Hierarchically porous ternary Au/ZnO@ZIF-8 nanocomposite: spatial in situ Au encapsulation and catalytic activity for the reduction of p-nitrophenol

Abstract

A hierarchically porous ternary Au/ZnO@ZIF-8 nanocomposite with flower-like morphology has been prepared using a new in situ encapsulation approach. In particular, Au nanoparticles (NPs) were spatially encapsulated from the ZnO surface into the ZIF-8 crystal matrix during its nucleation process. The ternary composite exhibited high catalytic activity for the reduction of p-nitrophenol.

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During the past few decades, metal nanoparticles (NPs) have attracted much attention owing to their unique properties and wide range of applications in various research fields, particularly in catalysis.¹ However, aggregation of NPs always occurs during catalytic processes, which significantly reduces the number of accessible active surface sites and thus decreases the catalytic activity.² Recently, metal–organic frameworks (MOFs) have been proved to be outstanding candidates as a new class of solid templates for loading metal NPs such as Cu, Pd, Pt, Ag and Au.³ The unique features of MOFs such as tuneable dimensionality and the ability to tailor their inner channels or cavities allow control over the particle size and prevent aggregation of NPs. Although numerous NP loaded MOF systems have been reported, it is still a difficult to load metal NPs inside the cavities of MOFs⁴ than on the outer surface of MOFs,⁵ which is strongly associated with the methodologies of encapsulation and the natures of the cavities in the MOFs.

Among the existing strategies, in situ encapsulation of pre-synthesized NPs during the formation process of MOFs is the most powerful method for the preparation of NP@MOF composites.⁶ For example, Hou and co-workers⁷ demonstrated an effective strategy that allowed complete incorporation of several types of NPs into ZIF-8 (zeolitic imidazolate framework-8, [Zn(MeIm)₂]_n, MeIm = 2-methylimidazole) nanocrystals, even when the particle sizes of the NPs were much larger than the pore diameters in the ZIF-8 structure. By adjusting the time of the addition of NPs during the synthesis of ZIF-8, they could control the spatial distribution of NPs within the ZIF-8 matrix. The spatial location of NPs can directly influence the catalytic properties of the composite. As demonstrated by Jiang and co-workers,⁸ Pt@UiO-66-NH₂ (Pt NPs incorporated inside MOF) enables much better separation of charge carriers, and thus significantly higher photocatalytic activity for the production of hydrogen than Pt/UiO-66-NH₂ is observed (Pt NPs supported on MOF).

Besides being utilized as functional active sites within MOFs, single metal or metal oxide NPs with various morphologies and particle sizes can also be used as seeds for the nucleation of MOFs to prepare core–shell NP@MOF structures.⁹ In particular, when ZnO NPs were used as seeds, core–shell structure ZnO@ZIF-8 could be fabricated via a solvothermal reaction, in which ZnO was not only a template but also provided zinc ions for the nucleation of ZIF-8.¹⁰

With the two abovementioned concepts in mind, ZnO NPs (such as nanorods or nanospheres) decorated with Au NPs would be a desirable template and precursor for preparing an Au/ZnO@ZIF-8 nanocomposite, in which Au NPs could be encapsulated in situ and, most importantly, Au NPs are spatially encapsulated from the ZnO surface into the ZIF-8 crystal matrix.

Herein, we demonstrate an alternative in situ encapsulation approach for obtaining a ternary Au/ZnO@ZIF-8 flower-like nanocomposite using a two-step synthesis method (Scheme 1). In such an approach, a facile hydrothermal reaction of Zn(CH₃COO)₂·2H₂O and HAuCl₄·6H₂O results in well-defined Au/ZnO nanoflowers, which are further used as self-sacrificing templates to fabricate core–shell flower-like Au/ZnO@ZIF-8 composites in the presence of 2-methylimidazole via a solvothermal reaction. During the epitaxial growth of ZIF-8 on the Au/ZnO surface, well-dispersed Au NPs are encapsulated in situ into the crystal matrix of the ZIF-8 shell, as the ZnO core simultaneously supplies zinc ions for the nucleation of ZIF-8. The resulting ternary Au/ZnO@ZIF-8 composite containing an optimized amount of Au exhibited high catalytic activity for the reduction of p-nitrophenol by sodium borohydride (NaBH₄) in an aqueous solution.

Scheme 1 Schematic of the preparation of a ternary Au/ZnO@ZIF-8 nanocomposite using a two-step synthesis method.

Monodisperse Au/ZnO nanoflowers with different contents of Au NPs were prepared via a one-step modified hydrothermal reaction.¹¹ The crystal structure of Au/ZnO samples was characterized by PXRD (Fig. S1, ESI†). As shown in Fig. S1,† the diffraction peaks of all the samples at 31.74°, 34.39°, 36.21°, 47.52°, 56.56°, 62.81°, 66.36°, 67.92°, and 69.05° are indexed to the (100), (002), (101), (102), (110), (103), (200), (112), and (201) planes, respectively, of ZnO crystals as given by the standard data file (JCPDS file no. 36-1451). It can be seen that as the content of Au NPs increased the intensity of the PXRD peaks of Au at 38.19°, 44.36°, and 66.42°, which are indexed to its (111), (200), and (220) planes, respectively, increased markedly. SEM images of Au/ZnO (7.5%) (Fig. 1a and b) and Au/ZnO (5%, 10%, and 12.5%) (Fig. S2, ESI†) show a large amount of monodisperse ZnO nanoflowers with uniformly sized multiple arms. The rod-like structures of each arm have an average length of 1.5–2.5 μm with a diameter of 250–500 nm. At low Au contents, no agglomeration of Au NPs could be observed on the basis of SEM measurements (Fig. 1b and S2a, ESI†), whereas in the cases of Au/ZnO (10% and 12.5%) the Au NPs formed large agglomerates (Fig. S2b and c, ESI†). Therefore, an Au/ZnO sample without agglomerated Au particles and with an appropriate Au content of 7.5% was chosen for further characterization and catalytic tests.

Fig. 1 SEM images (a, b), TEM images (c, d, e), and a high-resolution TEM (HRTEM) image (f) of Au(7.5%)/ZnO nanoflowers.

TEM images of Au(7.5%)/ZnO clearly also show the flower-like morphology (Fig. 1c). Fig. 1d and e are TEM images of one branch of a nanoflower, which show that Au NPs with diameters of 3–20 nm are uniformly dispersed on the surface of the branch. Atomic lattice fringes with a spacing of 0.23 nm corresponding to Au (111) planes were observed,¹² as indicated on the high-resolution TEM (HRTEM) image (Fig. 1f).

Fig. 2a shows a typical SEM image of the product after the synthesis of ZIF-8. The sample retains the flower-like morphology of Au(7.5%)/ZnO. In comparison to that of Au/ZnO, the average diameter of the arms with roughened surfaces increased to 600–800 nm. Fig. 2b–d show typical TEM images of the prepared nanoflowers with a ZIF-8 shell. The TEM images not only show that Au NPs are encapsulated within the ZIF-8 crystal matrix but also reveal that the prepared nanocomposite contains mesopores, which were generated in situ during the formation process of ZIF-8. The mesoporous feature was further confirmed by an N₂ sorption study. As shown in Fig. 3a, in contrast to the low adsorption of N₂ on Au/ZnO, the large uptake at a low relative pressure (<0.05) on Au(7.5%)/ZnO@ZIF-8 indicates the presence of micropores, which is mainly related to the formation of a microporous ZIF-8 shell. The Brunauer–Emmett–Teller (BET) surface areas of Au/ZnO and Au(7.5%)/ZnO@ZIF-8 are 12 m² g⁻¹ and 228 m² g⁻¹, respectively. The N₂ sorption isotherms exhibit a hysteresis loop in the relative pressure range from 0.5 to 1.0, which is due to the capillary condensation of N₂ in mesopores with a wide size distribution (Fig. 3b). The co-existence of Au, ZnO, and ZIF-8 in the ternary composite was confirmed by PXRD. As shown in Fig. 3c, the PXRD pattern of Au(7.5%)/ZnO@ZIF-8 reveals that the product is composed of three types of materials with distinctly different crystal structures. Besides the peaks of ZnO and Au NPs mentioned above, the residual strong diffraction peaks coincide well with the peaks of as-synthesized ZIF-8.

Fig. 2 SEM image (a), TEM images (b, c), and HRTEM image (d) of Au(7.5%)/ZnO@ZIF-8 nanoflowers.

Fig. 3 (a) N₂ sorption isotherms of Au(7.5%)/ZnO (red) and Au(7.5%)/ZnO@ZIF-8 (blue), respectively, at 77 K. (b) Pore size distribution of Au(7.5%)/ZnO@ZIF-8. (c) PXRD patterns of as-synthesized ZIF-8 (black), Au(7.5%)/ZnO (red), and Au(7.5%)/ZnO@ZIF-8 (blue), respectively.

As a reference material for catalytic tests, the microporous binary composite Au/ZIF-8 was also prepared using a simple aqueous solution reduction method. Fig. S3† shows that the PXRD pattern of Au/ZIF-8 is basically identical to that of ZIF-8, which suggests that the integrity of MOFs was maintained after reduction by NaBH₄. As shown in Fig. S4,† the typical type I N₂ sorption isotherm reveals that ZIF-8 retained its microporous structure after Au NPs were loaded. Au/ZIF-8 has a BET surface area of 1342 m² g⁻¹, which is slightly smaller than that of ZIF-8 (1460 m² g⁻¹). The TEM images in Fig. S5† reveal that the ZIF-8 template has a particle size of ∼120 nm. The Au NPs are highly dispersed on the template with a uniform particle size distribution, and the average particle size is 3.5 ± 0.5 nm. The majority of the Au NPs are encapsulated in the cavities of ZIF-8, where the Au NPs are suggested to exist not in single pores, but in merged neighboring pores of its three-dimensional structure. Some of the Au NPs are located on the surfaces and edges of ZIF-8 particles, as indicated by TEM. Therefore it is clear that with this aqueous impregnation method the encapsulation of all the NPs in ZIF-8 is not guaranteed. In sharp contrast, the Au NPs in the Au/ZnO@ZIF-8 sample were exclusively embedded in the ZIF-8 matrix, which highlights the advantage of the in situ encapsulation approach.

The catalytic activities of ternary and binary gold nanocomposites were examined via the reduction of p-nitrophenol. The catalytic reduction reaction of p-nitrophenol by NaBH₄ is not only an effective approach for the degradation of toxic organic pollutants, but also serves as a model reaction for analyzing the catalytic properties of catalysts loaded with Au NPs.¹³ Fig. 4a and b show typical UV-vis spectra recorded during catalytic tests. The absorption peak at 400 nm, which is the characteristic absorption band of p-nitrophenol, gradually decreased in intensity, which indicates the reduction of p-nitrophenol as the reaction time increased after the addition of Au/ZnO@ZIF-8 and Au/ZIF-8, while a new absorption peak appeared at 315 nm owing to the formation of p-aminophenol. The change in the p-nitrophenol concentration (C_t/C_o, C_o = 7.4 mM and C represents the concentration of p-nitrophenol) is proportional to the normalized absorption value (A_t/A_o) of the peak at 400 nm. The reduction did not proceed in the absence of a Au catalyst, as shown in Fig. 4c. Interestingly, we found that the catalytic reaction was initiated immediately upon the addition of Au(7.5%)/ZnO@ZIF-8. In contrast, it took a short period of time for Au/ZIF-8 to catalyse the reaction, i.e. a few minutes were required for p-nitrophenol to be adsorbed on the catalyst surface to react. Such a difference can be ascribed to the morphological and structural differences between Au/ZIF-8 and Au/ZnO@ZIF-8. Because the majority of Au NPs were embedded in the cavities of microporous ZIF-8, the diffusion of p-nitrophenol to the active sites was hindered, whereas the flower-like Au/ZnO@ZIF-8 catalyst possessed hierarchical pores spanning the micro- to mesoscale, which were centred around 0.48, 12, 18 and 23 nm (Fig. 3b); such a multimodal pore structure greatly facilitated the diffusion of the reactant to the active sites. As a result, Au/ZnO@ZIF-8 even had slightly higher catalytic activity than Au/ZnO, on which the Au NPs were in intimate contact with the reactant solution. The approximately linear relationship of −ln(C_t/C_o) vs. time is plotted in Fig. 4d, which suggests that this reaction system follows first-order kinetics. On the basis of the abovementioned observations, Au(7.5%)/ZnO@ZIF-8 can be considered as a highly effective Au NPs-based heterogeneous catalyst.¹⁴

Fig. 4 Representative time-dependent UV-vis absorption spectra for the reduction of p-nitrophenol over (a) Au(7.5%)/ZnO@ZIF-8 and (b) Au/ZIF-8 in aqueous solution at room temperature. (c) Extinction (normalized against the initial point) at the peak position for p-nitrophenol (400 nm) as a function of time. (d) Plot of –ln(C_t/C₀) against time for Au(7.5%)/ZnO@ZIF-8.

Conclusions

In summary, we have demonstrated a new in situ encapsulation approach for the preparation of a hierarchically porous ternary Au/ZnO@ZIF-8 nanoflower composite using a two-step synthesis method, which guarantees that Au NPs are spatially encapsulated from the ZnO surface into the ZIF-8 crystal matrix. With a flower-like morphology and multimodal pore structure, the resulting Au/ZnO@ZIF-8 nanocomposite exhibited higher catalytic activity than that of a microporous binary Au/ZIF-8 composite loaded with Au NPs in the reduction of p-nitrophenol. This new synthetic method can be used as a general strategy for the synthesis of various ternary NPs/ZnO@ZIF-8 with different metal NPs, tuneable particle morphologies, and multifunctional properties.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant No. 21401083 and 21373103).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, PXRD, SEM and TEM images, N₂ sorption studies and recycling tests. See DOI: 10.1039/c6ra23401a

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