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Synthesis of one-dimensional porous Co3O4 nanobelts and their ethanol gas sensing properties
Materials Research Bulletin 59 (2014) 69–76
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Synthesis of one-dimensional porous Co3O4 nanobelts and their ethanol gas sensing properties Hongwei Che a, *, Aifeng Liu a, *, Junxian Hou a , Xiaoliang Zhang a , Yongmei Bai a , Jingbo Mu a , Renliang Wang b, * a b
Department of Composite Materials and Engineering, College of Equipment Manufacturing, Hebei University of Engineering, Handan 056038, PR China Chemical Engineering Department, Taishan Medical University, Taian, Shandong 271016, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 23 April 2014 Received in revised form 3 June 2014 Accepted 30 June 2014 Available online 2 July 2014
In this paper, one-dimensional porous Co3O4 nanobelts were synthesized via a facile template-free hydrothermal method and subsequent the thermal decomposition. Their microstructures and morphologies were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and N2 adsorption–desorption techniques. The results indicate that the reaction parameters such as the molar ratio of Co(NO3)26H2O to C2H4N4, the amount of Co(NO3)26H2O, the hydrothermal temperature and time play crucial rules in controlling the microstructures and morphologies of the as-prepared cobalt precursors. A possible formation mechanism was proposed. Moreover, the obtained porous Co3O4 nanobelts exhibit ethanol gas sensing properties superior to the commercial Co3O4 powders at a working temperature of 200 C, suggesting their potential applications as nanosensors. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: Nanostructures Oxides
1. Introduction One-dimensional (1D) nanostructures such as nanotubes, nanorods, nanowires and nanobelts have extensively drawn much interest for potential applications being prior to bulky materials owing to their remarkable physicochemical properties [1–13]. Among them, 1D nanostructures composed of metal oxides reveal outstanding performances in catalysis, electrochemistry, sensors, etc., because of their high surface-to-volume ratios and special physiochemical properties such as the unique quantum confinement effect and anisotropic electron and photon transport [14–17]. 1D cobalt oxide (Co3O4) nanostructures have attracted great attention over the past decade due to their fascinating properties and potential applications as heterogeneous catalysts, electrode materials as lithium ion batteries, gas sensors, magnetic materials, and so on [18–24]. For instance, Co3O4 nanorods showed surprisingly high catalytic activity in CO oxidation at temperature as low as 77 C [19]. Co3O4 nanotubes displayed excellent activity and durability in catalytic combustion of CH4, showing a 100% CH4
* Corresponding authors. Tel.: +86 0310 8577973. E-mail addresses: [email protected] (H. Che), afl[email protected] (A. Liu), [email protected] (R. Wang). http://dx.doi.org/10.1016/j.materresbull.2014.06.033 0025-5408/ ã 2014 Elsevier Ltd. All rights reserved.
conversion at 325 C and a durability at T90 over 30 h [21]. Co3O4 nanobelts were reported to have excellent electrochemical performances, possessing a reversible capacities of up to 1400 mA h g1 much larger than the theoretical capacity of bulk Co3O4 (892 mA h g1) [22]. Moreover, Co3O4 nanofibers exhibited high responses to 100 ppm of C2H5OH at 301 C [24]. Therefore, much research effort has been focused on designing effective procedures to synthesize 1D Co3O4 with controllable microstructures. Up to now, various methods, such as the solvothermal/hydrothermal reaction methods, template-assisted method, precipitation reaction, microemulsion-based routes, etc., have been employed to synthesize 1D Co3O4 nanostructures with different morophologies including nanorods, nanotubes, nanobelts, nanowires [25–33]. Although great progress has been made, the synthesis of 1D Co3O4 nanostructures remains a huge challenge in view of developing a more environment-benign, economic and large-scale synthetic route. Herein, we reported a facile route to synthesize porous Co3O4 nanobelts without use of any surfactant or organic solvent. 1D cobalt precursor compounds nanobelts were firstly prepared via the hydrothermal method, in which cobalt nitrate and dicyandiamide were used as the reactants. Subsequently, the calcination was performed to obtain porous Co3O4 nanobelts. The effects of reaction parameters including the amount of cobalt nitrate, the
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molar ration of cobalt nitrate to dicyandiamide, the hydrothermal temperature and time on the morphology of cobalt precursor compounds were explored. And a possible formation mechanism was discussed. Furthermore, the ethanol gas-sensing performance of the obtained porous Co3O4 nanobelts was also investigated. 2. Experimental 2.1. Synthesis All of the reagents were of analytical grade and used without further purification. In a typical synthesis of porous Co3O4 nanobelts, 1.7 mmol (0.50 g) of cobalt nitrate (Co(NO3)26H2O) and 5.1 mmol (0.43 g) dicyandiamide (C2H4N4) were dissolved in 30 ml of water under stirring at 35 C. After stirring 30 min, the homogeneous solution was transferred into a 50 ml Teflon-lined stainless autoclave, which was sealed and maintained at 180 C for 12 h. The final pale product was collected by filtration, washed with alternately with deionized water and alcohol several times and dried by vacuum-drying. Subsequently, the as-prepared precursor was calcined at 450 C for 4 h to obtain porous Co3O4 nanobelts. For comparison, the commercial cobalt oxide nanopowders were obtained from Aladdin Industrial Corporation. 2.2. Characterization X-ray diffraction (XRD) patterns were recorded by a Bruker AXSD8 Advance X-ray diffractometer using Cu-Ka radiation (l = 1.5418 Å). Field emission scanning electron microscopy (FESEM) images were taken with a JSM-7001F field emission instrument. High resolution transmission electron microscopy (HRTEM) images were recorded using a JEM-2100 electron microscope operating at 200 kV. Fourier transformed infrared spectroscopy (FT-IR) spectra were recorded using an IRPrestige-21 FTIR spectrometer. X-ray photoelectron spectroscopy (XPS, ESCLAB 250) was applied to determine the surface composition of the products. Nitrogen adsorption–desorption isotherms of the products were determined at 77 K by a conventional volumetric technique with a Quantachrome Autosorb-1MP sorption analyser. Weight changes of the products were measured out on an EXSTAR TG/DTA 6300 using a heating rate of 5 C/min in air (200 ml min1). 2.3. Gas-sensing measurements
obtained products, the XPS measurements were performed and shown in Fig. 2. The binding energies in the XPS analyses were corrected for specimen charging by referring the C 1s peak to 284.8 eV. Fig. 2a shows a wide-scan spectrum of the Co3O4 products. The sharp peaks at 284.8, 529.8, and 780.8 eV are indexed as the characteristic peaks of C 1s, O 1s, and Co 2p, respectively, suggesting the existence of carbon, oxygen and cobalt elements on the surfaces of Co3O4 products. The outstanding O 1s peak at 529.8 eV shown in Fig. 2b corresponds to the lattice oxygen of Co3O4 [34]. The other two peaks at 531.0 and 532.2 eV can be ascribed to oxygen species in CoO and H2O, respectively [35]. The two major peaks at 780.8 and 795.7 eV in Fig. 2c correspond to the Co 2p3/2 and Co 2p1/2, respectively. The gap value of about 15 eV is also well consistent with the standard Co3O4 spectra [36]. On the basis of the above XRD/XPS analysis, the pure Co3O4 can be obtained in the current synthesis conditions. The morphology and microstructure of the Co3O4 products were investigated by FESEM, TEM and HRTEM. As is shown in Fig. 3a, 1D nanobelt-like morphology is displayed, and these nanobelts have a width of 100–500 nm. The high-magnification SEM image in Fig. 3b shows that these nanobelts are composed of nanoparticles, which are self-assembled to generate a large number of porous structures on the surfaces of Co3O4 nanobelts. Such porous structures will be favorable for effective diffusion of reactive gas onto the surfaces of Co3O4 nanobelts, improving its physical or chemical properties, for example, the gas sensing property. The microstructure of Co3O4 nanobelts was further characterized by TEM and HRTEM. The TEM images in Fig. 4a and b demonstrate that Co3O4 nanobelts are formed from the interconnection of plate-like nanocrystals with sizes of ca. 50 nm in width and 100 nm in length. And the highly porous structures are also generated. These results agree with the SEM images. The locally magnified HRTEM image in Fig. 4c, marked with the square area in Fig. 4b, shows the distinct lattice stripes, meaning that the walls of porous Co3O4 nanobelts are well crystallized. The spacing of lattice fringes is 0.467 nm, which can be indexed as the (111) plane of cubic spinel Co3O4. The selected area electron diffraction (SAED) in Fig. 4d further confirms the high crystallinity and suggests the quasi-single-crystalline nature of Co3O4 pore walls. Nitrogen isothermal absorption-desorption analyses were performed to evaluate the Brunauer–Emmett–Teller (BET) surface area and the porous feature of porous Co3O4 nanobelts. A typical type IV curve was observed in Fig. 5, having a hysteresis loop at relative pressure of 0.8–1.0. The BET specific surface area of porous
First, a gas sensor was fabricated as follows: the obtained Co3O4 powder was mixed with alcohol in an agate mortar, and then coated onto an Al2O3 tube on which two platinum wires had been installed at each end. The operating temperature was controlled by adjusting the heating voltage using a Ni–Cr alloy coil placed through the tube for heating the sensor. The sensor was aged at 300 C for 3 days in order to improve its stability and repeatability. Subsequently, gas sensing properties of the sensor were determined using a China HW-30A gas sensitivity instrument. The sensor response is defined as S = Rgas/Rair, where Rgas and Rair are the electrical resistances of the sensor in dry air mixed with the test gas and in dry air, respectively. 3. Results and discussion The XRD pattern of the obtained products after calcined at 450 C for 4 h was presented in Fig. 1. All the diffraction peaks can be well indexed as the cubic spinel Co3O4 phase with lattice constant of a = 8.08 Å (JCPDS No. 42-1467). No other peaks for impurities are detected, confirming that the products are composed of pure Co3O4 phase. In order to further investigate the surface information on the purity and composition of the
Fig. 1. XRD pattern of the obtained products after calcined at 450 C.
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correspond to the asymmetric and symmetric stretching vibration of y(OCO) [37,38]. Moreover, the bands at about 970, 837, 760, and 661 cm1 are assigned to the stretching vibrations of y(CQO), d(CO3), d(OCO), and r(OCO) in the carbonate anion, respectively [35,38]. All of these peaks confirm the presence of CO32 in the cobalt precursors. Fig. 8 presents the TGA and DrTGA curves of the cobalt precursors. A significant weight loss peak ranging from 200 C to 350 C and a weak weight loss peak ranging from 350 C to 500 C are displayed in the DrTGA curve. The corresponding weight loss is about 16.5% and 7.2%, respectively. The first weight loss can be ascribed to the removal of structural water and carbon dioxide from the dehydroxylation and decomposition of carbonate groups. And the second weight loss may be due to the further decomposition of residual carbonate groups. As described above, the formation and thermal decomposition of Co2(OH)2CO3 can be described in the following steps: C2 H4 N4 þ 8H2 O ! 4NH4 þ þ2CO2 þ 4OH CO2 þ 2OH ! CO3 2 þH2 O 2CO2þ þCO3 2 þ2OH ! CO2 ðOHÞ2 CO3 3CO2 ðOHÞ2 CO3 þ O2 ! 2Co3 O4 þ 3CO2 þ 3H2 O The morphology of cobalt precursors was shown in Fig. 9. Beltlike morphology with smooth surface is observed, possessing a size of 100–500 nm in width. We found that the reaction conditions,
Fig. 2. XPS spectra of the Co3O4 products: (a) full survey scan spectrum; slow scan spectra of (b) O 1s, (c) Co 2p peaks.
Co3O4 nanobelts is calculated to be about 31.6 m2/g. In addition, according to the corresponding Barrett–Joyner–Halenda (BJH) pore size distribution plot, the pore size is measured to be 28.6 nm, as shown the inset in Fig. 5. Fig. 6 presents the XRD pattern of the cobalt precursors after the hydrothermal treatment at 180 C for 10 h. All diffraction peaks can be indexed as the monoclinic cobalt hydroxide carbonate phase (Co2(OH)2CO3, JCPDS No. 29-1416). To further reveal the composition of the cobalt precursors, FTIR and TGA analyses were carried out. As is shown in Fig. 7, the strong peak at 3499 cm1 can be attributed to the stretching vibration of the OH group of molecular water and of hydrogen-bonded OH groups. Another intense peak at 3382 cm1 can be ascribed to the OH groups interacting with carbonate anions. The peaks at 1557 cm1 and 1353 cm1
Fig.
3. Low- (a) and high-magnification (b) SEM images of the Co3O4 products.
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Fig. 4. TEM images of the Co3O4 nanobelts (a) low magnification, (b) high magnification; HRTEM image (c) and SAED pattern (d) of the area marked with the square in (b).
such as the molar ratio (R) of Co(NO3)26H2O to C2H4N4, the amount of Co(NO3)26H2O and the hydrothermal temperature, have significant effects on the morphologies of cobalt precursors. Fig. S1 shows the SEM images of cobalt precursors prepared with different ratios of R while keeping all other experimental
parameters unchanged. The morphology of cobalt precursors transformed from irregular plate-like microstructures (Fig. S1a,b; R = 1:1 and 1:2) to belt-like nanostructures (Fig. 9a, R = 3), finally to the existence of nanobelts and irregular bulks (Fig. S1c, R = 4). The effect of the amount of Co(NO3)26H2O on the morphologies of
Fig. 5. Typical nitrogen adsorption–desorption isotherm and BJH pore size distribution plots (inset) of porous Co3O4 nanobelts.
Fig. 6. XRD pattern of the cobalt precursors after the hydrothermal treatment at 180 C for 12 h.
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Fig.
7. FT-IR spectrum of the obtained cobalt precursors.
cobalt precursors was also investigated. As is shown in Fig. S2a and Fig. 9a, irregular plates and bulks are gradually changed into beltlike nanostructures with increasing the amount of Co(NO3)26H2O form 0.15 g to 0.5 g. And the polyhedron-like microstructures (Fig. S2b) are formed when the amount of Co(NO3)26H2O was further added to 1.5 g. Fig. S3 investigated the effect of the hydrothermal temperature on the morphologies of cobalt precursors. The morphologies of the cobalt precursors (Fig. S3a, b) display mostly the rod-like nanostructures when the hydrothermal temperature was 100 C and 130 C. However, aggregates composed of fine stripes and small amount of rods were observed in Fig. S3c when the hydrothermal temperature was enhanced to 150 C. Therefore, as described above, the morphologies of the cobalt precursors can be tuned via adjusting the reaction parameters including the molar ratio of Co(NO3)26H2O to C2H4N4, the amount of Co(NO3)26H2O and the hydrothermal temperature. In order to make out the formation mechanism of porous Co3O4 nanobelts, a series of time-dependent experiments were carried out and the intermediate products were characterized by SEM. Fig. 10 presents the SEM images of the intermediate products collected after the hydrothermal reaction performed for 3 h, 6 h, 8 h at 180 C. These images reveal a morphological evolution of Co3O4 structures from flower-like microstructures to wire-like nanostructures. In the earlier reaction stage (3 h), 3D flower-like microstructures in Fig. 10a with a size of about 10 um were formed from the self-assembly of microplates. However, the obtained products were mainly composed of 2D microplate fragments
Fig.
8. TGA and DrTGA curves of the obtained cobalt precursors.
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(Fig. 10b) when the hydrothermal time was prolonged to 6 h. With further increasing the reaction time to 8 h, aggregated nanowires mixed with small amount of microplate fragments were observed, as shown in Fig. 10c,d. Finally, nanobelts were formed shown in Fig. 9a when the hydrothermal time was enhanced to 12 h. On the basis of the investigations described above, a possible formation mechanism of porous Co3O4 nanobelts was proposed and schematically illustrated in Scheme 1. At the first stage, the cobalt precursor nuclei were formed from the precipitation reaction between Co2+ and CO32 in the presence of OH in the solution. Then, these nuclei began to grow and tended to aggregate to nanoplates owing to high surface energies. Afterwards, these nanoplates were assembled into 3D flower-like microstructures. But it was difficult to maintain the 3D flower-like structures in the growth and re-crystallization of crystallites with prolonging the reaction time. As a result, the 2D microplate fragments were obtained and further transformed to 1D nanowires. Subsequently, 1D nanobelts were formed via the aggregative lateral attachment among nanowires. Finally, porous Co3O4 nanobelts were obtained from the thermal decomposition of cobalt precursor nanobelts, accompanied by the release of CO2 and H2O gases during the calcination process. The detailed mechanism for the formation of porous Co3O4 belts is still under further investigation. To demonstrate the gas sensing performance of the obtained porous Co3O4 nanobelts, the sensitivity of their response to ethanol was investigated. Fig. 11 presents an ethanol (200 ppm) sensing curve of porous Co3O4 nanobelts at different operating temperatures. The maximum sensitivity to ethanol is observed at 200 C,
Fig. 9. Low- (a) and high-magnification (b) SEM images of the obtained cobalt precursors.
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Fig. 10. SEM images of the intermediate products collected after the hydrothermal reaction performed for 3 h (a), 6 h (b), 8 h (c, d) at 180 C.
Scheme
1. Illustration of the formation process of porous Co3O4 nanobelts.
Fig. 11. Gas response of porous Co3O4 nanobelts as a function of different operating temperatures to 200 ppm ethanol.
indicating the optimum working temperature for the porous Co3O4 nanobelts sensor. Fig. 12a shows the real-time response–recovery curves for both porous Co3O4 nanobelts and commercial Co3O4 powders upon exposure to different concentrations of ethanol at a working temperature of 200 C. It can be seen that the output voltages abruptly decreased and then quickly reached a relatively stable value when porous Co3O4 nanobelts and commercial Co3O4 powders were exposed to alcohol gas with a concentration ranging from 5 ppm to 500 ppm. The decreased voltage values of the former are almost twice those of the latter, meaning that porous Co3O4 nanobelts exhibit higher gas sensing capabilities towards alcohol than those of commercial Co3O4 powders. It is also proved by the corresponding sensitivity curves (Fig. 12b) for both porous Co3O4 nanobelts and commercial Co3O4 powders. The higher sensitivity of porous Co3O4 nanobelts can be attributed to their special structures, i.e., high surface-to-volume ratio and porous structures, which is favorable for increasing the electron transfer
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References
Fig. 12. Real-time gas response curves (a) and the corresponding sensor response curves (b) of both porous Co3O4 nanobelts and commercial Co3O4 powders towards different concentrations of ethanol at a working temperature of 200 C.
caused by the chemical reaction between the active sites and alcohol gas. 7. Conclusions In summary, porous Co3O4 nanobelts have been successfully synthesized via a facile hydrothermal method followed by the direct thermal decomposition. The belt-like morphology can be finely controlled via adjusting the reaction parameters including the molar ratio of Co(NO3)26H2O to C2H4N4, the amount of Co (NO3)26H2O, the hydrothermal temperature and time. Ethanol gas sensing measurements indicate that the obtained porous Co3O4 nanobelts show higher sensitivity at a working temperature of 200 C compared with the commercial Co3O4 powders, suggesting their potential applications in nanosensors. Acknowledgements The authors gratefully acknowledge the financial supports from the National Natural Science Foundation of China (grant no. 21206025) and the Natural Science Foundation of Hebei Province (grant no. B2013402008). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.materresbull.2014.06.033.
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Synthesis of one-dimensional porous Co3O4 nanobelts and their ethanol gas sensing properties Hongwei Che a, *, Aifeng Liu a, *, Junxian Hou a , Xiaoliang Zhang a , Yongmei Bai a , Jingbo Mu a , Renliang Wang b, * a b
Department of Composite Materials and Engineering, College of Equipment Manufacturing, Hebei University of Engineering, Handan 056038, PR China Chemical Engineering Department, Taishan Medical University, Taian, Shandong 271016, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 23 April 2014 Received in revised form 3 June 2014 Accepted 30 June 2014 Available online 2 July 2014
In this paper, one-dimensional porous Co3O4 nanobelts were synthesized via a facile template-free hydrothermal method and subsequent the thermal decomposition. Their microstructures and morphologies were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and N2 adsorption–desorption techniques. The results indicate that the reaction parameters such as the molar ratio of Co(NO3)26H2O to C2H4N4, the amount of Co(NO3)26H2O, the hydrothermal temperature and time play crucial rules in controlling the microstructures and morphologies of the as-prepared cobalt precursors. A possible formation mechanism was proposed. Moreover, the obtained porous Co3O4 nanobelts exhibit ethanol gas sensing properties superior to the commercial Co3O4 powders at a working temperature of 200 C, suggesting their potential applications as nanosensors. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: Nanostructures Oxides
1. Introduction One-dimensional (1D) nanostructures such as nanotubes, nanorods, nanowires and nanobelts have extensively drawn much interest for potential applications being prior to bulky materials owing to their remarkable physicochemical properties [1–13]. Among them, 1D nanostructures composed of metal oxides reveal outstanding performances in catalysis, electrochemistry, sensors, etc., because of their high surface-to-volume ratios and special physiochemical properties such as the unique quantum confinement effect and anisotropic electron and photon transport [14–17]. 1D cobalt oxide (Co3O4) nanostructures have attracted great attention over the past decade due to their fascinating properties and potential applications as heterogeneous catalysts, electrode materials as lithium ion batteries, gas sensors, magnetic materials, and so on [18–24]. For instance, Co3O4 nanorods showed surprisingly high catalytic activity in CO oxidation at temperature as low as 77 C [19]. Co3O4 nanotubes displayed excellent activity and durability in catalytic combustion of CH4, showing a 100% CH4
* Corresponding authors. Tel.: +86 0310 8577973. E-mail addresses: [email protected] (H. Che), afl[email protected] (A. Liu), [email protected] (R. Wang). http://dx.doi.org/10.1016/j.materresbull.2014.06.033 0025-5408/ ã 2014 Elsevier Ltd. All rights reserved.
conversion at 325 C and a durability at T90 over 30 h [21]. Co3O4 nanobelts were reported to have excellent electrochemical performances, possessing a reversible capacities of up to 1400 mA h g1 much larger than the theoretical capacity of bulk Co3O4 (892 mA h g1) [22]. Moreover, Co3O4 nanofibers exhibited high responses to 100 ppm of C2H5OH at 301 C [24]. Therefore, much research effort has been focused on designing effective procedures to synthesize 1D Co3O4 with controllable microstructures. Up to now, various methods, such as the solvothermal/hydrothermal reaction methods, template-assisted method, precipitation reaction, microemulsion-based routes, etc., have been employed to synthesize 1D Co3O4 nanostructures with different morophologies including nanorods, nanotubes, nanobelts, nanowires [25–33]. Although great progress has been made, the synthesis of 1D Co3O4 nanostructures remains a huge challenge in view of developing a more environment-benign, economic and large-scale synthetic route. Herein, we reported a facile route to synthesize porous Co3O4 nanobelts without use of any surfactant or organic solvent. 1D cobalt precursor compounds nanobelts were firstly prepared via the hydrothermal method, in which cobalt nitrate and dicyandiamide were used as the reactants. Subsequently, the calcination was performed to obtain porous Co3O4 nanobelts. The effects of reaction parameters including the amount of cobalt nitrate, the
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molar ration of cobalt nitrate to dicyandiamide, the hydrothermal temperature and time on the morphology of cobalt precursor compounds were explored. And a possible formation mechanism was discussed. Furthermore, the ethanol gas-sensing performance of the obtained porous Co3O4 nanobelts was also investigated. 2. Experimental 2.1. Synthesis All of the reagents were of analytical grade and used without further purification. In a typical synthesis of porous Co3O4 nanobelts, 1.7 mmol (0.50 g) of cobalt nitrate (Co(NO3)26H2O) and 5.1 mmol (0.43 g) dicyandiamide (C2H4N4) were dissolved in 30 ml of water under stirring at 35 C. After stirring 30 min, the homogeneous solution was transferred into a 50 ml Teflon-lined stainless autoclave, which was sealed and maintained at 180 C for 12 h. The final pale product was collected by filtration, washed with alternately with deionized water and alcohol several times and dried by vacuum-drying. Subsequently, the as-prepared precursor was calcined at 450 C for 4 h to obtain porous Co3O4 nanobelts. For comparison, the commercial cobalt oxide nanopowders were obtained from Aladdin Industrial Corporation. 2.2. Characterization X-ray diffraction (XRD) patterns were recorded by a Bruker AXSD8 Advance X-ray diffractometer using Cu-Ka radiation (l = 1.5418 Å). Field emission scanning electron microscopy (FESEM) images were taken with a JSM-7001F field emission instrument. High resolution transmission electron microscopy (HRTEM) images were recorded using a JEM-2100 electron microscope operating at 200 kV. Fourier transformed infrared spectroscopy (FT-IR) spectra were recorded using an IRPrestige-21 FTIR spectrometer. X-ray photoelectron spectroscopy (XPS, ESCLAB 250) was applied to determine the surface composition of the products. Nitrogen adsorption–desorption isotherms of the products were determined at 77 K by a conventional volumetric technique with a Quantachrome Autosorb-1MP sorption analyser. Weight changes of the products were measured out on an EXSTAR TG/DTA 6300 using a heating rate of 5 C/min in air (200 ml min1). 2.3. Gas-sensing measurements
obtained products, the XPS measurements were performed and shown in Fig. 2. The binding energies in the XPS analyses were corrected for specimen charging by referring the C 1s peak to 284.8 eV. Fig. 2a shows a wide-scan spectrum of the Co3O4 products. The sharp peaks at 284.8, 529.8, and 780.8 eV are indexed as the characteristic peaks of C 1s, O 1s, and Co 2p, respectively, suggesting the existence of carbon, oxygen and cobalt elements on the surfaces of Co3O4 products. The outstanding O 1s peak at 529.8 eV shown in Fig. 2b corresponds to the lattice oxygen of Co3O4 [34]. The other two peaks at 531.0 and 532.2 eV can be ascribed to oxygen species in CoO and H2O, respectively [35]. The two major peaks at 780.8 and 795.7 eV in Fig. 2c correspond to the Co 2p3/2 and Co 2p1/2, respectively. The gap value of about 15 eV is also well consistent with the standard Co3O4 spectra [36]. On the basis of the above XRD/XPS analysis, the pure Co3O4 can be obtained in the current synthesis conditions. The morphology and microstructure of the Co3O4 products were investigated by FESEM, TEM and HRTEM. As is shown in Fig. 3a, 1D nanobelt-like morphology is displayed, and these nanobelts have a width of 100–500 nm. The high-magnification SEM image in Fig. 3b shows that these nanobelts are composed of nanoparticles, which are self-assembled to generate a large number of porous structures on the surfaces of Co3O4 nanobelts. Such porous structures will be favorable for effective diffusion of reactive gas onto the surfaces of Co3O4 nanobelts, improving its physical or chemical properties, for example, the gas sensing property. The microstructure of Co3O4 nanobelts was further characterized by TEM and HRTEM. The TEM images in Fig. 4a and b demonstrate that Co3O4 nanobelts are formed from the interconnection of plate-like nanocrystals with sizes of ca. 50 nm in width and 100 nm in length. And the highly porous structures are also generated. These results agree with the SEM images. The locally magnified HRTEM image in Fig. 4c, marked with the square area in Fig. 4b, shows the distinct lattice stripes, meaning that the walls of porous Co3O4 nanobelts are well crystallized. The spacing of lattice fringes is 0.467 nm, which can be indexed as the (111) plane of cubic spinel Co3O4. The selected area electron diffraction (SAED) in Fig. 4d further confirms the high crystallinity and suggests the quasi-single-crystalline nature of Co3O4 pore walls. Nitrogen isothermal absorption-desorption analyses were performed to evaluate the Brunauer–Emmett–Teller (BET) surface area and the porous feature of porous Co3O4 nanobelts. A typical type IV curve was observed in Fig. 5, having a hysteresis loop at relative pressure of 0.8–1.0. The BET specific surface area of porous
First, a gas sensor was fabricated as follows: the obtained Co3O4 powder was mixed with alcohol in an agate mortar, and then coated onto an Al2O3 tube on which two platinum wires had been installed at each end. The operating temperature was controlled by adjusting the heating voltage using a Ni–Cr alloy coil placed through the tube for heating the sensor. The sensor was aged at 300 C for 3 days in order to improve its stability and repeatability. Subsequently, gas sensing properties of the sensor were determined using a China HW-30A gas sensitivity instrument. The sensor response is defined as S = Rgas/Rair, where Rgas and Rair are the electrical resistances of the sensor in dry air mixed with the test gas and in dry air, respectively. 3. Results and discussion The XRD pattern of the obtained products after calcined at 450 C for 4 h was presented in Fig. 1. All the diffraction peaks can be well indexed as the cubic spinel Co3O4 phase with lattice constant of a = 8.08 Å (JCPDS No. 42-1467). No other peaks for impurities are detected, confirming that the products are composed of pure Co3O4 phase. In order to further investigate the surface information on the purity and composition of the
Fig. 1. XRD pattern of the obtained products after calcined at 450 C.
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correspond to the asymmetric and symmetric stretching vibration of y(OCO) [37,38]. Moreover, the bands at about 970, 837, 760, and 661 cm1 are assigned to the stretching vibrations of y(CQO), d(CO3), d(OCO), and r(OCO) in the carbonate anion, respectively [35,38]. All of these peaks confirm the presence of CO32 in the cobalt precursors. Fig. 8 presents the TGA and DrTGA curves of the cobalt precursors. A significant weight loss peak ranging from 200 C to 350 C and a weak weight loss peak ranging from 350 C to 500 C are displayed in the DrTGA curve. The corresponding weight loss is about 16.5% and 7.2%, respectively. The first weight loss can be ascribed to the removal of structural water and carbon dioxide from the dehydroxylation and decomposition of carbonate groups. And the second weight loss may be due to the further decomposition of residual carbonate groups. As described above, the formation and thermal decomposition of Co2(OH)2CO3 can be described in the following steps: C2 H4 N4 þ 8H2 O ! 4NH4 þ þ2CO2 þ 4OH CO2 þ 2OH ! CO3 2 þH2 O 2CO2þ þCO3 2 þ2OH ! CO2 ðOHÞ2 CO3 3CO2 ðOHÞ2 CO3 þ O2 ! 2Co3 O4 þ 3CO2 þ 3H2 O The morphology of cobalt precursors was shown in Fig. 9. Beltlike morphology with smooth surface is observed, possessing a size of 100–500 nm in width. We found that the reaction conditions,
Fig. 2. XPS spectra of the Co3O4 products: (a) full survey scan spectrum; slow scan spectra of (b) O 1s, (c) Co 2p peaks.
Co3O4 nanobelts is calculated to be about 31.6 m2/g. In addition, according to the corresponding Barrett–Joyner–Halenda (BJH) pore size distribution plot, the pore size is measured to be 28.6 nm, as shown the inset in Fig. 5. Fig. 6 presents the XRD pattern of the cobalt precursors after the hydrothermal treatment at 180 C for 10 h. All diffraction peaks can be indexed as the monoclinic cobalt hydroxide carbonate phase (Co2(OH)2CO3, JCPDS No. 29-1416). To further reveal the composition of the cobalt precursors, FTIR and TGA analyses were carried out. As is shown in Fig. 7, the strong peak at 3499 cm1 can be attributed to the stretching vibration of the OH group of molecular water and of hydrogen-bonded OH groups. Another intense peak at 3382 cm1 can be ascribed to the OH groups interacting with carbonate anions. The peaks at 1557 cm1 and 1353 cm1
Fig.
3. Low- (a) and high-magnification (b) SEM images of the Co3O4 products.
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Fig. 4. TEM images of the Co3O4 nanobelts (a) low magnification, (b) high magnification; HRTEM image (c) and SAED pattern (d) of the area marked with the square in (b).
such as the molar ratio (R) of Co(NO3)26H2O to C2H4N4, the amount of Co(NO3)26H2O and the hydrothermal temperature, have significant effects on the morphologies of cobalt precursors. Fig. S1 shows the SEM images of cobalt precursors prepared with different ratios of R while keeping all other experimental
parameters unchanged. The morphology of cobalt precursors transformed from irregular plate-like microstructures (Fig. S1a,b; R = 1:1 and 1:2) to belt-like nanostructures (Fig. 9a, R = 3), finally to the existence of nanobelts and irregular bulks (Fig. S1c, R = 4). The effect of the amount of Co(NO3)26H2O on the morphologies of
Fig. 5. Typical nitrogen adsorption–desorption isotherm and BJH pore size distribution plots (inset) of porous Co3O4 nanobelts.
Fig. 6. XRD pattern of the cobalt precursors after the hydrothermal treatment at 180 C for 12 h.
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Fig.
7. FT-IR spectrum of the obtained cobalt precursors.
cobalt precursors was also investigated. As is shown in Fig. S2a and Fig. 9a, irregular plates and bulks are gradually changed into beltlike nanostructures with increasing the amount of Co(NO3)26H2O form 0.15 g to 0.5 g. And the polyhedron-like microstructures (Fig. S2b) are formed when the amount of Co(NO3)26H2O was further added to 1.5 g. Fig. S3 investigated the effect of the hydrothermal temperature on the morphologies of cobalt precursors. The morphologies of the cobalt precursors (Fig. S3a, b) display mostly the rod-like nanostructures when the hydrothermal temperature was 100 C and 130 C. However, aggregates composed of fine stripes and small amount of rods were observed in Fig. S3c when the hydrothermal temperature was enhanced to 150 C. Therefore, as described above, the morphologies of the cobalt precursors can be tuned via adjusting the reaction parameters including the molar ratio of Co(NO3)26H2O to C2H4N4, the amount of Co(NO3)26H2O and the hydrothermal temperature. In order to make out the formation mechanism of porous Co3O4 nanobelts, a series of time-dependent experiments were carried out and the intermediate products were characterized by SEM. Fig. 10 presents the SEM images of the intermediate products collected after the hydrothermal reaction performed for 3 h, 6 h, 8 h at 180 C. These images reveal a morphological evolution of Co3O4 structures from flower-like microstructures to wire-like nanostructures. In the earlier reaction stage (3 h), 3D flower-like microstructures in Fig. 10a with a size of about 10 um were formed from the self-assembly of microplates. However, the obtained products were mainly composed of 2D microplate fragments
Fig.
8. TGA and DrTGA curves of the obtained cobalt precursors.
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(Fig. 10b) when the hydrothermal time was prolonged to 6 h. With further increasing the reaction time to 8 h, aggregated nanowires mixed with small amount of microplate fragments were observed, as shown in Fig. 10c,d. Finally, nanobelts were formed shown in Fig. 9a when the hydrothermal time was enhanced to 12 h. On the basis of the investigations described above, a possible formation mechanism of porous Co3O4 nanobelts was proposed and schematically illustrated in Scheme 1. At the first stage, the cobalt precursor nuclei were formed from the precipitation reaction between Co2+ and CO32 in the presence of OH in the solution. Then, these nuclei began to grow and tended to aggregate to nanoplates owing to high surface energies. Afterwards, these nanoplates were assembled into 3D flower-like microstructures. But it was difficult to maintain the 3D flower-like structures in the growth and re-crystallization of crystallites with prolonging the reaction time. As a result, the 2D microplate fragments were obtained and further transformed to 1D nanowires. Subsequently, 1D nanobelts were formed via the aggregative lateral attachment among nanowires. Finally, porous Co3O4 nanobelts were obtained from the thermal decomposition of cobalt precursor nanobelts, accompanied by the release of CO2 and H2O gases during the calcination process. The detailed mechanism for the formation of porous Co3O4 belts is still under further investigation. To demonstrate the gas sensing performance of the obtained porous Co3O4 nanobelts, the sensitivity of their response to ethanol was investigated. Fig. 11 presents an ethanol (200 ppm) sensing curve of porous Co3O4 nanobelts at different operating temperatures. The maximum sensitivity to ethanol is observed at 200 C,
Fig. 9. Low- (a) and high-magnification (b) SEM images of the obtained cobalt precursors.
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Fig. 10. SEM images of the intermediate products collected after the hydrothermal reaction performed for 3 h (a), 6 h (b), 8 h (c, d) at 180 C.
Scheme
1. Illustration of the formation process of porous Co3O4 nanobelts.
Fig. 11. Gas response of porous Co3O4 nanobelts as a function of different operating temperatures to 200 ppm ethanol.
indicating the optimum working temperature for the porous Co3O4 nanobelts sensor. Fig. 12a shows the real-time response–recovery curves for both porous Co3O4 nanobelts and commercial Co3O4 powders upon exposure to different concentrations of ethanol at a working temperature of 200 C. It can be seen that the output voltages abruptly decreased and then quickly reached a relatively stable value when porous Co3O4 nanobelts and commercial Co3O4 powders were exposed to alcohol gas with a concentration ranging from 5 ppm to 500 ppm. The decreased voltage values of the former are almost twice those of the latter, meaning that porous Co3O4 nanobelts exhibit higher gas sensing capabilities towards alcohol than those of commercial Co3O4 powders. It is also proved by the corresponding sensitivity curves (Fig. 12b) for both porous Co3O4 nanobelts and commercial Co3O4 powders. The higher sensitivity of porous Co3O4 nanobelts can be attributed to their special structures, i.e., high surface-to-volume ratio and porous structures, which is favorable for increasing the electron transfer
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References
Fig. 12. Real-time gas response curves (a) and the corresponding sensor response curves (b) of both porous Co3O4 nanobelts and commercial Co3O4 powders towards different concentrations of ethanol at a working temperature of 200 C.
caused by the chemical reaction between the active sites and alcohol gas. 7. Conclusions In summary, porous Co3O4 nanobelts have been successfully synthesized via a facile hydrothermal method followed by the direct thermal decomposition. The belt-like morphology can be finely controlled via adjusting the reaction parameters including the molar ratio of Co(NO3)26H2O to C2H4N4, the amount of Co (NO3)26H2O, the hydrothermal temperature and time. Ethanol gas sensing measurements indicate that the obtained porous Co3O4 nanobelts show higher sensitivity at a working temperature of 200 C compared with the commercial Co3O4 powders, suggesting their potential applications in nanosensors. Acknowledgements The authors gratefully acknowledge the financial supports from the National Natural Science Foundation of China (grant no. 21206025) and the Natural Science Foundation of Hebei Province (grant no. B2013402008). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.materresbull.2014.06.033.
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