Rapid, Room Temperature Synthesis of Eu3+ Doped NaBi(MoO4)2

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Rapid, room temperature synthesis of Eu3+ doped NaBi(MoO4)2 nanomaterials: Structural, optical and photoluminescence properties Pushpendra *, Ravi K. Kunchala, Srungarpu N. Achary, Avesh K. Tyagi, and Boddu S. Naidu Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.9b00267 • Publication Date (Web): 24 Apr 2019 Downloaded from http://pubs.acs.org on April 25, 2019

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Crystal Growth & Design

Rapid, room temperature synthesis of Eu3+ doped NaBi(MoO4)2 nanomaterials: Structural, optical and photoluminescence properties Pushpendra†, Ravi K. Kunchala†, Srungarpu N. Achary‡, Avesh K. Tyagi‡, Boddu S. Naidu†,* †Institute

of Nano Science and Technology (INST), Phase 10, Sector 64, Mohali, Punjab, India160062, E-mail: [email protected], [email protected] ‡Chemistry

Division, Bhabha Atomic Research Centre, Trombay, India

ABSTRACT Bismuth based host materials for doping rare earth ions gained considerable interest due to the possibility of forming crystalline homogeneous solid solution phases over a wide range and their excellent luminescent properties. In the present manuscript, a facile synthesis method for preparation of nanocrystalline NaBi(MoO4)2 and their optical properties are presented. Influence of various synthetic parameters such as precursor concentration, reaction time and reaction temperature on phase formation have been investigated and revealed that these nanoparticles can be synthesized even at 5°C within 5 minutes with Bi:Mo ratio of 1:4. All the doped and undoped samples show scheelite type tetragonal structures. Formation of solid solution between NaBi(MoO4)2 and NaEu(MoO4)2 over the complete range of compositions could be achieved. A systematic decrease in the unit cell parameters is observed with increasing concentration of Eu3+ ion in NaBi1-xEux(MoO4)2. Investigations on the luminescence properties of europium doped samples shows excellent red luminescence upon excitation at 465 nm. Quantum efficiency calculated from experimental luminescence studies shows optimum photoluminescence properties in NaBi0.9Eu0.1(MoO4)2 nanoparticles with highest quantum efficiency of 50%. Key words: NaBi(MoO4)2, Scheelite, Photoluminescence, solid solutions, Europium doping, Nanomaterials

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1. INTRODUCTION Rare Earth ion doped luminescent materials drag considerable attention due to their potential applications in several technologies such as, lighting and display devices, lasers, solar cells, fluorescent probes for biological labeling, imaging and therapy, catalysis, nano-electronics and nano-photonics.1-6 Lanthanide based materials known to be best host materials for doping rare earth ions due to their similarities in charge, size and co-ordination number.3 Though compounds of rare-earths can be excellent host material for luminescent lanthanide ions, the requirements of large quantities of high pure lanthanides become expensive due to their less abundance and difficulty for obtaining high pure lanthanides. As a result, there is a growing interest in this field to replace rare earth based host materials with low cost and highly abundant elements. Main group elements such as antimony, bismuth and gallium, etc., are known to have similar properties with lanthanide ions. Thus in recent times, large numbers of studies are being carried out on rare earth ions doped antimony, bismuth, gallium and indium based materials.7-15 NaBi(MoO4)2 is a technologically important inorganic double molybdate which exhibits excellent

sensing,

acoustic,

photoelastic,

photocatalytic,

luminescence

properties.16-21

NaBi(MoO4)2 possess scheelite (CaWO4, tetragonal, (C4h) space group: I41/a) type structure where Bi and Na are randomly distributed at Ca site and Mo is occupied at W site of CaWO4. 22 The structure consists of (Na,Bi)O8 square antiprisms and isolated tetrahedral MoO4 units. Since NaBi(MoO4)2 is isostructural to lanthanide based double molybdates, and Bi3+ is having similar size, charge and coordination number as those of rare earth ions, it is expected to be a better host material for rare earth ions.23 Single crystals of NaBi(MoO4)2 doped with Pr3+, Nd3+, Ho3+, Er3+ ions grown by Czochralski process show excellent photoluminescence properties.24-28 Mazurak et al., grown pure NaBi(MoO4)2 crystals by Czochralski process from the melt and studied photoluminescence properties of these crystals.21 Lin et al. have synthesized nanocrystalline NaBi(MoO4)2 by hydrothermal method at high temperature and high pressure, and utilized them for gas sensing applications.29 Gan, et al. synthesized Eu3+ doped powders by sol-combustion method.30 However, studies on rare earth ions doped NaBi(MoO4)2 nanomaterials still lack in literatures. In this present manuscript, nanoparticles of NaBi(MoO4)2 are prepared by rapid, facile co-precipitation method at room temperature. Here, Eu3+ has been chosen as a dopant to understand the possibility and extent of doping in the host material. Various factors effecting on the formation of crystal phase and particle size were investigated. The formation of solid solution over the 2 ACS Paragon Plus Environment

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complete range of compositions in between NaBi(MoO4)2 and NaEu(MoO4)2 is concluded. Structural, optical and photoluminescence properties of these nanomaterials are also studied. To the best of authors’ knowledge this is the first detailed study on simple precipitation and optical properties on NaBi(MoO4)2 and Eu3+ doped NaBi(MoO4)2 nanomaterials in wider range of dopant concentration. 2. EXPERIMENTAL SECTION Ethylene glycol (C2H4O2, Emparta grade) was purchased from Merck Millipore, India. Bi(NO3)3.5H2O, 99.9%, Eu(NO3)3.5H2O, 99.9% were procured from Sigma Aldrich and Na2MoO4.2H2O was obtained from S D Fine-Chem limited, India. 2.1. Synthesis of NaBi(MoO4)2 NaBi(MoO4)2 nanoparticles were prepared by facile co-precipitation method at room temperature and the detailed procedure is as follow. For the synthesis of un-doped nanoparticles, about 1g of Bi(NO3)3.5H2O was dissolved in 20 ml of ethylene glycol by heating while stirring. Then cool down to room temperature, naturally. A white precipitate was obtained immediately on addition of aqueous Na2MoO4.2H2O solution and the solution was stirred for 2 hours. After that, the precipitate was separated by centrifugation and it was washed several times with ethanol, distilled water and acetone. The obtained material was dried under ambient conditions for overnight. Effect of various parameters such as relative molar ratios of Bi:Mo, reaction time and reaction temperature were also varied to understand the crystal growth. 2.2. Synthesis of NaBi1-xEux(MoO4)2 solid solutions For the synthesis of NaBi1-xEux(MoO4)2 solid solutions, a similar method that used for NaBi(MoO4)2 has been followed other than the addition of Eu3+ source. The detailed method is as follows: Eu(NO3)3.5H2O and Bi(NO3)3.5H2O in required mole ratio were dissolved in 20 ml of ethylene glycol by heating while stirring. This solution was cool down to room temperature and then aqueous Na2MoO4.2H2O solution was added while stirring. An immediate white precipitate was formed on addition of Na2MoO4.2H2O solution. The total mixture was stirred continuously for two hours. The precipitate was centrifuged and washed several times with ethanol, distilled water and acetone. The obtained material was dried under ambient conditions for overnight. 2.3. Characterization Powder X-ray diffraction (XRD) patterns of prepared nanomaterials were obtained using Bruker D8 Advance diffractometer equipped with Ni-filtered Cu- K radiation (wavelength, = 1.5418Å). 3 ACS Paragon Plus Environment


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The operating current and voltage were 25mA and 40KV, respectively. Scherrer's formula was used to calculate average crystallite size. Structural analyses of these nanomaterials were carried out using Fullprof software. All Raman spectra were recorded using WI Tech Raman spectrometer. Samples were excited with unpolarized He-Ne laser (wavelength 633 nm and, max. CW power 75 mW) with spot size 1 m and collected in backscattered configuration at 600 grooves/mm grating by Peltier cooled (-600C) charged coupled device detector. We have considered 10x magnification objective to focus the laser on the sample. Fourier transform infrared (FTIR) spectra were obtained using thin pellets of the homogenous mixture of sample and KBr on a Carry-660 FTIR spectrometer. Shimadzu UV-2600 spectrophotometer was used to record UV-Visible diffuse reflectance measurements. Scanning electron microscopic (SEM) images were obtained using JEOL 7600F FESEM microscope. Transmission electron microscopic (TEM) measurements were performed using JEOL 2100 TEM microscope operated at 200 KeV. The photoluminescence spectra were recorded with Edinburgh Instruments FS5 fluorimeter equipped with 500W Xenon Lamp, and 100 W Xenon microsecond flashlight as an excitation source for steady state and lifetime measurements, respectively. 3. RESULTS AND DISCUSSION Nanoparticles of NaBi(MoO4)2 are prepared by facile co-precipitation method. The XRD pattern of these nanoparticles is shown in Figure1(a).These nanoparticles are crystallized in scheelite (CaWO4) type structure and are in agreement with reported data (JCPDS file no. 01-074-8694). No crystalline secondary phases were detected in the XRD data of the sample. The average crystallite size of these nanomaterials calculated from XRD patterns using Scherrer formula is 26 nm. UV-Vis DRS spectrum of these nanomaterials is recorded and shown in Figure 1(b). It shows very strong absorption in the UV region. Inset of Figure 1(b) presents the Tauc plot of NaBi(MoO4)2 nanoparticles. The band gap of NaBi(MoO4)2 nanoparticles is estimated from the Tauc plot and is found to be 3.36eV FTIR and Raman spectra of these nanoparticles are shown in Figure 1 (c & d). On the basis of

group

theory,

possible

modes

are

3Ag+5Bg+5Eg+5Au+3Bu+5Eu,

where,

Au+Eu,

2Bg+2Eg+Au+Eu, Ag+Eg+Bu+Eu and 2Ag+3Bg+2Eg+3Au+2Bu+2Eu, are acoustic modes, translator lattice modes, rotatory lattice modes and internal vibrations of MoO42- tetrahedra, respectively31. All the ‘g’ vibrations are Raman active whereas ‘u’ vibrations are Infrared active, respectively. FTIR spectrum of NaBi(MoO4)2 comprises of vibrational bands at 404, 1633, 2338, 2360, two 4 ACS Paragon Plus Environment

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broad bands with shoulders at 700, 750, 840, 914 and 3215, 3415 cm-1. The vibrational band at 404 cm-1 corresponds to the asymmetric bending of MoO42-tetrahedra. Vibrational bands at 700, 750 cm-1 and 840, 914 cm-1 are attributed to the symmetric and asymmetric stretching vibrations of MoO42- tetrahedra, respectively. All the vibrational modes are very well matched with previous reports31-32. Vibrational bands at 1633 and 2338, 2360 cm-1 are attributed to O-H bending and symmetric, asymmetric stretching of C-H bonds of ethylene glycol molecule present on the surface of the nanoparticles. Broad band at 3415 with shoulder peak at 3220 cm-1 are corresponds to free and bounded O-H stretching frequencies of water molecules present on the surface of NaBi(MoO4)2 nanoparticles. These results shows that ethylene glycol molecules are attached to the surface of NaBi(MoO4)2 nanoparticles. Raman spectrum of NaBi(MoO4)2 consist of vibrational bands at 409, 384, 340, 250, 200, 131 cm-1 and a group of bands in the range of 774 - 940 cm-1. The bands in the range of 774 - 940 cm-1 correspond to the symmetrical vibrational mode of MoO42- tetrahedra. The vibrational mode at 409 cm-1 is attributed to the asymmetric vibration of the MoO42-. Vibrational modes at 384, 340 cm-1 are attributed to the asymmetric and symmetric bending of MoO42-, respectively. The lattice modes below 250 cm-1 are strongly rely on cation and anion motions. Therefore, the mode at 200 cm-1 is the combination of both the translation motion of Na+ ions and rotational motion of MoO42-. The mode at 131 cm-1 is correspond to the translation motion of Bi3+ ions and MoO42. All these modes are consistent with reported in the literature.31-32 3.1 Factors effecting on the formation and size of the NaBi(MoO4)2 nanoparticles 3.1.

1 Effect of Bi:Mo mole ratio

Effect of mole ratio of Bi:Mo sources on the formation of NaBi(MoO4)2 has been studied and the ratio is varied from 2:1 to 1:4. The reaction is carried out at room temperature for two hours. XRD patterns of these materials are shown in Figure 2. These results revealed the formation of amorphous material when the ratio is 2:1, 1:1, while highly crystalline phases when the ratio is 1:2 and higher. Even at 1:4, the crystalline phase is remain same without any extra peaks. These XRD patterns of crystalline phases matches with the tetragonal phase (PDF 01-074-8694, Space group I41/a) of NaBi(MoO4)2. Samples synthesized with 2:1, 1:1, 1:2 are yellow in color and all others are white in color as shown in inset of corresponding XRD in Figure 2. In order to find out the nature of these amorphous samples, FTIR, UV-vis diffused reflectance spectra have been recorded and are shown in Figure 3. FTIR spectra shown in Figure 3(a) demonstrate that samples synthesized with 2:1, 1:1, 1:2 are entirely different from NaBi(MoO4)2 nanoparticles where they 5 ACS Paragon Plus Environment


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do not show clear asymmetric stretching and bending bands of MoO42- tetrahedra. Samples synthesized with 2:1 ratio shows an additional band at 500 cm-1 which is a characteristic peak of the O4

33

2O3.

With increase in molybdenum concentration, another band appears at 590cm-1 and

its intensity increase for 1:2 sample. These two peaks at 500, 590 cm-1 are characteristic peaks of the P4 P4

2O3

33-34. This suggests that at 2:1 amorphous O4

2O3

2O3 phase and at 1:1, 1:2 ratio amorphous

phases are preferentially formed. Thus an excess of MoO42- ions are desired for the

formation of the desired molybdates. FTIR spectra of samples synthesized with 1:2.5, 1:3 and 1:4 are in agreement with the spectrum of NaBi(MoO4)2. Normalized diffused reflectance spectra shows that absorption edge shifts toward higher wavelength with increase in Bi:Mo ratio up to 1:2 and then to lower wavelength at 1:2.5. There after it is constant and the absorption spectra matches with NaBi(MoO4)2 (Figure 3(b)). It shows that the nature of samples with Bi:Mo ratio 2:1, 1:1, 1:2 are different from NaBi(MoO4)2 and is similar to Bi2O3. It is known that -Bi2O3 have higher band gap than P4

33,35

2O3.

From the nature of the absorption spectra and FTIR spectra it is confirmed

that it forms amorphous -Bi2O3 at 2:1, P4

2O3

at 1:1, 1:2 and NaBi(MoO4)2 phase at 1:2.5 and

above. Based on XRD, FTIR, Raman, DR UV-Vis studies, the possible phase formation mechanism may be as follows. When Bi:Mo ratio is 2:1, amount of molybdenum and sodium are not sufficient to form the pure NaBi(MoO4)2 phase. The excess Bismuth react with water quickly to form Bi(OH)3 due to its low solubility product and converted into O4

2O3.

As result, O4

2O3

and NaBi(MoO4)2 are formed simultaneously and they hinder their further growth. Hence, amorphous phases of both of them resulted. When Bi:Mo ratio is 1:1 and 1:2, reactant concentration may favors the reaction kinetics of formation of P4

2O3

along with NaBi(MoO4)2.

The amount of Na and Mo are good enough to form the pure crystalline phase of NaBi(MoO4)2 nanomaterials at Bi:Mo ratio of 1:2.5 and above. The excess amount of sodium, molybdenum will remain in solution phase and they can be separated by centrifugation. 3.1.2 Effect of reaction time Reaction time at room temperature has been varied from one minute to five hours by keeping Bi(NO3)3.5H2O:Na2MoO4.2H2O ratio constant at 1:4. Each reaction is done independently and the product was separated immediately by centrifugation. Powder XRD patterns of these products are shown in Figure 4. It shows that pure form of NaBi(MoO4)2 is forming even within one minute time and the phase remains unchanged for five hours of reaction also. The calculated average crystallite sizes of these nanomaterials are around 26 nm and are almost similar for all samples 6 ACS Paragon Plus Environment

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prepared at different reaction time scales. Thus, it can be concluded that the formation and crystallization of moybdate phase is instantaneous and occur by reaction of Bi3+ with solutions of Na+ and MoO42- ions. FTIR and Raman spectra also confirm the absence of amorphous impurity in these samples (supplementary information, Figure S1-S2). 3.1.3 Effect of reaction temperature NaBi(MoO4)2 has been synthesized at different temperatures from 30°C to 180°C for 2hr and their powder XRD patterns are shown in Figure 5. As increasing reaction temperature, XRD peaks become sharper and forms highly crystalline NaBi(MoO4)2. In order to check the possibility of phase formation at low temperature, reactions were carried out at 5 and 10°C for five minutes. XRD patterns show that crystalline NaBi(MoO4)2 phase is forming even at 5°C in five minutes time and but the peaks are much broader than the samples synthesized at higher temperatures. This is may be due to the smaller crystallite size of nanomaterials synthesized at lower temperatures. Average crystallite size of these materials is calculated and shown in Table1. The value increases from 16 nm to 58 nm with temperature from 5°C to 180°C. This may be due to Ostwald ripening, i.e., growing bigger crystals at the expense of smaller ones. This suggests temperature is more effective than time for the growth of nanocrystals. FTIR and Raman spectra show that there is no amorphous impurities present in these samples (Figure S3-S4 of supplementary information). FESEM images of samples synthesized at room temperature and 180°C for two hours shows that nanoparticles are agglomerated and forms supraparticles (Figure S5, supplementary information). Boundaries between particles are not clear in these suprapaticles. TEM images of samples prepared at 30°C for 5 minutes, 30°C for 2 hours and 180°C for 2 hours are shown in Figure 6. It has been observed from the TEM and HRTEM images that samples prepared at room temperature for 5 minutes and 2 hours shows nearly same sized particles, i.e., 10-35 nm. From the HRTEM, the inter planar distance has been measured and are found to be 2.42Å, 2.01Å for 5minutes, 2hour samples whose are corresponding to (202), (206) planes, respectively of the NaBi(MoO4)2. Whereas samples prepared at 180°C shows much bigger particle with larger range of size distribution (20-60 nm range). This increase in size can be attributed to Ostwald ripening. The inter planar distance is found to be 1.54Å corresponding to the (224) plane of NaBi(MoO4)2. From the above results, a schematic representation of factors effecting on the synthesis of NaBi(MoO4)2 nanomaterials has been illustrated in the scheme 1.



3.2. Photoluminescence properties of Eu3+ doped NaBi(MoO4)2 nanoparticles Photoluminescence properties of 10 atom % Eu3+ doped NaBi(MoO4)2 nanoparticles synthesized at different temperature are measured by exciting at 465 nm and results are shown in Figure 7(a). The intensity of emission spectra increases with increase in the synthesis temperature, which can be attributed to the enhancement in the crystallinity of samples. The emission spectra consist of sharp and intense emission peaks in the range of 580 - 750 nm. These emission peaks are assigned to f-f transitions of Eu3+ ion, i.e., 590, 615, 650, 700 nm peaks are assigned to 5D0 Q 7F1, 5D0 Q 7F

2,

5D

0

Q 7F3 and 5D0 Q 7F4 transitions, respectively. The 5D0 Q 7F1 transition is allowed

magnetic dipole transition and its intensity is highest if Eu3+ ion occupy at an inversion center in the lattice. This transition is independent of the local environment in the crystal and it can be used to calibrate the intensity of Eu3+ luminescence spectra, as the total intensity of this transition remains constant. On the contrary, the 5D0 Q 7F2 transition is an electric dipole transition and is hypersensitive to symmetry of the Eu3+ ion in the crystal lattice. The ratio between these two transitions called as asymmetric ratio (5D0 Q 7F2 to 5D0 Q 7F1) and it reveals the information about the local symmetry of the Eu3+ ion in the crystal structure. The asymmetric ratio was calculated and found to be ~11. This high asymmetric ratio suggests that Eu3+ ion located at an asymmetric environment in the crystal lattice. The excitation spectra monitored to 615 nm emission are shown in Figure 7(b). The excitation spectra consist of a weak, very broad band in the region of 250 to 350 nm and sharp peaks centered at 363, 383, 394, 416, and 465 nm. The broad band corresponds to the combination of charge transfer bands of Eu3+-O2-, Mo6+Q O2- and s-p transition of Bi3+. Observation of Eu3+ emission upon excitation of Mo6+Q O2- suggests that there is a weak energy transfer from the host material to Eu3+ ions. Sharp peaks in the region 350 - 470 nm are assigned to f-f transitions of Eu3+ ion in the host lattice. These peaks at 363, 383, 394, 416 and 465 nm are attributed to 7F0 Q 5D4, 7F

0

Q 5G2, 7F0 Q 5L6, 7F0 Q 5D3 and 7F0 Q 5D2 transitions of Eu3+ ion, respectively. The strong

absorption at near Visible (394 nm) and blue (465 nm) region, as seen in the excitation spectra, are consistent with the output wavelength of commercial GaN based LEDs. This suggests that these phosphors are promising red light emitting material for white light emitting LEDs. The 5D0 excited state decay profiles of NaBi(MoO4)2 nanomaterials synthesized at various temperatures are measured and shown in Figure 7(c). Excited state lifetime values for samples synthesized at


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30, 100, 120, 180°C are 670, 763,778, 816 s, respectively. Increase in the 5D0 lifetime with increase in the synthesis temperature is attributed to increase in the crystallinity of nanoparticles. 3.3. NaBi1-xEux(MoO4)2 Solid solution In order to find out possibility of homogeneous mixing and optimum Eu3+concentration for better luminescence properties, a series of compounds, i.e., NaBi1-xEux(MoO4)2 (x =0.0, 0.02, 0.1, 0.25, 0.5, 0.75, 1.0) have been synthesized and their XRD patterns are shown in Figure 8(a-g). All these compounds are crystallizing in the scheelite type structures. There is no separate crystalline phase observed in the XRD patterns. These patterns are subjected to Rietveld refinement to elucidate any structural changes due to replacement of Bi3+ with Eu3+ in these nanomaterials. The resultant unit cell parameters and refinement parameters are shown in Table 2. The unit cell parameters and cell volume are plotted as a function of Eu3+ concentration in Figure 9(a). It shows that these values decrease linearly as increase in x value. This has been attributed to the lattice compression due to the smaller ionic radii of Eu3+ compared to Bi3+. The linear relation between the unit cell volume and composition shows that random solid solution formation between the NaBi(MoO4)2 and NaEu(MoO4)2 phases occur. No additional peaks other than those expected for scheelite phase indicate no cation ordering or separation of phases in between these two systems. The crystal structure analysis of the phase is described as follows. The unit cell has two sites for cations (4a and 4b) and one type (16f) site for O2- ions. The analyses of crystal structure indicated that the Na+ and Bi3+ are randomly distributed over the 4b site of the space group I41/a, and they are connected to eight O atoms. Typical M-O (M = Na0.5+Bi0.5) bond lengths are: M-O = 2.437(2) Å× 4 and 2.516(2) Å× 4. The Mo atoms are connected to four oxygen atoms (Mo-O bond lengths are: 1.806(2) Å× 2 and 1.806(1) Å× 2) forming regular MoO4 tetrahedra. The crystal structure of the unit cell is shown in Figure 9(b). The scheelite type structure of NaBi(MoO4)2 is formed (Na/Bi)O8 square antiprisms and MoO4 tetrahedral units. Each of the (Na/Bi)O8 square antiprisms share four of its edges with other four ((Na/Bi)O8 square antiprisms forming the three dimensional structure. Eight of MoO4 units are connected to (Na/Bi)O8 square antiprisms by sharing the corner oxygen of the later. Thus each of the MoO4 connected to four (Na/Bi)O8 units. The typical inter-cation distances in NaBi(MoO4)2 are: M…M: 3.919 Å× 4, and .M…Mo: 3.919 Å× 4 and 3.735 Å× 4. The structural analyses of NaEu(MoO4)2 indicates similar structural arrangements, except Na+ and Eu3+ are randomly distributed over the 4b (M) sites. The M-O bond lengths in NaEu(MoO4)2 are 2.465(2) Å× 4 and 2.485(2) Å× 4, suggesting that the (Na/Eu)O8 is more regular compared to 9 ACS Paragon Plus Environment


(Na/Bi)O8 units. However, the Mo-O bonds (1.768(2) Å× 4) in NaEu(MoO4)2 are relatively smaller compared to those in NaBi(MoO4)2. As the structure is mainly built by the MO8 units, the smaller polyhedra formed by Na/Eu pair lead to a compact structure. Typical inter-cation separation in NaEu(MoO4)2 are: M…M: 3.888 Å× 4, and .M…Mo: 3.888 Å× 4 and 3.714 Å× 4. Thus increasing the Eu3+ in the NaBi1-xEux(MoO4)2 leads to a appreciably decreasing trend of MM separation than the M-Mo- separation. However no additional differences in crystal structure are observed. The analyses of unit cell parameters NaBi1-xEux(MoO4)2 with concentrations of Eu3+ ions indicates decreasing trend for both a- and c-axis, and the variation of unit cell parameters with x can be represented as: a (Å) = 5.282(1) -0.030(1) × [x]; c (Å) = 11.579(1) -0.114(3) × [x]; V (Å)3 = 322.98(8) -6.8(2) × [x]. This indicates the c-axis decreases faster compared to the a-axis and thus the M-M and M-Mo (axial) separations decreases faster. FTIR, Raman spectra of these solid solutions in different regions have been recorded and they are shown in Figure 10. FTIR spectra of all these materials are similar to that of NaBi(MoO4)2 spectrum (Figure 10(a & b)) and it suggest that there are no amorphous secondary phases formed during the precipitation process. All the peaks corresponding to translational, bending and stretching frequencies are shifted towards higher energy side with increasing the Eu3+ concentration in the solid solution. It is expected that when heavier metal ion (Bi3+, M. Wt. =209) is replacing with lighter metal ion (Eu3+, M. Wt. =152) in the crystal lattice, the vibrational frequency shifts towards higher energy. In case of FTIR spectra, the shift is more predominant in case of bending frequencies as compared to stretching frequencies. Therefore, the MoO42- bending frequency is plotted as a function of composition (Figure 10(e)) and it shows a linear relationship. In case of Raman spectra, shift in the stretching frequencies are more prominent compared to bending and translational frequencies (Figure 10(c&d)). So, the Raman shift of stretching frequency is plotted as a function of Eu3+ composition (Figure 10(f)) and it shows a linear relationship. The linear relationship of Raman and FTIR frequencies with Eu3+ composition further confirm the solid solution formation between NaBi(MoO4)2 and NaEu(MoO4)2 Phases. Diffused reflectance spectra of these solid solutions shows that the absorption band shifts toward higher energy side with increase in the Eu3+ concentration (Figure S6 in supplementary information). The band gap of these solid solutions obtained from Tauc plot and these results show that band gap increases from 3.36eV to 4.42eV with increase in Eu3+ concentration from 0.0 to 1.0.


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Photoluminescence properties of these solid solutions have been measured by exciting at 465 nm and results are shown in Figure 11. They show very strong red emission with increasing intensity with Eu3+ composition till x=0.50 and after that it starts decreasing (Figure 11(a)). Initially increase in the intensity is due to the increasing the number of Eu3+ ions gets excited. However, as the concentration increases further, cross relaxations of excited Eu3+ ions dominate due to shorter distance between the europium ions and hence render a decreasing trend in emission intensity. The 5D0 excited state decay profiles (Figure 11(b)) shows that in all the samples, excited state decays single exponentially. The excited state lifetime values are shown in Table 3. Even though, the emission intensity increases till x=0.5, the lifetime values continuously decreases with increase in the Eu3+ concentration from 700 to 190 s with highest lifetime for x=0.02. This has been attributed to increase in the non-radiative decay with Eu3+ concentration due to cross relaxation and inter energy transfer between Eu-Eu ions in the lattice. In order to find the optimum concentration of Eu3+ in the lattice, quantum yield has been calculated from emission spectra and lifetime values as described below. Spontaneous emission cross-sections (A0-J) corresponding to 5D0 Q 7FJ transitions of Eu3+ ions are calculated from emission spectrum based on the following expression.36 0

=

0

1 0

0

0

1

0

1

-------------- (1)

where X0-J and I0-J are the frequency and integrated area under particular spectral line of Eu3+ emission spectrum. A0-1 is nearly constant and is 50 sY/.7, 36 Radiative decay rate (Arad) is calculated by adding A0-J values of all the 5D0 Q 7FJ transitions. Total radiative decay rate (Atot) is obtained from the experimentally measured lifetime values using the following expression, 1

=

=

+

------------- (2)

Anonrad is obtained by subtracting Arad from Atot. Quantum efficiency $Z( of 5D0 luminescence is calculated using following relation, =

+

--------------- (3)

Radiative decay rate (Arad), non-radiative decay rate (Anonrad), total decay rate (Atot), quantum efficiency $Z( for all samples are given in Table 3 and these values are plotted as a function of Eu3+ composition in Figure 12. Integrated intensity under spectral lines indicates that it has been 11 ACS Paragon Plus Environment


increases with Eu3+ concentration till x=0.5 the decreases, whereas lifetime values decreases continuously. The change in radiative decay rate is very less when compared to non-radiative decay rate. Up to x=0.1 the change in non-radiative decay rate is very less but after that it increases at very high rate. As a result, the quantum efficiency increases till x=0.1 (50%) and then it decreases continuously with increase in Eu3+ content and reached to 14% for NaEu(MoO4)2 nanomaterials. Thus, NaBi0.9Eu0.1(MoO4)2 shows optimum photoluminescence properties with highest quantum efficiency of 50%. 4. Conclusions NaBi(MoO4)2 nanoparticles have been synthesized at room temperature by facile co-precipitation method. Particle sizes of these nanomaterials have been tuned by changing reaction temperature. The formation of NaBi(MoO4)2 is found to be instantaneous and the growth of crystallites is more dependent on temperature than time. The initial concentration of MoO42- plays a crucial role for the formation of double molybdate and to restrict formation of Bi2O3 phases. Complete solid solution formation between NaBi(MoO4)2 and NaEu(MoO4)2 nanoparticle has been observed which can help in tuning the properties of these materials. The band gap of these solid solutions tuned from 3.36eV to 4.42eV. Europium gives strong red emission upon excitation at 465 nm light with an asymmetric ratio of ~11. NaBi0.9Eu0.1(MoO4)2 shows optimum photoluminescence properties with highest quantum yield of 50%. This material may be a suitable candidate for white light emitting LEDs. 5. Acknowledgement This research work is carried out with support from DST-SERB, India (Early Career Research Award to BSN, File number: ECR/2015/000333). Pushpendra and RKK are thankful to INST, Mohali, India for providing Fellowship. 6. Supporting Information The supporting information contains: FTIR, Raman spectra and FESEM images of NaBi(MoO4)2 nanomaterials synthesizes at different temperatures and reaction times. DR UV-Visible spectra and Tauc plots of solid solutions. 7. References 1. Wang, F.; Liu, X. Recent advances in the chemistry of lanthanide-doped upconversion nanocrystals. Chem. Soc. Rev. 2009, 38, 976-989.


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2. Liu, Y.; Tu, D.; Zhu, H.; Chen, X. Lanthanide-doped luminescent nano-probes: controlled synthesis, optical spectroscopy, and bio-applications. Chem. Soc. Rev. 2013, 42, 6924-6958. 3. Jüstel, Th.; Nikol, H.; Ronda, C. New developments in the field of luminescent materials for lighting and displays. Angew. Chem., Int. Ed. 1998, 37, 3084-3103. 4. Zhou, B.; Shi, B.; Jin, D.; Liu, X. Controlling up-conversion nanocrystals for emerging applications. Nat. Nanotechnol. 2015, 10, 924–936. 5. Wang, G.; Peng, Q.; Li, Y. Lanthanide-doped nanocrystals: synthesis, optical-magnetic properties, and applications. Acc. Chem. Res. 2011, 44, 322-332. 6. Wang, F.; Banerjee, D.; Liu, Y.; Chen, X.; Liu, X. Up-conversion nanoparticles in biological labeling, imaging, and therapy. Analyst 2010, 135, 1839-1854. 7. Naidu, B. S.; Vishwanadh, B.; Sudarsan, V.; Vatsa, R. K. BiPO4: a better host for doping lanthanide ions. Dalton Trans. 2012, 41, 3194-3203. 8. Naidu, B. S.; Pandey, M.; Nuwad, J.; Sudarsan, V.; Vatsa, R. K.; Kshirsagar, R. J.; Pillai, C. G. S. Lanthanide-ion-assisted structural collapse of layered GaOOH lattice. Inorg. Chem. 2011, 50, 4463-4472. 9. Naidu, B. S.; Pandey, M.; Sudarsan, V.; Ghattak, J.; Vatsa, R. K. Investigations of Ce3+ CoDoped SbPO4: Tb3+ Nano-Ribbons and Nanoparticles by Vibrational and Photoluminescence Spectroscopy. J. Nanosci. Nanotechnol. 2011, 11, 3180-3190. 10. Naidu, B. S.; Sudarsan, V.; Vatsa, R. K. Luminescence Studies on SbPO4: Ln3+ (Ln= Eu, Ce, Tb) Nanoparticles and Nanoribbons, J. Nanosci. Nanotechnol. 2009, 9, 2997-3004. 11. Sarkar, S.; Dash, A.; Mahalingam, V. Strong Stokes and Upconversion Luminescence from Ultrasmall Ln3+ Doped BiF3 (Ln= Eu3+, Yb3+/Er3+) Nanoparticles Confined in a Polymer Matrix. Chem. - Asian J. 2014, 9, 447-451. 12. Lei, P.; An, R.; Yao, S.; Wang, Q.; Dong, L.; Xu, X.; Du, K.; Feng, J.; Zhang, H. Ultrafast synthesis of novel hexagonal phase NaBiF4 upconversion nanoparticles at room temperature. Adv. Mater. 2017, 29, 1700505. 13. Du, P.; Huang, X.; Yu, J. S. Facile synthesis of bi-functional Eu3+-activated NaBiF4 redemitting nanoparticles for simultaneous white light-emitting diodes and field emission displays. Chem. Eng. J. 2018, 337, 91-100.



Page 14 of 34

14. Yu, C.; Cao, M.; Yan, D.; Lou, S.; Xia, C.; Xuan, T.; Xie, R.-J.; Li, H. Synthesis of Eu2+/Eu3+ Co-Doped Gallium Oxide Nanocrystals as a Full Colour Converter for White Light Emitting Diodes. J. Colloid Interface Sci. 2018, 530, 52–57. 15. Dutta, D. P.; Sudarsan, V.; Srinivasu, P.; Vinu, A.; Tyagi, A. K. Indium Oxide and Europium/Dysprosium

Doped

Indium

Oxide

._

Sonochemical

Synthesis,

Characterization, and Photoluminescence Studies. J. Phys. Chem. C, 2008, 112, 6781-6785. 16. Xu, M.; Lin, Z.; Hong, Y.; Chen, Z.; Fu, P.; Tang, D. Preparation and hydrogen sulfide gassensing performances of RuO2/NaBi(MoO4)2 nanoplates, J. Alloys Compd. 2016, 688, 504-509. 17. Liu, L.; Wang, H.; Zhang, X.; Lin, Z. Synthesis of novel RuO2/NaBi(MoO4)2 nano-sheets composite and its gas sensing performances towards ethanol. Sens. Actuators, B. 2016, 237, 275– 283. 18. Krupych, O.; Kushnirevych, M.; Mys, O.; Vlokh, R. Photoelastic properties of NaBi(MoO4)2 crystals. Appl. Opt. 2015, 54, 5016-5023. 19. L. Khusravbekov, A. Kholov, E. V. Charnaya, Acoustic investigation of NaBi(MoO4)2 and NaBi(WO4)2 crystals at high temperatures, Bull. Russ. Acad. Sci.: Phys. 79 (2015) 1306–1309. 20. Liu, J.; Wei, R.; Hu, J.; Li, L.; Li, J. Novel Bi2O3/NaBi(MoO4)2 heterojunction with enhanced photocatalytic activity under visible light irradiation, J. Alloys Compd. 2013, 580, 475–480. 21. Mazurak, Zb.; Blasse, G.; Liebertz, J. The luminescence of the scheelite NaBi(MoO4)2, J. Solid State Chem. 1987, 68, 181-184. 22. Waskowska, A.; Gerward, L.; Olsen, J. S.; Maczka, M.; Lis, T.; Pietraszko, A.; Morgenroth, W. Low-temperature and high-pressure structural behaviour of NaBi(MoO4)2—an X-ray diffraction study. J. Solid State Chem. 2005, 178, 2218–2224. 23. Shannon, R. D. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallogr., Sect. A: Found. Adv. 1976, A32, 751–767. 24. Méndez-Blas, A.; Rico, M.; Volkov, V.; Zaldo, C.; Cascales, C. Crystal field analysis and emission cross sections of Ho3+ in the locally disordered single-crystal laser hosts M+Bi(XO4)2 (M+= Li, Na; X= W, Mo), Phys. Rev. B. 2007, 75, 174208. 25. Volkov, V.; Rico, M.; Mendez-Blas, A.; Zaldo, C. Preparation and properties of disordered NaBi(XO4)2, X= W or Mo, crystals doped with rare earths, J. Phys. Chem. Solids 2002, 63, 95105.


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26. Tsydypova, B. N.; Gusakova, N. V.; Pavlyuk, A. A.; Yasyukevich, A. S.; Kuleshov, N. V.; Grigorev, S. V.; Solodovnikov, S. F. Growth and spectroscopic characteristics of Yb3+-Doped NaBi(MoO4)2 crystals, Inorg. Mater. 2014, 50, 617–621. 27. Rico, M.; Volkov, V.; Zaldo, C. Photoluminescence and up-conversion of Er3+ in tetragonal NaBi(XO4)2, X= Mo or W, scheelites, J. Alloys Compd. 2001, 323-324, 806–810. 28. Cascales, C.; Blas, A. M.; Rico, M.; Volkov, V.; Zaldo, C. The optical spectroscopy of lanthanides R3+ in ABi(XO4)2 (A= Li, Na; X= Mo, W) and LiYb(MoO4)2 multifunctional single crystals: Relationship with the structural local disorder. Opt. Mater. 2005, 27, 1672–1680. 29. Lin, Z.; Xu, M.; Hong, Y.; Wang, X.; Fu, P. Surfactant-free hydrothermal synthesis and gassensing properties of NaBi(MoO4)2 nanocrystals. Mater. Lett. 2016, 168, 72–75. 30. Gan, Y.; Liu, W.; Zhang, W.; Li, W.; Huang, Y.; Qiu, K. Effects of Gd3+ co-doping on the enhancement of the luminescent properties of a NaBi(MoO4)2:Eu3+ red-emitting phosphors, J. Alloys Compd. 2019, 784, 1003–1010. 31. Hanuza, J.; Maczka, M. Vibrational properties of the double molybdates MX(MoO4)2 family (M= Li, Na, K, Cs; X= Bi, Cr): Part I. Structure and infrared and Raman spectra in the polycrystalline state. Vib. Spectrosc. 1994, 7, 85-96. 32. Hanuza, J.; Haznar, A.; MaÓczka, M.; Pietraszko, A. Lemiec, A.; van der Maas, J. H.; Lutz, E. T. G. Structure and vibrational properties of tetragonal scheelite NaBi(MoO4)2, J. Raman Spectrosc. 1997, 28, 953-963. 33. Yan, Y.; Zhou, Z.; Cheng, Y.; Qiu, L.; Gao, C.; Zhou, J. Template-free fabrication of O4

P4

Bi2O3 hollow spheres and their visible light photocatalytic activity for water purification. J. Alloy Compd. 2014, 605, 102-108. 34. Liu, L.; Jiang, J.; Jin, S.; Xia, Z.; Tang, M. Hydrothermal synthesis of P4

oxide nanowires

from particles. CrystEngComm 2011, 13, 2529-2532. 35. Huang, Q.; Zhang, S.; Cai, C.; Zhou, B. P4

O4

2O3

nanoparticles synthesized via

microwave-assisted method and their photocatalytic activity towards the degradation of rhodamine B. Mater. Lett. 2011, 65, 988-990. 36. de Sa, G.F.; Malta, O.L.; de Mello Donega C.; Simas, A.M.; Longo, R.L.; Santa-Cruz, P.A.; da Silva Jr. E.F. Spectroscopic properties and design of highly luminescent coordination complexes. Coord. Chem. Rev. 2000, 196, 165–195.


lanthanide


8. Figure captions Figure 1.

(a) XRD pattern, JCPDS file, (b) UV-Visible diffused reflectance spectrum, (c) FTIR spectrum, (d) Raman spectrum of NaBi(MoO4)2 nanoparticles. Inset of figure (b) shows the Tauc plot of DRS spectrum

Figure 2.

(a) XRD patterns of nanomaterials synthesized with various mole ratios of precursors at room temperature for 2 hours. Inset of the figure shows photographs of the corresponding nanomaterial

Figure 3.

(a) FTIR spectra and (b) DR UV-Vis spectra of nanomaterials prepared with different mole ratios of precursors at room temperature for 2 hours. Normalisation of reflectance was done by dividing with highest reflectance value.

Figure 4.

XRD patterns of NaBi(MoO4)2 nanomaterials synthesized at different reaction time scales at room temperature

Figure 5.

XRD patterns of NaBi(MoO4)2 nanomaterials synthesized at various temperatures.

Figure 6.

TEM and HRTEM images of NaBi(MoO4)2 nanomaterials synthesized at room temperature for 5 minutes (a), (d); room temperature for 2 hours (b), (e) and 180°C for 2 hours (c), (f).

Scheme 1

Schematic representation of various factors effecting on the synthesis of NaBi(MoO4)2 nanoparticles

Figure 7.

(a) Emission spectra, (b) Excitation spectra and (c) 5D0 excited state decay profiles of Eu3+ doped NaBi(MoO4)2 nanoparticles prepared at various temperatures.

Figure 8.

(a-h) Reitvield refined XRD patters of NaBi1-xEux(MoO4)2 nanoparticles.

Figure 9

(a) Variation of unit cell parameters and cell volume as function of Europium composition, (b) crystal structure of NaBi(MoO4)2 nanoparticles. Color code: Red is oxygen, Blue is Molybdenum, pink/cyan is sodium/Bismuth.

Figure 10

FTIR spectra in the (a) 400-500 cm-1, (b) 500-1000cm-1 regions, Raman spectra in the (c) 50-250 cm-1, (d) 800-1000cm-1 regions of NaBi1-xEux(MoO4)2 solid solutions. (e) Variation in the IR frequency and (f) variation of Raman shift as a function of Eu3+ composition.

Figure 11

(a) Emission spectra, (b) 5D0 excited state decay profiles of NaBi1-xEux(MoO4)2 solid solutions.


Page 16 of 34

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Figure 12

(a) Integrated peak intensity, (b) 5D0 excited state lifetime, (c) excited state decay rate, (d) quantum efficiency of NaBi1-xEux(MoO4)2 solid solutions as a function of Eu3+ concentration



Page 18 of 34

Table. 1. Average crystallite size of nanomaterials synthesized at different temperatures.

Compound

Temperature (°C)

Time

Crystallite Size (nm)

NaBi(MoO )

5

5 min

16

NaBi(MoO )

10

5 min

18

NaBi(MoO )

30

2h

26

NaBi(MoO )

60

2h

30

NaBi(MoO )

100

2h

43

NaBi(MoO )

120

2h

55

NaBi(MoO )

180

2h

58

4 2 4 2 4 2 4 2 4 2 4 2 4 2


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Table 2: Lattice parameters and refinement parameters of NaBi1-xEux(MoO4)2 solid solutions from Rietveld refinement of their XRD patterns.

Sample Name

a=b (Å)

c (Å)

Cell Volume(Å3)

@2

Rp

Rwp

Rf

NaBi(MoO4)2

5.2823

11.5794

323.09

1.99

5.77

7.48

6.08

NaBi0.98Eu0.02(MoO4)2

5.2822

11.5779

323.05

1.72

5.26

6.72

8.77


5.2787

11.5644

322.03

1.67

5.51

6.96

3.58


5.2735

11.5554

321.36

1.67

5.51

6.96

3.73


5.2662

11.5212

319.52

1.23

4.94

6.22

5.48


5.2591

11.4887

317.75

1.46

4.59

5.75

3.93

NaEu(MoO4)2

5.2521

11.4672

316.32

1.18

3.69

4.68

4.97



Page 20 of 34

Table 3. Radiative decay rate (Arad), non-radiative decay rate (Anon-rad), total decay rate (Atot), excited state lifetime values and quantum efficiency (Z) of NaBi1-xEux(MoO4)2 solid solutions. ( em = 615 nm & ex = 465 nm) A'

A'

A'

Sample Name

Arad(s )

Anon-rad(s )

Atot(s )

BC


662

740

1402

713

47


755

737

1492

670

50


691

1423

2114

473

33


710

1729

2439

410

29


778

3514

4292

233

18

NaEu(MoO4)2

756

4507

5263

190

14


(%)

100

Intensity (a.u.)

(a)

PDF 01-074-8694

Reflectance (%)

NaBi(MoO4)2

(b) 80 60 40

3.36 eV 2.0

20

100

20

30

40 50 60 2 (degree)

70

200

80

2.5

3.0

3.5

4.0

4.5

5.0

h eV)

300

400

500

600

700

800

Wavelength (nm)

(d)

(c) MoO24

80

O H

Intensity (a.u.)

10

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[F(R).h ]^2

Page 21 of 34

C H

60 O H

40 20 0

MoO24

1000

2000

3000

4000

200 400 600 800 Raman shift (cm-1)

Wavenumber (cm-1)

1000

Figure 1.(a) XRD pattern, JCPDS file, (b) UV-Visible diffused reflectance spectrum, (c) FTIR spectrum, (d) Raman spectrum of NaBi(MoO4)2 nanoparticles. Inset of figure (b) shows the Tauc plot of DRS spectrum



Bi(NO3)3:Na2MoO4 = 1:4

Bi(NO3)3:Na2MoO4 = 1:3 Bi(NO3)3:Na2MoO4 = 1:2.5

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 34




PDF 01-074-8694

10

20

30

40

50

60

70

80

2 (degree) Figure 2. XRD patterns of nanomaterials synthesized with various mole ratios of precursors at room temperature for 2 hours. Inset of the figure shows photographs of the corresponding nanomaterial


Page 23 of 34

1.0

1:4

Normalised Reflectance (a.u)

Bi(NO3)3 : Na2MoO4 Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

1:3 1:2.5 1:2 1:1 2:1

(b) 0.8 0.6

Bi(NO3)3 : Na2MoO4 2:1 1:1 1:2 1:2.5 1:3 1:4

0.4 0.2 0.0

400

600 800 1000 -1 Wavenumver (cm )

200

1200

300

400 500 Wavelenght (nm)

600

700

Figure 3. (a) FTIR spectra and (b) DR UV-Vis spectra of nanomaterials prepared with different mole ratios of precursors at room temperature for 2 hours. Normalisation of reflectance was done by dividing with highest reflectance value.



5h

2h

30 min

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 34

15 min

5 min

2 min

1 min

PDF 01-074-8694

10

20

30 40 2 (degree)

50

60

Figure 4. XRD patterns of NaBi(MoO4)2 nanomaterials synthesized at different reaction time scales at room temperature 24 ACS Paragon Plus Environment

Page 25 of 34

0

180 C-2h 0

120 C-2h

0

100 C-2h

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

60 C-2h

0

30 C-2h

0

10 C-5min

0

5 C-5min PDF 01-074-8694

20

30

40 2 (degree)

50

60

Figure 5. XRD patterns of NaBi(MoO4)2 nanomaterials synthesized at various temperatures for 2 hours.



ACS Paragon Plus Environment

Page 26 of 34

Page 27 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


= H-Bi2O3 & NaBi(MoO4)2 composite

Na2MoO4.2H2O; Bi:Mo=1:2 RT, Stirring, 2h

Bi(NO3)3.5H2O in Ethylene glycol (EG)

=J-Bi2O3 & NaBi(MoO4)2 composite

= crystalline NaBi(MoO4)2 Nanoparticles

= crystalline NaBi(MoO4)2 Nanoparticles

= highly crystalline NaBi(MoO4)2 Nanoparticles

Scheme 1. Schematic representation of various factors effecting on the synthesis of NaBi(MoO4)2 nanoparticles



(a)

NaBi0.9Eu0.1(MoO4)2

(c)

NaBi0.9Eu0.1(MoO4)2

= 465nm

Intensity (a.u

)

ex

W

18 0

0 70

12 0

av 0 ele 65 ng 00 th (n 6 m 0 ) 55

(b)

10 0

30

m Te

e ur at r pe

= 615nm

0 48

C)

0

180 C

0

120 C

0

100 C

0

30 C

18 0

0 44

av 00 el 4 60 en 3 0 gt 32 h 0 (n 28 m )

(

NaBi0.9Eu0.1(MoO4)2 em

W

0

= 465nm = 615nm em ex

Normalized Intensity (a.u.)

0 75

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 34

12 0

10 0

30

m Te

t ra pe

) (C

0

e ur

1

2

3

4

5

6

Time (ms)

Figure 7. (a) Emission spectra, (b) Excitation spectra and (c) 5D0 excited state decay profiles of Eu3+ doped NaBi(MoO4)2 nanoparticles prepared at various temperatures.


Page 29 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



°

324

320

(b)

NaBi1-xEux(MoO4)2

322

(a)

318 316

11.58

c (A)

11.55 ° 11.52 11.49 11.46 5.28

a (A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Unit Cell Volume (A )


°

5.27 5.26 5.25 0.0

0.2 0.4 0.6 Composition (x)

0.8

1.0

Figure 9. (a) Variation of unit cell parameters and cell volume as function of Europium composition, (b) crystal structure of NaBi(MoO4)2 nanoparticles. Color code: Red is oxygen, Blue is Molybdenum, pink/cyan is sodium/Bismuth.


Page 30 of 34

Page 31 of 34

(a)

Transmittance (%)

Transmittance (%)

(b)

X=1.0

X=1.0 X=0.75 X=0.50 X=0.25 X=0.10

NaBi1-XEuX(MoO4)2

X=0.75 X=0.50 X=0.25 X=0.10 X=0

X=0 NaBi1-XEuX(MoO4)2

400

420

440

460

480

Wavenumber (cm-1)

500 500

NaBi1-xEux(MoO4)2

(c)

600

700

800

900

Wavenumber (cm-1)

(d)

1000

NaBi1-xEux(MoO4)2

x=0.75 x=0.50 x=0.25

Intensity (a.u.)

Intensity (a.u.)

x=1.0 x=1.0 x=0.75 x=0.50 x=0.25

x=0.10

x=0.10

x=0.0 x=0.0

50

100

150

200

Raman shift (cm-1)

430

250 800

850

900

950

Raman shift (cm-1)

1000

875

(f)

(e) Raman Shift (cm-1)

IR frequency (cm-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


420

410

400

870

865

860

855 0.00

0.25

0.50 X

0.75

1.00

0.00

0.25

0.50

0.75

1.00

X

Figure 10. FTIR spectra in the (a) 400-500 cm-1, (b) 500-1000cm-1 regions, Raman spectra in the (c) 50-250 cm-1, (d) 800-1000cm-1 regions of NaBi1-xEux(MoO4)2 solid solutions. (e) Variation in the IR frequency and (f) variation of Raman shift as a function of Eu3+ composition. 31 ACS Paragon Plus Environment


ex

Intensity (a.u.)

NaBi1-xEux(MoO4)2 = 465 nm

(a)

0 75

x= 1.0 x= 0 0 .75 x= X) 0.5 ( x= 0 n 0

0 70

) nm h( gt en el av W 0 65

.25 x= t io a 0 r .10 x= nt 0.0 e c

0 60

0 55

1

Normalized Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 34

2

n

Co

NaBi1-xEux(MoO4)2 = 465 nm ex = 615 nm em

0.1

0.02 0.10 0.25 0.50 0.75 1.00

0.01 (b)

0.001

0

1

2

3

4

5

6

Time (ms) Figure 11. (a) Emission spectra and (b) 5D0 excited state decay profiles of NaBi1-xEux(MoO4)2 solid solutions. 32 ACS Paragon Plus Environment

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For Table of Contents Use Only Rapid, room temperature synthesis of Eu3+ doped NaBi(MoO4)2 nanomaterials: Structural, optical and photoluminescence properties Pushpendra†, Ravi K. Kunchala†, Srungarpu N. Achary‡, Avesh K. Tyagi‡, Boddu S. Naidu†,* †Institute

of Nano Science and Technology (INST), Phase 10, Sector 64, Mohali, Punjab, India160062, E-mail: [email protected], [email protected] Division, Bhabha Atomic Research Centre, Trombay, India

)

‡Chemistry

Unit Cell Volume (

°

324 NaBi1-xEux(MoO4)2

321 318 315

c (A)

11.55 °

11.50 11.45 5.280

a (A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

°

5.265 5.250

0.0

0.2 0.4 0.6 Composition (x)

0.8

1.0

Synopsis: Eu3+ doped NaBi(MoO4)2 nanomaterials were synthesized at room temperature by simple co-precipitation method within 5 minutes. Solid solution formation between NaBi(MoO4)2 and NaEu(MoO4)2 has been achieved in the complete range of composition. Excellent red luminescence has been observed from these samples upon 465nm excitation. Optimum photoluminescence properties are obtained for NaBi0.9Eu0.1(MoO4)2 with highest quantum efficiency of 50%.


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Rapid, Room Temperature Synthesis of Eu3+ Doped NaBi(MoO4)2

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