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This page will be titled Platinum Nanocubes

Advances in nano-scale chemistry have led to the discovery of a wide variety of nanostructures and devices with interesting properties.

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

One such structure is the nanocube. 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). These different nanocubes have shown promise in multiple areas of catalysis, as biosensors, and as optical devices. Catalysis is an important area when talking about platinum nanocubes. Specifically for catalytic activity, platinum nanocubes have demonstrated three times higher when compared to bulk platinum and other nanoparticles.

The catalytic activity of Pt nanocubes is highly related to the nanostructure, size, shape and interactions of the species. Of the various morphologies of Pt nanocubes (cubic, octahedral and cuboctahedra), cubic is more preferable for catalytic study due to its peculiar symmetries and specific superstructures. Also because the cubic structure contains [100] surface plane and face-centered-cubic crystal structure. In addition, due to the high surface area to mass ratio, nanocubes require significantly less platinum metal which makes these structures a viable economic option. The enhanced catalytic abilities of platinum nanocubes make this material an ideal catalyst for reactions that occur within a fuel cell; such as the oxidation of hydrogen and the reduction of oxygen to water. This a particularly important application due to the environmental implications of generating energy using green chemistry. Through these fuel cells energy can be effectively produced without introducing more carbon dioxide into the atmosphere.

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. Nanocubes also have an interesting background. Some early background work includes surface reaction interaction understandings, which is important in catalyzing 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, nanocubes are used in detection, storage, sensing, catalysis, electronics, and much more. Properties of nanoparticles are tuned by their size, shape, and crystal structure. 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. 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. This is true for platinum also. The activity on the surface of platinum nanocubes has been measured and is up to three times higher than commercially available platinum. This page will focus on platinum nanocubes and their uses and properties as seen from a current research perspective.

Properties

The controlled synthesis of platinum 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 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.

Advantages of Platinum as a Catalyst

Platinum is commonly regarded as good catalyst for organic reactions, especially dehydrogenation and hydrosilation reactions. A metal is defined to be a “good catalyst” if it has the following characteristics:

(1) a satisfying activity, or the ability to maintain a reaction at a high rate

(2) good selectivity for one form of a desire product over other forms

(3) a long active lifetime, or the ability to retain its activity and mechanical stability

Platinum demonstrates particularly good activity and has a long active lifetime. These properties allow platinum to provide an alternative, low-activation-energy path by which reactions can occur. The intrinsic catalytic capability of platinum roots from its electron configuration (5d⁹ 6s¹), and its ability to form d¹⁰ complexes with four coordinate ligands. From a material point of view, if enough control over the sizes and shapes of the Pt nanoparticles could be attained, this would increase platinum’s value as a catalyst.

Advantages of Platinum as a Nanocube

Besides the intrinsic characteristics of platinum, other important factors need to be taken into consideration when assessing the usefulness of a catalyst for a given reaction. There are two types of catalytic reactions: homogeneous reaction and heterogeneous. Homogeneous reactions happen when catalyst, reactants and products are all in the same phase. On the contrary, heterogeneous reaction refers to those occurring in solution with a catalyst of solid phase. Most Pt-catalyzed reactions are heterogeneous reactions. For heterogeneous reactions, large contact area of catalyst and reactants is one of the key issues. With nanocubes, the surface area is enlarged so that enough reactants could be absorbed to react with great efficiency. The reactants should also be absorbed with the right configuration for reaction to happen. In this case, by absorption, we do not simply refer to physical absorption. Chemical bonds are formed when reactants are absorbed onto the surface of Pt nanoparticles. In a word, controlling the sizes, shapes and surface properties is significant for making a successful Pt catalyst.

A single nanocube has six facets, eight corners with approximately 90^o angles and twelve edges. Different types of bonding could occur on each of those facets, corners and edges. As the bonding is different, so are the reactants that are influenced by the catalyst, hence. Unlike totally spherically symmetric nanoparticles, nanocubes will greatly enhance the selectivity of catalytic reactions.

Moreover, cubic shaped nanoparticles will demonstrate great ability for self-assembly because of its low size and shape polydispersity. 2-D and 3-D supercrystals with sizes on the order of 10 μm are formed with Pt nanocubes. The building of supercrystals with Pt nanocubes will not only lay out the possibility of improving catalytic activity, but also influence the conducting properties potentially due to doping and defect of the supercrystals.

Synthesis

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

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

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:

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

•Atomic Layer Deposition

•Precipitation

•Template sacrifice

The simplest way of production is the molecular self assembly. In metals, the salt that is used can be reduced and creates the nanostructure without 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 creation process is allowed a smaller amount of time to complete.

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 used to coat a surface, but instead of just coating the surface with a flat coat, it was found that using platinum, small nanocubes had formed on the surface. This also 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. The interfacial rigidity between the nanoparticles and alkyl chains have been supposed to 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 and biological sensors, require large amounts of well-defined nanocrystals.

Current

Platinum nanocubes have demonstrated exceptional potential for catalyzing carbon monoxide oxidation on platinum, which is important for reducing emissions from automobile use. Nanometer-sized noble-metal particles supported on metal oxide surfaces can serve as effective heterogeneous catalysts for accelerating electrochemical, photochemical, and thermal processes. It is possible to utilize the cubic shape of the particle to control assembly texture and magnetic alignment.

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

The catalytic activity of nanocubes for oxygen reduction was evaluated electrochemically and compared to commercially available spherical Pt nanoparticles. The electrochemical measurements indicate an enhanced catalytic activity from the nanocubes. For a range of potentials, the Pt nanocubes demonstrate more than twice the catalytic activity.

Based on this , 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. In addition to Pt nanocubes, alloy Pt nanocubes (Pt-Fe3O4) are also used in catalytic study for oxygen reduction in a fuel cell reaction conditions. The alloy Pt nanocubes play a key role in the stability and shape-control of the nanocubes. In comparison to single component of Pt nanoparticles and commericial Pt particles, Pt nanoparticles in Pt- Fe3O4 show a 20-fold increase in mass activity toward oxygen reduction reaction.

Due to thermal motion and impurities, defects exist in Pt nanocubes, which will increase its surface activity. With different defect sites, the surface activity of Pt nanocubes varies so that the binding of reactants to Pt nanocubes is changing. Research on defects of Pt nanoparticles aims at knowing the influence of different types of defect sites on the catalytic activity, and by controlling the sizes and shapes of Pt nanocrystals can attain better reactivity. Just like defects in other materials, defects in Pt nanocubes could effectively tune the band gap together with the sizes of the particle. The effect of defects is even more obvious if Pt forms nanocubes with other metals, e.g. Fe, Ag, etc. because interface defects can pin the Fermi level and consequently change the energy barrier height compared to a defect-free interface.

Future

By adding defects on interfaces, optical, mechanical, catalytic properties might be improved on platinum nanocubes. This is also one of the fields scientists could work on in the future.

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. These property changes, along with the good surface Raman signals of platinum, have promising properties 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.