Unveiling the Growth Mechanism of Ordered‐Phase within Multimetallic Nanoplates

Abstract Tuning the crystal phase of alloy nanocrystals (NCs) offers an alternative way to improve their electrocatalytic performance, but, how heterometals diffuse and form ordered‐phase remains unclear. Herein, for the first time, the mechanism for forming tetrametallic ordered‐phase nanoplates (NPLs) is unraveled. The observations reveal that the intermetallic ordered‐phase nucleates through crystallinity alteration of the seeds and then propagates by reentrant grooves. Notably, the reentrant grooves act as intermediate NCs for ordered‐phase, eventually forming intermetallic PdCuIrCo NPLs. These NPLs substantially outperform for oxygen evolution reaction (221 mV at 10 mA cm−2) and hydrogen evolution reaction (19 mV at 10 mA cm−2) compared to commercial Ir/C and Pd/C catalysts in acidic media. For OER at 1.53 V versus RHE, the PdCuIrCo/C exhibits an enhanced mass activity of 9.8 A mg−1 Pd+Ir (about ten times higher) than Ir/C. For HER at ‐0. 2 V versus RHE, PdCuIrCo/C shows a remarkable mass activity of 1.06 A mg−1 Pd+Ir, which is three‐fold relative to Pd/C. These improvements can be ascribed to the intermetallic ordered‐structure with high‐valence Ir sites and tensile‐strain. This approach enabled the realization of a previously unobserved mechanism for ordered‐phase NCs. Therefore, this strategy of making ordered‐phase NPLs can be used in diverse heterogeneous catalysis.


Electrochemical Measurements:
The preparation of catalysts for electrochemical characterizations: The as-prepared samples were loaded on carbon (Vulcan XC-72) by sonicating in cyclohexane for 2 h (20 wt% of the precious metal is loaded on Vulcan XC-72). 2 mg of as prepared PdCuIr/C, PdCuIrCo/C, and PdCu/C catalysts were then dispersed in a mixed solvent containing isopropanol, ultrapure water, and Nafion (the volume ratio is 6/4/0.02)by sonication for 30 min to obtain a homogeneous ink with a concentration of 1 mg mL −1 .
Electrochemical characterization: All the electrochemical measurements were performed using an AUTOLAB PGSTAT302N potentiostat (Metrohm AG).A Pt wire and a reversible hydrogen electrode were applied as the counter and reference electrodes, respectively.The working electrode was prepared by dropping 10 µL of the catalyst ink onto a glassy-carbon electrode (GCE, 5 mm, 0.196 cm 2 ).The cyclic voltammetry (CV) curves of the catalysts were recorded at 30℃ in an N2saturated 0.1 M HClO4 solution in the potential range of 0.05−0.8V at a scanning rate of 50 mV•s −1 .
The electrochemically active surface area (ECSA) of all catalysts were determined from the charges associated with desorption of hydrogen in the potential range of 0.05−0.4V after double-layer correction.(cited by DOI: 10.1021/acs.chemmater.9b02011)The OER and HER polarization curves of the catalysts were collected at 30℃in an O2-saturated 0.1 M HClO4 solution via the RDE method at an scanning rate of 5 mV•s −1 with a rotating speed of 2500 rpm with 95% Ohmic iR drop compensation.The chronopotentiometry curves were recorded at a constant current density of 10 mA cm -2 without IR-compensation.According to the former study, a certain amount of the catalyst ink was dropped onto the carbon fiber paper with a geometric area of 1 cm × 1 cm for preparing the electrodes for overall water splitting.The mass loading density of Ir in carbon paper was ~100 ug cm −2 .The mass loading density of Pt in carbon paper was ~50 ug cm −2 .The overall water splitting performance was collected in 0.1 M HClO4 solution using the catalysts-modified carbon paper as both the anode and cathode in a two-electrode system.

Computational Methodology:
We used the Vienna ab initio simulation package (VASP 5.4.4) to execute all calculations based on spin-polarized density functional theory (DFT) [1][2][3][4] .A projector-augmented wave (PAW) method was used to describe electron-ion interactions.A generalized gradient approximation (GGA) with the Perdew-Burke Ernzerh (PBE) functional was employed to calculate electron exchange and correlation energy. 5To avoid potential interactions between consecutive periodic images, the 15 Å vacuum space was used along the z-direction.A kinetic-cutoff energy of 400 eV was chosen for the plane wave basis.A convergence criterion of 1.0 ×10 -6 eV/atom was set for the electronic energy, and ionic relaxation continued until the atomic force was less than 0.01 eV/Å.A semiempirical dispersion-corrected DFT+D3 scheme was used by Grimme to calculate the van der Waals interaction.The first Brillouin zone was sampled in the Monkhorst−Pack grid.The 3×3×1 k-point mesh was employed for geometric optimization and electronic structures analysis. 6Based on the computational hydrogen electrode model, the Gibbs free energy change for each elementary step of HER and OER was calculated.
Steps for OER processes can be summarized as follows: For the above elementary steps (∆GOOH*, ∆GO*, ∆GOH*, ∆GH*) containing electron transfers, the Gibbs free energy difference would be calculated using the equations below: where ΔE is the total energy change of intermediates adsorbed on the catalyst surface based on the DFT calculations, ΔZPE is the correction of zero-point energy and ΔS represents entropy contribution, respectively, and T is room temperature (298.15K).9] The entropy of the H2 is taken from the NIST 8 database, while the entropies of other intermediates were calculated from the vibrational frequencies.
Under electrode potential U = 0 V, the ∆G for all the four elementary steps can be calculated by:

D-band
To investigate the effects of multimetallic alloy nanocrystals, the d-band center of the added Ir metal atoms on the nanocrystals is calculated by using the following equation: Where  the marked box in (a) and the surface strain mapping for marked box with in-plan strain tensors εxx, εyy, εxy, through GPA, the color regions ranging from green to dark blue denote the compressive strain, while the regions from red to bright yellow represent the tensile strain. 10,11Note that the signal from regions outside the areas marked by two white quadrilaterals is the noise caused by blurring the STEM image. 12,13(c) Strain distribution along the white arrow in panel εxx for PdCuIrCo nanoplate.
Results shows that nanoplates are mostly dominated by the tensile-strain. 10,11,12,13 through GPA, the color regions ranging from green to dark blue denote the compressive strain, while the regions from red to bright yellow represent the tensile strain. 10,11Note that the signal from regions outside the areas marked by two white quadrilaterals is the noise caused by blurring the STEM image. 12,13                      taken from the marked box in (a) and the surface strain mapping for marked box with in-plan strain tensors εxx, εyy, εxy, through GPA, the color regions ranging from green to dark blue denote the compressive strain, while the regions from red to bright yellow represent the tensile strain. 10,11Note that the signal from regions outside the areas marked by two white quadrilaterals is the noise caused by blurring the STEM image. 12,13(c) Strain distribution along the white arrow in panel εxx for PdCuIrCo nanoplate.Results shows that nanoplates are mostly dominated by the tensilestrain. 10,11,12,13re S30.Schematic of the 4e − OER pathway of the active site over the PdCu (111) NC with the optimized configurations for intermediates.
Figure S1.TEM images of PdCuIrCo nanoplates at different magnification.

Figure S3 .
Figure S3.TEM topography showing the flat-top morphology of PdCuIrCo nanoplates with a line profile of the nanoplate.The thickness of a nanoplate is about 2nm.

Figure S 4 .
Figure S 4. Structural and surface strain characterizations of the PdCuIr nanoplates: (a) TEM image, (b) HAADF-STEM image, (c) HAADF-STEM image and the corresponding EDS elemental mapping, (d) line-scanning profiles across the yellow arrow shown in the image of figure c, (e) PXRD pattern, (f) FFT pattern of PdCuIr nanoplate and (g) maps of the in-plan strain tensors εxx, εyy, εxy, processed via GPA taken from the yellow frame area in figure e, (the color regions ranging from green to dark blue denote the compressive strain, while the regions from red to bright yellow represent the tensile strain).

Figure S 5 .
Figure S 5. (a) Abreaction-corrected HAADF-STEM image of PdCuIr nanoplate, (b) image taken from the marked box in (a) and the surface strain mapping for marked box with in-plan strain tensors εxx, εyy, εxy,

Figure S 12 .
Figure S 12. Morphological characterizations of the CuIr nanocrystals: (a) TEM image, (b) HAADF-STEM image and the corresponding EDS elemental mapping of CuIr nanocrystals and (c) TEM-EDX spectrum image of CuIr nanocrystals.

Figure S 13 .
Figure S 13.PdCuIr nanocrystals synthesized at 30 min reaction time: (a) TEM image, (b) HAADF-STEM image, (c) HAADF-STEM image and the corresponding EDS elemental mapping of CuIr nanocrystals and (d) TEM-EDX spectrum image of PdCuIr nanocrystals.

Figure S 18 .
Figure S 18. Characterization of the PdCuIrCo nanocrystals synthesized at (a) low and (b) high concentration of Ir.

Figure S 20 .
Figure S 20.Structural characterizations of the PdCuIrCo nanocrystals synthesized without gel: (a) TEM image, (b) HAADF-STEM image and the corresponding EDS elemental mapping of PdCuIrCo nanocrystals.

Figure S 21 .
Figure S 21.Structural characterizations of the PdCuIrCo nanocrystals synthesized by mixing all the gel components together with precursors in one-step: (a) TEM image, (b) HAADF-STEM image and the corresponding EDS elemental mapping of PdCuIrCo nanocrystals.

Figure
Figure S24.(a) The OER polarization curves recorded with a linear scan of potential at 5 mV s −1 from the PdCuIrCo/C with different loading.(b) The overpotential at a current density of 10 mAcm -2 (left) and mass activity at an overpotential of 300 mV (vs.RHE) of PdCuIrCo/C with different loading(right).(c) The HER polarization of PdCuIrCo/C with different loading curves recorded with a linear scan of potential at 5 mV s −1 .(d) Tafel plot of PdCuIrCo/C with different loading.

Figure
Figure S26.(a) The electrolyzer in a working condition.

Figure
Figure S27.(a) The OER polarization curves recorded with a linear scan of potential at 5 mV s −1 from the PdCuIrCo/C, PdIrCo/C and PdIr/C , respectively.(b) The overpotential at a current density of 10 mAcm -2 (left) and mass activity at an overpotential of 300 mV (vs.RHE) of different electrocatalysts (right).(c) The HER polarization of different electrocatalysts curves recorded with a linear scan of potential at 5 mV s −1 .(d) The overpotential (left) and mass activity(right).

Figure
Figure S28.(a) The OER polarization curves recorded with a linear scan of potential at 5 mV s −1 from the PdCuIrCo (without carbon) and PdCuIrCo/C , respectively.(b) The overpotential at a current density of 10 mAcm -2 (left) and mass activity at an overpotential of 300 mV (vs.RHE) of different electrocatalysts (right).

Figure S 29
Figure S 29.(a) Abreaction-corrected HAADF-STEM image of PdCuIrCo nanoplate, (b) imagetaken from the marked box in (a) and the surface strain mapping for marked box with in-plan strain tensors εxx, εyy, εxy, through GPA, the color regions ranging from green to dark blue denote the compressive strain, while the regions from red to bright yellow represent the tensile strain.10,11Note that the signal from regions outside the areas marked by two white quadrilaterals is the noise caused by blurring the STEM image.12,13(c) Strain distribution along the white arrow in panel εxx for PdCuIrCo nanoplate.Results shows that nanoplates are mostly dominated by the tensilestrain.10,11,12,13

Figure S31 .
Figure S31.Schematic of the 4e − OER pathway of the active site over the PdCuIr (110) NC with the optimized configurations for intermediates.

Figure S 32 .
Figure S 32.The relationship between the overpotential and d-band.
c σ 2 , the Debye Waller factor value; d ΔE0, inner potential correction to account for the difference in the inner potential between the sample and the reference compound; R factor indicates the goodness of the fit.S0 2 was fixed to 0.838 and 0.787, according to the experimental EXAFS fit of Pd foil and Ir_powder by fixing CN as the known crystallographic value.* This value was fixed during EXAFS fitting, based on the known structure of Pd and Ir.Fitting conditions: k range：3.0-12.5;R range: 1.0-3.5 (Pd_PdCuIr and Pd_PdCuIrCo); k range：2.0-10.5;R range: 1.2-3.5 (Ir_PdCuIr and Ir_PdCuIrCo); fitting space: R space; k-weight = 3.A reasonable rangeof EXAFS fitting parameters: 0.800 < Ѕ0 2 < 1.000; CN > 0; σ 2 > 0 Å 2 ; |ΔE0| < 10 eV; R factor < 0.02.

Table S1 .
The wt.% of Pd, Cu and Ir measured by ICP-MS in the PdCuIr samples at different reaction

Table S2 .
The wt.% of Pd, Cu, Ir and Co measured by ICP-MS in the PdCuIrCo samples at different

Table S3 .
The wt.% of elements measured by ICP-MS for the Pd, Cu, Ir and Co contents in the catalysts.

Table S4 .
Comparisons of OER activity of the as-prepared and commercial Ir/C and Pd/C catalysts.

Table S5 .
The OER performance of different noble metal based catalysts in acidic electrolyte solution.

Table S6 .
Comparisons of HER activity of the as-prepared and commercial Ir/C and Pd/C catalysts.

Table S7 .
The HER performance of different noble metal based catalysts in acidic electrolyte solution.

Table S8 .
EXAFS data fitting results of Samples.
a CN, coordination number; b R, the distance between absorber and backscatter atoms;