Ultrasonics Sonochemistry 21 (2014) 1707–1713
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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Template-free sonochemical synthesis of hierarchically porous NiO microsphere Wenli Zhu, Anze Shui ⇑, Linfeng Xu, Xiaosu Cheng, Pingan Liu, Hui Wang College of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China
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Article history: Received 14 October 2013 Received in revised form 20 February 2014 Accepted 24 February 2014 Available online 7 March 2014 Keywords: NiO Hierarchically porous microsphere Pore Sonochemical synthesis
a b s t r a c t A novel template-free sonochemical synthesis technique was used to prepare NiO microspheres combined with calcination of NiO2.45C0.74N0.25H2.90 precursor at 500 °C. The NiO microspheres samples were systematically investigated by the thermograviometric/differential scanning calorimetry (TG/DSC), X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), fourier-transformed infrared spectroscopy (FT-IR), Brunnauer–Emmett–Teller (BET) nitrogen adsorption–desorption isotherms, laser particle size analyzer, and ultraviolet–visible spectroscopy (UV–Vis). The morphology of the precursor was retained even after the calcination process, and exhibited hierarchically porous sphericity. The morphology changed over the ultrasonic radiation time, and the shortest reaction time was 70 min, which was much less than 4 h for the mechanical stirring process. The mechanical stirring was difficult to form the complete hierarchically porous microsphere structure. The BET specific surface area and the median diameter of the hierarchically porous NiO microspheres were 103.20 m2/g and 3.436 lm, respectively. The synthesized NiO microspheres were mesoporous materials with a high fraction of macropores. The pores were resulted from the intergranular accumulation. The ultraviolet absorption spectrum showed a broad emission at the center of 475 nm, and the band gap energy was estimated to be 3.63 eV. Ó 2014 Elsevier B.V. All rights reserved.
1. Instruction Porous nano- and micro-materials have been received considerable attention due to their low density, high surface urea, good permeation and unique optical, electrical and catalytic properties [1,2]. NiO, as one of the most important transition metal oxides, is of particular interesting due to its high surface area, high specific capacitance, and low cost [3]. Furthermore, NiO is being studied for the applications in various fields, including catalyst [4,5], electrochromic film [6], gas sensor [7], battery cathode [8], fuel cell electrode [9], photovoltaic device [10], magnetic material [11], electrochemical capacitor [12], and smart window [13]. NiO nanoparticles are expected to possess more improved properties and advanced applications than the bulk-sized NiO particles owing to the volume effect, the surface effect and the quantum size effect [14]. For example, NiO nanoparticles exhibit particular superparamagnetic, superantiferromagnetic and ferromagnetic properties, whereas bulk-sized NiO is antiferromagnetic insulator with a Neel temperature (TN) of 250 °C [15,16].
⇑ Corresponding author. Tel.: +86 20 87110290; fax: +86 20 87110273. E-mail address: [email protected] (A. Shui). http://dx.doi.org/10.1016/j.ultsonch.2014.02.026 1350-4177/Ó 2014 Elsevier B.V. All rights reserved.
Generally, NiO nanoparticles could be fabricated chiefly by ultrasonic radiation [17], coprecipitation method [18,19], sol–gel method [20,21], chemical vapor deposition [22], microemulsion method [23], solid-state method [24], and so forth. Among them, sonochemical process has been recently proven to be a versatile and favorable technique for the preparation of various nanomaterials including metal, oxide, carbide, and sulfide [25]. Sonochemistry takes the advantages of the acoustic cavitation phenomenon arising during the compression and rarefaction cycles. Compression and rarefaction cycles are generated as the ultrasound waves propagate through the liquid media. The tiny micro-bubbles are created during the rarefaction cycles due to the low pressure regions produced in the liquid, and collapsed intensely through the compression cycles [29]. The very high temperature (>5000 K) and pressure (>20 MPa) are obtained upon the collapse of the bubbles, resulting in the variation in the morphology of the sonochemically synthesized micro- and nano-particles. Since the collapse occurs in less than a nanosecond, the extreme high cooling rate (1010 K s1) is also obtained. The high cooling rate hinders the organization and crystallization of the products, for this reason, the amorphous nanoparticles are obtained [26–30]. The excessive high temperature, pressure and cooling rate due to the cavitation can supply enough energy to drive the formation of novel micro- or nanomaterials under the ultrasonic irradiation.
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Thus, the sonochemical technique can accelerate the reaction, shorten the reaction time and eliminate the need of additives, however, the detailed mechanism of sonochemical actions is not understood completely [31]. It is the intrinsic superiority that the sonochemical technique is fast, facile, economical and environmentaly benign [32]. A few literatures have been reported the utilization of the ultrasound to obtain the NiO nanoparticles. Gandhi et al. [33] fabricated NiO nanoparticles via the sonochemical technique and researched their surface catalytic effect on the PVA structure. Meybodi et al. [29] synthesized a wide band gap NiO nanoparticles from Ni(OH)2 precursor by the sonochemical technique. However, to the best of our knowledge, the preparation of the hierarchically porous NiO microsphere via the sonochemical technique has rarely been reported. Herein, a template-free sonochemical technique was used to transform the mixed aqueous solution of nickle sulfate hexahydrate and urea to the precursor NiO2.45C0.74N0.25H2.90. The hierarchically porous NiO microsphere was obtained after the calcination of the precursor at 500 °C. 2. Experimental 2.1. Materials The chemicals such as NiSO4 6H2O and urea were of analytical grade and purchased from Fuchen Chemical Reagent Company, Tianjin, China. The received chemicals were used without any further purification. Distilled water was used throughout the experiments. 2.2. Preparation In a typical experimental procedure, aqueous solution of NiSO4 6H2O (0.1 M, 50 ml) was added into aqueous solution of urea (0.2 M, 250 ml) in a beaker, and stirred for 10 min to make the homogeneous precursor solution. Then the beaker was covered with polyethylene film and transferred into an ultrasonic bath at 80 °C without stirring. After the reaction performed for 80 min, the reaction mixture solution was cooled naturally overnight. The resulting green precipitate was filtered by a microporous membrane with the pore size of 0.45 lm, and washed by deionized water and anhydrous ethanol for several times successively to remove impurities, then dried at 60 °C for 12 h. The black products were finally obtained after the calcination of the green powder in air at 500 °C for 3 h. 2.3. Characterization The sonication was performed at 50 w, and 40 kHz by a locally supplied ultrasonicator (KQ50-B, China). The composition and structure of the products were analyzed by X-ray diffraction (XRD, Philips PW-1710 model X-ray diffractometer, the Netherlands) equipped with Cu-Ka radiation. The surface morphology and size of the powder were observed by a field emission scanning electron microscopy (FESEM, Nova NanoSEM 430, the Netherlands). The thermal decomposition behavior of the precursor was conducted on a thermograviometric/differential scanning calorimetry (TG/DSC, NETZSCH STA449C, Germany) at a heating rate of 10 K min1 in air to determine the proper heat temperature. Infrared absorption spectra were used to determine the structures of the precursor and the final product using the KBr pellet technique (FT-IR, Vector 33 spectrometer, Germany). The particle size distribution was taken on a laser particle size analyzer (BT-9300S, Dandong, China). The specific surface area was calculated with the Brunauer–Emmett–Teller (BET) method on a surface area analyzer
(3H-2000 III, China). The UV–Vis absorption spectrum was recorded by a UV/visible/near infrared spectrophotometer (LAMBDA950, U.S.A) with the wavelength range of 200–2500 nm at room temperature.
3. Results and discussion 3.1. Crystal structure of the as-synthesized powder Fig. 1 shows the XRD pattern of the green precipitate after drying and before the calcination. The precursor is identified as the nickle compound and the molecular formula is NiO2.45C0.74N0.25 H2.90, which is consistent with the result of Hu et al. [34]. The pattern is ‘‘saw-tooth’’ like and all peaks are broad. They are typical of turbostratic phases, which are ordered in two dimensions, but their layers are orientationally disordered, resulting in the ‘‘sawtooth’’-like reflections [35]. Broadening of XRD diffraction peaks could be attributed to the reduced grain sizes or microstructural distortion [36]. As the ultrasonic cavitation effect, an intensively mechanical shock may produce when the bubbles collapse to act on the materials. Consequently, both the small grain sizes and the microstructural strains can be considered as the influencing factors of XRD peaks broadening of the sonochemically synthesized powder. The intensity of the peak at 2h 12.5° is very strong, indicating the high crystalline of the as-synthesized precursor. 3.2. TG/DSC characterization Fig. 2 shows the TG/DSC curves of NiO2.45C0.74N0.25H2.90 at a constant heating rate of 10 K min1 in the temperature range of 40–600 °C. As it can be observed in the figure, a major weight loss which is 21.50% starting at about 300 °C. The exothermic peak at 337 °C can be explained by the decomposition of NiO2.45C0.74N0.25H2.90 to release NH3 and CO2. Based on the thermal analysis, the precursor was heated at 500 °C in air for 3 h to form NiO particles. 3.3. Crystal structure of the heat treated powder The green precursor was converted to the black powder after the heat treatment. The XRD pattern of the black powder confirms the formation of NiO, as displayed in Fig. 3. All diffraction peaks could be assigned to the face-centered cubic phase NiO (space group Fm3/m, a = 4.176, JCPDS 65-2901) at 2h 37.40°, 43.65°, 62.95°, 75.40°, and 79.45°, which is in good agreement with (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) crystal planes, respectively. The pattern is ‘‘saw-tooth’’ like and all peaks are broad. It should be mentioned that no other phases can be detected in the final
Fig. 1. XRD pattern of the precursor.
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from the hydrolysis of urea. The sharp peak at 635 cm1 corresponds to the band of dNi–O–H vibration. A small peak at 483 cm1 is attributed to the Ni–O stretching vibration. From these data, it can be concluded that the precursor is alkali nickle carbonate compound which is consistent with the phase analysis of XRD. The FT-IR spectrum of the final products is shown in Fig. 4(b). All the bands of organic groups disappear, indicated that the decomposition is completely. The intensive band at around 411 cm1 is observed which is related to the Ni–O stretching vibration of NiO6 in the face-centered cubic NiO. 3.5. Electron microscopy characterization
Fig. 2. TG/DSC curves of the nickle carbonate.
Fig. 3. XRD pattern of NiO.
products, indicated that the precursor decomposed to nickle oxide at 500 °C completely.
3.4. FT-IR spectroscopy The FT-IR spectrum of the precursor is depicted in Fig. 4(a). The absorption peak located at 3642 cm1 is attributed to O–H stretching vibration of interlayer water molecules and H-bond of OH group. The peak located at 1630 cm1 is related to the bending mode (H–O–H) of the water molecules. The broaden peak at 3574 cm1 is the absorbed band of crystal water in the complex. The strong absorption peak at 2226 cm1 is the characteristic stretching band of C„N. The peak at 1122 cm1 suggests the presence of surface carbonate or bicarbonate species that may result
To investigate the surface morphologies and sizes of the precursor and the decomposition products, FESEM measurement was performed. Fig. 5(a and b) show the FESEM images of the precursor complex at two different scales. The morphology of the precursor is hierarchically porous sphericity, which is resulted from the disordered integration of the thin and highly vented nanosheets. Fig. 5(c and d) are the FESEM images of NiO. The morphology of the alkali nickle carbonate precursor is retained after the calcination process, but the size of the NiO microspheres is larger. The estimated diameter of the NiO microspheres is 2–4 lm. To further study the formation of the NiO hierarchically porous microsphere structure, a series of contrast experiments were carried out at various ultrasonic radiation times under the same other conditions. Under the ultrasound radiation for 60 min, the precursor solution did not react, and was still transparent. The porous morphology is observed on the surface of NiO particles when the reaction performs for 70 min (Fig. 6(a)), and as the reaction time proceeds to 80 min, more pores are observed, as seen in Fig. 6(b). The excessive high temperature, pressure and cooling rate due to the cavitation can supply enough energy to drive the growth of the particles under the ultrasonic irradiation. At the same time, through the attachment and coalescence of particles with favorable crystallographic planes, oriented attachments proceed, resulting in the formation of large aggregated microspheres. Further increasing the reaction time to 100 min, the FESEM image of the resulted NiO particles displays hierarchically porous morphology as well, besides slight aggregation and structural destruction (Fig. 6(c)). However, as the reaction proceeds for another 20 min, lots of NiO microspheres are collapsed (Fig. 6(d)). Compared to the porous surfaces, the densely packed regions are observed in the interior, and some nanosheets are scattered around. For the ultrasonic cavitation effect, an intensively mechanical shock may produce when the bubbles collapse to ravage the structure of the materials. In order to compare the sonication effect, the precursor solution was also treated by mechanical stirring without the ultrasound
Fig. 4. FT-IR spectra of (a) the precursor and (b) NiO microspheres.
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Fig. 5. FESEM images of the precursor complex (a and b) and NiO (c and d) at different scales.
Fig. 6. FESEM images of NiO products obtained at different ultrasonic radiation time ((a) 70 min, (b) 80 min, (c) 100 min, and (d) 120 min).
radiation at the same temperature and under the same posttreatments. From the precursor solution immersing in the water bath to generating the sediment, the reaction time was at least
4 h. However, in the presence of the sonication, the shortest reaction time was 70 min, as shown in Fig. 6(a). The ultrasound can accelerate the decomposition of urea. Fig. 7(a–c) show the
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Fig. 7. FESEM images of NiO products obtained with mechanical stirring in contrastive conditions ((a) 250 rpm/open system, (b) 375 rpm/open system, (c) 500 rpm/open system, (d) 250 rpm/closed system).
morphology of NiO particles treated by the mechanical stirring in the open system without the ultrasound radiation at 250, 375, 500 rpm, respectively. It was found that the microstructures at the stirring speed of 250 rpm were relatively close to that with the ultrasound treatment, except the severe agglomeration. As the speed increased, the hierarchically porous microstructures were destroyed gradually, even collapsed, as shown in Fig. 7(b and c). While the precursor solution was sealed and immersed in the water bath at the speed of 250 rpm, the hollow porous microspheres and a few unshaped NiO particles were prepared, as seen in Fig. 7(d). So it can be concluded that the mechanical stirring is hardly enough to form the complete hierarchically porous microsphere structure, and the effect is much less than the sonication. 3.6. Particle size distribution and BET measurements Fig. 8 shows the size distribution of the hierarchically porous NiO microsphere. The results display that the median diameter of
Fig. 9. Nitrogen adsorption–desorption isotherms and the corresponding pore size distribution of the hierarchically porous NiO microspheres (inset).
Fig. 8. The size distribution of hierarchically porous NiO microspheres.
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Fig. 10. (a) UV–Vis absorption spectrum and (b) (Aht)2–ht curves of the hierarchically porous NiO microsphere.
the hierarchically porous NiO microsphere is 3.436 lm which is well in accord with the estimated result from the FESEM images. In order to characterize the pores of the as-synthesized NiO microspheres, the BET gas sorptometry measurement was used to measure the specific surface area, the pore volume and the pore diameter, as shown in Fig. 9. According to the IUPAC classification, the N2 adsorption/desorption isotherms of the hierarchically porous NiO microspheres can be classified as H3 type of hysteresis. Although the effect of various factors on adsorption hysteresis is not fully understood, the specific pore structures have often been identified with the shape of hysteresis loops. Thus H3, which exhibits an extremely high quantity adsorbed at high P/P0, is often associated with aggregates (i.e., an assembly of particles, which are loosely coherent) having slit shaped pores. The pore diameter distribution curve is inset in Fig. 9, which is obtained according to the Barrett–Joyner–Halenda (BJH) method, using the Halsey equation for multi-layer thickness [37]. This plot shows that the majority of the pores correspond to the macropores (>50 nm), but still some pore volume corresponds to the mesoporous pores. This demonstrates that NiO microspheres are mesoporous materials with a high fraction of macropores. The pores including mesopores and macropores are resulted from the intergranular accumulation. The BET specific surface area of the hierarchically porous NiO microsphere was 103.20 m2/g, which is also due to the external surface area of the NiO microspheres.
The hierarchically porous NiO microsphere with a median diameter of 3.436 lm was successfully synthesized by a novel template-free sonochemical technique. The NiO microspheres could be formed via the thermal decomposition of the precursor NiO2.45C0.74N0.25H2.90. The NiO samples exhibit unique hierarchically porous morphology which is changed over reaction time under the ultrasound treatment. The mechanical stirring is hardly enough to form the complete hierarchically porous microsphere structure, and the efficiency is much less than the sonication. The N2 adsorption/desorption isotherms of the hierarchically porous NiO microspheres can be classified as H3 type of hysteresis, which shows that the pores are the silt shaped pores. The pores are mainly macropores with a small fraction of mesopores. The band gap of the hierarchically porous NiO microsphere was estimated to be 3.63 eV, which showed red shift compared with that of the bulk-sized NiO, hence, the uniform hierarchically porous NiO microsphere may become an advanced material for its potential applications in optical materials, catalysts, photovoltaic devices, etc.
3.7. UV–Vis spectroscopy
Acknowledgments
Fig. 10(a) is the UV–Vis spectrum of the hierarchically porous NiO microsphere, with a broad absorption band at the wavelength about 475 nm. The broad absorption in the visible light region is due to the electronic transition from the valence band to the conduction band in the NiO semiconductor [38]. It is known to all that optical band gap (Eg) can be calculated on the basis of the optical absorption spectrum by the following equation [Eq. (5)]: where A is absorbance, ht is the photon energy, B is a constant relative to the material and n is either 1/2 for direct transition or two for indirect transition [39].
This work was supported by the National Natural Science Foundation of China (No. 50872034), Major Scientific and Technological Projects of Guangdong Province (No. 2010A080405004), Project on the Integration of Industry, Education and Research of Guangdong Province (2012B091100451) and Applied Basic Research Program of Guangzhou (No. 2012J4100006).
n
ðAhtÞ ¼ Bðht EgÞ
ð5Þ
Hence, the optical band gap for the absorption peak can be obtained by the extrapolation of the linear portion of the (Aht)n versus ht curve to zero. The iabsorption of the nanomaterial is indirect transition, so n is two. Fig. 10(b) shows the (Aht)2–ht curve for the hierarchically porous NiO microsphere. The band gap of the as-synthesized NiO samples was estimated to be 3.63 eV according to the extrapolation of the above equation. Compared with that of the bulk-sized NiO (4.0 eV), the obvious red shift
is due to the quantum size effect [40,39], indicated that the hierarchically porous NiO microsphere synthesized by this technique could be a promising photocatalytic material. 4. Conclusions
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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Template-free sonochemical synthesis of hierarchically porous NiO microsphere Wenli Zhu, Anze Shui ⇑, Linfeng Xu, Xiaosu Cheng, Pingan Liu, Hui Wang College of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China
a r t i c l e
i n f o
Article history: Received 14 October 2013 Received in revised form 20 February 2014 Accepted 24 February 2014 Available online 7 March 2014 Keywords: NiO Hierarchically porous microsphere Pore Sonochemical synthesis
a b s t r a c t A novel template-free sonochemical synthesis technique was used to prepare NiO microspheres combined with calcination of NiO2.45C0.74N0.25H2.90 precursor at 500 °C. The NiO microspheres samples were systematically investigated by the thermograviometric/differential scanning calorimetry (TG/DSC), X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), fourier-transformed infrared spectroscopy (FT-IR), Brunnauer–Emmett–Teller (BET) nitrogen adsorption–desorption isotherms, laser particle size analyzer, and ultraviolet–visible spectroscopy (UV–Vis). The morphology of the precursor was retained even after the calcination process, and exhibited hierarchically porous sphericity. The morphology changed over the ultrasonic radiation time, and the shortest reaction time was 70 min, which was much less than 4 h for the mechanical stirring process. The mechanical stirring was difficult to form the complete hierarchically porous microsphere structure. The BET specific surface area and the median diameter of the hierarchically porous NiO microspheres were 103.20 m2/g and 3.436 lm, respectively. The synthesized NiO microspheres were mesoporous materials with a high fraction of macropores. The pores were resulted from the intergranular accumulation. The ultraviolet absorption spectrum showed a broad emission at the center of 475 nm, and the band gap energy was estimated to be 3.63 eV. Ó 2014 Elsevier B.V. All rights reserved.
1. Instruction Porous nano- and micro-materials have been received considerable attention due to their low density, high surface urea, good permeation and unique optical, electrical and catalytic properties [1,2]. NiO, as one of the most important transition metal oxides, is of particular interesting due to its high surface area, high specific capacitance, and low cost [3]. Furthermore, NiO is being studied for the applications in various fields, including catalyst [4,5], electrochromic film [6], gas sensor [7], battery cathode [8], fuel cell electrode [9], photovoltaic device [10], magnetic material [11], electrochemical capacitor [12], and smart window [13]. NiO nanoparticles are expected to possess more improved properties and advanced applications than the bulk-sized NiO particles owing to the volume effect, the surface effect and the quantum size effect [14]. For example, NiO nanoparticles exhibit particular superparamagnetic, superantiferromagnetic and ferromagnetic properties, whereas bulk-sized NiO is antiferromagnetic insulator with a Neel temperature (TN) of 250 °C [15,16].
⇑ Corresponding author. Tel.: +86 20 87110290; fax: +86 20 87110273. E-mail address: [email protected] (A. Shui). http://dx.doi.org/10.1016/j.ultsonch.2014.02.026 1350-4177/Ó 2014 Elsevier B.V. All rights reserved.
Generally, NiO nanoparticles could be fabricated chiefly by ultrasonic radiation [17], coprecipitation method [18,19], sol–gel method [20,21], chemical vapor deposition [22], microemulsion method [23], solid-state method [24], and so forth. Among them, sonochemical process has been recently proven to be a versatile and favorable technique for the preparation of various nanomaterials including metal, oxide, carbide, and sulfide [25]. Sonochemistry takes the advantages of the acoustic cavitation phenomenon arising during the compression and rarefaction cycles. Compression and rarefaction cycles are generated as the ultrasound waves propagate through the liquid media. The tiny micro-bubbles are created during the rarefaction cycles due to the low pressure regions produced in the liquid, and collapsed intensely through the compression cycles [29]. The very high temperature (>5000 K) and pressure (>20 MPa) are obtained upon the collapse of the bubbles, resulting in the variation in the morphology of the sonochemically synthesized micro- and nano-particles. Since the collapse occurs in less than a nanosecond, the extreme high cooling rate (1010 K s1) is also obtained. The high cooling rate hinders the organization and crystallization of the products, for this reason, the amorphous nanoparticles are obtained [26–30]. The excessive high temperature, pressure and cooling rate due to the cavitation can supply enough energy to drive the formation of novel micro- or nanomaterials under the ultrasonic irradiation.
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Thus, the sonochemical technique can accelerate the reaction, shorten the reaction time and eliminate the need of additives, however, the detailed mechanism of sonochemical actions is not understood completely [31]. It is the intrinsic superiority that the sonochemical technique is fast, facile, economical and environmentaly benign [32]. A few literatures have been reported the utilization of the ultrasound to obtain the NiO nanoparticles. Gandhi et al. [33] fabricated NiO nanoparticles via the sonochemical technique and researched their surface catalytic effect on the PVA structure. Meybodi et al. [29] synthesized a wide band gap NiO nanoparticles from Ni(OH)2 precursor by the sonochemical technique. However, to the best of our knowledge, the preparation of the hierarchically porous NiO microsphere via the sonochemical technique has rarely been reported. Herein, a template-free sonochemical technique was used to transform the mixed aqueous solution of nickle sulfate hexahydrate and urea to the precursor NiO2.45C0.74N0.25H2.90. The hierarchically porous NiO microsphere was obtained after the calcination of the precursor at 500 °C. 2. Experimental 2.1. Materials The chemicals such as NiSO4 6H2O and urea were of analytical grade and purchased from Fuchen Chemical Reagent Company, Tianjin, China. The received chemicals were used without any further purification. Distilled water was used throughout the experiments. 2.2. Preparation In a typical experimental procedure, aqueous solution of NiSO4 6H2O (0.1 M, 50 ml) was added into aqueous solution of urea (0.2 M, 250 ml) in a beaker, and stirred for 10 min to make the homogeneous precursor solution. Then the beaker was covered with polyethylene film and transferred into an ultrasonic bath at 80 °C without stirring. After the reaction performed for 80 min, the reaction mixture solution was cooled naturally overnight. The resulting green precipitate was filtered by a microporous membrane with the pore size of 0.45 lm, and washed by deionized water and anhydrous ethanol for several times successively to remove impurities, then dried at 60 °C for 12 h. The black products were finally obtained after the calcination of the green powder in air at 500 °C for 3 h. 2.3. Characterization The sonication was performed at 50 w, and 40 kHz by a locally supplied ultrasonicator (KQ50-B, China). The composition and structure of the products were analyzed by X-ray diffraction (XRD, Philips PW-1710 model X-ray diffractometer, the Netherlands) equipped with Cu-Ka radiation. The surface morphology and size of the powder were observed by a field emission scanning electron microscopy (FESEM, Nova NanoSEM 430, the Netherlands). The thermal decomposition behavior of the precursor was conducted on a thermograviometric/differential scanning calorimetry (TG/DSC, NETZSCH STA449C, Germany) at a heating rate of 10 K min1 in air to determine the proper heat temperature. Infrared absorption spectra were used to determine the structures of the precursor and the final product using the KBr pellet technique (FT-IR, Vector 33 spectrometer, Germany). The particle size distribution was taken on a laser particle size analyzer (BT-9300S, Dandong, China). The specific surface area was calculated with the Brunauer–Emmett–Teller (BET) method on a surface area analyzer
(3H-2000 III, China). The UV–Vis absorption spectrum was recorded by a UV/visible/near infrared spectrophotometer (LAMBDA950, U.S.A) with the wavelength range of 200–2500 nm at room temperature.
3. Results and discussion 3.1. Crystal structure of the as-synthesized powder Fig. 1 shows the XRD pattern of the green precipitate after drying and before the calcination. The precursor is identified as the nickle compound and the molecular formula is NiO2.45C0.74N0.25 H2.90, which is consistent with the result of Hu et al. [34]. The pattern is ‘‘saw-tooth’’ like and all peaks are broad. They are typical of turbostratic phases, which are ordered in two dimensions, but their layers are orientationally disordered, resulting in the ‘‘sawtooth’’-like reflections [35]. Broadening of XRD diffraction peaks could be attributed to the reduced grain sizes or microstructural distortion [36]. As the ultrasonic cavitation effect, an intensively mechanical shock may produce when the bubbles collapse to act on the materials. Consequently, both the small grain sizes and the microstructural strains can be considered as the influencing factors of XRD peaks broadening of the sonochemically synthesized powder. The intensity of the peak at 2h 12.5° is very strong, indicating the high crystalline of the as-synthesized precursor. 3.2. TG/DSC characterization Fig. 2 shows the TG/DSC curves of NiO2.45C0.74N0.25H2.90 at a constant heating rate of 10 K min1 in the temperature range of 40–600 °C. As it can be observed in the figure, a major weight loss which is 21.50% starting at about 300 °C. The exothermic peak at 337 °C can be explained by the decomposition of NiO2.45C0.74N0.25H2.90 to release NH3 and CO2. Based on the thermal analysis, the precursor was heated at 500 °C in air for 3 h to form NiO particles. 3.3. Crystal structure of the heat treated powder The green precursor was converted to the black powder after the heat treatment. The XRD pattern of the black powder confirms the formation of NiO, as displayed in Fig. 3. All diffraction peaks could be assigned to the face-centered cubic phase NiO (space group Fm3/m, a = 4.176, JCPDS 65-2901) at 2h 37.40°, 43.65°, 62.95°, 75.40°, and 79.45°, which is in good agreement with (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) crystal planes, respectively. The pattern is ‘‘saw-tooth’’ like and all peaks are broad. It should be mentioned that no other phases can be detected in the final
Fig. 1. XRD pattern of the precursor.
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from the hydrolysis of urea. The sharp peak at 635 cm1 corresponds to the band of dNi–O–H vibration. A small peak at 483 cm1 is attributed to the Ni–O stretching vibration. From these data, it can be concluded that the precursor is alkali nickle carbonate compound which is consistent with the phase analysis of XRD. The FT-IR spectrum of the final products is shown in Fig. 4(b). All the bands of organic groups disappear, indicated that the decomposition is completely. The intensive band at around 411 cm1 is observed which is related to the Ni–O stretching vibration of NiO6 in the face-centered cubic NiO. 3.5. Electron microscopy characterization
Fig. 2. TG/DSC curves of the nickle carbonate.
Fig. 3. XRD pattern of NiO.
products, indicated that the precursor decomposed to nickle oxide at 500 °C completely.
3.4. FT-IR spectroscopy The FT-IR spectrum of the precursor is depicted in Fig. 4(a). The absorption peak located at 3642 cm1 is attributed to O–H stretching vibration of interlayer water molecules and H-bond of OH group. The peak located at 1630 cm1 is related to the bending mode (H–O–H) of the water molecules. The broaden peak at 3574 cm1 is the absorbed band of crystal water in the complex. The strong absorption peak at 2226 cm1 is the characteristic stretching band of C„N. The peak at 1122 cm1 suggests the presence of surface carbonate or bicarbonate species that may result
To investigate the surface morphologies and sizes of the precursor and the decomposition products, FESEM measurement was performed. Fig. 5(a and b) show the FESEM images of the precursor complex at two different scales. The morphology of the precursor is hierarchically porous sphericity, which is resulted from the disordered integration of the thin and highly vented nanosheets. Fig. 5(c and d) are the FESEM images of NiO. The morphology of the alkali nickle carbonate precursor is retained after the calcination process, but the size of the NiO microspheres is larger. The estimated diameter of the NiO microspheres is 2–4 lm. To further study the formation of the NiO hierarchically porous microsphere structure, a series of contrast experiments were carried out at various ultrasonic radiation times under the same other conditions. Under the ultrasound radiation for 60 min, the precursor solution did not react, and was still transparent. The porous morphology is observed on the surface of NiO particles when the reaction performs for 70 min (Fig. 6(a)), and as the reaction time proceeds to 80 min, more pores are observed, as seen in Fig. 6(b). The excessive high temperature, pressure and cooling rate due to the cavitation can supply enough energy to drive the growth of the particles under the ultrasonic irradiation. At the same time, through the attachment and coalescence of particles with favorable crystallographic planes, oriented attachments proceed, resulting in the formation of large aggregated microspheres. Further increasing the reaction time to 100 min, the FESEM image of the resulted NiO particles displays hierarchically porous morphology as well, besides slight aggregation and structural destruction (Fig. 6(c)). However, as the reaction proceeds for another 20 min, lots of NiO microspheres are collapsed (Fig. 6(d)). Compared to the porous surfaces, the densely packed regions are observed in the interior, and some nanosheets are scattered around. For the ultrasonic cavitation effect, an intensively mechanical shock may produce when the bubbles collapse to ravage the structure of the materials. In order to compare the sonication effect, the precursor solution was also treated by mechanical stirring without the ultrasound
Fig. 4. FT-IR spectra of (a) the precursor and (b) NiO microspheres.
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Fig. 5. FESEM images of the precursor complex (a and b) and NiO (c and d) at different scales.
Fig. 6. FESEM images of NiO products obtained at different ultrasonic radiation time ((a) 70 min, (b) 80 min, (c) 100 min, and (d) 120 min).
radiation at the same temperature and under the same posttreatments. From the precursor solution immersing in the water bath to generating the sediment, the reaction time was at least
4 h. However, in the presence of the sonication, the shortest reaction time was 70 min, as shown in Fig. 6(a). The ultrasound can accelerate the decomposition of urea. Fig. 7(a–c) show the
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Fig. 7. FESEM images of NiO products obtained with mechanical stirring in contrastive conditions ((a) 250 rpm/open system, (b) 375 rpm/open system, (c) 500 rpm/open system, (d) 250 rpm/closed system).
morphology of NiO particles treated by the mechanical stirring in the open system without the ultrasound radiation at 250, 375, 500 rpm, respectively. It was found that the microstructures at the stirring speed of 250 rpm were relatively close to that with the ultrasound treatment, except the severe agglomeration. As the speed increased, the hierarchically porous microstructures were destroyed gradually, even collapsed, as shown in Fig. 7(b and c). While the precursor solution was sealed and immersed in the water bath at the speed of 250 rpm, the hollow porous microspheres and a few unshaped NiO particles were prepared, as seen in Fig. 7(d). So it can be concluded that the mechanical stirring is hardly enough to form the complete hierarchically porous microsphere structure, and the effect is much less than the sonication. 3.6. Particle size distribution and BET measurements Fig. 8 shows the size distribution of the hierarchically porous NiO microsphere. The results display that the median diameter of
Fig. 9. Nitrogen adsorption–desorption isotherms and the corresponding pore size distribution of the hierarchically porous NiO microspheres (inset).
Fig. 8. The size distribution of hierarchically porous NiO microspheres.
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Fig. 10. (a) UV–Vis absorption spectrum and (b) (Aht)2–ht curves of the hierarchically porous NiO microsphere.
the hierarchically porous NiO microsphere is 3.436 lm which is well in accord with the estimated result from the FESEM images. In order to characterize the pores of the as-synthesized NiO microspheres, the BET gas sorptometry measurement was used to measure the specific surface area, the pore volume and the pore diameter, as shown in Fig. 9. According to the IUPAC classification, the N2 adsorption/desorption isotherms of the hierarchically porous NiO microspheres can be classified as H3 type of hysteresis. Although the effect of various factors on adsorption hysteresis is not fully understood, the specific pore structures have often been identified with the shape of hysteresis loops. Thus H3, which exhibits an extremely high quantity adsorbed at high P/P0, is often associated with aggregates (i.e., an assembly of particles, which are loosely coherent) having slit shaped pores. The pore diameter distribution curve is inset in Fig. 9, which is obtained according to the Barrett–Joyner–Halenda (BJH) method, using the Halsey equation for multi-layer thickness [37]. This plot shows that the majority of the pores correspond to the macropores (>50 nm), but still some pore volume corresponds to the mesoporous pores. This demonstrates that NiO microspheres are mesoporous materials with a high fraction of macropores. The pores including mesopores and macropores are resulted from the intergranular accumulation. The BET specific surface area of the hierarchically porous NiO microsphere was 103.20 m2/g, which is also due to the external surface area of the NiO microspheres.
The hierarchically porous NiO microsphere with a median diameter of 3.436 lm was successfully synthesized by a novel template-free sonochemical technique. The NiO microspheres could be formed via the thermal decomposition of the precursor NiO2.45C0.74N0.25H2.90. The NiO samples exhibit unique hierarchically porous morphology which is changed over reaction time under the ultrasound treatment. The mechanical stirring is hardly enough to form the complete hierarchically porous microsphere structure, and the efficiency is much less than the sonication. The N2 adsorption/desorption isotherms of the hierarchically porous NiO microspheres can be classified as H3 type of hysteresis, which shows that the pores are the silt shaped pores. The pores are mainly macropores with a small fraction of mesopores. The band gap of the hierarchically porous NiO microsphere was estimated to be 3.63 eV, which showed red shift compared with that of the bulk-sized NiO, hence, the uniform hierarchically porous NiO microsphere may become an advanced material for its potential applications in optical materials, catalysts, photovoltaic devices, etc.
3.7. UV–Vis spectroscopy
Acknowledgments
Fig. 10(a) is the UV–Vis spectrum of the hierarchically porous NiO microsphere, with a broad absorption band at the wavelength about 475 nm. The broad absorption in the visible light region is due to the electronic transition from the valence band to the conduction band in the NiO semiconductor [38]. It is known to all that optical band gap (Eg) can be calculated on the basis of the optical absorption spectrum by the following equation [Eq. (5)]: where A is absorbance, ht is the photon energy, B is a constant relative to the material and n is either 1/2 for direct transition or two for indirect transition [39].
This work was supported by the National Natural Science Foundation of China (No. 50872034), Major Scientific and Technological Projects of Guangdong Province (No. 2010A080405004), Project on the Integration of Industry, Education and Research of Guangdong Province (2012B091100451) and Applied Basic Research Program of Guangzhou (No. 2012J4100006).
n
ðAhtÞ ¼ Bðht EgÞ
ð5Þ
Hence, the optical band gap for the absorption peak can be obtained by the extrapolation of the linear portion of the (Aht)n versus ht curve to zero. The iabsorption of the nanomaterial is indirect transition, so n is two. Fig. 10(b) shows the (Aht)2–ht curve for the hierarchically porous NiO microsphere. The band gap of the as-synthesized NiO samples was estimated to be 3.63 eV according to the extrapolation of the above equation. Compared with that of the bulk-sized NiO (4.0 eV), the obvious red shift
is due to the quantum size effect [40,39], indicated that the hierarchically porous NiO microsphere synthesized by this technique could be a promising photocatalytic material. 4. Conclusions
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