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Synthesis of NiO nanostructures from 1D to 3D and researches of their gas-sensing properties
Materials Research Bulletin 48 (2013) 449–454
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Synthesis of NiO nanostructures from 1D to 3D and researches of their gas-sensing properties Liyang Lin a,b, Tianmo Liu a,b,*, Bin Miao a,b, Wen Zeng a,b,c,* a
College of Materials Science and Engineering, Chongqing University, Chongqing 400030, China National Engineering Research Center for Magnesium Alloys, Chongqing 400030, China c State Key Laboratory of Power Transmission Equipment and System Security and New Technology, Chongqing University, Chongqing 400030, China b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 23 July 2012 Received in revised form 17 September 2012 Accepted 31 October 2012 Available online 12 November 2012
We successfully synthesized four kinds of NiO from 1D to 3D as follows: NiO nanowires, nanosheets, nanobulks and nanospheres. With our careful preparation and accurate reagent ratio, different and beautiful morphologies were obtained through the hydrothermal process only by changing the active agent. We mainly chose NiC2O42H2O as the precursors of NiO and found that the existence of ethylene glycol (EG), polyethylene glycol (PEG) and glycine played an important role in synthesis of disparate NiO nanostructures. The gas-sensing performances of the as-prepared NiO nanostructures with different dimensions were investigated toward ethanol at 350 8C. NiO nanowires present the better gas sensing properties and the corresponding response value (Rg/Ra), response time and recovery time were about 3.4, 4 s, and 5 s to 50 ppm ethanol, respectively. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures A. Oxides C. Electron microscopy D. Crystal structure
1. Introduction As an important p-type semiconductor with wide band gap energy in the range of 3.6–4.0 eV [1], NiO nanomaterials have been the eye-catching in the fields of magnetic materials [2], electrochemical catalysts [3], fuel cell electrodes [4], photocatalyst [5] and gas sensors [6–14,30,31]. Moreover, NiO is also a promising material for detecting some inconspicuous gases, for example, CO [15], H2 [16], alcohol [17,18], toluene [19], and methanol [20]. Currently, large researches have been done toward how to fabricate NiO with powerful performances, which is determined by their nanostructures. Many preparation methods such as vapor deposition process (VDP) [21], sol–gel process [22] and reactive gas deposition [23] are used. However, most of preparation techniques are confined to rigorous terms like high temperature, high vacuum and complex reactions, which lead to expensive cost and prevent the preparation from wide applications. Thus, it is necessary to find a particular method that can not only synthesize NiO nanostructure with novel morphologies but can also have the characteristics of briefness and handling ability. Compared with other methods, hydrothermal synthetic method
* Corresponding authors at: College of Materials Science and Engineering, Chongqing University, Chongqing 400030, China. Tel.: +81 22 217 5933; fax: +81 22 217 5930. E-mail addresses: [email protected] (T. Liu), [email protected] (W. Zeng). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.10.046
has attracted the broadest attention for its simple operation and low power consumption. Due to the widespread adhibition of hydrothermal technique, single dimensional or multidimensional morphologies could be prepared. Here, we give a definition of 1D to 3D. Generally, 1D structure refers to the line that was arranged by the countless point. Moreover, we regard the 2D structure as one kind of morphology that involves two dimensions including plate, disk, flake, etc. The 3D structure involves or relates to three dimensions or aspects including flower-like, microsphere, etc. As is well known, NiO could not be synthesized directly by hydrothermal method, but thermal decomposition of nickelous precursors like Ni(OH)2 and NiC2O42H2O made it possible. Ni et al. [24] reported the synthesis of NiO flowers using the former b-Ni(OH)2 as the precursor. Liu et al. [25] reported that they could obtain the urchinlike NiO by heating Ni2(CO3)(OH)2 precursors which was obtained under the Ni(NO3)26H2O-CO(NH2)2–PEG–H2O hydrothermal system. It is not difficult for us to make different morphologies and stabilized nanomaterials by controlling the reaction time, temperature, concentration, pH and active agents [26]. But so many influencing factors make it difficult to control the final stable morphology, and it is rare to get a variety of morphologies only by changing the active agent. Herein, we report the preparation of NiO nanowires, nanosheets, nanobulks and nanospheres via hydrothermal method. Particularly, NiO nanospheres were firstly reported in the field of NiO nanostructures by hydrothermal technique. The functions of active agents such as EG, PEG and glycine that make different
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Fig. 1. (a) Schematic of the gas sensor configuration and (b) the electric circuit which is used to test the gas-sensing properties.
morphologies of NiO would be discussed. Furthermore, the gassensing performances of the as-prepared NiO nanostructures with different dimensions were investigated toward ethanol.
2. Experiments 2.1. Preparation of NiO with different morphologies All reagents used were of analytical purity and were directly used without further purification.
2.1.1. NiO nanowires In a typical procedure, 0.476 g (2.0 mmol) of NiCl26H2O (98.0%) was dissolved into 15.0 ml distilled water to form a green clear solution in a beaker of 50 ml capacity. Then 0.134 g (1.0 mmol) of NaC2O4 (99.8%), 0.1 g of PEG (MW = 6000) and 25.0 ml of EG (99.5%) were added to the beaker. The mixture was magnetically stirred for 15 min to give a transparent solution and transferred into a Teflon-lined stainless steel autoclave, sealed and maintained at 180 8C for 15 h. After the reaction was completed, the autoclave was cooled to the room temperature naturally. The blue-green products were harvested by centrifugation, washed
Fig. 2. SEM images of NiO samples with different morphologies: (a) nanowires, (b) nanosheets, (c) nanobulks and (d) nanospheres. The corresponding enlarged SEM images are also shown at the left bottom.
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with distilled water and ethanol three times, respectively, to remove unexpected ions, and then dried at 60 8C in vacuum. The powder was heated to 400 8C with a rate of 1.0 8C min1 and then calcined at 400 8C for 2 h.
were collected by centrifugation and washed three times with distilled water and ethanol, respectively. The powder was heated to 400 8C with a rate of 1.0 8C min1 and then calcined at 400 8C for 2 h.
2.1.2. NiO nanosheets 0.237 g (1.0 mmol) of NiCl26H2O, 0.067 g (0.5 mmol) of NaC2O4 and 16.0 ml distilled water were added into a beaker of 50 ml capacity. Then 0.375 g (5.0 mmol) of glycine (99.0%) and 30 ml of EG were added to the beaker. The mixture was magnetically stirred for 30 min to form a clear solution and transferred into a Teflonlined stainless steel autoclave, sealed and maintained at 180 8C for 12 h. After the heating treatment, autoclave was cooled to room temperature naturally. The products were collected by centrifugation and washed three times with distilled water and ethanol, respectively. The as-prepared products were heated to 400 8C with a rate of 1.0 8C min1 and then maintained at 400 8C for 2 h.
2.2. Structure characterization
2.1.3. NiO nanobulks The reaction process is similar to that we mentioned in Section 2.1.1 but without adding the EG and PEG. 2.1.4. NiO nanospheres 0.237 g (1.0 mmol) of NiCl26H2O was dissolved into 20.0 ml distilled water to form a green clear solution in a beaker of 50 ml capacity. Then 0.294 g (1.0 mmol) of C6H5Na3O72H2O (99.5%) and 25.0 ml EG were added to the beaker. The mixture was magnetically stirred for 15 min to give a transparent solution and transferred into a Teflon-lined stainless steel autoclave, sealed and maintained at 190 8C for 12 h. After the heating treatment, autoclave was cooled to room temperature naturally. The products
The as-prepared products were characterized by the X-ray diffraction (XRD), a Rigaku D/Max-1200X diffractometry with the Cu Ka radiation, employing a scanning rate 0.028 s1 in the 2u ranging from 108 to 808. The microstructures of as-prepared samples were characterized with a Nova 400 Nano field emission scanning electronic microscopy (FE-SEM). 2.3. Gas-sensing property characterization Firstly, it is necessary to prepare the gas sensors. The schematic of the gas sensor configuration is shown in Fig. 1(a). It can be seen that gas sensor is made up of four kinds of significant components. The gray tube-like structure is alumina ceramic tube as the matrix and there are two annular substances made of Au electrode on each side of it. The Ni–Cr heater is inserted into the alumina ceramic tube to control the temperature when the test begins. Four Pt wires were connected to the Au electrodes closely. The coating on the surface of the matrix consists of as-prepared powders. The powders were dispersed in distilled water and then added into the ultrasonic oscillator. After a long time, the obtained pastes would be coated onto the matrix and Au electrodes. To enhance mechanical adhesion of the pastes to the matrix, a small amount of sodium carboxy methylcellulose was added into the mixed powders as organic binder [27]. The thickness of the coating
Fig. 3. SEM images of precursors with different morphologies: (a) nanowires, (b) nanosheets, (c) nanobulks and (d) nanospheres. The corresponding enlarged SEM images are also shown at the left bottom.
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should be controlled to about 1 mm. At the end, the organic binder must be removed from the matrix by heat treatment. We set the heat treatment temperature at 300 8C that remained for about 2 h. Gas-sensing properties were measured with HW-30A gas sensitivity instrument (Hanwei Electronics Co., Ltd.). Fig. 1(b) reveals the main circuit diagram. The resistance (Rs) value of gas sensor can be obtained via the following formula: Rs = Rl(Vm Vout)/Vout, where Vm is the circuit voltage and Vout is the output voltage. The sensor response (Sr) to the test gas was defined as Rg/Ra, where Rg and Ra were the resistance of the sensors in target gases and air, respectively. The response and recovery time was counted as the interval between when the response reached 90% of its maximum and when it dropped to 10% of its maximum. 3. Results and discussion 3.1. SEM and XRD analysis The morphologies of NiO nanowires, nanosheets, nanobulks and nanospheres were successfully observed via the FE-SEM and shown in Fig. 2 from (a) to (d), respectively. Fig. 2(a) shows that the products consist of wire-like structures with the lengths of about 12 mm and the diameters of 80–100 nm. The nanowires do not coagulate and can be observed distinctly. Fig. 2(b) shows the sheetlike structures. Irregular sheets with smooth surface distribute layer-by-layer uniformly can be seen. The chaotic nanobulks with the length of 300–500 nm, the width of 200–300 nm and the thickness of about 100 nm are shown in Fig. 2(c). In Fig. 2(d), beautiful nanospheres with the diameter of 3 mm are uniform and dispersed. When compared with Fig. 3, we could find that these four nanostructures almost did not have a change even if we add as-prepared precursors into the calciner at 400 8C for 2 h. Fig. 4 shows the XRD patterns, which characterize crystalline structures of the as-prepared precursors. Fig. 4a(I)–a(III), corresponds to the nanowires, nanosheets and nanobulks, respectively; the diffraction data are all in perfect agreement with the standard spectrum (JCPDS No. 25-0581), demonstrating that the precursors must be NiC2O42H2O. It can be seen that the obvious diffraction of the peaks at 2u = 18.88, 22.78, 25.18 and 30.48 correspond to the characteristic diffraction of the (1¯ 0 2), (0 0 2), (1¯ 1 2) and (4¯ 0 2) crystal planes of the NiC2O42H2O, respectively. And the X-ray diffractometer of the spheres-like sample is shown in Fig. 4a(IV).
Fig. 4. (a) XRD spectra of the precursors with different morphologies: (I) nanowires, (II) nanosheets, (III) nanobulks and (IV) nanospheres. (b) XRD spectra of NiO samples with different morphologies: (I) nanowires, (II) nanosheets, (III) nanobulks and (IV) nanospheres.
Fig. 5. Schematic diagrams of the formation process of wire-like, sheet-like, bulk-like and sphere-like products nanostructures.
L. Lin et al. / Materials Research Bulletin 48 (2013) 449–454
3.2. Growth mechanisms
4.5 4.0 3.5
Voltage (V)
Although there were two kinds of precursors, the products we obtained after calcining the precursors at 400 8C were both the pure NiO. In accordance with the standard spectrum (JCPDS No. 040835), the result was indicated in the XRD pattern in Fig. 4(b). The diffraction peaks at the angle of 2u = 37.28, 43.28, 62.88, 75.28 and 79.48 in the four products are similar to each other, which can be indexed as (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) lattice planes, respectively.
453
3.0 2.5 2.0 1.5
The above-mentioned different morphologies (in Fig. 2) of samples had been analyzed based on these experimental results, and a series of possible mechanisms of the growth process were described by constructing schematic diagram as shown in Fig. 5. Fig. 5(a) corresponds to the formation process of nanowires. As a kind of organic polymer with a long chain, PEG is usually used as a surface-active agent. In this condition, we could regard the PEG as the substrate offering an embeddable long-chain. Due to the master plate, NiC2O42H2O nanoparticles can be easily grown at the surface of the PEG. And at the other face of the crystal, EG preferentially connects to the surface keeping from the growth of NiC2O42H2O nanoparticles. With the contribution of both PEG and EG, the well-dispersed nanowires were successfully obtained. Without the addition of PEG but by adding the glycine, the obtained structure changed from wires to sheets. As shown in Fig. 5(b), it could be considered that the EG limited the growth along one direction and the glycine agglomerated the grains to form smooth plates. When compared with nanowires, the nanobulks could be illustrated distinctly. With the absence of PEG and EG, NiC2O42H2O grow along six directions, which we could see in Fig. 5(c). In Fig. 5(d), it could be seen that citrate ion played an important role in reducing surface energy. The nanoparticles of precursor formed spherical structure and then was grown layer-by-layer.
Nanowire Nanosheet Nanobulk Nanosphere
1.0 0.5 0.0 0
30
60
90
120
150
180
Time (s) Fig. 6. The response–recovery characteristics of NiO sensors to the ethanol gas.
3.3. Gas-sensing properties and mechanism We fabricated four kinds of gas sensors by using NiO nanowires, nanosheets, nanobulks and nanospheres separately and the gas sensing properties are measured at 350 8C under 50 ppm of ethanol. Fig. 6 shows the response–recovery characteristics of NiO sensors to the ethanol gas. From the graph, we can clearly see that the voltage value changed less abruptly when the test gas was injected into the system and increased fleetly to its initial value after the ethanol was released. As displayed by the experimental data curves, this phenomenon corresponded with the characteristics of NiO, one of the p-type semiconductor
Fig. 7. Sensitivity of the NiO sensors to the ethanol gas.
sensors [28,29]. According to the above definitions of response time and recovery time, it can be evaluated to be 4 and 5 s for the nanowires, 6 and 9 s for the nanosheets, 5 and 7 s for the nanobulks, 8 and 5 s for the nanospheres. Fig. 7 shows that gas response of four sensors to the ethanol is estimated to be 3.4, 1.8, 1.5, and 2.5. Based on the response–recovery time and sensitivity
Fig. 8. Schematic of an electron transfer process on the surface of NiO.
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values, NiO nanowires presented the best gas sensing properties among four sensors. We described the reaction processes as follows: O2 gas þ e $ O2 ads
(1)
frac12;O2 þ e $ O ads
(2)
2
(3)
Acknowledgement This work was supported by the Fundamental Research Funds for the Central Universities (No. CDJZR12110051). References
frac12;O2 þ 2e $ O
ads
CH3 CH2 OHads þ O ads $ H2 O þ CO2 þ e
(4)
CH3 CH2 OHads þ O2 ads $ H2 O þ CO2 þ 2e
(5)
As we know, the oxygen in the air content is 19% and this means that the air should have a certain ability of oxidation. On the atomic scale, the oxygen can receive electrons from the Ni2+ and absorb at the surface in the form of O or O2; then, the Ni2+ will be oxidized to Ni3+, as shown in Fig. 8. The increase of the holes contributes to the electrical conductivity, that is, the resistance of NiO samples is low in the air. When we added the target gas into the system, the ethanol would seize the electrons back from oxygen atoms owing to its stronger reducibility. It is no doubt that the resistance goes up abruptly. 4. Conclusion In summary, we had successfully synthesized four kinds of NiO nanostructures from 1D to 3D by the hydrothermal reaction. The morphology of NiO nanospheres which was shown in this article had never been observed in the field of NiO nanostructures via the hydrothermal method until now. We kept the same range of circumstance just by changing the active agents to find that PEG, EG and glycine have obvious effects on the formation of different products; moreover, their existences affect the final NiO morphologies directly. By the measurement of gas sensing properties, NiO nanowires showed the perfect performance toward the ethanol.
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Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Synthesis of NiO nanostructures from 1D to 3D and researches of their gas-sensing properties Liyang Lin a,b, Tianmo Liu a,b,*, Bin Miao a,b, Wen Zeng a,b,c,* a
College of Materials Science and Engineering, Chongqing University, Chongqing 400030, China National Engineering Research Center for Magnesium Alloys, Chongqing 400030, China c State Key Laboratory of Power Transmission Equipment and System Security and New Technology, Chongqing University, Chongqing 400030, China b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 23 July 2012 Received in revised form 17 September 2012 Accepted 31 October 2012 Available online 12 November 2012
We successfully synthesized four kinds of NiO from 1D to 3D as follows: NiO nanowires, nanosheets, nanobulks and nanospheres. With our careful preparation and accurate reagent ratio, different and beautiful morphologies were obtained through the hydrothermal process only by changing the active agent. We mainly chose NiC2O42H2O as the precursors of NiO and found that the existence of ethylene glycol (EG), polyethylene glycol (PEG) and glycine played an important role in synthesis of disparate NiO nanostructures. The gas-sensing performances of the as-prepared NiO nanostructures with different dimensions were investigated toward ethanol at 350 8C. NiO nanowires present the better gas sensing properties and the corresponding response value (Rg/Ra), response time and recovery time were about 3.4, 4 s, and 5 s to 50 ppm ethanol, respectively. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures A. Oxides C. Electron microscopy D. Crystal structure
1. Introduction As an important p-type semiconductor with wide band gap energy in the range of 3.6–4.0 eV [1], NiO nanomaterials have been the eye-catching in the fields of magnetic materials [2], electrochemical catalysts [3], fuel cell electrodes [4], photocatalyst [5] and gas sensors [6–14,30,31]. Moreover, NiO is also a promising material for detecting some inconspicuous gases, for example, CO [15], H2 [16], alcohol [17,18], toluene [19], and methanol [20]. Currently, large researches have been done toward how to fabricate NiO with powerful performances, which is determined by their nanostructures. Many preparation methods such as vapor deposition process (VDP) [21], sol–gel process [22] and reactive gas deposition [23] are used. However, most of preparation techniques are confined to rigorous terms like high temperature, high vacuum and complex reactions, which lead to expensive cost and prevent the preparation from wide applications. Thus, it is necessary to find a particular method that can not only synthesize NiO nanostructure with novel morphologies but can also have the characteristics of briefness and handling ability. Compared with other methods, hydrothermal synthetic method
* Corresponding authors at: College of Materials Science and Engineering, Chongqing University, Chongqing 400030, China. Tel.: +81 22 217 5933; fax: +81 22 217 5930. E-mail addresses: [email protected] (T. Liu), [email protected] (W. Zeng). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.10.046
has attracted the broadest attention for its simple operation and low power consumption. Due to the widespread adhibition of hydrothermal technique, single dimensional or multidimensional morphologies could be prepared. Here, we give a definition of 1D to 3D. Generally, 1D structure refers to the line that was arranged by the countless point. Moreover, we regard the 2D structure as one kind of morphology that involves two dimensions including plate, disk, flake, etc. The 3D structure involves or relates to three dimensions or aspects including flower-like, microsphere, etc. As is well known, NiO could not be synthesized directly by hydrothermal method, but thermal decomposition of nickelous precursors like Ni(OH)2 and NiC2O42H2O made it possible. Ni et al. [24] reported the synthesis of NiO flowers using the former b-Ni(OH)2 as the precursor. Liu et al. [25] reported that they could obtain the urchinlike NiO by heating Ni2(CO3)(OH)2 precursors which was obtained under the Ni(NO3)26H2O-CO(NH2)2–PEG–H2O hydrothermal system. It is not difficult for us to make different morphologies and stabilized nanomaterials by controlling the reaction time, temperature, concentration, pH and active agents [26]. But so many influencing factors make it difficult to control the final stable morphology, and it is rare to get a variety of morphologies only by changing the active agent. Herein, we report the preparation of NiO nanowires, nanosheets, nanobulks and nanospheres via hydrothermal method. Particularly, NiO nanospheres were firstly reported in the field of NiO nanostructures by hydrothermal technique. The functions of active agents such as EG, PEG and glycine that make different
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L. Lin et al. / Materials Research Bulletin 48 (2013) 449–454
Fig. 1. (a) Schematic of the gas sensor configuration and (b) the electric circuit which is used to test the gas-sensing properties.
morphologies of NiO would be discussed. Furthermore, the gassensing performances of the as-prepared NiO nanostructures with different dimensions were investigated toward ethanol.
2. Experiments 2.1. Preparation of NiO with different morphologies All reagents used were of analytical purity and were directly used without further purification.
2.1.1. NiO nanowires In a typical procedure, 0.476 g (2.0 mmol) of NiCl26H2O (98.0%) was dissolved into 15.0 ml distilled water to form a green clear solution in a beaker of 50 ml capacity. Then 0.134 g (1.0 mmol) of NaC2O4 (99.8%), 0.1 g of PEG (MW = 6000) and 25.0 ml of EG (99.5%) were added to the beaker. The mixture was magnetically stirred for 15 min to give a transparent solution and transferred into a Teflon-lined stainless steel autoclave, sealed and maintained at 180 8C for 15 h. After the reaction was completed, the autoclave was cooled to the room temperature naturally. The blue-green products were harvested by centrifugation, washed
Fig. 2. SEM images of NiO samples with different morphologies: (a) nanowires, (b) nanosheets, (c) nanobulks and (d) nanospheres. The corresponding enlarged SEM images are also shown at the left bottom.
L. Lin et al. / Materials Research Bulletin 48 (2013) 449–454
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with distilled water and ethanol three times, respectively, to remove unexpected ions, and then dried at 60 8C in vacuum. The powder was heated to 400 8C with a rate of 1.0 8C min1 and then calcined at 400 8C for 2 h.
were collected by centrifugation and washed three times with distilled water and ethanol, respectively. The powder was heated to 400 8C with a rate of 1.0 8C min1 and then calcined at 400 8C for 2 h.
2.1.2. NiO nanosheets 0.237 g (1.0 mmol) of NiCl26H2O, 0.067 g (0.5 mmol) of NaC2O4 and 16.0 ml distilled water were added into a beaker of 50 ml capacity. Then 0.375 g (5.0 mmol) of glycine (99.0%) and 30 ml of EG were added to the beaker. The mixture was magnetically stirred for 30 min to form a clear solution and transferred into a Teflonlined stainless steel autoclave, sealed and maintained at 180 8C for 12 h. After the heating treatment, autoclave was cooled to room temperature naturally. The products were collected by centrifugation and washed three times with distilled water and ethanol, respectively. The as-prepared products were heated to 400 8C with a rate of 1.0 8C min1 and then maintained at 400 8C for 2 h.
2.2. Structure characterization
2.1.3. NiO nanobulks The reaction process is similar to that we mentioned in Section 2.1.1 but without adding the EG and PEG. 2.1.4. NiO nanospheres 0.237 g (1.0 mmol) of NiCl26H2O was dissolved into 20.0 ml distilled water to form a green clear solution in a beaker of 50 ml capacity. Then 0.294 g (1.0 mmol) of C6H5Na3O72H2O (99.5%) and 25.0 ml EG were added to the beaker. The mixture was magnetically stirred for 15 min to give a transparent solution and transferred into a Teflon-lined stainless steel autoclave, sealed and maintained at 190 8C for 12 h. After the heating treatment, autoclave was cooled to room temperature naturally. The products
The as-prepared products were characterized by the X-ray diffraction (XRD), a Rigaku D/Max-1200X diffractometry with the Cu Ka radiation, employing a scanning rate 0.028 s1 in the 2u ranging from 108 to 808. The microstructures of as-prepared samples were characterized with a Nova 400 Nano field emission scanning electronic microscopy (FE-SEM). 2.3. Gas-sensing property characterization Firstly, it is necessary to prepare the gas sensors. The schematic of the gas sensor configuration is shown in Fig. 1(a). It can be seen that gas sensor is made up of four kinds of significant components. The gray tube-like structure is alumina ceramic tube as the matrix and there are two annular substances made of Au electrode on each side of it. The Ni–Cr heater is inserted into the alumina ceramic tube to control the temperature when the test begins. Four Pt wires were connected to the Au electrodes closely. The coating on the surface of the matrix consists of as-prepared powders. The powders were dispersed in distilled water and then added into the ultrasonic oscillator. After a long time, the obtained pastes would be coated onto the matrix and Au electrodes. To enhance mechanical adhesion of the pastes to the matrix, a small amount of sodium carboxy methylcellulose was added into the mixed powders as organic binder [27]. The thickness of the coating
Fig. 3. SEM images of precursors with different morphologies: (a) nanowires, (b) nanosheets, (c) nanobulks and (d) nanospheres. The corresponding enlarged SEM images are also shown at the left bottom.
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should be controlled to about 1 mm. At the end, the organic binder must be removed from the matrix by heat treatment. We set the heat treatment temperature at 300 8C that remained for about 2 h. Gas-sensing properties were measured with HW-30A gas sensitivity instrument (Hanwei Electronics Co., Ltd.). Fig. 1(b) reveals the main circuit diagram. The resistance (Rs) value of gas sensor can be obtained via the following formula: Rs = Rl(Vm Vout)/Vout, where Vm is the circuit voltage and Vout is the output voltage. The sensor response (Sr) to the test gas was defined as Rg/Ra, where Rg and Ra were the resistance of the sensors in target gases and air, respectively. The response and recovery time was counted as the interval between when the response reached 90% of its maximum and when it dropped to 10% of its maximum. 3. Results and discussion 3.1. SEM and XRD analysis The morphologies of NiO nanowires, nanosheets, nanobulks and nanospheres were successfully observed via the FE-SEM and shown in Fig. 2 from (a) to (d), respectively. Fig. 2(a) shows that the products consist of wire-like structures with the lengths of about 12 mm and the diameters of 80–100 nm. The nanowires do not coagulate and can be observed distinctly. Fig. 2(b) shows the sheetlike structures. Irregular sheets with smooth surface distribute layer-by-layer uniformly can be seen. The chaotic nanobulks with the length of 300–500 nm, the width of 200–300 nm and the thickness of about 100 nm are shown in Fig. 2(c). In Fig. 2(d), beautiful nanospheres with the diameter of 3 mm are uniform and dispersed. When compared with Fig. 3, we could find that these four nanostructures almost did not have a change even if we add as-prepared precursors into the calciner at 400 8C for 2 h. Fig. 4 shows the XRD patterns, which characterize crystalline structures of the as-prepared precursors. Fig. 4a(I)–a(III), corresponds to the nanowires, nanosheets and nanobulks, respectively; the diffraction data are all in perfect agreement with the standard spectrum (JCPDS No. 25-0581), demonstrating that the precursors must be NiC2O42H2O. It can be seen that the obvious diffraction of the peaks at 2u = 18.88, 22.78, 25.18 and 30.48 correspond to the characteristic diffraction of the (1¯ 0 2), (0 0 2), (1¯ 1 2) and (4¯ 0 2) crystal planes of the NiC2O42H2O, respectively. And the X-ray diffractometer of the spheres-like sample is shown in Fig. 4a(IV).
Fig. 4. (a) XRD spectra of the precursors with different morphologies: (I) nanowires, (II) nanosheets, (III) nanobulks and (IV) nanospheres. (b) XRD spectra of NiO samples with different morphologies: (I) nanowires, (II) nanosheets, (III) nanobulks and (IV) nanospheres.
Fig. 5. Schematic diagrams of the formation process of wire-like, sheet-like, bulk-like and sphere-like products nanostructures.
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3.2. Growth mechanisms
4.5 4.0 3.5
Voltage (V)
Although there were two kinds of precursors, the products we obtained after calcining the precursors at 400 8C were both the pure NiO. In accordance with the standard spectrum (JCPDS No. 040835), the result was indicated in the XRD pattern in Fig. 4(b). The diffraction peaks at the angle of 2u = 37.28, 43.28, 62.88, 75.28 and 79.48 in the four products are similar to each other, which can be indexed as (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) lattice planes, respectively.
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3.0 2.5 2.0 1.5
The above-mentioned different morphologies (in Fig. 2) of samples had been analyzed based on these experimental results, and a series of possible mechanisms of the growth process were described by constructing schematic diagram as shown in Fig. 5. Fig. 5(a) corresponds to the formation process of nanowires. As a kind of organic polymer with a long chain, PEG is usually used as a surface-active agent. In this condition, we could regard the PEG as the substrate offering an embeddable long-chain. Due to the master plate, NiC2O42H2O nanoparticles can be easily grown at the surface of the PEG. And at the other face of the crystal, EG preferentially connects to the surface keeping from the growth of NiC2O42H2O nanoparticles. With the contribution of both PEG and EG, the well-dispersed nanowires were successfully obtained. Without the addition of PEG but by adding the glycine, the obtained structure changed from wires to sheets. As shown in Fig. 5(b), it could be considered that the EG limited the growth along one direction and the glycine agglomerated the grains to form smooth plates. When compared with nanowires, the nanobulks could be illustrated distinctly. With the absence of PEG and EG, NiC2O42H2O grow along six directions, which we could see in Fig. 5(c). In Fig. 5(d), it could be seen that citrate ion played an important role in reducing surface energy. The nanoparticles of precursor formed spherical structure and then was grown layer-by-layer.
Nanowire Nanosheet Nanobulk Nanosphere
1.0 0.5 0.0 0
30
60
90
120
150
180
Time (s) Fig. 6. The response–recovery characteristics of NiO sensors to the ethanol gas.
3.3. Gas-sensing properties and mechanism We fabricated four kinds of gas sensors by using NiO nanowires, nanosheets, nanobulks and nanospheres separately and the gas sensing properties are measured at 350 8C under 50 ppm of ethanol. Fig. 6 shows the response–recovery characteristics of NiO sensors to the ethanol gas. From the graph, we can clearly see that the voltage value changed less abruptly when the test gas was injected into the system and increased fleetly to its initial value after the ethanol was released. As displayed by the experimental data curves, this phenomenon corresponded with the characteristics of NiO, one of the p-type semiconductor
Fig. 7. Sensitivity of the NiO sensors to the ethanol gas.
sensors [28,29]. According to the above definitions of response time and recovery time, it can be evaluated to be 4 and 5 s for the nanowires, 6 and 9 s for the nanosheets, 5 and 7 s for the nanobulks, 8 and 5 s for the nanospheres. Fig. 7 shows that gas response of four sensors to the ethanol is estimated to be 3.4, 1.8, 1.5, and 2.5. Based on the response–recovery time and sensitivity
Fig. 8. Schematic of an electron transfer process on the surface of NiO.
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values, NiO nanowires presented the best gas sensing properties among four sensors. We described the reaction processes as follows: O2 gas þ e $ O2 ads
(1)
frac12;O2 þ e $ O ads
(2)
2
(3)
Acknowledgement This work was supported by the Fundamental Research Funds for the Central Universities (No. CDJZR12110051). References
frac12;O2 þ 2e $ O
ads
CH3 CH2 OHads þ O ads $ H2 O þ CO2 þ e
(4)
CH3 CH2 OHads þ O2 ads $ H2 O þ CO2 þ 2e
(5)
As we know, the oxygen in the air content is 19% and this means that the air should have a certain ability of oxidation. On the atomic scale, the oxygen can receive electrons from the Ni2+ and absorb at the surface in the form of O or O2; then, the Ni2+ will be oxidized to Ni3+, as shown in Fig. 8. The increase of the holes contributes to the electrical conductivity, that is, the resistance of NiO samples is low in the air. When we added the target gas into the system, the ethanol would seize the electrons back from oxygen atoms owing to its stronger reducibility. It is no doubt that the resistance goes up abruptly. 4. Conclusion In summary, we had successfully synthesized four kinds of NiO nanostructures from 1D to 3D by the hydrothermal reaction. The morphology of NiO nanospheres which was shown in this article had never been observed in the field of NiO nanostructures via the hydrothermal method until now. We kept the same range of circumstance just by changing the active agents to find that PEG, EG and glycine have obvious effects on the formation of different products; moreover, their existences affect the final NiO morphologies directly. By the measurement of gas sensing properties, NiO nanowires showed the perfect performance toward the ethanol.
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