The facile ionic liquid-assisted synthesis of hollow and porous platinum nanotubes with enhanced catalytic performances

Abstract

A facile one-pot synthetic method was developed to synthesize hollow and porous Pt nanotubes (Pt-HPNTs) using silica nanorods as a template, ionic liquid as a precipitator and reductant, and in situ generated KCl as an etching agent. The work reported here might open a new route for the synthesis of hollow nanostructures.

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The hollow noble-metal nanostructures (HNMNs) that consist of a thin noble-metal shell with a hollow cavity have shown many fascinating physical/chemical properties that are substantially different from their solid counterparts.^1–12 In particular, the HNMNs generally display significantly improved catalytic activity and durability due to their high surface-to-volume ratio, abundant edge and corner atoms with open coordination sites, special confinement effect of reaction intermediates in the cavity, favorable mass transfer, and less Ostwald ripening/aggregation.^13–29 At present, the sacrificial template method has been successfully developed to fabricate the HNMNs using various inorganic or organic sacrificial templates, such as polystyrene,²⁹ silica,^{8,25,27,30–37} and metal nanocrystals.^{22–25,38,39} Among them, silica nanostructures are one of most common templates due to easy synthesis.^8,25,31,33 However, the silica template method generally suffers from the tedious template dissolution post-treatment procedure using acid/alkali. Thus, it is highly desirable to develop a facile in situ silica template removal method to synthesize the HNMNs.

Recently, Wang demonstrated that silica nanoparticles could be etched by salt in weak acidic solution under hydrothermal conditions,⁴⁰ which might open a new approach for the one-pot synthesis of the HNMNs. Here, we reported a facile one-pot synthetic method to prepare the hollow and porous Pt nanotubes (Pt-HPNTs) using K₂PtCl₄ as reaction precursor, solid core SiO₂ nanorods as template, 1-(2′-hydroxylethyl)-3-methylimidazolium tetrafluoroborate ionic liquid (HOEtMIMBF₄, Scheme S1 in ESI†) as precipitator and reductant, and in situ generated KCl as etching agent. Compared with the commercial Pt black, the as-prepared Pt-HPNTs exhibited significantly enhanced catalytic activity for the reductions of 4-nitrophenol and K₃[Fe(CN)₆].

In a typical synthesis, HOEtMIMBF₄ ionic liquid, K₂PtCl₄, and SiO₂ nanorods were mixed and heated to 80 °C for 10 h under magnetic stirring. After further elevating temperature to 180 °C for 15 h, the Pt-HPNTs were directly obtained (see Experimental in ESI for details†). The chemical composition of the products was first analyzed by energy dispersive X-ray (EDX) measurement. The EDX spectrum clearly shows the characteristic peak of the Pt element but no characteristic peak of the Si element is detected (Fig. 1A), indicating the SiO₂ nanorods have been removed during the synthesis of the products. In X-ray diffraction (XRD) measurement, the diffraction peaks of the final products well match with the standard value (JCPDS standard 04-0802) of the Pt crystal (Fig. 1B), indicating the Pt^II precursor has been reduced successfully and the as-prepared Pt nanostructures have the face centered cubic structure. According to Scherrer equation, the average size of the Pt grains in the Pt nanostructures is calculated to be 4.7 nm using Pt(111) peak data. The reduction of the Pt^II precursor was further confirmed by X-ray photoelectron spectroscopy (XPS). According to peak-fitting curves (Fig. 1C), the percentage of metallic Pt in the as-prepared Pt nanostructures is calculated to be 79.55%, demonstrating the successful reduction of Pt^II precursor. The existence of Pt oxide mainly originates from the oxidation of Pt nanoparticles by O₂ in the air. The architecture feature of the as-prepared Pt nanostructures was investigated by N₂ adsorption/desorption test. N₂ adsorption/desorption isotherm shows type II isotherms with H3 hysteresis loop, which is typical characteristic of porous nanomaterials (Fig. 1D). The pore size distribution curve shows the as-prepared Pt nanostructures have wide pore size distributions from 7 to 220 nm (insert in Fig. 1D). Due to their porous structure, the as-prepared Pt nanostructures show the big Brunauer–Emmett–Teller (BET) surface area (17 m² g⁻¹), which is comparable to commercial Pt black (ca. 18 m² g⁻¹).^41–43 In XPS measurement, the characteristic N 1s and F 1s signals are detected (Fig. S1 in ESI†), indicating HOEtMIMBF₄ ionic liquid (or its oxidation products) still adsorb on the surface of the Pt-HPNTs, which may reduce the measurement value of BET surface area.

Fig. 1 (A) EDX spectrum, (B) XRD pattern, (C) Pt 4f XPS spectrum, and (D) N₂ adsorption/desorption isotherm curve and pore diameter distribution curve of the products.

The morphology and structure of the as-prepared Pt nanostructures were investigated by scanning electronic microscopy (SEM) and transmission electron microscopy (TEM). SEM image clearly shows the as-prepared Pt nanostructures have typical hollow tube structure, and the Pt nanotubes are approximately 60 nm in diameter and 140 nm in length (Fig. 2A), matching the average diameter and length of the SiO₂ nanorods template (Fig. S2 in ESI†). TEM image clearly shows the tube wall of the Pt nanotubes is a porous structure and the wall thickness is ca. 10 nm (Fig. 2B). Further high resolution TEM (HRTEM) image indicates the as-prepared hollow and porous Pt nanotubes (Pt-HPNTs) are consist of homogeneous Pt nanocrystals with 4–6 nm size (Fig. 2C). The corresponding selected area electron diffraction (SAED) pattern indicates the as-synthesized Pt-HPNTs are polycrystalline (insert in Fig. 2C). The structural details of Pt grains in tube wall are further analyzed by magnified HRTEM image (Fig. 2D). The lattice fringes with interplanar spacing of 0.228 and 0.192 nm are clearly observed, which can be readily indexed to the Pt(111) and Pt(100) facets, respectively. Meanwhile, a great deal of defects (such as boundary, vacancy and dislocation) also are observed at magnified HRTEM images (Fig. 2D).

Fig. 2 (A) SEM, (B) TEM, (C) HRTEM, and (D) magnified HRTEM images of the Pt-HPNTs. Insert in (C) is corresponding SAED pattern.

The fabrication process and formation mechanism of the Pt-HPNTs are illustrated in Scheme 1. Firstly, Pt^II precursor was anchored at the SiO₂ nanorods surface with assistance of HOEtMIMBF₄ ionic liquid at 80 °C. Then, Pt^II precursor was reduced by HOEtMIMBF₄ ionic liquid and SiO₂ nanorods were simultaneously removed by salt etching at 180 °C.

Scheme 1 Schematic illustration for the synthesis of the Pt-HPNTs.

Photographs of actual system show HOEtMIMBF₄ ionic liquid can react with K₂PtCl₄ to generate gray HOEtMIMBF₄–Pt^II complex precipitate (Fig. 3A). Meanwhile, no characteristic diffraction peaks of Pt crystal are observed in XRD pattern of HOEtMIMBF₄–Pt^II complex precipitate (Fig. S3 in ESI†), indicating Pt^II precursor can't be reduced by HOEtMIMBF₄ ionic liquid at 80 °C. Interestingly, the insoluble HOEtMIMBF₄–Pt^II complex precipitate has good affinity for the SiO₂ nanorods. As shown in TEM image, a layer of HOEtMIMBF₄–Pt^II complex precipitate with a thickness of 13–17 nm uniformly coats the SiO₂ nanorods after heating the mixture of HOEtMIMBF₄ ionic liquid, K₂PtCl₄ and SiO₂ nanorods at 80 °C for 10 h (Fig. 3B). The large-area EDX mapping images show N, O, Cl, and Pt element patterns are similar and bigger than Si element pattern (Fig. 3C), confirming the uniform distribution of HOEtMIMBF₄–Pt^II complex precipitate on the SiO₂ nanorods. Generally, SiO₂ nanoparticles are negative charged due to the hydroxyl groups. Zeta potential test shows HOEtMIMBF₄–Pt^II complex precipitate is positive charged (zeta potential: 11.2 mV). Thus, the electrostatic interaction may be responsible for the uniform deposition of HOEtMIMBF₄–Pt^II complex precipitate on the SiO₂ nanorods. After evaluating reaction temperature to 180 °C, –OH groups at HOEtMIMBF₄ ionic liquid begin to show the reducibility. As confirmed by XRD measurement, Pt nanoparticles can be obtained by heating HOEtMIMBF₄–Pt^II complex precipitate at 180 °C for 15 h (Fig. 3D). In contrast, Pt nanoparticles can't be obtained by heating the mixture of K₂PtCl₄ and 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid without –OH group (BMIMBF₄, Scheme S2 in ESI†) under same conditions (Fig. S4 in ESI†), confirming the reduction of HOEtMIMBF₄–Pt^II complex precipitate originates from the reducibility of –OH group in HOEtMIMBF₄ ionic liquid. Meanwhile, the pH value of reaction system decreases to 4.5 from initial 6.0 after reaction, which may be ascribed to the formation of –COOH group due to the oxidation of –OH group. Upon HOEtMIMBF₄–Pt^II reduction, Cl⁻ ion is released, resulting in the existence of KCl in reaction system. Photographs of actual system show KCl can effectively etch SiO₂ nanorods at 180 °C under weak acidic conditions, resulting in the dissolution of the SiO₂ nanorods (Fig. 3E). Thus, during synthesis of the Pt-HPNTs, the in situ removal of the SiO₂ nanorods originates from the KCl etching. In particular, hollow and porous Pd nanotubes can also be easily obtained by replacing K₂PtCl₄ with K₂PdCl₄ as reaction precursor under same experimental conditions (Fig. S5 in ESI†), indicating the ionic liquid-assisted one-pot synthetic method has good universality.

Fig. 3 (A) Representative photographs of mixture of HOEtMIMBF₄ ionic liquid and K₂PtCl₄ (a) before and (b) after heating 10 h at 80 °C. (B) TEM images of (a) pristine SiO₂ nanorods and (b) HOEtMIMBF₄–Pt^II complex@SiO₂ nanorods composites prepared by heating mixture of HOEtMIMBF₄ ionic liquid, K₂PtCl₄, and SiO₂ nanorods at 80 °C for 10 h. (C) EDX maps of HOEtMIMBF₄–Pt^II complex@SiO₂ nanorods composites. (D) XRD pattern of the Pt nanoparticles prepared by heating HOEtMIMBF₄–Pt^II complex precipitate at 180 °C for 15 h. (E) Representative photographs of mixture of the SiO₂ nanorods and KCl (a) before and (b) after heating 15 h at 180 °C.

The catalytic reductions of 4-nitrophenol and K₃[Fe(CN)₆] by NaBH₄ were used as model heterogeneous catalytic reaction^44,45 and electron transfer reaction^46,47 to examine the catalytic performances of the Pt-HPNTs. The advantage of the two model reactions is the reaction rate can be easily evaluated by the ultraviolet-visible (UV-vis) spectra. Fig. 4A and B display the peak intensity changes of 4-nitrophenol at 400 nm after the injection of the Pt black and Pt-HPNTs, respectively. As shown in Fig. 4, the reduction of 4-nitrophenol at the Pt-HPNTs is finished within 20 min, which is 3 times faster than that at Pt black. Compared with the Pt black, the as-prepared Pt-HPNTs exhibit enhanced catalytic activity for the reduction of 4-nitrophenol. The reaction rate constant (κ) is calculated according to the linear plots of ln(C_t/C₀) versus time t using the equation −kt = ln(C_t/C₀), where C_t and C₀ represent the initial concentration of 4-nitrophenol at t = 0 and the concentration of 4-nitrophenol at time t, respectively. Calculation results demonstrate the Pt-HPNTs exhibit the superior catalytic activity for 4-nitrophenol reduction than commercial Pt black (κ_{the Pt-HPNTs}: 0.100 min⁻¹ vs. κ_{Pt black}: 0.041 min⁻¹). Meanwhile, the κ value (0.100 min⁻¹) is also higher than those of the Au/Au–polythiophene core–shell nanospheres (κ: 0.039 min⁻¹),⁴⁸ Pt–NCs/rGO (κ: 0.0545 min⁻¹),⁴⁹ and Pt nanocubes (κ: 0.064 min⁻¹),⁵⁰ under the same reaction conditions, further confirming the Pt-HPNTs have excellent reactivity for the 4-nitrophenol reduction. Additionally, it is observed that the as-prepared Pt-HPNTs also exhibit enhanced catalytic activity for K₃[Fe(CN)₆] reduction compared to commercial Pt black (Fig. S6 in ESI†). As shown in HRTEM image (Fig. 2D), the interconnected and porous structures endow the Pt-HPNTs with larger amounts of defect atoms, which can effectively enhance reactivity due to the particular open coordination nature of defect atoms.^51–53

Fig. 4 Successive UV-vis spectra of 4-nitrophenol reduction by NaBH₄ using (A) Pt black and (B) the Pt-HPNTs as catalysts. Inserts in (A) and (B): ln(C_t/C₀) versus time plot for 4-nitrophenol reduction.

In summary, a facile ionic liquid-assisted one-pot synthetic route was first developed to synthesize the Pt-HPNTs using SiO₂ nanorods as template. Meanwhile, hollow and porous Pd nanotubes can also be synthesized by using the ionic liquid-assisted one-pot synthetic route, showing its universality. The experimental results demonstrate the salt etching at high temperature is responsible for the in situ removal of SiO₂ template during the synthesis of the HNMNs. Additionally, the as-prepared Pt-HPNTs exhibit improved catalytic performance for reductions of 4-nitrophenol and K₃[Fe(CN)₆] compared to Pt black, demonstrating their potential applications in catalysis.

Acknowledgements

This work was supported by National Natural Science Foundation of China (21301114), Foundation of the Jiangsu Education Committee (14KJD150002), Environmental Science and Technology Innovation Team of Education department of Jiangsu province & Jiangsu Second Normal University, the Academic Research Fund of the Ministry of Education in Singapore (RGT27/13).

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Footnotes

† Electronic supplementary information (ESI) available: Experimental procedure and additional figures. See DOI: 10.1039/c6ra13451c

‡ These two authors made an equal contribution to this work.

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