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Vanadium pentoxide: Synthesis and characterization of nanorod and nanoparticle V2O5 using CTAB micelle solution
Microporous and Mesoporous Materials 120 (2009) 397–401
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Vanadium pentoxide: Synthesis and characterization of nanorod and nanoparticle V2O5 using CTAB micelle solution Nilofar Asim a,*, Shahidan Radiman b, Mohd Ambar Yarmo c, M.S. Banaye Golriz b a
Solar Energy Research Institute (SERI), Faculty of Engineering, University Kebangsaan Malaysia (UKM), 43600 Bangi, Selangor Darul Ehsan, Malaysia School of Applied Physics, Faculty of Science and Technology, University Kebangsaan Malaysia (UKM), 43600 Bangi, Selangor Darul Ehsan, Malaysia c School of Chemical Science and Food Technology, Faculty of Science and Technology, University Kebangsaan Malaysia (UKM), 43600 Bangi, Selangor Darul Ehsan, Malaysia b
a r t i c l e
i n f o
Article history: Received 28 October 2008 Received in revised form 10 December 2008 Accepted 10 December 2008 Available online 25 December 2008 Keywords: Nanostructures Vanadium pentoxide Surfactant-mediated method Temperature programmed reduction Morphology
a b s t r a c t Using a surfactant-mediated method (surfactant based on cetyltrimethyl ammonium bromide, CTAB) V2O5 nanorod and nanoparticles have been successfully prepared. Morphologies of V2O5 nanostructures can be controlled by applying different precursors and by varying reaction conditions within the CTAB soft template. With ammonium metavanadate and sulfuric acid as precursors, nanoparticles are synthesized in the size range of 45–160 nm. Precursors of vanadyl sulfate hydrate and sodium hydroxide yield vanadium pentoxide nanorods with diameters of 30–90 nm and lengths of 260–600 nm. The resulting products are characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), variable pressure scanning electron microscopy (VPSEM) and X-ray photoelectron spectroscopy (XPS). Temperature programmed reduction (TPR) is included to test catalytic performance. The results show that V2O5 nanoparticles and nanorods achieve better catalytic performance compared to bulk V2O5, i.e. lower onset temperature, workability at lower temperatures, and higher H2 consumption (lmol/g). Ó 2008 Elsevier Inc. All rights reserved.
1. Introduction Nanoscale oxide particles have gained increasing technical importance in classical applications such as catalysts, passive electronic components, and ceramic materials [1]. Vanadium pentoxide V2O5 is widely used as a catalyst [2–4], as a cathode material for solid-state batteries [5–7], in windows for solar cells [8–9] for electrochromic devices [10] and for electronic and optical switches [11]. Various interrelated electronic and structural factors induce the special chemistry of vanadium oxides and their catalysts. Vanadium has partially filled d-orbitals that are responsible for a wide variety of electronic, magnetic, and catalytic properties. The ability of vanadium atoms to exist in multiple stable oxidation states results in the easy conversion between oxides of different stoichiometry by oxidation or reduction and is believed to be an important factor for the oxide to function as a catalyst in selective oxidation. The pronounced anisotropy of chemical bonds in the crystal lattice of V2O5 results in the structure sensitivity of catalytic reactions on vanadium oxide-based catalysts. Two types of crystal planes, which differ in the type of bonds and the degree of coordinative unsaturation, are exposed by V2O5 crystallites. The (0 1 0) basal plane of V2O5, which has all chemical bonds is almost fully * Corresponding author. Tel.: +60 3 89214592; fax: +60 3 89214593. E-mail address: [email protected] (N. Asim). 1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.12.013
saturated. The non-bonding d-orbitals of V ions have the LUMO character and act as Lewis acid sites, whereas the lone electron pairs of bridging oxygen atoms have the HOMO characteristic and behave as Lewis basic sites [12]. It is well known that many nanomaterial properties depend on their size and shape. Control of the synthesis is generally a prerequisite for fabricating the desired size and morphology of a nanomaterial. Various chemical methods have been adopted for the preparation of metal oxide nanoparticles including gas-phase methods, sol–gel methods, evaporative decomposition of solutions, and wet chemical synthesis. In this study, synthesis and characterization of V2O5 nanoparticles and nanorods have been investigated using CTAB micelle solutions with different precursors and conditions. 2. Materials and methods 2.1. Materials A number of materials are used in this research. Hexadecyltrimethyl ammonium bromide (CTAB) (purity 99%) and 1-hexanol were purchased from Sigma and Fluka, respectively. Vanadyl sulfate hydrate was obtained from Aldrich and ammonium metavanadate (99%) was purchased from Sigma–Aldrich. Both the ammonia solution (25%) and sulfuric acid (sp.gr.1.84) were purchased from BDH. Deionized and double distilled water was used for micelle solution and solution preparation. All the chemicals and solvents
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N. Asim et al. / Microporous and Mesoporous Materials 120 (2009) 397–401
were used as received without further purifications. The bulk V2O5 was prepared by heating ammonium metavanadate in a furnace at 450 °C for 3 h under air atmosphere. In this study, the morphology and composition of the calcined nanoscale vanadium pentoxide were examined under a variable pressure scanning electron microscope (VPSEM) (model Leo 1450VP, accelerating voltage at 30 kV), and a transmission electron microscopy (TEM) (model Phillips, CM12) was operated at 100 kV. X-ray diffraction (XRD) measurements were taken to study the crystalinity and phase of the prepared V2O5. These measurements were performed by a Bruker D8 advance X-ray diffractometer with running step = 0.02° in the range of 10–60° 2-h, using a monochromatized Cu Ka radiation (k = 0.154 nm). The XPS analyses were performed using XSAM-HS KRATOS X-ray photoelectron spectroscopy. X-ray source type Mg Ka was used with 10 mA current and 12 kV voltage to run XPS analysis for samples at 109 torr pressure. The pass energy was set at 160 eV for the survey spectra and at 40 eV for the high resolution spectra of all elements of interest. Data processing was performed using the Kratos software after Shirley baseline subtraction and using Schofield sensitivity factors corrected for instrumentation transmission function. Temperature programmed reduction (TPR) with hydrogen is a widely used technique for the characterization of reducible solids and catalysts. In TPR, a reducible catalyst or catalyst precursor is exposed to a flow of a reducing gas mixture while the temperature is progressively increased. The rate of reduction is continuously followed by measuring the composition (H2 content) of the reducing gas mixture at the outlet of the reactor. The experiment permits the determination of the total amount of hydrogen consumed, from which the degree of reduction and thus, the average oxidation state of the solid after reduction can be calculated [13]. TPR experiments were carried out on a TPR 1100 Series (Thermo Finnigan) instrument.
3. Results and discussion The VPSEM and TEM results (Figs. 1 and 2) reveal ammonium metavanadate and sulfuric acid precursors yield nanoparticles of 45–160 nm in diameter (Sample 1) while vanadyl sulfate hydrate and sodium hydroxide as precursors yield vanadium pentoxide nanorods with diameters of 30–90 nm and the lengths of 260– 600 nm (Sample 2). The nanoparticle morphology is as a function of the reaction parameters: surfactant ratio, water concentration, time, temperature, and precursor. Some mechanisms have been suggested to explain the formation of the rod shape. It was found that precursors affect surfactant assembly e.g. OH from NaOH [15]. One of mechanism is that, due to the interaction between CTAB and OH it is possible that CTAB referentially adsorb some planes of growing V2O5 leading to the growth of V2O5 along 1D direction. However, different precursors have different thermal decomposition temperatures and hydroxyl compound precursors are difficult to completely thermally decompose at low temperatures and in a short time. During the thermally decomposed reaction or calcinations using precursor containing OH, the nanoparticles will aggregate and agglomerate to nanorod [16]. The XRD patterns in Fig. 3 show that the vanadium pentoxide nanoparticles and nanorods are in the crystalline form of an orthorhombic system. The X-ray photoelectron spectroscopy (XPS) studies of synthesized nano V2O5 show slightly blue shift in binding energy compared to bulk V2O5. The wide scan XPS spectrum of the V2O5 nanoparticles (not shown) only suggests C impurity in the sample.
2.2. Preparation of nanoparticles The CTAB micelle solution has been employed for the nanoscale synthesis of vanadium pentoxide. The typical micelle solution used this present study had a composition of 30 wt% CTAB, 54 wt% 1hexanol, and 16 wt% of aqueous solution. This composition was found to belong to reverse class of micelles solutions [14]. For the first preparation of nanoscale V2O5, micelle solutions of two different compositions were used: the aqueous composition in the first micelle solution was 6 g CTAB, 10.8 g 1-hexanol, and 3 g water which contained 0.2 g ammonium metavanadate, while the second micelle solution contained 6 g CTAB, 10.8 g 1-hexanol, 3.1 g water, and 0.1 g sulfuric acid. After stirring and getting clear solutions, the micelle solution containing sulfuric acid was added to another micelle solution containing ammonium metavanadate. The mixed micelle solution was stirred for 3 h at about 50 °C; the mixed micelle solution was then left for two days at room temperature to allow precipitation (hereafter denoted as Sample 1). A similar procedure was followed for the second nanoscale V2O5 preparation, this time varying the precursor in the aqueous composition, and reaction conditions. Two micelles solutions were also used: the first of which had an aqueous composition of 6 g CTAB, 10.8 g 1-hexanol, and 3.1 g water which contained 0.07 g vanadyl sulfate hydrate, and the second of which had 6 g CTAB, 10.8 g 1hexanol, and 3.2 g water which contained 0.02 g sodium hydroxide. The mixed micelle solution was stirred for 1.5 h at about 35 °C. Then it was kept at room temperature for two days in order to precipitate (hereafter denoted as Sample 2). The precipitates were washed with deionized water and absolute ethanol several times in order to remove the surfactant, residual reactants, and byproducts. Finally, the precipitates were heated in a furnace at 400 °C for 2 h under air.
Fig. 1. VPSEM images of nanoparticles and nanorods V2O5 calcinated at 400 °C for 2 h.
N. Asim et al. / Microporous and Mesoporous Materials 120 (2009) 397–401
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Fig. 2. TEM images of nanoparticles and nanorods V2O5 in nanoscale (after calcinations) and V2O5 bulk.
Fig. 3. The XRD pattern for vanadium pentoxide (a) bulk, (b) Sample 1, and (c) Sample 2, respectively.
The narrow scans and peak-fitted analysis of C1s,V2p3/2 and O1s for the nanoparticles and bulkV2O5 have carried out for chemical state investigation and the data are depicted in Fig. 4. The C1s peak in the XPS results is from carbon contamination which is very common, in fact, it is often used to calibrate peak position and in this case we assumed it had come from residual surfactant or the ambient air. The O1s band was deconvoluted into three components. The terminal oxygen (@O) and the linkage oxygen (–O–) were associated to the O2 state. The peak-fitted, which is around 533 eV, is assumed to come from different sources, probably coming from rooted OH groups or from ambient humidity. The V2p3/2 peak can be attributed to the oxide form of V [17]. The details of this chemical state analysis are tabulated in Table 1. In this study, the binding energy of nanosized vanadium pentoxide showed a slight blue shift compared to bulk vanadium pentoxide; this is related to particle size effects, which is in agreement in the literature [18,19]. Prior to TPR studies, the sample was pretreated under N2 in order to eliminate the moisture and to clean the samples surfaces. Carrier gas consisting of 5.1% hydrogen balance argon was allowed to pass over the sample. The temperature-programmed reduction profiles for nanoparticles, nanorods, and bulk V2O5 are depicted in Fig. 5. Similar to previous reports [20,21], the peaks in this study overlapped in the bulk V2O5 so that two peaks are seen instead of three. This phenomenon can be attributed to the reduction sequence;
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N. Asim et al. / Microporous and Mesoporous Materials 120 (2009) 397–401
Fig. 4. The peak-fitted C1s, V2p3/2 and O1s signals (a) Sample 2, (b) Sample 1, and (c) bulk V2O5.
Table 1 Binding energy (eV) for relative peak of nano V2O5 (corrected with considering of C1s = 285.0 eV as a reference). Sample
V2O5 (bulk) V2O5 (Sample 1) V2O5 (Sample 2)
B.E/eV
Particle size, shape
O1s (1) (terminal oxygen)
O1s (2) (linkage oxygen)
O1s (3) (rooted OH groups)
V2p3/2
530.0 530.3 530.4
531.9 532.2 532.3
533.3 533.7 533.9
516.9 517.2 517.2
200–500 nm, Different shape 45–160 nm, Spherical d = 30–90, l = 260–600 nm, Rod
Table 2 The TPR results for nano V2O5 synthesized in this study compared with bulk V2O5. Sample
Peak (1) (temperature, °C)
Peak (2) (temperature, °C)
H2 consumption (lmol/g)
Onset (temperature, °C)
Shape
V2O5 (bulk) V2O5 (Sample 1) V2O5 (Sample 2)
645 637 577
679 661
3159.99 4508.07 4680.04
500 455 306
Different shapes Spherical Rod
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401
4. Conclusion In this study, V2O5 nanoparticles and nanorod have been successfully prepared using CTAB micelle solution as a soft template. By applying different precursors and conditions in the same CTAB soft template, morphologies of V2O5 nanostructures can be controlled. Ammonium metavanadate and sulfuric acid precursors yield nanoparticles with a size range of 45–160 nm; vanadyl sulfate hydrate and sodium hydroxide as precursors yield V2O5 nanorod with diameters of 30–90 nm and lengths of 260–600 nm. Some suggested mechanisms for rod shape formation are described. In this study, the TPR results showed better performance in V2O5 catalytic behavior for both nanorods and nanoparticles compared to bulk V2O5. Acknowledgments The author would like to thank Mr. Ahmad Zaki Bin Zaini, Mr. Zailan Bin Mohd. Yusof, Ms. Normalawati Bt. Shamsudin, and Mr. Said Abd Ghani for helping with the use of SEM, XRD, TEM, and XPS, respectively. The author wishes to especially thank Prof. Taufiq Yap Yun Hin of Universiti Putra Malaysia for his fruitful discussions of TPR results. References Fig. 5. Temperature-programmed reduction profiles for: (a) bulk V2O5, (b) Sample 1, and (c) Sample 2.
V2 O5 ! V6 O13
ð1Þ
V6 O13 ! V2 O4
ð2Þ
V2 O 4 ! V 2 O 3
ð3Þ
The TPR profiles for V2O5 nanoparticles also show two peaks but at different temperatures. Nanorod V2O5 has shown only one prominent maximum (Tmax) and it appears to be overlapped. Details about the peak’s temperature and the amount of hydrogen consumed during TPR are shown in Table 2. The results showed lower onset temperature, higher H2 consumption (lmol/g) and work on lower temperature (peaks) for V2O5 nanoparticles and nanorod compared to the bulk. This indicates that the nano V2O5 have better catalytic performance. This is because of the high specific surface area in nanomaterials compared to bulk. A rod shape can confer another advantage, which of having a large surface area per unit volume. Therefore having a large surface area in rod shape lead to better catalytic performance compared to spherical shape.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
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Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Vanadium pentoxide: Synthesis and characterization of nanorod and nanoparticle V2O5 using CTAB micelle solution Nilofar Asim a,*, Shahidan Radiman b, Mohd Ambar Yarmo c, M.S. Banaye Golriz b a
Solar Energy Research Institute (SERI), Faculty of Engineering, University Kebangsaan Malaysia (UKM), 43600 Bangi, Selangor Darul Ehsan, Malaysia School of Applied Physics, Faculty of Science and Technology, University Kebangsaan Malaysia (UKM), 43600 Bangi, Selangor Darul Ehsan, Malaysia c School of Chemical Science and Food Technology, Faculty of Science and Technology, University Kebangsaan Malaysia (UKM), 43600 Bangi, Selangor Darul Ehsan, Malaysia b
a r t i c l e
i n f o
Article history: Received 28 October 2008 Received in revised form 10 December 2008 Accepted 10 December 2008 Available online 25 December 2008 Keywords: Nanostructures Vanadium pentoxide Surfactant-mediated method Temperature programmed reduction Morphology
a b s t r a c t Using a surfactant-mediated method (surfactant based on cetyltrimethyl ammonium bromide, CTAB) V2O5 nanorod and nanoparticles have been successfully prepared. Morphologies of V2O5 nanostructures can be controlled by applying different precursors and by varying reaction conditions within the CTAB soft template. With ammonium metavanadate and sulfuric acid as precursors, nanoparticles are synthesized in the size range of 45–160 nm. Precursors of vanadyl sulfate hydrate and sodium hydroxide yield vanadium pentoxide nanorods with diameters of 30–90 nm and lengths of 260–600 nm. The resulting products are characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), variable pressure scanning electron microscopy (VPSEM) and X-ray photoelectron spectroscopy (XPS). Temperature programmed reduction (TPR) is included to test catalytic performance. The results show that V2O5 nanoparticles and nanorods achieve better catalytic performance compared to bulk V2O5, i.e. lower onset temperature, workability at lower temperatures, and higher H2 consumption (lmol/g). Ó 2008 Elsevier Inc. All rights reserved.
1. Introduction Nanoscale oxide particles have gained increasing technical importance in classical applications such as catalysts, passive electronic components, and ceramic materials [1]. Vanadium pentoxide V2O5 is widely used as a catalyst [2–4], as a cathode material for solid-state batteries [5–7], in windows for solar cells [8–9] for electrochromic devices [10] and for electronic and optical switches [11]. Various interrelated electronic and structural factors induce the special chemistry of vanadium oxides and their catalysts. Vanadium has partially filled d-orbitals that are responsible for a wide variety of electronic, magnetic, and catalytic properties. The ability of vanadium atoms to exist in multiple stable oxidation states results in the easy conversion between oxides of different stoichiometry by oxidation or reduction and is believed to be an important factor for the oxide to function as a catalyst in selective oxidation. The pronounced anisotropy of chemical bonds in the crystal lattice of V2O5 results in the structure sensitivity of catalytic reactions on vanadium oxide-based catalysts. Two types of crystal planes, which differ in the type of bonds and the degree of coordinative unsaturation, are exposed by V2O5 crystallites. The (0 1 0) basal plane of V2O5, which has all chemical bonds is almost fully * Corresponding author. Tel.: +60 3 89214592; fax: +60 3 89214593. E-mail address: [email protected] (N. Asim). 1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.12.013
saturated. The non-bonding d-orbitals of V ions have the LUMO character and act as Lewis acid sites, whereas the lone electron pairs of bridging oxygen atoms have the HOMO characteristic and behave as Lewis basic sites [12]. It is well known that many nanomaterial properties depend on their size and shape. Control of the synthesis is generally a prerequisite for fabricating the desired size and morphology of a nanomaterial. Various chemical methods have been adopted for the preparation of metal oxide nanoparticles including gas-phase methods, sol–gel methods, evaporative decomposition of solutions, and wet chemical synthesis. In this study, synthesis and characterization of V2O5 nanoparticles and nanorods have been investigated using CTAB micelle solutions with different precursors and conditions. 2. Materials and methods 2.1. Materials A number of materials are used in this research. Hexadecyltrimethyl ammonium bromide (CTAB) (purity 99%) and 1-hexanol were purchased from Sigma and Fluka, respectively. Vanadyl sulfate hydrate was obtained from Aldrich and ammonium metavanadate (99%) was purchased from Sigma–Aldrich. Both the ammonia solution (25%) and sulfuric acid (sp.gr.1.84) were purchased from BDH. Deionized and double distilled water was used for micelle solution and solution preparation. All the chemicals and solvents
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N. Asim et al. / Microporous and Mesoporous Materials 120 (2009) 397–401
were used as received without further purifications. The bulk V2O5 was prepared by heating ammonium metavanadate in a furnace at 450 °C for 3 h under air atmosphere. In this study, the morphology and composition of the calcined nanoscale vanadium pentoxide were examined under a variable pressure scanning electron microscope (VPSEM) (model Leo 1450VP, accelerating voltage at 30 kV), and a transmission electron microscopy (TEM) (model Phillips, CM12) was operated at 100 kV. X-ray diffraction (XRD) measurements were taken to study the crystalinity and phase of the prepared V2O5. These measurements were performed by a Bruker D8 advance X-ray diffractometer with running step = 0.02° in the range of 10–60° 2-h, using a monochromatized Cu Ka radiation (k = 0.154 nm). The XPS analyses were performed using XSAM-HS KRATOS X-ray photoelectron spectroscopy. X-ray source type Mg Ka was used with 10 mA current and 12 kV voltage to run XPS analysis for samples at 109 torr pressure. The pass energy was set at 160 eV for the survey spectra and at 40 eV for the high resolution spectra of all elements of interest. Data processing was performed using the Kratos software after Shirley baseline subtraction and using Schofield sensitivity factors corrected for instrumentation transmission function. Temperature programmed reduction (TPR) with hydrogen is a widely used technique for the characterization of reducible solids and catalysts. In TPR, a reducible catalyst or catalyst precursor is exposed to a flow of a reducing gas mixture while the temperature is progressively increased. The rate of reduction is continuously followed by measuring the composition (H2 content) of the reducing gas mixture at the outlet of the reactor. The experiment permits the determination of the total amount of hydrogen consumed, from which the degree of reduction and thus, the average oxidation state of the solid after reduction can be calculated [13]. TPR experiments were carried out on a TPR 1100 Series (Thermo Finnigan) instrument.
3. Results and discussion The VPSEM and TEM results (Figs. 1 and 2) reveal ammonium metavanadate and sulfuric acid precursors yield nanoparticles of 45–160 nm in diameter (Sample 1) while vanadyl sulfate hydrate and sodium hydroxide as precursors yield vanadium pentoxide nanorods with diameters of 30–90 nm and the lengths of 260– 600 nm (Sample 2). The nanoparticle morphology is as a function of the reaction parameters: surfactant ratio, water concentration, time, temperature, and precursor. Some mechanisms have been suggested to explain the formation of the rod shape. It was found that precursors affect surfactant assembly e.g. OH from NaOH [15]. One of mechanism is that, due to the interaction between CTAB and OH it is possible that CTAB referentially adsorb some planes of growing V2O5 leading to the growth of V2O5 along 1D direction. However, different precursors have different thermal decomposition temperatures and hydroxyl compound precursors are difficult to completely thermally decompose at low temperatures and in a short time. During the thermally decomposed reaction or calcinations using precursor containing OH, the nanoparticles will aggregate and agglomerate to nanorod [16]. The XRD patterns in Fig. 3 show that the vanadium pentoxide nanoparticles and nanorods are in the crystalline form of an orthorhombic system. The X-ray photoelectron spectroscopy (XPS) studies of synthesized nano V2O5 show slightly blue shift in binding energy compared to bulk V2O5. The wide scan XPS spectrum of the V2O5 nanoparticles (not shown) only suggests C impurity in the sample.
2.2. Preparation of nanoparticles The CTAB micelle solution has been employed for the nanoscale synthesis of vanadium pentoxide. The typical micelle solution used this present study had a composition of 30 wt% CTAB, 54 wt% 1hexanol, and 16 wt% of aqueous solution. This composition was found to belong to reverse class of micelles solutions [14]. For the first preparation of nanoscale V2O5, micelle solutions of two different compositions were used: the aqueous composition in the first micelle solution was 6 g CTAB, 10.8 g 1-hexanol, and 3 g water which contained 0.2 g ammonium metavanadate, while the second micelle solution contained 6 g CTAB, 10.8 g 1-hexanol, 3.1 g water, and 0.1 g sulfuric acid. After stirring and getting clear solutions, the micelle solution containing sulfuric acid was added to another micelle solution containing ammonium metavanadate. The mixed micelle solution was stirred for 3 h at about 50 °C; the mixed micelle solution was then left for two days at room temperature to allow precipitation (hereafter denoted as Sample 1). A similar procedure was followed for the second nanoscale V2O5 preparation, this time varying the precursor in the aqueous composition, and reaction conditions. Two micelles solutions were also used: the first of which had an aqueous composition of 6 g CTAB, 10.8 g 1-hexanol, and 3.1 g water which contained 0.07 g vanadyl sulfate hydrate, and the second of which had 6 g CTAB, 10.8 g 1hexanol, and 3.2 g water which contained 0.02 g sodium hydroxide. The mixed micelle solution was stirred for 1.5 h at about 35 °C. Then it was kept at room temperature for two days in order to precipitate (hereafter denoted as Sample 2). The precipitates were washed with deionized water and absolute ethanol several times in order to remove the surfactant, residual reactants, and byproducts. Finally, the precipitates were heated in a furnace at 400 °C for 2 h under air.
Fig. 1. VPSEM images of nanoparticles and nanorods V2O5 calcinated at 400 °C for 2 h.
N. Asim et al. / Microporous and Mesoporous Materials 120 (2009) 397–401
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Fig. 2. TEM images of nanoparticles and nanorods V2O5 in nanoscale (after calcinations) and V2O5 bulk.
Fig. 3. The XRD pattern for vanadium pentoxide (a) bulk, (b) Sample 1, and (c) Sample 2, respectively.
The narrow scans and peak-fitted analysis of C1s,V2p3/2 and O1s for the nanoparticles and bulkV2O5 have carried out for chemical state investigation and the data are depicted in Fig. 4. The C1s peak in the XPS results is from carbon contamination which is very common, in fact, it is often used to calibrate peak position and in this case we assumed it had come from residual surfactant or the ambient air. The O1s band was deconvoluted into three components. The terminal oxygen (@O) and the linkage oxygen (–O–) were associated to the O2 state. The peak-fitted, which is around 533 eV, is assumed to come from different sources, probably coming from rooted OH groups or from ambient humidity. The V2p3/2 peak can be attributed to the oxide form of V [17]. The details of this chemical state analysis are tabulated in Table 1. In this study, the binding energy of nanosized vanadium pentoxide showed a slight blue shift compared to bulk vanadium pentoxide; this is related to particle size effects, which is in agreement in the literature [18,19]. Prior to TPR studies, the sample was pretreated under N2 in order to eliminate the moisture and to clean the samples surfaces. Carrier gas consisting of 5.1% hydrogen balance argon was allowed to pass over the sample. The temperature-programmed reduction profiles for nanoparticles, nanorods, and bulk V2O5 are depicted in Fig. 5. Similar to previous reports [20,21], the peaks in this study overlapped in the bulk V2O5 so that two peaks are seen instead of three. This phenomenon can be attributed to the reduction sequence;
400
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Fig. 4. The peak-fitted C1s, V2p3/2 and O1s signals (a) Sample 2, (b) Sample 1, and (c) bulk V2O5.
Table 1 Binding energy (eV) for relative peak of nano V2O5 (corrected with considering of C1s = 285.0 eV as a reference). Sample
V2O5 (bulk) V2O5 (Sample 1) V2O5 (Sample 2)
B.E/eV
Particle size, shape
O1s (1) (terminal oxygen)
O1s (2) (linkage oxygen)
O1s (3) (rooted OH groups)
V2p3/2
530.0 530.3 530.4
531.9 532.2 532.3
533.3 533.7 533.9
516.9 517.2 517.2
200–500 nm, Different shape 45–160 nm, Spherical d = 30–90, l = 260–600 nm, Rod
Table 2 The TPR results for nano V2O5 synthesized in this study compared with bulk V2O5. Sample
Peak (1) (temperature, °C)
Peak (2) (temperature, °C)
H2 consumption (lmol/g)
Onset (temperature, °C)
Shape
V2O5 (bulk) V2O5 (Sample 1) V2O5 (Sample 2)
645 637 577
679 661
3159.99 4508.07 4680.04
500 455 306
Different shapes Spherical Rod
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4. Conclusion In this study, V2O5 nanoparticles and nanorod have been successfully prepared using CTAB micelle solution as a soft template. By applying different precursors and conditions in the same CTAB soft template, morphologies of V2O5 nanostructures can be controlled. Ammonium metavanadate and sulfuric acid precursors yield nanoparticles with a size range of 45–160 nm; vanadyl sulfate hydrate and sodium hydroxide as precursors yield V2O5 nanorod with diameters of 30–90 nm and lengths of 260–600 nm. Some suggested mechanisms for rod shape formation are described. In this study, the TPR results showed better performance in V2O5 catalytic behavior for both nanorods and nanoparticles compared to bulk V2O5. Acknowledgments The author would like to thank Mr. Ahmad Zaki Bin Zaini, Mr. Zailan Bin Mohd. Yusof, Ms. Normalawati Bt. Shamsudin, and Mr. Said Abd Ghani for helping with the use of SEM, XRD, TEM, and XPS, respectively. The author wishes to especially thank Prof. Taufiq Yap Yun Hin of Universiti Putra Malaysia for his fruitful discussions of TPR results. References Fig. 5. Temperature-programmed reduction profiles for: (a) bulk V2O5, (b) Sample 1, and (c) Sample 2.
V2 O5 ! V6 O13
ð1Þ
V6 O13 ! V2 O4
ð2Þ
V2 O 4 ! V 2 O 3
ð3Þ
The TPR profiles for V2O5 nanoparticles also show two peaks but at different temperatures. Nanorod V2O5 has shown only one prominent maximum (Tmax) and it appears to be overlapped. Details about the peak’s temperature and the amount of hydrogen consumed during TPR are shown in Table 2. The results showed lower onset temperature, higher H2 consumption (lmol/g) and work on lower temperature (peaks) for V2O5 nanoparticles and nanorod compared to the bulk. This indicates that the nano V2O5 have better catalytic performance. This is because of the high specific surface area in nanomaterials compared to bulk. A rod shape can confer another advantage, which of having a large surface area per unit volume. Therefore having a large surface area in rod shape lead to better catalytic performance compared to spherical shape.
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