Structural analysis and enhanced photoluminescence via host sensitization from a lanthanide doped BiVO4 nano-phosphor

Accepted Manuscript Structural analysis and enhanced photoluminescence via host sensitization from a lanthanide doped BiVO4 nano-phosphor Ram Sagar Yadav, Shyam Bahadur Rai PII:

S0022-3697(17)30637-6

DOI:

10.1016/j.jpcs.2017.06.019

Reference:

PCS 8102

To appear in:

Journal of Physics and Chemistry of Solids

Received Date: 11 April 2017 Revised Date:

31 May 2017

Accepted Date: 10 June 2017

Please cite this article as: R.S. Yadav, S.B. Rai, Structural analysis and enhanced photoluminescence via host sensitization from a lanthanide doped BiVO4 nano-phosphor, Journal of Physics and Chemistry of Solids (2017), doi: 10.1016/j.jpcs.2017.06.019. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT Structural analysis and enhanced photoluminescence via host sensitization from a lanthanide doped BiVO4 nano-phosphor

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Ram Sagar Yadav and Shyam Bahadur Rai* Laser & Spectroscopy Laboratory, Department of Physics Institute of Science, Banaras Hindu University Varanasi 221 005, India

Abstract

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This paper reports the enhanced red photoluminescence from a Eu3+ doped BiVO4 nanophosphor synthesized through solution combustion method. The XRD and TEM

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measurements reveal nano-crystalline nature of the sample. The SEM and TEM micrographs show an increase in the particles size on annealing. The EDS measurement confirms the presence of Eu, Bi, V and O elements in the sample. The sample emits red photoluminescence at 615 nm due to 5D0 → 7F2 transition of Eu3+ upon 266 and 355 nm

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excitations. The emission intensity increases with the increase in the concentration of Eu3+ ion and it is maximum at 1.0 mol%. When the as-synthesized sample is annealed at higher temperature; the emission intensity of the sample enhances upto eight times. The

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enhancement in the emission intensity is due to an increase in crystallinity and significant reduction in the multi-phonon relaxation of the ions in the excited state, which has been

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verified by lifetime measurements. The emission intensity is larger for 266 nm excitation (upto two times), which is due to energy transfer from the host (1P1 and 3P1 levels of Bi3+ and CTS of VO43+ ions) to Eu3+ ion. Therefore, the Eu3+ doped BiVO4 nano-phosphor may be a suitable candidate for displays devices.

Keywords: Europium ion; annealing; energy transfer; red emission; nano-phosphor. ___________________________________________________________________________ Corresponding Author: [email protected] (S. B. Rai)

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ACCEPTED MANUSCRIPT 1. Introduction The rare earth doped materials have attracted much attention due to their wide applications in various fields such as cathode ray tubes, color TV screens, luminescent lamps, X-ray

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detectors, displays devices, light emitting diodes (LEDs), red emitting phosphor, etc. They have good structural, chemical and thermal stability compared to conventional materials. They show strong brightness, non-toxicity and long lifespan behavior [1-6]. The rare earths (RE) doped vanadates have been extensively studied due to their broad optical transparency

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[7-12]. Among these materials, bismuth vanadate (BiVO4) has attracted more attention

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because it has interesting optical properties under UV and visible light illuminations and it is crystalline in nature [13-17]. It has three crystalline phases’ viz. tetragonal zircon, monoclinic scheelite and tetragonal scheelite. It undergoes phase transition under different thermal conditions. Out of these three crystalline phases, the monoclinic scheelite phase of BiVO4 is more stable and efficient for optical properties. It is a non-toxic yellow pigment and shows

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coloristic properties on cooling. The similar results have been observed when the sample is placed for cooling after heat treatment [18-21].

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The rare earth ion behaves as optically active center and contains large number of energy levels in which many of them are meta-stable and gives emissions from ultraviolet

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(UV) to near infrared (NIR) regions [22-24]. The Eu3+ activated YVO4 phosphor has been developed by Uitert et al., which gives red emission [5]. Rare earth doped several other vanadate phosphors have been extensively studied by different researchers and observed intense photoluminescence via energy transfer [9, 12, 25-28]. The emission intensity of the Eu3+ bands could be improved significantly via energy transfer process. The Eu3+, Sm3+ codoped Ca2(VO4)3 phosphor has been studied by Huang et al. [25] and observed intense emission due to energy transfer. The color tunable Ln3+:YVO4 (Ln= Eu, Sm, Dy, Ho) phosphor has been reported by Chen et al. [26]. It gives enhanced photoluminescence via 2

ACCEPTED MANUSCRIPT efficient energy transfer under near ultra-violet (NUV) excitation. It has been also observed that there is prolonged aging through ion exchange in the nano-phosphor with increasing Bi3+ ion contents and the emission intensity decreases after certain duration due to concentration quenching [9,12]. A red emitting BiLnVO4:Eu3+, Sm3+ (Ln = Y, Gd) phosphor has been also

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studied by Neeraj et al. in which Bi3+ transfers its energy to rare earths under UV excitation [27]. Furthermore, an energy transfer from Bi3+ to Eu3+ enhances the emission intensity of the Eu3+ bands significantly [28]. From these studies, it is observed that the emission intensity

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could be enhanced greatly via energy transfer.

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The Bi3+ ion in different materials gives intense emission due to 6s2 → 6s6p excitation, which can be used to harvest the UV and visible light irradiation [26, 28]. It is interesting to note that Bi3+ ion in BiVO4 host has 6s2 electronic configuration, which results broad absorption band in the UV region [12]. The absorption band contains various peaks due to Bi3+ and charge transfer (CT) transitions of VO43+ and Eu3+ ions, respectively. It has been

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observed that an efficient energy transfer takes place from Bi3+ and CTS of VO43+ to Eu3+ ions. The host sensitization followed by energy transfer from the excited host to the Eu3+ ion

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is an alternative way to achieve highly efficient phosphor materials. The host sensitization promotes larger energy transfer than the usual direct absorption of photons to a particular

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level [26]. This flexible property of BiVO4 host has encouraged us to design a phosphor using it for larger emission intensity. The Eu3+ doped BiVO4 has been studied by Zhang et al. and reported the photocatalytic behavior of BiVO4 with different concentration of Eu3+ ions [29]. Similarly, Xu et al. have also studied the photocatalytic activities of rare earth loaded BiVO4 catalysts [20]. The optical properties of Eu3+ ion in the BiVO4 host have been reported by our group [24]. However, the optical properties of Eu3+ doped BiVO4 under different excitation wavelengths can be motivating to understand the pathways involved in the energy transfer.

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ACCEPTED MANUSCRIPT The host sensitized energy transfer from this combination of the phosphor is not investigated to our knowledge, which needs a further attention. In this paper, we have synthesized a novel red emitting solid state Eu3+ doped BiVO4

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nano-phosphor through solution combustion method. The XRD and TEM measurements have been carried out to know the crystalline nature of the sample. The surface morphology of the samples has been studied using scanning electron microscopy (SEM) technique. The photoluminescence spectra of the synthesized samples have been recorded on excitations

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with 266 and 355 nm and the emission intensity thus obtained has been compared to each

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other. The possible mechanisms involved in the energy transfer have been discussed in detail using schematic energy level diagram. The lifetime measurement has been carried out to confirm an enhancement in the emission intensity on annealing.

2.1 Sample Preparation

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2. Experimental

The Eu3+ doped BiVO4 nano-phosphors with different concentration of Eu3+ ions (0.5, 1.0

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and 1.5 mol%) have been synthesized using solution combustion method. The Eu2O3, NH4VO3 and Bi(NO3)3.5H2O with 99.99% purity have been used as starting materials. The

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stoichiometric amounts of these materials were dissolved in 5 ml of concentrated nitric acid separately. These solutions were mixed together and stirred for half an hour to mix them properly. The solution was then diluted with distilled water under constant stirring. Urea has been used as an organic fuel and was dissolved in 10 ml of distilled water. The urea solution was then added in the mixed solution. The final solution was stirred for 4 hours at 600C to obtain a sticky gel solution. The obtained gel was then placed in a closed furnace maintained at 6500C. Combustion took place within few minutes evolving different gases due to exothermic reactions. The powder product thus obtained is known as the as-synthesized 4

ACCEPTED MANUSCRIPT sample. The as-synthesized sample was then annealed at 400 0C for 5 hours in order to improve the structural and optical properties of the samples [20, 24]. 2.2 Characterization

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The XRD patterns of the synthesized samples have been recorded with Cu, Kα radiation (λ=0.15406 nm) from a RINT/DMAX 2200 H/PC (Rigaku, Japan) machine under 20/min scan speed. The crystallized phase of the sample was confirmed by the data from

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International Center for Diffraction Data (ICDD). The transmission electron microscopy (TEM) technique was used to record the micrographs of the samples using a Technai 20G2,

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Philips unit. The structural morphology of the powder samples were monitored with scanning electron microscopy (SEM) technique using JEOL-TM Model JSM 5410 system. The EDS measurements have been carried out to verify the presence of different elements in the synthesized sample. The photoluminescence excitation (PLE) spectrum of the phosphor was

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monitored using flurolog-3 450W fluorescence spectrofluorometer (Horiba Jobin Yvon, Model no. FL3-11). The photoluminescence (PL) spectra of the synthesized samples have been monitored using 266 and 355 nm excitations from a Nd:YAG laser. The Horiba iHR320

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monochromator attached with a photomultiplier tube (Model no. 1424 M) has been used to record the dispersed photoluminescence in the two cases. The lifetime measurement of the

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doped nano-phosphors has been carried out on excitation with 266 nm using 150 MHz digital oscilloscope (Hameg Instruments, Model no. HM 1507). 3 Results and discussion

3.1 Structural characterization 3.1.1 X-ray diffraction (XRD) measurements

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ACCEPTED MANUSCRIPT The XRD patterns of the as-synthesized (1.0 mol %) and the annealed (0.5, 1.0 and 1.5 mol %) Eu3+ doped BiVO4 nano-phosphor samples recorded in the range of 25o – 65o are shown in Fig. 1. The XRD patterns of the samples match well with the JCPDS File No. 14-0688. The phase of the sample is identified to be a monoclinic scheelite phase with space group I2/a

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(15) and cell parameters a = 5.195 Å, b = 11.70 Å and c = 5.092 Å [20, 24]. No other impurity peaks were detected in the XRD pattern. The indexing of the XRD patterns has been done using JCPDS file. The average crystallite size (Dhkl) of both the samples for (121), (040)

Kλ β Cosθ

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Dhkl =

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and (161) lattice planes have been calculated with the help of Debye-Scherrer equation [22]:

where K = 0.90, is a constant; λ is the wavelength of the X-ray; β and θ are the full width at half maxima (FWHM) and diffraction angle of the diffracted peaks, respectively. The average crystallite size has been calculated and it was found to be 11 and 23 nm for the

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as-synthesized and the annealed samples, respectively. The inset in Fig. 1(d) shows a slight change in FWHM of lattice plane (121) for the as-synthesized (1.0 mol %) and the annealed (0.5, 1.0 and 1.5 mol %) samples. When the as-synthesized sample is annealed at higher

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temperature the FWHM of the annealed sample is reduced slightly compared to the as-

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synthesized sample. Since the concentration of the Eu3+ ion is very low compared to BiVO4, therefore, it does not affect the crystallinity and phase of the xEu3+ doped BiVO4 nanophosphor samples (i.e. x= 0.5, 1.0 and 1.5 mol%). Thus, the reduction of FWHM in the XRD peaks reveals an improvement in crystallinity due to annealing, which is favorable for large photoluminescence of the samples [23].

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ACCEPTED MANUSCRIPT 3.1.2 TEM and SAED measurements The transmission electron micrograph reveals the distribution of nano-structured particles in the sample. The TEM micrographs of the as-synthesized and the annealed 1.0 mol% Eu3+

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doped BiVO4 nano-phosphor are shown in Fig. 2(a & b). The figures show that the particles are spherical and distributed randomly. The particle size is found to increase on annealing the as-synthesized sample (see Fig. 2(b)). The particle size is estimated to be ~104 and ~145 nm for the as-synthesized and the annealed samples, respectively. The inset in Fig. 2(b) shows

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the selected area electron diffraction (SAED) pattern of the annealed sample, which confirms

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poly-crystalline nature of the sample. 3.1.3 EDS measurement

The energy dispersive spectroscopic (EDS) measurement has been carried out in order to verify the presence of used and the impurity elements during synthesis of the sample. It has

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been seen that the presence of desired elements in the sample proves the appropriateness of the synthesis process. The EDS spectrum of the annealed 1.0 mol% Eu3+ doped BiVO4 nanophosphor is shown in Fig. 3. The spectrum reveals the presence of Eu, Bi, V and O elements

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in the synthesized sample. The EDS spectrum is recorded with the marked position in the

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TEM micrograph (see Fig. 2(b)). The peaks observed at 8.0, 9.0 and 13.2 eV is due to Cu element, whose grid mesh has been used in the experiment [22]. The weight% and atomic% of different elements viz. Eu, Bi, V and O, present in the sample are summarized in Table 1. 3.1.4 Scanning electron microscopy (SEM) measurements The surface micrographs of the as-synthesized and the annealed 1.0 mol % Eu3+ doped BiVO4 nano-phosphor sample recorded using scanning electron microscope (SEM) are shown in Fig. 4 (a & b). The particles in the micrographs are oriented in different directions throughout the

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ACCEPTED MANUSCRIPT sample. Though the shape of the particles is not uniform due to agglomeration, however, most of the particles are spheroidal in shape. Actually, at higher temperature the smaller nano-particles combine to each other and form larger agglomerated complex structure [23]. In this case, the annealed sample further shows an increase in the particle size than the as-

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synthesized sample (see Fig. 4(b)). 3.2 Optical characterization

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3.2.1 Excitation spectrum

The photoluminescence excitation (PLE) spectrum of the annealed 1.0 mol% Eu3+ doped

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BiVO4 nano-phosphor sample is shown in Fig. 5. The PLE spectrum has been recorded in the range of 200 - 450 nm and the emission is monitored at λemi = 615 nm for 5D0 → 7F2 transition of Eu3+ ion. The spectrum contains several peaks in the 325 - 450 nm region due to 1S0 → 3P1 and 4f-4f transitions of Bi3+ and Eu3+ ions, respectively [30]. The broad band observed in the

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200 - 325 nm region is due to overlapping of 1S0 → 1P1 transition (at 264 nm) of Bi3+ ion [31, 32] and charge transfer states (CTS) of O2- – V5+ (at 277 nm) and O2- – Eu3+ ions (at 280 nm), respectively [7-11].

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3.2.2 Emission spectra on excitation with 355 nm laser

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Fig. 6 shows the emission spectra of the as-synthesized (1.0 mol%) and the annealed (0.5, 1.0 and 1.5 mol%) Eu3+ doped BiVO4 nano-phosphor samples monitored on excitation with 355 nm in the range of 500 - 750 nm. The emission spectra contain various emission bands centered at 565, 596, 615, 665 and 705 nm and they are assigned to arise due to 5D0 → 7Fi (i= 0,1,2,3,4) electronic transitions of Eu3+ ion, respectively [4-5, 8-12, 25-28, 33-35]. These transitions are attributed due to (4f - 4f) electronic transitions. The most intense red band is observed at 615 nm due to 5D0 → 7F2 transition. It is well known that the 5D0 → 7F1 transition at 596 nm is typically a magnetic dipole type transition, following the selection rule ∆J =1. 8

ACCEPTED MANUSCRIPT On the other hand, the 5D0 → 7F2 transition is an electric dipole type transition, which satisfies the selection rule ∆J = 2. It is hypersensitive transition; hence, it gives most intense emission. Sun et al. have studied the luminescence properties of Eu3+, Bi3+ co-doped Y(V,P)O4 phosphor and also observed electric and magnetic type of transitions in Eu3+ ion [35]. The D0 → 7F2 transition strongly depends on the local symmetry of the environment around Eu3+

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ion in the host [34]. Therefore, the intensity ratio of these transitions gives an idea about the asymmetry of the sample environment i.e. I(5D0 → 7F2) to I(5D0 → 7F1). The 5D0 → 7F0

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transition is very sharp and is called as zero phonon width transition [35].

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The Eu3+ doped BiVO4 samples have been synthesized with different concentration of Eu3+ ions (i.e. 0.5, 1.0 and 1.5 mol%). It has been found that the emission intensity of the samples increases as the concentration of Eu3+ ion increases. Initially, when the concentration of Eu3+ increases from 0.5 mol% to 1.0 mol%; the emission intensity is found to increase simultaneously. When the concentration of Eu3+ ion is increased from 1.0 to 1.5 mol%; the

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emission intensity of the sample starts decreasing. The decrease in the emission intensity is due to concentration quenching effect. In fact, at higher concentration the distance between

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the ions becomes smaller than the critical value and the excitation energy is transferred to the optical quenching sites, which is lost non-radiatively through multi-phonon relaxations [26,

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36]. The effect of concentration on the emission intensity of Eu3+ is shown in Fig. 6. The sample annealed at higher temperatures also plays a crucial role in the emission

intensity. The as-synthesized sample contains various impurities such as CO32-, OH-, etc. [36, 37]. These impurities are termed as optical quenching centers and reduce the emission intensity considerably. The emission intensity of the as-synthesized sample can be improved significantly by annealing it. When the as-synthesized sample is annealed at higher temperatures the optical quenching centers present in it are reduced appreciably. This also increases the crystallinity of the sample. As discussed earlier in the XRD, the annealing of the 9

ACCEPTED MANUSCRIPT sample results an increase in the crystallite size i.e. from 11 nm to 23 nm [22-23, 36]. It is also clear from the SEM and TEM micrographs that the particle size of the sample is increased on annealing. As the crystallinity of the sample increases the multi-phonon relaxations of the ions are reduced significantly in the excited state. The effect of annealing

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temperature on the emission intensity of the phosphor has been studied by Pandey et al. and found an enhancement in the emission intensity due to reduction in the optical quenching centers and an improvement in crystallinity [37]. In our case, we have also observed an

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increase in the photoluminescence intensity of the as-synthesized sample upto eight times on annealing it (see Fig. 6).

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The processes involved in the excitation and emission can be easily understood using schematic energy level diagram as shown in Fig. 7. When the Eu3+ doped BiVO4 is excited by 355 nm laser the Eu3+ and Bi3+ ions are excited simultaneously. The Eu3+ ions are excited to 5D4 level, which through non-radiative relaxations populate 5D0 level. Similarly, the Bi3+

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ions are also excited to their 3P1 level. In a similar study, Yadav et al. have reported in Bi3+/Yb3+ co-doped phosphor that the Bi3+ ions are promoted to 3P1 level on excitation with 355 nm and give blue to red emissions due to 3P1 → 1S0 transition [38]. Since the energy of P1 level of Bi3+ lies close to the energy of 5D4 level of Eu3+ ions there is an energy transfer

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from 3P1 level of Bi3+ to the 5D4 level of Eu3+ ions, which ultimately populates the 5D0 level

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of Eu3+ ion non-radiatively. As a result, the Eu3+ ions emit large number of intense photons to the low lying levels (i.e. 565, 596, 615, 665 and 705 nm) [25-28, 33-35]. 3.2.3 Emission spectra on excitation with 266 nm laser The emission spectra of the as-synthesized (1.0 mol%) and the annealed (0.5, 1.0 and 1.5 mol%) Eu3+ doped BiVO4 nano-phosphor samples monitored on excitation with 266 nm in the 500 - 750 nm region are shown in Fig. 8. The emission spectra contain similar emission bands (i.e. 565, 596, 615, 665 and 705 nm) as upon 355 nm excitation. In this case, the 10

ACCEPTED MANUSCRIPT emission intensity thus obtained is two times larger than the earlier case. The most intense band is observed at 615 nm due to 5D0 → 7F2 transition of the Eu3+ ion [24, 35]. The annealed sample further shows an enhancement in the emission intensity. The emission intensity is maximum for the sample with Eu3+ ion concentration (at 1.0 mol%) and decreases for higher

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concentrations. Herein, the emission intensity of Eu3+ ions is enhanced significantly on excitation with 266 nm compared to the 355 nm excitation due to energy transfer to Eu3+ ions through several channels [26, 35]. These energy transfer processes can be easily understood

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with the help of energy level diagram (see Fig. 7).

When the Eu3+ doped BiVO4 nano-phosphor is energized by 266 nm laser, the 1P1 state of

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Bi3+ ion [31], charge transfer states (CTS) of O2- – V5+ and O2- – Eu3+ ions [35] are excited simultaneously as they have energy states corresponding to the 266 nm excitation. The 1P1 state of Bi3+ ion; charge transfer states of O2- – V5+ and O2- – Eu3+ ions are overlapped to each other (see Figs. 5 & 7). This arises several possibilities of energy transfer to Eu3+ ions.

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Firstly, some of the ions in the 1P1 level of Bi3+ transfer their energy to Eu3+ ions in the 5D4 level. The remaining ions relax non-radiatively to 3P1 level and due to matching of energy levels with Eu3+, they transfer their energy to Eu3+ ions in the 5D4 level [35]. Secondly, the

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ions in the CTS of O2- – V5+ transfer their energy to Eu3+ ions in the 5D4 level [7-12]. Finally, the ions in the CTS of O2- – Eu3+ ion also relax non-radiatively to 5D4 level. It is interesting to

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note that due to all these processes the populations of the ions in the 5D4 level are very large. The ions in this level relax non-radiatively to populate the 5D0 level of Eu3+ ion [26]. These ions relax radiatively to different low lying levels and emit very intense photoluminescence. Since only one process is involved in the fluorescence for the 355 nm excitation, therefore, the fluorescence intensity is found to be relatively weak. It is worth noting that the photoluminescence properties of Y(V,P)O4: Eu3+, Bi3+ phosphors have been studied by Sun et al. These workers have observed intense red emission

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ACCEPTED MANUSCRIPT due to energy transfer under 254 and 365 nm excitations [35]. They have reported an energy transfer from O2- – V5+ charge transfer band to Eu3+ ions and charge transfer transition from Bi3+ to VO43+ instead of 6s2 → 6s6p (1S0 → 3P1) transition of Bi3+, followed by the energy transfer to Eu3+ ions. Similarly, Chen et al. have also observed an intense red emission due to

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energy transfer from charge transfer transitions of O2- – V5+ and Bi3+ to VO43+ ions, followed by the energy transfer to Eu3+ ions [26]. However, in our case the broad band observed in the range of 200 - 325 nm is due to overlapping of the CT states of Eu3+ and VO43+ ions [7-12,

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26, 35] as well as absorption band at 264 nm due to 1S0 → 1P1 transition of Bi3+ ion [31]. Similarly, Wu et al. have studied the photoluminescence properties of Eu, Bi co-doped Y2O3

due to the

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phosphor. They have also reported that the excitation band observed at 263 nm is ascribed S0 → 1P1 transition of Bi3+ ion [32]. Therefore; the energy transfer is expected to

take place from the 1P1 and 3P1 levels of Bi3+ and CTS of VO43+ ions to Eu3+ ions in our case. The effect of the excitation wavelengths i.e. 266 and 355 nm, on the emission intensity of the

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Eu3+ ion are shown in Fig. 9. The figure clearly reveals that the emission intensity of the sample excited by 266 nm is enhanced upto two times compared to the 355 nm excitation. Thus, one could clearly understand an intense red photoluminescence from the Eu3+ doped

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BiVO4 nano-phosphor sample upon 266 nm excitation.

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In order to verify the color emitted by the sample, the human eye is the best tool to confirm it. The color emitted by the sample can be also represented mathematically in terms of two coordinate systems i.e. commission internationale de l’e’clairage (CIE) coordinates. This can verify the color purity of the sample [22-23]. The CIE plots for 1 mol% Eu3+ doped BiVO4 nano-phosphor on excitations with 266 and 355 nm are shown in Fig. 10. The CIE coordinates are found to be (0.63, 0.31) and (0.63, 0.32) for 266 and 355 nm excitations, respectively [28]. It suggests that the excitation wavelength does not change the color emitted

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ACCEPTED MANUSCRIPT by the sample, however, there is a large difference in the emission intensity of the sample in the two cases. 3.2.4 Lifetime measurements The lifetime of the as-synthesized and the annealed 1.0 mol% Eu3+ doped BiVO4 nano-

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phosphor for 5D0 → 7F2 transition (at 615 nm) has been measured on excitation with 266 nm. The decay curves thus obtained in the two cases are shown in Fig. 11(a & b). The lifetime (τ) of the level could be determined by exponential fitting of the decay curves. It was found that

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t I = I 0 exp(− )

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the decay curve fits well with mono-exponential fitting by the following equation [23]:

τ

where I0 and I refers the intensity at time zero and t seconds, respectively. The lifetime (τ) of 5

D0 level has been calculated in the two cases and the obtained values are 225 and 240 µs for

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the as-synthesized and the annealed samples, respectively. Interestingly, the lifetime of the annealed sample is found to be larger than the as-synthesized sample, which is due to reduction in quenching centers and an increase in crystallinity. This induces large reduction

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in the multi-phonon relaxation of the ions in the excited states. As a result, the emission intensity of the annealed sample is enhanced significantly.

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4. Conclusions

The Eu3+ doped BiVO4 nano-phosphor has been synthesized through solution combustion method. The XRD and the TEM measurements show nano-crystalline nature of the sample. The SEM and TEM micrographs reveal an increase in the particles size on annealing. The sample emits intense red photoluminescence at 615 nm on excitations with 266 and 355 nm. The emission intensity is optimum for 1.0 mol% concentration of Eu3+ ion. The annealed sample shows larger emission intensity due to reduction in optical quenching centers and an 13

ACCEPTED MANUSCRIPT increase in crystallinity. The lifetime of the annealed sample is larger due to reduction in the multi-phonon relaxation of the ions in the excited states. The sample excited by 266 nm emits larger photoluminescence up to two times compared to the 355 nm excitation, which is due to energy transfer from the host (i.e. Bi3+ and VO43+ ions) to Eu3+ ion. Thus, the host sensitized

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Eu3+ doped BiVO4 nano-phosphor may be used as red phosphor for displays devices.

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Acknowledgements

Prof. S. B. Rai wishes to acknowledge to University Grants Commission (UGC), India for

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financial assistance as BSR Faculty Fellow (F. No. 18-1(84)/2015(BSR)). The authors are thankful to Prof. O. N. Srivastava (Department of Physics, BHU) for extending experimental

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facilities for XRD, SEM and TEM measurements.

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doping process on photoluminescence properties of YVO4:Bi3+, Eu3+ nanophosphor, J. Lumin. 129 (2009) 1067–1072.

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[13] I. Grigioni, K. G. Stamplecoskie, E. Selli, P. V. Kamat, Dynamics of Photogenerated Charge Carriers in WO3/BiVO4 Heterojunction Photoanodes, J. Phys. Chem. C 119 (2015) 20792−20800.

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Patrocínio, A. C. Silva, L. C. A. Oliveira, J. D. Fabris, M. C. Pereira, A hole inversion layer at the BiVO4/Bi4V2O11 interface produces a high tunable photovoltage for water splitting, Sci. Rep. 6 (2016) 31406 pp 1-9.

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Structure of BiVO4: Elucidating the Role of the Bi s and V d Orbitals, Chem. Mater. 21 (2009) 547–551.

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ACCEPTED MANUSCRIPT [17] J. S. Yang J. J. Wu, Low-potential driven fully-depleted BiVO4/ZnO heterojunction nanodendrite array photoanodes for photoelectrochemical water splitting, Nano Energy 32 (2017) 232–240.

photocatalysts, Mater. Let. 62 (2008) 926–928.

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photocatalytic activities of rare earth-loaded BiVO4 catalysts, Appl. Surf. Sci. 256 (2009) 597–602.

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[22] R. S. Yadav, R. K. Verma, S. B. Rai, Intense white light emission in Tm3+/Er3+/Yb3+ codoped Y2O3–ZnO nano-composite, J. Phys. D: Appl. Phys. 46 (2013) 275101(pp 1-8). [23] R. S. Yadav, R. V. Yadav, A. Bahadur, T. P. Yadav, S. B. Rai, Role of Li+ on white light

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emission from Sm3+ and Tb3+ co-doped Y2O3 nano-phosphor, Mater. Res. Exp. 3 (2016) 036201 (pp 1-13).

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ACCEPTED MANUSCRIPT [26] D. Chen, Y. Yu, P. Huang, H. Lin, Z. Shan, L. Zeng, A. Yang, Y. Wang, Color-tunable luminescence for Bi3+/Ln3+:YVO4 (Ln = Eu, Sm, Dy, Ho) nanophosphors excitable by near-ultraviolet light, Phys. Chem. Chem. Phys. 12 (2010) 7775-7778. [27] S. Neeraj, N. Kijima, A. K. Cheetham, Novel red phosphors for solid state lighting: the

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system BixLn1-xVO4; Eu3+/Sm3+ (Ln=Y, Gd), Solid State Comm. 131 (2004) 65-69.

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BiVO4, J. Hazar. Mater. 173 (2010) 265–272.

[30] M. N. Huang, Y. Y. Ma, F. Xiao, Q. Y. Zhang, Bi3+ sensitized Y2WO6:Ln3+ (Ln = Dy, Eu, and Sm) phosphors for solar spectral conversion, Spectrochim. Acta Part A 120 (2014) 55–59.

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of Y2O3:Eu, Bi phosphors by molten salt synthesis for white light-emitting diodes, J. Mater. Sci. 46 (2011) 5581–5586. [33] Y. H. Zhou, J. Lin, Morphology control and luminescence properties of YVO4:Eu phosphors prepared by spray pyrolysis, Opt. Mater. 27 (2005) 1426–1432. [34] R. S. Yadav, Y. Dwivedi, S. B. Rai, Structural and optical properties of Eu3+, Sm3+ codoped La(OH)3 nano-crystalline red emitting phosphor, Spectrochim. Acta Part A 132 (2014) 599-603.

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ACCEPTED MANUSCRIPT [35] J. Sun, J. Xian, Z. Xia, H. Du, Synthesis, structure and luminescence properties of Y(V,P)O4: Eu3+,Bi3+ phosphors, J. Lumin. 130 (2010) 1818–1824. [36] R. S. Yadav, R. K. Verma, A. Bahadur, S. B. Rai, Structural characterizations and

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intense green upconversion emission in Yb3+, Pr3+ co-doped Y2O3 nano-phosphor,

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[38] R. V. Yadav, R. S. Yadav, A. Bahadur, A. K. Singh, S. B. Rai, Enhanced quantum

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Chem. 55 (2016) 10928−10935.

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cutting via Li+ doping from a Bi3+/Yb3+-codoped gadolinium tungstate phosphor, Inorg.

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Figures

Figure 1. X-ray diffraction patterns of (a) the as-synthesized (1.0 mol%) and (b-d) the annealed (0.5, 1.0 and 1.5 mol%) Eu3+ doped BiVO4 nano-phosphor samples. The inset in the figure (d) shows a change in FWHM of lattice plane (121) for the as-synthesized and the annealed samples.

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Figure 2. Transmission electron micrographs of (a) the as-synthesized and (b) the annealed 1.0 mol% Eu3+ doped BiVO4 nano-phosphor samples. The inset in figure (b) shows the

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SAED pattern of the annealed 1.0 mol% Eu3+ doped BiVO4 nano-phosphor sample.

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Figure 3. EDS spectrum of the annealed 1.0 mol% Eu3+ doped BiVO4 nano-phosphor

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sample.

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Figure 4. Scanning electron micrographs of (a) the as-synthesized and (b) the annealed 1.0

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mol% Eu3+ doped BiVO4 nano-phosphor samples.

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Figure 5. Excitation spectrum of the annealed 1.0 mol% Eu3+ doped BiVO4 nano-phosphor

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monitored at λemi = 615 nm.

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Figure 6. Emission spectra of the as-synthesized (1.0 mol%) and the annealed x mol% Eu3+

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doped BiVO4 nano-phosphors on excitation with 355 nm laser (i.e. x = 0.5, 1.0 and 1.5

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mol%); effect of annealing and concentration on the emission intensity.

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Figure 7. Energy level diagrams of Bi3+, V5+ and Eu3+ ions on excitations with 266 and 355

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nm and energy transfer mechanisms.

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Figure 8. Emission spectra of the as-synthesized (1.0 mol%) and the annealed x mol% Eu3+ doped BiVO4 nano-phosphors (i.e. x = 0.5, 1.0 and 1.5 mol%) on excitation with 266 nm

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laser; the effect of concentration of Eu3+ ions and annealing on the emission intensity.

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Figure 9. Effect of the excitation wavelengths (i.e. 266 and 355 nm) on the emission intensity

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of 1.0 mol% Eu3+ doped BiVO4 nano-phosphor sample.

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Figure 10. CIE diagram for the annealed 1.0 mol% Eu3+ doped BiVO4 nano-phosphor on

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excitations with 266 and 355 nm.

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Figure 11. Decay curves of 5D0 level for (a) the as-synthesized and (b) the annealed 1.0 mol% Eu3+ doped BiVO4 nano-phosphor of 5D0 → 7F2 transition (at 615 nm) upon 266 nm

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excitation.

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Table 1: The weight% and atomic% of different elements present in the annealed 1.0 mol%

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Atomic% 15.20 33.60 1.00 50.20

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Weight % 11.75 36.60 1.05 50.60

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Elements OK VK EuL BiL

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Eu3+ doped BiVO4 nano-phosphor.

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Highlights 3+


The Eu

doped BiVO4 nano-phosphor has been synthesized through solution combustion

method.

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The XRD and TEM measurements reveal nano-crystalline nature of the sample.
The EDS analysis confirms the presence of Eu, Bi, V and O elements in the sample.

Eu3+ ion upon 266 and 355 nm excitations.

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The sample emits intense red photoluminescence at 615 nm due to 5D0 → 7F2 transition of


The emission intensity is larger for 266 nm excitation (upto two times), which is due to

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energy transfer from the host to Eu3+ ion.