Influence of Bi doping on the structure and photoluminescence of ZnO phosphor synthesized by the combustion method

Accepted Manuscript Influence of Bi doping on the structure and photoluminescence of ZnO phosphor synthesized by the combustion method

Trilok K. Pathak, H.C. Swart, R.E. Kroon PII: DOI: Reference:

S1386-1425(17)30745-X doi: 10.1016/j.saa.2017.09.026 SAA 15460

To appear in:

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Received date: Revised date: Accepted date:

27 June 2017 22 August 2017 12 September 2017

Please cite this article as: Trilok K. Pathak, H.C. Swart, R.E. Kroon , Influence of Bi doping on the structure and photoluminescence of ZnO phosphor synthesized by the combustion method, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2017), doi: 10.1016/j.saa.2017.09.026

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ACCEPTED MANUSCRIPT Influence of Bi doping on the structure and photoluminescence of ZnO phosphor synthesized by the combustion method Trilok K. Pathaka*, H. C. Swarta and R. E. Kroona* a

Department of Physics, University of the Free State, P.O. Box 339, Bloemfontein, ZA 9300,

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South Africa

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Abstract

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Bismuth doped ZnO (BZO) phosphors have been synthesized by the combustion method. The effect of Bi doping up to 4 mol% on the structural, morphological, optical and

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photoluminescence (PL) properties have been investigated. X-ray diffraction analysis revealed

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that the BZO phosphors had the hexagonal wurtzite structure. The nanocrystallite size decreased from 75 to 38 nm as the Bi concentration increased up to 3 mol%, but then increased slightly for

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4 mol% Bi. The chemical states of the synthesized BZO phosphors were investigated using X-

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ray photoelectron spectroscopy and revealed the presence of both Bi3+ and Bi2+ charge states. The surface morphology showed spherical grains with some small particle agglomeration. The

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grain agglomeration and irregular shapes increased with increasing Bi concentration in the BZO

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phosphor. The absorption spectra were calculated from the reflection spectra using the KubelkaMunk function and a blue shift in the absorption was obtained. The optical bandgap varied from

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3.08 to 3.11 eV for increasing Bi doping concentration. The PL spectra showed a blue emission at 410-500 nm and a broad red peak at 650 nm. These peaks are attributed to oxygen related defects in the ZnO host. The addition of Bi decreased the red emission and enhanced the blue emission. Keywords: ZnO, Bi, phosphor, defects.

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ACCEPTED MANUSCRIPT 1. Introduction Zinc oxide (ZnO) nanostructures have attracted much attention due to their potential applications in solar cells, sensors, spintronic devices, luminescence applications, biomedical fields, gas sensing and optoelectronic devices [1-6]. ZnO is at the forefront of the research

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because of its potential application in optoelectronic devices such as the development of low

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energy, environmentally friendly and low cost white light emitting diodes (LEDs) operating at

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room temperature [7]. White LEDs that have been fabricated using a near ultraviolet (UV) LED (380-420 nm) coupled with red, green and blue phosphor have attracted much attention [8]. The

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synthesis of blue phosphors has gained research due to its application in near UV LEDs [9]. The

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luminescence properties of Bi3+ ions in different hosts are of interest due to the emission wavelength which varies from the UV to the visible region with different hosts. The diversity of

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possible Bi valence states (Bi3+, Bi2+, Bi+ and Bi0) in a single host makes Bi doped materials

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interesting candidates for inorganic single emitting component (ISEC) applications. The conversion between Bi valance states or the existence of many Bi valance states in different

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hosts have made Bi a potential dopant for various applications.

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ZnO is a wide bandgap semiconductor (3.37 eV) with a large exciton binding energy (60

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meV) at room temperature. It has good chemical stability and biocompatibility [10]. This suggests a number of possible practical applications, notably in the area of violet emission devices. The luminescence properties of Bi3+ ions doped in different host matrices such as CaO [11], Y2O3 [12], MgAl2O4 [13] and ZnGa2O4 [14] have been reported by several researchers but the luminescence properties of Bi doped ZnO are still under investigation. Congkang et al. [15] investigated the near band edge emission in the photoluminescence spectrum of Bi–ZnO nanowires and found that it was red shifted relative to undoped ZnO nanorods as a result of 2

ACCEPTED MANUSCRIPT enhanced carrier concentration. The effect of Bi and Mn doping on the green luminescence in the ZnO matrix was investigated by Garcia et al. [16]. They discussed the effect of the presence of impurities on the green luminescence band and also predicated that the luminescence depends on the concentration of intrinsic defects. A variety of techniques have been employed for the

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synthesis of Bi doped ZnO powders such as the co-precipitation method [17], the sol-gel

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technique [18], solution combustion [19] and solid state reaction [20], etc. Among all these

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methods, the combustion technique is noted to be a unique method to synthesize nanocrystalline materials in as-synthesized form with large surface area without further need of heat treatment.

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The main advantage of this method is the high temperature of reaction assures high purity and

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well crystallized powder. Nanocrystalline oxides are produced through the redox reaction between an oxidizer and a fuel at a moderate initiation temperature of around 350–600 ºC within

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a few minutes [21].

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In the present work, we report the synthesis of Bi doped ZnO (BZO) phosphor powders by

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the combustion method. The purpose of the present study was to investigate the effect of Bi doping on ZnO photoluminescence properties to assess the potential use of the doped

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nanostructures for blue phosphor applications. The innovation of this work was the study of the

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effect of Bi doping on the defect luminescence of the BZO phosphor. X-ray photoelectron spectroscopy results are reported to support the existence of Bi2+ and Bi3+ ions. The morphology of the BZO phosphors was investigated using scanning electron microscopy. The changes in absorbance and optical bandgap with doping concentrations are reported. These results represent an important contribution to the observations of the luminescence properties of Bi in the ZnO matrix and for developing novel ISEC materials for white LEDs.

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2. Experimental details Zinc nitrate (Zn(NO3)2.6H2O, 99.99%) and bismuth nitrate (Bi(NO3)3 5H2O, 99.99%) with fuel (urea, NH2CONH2) were dissolved in 5 ml of double distilled water and stirred thoroughly

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to obtain a transparent solution. This solution was heated at 80 ºC for 30 min for a gel to form.

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The gel was placed inside a pre-heated muffle furnace at 600 ºC to initiate the combustion

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process. Within a short time (less than 5 min) the mixture ignited with a flame and the rapid evolution of enormous amounts of gases produced a voluminous foamy product (ash). This was

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ground using an agate pestle and mortar to produce the final powder, without any additional heat

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treatment. The synthesis process is illustrated in Fig. 1. The prepared BZO powder was characterized by X-ray diffraction (XRD) using a Bruker D8 Advance diffractometer with Cu Kα

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radiation to assess the crystal structure. The Williamson-Hall method was used to determine the

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crystallite size and strain from the diffraction peaks. A PHI 5000 Versaprobe system was used to examine the surface chemistry with X-ray photoelectron spectroscopy (XPS) analysis using Al

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Kα X-rays of energy 1486.6 eV. The morphology and elemental analysis of the samples was

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investigated by scanning electronic microscopy (Jeol JSM-7800F microscope with field emission electron gun, equipped with an X-MaxN80 energy dispersive X-ray spectroscopy (EDS) detector

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from Oxford Instruments). Diffuse reflectance spectra were collected using a Lambda 950 UV– vis spectrophotometer from PerkinElmer equipped with a spectralon integrating sphere. The photoluminescence (PL) data was measured using a FLS980 Spectrometer (Edinburgh Instruments). Fig.1

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3. Results and discussion 3.1 X-ray diffraction pattern analysis The XRD patterns of the ZnO and BZO phosphor are depicted in Fig. 2. The XRD peaks are

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indexed using JCPDS card 01-075-0576. The XRD patterns are characteristic of polycrystalline

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ZnO having the hexagonal wurtzite structure. The diffraction peaks shifted to lower angle for

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lower concentrations of Bi (1% – 3%), which is consistent with a lattice expansion if large Bi ions replaced Zn ions in the crystal. At high Bi concentration (4 mol%) this shift was reversed.

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This might suggest a different location for the Bi ions is created, although no extra peaks of an

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impurity phase were detected. Fig. 2

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The crystallite size was affected by the Bi doping and strain also developed in the BZO

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phosphor. In order to calculate the strain (ε) and the average crystallite size (D), the Williamson-

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Hall (W-H) equation [22] was used:

Ks  4 sin  D

(1)

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 cos  

where β is the 2θ-FWHM (in radians) of the diffraction peaks at angle θ, λ is the X-ray

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wavelength and Ks is a shape factor taken as 0.9. Fig. 3(a) shows the W-H plots for different Bi doping concentrations. The plots of βcosθ versus 4sinθ have slopes equal to the microstrain and an intercept of Ksλ/D from which the crystallite size can be determined. Kumar et al. [23] calculated the strain on BZO thin films at different annealing temperature deposited by spray pyrolysis technique using W-H plots. They concluded that the strain decreased with increasing crystallinity of the sample. We observed that strain varied from 0.18 % to 0.03% with different 5

ACCEPTED MANUSCRIPT Bi concentration in the BZO phosphor. The crystallite size varied from 38 to 75 nm as the Bi doping varied (Table 1) and was a minimum for the 3 mol% BZO phosphor. The crystallite sizes as well as microstrain obtained by W-H plots with changing Bi concentration are shown in Fig 3(b). Both the crystallite size and strain varied with different Bi concentration in the BZO

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phosphor. In spite of this the BZO phosphor retained the hexagonal wurtzite structure. The

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reduction in strain with Bi content may be linked with the reduction in crystallite size due to the

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addition of Bi. For the sample doped with the high concentration of Bi (4%) this trend no longer continues, which suggests that the Bi may be incorporated in a different way.

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Fig. 3 (a, b)

3.2 X-ray photoelectron spectroscopy

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Table 1

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In order to examine the surface chemical properties of the BZO, the sample doped with 2

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mol% Bi was analyzed by XPS. The survey scan in Fig. 4(a) shows Zn, O, Bi and C peaks. The C is related to impurities adsorbed on the surface during the exposure of the sample to the

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ambient atmosphere. All binding energies were corrected for the charge shift using the C 1s peak

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of graphitic carbon (binding energy 284.6 eV) as a reference [24]. Fig. 4(b) shows the high resolution XPS spectrum of the Zn 2p region. The two strong peaks located at 1044.7 and 1021.6

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eV correspond to the binding energy of Zn 2p1/2 and Zn 2p3/2, respectively. The binding energy difference between the two lines is 23.1 eV, which is close to the standard reference value of ZnO [25]. The Bi 4f XPS spectrum shown in Fig. 4(c) exhibits two peaks with shoulders which suggest that Bi was in two different oxidation states. The main two peaks are centred at 164.2 eV and 158.9 eV and correspond to the Bi 4f5/2 and Bi 4f7/2 binding energies of Bi3+ as in Bi2O3 [26]. The shoulders centred at 161.7 eV and 156.4 eV may be attributed to the Bi 4f5/2 and Bi 4f7/2 6

ACCEPTED MANUSCRIPT binding energies of Bi2+ [26]. The area under Bi2+ peaks is small compared to the Bi3+ peaks so the BZO phosphor contain a higher concentration of Bi3+ ions. Fig. 4 (a-d) The broad and asymmetric O 1s profile is shown in Fig. 4(d) and can be fitted to five peaks

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located at 530.1 eV, 530.8 eV, 531.4 eV, 532.2 and 532.7 eV, respectively, which represent five

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different kinds of O species in the samples. The component at 530.8 eV can be attributed to

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lattice oxygen in ZnO and it has the maximum area of 65.6%. The peak at 531.4 eV is associated with O2- ions that are in the oxygen-deficient regions within the matrix [27]. These

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oxygen related defects were also apparent from the luminescence of the phosphors, as reported

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later. The component with higher binding energy at 532.7 eV is usually attributed to O2 adsorbed onto the surface [28]. The remaining peaks at 530.1 eV and 532.2 eV are due to O bonded to Bi

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ions in the BZO phosphor, with the lower binding energy corresponding to O-Bi2+ and the higher

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binding energy corresponding to O-Bi3+ i.e. following the same trend as for the Bi ions in Fig. 4(c) with the binding of both Bi and O both stronger in Bi2O3.

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3.3 Scanning electron microscopy

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The SEM images in Fig. 5 reveal the morphology at different doping concentrations of the BZO phosphor. The morphology of the sample doped with 1 mol% Bi shows agglomerates

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composed of small rounded particles. As the Bi concentration increased to 2 mol% uniformly spherical grains were obtained with some small particle agglomeration as shown in the inset image. Further increasing the Bi concentration resulted in the grains forming agglomerates and irregular shapes in the BZO phosphor. The shape of the obtained particles depended on many complex factors, such as the nucleation rate as well as the crystal growth rate. Although the samples were ground before imaging by SEM, most particles are less than 1 µm and their 7

ACCEPTED MANUSCRIPT morphology is unlikely to have been strongly influenced by the grinding process. Prakash et al. [29] obtained similar results of Bi doped ZnO nanoparticles synthesized by a microwave irradiation method. They reported a needle like morphology for undoped ZnO nanoparticles and agglomerated small rounded particles for Bi doped ZnO. Kumar et al. [30] also investigate the

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morphology of Bi doped ZnO thin films synthesized by the spray pyrolysis technique and

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observed that ZnO films showed marked changes on Bi doping. The irregular morphology with

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non-uniform size distribution is seen in each micrograph. At the higher magnification (Fig. 6) spherical particles are observed with some agglomeration for the sample doped with 1% Bi,

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Fig.5

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while for higher doping concentration (3 %, 4%) agglomeration is inhibited.

Fig.6

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X-ray diffraction also revealed that the maximum strain of 0.14% was obtained for the 1

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mol% doped BZO phosphor, which may be associated with agglomeration of the particles. The presence of Bi in the ZnO nanopowder was confirmed by EDS analysis and element mapping

Fig. 7

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shown in Fig. 7. The element mapping shows a uniform distribution of Bi in the ZnO matrix.

3.4 Optical studies

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3.4.1 Reflectance spectra, absorbance and optical bandgap Diffuse reflectance spectroscopy (DRS) is an excellent sampling tool for powdered or crystalline materials to find material characteristics in different wavelength regions. In this study we have measured DRS spectra of the BZO phosphor (powder) at room temperature in reflectance mode with the step size of 1 nm. The sample holder was filled completely with phosphor and covered with UV transparent silica, before being placed flush against the port of 8

ACCEPTED MANUSCRIPT the integrating sphere. A baseline curve was first recorded using a spectralon standard sample, after which the sample spectrum was recorded. All samples were measured consecutively using identical experimental conditions. The DRS spectra were investigated in the 300-800 nm wavelength range and are shown in Fig. 8.

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Fig. 8

Munk function (1  R) 2 2R

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K

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The corresponding absorbance spectra of the samples were calculated using the Kubelka-

(2)

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where R is the reflectance and K, proportional to the absorbance, is shown in fig. 9(a). The sharp increase in the absorption for all the samples at wavelengths less than 400 nm is well known and

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can be assigned to the optical bandgap absorption of ZnO. The optical bandgap may be affected

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by the morphology, particle size and surface microstructure or the quantum confinement effects [31]. The absorption band near 420 nm occurred only for the doped ZnO powders and varied

Fig.9 (a, b)

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with different Bi doping concentration. This is believed to be due to Bi ions in doped ZnO.

ZnO has a direct transition and the corresponding bandgap changed with different Bi doping

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concentrations in the ZnO. The optical bandgap calculation was possible by extrapolation of the linear portion to K=0 in a plot of (Khν)2 against hν as shown in Fig 9(b). The bandgaps of the samples were similar (3.08-3.11 eV) because the absorption was close to the band edge of the ZnO. Keskenler et al. [32] observed that the bandgap fluctuated (both decreased and increased) as the Bi content increased in ZnO thin films deposited by the sol-gel method. They concluded that the optical bandgap changed due to the broadening of the valance and conduction bands and 9

ACCEPTED MANUSCRIPT was related to the interaction among the electrons of Bi and host atoms, respectively. Stengl et al. [33] synthesised zinc-bismuth oxide-peroxide nanorod-like particles by thermal hydrolysis and observed that the bandgap energy decreased with increasing Bi3+ concentration. The narrowing of the optical bandgap to about 2.7 eV (fitted in bandgap diagram, Fig. 9b) may be

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attributed to the formation of the Bi impurity energy level.

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3.4.2 Photoluminescence

The PL spectra of the undoped and Bi doped ZnO samples were investigated at room

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temperature. Fig. 10 (a) shows the PL excitation spectra obtained with an emission wavelength

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of 438 nm. Fig. 10 (b) shows the PL emission spectra for the excitation peaks in Fig. 10(a), namely at 392 nm.

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Fig. 10 (a, b)

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In the emission spectra of Fig. 10 (b) two emission regions were obtained, one between 400-500 nm and another broad peak at 650 nm. The broad peak centred at 650 nm is related to zinc oxide

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defects [34]. Generally, ZnO has six kinds of intrinsic point defects, namely oxygen vacancy

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(VO), oxygen interstitial (Oi), oxygen antisite (ZnO), zinc vacancy (VZn), zinc interstitial (Zni) and zinc antisite (OZn) [35]. The luminescence peak at 650 may be related to oxygen interstitial

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defects in ZnO [28]. Dijken et al. [36] investigated that the particle surface played an important role in the visible emission. They offered a model that the valence band hole can be trapped by surface states and then tunnels back into the oxygen vacancies containing one electron VO+ to form a VO++ recombination centre. The recombination of a shallowly trapped electron with a deeply trapped hole in a VO+ centre causes the visible emission. The transitions from the surface traps to the deep level defects were significantly quenched on doping with Bi ions, as indicated 10

ACCEPTED MANUSCRIPT by a marked reduction of the emission feature at 650 nm in the present study. For the Bi doped samples this red defect luminescence band was quenched. Labib et al. [20] concluded that oxygen vacancies are known to be the most common defects in ZnO and usually act as radiative centers in the luminescence process. Especially the different states of oxygen vacancies are

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located in different energy levels in ZnO band-gap confirming that these different charge states

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are responsible for the different wavelength luminescence in the visible region. The XPS results

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also shows that Bi atoms play a crucial role to formed different oxygen related defects in ZnO. Since additional Bi dopant cations would compete with Zn cations for oxygen, it is expected in

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general that doping would increase the oxygen vacancies and reduce the oxygen interstitials.

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Although Bi3+ ions have been reported to give luminescence in the blue region [37], their emission is host dependent and not fixed as in the case of f-f transitions of rare-earth ions. In this

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study the blue emission is not considered to be from Bi3+ ions. The absorption associated with Bi

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from the diffuse reflectance measurements (Fig. 8) was near 420 nm, which does not match the excitation spectra in Fig. 10(a). Instead it more resembles the excitation region of the ZnO host

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above the optical bandgap. The emission is also clearly observed in the undoped sample. The

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origin of the luminescence band in the blue region (400-500 nm) can be attributed to the radiative recombination of a photo-generated hole with an electron occupying the oxygen

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vacancy [38]. Zhang et al. [39] reported that strong blue emission located at 446 nm decreased with increasing oxygen pressure and substrate temperature. The variation in the blue emission peak may be due the effect of Bi ions on the defects level in the ZnO lattice. Deng et al. [40] reported on the optical properties of the Bi doped ZnO nanomaterials. They observed a broad violet emission at 405-430 nm caused by impurities and structural defects in the grown nanostructures. Recently, Thangeeswari et al. [41] synthesised Co-Bi doped ZnO nanoparticles 11

ACCEPTED MANUSCRIPT and observed a similar PL violet emission peak at 430 nm. They suggested that the violet emission in the PL spectra of the Bi co-doped ZnO nanostructure can be directly assigned to the recombination from the conduction band to the energy level of deep traps or of surface and interface states originating from crystal defects inside or at the surfaces. The intensity of the blue

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emission in our results increased with Bi doping up to 3 mol% and decreased at higher values of

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the Bi doping. This confirms that although the Bi ions are not responsible for the luminescence,

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they play an important role in the defect luminescence spectra related to the oxygen vacancy in the ZnO lattice, as well as affecting the crystallite size and strain noted earlier when discussing

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the XRD results.

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Fig.11

The colour coordinates were estimated using the Commission of International del’Eclairage

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(CIE) 1931 color matching functions. The CIE diagrams presented in Fig. 11 indicate that the

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phosphor emission color changed with the Bi doping in the ZnO matrix. The CIE coordinates for different doping concentrations were calculated and are given in Table 1. For all the BZO

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phosphors the emission was near the blue region, while it changed continuously with different Bi

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concentrations.

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4. Conclusions

Blue emitting BZO phosphor powder was successfully prepared by the sol-gel combustion method. XRD patterns showed a polycrystalline hexagonal wurtzite structure. The crystallite size varied from 38 nm to 75 nm and was affected by the Bi doping. The minimum strain of 0.03% was obtained for the 3 mol% Bi doped BZO phosphor, which also corresponded to the minimum crystallite size and maximum blue luminescence. The morphology of the BZO phosphor powder 12

ACCEPTED MANUSCRIPT also changed with different doping concentration. XPS results confirmed Bi3+ and Bi2+ present in the BZO phosphor powder. PL results obtained at excitation energies of 3.16 eV (392 nm) produced emission peaks at 2.83 eV (438 nm) and a broad band at 1.90 eV (650 nm) which are attributed to defect emission from oxygen vacancy and oxygen interstitial defects in ZnO,

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respectively. The CIE diagram indicated that the BZO phosphor (Bi 3%) emission was in the

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blue region which is useful in near UV LED applications.

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ACCEPTED MANUSCRIPT Acknowledgement This work is based on the research supported in part by the National Research Foundation of South Africa (R.E. Kroon, Grant Number 93214). This work is supported both by the South African Research Chairs Initiative of the Department of Science and Technology, the National

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Research Foundation of South Africa (84415). Dr. Liza Coetsee-Hugo is acknowledged for XPS

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measurements. The University of the Free State is acknowledged for financial support.

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ACCEPTED MANUSCRIPT References 1. V. Kumar, R.G. Singh, F. Singh, L.P. Purohit, Highly transparent and conducting boron doped zinc oxide films for window of Dye Sensitized Solar Cell applications, J. Alloys Compd. 544 (2012) 120-124.

T

2. V. Cretu, V. Postica, A.K. Mishra, M. Hoppe, I. Tiginyanu, Y.K. Mishra, L. Chow, Nora H.

IP

de Leeuw, R. Adelung, O. Lupan, Synthesis, characterization and DFT studies of zinc doped

CR

copper oxide nanocrystals for gas sensing applications, J. Mater. Chem. A 4 (2016) 65276539.

US

3. T. Reimer, I. Paulowicz, R. Roder, S. Kaps, O. Lupan, S. Chemnitz, W. Benecke, C.

AN

Ronning, R. Adelung, Y.K. Mishra, Single step integration of ZnO nano-and microneedles in Si trenches by novel flame transport approach: Whispering gallery modes and

M

photocatalytic properties, ACS Appl. Mater. Inter. 6 (2014) 7806-7815.

ED

4. Y.K. Mishra, R. Adelung, C. Rohl, D. Shukla, F. Spors, V. Tiwari, Virostatic potential of

2 (2011) 305-312.

PT

micro-nano filopodia-like ZnO structures against herpes simplex virus-1, Antiviral Res. 92,

CE

5. Y.K Mishra, G. Modi, V. Cretu, V. Postica, O. Lupan, T. Reimer, I. Paulowicz, V. Hrkac, W. Benecke, L. Kienle, R. Adelung, Direct growth of freestanding ZnO tetrapod networks

AC

for multifunctional applications in photocatalysis, UV photodetection and gas sensing, ACS Appl. Mater. Inter. 7 (2015) 14303-14316. 6. A.B. Djurisic, A.M.C. Ng, X.Y. Chen, ZnO nanostructures for optoelectronics: Material properties and device applications, Prog. Quantum Electron. 34, 4 (2010) 191–259.

15

ACCEPTED MANUSCRIPT 7.

J. Bian, W. Liu, J. Sun, H. Liang, Synthesis and defect-related emission of ZnO based light emitting device with homo- and heterostructure, J. of Mater. Process. Tech. 184, 1-3 (2007) 451–454.

8. D. Hou, C. Liu, X. Ding, X. Kuang, H. Liang, S. Sun, Y. Huang, Y. Tao, A high efficiency

T

blue phosphor BaCa2MgSi2O8:Eu2+ under VUV and UV excitation, J. Mater. Chem. C 1

IP

(2013) 493–499.

CR

9. Y. Chiu, W. Liu, C. Chang, C. Liao, Y. Yeh, S. Janga, T. Chen, Ca2PO4Cl : Eu2+: an intense near-ultraviolet converting blue phosphor for white light-emitting diodes, J. Mater. Chem.

US

20 (2010) 1755-1758.

AN

10. U. Ozgur, Ya. I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Doğan, V. Avrutin, S.J. Cho, H. Morkoç, A comprehensive review of ZnO materials and devices, J. Appl. Phys. 98

M

(2005) 041301-103.

ED

11. A. Yousif, R.E. Kroon, E. Coetsee, O.M. Ntwaeaborwa, H.A.A. Seed Ahmed, H.C. Swart, Luminescence and electron degradation properties of Bi doped CaO Phosphor, Appl. Surf.

PT

Sci. 356 (2015) 1064–1069.

CE

12. R.M. Jafer, E. Coetsee, A. Yousif, R.E. Kroon, O.M. Ntwaeaborwa, H.C. Swart, X-ray photoelectron spectroscopy and luminescent properties of Y2O3:Bi3+ phosphor, Appl. Surf.

AC

Sci. 332 (2015) 198–204. 13. W.A I. Tabaza, H.C. Swart, R.E. Kroon, Optical properties of Bi and energy transfer from Bi to Tb in MgAl2O4 phosphor, J. Lumin. 148 (2014) 192-197. 14. Y. Zhuang, J. Ueda, S. Tanabe, Photochromism and white long-lasting persistent luminescence in Bi3+-doped ZnGa2O4 ceramics, Opt. Mater. Exp. 2, 10 (2012) 1379-1383.

16

ACCEPTED MANUSCRIPT 15. C. Xu, J. Chun, D.E. Kim, J.J. Kim, B. Chon, T. Joo, Electrical properties and near band edge emission of Bi-doped ZnO nanowires, Appl. Phys. Lett. 90 (2007) 083113-3. 16. J.A. Garcia, A. Remón, J. Piqueras, Influence of Bi and Mn on the green luminescence of ZnO ceramics, J. Appl. Phys. 62 (1987) 3058-3 .

T

17. I. Guy, A. Peigney, H. Andrianjatova, A. Rousse, Texture control of spherical undoped and

IP

Bi-doped zinc oxide powders, J. Mater. Process. Technol. 56 (1996) 98-107.

CR

18. R. Puyane, F. Toal, S. Hampshire, Production of doped ZnO powders for varistor applications using sol-gel techniques, J. Sol-Gel Sci. Technol. 6, 3 (1996) 219–225.

US

19. P.K. Roy, Synthesis of nano ZnO powder and study of its varistor behavior at different

AN

temperatures, J. Mater. Sci. Technol. Res. 1, 4 (2012) 28-34.

Werkstofftech, 47, 1 (2016) 19–28.

M

20. S. Labib, Study of photocatalytic properties of pure and doped ZnO powders, Mat.-wiss. u.

ED

21. K.C. Patil, S.T. Aruna, T. Mimani, Combustion synthesis: an update, Curr. Opin. Solid State Mater. Sci. 6 (2002) 507–512.

CE

(1953) 22–31.

PT

22. G. K. Williamson, W.H. Hall, X-ray line broadening from field Al and W, Acta Metall. 1

23. N.S. Kumar, K.V. Bangera, G.K. Shivakumar, Effect of annealing on the properties of Bi

AC

doped ZnO thin films grown by spray pyrolysis technique, Superlattices Microstruct. 75 (2014) 303–310.

24. J.F. Moulder, W.F. Strickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, ULVAC-PHI Inc, 1995.

17

ACCEPTED MANUSCRIPT 25. V. Kumar, F. Singh, O. M. Ntwaeaborwa, H.C. Swart, Effect of Br+6 ions on the structural, morphological and luminescent properties of ZnO/Si thin films, Appl. Surf. Sci. 279 (2013) 472-478. 26. A. Yousif, R.M. Jafer, S. Som, M.M. Duvenhage, E. Coetseea, H.C. Swart, Ultra-

T

broadband luminescent from a Bi doped CaO matrix, RSC Adv. 5, 2015, 54115-54122.

IP

27. C. Zhang, J. Lin, Visible-light induced oxo-bridged ZrIV–O–CeIII redox centre in tetragonal

CR

ZrO2–CeO2 solid solution for degradation of organic pollutants, Phys. Chem. Chem. Phys. 13 (2011) 3896

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28. V. Kumar, H.C. Swart, O.M. Ntwaeaborwa, R.E. Kroon, J.J. Terblans, S.K.K. Shaat,

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A. Yousif, M.M. Duvenhage, Origin of the red emission in zinc oxide nanophosphors, Mater. Lett. 101 (2013) 57-60.

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29. T. Prakash, G. Neri, A. Bonavita, E. Ranjith, K. Gnanamoorthi, Structural, morphological

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and optical properties of Bi-doped ZnO nanoparticles synthesized by a microwave irradiation method, J. Mater. Sci.: Mater. Electron., 26 (2015) 4913-4921.

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30. N. Sadananda Kumar, Kasturi V. Bangera, C. Anandan, G.K. Shivakumar, Properties of

613–619.

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ZnO:Bi thin films prepared by spray pyrolysis technique, J. Alloys Compd. 578 (2013)

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31. M. Dressel, G. Gruner, Electrodynamics of Solids, Optical Properties of Electron in Matter, Cambridge University Press, Cambridge, 2002, 159-165. 32. E.F. Keskenler, S. Aydin, G. Turgut, S. Dogan, Optical and structural properties of bismuth doped ZnO thin films by sol-gel method: Urbach rule as a function of crystal defects, Acta Phys. Pol. A, 126, 3 (2014) 782-786.

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ACCEPTED MANUSCRIPT 33. V. Stengl, J. Henych, M. Slusna, J. Tolasz, K. Zetkova, ZnO/Bi2O3 nanowire composites as a new family of photocatalysts, Powder Technology 270 (2015) 83-91. 34. M.H. Huang, Y. Wu, H. Feick, N. Tran, E. Weber, P. Yang, Catalytic growth of zinc oxide nanowires by vapor transport, Adv. Mater. 13, 2 (2001) 113–116.

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35. T.K. Pathak, V. Kumar, H.C. Swart, L.P. Purohit, Electrical and optical properties of p-type

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codoped ZnO thin films prepared by spin coating technique, Physica E 77 (2016) 1–6.

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36. A.V. Dijken, E.A. Meulenkamp, D. Vanmaekelbergh, A. Meijerink, The kinetics of the radiative and nonradiative processes in nanocrystalline ZnO particles upon photoexcitation,

US

J. Phys. Chem. B 104 (2000) 1715-1723.

AN

37. M. Back, E. Trave, R. Marin, N. Mazzucco, D. Cristofori, P. Riello, Energy Transfer in Biand Er-Codoped Y2O3 Nanocrystals: An Effective System for Rare Earth Fluorescence

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Enhancement, J. Phys. Chem. C 118 (2014) 30071−30078.

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38. P.B. Taunk, R. Das, D.P. Bisen, R.K. Tamrakar, Structural characterization and photoluminescence properties of zinc oxide nano particles synthesized by chemical route

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method, J. Rad. Res. Appl. Sci. 8 (2015) 433-438.

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39. D.H. Zhang, Z.Y. Xu, Q.P. Wang, The mechanisms of blue emission from ZnO films deposited on glass substrate by r.f. magnetron sputtering, J. Phys. D: Appl. Phys. 35, 21

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(2002) 2837-2840.

40. D. Deng, Z. Wu, G. Zhao, J. Zhao, Morphologies and photoluminescence of Bi-doped ZnO materials synthesized by sonochemical method, Adv. Mater. Res., 740 (2013) 535-539. 41. T. Thangeeswari, J. Velmurugan, M. Priya, Structural and optical properties of Co-Bi doped ZnO nano particles opto electronic devices, Int. J. ChemTech Res. 7, 3 (2015) 15481552. 19

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Fig.1 Flowchart for synthesis of BZO nanomaterials.

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Fig. 2 XRD patterns of Bi doped ZnO nanomaterials.

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Fig. 3 (a) W-H plot of Bi doped ZnO nanomaterials (offset vertically for clarity) (b) Crystallite

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size and strain as a function of Bi concentration.

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Fig. 4 XPS spectra of 2 mol% Bi doped ZnO nanomaterial (a) survey scan, (b, c ,d) High

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Fig.5 SEM image of the BZO phosphors for different Bi concentrations.

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Fig.6 High magnification SEM images of the BZO phosphors.

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Fig. 7 Element mapping and EDS spectrum of the BZO (Bi 2%) phosphor.

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Fig. 8 Diffuse reflectance spectra of the BZO phosphors.

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Fig.9 (a) Absorbance and reflectance of BZO phosphor (b) Plot between (Khν)2 vs hν.

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Fig. 10 (a) Excitation spectra of BZO phosphors at 438 nm emission wavelength. (b) Emission

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Fig.11 CIE chromaticity coordinates of BZO phosphor

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Graphical Abstract

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