Journal of Science: Advanced Materials and Devices (2017) 51e58 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article ZnO nanostructures with tunable visible luminescence: Effects of kinetics of chemical reduction and annealing R Raji, K.G Gopchandran* Department of Optoelectronics, University of Kerala, Thiruvananthapuram 695581, India a r t i c l e i n f o a b s t r a c t Article history: Received 20 December 2016 Received in revised form February 2017 Accepted February 2017 Available online 13 February 2017 Highly crystalline ZnO nanoparticles were synthesized using a co-precipitation method The morphology and optical properties of these nanoparticles are found to be highly sensitive to the growth parameters such as the concentration of reducing agent and annealing temperature Indeed, the concentration of the reducing agent can alter the morphology of nanoparticles from quasi-spherical to rod-like and then to flower-like structures Attempts were made to tune the emission wavelength over the visible region by varying the kinetics of chemical reduction and annealing The possibility of tuning the emission in a visible range from orange to red and then to green by changing the nature of defects by annealing is also reported Analysis of the Raman spectrum, with its intensity observed at 580 cmÀ1 corresponding to E1 (LO) mode, revealed that the kinetics and thermodynamics of formation and growth of these nanoparticles determined the nature and density of the probable defects such as oxygen vacancies, interstitial zinc atoms and their complexes © 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Semiconductor Zinc oxide Defects Raman spectroscopy Luminescence Introduction Over the past few decades, the scientific community has devoted considerable attention in the design and development of semiconductor nanostructures, which can show enhanced optical, electrical, mechanical and sensing properties owing to their quantum size effect [1,2] Zinc oxide (ZnO), a wide band gap (3.37 eV) oxide semiconductor gained substantial interest due to their tremendous demand for wide range of applications in photonic crystals, light emitting devices, photo detectors, photo diodes, solar cells, piezoelectric transducers, gas sensors, biological and chemical sensors etc., [3e5] Large exciton binding energy (60 meV) of ZnO at room temperature makes ZnO a promising material for the development of the blue and ultra-violet lasers and LEDs [6] Due to its unique and fascinating features, ZnO is considered as an appropriate substitute to GaN for the next generation light emitting devices [7] Extensive researches have been carried out to study the luminescence mechanism in ZnO nanostructures [8,9] In most cases, luminescence spectra of ZnO nanoparticles has two emission bands: one is the typical band edge transition or the exciton * Corresponding author E-mail address: gopchandran@yahoo.com (K.G Gopchandran) Peer review under responsibility of Vietnam National University, Hanoi combination and the other is the defect emission in the visible region due to trap states in ZnO [10] The origin of the defect related emission in the visible region is still a controversial question [11,12] Recently Ozgur et al reported that oxygen vacancies are responsible for visible luminescence from ZnO [9] The defect states arising from zinc or oxygen vacancies and the electrons or holes from the shallow trap states within the bandgap of ZnO are responsible for the broad visible luminescence in the region from 400 to 700 nm [11,13] In order to improve the luminescence efficiency and to tune the wavelength over a wide range from blue to red, size and shape controlled synthesis of ZnO nanocrystals has been widely employed [14] Recently, nanostructured ZnO materials with tunable particle size and shape have been prepared by adopting several physical or chemical synthetic methods such as thermal evaporation [15], pulsed laser deposition [16], thermal decomposition [17], solegel technique [18], hydrothermal process [19] and co-precipitation method [20] Among the various methods, we adopt co-precipitation method because of its simplicity and flexible post synthesis process, which offers a possibility of large area yield at low cost In this work, we studied the effect of reaction kinematics such as concentration of KOH and annealing temperature on the structural, morphological and optical properties of ZnO nanoparticles synthesized by simple co-precipitation method Attempts were made to tune the emission intensity and wavelength over the entire http://dx.doi.org/10.1016/j.jsamd.2017.02.002 2468-2179/© 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) 52 R Raji, K.G Gopchandran / Journal of Science: Advanced Materials and Devices (2017) 51e58 visible region by varying the concentration of KOH and annealing temperature The observed change in color and the intensity of photoemission of ZnO nanoparticles with the size and morphology are discussed in detail Experimental Zinc nitrate hexahydrate (Zn (NO3)2$6H2O) and potassium hydroxide (KOH) were procured from SigmaeAldrich All the chemicals were used in analytical grade and without further purification Deionized water was used throughout the experiment ZnO nanoparticles were prepared by co-precipitation method A volume of 120 ml of 0.1 M zinc nitrate hexahydrate precursor aqueous solution was prepared under vigorous stirring for 20 20 ml aqueous solution of 0.2 M KOH was added drop wise to 20 ml of zinc salt precursor solution under constant stirring and the reaction was continued for 40 min, yielding a white precipitate Thereafter, the solution was allowed to settle for h, the particle suspension was transferred to centrifuging tubes and subjected to centrifuging at 3000 rpm for 10 In each centrifugation, the reaction medium was changed using distilled water and ethanol and is repeated thrice Then, the precipitates were dried at room temperature for overnight The precursor powder thus obtained were fully grounded and then subjected to heat treatment in a muffle furnace at 300 C at the rate of C/minutes for 120 In order to study the influence of KOH on the formation of ZnO nanoparticles, concentration of KOH was varied from 0.2 to 1.2 M, in steps of 0.2 M Also, to investigate the effect of annealing on the luminescence properties of ZnO nanoparticles, sample with enhanced emission was subjected to annealing at various temperatures from 300 to 900 C The crystal structure and phase analysis of the nanostructures were investigated using an X-ray diffractometer (Philips PANanalyticalX’Pert Pro) with CuKa radiation (l ¼ 1.54056 A ) in the angular range of 2q from 20 to 80 The surface morphological analysis of the nanoparticles was studied using FE-SEM (JEOL-JSM 5600) The Raman spectra of the samples were recorded using a Horiba JobinYvon LABRAM-HR 800 spectrometer equipped with an Arỵ ion laser having 514 nm emissions Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) of the as-synthesized samples were recorded using TA instrument (Q600 SDT and Q 20 DSC) in the temperature range room temperature 28 C to 900 C UV-visible absorption and reflectance spectra of all the samples were recorded using a UV-visible spectrophotometer (JascoV550) A Horiba JobinYvonFluorolog (FL III) spectrofluorophotometer modified and equipped with HeeCd laser (325 nm) and R928P photomultiplier tube in photon counting mode as detector was used to record photoluminescence emission spectra at room temperature Fig TGA and DTA curve of the as synthesized ZnO nanoparticles The DTA curve shows an endothermic peak at 53 C and is due to the transition of ZnO nanoparticles Also, an exothermic peak was observed in the range from 55 to 296 C and it appears due to thermal lattice vibrations [22] There was no further weight loss above 300 C So the optimum annealing temperature taken for this study is 300 C 3.2 XRD analysis To study the effect of concentration of the reducing agent (KOH) on the formation of ZnO nanoparticles, it was varied from 0.2 to 1.2 M by keeping the concentration of Zn (NO3)$6H2O constant The corresponding X-ray diffraction patterns are shown in Fig All the peaks in the diffraction patterns are indexed according to the wurtzite structure of ZnO (hexagonal phase, space group P63mc), corresponding to JCPDS Card No 79-2205 The preferential growth was found to be along (101) crystal plane The other prominent peaks were (100) and (002) No excess peaks, such as Zn (OH)2 were detected, which indicated that the crystalline ZnO was formed at 300 C The phase of ZnO is found to be same for all the samples i.e., concentration of KOH has not influenced the formation of crystallite phase of ZnO The crystallite size (D) of the samples is calculated from the XRD data using DebyeeScherrer formula [23]: Results and discussion 3.1 TGA-DTA analysis The thermal behavior of as prepared ZnO nanocrystals has been investigated by differential thermal analysis (DTA) and thermo gravimetric analysis (TGA) and is shown in Fig Two weight losses were observed in the TGA curve The first step is in the temperature range 30e72 C indicating the evaporation of water adsorbed at the surface The second weight loss occurs in the range 72e288 C and may be due to the decomposition of residual compounds [21] In the TGA curve, a flat terrain is observed between 290 and 700 C indicating the formation of the ZnO nanoparticles as a decomposition product Fig XRD patterns of ZnO nanoparticles, prepared with different KOH concentrations; (a) 0.2, (b) 0.4, (c) 0.6, (d) 0.8, (e) and (f) 1.2 M [Inset: Typical HalleWilliamson plots of ZnO nanoparticles prepared with different KOH concentrations] R Raji, K.G Gopchandran / Journal of Science: Advanced Materials and Devices (2017) 51e58 D¼ 0:9l b cos q 53 (1) where, D represents crystallite size in nm, l is the wavelength (0.154178 nm) of the Cu Ka X-ray radiation used, q is the Bragg angle and b is the full-width at half-maximum (FWHM) corresponding to most prominent peak (101) measured in radians It is observed that the crystallite size of the ZnO nanoparticles decreases from 36 to 26 nm with increase of the concentration of KOH from 0.2 to M, beyond that the size increases (Table 1) The increase in size of particles with excess KOH may be due to higher precipitation rate On increasing the concentration of KOH, the diffraction peaks shift towards smaller Bragg's angles and it indicates a decrease in lattice parameters The lattice parameters of the prepared nanoparticles were calculated from the equation of lattice d spacing of the (h k l) planes for hexagonal crystal system and is given by [23]: h2 ỵ hk ỵ k2 l2 ỵ ẳ 2 a c dhkl (2) where dhkl is the interplanar separation corresponding to Miller indices h, k, l; a, b, and c are lattice parameters Table shows calculated values of lattice parameters of all the samples corresponding to (101) and (002) planes The cell volume and the number of atoms per unit cell for the ZnO samples with hexagonal form are estimated using equations [23]: pffiffiffi 3 a c V¼ n¼ (3) 4p D 3V (4) where a and c are lattice parameters, D is the crystallite size (nm) All the samples shows positive or extensive strain and may be due to the incorporation of defects in the form of interstitial oxygen or zinc [24] The strain (ε) and crystallite size (D) of the samples are also calculated by WilliamsoneHall (WÀH)method with the following relation [25]: b cos q ẳ Kl ỵ sin q D (5) where K (0.9) is crystallite shape constant The inset of Fig shows the typical WeH plots, i.e., by plotting ðb cos q=lÞ as a function of ðsin q=lÞ of samples prepared with KOH concentrations of and 1.2 M The intercept on the ðb cos q=lÞ axis gives the crystallite size corresponding to zero strain and slope of the line gives strain The sample with enhanced luminescence, prepared with M KOH, was selected for understanding the influence of heat Fig XRD patterns of ZnO nanoparticles annealed at different temperatures; (a) 300, (b) 400, (c) 500, (d) 600, (e) 700, (f) 800 and (g) 900 C [Inset: Typical HalleWilliamson plot of ZnO nanoparticles at 400 and 600 C ] treatment on the structural and optical properties The XRD patterns of this sample subjected to annealing at different temperatures is shown in Fig and the structural properties of these samples derived from XRD data is also provided in Table It is found that the intensity of all the peaks in the XRD patterns and the crystallite size increases with increase of annealing temperature (Fig 4) It is also observed that there is an increase in the number of unit cell with annealing temperature The observed size increment at high annealing temperature may be due to the migration of grain boundaries and the coalescence of grains [26] The lattice parameters are found to vary with temperature The change in lattice parameters indicates the presence of strain in the lattice of ZnO 3.3 Micro-Raman spectroscopy Raman spectroscopy is one of the effective methods to investigate the phase and purity of semiconductor nanocrystals ZnO is having two formula hexagonal structured with a space group C6v units per primitive cell, where all the atoms occupy the C3v sites [27] The group theoretical calculation predicts nine optical modes which are distributed as follows: Goptical ẳ A1 R; Rị þ 2B1 þ E1 ðIR; RÞ þ 2E2 ðRÞ (6) The B1 are silent modes, the A1 and E1 mode are active in both Raman and infrared spectra and split into transverse optical (A1T & E1T) and longitudinal optical (A1L & E1L) phonons with different frequencies due to microscopic electric field associated with the LO phonons E2 modes are non polar and Raman active only E2 mode split into E2 (high) and E2 (low) modes [27,28] Based on earlier Table Structural and optical properties of ZnO nanoparticles prepared with different concentrations of KOH [For bulk ZnO a ¼ b ¼ 0.3250 nm, c ¼ 0.5207 nm] Con: of KOH (M) 0.2 0.4 0.6 0.8 1.2 FWHM ( ) 0.2273 0.2772 0.2772 0.2922 0.2922 0.2598 Crystallite size Dhkl (nm) Lattice parameter DebyeeScherrer WeH plot a (A ) c (A ) 36.7 30 30 28 26 32 38 29 34 31 28 35 3.2557 3.2543 3.2521 3.2526 3.2503 3.2491 5.2421 5.2324 5.2265 5.2155 5.2245 5.2098 Strain ε Band gap (eV) No.of atoms/unit cell (n) 0.0027 0.0032 0.0038 0.0043 0.0051 0.0029 3.18 3.19 3.22 3.23 3.25 3.20 97 54 54 46 46 65 54 R Raji, K.G Gopchandran / Journal of Science: Advanced Materials and Devices (2017) 51e58 Table Structural and optical parameters of rod like ZnO nanoparticles annealed at different temperatures Temperature ( C) 300 400 500 600 700 800 900 FWHM ( ) 0.3168 0.2772 0.2772 0.2376 0.1980 0.1980 0.1584 Crystallite size Dhkl (nm) Lattice parameter DebyeeScherrer WeH plot a (A ) c (A ) 26 30 30 35 42 42 52 31 33 32 36 48 46 57 3.2503 3.2491 3.2481 3.2450 3.2476 3.2455 3.2483 5.2245 5.2132 5.2240 5.2084 5.2040 5.2048 5.2061 Fig Variation of intensity of (101) plane in XRD pattern with annealing temperature investigations, the frequencies of fundamental optical modes in ZnO can be assigned as follows, E2(low) ¼ 100 cmÀ1, E2(low)(TA) ¼ 208 cmÀ1, E2(high) ¼ 437 cmÀ1, E2(high) À E2(low)) ¼ 339 cmÀ1, E1(LO) ¼ 584 cmÀ1, A1(TO) ¼ 388 cmÀ1 and 2A1(LO), 2E1(LO) ¼ 1050 e 1200 cmÀ1 [27e29] Fig 5(a) represents micro-Raman spectra of ZnO nanoparticles over the spectral range 50e1300 cmÀ1 The intense peak located at 99 cmÀ1 is assigned to E2 (low) mode and is due to the vibration of Strain ε Band gap (eV) No.of atoms/unit cell (n) 0.0051 0.0044 0.0048 0.0040 e e e 3.25 3.23 3.21 3.20 3.17 3.16 3.15 46 54 54 85 147 147 290 Zn sub lattice The E2 (high) mode observed at 438 cmÀ1 is mainly due to the vibration of oxygen sub lattice and is the characteristic peak of hexagonal wurtzite phase of ZnO The peaks at 205 and 331 cmÀ1 correspond to multi phonon process and are assigned to TA (M) and E2 (M) or E2higheE2low modes The peak at 382 cmÀ1 corresponds to A1 (TO) mode The peak with medium intensity observed at 581 cmÀ1 corresponds to E1 (LO) mode and may be caused by the formation of oxygen vacancies and interstitial zinc and their complexes The broad feature between 1100 and 1200 cmÀ1 is assigned to the two phonon modes (2LO), characteristic of II-IV semiconductors [27e33] The observed small shift (few cmÀ1) in Raman modes at higher wave numbers compared to that of the bulk may be due to the strain existing in the samples and is confirmed from X-ray diffraction patterns All the phonon modes in the spectra of ZnO nanoparticles turn out to be stronger and sharper with increase of annealing temperature (Fig 5(b)), indicating the improvement of crystal quality as evident from the XRD data Also, at high annealing temperature, the E2 (high) mode is slightly red shifted indicating the optical phonon confinement At low temperature, the observed E1 (LO) mode is associated with the formation of structural defects whereas at high annealing temperature the intensity of E1 (LO) mode enhances and is related to the formation of surface defects [32,33] 3.4 FE-SEM analysis The morphology is found to be highly sensitive to the concentration of KOH (Fig 6) The shape of nanoparticles is found to vary first from quasi-spherical to rod like structures and then to flowerlike nanostructures with increase in concentration of KOH Quasi spherical nanoparticles with a diameter of 31e35 nm were formed Fig Micro-Raman spectra of ZnO nanoparticles prepared at; (a) different concentrations of KOH (a) 0.2, (b) 0.4, (c) 0.6, (d) 0.8, (e) and (f) 1.2 M; and (b) different annealing temperatures; (a) 300, (b) 400, (c) 500, (d) 600, (e) 700, (f) 800 and (g) 900 C R Raji, K.G Gopchandran / Journal of Science: Advanced Materials and Devices (2017) 51e58 when the concentration of KOH used was low (0.2 M) On increasing the concentration, it was found that the shape of the particles changes to elongated type with confinement along two directions (not shown) and a progressive increase in the intensity of photo emission was also accompanied this change as described in Section 3.6 For particles prepared with a KOH concentration of M, rod like structures with maximum emission intensity was obtained However, further increase in concentration of KOH led to the formation of flower-like structure resulting from the clustering of highly confined two dimensional layers Hence, it is evident from this work that KOH plays two roles viz., confinement making layers and agglomeration in a periodic manner so that the morphology look like that of flowers The observed decrease in luminescence from flower-like structures may be attributed to the lesser surface area exposed to the radiation when flower like nanoparticles were formed In order to prepare samples with tunable luminescence in the visible region the sample with intense visible emission having a morphology consisting of rod shaped particles was subjected to annealing at different temperatures The morphology of the samples undergoes various transformations as shown in Fig Generally, looking at the morphology of the samples (Figs and 10) obtained in this work, exhibiting tunable luminescence in the visible region, they are similar to that of morphology obtained for ZnO based phosphors reported by Hameed et al [34] At high temperature, atoms have large activation energy for diffusion and can stimulate the coalescence of smaller particles and may lead to an increase in size of the particle [26,35] Thus, it is concluded that concentration of KOH (OHÀ ions) and annealing temperature played essential role in the growth habits of ZnO nanostructures 3.5 UV-visible absorption spectroscopy Fig 8(a) shows the absorption spectra of the ZnO nanoparticles prepared by varying the concentration of KOH The peaks observed in the absorption spectra are attributed to the transition of electrons between the valence band, conduction band and the intrinsic defect levels [1] The synthesized ZnO nanoparticles exhibit blue shifted absorption peaks with respect to their bulk counterpart having the absorption peak at 386 nm [24,36] With increase in concentration of KOH from 0.2 to 1.2 M, the absorption peak was found to shift from 377 to 371 nm progressively and can be attributed to the changes in their surface morphologies and particle size From the diffuse reflectance spectrum taken in the 220e850 nm region, the optical band gap of ZnO nanoparticles were calculated using KubelkaeMunk relation [37,38] K ð1 À R∞ Þ2 ¼ ¼ FðR∞ Þ2 S 2R∞ (7) À Án FR ịhy ẳ hy Eg 55 (8) where FR Þ is called remission or KubelkaeMunk function, R∞ ¼ Rsample =Rstandard , K and S are absorption and scattering coefficients, hy is the energy of the incident photon and the exponent n depends on the nature of the optical transition caused by the photon absorption, for direct allowed transition n ¼ 1/2 [37] When the material scatters in a perfectly diffuse manner, the dependence of S on the energy becomes weak and can be assumed that FðR∞ Þ is proportional to absorption coefficient The band gap energy obtained from KubelkaeMunk plot was found to be in the range 3.25e3.18 eV and is in accordance with the particle size variation estimated from X-ray diffraction patterns The observed red shift in the optical band gap energy of the ZnO nanoparticles with respect to the bulk value can be due to the band bending effect caused by smaller size of the crystallites In the nanoregime, surface to volume ratio of particles is larger than that of the bulk material and can increase the effect due to band bending [24] The absorption spectra of ZnO nanoparticles (Fig 9(a)) annealed at different temperatures shows sharp peak for samples annealed up to 500 C, beyond that peak get broadened and may be due to large particle size distribution As the annealing temperature increases, the absorption peak gradually shifts from 371 to 387 nm and is attributed to increased crystallite size The optical band gap energy of the samples annealed at different temperatures obtained from KubelkaeMunk method was found to decreases with increase in annealing temperature and are reported in Table 3.6 Photoluminescence studies Fig 10(a) shows the room temperature photoluminescence spectra of ZnO nanoparticles prepared with different concentrations of KOH measured at an excitation wavelength of 325 nm using HeeCd laser The excitation energy (3.8 eV) used is higher than the band gap energy of ZnO (3.4 eV); therefore, it is easy for an electron in the valence band to be directly excited to the conduction band; in addition, excitation to the deep levels within the band gap was also possible [38] The PL spectra of ZnO nanoparticles exhibit two emission bands: one is in the UV region (389 nm) and the other is in the visible region (400e650 nm) The UV emission peak at 389 nm corresponds to the near bandedge (NBE) emission of ZnO and is due to the radiative recombination of free excitons [9,39] The intensity of UV emission was found to increase with concentration of KOH up to M and can be related to the decrease in size of the particles, beyond which intensity diminishes Size reduction causes more atoms to be closer to the surface and thereby increasing the rate of trapping of photogenerated holes at the surface, which in turn enhances the emission intensity [40] Photoluminescence spectra reveal that for all Fig FE-SEM images of ZnO nanoparticles synthesized with different concentrations of KOH 56 R Raji, K.G Gopchandran / Journal of Science: Advanced Materials and Devices (2017) 51e58 Fig FE-SEM images of ZnO nanoparticles prepared at different annealing temperatures Fig (a) Absorption spectra and (b) KubelkaeMunk plots of ZnO nanoparticles prepared with different KOH concentrations; (a) 0.2, (b) 0.4, (c) 0.6, (d) 0.8, (e) and (f) 1.2 M Fig (a) Absorption spectra and (b) KubelkaeMunk plots of ZnO nanoparticles annealed at different temperatures; (a) 300, (b) 400, (c) 500, (d) 600, (e) 700, (f) 800 and (g) 900 C the samples visible emission is observed at orange region (~582 nm) except for the sample prepared with a KOH concentration of 1.2 M, which shows emission band at yellow region (~570 nm) The radiative recombination of localized electrons with deeply trapped holes in the oxygen interstitials (Oi) located at 2.14 and 2.2 eV below conduction band results in orange (OL) and yellow luminescence (YL) bands respectively [41] Furthermore, it can be concluded that, higher concentration of reducing agent will result in the shifting of defect levels towards higher energies The Gaussian deconvolution of the emission spectrum of the sample with enhanced intensity is shown in Fig 10(b) Five bands were reproduced from the spectra without deviations at 389, 422, 516, 550 and 609 nm The emission due to band gap transition is observed at 389 nm without any deviation and can be attributed to the radiative recombination of free excitons [9,10] The violet emission (VL) band at 422 nm can be ascribed to the transition of an electron from Zni level located at 0.46 eV below conduction band to the valence band [10,42] The green luminescence (GL) band observed at 516 nm can be related to recombination of electrons in the singly ionized oxygen vacancies with photo excited holes in the valence band [9,39e43] YL band at 550 nm can be due to recombination of electron with deeply trapped holes in the oxygen interstitials (Oi) located at ~2.2 eV below conduction band [41,44] The orange luminescence (OL) band at 609 nm can be attributed to the transition of electrons from conduction band to oxygen interstitials located at 1.34 eV above the valance band [12,41] R Raji, K.G Gopchandran / Journal of Science: Advanced Materials and Devices (2017) 51e58 57 Fig 10 (a) Photoluminescence spectra of ZnO nanoparticles prepared with different concentrations of KOH; (a) 0.2, (b) 0.4, (c) 0.6, (d) 0.8, (e) and (f) 1.2 M excited at l ¼ 325 nm and (b) Deconvolution bands of sample prepared with M KOH Fig 11 represents the room temperature PL spectra of the ZnO nanoparticles annealed at different temperatures The intensity of emission, irrespective of whether it is UV or visible, is found to decrease with annealing temperature up to 600 C and beyond that it increases All the samples showed the typical band edge emission at 389 nm It can be seen that the intensity of UV band decreases initially with annealing temperature and may be due to the partial dissociation of donor bound exciton, supporting the assignment of Teke et al [39] The increase in intensity of UV band at higher temperature can be attributed to the dissociation of donor bound excitons into free excitons and neutral donors It can lead to an increase in probability of recombination of free excitons and thereby increasing the intensity of UV emission [9,44] It is of interest that the visible emission band first shifted from orange luminescence (OL) to red luminescence (RL) region upon increasing the annealing temperature from 300 to 700 C and then to green luminescence (GL) when annealed at temperatures above 700 C These shifts of visible emission band on annealing, may be due to changes in the local environments of the defect centers in the samples [9,43] The origin of OL, RL and GL bands are opposite in nature OL and RL bands are attributed to oxygen interstitials (Oi) whereas GL band is attributed to singly ionized oxygen vacancies (VO) [10,12] The nature of green luminescence in ZnO is the most controversial and many hypotheses have been proposed for this emission [9,11,41e44] Annealing from 300 to 700 C reduces the oxygen vacancies while increases the amount of oxygen interstitials in the sample [45,46] Depending on the energy levels of oxygen interstitials formed in the band gap, OL and RL emissions appears in the spectra OL band is ascribed to the transition of electron from the conduction band to oxygen interstitials located at 2.14 eV below the conduction band [9,46] RL band is attributed to the transition of electron from conduction band to oxygen interstitials located at 1.95 eV below the conduction band At elevated temperatures, GL band is observed at around 527 nm and can be related to oxygen interstitials or oxygen antisites [47] Thus, it is evident from the emission spectra that, the annealing of ZnO nanoparticles not only purges the moisture and OHÀ ions from the material but also influence various channels of optical recombination and it indicates that visible emission spread is altered by annealing treatment [48] Hence, this work also provides a method to control polychromic visible emission of ZnO nanoparticles; covering almost the whole visible region, with limitations Such adjustments in luminescent properties can pave ways open for applications of ZnO nanoparticles in the fabrication of white light emitting diodes, display devices, biological labeling etc Conclusion Fig 11 Photoluminescence spectra of ZnO nanoparticles prepared at different annealing temperatures; (a) 300, (b) 400, (c) 500, (d) 600, (e) 700, (f) 800 and (g) 900 C Highly crystalline ZnO nanoparticles were synthesized by coprecipitation method In this work, attempts were made to tune the visible luminescence of ZnO nanoparticles by controlling the growth parameters such as concentration of reducing agent and heat treatment The XRD and micro-Raman analysis of the samples confirmed its hexagonal wurtize phase FE-SEM micrographs showed the transformation of morphology of nanoparticles from quasi-spherical to rod-like and then to flower-like structures with increase in concentration of the reducing agent KOH from 0.2 to 1.2 M SEM images also indicated that morphology of these particles is highly sensitive to annealing temperature We have attempted to correlate the observed change in color and intensity of photoemission of ZnO nanoparticles with the size and morphology of the nanoparticles The UV emission was predominant in the emission spectra of all the samples but the visible emission was 58 R Raji, K.G Gopchandran / Journal of Science: Advanced Materials and Devices (2017) 51e58 found to vary in color with both concentration of KOH and annealing temperature It was found that annealing temperature alter the position of visible emission band from orange to red and then to green luminescence region; this shift in the visible emission band is attributed to change in the local environment of the defect centers The present study reveals that the emission intensity can be finely tuned by suitably selecting the amount of KOH during synthesis and emission color can be tuned by varying the annealing temperature Thus, this study provides a method to control visible emission of ZnO nanoparticles covering almost the whole visible region The results obtained are fruitful and the synthesized ZnO nanostructures can be used for the fabrication of white light emitting diodes, display devices, biological labeling, etc., Acknowledgments One of authors Raji.R wishes to express her gratitude to KSCSTE, Govt of Kerala, India (No (T)010-20/FSHP/10/CSTE) for providing the financial support to this work References [1] Q Zhu, C Xie, H Li, C Yang, S Zhang, D Zeng, Selectively enhanced UV and NIR photoluminescence from a degenerate ZnO nanorod array film, J Mater Chem C (2014) 4566e4580 [2] N Park, K Sun, Z.L Sun, Y Jing, D.L Wang, High efficiency NiO/ZnO heterojunction UV photodiode by solegel processing, J Mater Chem C (2013) 7333e7338 [3] Y.I Alivov, E.V Kalinina, A.E Cherenkov, D.C Look, B.M Ataev, A.K Omaev, M.V Chukichev, D.M Bagnall, Fabrication and 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Materials and Devices (2017) 51e58 found to vary in color with both concentration of KOH and annealing temperature It was found that annealing temperature alter the position of visible emission band... near bandedge (NBE) emission of ZnO and is due to the radiative recombination of free excitons [9,39] The intensity of UV emission was found to increase with concentration of KOH up to M and can