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Synthesis and enhanced photocatalytic property of feather-like Cd-doped CuO nanostructures by hydrothermal method
Accepted Manuscript Title: Synthesis and enhanced photocatalytic property of feather-like Cd-doped CuO nanostructures by hydrothermal method Author: Yongqian Wang Tingting Jiang Dawei Meng Dagui Wang Meihua Yu PII: DOI: Reference:
S0169-4332(15)01689-X http://dx.doi.org/doi:10.1016/j.apsusc.2015.07.122 APSUSC 30849
To appear in:
APSUSC
Received date: Revised date: Accepted date:
6-5-2015 14-7-2015 17-7-2015
Please cite this article as: Y. Wang, T. Jiang, D. Meng, D. Wang, M. Yu, Synthesis and enhanced photocatalytic property of feather-like Cd-doped CuO nanostructures by hydrothermal method, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.07.122 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis and enhanced photocatalytic property of feather-like Cd-doped CuO nanostructures by hydrothermal method Yongqian Wanga*, Tingting Jianga, Dawei Menga, Dagui Wanga, Meihua Yub a
Faculty of Material Science and Chemisty,China University of Geoscience, Wuhan 430074, China
b
School of Materials Science and Engineering, Guangxi University, Nanning 530004, China
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Correspondence Author: Yongqian Wang E-mail: [email protected] Tel: +86 138 7137 9285
1. Introduction
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Abstract: Feather-like Cd-doped CuO nanostructures were fabricated by a one-step hydrothermal method. X-ray diffraction pattern (XRD) and field emission scanning electron microscopy (FESEM) demonstrated that Cd2+ entered the crystal lattice of CuO and substituted Cu2+ without destroying crystal structures to form feather-like CuO nanostructures. The optical property of Cd-doped CuO was investigated by using UV-vis spectrophotometer. A slight blue-shift of optical band gap was observed because of quantum confinement effect. The doped samples exhibited obviously higher absorbance in UV light region and better photocatalytic activity for the photodegradation of methyl blue than the pure CuO nanosheets. Keywords: Cd-doped CuO, feather-like, optical property, photocatalytic performance
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In recent yeas, doping process has been widely used to change the structure of CuO to realize novel or enhanced properties[1]. Many previous references have reported the improvement of characteristic properties in various materials by doping[2-5]. Among the multifarious nanomaterials of transition-metal oxides, cupric oxide (CuO) with a narrow bandgap of 1.2-1.9 eV has attracted much interest due to its unique properties. CuO and doped CuO have become important materials in technological applications for high TC superconductors[6], hydration detection[7], nonenzymatic glucose sensing[8], gas sensors[9], lithium-ion batteries[10], magnetic storage media[11] and so on. As reported, optical property of semiconductors can be considerably altered by adding appropriate amount of dopants, which change the concentration of mobile charge carrier by many orders of magnitude[1]. In addition, introduced impurity levels and generated lattice defects can have a significant influence on photocatalytic performance of materials. However, the chemical and physical properties of CuO strongly depends on its size, morphology, specific surface area, which can be controlled by preparation methods such as hydrothermal method[12], electrodeposition[13], ultrasonic spray pyrolysis[14], sol-gel method[15], pulsed laser deposition[16], direct oxidation method[17] and molecular-beam epitaxy[18]. In present works, feather-like Cd-doped CuO nanostructures are synthesized by hydrothermal method without any template or surfactants. Compared to the pure CuO nanosheets, its UV light absorption and photocatalytic activity have notable enhancement with a suitable doping amount. There is not much change in calculative optical band gap and slight blue shift can be observed. The research about Cd-doped CuO has not been reported in the literatures.
2. Experiment
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2.1 Experimental procedure
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The hydrothermal method was employed to synthesize Cd-doped CuO nanostructures. Firstly, 50 mL of 0.1 M sodium hydroxid (NaOH) was added slowly into the 40 mL of 0.4 M copper (II) acetate (C4H6CuO4•H2O) under the continuous magnetic stirring to form homogeneous solution. An appropriate amount of cadmium nitrate (Cd(NO3)2•4H2O) was introduced into the above solution. Secondly, the suspension was transferred to a Teflon lined stainless steel autoclave, which was then heated at 110 ℃ for 2 h in a hot-air oven with the heating rate of 5 ℃/min. Finally, by centrifugally separated repeatedly with deionized water and ethanol, black product can be obtained and later be dried in a vacuum oven at 90 ℃ for 12 h.
2.2 Characterization
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Powder X-ray diffraction (XRD) analysis of the samples was carried out on a diffractometer (D8 Advanced, Bruker AXS, Germany) with Cu Kα radiation (λ= 0.154178 nm) at a scanning rate of 0.02 °/s in the 2θ range from 5 ° to 70 °. The microstructure of the products was observed with a field emission scanning electron microscopy (FESEM, SU8010, Hitachi, Japan), equipped with energy dispersive spectroscopy (EDS). UV-Vis absorbance spectra were measured on a spectrophotometer (UV-2550, Shimadzu, Japan).
2.3 Photocatalytic test
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Methyl blue (MB) is commonly adopted as a representative organic pollutant to evaluate the photocatalytic performance of the samples. The dye solution was prepared by dissolving MB in de-ionized water with concentration of 12 mg/L. And the required amount of the photocatalyst (0.01 g) was added into it. Before irradiation, the suspensions were stirred in the dark for 30 min to establish an adsorption-desorption equilibrium. Then the above suspension was exposed to light irradiation by a Xe lamp with the wavelength range from 200 nm to 800 nm. The solution with fixed amount was taken during the experiments every 30 min and was analyzed by a UV-Vis spectrophotometer (UV-2550, Shimadzu, Japan).
3. Results and discussion 3.1 Structural studies
Fig. 3 shows the XRD patterns of Cd-doped CuO powder samples with different doping amount. The results show typical diffraction peaks at 32.51 °, 35.55 °, 38.76 °, 49.71 °, 58.24 °, 61.60 °, 66.23 ° and 68.03 °, which correspond to the crystal planes (marked in Fig. 3) of monoclinic CuO phase (JCPDS No.89-5899) respectively. The peaks at 18.84 ° and 29.65 ° correspond to (001) and (100) reflections of Cd(OH)2 (JCPDS No.13-0226). It was observed that the Bragg’s angles (2θ) of (002) and (111) major diffraction peaks of doped CuO gradually and slightly shift towards lower angle with the increase of dopant, indicating the increase in lattice parameter values. The increase of lattice constant is due to the ionic radius of Cd2+ (0.95 Å) is larger than that of Cu2+ (0.73 Å) and therefore the substitution of Cu2+ ions by Cd2+ induces a lattice expansion[19]. Only when the doping amount exceeds 0.8 mmol, there are obvious impurity phase of Cd(OH)2 is observed in the XRD patterns. The crystallite size of the samples are calculated using Scherrer’s formula[20] and shown in Fig.2. D=
kλ β cos θ
(1)
Where k is the shape factor (~0.89), λ is the wavelength of Cu Kα radiation, θ Bragg’s angle of the
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peaks and β is is the full width at half maximum (FWHM) of diffraction peak. The doping of Cd2+ promotes grain size decrease from 19.64 nm to 13.77 nm along the crystal plane of (002) and from 17.03 nm to 12.44 nm along the crystal plane of (111) respectively. The grain size decreases dramatically at first, yet does not have much change when doping amount is more than 0.8 mmol, which illustrates that a small amount of doping can have a great influence on the grain size and excess may generate impurity phase instead of entering into the intracell of CuO.
Fig.1 XRD patterns of Cd-doped CuO nanosheets with different doping amount. The right figure shows the
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magnified XRD patterns
Fig.2 Crystallite size of Cd-doped CuO nanosheets along the major crystal faces
3.2 Morphological studies
The surface morphology and composition of the samples are determined by field emission scanning electron microscopy (FESEM) equipped with energy dispersive spectroscopy (EDS) as shown in Fig.3. In general, Cd2+ doping does not have much influence on the micromorphology of CuO. For pure CuO, irregular and thin nanosheets can be observed in Fig.3a and the size is about 300 nm in length and width. Cd2+ doping could make the CuO nanosheets smaller and be conducive to the formation of long and narrow nanostructures whose morphology are similar to feather. And batt-like aggregation is easier to form. the detailed actual doping amount of Cd is given in Table 1. The EDS spectra of CuO presented in Fig.3a shows the presence of Cu and O elements alone in the sample. And the EDS spectra of doped CuO samples reveal that the powders consist of Cu, O and Cd, which confirms the substitution of cadmium in CuO. The peak
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corresponding to C is also obtained in the spectra because since carbon is sprayed on the samples for conductivity.
Fig.3 FESEM images and EDS results of undoped and Cd-doped CuO nanostructures (a) undoped, (b) 0.4 mmol, (c) 0.8 mmol, (d) 1.2 mmol, (e)1.6 mmol, (f) 2.0 mmol
Table 1
The doping amount of Cd2+ in CuO Sample
Experimental doping
amount of Cd (mmol)
Actual mass percent (wt%)
Actual atomic percent (at%)
O
Cd
Cu
O
Cd
Cu
A
0
20.05
0
79.95
49.89
0
50.11
B
0.4
28.82
2.93
68.25
62.08
0.90
37.02
C
0.8
33.49
4.82
61.69
67.37
1.38
31.25
D
1.2
29.91
7.94
62.15
64.06
2.42
33.52
E
1.6
33.73
8.83
57.44
68.21
2.54
29.25
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F
2.0
32.52
8.45
59.03
66.94
2.48
30.59
3.3 Optical studies
= B (hv − E g )
us
(αhv )1 / r
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Fig. 4 shows the UV-vis spectra of Cd-doped CuO nanostructures with different doping amount. Typical optical absorption spectra can be observed and light absorption exists the entire test range from 200 to 800 nm, showing the good ability of light absorption both in UV and visible region. All the synthesized samples exhibit strong characteristic absorption peak at around 250-300 nm in the UV region. Obviously, Cd2+ doping can significantly enhance absorbance especially in UV region, which is caused by the improvement of carrier mobility because of lattice distortion and impurity levels. The optical band gaps of the products are determined on the basis of absorbance measurements. The right of Fig.4 shows the dependence of the absorption coefficient (αhν)2 on the photon energy hν for the samples deposited with various Cd dopant. The optical band gap energy (Eg) can be calculated by the equation[21] as follows. (2)
πa B
)2
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E ( R ) = E g (∞ ) + E b (
d
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where Eg is the band gap energy, B is a constant, ʋ is the frequency of the incident radiation, h is Planck’s constant, α is the absorption coefficient, r is determined by the type of optical transition of a semiconductor. Better linearity for (αhν)2 vs. hν was observed for direct transitions. Eg can be obtained by extrapolating the linear part of the curves to the energy axis. The measured optical band gap Eg values are 1.50 eV, 1.52 eV, 1.53 eV, 1.54 eV, 1.54 eV and 1.57 eV respectively with the increasing doping level. According to the statement of Marotti, an inverse dependence between the absorption edge energy Eg and crystallite size D is found[22]. Quantum confinement effect leads to this typical dependence in very small nanoparticles. Eq.(3) shows the actual relationship between optical ban gap and crystallite size. R
(3)
Eg( ∞ ) is the band gap energy of bulk materials, Eb is the exciton binding energy and aB is the exciton Bohr radius which only appears in the range of 6.6-28.7 nm[23]. Both Eb and aB are a constant for CuO. The smaller crystallite size is, the higher of Eg becomes due to quantum size effect. So, the measured optical band gap Eg values of our samples is slightly blueshifted.
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ip t cr us an M d Ac ce pt e Fig.4 UV-vis absorption spectra (the left ) and plot of (αhʋ)2 vs hʋ ( the right) of Cd-doped CuO with different doping amount (a) undoped, (b) 0.4 mmol, (c) 0.8 mmol, (d) 1.2 mmol, (e)1.6 mmol, (f) 2.0 mmol
3.4 Photocatalytic studies The photocatalytic activity of Cd-doped CuO nanostructures was evaluated via photo degradation of MB under the irradiation of UV-vis light in the range of 200-800 nm. Fig.5 shows the photodegradation of MB dyes of all samples with different doping amount. The curves reveal
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the highest photodegradations respectively are 84.26 %, 89.50 %, 92.33 %, 90.89 %, 89.22% and 89.17 %. Significant enhancement of photocatalytic activity can be obtained by doping Cd2+ of 0.8 mmol (1.38 at%). This is because lattice expansion would bring in the increase of carrier mobility, which can effectively inhibit the recombination of photogenerated electrons and holes. Furthermore, there are a large number of oxygen vacancies in the CuO crystal lattices and these or other lattice defects are prone to the oxidation center in photocatalytic reaction. And the impurity levels generated by doping can make the utilization rate of photons improve. Smaller specific surface area of doped samples also results in the more chances of photocatalytic reaction.
4. Conclusion
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Fig.5 MB degradation curves over undoped and Cd-doped CuO nanostructures
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In summary, feather-like Cd-doped CuO nanostructures were successfully prepared by using hydrothermal method. X-ray diffraction patterns confirmed the formation of CuO monoclinic phase within Cd content up to 2.54at%, where Cd2+ replaced Cu2+ ions. The crystallite size varies in the range of 19.64-13.77 nm along (002) crystal planes and 17.03-12.44 nm along (111) crystal planes respectively. The optical band gap of CuO slightly increases from 1.50 eV to 1.57 eV with increasing Cd doping levels from 0 to 8.5 wt%, which is mainly attributed to the quantum confinement effect. And Cd2+ doping can lead to a remarkable improvement in photocatalytic activity (the photodegradation of MB dyes from 84.26% to 92.33%) compared to the undoped CuO nanostructures, which can be expected to find promising application in the environmental field.
5. Acknowledgments
This work is supported by Guangxi Experiment Centre of Science and Technology with open project: YXKT2014027. Another financial support is the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan, CUG120118). They are all gratefully appreciated.
References [1] P. Chand, A. Gaur, A. Kumar, U.K. Gaur. Structural and optical study of Li doped CuO thin films on Si (1 0 0) substrate deposited by pulsed laser deposition. Appl Surf Sci, 307(2014), pp. 280-286. [2] A. Soukiassian, A. Tagantsev, N. Setter. Anomalous dielectric peak in Mg and Li doped ZnO
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ceramics and thin films. Appl Phys Lett, 97(2010), pp. 192903-192913. [3] N. Sharma, A. Gaur, R.K. Kotnala. Signature of weak ferroelectricity and ferromagnetism in Mn doped CuO nanostructures. J Magn Magn Mater, 377(2015), pp. 183-189. [4] J. Yang, X.X. Wang, X.J. Yang, J.X. Li, X.H. Zhang, J.L. Zhao. Energy storage ability and anti-corrosion properties of Bi-doped TiO2 nanotube arrays. Electrochim Acta, 169(2015), pp. 227-232. [5] W.X. Jin, S.Y. Ma, Z.Z. Tie, J.J. Wei, J. Luo, X.H. Jiang, T.T. Wang, W.Q. Li, L. Cheng, Y.Z. Mao. One-step synthesis and highly gas-sensing properties of hierarchical Cu-doped SnO2 nanoflowers. Sensor Actuat B-Chem, 213(2015), pp. 171-180. [6] T. Jarlborg. Effects of spin-phonon interaction within the CuO plane of high-TC superconductors. Physica C, 454(2007), pp. 5-14. [7] B. Sahin, M. Alomari, T. Kaya. Hydration detection through use of artificial sweat in dopedand partially-doped nanostructured CuO films. Ceram Int, 41(2015), pp.8002-8007. [8] Y. Tian, Y. Liu, W.P. Wang, X. Zhang, W. Peng. CuO nanoparticles on sulfur-doped graphene for nonenzymatic glucose sensing. Electrochim Acta, 156(2015), pp. 244-251. [9] X. Gou, G. Wang, J. Yang, J. Park, D. Wexler, Chemical synthesis, characterization and gas sensing performance of copper oxide nanoribbons, J Mater Chem, 2008, 18, 965-969. [10] J.C. Park, J. Kim, H. Kwan, H. Song, Gram-scale synthesis of Cu2O nanocubes and subsequent oxidation to CuO hollow nanostructures for lithium-ion battery anode materials, Adv Mater, 2009, 21, 803-807. [11] R.V. Kumar, Y. Diamant, A. Gedanken, Sonochemical synthesis and characterization of nanometer-size transition metal oxides from metal acetates, Chem Mater, 2000, 12, 2301-2305. [12] T.Y. Jiang, Y.Q. Wang, D.W. Meng, X.L. Wu, J.X. Wang, J.Y. Chen. Controllable fabrication of CuO nanostructure by hydrothermal method and its properties. Appl Surf Sci, 311(2014), pp. 602-608. [13] Y.Q. Wang, T.T. Jiang, D.W. Meng, J. Yang, Y.C. Li, Q. Ma, J. Han, Fabrication of nanostructured CuO films by electrodeposition and their photocatalytic properties, Appl Surf Sci, 317(2014), pp. 414-421. [14] S. Kose, F. Atay, V. Bilgin, I. Akyuza, Some physical properties of copper oxide films: The effect of substrate temperature. Mater Chem Phys, 2008, 111, 351-358. [15] Y.K. Su, C.M. Shen, H.T. Yang, H.L. Li, H.J. Gao. Controlled synthesis of highly ordered CuO nanowire arrays by template-based sol-gel route, Trans Nonferrous Met Soc China, 2007, 17, 783-786. [16] A.P. Chen, H. Long, X.C. Li, Y.H. Li, G. Yang, P.X. Lu, Controlled growth and characteristics of single-phase Cu2O and CuO films by pulsed laser deposition, Vacuum, 2009, 83, 927-930. [17] Y.Q. Wang, T.T. Jiang, D.W. Meng, J.H Kong, H.X. Jia, M.H. Yu. Controllable fabrication of nanostructured copper compound on a Cu substrate by a one-step route. RSC ADV, 2015, pp. 16277-16283. [18] K.P. Muthe, J.C. Vyas, S.N. Narang, D.K. Aswal, S.K. Gupta, D. Bhattacharya, R. Pinto, G.P. Kothiyal, S.C. Sabharwal, A study of the CuO phase formation during thin film deposition by molecular beam epitaxy. Thin Solid Films, 1998, 324, 37-43. [19] A. Fouzri, M.A. Boukadhaba, A. Touré, N. Sakly, A. Bchetnia, V. Sallet. Structural, morphological and optical properties of Cd doped ZnO film grown on a- and r-plane sapphire substrate by MOCVD. Appl Surf Sci, 311(2014), pp. 648-658.
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[20] D. Sivalingam, J. B. Gopalakrishnan, J. Bosco, B. Rayappan. Structural, morphological, electrical and vapour sensing properties of Mn doped nanostructured ZnO thin films. Sensor actuat B-Chem, 166-167(2012), pp. 624-631. [21] J. Tauc, R. Grigorovici, A. Vancu, Optical Properties and electronic structure of amorphous germanium, physica status solidi (b), (15) 1966, pp. 627-637. [22] R.E. Marotti, P. Giorgi, G. Machado, E.A. Dalchiele. Crystallite size dependence of band gap energy for electrodeposited ZnO grown at different temperatures. Sol Energy Mater Sol Cells, 90(2006), pp. 2356-2361. [23] K. Borgohain, S. Mahamuni. Formation of single-phase CuO quantum particles, J Mater Res, 2002, 17, pp.1220-1223.
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S0169-4332(15)01689-X http://dx.doi.org/doi:10.1016/j.apsusc.2015.07.122 APSUSC 30849
To appear in:
APSUSC
Received date: Revised date: Accepted date:
6-5-2015 14-7-2015 17-7-2015
Please cite this article as: Y. Wang, T. Jiang, D. Meng, D. Wang, M. Yu, Synthesis and enhanced photocatalytic property of feather-like Cd-doped CuO nanostructures by hydrothermal method, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.07.122 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis and enhanced photocatalytic property of feather-like Cd-doped CuO nanostructures by hydrothermal method Yongqian Wanga*, Tingting Jianga, Dawei Menga, Dagui Wanga, Meihua Yub a
Faculty of Material Science and Chemisty,China University of Geoscience, Wuhan 430074, China
b
School of Materials Science and Engineering, Guangxi University, Nanning 530004, China
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Correspondence Author: Yongqian Wang E-mail: [email protected] Tel: +86 138 7137 9285
1. Introduction
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Abstract: Feather-like Cd-doped CuO nanostructures were fabricated by a one-step hydrothermal method. X-ray diffraction pattern (XRD) and field emission scanning electron microscopy (FESEM) demonstrated that Cd2+ entered the crystal lattice of CuO and substituted Cu2+ without destroying crystal structures to form feather-like CuO nanostructures. The optical property of Cd-doped CuO was investigated by using UV-vis spectrophotometer. A slight blue-shift of optical band gap was observed because of quantum confinement effect. The doped samples exhibited obviously higher absorbance in UV light region and better photocatalytic activity for the photodegradation of methyl blue than the pure CuO nanosheets. Keywords: Cd-doped CuO, feather-like, optical property, photocatalytic performance
Ac ce pt e
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In recent yeas, doping process has been widely used to change the structure of CuO to realize novel or enhanced properties[1]. Many previous references have reported the improvement of characteristic properties in various materials by doping[2-5]. Among the multifarious nanomaterials of transition-metal oxides, cupric oxide (CuO) with a narrow bandgap of 1.2-1.9 eV has attracted much interest due to its unique properties. CuO and doped CuO have become important materials in technological applications for high TC superconductors[6], hydration detection[7], nonenzymatic glucose sensing[8], gas sensors[9], lithium-ion batteries[10], magnetic storage media[11] and so on. As reported, optical property of semiconductors can be considerably altered by adding appropriate amount of dopants, which change the concentration of mobile charge carrier by many orders of magnitude[1]. In addition, introduced impurity levels and generated lattice defects can have a significant influence on photocatalytic performance of materials. However, the chemical and physical properties of CuO strongly depends on its size, morphology, specific surface area, which can be controlled by preparation methods such as hydrothermal method[12], electrodeposition[13], ultrasonic spray pyrolysis[14], sol-gel method[15], pulsed laser deposition[16], direct oxidation method[17] and molecular-beam epitaxy[18]. In present works, feather-like Cd-doped CuO nanostructures are synthesized by hydrothermal method without any template or surfactants. Compared to the pure CuO nanosheets, its UV light absorption and photocatalytic activity have notable enhancement with a suitable doping amount. There is not much change in calculative optical band gap and slight blue shift can be observed. The research about Cd-doped CuO has not been reported in the literatures.
2. Experiment
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2.1 Experimental procedure
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The hydrothermal method was employed to synthesize Cd-doped CuO nanostructures. Firstly, 50 mL of 0.1 M sodium hydroxid (NaOH) was added slowly into the 40 mL of 0.4 M copper (II) acetate (C4H6CuO4•H2O) under the continuous magnetic stirring to form homogeneous solution. An appropriate amount of cadmium nitrate (Cd(NO3)2•4H2O) was introduced into the above solution. Secondly, the suspension was transferred to a Teflon lined stainless steel autoclave, which was then heated at 110 ℃ for 2 h in a hot-air oven with the heating rate of 5 ℃/min. Finally, by centrifugally separated repeatedly with deionized water and ethanol, black product can be obtained and later be dried in a vacuum oven at 90 ℃ for 12 h.
2.2 Characterization
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Powder X-ray diffraction (XRD) analysis of the samples was carried out on a diffractometer (D8 Advanced, Bruker AXS, Germany) with Cu Kα radiation (λ= 0.154178 nm) at a scanning rate of 0.02 °/s in the 2θ range from 5 ° to 70 °. The microstructure of the products was observed with a field emission scanning electron microscopy (FESEM, SU8010, Hitachi, Japan), equipped with energy dispersive spectroscopy (EDS). UV-Vis absorbance spectra were measured on a spectrophotometer (UV-2550, Shimadzu, Japan).
2.3 Photocatalytic test
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Methyl blue (MB) is commonly adopted as a representative organic pollutant to evaluate the photocatalytic performance of the samples. The dye solution was prepared by dissolving MB in de-ionized water with concentration of 12 mg/L. And the required amount of the photocatalyst (0.01 g) was added into it. Before irradiation, the suspensions were stirred in the dark for 30 min to establish an adsorption-desorption equilibrium. Then the above suspension was exposed to light irradiation by a Xe lamp with the wavelength range from 200 nm to 800 nm. The solution with fixed amount was taken during the experiments every 30 min and was analyzed by a UV-Vis spectrophotometer (UV-2550, Shimadzu, Japan).
3. Results and discussion 3.1 Structural studies
Fig. 3 shows the XRD patterns of Cd-doped CuO powder samples with different doping amount. The results show typical diffraction peaks at 32.51 °, 35.55 °, 38.76 °, 49.71 °, 58.24 °, 61.60 °, 66.23 ° and 68.03 °, which correspond to the crystal planes (marked in Fig. 3) of monoclinic CuO phase (JCPDS No.89-5899) respectively. The peaks at 18.84 ° and 29.65 ° correspond to (001) and (100) reflections of Cd(OH)2 (JCPDS No.13-0226). It was observed that the Bragg’s angles (2θ) of (002) and (111) major diffraction peaks of doped CuO gradually and slightly shift towards lower angle with the increase of dopant, indicating the increase in lattice parameter values. The increase of lattice constant is due to the ionic radius of Cd2+ (0.95 Å) is larger than that of Cu2+ (0.73 Å) and therefore the substitution of Cu2+ ions by Cd2+ induces a lattice expansion[19]. Only when the doping amount exceeds 0.8 mmol, there are obvious impurity phase of Cd(OH)2 is observed in the XRD patterns. The crystallite size of the samples are calculated using Scherrer’s formula[20] and shown in Fig.2. D=
kλ β cos θ
(1)
Where k is the shape factor (~0.89), λ is the wavelength of Cu Kα radiation, θ Bragg’s angle of the
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peaks and β is is the full width at half maximum (FWHM) of diffraction peak. The doping of Cd2+ promotes grain size decrease from 19.64 nm to 13.77 nm along the crystal plane of (002) and from 17.03 nm to 12.44 nm along the crystal plane of (111) respectively. The grain size decreases dramatically at first, yet does not have much change when doping amount is more than 0.8 mmol, which illustrates that a small amount of doping can have a great influence on the grain size and excess may generate impurity phase instead of entering into the intracell of CuO.
Fig.1 XRD patterns of Cd-doped CuO nanosheets with different doping amount. The right figure shows the
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Fig.2 Crystallite size of Cd-doped CuO nanosheets along the major crystal faces
3.2 Morphological studies
The surface morphology and composition of the samples are determined by field emission scanning electron microscopy (FESEM) equipped with energy dispersive spectroscopy (EDS) as shown in Fig.3. In general, Cd2+ doping does not have much influence on the micromorphology of CuO. For pure CuO, irregular and thin nanosheets can be observed in Fig.3a and the size is about 300 nm in length and width. Cd2+ doping could make the CuO nanosheets smaller and be conducive to the formation of long and narrow nanostructures whose morphology are similar to feather. And batt-like aggregation is easier to form. the detailed actual doping amount of Cd is given in Table 1. The EDS spectra of CuO presented in Fig.3a shows the presence of Cu and O elements alone in the sample. And the EDS spectra of doped CuO samples reveal that the powders consist of Cu, O and Cd, which confirms the substitution of cadmium in CuO. The peak
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corresponding to C is also obtained in the spectra because since carbon is sprayed on the samples for conductivity.
Fig.3 FESEM images and EDS results of undoped and Cd-doped CuO nanostructures (a) undoped, (b) 0.4 mmol, (c) 0.8 mmol, (d) 1.2 mmol, (e)1.6 mmol, (f) 2.0 mmol
Table 1
The doping amount of Cd2+ in CuO Sample
Experimental doping
amount of Cd (mmol)
Actual mass percent (wt%)
Actual atomic percent (at%)
O
Cd
Cu
O
Cd
Cu
A
0
20.05
0
79.95
49.89
0
50.11
B
0.4
28.82
2.93
68.25
62.08
0.90
37.02
C
0.8
33.49
4.82
61.69
67.37
1.38
31.25
D
1.2
29.91
7.94
62.15
64.06
2.42
33.52
E
1.6
33.73
8.83
57.44
68.21
2.54
29.25
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F
2.0
32.52
8.45
59.03
66.94
2.48
30.59
3.3 Optical studies
= B (hv − E g )
us
(αhv )1 / r
cr
ip t
Fig. 4 shows the UV-vis spectra of Cd-doped CuO nanostructures with different doping amount. Typical optical absorption spectra can be observed and light absorption exists the entire test range from 200 to 800 nm, showing the good ability of light absorption both in UV and visible region. All the synthesized samples exhibit strong characteristic absorption peak at around 250-300 nm in the UV region. Obviously, Cd2+ doping can significantly enhance absorbance especially in UV region, which is caused by the improvement of carrier mobility because of lattice distortion and impurity levels. The optical band gaps of the products are determined on the basis of absorbance measurements. The right of Fig.4 shows the dependence of the absorption coefficient (αhν)2 on the photon energy hν for the samples deposited with various Cd dopant. The optical band gap energy (Eg) can be calculated by the equation[21] as follows. (2)
πa B
)2
Ac ce pt e
E ( R ) = E g (∞ ) + E b (
d
M
an
where Eg is the band gap energy, B is a constant, ʋ is the frequency of the incident radiation, h is Planck’s constant, α is the absorption coefficient, r is determined by the type of optical transition of a semiconductor. Better linearity for (αhν)2 vs. hν was observed for direct transitions. Eg can be obtained by extrapolating the linear part of the curves to the energy axis. The measured optical band gap Eg values are 1.50 eV, 1.52 eV, 1.53 eV, 1.54 eV, 1.54 eV and 1.57 eV respectively with the increasing doping level. According to the statement of Marotti, an inverse dependence between the absorption edge energy Eg and crystallite size D is found[22]. Quantum confinement effect leads to this typical dependence in very small nanoparticles. Eq.(3) shows the actual relationship between optical ban gap and crystallite size. R
(3)
Eg( ∞ ) is the band gap energy of bulk materials, Eb is the exciton binding energy and aB is the exciton Bohr radius which only appears in the range of 6.6-28.7 nm[23]. Both Eb and aB are a constant for CuO. The smaller crystallite size is, the higher of Eg becomes due to quantum size effect. So, the measured optical band gap Eg values of our samples is slightly blueshifted.
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ip t cr us an M d Ac ce pt e Fig.4 UV-vis absorption spectra (the left ) and plot of (αhʋ)2 vs hʋ ( the right) of Cd-doped CuO with different doping amount (a) undoped, (b) 0.4 mmol, (c) 0.8 mmol, (d) 1.2 mmol, (e)1.6 mmol, (f) 2.0 mmol
3.4 Photocatalytic studies The photocatalytic activity of Cd-doped CuO nanostructures was evaluated via photo degradation of MB under the irradiation of UV-vis light in the range of 200-800 nm. Fig.5 shows the photodegradation of MB dyes of all samples with different doping amount. The curves reveal
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an
us
cr
ip t
the highest photodegradations respectively are 84.26 %, 89.50 %, 92.33 %, 90.89 %, 89.22% and 89.17 %. Significant enhancement of photocatalytic activity can be obtained by doping Cd2+ of 0.8 mmol (1.38 at%). This is because lattice expansion would bring in the increase of carrier mobility, which can effectively inhibit the recombination of photogenerated electrons and holes. Furthermore, there are a large number of oxygen vacancies in the CuO crystal lattices and these or other lattice defects are prone to the oxidation center in photocatalytic reaction. And the impurity levels generated by doping can make the utilization rate of photons improve. Smaller specific surface area of doped samples also results in the more chances of photocatalytic reaction.
4. Conclusion
M
Fig.5 MB degradation curves over undoped and Cd-doped CuO nanostructures
Ac ce pt e
d
In summary, feather-like Cd-doped CuO nanostructures were successfully prepared by using hydrothermal method. X-ray diffraction patterns confirmed the formation of CuO monoclinic phase within Cd content up to 2.54at%, where Cd2+ replaced Cu2+ ions. The crystallite size varies in the range of 19.64-13.77 nm along (002) crystal planes and 17.03-12.44 nm along (111) crystal planes respectively. The optical band gap of CuO slightly increases from 1.50 eV to 1.57 eV with increasing Cd doping levels from 0 to 8.5 wt%, which is mainly attributed to the quantum confinement effect. And Cd2+ doping can lead to a remarkable improvement in photocatalytic activity (the photodegradation of MB dyes from 84.26% to 92.33%) compared to the undoped CuO nanostructures, which can be expected to find promising application in the environmental field.
5. Acknowledgments
This work is supported by Guangxi Experiment Centre of Science and Technology with open project: YXKT2014027. Another financial support is the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan, CUG120118). They are all gratefully appreciated.
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