Oct 7, 2013 - unique properties distinct from those of free molecules and larger- sized nanoparticles. ..... Free (CdSe)13 nanoclusters were ... and sharp tips.
Review pubs.acs.org/cm
Synthesis, Characterization, and Application of Ultrasmall Nanoparticles Byung Hyo Kim,†,‡ Michael J. Hackett,†,‡ Jongnam Park,*,§ and Taeghwan Hyeon*,†,‡ †
Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 151-742, Korea School of Chemical and Biological Engineering, Seoul National University, Seoul 151-742, Korea § Interdisciplinary School of Green Energy, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Korea ‡
ABSTRACT: Nanomaterials in the size range of 3−50 nm have received increased attention in the last few decades because they exhibit physical properties that are intermediate to those of individual molecules and bulk materials. Similarly, ultrasmall nanoparticles (USNPs), with sizes in the 1−3 nm range, exhibit unique properties distinct from those of free molecules and largersized nanoparticles. These properties are greatly sensitive to both the composition and size of the particles, and thus, the ability to control the synthesis for both of these variables is of paramount importance. This review summarizes various methods for the synthesis of USNPs of metals, metal oxides, and metal chalcogenides as well as recent advances in the development of unique characterization methods for these USNPs. Last is a discussion of several novel applications of USNPs in biomedical imaging, catalysis, and semiconductor development, all of which benefit from the large surface-to-volume ratio and/or other characteristic properties inherent in USNPs. KEYWORDS: ultrasmall nanoparticles, surface-to-volume ratio, spin-canting effect, magic-sized nanocrystals, mass spectrometry
1. INTRODUCTION Nanoparticles (NPs) with sizes of 3−50 nm have garnered a great deal of attention from the perspective of both basic and developmental science in a vast range of fields. This is due to the fact that these NPs exhibit size-dependent electrical, optical, magnetic, and catalytic phenomena that cannot be realized by their bulk counterparts. Iron oxide NPs, as an example, exhibit superparamagnetism at room temperature, while semiconductor “quantum dots” exhibit the quantum confinement effect.1−4 For spherical particles, the surface area/volume ratio is inversely proportional to the radius, so a substantial reduction in particle size leads to a dramatic increase in surface area. This increased surface/volume ratio is what gives rise to the specific physical properties.5−7 USNPs lie in between complete molecular dispersions and larger-sized NPs and consequently exhibit intermediate structural, optical, electrical, catalytic, and magnetic properties. In this size range, iron oxide USNPs become nearly paramagnetic,7 and USNPs of some noble metals (e.g., gold, silver) become fluorescent.8 Some of these unique properties are summarized in Figure 1. The extremely narrow size range of USNPs necessitates nearly monodisperse size distributions and precise characterization techniques to utilize these physical phenomena. Consequently, a firm understanding of the NP formation mechanism is critical to ensure that NPs with the desired structures and characteristics can be obtained without the need for extensive size-dependent purification.9 In view of the nascent nature of nanomaterials, specifically ultrasmall nanoparticles, new techniques to accurately measure the sizes of © 2013 American Chemical Society
Figure 1. Schematic diagram juxtaposing the differences in the sizes of particles and their resultant properties.
USNPs have not yet been developed, and conventional techniques such as transmission electron microscopy (TEM) are notably insufficient.10 Special Issue: Celebrating Twenty-Five Years of Chemistry of Materials Received: July 5, 2013 Revised: September 25, 2013 Published: October 7, 2013 59
dx.doi.org/10.1021/cm402225z | Chem. Mater. 2014, 26, 59−71
Chemistry of Materials
Review
Since myriad excellent research has been reported in the field of nanotechnology, this review will focus specifically on USNPs. Sub-nanometer aggregates consisting of less than 20 atoms are better defined as clusters than particles or crystals. These clusters require different synthetic and characterization methods and exhibit different physicochemical properties.11 Thus, clusters will not be categorized as USNPs for the purpose of this review. Instead, this review will mainly focus on the uniqueness of USNPs with sizes of 1−3 nm. USNPs have been thoroughly examined and reviewed in many scientific disciplines,5,6,12−18 so this review will attempt to unify the results across the scientific gamut. Specifically, this review will summarize USNPs of noble metals, magnetic materials, and semiconductors in terms of their extremely large surface areas and ultrasmall volumes. These USNPs will be examined individually, detailing both the inherent attributes of specific elements and recent developments in synthetic methodologies. Subsequently this review will examine advances in characterization methods and conclude with a discussion of the various technological applications being explored in this emerging field.
to unique physical properties. For example, when iron oxide USNPs become nearly paramagnetic, it is due to the disordered surface spin. Additionally, the surrounding matter, such as surface ligands, can have a dramatic effect on the overall properties of these particles. Au USNPs can exhibit ferromagnetism when the surface atoms are coated with thiol ligands. In addition, CdSe USNPs modulate their emission spectra on the basis of the attached ligand as well. Additionally, USNPs have different quantum states compared with larger NPs because of their small volumes and small numbers of atoms.17,20 The controlled energy state modulates the reactivity compared with larger NPs.21 Noble-metal USNPs show attenuated surface plasmon resonance and exhibit molecular-like optical properties due to loss of their metallic properties.22 Then the generation of an energy gap near the Fermi energy induces the unique optical properties of metal USNPs. Similarly, iron oxide USNPs possess quantized spin states, while larger iron oxide NPs follow a continuum.17 This section will briefly introduce the unique properties of USNPs originating from the extremely large surface area and ultrasmall volume. The first four properties are derived from the surface effect, and the last two properties are derived from the volume effect. However, these two effects can be difficult to differentiate. For instance, catalysis is modulated through surface effects, but the selectivity is tuned by the reformed structure and energy state. 2.1. Ferromagnetism of Noble-Metal USNPs. Kubo developed the theory of paramagnetism of small particles of group 11 metals,23 although those metals (Au, Ag, and Cu) are well-known diamagnetic materials. He hypothesized that if a metal NP has an odd number of atoms, one electron in the particle must exist as an unpaired electron in the highest occupied state. When the number of atoms in a metal is small enough, the odd-number effect becomes significant and the particles begin exhibiting paramagnetism instead of diamagnetism as a result. In fact, this effect has been observed in both metal and semiconductor USNPs.24 However, ferromagnetic behavior was observed for thiolcapped Au USNPs (Figure 3d)25−27 with similar results for USNPs of Ag and Cu.28 The Kubo theory cannot explain these ferromagnetic properties because it is hard to say that only one spin per particle induces exchange coupling. According to Crespo et al.,27 this is a ligand-dependent phenomenon, as 1.4 nm Au NPs stabilized by weakly interacting amine ligands exhibited diamagnetism whereas those stabilized by thiol ligands exhibited ferromagnetism (Figure 3). The aforementioned thiol ligands on the Au surface can induce 5d-localized holes. These holes cause localized frozen magnetic moments due to the symmetry reduction from the two types of bonding (Au−Au and Au−S) and strong spin−orbit coupling. Consequently, the local structure of the Au−S bond can account for the observed ferromagnetism. Because of the extremely large surface area of metal USNPs, the ferromagnetic properties originating from the surface become more dominant than the diamagnetic properties originating from the greatly diminished metal core. 2.2. Paramagnetism in Magnetic USNPs. Iron oxide NPs typically show size-dependent superparamagnetic properties by the Neel and Brown relaxation effect induced by thermal fluctuation. This occurs when the thermal energy exceeds the anisotropic energy.3,4,29,30 However, the spins of surface atoms are disordered because of the differences between the states of the surface atoms and the bulk atoms. This is called the “spin-
2. PROPERTIES OF USNPS When the size of a material decreases to 1−3 nm, the number of atoms constituting the material falls to less than 500. Consequently, USNPs can be regarded as large molecules in which the majority of the component atoms are located at the interface with the solvent.18 This means that a greater number of the constituent atoms of USNPs are exposed to the outer environment. This tendency is shown in Figure 2, where the
Figure 2. Percentage of atoms on the surface of Pd NPs as a function of NP size. Reprinted with permission from ref 19. Copyright 2000 Springer.
smallest USNPs are almost entirely exposed to the solvent and thus have essentially no true core. When the range of USNP sizes is considered, the percentage of atoms on the surface of a 1.2 nm particle is 76%, while a 2.5 nm particle exposes only 45% of its atoms.19 Below 1 nm, the particles are almost complete molecular dispersions, which is a partial reason for the differences in the macroscopic properties of USNPs compared with clusters. Additionally, as many properties are derived from interfacial interactions of the surface atoms with the solvent, it is easy to see why USNPs accentuate these properties compared with their bulk counterparts. Dominant surface states and the surrounding environment in USNPs can also lead 60
dx.doi.org/10.1021/cm402225z | Chem. Mater. 2014, 26, 59−71
Chemistry of Materials
Review
core and a magnetically disordered shell.7 The magnetically disordered shell is considered as a paramagnetic compound, so the total magnetization (M) of the NPs is represented as a function of magnetic field (H) by eq 1: M(H ) = Ms + χH
(1)
where Ms represents the saturation magnetization originating from magnetic core and χH is the magnetization from the paramagnetic shell. The magnetic susceptibility (χ) is obtained by extrapolation of the difference of the magnetization (M) and magnetic field (H) curves at high field. Because the magnetic core fraction of magnetic USNPs is small, the particles exhibit a smaller saturation magnetization than larger-sized NPs. In the case of iron oxide, USNPs that have an exceptionally large surface area also have significant paramagnetic activity. If it is assumed that the thickness of the spin-canted layer is 0.9 nm,32 iron oxide USNPs with diameters of
