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. 2020 Nov 13;6(12):2347–2353. doi: 10.1021/acscentsci.0c01262

Synthesis and Crystal-Phase Engineering of Mesoporous Palladium–Boron Alloy Nanoparticles

Hao Lv †, Dongdong Xu ‡, Chuncai Kong §, Zuozhong Liang ∥, Haoquan Zheng ∥, Zhehao Huang ⊥,*, Ben Liu †,‡,*
PMCID: PMC7760460  PMID: 33376796

Abstract

graphic file with name oc0c01262_0005.jpg

Rational design and synthesis of noble metal nanomaterials with desired crystal phases (atomic level) and controllable structures/morphologies (mesoscopic level) are paramount for modulating their physiochemical properties. However, it is challenging to simultaneously explore atomic crystal-phase structures and ordered mesoscopic morphologies. Here, we report a simple synergistic templating strategy for the preparation of palladium–boron (Pd–B) nanoparticles with precisely controllable crystal-phases and highly ordered mesostructures. The engineering of crystal-phase structures at atomic levels is achieved by interstitially inserting metallic B atoms into face-centered cubic mesoporous Pd (fcc-mesoPd) confined in a mesoporous silica template. With the gradual insertion of B atoms, fcc-mesoPd is transformed into fcc-mesoPd5B, hcp-mesoPd2B with randomly distributed B atoms (hcp-mesoPd2B-r), and hcp-mesoPd2B with an atomically ordered B sequence (hcp-mesoPd2B-o) while preserving well-defined mesostructures. This synergistic templating strategy can be extended to engineer crystal-phase structures with various mesostructures/morphologies, including nanoparticles, nanobundles, and nanorods. Moreover, we investigate the crystal-phase-dependent catalytic performance toward the reduction reaction of p-nitrophenol and find that hcp-mesoPd2B-o displays much better catalytic activity. This work thus paves a new way for the synthesis of hcp-Pd2B nanomaterials with mesoscopically ordered structure/morphology and offers new insights of fcc-to-hcp evolution mechanisms which could be applied on other noble metal-based nanomaterials for various targeted applications.

Short abstract

A synergistic template method is reported to synthesize mesoporous Pd−B nanoparticles with atomically ordered hexagonal close-packed crystal-phase structure and mesoscopically ordered double gyriod Ia3̅d morphology.

1. Background

Noble metal (NM) nanocrystals have received a great deal of attention for their widespread applications in catalysis, photonics, electronics, and biomedicine.1−5 Among various NMs, palladium (Pd) is one of the predominantly used NMs because of its optimum affinity for hydrogen, which allows its potential application toward various hydrogen-related catalytic reactions.6−9 Pd is a 4d transition metal that adopts thermodynamically stable face-centered cubic (fcc) crystal-phase structure in the bulk (determined by the total electronic energy of Pd).10,11 Although Pd-based nanocrystals with different shapes, nanostructures, and compositions have been rationally designed and synthesized, it remains a major challenge to explore their novel crystal-phase structures and further understand the evolution mechanisms at atomic levels.12−14 This is because of the inevitable rearrangement of atoms during the evolutions/transformations of crystal-phase structures. When downsizing to nanodimensions, in contrast, the surface energy of NM nanocrystals becomes the important factor, making it possible to explore novel crystal-phase structures (with different atomic packing sequences) that cannot be usually achieved in the bulk counterpart materials with the high total bulk energy.4,15

Periodically ordered mesoporous materials, with pore sizes of 2–50 nm, have attracted great research interest in the past three decades because of their high porosities, large surface areas, and tunable pore sizes.16−20 In comparison to traditional mesoporous carbons and oxides, mesoporous NMs (for example, mesoPd, mesoPt, and mesoAu) present many novel physicochemical properties that are inherent to metal frameworks (e.g., good electrical and thermal conductivities, high catalytic activities) along with structural features (mesoporosities). This thus endows their better performances toward various catalytic and optical applications.21−24 Several strategies have been developed to synthesize mesoporous NMs with different nanostructures, morphologies, and compositions.20,25−29 However, to the best of our knowledge, simultaneously exploring novel atomic crystal-phase structures and mesoscopic morphologies has never been achieved for NM nanomaterials. Considering the importance of Pd-based nanomaterials in catalysis, therefore, the availability of a simple yet effective synthetic strategy to precisely synthesize mesoPd with different atomic crystal-phase structures is highly desirable for fundamentally understanding the relations between crystal-phase/structure and catalytic performance.

Herein we report experimentally the discovery of a hexagonal close-packed and mesoporous Pd2B that simultaneously discloses a novel hcp-crystal-phase structure with atomically ordered sequences of Pd/B atoms and a mesoscopically ordered Ia3̅d structure through a simple synergistic templating method (defined as hcp-mesoPd2B-o hereafter). Compared to traditional soft/hard-templating method, we propose a synergistic template of mesoPd/KIT-6 hybrids to explore and further engineer novel crystal-phase structures of Pd2B while retaining ordered mesoporous structure. Crystal-phase characterizations reveal that B atoms occupy a half number of the Pd octahedrons and form a highly ordered sequence in hcp-mesoPd2B-o, which are distinct from the reported hcp ones where B atoms arrange randomly.14,30 Mechanism studies on crystal-phase evolutions find that, with gradual insertion of B into Pd–Pd interatomic spacings, initial fcc-mesoPd first transfers to fcc-mesoPd5B, then to hcp-mesoPd2B-r (randomly distributed B), and finally to hcp-mesoPd2B-o (atomically ordered B) that is thermodynamically most stable at the nanoscale. We also demonstrate the generality of this synergistic templating strategy for controlling sizes and structures/morphologies of hcp-Pd2B-o (nanobundles and nanorods). Finally, the reduction reaction of p-nitrophenol by NaBH4 is studied as a model reaction to evaluate crystal-phase-dependent catalytic performance of as-resulted mesoPd–B nanoparticles.

2. Results and Discussion

Binary mesoPd–B nanoparticles are synthesized using a synergistic templating method (see synthetic scheme in Figure S1). Typically, mesoPd/KIT-6 hybrids were first obtained by a classic hard-templating method (see KIT-6 in Figure S2).28,31 Then, as-synthesized mesoPd/KIT-6 hybrids as the synergistic template were further mixed with dimethylamine (DMAB, B source) and solvothermally treated under a low boiling point solvent, for example tetrahydrofuran, with different reaction conditions. As-resulted mesoPd–B nanoparticles were finally obtained by etching with HF to remove KIT-6 template. Inductively coupled plasma-mass spectrometry (ICP-MS) indicates that Pd/B ratios of mesoPd–B with fcc- and hcp-crystal-phase structures are 84/16 (∼5:1) and 67/33 (∼2:1), respectively. With the structural analysis results, the mesoPd–B nanoparticles are denoted as fcc-mesoPd5B and hcp-mesoPd2B-o hereafter.

X-ray diffraction (XRD) was first characterized to reveal the atomic crystal-phase structures of obtained mesoPd, mesoPd5B, and mesoPd2B (Figure 1). Five typical diffraction peaks are observed from mesoPd in the 2θ range of 35–90°. The peaks can be indexed as 111, 200, 220, 311, and 222 reflections of a cubic Fm-3m crystal-phase structure (Figure 1a), where Pd atoms arrange in a typical fcc packing sequence of ABCABC (Figure 1b,c). All five XRD peaks preserve well for mesoPd5B, implying that alloying metalloid B does not change the fcc-crystal-phase structure of Pd. In comparison to fcc-mesoPd, however, all the XRD peaks of mesoPd5B become broader and shift toward the lower angles (i.e. ∼1.5° for 111 reflection). They suggest a larger unit cell parameter of mesoPd5B, where metalloid B atoms with smaller atomic radius randomly insert in between Pd–Pd atoms. The Pawley method was further applied to refine the unit cell parameters of mesoPd and mesoPd5B. The a-parameter is converged to 3.8890(1) and 4.0233(2) Å for mesoPd and mesoPd5B, respectively (Figure S3 and S4 and Table S1). In contrast, mesoPd2B shows a set of very interesting diffraction peaks (Figures 1d and S5), which are totally different to traditional fcc-crystal-phase structure of Pd and previously reported Pd2B.30−32 XRD analysis reveal that mesoPd2B has a novel but very rarely reported orthorhombically distorted hcp structure (Table S1). While Pd atoms have a common hcp packing sequence of ABABAB, interestingly, B atoms fill half of the octahedral holes (B–Pd6) between hcp-Pd atoms (see inset in Figure 1d) and form an atomically ordered sequence (Figure 1e and f). This ordered B sequence is distinct from any other PdxB reported before, where B atoms are randomly distributed.30,32−34 The simulated XRD pattern discloses great agreements with the experimental one, in both peak positions and peak intensities, further validating the structural model of hcp-mesoPd2B-o at atomic levels (Figure S6).

Figure 1.

Figure 1

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XRD characterizations and crystal-phase structures. (a) Experimental and simulated XRD patterns (λ = 1.5406 Å) and (b, c) atomic crystal-phase structural models of fcc-mesoPd and/or fcc-mesoPd5B. (d) Experimental and simulated XRD patterns (λ = 1.5406 Å) and (e, f) atomic crystal-phase structural models of hcp-mesoPd2B-o: green, red, and blue spheres Pd (in different packing layers); pink spheres B.

The low-magnification scanning electron microscopy (SEM) images display that this novel hcp-mesoPd2B-o is morphologically spherical with a good uniformity and homogeneity (Figures 2a and S7). The average size of hcp-mesoPd2B-o is 108 ± 10 nm, which is similar to the results observed from fcc-mesoPd and fcc-mesoPd5B (Figure S8). hcp-mesoPd2B-o is perfectly replicated the initial double gyroid mesostructure of KIT-6 template with the inverse double gyroid structure of Ia3̅d (Figure S9). The transmission electron microscopy (TEM) and high-angle annular dark-field scanning TEM (HAADF-STEM) images observed along the mesoscopic [100] and [111] directions disclose that interconnected mesopores with typical Ia3̅d mesostructures penetrate entire hcp-mesoPd2B-o nanoparticles (Figures 2b,c, and S10; see 3D tomography in Movie S1). In addition, small-angle X-ray scattering (SAXS) shows that mesoPd2B has mesoscopic unit cell parameters similar to those of KIT-6 (Figure S11).

Figure 2.

Figure 2

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Electron microscopy characterizations. (a) Low-magnification SEM and (b) TEM images, (c) HAADF-STEM images along the mesostructural [100] and [111] directions of Ia3̅d symmetry, (d) high-resolution HAADF-STEM image, and (e) STEM EDS mapping images of hcp-mesoPd2B-o. (f) High-resolution HAADF-STEM image of fcc-mesoPd5B. (g) High-resolution TEM image of fcc-mesoPd.

We further characterized the atomic crystal-phase structures of as-resulted samples with high-resolution HAADF-STEM. The typical ABABAB stacking sequence of Pd atoms is observed, indicating an hcp-crystal-phase structure of mesoPd2B-o with d020 = 2.50 Å and d200 = 2.23 Å (Figure 2d). Meanwhile, energy dispersive X-ray spectroscopy (EDS) mappings disclose uniformly distributed Pd and B elements through the entire mesoporous nanoparticles (Figure 2e). In contrast, both mesoPd5B and mesoPd have the fcc-crystal-phase structures with a typical ABCABC stacking sequence of Pd atoms (Figure 2f, g). A clear d111 of 2.32 Å for mesoPd5B is larger than that of 2.24 Å for mesoPd, further confirming that metalloid B has interstitially inserted in between Pd–Pd atoms. These results are highly consistent to the ones observed from XRD patterns.

In contrast to having atomically ordered B sequence, we also synthesize hcp-mesoPd2B with randomly distributed B atoms (Figure S12). It has ordered mesostructure upon the hcp phases reported before.14,30 With a medium synthetic temperature of 140 °C, we found that the as-resultant product includes major hcp-Pd2B and minor fcc-Pd5B phases (Figure S13). Interstitial insertion of B does not change the traditional hcp-crystal-phase structure of Pd, in which B atoms are randomly distributed in between Pd–Pd interatomic spacings (defined hcp-mesoPd2B-r, Figure S14), as revealed by XRD analysis (Figure S13 and Table S1). These results indicate that the gradual insertion of B into Pd crystals triggers an atomic structural evolution, from fcc-mesoPd to fcc-mesoPd5B, hcp-mesoPd2B-r with randomly distributed B, and finally hcp-mesoPd2B-o with an atomically ordered B sequence.

The above atomic and mesoscopic observations suggest that binary Pd–B alloys with precisely controllable crystal phases and highly ordered mesoporous structures are successfully obtained by a gradual insertion of B atoms into fcc-mesoPd. Remarkably, to the best of our knowledge, this is the first successful synthesis of hcp-Pd2B-o with an atomically ordered crystal-phase sequence and, simultaneously, a mesoscopically ordered mesostructure/morphology. We emphasize that mesoPd/KIT-6 hybrids as the synergistic template are critically important to preserve mesoporous structures during the fcc-to-hcp crystal-phase evolution. As a reference experiment, mesoporous structure gradually collapses during the atomic crystal-phase evolution (fcc-to-hcp) when using solo mesoPd instead of mesoPd/KIT-6 hybrids (Figure S9). This is similar to the result observed from mesoPd–B synthesized by the soft-templating method (Figure S15).35 Moreover, other solvents, such as phenylmethane, hexane, and ether, are also suitable for the crystal-phase-engineering formation of hcp-mesoPd2B-o (Figure S16), further indicating the importance of the strong nanoconfinement effect of mesoPd/KIT-6 hybrids.

This synthetic approach also allows the controllable tuning of the sizes of hcp-mesoPd2B-o nanoparticles by simply adjusting the amounts of reducing agent during the synthesis of mesoPd/KIT-6 hybrids. The average diameter of hcp-mesoPd2B-o nanoparticles can be tuned in the range of 65–170 nm, although the smaller nanoparticles have a slightly disordered mesoporous morphology (Figure S17).28 Moreover, other mesoscopic hcp-Pd2B-o nanostructures/morphologies can be also formed using the same procedures. As shown in Figure 3a, the synergistic templating of hcp-mesoPd2B-o with a single gyroid structure (and larger pore size) is achieved, when Pd precursor precisely inserts in one set of mesoporous Ia3̅d channel of KIT-6 (see the synthetic scheme in Figure S18).31,35 Besides, hcp-mesoPd2B-o nanobundles and hcp-Pd2B-o nanorods are formed by templating with mesoPd/SBA-15 hybrids (Figure 3c, e; see the synthetic schemes in Figures S19 and S20).25,36,37 Corresponding XRD patterns further disclose the fcc-to-hcp evolution of atomic crystal-phase structures (fcc-Pd, fcc-Pd5B, and hcp-Pd2B-o nanocrystals) (Figure 3b, d, f). These results highlight the synthetic capability of our method for precisely engineering crystal phases and mesostructures/morphologies of Pd–B nanomaterials on both atomic and mesoscopic levels.

Figure 3.

Figure 3

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Structure and morphology engineering. (a) TEM image and (b) corresponding evolved XRD patterns of hcp-mesoPd2B-o with a single gyriod mesostructure. (c) TEM image and (d) corresponding evolved XRD patterns of hcp-mesoPd2B-o nanobundles. (e) TEM image and (f) corresponding evolved XRD patterns of hcp-Pd2B-o nanorods.

Our successful synthesis provides an opportunity to evaluate the crystal-phase-dependent catalytic activities of mesoPd–B nanoparticles by using the reduction of p-nitrophenol by NaBH4 as a model reaction (Figure 4a). Figure 4b shows the time-dependent ultraviolet–visible (UV–vis) spectra of hcp-mesoPd2B-o, displaying a continuous decrease at the absorption band of ∼400 nm (p-nitrophenol + NaBH4) and an increase at the band of ∼303 nm (p-aminophenol). Notably, it only needs ∼80 s for full conversion of p-nitrophenol. In contrast, the conversion time is prolonged to ∼105 s for fcc-mesoPd5B, ∼160 s for fcc-mesoPd, and ∼235 s for commercial Pd black (Figure S21). The kinetic studies indicate, in the initial stage, that the reduction reaction follows a pseudo-first-order kinetics with the classic Langmuir–Hinshelwood mechanism (Figure 4c).38 Among them, hcp-mesoPd2B-o holds a highest slope (ln(A/A0 vs time)), indicating the quickest reaction kinetics. Last, the kinetic rate constant (k) is accordingly calculated. The k values for hcp-mesoPd2B-o, fcc-mesoPd5B, fcc-mesoPd, and Pd black are −0.076 ± 0.003, −0.045 ± 0.003, −0.032 ± 0.002, and −0.015 ± 0.002 s–1, respectively (Figure 4d). Remarkably, hcp-mesoPd2B-o is 1.70, 2.42, and 4.99 times more active than fcc-mesoPd5B, fcc-mesoPd, and Pd black. The results, again, suggest that hcp-mesoPd2B-o with novel hcp-crystal-phase and highly ordered mesoporous structure is catalytically more active than its counterpart catalysts. Moreover, hcp-mesoPd2B-o discloses a good catalytic stability (Figure S22). The better catalytic activity of hcp-mesoPd2B-o is possibly attributed to the global minimum and the expansion of Pd–Pd lattice of hcp-Pd2B-o, which lowers the binding energy of surface species.30 Meanwhile, the interconnected mesoporous framework enlarges the exposed surface sites and facilitates the mass transport, which also contributes to enhanced catalytic activity.22

Figure 4.

Figure 4

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Catalytic performance. (a) Reaction routes of the reduction of p-nitrophenol by NaBH4. (B) UV–vis spectra for the reduction of p-nitrophenol over hcp-mesoPd2B-o. (c) Relationship between ln(A/A0) and time. (d) Summarized k values over hcp-mesoPd2B-o, fcc-mesoPd5B, fcc-mesoPd, and Pd black.

3. Conclusion

We develop a facile synergistic templating strategy for precisely engineering atomic crystal-phase structures and mesoscopic nanostructures/morphologies of binary Pd–B nanoparticles. The fcc-to-hcp crystal-phase transformation of mesoPd–B at atomic levels is achieved by interstitially inserting metallic B atoms into Pd crystals that are confined within a mesoporous silica template. The changes in the types of the hard-templating mesoporous silicas also result in different mesopore sizes, mesostructures, and morphologies of the Pd–B crystals at mesoscopic levels. Lastly, crystal-phase-dependent catalytic activity of mesoPd and mesoPd–B nanoparticles is examined by the reduction reaction of p-nitrophenol. We find that hcp-mesoPd2B-o discloses much better catalytic activity than fcc-mesoPd5B, fcc-mesoPd, and commercial Pd black. This work represents the first successful example for synthesis and crystal-phase engineering of binary mesoPd–B nanomaterials at both atomic and mesoscopic levels, with precisely controllable morphologies. We believe that this synthetic strategy provides new insights in the crystal-phase engineering of mesoporous noble metals with defined nanostructures, mesostructures, and morphologies, which could display desired physicochemical properties for various potential applications.

Acknowledgments

We thank Mr. Chang Huang at Instrument Analysis Center of Xi’an Jiaotong University for their assistance with SAXS analysis. We thank Natural Science Foundation of Jiangsu Province (BK20180723 and BK20191366), Jiangsu Specially Appointed Professor Plan, the program of Jiangsu Province Innovation Team, the research fund from Priority Academic Program Development of Jiangsu Higher Education Institutions, National, Local Joint Engineering Research Center of Biomedical Functional Materials, and the Swedish Research Council (VR, 2016-04625) for their financial support.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acscentsci.0c01262.

  • Synthetic details and schemes, more mesostructures/morphologies and crystal-phase characterizations, and catalytic tests (PDF)

  • Mesoscopic 3D tomography of hcp-mesoPd2B-o nanoparticles (MP4)

Author Contributions

B.L. conceived the project and directed the experiments. H.L. synthesized and characterized the materials and performed the catalysis tests. D.X. assisted the high-resolution TEM characterizations. C.K. conducted SAXS test and analysis. Z.L. and H.Z. performed 3D tomography studies. Z.H. performed HAADF-STEM characterizations and analyzed atomic crystal-phase structures. All authors analyzed the results and cowrote the manuscript.

The authors declare no competing financial interest.

Supplementary Material

oc0c01262_si_001.pdf (1.7MB, pdf)
oc0c01262_si_002.mp4 (1.1MB, mp4)

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