ANALYSIS TECHNOLOGY

Effects of Inorganic Ions for Reduction and Growth ofCopper Oxides
Shigeyoshi NAKAYAMA---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------For the quantitative characterization of copper corrosion products, the author has developed a new voltammetric method that uses
a strongly alkaline solution (6 M KOH + 1 M LiOH) as the supporting electrolyte. Using this method, he investigated the growthmechanism of copper oxides (i.e., Cu2O and CuO). When copper was heated with no adherent ions supplied, both Cu2O and CuO
formed and grew. However, they showed different growth mechanisms when pre-immersed in salt solutions, and heated underhigh humidity. Cu2O grew selectively when copper was pre-immersed in LiCl and LiBr solutions, most likely due to Li+ ionspermeating into the Cu2O layer and continuing proportionation (Cu + CuO → Cu2O). On the contrary, CuO grew at the early stageof heating when copper was pre-immersed in a KOH solution presumably because of the high OH– concentration in the water layer
deposited on the copper surface.
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Keywords: Copper Oxides, Voltammetry, Strongly Alkaline Electrolyte, Li+ Ion

1. Introduction
Beginning thousands of years ago, copper ismankind’s longest used metal. Besides its relatively highcorrosion resistance, copper has high electrical and thermalconductivity. Owing to these characteristics, copper and its
alloys have been widely used to fabricate industrial products, coins, and artifacts. When copper is exposed to air
containing moisture and pollutants, however, various corrosion products (e.g., Cu2O and CuO) are formed on copper
surfaces. Copper corrosion products often have a bad influence on industrial products. For example, tarnishing viaoxidation frequently reduces the commercial value ofcopper products.
While this is true, copper oxides have been actively
utilized for various applications, such as the adhesion of acopper product and another material,(1) the sealing of electronic packaging,(2) and the electrodes of solar cells.(3) In
the study of solar cells, layers of Cu2O (bandgap: 2.1 eV)(4)and CuO (bandgap: 1.35 eV)(5) were prepared by cathodic
deposition from aqueous solutions. For these applications
as well as for corrosion management, it is important toperform qualitative and quantitative analyses on different
copper species, which have different chemical properties.
A new voltammetric method for the quantitative characterization of copper oxides and other copper species(6),(7)was developed. In this method, the use of a strongly alkaline electrolyte (SAE; i.e., 6 M KOH + 1 M LiOH) allowed
perfect separation between two current peaks due to thereduction of Cu2O and CuO. The specific inhibitory effectof Li+ ions on Cu2O reduction was discovered.
In the atmosphere, Cu(OH)2 was the initial corrosion
product formed on a copper surface. However, the surface
Cu(OH)2 layer did not grow much but dehydrated tobecome a layer of CuO. For further progress of corrosion,
an inner Cu2O layer was generated by the proportionation
reaction between the CuO layer and the base metal Cu. The
inner Cu2O layer grew for the subsequent oxidation until
the thickness reached a certain value (~35 nm). For furtheroxidation, the top CuO layer grew again preferentially over

68 · Effects of Inorganic Ions for Reduction and Growth of Copper Oxides

the inner Cu2O layer.(8) It was difficult to generate a single
layer of Cu2O or CuO under normal atmosphere conditions.
Several preparation procedures for copper oxides havebeen reported.(9),(10) However, no research has shown theformation mechanism of copper oxide.
This paper mainly describes the effects of inorganic
ions for the growth of copper oxides. In particular, it
reveals that Li+ ions had an inhibitory effect(11) of furtheroxidation of Cu2O in addition to a specific effect on thereduction behavior of Cu2O.

2. Experimental
2-1 Samples
For the clarification of the effect of alkali metal ionsin the reduction mechanism of copper oxides, three standard samples (i.e., Cu2O/Cu(12),(13), CuO/Cu(13) and CuO/Cu2O/Cu(12)), each having an almost constant thickness(~1 μm) of Cu2O and/or CuO layer(s) formed on metal Cu
were prepared.
Oxide-free copper plates (0.5 mm thick: 99.99%purity) were used for the copper substrate for the growth ofoxide layers. Each copper plate was cut into 20 × 50 mmpieces and then etched in 3 M sulfuric acid, followed byrinsing successively with distilled water. Copper plates
were heated at 300°C in a muffle furnace, or immersed invarious inorganic solutions for 1 hour and then subjected toheating in a temperature/humidity-controlled chamber at80°C and relative humidity (RH) of 90%.
2-2 Electrochemical measurement
A conventional three electrode electrolytic cell wasemployed for voltammetric measurements. An Ag/AgCl (in3 M NaCl) electrode was used as the reference electrode, towhich all potentials are referred to in this paper. A platinum
wire was used as the counter electrode. Unless otherwise
noted, the electrolyte solution used was SAE (6 M KOH +1 M LiOH), which was not deaerated. All of the reagentsused for the preparation of the electrolyte solutions were ofanalytical grade. Linear sweep voltammetric (LSV)
measurements were carried out using an electrochemicalmeasurement system (Hokuto Denko Co. Ltd., Japan,
HZ-5000). A potential sweep was performed from a rest
potential to the potential for the hydrogen evolution at
various sweep rates in consideration of the thickness of thefilm of copper oxides. The measurement area was 1.0 or
2.0 cm2.

3. Result and Discussion
3-1 Effect of Li+ ions in reduction of copper oxides
Adequate separation of the reduction potentials ofCu2O and CuO was successfully achieved in SAE as asupporting electrolyte.(6)Figure 1 shows current–potential curves obtained by
LSV for three standard samples. As clearly demonstrated inFig. 1. Current–potential curves recorded by LSV for (a) Cu2O/Cu,(b) CuO/Cu, and (c) CuO/Cu2O/Cu samples.
Measurement area: 1.0 cm2. Scan rate: 1 mV/s.

the figure, CuO is firstly reduced, prior to the reduction ofCu2O. The mass of each oxide layer can be calculated using
Faraday’s Law from each peak area, i.e., the quantity ofelectricity required for the reduction of the oxide layer. A
highly selective determination of Cu2O and CuO can be
performed using SAE.
Figure 2A(14) shows current–potential curves recordedby LSV for the Cu2O/Cu and CuO/Cu in 1.0 M of alkalihydroxides (i.e., LiOH, NaOH, and KOH).
As can be seen in the figure, the reduction of Cu2O isthermodynamically easier in the order of KOH > NaOH >LiOH. On the other hand, the reduction potential of CuO ishardly affected by the kind of alkali hydroxide, as also
shown in Fig. 2A. As a result, a good separation of thereduction potentials of Cu2O and CuO could be achieved in
1.0 M LiOH (see Fig. 2B). Also, when KOH is used, thereduction potential of Cu2O shifted to lower values with anincrease in KOH concentration.(15) A sufficient separation
between the two reduction potentials is achieved withLiOH rather than with KOH. It has thus been found that,
although the solution alkalinity has a definitive effect, Li+ions have a specific effect on the reduction behavior ofCu2O. Taking into consideration the solubility of reagents,
SAE has been proposed as the best electrolyte for the selective determination of copper oxides.
3-2 Normal heating without pre-immersion
In the absence of corrosive gases, the major corrosion
products of copper are its oxides (i.e., Cu2O and CuO). In
the atmosphere, Cu2O is first formed, followed by theformation of CuO and its growth by further oxidation.(6),(8)Figure 3 shows current–potential curves for the copper
plates heated at 300°C for 10–30 minutes. The peaks ofCu2O and CuO appeared and grew together. In the initialstage of corrosion, however, Cu2O seems to have formed
before CuO, because Cu2O was the main oxide species
formed by heating at a lower temperature, i.e., 250°C for
10 minutes (data not shown). The corrosion behavior at
higher temperatures such as 300°C was relatively simple.
The corrosion products were only CuO and Cu2O.

Fig. 2. Current–potential curves recorded by LSV for (A) Cu2O/Cu (dashed line) and CuO/Cu (solid line) and (B) CuO/Cu2O/Cu in 1.0 M of three alkali hydroxides.
Measurement area: 1.0 cm2. Scan rate: 1 mV/s.

SEI TECHNICAL REVIEW · NUMBER 85 · OCTOBER 2017 · 69

Fig. 3. Current–potential curves recorded by LSV for copper plates heated at300 °C for 10, 20, 30 min. Measurement area: 2.0 cm2.
Scan rate: 10 mV/s.

Figure 4 shows current-potential curves recorded byLSV for copper plates heated at 80°C and RH 90%.(8)A current peak is observed around -0.8 V for thesample treated for a short period (i.e., 10 minutes).
Referring to the results for standard powder samples,(8) thepeak may be ascribed to the reduction of Cu(OH)2.
Prolonging the treatment period around 30 minutes, thereduction peak of Cu2O appeared at -1.3 to -1.4 V and thengrew with an increase in the treatment period around 1 day.
The limiting thickness of the Cu2O layer is likely the result
from the competition of the proportionation reaction (Cu +CuO → Cu2O)(16) and the CuO formation (Cu2O + (1/2)O2→ 2CuO). The threshold value of the Cu2O-layer thickness(~35 nm) probably depends on unknown factors thatgovern the kinetics of the competing reactions.

3-3 Growth mechanism of copper oxides after
pre-immersion in salt solutions
Copper plates were prepared by pre-immersion invarious salt solutions, followed by heating in air at 80°C
and RH 90%. The formation kinetics of each oxide film
followed the parabolic rate law with a few exceptions.(6)The effects of pre-immersion of copper samples in various
0.1 M sodium salt solutions (i.e., NaCl, Na2SO4 andNaNO3) on the growth of oxide layers were studied.(17) The
growth rate of copper oxides on a copper surface wasfastest with pre-immersion in 0.1 M NaCl. As seen in Fig. 4
as well, there was a time lag after the initiation of Cu2O
formation before the CuO began to grow. In the case ofpre-immersion in NaNO3, CuO grew selectively.
Based on the specific effect of Li+ ions on the reduction behavior of Cu2O, other salts were used for preimmersion. Figure 5 shows current-potential curves
recorded by LSV for copper plates heated at 80°C and RH
90%. Three 0.1 M salt solutions (i.e., KCl, LiCl andLi2SO4) were used for pre-immersion. In Fig. 5(a), bothCu2O and CuO grew with the heating period. On thecontrary, Cu2O selectively grew in (the case of using) LiClsolution for pre-immersion. About 1 μm of Cu2O layer wasconfirmed after a heating period of 3 days. In Fig. 5(c),reduction peaks of copper oxides were not detected on thiscurrent scale. The difference was observed in oxidation
performance on copper surfaces between LiCl and Li2SO4.
This must have been due to the difference in complex
forming power between chloride ions and sulfate ions.

Fig. 5. Current–potential curves recorded by LSV for samples prepared by
pre-immersion of copper plates in (a) 0.1 M KCl, (b) 0.1 M LiCl, and(c) 0.1 M Li2SO4, followed by heating in air at 80°C and RH 90% for
8, 16 h and 1 day, and 3 days.
Measurement area: 2.0 cm2. Scan rate: 2 mV/s.

Fig. 4. C urrent–potential curves recorded by LSV for copper plates heated at80°C and RH 90% for (a) 10 min–4 h and for (b) 1–4 days.
Measurement area: 2.0 cm2. Scan rate: 100 mV/s.
Scan rate: 100 mV/s.

70 · Effects of Inorganic Ions for Reduction and Growth of Copper Oxides

Figure 6 shows current-potential curves recorded by
LSV heated at 80°C and RH 90%. Two 0.1 M salt solutions(i.e., BrCl and LiCl) were used for pre-immersion. As
shown in Fig. 6 (a), Cu2O grew selectively when using LiClfor pre-immersion as well. Bromide ions probably possess
complex forming power in the same manner as chloride2CuCl + H2O → Cu2O + 2Cl– + 2H+ ...................... (3)Current-potential curves in Fig. 4 show that CuO grew
rapidly when the Cu2O-layer was more than a certain level
under normal heating treatments. On the contrary, Cu2O
grew selectively when a 0.1 M LiCl solution was used forpre-immersion. As the ionic radius of Li+ ions is verysmall, Li+ ions probably entered the Cu2O-layer, and thencould continue the proportionation reaction (Cu + CuO →Cu2O) by the high ion conductivity of inserting Li+ ions.

Fig. 6. Current–potential curves recorded by LSV for samples prepared bypre-immersion of copper plates in (a) 0.1 M BrCl and, (b) 0.1 M LiCl,followed by heating in air at 80°C and RH 90% for 8, 16 h and 1 day,and 3 days. Measurement area: 2.0 cm2. Scan rate: 2 mV/s.

ions. However, the amounts of Cu2O were half comparedwith using LiCl.
Figure 7 shows current-potential curves recorded by
LSV for copper plates heated at 80°C and RH 30%, 60%,90%. 0.1 M LiCl was used for pre-immersion. Under low
RH conditions, the Cu2O-layer did not grow too much.
Fig. 8. Current–potential curves recorded by LSV for copper plates(A) incubated in 1 M NaOH at 30°C for 8-32 hours, and (B) preparedby pre-immersion in (a) 0.1 M NaOH, followed by heating in air at80°C and RH 90% for 8 hours, 1 day, and 3 days.
Measurement area: 2.0 cm2. Scan rate: (A) 10 mV/s, (B) 2 mV/s.

Fig. 7. Current-potential curves recorded by LSV for copper plates heated at80°C and RH 30, 60, 90% for 2 days.
Measurement area: 2.0 cm2. Scan rate: 2 mV/s.

In the presence of chloride ions, dissolved species ofmonovalent Cu+ would react with Cl– to form CuCl, which
is more stable than CuOH. CuCl would dissolve in thesolution as a Cu chloride complex ion, and at the same time
would yield Cu2O through hydrolysis. This reaction seemed
to be accelerated under high humidity conditions.
Cu+ + Cl– → CuCl ................................................... (1)CuCl + Cl– → CuCl2– .............................................. (2)Figure 8 (a) shows the current–potential curves
recorded for the copper plates incubated in 1 M NaOH at30°C for 8-32 hours. Cu2O grew with an increase in theimmersion period around 8 hours. However, the reduction
peak of Cu2O at around -1.4 V was almost constant inheight for the immersion periods of 8-32 hours. It wasspeculated that with heating treatment of more than 8hours, the oxide film had a homogeneously covered
CuO-layer (CuO/Cu2O/Cu(12)) in which the inner Cu2Olayer was stabilized. After the growth of Cu2O stopped, aCuO-layer grew rapidly, as can be seen in the figure. After
16 hours of incubation, the copper surface became black.
Figure 8 (b) shows current-potential curves recordedby LSV for heated copper plates previously immersed in
0.1 M NaOH. Differing from the current-potential curves
in Fig. 8 (a), CuO grew selectively for increasing heatingperiods. The early start of CuO formation was probably
due to a high OH– concentration in the water layer deposited on the Cu surface and the resultant quick saturation ofCu(OH)2. Cu(OH)2 is converted by dehydration to CuO
according to the reaction
Cu(OH)2 → CuO + H2O ......................................... (4)As mentioned above, Cu2O or CuO grew selectively

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on copper surfaces heated under high humidity conditions(e.g., 80°C + RH90%) on copper plates previously immersed
in specific salt solutions.

4. Conclusion
A new voltammetric method using SAE as thesupporting electrolyte was mainly applied for the investigation of the growth mechanism of copper oxides. Effects ofvarious adherent inorganic ions were studied, and thefollowing conclusions were drawn.
1. Under normal heating treatments using no adherent
ions on a copper surface, both Cu2O and CuO
formed and grew. It was difficult to generate asingle layer of Cu2O or CuO. At an early stage ofthe oxidation of copper, Cu2O grew selectively.
However, the growth of the inner Cu2O layer
appeared to stop after a certain time lag under thegiven conditions. Instead, the surface CuO layer
grew rapidly.
2. Different growth mechanisms of copper oxidesunder a high humidity condition using various salts
for pre-immersion of copper plates were shown.
Cu2O grew selectively on copper surfaces due toadherent Li+ ions and halogen ions (i.e., Cl– andBr–). Li+ ions probably enter in the Cu2O-layer, andthen could continue the proportionation reaction
inhibiting the growth of CuO. In the case of using aKCl solution, both Cu2O and CuO grew. On thecontrary, CuO grew selectively from the early stageof heating on copper plates pre-immersed in a KOHsolution. This behavior was perhaps due to the highOH– concentration in the water layer deposited on
the copper surface.

5. Acknowledgements
In the course of this study, the author received valuable advice from Dr. Takenori Notoya, Technical Adviser
at Japan Copper and Brass Association (JCBA), whoprovided detailed information regarding the mechanism ofcopper corrosion, and Dr. Toshiyuki Osakai, AssistantProfessor at Kobe University, who shared insight into datacollected via voltammetric reduction.

Technical Term

＊1	Voltammetry: Voltammetry is one of the most applied
electrochemical method for analyzing redox reactions
on a working electrode. By sweeping a potential of
the electrode, a current responding to the potential ismeasured.

72 · Effects of Inorganic Ions for Reduction and Growth of Copper Oxides

References(1)	Y.-Y. Su and R. M. Shemenski, Appl. Surf. Sci., 161, 355 (2000)(2)	M. Lebbai, J. K. Kim, W. K. Szeto, M. M. F. Yuen, and P. Tong,
J. Electron. Mater., 32, 558 (2003)(3)	T. Mahalingam, J. S. P. Chitra, J. P. Chu, H. Moon, H. J.Kown, andY. D. Kim, J. Mater. Sci.: Mater. Electron., 17, 519 (2006)(4)	K. Mizuno, M. Izaki, T. Shinagawa, M. Chigane, K. Murase, M. Inaba,
A. Tasaka, and Y. Awakura, J. Electrochem. Soc., 152, C179 (2005)(5)	M. Izaki, M. Nagai, K. Macda, B.M. Fariza, K. Motomura, J. Sasano,T. Shinagawa, S. Watase, J. Electrochem. Soc., 158, D578 (2011)(6)	S. Nakayama, A. Kimura, M. Shibata, S. Kuwabata, and T.Osakai,
J. Electrochem. Soc., 148, B467 (2001)(7)	S. Nakayama, T. Notoya, and T. Osakai, Anal. Sci., 28, 323 (2012)(8)	S. Nakayama, T. Notoya, and T. osakai, J. Electrochem. Soc., 157,C289 (2010)(9)	W. H. Safranek, Dou-to-Gijutsu, 5, 13 (1968) (in Japanese)(10)	T. kojima, J. Surf. Fin. Soc. JPN, 50, 322 (1999)(11)	S. Nakayama, JP Patent, No. 2013-177257(12)	S. Nakayama, T. Kaji, T. Notoya, and T. Osakai, J. Electrochem. Soc.,
154, C1 (2007)(13)	S. Nakayama, M. Shibata, T. Notoya, and T. Osakai, Bunseki Kagaku
51, 1145 (2002) (in japanese)(14)	S. Nakayama, SEI. Tec. Rev., 82, 97 (2016)(15)	S. Nakayama, T. Kaji, T. Notoya, and T. Osakai, Electrochim. Acta, 53,3493 (2008)(16)	J. Kruger, J. Electrochem. Soc., 108, 503 (1961)(17)	S. Nakayama, M. Shibata, S. Kuwabata, T. Osakai, and T. Noyoya,
Zairyo-to-Kankyo, 51, 566 (2002)

Contributor

S. NAKAYAMA• ‌Ph. D.
Senior Specialist
Senior Assistant General Manager, Analysis
Technology Research Center