Magnetic nanocomposites: Preparation and characterization of Co-ferrite nanoparticles

Colloids and Surfaces A: Physicochem. Eng. Aspects 281 (2006) 8–14

Magnetic nanocomposites: Preparation and characterization of Co-ferrite nanoparticles M.H. Khedr a,∗ , A.A. Omar b , S.A. Abdel-Moaty a a

b

Chemistry Department, Faculty of Science, Beni-Suef University, Egypt Mechanical Engineering Department, Benha High Institute of Technology, Egypt

Received 26 September 2005; received in revised form 5 January 2006; accepted 6 February 2006 Available online 24 March 2006

Abstract Cobalt ferrite nanoparticles, CoFe2 O4 , are one of the important spinel ferrites due to their high cubic magnetocrystalline anisotropy, high coercivity and moderate saturation magnetization. CoFe2 O4 nanoparticles have been known to be a photomagnetic material which shows an interesting light induced coercivity change. In this study, various preparation techniques were used to produce cobalt ferrite nanoparticles namely, (i) ball milling of a homogeneous mixture of cobalt(II) acetate, and iron(III) acetate (basic) treated by a novel self flash combustion, (ii) precipitation of cobalt(II) chloride (CoCl2 ·6H2 O) and iron(III) chloride (FeCl3 ), and (iii) ceramic method by firing of cobalt oxide (CoO) and iron oxide (Fe2 O3 ). These techniques help to obtain particle sizes ranging from a few micrometers to about 20 nm. Thermal analysis (TGA and DTA), X-ray diffraction, SEM, TEM, magnetic and surface area measurements have been used for characterization of the prepared samples. Results showed that saturation magnetic flux density (Bs) and remnant magnetic flux density (Br) varied with crystallite size from 6.929 to 14.91 and 2.73 to 8.146 emu/g, respectively. The measured surface area (SBET ) for the prepared Co-ferrite particles ranged from 5.327 to 47 m2 /g. The effect of different nanosizes on the total pore volume, adsorption energy, average pore diameter, micro pore volume, have been studied. Nanocrystalline CoFe2 O4 showed a catalytic activity towards CO2 decomposition with the formation of carbon nanotubes. © 2006 Elsevier B.V. All rights reserved. Keywords: Ferrites; Nanocrystalline; Magnetic; Surface area; Nanotubes

1. Introduction Nanocrystalline materials are showing great promise in industry and technology [1,2]. This is mainly because they have some unique properties which are not showed by the bulk crystalline materials [3,4]. The synthesis of spinel ferrite nanoparticles has been investigated intensively in recent years because of their remarkable electrical and magnetic properties and wide practical applications in information storage systems, ferrofluid technology, magnetocaloric refrigeration and medical diagnostics [5,6]. Various preparation techniques, such as sol–gel method [7], citrate precursor technique [8] and mechanical alloying [9,10], are used to produce ferrite nanoparticles. As it is well known,

∗

Correspondence to: 57,103, St. Maadi P.O. Box 41, Cairo, Egypt. Tel.: +20 25252053; fax: +20 822334551. E-mail address: [email protected] (M.H. Khedr). 0927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2006.02.005

chemical precipitation is an economical way to produce ultra fine powders [11,12]. Yeong et al. studied the magnetic properties of CoFe2 O4 nanoparticles which have been synthesized in a homogeneous aqueous solution without any template and subsequent heat treatment. The average particle size could be varied in the range of 2–14 nm by controlling co-precipitation temperature of Co+2 and Fe+3 ions in alkaline solution although the size distribution is pretty wide. As the precipitation temperature increased in the range of 20–80 ◦ C, the average particle size also increased. However, there was a considerable change in XRD crystallinity and the average size of the nanoparticles at the precipitation temperature between 40 and 60 ◦ C. While the nanoparticles prepared at temperatures below 40 ◦ C showed superparamagnetic relaxation at room temperature with blocking temperatures between −198 and −73 ◦ C. The samples prepared at temperatures higher than 60 ◦ C were consisted of both superparamagnetic and ferromagnetic nanoparticles that result in magnetic coercivity at room temperature. M¨ossbauer spectra of the samples also confirmed

M.H. Khedr et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 281 (2006) 8–14

their magnetic properties and wide size distribution in each sample. The analysis of the spectra gave a rough estimation of the ratio of superparamagnetic and ferromagnetic nanoparticles in each sample at various temperatures [13]. Bensebaa et al. studied stable CoFe2 O4 nanoparticles which have been obtained by co-precipitation using a microwave heating system. TEM images analysis showed the agglomeration of particles with an average size of about 5 nm, and XRD revealed the presence of a pure ferrite nanocrystalline phase. XRPS and thermal gravimetric analysis showed the presence of organic matter in the range of about 16 wt%. The magnetic response in DC fields was typical for an assembly of singledomain particles. The measured saturation magnetization was slightly larger than the bulk value, probably due to the presence of small amounts of Co and Fe. AC magnetization data indicated the presence of magnetic interactions between the nanoparticles [14]. Shi et al. assigned cobalt ferrite nanoparticles which have been prepared by the combination of chemical precipitation, mechanical alloying and subsequent heat treatment. Sodium chloride was added before milling in order to avoid agglomeration. From XRD measurements it was found that the cobalt ferrite phase could form directly during mechanical milling of the precipitated hydroxide/oxide precursor. Long-term milling caused contamination and growth in particle size. In order to reduce these two undesirable effects, a revised processing rout, milling at a lower speed for a relatively shorter time and further heat treatment, was adopted. CoFe2 O4 nanoparticles were obtained after a simple washing process with deionised water. TEM measurements showed that the nanoparticles had a fairly uniform structure with a mean particle size of approximately 10 nm. Anisotropic nanoparticles were obtained after magnetic annealing at 300 ◦ C [15]. Eun et al. studied CoFe2 O4 nanoparticles synthesized by a microemulsion method and characterized by XRD, TEM, SQUID magnetometry and M¨ossbauer spectroscopy. All peaks of X-ray diffraction patterns could be attributed to a cubic ˚ The averspinel structure with the lattice constant a0 = 8.39 A. age size of the particles, determined by transmission electron microscopy, was 7.8 nm. Super paramagnetic behavior of the particles was confirmed by the coincidence of plots of the magnetization versus field divided by temperature. As the temperature increased toward the Neel point, Mossbauer line broadening and a pronounced central doublet appeared, suggesting superparamagnetic relaxation. As the temperature increased, the relaxation rate increased rapidly as the seventh power of the absolute temperature [16]. CO2 is the major component of the greenhouse effect gases which caused the global warming. In order to decrease CO2 , the reduction of CO2 using chemical, photochemical, and biological methods have been studied [17–19]. Tabata et al. [20] reported that CO2 could be decomposed completely into carbon with oxygen-deficient ferrites (ODFs) at low temperature near 300 ◦ C. It is known that the efficiency of CO2 decomposition will be improved significantly by developing ultra fine ferrite particles with high specific surface areas [21,22].

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The present work studies the preparation of Co-ferrite nanoparticles by different techniques; namely chemical precipitation, ceramic method and a novel self flash combustion method. The prepared ferrite particles are investigated for their magnetic properties as well as their efficiency as a catalyst for CO2 decomposition. 2. Experimental Different preparation techniques were used to produce cobalt ferrite nanoparticles with different crystallite sizes. Cobalt ferrite with large crystallite size (188 nm) was prepared by the ceramic method where 1 mol of Fe2 O3 was mixed with 1 mol of CoO in a ball mill for 4 h. The obtained powder was moistened with 10% water and equal weights of 2 g were then pressed in cylindrical mould of 1.5 cm diameter at 200 kg/cm2 . The produced compacts were left overnight in open air, then dried at 100 ◦ C for 24 h. The dry compacts were gradually heated in a muffle furnace up to 1100 ◦ C and kept at this temperature for 3 h, then left to cool in a gradual way in the furnace to avoid cracking due to thermal shocks. Cobalt ferrite nanoparticles having average crystallite size of 85.5 nm were prepared by the precipitation method, where 2 mol of ferric chloride (FeCl3 ) solution were well mixed with 1 mol of cobalt(II) chloride hexahydrate, CoCl2 ·6H2 O. Ammonium hydroxide was then added until the pH value equals 10 and left for complete precipitation, then the mixture was washed with distilled water several times and filtered. The precipitate was dried in an oven at 100 ◦ C and fired at 500 ◦ C for 2 h. CoFe2 O4 with lower crystallite size was prepared by a novel self flash combustion method; 2 mol of pure fine powder ferric acetate basic (Fe(CH3 COO)2 ·OH) were well mixed in a ball mill for 30 min with 1 mol of cobalt(II) acetate tetrahydrate, (CH3 COO)2 Co·4H2 O. The mixture was then heated on a hot plate for 12 h, then mixed in a ball mill with a speed 170 r/m for 10 h. 21 porcelain balls were used; 10 of them have 1.7 cm diameter and 9.9 g weight, the other 11 balls are 0.8 cm in diameter and 1.54 g weight. Samples were then fired at 500 ◦ C for 2 h to obtain Co-ferrite particles with crystallite size of 38.5 nm. Finally, 2 moles of pure fine powder ferric acetate basic, (CH3 COO)2 Fe·OH were well mixed in a ball mill for 30 min with 1 mol of cobalt(II) acetate tetrahydrate, (CH3 COO)2 Co·4H2 O, the mixture was heated on a hot plate for 12 h, then mixed in a ball mill for 20 h. The average crystallite size of the obtained cobalt ferrite was 19.5 nm. The different nanosizes were identified by X-ray diffraction technique using a JSX-6OP JEOL diffractometer. The average crystallite size was calculated from the X-ray diffraction peaks using the following Scherer’s formula [23]: D=

0.9λ (β − β1 )cosθ

where D is the crystallite size, λ the X-ray wave length, β the broadening of the diffraction peak and θ is the diffraction angle. Microstructure was studied by a JEOL JSM-5410 SEM, and a JEOL JSM-5410 TEM. The magnetic properties were investi-

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gated by a Vibrating Sample Magnetometer model 9600 while the thermal analysis was performed using MAC-Science model DTA-TGA 2000. FTIR JASCO 410 was used for IR solid phase spectra. The surface area measurements were determined volumetrically using a Quantachrome NOVA Automated Gas Sorption System Report. Catalytic reactions were carried out in a continuous-flow fixed bed. Around 1 g portion of the different crystallite sizes of cobalt ferrite catalyst was packed in a quartz tube with a diameter of 1 cm and length of 8.8 cm. The reaction tube was heated in an electric furnace to allow reaction with gases [24]. CO2 gas was allowed to flow over the different samples of cobalt ferrite nanoparticles at a flow rate of 100 ml/min at 700 ◦ C for 1 h. The obtained specimens were examined by TEM. 3. Results and discussion 3.1. Thermal analysis Fig. 1 shows the thermal behaviour of cobalt(II) acetate tetrahydrate, iron(III) acetate basic and a mixture of 1 mol of the former to 2 mol of the later. Table 1 shows the different phases expected to be obtained from the decomposition process. Cobalt(II) acetate tetrahydrate (Co(CH3 COO)2 ·4H2 O), decomposes thermally, in dynamic air flow, in three steps. The first step at 179 ◦ C with a percent of weight loss (%WL) = 31.315 which is close to the theoretical %WL (31.474) involving elimination of 1 mol H2 O, 1 mol CO2 and 1 mol CH4 . Step II at 298.23 ◦ C with %WL = 13.65 which is close to the theoretical %WL (11.88) leading to removal of 1 mol H2 O and 1 mol CH4 . The last step (III) at 403 ◦ C with %WL = 24.89 which is close to the theoretical %WL (25.389) leading to removal of 1 mol of CO2 and the rest of water. The complete dehydration and formation of CoO can be summarized as follows: Co(CH3 COO)2 ·4H2 O (179 ◦ C) → CH3 COO · Co · OH · 2H2 O + CO2 + H2 O + CH4 (1)

Fig. 1. (a) TGA curves of CoAc, FeAc, and their mixture with 1:2 mmol ratio. (b) DTA curves of acetate mixture in dynamic air flow.

CH3 COO · Co · OH · 2H2 O (298.23 ◦ C) → CoCO3 ·H2 O + H2 O + CH4

(2)

CoCO3 ·H2 O (403.19 ◦ C) → CoO + H2 O + CO2

(3)

Iron(III) acetate basic (Fe(CH3 COO)2 ·OH) decomposes in two steps, the first is endothermic, at 160 ◦ C with 23%WL which is nearly similar to the theoretical %WL value (22%). The second step is exothermic, at 285 ◦ C and is accompanied by 33%WL

Table 1 Theoretical and observed weight loss and X-ray phase identification at different thermal steps Process

Step

X-ray phase identification

Observed WL%

Theoretical WL%

CoAc decomposition

I II III

(CH3 COO)2 Co·4H2 O → CH3 COO·Co·OH·2H2 O CH3 COO·Co·OH·2H2 O → CoCO3 ·H2 O CoCO3 ·H2 O → CoO

31.315 13.65 24.89

31.474 11.88 25.389

FeAc decomposition

I II

Fe(CH3 COO)2 ·OH → Fe(OH)2 CH3 COO Fe(OH)2 CH3 COO → FeOCO3

23 33

22 32

CoAc + FeAc decomposition

I II III IV V VI

(CH3 COO)2 Co.4H2 O + 2Fe(CH3 COO)2 ·OH → (CH3 COO)2 Co.3H2 O + 2Fe(CH3 COO)2 ·OH (CH3 COO)2 Co·3H2 O + 2Fe(CH3 COO)2 ·OH → (CH3 COO)2 Co·2H2 O + 2Fe(CH3 COO)2 ·OH (CH3 COO)2 Co·2H2 O + 2Fe(CH3 COO)2 ·OH → (CH3 COO)2 Co·1·5H2 O + 2Fe(CH3 COO)2 ·OH (CH3 COO)2 Co.1.5H2 O + 2Fe(CH3 COO)2 ·OH → CH3 COO.Co.OH + 2Fe(CH3 COO)2 ·OH CH3 COO·Co·OH + 2Fe(CH3 COO)2 ·OH → CH3 COO·Co·OH + 2CH3 COO·Fe(OH)2 CH3 COO·Co·OH + 2CH3 COO·Fe(OH)2 → CoFe2 O4

2.85 2.85 1.4 10.93 13.3 31.3

2.608 2.608 1.91 11.635 11.927 31.351

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C O and OH at vibration mode region. The V and VI steps are exothermic and this may be due to the burning of CH2 C O, CH3 COOH and disappearance of all water molecules to form CoFe2 O4 [25]. In support, the IR-solid phase shows at 360 and 500 ◦ C disappearance of all bands except M O with shift in its values due to the formation of CoFe2 O4 [25]. 3.2. X-ray analysis and microstructure Fig. 3 shows XRD patterns of cobalt ferrite powder prepared by different methods. It is obvious that for samples prepared by self flash combustion, amorphous phase of cobalt ferrite (19.5 and 38.5 nm), was formed with minor detection of iron oxide and cobalt oxide. Using wet method crystalline cobalt ferrite (85.5 nm) was formed with minor contribution to iron oxide and cobalt oxide. Finally, for sample prepared by ceramic method larger cobalt ferrite (188 nm) with high crystallinity was formed

Fig. 2. FTIR-solid phase spectra obtained for CoAc + FeAc with 1:2 mol calcined at different temperatures for 2 h.

which is equivalent to that calculated theoretically (31%) for the formation of the iron oxycarbonate as shown in Table 1 [25]. Fig. 2 shows IR-solid phase of CoAc and FeAc mixture which displays bands at 3860, 3742, 3679 and 3340 cm−1 indicating the presence of OH, which are assignable to the stretching mode. However, the bands at 2361, 2335, 1916 and 1873–1676 cm−1 indicate the presence of C O. On the other hand C O is shown at 1550, 1331 and 1029 cm−1 , CH3 at 1440 cm−1 . While bands at 885 and 798 cm−1 indicate the presence of M O, which are assignable to the vibration modes of acetates [26]. TGA and DTA curves of a homogeneous mixture of 1 mol Co(CH3 COO)2 ·4H2 O and 2 mol of Fe(CH3 COO)2 ·OH, shown in Fig. 1 illustrate that the total percent weight loss obtained from the TGA curves is 62.8 which is near to the value calculated theoretically (62.02) for the formation of cobalt ferrite (CoFe2 O4 ). The formation of cobalt ferrite is also confirmed by XRD patterns and IR spectra. The DTA curves illustrate that the acetate mixture with the proper mole ratio decomposes thermally in several steps. I–III steps are endothermic revealed by, peaks at 57.7, 87.97 and 115.76 ◦ C due to the dehydration of CoAc. In support of the above results, IR-solid phase for samples treated at 60–90–120 ◦ C (Fig. 2) bears a great deal of similarity to those obtained for unfired FeAc and CoAc. The main difference is the disappearance of the bands assignable to water at stretching mode region. The IV step is exothermic due to removal of 1 mol acetic acid and 0.5 mol water from CoAc. This probability is supported by the IR solid spectra obtained at 210 ◦ C which indicated the disappearance of the bands assignable to CH3 , C O,

Fig. 3. XRD patterns for different CoFe2 O4 nanosizes. (1) CoFe2 O4 , (2) Fe2 O3 and (3) CoO.

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Fig. 4. TEM micrograph of CoFe2 O4 nanoparticles prepared by self combustion from acetate precursor method (19.5 nm).

Fig. 6. B–H hysteresis loop for different CoFe2 O4 nanosizes.

with nearly no contribution to iron oxide and cobalt oxide. Fig. 4 shows the TEM image of the sample prepared by self flash combustion from acetate precursor (19.5 nm). The image shows small agglomerates of cobalt ferrite nanoparticles. SEM for the sample prepared by ceramic method (188 nm) shown in Fig. 5 indicates a dense agglomerates of cobalt ferrite with micro and macropores. 3.3. Magnetic properties Magnetic measurements were done at room temperature on a VSM with a peak field of 5 kOe. The hystersis loops for samples prepared by different methods are shown in Figs. 6 and 7

Fig. 7. Magnetic properties for different CoFe2 O4 nanosizes.

and summarized in Table 2. The coercivity first increases as the crystallite size decreases to reach a value of 1786 Oe at 38.5 nm, then decreases by further decrease in crystallite size. The squareness ratio (Br/Bs) of the particles exhibiting the highest coercivity is 0.54. The value of 1786 Oe for the coercivity at 25 ◦ C is a relatively high value through the low temperature synthesis technique. As the single domain size of cobalt ferrite is reported to be around 70 nm, the magnetization mechanism for particles below and above the single domain size will be differTable 2 Magnetic properties of different CoFe2 O4 nanosizes

Fig. 5. SEM micrograph of CoFe2 O4 nanoparticles prepared by ceramic method (188 nm).

Br/Bs Bs (emu/g) Hc (Oe)

19.5 nm

38.5 nm

85.5 nm

188 nm

0.48016 12.2 1110

0.54634 14.91 1786

0.25684 10.59 322

0.39688 6.929 790.5

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Table 3 Effect of different nanosizes of cobalt ferrite on surface area

(m2 /g)

Surface area Total pore volume (cc/g) Adsorption energy (kJ/mol) Average pore diameter (nm) Micro pore volume (cc/g)

Fig. 8. Hysteresis loop of surface area of different sizes of CoFe2 O4 .

ent. This mechanism also contributes to the variation observed in the coercivity values. Apparent ferromagnetic character by a Br/Bs ratio ≈0.256 is also observed for samples prepared by ceramic method (188 nm). Superparamagnetic behavior is always observed for other samples at room temperature and the squarness ratio values are 0.396, 0.54 and 0.48 for crystallite sizes 85.5, 38.5 and 19.5 nm, respectively. The saturation magnetization (Bs) ranges from 6.9 to 14.9 emu/g which are in all cases far from the reported values for bulk CoFe2 O4 (80 emu/g) [27]. Finite size effects have been reported as being responsible for the reduction of saturation magnetization of nano CoFe2 O4 [28]. Samples which were synthesized by self flash combustion from acetates yield particles with an average size of about 19.5 and 38.5 nm which are much smaller than the critical size of single domain CoFe2 O4 estimated at about 70 nm giving more evidence for superparamagnetism assembly of single domain particles [29].

19.5 nm

38.5 nm

85.5 nm

188 nm

47 0.02342 2.795 21.21 0.04279

23.43 0.01161 5.527 20.24 0.01685

36.08 0.01933 5.712 21.43 0.03031

5.327 0.001786 1.749 13.41 0.003624

The isotherms generally belong to type IV of the BET classification [30] for all samples except the sample prepared by ceramic method (188 nm) and this is attributed to the relatively high sintering. These are associated with capillary condensation in mesopores, indicated by the steep slop at higher relative pressures. The average pore diameter for all samples indicates mesopore structure (average pore diameter >20 nm) except for the sample of the highest crystallite size which shows a microporous structure (average pore diameter = 13.5 nm). The initial part of the IV isotherm follows the same path as the type II that represents unrestricted monolayer–multilayer adsorption. The hysteresis loops are nearly of type H3, in the general features primarily indicated that the pore structures are wedge-shaped pores with open ends. The measured values of surface area (SBET ) listed in Table 3 indicates that by increasing crystallite

3.4. Surface area measurements and catalytic activity The N2 adsorption study on the different nanosized cobalt ferrite samples are shown in Figs. 8 and 9 and listed in Table 3.

Fig. 9. The effect of different nanosizes on surface area, adsorption energy and total pore volume in cobalt ferrite.

Fig. 10. (a) Carbon nano-tubes formed by decopmsition of CO2 on CoFe2 O4 with 19.5 nm. (b) Carbon nano-tubes formed by decopmsition of CO2 on CoFe2 O4 with 85.5 nm.

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size, surface area decreases except for the sample prepared by wet method which shows a relatively high surface area. The relatively high surface area of the sample prepared by wet method may be attributed to its more uniform structure and homogeneity. The adsorption energy for all samples are ranged from 1.7 to 5.7 kJ/mol which is an indication of physisorption. The CO2 decomposition to C was studied at 100 ml/min at 700 ◦ C for 1 h over CoFe2 O4 nanoparticles. CO2 decomposes to carbon nano-tubes with variable amounts of CO. The XRD analysis of the black fine powder remained after the CO2 decomposition process reveals the presence of crystalline carbon. The intensity of the carbon peaks increases with the decrease in the crystallite size of the original CoFe2 O4 nanoparticles. TEM of CoFe2 O4 nanoparticles with crystallite size 19.5 nm after the CO2 decomposition process shows the maximum amount with the formation of multiwalled carbon nano-tubes (Fig. 10a). This is attributed to the large surface area compared to that of the CoFe2 O4 nanoparticles with larger crystallite size (85.5 nm), which shows a single walled carbon nano-tubes (Fig. 10b) [31]. It is also clear from the TEM photomicrographs that the diameter of the single walled nano-tubes is approximately 25 nm compared to 120 nm for the multiwalled carbon nano-tubes.

except sample prepared by ceramic method (188 nm) and this is attributed to the relatively high sintering. We noticed that for particles with 38.5 nm showed completely unexpected results, all the nanosizes of cobalt ferrite are mesopores. CO2 decomposes to carbon nano-tubes during the flow over the different samples of cobalt ferrite nanoparticles. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

4. Conclusion CoFe2 O4 nanoparticles (19.5 and 38.5 nm) were successfully prepared by a novel self flash combustion from acetate precursors at low temperature and also prepared by co-precipitation (85.5 nm) and by ceramic method (188 nm). From thermal analysis (TGA, DTA) and IR spectra, we noticed the thermal decomposition of CoAc, FeAc and the mixture of them with 1:2 mol ratio in six steps, three of them are endothermic and the last three steps are exothermic to form CoFe2 O4 at nearly 600 ◦ C. Magnetic measurements show that, the coercivity first increases as the crystallite size decreases, and reaches a maximum value of 1786 Oe at 38.5 nm, then decreases by a further decrease in crystallite size. Results show that saturation magnetic flux density (Bs) and remnant magnetic flux density (Br) varied with crystallite size from 6.929 to 14.91 and 2.73 to 8.146 emu/g, respectively. Surface area measurements, show that the isotherms generally belong to type IV of the BET classification for all samples

[15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]

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