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Invited Feature Article
Digestive Ripening Facilitated ‘NanoEngineering’ of Diverse Bimetallic Nanostructures Chirasmita Bhattacharya, Neha Arora, and Balaji R. Jagirdar Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02208 • Publication Date (Web): 10 Sep 2018 Downloaded from http://pubs.acs.org on September 10, 2018
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Digestive ripening facilitated 'nano-engineering' of diverse bimetallic nanostructures Chirasmita Bhattacharya, † Neha Arora,†‡ and Balaji R. Jagirdar* † †
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India (Dedicated in memory of Professor Kenneth J Klabunde)
ABSTRACT From an ingenious methodology for obtaining monodispersity, digestive ripening has advanced to become an outstanding solution based synthetic route to realizing various bimetallic heterostructures. This feature article attempts to provide an overview of the various facets of co-digestive ripening process and the array of heterostructures that could be achieved by this technique. We briefly discuss the mechanism of digestive ripening in the case of monometallic elements and use that understanding to elucidate the mechanism of the less established co-digestive ripening strategy for designing bimetallic nanostructures. The systems studied by our group in the last decade towards the fabrication of diverse heterostructures are highlighted in this article. Exploitation of digestive ripening to realize monodisperse bimetallic nanostructures by several other groups are also featured. In addition to digestive ripening agents, the significance of tuning various reaction parameters and its consequences on the final structure and morphology has also been discussed. Additionally, efforts based on theoretical studies to get
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an insight into the factors which dominate the mechanism of digestive ripening process have also been covered. This article is a contribution towards an understanding of the co-digestive ripening methodology and a demonstration of its tremendous potential in achieving desired bimetallic hetero nanostructures.
INTRODUCTION
Synthesis of metal nanoparticles has been intensively pursued over the last three decades, primarily due to their unique properties which are considerably different from those of their bulk counterparts and their diverse applications.1,2 Metal nanoparticles exhibit various prominent properties including optical, electronic, magnetic, and catalytic which could be associated with the quantum-confinement effects and high specific surface area realized upon approaching the nanosize regime. 3-6 In addition to metal nanoparticles, there has been significant interest in the synthesis of welldefined bimetallic nanostructures.7 The bimetallic systems usually exhibit synergistic properties arising from the corresponding monometallic components, endowing them with multifunctional applications.8-10 The physicochemical properties of bimetallic nanostructures are highly dependent on the nature and atomic ordering of the different metal atoms giving rise to different bimetallic architectures, such as core-shell, heterostructure, ordered intermetallic and random alloy structures.11,12 Advances in controlling the size and shape of different nanoparticles have been significant, however, a more precise and synchronized control over nucleation and growth processes of two discrete metals is required to realize the desired bimetallic nanostructures. In fact, to exploit the true potential of bimetallic nanoparticles, it is essential to have a precise control over their size. Consequently, synthesis of monodisperse bimetallic nanoparticles has
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been widely investigated.13 In general, bimetallic structures are obtained via complex solid-state methods involving high-temperature annealing protocols for long duration with a poor control over phase as well as size and shape.14,15 Bimetallic nanoparticles have also been obtained using bimetallic cluster complexes as precursors via controlled reduction or thermolization for removal of ligand/ ligand fragment moieties.16-19 The gaseous state methods including laser beam, pulsed arc, ion-sputtering are tedious and not cost-effective.20,21 Solution-based methods arguably have the advantage of better control over the nucleation and growth processes. LaMer and Dinegar described that “the separation of nucleation and growth stages” is necessary for the synthesis of monodisperse nanoparticles.22 Since the pioneering work by Klabunde and co-workers describing highly ordered 2D and 3D superlattices of gold nanocrystal colloids,23 the synthesis of monodisperse nanoparticles of dimensions on the order of a few nanometers (nm) using a post synthetic protocol termed as digestive ripening, has come a long way.24 Addition of dodecanethiol ligand to a polydisperse gold colloid followed by heating under reflux, afforded thiolate capped monodisperse gold nanoparticles. Digestive ripening process essentially involves first, the addition of surface active ligand, also termed as digestive ripening agent to a polydisperse colloid and finally refluxing the colloid at or near the solvent boiling point. The larger particles break down to smaller particles which eventually attain an equilibrium size that is dictated by how well the digestive ripening or capping agent fits in the crystal lattice, thus leading to the formation of a nearly monodisperse colloid. Digestive ripening is not a sacrificial process and monodisperse colloids are obtained without material loss.25 Prasad et al. demonstrated that in addition to thiols, amines, silanes, and phosphines can be used efficaciously for the digestive ripening process.26 Further modifications and processing of digestive ripening led to the realization of a variety of nanostructures
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exhibiting different compositions.27,28 Recently, the digestive ripening procedure has been discussed in a comprehensive review by Prasad and co-workers.29 Our group also demonstrated the efficiency of digestive ripening process in producing a wide variety of bimetallic nanostructures.30-32 In recent years, tremendous research efforts have been devoted to the development of controllable synthesis of bimetallic nanostructures. In this article, we will focus on the solution based digestive ripening process for bimetallic nanostructures. Firstly, we summarize the mechanism underpinning the formation of monodisperse nanoparticles using digestive ripening process. In the following sections, we review the synthesis of various kinds of bimetallic nanostructures. Furthermore, theoretical studies addressing the digestive ripening phenomenon will also be discussed.
An insight into the mechanism of digestive ripening Ever since the introduction of digestive ripening process by Klabunde and co-workers,23 there have been significant efforts by researchers to understand its underlying mechanism. Influence of several aspects such as digestive ripening agent, solvent, temperature, time and electric field on the digestive ripening process has been investigated in the past to understand the reaction pathways involved.26-28, 33-35 Although a majority of the studies have been conducted using Au nanoparticles, the versatility of this process was proven by achieving monodisperse nanoparticles of other monometallic systems such as Ag, Cu, In, and Pd.24,36-38 In addition to monometallic systems, digestive ripening approach has been successfully extended to a wide variety of nanomaterials like CdSe, CdTe, CuO, FeS2, FeCoS2, lanthanide oxides, etc.39-44 These studies have assisted in getting an insight into the mechanism of digestive ripening process.
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A systematic study was carried out by Bhaskar and Jagirdar using Au nanoparticles as a model system to elucidate the mechanism of digestive ripening.45 They described a combination of solvated metal atom dispersion (SMAD) method and digestive ripening to study the effects of alkyl chain length, concentration of capping agent, and room temperature ripening on the course of the digestive ripening process. In this study, amines of different chain lengths, octadecyl amine (ODA), hexadecyl amine (HDA), and dodecyl amine (DDA) were used as digestive ripening agents. It was noted that DDA owing to its shorter alkyl chain length compared to those of ODA and HDA, was not found to be an effective ripening agent. Although complete precipitation of particles was observed with lower metal:DDA ratio (1:10), raising it to 1:20 and 1:30 did lead to the formation of nearly monodisperse nanoparticles. The final size range of nanoparticles obtained with DDA and ODA were comparable (10-13 nm), whereas, smaller sized nanoparticles were realized using HDA (7.5-9.5 nm). Two studies carried out by Klabunde and co-workers, one involving amines and the other involving thiols to understand the effect of chain length showed contrasting trends in the final size of nanoparticles upon getting ripened.46,47 Thiols with longer chain lengths (C16) result in the formation of well separated nanoparticles, while the ones with shorter chain lengths like C8 and C10 lead to aggregates to form superlattices (Figure 1). This was rationalized by taking into consideration the reduction in attraction energies between particles with an increase in the chain length, thus forming separate particles. An increase in the chain length of thiols resulted in an increase in the average particle size, 46 whereas, a variation of chain length of amines from C4 to C18 lead to a decrease in the particle size of gold nanoparticles.47 Thus, the final size distribution is a manifestation of two opposing forces: curvature-dependent surface energy and ligand−metal binding energy.
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Figure 1. Bright-field TEM images of gold colloid after digestive ripening with (a) hexadecanethiol and (b) dodecanethiol. Adapted with permission from ref 46 (Copyright 2002 American Chemical Society) Monodisperse nanoparticles of highly reactive metals like Mg, Ca and Al have been synthesized by simple ‘room temperature’ digestive ripening in presence of capping agent.48-50 This is a manifestation of the arrest of the growth of small clusters by the capping agent before uncontrolled growth could take place, thereby resulting in highly monodisperse particles. Attempts were made to extend this to as-prepared Au nanoparticles.45 An interesting observation in the digestive ripening process of Au nanoparticles carried out at room-temperature was the observation of ‘dot particles’ (
