Review
Recent trends on nanocomposites based on Cu, Ag and Au clusters: A closer look

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Abstract

Nanocomposite materials based on metal nanoparticles (NPs, guest) in/on oxide matrices (host) have attracted increasing attention thanks to their intriguing chemico-physical properties that can be tailored as a function of NP size, shape and mutual interactions. The possibility to obtain a controlled dispersion of metal particles in/on suitable oxides (inside- and outside-cluster systems, respectively) paves the way to a broad spectrum of technological applications, ranging from heterogeneous catalysis, to gas sensing and non-linear optics. The control of functional performances relies on tailoring the system properties by design through a suitable choice of the synthesis and processing routes.

In this context, the present review provides a synoptic overview on our recent research activity concerning nanocomposites containing 11th group metal clusters (Cu, Ag and Au), dispersed in/on oxide matrices (SiO2, TiO2, Al2O3). We begin by briefly outlining the interest and size-dependent properties of such systems. Subsequently, the attention is switched to a survey on the preparation strategies previously adopted in the literature, focusing in particular on the use of soft methods, such as sol–gel (SG), RF-sputtering and their innovative combination. The most relevant results on M′/MxOy nanocomposites (where M′ = Cu, Ag, Au and M = Si, Ti, Al) obtained by these routes are then comparatively discussed, highlighting analogies and differences between them. Finally, the most attractive research perspectives in the field are briefly presented.

Introduction

The last decade has witnessed an exponential growth of research activities on composite materials based on metal NPs (guest) in/on suitable oxide matrices (host), thanks to the possibility of tailoring their chemico-physical properties as a function of particle size, shape and distribution [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. This widespread scientific and technological interest has been mainly driven by the concurrence of two causes, namely the full understanding of nanocomposite behavior and the potential hope for functional applications and economic impact. As a matter of fact, the novel features acquired on the nano-dimensional regime are not a mere result of scaling factors [15], but rather stem from the confinement of charge carriers [16] and the enhancement of the surface-to-volume ratio on decreasing NPs dimensions [8], [15], [17], [18], resulting in a spectrum of size-dependent properties widely diversified from the corresponding bulk counterparts [3], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. Studies on this evolution might help to elucidate how properties depend on size in the regime between molecules and solid-state materials [19], [29].

Unique characteristics are also derived from interfacial NP/matrix interactions, that might appreciably depend on the presence of defects, compositional gradients and roughness and play a pivotal role on the system functional performances [7], [8], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40]. Moreover, the chemical composition and surface structure of guest NPs might result unstable due to their high reactivity and energy content [16], leading thus to a progressive alteration of the system properties under prolonged use. As a consequence, a full exploitation of the nanosystem potential in advanced device structures relies on a thorough understanding of their stability and general properties, with particular regard to their structure, chemical composition and morphology.

In this context, materials based on clusters of 11th group metals (Cu, Ag, Au) have fascinated people for centuries, providing colors for the Medieval cathedral windows, as well as for vases, pottery and other ornaments [15], [16], [19], [41], [42], [43], [44], [45]. Actually, these systems stimulate considerable interest in various research fields, due to their intriguing chemico-physical characteristics and their potential application in catalysis, microelectronics, sensing, magnetism, photonics and energetics [12], [16], [25], [46]. More specific examples include labels for biomolecules, bio- and gas sensors, ultrafast optical switches, optical filters and tweezers [8], [11], [19], [20], [26], [47], [48], [49]. In particular, as metal particles are reduced in size to tenths of nanometers, a dramatic change in their optical properties occurs, resulting from the collective oscillation of electrons in the conduction band [50]. This phenomenon, the so-called SPR, leads to a characteristic absorption of the visible light [16], [43], [47]. The frequency and intensity of the SPR signal are typical of the type of hostguest materials [50] and are highly sensitive to the size, size distribution and shape of the NPs [51], as well as to the nature of the surrounding medium [52]. This direct correspondence between NP features and their optical response, resulting in the so-called MDRs [53], has prompted the ongoing intense interest in the optical properties of Cu, Ag and Au clusters dispersed in/on various media, fueling the construction of SPR-based sensors and devices in ever increasing variety.

Among the various supporting/embedding hosts for such metal NPs, metal oxides have received a significant attention [54], owing to their different structures and chemico-physical characteristics. In particular, oxide matrices such as SiO2, TiO2 and Al2O3 have been thoroughly investigated thanks to their amenable features, such as optical transparency in the visible range, insulating character, structural and thermal stability and weak interactions with guest particles [11], [30], [31], [55], [56], [57], [58].

Hostguest systems containing gold and silver NPs have been the subject of several investigations such as heterogeneous catalysis [4], [5], [20], [35], [36], [38], [48], [59], [60], [61], [62], [63], [64], [65], [66], [67], [68], gas sensing [6], [35], [49], [69], [70], as well as optics and optoelectronics [2], [5], [14], [57], [61], [71], [72], [73], [74], [75], [76], [77], [78], [79], thanks to the peculiar properties arising when their size is comparable or lower than the electron mean free path [16], [18], [80], [81], [82], [83]. Specific examples in these fields concern SERS experiments, where such NPs are frequently used for signal enhancement [26], [46], [51], [84], [85]. Au- and Ag-based nanocomposites have also been employed in a variety of catalytic processes such as CO oxidation, NOx reduction and water gas shift reaction [13], [20], [21], [33], [36], [48], [70], [86], [87], [88], [89], [90], [91], [92]. Furthermore, oxide-based materials containing Cu, Ag and Au nanoclusters are attractive candidates for the design and development of integrated all-optical devices with controllable non-linear properties [2], [27], [74], [75], [83], [93], [94], [95], [96], [97], [98], [99], [100], thanks to their fast response times and large third-order non-linear susceptibility (χ(3)) values, that are strongly enhanced near the SPR frequency [74], [75], [76], [77], [78], [79], [81], [94], [96], [101], [102], [103], [104], [105], [106].

Beyond non-linear optical devices, Au/TiO2 have been increasingly employed in photoelectrochemical solar cells, thanks to the well-known photocatalytic properties of TiO2 [107], [108] that are further enhanced by gold dispersion [109]. Dye-sensitized nanocrystalline TiO2 solar cells have achieved sunlight-to-electrical power conversion efficiencies greater than 9% and photocurrents >16 mA cm−2 [110].

In this review, we present the synthesis and properties of oxide-based nanocomposites containing 11th group NPs, focusing on the most relevant results obtained by our research team in the last years [53], [111], [112], [113], [114], [115], [116], [117], [118]. A schematic representation of the nanocomposites that are the subject of the present work is proposed in Fig. 1. The major difference between inside-cluster (Section 3, Ag/SiO2 and Cu/SiO2) and outside-cluster systems (Section 4, Ag/SiO2 and Au/SiO2) is represented by the spatial distribution of guest metal NPs in the host matrix. While in the first case, the aggregates are dispersed inside the SiO2 host that also serves as a protective medium for them, in the latter case they are distributed only on the silica surface. Au/TiO2 nanocomposites are mid-way between these two groups, gold NPs being present both on the surface and sub-surface titania layers. Obviously, the technological applications and intrinsic features of these systems are different. Whereas inside-cluster composites are mainly interesting for optical devices, outside-cluster ones are also of relevance in the field of heterogeneous catalysis and sensing.

A further system modulation can be achieved by the controlled assembly of metal nanoparticles into more complex shaped structures [16], [119]. In a similar philosophy, NPs are regarded as small “building blocks” that are hierarchically organized in super-structures by means of suitable preparation routes [19]. Among the various kinds of NPs-based architectures, specific examples are represented by 0D, 1D and 2D nanosystems including, for instance, quantum-dots, nanowires/nanotubes and 2D arrays [119], [120], [121]. To this regard, other fascinating systems, briefly discussed in the final part of this review, are represented by metal NPs dispersed in the pores of oxide membranes provided with 1D channels, resulting in the formation of Au NTs (Fig. 1). These mono-dimensional metal arrays are promising candidates in many research fields thanks to their anisotropic properties [121], [122], [123], that can be exploited in several advanced applications, such as microelectronic devices, sensors with improved performances [123], [124], [125], [126], alignment features in liquid crystal displays [127], novel heterogeneous catalysts [128], magnetic storage media with high capacities [129], [130], field-emission displays and optical energy transport systems [131].

Specifically, among the various metals, gold-based 1D nanosystems are attractive elements in device structures for their low resistivity and chemical stability, as well as in thiol-based bio-sensors in DNA chips [132] and advanced optical systems [123].

The present contribution starts with a brief introduction on the different preparation routes proposed in the literature for the synthesis of Cu-, Ag- and Au-containing nanocomposites. Due to the huge number of scientific publications in this field, this work is far from providing an exhaustive review on the previously performed research activities. Our aim is rather to present a survey of the more significant results obtained in the bottom-up synthesis of nanocomposites, focusing in particular on SG, RF-sputtering and their innovative combination. For a more detailed discussion on these methodologies, the reader can refer to specific pertinent literature.

In particular, we will comparatively report the most relevant features pertaining to the synthesis and tailoring of M′/MxOy nanocomposites (M′ = Cu, Ag, Au; MxOy = SiO2, TiO2, Al2O3), focusing on their structure, composition, morphology, optical properties and their mutual interrelations with the processing parameters. Emphasis will be given to the possibility of obtaining prescribed nanosystem properties by a proper choice of the synthesis and treatment conditions. The nanocomposite peculiarities arising from the NPs distribution and/or reactivity with the surrounding medium, also depending on the nanometric particle size, are outlined and discussed.

Section snippets

Preparation strategies for nanocomposites based on metal clusters: the role of sol–gel, RF-sputtering and hybrid RF-sputtering/sol–gel routes

An open challenge in the field of nanotechnology is the development of versatile synthetic strategies to control the nucleation of the material building blocks and their subsequent assembly on the nanometric scale into ordered 0D-, 1D-, 2D- and 3D-structures [15], [119], [123], [133], [134]. As a matter of fact, tailoring of nucleation/growth processes is a key step to improve specific system characteristics, thus allowing the design of new and more efficient functional devices provided with

Inside-cluster systems: Ag/SiO2 and Cu/SiO2 by sol–gel

The present section is devoted to the presentation of relevant results for the SG synthesis of silica coatings doped with silver [118], [212] and copper NPs [117], [213]. The as-prepared samples only contain the Ag(I) and Cu(II) cations, which can evolve to metallic particles after suitable ex situ thermal treatments. In this regard, whereas in the synthesis of Ag–silica hostguest composites metallic silver clusters are easily obtained, an important issue for the analogous copper-containing

Outside-cluster systems: Ag/SiO2 and Au/SiO2 by RF-sputtering

In the previous section, attention has been focused on the synthesis of Ag/SiO2 and Cu/SiO2 inside-cluster systems by the SG route, interesting in the field of optics. Nevertheless, applications in catalysis and gas sensing require the metal nanoparticles to be completely, or at least partially, exposed to the ambient in order for the reagents to reach the catalytic centers. While the previously proposed SG strategies are not a viable alternative for the preparation of such composites,

Between inside- and outside-clusters: Au/TiO2 by an hybrid RF-sputtering/sol–gel approach

We focus now the attention on Au/TiO2 nanosystems synthesized by an original hybrid RF-sputtering/SG route [111], as described in Section 2. In particular, titania xerogels (host) were deposited on silica slides by dip-coating from ethanolic solutions containing titanium(IV) isopropoxyde and acetylacetone. The obtained specimens, free from any detectable crystalline phase as proved by GIXRD analyses, were subsequently used in RF-sputtering depositions of gold (guest) from Ar plasmas without

From metal nanoclusters to nanotubes: RF-sputtering of gold into porous membranes

In the previous paragraphs, attention has been focused on nanocomposites containing both supported and embedded metal NPs. To this aim, tailoring of the particle size and shape [123], as well as of their surface and in-depth distribution, has been shown to be particularly important in order to control the system properties and achieve specific functional performances. In this context, several research efforts have been addressed to obtain metal 1D nanosystems, especially based on gold [123],

Concluding remarks and future outlook

This review was focused on functional nanocomposites based on 11th group metal NPs (Cu, Ag and Au, guest) in/on oxide matrices (SiO2, TiO2, Al2O3, host). Although the tailoring of elemental composition and size for NPs has been a key issue over the last decade, their controlled assembly into hierarchically organized structures is nowadays an important tool to tune the chemistry and physics of these systems. As a consequence, the research activities dedicated to the development of bottom-up

Acknowledgments

National Research Council (CNR), University of Padova and INSTM are acknowledged for financial support. We are also indebted to research programs FISR-MIUR “Molecular nanotechnologies for information storage and transmission”, FISR-MIUR “Inorganic and hybrid nanosystems for the development and innovation of fuel cells” and FIRB-MIUR-RBNE019H9K “Molecular manipulation for nanometric machines”. Thanks are also due to Prof. S. Gialanella (Department of Materials Engineering and Industrial

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