Solvothermal Synthesis of Monodisperse PtCu Dodecahedral

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Solvothermal Synthesis of Monodisperse PtCu Dodecahedral Nanoframes with Enhanced Catalytic Activity and Durability for Hydrogen Evolution Reaction Xiao-Fang Zhang, Ai-Jun Wang, Lu Zhang, Junhua Yuan, Zhengquan Li, and Jiu-Ju Feng ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01065 • Publication Date (Web): 22 Aug 2018 Downloaded from http://pubs.acs.org on August 23, 2018

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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.

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ACS Applied Energy Materials

Solvothermal Synthesis of Monodisperse PtCu Dodecahedral Nanoframes with Enhanced Catalytic Activity and Durability for Hydrogen Evolution Reaction Xiao-Fang Zhang, Ai-Jun Wang, Lu Zhang, Junhua Yuan, Zhengquan Li, Jiu-Ju Feng* Key laboratory of the Ministry of Education for Advanced Catalysis Materials, College of Chemistry and Life Sciences, College of Geography and Environmental Sciences, Zhejiang Normal University, Jinhua, 321004, China *Corresponding author: [email protected] (J. J. Feng).

ABSTRACT: Nanoframes (NFs) with highly open structures have attracted substantial attention due to their large surface area, three-dimensional structure and good stability, which are of significance to enhance the catalytic characters. However, it is still challenging to develop facile and efficient method to construct Pt-based NFs with tailored structures. Herein, a simple solvothermal method was developed to prepare monodisperse PtCu dodecahedral nanoframes (PtCu DNFs), where cetyltrimethylammonium chloride (CTAC) served as the structure-director and dispersing agent, oleylamine as reductant, allantoin as the co-structure-director and co-reductant, respectively. The PtCu DNFs displayed remarkably improved catalytic performances for hydrogen evolution reaction (HER) in both acid and alkaline electrolytes relative to home-made PtCu nano-polyhedrons (NPHs), commercial Pt/C (20 wt%) and Pt black catalysts. The work offers some instructive guidelines for synthesis of novel metallic NFs as efficient HER catalysts.

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ACS Paragon Plus Environment

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KEYWORDS:

Solvothermal

method;

Allantoin;

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Dodecahedral

nanoframes;

Three-dimensional structure; Oleylamine; Hydrogen evolution reaction

INTRODUCTION Nowadays, numerous efforts have been conducted to explore renewable and green energy resources to solve energy crisis and environmental pollution.1 Hydrogen energy is a clean reproducible energy source, which is regarded as one of the best substitutes for fossil fuels.2 Hydrogen evolution reaction (HER) is well recognized as the most promising hydrogen generation from water electrolysis.2 Pt catalysts are essential in HER owing to their excellent catalytic performances.3 However, the soaring price, easy deactivation and poisoning of Pt itself are the main obstacles for large-scaled practical applications.3 To overcome these issues, some strategies have been adopted: (1) Alloying Pt with non-noble metals can greatly reduce the usage of Pt, improve the durable ability, and enhance the catalytic efficiency;4

(2)

Hollow

nanoparticles

with

interconnected

edges,

large

surface-to-volume ratio, high atomic utilization and superior durability make the reactants accessibility to both the inner- and external-surfaces, increasing the utilization of the noble metals.5 Apart from the improved catalytic activity, the stability is also crucial for electrocatalysts.6 Tremendous researches have been conducted to develop Pt-based nanocatalysts with high catalytic efficiency and stability.7-9 Impressively, Pt-based nanoframes (NFs) with highly open structures have been attracted remarkable

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attention because of their large surface area, three-dimensional (3D) structures, good physical and chemical stability.10 For example, polyhedral Pt3Ni NFs showed enhanced catalytic features towards oxygen reduction reaction (ORR) and HER.11 In another example, rhombic dodecahedral PtCu NFs displayed enhanced catalytic properties for ORR.12 However, it is still scarce for good HER performance occurred on Pt-based NFs, albeit with most of them displaying dramatical improvement in catalytic features for ORR and alcohol oxidation reactions.13, 14 To this regard, it has great interest to explore novel Pt-based NFs. Now, various strategies have been employed for construction of noble metal NFs, including chemical etching,15 template16 and Kirkendall effects17, which provide the possibilities for scalable synthesis of Pt-based NFs. However, most of their preparation procedures involve the removal of micro- or nano-beads,16, 18 coupled with the treatments based on Kirkendall effects,17 which are complicated and/or time-consuming. Thereby, it is urgent to develop facile and straightforward methods in this context. Allantoin is a green nitrogen-rich heterocyclic compound derived from purine catabolism (Figure S1, Supporting Information, SI).19 Its carbonyl- and amino-groups endow it possibility as the reductant and stabilizing agent,20 as well as a good chelating agent with the metal precursors to decrease the reduction kinetics.21 Herein, a facile solvothermal strategy was utilized to fabricate PtCu dodecahedral nanoframes (PtCu DNFs) in the presence of cetyltrimethylammonium chloride (CTAC), allantoin and oleylamine (OAm). As a proof-of-concept application, the catalytic features of the

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synthesized PtCu DNFs were evaluated through HER in both acid and alkaline conditions.

EXPERIMENTAL Preparation of PtCu DNFs For typical synthesis of PtCu DNFs, 160 mg of CTAC was dissolved into 20 mL of OAm under stirring. Then, 16 mg of Pt(acac)2, 7 mg of CuCl2 and 79 mg of allantoin were successively put into the mixed solution under stirring, followed by ultrasonication for 40 min. Afterwards, the mixture was transferred into a Teflon-lined autoclave (25 mL) and reacted at 180°C for 10 h, and then cooled down naturally. The product was obtained by centrifugation, sufficiently washing with cyclohexane and ethanol, accompanied by drying at 60°C for further use. For comparison, PtCu nano-polyhedrons (NPHs) were synthesized by only extending the reaction time to 12 h, without changing the other experimental parameters. More information about Chemicals, characterization and Electrochemical measurements were displayed in SI.

RESULTS AND DISCUSSION Characterization The morphology of the typical nanocrystals was investigated by transmission electron microscopy (TEM). As illustrated in Figure 1A and B, there are many

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well-defined dodecahedral frame-like nanostructures with a mean diameter of 17.68 nm (Figure S2, SI).

A

B

100 nm

20 nm

C

D d = 0.21 nm 2 nm

E d = 0.19 nm

2 nm

2 nm

Figure 1. Low- (A), medium- (B), and high-resolution (C-E) TEM images of PtCu DNFs. The insets in C-E show the structure models.

In order to verify the detailed morphology and structures, Figure 1C, D and E show the high-resolution TEM (HRTEM) images of individual nanoframe. Clearly, each nanoframe has distinct edges and corners, in good accordance with the geometric model (insets in Figure 1C, D and E). Impressively, the inter-planar spacing distances are roughly 0.21 and 0.19 nm (Figure 1C), which coincide well with the (111) and

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(200) crystal planes of the face centered cubic (fcc) PtCu alloy, suggesting the formation of the PtCu alloy.13

B

E

Cu

Counts

A

Pt Pt

0

D

C

100

F Counts

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

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C

400

200 300 Distance / nm

Element

Weight%

Atomic%

Pt (L) Cu (K)

65.98 34.01

38.72 61.27

Mo Cu Mo

Cu Pt Pt

Cu

Overlap 0

4

Pt

8 12 Energy / KeV

16

Figure 2. HAADF-STEM image (A), HAADF-STEM-EDS mappings (B-D), line scanning profiles (E) and the EDS spectrum (F) of PtCu DNFs. Insets in E and F show the HAADF-STEM image, the weight and atomic ratios of Cu and Pt, respectively.

The high-angle annular dark-field scanning TEM (HAADF-STEM, Figure 2A-D and Figure S3, SI) images further confirm the generation of the frameworks with open nanostructures. The HAADF-STEM images reveal the elemental distributions of the prepared PtCu DNFs.22 As shown in Figure 2A-D, Pt and Cu atoms are evenly distributed throughout the single nanoframe, further confirming the formation of the

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PtCu alloy,23 as also identified by the uniform dispersion of Pt and Cu atoms in the elemental line scanning profiles (Figure 2E).24 In addition, the signals of Mo originated from Mo grids is also observed in the EDS pattern (Figure 2F), and the atomic ratio of Pt/Cu is roughly 38.72:61.27. To measure the chemical compositions and oxidation states, X-ray photoelectron spectroscopy (XPS) measurements were carried out.25 Figure 3 shows the high-resolution Pt 4f and Cu 2p XPS spectra. Obviously, the couple at 72.57 and 75.92 eV corresponds to Pt 4f7/2 and Pt 4f5/2 (Figure 3A), which has the upward shifts relative to pure Pt (71.0 and 74.4 eV), thanks to the lattice compression of PtCu DNFs.26 Most importantly, the downshift of Pt and d-band broadening induced by the lattice compression would reduce the binding energy to the reactive intermediates, thereby improving the HER performance.26 Meanwhile, the high-resolution Pt 4f XPS spectrum is divided into four peaks at 72.60, 73.23, 75.94 and 76.1 eV, confirming the coexistence of Pt (0) and Pt(II) species.27 By comparing the peak areas, Pt (0) is the predominant in PtCu DNFs, which is beneficial to the sequential catalysis.27 Figure 3B exhibits the high-resolution Cu 2p XPS region. The peak intensities of Cu0 (932.07 and 951.82 eV) are largely higher than those of Cu (II) (933.97 and 954.40 eV), revealing the efficient reduction of the Cu precursor.28 Strikingly, the Cu 2p3/2 peak (932.07 eV) has the downshift (ca. 0.73 eV) alternative to pure Cu (932.8 eV), owing to the strong interactions between Cu and Pt, suggesting efficient migration of the electrons from Pt to Cu.26

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A

B

Pt 4f5/2

Intensity / a.u.

Pt 4f7/2

Intensity / a.u.

0

Pt

Cu 2P3/2 0

Cu Cu 2P1/2 II

Cu

II

Pt

81

960

78 75 72 69 Binding Energy / eV

952

944

936

928

Binding Energy / eV

Figure 3. The high-resolution Pt 4f (A) and Cu 2p (B) XPS spectra of PtCu DNFs.

X-ray diffraction (XRD) measurements were conducted to determine the crystal structures of PtCu DNFs (Figure 4).29 The diffraction peaks at 41.88, 48.78, 71.55 and 86.39° are well assigned to the (111), (200), (220) and (311) facets of the fcc PtCu alloy, respectively.27 These peaks coincidentally show up between bulk Pt (JCPDS-04-0802) and Cu (JCPDS-04-0836), further confirming the formation of the PtCu alloy again.30 Besides, the peaks are close to those of pure Cu, indicating the greatly higher content of Cu relative to Pt, in good agreement with the above EDS data.31

30

(220)

(200)

45

60 75 2θ / Degree

8


(311)

Pt (JCPDS-04-0802) Cu (JCPDS-04-0836) PtCu DNFs

(111) Intensity / a.u.

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

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90

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Figure 4. The XRD pattern of PtCu DNFs. Standard XRD spectra of pure Pt (JCPDS-04-0802) and Cu (JCPDS-04-0836) were provided for comparison.

Formation mechanism To investigate the formation mechanism of PtCu DNFs, some controllable experiments were conducted. The CTAC concentrations are closely correlated with the morphology evolution (Figure 5). Some inhomogeneous solid nanoparticles are formed at 10 mM CTAC (Figure 5A) in contrast with the full-grown frame-like architectures in the standard process (25 mM CTAC, Figure 1). Adversely, excess CTAC (e.g. 35 mM, Figure 5B) yields the solid products with aggregation. Briefly, well-defined nanoframes emerge only in the narrow range of the CTAC concentrations. These phenomena demonstrate the important role of proper CTAC concentration as the structure-directing agent and dispersant.32 Commonly, insuficient CTAC would sparsely adsorb onto the crystal planes and fail to act as a guiding agent or dispersant in the synthetic process, causing the emergence of inhomogeneous nanoparticles.33 Inversely, excess CTAC would aggregate and fully cover the crystal surfaces, and seriously prevent the displacement reaction and/or oxidative etching process, leading to the formation of solid architectures with serious aggregation.34 Also, the vital role of allantoin as the co-structure-director is further certified by replacing

allantoin

with

4-amino-2,6-dihydroxypyrimidine

9


in

the

contrary


experiments, where numerous PtCu nanoparticles appear (Figure S4, SI).

B

A

50 nm

50 nm

Figure 5. TEM images of the PtCu products obtained with 10 mM (A) and 35 mM (B) CTAC.

It is noteworthy that the reaction temperature is another significant factor on the morphology of the products. Specifically, small solid nanoparticles show up at the relatively lower temperature (e.g. 160°C, Figure S5A, SI). On the contrary, the cavities on the nanoframes almost disappear at the higher temperature up to 200°C, accompanied by severe aggregation (Figure S5B, SI), owing to the fast reaction kinetics in this research.35 Therefore, only appropriate temperature can successfully generate the well-defined nanoframes. Besides, the time-dependent intermediates were taken at different reaction stages (Figure S6, SI). Clearly, small solid particles emerge at the reaction time of 4 h, with an average diameter of 12.60 nm (Figure S6A and B, SI). As the time prolongs to 10 h, well-defined nanoframes are generated (Figure 1). Moreover, the architectures gradually convert into solid NPHs by further proceeding the time up to 12 h (Figure S6C, SI), with an enlarged mean diameter of 26.11 nm (Figure S6D, SI).

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Displacement

Reduction

Ostwald ripening Displacement

Pt precursor

Cu precursor

Figure 6. The schematic illustration of the formation mechanism of PtCu DNFs.

Based on the aforementioned analysis, Figure 6 exhibits the formation mechanism of PtCu DNFs. As reported previously, the presence of CTAC would weaken the interactions between Pt(acac)2 and OAm.35 Based on this, Cu2+ ions are preferentially reduced to metallic Cu atoms by allantoin and OAm served as the co-reductants, followed by the quick coverage of the adjacent CTAC and allantoin molecules. When the Cu atoms reached hyper-saturated states, they would undergo nucleation and crystal growth to generate solid dodecahedrons by self-assembly driving by the strong interactions of the amino- and carbonyl-groups in allantoin with the crystal surfaces.36 Meanwhile, the presence of CTAC promotes the formation of dodecahedrons consisting of the {110} facets, due to the selective adsorption of Cl– onto the crystal planes.37 Then, a galvanic replacement reaction occurs between Cu atoms and the Pt precursor on the Cu nanostructures surfaces, leading to the appearance of some small cavities. Eventually, mature PtCu DNFs appear via Ostwald ripening and displacement reaction.35 However, Cu atoms would regenerate again when the reaction time further extends up to 12 h, resulting in negligible cavities left on the surfaces of the PtCu dodecahedrons.35 In the whole synthesized process, CTA+

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is of crucial significance as a good dispersant owing to its long alkyl chain.32

Electrocatalytic performances Figure 7 displays the cyclic voltammetry (CV) curves of PtCu DNFs, PtCu NPHs, Pt/C and Pt black catalysts. Evidently, the oxidation/reduction peaks of PtCu DNFs are the largest between 0.8 and 1.0 V, which can be explained by the dissolution of Cu from the inner layer.38 Normally, the electrochemically active surface area (EASA) of a catalyst is utilized to assess the catalytic activity.39 The EASA values were evaluated according to the previous work.39 As displayed in Figure S7 (SI), the EASA of PtCu DNFs is roughly 27.7 m2 g–1Pt, which is obviously larger than those of PtCu NPHs (7.0 m2 g–1Pt) and Pt black (12.5 m2 g–1Pt). However, this value is smaller than that of Pt/C (47.6 m2 g–1Pt), due to the fact that PtCu DNFs provide the smaller active area accessible for the reactants.40 4

-2

2 j / mA cm

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

0

-2

PtCu DNFs PtCu NPHs Pt/C Pt black

-4

0.2 0.4 0.6 0.8 1.0 1.2 1.4 E / V vs. RHE

Figure 7. The CV plots of PtCu DNFs, Pt/C, PtCu NPHs and Pt black catalysts in 0.5 M KOH at 50 mV s–1.

Figure 8A depicts the HER polarization curves without IR compensation in 0.5

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M H2SO4. Clearly, the onset potential (Eonset) of PtCu DNFs (0 mV) is determined by the initially linear segment of the Tafel plots,41 which is more positive than the contrary Pt/C (–20 mV), PtCu NPHs (–21 mV) and Pt black (–36 mV), reflecting the greatly improved HER activity of PtCu DNFs.42 Furthermore, the overpotential (η) at 10 mA cm–2 for PtCu DNFs (27 mV) is smaller than Pt/C (37 mV), PtCu NPHs (66 mV) and Pt black (78 mV), verifying the fast and efficient electron transfer occurred at PtCu DNFs.43 0.12

-0.04 -0.4

c

0.02

b a

-100 -0.36 -0.24 -0.12 0.00 Potential / V vs. RHE

0.00

-45 -0.8

D

-0.02

-0.6 -0.4 -0.2 0.0 Potential / V vs. RHE

0.2

d 0.00

-1

c de V -1

cb

a 55

m

V

de

m

m

d

1.2

64

-1 -2 m m V 0m V V

-75

0.4 0.8 -2 log (-j / mA cm )

0.04

V

-50

Overpotential / V

-30

-25

-4

-15

j / mA cm

-2

0

-2

0

-1

c

-1

0.06

C

0.0

-1

0.0

3

de 4 mV

c

15

-0.6 -0.4 -0.2 Potential / V vs. RHE

b a

c

-0.14 -0.07 0.00 0.07 Potential / V vs. RHE

-45 -0.8

c

0.00

de

d b a

V

-36

0.04

m

m

de

-75

-100

1

V

-50

c

-1

ec V d -1 m -1 45 ec d V d m V d ec 49 m 31

m

-2

mV -20 mV 0 mV

-25

V

B

81

-30

0.08

76

0

-2

-15

Overpotential / V

A

0 j / mA cm

j / mA cm

-2

15

j / mA cm

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

0.35 0.70 -21.05 log (-j / mA cm )

1.40

Figure 8. The HER polarization curves and Tafel plots of PtCu DNFs (curve a), Pt/C (curve b), PtCu NPHs (curve c) and Pt black (curve d) catalysts in 0.5 M H2SO4 (A, B) and 0.5 M KOH (C, D). Insets in A and C show the onset potentials. Scan rate, 5 mV s–1; rotation rate, 1600 rpm.

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Usually, the Tafel slope is applied to investigate the catalytic features for HER and the reaction kinetics of nanocatalysts.44 As shown in Figure 8B, the Tafel slope of PtCu DNFs (34 mV dec–1) is nearly equal to Pt/C (31 mV dec–1), but much smaller than those of PtCu NPHs (42 mV dec–1) and Pt black (45 mV dec–1), demonstrating the faster reaction kinetics and efficient HER.43 Furthermore, the smaller Tafel slope (34 mV dec–1) indicates that the recombination of the adsorbed H* species is the rate-determined step and thus HER occurred on PtCu DNFs follows the Volmer-Tafel reaction mechanism.45 The mass activity values of PtCu DNFs, Pt/C, PtCu NPHs and Pt black catalysts are 252.3, 589.6, 33.2 and 15.2 mA mg–1Pt at -0.05 V, respectively (Figure S8, SI). Table S1 (SI) lists the comparison of these catalysts with other Pt-based materials in the literature, reflecting the enhanced HER performance for PtCu DNFs. In order to further study the kinetics of HER, the electrochemical impedance spectroscopy measurements were carried out in 0.5 M H2SO4 (Figure S9A, SI) and 0.5 M KOH (Figure S9B, SI) at –0.05 V.46 The charge transfer resistance (Rct) determined by the semicircle is closely correlated with the electron transfer and thereby the lower Rct means the faster reaction kinetics in this regard.47 As revealed in Figure S9 (SI), the Rct of PtCu DNFs is distinctly smaller than those of Pt black and PtCu NPHs under the identical conditions, showing the faster HER kinetics for PtCu DNFs.21 This is ascribed to the nanoframes with highly open structures, which promote the reactants accessible to the external and/or inner catalyst surfaces. As a result, the electron transfer and gas diffusion rates are effectively

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facilitated in the catalytic process.5 Remarkably, the Rct of Pt/C is distinctly larger than that of PtCu DNFs, indicating the smaller charge transfer resistance of PtCu DNFs relative to Pt/C.21 It certifies the superior electrical conductivity of PtCu DNFs. This assumption is in good accordance well with the previous work.21 Figure 8C exhibits the correlated HER polarization curves in 0.5 M KOH. The Eonset and η at 10 mA cm–2 are 0 mV and 36 mV, -2 mV and 45 mV, -3 mV and 61 mV, -5 mV and 80 mV for PtCu DNFs, Pt/C, PtCu NPHs and Pt black catalysts, respectively. Clearly, the as-prepared PtCu DNFs catalyst owns the most positive Eonset and the smallest η among the investigated catalysts, showing the best HER performance of PtCu DNFs in the alkaline solution.48 Figure 8D offers the Tafel plots in the alkaline media. The Tafel slopes are estimated to be 55, 64, 76 and 81 mV dec–1 for PtCu DNFs, Pt/C, PtCu NPHs and Pt black catalysts, respectively. Besides, the value of PtCu DNFs is much lower than that of Pt-Ni excavated multipods (78 mV dec–1).49 The smaller Tafel slope means the higher HER activity for PtCu DNFs, outperforming the contrary PtCu NPHs, Pt/C and Pt black catalysts. Besides, the Tafel slope (55 mV dec–1) demonstrates that the PtCu DNFs-catalyzed HER undergoes the Volmer-Heyrovsky mechanism.50 The mass activity values of PtCu DNFs, Pt/C, PtCu NPHs and Pt black catalysts are 150.3, 332.4, 47.0 and 26.9 mA mg–1Pt, respectively (Figure S8, SI), exhibiting the similar trend found in 0.5 M H2SO4. Meanwhile, the smaller Rct of PtCu DNFs (Figure S9B, SI) means the faster HER kinetics. It is generally known that H2O/OH– is the reacting species in the alkaline

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circumstance rather than H+ in the acid electrolyte. Thus, the cleavage of HO-H bond in water is vital for the HER performance in the alkaline conditions alternative to that observed in the acid solution. It is mainly explained by the fact that water splitting into H* species is very difficult in the alkaline media, although Pt has strong adsorption with H* species who is inclined to recombine together to form H2 again (Figure 9).51 Fortunately, a certain amount of the doped Cu atoms would change the electronic structures of Pt, affect the affinity of the adsorbed H* and OH species, eventually showing the great improvement in the HER catalytic features52 when compared to those of Pt/C and Pt black catalysts. The smaller size of the highly open architecture is also a critical factor for their enhanced performance in contrast with PtCu NPHs. H+

e

H2

e

-

-

H+

H2O

H2

H2 + OH-

e

-

e

H2 O

- H2 + OH-

Alkaline medium

Acid medium

Figure 9. Schematic illustration of the HER mechanism occurred on PtCu DNFs in the acid and alkaline media. The stability of PtCu DNFs was further investigated by the accelerated durability test (ADT). There is almost no change observed for PtCu DNFs after 1000 cycles in

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the acidic media (Figure 10A) and negligible change detected in the alkaline electrolyte (Figure 10C) alternative to that of Pt/C (Figure S10, SI). After the ADT conducted in 0.5 M H2SO4 (Figure S11A, SI) or 0.5 M KOH (Figure S11B, SI), the overall outlines are almost retained for PtCu DNFs as observed from the TEM images, except the broken of some ultrathin edges. Besides, the superior stability of PtCu DNFs was further confirmed by chronoamperometry (Figure 10B and D). Evidently, the current densities exhibit a little decline (dropped by 6.4% in 0.5 M H2SO4 and 16% in 0.5 M KOH, respectively) after 10,000 s for PtCu DNFs, which are smaller than those of Pt/C (9.3% and 47.2%), PtCu NPHs (31.0% and 66.1%) and Pt black (44.7% and 73.8%), suggesting the best stability of PtCu DNFs among the investigated catalysts. 4

15

A j / mA cm-2

-30 Initial th 1000 cycle

-45

-2

-4

c

-8 b a

-16 -20


15

0.1

0

2500

5

C

-15

5000 7500 Time / s

10000

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0

c d b

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-10

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a Initial

-45

d

-12

j / mA cm-2

j / mA cm

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0

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0

-2

0

j / mA cm

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


-15

1000th cycle

-0.4

0


2500

17


5000 Time / s

7500

10000


Figure 10. The polarization curves of PtCu DNFs before and after 1000 cycles in 0.5 M H2SO4 (A) and 0.5 M KOH (C). The chronoamperometric curves of PtCu DNFs (curve a), Pt/C (curve b), PtCu NPHs (curve c) and Pt black (curve d) catalysts in N2-saturated 0.5 M H2SO4 (B) and 0.5 M KOH (D) at –0.05 V (vs. RHE). Insets in A and C show the Eonset values. Scan rate, 5 mV s–1; rotation rate, 1600 rpm.

The remarkable enhancement in the catalytic characters of PtCu DNFs is ascribed to the three factors: 1) The highly open structures, 3D accessible surfaces and large specific surface area of nanoframes;53 2) The synergic effects between the bimetals. Specifically, some Cu atoms would change the electronic structures and lower the d band center of Pt, affect the affinity of the adsorbed H* and OH species, eventually showing distinguishable improvement in the HER catalytic features.54, 55 3) The high utilization of Pt benefited from the incorporation of Cu atoms. The introduced Cu atoms are more active than those of Pt, which are readily eroded in the electrolyte, prevent the oxidization of Pt and liberate more Pt active site again, resulting in the largely enhanced durability of PtCu DNFs.54

CONCLUSIONS To summarize, the well-defined PtCu DNFs were constructed by a simple solvothermal method, using CTAC as the structure-director and dispersing agent, OAm as the reductant, allantoin as the co-structure-director and co-reductant, respectively. The contrary experiments demonstrated that the concentrations of CTAC,

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the kinds of reducing agent, reaction time and temperature were very critical in the synthesis. The prepared PtCu DNFs exhibited dramatical enhancement in the HER activity and durability in both acid and alkaline media in contrast with the references, owing to their highly open architectures. The results verify the promising applications of the designed catalyst in energy conversion and storage. The synthetic method offers a feasible way for development of frame-like alloy nanocatalysts.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: *****. Supporting Information includes Experimental section, Figure S1-S11, and Table S1.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]

ORCID: 0000-0002-7954-0573(J.J. Feng); 0000-0002-1569-7491 (J. Yuan); 0000-0002-0084-5113(Z. Li) Notes The authors declare no competing financial interest.

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Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 21475118) and Basic public welfare research project of Zhejiang Province (LGG18E010001).

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TOC

Pt

20 nm

Reduction

Cu

Displacement

Ostwald ripening Displacement

Pt precursor

Cu precursor

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Overlap

Solvothermal Synthesis of Monodisperse PtCu Dodecahedral

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