Color of bulk gold

It is comparatively well known that nanoparticles of gold have a very different color to bulk gold. This is the localized surface plasmon resonance. Localized surface plasmon resonances in gold... 

Synthesis of novel materials with desired and tunable physical and chemical properties continues to draw wide interest. Nanomaterials with a variety of shapes and sizes have been synthesized as they offer numerous possibilities to study size and shape-dependent variations of electronic, optical, and chemical properties. Nanomaterials of a particular element show drastic differences in physical and chemical properties when compared with the bulk state. For example, bulk gold, a metal that is insoluble in water can be made dispersible when it is in the nanoparticle form. There are drastic changes in the optical properties as well. Bulk gold appears yellow in color, but when it is in the nanoparticle form with an average core diameter of 16 nm, it appears wine red. Likewise, the chemistry of gold, such as catalysis, also shows a drastic change when the constituent units are in the nanometer range. 

In the recent years, nanosized materials have attained a certain attention. The reason for this was the observation that the properties of many materials change dramatically compared to macroscopic bulk materials when they exist as particles in the dimension of several nanometers, which corresponds to an amount of a few atoms. For instance, the color of gold changes to red when it exists as a powder whose particles have a diameter of less than 100 nm. At even smaller particle sizes, the visible light does not interact with the particles any more - they are becoming invisible. In order to be a nanostmctured material, at least one dimension of the structure must be smaller than 100 nm. 

In general, the properties of bulk materials vary when the structure of that material takes shape in nanodimension. Certain physical and mechanical properties change drastically with the change of materials size. Similar phenomena have been observed in the cases of optical and electrical properties. For example, gold nanoparticles exhibit a variety of colors depending on its shape and size. Similarly, boron nanomaterials possess a variety of interesting properties that are suitable for a... 

Metal nanoparticles have been studied mainly because of their unique optical properties especially nanoparticles of the noble metals copper, silver, and gold have a broad absorption band in the visible region of the electromagnetic spectrum. Solutions of these metal nanoparticles show a very intense color, which is absent in the bulk material and atoms. The origin of the intense color of noble metal nanoparticles is attributed to the collective oscillation of the free conductive electrons induced by an interacting electromagnetic field. These resonances are also denoted as siuface plasmons [15]. 

Both silver (m.p. 962°C, b.p. 2212°C) and gold (m.p. 1065°C, b.p. 2807°C) have characteristic brilliant white and yellow colours in bulk but when finely divided are black or, in the case of gold, can be purple, ruby red or blue. Thus reduction of gold compounds by SnCl2 gives the colloid known as Purple of Cassius , which is used as a ceramic colorant. 

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