Carbon nanofibers (CNFs) were synthesized through nickel ion (Ni2+) impregnation of powdered activated carbon (PAC). Chemical Vapor Deposition (CVD) using acetylene gas, in the presence of hydrogen gas, was employed for the synthesis process. Various percentages (1, 3, 5, and 7 wt. %) of Ni2+ catalysts were used in the impregnation of Ni2+ into PAC. Field Emission Scanning Electron Microscope (FESEM), Fourier Transform Infrared (FTIR) Spectroscopy, Energy Dispersive X-Ray Analyzer (EDX), Transmission Electron Microscopy (TEM), Thermal Gravimetric Analysis (TGA), zeta potential, and Brunauer, Emmett, and Teller (BET) were utilized for the characterization of the novel composite, which possessed micro and nanodimensions. FESEM and TEM images revealed that the carbonaceous structure of the nanomaterials was fibrous instead of tubular with average width varying from 100 to 200 nanometers. The PAC surface area increased from 101 m2/g to 837 m2/g after the growth of CNF. TGA combustion temperature range was within 400°C and 570°C, while the average zeta potential of the nanocomposite materials was −24.9 mV, indicating its moderate dispersive nature in water.
Activated carbon (AC) is an adsorbent used on an enormous scale in gaseous and aqueous purifications, metal extraction, and medicine, amongst other applications [
Carbon nanofibers (CNFs) are hydrophobic, generally, without significant functional groups [
Several methods have been used in the synthesis of CNF, such as the utility of various catalysts and carbon sources [
The process parameters that are important for the synthesis of CNF are the ratio of hydrogen to carbon source flow, reaction time, and temperature. The effect of hydrogen gas can be acceleration and suppression. Effect of acceleration on the formation of carbon may be interpreted in two ways. The first theory suggests decomposition of inactive metal carbides by hydrogen to form active metal [
The growth time amongst other factors plays a major role in tailoring the morphology of nanomaterials in terms of diameter, dense of growth, and so forth. [
The main objective of this study was to use oil palm kernel shell based PACs as a solid substrate for the growth of CNF on nickel impregnated PACs by CVD method. Determination of various physicochemical properties of the new nanomicrocomposite carbon material was done as well.
The following materials and equipment were used for the synthesis and characterization processes.
Palm kernel shell based granular activated carbon (GAC) was obtained from Effigen Carbon Sdn. Bhd., Malaysia. The GAC was bought in 20 kg bags and grinded to powdered activated carbon (PAC) of sizes ranging from 100 to 250
Philips grinder was used to crush the raw granular activated carbon to form PACs. Retsch sieve shakers model AS200 of German product, digit, and Max amplitude of 100 was used to sieve the grinded activated carbon to desired fraction size. A&D four digits weighing balance (HR-202i, Japan) was used for all the weighing measurements. The weighing range of the balance is from 0.001 to 220 g. Ultrasonic bath model (JAC 2010 P) from South Korea was used for the mixing of PAC and nickel solution to impregnate the metal onto the substrate. The bath is equipped with heater (max. 70°C), timer for 99 seconds, and three sonication levels. Lenton, England triple stage horizontal tubular ceramic reactor (diameter, 50 mm; length, 1500 mm), was used for the CVD process for the growth of CNF. The maximum heating temperature for the furnace is 1200°C. At the heating area, the ceramic tube utilized a resistance heating glass wool from Isolite Ceramic Fiber Sdn. Bhd. Malaysia. A drying oven model 600 from Memmert, Germany, of maximum temperature 220°C was used to dehumidify the samples of PAC and CNF-PAC.
FESEM model 6700F from JEOL Company (Japan) was used to determine the morphological features of the CNF. Energy Dispersive X-ray Analyzer was attached to the FESEM for the determination of structure of the PAC and CNF. High-resolution TEM model JEM-2010 from JEOL Company (Japan) was used to determine the diameter and length of the CNF. Automated Gas Sorption System (Quantachrome Autosorb) from Quantachrome Company (USA) was used to calculate the surface area of the powdered activated carbon before and after the CNF growth. FTIR model 100 from Perkin Elmer (USA) was used for the classification of the functional groups on the PAC, impregnated PAC, and CNF. Perkin Elmer Instruments Pyris diamond TG/DTA (USA) was used to study the thermogravimetric behavior of PAC and CNF-PAC.
Palm kernel shell based granular activated carbon (GAC) was grinded and sieved. Powdered activated carbon (PAC) with particle size between 100 and 250
The incipient wetness impregnation was used to impregnate nickel nitrate salt onto PAC by dissolving the nickel salt in acetone adopting method described elsewhere [
The catalyst was mixed with specified amount of PAC. The mixture was then sonicated in the ultrasonic bath for 30 min at room temperature and with high sonication speed. The nickel impregnated PAC was left in the water bath at temperature of 56°C and sonicated for 12 h to ensure complete impregnation process and evaporation of any excess acetone.
Catalyst substrate was fixed in a ceramic alumina boat in the tubular ceramic reactor (Figure
Schematic diagram of multipurpose triple stage CVD system.
After the impregnation, calcination, and reduction of the catalyst and the growth of the CNF, the product was weighed and the yield, %, of the grown CNF was determined by using the formula (
Yield percentage of the CNF growth on impregnated PAC.
Catalyst percentage (%) | Weight (g) of sample before CNF growth ( |
Weight (g) of sample after CNF growth ( |
Yield (%) |
---|---|---|---|
1 | 1.2 | 1.56 | 30 |
3 | 1.2 | 2.16 | 68 |
5 | 1.3 | 2.02 | 55 |
7 | 1.3 | 1.86 | 43 |
The yield calculations (%) showed that the highest yield of CNF was achieved from the 3% catalyst and the process condition was used for further production of PAC-CNF samples. A percentage weight catalyst higher than 3% gave less percentage yield, and this is attributable to the agglomeration of Ni, while 1% gave lesser yield when compared with that of 3%. This effect may be attributed to the lesser amount of nickel exposed to the carbon as concluded in previously reported similar investigation [
Field Emission Scanning Electron Microscope image of the CNF for (a) 1% catalyst, (b) 3% catalyst, (c) 5% catalyst, and (d) 7% catalyst.
The FESEM images showed that the catalyst of 3 wt.% gave the best growth of CNF and it matches with the yield calculations showed in Table
Field Emission Scanning Electron Microscopy (FESEM) technique was used for the analysis of physical morphology of the surface of the prepared samples. Figures
Scanning Electron Microscope images: (a) raw PAC, (b) Ni impregnated PAC, and (c) (d) CNF grown on PAC at optimized conditions.
The Transmission Electron Microscopy (TEM) analysis proved the formation of the as-prepared CNF. Figures
Transmission Electron Microscopy images CNF.
Transmission Electron Microscopy images of layered structure of CNF.
Using computer-monitoring system, the adsorbed nitrogen gas volume, equilibrium pressures, and BET surface area of the sample were determined. The specific BET surface area of the PAC was found to be 101.1 m2/g. After sonication, the Ni2+ impregnated PAC surface area increased to 131.2 m2/g due to production of additional fine particles during the sonication process. On the other hand, the CNF grown on PAC exhibited 836.7 m2/g, which was more than eight times surface area compared to that of PAC.
Plot of volume of nitrogen gas adsorbed on raw PAC and PAC-CNF versus
BET isotherm parameters for nitrogen gas adsorption by PAC and PAC-CNF.
Compound | Parameter | ||
---|---|---|---|
Total pore volume (cc/g) |
|
| |
PAC |
|
0.029 | 0.9999 |
Impregnated PAC |
|
0.029 | 0.9996 |
PAC-CNF |
|
0.24 | 0.9985 |
Adsorption/desorption of nitrogen gas on PAC.
Adsorption/desorption of nitrogen gas on PAC-CNF.
The zeta potential of the materials was measured for PAC and PAC-CNF in water. Test results showed that the zeta potential for the samples was negative and the peaks shifted towards zero point from PAC value (−30.9) to the PAC-CNF value (−24.9). The negative sign for the zeta potentials was indicative of the hydrophilicity of the composite. The high negative charge is an indication that the adsorbent is more hydrophilic [
The spectrum for the PAC, impregnated PAC, and CNF samples displayed bands with their respective functional groups as shown in Figure
FTIR diagram of PAC, impregnated PAC, and PAC-CNF.
The presence of these bands shows the formation of new carbon groups on the surface of PAC which is due to CNF [
Functional groups detected by FTIR on PAC, impregnated PAC, and PAC-CNF.
Wave number at peak (cm−1) | Functional group | Type of vibration |
---|---|---|
3859–2519 | –OH (un-bonded), –OH (bonded) | No interferences such as OH in lattice, –OH stretch |
2400–2250 & 1965, 1746 | Aldehyde, ketone, carboxylic acid | C=O, carbonyl group |
2343 | CO2 | |
1573 | C=C aromatic ring | C=C stretch |
996 | Methyl group | Saturated –CH3 |
766 & 702 | NO2 | Bending |
739 | Hydrogen on an aromatic ring | Adjacent hydrogens on an aromatic ring (Ortho) |
The EDX imaging analysis revealed the materials’ surface structure and the local elements distribution of PAC, impregnated PAC, and PAC-CNF were examined using the EDX analyzer as depicted in Figure
EDX spectrums for (a) PAC, (b) Ni impregnated PAC, and (c) PAC-CNF.
It was observed in Figure
Compositions of elements in PAC, Ni impregnated PAC, and PAC-CNF.
Element | Average weight |
Average atomic composition (%) | ||||
---|---|---|---|---|---|---|
PAC | Ni-PAC | PAC-CNF | PAC | Ni-PAC | PAC-CNF | |
C | 89.98 | 78.85 | 87.37 | 92.10 | 86.69 | 94.05 |
O | 4.88 | 13.96 | 3.95 | 7.20 | 11.52 | 6.66 |
Na | 0.18 | — | — | 0.10 | — | — |
Si | 0.71 | 0.37 | 0.69 | 0.32 | 0.18 | 0.32 |
Cl | 0.32 | — | — | 0.40 | — | — |
K | 0.33 | 0.36 | 0.55 | 0.11 | 0.12 | 0.18 |
Ca | 0.43 | 0.26 | 0.31 | 0.13 | 0.08 | 0.22 |
Ni | — | 6.16 | 5.84 | — | 1.39 | 1.29 |
Thermogravimetric analysis (TGA) revealed the temperature effect on the PAC and CNF. The TGA profiles obtained under the inert gas condition are presented in Figure
Thermal stability of PAC and PAC-CNF.
High temperatures (between 400 and 480°C) for CNF and PAC, respectively, led to the start of combustion. Decrease in weight is commonly considered to be due to the vaporization of volatile organic compounds [
The morphological property of the oil palm kernel shell-based PAC was modified through the impregnation of Ni catalyst, followed by the synthesis of CNF with the presence of acetylene as a carbon source. Dispersion of Ni2+ catalyst was successfully carried out with the use of acetone sonication for nickel (II) nitrate hexahydrate to arrive at the successful growth of CNF. Several concentrations of Ni2+ catalysts were tested and the best growth of CNF was achieved at 3% Ni2+ (w/w). The FESEM and TEM showed the graphitic structure of the PAC-CNF, BET surface area 836.7 m2/g, zeta potential −24.9 mV, and TGA combustion temperature range between 400°C and 570°C.
The authors declare that they have no competing interests.
The authors are grateful to the Ministry of Higher Education (MOHE), Government of Malaysia, for funding this project by the Fundamental Research Grant Scheme with Grant no. FRGS 0106-42 administrated by the ministry (MOHE). The authors duly appreciate the assistance rendered by Sajid and Imtiaz in typing and formatting the manuscript. Assistance from the laboratory technicians of IIUM and University Malaya (UM) is also acknowledged.