CuO nanoparticles: Novel synthesis by exploding wire technique and extensive characterization

Accepted Manuscript Title: Cu/Cu2 O/CuO Nanoparticles: Novel Synthesis by Exploding Wire Technique and Extensive Characterization Author: Anshuman Sahai Navendu Goswami S.D. Kaushik Shilpa Tripathi PII: DOI: Reference:

S0169-4332(16)31861-X http://dx.doi.org/doi:10.1016/j.apsusc.2016.09.005 APSUSC 33933

To appear in:

APSUSC

Received date: Revised date: Accepted date:

31-5-2016 30-8-2016 3-9-2016

Please cite this article as: Anshuman Sahai, Navendu Goswami, S.D.Kaushik, Shilpa Tripathi, Cu/Cu2O/CuO Nanoparticles: Novel Synthesis by Exploding Wire Technique and Extensive Characterization, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.09.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Cu/Cu2O/CuO Nanoparticles: Novel Synthesis by Exploding Wire Technique and Extensive Characterization Anshuman Sahai1, Navendu Goswami1a), S. D. Kaushik2, Shilpa Tripathi3,4 1

Department of Physics and Materials Science and Engineering, Jaypee Institute of

Information Technology, A-10, Sector-62, Noida-201307, India. 2

UGC-DAE-Consortium for Scientific Research Mumbai Centre, R5 Shed, BARC, Mumbai

400085, India 3

Previous: UGC-DAE Consortium for Scientific Research, Indore (M.P.), India

4

Present Address: Optics and Thin Film Laboratory, Bhabha Atomic Research Centre,

Vishakhapatnam, India (A. P.)- 530012, India a)

Corresponding Author’s email: [email protected], Tel :+91 (120) 2594364,

Fax : +91 (120) 2400986.

1

Graphical Abstract

2

Highlights The salient features of this research article are following:  Mixed phase synthesis of Cu/Cu2O/CuO nanoparticles by novel adaptation of Exploding Wire Technique.  Predominant existence of Cu/Cu2O phases confirmed through XRD and TEM analyses.  Minor phase of CuO was revealed during the analysis of Raman and IR active vibrational modes.  Modified energy levels and optical processes occurring therein manifest from quantum confinement and surface effects of nanoparticles.  XPS analysis provide direct evidences of Cu2+ and Cu+ alongwith O deficiency in lattice structure.  Room temperature weak ferromagnetic behaviour of nanoparticles was established.

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Abstract In this article, we explore potential of Exploding Wire Technique (EWT) to synthesize the copper nanoparticles using the copper metal in a plate and wire geometry. Rietveld refinement of X-ray diffraction (XRD) pattern of prepared material indicates presence of mixed phases of copper (Cu) and copper oxide (Cu2O). Agglomerates of copper and copper oxide comprised of ~20nm average size nanoparticles observed through high resolution transmission electron microscope (HRTEM) and energy dispersive x-ray (EDX) spectroscopy. Micro-Raman (μR) and Fourier transform infrared (FTIR) spectroscopies of prepared nanoparticles reveal existence of additional minority CuO phase, not determined earlier through XRD and TEM analysis. µR investigations vividly reveal cubic Cu2O and monoclinic CuO phases based on the difference of space group symmetries. In good agreement with µRaman analysis, FTIR stretching modes corresponding to Cu2-O and Cu-O were also distinguished. Investigations of µR and FTIR vibrational modes are in accordance and affirm concurrence of CuO phases besides predominant Cu and Cu2O phase. Quantum confinement effects along with increase of band gaps for direct and indirect optical transitions of Cu/Cu2O/CuO nanoparticles are reflected through UV-visible (UV-vis) spectroscopy. Photoluminescence (PL) spectroscopy spots the electronic levels of each phase and optical transitions processes occurring therein. Iterative X-ray photoelectron spectroscopy (XPS) fitting of core level spectra of Cu (2p3/2) and O (1s), divulges presence of Cu2+ and Cu+ in the lattice with an interesting evidence of O deficiency in the lattice structure and surface adsorption. Magnetic analysis illustrates that the prepared nanomaterial demonstrates ferromagnetic behaviour at room temperature.

PACS: 81.07.-b, 78.30.Fs, 61.05.cp, 68.37.Og, 78.30.-j, 78.67.-n, 33.60.+q Keywords: Nanoscale materials and structures: fabrication and characterization, III-V and IIVI semiconductors, x-ray diffraction, High-resolution transmission electron microscopy (HRTEM), Infrared and Raman spectra, Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures, X-ray photoelectron spectra of molecules

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1. Introduction There has been a surge in past decades to find low cost, high yield synthesis methods which lead to more productive applications [1-9]. Among the most investigated semiconducting materials, group III-V and II-VI and other metal oxides semiconductors have attracted attention of large number of researchers [1-9]. Compared with any conventional bulky material, nanomaterial possesses superior or novel physio-chemical properties due to their high aspect ratio and quantum size confinement [10]. Among various nanostructures, oxide nanoparticles of copper and zinc (i.e. CuO, Cu2O, ZnO) have gained extensive interest of scientific community [1, 4, 5, 11, 12]. Among these, copper oxides are preferably utilized for industrial applications in magnetic devices, catalysis and solar cell [13]. Cuprous oxide (Cu2O) and cupric oxide (CuO) are two principal semiconductor phases of copper oxide [10, 13]. Cupric Oxide, possessing a monoclinic crystal structure with indirect band gap (~1.41.85eV) is advantageous since its lower surface potential barrier, as compared to metals and hence, modifies its field emission properties [10, 13]. Cuprous oxide, a p-type semiconductor possessing cubic structure with direct band gap of 2.2eV, is widely applied for solar cell fabrications and catalysis [4, 13]. The electronic structure of Cu2O, with large excitonic binding energy (140meV), allows slow temperature absorption and luminescence [4]. Earlier, it has been observed that small CuO nanoparticles are not stable when synthesized by chemical route [14]. As observed in XPS spectra, the satellite peaks in the high energy side of core and valence levels are peculiar as they provide crucial insight of their electronic structure [13]. The synthesis methods, required to achieve, pure nanoparticles are for example, Metal−Organic Chemical Vapor Deposition, Plasma-Assisted Molecular Beam Epitaxy, electro-deposition followed by a gas-solid reaction, RF sputtering etc. not cost effective for commercial production. [3, 6, 10, 15]. In present work, one of the prime objectives has been to devise a low cost, high yield synthesis route where we employ the potential of relatively less popular synthesis technique namely, Exploding Wire Technique (EWT) or Electro Explosion of Wire (EEW) method, by exploding the copper in wire-plate geometry, in a vessel filled with deionised (DI) water [16]. Pure Cu nanoparticles were obtained by employing the EWT [17]. Since this is an underwater process, it could be interesting to investigate that how only the metal nanoparticles were formed without forming any oxide/oxide layer and/or without using any capping agent. Of late, morphological evolution of ZnO nanostructures prepared adopting EWT method was reported [18]. Through present 5

work, we demonstrate that employing EWT, we obtain copper metal nanoparticles as primary phase along with secondary phases of copper oxide (CuO/Cu2O) nanoparticles and systematically investigated to determine their structural, optical, vibrational, electrical and magnetic properties.

2. Materials and Method Using the wire and plate of pure copper metal, nanoparticles were synthesised by Exploding Wire Technique (EWT) (as shown in Fig. 1 (a) and (b)). The theory behind the formation of nanoparticles through EWT can be found elsewhere [16, 17].

Fig. 1(a) and (b) Cu plate and wire utilized for synthesis of nanoparticles via Exploding Wire Technique (EWT). The vessel designed to carry out synthesis are shown through (c) and (d).

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The particles, as obtained in the form of colloidal suspension and dried nanopowder are shown in (e) and (f), respectively

The vessel designed to carry out synthesis, is shown in Fig. 1 (c) and (d). The nanoparticles, as obtained initially in the form of colloidal suspension and afterwards, dried nanopowder are shown through Fig. 1 (e) and (f) respectively. Copper plate (99.9% pure, Goodfellow, UK) and Copper wire (99.9% pure, Goodfellow, UK with 1.0mm diameter) used for explosion. Fig. 1 (c) and (d) depicts the designed vessel consisting of copper plate (mounted on Teflon holder) and Cu wire (dipped in DI water having resistivity of 18MΩ.cm) are connected respectively to the negative and positive terminals of a 12V battery. Explosion of copper metal due to contacts of wire with copper plate resulted in formation of nanoparticles. Repetition of explosion process produces copious of tiny particles so as to form colloidal suspensions where nanoparticles remains suspended for several weeks (Fig. 1(e)). To obtain the powder from suspension, the colloidal particles were centrifuged for 20 minutes at 20000 rpm. The powder was segregated and dried in ambient environment in the oven at 60°C (Fig. 1(f)). However, a portion of colloidal particles was utilized for optical characterizations (Fig. 1(f)). Structural analysis and estimation of size of prepared nanoparticles performed employing powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). For this purpose, X-ray Diffractometer 6000 Shimadzu Analytical, Japan having incident Cu Kα radiation wavelength of 1.54Å and JOEL JEM-2100F HRTEM incorporated with Energy Dispersive X-ray (EDX) spectroscope were employed. The XRD data were acquired at a scan speed of 0.5°/min having the sampling pitch of 0.02°. The further refinement of XRD data was carried out by using FullProf Rietveld Suit Program refinement software. In order to probe inelastic scattering processes in these materials, room temperature micro-Raman spectroscopy was performed employing a Jobin Yvon LabRAM HR 800UV micro-Raman system using the 488 nm line of Ar+ laser as an excitation source. The vibrational modes present in prepared nanomaterials were probed by a Perkin-Elmer BXII FTIR spectrophotometer, Germany. FTIR spectra of nanoparticles were recorded at room temperature, and the particles were embedded in the KBr matrix to examine the vibrational modes in the range of 440-660cm-1.The processes of electronic absorption in synthesized nanoparticles

were

examined

through

Perkin

Elmer

Lambda

35

UV–visible

spectrophotometer. The absorption spectra were recorded for the dispersion of nanoparticles in DI water. Same dispersion was used to acquire photoluminescence excitation and emission 7

spectra using Perkin-Elmer LS55 Fluorescence spectrometer. Here, a Xenon source of high energy pulsed was used for excitation. To attain the PL data with optimum resolution, we fixed slit width of 2.5 nm for both monochromators. For X-ray photoelectron spectra (XPS) measurements, VSW scientific instrument (UK) with twin anode facility was used. For the present samples, Al Kα source (resolution ~0.9 eV) was employed. Before collecting the data, sample was vacuum dried so as to remove extraneous impurities and moisture. Binding energy correction was done by applying graphitic C 1s peak (B.E. 284.5eV) reference, whereas Au sample acted as external reference for resolution. The data fitting for C 1s and O 1s peaks performed using XPS peak 4.1 software. Magnetization study was carried out using a Quantum Design make cryogen free re-liquefier based 9T physical properties measurement system (PPMS), with a vibrating sample magnetometer VSM option.

3. Results and Discussion 3.1 X-ray Diffraction Analysis The XRD θ-2θ scans of copper plate/wire and prepared nanopowder were recorded in the range of 30-80° and shown respectively in Fig. 2 (a) and (b).

8

Fig. 2 (a) XRD pattern of copper plate/wire used for synthesis, (b) Rietveld refinement of synthesized nanoparticles with Bragg positions shown by green ticks (|) respectively for observed and calculated positions.

Fig. 2(a) illustrates that the peak positions of Cu plate/wire (used for explosion) are in good conformity with the pure cubic copper phase (JCPDF No. 04-0836) [19]. This confirms the purity of copper plate/wire (99.9%) used for the synthesis of nanoparticles through EWT. In Fig. 2(b), the Rietveld refinement results for XRD data of prepared nanoparticles are shown. Since the experiment was performed in DI water, formation of mixed phases of Cu, Cu2O and CuO could be anticipated. Among these three phases, Cu and Cu2O possess highly symmetric cubic phase while CuO possess low-symmetric monoclinic structure [14]. Previously, it is reported that during the synthesis of CuO, Cu2O is the intermediate phase formed due to oxidation of Cu nanocrystals [20]. Fitting of the experimental XRD data of the synthesized powder was performed as per the structure model considered for the peak profile refinement employing Rietveld analysis [21]. Deviation between experimental and theoretical patterns is minimized by the least-squares method. After numerous recursive refinements, we finally obtain the possible best refined lattice parameters and following results. For prepared sample, the prominent XRD peaks at 43.31°, 50.45° and 74.12° are indexed as (111), (200) and (220) hkl planes of Cu phase having space group symmetry of Fm-3m. Three starred peaks for copper phases are marked with ‘*’ as to depict the Cu phase in Fig. 2 (b). FCC phase in our case, has lattice constant a=b=c = 3.615Å; α=90°(=β=γ) and cell volume of 47.25Å3. Other peaks positions at 36.43°, 42.31°, 52.47°, 61.38°, 65.56°, 69.59°, 73.53° and 77.39° correspond to (111), (200), (211), (220), (221), (310), (311) and (222) planes, respectively of cubic phase Cu2O marked with ‘#’ with space group Pn-3m (JCPDS card 77-0199) [4, 14, 20]. In general, refinement data (Icalc) is in good agreement with experimental data (Iobs), as Iobs-Icalc (i.e. the difference in mismatch) is less without any variations. Thus, the formation of Cu and Cu2O phase is predominant in prepared material. Furthermore, Williamson-Hall (W-H) plots obtained from Rietveld analysis are shown in Fig. 3.

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Fig 3. Williamson-Hall (W–H) Plots for Cu and Cu2O phases obtained from Rietveld refined XRD parameters

The W-H method is reportedly used to de-convolute the peaks by taking into account the size and strain induced broadening [7, 22-25]. For the prominent diffraction peaks of Cu and Cu2O nanoparticles, the details about the method can be found elsewhere [4, 14, 19-21]. For W-H plots, the βcosθ terms are plotted with respect to 4sinθ. The graphs for W-H plot are shown in Fig. 3. The lattice parameters, refinement factors, quantitative phase analysis of Cu and Cu2O phases and crystallite size calculated from W-H plots are tabulated in Table 1. Table 1.Rietveld refined XRD parameters for Cu and Cu2O phases Phases

Cell parameter

R-factors

Phase Crystallite

Strain

a(=b=c) (Å)

V(Å3)

Rp

Rwp

Re

χ2

(%)

Size (nm)

Cu

3.6

47.2

13.5

10.5

5.6

3.4

66.85

93.7

1.75×10-4

Cu2O

4.2

77.6

11.4

9.84

4.92

4.0

33.15

150.1

5.04×10-3

Combined χ2 (Bragg contribution for both phases) 3.48

100

The large crystallite size for Cu2O and smaller size for Cu nanoparticles are revealed through W-H plots. The better χ2 reveals that the Rietveld refinement exercise is in good fit with experimental data and Cu and Cu2O phases are present within the detectable limit of XRD (as total Phase (%) equals 100) and therefore, demonstrates that the prepared samples are free from any extraneous impurities. Having analyzed the preliminary diffraction data, it is of significance to directly measure the particle size of synthesized nanoparticles. Therefore, we 10

performed the transmission electron microscopy and energy dispersive x-ray analysis to reveal the purity of our synthesized particles.

3.2 High Resolution Transmission Electron Microscopy (HRTEM) and Energy Dispersive X-ray (EDX) Analysis As intimated through XRD analysis, a report suggested that Cu and Cu2O system consists of particles having Cu shells and Cu2O core [26]. Also, to directly measure the shape and size of prepared nanoparticles, we performed the high resolution transmission electron microscopy. Previously, it has been reported that annealing may result in size and structure variations [5]. The electron micrograph of prepared nanoparticles is presented in Fig. 4.

Fig. 4. Transmission electron micrograph of (a) spherical shaped Cu/Cu2O nanoparticles. Inset (b) a single particle with approximate 10nm diameter (c) EDX of prepared sample

Agglomerated nanoparticles of approximately 100nm sizes are observed in Fig. 4(a). These agglomerates are comprised of smaller nanoparticles of ~20nm diameter (enlarged view shown in Fig. 4(b). EDX of the prepared nanoparticles (Fig. 4(c)) is also indicative of pure nanoparticles, free from any extraneous impurity.

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Fig. 5. HRTEM images of synthesized nanoparticles depicting the d-spacings of the two phases present in the samples

The HRTEM images, depicting the lattice fringes of the two phases, are presented through Fig. 5. In our prepared samples, dhkl of 0.147nm, 0.136nm and 0.172nm respectively depict (220), (310) and (211) planes of Cu2O nanoparticles (JCPDS card 77-0199) whereas 0.168nm correspond to (021) planes of CuO nanoparticles (JCPDS card 050661). Thus, HRTEM results further supports the XRD analysis regarding the presence of less intense Cu2O phases. In addition to Cu2O phase, HRTEM analysis brings out the first inception of the existence of CuO phase as a result of partial oxidation occurring in our samples. Having confirmed the nanosize and multiple phase of prepared sample, it is crucial to investigate the electronic and vibrational properties to probe the influence of quantum confinement and high aspect ratio of prepared nanoparticles [27]. Aiming this and in order to further correlate the crystal structure, space groups symmetries and vibrational states of prepared nanocrystals, we perform the micro-Raman and infrared spectroscopy on our samples.

\ 12

3.3 Micro-Raman Spectroscopy (µRS) Analysis Diverse novel applications are realized when the dimensions of material shrink to nanometer [27]. Aforesaid applications could be accomplished only after precise measurement and perfect control on nano-scale related properties, which still remains a challenge and principal motive to investigate tailor made nanomaterials [27]. Valuable information for a nanocrystalline system could be deduced through size dependent broadening and energy shifts of Raman peak profiles [28]. The μRS experiments also include the contributions from shape and size distribution of nanostructures, straining of bonds, thermal effects, surface-enhanced effects etc., which result in profile broadening and shifts in the Raman frequency [28]. Therefore μRS has been carried out on prepared nanostructures employing 488nm line of an Ar ion laser. Here, Raman spectrum acquired in range of 90-1325 cm-1 is shown as Fig. 6. The presence of Cu nanoparticles could not be ascertained from µRS due to instrumental limitations due to the reason that the materials possessing negative real and imaginary positive dielectric constant (e.g., metals) exhibit surface plasmon resonance (SPR) [29]. In case of metals, local SPR held responsible for field enhancement and therefore, leads to surface-enhanced processes (such as, SERS) [29]. Such processes are only visible employing surface enhanced Raman spectroscopy (SERS), helpful for characterising metallic samples.

Fig.6. Raman spectrum of copper oxide nanostructure showing peak fit (shown by line ISim), and experimental data (shown by dotted line IObs)

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Nevertheless, copper oxide nanoparticles are expected to provide response in µRS, as the group theory suggests that Raman response is function of space group symmetry of crystal [30]. Raman spectrum provides broad range of information about strength of bonds, strain and the degree of crystallinity [31]. Because of higher sensitivity of Raman Spectroscopy, basic modes of CuO were observed that were earlier not detected through XRD and TEM. The lattice dynamics of CuO and Cu2O has been previously studied in literature [28, 32, 33-37]. Theoretically, CuO possesses monoclinic structure with space 6 group symmetry C 2h . There are twelve zone-center of optically active phonon modes, 4Au +

5Bu + Ag + 2Bg; three of which Ag + 2Bg are Raman active [33]. From an experimental point of view, CuO crystallization occurs in a monoclinic y-unique structure with a space group of C2/c [28]. Experimentally, they identified the Raman peaks for the different modes corresponding to Ag, B1g , and B2g at 303, 350, and 636 cm-1, respectively. The properties of Cu2O, which are of most interest and most appropriate in present context are briefly stated ahead. Cu2O crystal belongs to space group Oh4 , or Pn3m (point symmetry Oh , or m3m) [35]. Two formula units are contained in a unit cell. Thus, optical phonons exists at 15 zone centers. The symmetry of optical phonons are represented as 2 + 3 + 24 + 5 + 5 . The normal modes of phonons are labelled A to F such that A( 5 ) symbolizes rotation of tetrahedron of Cu at its own centre; the motion of B ( 3 ) is same as that of A, with a difference in upper two atoms moving out of phase with respect to the lower two atoms . The C ( 4 ) signifies out-of-phase beating motion of two Cu pairs [e.g., in the (110) 

and ( 11 0 ) planes signifies in-plane beating]; D ( 2 ) is attributed to breathing mode of tetrahedron of Cu. The E ( 5 ) represents shearing motion of the O planes; and finally F ( 4 ) denotes beating of Cu and O sublattices. Since, phonons C and F possess same symmetry, finite mixing between these two modes would take place . Since these phonons are IR active, their splitting into transverse (TO) (C', F') and longitudinal (LO) (C,F) phonons takes place [35]. It is also reported that Cu2O has six zone centres of optically active phonons out of which two are IR active. The mode ( 15 ) are observed in infrared absorption and reflectivity figures with LO and TO wave numbers of 143 and 160 cm-1 and 608 and 640 cm-1 respectively; and the wave number of the 12 mode is observed as the phonon-assisted  absorption and exhibit luminescence at 110 cm-1 [35]. Three modes at 25 (99 cm-1), 2 (307

14

 cm-1), and 25 (550 cm-1) were predicted by previous calculations of rigid ion dynamics of  lattice [36]. Out of above six, only 25 mode is reported to be Raman active and is expected

in single-phonon scattering process. However, its polarization properties and frequency could not be evidently experimentally identified despite numerous attempts [37]. It is also well established that Cu2O has fifteen zone centers optically active phonons    with symmetry: 15 (1) , 12 ( 2) (infrared active); 25 (Raman active); and 25 , 2 and 12 (silent

modes).

Group-theoretical

analysis

predicted

that

phonons

with

symmetry

 25 , 15 , 2 , and12 could assist the excitation of 1s exciton. It was found experimentally that

phonon-assisted absorption of Cu2O was predominated by the involvement of 12 phonon. Through the absorption measurements, the frequency of the zone-centre 12 phonon was found to be ~105 cm-1[38]. The Raman frequencies at 105, 147, 219, 298, 348, 410, 535, 636 and 1122 cm-1 were vividly observed in our spectrum of Cu/Cu2O/CuO nanoparticles (Fig. 6) that are ascribed respectively based upon the descending order of intensity. Most intense peak at 219 cm-1 was assigned to second order overtone of Cu2O; corresponding to 212 mode [39]. Second intense peak at 636cm-1 symbolize B g2 mode of CuO [28, 33, 34] while in another report, a broad peak at 640cm-1 corresponds to 15 ( 2 ) mode for Cu2O. The third intense but low energy peak at 147 cm-1 denotes 15(1) ( LO) for Cu2O nanocrystals. Broad peak at 298 cm-1; in a close agreement with 308 cm-1; is assigned by Carranco et. al. for second order overtone 215(1) . Two other peaks at 410 cm-1 and 105 cm-1 are referred to 412 and 12 modes of Cu2O respectively [39]. Small peaks around 350 cm-1 are assigned to B 1g corresponding to

212  25  2 mode of phonon vibration of CuO [28, 33, 34]. To investigate other possible vibrational modes of prepared Cu/Cu2O/CuO nanoparticles we employ infrared spectroscopy, as discussed ahead.

3.4 Fourier Transform Infrared (FTIR) Spectroscopy Analysis Infrared and Raman, being the most common vibrational spectroscopic techniques, are useful for determining structural properties for example, vibrational modes of molecules and phonons [40]. These techniques are complementary to each other due to usually different selection rules for vibrational transitions [40]. Interactions among ions/molecules of surfaces with the nanostructures cause alteration in vibrational frequencies as compared to isolated 15

molecules or pure nanostructures [40]. Thus, one can utilize the frequency changes detected in FTIR spectrum to probe the interactions between surface molecules and the nanostructures [40]. This is crucial for gaining a better understanding of the behaviour and strength of interactions, so as to exploit applications of nanostructures [40]. FTIR spectrum of prepared nanoparticles is shown as Fig. 7.

Fig. 7. FTIR spectrum of synthesized nanoparticlesrecorded in ambient comditions

Four distinct vibrational modes at 468, 530, 590 and 630cm-1 are observed in FTIR spectrum. The high frequency modes around 468cm-1 and 590cm-1 are reported as stretching

  modes along 1 01 direction and the mode at 530cm-1 is attributed to stretching along [101]   direction, both for Cu—O [14, 40]. A mode at 630cm-1 is ascribed to Cu2O stretching mode and its broadening indicates presence of mixed phases in the prepared system, as earlier examined by XRD [14]. Moreover, the mode due to Cu2–O is seen at 630cm-1 which was previously reported the IR active modes of Cu2O [14, 40]. The broadening observed in the IR peak in our case is basically due to mixed phases and this further affirms that the prepared nanoparticles are predominately Cu2O along with traces of CuO nanoparticles [41, 42]. Thus,

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FTIR interpretations, in line with µRS analysis, suggest that besides Cu/Cu2O, Cu-O is also present in prepared samples. Having investigated the details of the vibrational properties, we now attempt to examine the optical properties to unravel the crucial insight of electronic structure of prepared nanoparticles [43]. Aiming this, UV-visible and PL spectroscopies were employed and presented ahead.

3.5 Ultraviolet-visible (UV-vis) and Photoluminescence (PL) Spectroscopy Analysis Ultraviolet-visible (UV-vis) and Photoluminescence (PL) spectroscopy are ubiquitous characterization techniques to analyze the extrinsic electronic structure of semiconducting and semi-insulating materials [44]. At low temperatures, PL spectrum can determine the concentration of impurities; probe the defects and their complexes and also measure the band gap of semiconductors [44]. Fig. 8 and its inset depict respectively PL and UV-visible spectrum of synthesized nanoparticles.

Fig. 8. Photoluminescence and UV-visible spectra (inset) of synthesized nanoparticles

Earlier, in a report on optical absorption spectrum of CuO/Cu2O nanocomposite, the variation of absorption coefficient (αhν) 1/n with photon energy (hν), was studied [45]. In that 17

report, the values of n used were ½ and 2 showing respectively the existence of direct and indirect absorption bands in Cu2O/CuO nanocomposites [45]. Similarly, in our case too, energy band gap studies of the CuO/Cu2O nanoparticles reveal the existence of both, direct and indirect optical transitions. The band gaps of bulk Cu2O and CuO are 2.13 and 1.2eV respectively [14, 47]. In our case, we observe three sharp absorption peaks (inset of Fig. 8) at 265, 380 and 668nm corresponding to 4.6, 3.26 and 1.8eV energies, respectively. The peak around 4.6eV represents the blue shift for prepared Cu nanoparticles, observed typically due to quantum confinement effect. As indirect band gap of bulk CuO is 1.45eV, the band edge absorptions at 3.26eV (~380nm) and 1.8eV (~668nm) could be attributed to direct and indirect band gap of Cu2O/CuO nanoparticles [20, 45]. In context of PL spectroscopy, characteristic PL emissions for Cu2O nanostructures previously at 388.2nm and 753.15nm are reported [26]. In a prior report on nanoparticles synthesized by EWT, disappearance of plasmons and appearance of distinct resonant absorptions demonstrated electronic levels occurring in Cu nanoparticles [17]. In present case, two bands in the region of 350-475nm and 725-850 nm are observed and attributed to Cu2O phase [42]. It was earlier published that PL emission takes place at 517nm, for CuO crystals, and at 856nm, for Cu2O crystals and at 850nm due to CuO nanowires [42]. Broadening observed for these peaks is due to concurrence of mixed phases such as Cu/Cu2O crystals. In line with above literature reports, we also observe the peak around 528nm and a 725-850nm, both due to Cu2O phase. Further, Oleg et. al .suggested that peaks ranging from 450-475nm and 560nm corresponds to Cu nanoparticles [25]. This is in good agreement with our observation of 475nm band and 599nm peak, and hence attributed to Cu nanoparticles. A peak at 528nm is clearly observed in our PL spectrum. Das et. al. earlier attributed this peak to the recombination of electron-hole between d-band and sp-conduction band [47]. The evidences of CuO phase along with Cu/Cu2O nanoparticles as revealed through previous characterizations demands for final in-depth analysis of chemical compositions of prepared samples and therefore, X-ray spectroscopy was performed [48].

3.6 X-ray Photoelectron Spectroscopy (XPS) Analysis XPS is frequently used to investigate transition metal compounds with localized valence d-orbitals [4]. It is known that the quantitative data can be estimated through profile analysis along with identification of chemical states of constituents [48].

Moreover,

photoelectron emission from pure metal exhibit asymmetric peaks due to coupling with

18

conduction band electron which leads to identify multiple phases, as is the case in our sample [48]. Lot of literature and reports are available on functional, structural and electronic properties of oxides on of d block metal elements [4, 13, 20, 43, 49]. Among the d-orbital elements, the most crucial is copper and therefore, its oxides are much important [4, 13, 15, 20, 42, 46]. It has been reported previously that copper in CuO existed in divalent state with d9 character while Cu in Cu2O is expected to have full 3d shell [4]. The XPS studies also reveal the dependence of ionicity of Cu2O system on the size of particle [49]. Depth profiling of XPS spectra could earlier reveal that the Cu2O nanoparticles were capped with CuO surface which confirmed the stabilization of cubic Cu2O nanophase [49]. XPS could also endorse the intermediate formation of Cu2O nanoparticles during the synthesis of CuO phase [20]. In Fig. 9 (a), the XPS survey scans of prepared copper oxide nanoparticles in the full range of 0-1200eV is shown. The data of the binding energy (B.E.) has been corrected as per C 1s (285.5eV) reference peak. In Fig. 9(b) and 9(c) the core level scans of Cu and O and peaks profile fitting are shown. The details analyzed from Fig. 9 are summarized in Table 2 and supported by previous literature reports [4, 13, 14, 20, 42].

19

Fig. 9. (a) Survey spectrum of copper oxide nanoparticles. De-convoluted XPS core spectra of nanoparticles: (b) Depiction of Cu 2p3/2 and Cu 2p1/2 peaks. The peak fit of Cu 2p3/2 peak revealing a main peak of Cu2+ at 933.9 ±0.1eV, Cu+ at 935.3 ±0.1eV accompanied by a series of satellites peaks on higher B.E. side at 942.2 ±0.1eV and 944.7±0.1eV and (c)

20

Depiction of Oxygen 1s with peak position of O2- phase at 529.8±0.1eV and 1s peak of oxygen vacancy (VO) / adsorbed O0 phase at 531.5±0.1eV.

Table 2: Parameters obtained through XPS profile fitting Peak

Position

Area

(eV)

FWHM

%Gaussian-

(eV)

Laurentzian

Phases

(%) 933.9±0.1

13417.0

2.5

0

Cu/CuO (Cu2+)

935.3±0.1

35904.8

3.6

4

Cu2O(Cu+)

Satellite

942.2±0.1

19991.7

3.1

73

Cu/CuO (Cu2+)

Peaks

944.7±0.1

9677.5

2.5

0

Cu 2p1/2

954.3±0.1

14459.0

3.0

0

956.1±0.1

17810.4

4.4

100

963.9±0.1

13123.9

3.1

12

529.8±0.1

6800.7

1.81

0

O2-

531.5±0.1

7714.50

2.55

12

O0 or VO

Cu 2p3/2

O 1s

Cu/CuO (Cu2+)

First of all, no external impurity was observed for prepared sample through the XPS survey spectrum (Fig. 9(a)). The XPS peaks corresponding to Cu (Auger, 3s, 3p and 3d) and O (Auger and 1s) are clearly found in Fig. 9(a). De-convoluted XPS core spectra of Cu 2p3/2 and Cu 2p1/2 peaks are shown in Fig. 9(b). Here, XPS peak fit of Cu 2p3/2 peak revealed a prominent emission of Cu2+ at 933.9 ±0.1eV and Cu+ at 935.3±0.1eV accompanied with a series of satellites peaks on the higher B.E. side at 942.2±0.1eV and 944.7±0.1eV corresponding to Cu2+ [4, 13]. Further, the Cu 2p1/2 peaks are de-convoluted at 954.3±0.1eV, 956.1±0.1eV and 963.9±0.1eV. It has been reported that the shakeup satellites peaks are reported to be indicator of CuO presence at the surface [4]. It was suggested that while XRD does not reveal the CuO phase, XPS analysis showed the presence of surface Cu2+ ions and therefore suggested that CuO was present at the surface of either Cu or Cu2O nanoparticles. It formed an outer amorphous shell when the sample was oxidised in air during characterization [20]. The difference between the XRD and XPS interpretations is attributed to their difference in experimental mechanisms [20]. Generally, XPS measurements predominantly show the surface information within a depth of approximately 5nm, whereas, XRD detects the bulk of the material [20]. In our analysis, the XRD results reveal the main

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phase of Cu and Cu2O nanoparticles whereas oxides of Cu2+ and Cu+ are accurately determined by XPS. Literature reports that O 1s peak can be divided into two parts. First, the lower B.E. for O2- phase (around 529.8eV) and second, higher B.E. (around 532.9eV) for absorbed O atom [20]. Further, O1s spectra were de-convoluted into two main peaks at 529.8±0.1eV (labelled S1) and 531.5±0.1eV (labelled S2), corresponding respectively to O 1s and surface absorbed Oo or oxygen vacancy (VO) [13]. The FWHM of S1 is 1.81eV, and is larger than the value reported for single crystal of CuO (0.8eV), whereas the satellite component (S2) almost disappeared [13]. The area ratio of S1/S2 found to be 0.88, represents of larger satellite component. It is well established that main peak (S1 in present case) and satellite peak (S2 in present case) of O 1s is corresponding to oxygen atoms of altered chemical states [13]. The lower B.E. peak S1, is attributed to the normal O2-, interacting with the copper atoms to form the chemical Cu–O bonds [13]. The low ratio S1/S2 is due to low lattice incorporated oxygen and hence, the satellite S2 is attributed to VO. This suggests the nonstoichiometric ratio of prepared samples that resulted in the formation of mixed phases (Cu/Cu2O and CuO). This result is in excellent agreement with our data and also with previous publications [13, 43]. The existence of satellite peaks is an indication of material having partially filled d9 shell configuration in the ground state, for example, copper di-halides, metallic nickel, or CuO [11]. For Cu2O, with a completely filled shell (d10), this peak is reportedly absent due to the fact that screening via charge transfer in to the d-states is not allowed [11]. Thus in our prepared nanoparticles too, we are affirmative of obtaining CuO phase, although in trace amount. Since, the mixed phases of Cu and Cu2O and CuO were detected through various characterizations, it is crucial to check the collective magnetic behaviour emerging from individual contribution of phases. Finally, we present the magnetic properties of prepared nanoparticles in next section.

3.7 Magnetization Analysis In order to understand the magnetic behaviour of the prepared nanoparticles, the detailed magnetization study as a function of temperature was carried out on prepared sample employing a 9T physical property measurement system (PPMS) in the vibrating sample magnetometer (VSM) mode. The temperature dependent variation of the dc magnetic susceptibility under ZFC (zero-field-cooled) and FC (field-cooled) at 1000 Oe is shown in

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Fig 10. In the inset to Fig. 10, the difference in susceptibility [χ(FC)-χ(ZFC)] is plotted against temperature.

Fig. 10. Temperature dependent variation of the magnetic susceptibility χ under ZFC (zerofield-cooled) and FC (field-cooled) at 1000 Oe.

As can be seen from the Fig. 10 that Cu/Cu2O/CuO nanoparticles demonstrates finite amount of magnetism present at room temperature [50]. M-T curve reveals the distinct feature of anti-ferromagnetic transition. [51]. The ordering temperature (TN) is reduced from bulk value of 230K to 50K as the size of the synthesized particles reduced to ~20nm, as revealed through TEM analysis [51]. Below, TN, field cooled magnetization displays anomalous behavior with an increase along the susceptibility value that rises beyond 1.5×10-6emu/g. The ZFC and FC retraces up to 50 K, below which anomalous behavior is observed, which could have possibly related to surface uncompensated spins or may be due to ferromagnetic clustering, arising because of secondary magnetic system. These observations are concomitant to the previously published reports [51]. Furthermore, measured magnetization (M) of the Cu/Cu2Oand CuO nanoparticles, as a function of applied field (H) from 0-90 kOe, at selected temperatures between 5 and 300 K, 23

is shown in Fig 11. In the inset of Fig. 11, first positive quadrant of M-H curves at 150, 200, 250 and 300K are shown separately.

Fig. 11. M-H plots of Cu/Cu2O/CuO nanoparticles, measured at the applied field of 90 kOe, at selected temperature. Weak ferromagnetic behaviour of prepared nanoparticles is depicted in inset figures.

Out of literature available on copper oxide nanoparticles majority of them are based on the study of CuO nanoparticles whereas few reports are available for Cu and Cu2O nanoparticles, still there is no consensus on the magnetic behaviour has been arrived, probably because of complexity involved in terms of secondary phase [50, 52, 53]. In our case also the M-H plots show linear behavior up to 90 kOe except for a weak ferromagnetic (WF) component at lower fields up to ~ 5kOe. This points out towards ferromagnetic kind of behavior. As depicted in Fig. 10, upon going down in the temperature, the non-linearity starts increasing below 80 K and thus showing anti-ferromagnetic behavior in this temperature as evident from temperature dependent magnetization as well. The values of saturation magnetization (Ms), magnetic coercivity (Hc) and remanent magnetization (Mr) were 24

measured to be 0.0096emu/g, 237.5Oe and 7.3048×10-5 emu/g respectively. The presence of hysteresis loop in the M–H curve, as observed below 80K, exhibits weak ferromagnetic characteristics of the synthesized nanoparticles [53]. These ferromagnetic interactions could have been originated due to uncompensated surface spins of Cu2O/CuO nanoparticles as explained previously [51].

4. Conclusions In this article, we report the production of copper/copper oxide nanoparticles employing the novel exploding wire technique. Rietveld refinement of XRD patterns reveals that sample is composed of mixed phases of Cu and Cu2O nanoparticles without any extraneous impurities. However, minority phase of CuO nanoparticles, which could not be detected due to XRD limitations, was revealed in TEM/EDX analysis. Phase of CuO, absent in XRD, was also revealed through vibrational modes occurring of Cu and copper oxide (Cu2O/CuO) nanostructures in µR and FTIR spectroscopies. Various optical channels of electronic absorption and emissions of prepared nanostructures were expatiated through UVvis and PL spectroscopies. X-ray photoelectron and Auger processes elucidate the core level emission from constituent elements of nanostructures, while endorsing the presence of mixed phases and oxygen deficiency in the lattice. Finally, magnetic characterization reveal that non-stoichiometry of O atom led the formation of mixed phases, resulted the low temperature anti-ferromagnetic ordering of prepared Cu/Cu2O/CuO nanoparticles.

Acknowledgements Authors acknowledge Dr. P. K. Siwach, National Physical Laboratory (NPL), New Delhi (India) for his continuous support. We acknowledge Advanced Instrumentation Research Facility (AIRF), JNU for HRTEM characterization. Anshuman Sahai is grateful to Department of Science and Technology (DST), India for INSPIRE Fellowship (IF#120042).

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