Shape Controls

The question remains whether these catalysts retain their shape and/or what shape they will adopt during exposure to electrocatalytic reactions. 

In the past, metal nanoparticle synthesis has been mainly focused on a limited size distribution, with the value of any new procedure consisting mainly of improvements in size distribution. Except for a few examples of discrete size (as in the full-shell clusters M55, M309 or M561), particles of 10% in size are generally also referred to as monodisperse.  

The formation of Ag triangular nanoprisms from spherical nanopartides in the presence of trisodium citrate and bis(p-sulfonatophenyl)phenylphosphine (BSPP) (dipotassium salt) with fluorescent light [245] has been described, as has that of Au nanoprisms [246]. 

Shape control can be directed by the use of molecular capping agents, which adsorb to specific crystal planes such that growth is limited on those crystal planes without capping molecules, or which have only weakly coordinated molecules. Surfactants, polymers and biomolecules, small gas molecules and even different metal ions have each been shown capable of controlling nanocrystal growth [219]. 

The formation of a silver octahedron via different geometries, beginning with a cube 

Finally, bioinspired techniques to control shape of metal nanocrystals are briefly described. Proteins, peptides or nucleic acids invariably possess sulfide, amide or carbonyl functions, while polypeptide sequences may initiate Au particle growth, resulting in thin platelets [263]. The shape control of AgNPs has also been observed with phage-display peptides [264]. 

4 Mesoscopic Assembly and Other Properties of Metal and Semiconductor Nanocrystals 

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The various stages of this process depend critically on the type of gas, its pressure, and the configuration of the electrodes. (Their distance apart and their shapes control the size and shape of the applied electric field.) By controlling the various parameters, the discharge can be made to operate as a corona, a plasma, or an arc at atmospheric pressure. All three discharges can be used as ion sources in mass spectrometry. 

Shape anisotropy Shape control Shape factors Shape-memory alloys Shape-selective catalysis Shape selectivity Sharpless catalyst Shaving cream Shaving creams... 

Ladle metallurgy, the treatment of Hquid steel in the ladle, is a field in which several new processes, or new combinations of old processes, continue to be developed (19,20). The objectives often include one or more of the following on a given heat more efficient methods for alloy additions and control of final chemistry improved temperature and composition homogenisation inclusion flotation desulfurization and dephosphorization sulfide and oxide shape control and vacuum degassing, especially for hydrogen and carbon monoxide to make interstitial-free (IF) steels. Electric arcs are normally used to raise the temperature of the Hquid metal (ladle arc furnace). 

EinaHy, hydrogen in relatively small amounts can lead to cracking, especially as the strength increases. If the presence of hydrogen caimot be adequately lowered by the ladle treatment, combinations of low carbon and siHcon, inclusion shape control, and more than 0.25% copper can be helpful in minimizingloss of toughness. 

W. G. Wilson and A. McLean, "Desulphurization of Iron and Steel and Sulfide Shape Control," The Iron and Steel Society of AIME, Warrendale, Pa.,... 

Adam, Z. and Peng, X. G. (2002). Nearly Monodisperse and Shape-controlled CdSe nanociystals via alternative Routes Nucleation and growth. /. Am. Chem. Soc., 124, 3343-3353. 

Peng, X. G. (2003). Mechanisms for the Shape-control and Shape-Evolution of Colloidal Semiconductor Nanocrystals. Adv. Mater., 15,459-463. 

Peng ZA, Peng X (2001) Nearly monodisperse and shape-controlled CdSe nanocrystals via alternative routes Nucleation and growth. J Am Chem Soc 124 3343-3353... 

Kongkanand A, Tvrdy K, Takechi K, Kuno M, Kamat PV (2008) Quantum dot solar cells. Tuning photoresponse through size and shape control of CdSe-Ti02 architecture. J Am Chem Soc 130 4007-4015... 

Scheme 1 Illustration of the general synthetic method followed in our group for the synthesis of metal nanoparticles i decomposition of the precimsor, nucleation ii first growth process in ripening or coalescence leading to size and shape controlled objects through addition of stabilizers which prevent the full precipitation of the metal (iv)...

In some cases shape-control has also been achieved tetra( -octyl)ammonium glycolate transforms Pd(N03)2 predominantly into trigonal Pd particles [186]. Recent work has confirmed that the colloidal protective agents not only prevent particle agglomeration but even provide control of the crystal growth during particle synthesis (see e.g., Ref. [187-191]). The drawbacks of this route are the restriction to noble metal salts and the limited industrial availability of A-(octyl)j RC02. 

Figure 11. Size- and shape-control of nanoparticles via salt reduction in (a) the hydrophobic core of a surfactant oil in water micelle and (b) the hydrophilic core of a water-in-oil reverse micelle.

Methods for the design of size- and even shape-controlled [186,190,191,370-372] metallic nanoparticles have reached a rather mature stadium thanks to the contributions of the pioneer groups of the last 25 years. Applications in a number of fields of practical Nanotechnology are now moving fast into the focus of R D [203,373]. For an overview on the potential application of metal nanoparticles in the rapidly growing fields of quantum dots, self-assembly, and electrical properties, the reader is advised to consult recently published specialist review articles, e.g.. Refs. [160,281] and book chapters (cf Chapters 2, 4, and 5 in Ref. [60]). In the following three sub-sections the authors restrict themselves to a brief summary of a few subjects of current practical interest in fields with which they are most familiar. 

Table 1. Yield and average size of shape-controlled Pt nanoparticles [15].

Scheme 2. Encapsulation of size- and shape-controlled Pt nanoparticles under neutral hydrothermal synthesis conditions of SBA-15. Silica templating block copolymers and silica precursors were added to PVP-protected Pt nanoparticle solutions and subjected to the standard SBA-15 silica synthesis conditions. Neutral, rather than acidic pH conditions were employed to prevent particle aggregation and amorphous silica formation [16j. (Reprinted from Ref. [16], 2006, with permission from American Chemical Society.)...

The shape-controlled nanoparticles were incorporated into the silica framework by NE by the procedure outlined in Scheme 2. These catalysts are denoted as Pt(X )/SBA-15-NE where X = cubes, cuboctahedra, and octahedra. 

Table 4. Ag/Pt molar ratio and its influence on ethylene hydrogenation rates and apparent activation energy for nanoparticle encapsulated shape-controlled Pt nanoparticles [17].

Shape Control Based on the Crystal Growth Habit... 

Among the several strategies used to achieve size and shape control, by far the most common one is to employ... 

Zhu J, Liu S, Palchik O et al (2000) Shape-controlled synthesis of silver nanoparticles by pulse sonoelectrochemical methods. Langmuir 16 6396-6399... 

Sun YG, Xia YN (2002) Shape-controlled synthesis of gold and silver nanoparticles. Science 298 2176-2179... 

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