Catalyzing cubic-to-hexagonal phase transition in NaYF4 via ligand

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Catalyzing cubic-to-hexagonal phase transition in NaYF4 via ligand enhanced surface ordering Sulay Saha, Raj Ganesh S Pala, and Sri Sivakumar Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00535 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 14, 2018

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Crystal Growth & Design

Catalyzing cubic-to-hexagonal phase transition in NaYF4 via ligand enhanced surface ordering Sulay Saha, Raj Ganesh S. Pala*†, Sri Sivakumar*†# †Department of Chemical Engineering, Indian Institute of Technology, Kanpur, 208016, India †Materials Science Programme, Indian Institute of Technology, Kanpur, 208016, India #

Centre for Environmental Science and Engineering, Indian Institute of Technology, Kanpur,

208016, India #

Centre for Nanoscience and Soft Nanotechnology, Indian Institute of Technology, Kanpur,

208016, India *Corresponding Authors: [email protected], [email protected]

1 ACS Paragon Plus Environment

Crystal Growth & Design 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

Abstract: The phase-pure hexagonal lanthanide-doped NaYF4 nanocrystals possesses enhanced luminescence properties due to its low phonon energy and chemical stability compared to its cubic phase counterpart. However, the phase-pure hexagonal NaYF4 nanoparticle forms only at high temperature (≥ 180 ̊C) and in the presence of certain ligands (e.g. oleic acid) whereas cubic phase forms at low reaction temperature. To this end, we report low temperature (~48 ̊C) synthesis of phase-pure hexagonal NaYF4 nanoparticle by converting cubic NaYF4 nanoparticle into hexagonal phase through surface oxidation of oleic acid (C-18) ligand to azelaic acid (C-9). The mechanism behind the transformation is investigated using density functional theory (DFT) based studies. DFT studies reveal that the phase transformation from cubichexagonal phase involves a disorderorder phase transition since all cationic sites are occupied by either Na or Y in cubic phase while hexagonal phase has more ordered structure with one Y having full occupancy at its cationic site. As carboxyl groups of adsorbents strongly adsorp on Y-terminated surfaces while having no interaction with Na-terminated surfaces of the cubic phase, the surfaceligand interaction promotes surface segregation of Y. This biasing of occupancy towards more ordered arrangement promotes the formation of more ordered phase and correspondingly reduces the energy barrier for transformation to the more ordered hexagonal phase. The surface coverage achieved by oleic acid is low due to its bulky long alkyl chains (C-18) imposing steric restriction that is removed on shortening of alkyl chain length. As a consequence, there is greater surfaceligand interaction leading to further surface segregation of Y and cationic ordering in cubic phase and making the phase transformation to hexagonal phase less energy intensive. In addition, oxidation of oleic acid makes nanoparticles hydrophilic making it suitable for biological applications.


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1. Introduction NaYF4 is one of the most commonly employed phosphor host matrix for lanthanide doping which has found wide application in variety of the fields such as bioprobes1, 2, flat-panel displays3, 4, solar-energy harvesting5, sensing6, 7, photocatalysis8-11 and theronaustic12, 13. It is one of the best matrices available for lanthanide doping since it possesses (1) exceptional quantum efficiency14, (2) high chemical stability with low hygroscopic property with good dispersibility in a wide range of solvents15, 16, (3) optical transparency over wide wavelength17, 18 and (4) low phonon energy (350 cm-1)19. For optical applications, hexagonal phase is preferred over the cubic phase since it gives better photoluminescence properties like upconversion, downconversion and Stoke’s shift luminescence upon doping with lanthanide ions with lower energy loss during energy transfer process due to presence of lower numbers of non-harmonic phonon modes in hexagonal phase14,

20-22

. However, low temperature synthesis strategies to prepare

NaYF4 yield a mixture of polymorphs with cubic phase predominating over optically efficient hexagonal phase23, and hence, the phase purification adopted should also necessarily be capable of retaining the nanostructure. The phase transformation from cubichexagonal involves a disorderorder phase transition. The cationic sites of cubic phase are disordered with equal probability of six-fold Na and Y atom for occupation (Fig. 1a). In hexagonal NaYF4, there are three types of cationic sites: 1) a nine-fold coordinated position occupied by Y3+, 2) nine-fold coordinated position occupied randomly by 1/2 Na+ and 1/2 Y3+ and 3) a six-fold coordinated position occupied by 1/2 Na+ (Fig. 1b)4. The phase transformation (cubichexagonal) involves lowering of randomness in selectivity of cationic positions as all the cationic position of cubic phase can be occupied by either Na+ and Y3+ whereas in hexagonal phase nine-fold coordinated position comprises only 3 ACS Paragon Plus Environment


Y3+. Hence the phase transformation from cubic to hexagonal polymorph can be interpreted as disorderorder transformation in terms of cationic site arrangement. Decrease of randomness in cationic positions in NaYF4 structure would lead to lowering of energy barrier of cubichexagonal polymorphic phase transitions. The prevalent strategies for phase transformation are either increasing reaction temperature or reaction duration24-28. However, during these processes, the crystals are susceptible to Ostwald ripening to yield larger crystals with broad size distribution and varied morphology. To overcome these issues, alternative synthetic procedures like self-focusing29, 30 and doping31-33 have been proposed. In self-focusing methodology, cubic phase nanocrystals gets transformed to hexagonal phase over a hexagonal phase seed29,

30

. Addition of dopants larger than Y atom yields hexagonal phase at lower

temperatures31, 32. However, addition of dopants may also alter the phonon energy at local sites thereby promoting the multi-phonon relaxation process and inter-dopant energy transfer. Among other methodologies to decrease the activation barrier includes tuning ligand to precursor ratio or varying ligands25,

34-36

hexagonal phase35,

36

. Increasing ligand concentration has found to yield greater amount of

. However increasing ligand concentration beyond a limit has found to

hinder the reaction kinetics thereby lowering the yield of hexagonal phase. In this context, we demonstrate the manipulation of surface-ligand interaction as an effective way of phase purification strategy to obtain sub-20 nm hexagonal phase nanoparticles. The proposed approach has been extended to phase transformation of pure cubic phase or mixed phase to pure hexagonal phase at low temperature (~48 oC). In the first step, suppression of Ostwald ripening is achieved by using a long-chained ligand (i.e. oleic acid) at the expense of low-ligand coverage. In this step, a mixture of cubic and hexagonal phase of NaYF4 is obtained whereas pure cubic phase is obtained upto 140 oC temperature. In the next step, long-chain 4 ACS Paragon Plus Environment

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ligand of oleic acid is cleaved by Lemiux-von Rudolf (LVR) reagent at an elevated temperature (>48 oC). The cleaving of ligand has two-fold effect on crystal phase transformation, (1) cleaved oleic ligand yields azelaic acid which imposes less steric hindrance and higher ligand coverage can be achievable and (2) the carboxylic group of azelaic acid imposes higher degree of ordering along the surface thereby reducing the energy barrier for cubichexagonal phase transformation. As a result, the phase transformation from cubichexagonal can be achieved at lower temperature (>48 oC). DFT studies indicate that carboxylic group favors Y-termination at the surface and does not adsorb at Na-terminated surface. Thus, increased ligand coverage induces less randomness in the cubic phase and transformation to the more ordered hexagonal phase would entail less activation energy. The results give an effective way of lowering the phase transformation energy for disorderorder phase transition by controlling the surface terminations through surface-ligand interactions. 2. Experimental and Computational Details: 2.1. Synthesis 2.1.1. Materials: Y(NO3)3, 4H2O salt [99% pure, (Sigma-Aldrich)], Eu(NO3)3, 5H2O salt [99% pure, Sigma-Aldrich], NaF (Fischer-Scientific) were used as precursors for NaYF4. As a solvent and ligands, oleic acid, ethanol, t-butyl alcohol, cyclohexane and deionized (DI) water were used. To prepare LVR reagent, NaIO4 and KMnO4 were used37. 2.1.2. Synthesis of hydrophobic and pure cubic or mixed phase of NaYF4:Eu3+ nanoparticles: NaOH (0.7 g), water (9 mL), ethanol (10 mL), and oleic acid (20 mL) were mixed under agitation to form a homogeneous solution. Subsequently, 1.0 M aqueous solution of NaF (2.5 mL) solution was added dropwise to the above solution (F-/Y3+=5:1). Then 0.6 mmol of rare-earth nitrate [1.2 mL, 0.5 mol Y (NO3)3, 4 H2O] and 5 mol% Eu(NO3)3, 5 H2O (21 mg)



aqueous solution was added under stirring conditions. The mixture was agitated for about 20 min, and then transferred to a 50 mL autoclave, sealed and treated at different temperature (180 ºC, 150 ºC and 130 ºC) temperature for 8 hrs. After 8 hrs, the system was allowed cool to room temperature. The product was washed with ethanol thrice and centrifuged in 10,000 rpm for 10 minutes. Pure powder sample was obtained by purifying the samples with ethanol several times to remove excess oleic acid, sodium oleate, and other remnants and drying the sample under vacuum for 4 hours. 2.1.3. Conversion of hydrophobic and pure cubic or mixed phase to hydrophilic and phase pure NaYF4:Eu3+ nanoparticles: A mixture of prepared sample (70mg), cyclohexane (70 mL), t-butanol (50mL), water (7.5 mL) and 5 wt % K2CO3 aqueous solutions (3.5 mL) was stirred at room temperature for about 20 min and then 15 mL of LVR reagent (5.7 mM KMnO4 and 0.105 M NaIO4 aqueous solution) was added dropwise38. The pH of the solution was maintained at 8. The resultant mixture was stirred at about 48 °C for over 48 h. The product was then isolated by centrifugation and washed with deionized water, acetone, and ethanol. Subsequently, the product was dispersed in HCl (25 mL) of pH 4-5, and the mixture was stirred for 30 min. At last, the oxidized product was obtained by centrifugation, washed with deionized water followed by drying under vacuum. 2.2. Material Characterization: The crystalline structures of the sample were examined by an X-ray diffraction apparatus (XPERT-PRO) with Cu Kα radiation (λ = 1.54 Å) operating at 45 kV and 40 mA. The XRD were taken of dried powdered samples. The photoluminescence measurements were done using a steady-state and time-resolved fluorescence spectrometer (Edinburgh instruments FLSP 920) attached with a red-sensitive Peltier element cooled Hamamatsu R928-P PMT detector. Steady-state emission spectra were recorded by exciting the


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samples with a steady-state Xe lamp. Decay curves were recorded by exciting the nanocrystals with a Nd: YAG laser, attached with an optical parametric oscillator (OPO) with an optical range of 210–2400 nm. The hydrophobic particles are dispersed in cyclohexane while the hydrophilic particles are dispersed in water while measuring the luminescence property. The effective luminescence lifetime of 5D0 level of Eu3+ ions has been estimated by recording the decay curve of 615 nm emission and performing double exponential fitting of the curve39. Transmission electron micrographs (TEM) were obtained from an FEI Technai G2 U-Twin (200 KeV) instrument. The hydrophobic particles were dispersed in cyclohexane while hydrophilic particles were dispersed in water, sonicated and then drop-cast in a 200-mesh size Cu-grid to transmission electron micrograph. To examine the surface-ligand interaction, FTIR (Perkin Elmer Spectrum Two Spectrophotometer) was being done. 2.3. Computational Methodology: All Density Functional Theoretical (DFT) simulations were performed using plane-augmented wave as implemented in Vienna Ab initio Simulation Package (VASP) software and the exchange-correlation contributions are treated according to Perdew, Burke and Ernzerhof (PBE)40-42. A kinetic energy cutoff of 400 eV was used for all calculations. For bulk calculation of a cubic polymorph, a supercell of 2x2x1 was prepared. Gamma sampling of Bruillion zone with the k-point mesh of 6x6x12 was used. Geometric optimizations of bulk lattice constant of cubic polymorph is found to be 5.44 Å which is close to the experimental lattice constant of 5.45 Å. A vacuum layer of ∼15 Å has been used for surface calculation and 4x4x1 k-point mesh was used during calculation. The (001) surface of cubic polymorphs was modelled by a periodic stack of polar slabs, containing three stoichiometric Na/Y-F layers with only bottom slab fixed. During the adsorption energy calculations, considering the length of the long carbon chain of oleic acid or azelaic acid and their sodium salt, acetate adsorbents were



used to reflect the carboxylate containing acids assuming the electronic effect of the long carbon chains could be summed by a single methyl group35. While considering the olefinic bond effect of oleic acid on the surface, an ethylene model molecule was used assuming the electronic effects of long carbon chain does not interact with the surface. The adsorption energy was determined by following equation, ‫ܧ‬௔ௗ௦௢௥௣௧௜௢௡ = ‫ܧ‬௦௟௔௕ା௔ௗ௦௢௥௕௔௧௘ − ‫ܧ‬௦௟௔௕ − ‫ܧ‬௔ௗ௦௢௥௕௔௧௘ where Eslab+adsorbate is the energy of the system comprising the crystal surface and adsorbate molecule, Eslab is the energy of the bare crystal surface and Eadsorbate is the energy of the isolated adsorbate. The electronic properties of the bare surface influence the adsorption site and ability of bonding with ligands. To understand the influence, Bader charge analysis was being also done43. 3. Results and Discussion In the present work, hydrophobic NaYF4 nanoparticles were synthesized using autoclave route in presence of oleic acid at various temperatures (130 °C-180 °C) to prepare pure cubic and mixture of cubic/hexagonal phases at different ratios. After preparation of oleic acid stabilized hydrophobic nanoparticles, the surface ligands were oxidized to azelaic acid by addition of LVR reagent at a temperature of 48 °C. Upon this oxidation, NaYF4 nanoparticles were converted into hydrophilic nanoparticles which can be easily dispersed in water. 3.1. Phase evolution with change in temperature and surface moiety: Fig. 2a shows the XRD pattern of the hydrophobic NaYF4 nanoparticles prepared at four different temperatures (130 ºC, 150 ºC and 180 ºC). At a reaction temperature of 130 ºC, all the diffraction peaks matches with the cubic phase (Fm-3m) of NaYF4, while no peaks of hexagonal phase (P-6) of NaYF4 are observed. This implies that kinetically stabilized disordered cubic phase forms over ordered


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hexagonal phase at lower temperature which is also corroborated in literature25. XRD pattern shows appearance of hexagonal phase from a temperature of 150 ºC. As the reaction temperature increases, hexagonal polymorph becomes a dominant phase. However, presence of small percentage of cubic polymorphs in the samples prepared at reaction temperature of 180 oC suggests that this transformation to hexagonal phase incomplete even at reaction temperature of 180 oC. Fig. 2b shows the XRD pattern of the hydrophilic particles by oxidizing the surface oleic acid ligand of hydrophobic particles. XRD pattern clearly suggests that all the hydrophilic particles possess pure hexagonal polymorph with complete disappearance of cubic phase of NaYF4. This suggests that phase transformation in the nanoparticles has occurred with the modification of ligands. The XRD peak width of hydrophilic particles is higher as compared to hydrophobic particles at same peak position, suggesting enhancement of crystallinity during phase transformation process. 3.2. Identification of surface ligands: The ligands on the surface of particles are identified by FTIR spectra as shown in Fig. 3. A small crest at 3007 cm-1 is found in the hydrophobic sample which is attributed to the –HC=CH– stretching vibration. This feature is absent in the spectrum of the hydrophilic sample suggesting cleaving of the –HC=CH– group. Both samples contain peaks a broad band around 3450 cm-1, two sharp peaks around at 2924 cm-1 and other peaks around 2854 cm-1, 1638 cm-1, 1564 cm-1, 1532 cm-1 and 1448 cm-1. The broad band around 3450 cm-1 corresponds to O-H stretching vibration. The sharp peaks around at 2924 and 2854 cm-1 correspond to asymmetric (Vasym) and symmetric (Vsym) stretching vibrations of methylene (CH2) in the long alkyl chain respectively. The bands around at 1564 cm-1 and 1448 cm-1 correspond to asymmetric (Vasym) and symmetric (Vsym) stretching vibrations of the -COOH



group respectively.

The bands corresponding to the -C=O stretching vibration of –COOH

groups are observed at 1638 cm-1 and 1532 cm-1. FTIR spectra clearly suggest formation of azelaic acid from oleic acid. The oleic acid ligand is attached to the surface of the nanoparticles through unsaturated carboxylic group and olefinic groups. The nanoparticles synthesized from the autoclave route show hydrophobicity since oleic acid has a –CH3 as terminal group which does not interact with either the surface or water while the other terminal group, –COOH interacts with the nanoparticles surface (see DFT results). During ligand modification oleic acid ligands on the surface of nanoparticles undergoes cleaving of olefinic bonds to convert into azelaic acids which has a two terminal polar –COOH groups. One of the -COOH groups is strongly bounded to surface of the nanoparticles through surface-ligand interaction while the other group renders the particle hydrophilic through its interaction with water. 3.3. Microstructural evolution with change in temperature and surface moiety: To investigate the size and morphological evolution of hydrophobic and hydrophilic nanoparticles at different reaction temperature, TEM has been performed. The hydrophobic nanoparticles prepared at 150 ºC and 130 ºC, have a spherical morphology with a size range of 10-15 nm (Fig. 4b-c). However with the increase in temperature, smaller nanoparticles begin to disappear with the appearance of long rod hexagonal prism like particles and subsequent increase in sizes of nanoparticles are observed (Fig. 4a). These hexagonal prism nanorods have high aspect ratio (L: D~10:1) with sharp edges and almost no overgrowth at the edge is observed. Hydrophilic particles which are prepared from hydrophobic particles of 130 ºC do not show much change in morphology and size (Fig. 4f). Hence, the demonstrated method is a good way to synthesize hexagonal phase at lower temperature. The hydrophilic samples, prepared from 180 ºC sample,


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consist of long nanorods having a hexagonal base (Fig.4d). But they do not have sharp edges as observed in hydrophobic samples. Also the small spherical nanoparticles have disappeared during this phase transformation. The spherical particle of cubic phase tends to grow over the rods of hexagonal phase to recrystallize as hexagonal phase which is well reported in literature29, 30

. The relative amount of cubic phase decreases with increasing temperature. Hence the loss of

sharp edges among the hexagonal nanorods is more pronounced in case of hydrophilic sample of 150 ᵒC as compared to 180 ᵒC. 3.4. Luminescence changes with phase evolution: To further study the phase transformation phenomenon in NaYF4, Eu3+ is used as an optical probe since (1) it substitutes Y3+ as it has the same charge and ionic radii and (2) upon doping, Eu3+ can be used to investigate the local structures and polymorph-sensitive luminescence properties that follow phase evolution. The magnetic dipole transition intensity is insensitive to its surrounding local structural environment. However the electric dipole transition intensity is hypersensitive to the local field symmetry around the probe atom and the ratio with magnetic to electric dipole transition intensity is significantly influenced by change in local environment around Eu3+ atom44, 45. It is well reported in the literature that 10% doping does not induce any kind of phase transformation in NaYF4 matrix31. Hence to study the phase transformation phenomena, the dopant concentration has been taken as 5%. Fig. 5 shows the comparison of the photoluminescence spectra of hydrophobic and hydrophilic particles at different temperatures. The strong emission bands appearing at ~ 592 nm and ~ 615 nm are associated with magnetic dipole induced transition (5D07F1) and electrical dipole induced transition (5D07F2), respectively. From the Table 2, it can be seen that in the pure hydrophobic cubic polymorph, the peak intensity ratio of 615 nm: 592 nm emissions which



is also known asymmetry ratio, is almost unity while it is the maximum in the pure hexagonal polymorph. It is also observed that with increase in the reaction temperature, the asymmetry ratio increases. This may be attributed to increase in asymmetric environment surrounding the Eu3+ atom in hexagonal phase causing electric dipole induced transition to dominate i.e. more cubic nanocrystals are getting phase transformed into hexagonal nanoparticles. The lifetime of hydrophobic particles 5D0 level increases from 1.92 ms in pure cubic polymorph 3-5.96 ms in pure hexagonal polymorph. The variance of lifetimes in different hydrophilic hexagonal samples are attributed to size of the nanoparticles. For all hydrophobic NaYF4 samples, the asymmetry ratio has increased from its hydrophobic counterpart indicating the increase of asymmetric environment surrounding the probe atom which can be correlated with an increase of hexagonal polymorph among the nanoparticles. However the lifetime of Eu3+ emission in hydrophilic nanoparticles reduce from their parent hydrophobic particle. The reduced lifetime of hydrophilic particle is ascribed to quenching of photoluminescence by the water molecules46. 3.5. Role of ligands during phase transformation: Density Functional Theory (DFT) studies are performed to study the effect of ligands on phase transformation phenomena. In this context, it is to be noted that FTIR studies show that oleic acid get adsorbed onto the surface of nanoparticles as already discussed in previous section. Oleic acid can interact with the surface via carboxylate or olefinic groups. The adsorption energies of acetate adsorbents with different terminations of (001) facet of NaYF4 are listed in Table 3. Computational studies indicate that both carboxylate and olefinic group of oleate ion can interact with the surface of the nanoparticles. The mode of the interaction of the acetate can be monodentate (single O atom for adsorption) or bidentate (both O atoms binds for adsorption) as shown in Fig. 6(d-g). Though both monodentate and bidentate adsorption is possible, the bidentate adsorption is energetically


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more favorable as compared to monodentate as can be observed from Table 3. This might be due to their steric structure of acid during surface interaction. The C–C bonds rather than –C=O bonds of adsorbate align themselves perpendicular to the surface to ensure that the two O atoms of ligand can interact as much as with cations of the surface. The interaction with acetate and surface is also dependent on the surface terminations. The carboxylate group does not interact with surface Na atoms as the interactions with them are energetically unfavorable. However they can easily adsorp with Y-terminated surfaces. It is found from Bader charge analysis that electron deficient regions are centered on Y-atom rather than Na-atom on surfaces. The formal charge over Y-atom on the (001)-surface is 1.5e lower than corresponding surface Na-(001) atom in (001) surface of cubic phase. Hence electron donor ligand like oleate ion tend to adsorp into Y-terminated surfaces. These results indicate that Y-terminated surfaces are preferred by the ligands containing a –COOH group. The surface segregation of Y3+ leads to greater ionic interaction between surface Y-F bonds in absence of ligands as exemplified by decrease of bonddistance of surface Y-F. The surface Y-F bond distance is 2.18 A° in absence of ligands as opposed to 2.20 A° in presence of ligands. It is also observed that surface coverage by acetate cannot exceed 0.75 monolayer due to steric considerations as two adjacent adsorbents in neighboring sites start to interact with each other. It is noted that the steric interaction arising from oleate (C-18) would be far higher than the computed acetate (C-2) leading to lower surface coverage. Similar types of preferential adsorption of ligands on Y-sites over Na-sites are also observed for (110) surfaces (see section S1 of ESI). In addition to hydrogen passivation of the anionic terminations, olefinic bond of oleic acid or oleate tend to interact with F-terminations of polar (001) surfaces of cubic phase. This is observed on adsorbing model ethylene adsorbate with F-terminated (001) surfaces of cubic surfaces (Fig. 7). The strong interaction between C



atom of ethylene molecule and F-termination of (001) surface lowers the surface energies of cubic phase (adsorption energy=-2.5 eV). During oleate-mediated synthesis of NaYF4 in presence of NaOH, oleic acid generates oleate ion which gets adsorped on surfaces during the growth stage as evident from FTIR studies. As per computational studies, oleate would favorably adsorp to the places where Y-terminated surfaces are present due to strong interaction between Y-carboxylate while rejecting Naterminated surface which in turn induce Y-atoms to surface segregate. Hence, to adsorp oleate and lower its surface energy, the surface of cubic polymorph has to undergo cationic reorientation to form stable surface-adsorbate configurations. To see the effect of neighboring surface cationic position on adsorption of acetate on Y-site, two cases are considered where the neighboring surface position is Na in one case (mixed surface termination) (Fig. 6c) and in another case it is Y-site (single surface termination) (Fig. 6a). It is found that acetate-Y interaction is more stable (by -2.8 eV) in Y-terminated surfaces than mixed surface terminations. This indicates the probability of stabilizing a random cationic terminated surface gets reduced in the adsorbate environment due to preference of ligands for specific surface termination. Thus, oleate ligand promotes cationic rearrangement in cubic phase with surface segregation of Yatoms while Na-atoms get clustered in the bulk sites. This leads to reduction in the activation energy of cubic to hexagonal phase transformation pathway as lowering of randomness Na/Y atoms reduces the stability of cubic phase (see section S2 of ESI). This is also supported by the experimental observations that excess of oleate favors the formation of hexagonal polymorph25, 35

. The achievable ligand coverage is less in hydrophobic particles due to the presence of long

chain alkyl group present in the oleic acid. The steric interaction of alkyl chain of the adsorped ligand in the surface prevents particle-particle interaction as well as surface-ligand interaction.


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This entails lowering of rate of Ostwald ripening enforced growth kinetics. The olefinic bond of oleic acid stabilizes F-terminated surface through interaction of π-bonds. However on cleaving of olefinic bond to carboxylate reduces the steric interaction of alkyl chains of the ligands. Oxidation of oleic acid to azelaic acid has twin effect on the growth kinetics; (1) stability of Fterminated surface gets reduced exposing it towards other chemical species in the solution and (2) more adsorption of species due to reduced alkyl chain steric interaction. The consequence is two-fold: ripening of these facets of cubic polymorph surface or adsorption of more ligands to induce cation rearrangement by increase surface selectivity thereby lowering the activation energy for cubic to hexagonal phase transformation. The particle-particle interaction/ripening effect is observed from TEM image that sharp-edged hydrophobic particle has gone through irregular growth on edges with the disappearance of smaller particle. However, NaYF4 did not form in presence of azelaic acid ligand at 48 °c temperature possibly due to the fact that initial transition barrier of formation of NaYF4 from precursors are much higher than transition barrier of cubic hexagonal NaYF4 transformation. The increase in ligand coverage decreases the activation barrier such that phase transformation is achieved at far lower (48 °C) temperature. 4. Conclusion In summary, we have demonstrated an approach towards reducing activation barrier for phase transformation of disordered cubic phase ordered hexagonal phase through modification of the ligand adsorbed on surface. The uniqueness of this method is that the phase transformation is carried out at a temperature as low as 48 ºC as opposed to more than 180 ºC temperature in conventional synthesis procedure. The lowering of activation barrier is due to increase in adsorbent coverage of cation-terminated surfaces where Y-terminated surfaces are favored over Na-terminated surfaces due to adsorbent-surface interaction leading to lowering of randomness



in occupancy of cationic position by Na and Y atom. The cleavage of bulky oleic acid during ligand oxidation lowers the steric hindrance thereby increasing the adsorbent coverage along the surface. The cubichexagonal phase transformation of NaYF4 nanoparticle has insignificant effects on the size except some overgrowth along the sharp edges of the nanoparticles as observable from TEM images. The enrichment of hexagonal phase is observed in mixture with an increase of luminescence intensity of Eu3+ dopant. The particles are also converted into hydrophilic particle making them more suitable for other applications such as bio-labels or photodynamic therapy. Supporting Information Comparison of bulk energies upon clusterization of Na- and Y-atoms in cubic NaYF4 phase and adsorption energies of acetate on (110) surface of cubic NaYF4 phase are provided in the supporting information. Acknowledgement We are grateful to Department of Science and Technology (DST/CHE/20090220 and DST/TSG/SH/2011/106), Govt. of India for providing the funding for research. Additional thanks to Deb Sankar De for discussion regarding computational aspect of the work. References (1)

Cheng, Z.; Chai, R.; Ma, P.; Dai, Y.; Kang, X.; Lian, H.; Hou, Z.; Li, C.; Lin, J.,

Multiwalled Carbon Nanotubes and NaYF4:Yb3+/Er3+ Nanoparticle-Doped Bilayer Hydrogel for Concurrent NIR-Triggered Drug Release and Up-Conversion Luminescence Tagging. Langmuir 2013, 29, 9573-9580.


Page 16 of 33

Page 17 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


(2)

Wang, M.; Mi, C.-C.; Wang, W.-X.; Liu, C.-H.; Wu, Y.-F.; Xu, Z.-R.; Mao, C.-B.; Xu,

S.-K., Immunolabeling and NIR-excited fluorescent imaging of HeLa cells by using NaYF4: Yb, Er upconversion nanoparticles. ACS Nano 2009, 3, 1580. (3)

Deng, R.; Qin, F.; Chen, R.; Huang, W.; Hong, M.; Liu, X., Temporal full-colour tuning

through non-steady-state upconversion. Nat. Nanotechnol. 2015, 10, 237-242. (4)

Yang, J.; Song, L.; Wang, X.; Dong, J.; Gan, S.; Zou, L., Facile synthesis and color-

tunable properties of monodisperse b- NaYF4:Ln3+ (Ln = Eu, Tb, Tm, Sm, Ho) microtubes. Dalton Trans. 2018, 47, 1294-1302. (5)

Ramasamy, P.; Kim, J., Combined plasmonic and upconversion rear reflectors for

efficient dye-sensitized solar cells. Chem. Comm. 2014, 50, 879-881. (6)

Rodríguez‐Sevilla, P.; Zhang, Y.; Haro‐González, P.; Sanz‐Rodríguez, F.; Jaque, F.;

Solé, J. G.; Liu, X.; Jaque, D., Thermal Scanning at the Cellular Level by an Optically Trapped Upconverting Fluorescent Particle. Adv. Mater. 2016, 28, 2421-2426. (7)

Wang, J.; Wei, T.; Li, X.; Zhang, B.; Wang, J.; Huang, C.; Yuan, Q., Near-Infrared-

Light-Mediated Imaging of Latent Fingerprints based on Molecular Recognition. Angew. Chem. 2014, 126, 1642-1646. (8)

Bai, L.; Jiang, W.; Gao, C.; Zhong, S.; Zhao, L.; Li, Z.; Bai, S., Facet engineered

interface design of NaYF4: Yb, Tm upconversion nanocrystals on BiOCl nanoplates for enhanced near-infrared photocatalysis. Nanoscale 2016, 8, 19014-19024. (9)

Tou, M.; Mei, Y.; Bai, S.; Luo, Z.; Zhang, Y.; Li, Z., Depositing CdS nanoclusters on

carbon-modified NaYF4: Yb, Tm upconversion nanocrystals for NIR-light enhanced photocatalysis. Nanoscale 2016, 8, 553-562.



(10)

Tang, Y.; Di, W.; Zhai, X.; Yang, R.; Qin, W., NIR-responsive photocatalytic activity

and mechanism of NaYF4: Yb, Tm@ TiO2 core–shell nanoparticles. ACS Catal. 2013, 3, 405412. (11)

Qin, W.; Zhang, D.; Zhao, D.; Wang, L.; Zheng, K., Near-infrared photocatalysis based

on YF3: Yb3+, Tm3+/TiO2 core/shell nanoparticles. Chem. Comm. 2010, 46, 2304-2306. (12)

Wang, C.; Cheng, L.; Liu, Z., Upconversion nanoparticles for photodynamic therapy and

other cancer therapeutics. Theranostics 2013, 3, 317-330. (13)

Rammohan, A.; Mishra, G.; Mahaling, B.; Tayal, L.; Mukhopadhyay, A.; Gambhir, S.;

Sharma, A.; Sivakumar, S., PEGylated Carbon Nanocapsule: A Universal Reactor and Carrier for In Vivo Delivery of Hydrophobic and Hydrophilic Nanoparticles. ACS Appl. Mater. Interfac. 2015, 8, 350-362. (14)

Boyer, J.-C.; van Veggel, F. C. J. M., Absolute quantum yield measurements of colloidal

NaYF4: Er3+, Yb3+ upconverting nanoparticles. Nanoscale 2010, 2, 1417-1419. (15)

Wilhelm, S.; Kaiser, M.; Würth, C.; Heiland, J.; Carrillo-Carrion, C.; Muhr, V.;

Wolfbeis, O. S.; Parak, W. J.; Resch-Genger, U.; Hirsch, T., Water dispersible upconverting nanoparticles: effects of surface modification on their luminescence and colloidal stability. Nanoscale 2015, 7, 1403-1410. (16)

Heer, S.; Kömpe, K.; Güdel, H. U.; Haase, M., Highly Efficient Multicolour

Upconversion Emission in Transparent Colloids of Lanthanide‐Doped NaYF4 Nanocrystals. Adv. Mater. 2004, 16, 2102-2105. (17)

Chan, E. M.; Han, G.; Goldberg, J. D.; Gargas, D. J.; Ostrowski, A. D.; Schuck, P. J.;

Cohen, B. E.; Milliron, D. J., Combinatorial discovery of lanthanide-doped nanocrystals with spectrally pure upconverted emission. Nano Lett. 2012, 12, 3839-3845.


Page 18 of 33

Page 19 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


(18)

Meruga, J. M.; Baride, A.; Cross, W.; Kellar, J. J.; May, P. S., Red-green-blue printing

using luminescence-upconversion inks. J. Mater. Chem. C 2014, 2, 2221-2227. (19)

Kramer, K. W.; Biner, D.; Frei, G.; Gudel, H. U.; Hehlen, M. P.; Lathi, S. R., Hexagonal

sodium yttrium fluoride based green and blue emitting upconversion phosphors. Chem. Mater. 2004, 16, 1244. (20)

Boyer, J.-C.; Cuccia, L. A.; Capobianco, J. A., Synthesis of Colloidal Upconverting

NaYF4:Er3+/Yb3+ and Tm3+/Yb3+ Monodisperse Nanocrystals. Nano Lett. 2007, 7, 847. (21)

Yi, G. S.; Chow, G. M., Synthesis of Hexagonal-Phase NaYF4:Yb,Er and NaYF4:Yb,Tm

Nanocrystals with Efficient Up-Conversion Fluorescence. Adv. Func. Mater. 2006, 16, 23242329. (22)

Klier, D. T.; Kumke, M. U., Analysing the effect of the crystal structure on upconversion

luminescence in Yb3+,Er3+-co-doped NaYF4 nanomaterials. J. Mater. Chem. C 2015, 3, 1122811238. (23)

Eliseeva, S. V.; Banzli, J.-C. G., Lanthanide luminescence for functional materials and

bio-sciences. Chem. Soc. Rev. 2010, 39, 189. (24)

Ostrowski, A. D.; Chan, E. M.; Gargas, D. J.; Katz, E. M.; Han, G.; Schuck, P. J.;

Milliron, D. J.; Cohen, B. E., Controlled Synthesis and Single-Particle Imaging of Bright, Sub-10 nm Lanthanide-Doped Upconverting Nanocrystals. ACS Nano 2012, 6, 2686-2692. (25)

Zhang, F.; Li, J.; Shan, J.; Xu, L.; Zhao, D., Shape, Size, and Phase-Controlled Rare-

Earth Fluoride Nanocrystals with Optical Up-Conversion Properties. Chem.-Eur. J. 2009, 15, 11010-11019. (26)

Liang, X.; Wang, X.; Zhuang, J.; Peng, Q.; Li, Y., Synthesis of NaYF4 Nanocrystals with

Predictable Phase and Shape. Adv. Func. Mater. 2007, 17, 2757-2765.



(27)

Mai, H.-X.; Zhang, Y.-W.; Si, R.; Yan, Z.-G.; Sun, L.-d.; You, L.-P.; Yan, C.-H., High-

Quality Sodium Rare-Earth Fluoride Nanocrystals: Controlled Synthesis and Optical Properties. J. Am. Chem. Soc. 2006, 128, 6426-6436. (28)

Schafer, H.; Ptacek, P.; Eickmeier, H.; Haase, M., Synthesis of Hexagonal Yb3+, Er3+

Doped NaYF4 Nanocrystals at Low Temperature. Adv. Func. Mater. 2009, 19, 3091-3097. (29)

Johnson, N. J. J.; Korinek, A.; Dong, C.; van Veggel, F. C. J. M., Self-focusing by

Ostwald ripening: a strategy for layer-by-layer epitaxial growth on upconverting nanocrystals. J. Am. Chem. Soc. 2012, 134, 11068-11071. (30)

Rinkel, T.; Nordmann, J.; Raj, A. N.; Haase, M., Ostwald-ripening and particle size

focussing of sub-10 nm NaYF4 upconversion nanocrystals. Nanoscale 2014, 6, 14523-14530. (31)

Wang, F.; Han, Y.; Lim, C. S.; Lu, Y.; Wang, J.; Xu, J.; Chen, H.; Zhang, C.; Hong, M.;

Liu, X., Simultaneous phase and size control of upconversion nanocrystals through lanthanide doping. Nature 2010, 463, 1061-1065. (32)

Chen, D.; Huang, P.; Yu, Y.; Huang, F.; Yang, A.; Wang, Y., Dopant-induced phase

transition: a new strategy of synthesizing hexagonal upconversion NaYF4 at low temperature. Chem. Comm. 2011, 47, 5801-5803. (33)

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. (34)

Sarkar, D.; Meesaragandla, B.; Samanta, T.; Mahalingam, V., A Greener Approach

towards Making Highly Luminescent Ln3+-Doped NaYF4 Nanoparticles with Ligand-Assisted Phase Control. ChemistrySelect 2016, 1, 4785-4793.


Page 20 of 33

Page 21 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


(35)

Sui, Y.; Tao, K.; Tian, Q.; Sun, K., Interaction Between Y3+ and Oleate Ions for the

Cubic-to-Hexagonal Phase Transformation of NaYF4 Nanocrystals. J. Phys. Chem. C 2012, 116, 1732-1739. (36)

Li, D.; Shao, Q.; Dong, Y.; Jiang, J., A facile synthesis of small-sized and monodisperse

hexagonal NaYF4:Yb3+,Er3+ nanocrystals. Chem. Comm. 2014, 50, 15316-15318. (37)

Lemieux, R. U.; Rudlo, E. V., Periodate-permangante oxidations of olefins. Can. J.

Chem. 1956, 33, 1701-1709. (38)

Chen, Z.; Chen, H.; Hu, H.; Yu, M.; Li, F.; Zhang, Q.; Zhou, Z.; Yi, T.; Huang, C.,

Versatile Synthesis Strategy for Carboxylic Acid-functionalized Upconverting Nanophosphors as Biological Labels. J. Am. Chem. Soc. 2008, 130, 3023-3029. (39)

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. (40)

Kresse, G.; Joubert, D., From ultrasoft pseudopotentials to the projector augmented-wave

method. Phys. Rev. B 1999, 59, 1758-1775. (41)

Kresse, G.; Furthmuller, J., Efficient iterative schemes for ab initio total-energy

calculations using a plane-wave basis set. Phys. Rev. B 1996, 54, 11169-11186. (42)

Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized gradient approximation made

simple. Phys. Rev. Lett. 1996, 77, 3865-3868. (43)

Sanville, E.; Kenny, S. D.; Smith, R.; Henkelman, G., Improved grid-based algorithm for

Bader charge allocation. J. Comput. Chem 2007, 28, 899-908.



(44)

Page 22 of 33

Yang, L.; Li, L.; Zhao, M.; Li, G., Size-induced variations in bulk/surface structures and

their impact on photoluminescence properties of GdVO4: Eu3+ nanoparticles. Phys. Chem. Chem. Phys. 2012, 14, 9956-9965. (45)

Werts, M. H. V.; Jukes, R. T. F.; Verhoeven, J. W., The emission spectrum and the

radiative lifetime of Eu3+ in luminescent lanthanide complexes. Phys. Chem. Chem. Phys. 2002, 4, 1542-1548. (46)

Blasse, G.; Sabbatini, N., The quenching of rare-earth ion luminescence in molecular and

non-molecular solids. Mater. Chem. Phys. 1987, 16, (3-4), 237-252.

Table 1. Structural data of cubic and hexagonal NaYF4 polymorphs Cubic (Fm-3m) a=5.45Aº Atom

Occupancy

Wycoff position

Na or Y

0.5 of Na or 0.5 of Y

4a (0,0,0)

F

1.0 of F

8c (0.25, 0.25,0.25)

Hexagonal (P-6): a=5.92Aº, c=3.54Aº Atom

Occupancy

Wycoff position

Na (1) or Y (1)

0.5 of Na or 0.5 of Y

1a(0,0,0)

Na (2) or vacant site

0.5 of Na or 0.5 of vacant site

2h(2/3, 1/3,0.156)

Y (2)

1.0 of Y

1f (1/3, 2/3,0.5)

F (1)

1.0 of F

3j (0.290,0.257,0.5)


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F (2)

1.0 of F


3k (0.067,0.422,0.00)


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Table 2. The effective luminescence lifetime of 5D0 level of Eu3+ ions (615 nm) and asymmetry ratio in hydrophobic and hydrophilic NaYF4 nanoparticles. Reaction temperature(°C)

Surface moiety

Lifetime (ms)

Asymmetry ratio

180ºC

Hydrophobic

5.96

1.56

Hydrophilic

3.28

1.69

Hydrophobic

3.08

1.08

Hydrophilic

3.26

1.25

Hydrophobic

1.92

1.01

Hydrophilic

1.94

1.46

150ºC

130ºC


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Table 3. Adsorption energies of acetate on (001)-facet of cubic phase Mode of

Y site of (Y+Y)-

Na-site of Na-

Y-site of (Na+Y)-

adsorption

clustered (001) facet



Bidentate

-6.5 eV

Not possible

-3.7 eV

-1.6 eV

Not possible

-1.9 eV

adsorption Monodentate adsorption



Figure 1: (a) Single cell of cubic polymorph and (2) 2x2x1 supercell of hexagonal polymorph of NaYF4.


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Figure 2. XRD pattern of (a) the hydrophobic NaYF4 nanoparticles that were synthesized at 180 °C, 150 °C and 130 °C temperature respectively, (b) the hydrophilic nanoparticles of NaYF4 after oxidation of hydrophobic NaYF4 nanoparticles that were synthesized at 180 °C, 150 °C and 130 °C temperature. 27 ACS Paragon Plus Environment


Figure 3. FTIR of hydrophobic and hydrophilic NaYF4 nanoparticles before and after ligand modification respectively.


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Figure 4. TEM images of the hydrophobic NaYF4 product with changing reaction temperature .Reaction temperatures were (a) 180 °C, (b) 150 °C and (c) 130 °C respectively. TEM images of the hydrophilic NaYF4 product after oxidation of hydrophobic NaYF4 particles that were produced at (d) 180 °C, (e) 150 °C, and (f) 130 °C.



Figure 5. Emission spectra of the hydrophilic and hydrophobic NaYF4 nanoparticles with changing reaction temperature. Reaction temperatures was (a) 180 ºC, (b) 150 ºC and (c) 130 ºC respectively. The emission spectra are normalized with respect to 592 nm emission (λex = 395 nm).


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Figure 6. (a-c) Different probable surface slab configurations of (001) facet of cubic NaYF4 are depicted : Top-view of (a) Y-clustered surface termination, (b) Na-clustered surface termination, (c) No clustering of individual atoms at the surface with probability of both atoms at the surface. (d) Top-view of bidentate acetate adsorption on Y-clustered surfaces of (001) facet of cubic phase. The subsurface Na atoms are visible in this figure. (e) Side-view of bidentate acetate adsorption on Y-clustered surfaces of (001) facet of cubic phase. (f) Top-view of monodentate acetate adsorption on Y-clustered surfaces of (001) facet of cubic phase. The subsurface Na atoms are visible in this figure. (g) Side-view of monodentate acetate adsorption on Y-clustered surfaces of (001) facet of cubic phase.



Figure 7. Ethylene adsorption on (001) facet of cubic phase: (a) top view, (b) side view.


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For Table of Contents Use Only: Catalyzing cubic-to-hexagonal phase transition in NaYF4 via ligand enhanced surface ordering Sulay Saha, Raj Ganesh S. Pala, Sri Sivakumar TOC Graphic:

Synopsis: A low temperature phase transformation strategy aimed towards producing phase-pure hexagonal NaYF4 from mixed phase/ phase-pure cubic NaYF4 is demonstrated. Instead of synthesizing phase-pure hexagonal NaYF4 at 180 ̊C through solvothermal/hydrothermal synthesis methodology from precursors, a lower phase transformation temperature of 48 ̊C is achieved through increased ligands/adsorbents surface coverage by ligand modification which increases Na/Y clusterization, thus reducing the stability of cubic phase.


Catalyzing cubic-to-hexagonal phase transition in NaYF4 via ligand

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