Probing the dominance of interstitial oxygen defects in ZnO nanoparticles through structural and optical characterizations

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CERAMICS INTERNATIONAL

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Probing the dominance of interstitial oxygen defects in ZnO nanoparticles through structural and optical characterizations Anshuman Sahai, Navendu Goswamin Department of Physics and Materials Science and Engineering, Jaypee Institute of Information Technology, A-10, Sector-62, Noida 201307, India Received 3 May 2014; received in revised form 6 June 2014; accepted 6 June 2014

Abstract ZnO nanoparticles, synthesized adopting a facile chemical precipitation route, are studied here. The structural, optical and electronic properties of prepared ZnO nanoparticles were extensively investigated employing X-ray diffraction (XRD), transmission electron microscope (TEM), energy dispersive analysis by X-rays (EDAX), X-ray photoelectron spectroscopy (XPS), UV–vis absorption and fluorescence (FL) spectroscopy. The XRD analysis revealed hexagonal wurtzite phase 26.1–29.6 nm size ZnO nanocrystallites. This observation gets further support from TEM images where particles of 25–30 nm size are vividly seen. Interestingly, oxygen rich stoichiometry of nanoparticles is detected via zinc and oxygen emission lines of EDAX spectrum. XPS analysis establishes coexistence of lattice oxygen (OL), interstitial oxygen (Oi) and oxygen vacancy (VO) in ZnO nanoparticles. In line with EDAX analysis, XPS investigations substantiate interstitial oxygen rich composition of nanoparticles. Blue shift of absorption energy, as observed in the UV–vis spectrum of ZnO nanoparticles, typically manifests quantum confinement effect. Such transitions indicate the occurrence of various discrete energy states of prepared nanoparticles. FL spectroscopic investigations ascertain the existence of these discrete states by probing the radiative transitions arising among such states. Finally, FL study not only demonstrates visible emissions emanating from the oxygen defect states but more remarkably, in concurrence with EDAX and XPS analysis, establishes the excess of interstitial oxygen defects in prepared ZnO nanoparticles. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: B. Defects; B. Spectroscopy; C. Optical properties; D. ZnO

1. Introduction Zinc oxide (ZnO) becomes of greater significance as the novel and improved technologically potent properties arise when the material attains the nanosize low-dimensional structures. The unflinching interest of researchers for study of various nanostructures of ZnO is driven not only due to its enormous applications but also due to the fact that this is a suitable wide band gap semiconductor material to investigate the rich science pertaining to the fundamental issues of synthesis, characterization and subsequently study of its novel properties. A huge wealth of literature dealing with the fabrication and characterization of ZnO nanoparticles is available. The subfield of synthesis and characterization of ZnO nanoparticles remains unabated as the scope of n

Corresponding author. Tel.: þ91 120 2594364; fax : þ91 120 2400986. E-mail address: [email protected] (N. Goswami).

exploring new fundamental properties still unexhausted [1–3]. Out of several properties, luminescence of ZnO nanoparticles has been the foremost investigated issue because of its direct band gap semiconductor behavior which enables ZnO nanoparticles to be used for phosphor, opto-electronic and display devices [4–6]. The fluorescence properties of ZnO are significantly influenced as the dimension of material is reduced. The high exciton binding energy (60 meV) in zinc oxide crystal can ensure efficient excitonic emission at room temperature and consequently room temperature ultraviolet (UV) luminescence has been reported in disordered nanoparticles and thin films [7]. A number of theoretical reports are published to elucidate the size-dependent optical properties of semiconductor clusters [8]. The experimental verification of these theoretical predictions can be achieved with the development of novel approaches of synthesis, processing and characterizations of nanoparticles. The size confinement and surface effects of

http://dx.doi.org/10.1016/j.ceramint.2014.06.041 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: A. Sahai, N. Goswami, Probing the dominance of interstitial oxygen defects in ZnO nanoparticles through structural and optical characterizations, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.06.041

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nanoparticles not only cause the increase in band gap but also extraordinarily influence the various optical transitions, such as luminescence [6]. The photoluminescence spectrum of ZnO may exhibit three distinct emission regions: a band in the UV (around 380 nm) that is called the near band edge emission, a band of violet– blue emission (400–440 nm approx.) and a band encompassing green, yellow or orange emission (500–750 nm approx.) [9– 15]. The near band edge emission in the UV region is the consequence of direct recombination of free or bound excitons triggered by exciton–exciton collision process. This is the reason behind high efficiency UV emission in powders and thin films of ZnO. The origin of the visible luminescence (say green emission) is usually attributed to the presence of various defects such as oxygen vacancies (VO), zinc interstitials (Zni) and oxygen interstitials (Oi) [7]. The visible emission of ZnO nanoparticle could be influenced by various parameters. There are reports mentioning the dissimilar fluorescence of ZnO nanoparticles of the same size prepared by different methods [4,16]. Researches indicate that the surface passivation via surfactant and polymer capping is also an effective scheme to quench the defect-related visible photoluminescence (PL) from nanosized ZnO [17]. The effect of synthesis methodology and surface passivation indicate that green emission is due to surface [16]. In our previous study we reported that although size of nanoparticles remains unaltered, the process of annealing give rise to some changes in the emission spectra of ZnO nanoparticles. It was further revealed by our previous fluorescence studies that the green emission spread and other visible emissions are altered by annealing treatment without affecting the structural parameters of ZnO nanoparticles [6]. Despite the worldwide intensive research on luminescence of ZnO nanostructures and the effect of various parameters; such as synthesis process, size, shape and distribution of prepared ZnO nanostructure on luminescence; there are still few unexplored issues to understand the fluorescence of nanostructured ZnO [1–17]. To the best of our literature survey, one such unexplored issue is to systematically investigate the effect of fluorescence excitation energy on the fluorescence of ZnO nanoparticles. In this paper, we report systematic investigation of structural and optical properties of ZnO nanoparticles prepared employing a facile chemical precipitation method. Details of the procedure and underlying mechanism for synthesis of ZnO nanoparticles are provided. Structural and optical properties of prepared ZnO nanoparticles were probed employing XRD, TEM, EDAX, XPS, UV–vis absorption and FL spectroscopies. The structural investigations revealed the hexagonal phase of 25–30 nm size of ZnO nanoparticles. Oxygen rich stoichiometery of prepared nanoparticles is attributed to the dominance of Oi defects in present case. The radiative transitions occurring among the various discrete energy states of prepared nanoparticles were extensively studied through optical characterizations. In contrast to the usually reported size-dependent luminescence of nanoparticles, our FL analysis demonstrates the presence of lattice oxygen along with the coexistence of majority of Oi defects and minority of VO defects in ZnO nanoparticles, prepared by us.

2. Materials and methods Zinc Acetate [Zn(CH3COO)2, Sigma-Aldrich, purity 99.99%], Methyl Alcohol [CH3OH, Sigma-Aldrich, purity 99.9%] and Sodium Hydroxide [NaOH, Sigma-Aldrich, purity 99.99%] were used as received with further purification. For synthesis of ZnO nanoparticles, we followed previously well established chemical precipitation route [6,18]. In brief, aqueous solutions of 1 M Zn(CH3COO)2 and 2 M CH3OH were stirred in a beaker for 30 min as to attain a uniform solution. To this homogeneous mixture, drop by drop addition of freshly prepared 1 M NaOH solution was added via a buret along with continuous magnetic stirring to this product solution for 12 h so as to acquire the white precipitate of ZnO nanoparticles. The precipitate of ZnO nanoparticles was repeatedly washed with distilled water and then dried at 100 1C for 18 h to remove moisture. Furthermore, as reported earlier the ZnO nanoparticles were annealed at 400 1C for 3 h to help us in removing any extraneous impurities (e.g. carbonates etc.) but also assists in formation of pure stable nanophase of ZnO without varying the size of nanocrystals [6,18,19]. The annealed powder of ZnO nanoparticles thus obtained was subjected to various characterizations, as described ahead. The schematic of synthesis procedure is depicted through a flow chart in Fig. 1. The details pertaining to the process, chemical reaction and underlying mechanism for the formation of ZnO nanoparticles can be found elsewhere [6,18]. As elaborated in our previous reports, thermodynamics and kinetics of synthesis method ensures the nanophase formation of ZnO [18,19]. 3. Characterization The X-ray diffraction patterns of prepared powder of ZnO nanoparticles were verified employing Rigaku MiniFlex II benchtop XRD system in continuous mode, operating at 30 kV/15 mA to generate the Cu Kα line (1.5419 Å). Transmission electron micrograph (TEM) of ZnO nanoparticles was obtained employing a Tecnai G2 Model T30 High-resolution

Fig. 1. Flowchart for synthesis of ZnO nanoparticles via chemical precipitation method.

Please cite this article as: A. Sahai, N. Goswami, Probing the dominance of interstitial oxygen defects in ZnO nanoparticles through structural and optical characterizations, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.06.041

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Transmission electron microscope (HRTEM) operating at the principal maximum acceleration voltage 300 kV. Utilizing the same instrument, Energy dispersive analysis by X-rays (EDAX) and Selected Area Electron Diffraction (SAED) data of ZnO nanoparticles were collected. The X-ray photoelectron spectra (XPS) acquired using a Perkin Elmer-1257 X-ray photoelectron spectrometer using Al Kα (E=1486.6 eV) radiation and a hemispherical section analyzer with 25 meV resolution. The UHV chamber contains a 0–5 keV differentially pumped Ar þ ion gun. The resolution provided by this setup is 0.1 eV at 90% of the peak height. Before collecting the data, the sample was vacuum dried so as to remove extraneous impurities and moisture. XPS spectra for the ‘as deposited’ samples were collected. Survey scans and core level spectra were obtained using pass energies of 100 eV and 40 eV respectively. The radiative absorptions in ZnO nanoparticles were investigated employing Perkin Elmer Lambda 35 UV–vis spectrophotometer. The absorption spectra were recorded for the methanol based dispersions of ZnO nanoparticles. The same dispersions were further used to acquire fluorescence excitation and emission spectra of ZnO nanoparticles employing Perkin-Elmer LS55 Fluorescence spectrometer. In this monochromator based spectrometer, a high energy pulsed Xenon source is used for excitation. The fluorescence spectra are obtained for ZnO nanoparticles suspended in the methanol and placed in the quartz cuvettes of 1 cm path length. The fluorescence excitation and emission data is collected at the scanning interval of 0.5 nm with a low scan speed of 200 nm/s so as to have sufficient data integration time; a prerequisite for optimal signal-to-noise ratio. In order to attain the fluorescence data with the best possible resolution, we fixed a narrow slit width of 2.5 nm of excitation and emission monochromators. 4. Results and discussion 4.1. X-ray diffraction (XRD) The X-ray diffraction pattern of prepared ZnO nanoparticles is recorded in θ–2θ geometry in the range 30–801 and shown in Fig. 2. Fig. 2 represents the XRD pattern of pure ZnO nanoparticles. It clearly shows all the prominent Bragg reflections and corresponding planes at 31.791 (100), 34.441 (002), 36.271 (101), 47.571 (102), 56.631 (110), 62.901 (103), 66.421 (200), 68.001 (112), 69.141 (201), 72.621 (004) and 77.021 (202). These reflections are close tin good agreement with the peak positions usually reported for hexagonal wurtzite phase of ZnO having space group P63mc (JCPD 36-1451) [6,18,19]. The average values of calculated lattice constants are a=3.2475 Å and c=5.2029 Å with the c/a ratio=1.6021. The unit cell volume for prepared ZnO nanocrystals estimated to be 47.24 nm3. Therefore, the lattice parameters (a, c and V) estimated for prepared nanoparticles are in good agreement with the published crystallographic data for ZnO [6,18]. The particle sizes were calculated through XRD data of ZnO nanoparticles using Debye–Scherrer method, Williamson– Hall(W–H) analysis employing Uniform Deformation Model

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Fig. 2. The X-ray diffractogram of prepared nanoparticles. Miller indices corresponding to hexagonal ZnO phase are also labeled.

(UDM) and Uniform Stress Deformation Model (USDM) comes out to be 26.1 nm, 29.6 nm and 27.5 nm. The details of the factors governing the variation in size due to dependency on stress and strain can be found elsewhere [18]. Details of the calculations and results pertaining to the peak indexing, crystallinity index, dislocation density, Zn–O bond length and specific surface area of ZnO nanostructures also already published [18]. 4.2. Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDAX) The TEM image of prepared nanoparticles is shown in Fig. 3. The irregular shaped ZnO nanoparticles of average size 30 nm are vividly seen here. The EDAX spectrum of prepared ZnO nanoparticles is also presented in the inset of Fig. 3. This observation, in concomitant with XRD and EDAX results, confirms the fabrication of hexagonal ZnO nanocrystallites of 25–30 nm size. In EDAX spectrum, the peaks at 0.296 keV and 7.851 keV denote the X-ray emissions from K-shell of C and Cu due to carbon coated TEM grid, used for mounting the nanoparticles [20]. Other prominent peaks at 0.592 keV, 1.037 keV, 8.44 keV and 9.33 keV are vividly observed in EDAX pattern of ZnO nanoparticles prepared by us. Out of these peaks, the X-ray energies 0.592 keV and 1.037 keV respectively represent the emissions from the K-shell of oxygen and L-shell of zinc [21,22]. Infact, the L-shell emission at 1.037 keV, as observed here, can be seen as the convolution of Zn 2p3/2 and Zn 2p1/2 photoelectron transitions reported usually at 1.021 keV and 1.044 keV respectively [23,24]. The X-ray energies 8.44 keV and 9.33 keV are additional emissions from Zn core levels [21,25]. Having identified the X-ray emissions from nanoparticle constituents; the elemental composition of prepared nanoparticles can also be deduced from EDAX spectroscopic data. The atomic and weight percent compositions (say at% and wt%) of the major constituents of ZnO nanoparticles i.e. Zn and O atoms and the R-value (i.e., at%

Please cite this article as: A. Sahai, N. Goswami, Probing the dominance of interstitial oxygen defects in ZnO nanoparticles through structural and optical characterizations, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.06.041

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Fig. 3. Transmission electron micrograph and EDAX spectrum (inset) of prepared ZnO nanoparticles.

Table 1 Weight, atomic and R percentage composition of prepared nanoparticles. Elements

wt%

at%

R-value (%)

O (K) Zn (L)

27.15 72.85

60.36 39.64

152.3

ratio of O2  to Zn2 þ ), thus calculated, are summarized in Table 1. Since the Zn:O at% ratio comes out to be 40:60; the R-value for prepared ZnO nanoparticles our sample is 152.3%. Generally ZnO is considered as an n-type semiconductor where most defects are zinc interstitials (Zni) and VO [6,26]. On the contrary, we obtain deficiency of zinc and excess of oxygen in the ZnO nanoparticles prepared by us. Therefore, the excess of oxygen, as determined in our case, could possibly be attributed to the Oi in ZnO nanoparticles. The excess of oxygen may give rise to less observed p-type semiconducting behavior in ZnO nanoparticles prepared by us [27]. Nonetheless, EDAX analysis not only ascertains the purity and stoichiometric composition of ZnO nanoparticles prepared. These results, in general; and the oxygen rich composition of prepared ZnO nanoparticles, in particular; encouraged us to carefully examine the electronic structure of prepared nanoparticles employing XPS spectroscopy as the photoelectron and Auger emission processes within the semiconductor nanostructure are sensitive to its size, stoichiometry and defects. 4.3. X-ray photoelectron spectroscopy (XPS) The X-ray photoemission and X-ray absorption spectroscopies are unique techniques to investigate intricacies of the

electronic structure of solids [28]. Due to the interaction of the positive hole created during process, with the mobile conduction electrons; the spectra produced by these techniques do not in general represent simply the electronic states of the material under investigation [28]. The resultant XPS spectra may contain additional information about the structure produced through this core-hole conduction–electron interaction [28]. In view of immense potential of XPS technique to unravel the intricate structural and electronic properties, a wealth of published reports is available on XPS investigations of ZnO [28–33]. XPS studies of nanophase ZnO are of particular interest due to the unprecedented capability of XPS to reveal insights of electronic structure modification due to diverse cases of doping and defects in ZnO based nanostructures [28– 33]. In this context, XPS studies are capable of revealing the presence of oxygen interstitials (Oi) and oxygen vacancies (VO) point defects in ZnO [29]. On the basis of XPS study, it was earlier reported that O 1s state always contains three binding energies components corresponding respectively to low binding energy peak (LP) at 530.15 eV, middle binding energy peak (MP) at 531.25 eV and high binding energy peak (HP) at 532.40 eV [30]. The LP was attributed to O2  ions at the intrinsical sites and the MP was attributed to the O2  ions in the oxygen deficient region. The HP emission was assigned to the chemisorbed oxygen (Oi) [30]. Similarly, the higher binding energy (531.7 eV) peak is attributed to chemisorbed or dissociated oxygen or hydroxyl (OH) species on the surface of the ZnO thin film [31]. In another report [29], three distinct Gaussian curves centered around 530.28 eV, 531.30 eV and 532.36 eV were attributed to O atoms at regular lattice site (OL), oxygen deficient regions (e.g. VO) and interstitial O (Oi) or surface oxygen atoms, respectively. However, Chen et al. [32] and Wang et al. [33] did not attribute the low binding energy peak at 530 eV to oxygen states i.e. OL or Oi rather attributed it to Zn–O bonds. This infers that although some discrepancy was observed in attribution of LP; however attributions of MP and HP are in good agreement among various reports [30]. In our case, oxygen rich stoichiometery of prepared ZnO nanoparticles is already suggested through EDAX analysis and this prompted us to perform XPS measurements. We perform XPS analysis not only to affirm the purity of prepared nanoparticles but more crucially to recognize the chemical states of O 1s and Zn 2p and finally ascertain the types of defects existing in the prepared nanoparticles. Aiming this XPS survey spectrum as well as core spectra of O 1s and Zn 2p levels of prepared ZnO nanoparticles were acquired and shown respectively as Fig. 4 (a)–(c). Here binding energies are calibrated considering the C 1s emission centered at 284.5 eV. Various X-ray photoelectron and Auger emissions from zinc and oxygen are observed in Fig. 4(a). Thus, XPS survey spectrum establishes that zinc and oxygen are the main constituents and prepared nanoparticles are absolutely free from extraneous impurity. In order to probe the oxygen rich stoichiometry of prepared nanoparticles and furthermore, to ascertain the nature of oxygen defects, it is significant to analyze the XPS scan of O 1s core level (Fig. 4(b)).

Please cite this article as: A. Sahai, N. Goswami, Probing the dominance of interstitial oxygen defects in ZnO nanoparticles through structural and optical characterizations, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.06.041

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Fig. 4. (a) XPS survey spectrum of ZnO nanoparticles. (b). XPS core level scan of O 1s. Existence of OL, VO and Oi states is shown through deconvoluted profiles (c) Zn 2p1/2 and 2p3/2 core level scans. Simulated profiles (blue and pink solid squares) also shows individual peaks.

Deconvolution of this peak profile through simulation reveals three distinct Lorentzian–Gaussian (%80:20) profiles centered around binding energies 530.23 eV (blue solid triangular curve), 531.57 eV (pink solid squares curve), and 532.60 eV (brown solid circle curve). As per the published reports, the LP at 530.23 eV, MP at 531.57 eV and HP at 532.60 eV are attributed to oxygen at lattice site (OL), oxygen vacancy (VO) and interstitial oxygen (Oi), respectively [29,30]. Similarly the peak profiles fitted around the Zn 2p3/2 and Zn 2p1/2 peaks centered around 1022.25 eV and 1045.31 eV respectively are shown in Fig. 4(c). These emissions correspond to Zn atom at the regular lattice site in ZnO [29]. Here, the binding energies difference between Zn 2p3/2 and Zn 2p1/2 emissions found to be 23.06 eV, which is the characteristic value for ZnO [29]. It is earlier reported that Zn 2p3/2 peak shape does not always give an asymmetric feature, so the Zn LMM Auger peak analysis is often used to identify the chemical states of the Zn species [31]. This is due to the fact that Auger peaks usually exhibit larger shape changes than XPS peaks with the varying chemical states since a single Auger transition involves three electron and many body effects [31]. In our case too, an Auger Zn L3M4,5M4,5 emission centered at 497.49 eV is detected through careful inspection of Fig. 4(a) and attributed to the interstitial Zn–O bonds, as per the literature reports [31]. The peak at 493.8 eV, earlier reported for interstitial zinc (Zni) is absent in our case [31].

This suggests that ZnO nanoparticles prepared by us do not possess Zni defects and therefore, zinc deficient condition could consequently give rise to oxygen rich stoichiometry of prepared nanoparticles. Nevertheless, we attempt to directly estimate the Zn/O stoichiometric ratio using the parameters obtained through the individual peak profile fittings (Fig. 4(b), (c)), as summarized in the Table 2. Using the peak areas for most intense zinc (i.e., Zn 2p3/2) and oxygen (OT) emissions, the ratio of Zn(2p3/2)/OT was calculated and found to be 0.68 which is in agreement with Zn/O ratio of 0.66, as estimated through abovementioned EDAX analysis. Thus abovementioned XPS analysis based interpretations, in concomitant with our EDAX analysis, suggest that oxygen rich stoichiometry of prepared ZnO nanoparticles in our case is due to dominance of Oi defects. However, in order to further affirm the Oi prominence albeit coexistence of Vo defects, we further investigate the optical properties. Aiming this, UV–vis and fluorescence spectroscopies were employed, owing to their unique capability for unraveling the crucial insight of discrete, defect and surface states in ZnO nanostructures [6]. 4.4. UV–vis absorption spectroscopy The optical processes of absorption and emission in ZnO become increasingly important as the size and dimensionality of structures is reduced to nanoscale [6] The UV–vis spectroscopy

Please cite this article as: A. Sahai, N. Goswami, Probing the dominance of interstitial oxygen defects in ZnO nanoparticles through structural and optical characterizations, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.06.041

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6 Table 2 Parameters obtained through peak profile fitting. Peaks

Position

Intensity

Area

OL VO Oi OT Zn(2p3/2) Zn(2p1/2)

530.23 531.57 532.60 – 1022.25 1045.31

3472.81 654.01 2756.27 – 14265 12268.13

15559.89 526.82 4720.45 21084.55 14423.67 899.46

was performed to investigate the radiative absorptions in ZnO nanoparticles. The UV–vis absorption spectrum of ZnO nanoparticles, in the scanning range 300–450 nm, is shown as Fig. 5. The optical absorption seem to be enhanced in 325–375 nm region. This is indicative of the blue shift in the absorption energies in ZnO nanoparticles due to size confinement [6,18]. In order to exactly determine the value of band edge absorption energy of prepared nanoparticles, (αhν)2 vs. hν plot was obtained employing Tauc's relation to UV–vis absorption spectrum [27]. The increased value of band edge absorption for ZnO nanoparticles estimated to be 3.54 eV ( 350.7 nm) through (αhν)2 vs. hν plot; as shown in the inset of Fig. 5. This represents a significant blue shift in the value of absorption energy levels of ZnO nanoparticles as compared to that of ZnO bulk band gap energy (i.e. 3.2 eV) [6]. The enhanced radiative absorptions in 325–375 nm region could be due to the interband absorptions occurring among the far separated discrete energy states of conduction and valence band. The existence of discrete energy states and subsequently the blue shift in absorption energy are the manifestations of quantum size confinement in ZnO nanoparticles [6,26,34]. The broadening of absorption region (3.81–3.30 eV) in our case is due to availability of various discrete energy states of conduction band (CB) and valence band (VB) and subsequently the interband absorption occurring among these states. The existence of several such discrete energy states in prepared ZnO nanoparticles could be due to two reasons. The first reason being the presence of additional surface or defect states (e.g., vacancy, interstitial, antisites of zinc or oxygen) or both. These new surface states appear due to large aspect ratio for nanoparticles whereas the defect states in our case may emerge due to excess of oxygen in prepared ZnO nanoparticles, as suggested previously by EDAX analysis. Secondly, each finite size nanoparticle has a corresponding definite band structure where discrete far spaced energy states are available. Since the ZnO nanoparticles in our case exhibit a size distribution (i.e. 25–30 nm); the different sized nanoparticles would provide different band structures/energy states. Therefore, due to the collective effect of above-mentioned two reasons, in our system of oxygen rich 25–30 nm ZnO nanoparticles discrete energy states for conduction and valance band region appear, along with defect states within this region. In principle, these various energy states could be available for radiative transitions (here optical absorption) of different energy values. Thus UV–vis spectroscopic analysis of synthesized ZnO nanoparticles reveal crucial information regarding the radiative

Fig. 5. UV–vis absorption spectrum of ZnO nanoparticles. Plot for (αhν)2 vs. hν is also shown in the inset.

absorptions of enhanced energy occurring among the discrete energy states from and around the edges of VB and CB. These interesting results motivated us to employ fluorescence spectroscopy so as to obtain a more comprehensive picture of band structure of prepared ZnO nanoparticles and optical processes occurring therein. 4.5. Fluorescence spectroscopy In order to study the band structure, constituent discrete states and optical processes of excitation and emission in nanoparticles and more significantly, to probe the variation in the aforesaid properties due to size distribution (i.e. 25–30 nm) accompanied with the possible presence of defect states in prepared nanoparticles; we performed fluorescence (FL) spectroscopy of methanol suspended ZnO nanoparticles, The FL emission spectrum of ZnO nanoparticles, excited with 275 nm wavelength, is shown in Fig. 6. FL data recorded in the emission range 285–700 nm while keeping the excitation and emission slit widths fixed at 2.5 nm. Various FL emissions of ZnO nanoparticles can be suitably identified and investigated by de-convoluting the experimental FL emission spectrum. As shown in Fig. 6, five individual emission spectra, centered respectively around 350.7 nm, 400 nm, 433 nm, 521 nm and 572 nm, and the convolution of these individual spectra fits well with the experimentally acquired FL emission spectrum of ZnO nanoparticles. In the forthcoming sections, we attempt to explicate these fluorescence emissions. Moreover, on the basis of our previously discussed EDAX, XPS and UV–vis analysis and in view of observed emissions in FL spectrum; we shall attempt to collectively comprehend the electronic structure and optical processes occurring within prepared ZnO nanoparticles. A prominent yet less intense peak at 521 nm is observed in the FL emission spectrum of nanoparticles and is usually reported green emission for ZnO [6,16]. Among more intense emissions, a broad emission region around 400–433 nm and an emission at 572 nm are noticed. The origin, nature and

Please cite this article as: A. Sahai, N. Goswami, Probing the dominance of interstitial oxygen defects in ZnO nanoparticles through structural and optical characterizations, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.06.041

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Fig. 6. Fluorescence emission spectrum of ZnO nanoparticles (labeled through black square boxes) depicting a good match with simulated data (red color solid circle curve) along with de-convoluted five emission spectra centered around 350.7 nm (violet color solid triangular curve), 400 nm (indigo color square solid curve), 433 m (blue color inverted solid triangular curve), 521 nm (green color inverted soils square curve) and 572 nm (greenish-yellow color hexagon curve) respectively.

Fig. 7. A schematic of various optical processes of absorption, excitation and subsequent radiative recombinations among discrete energy states of ZnO nanoparticles. The energy values of optical transitions are in electron-volts.

attributes of various radiative transitions are discussed ahead and illustrated with the help of the schematic (Fig. 7) depicting the electronic energy structure and optical processes occurring within ZnO nanoparticles. It is well known that the size quantization and existence of surface and defect states in nanoparticles not only increase the band gap but also influence various radiative transitions, such as fluorescence [4,9,35]. It is well established that abundant discrete energy states exist in 0-dimensional nanostructures such as ZnO nanoparticles [6]. The emergence of these new discrete energy states in a nanoparticle could be due to either quantum confinement effect or surface/defect effects or both [6].

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It is evident through previous density functional theory (DFT) calculations that oxygen vacancies (VO) and zinc vacancies (VZn) are the lowest energy defects, followed by the Zni and the Zinc antisite (ZnO). Oi and OZn were found to be further higher in energy. The defects that are favored under Zn-rich conditions could be VO, Zni and ZnO and these all act as donors, whereas defects that are favored under O-rich conditions could be VZn, Oi and OZn and these all act as acceptors [36]. Thus, Oi could be considered as deep acceptor levels, in vicinity of valance band (VB); whereas VO are generally considered as deep donor levels, located just below the conduction band (CB) [37]. These states for ZnO nanoparticles are vividly shown in the schematic (Fig. 7). For ZnO nanoparticles prepared by us, the green emission around 521 nm is observed however, as compared to various other luminescence lines of ZnO, the origin of the green emission is the most controversial. The green emission around 521 nm is widely reported and attributed to VO however, green emission centered around 490 nm, 500 nm 510 nm and 530 nm is also reported [30]. Not only the position of green emission but also the cause of this emission are presented differently, for example, few groups attributed this phenomenon to oxide antisites defect OZn. Vanheusden et al. [6,37] elucidated the mechanism behind green photoluminescence and claimed that VO are responsible for the green luminescence in ZnO. Of late, Thareja et. al. [6] also studied the photoluminescence from ZnO nanoparticles and nanowires grown by pulsed laser deposition and observed that luminescence due to defects, in particular, VO, is not observed in ZnO clusters formed in vapor phase [6,38,39]. Therefore, green emission at 521 nm observed for prepared nanoparticles is most widely reported emission for ZnO and this emission from Vo is shown in the proposed schematic [6]. Having explained the green emission, we now attempt to analyze the fluorescence in the prominent broad region 400– 433 nm. De-convolution of FL spectrum reveals that this broad region is actually comprised of three emission spectra centered around 350.7 nm (i.e. 3.5 eV), 400 nm (i.e. 3.07 eV) and 433 nm (i.e. 2.86 eV), as shown in Fig. 7. The FL emission at 350.7 nm (3.5 eV) is nothing else but the de-excitation from the band edge of ZnO nanoparticles and is in agreement with the UV–vis absorption observed earlier at 350.3 nm. Near UV–vis fluorescence around 400 nm is another popularly reported emission for ZnO nanoparticles [6]. Generally, visible luminescence in ZnO is caused primarily by the transition from deep donor level to valance band (VB) and by the transition from CB to deep acceptor level [6,16]. Therefore, emergence of violet–blue fluorescence at 400 nm divulges the existence of blue shifted near band edge defect/surface states in prepared nanoparticles [6]. Now we discuss remaining two FL emissions at 572 nm and 433 nm. As explained earlier, contrary to generally reported zinc rich stoichiometry for n-type ZnO, our EDAX analysis discloses oxygen rich stoichiometry of prepared ZnO nanoparticles. Oxygen rich stoichiometry of ZnO nanoparticles could be due to the presence of OZn or Oi defects [36]. High formation energy is required for OZn whereas relatively less

Please cite this article as: A. Sahai, N. Goswami, Probing the dominance of interstitial oxygen defects in ZnO nanoparticles through structural and optical characterizations, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.06.041

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formation energy is required for Oi [36,40,41]. It was also found that O on the ideal Zn site is unstable and spontaneously relaxes to an off-site configuration [36]. Hence, it is very unlikely that OZn would be present in equilibrium [36]. Therefore, occurrence of OZn seems unlikely in ZnO nanoparticles prepared by us. On the other hand, Oi can exist either as electrically inactive O0i (split) in semi-insulating and p-type materials, or as deep acceptors at the octahedral interstitial site O2i  (oct) in n-type materials (EF Z2.8 eV) [36]. Therefore, Oi can either occupy the octahedral/ tetrahedral interstitial sites or form split interstitials [42]. DFT calculations further elucidated that the Oi at the tetrahedral site is unstable and spontaneously relaxes into a split-interstitial configuration in which the extra O atom shares a lattice site with one of the nearest-neighbor O atoms [36]. Oi may also exist as electrically active interstitials occupying the octahedral site Oi (oct), introducing states in the lower part of the band gap that can accept two electrons. Based on a comparison with the migration energies of VO, it is earlier reported that Oi is the species responsible for defect recombination on the oxygen sublattice, especially under n-type conditions [36]. It is informative here to note that the OZn is an acceptor-type defect with very high formation energy, even under the most favorable O-rich conditions [36–38]. This suggests that Oi defects may act as donor as well as acceptor states. More precisely in our case, Oi acts as deep acceptor level since this lies near to the reported value of Fermi energy (i.e., 2.8 eV) for ZnO [36]. On the basis of aforesaid explanation pertaining to the formation of Oi and subsequently radiative transitions occurring from Oi defect states, we affirm that Oi defects exist in prepared ZnO nanoparticles. Accordingly, 433 nm (  2.86 eV) emission in our case are attributed to Oi defect states, as presented in Fig. 7. We therefore infer that oxygen rich stoichiometry in our case is basically due to the existence of Oi defects in prepared ZnO nanoparticles. At last, we attempt to understand the greenish yellow 572 nm FL emission for ZnO nanoparticles. In fact varied positions and consequently different causes of yellow or greenish-yellow emission in ZnO are suggested. To name a few, yellow luminescence around 590 nm due to Oi, 560 nm emission due to Vzn are reported [30]. Greenish-yellow luminescence at 570 nm earlier reported to be arising from Oi [30]. Therefore, between two optional channels of greenish yellow emission, namely between Oi and VZn, the emission at 572 nm in our case could preferably due to Oi rather than VZn. This assumption is in agreement with aforesaid discussion too where 433 nm emission is assigned to Oi. However, in order to further ascertain that Oi exist in prepared ZnO nanoparticles and these Oi defects are the actual cause of 572 nm emission in present case, one more justification could be provided. As per the published reports it can be inferred that annealing of ZnO nanoparticles at 400 1C, as performed by us, not only assist in removing carbonate impurities but also leads to the formation of VO following oxygen desorption [6,30]: 1 ZnO-VO þ ZnZn þ O2 : 2 The desorbed oxygen atoms leave the interstitial states and settle into other interstitial sites and due to this hopping process of

oxygen atoms, VO and Oi are created in the ZnO lattice [30]. Thus, VO and Oi related luminescence bands appear in ZnO, as observed in our case too [30]. Substantiated through aforesaid three interpretations, the FL emission at 572 nm in prepared ZnO nanoparticles is attributed to Oi and accordingly presented in Fig. 7. It is notable here that further increasing the annealing temperature to 600 1C and onwards, above mentioned processes enhance and the intensity of VO and Oi increases [30]. Due to further annealing at 625 1C, the oxygen atoms leave from the interstitial sites and hop to other interstitial sites and form VO–Oi pairs [30]. But, in this annealing process at 625 1C, due to the presence of ambient oxygen and Oi, the absorption of oxygen atoms counteracts the formation of the VO–Oi pairs and pushes the Oi atoms back to the sites of the VO [30]. These positive and negative processes can be expressed using the equilibrium: Oo ¼ VO þ Oi, where Oo denotes the oxygen atoms at the interstitial sites [30]. Thus annealing of ZnO at 625 1C cause increase of Oi and decrease of VO which eventually leads to the enhancement of the yellow luminescence (at 570 nm) and the quenching of the green luminescence (at 521 nm) [30]. When the annealing temperature becomes farther higher (650 1C), the desorption of oxygen atoms is extremely intensive so that the Oi cannot remain and diffuses off from ZnO surface [30]. Based on this mechanism of annealing temperature dependent variation of intensity of yellow/green emissions (572/521 nm) we suggest that the intensity of Oi defect related emissions (e.g., 433 nm, 572 nm etc.) in ZnO nanoparticles could possibly be fine tuned through variation of annealing temperature and a systematic study on this crucial issue is worth performing [30]. Therefore, FL and UV–vis absorption spectroscopies collectively provide significant insight of the energy states of prepared ZnO nanoparticles and radiative processes occurring therein. It is worth noticing here that not only VO exist and originate 521 nm green emission but more importantly Oi defects are dominant in our system and engender 433 nm and 572 nm emissions. It was already evident through XPS analysis that the point defects, dominant in prepared ZnO nanoparticles, are Oi rather than VO. This is further justified through our FL analysis since the intensity of blue and greenish-yellow emissions at 433 nm and 572 nm wavelengths due to Oi is more prominent than the green emissions at 521 nm wavelength due to VO. Thus, contrary to usually reported zinc rich or oxygen deficient stoichiometry of n-type ZnO, we not only establish the coexistence of VO and Oi in prepared ZnO nanoparticles but more significantly demonstrate that oxygen rich stoichiometry of prepared nanoparticles is due to dominance of Oi defects. 5. Conclusion In summary, hexagonal phase ZnO nanoparticles of 25–30 nm size were prepared adopting a facile chemical precipitation method. The oxygen rich composition of prepared nanoparticles was indicated through EDAX analysis. XPS analysis ascertains the dominance of interstitial oxygen defects albeit coexistence of

Please cite this article as: A. Sahai, N. Goswami, Probing the dominance of interstitial oxygen defects in ZnO nanoparticles through structural and optical characterizations, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.06.041

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oxygen vacancy defects. The correlation of structural and electronic properties, as determined through TEM, EDAX and XPS analysis, with the optical properties of ZnO nanoparticles was well established through FL analysis in conjunction with UV–vis spectroscopic investigations. The blue shift in the band edge absorption in prepared nanoparticles was evident. Analysis of radiative recombinations reveal the emergence of discrete energy states of ZnO nanoparticles and oxygen defects. FL emissions in prepared nanoparticles were shown to occur from the band edge and oxygen defect states (VO and Oi). However, contrary to usually reported VO defects and subsequent emissions in case of n-type ZnO, Oi defects were majorly responsible for various prominent FL emissions in present case. We provided substantial evidences of coexistence of VO and Oi defects in prepared ZnO nanoparticles although Oi defects and related emissions dominate the FL emissions in our case. As a future course, annealing temperature dependent variation of Oi defects in ZnO, the control on Oi defects and eventually fine tuning the Oi related fluorescence in ZnO nanoparticles, could possibly be investigated. Interestingly, present report indicates the possibility of attaining p-type semiconducting properties for prepared ZnO nanoparticles. The electrical characterization to determine the type of semiconductor and other associated properties of prepared ZnO nanoparticles are therefore, of great significance.

Acknowledgment Anshuman Sahai gratefully acknowledges Department of Science and Technology (DST), Ministry of Science and Technology (MST), India for INSPIRE fellowship.

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