Hydrothermal Synthesis and Catalytic Application of Ultrathin

Subscriber access provided by United Arab Emirates University | Libraries Deanship

Article

Hydrothermal Synthesis and Catalytic Application of Ultrathin Rhodium Nanosheet Nanoassemblies Juan Bai, Guang-Rui Xu, Shi-Hui Xing, Jing Hui Zeng, Jia-Xing Jiang, and Yu Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11210 • Publication Date (Web): 17 Nov 2016 Downloaded from http://pubs.acs.org on November 20, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 8

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

ACS Applied Materials & Interfaces

Hydrothermal Synthesis and Catalytic Application of Ultrathin Rhodium Nanosheet Nanoassemblies Juan Bai,§ Guang-Rui Xu,§ Shi-Hui Xing, Jing-Hui Zeng, Jia-Xing Jiang, and Yu Chen* Key Laboratory of Macromolecular Science of Shaanxi Province, Shaanxi Key Laboratory for Advanced Energy Devices, Shaanxi Engineering Lab for Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi Normal University, Xi'an 710062, China ABSTRACT: Ultrathin noble metal nanosheets with atomic thickness exhibit abnormal electronic, surfacial, and photonic properties due to the unique two-dimensional (2D) confinement effect, which have attracted intensive research attention in catalysis/electrocatalysis. In this work, the well-defined ultrathin Rh nanosheet nanoassemblies with dendritic morphology are synthesized by a facile hydrothermal method with assistance of polyallylamine hydrochloride (PAH), where PAH effectively acts as the complexant and shape-directing agent. Transmission electron microscopy and atomic force microscopy images reveal the thickness of 2D Rh nanosheet with (111) planes is only ca. 0.8~1.1 nm. Nitrogen adsorption-desorption measurement displays the specific surface area of the as-prepared ultrathin Rh nanosheet nanoassemblies is 139.4 m2 g−1, which is much bigger than that of homemade Rh black (19.8 m2 g−1). Detailed catalytic investigations display the as-prepared ultrathin Rh nanosheet nanoassemblies have nearly 20.4-fold enhancement in mass-activity for the hydrolysis of ammonia borane as compared with homemade Rh black. KEYWORDS: Rh nanoassemblies, nanosheets, catalytic activity, hydrogen generation, ammonia borane

and Rh39-40) show fascinating reactivity in heterogeneous catalysis/electrocatalysis, owing to the combined advantages of 3D nanoassemblies and 2D ultrathin nanosheets. For example, 3D Ir nanosheet nanoassemblies exhibited very high catalytic activity and durability for the oxygen evolution reaction in both acidic and alkaline media.34 3D Rh nanosheet nanoassemblies exhibited significantly improved catalytic performance in the hydrogenation of phenol and cyclohexene compared with commercial Rh black and 2D Rh nanosheets.39

1. INTRODUCTION Manipulating the morphology of noble metal nanocrystals is an extremely interesting research subject in improving catalytic activity/durability and/or understanding structure−activity relationship.1-6 Inspired by the amazing physical properties of two-dimensional (2D) nanomaterials,7-9 the synthesis of freestanding 2D noble metal nanosheets with atomic thickness has attracted extensive attention due to their unordinary electronic structure and physical/chemical properties arising from 2D effects.10-23 Specific to the catalysis field, the atomically thick 2D noble metal ultrathin nanosheets not only offer tremendously large surface area but also significantly improve the utilization of noble metal because nearly all atoms are exposed at the outside surface of nanosheets. Meanwhile, the interaction between metal atoms and reactants can be dramatically changed due to the unordinary electronic confinement in 2D structure.10-23 Unfortunately, similar to the planar stacking of 2D graphene, 2D ultrathin noble metal nanosheets may agglomerate easily to decrease the catalytic activity. Recently, three-dimensionally (3D) branched noble metal nanoassemblies consisted of primary noble metal nanocrystals as building blocks are attracting more attention, which generally provide much improved catalytic activity and durability due to their unordinary structural features, such as large internal reactive surface area, particular interconnected open-pore structure, fast mass/electron transfer, and less Ostwald ripening/aggregation.24-33 In particular, 3D branched noble metal nanoassemblies constructed of 2D ultrathin nanosheets (such as Ir,34 Ru,35 Pt,36-38

Rh nanocrystals, one kind of noble metal nanocrystals, have very wide applications in fine chemicals synthesis, hydrogen generation, automobile catalytic conversion, and energy conversion.41-48 In general, the synthesis of 2D noble metal nanosheets is very difficult because of the lack of inherent driving force for anisotropic growth.10-23 Furthermore, Rh metal has the very higher surface free energy than other noble metals, which results in the huge challenge in their morphology control.47 Herein, we demonstrated a facile hydrothermal method to synthesize the ultrathin Rh nanosheet nanoassemblies with dendritic morphology (Rh-3D-2D), using polyallylamine hydrochloride (PAH, Scheme S1 in Supporting Information) as a complexant and shapedirecting agent. When used as catalyst for the hydrolysis of ammonia borane (AB, NH3BH3), the as-prepared Rh-3D-2D displayed a 20.4-fold increase of mass activity for the AB hydrolysis compared with homemade Rh black.

2. EXPERIMENTAL SECTION 1

ACS Paragon Plus Environment


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

2.1. Reagents and Chemicals. PAH (Scheme S1 in Supporting Information, average molecular weight 5000) was supplied from Nitto Boseki Co., Ltd. Ammonia borane (AB, NH3BH3) was supplied from Aladdin Industrial Corporation. Other chemicals used in this work were of analytical reagent grade. 2.2. Synthesis of Rh-3D-2D. Rh-3D-2D were obtained by a facile hydrothermal method. Typically, RhCl3 (63 mg) and PAH (200 mg) were dissolved in 100 mL of water under vigorous stirring. After adding 10 mL of 40% HCHO solution, the homogeneous mixture solution was transferred into a Teflon-lined high-pressure vessel, and heated at 120 °C for 6 h. Then, the obtained Rh-3D-2D were separated by centrifugation, and washed with CH3COOH for 10 h.49-51 For comparison, homemade Rh black was also synthesized in absence of PAH under same experimental conditions.

Page 2 of 8

The selected-area electron diffraction (SAED) pattern of an individual Rh-3D-2D shows a series of diffraction rings (Figure 1D), revealing its polycrystalline structure. To determine the thickness of 2D Rh nanosheet subunits, further high-resolution TEM (HRTEM) investigation focusing on the edge of an individual Rh-3D-2D was performed (Figure 1E). The transparent and dark fringe-like regions correspond to nanosheets parallel and perpendicular to the support substrate, respectively.34, 39, 54 The widths of the very dark regions are determined to be 0.9~1.1 nm. After strong ultrasonic treatment, the few Rh nanosheets are obtained. AFM measurement is performed to determine the thickness of nanosheets (Figure 1F). The height profile shows that the thickness of Rh nanosheets is ca. 0.8 nm, in consistent with TEM results. Given that Rh has a covalent radius of ca. 0.135 nm, 2D Rh nanosheet is consisted of only ca. 6 atomic layers. The transparent regions in HRTEM image exhibit regular lattice fringes with a lattice spacing of 0.225 nm (Figure 1G and Figure S3 in Supporting Information), corresponding to Rh(111) planes. Meanwhile, we observed that the lattice fringes are discontinuous, indicating the polycrystalline nature of nanosheets, in consistent with SAED result.

2.3. Hydrolysis of AB for Hydrogen Generation. Catalytic reaction was performed according to the previously reported method with slight modification.52-53 In a typical experiment, 250 µL of well-dispersed Rh-3D-2D suspension − (2 mg mL 1) was added in a two-necked round-bottom flask with vigorous stirring at room temperature. And then 15 mg of AB was added into the above round-bottom flask. The volume of released gas was measured by a water filled gas burette. The activation energy was calculated through the Arrhenius equation.52-53 For the durability test, the catalytic reactions were repeated three times by adding an additional AB into the mixture after the previous cycle. For comparison, homemade Rh black was also used as catalyst to perform the catalytic reaction under same experimental conditions. 2.4. Instruments. The morphology, size, crystalline structure, and chemical composition were investigated using scan electron microscopy (SEM, SU-8020) with energy-dispersive X-ray (EDX) accessory, atomic force microscopy (AFM, Dimension Icon), transmission electron microscopy (TEM, JEM-2100F), powder X-ray diffraction (PXRD, DX-2700), X-ray photoelectron spectroscopy (XPS, Thermo VG Scientific ESCALAB 250), and fourier transform infrared spectroscopy (FT-IR, Tensor 27). The surface area of sample was measured by N2 adsorption-desorption measurement at 77 K on a physical adsorption instrument (Micromeritics ASAP 2020 HD88 system). The interaction between RhCl3 and PAH was investigated by ultraviolet and visible spectroscopy (UV-vis, Shimadzu UV-3600U). Linear scan voltammetry (LSV) measurements were performed on CHI 660E electrochemical analyzer at room temperature.

A

B

sheets 50 nm

C

50 nm

97 nm

D

E

d=1.1nm

20 nm

F

0.8 nm

G

d=0.9 nm

H d=0.223nm

50 nm

2 nm

d=0.225nm

Figure 1. Morphological and configurable characterizations for Rh-3D-2D. (A) TEM image, (B) SEM image, (C) particle size distribution histogram, and (D) SAED pattern of Rh-3D-2D. (E) HRTEM image of an individual Rh-3D-2D on the edge regain. (F) AFM image of Rh-3D-2D after strong ultrasonic treatment. (G) The enlarged HRTEM image of an individual Rh-3D-2D on the edge regain. (H) N2 adsorption−desorption isotherm of Rh-3D2D.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of Rh-3D-2D. Typically, Rh-3D-2D were synthesized by heating a mixture solution of RhCl3, PAH and HCHO at 120 °C for 6 h (see the Experimental section for details). TEM images (Figure 1A and Figure S1A in Supporting Information), SEM images (Figure 1B and Figure S1B in Supporting Information), and EDX spectrum (Figure S2 in Supporting Information) indicate the black products are Rh nanoassemblies, which have spherical dendritic morphology and are consisted of numerous nanosheets. The average size of overall Rh-3D-2D is about 97 nm (Figure 1C).

N2 adsorption−desorption isothermal measurement was performed to investigate the structure and surface area of Rh-3D2D (Figure 1H). The obtained nitrogen adsorption−desorption isotherm belongs to a type IV with a type H3 hysteresis loop, which is indicative of porous nanostructures. Brunauer-EmmettTeller (BET) specific surface area of Rh-3D-2D is measured to be 139.4 m2 g−1. Such high BET value originates from the particular 2D structure of the atomically thick ultrathin nanosheets. By the 2


Page 3 of 8

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


way, TEM images show that solution pH (Figure S4 in Supporting Information), reaction temperature (Figure S5 in Supporting Information), and PAH/RhIII feeding ratio (Figure S6 in Supporting Information) have very negligible effect for morphology and size of Rh-3D-2D, indicating our synthesis conditions is very robust. PXRD measurement was performed to investigate the phase structure of Rh-3D-2D. The diffraction peaks at a 2θ value of 41.1o, 47.8o, 69.9o, and 84.4o are close to (111), (200), (220) and (311) reflections of face centered cubic (fcc) Rh crystal (JCPDS No. 05-0685). After calcination in N2 flow for 2 h at 300 oC, the XRD diffraction peaks of sample match well the standard reflections of metallic fcc Rh crystal (Figure S7 in Supporting Information), confirming the formation of metallic Rh nanocrystals. According to the position of (111) peak (Figure 2A), the lattice spacing (i.e., the distance between the Rh layers) is calculated to be 0.223 nm, slightly bigger than that (0.2196 nm) of fcc Rh crystal (JCPDS No. 05-0685). The previous density functional theory calculations have demonstrated the average distance between Au layers in 2D Au nanosheets increases with decreasing the number of layers due to the weakness of formation energy per atom in multilayers.55 A similar phenomenon is also observed at 2D Pt nanosheets.55 Thus, the lattice expansion of Rh3D-2D originates from the low atomic layers of 2D Rh nanosheets. To further confirm the metallic nature of Rh-3D-2D, XPS measurement was performed (Figure 2B). The Rh 3d signal of Rh-3D-2D is resolved into two components. The main peaks at 307.3 eV and 312.05 eV are assigned to 3d5/2 and 3d3/2 of metallic Rh, respectively, whereas their shoulder peaks at higher binding energies are attributed to 3d5/2 and 3d3/2 of Rh oxide. Thus, XPS analysis confirms that the metallic Rh is the predominant species in Rh-3D-2D (93.8%). Compared with the standard Rh 3d values of Rh 3d3/2 (311.75 eV) and Rh 3d5/2 (307.0 eV),56 Rh 3d values of Rh-3D-2D have the slight upshift of binding energies, which may originate from the particularly electronic structure of ultrathin 2D nanosheets. Meanwhile, the Rh 3d values of Rh-3D-2D are also higher than that of homemade Rh black (Figure S8 in Supporting Information), confirming the morphologies of Rh nanostructures affect their electronic properties.

A

(i.e., homemade Rh black) seriously aggregate (Figure S9 in Supporting Information), suggesting PAH efficiently serves as capping agent due to its good hydrophilic property and bulky molecular size.50-51 For Rh-3D-2D sample without CH3COOH treatment, EDX maps show N element pattern is very similar to Rh element pattern (Figure 3A). Thus, EDX maps indicate PAH uniformly bind on Rh-3D-2D surface due to the strong Rh-N bond interaction,18, 40 confirming its function of capping agent. Our previous works have demonstrated that PAH has the strong coordination for metal ions due to –NH2 groups.50-51 UV–vis measurements show the UV–vis absorption spectrum of PAH+RhCl3 solution is different from the UV–vis absorption spectrum of RhCl3, indicating PAH can interact with RhCl3 due to the strong coordination of PAH (Figure 3B). Further LSV measurements indicate the generation of PAH–RhIII complex can significantly decreases the reduction potential of RhIII precursor (Figure 3C). The change in thermodynamics will slow the reduction rate of RhIII precursor, allowing kinetically controlled synthesis.

A 100 nm

B

Rh

N

C

Figure 3. The interaction between PAH and RhIII. (A) EDX maps of Rh-3D-2D. (B) UV–vis absorption spectra of PAH, RhCl3, and PAH+RhCl3 solution. (C) LSV curves of RhCl3 and PAH+RhCl3 at the glassy carbon electrode at 0.1 V s–1 in 0.1 M KCl electrolyte. For the formation of ultrathin noble metal nanosheets, the confinement growth is a main mechanism (Figure 4A).14, 17-18 The adsorption of capping molecules on the certain basal planes of noble metal nanosheets inhibits the growth along this direction. As a result, the noble metal atoms on other planes serve as active sites for further deposition of freshly reduced metal noble atoms, resulting in the continuous growth of noble metal nanosheets with a uniform thickness. Besides EDX maps data (Figure 3A), TGA experiment also confirm the adsorption of PAH on Rh surface (Figure S10 in Supporting Information). Thus, the strong adsorption of PAH on Rh (111) planes18 results in the formation 2D Rh nanosheets under kinetic control conditions. To track the evolution of Rh-3D-2D, we monitored the morphology of reaction intermediates at different times by TEM (Figure 4B). In the early stage of the reaction (30 min), ultrafine Rh nanoparticles with ca. 3 nm size are observed (Figure 4B-a). At 50 min, some Rh nanosheet nanoassemblies with dendritic morphology generate (Figure 4B-b). At 1.5 h, ultrafine Rh nanoparticles disappear completely and Rh nanosheet nanoassemblies with ca. 70 nm size are main products (Figure 4B-

B

Figure 2. Structural and component characterizations for Rh-3D2D. (A) PXRD pattern and (B) Rh 3d XPS spectrum of Rh-3D2D. The vertical black line in Figure 2B denotes the standard value of Rh 3d5/2.

3.2. The formation mechanism of Rh-3D-2D. To explore the formation mechanism of Rh-3D-2D, a serious of controlled experiments were carried out. In the absence of PAH, the products 3



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

c). Further increasing time to 6 h, Rh nanosheet nanoassemblies with ca. 95 nm size are obtained (Figure 4B-d). After extending time to 8 h, the morphology and size of Rh-3D-2D retain constant (Figure S11 in Supporting Information), confirming complete consumption of the RhIII precursor with 6 h. During the growth process, the thickness of Rh nanosheets retain unchanged (inserts in Figure 4B-bcd), which is indicative of nanosheet epitaxial growth. Importantly, the size of the performed Rh seed at 30 min (ca. 3.5 nm) is much bigger than that the thickness of Rh nanosheets (ca. 1 nm), implying no monodisperse Rh nanosheets generate initially during the synthesis. Furthermore, the asprepared Rh-3D-2D have flowers-like structure with obvious radial symmetry. Thus, we reasonably infer that the generation of 3D Rh nanosheet nanoassemblies originates from the initial formation of nanosheet on Rh seeds in all directions and succedent epitaxial growth of nanosheets, as schematically illustrated in Figure 4C.

A

can be effectively removed by CH3COOH washing (Figure S12 in Supporting Information), in consistent with FT-IR (Figure S13 in Supporting Information) and XPS (Figure S14 in Supporting Information) measurements. Catalytic hydrolysis of AB for hydrogen generation was preformed to investigate the catalytic activity of the catalysts. Hydrogen generation profiles indicate that the hydrolysis of AB on Rh-3D-2D and homemade Rh black finish in 2.2 and 46 min, respectively (Figure 5A), indicating Rh3D-2D has much better catalytic performance than homemade Rh black. The turnover frequency (TOF) value of Rh-3D-2D is calculated to be about 136.4 min−1 (molH2·molmetal·min−1) at 303 K, which is much 20.4 times bigger than that (6.67 min−1) of homemade Rh black (Figure 5B). Meanwhile, the TOF value (136.4 min−1) is also higher than most of the reported values for AB hydrolysis on various noble metal nanocrystals without supporting materials, such as Pd/polypyrrole/polyacrylonitrile catalyst (TOF: 3.9 min−1 at 298 K),61 Pd-Pt nanoparticles (TOF: 125 min−1 at 298 K),62 Rh nanoclusters (TOF: 92 min−1 at 298 K),63 PtO2 catalyst (TOF: 20.8 min−1 at 298 K)64 and AuCo nanoparticles (23.5 min−1 at 298 K),65 confirming the high activity of Rh-3D-2D, again. In addition, the catalytic performance of Rh3D-2D for the AB is much better than these of commercial Rh black, Pt black, and Pd black (Figure S15 in Supporting Information), indicating the potential practical application.

M M n+

a

B

Page 4 of 8

b

A

B

d=0.97 nm

100 nm

20 nm

c d=1 nm

100 nm

C reduction

Figure 5. The catalytic activity of Rh-3D-2D. (A) Time course plots for hydrogen generation from AB aqueous solution in the presence of Rh-3D-2D and homemade Rh black under ambient atmosphere at 303 K. (B) TOF for the hydrolysis of AB on Rh3D-2D and homemade Rh black at 303 K.

d=0.98 nm

100 nm

Rh 3+

d

growth

Since BET surface area (139.4 m2 g−1) of Rh-3D-2D is only 7.2 times bigger than that (19.2 m2 g−1) of homemade Rh black (Figure S16 in Supporting Information), the sole area effect can not explain the drastic catalytic activity enhancement (TOFRh-3D2D/TOFRh=20.4). To better understand the improved catalytic activity, we measure the activation energy (Ea) of AB hydrolysis by performing catalytic reaction at at different temperatures (Figure 6AB). As expected, the hydrogen generation rate monotonically increases with reaction temperature. According to the slope of straight line in Arrhenius plot, the Ea values of AB hydrolysis on Rh-3D-2D and Rh black are calculated to be 19.9 − and 54.9 kJ mol 1, respectively. The smaller Ea value is responsible for the improved catalytic activity. For the AB hydrolysis, the catalytic activity of noble metal nanocrystals highly relates to the adsorption affinity of reaction intermediates on metal surface. The strong adsorption affinity generally sluggish the active sites.66 The d-band center theory developed by Hammer and Nørskov has demonstrated the downshift in the d-band center of metal atom effectively decreases the chemisorption energy of reaction intermediates.67-69 Thus, the positive shift of Rh binding

continuous growth

Figure 4. The formation mechanism of Rh-3D-2D. (A) Schematic illustration on the confinement growth process of ultrathin noble metal nanosheets. (B) TEM images of reaction intermediates collected at (a) 30 min, (b) 50 min, (c) 1.5 h and (d) 6 h. (C) Schematic illustration on the growth mechanism of Rh-3D-2D.

3.3. Hydrogen Generation from AB. AB (NH3BH3), one of solid hydrogen storage materials, has attracted enormous attention as a excellent chemical hydrogen storage material because of its low molecular weight, big hydrogen content (19.6 wt%), and environmentally friendly property.52-53, 57-60 For the catalytic hydrolysis of AB, 1 mol AB can release as much as 3 mol of H2 in the presence of a suitable catalyst (NH3BH3 + H2O → NH4+ +3H2). For the catalytic reaction, the clean surface of the noble metal nanocrystals is very important. Although the detailed mechanism is not clear, SEM images show PAH on Rh-3D-2D 4


Page 5 of 8

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


energies in Rh-3D-2D (Figure 2B) may reduce the strength of transient Rh−H bond and adsorption affinity of intermediates, resulting in increase of the catalytic activity. Obviously, the theoretical prediction is consistent with the actual result for Ea measurements.

electronic structure of 2D Rh nanosheet subunits. The investigation on structure−activity relationship indicated that atomically thick 2D ultrathin noble metal nanosheet nanoassemblies should be the highly promising catalyst for specific applications in liquid phase reaction after improving their stability.

B

A

ASSOCIATED CONTENT Ea=19.9 kJ/mol

Supporting Information

Ea=54.9 kJ/mol

Experimental section and characterization details are available in Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Additional information for TEM images of Rh-3D-2D with different solution pH, reaction temperature and PAH/RhIII feeding ratio. EDX pattern of Rh-3D-2D. XRD of the heat-treated Rh-3D2D at 300 oC. FT-IR spectra of pure PAH, as-prepared Rh-3D-2D, and CH3COOH washed Rh-3D-2D. Time course plots for hydrogen generation from AB aqueous solution in the presence of Rh-3D-2D and commercial Rh black, Pt black, and Pd black under ambient atmosphere at 303 K.

Figure 6. The measurement of activation energy. Time course plots for hydrogen generation from AB aqueous solution in the presence of (A) Rh-3D-2D and (B) homemade Rh black under ambient atmosphere at different temperature. Insert: Arrhenius plot of Ln k versus 1/T.

3.4. Durability of Rh-3D-2D. High stability is a key requirement for practical application of catalysts. The durabilities of Rh-3D-2D and homemade Rh black were investigated by adding an additional aliquot of AB into the two-necked roundbottom flask after the completion of the previous run (Figure 7AB). As observed, the activity of catalyst decrease with cycle numbers. After 3th run, TOF values of Rh-3D-2D and homemade Rh black decrease to 6.04 and 1.1 min−1 (molH2·molmetal·min−1), respectively. Although the reactivity of Rh-3D-2D is still better than that of homemade Rh black, SEM images (Figure S17 in Supporting Information) and XRD patterns (Figure S18 in Supporting Information) show that both morphology and particle size of Rh-3D-2D obviously change after the catalytic reaction. Thus, the present results indicate the improvement in stability is still a huge challenge for practical application of ultrathin noble metal nanosheets.

A

AUTHOR INFORMATION Corresponding Author * Email: [email protected] (Y. Chen)

Author Contributions §Dr J. Bai and G. R. Xu contributed equally to this work.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This research was sponsored by the National Natural Science Foundation of China (21473111), Natural Science Foundation of Shaanxi Province (2015JM2043), Fundamental Research Funds for the Central Universities (GK201602002 and 2016TS063), and Innovation Funds of Graduate Programs at Shaanxi Normal University (2015CXS048).

B

REFERENCES 1. Chen, M.; Wu, B.; Yang, J.; Zheng, N. Small Adsorbate‐Assisted Shape Control of Pd and Pt Nanocrystals. Adv. Mater. 2012, 24, 862– 879. 2. Chen, Q.; Jia, Y.; Xie, S.; Xie, Z. Well-faceted Noble-metal Nanocrystals with Nonconvex Polyhedral Shapes. Chem. Soc. Rev. 2016, 45, 3207–3220. 3. Gu, J.; Zhang, Y. W.; Tao, F. Shape Control of Bimetallic Nanocatalysts Through Well-designed Colloidal Chemistry Approaches. Chem. Soc. Rev. 2012, 41, 8050–8065. 4. Ouyang, J. J.; Pei, J.; Kuang, Q.; Xie, Z. X.; Zheng, L. S. Supersaturation-Controlled Shape Evolution of α-Fe2O3 Nanocrystals and Their Facet-Dependent Catalytic and Sensing Properties. ACS Appl. Mater. Interfaces 2014, 6, 12505–12514. 5. Ding, J.; Bu, L.; Guo, S.; Zhao, Z.; Zhu, E.; Huang, Y.; Huang, X. Morphology and Phase Controlled Construction of Pt–Ni Nanostructures for Efficient Electrocatalysis. Nano Lett. 2016, 16, 2762–2767. 6. Gong, M. X.; Fu, G. T.; Chen, Y.; Tang, Y. W.; Lu, T. H. Autocatalysis and Selective Oxidative Etching Induced Synthesis of Platinu-Copper Bimetallic Alloy Nanodendrites Electrocatalysts. ACS Appl. Mater. Interfaces 2014, 6, 7301–7308.

Figure 7. The durability of Rh-3D-2D. Time course plots for hydrogen generation from AB aqueous solution in the presence of (A) Rh-3D-2D and (B) homemade Rh black at sequential runs at 303 K

4. CONCLUSIONS In summary, we have demonstrated a facile and robust hydrothermal method approach for the synthesis of ultrathin Rh nanosheet nanoassemblies with dendritic morphology. Herein, PAH play an important role in constructing the new lamellar structure. When evaluated as the self-supported catalyst for the AB hydrolysis under mild conditions, such ultrathin Rh nanosheet nanoassemblies displayed exceedingly high catalytic activity for hydrogen generation due to big surface area and particular 5



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

7. Ren, X.; Qi, X.; Shen, Y.; Xiao, S.; Xu, G.; Zhang, Z.; Huang, Z.; Zhong, J. 2D Co-Catalytic MoS2 Nanosheets Embedded with 1D TiO2 Nanoparticles for Enhancing Photocatalytic Activity. J. Phys. D: Appl. Phys. 2016, 49, 315304. 8. Zhang, Z.; Liu, Y.; Ren, L.; Zhang, H.; Huang, Z.; Qi, X.; Wei, X.; Zhong, J. Three-Dimensional-Networked Ni-Co-Se Nanosheet/Nanowire Arrays on Carbon Cloth: A Flexible Electrode for Efficient Hydrogen Evolution. Electrochim. Acta 2016, 200, 142151. 9. Wan, P.; Zhang, H.; Sun, X.; Huang, Z.; Qi, X.; Zhang, Z.; Ren, X.; Xu, G. Wall-like Hierarchical Metal Oxide Nanosheet Arrays Grown on Carbon Cloth for Excellent Supercapacitor Electrodes. Nanoscale 2016, 8, 13273-13279 . 10. Funatsu, A.; Tateishi, H.; Hatakeyama, K.; Fukunaga, Y.; Taniguchi, T.; Koinuma, M.; Matsuura, H.; Matsumoto, Y. Synthesis of MonoLayer Platinum Nanosheets. Chem. Commun. 2014, 50, 8503– 8506. 11. Cao, J.; Mo, S.; Jing, X.; Yin, J.; Li, J.; Zheng, N. Trace Surface-clean Palladium Nanosheets as a Conductivity Enhancer in Holetransporting Layers to Improve the Overall Performances of Perovskite Solar Cells. Nanoscale 2016, 8, 3274–3277. 12. Pan, Y. T.; Smith, C. E.; Kwok, K. S.; Chen, J.; Kong, H.; Yang, H. Functionalized Ultrathin Palladium Nanosheets as Patches for HepG2 Cancer Cells. Chem. Commun. 2015, 51 , 14171–14174. 13. Li, H.; Chen, G.; Yang, H.; Wang, X.; Liang, J.; Liu, P.; Chen, M.; Zheng, N. Shape-Controlled Synthesis of Surface-Clean Ultrathin Palladium Nanosheets by Simply Mixing a Dinuclear Pd-I Carbonyl Chloride Complex with H2O. Angew. Chem. Int. Edit. 2013, 52, 8368– 8372. 14. Huang, X.; Tang, S.; Mu, X.; Dai, Y.; Chen, G.; Zhou, Z.; Ruan, F.; Yang, Z.; Zheng, N. Freestanding Palladium Nanosheets with Plasmonic and Catalytic Properties. Nat. Nanotechnol. 2011, 6, 28–32. 15. Saleem, F.; Xu, B.; Ni, B.; Liu, H.; Nosheen, F.; Li, H.; Wang, X. Atomically Thick Pt-Cu Nanosheets: Self-Assembled Sandwich and Nanoring-Like Structures. Adv. Mater. 2015, 27, 2013–2018. 16. Saleem, F.; Zhang, Z.; Xu, B.; Xu, X.; He, P.; Wang, X. Ultrathin PtCu Nanosheets and Nanocones. J. Am. Chem. Soc. 2013, 135, 18304– 18307. 17. Hou, C.; Zhu, J.; Liu, C.; Wang, X.; Kuang, Q.; Zheng, L. Formaldehyde-assisted Synthesis of Ultrathin Rh Nanosheets for Applications in CO Oxidation. CrystEngComm 2013, 15, 6127–6130. 18. Jang, K.; Kim, H. J.; Son, S. U. Low-temperature Synthesis of Ultrathin Rhodium Nanoplates via Molecular Orbital Symmetry Interaction between Rhodium Precursors. Chem. Mater. 2010, 22, 1273–1275. 19. Zhao, L.; Xu, C.; Su, H.; Liang, J.; Lin, S.; Gu, L.; Wang, X.; Chen, M.; Zheng, N. Single-Crystalline Rhodium Nanosheets with Atomic Thickness. Adv. Sci. 2015, 2. 1500100. 20. Duan, H.; Yan, N.; Yu, R.; Chang, C. R.; Zhou, G.; Hu, H. S.; Rong, H.; Niu, Z.; Mao, J.; Asakura, H.; Tanaka, T.; Dyson, P. J.; Li, J.; Li, Y. Ultrathin Rhodium Nanosheets. Nat. Commun. 2014, 5, 1–8. 21. Cheong, W. C.; Liu, C.; Jiang, M.; Duan, H.; Wang, D.; Chen, C.; Li, Y. Free-standing Palladium-nickel Alloy Wavy Nanosheets. Nano. Res. 2016, 9, 2244–2250. 22. Qin, H. L.; Wang, D.; Huang, Z. L.; Wu, D. M.; Zeng, Z. C.; Ren, B.; Xu, K.; Jin, J. Thickness-Controlled Synthesis of Ultrathin Au Sheets and Surface Plasmonic Property. J. Am. Chem. Soc. 2013, 135, 12544– 12547. 23. Yin, X.; Liu, X.; Pan, Y. T.; Walsh, K. A.; Yang, H. Hanoi Tower-like Multi layered Ultrathin Palladium Nanosheets. Nano Lett. 2014, 14, 7188–7194. 24. Zhu, C.; Du, D.; Eychmüller, A.; Lin, Y. Engineering Ordered and Nonordered Porous Noble Metal Nanostructures: Synthesis, Assembly, and Their Applications in Electrochemistry. Chem. Rev. 2015, 115, 8896–8943. 25. Wang, L.; Yamauchi, Y. Metallic Nanocages: Synthesis of Bimetallic Pt-Pd Hollow Nanoparticles with Dendritic Shells by Selective Chemical Etching. J. Am. Chem. Soc. 2013, 135, 16762–16765.

Page 6 of 8

26. Lim, B.; Jiang, M.; Camargo, P. H.; Cho, E. C.; Tao, J.; Lu, X.; Zhu, Y.; Xia, Y. Pd-Pt Bimetallic Nanodendrites with High Activity for Oxygen Reduction. Science 2009, 324, 1302–1305. 27. Xia, B. Y.; Ng, W. T.; Bin Wu, H.; Wang, X.; Lou, X. W. SelfSupported Interconnected Pt Nanoassemblies as Highly Stable Electrocatalysts for Low-Temperature Fuel Cells. Angew. Chem. Int. Edit. 2012, 51, 7213–7216. 28. Gao, M. R.; Zhang, S. R.; Xu, Y. F.; Zheng, Y. R.; Jiang, J.; Yu, S. H. Self‐Assembled Platinum Nanochain Networks Driven by Induced Magnetic Dipoles. Adv. Funct. Mater. 2014, 24, 916–924. 29. Bai, J.; Fang,C. L; Liu, Z. H.; Chen, Y. One Pot, Gold Seed-assisted Synthesis of Gold/Platinum Wire Nanoassemblies and Their Enhanced Electrocatalytic Activity for The Oxalic Acid Oxidation. Nanoscale 2016, 8, 2875–2880. 30. Fu, G.; H, L.; You, N.; Wu, J.; Sun, D.; Xu, L.; Tang, Y.; Chen, Y. Dendritic Platinum–Copper Bimetallic Nanoassemblies with Tunable Composition and Structure: Arginine-driven Self-assembly and Enhanced Electrocatalytic Activity. Nano Res. 2016, 9, 755–765. 31. Fu, G.; Zhang, Q.; Wu, J.; Sun, D.; Xu, L.; Tang, Y.; Chen, Y., Arginine-mediated Synthesis of Cube-like Platinum Nanoassemblies as Efficient Electrocatalysts. Nano Res. 2015, 8, 3963–3971. 32. Ding, L. X.; Wang, A. L.; Ou, Y. N.; Li, Q.; Guo, R.; Zhao, W. X.; Tong, Y.-X.; Li, G. R. Hierarchical Pd-Sn Alloy Nanosheet Dendrites: an Economical and Highly Active Catalyst for Ethanol Electrooxidation. Sci. Rep. 2013, 3, 1181. 33. Cao, Y.; Yang, Y.; Shan, Y.; Huang, Z. One-Pot and Facile Fabrication of Hierarchical Branched Pt-Cu Nanoparticles as Excellent Electrocatalysts for Direct Methanol Fuel Cells. ACS Appl. Mater. Interfaces 2016, 8, 5998–6003. 34. Pi, Y.; Zhang, N.; Guo, S.; Guo, J.; Huang, X. Ultrathin Laminar Ir Superstructure as Highly Efficient Oxygen Evolution Electrocatalyst in Broad pH Range. Nano Lett. 2016, 16, 4424–4430. 35. Kong, X.; Xu, K.; Zhang, C.; Dai, J.; Norooz Oliaee, S.; Li, L.; Zeng, X.; Wu, C.; Peng, Z. Free-Standing Two-Dimensional Ru Nanosheets with High Activity toward Water Splitting. ACS Catal. 2016, 6, 1487– 1492. 36. Song, Y.; Hickner, M. A.; Challa, S. R.; Dorin, R. M.; Garcia, R. M.; Wang, H.; Jiang, Y.-B.; Li, P.; Qiu, Y.; van Swol, F.; Medforth, C. J.; Miller, J. E.; Nwoga, T.; Kawahara, K.; Li, W.; Shelnutt, J. A. Evolution of Dendritic Platinum Nanosheets into Ripening-Resistant Holey Sheets. Nano Lett. 2009, 9, 1534–1539. 37. Zhuang, L.; Wang, W.; Hong, F.; Yang, S.; You, H.; Fang, J.; Ding, B. Porous Platinum Mesoflowers with Enhanced Activity for Methanol Oxidation reaction. J. Solid State Chem. 2012, 191, 239–245. 38. Tong, X.; Zhao, G.; Liu, M.; Cao, T.; Liu, L.; Li, P. Fabrication and High Electrocatalytic Activity of Three-dimensional Porous Nanosheet Pt/boron-doped Diamond Hybrid Film. J. Phys. Chem. C 2009, 113, 13787–13792. 39. Jiang, Y.; Su, J.; Yang, Y.; Jia, Y.; Chen, Q.; Xie, Z.; Zheng, L. A Facile Surfactant-free Synthesis of Rh Flower-like Nanostructures Constructed from Ultrathin Nanosheets and Their Enhanced Catalytic Properties. Nano Res. 2016, 9, 849–856. 40. Kang, Y.; Li, F.; Li, S.; Ji, P.; Zeng, J.; Jiang, J.; Chen, Y., Unexpected catalytic activity of rhodium nanodendrites with nanosheet subunits for methanol electrooxidation in an alkaline medium. Nano Res. DOI:10.1007/s12274–016–1258–8. 41. Zhang, H.; Xia, X.; Li, W.; Zeng, J.; Dai, Y.; Yang, D.; Xia, Y. Facile Synthesis of Five-fold Twinned, Starfish-like Rhodium Nanocrystals by Eliminating Oxidative Etching with a Chloride-Free Precursor. Angew. Chem. Int. Edit. 2010, 49, 5296–5300. 42. Yu, N. F.; Tian, N.; Zhou, Z. Y.; Huang, L.; Xiao, J.; Wen, Y. H.; Sun, S. G. Electrochemical Synthesis of Tetrahexahedral Rhodium Nanocrystals with Extraordinarily High Surface Energy and High Electrocatalytic Activity. Angew. Chem. Int. Edit. 2014, 53, 5097– 5101. 43. Ozhava, D.; Ozkar, S. Rhodium(0) Nanoparticles Supported on Nanosilica: Highly Active and Long Lived Catalyst in Hydrogen Generation from the Methanolysis of Ammonia Borane. Appl. Catal., B 2016, 181, 716–726.

6


Page 7 of 8

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


44. Delpeuch, A. B.; Maillard, F.; Chatenet, M.; Soudant, P.; Cremers, C. Ethanol Oxidation Reaction (EOR) Investigation on Pt/C, Rh/C, and Pt-based Bi- and Tri-metallic Electrocatalysts: A DEMS and in Situ FTIR Study. Appl. Catal., B 2016, 181, 672–680. 45. Zhang, Z. P.; Zhu, W.; Yan, C. H.; Zhang, Y. W. Selective Synthesis of Rhodium-based Nanoframe Catalysts by Chemical Etching of 3d Metals. Chem. Commun. 2015, 51, 3997–4000. 46. Vermisoglou, E. C.; Romanos, G. E.; Karanikolos, G. N.; Kanellopoulos, N. K. Catalytic NO x Removal by Single-wall Carbon Nanotube-supported Rh Nanoparticles. J. Hazard. Mater. 2011, 194, 144–155. 47. Xie, S.; Liu, X. Y.; Xia, Y. Shape-controlled Syntheses of Rhodium Nanocrystals for the Enhancement of Their Catalytic Properties. Nano Res. 2015, 8, 82–96. 48. Guo, W.; Pleixats, R.; Shafir, A.; Parella, T. Rhodium Nanoflowers Stabilized by a Nitrogen-Rich PEG-Tagged Substrate as Recyclable Catalyst for the Stereoselective Hydrosilylation of Internal Alkynes. Adv. Synth. Catal. 2015, 357, 89–99. 49. Mazumder, V.; Sun, S. Oleylamine-Mediated Synthesis of Pd Nanoparticles for Catalytic Formic Acid Oxidation. J. Am. Chem. Soc. 2009, 131, 4588–4589. 50. Fu, G.; Jiang, X.; Gong, M.; Chen, Y.; Tang, Y.; Lin, J.; Lu, T. Highly Branched Platinum Nanolance Assemblies by Polyallylamine Functionalization as Superior Active, Stable, and Alcohol-tolerant Oxygen Reduction Electrocatalysts. Nanoscale 2014, 6, 8226–8234. 51. Gong, M.; Fu, G.; Chen, Y.; Tang, Y.; Lu, T. Autocatalysis and Selective Oxidative Etching Induced Synthesis of Platinum–copper Bimetallic Alloy Nanodendrites Electrocatalysts. ACS Appl. Mater. Interfaces 2014, 6, 7301–7308. 52. Xi, P.; Chen, F.; Xie, G.; Ma, C.; Liu, H.; Shao, C.; Wang, J.; Xu, Z.; Xu, X.; Zeng, Z. Surfactant Free RGO/Pd Nanocomposites as Highly Active Heterogeneous Catalysts for The Hydrolytic Dehydrogenation of Ammonia Borane for Chemical Hydrogen Storage. Nanoscale 2012, 4, 5597–5601. 53. Yao, Q.; Huang, M.; Lu, Z. H.; Yang, Y.; Zhang, Y.; Chen, X.; Yang, Z. Methanolysis of Ammonia Borane by Shape-controlled Mesoporous Copper Nanostructures for Hydrogen Generation. Dalton. Trans 2015, 44, 1070–1076. 54. Zhu, Y.; Guo, H.; Zhai, H.; Cao, C., Microwave-assisted and gramscale synthesis of ultrathin SnO2 nanosheets with enhanced lithium storage properties. ACS Appl. Mater. Interfaces 2015, 7, 2745–2753. 55. Takahashi, K.; Hussain, T.; Takahashi, L.; Baran, J. D. Designing Square Two-Dimensional Gold and Platinum. Cryst. Growth Des. 2016, 16, 1746–1750. 56. Moulder, J.; Stickle, W.; Sobol, P.; Bomben, K. Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corporation Physical Electronics Division: Eden Prairie, MN, 1992. 57. Sun, D. H.; Mazumder, V.; Metin, O.; Sun, S. H. Catalytic Hydrolysis of Ammonia Borane via Cobalt Palladium Nanoparticles. ACS Nano 2011, 5, 6458–6464. 58. Kim, S. K.; Kim, T. J.; Kim, T. Y.; Lee, G.; Park, J. T.; Nam, S. W.; Kang, S. O. Tetraglyme-mediated Synthesis of Pd Nanoparticles for Dehydrogenation of Ammonia Borane. Chem. Commun. 2012, 48, 2021–2023. 59. Yao, Q.; Lu, Z. H.; Jia, Y.; Chen, X.; Liu, X. In Situ Facile Synthesis of Rh Nanoparticles Supported on Carbon Nanotubes as Highly Active Catalysts for H2 Generation from NH3BH3 Hydrolysis. Int. J. Hydrogen Energ 2015, 40, 2207–2215. 60. Kang, J. X.; Chen, T. W.; Zhang, D. F.; Guo, L. PtNiAu Trimetallic Nanoalloys Enabled by a Digestive-assisted Process as Highly Efficient Catalyst for Hydrogen Generation. Nano Energy 2016, 23, 145–152. 61. Sun, W. N.; Lu, X. F.; Tong, Y.; Lei, J. Y.; Nie, G. D.; Wang, C. A one-pot Synthesis of a Highly Dispersed Palladium/polypyrrole/polyacrylonitrile Nanofiber Membrane and Its Recyclable Catalysis in Hydrogen Generation from Ammonia Borane. J. Mater. Chem. A 2014, 2, 6740–6746 62. Rakap, M. Poly (N-vinyl-2-pyrrolidone)-stabilized Palladium– Platinum Nanoparticles-catalyzed Hhydrolysis of Ammonia Borane for Hydrogen Generation. J. Power Sources 2015, 276, 320–327.

63. Zahmakıran, M.; Özkar, S. Zeolite Framework Stabilized Rhodium (0) Nanoclusters Catalyst for the Hydrolysis of Ammonia-borane in Air: Outstanding Catalytic Activity, Reusability and Lifetime. Appl. Catal., B 2009, 89, 104–110. 64. Xu, Q.; Chandra, M. A Portable Hydrogen Generation System: Catalytic Hydrolysis of Ammonia–borane. J. Alloy. Compd. 2007, 446, 729–732. 65. Li, J.; Zhu, Q. L.; Xu, Q. Highly Active AuCo Alloy Nanoparticles Encapsulated in the Pores of Metal-organic Frameworks for Hydrolytic Dehydrogenation of Ammonia Borane. Chem. Commun. 2014, 50, 5899–5901. 66. Qi, X.; Li, X.; Chen, B.; Lu, H.; Wang, L.; He, G. Highly Active Nanoreactors: Patchlike or Thick Ni Coating on Pt Nanoparticles Based on Confined Catalysis. ACS Appl. Mater. Interfaces 2016, 8, 1922–1928 67. Hammer, B.; Nørskov, J. Electronic Factors Determining the Reactivity of Metal Surfaces. Surf. Sci. 1995, 343, 211–220. 68. Mavrikakis, M.; Hammer, B.; Nørskov, J. K. Effect of Strain on the Reactivity of Metal Surfaces. Phys. Rev. Lett. 1998, 81, 2819–2822. 69. Hammer, B.; Morikawa, Y.; Nørskov, J. K. CO Chemisorption at Metal Surfaces and Overlayers. Phys. Rev. Lett. 1996, 76, 2141–2144.

7



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

Table of Contents artwork

8


Page 8 of 8

Hydrothermal Synthesis and Catalytic Application of Ultrathin

Recommend Documents