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This page will be titled Nanocubes

Figure 1: An image of a single nanocube and its facets, corners, and edges

Advances in nano-scale chemistry have led to the discovery of a wide variety of nanostructures and devices with interesting properties. One such structure is the nanocube, which has shown promise in areas of catalysis, as biosensors, and as optical devices. Nanocubes are cubic shape nanostructures with six facets, eight corners (90^o angle) and twelve edges. The size of a nanocube ranges from 10 - 100nm. There are various types of metal nanocubes, palladium (Pd), copper (Cu), Silver (Ag), gold (Au) and platinum (Pt).

History and Background

Nanotechnology is defined as a technique that involves working with nanoscale materials. While these materials have been around for centuries, until recently there has not been the technology available for research on such a small scale. As the technology becomes more sophisticated, so has the fabrication and multitude of uses for such a novel technique.

The history of nanotechnology only began in relatively recent years, but the work done on this technology is extensive. Work on surface science won the Nobel Prize in physics in 1932, 1937, and 1956, but at these times, the use of the term ‘nano’ was fairly uncommon. Achievements in this area sparked an interest and advanced work into the nanoscale phenomena. This lead to the characterization and exploration of the surfaces of nanoparticles, which in turn lead to the discovery of unique properties independent of macroscale materials. In 2007, the Nobel Prize for Chemistry was won for contributions to understanding mechanisms at the nanoscale which included catalysis and other surface-specific procedures. This work was mainly focused on the adsorption of simple molecules on well-defined surfaces, and the relationship of the related surface structures/energetics to industrial-scale reactions. One of the significant pieces of the work in this field involved the carbon monoxide oxidation on platinum, which is important for reducing emissions from auto use. Additionally, nanostructures are used in detection, storage, sensing, catalysis, electronics, and much more. Properties of nanoparticles are tuned by their size, shape, and crystal structure. With such a small material, the properties can be much different than a bulk material of the same composition, such as reactivity and catalytic activity. There are various kinds of nanoparticles and nanostructures, such as nanowires, nanorings, nanocages, and even nanoflowers, within which nanocubes is an important type. This page will focus on nanocubes and their uses and properties as seen from a current research perspective.

Metallic nanostructures have a specific importance, as the bulk material of a metal will have different properties. The smaller size can enhance catalytic reaction rates over the bulk materials and well designed nanostructures are essential for producing efficient catalysts. Size, shape, and structure of metal nano structures have significant effect on catalytic reactions. The greater amount of surface area to mass ratio can give an increase in the catalytic reactivity. This is especially important as catalysts, like platinum, are expensive. Using less platinum by increasing the surface area is an important use of nanotechnology.

Properties

The controlled synthesis of nanocubes of a specific size and shape has been of great synthetic interest, yet still remains a challenge. The catalytic activity of nanocubes is directly related to the facets, corners and edge sites on the surface of the cube, slight modifications to these sites have resulted in drastic changes in the properties. Variations in reaction conditions have produced different nano-structures and sizes. Indeed, these two things are focused on when synthesizing nanocubes. Depending on the reaction conditions, other shapes such as octahedral and cuboctahedral can be formed. These have different crystal packing planes at the surface, which changes properties of the nanoparticle. The nanocubic structure is of the most interest due to its specific packing planes that are on the surface. These are six identical [100] faces, which is important in catalysis where uniformity causes selectivity. It has been shown that the [100] plane is more catalytically active than the [111] plane.

Synthesis

The assembly of the nanocubes into long-ordered suprastructures is necessary for macroscale catalytic use of this material, and requires both uniform shape and narrow size distribution. Common methods for the synthesis of nanocubes include:

Figure 2: A depiction of a 2D array of self assembled nanocubes.

Figure 3: A depiction of a 3D superlattice of self assembled nanocubes.

•Self assembly in 2-D and 3-D superlattices

•Atomic Layer Deposition

•Precipitation

•Template sacrifice

The simplest method of nanocube production is the molecular self assembly. In solution the metal salt that is reduced creates the nanocubic structure without outside influence. Depending on the reaction process of the reduction of metal salts, the shape and size can be controlled. The smaller nanocubes are desired as there is a higher surface to weight ratio, which means that there can be an increase of the surface activity with a smaller total amount of the material. Smaller particles are created when the nucleation step of the reaction is allowed a short amount of time.

Atomic Layer Deposition (ALD) has been used both to deposit nanocubes on a substrate and to coat the surface of other nanocubes. Using a nanocube base of a different material can also change the properties that are available. For example, if platinum is plated on a perovskite nanocube base the catalytic properties of the cube increase. ALD has also been found to produce nanocubic layers on substrate materials instead of just coating the surface with a flat coat. This increases surface area that can be used for catalysis.

Some research groups have suggested post treatments for size or shape control, but additives can perform this task during the synthesis without needing a secondary step. Multiple additives, including silver and alkyl amines, have been used to do this. Alkyl amines have been used to not only control the shape but also the size of the nanoparticles. It has been hypothesized that the interfacial rigidity between the nanoparticles and alkyl chains can control the size, and depending on the size of the alkyl chain, these interactions can be used as capping agents to change the shape of the particle. The length of the alkyl chains can also change the shape of a particle. In one study it was found that very short alkyl chains in the reaction mix result in a larger percent of nanocubic structures. In addition to the influence of alkyl chains, the concentration of silver ions in the precipitating solution can also affect the particle shape. Another study found that the silver ions greatly increase the ease of shape control for platinum nanocubes. With a lower amount of silver ions in the solution, cubic shapes were prevalent. With a higher amount of silver concentration, octahedra and cuboctahedra were present, along with some tetrahedra shaped nanoparticles.

The addition of previously discussed additives has shown that a degree of chemical control can be demonstrated over the synthesis of catalytic particles. In addition to solid nanocubes, hollow nanocubes have also been synthesized using sacrificial templates. These cubic templates were coated with platinum and a reducing agent was used to etch the template from the cube. This gives a great deal of surface area for the catalyst, a desirable property.

Applications

Many applications in areas of catalysis, electronics, data storage, oxidation and reduction reactions, biological sensors, and drug delivery require large amounts of well-defined nanocrystals. Nanocubes of various metals (platinum, copper, silver, gold) have well defined facets, edges, and corners which are ideal for these uses. Some examples of the uses of theses metal nanocubes are:

Current

Figure 4:A depiction of how a fuel cell works and where platinum nanocubes can be used in them.

Platinum (Pt) Nanocubes:

Pt nanocubes could be used for the oxygen reduction reactions that occur in fuel cells. In this process, platinum assists in transferring electrons from hydrogen atoms to oxygen atoms, creating electrical energy. This is an important reaction because the only byproduct is water, instead of carbon dioxide, a green house gas. The catalytic activity of platinum can also be applied to other reactions.

Platinum also shows good surface Raman signals which is promising for sensing technologies. On a nanoscale, platinum has higher surface stability than other metals, which is the reason for the favorable Raman signals. This Surface Enhanced Raman Scattering (SERS) has the possibility of detecting very small amounts of atoms. Platinum nanocubes even have higher SERS activity than randomly oriented or rough Pt particles.

Copper (Cu) Nanocubes:

Minuscule metallic (Cu cubes coated with gold) nanocubes can be used as a potential new method of drug delivery treatment. Two reasons why they are very useful: first, the location of the nanocubes can be traced with magnetic resonance imaging inside the body. Second, mass production of these nanocubes are inexpensive because techniques from electronic chip making and self –assemble can be utilized in producing these cubes. For cell therapy, microbeads can be inserted into these hollow nanocubes using micropipettes to carry and release to the desire sites of the cell.

Silver (Ag) Nanocubes:

Optical and sensing properties of small-scale materials increase sensing usage possibilities for future work. Depending on the surface and shape of the nanoparticle, the optical extinction, absorption, and scattering properties are influenced. Rotation of a particle can also influence these optical properties.

Future

The previously mentioned uses of the nanocubes can be further studied and enhanced both by alloying the nanocubes with different materials and by changing the physical properties. The changes of physical and chemical properties of the nanocubes will allow boundless ways to use them in new technologies.