Influence of the sol gel synthesis parameters on the photoluminescence properties of ZnO nanoparticles

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A B S T R A C T In this work, ZnO NPs were successfully synthesized by the sol–gel method without any organic additives or post annealing. The effect of the preparation process on the structural and optical properties of the resulting NPs was
  Contents lists available at ScienceDirect Materials Science in Semiconductor Processing  journal homepage: In fl uence of the sol gel synthesis parameters on the photoluminescenceproperties of ZnO nanoparticles R. Bekkari a, ⁎ , L. laânab a , D. Boyer b , R. Mahiou b , B. Jaber c a  LCS, Faculty of Sciences. Mohammed V University of Rabat, Rabat, Morocco b University of Clermont Auvergne, Chemistry Institute of Clermont-Ferrand, UMR 6296 CNRS / UBP / Sigma Clermont, 63171 Aubiere, France c CNRST, Angle Allal El Fassi / FAR, B.P. 8027, Hay Riyad, 10000 Rabat, Morocco A R T I C L E I N F O  Keywords: ZnONanoparticlesSol-gelPhotoluminescence A B S T R A C T In this work, ZnO NPs were successfully synthesized by the sol – gel method without any organic additives or postannealing. The e ff  ect of the preparation process on the structural and optical properties of the resulting NPs wasinvestigated by means of X-ray di ff  raction (XRD), transmission electron microscopy (TEM) and photo-luminescence (PL) spectroscopy. The structural characterization demonstrated clearly that the NPs crystallize inpure ZnO würtzite structure without any other secondary phases. Furthermore, we show that it is possible toperform the control of the crystalline growth orientation of ZnO NPs, which is a key parameter when seeking todevelop ZnO NPs with piezoelectric properties for nano-transducer applications. In fact, TEM observations showthat the reduction of the NaOH  fl ow changes the NPs shape from hexagonal NPS to short nanorods grown alongthe c-axis. The PL spectra of the obtained NPs excited at 280 nm, present an UV emission centered at ap-proximately 380 nm with a slight shift when varying the synthesis temperature and/or the NaOH  fl ow.Moreover, as the visible region (from 400 to 650 nm) is concerned, it was shown that the increasing of thesynthesis temperature a ff  ects strongly the kind of interstitial defects (O i , Zn i  and V o Zn i ) formed in ZnO nanos-tructures. However, the excitation at 320 nm revealed a broad deep-level emission for all the samples that can bedeconvoluted into two Gaussian peaks centered at 514 nm (P 1 ) and 581 nm (P 2 ). These last results have beendiscussed in the light of a physical mechanism based on the Schottky barrier. 1. Introduction ZnO is a wide-band-gap energy (3.37 eV) semiconductor with manyuseful properties such as piezoelectricity [1 – 3], conductivity [4 – 6] andcatalytic activity [7 – 9]. These properties depend strongly on the mi-crostructural characteristics of the material such as orientation, crystalsize and morphology. As piezoelectric application (transducers) isconcerned [10,11], the synthesis of ZnO nanoparticles with controlledgrowth orientation and the understanding of their physical propertiesare the key parameters to develop this  fi eld [12,13]. One of the aims of this work is to show that it is possible to perform this control only byadjusting NaOH  fl ow during the process.In the optoelectronic side, ZnO nanoparticles exhibit two types of emissions: one is in the ultraviolet UV domain, centered at approxi-mately 380 nm; and the other is in the visible region in the range of 450 – 765 nm [14,15]. The UV emission band is related to a near band-edge transition of ZnO namely, the recombination of the free excitons[16,17], while the visible luminescence has been attributed to variousdefects in the crystal structure such as O-vacancy (V O ) [18,19], Zn-vacancy (V Zn ) [20,21], O-interstitial (O i ) [22,23], Zn-interstitial (Zn i )[24,25] and/or extrinsic impurities [26,27]. However, the suggested explanations for the di ff  erent visible emissions in ZnO are frequentlycon fl icting. Therefore, a complete understanding of the di ff  erent lu-minescence emissions and there srcins is a stimulating subject in theZnO emission studies.Several studies have been achieved in order to prepare ZnO nano-particles in di ff  erent sizes and morphologies using various developingmethods including the sol – gel method [28 – 30], the metal organicchemical vapor deposition (MOCVD) [31], the  fl ame spray pyrolysis[32], the thermal decomposition [33] and the precipitation [34]. It comes out that the kind and the concentration of the defects generatedin a material depend greatly on the preparation method and impactsdirectly the photoluminescence properties [14,35]. Up to now, no re-levant physical mechanism, explaining the relationship between thephotoluminescence properties and these defects, has been developed.In the present study, di ff  erent morphologies of ZnO nanoparticleshave been prepared by sol-gel method. The e ff  ects of the NaOH  fl owand the synthesis temperature on the morphological and 15 June 2017; Received in revised form 28 July 2017; Accepted 29 July 2017 ⁎ Corresponding author.  E-mail address: (R. Bekkari). Materials Science in Semiconductor Processing 71 (2017) 181–1871369-8001/ © 2017 Elsevier Ltd. All rights reserved. M R  photoluminescence properties of the resulting ZnO nanoparticles havebeen investigated. A physical mechanism has been developed to explainthe obtained photoluminescence results. 2. Experimental procedure Zinc Oxide nanostructure is synthesized using sol-gel method,without any additional organic additives or subsequent annealingtreatment. In all experiment, 0.1 M aqueous solution of zinc acetate(ZnAc 2 , 2H 2 O, minimum 98.5%) is prepared in 100 ml of ultrapurewater. Then, NaOH is added under  fl ow control and slow magneticstirring, respecting an alkaline ratio ([OH - ]/ [Zn 2+ ]) of 2.5:1. The re-action mixture is maintained for 1 h at selected temperature (rangingfrom 0 to 80 °C) then cooled to room temperature. After that, the pre-cipitate is  fi ltered by centrifugation, washed with ultrapure water anddried by lyophilization.Crystalline powder structure was checked by X-ray Di ff  raction(XRD) using the Panalytical X PERT-PRO powder di ff  ractometer withthe Cu-K α  radiation (45 kV, 40 mA) working in Bragg-Brentano geo-metry. The morphology of the obtained NPs was examined with the FEITransmission Electron Microscopy (TEM, Tecnai G2 12 TWIN, 120 kV)with a LaB 6  fi lament. The optical properties were investigated by re-cording the photoluminescence spectra with a Jobin-Yvon set-up con-sisting of a Xenon lamp operating at 400 W and two monochromators(Triax 550 and Triax 180) combined with a cryogenically cold chargecoupled device (CCD) camera (Jobin-Yvon Symphony LN2 series) foremission spectra and with a Hamamatsu 980 photomultiplicator forexcitation ones. 3. Results and discussion 3.1. E   ff  ect of NaOH   fl ow3.1.1. X-ray di  ff  raction (XRD) analysis Fig. 1 shows the XRD patterns of the ZnO nanoparticles elaboratedat three NaOH  fl ows; the alkaline ratio ([OH - ]/ [Zn 2+ ]) and reactiontemperature have been maintained at 2.5&50 °C, respectively. Therecorded peaks show that the particles crystallize in pure ZnO würtzitestructure (JCPDS 36-1451) without any other secondary phases. TheFig. 1 shows also that peaks intensities decrease with the increase of NaOH  fl ow, indicating that the crystallinity is improved when NaOH isslowly added to the solution i.e. the reaction time is increased.To quantify the preferred orientation tendency of the obtained ZnONPs, we compute the relative intensity ratio (I r ) using the followingformula [36]: =∑  I  I  I  r hklhkl  (1)As can be deduced from Fig. 2, the relative intensity ratio of (002)peak decreases when varying NaOH  fl ow up to 20 ml/min, implyingthat the ZnO NPs behave like the bulk with no preferential orientationat high NaOH  fl ow.The size of the nanoparticles (D) is estimated by using the Debye-Scherrer formula: =  D  0, 9λ βcos  (2)where  λ  is the wavelength of the used X-ray radiation;  β  is the FullWidth at Half Maximum (FWHM) and 2 θ  is the highest di ff  ractionangle.In the same way, the lattice constants a and c of the ZnO wurtzitestructure are calculated using Bragg's law: = a λ 3.Sin hkl  (3) = c λ Sin hkl  (4)Table 1 exhibits the crystallite sizes and the lattice parameters forZnO NPs elaborated at di ff  erent NaOH  fl ows. No signi fi cant di ff  erenceis observed between the ZnO NPs lattice constant values compared withthe bulk and the crystallite size remains almost constant whatever thesynthesis conditions. 3.1.2. Transmission electron microscopy (TEM) studies In order to elucidate the morphology and size distribution of theZnO NPs, transmission electron microscopy (TEM) micrographs arepresented in Fig. 3. The grain sizes measured from this  fi gure matchedwell with the obtained values by the Debye-Scherrer formula. Thesemicrographs show also that the particles take di ff  erent morphologieswhich depend signi fi cantly on the NaOH  fl ow. The particles take: i) ahexagonal NPs shape (Fig. 3(a)) with an average diameter of about37 nm at relatively high  fl ow (20 ml/min); ii) a self-assembled spindle-shape nanorods of about 35 nm at 2.5 ml/min, (Fig. 3(b)) and iii) a Fig. 1.  X-Ray di ff  raction patterns of ZnO NPs obtained at various NaOH  fl ows. Fig. 2.  Relative intensity ratio of ZnO NPs prepared at di ff  erent NaOH  fl ows. Table 1 Evaluated structural parameters of ZnO nanoparticles elaborated at di ff  erent growthconditions. Samples NaOH  fl ow (ml/min) size (nm) a (Å) c (Å)(a)  20 32 3.242±0.001 5.203±0.001 (b)  2.5 30 3.241±0.001 5.201±0.001 (c)  0.17 36 3.242±0.001 5.203±0.001 ZnO Bulk  – –  3.249 5.206  R. Bekkari et al.  Materials Science in Semiconductor Processing 71 (2017) 181–187  182  short nanorods oriented along the c-axis (Fig. 3(c)) with a mean dia-meter of about 40 nm at relatively low NaOH  fl ow (0.17 ml/min). Thisimplies that the high NaOH  fl ow favors the nucleation process andprevents the NPs growth. However, the low NaOH  fl ow favors the ZnONPs growth along the c axis taking advantage of the energy provided bystirring and heating [28]. 3.1.3. Photoluminescence (PL) studies Photoluminescence (PL) spectra of the powdered ZnO nanos-tructures, synthetized at di ff  erent NaOH  fl ows, are presented inFig. 4(a). The measurements were performed at room temperature withan excitation wavelength of 280 nm. Three peak positions are clearly identi fi ed on the  fi gure and are associated to the three NaOH  fl ows.More details are shown in Table 2.Fig. 4(b) and Table 2 show strong UV peaks centered at about 375, 380 and 381 nm with a slight shift ascribed to the variation of NaOH fl ow. The FWHM is less than 30 nm for the three samples, indicatingthat the system is monodisperse and has a narrow size distribution [37],which is consistent with the earlier experimental XRD observations(Table 1). As can be seen from Fig. 4(a)&(b), the intensity of the UV peak depends clearly on the NaOH  fl ow. The peak intensity is higher at2.5 mol/min and lower at 20 mol/min. Such behavior can be explainedby the enhancement of non-radiative recombination processes linked tothe creation of defects. It is quite common that the surface defects and/or native defects in the bulk (oxygen vacancies et al.) produce non-radiative recombination centers (NRCs) [38]. These defects have anenergy level structure that is di ff  erent from substantial semiconductoratoms; and can create one or several energy levels within the forbiddengap of the semiconductor (Fig. 5) [39]. These deep level defects called also recombination centers are responsible of the decrease in the ra-diative lifetime [40] and then the reduction of the internal quantume ffi ciency of the UV emission. In the non-radiative recombination Fig. 3.  Representative TEM images and particle size distribution of the ZnO nanoparticles obtained at di ff  erent NaOH fl ows: (a, a ′ ) hexagonal, (b, b ′ ) elliptical, (c, c ′ ) nano-columns shape. Fig. 4. (a)  The emission bands of the ZnO nanostructures obtained at di ff  erent NaOH fl ows. Upon excitation at 280 nm;  (b)  PL spectra of various nanostructures in the UVrange: 1) short nanorods, 2) spindle-shape, 3) hexagonal NPs. Table 2 The UV peak positions and the FWHM for the three samples obtained at di ff  erent NaOH fl ows. ParticlesmorphologiesNaOH  fl ow(ml/min)Peakposition(nm)Energy (eV) FWHM(nm) short nanorods 0.17 380 3.26 24spindle-shape 2.5 375 3.30 29Hexagonal NPs 20 381 3.25 26  R. Bekkari et al.  Materials Science in Semiconductor Processing 71 (2017) 181–187  183  process, the electron energy is converted to phonons (vibrational en-ergy of lattice atoms). We think that this phenomenon is the srcin of the observed decrease in the UV peak intensity. Owing to that, thesample prepared at 2.5 mol/min of NaOH  fl ow with the highest peakintensity would have the lowest non-radiative defect concentration.As mentioned before, a slightly red-shift of the UV emission peakfrom 3.30 to 3.25 eV is observed when varying NaOH  fl ow. It is usuallyassumed that the native defects are responsible for red shifted emission[41 – 43]. Generally ZnO shows n-type semiconducting properties inwhich most defects are Zn interstitials and oxygen vacancies that act asdonors in the ZnO lattice [44,45]. These defects give rise to shallowlevels closer to conduction band (0.05 eV) and provide a path for anemissive transition from a donor level band to the valence band [46].Therefore, we think that the observed shift indicates that the NaOH fl ow a ff  ects clearly the concentration of these shallow defects whichleads to the observed variation of the bandgap.In another hand, the Fig. 4(a) exhibits also two peaks centered at514 and 581 nm in the visible region, which have been attributed to thedi ff  erent defects emissions.Fig. 5(a) illustrates the photoluminescence spectra of the preparedsamples with the excitation wavelength at 320 nm. All samples exhibit,in addition to the UV emission, a broad deep level emission (DLE) bandranged from 500 to 650 nm. The relative integrated PL intensity ratiobetween the UV emission (I UV ) and DLE emission (I DLE ) is used tocharacterize the photoluminescence properties of ZnO as presented onTable 3. The larger the intensity ratio is, the better the optical prop-erties are and then the fewer deep level defects are formed. FromTable 3, the maximum of I uv  /I DLE  is obtained when the NaOH  fl ow is0.17 ml/min which is consistent with the XRD analysis. The deconvo-lution of the DLE band (Fig. 5(b) – (d)) shows that the experimentalspectra are composed of two Gaussians curves marked as P 1  and P 2  andcentered at 514 and 581 nm, respectively. Results presented in Table 3show that the relative intensity ratio (I P1 /I P2 ) is independent of theNaOH  fl ow.To explain the origin of the observed visible emissions each Fig. 5.  Schematic diagram of radiative and non-radiative recombination process in a n-type material. Table 3 The relative ratio of I uv /I DLE  and I P1 /I P2  for obtained ZnO NPs. ParticlesmorphologiesNaOH  fl ow(ml/min)I uv /I DLE  relativeratioI P1 /I P2  relativeratio Short nanorods 0.17 1.54 0.38Spindle-shape 2.5 0.81 0.37Hexagonal NPs 20 1.31 0.40 Fig. 6. (a)  The emission bands of the ZnO nanostructures prepared at di ff  erent NaOH  fl ows. Upon excitation at 320 nm;  (b-d)  deep level emissions  fi tted by Gaussian functions.  R. Bekkari et al.  Materials Science in Semiconductor Processing 71 (2017) 181–187  184  boundary can be regarded as a pair of back to-back Schottky barriers,i.e. metal-semiconductor junctions. This model supposes donor likepositively charged defects in the depletion layer and acceptor like ne-gatively charged defects in the grain-boundary interface (Fig. 6). Adepletion region of width W is then formed at the particle surface. Thisleads to the formation of a potential barrier associated to the bandbending, which plays a signi fi cant role in the variation of defectchemistry. Because of the high electrical  fi eld, all the oxygen vacancieswill be in the diamagnetic V O++ state in the positively chargeddepletion region; while in the bulk region of the grain, the majoritydefects are expected to be in their paramagnetic V O+ states.The width of the depletion region is given by: = W  ε V qN  2  ZnO bi D  (5)where  V  bi  is the barrier potential,  q  is the electronic charge,  N   D  is thedonor density and  ε ZnO  is the static dielectric constant of ZnO.In ZnO, the defect responsible for the broad visible emission isoxygen vacancy denoted as V O  in di ff  erent state of charge [48,49]. Thisfault takes di ff  erent states of charge depending on the region where it islocated. Because of the intense electrical  fi eld in the depletion regionthe singly charged center V O+ becomes doubly charged V O++ (1electron is ripped o ff  ). The transition of one electron from conductionband to the V O++ deep level leads to an emission in the surroundings of 2.2 eV (E P2 ); that is represented by the P 2  line in Fig. 6. Meanwhile, theoxygen vacancy defect is singly charged center (V O+ ) when it is locatedfar from the grain surface. It becomes neutral center (V OX ) by capturingan electron from the conduction band which then recombines with ahole in the valence band resulting to an emission (P 1 ) at 2.5 eV (E P1 ).From this model it is expected that the P 1 /P 2  ratio depends strongly onthe electronic properties of the grain surface that a ff  ect extension of thedepletion region and then the band bending. 3.2. Optimization of the synthesis temperature3.2.1. Photoluminescence (PL) studies To study the in fl uence of the synthesis temperature on the mor-phological (shape and size) and optical properties many samples wereelaborated at di ff  erent temperatures (from 0 to 80 °C). The structural,microstructural and optical analysis of these ZnO NPs has been pub-lished in previous work [29]. Here we are interested only on the pho-toluminescence behavior of these NPs that is illustrated in Fig. 7. This fi gure shows that all the NPs prepared at di ff  erent temperatures exhibitonly one emission in the UV region and many emissions in the visibleone. A slight red shift is observed in the UV region when the synthesistemperature is increased from 10 to 80 °C. This behavior, reported alsoby others [50,51], is due to the increase of the particle size which leadsto a reduced band gap, as the temperature increases.Meanwhile, in the visible region (from 400 to 650 nm), the in-creasing of the synthesis temperature a ff  ects strongly the kind of defects(deep level) formed in ZnO nanostructures. It is well documented thatthe emissions in the visible region in ZnO nanostructures are attributed Fig. 7.  Schematic diagram depicting the srcin of P1 and P2emission bands. Ec, Ev and EF are the conduction band, valanceband and Fermi energy levels respectively [47]. Fig. 8.  Photoluminescence spectra of ZnO nanoparticles prepared at di ff  erent synthesistemperatures and excited by 280 nm.  R. Bekkari et al.  Materials Science in Semiconductor Processing 71 (2017) 181–187  185
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