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Synthesis of nanosized Cu2O decorated single-walled carbon nanotubes and their superior catalytic activity
Colloids and Surfaces A 581 (2019) 123933
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Synthesis of nanosized Cu2O decorated single-walled carbon nanotubes and their superior catalytic activity
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M. A. Majeed Khana, , Wasi Khanb, Avshish Kumarc, Abdulaziz N. Alhazaaa,d a
King Abdullah Institute for Nanotechnology, King Saud University, Riyadh 11451, Saudi Arabia Department of Physics, Aligarh Muslim University, Aligarh 202002, India c Amity Institute for Advanced Research and Studies (M & D), Amity University, Noida 201313, India d Physics and Astronomy Department, King Saud University, Riyadh 11451, Saudi Arabia b
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Single walled carbon nanotubes PECVD Raman spectroscopy XPS Electron microscopy Photocatalytic activity
In this report, a parametric study was carried out on the single walled carbon nanotubes (SWCNTs) based Cu2O/ SWCNTs nanocomposite. The SWCNTs were synthesized via plasma enhanced chemical vapour deposition (PECVD) process and nanocomposite through RF sputtering technique. The prepared samples have been characterized through Fourier transform infrared (FTIR) spectroscopy, x-ray diffractometry (XRD), electron microscopy (SEM/HRTEM), energy dispersive x-ray (EDX) spectroscopy, x-ray photoelectron spectroscopy (XPS) and Raman scattering techniques for the detailed structural information. The analysis of XRD data affirms the amorphous nature of the nanotubes and crystalline phase of the Cu2O sample. HRTEM micrographs reveal dispersion of Cu2O nanoparticles of an average size of ∼3.2 nm onto the surface of SWCNTs. The optical properties were studied through UV–vis spectroscopy. Using Tauc’s relation, band gap was estimated and found to be 2.7 and 2.51 eV for Cu2O and Cu2O/SWCNTs nanocomposite respectively. The photocatalytic activity of the nanoparticles and nanocomposite was explored by the photodegradation of methylene blue solution under visible light irradiation. Field dependent magnetization exhibits ferromagnetic nature of both the samples at room temperature that opens a new window for the future electronic devices based on carbon nanotubes.
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Corresponding author. E-mail address: [email protected] (M.A. Majeed Khan).
https://doi.org/10.1016/j.colsurfa.2019.123933 Received 7 May 2019; Received in revised form 3 September 2019; Accepted 4 September 2019 Available online 20 September 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.
Colloids and Surfaces A 581 (2019) 123933
M.A. Majeed Khan, et al.
1. Introduction
purged several times with nitrogen gas and then Fe coated catalyst substrate was pre-treated in hydrogen (H2) atmosphere at a constant temperature of 400 0C. Thereafter, acetylene (source gas) with H2 gas was introduced into the system for 15 min at the rate of 20 sccm. During the growth process, temperature and pressure inside the chamber were maintained at 700 0C and 200 torr respectively. Finally, PECVD system was switched off and the product was cooled down to the room temperature.
In recent years, carbon nanotubes (CNTs) have been widely investigated because of their one-dimensional structure, exclusive properties and various applications in different fields [1–3]. According to the number of walls, CNTs are of two types i.e. single walled carbon nanotubes (SWCNTs) that comprise with a single sheet of graphite seamlessly folded into a cylindrical tube whereas multi walled carbon nanotubes (MWCNTs) consist of collection of coaxial nanotubes that nested like rings of a tree trunk [4,5]. Both types of CNTs have been widely explored as high-power electrode materials due to their excellent mechanical properties, high electrical conductivity, large intrinsic area with chemical stability. Presently, CNTs have been widely studied for binding the metal or metal oxide particles on the nanotube’s surface that assembled an inorganic nanocomposite. In the past, different metals or metal oxides like TiO2, Fe2O3, MnO2, SnO2, ZnO, RuO2 etc. have been used to alter the properties of CNTs [6–11]. Due to unique properties, cuprous oxide (Cu2O) has been considered as a suitable metal oxide to improve the properties of nanotubes for potential applications. Cu2O is a direct band gap natural semiconductor (p-type) having an energy gap of ∼2.17 eV. It has low cost, non-toxicity and good environmental acceptability that make it good alternative candidate for photocatalytic [12,13] and photovoltaic applications [14]. More attractively, Cu2O has notable physical and chemical properties [15]. In this view, an excellent catalytic performance was observed for the reduction of P-nitrophenol by the CNT/Cu2O nanocomposites [16]. Additionally, a variety of new CNT based nanocomposites were synthesized to determine photocatalytic performance of the system [17,18]. Zuo et al. have observed a high photocurrent effect and the minimum charge-transfer resistance in g-C3N4-Cu2O heterojunction composite. Further, they also noticed highest photodegradation rate for the methyl orange dye [19].-On the other hand, the same compound was also studied by Li et al. in the presence of glutamate and PEG-400 surfactant. They observed significant effect of these surfactants on the surface morphology and other properties including photocatalytic performance of the system [20,21]. Meanwhile, SWCNTs coated with Cu2O nanoparticles have been paid attention because of their inordinate utilization in various devices including solar cells, gas sensors, catalysis, magnetic storage etc [22]. This work demonstrates a facile approach to prepare Cu2O nanoparticles attached with SWCNTs. The microstructure, morphology and optical properties of the hybrid structure have been discussed in detail. Earlier reported work reveals improvement in the optical properties on the attachment of Cu2O nanoparticles with the CNTs, such as enhancement in the absorption and band gap energy of the system. On the other hand, carbon nanotubes have been synthesized through many reliable techniques such as arc discharge [23], laser ablation [24] and chemical vapor deposition (CVD) [25]. Among these methods, CVD is more easy and convenient because of several merits including low cost and easily control diameter of nanotubes. In this work, we have grown SWCNTs on Si < 100 > substrate by plasma-enhanced chemical vapour deposition (PECVD) process and then Cu2O nanoparticles were decorated on the SWCNTs. The microstructure, optical, magnetic and photocatalytic properties have been explored through various analytical techniques.
2.2. Synthesis of cuprous oxide/SWCNTs nanocomposite Cuprous oxide nanoparticles were decorated with as-grown SWCNTs using RF-sputtering system at low temperature because it is an excellent technique for the rapid and uniform deposition of nanoparticles over the substrate. In this process, a highly pure (99.99%) copper (Cu) target of 2″ diameter was used. Argon gas was injected into the chamber to get an inert environment while oxygen gas has been utilized as a reactive gas during the process in order to decorate Cu2O nanoparticles on the surface of SWCNTs. The separation between Cu target and the substrate was maintained as ∼10 cm. A high vacuum of the order of ∼10−6 torr was produced in the deposition chamber by a turbo molecular pump (TMP) and backed by the chemical rotary pump (RP). Thereafter, two step processes were followed: in the first step, a presputtering process was performed for 5 min to eliminate the contamination or any oxide layer present on the surface of the Cu target. In the next step, the argon and oxygen gases were injected into the deposition chamber via mass flow controllers (MFCs). The flow of argon and oxygen gases has been maintained at 50 and 15 sccm respectively. During deposition, the RF power of 100 W and pressure of 10−4 torr were kept constant inside the chamber. The deposition process was continued for 5 min. 2.3. Characterization The crystallographic phases of the growth film of Cu2O/SWCNTs were investigated via x-ray diffractometer (PANanalytic X’Pert Pro) using Cu-Kα radiation of wavelength 1.5405 Å in the 2θ angle of 20 to 80° at a scan rate of 10° min−1 and step width of 0.02/s. The morphology and crystallinity measurements have been determined by a field-emission scanning electron microscopy (FESEM) and high resolution transmission electron microscopy (HRTEM) JEOL, JEM-2100 F equipped with an energy dispersive x-ray (EDX) spectrometer. XPS measurement has been carried out using ESCA system (model VG 3000) with monochromatic Mg-Kα line (1253.6 eV) radiation. The Raman and photoluminescence (PL) spectra of the samples were recorded on a Horiba-T64000 spectrometer with a continuous wave laser operating at 325 nm having an excitation source of 200 to 1000 cm−1. The magnetic properties were studied using vibrating sample magnetometer (VSM) at room temperature. The Fourier transform infrared (FTIR) spectrum was registered through FTIR spectrometer (Perkin-Elmer) with the ATR device (400-4000 cm−1). Optical measurements were performed by a dual beam spectrophotometer (UV-2550, Shimadzu, Japan) over a wavelength range of 200–900 nm.
2. Experimental details
2.4. Photocatalytic performance
2.1. Growth of SWCNTs
The photocatalytic properties of the Cu2O/SWCNTs nanocomposite is examined for the degradation of methylene blue (MB) dye under irradiation by visible light at room temperature. The visible light of wavelength 400 nm is selected as a light source by a 400 W sodium lamp, Philips, with wavelength range of 400–900 nm. In a typical reaction, 2 mg of the catalytic was mixed into 10 ml of MB dye solution. Before the photocatalytic performance test, the suspension was stirred for 40 min in the dark to establish adsorption-desorption equilibrium between MB and the photocatalyst and then analyze by an UV–vis
SWCNTs have been grown onto a Si < 100 > substrate (n-type) by PECVD process. Prior to deposition, substrate of Si was washed with acetone ultrasonically and dried at room temperature. Then the substrate was placed inside a RF sputtering chamber for the deposition of iron as a catalyst. Now, Fe coated Si substrate has been positioned on the substrate holder inside a quartz tube of PECVD system. The chamber was then evacuated to 10−3 torr. Initially, quartz tube was 2
Colloids and Surfaces A 581 (2019) 123933
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3.2. X-ray photoelectron spectroscopy X-ray photoelectron spectroscopy (XPS) has been utilized to ensure chemical state and crystal structure at the interface of SWCNTs and Cu2O nanoparticles. The XPS signals of Cu+ in Fig. 3(a) having Cu 2p3/2 and Cu 2p1/2 components at about 934.4 and 954.6 eV binding energies (BE) respectively [27] that attribute uniform distribution and chemical bonding of Cu2O nanoparticles on the surface of SWNTs. Our results are well consistent with earlier reports on Cu2O [28,29]. Further, a peak located at 529.6 eV (Fig. 3b) is related to the oxygen atoms in Cu2O. The C 1s spectra of Cu2O/SWCNTs shown in Fig. 3(c) can be deconvoluted into two components located at 285.4 eV. Moreover, the peak located at 285.9 eV is related to the sp3 hybridized carbon and may be originated from the defects on the tube walls [30]. Thus, these results imply that the sample has Cu, O and C elements in Cu2O/SWCNTs composite. We have also obtained the quantitative structural information from the analysis of XPS data. 3.3. Raman and FTIR spectroscopy
Fig. 1. XRD pattern of Cu2O/SWCNTs nanocomposite. Inset shows the same for SWCNTs.
Raman scattering is one of the unique tools to study electronic structure of carbon nanotubes. In the present work, radial breathing mode (RBM) of the SWCNTs is investigated and analysis reveals coherent vibration of the carbon atoms in a radial direction with highly sensitive to the nanotube diameter [31]. These characteristic features may be found in the spectra at a wide range of Raman shift ∼ 120285 cm−1 as given in Fig. 4(a,b). We have also calculated diameter (d) of the prepared SWCNTs from the Raman scattering data with the help of RBM frequency by the relation, ωRBM = 248/ d (nm) , here ω is the Raman shift in cm-1. In the present spectrum, the RBM is observed at 192 cm-1 and 248 cm-1 wavenumbers that correspond to the diameters of 1.3 and 1.0 nm, respectively, indicating the existence of SWCNTs. The other peak at 300 cm−1 is attributed to the existence of Cu2O phase in the nanocomposite [32,33]. The peaks appear at ∼1300 and 1591 cm−1 are related to the well-documented characteristic D and G bands of SWCNTs, whose intensity ratio indicates quality of the tubes. Moreover, the band at 2595 cm−1 attributed to the photon-secondphonon interactions [34,35]. Due to the attachment of Cu2O nanoparticles on the SWCNTs, the ratio of intensities of the G and D bands (IG/ID) is obtained as 2.41 that signifies the deformation in the structure of SWCNTs and excludes the possibilities of other forms of non-graphitic carbon [36]. Further, in order to know deep structural information of Cu2O/SWCNTs nanocomposite composite, FTIR spectrum was recorded in the wavenumber range of 400 to 4000 cm−1 as illustrated in the inset of Fig. 4(b). The peak appears at 623 cm−1 can be ascribed to the Cu-O vibrations in the Cu2O structure that agrees with the previous reported value [37]. While the peaks at 3445 and 1652 cm−1 belong to the stretching and bending vibration of the OeH group mainly due to absorbed water. However, the signal at 2360 cm−1 is attributed to an asymmetrical OH stretch of hydrogen bonded carboxylic acid [38].
spectrometer (Shimadzu UV-1800) at 664 nm, which corresponds to the maximum absorbance wavelength for the MB dye. Photodegradation efficiency has been evaluated by the concentration (C/Co) according to the absorbance (A/Ao) at 664 nm, where Co and Ao are the concentration and adsorption of aqueous MB solution after the adsorption–desorption equilibrium.
3. Results and discussion 3.1. Structural studies The crystalline structure and formation of the SWCNTs and Cu2O/ SWCNTs nanocomposite have been examined through the XRD patterns and shown in Fig. 1. The diffraction peaks appeared at 29.7, 36.7, 42.6, 53.2, 61.8, 74.0 and 77.8° assign to the (110), (111), (200), (211), (220), (311) and (222) planes of cuprous oxide (JPCDS-077-0199). Whereas, the typical peaks at approximately 26.2, 43.2, 44.1 and 56.3 correspond to (002) (100), (101) and (004) planes of the graphitic phase of SWCNTs (JCPDS-75-1621) (Inset of Fig. 1) [26]. No other peaks related to any impurity or secondary phase exist in the patterns that signify successful synthesis of SWCNTs and its nanocomposite. Further, morphology, shape and size of the grown SWCNTs and Cu2O/SWCNTs were explored by SEM as illustrated in Fig. 2(a,b). It is evident from the micrographs that the uniform SWCNTs have been grown with random orientation. The synthesis of the nanotubes is in accordance with the growth mechanism. In addition, the diameter of the SWCNTs is about 0.82–1.5 nm, while the length can reach more than several micrometres. However, it is difficult to measure the exact length because bundle always found in the bending form whose both ends are not in the same line. In order to ensure crystallinity, precise value of diameters and lattice planes, HRTEM measurements were carried out and displayed in Fig. 2(c,d). It again reveals that the SWCNTs produced in bundles with small diameters which are clearly visible in Fig. 2(c). The analysis of HRTEM micrograph depicts the SWCNTs diameter distribution in the range of 0.76–1.2 nm. Fig. 2(d) indicates the formation of Cu2O/SWCNTs nanocomposite as nanoparticles of Cu2O can be seen clearly on the nanotubes that implies the weak interactions with the CNTs. Further, EDX spectrum is used to analyze the elemental composition of the Cu2O/SWCNTs nanocomposite as shown in Fig. 2(e). It indicates the presence of the expected elements in the structure of the nanocomposite, namely copper, carbon and oxygen.
3.4. Magnetic properties Room temperature magnetic behaviour of the Cu2O nanoparticles and Cu2O/SWCNTs nanocomposite have been studied via vibrating sample magnetometer (VSM) under an applied magnetic field of ± 6000 Oe and the obtained hysteresis loops are represented in Fig. 5. Both loops signify ferromagnetic nature of the samples with saturation magnetization (Ms) 2.05 × 10−3 emu/g and coercivity (Hc) of 1.55 kOe for Cu2O nanoparticles while the value of these parameters slightly reduce for the nanocomposite and becomes Ms ∼ 1.95 × 10−3 emu/g and Hc ∼ 1.40 kOe. This may be attributed to the defects or oxygen vacancies created in the system associated with the strong coupling between Cu2O nanoparticles and SWCNTs sheets [39]. 3
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Fig. 2. a, b) SEM micrographs, (c,d) HRTEM images of SWCNTs and Cu2O/SWCNTs nanocomposite, and (e) EDX spectrum of the Cu2O/SWCNTs.
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Fig. 3. High resolution XPS spectra of (a) Cu2p, (b) O1 s, and (c) C1 s components for Cu2O/SWCNTs nanocomposite.
3.5. Optical studies
(αhν )2 = K (hν − Eg )
Optical properties of the synthesized Cu2O/SWCNTs were studied via UV–vis absorption spectroscopy in the wavelength range of 200–900 nm (Fig. 6). A shift in the absorption peak is observed for the nanocomposite i.e. ∼230 nm for Cu2O to ∼240 nm for the Cu2O/ SWCNTs that signifies a red shift in the absorption edge. The band gap (Eg) has been calculated with the help of absorption coefficient (α = 2.303A/t, here A is the absorbance and t stands for the sample thickness) and photon energy (hν) [40], The direct band gap of SWCNTs is determined from the graph of (αhν)2 versus hν by employing the relation given as [41]
where K is a constant related to the optical absorption edge width parameter. The energy band gap can be obtained by extrapolation of the straight line in the graph of (αhν)2 on the x-axis to (αhν)2→0, as shown in the inset of Fig. 6. The estimated band gap was found to be 2.70 eV and 2.51 eV for Cu2O and Cu2O/SWCNTs respectively. The band gap of Cu2O is found to be much larger than that of the theoretical bulk value 2.17 eV [42], may be due to the quantum confinement effects in the nanostructures [43]. However, the obtained band gap of Cu2O is quite closer to the reported value [44]. We have also characterized our samples via photoluminescence (PL) technique to reveal the efficiency of charge carrier trapping, migration and recombination
Fig. 4. Raman spectra of (a) SWCNTs, and (b) Cu2O/SWCNTs nanocomposite. Inset shows FTIR spectrum of the nanocomposite. 5
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Fig. 7. Room temperature PL spectra of the Cu2O and Cu2O/SWCNTs samples. Fig. 5. Magnetic hysteresis loops for Cu2O and Cu2O/SWCNTs samples.
SWCNTs) was also performed. The time dependent absorption spectra of MB solution under light illumination in the existence of Cu2O and Cu2O/CNTs have shown in Fig. 8(a,b). It is evident from the figure that the intensity of absorption peak at 663 nm gradually decreases by increasing of irradiation time and almost vanish in 150 min. This infers complete deterioration of MB dye. The degradation ability of Cu2O and Cu2O/SWCNTs nanocomposite after 150 min has been calculated and found to be 81% and 94% (Fig. 8(c)). It is worth noting here that the photocatalytic performance of semiconductors strongly depends on its morphology, microstructure, composition, surface properties etc. [46]. It has also been reported that the photocatalytic performance is influenced by the photo-generated charge carriers (electrons and holes). In this study, nanocomposite exhibits high electron-hole recombination, an appropriate surface defect mechanism is required for high transportation rate of charge carriers and the occurrence of redox reactions. Hence, the high specific surface area of Cu2O/CNTs photocatalyst facilitates dense adsorption of the dye molecules. The prepared
processes of the photogenerated electron–hole pairs. Room temperature PL spectra of the Cu2O nanoparticles and Cu2O/SWCNTs nanocomposite in the wavelength range of 500–750 nm with an excitation wavelength of 480 nm is shown in Fig. 7. It is observed that the intensity in the spectra reduces for the nanocomposite. This may be due to reduction in recombination of the photoinduced charge carriers that suggests enhancement in the photocatalytic activity of the Cu2O/SWCNTs nanocomposite. The peaks appear in the luminescence spectrum are attributed to the localized surface states associated to the recombination of photogenerated charge carriers (electrons/holes) [45].
3.6. Photocatalytic activity Photocatalytic performance of the Cu2O and Cu2O/SWCNTs was explored for the deterioration of MB dye under visible light irradiation. A blank experiment in the absence of the photocatalyst (Cu2O/
Fig. 6. UV–vis absorption spectra of Cu2O and Cu2O/SWCNTs. Inset shows the Tauc’s plots for the same. 6
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Fig. 8. (a,b) UV–vis absorbance spectra of MB dye at different irradiation times for Cu2O and Cu2O/SWCNTs as photocatalysts, (c) Photocatalytic degradation of MB dye as a function of irradiation time under visible light in the presence of Cu2O and Cu2O/SWCNTs as catalysts.
cycle of the photocatalytic reaction was maintained for 150 min. The reduction in photocatalytic degradation rate after six cycles was less than 3.5%, indicating that the prepared nanocomposite has a desirable stability and maintains a high photocatalytic activity. 4. Conclusions In summary, SWCNTs and Cu2O/SWCNTs nanocomposite were synthesized using plasma enhanced chemical vapour deposition (PECVD) and RF sputtering techniques respectively. XRD analysis confirms the Cu2O nanoparticles coated on the SWCNTs in pure crystalline phase with face-centered cubic structure. SEM and HRTEM results reveal decoration of Cu2O nanoparticles of size of ∼3.2 nm on the surface of the SWCNTs. The optical band gap is estimated as 2.70 eV and 2.51 eV for Cu2O and Cu2O/SWCNTs nanocomposite respectively. The photocatalytic performance of the samples was evaluated for the methylene blue dye under visible light irradiation and degradation efficiency was found to be 81% and 94% after irradiation within 150 min for the Cu2O nanoparticles and Cu2O/SWCNTs nanocomposite respectively. Therefore, the results on Cu2O-coated SWCNTs signify that the synthesized nanocomposite offers a promising candidate for the high efficiency sunlight photocatalysis. Room temperature ferromagnetism has been observed in both the samples. However, ferromagnetic
Fig. 9. Reusability assay of Cu2O/SWCNTs nanocomposite for photodegradation of MB dye under visible light irradiation.
nanocomposite photocatalyst offers relatively high surface area that contributes the improvement of the photocatalytic activities. The recyclability and stability of the catalyst during photodegradation reaction were also investigated through the degradation of MB under visible light irradiation and shown in Fig. 9. The reusability of the Cu2O/SWCNTs nanocomposite was tested for six times and each 7
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ordering slightly suppresses in the nanocomposite attributed to the strong coupling between Cu2O nanoparticles and SWCNTs.
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement The authors would like to extend their sincere appreciation to the Deanship of Scientific Research at King Saud University Riyadh for its funding of this research through the research Group Project No. RGP1437-023. References [1] P. Avouris, Z. Chen, V. Perebeinos, Carbon-based electronics, Nat. Nanotechnol. 2 (2007) 605–615. [2] P.M. Ajayan, J.M. Tour, Nanotube composites, Nature 447 (2007) 1066–1068. [3] N. Narita, Y. Kobayashi, H. Nakamura, K. Maeda, A. Ishihara, T. Mizoguchi, Y. Usui, K. Aoki, M. Simizu, H. Kato, H. Ozawa, N. Udagawa, M. Endo, N. Takahashi, N. Saito, Multiwalled carbon nanotubes specifically inhibit osteoclast differentiation and function, Nano Lett. 9 (2009) 1406–1413. [4] X. Ji, C.E. Banks, A. Crossley, R.G. Compton, Oxygenated edge plane sites slow the Electron transfer of the Ferro/Ferricyanide redox couple at graphite electrodes, Z. Fã¼r Phys. Chemie/International J. Res. Phys. Chem. Chem. Phys. 7 (2006) 1337–1344. [5] T. Kolodiazhnyi, M. Pumera, Towards an ultrasensitive method for the determination of metal impurities in carbon nanotubes, Small 9 (2008) 1476–1484. [6] C.S. Kuo, Y.H. Tseng, H.Y. Lin, C.H. Huang, C.Y. Shen, Y.Y. Li, S.I. Shah, C.P. Huang, Synthesis of a CNT-grafted TiO2nanocatalyst and its activity triggered by a DC voltage, Nanotechnology 18 (2007) 465607. [7] W.Q. Han, A. Zettl, Coating single-walled carbon nanotubes with tin oxide, Nano Lett. 3 (2003) 681–683. [8] J.W. Liu, X.J. Li, L.M. Dai, Water-assisted growth of aligned carbon nanotube–ZnO heterojunction arrays, Adv. Mater. 18 (2006) 1740–1743. [9] X.B. Fan, F.Y. Tan, G.L. Zhang, F.B. Zhang, A novel strategy to fabricate -Fe2O3MWCNTs hybrids with selectively ferromagnetic or superparamagnetic properties, Mater. Sci. Eng. A 454–455 (2007) 37–42. [10] Z. Fan, J.H. Chen, B. Zhang, B. Liu, X.X. Zhong, Y.F. Kuang, Role of iron catalyst particles density in the growth of forest-like carbon nanotubes, Diamond Relat. Mater. 17 (2008) 1936–1943. [11] Z.Y. Wang, G. Chen, D.G. Xia, Coating of multi-walled carbon nanotube with SnO2 films of controlled thickness and its application for Li-ion battery, J. Power Sources 184 (2008) 432–436. [12] S. Chu, X.M. Zheng, F. Kong, G.H. Wu, L.L. Luo, Y. Guo, H.L. Liu, Y. Wang, H.X. Yu, Z.G. Zou, Architecture of Cu2O@TiO2 core–shell heterojunction and photodegradation for 4-nitrophenol under simulated sunlight irradiation, Mater. Chem. Phys. 129 (2011) 1184–1188. [13] M. Deo, D. Shinde, A. Yengantiwar, J. Jog, B. Hannoyer, X. Sauvage, M. More, S. Ogale, Cu2O/ZnO hetero-nanobrush: hierarchical assembly, field emission and photocatalytic properties, J. Mater. Chem. 22 (2012) 17055–17062. [14] K. Han, M. Tao, Electrochemically deposited p–n homojunction cuprous oxide solar cells, Sol. Energy Mater. Sol. Cells 93 (2009) 153–157. [15] M. Wang, J. Lu, J. Ma, Z. Zhang, F. Wang, Cuprous oxide catalyzed oxidative C-C bond cleavage for C-N bond formation: synthesis of cyclic imides from ketones and amines, Angew. Chem. Int. Ed. 54 (2015) 14061–14065. [16] Y. Feng, T. Jiao, J. Yin, L. Zhang, L. Zhang, J. Zhou, Q. Peng, Facile preparation of carbon nanotube- Cu2O nanocomposites as new catalyst materials for reduction of P-Nitrophenol, Nanoscale Res. Lett. 14 (2019) 78. [17] C. Wang, S. Sun, L. Zhang, J. Yin, et al., Facile preparation and catalytic performance characterization of AuNPs loaded hierarchical electrospun composite fibers by solvent vapor annealing treatment, Colloids Surf. A Physicochem. Eng. Asp. 561 (2019) 283–291. [18] K. Liu, R. Xing, C. Chen, G. Shen, L. Yan, et al., Peptide-induced hierarchical longrange order and photocatalytic activity of porphyrin assemblies, Angew. Chem. Int. Ed. 54 (2015) 500–505. [19] H.X. ShiyuZuo, W. Liao, X. Yuan, L. Sun, Q. Li, D.L. JieZan, D. Xia, Molten-salt synthesis of g-C3N4-Cu2O heterojunctions with highly enhanced photocatalytic performance, Colloids Surf. A Physicochem. Eng. Asp. 546 (2018) 307–315. [20] D. Li, L.W. JieZan, H.X. ShiyuZuo, D. Xia, Heterojunction tuning and catalytic efficiency of g‑C3N4−Cu2O with glutamate, Ind. Eng. Chem. Res. 58 (2019) 4000–4009.
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Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Synthesis of nanosized Cu2O decorated single-walled carbon nanotubes and their superior catalytic activity
T
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M. A. Majeed Khana, , Wasi Khanb, Avshish Kumarc, Abdulaziz N. Alhazaaa,d a
King Abdullah Institute for Nanotechnology, King Saud University, Riyadh 11451, Saudi Arabia Department of Physics, Aligarh Muslim University, Aligarh 202002, India c Amity Institute for Advanced Research and Studies (M & D), Amity University, Noida 201313, India d Physics and Astronomy Department, King Saud University, Riyadh 11451, Saudi Arabia b
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Single walled carbon nanotubes PECVD Raman spectroscopy XPS Electron microscopy Photocatalytic activity
In this report, a parametric study was carried out on the single walled carbon nanotubes (SWCNTs) based Cu2O/ SWCNTs nanocomposite. The SWCNTs were synthesized via plasma enhanced chemical vapour deposition (PECVD) process and nanocomposite through RF sputtering technique. The prepared samples have been characterized through Fourier transform infrared (FTIR) spectroscopy, x-ray diffractometry (XRD), electron microscopy (SEM/HRTEM), energy dispersive x-ray (EDX) spectroscopy, x-ray photoelectron spectroscopy (XPS) and Raman scattering techniques for the detailed structural information. The analysis of XRD data affirms the amorphous nature of the nanotubes and crystalline phase of the Cu2O sample. HRTEM micrographs reveal dispersion of Cu2O nanoparticles of an average size of ∼3.2 nm onto the surface of SWCNTs. The optical properties were studied through UV–vis spectroscopy. Using Tauc’s relation, band gap was estimated and found to be 2.7 and 2.51 eV for Cu2O and Cu2O/SWCNTs nanocomposite respectively. The photocatalytic activity of the nanoparticles and nanocomposite was explored by the photodegradation of methylene blue solution under visible light irradiation. Field dependent magnetization exhibits ferromagnetic nature of both the samples at room temperature that opens a new window for the future electronic devices based on carbon nanotubes.
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Corresponding author. E-mail address: [email protected] (M.A. Majeed Khan).
https://doi.org/10.1016/j.colsurfa.2019.123933 Received 7 May 2019; Received in revised form 3 September 2019; Accepted 4 September 2019 Available online 20 September 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.
Colloids and Surfaces A 581 (2019) 123933
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1. Introduction
purged several times with nitrogen gas and then Fe coated catalyst substrate was pre-treated in hydrogen (H2) atmosphere at a constant temperature of 400 0C. Thereafter, acetylene (source gas) with H2 gas was introduced into the system for 15 min at the rate of 20 sccm. During the growth process, temperature and pressure inside the chamber were maintained at 700 0C and 200 torr respectively. Finally, PECVD system was switched off and the product was cooled down to the room temperature.
In recent years, carbon nanotubes (CNTs) have been widely investigated because of their one-dimensional structure, exclusive properties and various applications in different fields [1–3]. According to the number of walls, CNTs are of two types i.e. single walled carbon nanotubes (SWCNTs) that comprise with a single sheet of graphite seamlessly folded into a cylindrical tube whereas multi walled carbon nanotubes (MWCNTs) consist of collection of coaxial nanotubes that nested like rings of a tree trunk [4,5]. Both types of CNTs have been widely explored as high-power electrode materials due to their excellent mechanical properties, high electrical conductivity, large intrinsic area with chemical stability. Presently, CNTs have been widely studied for binding the metal or metal oxide particles on the nanotube’s surface that assembled an inorganic nanocomposite. In the past, different metals or metal oxides like TiO2, Fe2O3, MnO2, SnO2, ZnO, RuO2 etc. have been used to alter the properties of CNTs [6–11]. Due to unique properties, cuprous oxide (Cu2O) has been considered as a suitable metal oxide to improve the properties of nanotubes for potential applications. Cu2O is a direct band gap natural semiconductor (p-type) having an energy gap of ∼2.17 eV. It has low cost, non-toxicity and good environmental acceptability that make it good alternative candidate for photocatalytic [12,13] and photovoltaic applications [14]. More attractively, Cu2O has notable physical and chemical properties [15]. In this view, an excellent catalytic performance was observed for the reduction of P-nitrophenol by the CNT/Cu2O nanocomposites [16]. Additionally, a variety of new CNT based nanocomposites were synthesized to determine photocatalytic performance of the system [17,18]. Zuo et al. have observed a high photocurrent effect and the minimum charge-transfer resistance in g-C3N4-Cu2O heterojunction composite. Further, they also noticed highest photodegradation rate for the methyl orange dye [19].-On the other hand, the same compound was also studied by Li et al. in the presence of glutamate and PEG-400 surfactant. They observed significant effect of these surfactants on the surface morphology and other properties including photocatalytic performance of the system [20,21]. Meanwhile, SWCNTs coated with Cu2O nanoparticles have been paid attention because of their inordinate utilization in various devices including solar cells, gas sensors, catalysis, magnetic storage etc [22]. This work demonstrates a facile approach to prepare Cu2O nanoparticles attached with SWCNTs. The microstructure, morphology and optical properties of the hybrid structure have been discussed in detail. Earlier reported work reveals improvement in the optical properties on the attachment of Cu2O nanoparticles with the CNTs, such as enhancement in the absorption and band gap energy of the system. On the other hand, carbon nanotubes have been synthesized through many reliable techniques such as arc discharge [23], laser ablation [24] and chemical vapor deposition (CVD) [25]. Among these methods, CVD is more easy and convenient because of several merits including low cost and easily control diameter of nanotubes. In this work, we have grown SWCNTs on Si < 100 > substrate by plasma-enhanced chemical vapour deposition (PECVD) process and then Cu2O nanoparticles were decorated on the SWCNTs. The microstructure, optical, magnetic and photocatalytic properties have been explored through various analytical techniques.
2.2. Synthesis of cuprous oxide/SWCNTs nanocomposite Cuprous oxide nanoparticles were decorated with as-grown SWCNTs using RF-sputtering system at low temperature because it is an excellent technique for the rapid and uniform deposition of nanoparticles over the substrate. In this process, a highly pure (99.99%) copper (Cu) target of 2″ diameter was used. Argon gas was injected into the chamber to get an inert environment while oxygen gas has been utilized as a reactive gas during the process in order to decorate Cu2O nanoparticles on the surface of SWCNTs. The separation between Cu target and the substrate was maintained as ∼10 cm. A high vacuum of the order of ∼10−6 torr was produced in the deposition chamber by a turbo molecular pump (TMP) and backed by the chemical rotary pump (RP). Thereafter, two step processes were followed: in the first step, a presputtering process was performed for 5 min to eliminate the contamination or any oxide layer present on the surface of the Cu target. In the next step, the argon and oxygen gases were injected into the deposition chamber via mass flow controllers (MFCs). The flow of argon and oxygen gases has been maintained at 50 and 15 sccm respectively. During deposition, the RF power of 100 W and pressure of 10−4 torr were kept constant inside the chamber. The deposition process was continued for 5 min. 2.3. Characterization The crystallographic phases of the growth film of Cu2O/SWCNTs were investigated via x-ray diffractometer (PANanalytic X’Pert Pro) using Cu-Kα radiation of wavelength 1.5405 Å in the 2θ angle of 20 to 80° at a scan rate of 10° min−1 and step width of 0.02/s. The morphology and crystallinity measurements have been determined by a field-emission scanning electron microscopy (FESEM) and high resolution transmission electron microscopy (HRTEM) JEOL, JEM-2100 F equipped with an energy dispersive x-ray (EDX) spectrometer. XPS measurement has been carried out using ESCA system (model VG 3000) with monochromatic Mg-Kα line (1253.6 eV) radiation. The Raman and photoluminescence (PL) spectra of the samples were recorded on a Horiba-T64000 spectrometer with a continuous wave laser operating at 325 nm having an excitation source of 200 to 1000 cm−1. The magnetic properties were studied using vibrating sample magnetometer (VSM) at room temperature. The Fourier transform infrared (FTIR) spectrum was registered through FTIR spectrometer (Perkin-Elmer) with the ATR device (400-4000 cm−1). Optical measurements were performed by a dual beam spectrophotometer (UV-2550, Shimadzu, Japan) over a wavelength range of 200–900 nm.
2. Experimental details
2.4. Photocatalytic performance
2.1. Growth of SWCNTs
The photocatalytic properties of the Cu2O/SWCNTs nanocomposite is examined for the degradation of methylene blue (MB) dye under irradiation by visible light at room temperature. The visible light of wavelength 400 nm is selected as a light source by a 400 W sodium lamp, Philips, with wavelength range of 400–900 nm. In a typical reaction, 2 mg of the catalytic was mixed into 10 ml of MB dye solution. Before the photocatalytic performance test, the suspension was stirred for 40 min in the dark to establish adsorption-desorption equilibrium between MB and the photocatalyst and then analyze by an UV–vis
SWCNTs have been grown onto a Si < 100 > substrate (n-type) by PECVD process. Prior to deposition, substrate of Si was washed with acetone ultrasonically and dried at room temperature. Then the substrate was placed inside a RF sputtering chamber for the deposition of iron as a catalyst. Now, Fe coated Si substrate has been positioned on the substrate holder inside a quartz tube of PECVD system. The chamber was then evacuated to 10−3 torr. Initially, quartz tube was 2
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3.2. X-ray photoelectron spectroscopy X-ray photoelectron spectroscopy (XPS) has been utilized to ensure chemical state and crystal structure at the interface of SWCNTs and Cu2O nanoparticles. The XPS signals of Cu+ in Fig. 3(a) having Cu 2p3/2 and Cu 2p1/2 components at about 934.4 and 954.6 eV binding energies (BE) respectively [27] that attribute uniform distribution and chemical bonding of Cu2O nanoparticles on the surface of SWNTs. Our results are well consistent with earlier reports on Cu2O [28,29]. Further, a peak located at 529.6 eV (Fig. 3b) is related to the oxygen atoms in Cu2O. The C 1s spectra of Cu2O/SWCNTs shown in Fig. 3(c) can be deconvoluted into two components located at 285.4 eV. Moreover, the peak located at 285.9 eV is related to the sp3 hybridized carbon and may be originated from the defects on the tube walls [30]. Thus, these results imply that the sample has Cu, O and C elements in Cu2O/SWCNTs composite. We have also obtained the quantitative structural information from the analysis of XPS data. 3.3. Raman and FTIR spectroscopy
Fig. 1. XRD pattern of Cu2O/SWCNTs nanocomposite. Inset shows the same for SWCNTs.
Raman scattering is one of the unique tools to study electronic structure of carbon nanotubes. In the present work, radial breathing mode (RBM) of the SWCNTs is investigated and analysis reveals coherent vibration of the carbon atoms in a radial direction with highly sensitive to the nanotube diameter [31]. These characteristic features may be found in the spectra at a wide range of Raman shift ∼ 120285 cm−1 as given in Fig. 4(a,b). We have also calculated diameter (d) of the prepared SWCNTs from the Raman scattering data with the help of RBM frequency by the relation, ωRBM = 248/ d (nm) , here ω is the Raman shift in cm-1. In the present spectrum, the RBM is observed at 192 cm-1 and 248 cm-1 wavenumbers that correspond to the diameters of 1.3 and 1.0 nm, respectively, indicating the existence of SWCNTs. The other peak at 300 cm−1 is attributed to the existence of Cu2O phase in the nanocomposite [32,33]. The peaks appear at ∼1300 and 1591 cm−1 are related to the well-documented characteristic D and G bands of SWCNTs, whose intensity ratio indicates quality of the tubes. Moreover, the band at 2595 cm−1 attributed to the photon-secondphonon interactions [34,35]. Due to the attachment of Cu2O nanoparticles on the SWCNTs, the ratio of intensities of the G and D bands (IG/ID) is obtained as 2.41 that signifies the deformation in the structure of SWCNTs and excludes the possibilities of other forms of non-graphitic carbon [36]. Further, in order to know deep structural information of Cu2O/SWCNTs nanocomposite composite, FTIR spectrum was recorded in the wavenumber range of 400 to 4000 cm−1 as illustrated in the inset of Fig. 4(b). The peak appears at 623 cm−1 can be ascribed to the Cu-O vibrations in the Cu2O structure that agrees with the previous reported value [37]. While the peaks at 3445 and 1652 cm−1 belong to the stretching and bending vibration of the OeH group mainly due to absorbed water. However, the signal at 2360 cm−1 is attributed to an asymmetrical OH stretch of hydrogen bonded carboxylic acid [38].
spectrometer (Shimadzu UV-1800) at 664 nm, which corresponds to the maximum absorbance wavelength for the MB dye. Photodegradation efficiency has been evaluated by the concentration (C/Co) according to the absorbance (A/Ao) at 664 nm, where Co and Ao are the concentration and adsorption of aqueous MB solution after the adsorption–desorption equilibrium.
3. Results and discussion 3.1. Structural studies The crystalline structure and formation of the SWCNTs and Cu2O/ SWCNTs nanocomposite have been examined through the XRD patterns and shown in Fig. 1. The diffraction peaks appeared at 29.7, 36.7, 42.6, 53.2, 61.8, 74.0 and 77.8° assign to the (110), (111), (200), (211), (220), (311) and (222) planes of cuprous oxide (JPCDS-077-0199). Whereas, the typical peaks at approximately 26.2, 43.2, 44.1 and 56.3 correspond to (002) (100), (101) and (004) planes of the graphitic phase of SWCNTs (JCPDS-75-1621) (Inset of Fig. 1) [26]. No other peaks related to any impurity or secondary phase exist in the patterns that signify successful synthesis of SWCNTs and its nanocomposite. Further, morphology, shape and size of the grown SWCNTs and Cu2O/SWCNTs were explored by SEM as illustrated in Fig. 2(a,b). It is evident from the micrographs that the uniform SWCNTs have been grown with random orientation. The synthesis of the nanotubes is in accordance with the growth mechanism. In addition, the diameter of the SWCNTs is about 0.82–1.5 nm, while the length can reach more than several micrometres. However, it is difficult to measure the exact length because bundle always found in the bending form whose both ends are not in the same line. In order to ensure crystallinity, precise value of diameters and lattice planes, HRTEM measurements were carried out and displayed in Fig. 2(c,d). It again reveals that the SWCNTs produced in bundles with small diameters which are clearly visible in Fig. 2(c). The analysis of HRTEM micrograph depicts the SWCNTs diameter distribution in the range of 0.76–1.2 nm. Fig. 2(d) indicates the formation of Cu2O/SWCNTs nanocomposite as nanoparticles of Cu2O can be seen clearly on the nanotubes that implies the weak interactions with the CNTs. Further, EDX spectrum is used to analyze the elemental composition of the Cu2O/SWCNTs nanocomposite as shown in Fig. 2(e). It indicates the presence of the expected elements in the structure of the nanocomposite, namely copper, carbon and oxygen.
3.4. Magnetic properties Room temperature magnetic behaviour of the Cu2O nanoparticles and Cu2O/SWCNTs nanocomposite have been studied via vibrating sample magnetometer (VSM) under an applied magnetic field of ± 6000 Oe and the obtained hysteresis loops are represented in Fig. 5. Both loops signify ferromagnetic nature of the samples with saturation magnetization (Ms) 2.05 × 10−3 emu/g and coercivity (Hc) of 1.55 kOe for Cu2O nanoparticles while the value of these parameters slightly reduce for the nanocomposite and becomes Ms ∼ 1.95 × 10−3 emu/g and Hc ∼ 1.40 kOe. This may be attributed to the defects or oxygen vacancies created in the system associated with the strong coupling between Cu2O nanoparticles and SWCNTs sheets [39]. 3
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Fig. 2. a, b) SEM micrographs, (c,d) HRTEM images of SWCNTs and Cu2O/SWCNTs nanocomposite, and (e) EDX spectrum of the Cu2O/SWCNTs.
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Fig. 3. High resolution XPS spectra of (a) Cu2p, (b) O1 s, and (c) C1 s components for Cu2O/SWCNTs nanocomposite.
3.5. Optical studies
(αhν )2 = K (hν − Eg )
Optical properties of the synthesized Cu2O/SWCNTs were studied via UV–vis absorption spectroscopy in the wavelength range of 200–900 nm (Fig. 6). A shift in the absorption peak is observed for the nanocomposite i.e. ∼230 nm for Cu2O to ∼240 nm for the Cu2O/ SWCNTs that signifies a red shift in the absorption edge. The band gap (Eg) has been calculated with the help of absorption coefficient (α = 2.303A/t, here A is the absorbance and t stands for the sample thickness) and photon energy (hν) [40], The direct band gap of SWCNTs is determined from the graph of (αhν)2 versus hν by employing the relation given as [41]
where K is a constant related to the optical absorption edge width parameter. The energy band gap can be obtained by extrapolation of the straight line in the graph of (αhν)2 on the x-axis to (αhν)2→0, as shown in the inset of Fig. 6. The estimated band gap was found to be 2.70 eV and 2.51 eV for Cu2O and Cu2O/SWCNTs respectively. The band gap of Cu2O is found to be much larger than that of the theoretical bulk value 2.17 eV [42], may be due to the quantum confinement effects in the nanostructures [43]. However, the obtained band gap of Cu2O is quite closer to the reported value [44]. We have also characterized our samples via photoluminescence (PL) technique to reveal the efficiency of charge carrier trapping, migration and recombination
Fig. 4. Raman spectra of (a) SWCNTs, and (b) Cu2O/SWCNTs nanocomposite. Inset shows FTIR spectrum of the nanocomposite. 5
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Fig. 7. Room temperature PL spectra of the Cu2O and Cu2O/SWCNTs samples. Fig. 5. Magnetic hysteresis loops for Cu2O and Cu2O/SWCNTs samples.
SWCNTs) was also performed. The time dependent absorption spectra of MB solution under light illumination in the existence of Cu2O and Cu2O/CNTs have shown in Fig. 8(a,b). It is evident from the figure that the intensity of absorption peak at 663 nm gradually decreases by increasing of irradiation time and almost vanish in 150 min. This infers complete deterioration of MB dye. The degradation ability of Cu2O and Cu2O/SWCNTs nanocomposite after 150 min has been calculated and found to be 81% and 94% (Fig. 8(c)). It is worth noting here that the photocatalytic performance of semiconductors strongly depends on its morphology, microstructure, composition, surface properties etc. [46]. It has also been reported that the photocatalytic performance is influenced by the photo-generated charge carriers (electrons and holes). In this study, nanocomposite exhibits high electron-hole recombination, an appropriate surface defect mechanism is required for high transportation rate of charge carriers and the occurrence of redox reactions. Hence, the high specific surface area of Cu2O/CNTs photocatalyst facilitates dense adsorption of the dye molecules. The prepared
processes of the photogenerated electron–hole pairs. Room temperature PL spectra of the Cu2O nanoparticles and Cu2O/SWCNTs nanocomposite in the wavelength range of 500–750 nm with an excitation wavelength of 480 nm is shown in Fig. 7. It is observed that the intensity in the spectra reduces for the nanocomposite. This may be due to reduction in recombination of the photoinduced charge carriers that suggests enhancement in the photocatalytic activity of the Cu2O/SWCNTs nanocomposite. The peaks appear in the luminescence spectrum are attributed to the localized surface states associated to the recombination of photogenerated charge carriers (electrons/holes) [45].
3.6. Photocatalytic activity Photocatalytic performance of the Cu2O and Cu2O/SWCNTs was explored for the deterioration of MB dye under visible light irradiation. A blank experiment in the absence of the photocatalyst (Cu2O/
Fig. 6. UV–vis absorption spectra of Cu2O and Cu2O/SWCNTs. Inset shows the Tauc’s plots for the same. 6
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Fig. 8. (a,b) UV–vis absorbance spectra of MB dye at different irradiation times for Cu2O and Cu2O/SWCNTs as photocatalysts, (c) Photocatalytic degradation of MB dye as a function of irradiation time under visible light in the presence of Cu2O and Cu2O/SWCNTs as catalysts.
cycle of the photocatalytic reaction was maintained for 150 min. The reduction in photocatalytic degradation rate after six cycles was less than 3.5%, indicating that the prepared nanocomposite has a desirable stability and maintains a high photocatalytic activity. 4. Conclusions In summary, SWCNTs and Cu2O/SWCNTs nanocomposite were synthesized using plasma enhanced chemical vapour deposition (PECVD) and RF sputtering techniques respectively. XRD analysis confirms the Cu2O nanoparticles coated on the SWCNTs in pure crystalline phase with face-centered cubic structure. SEM and HRTEM results reveal decoration of Cu2O nanoparticles of size of ∼3.2 nm on the surface of the SWCNTs. The optical band gap is estimated as 2.70 eV and 2.51 eV for Cu2O and Cu2O/SWCNTs nanocomposite respectively. The photocatalytic performance of the samples was evaluated for the methylene blue dye under visible light irradiation and degradation efficiency was found to be 81% and 94% after irradiation within 150 min for the Cu2O nanoparticles and Cu2O/SWCNTs nanocomposite respectively. Therefore, the results on Cu2O-coated SWCNTs signify that the synthesized nanocomposite offers a promising candidate for the high efficiency sunlight photocatalysis. Room temperature ferromagnetism has been observed in both the samples. However, ferromagnetic
Fig. 9. Reusability assay of Cu2O/SWCNTs nanocomposite for photodegradation of MB dye under visible light irradiation.
nanocomposite photocatalyst offers relatively high surface area that contributes the improvement of the photocatalytic activities. The recyclability and stability of the catalyst during photodegradation reaction were also investigated through the degradation of MB under visible light irradiation and shown in Fig. 9. The reusability of the Cu2O/SWCNTs nanocomposite was tested for six times and each 7
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ordering slightly suppresses in the nanocomposite attributed to the strong coupling between Cu2O nanoparticles and SWCNTs.
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