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CHEMICAL PROBLEMS 2024 no. 4 (22) ISSN 2221-8688
447
UDC 544.654.2
SIMULTANEOUS ANODIC AND CATHODIC ELECTRODEPOSITION OF Cu2O FOR
SOLAR ENERGY CONVERSION
Kevin F. Genaro-Saldivar1, Geovanni R. Negrete-Reyes1, Esther Ramirez-Meneses1, Carlos Juarez-Balderas2, Flor Avendano-Sanjuan1, Diana A. Garcia-Najera1, Jorge G. Ibanez1'*
1 Department of Chemical, Industrial, and Food Engineering, Iberoamerican University, Mexico City,
Mexico
2 Department of Engineering Studies for Innovation, Iberoamerican University, Mexico City, Mexico
*Corresponding author: _jorge.g. ibanez@gmail.com
Received 28.03.2024 Accepted 06.06.2024
Abstract: Copper oxide, Cu2O is a highly versatile product that can obtained by different techniques including high/low temperature thermal oxidation, sputtering, chemical oxidation, anodic oxidation, electrodeposition, among others. Until now, the most efficient synthetic processes to obtain Cu2O require relatively expensive equipment; therefore, we have developed a novel method based on simultaneous convergent oxidation-reduction reactions in an electrolytic cell at ambient pressure and relatively low temperature. In this work, a Cu(0) plate is oxidized to Cu(I) in the anodic compartment, while a Cu(II)-lactate complex is formed in the cathodic compartment of the cell and then Cu(II) is reduced to Cu(I), all in aqueous media.
Keywords: Cu2O synthesis, convergent synthesis, copper (I) oxide, electrodeposition, simultaneous processes.
DOI: 10.32737/2221-8688-2024-4-447-457
1. Introduction
Cu(I) oxide (Cu2O, copper oxide) is found in nature as the reddish mineral cuprite that crystallizes in a face-centered cubic crystal lattice (FCC), similar to the NaCl structure, with lattice parameters a = b = c = 4.27 A [1]. Cu2O represents a highly versatile material. Many antifouling paints contain Cu2O as the main active compound [2]. It is also used in agricultural products [3], fungicides [4], glucose sensors [5,6], photovoltaic devices [7,8], photocatalytic decomposition of water [9,10], dye-sensitized solar cells [11], piezoelectric and thermoelectric sensors [12], electrodes in Li+ batteries [13,14], fuel cell catalysis [15], pigments for ceramics [3], and for the removal of pollutants like I- [16], O3 [17], and dyes [18, 19]. For this last application, Cu2O recyclability may become an invaluable asset [20]. On the other hand, its use in glucose sensors represents
one of the most important niches regarding health, especially for those with diabetes; for example, a Ti3C2/Cu2O composite is used for the non-enzymatic photoelectrochemical glucose detection [6]. Alternatively, Cu2O is electrodeposited to provide a sensitive response to glucose and offers the advantage of low-cost production as compared to other alternatives [5].
Some of Cu2O's most promising applications rely on its properties as a semiconductor that allows the flow of electrons with an electrical conductivity smaller than a metallic conductor, and higher than an insulating material; excess electrons or holes yield the n-type or p-type semiconductor, respectively. In addition to its intrinsic characteristic as an ionic, non-stoichiometric p-type semiconductor, Cu2O can be artificially doped by a trivalent atom (acceptor impurity),
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CHEMICAL PROBLEMS 2024 no. 4 (22)
where each of the "holes" contributed by this trivalent species can accept a free electron [21, 22]. Its direct band gap energy, Eg = 2.17 eV [23] with an acceptor level at 0.4 eV above its valence band and an electron donor level at 1.1 eV below its conduction band —together with its low cost and toxicity, high optical absorption coefficient, and short minority carrier diffusion length make Cu2O a good candidate for solar conversion [7, 21].
Another potentially important use that takes advantage of Cu2O's semiconductivity is its application as a photocatalyst for the decomposition of water due to its favorable energetics and high chemical stability [24]. Photocatalysis in a p-type semiconductor implies that the majority charge carriers (i.e., holes) oxidize water to molecular oxygen, whereas the photogenerated minority charge carriers (i.e., electrons) reduce water to molecular hydrogen [9, 25]. In the case of Cu2O, its conduction band-edge is favorably located at 0.7 V more negative than the reduction potential of H+(aq) [26].
One of the current topics of greatest interest is sustainable energy, where the use of Cu2O in Li+ battery electrodes is gaining importance [14]. The benefits of these cells include high interconversions and a low degree of electrolyte decomposition. Due to its semiconducting nature, Cu2O is useful in these electrodes as it offers easy charging and discharging during electronic exchange with Li+ ions. Furthermore, due to its vacancies, it has viable sites for the Li+-promoted redox reactions to occur [14].
1.1. Cu2O Production
Cu2O production is achieved by different physical, chemical, and electrochemical methods such as pyrolysis [27], spraying or sputtering [28], vapor phase chemical deposition [29], thermal, hydrothermal, and chemical oxidation [16, 30], Cu(II) reduction [31], microgrowth within the pores of ion exchange resins [32], ultrasound and microwave assisted precipitation [24], and electrodeposition [13, 33]. The sputtering method, in addition to its high cost, has difficulties in the control of stoichiometry and the need for long deposition times to obtain coatings [34]. Pyrolytic spraying is an important alternative for industrial use and does not require a large investment, since the use of vacuum is avoided; however, the surface
deposits obtained are not entirely clean, which significantly influences the purity of the synthesized material [35]. Even at a considerable economic cost, composites of Cu2O thin films on carbon networks (CNW) have been designed with flexible structures [20].
A promising alternative for several of the above applications is the electrochemical pathway due to its low cost and eases of control. Here, Cu2O is produced by passing an electric current through a circuit consisting of two electrodes separated by an ionic aqueous medium (i.e., electrolyte), promoting oxidation-reduction reactions at the electrodes that cause the formation of insoluble deposits of Cu2O. This is possible due to the transfer of electrons between the electrode-substrate interface and the adjacent electrolyte [36]. Further discussion of these principles and the results of our preliminary tests to achieve the simultaneous production of Cu2O on both sides of an electrolytic cell now follow.
1.2. Electrochemical Synthesis of Cu2O
Electrodeposition has several practical advantages as it is performed in aqueous solutions at ambient pressure and relatively low temperatures [37], normally limited only by the boiling point of the electrolyte. The electrochemical production of Cu2O has been reported both cathodically and anodically [7, 22, 38-40]. Both the galvanostatic and the potentiostatic modes have been used [7, 21, 37, 40]. When working in the galvanostatic mode, there is a linear relationship between the deposition rate and time, and the deposits are essentially not affected by temperature, whereas in the potentiostatic mode the growth rate changes exponentially with deposition time and it is temperature dependent [38]. The temperature range is typically between 30 °C [38], and 86 °C [7]. The deposition requires a substrate that can vary among different metals such as Au [40], Cu [5, 7], Pt [38, 40], Ti [38], and stainless steel [21, 38].
While anodic electrosynthesis typically produces p-Cu2O [39], Cu2O can also acquire an «-type character by varying the pH of the electrodeposition bath since oxygen and copper vacancies are thereby generated. Excess copper in the lattice is believed to be responsible for the «-type behavior [39]. The pH and temperature can impact the final morphology of the deposit,
where the exposed atomic planes of the polycrystals can experience preferential orientation [41] probably due to alternate kinetic pathways [21, 42]. Electrodeposition has been reported at various pH values from 3.6 [38], to 12 [21]. For example, sheets deposited at pH = 9 show a uniform deposit with a preferential (100) crystal orientation of Cu2O grains, whereas at higher pH the grains show a (111) crystal orientation [38]. Current cycling is an alternative strategy to obtain single-phase, uniform and compact Cu2O cathodic films [43].
The electrolytic baths most commonly used involve CuSO4 at molarities ranging from 0.01 M to 0.4 M [7, 40]. The use of CuNO3 [44], and Cu(CH3COO)2 [38,39], as electrolytes has also been reported. For deposition at basic pH it is necessary to avoid Cu(OH)2 precipitation by using complexing agents such as lactate [21, 26, 41], citrate [14, 40], 1,3,5-benzentricarboxylate [5], among others. An essential factor in an electrochemical synthesis is the imposed potential (E) because, according to the corresponding Pourbaix diagram, this and the pH of the electrolyte determine the existence of the predominant species. The stability region for the formation of Cu2O under standard conditions in the Cu-Pourbaix diagram is rather small [46], which call for the need of complexing agents to amplify it and favor the formation of the semiconductor.
For CuNO3 in acidic media, the reaction mechanism involves a local pH increase prior to
the formation of Cu2O due to the OH- ions produced after the reduction of NO3- to NO2-[47]. On the other hand, the anodic deposition is simpler; it consists of performing the oxidation of a Cu(0) electrode at temperatures ranging from room temperature to 85 °C, where the required electrolyte is simply an ionic solution at different possible pH values [5]. The photovoltaic ability of Cu2O can be incremented by forming a heterojunction between CuO and Cu2O through the initial deposition of Cu2O on a Pt electrode and the subsequent application of heat treatment up to approximately 500 °C to convert part of the Cu2O to CuO [44].
1.3. Convergent Electrochemical Synthesis
The simultaneous convergent electrosynthesis of materials is very rarely attained [48, 49], and that of Cu2O has not been reported yet. By using an electrochemical cell divided by a cation exchange membrane, and adjusting the solution variables on both sides (i.e., pH, concentration, and temperature) as well as the applied potential, it is possible to form a Cu(II) lactate complex from which the cathodic deposition of Cu2O is enabled at the same time as the direct anodic conversion of Cu(0) to Cu2O. Using this rationale, we were able to obtain this oxide in both - the anodic and cathodic compartments - which opens a new frontier of possibilities for its production in a simple and economical way as described below.
2. Experimental part
The convergent synthesis of Cu2O was enabled in an H-type glass reactor divided by a Nafion membrane (Nafion 417, Aldrich). For the cathodic deposition, two solutions were prepared: a) 2.5 M lactic acid, C3H6O3 (as complexing agent), and b) 0.3 M CuSO4. The latter solutions were placed in the catholyte in equal volumes, and the pH of the resulting mixture was adjusted to 9 by adding 3.0 M NaOH, as necessary. The reported structure for the Cu(II) lactate complex is given in Fig. 1
[45].
For the anodic deposition, a 0.5 M Na2SO4 solution was prepared and its final pH adjusted to 9 - 11.3 with 0.1 M NaOH (see Fig. 2). All substances used were reagent grade (NaCl, J.T. Baker; Na2SO4, Sigma Aldrich; CuO, J.T. Baker; Cu2O, J.T. Baker; NaOH, Meyer), except lactic acid (80% pure, J.T. Baker), without further treatment and the solutions were prepared using deionized water.
Fig. 1. Structure of the Cu(II) lactate complex. Adapted with permission from Ref. [45].
Fig. 2. Schematic representation of the H-type glass cell used in this work for the convergent
electrosynthesis of Cu2O.
All experiments were performed at 65 °C for both the catholyte and the anolyte. The potentials were imposed with a TES-6100 power source and measured with a high-precision Steren model Mul-630 multimeter (Steren, Mexico). The electrodes used were 5 cm x 1 cm Cu plates, electrolytic grade
(minimum purity: 99.99%), gauge 26, pretreated with 50% HNO3 and then washed with distilled water. Current density stayed essentially constant at approx. 20 mA/cm2. An Ag/AgCl reference electrode (Bioanalytical Systems, USA) was used throughout the procedure.
3. Results and discussion
Plausible reactions occurring at each electrode are as follows: a) Anode [50, 51]
2Cu(s) + 4H2O(l) ^ 2Cu(OH)f (aq) + 4H+(aq) + 2e- (1)
2Cu(OH)2-(aq) ^ Cu2O(s) + 2OH~(aq) + H2OO) (2)
The excess Cu(II) present in the anolyte can migrate together with H+ ions through the Nafion membrane towards the cathode where
they become reduced as follows. b) Cathode [45]
2e-
Cu(II)(aq) Cu(s) (3)
Cu(s) + 2OH(aq) + [Cu(C3H5O3)4]2(aq) ^ Cu2O(s) + 4 C3H5Û3-(aq) + H2O(l) (4)
Fig. 3 shows optical images of the oxide The darker hue observed in the anodic deposit layers deposited on the anode and cathode of the may be due to disturbances in the crystal lattice. cell. The reddish hue is characteristic of Cu2O.
Fig. 3. Images of Cu plates with Cu2O deposited: (a) at the anode, and (b) at the cathode of the H-
cell.
Fig. 4. XRD of the Cu2O deposits from the anodic and cathodic convergent synthesis, and those obtained with a pure Cu2O sample. The conditions for the anodic deposits were: pHi = 11.38, pHf = 5.2, Vceiii = 4.55 V, Vceiif = 5.50 V, and for the cathodic deposits they were: pHi = 10.4, pHf = 9.18; Vceii,i = 0.40 V, Vceiif = 0.46 V. t= 15 min, and T = 65 °C.
To ascertain the nature and characteristics of the as-obtained deposits, structural and morphological characterizations were performed with a Hitachi model SU3500 Scanning Electron Microscope (SEM) and a Bruker model D8 Advance ECO X-Ray Diffractometer (XRD).
The initial and final pH values (pHh pHf), and the initial and final cell voltages (Vceii,i, Vcell,f ) are given below.
Fig. 4 shows the obtained XRD results of the Cu2O deposits from the anodic and cathodic convergent syntheses, and those obtained with a
pure Cu2O sample. Fig. 5 shows the summary comparison. The peaks at 20 = 29.8, 36.6, 42.7, 52.6, 61.7, 66.1, 69.9, and 73.6 ° correspond to the (110), (111), (200), (211), (220) (221), (310), and (311) planes, respectively, in good agreement with the corresponding JCPDS data for polycrystalline Cu2O (JCPDS card No. 050667). Concerning the possible presence of CuO, the lack of peaks at 20 = 37.1 61.8 and 74.3 ° that correspond to the (200), (220), and (311) CuO planes indicates their absence, although the peak at 20 = 36.6 ° may indicate that some CuO is present in the (111) phase.
i 4000 | 2000 1101 4 III) 1 1 . • (a) Aikiilu: ckfKtxil
1 | | I I I |(»m
25 30 35 40 45 50 55 60 20(J) 65 70 75 SO K5 90
8000 g-6000 g 4000 3 2000 (b) Caibodx deposit
25 30 35 40 45 50 55 60 2Bf >1 65 70 75 SO S5 90
6ooo
§ 4000 § j| 2000 0 25 It 1 1 1 1 • <c) C»:0
(31 pO)
■ (•20)
1 30 35 1 40 . 45 50 1 55 60 200 1 65 1 70 1 75 SO 85 90
«000 €'4000 g J 2000 (1 i" 1 <200) • (d) CuO
1 1 (2101 1 (711) |
0 30 40 45 <0 55 60 20 i'l 65 70 75 SO R5 90
2000 g-1500 g 1000 3 500 0 25 I' 1 1 • <«> Cu
1 1 1
30 35 40 45 50 55 60 29 (*) 65 7« SO S5 90
Fig. 5. XRD summary of the Cu2O deposits from the anodic and cathodic convergent synthesis. The conditions for the anodic deposits were: pHi = 11.38, pHf = 5.2, Vcell,i = 4.55 V, Vcell,f = 5.50 V, and for the cathodic deposits they were: pH, = 10.4, pHf = 9.18; Vceii,, = 0.40 V, Vceiif = 0.46 V. t = 15 min, and T = 65 °C. XRD line patterns of pure Cu2O, CuO and Cu are also shown for comparison.
a) b)
Fig. 6. SEM images of Cu2O prepared by convergent electrosynthesis. (a) Anodic deposit (pHt = 11.38, pHf = 5.20, Vceti,i = 4.55 V, Vceii,/ = 5.50 V), and (b) cathodic deposit (pHt = 10.40, pHf = 9.18; Vceii, t = 0.40 V, Vceiif= 0.46 V). t = 15 min, and T = 65 °C.
4. Conclusions
Adequate conditions were found for the simultaneous convergent deposition of Cu2O at the anode and cathode of an electrochemical cell. Using SEM and XRD, the presence of Cu2O was verified in both compartments, with some possible CuO contamination. The use of
an electrochemical process as an alternative synthetic route is very attractive and has several advantages, including the low cost of the equipment and reagents, and its ease of application.
Acknowledgments
K. G., G. N. and F. A. acknowledge student assistantships provided by the National Council for Science and Technology (CONACYT, Mexico) numbers 893221, 843910, and 843508 respectively.
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GÜN9§ ENERJiSi ÇEVÎRÎCiLaRÎ ÛÇÛN Cu2O-iN EYNi ZAMANDA ANOD V9 KATOD ELEKTROÇÔKDÛRÛLMaSi
Kevin F. Genaro-Saldivar1, Geovanni R. Neqret-Reyes1, Ester Ramirez-Meneses1, Karlos Xuarez-Balderas2, Flor Avendano-Sanjuan1, Diana A. Qarsiya-Najera1, Xorxe Q. ibanez1'*
1Kimya, Ssnaye vs Qida Mühsndisliyi Departamenti, iberoamerikan Universiteti, Mexi ko, Meksika
2innovasiya üzrs Mühsndislik Ara§dirmalari Departamenti, iberoamerikan Universiteti, Mexiko, Meksika
*e-mail: jorge.g.ibanez@gmail.com
Xulasa: Mis oksid Cu2O yuksak/a§agi temperaturlu termiki oksidla§ma, puskurtma, kimyavi oksidla§ma, anodda oksidla§ma, elektro9okma va s. muxtalif usullarla alina bilan 90X universal mahsuldur. indiya qadar Cu2O almaq u9un an samarali sintetik proseslar kifayat qadar bahali avadanliq talab edir. Bunu nazara alaraq, biz bu i§da normal tazyiq va nisbatan a§agi temperaturlarda elektrolitik dovrada hayata ke9irilan eyni vaxtda konvergent oksidla§ma-reduksiya reaksiyalarina asaslanan yeni metod i§layib hazirlami§iq. Cu(0) lovhasi anodda Cu(I)-a qadar oksidla§ir, dovranin katod hissinda isa Cu(II)-laktat kompleksi amala galir. Sonra Cu(II) ionu Cu(I)-a qadar reduksiya olunur (butun proseslar sulu muhitda aparilir).
ОДНОВРЕМЕННОЕ АНОДНОЕ И КАТОДНОЕ ЭЛЕКТРООСАЖДЕНИЕ Си20 ДЛЯ ПРЕОБРАЗОВАТЕЛЕЙ СОЛНЕЧНОЙ ЭНЕРГИИ
Кевин Ф. Хенаро-Сальдивар1, Джованни Р. Негрете-Рейес1, Эстер Рамирес-Менесес1,
2 11
Карлос Хуарес-Бальдерас , Флор Авендано-Санхуан , Диана А. Гарсиа-Нахера , Хорхе
Г. Ибанез1'*
1 Факультет химической, промышленной и пищевой инженерии, Ибероамериканский университет,
Мехико, Мексика
2Факультет инженерных исследований инноваций, Ибероамериканский университет, Мехико,
Мексика *е-таИ: ibanez@gmail.com
Резюме: Оксид меди Си20 представляет собой универсальный продукт, который можно получить различными методами, включая высоко/низкотемпературное термическое окисление, напыление, химическое окисление, анодное окисление, электроосаждение и другие. До сих пор наиболее эффективные синтетические процессы получения Си20 требуют относительно дорогого оборудования; поэтому мы разработали новый метод, основанный на одновременных конвергентных окислительно-восстановительных реакциях в электролитической ячейке при атмосферном давлении и относительно низкой температуре. В данной работе пластина Си(0) окисляется до Си(1) в аноде, а в катоде ячейки образуется комплекс Си(11)-лактат, а затем Си(11) восстанавливается до Си (I) (все реакции протекают в водных средах).
