One-step transformation of Cu to Cu₂O in alkaline solution

Abstract

Cu can be directly transformed to Cu₂O before forming CuO in high concentration NaOH aqueous solution by facile surface oxidation reaction without additional oxidant. By using different Cu substrates, pure Cu₂O films with different dominant facets can be obtained.

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Copper (Cu), an abundant and ductile metal element with very high electrical and thermal conductivity, has two oxides: cuprous oxide (Cu₂O) and cupric oxide (CuO). Both of them are p-type semiconductors with narrow band gaps of 1.9–2.2 eV and 1.2–1.7 eV, respectively;¹ they are good candidates for solar energy conversion photocatalysts, sensor materials, stable electron sources and optoelectronic device materials. Normally, CuO is dark color since it can absorb all of the visible light even in the infrared range. Although both of them as photocathode materials have poor chemical stabilities for water oxidation and reduction in aqueous solutions, CuO is more unstable than Cu₂O because its redox potential of E_CuO/Cu2O (+0.60 V vs. NHE) is more positive than the redox potential of E_Cu2O/Cu (+0.47 V vs. NHE);² CuO will become Cu₂O followed by Cu₂O reduction to Cu. Therefore, Cu₂O is more valuable than CuO in water splitting area and researchers have focused on improving the stability of Cu₂O by depositing protective layers such as ZnO/Al₂O₃/TiO₂.²

Many works have been done for the synthesis of copper oxides regardless of one intractable issue of chemical stability in electrolyte. The copper oxides/hydroxide (CuO, Cu₂O and Cu(OH)₂) and Cu can be transformed to one of them by different methods. Cu can be transformed to CuO and Cu₂O by thermal oxidation at high temperature in oxygen atmosphere. Cu₂O is stable at limited ranges of temperature and oxygen pressure and during thermal oxidation of Cu at atmospheric pressure, Cu₂O can be transformed to CuO after a sufficient oxidation time.³ Musa et al.⁴ reported that the oxide layer resulting from oxidation at 1050 °C consist only of Cu₂O and those grown below 1040 °C gave mixed oxides of Cu₂O and CuO; they also observed that, in general, the lower temperature of oxidation is, the lower amount of Cu₂O is formed. Choi's group⁵ also reported that Cu₂O film can be converted to transparent CuO film by heating in air. For morphology control, CuO wires can also be easily synthesized by heating copper substrates such as Cu TEM grid, foil and conventional electrical wire, even the electrodeposited Cu particle films.^1c,6 Wang's group⁷ reported the growth of a scroll-type nanotube structure of Cu(OH)₂ arrayed on copper foil at ambient temperature and pressure by surface oxidation of the Cu foil in an alkaline aqueous solution with the oxidant (NH₄)₂S₂O₈. Later, Yat Li's group⁸ reported that the Cu(OH)₂ nanowires obtained via the method mentioned above can be converted into Cu₂O nanowires with a small fraction of CuO nanowires by thermal treatment at 450 °C for 1 h in air. Pike et al.⁹ have shown, using in situ time-resolved X-ray diffraction (TR-XRD), that bulk CuO was reduced directly to metallic Cu; and nanoscale CuO can be reduced completely to Cu₂O by isothermal reduction.

As described above, Cu → Cu₂O and/or CuO, Cu₂O → CuO, Cu(OH)₂ → Cu₂O and CuO, CuO → Cu₂O can be achieved by thermal oxidation or reduction. However, for the complete conversion of Cu to Cu₂O it requires very high temperature.⁴ To the best of our knowledge, there are very few reports on the transformation of Cu to Cu₂O under mild experimental conditions. The Cu surface can be electrochemically oxidized to Cu₂O, CuO and Cu(OH)₂ at different potentials in alkaline solution.¹⁰ However, the layer of Cu₂O films was very thin. In addition, Allam and Grimes¹¹ reported that the various copper oxide nanostructured thin films were synthesized by anodization of Cu foil in aqueous and non-aqueous electrolytes containing hydroxide, chloride and/or fluoride ions at room temperature. Chu et al.¹² synthesized CuO crystals with different morphologies such as nanoplates, nanoribbons, nanowires, micro-polyhedrons, mat-like and chrysanthemum-like nanostructures via hydrothermal reactions with different types of surfactants and (NH₄)₂S₂O₈ in high concentration of NaOH aqueous solution (5.0 M). Liu et al.¹³ demonstrated a facile fabrication of flower-like and spherical CuO architectures on Cu substrates by immersing a Cu substrate into low concentrations (30.0 or 65.0 mM) of NaOH and NH₃·H₂O solutions at 60 °C. Very recently, Luo et al.¹⁴ showed that Cu₂O polyhedrons can be directly grown from Cu foil in 0.3 M NaOH at 60 °C. Gao et al.¹⁵ have reported an introduction of Cu₂O layer on Cu plate by immersing it into CuSO₄ solution. To our surprise, there is no detailed report on the mechanism and process of synthesis and morphological control of pure Cu₂O by simply immersing the different Cu substrates into NaOH solution. Herein, we report a facile process for transformation of Cu foil and Cu film^1c to pure Cu₂O layer and films without impurity of CuO or Cu(OH)₂ in high concentration of NaOH aqueous solution without any surfactants and additives. On the factors influencing the Cu₂O film formation, such as alkali concentration, temperature, reaction time and, especially, the most critical factor for Cu substrates with different dominant crystal facets have been discussed.

It is known that the relation of the surface energy (γ) of bulk face-centered cubic (fcc) metals is γ₁₁₁ < γ₁₀₀ < γ₁₁₀.¹⁶ The relation of the surface energy of fcc Cu is also a decreasing order as γ_Cu(111) < γ_Cu(100) < γ_Cu(110).¹⁷ Therefore, the different dominant facets of bulk fcc Cu in a solution will have different adsorption–desorption rate and chemical reaction rate. Herein, three different kinds of Cu foil and Cu/FTO film substrates, as their XRD patterns indexed as fcc-phase copper (JCPDS no. 65-9743) shown in Fig. 1(a) and 4(a2), were conducted for the experiments. The peak intensity ratios of I₍₁₁₁₎/I₍₂₀₀₎ and I₍₂₂₀₎/I₍₂₀₀₎ are 0.66 and 0.39 for Cu₁, 0.04 and 0.13 for Cu₂ and 2.92 and 0.52 for Cu/FTO, respectively. A typical experimental process of one-step transformation of Cu to Cu₂O was conducted by immersion of Cu substrates in NaOH aqueous solution at 80 °C for 1 h and kept at room temperature (24 °C) during various aging times, such as 14 h, as shown in Scheme 1(a). After the reactions, the as-synthesized film was washed by deionized water, ethanol and then dried under N₂ gas flow. The brownish red colored film was covered on the surface of Cu foil and the transparent film with darker orange color was formed on FTO glass as shown in Fig. 1(b). According to the description in ref. 12, 13 and 18, the possible reaction processes are given as following (see the detail explanation; ESI†): Cu foil or Cu/FTO can continuously release Cu²⁺ ions into NaOH solution by the naturally-dissolved O₂ oxidization

2 Cu + O₂ + 2H₂O → 2Cu²⁺ + 4OH⁻ (1)

Fig. 1 (a) XRD patterns of two different copper foils (Cu₁ and Cu₂) with different intensity ratios of crystal facets; (b) the photographs to show the typical experimental processes of the one-step formation of Cu₂O film on Cu foil and FTO coated glass surface by immersion in NaOH solution.

Scheme 1 Schematic illustration of (a) the preparation process and (b) the growth mechanism.

Then, some of Cu⁺ ions can be produced by the process of

Cu²⁺ + Cu → 2Cu⁺ (2)

The maximum concentration of [Cu⁺] is determined by the equation of

log [Cu⁺]_max = −0.84 – pH (ref. 18c) (3)

If the real concentration of Cu⁺ exceeds the [Cu⁺]_max given by eqn (3), the following process will be done.

2Cu⁺ + 2OH⁻ → Cu₂O + H₂O (4)

This process is similar to that Cu²⁺ ions can be reduced to Cu⁺ ions by Cu plate then form Cu₂O layer on Cu surface through in situ redox reaction according to eqn (5) in the ref. 15.

Cu²⁺ + Cu + H₂O → Cu₂O + 2H⁺ (5)

The transformation of Cu to Cu₂O in our experiment can be carried out by the net reaction as follows:

4Cu + O₂ → 2Cu₂O (6)

In summary, we present a growth mechanism of Cu₂O from Cu in high concentration alkaline solution with 5 steps as shown in Scheme 1(b). Step 1, the O₂ in air is dissolved in alkaline solution. Step 2, the dissolved O₂ is adsorbed on the surface of Cu. Steps 3, the Cu atoms in surface are oxidized to Cu²⁺ ions by the adsorbed O₂ as the reaction (1). Step 4, the Cu²⁺ ions can be reduced to Cu⁺ ions by neighboring Cu atoms during very short time as the reaction (2). Step 5, the Cu⁺ ions will react with OH⁻ in solution to form Cu₂O nucleation sites and H₂O molecules are released from the Cu surface as the reaction (4). The obtained Cu₂O nucleation sites will grow according to reaction time. Finally, the nano/micro-sized Cu₂O crystals can be formed. In the whole Cu₂O growth process, steps 3 to 5 are happened in situ on Cu surface. The step 2 is a key to open the next reactions, which depends on the adsorption of O₂ by Cu surface. Therefore, the surface property of Cu substrate is a critical factor for the experiment.

The XRD pattern of Cu₂O/Cu₁ can be fully indexed to a pure cubic phase with space group of Pn3̄m (no. 224) of Cu₂O (JCPDS no. 65-3288) except the peaks of Cu₁ substrate as shown in Fig. 2(a). The physicochemical properties of crystals such as adsorption, catalytic reactivity and selectivity etc., significantly depend on the surface atomic configuration and the degree of exposure of reactive crystal facet.¹⁹ The dominant crystal facet of Cu₂O/Cu₁ is (111). The corresponding SEM image as shown in Fig. 2(b) indicates that the compact and homogenous Cu₂O octahedrons particle film is covered on the Cu₁ foil. The low magnification SEM images, TEM image and SEAD pattern in Fig. S1 (ESI†) further demonstrate that the Cu₂O octahedron film with the average particle size of less than 1.5 μm is in large area scale and the particles are single crystalline. Since the surface energy order is γ_Cu(111) < γ_Cu(100) < γ_Cu(110) as previously stated, the Cu₁ with high I₍₁₁₁₎/I₍₂₀₀₎ needs longer reaction time (3 days) to form regular Cu₂O octahedron film.

Fig. 2 (a) XRD pattern and (b) SEM image of Cu₂O/Cu₁ obtained by adding Cu₁ foils in 20 ml 4 M NaOH aqueous solution followed by heating at 80 °C for around 1 h and then kept at room temperature for 4 days.

In contrast, it is assumed that the Cu substrate with very low I₍₁₁₁₎/I₍₂₀₀₎ should have shorter reaction time to form Cu₂O film by immersion in NaOH aqueous solution. To verify the assumption, the Cu₂ substrates were used for experiments. The Cu₂ foil was immersed in a 4.0 M hot NaOH aqueous solution of 80 °C and then kept at room temperature for 14 h. As shown in Fig. 3(a), the compact imperfect 26-facet polyhedral architecture (a perfect one with exposed 8 {111}, 6 {100} and 12 {110} facets as the inset shown in Fig. 3(a)) was obtained.²⁰ The size of particles is in the range of several microns and the thickness of film is ca. 3.5 μm as shown in Fig. S2 (ESI†). The {110} facets are exposed inhomogeneously. The dominant facet is {200} as shown in XRD pattern of Fig. 3(b). The purity of the as-obtained Cu₂O was further identified by X-ray photoelectron spectroscopy (XPS). Normally, the CuO and Cu(OH)₂ can be characterized by intense Cu(2p) XPS shake-up satellites and a broad O(1s) peak; Cu₂O and metallic Cu show weaker/no Cu(2p) satellites and a narrower O(1s) peak.²¹

Fig. 3 (a) SEM image, (b) XRD pattern and XPS spectra of (c) Cu2p and (d) O1s, (e) photocurrent and photoresponse curves under 1 sun illumination of Cu₂O/Cu₂ obtained by adding Cu₂ foils in 20 ml 4.0 M NaOH solution with initial temperature of 80 °C and then kept at room temperature for 14 h. (f) SEM images of Cu₂O/Cu₂ after checking (e). The inset in (a) is a perfect 26-facet polyhedral architecture with exposed {111} × 8, {100} × 6 and {110} × 12 facets. The inset of (f) is the photograph of Cu₂O/Cu₂ film after checking (e).

Unfortunately, it is difficult to distinguish the Cu₂O and the metallic Cu by XPS since their Cu(2p) binding energies are similar.²² However, herein, we just need to ensure non-existences of CuO and Cu(OH)₂ by XPS. Some very weaker shake-up satellites peaks rather than intense satellites peaks indicate that there is no impurities of CuO and Cu(OH)₂ in our samples as shown in Fig. 3(c). Considering the depth of detection of XPS (∼1 to 10 nm), the peaks of Cu(2p_3/2) at 932.6 eV and Cu(2p_1/2) at 952.4 eV can be attributed to Cu₂O.²² The corresponding O(1s) XPS spectra show two peaks at 530.3 and 531.5 eV, attributed to Cu₂O and surface hydroxide (–OH).²³ The surface hydroxide comes from the adsorbed and/or residual NaOH on the surface of Cu₂O. The pure Cu₂O crystals can also be obtained by immersing Cu₂ foils at different concentrations of NaOH solution even at room temperature (24 °C) as shown in Fig. S3 and S4.† At low concentration (2.0 M), the size of particles is relatively decreased; when the concentration was increased, the films become loose and sparse. At room temperature, the Cu₂O particles are much irregular and cannot be formed at very high concentration (8.0 M). The Cu₂O/Cu can be further oxidized to CuO/Cu as the immersion time was extended as shown in Fig. S5.† In general, the products can be affected by several parameters such as temperature, NaOH concentration and reaction time.

The photoelectrochemical photocurrent switching (PEPS) effect,^1c,24 which can switch photocurrent polarity by changing photoelectrode potential, was observed with Cu₂O/Cu₂ sample as shown in Fig. 3(e). The cathodic photocurrent gradually decreases as the applied potential is swept from −0.7 V to the positive direction and is converted to anodic photocurrent at −0.48 V. Herein, the performance of the current-potential under chopped light illumination indicates that Cu/Cu₂O and Cu₂O/electrolyte Schottky type junctions exist in the Cu/Cu₂O/electrolyte system. Similar results have been investigated earlier in the Ti/Cu₂O/electrolyte,²⁵ ITO/Cu₂O/electrolyte²⁶ and ITO/Cu₂O/Cu_xS²⁷ systems. When the potential is in the range of −0.7 to −0.48 V, the Cu/Cu₂O junction is dominant and therefore a p-type photosignal (cathodic photocurrent) is produced; when the potential is in the range of −0.48 to +0.1 V, the Cu₂O/electrolyte junction becomes dominant and therefore a n-type photosignal (anodic photocurrent) is obtained.²⁵ The anodic and cathodic photocurrents were observed by photoresponses at 0 V and −0.6 V, respectively, as the inset in Fig. 3(e). The cathodic photocurrent decreased gradually as time was increased under −0.6 V. It indicates that the film is not so stable. After the photocurrent and photoresponse checking, the color of the surface of film, which was exposed in electrolyte, was changed to dark as shown in the inset in Fig. 3(f); it indicates that Cu₂O was reduced to Cu on the surface as a similar result reported by Paracchine et al.² As shown in the SEM image in Fig. 3(f) and S6 (ESI†), the {110} crystal facets are partially dissolved while the others are stable. Many nano-sized Cu particles are deposited onto the {111} facets. This process is a decomposition–redeposition process. The relative surface energies of Cu₂O crystals are in the order of γ{111} < γ{100} < γ{110} and the order of photocatalytic activity of facets in Cu₂O is {110} > {111} > {100}.²⁸ Therefore, the {110} facet possesses the highest surface energy and is the most photoactive facet here. The {110} should be the most active facet for the reaction such as water splitting under light illumination. However, the {110} facet is the most unstable facet. For comparison, the Cu/FTO substrate with high I₍₁₁₁₎/I₍₂₀₀₎ ratio of 2.92 was chosen for experiment. After 5 day immersion, the Cu films can be completely transformed to Cu₂O as the XRD patterns shown in Fig. 4(a). The dominant crystal facet is (111). However, the film is composed of crystals of irregular polyhedron particles as shown in Fig. 4(b). The Cu₂O/FTO is not perfect because the Cu/FTO is composed of Cu particles rather than continuous and compact film as shown in Fig. S7 (ESI†).

Fig. 4 (a) XRD patterns of (a1) bared FTO, (a2) Cu/FTO and (a3) Cu₂O/FTO; (b) SEM image of Cu₂O/FTO obtained by immersing Cu/FTO substrate into 20 ml 4.0 M NaOH solution followed by heating at 80 °C for around 1 h and then kept at room temperature for 5 days.

To show further potential applications, the hydrophobicity properties of two kinds of Cu₂O films in Fig. 2(b) and 3(a) were tested via water contact angle as shown in Fig. 5. It is found that the surfaces of Cu₁ (Fig. 5(a)) and Cu₂ (Fig. 5(c)) substrates are hydrophobic with similar water contact angle of 96.8 ± 3° and 92.7 ± 3°, respectively. After formed Cu₂O film on the surface of Cu substrates, the surface of Cu₂O/Cu₁, with water contact angle of 125.6 ± 2°, is more hydrophobic than the bare Cu₁ substrate. In contrast, the Cu₂O/Cu₂ presents hydrophilic property with water contact angle of 64.6 ± 2°. In theory, the hydrophobic solid surface has low surface energy and the hydrophilic solid surface has high surface energy. Since the relative surface energies of Cu₂O crystals are in the order of γ{111} < γ{100}, the water contact angle of Cu₂O/Cu₁ film with dominant {111} facets should be larger than that of Cu₂O/Cu₂ film with dominant {100} facets. This is matched with the experimental results regardless of the other factors such as the morphology and the surface adsorption.

Fig. 5 Contact angle measurement results of (a) bare Cu₁ foil, (b) Cu₂O/Cu₁ film, (c) bare Cu₂ foil and (d) Cu₂O/Cu₂ film.

In summary, Cu can be directly transformed to Cu₂O before the formation of CuO in high concentrated NaOH aqueous solution by facile immersion process without any additional oxidant. According to the different type of Cu substrates, the different dominant Cu₂O films can be obtained. The process of Cu oxidation in high concentration of aqueous NaOH is from Cu to Cu₂O, and then Cu₂O to CuO as the immersion time is prolonged. Further work will be reported for the better understanding of the mechanism of growth in the following report. However, this work gives a good way to convert Cu particle or Cu film to crystalline Cu₂O particle or film and it can be a facile way to synthesize Cu₂O films.

Acknowledgements

This work was supported by the Korea Center for Artificial Photosynthesis (KCAP) located in Sogang University funded by the Minister of Science, ICT and Future Planning (MSIP) through the National Research Foundation of Korea (no. 2009-0093885), and the Brain Korea 21 Plus Project 2014.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and characterization (Fig. S1–S7). See DOI: 10.1039/c4ra01174k

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