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?

1. Setting the Scope of the Challenge 6474 1.1. The Need for Solar Energy Supply and Storage 6474 1.2. An Imperative for Discovery Research 6477 1.3. Scope of Review 6478 2. Large-Scale Centralized Energy Storage 6478 2.1. Pumped Hydroelectric Energy Storage (PHES) 6479 2.2. Compressed Air Energy Storage (CAES) 6480 3. Smaller Scale Grid and Distributed Energy Storage 6481 3.1. Flywheel Energy Storage (FES) 6481 3.2. Superconducting Magnetic Energy Storage 6482 4. Chemical Energy Storage: Electrochemical 6482 4.1. Batteries 6482 4.1.1. Lead-Acid Batteries 6483 4.1.2. Alkaline Batteries 6484 4.1.3. Lithium-Ion Batteries 6484 4.1.4. High-Temperature Sodium Batteries 6484 4.1.5. Liquid Flow Batteries 6485 4.1.6. Metal-Air Batteries 6485 4.2. Capacitors 6485 5. Chemical Energy Storage: Solar Fuels 6486 5.1. Solar Fuels in Nature 6486 5.2. Artificial Photosynthesis and General Considerations of Water Splitting 6486

Solar Energy Supply and Storage for the Legacy and Nonlegacy Worlds

The increasing human need for clean and renewable energy has stimulated research in artificial photosynthesis, and in particular water photoelectrolysis as a pathway to hydrogen fuel. Nanostructured devices are widely regarded as an opportunity to improve efficiency and lower costs, but as a detailed analysis shows, they also have considerably disadvantages. This article reviews the current state of research on nanoscale-enhanced photoelectrodes and photocatalysts for the water splitting reaction. The focus is on transition metal oxides with special emphasis of Fe2O3, but nitrides and chalcogenides, and main group element compounds, including carbon nitride and silicon, are also covered. The effects of nanostructuring on carrier generation and collection, multiple exciton generation, and quantum confinement are also discussed, as well as implications of particle size on surface recombination, on the size of space charge layers and on the possibility of controlling nanostructure energetics via potential determining ions. After a summary of electrocatalytic and plasmonic nanostructures, the review concludes with an outlook on the challenges in solar fuel generation with nanoscale inorganic materials.

Inorganic nanostructures for photoelectrochemical and photocatalytic water splitting

Earth-abundant first-row (3d) transition metal-based catalysts have been developed for the oxygen-evolution reaction (OER); however, they operate at overpotentials substantially above thermodynamic requirements. Density functional theory suggested that non-3d high-valency metals such as tungsten can modulate 3d metal oxides, providing near-optimal adsorption energies for OER intermediates. We developed a room-temperature synthesis to produce gelled oxyhydroxides materials with an atomically homogeneous metal distribution. These gelled FeCoW oxyhydroxides exhibit the lowest overpotential (191 millivolts) reported at 10 milliamperes per square centimeter in alkaline electrolyte. The catalyst shows no evidence of degradation after more than 500 hours of operation. X-ray absorption and computational studies reveal a synergistic interplay between tungsten, iron, and cobalt in producing a favorable local coordination environment and electronic structure that enhance the energetics for OER.

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Homogeneously dispersed, multimetal oxygen-evolving catalysts

Hydrogen from solar-driven water splitting has the potential to provide clean energy. Current progress towards artificial photosynthetic devices is reviewed, with particular focus on visible light active nanostructures. A vision for a future sustainable hydrogen fuel community based on artificial photosynthesis is outlined.

Artificial photosynthesis for solar water-splitting

nonuniform strain during delithiation produces a new-class of cathode materials, which are extremely tolerant to electrochemical cycling. Note that the lattice constant c of the ZrO2coated LiCoO2 exhibits negligible shift in the range 0< x< 0.7, which indicates that the original hexagonal symmetry is unaffected by delithiation. A sol ± gel coating of ZrO2, which has the highest fracture toughness, leads to the formation of a fracture-toughened thin-film solid solution (tens of nm thick) near the particle surface. This film significantly improves the structural stability of the cathode material by suppressing c-axis expansion (or phase transition), thereby preventing capacity fading during electrochemical cycling. In conclusion, the encapsulation of LiCoO2 powders by thin-film coating of high-fracture-toughness oxides effectively suppresses the lattice-constant changes during electrochemical cycling and thereby suppresses phase transitions. The order of capacity retention correlates well with the fracture toughness of the coated thin-film oxides. This innovative technology can be applied to any cathode material that has an accompanying lattice strain (or phase transition) during cycling. The commercial potential of a rechargeable Li-ion battery is enormous.

http://casey.brown.edu/chemistry/research/LSWang/publications/124.pdf

Aromatic Mercury Clusters in Ancient Amalgams

We have recently advanced the aromaticity concept into all-metal molecules containing Al(4)(2-), XAl(3)(-), Ga(4)(2-), In(4)(2-), Hg(4)(6-), Al(3)(-), and Ga(3)(-) aromatic units. All these systems are electron deficient species compared to the corresponding aromatic hydrocarbons. The electron deficiency results in an interesting new feature in all-metal aromatic systems, which should be considered as having both pi- and sigma-aromaticity, and that should result in their additional stability. In this work, we obtain crude evaluations of the resonance energies for Na(2)Al(4) and Na(2)Ga(4) all-metal aromatic molecules. The resonance energies were found to be unusually high: 30 kcal/mol (B3LYP/6-311+G*) and 48 kcal/mol (CCSD(T)/6-311+G(2df)) for Na(2)Al(4) and 21 kcal/mol (B3LYP/6-311+G*) for Na(2)Ga(4) compared to 20 kcal/mol in benzene. We believe that the high resonance energies in Na(2)Al(4) and in Na(2)Ga(4) are due to the presence of three completely delocalized bonds, one pi-bond and two sigma-bonds, thus confirming the presence of pi- and sigma-aromaticity.

On the resonance energy in new all-metal aromatic molecules.

We investigated the electronic structure and chemical bonding of the B3−, Al3−, and Ga3− anions, and the gas phase NaB3, NaAl3, and NaGa3 molecules We found that the ground state of the neutral gas phase salts contains an equilateral triangular anion interacting with a Na+ cation The B3−, Al3−, and Ga3− anions possess two delocalized π electrons and are found to be aromatic The triangular anions have been shown to be related to recently synthesized organometallic compound containing an aromatic -Ga32− unit, but they are differ from them by four valence electrons The reason for earlier appearance of the π-orbital in the B3−, Al3−, and Ga3− anions is discussed

Theoretical Evidence of Aromaticity in X3− (X = B, Al, Ga) Species

Photoelectron spectroscopy is combined with ab initio calculations to elucidate the structure and chemical bonding of a series of MAl6- (M = Li, Na, K, Cu, and Au) bimetallic clusters. Well-resolved photoelectron spectra were obtained for MAl6- (M = Li, Na, Cu, and Au) at several photon energies. The ab initio calculations showed that all of the MAl6- clusters can be viewed as an M+ cation interacting with an Al62- dianion. Al62- was found to possess an Oh ground-state structure, and all of the MAl6- clusters possess a C3v ground-state structure derived from the Oh Al62-. Careful comparison between the photoelectron spectral features and the ab initio one-electron detachment energies allows us to establish firmly the C3v ground-state structures for the MAl6- clusters. A detailed molecular orbital (MO) analysis is conducted for Al62- and compared with Al3-. It was shown that Al62- can be considered as the fusion of two Al3- units. We further found that the preferred occupation of those MOs derived from the...

Al62- − Fusion of Two Aromatic Al3- Units. A Combined Photoelectron Spectroscopy and ab Initio Study of M+[Al62-] (M = Li, Na, K, Cu, and Au)

The electronic structure and chemical bonding of the pentapnictogen cluster anions, Pn5- (Pn = P, As, Sb, and Bi), were investigated using both photoelectron spectroscopy and ab initio calculations. Well-resolved photoelectron spectra were obtained for the anions at several photon energies and were analyzed according to the theoretical calculations. The ground state of all the Pn5- species was found to be the aromatic cyclic D5h structure with a C2v low-lying isomer. We found that the C2v isomer gains stability from P5- to Sb5-, consistent with the experimental observation of the coexistence of both isomers in the spectra of Sb5-. The valence molecular orbitals (MOs) of the D5h Pn5- were analyzed and compared to those of the aromatic C5H5- hydrocarbon. The same set of π-MOs is shown to be occupied in the D5h Pn5- and C5H5- species, except that the MO ordering is slightly different. Whereas the three π-MOs in C5H5- all lie above the σ-MOs, the third π orbital (1a2‘ ‘ in Pn5-) lies below the σ-MOs. The stab...

Probing the Electronic Structure and Aromaticity of Pentapnictogen Cluster Anions Pn5- (Pn = P, As, Sb, and Bi) Using Photoelectron Spectroscopy and ab Initio Calculations

Aleksey E. Kuznetsov

Papers

Aromatic Mercury Clusters in Ancient Amalgams

On the resonance energy in new all-metal aromatic molecules.

Theoretical Evidence of Aromaticity in X3− (X = B, Al, Ga) Species

Al62- − Fusion of Two Aromatic Al3- Units. A Combined Photoelectron Spectroscopy and ab Initio Study of M+[Al62-] (M = Li, Na, K, Cu, and Au)

Probing the Electronic Structure and Aromaticity of Pentapnictogen Cluster Anions Pn5- (Pn = P, As, Sb, and Bi) Using Photoelectron Spectroscopy and ab Initio Calculations