title: Unraveling the Reactivity of Minium toward Bicarbonate and the Role of Lead Oxides Therein.
abstract: Understanding the reactivity of (semiconductor) pigments provides vital information on how to improve conservation strategies for works of art to avoid rapid degradation of the pigments. This study focuses on the photoactivity of minium (Pb3O4), a semiconductor pigment that gives rise to strong discoloration phenomena upon exposure to various environmental conditions. For demonstrating its photoactivity, an electrochemical setup with a minium-modified graphite electrode (C|Pb3O4) was used. It is confirmed that minium is a p-type semiconductor that is photoactive during illumination and becomes inactive in the dark. Raman measurements confirm the formation of degradation products. The photoactivity of a semiconductor pigment is partly defined by the presence of lead oxide (PbO) impurities; these introduce new states in the original band gap. It will be experimentally evidenced that the presence of PbO particles in minium leads to an upward shift of the valence band that reduces the band gap. Thus, upon photoexcitation, the electron/hole separation is more easily initialized. The PbO/Pb3O4 composite electrodes demonstrate a higher reductive photocurrent compared to the photocurrent registered at pure PbO or Pb3O4-modified electrodes. Upon exposure to light with energy close to and above the band gap, electrons are excited from the valence band to the conduction band to initialize the reduction of Pb(IV) to Pb(II), resulting in the initial formation of PbO. However, in the presence of bicarbonate ions, a significantly higher photoreduction current is recorded because the PbO reacts further to form hydrocerussite. Therefore, the presence of bicarbonates in the environment stimulates the photodecomposition process of minium and plays an important role in the degradation process.
authors.affiliation: AXES Research Group, Department of Chemistry, University of Antwerp , Groenenborgerlaan 171, B-2020 Antwerp, Belgium.
authors.forename: Eyasu
authors.initials: E
authors.lastname: Ayalew
authors.affiliation: AXES Research Group, Department of Chemistry, University of Antwerp , Groenenborgerlaan 171, B-2020 Antwerp, Belgium.
authors.forename: Koen
authors.initials: K
authors.lastname: Janssens
authors.affiliation: AXES Research Group, Department of Chemistry, University of Antwerp , Groenenborgerlaan 171, B-2020 Antwerp, Belgium.
authors.forename: Karolien
authors.initials: K
authors.lastname: De Wael