Mingshan Wuab,
Jianfeng Maa,
Zhiyong Caic,
Genlin Tiana,
Shumin Yanga,
Youhong Wangb and
Xing'e Liu*a
aInternational Centre for Bamboo and Rattan, Beijing, 100102, China. E-mail: liuxe@icbr.ac.cn; Tel: +86 1084789712
bSchool of Forestry and Landscape Architecture, Anhui Agricultural University, Hefei, 230036, China
cForest Products Laboratory, Forest Service, Madison, WI 53726-2398, USA
First published on 13th October 2015
The synthesis of magnetic biochar composites is a major new research area in advanced materials sciences. A series of magnetic bamboo charcoal composites (MBC800, MBC1000 and MBC1200) with high saturation magnetization (Ms) was fabricated in this work by mixing bamboo charcoal powder with an aqueous ferric chloride solution and subsequently pyrolyzing under different temperatures (800, 1000 and 1200 °C) in a tube furnace. All the products were characterized using X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectrometer (EDAX), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy and a superconducting quantum interference device (SQUID). The results show that the pyrolytic temperature plays a significant role in determining the final structures and magnetic properties of the products. The magnetite (Fe3O4) and a certain amount of amorphous hematite (α-Fe2O3) coexist in MBC800. However, zerovalent iron (ZVI) is the only detectable magnetic phase in both MBC1000 and MBC1200. The Ms values of MBC1000 and MBC1200 are 118.1 and 122.7 emu g−1, respectively. Excellent magnetic properties of the two ZVI/bamboo charcoal composites not only facilitate the separation of solid phase, but also indicate that these materials could have high potential for other applications, such as in the biomedical or ferrofluid fields.
Various types of magnetic particles have been employed to attain magnetic properties of biochar in recent years, such as Fe3O4,8 γ-Fe2O3,9 CoFe2O4.10 Among these media, zerovalent iron (ZVI) has attracted some of the greatest interest due to its powerful magnetic and reducing properties. So far, biochar supported ZVI composites have been successfully synthesized and demonstrated to be excellent adsorbents for the remediation of wastewater contaminated with acid orange 7, trichloroethylene and pentachlorophenol.11–13 However, to date, the preparation of these ZVI/biochar composites has mainly been carried out using a series of time-consuming and costly steps, including wet impregnation of preprocessed biochar, liquid-phase reduction of ferrous iron salt by borohydride. The complicated nature of these synthetic processes limited the large-scale application of ZVI/biochar composites. Therefore, developing simple synthesis procedures for ZVI/biochar composites is an area of high priority research.
Moreover, there is currently an interest in the use of magnetic carbon materials with high saturation magnetization (Ms), which are not only sufficient for conventional magnetic separation, but also desirable for use in high-end biomedical, ferrofluid and other applications.14,15 However, to the authors' knowledge, there is no report on the synthesis of magnetic biochar with high Ms. As biochar is carbon-enriched, low-cost and easily available, using biochar as both a starting material and an iron-reduced agent to attain ZVI/biochar composites with high Ms could be a very promising synthesis strategy that has yet to be explored.
In this research, three magnetic bamboo charcoal composites were prepared by mixing Moso bamboo charcoal powder with aqueous ferric chloride solution and subsequently pyrolyzing under different high temperatures (800, 1000 and 1200 °C), then characterized using X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDAX), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy and superconducting quantum interference device (SQUID). The former is a Fe3O4/α-Fe2O3/bamboo charcoal composite and the latter two are both ZVI/bamboo charcoal composites with high Ms. Based on the observation of structures and morphologies of different products, a potential formation mechanism of ZVI/bamboo charcoal composites is presented.
The suspension of bamboo charcoal with Fe3+ was prepared by dissolving four grams of the resulting charcoal powder and forty grams of FeCl3·6H2O in 60 mL DI water in a breaker first, then agitated at 350 rpm with a magnetic stirrer to maintain a uniform concentration and dried at 80 °C for 3 h in an oven. The obtained solid mixture was ground with a mortar to obtain a very fine powder.
Three pre-treated specimens of four grams each were finally magnetized. The sample was pyrolyzed at 800 °C (or 1000 and 1200 °C) for one hour at the heating rate of 5 °C min−1 with the above-mentioned flow rate of Ar gas. After being cooled naturally to room temperature inside the furnace, the resulting samples were collected and stored in an air-tight container for further use. These obtained products are referred to as MBC800, MBC1000 and MBC1200, respectively, where the suffix number represents the pyrolytic temperature.
Fig. 1a gives the XRD pattern of MBC800. Major diffraction peaks can be seen at 2θ = 18.4°, 30.2°, 35.5°, 37.1°, 43.2°, 53.5°, 57.3°, 62.9°. These distinct peaks correspond to eight indexed planes (111), (220), (311), (222), (400), (422), (511) and (440) of magnetite (JCPDS card no. 74-0748). However, several planes, i.e. (220), (311), (400), (422), (511) and (440), can also be indexed as maghemite with a spinel structure.16,17 To further distinguish the valence state of iron in MBC800, the XPS spectra were measured. Several peaks shown in the survey spectrum (Fig. 2a) imply the presence of carbon (C 1s), oxygen (O 1s) and iron (Fe 2p) elements, which would have come from the carbon and iron oxide in MBC800, respectively.18 As shown in Fig. 2b, the two peaks at 724.32 and 711.02 eV can be assigned to the binding energies of Fe 2p1/2 and Fe 2p3/2, respectively, which are close to the reported values of Fe3O4 in other available literature.19 Simultaneously, there is no charge transfer satellite structure of Fe 2p3/2 at around 720 eV indicating the presence of a mixed oxide of Fe(II) and Fe(III).20 Additionally, a nonlinear curve fit, with a Gaussian–Lorentzian mix function and Shirley background subtraction, was used to deconvolute the Fe 2p XPS spectrum. The investigation of multiplet splitting shows that Fe 2p envelope can be well fitted using peaks from both Fe(II) and Fe(III) constrained to be at 709.8 and 722.8 eV as well as 711.2 and 724.3 eV, respectively, based on the binding energies as reported by Graat and Somers.21 Thus, these results provide more evidence for the argument that the detected crystalline iron oxide in MBC800 by XRD analysis is multivalent Fe3O4 rather than γ-Fe2O3.
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Fig. 2 XPS spectrum of the MBC800: (a) survey spectrum and (b) high-resolution Fe 2p binding energy spectrum. |
The XRD patterns of MBC1000 and MBC1200 are almost identical and ZVI is identified as the major crystalline phase. The peaks at 44.8° and 65.1° corresponding to the crystal planes of (110) and (200) respectively indicate the formation of ZVI,22 while the peak centered at 2θ = 26.5° can be attributed to the (002) reflex of partially graphitized carbon,23 which in turn is due to the carbonization of bamboo char and the presence of iron giving carbon structures with some degree of graphitic order.24 What is more, the full width at half maximum (FWHM) of the most intense (110) reflection can be used as an indicator of crystallinity (crystal size/disorder, which is inversely related to FWHM). XRD data shows that the crystallinity of ZVI is better in MBC1000 (FWHM = 0.150° in 2θ) than in MBC1200 (FWHM = 0.185° in 2θ). Moreover, the ZVI crystallite dimension was determined from the XRD patterns using the Debye–Scherrer equation:
![]() | (1) |
The Raman spectra of all the specimens shown in Fig. 3 indicate the presence of the graphite and disordered amorphous carbon in the samples. The Raman spectrums obtained with all MBC samples mainly exhibit two strong peaks at the D- and G-bands. The D-band was the Raman band at a shift of around 1330 cm−1 corresponding to a stretching vibration mode of amorphous carbon and the G-band at a shift of around 1591 cm−1 was attributed to a stretching vibration mode of graphite CC bonds.25 The peak area ratio of G-band to D-band (AG/AD) for MBC800, MBC1000 and MBC1200 were calculated to be 7.8%, 36.7% and 38%, respectively. The AG/AD ratios in the Raman spectra of MBC1000 and MBC1200 are significantly higher than that of MBC800, implying that a more uniform carbonaceous structure (graphite) formed after thermal treatment. These results also agree with the XRD data.
Finally, the Raman diffraction peaks of MBC800 from 200 to 800 cm−1 provide additional information on the structure. The characteristic peak at 661 cm−1 corresponding to one A1g vibration mode of Fe3O4 creates a consensus on the former XRD and XPS analyses. However, the four Raman scattering peaks at 228, 294, 407 and 497 cm−1, can be accredited to the A1g, Eg, Eg and A1g modes respectively and these peaks clearly designate α-Fe2O3.26 It needs to be pointed that the absence of any α-Fe2O3 diffraction peak from the XRD pattern of MBC800 (Fig. 1a) does not rule out the formation of α-Fe2O3, due to detection limitations of the XRD technique and, or, the amorphous structure of α-Fe2O3.27
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Fig. 4 FESEM and EDAX elemental mapping images of (a and b) MBC800, (c and d) MBC1000 and (e and f) MBC1200. |
It can be seen from Fig. 4a that quantities of sphere-like aggregates (mean size of approximately 5.0 μm) attached onto the surface of bamboo charcoal in MBC800. The majority of these aggregates appear to be formed by some smaller particles, which should be hybrids of Fe3O4 and α-Fe2O3. Fine ZVI particles with diameter changing from micro-scale to nano-scale are well defined and embedded in the char matrix of MBC1000, as shown in Fig. 4c, thereby indicating a better adhesion of magnetic particles than MBC800. With regard to MBC1200, Fig. 4e illustrates that bigger spheres with diameter changing from 20.0 to 50.0 μm have been grown, which are independent of bamboo charcoal matrix. On top of that, Fig. 4f shows that there are a few smaller non-spherical particles with irregular surface still stuck to the original char.
In truth, nano-sized crystalline particles are clearly observed from HRTEM images (Fig. S1†), though FESEM images indicate that the dimension of magnetic particles in all the samples is dominantly in the microscale. This should be due to the aggregation of abundant nano-sized particles with varying degrees in the final products.
Since the iron chloride was dissolved with the bamboo charcoal in water under ambient conditions, i.e., atmospheric 20% O2, it was probable that the formation of colloidal iron hydroxides and oxides occurred during the dissolution and agitation in water, which was modulated by the acidic nature of the iron chloride, possible pH effects of the charcoal and a continuous process of drying and crystallization. Therefore, the Fe species in the forms of hydroxides and oxides were uniformly dispersed on the surface of bamboo charcoal matrix at the end of oven drying.
Different chemical reactions between the precursors and char matrix took place in the course of anaerobic pyrolysis. EDAX analysis (Fig. S2†) suggests the presence of O and small amounts of remaining Cl in MBC800. However, only carbon and iron can be found in MBC1000 and MBC1200. Hence, it is possible to infer that, the Fe species at an early stage decomposed into solid Fe3O4, α-Fe2O3 and gases such as HCl, CO2, which were directly blown away by the flowing Ar gas. All the iron oxides turned into ZVI when the temperature is high enough (e.g. 1000 °C), due to the constantly increased activity of carbon atoms with good reducibility and contact area between bamboo charcoal and magnetic particles. It has been reported that Fe2O3 can be converted to other reduced species in the sequential reactions with solid carbon,28–30 the equations (eqn (2) and (3)) of which may be applicable to this work.
6Fe2O3(s) + C(s) → 4Fe3O4(s) + CO2(g) | (2) |
Fe3O4(s) + 2C(s) → 3Fe0(s) + 2CO2(g) | (3) |
While the pyrolysis temperature reached 1000 °C, ZVI particles grew and partially embedded in bamboo charcoal due to the reduction of the iron oxides. With further temperature rise, these particles aggregated spontaneously into spherical ones with fewer number and larger volume, most of which ultimately broke away from char matrix. Part of the reason some irregular ZVI particles in MBC1200 were kept attached to the charcoal is the inadequate residence time or pyrolytic temperature.
Samples | Magnetic parameters | ||
---|---|---|---|
Ms (emu g−1) | Mr (emu g−1) | Hc (Oe) | |
MBC800 | 63.6 | 6.3 | 83.4 |
MBC1000 | 118.1 | 1.3 | 17.2 |
MBC1200 | 122.7 | 0.3 | 8.9 |
The Ms value of MBC800 is lower than that of bulk Fe3O4 (92 emu g−1)33 due to the smaller Fe3O4 particle size and the existence of non-magnetic carbon and weakly magnetic α-Fe2O3. According to Tao et al., the increase in the Ms is attributable to an increase in crystallinity.34 However, the Ms value of MBC1200 is slightly higher than that of MBC1000, even though the crystallinity of ZVI in the latter is higher. The reason leading to such a result might be the aggregation effect of ZVI nanoparticles.35 Besides, taking into account the relatively low Ms values of magnetic cottonwood biochar (69.2 emu g−1),36 magnetic undigested sludge biochar (13.6 emu g−1),28 magnetic oak wood and oak bark biochar (8.87, 4.47 emu g−1),37 magnetic corn stover biochar (5.1 emu g−1),38 the magnetic properties of MBC1000 and MBC1200 are rather high. Furthermore, each product uniformly dispersed in DI water exhibited prompt response to the external magnet (Fig. 6), highlighting the practicality of this technique for potential commercial applications in the future. It is notable that the aqueous solution of MBC1200 under the affinity of magnet was still slightly turbid, as shown in Fig. 6f. This is because a certain portion of bamboo charcoal in MBC1200 is separated from ZVI particles and not magnetized, which is also consistent with the FESEM results.
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Fig. 6 Photographs of magnetic separation process of (a and b) MBC800, (c and d) MBC1000 and (e and f) MBC1200. |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra13236c |
This journal is © The Royal Society of Chemistry 2015 |