A Photoluminescence Study of the Changes Induced in the Zinc White Pigment by Formation of Zinc Complexes

Abstract

It is known that oil paintings containing zinc white are subject to rapid degradation. This is caused by the interaction between the active groups of binder and the metal ions of the pigment, which gives rise to the formation of new zinc complexes (metal soaps). Ongoing studies on zinc white paints have been limited to the chemical mechanisms that lead to the formation of zinc complexes. On the contrary, little is known of the photo-physical changes induced in the zinc oxide crystal structure following this interaction. Time-resolved photoluminescence spectroscopy has been applied to follow modifications in the luminescent zinc white pigment when mixed with binder. Significant changes in trap state photoluminescence emissions have been detected: the enhancement of a blue emission combined with a change of the decay kinetic of the well-known green emission. Complementary data from molecular analysis of paints using Fourier transform infrared spectroscopy confirms the formation of zinc carboxylates and corroborates the mechanism for zinc complexes formation. We support the hypothesis that zinc ions migrate into binder creating novel vacancies, affecting the photoluminescence intensity and lifetime properties of zinc oxide. Here, we further demonstrate the advantages of a time-resolved photoluminescence approach for studying defects in semiconductor pigments.

On the photoluminescence changes induced by ageing processes on zinc white paints

“…Such a surface functionalization has not been experimentally demonstrated for paint materials, but it is widely observed between metal oxide surfaces and organic compounds [14] [15]. Despite the lack of experimental evidence, the functionalization of metal oxide particle is highly plausible even in paint materials and in our previous study we supported this hypothesis [16]. In fact, by employing Time Resolved Photoluminescence (TRPL) spectroscopy, we detected a significant modification of the photoluminescence (PL) emission of the zinc white mixed with binder with respect to the pigment powder.…”

“…This emission is the superposition of several intra-band transitions, which result in a broad spectrum. In previous work we showed that the typical photoluminescence emission of zinc white powder is modified by binding media [16]. Superimposed on the typical broad green emission centred at 530 nm (Green Luminescence, GL), microsecond TRPL analysis highlighted the presence of another distinct emission in the blue region peaked at 430 nm (Blue Luminescence, BL).…”

“…The system is based on a Q-switching laser source (FTSS 355-50, Crylas GmbH, Berlin, Germany, pulse energy = 70 μJ, pulse duration = 1.0 ns, repetition rate = 100 Hz), emitting light pulses at 355 nm focused onto the sample with 1 mm spot-size. Additional details of the TRPL device are described in detail elsewhere [16] [18]. Here, the measurement and protocol adopted in this work are briefly reported.…”

“…The kinetic data were fitted to a multi-exponential decay model (with a maximum of three components) [19] using a nonlinear least square method. As synthetic parameter, the effective lifetime was then calculated as the average of the lifetimes weighted over the number of photons originating from each decay path [16] and the same parameter is applied for comparison purpose amongst different samples.…”

“…The presence of this second peak is evident in all the mockup samples and in case of gum arabic it is even more intense than the GL. The features and the decay kinetic of this BL band in pristine samples have been fully characterized [16].…”