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Wednesday, 12 October 2016
Semiconductor carbon composite from coir dust and sepiolite
Published Date
doi:10.1016/j.matchar.2010.10.010
Open Access, Elsevier user license
Short communication
Author
Thalita Santos Bispo
Gabriela Borin Barin
Iara F. Gimenez
Ledjane Silva Barreto,
Programa de Pós-graduação em Ciência e Engenharia de Materiais, Centro de Ciências Exatas e de Tecnologia, Universidade Federal de Sergipe, Av. Marechal Rondon s/n, Jd. Rosa Elze 49100-000, São Cristovão, SE, Brazil
Received 26 April 2010. Revised 17 September 2010. Accepted 9 October 2010. Available online 26 October 2010. Research Highlights
►Sepiolite is added to coconut coir dust previous to thermal treatment. ►This favors formation of activated carbon monoliths without conventional binders.
►Compact aggregates in TEM images suggest a good affinity between components.
Composites based on activated carbons and clays minerals have been applied as adsorbents, sensors and catalysts [1] and [2]. In addition to relevant technological properties, these materials may provide alternatives for the reuse of residues from the petrochemical [3] and [4] and agricultural activities [5]. In one of the earliest descriptions of carbonaceous/clay composites, Gonzales and co-workers reported about carbon-sepiolite pellets using activated carbon from olive stones [6]. Those composites were developed aiming to combine the adsorptive properties of activated carbons with those of sepiolite, Si12Mg8O30(OH)4(H2O)4, a fibrous silicate with microporous channels parallel to the fiber axis. The composites were used in humidity controllers taking advantage of the water adsorption capacity of sepiolite at high relative humidity and that of activated carbon [7], being also used as adsorbent for NH3 and H2S [8]. Here we report the preparation of a carbon composite by the carbonization of pellets containing coconut coir dust [9] as carbon source and sepiolite as a binder. It is known that carbonaceous materials present good electronic conductivity in addition to high surface areas [10] and [11], thus being ideal candidates for use as electrodes in systems such as sensors, energy storage and generation devices [12]. Monoliths are required for some of these applications, such as electrodes for supercapacitors, as specifically discussed by Rojo and co-workers [13] and [14], so here we propose a possibility of obtaining carbon monoliths from coconut coir with semiconducting properties.
2 Experimental
Coconut coir was dried at 110 °C overnight and sieved to a particle size of approximately 300 μm. 2:1 coir:sepiolite (m/m) mixtures were ground for 30 min and then pressed into 5 mm diameter pellets. The pellets were heated in a tube furnace (10 °C min−1 heating rate under 100 mL min−1 N2 flow) for 2 h at 800 °C. The samples before and after heat treatment were characterized as follows. Fourier Transform Infrared Spectroscopy (FTIR) measurements were carried out for samples prepared as KBr discs, using a Perkin Elmer Spectrum BX instrument (4000–400 cm−1 range, 4 cm−1 resolution). Thermogravimetric (TG) Analysis (TA Instruments STD 2960) was performed using platinum pans, 10 °C min−1 heating rate under 100 mL min−1 N2flow). For X-ray diffraction (XRD) measurements we used a Rigaku instrument with Cu-Kα source in the 5–50° 2θ range and 3° min−1 scan rate. For the acquisition of Transmission electron microscopy (TEM) images we used a LEO-910 microscope, at an accelerating voltage of 80 kV, while a Jeol JSM 6360 LV was used for the scanning electron microscopy images. Electrochemical impedance spectroscopy measurements were made in the frequency range of 1–13 MHz with a Solartron 1260 analyzer coupled to a Eurotherm temperature controller. Pellets with 2 mm thickness were Au sputtered prior to the analysis, and during the measurements the temperature was changed from 200 °C to 500 °C under N2 flow, with the application of a 1000 mV amplitude signal.
3 Results and Discussion
The thermal decomposition evidenced by TG curves of coconut coir and sepiolite, Fig. 1(a), are well described in the literature [4] and [15]. Decomposition of coconut coir proceeds by the decomposition of hemicelluloses (200–260 °C), cellulose (240–350 °C) and lignin (280–500 °C) [5]. For sepiolite, the four steps observed are related to elimination of adsorbed water (66 °C), hydration water (227 °C), coordination water (600 °C) and water molecules from dehydroxylation (808 °C), as described in previous works that report the formation of a mixture of enstatite MgSiO3 and cristobalite (SiO2) as residues above 830 °C [15].
Fig. 1. TG curves for: a) coconut coir; b) sepiolite; c) 2:1 coir:sepiolite mixture before the heat treatment; and d) 2:1 coir:sepiolite mixture after the heat treatment at 800 °C.
The curve for 2:1 coir:sepiolite mixture before the heat treatment shows mainly events present in the coconut coir curve. The most evident quantitative observation when comparing the TG curves for the coir:sepiolite mixture and those of the starting materials is the difference in the mass % of the final residues. For the coir dust, a 16.8% residue is found, while the values for sepiolite and for the mixture are 79.6% and 42.8%, respectively. For the mixture, a residual mass of 37.7% could be estimated on the basis of starting materials, which suggests that both precursors can be stabilized in the mixture. Another change is the lowering in the resolution of the shoulder in DTG curve, Fig. 2(b), at 270 °C for the mixture, suggesting the contribution of the sepiolite decomposition event at 275 °C in the DTG curve.
Fig. 2. XRD powder diffraction patterns for: (a) coconut coir; b) original sepiolite; (c) 2:1 coir:sepiolite mixture before heat treatment; (d) sepiolite heated at 500 °C; (e) 2:1 coir:sepiolite mixture after treatment at 500 °C; f) sepiolite heated at 800 °C; and (g) 2:1 coir:sepiolite mixture after treatment at 800 °C.
The heat-treated sample, Fig. 1d, probably composed of carbon residues and products of the partial decomposition of sepiolite was found to lose mass gradually up to 800 °C, when a more pronounced mass loss indicates carbon pyrolysis.
According to XRD data, Fig. 2, the presence of coir dust modifies the progress of thermal decomposition of sepiolite. We first heated the mineral phase without the lignocellulosic material and observed that all changes in the diffraction patterns of sepiolite on heating are in very well agreement with the literature [16]. Upon heating at 500 °C, the peak at 2θ = 7.7° from sepiolite disappears and peaks at 2θ = 8.9, 11, 17.8, 19.2 and 20.3 have their intensity increased, which is assigned to a crystal phase known as sepiolite II or sepiolite anhydride, which is observed up to 730 °C. Formation of enstatite and cristobalite is observed above 860 °C. Here we heated sepiolite at 500 °C and 800 °C, observing only the intermediate phase sepiolite II. When mixed with dust, which exhibits a peak assigned to quartz at 2θ = 26.6° coir dust, the structure of sepiolite seem to be preserved until higher temperatures, since the sepiolite peak at 2θ = 7.7° can still be observed in the diffractogram of the mixture heated at 500 °C. Finally the XRD pattern for the mixture heated at 800 °C exhibits only peaks related to intermediate phases, without original sepiolite.
The morphology of the coconut coir as observed from TEM images, Fig. 3a, is predominantly fibrous [17], and so is sepiolite, Fig. 3b, as well documented in the literature [18]. Comparing the two kinds of morphologies, however, sepiolite fibers appear more separated from each other and also slightly thinner than coconut coir's. Coir fibers on the other hand seem to be aligned as bundles in the length direction. When mixed together, Fig. 3c, both fibrous solids are indistinguishable from TEM image and seem to mix very well with the formation of cohesive aggregates that suggest a good affinity. The 2:1 coir:sepiolite mixture contains aggregates of fibers apparently shorter than the original ones, probably due to fragmentation of longer fibers during grinding. A SEM image of the 2:1 mixture is also shown for comparison, Fig. 3d, and suggests that those aggregates assemble together by an effect of fiber intercrossing.
Fig. 3. a–c) TEM images: a) coconut coir (31,500 magnification); b) sepiolite (25,000 magnification); c) 2:1 coir:sepiolite mixture after heat treatment at 800 °C (25,000 magnification); and d) SEM image of the 2:1 coir:sepiolite mixture after heat treatment at 800 °C (5000 magnification).
Electrochemical Impedance spectroscopy was performed at temperatures from 200 to 500 °C for electrical conductivity evaluation of pellets heated at 500 °C and 800 °C. The specific conductivity C was calculated from the impedance value Z using the equation:
Where:
d
sample thickness (cm)
A
area of the gold circle covering the particle (cm2)
Z
impedance (Ω).
Both the conductivity values, Table 1, and the conductivity dependence with temperature characterize the semiconductor nature of the material [11]. It is also evident that the increase in the temperature of treatment of the pellets has resulted in a higher electrical conductivity of the samples, probably as a result of the increasing degree of conjugation of the carbon phase with temperature.
Table 1. Values of electrical conductivity (Ω−1 cm−1) with increasing temperature calculated from Nyquist plots.
Temperature (°C)
Conductivity (Ω−1 cm−1)
Composite prepared at 500 °C
Composite prepared at 800 °C
200
5.32 × 10−9
2.51 × 10−8
300
1.02 × 10−7
8.62 × 10−7
400
4.54 × 10−7
1.29 × 10−6
500
4.24 × 10−6
2.31 × 10−5
4 Conclusions
The affinity of the raw materials ensured the use of sepiolite as a binder for coir particles in order to yield monolithic composites. This was accomplished probably owing to the fact that coconut coir and sepiolite were mixed in the fiber level as suggested by TEM images. The thermal decomposition of sepiolite is apparently retarded by the presence of the lignocellulosic particles. Finally the electrical conductivity of the samples was found to be in the order of semiconduction, which points out potential applications in electrical and sensor devices.
Acknowledgements
Authors are grateful to CNPq, Fapitec and Capes for the financial support. Authors also acknowledge Prof. E. Ruiz–Hitzky and Dr. P. Aranda from ICMM and Prof. O. L. Alves from Unicamp.
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