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1. INTRODUCTION

Polyoxometalates (POMs) with Keggin structure are metal ox-ide clusters with a formula [XM12O40]-3 formed by a central tetra-hedral XO4 surrounded by 12 edge-sharing metal-oxygen MO6. This polyanions can be balanced either by protons or by other cations, and can undergo electrochemically reversible multielec-tron redox processes. Nevertheless, they need to be anchored or immobilized to a solid framework in order to increase their disper-sion and potentiality for solid-state applications. POMs are of con-siderable interest, they are cheap and non-toxic compounds, and recently they are finding their use in several potential solid state applications as in heterogeneous catalysis [1-4], electrocatalytic properties for amperometric sensors and fuel cells [5-13], and supercapacitors [14-16]. The main drawback for these solid state applications is finding an effective matrix for the immobilization of POMs where no desorption takes place simplifying their elec-trochemical study.

POMs have been immobilized to different solid matrices as in zirconia [17], silica [18], alumina [19], TiO2 [20], and Zeolites [1]

with desorption problems, electrodeposited on to metals [21-22], and in conducting polymers where POM properties are modified [23-32]. Also, it has been reported the immobilization of POMs on the surface of a great diversity of carbon matrices [1-7, 33-35], and more recently in multiwalled carbon nanotubes (NTs) [8-14,16,36-39]. It is well known that the immobilization or anchoring of POMs in these carbon matrices involves a strong spontaneous and irreversible adsorption based on a charge transfer [9,33-34] which is enhanced by the matrix microporosity [1,4,35], the hydrophilic nature of the carbon matrix [2], and the creation of functional groups on the surface of carbon nanotubes by oxidation treatments [12,14,16,36,39]. Also, the use of surfactants has been carried out to anchor POMs on to carbon nanotubes [8,38], but the electro-chemical properties are decreased [10]. In previous work carried out in our group [12,39] related with the anchoring of Cs POM salts on the surface of oxidized carbon nanotubes, it was found that the oxidation degree of the nanotubes resulted in the key factor for the improved dispersion of insoluble CsPOMs particles. Neverthe-less, segregation of these particles was observed in all hybrid ma-terials. Therefore, in order to avoid the interference of these in-soluble segregated CsPOM particles, the synthesis of hybrid mate-rials with no particle segregation was carried out with soluble *To whom correspondence should be addressed: Email: [email protected]

Phone: +52 (55) 56229840 Fax.: +52 (55) 56229742

Influence of the Functionalization Degree of Multiwalled Carbon Nanotubes on the Immobilization of Polyoxometalates and Its Effect on their Electrochemical Behavior

Ana Karina Cuentas-Gallegos*, Sandra Jimenez-Penaloza, Dulce A. Baeza-Rostro and Andrea German-Garcia

Centro de Investigación en Energía-Universidad Nacional Autónoma de México, Depto. Materiales Solares, Priv. Xochicalco s/n Col Centro, AP 34, CP 62580 Temixco, Morelos, México

Received: May 27, 2010, Accepted: October 22, 2010, Available online: December 07, 2010

Abstract: Hybrid materials based on the anchoring polyoxometalate particles POM on several multiwalled carbon nanotubes (NTs)

matrices with different oxidation degree are characterized and electrochemically studied, in order to correlate the dispersion degree of POM particles on these matrices with their electrochemical behavior. These hybrids synthesized with soluble PMo12 particles resulted in truly homogenous hybrids and are analyzed by XRD, SEM, EDS, TEM, and N2 adsorption isotherms in order to confirm the presence of their components and their dispersion properties. Higher dispersion of PMo12 nanoparticles is the main factor contributing to an improved electrochemical behavior in these types of hybrid materials, making hybrid NT-S-PMo12 with this property the one resulting in the best electrochemical performance where the redox contribution of PMo12 particles are clearly observed as well as the improved double layer contribution of its nanotube matrix (NT-S, highly oxidized).

Keywords: Hybrid Materials, Carbon Nanotubes, Polyoxometalates, Surface Modification, particle dispersion.

Journal of New Materials for Electrochemical Systems 13, 369-376 (2010) © J. New Mat. Electrochem. Systems

370 Ana Karina Cuentas-Gallegos et al. / J. New Mat. Electrochem. Systems

PMo12 polyanion. The main objective of this work is to understand how the functionalization degree on multiwalled carbon nanotubes (NT) surface affects the interaction properties with these POM nanoparticles, and how this is correlated with their electrochemical behavior.

2. EXPERIMENTAL SECTION

2.1. Hybrid Materials Hybrid materials are synthesized using multiwalled carbon nano-

tubes (NT) from Nanostructured & Amorphous Materials (95%, D: 3-20nm, Length 0.1-10µm) and PMo12 polyanion in its acidic form from Aldrich (H3PMo12O4014H2O, PM= 2080.67g/mol). NT are previously purified and oxidized with different acid treatments before proceeding with the dispersion of POM particles [12,39]. These pretreatments consist on first treating the NTs in concen-trated HCl and then oxidizing by using three different procedures: mild treatment in 2.5M HNO3 (NT-N), intermediate treatment in 0.5M H2SO4 + 2.5M H2SO4 (NT-NS), and strong treatment in 2.5M H2SO4 (NT-S). The hybrids are synthesized by dispersing the dif-ferent oxidized nanotube matrices (40 mg) in a PMo12 solution (30ml, 1.15mM) using an ultrasound bath for 3 hours. Then the solids are filtered and washed with an acidic solution (pH=2), and dried for 1 hour (100°C) to obtain the three NT/PMo12 hybrid sam-ples: NT-N-PMo12 (using NT-N matrix, mild treatment), NT-NS-PMo12 (NT-NS matrix, intermediate treatment), and NT-S-PMo12 (NT-S matrix, strong treatment). In previous reports, the reaction time used to obtain these types of hybrids was of 12 hours with a similar procedure [13]. Therefore, in our case the purification and oxidation procedure of NTs prior to the hybrid synthesis is also carried out to reduce considerably the reported reaction time and to obtain diluted POM hybrids.

2.2. Characterization Techniques Different characterization techniques are used to analyze the

different oxidized carbon nanotube matrices and hybrid materials. TGA analyses are carried out in a TGA Q-500 thermo-balance under nitrogen using a heating rate of 15°C min-1 up to 900°C to evaluate the oxidation degree in the nanotubes matrices. Powder XRD is used to determine crystalline size of the different matrices using the Sherrer formula and to determine the presence of PMo12 particles in all hybrid materials using a Rigaku Ultima+Dmax-2200 difractometer with a Cu-Kα= 1.54Å. SEM and EDS analyses are carried out to confirm the microstructure homogeneity and the presence of PMo12 in hybrid materials, using a JEOL electronic microscope model JSM-5400LV. TEM images are obtained using a JEOL JEM-2010 HRTM at 200Kv in order to calculate the average particle size of PMo12 immobilized on to the surface of carbon nanotubes, using a great number of images. Nitrogen adsorption porosimetry measurements for all the hybrids are carried out at 77°K with a Quantachrome NOVA instrument, by first purging the samples with vacuum at 120°C during 16 hours in order to prevent POM degradation. The N2 adsorption isotherms are analyzed by the Brunauer-Emmett-Teller (BET) Theory between a 0.02 and 0.3 P/Po to obtain the surface area (SBET). All hybrid materials are elec-trochemically characterized by cyclic voltammetry in an AUTO-LAB potentiostat-galvanostat PGSTAT302M, using a 3-electrode cell with a H2SO4 electrolyte (0.5M) a Pt gauze as the counter elec-trode, and our hybrid materials as the working electrode. Our mate-

rials (60%) are mixed with Teflon (10%) and conducting carbon (30%) in ethanol, following the procedure already reported [40] and using a stainless steel grid (316L, chemically resistant to acidic media) as the current collector. The cyclic voltammetry (CV) cur-rent is normalized by the active material weight of the composite electrode and the potential range vs. a sulfate saturated electrode (SSE).

3. RESULTS AND DISCUSSION

3.1. Oxidized Multiwalled Carbon Nanotubes (NT) In previous work from our group [39] related with the purifica-

tion and oxidation of NT, the ones treated with the strongest treat-ment (NT-S) resulted in the highest concentration of carbonyl and sulfide groups required for POM particles dispersion. This informa-tion is based only on FTIR analysis, therefore in this work results from XRD and TGA techniques, as well as the electrochemical behavior are shown in order to detect the influence of the oxidation procedure on the crystallite size and oxidation degree. Figure 1 shows the diffraction patterns of the different NTs samples where the characteristic [002] diffraction plane of carbon nanotubes is present around 2θ = 26° in all samples, confirming the presence of the graphitic planes of their structure. The Sherrer formula is ap-plied to calculate the crystallite size based on this [002] diffraction peak of the NTs diffractograms showing no significant change (between 39 and 42Å).

In order to determine the oxidation degree in these different NT samples, TGA analyses are carried out based on the procedure establish by NORIT N.V.[41] Figure 2 shows the results of this

Figure 1. Powder XRD patterns for all untreated (NT), purified (NT-HCl), and oxidized multiwalled carbon nanotubes (NT-N, NT-NS, NT-S).

371 Influence of the Functionalization Degree of Multiwalled Carbon Nanotubes on the Immobilization of Polyoxometalates and Its Effect on their Electrochemical Behavior / J. New Mat. Electrochem. Systems

analysis where different weight losses are detected with more clar-ity in Figure 2b. In general, the weight losses below 500° are attrib-uted to CO2 liberation, and are related with a higher degree of sur-face oxidation in carbon [42]. It is well known that functional groups derived from carboxylic acids like anhydrides, lactones, and carboxylic acid itself decomposes to give mainly CO2, while CO is formed during decomposition of functional groups with only one atom of oxygen (ketone, ether, alcohol, aldehyde) at higher tem-peratures (>500°) [41-44]. Therefore, weight losses below 500°C (more clearly around 250°C) are more evident for oxidized carbon nanotubes (NT-N, NT-NS, NT-S) as expected, indicating a higher oxidation degree for NT-S obtained with the strongest oxidation treatment in good agreement with previous results (FTIR) [39]. For the case of NT-N (mild treatment) and NT-NS (intermediate treat-ment) matrices an additional weight loss around 450°C is detected suggesting the existence of a different type of functional group. Even though all nanotubes are dried before the analyses, the libera-tion of residual solvent (H2O) and/or absorbed species below 200°C is observed with more evidence in NT-S matrix (stronger treatment), due to the creation of more hydrophilic functional

groups on its surface transforming them to a more hydrophilic na-ture needed for POM anchoring and dispersion [39-46]. On the other hand, in the case of untreated and unpurified nanotubes (NT, pristine) the small weight loss at lower temperatures is due to amorphous carbon, but the major weight loss is shown around 850°C indicating the poor oxidation degree and their higher ther-mal stability. Finally, purified nanotubes (NT-HCl) do not show any main weight loss, suggesting the absence of functional groups and amorphous carbon [39].

Cyclic voltammetry is carried out in order to determine the op-erational voltage window and charge transfer processes related with functional groups at 20mV/s. Figure 3 shows only the voltammo-grams of the pristine (NT) and the highly oxidized nanotubes (NT-S) for comparison. In general, they show the typical rectangular profile of carbon materials related with a capacitive behavior and in NT-S voltammogram a redox process at -0.017V/-0.022V vs SSE is observed confirming the existence of electrochemical active functional groups [47-49] on these highly oxidized nanotubes. In addition, during oxidation the evolution of O2 is more evident for NT-S with highly oxidized surface reducing the operational voltage window.

3.2. Hybrid Materials In order to avoid the influence of POM segregation in the elec-

trochemical behavior as mentioned in the introduction section and truly try to understand the effect of POM dispersion onto different NTs matrices, soluble PMo12 particles are used to obtain the hybrid materials. These synthesized hybrids are first characterized by dif-ferent techniques to confirm the presence of both components, their microstructural properties (dispersion, surface area, particle size of PMo12), and finally their electrochemical behavior. Figure 4 shows the diffraction patterns of all hybrid materials and the correspond-ing polyoxometalate, where the characteristic [002] diffraction peak of the nanotubes matrices are observed at 2θ = 26° in all hy-

Figure 3. Cyclic Voltammetry comparing the profiles of pristine NTs and the highly oxidized carbon nanotubes (NT-S), using a scanning rate of 20mV/s.

Figure 2. TGA analyses carried out for all NTs using a heating rate of 15°C/min up to 900°C (a), with their correspondent deviation graph (b).


brids. A calculation from this diffraction peak is made by using the Sherrer formula where the nanocrystilline nature is detected, but where no important differences between them and with their corre-spondent matrices are observed (40-41 Å). In addition, the main peak of PMo12 structure around 2θ = 7° is present as a week shoul-der in all hybrid materials suggesting its presence, its nanocrystal-line nature, and good PMo12 dispersion in the different nanotube matrices. EDS analyses (not shown here) are carried out on these hybrid samples resulting in the detection of C from the nanotubes, and of the atoms from the polyoxometalate (P, Mo, and O) in good agreement with XRD results regarding the presence of PMo12. In addition, this semi-quantitative technique indicated a similar amount of Mo from PMo12 in all diluted hybrid materials.

Figure 5 shows the SEM images obtained at 20000x magnifica-tion for all hybrid materials, where a fibrous microstructure related with the nanotubes microstructure and no PMo12 segregation is observed in all samples. Therefore, a more homogeneous micro-structure in these hybrids is confirmed compared to hybrids made with CsPMo particles [39]. Nevertheless, hybrids NT-NS-PMo12 and NT-S-PMo12 with a medium (NT-NS) and high (NT-S) level of oxidation on their matrices, respectively, showed some agglomera-tions probably related with a more nanometer microstructure. TEM analyses are carried out for all hybrid materials in order to confirm the immobilization or anchoring of PMo12 on to the surface of the oxidized nanotubes. Figure 6 shows only some of the images ob-tained from TEM analyses, where the focus was chosen in order to enhance the visualization of the nanoparticles detected over the nanotubes surface while the carbon nanotubes appear defocused in some images. These detected nanoparticles have been assigned to

PMo12 particles and seemed to be linked on to the different nano-tube surface showing a truly nanoscale dispersion, which is in good agreement with XRD analyses (Fig. 4). However, the average parti-cle size of PMo12 nanoparticles differs in the three hybrid samples suggesting an influence of the different nanotube matrices. Carbon nanotubes treated with a mild oxidation treatment (NT-N) with lower degree of functional groups (TGA, Fig.2) resulted in the immobilization of the biggest PMo12 nanoparticles with an average size of 7-8nm (Fig. 6a). As the degree of oxidation increases on the nanotube matrices (NT-NS= intermediate treatment, Fig. 6b) a decrease of the average nanoparticle size of PMo12 is observed,

(a)

(b)

(c)

Figure 5. SEM images obtained at a 20000x magnification for NT/PMo hybrid materials: (a) NT-N-PMo (matrix treated with a mild oxidation treatment, NT-N), (b) NT-NS-PMo (hybrid with NT-NS matrix, treated with an intermediate treatment), and (c) NT-S-PMo (matrix treated with a strong oxidation treatment)

Figure 4. Powder XRD patterns for all hybrid materials obtained from PMo12 solution (NT/PMo). The diffraction pattern of PMo has been included as reference.


detecting 2-4nm nanoparticles in NT-NS-PMo12 hybrid. Although, smaller particles (»1.5nm, Fig. 6c) are detected in NT-S-PMo12 hybrid synthesized with a highly oxidized nanotube matrix (NT-S), no clear TEM images are obtain to calculate the average particle size. Detail TEM analyses of these smaller nanoparticles are under

study and will be included in future work related with the coverage of carbon nanotubes with POM nanoparticles. However, all charac-terization techniques indicate the presence of PMo12 on this particu-lar hybrid (NT-S- PMo12). The reduction size of PMo12 particles confirms the dispersion degree improvement as the oxidation de-

Figure 6. TEM images obtained at different magnifications for NT-N-PMo (a), NT-NS-PMo (b), and NT-S-PMo (c) hybrid materials, where the focus was chosen in order to enhance the visualization of the nanoparticles detected over the NTs surface while some NTs appear defocused.


gree on the NT matrices enhances, confirming previous results with CsPOM hybrids [12,39]. Based on the particle size variation of PMo12 nanoparticles and the degree of oxidation of the different nanotube matrices, a sort of a nucleation-growth mechanism can be suggested for the formation of these hybrids as observed previously for similar materials [50], taking into account that the same amount of 1.15mM PMo12 solution is used for their synthesis. Therefore, PMo12 particles could be anchored first to oxidized NTs through an electron transfer from defect sites or functional groups in NTs wall forming a nuclei [39,50] and some of the remaining PMo12 in solu-tion could be used for the nuclei growth. In Figure 7 a schematic representation of these hybrids is presented, where the highest dis-persion of PMo12 particles on NT walls is shown for NT-S-PMo12 hybrid. In order to confirm this hypothesis and provide a detail mechanism more work is being carried out in our research group. This variation in the particle size of PMo12 anchored and dispersed on to oxidized nanotubes matrices must have an influence in the BET surface area of these hybrids.

From nitrogen isotherms BET surface area is calculated, obtain-ing for the pristine nanotubes an area of 264m2/g, and for the differ-ent hybrid materials the following values: 424 m2/g for NT-N-PMo12 hybrid, 563 m2/g for NT-NS-PMo12, and 570 m2/g for NT-S-PMo12. In general it is observed that as the oxidation degree of NT matrices increases and the particle size of PMo12 decreases (higher dispersion) to form the hybrid materials (Figure 7) the SBET also increases, confirming the dispersive effect of the anchored PMo12 particles on functionalized nanotube matrices. Nevertheless, the slight increment of this area (SBET) observed in hybrids obtained with the intermediate and highly oxidized matrices (NT-NS- PMo12 and NT-S- PMo12) could be related with the slight decrease of the PMo particle size, from 2-4nm in NT-NS- PMo12 to 1.5 nm in NT-S-PMo. These hybrids with different SBET are reviled on their elec-trochemical behavior as follows. Figure 8 shows cyclic voltam-metry (CV) experiments carried out at 5mV/s for all hybrid materi-als. In general, the typical rectangular CV profile of the nanotube matrix can be detected, and over this profile the three typical redox waves from PMo12 are observed confirming its diluted presence in all hybrid materials [5,9,13-14,39] around -0.18V/-0.11V (I), -

0.3V/-0.23V (II), and -0.55V/-0.48V. However, differences are observed in their current range and CV profiles. For NT-N-PMo12 hybrid electrode with the lowest SBET (424 m2/g) and bigger and less disperse PMo12 particles (Fig. 6a and 7), the current range is the lowest revealing its poorer electrochemical behavior. On the other hand, NT-NS- PMo12 and NT-S-PMo12 hybrid electrodes with more similar SBET (563 m2/g and 570 m2/g), show a similar current range but some differences on their CVs profiles. NT-NS-PMo12 hybrid electrode with PMo12 particles around 2-4nm (Fig. 6b) shows a CV profile more incline, indicating a higher ionic resis-tance. In contrast, NT-S-PMo12 hybrid electrode with the smallest and highly dispersed PMo12 particles and additional functional groups based on sulfur, shows the best electrochemical perform-ance where a rectangular profile indicative of a double layer contri-bution from the NT-S matrix and an additional redox pair at -0.076V/-0.02V related with functional groups [43] as in Figure 3 is observed. These results confirm the importance of the dispersion effect of POM particles on the surface of highly oxidized NTs (NT-S) on their electrochemical behavior.

4. CONCLUSIONS

Carbon nanotubes (purified and oxidized) were characterized before their interaction with the different POM particles to form their correspondent hybrid material. The graphitic nature of all nanotubes matrices were confirmed by XRD and no effect on their crystalline size was detected. TGA analyses as well as the electro-chemical behavior of the oxidized NT matrices revealed the pres-ence of functional groups needed for the anchoring of POM parti-cles in a higher degree for NT-S matrix oxidized with a stronger treatment, confirming previous FTIR results [39]. These nanotube matrices were used for the dispersion of soluble (PMo12) POM nanoparticles, where the dispersion effect had a strong influence on their electrochemical behavior. These hybrid materials were syn-thesized with all functionalized nanotube matrices and the presence

Figure 7. Schematic representation of the oxidation degree (presence of functional groups) in NTs and PMo particle size in the formation of the three NT/PMo hybrid material samples.

Figure 8. Cyclic voltammetry carried out for NT/PMo hybrid ma-terials at 5mV/s, using a 0.5M H2SO4 solution as the electrolyte:

NT-N-PMo (⎯ ), NT-NS-PMo (-----), and NT-S-PMo (….)


of both components was confirmed by XRD, EDS, TEM, and elec-trochemical characterization with cyclic voltammetry. XRD analy-ses revealed the nanocrystalline nature of the PMo12 particles de-tected by EDS and TEM analyses, and their homogeneity (no seg-regation) was confirmed by SEM images. Based on TEM images the different nano sized particles of PMo12 was confirmed over the surface of the different oxidized NT matrices. A clear effect of the functionalization degree of these matrices was observed on the average PMo12 particle size, where a nucleation-growth mechanism was suggested. The reduction size of PMo12 particles confirmed the dispersion degree improvement as the oxidation degree of the NT matrices enhanced resulting in an increase on their BET surface area, confirming previous results observed on SEM images for CsPOM hybrids. All these results were revealed on the electro-chemical behavior of these hybrid materials, where a clear impor-tance of the dispersion degree of PMo12 nanoparticles was the key factor for the improved electrochemical results.

5. ACKNOWLEDGMENTS

We thank the technicians from CNyN-UNAM, Israel Gradilla Martínez for the excellent SEM and EDS work, and Francisco Ruiz Medina for the TEM images. For XRD and TGA assistance, we acknowledge the technical work of Maria Luisa Ramón and Patricia Altuzar-Coello respectively. Finally, we thank the financial support granted from CONACYT (project 54761), UNAM-México (project IN102807-3, PAPIIT), and project PUNTA-UNAM.

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