Ultra-fast self-assembly and stabilization of reactive nanoparticles in reduced graphene oxide films

Abstract

Nanoparticles hosted in conductive matrices are ubiquitous in electrochemical energy storage, catalysis and energetic devices. However, agglomeration and surface oxidation remain as two major challenges towards their ultimate utility, especially for highly reactive materials. Here we report uniformly distributed nanoparticles with diameters around 10 nm can be self-assembled within a reduced graphene oxide matrix in 10 ms. Microsized particles in reduced graphene oxide are Joule heated to high temperature (∼1,700 K) and rapidly quenched to preserve the resultant nano-architecture. A possible formation mechanism is that microsized particles melt under high temperature, are separated by defects in reduced graphene oxide and self-assemble into nanoparticles on cooling. The ultra-fast manufacturing approach can be applied to a wide range of materials, including aluminium, silicon, tin and so on. One unique application of this technique is the stabilization of aluminium nanoparticles in reduced graphene oxide film, which we demonstrate to have excellent performance as a switchable energetic material.

Introduction

From the Lycurgus cup as early as the fourth century to modern cancer sensing and energy storage, nanoparticles are widely used in physical1,2,3, chemical4,5,6, environmental7,8, biological9,10 and medical applications11,12, owing to their interesting size-dependent properties. Numerous scalable and low-cost methods for making high-quality nanoparticles have been reported and many types of nanoparticles are commercially available13,14. These synthesis methods can be generally classified into two categories. The first category features ‘top-down’ approaches, usually solid-phase methods, including ball milling15,16, pulsed laser ablative deposition17,18,19, exploding wire13,20,21 and so on. The second category features ‘bottom-up’ approaches, such as chemical and physical vapour deposition22,23 and liquid-phase chemical synthesis24,25,26,27,28,29,30,31,32,33. Although many of these approaches have been quite successful, there are significant challenges that still exist. This is especially true for non-noble metals that can be highly reactive and tend to suffer from surface oxidation and agglomeration, which can significantly constrain nanoparticle functionality34,35,36. A fast, low-cost and scalable approach to manufacturing uniformly distributed nanoparticles, especially for energetic nanoparticles, without these impairments is urgently needed. At the same time, nanoparticles hosted in a conductive matrice are ubiquitous in electrochemical energy storage, catalysis and energetic devices, where a conductive matrix can provide fast electron transport for the operations of the devices. Usually, nanoparticles are synthesized first and then assembled into a conductive carbon matrix37. For reactive nanoparticles, significant oxidation often occurs during the two steps, which presents a major challenge to the effective use of these nanoparticles38.

Here we report an ultra-fast (as fast as 2 ms) process to produce uniformly distributed nanoparticles in a conductive reduced graphene oxide (RGO) matrix by directly Joule heating a metal/semiconductor-RGO film (Fig. 1a) to a high temperature (∼1,700 K or higher). Microsized metal or semiconductor particles used as precursors are melted under high-temperature Joule heating, then self-assemble into nanoparticles on cooling. Agglomeration and coalescence of the nanoparticles, which is driven by surface energy minimization, is suppressed by the defects of RGO in-plane and RGO layers out-of-plane (Fig. 1b). These defects serve as barriers for atom migration, to keep the uniformly distributed nanoparticles separated. The particles within the matrix during heating are protected from oxidation by the RGO sheets, owing to the impermeability of RGO to O2 and H2O39. Thus, RGO sheets serve as a perfect host material for such unique high-temperature process, owing to their defect sites and high melting temperature (stable up to 3,300 K)40,41,42,43,44,45. Furthermore, this high-temperature process for in situsynthesis of nanoparticles without agglomeration and surface oxidation is applicable to any materials with a lower melting point than 3,300 K. In this study, we demonstrate fast nanoparticle formation for aluminium (Al), silicon (Si), tin (Sn), gold (Au) and palladium (Pd). These materials in their nanoparticle form have been widely used in energetics, energy storage, optical, sensing and catalytic applications. We choose nAl-RGO to demonstrate its superior properties as an energetic material. Furthermore, we combined three-dimensional (3D) printing with this technique for both precise shape control and scalable manufacturing of nanoparticles-RGO architectures (Fig. 1c).

Figure 1: Schematic of in situ nanoparticles self-assembly process.

(a) Microsized particles in a conductive RGO network matrix (left) are self-assembled into nanoparticles (right) driven by direct Joule heating for ∼10 ms. (b) Proposed mechanism for nanoparticle formation, where a microsized particle melts on heating and self-assembles into nanoparticles due to confinement by the defects of the RGO sheet. (c) The raw materials are 3D printable, where nanoparticles can be formed in a conductive RGO matrix.