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Cu0-decorated, carbon-doped rutile TiO2 nanofibers via one step electrospinning: Effective photocatalyst for azo dyes degradation under solar light
Chemical Engineering and Processing 95 (2015) 202–207
Contents lists available at ScienceDirect
Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Cu0-decorated, carbon-doped rutile TiO2 nanofibers via one step electrospinning: Effective photocatalyst for azo dyes degradation under solar light Ayman Yousefa,b,* , Robert M. Brooksc, M.M. El-Halwanyd , Nasser A.M. Barakatb,* , Mohamed H. EL-Newehye, Hak Yong Kimb,* a
Mathematics and Physics Engineering Department, Faculty of Engineering in Matteria, Helwan University, Cairo, Egypt Organic Materials and Fiber Engineering Department, Chonbuk National University, Jeonju 561-756, South Korea c Civil and Environmental Engineering Department, Temple University, 1947 N. 12th Street, Philadelphia, PA 19122, USA d Engineering Mathematics and Physics Department, Faculty of Engineering, Mansoura University, El-Mansoura, Egypt e Petrochemical Research Chair, Department of Chemistry, College of Science, King Saud University, Riyadh 11451, Saudi Arabia b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 9 March 2015 Received in revised form 16 June 2015 Accepted 18 June 2015 Available online 22 June 2015
In this study, zero valent copper nanoparticles (Cu0 NPs)-decorated, carbon-doped titania nanofibers were successfully prepared by electrospinning of a solution composed of titanium isopropoxide (TIP), polyvinylpyrroliodine (PVP), and copper(II) acetate tetrahydrate. The calcination of the formed nanofiber mats in Ar atmosphere at 850 C led to produce good morphology nanofibers. The produced powder was characterized by X-ray diffraction (XRD), high resolution transmission electron microscopy (HR-TEM), and field-emission scanning electron microscopy (FESEM). Characterization techniques indicated that the obtained material is Cu0 NPs-decorated, carbon-doped TiO2 nanofibers. The synthesized nanofibers were used as photocatalyst for the effective degradation of reactive black 5 (RB5) and methyl red (MR) azo dyes in aqueous solution. The introduced nanofibers showed good photodegradation activity under sunlight. Overall, introduced nanofibers revealed a better photocatalytic activity when compared to pristine TiO2 nanofibers. The better performance was due to the removal of RB5 and MR at 83% and 65%, respectively. This study opens a new avenue to produce transition metal-decorated, carbon-doped titania nanofibers in a single production step for the use in different catalytic reactions. ã 2015 Elsevier B.V. All rights reserved.
Keywords: Copper nanoparticles Titania carbon nanofibers Electrospinning Azo dyes
1. Introduction Azo dyes are used in many industries such as textiles, additives, papers, foodstuffs, laser materials, leathers, etc. [1,2]. Nowadays, these dyes are used for colorizing the final products in many industrial applications. However, these industries generate a huge amount of effluent wastewater containing dye [3]. Most of the produced effluents are toxic and can undergo anaerobic decolonization forming potential carcinogens [2,4]. Moreover, the color of the dyes is harmful to aquatic life due to the reduction of dissolved oxygen and sunlight [3,5]. Thus, degradation of these dyes from effluent waste water before discharging to various water bodies becomes an urgent issue [6–8]. RB5 and MR dyes are water soluble
* Corresponding authors at: Organic Materials and Fiber Engineering Department, Chonbuk National University, Jeonju 561-756, South Korea. Fax: +82 632702348. E-mail addresses: [email protected] (A. Yousef), [email protected] (N.A.M. Barakat), [email protected] (H.Y. Kim). http://dx.doi.org/10.1016/j.cep.2015.06.015 0255-2701/ ã 2015 Elsevier B.V. All rights reserved.
and characterized by nitrogen to nitrogen double bond ( N¼N); the principal characteristic of Azo dye. They cannot easily be decolorized and degraded [5]. Thus, many routes are used such as adsorption, sedimentation and coagulation to treat this kind of dyes [9–11]. However, these methods do not lead to the complete elimination of the dye molecules from water, but only provide separation of the dyes. Besides, these traditional techniques create a waste disposal problem that leads to the generation of harmful secondary byproducts [5]. Consequently, after the traditional treatment methods, an additional step is needed to separate the sludge from the treated water. Accordingly, the researchers are investigating an environmentally and economically viable technology to eliminate toxic pollutants in wastewater effluents from various industries. One of the most interesting approaches is the photocatalytic oxidation over titania. This approach is considered a promising green technology method for treating dye wastewater [12–16]. This process does not create secondary byproducts and energy crisis due to utilization of the solar energy for its activation.
A. Yousef et al. / Chemical Engineering and Processing 95 (2015) 202–207
Furthermore, titania possesses many interesting characteristics including non-toxicity, chemical inertness, and high thermal stability [17,18]. However, titania suffers from low photocatalytic efficiency under the solar radiation due to the large band-gap energy and fast recombination rate of the photogenerated electrons/holes pairs. These two characteristics decrease the formation of free radicals in the solution [19– 21]. The aforementioned problems constrain the practical application of photocatalytic oxidation over TiO2 for dye removal from wastewater under the solar radiation. Thus, foreign elements (metal or non-metal) are doped on TiO2 matrix for enhancing the photocatalytic activity of pristine TiO2 [13,22– 24]. These dopant elements reduce the TiO2 band gap, enhance the absorption of the solar spectrum, and act as traps for the photogenerated charges. These activities lead to the fast separation of electrons and holes, subsequently enhancing the production of OH free radicals in the aqueous solutions [23,25,26]. This enhancement is responsible for the degradation of dye molecules in waste water. Among dopant materials, zero-valent copper and carbon are good candidates because of some salient characteristics such as relative abundance, low cost, low toxicity and good environmental compatibility. Many researchers study the effect of copper NPs and carbon doping in titania nanostructures separately; this strategy showed a good photocatalytic activity. However, synthesis of zero-valent copper-decorated, carbon-doped titania nanostructure is not an easy task. In the present work, we introduce Cu0 NPs-decorated, carbondoped TiO2 nanofibers as novel titania-based nanofibers having double function materials. The introduced nanofibers were synthesized by a simple and low cost electrospinning technique. Typically, calcinations of electrospun nanofiber mats consisting of titanium isopropoxide, polyvinylpyrroliodine (PVP) and copper (II) acetate in Ar atmosphere led to produce Cu0 NPs-decorated, carbon-doped TiO2 nanofibers. The final products were used as photocatalysts for color removal and degradation of two azo dyes. 2. Experimental 2.1. Materials RB5, MR, PVP, copper(II) acetate tetrahydrate (CuAc, 98%), ethanol, and TIP (97%) were purchased from Sigma–Aldrich. Distilled water and analytical grade ethanol were used as solvents. All the materials were used without any further purification.
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2.3. Activity of sunlight photocatalytic degradation RB5 and MR were subjected to photocatalytic degradation for 120 min between 9 AM and 12 AM in August when the ambient temperature was between 24 and 30 C. Typically, 25 mg of nanocatalysts (Cu0 NPs-decorated, carbon-doped TiO2 nanofibers and Cu-free carbon-doped TiO2 nanofibers) was added to 50 ml (10 mg/l) of the prepared azo dyes. These aqueous solutions were magnetically stirred under the sunlight radiation. 2 ml samples were withdrawn every 5 min radiation time, then samples were drawn every 15 min. These samples were centrifuged for the separation of the nanofibers catalyst. The sample dye concentration was measured by an UV–vis spectrophotometer [25,27]. 2.4. Samples characterization The surface morphology of the prepared catalytic NFs was observed by field-emission scanning electron microscope (FESEM, Hitachi S-7400, Japan). The crystallinity of NFs was confirmed by selected area electron diffraction patterns and high-resolution images were observed by a JEOL JEM-2200FS transmission electron microscope (TEM) operating at 200 kV equipped with EDX (JEOL Ltd., Japan). The crystalline structure and size of the catalysts were determined by X-ray diffraction (Rigaku Co., Japan) with Cu Ka (l = 1.54056 Å). 3. Results and discussion 3.1. Structural study Synthesis of copper metal from calcinations of copper(II) acetate tetrahydrate in an argon atmosphere was discussed in detail in other studies [28,29]. Typically, due to the abnormal decomposition of metallic acetate anions lead to the production of reducing gases (CO + H2). The gases are responsible for the formation of pure copper as given below [30]: Cu(CH3COO)Cu23H2O ! Cu(CH3COO)2 + 3H2O
(1)
Cu(CH3COO)2Cu2 ! 2Cu + 3CH3COOH + CO2 + H2 + C
(2)
XRD is an excellent analytical technique for investigating the composition of the crystalline materials. The typical XRD pattern of the sintered powder is presented in Fig. 1. As shown in the figure, the strong diffraction peaks at 2u values of 27.446 , 36.085 ,
2.2. Preparation of zero valent Cu-doped TiO2 CNFs Cu0 NPs-decorated, carbon-doped TiO2 nanofibers as well as Cu-free carbon-doped TiO2 nanofibers were fabricated using a sol– gel process. Typically, to prepare Cu-free carbon-doped TiO2 nanofibers, 1 g TIIP was added to a solution containing 2 g acetic acid and 2 g ethanol after 10 min stirring. Then, the obtained solution was added to a mixture contains 6 g ethanol and 1 g PVP. Then, the solution was stirred until a yellow transparent sol–gel was achieved. To get Cu0 NPs-decorated, carbon-doped TiO2 nanofibers, 10 %wt CuAc was added to the aforementioned solution followed by stirring at 50 C for 1 h to obtain the clear and consistent mixture. The prepared sol–gels were electrospun in a 15 ml plastic syringe at 18 kV and 15 cm distance. Later on, the electrospun mats were dried for 24 h at 60 C in vacuum, and then calcined in argon atmosphere at 850 C for 3 h at 2.3 C/min heating rate. Fig. 1. XRD patterns of electrospun mats after calcination at 850 C for 3 h in Ar.
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Fig. 2. FESEM images of the electrospun CuAc/TIIP/PVP nanofibers mat after calcination (panel A–C) and frequency distribution of the diameters for the prepared powder (panel D). (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)
54.322 , 65.478 , and 69.788 corresponding to 11 0, 1 0 1, 2 11, 2 2 1 and 11 2 crystal planes, respectively indicate formation of rutile phase of TiO2 (JCPDS-21-1276). There are three polymorphs for titania, anatase, brookite, and rutile; the anatase and brookite convert to rutile by heating above 700 C. Accordingly, one can claim that formation of rutile phase can be attributed to the high utilized calcination temperature (850 C). Furthermore, XRD analysis indicates that copper(II) acetate has been decomposed to Cu metal as the strong diffraction peaks appeared at 2u values of
43.297, 50.433 , and 74.130 agree with 111, 2 0 0 and 2 2 0 crystal planes, respectively (JCPDS-04-0836). It is worth mentioning that no foreign peaks were observed denoting to the existence of graphite. This might be attributed to the amorphous structure of carbon and/or its small content. It is expected that calcination of electrospun nanofiber mats consisting of copper(II) acetate tetrahydrate, titanium isopropoxide, and PVP in argon atmosphere. The calcination conditions will lead to partial decomposition of the utilized polymer and abnormal decomposition of the metallic
Fig. 3. Normal TEM images and HR-TEM images of produced powder.
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Fig. 6. Effect of recyclying of Cu0-decorated, carbon-doped rutile TiO2 nanofibers on the catalytic activity.
maximum (radian) of the diffraction peak. The characteristic peak of Cu at 2u = 43.297 and plane of (111) was chosen to calculate D. The size of the synthesized nanoparticle was found to be 17.34 nm. 3.2. Morphological study Good morphology nanofibers can be obtained from electrospinning technique [31–32]. Fig. 2(A–C) shows FESEM images of sintered nanofiber powder obtained after calcination process. It can be seen from images that the obtained nanofibers are smooth, continuous, and beads-free.
Fig. 4. TEM–EDX analysis result of the one nanofiber after calcination.
acetate to finally produce Cu0 NPs-decorated, carbon-doped TiO2 nanofibers. One of the most important factors affect the catalyst performance is the size of particles. Accordingly, the crystallite size of Cu nanoparticles is calculated from Scherrer’s model [31] as follows:-. D¼
Kl b cos u
(3)
where K = 0.89, D represents the crystallite size, l is the wavelength of Cu (k = 1.5405 Å), and b is the full width at half
Fig. 5. The degradation profile of MR and RB5 dyes under sunlight, (A) without photocatalyst (RB5), (B) without photocatalyst (MR), (C) TiO2 CNFs (RB5), (D) TiO2 CNFs (MR), (E) Cu-doped TiO2 CNFs (RB5), and (F) Cu-doped TiO2 CNFs (MR).
Fig. 7. Pseudo first-order for photodegradation of RB5 dye (A) and MR dye (B).
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Table 1 Pseudo first order constants for MR and RB5 dyes. MR Dye
k1 R2
RB5 Dye
Without catalyst
TiO2 CNFs
Cu0–TiO2 CNFs
Without catalyst
TiO2 CNFs
Cu0–TiO2 CNFs
6.91 104 0.9139
4.61 103 0.9819
8.06 103 0.9858
9.21 104 0.9712
7.6 103 0.97
1.336 103 0.9846
The internal structure of the prepared nanofibers was observed by a normal (TEM) and high resolution transmission electron microscope (HR-TEM). Fig. 3A and B displays the low and high resolution images, respectively. As shown in the figures, the nanofibrous morphology is clear, and crystalline structure appears on the surface of the NFs as well as the outer edge of NFs is amorphous. The crystalline phases on the surface of NFs would be attributed to TiO2 and Cu nanoparticles while the amorphous structure is graphite. The TiO2, Cu nanoparticles, and graphite are marked by blue, red, and orange dashed circles, respectively. Moreover, the space lattice indicated that there are different kinds of crystalline structures. The average diameter (267.5 nm) of the prepared nanofibers was estimated according to the results in Fig. 2D. To affirm and understanding the formation of Cu0 NPsdecorated, carbon-doped TiO2 nanofibers, TEM–EDX was carried out. Fig. 4A indicates the line EDX analysis that performed at a randomly selected line. As shown in Fig. 4(B–E), titanium, carbon, oxygen, and copper could be detected. A different distribution can be seen for C, Cu, and TiO2 in the figure. This may be due to the large difference in the crystal structures. Consequently, the three materials cannot form an alloy, but these can be physical blended.
of all the sites on catalyst surface may occupy by the adsorbed reactant molecules. 1 1 Co ¼ þ kobs kc K LH kc
(4)
where kobs is the apparent pseudo-first-order rate constant, KLH is the Langmuir–Hinshelwood adsorption equilibrium constant (L mg1), kc the rate constant of surface reaction (mg L1 min1) and Co is the initial concentration (mg L1). Pseudo first order (plotting log (C/Co) vs. time) results were shown in Table 1 and Fig. 7A and B. The application of Langmuir– Hinshelwood model for RB5 and MR dyes in the presence of Cu0 NPs-decorated, carbon-doped TiO2 nanofibers was revealed a linear relationship with a correlation factor of 0.98. The kinetic analysis revealed that the pseudo-first-order kinetics for the photocatalytic degradation of the two dyes in the presence of Cu0 NPs-decorated, carbon-doped TiO2 nanofibers is compatible with the results in the literature. This indicates the number of catalytic sites will not be a limiting factor at low dye concentration and the rate of degradation is proportional to the substrate concentration [32]. Finally, the present study investigated the potential use of synthesized Cu0 NPs-decorated, carbon-doped TiO2 nanofibers for the removal of toxic organic pollutants.
3.3. Azo dyes photocatalytic degradation 4. Conclusion In this study, photodegradation of RB5 and MR were used to evaluate the photocatalytic activity of synthesized nanocatalysts. The introduced Cu0 NPs-decorated, carbon-doped TiO2 nanofibers were compared with Cu-free, carbon-doped TiO2 nanofibers. The results are shown in Fig. 5. It can be seen that a fast and good oxidation of RB 5 and MR was achieved under sunlight. Moreover, in comparative experiments, the prepared Cu0 NPs-decorated, carbon-doped TiO2 nanofibers showed higher photocatalytic activity than Cu-free ones. This is due to the presence of copper as a co-catalyst enhancing the separation of electrons and holes that leads to an increase of the free radicals in the solution. The obtained result indicated that, after 120 min, the achieved removal of RB5 dye under solar radiation was up to 13% (without catalyst), 62% (Cu-free, carbon-doped TiO2 nanofibers) and 83% (Cu0 NPsdecorated, carbon-doped TiO2 nanofibers). While, it was up to 9% (without catalyst), 47% (Cu-free, carbon-doped TiO2 nanofibers), and 65% (Cu0 NPs-decorated, carbon-doped TiO2 nanofibers) for MR dye. As shown in Fig. 6, the synthesized NFs were reused for three times without regeneration to study the photocatalytic activity. Excellent photocatalytic activities of NFs toward dyes degradation were obtained. Mostly little change in the catalytic activity of NFs was observed. 3.4. Kinetics study There are different models used to describe kinetic behavior reactions. The pseudo first order (Langmuir–Hinshelwood, Eq. (4)) and pseudo second order models are the commonly used ones. The Langmuir–Hinshelwood (L–H) model was used in the most literature to describe the photocatalytic reaction. Zero-order kinetics was not used for high substrate concentration because
Cu0 NPs-decorated, carbon-doped TiO2 nanofibers can be prepared by electrospinning of sol–gel composed of copper(II) acetate tetrahydrate, titanium isopropoxide, and poly(vinylpyrroliodine) (PVP). Typically, calcination of electrospun mats in Ar atmosphere lead to the production Cu0 NPs-decorated, carbondoped TiO2 nanofibers. Interestingly, the prepared nanofibers were covered by a thin layer from graphite which improves the chemical resistivity. Furthermore, the synthesized nanofibers exhibited higher photocatalytic activities under solar light due to red shifts to longer wavelengths. The introduced Cu0 NPs-decorated, carbondoped TiO2 nanofibers revealed satisfactory photodegradation of azo dyes. The prepared NFs removed 83% and 65% from RB5 and MR, respectively after 2 h of reaction. We propose this process as a simple and low cost route to prepare metal-decorated, carbondoped metal oxide nanofibers for different catalytic reactions. This nanocatalyst can be used for the production of clean energy for the used in different industries such as transportation industry. Acknowledgement This work was supported by the IT R&D Program of MKE/KEIT (10041957, Design and Development of Fiber-based Flexible Display) (No. 10041947). The authors would like to extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for funding this research group no. (RGP#.021). References [1] T. Amna, et al., Inactivation of foodborne pathogens by NiO/TiO2 composite nanofibers: a novel biomaterial system, Food Bioprocess Technol. 6 (4) (2013) 988–996.
A. Yousef et al. / Chemical Engineering and Processing 95 (2015) 202–207 [2] M. Shamshi Hassan, T. Amna, M.-S. Khil, Synthesis of high aspect ratio CdTiO3 nanofibers via electrospinning: characterization and photocatalytic activity, Ceram. Int. 40 (1, Part A) (2014) 423–427. [3] S.S. Lee, et al., Novel-structured electrospun TiO2/CuO composite nanofibers for high efficient photocatalytic cogeneration of clean water and energy from dye wastewater, Water Res. 47 (12) (2013) 4059–4073. [4] M. Rauf, M. Meetani, S. Hisaindee, An overview on the photocatalytic degradation of azo dyes in the presence of TiO2 doped with selective transition metals, Desalination 276 (1) (2011) 13–27. [5] A. Di Paola, et al., A survey of photocatalytic materials for environmental remediation, J. Hazard. Mater. 211 (2012) 3–29. [6] C. O’Neill, et al., Azo-dye degradation in an anaerobic-aerobic treatment system operating on simulated textile effluent, Appl. Microbiol. Biotechnol. 53 (2) (2000) 249–254. [7] M. Zubair Alam, et al., Mutagenicity and genotoxicity of tannery effluents used for irrigation at Kanpur, India, Ecotoxicol. Environ. Saf. 73 (7) (2010) 1620– 1628. [8] S. Parsons, Advanced Oxidation Processes For Water And Wastewater Treatment, IWA Publishing, 2004, 2015. [9] S. Zodi, et al., Treatment of the industrial wastewaters by electrocoagulation: optimization of coupled electrochemical and sedimentation processes, Desalination 261 (1) (2010) 186–190. [10] K. Shakir, et al., Removal of rhodamine B (a basic dye) and thoron (an acidic dye) from dilute aqueous solutions and wastewater simulants by ion flotation, Water Res. 44 (5) (2010) 1449–1461. [11] M. Riera-Torres, C. Gutiérrez-Bouzán, M. Crespi, Combination of coagulation– flocculation and nanofiltration techniques for dye removal and water reuse in textile effluents, Desalination 252 (1) (2010) 53–59. [12] J. Yi, et al., AgI/TiO2 nanobelts monolithic catalyst with enhanced visible light photocatalytic activity, J. Hazard. Mater. 284 (0) (2015) 207–214. [13] G. Panthi, et al., Mn2O3/TiO2 nanofibers with broad-spectrum antibiotics effect and photocatalytic activity for preliminary stage of water desalination, Ceram. Int. 39 (3) (2013) 2239–2246. [14] A. Yousef, et al., Cu0-doped TiO2 nanofibers as potential photocatalyst and antimicrobial agent, J. Ind. Eng. Chem. (2015) . [15] A. Yousef, et al., Photocatalytic release of hydrogen from ammonia boranecomplex using Ni(0)-doped TiO2/C electrospun nanofibers, Colloids Surf. A Physicochem. Eng. Aspects 410 (0) (2012) 59–65. [16] A. Yousef, N.A.M. Barakat, H.Y. Kim, Electrospun Cu-doped titania nanofibers for photocatalytic hydrolysis of ammonia borane, Appl. Catal. A Gen. 467 (0) (2013) 98–106.
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Contents lists available at ScienceDirect
Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Cu0-decorated, carbon-doped rutile TiO2 nanofibers via one step electrospinning: Effective photocatalyst for azo dyes degradation under solar light Ayman Yousefa,b,* , Robert M. Brooksc, M.M. El-Halwanyd , Nasser A.M. Barakatb,* , Mohamed H. EL-Newehye, Hak Yong Kimb,* a
Mathematics and Physics Engineering Department, Faculty of Engineering in Matteria, Helwan University, Cairo, Egypt Organic Materials and Fiber Engineering Department, Chonbuk National University, Jeonju 561-756, South Korea c Civil and Environmental Engineering Department, Temple University, 1947 N. 12th Street, Philadelphia, PA 19122, USA d Engineering Mathematics and Physics Department, Faculty of Engineering, Mansoura University, El-Mansoura, Egypt e Petrochemical Research Chair, Department of Chemistry, College of Science, King Saud University, Riyadh 11451, Saudi Arabia b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 9 March 2015 Received in revised form 16 June 2015 Accepted 18 June 2015 Available online 22 June 2015
In this study, zero valent copper nanoparticles (Cu0 NPs)-decorated, carbon-doped titania nanofibers were successfully prepared by electrospinning of a solution composed of titanium isopropoxide (TIP), polyvinylpyrroliodine (PVP), and copper(II) acetate tetrahydrate. The calcination of the formed nanofiber mats in Ar atmosphere at 850 C led to produce good morphology nanofibers. The produced powder was characterized by X-ray diffraction (XRD), high resolution transmission electron microscopy (HR-TEM), and field-emission scanning electron microscopy (FESEM). Characterization techniques indicated that the obtained material is Cu0 NPs-decorated, carbon-doped TiO2 nanofibers. The synthesized nanofibers were used as photocatalyst for the effective degradation of reactive black 5 (RB5) and methyl red (MR) azo dyes in aqueous solution. The introduced nanofibers showed good photodegradation activity under sunlight. Overall, introduced nanofibers revealed a better photocatalytic activity when compared to pristine TiO2 nanofibers. The better performance was due to the removal of RB5 and MR at 83% and 65%, respectively. This study opens a new avenue to produce transition metal-decorated, carbon-doped titania nanofibers in a single production step for the use in different catalytic reactions. ã 2015 Elsevier B.V. All rights reserved.
Keywords: Copper nanoparticles Titania carbon nanofibers Electrospinning Azo dyes
1. Introduction Azo dyes are used in many industries such as textiles, additives, papers, foodstuffs, laser materials, leathers, etc. [1,2]. Nowadays, these dyes are used for colorizing the final products in many industrial applications. However, these industries generate a huge amount of effluent wastewater containing dye [3]. Most of the produced effluents are toxic and can undergo anaerobic decolonization forming potential carcinogens [2,4]. Moreover, the color of the dyes is harmful to aquatic life due to the reduction of dissolved oxygen and sunlight [3,5]. Thus, degradation of these dyes from effluent waste water before discharging to various water bodies becomes an urgent issue [6–8]. RB5 and MR dyes are water soluble
* Corresponding authors at: Organic Materials and Fiber Engineering Department, Chonbuk National University, Jeonju 561-756, South Korea. Fax: +82 632702348. E-mail addresses: [email protected] (A. Yousef), [email protected] (N.A.M. Barakat), [email protected] (H.Y. Kim). http://dx.doi.org/10.1016/j.cep.2015.06.015 0255-2701/ ã 2015 Elsevier B.V. All rights reserved.
and characterized by nitrogen to nitrogen double bond ( N¼N); the principal characteristic of Azo dye. They cannot easily be decolorized and degraded [5]. Thus, many routes are used such as adsorption, sedimentation and coagulation to treat this kind of dyes [9–11]. However, these methods do not lead to the complete elimination of the dye molecules from water, but only provide separation of the dyes. Besides, these traditional techniques create a waste disposal problem that leads to the generation of harmful secondary byproducts [5]. Consequently, after the traditional treatment methods, an additional step is needed to separate the sludge from the treated water. Accordingly, the researchers are investigating an environmentally and economically viable technology to eliminate toxic pollutants in wastewater effluents from various industries. One of the most interesting approaches is the photocatalytic oxidation over titania. This approach is considered a promising green technology method for treating dye wastewater [12–16]. This process does not create secondary byproducts and energy crisis due to utilization of the solar energy for its activation.
A. Yousef et al. / Chemical Engineering and Processing 95 (2015) 202–207
Furthermore, titania possesses many interesting characteristics including non-toxicity, chemical inertness, and high thermal stability [17,18]. However, titania suffers from low photocatalytic efficiency under the solar radiation due to the large band-gap energy and fast recombination rate of the photogenerated electrons/holes pairs. These two characteristics decrease the formation of free radicals in the solution [19– 21]. The aforementioned problems constrain the practical application of photocatalytic oxidation over TiO2 for dye removal from wastewater under the solar radiation. Thus, foreign elements (metal or non-metal) are doped on TiO2 matrix for enhancing the photocatalytic activity of pristine TiO2 [13,22– 24]. These dopant elements reduce the TiO2 band gap, enhance the absorption of the solar spectrum, and act as traps for the photogenerated charges. These activities lead to the fast separation of electrons and holes, subsequently enhancing the production of OH free radicals in the aqueous solutions [23,25,26]. This enhancement is responsible for the degradation of dye molecules in waste water. Among dopant materials, zero-valent copper and carbon are good candidates because of some salient characteristics such as relative abundance, low cost, low toxicity and good environmental compatibility. Many researchers study the effect of copper NPs and carbon doping in titania nanostructures separately; this strategy showed a good photocatalytic activity. However, synthesis of zero-valent copper-decorated, carbon-doped titania nanostructure is not an easy task. In the present work, we introduce Cu0 NPs-decorated, carbondoped TiO2 nanofibers as novel titania-based nanofibers having double function materials. The introduced nanofibers were synthesized by a simple and low cost electrospinning technique. Typically, calcinations of electrospun nanofiber mats consisting of titanium isopropoxide, polyvinylpyrroliodine (PVP) and copper (II) acetate in Ar atmosphere led to produce Cu0 NPs-decorated, carbon-doped TiO2 nanofibers. The final products were used as photocatalysts for color removal and degradation of two azo dyes. 2. Experimental 2.1. Materials RB5, MR, PVP, copper(II) acetate tetrahydrate (CuAc, 98%), ethanol, and TIP (97%) were purchased from Sigma–Aldrich. Distilled water and analytical grade ethanol were used as solvents. All the materials were used without any further purification.
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2.3. Activity of sunlight photocatalytic degradation RB5 and MR were subjected to photocatalytic degradation for 120 min between 9 AM and 12 AM in August when the ambient temperature was between 24 and 30 C. Typically, 25 mg of nanocatalysts (Cu0 NPs-decorated, carbon-doped TiO2 nanofibers and Cu-free carbon-doped TiO2 nanofibers) was added to 50 ml (10 mg/l) of the prepared azo dyes. These aqueous solutions were magnetically stirred under the sunlight radiation. 2 ml samples were withdrawn every 5 min radiation time, then samples were drawn every 15 min. These samples were centrifuged for the separation of the nanofibers catalyst. The sample dye concentration was measured by an UV–vis spectrophotometer [25,27]. 2.4. Samples characterization The surface morphology of the prepared catalytic NFs was observed by field-emission scanning electron microscope (FESEM, Hitachi S-7400, Japan). The crystallinity of NFs was confirmed by selected area electron diffraction patterns and high-resolution images were observed by a JEOL JEM-2200FS transmission electron microscope (TEM) operating at 200 kV equipped with EDX (JEOL Ltd., Japan). The crystalline structure and size of the catalysts were determined by X-ray diffraction (Rigaku Co., Japan) with Cu Ka (l = 1.54056 Å). 3. Results and discussion 3.1. Structural study Synthesis of copper metal from calcinations of copper(II) acetate tetrahydrate in an argon atmosphere was discussed in detail in other studies [28,29]. Typically, due to the abnormal decomposition of metallic acetate anions lead to the production of reducing gases (CO + H2). The gases are responsible for the formation of pure copper as given below [30]: Cu(CH3COO)Cu23H2O ! Cu(CH3COO)2 + 3H2O
(1)
Cu(CH3COO)2Cu2 ! 2Cu + 3CH3COOH + CO2 + H2 + C
(2)
XRD is an excellent analytical technique for investigating the composition of the crystalline materials. The typical XRD pattern of the sintered powder is presented in Fig. 1. As shown in the figure, the strong diffraction peaks at 2u values of 27.446 , 36.085 ,
2.2. Preparation of zero valent Cu-doped TiO2 CNFs Cu0 NPs-decorated, carbon-doped TiO2 nanofibers as well as Cu-free carbon-doped TiO2 nanofibers were fabricated using a sol– gel process. Typically, to prepare Cu-free carbon-doped TiO2 nanofibers, 1 g TIIP was added to a solution containing 2 g acetic acid and 2 g ethanol after 10 min stirring. Then, the obtained solution was added to a mixture contains 6 g ethanol and 1 g PVP. Then, the solution was stirred until a yellow transparent sol–gel was achieved. To get Cu0 NPs-decorated, carbon-doped TiO2 nanofibers, 10 %wt CuAc was added to the aforementioned solution followed by stirring at 50 C for 1 h to obtain the clear and consistent mixture. The prepared sol–gels were electrospun in a 15 ml plastic syringe at 18 kV and 15 cm distance. Later on, the electrospun mats were dried for 24 h at 60 C in vacuum, and then calcined in argon atmosphere at 850 C for 3 h at 2.3 C/min heating rate. Fig. 1. XRD patterns of electrospun mats after calcination at 850 C for 3 h in Ar.
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Fig. 2. FESEM images of the electrospun CuAc/TIIP/PVP nanofibers mat after calcination (panel A–C) and frequency distribution of the diameters for the prepared powder (panel D). (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)
54.322 , 65.478 , and 69.788 corresponding to 11 0, 1 0 1, 2 11, 2 2 1 and 11 2 crystal planes, respectively indicate formation of rutile phase of TiO2 (JCPDS-21-1276). There are three polymorphs for titania, anatase, brookite, and rutile; the anatase and brookite convert to rutile by heating above 700 C. Accordingly, one can claim that formation of rutile phase can be attributed to the high utilized calcination temperature (850 C). Furthermore, XRD analysis indicates that copper(II) acetate has been decomposed to Cu metal as the strong diffraction peaks appeared at 2u values of
43.297, 50.433 , and 74.130 agree with 111, 2 0 0 and 2 2 0 crystal planes, respectively (JCPDS-04-0836). It is worth mentioning that no foreign peaks were observed denoting to the existence of graphite. This might be attributed to the amorphous structure of carbon and/or its small content. It is expected that calcination of electrospun nanofiber mats consisting of copper(II) acetate tetrahydrate, titanium isopropoxide, and PVP in argon atmosphere. The calcination conditions will lead to partial decomposition of the utilized polymer and abnormal decomposition of the metallic
Fig. 3. Normal TEM images and HR-TEM images of produced powder.
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Fig. 6. Effect of recyclying of Cu0-decorated, carbon-doped rutile TiO2 nanofibers on the catalytic activity.
maximum (radian) of the diffraction peak. The characteristic peak of Cu at 2u = 43.297 and plane of (111) was chosen to calculate D. The size of the synthesized nanoparticle was found to be 17.34 nm. 3.2. Morphological study Good morphology nanofibers can be obtained from electrospinning technique [31–32]. Fig. 2(A–C) shows FESEM images of sintered nanofiber powder obtained after calcination process. It can be seen from images that the obtained nanofibers are smooth, continuous, and beads-free.
Fig. 4. TEM–EDX analysis result of the one nanofiber after calcination.
acetate to finally produce Cu0 NPs-decorated, carbon-doped TiO2 nanofibers. One of the most important factors affect the catalyst performance is the size of particles. Accordingly, the crystallite size of Cu nanoparticles is calculated from Scherrer’s model [31] as follows:-. D¼
Kl b cos u
(3)
where K = 0.89, D represents the crystallite size, l is the wavelength of Cu (k = 1.5405 Å), and b is the full width at half
Fig. 5. The degradation profile of MR and RB5 dyes under sunlight, (A) without photocatalyst (RB5), (B) without photocatalyst (MR), (C) TiO2 CNFs (RB5), (D) TiO2 CNFs (MR), (E) Cu-doped TiO2 CNFs (RB5), and (F) Cu-doped TiO2 CNFs (MR).
Fig. 7. Pseudo first-order for photodegradation of RB5 dye (A) and MR dye (B).
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Table 1 Pseudo first order constants for MR and RB5 dyes. MR Dye
k1 R2
RB5 Dye
Without catalyst
TiO2 CNFs
Cu0–TiO2 CNFs
Without catalyst
TiO2 CNFs
Cu0–TiO2 CNFs
6.91 104 0.9139
4.61 103 0.9819
8.06 103 0.9858
9.21 104 0.9712
7.6 103 0.97
1.336 103 0.9846
The internal structure of the prepared nanofibers was observed by a normal (TEM) and high resolution transmission electron microscope (HR-TEM). Fig. 3A and B displays the low and high resolution images, respectively. As shown in the figures, the nanofibrous morphology is clear, and crystalline structure appears on the surface of the NFs as well as the outer edge of NFs is amorphous. The crystalline phases on the surface of NFs would be attributed to TiO2 and Cu nanoparticles while the amorphous structure is graphite. The TiO2, Cu nanoparticles, and graphite are marked by blue, red, and orange dashed circles, respectively. Moreover, the space lattice indicated that there are different kinds of crystalline structures. The average diameter (267.5 nm) of the prepared nanofibers was estimated according to the results in Fig. 2D. To affirm and understanding the formation of Cu0 NPsdecorated, carbon-doped TiO2 nanofibers, TEM–EDX was carried out. Fig. 4A indicates the line EDX analysis that performed at a randomly selected line. As shown in Fig. 4(B–E), titanium, carbon, oxygen, and copper could be detected. A different distribution can be seen for C, Cu, and TiO2 in the figure. This may be due to the large difference in the crystal structures. Consequently, the three materials cannot form an alloy, but these can be physical blended.
of all the sites on catalyst surface may occupy by the adsorbed reactant molecules. 1 1 Co ¼ þ kobs kc K LH kc
(4)
where kobs is the apparent pseudo-first-order rate constant, KLH is the Langmuir–Hinshelwood adsorption equilibrium constant (L mg1), kc the rate constant of surface reaction (mg L1 min1) and Co is the initial concentration (mg L1). Pseudo first order (plotting log (C/Co) vs. time) results were shown in Table 1 and Fig. 7A and B. The application of Langmuir– Hinshelwood model for RB5 and MR dyes in the presence of Cu0 NPs-decorated, carbon-doped TiO2 nanofibers was revealed a linear relationship with a correlation factor of 0.98. The kinetic analysis revealed that the pseudo-first-order kinetics for the photocatalytic degradation of the two dyes in the presence of Cu0 NPs-decorated, carbon-doped TiO2 nanofibers is compatible with the results in the literature. This indicates the number of catalytic sites will not be a limiting factor at low dye concentration and the rate of degradation is proportional to the substrate concentration [32]. Finally, the present study investigated the potential use of synthesized Cu0 NPs-decorated, carbon-doped TiO2 nanofibers for the removal of toxic organic pollutants.
3.3. Azo dyes photocatalytic degradation 4. Conclusion In this study, photodegradation of RB5 and MR were used to evaluate the photocatalytic activity of synthesized nanocatalysts. The introduced Cu0 NPs-decorated, carbon-doped TiO2 nanofibers were compared with Cu-free, carbon-doped TiO2 nanofibers. The results are shown in Fig. 5. It can be seen that a fast and good oxidation of RB 5 and MR was achieved under sunlight. Moreover, in comparative experiments, the prepared Cu0 NPs-decorated, carbon-doped TiO2 nanofibers showed higher photocatalytic activity than Cu-free ones. This is due to the presence of copper as a co-catalyst enhancing the separation of electrons and holes that leads to an increase of the free radicals in the solution. The obtained result indicated that, after 120 min, the achieved removal of RB5 dye under solar radiation was up to 13% (without catalyst), 62% (Cu-free, carbon-doped TiO2 nanofibers) and 83% (Cu0 NPsdecorated, carbon-doped TiO2 nanofibers). While, it was up to 9% (without catalyst), 47% (Cu-free, carbon-doped TiO2 nanofibers), and 65% (Cu0 NPs-decorated, carbon-doped TiO2 nanofibers) for MR dye. As shown in Fig. 6, the synthesized NFs were reused for three times without regeneration to study the photocatalytic activity. Excellent photocatalytic activities of NFs toward dyes degradation were obtained. Mostly little change in the catalytic activity of NFs was observed. 3.4. Kinetics study There are different models used to describe kinetic behavior reactions. The pseudo first order (Langmuir–Hinshelwood, Eq. (4)) and pseudo second order models are the commonly used ones. The Langmuir–Hinshelwood (L–H) model was used in the most literature to describe the photocatalytic reaction. Zero-order kinetics was not used for high substrate concentration because
Cu0 NPs-decorated, carbon-doped TiO2 nanofibers can be prepared by electrospinning of sol–gel composed of copper(II) acetate tetrahydrate, titanium isopropoxide, and poly(vinylpyrroliodine) (PVP). Typically, calcination of electrospun mats in Ar atmosphere lead to the production Cu0 NPs-decorated, carbondoped TiO2 nanofibers. Interestingly, the prepared nanofibers were covered by a thin layer from graphite which improves the chemical resistivity. Furthermore, the synthesized nanofibers exhibited higher photocatalytic activities under solar light due to red shifts to longer wavelengths. The introduced Cu0 NPs-decorated, carbondoped TiO2 nanofibers revealed satisfactory photodegradation of azo dyes. The prepared NFs removed 83% and 65% from RB5 and MR, respectively after 2 h of reaction. We propose this process as a simple and low cost route to prepare metal-decorated, carbondoped metal oxide nanofibers for different catalytic reactions. This nanocatalyst can be used for the production of clean energy for the used in different industries such as transportation industry. Acknowledgement This work was supported by the IT R&D Program of MKE/KEIT (10041957, Design and Development of Fiber-based Flexible Display) (No. 10041947). The authors would like to extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for funding this research group no. (RGP#.021). References [1] T. Amna, et al., Inactivation of foodborne pathogens by NiO/TiO2 composite nanofibers: a novel biomaterial system, Food Bioprocess Technol. 6 (4) (2013) 988–996.
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