Nanocrystals are fundamental to modern science and technology. Mastery over the shape of a nanocrystal enables control of its properties and enhancement of its usefulness for a given application. Our aim is to present a comprehensive review of current research activities that center on the shape-controlled synthesis of metal nanocrystals. We begin with a brief introduction to nucleation and growth within the context of metal nanocrystal synthesis, followed by a discussion of the possible shapes that a metal nanocrystal might take under different conditions. We then focus on a variety of experimental parameters that have been explored to manipulate the nucleation and growth of metal nanocrystals in solution-phase syntheses in an effort to generate specific shapes. We then elaborate on these approaches by selecting examples in which there is already reasonable understanding for the observed shape control or at least the protocols have proven to be reproducible and controllable. Finally, we highlight a number of applications that have been enabled and/or enhanced by the shape-controlled synthesis of metal nanocrystals. We conclude this article with personal perspectives on the directions toward which future research in this field might take.

Shape‐Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics?

Coinage metals, such as Au, Ag, and Cu, have been important materials throughout history.1 While in ancient cultures they were admired primarily for their ability to reflect light, their applications have become far more sophisticated with our increased understanding and control of the atomic world. Today, these metals are widely used in electronics, catalysis, and as structural materials, but when they are fashioned into structures with nanometer-sized dimensions, they also become enablers for a completely different set of applications that involve light. These new applications go far beyond merely reflecting light, and have renewed our interest in maneuvering the interactions between metals and light in a field known as plasmonics.2–6

In plasmonics, the metal nanostructures can serve as antennas to convert light into localized electric fields (E-fields) or as waveguides to route light to desired locations with nanometer precision. These applications are made possible through a strong interaction between incident light and free electrons in the nanostructures. With a tight control over the nanostructures in terms of size and shape, light can be effectively manipulated and controlled with unprecedented accuracy.3,7 While many new technologies stand to be realized from plasmonics, with notable examples including superlenses,8 invisible cloaks,9 and quantum computing,10,11 conventional technologies like microprocessors and photovoltaic devices could also be made significantly faster and more efficient with the integration of plasmonic nanostructures.12–15 Of the metals, Ag has probably played the most important role in the development of plasmonics, and its unique properties make it well-suited for most of the next-generation plasmonic technologies.16–18

1.1. What is Plasmonics?
Plasmonics is related to the localization, guiding, and manipulation of electromagnetic waves beyond the diffraction limit and down to the nanometer length scale.4,6 The key component of plasmonics is a metal, because it supports surface plasmon polariton modes (indicated as surface plasmons or SPs throughout this review), which are electromagnetic waves coupled to the collective oscillations of free electrons in the metal.

While there are a rich variety of plasmonic metal nanostructures, they can be differentiated based on the plasmonic modes they support: localized surface plasmons (LSPs) or propagating surface plasmons (PSPs).5,19 In LSPs, the time-varying electric field associated with the light (Eo) exerts a force on the gas of negatively charged electrons in the conduction band of the metal and drives them to oscillate collectively. At a certain excitation frequency (w), this oscillation will be in resonance with the incident light, resulting in a strong oscillation of the surface electrons, commonly known as a localized surface plasmon resonance (LSPR) mode.20 This phenomenon is illustrated in Figure 1A. Structures that support LSPRs experience a uniform Eo when excited by light as their dimensions are much smaller than the wavelength of the light.



Figure 1

Schematic illustration of the two types of plasmonic nanostructures discussed in this article as excited by the electric field (Eo) of incident light with wavevector (k). In (A) the nanostructure is smaller than the wavelength of light and the free electrons ...



In contrast, PSPs are supported by structures that have at least one dimension that approaches the excitation wavelength, as shown in Figure 1B.4 In this case, the Eo is not uniform across the structure and other effects must be considered. In such a structure, like a nanowire for example, SPs propagate back and forth between the ends of the structure. This can be described as a Fabry-Perot resonator with resonance condition l=nλsp, where l is the length of the nanowire, n is an integer, and λsp is the wavelength of the PSP mode.21,22 Reflection from the ends of the structure must also be considered, which can change the phase and resonant length. Propagation lengths can be in the tens of micrometers (for nanowires) and the PSP waves can be manipulated by controlling the geometrical parameters of the structure.23

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Controlling the synthesis and assembly of silver nanostructures for plasmonic applications

Generations Yi Ma,† Xiuli Wang,† Yushuai Jia,† Xiaobo Chen,‡ Hongxian Han,*,† and Can Li*,† †State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences and Dalian National Laboratory for Clean Energy, 457 Zhongshan Road, Dalian 116023, China ‡Department of Chemistry, College of Arts and Sciences, University of Missouri-Kansas City, 5100 Rockhill Road, Kansas City, Missouri 64110, United States

Titanium dioxide-based nanomaterials for photocatalytic fuel generations.

Carbonaceous nanomaterials for the enhancement of TiO2 photocatalysis

A ZnO photocatalyst was hybridized with graphite-like C3N4via a monolayer-dispersed method. After hybridization with C3N4, the photocurrent of ZnO was enhanced by 5 times under UV irradiation and a photocurrent under visible light irradiation was observed. The photocatalytic activity of C3N4/ZnO under UV irradiation was increased by 3.5 times, the visible light photocatalytic activity was generated and the photocorrosion of ZnO was suppressed completely after ZnO was hybridized with C3N4. The enhancement in performance and photocorrosion inhibition under UV irradiation was induced by the high separation efficiency of photoinduced holes from ZnO to the HOMO of C3N4. Under visible light irradiation, the electron excited from the HOMO to the LUMO of C3N4 could directly inject into the CB of ZnO, making C3N4/ZnO present visible light photocatalytic activity. The optimum synergetic effect of C3N4/ZnO was found at a weight ratio of 3%, which corresponded to a monolayer dispersion of C3N4 on the surface of ZnO.

Enhancement of photocurrent and photocatalytic activity of ZnO hybridized with graphite-like C3N4

Optical properties of polymer-stabilized aqueous colloids of silver sulfide were studied. Nature and energies of optical transitions responsible for the absorption of visible light by silver sulfide nanoparticles were determined. Catalytic properties of Ag 2 S nanoparticles in methylviologen reduction by sodium sulfide were examined. It was shown that this reaction is reversible, the equilibrium caused by a reverse reaction between the product of direct catalytic process – sulfur (polysulfide anions) and cation-radical of methylviologen. It was found that Ag 2 S nanoparticles can catalyze reduction of Ag + ions by various reductants, in particular, hydroquinone or sodium sulfite, with the formation of silver particles having characteristic plasmon absorption bands in the visible spectral domain. Effect of the conditions of Ag + catalytic reduction on the shape and intensity of plasmon resonance bands of metallic silver were studied, schematic mechanism of this reaction was proposed. It was shown that at the irradiation of solutions containing Ag 2 S nanoparticles, Ag + cations and a reducing agent with the light corresponding to the fundamental absorption band of nanoparticulate silver sulfide, the rate of Ag + catalytic reduction increased whereas kinetic features of catalytic reaction and characteristics of plasmon resonance absorption bands of silver nanoparticles changed. A mechanism of photochemical activation of the catalytic reduction of silver ions with the participation of Ag 2 S nanoparticles was proposed.

Optical and catalytic properties of Ag2S nanoparticles

Photochemical properties of zinc oxide nanoparticles and their photocatalytic activity in the processes of the reduction of Cu 2+ and Pb 2+ ions as well as mixtures of Cu 2+ –Ag + and Cd 2+ –Ag + ions have been investigated. Optical properties of ZnO/Cu, ZnO/Pb, ZnO/Ag/Cu, ZnO/Ag/Cd and ZnO/Ag/Zn nanocomposites have been examined. It has been found that a decrease in the size of ZnO nanoparticles from 6.0 nm to 4.4 nm results in the strengthening of quantum confinement of photogenerated charge carriers, growth of the band gap and the potentials of the conduction band and the valence band of the semiconductor. It has been found that ZnO nanoparticles smaller than 2 R  = 6.0 nm photocorrode with the formation of metallic zinc when the colloidal solution is irradiated with the mild UV light. An increase in the potential of the conduction band electrons and the ability of ZnO nanoparticles to accumulate excessive negative charge have been supposed to be the principle reasons of advanced photochemical activity of ZnO nanoparticles with 2 R Mie theory has been applied to analyze the parameters of surface plasmon resonance bands of metallic copper in ZnO/Cu nanocomposites as well as to estimate an average size of Cu 0 nanoparticles formed on the surface of the semiconductor. It has been found that Cu(II) photoreduction rate increases substantially when Ag + ions are present in the irradiated systems. Photocatalytic reactions in these systems result in the formation of ternary bimetallic ZnO/Ag/Cu nanocomposites. It has been shown that Cd(II) ions, which are stable towards photoreduction on the surface of ZnO nanoparticles, can be co-reduced with Ag + cations giving ternary ZnO/Ag/Cd nanocomposites with peculiar optical properties. Correlations between the conditions of co-reduction of metal cations and the parameters of partially disturbed silver plasmon bands in the optical spectra of colloidal ZnO/Ag/Cd and ZnO/Ag/Zn have been established.

Photochemical synthesis and optical properties of binary and ternary metal-semiconductor composites based on zinc oxide nanoparticles

Photophysical characteristics of colloidal CuS nanoparticles synthesized in various conditions and stabilized in aqueous solutions with sodium polyphosphate (SPP) were studied. A correlation between the band gap of CuS nanoparticles and their average diameter was established. Catalytic activity of colloidal CuS nanoparticles in sodium sulfide air oxidation in aqueous solutions at atmospheric pressure and room temperatures was established and thoroughly investigated. Kinetics of HS − oxidation and the nature of principal products of this reaction (thiosulfate and sulfate ions) were elucidated. A scheme for the mechanism of hydrosulfide ions catalytic oxidation was proposed. Accordingly to the scheme, HS − oxidation is a chain-radical reaction initiated on the surface of CuS nanoparticles and propagating further in the bulk of a solution.

Catalytic activity of CuS nanoparticles in hydrosulfide ions air oxidation

Nanoparticles of zinc and iron oxides as well as cadmium sulfide of different size were synthesized in 2-propanol, their band gaps and allowed bands positions were determined. The foregoing nanoparticles turned out to be efficient photoinitiators of butylmethacrylate polymerization. On the basis of the kinetic parameters of the process a free radical mechanism of the butylmethacrylate photopolymerization induced by investigated nanosized semiconductors was inferred. An increase in the specific rate of the butylmethacrylate photopolymerization with the decrease in the average CdS nanoparticles diameter was explained in terms of the theory of quantum confinement effects in semiconductors and was attributed to an increase in the probability of monomer reduction by photogenerated electrons of CdS nanocrystals conduction band with the diminishing of crystals size. It was shown that a number of xanthene dyes—fluorescein and its halogenated derivatives are efficient sensitizers of ZnO nanoparticles to the visible light. Butylmethacrylate photopolymerization rates grow at the presence of ZnO nanoparticles and sensitizers symbately with an increase in singlet and triplet-excited states energy of dyes studied. Such relation indicates the determining role of the process of electron transfer from photoexcited sensitizers to the ZnO nanoparticles conduction band in the initiation stage of polymerization. Flash photolysis was used for the investigation of the nature and decay characteristics of short-lived intermediates generated in systems consisting of ZnO nanoparticles, sensitizers and monomer. Schemes of the mechanism of the butylmethacrylate photopolymerization induced by investigated nanosized semiconductors were proposed.

Photoinitiation of buthylmethacrylate polymerization by colloidal semiconductor nanoparticles

The characteristics of photocatalytic systems based on nanostructural semiconductors, characterized by quantum size effects, are discussed. An analysis is made of the consequences of exciton quantum confinement in the volume that are significant for photocatalysis and, in particular, the increase in the energy of photogenerated charges with decrease in the particle size, the photoinduced polarization processes, and also the simultaneous display of these effects. Possible ways of further increasing the effectiveness of systems based on nanostructural semiconductors are examined.

S. Ya. Kuchmii

Papers

Optical and catalytic properties of Ag2S nanoparticles

Photochemical synthesis and optical properties of binary and ternary metal-semiconductor composites based on zinc oxide nanoparticles

Catalytic activity of CuS nanoparticles in hydrosulfide ions air oxidation

Photoinitiation of buthylmethacrylate polymerization by colloidal semiconductor nanoparticles

Quantum Size Effects in Semiconductor Photocatalysis