Identification of Copper(II)-Lactate Complexes in Cu₂O Electrodeposition Baths: Deprotonation of the alpha-Hydroxyl Group in Highly Concentrated Alkaline Solution

Abstract

Unveiling dissolved species in electrodeposition baths helps our understanding of electrodeposition behavior, such as growth orientation. A highly concentrated aqueous alkaline copper(II)–lactate solution is used for the electrodeposition of copper(I) oxide (Cu₂O) thin films with <111> orientations; the semiconductor properties of these films facilitate their use in solar-cell materials, photocathodes, and photocatalysts. However, the dissolved species, presumably copper(II)–lactate complexes, cannot be deduced on the basis of known thermodynamic data, and have not been convincingly determined yet. In this work, we determine these cupric complexes by pH titration, ultraviolet–visible spectroscopy, and electrospray-ionization mass spectrometry (ESI-MS), including probe-ESI-MS (PESI-MS). Using PESI-MS, we successfully analyzed a highly concentrated solution without sample dilution. The determined complexes are Cu(H₋₁L)L⁻ and Cu(H₋₁L)₂²⁻ (CH₃CH(O⁻)COO⁻) is a lactate ion with a deprotonated α-hydroxyl group. As far as we know, this is the first direct experimental observation of H₋₁L²⁻ ions in a highly concentrated aqueous alkaline copper(II)–lactate solution. We also propose that H₋₁L²⁻ is stabilized by the high concentration and through coordination to copper(II) ions.