Abstract
A detailed comparative study on the synthesis process of coral-like CuO/Cu2O nanorods (NRs) and nanopolycrystals (NPCs) fabricated on Cu foil employing aqueous electrolyte via potentiostatic (POT) and galvanostatic (GAL) modes is discussed. The structural, morphological, thermal, compositional, and molecular vibration of the prepared CuO/Cu2O nanostructures was characterized by XRD, HRSEM, TG/DTA, FTIR, and EDX techniques. XRD analysis confirmed the crystalline phase of the formation of monoclinic CuO and cubic Cu2O nanostructures with well-defined morphology. The average particle size was found to be 21.52 nm and 26.59 nm for NRs (POT) and NPCs (GAL), respectively, and this result is corroborated from the HRSEM analysis. POT synthesized nanoparticle depicted a higher thermal stability up to 600°C implying that the potentiostatically grown coral-like NRs exhibit a good crystallinity and well-ordered morphology.
1. Introduction
Development of efficient energy storage devices has gained a tremendous attention in recent years and. In this scenario, supercapacitors are emerging electrochemical energy storing devices due to its enormous properties like high power density, safe operational quality, fast charging/discharging rate, faster response time, long-term cycle stability and ecofriendliness [1–8]. Transition metal oxide/hydroxide such as CoO [9, 10], RuO2 [11], NiO [12, 13], MgO, CuO [14–18], TiO2 [19, 20], and FeO [21, 22] is the most commonly used electrode material in a electrochemical setup, and they determine the electrochemical performance of the supercapacitors [23]. Among these transition metal oxides, CuO is a multifunctional material, and it has versatile properties like inexpensiveness, low-toxicity, high theoretical capacity (670 mAhg−1), and low electrical conductivity [24–26].
The two forms of Cu, namely, cuprous oxide (Cu2O) that has a bandgap of ~2.17 eV and copper oxide (CuO) that has a bandgap of ~1.2–1.5 eV, are identified as excellent p-type semiconductors. Therefore, these materials can be used as electrodes in supercapacitors [27, 28], infrared photo detectors [29], lithium ion batteries [30], and photovoltaic solar cells [31]. Previous studies have been stimulated by these excellent properties to synthesizeCuO-Cu2O nanoparticles [32]. It is evident from literature reviews that the bicomponent functional materials have improved the properties of super capacitors to greater extent than those of single component with morphologies such as CuO-Cu2O nanowires [33], CuO-Cu2O microspheres [34], nanorods [35, 36], CuO-Cu2O nano-flowers [36], Leaf-Like CuO-Cu2O [37], Cu2O films [38], nanoribbons [39], Cu2O nanocorals [40], and Cu2O polycrystal [41]. The widely used processing routes among the various methods reported in the literature to fabricate CuO-Cu2O are the hydrothermal method, electrochemical deposition [42], electrostatic spray deposition (ESD), sonochemical methods [43], and chemical bath method.
The one-step electrochemical deposition method is adopted for the fabrication of CuO/Cu2O in the present work because (i)It is a low-temperature growth process(ii)Unbinding structure [44](iii)Constant temperature bath maintenance during phase composition(iv)pH dependant(v)Low applied potential to attain high degree of crystalline(vi)High conductivity [45]
Although CuO/Cu2O is abundant in nature, the most challenging issue is that they are highly unstable in aqueous phases. An attempt is therefore made in the present work to study the formation mechanism of CuO/Cu2O nanostructures with various morphologies on Cu surface based on potentiostatic (POT) and galvanostatic(GAL) modes in NaOH, an aqueous electrolyte with [46]. The comparative study fascinatingly helps to identify the prominent structures in which large amount of electrons could be packed into a small surface area that may help promoting the applications of the super capacitors.
2. Experimental Details
2.1. Materials
All the chemicals are of analytical grade and were used without further purification. High-purity copper foil (99.99%, 0.25 mm thick) was purchased from Sigma-Aldrich. NaOH from (Merck, India). Deionized water (DI) was obtained from the Deionizer Millipore Simplicity UV system, alumina powder has been procured from Merck, and acetone (99.5% purity), isopropyl alcohol (98% purity), and ethanol (96% purity) were purchased from Sigma-Aldrich. Hydrochloric acid (37%) was purchased from Emplura Merck. Pt mesh was bought from Sigma-Aldrich.
2.2. Synthesis of CuO/Cu2O Coral-like Nanorods and Nanopolycrystals
Copper foil was uniformly cut into small pieces in the dimension of and then polished with 0.2 μm alumina powder for the removal of the native oxide followed by a DI water rinse. Cu foils were ultrasonically cleaned in acetone, isopropyl alcohol, ethanol, and deionized water consecutively for 15 min and then immersed in 1.0 mold m-3 of HCL (35%) solution to remove surface impurities. The surface of the copper foil turned bright and smooth after the treatment. The precleaned Cu foils were then dried in air, and teflon tape was used to cover for one-sided anodization. In typical synthesis procedure, potentiostatic (POT) and galvanostatic (GAL) anodization was performed using two-electrode cell with Cu foil as the working electrode (WE). Pt mesh was used as a counter electrode (CE), and 1 M NaOH aqueous solution is used as electrolyte. Elico pH meter was used to confirm the alkaline nature (pH =12).
The working electrode and the counter electrode are placed at a constant distance of 3 cm to provide better dissemination of heat formed at the bottom of the pores. The distance between the WE and CE is maintained in order to attain a highly ordered pore diameter, wall thickness, and rod dimension. During the anodization process, a constant voltage of about 20 V and a constant current density of about 10 mA cm-2 were set to using Keithley 2400 as DC power source for both POT and GAL approaches, respectively. The anodization took place for 480 seconds at room temperature (∼28°C). Once the anodization time is complete, the foil was cleaned with ethanol to obtain the exfoliated nanoparticles. Subsequently, the amorphous samples were then crystallized by annealing at 350°C for 1 hour. Finally, a black colored uniform film on the Cu foils is obtained which was taken for further characterizations.
2.3. Instrumentation
The structure, phase, and crystalline of the CuO/Cu2O of POT and GAL were investigated by the X-ray diffraction system (Bruker XRD 3003 TT) using monochromatic nickel filtered CuKα () radiation. The Fourier transform infrared (FT-IR) spectral analysis was carried out using a Perkin Elmer Spectrum Two. Scanning electron microscope (SEM Quanta 200 FEG) was employed for morphological study. The instrument is attached with an energy dispersive X-ray spectrometry (EDX) for performing crystalline information from the few nanometer depths of the material surface.
3. Results and Discussion
Figure 1 illustrates the one-step electrochemical anodization process of fabricating the CuO/Cu2O on Cu electrode. Under the effect of POT and GAL, the copper substrate was made to oxidize and release Cu2+ and Cu+ ions into the NaOH solution respectivley, while OH- in the solution captured the Cu2+ and Cu+ ions to form Cu(OH)2 and CuOH nuclei with the following reactions [31],
During the process of anodization, the Cu surface on interaction with the OH– ions that form the electrolyte under the influence of potential will change from the brown color to a faint blue color due to the formation copper II hydroxide. Cuprate ions in the form of the complex Cu(OH)2−4 were generated at the substrate–electrolyte interface, which creates nucleating sites on the copper substrate. Because of the negative charge on Cu(OH)2−4, it gets attracted rapidly toward the copper anode, where this combination precipitates the creation of the Cu(OH)2 film on Cu at the anode. The obtained copper hydroxide film being crystalline is engineered to enhance the band gap or the phase formation by calcination in the presence of oxygen to yield a black precipitate indicating the presence of both CuO and Cu2O [6, 45, 47].
XRD studies were carried out in order to understand the structural property of the prepared samples. The phase identification of a crystalline material and crystal structure of the as-prepared CuO-Cu2O POT and GAL were analyzed by the XRD (Figure 2). The four peaks marked with diamond shape which can be indexed to the (111), (200), (111), and (-311) planes are presented in (Figure 2(a) and 2(b)) of the cubic Cu substrate (JCPDS No. 01-1241). The peaks are marked with clover shape which can be perfectly indexed to (002), (111), (-202), (202), (-311), and (220) planes of monoclinic CuO (JCPDS no. 89-2530), while the peaks marked with spade shape that can be indexed to the (111), (200), and (220) are the planes of the cubic Cu2O (JCPDS no.77-0199), and no other crystalline peaks of impurities were observed which indicates that the as-prepared sample was highly pure. By using the Debye Scherrer formula, we could find average crystallite size () is calculated for both POT and GAL and was found to be ~21.52 nm and~26.59 nm, respectively. It is clearly evident that the sample obtained by the potentiostatic anodization (POT) shows small crystal size than that of the galvanostatic (GAL) method.
Figure 3 depicts the morphologies of nanostructures of CuO/Cu2O investigated with scanning electron microscopy (SEM). Figures 3(a) and 3(b) exhibit coral-like CuO/Cu2O nanorods (NRs) which are formed during the potentiostatic modes of anodization carried out at the rate of about 20 V, whereas (Figures 3(c) and 3(d)) represent high-magnification images of CuO/Cu2O nanopolycrystals (NPCs) formed during galvanostatic modes of anodization at the rate of about 10 mA cm-2. The coral-like nanorods have a pointing tips that are around 26.9 nm and 29.3 nm presented in Figure 3(b) indicating a well-ordered morphology, demonstrating a controlled-size and rod-like structure which may help to enhance for the supercapacitor applications [48]. Figure 3(d) represents the size and shape of the CuO/Cu2O nanopolycrystals (NPCs) that are around 50.3 nm and 57.2 nm which are in general larger in size compared to its counterpart.
(a)
(b)
(c)
(d)
The thermal stability of the nanomaterials was determined by thermogravimetry and differential thermal analysis (TG/DTA). The TG/DTA traces of CuO/Cu2O nanoparticles are shown in (Figure 4). A small weight loss appears room temperature to 100°C recognized due to dehydration of surface moisture. The POT samples gradually lose weight but is almost stable until 600°C with an estimated weight loss of 10%; on the other hand, GAL depicted a weight loss of 5% at 200°C and another 5% at 500°C and a slope nearing 20% at 600°C. From the TG graphs, it is clearly evident that the POT route synthesized samples were more stable at higher temperature than the GAL route.
FT-IR spectra of the CuO/Cu2O nanostructures prepared in different modes are shown in (Figure 5). The broad absorption peaks at 3444 cm−1 and 3437 cm−1 belong to the symmetric or asymmetric stretching of O-H bonds. The peaks were observed at 1630 cm−1 and 1629 cm−1 indicate the formation of CuO nanoparticles. The stretching vibration of Cu-O bonds of Cu2O nanoparticles is found at 1100-1400 cm−1and shown in (Figure 5). The two infrared absorption peaks reveal the vibrational modes in the range of 500-700 cm−1. The peaks observed at 530 cm−1 represent the formation of CuO/Cu2O (NRs) and 554 cm−1 for CuO/Cu2O (NPCs), respectively. Therefore, the metal-oxygen frequencies observed for CuO nanoparticles are in close agreement with those reported in the literature. Figure 6 shows the energy dispersive X-ray (EDX) analysis of POT (NRs) and GAL (NPCs) annealed at 350°C for 1 hr. Graphical representation reveals the presence of copper (Cu) and oxygen (O) elements in nanoparticals, and the data indicate that the nanocomposites are nearly stoichiometric. The weight percent of copper and oxide calculated from EDX analysis is shown in Figures 6(a) and 6(b). No other elemental impurities are detected in the EDX spectra. This result confirmed that the formation of as-prepared metal oxides was CuO/Cu2O nanoparticals. Table 1 describes a list of materials fabricated by a similar procedure.
(a)
(b)
4. Conclusion
In this paper, we have demonstrated a facile and cost-effective potentiostatic and galvanostatic modes of the anodization method to synthesize the coral-like CuO/Cu2O nanorods and CuO/Cu2O nanopolycrystals on copper foil. Fascinatingly, the comparative studies from the XRD patterns revealed that the galvanostatically anodized CuO/Cu2O NPCs have a chaotic structure and large crystallite size on comparison with POT mode, whereas potentiostatically anodized coral-like NRs have a well-layered structure and binder less and smaller crystallite size as compared to the galvanostatic technique from HRSEM analysis. The thermal studies indicate that the POT mode fabricated samples were found to be more stable than the GAL mode. EDX analysis depicted a higher purity of both the samples.
Data Availability
The data supporting this work is available from the corresponding author upon request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.