Luminescent and thermal properties of novel orange–red emitting MgNb2O6:Sm3+ phosphors for displays, photo catalytic and sensor applications

Novel Sm3+ doped columbite-type orthorhombic structured MgNb2O6 (MNO) orange-red emitting phosphors were prepared by solution combustion method using ODH as a fuel. The powder phase purity, particle morphology, size, elemental composition, luminescence properties, photocatalytic behaviors and electrochemical studies of prepared samples were studied in detail. Photoluminescence emission spectra of MNO:Sm3+ nanophosphors show all the characteristic emissions of Sm3+ cations corresponds to the transitions 4G5/2 → 6Hj/2(j=5,7,9,11) when excited at 463 nm energy. Among these the strongest emission peak was at 608 nm which corresponds to 4G5/2 → 6H7/2 transition of Sm3+ cations in the host lattice. The luminescence quenching was confirmed by the dipole–dipole interaction among Sm3+ ions. As a result of J-O analysis the branching ratio (~ 58% > 50%) show that the phosphor was highly suitable for color display devices. Photocatalytic activity of MNO:Sm3+ (5 mol%) under UV light shows 99% degradation of AR-88 dye. Electrochemical Impedance Spectroscopy (EIS) confirms the reversibility of the redox reaction, which helps in sensing the presence of paracetamol and alcohol. Thus, MNO:Sm3+ phosphors have great potential applications in display, catalytic, photonic, chemical and thermal sensor applications.


Introduction
Currently phosphor materials are extensively used in various applications relating to the production of synthetic light for liquid crystal displays, cathode ray tubes, lightemitting diodes, scintillators and many more [1][2][3]. However, these phosphor oxide materials have more significant and attained high potential interest to researchers due to their enhanced physico-chemical stability. Nowadays, developed oxide phosphors are widely used in the various applications like health and eco-friendly progress [4]. Therefore, the researchers are focusing on the preparation of various structural oxide materials. Metal niobates (MNb 2 O 6 where M = Mg, Zn, Ni, Mn Co, etc.) are most significant phosphor compounds which exhibit excellent properties [5,6]. It is impressive to study on the luminescence properties and energy transfer process between metal ion and Nb 2 O 6 groups of MNb 2 O 6 nanoparticles which have great attention for their wide range of potential applications including solid state lightening [7][8][9].
The materials doped with rare earth ions known as phosphors, used for white light-emitting diodes (wLEDs) are more potential materials and create high interest to the researchers for their luminescent features [10,11]. Among the rare earth metal ions, Sm 3+ is generally adapted as a orange red emitting material because of its distinctive 4f-4f structure that can be stimulated by NUV or visible (blue) rays efficiently and produce high colour purity [12,13]. Therefore, MNO:Sm 3+ phosphor plays a vital role in these solid-state lighting applications. In addition, the significant movement has been achieved for the Sm 3+ doped MNO nanoparticles in the sensor studies. Thus, the sensor activities of prepared material can be used for chemical monitoring applications and is of great use for sustaining human health and also in different detection practices [14,15]. The Cyclic Voltammetric investigation was carried out for Sm 3+ doped MNO to illustrate the modified graphite paste electrode which can be used as sensors for the detection of chemical and biochemicals such as Paracetamol and human consumable Alcohol (Red Wine). The nanoparticles with variable energy band gaps are confirmed to be promising candidates for purification of water, toxic waste remediation and undesirable pollutants obtained from various industries [16,17]. However, it needs to initiate a photocatalyst that potentially response in photocatalytic reactions for eco-friendly progress. In this work, Sm 3+ doped MNO with single-phase columbite structure by novel synthetic combustion method has been studied for the first time and it is found to be a potential material for multiple applications.

Synthesis of the Sm 3+ doped MNO phosphor
Magnesium niobate doped with Sm 3+ (1-11 mol%) nanophosphors were synthesized by solution combustion route [15]. The precursors used in the synthesis are Niobium pentoxide (Nb 2 O 5 ) (99.95%, Sisco research laboratories (SLR)), Ammonium nitrate (NH4(NO 3 ) 2 ), Magnesium nitrate (Mg(NO 3 ) 2 (99%, SLR), Samarium oxide (Sm 2 O 3 ) (99.99% Sigma Aldrich Ltd) and Oxalyl dihydrazide (ODH-C 2 H 6 N 4 O 2 ) (lab made) as a fuel [18]. Initially the Sm 2 O 3 is converted into Sm(NO 3 ) 2 by using a small amount of 1:1 HNO 3 and subjected to heating followed by drying. Stoichiometric ratios of nitrates and ODH were well dispersed in doubled distilled water using magnetic stirrer. Finally, the above mixture was subjected to combustion in preheated muffle furnace at ~ 750 ± 10 °C, the entire reaction completed in less than 10 min producing a transperant gel, which boils to yield a white final product. The final product obtained was calcined at 950 °C for 6 h to obtain pure single phase. Schematic representations of the steps of synthesis are shown in Fig. 1.

Characterizations
Powder X-ray diffractometer (PXRD) (Shimadzu) (50 kV, 20 mA, CuK α -1.541Å with a nickel filter) was used to get the crystalline nature of the prepared samples. Transmission Electron Microscopy (TEM) analysis was performed on a JEOL, JEM-2100 (200 kV, LaB 6 filament, EDS-1.5 Å) to obtain the microstructure and lattice parameters. Hitachi SU1510 Scanning Electron Microscopy (SEM) was used to observe the morphology of the samples. Shimadzu UV-Vis spectrophotometer model 2600 (range 200-800 nm) was used for the diffused reflection spectra (DRS) of the samples. Photoluminescence (PL) studies were made using Horiba, (model Fluorolog-3) Spectrofluorimeter with Fluor Essence™ software at RT (Xe-450 W). Photocatalytic studies (PCA) were conducted with lab made photocatalytic setup. Electrochemical Analyzer model-604E (CH-Instruments) with try-electrode system was used for electrochemical impedance studies.

Results and discussion
The phase and structural arrangement of the prepared host and Sm-doped MNO samples calcinated at 950 °C were investigated by PXRD analysis. All strong and sharp intensity peaks with columbite type orthorhombic phase match well with the JCPDS card number 33-0875 and no impurity parts were observed, representing the same crystal structure even after adding samarium into the host ( Fig. 2a) with lattice parameters a = 5.7 Å, b = 14.19 Å, and c = 5.03 Å [19][20][21], their average crystallite sizes were calculated by Debye-Scherrer's formula [22]. Significant extent of strains were associated with nanoparticles because they are known to have a number of surface atoms which have unsaturated in co-ordinations. Various structural parameters namely Crystallite size (D), micro strain (ε), Stress (σ) and stacking fault (SF) were estimated and all the crystallite values are tabulated in Table 1. Increase in doping concentration influences the expansion of the unit cell volume, resulting in tensile stress and micro strain, hence the peaks of PXRD shifts slightly to the lower angle side (Fig. 2b) [15]. FULLPROF program was used to estimate the lattice parameters for MNO nanophosphor. Thomson-Cox-Hasting Pseudovoigt function [15,22] was used to fit the several parameters to the data point. The experimental and calculated PXRD pattern obtained by the Rietveld refinement is shown in Fig. 2c. In general, the Rietveld method utilizes the least-squares refinement for obtaining the best fit between the experimental data and the calculated pattern based on the simultaneously refined models. The fitting parameters (R p , R wp and χ 2 ) indicate a good agreement between the experimental relative intensity (observed PXRD intensities) and stimulated intensity (calculated PXRD intensities) from the model for the orthorhombic MNO phase. Diamond software was utilize for extracting the possible packing diagram (inset of Fig. 2c) and the refined parameters were tabulated in Table 2.
The microstructure, lattice parameters and selected area electron diffraction (SAED) pattern of Sm (5 mol%) doped MNO samples were analyzed by TEM & High Resolution (HR) TEM spectral technique. The microstructures of the samples confirm the consistent distribution of irregular particles, forms agglomeration with average particle size to be around 25 nm confirms the Sm doping and purity of the sample [18]. SEM was predominantly utilized to contemplate the composition, porous, voids, agglomerated morphologies of the powders prepared via combustion process and the same was observed at different magnifications ( Fig. 3b) in the present case [23].
The bonding nature of metal-metal ions or functional groups of prepared host and Sm doped MNO samples were recorded by FT-IR spectra (Fig. 4). The appearance of broad band around at 3417 cm −1 can be represented by O-H stretching and attributed to the presence of water molecule. Whereas the smaller peaks appear at 2338 cm −1 was generally allotted to adsorption of CO 2 molecules [24,25]. The symmetric and asymmetric stretching vibrations of samples were found at 1462 cm −1 indicating metal oxygen [Mg-O, Sm-O, Nb-O] vibrations. Also, the presence of peaks in the range 550-850 cm −1 was indicating to the coordination between metal ions [18].
The conducting nature of prepared series of different moles of Sm-doped MNO are successfully investigated for finding the energy band gap by measuring optical absorbance by means of DRS from 200 to 800 nm    [26,27] and Eg values were found to be in the range 4.0-4.3 eV.
The decrease in Eg for doped samples can be related to the degree of structural order-disorder in the lattice which could affect the distribution of the intermediate energy-level within the band gap. Shallow level donor impurities create energy levels in the band gap near the conduction band edge and shallow acceptor impurities create energy levels near the valence band edge. With an increase in the doping concentration, the density of states of these dopants increased and formed a continuum of states. So, the effective band gap has decreased with an increase in doping concentration. Also, the same may be due to the increase of carrier concentrations or due to the influence of [Sm-O] clusters on the electronic structure of the host materials or the formation of non-bridging oxygen's (NBO) as per the literature [28][29][30].
Undoped and Sm 3+ doped (5 mol%) MNO photocatalyst were successfully used for photo-dye degradation activity under UV light irradiation. Presently, the wide usage of organic dyes in various regions releases toxic pollutants such as dyes containing azo group, phenol derivatives, etc., which are difficult to biodegrade and hence, significantly affects the environmental systems [31]. However, the synthesized host and Sm (5 mol%) doped MNO photocatalysts are potentially practiced for the heterogeneous photodegradation of AR-88 dye measured under UV light irradiation (Fig. 6a, b) and doped sample show high photocatalytic performance (99%) than the host (88%) sample (Fig. 6c). It was reported that the addition of known amount of doping elements into the catalyst resulted in higher surface barrier, narrower space charge layer by increasing the penetration depth of light and makes separation of charge carriers (produced e − − h + pairs) effectively. This leads to increase in catalytic activity of the doped sample when compared to the undoped sample [30]. The AR-88 dye gets photosensitized by irradiated photons and furthermore, the presence of O 2 and H 2 O molecules captured irradiated light on the surface of the photocatalysts, which creates electron-hole pairs. However, number of active radicals (O 2·− and OH · ) are formed by the effect of obtained electron-hole pairs on the surface of the prepared photocatalysts (Scheme 1) [31]. These superoxide and hydroxyl radicals are responding for the ionization of dye molecules and resulting into the less toxic chemicals like H 2 O, CO 2 , and mineral acids. The schematic representation of degradation mechanism in the present case is as shown in the Fig. 6d [32][33][34].
Tuning of the light emission by adjusting the compositions of the nanophosphors allow us to investigate the photoluminescence properties of MNO with varied doping concentrations samarium ions, the excitation was studied by monitoring emission at 608 nm ( Fig. 7), it has PL excitation maximums at 362, 384, 395, 416, ~ 440, 463, and ~ 480 nm because of the transitions 6 H 5/2 → 4 D 3/2,5/2 , 4 D 1/2 , 4 F 7/2 , 4 M 19/2 , 4 G 9/2 , 4 I 13/2 , and 4 I 11/2 respectively which are the excitations from f-f shell transition of Sm 3+ ions [12]. Among these the transition due to 6 H 5/2 → 4 F 7/2 is intense one at 395 nm which imply that the interaction between O 2− and Sm 3+ was stronger [13,15]. The PL intensity of 4 G 5/2 -6 H 7/2 (608 nm) transition was stronger under 463 nm excitation when compared to 395 nm (Fig. 8), even though the excitation intensity was stronger for the later one, this may be due strong energy transfer between the host and lanthanide ions or due to resonance transfer of energy [22]. Hence, visible energy (463 nm) was used to excite the samples in the present case. It shows that the present phosphor can be effectively excited by radiations   (Fig. 9) show all the characteristic emission at 564, 608, 656 and 718 nm, assigned to transitions Fig. 3 a TEM (i), SAED pattern (ii), HRTEM (iii), EDX (iv) of (5 mol%) MNO:Sm 3+ NPs. b SEM images of MNO:Sm 3+ (5 mol%) NPs at various magnifications 4 G 5/2 → 6 H 5/2 , 6 H 7/2 , 6 H 9/2 and 6 H 11/2 respectively [13,15,35] since photo excited carriers transfer energy to Sm 3+ ions from the host lattice, resulting in the 4 G 5/2 → 6 H J/2 transitions and all the transitions were reflected in energy level diagram (Fig. 10). In MNO, Mg 2+ occupies the sites (M1 & M2) i.e., inversion symmetry (CI) and mirror symmetry (CS), two non equivalent octahedral sites. The emission at 608 nm was hyper sensitive in nature when dopant ions occupies CS site results in characteristic orange red emission which was partly magnetic and partly electric dipole transition with shoulder peaks may be due to (2 J + 1) splitting, the transition at 656 nm was due to purely electric dipole (ED) sensitive to the crystal field [36,37], no emission peak positions shifted while doping concentrations were relatively varied, suggesting that the nature of Sm 3+ activators remained unchanged, whereas the intensity reaches maximum at 5 mol% and falls afterwards (inset of Fig. 9) due to concentration quenching [18].
The octahedral sites MgNb 2 O 6 are occupied by dopants in the host matrix, charge imbalance will be compensated by oxygen vacancies, if the dopants are excess then they will be appear on the surfaces or grain boundaries to yield optimum strain relief in their oxide form. The defect reaction can be represented in the following way,  [38,39].
According to Blasse, the critical distance (Rc) between two Sm 3+ activators can be calculated as [12,40]: where C; the critical dopant concentration = 0.05, N; the number of Mg ion in the unit cell = 8, and V; the volume of the unit cell = 407.02 Å 3 . Rc was found to be 12.46 Å, which is larger than 5 Å, for the present phosphor confirms that the non-radiative energy transfer is due to multipole-multipole interaction and is the main reason for quenching, in order to know the type of interaction like dipole-dipole (Q-6), dipole-quadrupole (Q-8) or quadrupole-quadrupole (Q-10), we have to utilize Dexter's theory and Van-Uitert equation [41] and by ploting log (X) V/s log (I/X) (Fig. 11): where 'x'; activator concentration, 'k' and 'β' are the constants for the given host, and the value of Q was identified to be around 6, hence dipole-dipole (d-d) interaction was responsible for concentration quenching in the present case.
The quantum efficiency (QE) of the optimized MNO:Sm 3+ (5 mol%) was calculated by the method described by De Mello [42] and Palsson [43] (2)   where E C ; the integrated luminescence of the phosphor caused by direct excitation, E a ; the integrated luminescence from the empty integrating sphere (blank, without sample), L a ; the integrated excitation profile from the empty integrating sphere, L C ; the integrated excitation profile when the sample is directly excited by the incident beam. The QE for the MNO:Sm 3+ (5 mol%) nanophosphor   [44][45][46] to study the site symmetry and luminescence behavior of the Sm 3+ ions in MNO host crystal to understand site symmetry and luminescence behavior using emission data, the adopted procedure is similar to our earlier work [18]. Spontaneous emission probability (A R ), the experimental intensity of 4 G 5/2 to 6 H 7/2 transitions Ω λ (λ = 2, 4 and 6), Total radiative rate (A T ), Non-radiative rates (A NR ), luminescence quantum efficiency ( ), fluorescence branching ratio (β), radiative lifetime ( rad ) were estimated.  Table 3 and all the obtained values uphold this system a promising material for displays [46][47][48]. The luminescence decay curve of MNO:Sm 3+ (5 mol%) phosphors (Fig. 12) due to possible interactions between Sm 3+ ions was well fitted by triple exponential and the average decay time (τ r ) was estimated to be 7.68 ms [46].
Inset of Fig. 12 shows the decay curve at different temperatures, decrease in decay time with increase of temperature was due to the forbidden nature of the f-f rad transition or self-absorption rate among dopants. Also, it was affected by the existence of defects, which could shorten the decay time by capture electrons, hence the values are in millisecond and were reasonably good to meet the purpose of the w-LEDs [12], a probable candidate for temperature sensor and the decay time quenching can be estimated as discussed elsewhere and the value of E CTS and b were found to be 2650 cm −1 and 1.7 × 10 6 s −1 respectively [24,46]: These properties of the present material confirms good to be used as a temperature sensor operated in the temperature interval of between 50 and 400 °C where the decay time is decreasing. When temperature increases, the pathways of non-radiative transition will enhance which decrease the decay times. The emission intensities of MgNb 2 O 6 :Sm 3+ (1-11 mol %) where characterized by using Commission International de I' Eclairage (CIE) chromaticity coordinates spectra shown in Fig. 13. The method estimate CIE chromaticity coordinates (x, y) and correlated colour temperature (CCT) coordinates (u' , v') as discussed elsewhere [15,[48][49][50][51]. Besides, the average CCT of nanophosphor was (Fig. 14) found to be 1772 K (warm white light). The color purity of the nano phosphor can be estimated as: where, (x ee , y ee ) and (x d , y d ) are the coordinates of standard white point and dominate wavelength point respectively, considering pure white point at x = 0.3127 and y = 0.3290 (D65) [18] and it was found to be 99.2%, which was very close to 98.88% in our previous work on MNO:Eu 3+ [18].
These values indicate that present phosphor was highly useful for the production of artificial white light, wLEDs and solid state display applications with near-UV or blue light chips [12]. Figure 15a shows the CV curve of MNO:Sm 3+ (5 mol%) electrode at different scan rates in 1 M KOH using homemade carbon paste electrode [52]. The presence of redox peaks in the anodic and cathodic scan show the noteworthy contributions in the electrochemical process of pseudo-capacitance. The peak potentials were 0.296 V at 10 mVs −1 and 0.401 V at 50 mVs −1 respectively, which confirms reversibility of redox reaction is more [53]. Figure 15b shows the Nyquist plot and equivalent circuit (inset of Fig. 15b) of the MNO:Sm 3+ (5 mol%) electrode. With the support on the frequency, the Nyquist plot has basically  Table 4. In the given circuits, solution resistance (Rs), charge transfer resistance (Rct) and the double-layer capacitance (C) were calculated by intercepting the semicircle on the real axis and electrode-electrolyte interface in high-frequency region, whereas the Warburg element (W) was calculated by straight line in the low-frequency region. Q 1 : constant phase element which was parallel to Rct and Q 2 : the low frequency capacitance parallel to the leakage resistance (R l ) [54][55][56]. Figure 16a,  A drastic variation of oxidation and reduction peak positions confirms that, MNO:Sm 3+ (5 mol%) was an effective material for chemical sensor applications in alkaline medium for quantities of 1-8 mM concentrations (Fig. 17).

Conclusions
Single phase columbite type orthorhombic structured MNO:Sm 3+ (1-11 mol%) nanophosphors were prepared in  the present work, TEM results uphold the nano and polycrystalline nature of the samples, having energy gap in the range of 4 eV as confirmed by DRS. Better photocatalytic activity of 99% degradation for AR-88 dye under UV light irradiation was observed in this work. All the characteristic transitions were observed with average decay time (τ r ) of 7.68 ms, it was found to decrease with increase in temperature indicating the thermal sensing property of this material. CIE n CCT coordinates confirms orange red emission and ~ 1772 K respectively. Electrochemical studies show high reversible electrode reaction, high charge transfer resistance and exhibit high sensitivity for detection of paracetamol and alcohol. All the above results validate that; the present material can be a good candidate for solid state display, photocatalytic, thermal and chemical sensor applications.

Compliance with ethical standards
Conflict of interest We wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.
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