Self-assembly procedure is employed to synthesize colloidal copper nanoparticles (ccNPs) with cationic surfactant in an environmentally friendly method. Scanning electron microscopy images provide a clear view of the ccNPs formed having an approximate size of 15 nm. The X-ray diffraction reveals that the ccNPs have the two types of copper oxide as well as the metallic copper. The new procedure shows that the cationic surfactant CTAB plays an important role in the understanding and development of self-assembly. There is a strong relationship between the ccNPs formation with the critical micelle concentration of the CTAB which influences both shape and size. The outcomes allowed the development of a molecular model for the ccNPs synthesis showing that the CTAB monomer on the surface has the function of a molecular velcro making the linkage of ccNPs to form an agglomerate with size around 600 nm. Finally, with the emerging new technologies, the synthesis of copper oxide takes a new perspective for their applicability in diverse integrated areas such as the flexible electronics and energy.
Advances in nanotechnology are increasingly dependent on the processes of synthesis to obtain nanoparticles of uniform size and shape [
The self-assembly involving metals, reducing agents, and stabilizers such as surfactants hava attracted interest due to the ability of controlling the properties of NPs as uniform size and distribution [
The growing interest in environmentally friendly processes is due to pressure on industries in reducing costs and environmental damage with the waste process caused by traditional methods [
NPs have physicochemical properties that need to be better understood, principally to maintain a standard procedure in obtaining these nanocomposites. However, the size control of nanoparticles is complex and dependent upon parameters such as temperature, ionic strength, pH, and the sequence of reagents addition [
In this work, we report a new strategy, simple and versatile to synthesize colloidal copper NPs (ccNPs) using a cationic surfactant. The synthesis compared with other processes [
The ccNPs were synthesized in aqueous solution, under uncontrolled atmospheric conditions. In a beaker containing ultrapure water, ascorbic acid (Sigma) approximately 420 mM sufficient to adjust the pH at 2.8 was added. In the solution 1 mM of (1-hexadecyl) trimethylammonium bromide (CTAB—Alfa Aesar) value below critical micelle concentration (CMC) was added. The molar ratio was fixed at 22 times for the above CMC condition. The solution was heated at 55°C under constant agitation allowing the equilibrium for 60 min and then was added slowly to 720 mM of CuSO4·5H2O (Alfa Aesar). During the process there is fast and intense color change ranging from yellow, orange, until red-brown indicating the end of the reaction at which point the mixture was left to rest at room temperature for 12 hours. The solution was then centrifuged at 20,000 rpm for 20 minutes at 20°C, the supernatant was discarded, and the pellet was washed in ethanol, then again centrifuged for five times and dried at room temperature. For the experiment in the absence of CTAB 420 mM of ascorbic acid and 720 mM of CuSO4·5H2O was used for the same experimental conditions. All reagents were of analytical grade and used without further purification.
SEM measurements were carried out with a JEOL SEM-FEG JSM 6330F microscope with a field emission gun and a ZEISS SUPRA 55 microscope with ultrahigh resolution field emission (FESEM). The samples were not sonicated and were prepared by diluting a resultant colloidal solution two-hundredfold with n-isopropanol, placing a drop of the solution onto the silicon wafer and drying it under low vacuum. XRD patterns were collected using a Shimadzu XRD-7000 diffractometer with Cu K
The principal result for the ccNPs was obtained in the presence of ascorbic acid (AscAc) with the cationic surfactant CTAB concentration below the CMC resulting in the formation of nanostructures containing CuO, Cu2O, and Cu NPs. The SEM images (Figures
SEM images of the ccNPs with CTAB surfactant concentration below the CMC. (a) The SEM images and further magnified (b and c) show homogeneous and nearly spherical structures with size around 600 nm. Image (c) shows more details in the interior and the surface of the sphere with the top cover removed. (d) FESEM images show details in the enlargement of the highlighted area (inset) with the interior of structures filled with ccNPs showing that copper is incorporated inside the ccNPs with a size around 15 nm.
A new experiment was performed to evaluate the contribution of CTAB concentration in the formation of ccNPs. Figure
SEM images of the ccNPs with CTAB surfactant concentration above the CMC. All pictures present a distribution of NPs with irregular surfaces and nonspherical forms as well as random size.
To explore the real contribution of AscAc in the ccNPs formation, an experiment under the same conditions was performed in the absence of CTAB surfactant. This condition produced clusters of NPs with an undefined shape as seen in Figure
SEM images of the ccNPs without CTAB surfactant. (a) This condition produced structures with irregular shape and further enlarged picture (b) shows structures formed by clusters of NPs.
In order to further characterize the ccNPs, XRD was used to identify the formation of crystalline phases. Figure
(a) XRD spectrum after 3 and 120 days with CTAB below the CMC shows the diffraction peaks of Cu2O, CuO, and Cu. (b) The absorbance spectrum below and above CMC.
Figure
Self-assembly is a complex process; thereby a comparative analysis of results could contribute to our understanding of the steps involved in the process of synthesis. The formation of the ccNPs begins with the addition of AscAc in water. The initial nucleus is formed in accordance with the supplied concentration and was used in the experiment 420 mM sufficient to maintain pH at 2.8; however, the AscAc does not have a buffer effect. The growth conditions are a function of Brownian motion, the concentration of AscAc, the agitation, and constant temperature of the medium promoting the formation of symmetric structures which are very similar to the dendrimers [
In solution, initially, the electrostatic interaction is predominant and an unequal distribution of surfactant monomers occurs on the periphery of AscAcDen. At this stage, the structures remain formless and the surfactant monomers are attracted electrostatically to the AscAcDen edges, where the polar heads of CTAB bind to the –OH groups of the AscAc and then the tails accommodate on the dendrimertype in a more hydrophobic region.
The self-assembly of the final shape of ccNPs begins with the addition of copper resulting in an increase of positive charges in the solution. The CTAB alone does not attract copper; however the possibilities of interaction between copper and AscAc are larger and the structures formed are held together only by hydrogen bridges of –OH groups [
As the synthesis procedure was done in an acid environment, the –OH groups of AscAc allow bonds and thus the possibility of oxygen binding to copper is increased, although it has a limited number of internal binding sites in each AscAcDen. Copper may also be located inside the structure and maintained by hydrogen bridges. The cooper oxide surface layer on ccNPs appears to be inevitable because the oxide phases are thermodynamically more stable [
Figure
Previous studies described similar syntheses for obtaining copper NPs with cationic surfactant [
The CTAB molecules acting as a monomer and their hydrophilic and hydrophobic properties are very important to control the size and shape of the NPs. To complete the studies of the synthesis procedures a model was developed for the ccNPs with surfactant concentration below the CMC based on the images of scanning electron microscopy. The proposed model was generated with the DS Visualizer software [
Self-assembly model of the ccNPs. Illustration of the interaction between AscAcDen and CTAB below CMC to form the ccNPs with subsequent addition of Cu. The enlarged pictures show the CTAB located on the AscAcDen and the Cu distributed from the surface into the deeper region. The interactions between the ccNPs were made by a molecular velcro with CTAB tail forming AccNPs. For a better visualization of the structures size relationships were not considered in the molecular ratio.
We developed colloidal copper nanostructures in a new environmentally friendly synthesis. Our results strongly suggest that the self-assembly of the AscAc with CTAB below CMC form dendrimer-type structures allowing to control the size and homogeneous distribution of the ccNPs. The self-assembly clearly shows that the concentration of cationic surfactant CTAB plays an important role in controlling the size and shape of the ccNPs. The synthesis procedure resulted in copper oxides NPs that could be used in applications such as sensor sand energy conversion and mainly in the emerging flexible electronics with the possibility of printing directly on the substrate materials with specific properties. The advancement of new technologies such as the IPL sintering technique the formation of oxide stops being a problem and assumes particular attention in the development process. An efficient synthesis can be only considered when producing uniform NPs in size and shape because their physicochemical properties are directly related to these characteristics inasmuch as heterogeneous NPs do not have homogeneous properties. To strengthen the results obtained a model of ccNPs formation is proposed showing the action of molecular forces in the self-assembly with the CTAB monomer on the surface making the linkage, a molecular velcro. Our expectations with relation to the process developed are towards the application of the knowledge of self-assembly into new technologies.
R. K. Bortoleto-Bugs and M. R. Bugs contributed equally to this work.
The work was supported by the CI BRASIL program of the Ministry of Science, Technology and Innovation under Grant no. 301464/2011-1 (R. K. Bortoleto-Bugs) and 301105/2011-1 (M. R. Bugs). The authors gratefully acknowledge the financial support from the INCT NAMITEC (CNPq no. 573738/2008-4 and FAPESP no. 2008/57862-6), CNPq no. 477367/2011-9, the financial support from CTI Renato Archer/OGU, and the use of SEM facility of the Electron Microscopy Laboratory—LNLS, Campinas—SP, Brazil.