UV–Vis-NIR and micro Raman spectroscopies for the non destructive identification of Cd1−xZnxS solid solutions in cadmium yellow pigments

Commercially available since the 1840s, cadmium zinc sulfides were extensively employed by the nineteenth and twentieth century artists. The term cadmium sulfide refers to a number of yellow-orange-red compounds sharing a common structure based on CdS where various elements including Zn, Hg, Se, may substitute for Cd or S, producing a variety of hues. In fact, their chromatic properties are linked to the fact that CdS is a semiconductor with a direct band gap of 2.42eV that can be modulated by the progressive substitution of Cd e.g. with Zn to give pale yellow cadmium zinc sulfide pigments. The aim of this paper was to develop an analytical methodology based on the combination of electronic and vibrational spectroscopies for the non destructive identification of different forms of yellow Cd1-xZnxS solid solutions. The methodology was optimized on six commercial CdS-based pigments and then applied and tested on four historical pigments from the early twentieth century. The complete set of commercial and historical samples has been investigated through X-ray fluorescence, microRaman, UV-Vis-NIR reflection, Vis-NIR emission spectroscopies and X-ray diffraction. The optical properties were investigated exploring i) the reflection behavior in the visible range and ii) the emission profiles in the visible and near infrared ranges correlating the results with the pigment composition. On the other hand, microRaman spectroscopy proved to be a sensitive probe of the short range disorder generated by Zn substitution. The availability of different Raman laser excitations (488, 514, 633 and 785nm) permitted, depending on the optical band gap value of each pigment, to record both on- and off-resonance Raman spectra giving information on the stoichiometry of the solid solutions and on their local structure in fairly good agreement with XRD analysis.