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Effects of gas pressure and temperature on the synthesis of hollow carbon spheres in argon atmosphere
Materials Chemistry and Physics 114 (2009) 391–397
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Effects of gas pressure and temperature on the synthesis of hollow carbon spheres in argon atmosphere Boyang Liu, Dechang Jia ∗ , Yingfeng Shao, Jiancun Rao Institute for Advanced Ceramics, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 7 September 2007 Received in revised form 12 August 2008 Accepted 19 September 2008 Keywords: Non-crystalline materials Inorganic compounds Chemical synthesis Differential scanning calorimetry
a b s t r a c t Effects of gas pressure and temperature on the synthesis and formation mechanism of hollow carbon spheres (HCSs) prepared in argon atmosphere using ferrocene and ammonium chloride as reactants are investigated. The reactions occurred in the process as well as the formation mechanism of the HCSs is also studied. Samples are characterized by X-ray diffraction, field-emission scanning electron microscopy, differential thermal analysis, differential scanning calorimetry analysis and thermogravimetric analysis. It is found that the quasi-static atmosphere is crucial for the formation of the HCSs. The yield of HCSs can be enhanced by increasing the initial argon pressure and reaches a maximum value at 2 MPa. When the initial argon pressure is 2 MPa, HCSs can be fabricated at 450 ◦ C and yield of the HCSs is increased with temperature up to 600 ◦ C. However, the diameters of the HCSs are several micrometers and independent on the argon pressure and temperature in the given ranges. Several iron compounds, including Fe(NH3 )2 Cl2 , Fe(NH3 )6 Cl2 , (NH4 )FeCl5 , NH4 FeCl3 and FeCl2 , are formed in different temperature ranges. The spherical Fe(NH3 )2 Cl2 droplets formed in the process is thought to serve as the core templates for the formation of the HCSs. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Among many inorganic materials with distinct structural and geometrical features, hollow nano/microspheres currently represent one of the fastest growing areas of materials research because of their wide range applications including delivery and controlled release systems, bioencapsulation, medical diagnostics, catalysis, photonic crystals, adsorption, microreactors, energy storage and conversion [1–3]. Therefore, in the last decade, fabrication of hollow spheres has attracted considerable interest. Varieties of hollow spheres have been successfully prepared by different routes, such as solvothermal [4], template assisted [5], chemical vapor deposition (CVD) [6], ultrasonic irradiation [7], Ostwald ripening [8] and Kirkendall effect [9], etc. Hollow carbon spheres (HCSs) exhibit excellent properties such as outstanding biocompatibility [10], high chemical inertness, thermal insulation, high specific surface area, low effective density and high compressive strength [11], which make them appropriate systems to be used in drug delivery, active material encapsulation, lithium battery and hydrogen storage [12–14], hollow sphere composites [15], damping materials [16] and surface functionalization [17], etc. In recent years, numerous efforts have been devoted to the
∗ Corresponding author. Tel.: +86 451 86414291; fax: +86 451 86414291. E-mail address: [email protected] (D. Jia). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.09.047
exploration of various synthesis approaches for HCSs with emphasis on solvothermal and template assisted method. Generally, for the solvothermal method, reactants in organic solvent react under a high pressure and at a moderate temperature for several hours in an autoclave. For example, Wu et al. prepared necklace-like HCSs with a high specific surface area of 594.32 m2 g−1 using ferrocene and hexachlorocyclopentadiene [18]. Liu et al. used Mg, NaCO3 and CCl4 as starting materials and kept the autoclave at 450 ◦ C for 10 h to fabricate HCSs [19]. The diameters of the HCSs prepared in this method range from tens to several hundred nanometers. However, efficiency of this method is low due to long reaction duration and high pressure in the autoclave. For the template assisted process, nanospheres with alternative sizes, serving as templates, are first coated either by CVD of pyrolytic carbon or through surface reactions of polymers on their surfaces to obtain core–shell structures. Then the templates are subsequently removed by wet chemical etching in an appropriate solvent or calcination at elevated temperature in an inert atmosphere to create HCSs. For example, Su et al. prepared HCSs with single and double shells by CVD coating process using silica spheres as the sacrificial templates [20]. Yang et al. first fabricated phenolic formaldehyde (PF) resin-based composite hollow spheres by absorption and catalytic crosslinking of (PF) resin within sulfonated polystyrene gel layers of the hollow sphere templates. Then HCSs were derived by calcination of the PF composite hollow spheres at 800 ◦ C in nitrogen [21]. The templateassisted method is simple and HCSs with uniform diameters can
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Fig. 1. The schematic illustration of the chamber structure of the gas pressure furnace.
be obtained by the monodisperse spherical templates. But it may suffer from scale-up problem and uniformity of the carbon layer on the surfaces of small templates is difficult to control. Besides the methods above, shock-compression [22] and CVD with catalyst methods [23] can also be employed to prepare HCSs. But the low yield and complex process are still the main factors that hinder the applications of HCSs for all the methods. In our previous paper [24], a convenient and efficient method has been suggested for preparation of HCSs with several micrometers in diameter using ferrocene and ammonium chloride as starting materials in argon atmosphere. The spherical droplets of iron amine complexes are regarded as the self-generated core templates for carbon enwrapping to form HCSs. Moreover, the yield of HCSs is also considerably high, which is suitable for large-scale synthesis. In this paper, gas pressure dependent and temperature dependent experiments are carried out to investigate their effects on the synthesis and formation mechanism of the HCSs. The effects of gas pressure and temperature on the synthesis of HCSs should be a valuable addition to the research of HCSs, which are indispensable for preparation control and commercial applications.
High density graphite crucible in the center of the hot zone is used as a container and reactor for the well mixed starting materials. Ferrocene and ammonium chloride, which are reagent grade without further purification, are used to prepare HCSs. As the sublimation and boiling points of the reactants are low, the crucible is covered by a graphite lid to prevent the reactants vapor from escaping rapidly during the heating process. 2.2. Preparation of HCSs under different gas pressures
2. Experimental
Synthesis conditions of the HCSs under different gas pressures are summarized in Table 1. In brief, 4 g ferrocene and 2.4 g ammonium chloride were mixed and put into a small graphite crucible (Ø80 mm × 90 mm) with a lid, and then they were placed into the furnace. After being pumped to 0.1 Pa, the furnace was filled with Ar up to a given gas pressure. Next the crucible was heated to 700 ◦ C at a ramp rate of 20 ◦ C min−1 . The gas pressure in the furnace increased accordingly with temperature because the furnace chamber was a closed system. After being held for 30 min, the furnace was cooled down naturally (about 5 ◦ C min−1 ). The as-prepared products were thoroughly washed with HCl solution and deionized water for several times in sequence, and dried in an oven at 80 ◦ C for 24 h. In order to investigate if the HCSs could be synthesized in a flow atmosphere, an additional experiment was carried out in a tube furnace. Ferrocene and ammonium chloride were mixed and placed in a quartz boat (30 ml in capacity) with a lid. Throughout the experiment, a slow flow of Ar about 10 sccm was passed through the quartz tube while other synthesis parameters were identical with those of the gas pressure furnace. Because there was only a little black powder left in the quartz boat eventually, the product was analyzed without being washed.
2.1. Experimental apparatus and materials
2.3. Preparation of HCSs at different temperatures
Fig. 1 shows the schematic illustration of the chamber structure of the gas pressure furnace used in the paper. The cylindrical graphite heater (Ø220 mm × 310 mm) provides a large hot zone, which is a promise for a large scale preparation of HCSs.
Synthesis conditions at different temperatures are summarized in Table 2. Compared with the above procedures, 40 g ferrocene and 24 g ammonium chloride were mixed and put into a large graphite crucible (Ø190 mm × 270 mm) instead. Initial
Table 1 Synthesis conditions under different gas pressures and yield of the product. Furnace type
Starting materials (g) (C5 H5 )2 Fe
4
Final gas pressure (MPa)
0.1
0.1
0.5 1 2 3
0.7 1.5 2.8 4.3
Final temperature (◦ C)
Ramp rate (◦ C min−1 )
Holding time (h)
Product in crucible (g)
NH4 Cl
Tube furnace Gas pressure furnace
Initial gas pressure (MPa)
2.4
700
20
0.5
Almost none 0.35 2.33 3.25 3.31
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Table 2 Synthesis conditions at different temperatures and yield of product. Starting materials (g) (C5 H5 )2 Fe
NH4 Cl
40
24
Initial gas pressure (MPa)
Final gas pressure (MPa)
Final temperature (◦ C)
2
2.5 2.6 2.7 2.8
450 500 600 700
gas pressure was controlled at 2 MPa by filling with Ar to synthesize HCSs at different temperatures while other parameters for the process were the same as those above. After the product prepared at 600 ◦ C was thoroughly washed with HCl solution and deionized water in sequence and dried, 11.1 g of high purity HCSs were totally obtained. 2.4. Characterization of the samples Structure of the products was characterized by an X-ray diffractometer (XRD, Shimadzu xrd-6000, operated at 30 kV and 40 mA) with Cu K␣ radiation ( = 1.5418 Å). Morphology of the washed products was recorded on a field emission scanning electron microscopy (S-4700, Hitachi, operated at 15 kV). Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) of the as-prepared products without being washed were conducted on a Netzsch STA 449C in Ar atmosphere using alumina pans at a ramp rate of 10 ◦ C min−1 . And the differential scanning calorimetry (DSC) analysis of the as-prepared iron amine complexes was also conducted on this equipment using a high-pressure pan which was a sealed system.
3. Results and discussion 3.1. Effects of gas pressure In our previous work, HCSs are successfully prepared at 700 ◦ C at 2 MPa, thus in this paper, effects of gas pressure on the synthesis of HCSs at the same temperature are investigated first. As shown in Table 1, the weight of as-prepared product is much lower than that of the starting materials, but it increases greatly with initial gas pressure below 2 MPa. And when the initial gas pressure is further raised, the weight increases only slightly. Due to the very low boiling and sublimation points of the starting materials (ferrocene: sublimation point 100 ◦ C, melting point 174 ◦ C, boiling point 249 ◦ C; ammonium chloride: sublimation point 338 ◦ C) [25], they begin to evaporate at low temperature and can completely change to gas at a temperature well below 400 ◦ C, and this is proved by their TG curves recorded in a flow of Ar as shown in Fig. 2. Furthermore, the gas pressure furnace is a closed system, so the atmosphere in graphite crucible is regarded as a quasi-static atmosphere, i.e. the gases inside could come out through diffusion gradually. Conse-
Ramp rate (◦ C min−1 )
Holding time (h)
Product in crucible (g)
20
0.5
24 45 37 34
quently, when the initial gas pressure is 0.5 MPa, the great weight loss should be mainly attributed to the evaporation loss of the starting materials, and this is further proved by the observation of carbon layer deposited on the outside wall of the crucible after the experiment. In comparison, when the initial gas pressure is high, the sublimation and boiling points of the reactants will be raised correspondingly, thus the evaporation loss of the reactants can be greatly reduced, which is beneficial to get a higher yield. As a result, when the initial gas pressure is above 2 MPa, the loss of reactants can be ignored and the weight loss should be mainly attributed to the generation of gases from the reactions in the process, and this will be discussed in the next section based on chemical reaction equations. Fig. 3 shows typical SEM images of the HCSs synthesized at 700 ◦ C under different initial gas pressures. For each initial gas pressure, both a low and a high magnification image are provided to describe the yield and diameter distribution of the HCSs in the final product, respectively. It is indicated that HCSs can be successfully fabricated for all the cases selected. The hollow structure of the HCSs can be clearly demonstrated by the broken spheres shown in the SEM images. The diameters of HCSs prepared under different gas pressures do not vary significantly and mostly range from 1 to 10 m, as shown in high magnification images. However, the yield of HCSs increases with the initial gas pressure, as shown in low magnification images. This may be attributed to the loss of ferrocene and ammonium chloride in the crucible at low initial gas pressure due to the evaporation loss before their chemical reaction taking place as discussed earlier. Hence, it is concluded that the gas pressure is crucial for the formation of the HCSs in the quasi-static atmosphere which can be obtained in the gas pressure furnace. To be more exact, the purpose of higher initial gas pressure used in the experiment is helpful to put off the sublimation and boiling points of the reactants to their reaction temperature range, thus to reduce the evaporation loss of reactants and obtain a higher yield of the HCSs. When the experiment is carried out in a tube furnace using ferrocene and ammonium chloride as starting materials, most of the reactants are lost even in a slow flow of Ar. The powder left in the quartz boat does not have any signs of HCSs as shown in Fig. 4, suggesting that the flow atmosphere may not be available to prepare HCSs through the reaction between ferrocene and ammonium chloride, and this is further proved by the XRD and DSC curves of the as-prepared product as discussed below. So the quasi-static atmosphere, which can be obtained in a gas pressure furnace using a graphite crucible as the reactor, should be crucial to for the preparation of HCSs. 3.2. Effects of temperature
Fig. 2. TG curves of the starting materials in a flow of Ar.
In order to study the formation temperature of the HCSs and chemical reactions occurring in the process, temperaturedependent experiments are carried out. Fig. 5 shows the SEM images of HCSs prepared under an initial gas pressure of 2 MPa at different temperatures. For each case, both a low and a high magni-
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Fig. 3. SEM images of HCSs synthesized at 700 ◦ C under different initial gas pressures: (a and b) 0.5 MPa; (c and d) 1 MPa; (e and f) 2 MPa and (g and h) 3 MPa.
Fig. 4. SEM images of (a) low magnification image of the product prepared in the tube furnace and (b) high magnification image of the same sample.
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Fig. 5. SEM images of the HCSs synthesized at different temperatures under the same initial gas pressure of 2 MPa: (a and b) 450 ◦ C; (c and d) 500 ◦ C; (e and f) 600 ◦ C and (g and h) 700 ◦ C.
fication image are recorded. As indicated in Fig. 5(a and b), HCSs can even be synthesized at 450 ◦ C though the yield of HCSs is still quite low. Moreover, the yield of HCSs in the final product can be greatly enhanced by increasing temperature and approximately reaches a maximum value at 600 ◦ C, suggesting that 600 ◦ C might be the optimum temperature for a large-scale preparation of HCSs at the initial gas pressure of 2 MPa. Similarly, temperature does not evidently influence the diameter of the HCSs, which lies in the same range of 1–10 m. XRD patterns of the as-prepared products without being washed at different temperatures are shown in Fig. 6. At 450 ◦ C even held for 0.5 h (Fig. 6(a)), ferrocene and ammonium chloride still exist in the final product, suggesting that the reaction rate between ferrocene and ammonium chloride is slow. The residual starting materials sticking onto the bottom of the crucible are difficult to scrape off, leading to the low yield of the product as shown in Table 2. Most of
the diffraction peaks can be indexed to Fe(NH3 )2 Cl2 (according to Bremm and Meyer’s work [26]) and Fe(NH3 )6 Cl2 (JCPDS 85-2094), respectively. In addition, some island-shaped pale brown lumps sticking onto the bottom of the graphite crucible are also characterized. The XRD pattern and DTA curves proves that it is mainly composed of (NH4 )3 FeCl5 (JCPDS 71-1419, Supplementary material, Fig. S1). So the chemical reactions took place in this temperature range are proposed as follows: (C5 H5 )2 Fe → C + Fe + H2 + CH4 + C5 H6 + otherhydrocarbons
(1)
Fe + 2NH4 Cl → Fe(NH3 )2 Cl2 + 2H2
(2)
Fe(NH3 )2 Cl2 + 3NH4 Cl ↔ (NH4 )3 FeCl5 + 3NH3
(3)
Fe(NH3 )2 Cl2 + 4NH3 ↔ Fe(NH3 )6 Cl2
(4)
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Fig. 6. XRD patterns of the as-prepared products in the graphite crucible synthesized at 2 MPa (initial gas pressure) at different temperatures: (a) 450 ◦ C; (b) 500 ◦ C; (c) 600 ◦ C and (d) 700 ◦ C.
It is well known that ferrocene will decompose at about 400 ◦ C [27,28] to form carbon and iron which will react with ammonium chloride to form Fe(NH3 )2 Cl2 around 450 ◦ C [26]. Thus the reaction rate should be dominated by the decomposition of ferrocene. In this experiment, as the decomposition of ferrocene is a slow process, excessive NH4 Cl will further react with Fe(NH3 )2 Cl2 to form (NH4 )3 FeCl5 and ammonia. Moreover, the atmosphere in the crucible is a quasi-static atmosphere, so the ammonia generated by Eq. (3) could not quickly diffuse out of the graphite crucible to the gas pressure furnace. Therefore, the generated ammonia will simultaneously react with Fe(NH3 )2 Cl2 to form Fe(NH3 )6 Cl2 . At the end of the experiment, an alkaline gas was detected in the furnace when we released the gas pressure, suggesting the definitely generation of ammonia by the reactions above. At 500 ◦ C, as shown in Fig. 6(b), the starting materials completely react and the main products are Fe(NH3 )2 Cl2 , Fe(NH3 )6 Cl2 and Fe3 N. Intensity of the peaks corresponding to Fe(NH3 )2 Cl2 are stronger than those at 450 ◦ C, suggesting that the amount of Fe(NH3 )2 Cl2 is relatively higher in the final product prepared at 500 ◦ C. And the existence of Fe3 N is probably ascribed to the decomposition of Fe(NH3 )2 Cl2 in ammonia atmosphere at high temperature, which is consistent with Bremm and Meyer’s work [26], indicating that the generated ammonia does not quickly diffuse out of the graphite crucible even at 500 ◦ C. The pale brown island-shaped lumps found at the bottom of the crucible are proved to be (NH4 )3 FeCl5 by XRD pattern and DTA–TG curves. Thus at a temperature around 500 ◦ C an additional chemical reaction is proposed as follows: 6Fe(NH3 )2 Cl2 → 2Fe3 N + N2 + 8NH4 Cl + 4HCl
Fe(NH3 )2 Cl2 ↔ FeCl2 + 2NH3
(7)
NH4 FeCl3 ↔ FeCl2 + NH3 + HCl
(8)
But Fe3 N is not detected as indicated in Fig. 6(c), this may be caused by the rapid ramp rate and most of the generated ammonia came out of the crucible rapidly owing to the vigorous reaction between ferrocene and ammonium chloride at high temperature, resulting in a relatively low content of ammonia in the crucible, which may be not high enough for the decomposition of Fe(NH3 )2 Cl2 to form Fe3 N. Furthermore, in a flow of argon, Fe(NH3 )2 Cl2 and NH4 FeCl3 are not stable at such a high temperature according to Bremm and Meyer’s results. Fe(NH3 )2 Cl2 and NH4 FeCl3 will release NH3 and NH4 Cl, respectively, to form FeCl2 below 400 ◦ C. However, due to the quasi-static atmosphere in the graphite crucible (the gas pressure furnace is a sealed system like autoclave), Fe(NH3 )2 Cl2 and NH4 FeCl3 can be maintained up to a higher temperature range though some of them convert to FeCl2 . And the process above is described by Eqs. (7) and (8), the only strong diffraction peak of FeCl2 may be caused by its preferred orientation growth due to the complicated transformations of the iron compounds. At 700 ◦ C, (NH4 )FeCl3 and FeCl2 are the main products, suggesting that most of the Fe(NH3 )2 Cl2 is transformed to FeCl2 . In our previous paper, we suggest that the spherical droplets of iron amine complexes served as the self-generated core templates for carbon enwrapping to form HCSs. Unfortunately, all the iron amine complexes, usually prepared in autoclaves [26,29–31], will decompose below 400 ◦ C in an open atmosphere as stated above, thus it is difficult to show their thermal properties at high temperature. In order to overcome the problem and simulate the environment in the gas pressure furnace, DSC analysis of the brown lumps of iron amine complexes obtained at 600 ◦ C is carried out using a high-pressure pan. In the analysis, the pan is a sealed system which is similar to the autoclave, and both the heating and cooling curves are recorded, as shown in Fig. 7. It shows that no evident peaks appear below 400 ◦ C, suggesting the decomposition processes of iron amine complexes are restrained by the partial pressures of the decomposed gases (ammonia or ammonium chloride above 300 ◦ C) in the sealed pan. As a result, the iron amine complexes could be maintained to a higher temperature range. Based on the discussion above, the brown iron amine complexes prepared at 600 ◦ C are mainly consisted of Fe(NH3 )2 Cl2 and NH4 FeCl3 , which are unstable and decomposed below 400 ◦ C. Therefore, the endothermal peak at 420 ◦ C in the heating cure and the corresponding exothermal peak in the cooling curve at about 400 ◦ C should represent the thermal property of the iron amine
(5)
When synthesis temperature is increased to 600 ◦ C (Fig. 6(c)), Fe(NH3 )6 Cl2 is diminished while (NH4 )FeCl3 (JCPDS 26-0072) and FeCl2 (JCPDS 89-3732) appear instead as the products, which indicates that both Fe(NH3 )6 Cl2 and (NH4 )3 FeCl5 are not preferred iron amine complexes in a high temperature range in the quasi-static atmosphere. XRD pattern of the pale brown island-shaped lumps is the same as Fig. 6(c). When the ramp rate is 20 ◦ C min−1 and the decomposition rate of ferrocene is slow, the main reactions took place in a higher temperature range are described by Eqs. (6)–(8) instead of (3) and (4). Fe(NH3 )2 Cl2 + NH4 Cl ↔ NH4 FeCl3 + 2NH3
(6)
Fig. 7. The DSC curves of the as-prepared iron amine complexes obtained at 600 ◦ C in argon atmosphere using a high-pressure pan.
B. Liu et al. / Materials Chemistry and Physics 114 (2009) 391–397
complexes. And the two peaks probably represent a physical change and are much like that in melt-freeze transition. The temperature difference between the two peaks should be attributed to the degree of supercooling. The iron amine complexes, which have lump form before the measurement, stick on the bottom of the pan after the DSC analysis (Supplementary material, Fig. S2). So it can be deduced that the Fe(NH3 )2 Cl2 or NH4 FeCl3 had melted during the heating process and the droplets will appear in the preparation of HCSs in argon atmosphere. As stated above, the Fe(NH3 )2 Cl2 is formed well after the decomposition of ferrocene and exists between 450 ◦ C and 600 ◦ C according to the XRD patterns (Fig. 6). Both its content and the yield of HCSs increase with temperature. Additionally, the NH4 FeCl3 is not detected at 450 and 500 ◦ C, so the formation of Fe(NH3 )2 Cl2 should be the crucial factor for preparation of HCSs and the endothermal peak at 420 ◦ C in Fig. 7 should correspond to the melting process of Fe(NH3 )2 Cl2 . Thus, the droplets of Fe(NH3 )2 Cl2 could serve as the core templates for carbon enwrapping in the gas pressure furnace above 450 ◦ C. In summary, during the process, carbon and iron can be formed in advance by the decomposition of ferrocene vapor at about 400 ◦ C and the nanosized iron particles can grow quickly at such a high temperature and gas pressure due to aggregation. As temperature is increased further, iron particles soon react with NH4 Cl to form Fe(NH3 )2 Cl2 , which are spherical droplets rather than particles as the temperature is higher than its melting point. The droplets serve as the templates, whose surface have a relatively higher sticking coefficient and are therefore preferred adsorption sites for the arriving carbon feedstock. However, when the temperature is relatively low (i.e. 450 ◦ C), other iron amine complexes, such as (NH4 )3 FeCl5 and Fe(NH3 )6 Cl2 will be formed simultaneously through consuming Fe(NH3 )2 Cl2 , leading to the low yield of HCSs. When synthesis temperature is set at 500 ◦ C, the yield of HCSs increases with the yield of Fe(NH3 )2 Cl2 , but Fe3 N is also formed. At a higher temperature (600 ◦ C), due to the ramp rate is rapid and the decomposition of ferrocene is slow at a low temperature, reactions consuming Fe(NH3 )2 Cl2 are restrained and Fe(NH3 )2 Cl2 is the preferred product in high temperature ranges in spite of the existence of NH4 FeCl3 . Moreover, almost all the reactions are accomplished synchronously so that the carbon derived from ferrocene can be completely adsorbed for the formation of HCSs to prevent other shapes of carbon to be formed and the yield of HCSs reaches a maximum. The process can be considered as a self-template method because of the self-generated spherical Fe(NH3 )2 Cl2 droplets during the reactions. Finally at 700 ◦ C, although most of the Fe(NH3 )2 Cl2 decomposes, the HCSs have already been synthesized during the heating process. Consequently, the HCSs with several micrometers in diameter can be prepared in a large scale at 600 ◦ C under an initial gas pressure of 2 MPa. And this type of HCSs could not be synthesized in a flow atmosphere because Fe(NH3 )2 Cl2 droplets serving as the templates will decompose at high temperature before they could adsorb carbon feedstock. Furthermore, due to the Fe(NH3 )2 Cl2 droplets serve as the core templates, HCSs with core–shell structure prepared at 600 ◦ C have already been used to fabricate magnetite nanoparticles encapsulated HCSs by a hydrolysis treatment, and it has been discussed elsewhere [32]. 4. Conclusions The effects of gas pressure and temperature on large-scale synthesis of HCSs in argon atmosphere are studied. The yield of HCSs
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can be enhanced by increasing initial gas pressure and reaches a maximum value at 2 MPa. When initial gas pressure is fixed at 2 MPa, HCSs can be fabricated between 450 and 700 ◦ C. Several iron compounds including Fe(NH3 )2 Cl2 , Fe(NH3 )6 Cl2 , (NH4 )FeCl5 , NH4 FeCl3 and FeCl2 are formed in different temperature ranges. The yield of HCSs increases with temperature and reaches to a maximum value at 600 ◦ C. The diameter of the HCSs is independent on the gas pressure and temperature. The quasi-static atmosphere is crucial for the formation of HCSs. The spherical Fe(NH3 )2 Cl2 droplets is thought to be the self-generated core templates for the formation of the HCSs. Acknowledgements The authors would like to thank Prof. Gerd Meyer for providing the standard XRD pattern of Fe(NH3 )2 Cl2 and Xiaodong Zhang for operating the gas pressure furnace. This work is supported by the Program for New Century Excellent Talents in University (No. 04-0327) and Program of Excellent Team in Harbin Institute of Technology. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.matchemphys.2008.09.047. References [1] W.S. Wang, L. Zhen, C.Y. Xu, B.Y. Zhang, W.Z. Shao, J. Phys. Chem. B 110 (2006) 23154. [2] Z.Y. Zhong, Y.D. Yin, B. Gates, Y.N. Xia, Adv. Mater. 12 (2000) 206. [3] X.L. Li, T.J. Lou, X.M. Sun, Y.D. Li, Inorg. Chem. 43 (2004) 5442. [4] X.X. Li, Y.J. Xiong, Z.Q. Li, Y. Xie, Inorg. Chem. 45 (2006) 3493. [5] C.L. Yan, D.F. Xue, J. Phys. Chem. B 110 (2006) 7102. [6] J. Etzkorn, H.A. Therese, F. Rocker, N. Zink, U. Kolb, W. Tremel, Adv. Mater. 17 (2005) 2372. [7] N.A. Dhas, K.S. Suslick, J. Am. Chem. Soc. 127 (2005) 2368. [8] H.G. Yang, H.C. Zeng, J. Phys. Chem. B 108 (2004) 3492. [9] Y.D. Yin, R.M. Rioux, C.K. Erdonmez, S. Hughes, G.A. Somorjai, A.P. Alivisatos, Science 304 (2004) 711. [10] J.C. Bokros, Carbon 15 (1977) 353. [11] L.C. Li, H.H. Song, X.H. Chen, Carbon 44 (2006) 596. [12] Y. Wang, F.B. Su, J.Y. Lee, X.S. Zhao, Chem. Mater. 18 (2006) 1347. [13] K.T. Lee, Y.S. Jung, S.M. Oh, J. Am. Chem. Soc. 125 (2003) 5652. [14] H. Tamai, T. Sumi, H. Yasuda, J. Colloid Interf. Sci. 177 (1996) 325. [15] E. Baumeister, S. Klaeger, Adv. Eng. Mater. 5 (2003) 673. [16] J. Zhang, R.J. Perez, E.J. Lavernia, J. Mater. Sci. 28 (1993) 2395. [17] M. Baibarac, I. Baltog, C. Godon, S. Lefrant, O. Chauvet, Carbon 42 (2004) 3143. [18] C.Z. Wu, X. Zhu, L.L. Ye, C.Z. OuYang, S.Q. Hu, L.Y. Lei, Y. Xie, Inorg. Chem. 45 (2006) 8543. [19] J.W. Liu, M.W. Shao, Q. Tang, X.Y. Chen, Z.P. Liu, Y.T. Qian, Carbon 41 (2003) 1682. [20] F.B. Su, X.S. Zhao, Y. Wang, L.K. Wang, J.Y. Lee, J. Mater. Chem. 16 (2006) 4413. [21] M. Yang, J. Ma, S.J. Ding, Z.K. Meng, J.G. Liu, T. Zhao, L.Q. Mao, Y. Shi, X.G. Jin, Y.F. Lu, Z.Z. Yang, Macromol. Chem. Phys. 207 (2006) 1633. [22] K. Niwase, T. Homae, K.G. Nakamura, K. Kondo, Chem. Phys. Lett. 362 (2002) 47. [23] Z.L. Wang, J.S. Yin, Chem. Phys. Lett. 289 (1998) 189. [24] B.Y. Liu, D.C. Jia, Q.C. Meng, J.C. Rao, Carbon 45 (2007) 668. [25] H. Guttmann, D. Hoehr, H.K. Schaedlich, K.P. Schug, W. Thuenker, US Patent 5,386,804 (1995). [26] S. Bremm, G. Meyer, Z. Anorg. Allg. Chem. 629 (2003) 1875. [27] H.Q. Hou, A.K. Schaper, F. Weller, A. Greiner, Chem. Mater. 14 (2002) 3990. [28] A. Barreiro, S. Hampel, M.H. Rümmeli, C. Kramberger, A. Grüneis, K. Biedermann, A. Leonhardt, T. Gemming, B. Büchner, A. Bachtold, T. Pichler, J. Phys. Chem. B 110 (2006) 20973. [29] M. Amit, A. Zodkevitz, J. Makovsky, Israel J. Chem. 8 (1970) 737. [30] M. Amit, A. Horowitz, E. Ron, J. Makovsky, Israel J. Chem. 11 (1973) 749. [31] R. Eßmann, G. Kreiner, A. Niemann, D. Rechenbach, A. Schmieding, T. Sichla, U. Zachwiejab, H. Jacobsb, Z. Anorg. Allg. Chem. 622 (1996) 1161. [32] B.Y. Liu, D.C. Jia, H.B. Feng, Q.C. Meng, Y.F. Shao, J. Mater. Res. 23 (2008) 1980.
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Effects of gas pressure and temperature on the synthesis of hollow carbon spheres in argon atmosphere Boyang Liu, Dechang Jia ∗ , Yingfeng Shao, Jiancun Rao Institute for Advanced Ceramics, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 7 September 2007 Received in revised form 12 August 2008 Accepted 19 September 2008 Keywords: Non-crystalline materials Inorganic compounds Chemical synthesis Differential scanning calorimetry
a b s t r a c t Effects of gas pressure and temperature on the synthesis and formation mechanism of hollow carbon spheres (HCSs) prepared in argon atmosphere using ferrocene and ammonium chloride as reactants are investigated. The reactions occurred in the process as well as the formation mechanism of the HCSs is also studied. Samples are characterized by X-ray diffraction, field-emission scanning electron microscopy, differential thermal analysis, differential scanning calorimetry analysis and thermogravimetric analysis. It is found that the quasi-static atmosphere is crucial for the formation of the HCSs. The yield of HCSs can be enhanced by increasing the initial argon pressure and reaches a maximum value at 2 MPa. When the initial argon pressure is 2 MPa, HCSs can be fabricated at 450 ◦ C and yield of the HCSs is increased with temperature up to 600 ◦ C. However, the diameters of the HCSs are several micrometers and independent on the argon pressure and temperature in the given ranges. Several iron compounds, including Fe(NH3 )2 Cl2 , Fe(NH3 )6 Cl2 , (NH4 )FeCl5 , NH4 FeCl3 and FeCl2 , are formed in different temperature ranges. The spherical Fe(NH3 )2 Cl2 droplets formed in the process is thought to serve as the core templates for the formation of the HCSs. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Among many inorganic materials with distinct structural and geometrical features, hollow nano/microspheres currently represent one of the fastest growing areas of materials research because of their wide range applications including delivery and controlled release systems, bioencapsulation, medical diagnostics, catalysis, photonic crystals, adsorption, microreactors, energy storage and conversion [1–3]. Therefore, in the last decade, fabrication of hollow spheres has attracted considerable interest. Varieties of hollow spheres have been successfully prepared by different routes, such as solvothermal [4], template assisted [5], chemical vapor deposition (CVD) [6], ultrasonic irradiation [7], Ostwald ripening [8] and Kirkendall effect [9], etc. Hollow carbon spheres (HCSs) exhibit excellent properties such as outstanding biocompatibility [10], high chemical inertness, thermal insulation, high specific surface area, low effective density and high compressive strength [11], which make them appropriate systems to be used in drug delivery, active material encapsulation, lithium battery and hydrogen storage [12–14], hollow sphere composites [15], damping materials [16] and surface functionalization [17], etc. In recent years, numerous efforts have been devoted to the
∗ Corresponding author. Tel.: +86 451 86414291; fax: +86 451 86414291. E-mail address: [email protected] (D. Jia). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.09.047
exploration of various synthesis approaches for HCSs with emphasis on solvothermal and template assisted method. Generally, for the solvothermal method, reactants in organic solvent react under a high pressure and at a moderate temperature for several hours in an autoclave. For example, Wu et al. prepared necklace-like HCSs with a high specific surface area of 594.32 m2 g−1 using ferrocene and hexachlorocyclopentadiene [18]. Liu et al. used Mg, NaCO3 and CCl4 as starting materials and kept the autoclave at 450 ◦ C for 10 h to fabricate HCSs [19]. The diameters of the HCSs prepared in this method range from tens to several hundred nanometers. However, efficiency of this method is low due to long reaction duration and high pressure in the autoclave. For the template assisted process, nanospheres with alternative sizes, serving as templates, are first coated either by CVD of pyrolytic carbon or through surface reactions of polymers on their surfaces to obtain core–shell structures. Then the templates are subsequently removed by wet chemical etching in an appropriate solvent or calcination at elevated temperature in an inert atmosphere to create HCSs. For example, Su et al. prepared HCSs with single and double shells by CVD coating process using silica spheres as the sacrificial templates [20]. Yang et al. first fabricated phenolic formaldehyde (PF) resin-based composite hollow spheres by absorption and catalytic crosslinking of (PF) resin within sulfonated polystyrene gel layers of the hollow sphere templates. Then HCSs were derived by calcination of the PF composite hollow spheres at 800 ◦ C in nitrogen [21]. The templateassisted method is simple and HCSs with uniform diameters can
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Fig. 1. The schematic illustration of the chamber structure of the gas pressure furnace.
be obtained by the monodisperse spherical templates. But it may suffer from scale-up problem and uniformity of the carbon layer on the surfaces of small templates is difficult to control. Besides the methods above, shock-compression [22] and CVD with catalyst methods [23] can also be employed to prepare HCSs. But the low yield and complex process are still the main factors that hinder the applications of HCSs for all the methods. In our previous paper [24], a convenient and efficient method has been suggested for preparation of HCSs with several micrometers in diameter using ferrocene and ammonium chloride as starting materials in argon atmosphere. The spherical droplets of iron amine complexes are regarded as the self-generated core templates for carbon enwrapping to form HCSs. Moreover, the yield of HCSs is also considerably high, which is suitable for large-scale synthesis. In this paper, gas pressure dependent and temperature dependent experiments are carried out to investigate their effects on the synthesis and formation mechanism of the HCSs. The effects of gas pressure and temperature on the synthesis of HCSs should be a valuable addition to the research of HCSs, which are indispensable for preparation control and commercial applications.
High density graphite crucible in the center of the hot zone is used as a container and reactor for the well mixed starting materials. Ferrocene and ammonium chloride, which are reagent grade without further purification, are used to prepare HCSs. As the sublimation and boiling points of the reactants are low, the crucible is covered by a graphite lid to prevent the reactants vapor from escaping rapidly during the heating process. 2.2. Preparation of HCSs under different gas pressures
2. Experimental
Synthesis conditions of the HCSs under different gas pressures are summarized in Table 1. In brief, 4 g ferrocene and 2.4 g ammonium chloride were mixed and put into a small graphite crucible (Ø80 mm × 90 mm) with a lid, and then they were placed into the furnace. After being pumped to 0.1 Pa, the furnace was filled with Ar up to a given gas pressure. Next the crucible was heated to 700 ◦ C at a ramp rate of 20 ◦ C min−1 . The gas pressure in the furnace increased accordingly with temperature because the furnace chamber was a closed system. After being held for 30 min, the furnace was cooled down naturally (about 5 ◦ C min−1 ). The as-prepared products were thoroughly washed with HCl solution and deionized water for several times in sequence, and dried in an oven at 80 ◦ C for 24 h. In order to investigate if the HCSs could be synthesized in a flow atmosphere, an additional experiment was carried out in a tube furnace. Ferrocene and ammonium chloride were mixed and placed in a quartz boat (30 ml in capacity) with a lid. Throughout the experiment, a slow flow of Ar about 10 sccm was passed through the quartz tube while other synthesis parameters were identical with those of the gas pressure furnace. Because there was only a little black powder left in the quartz boat eventually, the product was analyzed without being washed.
2.1. Experimental apparatus and materials
2.3. Preparation of HCSs at different temperatures
Fig. 1 shows the schematic illustration of the chamber structure of the gas pressure furnace used in the paper. The cylindrical graphite heater (Ø220 mm × 310 mm) provides a large hot zone, which is a promise for a large scale preparation of HCSs.
Synthesis conditions at different temperatures are summarized in Table 2. Compared with the above procedures, 40 g ferrocene and 24 g ammonium chloride were mixed and put into a large graphite crucible (Ø190 mm × 270 mm) instead. Initial
Table 1 Synthesis conditions under different gas pressures and yield of the product. Furnace type
Starting materials (g) (C5 H5 )2 Fe
4
Final gas pressure (MPa)
0.1
0.1
0.5 1 2 3
0.7 1.5 2.8 4.3
Final temperature (◦ C)
Ramp rate (◦ C min−1 )
Holding time (h)
Product in crucible (g)
NH4 Cl
Tube furnace Gas pressure furnace
Initial gas pressure (MPa)
2.4
700
20
0.5
Almost none 0.35 2.33 3.25 3.31
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Table 2 Synthesis conditions at different temperatures and yield of product. Starting materials (g) (C5 H5 )2 Fe
NH4 Cl
40
24
Initial gas pressure (MPa)
Final gas pressure (MPa)
Final temperature (◦ C)
2
2.5 2.6 2.7 2.8
450 500 600 700
gas pressure was controlled at 2 MPa by filling with Ar to synthesize HCSs at different temperatures while other parameters for the process were the same as those above. After the product prepared at 600 ◦ C was thoroughly washed with HCl solution and deionized water in sequence and dried, 11.1 g of high purity HCSs were totally obtained. 2.4. Characterization of the samples Structure of the products was characterized by an X-ray diffractometer (XRD, Shimadzu xrd-6000, operated at 30 kV and 40 mA) with Cu K␣ radiation ( = 1.5418 Å). Morphology of the washed products was recorded on a field emission scanning electron microscopy (S-4700, Hitachi, operated at 15 kV). Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) of the as-prepared products without being washed were conducted on a Netzsch STA 449C in Ar atmosphere using alumina pans at a ramp rate of 10 ◦ C min−1 . And the differential scanning calorimetry (DSC) analysis of the as-prepared iron amine complexes was also conducted on this equipment using a high-pressure pan which was a sealed system.
3. Results and discussion 3.1. Effects of gas pressure In our previous work, HCSs are successfully prepared at 700 ◦ C at 2 MPa, thus in this paper, effects of gas pressure on the synthesis of HCSs at the same temperature are investigated first. As shown in Table 1, the weight of as-prepared product is much lower than that of the starting materials, but it increases greatly with initial gas pressure below 2 MPa. And when the initial gas pressure is further raised, the weight increases only slightly. Due to the very low boiling and sublimation points of the starting materials (ferrocene: sublimation point 100 ◦ C, melting point 174 ◦ C, boiling point 249 ◦ C; ammonium chloride: sublimation point 338 ◦ C) [25], they begin to evaporate at low temperature and can completely change to gas at a temperature well below 400 ◦ C, and this is proved by their TG curves recorded in a flow of Ar as shown in Fig. 2. Furthermore, the gas pressure furnace is a closed system, so the atmosphere in graphite crucible is regarded as a quasi-static atmosphere, i.e. the gases inside could come out through diffusion gradually. Conse-
Ramp rate (◦ C min−1 )
Holding time (h)
Product in crucible (g)
20
0.5
24 45 37 34
quently, when the initial gas pressure is 0.5 MPa, the great weight loss should be mainly attributed to the evaporation loss of the starting materials, and this is further proved by the observation of carbon layer deposited on the outside wall of the crucible after the experiment. In comparison, when the initial gas pressure is high, the sublimation and boiling points of the reactants will be raised correspondingly, thus the evaporation loss of the reactants can be greatly reduced, which is beneficial to get a higher yield. As a result, when the initial gas pressure is above 2 MPa, the loss of reactants can be ignored and the weight loss should be mainly attributed to the generation of gases from the reactions in the process, and this will be discussed in the next section based on chemical reaction equations. Fig. 3 shows typical SEM images of the HCSs synthesized at 700 ◦ C under different initial gas pressures. For each initial gas pressure, both a low and a high magnification image are provided to describe the yield and diameter distribution of the HCSs in the final product, respectively. It is indicated that HCSs can be successfully fabricated for all the cases selected. The hollow structure of the HCSs can be clearly demonstrated by the broken spheres shown in the SEM images. The diameters of HCSs prepared under different gas pressures do not vary significantly and mostly range from 1 to 10 m, as shown in high magnification images. However, the yield of HCSs increases with the initial gas pressure, as shown in low magnification images. This may be attributed to the loss of ferrocene and ammonium chloride in the crucible at low initial gas pressure due to the evaporation loss before their chemical reaction taking place as discussed earlier. Hence, it is concluded that the gas pressure is crucial for the formation of the HCSs in the quasi-static atmosphere which can be obtained in the gas pressure furnace. To be more exact, the purpose of higher initial gas pressure used in the experiment is helpful to put off the sublimation and boiling points of the reactants to their reaction temperature range, thus to reduce the evaporation loss of reactants and obtain a higher yield of the HCSs. When the experiment is carried out in a tube furnace using ferrocene and ammonium chloride as starting materials, most of the reactants are lost even in a slow flow of Ar. The powder left in the quartz boat does not have any signs of HCSs as shown in Fig. 4, suggesting that the flow atmosphere may not be available to prepare HCSs through the reaction between ferrocene and ammonium chloride, and this is further proved by the XRD and DSC curves of the as-prepared product as discussed below. So the quasi-static atmosphere, which can be obtained in a gas pressure furnace using a graphite crucible as the reactor, should be crucial to for the preparation of HCSs. 3.2. Effects of temperature
Fig. 2. TG curves of the starting materials in a flow of Ar.
In order to study the formation temperature of the HCSs and chemical reactions occurring in the process, temperaturedependent experiments are carried out. Fig. 5 shows the SEM images of HCSs prepared under an initial gas pressure of 2 MPa at different temperatures. For each case, both a low and a high magni-
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Fig. 3. SEM images of HCSs synthesized at 700 ◦ C under different initial gas pressures: (a and b) 0.5 MPa; (c and d) 1 MPa; (e and f) 2 MPa and (g and h) 3 MPa.
Fig. 4. SEM images of (a) low magnification image of the product prepared in the tube furnace and (b) high magnification image of the same sample.
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Fig. 5. SEM images of the HCSs synthesized at different temperatures under the same initial gas pressure of 2 MPa: (a and b) 450 ◦ C; (c and d) 500 ◦ C; (e and f) 600 ◦ C and (g and h) 700 ◦ C.
fication image are recorded. As indicated in Fig. 5(a and b), HCSs can even be synthesized at 450 ◦ C though the yield of HCSs is still quite low. Moreover, the yield of HCSs in the final product can be greatly enhanced by increasing temperature and approximately reaches a maximum value at 600 ◦ C, suggesting that 600 ◦ C might be the optimum temperature for a large-scale preparation of HCSs at the initial gas pressure of 2 MPa. Similarly, temperature does not evidently influence the diameter of the HCSs, which lies in the same range of 1–10 m. XRD patterns of the as-prepared products without being washed at different temperatures are shown in Fig. 6. At 450 ◦ C even held for 0.5 h (Fig. 6(a)), ferrocene and ammonium chloride still exist in the final product, suggesting that the reaction rate between ferrocene and ammonium chloride is slow. The residual starting materials sticking onto the bottom of the crucible are difficult to scrape off, leading to the low yield of the product as shown in Table 2. Most of
the diffraction peaks can be indexed to Fe(NH3 )2 Cl2 (according to Bremm and Meyer’s work [26]) and Fe(NH3 )6 Cl2 (JCPDS 85-2094), respectively. In addition, some island-shaped pale brown lumps sticking onto the bottom of the graphite crucible are also characterized. The XRD pattern and DTA curves proves that it is mainly composed of (NH4 )3 FeCl5 (JCPDS 71-1419, Supplementary material, Fig. S1). So the chemical reactions took place in this temperature range are proposed as follows: (C5 H5 )2 Fe → C + Fe + H2 + CH4 + C5 H6 + otherhydrocarbons
(1)
Fe + 2NH4 Cl → Fe(NH3 )2 Cl2 + 2H2
(2)
Fe(NH3 )2 Cl2 + 3NH4 Cl ↔ (NH4 )3 FeCl5 + 3NH3
(3)
Fe(NH3 )2 Cl2 + 4NH3 ↔ Fe(NH3 )6 Cl2
(4)
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Fig. 6. XRD patterns of the as-prepared products in the graphite crucible synthesized at 2 MPa (initial gas pressure) at different temperatures: (a) 450 ◦ C; (b) 500 ◦ C; (c) 600 ◦ C and (d) 700 ◦ C.
It is well known that ferrocene will decompose at about 400 ◦ C [27,28] to form carbon and iron which will react with ammonium chloride to form Fe(NH3 )2 Cl2 around 450 ◦ C [26]. Thus the reaction rate should be dominated by the decomposition of ferrocene. In this experiment, as the decomposition of ferrocene is a slow process, excessive NH4 Cl will further react with Fe(NH3 )2 Cl2 to form (NH4 )3 FeCl5 and ammonia. Moreover, the atmosphere in the crucible is a quasi-static atmosphere, so the ammonia generated by Eq. (3) could not quickly diffuse out of the graphite crucible to the gas pressure furnace. Therefore, the generated ammonia will simultaneously react with Fe(NH3 )2 Cl2 to form Fe(NH3 )6 Cl2 . At the end of the experiment, an alkaline gas was detected in the furnace when we released the gas pressure, suggesting the definitely generation of ammonia by the reactions above. At 500 ◦ C, as shown in Fig. 6(b), the starting materials completely react and the main products are Fe(NH3 )2 Cl2 , Fe(NH3 )6 Cl2 and Fe3 N. Intensity of the peaks corresponding to Fe(NH3 )2 Cl2 are stronger than those at 450 ◦ C, suggesting that the amount of Fe(NH3 )2 Cl2 is relatively higher in the final product prepared at 500 ◦ C. And the existence of Fe3 N is probably ascribed to the decomposition of Fe(NH3 )2 Cl2 in ammonia atmosphere at high temperature, which is consistent with Bremm and Meyer’s work [26], indicating that the generated ammonia does not quickly diffuse out of the graphite crucible even at 500 ◦ C. The pale brown island-shaped lumps found at the bottom of the crucible are proved to be (NH4 )3 FeCl5 by XRD pattern and DTA–TG curves. Thus at a temperature around 500 ◦ C an additional chemical reaction is proposed as follows: 6Fe(NH3 )2 Cl2 → 2Fe3 N + N2 + 8NH4 Cl + 4HCl
Fe(NH3 )2 Cl2 ↔ FeCl2 + 2NH3
(7)
NH4 FeCl3 ↔ FeCl2 + NH3 + HCl
(8)
But Fe3 N is not detected as indicated in Fig. 6(c), this may be caused by the rapid ramp rate and most of the generated ammonia came out of the crucible rapidly owing to the vigorous reaction between ferrocene and ammonium chloride at high temperature, resulting in a relatively low content of ammonia in the crucible, which may be not high enough for the decomposition of Fe(NH3 )2 Cl2 to form Fe3 N. Furthermore, in a flow of argon, Fe(NH3 )2 Cl2 and NH4 FeCl3 are not stable at such a high temperature according to Bremm and Meyer’s results. Fe(NH3 )2 Cl2 and NH4 FeCl3 will release NH3 and NH4 Cl, respectively, to form FeCl2 below 400 ◦ C. However, due to the quasi-static atmosphere in the graphite crucible (the gas pressure furnace is a sealed system like autoclave), Fe(NH3 )2 Cl2 and NH4 FeCl3 can be maintained up to a higher temperature range though some of them convert to FeCl2 . And the process above is described by Eqs. (7) and (8), the only strong diffraction peak of FeCl2 may be caused by its preferred orientation growth due to the complicated transformations of the iron compounds. At 700 ◦ C, (NH4 )FeCl3 and FeCl2 are the main products, suggesting that most of the Fe(NH3 )2 Cl2 is transformed to FeCl2 . In our previous paper, we suggest that the spherical droplets of iron amine complexes served as the self-generated core templates for carbon enwrapping to form HCSs. Unfortunately, all the iron amine complexes, usually prepared in autoclaves [26,29–31], will decompose below 400 ◦ C in an open atmosphere as stated above, thus it is difficult to show their thermal properties at high temperature. In order to overcome the problem and simulate the environment in the gas pressure furnace, DSC analysis of the brown lumps of iron amine complexes obtained at 600 ◦ C is carried out using a high-pressure pan. In the analysis, the pan is a sealed system which is similar to the autoclave, and both the heating and cooling curves are recorded, as shown in Fig. 7. It shows that no evident peaks appear below 400 ◦ C, suggesting the decomposition processes of iron amine complexes are restrained by the partial pressures of the decomposed gases (ammonia or ammonium chloride above 300 ◦ C) in the sealed pan. As a result, the iron amine complexes could be maintained to a higher temperature range. Based on the discussion above, the brown iron amine complexes prepared at 600 ◦ C are mainly consisted of Fe(NH3 )2 Cl2 and NH4 FeCl3 , which are unstable and decomposed below 400 ◦ C. Therefore, the endothermal peak at 420 ◦ C in the heating cure and the corresponding exothermal peak in the cooling curve at about 400 ◦ C should represent the thermal property of the iron amine
(5)
When synthesis temperature is increased to 600 ◦ C (Fig. 6(c)), Fe(NH3 )6 Cl2 is diminished while (NH4 )FeCl3 (JCPDS 26-0072) and FeCl2 (JCPDS 89-3732) appear instead as the products, which indicates that both Fe(NH3 )6 Cl2 and (NH4 )3 FeCl5 are not preferred iron amine complexes in a high temperature range in the quasi-static atmosphere. XRD pattern of the pale brown island-shaped lumps is the same as Fig. 6(c). When the ramp rate is 20 ◦ C min−1 and the decomposition rate of ferrocene is slow, the main reactions took place in a higher temperature range are described by Eqs. (6)–(8) instead of (3) and (4). Fe(NH3 )2 Cl2 + NH4 Cl ↔ NH4 FeCl3 + 2NH3
(6)
Fig. 7. The DSC curves of the as-prepared iron amine complexes obtained at 600 ◦ C in argon atmosphere using a high-pressure pan.
B. Liu et al. / Materials Chemistry and Physics 114 (2009) 391–397
complexes. And the two peaks probably represent a physical change and are much like that in melt-freeze transition. The temperature difference between the two peaks should be attributed to the degree of supercooling. The iron amine complexes, which have lump form before the measurement, stick on the bottom of the pan after the DSC analysis (Supplementary material, Fig. S2). So it can be deduced that the Fe(NH3 )2 Cl2 or NH4 FeCl3 had melted during the heating process and the droplets will appear in the preparation of HCSs in argon atmosphere. As stated above, the Fe(NH3 )2 Cl2 is formed well after the decomposition of ferrocene and exists between 450 ◦ C and 600 ◦ C according to the XRD patterns (Fig. 6). Both its content and the yield of HCSs increase with temperature. Additionally, the NH4 FeCl3 is not detected at 450 and 500 ◦ C, so the formation of Fe(NH3 )2 Cl2 should be the crucial factor for preparation of HCSs and the endothermal peak at 420 ◦ C in Fig. 7 should correspond to the melting process of Fe(NH3 )2 Cl2 . Thus, the droplets of Fe(NH3 )2 Cl2 could serve as the core templates for carbon enwrapping in the gas pressure furnace above 450 ◦ C. In summary, during the process, carbon and iron can be formed in advance by the decomposition of ferrocene vapor at about 400 ◦ C and the nanosized iron particles can grow quickly at such a high temperature and gas pressure due to aggregation. As temperature is increased further, iron particles soon react with NH4 Cl to form Fe(NH3 )2 Cl2 , which are spherical droplets rather than particles as the temperature is higher than its melting point. The droplets serve as the templates, whose surface have a relatively higher sticking coefficient and are therefore preferred adsorption sites for the arriving carbon feedstock. However, when the temperature is relatively low (i.e. 450 ◦ C), other iron amine complexes, such as (NH4 )3 FeCl5 and Fe(NH3 )6 Cl2 will be formed simultaneously through consuming Fe(NH3 )2 Cl2 , leading to the low yield of HCSs. When synthesis temperature is set at 500 ◦ C, the yield of HCSs increases with the yield of Fe(NH3 )2 Cl2 , but Fe3 N is also formed. At a higher temperature (600 ◦ C), due to the ramp rate is rapid and the decomposition of ferrocene is slow at a low temperature, reactions consuming Fe(NH3 )2 Cl2 are restrained and Fe(NH3 )2 Cl2 is the preferred product in high temperature ranges in spite of the existence of NH4 FeCl3 . Moreover, almost all the reactions are accomplished synchronously so that the carbon derived from ferrocene can be completely adsorbed for the formation of HCSs to prevent other shapes of carbon to be formed and the yield of HCSs reaches a maximum. The process can be considered as a self-template method because of the self-generated spherical Fe(NH3 )2 Cl2 droplets during the reactions. Finally at 700 ◦ C, although most of the Fe(NH3 )2 Cl2 decomposes, the HCSs have already been synthesized during the heating process. Consequently, the HCSs with several micrometers in diameter can be prepared in a large scale at 600 ◦ C under an initial gas pressure of 2 MPa. And this type of HCSs could not be synthesized in a flow atmosphere because Fe(NH3 )2 Cl2 droplets serving as the templates will decompose at high temperature before they could adsorb carbon feedstock. Furthermore, due to the Fe(NH3 )2 Cl2 droplets serve as the core templates, HCSs with core–shell structure prepared at 600 ◦ C have already been used to fabricate magnetite nanoparticles encapsulated HCSs by a hydrolysis treatment, and it has been discussed elsewhere [32]. 4. Conclusions The effects of gas pressure and temperature on large-scale synthesis of HCSs in argon atmosphere are studied. The yield of HCSs
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can be enhanced by increasing initial gas pressure and reaches a maximum value at 2 MPa. When initial gas pressure is fixed at 2 MPa, HCSs can be fabricated between 450 and 700 ◦ C. Several iron compounds including Fe(NH3 )2 Cl2 , Fe(NH3 )6 Cl2 , (NH4 )FeCl5 , NH4 FeCl3 and FeCl2 are formed in different temperature ranges. The yield of HCSs increases with temperature and reaches to a maximum value at 600 ◦ C. The diameter of the HCSs is independent on the gas pressure and temperature. The quasi-static atmosphere is crucial for the formation of HCSs. The spherical Fe(NH3 )2 Cl2 droplets is thought to be the self-generated core templates for the formation of the HCSs. Acknowledgements The authors would like to thank Prof. Gerd Meyer for providing the standard XRD pattern of Fe(NH3 )2 Cl2 and Xiaodong Zhang for operating the gas pressure furnace. This work is supported by the Program for New Century Excellent Talents in University (No. 04-0327) and Program of Excellent Team in Harbin Institute of Technology. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.matchemphys.2008.09.047. References [1] W.S. Wang, L. Zhen, C.Y. Xu, B.Y. Zhang, W.Z. Shao, J. Phys. Chem. B 110 (2006) 23154. [2] Z.Y. Zhong, Y.D. Yin, B. Gates, Y.N. Xia, Adv. Mater. 12 (2000) 206. [3] X.L. Li, T.J. Lou, X.M. Sun, Y.D. Li, Inorg. Chem. 43 (2004) 5442. [4] X.X. Li, Y.J. Xiong, Z.Q. Li, Y. Xie, Inorg. Chem. 45 (2006) 3493. [5] C.L. Yan, D.F. Xue, J. Phys. Chem. B 110 (2006) 7102. [6] J. Etzkorn, H.A. Therese, F. Rocker, N. Zink, U. Kolb, W. Tremel, Adv. Mater. 17 (2005) 2372. [7] N.A. Dhas, K.S. Suslick, J. Am. Chem. Soc. 127 (2005) 2368. [8] H.G. Yang, H.C. Zeng, J. Phys. Chem. B 108 (2004) 3492. [9] Y.D. Yin, R.M. Rioux, C.K. Erdonmez, S. Hughes, G.A. Somorjai, A.P. Alivisatos, Science 304 (2004) 711. [10] J.C. Bokros, Carbon 15 (1977) 353. [11] L.C. Li, H.H. Song, X.H. Chen, Carbon 44 (2006) 596. [12] Y. Wang, F.B. Su, J.Y. Lee, X.S. Zhao, Chem. Mater. 18 (2006) 1347. [13] K.T. Lee, Y.S. Jung, S.M. Oh, J. Am. Chem. Soc. 125 (2003) 5652. [14] H. Tamai, T. Sumi, H. Yasuda, J. Colloid Interf. Sci. 177 (1996) 325. [15] E. Baumeister, S. Klaeger, Adv. Eng. Mater. 5 (2003) 673. [16] J. Zhang, R.J. Perez, E.J. Lavernia, J. Mater. Sci. 28 (1993) 2395. [17] M. Baibarac, I. Baltog, C. Godon, S. Lefrant, O. Chauvet, Carbon 42 (2004) 3143. [18] C.Z. Wu, X. Zhu, L.L. Ye, C.Z. OuYang, S.Q. Hu, L.Y. Lei, Y. Xie, Inorg. Chem. 45 (2006) 8543. [19] J.W. Liu, M.W. Shao, Q. Tang, X.Y. Chen, Z.P. Liu, Y.T. Qian, Carbon 41 (2003) 1682. [20] F.B. Su, X.S. Zhao, Y. Wang, L.K. Wang, J.Y. Lee, J. Mater. Chem. 16 (2006) 4413. [21] M. Yang, J. Ma, S.J. Ding, Z.K. Meng, J.G. Liu, T. Zhao, L.Q. Mao, Y. Shi, X.G. Jin, Y.F. Lu, Z.Z. Yang, Macromol. Chem. Phys. 207 (2006) 1633. [22] K. Niwase, T. Homae, K.G. Nakamura, K. Kondo, Chem. Phys. Lett. 362 (2002) 47. [23] Z.L. Wang, J.S. Yin, Chem. Phys. Lett. 289 (1998) 189. [24] B.Y. Liu, D.C. Jia, Q.C. Meng, J.C. Rao, Carbon 45 (2007) 668. [25] H. Guttmann, D. Hoehr, H.K. Schaedlich, K.P. Schug, W. Thuenker, US Patent 5,386,804 (1995). [26] S. Bremm, G. Meyer, Z. Anorg. Allg. Chem. 629 (2003) 1875. [27] H.Q. Hou, A.K. Schaper, F. Weller, A. Greiner, Chem. Mater. 14 (2002) 3990. [28] A. Barreiro, S. Hampel, M.H. Rümmeli, C. Kramberger, A. Grüneis, K. Biedermann, A. Leonhardt, T. Gemming, B. Büchner, A. Bachtold, T. Pichler, J. Phys. Chem. B 110 (2006) 20973. [29] M. Amit, A. Zodkevitz, J. Makovsky, Israel J. Chem. 8 (1970) 737. [30] M. Amit, A. Horowitz, E. Ron, J. Makovsky, Israel J. Chem. 11 (1973) 749. [31] R. Eßmann, G. Kreiner, A. Niemann, D. Rechenbach, A. Schmieding, T. Sichla, U. Zachwiejab, H. Jacobsb, Z. Anorg. Allg. Chem. 622 (1996) 1161. [32] B.Y. Liu, D.C. Jia, H.B. Feng, Q.C. Meng, Y.F. Shao, J. Mater. Res. 23 (2008) 1980.