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Photocatalytic properties of different morphologies of CuO for the degradation of Congo red organic dye
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CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 11311–11317 www.elsevier.com/locate/ceramint
Photocatalytic properties of different morphologies of CuO for the degradation of Congo red organic dye Azar Sadollahkhania,b,n, Zafar Hussain Ibupotoa, Sami Elhaga, Omer Nura, Magnus Willandera a
Department of Science and Technology, Campus Norrköping, Linköping University, SE-60174 Norrköping, Sweden b Nanotechnology Lab, Department of Physics, Shahid Chamran University, Ahvaz, Iran Received 13 February 2014; received in revised form 13 March 2014; accepted 24 March 2014 Available online 29 March 2014
Abstract In this study, Congo red organic dye was degraded by different morphologies of CuO and it was found that CuO nanorods are more favorable for the degradation of Congo red due to their more specific surface area and sensitive surface for the Congo red. All the CuO nanostructures were prepared by low temperature aqueous growth method. Scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) techniques were used for the morphological and structural characterization of CuO nanostructures. The relative degradation of Congo red for nanorods, nanoleaves and nanosheets was in order 67%, 48% and 12% respectively. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: A. Powders: Chemical preparation; B. Spectroscopy; D. Metal oxides
1. Introduction Since last decade, the preparation of metal oxide nanostructures with different favorable morphologies has caught the attention of the scientific community because of their potential applications in different fields [1]. Among the various metal oxides, cupric oxide (CuO) is a p-type compound semiconductor with a narrow band gap of 1.2 eV at room temperature [2]. The uniqueness of synthesis and applications of CuO nanostructures are very crucial from fundamental importance point of view. The morphology and dimension of CuO nanomaterial play a critical role for each specific application [3]. CuO is versatile nanomaterial due to its wide range of applications like e.g. gas sensors [4], solar energy conversion [5], and lithium ion batteries [6], filed emitter [7] and is well known as useful heterogeneous catalyst [7]. Because n Corresponding author at: Department of Science and Technology, Campus Norrköping, Linköping University, SE-60174 Norrköping, Sweden. E-mail addresses: [email protected], [email protected] (A. Sadollahkhani).
http://dx.doi.org/10.1016/j.ceramint.2014.03.132 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
of the size and morphology dependent properties of CuO nanostructures, different types of CuO morphologies has been synthesized such as nanorods, nanosheets, and nanodendrites, honey comb like, urchin like and dumbbell like nanostructures [8–10]. Several preparation methods have been adopted for the synthesis of these morphologies including chemical bath method, sol–gel method, gas phase oxidation, micro emulsion etc. [11]. However, the hydrothermal growth technique has several advantages for the synthesis of CuO nanostructures and microstructures such as low growth temperature, environment friendly, safe, inexpensive and gives product on large scale of the desired nanomaterial [12]. CuO has been obtained in various nanostructures using the hydrothermal growth technique including nanorods, and nanotubes. The industrial dye materials are based on a wide range of groups of organic compounds which makes discolouration of water and consequently becomes the cause of aquatic life loss at large scale. Therefore, it is an upmost priority and a demand to remove these colors and other organic stuff for the provision of safe and clean environment. Advanced oxidation process has been used as a handy tool since last decade for the degradation of
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A. Sadollahkhani et al. / Ceramics International 40 (2014) 11311–11317
dyes in the aqueous medium without the production of hazardous by products [13]. The advanced oxidation process involves the formation of very reactive substances including hydroxyl radicals (OH) which easily oxidize a wide range of pollutants in short time. Such processes are limited by the high cost and it is the main drawback of these methods [14]. The irradiation processes actively improve the catalytic properties of a particular catalyst. CuO is among the various metal oxides a cheap, that is to produce and environment friendly catalysts have been used effectively. The literature provides the idea that the catalytic reaction of CuO is mostly morphology sensitive process and the oxygen surface lattices of CuO are participating in the given processing chemical reaction. The catalytic property of CuO nanomaterial is morphology dependent and the disclose crystal planes for the degradation of dyes. This is the reason which provides the solid platform for the synthesis of new CuO nanostructures with the desired and enhanced catalytic activity for the degradation of dyes and organic pollutants [15]. In this study, catalytic properties of different CuO nanostructures were investigated by the degradation of Congo red dye in the presence of UV light. The synthesized CuO nanostructures were found highly active for the degradation of Congo red dye in a relatively short interval of time. This investigation indicates that CuO nanorods prepared by hydrothermal method can be used efficiently for the degradation of Congo red. 2. Experimental section The copper nitrate pentahydrate and hexamethylenetetramine of analytical grade and were used without any further purification. The synthesis of CuO nanostructures was carried out by using the hydrothermal growth technique. Equimolar 25 mM concentration of copper nitrate pentahydrate and hexamethylenetetramine was used for the preparation of CuO nanomaterial in 100 mL of deionized water. After achieving a homogeneous solution of copper nitrate pentahydrate and hexamethylenetetramine by continuous stirring for a period of 15 min, the beaker was tightly sealed with aluminum foil and kept in a preheated electric oven for a period of 4–8 h. After the completion of the growth duration, CuO nano powder was settled down by natural cooling of the growth solution, and the nano powder was collected by removing the liquid solution from the bottom of beaker and then the beaker was left for drying at 60 1C overnight. Finally CuO nanomaterial was taken out from the bottom of beaker with the help of a spoon. The morphology and structural characterization of CuO nanostructures were investigated by LEO 1550 Gemini field emission scanning electron microscope working at 15 kV. The nanocrystalline phase of CuO nanostructures was examined by X-ray powder diffraction (XRD) using a Phillips PW 1729 powder diffractometer equipped with CuKα radiation (λ¼ 1.5418 Å) operating at a generator voltage of 40 kV and a current of 40 mA. Fourier transforms infrared (FT-IR) analysis was performed using Equinox 55. A PerkinElmer Lambda 900
UV–visible spectrophotometer was employed for the measurement of the degradation of Congo red. The degradation experiments were performed with a homemade photo reactor equipped with four 18 W UV lamps, whit a wavelengths of 256 nm. The experiment was performed with an initial dye concentration of 20 mg l 1 and 0.05 g of identical samples. The photo catalyst was mixed with 100 mL of the aqueous Congo red dye solution. The solution was put on stirring until the adsorption equilibrium was achieved. The maximum equilibrium time was found to be 30 min. Sample was kept in the reactor and exposed to UV lamps. In order to determine the dye concentration by UV–vis spectrophotometer, some irradiated solution was withdrawn from the beaker for a specific interval of time and their suspended CuO nanostructures were collected by centrifugation. 3. Results and discussion 3.1. The morphological crystal structure studies of CuO nanostructures The nanocrystalline phase of these CuO morphologies was studied by X-ray diffraction technique and the measured XRD spectrum for different CuO nanostructures are shown in Fig. 1. The observed main peaks at 2θ values of 32.51, 35.51, and 38.51 are indexed to the CuO crystal planes (110), (11 1)(002) and (111), (200) respectively. All the peaks are fully agreed with the JCPDS (card no. 05-661) and assigned to pure monocrystalline phase of CuO and no any other impurity such as Cu(OH)2 or Cu2O was observed during the measurement. The crystalline size was determined using the Scherrer formula: D¼
kλ βhkl cos θ
where D is the crystalline size, λ is the X-ray wavelength, θ is the Bragg diffraction angle of the peak, and β is the full-width at halfmaximum of θ. The crystallite size for the
Fig. 1. XRD pattern of different morphologies of CuO.
A. Sadollahkhani et al. / Ceramics International 40 (2014) 11311–11317
nanorods, nanoleaves and nanosheets was in around 16 nm, 14 nm and 18 nm, respectively. Further information about the nature of products and their surfaces was obtained via FTIR spectroscopy. Fig. 2 shows the FTIR spectra of CuO nanorods. The peak occurring at approximately 480 cm 1 can be attributed to the vibrations of Cu–O [16]. A weak band at around 2300 cm 1 can be attributed to the vibration of C–O. The distinctive SEM images of different CuO nanostructures are shown in Fig. 3 and three different morphologies were investigated by SEM study. Fig. 3(a) shows the CuO nanorods and Fig. 3(b) shows the nanoleaves like morphology obtained by the use of polyethylene glycol as a growth template. CuO nanorods and nanoleaves were studied in our previous studies [17,18]. Fig. 3(c) shows a typical SEM image of CuO
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nanosheets synthesized in an alkaline pH medium and these nanostructures have been used in our published work [19]. However, the catalytic effect of three of these nanostructures is investigated in this study through the degradation of Congo red. Fig. 4(a) shows a high resolution electron microscopy image of the nanorods clearly demonstrating the growth direction of the nanorods is to be along the [010] direction in addition to the selective area electron diffraction (SAED) studies. SAED investigation has shown the monocrystalline phase of CuO which agree fully with the XRD results. The TEM analysis has clearly shown that the CuO nanostructures exhibit morphology of nanorods. For CuO nanoleaves, high resolution transmission electron microscopy is shown in Fig. 3(b) with SAED pattern as an inset. The TEM image of a single nanoleaf shows the perpendicular growth pattern of CuO nanostructures to the substrate. The SAED pattern study was done for single nanoleaf and is shown in the inset of Fig. 4(b) and the diffraction points are demonstrating the monoclinic structure of CuO and the prepared CuO nanoleaves are single crystalline. Fig. 4(c) shows a HRTEM image of CuO nanosheets with upper right inset of SAED. SAED pattern revealed that the CuO nanosheets exhibits a monoclinic crystalline structure and that the growth direction is along the [010] and [100] direction. 3.2. The catalytic performance of CuO nanostructures for the degradation of Congo red The catalytic activity of different morphologies of CuO including bundle of nanorods, nanoleaves and nanosheets was observed for the degradation of Congo red organic dye at room temperature using UV–visible spectroscopy technique. The chemical structure of the Congo red is shown in Fig. 5.
Fig. 2. FTIR spectra of CuO nanorods.
Nanorod
Nanolea
Nanoshe ets
Fig. 3. SEM images of different CuO morphologies.
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Nanorods
Nanosheets
Nanoleaves
Fig. 4. TEM images of different CuO morphologies.
either H2O or OH to produce the HOd through the following reactions [20]: h þ þ H2 O-OHd þ H þ
ð1Þ
h þ þ OH -OHd
ð2Þ
Fig. 5. Chemical structure of Congo red dye.
Photogenerated electron–hole pairs are responsible for degradation of dye pollutant in photocatalytic degradation process. Photogenerated h þ in the valence band reacts with
while e in the conduction band reacts with adsorbed O2 on the surface of particle to generate Od2 and according to the following steps leads to generate HOd radicals [20]: e þ O2 -Od2 þ H þ -HOd2 þ Od2 -HOd2 þ O
ð3Þ
HOd2 -H2 O2 þ O2
ð4Þ
A. Sadollahkhani et al. / Ceramics International 40 (2014) 11311–11317
H2 O2 þ Od2 -HOd þ OH þ O2
ð5Þ
H2 O2 þ e -HOd þ OH
ð6Þ
H2 O2 þ hν-2HOd
ð7Þ
Dye þ ðOd2 or HOd or OHOd2 Þ- int ermediate-product ð8Þ
Fig. 6. Adsorption of Congo red on the surface of different morphologies of CuO.
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Fig. 6 shows the adsorption Congo red dye on the surface of different CuO nanostructures. In view of the fact that decomposition of some dyes takes place on the surface of photocatalyst, the adsorption of dye is a crucial step in a photocatalytic degradation process. Although the dye with high adsorption degrades faster, the effect sites for adsorbing the UV light decreases with increasing the adsorption [20]. As it can be seen from Fig. 6 the maximum adsorption of the dye is around 46% and take place for CuO nanorods. Fig. 7 presents the photocatalytic removal of Congo red dye after 210 min of UV irradiation for different morphologies. More details are shown in the C/C0 versus time plot which is given in Fig. 7(d), where C0 is the initial concentration of the dye, C is the concentration of the dye after irradiation time t (min). It can be seen that almost 67%, 48%, and 12% of Congo red dye were degraded after 210 min UV illumination for nanorods, nanoleaves and nanosheets, respectively. It is obvious that the photocatalytic activity of CuO nanorods with high adsorption is better compared to nanosheets and nanoleaves. The other possible reason for the high percentage of degradation of Congo red shown by CuO nanorods could be associated to their high specific surface area and exhibiting large specific surface sites which have been exposed for the Congo red molecules compared to other investigated morphologies of CuO.
Fig. 7. UV–vis spectra of Congo red dye solution in 30 min intervals (a) nanorods, (b) nanoleaves and (c) nanosheets. (d) The plot of C/C0 versus reaction time for different CuO morphologies.
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reaction follows first order kinetics with the three CuO morphologies. 4. Conclusion
Fig. 8. The plot of ln(C0/C) versus time for different CuO morphologies.
Moreover, other possibility for rapid degradation of Congo red might be attributed to the dissociation of water molecules on the surface of CuO nanostructures. It has been proved that metal oxides prepared by the hydrothermal growth technique have sufficient oxygen vacancies. Schaub R and co-workers investigated the splitting of water on oxygen vacancies for the rutile phase of TiO2. The dissociative portion of water lies on the oxygen vacancies through proton transfer to a next oxygen atom which produces two bridging hydroxyl groups for a single vacancy created at the beginning. Therefore, it can be estimated that oxygen vacancies serve as active for sites the splitting of water on the surface of CuO nanostructures. The logic behind the reactivity of the vacancies is accompanied by the large energy of defects [21]. CuO nanorods are associated with large number of oxygen vacancies in their crystal structure and consequently results high magnitude of hydroxyl ions which are actively involved for the degradation of Congo red. Congo red is degraded more and faster by CuO nanorods in given interval of time compared to other morphologies as shown in Fig. 7. This study indicates that CuO nanorods are providing high number of specific surface active sites for the favorable degradation of Congo red dye. The kinetics of these morphologies of CuO for the degradation of Congo red was studied and it was found that the degradation reactions of all three morphologies with Congo red showed pseudo first order kinetics for specific time of degradation. All the obtained kinetics information agree fully with the linear equation [20]. ln ðC=C0 Þ ¼ kt Here C0 is the initial concentration of Congo red dye and C is the concentration at interval of time t, k is specific rate constant for the first order kinetics reaction. For the first order kinetics a graph between the values of ln(C/C0) versus time for the degradation of Congo red is plotted using three of the CuO nanostructures as shown in Fig. 8. Fig. 8 illustrates a linear correlation and it indicates that the Congo red degradation
In summary, CuO bundle of nanorods, nanoleaves and nanosheets were obtained under slight different conditions by hydrothermal growth technique. The nanostructures are characterized by SEM, HRTEM and XRD techniques. The potential application of the three nanostructures for the degradation of Congo red was observed and the observed results suggest better photocatalytic properties of CuO nanorods compared to the other two morphologies. The possible cause behind the better catalytic property of CuO nanorods is lying in the fact of its higher rate of specific surface area in comparison to the two other morphologies of CuO. The CuO nanorods have a high potential application for the degradation of Congo red at room temperature. References [1] S. Rackauskas, A.G. Nasibulin, H. Jiang, Y. Tian, V.I. Kleshch, J. Sainio, E.D. Obraztsova, S.N. Bokova, A.N. Obraztsov, E. Kauppinen, Nanotechnology 20 (2009) 165603. [2] M. Yin, C.K. Wu, Y.B. Lou, C. Burda, J.T. Koberstein, Y.M. Zhu, S. O’Brien, Copper oxide nanocrystals, J. Am. Chem. Soc. 127 (2005) 9506–9511. [3] J.Y. Xiang, J.P. Tu, L. Zhang, Y. Zhou, X.L. Wang, S.J. Shi, Selfassembled synthesis of hierarchical nanostructured CuO with various morphologies and their application as anodes for lithium ion batteries, J. Power Sour. 195 (2010) 313–319. [4] D. Li, J. Hu, R. Wu, J.G. Lu, Conductometric chemical sensor based on individual CuO nanowires, Nanotechnology 21 (2010) 485502. [5] Y. Xu, D. Chen, X. Jiao, Fabrication of CuO pricky microspheres with tunable size by a simple solution route, J. Phys. Chem. B 109 (2005) 13561–13566. [6] S. Venkatachalam, H.W. Zhu, C. Masarapu, K.H. Hung, Z. Liu, K. Suenaga, B.Q. Wei, In-situ formation of sandwiched structures of nanotube/CuxOy/Cu composites for lithium battery applications, ACS Nano 3 (2009) 2177–2184. [7] C.T. Hsieh, J.M. Chen, H.H. Lin, H.C. Shih, Field emission from various Cuo nanostructures, Appl. Phys. Lett. 83 (2003) 3383–3385. [8] D.P. Singh, A.K. Ojha, O.N. Srivastava, Synthesis of different Cu(OH)2 and CuO (nanowires, rectangles, seed-, belt-, and sheetlike) nanostructures by simple wet chemical route, J. Phys. Chem. C 113 (2009) 3409–3418. [9] Y. Liu, Y. Chu, Y.J. Zhuo, M.Y. Li, L.L. Li, L.H. Dong, Anioncontrolled construction of CuO honeycombs and flowers-like assemblies on copper foils, Cryst. Growth Des. 7 (2007) 467–470. [10] Y. Zhao, J. Zhao, Y. Li, D. Ma, S. Hou, L. Li, X. Hao, Z. Wang, Room temperature synthesis of 2D CuO nanoleaves in aqueous solution, Nanotechnology 22 (2011) 115604. [11] L. Armelao, D. Barreca, M. Bertapelle, G. Bottaro, C. Sada, E. Tondello, A sol–gel approach to nanophasic copper oxide thin films, Thin Solid Films 442 (2003) 48–52. [12] D. Li, Y.H. Leung, A.B. Djurisic, Z.T. Liu, M.H. Xie, J. Gao, W. K. Chan, CuO nanostructures prepared by a chemical method, J. Cryst. Growth 282 (105) (2005). [13] S.F. Yin, B.Q. Xu, C.F. Ng, C.T. Au, Nano Ru/CNTs: a highly active and stable catalyst for the generation of COx-free hydrogen in ammonia decomposition, Appl. Catal. B: Environ. 48 (2004) 237–241. [14] S. Malato, J. Blanco, A. Vidal, C. Richter, Photocatalysis with solar energy at a pilot-plant scale: an overview, Appl. Catal. B: Environ. 37 (2002) 1–15.
A. Sadollahkhani et al. / Ceramics International 40 (2014) 11311–11317 [15] R. Narayanan, Mostafa A. El-Sayed, , Shape-dependent catalytic activity of platinum nanoparticles in colloidal solution, Nano Lett. 4 (2004) 1343–1348. [16] M.R. Johan1, M.S. Mohd Suan, N.L. Hawari, H.A. Ching, Annealing effects on the properties of copper oxide thin films prepared by chemical deposition, Int. J. Electrochem. Sci. 6 (2011) 6094–6104. [17] Z.H. Ibupoto, K. Khun, X. Liu, M. Willander, Low temperature synthesis of seed mediated CuO bundle of nanowires, their structural characterisation and cholesterol detection, Mater. Sci. Eng. C 33 (2013) 3889–3898. [18] Z.H. Ibupoto, K. Khun, J. Lu, M. Willander, The synthesis of CuO nanoleaves, structural characterization, and their glucose sensing application, Appl. Phys. Lett. 102 (2013) 103701.
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[19] Zafar Hussain Ibupoto, Kimleang Khun, Valerio Beni, Xianjie Liu, Magnus Willander, Synthesis of novel CuO nanosheets and their nonenzymatic glucose sensing applications, Sensors 13 (2013) 7926–7938. [20] I Kazeminezhad, A. sadollahkhani, Photocatalytic degradation of Eriochrome black-T dye using ZnO nanoparticles, Mater. Lett. 120 (2014) 267–270. [21] S. Warren, W.R. Flavell, A.G. Thomas, J. Hollingworth, P.M. Dunwoody, S. Downes, C. Changkang, Adsorption of H2O on single crystal CuO, Surf. Sci. 436 (1999) 1–8.
CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 11311–11317 www.elsevier.com/locate/ceramint
Photocatalytic properties of different morphologies of CuO for the degradation of Congo red organic dye Azar Sadollahkhania,b,n, Zafar Hussain Ibupotoa, Sami Elhaga, Omer Nura, Magnus Willandera a
Department of Science and Technology, Campus Norrköping, Linköping University, SE-60174 Norrköping, Sweden b Nanotechnology Lab, Department of Physics, Shahid Chamran University, Ahvaz, Iran Received 13 February 2014; received in revised form 13 March 2014; accepted 24 March 2014 Available online 29 March 2014
Abstract In this study, Congo red organic dye was degraded by different morphologies of CuO and it was found that CuO nanorods are more favorable for the degradation of Congo red due to their more specific surface area and sensitive surface for the Congo red. All the CuO nanostructures were prepared by low temperature aqueous growth method. Scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) techniques were used for the morphological and structural characterization of CuO nanostructures. The relative degradation of Congo red for nanorods, nanoleaves and nanosheets was in order 67%, 48% and 12% respectively. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: A. Powders: Chemical preparation; B. Spectroscopy; D. Metal oxides
1. Introduction Since last decade, the preparation of metal oxide nanostructures with different favorable morphologies has caught the attention of the scientific community because of their potential applications in different fields [1]. Among the various metal oxides, cupric oxide (CuO) is a p-type compound semiconductor with a narrow band gap of 1.2 eV at room temperature [2]. The uniqueness of synthesis and applications of CuO nanostructures are very crucial from fundamental importance point of view. The morphology and dimension of CuO nanomaterial play a critical role for each specific application [3]. CuO is versatile nanomaterial due to its wide range of applications like e.g. gas sensors [4], solar energy conversion [5], and lithium ion batteries [6], filed emitter [7] and is well known as useful heterogeneous catalyst [7]. Because n Corresponding author at: Department of Science and Technology, Campus Norrköping, Linköping University, SE-60174 Norrköping, Sweden. E-mail addresses: [email protected], [email protected] (A. Sadollahkhani).
http://dx.doi.org/10.1016/j.ceramint.2014.03.132 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
of the size and morphology dependent properties of CuO nanostructures, different types of CuO morphologies has been synthesized such as nanorods, nanosheets, and nanodendrites, honey comb like, urchin like and dumbbell like nanostructures [8–10]. Several preparation methods have been adopted for the synthesis of these morphologies including chemical bath method, sol–gel method, gas phase oxidation, micro emulsion etc. [11]. However, the hydrothermal growth technique has several advantages for the synthesis of CuO nanostructures and microstructures such as low growth temperature, environment friendly, safe, inexpensive and gives product on large scale of the desired nanomaterial [12]. CuO has been obtained in various nanostructures using the hydrothermal growth technique including nanorods, and nanotubes. The industrial dye materials are based on a wide range of groups of organic compounds which makes discolouration of water and consequently becomes the cause of aquatic life loss at large scale. Therefore, it is an upmost priority and a demand to remove these colors and other organic stuff for the provision of safe and clean environment. Advanced oxidation process has been used as a handy tool since last decade for the degradation of
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A. Sadollahkhani et al. / Ceramics International 40 (2014) 11311–11317
dyes in the aqueous medium without the production of hazardous by products [13]. The advanced oxidation process involves the formation of very reactive substances including hydroxyl radicals (OH) which easily oxidize a wide range of pollutants in short time. Such processes are limited by the high cost and it is the main drawback of these methods [14]. The irradiation processes actively improve the catalytic properties of a particular catalyst. CuO is among the various metal oxides a cheap, that is to produce and environment friendly catalysts have been used effectively. The literature provides the idea that the catalytic reaction of CuO is mostly morphology sensitive process and the oxygen surface lattices of CuO are participating in the given processing chemical reaction. The catalytic property of CuO nanomaterial is morphology dependent and the disclose crystal planes for the degradation of dyes. This is the reason which provides the solid platform for the synthesis of new CuO nanostructures with the desired and enhanced catalytic activity for the degradation of dyes and organic pollutants [15]. In this study, catalytic properties of different CuO nanostructures were investigated by the degradation of Congo red dye in the presence of UV light. The synthesized CuO nanostructures were found highly active for the degradation of Congo red dye in a relatively short interval of time. This investigation indicates that CuO nanorods prepared by hydrothermal method can be used efficiently for the degradation of Congo red. 2. Experimental section The copper nitrate pentahydrate and hexamethylenetetramine of analytical grade and were used without any further purification. The synthesis of CuO nanostructures was carried out by using the hydrothermal growth technique. Equimolar 25 mM concentration of copper nitrate pentahydrate and hexamethylenetetramine was used for the preparation of CuO nanomaterial in 100 mL of deionized water. After achieving a homogeneous solution of copper nitrate pentahydrate and hexamethylenetetramine by continuous stirring for a period of 15 min, the beaker was tightly sealed with aluminum foil and kept in a preheated electric oven for a period of 4–8 h. After the completion of the growth duration, CuO nano powder was settled down by natural cooling of the growth solution, and the nano powder was collected by removing the liquid solution from the bottom of beaker and then the beaker was left for drying at 60 1C overnight. Finally CuO nanomaterial was taken out from the bottom of beaker with the help of a spoon. The morphology and structural characterization of CuO nanostructures were investigated by LEO 1550 Gemini field emission scanning electron microscope working at 15 kV. The nanocrystalline phase of CuO nanostructures was examined by X-ray powder diffraction (XRD) using a Phillips PW 1729 powder diffractometer equipped with CuKα radiation (λ¼ 1.5418 Å) operating at a generator voltage of 40 kV and a current of 40 mA. Fourier transforms infrared (FT-IR) analysis was performed using Equinox 55. A PerkinElmer Lambda 900
UV–visible spectrophotometer was employed for the measurement of the degradation of Congo red. The degradation experiments were performed with a homemade photo reactor equipped with four 18 W UV lamps, whit a wavelengths of 256 nm. The experiment was performed with an initial dye concentration of 20 mg l 1 and 0.05 g of identical samples. The photo catalyst was mixed with 100 mL of the aqueous Congo red dye solution. The solution was put on stirring until the adsorption equilibrium was achieved. The maximum equilibrium time was found to be 30 min. Sample was kept in the reactor and exposed to UV lamps. In order to determine the dye concentration by UV–vis spectrophotometer, some irradiated solution was withdrawn from the beaker for a specific interval of time and their suspended CuO nanostructures were collected by centrifugation. 3. Results and discussion 3.1. The morphological crystal structure studies of CuO nanostructures The nanocrystalline phase of these CuO morphologies was studied by X-ray diffraction technique and the measured XRD spectrum for different CuO nanostructures are shown in Fig. 1. The observed main peaks at 2θ values of 32.51, 35.51, and 38.51 are indexed to the CuO crystal planes (110), (11 1)(002) and (111), (200) respectively. All the peaks are fully agreed with the JCPDS (card no. 05-661) and assigned to pure monocrystalline phase of CuO and no any other impurity such as Cu(OH)2 or Cu2O was observed during the measurement. The crystalline size was determined using the Scherrer formula: D¼
kλ βhkl cos θ
where D is the crystalline size, λ is the X-ray wavelength, θ is the Bragg diffraction angle of the peak, and β is the full-width at halfmaximum of θ. The crystallite size for the
Fig. 1. XRD pattern of different morphologies of CuO.
A. Sadollahkhani et al. / Ceramics International 40 (2014) 11311–11317
nanorods, nanoleaves and nanosheets was in around 16 nm, 14 nm and 18 nm, respectively. Further information about the nature of products and their surfaces was obtained via FTIR spectroscopy. Fig. 2 shows the FTIR spectra of CuO nanorods. The peak occurring at approximately 480 cm 1 can be attributed to the vibrations of Cu–O [16]. A weak band at around 2300 cm 1 can be attributed to the vibration of C–O. The distinctive SEM images of different CuO nanostructures are shown in Fig. 3 and three different morphologies were investigated by SEM study. Fig. 3(a) shows the CuO nanorods and Fig. 3(b) shows the nanoleaves like morphology obtained by the use of polyethylene glycol as a growth template. CuO nanorods and nanoleaves were studied in our previous studies [17,18]. Fig. 3(c) shows a typical SEM image of CuO
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nanosheets synthesized in an alkaline pH medium and these nanostructures have been used in our published work [19]. However, the catalytic effect of three of these nanostructures is investigated in this study through the degradation of Congo red. Fig. 4(a) shows a high resolution electron microscopy image of the nanorods clearly demonstrating the growth direction of the nanorods is to be along the [010] direction in addition to the selective area electron diffraction (SAED) studies. SAED investigation has shown the monocrystalline phase of CuO which agree fully with the XRD results. The TEM analysis has clearly shown that the CuO nanostructures exhibit morphology of nanorods. For CuO nanoleaves, high resolution transmission electron microscopy is shown in Fig. 3(b) with SAED pattern as an inset. The TEM image of a single nanoleaf shows the perpendicular growth pattern of CuO nanostructures to the substrate. The SAED pattern study was done for single nanoleaf and is shown in the inset of Fig. 4(b) and the diffraction points are demonstrating the monoclinic structure of CuO and the prepared CuO nanoleaves are single crystalline. Fig. 4(c) shows a HRTEM image of CuO nanosheets with upper right inset of SAED. SAED pattern revealed that the CuO nanosheets exhibits a monoclinic crystalline structure and that the growth direction is along the [010] and [100] direction. 3.2. The catalytic performance of CuO nanostructures for the degradation of Congo red The catalytic activity of different morphologies of CuO including bundle of nanorods, nanoleaves and nanosheets was observed for the degradation of Congo red organic dye at room temperature using UV–visible spectroscopy technique. The chemical structure of the Congo red is shown in Fig. 5.
Fig. 2. FTIR spectra of CuO nanorods.
Nanorod
Nanolea
Nanoshe ets
Fig. 3. SEM images of different CuO morphologies.
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Nanorods
Nanosheets
Nanoleaves
Fig. 4. TEM images of different CuO morphologies.
either H2O or OH to produce the HOd through the following reactions [20]: h þ þ H2 O-OHd þ H þ
ð1Þ
h þ þ OH -OHd
ð2Þ
Fig. 5. Chemical structure of Congo red dye.
Photogenerated electron–hole pairs are responsible for degradation of dye pollutant in photocatalytic degradation process. Photogenerated h þ in the valence band reacts with
while e in the conduction band reacts with adsorbed O2 on the surface of particle to generate Od2 and according to the following steps leads to generate HOd radicals [20]: e þ O2 -Od2 þ H þ -HOd2 þ Od2 -HOd2 þ O
ð3Þ
HOd2 -H2 O2 þ O2
ð4Þ
A. Sadollahkhani et al. / Ceramics International 40 (2014) 11311–11317
H2 O2 þ Od2 -HOd þ OH þ O2
ð5Þ
H2 O2 þ e -HOd þ OH
ð6Þ
H2 O2 þ hν-2HOd
ð7Þ
Dye þ ðOd2 or HOd or OHOd2 Þ- int ermediate-product ð8Þ
Fig. 6. Adsorption of Congo red on the surface of different morphologies of CuO.
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Fig. 6 shows the adsorption Congo red dye on the surface of different CuO nanostructures. In view of the fact that decomposition of some dyes takes place on the surface of photocatalyst, the adsorption of dye is a crucial step in a photocatalytic degradation process. Although the dye with high adsorption degrades faster, the effect sites for adsorbing the UV light decreases with increasing the adsorption [20]. As it can be seen from Fig. 6 the maximum adsorption of the dye is around 46% and take place for CuO nanorods. Fig. 7 presents the photocatalytic removal of Congo red dye after 210 min of UV irradiation for different morphologies. More details are shown in the C/C0 versus time plot which is given in Fig. 7(d), where C0 is the initial concentration of the dye, C is the concentration of the dye after irradiation time t (min). It can be seen that almost 67%, 48%, and 12% of Congo red dye were degraded after 210 min UV illumination for nanorods, nanoleaves and nanosheets, respectively. It is obvious that the photocatalytic activity of CuO nanorods with high adsorption is better compared to nanosheets and nanoleaves. The other possible reason for the high percentage of degradation of Congo red shown by CuO nanorods could be associated to their high specific surface area and exhibiting large specific surface sites which have been exposed for the Congo red molecules compared to other investigated morphologies of CuO.
Fig. 7. UV–vis spectra of Congo red dye solution in 30 min intervals (a) nanorods, (b) nanoleaves and (c) nanosheets. (d) The plot of C/C0 versus reaction time for different CuO morphologies.
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reaction follows first order kinetics with the three CuO morphologies. 4. Conclusion
Fig. 8. The plot of ln(C0/C) versus time for different CuO morphologies.
Moreover, other possibility for rapid degradation of Congo red might be attributed to the dissociation of water molecules on the surface of CuO nanostructures. It has been proved that metal oxides prepared by the hydrothermal growth technique have sufficient oxygen vacancies. Schaub R and co-workers investigated the splitting of water on oxygen vacancies for the rutile phase of TiO2. The dissociative portion of water lies on the oxygen vacancies through proton transfer to a next oxygen atom which produces two bridging hydroxyl groups for a single vacancy created at the beginning. Therefore, it can be estimated that oxygen vacancies serve as active for sites the splitting of water on the surface of CuO nanostructures. The logic behind the reactivity of the vacancies is accompanied by the large energy of defects [21]. CuO nanorods are associated with large number of oxygen vacancies in their crystal structure and consequently results high magnitude of hydroxyl ions which are actively involved for the degradation of Congo red. Congo red is degraded more and faster by CuO nanorods in given interval of time compared to other morphologies as shown in Fig. 7. This study indicates that CuO nanorods are providing high number of specific surface active sites for the favorable degradation of Congo red dye. The kinetics of these morphologies of CuO for the degradation of Congo red was studied and it was found that the degradation reactions of all three morphologies with Congo red showed pseudo first order kinetics for specific time of degradation. All the obtained kinetics information agree fully with the linear equation [20]. ln ðC=C0 Þ ¼ kt Here C0 is the initial concentration of Congo red dye and C is the concentration at interval of time t, k is specific rate constant for the first order kinetics reaction. For the first order kinetics a graph between the values of ln(C/C0) versus time for the degradation of Congo red is plotted using three of the CuO nanostructures as shown in Fig. 8. Fig. 8 illustrates a linear correlation and it indicates that the Congo red degradation
In summary, CuO bundle of nanorods, nanoleaves and nanosheets were obtained under slight different conditions by hydrothermal growth technique. The nanostructures are characterized by SEM, HRTEM and XRD techniques. The potential application of the three nanostructures for the degradation of Congo red was observed and the observed results suggest better photocatalytic properties of CuO nanorods compared to the other two morphologies. The possible cause behind the better catalytic property of CuO nanorods is lying in the fact of its higher rate of specific surface area in comparison to the two other morphologies of CuO. The CuO nanorods have a high potential application for the degradation of Congo red at room temperature. References [1] S. Rackauskas, A.G. Nasibulin, H. Jiang, Y. Tian, V.I. Kleshch, J. Sainio, E.D. Obraztsova, S.N. Bokova, A.N. Obraztsov, E. Kauppinen, Nanotechnology 20 (2009) 165603. [2] M. Yin, C.K. Wu, Y.B. Lou, C. Burda, J.T. Koberstein, Y.M. Zhu, S. O’Brien, Copper oxide nanocrystals, J. Am. Chem. Soc. 127 (2005) 9506–9511. [3] J.Y. Xiang, J.P. Tu, L. Zhang, Y. Zhou, X.L. Wang, S.J. Shi, Selfassembled synthesis of hierarchical nanostructured CuO with various morphologies and their application as anodes for lithium ion batteries, J. Power Sour. 195 (2010) 313–319. [4] D. Li, J. Hu, R. Wu, J.G. Lu, Conductometric chemical sensor based on individual CuO nanowires, Nanotechnology 21 (2010) 485502. [5] Y. Xu, D. Chen, X. Jiao, Fabrication of CuO pricky microspheres with tunable size by a simple solution route, J. Phys. Chem. B 109 (2005) 13561–13566. [6] S. Venkatachalam, H.W. Zhu, C. Masarapu, K.H. Hung, Z. Liu, K. Suenaga, B.Q. Wei, In-situ formation of sandwiched structures of nanotube/CuxOy/Cu composites for lithium battery applications, ACS Nano 3 (2009) 2177–2184. [7] C.T. Hsieh, J.M. Chen, H.H. Lin, H.C. Shih, Field emission from various Cuo nanostructures, Appl. Phys. Lett. 83 (2003) 3383–3385. [8] D.P. Singh, A.K. Ojha, O.N. Srivastava, Synthesis of different Cu(OH)2 and CuO (nanowires, rectangles, seed-, belt-, and sheetlike) nanostructures by simple wet chemical route, J. Phys. Chem. C 113 (2009) 3409–3418. [9] Y. Liu, Y. Chu, Y.J. Zhuo, M.Y. Li, L.L. Li, L.H. Dong, Anioncontrolled construction of CuO honeycombs and flowers-like assemblies on copper foils, Cryst. Growth Des. 7 (2007) 467–470. [10] Y. Zhao, J. Zhao, Y. Li, D. Ma, S. Hou, L. Li, X. Hao, Z. Wang, Room temperature synthesis of 2D CuO nanoleaves in aqueous solution, Nanotechnology 22 (2011) 115604. [11] L. Armelao, D. Barreca, M. Bertapelle, G. Bottaro, C. Sada, E. Tondello, A sol–gel approach to nanophasic copper oxide thin films, Thin Solid Films 442 (2003) 48–52. [12] D. Li, Y.H. Leung, A.B. Djurisic, Z.T. Liu, M.H. Xie, J. Gao, W. K. Chan, CuO nanostructures prepared by a chemical method, J. Cryst. Growth 282 (105) (2005). [13] S.F. Yin, B.Q. Xu, C.F. Ng, C.T. Au, Nano Ru/CNTs: a highly active and stable catalyst for the generation of COx-free hydrogen in ammonia decomposition, Appl. Catal. B: Environ. 48 (2004) 237–241. [14] S. Malato, J. Blanco, A. Vidal, C. Richter, Photocatalysis with solar energy at a pilot-plant scale: an overview, Appl. Catal. B: Environ. 37 (2002) 1–15.
A. Sadollahkhani et al. / Ceramics International 40 (2014) 11311–11317 [15] R. Narayanan, Mostafa A. El-Sayed, , Shape-dependent catalytic activity of platinum nanoparticles in colloidal solution, Nano Lett. 4 (2004) 1343–1348. [16] M.R. Johan1, M.S. Mohd Suan, N.L. Hawari, H.A. Ching, Annealing effects on the properties of copper oxide thin films prepared by chemical deposition, Int. J. Electrochem. Sci. 6 (2011) 6094–6104. [17] Z.H. Ibupoto, K. Khun, X. Liu, M. Willander, Low temperature synthesis of seed mediated CuO bundle of nanowires, their structural characterisation and cholesterol detection, Mater. Sci. Eng. C 33 (2013) 3889–3898. [18] Z.H. Ibupoto, K. Khun, J. Lu, M. Willander, The synthesis of CuO nanoleaves, structural characterization, and their glucose sensing application, Appl. Phys. Lett. 102 (2013) 103701.
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[19] Zafar Hussain Ibupoto, Kimleang Khun, Valerio Beni, Xianjie Liu, Magnus Willander, Synthesis of novel CuO nanosheets and their nonenzymatic glucose sensing applications, Sensors 13 (2013) 7926–7938. [20] I Kazeminezhad, A. sadollahkhani, Photocatalytic degradation of Eriochrome black-T dye using ZnO nanoparticles, Mater. Lett. 120 (2014) 267–270. [21] S. Warren, W.R. Flavell, A.G. Thomas, J. Hollingworth, P.M. Dunwoody, S. Downes, C. Changkang, Adsorption of H2O on single crystal CuO, Surf. Sci. 436 (1999) 1–8.