Identification of Copper(II)–Lactate Complexes in Cu₂O Electrodeposition Baths: Deprotonation of the α-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 (Cu 2 O) 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 -1 L)L -and Cu(H -1 L) 2 2-, where the H -1 L 2-(CH 3 CH(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 -1 L 2-ions in a highly concentrated aqueous alkaline copper(II)-lactate solution. We also propose that H -1 L 2-is stabilized by the high concentration and through coordination to copper(II) ions. Cuprous oxide (Cu 2 O) is a p-type semiconductor that is inexpensive and of low toxicity. Cu 2 O thin films are attracting increasing attention as solar-cell materials, 1-5 photocathodes, 6-9 and photocatalysts 10-12 for, for example, water splitting. Among thin-film fabrication methods, electrodeposition from an aqueous solution has advantages over conventional methods, such as chemical vapor deposition, because of its low cost and low environmental burden. Moreover, it is easy to obtain cubic Cu 2 O with the preferential <111> orientation by tuning the pH of the electrodeposition bath;13-17 this orientation relaxes lattice misfit with hexagonal ZnO, which is favorable for Cu 2 O-ZnO solar cells.Since the pioneering work of Rakhshani et al., 13 highly concentrated aqueous alkaline solutions have commonly been used for Cu 2 O electrodeposition;13-16 these solutions contain 0.4 M of a copper(II) salt and 3.0 M lactic acid (HL; CH 3 CH(OH)COOH) as the complexing agent, and the pH is adjusted to be in the 9.0-12.5 range. [13][14][15][16] The crystal orientation of the electrodeposited Cu 2 O is pH dependent; i.e. it is <100> at pH values of 9.0 and 9.5, and <111> at pH values of 12.0 and 12.5. [13][14][15][16][17] The pH dependence of the crystal orientation during Cu 2 O electrodeposition may be ascribable to changes in the dissolved copper(II)-lactate complexes. 6,15,16 Unfortunately, to the best of our knowledge, there are no available thermodynamic data for these copper(II)-lactate complexes under these alkaline conditions, especially for such concentrated solutions. Two previous reports have indicated the presence of dissolved copper(II)-lactate complexes in such concentrated alkaline solutions; Leopold et al. a...

Determination of Stability Constants of Copper(II)–Lactate Complexes in Cu2O Electrodeposition Baths by UV-vis Absorption Spectra Factor Analysis

“…This work precipitates at pH > 8.0, which is inconsistent with the experimental results. In contrast, we also confirmed the formation of the Cu(OH) 2 precipitate during the dilution of 0.4 M copper(II)-3.0 M lactate solution with deionized water above pH 8.0, 18 which corresponded with the data shown in Figure 7b. These results indicated that the reported thermochemical data were insufficient to discuss the concentrated copper(II)-lactate system in the alkaline region.…”

“…Preparation of sample solutions.-For recording the UV-vis absorption spectra, 30 sets of 15-mL aqueous solutions of 0.4 mol dm -3 copper(II)-3.0 M lactate 18 were used as the analytes. Each sample contained 0.06 mol (2.25 g) Cu(ClO 4 ) 2 •6H 2 O (99%, Nacalai Tesque) and 0.045 HL (3.75 mL) (91.1%, Nacalai Tesque).…”

“…As mentioned in our previous work, Cu(II)-lactate complexation required at least 24 h to reach chemical equilibrium. 18 To ensure completion of the complexation processes, these samples were then stored in 20-mL airtight screw tubes for one week. The pH values were measured at 25°C using a pH meter (HORIBA D-51; Horiba) with a glass electrode (HORIBA 9615-10D; Horiba); the results are summarized in Table I.…”

“…Three species, i.e., CuL 2 , Cu(H -1 L)L -, and Cu(H -1 L) 2 2-, are in equilibrium in this region, and the absorption spectra strongly overlap. 18 Therefore, a method that can analyze the copper(II)-lactate system while maintaining the original experimental conditions (i.e., concentration and equilibrium relationship) is desired.…”

“…In our previous work, we directly analyzed a concentrated solution via probe electrospray ionization mass spectrometry (PESI-MS). 18 The unknown complexes were shown to be Cu(H -1 L)Land Cu(H -1 L) 2 2-, in which H -1 L 2was the lactate ion bearing a deprotonated α-hydroxyl group. However, the stability constants of each complex could not be determined.…”