INTRODUCTION:

Since their discovery, carbon nanotubes (CNTs) have attracted great attention in the field of nanoscience and nanotechnology because of their exceptional properties and potential applications as breakthrough materials for energy storage, electronics and catalysis^1,2,3,4. Numerous studies have been undertaken to modify their surface to obtain the desired properties suitable for various applications^5,6,7. CNTs are considered as interesting support materials for catalytic properties because of their good water tolerance, unique hollow tubular structure, large surface area, large adsorption capacity, high chemical, thermal and mechanical stability and mesoporous character which favour the diffusion of reacting species^8,9. In this regard, many studies have been focused on depositing metal or metal oxide nanoparticles on their outer surface. CNTs have been extensively used as supports for various nanoparticles such as TiO₂, SiO₂, Fe₃O₄, CdS, Au, Pd, Pt and Ag etc.^10-15

Due to the turbostratic character of the hollow tubule nanostructures, it is possible for CNTs to function as supports for preparing nano-sized catalyst particles¹⁶. Since CNTs are largely inert, activating their surface is essential, and this has motivated numerous studies to improve metal dispersions on carbons, mainly through optimization of the metal supporting procedures or functionalization of the carbon surface¹⁷. The aromatic ring system of the CNTs can be disrupted by the application of oxidation reagents, such as HNO₃ or HNO₃/H₂SO₄ mixture, and therefore the nanotubes can be functionalized with groups such as hydroxyl (–OH), carboxyl (–COOH) and carbonyl (>C=O) that are necessary to anchor metal ions to the tube ¹⁸. As it is well known, CNTs have a strong tendency to agglomerate due to their nano-size and their respective high surface energy. However, the grafting of chemical functionalities on the CNTs surface, such as carboxylates, imparts negative charges and, therefore, creates the electrostatic stability required for a colloidal dispersion. Chemical functional groups, namely –COOH, –OH and >C=O derived from chemical oxidation processes act as anchoring sites for metal oxide nanoparticles and induce the impregnation of small and large particles¹⁹. As a result, the unique mechanical and electrical properties of CNTs can be transferred to the properties of CNT-based composites.

Among various metal oxide nanoparticles, increased attention has been focused on copper and copper oxide nanoparticles due to their redox chemistry, extraordinary electrical, thermal, catalytic and sensing properties^20,21. In this context, it is essential to control the nature, diameter, concentration and dispersion of the copper and copper oxide nanoparticles on the CNTs. Thus far, many attempts have been made to decorate copper and copper oxide nanoparticles on CNTs with a diameter ranging above 20 nm^22,23. The use of Cu(I) phenyl acetylide (CPA) as the precursor to deposit Cu₂O and Cu nanoparticles on thick MWCNTs having diameter ranging between 70 and 110 nm is recently reported²⁴. In the present study, we report the use of CPA as a source of copper, to achieve uniform dispersion of cuprous oxide nanocrystals on thin MWCNTs having an average diameter of 10 nm. Even though, deposition of copper and copper oxide nanoparticles on CNTs in the literature is reported ^25,26,27, the processes are either lengthy or tedious and in other cases surfactants and reducing agents have been used to control the size of the particles. Whereas an easy and efficient impregnation method to produce uniformly dispersed Cu₂O nanocrystals without any usage of surfactants and reducing agents, is reported here. Moreover, the content of Cu₂O nanocrystals decorated on MWCNTs can be controlled by varying the ratio of copper to MWCNTs.

Experimental details:

Origin of the thin MWCNTs:

The thin MWCNTs (NC-3100) for the study, procured from Nanocyl S.A., were synthesized by the decomposition of ethylene using the CCVD method. These MWCNTs have an average diameter of 10 nm. They are several micrometres (0.1–10) in length and have more than 95% purity.

Oxidation of the thin MWCNTs:

The as-received samples were treated with concentrated (H₂SO₄/HNO₃ : 3/1) acids for 8 h at 50 °C to generate oxygen functionalities on the surface of MWCNTs^28,29,30. In a typical experiment, 75 mL of concentrated H₂SO₄ (97%) and 25 mL of concentrated HNO₃ (65%) were carefully mixed together and added to 1 g of MWCNTs in a round-bottomed flask. The above suspension was then heated under constant agitation at 50 °C for 8 h. Afterwards, the mixture was allowed to cool down to room temperature, filtered and the residue was washed several times with deionised water until neutral pH was attained. The residue was then filtered and freeze dried for overnight. The weight of the residue was found to be 0.812 g.

Preparation of copper-containing nanoparticles on the oxidized thin MWCNTs:

A 100 mg of oxygen-functionalized MWCNTs was sonicated in 50 mL of xylene for an hour to form a good suspension. Similarly 2.6 × 10⁻³ M of CPA (4.76 wt% of Cu) in 30 mL of xylene was also sonicated for the same amount of time. While the suspensions were still under ultrasonication, CPA in xylene was added to the MWCNTs in xylene suspension. The mixture was further sonicated for a few minutes and then refluxed for 12 h under magnetic stirring. The mixture was cooled to room temperature, filtered and the residue was washed three times with acetone. The copper-treated MWCNTs thus obtained were finally dried under vacuum at room temperature. The weight of the residue was found to be 110.05 mg.

Formation of Cu₂O nanocrystals on the oxidized thin MWCNTs:

A 50 mg of the copper-treated MWCNTs was then placed on a quartz boat and heated in a quartz tube at 400 °C for 3 h in a nitrogen atmosphere. The weight of the residue was found to be 45.10 mg. The copper-treated MWCNTs, thus obtained, were finally dried under vacuum at room temperature. It is noted that necessary oxygen to produce Cu₂O nanocrystals is produced by thermal decomposition of the functional oxygenated groups attached to the CNTs. Hence the key to obtain only Cu₂O instead of metallic Cu, as well as a greater amount of CuO, is the excess of oxygen contained in the treated CNTs. The above procedure was carried out with different weight percentages of copper (9.09, 13.04 and 16.67) on MWCNTs to study the extent of deposition and Cu₂O nanocrystals distribution on the MWCNTs surface.

Characterization:

Transmission electron microscopy (TEM) was used to obtain information on the dispersion of the active phase, its morphology, particle size and distribution of copper-containing nanoparticles on MWCNTs. This was done by low resolution TEM using a TECNAI 10 PHILIPS microscope. Samples were dispersed in ethanol, and a drop of the suspension was deposited on a holey carbon-coated copper grid for TEM analysis. X-ray powder diffraction (XRD) analysis was performed to determine the structure and crystallographic phase of the sample. The samples were characterized by XRD in PHILIPS-7602EA operating with 40 kV of voltage and 30 mA of tube current to obtain CuKα radiation (wave length 0.154 nm) and with 1 h scanning from 10° to 80°. Field-emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray analysis (EDX) of the copper-containing nanoparticles on MWCNTs were carried out to know the nature of the distribution and the morphology of the nanoparticles on CNTs surface. This was carried out by a FE-SEM equipped with an EDX in a JEOL JSM-7500F operating at 20 kV at a working distance of 8 mm. X-ray photoelectron spectroscopy (XPS) spectra were recorded at 35° take-off angle (relative to the surface normal) with an SSX-100 spectrometer using monochromatized AlKα radiation (1486.6 eV). The analyzed core-level lines were calibrated with respect to the component C1s of binding energy set at 284.6 eV. Thermogravimetric analysis (TGA) measurements were performed on a METTLER TOLEDO TGA/SDTA 851e. The samples were heated from 30 to 800 °C at 10 °C/min in nitrogen. The residual mass loss was recorded as a function of temperature.

RESULTS AND DISCUSSION:

XRD analysis of Cu₂O nanocrystals on MWCNTs:

The crystallinity of the copper-containing nanoparticles on MWCNTs, introduced by the CPA treatment, was assessed from their XRD patterns. The XRD patterns of the samples, dried at room temperature and after heat-treatment at 400 °C in a nitrogen atmosphere are represented in Fig. 1a and b, respectively. In each case, the four XRD spectra are stacked together starting from 4.76 up to 16.67 wt% of copper on MWCNTs, in an ascending order. There is a substantial difference seen in the diffractograms recorded before and after the heat-treatment. The room temperature dried samples in Fig. 1a show peaks corresponding to only the graphitic structures of the CNTs, whereas the heat-treated samples in addition show peaks (Fig. 1b) for Cu₂O nanocrystals as well. This significant difference in case of heat-treated samples in XRD patterns noticed may be due to the complete decomposition of organic part of the CPA at 400 °C in nitrogen to form well crystallized Cu₂O nanoparticles on MWCNTs. The diffractograms shown in Fig. 1a and b reveal the phase purity of the material. Despite the small size (11 nm) of Cu₂O nanoparticles, they were well crystallized. The diffraction peaks at 26° and 43° correspond to the graphitic structure of the CNTs. The diffraction peaks at 29.71°, 36.58°, 42.50°, 61.66°, 73.90° and 77.72° corresponding to (110), (111), (200), (220), (311) and (222) crystal planes of Cu₂O, respectively, indicate the formation of Cu₂O nanocrystals ²⁰. The result indicates that Cu₂O nanoparticles are well crystallized on MWCNTs after the heat-treatment. The Cu₂O deposited on the MWCNTs are composed of pure crystalline Cu₂O nanoparticles, since no other significant impurity peaks were observed in the XRD patterns.

Fig. 1 a XRD patterns of copper-containing nanoparticles on MWCNTs, b XRD patterns of Cu₂O nanocrystals on MWCNTs, heat-treated in nitrogen

The mean size of the crystalline Cu₂O nanoparticles were calculated from the major diffraction peak (111) using the Debye-Scherrer formula,L = kλ/βcosθ, where L is the mean dimension of the particle, β is the full width at half maximum of the diffraction peak, θ is the Bragg angle, λ is the wavelength of CuKα radiation (0.154 nm) and k is constant dependent on crystallite shape equal to 0.89²⁶. From the diffraction peak (111), taken for calculation, it was found that the Cu₂O nanocrystals on MWCNTs have an average particle size of about 11 nm.

TEM analysis of copper-containing nanoparticles on MWCNTs:

The morphologies of the copper-treated MWCNTs were investigated by TEM as shown in Fig. 2a–f. The room temperature dried sample images displayed in Fig. 2a and b, depict clearly copper-containing nanoparticles well dispersed on the outer surface of the MWCNTs. Figure 2c and d show the low magnification TEM images, and Fig. 2e and f represent the high magnification TEM images of the heat-treated samples at 400 °C in a nitrogen atmosphere. It can be seen in Fig. 2c–f that Cu₂O nanocrystals with irregular shapes are homogeneously dispersed on MWCNTs surface. The copper-containing nanoparticle sizes estimated from TEM fall between the ranges of 10–80 nm. TEM images have been taken, before and after heat-treatment at 400 °C in nitrogen, to ensure the observation of the structure and morphology changes. There is a significant observation made, in either case, in terms of the deposition and size of the nanoparticles. The room temperature dried samples show small size particles well decorated on the outer surface of the tubes as shown in Fig. 2a and b. However, some particles are quite large and isolated. There is a high degree of spheroid particles observed, despite the difference in their sizes. The heat-treated samples at 400 °C show a highly homogeneous distribution of cuprous oxide nanocrystals with compact shapes and almost uniform sizes in a similar pattern all over the MWCNTs surface. However, after heat treatment the average size of the particles was larger than the average size of the particles produced at room temperature. This could be as a result of heating at high temperature. The small particles due to the capillary action have assembled together and formed a slightly large mass with compact shape, conglomerated and well distributed all over the surface in a similar pattern³¹. This is a unique phenomenon we observed as shown in the low magnification images in Fig. 2c and d. There seems to be two main functions of the aromatic ring of CPA which may have also influenced the interfacial adhesion and homogeneous distribution. The aromatic ring interacts with CNTs via π–π interaction and secondly it links copper particles with CNTs and leads to the uniform dispersion of nanoparticles on CNTs^13,17. The Cu⁺ could be bound to the surfaces of the MWCNTs through combining the lone electron pairs of the oxygen atoms of carboxyl groups on sidewalls of the MWCNTs and the empty orbitals of the copper atom³².

Fig. 2 a and b TEM images of 4.76 wt% copper-containing nanoparticles on MWCNTs; c, d, e and f TEM images of Cu₂O nanocrystals on MWCNTs, heat-treated in nitrogen

FE-SEM and EDX analysis of Cu₂O nanocrystals on MWCNTs

FE-SEM was performed to detect possible morphological changes on MWCNT specimens depending on the treatment. Figure 3a shows the fragmentation of the MWCNTs surface in the formation of flakes, which is due to the oxidation treatment (H₂SO₄/HNO₃ : 3/1) before using them in deposition of metal oxide nanoparticles. The low magnification SEM images in Figure 3a clearly show the decoration of Cu₂O nanocrystals in all the flakes in a similar pattern. The Cu₂O nanocrystals are homogeneously distributed on the surface of MWCNTs as shown in Fig. 3b, c, and d. Figure 3d displays high magnification SEM images of Cu₂O spheroid particles of compact shapes, conglomerated on the surface of MWCNTs. EDX spectra in Fig. 4a and b represent the weight ratio of copper loaded on MWCNTs with reference to carbon. The mass percentages of copper on MWCNTs can be evaluated quantitatively as 5.11% and 9.03% (Fig. 4a and b) for the 4.76 and 9.09 wt% of copper initially utilized in the experiment. This shows that almost all the amount of copper loaded initially is accounted for in the product. Thus, the one-step impregnation method used with CPA as the source of copper, followed by the heat-treatment at 400 °C in nitrogen, is proved to be efficient in retaining the initial amount as well as uniform distribution of Cu₂O nanocrystals on the surface of MWCNTs, which could be useful in various applications.

Fig. 3 a, b, c and d FE-SEM images of 4.76 wt% Cu₂O nanocrystals on MWCNTs, heat-treated in nitrogen

Fig. 4 a and b EDX spectra of Cu₂O nanocrystals on MWCNTs containing 4.76 and 9.09 wt% of Cu, respectively

TGA of copper-containing nanoparticles on MWCNTs

The thermal decomposition was investigated by TGA in a flowing nitrogen atmosphere at the heating rate of 10 °C/min as shown in Fig. 5a, b and c. Figure 5a displays the thermograms of the pristine and oxygen-functionalized MWCNTs. The weight loss as a function of temperature in the case of pristine MWCNTs was seen after 650 °C. Whereas in the case of oxygen-functionalized MWCNTs, the weight loss was observed in several steps starting from 100 °C up to 800 °C and further. The total weight loss observed was about 30% for oxygen-functionalized MWCNTs. This weight loss observed may be due to the degradation of different oxygen-functionalities on the MWCNTs. Figure 5c represents the thermograms of CPA and different amounts of copper-treated MWCNTs all together. In the case of CPA, two stages of degradation on weight loss were clearly seen, one at 230 °C having 15% weight loss that may correspond to the loss of the acetylene moiety, and the other at 405 °C having 10% weight loss. The other thermograms of 4.76, 9.09, 13.04 and 16.67 wt% content of Cu on MWCNTs show the same pattern with the different stages of degradation, as already noted in the thermograms of oxygen-functionalized MWCNTs. However, as shown in Fig. 5b, there is a slight difference in weight loss observed in the thermograms of copper-treated MWCNTs compared to the oxygen-functionalized MWCNTs.

Fig. 5 a, b and c TGA graphs of copper-containing nanoparticles on MWCNTs in nitrogen at 10 °C/m in heating rate

This difference in weight loss is attributed to the degradation of the organic part of CPA molecules. It is possible that around 400 °C, all the organic part of CPA would have decomposed and so all the samples were heat-treated at that temperature in the presence of nitrogen, to ensure complete degradation of the organic part of CPA. Notably, the different weight percentages of copper on MWCNTs had no substantial effect on weight loss pattern.

XPS analysis of Cu₂O nanocrystals on MWCNTs

XPS analyses were performed to determine the electronic structure, chemical bonding and composition of materials³³. The general survey of XPS spectra of the pristine MWCNTs and oxygen-functionalized MWCNTs are displayed in Fig. 6a and b. The main peak at 284.6 eV, in either case, is attributed to sp²-hybridized carbon in the graphitic layers of the CNTs. In Fig. 6b, there is an intense O1s peak, in addition to C1s peak (Fig. 6a) noticed, which is due to the treatment of MWCNTs in the mixture of concentrated acids (H₂SO₄/HNO₃ : 3/1)²⁹. It was calculated from the XPS spectra and found that there was about 18% of oxygen in comparison to 82% carbon on the surface of the oxygen-functionalized MWCNTs. Figure 7 shows the XPS general spectra, with copper 2p and copper Auger spectra enclosed, of the 16.67 wt% of copper on MWCNTs, taken after the treatment of MWCNTs with CPA, dried at room temperature. The spectra shown in Figure 7 suggest the presence of CuO on the surface layer (i.e. a Cu²⁺ peak in the spectrum) on Cu₂O nanocrystals, seen by XRD. The shake-up satellite peaks seen in Figure 7 are evident and characteristic of an open 3d⁹ shell, corresponding to a Cu²⁺ state. The relative intensities of the shake-up satellites from these levels are indicative of the presence of CuO at the surface. The presence of CuO on the surface layer is found in all the samples invariable of the different content of copper on MWCNTs and the heat-treatment of the samples. XPS, as a surface analyzing tool, is sensitive to information on surface layer of about 7 nm for inorganic materials. Thus, the XPS signal is proportional to the surface layer volumes. For the heat-treated samples, the fact that XRD does not show evidence of CuO phase, while XPS indicates the surface presence of Cu²⁺ ions, suggested that CuO is present only on the surface of the Cu₂O nanocrystals and forms a thin amorphous outer shell ³³.

Fig. 6 a and b XPS spectra of pristine MWCNTs and of oxygen-functionalized MWCNTs, respectively

Fig. 7 XPS general spectra with copper 2p and copper Auger spectra enclosed

CONCLUSIONS:

An easy, efficient and one-step impregnation method to produce a homogeneous distribution of copper-containing nanoparticles decorated on oxygen-functionalized MWCNTs having an average diameter of 10 nm is reported in the present study. The heat-treatment of the copper-treated MWCNTs leads to Cu₂O nanocrystals, of 10 to 80 nm, well distributed on the MWCNTs surface. It is also reported that by varying the ratio of copper to oxygen-functionalized MWCNTs, Cu₂O nanocrystals decorated on MWCNTs with different copper content can be obtained. The products were characterized by different analysis techniques, such as XRD, TEM, FE-SEM, EDX, XPS and TGA, to confirm the presence of cuprous oxide nanocrystals well decorated on MWCNTs. To the best of our knowledge, though CPA was used before in the literature²⁴, it is important to note that there are significant differences observed in terms of a homogeneous dispersion of copper-containing nanoparticles on MWCNTs and Cu₂O nanocrystals on MWCNTs surface. Moreover, the average diameter of MWCNTs (10 nm) used in the present study is much smaller than the diameter of MWCNTs (70–110 nm) used in the previous study, in addition to the oxidation treatment (H₂SO₄/HNO₃ : 3/1) and differences in several parametres in the experimental procedure. However, it is worth making further studies on the role of CPA in the deposition of copper and copper oxide nanoparticles on MWCNTs. It can be expected that as-obtained cuprous oxide nanocrystals decorated on MWCNTs would be of use in many potential applications, especially in catalysis.

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Received on 24.11.2011 Modified on 19.12.2011

Asian J. Research Chem. 5(1): January 2012; Page 116-124