All Precursors Are Not Equal: Morphology Control via Distinct

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All Precursors Are Not Equal: Morphology Control via Distinct Precursors-Facet Interaction in Eu3+-doped NaLa(WO4)2 Chandresh Kumar Rastogi, Sulay Saha, Vishal Kusuma, Raj Ganesh S. Pala, Jitendra Kumar, and Sri Sivakumar Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.9b00354 • Publication Date (Web): 28 May 2019 Downloaded from http://pubs.acs.org on May 28, 2019

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Crystal Growth & Design

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All Precursors Are Not Equal: Morphology Control via Distinct Precursors-Facet Interaction in Eu3+-doped NaLa(WO4)2 Chandresh Kumar Rastogi,Ϯ Sulay Saha,$ Vishal Kusuma,$ Raj Ganesh S. Pala,*,$ Jitendra Kumar*, and Sri Sivakumar*,Ϯ, $, £, € ϮMaterials

Science Programme, $Department of Chemical Engineering, £Centre for

Environmental Science and Engineering and €Thematic Unit of Excellence on Soft Nanofabrication, Indian Institute of Technology Kanpur, 208016, Kanpur, India School

of Materials Science and Technology, IIT (BHU), 221005, Varanasi, India

Tel.: +91-512-2597697. Fax: +91- 512-2597664; E-mail: [email protected], [email protected], [email protected]

Abstract Morphology of lanthanide (Ln3+)-doped nanostructures plays a crucial role in determining the crystal field dependent luminescent properties as that modulates the surface-to-bulk ratio of Ln3+ ions. In this work, we demonstrate a systematic morphology control in Eu3+-doped sodium lanthanum tungstate NaLa0.95Eu0.05(WO4)2 by varying lanthanide precursor salts and report variation in its photoluminescence properties. Chloride and nitrate salts of lanthanide produce nanoneedles and -cuboids, respectively whereas acetate and carbonate salts produce rugby ball-like morphology. Density functional theory-based simulations have been performed to study the effect of growth hindering species (Na salts of precursor anions, i.e., Cl-, NO3-, CO3-2) in controlling the evolution of low index (100) and (001) facets of NaLa0.95Eu0.05(WO4)2. The changes in morphology are correlated with the preferential development of low index (100) and (001) facets caused by 1 ACS Paragon Plus Environment

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differential adsorption of growth hindering species. Accordingly, they attach with La-sites of (100) rather than O-terminated (001) facet; binding selectivity order being chloride > nitrate > carbonate. Strong adsorption of chloride salts on (l00) causes anisotropic growth along [001] direction and leads to needle morphology due to vast stacking of basal planes. The minor binding difference in adsorption of nitrate salts observed on (100) and (001) facets is responsible for overall growth and evolution of cuboid morphology. The binding of carbonate (or acetate) salts on (100) is weak and combined evolution of (001), (011), and (100) facets leads to rugby ball morphology. Further, the needle, cuboidal and rugby shape particles produce different UV sensitized emission with the Commission Internationale de I’ Eclairage (CIE) chromaticity coordinates as (0.44, 0.25), (0.48, 0.26), (0.59, 0.32), respectively.

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1. INTRODUCTION Lanthanide-doped inorganic nanocrystals with distinct morphology and tunable size continue to be of research interest due to their unique luminescent properties.1-7 In fact, their large surface area-to-volume ratio allows the presence of many lanthanide (Ln3+) ions at surface sites, with some variation of course depending on the crystal morphology. Further, as the crystal field atmosphere is asymmetric at the surface, lanthanide ions display diverse emission characteristics with different size and shape of crystallites. For instance, intensity of Eu3+ ions red and orange emissions resulting due to (5D07F2) and (5D07F1) transitions, respectively, are greatly influenced by the nature of host crystal.8 It is known that the 5D07F2 transition is hyper-sensitive and rests on Eu3+ ion site-symmetry.9 In recent years, various lanthanide -doped materials such as -oxides,10 -fluorides,11 -vanadates,12 -phosphates,13 –tungstates,2, 14 and –molybdates15 have been explored for their morphology-dependent luminescence properties. In particular, tungstate based inorganic hosts such as MLn(WO4)2 [M = Li, Na, and K; Ln = lanthanide ions] has turned out to be a suitable host for lanthanide ions because of i) favourable size and charge matching of cations, ii) strong and broad absorption band in UV region for efficient sensitization of doped Ln3+ ions and iii) relatively low phonon energy -vis-à-vis other oxide matrices.2, 14, 16, 17 Size and morphology control is usually realized by varying the preparation conditions/parameters which facilitates or inhibit growth of certain facets of evolving crystallites. These include temperature,18 reaction time,2 pH,2, 19 ligands,20 additives21, 22 and precursor salts.2326

For example, reactions of short duration at low temperature favour kinetically controlled growth

resulting in nanoparticles whereas a high-temperature process is driven by thermodynamics and yields equilibrium structural shape bulk crystals. Ligands lower the surface energy, suppress growth kinetics, and facilitate development of morphologies like rods, cubes, tetrapod, etc. The ligand3 ACS Paragon Plus Environment


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surface interaction depends on polarity of solvent and pH of the medium. These considerations led to growth studies of NaLa1-xEux(WO4)2 crystals by varying temperature, pH, reaction time and ligands [viz.,cetyl-trimethyl ammonium bromide (CTAB), poly-vinyl-pyrrolidone (PVP), and ethylenediamine tetra-acetic acid (EDTA)]. 2, 14, 16, 17, 27 Also, precursor salt (with its unique ionic character, steric effect, and electronic nature) may itself be responsible for growth variation of facets.16, 24-26, 28-31 The amount of precursor also causes difference in reactivity of species in a given medium.32-34 The solvent polarity is yet another parameter that controls the reaction and dictates the optimum concentration requirement.35 In addition, the counter-ions in precursor salts lower the surface energy of nanoparticles through adsorption. The electronic and steric interactions among anions and with ligands determine the surface coverage and, in turn, overall growth of emerging nanoparticles.36 The reactivity may possibly be altered by changing the precursor salt itself. In this context, previous reports suggest that different anion precursor salts, having same charge but different polarizability (e.g., K+, Na+, NH4+), can produce distinct morphologies.24 For example, EuF3 nano-wires, -rods, -bundles, and branched tree-like particles could be obtained with fluoride precursor salts having different cations such as caesium, rubidium, sodium, and ammonium, respectively.24 Similar effects have also been observed for cation providing precursor salts possessing different anions.31 Though the literatures are available reporting the effect of precursor on the final morphology of the nanoparticles, nevertheless, the role of inorganic species in determining the morphology of growing nanoparticles is not well understood as yet and needs further investigation. The present paper addresses this issue and describes investigation with regard to morphology of NaLa1-x Eux (WO4)2 crystallites grown with different precursors (viz., chloride, nitrate, carbonate, and acetate of lanthanides) via colloidal route. Surface energy simulation has been carried out using density functional theory (DFT) to 4 ACS Paragon Plus Environment

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understand preferential crystal growth in certain directions in each case. Further, luminescence characteristics of NaLa1-x Eux (WO4)2 crystals having needle, cuboidal, and rugby ball morphology have been studied with the objective of producing white radiation in some situation for biological and solid-state lighting applications.

2. MATERIALS PREPARATION AND CHARACTERIZATION All the lanthanide salts (99.9 %) and 1-octadecene (99.9 %) (Sigma Aldrich), Oleic acid (90 %) and Na2(WO4)2 (99 %)

(Loba Chemie and CDH Pvt. Ltd.) were used without further

purification. Eu3+- doped sodium lanthanum tungstate NaLa0.95Eu0.05(WO4)2 samples have been prepared at 300 C by colloidal route using oleic acid as ligand. The synthesis scheme for the material preparation is shown in Figure S1 of the supporting information. In a typical synthesis, 2 ml aqueous solution of LaCl3.6H2O (0.75 mmol) and EuCl3.5H2O (0.04 mmol) salts, 6 ml oleic acid, and 15 ml 1-octadecene were added in a 250 mL three-neck round bottom flask. The resulting mixture was then heated at 80 C for 30 min under vacuum using a rotovap unit. The obtained solution was further heated to 150 ºC in oil bath, and after 30 minutes cooled down to 80 ºC before adding an aqueous solution of 1.125 mmol of Na2WO4. Thereafter, the obtained reaction mixture was heated at 125 ºC for 30 min to remove the water, and subsequently heated to 300 ºC with a rate of 10 C/min under an inert argon atmosphere. The temperature of the reaction medium is maintained at 300 C for a duration of 90 min, and subsequently cooled down to room temperature. The obtained product was then purified thrice with cyclohexane and precipitated using a mixture of ethanol and water to obtain the NaLa1-xEux(WO4)2 powder. Similarly, other samples of NaLa0.95Eu0.05(WO4)2

were prepared using nitrate, acetate and carbonate precursor salts of

lanthanide. 5 ACS Paragon Plus Environment


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Transmission electron microscope (FEI Technai G2 TEM) and a field emission scanning electron microscope (Carl Zeiss NTS GmbH-SUPRA 40VP SEM) were used to observe the microstructure of each sample. An X-ray diffractometer (PANalytical X’ Pert PRO) with Cu Kα1 radiation was employed for determining the phase(s) present. Rietveld refinement of the X-ray diffraction pattern was undertaken with a FullProof Suite Program. A Fourier transform infrared (FTIR) spectrometer (BRUKER vertex-70) was utilized for the identification of various stretching bonds existing in samples. Further, a fluorescence spectrophotometer (Edinburgh Instruments FLSP 920) [equipped with a double monochromator, 450 W Xenon lamp as excitation source, and Peltier element cooled Hamamatsu R928-P PMT detector] was used for studying the luminescence behavior of various products. The luminescence decay curve was recorded with a 100 W micro flash lamp (µF920H) as the excitation source to estimate the emission lifetime in each case.

3. COMPUTATIONAL METHODS To study the effect of lanthanide salts (viz., chloride, nitrate, acetate and carbonate) on the formation of NaLa1-xEux(WO4)2 crystals, surface energy calculations were made with density functional theory (DFT) and a plane-wave basis set (energy cut-off 400 eV) by a projectoraugmented method. Perdew, Burke and Ernzerhof (PBE) exchange-correlation functions, as implemented in Vienna Ab initio Simulation Package (VASP) software, were used.37-40 Ultrasoft pseudo-potentials are used. The pseudo-potentials for La, Na, W and O have 5s25p65d16s2, 2p63s1, 5p65d46s2 and 2s22p4 electronic configuration, respectively. For the bulk calculations, the Brillouin zone is sampled using a gamma centered mesh of k-points (883). A vacuum layer of 10 Å has been used for surface calculations to avoid any spurious interaction between adjacent slabs along the perpendicular direction surface. Appropriate stoichiometry is maintained throughout the slab 6 ACS Paragon Plus Environment

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for all the surface calculations. Geometric optimizations are carried out with all the surface layers relaxed. During surface calculations, the sampling of Brillouin zone is changed accordingly. For (100) and (001) bare surfaces, the Brillouin zone has been sampled using (143) and (441) respectively. The geometry has been relaxed by optimizing all structural parameters through conjugate gradient approximation method until the forces on each ion are smaller than 0.01 eV/Å. The ultra-soft pseudo-potentials chosen for surface energy calculations were based on electronic configurations 5s25p65d16s2, 2p63s1, 5p65d46s2 and 2s22p4 for lanthanum, sodium, tungsten and oxygen, respectively. For bulk, Brillouin zone is sampled with a gamma centered (8x8x3) mesh of k-points. The presence of a 10 Å thick vacuum layer was assumed at the surface for energy evaluation to avoid spurious interactions between the adjacent slabs. A proper stoichiometry is maintained throughout the slab for all the cases. Geometric optimization was done with the surface layers relaxed. Brillouin zones were sampled for (100) and (001) faces using (143) and (441) mesh, respectively. For geometry relaxation, structural parameters were optimized using a conjugate gradient approximation until force on each ion reached below 0.01 eV/Å. The surface energy (Es) was obtained using the relation41 1

(1)

𝐸𝑠 = 2𝐴(𝐸𝑡 ―𝑙 𝐸𝑏)

where Et and Eb represent the total energy of the slab and the bulk, respectively, A is the surface area of the slab and l is the number of slabs considered. It may be pointed out here that the traditional methods of partitioning energies fail in case of a polar surface because of its inherent nature, i.e., terminating cation or anion layer. In contrast, a non-polar surface contains both cations and anions in some stoichiometric ratio.41,42 However, equation (1) holds for polar surfaces also provided thin films containing l slabs of polar surfaces varies linearly with the total energy (Et).41 7 ACS Paragon Plus Environment


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A typical film with (100) slabs in NaLa(WO4)2 has either sodium and tungsten (Na and W) or lanthanum and tungsten (La and W) species at one end and oxygen at the other. For adsorption purposes, a five-layer slab is taken. Since the electronic nature of adsorbates effects the surface interactions, Bader charge decomposition scheme was adopted for the analysis.43

4. RESULTS AND DISCUSSION The lanthanide salts are soluble in water. Hence, there is appreciable reactivity of species under aqueous condition leading to high growth rates. This property poses difficulty in controlling size and shape of emerging NaLa1-xEux(WO4)2 crystallites. Hence, oleic acid and 1-octadecene are chosen for preparation as ligand and reaction medium, respectively. Non-polar solvents promote swelling of alkyl chain of oleic acid. Hence, steric interaction follows, and aggregation of ligand capped nanoparticles is curbed.44 Highly ionic precursor salts like nitrate and chloride do not dissociate easily in 1-octadecene and so reactions become quite slow. In contrast, the acetate and carbonate precursors, being weak ionic solids, are likely to display relatively higher reactivity.

4.1 Morphology and Phase formation Figures 1 and 2 show the electron microscopic images of NaLa0.95Eu0.05(WO4)2 crystals, prepared with various lanthanide salts (i.e., chloride, nitrate, acetate, or carbonate) and Na2WO4. Notice the formation of needle (average length ~300 nm), cuboidal (size ~150 nm), and rugby ball (length ~350 nm, middle width ~ 100 nm) morphology with different precursor salts, i.e., lanthanide chloride, nitrate, and acetate (or carbonate), respectively. Traces of spherical and needle shape particles were observed along with cuboidal particles in case of the samples prepared using nitrate precursors. These are attributed to the incomplete growth of nanoparticles which results in mixture of morphologies. The electron micrographs shown in Figure S2a of supporting information display 8 ACS Paragon Plus Environment

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a mixture of cuboidal and needle shape morphologies which are obtained when an additional amount of nitrate precursor is used along with the lanthanide chloride precursors. Further, aggregates of flower shape particles are obtained when an excess of sodium nitrate (NaNO3) is used with lanthanide chloride precursor (Figure S2b). The observed variations in the morphology are due to differences in the solubility and reactivity of lanthanide nitrate and sodium nitrate in reaction medium. The details of the growth mechanism responsible to produce a variety of morphologies under different experimental conditions are provided by density functional theory based simulations (discussed later). The high-resolution transmission electron microscope (HRTEM) images of a typical rugby ball NaLa0.95Eu0.05(WO4)2 nanocrystal (particularly of narrow end portion) depict lattice fringes of (002) and (011) planes of tetragonal-phase with spacing 0.582 and 0.486 nm, respectively (Figure 2d). The Fourier transform of HRTEM image (Figure 2e) belonging to rugby ball morphology shown in Fig. 2d correspond to [100] orientation (or b*- c* reciprocal net). The raw Fourier transform pattern is shown in Figure S3 of the supporting information. Its indexing as marked in Figure 2e match well with the lattice fringes of (002) and (011) planes having spacings 0.582 and 0.486 nm, respectively. The nose of rugby ball stands along [001] and the fringes lie normal to this direction (Fig 2d). The appearance of 002 spot is due to double diffraction as the systematic condition of space group (I41/a) for 00l reflections demands l = 4n. The rugby ball (or shuttle-like) and needle-type crystallites grow preferentially along their lengths in [001] direction. Further, the detailed information about the elemental composition of rugby shape NaLa0.95Eu0.05(WO4)2 nanocrystal as determined by high angle annular dark field (HDAAF) imaging technique is provided in Figure S4 and Table S1 of the supporting information. Figure 3 shows the XRD patterns of four NaLa0.95Eu0.05(WO4)2 powder samples prepared using chloride, nitrate, acetate, and carbonate salts of lanthanides (La, Eu) and Na2WO4. All these 9 ACS Paragon Plus Environment


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patterns are nearly similar and can be indexed with a tetragonal structure (lattice parameters ao = 0.5349 nm co = 1.1628 nm, Z = 2, and space group I4 1/a; JCPDS file No. 079-1118) of pure NaLa (WO4)2. The marginal difference in lattice parameters is observed for needle, cuboidal and rugbyball crystal morphologies with slight shift in peaks towards smaller or higher Bragg angles vis-àvis standard NaLa(WO4)2 data (Table S2). Rietveld refinement undertaken in the case of rugby balls reveals reasonable correspondence with fitting parameters 2 = 1. 52, Rp = 14.9, Rwp = 12.0, and Rexp = 9.7. A close observation of XRD patterns also indicates some variation in intensity of diffraction peaks arising due to changes in morphology, size and preferred orientation of crystallites produced during synthesis with different precursors. The crystallite size, deduced from the Scherrer formula using full width at half maximum (β) value of 112 diffraction peak intensity, lie in the range 15–19, 22–36, and 22 – 29 nm for samples prepared with chloride, nitrate, and acetate/ carbonate salts, respectively. Gu et al. synthesized La1-xREx (WO4)2 (RE = Eu or Tb, x = 0.05) nanocrystals (possessing tetragonal structure with a = 0.5349 nm, c = 1.1620 nm, Z = 2, and space group I41/a) using a solvothermal process and realized variation in their morphology and size by keeping the ethylene glycol (EG) to water volume ratio (VEG / Vwater) as 4, 3/2, 2/3, and 1/4.16 The precursors used were La(NO3).6H2O, RECl3.6H2O, and Na2WO4. 6H2O. They obtained shuttles of dimensions (length ~ 300 nm, middle width ~ 100 nm) and needles of size (length ~ 9000 nm, breadth ~ 1000 nm) with (VEG / Vwater) ratio 4 and 1/4, respectively. So, the decrease in EG content led to preferential growth of crystals along [001] direction with shape modification by limiting the development of breadth with significant rise in aspect ratio from 3 to 9. Obviously, ethylene glycol solvent was vital in synthesis and served as ligand. The underlying variation in viscosity and chelation is believed to control solubility, diffusion and reactivity of species. EG possibly forms complexes with rare earth 10 ACS Paragon Plus Environment

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initially and facilitates nucleation of NaLa1-xREx (WO4)2 crystals later via reaction of disjointed lanthanide ions with Na2WO4. The free EG component stabilizes the specific crystallographic planes by selective adsorption and regulates the growth process too. On the other hand, Wang et al. prepared microcrystals of NaLu1-xEux (WO4)2 (x = 0.4, 0.5, 0.7, 1.0) using ethylene-diamine-tetra-acetic acid (EDTA) assisted hydrothermal process with Lu2O3, Eu2O3, and Na2WO4. 2H2O precursors.3 The process involves emergence of intermediate compounds WO3, Na2W2O7 and Lu1-xEuxO(OH) with their reaction giving NaLu1-xEux (WO4)2 via nucleation and growth. All the crystals formed belonged to the tetragonal phase (mentioned above) but with different morphologies depending on the Eu3+ content (x). Thus, hexahedral (irregular and distinct boundaries), olive-like tetrahedron, and shuttle shape crystals with progressively higher aspect ratio were produced with x = (0.4 and 0.5), 0.7, and 1.0, respectively. The hexahedral morphology exhibits {100} terminated facets possibly due to reduction in their surface energies caused by high Lu3+ concentration (1-x) = 0.5-0.6 and optimal EDTA (being chelating agent with good capping ability) content. The polyhedron shuttle-like crystals grow preferentially along their length in [001] direction by gradual disappearance of high energy {100} facets with increasing amount of Eu3+ ions (x = 0.7 and 1.0). NaLa(WO4)2 microcrystals of tetragonal phase (JCPDS file # 79-1118) were also prepared with a hydrothermal process using La(NO3)3 and Na2WO4 as precursors and polyvinyl-pyrrolidone (PVP) as a surfactant with HNO3 and NaOH for pH adjustment.2 The process is shown to produce different morphologies, i.e., uniform spindles, flowers comprised of flakes, irregular parallelepiped, and thin plates at micrometer scale with pH 4, 7, 8, and 10, respectively. Besides, PVP was vital for compound formation and its amount determined the product morphology. As per the report, PVP molecules adsorb preferentially on (001) face, cap the entities/species, stabilizes the surface 11 ACS Paragon Plus Environment


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by reducing its energy, and, in turn, controls growth along [001]. So, spindles, quasi- or uniform cuboids, and cubes of NaLa(WO4)2 were formed with increasing PVP content (0.3 – 1.8 g). Since growth was severally affected along [001] with increasing solution pH, HNO3 and NaOH seem to act like PVP but much strongly. The factors responsible for morphology evolution in tungstate crystals include (i) organic ligands, (ii) reaction temperature and time, (iii) pH of precursor solution, (iv) lanthanide source, and (v) solvent. Accordingly, flake, spindle, sphere, sheet, flower, shuttle/ peanuts/ olive-like, rugby, belt, rod, bipyramid, star shape crystallites of wide size range (nano-micrometer) could be produced. The mechanism of formation is complex because of multiple components involved and so not well understood. In the present context, morphology variation is realized by use of different precursors namely, lanthanide chloride, nitrate, acetate, and carbonate. Each salt solution reacts with Na2WO4 differently and at diverse rates (depending on the availability of reactants) and form NaLa(WO4)2 nuclei which, in turn, grow with time. The quick removal of the byproduct (i.e., sodium salt) is essential as its elements may influence the growth process by preferential adsorption on facets (causing effective separation of reactants), changing solution pH, etc. The species/compounds present vary in each case and can be Cl1-, (NO3)1-, (CO3)2-, (OH)1-, H+, Na+, La3+, (WO4)2-, NaOH, HNO3, and sodium salt. The origin of the changes observed in morphology lies with the real role played by the species present. Table 1 summarizes the results obtained with NaLa(WO4)2 prepared using different lanthanide salts. Accordingly, needle, cuboidal, and rugby ball shape crystals emerge. Since all other ingredients remain unaltered, the morphology changes owe allegiance to the precursor and its interaction with the species present.


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4.2 Surface energetics, growth and morphology evolution The morphology of nano-crystallites depends on surface phenomena which essentially control the growth energetics. The exposed facet allows reaction of species/monomers and facilitates extension along its normal. The growth along a particular facet is not only dependent on its surface energy and interaction with reactants but also on its interaction with growth hindering species (Figure 4). This makes the potential sites devoid of reactants. The species can, therefore, be classified as (a) reactive - lanthanum salts and Na2WO4 with their monomers and (b) growth hindering or disruptive - ligand (oleic acid) and byproducts of reactions (as NaCl/ Na2CO3/ NaNO3). These growth hindering Na-salts are produced as follows: 𝐿𝑎𝐶𝑙3 + 2𝑁𝑎2(𝑊𝑂4)→𝑁𝑎𝐿𝑎(𝑊𝑂4)2 + 3𝑁𝑎𝐶𝑙 𝐿𝑎2𝐶𝑂3 + 4𝑁𝑎2(𝑊𝑂4)→2𝑁𝑎𝐿𝑎(𝑊𝑂4)2 + 3𝑁𝑎2𝐶𝑂3 𝐿𝑎𝑁𝑂3 + 2𝑁𝑎2(𝑊𝑂4)→𝑁𝑎𝐿𝑎(𝑊𝑂4)2 + 3𝑁𝑎𝑁𝑂3

To understand the effect of disruptive species on growth of crystallites, their binding energies with (100) and (001) facets were estimated using DFT based simulation and correlated with the morphology observed. Anion species of lanthanum salt complex remain on exposed faces [e.g., (100), (001)] for a while and thereafter produce sodium salt following recombination. The quick desorption of anions (or removal of sodium salt) provides additional reaction sites for monomers/ species to facilitate growth. The growth limiting kinetics of disruptive/ hindering species depends not just on their binding energy with the developing facet but on other factors too, viz., interactions with adsorbates of neighbouring surface sites, susceptibility of reactive species, etc. The ligand (oleic acid) also passivates the surface and reduces the growth rate of crystallites. 13 ACS Paragon Plus Environment


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The presence of oleic acid is confirmed by observing the signature peak at ~ 1410 and 1556 cm-1 due to symmetric and antisymmetric stretches of COO- group as shown in Fourier transformed infra-red spectra (Figure S5 of the supporting information). Since same amount of oleic acid is used in all experiments, therefore, its role in influencing shape is believed to remain nearly unaltered in each case. For this reason, its role is not accounted in the simulation studies. The structural optimization yields bulk lattice parameters of tetragonal NaLa(WO4)2 phase as a = b = 5.46 Å, c = 11.88 Å, i.e., close to those deduced using x-ray diffraction data (vide supra). It comprises of co-ordination eight (8) for lanthanum and sodium (both occupying cationic site 8c) and co-ordination four (4) for tungsten ions (site 4-a). The (001) facet contains O- terminated ions with two dangling bonds (Figure S6a-b of the supporting information) while the (100) facet comprises of W- and Na/La – terminated species with two and four dangling bonds, respectively (Figure S6c-d). Since both (100) and (001) facets can terminate differently, all possible surface arrangements have been considered. In (100) facet, (Na + W) combination (σ = 0.14 eV/Å2) is energetically more favourable than (La + W; σ = 0.17 eV/Å2) when no adsorbate interaction occurs. This may be attributed to valency (+1) of sodium with higher co-ordination (or better bond saturation) vis-à-vis lanthanum (3+). Among the two facets, O-terminated (001) is relatively more stable (σ = 0.08 eV/Å2). The cation-terminated (100) has high surface energy as its presence entails more number of bonds breaking per unit surface area than in (001) facet. The adsorption of growth hindering species NaCl/Na2CO3/NaNO3 is examined on different sites of (100) and (001) facets. Figure 5 depicts the passivation of top and side views of cationterminated (100) facet with chlorine-, nitrate- and carbonate-species. Typically, (100) facet is polar with one end having Na/La and W ions while the other has oxygen species. While the anions arising from lanthanum salt are adsorbed on cation-terminated (100), Na+/ La3+ species get attached on O14 ACS Paragon Plus Environment

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terminated end. There are two possible sites on (100) for anion adsorption: (i) Na/ La (ii) W. Since the distance between the two cation sites is large (4.02 Å), steric interaction between smaller anions such as chlorine, nitrate and carbonate -species can be ruled out. However, this situation allows adsorption of two chlorine ions (and not nitrate and carbonate species) at a cation-site (Na/La/ W) without any steric hindrances. On the contrary, the distance between two adjacent O-sites being 3.26 Å on (001) facet favours interaction between anions. Further, the steric interface between large anion like CO32- or NO3- and ligand (oleic acid) hinders complete surface coverage of adsorbents. Also, the interaction between precursor anions and surface cations of growing nanocrystals are affected by other factors such as solvent polarity and electronic interaction with neighbouring surface adsorbates.44 The adsorption energy of anions on (100) and (001) facets is estimated using the relation 𝐸𝑎𝑑𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛 = 𝐸𝑠𝑢𝑟𝑓𝑎𝑐𝑒 + 𝑁𝑎 ― 𝑠𝑎𝑙𝑡 𝑜𝑓 𝑎𝑛𝑖𝑜𝑛𝑠 ― 𝐸𝑠𝑢𝑟𝑓𝑎𝑐𝑒 ― 𝐸𝑁𝑎 ― 𝑠𝑎𝑙𝑡 𝑜𝑓 𝑎𝑛𝑖𝑜𝑛𝑠

(2)

The values of adsorption energy obtained with different anions from sodium salts (chlorine, nitrate, and carbonate) are listed in Table 2. These indicate weak adsorption of all anion species on (100) facet at Na-site. But, they get adsorb readily at La- and W-sites due to inhomogeneous distribution of electron deficiency prevailing around cations (evident from Bader charge analysis). The formal charge (i.e., valence-Bader) over Na, La and W in bulk is found to be +0.91, +2.22 and +2.86, respectively. This makes electron donor species (oleate ions and precursor anions) bind preferentially towards W and La-sites on (100) surfaces. The binding strength of different anion species on cation terminated (100) surface is in decreasing order of chlorine, nitrate, carbonate (Table 2). The anions binding on O-terminated (001) is however much weaker than at La- site on (100).



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16

During evolution of NaLa(WO4)2 crystallites, there is adsorption competition between the reactive monomers and disruptive species at various exposed facets. Strong adsorption of the latter (disruptive entities) suppresses reaction of monomers and facet species, leading to growth inhibition. The adsorption energies of various anions on (100) and (001) facets given in Table 2 explain the extent of hindrance present in each case. Since their adsorption can occur on both (100) and (001), the relative picture is judged by the difference of adsorption energies, i.e., La-site on (100) and O- site on (001), termed as Eselect in Table 2. A higher positive difference indicates strong attraction of species on (100) at La-site, resulting in preferential growth along [001] direction. A facet with low selectivity is less reactive and so meagre growth occurs in the perpendicular direction with its limited piling.45 The Eselectivity trend shows values in decreasing order of chlorine-, nitrate-, and carbonate species. It essentially means preferential adsorption of (i) anions occurs at La-sites of (100) rather than on O-terminated (001) and (ii) chlorine ions more than nitrate and carbonate species. Their strong adsorption prevents interaction of reactive monomers on (100) vis-a-vis (001) facet, leading to anisotropic growth along [001] and formation of crystallites with needle morphology. In contrast, since the selectivity of carbonate species is least, growth occurs in other directions (particularly ) as well giving rise to rugby ball morphology. Their coarse edges may result due to misalignment of identical facets. The removal of byproduct is also much easier in case of carbonate species vis-à-vis chlorine ions. The situation with nitrate salt is balanced and overall growth forms cuboidal shape crystallites. Incidentally, XRD pattern of this sample matches well with the powder data (JCPDS file 01-0583) where there is no preferred orientation. Figure 6 presents a schematic illustration demonstrating the growth of NaLa(WO4)2 crystal in presence of growth hindering


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species of chlorine, nitrate, and carbonate resulting in needle, cuboidal and rugby shape morphologies, respectively.

4.3 Photoluminescence studies The photoluminescence (PL) from Eu3+ ions are strongly dependent on their local environment in nanocrystals.46 Hence, PL spectrum is expected to contain signature of Eu3+ ion location, if exists differently in various morphologies described above. Figure 7a shows the emission spectra of NaLa0.95Eu0.05(WO4)2 samples displaying needle-, cuboidal- and rugby ballmorphology (excitation wavelength being 270 nm). It is to be noted that the emission spectra are normalized with respect to peak at ~ 615 nm for each case. The corresponding pre-normalized spectra are displayed in Figure S7 of the supporting information. As shown in Figure 7a, all the curves contain a broad emission band in the wavelength range of 400-550 nm, belonging to the host NaLa(WO4)2 itself (Figure S8 of the supporting information), and sharp peaks at 593, 615, 651, and 701 nm emanating from Eu3+ ions following 5D07FJ, J = 1, 2, 3, and 4 transitions, respectively. The emissions peaks at ~ 615 and 701 nm are due to electric- dipole transitions which are hypersensitive in nature, and strongly dependent on the local chemical environment around Eu3+ ions. Further, the observed splitting of peaks at ~ 615 nm and ~ 701 nm can be ascribed to crystal field effect. On the other hand, emission at 593 nm corresponds to 5D07F1 magnetic dipole transition and, if dominant in spectrum, is indicative of inversion symmetry for Eu3+ ion. Since red emission at 615 nm is invariably strong in the spectra, Eu3+ ions appear to be located away from the inversion symmetry site in NaLa0.95Eu0.05(WO4)2 crystallites. Nevertheless, there is gradual change occurring in local disposition of surrounding species as 593 nm emission building up progressively in samples depicting needle-, cuboidal- and rugby ballmorphology. Figure S9 of the supporting information



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displays the difference in Eu3+ ions peak emission intensity due to electric (5D07F2) and magnetic (5D07F1) dipole transitions. The change in Eu3+ ion emission arising from crystallites of different morphologies (presuming some variation in local environment) is evaluated with an asymmetry parameter, defined by red-to-orange luminescence intensity ratio, A21 = [ ∫I2 dλ2 /∫I1 dλ1]; λ2 and λ1 being 615 nm and 593 nm, respectively. The value of A21 deduced from integrating intensities turns out to be 9.7, 4.9, and 3.7 for samples having needle, cuboid, and rugby ball shape crystallites, respectively (Table S3). The high value of A21 in case of needles can be attributed to asymmetrical disposition of large number of Eu3+ ions lying on the surface possibly due to size effect (i.e., large surface area-to-volume ratio). Also, the relatively large intensity ratio of host emission band (range 400-550 nm) and red peak (615 nm) (Table S3) for needle and cuboidal crystallites vis-à-vis rugby ball shape sample suggests low energy transfer from NaLa(WO4)2 host to surface Eu3+ ions (evident as well from the excitation spectra discussed below). The change in ratio can, therefore, be taken as a measure of surface Eu3+ ion density, being high in needles and cuboidal morphology vis-à-vis bigger size rugby shape particles. Insets of Figure 7a show digital photographs displaying near white light emission induced by of 270 nm light irradiation in different samples. The CIE chromaticity coordinates for needle, cuboidal, and rugby shapes are found as (0.44, 0.25), (0.48, 0.26), (0.59, 0.32), respectively (Fig 7b). Figure S10 of the supporting information exhibits the emission spectra of three different morphologies of NaLa1-xEux(WO4)2 recorded using direct excitation of Eu3+ ions (ex = 395 nm). In case of rugby shape crystallite, the intensity of Eu3+ emission at ~615 nm is least in contrast to cuboidal and rugby shape particles. The difference in the emission intensities in all the three cases is related to the number of surface Eu3+ ions as discussed above.


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Figure 7c presents the excitation spectra of NaLa0.95Eu0.05(WO4)2 samples (having needles, cuboids and rugby ball morphology), as monitored with Eu3+ ion 615 nm emission. These reveal (i) intense and broad band in the wavelength range of 245-322 nm due to absorption by host and (ii) a series of sharp peaks centered around 363, 383, and 393 nm caused by 7F0  5D4, 5G3, and 5L6 transitions within 4f shell of Eu3+ ions, respectively. Moreover, the host excitation band is weak compared to 393 nm peak (7F0  5L6 transition Eu3+ ion) for needles and cuboids. These features suggest poor energy transfer from host to Eu3+ ions in needle and cuboidal morphologies vis-à-vis rugby balls shape due to higher density of emitter present near the surface.46 Figure 7d shows the decay curves for NaLa0.95Eu0.05(WO4)2 samples recorded by monitoring 615 nm emission of Eu3+ ions. These are well fitted with a bi-exponential function (expressed by equation SE1 of the supporting information) to give two different components of the lifetime values. The average luminescence lifetime of 5D0 state of Eu3+ ions is then deduced using equation SE2 of the supporting information, value being ~ 569, 698, and 783 µs for samples displaying needle, cuboid and rugby ball shape morphology, respectively. The low lifetime for needles is indicative of large population of Eu3+ ions present at the surface. Other surface defects, inhomogeneities, ligand, -OH groups serve as e-h recombination centres and cause luminescence quenching. These results demonstrate tuning of Eu3+ emission with morphology of NaLa0.95Eu0.05(WO4)2 host crystallites.

5. CONCLUSIONS The present study emphasizes that the choice of precursor plays a crucial role in determining the

morphology

of

nanoparticles.

This

has

been

demonstrated

by

synthesizing

NaLa0.95Eu0.05(WO4)2 nanophosphors with different morphologies such as needle, cuboidal, and 19 ACS Paragon Plus Environment


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20

rugby ball using chloride, nitrate, and carbonate/acetate precursor salts of lanthanides, respectively. The byproduct Na-salts of anions (i.e. chloride, nitrate, carbonate and acetate) of La-precursor interacts preferentially with surface La-sites of doped NaLa(WO4)2 and act as a growth hindering species. The competitive growth along (100) and (001) facets determines the final morphology of the nanoparticle. The binding selectivity of the growth hindering species towards (100) surface visa-vis (001) surface reduces in the order of chloride > nitrate > carbonate salts and the degree of anisotropic growth also reduces in the same order. Consequently, chloride salts yield needle morphology due to preferred growth of nanoparticles along [001] direction. A far more uniform growth in both the [100] and [001] directions is observed in case of nitrate salts resulting in cuboidal morphology. Further, in case of carbonate and acetate salts, the selectivity towards (100) facet is minimal. The produced morphologies exhibit UV sensitized emission with CIE chromaticity coordinates (0.44, 0.25), (0.48, 0.26), (0.59, 0.32) for needle, cuboidal and rugby ball morphologies, respectively. The observed variations in the luminescence characteristics of different morphologies of NaLa0.95Eu0.05(WO4)2 nanophosphors are related to the differences in the number of surface to bulk Eu3+ ions in each case.

 ASSOCIATED CONTENT Supporting Information S1: Synthesis scheme. S2: SEM images using mixture of chloride and nitrate precursor. S3: FFT Pattern. S4: Elemental Mapping. S5: FTIR spectra. S6: Top and side view of NaLa(WO4)2 surfaces. S7: Pre-normalized PL spectra (ex = 270 nm). S8: PL spectra of undoped sample (ex = 270 nm). S9: PL spectra for wavelength range 580-600 nm. S10: PL spectra for ex = 395 nm


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 AUTHOR INFORMATION Corresponding author *Email: [email protected], [email protected], [email protected] ORCID: Chandresh Kumar Rastogi: 0000-0003-4517-8622

Notes The authors declare no competing financial interest

 ACKNOWLEDGEMENTS Authors are grateful to Department of Science and Technology (DST/CHE/20090220), Govt. of India for providing funding for research. Thanks are due to DST for funding the Thematic Unit of Excellence. The authors are thankful to High Performance Computational Facility (HPC) at IIT Kanpur for providing the computational time.

 REFERENCES (1) Zhou, Y.; Yan, B.; He, X. H., Controlled Synthesis and Up/Down-Conversion Luminescence of SelfAssembled Hierarchical Architectures of Monoclinic AgRE(WO4)2:Ln3+ (RE = Y, La, Gd, Lu; Ln = Eu, Tb, Sm, Dy, Yb/Er, Yb/Tm). J. Mater. Chem. C 2014, 2, 848-855. (2) Huang, S.; Wang, D.; Li, C.; Wang, L.; Zhang, X.; Wan, Y.; Yang, P., Controllable Synthesis, Morphology Evolution and Luminescence Properties of NaLa(WO4)2 Microcrystals. CrystEngComm 2012, 14, 2235-2244. (3) Wang, Z.; Zhong, J.; Jiang, H.; Wang, J.; Liang, H., Controllable Synthesis of NaLu(WO4)2:Eu3+ Microcrystal and Luminescence Properties for LEDs. Cryst. Growth Des. 2014, 14, 3767-3773. (4) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A., Chemistry and Properties of Nanocrystals of Different Shapes. Chem. Rev. 2005, 105, 1025-1102. (5) Liu, Y.; Tu, D.; Zhu, H.; Chen, X., Lanthanide-Doped Luminescent Nanoprobes: Controlled Synthesis, Optical Spectroscopy, and Bioapplications. Chem. Soc. Rev. 2013, 42, 6924-6958. (6) Singh, S.; Tripathi, A.; Rastogi, C. K.; Sivakumar, S., White Light from Dispersible Lanthanide-Doped LaVO4 Core–Shell Nanoparticles. RSC Adv. 2012, 2, 12231-12236. 21 ACS Paragon Plus Environment


Page 22 of 33

22 (7) Rastogi, C. K.; Saha, S.; Sivakumar, S.; Pala, R. G. S.; Kumar, J., Kinetically Stabilized Aliovalent Europium-Doped Magnesium Oxide as a UV Sensitized Phosphor. Phys. Chem. Chem. Phys. 2015, 17, 4600-4608. (8) Rastogi, C. K.; Sharma, S. K.; Patel, A.; Parthasarathy, G.; Pala, R. G. S.; Kumar, J.; Sivakumar, S., Dopant Induced Stabilization of Metastable Zircon-Type Tetragonal LaVO4. J. Phys. Chem. C 2017, 121, 16501-16512. (9) Gangwar, P.; Pandey, M.; Sivakumar, S.; Pala, R. G. S.; Parthasarathy, G., Increased Loading of Eu3+ Ions in Monazite LaVO4 Nanocrystals via Pressure-Driven Phase Transitions. Cryst. Growth Des. 2013, 13, 2344-2349. (10) Silver, J.; Martinez-Rubio, M.; Ireland, T.; Fern, G.; Withnall, R., The Effect of Particle Morphology and Crystallite Size on the Upconversion Luminescence Properties of Erbium and Ytterbium Co-Doped Yttrium Oxide Phosphors. J. Phys. Chem. B 2001, 105, 948-953. (11) Na, H.; Woo, K.; Lim, K.; Jang, H. S., Rational Morphology Control of β-NaYF4:Yb,Er/Tm Upconversion Nanophosphors Using a Ligand, an Additive, and Lanthanide Doping. Nanoscale 2013, 5, 4242-4251. (12) Xu, Z.; Li, C.; Hou, Z.; Peng, C.; Lin, J., Morphological Control and Luminescence Properties of Lanthanide Orthovanadate LnVO4 (Ln= La To Lu) Nano-/Microcrystals via Hydrothermal Process. CrystEngComm 2011, 13, 474-482. (13) Rodriguez-Liviano, S.; Aparicio, F. J.; Rojas, T. C.; Hungría, A. B.; Chinchilla, L. E.; Ocaña, M., Microwave-Assisted Synthesis and Luminescence of Mesoporous RE-Doped YPO4 (RE= Eu, Ce, Tb, And Ce+ Tb) Nanophosphors with Lenticular Shape. Cryst. Growth Des. 2011, 12, 635-645. (14) Xue, N.; Fan, X.; Wang, Z.; Wang, M., Synthesis Process and the Luminescence Properties of Rare Earth Doped NaLa(WO4)2 Nanoparticles. J. Phys. Chem. Solids 2008, 69, 1891-1896. (15) Zhang, N.; Bu, W.; Xu, Y.; Jiang, D.; Shi, J., Self-Assembled Flowerlike Europium-Doped Lanthanide Molybdate Microarchitectures and Their Photoluminescence Properties. J. Phys. Chem. C 2007, 111, 50145019. (16) Gu, J.; Zhu, Y.; Li, H.; Zhang, X.; Qian, Y., Uniform Ln3+ (Eu3+, Tb3+) Doped NaLa(WO4)2 Nanocrystals: Synthesis, Characterization, and Optical Properties. J. Solid State Chem. 2010, 183, 497-503. (17) Liu, J.; Cano-Torres, J. M.; Cascales, C.; Esteban-Betegón, F.; Serrano, M. D.; Volkov, V.; Zaldo, C.; Rico, M.; Griebner, U.; Petrov, V., Growth and Continuous-Wave Laser Operation of Disordered Crystals of Yb3+:NaLa(WO4)2 and Yb3+:NaLa(MoO4)2. Phys. Status Solidi (a) 2005, 202, R29-R31. (18) Liu, L.; Zhuang, Z.; Xie, T.; Wang, Y. G.; Li, J.; Peng, Q.; Li, Y., Shape Control of Cdse Nanocrystals with Zinc Blende Structure. J. Am. Chem. Soc. 2009, 131, 16423-16429. (19) Li, P.; Zhao, X.; Jia, C.-J.; Sun, H.; Li, Y.; Sun, L.; Cheng, X.; Liu, L.; Fan, W., Mechanism Of Morphology Transformation of Tetragonal Phase LaVO4 Nanocrystals Controlled by Surface Chemistry: Experimental and Theoretical Insights. Cryst. Growth Des. 2012, 12, 5042-5050. (20) Peng, Z. A.; Peng, X., Mechanisms of the Shape Evolution of Cdse Nanocrystals. J. Am. Chem. Soc. 2001, 123, 1389-1395. (21) Deng, H.; Liu, C.; Yang, S.; Xiao, S.; Zhou, Z. K.; Wang, Q. Q., Additive-Mediated Splitting of Lanthanide Orthovanadate Nanocrystals in Water: Morphological Evolution from Rods to Sheaves and to Spherulites. Cryst. Growth Des. 2008, 8, 4432-4439. (22) Siegfried, M. J.; Choi, K.-S., Elucidating the Effect of Additives on the Growth and Stability of Cu2O Surfaces via Shape Transformation of Pre-Grown Crystals. J. Am. Chem. Soc. 2006, 128, 10356-10357. (23) Zhu, L.; Li, Q.; Liu, X.; Li, J.; Zhang, Y.; Meng, J.; Cao, X., Morphological Control and Luminescent Properties of CeF3 Nanocrystals. J. Phys. Chem. C 2007, 111, 5898-5903. (24) Wang, M.; Huang, Q. L.; Hong, J. M.; Chen, X. T.; Xue, Z. L., Controlled Synthesis and Characterization of Nanostructured EuF3 with Different Crystalline Phases and Morphologies. Cryst. Growth Des. 2006, 6, 2169-2173. (25) Filankembo, A.; Giorgio, S.; Lisiecki, I.; Pileni, M. P., Is the Anion the Major Parameter in the Shape Control of Nanocrystals?. J. Phys. Chem. B 2003, 107, 7492-7500. 22 ACS Paragon Plus Environment

Page 23 of 33 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


23 (26) Sayed, F. N.; Polshettiwar, V., Facile and Sustainable Synthesis of Shaped Iron Oxide Nanoparticles: Effect of Iron Precursor Salts on the Shapes of Iron Oxides. Sci. Rep. 2015, 5, 9733. (27) Su, Y.; Du, C.; Jia, Q.; Lv, L.; Liu, Q.; Wang, X., Self-Assembled Peanut-Like Ln3+ (Ln = Tb, Sm, Eu) Doped NaLa(WO4)2: Phase Control and its Relevance to Luminescence Modification. J. Nanosci. Nanotech. 2011, 11, 9855-9860. (28) Wang, M.; Jiang, G.; Tang, Y.; Shi, Y., LaF3 and LaF3:Ln3+ (Ln = Eu, Tb) Hierarchical Microstructures: Synthesis, Characterization and Photoluminescence. CrystEngComm 2013, 15, 1001-1006. (29) Li, C.; Quan, Z.; Yang, P.; Yang, J.; Lian, H.; Lin, J., Shape Controllable Synthesis and Upconversion Properties of NaYbF4/NaYbF4:Er3+ and YbF3/YbF3:Er3+ Microstructures. J. Mater. Chem. 2008, 18, 13531361. (30) Li, C.; Yang, J.; Quan, Z.; Yang, P.; Kong, D.; Lin, J., Different Microstructures of β-NaYF4 Fabricated by Hydrothermal Process: Effects of pH Values and Fluoride Sources. Chem. Mater. 2007, 19, 4933-4942. (31) Kim, H. B.; Jang, D. J., Precursor-Dependent Shape Variation of Wurtzite CdSe Crystals in a Microwave-Assisted Polyol Process. CrystEngComm 2012, 14, 6946-6951. (32) Li, X.; Shen, D.; Yang, J.; Yao, C.; Che, R.; Zhang, F.; Zhao, D., Successive Layer-by-Layer Strategy for Multi-Shell Epitaxial Growth: Shell Thickness and Doping Position Dependence in Upconverting Optical Properties. Chem. Mater. 2013, 25, 106-112. (33) Bhattacharya, D.; Saha, S.; Shrivastava, V. P.; Pala, R. G. S.; Sivakumar, S., Designing Coupled Quantum Dots with ZnS–CdSe Hybrid Structures for Enhancing Exciton Lifetime. J. Phys. Chem. C 2018, 122, 9198-9208. (34) Hendricks, M. P.; Campos, M. P.; Cleveland, G. T.; Jen-La Plante, I.; Owen, J. S., A Tunable Library of Substituted Thiourea Precursors to Metal Sulfide Nanocrystals. Science 2015, 348, 1226-1230. (35) Dutta, A.; Behera, R. K.; Pradhan, N., Solvent Polarity: How Does This Influence the Precursor Activation, Reaction Rate, Crystal Growth, and Doping in Perovskite Nanocrystals? ACS Energy Lett. 2019, 4, 926-932. (36) Bealing, C. R.; Baumgardner, W. J.; Choi, J. J.; Hanrath, T.; Hennig, R. G., Predicting Nanocrystal Shape Through Consideration of Surface-Ligand Interactions. ACS Nano 2012, 6, 2118-2127. (37) Blochl, P. E., Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953. (38) Kresse, G.; Furthmuller, J., Efficiency of Ab-initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15-50. (39) Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868. (40) Kresse, G.; Furthmuller, J., Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169. (41) Pala, R. G. S.; Metiu, H., The Structure And Energy of Oxygen Vacancy Formation in Clean and Doped, Very Thin Films Of ZnO. J. Phys. Chem. C 2007, 111, 12715-12722. (42) Zhang, S. B.; Wei, S. H., Surface Energy and the Common Dangling Bond Rule for Semiconductors. Phys. Rev. Lett. 2004, 92, 086102. (43) Bader, R. F. W., A Quantum Theory of Molecular Structure and its Applications. Chem. Rev. 1991, 91, 893. (44) Leekumjorn, S.; Gullapalli, S.; Wong, M. S., Understanding the Solvent Polarity Effects on SurfactantCapped Nanoparticles. J. Phys. Chem. B 2012, 116, 13063-13070. (45) Yin, Y.; Alivisatos, A. P., Colloidal Nanocrystal Synthesis and the Organic-Inorganic Interface. Nature 2005, 437, 664-670. (46) Sudarsan, V.; van Veggel, F. C. J. M.; Herring, R. A.; Raudsepp, M., Surface Eu3+ Ions Are Different Than "Bulk" Eu3+ Ions in Crystalline Doped LaF3 Nanoparticles. J. Mater. Chem. 2005, 15, 1332-1342.



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List of Figures

Figure 1. FESEM images of NaLa1-xEux(WO4)2 samples produced with (a) chloride (b) nitrate (c) acetate and (d) carbonate precursor salts of lanthanides


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Figure 2. TEM images of NaLa1-xEux(WO4)2 [x = 0.05] samples produced using (a) chloride (b) nitrate and (c) acetate precursor salts of lanthanides. (d) HRTEM images of rugby shape crystallite, and (e) Fourier transform (FT) electron diffraction pattern of rugby shape crystallite



(112)

26

NaLa1-xEux(WO4)2 [x=0.05]

Precursor (116) (215) (132) (107)

(213) (204) (220)

(121)

(200)

Morphology (004)

(101)

Chloride

Needle

Cuboidal

Nitrate

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

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Carbonate

Rugby ball Observed Calculated

Yobs-Ycal Bragg Position

Acetate

Rugby ball

NaLa(WO4)2

JCPDS PDF No- 01-0583

10

20

30

40

50

60

70

80

2 (degree) Figure 3. X-ray diffraction patterns of NaLa1-xEux(WO4)2 [x = 0.05] powder prepared with different lanthanide salts, Rietveld refinement in case of acetate precursor, and JCPDS data of NaLa(WO4)2 (File No: 01-0583) 26 ACS Paragon Plus Environment

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Figure 4. Schematic illustration of adsorption (ads)-desorption (des) kinetics of reactive monomer (R) and growth hindering species (H) occurring at various facets (e.g. fast growing f-plane and slow growing s-plane) of nanocrystal leading to differential growth. In the figure, 𝑟𝐴𝑐 ― 𝑏 notation signifies the rate of reaction of A species (i.e. R or H) on b plane (i.e. f- or s-plane) due to c process (i.e. ads. ―𝑓 ―𝑠 or des.). The conditions for growth are 𝑟𝑅𝑎𝑑𝑠 > 𝑟𝑅𝑑𝑒𝑠― 𝑓 and 𝑟𝑅𝑎𝑑𝑠 > 𝑟𝑅𝑑𝑒𝑠― 𝑠 while the conditions for ―𝑓 𝐻―𝑠 𝐻―𝑓 𝐻―𝑠 anisotropic growth are 𝑟𝐻 𝑎𝑑𝑠 < 𝑟𝑎𝑑𝑠 and 𝑟𝑑𝑒𝑠 > 𝑟𝑑𝑒𝑠 .



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Figure 5. (a) Top and (b) side views of chlorine passivated La-W-terminated (100) surfaces, (c) Top and (d) side views of nitrate passivated La-W-terminated (100) surfaces, (e) Top and (f) side views of carbonate passivated La-W-terminated (100) surfaces of NaLa(WO4)2. 28 ACS Paragon Plus Environment

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Figure 6. Schematic illustration showing growth of NaLa(WO4)2 crystal in presence of growth hindering species of (a) Chloride (b) Nitrate (c) carbonate salts resulting (d) needle (e) cuboidal and (f) rugby shape morphologies, respectively.



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Figure 7. (a) Photoluminescence emission spectra at λex = 270 nm (Insets show digital photographs displaying emission) (b) CIE coordinates of emissions (c) excitation spectra and (d) luminescence decay curves with em = 615 nm for different morphologies of NaLa1-xEux(WO4)2 [x = 0.05]. The emission spectra are normalized with respect to peak at 615 nm.


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Tables Table 1 Characteristics and morphologies of NaLa1-x Eux (WO4)2 (x = 0.05) crystallites produced with chloride, nitrate, and acetate/carbonate precursor salts of lanthanides (La, Eu) Parameter/ Property

Precursor salt and lanthanide source Chloride

Nitrate

Acetate/Carbonate

a < ao , c = co

a ~ ao , c ~ co

a > ao , c > co

Morphology and

Needle

Cuboidal

Rugby ball

size

Length ~ 200-300

Length~ 95-180

Length ~ 160 -380 nm

nm diameter ~ 8-10

nm Breadth ~ 40-

Middle Breadth ~ 70-150 nm

nm

80 nm

Enhanced along

Nearly uniform in

Sturdy along [001] and

[001] Suppressed

all directions

progressively weak along

Lattice parameters (tetragonal cell)

Crystal growth

along and

Ion size

Cl- 0.184 nm

(NO3)1- 0.179 nm

(CO3)2- 0.178 nm

NaLa(WO4)2 ►Tetragonal with ao = bo = 0.5349 nm co = 1.1628 nm, Z = 2, Space group I41/a (JCPDS file 079-1118); Ion size (Coordination): Na+ = 0.118 nm (8), La3+ = 0.116 nm (8), W6+ = 0.042 nm (4), O2- = 0.138 nm (4), Eu3+ = 0.1066 nm (8)



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Table 2. Interaction energy (eV) of different Na-salts of anions on cationic sites of (100) and oxygen-terminated (001) facets of NaLa(WO4)2 and their selectivity towards La-site of (100) surface w.r.t. O- terminated (001) surface Interaction energy (eV) (100) polar facet

ESelectivity = (001) facet

Adsorbents

𝑬𝟎𝟎𝟏 𝑶, 𝒂𝒅𝒔 𝑬𝟏𝟎𝟎 𝑳𝒂,𝒂𝒅𝒔

La-site

W-site

Na-site

O-Site

𝑬𝟏𝟎𝟎 𝑳𝒂,𝒂𝒅𝒔

𝑬𝟏𝟎𝟎 𝑾, 𝒂𝒅𝒔

𝑬𝟏𝟎𝟎 𝑵𝒂, 𝒂𝒅𝒔

𝑬𝟎𝟎𝟏 𝑶, 𝒂𝒅𝒔

Na2CO3

-1.5

-1.1

NP

-1.2

0.3

NaNO3

-2.7

-1.5

-0.1

-0.7

2.0

NaCl

-4.9

-3.3

-0.2

-1.4

3.5

NP=No attractive interaction is possible


(eV)

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For Table of Contents Use Only

All Precursors Are Not Equal: Morphology Control via Distinct PrecursorsFacet Interaction in Eu3+-doped NaLa(WO4)2 Chandresh Kumar Rastogi,Ϯ Sulay Saha,$ Vishal Kusuma,$ Raj Ganesh S. Pala,*,$ Jitendra Kumar*, and Sri Sivakumar*,Ϯ, $, £, €

Synopsis

Current work focusses on morphology control of NaLa0.95Eu0.05(WO4)2 by varying lanthanide precursor (chloride, nitrate, acetate/carbonate). DFT simulation suggests that changes in morphology are correlated with preferential development of (100) and (001) facets caused by differential adsorption of growth hindering species (Cl-, NO3-, CO32-). Prepared morphologies exhibit variations in photoluminescence characteristics which is attributed to differences in number of surface and bulk Eu3+ ions.

ACS Paragon Plus Environment

All Precursors Are Not Equal: Morphology Control via Distinct

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