Chemical and morphological characterization of the "silver photochloride"-based sensitized layer created by Edmond Becquerel's colour photographic process

Edmond Becquerel invented in 1848 the first colour photographic process and used it to record the solar spectrum with its own colours, as well as to take pictures in the camera obscura. The "photochromatic images", as he named them, raised several questions, the main being that of the origin of their colours. Here, we present the characterization of the sensitized layer, created at the surface of a silver plate or a silver foil according to Becquerel's process, which 19th century scientists identified as "silver photochloride", a combination of "silver subchloride" and silver chloride. The chemical composition of the sensitized layer has been identified by complementary spectroscopies (EDX, XPS, HAXPES and EXAFS), while its morphology has been studied by electron microscopies (SEM and STEM). These techniques involve X-ray or electron beams, which can have an impact on the silver chloride-based sensitized layer; a first part of this article hence introduces a study of the beam effects and of the way of reducing them. Analysis in beam damage-free conditions then allow us to identify for the first time the "silver photochloride" of the 19th century literature that constitutes the sensitized layer as silver nanoparticles dispersed in a micrometric silver chloride grains matrix. This makes the sensitized layer comparable to the Ag@AgCl photocatalysts, which have recently drawn a lot of attention.