Journal of Solid State Electrochemistry

, Volume 21, Issue 7, pp 1999–2020 | Cite as

Electrodeposition of Cu2O: growth, properties, and applications

  • I. S. Brandt
  • M. A. Tumelero
  • S. Pelegrini
  • G. Zangari
  • A. A. Pasa
Review
  • 278 Downloads

Abstract

For a long time, the world has been waiting for a sustainable, inexpensive, and efficient material for application in electronic and energy conversion purposes. Cu2O thin films made by electrodeposition clearly fulfill the sustainability and cost pre-requisites, and are broadly believed that they could lead to the fabrication of highly efficient devices if well prepared and designed. Here, we review the fundamentals for electrochemical synthesis and the electrodeposition aspects and procedures for growing Cu2O. The properties of electrodeposited Cu2O in thin films and nanostructures will be discussed in view of the literature, with emphasis on the electrical and optical properties and applications in photocatalysis and photovoltaics.

Keywords

Cuprous oxide Electrodeposition Electrochemical doping Photoelectrodes 

Introduction

Cu2O (cuprous oxide) is an oxide with cubic lattice (cuprite structure), space group Pn3m (224), with lattice parameter a = 4.27 Å, and shortest distances dCu-O = 1.84 Å, dO-O = 3.68 Å, and dCu-Cu = 3.02 Å [1]. Figure 1 shows the unit lattice of Cu2O, where the Cu atoms are arranged in a fcc sublattice and the O atoms in a bcc sublattice. It is an oxide usually considered as having fully occupied 3d10 states, p-type semiconducting properties, and direct band gap of about 2 eV. Cu2O has been prepared using different techniques, and as a consequence, the physical and chemical properties are strongly dependent on the preparation method.
Fig. 1

Unit cell of crystalline Cu2O

Cu2O has great potential in many technological applications, due to its electronic and optical properties, with the added advantage that the constituting elements are abundant and the associated production costs can be relatively low. In 1926, copper oxide was employed as one of the first semiconductors as a diode, which consisted of oxidized copper disks [2]. Subsequently, in 1927, with the observation of an emf between Cu and Cu2O when its interface was illuminated, the Cu/Cu2O Schottky barrier solar cells started to be investigated [3]. Since then, the possibilities for applying Cu2O have increased and encompassed, for example, photovoltaic and photoelectrochemical devices [4], as well as photocatalyst [5], memory [6], and nonlinear optical devices [7]. In addition, it is considered for electrochemical and gas sensors. More recently, with the advent of spintronics, Cu2O has been studied as a component of a magnetic metal-base transistor [8].

In this review, we will discuss the electrochemical deposition of Cu2O, its properties, and some of its applications. First, we will discuss the features and advantages of the technique when forming metal oxides, the ability to control doping, and morphology, and finally, we will describe some of the properties and applications of electrodeposited layers and provide a comparison with other preparation techniques.

Electrochemical growth of Cu2O

Various deposition techniques have been utilized to form Cu2O layers; most of these however, such as reactive sputtering, evaporation, or pulsed laser deposition, require vacuum systems and therefore high capital investment and limited opportunities for scale-up. Electrodeposition in contrast is recognized as an inexpensive and versatile technique that nonetheless provides close control of growth and structure and various opportunities for material modification, as discussed in the following.

General background on the electrodeposition of metal oxides

Electrochemical methods provide an advantageous route to the formation of metal oxides; in contrast to physical methods, in fact, the electrodeposition of oxides can be accomplished at temperatures below 100 °C. Furthermore, the direct formation of films on a conductive substrate is possible, favoring integration of the material within devices. The diffusive nature of the deposition process in addition facilitates deposition on substrates with complex shape, and the low deposition temperature also enables film formation on flexible substrates. Metal oxides can be formed electrochemically by anodic processes, i.e., direct oxidation, or via cathodic routes. Through the latter, oxides may be deposited via one of two mechanisms: (i) partial reduction of the metal ion to a lower valence state where it can react with water to precipitate and form the oxide, or (ii) through the electrogeneration of a base near the working electrode, which results in a local pH increase and again leads to precipitation of the oxide at the electrode [9].

Electrodeposition of Cu2O

The electrodeposition of Cu2O occurs through the mechanism (i) and can be described by the following schematic reaction:
$$ {2\mathrm{Cu}}^{2+}{+2\mathrm{e}}^{\hbox{-} }{+\mathrm{H}}_2\mathrm{O}\to {\mathrm{Cu}}_2\mathrm{O}\downarrow {+2\mathrm{H}}^{+} $$
(1)
The overall process consists of two main steps: an initial electrochemical reduction
$$ {\mathrm{Cu}}^{2+}{+\mathrm{e}}^{\hbox{-}}\to {\mathrm{Cu}}^{+},\kern0.5em \left({E}^0=+0.153{V}_{SHE}\right) $$
(2)
followed by chemical precipitation of the oxide when the concentration of Cu+ is larger than its solubility limit
$$ \begin{array}{ll}2{\mathrm{Cu}}^{+}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{Cu}}_2\mathrm{O}\downarrow +2{\mathrm{H}}^{+},\kern1em & \left({E}^0=-0.84-\mathrm{pH}\kern6pt {V}_{SHE}\right)\kern1em \end{array} $$
(3)

where E0 is the standard potential.

The growth kinetics of Cu2O has so far received little attention, limited only to p-type films. Golden et al. [10] observed under potentiostatic conditions a temperature-dependent limiting current; this was interpreted either as (i) a charge transfer process limited by a 0.6 eV rectifying barrier formed under cathodic conditions at a p-type semiconductor/solution interface, or (ii) a chemical rate-determining step thermally activated with an activation energy of 0.6 eV. Similar conclusions were reached by de Jongh et al. [11], who estimated this barrier to be 0.8 eV. To our knowledge, no reports are available discussing the growth kinetics of n-type Cu2O films.

The precipitation process can be understood by the solubility properties of Cu2+ and Cu+ in solution, which are described by a Pourbaix diagram and by the corresponding solubility curves. A Pourbaix diagram is a potential vs. pH diagram showing regions of stability of the various species formed by interaction of Cu ions and water, providing a practical means to estimate and predict the conditions under which the oxide of interest may be precipitated. For example, in Fig. 2, from [12], it is shown that Cu2O could be precipitated in a 200 mV range, dependent on pH. Note that this is a thermodynamic diagram, which can determine the conditions of precipitation, but cannot predict the rate of formation of the various species.
Fig. 2

Calculated Pourbaix diagram for Cu in aqueous solution at 25 °C. Reproduced with permission from [12]; copyright 1997, The Electrochemical Society

The precipitation of Cu2O nuclei may in principle occur homogeneously, i.e., in the electrolyte phase, or heterogeneously, i.e., on the substrate. The actual process is difficult to ascertain and has never been discussed in detail. Homogeneous precipitation is described by the DLVO (Derjaguin, Landau, Verwey, and Overbeek) theory [13], whereby the stability of an assembly of colloidal species is determined by a balance between an electrostatic repulsive force and an attractive van der Waals interaction. In such case, the precipitates being formed would migrate to the electrode due to electrophoresis, forming the film. Heterogeneous nucleation in contrast involves Cu2O nucleation directly at the substrate; in this case, the DLVO theory should be augmented to take into account the voltage drop at the double layer and the additional interactions with the substrate; such a theory is not yet available. Several authors have successfully used nucleation and growth concepts borrowed from electrodeposition to predict trends in the morphology and orientation of Cu2O films and microcrystals [14], suggesting that the electrochemical process (Eq. 2) may indeed be the rate-determining step.

Tailoring morphology and orientation by complexation, capping agents

The stability conditions described by the Pourbaix diagram in Fig. 2 can be tailored by adding to the electrolyte complexing agents, which by selectively binding with the Cu ions shift the regions of stability for the various species. In addition, complexing agents may change the applied potential necessary to form Cu2O and enable control over the rate of precipitation, resulting in changes in morphology and crystallographic orientation of the resulting films. Methods to construct Pourbaix diagram for Cu in presence of complexing agents are available [15], helping to predict and identify precipitation conditions for any given chemistry.

Electrodeposition of p-type Cu2O

As discussed in detail in the following, due to its electronic structure, Cu2O tends to be cation deficient and therefore exhibit hole conductivity and p-type doping [16]. N-type Cu2O could however still be formed by electrodeposition as discussed in the next section.

Complexation of Cu2+ with lactate C2H4OHCOO was used by Rakshani and Varghese [17] in 1987 to grow Cu2O, and this chemistry has been extensively used to obtain good-quality polycrystalline films, such that this is currently the most used complexing agent. Switzer and coworkers [10] used a 0.4 M CuSO4, 3 M lactic acid solution with a pH varying between 9 and 12 and temperatures from 25 to 65 °C to obtain Cu2O films. Pure Cu2O in the bulk cubic structure could be obtained in the potential range −0.35 to −0.55 VSCE (V vs. SCE (saturated calomel electrode)), with C and H impurities in the order of 0.39 and 0.1 wt%, respectively. At pH 9 growth along <100> was obtained, while at pH 12, a <111> orientation was reported.

Another complexing agent for Cu2+ is tartrate; it has been used in an electrolyte for cathodic electrodeposition of Cu2O [18]. This solution contains 0.2 M CuSO4 pentahydrate, 0.2 M tartrate, and 3 M NaOH. The pH of the solution was 13.2.

A minor number of works have reported the growth of p-type Cu2O layers using solutions containing copper(II) acetate and sodium acetate, and pH adjusted to a value equal or higher than 6.9 [19, 20]. Additionally, there are works that reported Cu2O electrodeposition from solutions containing Cu(NO3)2 [14, 21, 22, 23].

Electrodeposition of n-type Cu2O

The electrodeposition of n-type Cu2O layers have been obtained from solutions based on copper(II) sulfate [24, 25, 26] and copper(II) acetate [14, 19, 27, 28, 29] salts. The first salt is often mixed with copper(II) chloride in order to obtain n-type Cl-doped Cu2O layers [30].

N-type conduction in electroplated films was first reported by Siripala and Jayakody [24], where a slightly alkaline solution containing 1 mM CuSO4 and a small amount of NaOH to ensure basicity was used to obtain Cu2O at 5 mA/cm2; temperature was not mentioned. Based on reports by Switzer and coworkers [10] and de Jongh et al. [11], this current density should be much above the diffusion limit. The observed n-type behavior was related to oxygen vacancies, which could be understood in terms of pH, since a lower [OH] would be present with respect to the alkaline solutions generally used to make p-type Cu2O.

Kafi et al. [28] investigated Cu2O layers electrodeposited from copper(II) acetate solution (0.01 M copper(II) acetate and 0.1 M sodium acetate) at 55 °C. Solutions with pHs of 5.7 and 6.5 were tested. Cu2O films grown under both pH conditions showed n-type characteristic as checked by capacitance-voltage measurements. In other works, n-type Cu2O growth has been also reported for copper(II) acetate solutions at pH 4.9 [14, 31, 32]. Earlier, Siripala et al. [27] electrodeposited Cu2O layers using 16 mM Cu acetate and 0.1 M Na acetate solution at 55 °C. As grown and air-annealed films below 300 °C have shown n-type behavior, while a conversion to p-type was observed above 300 °C.

Cl-doped Cu2O layers with n-type conductivity were grown using a solution of 0.3 M copper(II) sulfate, 4 M sodium lactate, and variable concentration of copper(II) chloride [30]. The pH of the solution was 7.5, adjusted by a solution of NaOH. The Cl doping level could be controlled by the concentration of copper(II) chloride added to the solution.

In some cases, n-type Cu2O exhibits both n- and p-type conductivity as observed by photocurrent (PC) measurements; in order to minimize the p-type conductivity in these samples, various methods have been identified, including growth of a thin Cu film before Cu2O deposition at pH 6.1 and annealing at 100 °C for 24 h [33].

Electrodeposition of p-n Cu2O junctions

The possibility of preparing n-type Cu2O enabled in turn the growth of p-n homojunctions, which was achieved by depositing the layers at two different pH values. In the first effort at pH 11, lactate solution was used to grow p-Cu2O, and successively at pH 7.5 acetate solution for n-Cu2O. The dark I-V curves showed a rectifying characteristic, confirming formation of a p-n junction [26]. The most efficient, up to date, Cu2O p-n homojunction for photovoltaics (see “Photovoltaic cells” section) was electrodeposited from a lactate-stabilized copper sulfate solution [34]. The p- and n-type layers were doped with Na and Cl ions, respectively. The Na doping was obtained by adding a sodium aluminate solution to the copper(II) lactate solution, and the pH was adjusted to 9. During electrodeposition, the solution temperature was kept at 60 °C.

Structure and morphology of electrodeposited Cu2O layers

Control of crystal habit and preferential orientation

Changes in the preferential orientation and of the crystallographic facets exposed at the surface can strongly affect the physicochemical properties of Cu2O. Close to thermodynamic equilibrium, the shape of a crystal is determined by minimization of the total surface energy, using the so-called Wulff construction [35], mathematically described by the following relation:
$$ {\Sigma}_{hkl}{\sigma}_{s(hkl)}{A}_{(hkl)}= \min, $$
(4)

where σs(hkl) is the surface energy of the facet with Miller indices (hkl), A(hkl) is the area of the corresponding surface and the sum runs over all the existing facets.

The shape is dynamically achieved through an interplay of the growth of the distinct facets and is determined by the relative growth rate; fast growth directions tend to disappear, while slowly growing facets survive and end up dominating the crystal habit [36]. Exposed facets are those with the lowest surface energy, with the highest atomic packing and most strongly bonded. In contrast, under fast growth conditions (typically when growth rate is higher than mass transport in solution, approaching mass transport limitations), thermodynamic concepts do not generally hold; growth is dominated by metal ion diffusion and protrusions experience higher growth rates, resulting in the formation of high index facets and dendrites. Surface energies calculated for a crystal in vacuum however do not necessarily scale with the surface energy in solution; in this case, the polarity of the facet of interest must be considered, and the corresponding adsorption of water species and other compounds present in the electrolyte must be taken into account. In particular, the selective adsorption of compounds can slow down or block growth at a particular facet and favor the exposure of such facet(s). Figure 3 illustrates the crystal habit formation due to differential growth rate and preferential adsortion [23].
Fig. 3

a Crystal habit formation due a differential growth rate. b Crystal habit modification by preferential adsorption. Reprinted with permission from [23]

An exhaustive discussion of this phenomenon has been given by Siegfried [23]. Electrodeposition from a noncomplexing electrolyte containing 20 mM Cu(NO3)2 results in free growth of Cu2O, with the <100> direction exhibiting the slowest growth rate, eventually forming {100} facets. Addition of sodium dodecyl sulfate (SDS) to this solution results in preferential adsorption to the {111} planes, slowing the relative growth and resulting in the formation of octahedral crystals. The relative rate of growth R along <100> and <111> could be continuously varied to form arbitrary ratios of the {100} and {111} facets by using Cl to hinder growth along <100> and SDS to hinder growth along <111>, and varying the ratio of Cl to SDS, as illustrated in Fig. 4. This result was reproduced by Sun et al. [37], who showed a transition from star-like to cubic-like films with increasing Cl in solution. In addition, novel combinations of exposed facets can be achieved by injecting additives after a habit has already been formed. Specifically, if cubic {100} crystals are allowed to grow, successive addition of NH4+ results in the temporary appearance of {110} facets, overcome after some time by the {111} planes. This suggests that the relative growth rate in presence of NH4+ is <100> > <110> > <111>. Note that addition of NH4+ at the start of the growth process would result only in the formation of octahedral shapes, hindering the detection of an intermediate growth rate for the <110> directions. This method could be used to directly study the relative growth rate of distinct crystallographic facets.
Fig. 4

Evolution of crystal shape by continuously varying the ratio R = growth rate along <100>/growth rate along <111>. Figure reproduced with permission from [23]

It should be noted that these results have been observed during the formation of isolated Cu2O micron-sized particles, under conditions of low nucleation density. In order to be useful for film formation, these results must be adapted to the growth of continuous films; nucleation density was therefore increased by tuning the temperature, since potential variation would vary also the current density, destabilizing the growth morphology [23]. This approach is feasible, but the results are not as clear-cut as in the isolated particle case, making it difficult to obtain films with well-defined orientation throughout the whole film area.

In many instances, Cu2O films exhibit well-defined, large grains with sharp interfaces, each grain appearing to be a single crystal, as shown in Fig. 5 [38, 39]. This is in clear contrast with many other oxides, which only show a nanocrystalline or amorphous structure [40]. This is essentially due to the high solubility of Cu+ in the solution used, which enables a reversible dissolution/redeposition behavior.
Fig. 5

SEM images of p-type Cu2O layers electrodeposited from solutions containing a, c 0.02 M CuSO4 and 0.4 M lactic acid, b, d 0.4 M CuSO4 and 3 M lactic acid, and e, f 0.2 M CuSO4 and 3 M lactic acid. The pH of the solution was 11 (a, b), 13 (c, d), and 12.5 (e, f). The deposition time in e and f is 600 and 2000 s, respectively. ad Reprinted with permission from [38]; copyright 2010 American Chemical Society. e, f Reprinted from [39]; copyright 2017, with permission from Elsevier

The morphology of the Cu2O layers is affected by electrodeposition parameters. Essentially, it has been shown that the Cu2O grain size increases with the pH and temperature of the solution and decreases with the modulus of the current density or potential applied for deposition [11, 38, 41, 42, 43].

Depending on the pH of the solution, different crystal morphologies are obtained, e.g., from a solution containing 0.02 M copper(II) acetate and 0.1 M sodium acetate at pH 4.8 are grown Cu2O grains with star-like shape and at pH from 5.0 to 5.8 dendrites are grown [44]. In [45], using a solution consisting of 0.4 copper(II) sulfate and 3 M lactic acid, it is shown that at pHs 8 and 9, four-sided pyramid-shaped grains are obtained; meanwhile, at pHs 10 and 12, truncated pyramids are grown.

McShane et al. [38] demonstrated that Cu2O layers present cubic surface morphology with Cu2O {100} exposed planes if grown from an electrolyte containing 0.02 M CuSO4. Nonetheless, when this salt concentration is increased to 0.4 M, octahedral or cuboctahedral Cu2O crystals are observed, in which both {100} and {111} planes are exposed.

Cu2O film morphology also depends on the substrate, for instance, Cu2O grown by thermal oxidation can act as substrate for epitaxial growth of electrodeposited Cu2O with larger grain size if compared with electrodeposited Cu2O layers grown on FTO substrate [46]. In [47], it was shown that the morphology of Cu2O layers electrodeposited on ITO/glass, FTO/glass, and ITO/PET substrates present fernlike stellar dendrites, dendrite like-crystals, and polyhedral grained structures, respectively. Additionally, Kaur et al. [39] demonstrated that the grain shape of electrodeposited Cu2O layers dramatically changes as a function of deposition time.

Additives with more complex chemical structures have been utilized to further control of Cu2O morphology, without however providing additional insight; these include CTAB (cetyl trimethylammonium bromide, a cationic surfactant), the addition of which causes the formation of leaves initially extending in different directions to grow parallel and to finally form single stumps [48], and PVP (polyvinyl pyrrolidone, a water-soluble polymer) which is shown to progressively cause a transition from nanorods to spherical then cubic nanoparticles with increasing ratio PVP/Cu2+ [49].

Most reports on habit modification by use of capping agents have been rationalized in terms of the adsorption strength of these compounds at distinct facets. The report by Zhang et al. [50] in contrast suggests that the surface energy of such facets, and therefore the crystal habit, is dependent on overvoltage; the use of distinct complexing agents (lactic acid, citric acid, EDTA) results in unequal overpotentials for the formation of Cu2O, leading to the formation of different facets. Specifically, above a critical potential, the {111} planes grow faster, while below this potential, the {200} facets grow faster.

The texture of electrodeposited Cu2O layers also depends strongly on electrodeposition parameters such as pH and temperature of the solution, substrate, and applied potential/current. The pH strongly affects the out-of-plane orientation of Cu2O grown on top of In2O3 substrate: at pH 9.0, 9.9, and 12.0, the PO (preferential orientation) varies in the order <100>, <110>, and <111>, respectively [51]. This trend has been related to the number of oxygen atoms per unit area in the planes (100), (110), and (111) that are, respectively, 2.78, 5.89, and 8.83/nm2. As the pH increases, the adsorption of OH species is faster, making more O atoms available and favoring the growth of facets with higher O density. The growth of Cu2O was also investigated on n-Si(100) [52], and an abrupt growth transition from <100> at pH 10.0 to <111> at pH 10.1 was observed—a variation of only 1% in pH—suggesting a similar behavior but much more abrupt effect on OH adsorption than in [51].

Switzer et al. [53], using solutions at pH 9 and 12, demonstrated epitaxial growth of Cu2O on single crystal Au(111) and Au(100) as an alternative method for controlling orientation. Pole figure X-ray data clarified that the film was epitaxial both in- and out-of-plane. Additionally, Barton et al. [54] electroplated Cu2O from the same solution on various single crystal Cu surfaces, finding epitaxial growth on Cu(110) and Cu(111) up to 800 nm thickness, while on Cu(100) resulted in an initial growth along a <111> direction with a transition to <100> above 360 nm thickness. This transition was explained by a favorable epitaxial configuration along the <111> direction, overcome by the increasing stress with film thickness, resulting in the film being reoriented to <100>.

Cu2O electrodeposited at pH 10.0 on top of a thin polycrystalline Ni film with <111> PO is seen to display initially a preferential <111> growth [52]. Such a texture is due to the formation of a (1 × 2R90°)Cu2O(111)//(4 × 3)Ni(111) interface with a lattice mismatch of up to 6.7%, much lower than the 14.3% calculated for the (1 × 1)Cu2O(100)//(2 × 1)Ni(111) interface. The switch from <111> to <100> PO with the increase in thickness in layers grown at pH 10.0 is definitely a pH influence, privileging the growth of planes with lower oxygen content, as earlier discussed. If pH is raised to 10.1 or above, then Cu2O layers on top of Ni will present out-of-plane growth in the <111> direction in the whole range of investigated thicknesses, since in this case substrate and electrolyte favor <111> growth. These results are summarized in Table 1.
Table 1

Influence of pH and substrate on the growth of Cu2O [52]

Substrate

pH

T (°C)

Voltage (VxSCE)

Thickness (nm)

Orientation

Ni(111)

10.0

25

−0.5

250

30% in <111>

Ni(111)

10.0

25

−0.5

900

60% in <100>

Ni(111)

10.1

25

−0.5

250–1250

100% in <111>

The maximum lattice mismatch of 6.7% described above between Cu2O(111) and Ni(111) leads to a shrinkage of the Cu2O lattice parameter in the out-of-plane direction. Indeed, due to lattice mismatch, it was observed that Cu2O films deposited on Ni(111), Au(111), and Si(100) at pH 10.00 and 10.10 display a lattice parameter much lower than that expected for relaxed Cu2O, as shown in Fig. 6, from [52]. Additionally, from this figure, we see that as a function of thickness, the lattice parameter stays constant initially and then increases abruptly to a value that is still below the 4.27 Å characteristic of bulk Cu2O. In a recent study on the kinetic roughening of Cu2O surfaces grown on Si(100) and Ni(111) at pH 10.00, the authors concluded that despite the differences in PO, the growth dynamics of the layers in both systems is dominated by surface diffusion of adsorbed species [55].
Fig. 6

Lattice parameter as a function of thickness for Cu2O layers grown on three different substrates. Figure reprinted from [52]; copyright 2014, with permission from Elsevier

Influence of anions

In this section is discussed the influence of the anion Cl on the structure and morphology of Cu2O layers. The Cl anions are introduced into the electrolyte by adding controlled amounts of CuCl2. As observed by Jayathilaka et al. [56] for Ti substrates and pH 9.3 and Pelegrini et al. [57] for Au substrates and pH 7.5, the PO and the grain size are strongly affected by the Cl concentration in the electrolyte. Both reports show that by increasing the CuCl2 concentration the growth orientation changes to <100> independently of the substrate and that the grain size is significantly reduced. It was also observed in [57] that for both CuCl2 concentrations tested of 0.01 and 0.1 M, the lattice constant is the same with the average value of 4.269 Å, which is close to the value of powder Cu2O samples [58]. This result indicates that the Cl atoms are well accommodated in the Cu2O lattice.

Electrodeposition of dendritic Cu2O

As briefly mentioned earlier, the morphology of Cu2O can be also tuned by deposition under far-from-equilibrium conditions, where the growth rate is sufficiently high to generate an ion depletion zone at the solid/electrolyte interface, favoring growth at grain apexes and protrusions, and therefore dendritic growth. Control of morphology in this instance can in principle be achieved by balancing of growth rate and mass transport rate; unfortunately, in an electrochemical/chemical two-step process such as for Cu2O, the two parameters cannot be varied independently, in contrast to an electrodeposition process.

Control over formation of a dendritic structure may be of interest to increase the surface area of the material while avoiding formation of a high density of interfaces [14], increasing the ability of the material to transport charge carriers. Cu2O formation generates H+ as a side product (Eq. 3), leading to a pH decrease over time; in order to maintain stable growth conditions, it is thus essential to use a buffer. McShane et al. [14] selected an acetate buffer for this purpose, which selectively adsorbs on {111} facets, with the additional result of slowing down their growth and to decrease the growth rate along <111>. Branching growth was observed at pH 4.9, using a potential of 0.02 VSCE in a 0.02 M Cu(CH3COO)2 + 0.08 M acetate buffer. Film growth occurred preferably along the substrate due to the higher conductivity of the ITO substrate with respect to Cu2O. The dendritic films show anodic photocurrent PC (n-type behavior) without applying an external bias. In order to further increase PC, the charge transfer efficiency was further increased by decreasing the nucleation density via lowering the overvoltage; this was done through three different methods: decreasing overpotential, decreasing [Cu2+], and increasing acetate concentration. The fact that all methods invariably resulted in a PC enhancement is an evidence that this effect is the most important. PC increased from 0.02 to 0.45 mA/cm2 (~20×) with increasing lateral grain size, leading to an efficiency of ~1%. It should be noted that, despite the large enhancement, these values are less than those obtained by other researchers in thin films.

Properties of electrodeposited layers

Electrical properties

A fundamental step towards the application of electrodeposited Cu2O thin films in photovoltaic, photocatalysis, and electronic/spintronic devices is to determine and control the electrical properties, i.e., electrical resistivity, activation energy, and type of conductivity. Aiming for this knowledge, the role of defect chemistry and doping on Cu2O electrical properties have been investigated. These topics will be discussed in the next subsections.

Electrical resistivity

Several works reported on the electrical characterization of Cu2O layers were obtained by electrodeposition, essentially by measuring the electrical resistivity. Figure 7 shows a graph comparing values of resistivity gathered from literature for the most relevant growth methods, including electrodeposition. The high values of electrical resistivity observed for electrodeposited Cu2O places it in the semi-insulator category rather than in the semiconductor one, restraining the possibilities for applications. The origin of the discrepancy between electrodeposition and other preparation techniques is still open, and it is a major issue in this area of research.
Fig. 7

Graph presenting values of electrical resistivity at room temperature for Cu2O obtained by different growth techniques. ED electrodeposition [30, 34, 59, 60, 61, 62, 63, 64], THERM thermal oxidation [65], SPUTT-RF reactive sputtering [66, 67, 68], PLD pulsed laser deposition [69, 70, 71]

The wide range of electrical resistivity values found for electrodeposited Cu2O could be related to the strong dependence of resistivity on the growth parameters [61], such as pH, bath temperature, and deposition potential. Han et al. [61] observed that p-type Cu2O resistivity decreased as pH and temperature of the solution were increased and as the modulus of the applied potential for Cu2O deposition was decreased. The decrease of resistivity by changing these parameters is suggested to be associated to grain size and <111> crystal orientation (see “Control of crystal habit and preferential orientation” section), which enable higher defect concentration and enhancement of carrier density. The lowest measured resistivity they measured is about 3.5 × 105 Ω cm. Han et al. [61] also investigated n-type Cu2O layers and noticed that its resistivity reduces with pH and the lowest obtained value is about 2 × 107 Ω cm. This extremely high electrical resistivity for electrodeposited Cu2O is an important drawback for the use of such thin films in technological application. Nevertheless, the resistivity can be reduced in three orders of magnitude by thermal treatments after the electrodeposition of the dopants [59].

Defect chemistry of electrodeposited Cu2O

As discussed in the introduction, Cu2O exhibits a 2.1 eV band gap and semiconductive behavior; in most cases, it is a native p-type conductor, even though intrinsic n-type has also been reported [24, 26, 45, 72]. The electrical conduction is attributed to the presence of native carriers generated by point defects such as vacancies of Cu and O. The wide range of electrical resistivity, as well as the large variation in comparison to other deposition techniques could be directly correlated to such native defects. Theoretical DFT (density function theory)-based calculations have been used as a powerful tool to study defect chemistry of Cu2O [73]. Several works have been pointing to the high stability of copper vacancies, leading to hole carrier generation [16, 74, 75, 76, 77]. These computational works also agree that n-type carrier cannot be sourced by oxygen vacancies [76]. For the case of electroplated samples, Wang et al. [73] have presented calculations using pH-based equilibrium condition for defect nucleation rather than vacuum based, as in previous works. At high pH conditions, the presence of O antisites and mainly O interstitial become possible in electrochemical experiments, which do not occur in vacuum condition. Such defects are sources of deep acceptor levels that can enlighten the differences of electrical properties of electrodeposited Cu2O where compared with other preparation techniques.

n-type intrinsic conduction

The intrinsic n-type conduction has been consistently observed [24, 26, 45, 72]; however, the source of electron carriers is still debated in the scientific community, as the theoretical approach points to oxygen vacancies that cannot generate electron carriers [16, 73, 76]. The DFT result discussed above, using pH condition instead of vacuum [73], points to a new interpretation for the origin of n-type carriers, that for low pH conditions, defects such as copper antisites and copper interstitials become relevant (at variance with vacuum growth environment), generating donor levels that could induce the native n-type conduction found by lowering the pH experimentally. Another possibility was highlighted by Nian et al. [45] whereupon the n-type behavior originates from incorporation of Cu2+ ions at unstable Cu-deficient surface states, which would induce the formation of an inversion layer with n-type conductivity. Even though many groups have been seeking a comprehensive understanding of the n-carriers in Cu2O, there is still a lack in electrical data about n-Cu2O. This state of affairs is because most of the works are using capacitance curves and photocurrent measurements to characterize the electrical behavior of Cu2O, which are techniques more sensitive to the surface rather than bulk. For the latter set of data, experiments such as Hall effect, transient spectroscopies (DLTS (deep-level transient spectroscopy), TSIC (thermostimulated ionic conductivity), and PICTS (photoinduced current transient spectroscopy)) and local characterization with positron annihilation and X-ray/electron scattering are still missing.

Defect energies and activation energies: experimental vs. computational

The defect transition level calculated by DFT can be directly compared to the thermal activation energies of charge carries in electrodeposited Cu2O. The activation energies have been experimentally measured by several techniques like DLTS [78], TSIC [79], PICTS [80], and Hall or resistivity vs. temperature (ρxT) profile measurements [59, 66, 69, 81]; the results are summarized in Table 2. The usual characterization of activation energy through the Hall effect cannot be implemented in electrodeposited semiconductors, since the substrate has a smaller electrical resistance than the grown layer. The activation energies values for electrodeposited Cu2O films also differ from the ones obtained for Cu2O grown by vacuum-based techniques, see Table 2. Such differences strongly support the hypothesis of different origin for the various charge carriers, i.e., carrier coming from distinct types of point defect and deep levels. The deep energy level of 0.63 eV measured by PICTS [80] for electrodeposited Cu2O was assigned to a hole trap defect and occurs at same energy range proposed for the oxygen interstitial [16, 77, 84]. The activation energies of about 0.2 eV, measured in samples grown by vacuum-based method lies in the range of copper vacancy transition energies [16, 77, 84]. No activation energy studies were found for intrinsic n-type Cu2O.
Table 2

Activation energies of carriers in Cu2O as measured by several techniques

Growing technique

Measurement Technique

Activation energy

Electrodeposition

ρxT

0.88 eV [59]

0.42 [60]

Electrodeposition

TSIC

0.03 and 0.79 eV [79]

Electrodeposition

PICTS

0.12 and 0.63 eV [80]

Sputtering

Hall effect

0.19 eV [66]

Sputtering

DLTS

0.25 and 0.45 eV [78]

Pulsed laser deposition

ρxT and Hall effect

0.21 [69], 0.25 [70], and 0.15 eV [71]

Thermal

Hall (carrier density)

0.22 eV [81]

0.31 eV [82]

0.4 eV [83]

Electrochemical doping of p-Cu2O

Many attempts of doping were carried out to improve the charge carrier density and the electrical resistivity of p-Cu2O layers. Some of the elements already tested for doping Cu2O are Si [66], Ni [70], Mn [85], Co [69], N [86], and Na [34]. Table 3 summarizes the resistivity values found for doped Cu2O layers. The most significant result was obtained by Lee et al. [86] for layers prepared by sputtering, which with 1.7 at.% of nitrogen in Cu2O, reduced the electrical resistivity by a factor of 750. For the case of electrodeposition, the best doping so far was found by Elfadill et al. [34], which achieved a decrease by a factor 330 in the resistivity by doping with Na, which led to significant improvement of the p-n homojunction solar cell efficiency, as will be described in “Photovoltaic cells” section. Notwithstanding, for an ultimate energy or electronic device based on electrodeposited p-Cu2O, a much more robust improvement in the electrical properties is still needed.
Table 3

Electrical resistivity values measured for doped Cu2O layers

Element

Growth technique

Minimum resistivity (Ω cm) (300 K)

Maximum reduction factor

Si (~0.5%) [66]

Reactive sputtering

12

~13

Ni (~2%) [70]

Pulsed laser deposition

~250

~1

Mn (0.5%) [85]

Electrodeposition

~2 × 106

~3

Co (5%) [69]

Pulsed laser deposition

512

~0.5

N (1.7%) [86]

Sputtering

0.2

~750

N [87]

Thermal oxidation

100

~4

N (2.5%) [88]

Sputtering

1.1

~75

Na (1.7%) [34]

Electrodeposition

291

~330

Electrochemical doping of n-Cu2O

N-type doping of Cu2O layers is also desirable for applications. Han et al. [30], demonstrated for the first time the doping of Cu2O using electrodeposition and chlorine as dopant, reaching n-type conduction and resistivity down to 7 Ω cm [30]. The films were prepared by adding about 0.1 M CuCl2 in the electrolyte at a pH of 7.5. In contrast to this previous report for Cl doping, Lincot and coworkers [89] did not observe n-type doping in Cu2O films grown from solutions containing Cl.

Even though some disagreements have been noted about the Cl doping of Cu2O, several other works have been confirming the results of Han et al. [30]. This work demonstrated a real reduction of the electrical resistivity, an inversion of the carrier type and improvements in the electrical properties [56, 90, 91], such that the carrier densities could be controlled in the range of 1017 to 1019/cm3. More recently, n-type doping using other elements besides chlorine has been reported. Yu et al. [92] demonstrated n-type doping using fluorine (F-doped Cu2O) and electrodeposition; no information about the electric properties were given; the doping however seems to enhance the homojunction cell efficiency. Cai et al. [93] also reported n-type Cu2O samples grown by sputtering using indium as dopant, showing resistivity of about 310 Ω cm, which is a relatively high value when compared to the resistivity of samples grown by RF sputtering (see Table 3). Ab initio calculations based on DFT was used to confirm the capability of Cl and F to dope Cu2O [94]. In order to achieve a comprehensive understanding of electron doping in Cu2O, using either Cl and F or any other dopant, much more information about electrical phenomena is still needed.

Optical properties

The deposition technique has great influence on the optical properties of Cu2O. Figure 8 shows the Cu2O refractive index (n) as a function of wavelength (λ) of Cu2O films grown by reactive sputtering, thermal oxidation, sintering, and electrodeposition [95]. The dependence of n on the employed technique is apparent, but the general behavior is similar, except for curve b (reactive sputtering). For λ higher than 0.8 μm, n tends to be constant due to the low density of free electrons in Cu2O. At about 0.5, there is a maximum in n, which is related to the Cu2O band gap edge. The refractive index data is fitted by the following empirical equation:
$$ {n}^2=1+ A{\lambda}^2/\left({\lambda}^2- B\right), $$
(5)
Fig. 8

Refractive index of Cu2O prepared by a, b reactive sputtering, c thermal oxidation, d sintering, and e electrodeposition. Reprinted from [95], with the permission of AIP Publishing

where A and B are constants. The values of these constants found for electrodeposited Cu2O are, respectively, 4.81 ± 0.01 and 0.125 ± 0.001 (μm)2 [95].

Figure 9 compares n of Cu2O films with different thicknesses electrodeposited on Si(100), Au(111), and Ni(111) with electrolyte pH equal to 10.0 and 10.1 [52]. Such n values were obtained at λ = 1.5 μm, which is in the range where n is roughly constant. Independently of electrodeposition conditions, it is observed that n decreases as film thickness increases. However, for Cu2O electrodeposited on Au(111) and Si(100), such decrease is close to the linearity with similar slope coefficient and n is higher for films grown at pH 10.1 than at pH 10.0. On the other hand, the sample Cu2O/Ni(111) initially shows a more accentuated reduction of n with film thickness, and after 750 nm, this decrease is attenuated. The n values for pH 10.0 and 10.1 become much closer.
Fig. 9

Values of n at λ = 1500 nm as a function of film thickness for Cu2O grown on Ni/Si(100), Au/Si(100), and Si substrates. Two solutions of pH 10.00 and 10.10 were used for growing Cu2O. Reprinted from [52]; copyright 2014, with permission from Elsevier

The presence of point defects in the Cu2O affects its electrical properties (see “Electrical properties” section1); the same goes for its optical properties. In [52], the Cu2O n dispersion data (see Fig. 9) was analyzed in light of the Wemple and DiDomenico (WD) dispersion relationship [96], which is an empirical model and has been used by some authors to qualitatively describe the density of defects in crystal systems [52, 97]. It was shown in [52] that Cu2O layers grown on Au(111) and Si(100) substrates have a higher density of defects when the out-of-plane growth is in the <111> direction than in the <100> one. Films with <111> and <100> growth direction were deposited at pH 10.1 and 10.0, respectively. Indeed, different Cu2O surfaces or atomic planes present unequal energies for formation of defects. A previous study based on first principles calculations showed that the surface Cu2O(111) presents the lowest energy for the formation of VCu [98], indicating that films with <111> growth direction should have the highest density of this defect.

The Cu2O photoluminescence (PL)-related energy levels have been investigated for layers grown by several techniques and some of these levels are directly linked to defect sites (see [99] and therein). A PL band due to near band emission is observed at ~2.0 eV. Other PL bands are found in the near infrared region, which are at ~1.4, ~1.5, and 1.7 eV and attributed to VCu, VO+, and VO2+ defects, respectively. The defects VO+ and VO2+ are singly and doubly ionized oxygen vacancies, respectively. For electrodeposited Cu2O layers, the PL spectrum shows a dependence on deposition parameters such as substrate [100], surfactant concentration in the solution [48], and temperature of the solution [101].

The PL spectrum of electrodeposited Cu2O films is also affected by Mn doping as investigated in [85]. The films were doped with Mn ions by adding MnSO4 in the solution. Comparing undoped and Mn-doped Cu2O spectra, it could be concluded that the Mn-doped samples present an additional emission band at 1.97 eV and an intensity reduction of the luminescence peak at 1.33 eV, which was attributed to VCu.

The band gap energy (Eg) of electrodeposited Cu2O layers depends on the employed deposition parameters. As presented in [102], Eg increases from ~2.0 to ~2.5 eV as the electrical potential applied to electrodeposition is varied from −0.50 to −0.60 VAg/AgCl (V vs. silver chloride electrode). Such a behavior is explained by a variation in Cu2O stoichiometry, which is related to formation of lattice defects. The Eg dependence on the electrolyte pH, substrate, and thickness was investigated in [52]. The calculated Eg values are displayed in Fig. 10 and are in the expected range for Cu2O. An interestingly abrupt transition from higher to lower values is observed as a function of film thickness. The Eg transition is associated with the increase of the Cu2O lattice parameter already presented in Fig. 6. Comparing lattice parameter and Eg values for films with thickness up to 500 nm deposited on different substrates and pH values of 10.00 and 10.10, it was observed that these two quantities are inversely proportional. The same conclusion is obtained comparing films with thickness higher than 900 nm. The increase of the lattice parameter means an enlargement of the interatomic spacing and consequently an increase in Cu–Cu internetwork interactions. From theoretical studies, it is expected that a more intense Cu–Cu internetwork interaction reduces the Cu2O band gap [103, 104]. Additionally, in [105], a decrease of Eg as a function of electrodeposition time was observed. Similar result was obtained for ZnO/Ag/Cu2O nanorods [106].
Fig. 10

Eg of Cu2O films as a function of thickness. Reprinted from [52]; copyright 2014, with permission from Elsevier

Magnetic properties

Although pristine Cu2O is a diamagnetic material, ferromagnetic-like behavior in undoped [107, 108, 109] and doped [69, 85, 110, 111, 112] Cu2O layers has been observed. The results indicate that both defects and dopants can generate weak ferromagnetism in Cu2O, resulting in a new functionality and in particular providing a possible material for spintronic applications. The magnetic investigation of Cu2O has strongly contributed to the field of defect-mediated ferromagnetism [113], mainly in the case of electrodeposited samples, where a large variety of point defects can be found [73].

Liu et al. [85] reported an experiment in which electrodeposited thin films with about 0.2% of Mn concentration show magnetic moment of 0.6 μB/Mn and weak ferromagnetic behavior at room temperature (RT). In another investigation, ferromagnetism above RT was obtained in Co-doped electrodeposited Cu2O layers [111].

Applications of electrodeposited Cu2O layers

Photovoltaic cells

The maximum theoretical efficiency for solar cells based on Cu2O is ~23%; driven by this expected efficiency Cu2O became the most popular oxide material for photovoltaics [114]. Cu2O presents a high optical/UV absorption coefficient that is strongly dependent on the wavelength [115], a band gap well matched to the solar spectrum and high room temperature mobility (100 cm2/Vs for oxidized Cu sheets) [65, 115, 116, 117]. Schottky junction, homojunction, and heterojunction architectures have been used to build up Cu2O-based solar cells [31, 61, 118, 119, 120, 121, 122, 123, 124]. The best photovoltaic efficiency to date (8.1%) has been achieved using a Al:ZnO/Zn1 − XGeXO/Cu2O structure [125]. An open circuit voltage (VOC) of 1.1 V was measured for this solar cell. The Cu2O layer was grown by thermal oxidation of copper sheets, followed by a treatment with sodium. The role of this treatment was to produce Na-doped Cu2O layers of resistivity as low as 10−1 Ω cm. The interlayer Zn1 − XGeXO is a n-type oxide and the high efficiency of this photovoltaic cell was mainly attributed to a better band alignment between Al:ZnO and Cu2O provided by Zn1 − XGeXO.

The highest conversion efficiency among solar cells using electrodeposited Cu2O as active layer is of 4.41% [46]. The full structure of this solar cell is Al:ZnO/Ga0.975Al0.025O/Cu2O/Na:Cu2O, and the Cu2O electrodeposited layer was grown on top of the Na-doped Cu2O substrate, which was previously grown by thermal oxidation. The use of this substrate allows to grow epitaxially electrodeposited Cu2O with improved crystallographical properties and larger crystallites when compared to layers grown on Au thin films and FTO substrates [46, 126]. These features are beneficial for a higher photovoltaic efficiency.

Minami et al. [126] have investigated the efficiency of solar cells in which a thin film of Mn-doped Cu2O was electrodeposited on top of Na-doped Cu2O substrate and covered by Al:ZnO. This structure (Al:ZnO/Mn:Cu2O/Na:Cu2O) presented a photovoltaic efficiency of 4.21% and VOC of 0.78 V. The electrodeposited Mn-doped Cu2O layer was characterized as an intrinsic-type semiconductor.

A solar cell with the structure Al:ZnO/Ga2O3/Cu2O, in which the unique Cu2O layer was grown by electrodeposition, showed a photovoltaic efficiency of 3.97% [127]. The Cu2O layer was grown on Au(200 nm)/Ti(5 nm)/SiO2 substrate. Additionally, this device presented a VOC of 1.2 V that is the highest value up to now reported for a Cu2O-based solar cell.

The photovoltaic efficiency is more modest for Cu2O-based solar cells entirely built up with electrodeposited layers. The highest efficiency up to now is of 1.52% for an electrodeposited ZnO/Cu2O nanostructured solar cell [128]. This work will be further discussed in “Nanostructuring solar cells based on Cu2O” section. This same architecture (ZnO/Cu2O), but in planar geometry, has shown efficiency values up to 1.43% [129].

Electrochemically fabricated Cu2O homojunction solar cells have been reported [31, 34, 38, 61, 123, 124] and the obtained maximum efficiency is to date of 2.05% [34]. The VOC is 0.49 V, which is considered low if compared with other Cu2O-based solar cells. In this device the p- and n-type layers were electrodeposited Na- and Cl-doped Cu2O films, respectively.

One can notice from the above paragraphs that the efficiency of Cu2O-based solar cells decreases as more electrodeposition steps are added to the fabrication process. In fact, the lower efficiency found for electrodeposited Cu2O-based solar cells, compared to oxidized Cu2O-based solar cells, is attributed to a lower electronic mobility [130, 131]. This results in charge carrier lengths much lower than 1 μm. Whereas the light penetration depth in Cu2O is in the range of 1–10 μm. Therefore, photocarriers generated deep within the Cu2O layer will not be collected and the solar cell efficiency is diminished. Besides this, the limited understanding regarding the defect chemistry of n-type Cu2O layers hampers the development of Cu2O homojunction solar cells of high efficiency.

McShane et al. [38] investigated the effect of the surface morphology of p-type electrodeposited Cu2O layers on the efficiency of a Cu2O homojunction solar cell. The authors concluded that the high resistivity of the p- and n-type Cu2O is detrimental for Cu2O photovoltaic efficiency, which also strongly depends on the crystals faces exposed at the p-n junction.

In [31], it is shown that the photovoltaic efficiency is improved when the n- and p-type layers in a Cu2O homojunction solar cell have the same crystallographic orientation. The reason for the enhanced efficiency is related to the reduction of interfacial states. The growth of textured Cu2O layers also enhances the absorption of photons.

Photoelectrochemical cells

The conversion from sunlight to chemical fuels is a possible route to overcome the planet dependence on oil for energy production. Photoelectrochemical cells are devices able to drive redox reactions that will provide chemical fuels as end products [132]. These cells are composed of two electrodes, which are separated by a solution (liquid, gel, or organic solid). At least one of the electrodes consists in a semiconducting layer, which is used to absorb sunlight and generate electron-hole pairs, with the holes migrating towards the electrode/electrolyte interface providing the necessary charge for the redox reactions.

In the next subsections, results will be presented about Cu2O-based photoelectrochemical cells that could produce chemical fuels either from water splitting or by coupled proton and CO2 reduction.

Cu2O for photoelectrochemical hydrogen reduction

The thermodynamic redox potentials for proton reduction and water oxidation lie in the band gap of the Cu2O. Additionally, Cu2O layers can absorb a good portion of the solar spectrum due to its band gap of ~2 eV. Therefore, the production of H2 can occurs on the Cu2O photoelectrodes under solar illumination and immersed in water solutions. When the Cu2O layer presents p-type (n-type) conduction, it will act as photocathode (photoanode). The theoretical solar-to-hydrogen efficiency of a Cu2O solar water splitting system is ~18% [4]. A diagram of Cu2O bands and relevant redox reactions are shown in Fig. 11.
Fig. 11

Band structure of Cu2O in contact with water solutions. The position inside the gap of Cu2O and H redox reactions are also shown

One can notice that in Fig. 11, the oxidation and reduction potentials of Cu2O are also positioned within its band gap, making it not thermodynamically stable in aqueous solution. Several works have reported on the Cu2O degradation in aqueous environment and the consequent reduction of the photocurrent [4, 14, 133, 134, 135, 136, 137, 138, 139, 140, 141]. Although, Zangari and coworkers [142] have shown that electrodeposited Cu2O films are chemically stable within the voltage window where only the CuO ↔ Cu2O reaction can occur, the Cu2O morphology evolved from a dense structure to a more stable network with elongated leaf-like crystals. The photoelectrochemical experiments were carried out using a solution of 0.2 M sodium sulfate, 0.1 M sodium acetate, and 10% (v/v) methanol (pH 7.28).

The Cu2O degradation in aqueous solutions have been demonstrated in several works, and the surface coating of Cu2O with conformal layers of wide band gap materials appears as an efficient method to overcome the instability and enhance the photoelectrochemical response [4, 14, 133, 134, 135, 136, 137, 138, 139, 140, 141]. A remarkable result was reported in [141], where a photocurrent of −7.6 mA/cm2 at 0 VRHE (V vs. RHE (reversible hydrogen electrode)) in a Na2SO4 solution at pH 4.9 and under AM 1.5 illumination was shown. The cathodic current upon illumination indicated the p-type character of the Cu2O layer, which was electrodeposited from a lactate-CuSO4 solution at pH 12 and at 30 °C. The Cu2O photocathode was coated by multilayers of ZnO/Al2O3 and a top layer of Ti2O grown by ALD. On top of these oxide layers were electrodeposited Pt nanoparticles that worked as catalyst for proton reduction. The stability of such Cu2O photocathodes strongly depends on the deposition temperature of the TiO2 layer. Paracchino et al. [137] using the optimized temperature of 150 °C were able to measure a photocurrent of −4.5 mA/cm2 at 0 VRHE for a longer period.

In [139], it was demonstrated that by replacing the Pt nanoparticles by a porous amorphous RuO2 layer, the stability of the Cu2O photocathode is significantly improved. Photocurrent stability of 94% after 8 h of chronoamperometric measurement in chopped light, with an initial photocurrent of −5 mA/cm2 was observed. Better stability was obtained by exposing the photocathode to steam at 150 °C for 3 h inside of an autoclave before the RuO2 deposition [133]. The photocurrent reduced to 90% of its initial value of −5.5 mA/cm2 at 0 VRHE only after 50 h of chronoamperometric measurement in chopped light. The improvement was associated to smoothness of the TiO2 surface, which allowed a more homogeneous RuO2 electrodeposition.

Azevedo et al. [134] fabricated the most stable Cu2O photocathode up to date by replacing the TiO2 protecting layer by SnO2. With the photoelectrode structure RuO2/SnO2/Cu2O a photocurrent of −4.25 mA/cm2 was measured that reduced to 90% of its initial value after 57 h.

Molybdenum sulfide has been also used as catalyst for hydrogen evolution reaction (HER) in Cu2O photoelectrodes. Mo and S are earth-abundant and cheaper than Pt and RuO2. Layers of MoSx were grown on the TiO2/ZnO:Al/Cu2O structure discussed above and photocurrents up to −5.7 mA/cm2 at 0 VRHE were obtained for 7 h in a solution with pH 1.0 [135].

Cu2O photocathodes of high stability were also obtained by growing ultrathin NiFe-layered double hydroxide cocatalyst on Cu2O surface [138]. Applying an external bias of −0.2 VAg/AgCl, an excellent stability over 40 h of uninterrupted illumination was achieved with a photocurrent of about −0.42 mA.

The photocurrent of Cu2O photocathodes can be enhanced by using p-n Cu2O homojunction films as showed by Jiang et al. [136]. Electrodeposited p-type Cu2O films from three solutions with pHs 7.0, 8.0, and 9.0 and subsequently covered by layers with n-type Cu2O. The p-type Cu2O prepared at different pH values is expected to present distinct carrier concentrations [73]. The best photocurrent response obtained by the authors was −0.2 mA/cm2 at 0 VNHE for the p-n homojunction in which the p-type layer was grown from the solution with pH 9.0.

Photoanodes of electrodeposited n-type Cu2O have been reported by MacShane and Choi [14] exhibiting anodic photocurrent of up to 0.45 mA/cm2 at short-circuit condition in a two-electrode configuration. The solution for chronoamperometric measurements was 0.02 M potassium sulfate. The illumination was provided by a 300 W xenon lamp with UV filter delivering an intensity of 1 W/cm2. The Cu2O layers were grown by potentiostatic electrodeposition using solutions containing 0.005–0.02 M copper(II) acetate and acetic acid buffers. The obtained layers were composed of Cu2O dendritic crystals, and the photocurrent showed to be directly proportional to the size of these crystals. The applied potential, acetate buffer, and copper(II) acetate concentrations were altered to enhance the crystal size, and an improvement over 20 times on the photocurrent was achieved.

In another effort, the photoelectrochemical properties of n-type Cu2O film electrodeposited from a copper(II) acetate solution was also investigated [32]. The chronoamperometric measurement showed an initial anodic photocurrent of ~0.5 mA/cm2, but it dropped to ~0.1 mA/cm2 after 40 min. Additionally, in this investigation, p-type Cu2O films were obtained from the n-type Cu2O films by a controlled annealing process. The p-type Cu2O presented an initial cathodic photocurrent of about −1.0 mA/cm2, which dropped to −0.5 mA/cm2 after 40 min. The higher stability of the p-type Cu2O film over the n-type one is attributed to the formation of a CuO layer on top of the p-type Cu2O surface. This CuO acts as a protecting layer passivating the Cu2O surface.

Recently, Yang et al. [140] investigated the photoelectrochemical response of a Cu2O/CuO bilayer. A p-type Cu2O layer was electrodeposited and subsequently exposed to a thermal oxidation process. This photoelectrode showed high activity and good stability mainly at high potentials in solutions of high pH. The advantages of using Cu2O/CuO over a pristine Cu2O photocathode include a spectrum broadening for light absorption, formation of a larger depletion region, and enhancement of the majority carrier density.

Nanostructured Cu2O photoelectrodes have been also synthetized and presented high photoeletrochemical response and stability. This topic will be discussed in “Nanostructuring Cu2O photoelectrodes for photoeletrochemical cells” section.

Cu2O electrodes for CO2 reduction

The strong global demand for energy and the increasing atmospheric CO2 concentration led to extensive investigations on renewable energy systems. Artificial photosynthesis processes that are able to convert CO2 by solar energy into carbon-based fuels are of great interest.

Cu2O shows high electrocatalytic activity for CO2 [143, 144], including electrochemically grown Cu2O. More recently, in 2013, it was demonstrated for the first time the capability of Cu2O layers for photoreduction of CO2 [145]. The photocathodes were composed of p-type Cu2O layers electrodeposited on CuO nanorods walls. Such a photocathode achieved Faradaic efficiencies up to ~95% for photoreduction of CO2 into methanol.

Another topic with intense research nowadays is the photoreduction of CO2 into CO. Schreier et al. [5] investigated the efficiency of photocathodes fabricated by growing nanostructured TiO2 layers on top of Cu2O electrodes protected by Al:ZnO. The attachment of CO2 to the TiO2 surface was mediated by phosphonate linkers. The Faradaic efficiency achieved during CO generation was between 80 and 95%, which is comparable with values obtained using Au nanowires [146]. Additionally, the achieved photocurrent is significantly higher than the earlier values obtained for molecular catalysts dissolved in solution.

In [147], ultra-long carbon nanotubes (CNTs) with Cu2O nanocrystals were employed for the first time for photoreduction of CO2. This hybrid structure presented five orders of magnitude higher conductivity than bare Cu2O, and improved photocurrent as well as stability. A multistep electrodeposition procedure was developed for homogeneous growth of Cu2O layers on CNTs.

Photoelectrochemical cells based on electrodeposited Cu2O cathodes show a selectivity of 92.6% for carbonaceous products as demonstrated by Chang et al. [148]. This high selectivity was associated to a direct exposure of Cu2O layer to the solution. However, this direct exposure is detrimental for Cu2O stability. The authors improved the Cu2O stability by using it as a dark cathode, while the anode (TiO2 nanorods) was in charge of absorbing the incident radiation.

Spintronics

With the advent of spintronics, Cu2O started to be used for applications in magnetic devices, e.g., as a p-type semiconductor at the emitter electrode of a magnetic metal-base transistor (MBT) [8, 149]. In such MBT, the Cu2O layer was electrochemically deposited and the transistor presented a magnetocurrent of approximately 40% in a low emitter current condition [149].

Many of the spintronic devices proposed in the literature, have as a common feature the injection of spin-polarized carries into a nonmagnetic semiconductor [150, 151, 152, 153, 154]. Spin transport experiments in Cu2O grown by pulsed laser ablation were performed by Pallecchi et al. [71]. With the structure LSMO/Cu2O/Co, the Cu2O spin diffusion length was estimated to be 40 nm, which is hundred times larger than its carrier diffusion length. This result shows that Cu2O-based spintronic devices should work in theory; however, no experimental reports are found in the literature. Likewise, spin transport investigations in electrodeposited samples of Cu2O have not been reported yet.

A fundamental limitation for efficient spin injection in semiconductors is the impedance mismatch between the semiconductor and the ferromagnetic metal used as electrode for injecting spin-polarized current [155]. One of the possible solutions is to replace the ferromagnetic metal by a semiconductor with ferromagnetic ordering. It has been possible to induce spin ordering in semiconductors by doping it with ferromagnetic transition metals [156, 157, 158]. For Cu2O, several groups have reported ferromagnetic behavior, for instance, in Mn [68, 85, 112], Co [69, 111], and Fe [110] doped samples. Therefore, doped Cu2O may be able to generate spin-polarized currents at room temperature and inject it into another semiconductor or even another Cu2O layer. However, the origin of the spin ordering in Cu2O remains under discussion and may be related to point defects besides the doping ions, as previously discussed in “Magnetic properties” section.

Gas and glucose sensing

Cu2O is a competitive material for applications in gas [159, 160, 161, 162, 163] and glucose [164, 165, 166] sensing. It has been used for detecting different gases, e.g., H2S [160], NO2 [161, 162], gas oil [163], and ethanol [159]. For both purposes, gas and glucose sensing, several works report on Cu2O growth by chemical routes [159, 160, 161, 163, 164]. Nevertheless, aiming to these applications, electrochemically grown Cu2O layers have been also investigated [162, 165, 166, 167].

Yan et al. [162] electrodeposited Cu2O on porous silicon substrate and investigated this structure for NO2 gas sensing. For the Cu2O the deposition it was used a lactate-stabilized copper sulfate solution adjusted at pH 10. The Cu2O/porous silicon structure showed good performance, with gas response in the range of 4.5–1 ppm NO2, good selectivity and fast response-recovery characteristics.

In ref. [167] electrodeposited Cu2O nanotube arrays were fabricated and then converted to CuO. Gas sensing measurements indicated that these nanotubes can be potentially applied as gas sensors for alcohol detection.

The development of glucose sensors for effective detection of glucose in blood is of great interest since diabetes is a chronic disease, and by 2030, it will affect approximately 366 million people in the world [168]. Cu2O-based sensors can represent a low-cost alternative for glucose detection. Pagare et al. [165] electrodeposited Cu2O thin films on FTO using a solution of copper(II) sulfate and lactic acid. The sensor showed a high current sensitivity of 6.25 mA/mM cm2 for 7 mM glucose and a linear detection range from 1 to 7 mM. In a previous work, a linear electrical current regime from 0.01 to 5 mM glucose was observed [166].

Cu2O nanostructures (synthesis and applications)

Nanostructuring of Cu2O is of interest in various contexts, including tuning of optical properties via quantum confinement effects, modulation of charge carrier transport through the formation of anisotropic structures, tailor catalytic properties by control of the facets exposed to the reacting medium, and enable or facilitate drug delivery, fluid transport, and sensing by the synthesis of high surface area, hollow structures.

One of the problems in the synthesis of Cu2O nanostructures is that anisotropic structures with some dimensions in the nanoscale are difficult to obtain due to the high symmetry of the cubic Cu2O structure. For this reason, anisotropic structure are mostly obtained by synthesis into suitable templates, or by the use of appropriate surfactants or other molecules that may selectively adsorb on crystal facets, modulating the growth rate of the corresponding orientation, or otherwise form bilayer aggregates to separate aqueous from nonaqueous phases, generating possible nucleation sites for Cu2O and at the same time spatially limiting the region available for growth.

In the following, the discussion will be organized by the dimensionality D of the nanostructures: 0-D for nanoparticles, 1-D for nanowires, and 2/3-D for complex interfaces.

Nanoparticles and hollow structures

Early on, efforts have been focused on demonstrating quantum size effects in Cu2O; since however the Bohr excitonic radius of Cu2O is only 0.7 nm, the observation of these effects requires the reliable formation of nanoparticles (NPs) with size at or below about 5 nm, with minimal size dispersion. One such method consists in performing electrochemical Cu dissolution from a Cu anode in a mixture of acetonitrile and tetrahydofuran [169]. In this process, Cu2+ is immediately oxidized and passivated by the supporting electrolyte, leading to nanoparticles with 2–10 nm size. The as-made Cu2O NPs are green instead of reddish, the color of bulk Cu2O, suggesting quantum dot behavior. This is confirmed by the optical absorption shift with increasing NP size.

Cu2O NPs have been prepared by a number of different methods; small size and very limited dispersion have been reliably achieved however by decomposition and reduction of Cu-acetate precursors to metallic Cu in trioctylamine and oleic acid at 270 °C, and slowly oxidized by storage in hexane [170]. Size was varied from 3 to 10 nm by controlling the molar ratio of oleic acid to Cu precursor, and dispersion was 5–10%. A shift in the absorption peak was clearly observed between 2 and 10 nm.

The relative dimensions and the exposed crystallographic facets of octahedral Cu2O micro- and nanostructures can be controlled not only by surfactants but also using species exhibiting selective adsorption. It was found for example that in the reduction of Cu(OH)2 by hydrazine N2H4, the relative ratio of NH3, Cu2+, and OH enables control of morphology [171]. Specifically, the concentration of NH3 affects the relative growth rate in the <111> vs. <100> direction; an increase in concentration results in a progressive shift from spherical to cubes to octahedral facets. Octahedral particles could also be obtained at higher pH. In another work, Cu2O cubes with size 40–420 nm could be obtained by using CuSO4, Na-ascorbate as a reductant, and sodium dodecyl sulfate as a surfactant [172]. Progressively larger cubes were obtained by a cascade method whereby smaller cubes were used to seed the larger ones. The cubes actually show also (111) and (110) facets, similar to real dices. Optical absorption tails appear in the near-IR region, shifting towards the red when size increased.

Intriguing optical properties have been obtained by synthesizing core/shell NPs with a Au or Ag core and a Cu2O shell [173]. The NP cores were obtained by solution reduction, while the shells were grown by N2H4 reduction of CuNO3 in a solution containing the NPs. Cu2O nucleates at the NP seeds, and as the concentration of colloidal Au decreases, the outer radii of the shells increase due to the lower density of nucleation centers, until the formation of pure Cu2O NPs occurs. Epitaxial growth is possible due to the small lattice mismatch. Decreasing Cu2O thickness on 60 nm Au cores, the color of the solution changes from yellow to dark green (blue shift), confirmed by a blue shift in the Cu2O absorption peak; furthermore, the plasmonic resonances of Au undergo a red shift, demonstrating tunability due to changes in the shell geometry. With smaller Au colloids (15 nm), the optical properties can be tuned in the visible region; the plasmon tuning range is however limited, due to the electromagnetic field decaying over much smaller length scales. By keeping the core/shell NPs in solution, hollowing out and destruction of the shells occurs over time.

Hollow structures are of interest for drug delivery, artificial cells, catalysis, or chemical storage. Hollow Cu2O spheres with 100–200 nm diameter were obtained by reduction of Cu2+ to Cu+ in dimethylformamide while heating at 150–180 °C [174]. Crystallites initially agglomerate to form a sphere, which over time recrystallizes, evacuating the core through an Ostwald ripening process that decreases NP curvature. Temperature-dependent phase change from CuO to Cu2O occurs in parallel with the hollowing process. Multishell geometries with single crystal shells have been also synthesized, using cetyltrimethylammonium bromide (CTAB) multilamellar vesicles as soft templates and ascorbic acid as reductant [175]. In this work, surfactants were used to separate the aqueous from the organic phase, generating nucleation sites for seed growth. Bilayer templates tend to form hollow structures; multiple shells could however be obtained by using CTAB. An increase in CTAB concentration yields more growth sites, increasing the density of hollow structure as well as the percentage of the multiple-shelled structures. Sensing properties of hollow Cu2O microspheres towards ethanol detection were investigated; the detection mechanism involved the electron injection of ethanol to neutralize holes in Cu2O, decreasing conductivity. These materials were twice as sensitive with respect to similar microspheres [159]. The catalytic activity of Cu2O hollow microspheres towards the decomposition of methyl orange was also studied [176]; best catalytic activity was observed for mixtures CuO/Cu2O due to improved electron-hole separation.

Nanowires

Cu2O nanowires synthesis has been achieved by various methods. One example involves the reduction of Cu(CH3COO)2 by cycloalkanes; the oxidative polymerization of these compounds templates the 1-D growth of Cu2O single crystal wires (40–70 nm in diameter, >10 μm in length) growing along the <110> direction without the need of any structure-directing agent. Charge transport in single wires was studied, showing that polymer/Cu2O composites exhibit a better conductivity, probably due to electron injection from the polymer to Cu2O, resulting in electrons becoming majority carriers [177]. Alternatively, Cu2O NWs have been obtained via reduction of Cu2+ by hydrazine. The resulting NWs are intertwined, with diameter about 8 nm, lengths 10–20 μm. NWs are crystalline, and the growth direction is <111>, distinct from the former example. No explanation was given in this case for the anisotropic growth [178].

NWs can also be obtained through the use of templates, one of the most common being anodic aluminum oxide. Electrodeposition in such templates was performed from lactate solutions under current control, 0.1 to 0.5 mA/cm2. High currents led to a localized pH increase due to hydrogen evolution, accelerating growth and increasing the fraction of Cu2O vs. Cu; a similar effect could be obtained by increasing the bulk pH [179]. Note that above pH 8, the Al oxide template is etched, limiting the duration of the growth process. Cu2O nanotubes and nanorods have been also obtained by sputtering Pt on the backside of an open Al oxide template. The dependence of CuxO stoichiometry and preferential orientation on applied voltage and pH was summarized in a Pourbaix-type diagram [179].

Nanostructuring solar cells based on Cu2O

Solar cells based on a Cu2O absorber layer can be entirely synthesized by electrodeposition, resulting in low-cost and easily scalable devices. Conventional solar cells comprised of planar interfaces however exhibit a low efficiency, mainly due to the poor minority carrier transport. A nanowire configuration would decrease the distance the minority carriers must cover in order to reach the p-n junction, possibly leading to a significant increase in efficiency. The ideal configuration consists of an interpenetrated high aspect ratio structure with a length scale of the order of the mean free path, forming a high surface area p-n junction. This structure has been fabricated by electrodeposition of n-type ZnO nanowires, followed by electrodeposition of p-type Cu2O from lactate solutions. In order to achieve the target structure, ZnO nanowires must be well separated and Cu2O should grow conformally on ZnO, maximizing the contact area. An early effort [180] led to a solar cell with a short circuit current of 8.2 mA/cm2 (twice that of an equivalent planar junction) and efficiency of 0.88% under 100 mW/cm2 illumination. In another work, a complete filling of the spaces between ZnO nanowires by Cu2O was claimed [181], leading however to a short circuit current of 5.4 mA/cm2 and efficiency of at most 0.5%. Most recently [128], this design was improved by producing ordered patterns of ZnO nanorods. Patterning was carried out by two-beam laser interference lithography to define the nucleation sites for ZnO, followed by ZnO growth and Cu2O plating. Significant improvements were reported, yielding a short circuit current of 9.89 mA/cm2 and efficiency of 1.52%. Note however that these results are still far from the theoretical efficiency of Cu2O/ZnO solar cells, as well as those of commercial silicon photovoltaic cells of efficiency 25% and a short circuit current of 35 mA/cm2.

Nanostructuring Cu2O photoelectrodes for photoeletrochemical cells

As well as the nanostructuration of solar cells can improve its efficiency, the same goes with Cu2O photoelectrodes. Photoelectrodes made of structures with high aspect ratio are an ideal choice for efficient light harvesting, enabling high current densities. Furthermore, the nanostructuration of Cu2O photoelectrodes can be an effective solution to overcome the short electron diffusion length occurring in Cu2O.

Tsui et al. [182] investigated the photoelectrochemical response of Cu2O nanocrystals embedded into the surface of a TiO2 nanotube array (TNA). TNAs were synthetized by anodizing Ti foils. P-type and n-type Cu2O nanocrystals were grown by electrodeposition. The first layer from a solution containing 0.3 M CuSO4 and 3 M lactic acid, adjusted to pH 12, and the second one using a solution of 0.02 M Copper(II) acetate and 0.1 M sodium acetate adjusted to pH 5.7. An enhancement of photocurrent response was observed under visible light after Cu2O electrodeposition on the TNA surface. However, at λ = 350 nm, the photocurrent is suppressed due to the Cu2O on TNA surface, indicating that the dense coverage of Cu2O crystals blocks UV light to reach the TNA, where the efficiency for photoinduced charge generation at UV is higher. The authors suggested that an optimized system would consist of Cu2O crystals embedded in the TNA.

Santamaria et al. and Zhang et al. [183, 184] reported photoelectrochemical characterization of photoelectrodes formed by Cu2O crystals electrodeposited inside the TNA. In the first reference, a reduction (enhancement) in the photocurrent under UV (visible) illumination after Cu2O electrodeposition was again noticed. In [184], the Cu2O films were electrodeposited by square wave voltammetry method and the photocurrent showed large dependence on the deposition potential, which determines the loading content of Cu2O crystals. The chronoamperometric measurements were carried out with a 300 W Xe lamp with a UV filter as illumination source. Under this illumination, Cu2O-TNA photoelectrodes grown at −1.0 VAg/AgCl presented the highest photocurrent density (0.54 mA/cm2 at 0 VAg/AgCl); meanwhile, pure TNA produced one order of magnitude lower photocurrent. The photocurrent obtained from Cu2O-TNA nanostructures is about ten times lower than the one measured for unstructured Cu2O photoelectrodes [141].

Nevertheless, there are examples of nanostructured Cu2O photoelectrodes for HER which have shown comparable or even higher performance when compared to planar photoelectrodes. For instance, nanostructured Cu2O nanowires have shown photocurrents as high as 10 mA/cm2 and good stability over 50 h [185], the values of which are comparable with the ones obtained for planar architecture [4, 14, 133, 134, 135, 136, 137, 138, 139, 140, 141]. These Cu2O nanowires were fabricated by electrochemical anodization of Cu/FTO substrates followed by thermal annealing. Other examples of Cu2O-based nanostructures used for HER are Cu2O/Cu microcone arrays [186], 3-D metal/Cu2O micropillar arrays [187], Cu2O/WO3 nanorods [188], and NiOx/Cu2O nanowires [189]. In the first three architectures, the Cu2O layers were electrochemically grown and all the cited photoelectrodes presented higher or comparable photocurrent as its similar photoelectrodes in planar or pristine form.

Conclusion

Cu2O is an important material, since it is abundant, relatively cheap, nontoxic, and has been considered more recently for applications in devices for renewable energy. We have reviewed the semiconducting oxide Cu2O when prepared by electrochemical synthesis as thin films and nanostructures. The growth process and the properties, especially the electrical and optical ones, were described extensively. In addition, we have presented the application of Cu2O in photocatalysis and photovoltaics. One of the main conclusions of this work is that the understanding of the optical and electric/electronic properties of the material is still a work in progress. The growth of n-type Cu2O, the dependency of the resistivity on the growth process, defects, and activation energies are, for example, points that need to be addressed for a comprehensive description of the material.

Notes

Acknowledgements

This work was supported by the Brazilian funding agencies FAPESC, FINEP, CAPES, and CNPq.

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Copyright information

© Springer-Verlag Berlin Heidelberg 2017

Authors and Affiliations

  • I. S. Brandt
    • 1
    • 2
  • M. A. Tumelero
    • 3
  • S. Pelegrini
    • 1
  • G. Zangari
    • 4
  • A. A. Pasa
    • 1
  1. 1.Laboratório de Filmes Finos e SuperfíciesUniversidade Federal de Santa CatarinaFlorianópolisBrazil
  2. 2.Programa de Pós-Graduação em Ciência e Engenharia de Materiais, Departamento de Engenharia MecânicaUniversidade Federal de Santa CatarinaFlorianopolisBrazil
  3. 3.Instituto de FísicaUniversidade Federal do Rio Grande do Sul, UFRGSPorto AlegreBrazil
  4. 4.Department of Materials Science and EngineeringUniversity of VirginiaCharlottesvilleUSA

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