Method of Electrodeposition

There are many methods to prepare Cu₂O thin films. For example, physical vapor deposition (e.g., thermal evaporation, sputtering), chemical vapor deposition (CVD), and electrochemical deposition have been studied extensively. Among them, the electrodeposition is the most attractive approach. Electrodeposition is a versatile and low-cost technique for preparing thin films of oxide semiconductors. It is possible to grow uniform thin film over large areas in unique shape. Fabrication of Cu2O films by the electrochemical method has been successfully developed.

A. Cathodic Electrodeposition in Cuprous Oxides

Cathodic deposition of Cu2O has been extensively studied before. It is established that electrochemical deposition is able to adjust precisely the driving force for the reaction involved in deposition. This allows the control of structure and phase composition of the resulting films. Many different electrodeposition environments, substrates, and power supply methods have been explored. It is recognized that changing these conditions would alter structure, morphology, and photoelectro- chemical properties of Cu2O.

A commonly used bath was developed in 1987 by Rakhshoni et al, in which copper sulfide, lactic acid, and sodium hydroxide were used [12]. The Cu2O films were electrodeposited successfully under galvanostatic conditions on stainless steel cathodes. It was shown that uniformly oriented Cu2O films could be grown with robust adhesion.

The pH environment is recognized to critically influence the principal reaction route of electrodeposited Cu2O film. The reactions steps involved for the cathodic

deposition of Cu₂O are [13]; dissolve to copper(II) or reduce to copper metal in an acid environment.

In 1998, Switzes et al. identified a stable ratio of copper lactic solution, which contained 0.4 M cuprous sulfate and 3 M lactic acid [13]. The pH for the plating bath could be adjusted from 7 to 12. The copper ion would be stabilized by the lactic acid to form Cu(CH₃CHOHCOO)_2. As a result, it would not precipitate out in high pH environments. Potentiostatic and galvanostatic depositions both confirmed that the preferential orientation of Cu₂O film was [100] in a pH 9 bath, and [111] in a bath whose pH value was above 10 (Fig 2.8).

Cu₂O had been electrodeposited on several substrates including Cu, InP (001), ITO, Si, and stainless steel [14-15]. The out-of-plane orientation of film deposited on a polycrystalline substrate depended on the solution pH, and was only slightly affected by the single crystal substrate [16].

Wang et al. carefully examined the effect of pH value on the resulting crystal growth [17]. They observed a new preferred orientation, which is the (110) of Cu2O.

They concluded that the film could be obtained in a narrow pH range from 9.4 to 9.9 (Fig 2.9).

Lee et al. electrodeposited the Cu2O film in a weak acidic electrolyte of 10 mM Cu(NO3)2 by a constant current mode [18]. Their results confirmed that the Cu2O and

Cu were deposited simultaneously. Formation mechanism of Cu₂O was investigated by electrochemical analysis utilizing an electrochemical quartz crystal microbalance (EQCM). The superimposition of anodic current and mass change data suggested that the Cu exhibited a lower dissolution overpotential. Therefore, they supplied an anodic current to selectively dissolve the Cu, and obtained a pure Cu₂O film afterward. Their results provided a different preparation route for pure Cu2O phase.

Figure 2.8 Plot of relative intensity (I(200)/I(111)) and grain size as a function of bath pH, the applied potential is -0.4 V vs. SCE, and the bath temperature is 60 °C on a stainless steel substrate [13].

Figure 2.9 XRD of (110) oriented Cu2O film and corresponding SEM image [17].

B. Morphology Controlled by Electrodeposition

In 2002, Huang et al. synthesized the Cu₂O nanowires by electrodeposition from a lyotropic reverse hexagonal liquid crystalline phase, which was used as a soft template [19]. The liquid crystalline phase was polarized, and aligned in an electric field. Its alignment could be further improved by controlling the distance between the electrodes. The deposited Cu₂O nanowires had a diameter ranging from 25 to 100 nm with a high aspect ratio (Fig 2.10).

In 2004, Choi et al. pointed out that the shape of a crystal was determined by the crystallographic planes on its surface [20]. They used an additive, sodium dodecyl sulfate (SDS), in order to tailor the crystal habits of electrochemically grown Cu₂O crystals. The SDS was preferentially adsorbed onto {111} faces, and impeded the

crystal growth alone the <111> direction. Its growth process is shown in Fig 2.11.

Furthermore, the preferential adsorption of SDS was pH-dependence, which enabled selective tuning for the growth rate of Cu₂O crystals along the <111> directions.

In 2005, they further demonstrated a systematic and simultaneous tuning of the habit and degree of branching in the Cu₂O crystals by manipulating certain key conditions, such as electrodeposited voltage, current, temperature, and composition of the solution [21]. The deposition potential-current diagram, as shown in Fig 2.12, summarizes the effect of electrochemical conditions on branching and faceted growth of Cu₂O crystals. The crystal growth could be precisely controlled by adjusting the preference for branching or faceting growth conditions through suitable electrodeposition methods. In such way, the crystal shape could be designed and reproduced.

Adding different additives engendered a distinct preferential adsorption effect.

The additives include surfactants, polymers, and specific ions. Choi et al. had studied the pre-grown crystals carefully in 2006 [22]. Different ions exhibited various adsorbing strengths. By adjusting the additives, the growth process could be totally reversed (Fig 2.13). Wang et al. studied the additives effect by adding different amounts of CTAB and cations. The structure for the bulk Cu2O film was also affected by the additives (Fig 2.14) [23].

Figure 2.10 a) Structure of surfactant AOT molecule, and b) SEM image of electro- deposited Cu2O wires [19].

Figure 2.11 a) The scheme of crystal-habit control achieved by preferential orientation adsorption of additives during the crystal growth process and b) crystal shape evolution from cubic to octahedral [20].

Figure 2.12 a) A deposition potential–current diagram summarizing the effect of electrochemical conditions on branching (triangle) and faceting (diamond) growth and b) SEM images of branched Cu₂O crystals with varying crystal habits [21].

Figure 2.13 a) SEM images showing the transformation of pre-grown cubic Cu2O crystals over time in a 0.02 M Cu(NO3)2 solution containing 0.17 M (NH4)2SO4 and b) in a 0.02 M copper nitrate solution containing 0.17 M SDS and 0.004 M NaCl [22].

Figure 2.14 SEM images of Cu2O micro-nanostructures deposited on ITO substrates from electrolyte containing 0.02 M Cu(Ac)2, 0.1M NaAc and CTAB with different concentration; a) 0, b) 0.4, c) 0.8, and d) 2.8 mM [23].