The Colour due to the Dispersed Particles

7.4.3. The Colour due to the Dispersed Particles

The glass consists of a medium where it is possible to produce a variable precipitation reaction under the influence of heat treatment or action of light, i.e. the photo-sensitive reaction. The ions of certain metals like Cu, Au, Ag, Pt dissolving in the glass can be reduced to a metallic state by incorpo- rating reducing agents like tin oxide or antimony oxide in the composition.

7.4.3.1. The Gold Ruby Glass

The famous “Gold Ruby” glass is made by dissolving a small quantity of gold (0.01 to 0.02 wt%) in the glass. The glass containing Au 3+ ions is then heated at a temperature higher than the annealing point and the gold ions are reduced as :

Au 3+ + 3e – → Au 0

and the necessary electrons are supplied by the reactions as :

In course of this heat treatment, first of all, it produces an agglomeration of Au atoms in the form of a “colloidal mass” and then in the form of small crystals, which are in the nano range. The glass, initially colourless, then takes a tinge of lively “ruby” during the final stage of this heat treatment, which is called the “striking” treatment.

The same phenomenon occurs from the interaction of light with the metallic particles : It does not refer to a phenomenon of “scattering”, but an absorption by the gold sol. The corresponding theory is given by Mie [8] by using the Maxwell equation. Doremus [9] has shown that it refers to an effect of resonance of plasma, the electrons collectively oscillate at a characteristic frequency in the particles, whose dimensions are of the order of 20 nm.

Very small gold “nano-particles” were studied by absorption spectroscopy by Lindfors et al [10]. Such particles are an interesting model system to study the transition from individual atoms to the optical properties of bulk matter. The nano-particles also have important applications in ‘chemical sens- ing’ and as ‘labels’ in biology. These workers used supercontinuum light produced in a photonic crystal fibre to detect less than 10 nm gold particles [10] by a detailed analysis of the ‘confocal microscope images’. The cross section showed that the super-continuum light can be focused to almost a ‘diffraction limited spot’. The scattering spectrum displays a clear resonance that can be used to distinguish ‘gold nano-particles’ from other ‘scatterers’ in a biological medium, which has a strong implication in the detection of some defects in the biological systems of many technical applications.

7.4.3.2. The Silver and Copper Rubies

Similar absorption is produced in the glass containing silver in solid solution. The dissolving Ag + ions that was originally colourless can be reduced to a metallic state Ag 0 , the atoms are then fluoroscent. The agglomeration of the Ag atoms to a colloidal state makes the fluorescence disappear, but it provokes a yellow colouration in its place, which can again be explained by the theory of Mie. The

263 corresponding absorption around 396 nm has been used in the study of diffusion of hydrogen in the

OPTICAL PROPERTIES

glasses doped with silver, which serves as the “tracer”. The reduction of the ions like Cu + , Ag + , Au 3+ can be effected by the photo-sensitive reaction by adding a small quantity (0.05%) of photo-reducing agents like CeO 2 in the glass. Under the action of

UV irradiation at room temperature, there is an emission of an electron as :

Ce 3+ +h ν → Ce 4+ +e –

which serves to reduce the Cu + ions as :

Cu + +e – → Cu 0

The atoms of Cu serve as the nucleation centres and a heat treatment permits the development of the colour in the irradiated part. Such glasses, called “photo sensitive” glasses, containing Cu, Ag or Au are usually produced for commercial applications. It allows the execution of real photography by using the sensitivity exposure to the UV light, which is followed by a development of heat treatment at

a temperature of 500 - 600°C. It is possible to precipitate small ‘silver halide’ crystals in a suitable glass and to obtain a trans-

parent glass enjoying the property of ‘photochromism’. The typical glasses are boro-alumino-silicates containing AgCl, AgBr or AgI in the form of small crystals of 8 nm to 15 nm, which is precipitated by

a heat treatment between 400 and 800°C. The space between the particles is of the order of 100 nm. The sensitivity and the kinetics of the darkening process of such glasses and their return to normalcy are influenced by the following :

(a) The Glass Composition , (b) The Nature of the Halogen Ions, (c) The Particle Size (preferably in the nano range), and (d) The Heat Treatment Schedule. It is known that the addition of Cu increases the sensitivity to the light. The system function as a reversible “photographic plate” : the absorption of a photon provokes a

dissociation to Ag 0 and halogen. The metallic Ag 0 absorbs the light and colours the glass in grey. Con- trary to the usual photo-sensitive layer in photography, the pair can recombine as the light is removed, which produces a “whitening” or “Colour Leaching”.

Such systems are perfectly reversible and do not show the sign of “fatigue” up to about 300,000 cycles of ‘darkening-whitening’ cycles, which is contrary to the organic photochromic substances, which are found to be progressively degraded (i.e. ageing). These glasses find important applications in the glass lenses for spectacles.