Ultrathin Template Approach to Synthesize High‐Entropy Intermetallic Nanoparticles for Hydrogen Evolution Reaction

Intermetallic nanoparticles (i‐NPs) have received considerable attention as high‐performance catalysts for diverse catalytic applications. However, high‐entropy i‐NPs have been rarely reported due to particle growth and phase separation in the multicomponent alloy. In this study, for the first time, an integrated approach of template and structure transformation is introduced to synthesize a series of sub ≈ 5 nm multicomponent i‐NPs with varying compositions. Firstly, ultrathin PtCu nanosheets (NSs) are directly converted into bimetallic PtCu i‐NPs. Furthermore, these PtCu NSs are also used as templates for epitaxial growth of the other metals, followed by annealing to obtain high‐loading, novel compositions and uniform i‐NPs with ternary, quaternary, and quinary compositions, including high‐entropy i‐NPs. Interestingly, L10 intermetallic phases are obtained when Fe is introduced. As a proof‐of‐concept application, these high‐entropy i‐NPs showed superior catalytic performance for the hydrogen evolution reaction , which can be attributed to the negatively shifted d‐band center of Pt, thereby resulting in reduced H* adsorption energy at Pt sites as supported by density functional theory calculations. Overall, this research presents a promising method for the development of i‐NPs with various compositions.


Introduction
The high cost and low utilization efficiency of Pt, a cutting-edge electrocatalyst for electrochemical energy conversion, are currently obstacles. [1,2]Alloying Pt with other metals offers a practicable solution to address these challenges related to their potentials, which makes it hard to prepare these disordered high-entropy alloy NPs with the desired compositions and size uniformity.Recently, the templated approach has received a lot of attention for the synthesis of nanocrystals with controlled compositions and structures. [30,31]This template method involves reducing the number of metal precursors at a time (first core and then shell synthesis) when compared to direct colloidal synthesis.This facilitates the development of monodisperse and highly uniform NPs.However, such template approaches are limited to preparing disordered high-entropy NPs, even though high temperature is used for annealing. [31]To our knowledge, this approach has not been used for the synthesis of high-entropy i-NPs.The possible reason may be the selection of seeds, which are always spherical NPs with sizes larger than 5 nm when compared to ultrathin NSs that are only a few atoms thick.Further, during annealing, these NSs as seeds could easily change shape as a result of the diffusion and rearrangement of atoms, which is a very important factor for the formation of i-NPs.Therefore, using readily synthesized ultrathin NSs as a template to prepare more complex ultrathin NSs, followed by structure transformation, is a possibility for the preparation of multicomponent i-NPs.
Herein, we present a template approach for synthesizing multicomponent i-NPs with uniform size.Firstly, ultrathin bimetallic PtCu NSs were used as a starting material.These NSs were annealed at 500 °C and converted into corresponding bimetallic i-NPs.To further prepare the other compositions, PtCu NSs were used as templates for the epitaxial growth of other metals to form ultrathin multicomponent NSs.Subsequently, these NSs were annealed at 650 °C, resulting in the successful synthesis of a series of sub ≈ 5 nm i-NPs comprising ternary, quaternary, and quinary Pt-based i-NPs.Using this integrated method, we also prepared PtCuPdAgFe high-entropy intermetallic (PCPAF-HEI) NPs with a control size (average size < 5 nm).As a proof-of-concept application, when PCPAF-HEI NPs used as the catalyst for electrochemical hydrogen evolution reaction (HER) in acidic media, carbon-loaded PCPAF-HEI NPs showed a 10-fold increase (7.881A mg PtþPd À1 ) in mass activity compared to 20% commercial Pt/C (0.7665 A mg Pt À1 ) at À0.05 V (vs. the reversible hydrogen electrode, RHE) while maintaining high stability.Density functional theory (DFT) calculations show that the five elements have a significant role in optimizing H* adsorption and modifying the d-band center of Pt, which enables substantially enhanced HER activity.

Synthesis and Characterization of PtCu i-NPs
Ultrathin PtCu NSs (thickness ≈ 1 nm) were synthesized by the co-reduction of metal precursors, and PtCu i-NPs were obtained after annealing at 500 °C for 3 h in a N 2 atmosphere (Figure 1a).The transmission electron microscopy (TEM) images (Figure 1b,  c) indicate that PtCu NSs and PtCu i-NPs were successfully prepared.The HRTEM image (Figure 1d) shows that the spacing of the lattice fringe is 0.433 nm, corresponding to the (021) facet of L1 1 -PtCu phase.It also indicates that at this temperature, some bigger sizes or thicker NSs are not totally converted into spherical i-NPs, while some smaller NSs can be easily converted into spherical i-NPs (as shown in areas d 1 and d 2 in Figure 1d).The X-ray diffraction (XRD) patterns (Figure 1e and S1, Supporting Information) indicate that the peak positions of as-synthesized PtCu NSs can be well indexed to the face-centered cubic crystal structure according to PDF #48-1549 and the prepared PtCu i-NPs matched well with the card of the corresponding ordered L1 1 -PtCu phase (PDF #42-1326), the characteristic peaks of ordered intermetallic structure marked by asterisk.The elemental mapping (Figure 1f ) and line scanning profile (Figure 1g) reveal the uniform distribution of Pt and Cu elements, and EDS (Figure S2, Supporting Information) shows the atomic ratio of Pt/Cu of a single L1 1 -PtCu NPs is 52:48.The XRD pattern and HRTEM image confirm the successful formation of L1 1 -PtCu NPs from PtCu NSs.Interestingly, a thin carbon layer (≈0.5 nm) was formed in background and on the surface of the NPs, produced from ligands decomposition at high temperature, serving as a protective shell that halt the aggregation of NPs, [32] as revealed by HRTEM (Figure S3, Supporting Information).

Synthesis and Characterization of Multicomponent
Pt-Based i-NPs Importantly, our strategy can be further expended to prepare multicomponent Pt-based i-NPs with different elements (Pd, Ru, Ag, and Fe).As schematically shown in Figure 2a, multicomponent NSs are prepared by using PtCu NSs as templates to grow other metals and then multicomponent i-NPs were obtained via annealing.Line scanning profiles (Figure S4, Supporting Information) and TEM images (Figure S6a-S12a, Supporting Information) demonstrate the successful synthesis of multicomponent NSs.TEM images confirm their successful transition into multimetallic i-NPs (Figure 2b,d,f,h).Moreover, the lattice fringes of the L1 1 -PtCuAg, L1 1 -PtCuPdAg, and L1 1 -PtCuPdAgRu (the inset in Figure 2b,d,h) are 0.439 nm, 0.444 nm, and 0.443 nm, respectively, which correspond to the (021) facet of L1 1 -PtCu and have a crystal structure similar to that of L1 1 -PtCu intermetallic phase.One of the interesting findings from our study is the impact of introducing Fe, which resulted in an alteration in the lattice fringe of L1 0 -PtCuAgFe to 0.367 nm (as depicted in the inset of Figure 2f ), which corresponds to the (001) plane of the L1 0 -PtFe intermetallic phase, a significant change in the crystal phase.The XRD patterns (Figure 2c,e,g,  i) also confirmed that the obtained NPs have the intermetallic phase, and they all have distinct superlattice peaks.In addition, we have also prepared i-NPs of other compositions, which were confirmed by TEM and XRD (Figure S5, Supporting Information).All multicomponent i-NPs have a uniform size with a narrow distribution (Figure S6b-S12b, Supporting Information).The elemental mappings (Figure S13, Supporting Information) and line scan analysis (Figure S6c-S12c, Supporting Information) of multicomponent i-NPs confirm the uniform distribution of Pt and other elements.EDS shows the elemental content of multicomponent i-NPs (Figure S6d-S12d, Supporting Information).

Synthesis and Characterization of PCPAF-HEI NPs
Notably, by using the PtCu NSs as templates, structurally ordered PCPAF-HEI NPs were also prepared.The TEM image (Figure 3a) shows that PCPAF-HEI NPs have an average particle size sub ≈ 5 nm with a uniform distribution, and the corresponding HAADF-STEM image of obtained NPs (Figure 3b) indicates that the as-prepared PCPAF-HEI NPs have no obvious agglomeration.The atomic structure of PCPAF-HEI NPs was further verified by using the aberration-corrected HAADF-STEM (Figure 3c).Because the intensity in the HAADF-STEM image reflects the atomic number (Z) of materials, [9] so Pt/Pd/Ag atom columns will have a higher intensity than columns of metals (Cu/Fe) with a lower Z.From the alternating layers of brighter and darker atoms along the <001> direction, the structure of PCPAF-HEI NPs can be assigned as the ordered L1 0 typical phase and the spacing of the lattice fringe is 0.382 nm, which corresponds to the (001) facet.An enlarged HAADF-STEM pattern and atomic model reveal regularly positioned bright (Pt/Pd/Ag) and dark (Cu/Fe) dots (Figure 3d).In addition, this alternating stacking of brighter Pt/Pd/Ag columns and dark Cu/Fe columns is further supported by the intensity profile (Figure 3e), which exhibits an intensity contrast between the Pt/Pd/Ag columns and the Cu/Fe columns.Further, the intensity profiles of the brighter columns also show an intensity contrast, demonstrating that the brighter columns contain three different metals such as Pt, Pd and Ag rather than only Pt (Figure S14, Supporting Information).The XRD patterns (Figure 3f ) also confirm the successful synthesis of PCPAF-HEI NPs, which is generally consistent with L1 0 -PtFe (PDF#43-1359).The superlattice peak position of PCPAF-HEI NPs is located between L1 1 -PtCu and L1 0 -PtFe, demonstrating that the introduction of other elements leads to significant lattice changes.Figure 3g illustrates the simulated crystal structure of PCPAF-HEI NPs, which have a crystal structure similar to that of L1 0 -PtFe intermetallic phase (Figure 3h).The STEM-EDS elemental mapping (Figure 3j) also confirms the uniform distribution of all elements without phase segregation.The atomic ratio (Table S1, Supporting Information) of Pt/Cu/Pd/Ag/Fe was determined to be 29.5/27.9/10.8/6.5/25.3 by using the inductively coupled plasma-optical emission spectrometer, which is roughly in accordance with the EDS result (Figure S12d, Supporting Information).
Further, X-ray photoelectron spectroscopy (XPS) was used to confirm the electronic states of the Pt, compared to Pt/C, the PCPAF-HEI NPs showed a negative peak shift, which indicates transfer of electrons between the neighboring atoms and Pt atoms.The binding energy of zero-valent Pt in PCPAF-HEI NPs is also shifted, compared with zero-valent Pt in L1 1 -PtCu NPs, indicating the successful introduction of other elements and the resulting change in the electronic states of Pt (Figure S15, Supporting Information).Further, X-ray absorption fine structure (XAFS) measurements were used to characterize the oxidation state and electronic structure of Pt.The white line intensity of the Pt L 3 -edge is consistent with the corresponding Pt foils and far away from the PtO 2 reference samples, revealing that Pt element in PCPAF-HEIs are in metallic states (Figure 3i).The white line generally provides information about the number of d vacancy in metal catalysts.The white line shifts towards higher energies, which indicates an increase in d-orbital of Pt in PCPAF-HEIs. [33]

Electrochemical Performance of PCPAF-HEI/C toward HER
Prior to conducting the electrocatalytic HER activity, the PtCuPdAgFe NSs are loaded on the active carbon (PCPAF-NSs/C) (Figure S16a, Supporting Information) and subsequently annealed to form highly dispersed i-NPs (Figure S16b, Supporting Information).PtCuPdAgFe high-entropy alloy (PCPAF-HEA) NPs loaded on carbon (PCPAF-HEA/C) were annealed at a lower temperature (Figure S16c, Supporting Information).The HRTEM images (the inset in Figure S16b, c, Supporting Information) and XRD patterns (Figure S16d, Supporting Information) confirm the successful synthesis of PCPAF-HEI NPs and PCPAF-HEA NPs onto active carbon support, designated as PCPAF-HEI/C and PCPAF-HEA/C catalysts, respectively.
The HER performance of 20% commercial Pt/C, PCPAF-NSs/ C, PCPAF-HEI/C, and PCPAF-HEA/C was examined under the acidic conditions.The potential required to reach a current density of 10 mA cm À2 is a key indicator to evaluate the HER performance. [34]The polarization curves, normalized by geometric area without iR compensation, of 20% commercial Pt/C, PCPAF-HEA/C, and PCPAF-HEI/C (Figure 4a and S17, Supporting Information) indicate that the PCPAF-HEI/C shows a smaller overpotential (24 mV) at a current density of 10 mA cm À2 .To further explore the activity, the specific activities as shown in Figure 4b are normalized to the electrochemically active surface area (ECSA) obtained by calculating the hydrogen absorptionÀdesorption regions (Figure S18, Supporting Information).As expected, the PCPAF-HEI/C exhibits promising specific activities in the entire potential region.At a potential of À0.05 V (vs.RHE), the PCPAF-HEI/C shows an extremely high specific activity (34.9 mA cm À2 ) (Figure S19, Supporting Information), which is higher than the PCPAF-HEA/C (26.16 mA cm À2 ) and commercial Pt/C (1.681 mA cm À2 ).
Furthermore, the PCPAF-HEI/C displays lower Tafel slope (29 mV dec À1 ) compared with Pt/C (35 mV dec À1 ) and PCPAF-HEA/C (38 mV dec À1 ), indicating fast HER kinetics [35] (Figure 4c). ) and commercial Pt/C (0.7665 A mg Pt À1 ).To get further insight into the superior HER performance of the PCPAF-HEI/C, electrochemical impedance spectroscopy demonstrates that the PCPAF-HEI/C possesses a faster kinetic for HER (Figure S21, Supporting Information).The higher C dl of PCPAF-HEI/C (Figure S22, Supporting Information) than commercial catalysts indicates a larger number of exposed active sites. [35]Beside activity, stability is another very important parameter to evaluate the practicality of the catalyst, especially in acidic media.We performed an accelerated cyclic voltammetry cycling test to evaluate its stability.PCPAF-HEI/C exhibits excellent stability with a nearly negligible shift in the polarization curve after 10,000 cycles (Figure 4f ).A long-term stability test of the PCPAF-HEI/C was performed at 100 mA cm À2 for 140 h in the 0.5 M H 2 SO 4 solution (Figure 4h) and exhibits excellent stability with negligible voltage change.Additionally, the morphology (Figure S23a, Supporting Information) of PCPAF-HEI/C is well maintained after the stability test without obvious NP agglomeration and spacing of lattice fringes (Figure S23b, Supporting Information) remain same.The superior stability of PCPAF-HEI/C can be attributed to the high-entropy stabilization effect [36] and strong d-d interaction [8] in the HEI structure.In addition, we compared the overpotential and Tafel slope of PCPAF-HEI/C with previously reported Pt-based catalysts, and PCPAF-HEI/C exhibits comparable or even better catalytic performance than those of reported advanced electrocatalysts in acidic media (Figure 4g and Table S3, Supporting Information).To explore the potential application of the PCPAF-HEI/C catalyst in industry, we also fabricated a prototype proton exchange membrane electrolyser consists of PCPAF-HEI/C as the cathode catalyst that showed superior activity and stability as compared to commercial Pt/C, which proves the potential of PCPAF-HEI/C catalyst for practical application (Figure S24, Supporting Information).

DFT Calculations
To get further insight into the high activity of the PCPAF-HEI/C for HER, DFT calculations were performed.According to the   S2, Supporting Information), which might be beneficial for boosting HER activity. [36][39][40] Figure 4j shows the specifically calculated ΔG H* of HER for representative Pt sites on the PCPAF-HEI (111) surface and pure Pt (111) surface.The Pt site in the PCPAF-HEI/C with this structural model has a smaller absolute ΔG H* (À0.434 eV) than that in pure Pt (À0.587 eV).The optimized electronic structure of Pt in the PCPAF-HEI permits the Pt site to attain an optimized PtÀH binding value, which might lead to rapid H* adsorption, ultimately accelerating HER.
To further reveal the elevated activity of the PCPAF-HEI, the partial density of states (PDOS) of the Pt-d state of the investigated models and their corresponding d-band centers are plotted (Figure 4k and S26, Supporting Information).The PDOS of PCPAF-HEI shows a high occupied state of the d orbital of Pt near the Fermi level, which is associated with promoted electron transfer, [40] leading to a high conductivity of PCPAF-HEI.Based on the PDOS of Pt in PCPAF-HEI and pure Pt, the replacement of other metal atoms causes a redistribution of the electronic structure.The downshift of the d-band center further demonstrates that the electronic structure of Pt has a weaker adsorption ability for hydrogen atoms, relieving the excessive binding of hydrogen on the surface and thus shifting the hydrogenadsorption free energies (ΔG H* ) towards the optimized value. [36,41,42]The emergence of a new d-electronic state could be attributed to the hybridization between Pt and the neighboring metal atoms, which leads to an improved electron environment for boosting HER activity.

Conclusion
In summary, we report a template and structure transformation approach for the synthesis of multimetallic i-NPs.PtCu model system with additional metals (Pd, Ag, Ru, and Fe) was successfully used to develop binary, ternary, quaternary, and quinary intermettalic compositions.The precise control of composition was achieved by using ultrathin PtCu NSs with high surface energy as templates, facilitating the epitaxial growth of other metals.Subsequent annealing resulted in the successful synthesis of highly uniform multicomponent i-NPs.Furthermore, PCPAF-HEI NPs were successfully obtained using the same approach, which were used as an electrocatalyst for HER in acidic media, exhibited superior electrocatalytic activity comparable to the commercial Pt/C.Theoretical calculations, indicating the proximity of ΔG H* to zero and weaker hydrogen adsorption, supported the observed enhanced electrocatalytic performance of PCPAF-HEI/C.These findings reveal the significance of our template strategy in the controlled synthesis of novel, highly uniform, and small-sized multicomponent and high-entropy i-NPs.We believe that this strategy can potentially open new horizons for the preparation of intermetallic nanomaterials with desired compositions to tailor their electrical, optical, magnetic, and catalytic properties for various applications.

Figure 3 .
Figure 3. Characterization of the morphology, structure, and composition of structurally ordered PCPAF-HEI NPs.a) TEM image (inset showing the particle size distribution).b) HAADF-STEM image of PCPAF-HEI NPs.c) Aberration-corrected HAADF-STEM image and d) the corresponding enlarged view and atomic model of PCPAF-HEI NP. e) Intensity line profile taken along the pink line indicated in (c).f ) XRD pattern of PCPAF-HEI NPs.Structural model showing a crystalline unit of g) PCPAF-HEI NP and h) L1 0 -PtFe phase.i) X-ray absorption near edge structure spectra of PCPAF-HEIs for Pt L 3 -edge.j) HAADF-STEM image and STEM-EDS elemental mapping of PCPAF-HEI NPs.
The linear sweep voltammetry curves normalized by mass indicate that the PCPAF-HEI/C catalyst has an absolute advantage (Figure S20, Supporting Information).The mass activity of PCPAF-HEI/C is 9.531 A mg Pt À1 and 7.881 A mg PtþPd À1 at À0.05 V vs. RHE (Figure 4d,e), outperforming PCPAF-HEA/C (5.964A mg Pt À1 and 4.931 A mg PtþPd À1

Figure 4 .
Figure 4. Electrochemical performance of PCPAF-HEI/C toward HER in an N 2 -purged 0.5 M H 2 SO 4 solution.a) HER polarization curves of 20% commercial Pt/C, PCPAF-HEA/C, and PCPAF-HEI/C, normalized by geometric area.b) ECSA normalized HER polarization curves of 20% commercial Pt/C, PCPAF-HEA/C, and PCPAF-HEI/C.c) The corresponding Tafel plots were calculated from the HER polarization curves of 20% commercial Pt/C, PCPAF-HEA/C, and PCPAF-HEI/C normalized by geometric area.d,e) Quantitative comparisons of the mass activities of 20% commercial Pt/C, PCPAF-HEA/C, and PCPAF-HEI/C at different potentials.f ) Stability test of PCPAF-HEI/C recorded before and after 10,000 cycles, normalized by geometric area.g) Comparison of the both Tafel slope and the overpotential required to achieve 10 mA cm À2 for previously reported Pt-based catalysts.Values were plotted from references in Supporting Information (the expanded values are shown in TableS3, Supporting Information).All the results were not iR corrected.h) Time-dependent overpotential curve of PCPAF-HEI/C at constant current density of 100 mA cm À2 .All the results were not iR corrected.i) The view of the model used to describe PCPAF-HEI (111) and hydrogen atom absorption of Pt site in PCPAF-HEI (111).j) Calculated free energy profiles of HER at the equilibrium potential for the pure Pt and PCPAF-HEI.k) PDOS of the d-band for the surface Pt atoms in the pure Pt (111) and PCPAF-HEI (111) slab system, the vertical dashed lines indicate the calculated d-band center.
model of the PCPAF-HEI (111) and pure Pt surface and hydrogen atom absorption of the Pt site in PCPAF-HEI (111) and pure Pt (Figure4iand S25, Supporting Information), compared with pure Pt (111), the (111) surface of the PCPAF-HEI has a weaker adsorption capacity for hydrogen atoms (Table