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Potentiostatic deposition of Cu2O films as p-type transparent conductors at room temperature
Potentiostatic deposition of Cu 2 O films as p-type transparent conductors at room temperature M.M. Moharam, E.M. Elsayed, J.C. Nino, R.M. Abou-Shahba, M.M. Rashad PII: DOI: Reference:
S0040-6090(16)30593-4 doi: 10.1016/j.tsf.2016.10.005 TSF 35529
To appear in:
Thin Solid Films
Received date: Revised date: Accepted date:
20 June 2016 27 September 2016 3 October 2016
Please cite this article as: M.M. Moharam, E.M. Elsayed, J.C. Nino, R.M. Abou-Shahba, M.M. Rashad, Potentiostatic deposition of Cu2 O films as p-type transparent conductors at room temperature, Thin Solid Films (2016), doi: 10.1016/j.tsf.2016.10.005
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ACCEPTED MANUSCRIPT Potentiostatic deposition of Cu2O films as p-type transparent conductors at room temperature
Central Metallurgical Research and Development Institute (CMRDI), P.O. Box: 87 Helwan, Cairo, Egypt
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M.M. Moharam1,2, E. M. Elsayed1, J.C. Nino2 , R.M. Abou-Shahba3, M. M. Rashad1
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Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611
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Chemistry Department, Faculty of Science (Girls), Al-Azhar University, Cairo, Egypt
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Abstract
Single phase Cu2O films have been prepared via an electrodeposition technique onto ITO glass substrates at room temperature. Likewise, Cu2O films were deposited using a
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potentiostatic process from an alkaline electrolyte containing copper (II) nitrate and 1M
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sodium citrate. Single phase Cu2O films were electrodeposited at a cathodic deposition potential of 500mV for a reaction period of 90 min, and pH of 12 to yield a film thickness
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of 0.49 µm. The mechanism for nucleation of Cu2O films was found to vary with deposition potential. Applying the Scharifker and Hills model at -500 and -600 mV to describe the mechanism of nucleation for the electrochemical reaction, the nucleation mechanism consisted of a mix between instantaneous and progressive growth mechanisms at -500 mV, while above -600 mV the growth mechanism was instantaneous. Using deposition times from 30 to 90 min at -500 mV deposition potential, pure Cu2O films with different microstructures were electrodeposited. Changing the deposition time from 30 to 90 min varied the microstructure from cubic to more complex polyhedra. The transmittance of electrodeposited Cu2O films ranged from 20-70% in visible range, and samples exhibited a 2.4 eV band gap. The electrical resistivity for electrodeposited Cu2O films was found to decrease with increasing deposition time from 0.854 x 105 Ω-cm at 30 min to 0.221 x 105 Ω-cm at 90 min without any thermal treatment following the electrodeposition process.
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Keywords: Cu2O, Electrodeposition, Film thickness, Characterization, Optical properties
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1. Introduction: The growing demand for environmentally friendly and economical energy sources
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has led to intensive research into solar cells. Towards this, much attention is given to the research and development of the electrodes used in photovoltaic cells. Many
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materials have been employed as electrodes: crystalline silicon, carbon electrodes such as graphene [1], transparent conducting oxides (TCO) like indium tin oxide (ITO), ZnO,
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aluminum doped zinc oxide, and other ternary compounds such as Cd 2SnO4, CdSnO3, CdInO4, Zn2SnO4, MgIn2O4, and Sn3O12 [2]. Although, silicon based solar cells have
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achieved electricity conversion efficiencies ranging from 15% to 20%, the high fabrication cost and the use of toxic chemicals for silicone purification during the
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manufacturing process has motivated the search for a more environmentally friendly and low-cost alternative [3]. The popular transparent electrode ITO has high
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transmission (over 80%) and electrical sheet resistance of approximately 18 Ω/cm 2.
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However, in spite of its attractive properties there are arguments to find alternatives for ITO due to its high cost, the limited supply of indium, its fragility, and its instability toward acids or bases [3]. Recently, graphene has been developed as a transparent anode because it has shown many advantages over conventional electrode materials used in solar cells in terms of transparency, cost, stability, and flexibility. TCOs have also been extensively studied because of their exceptional electronic properties and their good transparency for electrodes. TCOs are very attractive for use in photocatalytic films, electrochromic devices, solar cells, organic photovoltaics (OPV), as well as in organic light emitting diode (OLED) devices [4].
Among transition metal oxides, cuprous oxide (Cu2O), is a p-type semiconductor which has been deemed as an attractive and promising photovoltaic material due to its relatively narrow band gap (2.0–2.2 eV), acceptable solar efficiency, good electrical properties and high optical absorption coefficient in the visible range [5-8]. These characteristics make Cu2O one of the most attractive candidates for applications 3
ACCEPTED MANUSCRIPT ranging from solar energy conversion, micro- and nano-electronics, fuel cells, magnetic storage devices, and catalysis [9,10]. Cu2O is also earth abundant, nontoxic, allows for low-cost fabrication processes, and exhibits structural transition behavior, anomalous
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elastic behavior, negative thermal expansion, mechanical stability, and good electrical properties [11]. Moreover, the long-term stability associated with oxides allows the
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possibility of constructing durable, long-lasting solar cells with Cu2O as the active light-
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absorbing component [12].
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The crystallographic structure of Cu2O (cuprite) is cubic, and resides in the
Pn3 m space group with a = b = c = 4.27 Å. Cu2O crystallizes in a simple cubic
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structure which can be viewed as a superposition of two sublattices: a face centered cubic (fcc) sublattice of Cu cations and a body-centered cubic (bcc) sublattice of O anions. The O atoms occupy tetrahedral interstitial positions relative to the Cu
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sublattice, whereas Cu is linearly coordinated by two neighboring O atoms [13,14].
Up to now, many synthesis routes have been intensively studied to produce
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Cu2O thin films with high homogeneity. Techniques such as hydrothermal processing [15], thermal oxidation [9], reactive DC magnetron sputtering [6], sol gel synthesis [16], chemical vapor deposition, anodic oxidation, spray pyrolysis, chemical oxidation, electrodeposition [17,18], and electroless deposition [19] have all been investigated to optimize Cu2O production. Among these methods, the electrochemical deposition technique has been favored due to its simplicity, low cost, and precise control of the film growth by manipulation of the deposition parameters [5]. In electrochemical deposition synthesis, the variation of parameters such as pH and temperature can be controlled to tailor the morphology of the film deposited [5,20-22]. Changes to these parameters have very significant effects on the electrical and optical properties of the deposited films. Despite what is understood about the relationship between synthesis parameters and properties, the effect of chelating agents, such as citrates, on the optical properties of Cu2O phases during the electrodeposition process is unknown. Sodium citrate
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ACCEPTED MANUSCRIPT nonahydrate (Na3C6H5O7·9H2O, Cit-) acts as chelating agent with Cu+ ions to prevent the precipitation of copper hydroxide at alkaline pH [23,8]. Cu(II) Cit2 + e- = Cu(I)Cit + Cit-
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(1)
2 Cu(I)Cit + 2OH- = Cu2O+2Cit+ e- = Cu+ Cit-
(3)
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Cu(I)Cit
(2)
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The role of varying chelating agent concentration on the optical properties of the electrodeposited Cu2O has not studied before. The purpose of this work is to fabricate
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highly transparent p-type Cu2O films at room temperature without the need for any thermal treatment during electrodeposition or post-process annealing. We postulate that
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this outcome can be achieved by changing deposition conditions as deposition potential, deposition time, the pH of the medium, and chelating agent concentration. The change in the optical and electrical properties at room temperature for
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2. Experimental
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electrodeposited Cu2O films will be studied.
2.1. Film Synthesis
Cu2O films were cathodically electrodeposited in a bath containing 0.1 M copper (II) nitrate trihydrate (Cu(NO3)2·3H2O) in the presence of 0.5 and 1.0 M [24] sodium citrate nonahydrate (Na3C6H5O7·9H2O) as chelating agent to avoid hydroxide precipitation. All solutions were prepared immediately prior to performing each experiment. Sodium hydroxide (1.0 M) was used to adjust pH values between 10 and 12. An ITO (sheet resistance of 10 Ω/sq) coated on a glass substrate (10 mm × 20 mm) was used as the working electrode, which was cleaned before deposition in ultra-sonic baths with acetone, ethanol, and deionized water for 20 min, respectively. Pt sheets (10 mm x 10 mm), were used as the counter electrode. Comparatively, silver/silver chloride Ag/AgCl (+220 mV versus normal hydrogen electrode) was employed as a reference electrode. Following synthesis, cyclic Voltammetry (CV) tests were performed at room temperature using a conventional three-electrode cell [25]. To compare to 5
ACCEPTED MANUSCRIPT the electrodeposited films, Cu2O films were also deposited potentiostatically at room temperature with cathodic potentials of -500 and-600 mV versus Ag/AgCl for reaction periods of 30, 60, and 90 min. After potentiostatic deposition, the prepared
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Cu2O film was washed with di-ionized water. All depositions were carried out using
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PGP201 potentiostat.
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2.2. Film Characterization
Cyclic voltammograms were performed with a computer controlled potentiostat (Volta- lab 21), PGP201 potentiostat, a galvanosatat 20V, and a 1A with general
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generator. The crystal structure of the films was identified through X-ray diffraction (XRD) using a Bruker axis D8 diffractometer using Cu-Kα (λ = 1.5406 KeV) radiation
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operating at 40 kV and 30 mA at a step rate of 2°/min. The diffraction data was recorded
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for 2θ values between 20° and 80°. The change in the microstructure of the formed of samples was inspected by field emission scanning electron microscopy (JEOL-JSM-
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5410, Japan). The UV-Vis transmittance spectrum was measured by UV-Vis-NIRscanning spectrophotometer (JASCO V-570 spectrophotometer, Japan). Film thickness was measured using Bruker optical profilometer. An Agilent precision semiconductor parameter analyzer (PSPA, 4156C) was used to measure the I-V response of the Cu2O films. The films were contacted with two probes of probe station. The bias voltage was applied in steps 0.05 V, and the current was measured in delay time of 5 s [18,24].
3. Results and discussion 3.1. Cyclic Voltammetry Fig. 1 presents cyclic voltammograms (CV) for electrolyte solutions consisting of 0.1M Cu(NO3)2·3H2O and 1.0 M Na3C6H5O7 at different pH values of 5.7, 10.0, and 12.0. The voltammetric data were taken in the potential range from +1.0 to -2.0 V vs. Ag/AgCl with a scan rate of 5 mV/s. Each voltammogram shows the presence of a 6
ACCEPTED MANUSCRIPT cathodic peak (C) in the potential window between -0.46 to -0.70 V. This peak can be attributed to the reduction of Cu2+ ions into copper metal as through the reaction
Cu, Cu2+ + 2e
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(4)
Where E0 is 0.34 V. At pH 5.7 (without addition of NaOH), the anodic peak A at 0.57 V
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was assigned to the dissolution of deposited copper metal [26].Upon sweeping the
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current density in the positive direction, it was found that three oxidation peaks, A, A 1, and A2, at anodic potentials 0.622,-0.065 and 0.28 V respectively exist for pH 10.
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However, the oxidation potentials change to 0.53, -0.38 and 0.10 V for the peaks when the pH was varied to 12. The anodic peak A 1 is ascribed to the oxidation of Cu(0) to
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Cu(I) to form Cu2O whereas peak A2 corresponds to CuO/Cu(OH)2 formation at pH values 10 and 12. Furthermore, it was observed that the peak A 1 was smaller than peak A2 as a result of the lower conductivity of Cu2O in comparison with CuO [26]. In
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the voltammogram of Cu2O films prepared using 0.5M Na3C6H5O7 as chelating agent, it
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was found that two oxidation peaks existed as appeared in Fig.2. These peaks, A2 and A, were found at 0.034, 0.87 V and were more positively shifted than the values of the
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peaks found at pH 10 and 12 using 1 M citrate.
Fig. 1. Cyclic Voltammograms (CV) for Cu2O deposited on ITO substrate, from 0.1 M Cu(NO3)2, 1.0 M sodium citrate at pH 5.7, 10 and 12. Inset of Fig.1: Indication the cathodic peak (C) in CV of Cu2O.
Fig. 2. Cyclic Voltammograms (CV) for Cu2O from of 0.1 M Cu(NO3)2 ,0.5 M sodium citrate at pH 12 3.2. Chronoamperometric study To study the nucleation mechanism for the electrochemical reaction, Scharifker and Hills [22, 25,26-31] suggested a model to describe the nucleation process during the initial few seconds using chronoamperometric techniques. They proposed that the nucleation process may be either progressive or instantaneous. Progressive nucleation 7
ACCEPTED MANUSCRIPT implies that the slow growth of nuclei occurs on a low number of active sites, and that all of these sites activated at the same time. Comparatively, instantaneous nucleation states that a fast growth of nuclei occurs on many active sites, all of which are activated
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during the course of electro reduction [22-28]. Fig. 3 represents the potentiostatic current as a function of time for nucleation and growth of Cu2O film at potentials of -
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500 and -600 mV. The transients are separated into three regions which are linked to
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distinct physical phenomena. The first region (Region I) occurs on a short time frame, less than (0.5 s), where a decrease in the cathodic current density signals the charging
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of a double layer. Following this, an increase in the cathodic current density occurs which marks the crystal nucleation process and growth of crystals. This reaction occurs max),
then it decays as a result of
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until the current density reaches its maximum (I
diffusion processes (Region II) [22-28]. The current transients for Cu2O at -500 and -600 mV are typical of three dimensional electro crystallization growth processes. The
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increase in current density is observed as the potential increases, which we postulate to
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be related to the formation of a higher density of Cu2O deposit. Finally, further increases to deposition time (more than 1 min) cause a stabilization in the current density value,
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which almost reaches a constant plateau as represented in Fig. 3 (Region III). In this stage the current levels off as the ITO substrate becomes completely covered by a Cu2O layer. The current in this regime is almost constant, with a value of -17 and -35 mA/cm2 at potentials of -500 and -600 mV, respectively.
Fig. 3. Potentiostatic transients for nucleation and growth of Cu 2O films at 500 mV and -600 mV To distinguish between the two nucleation/growth models, the data in Fig. 3 was first normalized to (I/I max) 2 and t/t max. The normalized experimental data from the entire experimental current–time
transient
were
then
compared to
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dimensionless theoretical curves derived for the three-dimensional (3D) instantaneous and progressive nucleation. These are expressed as
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where I
max
and t
max
2.2367 t t max 1.2254 1 e t max t
(5)
2
(6)
are the maximum current density observed and the maximum
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2 progress 2 max
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I I
1.2564 t t max 1.9542 1 e t max t
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2 inst 2 max
time [22]. It was found that by fitting the experimental curves with theoretical ones, as
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represented in Fig. 4A, that the mechanism for nucleation and growth of electrodeposited Cu2O film was instantaneous at the beginning of the nucleation, but
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then followed the progressive mechanism during the growth of the Cu 2O films at the deposition potential of -500 mV. Comparatively, at a cathodic potential -600 mV, as depicted in Fig. 4B, the Cu2O film growth was progressive at the beginning of the
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nucleation, but followed the instantaneous mechanism during the growth of the Cu 2O
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films [22,28].
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Fig. 4. Non-dimensional i2/i2max vs t/tmax plots for the electrodeposited Cu2O films at A - 500 mV, B - 600 mV The variation in the measured film thickness with the deposition time at a deposition potential of -500 m V is presented in Table 1. The film thickness was clearly observed to increase with increasing deposition time.
Table 1: Film thickness of Cu2O films at deposition potentials -500 mV for reaction times 30, 60 and 90 min.
3.3. Crystal structure XRD patterns for the electrodeposited Cu2O films using deposition potentials of 500 and -600 mV, with stable pH 12, are presented in Fig. 5A and Fig. 5B for different 9
ACCEPTED MANUSCRIPT deposition times (30, 60, 90 min). The characteristic diffraction peaks for Cu 2O films found in the 2θ range of 25– 80° were assigned to the (110), (111), (200), (220), and (311) planes of the Cu2O structure. The data falls in agreement with the Joint
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Committee on Powder Diffraction Standards (JCPDS) of cubic cupreous oxide (JCPDS card no. 05-0667) [19]. There were no observed diffraction peaks for other phases
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except for the ITO peaks belonging to the substrate. We can therefore conclude that
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single phase polycrystalline Cu2O film can be obtained through the electrodeposition route without any need for thermal treatment. Furthermore, the relative intensities of
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characteristic peaks of Cu2O enhance with increasing deposition time from 30 to 90 min as a result of the increase to the film thickness, as evidenced by the data
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presented in Table 1 [25]. Comparing the Cu2O films deposited at–500mV with that at 600 mV, it was found that films formed with the best crystallinity and thickness is at -
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500 mV for 90 min.
XRD patterns of the Cu2O films deposited at room temperature for deposition potential (A. -500, B. -600) mV with reaction times (30, 60, 90 min), and pH 12.
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Fig. 5.
To explore the effect of pH value on the deposition of Cu 2O films, Fig. 6 presents XRD profiles for films deposited at -500 mV for reaction times of 90 min at different pH values of 5.7 and 10. It is clear that the film formed at pH 5.7 was copper metal, which confirms the voltammogram explanation in the discussion of Fig. 1 The present findings oppose the previous investigation by Tang et al. [25] and Georgieva [29]. Tang et al. claimed that single phase Cu2O was electrodeposited at pH between 5.5 and 6.0 using a deposition potential of -245 mV [21]. Georgieva’s work stated that pure Cu2O film was prepared at pH 9 from a sulphate bath at 60 °C [29]. In this work, the films deposited at pH 10 with 1M citrate and at pH 12 using 0.5 M citrate resulted in Cu/Cu 2O composites as evidenced in Fig. 6. This discrepancy can be attributed to the faster growth rates originating from the increase of OH- intensity. Likewise, more Cu(NO3)2 in the electrolyte will hydrolyze at pH 12 to form Cu(OH)2, which decreases the 10
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Cu2O + 2 OH- + H2O 2 Cu(OH)2 + 2e-
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where E0 = -0.09 V [25]. Thus, we can conclude that the electrodeposition carried out at
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pH 12 yields good quality single phase of Cu2O thin films.
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Fig. 6. XRD patterns of the Cu2O films deposited using 1 M citrate at -500 mV as potential, 90 min time, for pH (5.7, 10, 12), and another films deposited using 0.5 M citrate at pH 12 under same potential and time.
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3.4. Microstructure
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The deposition time plays a key role in the morphology of the synthesized Cu2O
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films as confirmed in Fig. 7. The increase in the deposition time from 30 to 90 min changes the crystalline morphology for deposited Cu2O film from small cubic structure to
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aggregated polyhedron shape with 4-sided structure for film deposited for 90 min. Meanwhile, the average grain size for the formed particles is in the range from 400 to 700 nm.
Fig. 7. FESEM images of Cu2O films at a deposition potential of -500 mV, pH12, and at deposition times (A) 30, (B) 60, and (C) 90 min Fig. 8 (A) and (B) present FE-SEM images of the formed films at pH 5.7 and 10. The metallic Cu distinctively appears as lumps at the electrode surface with cracks in the deposited film as seen in Fig. 8A [27]. Upon increasing pH to 10, the film forms as 3 faced pyramids with small lumps of copper metal as presented in Fig. 8(B). No difference in the microstructure of the Cu2O films was observed for deposition at pH12 using 0.5 M citrate as shown between Fig.8 (C) and Fig. 7 (C).
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3.5. Optical properties
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The optical properties of the thin films were sensitive to modifications made to
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their composition and structure. Fig. 9 shows a transmittance measurement for Cu2O thin films deposited at -500 mV for various deposition times (30, 60 and 90 min ) as well
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as a film prepared with 0.5 M citrate for 90 min. The loss of transmission exhibited in the spectra for wavelengths lower than ~500 nm was due to the Cu2O band gap [5]. Up to
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nearly 500 nm, the Cu2O films were completely absorbent. The band located at 300-500 nm corresponds to the Cu2O absorption band [5-7]. As film thickness as shown in table 1 and the particle size increased (with deposition time), the band shifted progressively
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to the red region [25]. The absorption band of Cu2O was shape dependent. The
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recorded band positions at 480 and 510 nm, for the samples with the smallest cubes and polyhedron shapes, supports that the particle size, shape, and composition has an
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important role on the exhibited UV-Vis spectra of Cu2O [9]. Features at 290, 357 and 450 nm are most likely related to Van Hove singularities on the electronic structure of Cu2O [5]. The absorption thresholds were found to be at wavelengths 274, 271, 269, 266 nm for deposition times of 30, 60, 90 min, and 0.5 M citrate respectively. The transmittance increases quite strongly between 500 and 850 nm. The value was higher than 25–70 % in the visible range at long wavelengths. This value was better than the optical transmission found in literature for the Cu2O thin films (grown on glass substrates) observed when annealed at 700K in vacuum, which is 72% at 600 nm [7].
Fig. 9. UV-VIS transmission spectra for Cu2O films deposited at -500 mV potential, pH 12 at deposition times 30, 60, and 90 min (1M citrate), another Cu2O films deposited using (0.5M citrate) at 90 min and pH 12.
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inter atomic spacing and consequently an increase in Cu–Cu internetwork interactions
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[5]. Optical transmittance was found to be between 20–40 % in the visible region.
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Fig. 10. UV-VIS transmission spectra for Cu2O films deposited at -500 mV for 90 min at
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pH 10 and 12.
In order to obtain the direct band gap energy (Eg), absorption coefficients ʋ were
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fitted by the Tauc relation (αhʋ)2= hʋ - Eg [30], where α is the absorption coefficient, hʋ is the photon energy, and Eg is the band gap energy. The direct band gap energy was estimated by extrapolation of the plot (αhʋ)2 vs. photon energy (hʋ) as represented in
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Fig. 11 and Fig. 12. The Eg values were found to be within the expected range for Cu 2O.
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The direct band gap energy for Cu2O films were computed as 2.40, 2.30, and 2.20 eV for deposition times of 30, 60, and 90 min, respectively. The band gap was likewise
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found to be 1.6 eV for films of Cu2O deposited using 0.5 M Na3C6H5O7 at 90 min deposition time. Furthermore, the obtained values were nearly similar to those published before by Murali et al. [10] in which the band gap ranged between 2.0 and 2.4 eV for deposited Cu2O thin films from CuO using magnetron sputtering. Fig. 12 presents the band gap energy of the film deposited at -500 mV for 90 min at pH 10 and 12. Evidently, a more intense Cu–Cu internetwork interaction reduces the Cu2O band gap from 2.2 at pH 12 to 1.7 eV at pH 10 [5].
Fig. 11. Optical band gap energy of Cu2O films deposited at -500 mV potential for deposition times 30, 60, and 90 min, using 1 M citrate at pH 12 , inset (band gap energy for Cu2O films deposited at same conditions using 0.5 M citrate. Fig.12. Optical band gap energy of Cu2O films deposited at -500 mV for deposition time 90 min and pH 10, 12.
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3.6. Electrical properties Fig.13 shows the I-V curves of the Cu2O/ITO heterojunction for different
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deposition times (30, 60, 90 min). To produce an ohmic electrode, gold was evaporated on the top of the Cu2O films. The resulting structure of the photocell could be specified
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as ITO/Cu2O/Au. The electrical resistivity of the Cu2O layers were determined by using the area of the electrode and film thickness. The electrical resistivity for
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electrodeposited Cu2O films was found to decrease with increasing deposition time from 0.854 x 105 Ω-cm at 30 min to 0.221 x 105 Ω-cm at 90 min. the electrical conductivity for
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the Cu2O/ITO heterojunction increased with deposition time. This effect is due to the increase in the carrier density of Cu2O, based on the fact that copper vacancy and
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oxygen interstitials are controlled by the amount of hydroxyl concentration. Therefore, increasing reduction time likewise increased the possibility for the reduction of Cu(OH)2
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into Cu2O as represented in Eg. 7 [18]. These values of resistivity are smaller than other
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values reported in literature which range between 10 7-104 Ω-cm. Remarkably, the Cu2O films prepared in this work have higher conductivities than others prepared at higher temperatures [32,33].
Fig.13. I-V response of Cu2O film deposited at -500 mV, pH 12 for deposition
times 30, 60, and 90 min. The inset shows the change of electrical resistivities for Cu2O films with deposition times
4. Conclusions: Single phase Cu2O thin films were deposited from cupric nitrate solution on ITO substrates using an electrodeposition technique at pH 12. Sodium citrate was acted as complexing agent to prevent copper hydroxide precipitation through alkaline medium. Voltammetric studies were performed to determine the reduction potential of Cu 2O which was in cathodic potential window from 0.4 to 0.7 V . The nucleation mechanism for electrodeposition of Cu2O was found to be instantaneous at the initiation of the 14
ACCEPTED MANUSCRIPT reaction, and then evolved to be a progressive mechanism at -500 mV. XRD results proved that the existence of a cubic Cu2O phase which was deposited at -500 and -600 mV, pH 12 using deposition times 30, 60 and 90 min. The deposition time and pH
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values had significant effects on the surface morphology of the formed oxides. Cu metal was detected at pH 5.7 and 10. As the pH was changed to 12 and the reaction time
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was increased from 30 to 90 min, the microstructure of Cu2O transformed from cubic to
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polyhedron agglomerates at -0.5 V. The maximum transparency for the deposited Cu 2O was found to be 70% when deposited at -500 mV and pH 12, for 30 min deposition time.
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Band gap energy values of the films were varied from 1.60 to 2.40 eV with pH and reaction time changes. The resistivity of the Cu2O films was decreased on increasing
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the deposition time, therefore, it was found to be on the order of 105 Ω-cm, which was
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Acknowledgment
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smaller than aforementioned values depicted in literature at high temperatures.
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All authors are so grateful for financial support from the Ministry of Higher EducationEgypt, Project No. 00120805 to carry electrical properties measurements in Materials science and Engineering department, University of Florida, Florida.
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References:
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[1] Z. Liu, P. You, S. Liu, F. Yan, Neutral-color semitransparent organic solar cells with all-graphene eectrodes, ACS Nano, 9 (2015)12026 – 12034.
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[2] J. Meiss, C. L. Uhrich, K. Fehse, S. Pfuetzner, M.K. Riede, K. Leo, Transparent electrode materials for solar cells, Photonics for Solar Energy Systems II, 7002 (2008) 700210-1 - 700210-9.
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[3] S. Suhaimi , M. M. Shahimin , Z.A. Alahmed , J. Chyský ,A. H. Reshak, Materials for enhanced dye-sensitized solar cell performance: Electrochemical Application, Int. J. Electrochem. Sci., 10 (2015) 2859 - 2871.
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[4] L. Wu, Y. Yu, X. Han, Yuan Zhang, Y. Zhang, Y. Li , J. Zhi, An electroless-platinglike solution deposition approach for large-area flexible thin films of transition metal oxide nanocrystals, J. Mater. Chem. C, 2 (2014) 2266 - 2271. [5] M. Majumder, I. Biswas, S. Pujaru, A.K. Chakraborty. Cuprous oxide thin films grown
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741–749.
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by hydrothermal electrochemical deposition technique, Thin Solid Films, 589 (2015)
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[6] Mugwang F.K , Karimi P.K , Njoroge W.K , Omayio O and Waita S.M , Optical characterization of copper oxide thin films prepared by reactive dc magnetron sputtering for solar cell applications, Int. J. Thin Film Sci. Tec. 2 No. 1, (2013) 15 - 24. [7] T. Jiang, T. Xie, Y. Zhang, L. Chen, L. Peng, H. Li , D. Wang, Photoinduced charge transfer in ZnO/Cu2O heterostructure films studied by surface photovoltage technique, Phys. Chem. Chem. Phys.,12 (2010) 15476 – 15481. [8] S. Bijani, R. Schrebler, E. A. Dalchiele, M. Gab_as, L. Martínez, J. R. RamosBarrado, Study of the nucleation and growth mechanisms in the electrodeposition of micro- and nanostructured Cu2O thin films, J. Phys. Chem. C. 115 (2011) 21373 21382. [9] T. Minami, Y. Nishi, and T. Miyata, High-efficiency Cu2O-based heterojunction solar cells fabricated using a Ga2O3 thin Film as N-Type layer , Appl. Phys. Express 6 (2013) 044101 . [10] X. Zhanga, , G. Wanga, A. Gu, H. Wu , B. Fang, Preparation of porous Cu 2O octahedron and its application as L-Tyrosine sensors, Solid State Commun., 148 (2008) 525 - 528.
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ACCEPTED MANUSCRIPT [11] C.L. Liu, Y.M. Sui, W. B. Ren, B.H. Ma, Y. Li, N. Ning S, Q.L. Wang, Y. Q. Li, J.K. Zhang, Y. H. Han, Y.Z. Ma, C.X. Gao, Electrical properties and behaviors of cuprous
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oxide cubes under high pressure, Inorg. Chem. 51(2012) 7001−7003.
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[12] B. D. Yuhas, P. Yang, Nanowire-Based All-Oxide Solar Cells, J. Am. Chem. Soc. 131(2009) 3756 – 3761.
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[13] J. Luo, L. Steier, M.K. Son, M. Schreier, M. T. Mayer, M. Gratzel, Cu2O Nanowire photocathodes for efficient and durable solar water splitting, Nano Lett.,16 (2016) 1848 – 1857.
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[14] K. Chen, C.g Sun, S. Song, D. Xue, Polymorphic compound, Cryst.Eng.Comm. 16 (2014) 5257 – 5267.
crystallization of Cu 2O
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[15] X. Zhang, G. Wang, H. Wu, D. Zhang, X. Zhang, P. Li, H. Wu, Synthesis and photocatalytic characterization of porous cuprous oxide octahedra, Mater. Lett. 62 (2008) 4363 – 4365.
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[16] R.K. Gupta, M. Cavas, Ahmed A. Al-Ghamdi, Z.H. Gafer, F. El-Tantawy, F. Yakuphanoglu, Electrical and photoresponse properties of Al/p-CuFeO2/p-Si/Al MTCOS, Solar Energy, 92 (2013) 1-6.
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[17] X. Jiang, M. Zhang, S.Shi, G. He, X. Song, Z. Jiang et al, Microstructure and optical properties of nanocrystalline Cu2O thin films prepared by electrodeposition, Nanoscale Research Letters 9: 219 (2014) 1- 5. [18] Seung Ki B, Ki Ryong L, Hyung Koun, Oxide p-n Heterojunction of Cu2O/ZnO nanowires and their photovoltaic performance, J Nanomater., 2013 (2013) 1-7. [19] Z. Xiong, M. Zheng, H. Li, W. Shen, Fabrication and optical properties of silicon nanowire/Cu2O nano-heterojunctions by electroless deposition technique, Mater. Lett.112 (2013) 211-214. [20] Y. Song , M. Ichimura, H2O2 Treatment of electrochemically deposited Cu2O thin films for enhancing optical absorption. Int. J. Photoenergy, Article ID 738063 (2013) 16. [21] S. Ikeda, R. Kamai, S.M.Lee, T. Yagi, T. Harada, M. Matsumura, A superstrate solar cell based on In2(Se,S)3 and CuIn(Se,S)2 thin films fabricated by electrodeposition combined with annealing, Sol. Energ. Mat. Sol. C. 95 (2011) 1446-1451. [22] A. E. Saba , E. M. Elsayed , M. M. Moharam, M. M. Rashad , R. M. AbouShahba, Structure and magnetic properties of NixZn1-xFe2O4 thin films prepared through electrodeposition method , J. Mater.Sci. 46 (2011) 3574- 3582. 17
ACCEPTED MANUSCRIPT [23] Ruey- Shin J, Li-Chun L, Rates of metal electrodeposition from aqueous solutions in the presence of chelating agents, Sep. Sci. Technol, 35 (2000)1087–1098.
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[24] S. K. Baek, J. H. Shin, S. W. Cho, H. K. Cho, Electrodeposition of p-type cuprous oxide layers on n-type zinc oxide layers with different electrical resistivities, J. Vac. Sci. Technol. 33 (2015) 02B1041- 02B10416.
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[25] Y. Tang, Z. Chen, Z. Jia, L. Zhang, Electrodeposition and characterization of nanocrystalline cuprous oxide thin films on TiO2 films, J. Mater.Lett., 59 (2005) 434–
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438.
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[26] N. Fredj and T. D. Burleigh, Transpassive dissolution of copper and rapid formation of brilliant colored copper oxide films, J. Electrochem. Soc. 158 (4) (2011) C104-C110.
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[27] W. Septina , S. Ikeda , M. Alam Khan , T. Hirai , T. Harada , M. Matsumura, , L. M. Peter, Potentiostatic electrodeposition of cuprous oxide thin films for photovoltaic
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applications, Electrochim. Acta 56 (2011) 4882–4888.
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[28]Q. B. Zhang, Y. X. Hua, Electrochemical synthesis of copper nanoparticles using cuprous oxide as a precursor in choline chloride–urea deep eutectic solvent: nucleation
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and growth mechanism, Phys. Chem.16 (2014) 27088-27095. [29] V. Georgieva, , M. Ristov, Electrodeposited cuprous oxide on indium tin oxide for solar applications, Sol. Energ. Mat. Sol. C. 73 (2002) 67–73. [30] M. M. Moharam , M. M. Rashad , E. M. Elsayed , A facile novel synthesis of delafossite CuFeO2 powders. J Mater. Sci. 25 (2014) 1798–1803. [31] Z. Zhang, W.Hu, C. Zhong, Y. Deng, L. Liu, Y. Wu, Preparation of submicron-sized cuprous oxide crystallites by electrodeposition with polyethylene glycol as additive, J. of Cryst. Growth, 354 (2012) 193-197. [32] T. Mahalingam, J.S.P. Chitra, S. Rajendran, M. Jayachandran, Mary Juliana Chockalingam, Galvanostatic deposition and characterizationof cuprous oxide thin flms, J. of Cryst. Growth 216 (2000) 304-310.
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ACCEPTED MANUSCRIPT Highlights Cu2O films were electrodeposited at room temperature.
The nucleation mechanism of Cu2O films was studied
The microstructures were synthesis conditions dependent.
The maximum transparency of Cu2O films was 70%.
Low electrical resistivity of Cu2O films was achieved.
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S0040-6090(16)30593-4 doi: 10.1016/j.tsf.2016.10.005 TSF 35529
To appear in:
Thin Solid Films
Received date: Revised date: Accepted date:
20 June 2016 27 September 2016 3 October 2016
Please cite this article as: M.M. Moharam, E.M. Elsayed, J.C. Nino, R.M. Abou-Shahba, M.M. Rashad, Potentiostatic deposition of Cu2 O films as p-type transparent conductors at room temperature, Thin Solid Films (2016), doi: 10.1016/j.tsf.2016.10.005
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ACCEPTED MANUSCRIPT Potentiostatic deposition of Cu2O films as p-type transparent conductors at room temperature
Central Metallurgical Research and Development Institute (CMRDI), P.O. Box: 87 Helwan, Cairo, Egypt
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M.M. Moharam1,2, E. M. Elsayed1, J.C. Nino2 , R.M. Abou-Shahba3, M. M. Rashad1
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Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611
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Chemistry Department, Faculty of Science (Girls), Al-Azhar University, Cairo, Egypt
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Abstract
Single phase Cu2O films have been prepared via an electrodeposition technique onto ITO glass substrates at room temperature. Likewise, Cu2O films were deposited using a
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potentiostatic process from an alkaline electrolyte containing copper (II) nitrate and 1M
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sodium citrate. Single phase Cu2O films were electrodeposited at a cathodic deposition potential of 500mV for a reaction period of 90 min, and pH of 12 to yield a film thickness
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of 0.49 µm. The mechanism for nucleation of Cu2O films was found to vary with deposition potential. Applying the Scharifker and Hills model at -500 and -600 mV to describe the mechanism of nucleation for the electrochemical reaction, the nucleation mechanism consisted of a mix between instantaneous and progressive growth mechanisms at -500 mV, while above -600 mV the growth mechanism was instantaneous. Using deposition times from 30 to 90 min at -500 mV deposition potential, pure Cu2O films with different microstructures were electrodeposited. Changing the deposition time from 30 to 90 min varied the microstructure from cubic to more complex polyhedra. The transmittance of electrodeposited Cu2O films ranged from 20-70% in visible range, and samples exhibited a 2.4 eV band gap. The electrical resistivity for electrodeposited Cu2O films was found to decrease with increasing deposition time from 0.854 x 105 Ω-cm at 30 min to 0.221 x 105 Ω-cm at 90 min without any thermal treatment following the electrodeposition process.
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Keywords: Cu2O, Electrodeposition, Film thickness, Characterization, Optical properties
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1. Introduction: The growing demand for environmentally friendly and economical energy sources
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has led to intensive research into solar cells. Towards this, much attention is given to the research and development of the electrodes used in photovoltaic cells. Many
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materials have been employed as electrodes: crystalline silicon, carbon electrodes such as graphene [1], transparent conducting oxides (TCO) like indium tin oxide (ITO), ZnO,
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aluminum doped zinc oxide, and other ternary compounds such as Cd 2SnO4, CdSnO3, CdInO4, Zn2SnO4, MgIn2O4, and Sn3O12 [2]. Although, silicon based solar cells have
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achieved electricity conversion efficiencies ranging from 15% to 20%, the high fabrication cost and the use of toxic chemicals for silicone purification during the
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manufacturing process has motivated the search for a more environmentally friendly and low-cost alternative [3]. The popular transparent electrode ITO has high
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transmission (over 80%) and electrical sheet resistance of approximately 18 Ω/cm 2.
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However, in spite of its attractive properties there are arguments to find alternatives for ITO due to its high cost, the limited supply of indium, its fragility, and its instability toward acids or bases [3]. Recently, graphene has been developed as a transparent anode because it has shown many advantages over conventional electrode materials used in solar cells in terms of transparency, cost, stability, and flexibility. TCOs have also been extensively studied because of their exceptional electronic properties and their good transparency for electrodes. TCOs are very attractive for use in photocatalytic films, electrochromic devices, solar cells, organic photovoltaics (OPV), as well as in organic light emitting diode (OLED) devices [4].
Among transition metal oxides, cuprous oxide (Cu2O), is a p-type semiconductor which has been deemed as an attractive and promising photovoltaic material due to its relatively narrow band gap (2.0–2.2 eV), acceptable solar efficiency, good electrical properties and high optical absorption coefficient in the visible range [5-8]. These characteristics make Cu2O one of the most attractive candidates for applications 3
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elastic behavior, negative thermal expansion, mechanical stability, and good electrical properties [11]. Moreover, the long-term stability associated with oxides allows the
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possibility of constructing durable, long-lasting solar cells with Cu2O as the active light-
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absorbing component [12].
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The crystallographic structure of Cu2O (cuprite) is cubic, and resides in the
Pn3 m space group with a = b = c = 4.27 Å. Cu2O crystallizes in a simple cubic
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structure which can be viewed as a superposition of two sublattices: a face centered cubic (fcc) sublattice of Cu cations and a body-centered cubic (bcc) sublattice of O anions. The O atoms occupy tetrahedral interstitial positions relative to the Cu
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sublattice, whereas Cu is linearly coordinated by two neighboring O atoms [13,14].
Up to now, many synthesis routes have been intensively studied to produce
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Cu2O thin films with high homogeneity. Techniques such as hydrothermal processing [15], thermal oxidation [9], reactive DC magnetron sputtering [6], sol gel synthesis [16], chemical vapor deposition, anodic oxidation, spray pyrolysis, chemical oxidation, electrodeposition [17,18], and electroless deposition [19] have all been investigated to optimize Cu2O production. Among these methods, the electrochemical deposition technique has been favored due to its simplicity, low cost, and precise control of the film growth by manipulation of the deposition parameters [5]. In electrochemical deposition synthesis, the variation of parameters such as pH and temperature can be controlled to tailor the morphology of the film deposited [5,20-22]. Changes to these parameters have very significant effects on the electrical and optical properties of the deposited films. Despite what is understood about the relationship between synthesis parameters and properties, the effect of chelating agents, such as citrates, on the optical properties of Cu2O phases during the electrodeposition process is unknown. Sodium citrate
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ACCEPTED MANUSCRIPT nonahydrate (Na3C6H5O7·9H2O, Cit-) acts as chelating agent with Cu+ ions to prevent the precipitation of copper hydroxide at alkaline pH [23,8]. Cu(II) Cit2 + e- = Cu(I)Cit + Cit-
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2 Cu(I)Cit + 2OH- = Cu2O+2Cit+ e- = Cu+ Cit-
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Cu(I)Cit
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The role of varying chelating agent concentration on the optical properties of the electrodeposited Cu2O has not studied before. The purpose of this work is to fabricate
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highly transparent p-type Cu2O films at room temperature without the need for any thermal treatment during electrodeposition or post-process annealing. We postulate that
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this outcome can be achieved by changing deposition conditions as deposition potential, deposition time, the pH of the medium, and chelating agent concentration. The change in the optical and electrical properties at room temperature for
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2. Experimental
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electrodeposited Cu2O films will be studied.
2.1. Film Synthesis
Cu2O films were cathodically electrodeposited in a bath containing 0.1 M copper (II) nitrate trihydrate (Cu(NO3)2·3H2O) in the presence of 0.5 and 1.0 M [24] sodium citrate nonahydrate (Na3C6H5O7·9H2O) as chelating agent to avoid hydroxide precipitation. All solutions were prepared immediately prior to performing each experiment. Sodium hydroxide (1.0 M) was used to adjust pH values between 10 and 12. An ITO (sheet resistance of 10 Ω/sq) coated on a glass substrate (10 mm × 20 mm) was used as the working electrode, which was cleaned before deposition in ultra-sonic baths with acetone, ethanol, and deionized water for 20 min, respectively. Pt sheets (10 mm x 10 mm), were used as the counter electrode. Comparatively, silver/silver chloride Ag/AgCl (+220 mV versus normal hydrogen electrode) was employed as a reference electrode. Following synthesis, cyclic Voltammetry (CV) tests were performed at room temperature using a conventional three-electrode cell [25]. To compare to 5
ACCEPTED MANUSCRIPT the electrodeposited films, Cu2O films were also deposited potentiostatically at room temperature with cathodic potentials of -500 and-600 mV versus Ag/AgCl for reaction periods of 30, 60, and 90 min. After potentiostatic deposition, the prepared
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Cu2O film was washed with di-ionized water. All depositions were carried out using
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PGP201 potentiostat.
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2.2. Film Characterization
Cyclic voltammograms were performed with a computer controlled potentiostat (Volta- lab 21), PGP201 potentiostat, a galvanosatat 20V, and a 1A with general
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generator. The crystal structure of the films was identified through X-ray diffraction (XRD) using a Bruker axis D8 diffractometer using Cu-Kα (λ = 1.5406 KeV) radiation
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operating at 40 kV and 30 mA at a step rate of 2°/min. The diffraction data was recorded
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for 2θ values between 20° and 80°. The change in the microstructure of the formed of samples was inspected by field emission scanning electron microscopy (JEOL-JSM-
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5410, Japan). The UV-Vis transmittance spectrum was measured by UV-Vis-NIRscanning spectrophotometer (JASCO V-570 spectrophotometer, Japan). Film thickness was measured using Bruker optical profilometer. An Agilent precision semiconductor parameter analyzer (PSPA, 4156C) was used to measure the I-V response of the Cu2O films. The films were contacted with two probes of probe station. The bias voltage was applied in steps 0.05 V, and the current was measured in delay time of 5 s [18,24].
3. Results and discussion 3.1. Cyclic Voltammetry Fig. 1 presents cyclic voltammograms (CV) for electrolyte solutions consisting of 0.1M Cu(NO3)2·3H2O and 1.0 M Na3C6H5O7 at different pH values of 5.7, 10.0, and 12.0. The voltammetric data were taken in the potential range from +1.0 to -2.0 V vs. Ag/AgCl with a scan rate of 5 mV/s. Each voltammogram shows the presence of a 6
ACCEPTED MANUSCRIPT cathodic peak (C) in the potential window between -0.46 to -0.70 V. This peak can be attributed to the reduction of Cu2+ ions into copper metal as through the reaction
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Where E0 is 0.34 V. At pH 5.7 (without addition of NaOH), the anodic peak A at 0.57 V
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However, the oxidation potentials change to 0.53, -0.38 and 0.10 V for the peaks when the pH was varied to 12. The anodic peak A 1 is ascribed to the oxidation of Cu(0) to
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Cu(I) to form Cu2O whereas peak A2 corresponds to CuO/Cu(OH)2 formation at pH values 10 and 12. Furthermore, it was observed that the peak A 1 was smaller than peak A2 as a result of the lower conductivity of Cu2O in comparison with CuO [26]. In
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peaks found at pH 10 and 12 using 1 M citrate.
Fig. 1. Cyclic Voltammograms (CV) for Cu2O deposited on ITO substrate, from 0.1 M Cu(NO3)2, 1.0 M sodium citrate at pH 5.7, 10 and 12. Inset of Fig.1: Indication the cathodic peak (C) in CV of Cu2O.
Fig. 2. Cyclic Voltammograms (CV) for Cu2O from of 0.1 M Cu(NO3)2 ,0.5 M sodium citrate at pH 12 3.2. Chronoamperometric study To study the nucleation mechanism for the electrochemical reaction, Scharifker and Hills [22, 25,26-31] suggested a model to describe the nucleation process during the initial few seconds using chronoamperometric techniques. They proposed that the nucleation process may be either progressive or instantaneous. Progressive nucleation 7
ACCEPTED MANUSCRIPT implies that the slow growth of nuclei occurs on a low number of active sites, and that all of these sites activated at the same time. Comparatively, instantaneous nucleation states that a fast growth of nuclei occurs on many active sites, all of which are activated
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during the course of electro reduction [22-28]. Fig. 3 represents the potentiostatic current as a function of time for nucleation and growth of Cu2O film at potentials of -
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500 and -600 mV. The transients are separated into three regions which are linked to
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distinct physical phenomena. The first region (Region I) occurs on a short time frame, less than (0.5 s), where a decrease in the cathodic current density signals the charging
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of a double layer. Following this, an increase in the cathodic current density occurs which marks the crystal nucleation process and growth of crystals. This reaction occurs max),
then it decays as a result of
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until the current density reaches its maximum (I
diffusion processes (Region II) [22-28]. The current transients for Cu2O at -500 and -600 mV are typical of three dimensional electro crystallization growth processes. The
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increase in current density is observed as the potential increases, which we postulate to
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be related to the formation of a higher density of Cu2O deposit. Finally, further increases to deposition time (more than 1 min) cause a stabilization in the current density value,
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which almost reaches a constant plateau as represented in Fig. 3 (Region III). In this stage the current levels off as the ITO substrate becomes completely covered by a Cu2O layer. The current in this regime is almost constant, with a value of -17 and -35 mA/cm2 at potentials of -500 and -600 mV, respectively.
Fig. 3. Potentiostatic transients for nucleation and growth of Cu 2O films at 500 mV and -600 mV To distinguish between the two nucleation/growth models, the data in Fig. 3 was first normalized to (I/I max) 2 and t/t max. The normalized experimental data from the entire experimental current–time
transient
were
then
compared to
the
appropriate
dimensionless theoretical curves derived for the three-dimensional (3D) instantaneous and progressive nucleation. These are expressed as
8
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I
where I
max
and t
max
2.2367 t t max 1.2254 1 e t max t
(5)
2
(6)
are the maximum current density observed and the maximum
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I
2 progress 2 max
2
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I I
1.2564 t t max 1.9542 1 e t max t
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2 inst 2 max
time [22]. It was found that by fitting the experimental curves with theoretical ones, as
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represented in Fig. 4A, that the mechanism for nucleation and growth of electrodeposited Cu2O film was instantaneous at the beginning of the nucleation, but
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then followed the progressive mechanism during the growth of the Cu 2O films at the deposition potential of -500 mV. Comparatively, at a cathodic potential -600 mV, as depicted in Fig. 4B, the Cu2O film growth was progressive at the beginning of the
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nucleation, but followed the instantaneous mechanism during the growth of the Cu 2O
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films [22,28].
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Fig. 4. Non-dimensional i2/i2max vs t/tmax plots for the electrodeposited Cu2O films at A - 500 mV, B - 600 mV The variation in the measured film thickness with the deposition time at a deposition potential of -500 m V is presented in Table 1. The film thickness was clearly observed to increase with increasing deposition time.
Table 1: Film thickness of Cu2O films at deposition potentials -500 mV for reaction times 30, 60 and 90 min.
3.3. Crystal structure XRD patterns for the electrodeposited Cu2O films using deposition potentials of 500 and -600 mV, with stable pH 12, are presented in Fig. 5A and Fig. 5B for different 9
ACCEPTED MANUSCRIPT deposition times (30, 60, 90 min). The characteristic diffraction peaks for Cu 2O films found in the 2θ range of 25– 80° were assigned to the (110), (111), (200), (220), and (311) planes of the Cu2O structure. The data falls in agreement with the Joint
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Committee on Powder Diffraction Standards (JCPDS) of cubic cupreous oxide (JCPDS card no. 05-0667) [19]. There were no observed diffraction peaks for other phases
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except for the ITO peaks belonging to the substrate. We can therefore conclude that
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single phase polycrystalline Cu2O film can be obtained through the electrodeposition route without any need for thermal treatment. Furthermore, the relative intensities of
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characteristic peaks of Cu2O enhance with increasing deposition time from 30 to 90 min as a result of the increase to the film thickness, as evidenced by the data
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presented in Table 1 [25]. Comparing the Cu2O films deposited at–500mV with that at 600 mV, it was found that films formed with the best crystallinity and thickness is at -
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500 mV for 90 min.
XRD patterns of the Cu2O films deposited at room temperature for deposition potential (A. -500, B. -600) mV with reaction times (30, 60, 90 min), and pH 12.
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Fig. 5.
To explore the effect of pH value on the deposition of Cu 2O films, Fig. 6 presents XRD profiles for films deposited at -500 mV for reaction times of 90 min at different pH values of 5.7 and 10. It is clear that the film formed at pH 5.7 was copper metal, which confirms the voltammogram explanation in the discussion of Fig. 1 The present findings oppose the previous investigation by Tang et al. [25] and Georgieva [29]. Tang et al. claimed that single phase Cu2O was electrodeposited at pH between 5.5 and 6.0 using a deposition potential of -245 mV [21]. Georgieva’s work stated that pure Cu2O film was prepared at pH 9 from a sulphate bath at 60 °C [29]. In this work, the films deposited at pH 10 with 1M citrate and at pH 12 using 0.5 M citrate resulted in Cu/Cu 2O composites as evidenced in Fig. 6. This discrepancy can be attributed to the faster growth rates originating from the increase of OH- intensity. Likewise, more Cu(NO3)2 in the electrolyte will hydrolyze at pH 12 to form Cu(OH)2, which decreases the 10
ACCEPTED MANUSCRIPT concentration of Cu2+ and further reduces to form Cu2O via (7)
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Cu2O + 2 OH- + H2O 2 Cu(OH)2 + 2e-
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where E0 = -0.09 V [25]. Thus, we can conclude that the electrodeposition carried out at
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pH 12 yields good quality single phase of Cu2O thin films.
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Fig. 6. XRD patterns of the Cu2O films deposited using 1 M citrate at -500 mV as potential, 90 min time, for pH (5.7, 10, 12), and another films deposited using 0.5 M citrate at pH 12 under same potential and time.
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3.4. Microstructure
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The deposition time plays a key role in the morphology of the synthesized Cu2O
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films as confirmed in Fig. 7. The increase in the deposition time from 30 to 90 min changes the crystalline morphology for deposited Cu2O film from small cubic structure to
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aggregated polyhedron shape with 4-sided structure for film deposited for 90 min. Meanwhile, the average grain size for the formed particles is in the range from 400 to 700 nm.
Fig. 7. FESEM images of Cu2O films at a deposition potential of -500 mV, pH12, and at deposition times (A) 30, (B) 60, and (C) 90 min Fig. 8 (A) and (B) present FE-SEM images of the formed films at pH 5.7 and 10. The metallic Cu distinctively appears as lumps at the electrode surface with cracks in the deposited film as seen in Fig. 8A [27]. Upon increasing pH to 10, the film forms as 3 faced pyramids with small lumps of copper metal as presented in Fig. 8(B). No difference in the microstructure of the Cu2O films was observed for deposition at pH12 using 0.5 M citrate as shown between Fig.8 (C) and Fig. 7 (C).
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ACCEPTED MANUSCRIPT Fig. 8. FESEM for Cu2O films at deposition potential -500 mV for 90 min as reaction time, 1 M citrate with pH (A) 5.7, (B) 10, and (C) 0.5 M citrate at pH 12 using same potential and time.
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3.5. Optical properties
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The optical properties of the thin films were sensitive to modifications made to
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their composition and structure. Fig. 9 shows a transmittance measurement for Cu2O thin films deposited at -500 mV for various deposition times (30, 60 and 90 min ) as well
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as a film prepared with 0.5 M citrate for 90 min. The loss of transmission exhibited in the spectra for wavelengths lower than ~500 nm was due to the Cu2O band gap [5]. Up to
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nearly 500 nm, the Cu2O films were completely absorbent. The band located at 300-500 nm corresponds to the Cu2O absorption band [5-7]. As film thickness as shown in table 1 and the particle size increased (with deposition time), the band shifted progressively
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to the red region [25]. The absorption band of Cu2O was shape dependent. The
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recorded band positions at 480 and 510 nm, for the samples with the smallest cubes and polyhedron shapes, supports that the particle size, shape, and composition has an
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important role on the exhibited UV-Vis spectra of Cu2O [9]. Features at 290, 357 and 450 nm are most likely related to Van Hove singularities on the electronic structure of Cu2O [5]. The absorption thresholds were found to be at wavelengths 274, 271, 269, 266 nm for deposition times of 30, 60, 90 min, and 0.5 M citrate respectively. The transmittance increases quite strongly between 500 and 850 nm. The value was higher than 25–70 % in the visible range at long wavelengths. This value was better than the optical transmission found in literature for the Cu2O thin films (grown on glass substrates) observed when annealed at 700K in vacuum, which is 72% at 600 nm [7].
Fig. 9. UV-VIS transmission spectra for Cu2O films deposited at -500 mV potential, pH 12 at deposition times 30, 60, and 90 min (1M citrate), another Cu2O films deposited using (0.5M citrate) at 90 min and pH 12.
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ACCEPTED MANUSCRIPT Fig. 10 presents the UV-Vis transmittance spectra of the deposited Cu2O at different pH values of 10 and 12. From this spectra, the broad absorption band assigned at ~550 nm shifts to red region as expected. This effect is ascribed to an enlargement of
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inter atomic spacing and consequently an increase in Cu–Cu internetwork interactions
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[5]. Optical transmittance was found to be between 20–40 % in the visible region.
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Fig. 10. UV-VIS transmission spectra for Cu2O films deposited at -500 mV for 90 min at
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pH 10 and 12.
In order to obtain the direct band gap energy (Eg), absorption coefficients ʋ were
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fitted by the Tauc relation (αhʋ)2= hʋ - Eg [30], where α is the absorption coefficient, hʋ is the photon energy, and Eg is the band gap energy. The direct band gap energy was estimated by extrapolation of the plot (αhʋ)2 vs. photon energy (hʋ) as represented in
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Fig. 11 and Fig. 12. The Eg values were found to be within the expected range for Cu 2O.
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The direct band gap energy for Cu2O films were computed as 2.40, 2.30, and 2.20 eV for deposition times of 30, 60, and 90 min, respectively. The band gap was likewise
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found to be 1.6 eV for films of Cu2O deposited using 0.5 M Na3C6H5O7 at 90 min deposition time. Furthermore, the obtained values were nearly similar to those published before by Murali et al. [10] in which the band gap ranged between 2.0 and 2.4 eV for deposited Cu2O thin films from CuO using magnetron sputtering. Fig. 12 presents the band gap energy of the film deposited at -500 mV for 90 min at pH 10 and 12. Evidently, a more intense Cu–Cu internetwork interaction reduces the Cu2O band gap from 2.2 at pH 12 to 1.7 eV at pH 10 [5].
Fig. 11. Optical band gap energy of Cu2O films deposited at -500 mV potential for deposition times 30, 60, and 90 min, using 1 M citrate at pH 12 , inset (band gap energy for Cu2O films deposited at same conditions using 0.5 M citrate. Fig.12. Optical band gap energy of Cu2O films deposited at -500 mV for deposition time 90 min and pH 10, 12.
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3.6. Electrical properties Fig.13 shows the I-V curves of the Cu2O/ITO heterojunction for different
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deposition times (30, 60, 90 min). To produce an ohmic electrode, gold was evaporated on the top of the Cu2O films. The resulting structure of the photocell could be specified
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as ITO/Cu2O/Au. The electrical resistivity of the Cu2O layers were determined by using the area of the electrode and film thickness. The electrical resistivity for
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electrodeposited Cu2O films was found to decrease with increasing deposition time from 0.854 x 105 Ω-cm at 30 min to 0.221 x 105 Ω-cm at 90 min. the electrical conductivity for
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the Cu2O/ITO heterojunction increased with deposition time. This effect is due to the increase in the carrier density of Cu2O, based on the fact that copper vacancy and
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oxygen interstitials are controlled by the amount of hydroxyl concentration. Therefore, increasing reduction time likewise increased the possibility for the reduction of Cu(OH)2
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into Cu2O as represented in Eg. 7 [18]. These values of resistivity are smaller than other
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values reported in literature which range between 10 7-104 Ω-cm. Remarkably, the Cu2O films prepared in this work have higher conductivities than others prepared at higher temperatures [32,33].
Fig.13. I-V response of Cu2O film deposited at -500 mV, pH 12 for deposition
times 30, 60, and 90 min. The inset shows the change of electrical resistivities for Cu2O films with deposition times
4. Conclusions: Single phase Cu2O thin films were deposited from cupric nitrate solution on ITO substrates using an electrodeposition technique at pH 12. Sodium citrate was acted as complexing agent to prevent copper hydroxide precipitation through alkaline medium. Voltammetric studies were performed to determine the reduction potential of Cu 2O which was in cathodic potential window from 0.4 to 0.7 V . The nucleation mechanism for electrodeposition of Cu2O was found to be instantaneous at the initiation of the 14
ACCEPTED MANUSCRIPT reaction, and then evolved to be a progressive mechanism at -500 mV. XRD results proved that the existence of a cubic Cu2O phase which was deposited at -500 and -600 mV, pH 12 using deposition times 30, 60 and 90 min. The deposition time and pH
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values had significant effects on the surface morphology of the formed oxides. Cu metal was detected at pH 5.7 and 10. As the pH was changed to 12 and the reaction time
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was increased from 30 to 90 min, the microstructure of Cu2O transformed from cubic to
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polyhedron agglomerates at -0.5 V. The maximum transparency for the deposited Cu 2O was found to be 70% when deposited at -500 mV and pH 12, for 30 min deposition time.
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Band gap energy values of the films were varied from 1.60 to 2.40 eV with pH and reaction time changes. The resistivity of the Cu2O films was decreased on increasing
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the deposition time, therefore, it was found to be on the order of 105 Ω-cm, which was
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Acknowledgment
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smaller than aforementioned values depicted in literature at high temperatures.
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All authors are so grateful for financial support from the Ministry of Higher EducationEgypt, Project No. 00120805 to carry electrical properties measurements in Materials science and Engineering department, University of Florida, Florida.
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References:
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[1] Z. Liu, P. You, S. Liu, F. Yan, Neutral-color semitransparent organic solar cells with all-graphene eectrodes, ACS Nano, 9 (2015)12026 – 12034.
SC
RI
[2] J. Meiss, C. L. Uhrich, K. Fehse, S. Pfuetzner, M.K. Riede, K. Leo, Transparent electrode materials for solar cells, Photonics for Solar Energy Systems II, 7002 (2008) 700210-1 - 700210-9.
NU
[3] S. Suhaimi , M. M. Shahimin , Z.A. Alahmed , J. Chyský ,A. H. Reshak, Materials for enhanced dye-sensitized solar cell performance: Electrochemical Application, Int. J. Electrochem. Sci., 10 (2015) 2859 - 2871.
MA
[4] L. Wu, Y. Yu, X. Han, Yuan Zhang, Y. Zhang, Y. Li , J. Zhi, An electroless-platinglike solution deposition approach for large-area flexible thin films of transition metal oxide nanocrystals, J. Mater. Chem. C, 2 (2014) 2266 - 2271. [5] M. Majumder, I. Biswas, S. Pujaru, A.K. Chakraborty. Cuprous oxide thin films grown
TE
741–749.
D
by hydrothermal electrochemical deposition technique, Thin Solid Films, 589 (2015)
AC CE P
[6] Mugwang F.K , Karimi P.K , Njoroge W.K , Omayio O and Waita S.M , Optical characterization of copper oxide thin films prepared by reactive dc magnetron sputtering for solar cell applications, Int. J. Thin Film Sci. Tec. 2 No. 1, (2013) 15 - 24. [7] T. Jiang, T. Xie, Y. Zhang, L. Chen, L. Peng, H. Li , D. Wang, Photoinduced charge transfer in ZnO/Cu2O heterostructure films studied by surface photovoltage technique, Phys. Chem. Chem. Phys.,12 (2010) 15476 – 15481. [8] S. Bijani, R. Schrebler, E. A. Dalchiele, M. Gab_as, L. Martínez, J. R. RamosBarrado, Study of the nucleation and growth mechanisms in the electrodeposition of micro- and nanostructured Cu2O thin films, J. Phys. Chem. C. 115 (2011) 21373 21382. [9] T. Minami, Y. Nishi, and T. Miyata, High-efficiency Cu2O-based heterojunction solar cells fabricated using a Ga2O3 thin Film as N-Type layer , Appl. Phys. Express 6 (2013) 044101 . [10] X. Zhanga, , G. Wanga, A. Gu, H. Wu , B. Fang, Preparation of porous Cu 2O octahedron and its application as L-Tyrosine sensors, Solid State Commun., 148 (2008) 525 - 528.
16
ACCEPTED MANUSCRIPT [11] C.L. Liu, Y.M. Sui, W. B. Ren, B.H. Ma, Y. Li, N. Ning S, Q.L. Wang, Y. Q. Li, J.K. Zhang, Y. H. Han, Y.Z. Ma, C.X. Gao, Electrical properties and behaviors of cuprous
PT
oxide cubes under high pressure, Inorg. Chem. 51(2012) 7001−7003.
RI
[12] B. D. Yuhas, P. Yang, Nanowire-Based All-Oxide Solar Cells, J. Am. Chem. Soc. 131(2009) 3756 – 3761.
SC
[13] J. Luo, L. Steier, M.K. Son, M. Schreier, M. T. Mayer, M. Gratzel, Cu2O Nanowire photocathodes for efficient and durable solar water splitting, Nano Lett.,16 (2016) 1848 – 1857.
NU
[14] K. Chen, C.g Sun, S. Song, D. Xue, Polymorphic compound, Cryst.Eng.Comm. 16 (2014) 5257 – 5267.
crystallization of Cu 2O
MA
[15] X. Zhang, G. Wang, H. Wu, D. Zhang, X. Zhang, P. Li, H. Wu, Synthesis and photocatalytic characterization of porous cuprous oxide octahedra, Mater. Lett. 62 (2008) 4363 – 4365.
TE
D
[16] R.K. Gupta, M. Cavas, Ahmed A. Al-Ghamdi, Z.H. Gafer, F. El-Tantawy, F. Yakuphanoglu, Electrical and photoresponse properties of Al/p-CuFeO2/p-Si/Al MTCOS, Solar Energy, 92 (2013) 1-6.
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[17] X. Jiang, M. Zhang, S.Shi, G. He, X. Song, Z. Jiang et al, Microstructure and optical properties of nanocrystalline Cu2O thin films prepared by electrodeposition, Nanoscale Research Letters 9: 219 (2014) 1- 5. [18] Seung Ki B, Ki Ryong L, Hyung Koun, Oxide p-n Heterojunction of Cu2O/ZnO nanowires and their photovoltaic performance, J Nanomater., 2013 (2013) 1-7. [19] Z. Xiong, M. Zheng, H. Li, W. Shen, Fabrication and optical properties of silicon nanowire/Cu2O nano-heterojunctions by electroless deposition technique, Mater. Lett.112 (2013) 211-214. [20] Y. Song , M. Ichimura, H2O2 Treatment of electrochemically deposited Cu2O thin films for enhancing optical absorption. Int. J. Photoenergy, Article ID 738063 (2013) 16. [21] S. Ikeda, R. Kamai, S.M.Lee, T. Yagi, T. Harada, M. Matsumura, A superstrate solar cell based on In2(Se,S)3 and CuIn(Se,S)2 thin films fabricated by electrodeposition combined with annealing, Sol. Energ. Mat. Sol. C. 95 (2011) 1446-1451. [22] A. E. Saba , E. M. Elsayed , M. M. Moharam, M. M. Rashad , R. M. AbouShahba, Structure and magnetic properties of NixZn1-xFe2O4 thin films prepared through electrodeposition method , J. Mater.Sci. 46 (2011) 3574- 3582. 17
ACCEPTED MANUSCRIPT [23] Ruey- Shin J, Li-Chun L, Rates of metal electrodeposition from aqueous solutions in the presence of chelating agents, Sep. Sci. Technol, 35 (2000)1087–1098.
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[24] S. K. Baek, J. H. Shin, S. W. Cho, H. K. Cho, Electrodeposition of p-type cuprous oxide layers on n-type zinc oxide layers with different electrical resistivities, J. Vac. Sci. Technol. 33 (2015) 02B1041- 02B10416.
RI
[25] Y. Tang, Z. Chen, Z. Jia, L. Zhang, Electrodeposition and characterization of nanocrystalline cuprous oxide thin films on TiO2 films, J. Mater.Lett., 59 (2005) 434–
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438.
NU
[26] N. Fredj and T. D. Burleigh, Transpassive dissolution of copper and rapid formation of brilliant colored copper oxide films, J. Electrochem. Soc. 158 (4) (2011) C104-C110.
MA
[27] W. Septina , S. Ikeda , M. Alam Khan , T. Hirai , T. Harada , M. Matsumura, , L. M. Peter, Potentiostatic electrodeposition of cuprous oxide thin films for photovoltaic
D
applications, Electrochim. Acta 56 (2011) 4882–4888.
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[28]Q. B. Zhang, Y. X. Hua, Electrochemical synthesis of copper nanoparticles using cuprous oxide as a precursor in choline chloride–urea deep eutectic solvent: nucleation
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and growth mechanism, Phys. Chem.16 (2014) 27088-27095. [29] V. Georgieva, , M. Ristov, Electrodeposited cuprous oxide on indium tin oxide for solar applications, Sol. Energ. Mat. Sol. C. 73 (2002) 67–73. [30] M. M. Moharam , M. M. Rashad , E. M. Elsayed , A facile novel synthesis of delafossite CuFeO2 powders. J Mater. Sci. 25 (2014) 1798–1803. [31] Z. Zhang, W.Hu, C. Zhong, Y. Deng, L. Liu, Y. Wu, Preparation of submicron-sized cuprous oxide crystallites by electrodeposition with polyethylene glycol as additive, J. of Cryst. Growth, 354 (2012) 193-197. [32] T. Mahalingam, J.S.P. Chitra, S. Rajendran, M. Jayachandran, Mary Juliana Chockalingam, Galvanostatic deposition and characterizationof cuprous oxide thin flms, J. of Cryst. Growth 216 (2000) 304-310.
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Mater. Sci.17(2006) 519–523.
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20 10
pH 5.7 pH 10 pH 12
A A1
A2
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1 2 3
0 0
1
C
2
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-10 -20
-10 -20
2
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i, mA/ cm
i, mA/cm
2
3
-2.0 -1.5 -1.0 -0.5 0.0
-30 -40 -2.0
-1.5
-1.0
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E, V vs Ag, Agcl, Sat Kcl
-0.5
0.0
0.5
1.0
E, V vs Ag, AgCl, sat. KCl
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Fig. 1. Cyclic Voltammograms (CV) for Cu2O deposited on ITO substrate, from 0.1 M
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Cu(NO3)2, 1.0 M sodium citrate at pH 5.7, 10 and 12. Inset of Fig.1: Indication the cathodic peak (C) in CV of Cu2O.
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10 0.5 M citrate
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-10
-20
-30 -2.0
-1.5
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i, mA/cm
2
0
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A
A2
-1.0
-0.5
0.0
0.5
1.0
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E,V vs Ag, AgCl,Sat KCl
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Fig. 2. Cyclic Voltammograms (CV) for Cu2O from of 0.1 M Cu(NO3)2 ,0.5 M sodium citrate at pH 12
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0 500 mV
-30
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-20
Region I
600 mV
Region III
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-40 -50 Region II
-60
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2 Current density,uA/cm
-10
-70 max
0
5
10
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-80
15
20
25
30
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Time, min
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Fig. 3. Potentiostatic transients for nucleation and growth of Cu 2O films at -500 mV and -600 mV
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0.6
0.6
2
0.2
0.4
1 0.2 A
3
0.0
2
4
6
8
10 0
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0
0.0
t/tmax
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0.8
Experimental Instanteneous progressive
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0.8
0.4
1 2 3
1.0
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2 2
i /i max
Experimental Instanteneous progressive
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1 2 3
1.0
B 2
4
6
8
10
t/tmax
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Fig. 4. Non-dimensional i2/i2max vs t/tmax plots for the electrodeposited Cu2O films at A - 500 mV, B - 600 mV
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2000
30 min
+ (111)
A
60 min
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90 min + Cu O
1600
2
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1200
800 (200) +
400
* * *
0 30
40
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(110) +
(220) +
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Intensity (arb. units)
* ITO
(311) + (222) +
*
50
60
70
80
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2 (°)
B
(111) +
1200
60 min 90 min + Cu O 2
* ITO
(110)
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30 min
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1600
800
(200)
*
+
*
400
(220) +
(311)
* *
+ *
(222) +
0
30
40
50
60
70
80
2 (°)
Fig. 5. XRD patterns of the Cu2O films deposited at room temperature for deposition potential (A. -500, B. -600) mV with reaction times (30, 60, 90 min), and pH 12
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(111)
* ITO 0 Cu
pH 12 , 0.5 M citrate
(200)
(220) 0*
(311)
+
*+
+ +
*
0
*
pH 10, 1 M citrate + * *
0
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pH 5.7, 1M citrate
*, 0 *
*
40
50 2 (°) 60
70
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30
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pH 12, 1M citrate
+
*+
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+
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Intensity (arb. units)
(110)
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+ Cu2O
80
Fig. 6. XRD patterns of the Cu2O films deposited using 1 M citrate at -500 mV as
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potential, 90 min time, for pH (5.7, 10, 12), and another films deposited using 0.5 M citrate at pH 12 under same potential and time..
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Fig. 7. FESEM images of Cu2O films at a deposition potential of -500 mV, pH12, and at
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deposition times (A) 30, (B) 60, and (C) 90 min.
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Fig. 8. FESEM for Cu2O films at deposition potential -500 mV for 90 min as reaction
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time, 1 M citrate with pH (A) 5.7, (B) 10, and (C) 0.5 M citrate at pH 12 using same potential and time.
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300
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Transmittance, %
80
400
500
600
700
3
4
800
900
D
Wavelength, nm
TE
Fig. 9. UV-VIS transmission spectra for Cu2O films deposited at -500 mV potential, pH 12 at deposition times 30, 60, and 90 min (1M citrate), another Cu 2O
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films deposited using (0.5M citrate) at 90 min and pH 12
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NU
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PT
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D
MA
Fig. 10. UV-VIS transmission spectra for Cu2O films deposited at -500 mV for 90 min at pH 10 and 12, 1M citrate.
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1 2 3
30 min 60 min 90 min
2
PT
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RI 1.6
2.0
2.4
MA
h, eV
3
NU
1.2
SC
h eV/m
heV/m
1
3 2
1
h, eV
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D
1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8
Fig. 11. Optical band gap energy of Cu2O films deposited at -500 mV potential for
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deposition times 30, 60, and 90 min, using 1 M citrate at pH 12 , inset (band gap energy for Cu2O films deposited at same conditions using 0.5 M citrate.
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pH 12 pH 10
1.2
1.4
1
1.6
1.8
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2
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heV/m
1 2
2.0
2.2
2.4
2.6
2.8
h, eV
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Fig.12. Optical band gap energy of Cu2O films deposited at -500 mV for deposition time 90 min and pH 10, 12.
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-2
0.8
0.4
30min 60 min 90 min
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1 2 3
-0.8
1 2 -1.0
-0.5
1.0 0.8 0.6 0.4
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-0.4
5
Resistivity, Ohm.cm
x10
RI
3
0.0
0.2 20
40 60 80 100 Deposition time, min
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Current density, A/cm
2
x10
0.0
0.5
1.0
MA
Voltage, V
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Fig.13. I-V response of Cu2O film deposited at -500 mV, pH 12 for deposition times 30, 60, and 90 min. The inset shows the change of electrical resistivities for Cu2O films with deposition times.
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ACCEPTED MANUSCRIPT Table 1: Film thickness of Cu2O films at deposition potentials -500 mV for reaction times 30, 60 and 90 min Time (min) 30 60 90
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D
MA
NU
SC
RI
-500
Film thickness (µm) 0.21 0.41 0.49
PT
Deposition potential (mV)
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ACCEPTED MANUSCRIPT Highlights Cu2O films were electrodeposited at room temperature.
The nucleation mechanism of Cu2O films was studied
The microstructures were synthesis conditions dependent.
The maximum transparency of Cu2O films was 70%.
Low electrical resistivity of Cu2O films was achieved.
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PT
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