Weber Research Group ^{Ion Spectroscopy}

Highlights

Jesse C. Marcum and Sydney H. Kaufman, two graduate students in the Weber group, have characterized the photochemistry of the gold salt tetrachloroaurate, Au(III)Cl₄^-, and its dihydroxylated analog, Au(III)Cl₂(OH)₂^- in the gas phase. Gold salts are important species in a number of chemical areas.  In geochemistry, these gold salts represent ways of how gold is transported in water. Moreover (and perhaps more importantly), Au(III)Cl₄^- is the most important precursor for the production of gold nanoparticles and can be reacted to other gold salts replacing chlorine atoms by hydrody groups (OH)
(e.g. leading to Au(III)Cl₂(OH)₂^-) by decreasing the acidity of the solution. Metal nanoparticles hold enormous promise in applications ranging from drug delivery to new materials with unusual optical properties. Most of the current synthetic routes use wet chemistry to achieve nanoparticle formation. Photochemical approaches may offer additional control over the nanoparticle synthesis by choosing the best possible wavelengths and by achieving spatial control over where the reaction takes place in solution. In order to obtain information on the intrinsic properties of the salt and the influence of the solvent, it is important to characterize the photochemistry of the process both in solution and in vacuum (i.e. in the absence of solvent).

The Weber group has studied the photofragmentation of AuCl₄^- and Au(III)Cl₂(OH)₂^- in the absence of solvent. They found that the fragmentation of AuCl₄^- in vacuum is highly wavelength-specific. Wavelengths longer than 325 nm result at best in the loss of one chlorine atom, wavelengths shorter than 314 nm result in the loss of two chlorine atoms. the former process leaves behind AuCl₃^- as a product, while the latter process generates AuCl₂^-. Both are intermediates along the route to producing gold nanoparticles. Utilizing wavelength specific reactions may offer the prospect to tune the photochemistry of the salt by choosing the laser wavelength for the light employed in this process. At the same time, the new data show at which wavelengths AuCl₄^- absorbs ultraviolet light in vacuum. Comparing the absorption wavelengths from experiments under vacuum with those in solution may lead to similar control over the photochemistry of this gold salt in solution and therefore new routes to better control in the production of gold nanoparticles.

In addition, the researchers found that AuCl₄^- fragments by losing one or two ligands, while AuCl₂(OH)₂^- always loses two of the ligands. This may shed light on a little-used synthetic approach in this area, namely a combination of making the solution less acidic and using UV light to activate the gold salt used. This could lead to a new route in nanparticle synthesis.