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Facile synthesis of Montroseite VOOH, Paramontroseite VO2 and V2O3-VO2 carbonaceous core-shell microspheres
Solid State Sciences 13 (2011) 2049e2054
Contents lists available at SciVerse ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Facile synthesis of Montroseite VOOH, Paramontroseite VO2 and V2O3-VO2 carbonaceous core-shell microspheres Hailong Feia, Xiaokun Dinga, Mingdeng Weia, b, *, Kemei Weib a b
Institute of New Energy Technology and Nano-Materials, Fuzhou University, Fuzhou, Fujian 350002, China National Engineering Research Center for Fertilizer Catalyst, Fuzhou University, Fuzhou, Fujian 350002, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 February 2010 Received in revised form 19 August 2011 Accepted 13 September 2011 Available online 23 September 2011
Hydrothermal carbonization of sucrose was used to controllably synthesize Montroseite VOOH and Paramontroseite VO2 nanoparticles carbonaceous core-shell microspheres. After calcinations, V2O3-VO2C core-shell microspheres were obtained. When they were used as cathode materials in lithium-ion battery (LIB), it was found that Montroseite VOOH carbonaceous core-shell microspheres exhibited higher discharge capacity than Paramontroseite VO2 counterpart, while the content of V2O3 had some large effects on the electrochemical properties of V2O3-VO2C core-shell microspheres. Ó 2011 Elsevier Masson SAS. All rights reserved.
Keywords: Paramontroseite VO2 Montroseite VOOH Hydrothermal Carbonization
1. Introduction Montroseite VOOH was discovered by Cisney and Sherwood in 1953 at Colorado Plateau, which has analogous structure to diaspore AlO(OH) [1]. Montroseite is such an unstable material phase that it easily transforms to a new metastable phase Paramontroseite VO2, which is similar to the transformation of magnetite to maghemite, lepidocrocite to maghemite and goethite to hematite [2]. So far V2O5 with various morphologies and structure have been widely reported [3,4]. Among various VO2, monoclinic/rutile (M/R) VO2 was the most fascinating one for their potential application in gas sensor [5], NW-based solar cells [6] and light modulators and smart windows [7]. VO2 nanowire and film have important effects on VO2 properties for depressing phase transition temperature [8] and low optical constants, respectively [9]. Sub-micrometer VO2 thin film was first prepared from a precursor solution of VCl3 and NH4VO3 via ultrasonic nebulaspray pyrolysis [10]. Great interest was also drawn to develop novel method to prepare VO2. Recently, Wu et al. reported that Monoclinic VO2(M) was prepared from Paramontroseite VO2 via heating at 400 C for 1 min in a flowing N2 atmosphere [11]. But the low valence (eg. þ2 and þ3) vanadium compounds were seldom * Corresponding author. Institute of New Energy Technology and Nano-Materials, Fuzhou University, Fuzhou, Fujian 350002, China. Tel./fax: þ86 591 83753180. E-mail address: [email protected] (M. Wei). 1293-2558/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2011.09.009
reported for the difficulties of synthesis. It was in 1974 that VOOH of diaspore type containing mostly trivalent vanadium was synthesized under hydrothermal conditions at 200 C and 2 k bar by hydrolysis of NaVO3 previously reduced under hydrogen [12]. Recently, Wu et al reported a new phase VOOH hollow dandelion, and its structure is identical to orthorhombic FeOOH. They also synthesized Paramontroseite VO2 via a simple reaction of sodium orthovanadate (Na3VO412H2O) and thioacetamide (TAA) [13]. However, a simple route is still needed to synthesize them under control condition at the same time. Paramontroseite VO2 exhibited superior aqueous LIB performance in the VO2/LiMn2O4 aqueous LIB system for its good conductivity and tunnel structure. The first discharge capacity was 61.9 mAh g1 [14]. But it was not used as cathode materials in lithium-ion battery. As far as Montroseite VOOH is considered, there were few reports on its applications in electrochemistry and other fields because it was rather difficult to fabricate. So it would be significant to develop a simple way to prepare Montroseite VOOH and investigate its electrochemical properties. VO2 exhibit metal-insulator (MIT) phase transitions with changes in the electrical conductivity ranging up to 5 orders of magnitude [8] and have been widely investigated as well as Crdoped VO2 [15,16]. The MIT in VO2 is a structural phase transition from a low-temperature (semiconducting or insulating) monoclinic lattice to a high-temperature (metallic) tetragonal lattice [17,18]. The translation in V2O3 is distinguished from that in VO2 as only in
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H. Fei et al. / Solid State Sciences 13 (2011) 2049e2054
a
Montroseite 11-0152
b
Paramontroseite 25-1003
10
20
30
40
50
2Theta (Deg.) Fig. 1. XRD patterns of Montroseite VOOH (a) and Paramontroseite VO2 carbonaceous composites (b), the vertical lines are denoted as the standard Montroseite VOOH and Paramontroseite VO2, respectively.
the latter is there no antiferromagnetic phase [15]. V2O3 undergoes a metal-insulator at w 150 K marked by a jump in the resistivity by seven orders of magnitude hysteris loop of 10 K, exhibiting a smaller value of positron lifetime in the low temperature insulating phase related to the changes in the local structure across the rhombohedral to monoclinic structural transition [19]. But little attention was paid to prepare V2O3-VO2 composite as cathode materials. Our current research found that NH4VO3 could be reduced to NH4V3(OH)6(SO4)2 and Paramontroseite VO2 by oxalic acid in the presence of dimethyl sulphoxide (DMSO) and tetrahydrofuran (THF), respectively [20,21]. The S]O and etheric oxygen can form donoreacceptor with vanadium atom [22], which exerts large effects on both the morphology and crystalline structure of
products. Kanamori et al found that vanadium (IV) could be reduced to vanadium (III) by L-cysteine methyl ester (CysME) with the aid of moderately weak chelators glycylhistidine and glycylaspartic acid [23]. As is well known, saccharides (glucose, sucrose, and starch) can be carbonized to form coreeshell structure consisting of a hydrophobic aromatic nucleus and a hydrophilic shell containing a high concentration of reactive oxygen functional groups (i.e., hydroxyl/phenolic, carbonyl, or carboxylic) and aqueous soluble products (furfural, hydroxymethylfurfural, acids, and aldehydes) [24,25]. Saccharides have been used to synthesize metal oxide hollow sphere and carbon nanocomposites [26e31]. Based on our previous results, novel low covalent vanadium oxides might be formed in the presence of these active reactive oxygen functional groups. In the present work, hydrothermal carbonization of sucrose was used to prepare Montroseite VOOH and Paramontroseite VO2 nanoparticles carbonaceous core-shell microspheres controllably. When further carbonized in an argon oven, VO2-V2O3-C core-shell microspheres were formed. All the obtained core-shell microspheres were used as cathode material for LIB. 2. Experimental 2.1. Preparation of materials Chemicals (analytical grade) used in this study were obtained from commercial sources without further purification. For the synthesis of Montroseite VOOH and carbonaceous core-shell microsphere, 0.015 mol oxalic acid was dissolved in 40 ml H2O. 0.5 g sucrose and 0.008 mol ammonium metavanadate (NH4VO3) was added into the solution and then stirred for 2 h at room temperature. After that the mixture was transferred into a 50-ml Teflon-lined stainless autoclave, sealed, kept at 200 C for 24 h and cooled to room temperature. The black product was centrifugated, washed with deionized water and absolute ethanol, and dried at 70 C for 12 h. By variation of the amount of sucrose to 1.5 g, Paramontroseite VO2 carbonaceous core-shell microspheres were
Fig. 2. SEM images of Montroseite VOOH (a, b) and Paramontroseite VO2 carbonaceous composites (c, d).
H. Fei et al. / Solid State Sciences 13 (2011) 2049e2054
prepared under the identical condition. V2O3-VO2-C core-shell microspheres were obtained via calcining Montroseite VOOH/Paramontroseite VO2 carbonaceous core-shell microspheres at 900 C for 1 h with the flow of argon (Ar). 2.2. Characterizations X-ray diffraction (XRD) patterns were recorded on a diffractometer (Co Ka, PANalytical, X’Pert, data were convert into Cu Ka). Scanning electron microscopy (SEM) was carried out with a PhilipsFEI XL30 ESEM-TMP and a JEOL JSM-6700F. Transmission electron microscopy (TEM) was carried out on a FEI Tecnai G2 F20 S-TWIN electron microscopy instrument. 2.3. Electrochemical measurements The composite positive electrodes consisted of the active materials, conductive material (acetylene black) and binder (PTFE) in a weight ratio of 75/15/10. The Li metal was used as the counter electrode. The electrolyte was 1M LiPF6 in a 1/1/1 (volume ratio) mixture of ethylene carbonate (EC), propylene carbonate (PC) and ethylmethyl carbonate (EMC). Cell assembly was performed in a glove box with the concentration of moisture below 1%. A Land CA2001A battery tester was used to measure the roomtemperature electrode activities.
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3. Results and discussion In our experiments, it has been found that the crystalline structure of the products was sensitive to the amount of sucrose. When 0.5 g sucrose was added, the obtained product was ascribed to Montroseite VOOH (JCPDS 11-0152), as shown in Fig. 1a. When the dosage of sucrose was increased to 1.5 g, Paramontroseite VO2 (JCPDS 25-1003) was formed (Fig. 1b). Carbonization of sucrose is the precondition to form the two minerals. Our previous results showed that NH4VO3 was easily reduced to VO2(B) by oxalic acid in the absence of any carbohydrates [11,12]. Montroseite VOOH is apt to convert to Paramontroseite VO2 for structure similarity. In the process of Montroseite VOOH to Paramontroseite VO2, V3þ will be oxided to V4þ and OeHeO will be destroyed with the release of Hþ [11]. Since the carbonization products of different concentration sucrose solution was the same, the different dosage of sucrose leads to the Montroseite VOOH or Paramontroseite VO2, which might be because sucrose inhibited VOOH to Paramontroseite VO2 via VOOH-sucrose interactions. At high concentration of sucrose, there might be no VOOH-sucrose interaction, OeHeO will be destroyed so easily that Paramontroseite VO2 was obtained. Dölle and coworkers have investigated the reorientational dynamics of sucrose in aqueous solutions by measurement of 13C spin lattice relaxation times [32]. The results showed that the rotational motion of sucrose molecules in aqueous solution is anisotropic, and the anisotropy of the molecular reorientation turned out to be
Fig. 3. TEM images of Montroseite VOOH (a, b) and Paramontroseite VO2 carbonaceous composites (c, d), the inset in (c) is selective area electron diffraction(SAED)of Paramontroseite VO2 carbonaceous composites.
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♣ V2 O3 74-0325
♣
♣
•
• VO 74-1642 2
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♣ •
♣ ♣
•
♣ ♣
♣
• ♣ ♣ ♣
♣
a
b 10
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40 50 2Theta(Deg.)
60
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Fig. 4. XRD patterns of V2O3-VO2-C obtained from Montroseite VOOH (a) and Paramontroseite VO2 carbonaceous composites (b), respectively.
concentration-dependent. The intermolecular interactions of the sucrose are from mainly water-sucrose interactions to a mixture of water-sucrose and sucroseesucrose interactions. SEM image reveals that Montroseite VOOH carbonaceous composites are composed of microspheres with an average size of ca. 4 mm (Fig. 2a). The magnified SEM image identifies that the surface of microspheres are composed of particles about 300 nm in size (Fig. 2b). SEM image also displays that Paramontroseite VO2 carbonaceous composite were well dispersed microspheres about 6 mm in size (Fig. 2c). It can be observed from the magnified SEM image that these microspheres have a core-shell structure (Fig. 2d). The sphere-like core is the carbonization product of sucrose as the earlier performances reported [15]. Fig. 3 shows transmission electron microscopy (TEM) images of Montroseite VOOH and Paramontroseite VO2 carbonaceous composites. Fig. 3a is an image of sphere-like Montroseite VOOH
carbonaceous composite. As is well known that, sucrose can carbonize to sphere-like colloid carbon via hydrothermal treatment [24]. It could be observed that VOOH coated on the surface of colloid carbon. The high-resolution TEM image of the shell (Fig. 3b) shows that the lattice fringe with a spacing of 0.214 nm, ascribed to d220-spacing of Montroseite VOOH (JCPDS 11-0152), which further confirmed that the shell was VOOH, while the core was colloid carbon. Fig. 3c is a lower-magnification TEM image of Paramontroseite VO2 carbonaceous composite. It can be clearly observed that the gray colloid carbon sphere is covered with Paramontroseite VO2 to construct core-shell microspheres. The selected-area electron diffraction (SAED) pattern taken on the surface of spheres (the inset of Fig. 3c) is consistent with the structure of Paramontroseite VO2 (JCPDS 25-1003), as indicated by the diffraction spots ascribed to the (122) and (120) planes. The lattice fringe is 0.249 nm, corresponding to d021-spacing of Paramontroseite VO2 (JCPDS 25-1003) (Fig. 3d). The carbon core is amorphous, whose SAED pattern would be single ring characteristic of amorphous material. We also tried to do SAED, but it failed because the carbon core is too large to be done SAED. Sucrose can be carbonized to form coreeshell structure consisting of a hydrophobic aromatic nucleus and a hydrophilic shell containing a high concentration of reactive oxygen functional groups i.e., hydroxyl/phenolic, carbonyl, or carboxylic [24]. The negatively charged carbonyl or carboxylic groups on the surface of colloid carbon can interact with VO2þ, resulting in the denser Paramontroseite VO2 floating and coating the surface of less dense carbon particle. Extra experiments were also performed to further carbonize Montroseite VOOH and Paramontroseite VO2 carbonaceous coreshell microspheres at 900 C to obtain vanadium oxide and C composites with a flow of Ar. As depicted in Fig. 4, it clearly shows that Montroseite VOOH was converted to V2O3 (JCPDS 74-0325) and VO2 (JCPDS 74-1642) (Fig. 4a), and Paramontroseite VO2 was also converted to V2O3 (JCPDS 74-0325) and VO2 (JCPDS 74-1642) (Fig. 4b). The V2O3 is quite strong in equal magnitude to those of VO2. So VO2 is not quite the major phase in Fig. 4a.
Fig. 5. SEM images of V2O3-VO2-C formed from Montroseite VOOH (a, b) and Paramontroseite VO2 carbonaceous core-shell microspheres (c, d), respectively.
H. Fei et al. / Solid State Sciences 13 (2011) 2049e2054
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Fig. 6. TEM images of V2O3-VO2-C core-shell microspheres obtained from Paramontroseite VO2 carbonaceous composites.
Fig. 7. The initial and final charge-discharge curves of Montroseite VOOH (a), Paramontroseite VO2 carbonaceous core-shell microspheres (b), V2O3-VO2-C core-shell microspheres obtained with Montroseite VOOH (c) and Paramontroseite VO2 carbonaceous composite (d).
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former is VO2, while the latter consists of large amount of V2O3, as the above XRD patterns demonstrated.
4. Conclusion Carbonization of sucrose was first used to control the formation of Montroseite VOOH and Paramontroseite VO2 nanoparticles. The synthesized carbonization product was also used as a template to fabricate Montroseite VOOH, Paramontroseite VO2 and carbonaceous core-shell microspheres. Montroseite VOOH composite showed better electrochemical properties than Paramontroseite VO2 counterpart. The electrochemical properties of V2O3-VO2-C core-shell microspheres could be improved via optimizing the content of V2O3.
Acknowledgments This work was supported by the funds (2010J05025, 2010-XY-5 and XRC-0926).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
300
a
-1
Discharge capacities (mAhg )
Fig. 5a is a Scanning electron microscopy (SEM) image of V2O3VO2-C microsphere obtained from Montroseite VOOH. The magnified SEM image reveals that the surface of microsphere consists of small particles which are accumulated to form 3D-voids (Fig. 5b). After calcinations, Paramontroseite VO2 composites were also converted to sphere-like V2O3-VO2-C accumulated by particle (Fig. 5c and d). The crashed spheres in the inlet of Fig. 5d confirm that V2O3-VO2-C composites have a core-shell structure and the core was sphere-like C. The formation mechanisms of VOOH, VO2 and the V2O3-VO2-C core-shell composite is very similar to the V2O5-VO2 quantum dots wrapped in graphene layers making triangular envelops via carbonization of organic-inorganic hybrid materials to get oxide-carbon composite [33]. They succeeded in preparing dense VO2-V2O5 inorganic fullerenes wrapped by thin lighted carbon via laser photolysis of the mixture of V2O5, VCl3 and ethanol with ethanol as carbon source, while we prepared carbon materials wrapped by different vanadium oxide with sucrose as carbon source via hydrothermal treatment and calcination. It can be observed from TEM images that the surface of separate microsphere consists of particles and whisker-like sheet (Fig. 6a). The selected-area electron diffraction patterns (SAED) showed that the particle was polycrystalline (Fig. 6b) with a few diffraction spots, revealing it is a mixed phase. It can be carefully found that there are some small nanosheets around particles (Fig. 6c). However, the selected-area electron diffraction patterns of whisker-like sheet is only diffraction rings without diffraction pots. The Montroseite VOOH, Paramontroseite VO2 and V2O3-VO2-C core-shell microspheres were used as cathode materials in LIB. The initial and final charge-discharge profiles and corresponding cyclic performance are displayed in Fig. 7, and Fig. 8, respectively. The initial and final discharge capacities were (177.5, 60.1), (133.2, 57.7), (103.7, 22.3) and (418, 83.7) mAh g1, respectively. The discharge capacity was relative low because the active materials VO2 have a low percent of the composite positive electrodes. As shown in Fig. 8a, Montroseite VOOH composites have higher discharge capacity than Paramontroseite VO2 composites, but the discharge capacity was comparable after 15 cycles. On the contrary, V2O3-VO2 composite derived from Paramontroseite VO2 show higher discharge capacity for 30 cycles (Fig. 8b). The first and 29th discharge capacity was 173 and 83.7 mAh g1, respectively, which is much higher than those (37, 22.3 mAh g1) of V2O3-VO2-C composites derived from Montroseite. This might be attributed to the fact that V2O3 is inert electrode material. The major phase of the
200
[19] [20] [21]
100
[22]
0 0
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15
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b
400 300
[23] [24] [25] [26] [27] [28]
200 100 0 0
5
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Cycle Times Fig. 8. Cyclic performance of the cells with Montroseite VOOH (-) and Paramontroseite VO2 (C) carbonaceous composites (a), and V2O3-VO2-C composites obtained via calcining Montroseite VOOH (-) and .Paramontroseite VO2 (C) (b), respectively.
[29] [30] [31] [32] [33]
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Contents lists available at SciVerse ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Facile synthesis of Montroseite VOOH, Paramontroseite VO2 and V2O3-VO2 carbonaceous core-shell microspheres Hailong Feia, Xiaokun Dinga, Mingdeng Weia, b, *, Kemei Weib a b
Institute of New Energy Technology and Nano-Materials, Fuzhou University, Fuzhou, Fujian 350002, China National Engineering Research Center for Fertilizer Catalyst, Fuzhou University, Fuzhou, Fujian 350002, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 February 2010 Received in revised form 19 August 2011 Accepted 13 September 2011 Available online 23 September 2011
Hydrothermal carbonization of sucrose was used to controllably synthesize Montroseite VOOH and Paramontroseite VO2 nanoparticles carbonaceous core-shell microspheres. After calcinations, V2O3-VO2C core-shell microspheres were obtained. When they were used as cathode materials in lithium-ion battery (LIB), it was found that Montroseite VOOH carbonaceous core-shell microspheres exhibited higher discharge capacity than Paramontroseite VO2 counterpart, while the content of V2O3 had some large effects on the electrochemical properties of V2O3-VO2C core-shell microspheres. Ó 2011 Elsevier Masson SAS. All rights reserved.
Keywords: Paramontroseite VO2 Montroseite VOOH Hydrothermal Carbonization
1. Introduction Montroseite VOOH was discovered by Cisney and Sherwood in 1953 at Colorado Plateau, which has analogous structure to diaspore AlO(OH) [1]. Montroseite is such an unstable material phase that it easily transforms to a new metastable phase Paramontroseite VO2, which is similar to the transformation of magnetite to maghemite, lepidocrocite to maghemite and goethite to hematite [2]. So far V2O5 with various morphologies and structure have been widely reported [3,4]. Among various VO2, monoclinic/rutile (M/R) VO2 was the most fascinating one for their potential application in gas sensor [5], NW-based solar cells [6] and light modulators and smart windows [7]. VO2 nanowire and film have important effects on VO2 properties for depressing phase transition temperature [8] and low optical constants, respectively [9]. Sub-micrometer VO2 thin film was first prepared from a precursor solution of VCl3 and NH4VO3 via ultrasonic nebulaspray pyrolysis [10]. Great interest was also drawn to develop novel method to prepare VO2. Recently, Wu et al. reported that Monoclinic VO2(M) was prepared from Paramontroseite VO2 via heating at 400 C for 1 min in a flowing N2 atmosphere [11]. But the low valence (eg. þ2 and þ3) vanadium compounds were seldom * Corresponding author. Institute of New Energy Technology and Nano-Materials, Fuzhou University, Fuzhou, Fujian 350002, China. Tel./fax: þ86 591 83753180. E-mail address: [email protected] (M. Wei). 1293-2558/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2011.09.009
reported for the difficulties of synthesis. It was in 1974 that VOOH of diaspore type containing mostly trivalent vanadium was synthesized under hydrothermal conditions at 200 C and 2 k bar by hydrolysis of NaVO3 previously reduced under hydrogen [12]. Recently, Wu et al reported a new phase VOOH hollow dandelion, and its structure is identical to orthorhombic FeOOH. They also synthesized Paramontroseite VO2 via a simple reaction of sodium orthovanadate (Na3VO412H2O) and thioacetamide (TAA) [13]. However, a simple route is still needed to synthesize them under control condition at the same time. Paramontroseite VO2 exhibited superior aqueous LIB performance in the VO2/LiMn2O4 aqueous LIB system for its good conductivity and tunnel structure. The first discharge capacity was 61.9 mAh g1 [14]. But it was not used as cathode materials in lithium-ion battery. As far as Montroseite VOOH is considered, there were few reports on its applications in electrochemistry and other fields because it was rather difficult to fabricate. So it would be significant to develop a simple way to prepare Montroseite VOOH and investigate its electrochemical properties. VO2 exhibit metal-insulator (MIT) phase transitions with changes in the electrical conductivity ranging up to 5 orders of magnitude [8] and have been widely investigated as well as Crdoped VO2 [15,16]. The MIT in VO2 is a structural phase transition from a low-temperature (semiconducting or insulating) monoclinic lattice to a high-temperature (metallic) tetragonal lattice [17,18]. The translation in V2O3 is distinguished from that in VO2 as only in
2050
H. Fei et al. / Solid State Sciences 13 (2011) 2049e2054
a
Montroseite 11-0152
b
Paramontroseite 25-1003
10
20
30
40
50
2Theta (Deg.) Fig. 1. XRD patterns of Montroseite VOOH (a) and Paramontroseite VO2 carbonaceous composites (b), the vertical lines are denoted as the standard Montroseite VOOH and Paramontroseite VO2, respectively.
the latter is there no antiferromagnetic phase [15]. V2O3 undergoes a metal-insulator at w 150 K marked by a jump in the resistivity by seven orders of magnitude hysteris loop of 10 K, exhibiting a smaller value of positron lifetime in the low temperature insulating phase related to the changes in the local structure across the rhombohedral to monoclinic structural transition [19]. But little attention was paid to prepare V2O3-VO2 composite as cathode materials. Our current research found that NH4VO3 could be reduced to NH4V3(OH)6(SO4)2 and Paramontroseite VO2 by oxalic acid in the presence of dimethyl sulphoxide (DMSO) and tetrahydrofuran (THF), respectively [20,21]. The S]O and etheric oxygen can form donoreacceptor with vanadium atom [22], which exerts large effects on both the morphology and crystalline structure of
products. Kanamori et al found that vanadium (IV) could be reduced to vanadium (III) by L-cysteine methyl ester (CysME) with the aid of moderately weak chelators glycylhistidine and glycylaspartic acid [23]. As is well known, saccharides (glucose, sucrose, and starch) can be carbonized to form coreeshell structure consisting of a hydrophobic aromatic nucleus and a hydrophilic shell containing a high concentration of reactive oxygen functional groups (i.e., hydroxyl/phenolic, carbonyl, or carboxylic) and aqueous soluble products (furfural, hydroxymethylfurfural, acids, and aldehydes) [24,25]. Saccharides have been used to synthesize metal oxide hollow sphere and carbon nanocomposites [26e31]. Based on our previous results, novel low covalent vanadium oxides might be formed in the presence of these active reactive oxygen functional groups. In the present work, hydrothermal carbonization of sucrose was used to prepare Montroseite VOOH and Paramontroseite VO2 nanoparticles carbonaceous core-shell microspheres controllably. When further carbonized in an argon oven, VO2-V2O3-C core-shell microspheres were formed. All the obtained core-shell microspheres were used as cathode material for LIB. 2. Experimental 2.1. Preparation of materials Chemicals (analytical grade) used in this study were obtained from commercial sources without further purification. For the synthesis of Montroseite VOOH and carbonaceous core-shell microsphere, 0.015 mol oxalic acid was dissolved in 40 ml H2O. 0.5 g sucrose and 0.008 mol ammonium metavanadate (NH4VO3) was added into the solution and then stirred for 2 h at room temperature. After that the mixture was transferred into a 50-ml Teflon-lined stainless autoclave, sealed, kept at 200 C for 24 h and cooled to room temperature. The black product was centrifugated, washed with deionized water and absolute ethanol, and dried at 70 C for 12 h. By variation of the amount of sucrose to 1.5 g, Paramontroseite VO2 carbonaceous core-shell microspheres were
Fig. 2. SEM images of Montroseite VOOH (a, b) and Paramontroseite VO2 carbonaceous composites (c, d).
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prepared under the identical condition. V2O3-VO2-C core-shell microspheres were obtained via calcining Montroseite VOOH/Paramontroseite VO2 carbonaceous core-shell microspheres at 900 C for 1 h with the flow of argon (Ar). 2.2. Characterizations X-ray diffraction (XRD) patterns were recorded on a diffractometer (Co Ka, PANalytical, X’Pert, data were convert into Cu Ka). Scanning electron microscopy (SEM) was carried out with a PhilipsFEI XL30 ESEM-TMP and a JEOL JSM-6700F. Transmission electron microscopy (TEM) was carried out on a FEI Tecnai G2 F20 S-TWIN electron microscopy instrument. 2.3. Electrochemical measurements The composite positive electrodes consisted of the active materials, conductive material (acetylene black) and binder (PTFE) in a weight ratio of 75/15/10. The Li metal was used as the counter electrode. The electrolyte was 1M LiPF6 in a 1/1/1 (volume ratio) mixture of ethylene carbonate (EC), propylene carbonate (PC) and ethylmethyl carbonate (EMC). Cell assembly was performed in a glove box with the concentration of moisture below 1%. A Land CA2001A battery tester was used to measure the roomtemperature electrode activities.
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3. Results and discussion In our experiments, it has been found that the crystalline structure of the products was sensitive to the amount of sucrose. When 0.5 g sucrose was added, the obtained product was ascribed to Montroseite VOOH (JCPDS 11-0152), as shown in Fig. 1a. When the dosage of sucrose was increased to 1.5 g, Paramontroseite VO2 (JCPDS 25-1003) was formed (Fig. 1b). Carbonization of sucrose is the precondition to form the two minerals. Our previous results showed that NH4VO3 was easily reduced to VO2(B) by oxalic acid in the absence of any carbohydrates [11,12]. Montroseite VOOH is apt to convert to Paramontroseite VO2 for structure similarity. In the process of Montroseite VOOH to Paramontroseite VO2, V3þ will be oxided to V4þ and OeHeO will be destroyed with the release of Hþ [11]. Since the carbonization products of different concentration sucrose solution was the same, the different dosage of sucrose leads to the Montroseite VOOH or Paramontroseite VO2, which might be because sucrose inhibited VOOH to Paramontroseite VO2 via VOOH-sucrose interactions. At high concentration of sucrose, there might be no VOOH-sucrose interaction, OeHeO will be destroyed so easily that Paramontroseite VO2 was obtained. Dölle and coworkers have investigated the reorientational dynamics of sucrose in aqueous solutions by measurement of 13C spin lattice relaxation times [32]. The results showed that the rotational motion of sucrose molecules in aqueous solution is anisotropic, and the anisotropy of the molecular reorientation turned out to be
Fig. 3. TEM images of Montroseite VOOH (a, b) and Paramontroseite VO2 carbonaceous composites (c, d), the inset in (c) is selective area electron diffraction(SAED)of Paramontroseite VO2 carbonaceous composites.
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♣ V2 O3 74-0325
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• VO 74-1642 2
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a
b 10
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40 50 2Theta(Deg.)
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Fig. 4. XRD patterns of V2O3-VO2-C obtained from Montroseite VOOH (a) and Paramontroseite VO2 carbonaceous composites (b), respectively.
concentration-dependent. The intermolecular interactions of the sucrose are from mainly water-sucrose interactions to a mixture of water-sucrose and sucroseesucrose interactions. SEM image reveals that Montroseite VOOH carbonaceous composites are composed of microspheres with an average size of ca. 4 mm (Fig. 2a). The magnified SEM image identifies that the surface of microspheres are composed of particles about 300 nm in size (Fig. 2b). SEM image also displays that Paramontroseite VO2 carbonaceous composite were well dispersed microspheres about 6 mm in size (Fig. 2c). It can be observed from the magnified SEM image that these microspheres have a core-shell structure (Fig. 2d). The sphere-like core is the carbonization product of sucrose as the earlier performances reported [15]. Fig. 3 shows transmission electron microscopy (TEM) images of Montroseite VOOH and Paramontroseite VO2 carbonaceous composites. Fig. 3a is an image of sphere-like Montroseite VOOH
carbonaceous composite. As is well known that, sucrose can carbonize to sphere-like colloid carbon via hydrothermal treatment [24]. It could be observed that VOOH coated on the surface of colloid carbon. The high-resolution TEM image of the shell (Fig. 3b) shows that the lattice fringe with a spacing of 0.214 nm, ascribed to d220-spacing of Montroseite VOOH (JCPDS 11-0152), which further confirmed that the shell was VOOH, while the core was colloid carbon. Fig. 3c is a lower-magnification TEM image of Paramontroseite VO2 carbonaceous composite. It can be clearly observed that the gray colloid carbon sphere is covered with Paramontroseite VO2 to construct core-shell microspheres. The selected-area electron diffraction (SAED) pattern taken on the surface of spheres (the inset of Fig. 3c) is consistent with the structure of Paramontroseite VO2 (JCPDS 25-1003), as indicated by the diffraction spots ascribed to the (122) and (120) planes. The lattice fringe is 0.249 nm, corresponding to d021-spacing of Paramontroseite VO2 (JCPDS 25-1003) (Fig. 3d). The carbon core is amorphous, whose SAED pattern would be single ring characteristic of amorphous material. We also tried to do SAED, but it failed because the carbon core is too large to be done SAED. Sucrose can be carbonized to form coreeshell structure consisting of a hydrophobic aromatic nucleus and a hydrophilic shell containing a high concentration of reactive oxygen functional groups i.e., hydroxyl/phenolic, carbonyl, or carboxylic [24]. The negatively charged carbonyl or carboxylic groups on the surface of colloid carbon can interact with VO2þ, resulting in the denser Paramontroseite VO2 floating and coating the surface of less dense carbon particle. Extra experiments were also performed to further carbonize Montroseite VOOH and Paramontroseite VO2 carbonaceous coreshell microspheres at 900 C to obtain vanadium oxide and C composites with a flow of Ar. As depicted in Fig. 4, it clearly shows that Montroseite VOOH was converted to V2O3 (JCPDS 74-0325) and VO2 (JCPDS 74-1642) (Fig. 4a), and Paramontroseite VO2 was also converted to V2O3 (JCPDS 74-0325) and VO2 (JCPDS 74-1642) (Fig. 4b). The V2O3 is quite strong in equal magnitude to those of VO2. So VO2 is not quite the major phase in Fig. 4a.
Fig. 5. SEM images of V2O3-VO2-C formed from Montroseite VOOH (a, b) and Paramontroseite VO2 carbonaceous core-shell microspheres (c, d), respectively.
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Fig. 6. TEM images of V2O3-VO2-C core-shell microspheres obtained from Paramontroseite VO2 carbonaceous composites.
Fig. 7. The initial and final charge-discharge curves of Montroseite VOOH (a), Paramontroseite VO2 carbonaceous core-shell microspheres (b), V2O3-VO2-C core-shell microspheres obtained with Montroseite VOOH (c) and Paramontroseite VO2 carbonaceous composite (d).
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former is VO2, while the latter consists of large amount of V2O3, as the above XRD patterns demonstrated.
4. Conclusion Carbonization of sucrose was first used to control the formation of Montroseite VOOH and Paramontroseite VO2 nanoparticles. The synthesized carbonization product was also used as a template to fabricate Montroseite VOOH, Paramontroseite VO2 and carbonaceous core-shell microspheres. Montroseite VOOH composite showed better electrochemical properties than Paramontroseite VO2 counterpart. The electrochemical properties of V2O3-VO2-C core-shell microspheres could be improved via optimizing the content of V2O3.
Acknowledgments This work was supported by the funds (2010J05025, 2010-XY-5 and XRC-0926).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
300
a
-1
Discharge capacities (mAhg )
Fig. 5a is a Scanning electron microscopy (SEM) image of V2O3VO2-C microsphere obtained from Montroseite VOOH. The magnified SEM image reveals that the surface of microsphere consists of small particles which are accumulated to form 3D-voids (Fig. 5b). After calcinations, Paramontroseite VO2 composites were also converted to sphere-like V2O3-VO2-C accumulated by particle (Fig. 5c and d). The crashed spheres in the inlet of Fig. 5d confirm that V2O3-VO2-C composites have a core-shell structure and the core was sphere-like C. The formation mechanisms of VOOH, VO2 and the V2O3-VO2-C core-shell composite is very similar to the V2O5-VO2 quantum dots wrapped in graphene layers making triangular envelops via carbonization of organic-inorganic hybrid materials to get oxide-carbon composite [33]. They succeeded in preparing dense VO2-V2O5 inorganic fullerenes wrapped by thin lighted carbon via laser photolysis of the mixture of V2O5, VCl3 and ethanol with ethanol as carbon source, while we prepared carbon materials wrapped by different vanadium oxide with sucrose as carbon source via hydrothermal treatment and calcination. It can be observed from TEM images that the surface of separate microsphere consists of particles and whisker-like sheet (Fig. 6a). The selected-area electron diffraction patterns (SAED) showed that the particle was polycrystalline (Fig. 6b) with a few diffraction spots, revealing it is a mixed phase. It can be carefully found that there are some small nanosheets around particles (Fig. 6c). However, the selected-area electron diffraction patterns of whisker-like sheet is only diffraction rings without diffraction pots. The Montroseite VOOH, Paramontroseite VO2 and V2O3-VO2-C core-shell microspheres were used as cathode materials in LIB. The initial and final charge-discharge profiles and corresponding cyclic performance are displayed in Fig. 7, and Fig. 8, respectively. The initial and final discharge capacities were (177.5, 60.1), (133.2, 57.7), (103.7, 22.3) and (418, 83.7) mAh g1, respectively. The discharge capacity was relative low because the active materials VO2 have a low percent of the composite positive electrodes. As shown in Fig. 8a, Montroseite VOOH composites have higher discharge capacity than Paramontroseite VO2 composites, but the discharge capacity was comparable after 15 cycles. On the contrary, V2O3-VO2 composite derived from Paramontroseite VO2 show higher discharge capacity for 30 cycles (Fig. 8b). The first and 29th discharge capacity was 173 and 83.7 mAh g1, respectively, which is much higher than those (37, 22.3 mAh g1) of V2O3-VO2-C composites derived from Montroseite. This might be attributed to the fact that V2O3 is inert electrode material. The major phase of the
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[19] [20] [21]
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[23] [24] [25] [26] [27] [28]
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5
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Cycle Times Fig. 8. Cyclic performance of the cells with Montroseite VOOH (-) and Paramontroseite VO2 (C) carbonaceous composites (a), and V2O3-VO2-C composites obtained via calcining Montroseite VOOH (-) and .Paramontroseite VO2 (C) (b), respectively.
[29] [30] [31] [32] [33]
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