Structural, vibrational and electronic properties of CuO nanoparticles synthesized via exploding wire technique

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Structural, vibrational and electronic properties of CuO nanoparticles synthesized via exploding wire technique ⁎

Anshuman Sahaia,c, Navendu Goswamia, , Monu Mishrab, Govind Guptab a

Department of Physics and Materials Science and Engineering, Jaypee Institute of Information Technology, A-10, Sector-62, Noida 201307, India CSIR-National Physical Laboratory, Dr. K. S. Krishnan Marg, New Delhi 110012, India c Department of Physics, Faculty of Applied Sciences (FAS), Manav Rachna University, Sector-43, Aravalli Hills, Delhi-Surajkund Road, Faridabad 121004, Haryana, India b

A R T I C L E I N F O

A B S T R A C T

Keywords: Nanoscale materials and structures: fabrication and characterization Infrared and Raman spectra Photoemission and photoelectron spectra X-ray diffraction Transmission electron microscopy (TEM)

The study of mixed phase Cu/Cu2O/CuO nanoparticles synthesized by Exploding Wire Technique has been recently reported by us. Aiming to achieve single phase CuO nanoparticles, the mixed phase Cu/Cu2O/CuO nanoparticles were subjected to annealing at different temperature and time durations in oxygen environment. In this article, we discussed two samples; two phase Cu2O/CuO and single phase pure CuO nanoparticles obtained by annealing at 500 °C and 900 °C for 10 h. Rietveld refinement and Williamson-Hall analyses revealed formation of pure phase of CuO at 900 °C with an average crsytallite size of 27.6 nm. Irregular shape of nanoparticles with average size of ~8 nm was observed by Transmission Electron Microscopy. Selected Area Electron Diffraction pattern matches with standard interplanar distance of CuO. Fourier Transform Infrared and Micro-Raman (µR) spectra exhibit broadening of vibrational modes; indicative of pure phase CuO at 900 °C. Extensive X-ray Photoelectron Spectroscopy analysis revealed that the percentage contributions of Cu1+ and oxygen vacancy (VO) decreases whereas; Cu2+ and interstitial oxygen (Oi) enhances on increasing the annealing temperature from 500 °C to 900 °C and thus, resulting the pure phase formation of CuO nanoparticles. Notably, through our analyses we propose an electronic band structure diagram on the basis of valance band maximum, as obtained by XPS and the band gap energy as estimated via UV–visible spectroscopy for mixed phase of Cu2O/ CuO (1.6 ± 0.02 eV) and pure phase of CuO (1.3 ± 0.02 eV) nanoparticles.

1. Introduction Among transition metal oxide nanomaterials, copper oxide (also known as cupric oxide, CuO) nanostructured material has drawn great interest of wide group of researchers [1,2]. Since, CuO is a p-type semiconductor with a narrow band gap of 1.2 eV, it is widely reported for gas sensing, interconnection of microelectronic, biologically compatible processes involving DNA complexes, solar cell fabrication, high Tc superconductors, magnetic storage devices, magneto-resistance, catalysis, photothermal, photoconductive and electrode material for battery applications [2–4]. CuO also acts as a field emitter and as a cathodic material in dye-sensitized solar cells (DSSCs) [2]. The comparable work function of CuO (5.3 eV) and Pt (5.65 eV), arrays of CuO nanorods were cheaper substitute to replace Pt in DSSCs [4]. CuO also attracted much interest because of being the basis of several high-Tc superconductors [5]. Previously, Raman scattering experiments have been carried out to investigate the lattice dynamics in CuO powder and single crystal because Raman intensity was considered to be directly

⁎

related to electron-phonon interaction [5,6]. Earlier several researches were performed to study the reactions of copper atoms with dioxygen because of their important role in corrosion of materials and catalytic oxidation to oxygen transport in biological systems [7]. Further, the basic nature to weakly bound complex of Cu + O2 was not very clear for which different infrared absorption spectroscopies assigned several groups like cyclic Cu(O2) (C2v) and bent CuOO (Cs), but clear agreement is still debatable [7]. It was earlier known that phases of Cu/Cu2O and CuO nanophases coexisted with a slight variation in temperature and ambient environment which was very important to understand the oxidation states, electronic and optical properties of copper oxide as these properties vastly influence the performance of many devices [1,3]. On the basic of above mentioned promising applications as well as challenges encountered to achieve single phase CuO nanoparticles via facile and cost effective synthesis process, we attempt to achieve the pure CuO nanoparticles by Exploding Wire Technique (EWT). Advancing ahead from our previous published report, here we report

Corresponding author. E-mail address: [email protected] (N. Goswami).

http://dx.doi.org/10.1016/j.ceramint.2017.10.224 Received 15 April 2017; Received in revised form 30 October 2017; Accepted 30 October 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Sahai, A., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.10.224

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Fig. 1. (a) XRD spectrum of CuO/Cu2O nanoparticles annealed at 500 °C for 10 h phases of Cu2O, CuO are shown respectively by ‘#’, ‘Δ’, (b) Rietveld refined spectrum of pure CuO nanoparticles obtained through annealing at 900 °C for 10 h. The dots, solid line, and vertical ticks respectively represent the observed data, calculated profile, and Bragg positions for CuO phase. The difference pattern (YobsYcalc) is given below the vertical tics. (c) W-H plot of CuO nanoparticles annealed at 900 °C. (d) Crystal structure of the refined CuO unit cell.

Lambda 35 UV–visible spectrophotometer, Germany. The absorption spectra of nanoparticles dispersed in DI water were collected. X-ray Photoemission spectroscopic (XPS) experiments were performed using an Ultra-high vacuum (UHV) based system, OMICRON Multiprobe Surface Analysis System operating at a base pressure of 5 × 10−11 Torr. Samples were mounted on molybdenum plates using conductive tape to avoid charging. The XPS measurements were carried out by using Mg Kα (1253.6 eV) radiation source. The core level profile fitting was performed using a Shirley background and Voigt (mixed Lorentzian–Gaussian) line shape calibrated against the C (1 s) binding energy of 284.8 eV. The VB maximum (VBM) position was determined by extrapolating a linear fit to the leading edge of the valence band photoemission to the baseline [9,10].

the synthesis of pure CuO nanoparticles employing the EWT followed by systematic annealing in presence of oxygen rich environment [1]. The samples thus obtained were studied through an assortment of characterization techniques. Hereby, this study explains the synthesis mechanism as well as transformation of mixed phases CuO/Cu2O nanoparticles into pure single phase CuO nanoparticles and corresponding alterations in various structural, vibrational and electronic properties of the nanosystem. 2. Materials and methods We adopted Exploding Wire Technique (EWT) to obtain copper oxide nanoparticles [8]. The details for obtaining the mixed phase of Cu/Cu2O/CuO nanoparticles have been already reported by some of us [1]. Advancing on this work, Cu/Cu2O/CuO nanoparticles, initially collected in the powder form through previous approach, were now subjected to annealing in quartz tube at different temperatures ranges (from 100 to 1100 °C) for different time duration (1–10 h), respectively in the absence and/or presence of O2 rich supply. The powder annealed for 10 h was grinded to obtain fine nanopowder which was investigated by various characterization techniques.

4. Result and discussions 4.1. X-ray diffraction (XRD) analysis Based on the low temperature oxidation kinetics of copper and its dependence on thickness of oxide layer, a diffusion model was proposed earlier [11]. As per the previous reports, films of copper oxide show the formation of CuO through conversion of Cu2O at the surface due to oxygen abundance in the oxide-air interface along with the instability of Cu2O in the temperature region of 750–950 °C [12]. Also, it was reported earlier that unstable phase of Cu2O reacts with oxygen at 1020 °C to form more stable phase of CuO [13–15]. In view of aforesaid reports, the powder of mixed phase Cu/Cu2O/CuO nanoparticles obtained through EWT method was subjected to annealing at different temperatures ranges (100–1100 °C) for different time duration (1–10 h), in the absence and presence of O2 rich supply, respectively. In order to identify the phase formation, all samples were subjected to XRD characterization. The XRD spectra of material obtained through above mentioned annealing conditions are presented in the supplementary file. It is indicated in the XRD spectrum of the sample, annealed at 500 °C in presence of oxygen (Fig. 1(a)) that incomplete oxidation of Cu/Cu2O/CuO nanoparticles results in partial formation of CuO phase (marked by ‘Δ’) concurring with formation of minor Cu2O phase (marked by ‘#’). This experimental evidence encouraged the investigators to further anneal the sample at 900 °C in presence of

3. Characterizations The XRD data of all samples was acquired using X-ray Diffractometer (XRD) 6000 Shimadzu Analytical, Japan having incident Cu Kα radiation wavelength of 1.54 Å. Transmission electron micrographs (TEM) of nanoparticles were captured employing a Tecnai G20-Stwin, High-resolution Transmission electron microscope (HRTEM), operating at the principal maximum acceleration voltage 200 kV. Utilizing the same instrument, Energy Dispersive X-rays Spectroscopy (EDS) analysis and Selected Area Electron Diffraction (SAED) data of CuO nanoparticles was also collected. The vibrational modes of prepared nanosamples were probed through Perkin-Elmer BXII FTIR spectrophotometer, Germany. FTIR spectra of the prepared nanoparticles suspended in the KBr matrix, were recorded at room temperature in the low energy range of 440–660 cm−1. Raman scattering processes in prepared nanoparticles were examined through an InVia Raman microscope, Renishaw UK system consisting of an Ar+ ion laser. The electronic absorptions in prepared nanoparticles were investigated employing Perkin Elmer 2

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900 °C is free from any impurity. The presence of carbon and Ni peaks in the data however, is basically due to utilization of carbon coated Ni grid for the experiment. Thus, collectively from TEM, SAED and EDS analyses, it is ensured that the prepared sample is of CuO nanoparticles without any extraneous impurity.

Table 1 Rietveld refinement parameters used for atomic positions in FullProf Suite Program (ver. 2.05).

Atom #1 Atom #2

Element

X

Y

Z

Occ

Cu O

0.25 0

0.25 0.414

0 0.25

1.085 1.086

4.3. Fourier transform infrared (FTIR) spectroscopy analysis oxygen, the XRD data of the same has been presented in Fig. 1(b). Moreover, Williamson-Hall analysis (shown in Fig. 1(c)) was also performed to estimate the crystallite sizes and strain in the prepared nanoparticles. In Fig. 1(b), Rietveld refinement, profile matching and integrated intensity refinement of X-ray data was performed by using FullProf Suite Program ver. 2.05. The pseudo-voigt function was used to refine CuO xray data applying C1c1 space group and monoclinic symmetry with Lau class 2 /m. The parameters thus obtained for the atomic positions of Cu and O element are presented in Table 1. Other parameters for example, full width at half maximum (FWHM), shape parameters, linear interpolations etc. were refined iteratively. All the diffraction peaks could be suitably indexed to the CuO teronite phase [13–15]. Refinement results reveal a monoclinic lattice structure (JCPDS 141–0254), having lattice constants a = 4.68 Å, b = 3.42 Å and c = 5.13 Å alongwith α = γ = 90°, and β = 99.471°. The Rp (profile fitting R-value), Rwp (weighted profile Rvalue) and χ2 (quality factor for goodness-of-fit) parameters obtained after iterative refinement cycles are 18.6, 12.8 and 3.97, respectively. The crystallite size calculated from W-H plot (as shown in Fig. 1(c)) found to be 27.6 nm whereas, strain (ε) and unit cell volume (V) are found to be 0.00119 and 81.15 Å3, respectively. The refined crystal structure of CuO nanoparticles is illustrated through Fig. 1(d). Here, the crystallographic information file, as obtained by Rietveld refinement, was used in the Visualization for Electronic and Structure Analysis (VESTA) software, Version 3.3.2 to visualize the refined structure of CuO in our case. In usual CuO crystal, Cu atom is coordinated by four coplanar O atoms, forming an rectangular parallelogram while, O coordination polyhedron has four Cu atoms at the corners of a distorted tetrahedron [16]. Similar to previous report on CuO structure, it could be illustrated through Fig. 1(d), that while sharing the opposite edges, − two ribbons of parallelograms run in [110] and [110] directions which can easily differentiate between two types of –Cu-O-Cu- chains along − the [101] and [101] directions [16]. The energies of these surfaces were reportedly determined via density functional theory calculations and {001} facets found to be most active crystal planes that provide highly reactive sites, leading to enhanced performance for technological applications [16]. Therefore, CuO nanoparticles prepared by our approach could possibly be ideal for gas sensing and enhancing the efficiency of Li-ion batteries [16]. Having determined the basic lattice parameters, we now perform electron microscopy of single phase CuO sample to directly determine the real space lattice structure and purity of prepared nanoparticles.

To detect the bondings and interactions through examination of vibrational modes in the prepared nanomaterials, we performed FTIR spectroscopy. The FTIR data was recorded in the range of 400–4000 cm−1. However, for better clarity and in view of region of interest FTIR spectra are shown in a selective range of 400–650 cm−1 in Fig. 3. As per theoretical report, a set of six IR active bands exist for CuO at 147 (Bu), 161 (Au), 321 (Au), 478 (Au), 530(Bu) and 590 (Bu) cm−1 [14]. In Fig. 3, we too observe the Au vibrational mode at 478 cm−1, a Bu Cu-O stretching mode along [101] direction at 533 cm−1 and an− other Bu Cu-O stretching mode along [101] at 581 cm−1 for both the nanoparticle samples [14]. Interestingly, the modes broaden for the sample annealed at 900 °C (Fig. 3(a)) as compared to the sample annealed at 500 °C (Fig. 3(b)). The broadening of the peaks suggest that when the sample was annealed at high temperature, formation of pure phase CuO nanoparticles give rise to enhancement of intensity for Cu-O − stretching mode along [101] and [101] directions. As indirect evidence for reassuring the purity of the prepared nanomaterials the IR mode at 610 cm−1 is absent now which was previously attributed for the stretching mode of Cu2-O [14]. This finding further endorses that the annealing of the samples assist in achieving the pure materials. 4.4. Micro Raman spectroscopy (µRS) analysis The vibrational properties, micro-structural changes and possible defect states in prepared nanoparticles could be investigated through µRaman spectroscopy [5,6]. Copper (II) oxide (CuO) belongs to the C26h space group with two molecules per primitive cell [5,6]. There are nine zone-centre optical phonon modes with symmetries 4Au + 5Bu + Ag + 2Bg; only three Ag + 2Bg modes are Raman active [5]. In a previous report, the change in grain size was found to largely shift and broaden the Raman peaks [5]. The µRaman spectra of nanoparticles obtained by annealing at 500 °C and 900 °C are shown in Fig. 4. In Fig. 4, three Raman peaks at 294, 341 and 629 cm−1 are observed for the nanomaterials annealed at 900 °C. Here, second mode (at 341 cm−1) is more intense whereas, third mode (at 629 cm−1) is more broadened as compared to the respective nanoparticles at 500 °C. In comparison to the vibrational spectrum of CuO nanocrystals, the peak at 294 cm−1 can be assigned to Ag mode and the peaks at 341 and 629 cm-1 for the Bg modes [5]. Earlier, the Bg mode was attributed to the stretching vibration in the x2-y2 plane [6]. Here we notice that these wavenumbers observed in our study are lower than the reported literature values (i.e. at 298, 354 and 632 cm-1) and hence indicative of the size reduction effect in CuO nanoparticles [5]. Earlier, three Raman modes in CuO (as aforesaid three modes in our case too) were believed to originate from the vibrations of oxygen atoms only and it was experimentally shown later that any shift in Raman modes appears due to grain size [5]. Further as seen in Fig. 4(a) and (b), one can observe that by increasing the annealing temperature, the modes of sample annealed at 900 °C get intense and broadened as compared to those at 500 °C. These distinct changes demonstrate that the pure CuO phase formation at 900 °C as compared to CuO/Cu2O phase formed at 500 °C. However, more concrete and direct evidences of alterations in oxygen vacancies or defects so as to establish the transition of two phase CuO/Cu2O (at 500 °C) towards single phase CuO (at 900 °C) could be suitably addressed through x-ray photoelectron spectroscopy

4.2. Transmission electron microscopy (TEM), selected area electron diffraction (SAED) and energy dispersive X-ray spectroscopy (EDS) analysis TEM image of CuO nanoparticles annealed at 900 °C is shown in Fig. 2(a). Here, irregular shape CuO nanoparticles with average size of 8 nm are observed. The agglomeration of smaller nanoparticles is also indicative through appearance of larger nanostructures in Fig. 2(a). The interplanar distances (d) estimated through diffraction profile fitting in Fig. 2(b) were found to be 2.75, 2.54, 2.35, 1.95, 1.84, 1.66, and 1.55 Å. These values are in excellent match with the ones obtained earlier by Rietveld refinement respectively as, 2.75 Å (110), 2.53 Å − − (002), 2.32 Å (111), 1.96 (1 12), 1.86 Å (2 02), 1.62 Å (021) and 1.58 Å (202). EDS spectrum (Fig. 2(c)) confirms that the sample obtained at 3

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Fig. 2. (a) Transmission electron micrograph showing ~ 8 nm average size nanoparticles, (b) SAED pattern demonstrating the interplanar distance d for CuO planes and (c) EDS spectrum of nanoparticles obtained by annealing at 900 °C.

(XPS) analysis. Assuming this, we performed the detailed XPS analysis of nanoparticles annealed at 500 °C and 900 °C. 4.5. X-ray photoelectron spectroscopy (XPS) analysis The XPS spectra for the Cu (2p) and O (1s) core levels for the nanoparticles annealed at 500 °C and 900 °C are shown in Fig. 5. In Fig. 5(a) and (b), the Cu (2p3/2) peaks of both the samples were deconvoluted into two major components corresponding to Cu1+ and Cu2+ species along with their satellites [13,14,17,18]. Similarly, in Fig. 5(c) and (d), the O (1s) spectra were deconvoluted into four peaks corresponding to O2- and adsorbed oxygen with some chemisorbed hydroxyl species [13,14,17–21]. The percentage contributions of various emissions for Cu (2p3/2) and O (1s) are tabulated in Table 2. One can directly infer from Table 2 that with the increase in annealing temperature, the percentage of Cu2+ increases from 42.4% to 65.2%. Also, a decrease in Cu1+ phase was observed from 25.3% to 10.2%. Together, these findings confirm that Cu2O nanoparticles at 500 °C are oxidised completely to transform into CuO nanoparticles and hence, Cu2+ enhances at the expense of decrement of Cu1+. The negligible presence of Cu1+ phase in samples annealed at 900 °C, was attributed to the naturally oxidised Cu nano-crystalline layer [22]. As observed from Fig. 5(c, d) and Table 2, an asymmetric O (1s) XPS core level peaks could be deconvoluted adopting Voigt fitting into four components i.e. lattice oxygen of CuO and Cu2O (OL(Cu2+) and OL(Cu1+)), oxygen vacancy (VO), interstitial oxygen (Oi) and adsorbed oxygen (OC). The OC component of high binding energy (> 532.9 eV) is usually attributed to chemisorbed and dissociated oxygen species (O2-, O2- or O-) and OH- [21]. The total contribution of lattice oxygen (O2-) and chemisorbed and dissociated oxygen species (O2-, O2- or O-) and OH- remained almost same in both the samples. No significant change in the percentage contribution of lattice oxygen (O2-) is observed although, oxygen vacancy (VO) decreased and interstitial oxygen (Oi) increased with annealing at 900 °C in oxygen rich environment. This observation is complementary to above findings of enhancement of Cu2+ state with annealing. Jointly, the changes of Cu1+ into Cu2+ and decrease of VO to enhance Oi absolutely establish the oxidation of Cu2O nanoparticles to form CuO nanoparticles at 900 °C.

Fig. 3. FTIR spectra of nanoparticles obtained after annealing at (a) 500 °C and (b) 900 °C.

Fig. 4. Raman spectra of prepared nanoparticles annealed at (a) 500 °C and (b) 900 °C for 10 h.

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Fig. 5. Deconvoluted XPS peaks for the nanoparticles annealed at 500 °C (a and c) and 900 °C (b and d).

photoelectric cross-section and the density of states of the valance band of the crystal [23]. Moreover, it will be interesting to note the spectral width of VBM spectra of the samples annealed at 500 and 900 °C (Fig. 6(a) and (b), respectively). The broader plateau observed in Fig. 6(a) could be ascribed to overlapping of two density of states for the valence band of CuO/Cu2O nanoparticles. Hence, it is in line with coexisting two phases of Cu2O/CuO nanoparticles at 500 °C. However, contrary to broader hump of Fig. 6(a), the peak is quiet sharp in Fig. 6(b) and hence, vividly establishes the single phase formation of CuO nanoparticles. Understanding the coexistence of Cu2O, albeit in little quantity, along with the dominating phase at 500 °C and its transformation into CuO at 900 °C, has been of great significance. In view of this, we further propose the schematics of energy band diagrams of both types of nanoparticles systems illustrated through Fig. 7. Before elucidating aforesaid proposed schematics, the prerequisite brief background of the electronic band structure of semiconductor is as follows. The electronic structure of semiconductor is usually signified through its band gap (Eg), which is basically an energy gap with few electronic states (of low density of states) between valence band and conduction band (which have high density of states) [25]. The electrons undergo a transfer from the highest occupied molecular orbital (HOMO) known as valence band maximum (VBM) with energy EV, which is also a measure of the ionization potential I, of the bulk material [25]. The lowest unoccupied molecular orbital (LUMO) in most semiconductors coincides with the bottom of the conduction band (CB), having band energy EC, where EC is a measure of the electron affinity (EA) of the compound [25]. The Fermi level energy (EF) represents the chemical potential of electrons in a semiconductor [25]. In essence, the Fermi level is the absolute electronegativity –χ, of a pristine semiconductor, a value that corresponds to the energy halfway between the conduction and valence band edges [25]. The relationship between band edge energies and electronegativity can therefore be expressed as:

Table 2 Position and percentage contribution of various peaks occurring in Cu (2p3/2) and O (1 s) core level spectra of samples annealed at 500 °C and 900 °C. Sample name Peak

CuO/Cu2O @500 °C

CuO @900 °C

Position (eV)

%

Position (eV)

%

1 2 3 4 5 6 7

932.9 935.0 941.0 943.4 530.0 531.7 533.0

25.3 42.4 11.8 20.5 20 33.2 26.8

932.7 934.6 940.8 943.3 530.2 531.7 532.9

10.2 65.2 4.2 20.4 21.7 29.3 31

8

534.3

20

534.3

18

Attribution

Cu1+ Cu2+ Cu2+(Satellite Peak) Cu2+(Satellite Peak) OL(O2-) VO (Oxygen vacancy) Oi (Interstitial oxygen) OC (OH-/adsorbed moisture)

The study of valance band structure of graphite states that X-ray photoemission spectrum of valance band is related with density of states [23]. To analyse the electronic structure and semiconducting behaviour of nanoparticles (p- or n- type), the valence bands of both the samples were probed (shown in Fig. 6). We attempted to examine the valence band spectra of prepared nanoparticle employing XPS. The valence band maximum (VBM) to Fermi level (EFi) separation are illustrated through Fig. 6(a) and (b), as obtained for the two samples CuO/Cu2O (at 500 °C) and pure CuO (at 900 °C) nanoparticles, respectively. The VBM positions were found to be 0.13 and 0.5 eV, respectively for the sample obtained via annealing at 500 °C and 900 °C. As per literature, both Cu2O and CuO are p-type semiconductors with direct band gap of 2.2 and 3.39 eV, respectively [2–4,12–14,17,24]. Other investigations report the band gap of bulk CuO to be about 1.2 eV [24]. From UV–visible spectroscopy, we had already estimated the values of absorption energies from the band edge. The samples annealed at 500 °C and 900 °C, absorption energy values were found to be 1.6 ± 0.02 eV and 1.3 ± 0.02 eV, respectively (data not shown). The low value of VBM also indicates the p-type nature of the prepared nanomaterial [24]. It has been reported earlier that the intensity of the photoemission spectrum is directly proportional to average

EC = −EA = −χ + 0.5Eg

(1)

EV = −I = −χ – 0.5Eg

(2)

Incorporation of impurities in the structure of a semiconductor leads to the presence of electron acceptor state levels and/or donor state levels within the band gap [25]. The presence of donor or acceptor state 5

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Fig. 6. Valence band spectra of nanoparticles annealed at (a) 500 °C and (b) 900 °C.

close to VBM. Thus, this analysis unravelled the intricate depiction of electronic structure of the nanoparticles obtained by our novel process.

levels changes the position of EF so that EF lies just above EV for p-type semiconductors (presence of acceptor states) and EF lies just below EC for n-type semiconductors (presence of donor states) [25]. In our case, as estimated from valence band analysis of CuO/Cu2O nanoparticles (Fig. 7(a)), the EFi lies at 0.13 eV above the VBM while for CuO nanoparticles (Fig. 7(b)), the EFi lies at 0.5 eV above the VBM. This demonstrates that the Fermi level lies closer to VBM for the sample containing mixed phase of CuO/Cu2O as compared to that of pure single phase of CuO nanoparticles. Thus, pure CuO nanoparticles as well as two phase Cu2O/CuO nanoparticles prepared by us maintain their p-type behaviour [2–4,12–14,17,21]. Finally, our analysis also reveals that the sample obtained at 900 °C is pure single phase CuO nanoparticles as the close gap of 0.13 eV from VBM in case of CuO/ Cu2O nanoparticles only (Fig. 7(a)), is suggestive of the existence of degenerate p++-type semiconducting behaviour of mixed CuO/Cu2O nanoparticle which is absent for CuO nanoparticles as here EFi is not

5. Conclusions We establish that the facile synthesis method of EWT, capable of synthesizing Cu/Cu2O/CuO nanoparticles, can further be advanced to achieve pure single phase CuO nanoparticles. We developed and optimized the process to obtain pure phase CuO nanoparticles by annealing the mixed phase nanomaterials to 900 °C in oxygen rich environment. Irregular shapes of CuO nanoparticles with average diameter ~ 8 nm are vividly seen. The FTIR and Raman spectroscopy studies revealed the fundamental modes of copper oxide and endorsed the XRD interpretations. Although above analyses could only partially implicate towards the transformation of Cu2O into CuO with the increasing rate of annealing, yet the finer details of the oxygen – annealing driven Fig. 7. Schematic energy band diagrams for samples annealed at (a) 500 °C consisting of CuO/Cu2O nanoparticles and (b) 900 °C consisting of pure CuO nanoparticles. Symbols E0, EFi, Eg, EA and φ corresponds to Fermi level, intrinsic Fermi level, electron affinity and work function, respectively.

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conversion of Cu2O/CuO nanoparticles into CuO nanoparticles can be substantially elucidated through our detailed XPS analyses. By XPS analysis, not only the quantitative determination of enhanced Cu2+ phase was demonstrated but more notably the degenerate p++ type and simple p type behaviour, for CuO/Cu2O and CuO samples, respectively was proposed on the basis of VBM spectra based schematics.

[8]

[9] [10]

Acknowledgements

[11]

We are grateful to Dr. P.K. Siwach, NPL, New Delhi, for providing annealing resources and XRD facility and Prof. B.R. Mehta, IIT Delhi for providing crucial TEM characterization. Author (AS) acknowledges Department of Science and Technology (DST), New Delhi for INSPIRE Fellowship [IF#120042].

[12] [13] [14] [15]

Appendix A. Supplementary material

[16]

Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.ceramint.2017.10.224.

[17]

[18]

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