Single-wall carbon nanotube modified with copper-oxamate flat complex probed by synchrotron x-ray photoelectron and x-ray absorption spectroscopies
Introduction
Carbon nanotubes (CNTs) have gained immense popularity mainly because of their high mechanical strength, flexibility, and unique electronic properties [1,2]. Single-wall carbon nanotubes (SWCNTs) exhibit electrical conductivity similar to that of copper or silicon (if the SWCNTs are metallic or semiconductor, respectively). The thermal conductivity of SWCNT along the tube axis is comparable to that of diamond at a single-nanotube level [1,2]. SWCNTs may constitute the basis for a myriad of nanomaterials and devices [3].
The remarkable physical and chemical properties of CNTs have stimulated the investigation of new CNT-based nanostructures, mainly those formed from polymers, metal particles, and organic and biological molecules [[4], [5], [6], [7], [8]]. In the past few years, there has been an increased interest in understanding the electronic structure of single (SWCNTs) or double-walled carbon nanotubes (DWCNTs) post doping [5,6]. By doping nanotubes, it is possible to control and tune their electronic properties. In addition, these doped systems also open up the opportunity for studying the basic physical properties of nanotubes in a controlled way. In this regard, it is worth mentioning that the properties of carbon nanotubes can be modified by any kind of charge-transfer process, such as electrochemical and/or chemical doping [7,8]. The combination of carbon nanotubes with other molecules can lead to the formation of unique composites, with enhanced chemical and/or physical properties. The modified CNTs have many possible applications, such as in cylindrical molecular capacitors, GHz oscillators, nanocomposites, field emission sources, nanotube cables, and electronic devices [3].
In recent time, a revolution in electronics is gradually emerging in which spintronics and molecular electronics have combined with carbon nanotubes [9,10]. A fundamental link between these two fields can be established using molecular magnetic materials, and in particular, single-molecule magnets. These compounds can produce materials with a magnetization relaxation time, which is extremely long at low temperatures and has both high-density information storage and long coherence time for quantum computing [11]. There are many reports in the literature showing slow relaxation of the magnetization [12,13]. However, studies on the synthesis and characterization of molecule-based magnets supported on solid matrices are inadequate in the literature [14,15].
In a previous work, the electronic interactions between the carbon nanotubes and the building block [Cu(opba)]2− [where opba = orthophenylenebis(oxamate)] were studied by resonance Raman spectroscopy [9]. The results suggested a specific interaction between the complex and the SWCNTs with diameters larger than 1.21 nm. This reported diameter is within the diameter distribution of our current sample. The interaction is a function of the amount of complex present in the composite, and this can be related to the presence of a large amount of complex adsorbed on the carbon surface of the SWCNTs.
Nevertheless, in order to better characterize the charge-transfer mechanism between the SWCNTs and [Cu(opba)]2− molecules it is important to know which atoms are involved in the charge-transfer process. For this purpose, X-ray absorption near edge spectroscopy (XANES) and X-ray photoelectron spectroscopy (XPS) are very efficient techniques to study electronic changes for which atom present in a sample. Both techniques involve the excitation of “core” electrons of the atoms and each absorption energy edge is related to a specific atom present in the material [16,17]. XANES and XPS spectra are sensitive to the electronic density around the absorbing atom and can be used to study changes in the electronic distribution due to charge-transfer, oxidation or electronic delocalization via resonance and/or conjugation effects [[18], [19], [20]].
The goal of the present work is to characterize which atoms are involved in the charge transfer process between a molecular conductor (carbon nanotubes) and the precursor of molecule-based magnets [Cu(opba)]2− using synchrotron X-ray absorption and photoelectron spectroscopies. Hence, in the present work, the XANES and XPS at different edges (Carbon K, Nitrogen K and L edges, and Cu edge) were used to investigate the electronic interaction between the [Cu(opba)]2− anions and the SWCNTs tubes. In addition, ATR-FTIR data were also used to support the characterization of the nanocomposites. These materials can produce very different magnetic/electronic properties and open up the opportunity to study in a future some interesting spintronic effects.
Section snippets
Chemicals and materials
The compounds CuCl2·2H2O (Aldrich), CuI (Aldrich), Cu(CO2CH3)2 (Aldrich), sodium dodecyl sulfate (Merck), and KNO3 (Merck) were used as received. The molecule-based precursors (C4MIm)2[Cu(opba)]·3H2O and (NBu4)2[Cu(opba)] were prepared according to refs [20] and [21], respectively. The SWCNTs were prepared by the HiPCO process [22].
Preparation of SWCNTs modified with (NBu4)2[Cu(opba)]
The SWCNTs were dispersed in sodium dodecyl sulfate in 1:10 w/w ratio. The resulting suspension was sonicated in a tip ultrasonic bath for 2 h (titanium tip was
Results and discussion
Fig. 1 shows the XPS spectra acquired in large scan mode of powdered samples of pristine SWCNTs, crystalline sample of the complex (NBu4)2[Cu(opba)], and the nanocomposite formed by SWCNTs and the metal complex. The samples were excited with photons (Eo) having energy 800.0 eV (Eo, SGM beamline). Kα1 lines of light elements, e.g. carbon, nitrogen, and oxygen appeared, as indicated in Fig. 1 (carbon, 277 eV, nitrogen, 392.4 eV, and oxygen, 524.9 eV [37]). The presence of nitrogen atoms was
Conclusions
The (NBu4)2[Cu(opba)]-SWCNT nanocomposites were investigated by XANES and XPS data at different edges and by ATR-FTIR technique. The presence of copper(II) metal complex on SWCNTs was observed. Previously, from resonance Raman spectroscopy, only the characteristic bands of the nanotubes could be seen. In the current work, the presence of nitrogen and copper atoms in the complex and their changes due to interactions with the nanotubes were investigated. In addition, the micro- ATR-FTIR data also
Acknowledgment
CNPq, FAPEMIG, CAPES (Brazilian agencies), supported this work. G. M. do Nascimento acknowledges CAPES for his post-doctoral fellowship (PNPD program). The authors would like to thank the LNLS for the use of SGM beamline (Proposal No 10685 and 12607) and TGM beamline (Proposal No 15352). Thanks to the CEM (Central Experimental Multiusuário) of UFABC. Special thanks to Dr. Fabio Rodrigues and Dr. Douglas Galante for their helpful support during the XANES experiments. Y.A.K. acknowledges the
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