Surfactant assisted wet chemical synthesis of copper oxide (CuO) nanostructures and their spectroscopic analysis

Optik 127 (2016) 2740–2747

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Surfactant assisted wet chemical synthesis of copper oxide (CuO) nanostructures and their spectroscopic analysis Hafsa Siddiqui, M.S. Qureshi, Fozia Z. Haque ∗ Optical Nanomaterials Lab, Department of Physics, Maulana Azad National Institute of Technology (M.A.N.I.T.), Bhopal 462051, MP, India

a r t i c l e

i n f o

Article history: Received 11 June 2015 Accepted 23 November 2015 Keywords: CuO Nanoarchitectures Surfactant Wet chemical route

a b s t r a c t CuO nanocrystals and their nanoarchitectures with various morphologies were constructed via a simple surfactant-assisted wet chemical process. The introduction of sodium dodecyl sulphate (SDS) as anionic surfactant, cetyltrimethylammonium bromide (CTAB) as cationic surfactant and polyethylene glycol (PEG) as nonionic surfactant leads to the formation of cube, leaf and flower-like CuO nanostructures, respectively. The prepared samples were characterized by different characterization techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectroscopy, Raman, UV–visible diffused reflectance spectroscopy (DRS) and photoluminescence (PL) spectroscopy. Different spectroscopic results revealed that all the samples have crystalline single-phase CuO monoclinic structures. The average crystallite size for SDS, PEG and CTAB assisted CuO samples are around 20 nm, 19 nm and 16 nm respectively. It was observed that the size and shape of particle is affected by the nature of surfactant. Additionally, in all the samples, Raman and FTIR spectra shows three identical Cu–O vibration modes that indicate the presence of crystalline CuO monoclinic structure. Diffuse reflectance results revealed that the addition of surfactants significantly enhanced the optical bandgap value from 1.40 eV to 1.54 eV, due to quantum confinement effect. Photoluminescence (PL) spectra showed both UV as well as visible emission peaks indicating their good optical properties. The presence of strong visible light absorption in all the samples revealed that it can be used as absorbing material in solar cell. © 2015 Elsevier GmbH. All rights reserved.

1. Introduction Crystalline semiconductor nanoparticles have drawn considerable interest in recent years because of their special properties such as a large surface-to-volume ratio, higher activity, and special electronic and optical properties as compared to bulk materials. The oxides of transition metals are an important class of semiconductors having applications in magnetic storage, solar energy transformation electronics and catalysis [1]. Cupric oxide (CuO) is an important transition metal oxide. It is a P-type semiconductor material with a monoclinic crystal structure and indirect band gap Eg ∼1.2 eV [2,3]. It has excellent thermal stability and good electrical and optical properties and has many practical applications such as an antimicrobial, thermal conductor [1,4,5], gas sensor [6], catalysis [7]. CuO used as a passive and active factor in solar cell technology and photovoltaic applications [8,9] also used in lithium ion batteries [10], field emission emitters [11] etc. It makes the base of several interesting high temperature Tc-superconductors and

∗ Corresponding author. Tel.: +91 9300687943; fax: +91 755 2670562. E-mail address: [email protected] (F.Z. Haque). http://dx.doi.org/10.1016/j.ijleo.2015.11.220 0030-4026/© 2015 Elsevier GmbH. All rights reserved.

giant magnetoresistance materials [12,13]. CuO nanostructures with dimensionalities, such as wires, tubes, seeds, belts, sheets [3,11,14] rods, leaves, needles and platelets, [15–18] are obtained with success, synthesized via different paths. Among different methods, wet chemical route has drawn more attention due to its simplicity and easy chemical reactions, which can be conducted in an open container with a relatively low reaction temperature (normally below 100◦ C). This process can be simply defined as the chemical reaction between the precursors to produce monomers that subsequently aggregate into final resulting materials [19]. Moreover, compared with the other synthesis strategies, it has several advantages; this provides high surface stability and additionally has an important influence on the morphology and optical properties of CuO nanoparticles [14,19,20]. Lately, it has been noted that anionic, cationic and nonionic surfactants can be utilized to help the formation of nanoscale materials. Surfactants can modulate the kinetic growth and determine the subsequent morphologies of the final products the expansion of surfactants inside the development determination is phenomenal imperative their impact on morphology and physical properties of the materials [21]. The major role of the surfactant is to modify the morphology of the materials by virtue of site selective adsorption

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on the specific site during the growth process. They control the crystal shape and size of growing particles. Zou et al. [22] reported the cationic surfactant CTAB assisted flower-like nanocrystalline CuO was prepared by hydrothermal method, while Ibupoto et al. [23] explain the nonionic surfactant PEG as template assisted CuO nanoleaves on the gold-coated glass substrate. The nature of surfactant could influence crystal habit by selective adsorption processes that lead to preferential growth inhibition for distinct crystal faces. Addition strong, attractive interactions between the surfactants and the inorganic surface can arrest nucleation and change the shape and size of the primary clusters. Steric, Van der Waals and hydrophilic–hydrophobic interactions involving the pendent chains of the surfactants, as well as shape anisotropy, was influence the assembly of the primary clusters. In the present work, a simple wet chemical route was designed to prepare CuO nanoparticles by using different types of surfactants like anionic surfactant sodium dodecyl sulfate, cationic surfactant cetyltrimethylammonium bromide and nonionic surfactant polyethylene glycol. It was observed that surfactants can minimize the surface defects and enhances the optical properties of CuO nanoparticles. Moreover, the morphology and optical features of the nanoparticles are affected by the nature of surfactant. Surface modification using these surfactants provides chemical stability to react with Cu2+ ions, the molecular structures of all three surfactant are shown in Fig. 1. 2. Experimental details 2.1. Materials All the chemicals used in the experiment were analytical reagent grade and used without further purification. Cupric nitrate trihydrate [Cu(NO3 )2 ·3H2 O], sodium dodecyl sulfate (SDS) [CH3 (CH2 )11 OSO3 Na], ccetyltrimethylammonium bromide (CTAB) [C19 H42 BrN], poly ethylene glycol (PEG-400) [H(OCH2 CH2 )n OH] and sodium hydroxide pellets (NaOH). Double distilled water was used throughout the experiments. 2.2. Synthesis of various CuO nanostructures

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(ICDD # 45-0937). The morphologies of the samples were determined through, scanning electron microscopy [JEOL-JSM-6390]. The optical characterizations of the samples were carried using micro-raman system [Jobin–Yvon–Horibra LABRAM-HR] in visible range 200 cm−1 to 1000 cm−1 and UV–visible spectrometer (Perkin Elmer Lambda 950) in diffuse reflectance mode using integrating sphere attachments. Optical bandgap energies were calculated using the Kubelka–Munk function. Room temperature photoluminescence (PL) measurements were carried out on Hitachi instrument F-7000 spectrophotometer using xenon lamp as excitation source with an excitation wavelength (ex ) of 325 nm. 3. Results and discussion 3.1. Structural analysis 3.1.1. X-ray diffraction spectroscopic analysis The X-ray diffraction spectroscopy was used to examine the crystal structure, diffraction planes peak broadening, crystallite size and lattice strain due to dislocations. Fig. 2(a)–(d)) depicted the powder XRD patterns of the uncapped and various surfactants capped CuO samples calcined at 200 ◦ C for 5 h. All the reflections on the pattern can be indexed to monoclinic CuO phase, with peak positions corresponding to those reported by the ICDD # 450937. No diffraction peaks arising from Cu2 O or any impurity were detected in the patterns confirmed that the grown product were in pure phase. The ‘d’ values of monoclinic structure CuO were calculated, according to Bragg’s law: n = 2d sin 

(1)

where n is the order of diffraction (usually n = 1),  is the X-ray wavelength and ‘d’ is the spacing between giving miller planes indices h, k and l. The observed interplanar spacing, ‘d’ value of the samples with and without surfactant shifts to lower and higher side respectively as compared to the ‘d’ value reported in the ICDD card # 45-0937 of CuO, as shown in Table 1. The crystallite size (D) all prepared samples were determined from the XRD line broadening measurement using Scherrer’s equation: D=

k ˇhkl cos 

(2)

Cupric salt (cupric nitrate trihydrate), alkaline compound (NaOH), anionic (SDS), cationic (CTAB) and nonionic (PEG-400) surfactants were used to synthesized CuO nanoparticles. A typical process of the wet chemical method is as follows: 0.1 M of [Cu(NO3 )2 ·3H2 O] was dissolved in 150 ml deionized (DI) water and kept at 50 ◦ C with vigorous stirring. 25 ml aqueous solution of NaOH was added slowly (drop-wise) into the solution, resulting in the production of a large amount of black precipitates and the crystallization temperature of 60 ◦ C was maintained for 30 min. Next, the precipitate was heated at 110 ◦ C for 5 h. After completion of above reaction, the mixture was centrifuged to get the precipitate out. DI water was first used to wash the precipitate several times, followed by washing with ethanol to discard the remaining impurities. A similar procedure was adopted to prepare surfactant assisted CuO nanoparticles with the addition of 0.1 M of surfactants SDS, PEG and CTAB into the solution initially. Finally, all samples were calcined in air at 200 ◦ C for 5 h.

3.1.2. Lattice strain and dislocation density A shift in observed ‘d’ values suggested the accumulation of tensile strain in all the as synthesized samples. These observations were in good agreement with those reported in our previous paper [2]. The average crystallite size and the internal lattice strain ε = ˇh k l /4 tan  of all the samples were evaluated by the Williamson Hall (W–H) equation expressed as [15,21]:

2.3. Characterizations

ˇh l k cos  =

The powder X-ray diffraction (PXRD) data of uncapped and capped samples was collected through Bruker D8 Advance X-ray diffractrometer with CuK␣1 radiation. In each case, scanning was performed from 20◦ to 80◦ 2 range. Diffraction peaks of the crystalline phase were compared with those of the standard compound reported in the international center for diffraction data

The ‘D’ and ‘ε’ values were calculated from the least square fit of ˇh k l cos  vs. 4 sin  for all the prominent peaks having comparatively higher intensity as shown in Fig. 2((a)–(d)). The slope of the curve gives the value of internal lattice strain (ε) associated with the lattice dislocations. Other dislocations present in the samples obstruct the movement of dislocation, this was defined as the

˚ k = shape factor (0.9),  the CuK␣1 radiation of wavelength 1.5406 A, ˇh k l the full width at half maximum (FWHM) in radians and  the scattering angle. The crystallite size of uncapped CuO sample was ∼21 nm. However, in the case of SDS, PEG and CTAB assisted CuO samples the average crystallite size was 20 nm, 19 nm and 15 nm, respectively, and having the formation of nanocrystalline CuO phase.

k + 4ε sin  D

(3)

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Fig. 1. Molecular structures of (a) anionic surfactant SDS, (b) cationic surfactant CTAB, (c) nonionic surfactant PEG-400.

Fig. 2. XRD pattern with W–H plots of (a) uncapped CuO nanoparticles, (b) SDS capped CuO nanocube, (c) PEG capped CuO nanoleaf, (d) CTAB capped CuO nanoflower.

Table 1 The computed d values of uncapped and different surfactants capped CuO nanoparticles comparison with standard d values of CuO (JCPDS# 45-0937). JCPDS# 45-0937 hkl

d = /2sin 

110 −111 111 −202 202 −113 −331 −220

2.7530 2.5270 2.3230 1.8673 1.5805 1.5058 1.4094 1.3759

Uncapped CuO nanoparticles

SDS capped CuO nanoparticles

PEG capped CuO nanoparticles

CTAB capped CuO nanoparticles

2.757358 2.531501 2.327037 1.872034 1.584998 1.509312 1.415943 1.379997

2.763236 2.536566 2.331604 1.874525 1.58686 1.510929 1.416965 1.381343

2.755678 2.532094 2.325613 1.870186 1.585101 1.5089 1.413714 1.380379

2.761687 2.533511 2.328926 1.872347 1.586303 1.51038 1.41521 1.380162

dislocation density (ı), length of dislocation line per unit meter square of the crystal was evaluated using the following equation. ı=

1 D2

(4)

where ı is dislocation density and D is crystallite size. The overall effect of surfactant addition causes a decrease in the crystallite size and generates non-uniform strain in the CuO crystallites. The positive slope observed in all samples shows the presence of tensile strain. The higher magnitude of the slope in the CuO sample prepared with CTAB in comparison to SDS, and PEG surfactant suggests the enhancement in the strain. Table 2 display the comparison

of obtaining results from Scherrer’s formula, W–H analysis. The observations from Eqs. (3) and (4) revealed that introducing SDS, PEG and CTAB surfactants in reaction mixture resulted in a drastic decrease in average crystallite size. Addition of CTAB created maximum strain in the CuO lattice and smallest average crystallite size to 15.68 nm was observed. 3.2. Morphology and capping mechanism of various CuO nanostructures The mechanism of various CuO nanostructures (particle size, shape and composition) was studied by the detailed investigation

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Table 2 Comparison of crystallite size, lattice strain and dislocation density of uncapped and different surfactants capped CuO nanoparticles. Samples

Surfactant

Average crystallite size D (nm) Scherrer’s method (D)

Dislocation density Williamson–Hall analysis

(␦) line per m2

D

ε (unit less)

Uncapped CuO NP’s

Uncapped CuO NP’s

20.83

27.90

0.00419

0.0026

Capped CuO NP’s

SDS capped CuO NP’s PEG capped CuO NP’s CTAB capped CuO NP’s

19.85 19.24 15.68

26.65 25.81 20.93

0.00442 0.00458 0.00557

0.0030 0.0033 0.0045

Fig. 3. SEM images of (a) uncapped CuO nanoparticles, (b) SDS capped CuO nanocube, (c) PEG capped CuO nanoleaf, (d) CTAB capped CuO nanoflower.

using scanning electron microscopy along with energy dispersive X-ray spectroscopy. In Fig. 3 the images of CuO nanostructures with different morphologies i.e. nanoparticles, nanocubes, nanoleafs and nanoflowers were observed for SDS, PEG, and CTAB capped samples, respectively. Fig. 3(a) showed that CuO nanocrystals organized into mixed shape particle assemblies. The surface of CuO nuclei is either positively or negatively charged (OH− or Cu2+ ) [24]. In the nucleation step, a growth unit was needed for the crystals to grow on. Generally, cations are identified as a growth unit. In aqueous solutions, the coordination number of Cu2+ is normally six [14,15]. Consequently, growth units exist in Cu(OH)6 4− of the coordinating octahedron in the NaOH solution. In a Cu(OH)6 4− complex, two OH ligands are located at its axis, and four OH ligands are located at the square plane. The binding energies of four OH ligands are higher than the two axial ligands because of the interplanar distances are shorter compared with that of the two axial OH− . Therefore, the two axial OH ligands are easily dehydrated to form anisotropic CuO nanoparticles. The accessibility of the copper ion in this complex is favored, because the coordination with water is weak. At temperatures higher than room temperature, CuO was formed. SDS capped CuO nanoparticles (cube-like) The homogeneous nucleation process, that starts with the formation of nuclei in the presence of surfactant sodium dodecyl sulfate (SDS) as anionic surfactant [21]. The hydroxyl groups react to form dodecyl sulfate–copper hydroxide ion [DS-Cu(OH)4− 6 ] nuclei that form active sites to generate DS [Cu(OH)2 ] and structures. Accordingly, the growth units for the CuO nanocrystals are considered to be Cu(OH)4− 6 , which is a coordinating octahedron in the NaOH solution. When the temperature is high enough, CuO can be produced by the dehydration of Cu(OH)2 and the simultaneous

release of SDS− anions. The structural features and specific interactions of the Cu2+ ions with ligands in the solution, DS-Cu(OH)2 tends to form a cube-like structure as shown in Fig. 3(b). PEG400 capped CuO nanoparticles (leaf-like) Polyethylene glycol-PEG400 acts as a nonionic surfactant for the formation CuO leaf-nanostructures. In the initial stage, the aqueous solution of Cu2+ with PEG and NaOH leads to the formation of nucleation seeds, which act as nucleus for the particle growth [23]. When the particle reaches a critical dimension, PEG absorbs the small particles by the ended OH bonds acting as a surfactant for the formation of CuO leaf-like structures as shown in Fig. 3(c). This was observed in an intermediated growth alignment that occurs at the high-energy facets causing elimination of their high surface energy. CTAB capped CuO nanoparticles (flower-like) The introduction of cetyltrimethyl ammonium bromide (CTAB) as cationic surfactant, which controls the growth rates of the various faces of CuO nanoparticles. CTAB ionize completely and result in cation formation with tetrahedral structure. The electrostatic interaction takes place between CTA+ cations and Cu(OH)4− 6 anions, the cation CTA+ condense into aggregates in which counter ions Cu(OH)4− 6 are interrelated in the interfaces between the head group to form CTA+ –Cu(OH)4− 6 pair. The flower-like morphology was observed (Fig. 3(d)) in the presence of CTAB. CTAB has critically influenced the morphology of CuO particle, CuO nuclei slowly grew along the surfactant molecules, resulting in the formation of flower-like structure of the material [21,22]. 3.2.1. Energy dispersive X-ray (EDX) analysis EDX spectra of uncapped CuO and different surfactant capped CuO nanoparticles were shown in Fig. 4((a)–(d)). EDX spectra results indicated the presence of Cu and O. The elemental composition of pure CuO nanoparticles was clearly obtained for all the samples. The atomic ratio of copper and oxygen obtained from the EDX spectra are 43:57, 51:49, 53:47 and 48:52 for uncapped and SDS, PEG and CTAB capped samples, respectively, which could be assigned to the CuO phase. 3.3. Chemical composition analysis Fourier transform-infrared spectroscopy in transmission mode was used to determine the existence of organic species in the final products, as depicted in Fig. 5((a)–(d)). In Fig. 5 the inset arrows, indicated the peak positions for Cu–O vibrational modes. All the as synthesized samples exhibit peaks between 432 cm−1 and 606 cm−1 which corresponds to the characteristic stretching vibrations of Cu–O bond in the monoclinic crystal structure of CuO. The peaks positioned at around 606 cm−1 and 525 cm−1 observed due to Cu–O stretching along the (−2 0 2) direction and 432 cm−1 from Cu–O stretching along the (2 0 2) direction [1,6]. The band at 3400 cm−1 corresponds to the stretching and bending modes of hydroxyls of adsorbed water. The absorption peaks observed around 2352.5 cm−1 indicated the existence of CO2 molecules. Moreover, no active bands from Cu2 O or impurities were observed.

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Fig. 4. EDX spectra of (a) uncapped CuO nanoparticles, (b) SDS capped CuO nanocube, (c) PEG capped CuO nanoleaf, (d) CTAB capped CuO nanoflower.

Fig. 5. FT-IR spectra of (a) uncapped CuO nanoparticles, (b) SDS capped CuO nanocube, (c) PEG capped CuO nanoleaf, (d) CTAB capped CuO nanoflower.

The IR spectra revealed that the as synthesized CuO samples were of pure monoclinic crystal structure. 3.4. Raman spectroscopy analysis Raman spectroscopy has been widely used as a sensitive probe to investigate the micro/nanostructure, namely the local atomic arrangement and vibration, of the nano-sized materials [24]. CuO has a monoclinic structure with space group symmetry of C6 2h the present study was performed to confirm the crystal structure of CuO nanostructures by Raman investigation. The primitive cell contains nine zone-center optical phonon modes with symmetries  RA = 4Au + 5Bu + Ag + 2Bg . There are three acoustic modes (Au + 2Bu ), six infrared active modes (3Au + 3Bg ), and three Raman active modes (Ag + 2Bg ) [25]. The Raman spectra of the various CuO nanostructures were shown in Fig. 6((a)–(d)), based on the reported zone-center optical phonon frequencies in CuO [26]. The vibrational

Fig. 6. Raman spectra of the various CuO nanostructures (a) uncapped CuO nanoparticles, (b) SDS capped CuO nanocube, (c) PEG capped CuO nanoleaf, (d) CTAB capped CuO nanoflower.

spectra of uncapped CuO and capped CuO nanoparticles can assign the peak at 279–289 cm−1 for the Ag and the peaks at 336–330 cm−1 and 620–616 cm−1 to the Bg modes. The intensity of Raman spectra is related to the grain size, the stronger and sharper Raman peaks were observed. The broadening and downshifts of the Raman peaks are mainly attributed to the quantum confinement effect of CuO nanostructures or depicting the existence of a large number of defects (such as the formation of oxygen vacancies and interstitial defect states) in the lattice [27]. There is no vibrational mode

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Fig. 7. UV–visible diffuse reflectance spectra along with the plots of [F(R)·hv]0.5 vs. energy (Eg = hv) of (a) uncapped CuO nanoparticles, (b) SDS capped CuO nanocube, (c) PEG capped CuO nanoleaf, (d) CTAB capped CuO nanoflower.

Fig. 8. Photoluminescence spectra of (a) uncapped CuO nanoparticles, (b) SDS capped CuO nanocube, (c) PEG capped CuO nanoleaf and (d) CTAB capped CuO nanoflower.

belonging to any secondary phase and hence the CuO nanoparticles are in the pure monoclinic structure, which is consistent with the results of the XRD, and capping does not appear the new peak is observed. Among them, the CTAB assisted sample exhibits the enhanced intensity of broad peak, which can be ascribed to the smaller crystallite size.

3.5. Optical analysis of various CuO nanostructures 3.5.1. UV–visible spectroscopy analysis UV–visible analysis in diffuse reflectance (DR) mode was used to investigate the optical properties of various CuO nanostructures in the range of 400-1300 nm as shown in Fig. 7((a)–(d)). In order to

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resolve the excitonic or interband (valence conduction band) transition, which allows to calculate the band gap (Eg ) energies were estimated from the plot of the Kubelka-Munk remission function (converted from the diffuse reflection values) vs. energy (in eV) spectra. F (R) =

(1 − R)2 2R

(5)

here (R) is the absolute value of reflectance and F(R) is equivalent to the absorption coefficient. The indirect band gap of CuO was estimated by plotting [F(R)·hv]0.5 vs. the energy is presented [subset in Fig. 7((a)–(d))]. The linear part of the Tauc curve was extrapolated to [F(R)·hv]0.5 = 0 to get the indirect band gap energy. The band gap energy was calculated to be around 1.40 eV for the prepared uncapped CuO nanoparticles, which is greater than its bulk value of 1.2 eV [1] due to the quantum confinement effect exerted by the nano dimensional CuO. The band gap energy (Eg ) value obtained here is in good agreement with our previous report [2]. The CuO crystals with different morphologies exhibit different sensitivities to visible light. The corresponding band gaps of the SDS, PEG, CTAB capped CuO nanoparticles were estimated to be 1.45 eV, 1.46 eV and 1.54 eV, respectively. All these values were apparently greater than the obtained value of Eg for uncapped CuO (Eg = 1.40 eV). The cupric oxide crystals with different morphologies exhibit different sensitivities to visible light. The uncapped CuO crystal exhibits the strongest absorbency, the CTAB capped flower-shaped CuO crystal has the weakest one, strong visible light absorption thus the former samples are ideal solar selective absorbers. 3.5.2. Photoluminescence (PL) spectroscopy analysis The PL study is a powerful tool to investigate the optical properties of copper oxide nanoparticles. The room temperature PL spectra of various CuO nanostructures were shown in Fig. 8((a)–(d)). The PL spectra showed a broad emission band centered at 375–377 nm and visible emission peak in the violet region (400–402 nm), a small hump in blue regions 435–441 nm and green emission peak at 541–547 nm in all synthesized samples. The peak at 375–377 nm is ascribed to the UV emission of near band-edge (NBE) and originated because of electron–hole pair recombination in free-excitons [2]. The manifestation of sharp and broad shouldered UV peak in the PL spectra shows that the as synthesized samples have well crystallization worth with outstanding features of optical properties. The luminescence bands in the violet–blue region (400–402 nm) are caused by transition vacancy of oxygen and interstitial oxygen [24]. The PL peak at 541–547 nm corresponds to green emission arises from the singly ionized oxygen vacancy. PL spectra for capped CuO nanoparticles are observed low intensity emission peak in green region. All emission bands shifts shorter wavelength in comparison with that of the reported value for CuO nanostructures [24–26], which could arise from the quantum confinement effect. 4. Conclusion In conclusion, CuO nanocrystals and their nanoarchitectures with various morphologies were successfully synthesized via an anionic, cationic and nonionic surfactant assisted wet chemical process. The introduction of various types of surfactants generated new morphologies such as cube, leaf, flower-like CuO nanostructures with minimum agglomeration. The structural analysis clearly indicated the monoclinic phase and single crystalline nature was obtained in all CuO samples. The strain associated with the CuO samples due to lattice deformation was estimated by Williamson-Hall analysis. The average crystallite size for SDS, PEG and CTAB assisted CuO samples were around 20 nm, 19 nm and 16 nm, respectively. The FT-IR and Raman spectra confirmed

the single-phase formation of CuO monoclinic structure in all samples. Diffuse reflectance results illustrated that the addition of SDS, PEG and CTAB significantly enhanced the optical band gap energies ∼1.40 eV, 1.45 eV, 1.46 eV, and 1.54 eV, respectively, due to quantum confinement effect. Photoluminescence spectra showed both UV as well as visible emissions. All the samples exhibit strong visible light absorption, which might be very interesting for further applications in energy storage as well as in solar cell devices due to their excellent visible light absorption properties.

Acknowledgments The authors would like to acknowledge Director, Maulana Azad National Institute of Technology, Bhopal for giving internal facilities to carry out this research work. Authors are indebted to the Director, UGC-DAE-Consortium for Scientific Research, Indore centre (M.P.), India for performing X-ray diffraction, Raman, FT-IR and UV–visible spectroscopy measurements. The authors are also grateful to Dr. Mukul Gupta and Dr. U.P. Deshpandey (scientist, UGC-DAE-CSR, Indore) for their valuable suggestions during XRD and UV–visible Measurements. Ms. H. Siddiqui deeply acknowledges University Grants Commission (UGC), New Delhi, India for the financial support given in the form of Maulana Azad National Fellowship (F.N.2011/MANF-MUS-MAD-4694).

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