Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 662–668
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
CuO nanostructures: Optical properties and morphology control by pyridinium-based ionic liquids Maryam Sabbaghan a,⇑, Ashraf Sadat Shahvelayati b, Kamelia Madankar b a b
Chemistry Department, Faculty of Sciences, Shahid Rajaee Teacher Training University, PO Box 16785-163, Tehran, Iran Department of Chemistry, Yadegar-e-Imam Khomeini (RAH) Branch, Islamic Azad University, Tehran, Iran
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Amount of base on the morphology of
CuO nanostructures were affected. Different morphologies of CuO
nanostructures were obtained by changing the NaOH. The red shift of the band gap with using ionic liquids has been observed. The ionic liquids can be affected on PL pattern of the samples.
a r t i c l e
i n f o
Article history: Received 23 May 2014 Received in revised form 11 July 2014 Accepted 29 July 2014 Available online 8 August 2014 Keywords: CuO nanostructure Ionic liquid Nanosheet Green synthesis PL patterns Optical properties
a b s t r a c t Copper oxide nanostructures have been synthesized by a simple reflux method in aqueous medium of pyridinium based ionic liquids. The structural and optical properties of CuO nanostructures were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence spectroscopy (PL) and UV–visible. The morphologies of the nanostructures can be controlled by changing the amount of NaOH and ionic liquids. The results show that the use identical pyridinium based ionic liquids in ratio of 4:1 NaOH/Cu(OAc)2H2O yield minor differences in morphology of CuO nanostructures. Different morphologies of CuO nanostructures were obtained by changing the ratio NaOH/Cu(OAc)2H2O to 2:1. Ionic liquids play an important role on optical properties of CuO nanostructures. The results of optical measurements of the CuO nanostructures illustrate that band gaps are estimated to be 1.67–1.85 eV. PL patterns studies show that the ionic liquids can be effect on PL patterns of the samples. The reasons of these phenomena are discussed. Ó 2014 Published by Elsevier B.V.
Introduction In the recent decades, much effort has been made toward the synthesis and characterization of nanosized transition metal oxide particles. Among them, copper oxide (CuO) nanostructures have been of great interest due to their potential applications in many important fields of science and technology such as gas sensors,
⇑ Corresponding author. Tel.: +98 (21)22970060; fax: +98 (21)22970033. E-mail address: [email protected] (M. Sabbaghan). http://dx.doi.org/10.1016/j.saa.2014.07.097 1386-1425/Ó 2014 Published by Elsevier B.V.
magnetic storage media, solar energy transformation, semiconductors and catalysis [1–3]. With the decrease in the particle size, nano sized copper oxides may exhibit unique properties which can be significantly different from those of their bulk counterparts [4]. Many efficient approaches to prepare copper oxide thin film, nanotube, nanowire, nanorods, and nanoparticles have been reported [5–7]. Recently, the ionic liquids have been used as templates to prepare inorganic nanostructure materials. The use of them has been found to have specific advantages, such as convenience, economical, less energy and material consumption and high yield
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M. Sabbaghan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 662–668
Br
N
Br IL2
IL1
Br
N
N
Br
Br
N
IL4
IL3
Intensity
N
P8 P7 P6 P5 P4 P3 P2 P1 P0
Scheme 1. Chemical structures of ionic liquids.
[8,9]. Ionic liquids (ILs) are known as salts that are liquid at room temperature in contrast to high-temperature molten salts. They have unique properties which make them suitable in numerous applications in which conventional organic solvents are not sufficiently effective. As solvents, ILs possess several advantages of the ability to dissolve many different organic, inorganic and organometallic materials, high polarity, thermally stability, high thermal conductivity, and a large electrochemical window [10]. ILs can also act as templates for the synthesis of inorganic materials (such as metal oxides) with novel or improved properties [11]. The use of ionic liquids as reaction atmosphere for the preparation of inorganic nanoparticles has been mainly due to the high polarity of ionic liquids [12,13]. At present, a part of our continual efforts are focused on the development of novel, efficient and green procedures for the synthesis of nanomaterials [14,15]. In this research, the different pyridinium-based ionic liquids dissolved in water had been used as a reaction medium for synthesis of different morphologies of CuO nanostructures. Different morphology of CuO showed interesting optical properties. The pyridinium-based ionic liquids, used in this research, were N-pentylpyridinium bromide abbreviated as [PPy][Br] (IL1), N-octylpyridinium bromide ([OPy][Br] (IL2), N-benzylpyridinium bromide [BzPy][Br] (IL3), pentylen dipyridinium dibromide [PDPy][Br]2 (IL4) (Scheme 1). Experimental Synthesis and characterization ILs All the ILs used in this study was synthesized according to Menshutkin reaction [16]. The IR Spectra were recorded Shimadzu IR-460 spectrometer. 1H NMR was measured on Bruker DRX-500 AVANCE instrument in CDCl3 at 500.1 and 125.7 MHz, respectively d in ppm, J in Hz. N-pentyl pyridinium bromide (IL1) White powder, yield: 95%, m.p = 83 °C. IR cm1 (KBr): 3055, 2947, 2864, 1632, 1488 cm1 1HNMR (500 MHz, CDCl3): d = 0.82 (3 H, t, 3J = 6.8, CH3), 1.23–1.37 (4 H, m, 2 CH2), 1.95–2.08 (2 H, m, CH2), 4.95 (2 H, t, 3J = 7.4, CH2), 8.14 (2 H, t, 3J = 6.5, 2 CH-m py), 8.52 (1 H, t, 3J = 8.0, CH-p py), 9.54 (2 H, d, 3J = 4. 5, 2 CH-o py) ppm.
2θ Fig. 1. XRD pattern of CuO nanostructures.
n-Octyl pyridinium bromide (IL2) White powder, yield: (97%), m.p = 29 °C. IR cm1 (KBr): 3051, 2929, 2860, 1632, 1490 1HNMR (500 MHz, CDCl3) IL2: d = 0.82 (3 H, t, 3J = 6.5, CH3), 1.21–1.26 (6 H, m, 3CH2), 1.28–1.36 (4 H, m, 2CH2), 2.04 (2 H, quintet, 3J = 7.4, CH2), 4.99 (2 H, t, 3J = 7.4, CH2), 8.18 (2 H, t, 3J = 6.5, 2 CH-m py), 8.53 (1 H, t, 3J = 7.5, CH-o py), 9.55 (2 H, d,3J = 4.95, 2 CH-m py) ppm. n-Benzyl pyridinium bromide (IL3) White powder, yield: 97%, m.p = 86 °C. IR cm1 (KBr): 3041, 2963, 2863, 1629, 1488, 1HNMR (250 MHz, CDCl3): d = 6.20 (2 H, s, CH2), 7.17–7.20 (3 H, m, 3 CH-Aromatic), 7.58–7.62 (2 H, m, 2 CH-Aromatic), 7.93 (2 H, t, 3J = 7.0, 2 CH-m py), 8.38 (1 H, t,3J = 7.7, CH-p py), 9.58 (2 H, d, 3J = 5.3, 2 CH-o py) ppm. Pentylen dipyridinium dibromide (IL4) White powder, yield: (92%); m.p = 180 °C. IR cm1 (KBr): 3049, 2925, 2863, 1627, 1479 1HNMR (250 MHz, CDCl3): d = 1.56–1.68 (2 H, m, CH2), 2.06–2.19 (4 H, m, 2 CH2), 4.93 (4 H, t, 3J = 7.3, 2 CH2), 8.00 (4 H, t, 3J = 6.7, 4 CH-m py), 8.45 (2 H, t, 3J = 7.6, 2 CH-p py), 9.27 (4 H, d, 3J = 5.6, 4 CH-o py) ppm. Synthesis and characterization nanostructures of CuO Copper acetate dihydrate was purchased from Aldrich. Sodium hydroxide was used without further purification. Copper acetate was employed as a copper source. In a typical experiment, the definite amount of NaOH was dissolved in 25 mL of distilled water under vigorous stirring, followed by the addition of IL and 0.1995 g Cu(OAc)2H2O to the mixture. The mixture transfer to a round bottomed flask and was refluxed for 24 h. After cooling to
Table 1 Preparation of CuO nanostructures by reflux method in ILs and water with information on morphology, crystallite particle size and band gap energy. Sample
IL
NaOH/Cu(OAc)2H2O
Morphology
Size (nm)
Band gap (eV)
P0 P1 P2 P3 P4 P5 P6 P7 P8
– 1 1 2 2 3 3 4 4
4:1 4:1 2:1 4:1 2:1 4:1 2:1 4:1 2:1
Ununiform nanosheet Leaf like Oval like Leaf like Oval like Nanosheet Leaf like Nanosheet Sword like
14 16 14 16 14 17 16 17 16
1.85 1.75 1.79 1.75 1.83 1.67 1.79 1.68 1.79
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P0
P1
P2
P3
P4
P5
P6
P7
P8
Fig. 2. SEM images of CuO nanostructures.
room temperature, the precipitate was collected by filtration and washed by distilled water and ethanol (96%) several times. Finally, the CuO samples were obtained by centrifugation and drying of the precipitate in the air at room temperature. Also this experiment was done without template and was labeled as P0. The detailed synthesis conditions are summarized in Table 1.
The morphology of CuO nanostructure was determined using scanning electron microscopy (SEM) of a Holland Philips XL30 microscope. X-ray diffraction (XRD) analysis was carried out at room temperature using a Holland Philips Xpert X-ray powder diffractometer with Cu Ka radiation (k = 0.15406 nm), over the 2h collection range of 20–80°. Average crystallite sizes of products were
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Fig. 3. UV–vis spectra of CuO nanostructures with different morphologies.
calculated using Scherrer’s formula: D = 0.9k/b cos h [10], where D is the diameter of the nanoparticles, k (Cu Ka) = 1.5406 Å and b is the full-width at half-maximum of the diffraction lines. The band gaps of our samples were determined by a UV–visible spectrometer on an instrument PG T80/T80+ with drift and solid cells. The spectra were recorded at room temperature in the wavelength range of 200–900 nm and with the accuracy of 0.5 nm. The photoluminescence (PL) spectrum was recorded by applying a photoluminescence spectrophotometer (Avantes/Avaspec 2048) at room temperature in the wavelength range of 200–1100 nm with the measurement accuracy of 0.04–20 nm. The optical measurements were done in solid state. Results and discussion Structure and morphology The CuO samples were prepared from cupric acetate monohydrate as a cupric source, and the chemical reaction can be formulated as:
CuðOAcÞ2 þ 4NaOH ! Cu½OH2 4 ! DCuo 24 h
The XRD pattern of the as-obtained products is shown in Fig. 1. All the prominent peaks in the pattern corresponded to monoclinic-phase CuO (space group C2/c), which can be indexed on the basis of JCPDS Card No. 48-1548. The XRD patterns exhibit two main signals at 35.51° and 38.70° 2h that can be ascribed to the (1 1 1) and (1 1 1) reflections of CuO phase [17]. The structures of the ILs 1–4 used in this study are shown in Scheme 1. The morphologies of the products were analyzed by SEM images shown in Fig. 2. When only Cu2+ was used (without any IL), CuO non-uniform nanosheets were formed (P0). The proportion of NaOH to Cu(OAc)2H2O was 4:1. Leaf-like morphology (P1) was obtained in ratio of 4:1 NaOH/Cu(OAc)2H2O and 4:1 in IL1/Cu(OAc)2H2O (pH = 14). By changing the ratio of NaOH/ Cu(OAc)2H2O to 2:1, oval-like morphology (P2) was obtained (pH = 14). SEM image of P3 and P4 prepared in IL2 clearly shows homogeneous samples with uniform leaf-like and oval-like morphologies respectively in the same condition with P1 and P2. Nanosheet CuO (P5) was synthesized in IL3 by NaOH/Cu (OAc)2H2O and IL3/Cu(OAc)2H2O ratios 4:1. Apposite of mentioned results by changing the ratio NaOH/Cu(OAc)2H2O to 2:1, leaf-like morphology (P6) was obtained. Pentylen dipyridinium dibromide (IL4) in both of ratios 4:1 and 2:1 in NaOH/Cu(OAc)2H2O, nanosheet and sword-like morphologies were obtained (P7 and P8). It is
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Fig. 4. PL spectra of CuO nanostructures with different morphologies.
noticed that the use different pyridinium based ionic liquids in ratio of 4:1 NaOH/Cu(OAc)2H2O have minor different in morphology of CuO nanostructures. Different morphologies of CuO nanostructures were obtained by changing the ratio NaOH/Cu(OAc)2H2O to 2:1 (see Supplementary material). Possible mechanism of formation The ionic liquid played an important role in the control over the morphology of the product based on different mechanisms, including based on hydrogen bonds, p–p stack interactions, self-assembled mechanism, electrostatic attraction, and so on [18,19]. ILs were employed as a directing agent to control CuO nanostructure morphologies. It seems, depends on IL structure and concentration, various micelles in aqueous solution such as cylindrical or bilayer are formed [20]. An explanation for the formation of 2D CuO involves the role of Cu(OH)2 4 and IL to control the growth rate of various faces of the preformed nucleus. The ILs molecules firstly form bilayers in the solution that named bilayer-like micelles. At the moment, formation of Cu(OH)2 4 nuclei takes place in the head group regions of IL (pH = 14). The head group regions of the formed
micelles occupied by coulombic force with Cu(OH)2 4 . At the same time, formation of CuO nuclei takes place in the head group regions of the micelles, resulting uniform 2D CuO. It is intriguing to note that the surfaces of 2D-CuO in P1, P3 and P5 samples are rough. It means the structure is assembled from small nanoparticles. Interestingly, the CuO nanostructures in P7 are composed of many nanorods that they self-assembled into 2D-CuO. The effect of NaOH on the morphology of products in ILs is also investigated. It seems, the adsorption band related to the stretching vibration of C(2)-H in slightly weakened after NaOH was added in solution. It caused the change of the interaction between CuO surface and IL1–2, that is to say, it weakened the interaction between hydrogen and carbon atoms at position 2 of the pyridium ring. On the other hand, the hydrogen bond, formed between the hydrogen atom at position 2 of the pyridium ring and the oxygen atoms of O–Cu were enhanced, playing a crucial rule in the morphology of CuO nanostructures [21]. It seems benzyl group in IL3 has more steric effect in compare to alkyl group in IL1 or IL2 and influence of the amount of NaOH is not distinct. 2D CuO morphologies in different concentration of NaOH were obtained.
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By closer looking to the morphology of the P7 and P8 show that nanorods of CuO array and nanosheet morphology was obtained in excess NaOH, On the contrary, P8 sample CuO nanoparticles array and Sword like morphology was produced. It indicates the effect of NaOH concentration on primary morphology of CuO nanostructures. Thus, ionic liquid structure and amount of base in the same pH have critical roles in the morphology of CuO nanostructures.
667
without IL (1.85 eV). Photoluminescence spectra at 300 K showed that a dominant emission peak appeared at the about 330– 360 nm and sample with n-benzyl pyridinium bromide template appeared at the blue region. Also the samples with identical ionic liquids have the same PL patterns.
Appendix A. Supplementary material Optical properties To study the effect of ILs on the band gap of the CuO nanostructures, the UV–vis absorption spectra of all the samples P0-P8 were studied, which are shown in Fig. 3. For excitonic absorption peaks are observed at 332, 340, 346, 332, 333, 335, 338, 342 and 336 nm for the samples P0–P8, respectively Fig. 3. A higher increase in the optical absorption band of the P1 sample was observed in comparison the other samples. The band gap energies can be estimated on according to following equation [17]:
aEphoton ¼ KðEphoton Eg Þ1=2 where K is a constant, a is the absorption coefficient, Ephoton is the discrete photo energy and Eg is the band gap energy. The energy intercept of a plot of (aEphoton)2 vs. Ephoton yields Eg for a direct transition. The value of band gap energies present in Table 1. The band gap of nano CuO with various morphology ranges from 1.67 to 1.83 which are red shift with respect to the band gap for bulk CuO (3.25 eV) and with P0 [22,23]. The results show that the IL and the amount of NaOH greatly affect band gap energy of the CuO samples. Also all of samples may use in optoelectronic and photovoltaic application. The photoluminescence (PL) spectra of these samples were measured at room temperature and the excitation wavelengths were 300, 350, 400 and 450 nm. As shown in Fig. 4, is a strong peak at about 390–421 nm in all samples that can be attributed to the band–band PL phenomenon [24–26]. Besides this peak observed in all samples, the sharp blue emission peak of around 418– 470 nm is due to artifacts [27]. The emission band of around 541–570 nm can be attributed to the recombination of a photogenerated hole with a singly ionized electron in the valence band [28–30]. By closer looking at PL spectrum of P5 and P6 samples show the second emission enhanced comparing with other peaks and green emission intensity more than other samples. Also the emission peak position has been shifted from 420 to around 470 nm as well as the emission intensity has been increased by IL3 whereas ratio of NaOH/Cu(OAc)2H2O is 4:1. These luminescence blue bands at 470 nm may be caused by transition vacancy of oxygen and interstitial oxygen [31]. In another view to Fig. 4 shows that PL patterns of nanostructures are the same with identical ILs although they have different amounts of base. These results show that the ionic liquids structures can effect on the electron band of CuO nanostructures. The effect of ILs on optical properties of CuO nanostructures needs to be further investigated. Conclusion In summary, various morphologies of CuO nanostructures were successfully prepared using ionic liquids with different amount of sodium hydroxide through reflux method in water. The effect of pyridinium-based ionic liquids and sodium hydroxide on the structure and optical properties of the powders was studied. The optical band gap energies of CuO nanostructures, as determined from the absorbance spectrum, were 1.67–1.83 eV which are red shift with respect to the band gap for bulk CuO (3.25 eV) and CuO sample
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2014.07.097.
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Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
CuO nanostructures: Optical properties and morphology control by pyridinium-based ionic liquids Maryam Sabbaghan a,⇑, Ashraf Sadat Shahvelayati b, Kamelia Madankar b a b
Chemistry Department, Faculty of Sciences, Shahid Rajaee Teacher Training University, PO Box 16785-163, Tehran, Iran Department of Chemistry, Yadegar-e-Imam Khomeini (RAH) Branch, Islamic Azad University, Tehran, Iran
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Amount of base on the morphology of
CuO nanostructures were affected. Different morphologies of CuO
nanostructures were obtained by changing the NaOH. The red shift of the band gap with using ionic liquids has been observed. The ionic liquids can be affected on PL pattern of the samples.
a r t i c l e
i n f o
Article history: Received 23 May 2014 Received in revised form 11 July 2014 Accepted 29 July 2014 Available online 8 August 2014 Keywords: CuO nanostructure Ionic liquid Nanosheet Green synthesis PL patterns Optical properties
a b s t r a c t Copper oxide nanostructures have been synthesized by a simple reflux method in aqueous medium of pyridinium based ionic liquids. The structural and optical properties of CuO nanostructures were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence spectroscopy (PL) and UV–visible. The morphologies of the nanostructures can be controlled by changing the amount of NaOH and ionic liquids. The results show that the use identical pyridinium based ionic liquids in ratio of 4:1 NaOH/Cu(OAc)2H2O yield minor differences in morphology of CuO nanostructures. Different morphologies of CuO nanostructures were obtained by changing the ratio NaOH/Cu(OAc)2H2O to 2:1. Ionic liquids play an important role on optical properties of CuO nanostructures. The results of optical measurements of the CuO nanostructures illustrate that band gaps are estimated to be 1.67–1.85 eV. PL patterns studies show that the ionic liquids can be effect on PL patterns of the samples. The reasons of these phenomena are discussed. Ó 2014 Published by Elsevier B.V.
Introduction In the recent decades, much effort has been made toward the synthesis and characterization of nanosized transition metal oxide particles. Among them, copper oxide (CuO) nanostructures have been of great interest due to their potential applications in many important fields of science and technology such as gas sensors,
⇑ Corresponding author. Tel.: +98 (21)22970060; fax: +98 (21)22970033. E-mail address: [email protected] (M. Sabbaghan). http://dx.doi.org/10.1016/j.saa.2014.07.097 1386-1425/Ó 2014 Published by Elsevier B.V.
magnetic storage media, solar energy transformation, semiconductors and catalysis [1–3]. With the decrease in the particle size, nano sized copper oxides may exhibit unique properties which can be significantly different from those of their bulk counterparts [4]. Many efficient approaches to prepare copper oxide thin film, nanotube, nanowire, nanorods, and nanoparticles have been reported [5–7]. Recently, the ionic liquids have been used as templates to prepare inorganic nanostructure materials. The use of them has been found to have specific advantages, such as convenience, economical, less energy and material consumption and high yield
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M. Sabbaghan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 662–668
Br
N
Br IL2
IL1
Br
N
N
Br
Br
N
IL4
IL3
Intensity
N
P8 P7 P6 P5 P4 P3 P2 P1 P0
Scheme 1. Chemical structures of ionic liquids.
[8,9]. Ionic liquids (ILs) are known as salts that are liquid at room temperature in contrast to high-temperature molten salts. They have unique properties which make them suitable in numerous applications in which conventional organic solvents are not sufficiently effective. As solvents, ILs possess several advantages of the ability to dissolve many different organic, inorganic and organometallic materials, high polarity, thermally stability, high thermal conductivity, and a large electrochemical window [10]. ILs can also act as templates for the synthesis of inorganic materials (such as metal oxides) with novel or improved properties [11]. The use of ionic liquids as reaction atmosphere for the preparation of inorganic nanoparticles has been mainly due to the high polarity of ionic liquids [12,13]. At present, a part of our continual efforts are focused on the development of novel, efficient and green procedures for the synthesis of nanomaterials [14,15]. In this research, the different pyridinium-based ionic liquids dissolved in water had been used as a reaction medium for synthesis of different morphologies of CuO nanostructures. Different morphology of CuO showed interesting optical properties. The pyridinium-based ionic liquids, used in this research, were N-pentylpyridinium bromide abbreviated as [PPy][Br] (IL1), N-octylpyridinium bromide ([OPy][Br] (IL2), N-benzylpyridinium bromide [BzPy][Br] (IL3), pentylen dipyridinium dibromide [PDPy][Br]2 (IL4) (Scheme 1). Experimental Synthesis and characterization ILs All the ILs used in this study was synthesized according to Menshutkin reaction [16]. The IR Spectra were recorded Shimadzu IR-460 spectrometer. 1H NMR was measured on Bruker DRX-500 AVANCE instrument in CDCl3 at 500.1 and 125.7 MHz, respectively d in ppm, J in Hz. N-pentyl pyridinium bromide (IL1) White powder, yield: 95%, m.p = 83 °C. IR cm1 (KBr): 3055, 2947, 2864, 1632, 1488 cm1 1HNMR (500 MHz, CDCl3): d = 0.82 (3 H, t, 3J = 6.8, CH3), 1.23–1.37 (4 H, m, 2 CH2), 1.95–2.08 (2 H, m, CH2), 4.95 (2 H, t, 3J = 7.4, CH2), 8.14 (2 H, t, 3J = 6.5, 2 CH-m py), 8.52 (1 H, t, 3J = 8.0, CH-p py), 9.54 (2 H, d, 3J = 4. 5, 2 CH-o py) ppm.
2θ Fig. 1. XRD pattern of CuO nanostructures.
n-Octyl pyridinium bromide (IL2) White powder, yield: (97%), m.p = 29 °C. IR cm1 (KBr): 3051, 2929, 2860, 1632, 1490 1HNMR (500 MHz, CDCl3) IL2: d = 0.82 (3 H, t, 3J = 6.5, CH3), 1.21–1.26 (6 H, m, 3CH2), 1.28–1.36 (4 H, m, 2CH2), 2.04 (2 H, quintet, 3J = 7.4, CH2), 4.99 (2 H, t, 3J = 7.4, CH2), 8.18 (2 H, t, 3J = 6.5, 2 CH-m py), 8.53 (1 H, t, 3J = 7.5, CH-o py), 9.55 (2 H, d,3J = 4.95, 2 CH-m py) ppm. n-Benzyl pyridinium bromide (IL3) White powder, yield: 97%, m.p = 86 °C. IR cm1 (KBr): 3041, 2963, 2863, 1629, 1488, 1HNMR (250 MHz, CDCl3): d = 6.20 (2 H, s, CH2), 7.17–7.20 (3 H, m, 3 CH-Aromatic), 7.58–7.62 (2 H, m, 2 CH-Aromatic), 7.93 (2 H, t, 3J = 7.0, 2 CH-m py), 8.38 (1 H, t,3J = 7.7, CH-p py), 9.58 (2 H, d, 3J = 5.3, 2 CH-o py) ppm. Pentylen dipyridinium dibromide (IL4) White powder, yield: (92%); m.p = 180 °C. IR cm1 (KBr): 3049, 2925, 2863, 1627, 1479 1HNMR (250 MHz, CDCl3): d = 1.56–1.68 (2 H, m, CH2), 2.06–2.19 (4 H, m, 2 CH2), 4.93 (4 H, t, 3J = 7.3, 2 CH2), 8.00 (4 H, t, 3J = 6.7, 4 CH-m py), 8.45 (2 H, t, 3J = 7.6, 2 CH-p py), 9.27 (4 H, d, 3J = 5.6, 4 CH-o py) ppm. Synthesis and characterization nanostructures of CuO Copper acetate dihydrate was purchased from Aldrich. Sodium hydroxide was used without further purification. Copper acetate was employed as a copper source. In a typical experiment, the definite amount of NaOH was dissolved in 25 mL of distilled water under vigorous stirring, followed by the addition of IL and 0.1995 g Cu(OAc)2H2O to the mixture. The mixture transfer to a round bottomed flask and was refluxed for 24 h. After cooling to
Table 1 Preparation of CuO nanostructures by reflux method in ILs and water with information on morphology, crystallite particle size and band gap energy. Sample
IL
NaOH/Cu(OAc)2H2O
Morphology
Size (nm)
Band gap (eV)
P0 P1 P2 P3 P4 P5 P6 P7 P8
– 1 1 2 2 3 3 4 4
4:1 4:1 2:1 4:1 2:1 4:1 2:1 4:1 2:1
Ununiform nanosheet Leaf like Oval like Leaf like Oval like Nanosheet Leaf like Nanosheet Sword like
14 16 14 16 14 17 16 17 16
1.85 1.75 1.79 1.75 1.83 1.67 1.79 1.68 1.79
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P0
P1
P2
P3
P4
P5
P6
P7
P8
Fig. 2. SEM images of CuO nanostructures.
room temperature, the precipitate was collected by filtration and washed by distilled water and ethanol (96%) several times. Finally, the CuO samples were obtained by centrifugation and drying of the precipitate in the air at room temperature. Also this experiment was done without template and was labeled as P0. The detailed synthesis conditions are summarized in Table 1.
The morphology of CuO nanostructure was determined using scanning electron microscopy (SEM) of a Holland Philips XL30 microscope. X-ray diffraction (XRD) analysis was carried out at room temperature using a Holland Philips Xpert X-ray powder diffractometer with Cu Ka radiation (k = 0.15406 nm), over the 2h collection range of 20–80°. Average crystallite sizes of products were
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665
Fig. 3. UV–vis spectra of CuO nanostructures with different morphologies.
calculated using Scherrer’s formula: D = 0.9k/b cos h [10], where D is the diameter of the nanoparticles, k (Cu Ka) = 1.5406 Å and b is the full-width at half-maximum of the diffraction lines. The band gaps of our samples were determined by a UV–visible spectrometer on an instrument PG T80/T80+ with drift and solid cells. The spectra were recorded at room temperature in the wavelength range of 200–900 nm and with the accuracy of 0.5 nm. The photoluminescence (PL) spectrum was recorded by applying a photoluminescence spectrophotometer (Avantes/Avaspec 2048) at room temperature in the wavelength range of 200–1100 nm with the measurement accuracy of 0.04–20 nm. The optical measurements were done in solid state. Results and discussion Structure and morphology The CuO samples were prepared from cupric acetate monohydrate as a cupric source, and the chemical reaction can be formulated as:
CuðOAcÞ2 þ 4NaOH ! Cu½OH2 4 ! DCuo 24 h
The XRD pattern of the as-obtained products is shown in Fig. 1. All the prominent peaks in the pattern corresponded to monoclinic-phase CuO (space group C2/c), which can be indexed on the basis of JCPDS Card No. 48-1548. The XRD patterns exhibit two main signals at 35.51° and 38.70° 2h that can be ascribed to the (1 1 1) and (1 1 1) reflections of CuO phase [17]. The structures of the ILs 1–4 used in this study are shown in Scheme 1. The morphologies of the products were analyzed by SEM images shown in Fig. 2. When only Cu2+ was used (without any IL), CuO non-uniform nanosheets were formed (P0). The proportion of NaOH to Cu(OAc)2H2O was 4:1. Leaf-like morphology (P1) was obtained in ratio of 4:1 NaOH/Cu(OAc)2H2O and 4:1 in IL1/Cu(OAc)2H2O (pH = 14). By changing the ratio of NaOH/ Cu(OAc)2H2O to 2:1, oval-like morphology (P2) was obtained (pH = 14). SEM image of P3 and P4 prepared in IL2 clearly shows homogeneous samples with uniform leaf-like and oval-like morphologies respectively in the same condition with P1 and P2. Nanosheet CuO (P5) was synthesized in IL3 by NaOH/Cu (OAc)2H2O and IL3/Cu(OAc)2H2O ratios 4:1. Apposite of mentioned results by changing the ratio NaOH/Cu(OAc)2H2O to 2:1, leaf-like morphology (P6) was obtained. Pentylen dipyridinium dibromide (IL4) in both of ratios 4:1 and 2:1 in NaOH/Cu(OAc)2H2O, nanosheet and sword-like morphologies were obtained (P7 and P8). It is
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Fig. 4. PL spectra of CuO nanostructures with different morphologies.
noticed that the use different pyridinium based ionic liquids in ratio of 4:1 NaOH/Cu(OAc)2H2O have minor different in morphology of CuO nanostructures. Different morphologies of CuO nanostructures were obtained by changing the ratio NaOH/Cu(OAc)2H2O to 2:1 (see Supplementary material). Possible mechanism of formation The ionic liquid played an important role in the control over the morphology of the product based on different mechanisms, including based on hydrogen bonds, p–p stack interactions, self-assembled mechanism, electrostatic attraction, and so on [18,19]. ILs were employed as a directing agent to control CuO nanostructure morphologies. It seems, depends on IL structure and concentration, various micelles in aqueous solution such as cylindrical or bilayer are formed [20]. An explanation for the formation of 2D CuO involves the role of Cu(OH)2 4 and IL to control the growth rate of various faces of the preformed nucleus. The ILs molecules firstly form bilayers in the solution that named bilayer-like micelles. At the moment, formation of Cu(OH)2 4 nuclei takes place in the head group regions of IL (pH = 14). The head group regions of the formed
micelles occupied by coulombic force with Cu(OH)2 4 . At the same time, formation of CuO nuclei takes place in the head group regions of the micelles, resulting uniform 2D CuO. It is intriguing to note that the surfaces of 2D-CuO in P1, P3 and P5 samples are rough. It means the structure is assembled from small nanoparticles. Interestingly, the CuO nanostructures in P7 are composed of many nanorods that they self-assembled into 2D-CuO. The effect of NaOH on the morphology of products in ILs is also investigated. It seems, the adsorption band related to the stretching vibration of C(2)-H in slightly weakened after NaOH was added in solution. It caused the change of the interaction between CuO surface and IL1–2, that is to say, it weakened the interaction between hydrogen and carbon atoms at position 2 of the pyridium ring. On the other hand, the hydrogen bond, formed between the hydrogen atom at position 2 of the pyridium ring and the oxygen atoms of O–Cu were enhanced, playing a crucial rule in the morphology of CuO nanostructures [21]. It seems benzyl group in IL3 has more steric effect in compare to alkyl group in IL1 or IL2 and influence of the amount of NaOH is not distinct. 2D CuO morphologies in different concentration of NaOH were obtained.
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By closer looking to the morphology of the P7 and P8 show that nanorods of CuO array and nanosheet morphology was obtained in excess NaOH, On the contrary, P8 sample CuO nanoparticles array and Sword like morphology was produced. It indicates the effect of NaOH concentration on primary morphology of CuO nanostructures. Thus, ionic liquid structure and amount of base in the same pH have critical roles in the morphology of CuO nanostructures.
667
without IL (1.85 eV). Photoluminescence spectra at 300 K showed that a dominant emission peak appeared at the about 330– 360 nm and sample with n-benzyl pyridinium bromide template appeared at the blue region. Also the samples with identical ionic liquids have the same PL patterns.
Appendix A. Supplementary material Optical properties To study the effect of ILs on the band gap of the CuO nanostructures, the UV–vis absorption spectra of all the samples P0-P8 were studied, which are shown in Fig. 3. For excitonic absorption peaks are observed at 332, 340, 346, 332, 333, 335, 338, 342 and 336 nm for the samples P0–P8, respectively Fig. 3. A higher increase in the optical absorption band of the P1 sample was observed in comparison the other samples. The band gap energies can be estimated on according to following equation [17]:
aEphoton ¼ KðEphoton Eg Þ1=2 where K is a constant, a is the absorption coefficient, Ephoton is the discrete photo energy and Eg is the band gap energy. The energy intercept of a plot of (aEphoton)2 vs. Ephoton yields Eg for a direct transition. The value of band gap energies present in Table 1. The band gap of nano CuO with various morphology ranges from 1.67 to 1.83 which are red shift with respect to the band gap for bulk CuO (3.25 eV) and with P0 [22,23]. The results show that the IL and the amount of NaOH greatly affect band gap energy of the CuO samples. Also all of samples may use in optoelectronic and photovoltaic application. The photoluminescence (PL) spectra of these samples were measured at room temperature and the excitation wavelengths were 300, 350, 400 and 450 nm. As shown in Fig. 4, is a strong peak at about 390–421 nm in all samples that can be attributed to the band–band PL phenomenon [24–26]. Besides this peak observed in all samples, the sharp blue emission peak of around 418– 470 nm is due to artifacts [27]. The emission band of around 541–570 nm can be attributed to the recombination of a photogenerated hole with a singly ionized electron in the valence band [28–30]. By closer looking at PL spectrum of P5 and P6 samples show the second emission enhanced comparing with other peaks and green emission intensity more than other samples. Also the emission peak position has been shifted from 420 to around 470 nm as well as the emission intensity has been increased by IL3 whereas ratio of NaOH/Cu(OAc)2H2O is 4:1. These luminescence blue bands at 470 nm may be caused by transition vacancy of oxygen and interstitial oxygen [31]. In another view to Fig. 4 shows that PL patterns of nanostructures are the same with identical ILs although they have different amounts of base. These results show that the ionic liquids structures can effect on the electron band of CuO nanostructures. The effect of ILs on optical properties of CuO nanostructures needs to be further investigated. Conclusion In summary, various morphologies of CuO nanostructures were successfully prepared using ionic liquids with different amount of sodium hydroxide through reflux method in water. The effect of pyridinium-based ionic liquids and sodium hydroxide on the structure and optical properties of the powders was studied. The optical band gap energies of CuO nanostructures, as determined from the absorbance spectrum, were 1.67–1.83 eV which are red shift with respect to the band gap for bulk CuO (3.25 eV) and CuO sample
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2014.07.097.
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