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Facile synthesis of 2-dimensional CuO nanoleaves and their degradation behavior for Eosin Y
Author’s Accepted Manuscript Facile synthesis of 2-dimensional CuO nanoleaves and their degradation behavior for eosin Y Archita Bhattacharjee, M. Ahmaruzzaman
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S0167-577X(15)30420-1 http://dx.doi.org/10.1016/j.matlet.2015.08.064 MLBLUE19425
To appear in: Materials Letters Received date: 12 June 2015 Revised date: 5 August 2015 Accepted date: 12 August 2015 Cite this article as: Archita Bhattacharjee and M. Ahmaruzzaman, Facile synthesis of 2-dimensional CuO nanoleaves and their degradation behavior for eosin Y, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2015.08.064 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 galley proof before it is published in its final citable 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.
acile synthesis of 2-dimensional CuO nanoleaves and their degradation behavior for Eosin Y Archita Bhattacharjee, M. Ahmaruzzaman* Department of Chemistry National Institute of Technology, Silchar-788010, Assam, India ABSTRACT For the first time, 2D CuO nanoleaves (NLs) with average dimensions of ~350-450nm in length and ~60-90nm in width was successfully synthesized using NaOH and L-arginine. The as-obtained 2D CuO NLs were composed of CuO primary single crystal nanoparticles. The CuO NLs were characterized by XRD, TEM, SAED, FT-IR and UV analyses. A clear blue shift was observed in the band gap energy (~2.15eV) of synthesized CuO NLs. The prepared 2D CuO NLs act as an efficient and remarkable photocatalyst for the degradation of eosin Y by solar irradiation for the first time. The complete degradation takes place within 45 min. Keywords: CuO-nanoleaves, nanoparticles, nanocrystalline materials, Eosin Y, X-ray techniques, Spectroscopy. * Corresponding author: [email protected] (M. Ahmaruzzaman)
1. INTRODUCTION In recent years, semiconductor nanoparticles composed of nanorods, nanoleaves, nanowires, etc. have attracted a lot of interest. This is because of their potential applications in photocatalysis, gas sensors, electrodes and batteries. Among them, copper oxide (CuO), a p-type semiconductor, has been explored widely in photocatalysis, solar cells, sensors, batteries, field-emmiters, etc. [13]. This is because of the natural availability of starting material, low cost, stability, non-toxicity and high reactivity of CuO. The sizes and morphology of nanoparticles have a great impact on their physical and chemical properties. Therefore, many efforts were devoted for the fabrication of CuO nanostructures with different sizes and morphology to enhance their existing applications. Now-a-days, different CuO nanostructures, such as nonoribbons, nanorings, nanorods, nanobelts, nanowires, etc. have been synthesized [4]. Because of their potential applications, numerous methods were used to synthesize CuO nanostructures [1, 4-7]. Microwave methods are a promising route for the production of metal-oxide nanostructures. This method provides a more convenient, quite faster, energy saving and environmentally benign route for the synthesis of nanostructured metal oxides. In this paper, we developed green synthesis of 2-dimensional CuO nanoleaves by microwave heating method using L-arginine. It was evident from the literature that size, morphology and properties of nanoparticles were modified because of the presence of amino acids. Hence, we design the synthesis using aminoacid, L-arginine so that nanoparticles with different morphology and
1
improved properties can be obtained. To the best knowledge of the authors, microwave synthesis of CuO NLs using L-arginine has not been reported in the literature. Nanostructured metal oxide was utilized as a photocatalyst for the degradation of organic compounds from aqueous phase. Herein, for the first time, we evaluated the photocatalytic activity of the synthesized 2D CuO NLs in the degradation of eosin Y by solar irradiation. 2. EXPERIMENTAL Materials All the reagents, CuSO4.5H2O, sodium hydroxide, L-arginine, and eosin Y were procured from Merck and of analytical grade (AR). The reaction was carried out in a domestic microwave oven. Synthesis of CuO nanoleaves (NLs) CuO NLs were synthesized using aqueous solution of CuSO 4.5H2O, L-arginine and NaOH. In the particular experiment, 50ml of 0.01M aqueous solution of L-arginine was added slowly to 50ml, 0.01M solution of CuSO4.5H2O under vigorous stirring. To the above mixture, 50ml of 0.05M NaOH solution was added under constant stirring. The reaction mixture was then kept in a microwave oven and irradiated with thirty 10s shots. This resulted in the formation of black precipitate. The precipitate obtained was centrifuged and washed several times with distilled water. The final product was dried at 100 oC and collected for characterization. Characterization CuO NLs were characterized by powder X-ray diffraction (XRD) method using Phillips X’Pert PRO diffractometer with CuKα radiation of wavelength 1.5418Å. The size, morphology and diffracted ring pattern of CuO NLs were determined by JEM-2100 Transmission Electron Microscope. Infrared spectrum was recorded by Bruker Hyperion 3000 FTIR spectrometer. Absorption spectra were recorded on Cary 100 BIO UV-visible spectrophotometer. Photocatalytic Activity The photocatalytic activity of CuO NLs was evaluated by the degradation of eosin Y (EY) under direct sunlight. To evaluate the photocatalytic activity, 10 mg of CuO photocatalyst was dispersed in 200ml of 10 -4M aqueous solution of EY. The solution was kept in the dark for 30m to obtain the adsorption/desorption equilibrium. Afterwards, the reaction mixture was exposed to sunlight irradiation. The experiments were carried out on a sunny day at Silchar city between 10a.m–3p.m (outside temperature 350- 400C). During the degradation process, 5ml of aqueous suspension was taken at a regular interval of time and centrifuged. The UV-visible spectra of supernatants were recorded to determine the degree of degradation of EY.
2
3. RESULTS AND DISCUSSION FTIR studies FTIR spectrum is recorded to perceive the formation of CuO NLs (Fig. 1a). The peaks at 418, 498 and 610cm-1 correspond to characteristic stretching vibration of Cu-O bond in the monoclinic CuO [3, 6]. Hence, the presence of these bands confirmed the formation of CuO. FTIR spectrum was also recorded to perceive the presence of capping agent. The band around 1654 cm-1 indicates the presence of –COO group of L-arginine adsorbed on the surface of CuO NLs. The peaks around 2923 cm-1 and 2853 cm-1 are due to C-H asymmetric and symmetric stretch which further confirms that L-arginine acts as a capping agent in the synthesis of CuO NLs [8, 9]. XRD studies The crystal structure, purity and crystalline nature of CuO nanoparticles were investigated by XRD pattern (Fig. 1b). The XRD pattern clearly displays the monoclinic phase of CuO. The diffraction pattern was in good agreement with the JCPDS card of CuO (JCPDS 05-0661) [1]. However, no peaks for precursor molecules were detected in the XRD pattern. This indicates the purity and complete conversion of precursor molecules into CuO nanoparticles. The XRD pattern also depicts the highly crystalline nature of CuO NLs. UV-visible analysis The optical properties of CuO NLs were investigated by recording the absorption spectrum (Fig. 1c). The UV-visible spectra of CuO NLs showed a broad absorption band around 360 nm because of surface plasmon absorption of metal oxide [10, 3]. The optical band gap of CuO NLs can be obtained by using the equation: α(ν) hν = K (hν-Eg)n, where K is a constant, Eg is the band gap energy and exponent ‘n’ is the type of transition whose value is ½ due to allowed direct transition. The Fig. 1(c) depicts the plot of (αhν)2 versus (hν) for CuO NLs. The band gap energy is obtained by the extrapolation of the linear portion of the curve to zero absorption co-efficient and is found to be 2.15eV. A significant blue shift was observed in the band gap energy of synthesized CuO NLs from bulk CuO (1.2eV). This is because of enhancement of quantum confinement effect resulting from the decrease in the size of nanoparticles. TEM and SAED analysis TEM and HRTEM images were investigated to study the size distribution and morphology of the prepared CuO nanoparticles. Fig 2 (a) indicated the formation of 2-dimensional leaf-like morphology of the as-prepared CuO nanoparticles. It also showed that the CuO nanoparticles possess dimensions of ~350-450nm in length and ~60-90 nm in width. From the HRTEM image (Fig. 2b), the lattice spacing obtained is 0.182 nm and corresponds to (
) lattice plane of standard CuO nanoparticles. The monoclinic
3
phase of synthesized CuO (JCPDS 05-0661) was evidenced from the SAED pattern of CuO NLs (Fig. 2c). These results are also supported by the XRD pattern of CuO NLs [1]. In addition, the SAED pattern indicated that the synthesized CuO nanoleaves are similar to some extent with that of a single crystal. The near perfect alignment of primary crystals within each particle is also reflected from the spots observed in the SAED pattern. Role of aminoacid: In the synthesis of CuO NLs, arginine acts as a complexing as well as capping agent. Initially, L-arginine forms complex with Cu2+ ions. On treatment with NaOH, the complex breaks down to form Cu(OH)2 which on further heat treatment (100oC) decomposes to form CuO NLs. After the decomposition of Cu 2+-arginine complex, some molecules of L-arginine gets adsorbed on the surface of CuO NLs and thereby acts as a good capping agent [11]. The aminoacid acid L-arginine with guanidino group serves as capping agent in this reaction [12, 8-9]. Hence, the aminoacid, L-arginine acts as a good complexing and capping agent in the synthesis of CuO NLs.
[Cu2+-(L-Arginine)]
CuSO4 + L-Arginine
Formationof deep blue coloured complex
NaOH Immediate formation of Cu(OH)2 nuclei Aggregation of nuclei
OH Cu
-
Formation of individual leaf-like structure on heating at 100oC
2+
Formation of 2D CuO nanoleaves
Photocatalytic activity: The photocatalytic activity of the synthesized CuO NLs was evaluated by monitoring the changes in the optical absorption spectra of EY solution during its photodegradation process. During the course of degradation of EY (Fig. 3a), the characteristic absorption peak of EY observed at 517 nm decreases gradually and the color of the solution also fades away with the increase in irradiation time. This indicates strong oxidation of EY in presence of CuO NLs. The decomposition of EY takes place because of the formation of superoxides and hydroxyl radicals on the interface CuO NLs. The kinetic of the reaction was studied from the plot of ln At versus irradiation time. Fig. 3(b) represents the plot of ln A t versus irradiation time for the degradation of EY. This gives a linear relationship and implies that the photodegradation reaction followed pseudo first order kinetics. The slope of the
4
line represents the degradation rate constant (k) and was found to be 0.089 min-1 [13]. Fig. 3(c) graphically represents the percentage efficiency of photodegradation of EY with time. It is observed that 99.1% of EY was degraded within 45 min using CuO NLs. Hence, CuO NLs acts as an efficient photocatalyst for the degradation of EY. The higher rate of photodegradation of EY was due to the following facts: (1) Due to the special leaf-like morphology exhibited by the synthesized CuO nanoparticles, the exposed surface area will be higher than that of spherical CuO nanoparticles. CuO photocatalyst having large surface area is favourable for absorption of large amount of photons which generate photoexcited electron–hole pairs. The increase in band gap also facilitates absorption of photons in the visible region and hinder exitonic recombination to favour photocatalytic activity under direct sunlight [14]; (2) Another reason for higher degradation rate was the formation of aminoacid (arginine) capped CuO nanoparticles. Due to the presence of arginine on the surface of CuO nanoleaves, the electron-hole recombination will be hindered. Mechanism of photodegradation of Eosin Y under direct sunlight using CuO NLs as photocatalyst: The illumination of the catalyst surface (CuO NLs) with light energy (higher than its band-gap energy) leads to the formation of holes (h+) in the valence band and an electron (e-) in the conduction band of CuO nanoparticles. The photocatalytic activity of metal oxide depends strongly on the ability to inhibit electron-hole recombination which directly implies the availability of electrons and holes to react with O2 and H2O to produce highly reactive oxygen species. Due to the presence of arginine on the surface of CuO nanoleaves, the electron-hole recombination will be hindered and this will lead to the existence of larger number of photogenerated holes and electrons. The photogenerated holes oxidize the pollutant (Eosin Y) directly or react with OH− or H2O thereby oxidizing them into OH • radicals. While, the photogenerated electrons can reduce the dye or react with electron acceptors such as O2 adsorbed on the CuO surface or dissolved in water, reducing it to superoxide radical anion O 2− •. Less extent of electron-hole recombination makes the degradation process faster. This leads to the increase in the rate of degradation of eosin Y. [14, 15, 13]. The plausible mechanism for the photocatalytic degradation of EY dye can be shown as: CuO + hν → e- + h+ H2O + h+ → OH- + H+ OH- + h+→ .OH e- + O2 → .O2.
O2- + H+→ . OOH EY + hν → EY*
5
EY* + CuO → EY + CuO (e-) CuO (e-) + O2 → CuO + O2CuO (e-) + .O2- + H+ → CuO + H2O2 CuO (e-) + H2O2 → CuO + .OH + OHh+ + EY → degradation products EY* + O2 or .OH or .O2- → degradation products The schematic representation for the photodegradation of EY dye using CuO NLs is depicted in Scheme 1:
Conduction Band e-
Sunlight
Eg CuO NLs
+
h Valence Band
+ Organic dye 1.2
0m 15 m 30 m 45 m
Absorbance (a.u.)
1.0
0.8
0.6
0.4
0.2
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0.0 400
500
600
700
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Scheme 1. Schematic representation of photodegradation of Eosin Y dye using CuO NLs.
4. CONCLUSION This article illustrates a facile, cost-effective and green synthesis of CuO NLs using L-arginine by microwave heating method. The aminoacid L-arginine acts as a good complexing as well as a good capping agent. The TEM image showed the formation of 2D CuO nanoleaves with an average dimension of ~350-450nm in length and ~60-90nm in width. The XRD and SAED pattern revealed the single phase monoclinic crystal structure of CuO NLs. A significant blue shift was observed in the band gap energy of synthesized CuO NLs (2.15eV) from bulk CuO (1.2eV). The investigation showed that photocatalytic degradation of eosin Y takes place within 45 min by solar radiation. 6
References 1.
Chen H, Jhao G, Liu Y. Mater Lett 2013; 93:60-3.
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Zhang J, Guo Y, Fang H, Jia W, Li H, Yang L, Wang K. New J Chem 2015; DOI: 10.1039/C5NJ00674K.
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Lin TY, Chen DH. RSC Adv 2014;4:29357-64.
10. Das D, Nath BC, Phukon P, Dolui SK. Colloids Surf. B 2013; 101:430-3. 11. Srestha KM, Sorensen CM, Klabunde KJ. J Phys Chem. C 2010; 114:14368-76. 12. Lai Y, Yin W, Liu J, Xi R, Zhan J. Nanoscale Res Lett 2010; 5:302–307. 13. Bhattacharjee A, Ahmaruzzaman M. J Colloid Interface Sci 2015; 448:130-9. 14. Sharma A, Dutta RK. RSC Adv 2015; 5:43815-23. 15. Meshram SP, Adhyapak PV, Mulik UP, Amalnerkar DP. Chem Eng J 2012; 204-206:158-168.
Figures:
(a)
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Figure 1. (a) FT-IR spectrum, (b) XRD pattern, (c) Absorption spectrum of CuO NLs (upper right inset represents the corresponding plot of (αhν)2 versus photon energy (hν)
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Figure 2. (a) TEM microphotograph, (b) HRTEM image, (c) SAED pattern of CuO NLs.
1.2
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Fig. 3(a) Photodegradation of EY dye by solar irradiation using CuO NLs as photocatalyst, (b) Plot of ln A t versus irradiation time, (c) Percentage efficiency of photodegradation of EY dye with time.
HIGHLIGHTS 1. A facile, green method was developed for the synthesis of 2D CuO nanoleaves (NLs). 2. For the first time microwave heating method was designed using aminoacid L-arginine. 3. Formation of single crystalline CuO NL having ~350nm length and ~60nm width. 4. Optical band gap of CuO NLs (2.15eV) showed a clear blue shift from bulk CuO. 5. For the first time, Eosin Y was degraded using CuO NLs by solar irradiation.
10
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PII: DOI: Reference:
S0167-577X(15)30420-1 http://dx.doi.org/10.1016/j.matlet.2015.08.064 MLBLUE19425
To appear in: Materials Letters Received date: 12 June 2015 Revised date: 5 August 2015 Accepted date: 12 August 2015 Cite this article as: Archita Bhattacharjee and M. Ahmaruzzaman, Facile synthesis of 2-dimensional CuO nanoleaves and their degradation behavior for eosin Y, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2015.08.064 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 galley proof before it is published in its final citable 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.
acile synthesis of 2-dimensional CuO nanoleaves and their degradation behavior for Eosin Y Archita Bhattacharjee, M. Ahmaruzzaman* Department of Chemistry National Institute of Technology, Silchar-788010, Assam, India ABSTRACT For the first time, 2D CuO nanoleaves (NLs) with average dimensions of ~350-450nm in length and ~60-90nm in width was successfully synthesized using NaOH and L-arginine. The as-obtained 2D CuO NLs were composed of CuO primary single crystal nanoparticles. The CuO NLs were characterized by XRD, TEM, SAED, FT-IR and UV analyses. A clear blue shift was observed in the band gap energy (~2.15eV) of synthesized CuO NLs. The prepared 2D CuO NLs act as an efficient and remarkable photocatalyst for the degradation of eosin Y by solar irradiation for the first time. The complete degradation takes place within 45 min. Keywords: CuO-nanoleaves, nanoparticles, nanocrystalline materials, Eosin Y, X-ray techniques, Spectroscopy. * Corresponding author: [email protected] (M. Ahmaruzzaman)
1. INTRODUCTION In recent years, semiconductor nanoparticles composed of nanorods, nanoleaves, nanowires, etc. have attracted a lot of interest. This is because of their potential applications in photocatalysis, gas sensors, electrodes and batteries. Among them, copper oxide (CuO), a p-type semiconductor, has been explored widely in photocatalysis, solar cells, sensors, batteries, field-emmiters, etc. [13]. This is because of the natural availability of starting material, low cost, stability, non-toxicity and high reactivity of CuO. The sizes and morphology of nanoparticles have a great impact on their physical and chemical properties. Therefore, many efforts were devoted for the fabrication of CuO nanostructures with different sizes and morphology to enhance their existing applications. Now-a-days, different CuO nanostructures, such as nonoribbons, nanorings, nanorods, nanobelts, nanowires, etc. have been synthesized [4]. Because of their potential applications, numerous methods were used to synthesize CuO nanostructures [1, 4-7]. Microwave methods are a promising route for the production of metal-oxide nanostructures. This method provides a more convenient, quite faster, energy saving and environmentally benign route for the synthesis of nanostructured metal oxides. In this paper, we developed green synthesis of 2-dimensional CuO nanoleaves by microwave heating method using L-arginine. It was evident from the literature that size, morphology and properties of nanoparticles were modified because of the presence of amino acids. Hence, we design the synthesis using aminoacid, L-arginine so that nanoparticles with different morphology and
1
improved properties can be obtained. To the best knowledge of the authors, microwave synthesis of CuO NLs using L-arginine has not been reported in the literature. Nanostructured metal oxide was utilized as a photocatalyst for the degradation of organic compounds from aqueous phase. Herein, for the first time, we evaluated the photocatalytic activity of the synthesized 2D CuO NLs in the degradation of eosin Y by solar irradiation. 2. EXPERIMENTAL Materials All the reagents, CuSO4.5H2O, sodium hydroxide, L-arginine, and eosin Y were procured from Merck and of analytical grade (AR). The reaction was carried out in a domestic microwave oven. Synthesis of CuO nanoleaves (NLs) CuO NLs were synthesized using aqueous solution of CuSO 4.5H2O, L-arginine and NaOH. In the particular experiment, 50ml of 0.01M aqueous solution of L-arginine was added slowly to 50ml, 0.01M solution of CuSO4.5H2O under vigorous stirring. To the above mixture, 50ml of 0.05M NaOH solution was added under constant stirring. The reaction mixture was then kept in a microwave oven and irradiated with thirty 10s shots. This resulted in the formation of black precipitate. The precipitate obtained was centrifuged and washed several times with distilled water. The final product was dried at 100 oC and collected for characterization. Characterization CuO NLs were characterized by powder X-ray diffraction (XRD) method using Phillips X’Pert PRO diffractometer with CuKα radiation of wavelength 1.5418Å. The size, morphology and diffracted ring pattern of CuO NLs were determined by JEM-2100 Transmission Electron Microscope. Infrared spectrum was recorded by Bruker Hyperion 3000 FTIR spectrometer. Absorption spectra were recorded on Cary 100 BIO UV-visible spectrophotometer. Photocatalytic Activity The photocatalytic activity of CuO NLs was evaluated by the degradation of eosin Y (EY) under direct sunlight. To evaluate the photocatalytic activity, 10 mg of CuO photocatalyst was dispersed in 200ml of 10 -4M aqueous solution of EY. The solution was kept in the dark for 30m to obtain the adsorption/desorption equilibrium. Afterwards, the reaction mixture was exposed to sunlight irradiation. The experiments were carried out on a sunny day at Silchar city between 10a.m–3p.m (outside temperature 350- 400C). During the degradation process, 5ml of aqueous suspension was taken at a regular interval of time and centrifuged. The UV-visible spectra of supernatants were recorded to determine the degree of degradation of EY.
2
3. RESULTS AND DISCUSSION FTIR studies FTIR spectrum is recorded to perceive the formation of CuO NLs (Fig. 1a). The peaks at 418, 498 and 610cm-1 correspond to characteristic stretching vibration of Cu-O bond in the monoclinic CuO [3, 6]. Hence, the presence of these bands confirmed the formation of CuO. FTIR spectrum was also recorded to perceive the presence of capping agent. The band around 1654 cm-1 indicates the presence of –COO group of L-arginine adsorbed on the surface of CuO NLs. The peaks around 2923 cm-1 and 2853 cm-1 are due to C-H asymmetric and symmetric stretch which further confirms that L-arginine acts as a capping agent in the synthesis of CuO NLs [8, 9]. XRD studies The crystal structure, purity and crystalline nature of CuO nanoparticles were investigated by XRD pattern (Fig. 1b). The XRD pattern clearly displays the monoclinic phase of CuO. The diffraction pattern was in good agreement with the JCPDS card of CuO (JCPDS 05-0661) [1]. However, no peaks for precursor molecules were detected in the XRD pattern. This indicates the purity and complete conversion of precursor molecules into CuO nanoparticles. The XRD pattern also depicts the highly crystalline nature of CuO NLs. UV-visible analysis The optical properties of CuO NLs were investigated by recording the absorption spectrum (Fig. 1c). The UV-visible spectra of CuO NLs showed a broad absorption band around 360 nm because of surface plasmon absorption of metal oxide [10, 3]. The optical band gap of CuO NLs can be obtained by using the equation: α(ν) hν = K (hν-Eg)n, where K is a constant, Eg is the band gap energy and exponent ‘n’ is the type of transition whose value is ½ due to allowed direct transition. The Fig. 1(c) depicts the plot of (αhν)2 versus (hν) for CuO NLs. The band gap energy is obtained by the extrapolation of the linear portion of the curve to zero absorption co-efficient and is found to be 2.15eV. A significant blue shift was observed in the band gap energy of synthesized CuO NLs from bulk CuO (1.2eV). This is because of enhancement of quantum confinement effect resulting from the decrease in the size of nanoparticles. TEM and SAED analysis TEM and HRTEM images were investigated to study the size distribution and morphology of the prepared CuO nanoparticles. Fig 2 (a) indicated the formation of 2-dimensional leaf-like morphology of the as-prepared CuO nanoparticles. It also showed that the CuO nanoparticles possess dimensions of ~350-450nm in length and ~60-90 nm in width. From the HRTEM image (Fig. 2b), the lattice spacing obtained is 0.182 nm and corresponds to (
) lattice plane of standard CuO nanoparticles. The monoclinic
3
phase of synthesized CuO (JCPDS 05-0661) was evidenced from the SAED pattern of CuO NLs (Fig. 2c). These results are also supported by the XRD pattern of CuO NLs [1]. In addition, the SAED pattern indicated that the synthesized CuO nanoleaves are similar to some extent with that of a single crystal. The near perfect alignment of primary crystals within each particle is also reflected from the spots observed in the SAED pattern. Role of aminoacid: In the synthesis of CuO NLs, arginine acts as a complexing as well as capping agent. Initially, L-arginine forms complex with Cu2+ ions. On treatment with NaOH, the complex breaks down to form Cu(OH)2 which on further heat treatment (100oC) decomposes to form CuO NLs. After the decomposition of Cu 2+-arginine complex, some molecules of L-arginine gets adsorbed on the surface of CuO NLs and thereby acts as a good capping agent [11]. The aminoacid acid L-arginine with guanidino group serves as capping agent in this reaction [12, 8-9]. Hence, the aminoacid, L-arginine acts as a good complexing and capping agent in the synthesis of CuO NLs.
[Cu2+-(L-Arginine)]
CuSO4 + L-Arginine
Formationof deep blue coloured complex
NaOH Immediate formation of Cu(OH)2 nuclei Aggregation of nuclei
OH Cu
-
Formation of individual leaf-like structure on heating at 100oC
2+
Formation of 2D CuO nanoleaves
Photocatalytic activity: The photocatalytic activity of the synthesized CuO NLs was evaluated by monitoring the changes in the optical absorption spectra of EY solution during its photodegradation process. During the course of degradation of EY (Fig. 3a), the characteristic absorption peak of EY observed at 517 nm decreases gradually and the color of the solution also fades away with the increase in irradiation time. This indicates strong oxidation of EY in presence of CuO NLs. The decomposition of EY takes place because of the formation of superoxides and hydroxyl radicals on the interface CuO NLs. The kinetic of the reaction was studied from the plot of ln At versus irradiation time. Fig. 3(b) represents the plot of ln A t versus irradiation time for the degradation of EY. This gives a linear relationship and implies that the photodegradation reaction followed pseudo first order kinetics. The slope of the
4
line represents the degradation rate constant (k) and was found to be 0.089 min-1 [13]. Fig. 3(c) graphically represents the percentage efficiency of photodegradation of EY with time. It is observed that 99.1% of EY was degraded within 45 min using CuO NLs. Hence, CuO NLs acts as an efficient photocatalyst for the degradation of EY. The higher rate of photodegradation of EY was due to the following facts: (1) Due to the special leaf-like morphology exhibited by the synthesized CuO nanoparticles, the exposed surface area will be higher than that of spherical CuO nanoparticles. CuO photocatalyst having large surface area is favourable for absorption of large amount of photons which generate photoexcited electron–hole pairs. The increase in band gap also facilitates absorption of photons in the visible region and hinder exitonic recombination to favour photocatalytic activity under direct sunlight [14]; (2) Another reason for higher degradation rate was the formation of aminoacid (arginine) capped CuO nanoparticles. Due to the presence of arginine on the surface of CuO nanoleaves, the electron-hole recombination will be hindered. Mechanism of photodegradation of Eosin Y under direct sunlight using CuO NLs as photocatalyst: The illumination of the catalyst surface (CuO NLs) with light energy (higher than its band-gap energy) leads to the formation of holes (h+) in the valence band and an electron (e-) in the conduction band of CuO nanoparticles. The photocatalytic activity of metal oxide depends strongly on the ability to inhibit electron-hole recombination which directly implies the availability of electrons and holes to react with O2 and H2O to produce highly reactive oxygen species. Due to the presence of arginine on the surface of CuO nanoleaves, the electron-hole recombination will be hindered and this will lead to the existence of larger number of photogenerated holes and electrons. The photogenerated holes oxidize the pollutant (Eosin Y) directly or react with OH− or H2O thereby oxidizing them into OH • radicals. While, the photogenerated electrons can reduce the dye or react with electron acceptors such as O2 adsorbed on the CuO surface or dissolved in water, reducing it to superoxide radical anion O 2− •. Less extent of electron-hole recombination makes the degradation process faster. This leads to the increase in the rate of degradation of eosin Y. [14, 15, 13]. The plausible mechanism for the photocatalytic degradation of EY dye can be shown as: CuO + hν → e- + h+ H2O + h+ → OH- + H+ OH- + h+→ .OH e- + O2 → .O2.
O2- + H+→ . OOH EY + hν → EY*
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EY* + CuO → EY + CuO (e-) CuO (e-) + O2 → CuO + O2CuO (e-) + .O2- + H+ → CuO + H2O2 CuO (e-) + H2O2 → CuO + .OH + OHh+ + EY → degradation products EY* + O2 or .OH or .O2- → degradation products The schematic representation for the photodegradation of EY dye using CuO NLs is depicted in Scheme 1:
Conduction Band e-
Sunlight
Eg CuO NLs
+
h Valence Band
+ Organic dye 1.2
0m 15 m 30 m 45 m
Absorbance (a.u.)
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Scheme 1. Schematic representation of photodegradation of Eosin Y dye using CuO NLs.
4. CONCLUSION This article illustrates a facile, cost-effective and green synthesis of CuO NLs using L-arginine by microwave heating method. The aminoacid L-arginine acts as a good complexing as well as a good capping agent. The TEM image showed the formation of 2D CuO nanoleaves with an average dimension of ~350-450nm in length and ~60-90nm in width. The XRD and SAED pattern revealed the single phase monoclinic crystal structure of CuO NLs. A significant blue shift was observed in the band gap energy of synthesized CuO NLs (2.15eV) from bulk CuO (1.2eV). The investigation showed that photocatalytic degradation of eosin Y takes place within 45 min by solar radiation. 6
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Figures:
(a)
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Figure 1. (a) FT-IR spectrum, (b) XRD pattern, (c) Absorption spectrum of CuO NLs (upper right inset represents the corresponding plot of (αhν)2 versus photon energy (hν)
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Figure 2. (a) TEM microphotograph, (b) HRTEM image, (c) SAED pattern of CuO NLs.
1.2
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ln At
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Fig. 3(a) Photodegradation of EY dye by solar irradiation using CuO NLs as photocatalyst, (b) Plot of ln A t versus irradiation time, (c) Percentage efficiency of photodegradation of EY dye with time.
HIGHLIGHTS 1. A facile, green method was developed for the synthesis of 2D CuO nanoleaves (NLs). 2. For the first time microwave heating method was designed using aminoacid L-arginine. 3. Formation of single crystalline CuO NL having ~350nm length and ~60nm width. 4. Optical band gap of CuO NLs (2.15eV) showed a clear blue shift from bulk CuO. 5. For the first time, Eosin Y was degraded using CuO NLs by solar irradiation.
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