Correlation between Copper(II)-Lactate Complexes in Aqueous Alkaline Solutions and Copper(I) Oxide (Cu2O) Electrodeposited
Authors: Yuasa Ryusuke , Miura Ryutaro , Shinagawa Tsutomu , Fukami Kazuhiro , Murase Kuniaki 
Published: February 2024

Cuprous oxide (Cu2O) is a p-type semiconductor well-investigated as materials for photovoltaics and photoelectrodes due to its low cost. There is a number of reports on the Cu2O electrodeposition from aqueous alkaline electrolytes containing copper(II) salt and lactate ions as complex formers. Cu2O electrodeposition behaviors and several properties of resulting Cu2O are known to depend on the electrolyte pH, but mention of the reasons for this is limited. Recently, we have succeeded in identifying two different copper(II)-lactate complexes (1 and 2 in Table I) formed in the alkaline region depending on pH [1], and their formation constants determined made it possible to discuss the electrodeposition in terms of cathodic overpotential, or driving force, for the deposition. In addition, we found that the complexing took at least 1 day to reach equilibrium [2], suggesting that aging is mandatory after the preparation of electrolytes for reproducible electrodeposition. In this study, we prepared a set of thermodynamically well stabilized copper(II)-lactate electrolytes containing 0.4 M Cu(CH3COO)2·H2O and 3.0 M lactic acid, and compared the Cu2O electrodeposition at several different pH but unified overpotential from the CuII/Cu2O redox equilibrium at each pH. The effects of the pH-dependent complexes on the crystallographic orientation and electrical properties of Cu2O were rigorously investigated.
Table I summarizes the relationship between the two complexes, deposition current density, and the properties of resulting Cu2O layers. In concert with the change of the complex, distinct differences in the properties were found bordering about pH 9.5. Since Cu2O shows p-type properties with intrinsic Cu+ defects, the conductivity of Cu2O depends on the amount of introduced Cu+ defects, the origin of p-type carriers. If the reduction of the complexes occur exactly according to the stoichiometry:
2Cu(H–1L)L– (1) + H2O + 2e = Cu2O + 4L–

2Cu(H–1L)2
2– (2) + 3H2O + 2e = Cu2O + 4L– + 2OH–

then, there should be no Cu+ defects. In practice, however, the defects often occur in Cu2O, because O2– ions can be incorporated into the lattice through an acid-base reaction like OH– = O2– + H+, independently of the reduction reactions. The amount of Cu+ defects is thus influenced by how readily the reduction of the complexes occur. The lower carrier density of Cu2O deposited at lower pH can be due to the ease in reduction of lower symmetric complex 1 dominant in the electrolytes, because complex with lower symmetry is generally more favorable to be reduced than that with higher one (2 in this case). The formation of Cu+ defects can also indebted to OH– activity near the cathode. At lower pH, the independent incorporation of O2– is restricted due to lower OH– activities, while high pH electrolytes with higher OH– activities provide an environment conducive to Cu+ defects. As a result, Cu2O layers with few Cu+ defects, low carrier density, and high resistivity is deposited from the electrolytes with complex 1. Owing to the low carrier density, the deposition becomes limited by carrier diffusion showing a low deposition current density.
Although the deposited Cu2O crystals were randomly oriented at the early stage of electrodeposition irrespective of the pH, the deposits from lower and higher pH electrolytes became to have 〈100〉 and 〈111〉 orientations, respectively, as the electrodeposition proceed. Regarding the cubic Cu2O crystal, {100} planes have a higher surface energy than {111} planes. Therefore, under low current density (at lower pH), that is slow crystal growth with thermodynamic equilibrium control, Cu2O crystals with stable {111} facets are advantageous to appear. In contrast, at high current densities (at higher pH), the conditions favoring the appearance of {111} facets weaken and crystals with {100} facets appear. In such a growth mode, crystals whose preferential growth direction are not perpendicular to the planar substrate (cathode) were eliminated as the growth proceeds, because the directions interfere each other. As a result, the surface of thick Cu2O layers obtained from lower pH electrolytes is composed of square pyramidal grains with the vertex pointing perpendicular to the substrate; here, the faces of the pyramids correspond to four {111} facets. In contrast, the surface those obtained at higher pH is composed of triangular pyramidal grains with {100} facets.
Consequently, it is considered that the two different complexes directly affect the difference in carrier density and conductivity of electrodeposited Cu2O, and the deposition current density changes depending on these electrical properties. Then, the current density determines the crystallographic orientation of resulting Cu2O.
[1] T. Chen, et al., J. Electrochem. Soc., 166, D761 (2019); [2] T. Chen, et al., J. Electrochem. Soc., 165, D444 (2018).

Figure 1