Noncovalent Grafting of a DyIII2 Single-Molecule Magnet onto

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Noncovalent Grafting of a DyIII2 Single-Molecule Magnet onto Chemically Modified Multiwalled Carbon Nanotubes Vassilis Tangoulis,*,† Nikolia Lalioti,† John Parthenios,‡ Nikos Boukos,§ Ondřej Malina,∥ Jiří Tuček,∥ and Radek Zbořil∥ †

Department of Chemistry, University of Patras, GR-26504 Patras, Greece Foundation for Research and Technology, Hellas (FORTH), Institute of Chemical Engineering Sciences (ICE/HT), P.O. Box 1414, GR-26504 Patras, Greece § Institute of Nanoscience and Nanotechnology, National Centre for Scientific Research “Demokritos”, Patriarchoy Grigoriou & Neapoleos Str., GR-15310 Agia Paraskevi Attikis, Athens, Greece ∥ Regional Centre of Advanced Technologies and Materials, Departments of Physical Chemistry and Experimental Physics, Faculty of Science, Palacký University in Olomouc, Šlechtitelů 27, 78371 Olomouc, Czech Republic ‡

S Supporting Information *

ABSTRACT: While synthetic methods for the grafting of nanoparticles or photoactive molecules onto carbon nanotubes (CNTs) have been developed in the last years, a very limited number of reports have appeared on the grafting of singlemolecule magnets (SMMs) onto CNTs. There are many potential causes, mainly focused on the fact that the attachment of molecules on surfaces remains not trivial and their magnetic properties are significantly affected upon attachment. Nevertheless, implementation of this particular type of hybrid material in demanding fields such as spintronic devices makes of utmost importance the investigation of new synthetic protocols for effective grafting. In this paper, we demonstrate a new experimental protocol for the noncovalent grafting of DyIII2 SMM, [Dy2(NO3)2(saph)2(DMF)4], where H2saph = N-salicylidene-o-aminophenol and DMF = N,N-dimethylformamide, onto the surface of functionalized multiwalled CNTs (MWCNTs). We present a simple wet chemical method, followed by an extensive washing protocol, where the cross-referencing of data from high-resolution transmission electron microscopy combined with electron energy loss spectroscopy, conventional magnetic measurements (direct and alternating current), X-ray photoelectron spectroscopy, and Raman spectroscopy was used to investigate the physical properties, chemical nature, and overall magnetic behavior of the resulting hybrids. A key point to the whole synthesis involves the functionalization of MWCNTs with carboxylic groups, which proved to be a powerful strategy for enhancing the ability to process MWCNTs and facilitating the preparation of hybrid composites. While in the majority of analogous hybrid materials the raw carbon material (multiwalled or single-walled nanotubes) is heavily treated to minimize the contribution of contaminant traces of magnetic nanoparticles with important effects on their electronic properties, this method can lead easily to elimination of the largest part of the impurities and provide an effective way to investigate/discriminate the magnetic contribution of the SMM molecules. crystal field and strong spin−orbit coupling.13−15 Nowadays, the synthesis of multifunctional molecular materials, i.e., materials that combine multiple features, is quite appealing for synthetic chemists and materials scientists. These materials are mainly composites or nanocomposites, where various components are integrated in a matrix of a main component and their properties are modified accordingly.16−19 The multifunctional molecular materials combine two or more physical properties in the same crystal lattice and cover a

1. INTRODUCTION The discovery of single-ion (single-ion magnets, SIMs) or polynuclear complexes of paramagnetic 3d/4f metal ions (single-molecule magnets, SMMs) exhibiting properties of bulk magnets but at the molecular level has received outstanding recognition over the last 3 decades.1−8 In particular, for the case of lanthanide (Ln)-containing molecules, a wide range of applications have been envisaged in the area of quantum information processing9,10 as well as molecular spin valves11 and transistors.12 The origin of the SMM behavior observed for Ln complexes is closely related to the large magnetic anisotropy of the 4f ions, induced by the effects of the © XXXX American Chemical Society

Received: February 21, 2018

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DOI: 10.1021/acs.inorgchem.8b00472 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry plethora of different compounds.20−22 One of our goals in this interdisciplinary field of research is the synthesis of hybrid multifunctional materials where SMM molecules are deposited/ grafted/encapsulated onto/into suitable carbon substrates. The carbon nanotubes (CNTs) are excellent candidates for substrates in this context,23 based on (i) their high sensitivity to any change in their chemical environment and, hence, electronic properties and (ii) their extremely low spin−orbit and hyperfine couplings providing high-spin coherence lengths.24 Carbon-based hybrid multifunctional materials basically depend on the chemical functionalization of CNTs. This functionalization treatment is furthermore related to solubilization methods of the nanotubes.25 Considering the morphology of the CNTs, three possibilities can be considered: (a) Noncovalent f unctionalization of the CNTs through π−π stacking. Noncovalent functionalization appears as a very attractive method for the derivatization of CNTs because it provides an efficient way of attaching chemical species without affecting the electronic structure of the tubes.26 The noncovalent interactions are based on either π−π stacking or weak van der Waals forces. For example, Ruben and co-workers reported the binding of rare-earth-based SMMs over the nanotube wall, exploiting an advantage of well-known π−π affinity.27 In a similar manner, Bogani et al. obtained the hybrid composed of CNT@[Fe4L2(dpm)6] SMM (Hdpm = dipivaloylmethane), where the ligand L bears a pyrenyl moiety.28 The controlled organization of SMMs on single-walled nanotubes (SWNTs) was investigated also with the anionic polyoxometalate [Fe4(H2O)2(FeW9O34)2]6+.29 In all previous cases, the produced hybrid materials retained the magnetic bistability of the pristine SMM unit. These hybrid systems are particularly interesting in molecular spintronics, where the organization of a number of nanomagnets over an electronic nanodevice is vital. (b) Covalent derivatization of their wall. Coronado and coworkers30 showed the possibility of grafting electrostatically cationic SMMs onto functionalized anionic multiwalled carbon nanotubes (MWCNTs); here, the positively charged manganese tetranuclear complex [Mn4(O2CCH3)2(pdmH)6]4+ (pdmH = deprotonated pyridine-2,6-dimethanol) was chosen as the molecular nanomagnet. It was found that its magnetic behavior is considerably influenced by the grafting process via surface effects. (c) Endohedral f unctionalization of the CNT’s cavity where filling CNTs with foreign elements or compounds can lead to modification of the properties of both CNTs and the encapsulated species. A very interesting application comes from the encapsulation of Mn12 SMMs inside the cavity of MWCNTs.31 In this case, the CNTs serve to bind the nanometric magnets with the macroscopic world, eventually resulting in a hybrid that possesses the unique electronic properties of CNTs and the interesting magnetic characteristics of SMMs. Many different applications can be imagined considering the unique physical properties of CNTs. A very appealing example of a CNT-field-effect-transistor (FET) device was reported by Mallah et al.32 In this work, a binuclear paramagnetic Cu2 complex was attached to SWNTs through π−π interactions, and the final hybrid material exhibits CNTFET device properties based on charge transfer between the complex and the carbon material. The general grafting experimental methodology includes several “gray areas” and needs to be explored more thoroughly. More specifically, several questions remain unanswered, namely, the following: (a) Can the magnetic properties of the

SMMs be altered (possible enhanced) through the grafting processing? (b) The hybridization or environmental effects are responsible for these (if any) changes in the magnetic behavior of the SMMs; (c) Which of the three aforementioned methods of chemical functionalization is/are the most suitable for an effective grafting of SMMs onto/into the nanotubes without dramatically affecting the electrical conductivity of them? (d) Can the magnetic contribution of the functional SMM molecules be experimentally characterized in comparison to the general magnetic response of the hybrid material influenced (possibly) by the contribution of contaminant traces of magnetic nanoparticles? It is crucial for the magnetochemists to understand the adhesion of SMMs to a conducting surface structure, while controlling the inherent magnetic properties of the SMM molecule itself. In this work, we developed a new chemical strategy for the attachment of SMM molecules to chemically functionalized CNTs and the fabrication of a hybrid material where the grafted SMM molecule retains its magnetic characteristics. The functionalization process debundles MWCNTs, increasing their available surface area for the subsequent attachment of individual finely dispersed SMM molecules. In detail, our strategy involves the covalent chemical functionalization of MWCNTs, yielding carboxylic MWCNT derivatives. Effective grafting of DyIII2 SMM, [Dy2(NO3)2(saph)2(DMF)4], where H2L = N-salicylidene-o-aminophenol and DMF = N,Ndimethylformamide,1 onto the sidewalls of the chemically modified anionic MWCNT was possible due to π−π-stacking effects (noncovalent interaction via the N-salicylidene-o-aminophenol ligands). The main advantage of this experimental protocol is related to the fact that the MWCNT retains its integrity and electronic structure and therefore its electron conductivity. The freshly prepared hybrid materials were characterized by high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, X-ray photoelectron microscopy (XPS), and magnetic direct-current (dc) and alternating-current (ac) measurements. The washing protocol is clearly outlined involving continuous monitoring of the composite and filtrates after each washing cycle using a combination of IR/UV spectroscopy and electron microscopy. Although the magnetic behavior of the raw carbon materials is influenced by the contribution of contaminant traces of ferromagnetic nickel magnetic nanoparticles, it was possible to eliminate the largest part of the impurities and provide an effective way of investigating/discriminating the magnetic contribution of the SMM molecules in the final hybrid materials and calculating the molar concentration of the DyIII2 molecules directly from the magnetization data.

2. EXPERIMENTAL SECTION 2.1. Materials, Instrumentation, and Physical Measurements. MWCNTs (greater than 95% pure; diameter 20−30 nm; length 0.5−2 μm) were obtained from Sigma-Aldrich (CAS No. 308068-56-6). The manufacturer indicates that the CNTs contain less than 5% impurities, including an amorphous content of less than 2%, Fe < 0.1%, and Ni < 2%. The filtration was employed with a vacuum filter funnel of pore size number 3, using either poly(tetrafluoroethylene) (PTFE) membrane filters of 0.45 μm pore size or nitrate membrane filters of 0.45 μm pore size. 2.1.1. IR/UV−Vis/TGA. IR spectra (400−4000 cm−1) were recorded on a Nicolet FT-IR 6700 spectrometer with samples prepared as KBr disks, while the attenuated total reflection (ATR) spectra were recorded on a Bruker Optics Alpha-P Diamond ATR spectrometer from Bruker Optics GmbH. UV−vis spectra were recorded in various B


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Inorganic Chemistry solution concentrations in the range 10−5−10−3 M using a Hitachi U2001 dual-beam spectrophotometer. Elemental analysis was performed on a PerkinElmer 240B elemental analyzer. Thermogravimetric analysis (TGA) was carried out on ∼8 mg samples in alumina crucibles using a Labsys TM TG apparatus from Setaram under nitrogen and at a heating rate of 10 °C min−1. 2.1.2. Magnetic Measurements. A Quantum Design physical properties measurement system (PPMS Dynacool system) was used for the dc magnetic measurements of powder samples (T = 1.9−300 K at external magnetic field H = 5000 Oe). All of the corrections of the experimental data for the diamagnetism and the signal of the sample holder have been taken into consideration. The hysteresis loops were recorded at a temperature of 2 K in external magnetic fields ranging from −60 to +60 kOe. The ac magnetic measurements were measured using a Quantum Design MPMS-XL SQUID magnetometer at 2−300 K with a rate of 1 K min−1. For observation of the out-of-phase susceptibility signal, the nonzero static magnetic field (Hdc = 1000 Oe) for various frequencies (10, 100, 997, and 1488 Hz) of a small ac field (Hac = 3 Oe) was used. 2.1.3. XPS Measurements. The XPS measurements were carried out with a PHI 5000 VersaProbe II XPS system (Physical Electronics) with a monochromatic Al Kα source (15 kV, 50 W) and a photon energy of 1486.7 eV. Mounting of the powder samples on the sample holder was performed using double-sided tape (Scotch). The vacuum used for all spectra was 1.2 × 10−7 Pa, while all of the measurements were done at a temperature of 21 °C. The area of analysis for each sample was a spot having a diameter of 100 μm. The survey spectra were measured with the following technical details: (i) pass energy of 187.850 eV; (ii) electronvolt step of 0.8 eV. For the high-resolution spectra, technical details are as follows: (i) pass energy of 23.500 eV; (ii) electronvolt step of 0.2 eV. For all of the measurements, dual-beam charge compensation was used. MultiPak (Ulvac-PHI, Inc.) software was used for evaluation of the spectra. All of the values of the binding energy (BE) were referenced to the carbon peak C 1s at 284.80 eV. 2.1.4. HRTEM. Transmission electron microscopy (TEM) was performed using a FEI CM20 microscope, operating at 200 kV accelerating voltage, equipped with a Gatan GIF200 energy filter for electron energy loss spectroscopy (EELS) measurements and energy filter TEM (EFTEM) mapping. The sample was prepared by casting several drops of a hexane suspension of the hybrid material onto a copper-supported lacey carbon film, which was left to dry in air. 2.1.5. Raman Measurements. The Raman spectra were collected with a Renishaw InVia Reflex instrument at 785 nm (1.58 eV). The laser was focused on the sample using a 100× objective, while the laser power was kept below 1.5 mW to eliminate laser-heating effects on the probed materials. 2.2. Synthetic Protocols. 2.2.1. Synthesis of [Dy2(NO3)2(saph)2(DMF)4 (Dy2). All of the synthetic procedures described herein were performed under aerobic conditions using materials of reagent grade and solvents as received. The organic ligand saph and the Dy2 compound (Figure S1) were synthesized in typical yields of >70% following the reported method.1 Solid Dy(NO3)3· 6H2O (0.226 g, 0.5 mmol) was added to a stirred dark-yellow solution of H2saph (0.107 g, 0.5 mmol) and triethylamine (0.067 mL, 0.5 mmol) in a solvent mixture comprising acetonitrile (MeCN; 5 mL) and DMF (5 mL). The solid was dissolved easily, and the resulting yellow solution was further stirred for a time period of 10 min and finally filtered. The resulting solution was further layered using diethyl ether (Et2O; 20 mL). After 4−5 days, pale-yellow rhombohedral crystals of the product appeared and were collected by filtration, washed with MeCN (1 mL) and Et2O (3 × 2 mL), and dried in air. The yield was ∼65%. Anal. Calcd for C38H46Dy2N8O14 (found): C, 39.41 (39.18); H, 4.04 (4.12); N, 9.72 (9.57). IR bands (KBr, cm−1): 3142w, 3036w, 2931w, 1665s, 1647s, 1607s, 1583m, 1535m, 1481sh, 1464s, 1454sh, 1381s, 1342m, 1324w, 1285s, 1255m, 1243w, 1171m, 1149s, 1105m, 1043w, 1027m, 975w, 917m, 867m, 856sh, 827s, 821sh, 777sh, 761s, 679m, 663m, 599m, 539w, 507sh, 493m, 447w, 411w. 2.2.2. Shortening and Functionalization of MWCNTs. Shortening of MWCNTs was achieved by employing an ultrasonic processor (Hielscher UP400S 400W, 24 kHz). A sample of 3.5 g of MWCNTs

was suspended in 500 mL of 3DH2O and ultrasonicated at full power for 4 h. The sample was vacuum-filtered, employing a 0.40 μm polycarbonate membrane filter (Isopore Millipore), and dried overnight at 110 °C. Scanning electron microscopy characterization of the sample after sonication showed a length range between 500 and 700 nm, confirming a direct relationship of the ultrasonication time and the final length of the carbon material. Ultrasonication at full power for 2 h showed a length range between 1.5 and 2.0 μm. For functionalization of the MWCNTs with carboxylic groups, a sample of 2.4 g of MWCNTs was dispersed in 130 mL of 65% HNO3, employing an ultrasonic processor (Hielscher UP400S 400 W, 24 kHz) for 10 min at 180 W. The solution was then mechanically stirred on a hot plate at 90 °C for 3 h. Afterward, the heating was switched off, and the solution was kept stirring overnight. The sample was vacuum-filtered, employing a 0.40 μm polycarbonate membrane filter (Isopore Millipore) under 3DH2O reflux up to neutralized pH, and then dried overnight at 110 °C. For deprotonation of the carboxylic groups, the sample obtained was dispersed in 80 mL of a 10 wt % aqueous solution of NaOH, employing an ultrasonic processor for 5 min at 200 W. The solution was then vacuum-filtered, dispersed in 100 mL of a 10 wt % aqueous solution of NaOH, and stirred for 30 min. This procedure was repeated one more time, and finally the sample was vacuum-filtered using a 0.40 μm polycarbonate membrane filter (Isopore Millipore) under 3DH2O reflux up to between pH 7 and 8 and dried overnight at 110 °C. 2.2.3. Preparation of the Dy2-MWCNT Hybrid, Dy2@MWCNT. The functionalized MWCNTs (40 mg) were suspended in MeCN (20 mL) and subjected to sequential ultrasound cycles for 15 min. Next, Dy2 (100 mg) was added, and the orange-yellow mixture was magnetically stirred at room temperature for 3 subsequent days. Afterward, the solution was filtered (PTFE membrane, 0.45 μm pore size) and rinsed with MeCN. During filtration, a pellet of the Dy2@ MWCNT hybrid material formed on the PTFE membrane. The pellet was removed from the membrane, redispersed in MeCN, applying an ultrasonic treatment for 10 min, and filtered again. This cycle was repeated two subsequent times using ∼200 mL of solvent for each of the rinsing cycles until the filtrate became colorless. The fraction of Dy2 molecules removed during each of the subsequent rinsing cycles was investigated by absorption spectroscopy of the filtrate in order to validate that they retained their structures during the washing cycles. Before the first washing cycle, a portion of the filtrate was kept (Dy2_W0) as a reference sample and also to check the integrity of the structure of the Dy2 molecules. During the first washing cycle, the largest fraction of Dy2 molecules is present in the filtrate denoted as Dy2_W1, while in the second washing cycle, the fraction of Dy2 is close to zero, Dy2_W2 (the color of the filtrate became colorless). The Dy2/MWCNT powder pellet was also monitored during this washing procedure using IR spectroscopy. Before the first washing cycle, a fraction of the powder was kept (Dy2@MWCNT_W0) for comparison with samples after the first and second cycles (Dy2@ MWCNT_W1 and Dy2@MWCNT_W2). Finally, the success of the washing cycles was examined with electron microscopy, investigating the material before the washing, Dy2@MWCNT_W0, and after the second washing cycle, where the filtrate was colorless, Dy2@ MWCNT_W2.

3. RESULTS AND DISCUSSION 3.1. Synthetic Considerations. The synthetic strategies for the effective attachment of functional molecules on CNTs involve mainly (a) strong covalent bonds between prefunctionalized nanotubes and the molecular moiety of interest, (b) weaker π−π supramolecular interactions between predesigned molecules with polyaromatic/aromatic groups and the aromatic character of the sidewalls of the nanotubes and/or (c) interactions of the electrostatic character between the anionic/cationic nanotube and the cationic/anionic molecular unit. The hybrids produced from the first two strategies are weakly coupled because they force the molecular cluster to C


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Figure 1. (a) Raw MWCNTs. (b) Functionalization of the MWCNTs with carboxylic groups to avoid undesirable self-agglomeration and effective dispersion due to the electrostatic repulsion forces. (c) π−π supramolecular interactions between neutral molecules, Dy2, with aromatic groups and the sidewalls of well-dispersed and isolated anionic MWCNTs (the size of the solid sphere is exaggerated).

addition of 5% of −COOH functional groups after our functionalization protocol. The decomposition temperature of the nanotubes is significantly smaller (530 °C) than that reported for the noncarboxylated MWCNTs because the number of defects is larger in the structures of MWCNTs, decreasing their thermal stability.39 Finally, the deprotonated (carboxylated) MWCNTs were further suspended in MeCN, and several ultrasonication cycles were employed in order to accomplish a homogeneous dispersion of the mixture. At that time, the Dy2 molecules were added under continuous mechanical stirring for a period of 3 days. Effective grafting of the Dy2 molecules onto the sidewalls of the MWCNTs was possible due to noncovalent (π−π-stacking effects) interaction via N-salicylidene-o-aminophenol. This grafting was preliminarily confirmed through the TGA study of the Dy2@ MWCNT hybrid (Figure S3), which shows decomposition of the Dy2 molecules and the loss of the ligands involved in the coordination at 230 °C and, hence, the consequent formation of Dy2O3 above 550 °C, which represents approximately 10% of the sample weight. The residual weight remaining at 650 °C is attributed to Dy2O3, which does not decompose further. Thermal decomposition of the pristine Dy2 (Figure S3) reveals a similar first step in TGA, clearly indicating that the Dy2 molecules remain structurally intact upon grafting onto the MWCNTs. Also the decomposition temperature of the grafting molecules onto the nanotubes is slightly higher (ca. 230 °C) compared with free molecules (ca. 210 °C), indicating that the grafting of SMMs onto the surface of MWCNTs enhances their thermal stability. 3.2. Electron Microscopy. TEM was utilized in order to study the deposition of Dy2 molecules onto MWCNTs, the Dy2@MWCNT_W2 sample. A typical bright-field image of the hybrid material is shown in Figure 2a. MWCNTs with diameters in the range of 5−20 nm can be seen, and darkcontrast particles are indicated by arrows at the end of the MWCNTs that contain nickel, as evidenced by energydispersive X-ray microanalysis. These particles were utilized for growth of the MWCNTs. The background-subtracted EELS spectrum corresponding to the area imaged in Figure 1a is shown in Figure 2b. The Dy N-edge at 154 eV, in addition to the C K-edge at 280 eV originating from the CNTs, proves the existence of dysprosium in the hybrid material. A higher magnification of the composite material can be seen in Figure 2c. A high density of low-contrast nanoparticles, some of them indicated by arrows, can be observed on the surface and in the interior of MWCNTs. Using the Dy N-edge of the EELS

reside far from a direct contact with the nanotubes, and in this way, the electronic interaction between the nanotubes and the molecular unit is minimized, while the last one favors a direct contact between the molecular cluster and the MWCNTs and, therefore, a stronger electrostatic interaction. However, in all of the above-mentioned strategies, one of the most important challenges is the ability to generate hybrid materials with uniform coatings designed to avoid undesirable self-agglomeration33 especially when the interface and interaction between the hybrid materials are being exploited. It has been noted that nonuniform hybridization or simple mechanical mixing evolves upon degradation of the desired performance.34 The solubility and compatibility of MWCNTs can be improved by employing covalent functionalization with polymer chains, biomolecules, or long alkyl chains,35,36 while the main disadvantage of this particular functionalization is modification of the structure and the original properties of MWCNTs.37 In this work, we have designed a modified synthetic protocol, which can be seen in Figure 1 and is briefly described as a twostep experimental protocol: (a) functionalization of the MWCNTs with carboxylic groups to avoid their undesirable self-agglomeration and succeed effective dispersion due to the electrostatic repulsion forces between the deprotonated carboxylic acid groups in MWCNTs; (b) π−π supramolecular interactions between a neutral molecule with aromatic groups and the sidewalls of well-dispersed and isolated functionalized MWCNTs. MWCNTs were obtained commercially and refluxed in a concentrated HNO3 medium (see the Experimental Section for more details) in order to effectively (i) introduce carboxylic groups at the periphery of the carbon sidewalls, (ii) remove a percentage of metal particles (mainly nickel according to the manufacturer), which are commonly used in the production of MWCNTs as catalysts, and (iii) remove amorphous carbonaceous forms present in the raw material. Monitoring of the functionalization was accomplished with FT-IR and TGA. FTIR reveals a characteristic broad band at 1115 cm−1, which is related to the C−O stretching mode (Figure S2).38 This band is also visible in the raw MWCNTs, indicating the presence of a small percentage of carboxylic groups in the raw material. Figure S3 shows thermal decomposition of these materials in a nitrogen atmosphere. The “as-delivered” MWCNTs remain without any mass loss in the temperature range of 100−650 °C, while above this temperature, decomposition of the nanotubes occurs due to defects in their structure. The total weight loss for the carboxylated material is 5%, corresponding to an D


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3.3. Investigation of the Washing Protocol. The success of the washing protocol was investigated with (a) electron microscopy images of Dy2@MWCNT_W2 discussed before and (b) IR/UV spectroscopy of the hybrid material (Dy2@ MWCNT_Wi, where i = 0−2) and filtrates (Dy2_Wi, where i = 0, 2) after each washing cycle (i = 0−2), where 0 is the condition before the first washing cycle and 2 is the condition after the second and last washing cycle. A general overview of this procedure is shown in Figure 3. A typical bright-field image of the hybrid material before the washing protocols, Dy2@ MWCNT_W0, is shown in Figure S4a. MWCNTs with diameter in the range of 5−20 nm can be seen, decorated with low-contrast small precipitates. Large areas of low-contrast material between the nanotubes are also observed. The background-subtracted EELS spectrum corresponding to the area imaged in Figure S4a is shown in Figure S4c. The Dy Nedge at 154 eV, in addition to the C K-edge at 280 eV originating from the CNTs, proves the existence of dysprosium in the composite material. The distribution of Dy2 was examined by EFTEM mapping. The distribution of Dy2 in the area imaged in Figure S4a can be seen in Figure S4b, where regions with bright contrast correspond to high Dy2 concentration. Comparing parts a and b of Figure S4 indicates that Dy2 grafting of the nanotubes is accomplished, while, on the other hand, the low-contrast areas between the MWCNTs correspond to large agglomerates containing unreacted Dy2. A HRTEM image of a nanotube is shown in Figure S4d. The sidewalls of the MWCNTs corresponding to fringes of 0.33 nm spacing can be seen. At the left part of the HRTEM image, the edges of a large Dy2 agglomerate are shown by big arrows. On the surface and interior of the nanotube, low-contrast objects indicated by small arrows are observed. Their dimensions are about 1.5−2.0 nm, in excellent agreement with that of Dy2 molecules. These objects are found either isolated or in close proximity to one another. Consequently, TEM examination of the prewashed material shows that Dy2 molecules are attached

Figure 2. (a) Typical bright-field image of the composite material. (b) Background-subtracted EELS spectrum corresponding to the area imaged in Figure 1a. (c) Higher magnification of the composite material showing a high density of low-contrast Dy2 molecules. Some of them are indicated by arrows. (d) EFTEM map of Dy based on the Dy N-edge of the EELS spectrum.

spectrum, the EFTEM map of dysprosium was obtained and is shown in Figure 2d. By a comparison of parts c and d of Figure 2, it is evident that low-contrast nanoparticles contain dysprosium, while their size is about 1.5−2 nm, in excellent agreement with that of Dy2 molecules. Consequently, TEM examination of the hybrid material shows that Dy2 molecules are grafted onto the surfaces of MWCNTs.

Figure 3. (left) IR monitoring of the hybrid material after each washing cycle: (a) IR spectrum of the pristine Dy2 for comparison reasons; (b) Dy2@MWCNT_W0, hybrid prewashed (the EFTEM map of dysprosium based on the Dy N-edge of the EELS spectrum is also shown); (c) Dy2@ MWCNT_W1, hybrid washed after the first cycle; (d) Dy2@MWCNT_W2, hybrid washed after the second cycle (the EFTEM map of dysprosium based on the Dy N-edge of the EELS spectrum is also shown). (right) UV monitoring of the filtrates after each washing cycle: Dy2_W0 is the filtrate of the prewashed hybrid Dy2@MWCNT_W0, Dy2_W1 is the filtrate of the Dy2@MWCNT_W1 hybrid washed after the first cycle, Dy2_W2 is the filtrate of the Dy2@MWCNT_W2 hybrid washed after the second cycle. The optical spectrum of a MeCN solution of pristine Dy2, Dy2_MeCN, is also shown for comparison reasons. E


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Inorganic Chemistry onto MWCNTs, but large agglomerates are also formed between the nanotubes, while in the TEM images of the Dy2@ MWCNT_W2, these agglomerates vanished, suggesting an effective washing protocol. In order to monitor the amount of Dy2 molecules removed during the different washing steps, we performed optical absorption experiments of the filtrates. For the second washing step, the filtrate became colorless, while its optical absorption spectrum was also close to the zero-intensity values. UV monitoring of the filtrates is shown in the right part of Figure 3, where, for comparison reasons, the spectrum of a solution of pristine Dy2 in MeCN is also shown. The UV bands are most probably ligand-based, and the f−f transitions of the DyIII ions present are too weak to be observed. On the other hand, all of the spectra of the filtrate solutions are identical in comparison to the pristine Dy2 solution spectrum, denoting the integrity of its structure. IR monitoring of the hybrid material (portions of the powder pellet after each washing cycle) is also revealed in the same figure (left part). For comparison reasons, the IR spectrum of the pristine Dy2 molecules is also present. The main IR modes in the pristine Dy2 compound are (i) the ν(CO) and δ(OCN) IR modes of coordinated DMF at ∼1664, ∼1651, and ∼682 cm−1, respectively, (ii) the strong bands at 1604−1609 cm−1, which are assigned to the CN stretching vibration of the Schiff-base linkage, ν(CN), and (iii) the IR bands at ∼1464, ∼1286, and ∼1024 cm−1, which are assigned to the ν1(Α1) [ν(NO)], ν5(Β2) [να(ΝΟ2)], and ν2(A1) [νs(NO2)] vibrational modes of the bidentate chelating (C2v) nitrato group. Before the first washing cycle, the IR spectrum of the hybrid material, Dy2@MWCNT_W0, contains the majority of the IR bands of the pristine Dy2 compound, but their intensity is significantly reduced, denoting that the largest fraction of Dy2 molecules is present in the filtrate. As is shown in the EFTEM map of dysprosium based on the Dy N-edge of the EELS spectrum of this hybrid (Figure 2), there are large areas between the nanotubes where “unreacted” Dy2 compounds are formed as large agglomerates. The IR spectrum of the final hybrid material, Dy2@MWCNT_W2, after the second washing cycle, becomes featureless; an indication of an effective washing of the unreacted Dy2 agglomerates. This was further confirmed with the EFTEM map of dysprosium based on the Dy N-edge of the EELS spectrum of this hybrid where low-contrast nanoparticles containing dysprosium are adsorbed in the sidewalls and inside the MWCNTs with no sign of agglomerates between them. 3.4. XPS Study. XPS can provide direct proof for the elements grafted on the sidewalls of the MWCNTs, comparing their BEs with the expected bonding states. The hybrid material Dy2@MWCNT was investigated in comparison to a powder reference of Dy2, and the survey patterns recorded in the range of 0−1400 eV and high-resolution scans for the C 1s and Dy 3d5 peaks are shown in Figures S5 and S6. Figure S5 shows the survey spectrum of the Dy2 sample and its chemical composition, while similar information is given in Figure S6 about the Dy2@MWCNT hybrid material. The XPS spectrum of Dy2 3d is characterized by a double peak, namely, Dy 3d3/2 and Dy 3d5/2 at a BE of ca. 1297.0 and 1333.4 eV due to the spin−orbit splitting (Figure 4). The presence of Dy2 in the Dy2@MWCNT hybrid material is confirmed by the nearly identical values of ΔΕ = Ε(Dy 3d3/2) − Ε(Dy 3d5/2) = 37.4 eV and is shown in Figure 4. 3.5. Raman Spectroscopy. Raman spectroscopy provides the characterization of carbon-based nanostructures and

Figure 4. High-resolution XPS spectra of Dy 3d of the hybrid material and the powder reference of Dy2.

MWCNTs in particular.40 Characteristic Raman spectra of pristine and treated MWCNTs excited with the 785 nm laser line are shown in Figure 5. The most prominent features in

Figure 5. (a) Raman spectra of pristine and treated MWCNTs excited with the 785 nm laser line. (b) Profile characteristics of (D, G, and D′) bands along with the intensity ratios ID/IG deduced by Lorentzian fits (see the text and Table S1 for details).

each spectrum are the so-called D band at ∼1317 cm1, the G band at ∼1588 cm1, and the D′ band at ∼1616 cm1. The D and D′ bands are disorder-induced features arising from the doubleresonance Raman scattering process from a nonzero-center phonon mode.41 Both bands are also dispersive, and their frequencies shift to higher frequencies with the energy of the excitation laser.41 In addition, the intensity of the D′ band significantly decreases with the energy of the laser line. In general, the D and D′ bands are strongly related to the percentage of disordered or amorphous carbon in the MWCNT material. The structural disorder in the carbon material is influenced by finite or nanosized graphitic planes and other forms as well as defects on the sidewalls of the nanotube. The origin of the G band is the in-plane tangential stretching of the C−C bonds in the graphene sheets, and it is doubly degenerate. The profile characteristics of all Raman bands along with the intensity ratios ID/IG and ID′/IG were deduced by Lorentzian fits (Figure 5) and summarized in Table S1. Values greater than 2 concerning the ratio ID/IG are F


Article

Inorganic Chemistry observed for chemical-vapor-deposition-based MWCNTs.42 The high values of ID/IG for the MWCNT samples imply that a pristine material is highly disordered (Table S1). The values of the profile characteristics of the bands are approximately identical, suggesting that the chemical treatments do not alter the graphitic structure either by strain or doping, which is also direct evidence of the nondestructive character of the attachment (noncovalent) of Dy2. 3.6. Magnetic Measurements. 3.6.1. dc Magnetic Measurements. Figure 6 summarizes the magnetization

Figure 6. Magnetization curves of the functionalized MWCNTs (solid stars), pristine Dy2 (solid triangles), and hybrid Dy2@MWCNT material (solid spheres).

Figure 7. Temperature dependence of the dc magnetization of the hybrid material in the FC (red solid spheres) and ZFC (blue solid spheres) modes at (a) low external field (50 Oe) and (b) high external field (5000 Oe). Temperature dependence of the dc magnetization of functionalized MWCNT bundles in the FC (blue solid spheres) and ZFC (red solid spheres) modes at (c) low external field (50 Oe) and (d) high external field (5000 Oe).

measurements of the (a) functionalized MWCNTs, (b) pristine Dy2 compound, and (c) hybrid Dy2@MWCNT material at 4 K and in the field range from −60 to +60 kOe. The values of saturation magnetization, Ms, are 1.55, 66.22, and 29.82 emu/g, respectively. The dc magnetic response in the functionalized MWCNTs is determined by the contribution of a ferromagnetic nickel catalyst residues (as was already confirmed by electron microscopy). A decrease in magnetization at fields greater than 30 kOe is observed, indicating a diamagnetic contribution from the graphitic material (Figure S7).43 Moreover, this magnetic behavior of the raw material demonstrates the difficulties related to the purification of MWCNTs.44 It should be pointed out that the magnetization saturation value of the hybrid material is lower in comparison to the values obtained from the pristine Dy2 SMM compound, and possible reasons for this are (a) different electronic structures of the Dy2 molecules when grafted to the nanotubes and (b) interaction between the 2p orbitals of the MWCNTs’ closest carbon atoms and the 4f orbitals of the Dy2 molecules.45 In fact, previous work has already reported similar results with the grafting of Mn4 SMM molecules to CNTs.30 The temperature dependence of the dc magnetization of the functionalized MWCNTs, in the zero-field-cooled (ZFC) and field-cooled (FC) modes, shows significant irreversibility at low magnetic field, 50 Oe, and is shown in Figure 7c. The magnetization curves indicate that the particles are thermally stable without blocking or superparamagnetic behavior. The nickel particles are expected to be single-domain particles because their average size is 20 nm and the proportional temperature dependence of the ZFC magnetization can be ascribed mainly to the domain-wall pinning processes.46

The negative values of the ZFC magnetization curve at low temperatures can be explained by a low applied magnetic field, lower than the coercivity of nickel nanoparticles, thus resisting alignment in the direction of the field due to their large magnetic anisotropy. Alternatively, the negative ZFC magnetization can be observed as a consequence of the presence of magnetic exchange interactions, random orientation of the easy axis, intrinsic negative spin polarization, or crystal structure disorders.47 The temperature dependence of the dc magnetization of the functionalized MWCNTs, recorded under ZFC and FC conditions at high external fields (5000 Oe), is shown in Figure 7d, where an increase in the magnetization branches is evident because of the presence of a small fraction of unblocked nanoparticles and/or paramagnetic impurities or defects. The ZFC/FC magnetization behavior of the hybrid material is shown in Figure 7a for a low external field (50 Oe) and Figure 7b for a high external field (5000 Oe). In accordance with the functionalized MWCNTs, there is significant irreversibility for H = 50 Oe, and the blocking temperature of the nickel nanoparticles is also well above 300 K, indicating that no significant change of the nature of the nanoparticles occurred after the chemical treatment of MWCNTs. The main difference of the hybrid material, in comparison to the functionalized MWCNTs, is the increasing values of the magnetization data (ZFC/FC) in the very low temperature range due to attachment of the Dy2 compounds onto the sidewalls of the nanotubes. More explicitly, the positive values of ZFC magnetization are closely related to the paramagnetic contribution of the Dy2 molecules. The “paramagnetic character” of the hybrid material is also confirmed with the ZFC/FC magnetization measurements at high magnetic fields G


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Inorganic Chemistry

exponential increase in the in-phase signal, χ′ (Figure 8c), was detected, accompanied by a positive response in the imaginary component below 10 K (Figure 8d). Although no maximum is observed in the χ″ signal because of temperature limitations, there is a frequency dependence of the position of the susceptibility tail, denoting a dynamic relaxation process and providing additional support for the presence of Dy2 SMMs in the MWCNTs. The χ″ ac signals of the pristine Dy2 compound show two peaks that correspond to two well-defined, thermally activated processes around 6 K for a frequency range of 1500− 500 Hz and around 2.5 K for a frequency range of 200−10 Hz.1 According to ref 1, fitting to the Arrhenius law gave values of Ueff = 17.4 cm−1 and 3.0 × 10−6 s for the process at ∼6 K and Ueff = 16.2 cm−1 and = 4.4 × 10−7 s for the lower temperature process, while the relaxation time, τ, remains temperaturedependent at temperatures close to 2.2 K, a clear indication of a nonactivated pure quantum regime. The difference between the hybrid material and the pristine Dy2 compound may be related to the structural changes chemical distortion and loss of symmetryin the magnetic clusters after their attachment to the sidewalls of MWCNTs.30

(H = 5000 Oe), as evidenced by a coincidence of the two curves (Figure 7b). 3.6.2. Calculation of the Molar Concentration of Dy2. We attempted to calculate the molar concentration of Dy2 following the analysis of ref 31. According to this analysis, the total mass of the hybrid material mDy2@MWCNT is given by the following equation: mDy2@MWCNT = mMWCNT + mDy2 (1) and the total magnetic moment per mass is shown below: MDy2@MWCNT = mMWCNTM *MWCNT + mDy2M *Dy2

(2)

For evaluation of the mass of Dy2, the following equation can be used: mDy2 =

MDy2@MWCNT − mDy2@MWCNTM *MWCNT M *Dy2 − M *MWCNT

(3)

From the magnetic moment measured at 2 K and 60 kOe (Figures 6 and S6), the following parameters are taken: M*MWCNT = 1.55 emu/g; M*Dy2 = 66.22 emu/g, and MDy2@MWCNT = 29.82 emu/g. The mass of the hybrid material used for the magnetic measurements was mDy2@MWCNT = 2.97 mg, and the calculated mass of the Dy2 mass is mDy2 = 1.1(1) mg. 3.6.3. ac Magnetic Measurements. ac magnetic measurements were carried out under an external applied field of 0 Oe with Hac = 3 Oe for the pristine Dy2 compound (Figure 8a,b),

4. CONCLUSIONS A new method for the attachment of SMM molecules to MWCNTs is described here. Our synthetic protocol includes two steps: (a) functionalization of the MWCNTs with carboxylic groups to avoid their undesirable self-agglomeration and achieve effective dispersion due to the electrostatic repulsion forces between the deprotonated carboxylic acid groups of the MWCNT and (b) the π−π supramolecular interactions between the neutral SMM molecule with aromatic groups and the sidewalls of well-dispersed and isolated MWCNTs. In our method, special consideration was given to the washing protocol involving extensive monitoring of the powder hybrid material and the filtrates after each washing cycle using electron microscopy, optical absorption experiments, and IR spectroscopy. Monitoring of the effectiveness of our method was provided by (i) HRTEM analysis, revealing that the grafting of molecular components containing dysprosium with composition as well as size is very close with those expected for the Dy2 compound, (ii) XPS analysis, confirming the chemical integrity of these nanometric components, by showing good agreement between the spectra of the isolated hybrids and the bulk Dy2 compound. The magnetic behavior of the attached molecules was studied with conventional magnetometry, and the saturation values of the magnetization and the molar concentration of the Dy2 in the hybrid material indicated a large degree of grafting. ac dynamic magnetic measurements revealed a dynamic relaxation process, suggesting the presence of a relaxation state and thus providing additional support for the presence of Dy2 SMMs in the hybrid material. Nevertheless, deviation of the M versus H curves as well as dynamic ac susceptibility measurements of the hybrid material in comparison to bulk Dy2 compounds suggested that the magnetic behavior of the grafted molecules was influenced from the surface effects. This experimental protocol will be tested extensively with a large family of SMM compounds, investigating the conditions for an effective grafting and possible enhancement of the magnetic properties of the hybrid material. Furthermore, potential candidates for this grafting procedure will be SMM molecules with carboxylates as ligands because it is likely that the CO groups of the functionalized MWCNTs will introduce minor changes to the coordination

Figure 8. In-phase, χ′, and out-of-phase, χ″, signals under an external applied field of 0 Oe and Hac = 3 Oe for the pristine Dy2 compound (a and b) and the hybrid material (c and d) at ac-field frequencies of 10, 100, 997, and 1488 Hz.

the functionalized MWCNT (Figure S7), and the hybrid material (Figure 8c,d). An out-of-phase signal was detected for the hybrid material (Figure 8d) at the temperatures explored, while no signal was detected for the case of the functionalized MWCNTs (Figure S7). The signal observed in the case of the hybrid material is attributed to Dy2 molecules grafted to the sidewalls of the functionalized MWCNTs. More explicitly, an H


Article

Inorganic Chemistry

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sphere of the SMM molecules and eventually their SMM behavior.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00472. Molecular structure of the Dy2 molecule, IR spectra, TGA, additional XPS, HRTEM figures, Raman figures and tables, magnetization, and susceptibility measurements (ac and dc) PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Vassilis Tangoulis: 0000-0002-2039-2182 Jiří Tuček: 0000-0003-2037-4950 Radek Zbořil: 0000-0002-3147-2196 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge support from the Ministry of Education, Youth and Sports of the Czech Republic under Project LO1305 and assistance provided by the Research Infrastructure NanoEnviCz supported by the Ministry of Education, Youth and Sports of the Czech Republic, under Project LM2015073.

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REFERENCES

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Noncovalent Grafting of a DyIII2 Single-Molecule Magnet onto

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