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Microscopic investigations of Cu2O nanostructures
Journal of Alloys and Compounds 557 (2013) 152–159
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Microscopic investigations of Cu2O nanostructures Poonam Sharma ⇑, Shatendra K. Sharma University Science Instrumentation Centre, Jawaharlal Nehru University, New Delhi 110 067, India
a r t i c l e
i n f o
Article history: Received 27 August 2012 Received in revised form 13 December 2012 Accepted 18 December 2012 Available online 26 December 2012 Keywords: Cu2O Nanostructure Surfactant Electron microscopy
a b s t r a c t The present communication describes the synthesis of 1D and 3D cuprous oxide nanostructures as a function of precursor concentration, surfactant, and reducing agents. All these synthetic parameters influence the morphological variation of Cu2O nanostructures during the growth process. The shape of the surfactant micelles in the aqueous solution plays a decisive role in controlling the morphological growth of final products. The Cu2O nanostructures were investigated employing various analytical techniques such as powder X-ray diffraction, electron microscopy, energy dispersive spectroscopy, energy mapping, and UV/visible spectroscopy for their optical and morphological properties along with crystal structure phase purity. The present investigations aim to take an account of the effect of such parametric changes along with the structural changes of the nanoparticles that are discussed in detail. A tentative model for the mechanisms involved therein is also proposed for the better understanding of the morphological growth behavior. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction One-dimensional (1D) nanostructures, including nano-wires, nano-rods, quantum wires, are considered as an important building block because of their potential applications in nanoelectronics and opto-electronic devices. These structures with high aspect ratio (i.e. size confinement in two coordinates) offer better crystallinity, higher integration density, and also demonstrate high sensitivity to the surface processes due to their large surface-tovolume ratio and a comparable Debye length [1]. In this view, it’s a challenge to control not only the size, but also the shape and the morphology of the nanoparticles. Various approaches are best described for the synthesis of Cu2O nanostructures with variable shapes, such as wires [2], cages [3], cubes [4], whiskers [5], spheres [6], polyhedrons [7], star- and flower-like [8] structures. Among these morphologies, most widely used method for the synthesis of 1D and 3D nanostructures is physical and chemical method assisted growth through an appropriate porous ‘hard’ and versatile ‘soft’ templates such as surfactants. Cetyl trimethylammonium bromide (CTAB) is a cationic surfactant which aggregated to form micelles or vesicles or condense to bind in some ordered structures of complex morphologies through the electrostatic interactions [9]. Thus, CTAB is widely used to fabricate nanostructures with different morphologies as it exists in hexagonal, cubic, and lamellar structure [10] depending on the fabrication conditions. Selenium nano-tubes [11], ZnO nano-rods [12], Au nano-rods [13], dandelion
⇑ Corresponding author. Tel./fax: +91 99880 46328. E-mail address: [email protected] (P. Sharma). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.12.082
type hydroxyapatite nano-structure [14] have been synthesized employing CTAB as a soft template. Cu2O is a well recognized reddish color p-type semiconducting material, having direct band-gap of 2.172 eV at the centre of brillouin zone [15], for the study of excitonic transitions and luminescence in semiconductors. Now days, researchers show interest in Cu2O nanostructures due to the following reasons: (a) Excitonic excitations can be localized in the confined space of a nanoparticle that can be transformed back into light without diffraction losses. This idea of light was suggested by Snoke [16]. (b) Cu2O possesses several characteristics, which make this crystal as an ideal case for the study of a high-density free exciton gas [15]. (c) Cu2O tends to be more stable at nano-scale regime as decrease in the particle size increases the ionic nature of the material, which in turn makes the lattice less directional and thus, the higher symmetry phase become more stable [17]. Nanowhiskers are 1D nanostructure [18] which plays an important role in the fundamental research as well in the practical applications, such as light-emitting devices with extremely low power consumption. 1D and 3D-nanostructures of Cu2O have been synthesized using various approaches, such as electrochemical deposition [19], electro deposition [20], liquid deposition [18], solution phase [4], radiolytic [21], and template growth [3]. Ruo et al. [22] prepared hollow and porous morphology of the nanostructure by adjusting the concentration of cationic surfactant, CTAB, in the
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reaction solution. Guo and Murphy [23] reported the solutionphase synthesis of cubical Cu2O having an average edge length of 400 nm employing cetyl trimethylammonium bromide as soft template. In the present communication, the parameters responsible for the morphological alterations of various Cu2O nanostructures in the presence and absence of surfactant CTAB have been discussed. The interfacial interactions between nuclei and different shapes of the surfactant micelles provide some control over the structuring during the final growth process. The tentative mechanism involved during the growth of various morphologies of the product is proposed in detail.
½CuðOHÞ4 2 $ CuðOHÞ2 þ 2OH
ð3Þ
½CuðOHÞ4 2 þ N2 H4 ! Cuþ þ N2 " þ4H2 O
ð4Þ 2
It is documented that the intermediate complex [Cu(OH)4] obtained before the adding of hydrazine in the suspension, intended to initiate the back reaction process due to prolong reaction time which results in CuO precipitation in the colloidal solution.
CuðOHÞ2 ! CuO þ H2 O
ð5Þ
On the other hand, reduction via D-glucose results in the brick red color precipitation of Cu2O nanostructure. The redox reaction involved is given as:
½CuðOHÞ4 2 þ CH2 OH ðCHOHÞ4 CHO $ Cuþ þ CH2 OH ðCHOHÞ4 COOH þ 4H2 O
2. Methodologies 2.1. Growth methods All the chemicals utilized during the synthesis process and the characterizations are of analytical grade. Six sets of samples are prepared under various synthetic conditions, as shown in Table 1. Copper (II) chloride is used as a base material. In the typical synthesis process, 10 ml of 2.93 M precursor is dissolved in the aqueous media using magnetic stirrer until the transparent solution observed. 10 ml of 13.75 M NaOH solution is added drop-wise to the above stirring solution followed by the addition of 10 ml of 2.74 M CTAB micelles suspension. A deep blue color colloidal of Cu(OH)2 is soon produced with the continuous stirring for another 30 min. After that, 5 ml of 2 M N2H4 solution is added at the rate of 0.1 ml/s into the above stirring solution and the change in color from deep blue to yellow is clearly observed. On continuing stirring for another 1 h, the color of the colloidal solution further changes to orange and then to red. The red color precipitates of Cu2O are washed, filtered and dried in vacuum for 24 h at room temperature. The prepared sample is labelled as S1. The other sets of samples (S2–S6) are prepared in the similar manner as per described in the Table 1. All these samples are kept under vacuum till further characterizations. 2.2. Characterizations Estimation of the composition and the phase determination are carried out employing powder X-ray diffractometer as well transmission and scanning electron microscope. The powder X-ray diffraction measurements are performed employing PANAlytical X’Pert Pr o diffractometer, with Cu Ka radiation (k = 0.154187 nm) as an incident radiation, equipped with a secondary pyrolytic graphite monochromator. The step scan covers the angular range of 20–70° in the step size of 0.06°. The scanning electron microscope images of the samples are taken on (ZEISS EVO 40 operating at an accelerating voltages ranging from 10 to 30 kV. The transmission electron microscope images and electron diffraction (ED) pattern are taken on a JEOL – 2100 F transmission electron microscope operating at an acceleration voltage of 200 kV. Single slit low background sample holder is used for the TEM measurements. The room temperature optical absorption of the nanostructures is recorded using Hitachi model U – 2800 PC coupled spectrophotometer within the wavelength range varies from 400 to 550 nm, and the colloidal suspension is collected in the quartz cell. 2.3. Results and discussion In the reaction system, when hydrazine is used as a reducing agent the N2 gas is produced which creates the inert atmosphere within the reaction solution and prevents the further oxidation of Cu2O to CuO. The chemical reactions involved during the growth mechanism of Cu2O nanowhiskers are:
CuCl2 ! Cu2þ þ Cl
ð1Þ
Cu2þ þ 2NaOH þ 2H2 O ! ½CuðOHÞ4 2 þ 2Naþ þ 2Hþ
ð2Þ
ð6Þ
The powder X-ray diffraction scans, shown in Fig. 1, analyze the composition, phase purity and crystal structure of Cu2O nanostructures. All samples exhibits XRD reflection corresponding to h1 1 0i, h1 1 1i, h2 0 0i, h2 2 0i, h3 1 1i and h2 2 2i planes for cubic fcc structure (Space Group: pn3m), in the 2h range of 20–70°, of Cu2O identified from the International Center of Diffraction Data Card reflections [as JCPDS X-ray powder diffraction file 05-667]. The diffractogram consists of characteristically sharp peaks with no significant scattering background throughout range. The lattice parameter (a) for the nano-whiskers, octahedrons and cubes are 0.4267 nm, 0.4271 nm and 0.4264 nm, respectively. The h1 1 1i and h2 2 2i planes demonstrate the most and weak intense reflections within whole diffractogram due to the possible h1 1 1i planar orientation during growth. The diffractograms exhibit the purity of Cu2O. The variation of optical absorbance of S1, S5 and S6 as a function of wavelength are shown in Fig. 2. A broad band feature centered at 482 nm, 464 nm and 476 nm for samples S1, S5, and S6, respectively, is observed in all spectrographs which demonstrated the blue-shift effect compare to bulk Cu2O [570 nm]. The broad band absorption peak behavior is explained on the basis of band-to-band transitions in the nanocrystalline Cu2O. The synthetic parameters of the growth process are summarized in Table 1 which clearly shows that NaOH, surfactant, reaction temperature, and reducing agent plays an efficient role for the morphological alteration of the product. Fig. 3a–e shows the scanning electron micrograph (SEM) of the samples synthesized in the presence and absence of CTAB. All batches of synthesized sample represent different morphologies with minute alteration in the synthetic parameters. The sample S1, Fig. 3a, shows the well-defined whisker morphology of the red color precipitates having an average length of 1.5 lm and diameter of 10 nm. On reducing the molar concentration of NaOH to the half of it original value, the sample S2, Fig. 3b, demonstrates the agglomerated whisker morphology. The dual morphology of the product co-existed in sample S3, Fig. 3c, on reducing the quantity of reducing agent to its half of its original value with same molar concentration. The co-existence of the dual morphology is defended by the fact that the octahedrons grows before the final product shaped itself to whisker morphology. Prolong reaction time reduces the initial octahedron morphology with continuous slow addition of reducing agent to whisker morphology. On careful analysis of micrograph in Fig. 3d, for sample S4, the agglomerated octahedron morphology is observed as the molar concentration of the reducing agent is reduced to half of its original concentrations. A well-define octahedrons having an edge lengths varying from 100 nm to 1 lm are observed for sample S5 as shown in Fig. 3e when no CTAB is used during growth process. On replacing hydrazine to D-glucose no precipitation is initially observed at room temperature for sample S6. While on increasing the reaction temperature slowly at the rate of 5 °C/min up to 90 °C, a well-defined uniform cubical morphology is clearly observed as shown in Fig. 3f. Chemical analyses of the samples have been performed employing EDX spectroscopy technique. The well-cleaned dried final products comprised Cu and O as its main constituents as observed in Fig. 4. X-ray energy mapping is an important analytical technique to investigate the presence of elements under study. Same set of sample is used for the energy mapping on which EDX has been done. The presence of Cu and O atoms in the sample is clearly demonstrated in Fig. 5a. The individual presence of Cu (green) and O (red) atoms in the sample is shown in Fig. 5b and c.
Table 1 Summary of synthetic conditions, concentration of chemical used and corresponding morphology of the samples. Sample no.
CuCl2:CTAB
NaOH
Reducing agent
Temp
Morphology
S1 S2 S3 S4 S5 S6
2.93: 2.93: 2.93: 2.93: 2.93: 2.93:
13.75 M 6.88 M 13.75 M 13.75 M 13.75 M 13.75 M
5 ml (2 M) (N2H4H2O) 5 ml (2 M) (N2H4H2O) 2.5 ml (2 M) (N2H4H2O) 5 ml (1 M) (N2H4H2O) 5 ml (2 M) (N2H4H2O) 5 ml (2 M) (D-glucose)
25 °C 25 °C 25 °C 25 °C 25 °C 80 °C
Whiskers Agglomerate whiskers Whiskers + octahedrons Agglomerate octahedron Octahedrons Cubes
2.74 M 2.74 M 2.74 M 2.74 M 0M 2.74 M
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Fig. 1. PXRD scan of Cu2O nanowhiskers (S1), nanoctahedrons (S5) and nanocubes (S6) prepared in the presence of N2H4H2O and D-glucose at 25 °C and 80 °C, respectively.
Fig. 2. Optical absorption spectra of Cu2O nanowhiskers (S1), nanoctahedrons (S5) and nanocubes (S6) prepared in the presence of N2H4H2O and D-glucose at 25 °C and 80 °C, respectively.
To get the optimum information regarding the morphological growth and the mechanism involved, a detailed investigation on the samples are done employing low-resolution and high-resolution bright field transmission electron microscope and SAED. The typical low-resolution bright field TEM images of samples synthesized under various parametric conditions are shown in Fig. 6a–e and the inset shows the corresponding SAED pattern. A hollow interior with porous morphology is observed in Fig. 6a–c while the Cu nuclei adopted cubical morphology with solid interior in Fig. 6d as reducing agent control the interior growth of products. Fig. 6e shows the ribbon shaped hollow morphology of CuðOHÞ2 4 CTA complex obtained after the inter-catalation of surfactant and intermediate unstable product. This intermediate complex guided the morphology of various products during growth process. The selected area electron diffraction patterns obtained from individual nanostructures, shown in inset of Fig. 6a–d, are in well agreement with observed X-ray diffraction pattern. All the set of samples are having single phase with preferred h1 1 1i orientation. The observed electron diffraction spots can be indexed to pure cubic Cu2O structure. The weak diffraction spots are observed in the electron diffraction pattern which is defended due to the presence of small crystallites
absorbed on the surface that are not incorporated into the nanostructures. The present morphological results differ in contrast to the previous observations in the presence of CTAB [6,23]. Further, Fig. 6d, a remarkable morphological change of the product is observed on replacing hydrazine with D-glucose. The hollow morphology diminishes which prevents the penetration of electron beam inside the grain due to the increased thickness of the walls. This might be explained on the basis of layerby-layer growth arrangement of ribbon shaped Cu(OH)2 intermediate complex along some preferred direction, results into solidified cubical morphology. Certain double diffraction spots are also observed in the electron diffraction pattern due to the solid nature of the product. The morphology and crystal structure of Cu2O nanoparticles are investigated employing high resolution bright field transmission electron micrographs as shown in Fig. 7a–c. Fig. 7a shows the high resolution electron micrograph of nanowhiskers projecting along h1 1 0i direction. The arrow indicates the growth projection direction of the whiskers. The presence of Moire fringes, Fig. 7b, originated due to the superposition of small particles onto each other with small angle of inclinations in sample S3 confirms that the nanoparticles are highly orientated along some direction. Fig. 7c represents the hollow nature of sample S5 even though in the absence of surfactant. The small particles grows into cubical morphology that projected along h1 0 0i and h1 0 1i growth directions. The tentative mechanisms involved during the morphological growth of nanostructures under various synthetic conditions are schematically shown in Fig. 8. The mechanisms are explicitly explained on the basis of Ostwald ripening and oriented attachments as step 1, 2 and 3. In step 1, the Cu2+ species bonded with OH ions present within the reaction solution, provided by the dissociation of NaOH and results in the blue colloidal suspension of Cu(OH)2 having small needles like structure [18]. On the addition of CTAB, the needles joined together by electrostatic interaction to give flat ribbon shaped morphology as clearly shown in Fig. 6e. Initially, Cu2+ species are dispersed as small nucleates within the suspension which have high affinity for network like growth on bonding with other species present in the reaction solution. The abundance of OH species within the reaction solution gives rise to the formation of intermediate [Cu(OH)4]2 anion. The Cu2+ nuclei’s further undergoes crystallization process via Ostwald ripening which resulted to needle like morphology. Surfactant CTAB induces the phase separation on addition of inorganic salt and forms micelles in the reaction system. The shape and structure of micelles in the reaction solution provides the control over the morphology of the product during growth. CTAB micelles existed in lamellar structure at room temperature which allows the intermediate to takes onto needle like morphology along particular growth direction. The interface between micelles and their respective counter ions on certain crystallographic facets greatly affects the morphology of nanoparticles. These cationic micelles react electrostatically with [Cu(OH)4]2 an ions and results in the formation of unstable intermediate CuðOHÞ2 4 CTA complex. The [Cu(OH)4]2 anions adsorb preferentially on the surface of cationic micelles to give ribbon shaped morphology. This needle shape morphology of CuðOHÞ2 4 CTA complex starts diminishing as the reducing agent is added and octahedral morphology observed by the process of orientated attachment. Further on increasing the amount of reducing agent, the octahedral morphology changes
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Fig. 3. SEM images of samples prepared at different synthetic conditions (a) S1, (b) S2, (c) S3, (d) S4, (e) S5, and (f) S6.
to whisker shaped structures. The morphology of product is mainly depending on the presence of various chemical species within the reaction solution and the thermodynamic stability of the nanoparticles in the crystalline domain [24]. The possible mechanism for the synthesis of nanocubes is shown in Fig. 8 as step 2. This follows the similar mechanism up to the formation of CuðOHÞ2 4 CTA complex as this ribbon shaped complex is highly unstable and act as template for the growth of various shapes of final product. After the addition of reducing agent, D-glucose, at room temperature no reaction takes place which unable to make changes in the morphology as well in the product formation. While on increasing the reaction temperature, the electrostatic interactions present between surfactant and inorganic precursor increases which contributed to the polymorphic crystallization for the formation of ordered structure with solidified cubical morphology [25]. The octahedrons morphology of the product is described, step 3, on the basis of Ostwald ripening, i.e. the formation of smaller crystallites is kinetically favoured during the initial agglomeration and larger crystallites are thermodynamically fa-
vored and oriented attachment. This step follows same process as in step 1 and 2 as [Cu(OH)4]2 anions act as basic intermediate template for the shape orientated growth of the product without utilizing CTAB. The positive Cu2+ ions co-ordinate loosely with the OH ions initially present in the aqueous media. On the reduction of Cu2+ ions by hydrazine nucleate the tiny Cu+ nanocrystallites to provide a direct site for the growth of hollow and porous structures through planer growth mechanism.
2.3.1. Effect of cetryl methylammonium bromide (CTAB) surfactant Surfactant plays most important role in guiding the morphology of products. It has been observed that when no CTAB is used in the reaction system, the final product takes onto hollow and porous 3D morphology by the process of oriented attachment of needle shaped morphology of intermediate tamplate. On using CTAB and D-glucose as reducing agent, the morphology of the product changes to solidified 3D structure as the thickness of the walls of cube increases. The total number of
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Fig. 4. Typical EDS pattern of Cu2O nanowhiskers (S1) prepared in the presence of N2H4H2O.
Fig. 5. Energy mapping of Cu2O nanowhiskers (S1) prepared in the presence of N2H4H2O. Red and green color shows the presence of Cu and O, respectively(For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).
charge compensating cations decreases, when the concentration of [Cu(OH)4]2 anions is less in the reaction solution. Thus, the optimal value of the surfactant head group increases so that the system finds minimum energy configuration by adopting the 3D morphology.
the agglomerated whiskers morphology is observed (Fig. 3b). The existence of the same morphology of the ½CuðOHÞ4 2 species is attributed to the fact that the agglomeration increases in latter case due to the availability of fewer OH ions in the reaction solution.
2.3.2. Effect of NaOH It is found that the concentration of NaOH in the reaction solution plays an influential role throughout the shape-orientated growth process. When the alkaline concentration is high [Cu2+/OH = 1:5], the final product has well-defined whiskers (Fig. 3a) morphology. The concentration of OH ions remains more in the solution as OH ions are provided by the dissociation of NaOH as well by the reaction itself as whole reaction is carried out under aqueous medium. On the other hand when alkaline concentration reduces to half of its original value i.e. [Cu2+/OH = 1:2.5],
2.3.3. Effect of reducing agents In the present case the reducing strength of the hydrazine and D-glucose plays an effective role during morphology finalization of final products. Hydrazine is very strong reducing agent compare to D-glucose. The use of hydrazine as reducing agent resulting in the growth of whisker and octahedron structures (Fig. 3c) while D-glucose represents the well-defined solidified cubical structure (Fig. 3e). To investigate the detailed effect of the reducing agent, the high and low resolution electron microscopy analysis has been done. When 5 ml of 2 M N2H4 is used in sample S1,
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Fig. 6. Transmission electron micrograph of sample synthesize at different conditions (a) S1, (b) S3, (c) S5, (d) S6, and (e) intermediate Cu(OH)2 (inset shows the SAED pattern).
a well-defined whiskers are observed but on reducing the quantity of N2H4 to half of its original amount, the dual morphology i.e. whiskers as well octahedron, co-existed (Fig. 3c). Whereas, when the quantity of N2H4 remains same (5 ml) but the molar concentration reduces to half of its original value then the agglomerated octahedrons morphology are observed, as shown in Fig. 3d. D-glucose is a very weak reducing agent. The weak strength of the D-glucose is enhanced by increasing the reaction temperature. As the reaction temperature increases at the heating rate of 5 °C/min to 80 °C the red color, the solid and uniform cubical morphology of Cu2O nanostructure have been observed. This confirms that the change in the morphology of the product is strongly concentration and quantity dependent. Also, hydrazine gives rise to hollow morphology of the product while D-glucose increases the solid nature of the product. This hollowness is documented on the basis of enhances reducing strength of hydrazine compare to D-glucose.
3. Conclusions Cu2O nanostructures were successfully synthesized using wetchemical reduction route method. The mechanism involved during
the synthesis of Cu2O nanostructures is examined on the basis of reaction temperature, precursor concentrations, surfactant micelles, and reducing agents. Temperature and precursor/CTAB mixing ratio effectively determine the rate of the growth of crystal. An optical absorption study shows the blue shift effect compared to the bulk Cu2O [570 nm]. The colloidal solution gets precipitated into various morphologies depending upon different reaction conditions. We have found that the nanoparticles shaping is not only depend on the surfactant CTAB, but also, on the concentration of reducing agents under study. The interfacial interactions between the crystallite ions and the surfactant micelles present in the reaction solution determine the shapes of the final product. Mass production of Cu2O is achieved by present synthetic technique where we can have control over the shape and size of the product. These synthesized nanostructures can be effectively used for the biomedical and semiconductor applications.
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Fig. 7. High resolution bright field TEM images of sample synthesize at different conditions (a) S1, (b) S3, and (c) S5.
Fig. 8. Schematic representation of Cu2O nanostructures under different synthetic conditions.
Acknowledgements The authors are very thankful to Dr. P. Koshy from NIST, Kerala for providing SEM images. AIF-USIC, JNU, New Delhi for TEM and
XRD measurements and Bhupinder S. Rathor, RBS, Agra for providing Spectrophotometer scans is highly acknowledged. Financial assistance to P S from University Grant Commission, New Delhi, India is gratefully acknowledged.
P. Sharma, S.K. Sharma / Journal of Alloys and Compounds 557 (2013) 152–159
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Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Microscopic investigations of Cu2O nanostructures Poonam Sharma ⇑, Shatendra K. Sharma University Science Instrumentation Centre, Jawaharlal Nehru University, New Delhi 110 067, India
a r t i c l e
i n f o
Article history: Received 27 August 2012 Received in revised form 13 December 2012 Accepted 18 December 2012 Available online 26 December 2012 Keywords: Cu2O Nanostructure Surfactant Electron microscopy
a b s t r a c t The present communication describes the synthesis of 1D and 3D cuprous oxide nanostructures as a function of precursor concentration, surfactant, and reducing agents. All these synthetic parameters influence the morphological variation of Cu2O nanostructures during the growth process. The shape of the surfactant micelles in the aqueous solution plays a decisive role in controlling the morphological growth of final products. The Cu2O nanostructures were investigated employing various analytical techniques such as powder X-ray diffraction, electron microscopy, energy dispersive spectroscopy, energy mapping, and UV/visible spectroscopy for their optical and morphological properties along with crystal structure phase purity. The present investigations aim to take an account of the effect of such parametric changes along with the structural changes of the nanoparticles that are discussed in detail. A tentative model for the mechanisms involved therein is also proposed for the better understanding of the morphological growth behavior. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction One-dimensional (1D) nanostructures, including nano-wires, nano-rods, quantum wires, are considered as an important building block because of their potential applications in nanoelectronics and opto-electronic devices. These structures with high aspect ratio (i.e. size confinement in two coordinates) offer better crystallinity, higher integration density, and also demonstrate high sensitivity to the surface processes due to their large surface-tovolume ratio and a comparable Debye length [1]. In this view, it’s a challenge to control not only the size, but also the shape and the morphology of the nanoparticles. Various approaches are best described for the synthesis of Cu2O nanostructures with variable shapes, such as wires [2], cages [3], cubes [4], whiskers [5], spheres [6], polyhedrons [7], star- and flower-like [8] structures. Among these morphologies, most widely used method for the synthesis of 1D and 3D nanostructures is physical and chemical method assisted growth through an appropriate porous ‘hard’ and versatile ‘soft’ templates such as surfactants. Cetyl trimethylammonium bromide (CTAB) is a cationic surfactant which aggregated to form micelles or vesicles or condense to bind in some ordered structures of complex morphologies through the electrostatic interactions [9]. Thus, CTAB is widely used to fabricate nanostructures with different morphologies as it exists in hexagonal, cubic, and lamellar structure [10] depending on the fabrication conditions. Selenium nano-tubes [11], ZnO nano-rods [12], Au nano-rods [13], dandelion
⇑ Corresponding author. Tel./fax: +91 99880 46328. E-mail address: [email protected] (P. Sharma). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.12.082
type hydroxyapatite nano-structure [14] have been synthesized employing CTAB as a soft template. Cu2O is a well recognized reddish color p-type semiconducting material, having direct band-gap of 2.172 eV at the centre of brillouin zone [15], for the study of excitonic transitions and luminescence in semiconductors. Now days, researchers show interest in Cu2O nanostructures due to the following reasons: (a) Excitonic excitations can be localized in the confined space of a nanoparticle that can be transformed back into light without diffraction losses. This idea of light was suggested by Snoke [16]. (b) Cu2O possesses several characteristics, which make this crystal as an ideal case for the study of a high-density free exciton gas [15]. (c) Cu2O tends to be more stable at nano-scale regime as decrease in the particle size increases the ionic nature of the material, which in turn makes the lattice less directional and thus, the higher symmetry phase become more stable [17]. Nanowhiskers are 1D nanostructure [18] which plays an important role in the fundamental research as well in the practical applications, such as light-emitting devices with extremely low power consumption. 1D and 3D-nanostructures of Cu2O have been synthesized using various approaches, such as electrochemical deposition [19], electro deposition [20], liquid deposition [18], solution phase [4], radiolytic [21], and template growth [3]. Ruo et al. [22] prepared hollow and porous morphology of the nanostructure by adjusting the concentration of cationic surfactant, CTAB, in the
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reaction solution. Guo and Murphy [23] reported the solutionphase synthesis of cubical Cu2O having an average edge length of 400 nm employing cetyl trimethylammonium bromide as soft template. In the present communication, the parameters responsible for the morphological alterations of various Cu2O nanostructures in the presence and absence of surfactant CTAB have been discussed. The interfacial interactions between nuclei and different shapes of the surfactant micelles provide some control over the structuring during the final growth process. The tentative mechanism involved during the growth of various morphologies of the product is proposed in detail.
½CuðOHÞ4 2 $ CuðOHÞ2 þ 2OH
ð3Þ
½CuðOHÞ4 2 þ N2 H4 ! Cuþ þ N2 " þ4H2 O
ð4Þ 2
It is documented that the intermediate complex [Cu(OH)4] obtained before the adding of hydrazine in the suspension, intended to initiate the back reaction process due to prolong reaction time which results in CuO precipitation in the colloidal solution.
CuðOHÞ2 ! CuO þ H2 O
ð5Þ
On the other hand, reduction via D-glucose results in the brick red color precipitation of Cu2O nanostructure. The redox reaction involved is given as:
½CuðOHÞ4 2 þ CH2 OH ðCHOHÞ4 CHO $ Cuþ þ CH2 OH ðCHOHÞ4 COOH þ 4H2 O
2. Methodologies 2.1. Growth methods All the chemicals utilized during the synthesis process and the characterizations are of analytical grade. Six sets of samples are prepared under various synthetic conditions, as shown in Table 1. Copper (II) chloride is used as a base material. In the typical synthesis process, 10 ml of 2.93 M precursor is dissolved in the aqueous media using magnetic stirrer until the transparent solution observed. 10 ml of 13.75 M NaOH solution is added drop-wise to the above stirring solution followed by the addition of 10 ml of 2.74 M CTAB micelles suspension. A deep blue color colloidal of Cu(OH)2 is soon produced with the continuous stirring for another 30 min. After that, 5 ml of 2 M N2H4 solution is added at the rate of 0.1 ml/s into the above stirring solution and the change in color from deep blue to yellow is clearly observed. On continuing stirring for another 1 h, the color of the colloidal solution further changes to orange and then to red. The red color precipitates of Cu2O are washed, filtered and dried in vacuum for 24 h at room temperature. The prepared sample is labelled as S1. The other sets of samples (S2–S6) are prepared in the similar manner as per described in the Table 1. All these samples are kept under vacuum till further characterizations. 2.2. Characterizations Estimation of the composition and the phase determination are carried out employing powder X-ray diffractometer as well transmission and scanning electron microscope. The powder X-ray diffraction measurements are performed employing PANAlytical X’Pert Pr o diffractometer, with Cu Ka radiation (k = 0.154187 nm) as an incident radiation, equipped with a secondary pyrolytic graphite monochromator. The step scan covers the angular range of 20–70° in the step size of 0.06°. The scanning electron microscope images of the samples are taken on (ZEISS EVO 40 operating at an accelerating voltages ranging from 10 to 30 kV. The transmission electron microscope images and electron diffraction (ED) pattern are taken on a JEOL – 2100 F transmission electron microscope operating at an acceleration voltage of 200 kV. Single slit low background sample holder is used for the TEM measurements. The room temperature optical absorption of the nanostructures is recorded using Hitachi model U – 2800 PC coupled spectrophotometer within the wavelength range varies from 400 to 550 nm, and the colloidal suspension is collected in the quartz cell. 2.3. Results and discussion In the reaction system, when hydrazine is used as a reducing agent the N2 gas is produced which creates the inert atmosphere within the reaction solution and prevents the further oxidation of Cu2O to CuO. The chemical reactions involved during the growth mechanism of Cu2O nanowhiskers are:
CuCl2 ! Cu2þ þ Cl
ð1Þ
Cu2þ þ 2NaOH þ 2H2 O ! ½CuðOHÞ4 2 þ 2Naþ þ 2Hþ
ð2Þ
ð6Þ
The powder X-ray diffraction scans, shown in Fig. 1, analyze the composition, phase purity and crystal structure of Cu2O nanostructures. All samples exhibits XRD reflection corresponding to h1 1 0i, h1 1 1i, h2 0 0i, h2 2 0i, h3 1 1i and h2 2 2i planes for cubic fcc structure (Space Group: pn3m), in the 2h range of 20–70°, of Cu2O identified from the International Center of Diffraction Data Card reflections [as JCPDS X-ray powder diffraction file 05-667]. The diffractogram consists of characteristically sharp peaks with no significant scattering background throughout range. The lattice parameter (a) for the nano-whiskers, octahedrons and cubes are 0.4267 nm, 0.4271 nm and 0.4264 nm, respectively. The h1 1 1i and h2 2 2i planes demonstrate the most and weak intense reflections within whole diffractogram due to the possible h1 1 1i planar orientation during growth. The diffractograms exhibit the purity of Cu2O. The variation of optical absorbance of S1, S5 and S6 as a function of wavelength are shown in Fig. 2. A broad band feature centered at 482 nm, 464 nm and 476 nm for samples S1, S5, and S6, respectively, is observed in all spectrographs which demonstrated the blue-shift effect compare to bulk Cu2O [570 nm]. The broad band absorption peak behavior is explained on the basis of band-to-band transitions in the nanocrystalline Cu2O. The synthetic parameters of the growth process are summarized in Table 1 which clearly shows that NaOH, surfactant, reaction temperature, and reducing agent plays an efficient role for the morphological alteration of the product. Fig. 3a–e shows the scanning electron micrograph (SEM) of the samples synthesized in the presence and absence of CTAB. All batches of synthesized sample represent different morphologies with minute alteration in the synthetic parameters. The sample S1, Fig. 3a, shows the well-defined whisker morphology of the red color precipitates having an average length of 1.5 lm and diameter of 10 nm. On reducing the molar concentration of NaOH to the half of it original value, the sample S2, Fig. 3b, demonstrates the agglomerated whisker morphology. The dual morphology of the product co-existed in sample S3, Fig. 3c, on reducing the quantity of reducing agent to its half of its original value with same molar concentration. The co-existence of the dual morphology is defended by the fact that the octahedrons grows before the final product shaped itself to whisker morphology. Prolong reaction time reduces the initial octahedron morphology with continuous slow addition of reducing agent to whisker morphology. On careful analysis of micrograph in Fig. 3d, for sample S4, the agglomerated octahedron morphology is observed as the molar concentration of the reducing agent is reduced to half of its original concentrations. A well-define octahedrons having an edge lengths varying from 100 nm to 1 lm are observed for sample S5 as shown in Fig. 3e when no CTAB is used during growth process. On replacing hydrazine to D-glucose no precipitation is initially observed at room temperature for sample S6. While on increasing the reaction temperature slowly at the rate of 5 °C/min up to 90 °C, a well-defined uniform cubical morphology is clearly observed as shown in Fig. 3f. Chemical analyses of the samples have been performed employing EDX spectroscopy technique. The well-cleaned dried final products comprised Cu and O as its main constituents as observed in Fig. 4. X-ray energy mapping is an important analytical technique to investigate the presence of elements under study. Same set of sample is used for the energy mapping on which EDX has been done. The presence of Cu and O atoms in the sample is clearly demonstrated in Fig. 5a. The individual presence of Cu (green) and O (red) atoms in the sample is shown in Fig. 5b and c.
Table 1 Summary of synthetic conditions, concentration of chemical used and corresponding morphology of the samples. Sample no.
CuCl2:CTAB
NaOH
Reducing agent
Temp
Morphology
S1 S2 S3 S4 S5 S6
2.93: 2.93: 2.93: 2.93: 2.93: 2.93:
13.75 M 6.88 M 13.75 M 13.75 M 13.75 M 13.75 M
5 ml (2 M) (N2H4H2O) 5 ml (2 M) (N2H4H2O) 2.5 ml (2 M) (N2H4H2O) 5 ml (1 M) (N2H4H2O) 5 ml (2 M) (N2H4H2O) 5 ml (2 M) (D-glucose)
25 °C 25 °C 25 °C 25 °C 25 °C 80 °C
Whiskers Agglomerate whiskers Whiskers + octahedrons Agglomerate octahedron Octahedrons Cubes
2.74 M 2.74 M 2.74 M 2.74 M 0M 2.74 M
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Fig. 1. PXRD scan of Cu2O nanowhiskers (S1), nanoctahedrons (S5) and nanocubes (S6) prepared in the presence of N2H4H2O and D-glucose at 25 °C and 80 °C, respectively.
Fig. 2. Optical absorption spectra of Cu2O nanowhiskers (S1), nanoctahedrons (S5) and nanocubes (S6) prepared in the presence of N2H4H2O and D-glucose at 25 °C and 80 °C, respectively.
To get the optimum information regarding the morphological growth and the mechanism involved, a detailed investigation on the samples are done employing low-resolution and high-resolution bright field transmission electron microscope and SAED. The typical low-resolution bright field TEM images of samples synthesized under various parametric conditions are shown in Fig. 6a–e and the inset shows the corresponding SAED pattern. A hollow interior with porous morphology is observed in Fig. 6a–c while the Cu nuclei adopted cubical morphology with solid interior in Fig. 6d as reducing agent control the interior growth of products. Fig. 6e shows the ribbon shaped hollow morphology of CuðOHÞ2 4 CTA complex obtained after the inter-catalation of surfactant and intermediate unstable product. This intermediate complex guided the morphology of various products during growth process. The selected area electron diffraction patterns obtained from individual nanostructures, shown in inset of Fig. 6a–d, are in well agreement with observed X-ray diffraction pattern. All the set of samples are having single phase with preferred h1 1 1i orientation. The observed electron diffraction spots can be indexed to pure cubic Cu2O structure. The weak diffraction spots are observed in the electron diffraction pattern which is defended due to the presence of small crystallites
absorbed on the surface that are not incorporated into the nanostructures. The present morphological results differ in contrast to the previous observations in the presence of CTAB [6,23]. Further, Fig. 6d, a remarkable morphological change of the product is observed on replacing hydrazine with D-glucose. The hollow morphology diminishes which prevents the penetration of electron beam inside the grain due to the increased thickness of the walls. This might be explained on the basis of layerby-layer growth arrangement of ribbon shaped Cu(OH)2 intermediate complex along some preferred direction, results into solidified cubical morphology. Certain double diffraction spots are also observed in the electron diffraction pattern due to the solid nature of the product. The morphology and crystal structure of Cu2O nanoparticles are investigated employing high resolution bright field transmission electron micrographs as shown in Fig. 7a–c. Fig. 7a shows the high resolution electron micrograph of nanowhiskers projecting along h1 1 0i direction. The arrow indicates the growth projection direction of the whiskers. The presence of Moire fringes, Fig. 7b, originated due to the superposition of small particles onto each other with small angle of inclinations in sample S3 confirms that the nanoparticles are highly orientated along some direction. Fig. 7c represents the hollow nature of sample S5 even though in the absence of surfactant. The small particles grows into cubical morphology that projected along h1 0 0i and h1 0 1i growth directions. The tentative mechanisms involved during the morphological growth of nanostructures under various synthetic conditions are schematically shown in Fig. 8. The mechanisms are explicitly explained on the basis of Ostwald ripening and oriented attachments as step 1, 2 and 3. In step 1, the Cu2+ species bonded with OH ions present within the reaction solution, provided by the dissociation of NaOH and results in the blue colloidal suspension of Cu(OH)2 having small needles like structure [18]. On the addition of CTAB, the needles joined together by electrostatic interaction to give flat ribbon shaped morphology as clearly shown in Fig. 6e. Initially, Cu2+ species are dispersed as small nucleates within the suspension which have high affinity for network like growth on bonding with other species present in the reaction solution. The abundance of OH species within the reaction solution gives rise to the formation of intermediate [Cu(OH)4]2 anion. The Cu2+ nuclei’s further undergoes crystallization process via Ostwald ripening which resulted to needle like morphology. Surfactant CTAB induces the phase separation on addition of inorganic salt and forms micelles in the reaction system. The shape and structure of micelles in the reaction solution provides the control over the morphology of the product during growth. CTAB micelles existed in lamellar structure at room temperature which allows the intermediate to takes onto needle like morphology along particular growth direction. The interface between micelles and their respective counter ions on certain crystallographic facets greatly affects the morphology of nanoparticles. These cationic micelles react electrostatically with [Cu(OH)4]2 an ions and results in the formation of unstable intermediate CuðOHÞ2 4 CTA complex. The [Cu(OH)4]2 anions adsorb preferentially on the surface of cationic micelles to give ribbon shaped morphology. This needle shape morphology of CuðOHÞ2 4 CTA complex starts diminishing as the reducing agent is added and octahedral morphology observed by the process of orientated attachment. Further on increasing the amount of reducing agent, the octahedral morphology changes
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Fig. 3. SEM images of samples prepared at different synthetic conditions (a) S1, (b) S2, (c) S3, (d) S4, (e) S5, and (f) S6.
to whisker shaped structures. The morphology of product is mainly depending on the presence of various chemical species within the reaction solution and the thermodynamic stability of the nanoparticles in the crystalline domain [24]. The possible mechanism for the synthesis of nanocubes is shown in Fig. 8 as step 2. This follows the similar mechanism up to the formation of CuðOHÞ2 4 CTA complex as this ribbon shaped complex is highly unstable and act as template for the growth of various shapes of final product. After the addition of reducing agent, D-glucose, at room temperature no reaction takes place which unable to make changes in the morphology as well in the product formation. While on increasing the reaction temperature, the electrostatic interactions present between surfactant and inorganic precursor increases which contributed to the polymorphic crystallization for the formation of ordered structure with solidified cubical morphology [25]. The octahedrons morphology of the product is described, step 3, on the basis of Ostwald ripening, i.e. the formation of smaller crystallites is kinetically favoured during the initial agglomeration and larger crystallites are thermodynamically fa-
vored and oriented attachment. This step follows same process as in step 1 and 2 as [Cu(OH)4]2 anions act as basic intermediate template for the shape orientated growth of the product without utilizing CTAB. The positive Cu2+ ions co-ordinate loosely with the OH ions initially present in the aqueous media. On the reduction of Cu2+ ions by hydrazine nucleate the tiny Cu+ nanocrystallites to provide a direct site for the growth of hollow and porous structures through planer growth mechanism.
2.3.1. Effect of cetryl methylammonium bromide (CTAB) surfactant Surfactant plays most important role in guiding the morphology of products. It has been observed that when no CTAB is used in the reaction system, the final product takes onto hollow and porous 3D morphology by the process of oriented attachment of needle shaped morphology of intermediate tamplate. On using CTAB and D-glucose as reducing agent, the morphology of the product changes to solidified 3D structure as the thickness of the walls of cube increases. The total number of
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Fig. 4. Typical EDS pattern of Cu2O nanowhiskers (S1) prepared in the presence of N2H4H2O.
Fig. 5. Energy mapping of Cu2O nanowhiskers (S1) prepared in the presence of N2H4H2O. Red and green color shows the presence of Cu and O, respectively(For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).
charge compensating cations decreases, when the concentration of [Cu(OH)4]2 anions is less in the reaction solution. Thus, the optimal value of the surfactant head group increases so that the system finds minimum energy configuration by adopting the 3D morphology.
the agglomerated whiskers morphology is observed (Fig. 3b). The existence of the same morphology of the ½CuðOHÞ4 2 species is attributed to the fact that the agglomeration increases in latter case due to the availability of fewer OH ions in the reaction solution.
2.3.2. Effect of NaOH It is found that the concentration of NaOH in the reaction solution plays an influential role throughout the shape-orientated growth process. When the alkaline concentration is high [Cu2+/OH = 1:5], the final product has well-defined whiskers (Fig. 3a) morphology. The concentration of OH ions remains more in the solution as OH ions are provided by the dissociation of NaOH as well by the reaction itself as whole reaction is carried out under aqueous medium. On the other hand when alkaline concentration reduces to half of its original value i.e. [Cu2+/OH = 1:2.5],
2.3.3. Effect of reducing agents In the present case the reducing strength of the hydrazine and D-glucose plays an effective role during morphology finalization of final products. Hydrazine is very strong reducing agent compare to D-glucose. The use of hydrazine as reducing agent resulting in the growth of whisker and octahedron structures (Fig. 3c) while D-glucose represents the well-defined solidified cubical structure (Fig. 3e). To investigate the detailed effect of the reducing agent, the high and low resolution electron microscopy analysis has been done. When 5 ml of 2 M N2H4 is used in sample S1,
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Fig. 6. Transmission electron micrograph of sample synthesize at different conditions (a) S1, (b) S3, (c) S5, (d) S6, and (e) intermediate Cu(OH)2 (inset shows the SAED pattern).
a well-defined whiskers are observed but on reducing the quantity of N2H4 to half of its original amount, the dual morphology i.e. whiskers as well octahedron, co-existed (Fig. 3c). Whereas, when the quantity of N2H4 remains same (5 ml) but the molar concentration reduces to half of its original value then the agglomerated octahedrons morphology are observed, as shown in Fig. 3d. D-glucose is a very weak reducing agent. The weak strength of the D-glucose is enhanced by increasing the reaction temperature. As the reaction temperature increases at the heating rate of 5 °C/min to 80 °C the red color, the solid and uniform cubical morphology of Cu2O nanostructure have been observed. This confirms that the change in the morphology of the product is strongly concentration and quantity dependent. Also, hydrazine gives rise to hollow morphology of the product while D-glucose increases the solid nature of the product. This hollowness is documented on the basis of enhances reducing strength of hydrazine compare to D-glucose.
3. Conclusions Cu2O nanostructures were successfully synthesized using wetchemical reduction route method. The mechanism involved during
the synthesis of Cu2O nanostructures is examined on the basis of reaction temperature, precursor concentrations, surfactant micelles, and reducing agents. Temperature and precursor/CTAB mixing ratio effectively determine the rate of the growth of crystal. An optical absorption study shows the blue shift effect compared to the bulk Cu2O [570 nm]. The colloidal solution gets precipitated into various morphologies depending upon different reaction conditions. We have found that the nanoparticles shaping is not only depend on the surfactant CTAB, but also, on the concentration of reducing agents under study. The interfacial interactions between the crystallite ions and the surfactant micelles present in the reaction solution determine the shapes of the final product. Mass production of Cu2O is achieved by present synthetic technique where we can have control over the shape and size of the product. These synthesized nanostructures can be effectively used for the biomedical and semiconductor applications.
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Fig. 7. High resolution bright field TEM images of sample synthesize at different conditions (a) S1, (b) S3, and (c) S5.
Fig. 8. Schematic representation of Cu2O nanostructures under different synthetic conditions.
Acknowledgements The authors are very thankful to Dr. P. Koshy from NIST, Kerala for providing SEM images. AIF-USIC, JNU, New Delhi for TEM and
XRD measurements and Bhupinder S. Rathor, RBS, Agra for providing Spectrophotometer scans is highly acknowledged. Financial assistance to P S from University Grant Commission, New Delhi, India is gratefully acknowledged.
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