Synthesis of hierarchical copper oxide composites prepared via electrical explosion of the wire in liquids method

Colloids and Surfaces A: Physicochem. Eng. Aspects 482 (2015) 710–717

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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

Synthesis of hierarchical copper oxide composites prepared via electrical explosion of the wire in liquids method Eunju Park a , Hyung Wook Park b,∗ , Jinae Lee c a School of Materials Science and Engineering, Ulsan National Institute of Science and Technology, UNIST-gil 50, Eonyang-eup, Ulju-gun, Ulsan 689-798, Republic of Korea b Department of Mechanical Engineering, Ulsan National Institute of Science and Technology, UNIST-gil 50, Eonyang-eup, Ulju-gun, Ulsan 689-798, Republic of Korea c College of Medicine, Yonsei University, 50 Yonsei-Ro Seodaemun-gu, Seoul 120-752, Republic of Korea

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• We synthesized copper oxide composite via electrical explosion of the wire method. • The phase and shape of copper oxide particles can be controlled via additives. • Leaf-like, flower-like, and rod copper oxide structures synthesized via a single process. • The band gap of copper oxide (CuO/Cu2 O) was approximately 1.85–2.10 eV.

a r t i c l e

i n f o

Article history: Received 2 May 2015 Received in revised form 12 July 2015 Accepted 13 July 2015 Available online 18 July 2015 Keywords: Hierarchical copper oxide composites In situ synthesis Electrical explosion of wire in liquid (EEWL) Bandgap widening

a b s t r a c t Hierarchical copper oxide particles have attracted recent research interest, due to their structure, properties, and potential applications in catalysis, gas sensing, solar cells, biosensors, and lithium ion batteries. In this study, we synthesized hierarchical copper oxide (CuO/Cu2 O) structures with various morphologies via electrically induced explosions of Cu wire in a liquid. This method does not require a surfactant or template and can be completed at room temperature. In this process, particle size and shape can be controlled by varying the energy supplied in the solution during the explosion. Leaf-like, flowerlike, and rod structures were generated, composed of needle-like building blocks, via self-assembly and Ostwald ripening. Particle structure and morphology were investigated using X-ray diffraction, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and field emission scanning electron microscopy. The resistance of copper oxide film prepared via spin coating was measured. The optical band gap was determined using ultraviolet/visible spectroscopy. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Copper oxide is separated into two phases, cupric oxide (CuO) and cuprous oxide (Cu2 O). Cuprous oxide (Cu2 O) and cupric oxide

∗ Corresponding author. Fax: +82 52 217 2309. E-mail address: [email protected] (H.W. Park). http://dx.doi.org/10.1016/j.colsurfa.2015.07.029 0927-7757/© 2015 Elsevier B.V. All rights reserved.

(CuO) are p-type semiconductors with a bandgap of approximately 2.2 eV and 1.2 eV, respectively [1], depending on the method of fabrication and stoichiometry. However, CuO has also been reported to have n-type conductivity [2]. Due to their high absorption coefficients, favorable electronic structures, and low-cost producibility, Cu2 O and CuO are widely used in a variety of applications (e.g. catalysis, solar cells, sensors, magnetic data storage, lithium batteries [3–9]). For example, Cu2 O/CuO nanocomposite has been used as

E. Park et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 482 (2015) 710–717

an effective photocathode for water splitting [10]. To date, copper oxide research has focused on shape-controlled hierarchical copper oxide structures including nanorods, nanoplates, microflowers, and nanowires. Hierarchical copper oxide structures have attracted much interest because of their large surface area, electrical conductivity, and capacitance, which arise from their unique structural properties [11–13]. The properties of hierarchical copper oxide depend strongly on the microstructure (i.e. orientation, size, and morphology). Hierarchical copper oxide structures have been synthesized by various several techniques, including the hydrothermal method, the facile solution oxidative methodon, and electro-deposition, all of which are generally considered “bottom-up” approaches [14–17]. With such With regards to these approaches, it is important to understand the nuclei formation and growth mechanism. In a study performed by Jiang et al. [18], hydrothermal synthesis of flower-like, plume-like and spindle-like CuO nanostructures was reported, which allows structure control by varying the reaction parameters, including the reactant concentration, the temperature, and the reaction time. Chen et al. [19] prepared CuO nanorods with a small diameter via a low-temperature solution method. Mukherjee et al. [16] used an electrochemical technique, to convert the cubic shape of CuO thin films to a nanowhisker-like shape post-annealing. Most of these synthesis methods require a precursor, high temperatures, and a surfactant to promote self-assembly. However, methods involving electrical explosion of wires in liquids (EEWL) can synthesize a variety of copper oxide shapes. Fabrication by this method can be finished within a second, because it does not require a precursor or additional heating steps. EEWL is a single-step physical synthesis technique that can be used to fabricate metal oxide nanostructures [20]. This method involves the use of a pulsed electric current to heat wire, synthesizing high-purity nanoparticles without a surfactant. In this process, a metal wire is heated using a pulsed electric current. As the wire heats, it melts, at which time thermal expansion and evaporation of the wire surface occurs. Particle size is controlled by adjusting deposited energy in the wire. The required deposited energy is determined by all of the following: stored energy in capacitor, properties of the materials and wires, and the type of solution. During this process a shortlived solid–liquid plasma is generated, in which the temperature can reach approximately 4000 ◦ C. The resulting vapor-phase metal condenses at the interface with the surrounding liquid. The nucleation and growth of the particles occurs over a very short time period resulting in a variety of particles synthesized, including pure metals and metal oxides. In this study, we synthesized hierarchical copper oxide composites via EEWL. We observed a varied effect of Cu+ and Cu2+ pH on morphology. In detail, we explain the mechanism(s) of formation as well as the properties of copper oxide composites.

2. Experimental setup 2.1. Synthesis of copper oxide composites The EEWL system was a simple RLC circuit, as shown in Fig. 1. The capacitance C was proportional to the applied voltage, and the resistance R and inductance L were determined by the materials’ properties and dimensions of the wire. All copper oxide particles were produced using the same electrical conditions. Pure copper wire 0.1 mm in diameter was placed between the two electrodes. The capacitor was charged to 3 kV and allowed current to flow through the wire when the switch was closed. High temperature plasma was generated by the electrical energy deposited in the wire using a pulsed current and then condensed by the base fluid.

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Fig. 1. Schematic diagram of the EEWL system. Table 1 Conditions used for copper oxide particle synthesis. Contents

Values

Capacitance Charging voltage Material Wire diameter/length Base liquid Additives Liquid volume

30 ␮F 3 kV Copper wire (99%) 0.1 mm/30 mm Deionized water NaOH, NH3 ·H2 O 200 ml

Deionized (DI) water was used as the base liquid and 5, 10, 15 mL of 0.1 M sodium hydroxide (NaOH) and 25-30% ammonia solution (NH3 ·H2 O) were added for a total volume of 200 mL. Synthesis conditions are listed in Table 1. All copper oxide solutions were prepared using the same number of explosions and 0.001 vol% copper oxide. 2.2. Characterization of copper oxide composites The structure of the copper oxide particles was analyzed using X-ray diffraction (XRD) with a high-power X-ray diffractometer (Rigaku, Japan) at a scan rate of 0.02◦ s−1 , a range of 20–80◦ , and a Cu-K␣ source ( = 1.5406 Å). The morphology of hierarchical copper oxide particles was observed using field emission scanning microscopy (FE-SEM) (FEI Nanonova230) and high-resolution transmission electron microscopy (HR-TEM) (JEOL JEM-2100F). Elemental compositions were measured by X-ray photoelectron spectroscopy (XPS, K-alpha, UK) using an Al K␣ X-ray source. Electrical resistance was measured using a high-resistance digital multimeter (6517b, Keithley Instruments). Copper oxide films were formed on the glass by spin coating using the copper oxide solution prepared via EEWL. Five layers of film was deposited on ultraviolet (UV)-treated glass for 40 min, followed by coating for 40 s at 1200 rpm and annealing for 10 min at 110 ◦ C. Optical bandgap was determined by ultraviolet/visible (UV–vis) absorption spectroscopy (Agilent Cary5000) using a wavelength range of 250–1000 nm. 3. Results and discussion 3.1. Synthesis of hierarchical copper oxide structure All samples were prepared via EEWL and only the concentration of NaOH or NH3 H2 O was varied. Fig. 2 displays photographs of the copper oxide colloidal dispersions. The aqueous copper oxide colloidal dispersion was brown and changed to blue when ammonia

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Fig. 2. Photographs of (a) copper oxide colloid dispersions immediately following synthesis and (b) after one day.

was added, due to the more stable dispersion and low concentration of copper oxide particles. However, addition of ammonia caused copper oxide particles to agglomerate, which settled were agglomerated and settled after one day. Fig. 3 shows XRD patterns of the copper oxide composites synthesized via EEWL, which revealed a monoclinic structure (JCPDS 03-1005, JCPDS 05-0661, a = 4.684 Å, b = 3.425 Å, c = 5.129 Å, ˇ = 99.47◦ ). Diffraction peaks, shown in Fig. 3(a) and (b), indicated that the copper oxide particles were well-crystallized and did not possess impurities. Fig. 3(c) shows no diffraction peaks, for reasons that are currently unknown. Fig. 4 shows FE-TEM and TEM images of the copper oxide nanoparticles produced in water as well as the histograms of particle size distribution. Nanoparticles were 5–10 nm in diameter and approximately spherical, as shown in Fig. 4(b); however, some agglomeration occurred due to particles’ large surface area. Mean diameter of the copper oxide nanoparticles was 163.3 nm as measured via dynamic light scattering (DLS), a standard light scattering technique for measuring the size of nanoparticles (Fig. 4(c)). Nanoparticles randomly move because of collisions between particles and solvent molecules, a process known as Brownian motion. DLS can measure the speed of nanoparticles in Brownian motion. It is important to note, that a difference in particle size will be reported for DLS compared to TEM, a method in which particles can be individually measured.

Fig. 3. XRD pattern of (a) water-based copper oxide nanoparticles, (b) water and NaOH-based, and (c) water and NH3 OH-based copper oxide microspheres synthesized via EEWL process.

Fig. 4. Morphology and size distribution of copper oxide nanoparticles in the water: (a) SEM image, (b) TEM image, and (c) size distribution measured via DLS.

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Fig. 6. Morphologies of copper oxide composites utilizing different concentrations of NH3 ·H2 O: (a) 5 mL, (b) 10 mL, and (c) /(d) 15 mL. Table 2 Atomic concentration of CuO/Cu2 O composite in XPS spectra.

Cu2p O1s C1s Na1s Fig. 5. Morphologies of copper oxide composites obtained with (a) and (b) 5 mL of NaOH, (c) and (d) 10 mL of NaOH, and (e) and (f) 15 mL of NaOH.

Fig. 5 shows FE-SEM images of copper oxide prepared with various concentrations of NaOH. Addition of 5 mL of 0.1 M NaOH solution resulted in nanoparticles aggregation into needle-like structures, as shown in Fig. 5(a) and (b). Addition of 15 mL of NaOH resulted in the formation of nanoparticles with leaf-like and flowerlike structures, composed of needle-like building blocks whose size increased with increasing concentrations of NaOH. Fig. 5(c) and (e) show nanoparticles with leaf-like structures, which had smooth walls and a high aspect ratio. Hierarchical copper oxide nanoflowers composed of nanosheet building blocks were formed when 10 and 15 mL of NaOH were added, as shown in Fig. 5(d) and (f), respectively. Fig. 6 shows FE-SEM images of the copper oxide samples prepared with 5, 10, and 15 mL of NH3 ·H2 O. Morphology differed substantially between those prepared with NaOH versus NH3 ·H2 O. Addition of 5 mL of NH3 ·H2 O resulted in self-assembled foil structures, while 10 mL yielded hierarchical flower-like composites, consisting of leaf-like building blocks formed via a step-wise process: aggregation of nanoparticles into leaf-like particles, which self-assembled into flower-like structures, followed by Ostwald ripening into larger flower-like copper oxide structures. Fig. 6(c) and (d) show nanoparticle morphology with the addition of 15 mL of NH3 ·H2 O. Two different regions formed: one region was flowerlike and the second was 100-nm-diameter hexagonal copper oxide rods, which were approximately 916 nm long. The foil structure of nanoparticles shown in Fig. 6(a) was similar to results from Liu et al. [21]. Fig. 6 shows the flower-like copper oxide structures composed of hierarchical two-dimensional (2-D) nanosheets. However, we also observed the formation of spherical structures using different alkaline solutions (NaOH and NH3 ·H2 O).

No additives (%)

NaOH (%)

NH3 ·H2 O (%)

21.2 46.06 32.74 –

1.84 27.88 68.41 1.87

9.86 38.32 51.82 –

In Figs. 5 and 6, we observed that the various copper oxide structures were dependent on solution pH, where a higher pH, led to more compact structures. Fig. 7(a) and (b) show TEM images of copper oxide composites synthesized in NaOH and NH3 ·H2 O solutions, respectively, which were composed of crystalline nanoparticles. The absence of a diffraction peak observed in the Fig. 3(c) was attributed to a smaller number of particles. The XPS spectrum of the CuO/Cu2 O composite is presented in Fig. 8. In Fig. 8(a), the four peaks indicate the presence of Cu+ and Cu2+ , which contributed to the formation of CuO or Cu2 O, respectively. The addition of NaOH and NH3 ·H2 O shifted the binding energy of the Cu 2p3/2 peak to a higher binding energy (1.2 eV and 0.4 eV) compared to non- alkaline samples. The oxidation states of Cu can be distinguished by satellite peaks of Cu2p; CuO has a satellite peak at approximately 943 eV. Peak positions and intensities relate to the different compositions. O1s peaks, representing byproducts of copper oxide synthesis, of three samples are presented in Fig. 8(c); the inset indicates the distributable energy band of oxide (O2− ), hydroxide, or hydroxyl groups (OH− ), as well as water (H2 O) [22]. The O1s spectrum in the control sample was composed of two overlapping peaks corresponding to O2− and OH− . Copper oxide particles to which NaOH and NH3 ·H2 O were added displayed observed peaks for OH− and H2 O. For samples to which NaOH was added, peaks for H− and H2 O were significantly stronger than those observed in samples with NH3 ·H2 O. Detailed atomic concentrations of the copper oxide composite are listed in Table 2. 3.2. Growth mechanism Nanoparticle size distribution was affected by the absence of alkaline additives as shown in the TEM images in Fig. 9(b). Nanoparticles produced via EEWL without additives aggregated

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Fig. 7. TEM images of copper oxide composites obtained with (a) 10 mL NaOH and (b) 10 mL NH3 ·H2 O.

and stabilized by thermodynamic growth. Additives led to aggregation in specific orientations due to attachment of functional groups. The chemical reactions during the formation of the copper oxide structures were as follows: Cu2+ + 4OH− → [Cu(OH)4 ]2− [Cu(OH)4 ] Cu

2+

2−

(2)

2+

(3)

→ CuO + 2OH +H2 O

+ 4NH3 → [Cu(NH3 )4 ]

[Cu(NH3 )4 ]

2+

−

(1)

−

→ OH → CuO + 4NH4 OH

(4)

Copper wire and solutions were ionized, generating plasma during explosion of the wire, as shown in Fig. 9(a). Due to Cu dissociation, Cu+ and Cu2+ ions are found within the plasma. When NaOH is added, Cu2+ ions combine with OH− to form [Cu(OH)4 ]2− (Eq. (1)). The concentrations of Cu2+ and OH− determine the formation of [Cu(OH)4 ]2− and hence the CuO structure. Previously, it was reported that pH changes in response to the molar ratio of Cu2+ to OH− , and that a higher pH can accelerate the transformation process [23,24]. Primary CuO nuclei were condensed into nanoparticles, which aggregate spontaneously in liquids, forming

Fig. 8. XPS spectra of the copper oxide particles produced using different liquid conditions: (a) Cu2p spectrum, (b) C1s spectrum, (c) O1s spectrum, and (d) Na1s spectrum.

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Fig. 9. Schematic diagrams showing (a) explosions of the wire and (b) formation of copper oxide particles.

needle-like structures, because of the large surface area and van der Waals forces (Fig. 9(b)). Needle-like structures grew on the leaf-like structures via Ostwald ripening [25,26]. When NH3 ·H2 O was added, synthesis proceeded via [Cu(NH3 )4 ]2+ (Eq. (3)). A larger ratio of NH3 to Cu can accelerate crystal formation and favor a specific orientation [27]. According to Eqs. (2) and (4), the concentration of OH- will have a significant effect on the morphology of CuO structures. Greater pH translates to higher concentrations of OH− ions which are attracted to positively charge Cu+ ions. These ionic attractions promote a higher growth rate via Cu O bonding, which results in oriented structures. At pH 9–10, CuO needle-like structures assemble and become leaf-like structures. At pH 11, these leaf-like structures create a microsized hierarchical structure (Fig. 6(b)). The CuO structure grows in a certain direction as the OH- concentration increases (pH > 11), resulting in the formation of a rod structure. The Cu+ in plasma formed [Cu(OH)4 ]− and [Cu(NH3 )4 ]+ as an intermediate phase as shown in Eqs. (3) and (7), respectively. Cu2 O particles were formed in the intermediate phase and had a strong tendency to reduce to CuO. During these reactions, three-dimensional (3-D) structures were synthesized via anisotropic growth, as shown in Fig. 6. All processes occurred over a short time period, corresponding to the duration of the explosions. 3.3. Electrical properties Variation in the electrical resistivity of hierarchical copper oxide films is shown in Fig. 10. The resistance of copper oxide on glass was compared to that of pure glass because it is difficult to spin-coat copper oxide at sufficient thickness for measurement of resistance. Resistance increases with increasing particle size; the resistance

Fig. 10. Resistivity variation of CuO/Cu2 O composites at room temperature.

of the copper oxide nanoparticles synthesized in water was higher than that of microspheres synthesized by adding 10 mL NH3 ·H2 O. The increase in resistance is likely due to the nanoparticle’s high surface area-to-volume ratio. Resistance was also influenced by the copper oxide phase composition, where leaf-like 2-D copper oxide structures had the largest and hierarchical 3-D structures had the lowest resistance values.

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Fig. 11. UV–vis absorption spectra of CuO/Cu2 O composites synthesized in (a) water, (b) plus NaOH, and (c) plus NH3 ·H2 O.

Fig. 12. Variation of (a), (c), (e) (␣h)2 -h curves and (b), (d), (f) (␣h)0.5 -h curves for CuO/Cu2 O composites synthesized under different liquid conditions: (a and b) in water, (c and d) in NaOH solution, (e and f) in NH3 ·H2 O solution.

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3.4. Optical properties The UV–vis spectra of copper oxide colloids dispersed in different alkaline solutions are shown in Fig. 11. Absorption bands for copper oxide nanoparticles are reported to be within the range of 500–600 nm [28,29], because of surface plasmon resonance, which is strongly dependent on particle size and morphology. A broad absorption peak for the copper oxide nanoparticles was observed at 493 nm, which should blue-shift as particle size decreases due to quantum size effects [30]. The bandgap of the hierarchical copper oxide composites was estimated from the Tauc’s relation [31–33]; i.e.



˛h = h − Eg

n

,

(5)

where ␣, h, , and Eg are the absorption coefficient, Planck constant, photon frequency, and optical band gap of the material, respectively. The parameter n takes the value of 1/2, 3/2, 2, or 3 depending on whether the transition is direct allowed, direct forbidden, indirect allowed, or indirect forbidden, respectively [34]. Fig. 12 shows the variation of (˛h)n versus photon energy, h. The n value is reported to be 1/2 and 2 [35]. The direct bandgap, assuming ˛ = 0, of the copper oxide composite synthesized in water was 1.04 eV, which red-shifted to approximately 1.99 eV, compared to a bulk value of 3.25 eV [36]. The indirect bandgap of copper oxide composites was not determinable. Fig. 11(b) shows the absorption spectra of the leaf-like and flower-like copper oxide structures formed in aqueous NaOH solution. The maximum absorption peaks were observed at 370, 615, and 624 nm for NaOH quantities of 5, 10, and 15 m:, respectively. With 10 and 15 m: of NaOH, two plasmon bands were observed at 370 and 620 nm, respectively. The direct bandgap was approximately 1.95–2.10 eV. Indirect bandgaps of copper oxide composites synthesized in NaOH solutions were not determinable as shown in Fig. 12(d). The higher energy band corresponds to oscillations of electrons perpendicular to the long axis. Fig. 11(c) shows absorption spectra of the hierarchical copper oxide structures of various morphologies formed in NH3 ·H2 O solutions. The maximum absorption peaks observed were 627, 606, and 615 nm for structures formed in 5, 10, and 15 mL of NH3 ·H2 O, respectively. Direct and indirect bandgaps were approximately 4.02–4.75 eV and 3.56–4.56 eV, as shown in Fig. 12(e) and (f). Direct and indirect bandgaps had a blue shift of approximately 0.75–1.5 eV and 2.11–3.11 eV as compared to the bulk values of 3.25 and 1.45 eV, respectively. Synthesized copper oxide particles were approximately 1 ␮m in size; this increase in size compared to those formed in water led to the red shift. NH3 in copper oxide colloids increases the stability of the complex and separation of the energy levels [37]. Structures of copper oxide enhanced the valence position and conduction band edge. The optical bandgap energy strongly depended on the structure and composition of copper oxide composites. 4. Conclusions Copper oxide structures with various morphologies were synthesized via EEWL, with no surfactants or templates. Cu/CuO/Cu2 O synthesized in water were nanometer-scale and spherical. Hierarchical copper oxide particles were obtained by adding NaOH or NH3 ·H2 O to obtain an alkali solution. Cu+ and Cu2+ ions were combined with OH− and NH3 to form agglomerates with various morphologies. Leaf-like 2-D copper oxide structures were synthesized by adding NaOH, and increasing the quantity of NaOH accelerated growth of the agglomerate structures. The addition of NH3 ·H2 O resulted in the nucleation and growth of hierarchical 3-D copper oxide structures. Uniform copper oxide rods were synthesized by adding 15 mL of NH3 ·H2O and the size was controlled by varying the amount of energy provided to the wire via Joule heating.

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