Study on Cupric Lactate Complexes in Electrodeposition of <111>-Oriented Copper(I) Oxide

Copper(I) oxide (Cu₂O) with cubic cuprite structure is known as an intrinsic p-type semiconductor, and it has 2.1 eV band gap and high optical-absorption coefficient (~10⁴ cm⁻¹). Hence, Cu₂O is often used in connection with n-type ZnO to fabricate p-n junction solar cells. The Cu₂O-ZnO device consisting of ubiquitous elements has attracted increasing attention because of its low fabrication cost. Additionally, <111>-oriented Cu₂O is favorable considering the lattice match with {0001} plane of wurtzite ZnO, giving high permeability of light.

   Electrodeposition of Cu₂O from aqueous solutions containing lactic acid and hydrated copper(II) salt has been reported in previous works[1-3]. It was revealed that Cu₂O electrodeposited from pH 9.5 and 10.0 solution oriented <100> and those from pH 10.5, 11.5 and 12.5 solution oriented <111>, and <111>-oriented Cu₂O could be electrodeposited even from pH 9.5 and 10.0 solution by lowering the cathode potential (see Fig. 1(b)). However, according to the E-pH diagram (see Fig. 1(a)) based on given thermodynamic data[4], above pH~8, it is expected that Cu²⁺ ion hydrolyzes and Cu(OH)₂ precipitates. However, in our experiment, Cu₂O was electrodeposited without any precipitation, implying that the stability regions for cupric lactate complexes are larger.

   In this study, we performed titration and UV-vis absorption spectroscopy of solutions containing lactic acid and hydrated copper(II) salt. It is discussed what kinds of complexes exist in the solution and the orientation of Cu₂O.

   Aqueous solution containing lactic acid and hydrated copper(II) nitrate was examined by titration with potassium hydroxide aqueous solution. Above pH~4, deviation was observed from the calculated curve using given thermodynamic data[4]; moreover, a theoretical plateau centered at pH~7.2 was not seen. As a result, formation of Cu(OH)₂ seems not to be realistic.

   Figure 2(b) demonstrates UV-vis absorption spectra of aqueous solutions containing lactic acid and hydrated copper(II) acetate at 2.0 < pH < 13.5. As pH increased from 6.0 to 9.0, blue shift was observed between 500 nm and 1000 nm, while red shift was seen between 300 nm and 400 nm. In addition, as pH increased from 9.0 to 10.5, the absorption edge shifted from about 420 nm to 380 nm. These results strongly suggest some kinds of cupric lactate complexes in the region of 6.0 < pH < 13.5.

   Here, we assumed three complexes (Cu(OH)L²⁻, Cu(OH)₂L⁻, CuL₄²⁻ ) at 9.5 < pH < 12.5 and three Nernst equations with a corresponding number of protons and electrons (see Table I). Figure 1(b) shows relationship between the orientation of Cu₂O[3] and three assumed equations. For many cases, Cu₂O electrodeposited from pH 9.5 and 10.0 solution oriented <100> and those from pH > 10.5 solution oriented <111>. Exceptionally, in the case of Eq.3, even at pH 9.5 and 10.0, we obtained <111>-oriented Cu₂O when lowering the cathode potential because of increased current density proportional to v_(CuOH) = k[Cu][OH] [2,3]. Suppose Eq.3 agrees with the experimental fact, since current density cannot be increased so large when lowering cathode poential considering deposition overpotential of Cu₂O, Cu₂O obtained from pH 9.5 and 10.0 solution should orient <100>. In consequence, Eq.3 may be inconsistent with the experimental fact and the unidentified cupric lactate complexes should be assumed as Cu(OH)L²⁻ and/or Cu(OH)₂L⁻.

[1] K. Mizuno et al., J. Electrochem. Soc., 152, C179 (2005).

[2] T. Shinagawa et al., Cryst. Growth Des., 13(1), 52 (2013).

[3] Y. Seki et al., Abstract 58, The ECS Meeting Abstracts, Vol. 2013-02, CA, USA, Oct 29, 2013.

[4] R. M. Smith et al., Critical Stability Constants, Vol. 5, First Supplement, (Plenum Press, NewYork, 1982), p.291.