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mesoporous ZnO double-pyramids and their optical and photocatalytic properties
Journal of Alloys and Compounds 545 (2012) 176–181
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Ultrasound-assisted synthesis of macro-/mesoporous ZnO double-pyramids and their optical and photocatalytic properties Dong Xie a, Ling Chang a, Fengxian Wang a, Gaohui Du a,⇑, Bingshe Xu b a b
Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China Key Laboratory of Interface Science and Engineering in Advanced Materials, Taiyuan University of Technology, Taiyuan 30024, China
a r t i c l e
i n f o
Article history: Received 26 June 2012 Received in revised form 9 August 2012 Accepted 15 August 2012 Available online 22 August 2012 Keywords: Oxide materials Microstructure Luminescence Photocatalysis Transmission electron microscopy
a b s t r a c t We report a facile, surfactant-free method for synthesizing double pyramid-like ZnO architectures via an ultrasound-assisted route. The morphology, microstructures, and porosity of the materials have been studied and the formation mechanism has been discussed. Uniform octahedral ZnC2O4 precursors were first formed during ultrasound irradiation, and then transformed to macro-/mesoporous ZnO with the preservation of original shape after calcination at 350 °C. The obtained hierarchical ZnO double-pyramids were composed of ZnO nanoparticles of 35 nm and pores with the main distribution between 20 and 100 nm. Raman and photoluminescence measurements reveal the presence of intrinsic defects in the porous ZnO, which accounts for the narrowed band gap (3.16 eV) and the sharp blue emission at 472 nm. Moreover, the ZnO product possesses large specific surface area and exhibits extraordinary photocatalytic activity to degrade organic pollutants. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Environmental pollution problems put forward a challenge for humans, especially pollution problems relating to organic dyes industrialization. The traditional techniques for the treatment of dye waste effluents are usually non-destructive, inefficient and costly, or simply transfer the pollution from the water to another phase [1]. Fortunately, some nanostructure materials, such as ZnO, TiO2, CeO2, and Bi2O3, have been reported to be effective photocatalysts for degrading organic dye pollutants in water [2–4]. The degradation percentage, the complexity of the preparation, and the high cost are three factors limiting the industrialization of these photocatalysts [5,6]. Therefore, an indispensable and challenging issue in this field is the development of a simple, low cost, and highly efficient synthesis of nanostructures with high catalytic activity. The shape, size, and microstructure of nanomaterials play important roles in determining their electrical, optical, and catalytic properties. In general, small size and high surface area can be advantageous for catalysts to improve adsorption effects and activate reaction activity [6]. In addition, porous materials have advantages as catalysts in the photodegradation of organic dyes due to their large surface area and favorable adsorption. Therefore, the design and controllable fabrication of functional nanomaterials are necessary to meet an ever-increasing demand.
⇑ Corresponding author. Tel.: +86 579 82283897; fax: +86 579 82282595. E-mail address: [email protected] (G. Du). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.08.059
ZnO has a wide band-gap energy of 3.37 eV with a large binding energy, and other useful characteristics, such as a large piezoelectric constant, favorable biocompatibility, and excellent photocatalytic activity [6,7]. It attracts scholars to explore the preparations of various ZnO shapes, such as mesoporous nanostructures [6], nanocombs [8], nanorods [9], belts [10], nanosheets [11], nanowires [12], hollow microspheres [13], and flowerlike nanostructures [14] in applications of photocatalysts, photocurrent generation [15], gas sensitive photoconductivity [16], nanogenerators [17], and lithium-ion batteries. Ultrasound is composed of a series of density alternating with a longitudinal wave. When ultrasound energy is high enough, it will produce an ‘‘ultrasonic cavitation’’ phenomenon. Meybodi et al. reported the preparation of wide band-gap nanocrystalline NiO using a sonochemical method [18]. Geng et al. reported a sonochemical selective synthesis of ZnO/CdS core/shell nanostructures [19]. The dentritic ZnO and ZnO nanorods have also been prepared in ionic liquid via an ultrasound-assisted method [20,21]. In this study, we demonstrate the synthesis of ZnO double-pyramids with macro-/mesoporous structures via the ultrasound-assisted route. The porous ZnO exhibited outstanding photocatalytic and photoluminescence properties. 2. Experimental 2.1. Synthesis All the reagents used were of analytical purity and were used without further purification. Zn(NO3)26H2O and (NH4)2C2O4H2O were purchased from the Shanghai Zhanyun Chemical Ltd. Co. of China. During synthesis, 50 mL of Zn(NO3)2
D. Xie et al. / Journal of Alloys and Compounds 545 (2012) 176–181 solution (0.3 M) was prepared in a beaker; then 70 mL of (NH4)2C2O4 (0.1 M) solution was added to the beaker slowly under ultrasound irradiation (45 kHz and 60 W) at room temperature. White precipitation appeared after about 5 min, and the ultrasound irradiation continued for additional 30 min. Subsequently, the white precipitate was centrifuged, washed with distilled water and absolute ethanol, and dried at 60 °C. The formed precursors were put into a quartz boat and roasted at 350 °C for 1 h in a furnace to obtain the final products. 2.2. Characterization The products were characterized using X-ray powder diffraction (XRD) on a Philips PW3040/60 X-ray diffractometer with Cu Ka (k = 1.5418 Å) radiation for phase identification. The morphologies were examined using scanning electron microscopy (SEM) on a Hitachi S4800 microscope, and the microstructures were investigated using transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) performed on a JEOL 2100F instrument at an acceleration voltage of 200 kV. The sample for TEM analysis was prepared by dispersing the final sample in ethanol; the suspension was then dropped on a copper grid covered with an amorphous carbon film. N2 adsorption was measured at 77 K with an ASAP 2020 apparatus (Micromeritics). The samples were previously evacuated at 473 K for 2 h. The Brunauer–Emmett–Teller (BET) method was utilized to calculate the specific surface areas, and the pore size distributions were derived from the adsorption branches of the isotherms based on Barrett–Joyner–Halanda (BJH) model. The Raman spectrum was recorded by a Renishaw RM1000 micro-Raman system under Ar+ (514.5 nm) laser excitation. The ultraviolet–visible (UV–vis) absorption spectra were recorded on a Nicolet Evolution 500 spectrophotometer with a wavelength range of 250–800 nm. Photoluminescence (PL) experiments were measured at the excitation wavelength of 325 nm with an Edinburgh FLSP920 spectrophotometer using a Xenon lamp as the excitation source at room temperature. Fourier transform infrared (FT-IR) spectroscopy was recorded on Nicolet NEXUS670 instrument. 2.3. Photocatalytic measurements The photodegradation of Rhodamine B (Rh-B) and methyl orange (MO) was used to investigate the photocatalytic properties of ZnO products. The photocatalytic reactions in aqueous solution were carried out at room temperature in a closed system using a high pressure mercury lamp (125 W). 0.2 g of ZnO powders were transferred to 100 mL of Rh-B or MO (4 ppm) solution. The suspensions were exposed to UV irradiation under stirring. The concentrations of Rh-B and MO in solutions were determined using a UV–vis spectrophotometer (TU-1810).
3. Results and discussion Fig. 1(a) shows the XRD patterns of the as-prepared precursor and the product by calcining the precursor at 350 °C. The XRD peaks of the precursor could be indexed as zinc oxalate crystalline structure (JCPDS No. 25-1029). As for the calcined product, welldefined diffraction peaks are indicative of the hexagonal ZnO (JCPDS No. 36-1451) with cell parameters a = 3.249 Å and c = 5.206 Å. The sharp diffraction peaks reflect the excellent crystallinity of ZnO without any impurities. The FT-IR spectra of the precursor and the calcined product are shown in Fig. 1(b). The intense broad-band at 3385.3 cm1 represents the stretching vibration of H–O, which indicates the presence of crystallization water [22]. Another broad-band at 1629.5 cm1 is superposed by the
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stretching vibration of C = O and the bending vibration of H–O–H [22]. The bands at 1364 cm1 and 1320 cm1 are due to the stretching vibration of C–O; the strong peak at 492.4 cm1 is assigned to the vibrations of Zn–O bonding [22]. The absorption peak of the calcined product at 488.4 cm1 is assigned to the characteristic vibration of Zn–O [22,23]. In contrast with the characteristic vibration of bulk ZnO at 450 cm1, there is an obvious blue-shift in the FT-IR spectra, which is caused by the enhanced vibration frequency resulting from reduced particle size. The peaks around 3441.5 and 1629.5 cm1 are related to the vibrations of –OH residing in the surface of ZnO [24]. Based on the XRD and FT-IR analysis, we can definitely draw the conclusion that the obtained precursor is ZnC2O42H2O and the calcined product is pure ZnO. The morphology and microstructure of the products were examined using SEM and TEM. Fig. 2(a and b) includes the SEM images of the ZnC2O42H2O precursor and the ZnO product. The double pyramid-like products can be observed in the two images. However, there are evident differences in their surfaces. Fig. 2(a) shows that the surfaces of the precursor are basically smooth with few holes. In contrast, Fig. 2(b) shows the loose porous ZnO architectures. The lower-magnification SEM image of the ZnO product is shown in the Fig. 2(c). We can observe a large quantity of ZnO double-pyramids with a narrow size distribution around 3–5 lm. A high-magnification SEM image is shown in Fig. 2(d); many small spherical nanoparticles with diameters of approximately 35 nm and macro-/mesopores constitute the ZnO double-pyramids. Shown in Fig. 2(e) is a TEM image of a slice of a ZnO doublepyramid, further revealing that it is composed of ZnO nanoparticles of 35 nm with the presence of macro-/mesopores. A HRTEM image and the corresponding FFT pattern shown in Fig. 2(f) demonstrate that the ZnO nanoparticles are single crystalline. To investigate the influence of ultrasound irradiation on the product, a similar experiment was carried out without ultrasound irradiation. The SEM image of the obtained ZnO after calcination is shown in Fig. 3; no double pyramid-like morphology was observed. The product is still porous ZnO, but with irregular morphology, and particle sizes range from 5 to 12 lm. The chemical effects of ultrasonic irradiation arise from acoustic cavitation, namely the formation, growth and implosive collapse of bubbles in a liquid medium, which results in an instantaneously high temperature and pressure [25]. The mechanical effect of ultrasonic processing is that ultrasonic vibrations can efficiently break up the aggregates and prevent nanocrystals from agglomerating. These special conditions attained during ultrasound irradiation lead to the growth of the precursor with uniform morphology. On the contrary, the ZnC2O4 nuclei or nanocrystals tend to agglomerate to decrease surface energy during preparation without ultrasound irradiation. The agglomerate growth leads to irregular particles of larger size. So
Fig. 1. (a) XRD patterns and (b) FT-IR spectra of the as-prepared precursor and the calcined product.
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Fig. 2. SEM images of ZnC2O4 precursor (a) and ZnO double-pyramids (b–d); TEM (e) and HRTEM (f) images of ZnO double-pyramids and the corresponding FFT pattern (inset).
Fig. 3. Low-magnification (a) and high-magnification (b) SEM images of ZnO product obtained without ultrasound irradiation.
the ultrasound irradiation plays an important role in determining the octahedral morphology of the precursor and final ZnO product. We designed a schematic diagram to interpret the formation of the loose porous ZnO double-pyramids in Fig. 4 on the basis of the comprehensive analysis. First, the zinc cations (Zn2+) are known to readily react with oxalic ion (C2 O2 4 ) in the dilute concentration. As a result, the ZnC2O4 nuclei form. The growth habit of a crystal is mainly determined by its internal structure, and is controlled by external conditions, such as temperature, super-saturation, and solvent [26]. The structure of zinc oxalate is composed of ZnO6 octahedrons and oxalate units. ZnC2O4 nuclei are well dispersed in the solution and evolve into microcrystals due to the chemical
and mechanical effects of ultrasound irradiation. The formation of a double pyramid-like morphology could be attributed to the inherent ZnO6 octahedral units in ZnC2O4. Upon heating, the hydrous compound ZnC2O42H2O loses water, releases CO2, and converts to ZnO nanoparticle-assembled double-pyramids with macro-/mesoporous structures. To provide further insight into the specific surface area and the porous structure of ZnO double-pyramids, N2 adsorption–desorption isotherms were measured. A type IV hysteresis loop is observed in Fig. 5, which is characteristic of mesoporous materials. The specific surface area of the porous ZnO estimated from the BET method is 25.4 m2/g, which is larger than that of the previously reported
D. Xie et al. / Journal of Alloys and Compounds 545 (2012) 176–181
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Fig. 4. Schematic drawing of the formation process of porous ZnO double-pyramids via ultrasound-assisted route.
quasi-single crystal mesoporous ZnO nanostructures [6] and ZnO hexagonal architectures [27]. The pore-size distribution obtained from the BJH method is shown in the inset of Fig. 5. The plot indicates that the pores are distributed over a wide range of sizes, and the dominant peaks are in the mesoporous range with a wide main peak around 30–40 nm and a small narrow peak at 2 nm. It is clear that the macropores (>50 nm) are also present, which have been observed by SEM and TEM analysis. The high surface area of the as-prepared macro-/mesoporous ZnO double-pyramids will be of benefit to the improvement of many physicochemical properties, e.g. photocatalytic activity. Raman scattering is sensitive to the microstructure and defect of nanomaterials. Fig. 6 is a Raman spectrum of the ZnO doublepyramids recorded at room temperature. Based on the previous literature [28], all observed spectroscopic peaks could be assigned to wurtzite ZnO. The dominant sharp peak at 437 cm1 was observed and is known as a Raman-active optical phonon E2H mode, which is a characteristic of wurtzite hexagonal ZnO. The weak peak at 333 cm1 in the spectrum is assigned to be the E2H–E2L mode and the weak peak at 586 cm1 is attributed as the E1L mode, which is associated with presence of oxygen vacancies, interstitial zinc or their complexes [29,30]. It is well known that the optical and electronic properties of ZnO nanoparticles depend not only on the particular crystal structure, composition, and morphology of the oxide particles, but also on their defect structure [31]. Thus, the presence of defects might endow ZnO double-pyramids with extraordinary optical properties. The UV–vis absorption spectrum of the porous ZnO is shown in Fig. 7(a). The ZnO sample has strong adsorption from 300 to 380 nm in the UV region. It is well known that the optical band gap energy (Eg) can be calculated on the basis of the optical absorption spectrum using the following equation:
Fig. 5. Nitrogen adsorption–desorption isotherms and the corresponding pore size distribution (inset) for the as-prepared ZnO.
ðahmÞn ¼ kðhm Eg Þ where hm is the photon energy, a is the absorption coefficient, k is a constant relative to the material, and n is either 2 for a direct transition or 1/2 for an indirect transition. The value for a direct allowed optical transition of ZnO double-pyramids can be determined by extrapolating the linear portion of the absorption spectrum. The inset of Fig. 7(a) shows the plot of (ahm)2 versus photon energy, where h is Plank constant, and m is the frequency of the incident radiation. The relation yields a straight line, indicating the existence of the direct optical transition. The optical band gap was calculated to be Eg = 3.16 eV, which is red-shifted compared with that of bulk ZnO powder (Eg = 3.37 eV). In general, the absorption edges of nanoscale materials show a blue shift and their band gaps enlarge compared with the bulk due to quantum size effects. However, our ZnO sample exhibits a unique narrowed band gap. The effect may be attributed to the intrinsic defects in the ZnO octahedrons, which result in some electronic states in the band gap that reduce the band gap energy. The room-temperature PL spectrum of the porous ZnO measured using a 325 nm excitation wavelength is shown in Fig. 7(b). The PL
Fig. 6. Raman spectrum of the ZnO double-pyramids.
spectrum shows a weak UV emission around 396 nm, a sharp blue emission at 472 nm and a broad yellow emission around 525 nm. In general, the visible emission in ZnO is attributed to different intrinsic defects, such as oxygen vacancies, zinc vacancies, and zinc interstitials [32–34]. The emission band at 396 nm originates from the excitonic recombination corresponding to the band edge emission of ZnO. The blue emission of 472 nm is associated with defects resulting from the transition of zinc interstitial defects to zinc vacancy defects [3,32], in accordance with the defect’s energy level of ZnO calculated by FP-Muffin [33]. The green emission of 525 nm is related to oxygen vacancies in ZnO nanostructures [34]. Zheng et al. reported a similar porous ZnO prepared by a solvothermal synthesis of precursor and calcination [35]. But the sharp green
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Fig. 7. (a) UV–vis absorption and (b) PL spectra of the porous ZnO. Inset of (a) is the plot of (ahm)2 versus photon energy.
emission in our PL spectrum was not observed in their PL measurement. So our as-prepared porous ZnO exhibits distinct PL properties owing to the unique microstructures resulting from the ultrasoundassisted synthesis method. The visible emissions also provide evidence for the presence of intrinsic defects in ZnO double-pyramids in agreement with the Raman analysis. ZnO materials have shown interesting photodecomposition of organic compounds [7]. In our research, the photocatalytic decomposition of Rh-B and MO over the porous ZnO double-pyramids was evaluated and the results are shown in Fig. 8. For comparison, the ZnO with irregular morphology prepared without ultrasound irradiation (Fig. 3) was also measured to degrade Rh-B and MO. In the beginning of the experiments, the Rh-B/MO solution with 0.2 g of ZnO was aged in the dark for 20 min under stirring to examine the adsorption ability. We can observe that the ZnO samples both show poor adsorption ability to Rh-B and MO. However, the photocatalytic ability is admirable. Under UV irradiation, the degradation percentages to Rh-B are as high as 91% and 96% after 65 min and 100 min, respectively. The efficiency to degrade MO is 97.2% after 100 min, slightly better than degrading Rh-B. In contrast, it is obvious that the degradation efficiency of ZnO doublepyramids is much higher than the irregular ZnO. The degradation percentage of Rh-B and MO by irregular ZnO is only 52.3% and 53.1% after 100 min, respectively. Therefore, the porous ZnO double-pyramids have predominant photocatalytic activity to degrade organic dyes owing to their special structures. There are at least two possible reasons accounting for the better photocatalytic ability of the ZnO double-pyramids: (1) the irregular ZnO particles are
larger than the ZnO double-pyramids. Even though they are both porous, a larger size will lead to a smaller specific surface area and a long diffusion path for the Rh-B and MO molecules into ZnO particles, which can slow down the reaction rate. (2) The central crystals in large particles might not absorb photons, and thus have no contributions to photocatalytic degradation because of short photon penetration depth. 4. Conclusions In summary, we present an ultrasound-assisted approach to synthesize macro-/mesoporous ZnO double-pyramids. The role of ultrasound irradiation is proven to be important to the morphology and photocatalytic activity of the ZnO product. The ZnO doublepyramids are composed of plenty of nanoparticles and macro-/ mesopores in the range of 20–100 nm. The porous ZnO shows a narrowed band gap of 3.16 eV, and exhibits a sharp intensive blue emission at 472 nm. The remarkable optical properties originate from the special microstructure in the ZnO double-pyramids. Owing to the uniform morphology, narrowed band gap and large surface area, ZnO double-pyramids show a superior photocatalytic activity to degrade organic dyes. The synthesis method is simple, low cost and highly efficient, and we believe this method can be applied in preparation of other porous metal-oxides, such as NiO and Bi2O3. Acknowledgments This work was supported by the National Science Foundation of China (No. 10904129), and the Program for New Century Excellent Talents in University of Ministry of Education of China (NCET-111081). References
Fig. 8. Photodegradation of Rh-B and MO by the as-prepared ZnO double-pyramids. The irregular ZnO powders obtained without ultrasound irradiation were also measured to degrade Rh-B and MO for comparison.
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Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Ultrasound-assisted synthesis of macro-/mesoporous ZnO double-pyramids and their optical and photocatalytic properties Dong Xie a, Ling Chang a, Fengxian Wang a, Gaohui Du a,⇑, Bingshe Xu b a b
Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China Key Laboratory of Interface Science and Engineering in Advanced Materials, Taiyuan University of Technology, Taiyuan 30024, China
a r t i c l e
i n f o
Article history: Received 26 June 2012 Received in revised form 9 August 2012 Accepted 15 August 2012 Available online 22 August 2012 Keywords: Oxide materials Microstructure Luminescence Photocatalysis Transmission electron microscopy
a b s t r a c t We report a facile, surfactant-free method for synthesizing double pyramid-like ZnO architectures via an ultrasound-assisted route. The morphology, microstructures, and porosity of the materials have been studied and the formation mechanism has been discussed. Uniform octahedral ZnC2O4 precursors were first formed during ultrasound irradiation, and then transformed to macro-/mesoporous ZnO with the preservation of original shape after calcination at 350 °C. The obtained hierarchical ZnO double-pyramids were composed of ZnO nanoparticles of 35 nm and pores with the main distribution between 20 and 100 nm. Raman and photoluminescence measurements reveal the presence of intrinsic defects in the porous ZnO, which accounts for the narrowed band gap (3.16 eV) and the sharp blue emission at 472 nm. Moreover, the ZnO product possesses large specific surface area and exhibits extraordinary photocatalytic activity to degrade organic pollutants. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Environmental pollution problems put forward a challenge for humans, especially pollution problems relating to organic dyes industrialization. The traditional techniques for the treatment of dye waste effluents are usually non-destructive, inefficient and costly, or simply transfer the pollution from the water to another phase [1]. Fortunately, some nanostructure materials, such as ZnO, TiO2, CeO2, and Bi2O3, have been reported to be effective photocatalysts for degrading organic dye pollutants in water [2–4]. The degradation percentage, the complexity of the preparation, and the high cost are three factors limiting the industrialization of these photocatalysts [5,6]. Therefore, an indispensable and challenging issue in this field is the development of a simple, low cost, and highly efficient synthesis of nanostructures with high catalytic activity. The shape, size, and microstructure of nanomaterials play important roles in determining their electrical, optical, and catalytic properties. In general, small size and high surface area can be advantageous for catalysts to improve adsorption effects and activate reaction activity [6]. In addition, porous materials have advantages as catalysts in the photodegradation of organic dyes due to their large surface area and favorable adsorption. Therefore, the design and controllable fabrication of functional nanomaterials are necessary to meet an ever-increasing demand.
⇑ Corresponding author. Tel.: +86 579 82283897; fax: +86 579 82282595. E-mail address: [email protected] (G. Du). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.08.059
ZnO has a wide band-gap energy of 3.37 eV with a large binding energy, and other useful characteristics, such as a large piezoelectric constant, favorable biocompatibility, and excellent photocatalytic activity [6,7]. It attracts scholars to explore the preparations of various ZnO shapes, such as mesoporous nanostructures [6], nanocombs [8], nanorods [9], belts [10], nanosheets [11], nanowires [12], hollow microspheres [13], and flowerlike nanostructures [14] in applications of photocatalysts, photocurrent generation [15], gas sensitive photoconductivity [16], nanogenerators [17], and lithium-ion batteries. Ultrasound is composed of a series of density alternating with a longitudinal wave. When ultrasound energy is high enough, it will produce an ‘‘ultrasonic cavitation’’ phenomenon. Meybodi et al. reported the preparation of wide band-gap nanocrystalline NiO using a sonochemical method [18]. Geng et al. reported a sonochemical selective synthesis of ZnO/CdS core/shell nanostructures [19]. The dentritic ZnO and ZnO nanorods have also been prepared in ionic liquid via an ultrasound-assisted method [20,21]. In this study, we demonstrate the synthesis of ZnO double-pyramids with macro-/mesoporous structures via the ultrasound-assisted route. The porous ZnO exhibited outstanding photocatalytic and photoluminescence properties. 2. Experimental 2.1. Synthesis All the reagents used were of analytical purity and were used without further purification. Zn(NO3)26H2O and (NH4)2C2O4H2O were purchased from the Shanghai Zhanyun Chemical Ltd. Co. of China. During synthesis, 50 mL of Zn(NO3)2
D. Xie et al. / Journal of Alloys and Compounds 545 (2012) 176–181 solution (0.3 M) was prepared in a beaker; then 70 mL of (NH4)2C2O4 (0.1 M) solution was added to the beaker slowly under ultrasound irradiation (45 kHz and 60 W) at room temperature. White precipitation appeared after about 5 min, and the ultrasound irradiation continued for additional 30 min. Subsequently, the white precipitate was centrifuged, washed with distilled water and absolute ethanol, and dried at 60 °C. The formed precursors were put into a quartz boat and roasted at 350 °C for 1 h in a furnace to obtain the final products. 2.2. Characterization The products were characterized using X-ray powder diffraction (XRD) on a Philips PW3040/60 X-ray diffractometer with Cu Ka (k = 1.5418 Å) radiation for phase identification. The morphologies were examined using scanning electron microscopy (SEM) on a Hitachi S4800 microscope, and the microstructures were investigated using transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) performed on a JEOL 2100F instrument at an acceleration voltage of 200 kV. The sample for TEM analysis was prepared by dispersing the final sample in ethanol; the suspension was then dropped on a copper grid covered with an amorphous carbon film. N2 adsorption was measured at 77 K with an ASAP 2020 apparatus (Micromeritics). The samples were previously evacuated at 473 K for 2 h. The Brunauer–Emmett–Teller (BET) method was utilized to calculate the specific surface areas, and the pore size distributions were derived from the adsorption branches of the isotherms based on Barrett–Joyner–Halanda (BJH) model. The Raman spectrum was recorded by a Renishaw RM1000 micro-Raman system under Ar+ (514.5 nm) laser excitation. The ultraviolet–visible (UV–vis) absorption spectra were recorded on a Nicolet Evolution 500 spectrophotometer with a wavelength range of 250–800 nm. Photoluminescence (PL) experiments were measured at the excitation wavelength of 325 nm with an Edinburgh FLSP920 spectrophotometer using a Xenon lamp as the excitation source at room temperature. Fourier transform infrared (FT-IR) spectroscopy was recorded on Nicolet NEXUS670 instrument. 2.3. Photocatalytic measurements The photodegradation of Rhodamine B (Rh-B) and methyl orange (MO) was used to investigate the photocatalytic properties of ZnO products. The photocatalytic reactions in aqueous solution were carried out at room temperature in a closed system using a high pressure mercury lamp (125 W). 0.2 g of ZnO powders were transferred to 100 mL of Rh-B or MO (4 ppm) solution. The suspensions were exposed to UV irradiation under stirring. The concentrations of Rh-B and MO in solutions were determined using a UV–vis spectrophotometer (TU-1810).
3. Results and discussion Fig. 1(a) shows the XRD patterns of the as-prepared precursor and the product by calcining the precursor at 350 °C. The XRD peaks of the precursor could be indexed as zinc oxalate crystalline structure (JCPDS No. 25-1029). As for the calcined product, welldefined diffraction peaks are indicative of the hexagonal ZnO (JCPDS No. 36-1451) with cell parameters a = 3.249 Å and c = 5.206 Å. The sharp diffraction peaks reflect the excellent crystallinity of ZnO without any impurities. The FT-IR spectra of the precursor and the calcined product are shown in Fig. 1(b). The intense broad-band at 3385.3 cm1 represents the stretching vibration of H–O, which indicates the presence of crystallization water [22]. Another broad-band at 1629.5 cm1 is superposed by the
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stretching vibration of C = O and the bending vibration of H–O–H [22]. The bands at 1364 cm1 and 1320 cm1 are due to the stretching vibration of C–O; the strong peak at 492.4 cm1 is assigned to the vibrations of Zn–O bonding [22]. The absorption peak of the calcined product at 488.4 cm1 is assigned to the characteristic vibration of Zn–O [22,23]. In contrast with the characteristic vibration of bulk ZnO at 450 cm1, there is an obvious blue-shift in the FT-IR spectra, which is caused by the enhanced vibration frequency resulting from reduced particle size. The peaks around 3441.5 and 1629.5 cm1 are related to the vibrations of –OH residing in the surface of ZnO [24]. Based on the XRD and FT-IR analysis, we can definitely draw the conclusion that the obtained precursor is ZnC2O42H2O and the calcined product is pure ZnO. The morphology and microstructure of the products were examined using SEM and TEM. Fig. 2(a and b) includes the SEM images of the ZnC2O42H2O precursor and the ZnO product. The double pyramid-like products can be observed in the two images. However, there are evident differences in their surfaces. Fig. 2(a) shows that the surfaces of the precursor are basically smooth with few holes. In contrast, Fig. 2(b) shows the loose porous ZnO architectures. The lower-magnification SEM image of the ZnO product is shown in the Fig. 2(c). We can observe a large quantity of ZnO double-pyramids with a narrow size distribution around 3–5 lm. A high-magnification SEM image is shown in Fig. 2(d); many small spherical nanoparticles with diameters of approximately 35 nm and macro-/mesopores constitute the ZnO double-pyramids. Shown in Fig. 2(e) is a TEM image of a slice of a ZnO doublepyramid, further revealing that it is composed of ZnO nanoparticles of 35 nm with the presence of macro-/mesopores. A HRTEM image and the corresponding FFT pattern shown in Fig. 2(f) demonstrate that the ZnO nanoparticles are single crystalline. To investigate the influence of ultrasound irradiation on the product, a similar experiment was carried out without ultrasound irradiation. The SEM image of the obtained ZnO after calcination is shown in Fig. 3; no double pyramid-like morphology was observed. The product is still porous ZnO, but with irregular morphology, and particle sizes range from 5 to 12 lm. The chemical effects of ultrasonic irradiation arise from acoustic cavitation, namely the formation, growth and implosive collapse of bubbles in a liquid medium, which results in an instantaneously high temperature and pressure [25]. The mechanical effect of ultrasonic processing is that ultrasonic vibrations can efficiently break up the aggregates and prevent nanocrystals from agglomerating. These special conditions attained during ultrasound irradiation lead to the growth of the precursor with uniform morphology. On the contrary, the ZnC2O4 nuclei or nanocrystals tend to agglomerate to decrease surface energy during preparation without ultrasound irradiation. The agglomerate growth leads to irregular particles of larger size. So
Fig. 1. (a) XRD patterns and (b) FT-IR spectra of the as-prepared precursor and the calcined product.
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Fig. 2. SEM images of ZnC2O4 precursor (a) and ZnO double-pyramids (b–d); TEM (e) and HRTEM (f) images of ZnO double-pyramids and the corresponding FFT pattern (inset).
Fig. 3. Low-magnification (a) and high-magnification (b) SEM images of ZnO product obtained without ultrasound irradiation.
the ultrasound irradiation plays an important role in determining the octahedral morphology of the precursor and final ZnO product. We designed a schematic diagram to interpret the formation of the loose porous ZnO double-pyramids in Fig. 4 on the basis of the comprehensive analysis. First, the zinc cations (Zn2+) are known to readily react with oxalic ion (C2 O2 4 ) in the dilute concentration. As a result, the ZnC2O4 nuclei form. The growth habit of a crystal is mainly determined by its internal structure, and is controlled by external conditions, such as temperature, super-saturation, and solvent [26]. The structure of zinc oxalate is composed of ZnO6 octahedrons and oxalate units. ZnC2O4 nuclei are well dispersed in the solution and evolve into microcrystals due to the chemical
and mechanical effects of ultrasound irradiation. The formation of a double pyramid-like morphology could be attributed to the inherent ZnO6 octahedral units in ZnC2O4. Upon heating, the hydrous compound ZnC2O42H2O loses water, releases CO2, and converts to ZnO nanoparticle-assembled double-pyramids with macro-/mesoporous structures. To provide further insight into the specific surface area and the porous structure of ZnO double-pyramids, N2 adsorption–desorption isotherms were measured. A type IV hysteresis loop is observed in Fig. 5, which is characteristic of mesoporous materials. The specific surface area of the porous ZnO estimated from the BET method is 25.4 m2/g, which is larger than that of the previously reported
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Fig. 4. Schematic drawing of the formation process of porous ZnO double-pyramids via ultrasound-assisted route.
quasi-single crystal mesoporous ZnO nanostructures [6] and ZnO hexagonal architectures [27]. The pore-size distribution obtained from the BJH method is shown in the inset of Fig. 5. The plot indicates that the pores are distributed over a wide range of sizes, and the dominant peaks are in the mesoporous range with a wide main peak around 30–40 nm and a small narrow peak at 2 nm. It is clear that the macropores (>50 nm) are also present, which have been observed by SEM and TEM analysis. The high surface area of the as-prepared macro-/mesoporous ZnO double-pyramids will be of benefit to the improvement of many physicochemical properties, e.g. photocatalytic activity. Raman scattering is sensitive to the microstructure and defect of nanomaterials. Fig. 6 is a Raman spectrum of the ZnO doublepyramids recorded at room temperature. Based on the previous literature [28], all observed spectroscopic peaks could be assigned to wurtzite ZnO. The dominant sharp peak at 437 cm1 was observed and is known as a Raman-active optical phonon E2H mode, which is a characteristic of wurtzite hexagonal ZnO. The weak peak at 333 cm1 in the spectrum is assigned to be the E2H–E2L mode and the weak peak at 586 cm1 is attributed as the E1L mode, which is associated with presence of oxygen vacancies, interstitial zinc or their complexes [29,30]. It is well known that the optical and electronic properties of ZnO nanoparticles depend not only on the particular crystal structure, composition, and morphology of the oxide particles, but also on their defect structure [31]. Thus, the presence of defects might endow ZnO double-pyramids with extraordinary optical properties. The UV–vis absorption spectrum of the porous ZnO is shown in Fig. 7(a). The ZnO sample has strong adsorption from 300 to 380 nm in the UV region. It is well known that the optical band gap energy (Eg) can be calculated on the basis of the optical absorption spectrum using the following equation:
Fig. 5. Nitrogen adsorption–desorption isotherms and the corresponding pore size distribution (inset) for the as-prepared ZnO.
ðahmÞn ¼ kðhm Eg Þ where hm is the photon energy, a is the absorption coefficient, k is a constant relative to the material, and n is either 2 for a direct transition or 1/2 for an indirect transition. The value for a direct allowed optical transition of ZnO double-pyramids can be determined by extrapolating the linear portion of the absorption spectrum. The inset of Fig. 7(a) shows the plot of (ahm)2 versus photon energy, where h is Plank constant, and m is the frequency of the incident radiation. The relation yields a straight line, indicating the existence of the direct optical transition. The optical band gap was calculated to be Eg = 3.16 eV, which is red-shifted compared with that of bulk ZnO powder (Eg = 3.37 eV). In general, the absorption edges of nanoscale materials show a blue shift and their band gaps enlarge compared with the bulk due to quantum size effects. However, our ZnO sample exhibits a unique narrowed band gap. The effect may be attributed to the intrinsic defects in the ZnO octahedrons, which result in some electronic states in the band gap that reduce the band gap energy. The room-temperature PL spectrum of the porous ZnO measured using a 325 nm excitation wavelength is shown in Fig. 7(b). The PL
Fig. 6. Raman spectrum of the ZnO double-pyramids.
spectrum shows a weak UV emission around 396 nm, a sharp blue emission at 472 nm and a broad yellow emission around 525 nm. In general, the visible emission in ZnO is attributed to different intrinsic defects, such as oxygen vacancies, zinc vacancies, and zinc interstitials [32–34]. The emission band at 396 nm originates from the excitonic recombination corresponding to the band edge emission of ZnO. The blue emission of 472 nm is associated with defects resulting from the transition of zinc interstitial defects to zinc vacancy defects [3,32], in accordance with the defect’s energy level of ZnO calculated by FP-Muffin [33]. The green emission of 525 nm is related to oxygen vacancies in ZnO nanostructures [34]. Zheng et al. reported a similar porous ZnO prepared by a solvothermal synthesis of precursor and calcination [35]. But the sharp green
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Fig. 7. (a) UV–vis absorption and (b) PL spectra of the porous ZnO. Inset of (a) is the plot of (ahm)2 versus photon energy.
emission in our PL spectrum was not observed in their PL measurement. So our as-prepared porous ZnO exhibits distinct PL properties owing to the unique microstructures resulting from the ultrasoundassisted synthesis method. The visible emissions also provide evidence for the presence of intrinsic defects in ZnO double-pyramids in agreement with the Raman analysis. ZnO materials have shown interesting photodecomposition of organic compounds [7]. In our research, the photocatalytic decomposition of Rh-B and MO over the porous ZnO double-pyramids was evaluated and the results are shown in Fig. 8. For comparison, the ZnO with irregular morphology prepared without ultrasound irradiation (Fig. 3) was also measured to degrade Rh-B and MO. In the beginning of the experiments, the Rh-B/MO solution with 0.2 g of ZnO was aged in the dark for 20 min under stirring to examine the adsorption ability. We can observe that the ZnO samples both show poor adsorption ability to Rh-B and MO. However, the photocatalytic ability is admirable. Under UV irradiation, the degradation percentages to Rh-B are as high as 91% and 96% after 65 min and 100 min, respectively. The efficiency to degrade MO is 97.2% after 100 min, slightly better than degrading Rh-B. In contrast, it is obvious that the degradation efficiency of ZnO doublepyramids is much higher than the irregular ZnO. The degradation percentage of Rh-B and MO by irregular ZnO is only 52.3% and 53.1% after 100 min, respectively. Therefore, the porous ZnO double-pyramids have predominant photocatalytic activity to degrade organic dyes owing to their special structures. There are at least two possible reasons accounting for the better photocatalytic ability of the ZnO double-pyramids: (1) the irregular ZnO particles are
larger than the ZnO double-pyramids. Even though they are both porous, a larger size will lead to a smaller specific surface area and a long diffusion path for the Rh-B and MO molecules into ZnO particles, which can slow down the reaction rate. (2) The central crystals in large particles might not absorb photons, and thus have no contributions to photocatalytic degradation because of short photon penetration depth. 4. Conclusions In summary, we present an ultrasound-assisted approach to synthesize macro-/mesoporous ZnO double-pyramids. The role of ultrasound irradiation is proven to be important to the morphology and photocatalytic activity of the ZnO product. The ZnO doublepyramids are composed of plenty of nanoparticles and macro-/ mesopores in the range of 20–100 nm. The porous ZnO shows a narrowed band gap of 3.16 eV, and exhibits a sharp intensive blue emission at 472 nm. The remarkable optical properties originate from the special microstructure in the ZnO double-pyramids. Owing to the uniform morphology, narrowed band gap and large surface area, ZnO double-pyramids show a superior photocatalytic activity to degrade organic dyes. The synthesis method is simple, low cost and highly efficient, and we believe this method can be applied in preparation of other porous metal-oxides, such as NiO and Bi2O3. Acknowledgments This work was supported by the National Science Foundation of China (No. 10904129), and the Program for New Century Excellent Talents in University of Ministry of Education of China (NCET-111081). References
Fig. 8. Photodegradation of Rh-B and MO by the as-prepared ZnO double-pyramids. The irregular ZnO powders obtained without ultrasound irradiation were also measured to degrade Rh-B and MO for comparison.
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