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Ionic liquid-assisted synthesis of copper oxalate nanowires and their conversion to copper oxide nanowires
ARTICLE IN PRESS Journal of Crystal Growth 310 (2008) 4628–4634
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Ionic liquid-assisted synthesis of copper oxalate nanowires and their conversion to copper oxide nanowires Meng-Yuan Li a,b, Wen-Sheng Dong a,b,, Chun-Ling Liu a,b, Zhaotie Liu a,b, Feng-Qiang Lin a,b a b
Key Laboratory of Applied Surface and Colloid Chemistry, Shaanxi Normal University, Ministry of Education, Xi’an 710062, China School of Chemistry and Materials Science, Shaanxi Normal University, No. 199 Chang’an South Road, Xi’an 710062, China
a r t i c l e in f o
a b s t r a c t
Article history: Received 22 May 2008 Received in revised form 9 August 2008 Accepted 18 August 2008 Communicated by H. Fujioka Available online 22 August 2008
Copper oxalate nanowires were synthesized via the reaction of Cu(CH3COO)2 4H2O and oxalic acid in ethanol solution with the aid of 1-butyl-3-methylimidazolium tetrafluoroborate (BMImBF4) ionic liquid under solvothermal conditions at 180 1C. The as-synthesized nanowires were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), powder X-ray diffraction (XRD), thermogravimetric analysis and differential scanning calorimetric analysis (TG-DSC). The length of the nanowires reaches 10 mm, and the width 30 nm, giving an aspect ratio of a few hundreds. The formation of the nanowires follows a novel mechanism based on the reorganization of a copper oxalate solid phase. The copper oxalate nanowires can further transform from a dense structure (enclosed by a smooth surface) into highly porous CuO nanowires consisting of interconnected nanocrystallites. & 2008 Elsevier B.V. All rights reserved.
PACS: 81.07.Bc 81.16.Be 81.16.Dn Keywords: A1. Crystal morphology B1. Nanomaterials B1. Oxides B2. Semiconducting materials
1. Introduction In recent years, the synthesis of one-dimensional (1D) materials has received much attention due to their excellent physical and chemical properties in comparison to their bulk counterparts, and their potential applications in nanocomposite materials and nanoscale devices as interconnectors for nanoelectronics [1–4]. As a p-type semiconductor with a narrow band gap (1.2 eV) [5], CuO is a unique monoxide compound (in monoclinic phase, different from normal rock-salt-type structure) for both fundamental investigations and practical applications. CuO has been used as heterogeneous catalysts in many important chemical processes, such as degradation of nitrous oxide, selective catalytic reduction of nitric oxide with ammonia, and oxidation of carbon monoxide, hydrocarbon and phenol in supercritical water [6–8]. Recent studies have found that CuO could exist in as many as three different magnetic phases and form the basis for several high-Tc superconductors and materials with giant magnetoresis Corresponding author at: No. 199, Chang’an South Road, Xi’an, China. Tel./fax: +86 29 85303657. E-mail address: [email protected] (W.-S. Dong).
0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.08.032
tance [9,10]. CuO can also be used as gas sensors, optical switch, magnetic storage media, lithium batteries, and solar cells owing to its photoconductive and photochemical properties [11,12]. Diversified nanostructures such as nanoparticles, nanofilms, microspheres, nanorods, nanotube, and mesoporous structures have been synthesized to enhance its performance in currently existing applications [13–15]. In particular, CuO nanowires have been prepared by several groups using high-temperature approaches [16–20] and low-temperature wet chemical approaches [21–26]. For example, Xia et al. [16] prepared CuO nanowires by thermal oxidation of various copper substrates e.g. grids, foils, and wires in air and within the temperature range from 400 to 700 1C. Yang et al. [26] observed the formation of polycrystalline nanowires containing both CuO and Cu2O when CuS2 nanowires were oxidized by oxygen at elevated temperature. In wet chemical approaches 1D Cu(OH)2 nanostructures have been generally employed as the precursors for the fabrication of 1D CuO nanostructures. Room temperature ionic liquids (RTILs) have received much attention due to their advantageous chemical and physical properties, such as negligible vapor pressure, thermal stability, high polarity, etc., and significant progress has been made in applications to synthetic–organic chemistry, catalysis, separation,
ARTICLE IN PRESS M.-Y. Li et al. / Journal of Crystal Growth 310 (2008) 4628–4634
electrochemistry biopolymers, molecular self-assemblies, inorganic material synthesis, etc. [27–30]. For example, RTILs have been used as both solvent and template in the synthesis of aluminum phosphates or zeolitic analogues [31]. Supermicroporous lamellar and well-defined inverse opal microstructured silicas have been prepared in ionic liquids, which serve as a template [32]. Various metallic nanoparticles and metal oxide have also been synthesized in RTILs, displacing distinctive properties [33,34]. In a previous study, we reported a rapid solution route to synthesize rutile structure SnO2 microspheres using RTILs containing BF 4 as templates under microwave-assisted conditions [35]. In the present work, copper oxalate nanowires have been successfully prepared by a novel, facile solvothermal reaction between Cu(CH3COO)2 4H2O and oxalic acid in ethanol solution with the aid of 1-butyl-3-methylimidazolium tetrafluoroborate (BMImBF4) ionic liquid at 180 1C, and these copper oxalate nanowires can transform into CuO nanowires upon calcination in air.
2. Experimental procedure 2.1. Chemicals The ionic liquids, 1-butyl-3-methylimidazolium tetrafluoroborate (BMImBF4), 1-butyl-3-methylimidazolium hexafluorophosphate (BMImPF6), 1-octyl-3-methylimidazolium tetrafluoroborate (OMImBF4), 1-butyl-2,3-dimethylimidazolium tetrafluoroborate (BDMImBF4), 1-butyl-3-methylimidazolium chloride (BMImCl), were purchased from Chemer Company (Hangzhou, China) and their purities are higher than 97%. All reagent-grade chemicals were used as received without further purification.
2.2. Preparation In a typical synthesis, 0.8 g Cu(CH3COO)2 4H2O was dissolved into 20 ml of absolute ethanol under stirring to form a homogeneous solution, and then 2.0 g BMImBF4 was added. A stoichiometric amount of oxalic acid (0.51 g) dissolved in an equal volume of ethanol was dropwisely added into the above solution under magnetic stirring. Then the suspension colloid was transferred into a 50 mL Teflon-lined stainless steel autoclave, which was filled with ethanol up to 80% of the total volume, and then was sealed and heated at 180 1C for 48 h unless otherwise stated. The obtained solid product was centrifuged, washed with absolute alcohol and distilled water, and dried at 60 1C under vacuum.
2.3. Characterization Powder X-ray diffraction (XRD) patterns were recorded with a Rigaku D/MAX-III X-ray diffractometer (35 kV, 40 mA) using a Cu Ka source. The diffraction patterns were taken from 101 to 701 at a scan rate of 81/min. The morphology and size of the products were observed by scanning electron microscopy (SEM, Philips-FEI Quanta 200) and transmission electron microscopy (TEM, Hitachi H-600 electron microscopy at 75 kV). The high-resolution TEM (HRTEM) images and the corresponding selected-area electron diffraction (SAED) patterns were taken on a JEOL JEM-3010 electron microscopy at 200 kV. Thermogravimetric analysis (TG) and differential scanning calorimetric (DSC) analysis of samples were performed on a TGA Analyzer (TA-Q600SDT, USA) with a heating rate of 10 1C/min under a flow of air.
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3. Results and discussion 3.1. Synthesis and transformation of copper oxalate nanowires The morphology and structure of the as-synthesized product derived from the solvothermal reaction at 180 1C for 48 h was characterized by SEM, TEM, HRTEM, and powder XRD. From the SEM image (Fig. 1a) it is evident that the product entirely consists of very long (up to 10 mm) nanowires, and the nanowires tend to assemble into nanowire bundles. TEM image (Fig. 1b) reveals that the nanowires exhibit relatively uniform width (30 nm). The SAED pattern (Fig. 1c) taken from the nanowires, in which the primary ring pattern, diffraction induced by polycrystal, can be indexed with orthorhombic phase copper oxalate. HRTEM image (Fig. 1d) further confirms that the nanowires are polycrystalline and there is no specific orientation relationship among the grains. The energy dispersive analysis of X-rays (EDAX) in Fig. 1e shows that only Cu, O, C elements are detected in the product. FT-IR spectra (not shown) of the as-synthesized sample reveals that there is no IR adsorption peaks from BMImBF4 ionic liquid, confirming further that no BMImBF4 remains in the product. XRD analysis indicates that the sample is composed of orthorhombic phase copper oxalate together with a small amount of metallic Cu. Fig. 2 shows TG-DSC curves of the as-synthesized sample with a heating rate of 10 1C/min in air. A sharp peak of weight loss in the temperature range of 270–350 1C in TG curve and a strong exothermic peak at 321 1C in DSC curve, mainly attributed to the decomposition of the copper oxalate, are observed. The weight loss is about 46.4%, which is close to the theoretical value (47.5%) of CuC2O4 conversion to CuO. The as-synthesized copper oxalate nanowires were heated to 350 1C in air at a rate of 1 1C/min and held at the temperature for 3 h. Fig. 3 shows the SEM, TEM images, and XRD pattern of the obtained copper oxide nanostructure. Both SEM and TEM images reveal that each wire transforms from a dense structure (enclosed by a smooth surface) into a highly porous one consisting of interconnected nanocrystallites. The width of the nanowires is near 27 nm. SAED pattern (Fig. 3c) confirms that the nanocrystallites are polycrystalline, and purely made of monoclinic phase copper oxide. All the diffraction peaks of the sample in XRD pattern (Fig. 3d) can be indexed to the monoclinic phase copper oxide (JCPDS 48–1548). No characteristic peaks for other impurities are observed. The average size of crystallites calculated using the Scherrer method is 27 nm, which is in agreement with the data obtained from the TEM image.
3.2. Effect of the synthesis conditions on the morphology of copper oxalate The BMImBF4 ionic liquid was found playing an important role in the formation of the copper oxalate nanowires, although the exact role of BMImBF4 in the process is still unclear. SEM images in Fig. 4 shows that in the absence of BMImBF4 some short whiskers are grown directly from large copper oxalate block substrates after solvothermal treatment of the gel derived from the reaction of Cu(CH3COO)2 4H2O with oxalic acid, at 180 1C for 48 h. Even after solvothermal treatment of the gel at 180 1C for 96 h the amount of short whiskers does not change obviously. However, when 1.0 g BMImBF4 is present in the system, the amount of short whiskers grown from large copper oxalate block substrates, increases after solvothermal treatment of the gel at 180 1C for 48 h. With the addition of 2.0 g BMImBF4 the product is entirely consists of nanowires array (Fig. 4c). With further increase, the amount of BMImBF4 to 4.0 g, no apparent changes
ARTICLE IN PRESS M.-Y. Li et al. / Journal of Crystal Growth 310 (2008) 4628–4634
------ Copper Oxalate ------ Copper
120
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110
f
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111 220 200 121
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2 theta (degree) Fig. 1. SEM (a), TEM (b), SAED (c), HRTEM (d), EDAX images (e) and XRD pattern (f) of the as-prepared nanowires.
100
80 Exothermic
TG (%)
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70 60 50 100
200
300 400 Temperature (°C)
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Fig. 2. TG-DSC curves of the as-prepared nanowires.
in the nanowire product were observed. This suggests that BMImBF4 may serve as a mineralizing agent to promote recrystallization of copper oxalate gel to nanowires during the solvothermal process. The various ionic liquids e.g. BMImBF4, BMImPF6, BMImCl, OMImBF4, and BDMImBF4 were employed to investigate the effect of IL species. The results in Fig. 5 show that copper oxalate nanowires were obtained only in the presence of BMImBF4. In the presence of BMImCl nothing was obtained, indicating that the initial suspension copper oxalate colloid was dissolved in the solution after the solvothermal reaction, which is probably due to the formation of cupric chloride. Whereas, in the presence of BDMImBF4 the product is composed of large copper oxalate blocks together with some short whiskers grown directly from large copper oxalate block substrates. In the presence of BMImPF6 and
OMImBF4, only large aggregate was obtained; XRD (not shown) confirms that these products contain some metallic Cu, especially in the presence of OMImBF4. The results clearly suggest that both anion and cation of ionic liquids play an important role in the synthesis of copper oxalate nanowires. It was found that increasing the chain length of alkyl substituents on cations leads to greater lipophilicity of the ionic liquids [28]. While, the strength of H bonding between anion and water increases in the order PF 6 oBF4 oCl [30]. These facts suggest that among the used ionic liquids in the present study BMImBF4 has the strongest H bonding interaction with ethanol solvent besides BMImCl, which might provide a favorable environment for the growth of copper oxalate nanowires. Our experimental results reveal that the growth of the copper oxalate nanowires depends on the solvothermal temperature and reaction time; in particular, temperature plays a more important role. When the temperature was fixed at 120 and 140 1C with a reaction time of 48 h, large copper oxalate blocks with layer morphology were formed (see Fig. 6a and b). At 160 1C, some short whiskers were grown directly from large copper oxalate block substrates. At 180 1C, copper oxalate nanowires were formed. To achieve further insight of the whole growth process of copper oxalate nanowires, both temperature- and timedependant experiments were carried out. Fig. 7 shows the SEM images of the synthesized copper oxalate samples. Fig. 8 shows the corresponding XRD patterns. These images provide essential information about the changes and transformation taking place during the crystallization. The starting gel (Figs. 7a and 8a) prepared at room temperature is characterized by the appearance of well-crystalline copper oxalate blocks. The sample prepared at 180 1C is much rough than that prepared at 160 1C (Fig. 7b and c), and no nanowires were found in the products. At 180 1C, when reaction time was increased from 5 min to 1 h, large copper oxalate blocks together with some short whiskers grown directly from large copper oxalate block substrates were formed (see Fig. 7a), suggesting that nucleation of nanowires took place
ARTICLE IN PRESS M.-Y. Li et al. / Journal of Crystal Growth 310 (2008) 4628–4634
a
31-1 220
202 11-3
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d
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b
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40 50 2 theta (deg.)
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Fig. 3. SEM (a), TEM (b), SAED (c) images and XRD pattern (d) of the copper oxide nanowires.
Fig. 4. SEM images of copper oxalate prepared in the absence of BMImBF4 (a); and in the presence of different amounts of BMImBF4: (b) 1.00 g, (c) 2.00 g and (d) 4.00 g.
within the solid phase. XRD patterns for these four samples (see Fig. 8a and d) show that the intensity of the diffraction peak at 2y ¼ 22.91, assigned to (11 0) plane of copper oxalate decreases and the half-width of the peak increases, indicating that the
crystalline size is reduced with increasing the reaction temperature and time; in another words, the copper oxalate blocks were evolving gradually from the well-crystalline orthorhombic phase to amorphous. With further increasing the reaction time, the
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M.-Y. Li et al. / Journal of Crystal Growth 310 (2008) 4628–4634
Fig. 5. SEM images of samples prepared in the presence of different ionic liquids: (a) BMImBF4, (b) BDMImBF4, (c) OMImBF4 and (d) BMImPF6.
Fig. 6. SEM images of samples prepared by solvothermal reaction at different temperatures for 48 h: (a) 120 1C, (b) 140 1C, (c) 160 1C and (d) 180 1C.
ARTICLE IN PRESS M.-Y. Li et al. / Journal of Crystal Growth 310 (2008) 4628–4634
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Fig. 7. SEM images of samples prepared under different conditions: (a) room temperature, (b) 160 1C for 5 min, (c) 180 1C for 5 min, (d) 1 h, (e) 4 h, (f) 24 h, (g) 48 h and (h) 96 h.
amount of short whiskers grown directly from large copper oxalate block substrates is increased, and their length and width increased too. After 24 h of reaction, the product entirely consists
of copper oxalate nanowires with 5 mm in length. After 48 h of reaction both the length and width of the nanowires increase; in addition, metallic Cu impurity in copper oxalate nanowires was
ARTICLE IN PRESS M.-Y. Li et al. / Journal of Crystal Growth 310 (2008) 4628–4634
110
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g
to promote crystallization of copper oxalate to nanowires during the solvothermal process. The crystallization follows a novel mechanism based on the reorganization and solid–solid transformation of a copper oxalate solid phase. Upon the thermal decomposition at 350 1C in air, copper oxalate nanowires can be converted to copper oxide nanowires.
f
Acknowledgements
e d
This work was supported by the program for NCET (NCET-060871) and SRF for ROCS of State Education Ministry, China.
c b
References
------ Copper oxalate ------ Copper
Intensity (a.u.)
h
a
10
20
30
40 2 theta (deg.)
50
60
70
Fig. 8. XRD patterns of samples prepared under different conditions: (a) room temperature, (b) 160 1C for 5 min, (c) 180 1C for 5 min, (d) 1 h, (e) 4 h, (f) 24 h, (g) 48 h and (h) 96 h.
formed. XRD patterns (see Fig. 8d and g) reveal that the half-width of (11 0) diffraction peak of copper oxalate decreases as the recrystallization from the amorphous to well-crystalline nanowires progresses along the increase of reaction time. In most nanowire synthesis, it is generally assumed that the formation of nanowires takes place mainly from the liquid phase, with a first nucleation step followed by crystal growth through the progressive incorporation of dissolved species (Ostwald ripening) or through oriented attachment [22,36,37]. This crystallization mechanism seems to govern the nanowire synthesis, which start from clear solutions. However, when a crystalline solid is present in the starting reaction mixture other alternative routes are also possible. In the present system, it is evident that the formation of CuO nanowires does not follow a mechanism of nucleation and crystal growth from soluble species. The nanowires are formed directly from the copper oxalate solid phase through its reorganization and crystallization. Therefore, the formation of the copper oxalate nanowires in the present conditions takes place mainly by a solid–solid transformation mechanism. The existence of solid–solid transformation has been observed earlier during the crystallization of different zeolites [38]. The presence of BMImBF4 ionic liquid is one of the important factors influencing the recrystallization process and the growth of the copper oxalate nanowires. The addition of BMImBF4 may provide a favorable environment for the growth of the nanowires in this system.
4. Conclusion In summary, copper oxalate nanowires with high-aspect ratio were synthesized by a novel, facile solvothermal reaction between Cu(CH3COO)2 4H2O and oxalic acid in ethanol solution with the aid of BMImBF4 ionic liquid at 180 1C. Both temperature and time are the key parameters for the preparation of the nanowires. The BMImBF4 ionic liquid plays a unique role in the preparation of copper oxalate nanowires, and may serve as a mineralizing agent
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Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Ionic liquid-assisted synthesis of copper oxalate nanowires and their conversion to copper oxide nanowires Meng-Yuan Li a,b, Wen-Sheng Dong a,b,, Chun-Ling Liu a,b, Zhaotie Liu a,b, Feng-Qiang Lin a,b a b
Key Laboratory of Applied Surface and Colloid Chemistry, Shaanxi Normal University, Ministry of Education, Xi’an 710062, China School of Chemistry and Materials Science, Shaanxi Normal University, No. 199 Chang’an South Road, Xi’an 710062, China
a r t i c l e in f o
a b s t r a c t
Article history: Received 22 May 2008 Received in revised form 9 August 2008 Accepted 18 August 2008 Communicated by H. Fujioka Available online 22 August 2008
Copper oxalate nanowires were synthesized via the reaction of Cu(CH3COO)2 4H2O and oxalic acid in ethanol solution with the aid of 1-butyl-3-methylimidazolium tetrafluoroborate (BMImBF4) ionic liquid under solvothermal conditions at 180 1C. The as-synthesized nanowires were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), powder X-ray diffraction (XRD), thermogravimetric analysis and differential scanning calorimetric analysis (TG-DSC). The length of the nanowires reaches 10 mm, and the width 30 nm, giving an aspect ratio of a few hundreds. The formation of the nanowires follows a novel mechanism based on the reorganization of a copper oxalate solid phase. The copper oxalate nanowires can further transform from a dense structure (enclosed by a smooth surface) into highly porous CuO nanowires consisting of interconnected nanocrystallites. & 2008 Elsevier B.V. All rights reserved.
PACS: 81.07.Bc 81.16.Be 81.16.Dn Keywords: A1. Crystal morphology B1. Nanomaterials B1. Oxides B2. Semiconducting materials
1. Introduction In recent years, the synthesis of one-dimensional (1D) materials has received much attention due to their excellent physical and chemical properties in comparison to their bulk counterparts, and their potential applications in nanocomposite materials and nanoscale devices as interconnectors for nanoelectronics [1–4]. As a p-type semiconductor with a narrow band gap (1.2 eV) [5], CuO is a unique monoxide compound (in monoclinic phase, different from normal rock-salt-type structure) for both fundamental investigations and practical applications. CuO has been used as heterogeneous catalysts in many important chemical processes, such as degradation of nitrous oxide, selective catalytic reduction of nitric oxide with ammonia, and oxidation of carbon monoxide, hydrocarbon and phenol in supercritical water [6–8]. Recent studies have found that CuO could exist in as many as three different magnetic phases and form the basis for several high-Tc superconductors and materials with giant magnetoresis Corresponding author at: No. 199, Chang’an South Road, Xi’an, China. Tel./fax: +86 29 85303657. E-mail address: [email protected] (W.-S. Dong).
0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.08.032
tance [9,10]. CuO can also be used as gas sensors, optical switch, magnetic storage media, lithium batteries, and solar cells owing to its photoconductive and photochemical properties [11,12]. Diversified nanostructures such as nanoparticles, nanofilms, microspheres, nanorods, nanotube, and mesoporous structures have been synthesized to enhance its performance in currently existing applications [13–15]. In particular, CuO nanowires have been prepared by several groups using high-temperature approaches [16–20] and low-temperature wet chemical approaches [21–26]. For example, Xia et al. [16] prepared CuO nanowires by thermal oxidation of various copper substrates e.g. grids, foils, and wires in air and within the temperature range from 400 to 700 1C. Yang et al. [26] observed the formation of polycrystalline nanowires containing both CuO and Cu2O when CuS2 nanowires were oxidized by oxygen at elevated temperature. In wet chemical approaches 1D Cu(OH)2 nanostructures have been generally employed as the precursors for the fabrication of 1D CuO nanostructures. Room temperature ionic liquids (RTILs) have received much attention due to their advantageous chemical and physical properties, such as negligible vapor pressure, thermal stability, high polarity, etc., and significant progress has been made in applications to synthetic–organic chemistry, catalysis, separation,
ARTICLE IN PRESS M.-Y. Li et al. / Journal of Crystal Growth 310 (2008) 4628–4634
electrochemistry biopolymers, molecular self-assemblies, inorganic material synthesis, etc. [27–30]. For example, RTILs have been used as both solvent and template in the synthesis of aluminum phosphates or zeolitic analogues [31]. Supermicroporous lamellar and well-defined inverse opal microstructured silicas have been prepared in ionic liquids, which serve as a template [32]. Various metallic nanoparticles and metal oxide have also been synthesized in RTILs, displacing distinctive properties [33,34]. In a previous study, we reported a rapid solution route to synthesize rutile structure SnO2 microspheres using RTILs containing BF 4 as templates under microwave-assisted conditions [35]. In the present work, copper oxalate nanowires have been successfully prepared by a novel, facile solvothermal reaction between Cu(CH3COO)2 4H2O and oxalic acid in ethanol solution with the aid of 1-butyl-3-methylimidazolium tetrafluoroborate (BMImBF4) ionic liquid at 180 1C, and these copper oxalate nanowires can transform into CuO nanowires upon calcination in air.
2. Experimental procedure 2.1. Chemicals The ionic liquids, 1-butyl-3-methylimidazolium tetrafluoroborate (BMImBF4), 1-butyl-3-methylimidazolium hexafluorophosphate (BMImPF6), 1-octyl-3-methylimidazolium tetrafluoroborate (OMImBF4), 1-butyl-2,3-dimethylimidazolium tetrafluoroborate (BDMImBF4), 1-butyl-3-methylimidazolium chloride (BMImCl), were purchased from Chemer Company (Hangzhou, China) and their purities are higher than 97%. All reagent-grade chemicals were used as received without further purification.
2.2. Preparation In a typical synthesis, 0.8 g Cu(CH3COO)2 4H2O was dissolved into 20 ml of absolute ethanol under stirring to form a homogeneous solution, and then 2.0 g BMImBF4 was added. A stoichiometric amount of oxalic acid (0.51 g) dissolved in an equal volume of ethanol was dropwisely added into the above solution under magnetic stirring. Then the suspension colloid was transferred into a 50 mL Teflon-lined stainless steel autoclave, which was filled with ethanol up to 80% of the total volume, and then was sealed and heated at 180 1C for 48 h unless otherwise stated. The obtained solid product was centrifuged, washed with absolute alcohol and distilled water, and dried at 60 1C under vacuum.
2.3. Characterization Powder X-ray diffraction (XRD) patterns were recorded with a Rigaku D/MAX-III X-ray diffractometer (35 kV, 40 mA) using a Cu Ka source. The diffraction patterns were taken from 101 to 701 at a scan rate of 81/min. The morphology and size of the products were observed by scanning electron microscopy (SEM, Philips-FEI Quanta 200) and transmission electron microscopy (TEM, Hitachi H-600 electron microscopy at 75 kV). The high-resolution TEM (HRTEM) images and the corresponding selected-area electron diffraction (SAED) patterns were taken on a JEOL JEM-3010 electron microscopy at 200 kV. Thermogravimetric analysis (TG) and differential scanning calorimetric (DSC) analysis of samples were performed on a TGA Analyzer (TA-Q600SDT, USA) with a heating rate of 10 1C/min under a flow of air.
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3. Results and discussion 3.1. Synthesis and transformation of copper oxalate nanowires The morphology and structure of the as-synthesized product derived from the solvothermal reaction at 180 1C for 48 h was characterized by SEM, TEM, HRTEM, and powder XRD. From the SEM image (Fig. 1a) it is evident that the product entirely consists of very long (up to 10 mm) nanowires, and the nanowires tend to assemble into nanowire bundles. TEM image (Fig. 1b) reveals that the nanowires exhibit relatively uniform width (30 nm). The SAED pattern (Fig. 1c) taken from the nanowires, in which the primary ring pattern, diffraction induced by polycrystal, can be indexed with orthorhombic phase copper oxalate. HRTEM image (Fig. 1d) further confirms that the nanowires are polycrystalline and there is no specific orientation relationship among the grains. The energy dispersive analysis of X-rays (EDAX) in Fig. 1e shows that only Cu, O, C elements are detected in the product. FT-IR spectra (not shown) of the as-synthesized sample reveals that there is no IR adsorption peaks from BMImBF4 ionic liquid, confirming further that no BMImBF4 remains in the product. XRD analysis indicates that the sample is composed of orthorhombic phase copper oxalate together with a small amount of metallic Cu. Fig. 2 shows TG-DSC curves of the as-synthesized sample with a heating rate of 10 1C/min in air. A sharp peak of weight loss in the temperature range of 270–350 1C in TG curve and a strong exothermic peak at 321 1C in DSC curve, mainly attributed to the decomposition of the copper oxalate, are observed. The weight loss is about 46.4%, which is close to the theoretical value (47.5%) of CuC2O4 conversion to CuO. The as-synthesized copper oxalate nanowires were heated to 350 1C in air at a rate of 1 1C/min and held at the temperature for 3 h. Fig. 3 shows the SEM, TEM images, and XRD pattern of the obtained copper oxide nanostructure. Both SEM and TEM images reveal that each wire transforms from a dense structure (enclosed by a smooth surface) into a highly porous one consisting of interconnected nanocrystallites. The width of the nanowires is near 27 nm. SAED pattern (Fig. 3c) confirms that the nanocrystallites are polycrystalline, and purely made of monoclinic phase copper oxide. All the diffraction peaks of the sample in XRD pattern (Fig. 3d) can be indexed to the monoclinic phase copper oxide (JCPDS 48–1548). No characteristic peaks for other impurities are observed. The average size of crystallites calculated using the Scherrer method is 27 nm, which is in agreement with the data obtained from the TEM image.
3.2. Effect of the synthesis conditions on the morphology of copper oxalate The BMImBF4 ionic liquid was found playing an important role in the formation of the copper oxalate nanowires, although the exact role of BMImBF4 in the process is still unclear. SEM images in Fig. 4 shows that in the absence of BMImBF4 some short whiskers are grown directly from large copper oxalate block substrates after solvothermal treatment of the gel derived from the reaction of Cu(CH3COO)2 4H2O with oxalic acid, at 180 1C for 48 h. Even after solvothermal treatment of the gel at 180 1C for 96 h the amount of short whiskers does not change obviously. However, when 1.0 g BMImBF4 is present in the system, the amount of short whiskers grown from large copper oxalate block substrates, increases after solvothermal treatment of the gel at 180 1C for 48 h. With the addition of 2.0 g BMImBF4 the product is entirely consists of nanowires array (Fig. 4c). With further increase, the amount of BMImBF4 to 4.0 g, no apparent changes
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in the nanowire product were observed. This suggests that BMImBF4 may serve as a mineralizing agent to promote recrystallization of copper oxalate gel to nanowires during the solvothermal process. The various ionic liquids e.g. BMImBF4, BMImPF6, BMImCl, OMImBF4, and BDMImBF4 were employed to investigate the effect of IL species. The results in Fig. 5 show that copper oxalate nanowires were obtained only in the presence of BMImBF4. In the presence of BMImCl nothing was obtained, indicating that the initial suspension copper oxalate colloid was dissolved in the solution after the solvothermal reaction, which is probably due to the formation of cupric chloride. Whereas, in the presence of BDMImBF4 the product is composed of large copper oxalate blocks together with some short whiskers grown directly from large copper oxalate block substrates. In the presence of BMImPF6 and
OMImBF4, only large aggregate was obtained; XRD (not shown) confirms that these products contain some metallic Cu, especially in the presence of OMImBF4. The results clearly suggest that both anion and cation of ionic liquids play an important role in the synthesis of copper oxalate nanowires. It was found that increasing the chain length of alkyl substituents on cations leads to greater lipophilicity of the ionic liquids [28]. While, the strength of H bonding between anion and water increases in the order PF 6 oBF4 oCl [30]. These facts suggest that among the used ionic liquids in the present study BMImBF4 has the strongest H bonding interaction with ethanol solvent besides BMImCl, which might provide a favorable environment for the growth of copper oxalate nanowires. Our experimental results reveal that the growth of the copper oxalate nanowires depends on the solvothermal temperature and reaction time; in particular, temperature plays a more important role. When the temperature was fixed at 120 and 140 1C with a reaction time of 48 h, large copper oxalate blocks with layer morphology were formed (see Fig. 6a and b). At 160 1C, some short whiskers were grown directly from large copper oxalate block substrates. At 180 1C, copper oxalate nanowires were formed. To achieve further insight of the whole growth process of copper oxalate nanowires, both temperature- and timedependant experiments were carried out. Fig. 7 shows the SEM images of the synthesized copper oxalate samples. Fig. 8 shows the corresponding XRD patterns. These images provide essential information about the changes and transformation taking place during the crystallization. The starting gel (Figs. 7a and 8a) prepared at room temperature is characterized by the appearance of well-crystalline copper oxalate blocks. The sample prepared at 180 1C is much rough than that prepared at 160 1C (Fig. 7b and c), and no nanowires were found in the products. At 180 1C, when reaction time was increased from 5 min to 1 h, large copper oxalate blocks together with some short whiskers grown directly from large copper oxalate block substrates were formed (see Fig. 7a), suggesting that nucleation of nanowires took place
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Fig. 4. SEM images of copper oxalate prepared in the absence of BMImBF4 (a); and in the presence of different amounts of BMImBF4: (b) 1.00 g, (c) 2.00 g and (d) 4.00 g.
within the solid phase. XRD patterns for these four samples (see Fig. 8a and d) show that the intensity of the diffraction peak at 2y ¼ 22.91, assigned to (11 0) plane of copper oxalate decreases and the half-width of the peak increases, indicating that the
crystalline size is reduced with increasing the reaction temperature and time; in another words, the copper oxalate blocks were evolving gradually from the well-crystalline orthorhombic phase to amorphous. With further increasing the reaction time, the
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Fig. 5. SEM images of samples prepared in the presence of different ionic liquids: (a) BMImBF4, (b) BDMImBF4, (c) OMImBF4 and (d) BMImPF6.
Fig. 6. SEM images of samples prepared by solvothermal reaction at different temperatures for 48 h: (a) 120 1C, (b) 140 1C, (c) 160 1C and (d) 180 1C.
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Fig. 7. SEM images of samples prepared under different conditions: (a) room temperature, (b) 160 1C for 5 min, (c) 180 1C for 5 min, (d) 1 h, (e) 4 h, (f) 24 h, (g) 48 h and (h) 96 h.
amount of short whiskers grown directly from large copper oxalate block substrates is increased, and their length and width increased too. After 24 h of reaction, the product entirely consists
of copper oxalate nanowires with 5 mm in length. After 48 h of reaction both the length and width of the nanowires increase; in addition, metallic Cu impurity in copper oxalate nanowires was
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to promote crystallization of copper oxalate to nanowires during the solvothermal process. The crystallization follows a novel mechanism based on the reorganization and solid–solid transformation of a copper oxalate solid phase. Upon the thermal decomposition at 350 1C in air, copper oxalate nanowires can be converted to copper oxide nanowires.
f
Acknowledgements
e d
This work was supported by the program for NCET (NCET-060871) and SRF for ROCS of State Education Ministry, China.
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References
------ Copper oxalate ------ Copper
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Fig. 8. XRD patterns of samples prepared under different conditions: (a) room temperature, (b) 160 1C for 5 min, (c) 180 1C for 5 min, (d) 1 h, (e) 4 h, (f) 24 h, (g) 48 h and (h) 96 h.
formed. XRD patterns (see Fig. 8d and g) reveal that the half-width of (11 0) diffraction peak of copper oxalate decreases as the recrystallization from the amorphous to well-crystalline nanowires progresses along the increase of reaction time. In most nanowire synthesis, it is generally assumed that the formation of nanowires takes place mainly from the liquid phase, with a first nucleation step followed by crystal growth through the progressive incorporation of dissolved species (Ostwald ripening) or through oriented attachment [22,36,37]. This crystallization mechanism seems to govern the nanowire synthesis, which start from clear solutions. However, when a crystalline solid is present in the starting reaction mixture other alternative routes are also possible. In the present system, it is evident that the formation of CuO nanowires does not follow a mechanism of nucleation and crystal growth from soluble species. The nanowires are formed directly from the copper oxalate solid phase through its reorganization and crystallization. Therefore, the formation of the copper oxalate nanowires in the present conditions takes place mainly by a solid–solid transformation mechanism. The existence of solid–solid transformation has been observed earlier during the crystallization of different zeolites [38]. The presence of BMImBF4 ionic liquid is one of the important factors influencing the recrystallization process and the growth of the copper oxalate nanowires. The addition of BMImBF4 may provide a favorable environment for the growth of the nanowires in this system.
4. Conclusion In summary, copper oxalate nanowires with high-aspect ratio were synthesized by a novel, facile solvothermal reaction between Cu(CH3COO)2 4H2O and oxalic acid in ethanol solution with the aid of BMImBF4 ionic liquid at 180 1C. Both temperature and time are the key parameters for the preparation of the nanowires. The BMImBF4 ionic liquid plays a unique role in the preparation of copper oxalate nanowires, and may serve as a mineralizing agent
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