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Hydrothermal synthesis and photocatalytic property of porous CuO hollow microspheres via PS latex as templates
Solid State Sciences 20 (2013) 29e35
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Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Hydrothermal synthesis and photocatalytic property of porous CuO hollow microspheres via PS latex as templates Qian Shao*, Ling-Yun Wang, Xiao-Jie Wang, Meng-Chen Yang, Sheng-Song Ge, Xiao-Kun Yang, Jun-Xiang Wang College of Chemical and Environmental Engineering, Shandong University of Science & Technology, Qingdao 266590, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 November 2012 Received in revised form 2 February 2013 Accepted 6 March 2013 Available online 14 March 2013
Porous copper oxide (CuO) hollow microspheres have been fabricated through a simple hydrothermal method using PS latex as templates. The as-obtained samples were characterized by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffractometry (XRD) and Fourier transform infrared spectroscopy (FTIR). The influences of the mole ratio of Ethylenediamine (C2H8N2) and copper acetate (Cu(Ac)2$H2O), hydrothermal temperature and time on the size and morphologies of the final products have been investigated. The possible formation mechanism of porous CuO hollow microspheres has been proposed and the specific surface area of the hollow microspheres with 81.71 m2/g is measured by BET method. The band gap value calculated from a UVevis absorption spectrum of porous CuO hollow microspheres is 2.71 eV. The as-synthesized product exhibits high photocatalytic activity during the photodegradation of an organic dyestuff, rhodamine B (RhB), under UV-light illumination. Ó 2013 Elsevier Masson SAS. All rights reserved.
Keywords: CuO Porous hollow spheres PS latex Photocatalytic property Template
1. Introduction In the past decades, controlled synthesis of semiconductor materials of explicit structure and composition, with specific properties, has been of tremendous scientific and technological interest, as the physical and chemical properties of semiconductor nanomaterials are largely dependent on their size and shape [1e3]. As an important p-type semiconductor metal oxide with a narrow band gap of 1.2e1.5 eV copper oxide (CuO) is a novel material both in terms of basic studies and practical applications [4,5]. The peculiar properties of CuO have made it as an important multifunctional material for its diverse and fascinating applications which include gas sensors [6,7], lithium ion electrode materials [8], field emission emitted materials [9], magnetic storage media [10] and solar cells [11]. In regard to its commercial value, CuO has been used as heterogenous catalysts in many chemical fields, such as complete conversion of hydrocarbon and phenol into carbon dioxide and water [12], degradation of nitrous oxide and selective catalytic reduction of nitric oxide with ammonia [13]. To date, CuO nanostructures with different morphologies, such as nanoplates [14], nanorods [15], nanowires [16], nanotubes [17], spindle [18]
* Corresponding author. Tel.: þ86 532 86057567. E-mail address: [email protected] (Q. Shao). 1293-2558/$ e see front matter Ó 2013 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.solidstatesciences.2013.03.006
and hollow spheres [19], have been fabricated by a range of physical and chemical methods. To the best of our knowledge, although CuO nanostructures with different morphologies have been obtained through various methods, rare report was found in literature on the synthesis of porous CuO hollow spheres. Hollow spheres have attracted great interests because of their potential applications. Different methods have been used to prepare hollow spheres materials, among which, the templatedirected synthesis, as a valid method, has been widely used. More recently, a series of hard templates such as silica spheres [20], woelm alumina, polymethyl methacrylate(PMMA) [21], polystyrene(PS) [22], carbon spheres [23], spherobacterium [24] are employed to conduct the growth of oxide hollow spheres such as SiO2, ZnO, Al2O3, TiO2. Chen et al. have successfully obtained ellipsoidal complex hollow nanostructures with the shells assembled from anatase TiO2 nanosheets with exposed (001) facets by utilizing silica (SiO2)-coated hematite (a-Fe2O3) nanospindles as the starting templates [25]. They also reviewed the synthesis of hollow SnO2 nanostructures created by both templating and template-free approaches [26]. In addition, Wang et al. have reported a soft-templated hydrothermal method to synthesize aFe2O3 hollow spheres with sheet-like sub-units [27]. In their work, glycerol is dispersed in water to form oil-in-water quasiemulsion microdroplets, which serve as soft templates for the deposition of the a-Fe2O3 shell.
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Recently, our group has demonstrated a novel hydrothermal process to synthesis porous CuO hollow spheres using carbon spheres as templates [28]. In the current paper, we present a simple and effective synthesis pathway for porous CuO microspheres employing PS latex as templates under hydrothermal conditions based on electrostatic adsorption. In particular, the PS latex templates were prepared using emulsifier-free emulsion polymerization hence it is environment-friendly. The morphology and phase of the final products were availably dominated by modifying reaction conditions such as the mole ratio of C2H8N2/Cu(Ac)2$H2O, hydrothermal temperature and hydrothermal time. The possible formation mechanism of the samples has been proposed based on the observed results. Moreover, the optical property and the photocatalytic activity of the as-obtained porous CuO hollow microspheres have also been evaluated from the UVevis absorption spectrum and photodegradation of a model pollutant, Rhodamine B(RhB), respectively. 2. Experimental details 2.1. Materials Potassium persulfate(KPS), copper acetate(Cu(Ac)2$H2O) and absolute ethanol were purchased from Tianjin Benchmark Chemical Reagent Co. Ltd. (Tianjin, Chian). Styrene (St) and ethylenediamine (en) were provided by Tianjin Bodi Co. Ltd. (Tianjin, Chian). Rhodamine B(RhB) was purchased from Shanghai Chemical Reagent Co. Ltd. (Shanghai, China). All the chemical reagents used in the experiment were analytical grade and used as received without further purification except that styrene was washed by 5%(wt) sodium hydroxide solution. Distilled water was used throughout the experiment. 2.2. Preparation of PS latex PS spheres of 500 nm diameter, used as templates, were prepared by emulsifier-free emulsion polymerization using KPS as anionic initiator. A typical synthesis of PS latex was carried out as follows: Under gentle stirring, 25 mL styrene and 200 mL distilled water were added into a 250-mL round flask in sequence. The solution was purged with nitrogen to eliminate the inhibition effect of oxygen before the polymerization was initiated at 75 C. After 15 min stirring, 25 mL KPS solution with the concentration of 1.6 102 M was added to the above solution. The polymerization was continued for 5 h with mechanical stirring at 300 rpm. The surface of the PS emulsion particle will have a negative charge as it was obtained by emulsifier-free emulsion polymerization using KPS as anionic initiator and without any emulsifier.
2.4. Characterization The morphology and size of the PS latex were examined by transmission electron microscopy (TEM, JEM-2100, Japan) with an accelerating voltage of 200 kV. The sample used for TEM was prepared by dipping the PS latex (dispersed in distilled water) on carbon-film coated copper grids. Scanning electron microscopes, (KYKY2800B, China) with acceleration voltage of 10 kV and (SU-70, Japan) with acceleration voltage of 30 KV were used for morphological imaging analysis of the hydrothermal products. The average particle size and polydispersity index (PDI) were obtained with a high sensitivity laser particle analyzer (ZEN1690, Malvern) with distilled water as dispersion at room temperature (25 C). The crystallographic information of the prepared samples was analyzed by a Rigaku D/Max2500PC X-ray diffraction (XRD) equipped with graphite monochromatized Cu Ka radiation (l ¼ 0.15418 nm), using a scanning rate of 4 min1 in 2q rage from 5 to 80 . Fourier transform infrared (FT-IR) spectra were measured on a NICOLTE 380 FTIR spectrometer using KBr pellet. Each FTIR spectrum was collected after 40 scans at a resolution of 4 cm1 from 400 to 4000 cm1. The nitrogen adsorptionedesorption isotherm was obtained from a volumetric adsorption analyzer (SSA-4300, China) in which the sample was first degassed in vacuum at 200 C overnight. And then the measurement was carried out with nitrogen as adsorption gas under the pressure from 0 to 120 kPa. The surface area and the pore size distribution were calculated by the BrunauereEmmetteTeller (BET) method and BarreteJoynere Halenda (BJH), respectively. Band gap of the product was determined from UVevis absorption spectrum on a UVevisible diffuse reflectance Spectrophotometer (UV-2450, Shimadzu). 2.5. Photocatalytic activity test The photocatalytic activity of the porous CuO hollow microspheres was measured at room temperature (ca.25 C) by the photocatalytic decolorization of a model pollutant rhodamine B (RhB). The experimental process was as follows: 15 mg of the prepared samples were dispersed in 30 mL of RhB aqueous solution with a concentration of 10 mg L1, and the suspensions were stirred for 30 min in the dark for the sake of establishing the absorptione desorption equilibrium between the catalyst and the RhB. The UVlight lamp with a wavelength of 254 nm placed 8 cm above the beaker was used as a light source. After UVevisible irradiation for certain time, the reaction solution was filtered, and the absorption spectrum of the centrifuged solution was then recorded with a UVe visible spectrophotometer (UV-3200PC, Shanghai). 3. Results and discussion 3.1. Characterizations of the as-prepared samples
2.3. Preparation of porous CuO hollow microspheres In a typical reaction process, 0.15 g copper acetate was dissolved in 10.5 mL distilled water to form a clear solution. The solution was then added with 0.05 mL anhydrous ethylenediamine under magnetic stirring to form a Cu(en)2þ complex solution. Afterward, 15 mL of the as-obtained PS latex was added to the above mixture under slow stirring. After ultrasonication for 30 min, the resulting solution was sealed in a 30 mL Teflon-lined stainless steel autoclave and maintained at 160 C for 14 h, and then cooled down to room temperature naturally. The black products were centrifuged, and washed several times with distilled water and absolute ethanol to remove the impurities, and then dried in a drying oven at 80 C for 6 h. Finally, the products were calcined in air at 600 C (heating rate of 20 C/min) for 2 h.
Size distribution by intensity of the obtained PS latex is shown in Fig. 1(a). It demonstrates that the PS latex is in high monodispersity with the average particle size of 474.7 nm and polydispersity index (PDI) of 0.0013. The TEM image of the PS latex in Fig. 1(b) also shows that the diameters of the PS emulsion particles are ca. 500 nm. The morphologies of PS microspheres and Cu-adsorbed PS microspheres after hydrothermal reaction were examined using SEM and the images are shown in Fig.2. Fig. 2(a) reveals that the PS microspheres have smooth surface, and the diameters of the PS microspheres are ca. 500 nm. The results are also in good agreement with the particle size analysis results. More interestingly, the size dispersion range of the as-synthesized Cu-adsorbed PS microspheres after hydrothermal reaction is from 6 mm to 8 mm
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Fig. 1. (a) Size distribution by intensity of PS latex; (b) TEM image of PS latex.
(Fig. 2(b)). It is probably the high pressure and temperature under hydrothermal condition that changed the rheological behavior of PS so that some PS microspheres fusing together [29]. Fig. 2 shows the typical SEM images of the porous CuO hollow microspheres prepared by the mole ratio of C2H8N2/Cu(Ac)2$H2O of 1:1 at 160 C for 14 h. It is clearly seen that the diameters of the microspheres range from 10 mm to 15 mm from a low-magnification SEM images in Fig. 2(c). The hollow cavity porous structure can be evidently viewed from an individual broken sphere with the thickness of the sphere shell is ca. 2.5 mm, as is shown in Fig. 2(d).
Careful viewing of the hollow microsphere (Fig. 2(e) and (f)) reveals that the surface of the sphere is porous structure composed of irregular nano-polyhedra. The broken spheres obtained from the images are perhaps as a result of the escapement of CO2 during the calcinations. XRD analyzes have been carried out to confirm the structure and the phase composition of the samples. Fig. 3 shows the XRD spectra for pure PS emulsion particles, PS microspheres adsorbed with Cu2þ before calcination and the porous CuO hollow microspheres obtained after calcinations. Fig. 3(a) demonstrates that the obtained
Fig. 2. SEM images of (a) templating PS microspheres; (b) Cu-adsorbed PS microspheres after hydrothermal reaction; (c) porous CuO hollow microspheres; (d) an individual broken porous CuO hollow microsphere; (e) an individual intact porous CuO hollow microsphere; (f) the surface of the porous CuO hollow microsphere.
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absorbed water and surface hydroxyls, respectively. The bands at 1500e600 cm1 become weak even disappeared after calcination as shown in Fig. 4(c), indicating that most of the PS microspheres templates were removed. Moreover, the presence of strong bands at 522.61 cm1 and 485.62 cm1, which are corresponded to the Bu mode and Au mode of monoclinic phase of CuO respectively [30], demonstrate that the synthesized product is pure CuO with monoclinic phase. The FTIR results are also in good agreement with the XRD analysis results. 3.2. Influencing factors
Fig. 3. XRD patterns of (a) pure PS microspheres, (b) PS microspheres adsorbed with Cu2þ before calcinations and (c) the porous CuO hollow microspheres obtained after calcination.
PS latex is constituted of pure styrene as there only appears a broaden peak at about 22 . A series of peaks in conformity to the face-centered cubic (fcc) structure of metal copper, which perhaps because of the reduction of Cu2þ by ethylenediamine under the hydrothermal conditions, were surveyed from Fig. 3(b). After removal of PS cores by calcination (Fig. 3(c)), all peaks could be clearly indexed as monoclinic CuO phase (JCPDS Card NO.45-0937). Moreover, the major peaks located at 2q value of 35.199 and 38.341 indexed as ð111Þe(002) and (111)e(200) planes, respectively. Meanwhile, no peaks of impurities such as Cu or Cu2O were detected, indicating the high phase of the product. The quality and composition of as-obtained samples were further examined by the Fourier transform infrared (FTIR) spectroscopy. Fig. 4 shows the FTIR spectra of pure PS microspheres, PS microspheres adsorbed with Cu2þ before calcination and the porous CuO hollow microspheres obtained after calcination, respectively. From Fig. 4(a) and (b), we can see that the absorption bands of PS microspheres adsorbed with Cu2þ before calcination are similar to those of pure PS microspheres. The weak absorption band appeared at 3432.67 cm1 and 1600.63 cm1 are assigned to the stretching vibration of the OeH bond, which could be attributed to the stretching vibration and bending vibration of the
It is found that numerous factors such as the mole ratio of C2H8N2/Cu(Ac)2$H2O, the hydrothermal temperature and time have great influence on the size or morphology of the final products. The influence of the mole ratio of C2H8N2/Cu(Ac)2$H2O on the morphology of the products has been investigated under the hydrothermal temperature of 160 C for 14 h. A large amount of cavity bulk structure was obtained with the mole ratio of C2H8N2/ Cu(Ac)2$H2O of 0.5/1, as is shown in Fig. 5(a). When the mole ratio of C2H8N2/Cu(Ac)2$H2O was increased to 1/1, porous CuO hollow microspheres were obtained (Fig. 2(c)). However, further increasing the mole ratio of C2H8N2/Cu(Ac)2$H2O to 1.5/1, the products become unformed and no integrate sphere was found (Fig. 5(b)). From the above results, it can be suggested that ethylenediamine has a guiding effects on the formation of the products and CuO spheres were obtained at the mole ratio of C2H8N2/Cu(Ac)2$H2O of 1/1. The hydrothermal time also played a key role in the formation of porous CuO hollow spheres, while keeping other conditions unchanged. After 10 h of hydrothermal treatment, only a few porous CuO hollow spheres with the diameter of ca.10 mm were obtained (Fig. 6(a)). Huge amounts of porous CuO hollow spheres with the same diameter were gained when the hydrothermal time was extended to 14 h (Fig. 2(c)). Further extension of the hydrothermal time resulted in the formation of agminated and irregular products (Fig. 6(b)). The hydrothermal temperature is also an essential element influencing the size and morphology of the products. Porous CuO hollow spheres were synthesized at 160 C (Fig. 2(c)). When the temperature was kept at 140 C-lower than 160 C, the reaction could not be commenced and no products were obtained. Nevertheless, enhancing the temperature to 180 C, the morphologies of the products were not homogenous as that at 160 C and only a few spheres were obtained. 3.3. Formation mechanism
Fig. 4. FTIR patterns of (a) pure PS microspheres, (b) PS microspheres adsorbed with Cu2þ before calcinations and (c) the porous CuO hollow microspheres obtained after calcination.
From the observed SEM, XRD and FTIR results, we could predict the possible growth process for the formation of the porous CuO hollow microspheres. The whole encapsulation process can be defined as a three-step adsorption process as is shown in Fig. 7. In the first-step, the rheological behavior of PS was changed so that some PS microspheres fusing together high press and temperature is generated in hydrothermal condition. Meanwhile, Cu(en)2þ 2 during the complexation of C2H8N2 and Cu2þ in the solution of cupric acetate. In the following second-step, under hydrothermal condition, Cu(en)2þ 2 formed in the first step are adsorbed onto the glomerated PS spheres walls combining with negative charge on their surfaces. Finally, with the removal of PS spheres templates during the calcination process, shells with cross-linked and compact surfaces were obtained. In addition, the volume of the hollow spheres increased because of the escaping of CO2 during the calcination.
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Fig. 5. SEM images of the CuO products obtained with different mole ratio of C2H8N2/Cu(Ac)2$H2O: (a) 0.5:1, (b) 1.5:1.
Fig. 6. SEM images of the CuO products obtained for different hydrothermal time intervals: (a) 10 h, (b) 18 h.
Fig. 7. The schematic illustration of the formation mechanism of porous CuO hollow microspheres using PS latex as templates.
3.4. Measurement of surface area of porous CuO hollow microspheres The typical nitrogen adsorptionedesorption isotherm and BJH pore size distribution based on the desorption branch of porous CuO
hollow microspheres are presented in Fig. 8. They were typical typeII isotherms and displayed a little H3 hysteresis loop, according to the IUPAC classification. A clear adsorption step occurred at a relative pressure between 0 and 0.2, which is the characteristic of capillary condensation of N2 molecules in mesopores. The specific surface area
Fig. 8. Nitrogen adsorptionedesorption isotherm and pore size distribution (inset) of porous CuO hollow microspheres using PS latex as templates.
Fig. 9. UVevis absorption spectrum of porous CuO hollow microspheres using PS latex as templates. The inset is the (Ahv)2 vs. hv curve of the CuO hollow microspheres.
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of the porous CuO hollow microspheres measured by BET method is 81.71 m2/g. The corresponding PSD data calculated from the desorption branch of the nitrogen isotherms by the BJH method revealed that the average pore radius is 7.05 nm and total pore volume is 0.0683 cc/g, which matched well with the SEM image (Fig. 2(f)). 3.5. UVevis absorption spectrum To ascertain the band gap of the sample, UVevis absorption spectrum was carried out and presented in Fig. 9. It is clearly to see that there was an obvious and sharp peak appeared at ca.250 nm in the spectrum, which considerably blue-shifted from the value of 360 nm reported by Hong et al. [31]. The equation (ahv)2 ¼ K (hv Eg) could be used to calculate the band gap of the porous CuO hollow microspheres, where a is the absorption coefficient, K is a proportionality constant, and Eg is the band-gap energy [32]. The plot of (Ahv)2 vs. hv based on the direct transition is shown in the inset of Fig. 9. Interestingly, the extrapolated value (the straight lines to the x axis) of hv at Ahv ¼ 0 exhibits the band gap of 2.71 eV, which is larger than the reported value of bulk CuO (Eg ¼ 1.7 eV). The wellknown quantum confinement effects [33] perhaps be the reason for the blue-shift of the absorption peak and the increase of band gap. 3.6. Photocatalytic activity As an important p-type semiconductor, CuO has been widely doped into other semiconductors to improve their photocatalytic.
Yet, there is little research on the photocatalytic activity of cupric oxide alone. To explore the capability of the porous CuO hollow microspheres to remove pollutants from wastewater, we carried out an experiment on the degradation of organic dyestuff, rhodamine B (RhB) using the as-prepared CuO as catalyst. Experiments were carried out both with and without the porous CuO hollow microspheres as catalyst to discriminate the photocatalysis and just the photolysis of RhB. As an ionic compound dye, the absorption peak of RhB slightly decreased after it had been irradiated under 254 nm UV light for 120 min (Fig. 10(a)), indicating that only little amount of RhB was degraded without any catalyst. However, after use the as-synthesized CuO as catalysts, the absorption peak intensity obviously decreased (Fig. 10(b)). This is also identical in the degradation rate of RhB for different irradiation times. From Fig. 10(c), in 120 min, the degradation rate of RhB was more than 90% with porous CuO hollow microspheres as catalyst, however, it was no more than 20% when no catalyst was used. Similarly, the first-order apparent rate constants of porous CuO hollow microspheres catalysts for RhB photodegradation is much larger than that of its self-degradation, which is shown in Fig. 10(d). As recent report, one main reason for the high catalytic ability of the nanocatalysts could be ascribed to the exposed high surface-energy facets because they have better chemical activity than those of low surface energy [34]. Due to large surface area of porous CuO hollow microspheres, more active sites for the photocatalytic degradation were provided to degrade the RhB molecules.
Fig. 10. The UVevis absorption spectra of degradation of RhB solution (a) without catalysts and (b) with porous CuO hollow microspheres as catalyst; (c) The degradation rate of RhB for different irradiation times and (d) RhB photodegradation kinetic curves with and without porous CuO hollow microspheres as catalyst.
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4. Conclusions Porous CuO hollow microspheres with high specific surface area have been successfully synthesized under simple hydrothermal conditions using PS latex as templates. Many parameters including the mole ratio of C2H8N2/Cu(Ac)2$H2O, hydrothermal temperature and time could affect the size and morphology of the resulting products. The as-obtained porous CuO hollow microspheres exhibit high photocatalytic activity for the degradation of organic dyestuff RhB aqueous solution under UV light illumination. The unique hollow spheres of the CuO microspheres might have potential applications in the treatment of wastewater in industrial applications. References [1] C.E. Flynn, C. Mao, A. Hayhurst, J.L. Williams, Georgiou, B. Iverson, A.M. Belcher, J. Mater. Chem. 13 (2003) 2414e2421. [2] C.N.R. Rao, A. Govindaraj, F.L. Deepak, Appl. Phys. Lett. 78 (2001) 1853. [3] M.S. Gudiksen, C.M. Lieber, J. Am. Chem. Soc. 122 (2000) 8801e8802. [4] X. Liu, G. Yang, S. Fu, Mater. Sci. Eng. C 27 (2007) 750e755. [5] Y. Liu, L. Jiang, H. Dong, Small 7 (2011) 1412e1415. [6] J. Zhang, J. Liu, Q. Peng, Chem. Mater. 18 (2006) 867e871. [7] X. Zhang, G. Wang, X. Liu, J. Am. Chem. Soc. 112 (2008) 6845e6849. [8] P. Poizot, S. Laruelle, S. Grugeon, Nature 407 (2000) 496e499. [9] D. Shang, K. Yu, Y. Zhang, Appl. Surf. Sci. 255 (2009) 4093e4096. [10] R.D. Desautels, Y. Chen, H. Ouyang, S. Lo, J.W. Freeland, J. Appl. Phys. 111 (2012), 07B518e07B518-3. [11] R.P. Wijesundera, Semicond. Sci. Technol. 25 (2010).
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[12] H.A. Zaidi, K.K. Pant, J. Biofuels 1 (2010) 151e156. [13] M. Yang, J. He, J. Colloid Interface Sci. 355 (2011) 15e22. [14] Y. Xiang, J.P. Tu, J. Zhang, J. Zhong, D. Zhang, J.P. Cheng, Electrochem. Commun. 12 (2010) 1103e1107. [15] Y. Chang, H. Zeng, Cryst. Growth Des. 4 (2004) 397e402. [16] G.H. Du, G. Tendeloo, Chem. Phys. Lett. 393 (2004) 64e69. [17] M. Cao, C. Hu, Y. Wang, Y. Guo, C. Guo, E. Wang, Chem. Commun. 15 (2003). [18] J. Zhu, G. Zeng, F. Nie, X. Xu, S. Chen, Q. Han, X. Wang, Nanoscale 6 (2010). [19] Y. Zhang, S. Wang, Y. Qian, Solid State Sci. 8 (2006) 462e466. [20] Y. Wang, A. Yu, F. Caruso, Angew. Chem. Int. Ed. 117 (2005) 2948e2952. [21] B. Peng, F. Tang, D. Chen, J. Colloid Interface Sci. 329 (2009) 62e66. [22] K. Zhang, L. Zheng, X. Zhang, Colloid Surf. A 277 (2006) 145e150. [23] Y. Zhu, E. Kockrick, T. Ikoma, N. Hanagata, S. Kaskel, Chem. Mater. 21 (2009) 2547e2553. [24] H. Zhou, T. Fan, D. Zhang, Microporous Mesoporous Mater. 100 (2007) 322e327. [25] J.S. Chen, C.P. Chen, J. Liu, R. Xu, S.Z. Qiao, X.W. Lou, J. Mater. Chem. 47 (2011) 2631e2633. [26] J.S. Chen, L.A. Archer, X.W. (David) Lou, J. Mater. Chem. 21 9912e9924. [27] B. Wang, J.S. Chen, H.B. Wu, Z.Y. Wang, X.W. Lou, J. Am. Chem. Soc. 133 (2011) 1476e1478. [28] Q. Shao, X.J. Wang, Q.Y. Liu, L.Y. Wang, C. Kang, Q.Y. Wang, S.S. Ge, J. Nanosci. Nanotechnol. 11 (2011) 10271e10277. [29] J.T. Varkey, S.S. Rao, S. Thomas, J. Appl. Polym. Sci. 62 (1996) 2169e2180. [30] L. Chen, L. Li, G. Li, J. Alloys Compd. 464 (2008) 532e536. [31] J.M. Hong, J. Li, Y.G. Ni, J. Alloys Compd 481 (2009) 610e615. [32] H. Abdullah, N. Saadah, S. Shaari, A. Muchtar, Adv. Mater. 14 (2010) 149e152. [33] K. Takei, H. Fang, S. Kuma, R. Kapadia, Q. Gao, M. Madsen, H. Kim, C. Liu, Y. Chueh, E. Plis, S. Krishna, H. Bechtel, J. Guo, A. Javey, Nano Lett. 11 (2011) 5008e5012. [34] S.L. Wang, H. Xu, L.Q. Qian, X. Jia, J. Wang, Y. Liu, W. Tang, J. Solid State Chem. 182 (2009) 1088.
Contents lists available at SciVerse ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Hydrothermal synthesis and photocatalytic property of porous CuO hollow microspheres via PS latex as templates Qian Shao*, Ling-Yun Wang, Xiao-Jie Wang, Meng-Chen Yang, Sheng-Song Ge, Xiao-Kun Yang, Jun-Xiang Wang College of Chemical and Environmental Engineering, Shandong University of Science & Technology, Qingdao 266590, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 November 2012 Received in revised form 2 February 2013 Accepted 6 March 2013 Available online 14 March 2013
Porous copper oxide (CuO) hollow microspheres have been fabricated through a simple hydrothermal method using PS latex as templates. The as-obtained samples were characterized by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffractometry (XRD) and Fourier transform infrared spectroscopy (FTIR). The influences of the mole ratio of Ethylenediamine (C2H8N2) and copper acetate (Cu(Ac)2$H2O), hydrothermal temperature and time on the size and morphologies of the final products have been investigated. The possible formation mechanism of porous CuO hollow microspheres has been proposed and the specific surface area of the hollow microspheres with 81.71 m2/g is measured by BET method. The band gap value calculated from a UVevis absorption spectrum of porous CuO hollow microspheres is 2.71 eV. The as-synthesized product exhibits high photocatalytic activity during the photodegradation of an organic dyestuff, rhodamine B (RhB), under UV-light illumination. Ó 2013 Elsevier Masson SAS. All rights reserved.
Keywords: CuO Porous hollow spheres PS latex Photocatalytic property Template
1. Introduction In the past decades, controlled synthesis of semiconductor materials of explicit structure and composition, with specific properties, has been of tremendous scientific and technological interest, as the physical and chemical properties of semiconductor nanomaterials are largely dependent on their size and shape [1e3]. As an important p-type semiconductor metal oxide with a narrow band gap of 1.2e1.5 eV copper oxide (CuO) is a novel material both in terms of basic studies and practical applications [4,5]. The peculiar properties of CuO have made it as an important multifunctional material for its diverse and fascinating applications which include gas sensors [6,7], lithium ion electrode materials [8], field emission emitted materials [9], magnetic storage media [10] and solar cells [11]. In regard to its commercial value, CuO has been used as heterogenous catalysts in many chemical fields, such as complete conversion of hydrocarbon and phenol into carbon dioxide and water [12], degradation of nitrous oxide and selective catalytic reduction of nitric oxide with ammonia [13]. To date, CuO nanostructures with different morphologies, such as nanoplates [14], nanorods [15], nanowires [16], nanotubes [17], spindle [18]
* Corresponding author. Tel.: þ86 532 86057567. E-mail address: [email protected] (Q. Shao). 1293-2558/$ e see front matter Ó 2013 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.solidstatesciences.2013.03.006
and hollow spheres [19], have been fabricated by a range of physical and chemical methods. To the best of our knowledge, although CuO nanostructures with different morphologies have been obtained through various methods, rare report was found in literature on the synthesis of porous CuO hollow spheres. Hollow spheres have attracted great interests because of their potential applications. Different methods have been used to prepare hollow spheres materials, among which, the templatedirected synthesis, as a valid method, has been widely used. More recently, a series of hard templates such as silica spheres [20], woelm alumina, polymethyl methacrylate(PMMA) [21], polystyrene(PS) [22], carbon spheres [23], spherobacterium [24] are employed to conduct the growth of oxide hollow spheres such as SiO2, ZnO, Al2O3, TiO2. Chen et al. have successfully obtained ellipsoidal complex hollow nanostructures with the shells assembled from anatase TiO2 nanosheets with exposed (001) facets by utilizing silica (SiO2)-coated hematite (a-Fe2O3) nanospindles as the starting templates [25]. They also reviewed the synthesis of hollow SnO2 nanostructures created by both templating and template-free approaches [26]. In addition, Wang et al. have reported a soft-templated hydrothermal method to synthesize aFe2O3 hollow spheres with sheet-like sub-units [27]. In their work, glycerol is dispersed in water to form oil-in-water quasiemulsion microdroplets, which serve as soft templates for the deposition of the a-Fe2O3 shell.
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Recently, our group has demonstrated a novel hydrothermal process to synthesis porous CuO hollow spheres using carbon spheres as templates [28]. In the current paper, we present a simple and effective synthesis pathway for porous CuO microspheres employing PS latex as templates under hydrothermal conditions based on electrostatic adsorption. In particular, the PS latex templates were prepared using emulsifier-free emulsion polymerization hence it is environment-friendly. The morphology and phase of the final products were availably dominated by modifying reaction conditions such as the mole ratio of C2H8N2/Cu(Ac)2$H2O, hydrothermal temperature and hydrothermal time. The possible formation mechanism of the samples has been proposed based on the observed results. Moreover, the optical property and the photocatalytic activity of the as-obtained porous CuO hollow microspheres have also been evaluated from the UVevis absorption spectrum and photodegradation of a model pollutant, Rhodamine B(RhB), respectively. 2. Experimental details 2.1. Materials Potassium persulfate(KPS), copper acetate(Cu(Ac)2$H2O) and absolute ethanol were purchased from Tianjin Benchmark Chemical Reagent Co. Ltd. (Tianjin, Chian). Styrene (St) and ethylenediamine (en) were provided by Tianjin Bodi Co. Ltd. (Tianjin, Chian). Rhodamine B(RhB) was purchased from Shanghai Chemical Reagent Co. Ltd. (Shanghai, China). All the chemical reagents used in the experiment were analytical grade and used as received without further purification except that styrene was washed by 5%(wt) sodium hydroxide solution. Distilled water was used throughout the experiment. 2.2. Preparation of PS latex PS spheres of 500 nm diameter, used as templates, were prepared by emulsifier-free emulsion polymerization using KPS as anionic initiator. A typical synthesis of PS latex was carried out as follows: Under gentle stirring, 25 mL styrene and 200 mL distilled water were added into a 250-mL round flask in sequence. The solution was purged with nitrogen to eliminate the inhibition effect of oxygen before the polymerization was initiated at 75 C. After 15 min stirring, 25 mL KPS solution with the concentration of 1.6 102 M was added to the above solution. The polymerization was continued for 5 h with mechanical stirring at 300 rpm. The surface of the PS emulsion particle will have a negative charge as it was obtained by emulsifier-free emulsion polymerization using KPS as anionic initiator and without any emulsifier.
2.4. Characterization The morphology and size of the PS latex were examined by transmission electron microscopy (TEM, JEM-2100, Japan) with an accelerating voltage of 200 kV. The sample used for TEM was prepared by dipping the PS latex (dispersed in distilled water) on carbon-film coated copper grids. Scanning electron microscopes, (KYKY2800B, China) with acceleration voltage of 10 kV and (SU-70, Japan) with acceleration voltage of 30 KV were used for morphological imaging analysis of the hydrothermal products. The average particle size and polydispersity index (PDI) were obtained with a high sensitivity laser particle analyzer (ZEN1690, Malvern) with distilled water as dispersion at room temperature (25 C). The crystallographic information of the prepared samples was analyzed by a Rigaku D/Max2500PC X-ray diffraction (XRD) equipped with graphite monochromatized Cu Ka radiation (l ¼ 0.15418 nm), using a scanning rate of 4 min1 in 2q rage from 5 to 80 . Fourier transform infrared (FT-IR) spectra were measured on a NICOLTE 380 FTIR spectrometer using KBr pellet. Each FTIR spectrum was collected after 40 scans at a resolution of 4 cm1 from 400 to 4000 cm1. The nitrogen adsorptionedesorption isotherm was obtained from a volumetric adsorption analyzer (SSA-4300, China) in which the sample was first degassed in vacuum at 200 C overnight. And then the measurement was carried out with nitrogen as adsorption gas under the pressure from 0 to 120 kPa. The surface area and the pore size distribution were calculated by the BrunauereEmmetteTeller (BET) method and BarreteJoynere Halenda (BJH), respectively. Band gap of the product was determined from UVevis absorption spectrum on a UVevisible diffuse reflectance Spectrophotometer (UV-2450, Shimadzu). 2.5. Photocatalytic activity test The photocatalytic activity of the porous CuO hollow microspheres was measured at room temperature (ca.25 C) by the photocatalytic decolorization of a model pollutant rhodamine B (RhB). The experimental process was as follows: 15 mg of the prepared samples were dispersed in 30 mL of RhB aqueous solution with a concentration of 10 mg L1, and the suspensions were stirred for 30 min in the dark for the sake of establishing the absorptione desorption equilibrium between the catalyst and the RhB. The UVlight lamp with a wavelength of 254 nm placed 8 cm above the beaker was used as a light source. After UVevisible irradiation for certain time, the reaction solution was filtered, and the absorption spectrum of the centrifuged solution was then recorded with a UVe visible spectrophotometer (UV-3200PC, Shanghai). 3. Results and discussion 3.1. Characterizations of the as-prepared samples
2.3. Preparation of porous CuO hollow microspheres In a typical reaction process, 0.15 g copper acetate was dissolved in 10.5 mL distilled water to form a clear solution. The solution was then added with 0.05 mL anhydrous ethylenediamine under magnetic stirring to form a Cu(en)2þ complex solution. Afterward, 15 mL of the as-obtained PS latex was added to the above mixture under slow stirring. After ultrasonication for 30 min, the resulting solution was sealed in a 30 mL Teflon-lined stainless steel autoclave and maintained at 160 C for 14 h, and then cooled down to room temperature naturally. The black products were centrifuged, and washed several times with distilled water and absolute ethanol to remove the impurities, and then dried in a drying oven at 80 C for 6 h. Finally, the products were calcined in air at 600 C (heating rate of 20 C/min) for 2 h.
Size distribution by intensity of the obtained PS latex is shown in Fig. 1(a). It demonstrates that the PS latex is in high monodispersity with the average particle size of 474.7 nm and polydispersity index (PDI) of 0.0013. The TEM image of the PS latex in Fig. 1(b) also shows that the diameters of the PS emulsion particles are ca. 500 nm. The morphologies of PS microspheres and Cu-adsorbed PS microspheres after hydrothermal reaction were examined using SEM and the images are shown in Fig.2. Fig. 2(a) reveals that the PS microspheres have smooth surface, and the diameters of the PS microspheres are ca. 500 nm. The results are also in good agreement with the particle size analysis results. More interestingly, the size dispersion range of the as-synthesized Cu-adsorbed PS microspheres after hydrothermal reaction is from 6 mm to 8 mm
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Fig. 1. (a) Size distribution by intensity of PS latex; (b) TEM image of PS latex.
(Fig. 2(b)). It is probably the high pressure and temperature under hydrothermal condition that changed the rheological behavior of PS so that some PS microspheres fusing together [29]. Fig. 2 shows the typical SEM images of the porous CuO hollow microspheres prepared by the mole ratio of C2H8N2/Cu(Ac)2$H2O of 1:1 at 160 C for 14 h. It is clearly seen that the diameters of the microspheres range from 10 mm to 15 mm from a low-magnification SEM images in Fig. 2(c). The hollow cavity porous structure can be evidently viewed from an individual broken sphere with the thickness of the sphere shell is ca. 2.5 mm, as is shown in Fig. 2(d).
Careful viewing of the hollow microsphere (Fig. 2(e) and (f)) reveals that the surface of the sphere is porous structure composed of irregular nano-polyhedra. The broken spheres obtained from the images are perhaps as a result of the escapement of CO2 during the calcinations. XRD analyzes have been carried out to confirm the structure and the phase composition of the samples. Fig. 3 shows the XRD spectra for pure PS emulsion particles, PS microspheres adsorbed with Cu2þ before calcination and the porous CuO hollow microspheres obtained after calcinations. Fig. 3(a) demonstrates that the obtained
Fig. 2. SEM images of (a) templating PS microspheres; (b) Cu-adsorbed PS microspheres after hydrothermal reaction; (c) porous CuO hollow microspheres; (d) an individual broken porous CuO hollow microsphere; (e) an individual intact porous CuO hollow microsphere; (f) the surface of the porous CuO hollow microsphere.
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absorbed water and surface hydroxyls, respectively. The bands at 1500e600 cm1 become weak even disappeared after calcination as shown in Fig. 4(c), indicating that most of the PS microspheres templates were removed. Moreover, the presence of strong bands at 522.61 cm1 and 485.62 cm1, which are corresponded to the Bu mode and Au mode of monoclinic phase of CuO respectively [30], demonstrate that the synthesized product is pure CuO with monoclinic phase. The FTIR results are also in good agreement with the XRD analysis results. 3.2. Influencing factors
Fig. 3. XRD patterns of (a) pure PS microspheres, (b) PS microspheres adsorbed with Cu2þ before calcinations and (c) the porous CuO hollow microspheres obtained after calcination.
PS latex is constituted of pure styrene as there only appears a broaden peak at about 22 . A series of peaks in conformity to the face-centered cubic (fcc) structure of metal copper, which perhaps because of the reduction of Cu2þ by ethylenediamine under the hydrothermal conditions, were surveyed from Fig. 3(b). After removal of PS cores by calcination (Fig. 3(c)), all peaks could be clearly indexed as monoclinic CuO phase (JCPDS Card NO.45-0937). Moreover, the major peaks located at 2q value of 35.199 and 38.341 indexed as ð111Þe(002) and (111)e(200) planes, respectively. Meanwhile, no peaks of impurities such as Cu or Cu2O were detected, indicating the high phase of the product. The quality and composition of as-obtained samples were further examined by the Fourier transform infrared (FTIR) spectroscopy. Fig. 4 shows the FTIR spectra of pure PS microspheres, PS microspheres adsorbed with Cu2þ before calcination and the porous CuO hollow microspheres obtained after calcination, respectively. From Fig. 4(a) and (b), we can see that the absorption bands of PS microspheres adsorbed with Cu2þ before calcination are similar to those of pure PS microspheres. The weak absorption band appeared at 3432.67 cm1 and 1600.63 cm1 are assigned to the stretching vibration of the OeH bond, which could be attributed to the stretching vibration and bending vibration of the
It is found that numerous factors such as the mole ratio of C2H8N2/Cu(Ac)2$H2O, the hydrothermal temperature and time have great influence on the size or morphology of the final products. The influence of the mole ratio of C2H8N2/Cu(Ac)2$H2O on the morphology of the products has been investigated under the hydrothermal temperature of 160 C for 14 h. A large amount of cavity bulk structure was obtained with the mole ratio of C2H8N2/ Cu(Ac)2$H2O of 0.5/1, as is shown in Fig. 5(a). When the mole ratio of C2H8N2/Cu(Ac)2$H2O was increased to 1/1, porous CuO hollow microspheres were obtained (Fig. 2(c)). However, further increasing the mole ratio of C2H8N2/Cu(Ac)2$H2O to 1.5/1, the products become unformed and no integrate sphere was found (Fig. 5(b)). From the above results, it can be suggested that ethylenediamine has a guiding effects on the formation of the products and CuO spheres were obtained at the mole ratio of C2H8N2/Cu(Ac)2$H2O of 1/1. The hydrothermal time also played a key role in the formation of porous CuO hollow spheres, while keeping other conditions unchanged. After 10 h of hydrothermal treatment, only a few porous CuO hollow spheres with the diameter of ca.10 mm were obtained (Fig. 6(a)). Huge amounts of porous CuO hollow spheres with the same diameter were gained when the hydrothermal time was extended to 14 h (Fig. 2(c)). Further extension of the hydrothermal time resulted in the formation of agminated and irregular products (Fig. 6(b)). The hydrothermal temperature is also an essential element influencing the size and morphology of the products. Porous CuO hollow spheres were synthesized at 160 C (Fig. 2(c)). When the temperature was kept at 140 C-lower than 160 C, the reaction could not be commenced and no products were obtained. Nevertheless, enhancing the temperature to 180 C, the morphologies of the products were not homogenous as that at 160 C and only a few spheres were obtained. 3.3. Formation mechanism
Fig. 4. FTIR patterns of (a) pure PS microspheres, (b) PS microspheres adsorbed with Cu2þ before calcinations and (c) the porous CuO hollow microspheres obtained after calcination.
From the observed SEM, XRD and FTIR results, we could predict the possible growth process for the formation of the porous CuO hollow microspheres. The whole encapsulation process can be defined as a three-step adsorption process as is shown in Fig. 7. In the first-step, the rheological behavior of PS was changed so that some PS microspheres fusing together high press and temperature is generated in hydrothermal condition. Meanwhile, Cu(en)2þ 2 during the complexation of C2H8N2 and Cu2þ in the solution of cupric acetate. In the following second-step, under hydrothermal condition, Cu(en)2þ 2 formed in the first step are adsorbed onto the glomerated PS spheres walls combining with negative charge on their surfaces. Finally, with the removal of PS spheres templates during the calcination process, shells with cross-linked and compact surfaces were obtained. In addition, the volume of the hollow spheres increased because of the escaping of CO2 during the calcination.
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Fig. 5. SEM images of the CuO products obtained with different mole ratio of C2H8N2/Cu(Ac)2$H2O: (a) 0.5:1, (b) 1.5:1.
Fig. 6. SEM images of the CuO products obtained for different hydrothermal time intervals: (a) 10 h, (b) 18 h.
Fig. 7. The schematic illustration of the formation mechanism of porous CuO hollow microspheres using PS latex as templates.
3.4. Measurement of surface area of porous CuO hollow microspheres The typical nitrogen adsorptionedesorption isotherm and BJH pore size distribution based on the desorption branch of porous CuO
hollow microspheres are presented in Fig. 8. They were typical typeII isotherms and displayed a little H3 hysteresis loop, according to the IUPAC classification. A clear adsorption step occurred at a relative pressure between 0 and 0.2, which is the characteristic of capillary condensation of N2 molecules in mesopores. The specific surface area
Fig. 8. Nitrogen adsorptionedesorption isotherm and pore size distribution (inset) of porous CuO hollow microspheres using PS latex as templates.
Fig. 9. UVevis absorption spectrum of porous CuO hollow microspheres using PS latex as templates. The inset is the (Ahv)2 vs. hv curve of the CuO hollow microspheres.
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of the porous CuO hollow microspheres measured by BET method is 81.71 m2/g. The corresponding PSD data calculated from the desorption branch of the nitrogen isotherms by the BJH method revealed that the average pore radius is 7.05 nm and total pore volume is 0.0683 cc/g, which matched well with the SEM image (Fig. 2(f)). 3.5. UVevis absorption spectrum To ascertain the band gap of the sample, UVevis absorption spectrum was carried out and presented in Fig. 9. It is clearly to see that there was an obvious and sharp peak appeared at ca.250 nm in the spectrum, which considerably blue-shifted from the value of 360 nm reported by Hong et al. [31]. The equation (ahv)2 ¼ K (hv Eg) could be used to calculate the band gap of the porous CuO hollow microspheres, where a is the absorption coefficient, K is a proportionality constant, and Eg is the band-gap energy [32]. The plot of (Ahv)2 vs. hv based on the direct transition is shown in the inset of Fig. 9. Interestingly, the extrapolated value (the straight lines to the x axis) of hv at Ahv ¼ 0 exhibits the band gap of 2.71 eV, which is larger than the reported value of bulk CuO (Eg ¼ 1.7 eV). The wellknown quantum confinement effects [33] perhaps be the reason for the blue-shift of the absorption peak and the increase of band gap. 3.6. Photocatalytic activity As an important p-type semiconductor, CuO has been widely doped into other semiconductors to improve their photocatalytic.
Yet, there is little research on the photocatalytic activity of cupric oxide alone. To explore the capability of the porous CuO hollow microspheres to remove pollutants from wastewater, we carried out an experiment on the degradation of organic dyestuff, rhodamine B (RhB) using the as-prepared CuO as catalyst. Experiments were carried out both with and without the porous CuO hollow microspheres as catalyst to discriminate the photocatalysis and just the photolysis of RhB. As an ionic compound dye, the absorption peak of RhB slightly decreased after it had been irradiated under 254 nm UV light for 120 min (Fig. 10(a)), indicating that only little amount of RhB was degraded without any catalyst. However, after use the as-synthesized CuO as catalysts, the absorption peak intensity obviously decreased (Fig. 10(b)). This is also identical in the degradation rate of RhB for different irradiation times. From Fig. 10(c), in 120 min, the degradation rate of RhB was more than 90% with porous CuO hollow microspheres as catalyst, however, it was no more than 20% when no catalyst was used. Similarly, the first-order apparent rate constants of porous CuO hollow microspheres catalysts for RhB photodegradation is much larger than that of its self-degradation, which is shown in Fig. 10(d). As recent report, one main reason for the high catalytic ability of the nanocatalysts could be ascribed to the exposed high surface-energy facets because they have better chemical activity than those of low surface energy [34]. Due to large surface area of porous CuO hollow microspheres, more active sites for the photocatalytic degradation were provided to degrade the RhB molecules.
Fig. 10. The UVevis absorption spectra of degradation of RhB solution (a) without catalysts and (b) with porous CuO hollow microspheres as catalyst; (c) The degradation rate of RhB for different irradiation times and (d) RhB photodegradation kinetic curves with and without porous CuO hollow microspheres as catalyst.
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4. Conclusions Porous CuO hollow microspheres with high specific surface area have been successfully synthesized under simple hydrothermal conditions using PS latex as templates. Many parameters including the mole ratio of C2H8N2/Cu(Ac)2$H2O, hydrothermal temperature and time could affect the size and morphology of the resulting products. The as-obtained porous CuO hollow microspheres exhibit high photocatalytic activity for the degradation of organic dyestuff RhB aqueous solution under UV light illumination. The unique hollow spheres of the CuO microspheres might have potential applications in the treatment of wastewater in industrial applications. References [1] C.E. Flynn, C. Mao, A. Hayhurst, J.L. Williams, Georgiou, B. Iverson, A.M. Belcher, J. Mater. Chem. 13 (2003) 2414e2421. [2] C.N.R. Rao, A. Govindaraj, F.L. Deepak, Appl. Phys. Lett. 78 (2001) 1853. [3] M.S. Gudiksen, C.M. Lieber, J. Am. Chem. Soc. 122 (2000) 8801e8802. [4] X. Liu, G. Yang, S. Fu, Mater. Sci. Eng. C 27 (2007) 750e755. [5] Y. Liu, L. Jiang, H. Dong, Small 7 (2011) 1412e1415. [6] J. Zhang, J. Liu, Q. Peng, Chem. Mater. 18 (2006) 867e871. [7] X. Zhang, G. Wang, X. Liu, J. Am. Chem. Soc. 112 (2008) 6845e6849. [8] P. Poizot, S. Laruelle, S. Grugeon, Nature 407 (2000) 496e499. [9] D. Shang, K. Yu, Y. Zhang, Appl. Surf. Sci. 255 (2009) 4093e4096. [10] R.D. Desautels, Y. Chen, H. Ouyang, S. Lo, J.W. Freeland, J. Appl. Phys. 111 (2012), 07B518e07B518-3. [11] R.P. Wijesundera, Semicond. Sci. Technol. 25 (2010).
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