Emergent methods to synthesize and characterize semiconductor CuO nanoparticles with various morphologies – an overview

Abstract

Nanotechnology can be defined as the science and engineering involved in the design, synthesis and application of materials and devices on the nanometer scale. It is not in itself a single emerging discipline, but rather a meeting of several traditional sciences such as chemistry, physics, materials science, biology and engineering to bring together the required expertise needed to develop these new technologies. The unique properties of nanoscale materials provide benefits in remediation, pollution prevention, and efficient use of resources; however, the greatest contribution to green chemistry is likely to be the new manufacturing strategies available through nanoscience. So, the present review mainly focuses on the synthesis of semiconductor CuO nanomaterials from different starting materials by adopting various routes such as vapor-solid, vapor-liquid-solid, solid-liquid, etc. to reveal the morphological features of the precursor.

Keywords:

• CuO
• Nanomaterials
• Nanorods
• Nanotubes
• Nanobelts
• Nanowires
• Synthesis
• Alcohothermal
• SEM
• TEM
• XRD

1. Introduction

Nanomaterials have attracted much attention in recent years because they are both important and interesting: important, as nanomaterials can have useful properties not exhibited by either small molecular systems or larger particulate matter; interesting, as working on supramolecular scales raises exciting synthetic challenges and allows intimate exploration of the relationships among atoms, molecules, and extended systems. Hence in recent years, synthesis and fabrication of one-dimensional (1D) nanostructured materials have received much research attention, due to their outstanding properties and potential applications of this class of materials 1. For example, 1D nanobuilding blocks of transition metal oxides, hydroxides, sulfides, semiconductors, metals, and metal salts have now been prepared by various process techniques giving rise to the possibility of constituting a nanotoolbox for a “bottom-up” approach in nanoscience and nanotechnology 1–4. Among the various materials mentioned above, semiconductor nanoscale materials, such as nanowires, nanorods, and nanodiscs have stimulated great interest due to their importance in basic research and potential applications 5. They are quite attractive as components of electronic devices because many of their physical properties can be controlled through modification of the sizes and shapes of the building blocks 6, 7. To this end, various techniques including the vapor-phase transport, laser ablation, chemical vapor deposition, arc-discharge, template-based, solution-chemistry methods, etc., have been developed to prepare well-aligned 1D nanomaterials 8–13. However, it is still a challenge to develop simple methods for the fabrication of uniform nanomaterials that are cost-effective and allow large-scale production. Our main focus here is to review recent progress to establish convenient route (vapor–solid, vapor–liquid–solid, solid–liquid, etc.) to prepare robust semiconductor CuO nanostructured materials because this material has an advantage for application towards electronic and optoelectronic industry.

2. Preparation and characterization of CuO nanoparticles starting from the corresponding salts

In past decades, most of the research on synthesis of anisotropic nanocrystals has largely focused on II–IV semiconductor systems 14–20 other systems reported in the literature are very limited. Recently, copper-based nanomaterials have received attention because of their potential application in optoelectronic devices, catalysis and superconductors. CuO is a p-type semiconductor (E_g = 1.2 eV) with excellent photovoltaic, electrochemical, and catalytic properties 21-24. Also, it is inexpensive, non-toxic, and readily available.

Xu et al . 25, 26 reported the synthesis of CuO nanocrystals by one step solid state reaction under ambient conditions by grinding 2:5 molar ratios of CuCl₂ · 2H₂O and NaOH in an agate mortar followed by a short induction period of 30 min. The obtained black product was washed with an ultrasonic wave, distilled water (3 times), alcohol (2 times), and then dried in air. This method yielding polyhedral CuO nanocrystals with an average size of 12 nm was confirmed by TEM image (figure 1). XRD diffraction peaks (figure 2) also confirmed that the size of this CuO nanocrystal was found to be same by immediately pressing the powder as a pellet. Whereas an increase in grain size from 12.1 nm to 16.4 nm was observed on aging (15 days) of the pellet may be due to the more closely packed nanocrystals.

Figure 1. TEM micrograph of the CuO particles (×150,000) 25.

Figure 2. XRD curves of CuO pellet samples (a) just after pressed, (b) 4 days later, and (c) 15 days later at RT 25.

CuO belongs to the space group shows three acoustic modes (A_u + 2B_u), six infrared active modes (3A_u + 3B_u), and three Raman active modes (A_g + 2B_g). Thus, the Raman spectra of CuO nanocrystal show three peaks at 282 cm⁻¹, 332 cm⁻¹ and 618 cm⁻¹ (figure 3). With aging, Raman peaks become sharper and show a slight blue shift because the grain size increases from 12.1 nm to 16.4 nm. From this study, it is concluded that there was change in the grain size only in the pressed pellets (due to their being more closely packed), not in CuO powder from XRD and Raman measurements.

Figure 3. Raman spectra of CuO pellet samples (a) just after pressed, (b) 4 days later at RT, (c) 15 days later at RT, (d) annealed at 500°C for 2 h, and (e) annealed at 800°C for 2 h 25.

Wang et al . 27 prepared CuO nanowhiskers similar to Xu et al . 25 but in the presence of a nonionic surfactant. About 5.045 g of CuCl₂ · 2H₂O and 3 g of solid NaOH were ground for 5 min. To this mixture, 6 ml (6.78 g) of PEG 400 was added and the grinding continued for 30 min before washing the mixture several times with distilled water in an ultrasonic bath and then with ethanol to remove PEG 400, NaCl and unreacted precursors. Finally, the products were dried for 3 h in an oven at 60°C in air. The CuO nanowhiskers most likely form via the following reaction:

The ionic liquid (PEG 400) exposes more ethyleneoxide monomers to the surface than in aqueous solutions which in due course interacts with water molecules to form a chain structure 28. Thus, PEG 400 acts as a soft template leading to the formation of CuO nanowhiskers having an average size of ∼8 nm in diameter and length scales larger than 100 nm (figure 4).

Figure 4. TEM image of the CuO nanowhiskers sample 27.

In addition, they also carried out preparation of CuO nanorods in the presence of nonionic surfactant 29 as follows. About 200 mg of PEG (M_w 20,000) and 178.48 mg of CuCl₂ · 2H₂O were dissolved in 200 ml of H₂O, which was stirred with a magnetic stirrer continuously until both the PEG and the CuCl₂ dissolved completely. Then 2 ml of NaOH solution (6 M) was added to the above mixture drop wise under constant stirring. While stirring Cu(OH)₂ blue precipitate formed which upon heating by steam for 30 min, generates CuO. The black CuO precipitate was washed several times with distilled water and then with ethanol to remove the PEG completely, filtered and dried in an oven at 80°C for 5 h. TEM image (figure 5) revealed that these materials have a relatively straight rod-like shape (diameters of ∼10 nm and of length up to 400 nm) and smooth surfaces. The CuO lattice constants calculated for these nanorods by XRD data is consistent with the standard values of bulk CuO 30 and also results clearly indicate that the obtained nanorods were in fact crystalline CuO belonging to the monoclinic space group .

The CuO nanorods form here via two steps as shown below:

Here also, the nonionic surfactant (PEG) acts as a template which plays an important role in controlling the CuO shape.

Figure 5. TEM image of the CuO nanorods 29.

Similarly, Gao et al . 31 synthesized CuO nanorods starting from CuCl₂ as follows. First, Cu(OH)₂ precipitates were prepared by mixing 0.5 M solution of both CuCl₂ and NaOH. The obtained precipitates were dispersed in a 15 M NaOH solution of which one part of the dispersion was transferred into an autoclave with a PTFE container and kept at room temperature, while the second part was maintained at 100°C for 48 h. The obtained precipitates were washed in distilled water (until the solution pH ∼ 7) and then dried at 100°C in air for 1 day. The average crystalline sizes of obtained CuO samples at room temperature and 100°C were estimated to be 17 nm and 73 nm from the XRD patterns (using Sherrer equation). The room temperature product was uniform fine nanorods of about 10–20 nm thick and several hundred nanometers long which are even thinner than the precursor Cu(OH)₂ precipitate (40–60 nm). Whereas at a temperature of 100°C, bulk CuO nanorods were obtained with a diameter 60–100 nm in size and length of up to 1 µm (figure 6). Thus, the morphological features of the synthesized nanorods were highly dependent on temperature and caustic soda present in autoclaves. Also from the selected area electron diffraction (SAED) patterns it is concluded that bulk nanorods obtained at 100°C consist of single crystals while the fine nanorods obtained at room temperature have a polycrystalline structure (see inset of figure 6).

Figure 6. TEM images of Cu(OH)₂ precipitates dried at 100°C (a) and as-prepared nanorods at room temperature (b) and 100°C (c). The selected area electron diffraction of the as-prepared nanorods is inserted 31.

Secondly, Punnoose et al . 32 reported preparation of CuO nanoparticle by the sol-gel method. That is, mixing aqueous solutions of copper nitrate and sodium hydroxide at room temperature and by maintaining the pH ∼ 10. The resulting Cu(OH)₂ gel was washed several times with distilled water until free of nitrate ions, then centrifuged and finally dried in air. In order to get CuO nanoparticles, the Cu(OH)₂ gel obtained above was calcined in air for 3 h at the selected temperatures of 160, 200, 250, 300, 400, 600, 800 and 1000°C. The XRD patterns of the calcined samples (figure 7) show only lines due to CuO nanoparticles and also the particle size increases from 6.6 nm to 37 nm as the calcined temperature increases is determined by employing the Scherrer relation to the (202), (202) and (113) reflections 33. The increase in grain size that may be due to the fusion of the particles on sintering is confirmed further by TEM micrographs (figure 8). That is, upon sintering small particles get aggregated and, in addition, the occurrence of surface faceting is evident from TEM micrographs.

Figure 7. Room-temperature XRD patterns of CuO particles annealed at different temperatures shown. The particle size shown is the average value determined from the (20), (202), and (11) lines using the Scherrer relation, after correcting for the instrumental width. Temperatures are in °C 32.

Figure 8. TEM characterization reveals agglomeration of the nanoscale particles. A cluster of 6.6 nm particles, shown in (a), consists of highly faceted single particles (b). A cluster of the nominal 33 nm particles is shown in (c). These large particles have surfaces decorated with smaller particles (d), which exhibit defects such as multiple twinning (arrow) 32.

Viano et al . 34 confirmed the agglomeration of nanoscale particles by TEM analysis (figure 9) while following the same synthesis procedure of Punnoose et al . 32. They used electron spin resonance (ESR) technique to probe the calcination temperature effect on magnetic ordering in nano and bulk CuO powder. From the ESR results (figure 10), they concluded that (1) line shapes belong to typical polycrystalline material, (2) the appearance of broad signals are ascribed to weak antiferromagnetic ordering in CuO, and (3) the difference in line widths of the ESR signals between bulk and nanopowder indicates a high defect concentration in nanopowder, which must arise from the comparatively high surface area of nanoparticles. Thus the comparative study of bulk and nano CuO powder highlights the importance of particle size on the magnetic properties of nanoparticles.

Figure 9. TEM image of CuO nanopowder calcined at 200°C 34.

Figure 10. Overlay of ESR spectra of (a) CuO nanopowder and (b) CuO bulk calcined at different temperatures. The calcination temperature is written just above each curve 34.

Zhu et al . 35 reported preparation of CuO nanoribbons starting from Cu₂Cl(OH)₃ nanoribbons by template free method. First, Cu₂Cl(OH)₃ nanoribbons were prepared by mixing 1:2 molar ratio of CuCl₂ · 2H₂O and (NH₄)₂CO₃ solution and keeping the solution aside for 2–7 days. The jade-green precipitate formed was washed with distilled water and ethanol, and then dried at 60°C for 12 h yields Cu₂Cl(OH)₃ nanoribbons (see figure 11 for TEM image). The reaction that accounts for the formation of Cu₂Cl(OH)₃ nanoribbons probably proceed as follows:

In order to succeed in converting the Cu₂Cl(OH)₃ nanoribbon to CuO nanoribbon, the blue complex solution was transferred into a sealed Teflon-lined stainless-steel autoclave maintained at a relatively low temperature (105°C) so as to avoid the collapse of the ribbon morphology as much as possible. TEM image (figure 12) confirms the results for the conversions of Cu₂Cl(OH)₃ nanoribbon to CuO nanoribbon with an average width of 50 nm which corresponds well to the width of the sacrificial template. Also the SAED pattern confirms that the above-prepared CuO nanoribbons are polycrystalline (see inset of figure 12). Similarly, CuO nanostructures with nanoribbons, scroll-like structures etc. were prepared starting from single crystalline Cu₂(OH)₂CO₃ nanoribbons as precursors for sacrificial-template via heat treatments 36.

Figure 11. The TEM images of the Cu₂Cl(OH)₃ nanoribbon 35.

Figure 12. TEM image of CuO nanoribbon obtained by heat treatment of the Cu₂Cl(OH)₃ nanoribbon 35.

Song et al . 37 reported a different approach to the preparation for CuO nanowires starting from Cu(DEHP)₂ complex is as follows. First, Cu(DEHP)₂ complex were generated at pH 6 by mixing 2:1 molar ratio of bis(ethylhexyl) hydrogen phosphate (HDEHP) and CuCl₂ in heptane and water. Secondly, Cu(OH)₂ nanowire arrays were prepared at room temperature by injecting 20 ml of Cu(DEHP)₂ complex (0.05 M) in n-heptane into 20 ml of aqueous NaOH solution (0.05 M) taken in a 50 ml test tube. After several minutes, a blue colour gradually grew at the solution interface showing that the reaction occurs at the static organic/aqueous biphasic boundary. The product formed was taken out of the test tube after a given reaction time, washed with absolute alcohol, deionozied water, and dried in air. XRD and TEM analysis (figure 13) confirmed the formation of Cu(OH)₂ nanowires at the interface. Finally, black CuO nanowire arrays (figure 14) were obtained by allowing the above reactions to take place at (i) higher concentration of NaOH and (ii) more reaction time.

Figure 13. XRD and TEM image of Cu(OH)₂ prepared at the static interface between Cu(DEHP)₂ = 0.05 M in n-heptane and NaOH = 0.05 M 37.

Figure 14. TEM images of CuO obtained from the dehydration of Cu(OH)₂ at the different concentrations of NaOH (a) and (b) Cu(DEHP)₂ = 0.05 M and NaOH = 0.1 M at different magnifications. (c) Cu(DEHP)₂ = 0.05 M and NaOH = 0.2 M 37.

Lu et al . 38 reported a simple template-free solution route for the controlled synthesis of Cu(OH)₂ and CuO nanostructures starting from CuSO₄ solution. They followed two different methods for the synthesis of Cu(OH)₂ nanostructures, i.e., the KOH/NH₃ route and the NH₃/KOH route. In the first route (KOH/NH₃), about 0.8 ml of KOH solution (1M) was added into 2 ml of CuSO₄ solution (0.1 M) under rigorous stirring at ambient temperature followed by addition of 0.3 ml of ammonia solution (12.5 wt%, ∼7.0 M). Further stirring the sample for a while and then kept mixed solution static in a sealed vessel for 12 h followed by filtering, washing thoroughly with deionized water and dried in a vacuum at 25°C for 12 h generates Cu(OH)₂ nanowires (figure 15). For the second route (NH₃/KOH), the synthesis procedures are same but the feeding order of NH₃ and KOH are vice-versa compared to the first method. For this method Cu(OH)₂ nanoribbons were obtained which was confirmed by SEM and TEM images (figure 16). The exact reactions for the selective formation of Cu(OH)₂ nanowires and nanoribbons are as follows:

(a) KOH/NH₃ route

(b) NH₃/KOH route

The thermal dehydration of the 1D Cu(OH)₂ nanowires obtained through KOH/NH₃ route in the solution phase generates CuO nanoplatelets (figure 17), upon heating the original solution below 50°C after the addition of the two alkaline solutions, and aged at 50°C. But for the 1D Cu(OH)₂ nanostructures only bundles of CuO nanowires (figure 18) were obtained without any change in the morphology, when dehydrated the sample in the solid state at 120°C for 2 h. While, for the Cu(OH)₂ nanoribbons synthesized via NH₃/KOH route, a unique flower like aggregates consisting of CuO nanopetals (figure 19) were obtained, when the Cu(OH)₂ nanoribbons were dispersed in water and heated at 70°C for 12 h.

Figure 15. SEM (a, b), TEM (c, d), and HRTEM (e) images of Cu(OH)₂ nanowires prepared via the KOH/NH₃ route aged for 12 h. Inset shows ED pattern of the bundled nanowires. Scale bars: (a) 5 µm; (b) 500 nm; (c) 100 nm; (d) 50 nm; (e) 5 nm 38.

Figure 16. SEM (a, b) and TEM (c, d) images and ED (e) patterns of Cu(OH)₂ nanoribbons prepared via the NH₃/KOH route. The ED pattern shown in plate e corresponds to the single nanoribbon shown in plate d. Scale bars: (a) 5 µm; (b) 1 µm; (c) 200 nm; (d) 100 nm 38.

Figure 17. SEM images (a, b) of CuO nanoplatelets obtained by thermal dehydration of the Cu(OH)₂ nanowires prepared via the KOH/NH₃ route immediately after the addition of the KOH and ammonia solutions: (a) 30°C; (b) 50°C. Scale bars: 500 nm 38.

Figure 18. SEM (a) and TEM (b) images of CuO nanowires obtained by thermal dehydration of the as prepared Cu(OH)₂ nanowires in the solid state at 120°C for 2 h. Scale bars: (a) 500 nm; (b) 200 nm 38.

Figure 19. SEM images of CuO nanostructures obtained by thermal dehydration of the as prepared Cu(OH)₂ nanoribbons in water at 70°C for 12 h. Scale bars: (a) 2 µm; (b) 500 nm 38.

Zeng et al . 39 reported synthesis of CuO nanorods and nanoribbons by wet chemical method in which water--ethanol mixture is used as a solvent. 40 ml of Cu(NO₃)₂ · 3H₂O (0.01 M in pure ethanol) was slowly added at a rate of 0.1 ml/min using a mechanically pumped syringe to 40 ml of NaOH (0.5 M in ethanol) solution under stirring in a Teflon flask at 65°C and the resultant product was then aged together with mother liquor at the same temperature for 12–36 h respectively. Separation of CuO nanocrystals were done by centrifugation at a speed of 6000 rpm for 10 min followed by washing with de-ionized water repeatedly and then dried under vacuum overnight at room temperature. In addition, they found that the dimensions of the nanostructured CuO can be controlled across a wide spatial span from nanometer to micrometer regime via manipulating synthetic parameters (summarized in table 1) and most importantly desired morphology of CuO crystallites (rod, wire, ribbon, net, platelet or sheet) can be obtained by selecting simple changes of reaction conditions. To achieve this, NaOH is playing a number of roles in the synthesis. First it combines with Cu(NO₃)₂ to form Cu(OH)₂, which is then decomposed to CuO as follows:

Since, heating was used to accelerate the reaction, no Cu(OH)₂ intermediate precipitate was observed. Secondly, NaOH is a strong electrolyte, and it may neutralize the surface charges of the CuO nanorods, preventing them from possible crystallite aggregation. Finally, the high concentrated NaOH used may create diffusion layers on certain surfaces of CuO nanorods, which may create an additional growth anisotropy allowing only energetically favourable crystallographic planes to grow. Thus, from the simple wet-chemical methods the following points to be noted: (1) at low reaction temperatures and normal atmospheric pressure, single crystalline monodispersed CuO nanorods with a selected breadth in the range of 5–15 nm can be prepared by simply changing the starting copper ion concentration, (2) upon the ageing treatment, morphology of the CuO nanorods can be further modified, and (3) nanoplatelets or sheets of CuO can also be synthesized with a high concentration solution of NaOH.

Table 1. Experimental procedures and product results 39.

Expt	Preparation procedure	Result
A	40 mL of NaOH (0.5 M^a) + 40 mL of Cu^2+ (0.01 M^a) at 0.1 mL/min and 65°C, aged for 12–36 h	Amorphous product
B	40 mL of NaOH (0.5 M^a) + 40 mL of Cu^2+ (0.01 M^a) at 0.1 mL/min and 77°C + 40 mL of H2O at 0.2 mL/min and 77°C, aged for 0–60 h	Short nanorods, breadth = 5–8 nm
C	40 mL of NaOH (0.5 M^a) + 40 mL of Cu^2+ (0.03 M^a) at 0.2 mL/min and 78°C + 60 mL of H2O at 0.1 mL/min and 78°C, aged for 0–126 h	Short nanorods, breadth = 8–10 nm
D	20 mL of NaOH (0.5 M^a) + 20 mL of Cu^2+ (0.08 M^a) at 0.1 mL/min and 78°C + 40 mL of H2O at 0.1 mL/min and 78°C, aged for 0–16 h	Short nanorods, breadth = 10–15 nm
E	20 mL of NaOH (0.5 M^a) + 20 mL of Cu^2+ (0.01 M^a) at 0.2 mL/min and 81°C + 20 mL of H2O, aged for 3 h; then 40 mL of NaOH (0.5 M^a) + 40 mL of Cu^2+ (0.02 M) both at 0.1 mL/min and 81°C, aged for 0–39 h	Long nanoribbons, breadth = 5–15 nm, long nanorods (after aging)
F	40 mL NaOH (5 M^b) + 40 mL Cu^2+ (0.02 M^b) at 0.1 mL/min and 82°C + 40 mL Cu^2+ (0.02 M^b) at 0.3 mL/min and 82°C, during which 16 g of NaOH was also added, aged for 0–90 h	Nanoribbons, nanoplatelets, nanosheets (maximum area = 500 × 1000 nm)
^aIn pure ethanol; ^bin deionized water.

In addition, they reported how CuO crystallites self-organized into spherical assemblies or “dandelions” with a puffy appearance (figure 20) 40. The synthesis procedure for such samples is as follows: About 15–25 ml of Cu(NO₃)₂ · 3H₂O (1 M in pure ethanol solvent) was added to 15–25 ml of ammonia solution (32%), followed by an addition of 4–5 ml of aqueous NaOH (1 M) and 0–5 g of solid NaNO₃. The solution mixture was then transferred to a Teflon-lined stainless steel autoclave and heated at 100–180°C for 2–24 h in an electric oven. After reaction, the autoclave was cooled in tap water, and CuO products were washed via centrifugation-redispersion cycles, with each successive supernatant being decanted and replaced with deionized water and ethanol. Under the reported conditions, the prepared CuO products are all in the “dandelion”-like morphology (100%), with diameters ranging from 4–8 µm.

Figure 20. SEM images of CuO microspheres (prepared at 100°C, 24 h): (a) overall product morphology; (b and c) detailed views on average sized spheres; and (d) a detailed view on an individual sphere 40.

3. Preparation and characterization of CuO nanoparticles starting from the corresponding acids

In past decades, much effort has been made towards the synthesis and characterization of nanosized transition metal oxide particles. Among all transition metal oxides that have been investigated, much less time has been devoted to the preparation and characterization of CuO nanoparticles. For example, Yang and Gao 41 reported preparation of CuO nanocrystals with various morphologies via a simple and convenient hydrothermal decomposition route. That is, by mixing about 0.5 g of Cu(COOH)₂ in a series of mixture of anhydrous dimethyl sulphoxide (DMSO) and distilled water (total volume 70 ml and volume ratios were 50:3, 10:1, 5:1, 5:2 and 1:1) in a Teflon-lined autoclave. The whole mixture was then maintained at 110°C for 3 h and cooled to room temperature naturally yields CuO nanocrystals. The obtained CuO black precipitates were collected and washed with distilled water followed by absolute ethanol several times. The final products were dried in vacuum at 60°C for 6 h.

The following reactions supported such preparation methods.

From the TEM photographs (figure 21), it is concluded that morphology of CuO nanocrystals changes from spherical to leaf like morphology via nanorod structures with the increase of water in the reaction. This is because the surface tension of the DMSO--H₂O mixtures increases with water content 42. When the water content is low, the DMSO tends to absorb the water vapor more quickly which results in abundant small nuclei of CuO. In addition, DMSO acts as a dynamic stabilizer 43, whose molecules can be bound to the surface of the nuclei through donating a lone pair of electrons from oxygen atom to the empty orbitals of the copper atoms, which tend to hinder the more combinations of Cu⁺ and OH⁻. Further, the steric effect caused by the DMSO molecules and the generation of more OH⁻ with the increase of water content (identified from the pH value) can also limit the growth of the nuclei.

Figure 21. Representative TEM images of CuO nanocrystals synthesized at various volume ratios of DMSO and water: (a) 50:3; (b) 10:1; (c) 5:1; (d) 5:2; (e) 1:1 41.

Xu et al . 44 reported preparation of CuO nanorods by thermal decomposition of CuC₂O₄ similar to Yang and Gao 41 but they use copper acetate, oxalic acid and surfactant (nonyl phenyl ether) as starting compounds. That is, about 3.99 g of Cu(CH₃COO)₂ · H₂O and 2.53 g of H₂C₂O₄ · 2H₂O were mixed with 5 ml of nonyl phenyl ether 9/5 (NP-9/5) in a mortar, grind for several minutes and kept in a thermostat oven at 50–60°C for 6 h. Then the product was washed several times with distilled water and acetone to remove unreacted reactants and then dried in an oven at 70–80°C for 12 h to yield pure CuC₂O₄. To yield CuO nanorods, grind about 2 g of CuC₂O₄ as synthesized above with 8 g of NaCl powder, and then annealed at 950°C for 2 h in a porcelain crucible. Finally, the annealed sample was washed with distilled water one time, ethanol three times, ethyl ether one time using an ultrasonic bath and then centrifuge to yield CuO nanorods. TEM morphologies (figure 22) confirmed the as-synthesized CuO nanorods were around 30 nm in diameter and length of several micrometers. The existence of conical tip at the end of nanorod conforms that the growth mechanism of the CuO nanorods is most likely controlled by the vapour-solid (VS) growth mechanism.

Figure 22. TEM images of the as-prepared CuO nanorods. (a) Morphologies of the as prepared CuO nanorods. (b) Morphologies of the as prepared single CuO nanorod 44.

The following reactions supported such preparation methods.

The use of surfactant may act as a template to form fine particle and also provide a favorable environment for the growth of CuO nanorods, i.e., make a “shell” surrounding the particles to prevent them from aggregating to larger particles during the grinding process.

Hong et al . 45 also followed alcohothermal route to synthesize CuO nanoparticles starting from copper acetate. That is 50 ml of copper acetate alcoholic solution (0.05 M) was taken in a Teflon lined stainless steel autoclave and kept constant in an oven at various temperatures (90°C, 110°C, 130°C, 150°C and 180°C) for 20 h. After cooling the autoclave to room temperature, the products were recovered by centrifugation, washed with distilled water (twice), ethanol (three times) and then vacuum dried at room temperature. Through this approach, stable CuO nanoparticles as small as 3–9 nm have been successfully synthesized below 150°C. However, the rate of formation of CuO nanoparticle was slow when the synthesis temperature was below 90°C and on the other hand, if the temperature was raised above 180°C, reducing alcohol would readily reduce the Cu(II) to metallic copper nanoparticles with a relatively large size of 54.1 nm. This may be confirmed from the XRD patterns (figure 23).

Figure 23. X-ray diffraction patterns of the samples prepared using the alcohothermal route at different temperatures: (a) 90°C; (b) 110°C; (c) 150°C; and (d) 180°C 45.

Similarly, Wang et al . 46 reported synthesis of CuO nanoparticles by microwave irradiation of copper acetate. About 25 ml ethanolic solutions of both copper acetate (0.2 M) and NaOH (0.01 M) were taken in a round-bottomed flask with 0.5 g of polyethylene glycol −19,000 (PEG). The reaction was carried out under ambient air by keeping the total mixture under microwave irradiation. The microwave oven followed a working cycle of 6 s on and 24 s off and altogether the on/off heating procedure was repeated 20 times. After cooling the irradiated mixture to room temperature, the products were recovered by centrifugation, washed with distilled water, absolute ethanol and acetone, and then dried in air at room temperature.

By this microwave irradiation approach, the size of the CuO particles obtained can be in the range of 3–5 nm (figure 24) with the presence of dispersant (PEG). While in the absence of dispersant (PEG), CuO nanoparticles get agglomerated to form some quadrate aggregates of over 200 nm (figure 25). These can be explained by the stabilization effect of the surface caused by PEG. That is, the existence of dispersant PEG may reduce the Gibbs's free energy at the surface of the CuO nanoparticles and thus prevent the grains from merging into larger ones. The mechanism for this approach is proposed similar to Iwasaki et al . 47 as follows.

That is, nucleophilic attack by ethoxide produces an intermediate (EtO--Cu--Ac), which on hydrolysis produces another intermediate (Ac--Cu--OH). Cu--O--Cu bonds were formed by dehydration-condensation of this intermediate followed by decomposition leads to CuO nanoparticles under microwave irradiation.

Figure 24. (a) TEM and (b) electron diffraction pattern of the as-prepared CuO nanoparticles 46.

Figure 25. TEM image of the CuO nanoparticles prepared in the absence of PEG 46.

4. Preparation and characterization of CuO nanoparticles starting from the corresponding metal

Apart from the routine method of synthesis of CuO nanoparticles starting from their corresponding salts, there are also reports 1 available on syntheses of CuO nanoparticles from the corresponding metal. For example, Borgohain et al . 48 prepared stable CuO nanoparticles having size as small as 4 nm by novel electrochemical route using a copper metal electrode as sacrificial anode and platinum as cathode in an electrochemical bath comprised of tetraoctylammonium bromide (TOAB) in acetonitrile and tetrahydrofuran, in the ratio 4:1 as electrolyte. This method gives a high yield of sample, without any undesired side products compared to the synthetic routes involving corresponding salts as their starting material.

Secondly, Lo et al . 49 reported preparation of CuO nanofluid by submerged arc nanoparticle synthesis system (SANSS) using a copper electrode. A pure copper rod is deployed to prepare nanoparticles, submerged in dielectric liquid (deionized water) in the vacuum chamber. By generating an electric arc at high temperature (between 6000–12,000°C), the rod is melted and vaporized, undergoing nucleation, growth and condensation in the submerged deionized water to disperse nanoparticles. The morphology of the CuO nanoparticle prepared by this method revealed a needle-like structure (figure 26) with an average width of 20 nm and length of 80 nm.

Figure 26. SEM image of the CuO nanoparticle 49.

In the late 1950s, Nabarro and Jackson 5 reported preparation of CuO and Cu₂O nanowhiskers of short length (<5 µm) and large diameter (>100 nm) by oxidizing copper substrate. But recently, Jiang et al . 50 reported preparation of CuO nanowires of controllable diameter in the range of 30–100 nm with lengths of up to 15 µm by simply heating copper foils at different temperature and growth time. They followed the procedure as follows: First, the copper substrate was cleaned in an aqueous 1 M HCl solution, followed by repeated rinsing with distilled water. After it had been dried under a N₂ gas flow; it was placed in an alumina boat and immediately heated to the set-point temperature (at ambient pressure) in a furnace. The surfaces of the copper substrates were tarnished (when viewed by the naked eye) after they had been treated in air at elevated temperatures. Further examination of the sample under optical or electron microscope indicated the formation of wire-like nanostructures over the entire surfaces of these substrates. For example, TEM images (figure 27) conclude that the diameter of the CuO nanowires could be varied in the range of 30–100 nm by controlling the reaction temperature. The following reactions can be summarized for this entire synthesis, with the second one being as the rate-determining step for the formation of CuO vapour 51.

The slow rate for the formation of CuO ensures a relatively low vapour pressure is required for this material in the reaction chamber, which helps for a continuous growth mode and uniform diameter for the CuO nanowires.

Figure 27. TEM images of CuO nanowires prepared by heating copper grids in air for 4 h at various temperatures: (a) 400°C, (b) 500°C, and (c) 600°C. The corresponding size distributions of these nanowires are shown in graphs D to F. These results suggest that the diameter of the CuO nanowires could be varied in the range of 30–100 nm by controlling the reaction temperature 50.

Similarly, Yu et al . 52 reported preparation of CuO nanorods of diameter 20–150 nm and length of several micrometers by annealing the copper plate at 400°C for 24 h in air. However, Huang et al . 53 prepared large scale of CuO nanowires by thermal evaporation of copper foil at various temperatures (300–800°C) under an oxygen flow rate of about 50 ml/min. Also, they determined that the diameter and length of nanowires vary with the evaporation temperature and time. For example, SEM images (figure 28) show the morphologies of the samples prepared at various temperatures and they also proposed growth mechanism similar to Xia et al . 50.

Figure 28. SEM images of CuO nanowires prepared at various temperatures: (a) 300°C, (b) 400°C, (c) 500°C, (d) 600°C, (e) 750°C and (f) 800°C 53.

Though preparation of CuO nanoparticles by different approaches are mentioned above, Yang et al . 54 obtained CuO nanoribbon arrays on copper converted from Cu(OH)₂ nanoribbon arrays prepared by solution-solid reactions. That is, copper foil was washed with a 4 M HCl aqueous solution and subsequently with deionized water repeatedly to remove surface impurities. The washed copper foil was then immersed into a dilute solution of ammonia (3.3 × 10⁻² M, pH ≈ 10) for certain periods of time (12–96 h) and then taken out of the solution, washed with deionized water. The blue film covered uniformly on the Cu substrate was found to be Cu(OH)₂ nanoribbon arrays, which was confirmed by SEM analysis (figure 29). To get CuO nanoribbons, the above prepared Cu(OH)₂ nanoribbon sample was loaded into a quartz boat and heated in a furnace under constant flow of N₂ (∼25 sccm) at 120°C for 3 h to get dehydration and then heated at 180°C for 2 h to promote crystallization. Adopting this procedure, CuO nanoribbons (figure 30) were obtained with an average width of ∼50–60 nm and several nanometers long by varying the reaction time and temperature interval. The reactions that account for the formation of Cu(OH)₂ and CuO nanoribbons probably proceed as follows:

In addition, they also prepared CuO nanorod arrays 55 at ambient temperature conditions (without heating) as follows. Washed copper foil was immersed into 400 ml of water containing various amounts of ammonia (13 M) and NaOH (1 M) solutions at room temperature for certain period of time. After a given reaction time, the sample was taken out of the solution, washed with deionized water, and dried in air. A black film covered uniformly on the Cu substrate. This was found to be CuO nanorod, which was confirmed by SEM analysis (figure 31). The reactions are almost same as above but the third reaction occurs at room temperature for this system. The main reason for such effect is that the solution is more basic here, so that Cu(OH)₂ is unstable and thus immediately decomposed to CuO. By adopting this simple wet chemical method at room temperature vertically aligned CuO nanorod arrays on copper electrodes with diameters of ∼5 nm and lengths of hundreds of nanometers were obtained. Also, they used as-prepared CuO nanorod arrays as cathode for fabrication of dye-sensitized solar cells.

Figure 29. SEM images of a Cu(OH)₂ nanoribbon array. (a) Top view. (b) Side view 54.

Figure 30. TEM image (a), XRD pattern (b), SEM image (c), and HRTEM image (d) of CuO nanoribbons converted from the Cu(OH)₂ nanoribbons by heat treatment. Inset of part A: SAED pattern of an ensemble of CuO nanoribbons. Inset of part D: SAED pattern of a single CuO nanoribbon 54.

Figure 31. SEM images of p-CuO nanorods prepared at various pH and reaction time intervals: (a) pH 11 (96 h); (b) pH 11 (130 h); (c) pH 11.5 (96 h); (d) pH 11.5 (120 h); (e) pH 12 (96 h); (f) pH 12.3 (14 days) 55.

5. Conclusion

In summary, copper oxide nanoparticles of different crystalline structures and morphologies were synthesized successfully starting from the corresponding salts, acids and metal. Each method available here has its own significant advantages in preparing CuO nanoparticles of particular morphology and structure. But in order to identify which is the best method for large-scale production many factors (cost effective, preparation conditions, required facilities, etc) are to be taken into account. A great variety of procedures are mentioned in this paper, giving rise to the possibility of constituting a nanotoolbox for a “bottom-up” approach in nanoscience and nanotechnology. From the results, it is hoped that preparation of CuO nanoparticles will provide the impetus for solution-phase chemical synthesis to be considered as the most promising route in terms of cost, throughput, and the potential for large-scale production.

Overall, we are happy if the present survey gives researchers an opportunity to explore available methods to synthesize semiconductor CuO nanomaterials from different source materials. Thus, the given examples here illustrate the most impressive methods, which are available in the preparation of CuO nanomaterials in view of the nano-science. The investigation of nano-science will remain a challenging task in the near future until it is regarded as a genuine field of science.

Acknowledgements

Author (SA) wishes to thank DST, New Delhi for the Fast Track Young Scientist award and The Director of the National Institute of Technology, Trichy, for providing funding from the TEQIP programme to create a new lab (Nanomaterials and Solar Energy Conversion).