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Stoichiometry, Morphology and Size-Controlled Electrochemical Fabrication of CuxO (x=1,2) at Underpotential Cemile Kartal, Yesim Hanedar, Tuba Öznülüer, and Ümit Demir Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b00340 • Publication Date (Web): 09 Apr 2017 Downloaded from http://pubs.acs.org on April 10, 2017

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Stoichiometry, Morphology and Size-Controlled Electrochemical Fabrication of CuxO (x=1,2) at Underpotential

Cemile Kartal, Yeşim Hanedar, Tuba Öznülüer and Ümit Demir* Atatürk University, Sciences Faculty, Department of Chemistry, 25240, Erzurum-Turkey

Tel.: +90-442-2314434; Fax: +90-442-2360948; e-mail: [email protected] *Author to whom correspondence should be addressed.

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Abstract A new one-step electrochemical approach has been developed for the morphology, size and stoichiometry-controlled synthesis of Cu2O, CuO and Cu2O/CuO composite structures at room temperature without using surfactants, capping agents or any other additives. The electrochemical deposition of a Cu monolayer using underpotential deposition (UPD) and the flow rate of oxygen gas bubbled through the deposition solution used for oxidation of the Cu layer are the key parameters for controlling the stoichiometry of the CuxO (x=1,2) structures. The morphologies, crystallinity, stoichiometries, optical properties and photoelectrochemical properties of the as-electrodeposited Cu2O and CuO materials were analyzed using scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy dispersive spectroscopy (EDS), UV-Vis absorption and photoelectrochemical (PEC) techniques. The results indicated that the Cu2O and CuO materials electrodeposited on both indium tin oxide coated (ITO) quartz and gold electrodes using this new electrochemical technique exhibit high-quality single crystalline structures and high photoactivity with rapid photoelectrical response to light irradiation.

Keywords: Cu2O; CuO; Cu2O/CuO Composite; Electrochemical Deposition; Underpotential Deposition; Photoelectrode


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1. Introduction Cuprous oxide (Cu2O) and cupric oxide (CuO), which possess unique optical and magnetic properties, have been widely exploited for various applications in energy conversion and storage, magnetic storage, catalysis and sensors.1-5 Over the last decade, Cu2O- and CuObased nanostructures have been the focus of much research due to their potential application as p-type and n-type semiconductors with narrow energy band gaps of 2.1 eV (Cu2O) and 1.2 eV (CuO).1,6,7 The morphology, particle size, orientation, and nanostructure of metal oxides have a dramatic effect on the catalytic, magnetic and electronic properties and therefore must be controlled during the fabrication processes.8,9 There have been extensive efforts focused on developing new techniques for shape- and size-controlled synthesis of metal oxides for a variety of applications.10,11 Among the various synthesis methods, electrochemical deposition (ECD) is an economical and a simple approach for the preparation of metal oxides.12,13 By adjusting the electrochemical parameters (e.g., deposition time and potentials, current densities and composition of electrolyte solution), the growth rate, size and morphology of the electrodeposits can be easily controlled. In addition, the metal oxide structures can be fabricated directly on the substrate surface via an electrochemical method, which is required for most applications, such as energy conversion and storage as well as sensors. The electrodeposition of Cu2O and CuO has been extensively studied by Switzer and co-workers in aqueous solutions at different cathodic potentials.14-16 Choi and co-workers have also systematically investigated the fabrication and morphological transformation of Cu2O and CuO nanocrystals obtained by electrochemical deposition.17-19 All of their results indicate that the applied potential and electrolyte pH value have a strong effect on the oxidation state (Cu, CuO or Cu2O) and morphology of electrodeposited copper oxide structures. It has been shown that the more negative overpotentials favor the formation of metallic Cu deposition while moderate and less negative overpotentials favor the formation of Cu2O and CuO at a fixed


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electrolyte pH.20 Therefore, the deposition potential must be precisely controlled in an acceptable range such that the growth kinetics and mechanism do not vary during the growth process, leading to a repeatable and controllable fabrication of electrodeposits. All of the electrochemical deposition techniques used thus far for the formation of metal oxide compounds have been carried out using overpotential deposition (OPD, after the Nernst equilibrium potential also known as bulk deposition) where the growth kinetics are difficult to control. The underpotential deposition (UPD)21 method appears to be favorable for the production of nanostructured compound semiconductors with high purity and controllable morphology. UPD, which involves the electrodeposition of foreign metal ions onto a metallic substrate at potentials that are more positive than the Nernst potential, is a surface limited process. Due to the quantity of the deposit is limited to an atomic layer, UPD self-terminates when the surface of the electrode is covered by a monoatomic layer. Electrochemical atomic layer epitaxy (ECALE), which was introduced by Stickney et al., was the first UPD-based electrochemical deposition method performed by the sequential UPD of different elements.22 This technique has been successfully employed to the electrochemical growth of structurally well-ordered thin films of compound semiconductors.23-25 UPD has also been employed to grow metal monolayers using surface-limited redox replacement (SLRR) 26-29, electrochemical atomic layer deposition (E-ALD) with a “Bait and Switch30, and selective electrodesorption based atomic layer deposition (SEBALD)31 techniques. The electrodeposition of CdTe has been prepared by the co-deposition from an aqueous solution of CdSO4 and TeO2 by Kroger et al.32 Our group recently developed an UPD-based codeposition method for the electrodeposition of compound semiconductors from the same solution.33,34 This applicable and simple technique has been successfully used for the formation of nanostructured compound semiconductors by our group 35-39, as well as other groups.40,41 Recently, we have synthesized ZnO nanostructures using a simple and one-step


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electrochemical approach, which is based on the UPD of Zn from an aqueous suspension of ZnO powder.42 In this study, for the first time, we report a one-step electrochemical approach for the morphology, size and stoichiometry-controlled synthesis of Cu2O, CuO or Cu2O/CuO composite structures, which comprises Cu UPD followed by a spontaneous chemical oxidation with dissolved oxygen from an oxygenated aqueous solution of Cu2+ on indium tin oxide (ITO) and Au electrodes. The cubic Cu2O or flower-like CuO structures with a controllable size and composition can be easily and selectively prepared by adjusting the flow rate of the oxygen gas at room temperature without using surfactants, capping agents or any other additives. The as-electrodeposited Cu2O and CuO materials exhibited a high-quality single crystalline structure and a high photoactivity with rapid photoelectrical response to light irradiation. 2. Experimental. 2.1 Electrochemistry All of the electrochemical measurements and depositions were performed using a BAS 100B /W electrochemical workstation connected to a three-electrode cell (C3 Cell Stand, BAS). In all of the cases, an Ag/AgCl (3 M KCl) served as the reference electrode (0.210 V vs. SHE), and a platinum wire was used as the counter electrode. Polycrystalline Au, single crystalline Au(111) and ITO coated quartz (10 Ω cm-2) electrodes were used as the working electrode for electrochemical and photoelectrochemical measurements as well as morphological imaging. Cu2O and CuO electrodepositions were performed in a single compartment electrochemical glass cell (ca. 15 mL total volume) containing an aqueous solution of 0.05 M CuSO4 and 0.2 M Na2SO4 (pH: 4.6) in which oxygen gas was introduced at different flow rates. The UPD potentials for electrochemical deposition were determined by cyclic voltammetric measurement prior to each deposition experiments. The volumetric flow rate of the oxygen


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gas fed into an electrochemical cell was controlled by gas mass flow controllers (ranged of 2.0-430 mL/min). 2.2 Instrumentation Morphological characterizations of the Cu2O and CuO structures were performed with an FEI Quanta scanning electron microscope (SEM). An energy-dispersive X-ray spectroscopy (EDS) system attached to the SEM was employed to analyze the chemical composition of the Cu2O and CuO electrodeposits. X-ray photoelectron spectroscopy (XPS) measurements were performed on an on a PHI 5000 VersaProbe electron spectrometer using a monochromatized Al Kα excitation source. X-ray diffraction patterns were acquired with a Rigaku Miniflex diffractometer using Cu Kα radiation (1.5406 Å) to examine the crystal structure and orientation of the deposits. The absorption spectra of the deposits on the ITO-coated quartz plates were recorded using a Shimadzu UV-3101 UV-Vis-NIR spectrometer at room temperature. The photoelectrochemical response of Cu2O or CuO deposited ITO electrodes was investigated using a three-electrode electrochemical cell with a Pt wire as a counter electrode and an Ag/AgCl reference electrode in a 0.5 M Na2SO4 aqueous solution. The photocurrent density were recorded in the dark and under irradiation with an AM 1.5G simulated solar light (1 sun, 100 mW/cm2). 3. Results and Discussion 3.1 Electrochemical Deposition The electrochemical deposition technique used here involved the Cu UPD followed by a spontaneous chemical oxidation with dissolved oxygen (DO) from an oxygenated aqueous solution containing Cu2+. The nucleation and growth morphology of the electrodeposits depend on many factors, such as the nature of the substrate surface, precursor concentrations, and deposition potential. However, the deposition potential is the most important parameter in the electrochemical deposition process, and it must be precisely controlled. In principle, UPD,


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which is the electrochemical deposition of a metal onto a foreign substrate at potentials that are more positive than the Nernst potential, is typically restricted to the formation of one atomic layer of the deposited metal.21 Therefore using the UPD potential as a deposition potential enable us to control the deposition potential and to form a deposit that consists of one atomic layer on the substrate surface independent of the deposition time and deposition kinetics. Figure 1a shows the cyclic voltammograms for the OPD (bulk) and UPD of Cu electrodeposition in an oxygen-free aqueous solution of 1.0 mM CuSO4 and 0.2 M Na2SO4 (pH: 4.6) on a single crystalline Au (111) electrode. The broad cathodic peak at -65 mV is considered as bulk deposition of Cu. The following anodic scan resulted in a larger anodic peak at 70 mV corresponding for the stripping of the bulk Cu from the Au(111) electrode surface. Meanwhile, the smaller cathodic and anodic peaks in between 0-350 mV are ascribed to UPD deposition and stripping of Cu monolayer. It has been demonstrated that the shape and potential of Cu UPD peaks recorded on Au(111) electrodes are strongly influenced by the anions of the electrolyte which co-adsorb with Cu adatoms, the pH of the deposition solution and surface structure of the electrode.43-46 Figure 1b displays a comparison between CVs in oxygen free (solid line) and oxygenated (dotted line) solution recorded for Cu electrodeposition on Au(111) electrodes at the UPD region. In the absence of DO, two pairs of well-defined anodic and cathodic peaks (A1/C1 and A2/C2) are attributed to a corresponding two-stage deposition and desorption of the Cu atoms with 2/3 and 1/3 coverage, respectively. The shape of the CV obtained for the oxygen free solution compares favorably with the well-established form published in the literature. 43-46However, Figure 1 b represents the CVs obtained under the same conditions as above after pure oxygen gas was bubbled through the solution for 10 min. Two broad peaks for both cathodic (C1* and C2*) and anodic sweep (A1* and A2*) have been also observed at the UPD region prior to the onset of bulk deposition. The shape and positions of peaks in UPD region in the presence of


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DO are quite different from those recorded in the absence of DO. While the C2* and A2* peaks are shifted to a significantly more positive potentials, the C1* and A1* peaks are much broadened and slightly shifted to more negative potentials compared to the CVs in the absence of DO. We found that the currents of these peaks are proportional to v and have a linear dependence, indicating that these peaks are associated with the surface limited deposition and desorption processes like UPD. We also studied Cu electrodeposition on the CuxO covered Au(111) electrode in the absence of DO, which was not investigated before. The CuxO deposited Au(111) was prepared electrochemically by holding the Au(111) electrode at 50 mV (UPD deposition potential) in an oxygenated solution for 30 min. The corresponding CV (shown in Figure S1 in the Supporting Information) is similar to the one obtained on Au(111) for Cu UPD in oxygenated solution but the bulk deposition peak potential (Ebulk) shifted to more positive potentials on CuxO deposited Au(111) surfaces in the absence of DO. We found that the peak currents of C1*, C2*, A1*, and A2* have linear dependency on the scan rate at low scan rates up to 200 mV/s. In addition, the increase of Cu2+ concentration does not cause significant change of Cu coverage and the obtained Cu coverage inevitably reaches a constant value when holding the potential of CuxO covered Au(111) electrode at the Cu UPD region, which are consistent with the surface limiting nature of UPD process. Figure S1 reveal that the C1*, C2* A1*, and A2* peaks starts before the potential of OPD bulk Cu deposition, indicating the expected energetic gain of a Cu UPD. All of these electrochemical observations for the Cu electrodeposition at underpotential in the presence of DO might indicate that the change of structure and composition of Au(111) surface might be due to electrochemically irreversible new compound formation on the electrode surface. This hypothesis could simply be tested by running the electrochemical deposition under the same conditions both in the presence and absence of DO. If our hypothesis is correct, we would expect to see CuxO formation on the


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electrode surface, which could be imaged and characterized by SEM, EDS and XPS. Figure 2 shows the SEM and EDS results, which indicate that Cu UPD in the oxygenated solution results in the growth of some structures with different shapes composed of Cu and O elements. In contrast, no CuxO deposit was observed by SEM and EDS techniques when the electrodeposition was carried out in the oxygen-free solutions deaerated by purging with argon gas. If there is no DO in the deposition solution to react with the Cu deposited at the UPD potential, only one atomic layer of Cu will be formed on the electrode surface due to UPD being limited to only one atomic layer because the Cu atoms cannot deposit on top of each other at the UPD potential. If there is DO in the deposition solution, the DO could readily react with the pre-deposited Cu monolayer to form CuxO compound. Therefore, the electrode surface could be covered by a new compound resulting in the formation of a new surface. XPS measurements have been performed to analyze the chemical state of Cu on the Au(111) surface after Cu UPD at 50 mV in the absence and presence of DO in deposition solution. The main peak is observed at a binding energy of 932.7 eV for both Cu UPD deposited in the absence and presence of DO and could be attributed to reduced Cu species suggesting either Cu2O or metallic Cu. However, a new shoulder on the higher binding energy (934 eV) side of the Cu 2p main peak and a weak satellite feature around 945 eV appears when the electrodeposition is carried out in the presence of DO (Supporting Information, Figure S2). These changes in binding energies indicate the formation of Cu2O and CuO in the presence of DO on the electrode surface, resulting in the formation of CuxO covered new surface.47,48 If we set the electrode potential to the underpotential for the Cu electrodeposition in the presence of DO, we could expect that each deposited Cu atom at underpotential could be converted to CuxO, by the chemical oxidation with DO. Based on the electrochemical, SEM


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and XPS results, we propose the following reaction mechanisms for the electrochemical formation of Cu2O and CuO deposits at our experimental conditions. Cu 2+ (aq) + 2e − → Cu (UPD)

1

2Cu (UPD) + 1 / 2O2 (aq) → Cu 2 O( s)

2

2Cu (UPD) + O 2 ( g ) → 2CuO( s )

3

We also tested the electrochemical deposition of Cu in the presence and absence of DO using overpotential deposition (OPD) instead of the electrodeposition at underpotential. In both cases, we observed the same bulk Cu deposits on the surface of the electrode, which might indicate that the electrochemical bulk deposition of Cu is much faster than the chemical oxidation of the deposited Cu layers (Figure 3). Similar dendritic growth and morphology was observed by Zangari et al. for bulk copper electrodeposition from acidic sulfate solutions containing chlorides.49 Because UPD is limited to atomic layer deposition of Cu, subsequent Cu atoms will not be deposited until the previously deposited Cu atoms are converted to Cu2O or CuO. At the bulk deposition potential, Cu atoms can be deposited on top each other. If the Cu deposition rate at the bulk deposition potential is faster that the oxidation process, the deposited Cu will not have enough time to be oxidized. Therefore, electrochemical deposition at bulk deposition in both the absence and presence of DO result in bulk Cu deposit, as shown in Figure 3. In contrast, electrodeposition of Cu at the underpotential results in the formation of Cu2O or CuO structures in the presence of DO but an atomic layer of Cu in the absence of DO. 3.2 Morphological and structural characterization Our experimental results indicated that the morphology and chemical composition of the CuxO (x=1,2) structures strongly depend on the flow rate of oxygen gas through the electrodeposition solution. The deposition and growth kinetics during electrodeposition from electrolytes of different concentrations and mass transport rates are believed to play an


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especially important role in forming different shapes of the electrodeposits. Since the Cu UPD is a surface limited process, then the concentration and the mass transport rate of DO will be the most important parameters on the growth kinetics. We measured the concentration of DO in deposition solution by using a DO meter at different flow rates. Results revealed that the deposition solution (10 mL) was saturated with O2 within a 2 min, even at slow flow rates. Therefore, the concentration of DO could not be the only parameter effecting growth kinetics of CuxO. We systematically investigated the influence of the flow rate of oxygen gas by maintaining the deposition solution as saturated as possible and found out that the flow rate has the most important parameter effecting the growth morphology of CuxO structures. We also observed that a successful transition between Cu2O and CuO could be achieved by changing the oxygen flow rate during electrochemical growth. When O2 gas is bubbled in deposition solution, not only the O2 concentration increases slightly, causing an appreciable increase in the oxidation rate, but also increases the mass transport rate of DO from the deposition solution to the surface of electrode by convection. Therefore, we believe that it is important to control the flow rate of oxygen gas to achieve the desired morphology and chemical composition of the CuxO structures. Figure 4 shows the SEM micrographs obtained after electrodeposition of Cu at the UPD potential on an Au electrode from a solution in which oxygen gas was bubbled at a flow rate of 4 mL/min. As shown in Figure 4, welldefined cubic structures were uniformly distributed on the Au substrate. The inset of Fig. 4 clearly shows the morphology of the cubic structures, which have an edge size of ∼500 nm, formed after 60 minutes of electrodeposition. We discovered that this simple electrodeposition method enabled us to control the size and number of cubic structures by varying the deposition time. The XRD pattern of the electrodeposited cubic structures on the Au(111) substrates, which are shown in Figure 5 a, exhibits two sharp diffraction peaks at 36.8° and 38.2°, which are exclusively attributed to the cubic phase of Cu2O crystals (JCPDS


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File No. 05-0667) and the underlying Au(111) substrate used as the working electrode, respectively. The single and sharp diffraction peak at 36.8° indicates the (111)-preferred growth orientation of the cubic structured Cu2O. No peaks corresponding to other impurities (i.e., Cu(OH)2, CuO, Cu4O3 or metallic Cu) were detected in the XRD patterns, indicating the formation of pure and well-crystallized Cu2O under our experimental conditions. The EDS analysis of these cubic structures confirmed this speculation. The EDS spectrum in Figure 4 shows the peaks of O and Cu without any other peaks, and the Au peaks appearing in the spectrum are due to the working electrode. Additional evidence for the purity and composition of the Cu2O cubic structures was obtained by XPS measurements, which is a powerful technique to determine the oxidation state of the elements as well as whether the material is in the pure or alloy form. Figure 6 (solid line) shows the XPS spectrum of the electrodeposited cubic structures. The binding energies at 932.8 and 952.8 eV are attributed to the presence of Cu 2p3/2 and Cu 2p1/2 corresponding to Cu(I), which confirms the formation of pure Cu2O structures.47,48 When a higher flow rate of oxygen gas (i.e., higher than ∼12 mL/min) was bubbled through the same electrodeposition solution during the UPD, the morphology and chemical composition of the electrodeposits were totally different from that of the Cu2O cubic structures. The SEM image (Figure 7) indicates that the electrodeposits are composed of a large amount of flower-like structures, in which the flowers are composed of a large number of randomly arranged irregular ultrathin flakes due to aggregation. The XRD patterns of these flower-like structures exhibit only two diffraction peaks (Figure 5b). The first peak at 35.2º in the XRD pattern confirms the reflection from the (002) atomic planes of CuO phase (JCPDS Card No. 05-0661), and the second peak at 38.2° corresponds to the reflection from the (111) atomic planes of single crystalline Au (JCPDS Card No. 04-0784). The presence of a single peak corresponding to the (002) reflection of the CuO plate indicates the preferential


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electrochemical growth in the CuO (002) lattice direction. The EDS analysis of the flowerlike structures (inset of Figure 7) confirms the formation of CuO, and the elemental analysis indicates a 1:1 ratio of copper and oxygen. XPS analysis also confirms that the electrodeposited flower-like structures are a pure-phase of CuO. The binding energies observed at 934.2 and 953.6 eV are assigned to Cu2p3/2 and Cu2p1/2, which are consistent with those observed in CuO (Figure 6, dotted line).48 In addition, the two ‘‘shake up’’ satellite peaks at 943.8 and 962.4 eV further confirm that the oxide in the sample is CuO.50 It is important to note that the peak corresponding to Cu (I) arising from Cu2O was not detected. If the flow rate of the oxygen gas was maintained between 4 and 12 mL/min and all of the other electrodeposition conditions were constant as in the previous experiments, both cubic Cu2O and flower-like CuO plates were observed on the same surface, as shown in Figure 2. These experiments demonstrate that the formation of Cu2O and CuO can simply be tuned by controlling the oxygen gas flow rate during the electrochemical deposition. In addition, the Cu2O and CuO structures could be simultaneously electrodeposited on the same surface. This method was successfully used for the formation of layered structures containing both phases. The selective overgrowth of CuO on top of the Cu2O cubic structures was accomplished by simply increasing the flow rate of oxygen gas during the electrodeposition process. The thickness of each phase can be controlled by regulating the deposition time and flow rate of oxygen gas. Figure 8 shows the Cu2O nanocubes coated with CuO by performing the electrodeposition at a flow rate of 3 mL/min for 30 min followed by increasing the flow rate to 15 mL/min for 30 min. As shown in the SEM images, each nanocube of the Cu2O structure is coated by a thin layer consisting of a CuO film, and a new flower-like CuO structure started to grow on the bare surface of the substrate. This selective overgrowth is due to varying the flow rate of oxygen gas. All of these results suggest that the oxygen flow rate has a substantial influence on the morphology and chemical composition of CuxO deposits. A similar Cu2O-


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CuO structure was fabricated via etching oxidation of Cu2O cubic structures in NaOH solutions.5 3.3 Photoelectrochemical characterization For optical and electronic characterizations, electrodeposition of CuxO at the UPD potential was performed on an ITO-coated quartz electrode. As shown in the SEM images (Figures 9 a, b and c), the same Cu2O cube and CuO flower-like deposits were formed on the ITO electrodes using the same electrodeposition conditions that we used for the Au electrodes. Figure 10 shows the UV-Vis-NIR absorption spectrum recorded for the Cu2O cube (a) and CuO flower-like structures. While the prepared cubic structured Cu2O has an absorption maximum at ∼505 nm, the flower-like CuO structures exhibit a broad absorption in the visible range with a small shoulder at ∼510 nm. Using these absorption spectra, we can estimate the optical band gap energies of the Cu2O cube and CuO flower-like structures by simply plotting (αhν)2 as a function of hν according to the classical Tauc approach. The optical band gap energies determined from the intercept of (αhν)2 with the axis containing the photo energy (hν) were 2.1 and 1.3 eV for Cu2O and CuO, respectively, which is in good agreement with previously reported results.7,8,50 The photoelectrochemical behavior of the Cu2O and CuO electrodes were evaluated by measuring the photocurrent densities produced under chopped light irradiation (light on/off cycles: 8 s) at no bias in a 0.50 M Na2SO4 electrolyte without any sacrificial reagents or cocatalysts (Figures 11 a and b). Upon light irradiation, the photocurrent values of the aselectrodeposited Cu2O and CuO electrodes sharply increased to 40 and 55 µA cm-2, respectively. In comparison to the photocurrent response of cubic Cu2O structures (with limited surface area provided by micrometer-sized cubes), the flower-like CuO structures exhibit a considerably enhanced photocurrent response. This result indicates that the flowerlike plated structures of the CuO morphology efficiently increased the surface coverage and


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surface areas, as shown by the SEM images (Figures 9 a, b and c). However, CuO has a larger dark-current baseline drift compared to Cu2O, without a noticeable change in the net photo-current level. The origin for the change in dark current with time is not very clear in our study but it is commonly observed in photodetectors.51-53 These photo-induced photocurrents result from the reduction of protons involving the photo-generation of electrons under visible light irradiation. The photoresponsivity of both materials immediately decreased to zero when the sunlight was switched off. The fast rise and decay time in these photocurrent transients indicates that the charge transport in these material proceeds rapidly and may be associated with the single crystalline structure of Cu2O and CuO. These results indicate that the electrodeposited Cu2O and CuO structures can be used in optoelectronic and photovoltaic applications as well as in photoelectrode applications for water splitting under light irradiation in the absence of an external bias. Conclusions In conclusion, we have reported the stoichiometry and morphology controlled electrochemical fabrication of Cu2O and CuO structures by a simple and one-step electrochemical approach, which is based on the Cu UPD followed by a spontaneous chemical oxidation of Cu atomic layers with DO in an oxygenated aqueous solution. We determined that the underpotential deposition is the most critical factor affecting the growth of highly crystalline Cu2O and CuO structures with a desired size because it is limited to one atomic layer deposition. The oxygen gas flow rate has a substantial influence on the stoichiometry and morphology. We have successfully fabricated cubic structured Cu2O, flower-like CuO and Cu2O structures coated with CuO layers by controlling the oxygen flow rate through the deposition solution during the electrodeposition. These electrodeposited materials have excellent photocatalytic activity and may be good candidates in photocatalytic applications for efficient visible light conversion. The results revealed that this new electrochemical approach may be widely used


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to control the growth of other metal oxide nanostructures effectively without the use of surfactants, capping agents or any other additives.

Acknowledgment This work was supported financially by TÜBĐTAK (111T488).

Figure Captions: Figure 1. Cyclic voltammograms of Cu electrodeposition onto single crystalline Au (111) electrode from an oxygen-free solution containing 1.0 mM CuSO4 + 0.1M Na2SO4 (a) and Cu UPD region from the same solution in presence and absence of dissolved oxygen (b). Figure 2. SEM images of the Au surface recorded after electrodeposition of Cu at UPD potential (50 mV) for 30 min. from oxyganeted solutions. The inset of Figure shows the EDS patterns of electrodeposited CuxO structures. Figure 3. SEM images of Cu dendritic structures electrodeposited at OPD region (-50 mV) for 10 min. from oxygenated solution. The inset of figure shows the EDS patterns of Cu dendritic structure. Figure 4. SEM images and corresponding EDS (inset) spectra obtained after electrodeposition of Cu for 60 minutes at the UPD potential (50 mV) on an Au electrode from a solution in which oxygen gas was bubbled at a flow rate of 4 mL/min. Figure 5. XRD patterns of the cubic Cu2O and flower-like CuO structures deposited on Au(111) substrates. Figure 6. XPS spectrum of Cu 2p3/2 and Cu2p1/2 of Cu2O and CuO electrodeposits. Figure 7. SEM images and corresponding EDS (inset) spectra obtained after electrodeposition of Cu for 30 minutes at the UPD potential (50 mV) on an Au electrode from a solution in which oxygen gas was bubbled at a flow rate of 20 mL/min.


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Figure 8. SEM images of Cu2O nanocubes coated with CuO by performing the electrodeposition at a flow rate of 3 mL/min for 30 min followed by increasing the flow rate to 15 mL/min for 30 min. Figure 9. SEM images of cubic Cu2O and flower-like CuO structures electrodeposited on the ITO electrodes using the same electrodeposition condition as for the Au electrodes. Figure 10. UV-Vis absorption spectrum recorded for the cubic Cu2O (a) and CuO flower-like (b) structures. Figure 11. Photocurrents measured of the as-electrodeposited cubic Cu2O and flower-like CuO structures in 0.50 M Na2SO4 with AM 1.5 G illumination (100 mW/cm2), no bias voltage is applied.

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Figure 1a and b. Cyclic voltammograms of Cu electrodeposition onto single crystalline Au (111) electrode from an oxygen-free solution containing 1.0 mM CuSO4 + 0.1M Na2SO4 (a) and Cu UPD region from the same solution in presence and absence of dissolved oxygen (b). 85x92mm (300 x 300 DPI)


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Figure 2. SEM images of the Au surface recorded after electrodeposition of Cu at UPD potential (50 mV) for 30 min. from oxyganeted solutions. The inset of Figure shows the EDS patterns of electrodeposited CuxO structures. 85x73mm (300 x 300 DPI)


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Figure 3. SEM images of Cu dentritic structures electrodeposited at OPD region (-50 mV) for 10 min. from oxyganated solution. The inset of figure shows the EDS patterns of Cu dentritic structure. 73x63mm (300 x 300 DPI)


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Figure 4. SEM images and corresponding EDS (inset) spectra obtained after electrodeposition of Cu for 60 minutes at the UPD potential (50 mV) on an Au electrode from a solution in which oxygen gas was bubbled at a flow rate of 4 mL/min. 73x63mm (300 x 300 DPI)


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Figure 5. XRD patterns of the cubic Cu2O and flower-like CuO structures deposited on Au(111) substrates. 100x119mm (300 x 300 DPI)


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Figure 6. XPS spectrum of Cu 2p3/2 and Cu2p1/2 of Cu2O and CuO electrodeposits. 130x199mm (300 x 300 DPI)


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Figure 7. SEM images and corresponding EDS (inset) spectra obtained after electrodeposition of Cu for 30 minutes at the UPD potential (50 mV) on an Au electrode from a solution in which oxygen gas was bubbled at a flow rate of 20 mL/min. 73x63mm (300 x 300 DPI)


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Figure 8. SEM image of Cu2O nanocubes coated with CuO by performing the electrodeposition at a flow rate of 3 mL/min for 30 min followed by increasing the flow rate to 15 mL/min for 30 min. 73x63mm (300 x 300 DPI)


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Figure 9. SEM images of cubic Cu2O and flower-like CuO structures electrodeposited on the ITO electrodes using the same electrodeposition condition as for the Au electrodes. 130x200mm (300 x 300 DPI)


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Figure 10. UV-Vis absorption spectrum recorded for the cubic Cu2O and CuO flower-like structures. 85x87mm (300 x 300 DPI)


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Figure 11. Photocurrents measured of the as-electrodeposited cubic Cu2O and flower-like CuO structures in 0.50 M Na2SO4 with AM 1.5 G illumination (100 mW/cm2), no bias voltage is applied 100x118mm (300 x 300 DPI)


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