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Controllable fabrication of cadmium phthalocyanine nanostructures immobilized on electrospun polyacrylonitrile nanofibers with high photocatalytic properties under visible light
Catalysis Communications 12 (2011) 880–885
Contents lists available at ScienceDirect
Catalysis Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t c o m
Short Communication
Controllable fabrication of cadmium phthalocyanine nanostructures immobilized on electrospun polyacrylonitrile nanofibers with high photocatalytic properties under visible light Zengcai Guo, Changlu Shao ⁎, Jingbo Mu, Mingyi Zhang, Zhenyi Zhang, Peng Zhang, Bin Chen ⁎, Yichun Liu Center for Advanced Optoelectronic Functional Materials Research, and Key Laboratory of UV Light-Emitting Materials and Technology of Ministry of Education, and Department of Chemistry, Northeast Normal University, 5268 Renmin Street, Changchun 130024, PR China
a r t i c l e
i n f o
Article history: Received 9 December 2010 Received in revised form 27 January 2011 Accepted 1 February 2011 Available online 21 March 2011 Keywords: Phthalocyanine Immobilization Composite nanofibers Reuse Photocatalysis
a b s t r a c t A novel photocatalyst of nanostructured cadmium phthalocyanine (CdPc) immobilized on the surface of polyacrylonitrile (PAN) nanofibers had been successfully fabricated by a simple combination of electrospinning technique and the solvent-thermal process. FE-SEM micrographs indicated that the nanostructured CdPc uniformly immobilized on the surface of PAN nanofibers without agglomeration. And the obtained CdPc/PAN composite nanofibers exhibited high visible light photocatalytic activity for the degradation of rhodamine B. Moreover, this photocatalyst could be easily separated for reuse due to the one-dimensional nanostructural property of the CdPc/PAN composite nanofibers. © 2011 Elsevier B.V. All rights reserved.
1. Introduction As we all know, metal phthalocyanines are porphyrin derivatives consisting of a central metallic atom bound to a π-conjugated ligand, characterized by high symmetry, planarity, and electron delocalization [1]. With the development of nanoscience and nanotechnology, considerable attention has been focused on nanosized phthalocyanine owing to its unique properties and promising applications. Such as photovoltaic cell [2,3], chemical sensors [4], light-emitting diodes [5], organic field effect transistors [6,7], microelectronics [8] and photocatalysis [9–11]. Especially, the photocatalysis of metal phthalocyanines has been the focus of numerous investigations nowadays due to their intense absorption bands in the longer wavelength region of the visible light in a solar spectrum [12,13]. However, the photocatalysis of different phthalocyanine complexes in the homogeneous system has been greatly limited by the separation of the photocatalyst from the products, which is usually difficult and may incur high extra cost [14]. To overcome these hurdles, many efforts have been made to immobilize the photocatalysts on various supports [15–21]. The nanostructured supports stand out of other supports because of their extremely high surface
⁎ Corresponding authors. Tel.: +86 43185098803. E-mail addresses: [email protected] (C. Shao), [email protected] (B. Chen). 1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2011.02.004
area-to-volume ratios, which can provide large specific surface areas for highly efficient immobilization [22,23]. So far, different dimensional nanostructured materials have been used as supports, such as nanoparticles [17,18] and nanofilms [19–21]. However, these nanostructured materials have disadvantages that are difficult to overcome. For example, the small particle size of the nanoparticles lead to difficulty in subsidence, so the dispersion and the recycling are not satisfied for large-scale applications. The relatively small surface areas of the nanofilms decrease the specific surface area of the photocatalysts, so the efficiency of the photocatalysis is significantly decreased. On the contrary, electrospun polyacrylonitrile (PAN) nanofibers have a great potential to overcome these problems, and may be promising supports for the immobilization of phthalocyanine. Briefly, the qualification of electrospun PAN nanofibers as excellent supports is attributed to: (i) the more easily separation due to the one-dimensional nanostructural property, the randomly arrayed nanofibers forms a non-woven mesh would benefit for the separation and reuse of the photocatalysts; (ii) the high level exposure of the photocatalysts nanoparticles due to the large surface areas of the onedimensional nanofibers. To date, there has been no report on the fabrication of CdPc/PAN composite nanofibers. In this paper, we report a successful immobilization of nanostructured CdPc on the support of electrospun PAN nanofibers. The photocatalytic activities of the as-prepared CdPc/PAN composite nanofibers for the degradation of rhodamine B under visible-light irradiation were carefully investigated.
Z. Guo et al. / Catalysis Communications 12 (2011) 880–885
2. Experimental section 2.1. Fabrication of phthalonitrile/polyacrylonitrile (PN/PAN) nanofibers Firstly, 1.5 g of PAN powder was dissolved in 10 ml DMF with stirring for about 24 h at room temperature. Then 1 g of PN was added to the PAN solution over a 2 h period. Subsequently, the above mixture was stirred for another 14 h at room temperature to produce a transparent and viscous solution. After that, the obtained solution of PN/PAN composite was contained in a syringe pump for electrospinning. A copper pin connected to a high-voltage generator was placed in the solution, and the solution was kept in the syringe by adjusting the angle between syringe and the collector. A drum covered with an aluminum foil, served as the counter electrode. The operating voltage was 12 kV and the distance between the syringe tip and the collector was 12 cm, resulting in a dense web of electrospun PN/PAN nanofibers being collected on the aluminum foil. 2.2. Fabrication of CdPc/PAN composite nanofibers In this reaction, we chose ammonium molybdate as the catalyst. In a typical preparation, PN/PAN nanofibers (20 mg), Cd(NO3).24H2O (0.100 mmol), and ammonium molybdate (3 mg) were put into a 25 mL Teflon-lined stainless autoclave. The autoclave was maintained at 160 °C for 15 h with ethylene glycol up to 80% of the total volume, and then cooled to room temperature naturally. The obtained samples were washed with distilled water under ultrasound, then, dried at 60 °C for 8 h. By this method, three samples of CdPc/PAN composite nanofibers were fabricated, and then were denoted as S1, S2, and S3, respectively. Simplicity, the detailed experimental conditions were listed in Table 1. In addition, for contrast, the treated of PN/PAN nanofibers and the synthesis of pure CdPc were by solvent-thermal method and ready for further test. 2.3. Characterization The field emission SEM was performed on a Hitachi S-4800 FESEM field emission scanning electron microscope. Energy dispersive X-ray (EDX) spectroscopy being attached to scanning electron microscopy (SEM) was used to analyze the composition of samples. XRD patterns of the sample were recorded on a Rigaku, D/max-2500 X-ray diffractometer. UV–vis absorption spectra were recorded on a Cary 500 UV–vis-NIR SPECTROPHOTOMETER. IR spectra were recorded on an Aipha-Centuart FT-IR spectrometer (KBr). X-ray photoelectron spectroscopy (XPS) was performed on a VG ESCALAB LKII instrument with Mg KR-ADES (hν = 1253.6 eV) source at a residual gas pressure of below 10−8 Pa. 2.4. Photocatalytic test The photoreactor was designed with an internal xenon lamp (XHA 150 W and the average intensity was 28 mW/cm2) equipped with a cut-off glass filter transmitting N400 nm surrounded by a water-cooling quartz jacket to cool the lamp, where a 100 mL aliquot of the rhodamine B (RhB) solution with an initial concentration of 10 mg L−1 in the presence of CdPc/PAN nanocatalyst (0.1 g). The
Table 1 Experimental conditions and CdPc nanostructure characteristics of the prepared samples. Sample
Time/h
Temperature/°C
Cd(NO3)2.3H2O/mM
Morphology
S1 S2 S3
15 30 15
160 160 160
0.1 0.1 0.2
Nano-cubes Nano-cubes Nano-spindles
881
solution was stirred in the dark for 1 h to obtain a good dispersion and established adsorption–desorption equilibrium between the organic molecules and the catalyst surface. Decreases in the concentrations of dyes were analyzed by a Cary 500 UV–vis-NIR spectrophotometer. At given intervals of illumination, the samples of the reaction solution were taken out and analyzed. 3. Results and discussion Fig. 1a showed the SEM image of PN/PAN composite nanofibers. It could be clearly seen that the surface of the PN/PAN nanofibers were relatively smooth. The diameter of the fibers ranged from 200 to 300 nm. The SEM image of the sample S1 was shown in Fig. 1b, it indicated that the CdPc nanocubes were uniformly distributed across the surface of each nanofiber. But, there was essentially no change to the morphology of the nanofibers during the growth of the CdPc nanocubes. Moreover, the energy dispersive X-ray (EDX) analysis in Fig. 1e demonstrated that only N, C and O elements existed in the sample. While, in Fig. 1f Cd, N, C and O elements were detected in the sample, indicating that the growth of the CdPc on the PN/PAN nanofibers. We further explored the influence of different reaction conditions on the formation of CdPc nanostructures. By tuning the experimental parameters including precursor concentration and reaction temperature, the secondary CdPc nanostructures grown on PN/PAN nanofibers with different shapes and density could be further controlled. As observed in Fig. 1c the dimension of the cubes were dramatically increased when the growth time was increased to 30 h. As shown in Fig. 1d, with the increase of reaction concentration, the morphology of secondary CdPc nanostructures grown on the nanofibers changed significantly, and the grown density of CdPc nanostructures on PN/PAN nanofibers was increased. From the inset of Fig. 1d, we could clearly see that the CdPc nano-spindles were uniformly distributed across the surface of each fiber without aggregation, offering high level exposure of the nanocatalyst. The EDX of the samples S2 and S3 were shown in Fig. 1g and h, respectively. The X-ray diffraction (XRD) patterns of the sample CdPc/PAN (S3) were shown in Fig. 2. As observed in Fig. 2, all of the reflection peaks of S3 could be indexed as CdPc (JCPDS card number 14-904), indicating that the CdPc nanostructures were successfully fabricated on the surface of PAN nanofibers. The UV–vis absorption spectra of PN/PAN nanofibers, CdPc/PAN (S3 was selected as a sample) and pure CdPc were shown in Fig. 3. All the samples were dissolved in DMF, respectively. As observed in Fig. 3a there was no peak of PN/PAN nanofibers, while the UV–vis absorption spectra of S3 and pure CdPc in Fig. 3b and c exhibited two absorption region, which were the characteristic Q and B bands of phthalocyanines [24,25]. The absorption region at 550–750 nm was the typical Q-band electronic spectra of CdPc, which should be assigned to the π–π* transition of monomers from the HOMO to the LUMO of the Pc−2 ring [26]. Band of 300–450 nm arising from the deeper π levels-LUMO transition belonged to the typical B-band absorption of phthalocyanines [27,28]. FT-IR spectra were further utilized to detect the presence of all the typical features of the CdPc molecule in CdPc/PAN (S3). The FTIR spectra of PN/PAN nanofibers and CdPc/PAN (S3) were shown in Fig. S (see supplementary data for Fig. S). As observed in Fig. S, all the peaks in Fig. Sa were present in Fig. Sb, except the characteristic stretching vibration of C=N at 2240 cm−1. The disappearance of C=N in CdPc/PAN composite nanofibers might be originate from the interaction of PAN and CdPc. In Fig. Sb, the absorption peaks at 1558 cm−1, 1166 cm−1, and 858 cm−1 were assigned to phthalocyanine skeletal vibration [29]. Absorption peaks observed around 1405 cm−1, 1274 cm−1, 1079 cm−1 and 777 cm−1 were assigned to aromatic phenyl ring, C–N = stretch vibrations, C–H in-plane
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Z. Guo et al. / Catalysis Communications 12 (2011) 880–885
Fig. 1. SEM images of samples (a) PN/PAN, (b) S1, (c) S2, and (d) S3; EDX spectra of samples (e) PN/PAN, (f) S1, (g) S2, and (h) S3.
bending vibrations and C–H out-of-plane bending vibrations, respectively [30]. The broad signal at 3435 cm−1 was due to the adsorption of water. The above results revealed that CdPc was synthesized successfully on the surface of PAN nanofibers. The chemical composition of CdPc/PAN composite nanofibers (S3) was further studied and compared with that of PN/PAN nanofibers by XPS analysis. The fully scanned spectra in Fig. 4a demonstrated that C, N, and O elements exist in PN/PAN nanofibers, while C, N, O, and Cd
exist in CdPc/PAN composite nanofibers (S3), respectively. The increasing amount of O element observed in Fig. 4a might originate from the high hygroscopic nature of CdPc, which was in accordance with the result of the EDX and FT-IR spectra above. As shown in Fig. 4b, there were two peaks in the Cd 3d region. The peak located at 405 eV corresponded to the Cd 3d5/2 and another one located at 412 eV was assigned to Cd 3d3/2, indicating a normal state of Cd3+ in the CdPc molecule.
Z. Guo et al. / Catalysis Communications 12 (2011) 880–885
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Fig. 3. UV–vis absorption spectra of the samples: (a) PN/PAN nanofibers; (b) CdPc/PAN (S3); (c) pure CdPc. Fig. 2. XRD patterns of the sample CdPc/PAN (S3).
3.1. Photocatalytic activity To test the photocatalytic activity of CdPc/PAN composite nanofibers for the degradation of organic pollutants, rhodamine B (RhB) was selected as a representative dye pollutant of industrial wastewaters. An attempt was made to look at the decoloration of this dye in aqueous solution in the presence of CdPc/PAN composite nanofibers. The photocatalytic activities of the as-prepared samples with different morphologies and reference sample were illustrated in Fig. 5a. The change of absorption spectra of RhB aqueous solution showed the change of its concentration. The initial concentration (C0), the final concentration (C), and the degradation (D%) had a mathematical expression as follows: D% ¼
C0 −C 100% C0
tures on CdPc/PAN composite nanofibers enhanced the photocatalytic activity of CdPc/PAN composite nanofibers. Moreover, the stability of the CdPc/PAN composite nanofibers (S3) was examined for degradation of RhB during a three cycle experiment, which was very important for the CdPc/PAN composite nanofibers to apply in environmental technology. As shown in Fig. 5c, each experiment was carried out under identical conditions, after a three cycle experiment, the photocatalytic activity of S3 remained almost unchanged. It was indicated that the CdPc/PAN composite nanofibers displayed an efficient photoactivity for the degradation of RhB under visible light irradiation and could easily be separated for reuse.
ð1Þ
After 1 h of absorption without visible light, the solutions with different samples were then subjected to visible light. It was indicated that the 16, 54, 75, 85% degradation of RhB were observed after 7 h irradiation for PN/PAN, S1, S2, and S3, respectively, implying that the immobile CdPc/PAN composite nanofibers as a photocatalyst had excellent photocatalytic activity for degradation of dye pollutants. For a better comparison of the photocatalytic efficiency of the samples, the kinetic analysis of degradation of RhB was discussed. The kinetic linear simulation curves of the photocatalytic degradation of RhB over the above photocatalysts showed that the above degradation reactions followed the Langmuir– Hinshelwood apparent first-order kinetics. The apparent first-order model was described below: ln C0 = C = kKt = kapp t
ð2Þ
where kapp was the apparent first-order rate constant (min−1). The determined kapp values for different photocatalysts were summarized in Fig. 5b. The photocatalytic reactivity order was S3 N S2 N S1, which was well consistent with the content of CdPc. Further comparative experiments were carried out to compare the photocatalytic activity of the CdPc/PAN composite nanofibers with P-25 titania and CdPc powder under the same experimental conditions. As shown in Fig. 5c, P-25 titania had a low photocatalytic activity under visible light, and the degradation was only 7% in 7 h. For the CdPc powder, the degradation rate reached 43% in 7 h. In contrast, the CdPc/PAN composite nanofibers exhibited high photocatalytic activity. The corresponding degradation rates of RhB reached about 85% within 7 h, which illuminated that the large surface area of the CdPc/PAN composite nanofibers and the high dispersivity of CdPc nanostruc-
Fig. 4. (a) XPS fully scanned spectra of PN/PAN and CdPc/PAN composite nanofibers (S3); (b) XPS spectra of Cd 3d for CdPc/PAN composite nanofibers (S3).
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Fig. 5. (a) Degradation profiles of RhB over different samples. (b) Kinetic linear simulation curves of RhB photocatalytic degradation with different samples. (c) Degradation profiles of RhB over different samples (RhB = 10 mg/L, P25, S3 = 0.1 g, CdPc powder = 0.03 g). (d) The photocatalytic degradation of RhB over S3 under visible light irradiation for 3 cycles.
4. Conclusion The different morphologies of CdPc nanostructures had been successfully fabricated on the electrospun PAN nanofibers via the easy electrospinning and solvent-thermal methods. The photocatalysis of CdPc/PAN composite nanofibers for potential use sunlight as the energy source to degrade organic pollutions was effective. More importantly, the CdPc/PAN composite nanofibers photocatalysts with high photocatalytic activity could be easily separated and reused by sedimentation, which can greatly promote their industrial application to eliminate the organic pollutants from wastewater. We believed that the as-prepared CdPc/PAN composite nanofibers were of tremendous potential in practical use to eliminate the organic pollutants from wastewater. Supplementary materials related to this article can be found online at doi:10.1016/j.catcom.2011.02.004. Acknowledgements The present work is supported financially by the National Natural Science Foundation of China (Nos. 50572014, 50972027, and 10647108) and the Program for New Century Excellent Talents in University (NCET-05-0322). References [1] W.Y. Tong, A.B. Djurišić, M.H. Xie, A.C.M. Ng, K.Y. Cheung, W.K. Chan, Y.H. Leung, H.W. Lin, S. Gwo, The Journal of Physical Chemistry B 110 (2006) 17406–17413.
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Contents lists available at ScienceDirect
Catalysis Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t c o m
Short Communication
Controllable fabrication of cadmium phthalocyanine nanostructures immobilized on electrospun polyacrylonitrile nanofibers with high photocatalytic properties under visible light Zengcai Guo, Changlu Shao ⁎, Jingbo Mu, Mingyi Zhang, Zhenyi Zhang, Peng Zhang, Bin Chen ⁎, Yichun Liu Center for Advanced Optoelectronic Functional Materials Research, and Key Laboratory of UV Light-Emitting Materials and Technology of Ministry of Education, and Department of Chemistry, Northeast Normal University, 5268 Renmin Street, Changchun 130024, PR China
a r t i c l e
i n f o
Article history: Received 9 December 2010 Received in revised form 27 January 2011 Accepted 1 February 2011 Available online 21 March 2011 Keywords: Phthalocyanine Immobilization Composite nanofibers Reuse Photocatalysis
a b s t r a c t A novel photocatalyst of nanostructured cadmium phthalocyanine (CdPc) immobilized on the surface of polyacrylonitrile (PAN) nanofibers had been successfully fabricated by a simple combination of electrospinning technique and the solvent-thermal process. FE-SEM micrographs indicated that the nanostructured CdPc uniformly immobilized on the surface of PAN nanofibers without agglomeration. And the obtained CdPc/PAN composite nanofibers exhibited high visible light photocatalytic activity for the degradation of rhodamine B. Moreover, this photocatalyst could be easily separated for reuse due to the one-dimensional nanostructural property of the CdPc/PAN composite nanofibers. © 2011 Elsevier B.V. All rights reserved.
1. Introduction As we all know, metal phthalocyanines are porphyrin derivatives consisting of a central metallic atom bound to a π-conjugated ligand, characterized by high symmetry, planarity, and electron delocalization [1]. With the development of nanoscience and nanotechnology, considerable attention has been focused on nanosized phthalocyanine owing to its unique properties and promising applications. Such as photovoltaic cell [2,3], chemical sensors [4], light-emitting diodes [5], organic field effect transistors [6,7], microelectronics [8] and photocatalysis [9–11]. Especially, the photocatalysis of metal phthalocyanines has been the focus of numerous investigations nowadays due to their intense absorption bands in the longer wavelength region of the visible light in a solar spectrum [12,13]. However, the photocatalysis of different phthalocyanine complexes in the homogeneous system has been greatly limited by the separation of the photocatalyst from the products, which is usually difficult and may incur high extra cost [14]. To overcome these hurdles, many efforts have been made to immobilize the photocatalysts on various supports [15–21]. The nanostructured supports stand out of other supports because of their extremely high surface
⁎ Corresponding authors. Tel.: +86 43185098803. E-mail addresses: [email protected] (C. Shao), [email protected] (B. Chen). 1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2011.02.004
area-to-volume ratios, which can provide large specific surface areas for highly efficient immobilization [22,23]. So far, different dimensional nanostructured materials have been used as supports, such as nanoparticles [17,18] and nanofilms [19–21]. However, these nanostructured materials have disadvantages that are difficult to overcome. For example, the small particle size of the nanoparticles lead to difficulty in subsidence, so the dispersion and the recycling are not satisfied for large-scale applications. The relatively small surface areas of the nanofilms decrease the specific surface area of the photocatalysts, so the efficiency of the photocatalysis is significantly decreased. On the contrary, electrospun polyacrylonitrile (PAN) nanofibers have a great potential to overcome these problems, and may be promising supports for the immobilization of phthalocyanine. Briefly, the qualification of electrospun PAN nanofibers as excellent supports is attributed to: (i) the more easily separation due to the one-dimensional nanostructural property, the randomly arrayed nanofibers forms a non-woven mesh would benefit for the separation and reuse of the photocatalysts; (ii) the high level exposure of the photocatalysts nanoparticles due to the large surface areas of the onedimensional nanofibers. To date, there has been no report on the fabrication of CdPc/PAN composite nanofibers. In this paper, we report a successful immobilization of nanostructured CdPc on the support of electrospun PAN nanofibers. The photocatalytic activities of the as-prepared CdPc/PAN composite nanofibers for the degradation of rhodamine B under visible-light irradiation were carefully investigated.
Z. Guo et al. / Catalysis Communications 12 (2011) 880–885
2. Experimental section 2.1. Fabrication of phthalonitrile/polyacrylonitrile (PN/PAN) nanofibers Firstly, 1.5 g of PAN powder was dissolved in 10 ml DMF with stirring for about 24 h at room temperature. Then 1 g of PN was added to the PAN solution over a 2 h period. Subsequently, the above mixture was stirred for another 14 h at room temperature to produce a transparent and viscous solution. After that, the obtained solution of PN/PAN composite was contained in a syringe pump for electrospinning. A copper pin connected to a high-voltage generator was placed in the solution, and the solution was kept in the syringe by adjusting the angle between syringe and the collector. A drum covered with an aluminum foil, served as the counter electrode. The operating voltage was 12 kV and the distance between the syringe tip and the collector was 12 cm, resulting in a dense web of electrospun PN/PAN nanofibers being collected on the aluminum foil. 2.2. Fabrication of CdPc/PAN composite nanofibers In this reaction, we chose ammonium molybdate as the catalyst. In a typical preparation, PN/PAN nanofibers (20 mg), Cd(NO3).24H2O (0.100 mmol), and ammonium molybdate (3 mg) were put into a 25 mL Teflon-lined stainless autoclave. The autoclave was maintained at 160 °C for 15 h with ethylene glycol up to 80% of the total volume, and then cooled to room temperature naturally. The obtained samples were washed with distilled water under ultrasound, then, dried at 60 °C for 8 h. By this method, three samples of CdPc/PAN composite nanofibers were fabricated, and then were denoted as S1, S2, and S3, respectively. Simplicity, the detailed experimental conditions were listed in Table 1. In addition, for contrast, the treated of PN/PAN nanofibers and the synthesis of pure CdPc were by solvent-thermal method and ready for further test. 2.3. Characterization The field emission SEM was performed on a Hitachi S-4800 FESEM field emission scanning electron microscope. Energy dispersive X-ray (EDX) spectroscopy being attached to scanning electron microscopy (SEM) was used to analyze the composition of samples. XRD patterns of the sample were recorded on a Rigaku, D/max-2500 X-ray diffractometer. UV–vis absorption spectra were recorded on a Cary 500 UV–vis-NIR SPECTROPHOTOMETER. IR spectra were recorded on an Aipha-Centuart FT-IR spectrometer (KBr). X-ray photoelectron spectroscopy (XPS) was performed on a VG ESCALAB LKII instrument with Mg KR-ADES (hν = 1253.6 eV) source at a residual gas pressure of below 10−8 Pa. 2.4. Photocatalytic test The photoreactor was designed with an internal xenon lamp (XHA 150 W and the average intensity was 28 mW/cm2) equipped with a cut-off glass filter transmitting N400 nm surrounded by a water-cooling quartz jacket to cool the lamp, where a 100 mL aliquot of the rhodamine B (RhB) solution with an initial concentration of 10 mg L−1 in the presence of CdPc/PAN nanocatalyst (0.1 g). The
Table 1 Experimental conditions and CdPc nanostructure characteristics of the prepared samples. Sample
Time/h
Temperature/°C
Cd(NO3)2.3H2O/mM
Morphology
S1 S2 S3
15 30 15
160 160 160
0.1 0.1 0.2
Nano-cubes Nano-cubes Nano-spindles
881
solution was stirred in the dark for 1 h to obtain a good dispersion and established adsorption–desorption equilibrium between the organic molecules and the catalyst surface. Decreases in the concentrations of dyes were analyzed by a Cary 500 UV–vis-NIR spectrophotometer. At given intervals of illumination, the samples of the reaction solution were taken out and analyzed. 3. Results and discussion Fig. 1a showed the SEM image of PN/PAN composite nanofibers. It could be clearly seen that the surface of the PN/PAN nanofibers were relatively smooth. The diameter of the fibers ranged from 200 to 300 nm. The SEM image of the sample S1 was shown in Fig. 1b, it indicated that the CdPc nanocubes were uniformly distributed across the surface of each nanofiber. But, there was essentially no change to the morphology of the nanofibers during the growth of the CdPc nanocubes. Moreover, the energy dispersive X-ray (EDX) analysis in Fig. 1e demonstrated that only N, C and O elements existed in the sample. While, in Fig. 1f Cd, N, C and O elements were detected in the sample, indicating that the growth of the CdPc on the PN/PAN nanofibers. We further explored the influence of different reaction conditions on the formation of CdPc nanostructures. By tuning the experimental parameters including precursor concentration and reaction temperature, the secondary CdPc nanostructures grown on PN/PAN nanofibers with different shapes and density could be further controlled. As observed in Fig. 1c the dimension of the cubes were dramatically increased when the growth time was increased to 30 h. As shown in Fig. 1d, with the increase of reaction concentration, the morphology of secondary CdPc nanostructures grown on the nanofibers changed significantly, and the grown density of CdPc nanostructures on PN/PAN nanofibers was increased. From the inset of Fig. 1d, we could clearly see that the CdPc nano-spindles were uniformly distributed across the surface of each fiber without aggregation, offering high level exposure of the nanocatalyst. The EDX of the samples S2 and S3 were shown in Fig. 1g and h, respectively. The X-ray diffraction (XRD) patterns of the sample CdPc/PAN (S3) were shown in Fig. 2. As observed in Fig. 2, all of the reflection peaks of S3 could be indexed as CdPc (JCPDS card number 14-904), indicating that the CdPc nanostructures were successfully fabricated on the surface of PAN nanofibers. The UV–vis absorption spectra of PN/PAN nanofibers, CdPc/PAN (S3 was selected as a sample) and pure CdPc were shown in Fig. 3. All the samples were dissolved in DMF, respectively. As observed in Fig. 3a there was no peak of PN/PAN nanofibers, while the UV–vis absorption spectra of S3 and pure CdPc in Fig. 3b and c exhibited two absorption region, which were the characteristic Q and B bands of phthalocyanines [24,25]. The absorption region at 550–750 nm was the typical Q-band electronic spectra of CdPc, which should be assigned to the π–π* transition of monomers from the HOMO to the LUMO of the Pc−2 ring [26]. Band of 300–450 nm arising from the deeper π levels-LUMO transition belonged to the typical B-band absorption of phthalocyanines [27,28]. FT-IR spectra were further utilized to detect the presence of all the typical features of the CdPc molecule in CdPc/PAN (S3). The FTIR spectra of PN/PAN nanofibers and CdPc/PAN (S3) were shown in Fig. S (see supplementary data for Fig. S). As observed in Fig. S, all the peaks in Fig. Sa were present in Fig. Sb, except the characteristic stretching vibration of C=N at 2240 cm−1. The disappearance of C=N in CdPc/PAN composite nanofibers might be originate from the interaction of PAN and CdPc. In Fig. Sb, the absorption peaks at 1558 cm−1, 1166 cm−1, and 858 cm−1 were assigned to phthalocyanine skeletal vibration [29]. Absorption peaks observed around 1405 cm−1, 1274 cm−1, 1079 cm−1 and 777 cm−1 were assigned to aromatic phenyl ring, C–N = stretch vibrations, C–H in-plane
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Fig. 1. SEM images of samples (a) PN/PAN, (b) S1, (c) S2, and (d) S3; EDX spectra of samples (e) PN/PAN, (f) S1, (g) S2, and (h) S3.
bending vibrations and C–H out-of-plane bending vibrations, respectively [30]. The broad signal at 3435 cm−1 was due to the adsorption of water. The above results revealed that CdPc was synthesized successfully on the surface of PAN nanofibers. The chemical composition of CdPc/PAN composite nanofibers (S3) was further studied and compared with that of PN/PAN nanofibers by XPS analysis. The fully scanned spectra in Fig. 4a demonstrated that C, N, and O elements exist in PN/PAN nanofibers, while C, N, O, and Cd
exist in CdPc/PAN composite nanofibers (S3), respectively. The increasing amount of O element observed in Fig. 4a might originate from the high hygroscopic nature of CdPc, which was in accordance with the result of the EDX and FT-IR spectra above. As shown in Fig. 4b, there were two peaks in the Cd 3d region. The peak located at 405 eV corresponded to the Cd 3d5/2 and another one located at 412 eV was assigned to Cd 3d3/2, indicating a normal state of Cd3+ in the CdPc molecule.
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Fig. 3. UV–vis absorption spectra of the samples: (a) PN/PAN nanofibers; (b) CdPc/PAN (S3); (c) pure CdPc. Fig. 2. XRD patterns of the sample CdPc/PAN (S3).
3.1. Photocatalytic activity To test the photocatalytic activity of CdPc/PAN composite nanofibers for the degradation of organic pollutants, rhodamine B (RhB) was selected as a representative dye pollutant of industrial wastewaters. An attempt was made to look at the decoloration of this dye in aqueous solution in the presence of CdPc/PAN composite nanofibers. The photocatalytic activities of the as-prepared samples with different morphologies and reference sample were illustrated in Fig. 5a. The change of absorption spectra of RhB aqueous solution showed the change of its concentration. The initial concentration (C0), the final concentration (C), and the degradation (D%) had a mathematical expression as follows: D% ¼
C0 −C 100% C0
tures on CdPc/PAN composite nanofibers enhanced the photocatalytic activity of CdPc/PAN composite nanofibers. Moreover, the stability of the CdPc/PAN composite nanofibers (S3) was examined for degradation of RhB during a three cycle experiment, which was very important for the CdPc/PAN composite nanofibers to apply in environmental technology. As shown in Fig. 5c, each experiment was carried out under identical conditions, after a three cycle experiment, the photocatalytic activity of S3 remained almost unchanged. It was indicated that the CdPc/PAN composite nanofibers displayed an efficient photoactivity for the degradation of RhB under visible light irradiation and could easily be separated for reuse.
ð1Þ
After 1 h of absorption without visible light, the solutions with different samples were then subjected to visible light. It was indicated that the 16, 54, 75, 85% degradation of RhB were observed after 7 h irradiation for PN/PAN, S1, S2, and S3, respectively, implying that the immobile CdPc/PAN composite nanofibers as a photocatalyst had excellent photocatalytic activity for degradation of dye pollutants. For a better comparison of the photocatalytic efficiency of the samples, the kinetic analysis of degradation of RhB was discussed. The kinetic linear simulation curves of the photocatalytic degradation of RhB over the above photocatalysts showed that the above degradation reactions followed the Langmuir– Hinshelwood apparent first-order kinetics. The apparent first-order model was described below: ln C0 = C = kKt = kapp t
ð2Þ
where kapp was the apparent first-order rate constant (min−1). The determined kapp values for different photocatalysts were summarized in Fig. 5b. The photocatalytic reactivity order was S3 N S2 N S1, which was well consistent with the content of CdPc. Further comparative experiments were carried out to compare the photocatalytic activity of the CdPc/PAN composite nanofibers with P-25 titania and CdPc powder under the same experimental conditions. As shown in Fig. 5c, P-25 titania had a low photocatalytic activity under visible light, and the degradation was only 7% in 7 h. For the CdPc powder, the degradation rate reached 43% in 7 h. In contrast, the CdPc/PAN composite nanofibers exhibited high photocatalytic activity. The corresponding degradation rates of RhB reached about 85% within 7 h, which illuminated that the large surface area of the CdPc/PAN composite nanofibers and the high dispersivity of CdPc nanostruc-
Fig. 4. (a) XPS fully scanned spectra of PN/PAN and CdPc/PAN composite nanofibers (S3); (b) XPS spectra of Cd 3d for CdPc/PAN composite nanofibers (S3).
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Fig. 5. (a) Degradation profiles of RhB over different samples. (b) Kinetic linear simulation curves of RhB photocatalytic degradation with different samples. (c) Degradation profiles of RhB over different samples (RhB = 10 mg/L, P25, S3 = 0.1 g, CdPc powder = 0.03 g). (d) The photocatalytic degradation of RhB over S3 under visible light irradiation for 3 cycles.
4. Conclusion The different morphologies of CdPc nanostructures had been successfully fabricated on the electrospun PAN nanofibers via the easy electrospinning and solvent-thermal methods. The photocatalysis of CdPc/PAN composite nanofibers for potential use sunlight as the energy source to degrade organic pollutions was effective. More importantly, the CdPc/PAN composite nanofibers photocatalysts with high photocatalytic activity could be easily separated and reused by sedimentation, which can greatly promote their industrial application to eliminate the organic pollutants from wastewater. We believed that the as-prepared CdPc/PAN composite nanofibers were of tremendous potential in practical use to eliminate the organic pollutants from wastewater. Supplementary materials related to this article can be found online at doi:10.1016/j.catcom.2011.02.004. Acknowledgements The present work is supported financially by the National Natural Science Foundation of China (Nos. 50572014, 50972027, and 10647108) and the Program for New Century Excellent Talents in University (NCET-05-0322). References [1] W.Y. Tong, A.B. Djurišić, M.H. Xie, A.C.M. Ng, K.Y. Cheung, W.K. Chan, Y.H. Leung, H.W. Lin, S. Gwo, The Journal of Physical Chemistry B 110 (2006) 17406–17413.
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