Electrodeposited Cu2O as Photoelectrodes with Controllable

Nov 6, 2015 - Mechanistic investigation on tuning the conductivity type of cuprous oxide (Cu 2 O) thin films via deposition potential. Juan Han , Jing...
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Electrodeposited Cu2O as Photoelectrodes with Controllable Conductivity Type for Solar Energy Conversion Peng Wang,† Hao Wu,† Yiming Tang,† Rose Amal,*,† and Yun Hau Ng*,† †

Particles and Catalysis Research Group, School of Chemical Engineering, The University of New South Wales, Sydney, NSW 2052, Australia S Supporting Information *

ABSTRACT: Electrodeposition of copper acetate under mild acidic conditions followed by a controlled annealing process allowed the manipulation of the oxygen vacancies in the resultant Cu2O-based electrodes. The conduction type of the Cu2O-based semiconductor was, therefore, tunable, allowing the fabrication of n-type, p-type, and p−n junction photoelectrodes. A transformation of the original n-type conduction to the subsequent p-type nature was observed through the variation of annealing temperature and duration. The observation of anodic and cathodic photocurrents for n-type and p-type thin films confirmed their potential use as photoanodes and photocathodes, respectively, in liquid-junction photoelectrochemical systems. The high carrier densities of the electrodeposited n- and p-type Cu2O were estimated to be 8.9 × 1019 and 1.3 × 1020 cm−3, respectively, using Mott−Schottky analysis. Furthermore, the p−n junction photoelectrodes in a device configuration also exhibited diode behavior in current−voltage measurements, indicating their potential application in solid-state photovoltaic devices. can experience photocorrosion or electrodissolution issues.9,10 Among oxide-based photocathodes (e.g., SrTiO3, CuFe2O4, and LaFeO3),11−13 cuprous oxide (Cu2O) is attractive because it is a simple oxide, abundant (as compared with Sr, La, and so on), nontoxic, intrinsically p-type, and visible-light-active with a direct band gap of ca. 2 eV.14 It has valence- and conductionband energies capable of oxidizing and reducing water, respectively. More interestingly, Cu2O shows tunable semiconducting behavior between p-type and n-type, although its ntype form has received only limited attention.15−17 Thus, Cu2O offers the opportunity for constructing a low-cost bias-free photoelectrochemical water-splitting system using the same chemical component of Cu2O as both the photoanode and photocathode. Furthermore, the ability to fabricate p−n junction thin films based on Cu2O should also find broad interest in photovoltaic device applications. Although it is still debatable, the p-type semiconducting behavior of Cu2O originates from Cu vacancies (or excess oxygen).18 It is therefore possible to control the semiconducting type of Cu2O by manipulating its atomic vacancies through the imposition of different experimental conditions. Recently, n-type Cu2O thin films on titanium substrate, indium tin oxide (ITO) substrate, and fluorine-doped tin oxide (FTO) conductive glass substrate were prepared by the electrodeposition method.15−17,19,20 Electrodeposition is a simple

1. INTRODUCTION Sunlight-driven splitting of water into gaseous hydrogen and oxygen using a photocatalytic or photoelectrochemical approach has received great attention in the research community as it is a potential model for efficient artificial photosynthesis.1−4 Compared with photocatalytic water splitting, the advantage of using photoelectrochemical systems is the ready separation of the evolved H2 and O2.5 Furthermore, the ability to modulate the band bending at the photoelectrode−electrolyte interface can assist charge separation and transport to achieve better performance. An ideal photoelectrochemical water-splitting system would require a photoanode (n-type semiconductor) and a photocathode (p-type semiconductor) for water oxidation and reduction, respectively. External energy in the form of an applied voltage is not necessary when the band structures of the photoanode and photocathode are well-matched. Since the work of Fujishima and Honda, there have been extensive studies on the development of efficient n-type oxide semiconductors as photoanodes for water oxidation.1 BiVO4, modified TiO2, and WO3 are good examples of photoanodes capable of oxidizing water.6−8 Despite their promising photoactivity for oxygen evolution, these systems employ platinum as the counter electrode for water reduction. More recently, strategies for replacing platinum as the H2 evolution electrode have been formulated. The development of p-type semiconductors as photocathodes has emerged rapidly because p-type electrodes can be used directly for hydrogen evolution. Sulfide-based photocathodes are generally active in hydrogen generation but © 2015 American Chemical Society

Received: July 28, 2015 Revised: October 7, 2015 Published: November 6, 2015 26275

DOI: 10.1021/acs.jpcc.5b07276 J. Phys. Chem. C 2015, 119, 26275−26282

Article

The Journal of Physical Chemistry C

Figure 1. SEM images of (a,c) the electrodeposited Cu2O and (b,d) the electrodeposited-annealed Cu2O−CuO thin films [(a,b) top view, (c,d) cross section]. (e) XRD patterns of the electrodeposited Cu2O (top, black) and the electrodeposited-annealed Cu2O−CuO (bottom, red) thin films.

and practical method for fabricating large-area thin films. It can be used to control the thickness, morphology , crystallinity, and even orientation of electrodeposited oxides by varying the electrodeposition voltage/current, temperature, and electrolyte composition.21−23 A general trend in electrodeposited Cu2O thin films was found related to the pH value of the plating solutions. Most earlier works have indicated that slightly acidic conditions (pH range from 4.5 to 5.5) are essential to directly obtain n-type Cu2O thin films, whereas direct electrodeposition of p-type Cu2O can only be achieved in basic solutions (pH 7− 9).15−17,24,25 The necessity of using plating solutions in different pH ranges has effectively imposed challenges on the preparation of a single Cu2O thin film with p−n interfaces, as the preparation involves two different solutions with opposite pH values. Meanwhile, annealing of copper foil and Cu2O at high temperature (>400 °C) has been reported to yield p-type Cu2O with the presence of trace CuO.26 It is believed that the replenishment of oxygen vacancies from air at high temperature can induce p-type behavior. In this study, we present a simple method for electrodepositing p- or n-type Cu2O thin films using a standardized slightly acidic precursor solution and a controlled heat treatment. The as-electrodeposited n-type Cu2O was tuned into p-type upon simple annealing in air. The evolution of the transformation of the conduction type was observed through the modulation of the annealing temperature and duration. A thin layer of protective CuO was formed on the surface of the p-type Cu2O during the heat treatment, improving the chemical stability of the generally unstable ptype Cu2O. The presence of a thin CuO top layer minimizes the surface redox reactions of Cu2O with the aqueous electrolyte, thus improving its photostability. In addition to these individual p- and n-type Cu2O films, we also demonstrate the potential of employing this method to fabricate p−n junction thin films that exhibit photodiode behavior.

in a copper(II) acetate solution composed of 0.02 M copper acetate (≥98%, Ajax Finechem) and 0.08 M sodium acetate (≥99%, Ajax Finechem) with a pH of 5.0 ± 0.1 adjusted by acetic acid (≥99%, Ajax Finechem). The electrodeposition was performed potentiostatically at −0.1 V for 30 min at a constant temperature of 60 °C. The electrodeposited-annealed p-type thin film was fabricated by thermal treatment of the electrodeposited n-type Cu2O thin film in air at 400 °C for 1 h. For comparison, thermal treatments of electrodeposited Cu2O were performed at 200 and 300 °C in air for 1 h. In addition, thermal treatments of electrodeposited Cu2O at 400 °C in air were performed for 20, 40, and 60 min. The p−n junction thin film was fabricated by the secondary electrodeposition of n-type Cu2O thin film on the electrodepositedannealed p-type thin film under identical conditions. For a control experiment, a p−n Cu2O junction thin film (denoted as control p−n Cu2O junction) was prepared by means of a two-step electrodeposition of n-type Cu2O thin film (as described above) on top of an electrodeposited p-type Cu2O thin film grown on FTO substrate. The electrodeposited p-type Cu2O thin film was fabricated by electrodeposition in 0.4 M copper sulfate (≥98%, Sigma-Aldrich) and 1.0 M trisodium citrate (≥99%, Sigma-Aldrich) at a pH of 12.5 ± 0.1 adjusted by sodium hydroxide. The electrodeposition was conducted at −0.6 V for 30 min. 2.2. Material Characterization. The morphologies of the electrodeposited n-type and electrodeposited-annealed p-type thin films grown on FTO substrate were analyzed by scanning electron microscopy (SEM, NanoSEM 230, FEI Nova). The phase composition and crystallinity were measured by X-ray diffraction (XRD, X’pert Pro MRD, Philips, using Cu Ka radiation with λ = 1.54 Å). The surface compositions of the tuned p-type Cu2O−CuO thin film was recorded by X-ray photoelectron spectrometry (XPS, ESCALAB250Xi, Thermo Scientific) with monochromated Al Kα at 1486.6 eV. All XPS data were calibrated to the carbon 1s peak at 285.0 eV. The composition transition at different depths within the electrodeposited-annealed p-type thin film was measured by XPS depth profiling. 2.3. Photoelectrochemical Performance. All electrochemical measurements were carried out in a standard threeelectrode photoelectrochemical cell, with a Ag/AgCl reference electrode, a Pt counter electrode, and the prepared thin-film working electrode. K2SO4 solution (0.5 M) was used as the electrolyte. A 300-W xenon light with a cutoff filter (435 nm) was applied as the illumination source during the on−off

2. EXPERIMENTAL SECTION 2.1. Preparation of n-Type, p-Type, and p−n Junction Thin Films. The electrodeposition of n-type Cu2O thin films was carried out in a standard three-electrode chemical cell using a potentiostat (PG STAT-302N, Autolab). Fluorine-doped tin oxide (FTO) conductive glass was used as the working electrode. The FTO substrate was cleaned in ethanol, acetone, and Milli-Q water under mild sonication to remove organic contaminants and then dried before use. A Ag/AgCl electrode and a Pt electrode were use as the reference and counter electrodes, respectively. The n-type Cu2O was electrodeposited 26276

DOI: 10.1021/acs.jpcc.5b07276 J. Phys. Chem. C 2015, 119, 26275−26282

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Whereas the XRD measurement provides the composition of the bulk film, the surface structure analysis using X-ray photoelectron spectroscopy (XPS) (Figure 2) reveals that the

illumination cycles. The conduction types of the electrodeposited n-type and electrodeposited-annealed p-type thin films were characterized by Mott−Schottky (MS) plots at a frequency of 10 kHz in the dark using a potentiostat (PG STAT-302N, Autolab). The visible-light (>435-nm) photocurrent generation of the electrodeposited n-type and electrodeposited-annealed p-type thin films was investigated by chronoamperometry measurement at potentials of 0.2 and −0.36 V, respectively, under on−off illumination cycles. The photocurrent−potential curves were recorded from −0.5 to 0 V at a scan rate of 5 mV/s under on−off illumination cycles. The solid-state dark current−voltage (I−V) curve was measured with a SemiProbe MA8000 instrument.

3. RESULTS AND DISCUSSION During the electrodeposition of Cu2O under a mild cathodic voltage of −0.1 V at a constant temperature of 60 °C, a homogeneous and evenly coated brown layer gradually formed on the FTO substrate, suggesting the successful deposition of Cu2O (as Cu2O has a brown color). Under SEM analysis, this deposited film showed the typical morphological features of adendritic pattern with a film thickness of ∼14.2 μm (Figure 1a,c). The formation of these dendritic branches was previously encountered by McShane and Choi.15 Cu2+ ions from the precursor solution are driven to the FTO substrate by the negative bias and subsequently reduced to Cu+ ions. Because of the low solubility of Cu+ ions under acidic conditions, they eventually precipitate as Cu2O. As the precipitation/deposition proceeds, a depletion zone with a concentration gradient of fresh Cu2+ ion is formed near the growing Cu2O. This depletion zone creates a local environment with different concentrations of Cu2+ and leads to the anisotropic growth of Cu2O (formation of dendritic branches). The as-electrodeposited Cu2O was subsequently subjected to heat treatment at 400 °C in the air for 1 h to replenish the oxygen vacancies, thus introducing p-type behavior.27 The annealed Cu2O film retained the dendritic morphology, indicating the robustness of this surface structure (Figure 1b), whereas the thickness of the film increased slightly to ∼15.6 μm (Figure 1d). As shown in Figure S1 (Supporting Information), a Tauc plot of the aselectrodeposited thin film supports the presence of Cu2O by showing a typical optical bandgap of Cu2O (∼2.2 eV). Upon annealing, because of the formation of surface CuO, an optical band at ∼1.5 eV was observed. X-ray diffraction (XRD) patterns of the as-electrodeposited and electrodeposited-annealed Cu2O thin films are shown in Figure 1e. For the as-electrodeposited film, Cu2O is the only precipitate from the synthesis. The diffraction peaks at 29.6°, 36.5°, 42.3°, and 61.5° are indexed to cubic crystalline Cu2O with orientations of (110), (111), (200), and (220), respectively. In combination with the SEM images, it can be concluded that the dendritic branches deposited on the FTO substrate are crystalline Cu2O with a dominant facet of (111). The diffraction peaks assigned to SnO2 are attributed to the FTO substrate. Upon thermal treatment, the formation of monoclinic CuO was observed, as indicated by the new diffraction peaks at 35.5° and 38.7°. The growth of CuO was at the expense of Cu2O, as the peak intensity for Cu2O decreased accordingly. This is in good agreement with previous reports that Cu2O can be further oxidized at temperatures higher than 300 °C to form CuO. Because of the thin-film configuration and annealing duration, the surface Cu2O layer had the highest likelihood to undergo the mentioned oxidation process.

Figure 2. (a) XPS spectrum and (b) XPS depth profile of the electrodeposited-annealed p-type Cu2O−CuO thin film.

surface of the thin film was covered solely by CuO. Although the existence of Cu2O was indicated in the XRD diffractogram, Cu2O was not present on the surface. XPS depth profiles provide useful information about the composition of thin films at different depths. The profile showed a drastic decrease of Cu(II) and oxygen within the first 20 s of etching time, indicating that CuO was dominant on the surface of the thin film with a gradually decreased content in the bulk film. Correspondingly, a significant increase of Cu(I) was recorded, implying that Cu2O was embedded underneath the CuO surface layer. Even though the possible reduction of Cu(II) to Cu(I) due to the electron beams might occur during argon etching, this dramatic change in the ratio of Cu(II) to Cu(I) in the first 20 s was mainly attributed to the variation of the composition in the thin film. Combining the XRD and XPS results, it is apparent that the thin film was composed of a CuO surface layer and a composite of Cu2O−CuO in the bulk. The formation of a compact surface layer of CuO is important for Cu2O, especially in the environment of liquid-junction photoelectrochemical systems. Formation of such a layer hinders electron conduction at the Cu2O−electrolyte interface, which usually promotes the self-oxidation or self-reduction of photoexcited Cu2O because the redox potentials of Cu2O lie within its own optical band gap.28 Furthermore, owing to the matched band energy alignment, the presence of surface CuO 26277

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CuO film is slightly higher than the reported value for singlephase Cu2O films (∼1.4 × 1018 cm−3) and is comparable to that for p-type Cu2O−CuO composite prepared using a combined electrodeposition-anodization method (2.1 × 1019 cm−3).30 Although there have been limited investigations on n-type Cu2O, the estimated flat-band potential value for this p-type Cu2O−CuO thin film is 0.87 V versus reversible hydrogen electrode (RHE), which is in good agreement with the reported range.32,33 Strategies of using different pH values to control the rate of Cu2+ reduction (and, therefore, the rate of Cu2O precipitation, as the solubility of Cu+ depends on pH) were previously formulated. A rapid Cu2+ reduction process results in the fast growth of Cu2O accompanied by high oxygen vacancies. It therefore exhibited n-type semiconducting pattern. In this work, by simply replenishing the vacancies with excess oxygen from the annealed environment, Cu vacancies in the film, which are responsible for generating acceptor levels for ptype behavior, are created. The conduction types of the electrodeposited n-type Cu2O and the electrodeposited-annealed p-type Cu2O−CuO thin films were further verified in photoelectrochemical measurements. Figure 4a,b shows the visible-light photocurrent−voltage profiles of the two thin films in a standard three-electrode system using a Ag/AgCl reference electrode. Upon visible-light illumination (λ > 435 nm), Cu2O and Cu2O−CuO thin films are band-gap photoexcited to generate electron−hole pairs. Because of the upward bending of the band edges at the Cu2O−electrolyte interface, photoexcited electrons travel toward the counter electrode, thus generating an anodic photocurrent. The n-type nature of the electrodeposited Cu2O is verified by the repeatable anodic photocurrent observed over the measured voltage range (Figure 4a). In contrast, a p-type Cu2O−CuO thin film will exhibit a downward band bending at the interface with electrolyte, thus promoting the flow of electrons toward the electrolyte solution. Again, the repeatable cathodic photocurrent induced by this downward bending phenomenon confirmed the p-type behavior of the Cu2O−CuO electrode (Figure 4b). Both Cu2O and CuO components in this electrodeposited-annealed Cu2O−CuO thin film are p-type semiconductors, and it cannot be n-Cu2O/p-CuO. If n-Cu2O/p-CuO were present in this sample, the p−n junction at the Cu2O−CuO interface would be a charge recombination center and become the barrier for charge transfer. In this situation, negligible photocurrent would be observed in this liquid-junction photoelectrochemical system configuration. This is further proved by dark current−voltage curve obtained under the solid-state measurement conditions. The red line in Figure 7a (below) indicates no observation of junction photodiode behavior for the electrodepositedannealed Cu2O−CuO thin film. If the underlayer Cu2O is ntype and the top CuO is p-type, a diode pattern rectifying p−n junction would be observed. Panels c and d of Figure 4 show the amperometric visiblelight photocurrent generation of the n-type Cu2O and p-type Cu2O−CuO thin films, respectively. It is notable that both nand p-type Cu2O thin films underwent diminishing photocurrent over time, indicating the photocorrosion of Cu2O. This photoinstability originates from the redox potentials of Cu2O. The oxidation potential of Cu2O to CuO and the reduction potential of Cu2O to metallic Cu are located within its valence and conduction bands. The photoexcited charges in Cu2O tend to self-oxidize or self-reduce (depending on the applied bias) and lead to the photocurrent decay. Figure S2 (Supporting

would also be beneficial for electrons transfer if the underlayer Cu2O is p-type semiconducting.29,30 Otherwise, the interface between p-type CuO and n-type Cu2O will become a charge recombination center and diminish the photoelectrochemical performance. As the apparent capacitance of p- and n-type semiconductors depends on the potential under depletion conditions, a Mott− Schottky relationship was usually employed to verify the conduction type of a semiconductor.31 Figure 3 shows the

Figure 3. Mott−Schottky plots of (a) the electrodeposited Cu2O thin film and (b) the electrodeposited-annealed Cu2O−CuO thin film, measured at 10 kHz.

Mott−Schottky plots for the electrodeposited Cu2O and electrodeposited-annealed Cu2O−CuO thin films. Because the Fermi levels (Ef) of p- and n-type semiconductors are located close to the different band edges, scanning of the applied voltage will result in the opposite phenomenom in the formation of either an accumulation or depletion zone. A Mott−Schottky plot of the electrodeposited Cu2O thin film (Figure 3a) shows characteristic n-type behavior by having a “positive” slope value in the plot, that is, a linear increase of C−2 with higher applied voltage. Upon being annealed in air for 1 h, the Cu2O−CuO thin film demonstrates a p-type semiconducting pattern indicated by the “negative” slope in the Mott−Schottky plots. Comparable Mott−Schottky plots with similar slope patterns were obtained at different measured frequencies for these two films. Based on the Mott−Schottky equation, the carrier densities (NA) of the electrodeposited ntype Cu2O and electrodeposited-annealed Cu2O−CuO thin films were estimated to be 8.9 × 1019 and 1.3 × 1020 cm−3, respectively. The carrier density of the resultant p-type Cu2O− 26278

DOI: 10.1021/acs.jpcc.5b07276 J. Phys. Chem. C 2015, 119, 26275−26282

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Figure 4. (a,b) Visible-light current−voltage profiles of (a) electrodeposited n-type Cu2O and (b) electrodeposited-annealed p-type Cu2O−CuO. (c,d) Amperometric current−time curves for (c) electrodeposited n-type Cu2O and (d) electrodeposited-annealed p-type Cu2O−CuO.

Information) compares the photocurrent decay rates of the electrodeposited n-type Cu2O and electrodeposited-annealed ptype Cu2O−CuO thin films. Photocorrosion was less profound in the p-type Cu2O−CuO film compared with the n-type Cu2O film. We previously reported that surface compact oxide (e.g., CuO and TiO2) can passivate the redox activities of Cu2O at the interface with the electrolyte. The formation of CuO completely covering the Cu2O, as indicated in the XPS analysis, minimized the reaction with aqueous electrolyte solution, and thus, photocorrosion was suppressed. Apparently, however, more efforts toward stabilizing both photoelectrodes in liquidjunction photoelectrochemical systems are needed. In this work, the surface CuO also acts as the electron mediator for water reduction under a mild negative voltage. CuO has a conduction band more positive than that of the Cu2O. Therefore, the presence of CuO assists the electron transfer from Cu2O to CuO and suppresses the charge recombination within Cu2O. Transformation of n-type Cu2O to p-type Cu2O−CuO can be controlled upon thermal treatment at various temperatures. Figure 5 shows the photocurrent profiles (either anodic or cathodic signal) of the Cu2O-based thin films treated at room temperature and 200, 300, and 400 °C for 1 h. In a relatively broad window of measured voltage, the as-electrodeposited Cu2O film exhibited anodic photocurrent across the entire voltage range. This verifies the n-type conduction of the electrodeposited Cu2O film. Similarly, upon annealing at 400 °C, the Cu2O−CuO film undoubtedly indicated its p-type nature by generating a cathodic photocurrent throughout the measurement. It is interesting to note the transition of n-type to p-type behavior at temperatures of 200−300 °C. Samples annealed at 200 and 300 °C generate both anodic and cathodic photocurrents depending on the applied voltage. Although they both exhibited voltage-driven n- and p-type conduction behaviors, the apparent difference between the samples annealed at 200 and 300 °C is the obvious dominance of a

Figure 5. Visible-light current−voltage profiles of electrodeposited (denoted as ED) and electrodeposited-annealed photoelectrodes treated at 200, 300, and 400 °C for 1 h.

certain conduction type within the thin film, that is, n-type is dominant at relatively low temperature (200 °C) and p-type dominant at relatively high temperature (300 °C). This observation of the presence of a dominant component can be amplified by performing amperometric current measurements as shown in Figure 6. Even though the samples exhibited both conduction types, the n-type-dominant thin film (treated at 200 °C) generated only a moderate cathodic photocurrent at −0.36 V vs Ag/AgCl whereas a high anodic photocurrent was observed. On the other hand, the p-type-dominant thin film (annealed at 300 °C) showed amore promising cathodic photocurrent than its anodic photocurrent counterpart. In addition, a similar transformation of n-type to p-type was observed when the duration of annealing was controlled. (See 26279

DOI: 10.1021/acs.jpcc.5b07276 J. Phys. Chem. C 2015, 119, 26275−26282

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Figure 6. Amperometric current−time curves for electrodeposited-annealed photoelectrodes treated at (a) 300 and (b) 200 °C for 1 h. (Left) Anodic current profiles measured at 0.2 V, and (right) cathodic current profiles measured at −0.36 V.

Figure S3 in the Supporting Information.) The evidence of an n-type to p-type crossover upon heating in air supports the argument that the n-type conductivity of the as-electrodeposited Cu2O is convertible. The present investigation also suggests that the origin of the n-type conduction is the oxygen vacancies created in the crystal lattice during the electrodeposition. Because of the supply of oxygen to refill the vacancies by annealing in air, the semiconductor-type conversion is achieved. Following the investigation of n-type to p-type conversion, a p−n junction Cu2O-based film can be developed using a similar experimental procedure. An n-type Cu2O layer was directly electrodeposited on top of the electrodeposited-annealed ptype Cu2O−CuO photoelectrode to yield a single thin film with a p−n junction. (An SEM image of the p−n junction Cu2O thin film is presented in Figure S4, Supporting Information.) Figure 7 shows the dark current−voltage curves of electrodeposited ntype Cu2O, electrodeposited-annealed p-type Cu2O−CuO, and p−n junction thin films under the solid-state measurement conditions. No asymmetric current profile was observed for the individual n-type and p-type thin films, indicating no junction photodiode behavior. The absence of photodiode behavior in the electrodeposited-annealed p-type Cu2O−CuO also ruled out the possible formation of p-CuO and n-Cu2O in that sample. Figure 7b shows the observation of obvious asymmetry during the forward and reverse voltage scans, verifying the formation of a p−n junction at the interface. This asymmetric current profile is a characteristic of a diode and indicates that a reasonable-quality p−n interface was formed using this synthetic method. As a control experiment, a controlled p−n junction Cu2O thin film was prepared by electrodeposition of n-type Cu2O (in acidic precursor) on top of the electrodeposited p-type Cu2O (in alkaline precursor). (Please see the Experimental Section.) Not surprisingly, an asymmetric current

Figure 7. Solid-state measurements of dark current−voltage (I−V) patterns of (a) electrodeposited n-type Cu2O and electrodepositedannealed p-type Cu2O−CuO and (b) p−n junction thin films.

profile (Figure S5a, Supporting Information) was also observed, indicating photodiode behavior. However, the recorded current of the controlled p−n junction Cu2O thin film was 2 orders of 26280

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magnitude lower than that of the p−n junction created in our work (under the same pH conditions). In addition, the quality of the p−n junction film in this work can be further improved through parameter optimization to reduce the pin holes and to flatten the underlayer p-type Cu 2 O−CuO before the subsequent n-type Cu2O electrodeposition. However, this is a successful demonstration of the possibility of using the same electrodeposition recipe to prepare p−n junction thin films.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b07276. Kubelka−Munk-transformed diffuse reflectance spectra of electrodeposited n-type and electrodeposited-annealed p-type thin films; photocurrent decay curves of electrodeposited n-type and electrodeposited-annealed p-type thin films; visible-light current−voltage profiles for electrodeposited and electrodeposited-annealed photoelectrodes at 400 °C for 20, 40, and 60 min; SEM images of p−n junction Cu2O−CuO films; and solid-state measurements of dark current−voltage (I−V) patterns of controlled p−n junction thin films and p−n junction thin films. (PDF)

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REFERENCES

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4. CONCLUSIONS In summary, n-type Cu2O, p-type Cu2O−CuO, and p−n junction photoelectrodes can be prepared under identical electrodeposition conditions. Transformation of n-type to ptype behaviors of Cu2O-based thin film is induced by the replenishment of oxygen vacancies created in the crystal lattice during the electrodeposition. The n- and p-type Cu2O thin films demonstrate characteristic anodic and cathodic photocurrents, respectively, indicating their potential uses as photoanodes and photocathodes, respectively, in photoelectrochemical cells. Mott−Schottky analyses also revealed the presence of high donor densities within these n- and p-Cu2O samples. Combining the synthetic procedures, a p−n junction photoelectrode can be made. The ability to control the conduction type of Cu2O semiconductor through a simple electrodeposition offers great opportunities in Cu2O-based tandem photoelectrochemical water-splitting systems, as well as in the solid-state photovoltaic devices.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.H.N.). *E-mail: [email protected] (R.A.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Australia Research Council Discovery Project (DP110101638). The authors acknowledge the UNSW Mark Wainwright Analytical Centre for providing facility and technical support. Dr. Bill Gong from Solid State & Elemental Analysis Unit of UNSW is gratefully acknowledged for his help in XPS measurements and analysis. The authors thank Dr. Shujuan Huang and Prof. Gavin Conibeer from the Australian Centre for Advanced Photovoltaics UNSW for their assistance in the dark I−V measurements of the samples. 26281

DOI: 10.1021/acs.jpcc.5b07276 J. Phys. Chem. C 2015, 119, 26275−26282

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DOI: 10.1021/acs.jpcc.5b07276 J. Phys. Chem. C 2015, 119, 26275−26282