Rapid Sonochemical Synthesis and Characterisation of Copper Oxide Nanoparticles from Schweizer's Reagent

The present work reports the results of a study that investigated a sonochemical approach to synthesis copper oxide nanostructures from tetraamminediaquacopper dihydroxide. (Schweizer's Reagent). Ultrasonic irradiation ranging from 0 to 400 W over time periods ranging from 5 min to 15 min were performed on Schweizer's reagent. UV-visible spectroscopy has shown that copper nanoparticles are initially formed but soon oxidize in the ultrasonically treated reagent. Formation of copper oxide nanostructures was indicated by the original blue colour of the reagent turning brown at particular power settings. XRD analysis confirmed the presence of both copper (I) oxide (Cu2O) and copper (II) oxide (CuO) at the end of the ultrasonic treatment. SEM microscopy revealed particles sizes ranged from 200 nm up to 1μm and were predominantly granular and agglomerated in nature.


Introduction
Copper is an abundant metal with excellent thermal and electrical properties and because of its lower cost compared to noble metals such as gold and silver it has been extensively used in numerous industrial applications. However, in recent years there has been significant interest shown in developing onedimensional (1-D) nanometre scale copper oxide materials. There are two forms of copper oxide, the first is the red coloured cuprous oxide [copper (I) oxide, Cu2O] and the second is the black coloured cupric oxide [copper (II) oxide, CuO]. With the passage of time and in the presence of moist atmospheric air Cu2O with time decomposes to CuO. Both copper oxides in bulk form tend to be antiferromagnetic, ferroelectric and are p-type semiconductors with band gap of 1.2 eV for CuO and 2.137 eV for Cu2O respectively. Although other studies have shown that the band gap for CuO can range between 1.3 and 1.7 eV [1]. The band gap of both copper oxides makes them potential candidates for a number of applications such as solar cells, sensors electronic cathode materials, high temperature superconductors, optoelectronic devices and catalytic materials for converting hydrocarbons into carbon dioxide and water [2][3][4]. In general nanometre scale materials have significant properties differences compared to their bulk counterpart and copper oxide one-dimensional structures such as nanowires also display unique properties. The properties are largely dependent the structures size, shape and surface chemistry. These unique properties are derived from the extremely large surface areas and large aspect ratios, which tend to promote surface interactions and processes. This makes onedimensional copper oxide structures ideal candidates for chemical sensors [5,6], photovoltaic devices and solar cells [7,8]. And recent in years there has been many studies carried out using copper oxide nanowires to develop solar cells [9][10][11] and a wide range of chemical sensors [12], gas sensors [13], photo-electrochemical hydrogen generation [14] and catalytic oxidation of methane [15].
A wide range of techniques have been developed and used globally by researchers to synthesize copper oxide nanostructures. Techniques used to manufacture copper oxide nanostructures include wet chemical methods [16,17], direct plasma [18], electrochemical [19], hydrothermal [20] and thermal oxidation [21,22]. Among the wide range of synthesis techniques currently available, the use of ultrasound irradiation for the production of nanometre scale materials offers some unique reaction conditions which cannot be reproduced by the above-mentioned techniques.
During ultrasonic irradiation of a fluid medium, the alternating compressive and expansive acoustic waves create bubbles, which subsequently grow and ultimately undergo an implosive collapse. Throughout the growth period, the bubble accumulates ultrasonic energy until a critical size is reached, usually a few micrometres in size, before the bubble collapses. During the bubble implosion, the concentrated energy stored in the bubble is released within a very short duration producing a localize region of extremely high pressures and temperatures. The short-lived implosion can produce pressures of ~1000 bar and temperatures of ~5000 K, with heating and cooling rates greater than 1010 K s -1 [23]. Interestingly, it is the transient nature of the acoustically produced cavitations, produced by the ultrasonic irradiation that generates the unique effects of ultrasound.
During the last decade there has been significant progress in synthesizing a wide variety of nanometre scale materials via sonochemical processes using a variety of ultrasound based techniques [23]. The sonochemical approach has a number of features that clearly distinguish it from conventional nonultrasound based techniques. For most materials these features include: (1) faster primary nucleation; (2) relatively easy nucleation in materials that are usually difficult to nucleate otherwise; (3) the initiation of secondary nucleation; and (4) the production of smaller, purer crystals that are more uniform in size. It is due to these advantageous features that have made sonochemical synthesis an attractive technique for producing nanometre scale materials.
In the present paper, we report the synthesis of copper oxide nanostructures via a sonochemical technique that uses ultrasound irradiation as the driving mechanism in the process for Schweizer's reagent using ammonia as a base. The copper oxide nanostructures formed were subsequently investigated using advanced characterisation techniques such as X-ray diffraction (XRD) analysis to confirm the presence and type of copper oxides present, scanning electron microscopy (SEM) to determine particle size and morphology and UVvisible spectroscopy and Fourier Transform Infrared Spectroscopy (FT-IR) were used to monitor the formation of the nanostructures. and Bio Lab supplied the (25% w/w) analytical reagent grade Ammonia solution. The glass microscope slides were supplied by Sail, while all aqueous solutions were prepared with Milli-Q ® water (18.3 MΩ) produced by Thermo Scientific Water System (Barnstead Ultrapure System D11931).

Synthesis of copper oxide nanostructures
The procedure begins by preparing the glass microscope slides, which are used as sample substrates. Each glass slide was placed into Transtek ® Systems Soniclean 30A (20 W) ultrasonic bath containing acetone and clean for 15 minutes at room temperature. After ultrasonic treatment the glass slide was rinsed using Milli-Q ® water before being allowed to air dry. After drying the slides were then stored in airtight containers ready for use. A 1000 mL stock solution of Schweizer's reagent was used as the reactant mixture. The reagent consisted of a 50 ml solution of 0.01 M Urea, 400 ml solution of 0.01 M of CuSO4.7H2O, 50 ml solution of 25 % NH3 and 500 ml solution of 0.02 M NH4Cl. The ingredients were thoroughly mixed to produce a clear blue solution (0% power solutions) as seen in Figure 1 (b). The stock solution was then used to prepare three sets of test solutions. Each set consisted of a six small vials each containing 50 ml of the stack solution. In each set, one viral acted as the control and was not subjected to ultrasonic treatment. The glass slides were then placed into the vials and then subjected to varying amounts of ultrasonic power (20, 40, 60, 80 and 100 %,), with 100 % being equivalent to 400 W. The ultrasonic power was supplied by a Hielscher Ultrasonic processor UP400S [400 W, 24 kHz, H22 Sonotrode (22 mm dia.)]. The power output was controlled via the rotary regular as per manufacturer's specification. The procedure was repeated for three time periods 5 min, 10 min and 15 min. During ultrasonic treatment UV-visible spectroscopy and Fourier Transform Infrared Spectroscopy (FT-IR) was used to monitor copper oxide nanostructure formation. After ultrasonic treatment the glass slides were rinsed using Milli-Q ® water before being allowed to air dry and then stored in airtight containers ready for further advanced characterisation.
2.3. Advanced characterisation UV-visible spectroscopy was carried out after each set of ultrasonic treatments to monitor copper oxide nanostructure formation. Samples were examined using a UV-Visible spectrophotometer (Varian Cary 50 series, V3) over a spectral range from 200 nm up to 1100 nm (1 nm resolution) at room temperature (24 °C). A Perkin-Elmer Frontier Fourier Transform Infrared (FT-IR) spectrometer fitted with a Universal Single bounce Diamond ATR attachment (Range: 525 cm -1 to 4000 cm −1 in 4 cm -1 increments) was used to investigate functional groups and vibration modes present in the samples. A GBC ® eMMA X-ray Powder Diffractometer [Cu Kα = 1.5406 Å radiation source] was used to carry out the diffraction study and identify copper oxide phases present in the samples. The Diffractometer operated at 35 kV and 28 mA in flat plane geometry mode with each scan taking 2 seconds, while the diffraction patterns were collected over a 2θ range of 20° to 90°. A Cressington 208HR High Resolution Sputter coater was used to sputter coat samples with a 2 nm layer of gold to prevent charge build prior to Scanning electron microscope (SEM). SEM micrographs were taken using a JCM-6000 (NeoScopeTM) to determine size and morphology of copper oxide nanostructures formed.

Ultrasonic Treatment of Schweizer's Reagent and Spectroscopy Analysis
The experimental setup was designed around a Hielscher UP400S Ultrasonic processor as shown in Figure 1 (a). In turn, individual vials were placed on the sample tray and the sonotrode was lowered into the reagent. This was followed by the step of selecting a particular power setting and then switching on the processor. The ultrasonic treatment of each respective vial was carried out at various power settings and treatment periods as detailed in Figure 1 (b). The treatment was carried out at room temperature (24 °C), however during treatment solutions temperatures reached 85 °C. The vial on the extreme left was the control and was not subjected to ultrasonic treatment.  Figure 2. Inspection of Figure 2 (a) reveals the presence of major peaks located at 375 nm, 560nm and 785 nm, which indicate the formation of copper nanoparticle of various sizes and morphologies [24]. However, the ultrasonic induced oxidation processes produced in the reaction solution results in the rapid oxidation of the newly formed copper nanoparticles. Inspection of Figure 2 (b) reveals the presence of strong broadening of the plasmon resonances between 450 nm to 660 nm indicating the formation copper oxide nanoparticles [25].  Figure 3 (a). In summary, starting from the right hand side of Figure 3 (a), the analysis reveals the presence of a broad absorption band extending from 3700 to 3000 cm-1 and corresponds to O-H stretching [26]. The strong and broad band centred around 1274 cm-1 and the band located at 1073 cm -1 can be assigned to sulphate absorptions (ν3) and (ν1) respectively. While the bands located at 905, 933, 894, 854 and 675 cm −1 are due to sulphate compounds [27][28].

XRD analysis and SEM studies
XRD analysis was carried out on the respective samples and a representative spectrum for 15 min and 100 % ultrasonic power is presented in Figure 3 (b). Inspection of Figure 3   Representative XRD patterns were used to determine copper oxide crystalline sizes (t(hkl)) using the Debye-Scherrer equation [29,30] where, λ is the wavelength of the monochromatic X-ray beam, B is the Full-Width at Half Maximum (FWHM) of the peak at the maximum intensity, θ(hkl) is the peak diffraction angle that satisfies Bragg's law for the (h k l) plane and t(hkl) is the crystallite size. Analysis has revealed that time didn't have any major effect on crystallite size, but the level of ultrasonic power used during the treatment did influence crystallite size. Increasing ultrasonic power tended to reduce the size of the crystallites, with sizes ranging from 8 nm down to 72 nm as the power varied from 400 W (100 %) down to 80 W (20 %) respectively. Microscopy studies using (SEM) reveals the particle size and morphology of the particles produced during ultrasonic treatment. Figure 4 presents a representative micrograph of copper oxide nanoparticle produced via a 15 minutes period at 400 W (100 %). Micrograph reveals an array of agglomerations scattered over the substrate surface. While the enlarged image presented in Figure 4 (b) reveals that the agglomerations are composed of numerous particles with granular and some rod-like nanostructures.
The ultrasonic treatment has been able to synthesis copper oxide nanostructures with particle size control achieved through the power setting. The nanoparticles are predominantly granular in nature with some plate-like and rod-like structures in smaller numbers as seen in Figure 4 (d), with particles ranging in size from around 200 nm up to around 1µm. Future studies are planned to further investigate the ultrasonic processes and to optimise the experimental parameters and improve the yield of copper oxide nanostructures.

Conclusion
The present investigation has shown that a sonochemical approach using ultrasonic irradiation was capable of producing copper oxide nanostructures directly from Schweizer's reagent (tetraamminediaquacopper dihydroxide). The ultrasonic power was varied between 0 to 400W over time periods ranging from 5 min to 15 min. The reaction produced the initial blue colour of the reagent to turn brown. UV-visible spectroscopy revealed that copper nanoparticles were formed initially, but rapidly oxidized in the highly oxygenated ultrasonically induced reagent. Subsequent XRD analysis revealed the presence of both copper (I) oxide (Cu2O) and copper (II) oxide (CuO) particles after synthesis. While analysis of the SEM microscopy data revealed the copper oxide particles ranged in size from 200 nm up to 1µm and were predominantly granular in nature with some rod-like structures present in smaller numbers. The results of this study are very promising as a facile method and further studies are planned to investigate the nucleation process more closely and optimise the experimental parameters to improve the yield from the sonochemical process.