Ultrasound-assisted fabrication of metal nano-porous shells across polymer beads and their catalytic activity for reduction of 4-nitrophenol
Graphical abstract
Introduction
Nano-sized particles of noble metals have attracted wide attention in various fields of chemistry because of their unique physicochemical properties [1]. Leveraging on the characteristic of high surface-to-volume ratios, these nanoparticles (NPs) provide high reaction sites for the efficient chemical/physical adsorption with an outstanding chemical reactivity [2], [3], [4], [5]. On the other hand, the size of NPs restricts the direct application in the fields such as catalysis and sensors due to the difficulties associated with particle selection and separation that can lead to toxicity concerns [5], [6], [7].
The deposition of metal NPs onto templates is therefore a promising approach to gain control of NPs location while maintaining the advantages of the high reactivity of nano-sized material. Different methodologies have been developed to generate NPs onto solid substrate surfaces, such as chemical vapor deposition (CVD), sputtering, chemical reduction and lithography, leading to highly ordered 2D and 3D architectures [8]. Such design has resulted in complex structures formation such as core-shells, hollow spheres, nano-cages, or porous films [9].
Amongst the different techniques available, ultrasound-assisted or sonochemical methods were shown to be promising for NPs synthesis due to its facile operation, mild reaction condition and fast kinetics [1], [3], [4], [5], [10], [11], [12], [13], [14]. Synthetic routes have been developed to synthesize noble metal NPs made of silver [15], [16], [17], platinum [1], [18], [19], palladium [20], [21] or ruthenium [12], offering advantages over other traditional reduction methods such as reductant-free and ambient temperature reaction. Sonochemical synthesis of ultra-small NPs, with diameters below 10 nm, is particularly efficient since no additional chemical reducing agents are required for the reduction steps [22], [23]. Furthermore, ultrasound kinetics also offers a facile route for the fabrication of bimetallic structures sequentially or simultaneously [4], [11], [23], [24], to develop metal alloys or metal oxide nano-composites.
The synthesis of Au NPs under ultrasonic irradiation in aqueous medium has been well studied including the effect of ultrasound frequency, power input, types of atmospheric gas and reducing agent [3], [5], [14], [25], [26], [27], [28]. The presence of ultrasound irradiations not only affects the yield of Au-NPs production but also determines the pathways used to optimize cavitation-induced reactions [29]. However, the controlled reduction and location of these NPs remains a challenge and no systematic study has elucidated the impact of ultrasound irradiation and reaction parameters on the metal NPs deposition kinetics and control. The fabrication of noble metal NPs on PS spheres was demonstrated using 20 kHz horn transducer [30] while metal/metal oxide templates were also formed by iron oxide deposition across submicron-spherical alumina templates using a 20 kHz ultrasound [31]. In such studies, the low surface coverage, large NPs size distributions and the lack of tunability of the interface and deposition homogeneity were found to be challenges. Therefore, the full potential of sonochemical methods for metal NP deposition has not yet been revealed and novel approaches to evaluate the mechanisms of coating are required.
In this work, a cost-effective approach of fabricating Metal nanoporous Networks (MNM) architectures onto supportive discrete templates via ultrasonic irradiations using a gold and poly(styrene) (PS) beads system is demonstrated. The novel seeding and shell growth processes were conducted under two applied frequencies. The catalytic capability of the MNMs is demonstrated for the reduction of 4-nitrophenol as a model reactant but opens the route to the design of efficient nano-reactors highly controllable microstructures.
Section snippets
Materials
Gold(III) chloride trihydrate (HAuCl4·4H2O, 99.99%), isopropyl alcohol ((CH3)2CHOH, > 99.7%), amine-modified polystyrene beads solution (average diameter of 1 µm) were purchased from Sigma-Aldrich and used directly with no further purification. 4-nitrophenol (4-NP, ReagentPlus®, ≥99% purity) and sodium borohydride (NaBH4, ReagentPlus®, 99% purity) were obtained from Sigma-Aldrich (Milwaukee, WI, USA). Water used in all experiments was obtained from a Milli-Q Academic purification system with a
Sonochemical Au NPs seeding and shell growth
As shown across the TEM images in Fig. 1, Au-NPs were successfully synthesized and coated on the PS beads. Compared to virgin PS beads shown in Fig. 1C and D, a uniform deposition of Au NPs can be spotted across the beads surface. The average diameter of the Au-NPs was found to be around 7 nm, with a narrow size distribution from 4 to 10 nm. Conventional reduction methods usually involve a two-step process, where the Au NPs are required to be synthesized prior to the seeding step [7]. Such
Conclusions
In this work, a novel and cost-effective sonochemical metal template deposition method was developed to synthesize MNMs architectures. A one-pot seeding and fully sonochemical shell growth process was applied by two different frequencies. The seeding process shows a uniform and relatively high template surface coverage with a small average size around 7 nm and narrow size distribution. An increasing surface porosity and roughness can be achieved by employing 1 MHz ultrasonic irradiation. The
Acknowledgement
The financial support through the Australian Research Council for the DECRA (Discovery Early Career Research Award, DE120101567) is gratefully acknowledged.
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