**3. Electrochemical synthesis**

### **3.1 Electrodeposition**

Synthesis of Cu2O nanostructures by the methods described in the previous part demands complex process control, high reaction temperatures, long reaction times, expensive chemicals and specific method for specific nanostructures. A request for obtaining nanometer particles, cause complete change of technology in which Cu2O is formed on the cathode by reduction of Cu2+ ions from the organic electrolyte. The possible reactions during the cathodic reduction of copper (II) lactate solution are:

$$2\text{Cu}^{2+} + \text{H}\_2\text{O} + 2\text{e}^{\text{--}} = \text{Cu}\_2\text{O} + 2\text{H}^\* \tag{4}$$

$$\rm Cu^{2+} + 2e^{\circ} = Cu \tag{5}$$

$$\rm Cu\_2O + 2H^+ + 2e^- = 2Cu + H\_2O \tag{6}$$

The electrodeposition techniques are particularly well suited for the deposition of single elements but it is also possible to carry out simultaneous depositions of several elements and syntheses of well-defined alternating layers of metals and oxides with thicknesses down to a few nm. So, electrodeposition is a suitable method for the synthesis of semiconductor thin films such as oxides. This method provides a simple way to deposit thin Cu(I) oxide films onto large-area conducting substrates (Lincot, 2005). Thus, the study of the growth kinetics of these films is of considerable importance. In this section we present some results of electrochemical deposition of cuprous oxide obtained by various authors.

Rakhshani et al. (1987) cathodically electrodeposited Cu(I) oxide film onto conductive substrates from a solution of cupric sulphate, sodium hydroxide and lactic acid. Films of Cu2O were deposited in three different modes, namely the potentiostatic mode, the mode with constant WE potential with respect to the CE and the galvanostatic mode. The composition of the films deposited under all conditions was Cu2O with no traces of CuO. The optical band gap for electrodeposited Cu2O films was 1.95 eV. Deposition temperature played an important role in the size of deposited grains. Films were photoconductive with high dark resistivities. Also, Rakhshani & Varghese (1987) electrodeposited cuprous oxide thin films galvanostatically on 0.05 mm thick stainless steel substrates at a temperature of 600C. The deposition solution with pH 9 consisted of lactic acid (2.7 M), anhydrous cupric sulphate (0.4 M), and sodium hydroxide (4 M). Authors found that all the films deposited at 60 °C consisted only of Cu2O grains a few

All the obtained films have nanostructure with an average crystallite size lower than

Nair et al. (1999) deposited cuprous oxide thin films on glass substrate using chemical technique. The glass slides were dipped first in a 1 M aqueous solution of NaOH at the temperature range 50-90°C for 20 s and then in a 1 M aqueous solution of copper complex. X-ray diffraction patterns showed that the films, as prepared, are of cuprite structure with composition Cu2O. Annealing the films in air at 3500C converts these films to CuO. This conversion is accompanied by a shift in the optical band gap from 2.1 eV (direct) to 1.75 eV (direct). The films show p-type conductivity, ~ 5 x 10-4 Ω-1 cm-1 for a film of thickness

Synthesis of Cu2O nanostructures by the methods described in the previous part demands complex process control, high reaction temperatures, long reaction times, expensive chemicals and specific method for specific nanostructures. A request for obtaining nanometer particles, cause complete change of technology in which Cu2O is formed on the cathode by reduction of Cu2+ ions from the organic electrolyte. The possible reactions during

2Cu2+ + H2O + 2e-- = Cu2O + 2H+ (4)

Cu2+ + 2e-- = Cu (5)

 Cu2O + 2H+ + 2e-- = 2Cu + H2O (6) The electrodeposition techniques are particularly well suited for the deposition of single elements but it is also possible to carry out simultaneous depositions of several elements and syntheses of well-defined alternating layers of metals and oxides with thicknesses down to a few nm. So, electrodeposition is a suitable method for the synthesis of semiconductor thin films such as oxides. This method provides a simple way to deposit thin Cu(I) oxide films onto large-area conducting substrates (Lincot, 2005). Thus, the study of the growth kinetics of these films is of considerable importance. In this section we present some results of electrochemical deposition of cuprous oxide obtained by

Rakhshani et al. (1987) cathodically electrodeposited Cu(I) oxide film onto conductive substrates from a solution of cupric sulphate, sodium hydroxide and lactic acid. Films of Cu2O were deposited in three different modes, namely the potentiostatic mode, the mode with constant WE potential with respect to the CE and the galvanostatic mode. The composition of the films deposited under all conditions was Cu2O with no traces of CuO. The optical band gap for electrodeposited Cu2O films was 1.95 eV. Deposition temperature played an important role in the size of deposited grains. Films were photoconductive with high dark resistivities. Also, Rakhshani & Varghese (1987) electrodeposited cuprous oxide thin films galvanostatically on 0.05 mm thick stainless steel substrates at a temperature of 600C. The deposition solution with pH 9 consisted of lactic acid (2.7 M), anhydrous cupric sulphate (0.4 M), and sodium hydroxide (4 M). Authors found that all the films deposited at 60 °C consisted only of Cu2O grains a few

20 nm.

0.15 μm.

**3. Electrochemical synthesis** 

the cathodic reduction of copper (II) lactate solution are:

**3.1 Electrodeposition** 

various authors.

μm in size and preferentially oriented along (100) planes parallel to the substrate surface. A band gap was found and it was 1.90-1.95 eV.

Mukhopadhyay et al. (1992) deposited Cu2O films by galvanostatic method on copper substrates. An alkaline cupric sulphate (about 0.3 M) bath containing NaOH (about 3.2 M) and lactic acid (about 2.3 M) was used as the electrolyte at pH 9. The bath temperatures were 40, 50 and 60°C. XRD analysis indicated a preferred (200) orientation of the Cu2O deposited film. The deposition kinetics was found to be independent of deposition temperature and linear in the thickness range studied (up to about 20 μm). The electrical conductivity of Cu2O films was found to vary exponentially with temperature in the 145- 3000C range with associated activation energy of 0.79 eV.

Golden et al. (1996) found that the reflectance and transmittance of the electrodeposited films of cuprous oxide give a direct band gap of 2.1 eV. Namely, authors used electrodeposition method for obtaining the films of cuprous oxide by reduction of copper (II) lactate in alkaline solution (0.4 M cupric sulfate and 3 M lactic acid). Films were deposited onto either stainless steel or indium tin oxide (ITO) substrates. Deposition temperatures ranged from 25 to 65 °C. They found that the cathodic deposition current was limited by a Schottky-like barrier that forms between the Cu2O and the deposition solution. A barrier height of 0.6 eV was determined from the exponential dependence of the deposition current on the solution temperature. At a solution pH 9 the orientation of the film is [100], while at a solution pH 12 the orientation changes to [111]. The degree of [111] texture for the films grown at pH 12 increased with applied current density.

Siripala et al. (1996) deposited cuprous oxide films on indium tin oxide (ITO) coated glass substrates in a solution of 0.1 M sodium acetate and 1.6 x 10-2 M cupric acetate and the effect of annealing in air has been studied too. Electrodeposition was carried out for 1.5 h in order to obtain films of thicknesses in the order of 1 μm. Authors concluded that the electrodeposited Cu2O films are polycrystalline with grain sizes in the order of 1-2 μm and the bulk crystal structure is simple cubic. They concluded that there is no apparent change in the crystal structure when heat treated in air at or below 300°C. Annealing in air changes the morphology of the surface creating a porous nature with ring shaped structures on the surface. Annealing above 300°C causes decomposition of the yellow-orange colour Cu2O film into a darker film containing black CuO and its complexes with water.

Zhou & Switzer (1998) deposited Cu2O films on stainless steel disks by the cathodic reduction of copper (II) lactate solution (0.4 M cupric sulfate and 3 M lactic acid). The pH of the bath was between 7 and 12 and the bath temperature was 60°C. Authors concluded that the preferred orientation and crystal shape of Cu2O films change with the bath pH and the applied potential. They obtained pure Cu2O films at bath pH 9 with applied potential between -0.35 and -0.55 (SCE) or at bath pH 12.

Mahalingam et al. (2000) deposited cuprous oxide thin films on copper and tin-oxide-coated glass substrates by cathodic reduction of alkaline cupric lactate solution (0.45 M CuSO4, 3.25 M lactic acid and 0.1 M NaOH). The deposition was carried out in the temperature range of 60-800C at pH 9. Galvanostatic deposition on tin-oxide-coated glass and copper substrates yields reddish-grey Cu2O films. All the films deposited are found to be polycrystalline having grains in the range of 0.01 - 0.04 μm. The deposition kinetics is found to be linear and independent of the deposition temperature. From the optical absorption measurements, authors found that the deposit of cuprous oxide films has a refractive index of 2.73, direct band gap of 1.99 eV, and extinction coefficient of 0.195. After deposition on temperature of 700C, cuprous oxide films were annealed in air for 30 min at different temperatures (150, 250

Cuprous Oxide as an Active Material for Solar Cells 177

Tang et al. (2005) investigated the electrochemical deposition of nanocrystalline Cu2O thin films on TiO2 films coated on transparent conducting optically (TCO) glass substrates by cathodic reduction of cupric acetate (0.1 M sodium acetate and 0.02 M cupric acetate). Authors concluded that the pH and bath temperature strongly affect the composition and microstructure of the Cu2O thin films. The effect of bath pH on electrodeposition of Cu2O thin film was investigated by selecting a bath temperature of 300C and an applied potential of -245 mV (SCE). Authors found that the films deposited at pH 4 are mostly metallic Cu and only little Cu2O. In the region of pH 4 to pH 5.5, the deposited films are a composite of Cu and Cu2O, while the films deposited at pH between 5.5 and 6 are pure Cu2O. Pure Cu2O deposited at bath temperature between 0 and 300C produced spherically shaped grains with 40~50 nm in diameter. The bath temperature must be controlled in the range of 0-300C to obtain nanocrystalline Cu2O thin film. At a temperature of 45°C, a highly branched dendrite formed, and the grain size increased to 200–500 nm. At the temperature above 60°C, a ring-shaped structure with a porous surface was observed. Optical absorption measurements indicate that annealing at 2000C can improve the transmittance of the nanocrystalline Cu2O thin films. Figure 4 shows SEM photographs of Cu2O films deposited at various bath temperatures.

Fig. 4. SEM photographs of Cu2O films deposited at various bath temperatures: (A) 00C, (B)

Wijesundera et al. (2006) investigated the potentiostatic electrodeposition of cuprous oxide and copper thin films. Electrodeposition was carried out in an aqueous solution containing

300C, (C) 450C, and (D) 600C (Tang et al., 2005)

and 3500C) to obtain their room temperature resistivity. It showed a decrease in resistivity of Cu2O film of the order of 107 Ωcm to 104 Ωcm. The explanation of such behavior may be due to increase in hole conduction.

Georgieva & Ristov (2002) deposited the cuprous oxide (Cu2O) films using a galvanostatic method from an alkaline CuSO4 bath containing lactic acid and sodium hydroxide (64 g/l anhydrous cupric sulphate (CuSO4), 200 ml/l lactic acid (C3H6O3) and about 125 g/l sodium hydroxide (NaOH)). The electrodeposition temperature was 600C. Authors obtained polycrystalline films of 4–6 μm in thickness with optical band gap of 2.38 eV.

Daltin et al. (2005) applied potentiostatic deposition method to obtain cuprous oxide nanowires in polycarbonate membrane by cathodic reduction of alkaline cupric lactate solution (0.45 M Cu(II) and 3.25 M lactate). Authors found that the optimum electrochemical parameters for the deposition of nanowires are: pH 9.1, temperature 700C, and applied potential -0.9 V (SSE). The morphology of the nanowires was analyzed by SEM. The obtained nanowires had uniform diameters of about 100 nm and lengths up to 16 μm. Scanning electron micrograph of electrodeposited Cu2O nanowires are presented in Figure 3.

Liu et al. (2005) investigated the electrochemical deposition of Cu2O films onto three different substrates (indium tin oxide film coated glass, n-Si wafer with (001) orientation and Au film evaporated onto Si substrate). For the film grown on ITO, electrical current increases gradually during deposition, while for the films growth on both Si and Au substrates, the monitored current decreases monotonically. Authors considered that the continuous decrease in current reflects different deposition mechanisms. In the case of Si substrate, the decrease of the current may be the result of the formation of an amorphous SiO2 layer on the Si surface, which limits the current. For the Au surface, the decrease in measured current is due to the resistivity increase as a result of Cu2O film formation. Cu2O crystals with microsized pyramidal shape were grown on ITO substrate. Nanosized and pyramidal shaped Cu2O particles were formed on Si substrate and the film grown on Au substrate shows a (100) orientation with much better crystallinity.

Fig. 3. (a) Scanning electron micrograph of electrodeposited Cu2O nanowires. Bath temperature 700C, pH 9.1, E -1.69 V/SSE. (b) Enlarged (a) (Daltin et al., 2005)

and 3500C) to obtain their room temperature resistivity. It showed a decrease in resistivity of Cu2O film of the order of 107 Ωcm to 104 Ωcm. The explanation of such behavior may be due

Georgieva & Ristov (2002) deposited the cuprous oxide (Cu2O) films using a galvanostatic method from an alkaline CuSO4 bath containing lactic acid and sodium hydroxide (64 g/l anhydrous cupric sulphate (CuSO4), 200 ml/l lactic acid (C3H6O3) and about 125 g/l sodium hydroxide (NaOH)). The electrodeposition temperature was 600C. Authors obtained

Daltin et al. (2005) applied potentiostatic deposition method to obtain cuprous oxide nanowires in polycarbonate membrane by cathodic reduction of alkaline cupric lactate solution (0.45 M Cu(II) and 3.25 M lactate). Authors found that the optimum electrochemical parameters for the deposition of nanowires are: pH 9.1, temperature 700C, and applied potential -0.9 V (SSE). The morphology of the nanowires was analyzed by SEM. The obtained nanowires had uniform diameters of about 100 nm and lengths up to 16 μm. Scanning electron

Liu et al. (2005) investigated the electrochemical deposition of Cu2O films onto three different substrates (indium tin oxide film coated glass, n-Si wafer with (001) orientation and Au film evaporated onto Si substrate). For the film grown on ITO, electrical current increases gradually during deposition, while for the films growth on both Si and Au substrates, the monitored current decreases monotonically. Authors considered that the continuous decrease in current reflects different deposition mechanisms. In the case of Si substrate, the decrease of the current may be the result of the formation of an amorphous SiO2 layer on the Si surface, which limits the current. For the Au surface, the decrease in measured current is due to the resistivity increase as a result of Cu2O film formation. Cu2O crystals with microsized pyramidal shape were grown on ITO substrate. Nanosized and pyramidal shaped Cu2O particles were formed on Si substrate and the film grown on Au

polycrystalline films of 4–6 μm in thickness with optical band gap of 2.38 eV.

micrograph of electrodeposited Cu2O nanowires are presented in Figure 3.

substrate shows a (100) orientation with much better crystallinity.

Fig. 3. (a) Scanning electron micrograph of electrodeposited Cu2O nanowires. Bath temperature 700C, pH 9.1, E -1.69 V/SSE. (b) Enlarged (a) (Daltin et al., 2005)

to increase in hole conduction.

Tang et al. (2005) investigated the electrochemical deposition of nanocrystalline Cu2O thin films on TiO2 films coated on transparent conducting optically (TCO) glass substrates by cathodic reduction of cupric acetate (0.1 M sodium acetate and 0.02 M cupric acetate). Authors concluded that the pH and bath temperature strongly affect the composition and microstructure of the Cu2O thin films. The effect of bath pH on electrodeposition of Cu2O thin film was investigated by selecting a bath temperature of 300C and an applied potential of -245 mV (SCE). Authors found that the films deposited at pH 4 are mostly metallic Cu and only little Cu2O. In the region of pH 4 to pH 5.5, the deposited films are a composite of Cu and Cu2O, while the films deposited at pH between 5.5 and 6 are pure Cu2O. Pure Cu2O deposited at bath temperature between 0 and 300C produced spherically shaped grains with 40~50 nm in diameter. The bath temperature must be controlled in the range of 0-300C to obtain nanocrystalline Cu2O thin film. At a temperature of 45°C, a highly branched dendrite formed, and the grain size increased to 200–500 nm. At the temperature above 60°C, a ring-shaped structure with a porous surface was observed. Optical absorption measurements indicate that annealing at 2000C can improve the transmittance of the nanocrystalline Cu2O thin films. Figure 4 shows SEM photographs of Cu2O films deposited at various bath temperatures.

Fig. 4. SEM photographs of Cu2O films deposited at various bath temperatures: (A) 00C, (B) 300C, (C) 450C, and (D) 600C (Tang et al., 2005)

Wijesundera et al. (2006) investigated the potentiostatic electrodeposition of cuprous oxide and copper thin films. Electrodeposition was carried out in an aqueous solution containing

Cuprous Oxide as an Active Material for Solar Cells 179

Fig. 6. Current density vs. time curves for electrodeposition of Cu2O thin film on titanium

electrode (electrodeposition time: (A ) 6 s, (B) 10 s and (C) 60 s; t = 25 ºC, pH 9.22)

sodium acetate and cupric acetate. The results of their investigation show that the single phase polycrystalline Cu2O can be deposited from 0 to -300 mV (SCE). Also, co-deposition of Cu and Cu2O starts at - 400 mV (SCE). At the deposition potential from -700 mV (SCE) a single phase Cu thin films are produced. Single phase polycrystalline Cu2O thin films with cubic grains of 1–2 μm can be possible at the deposition potential around -200 mV (SCE).

Wang et al. (2007) cathodically electrodeposited cuprous oxide films from 0.4 M copper sulfate bath containing 3 M lactic acid. The bath pH was carefully adjusted between 7.5 and 12.0 by controlled addition of 4 M NaOH. The electrodeposition was done on Sn-doped indium oxide substrates. The influence of electrodeposition bath pH on grain orientation and crystallite shape was examinated. Authors found that three orientations, namely, (100), (110), and (111) dominate as the bath pH is increased from ~ 7.5 to ~ 12.

Recently, Hu et al. (2009) electrodeposited Cu2O thin films onto an indium tin oxide (ITO) coated glass by a two-electrode system with acid and alkaline electrolytes under different values of direct current densities. Copper foils were used as the anodes, and the current density between the anode and cathode varied between 1 mA cm−2 and 5 mA cm−2. It was obtained that the microstructure of Cu2O thin films produced in the acid electrolyte changes from a ring shape to a cubic shape with the increase of direct current densities. The microstructure of Cu2O thin films produced in the alkaline electrolyte has a typical pyramid shape. The electrocrystallization mechanisms are considered to be related to the nucleation rate, cluster growth, and crystal growth. To investigate the initial stage of nucleation and cluster growth, different current densities with the same deposition time were applied. Figure 5 shows that a relatively large cluster size and a relatively small number of nucleation sites were obtained under a current density of 1 mAcm−2. At a high current density of 5 mAcm−2, more nucleation sites and a small cluster size were obtained.

Fig. 5. The Cu2O films synthesized under different current densities with the same deposition time (Hu et al., 2009)

sodium acetate and cupric acetate. The results of their investigation show that the single phase polycrystalline Cu2O can be deposited from 0 to -300 mV (SCE). Also, co-deposition of Cu and Cu2O starts at - 400 mV (SCE). At the deposition potential from -700 mV (SCE) a single phase Cu thin films are produced. Single phase polycrystalline Cu2O thin films with cubic grains of 1–2 μm can be possible at the deposition potential around -200 mV (SCE). Wang et al. (2007) cathodically electrodeposited cuprous oxide films from 0.4 M copper sulfate bath containing 3 M lactic acid. The bath pH was carefully adjusted between 7.5 and 12.0 by controlled addition of 4 M NaOH. The electrodeposition was done on Sn-doped indium oxide substrates. The influence of electrodeposition bath pH on grain orientation and crystallite shape was examinated. Authors found that three orientations, namely, (100),

Recently, Hu et al. (2009) electrodeposited Cu2O thin films onto an indium tin oxide (ITO) coated glass by a two-electrode system with acid and alkaline electrolytes under different values of direct current densities. Copper foils were used as the anodes, and the current density between the anode and cathode varied between 1 mA cm−2 and 5 mA cm−2. It was obtained that the microstructure of Cu2O thin films produced in the acid electrolyte changes from a ring shape to a cubic shape with the increase of direct current densities. The microstructure of Cu2O thin films produced in the alkaline electrolyte has a typical pyramid shape. The electrocrystallization mechanisms are considered to be related to the nucleation rate, cluster growth, and crystal growth. To investigate the initial stage of nucleation and cluster growth, different current densities with the same deposition time were applied. Figure 5 shows that a relatively large cluster size and a relatively small number of nucleation sites were obtained under a current density of 1 mAcm−2. At a high current

density of 5 mAcm−2, more nucleation sites and a small cluster size were obtained.

Fig. 5. The Cu2O films synthesized under different current densities with the same

deposition time (Hu et al., 2009)

(110), and (111) dominate as the bath pH is increased from ~ 7.5 to ~ 12.

Fig. 6. Current density vs. time curves for electrodeposition of Cu2O thin film on titanium electrode (electrodeposition time: (A ) 6 s, (B) 10 s and (C) 60 s; t = 25 ºC, pH 9.22)

Cuprous Oxide as an Active Material for Solar Cells 181

of average crystallite size was obtained at pH 7.5, whereas the highest value was obtained at pH 9.62. They found a strong dependence of grain size and cupric oxide purity on current density. The average srystallite size increased from 45 nm (at a current density of 500 Am-2) to 400 nm (at a current density of 4000 Am-2), other conditions being as follows: pH 7.5,

There have been a number of papers on anodic formation of thin Cu2O layers (< 1 m) using alkaline solutions, but some work has been done with slightly acidic solutions. For example, backwall Cu2O/Cu photovoltaic cells have been prepared by Sears and Fortin (Sears & Fortin, 1983) with the Cu2O layer being about 1 m thick. They used and compared two methods of oxidation – thermal and anodic. The condition of the underlying copper surface is expected to influence the resulting parameters of thin solar cells, so they examined the influence of the surface preparation of the starting copper (i.e., polishing technique, thermal annealing). All this experience can help in researching the optimal way of production of

Recently, Singh et al. (2008) reported synthesis of nanostructured Cu2O by anodic oxidation of copper through a simple electrolysis process employing plain water as electrolyte. They found two different types of Cu2O nanostructures. One of them belonged to particles collected from the bottom of the electrolytic cell, while the other type was located on the copper anode itself. The Cu2O structures collected from the bottom consist of nanowires (length, ~ 600–1000 nm and diameter, ~ 10–25 nm). It may be mentioned that the total length of Cu2O nanothread and nanowire is comprised of several segments. These were presumably formed due to interaction between nanothreads/nanowires forming the network in which the Cu2O nanothread/nanowire configuration finally appears. When the electrolysis conditions were maintained at 10 V for 1 h, the representative TEM microstructure revealed the presence of dense Cu2O nanowire network (length, ~ 1000 nm, diameter, ~ 10–25 nm). The X-ray diffraction pattern obtained from these nanomaterials, could be indexed to a cubic system with lattice parameter, a = 0.4269 ± 0.005 nm. These tally quite well with the lattice parameter of Cu2O showing that the material formed under

In addition to the delaminated nanostructures, investigations of the copper anode, which were subjected to electrolysis runs, revealed the presence of another type of nanostructure of Cu2O. Authors propose that the higher applied voltage (e.g. 8 V or 10 V) for electrolysis represents the optimum conditions for the formation of nanocubes. These nanocubes reflect

Copper oxides, especially cuprous oxide, are of interest because of their applications in solar cell technology. The semiconductor cuprous oxide Cu2O film has been of considerable interest as a component of solar cells due to its band gap energy and high optical absorption coefficient. Since the properties of cuprous oxide not only depend upon the nature of the material but also upon the way they are synthesized, different methods and results obtained on the synthesis of cuprous oxide by various researchers are discussed in this chapter. The properties of the prepared cuprous oxide films related to surface morphology are presented too. In this chapter, the point is made on electrodeposition of cuprous oxide because electrodeposition techniques are particularly well suited for the deposition of metal oxides with thicknesses down to a few nm. The

temperature of 353 K and 1.5 M Na2SO4.

nanostructured Cu2O powders or films.

the basic cubic unit cell of Cu2O.

**4. Conclusion** 

electrolysis conditions consists of cubic Cu2O lattice structure.

Bugarinović et al. (2009) investigated the electrochemical deposition of thin films of cuprous oxide on three different substrates (stainless steel, platinum and copper). All experiments of Cu2O thin films deposition were performed at room temperature. Using experimental technique described elsewhere (Stević & Rajčić-Vujasinović, 2006; Stević & et al., 2009), electrodeposition was carried out in in a copper lactate solution as an organic electrolyte (0.4 M copper sulfate and 3 M lactic acid, pH 7-10 is set using NaOH). The conditions are adjusted so that the potentials which arise Cu2O and CuO are as different as possible. Characterization of obtained coatings was performed by cyclic voltammetry. The results indicate that the composition of the substrate strongly affects electrochemical reactions. Reaction with the highest rate took place on a copper surface, while the lowest rate was obtained on the platinum electrode. The results show that the co-deposit of Cu2O and Cu was obtained at - 800 mV (SCE) on stainless steel electrode. The same authors investigated the electrodeposition of cuprous oxide thin film on titanium electrode. The obtained results are presented in Figure 6.

Cuprous oxide thin films were deposited at potentials -0.6 V, -0.8 V, -1.0 V and 1.2 V with respect to SCE. All experiments were carried out for a duration of 6 s, 10 s and 1 minute. When the electrodeposition lasted 6 s (Fig. 6A), obtained currents depended on applied potentials. Lowest current of 1mA was obtained at the potential of -0.6 V vs. SCE, while the highest value of 17.9 mA was reached at -1.2 V (SCE). When the electrodeposition time was 10 s (Fig. 6B), curves current vs. time had similar shape as the previous, but when the process duration prolongates to 60 s (Fig. 6C), currents obtained at higher potentials (-1.0 V and -1.2 V vs. SCE) decrease after about 15 s and stabilise again after about 40 s at some lower value (nearly 80% of the previous ones). Maximum theoretical thicknesses of Cu2O film for every applied potential and all process durations were calculated. The lowest thickness of 7 nm was obtained for 6 s with potential of -0.6 V (SCE). More negative potentials and the increase of time lead to the increase of the film thickness. Theoretical value of the Cu2O film thickness for the longest time (60 s) and most negative potential (-1.2 V vs. SCE) is about 900 nm.

### **3.2 Anodic oxidation**

In spite of the simple equipment and easy process control, cathodic synthesis demands expensive chemicals as a big dissadventage. On the other hand, anodic oxidation of copper in alkaline solution is one of the standard methodologies for producing cuprous oxide powders used for marine paints and for plants preservation. Those powders are composed of particles of micrometer scale. However, solar sells, for their part, require particles or films of much smaller dimensions in order to achieve higher efficiency. Passive protecting layers formed on copper during anodic oxidation in alkaline solutions are widely investigated and described in electrochemical literature. The structure of those films formed on copper in neutral and alkaline solutions consists mainly of Cu2O and CuO or Cu(OH)2. Applying in situ electrochemical scanning tunneling microscopy (STM), Kunze et al. (2003) found that in NaOH solutions, a Cu2O layer is formed at E > 0.58-0.059 pH (V vs. SHE). A Cu2O/Cu(OH)2 duplex film is found for E > 0.78-0.059 pH (V vs. SHE). In borate buffer solutions, oxidation to Cu2O leads to non-crystalline grain like structure, while a crystalline and epitaxial Cu2O layer has been observed in 0.1 M NaOH indicating a strong anion and/or pH effect on the crystallinity of the anodic oxide film.

Stanković et al. (1998; 1999) investigated the effect of different parameters such as temperature, pH and anodic current density on CuO powder preparation. The lowest value

Bugarinović et al. (2009) investigated the electrochemical deposition of thin films of cuprous oxide on three different substrates (stainless steel, platinum and copper). All experiments of Cu2O thin films deposition were performed at room temperature. Using experimental technique described elsewhere (Stević & Rajčić-Vujasinović, 2006; Stević & et al., 2009), electrodeposition was carried out in in a copper lactate solution as an organic electrolyte (0.4 M copper sulfate and 3 M lactic acid, pH 7-10 is set using NaOH). The conditions are adjusted so that the potentials which arise Cu2O and CuO are as different as possible. Characterization of obtained coatings was performed by cyclic voltammetry. The results indicate that the composition of the substrate strongly affects electrochemical reactions. Reaction with the highest rate took place on a copper surface, while the lowest rate was obtained on the platinum electrode. The results show that the co-deposit of Cu2O and Cu was obtained at - 800 mV (SCE) on stainless steel electrode. The same authors investigated the electrodeposition of cuprous oxide thin film on titanium electrode. The obtained results

Cuprous oxide thin films were deposited at potentials -0.6 V, -0.8 V, -1.0 V and 1.2 V with respect to SCE. All experiments were carried out for a duration of 6 s, 10 s and 1 minute. When the electrodeposition lasted 6 s (Fig. 6A), obtained currents depended on applied potentials. Lowest current of 1mA was obtained at the potential of -0.6 V vs. SCE, while the highest value of 17.9 mA was reached at -1.2 V (SCE). When the electrodeposition time was 10 s (Fig. 6B), curves current vs. time had similar shape as the previous, but when the process duration prolongates to 60 s (Fig. 6C), currents obtained at higher potentials (-1.0 V and -1.2 V vs. SCE) decrease after about 15 s and stabilise again after about 40 s at some lower value (nearly 80% of the previous ones). Maximum theoretical thicknesses of Cu2O film for every applied potential and all process durations were calculated. The lowest thickness of 7 nm was obtained for 6 s with potential of -0.6 V (SCE). More negative potentials and the increase of time lead to the increase of the film thickness. Theoretical value of the Cu2O film thickness for the longest time (60 s) and most negative potential

In spite of the simple equipment and easy process control, cathodic synthesis demands expensive chemicals as a big dissadventage. On the other hand, anodic oxidation of copper in alkaline solution is one of the standard methodologies for producing cuprous oxide powders used for marine paints and for plants preservation. Those powders are composed of particles of micrometer scale. However, solar sells, for their part, require particles or films of much smaller dimensions in order to achieve higher efficiency. Passive protecting layers formed on copper during anodic oxidation in alkaline solutions are widely investigated and described in electrochemical literature. The structure of those films formed on copper in neutral and alkaline solutions consists mainly of Cu2O and CuO or Cu(OH)2. Applying in situ electrochemical scanning tunneling microscopy (STM), Kunze et al. (2003) found that in NaOH solutions, a Cu2O layer is formed at E > 0.58-0.059 pH (V vs. SHE). A Cu2O/Cu(OH)2 duplex film is found for E > 0.78-0.059 pH (V vs. SHE). In borate buffer solutions, oxidation to Cu2O leads to non-crystalline grain like structure, while a crystalline and epitaxial Cu2O layer has been observed in 0.1 M NaOH indicating a strong anion and/or pH effect on the

Stanković et al. (1998; 1999) investigated the effect of different parameters such as temperature, pH and anodic current density on CuO powder preparation. The lowest value

are presented in Figure 6.

(-1.2 V vs. SCE) is about 900 nm.

crystallinity of the anodic oxide film.

**3.2 Anodic oxidation** 

of average crystallite size was obtained at pH 7.5, whereas the highest value was obtained at pH 9.62. They found a strong dependence of grain size and cupric oxide purity on current density. The average srystallite size increased from 45 nm (at a current density of 500 Am-2) to 400 nm (at a current density of 4000 Am-2), other conditions being as follows: pH 7.5, temperature of 353 K and 1.5 M Na2SO4.

There have been a number of papers on anodic formation of thin Cu2O layers (< 1 m) using alkaline solutions, but some work has been done with slightly acidic solutions. For example, backwall Cu2O/Cu photovoltaic cells have been prepared by Sears and Fortin (Sears & Fortin, 1983) with the Cu2O layer being about 1 m thick. They used and compared two methods of oxidation – thermal and anodic. The condition of the underlying copper surface is expected to influence the resulting parameters of thin solar cells, so they examined the influence of the surface preparation of the starting copper (i.e., polishing technique, thermal annealing). All this experience can help in researching the optimal way of production of nanostructured Cu2O powders or films.

Recently, Singh et al. (2008) reported synthesis of nanostructured Cu2O by anodic oxidation of copper through a simple electrolysis process employing plain water as electrolyte. They found two different types of Cu2O nanostructures. One of them belonged to particles collected from the bottom of the electrolytic cell, while the other type was located on the copper anode itself. The Cu2O structures collected from the bottom consist of nanowires (length, ~ 600–1000 nm and diameter, ~ 10–25 nm). It may be mentioned that the total length of Cu2O nanothread and nanowire is comprised of several segments. These were presumably formed due to interaction between nanothreads/nanowires forming the network in which the Cu2O nanothread/nanowire configuration finally appears. When the electrolysis conditions were maintained at 10 V for 1 h, the representative TEM microstructure revealed the presence of dense Cu2O nanowire network (length, ~ 1000 nm, diameter, ~ 10–25 nm). The X-ray diffraction pattern obtained from these nanomaterials, could be indexed to a cubic system with lattice parameter, a = 0.4269 ± 0.005 nm. These tally quite well with the lattice parameter of Cu2O showing that the material formed under electrolysis conditions consists of cubic Cu2O lattice structure.

In addition to the delaminated nanostructures, investigations of the copper anode, which were subjected to electrolysis runs, revealed the presence of another type of nanostructure of Cu2O. Authors propose that the higher applied voltage (e.g. 8 V or 10 V) for electrolysis represents the optimum conditions for the formation of nanocubes. These nanocubes reflect the basic cubic unit cell of Cu2O.
