL of deoxygenated acetate buffer (100 mM, pH 4) containing 28 μM of Pt1Rh2/Rh (based on total metals) pre-saturated with N2 were co-incubated at room temperature under N2 for 1 h. Then the reaction solution was collected for UV-Vis absorbance measurement. The control experiment without Pt0.219/Rh was also conducted. ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Additional characterizations (TEM, SEM, AFM, STEM and EDX) of Pt1Rh2/Rh, characterizations (TEM) of Pt NPs and Rh NSs, experimental details and catalytic studies (PDF) AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected], [email protected] 17



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ORCID S. F. Cai: 0000-0002-8122-684X C. Lian: 0000-0003-4923-1347 Q. S. Han: 0000-0002-3473-4614 C. Qi: 0000-0002-7723-5890 Notes The authors declare no competing financial interest. Funding Sources This work was supported by National key research and development program from the Ministry of Science and Technology of China (2016YFC0207102) and National Natural Science Foundation of China (21501034, 21573050 and 21503053). Financial support from Chinese Academy of Sciences (XDA09030303) was also gratefully acknowledged. We thank Dr. Ruilong Zong and Chao Ma from Analysis Center of Department of Chemistry at Tsinghua University for help with TEM study. REFERENCES (1) Schauermann, S.; Nilius, N.; Shaikhutdinov, S.; Freund, H.-J. Nanoparticles for Heterogeneous Catalysis: New Mechanistic Insights. Acc. Chem. Res. 2013, 46, 1673–1681. (2) Lightcap, I. V.; Kamat, P. V. Graphitic Design: Prospects of Graphene-Based Nanocomposites for Solar Energy Conversion, Storage and Sensing. Acc. Chem. Res. 2013, 46, 2235–2243. (3) Su, S.; Xu, Y. Q.; Sun, Q.; Gu, X. D.; Weng, L. X.; Wang, L. H. Noble Metal Nanostructure-Decorated Molybdenum Disulfide Nanocomposites: Synthesis and Applications. J. Mater. Chem. B. 2018, 6, 5323–5334. (4) Li, Y. J.; Li, Y. J. Zhu, E. B.; McLouth, T.; Chiu, C.-Y.; Huang, X. Q.; Huang, Y. Stabilization of High-Performance Oxygen Reduction Reaction Pt Electrocatalyst Supported on Reduced Graphene Oxide/Carbon Black Composite. J. Am. Chem. Soc. 2012, 134, 12326–12329. (5) Zhao, M. T.; Yuan, K.; Wang, Y.; Li, G. D.; Guo, J.; Gu, L.; Hu, W. P.; Zhao, H. J.; Tang, Z. Y. Metal-Organic Frameworks as Selectivity Regulators for Hydrogenation Reactions. Nature 2016, 539, 76–80. (6) Zhang, G. G.; Lan, Z. A.; Lin, L. H.; Lin, S.; Wang, X. C. Overall Water Splitting by Pt/g-C3N4 Photocatalysts without Using Sacrificial Agents. Chem. Sci. 2016, 7, 3062–3066. 18


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Facile Strategy To Prepare Rh Nanosheet-Supported PtRh

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