# Incorporation of Upconverting LiYF4:Yb3+, Tm3+ Nanoparticles with High Quantum Yield in TiO2 Metallogels for Near Infrared-Driven Photocatalytic Dye Degradation

B. Purohit, E. Jeanneau, Y. Guyot, D. Amans, B. Mahler, M. Joubert, C.

Dujardin, G. Ledoux, S. Mishra

<figure><img src="image_1.png" /></figure>

## ► To cite this version:

B. Purohit, E. Jeanneau, Y. Guyot, D. Amans, B. Mahler, et al., Incorporation of Upconverting LiYF4:Yb3+, Tm3+ Nanoparticles with High Quantum Yield in TiO2 Metallogels for Near Infrared-Driven Photocatalytic Dye Degradation. ACS Applied Nano Materials, 2023, 6 (4), pp.2310. 10.1021/acsnm.2c04240. [https://doi.org/10.1021/acsnm.2c04240](https://doi.org/10.1021/acsnm.2c04240)

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Submitted on 22 Nov 2023

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<figure><img src="image_2.png" /></figure>
# Incorporation of Upconverting LiYF₄:Yb³+, Tm³+ Nanoparticles with High Quantum Yield in TiO₂ Metallogels for NIR-Driven Photocatalytic Dye Degradation

Bhagyesh Purohit,†,# Erwann Jeanneau,‡ Yannick Guyot,‡ David Amans,‡ Benoit Mahler,‡ Marie-France Joubert,‡ Christophe Dujardin,‡ Gilles Ledoux,‡ Shashank Mishra*,†

†Université Claude Bernard Lyon 1, CNRS, UMR 5256, Institut de Recherches sur la Catalyse et l'Environnement de Lyon (IRCELYON), 2 avenue Albert Einstein, 69626 Villeurbanne, France.

‡Université Claude Bernard Lyon 1, CNRS, UMR5306 Institut Lumière Matière, 69622, Villeurbanne, France.

‡Université Claude Bernard Lyon 1, Centre de Diffractométrie Henri Longchambon, 5 rue de La Doua, 69100 Villeurbanne, France.

**KEYWORDS:** anhydrous precursors • X-ray structure • nanoparticles • upconversion • quantum yield • NPs-embedded gel.

**ABSTRACT:** By using anhydrous conditions, we report here an optimized synthesis of small but highly emitting upconverting nanoparticles (UCNPs) of LiYF₄:Yb³+, Tm³+ UCNPs without any
surface passivation layers of silica or undoped matrix. For this, anhydrous precursors
[Li(TFA)(monoglyme)] and [Ln(TFA)₃(monoglyme)] (Ln = Y, Gd, Tm, Yb; TFA =
trifluoroacetate) were synthesized *via* a facile one-pot synthetic protocol, which showed
excellent thermal decomposition compatibility to afford Ln³⁺-doped LiLnF₄ NPs by
simultaneous decomposition in solution phase. A detailed study of the influence of many key
parameters such as the ratio of dopant ions and solvents, concentration of precursors and reaction
time and temperature over crystalline phase, size, shape, morphology and upconverting
properties of the NPs produced, followed by a mechanism for the formation of LiYF₄-based NPs
is presented. The high quality of these NPs is manifested by the fact that their upconversion
efficiency is quite comparable to that of a bulk single crystal of the same composition (bulk
materials are believed to have far better UC performance due to low surface area and less
defects). Successful incorporation of these UCNPs in transparent TiO₂ metallogel provided an
alternative strategy of highly homogeneous UCNPs-TiO₂ composites of variable composition (1-
10% of NPs) having very high specific surface area (435-580 m²/g).

Introduction

The lanthanide-based upconverting nanomaterials that convert low energy near IR photons to
high energy UV-visible ones are versatile materials for a whole gamut of applications ranging
from biological imaging and therapeutics to photovoltaics, photonics and photocatalysis. ¹⁻⁶ In
general, nanometric materials have numerous advantages over bulk materials in terms of
tunability of size-dependent properties and extremely high surface areas.⁷ However, the latter is
also responsible for surface-related emission quenching processes in lanthanide (Ln)-based
luminescent nanomaterials.⁸,⁹ As a result, the quantum yields of these upconverting nanoparticles
(UCNPs) are in general significantly low (typically <1%), an order of magnitude lower than those of the corresponding upconversion bulk materials. A holistic approach to enhance upconversion luminescence of these materials includes an appropriate selection of a host matrix with low phonon energy, optimization of the concentration of doping lanthanide ions, and a suitable synthetic approach to synthesize NPs with the defect-free, less strained and anhydrous surface.¹⁰ As the fluoride-based matrices enjoy the benefit of having relatively low phonon energy (< 400 cm⁻¹), they serve as an ideal host for lanthanide dopants like Yb³⁺ and Tm³⁺. Surface passivation either by forming a shell of silica or undoped matrix (e.g., NaYF₄, NaGdF₄, etc.) or incorporating hydrophobic ligands around the upconverting core is the usual strategy¹¹⁻¹⁴ to minimise the –OH functionality on the NP’s surface, the primary cause of luminescence quenching.¹⁵,¹⁶

However, this strategy cannot be used universally for all types of applications, especially for those requiring multi-layers including another material or necessitating short distances because of specific interactions with grafted species, e.g., in photocatalysis.² Since our interest in unconverting materials is motivated by the potential application of TiO₂@UCNPs type core-shell structure for IR-induced photocatalysis,¹⁷ energy transfer to TiO₂ is required. We have thus focused on the synthesis of smaller UCNPs with intense emission without any intermediate passivation layer such as silica or undoped matrix. Our synthesis requires thus appropriately designed anhydrous molecular precursors as a synthetic strategy to minimize –OH functionality on the surface as well as in the volume of the NPs.¹⁸⁻²² Indeed, it has been shown recently that, like surface hydroxyl groups, the internal hydroxyl impurity too has detrimental effect on the upconversion efficiency.¹⁶ The NaGdF₄-based UCNPs prepared using this strategy showed a much higher upconversion intensity in comparison to NPs of a similar composition produced
from hydrated inorganic salts.²² Previously, Homann et al. used anhydrous lanthanide acetate precursors to synthesize NaYF₄:Yb,Er/NaYF₄ core-shell NPs with high UC quantum yield, although the anhydrous nature of the precursors was not established structurally.²³ Here we extend this approach to Li-based UCNPs as the LiYF₄:Yb³⁺, Tm³⁺ system is reported to be one of the most efficient ones for the generation of UV photons by upconversion (and hence useful for IR-induced photocatalysis in TiO₂@UC).²⁴⁻²⁶ In TiO₂@UC for IR-induced photocatalysis, titania essentially requires UV radiation for their active participation.

Self-assembled hybrid materials containing both molecules and appropriately functionalized nanoparticles is a new exciting way of fabricating soft materials with significant advantages.²⁷⁻³⁰ This multi-scale self-assembly strategy makes it possible to obtain synergetic properties and to arrange the various components in a complex hierarchical architecture allowing, for example, the modulation of optical, electronic and mechanical performances. The possibility to enhance the upconversion-induced photocatalysis depends on the material’s ability to absorb and transfer as much light as possible. Both of these necessary criteria can be achieved by embedding UCNPs in transparent TiO₂ metallogel, followed by its soft drying. Metallogels are exceptionally versatile, soft materials formed by the cooperation of covalent and self-complementary interactions of the low-molecular weight species resulting in a self-assembled network capable of immobilizing the solvent, and therefore, yielding a solid-like appearance.³¹

As compared to Na-based host matrices such as NaYF₄ and NaGdF₄, there are relatively few studies on the synthesis of lithium-based host matrices to date.¹²,²⁵,³²⁻³⁷ The precursor-directed synthesis of nanomaterials has many advantages over traditional solid-phase synthesis.¹⁷⁻²¹ It not only provides numerous opportunities for the design and improvement in the properties of these materials but also paves the way for low temperature and sustainable syntheses, thus reducing
the fabrication cost and minimizing environmental pollution. While the influence of precursors on the structural and physical properties of the metal oxide and -chalcogenide nanomaterials has been demonstrated significantly,³⁸⁻⁴⁰ the precursor chemistry remains scarce for the metal fluoride-based UC nanomaterials ⁴¹,⁴² and, almost non-explored for the synthesis of LiYF₄-based UCNPs.³³ In this article, we describe anhydrous lithium and lanthanide trifluoroacetate complexes with monoglyme ligand, prepared *via* one-pot synthetic protocol using non-expensive commercially available reagents, as excellent precursors to bright upconverting LiYF₄: Yb³⁺, Tm³⁺ NPs by thermal decomposition (Scheme 1). A detailed study of the influence of many key parameters over crystalline phase, size, shape, morphology and upconverting properties of the NPs produced is achieved. Following an in-depth analysis of the influence of the doping concentration, the ratio between the solvents 1-octadecene and oleic acid, concentration of precursors, reaction time, and temperature, a mechanism for the formation of LiYF₄-based NPs is proposed. A detailed UC study reveals that the upconversion quantum yield of these NPs (15 ± 5 nm) increases to more than 3 times (in comparison to the best ever value observed previously on NPs) and becomes quite comparable to that of bulk single crystal, thus underlining the success of the anhydrous strategy employed here. Finally, these UCNPs were successful embedded in transparent TiO₂ metallogel, which on subsequent drying provided highly homogeneous UCNPs-TiO₂ composites of variable composition (1-10% of NPs) and very high specific surface area (435-580 m²/g).
<figure><img src="image_3.png" /><figcaption>**Scheme 1.** Anhydrous synthesis of LiYF₄ matrix-based UCNPs and their incorporation in TiO₂ metallogel for obtaining UCNPs-TiO₂ xerogel with high surface area for IR-induced photocatalysis.</figcaption></figure>

## Results and discussion

### 1) Synthesis and characterization of novel anhydrous molecular precursors

As mentioned in the Introduction section, there is a clear void in the literature for the well-characterized molecular precursors for LiLnF₄-based upconverting nanomaterials. Metal trifluoroacetate complexes are the most easily available and the cheapest source for the metal fluoride/oxofluoride nanomaterials.⁴¹ The hydrated Li(TFA)(H₂O) and Ln(TFA)₃(H₂O)ₓ are commercially available, but can also be synthesized easily by reacting LiOH or Ln₂O₃ with
trifluoroacetic acid (TFAH) under reflux. However, the presence of water molecules is detrimental to the luminescent properties of UC nanomaterials.<sup>15,16</sup> As thermal dehydration of these hydrated complexes have proved difficult,<sup>43</sup> the use of an ancillary co-ligand seems to be a preferred strategy to make them anhydrous. We and some other groups have previously demonstrated that the acyclic polyether glyme ligands, CH<sub>3</sub>O(CH<sub>2</sub>CH<sub>2</sub>O)<sub>x</sub>CH<sub>3</sub> [x = 1 (monoglyme), 2 (diglyme), 3 (triglyme), 4 (tetraglyme)], not only replace the water molecules of hydrated metal trifluoroacetates to afford anhydrous precursors, but also stabilize metal complexes by wrapping/shielding the metal cations efficiently and saturating their coordination spheres.<sup>17-21, 44-46</sup> Using this strategy, anhydrous glyme adducts of metal trifluoroacetates, such as [Li<sub>3</sub>(TFA)<sub>3</sub>(diglyme)<sub>2</sub>]<sup>45</sup> [Li<sub>5</sub>(TFA)<sub>5</sub>(tetraglyme)<sub>2</sub>]<sup>46</sup> [Ln(TFA)<sub>3</sub>(diglyme)] (Ln = Gd, Tm, Yb),<sup>20</sup> [Na<sub>2</sub>Y(TFA)<sub>5</sub>(tetraglyme)] and [NaLn(TFA)<sub>4</sub>(diglyme)] (Ln = Y, Gd)<sup>17-19</sup> have been synthesized and characterized structurally. These glyme ligands also act as a surfactant during decomposition and, therefore, help to get nanoparticles with controlled size and desired dispersibility in organic media. In this work, we describe anhydrous adducts of lithium and lanthanide trifluoroacetates with monoglyme and THF ligands as potential precursors for LiLnF<sub>4</sub>-based upconverting nanoparticles. As compared to the high boiling di-, tri- and tetraglyme (162-275 °C), employing a low boiling monoglyme (b. p. 85 °C) here has allowed us to use it both as a solvent and an ancillary co-ligand in a straightforward one-pot synthesis of the anhydrous precursors. This has important implications in terms of producing these precursors on large scale for industrial applications.

A simple stirring of Li(TFA)(H<sub>2</sub>O) in monoglyme affords an anhydrous precursor [Li(TFA)(monoglyme)] (1) in 86% yield (Scheme 2). When the reaction medium was a 1:1 mixture of THF and monoglyme, it afforded anhydrous THF adduct [Li(TFA)(THF)] (2). We
also synthesized anhydrous lanthanide monoglyme adducts $[Ln(TFA)_3(\text{monoglyme})] [Ln = Y (3), Gd (4), Tm (5), Yb (6)]$ by refluxing $Ln_2O_3$ with trifluoroacetic acid in THF in the presence of monoglyme. Alternatively, the derivatives (3)–(6) could also be synthesized by stirring $Ln(TFA)_3(H_2O)_x$ with monoglyme in the presence of THF. Only in the case of yttrium, it was necessary to avoid THF which promoted the formation of $[Y(TFA)_3(THF)_2]$. These air-sensitive complexes (1)–(6) were isolated as colourless crystals. They are readily soluble in THF, mildly soluble in monoglyme and sparingly soluble in diethyl ether, hexane and pentane. The synthetic details are provided in the supporting information.

<figure><img src="image_4.png" /><figcaption>**Scheme 2.** Synthesis of anhydrous precursors **1-6**.</figcaption></figure>

**X-ray structures.** Structural characterization of the precursors is critical to understand molecule-to-material transformation and, therefore, to optimize the properties of the final materials. Since the trifluoroacetate ligand can bind to a metal centre in a variety of fashions (Scheme S1), it is
difficult to predict the structure of metal trifluoroacetate complexes. Therefore, we have carried
out single-crystal X-ray structural studies on some representative compounds among newly
synthesized **1-6** (Table S1). The THF adduct [Li(TFA)(THF)]<sub>∞</sub> (**2**) crystallizes in the triclinic
space group *P*1 and forms a straight 1D polymeric chain along the a-axis (Figure 1a, S1). Each
TFA ligand bridges three lithium atoms in a µ<sub>3</sub>, η<sup>1</sup>(O):η<sup>1</sup>(O):η<sup>1</sup>(O) manner. Each lithium atom
bears a THF molecule to attain a 4-coordination number, which is further supplemented by
secondary Li···Li [2.827(7)-3.795(4) Å] and Li···F [2.921(8)-4.633(8) Å] interactions. The
overall structure can be seen as the [Li(µ, η<sup>1</sup>(O):η<sup>1</sup>(O)-TFA)(THF)]<sub>2</sub> dimers being joined via
four-membered Li<sub>2</sub>O<sub>2</sub> rings, thus forming 8- and 4-membered rings in an alternate manner
(Figure S1). The Li–O bond lengths range from 1.887(4) to 1.975(4) Å, which are consistent
with those observed in the related [Li<sub>3</sub>(TFA)<sub>3</sub>(diglyme)<sub>2</sub>]<sup>45</sup> and [Li<sub>5</sub>(TFA)<sub>5</sub>(tetraglyme)<sub>2</sub>].<sup>46</sup> As
expected, the Li···Li distance is significantly shorter in the 4-membered ring [2.827(7) Å] than in
the 8-membered ring [3.795(4) Å] (Table 1).

The lanthanide complexes [Ln(TFA)<sub>3</sub>(monoglyme)]<sub>∞</sub> [Ln = Gd (4), Yb (6)] are isostructural and they crystallize in the monoclinic space group C2/c and form 1D zig-zag polymer. The perspective views of 4 and 6 are shown in Figure 1b and 1c, respectively, whereas Figure S2 and S3 depict their extended structures. Table 1 summarizes their selected bond lengths (Å) and angles (°). For the sake of brevity, only the gadolinium complex (4) is described in detail here. The gadolinium centres in 4 are bridged by two and four TFA ligands alternatively, all bonded in µ,η¹:η¹ fashion. The Gd···Gd distance [5.5644(10) Å] is significantly longer when bridged by only two TFA ligands, as against 4.4526(7) Å for Gd atoms bridged by four TFA ligands. Each Gd atom bears one bidentate monoglyme ligand to attain an 8-coordination number, the geometry being square antiprismatic. The Gd–O (η²-monoglyme) and Gd–O (µ-TFA) bond
lengths range from 2.316(5) to 2.501(5) Å. As expected, the Gd-O (η²-monoglyme) bonds (av. 2.50 Å) are longer than the corresponding Gd-O (μ, η¹:η¹-TFA) ones (av. 2.357 Å). The structure of **4** and **6** can be related to that of [Y(TFA)₃(THF)(H₂O)]∞, where the metals are bridged by four and two μ,η¹:η¹-TFA ligands alternatively.⁴³

<figure><img src="image_5.png" /><figcaption>**Figure 1.** Perspective views of (a) [Li(TFA)(THF)] (2), (b) [Gd(TFA)₃(monoglyme)] (4), and (c) [Yb(TFA)₃(monoglyme)] (6) with displacement ellipsoids drawn at the 30% probability level. H-atoms on THF and monoglyme ligands have been omitted for clarity. Symmetry codes for 2: (i) 2-x, 1-y, -z; (ii) 2+x, -1+y, z. (d) TGA curves of the anhydrous metal trifluoroacetate monoglyme precursors.</figcaption></figure>
<table><caption>Table 1: Selected bond lengths (Å) and angles (°) for 2, 4 and 6.</caption><tbody><tr><td colspan="4"><b>2</b></td></tr><tr><td colspan="4">Bond length (Å)</td></tr><tr><td>O1<sup>i</sup>-Li1<sup>i</sup></td><td>1.887(4)</td><td>O2-Li1<sup>i</sup></td><td>1.956(4)</td></tr><tr><td>O2-Li1</td><td>1.975(4)</td><td>O3-Li1</td><td>1.927(5)</td></tr><tr><td>Li1---Li1<sup>i</sup></td><td>2.827(7)</td><td></td><td></td></tr><tr><td colspan="4">Bond angle (°)</td></tr><tr><td>O1<sup>ii</sup>-Li1<sup>ii</sup>-O2<sup>i</sup></td><td>121.4(2)</td><td>O1<sup>ii</sup>-Li1<sup>ii</sup>-O3<sup>ii</sup></td><td>107.2(2)</td></tr><tr><td>O2<sup>iii</sup>-Li1<sup>ii</sup>-O2<sup>i</sup></td><td>88.0(1)</td><td>O3<sup>ii</sup>-Li1<sup>ii</sup>-O2<sup>iii</sup></td><td>110.3(2)</td></tr><tr><td colspan="4"><b>4</b></td></tr><tr><td colspan="4">Bond length (Å)</td></tr><tr><td>Gd1-O2</td><td>2.499(5)</td><td>Gd1-O1</td><td>2.501(5)</td></tr><tr><td>Gd1-O3</td><td>2.316(5)</td><td>Gd1-O4</td><td>2.347(5)</td></tr><tr><td>Gd1-O5</td><td>2.325(5)</td><td>Gd1-O6</td><td>2.421(5)</td></tr><tr><td>Gd1-O7</td><td>2.358(5)</td><td>Gd1-O8</td><td>2.376(5)</td></tr><tr><td colspan="4">Bond angle (°)</td></tr><tr><td>O2-Gd1-O1</td><td>65.04(16)</td><td>O3-Gd1-O2</td><td>71.67(18)</td></tr><tr><td>O3-Gd1-O1</td><td>111.60(18)</td><td>O5-Gd1-O2</td><td>149.81(17)</td></tr><tr><td>O5-Gd1-O6</td><td>119.32(18)</td><td>O5-Gd1-O1</td><td>142.58(18)</td></tr><tr><td>O7-Gd1-O2</td><td>133.25(17)</td><td>O5-Gd1-O4</td><td>77.75(19)</td></tr><tr><td colspan="4"><b>6</b></td></tr><tr><td colspan="4">Bond length (Å)</td></tr><tr><td>Yb1-O1</td><td>2.282(4)</td><td>Yb1-O2</td><td>2.301(5)</td></tr><tr><td>Yb1-O3</td><td>2.251(4)</td><td>Yb1-O4</td><td>2.324(4)</td></tr><tr><td>Yb1-O5</td><td>2.251(4)</td><td>Yb1-O6</td><td>2.271(4)</td></tr><tr><td>Yb1-O7</td><td>2.428(5)</td><td>Yb1-O8</td><td>2.440(5)</td></tr><tr><td colspan="4">Bond angle (°)</td></tr><tr><td>O1-Yb1-O2</td><td>118.81(15)</td><td>O1-Yb1-O7</td><td>134.31(15)</td></tr><tr><td>O1-Yb1-O8</td><td>74.39(15)</td><td>O3-Yb1-O1</td><td>76.67(16)</td></tr><tr><td>O3-Yb1-O5</td><td>81.83(17)</td><td>O3-Yb1-O7</td><td>148.48(15)</td></tr><tr><td>O5-Yb1-O8</td><td>112.56(16)</td><td>O7-Yb1-O8</td><td>66.42(15)</td></tr></tbody></table>

Symmetry codes: (i) 2-x, 1-y, -z; (ii) 2+x, -1+y, z; (iii) 3-x, 1-y, -z.

**Spectroscopic characterization.** The FT-IR spectra of **1-6** account for anhydrous species (absence of vOH absorptions in the region 3000-3500 cm⁻¹) (Figure S4). The asymmetric and symmetric vibrations of the v(CO₂) group are indicative of binding modes of a carboxylate ligand (Scheme S1). The stretching frequency of the C=O bond differs for solely bridging, chelating or both bridging and chelating TFA ligand. The position of this frequency is affected
also by the number and type of donating atoms of the coordinating ligand on the metal centre.

The appearance of a single but relatively broad peak in the region 1550-1800 cm⁻¹ for the monoglyme complexes **1-6** is consistent with only one type of TFA ligand present in these precursors. The frequencies around 1110-1210 cm⁻¹ confirm the presence of C-F and C-O bonds. Some low-frequency peaks around 410-650 cm⁻¹ are characteristic vibration peaks for vM-O bonds. The ¹H NMR spectra of the diamagnetic complexes **1** and **3** show peaks of the coordinated monoglyme moiety at δ 3.28 and 3.46 ppm for CH₃ and CH₂ protons, respectively, whereas in the spectrum of **2**, the coordinated THF appears as two triplets centred at δ 1.80 and 3.65 ppm for the β- and α-hydrogens, respectively (Figure S5-S7). A single peak at δ ~ 76.2-77.2 ppm in the ¹⁹F NMR spectra of **1-3** confirmed the presence of TFA groups (Figure S5-S7). These spectroscopic results, along with the elemental analysis data, indicate that the bulk powders of **1-6** are consistent with the earlier described single-crystal structures.

**Thermogravimetric studies.** The thermal behaviour of the anhydrous lithium and lanthanide trifluoroacetate monoglyme precursors was investigated under an argon atmosphere in the 20-600 °C temperature range by thermogravimetric analysis (TGA). The TGA curves of the lanthanide precursors (Ln(TFA)₃(monoglyme)) show predominantly one-step decomposition in the range 270-315 °C, as indicated by a prominent DTG peak at ~300 °C (Figure 1d, S8). The remaining weight of the residues (37.2-37.7% at 350 °C) is consistent with the formation of LnF₃ (theoretical values ~36.5-38.2%) as the end product. The lithium derivative **1** also shows a similar 1-step decomposition, with an endothermic peak at 300 °C. The remaining residue weight (~18% at 350 °C) accounts for a little more than the expected yield of LiF materials (12.4%), indicating an incomplete decomposition due to the presence of argon atmosphere. As compared to the starting hydrated complexes Li(TFA)(H₂O) and Y(TFA)₃(H₂O)ₓ (Figure S9) the newly
synthesized monoglyme complexes not only show a better-defined decomposition pattern but also require a slightly lower temperature to decompose. These monoglyme complexes are also better than the THF adduct [Y(TFA)₃(THF)₂] which shows a clear 2-step decomposition.⁴³ More importantly, the good compatibility in the thermal behaviour of [Li(TFA)(monoglyme)] and [Ln(TFA)₃(monoglyme)] suggests that these precursors can be decomposed simultaneously to afford the LiLnF₄ matrix.

## 2) Synthesis of undoped and Yb³⁺, Tm³⁺ co-doped LiYF₄ nanoparticles from the thermal decomposition of anhydrous precursors

### a) Synthesis of undoped LiLnF₄ matrices.

The upconversion process is significantly dependent on the choice of host matrix, as it can exert a crystal field around the dopant ions and modify the excitation energy of dopant through lattice vibrations. Hence, it is necessary to choose a perfect host matrix that promises a very low phonon energy to minimize non-radiative relaxation emission. Many studies have shown the importance of fluoride matrices for obtaining high upconversion emission.²⁴ The lithium-based matrices LiLnF₄ (Ln = Y, Gd) are promising host matrices for NIR to UV upconversion. LiYF₄ is a good host for dopant because it is composed of optically inactive ions and hence cannot contribute to the energy transfer.²⁵,³⁶ Therefore, we synthesized LiYF₄ by simultaneous thermal decomposition of the anhydrous precursors [Li(TFA)(monoglyme)] (1) and [Y(TFA)₃(monoglyme)] (3) in the mixture of 1-octadecene (ODE) and oleic acid (OA) at 290 °C for 1 h (Table S2). This decomposition results in the formation of phase pure and well crystalline tetragonal LiYF₄
nanoparticles as indicated by its powder XRD pattern that corresponds to ICDD # 04-007-5501 (Figure 2a).

On the other hand, a similar reaction of Gd(TFA)₃(monoglyme)] and [Li(TFA)(monoglyme)] under identical synthetic conditions afforded a mixture of LiF and GdF₃ instead of LiGdF₄ (Figure S10), most probably because the thermodynamic barrier for the formation of tetragonal LiGdF₄ is higher than the LiYF₄ and, therefore, orthorhombic GdF₃ which has lower energetic barrier is readily formed.⁴⁷ Du et al reported previously that the heavier lanthanides (Dy to Lu including Y) tend to arrange themselves stoichiometrically into tetragonal LiLnF₄ phase, whereas a mixture of LiF and LnF₃ is formed for the lighter lanthanides (La to Gd).⁴⁸ To verify this hypothesis, we decomposed the mixture of the precursors [Li(TFA)(monoglyme)] (1) and [Yb(TFA)₃(monoglyme)] (6) under similar conditions which afforded well crystalline LiYbF₄ (ICDD # 04-010-3625, Figure 2b). The LiYbF₄ matrix is isostructural to LiYF₄, but unlike the Y³⁺ ion, Yb³⁺ ion could act as the sensitizer ion to transfer energy to the other active centres to enhance the luminescence of the upconverting nanoparticles. Indeed, LiYbF₄ has been reported as one of the brightest host matrices, if passivated efficiently by a shell.³⁷ The TEM images show that while the LiYF₄, nanoparticles are stacked over each other, the LiYbF₄ NPs present plate-like morphology (Figure 2c, d). The HRTEM image and the associated FFT confirm the well-crystalline, tetragonal phase for the LiYbF₄ NPs (Figure S11).
<figure><img src="image_6.png" /><figcaption>**Figure 2.** Powder x-ray diffraction patterns (a, b) and TEM images (c, d) of undoped LiLnF₄ (Ln = Y, Yb) NPs prepared by thermal decomposition of anhydrous molecular precursors at 290 °C for 1 h in 1:1 mixture of ODE and OA.</figcaption></figure>

**b) Synthesis of Yb³⁺, Tm³⁺ doped LiYF₄ nanoparticles under different synthetic conditions**

(i) **Influence of ratio of activator ions.** The ratio of sensitizer and activator ions is a crucial factor in a co-doped system like LiYF₄: Yb³⁺, Tm³⁺ and a fine balance of concentration of both dopants (sensitizer Yb³⁺ and activator Tm³⁺) is needed to maximize the upconversion emission.⁴⁹,⁵⁰ The optimum amount of the sensitizer is necessary to both absorb and transfer as much excitation as possible before quenching of upconversion starts due to a higher concentration of these ions. Similarly, the amount of activator thulium ions is also a key factor to get higher upconversion emission. Based on our previous studies,²² we kept the doping concentration of sensitizer (Yb³⁺) to be 16.5 mol% of the host matrix and varied the concentration of activator (Tm³⁺) as 0.4, 0.8 and 1.2% (Table S3). Generally, going beyond 2% of activator concentration leads to reduced upconversion emission due to cross-relaxation
between different excited energy states.⁴⁹ The synthesis of these nanoparticles was achieved by thermal decomposition of appropriate amounts of [Li(TFA)(monoglyme)] and [Ln(TFA)₃(monoglyme)] (Ln = Y, Yb, Tm) in a mixture of ODE and OA. Figure 3a depicts the Rietveld refinement of the powder X-ray diffraction patterns of a series of samples prepared by varying the doping ratio of the activator ions. All these samples are well-crystalline and correspond to the phase-pure tetragonal LiYF₄ (ICDD database file # 04-004-5501). Their TEM images show them to be diamond-shaped, plate-like nanoparticles stacked in a way to form star-shaped agglomerated nanoparticles (Figure 3b-d).

The upconversion studies on all these samples were performed at a power density of ~5W/cm². As reported, at a higher concentration of dopant ions, the average dopant-dopant separation becomes smaller, therefore increasing the rates and efficiencies of a variety of non-radiative energy-transfer processes.⁴⁹ The upconversion emission spectra, obtained on exciting the samples with 973 nm CW laser, are dominated by NIR ³H₄ → ³H₆ transitions centred at 794 nm (Figure 3e). Other transitions corresponding to 475 nm (¹G₄ → ³H₆) and 364 nm (¹D₂ → ³H₆) are also prominent in nearly all the samples. Among all the samples measured, the highest intensity of upconversion emission is obtained for the sample doped with 0.4% of Tm³⁺ ions, particularly for the deep UV emission arising from the ¹I₆ to the ground levels. Following this, LiYF₄ NPs doped with 16.5% and 0.4% for the sensitizer Yb³⁺ and activator Tm³⁺, respectively, was chosen as the optimized composition for the rest of the studies.
<figure><img src="image_7.png" /><figcaption>**Figure 3.** Rietveld refined X-ray diffraction patterns (a), TEM images (b-d) and upconversion emission spectra presented in semi-log scale (e) of LiYF₄: 16.5% Yb³⁺, x% Tm³⁺ (x = 0.4, 0.8 and 1.2) NPs prepared by decomposition of precursors at 290 °C for 1 h in a 1:1 mixture of ODE and OA. The UC emission spectra were measured on dry powder samples under similar excitation conditions.</figcaption></figure>

(ii) Influence of ratio of 1-octadecene (ODE) and oleic acid (OA). To understand the effect of the varying ratio of solvent during the thermal decomposition, we chose three different samples having a nearly similar concentration of the precursors but the different ratio of solvent (1-octadecene and oleic acid = 1:1, 1:3 and 3:1). The decomposition performed under these different conditions resulted in highly crystalline nanoparticles, the powder XRD of which revealed that they were consisted mainly of tetragonal LiYF₄ (ICDD # 04-007-5501), with the presence of a small amount of YF₃ or LiF in certain cases (Figure 4). On assessing the
composition of these nanoparticles using Rietveld analysis (Table S8), we observed 5.2% of orthorhombic YF₃ (ICDD # 00-032-1431) for the 1:3 ratio of ODE and OA. On the other hand, a small amount of cubic LiF (ICDD # 00-004-0857) was observed for the ODE/OA ratio of 3:1 (5.1%) or 1:1 (4.1%). These results indicate that the formation of LiYF₄ nanoparticles is achieved by the interaction of individual LiF and YF₃ at high temperatures. In bulk, LiF and YF₃ binary systems usually result in a eutectic mixture of LiF-YF₃, followed by the formation of single-crystal LiYF₄,<sup>51,52</sup> whereas in nanoparticles they either form well crystalline LiYF₄ single-crystal nanoparticles or a part of them may remain in the forms of LiF and YF₃ depending upon the reaction conditions (temperature, duration and solvent ratio).<sup>53,54</sup> In our study, we found that when the amount of OA in the decomposition solvent is more than ODE (OA:ODE = 3:1), it favours the formation of thermodynamically stable orthorhombic YF₃ first (due to feeble competition between Li⁺ and Y³⁺ ions to interact with F⁻ ions in the solution) and then depending on the reaction conditions (high temperature, long reaction time, etc.) the slow diffusion of Li⁺ ions causes the phase transformation from orthorhombic YF₃ into tetragonal LiYF₄. On the contrary, when the amount of OA is less than ODE (OA:ODE = 1:3), then the solubility of F⁻ ions in the solution reduces and supersaturation of Li⁺ increases, resulting in LiF crystals. These already precipitated LiF crystals then react with Y³⁺ and F⁻ present in the solution to yield LiYF₄ nanocrystals. With a longer reaction time, we can completely transform these LiF crystals and get pure LiYF₄ nanocrystals. These findings are consistent with the detailed study on the impact of pH on the formation of YF₃ nanoparticles by Ye et al.<sup>55</sup> The TEM images (Figure 4), demonstrate that the composition of solvents for the thermal decomposition of anhydrous precursors has a great impact on the morphology (size and shape) as usual but also on the phase of nanoparticles prepared. When the volumetric ratio of ODE and OA is kept at either 1:3 or 3:1,
we observed agglomerated, unevenly distributed non-uniform nanoparticles, whereas well-crystalline diamond-shaped nanoparticles are observed for the ratio 1:1. The use of an optimum ratio of these solvents is critical for the synthesis of these upconverting nanoparticles as it is one of the major factors deciding the morphology and composition of these upconverting nanoparticles.

<figure><img src="image_8.png" /><figcaption>**Figure 4.** Rietveld refinement performed on X-ray diffraction pattern of three different samples prepared by decomposition of precursors at 290 °C for 1 h using different ratios of ODE and OA and their respective TEM images.</figcaption></figure>

This agglomeration and uneven distribution of nanoparticles might result in higher surface defects on the nanoparticle surface, which can also be assessed by observing the intensity of upconversion. Under similar excitation conditions ($\lambda_{exc}$ = 972 nm; power density = 10.9 W/cm²), the samples prepared using different ratios of the solvents show varied photoluminescence emission profiles (Figure 5). For the nanoparticles prepared using a 1:1 ratio, we observe sharp and strong UV emission starting from 265 nm ($^3$P$_2$ → $^3$H$_6$) up to 362 nm ($^1$D$_2$ → $^3$H$_6$) including
290 nm ($^1$I$_6$ → $^3$H$_6$), 312 nm ($^3$P$_2$ → $^3$F$_4$), 347 nm ($^1$I$_6$ → $^3$F$_4$). The emission intensity for the same transitions shows a strong reduction for the samples prepared using 1:3 and 3:1 ratios of solvents. The dip in the upconversion emission intensity can be attributed to the combination of the following factors - lower crystallinity, higher cross-relaxation due to the phase transformation from orthorhombic YF$_3$ to the tetragonal LiYF$_4$$^{53,54}$ or higher surface defects.$^{56,57}$ Usually, the presence of -OH groups$^{58,59}$ on the surface is the major cause of upconversion intensity quenching. The important aspect of this study is that the UV component of the upconversion intensity is high and comparable to the near-infrared region, which is usually not observed in the case of small nanoparticles because of higher surface energies and defects.$^{32}$ This can be attributed to the high crystallinity of the nanoparticles prepared using the thermal decomposition of anhydrous precursors.$^{22}$ Following this study, we chose a 1:1 ratio for ODE and OA for the general synthesis of NPs for the rest of the studies.

<figure><img src="image_9.png" /><figcaption>**Figure 5.** The upconversion emission intensities (a) and CIE diagram (b) of the samples prepared using three different ratios of solvents (ODE and OA). The points A, B and C in the CIE graph correspond to the samples prepared using 1:1, 1:3 and 3:1 ratio, respectively, of ODE</figcaption></figure>
and OA. These spectra were measured from dry powder in the same excitation condition for all three samples.

**(iii) Influence of precursor's concentration.** The above studies indicated that the formation of $LiYF_4$ is progressed by the formation of $YF_3$ seeds, followed by the intercalation of $Li^+$ into the $YF_3$ crystal lattice. The study of variation of concentration of precursors further enhances our understanding of the formation of $LiYF_4$ nanoparticles. Here, we prepared two different precursor solutions, one having a concentration of yttrium precursor equal to 0.520 mmol/ml and another with a concentration of 0.175 mmol/ml. These solutions were then decomposed separately in the mixture of 1:1 ODE and OA at 290 °C for 1h under an inert atmosphere. The powder XRD patterns confirm the presence of phase-pure $LiYF_4$ for lower concentration (0.175 mmol/ml) and a mixture of $LiYF_4$ and $YF_3$ nanoparticles for higher concentration (0.520 mmol/ml) of precursors (Figure 6). The composition of different phases was then analysed using Rietveld refinement. The decomposition performed with a higher concentration of precursors results in a mixed-phase of ~19% orthorhombic $YF_3$ and ~81% tetragonal $LiYF_4$, whereas for a lower concentration of precursors, we obtained ~100% tetragonal $LiYF_4$. The complete refinement data is provided in Table S9. From the previously discussed section and available literature, we can say that formation of $LiYF_4$ is achieved by the synthesis of $YF_3$ nuclei seeds first and then incorporation of $Li^+$ ions into the crystal matrix leads to the formation of tetragonal $LiYF_4$.43,50 However, at higher concentration of precursors in the decomposition solution the diffusion of $Li^+$ ions (being already slow) into the $YF_3$ seeds now become more hindered (due to the presence of longer alkyl chains, which results in steric repulsion, therefore, keeping these particles well separated in the solutions) leading to precipitation of some $YF_3$. But if the reaction conditions are modified (e.g. prolonged reaction time), this problem of slow diffusion could be
overcome. In other words, at higher concentrations of these precursors, due to steric hindrance, the preferential growth of initial orthorhombic YF₃ nuclei into tetragonal LiYF₄ via Li⁺ intercalation is limited, hence, leading to the mixture of tetragonal LiYF₄ and some non-transformed orthorhombic YF₃. From the transmission electron microscopy data (Figure 7), we observe that the more the concentration of precursors is, the more non-uniformity in their shape is observed. The higher concentration of precursors leads to spherical nanoparticles of average size of ~15 nm (Figure S12), whereas at lower concentrations, we observe diamond-shaped (of varied sizes) nanoparticles. The HRTEM analysis on these particles confirmed their well-crystalline nature.

<figure><img src="image_10.png" /></figure>
**Figure 6.** Rietveld refined X-ray diffraction patterns of the LiYF₄: 16.5% Yb³⁺, 0.4% Tm³⁺ NPs prepared by varying the concentration of precursors (decompn. cond.: 290 °C, 1 h, 1:1 ratio of ODE and OA).

<figure><img src="image_11.png" /></figure>

**Figure 7.** TEM characterization of the LiYF₄: 16.5% Yb³⁺, 0.4% Tm³⁺ NPs prepared by varying the concentration of precursors (decompn. cond.: 290 °C, 1 h, 1:1 ratio of ODE and OA). Representative TEM image of the sample prepared using the concentration ratio of yttrium precursor = 0.175 mmol/ml (a), HRTEM showing the crystal structure of a large diamond-shaped particle (b), and its reciprocal lattice showing the tetragonal structure of LiYF₄ in the zone axis [110] (c). Representative TEM image of the sample prepared using the concentration ratio of yttrium precursor = 0.520 mmol/ml (d).

The upconversion emission intensity observed for the doped LiYF₄: 16.5% Yb³⁺, 0.4% Tm³⁺ NPs, prepared by varying the concentration of precursors, are quite different (Figure S13). Under similar excitation conditions (λexc = 972 nm; power density = 10.9 W/cm²), the decrease in the intensity by almost 1 order of magnitude can be accounted to differently shaped nanoparticles.⁶⁰
Intense and very sharp UV and visible emissions are observed for both samples. These emissions
justify that the prepared nanoparticles are of very good crystallinity and minimized quenching
processes including lower phonons cross relaxations, because these emissions require a transfer
of at least 3 or more photons to the higher excited states, which is not usual for the particles
synthesized using hydrated precursors in similar decomposition techniques. The absence of OH-
ions during decomposition and optimum ratio of solvents during decomposition, helps us to
analyse the effect of shape and morphology on the upconversion luminescence. Hence, we can
conclude that the optical properties of the nanoparticles can be tailored not only by changing the
size of nanoparticles but also by varying their morphology.

**(iv) Influence of decomposition temperature.** We investigated the transformation of anhydrous molecular precursors into desired LiYF₄ nanoparticles by decomposing them at different temperatures because temperature provides sufficient threshold energy for LiYF₄ formation and, therefore, it plays a crucial role in understanding the growth mechanism of LiYF₄ nanoparticles. Following the TGA-DTG studies of the precursors **1-6**, we varied the temperature of the decomposition in the range 270-315 °C. When the precursors were decomposed at 270 °C, the composition of the resulting powder was not pure tetragonal LiYF₄ but a small percent of YF₃ was also present (Figure 8). This indicates that the decomposition proceeds *via* nuclei formation of LiF and YF₃, followed by their interaction to give LiYF₄ phase.³³ The absence of LiF in powder XRD is probably due to its loss during the final step of washing with ethanol. At higher decomposition temperatures (290 °C and onwards), no impurity of YF₃ or LiF was observed, which indicated that the formation of LiYF₄ was almost completed at that temperature.
TEM images of these samples obtained at different temperatures showed significantly different morphologies. These TEM images, along with the results obtained in the previous sections, throw light on the key mechanism of formation of LiYF₄ nanoparticles and indicate that at lower decomposition temperature (270 °C), the spherical YF₃ and LiF nuclei are formed first (Figure S14) which arrange / organize themselves, as a function of reaction time and temperature, reactant's concentration or solvent ratio, in a particular thermodynamically favourable direction to get transformed into diamond-shaped tetragonal LiYF₄ nanoparticles (Figure 8). At temperatures higher than 290 °C, the transformation of spherical nanoparticles into diamond-shaped particles is observed alongside some agglomeration of these diamond-shaped nanoparticles into larger particles. At 315 °C, the decomposition leads to very highly crystalline nanocrystals. As expected, the average particle size of these NPs increased with the increase in decomposition temperature due to Ostwald ripening and arrangement of the smaller nanoparticles in the directions particularly favourable for a reduction in the surface energy and subsequent aggregation. The spherical NPs obtained at 270 and 290°C have average size of 14 and 21 nm, respectively, whereas the large diamond-shaped particles obtained at 315 °C, are ~165 nm in length and ~150 nm in width (Figure S15, S16).
<figure><img src="image_12.png" /><figcaption>**Figure 8.** Powder X-ray diffraction patterns and TEM images of Ln³⁺-doped LiYF₄ NPs prepared by decomposition of the anhydrous precursors at 270, 290 and 315 °C for 1 h in a 1:1 mixture of ODE and OA.</figcaption></figure>

The upconversion intensity is governed not only by the crystallinity of the nanoparticles but also by other aspects such as morphology or defects induced due to lattice strain.⁶¹ The fact that the samples synthesized at different temperatures show different morphologies and induced strain, perhaps explains the varied upconversion emission intensity of nanoparticles. The power-dependent upconversion study of these samples show a uniform trend of increase of upconversion emission with incident laser power until a threshold of 26 W/cm², after which we observed loss of intensity counts in all the samples for almost all emissions. The evolution of these different wavelengths with different incident laser intensities shows that up to a specific threshold value of excitation of these nanoparticles, their emission increases, which saturates at
higher laser excitations. The upconversion phenomenon is very much dependent on the crystalline nature of nanoparticles.⁶² Since the sample is more crystalline at high temperature, there would be fewer defects levels induced into the matrix, which usually compete with the intrinsic energy levels of material, leading to loss of energy as heat and unnecessary radiative losses.⁵⁶ Figure 9 shows a typical trend for the emission at 451 nm for LiYF₄: 16.5% Yb³⁺, 0.4% Tm³⁺ NPs prepared at different temperatures (270, 290 and 315 °C) and excited by a 973 nm CW laser. Expectedly, the upconversion emission of the sample decomposed at higher temperature (315 °C) is higher than those synthesised at lower temperature (290 and 270 °C). This laser power-dependent study shows that even at low excitation power densities, we can observe the UV emission from our upconverting nanoparticles. This indicates that this LiYF₄: 16.5% Yb³⁺, 0.4% Tm³⁺ upconverting system, if combined with an efficient UV photocatalyst (such as TiO₂), can result in NIR based photocatalysis operated in solar condition excitation.

<figure><img src="image_13.png" /></figure>
**Figure 9.** log-log plots of Incident power-dependent upconversion studies of LiYF₄: 16.5% Yb³⁺, 0.4% Tm³⁺ prepared by decomposing anhydrous molecular precursors at different temperatures (270, 290 and 315 °C). These samples were excited by a 973 nm CW laser.

(v) **Influence of reaction time.** The main objective of this study was to understand the growth mechanism of different morphology of LiYF₄: 16.5% Yb³⁺, 0.4% Tm³⁺ NPs. During decomposition with a 1:1 ratio of solvent ODE and OA at 290 °C, we isolated different samples at different time intervals (from 30 min to 2 h) in order to analyse the growth mechanism and the formation of different morphologies among these nanoparticles (Figure 10). The most commonly reported, and apparently thermodynamically preferred, the diamond-like morphology,²³ is obtained in the end. The initially obtained spherical seeds grow in size, guided by the surface energy of a particular lattice plane into specific morphology of nanoparticles within a course of two hours. It is well-known that understanding of surface energy (or surface-to-volume ratio) of the growing nanoparticles is very crucial in determining the shape and morphology of nanoparticles.⁵⁴,⁵⁵,⁶⁰ In the initial 30 min of reaction, the formation of very small nanoparticles (< 5 nm) is observed, which grow into larger size retaining the similar spherical shape until 60 min of the reaction. These nanoparticles then start to grow/arrange themselves into a particular (011) direction, leading to the formation of tetragonal diamond-shaped morphology of LiYF₄ nanoparticles. After two hours of decomposition, most of these nanoparticles are diamond-shaped. These images clearly show the formation of diamond morphology of nanoparticles is guided through small spherical seeds which aligns themselves in accordance to their minimum surface energy to get the most thermodynamic stable phase and morphology. The sample 120 min hot refers to the sample collected just after completion of the reaction after 2h, whereas the
120 min cooled refers to the sample taken after cooling down the reaction mixture after 2 h to room temperature. In the literature, both spherical<sup>65</sup> and diamond-shaped<sup>25</sup> LiYF<sub>4</sub> NPs have been reported, but their transformation has never been described.

<figure><img src="image_14.png" /><figcaption>**Figure 10.** TEM images of LiYF<sub>4</sub>: 16.5% Yb<sup>3+</sup>, 0.4% Tm<sup>3+</sup> samples taken at a different course of reaction from 30 min to 2 h during the decomposition of the precursors at 290 °C in a 1:1 mixture of ODE and OA.</figcaption></figure>

**3) Upconversion quantum yield measurements**

The upconversion quantum yield of the particles decreases drastically by lowering its size,<sup>64</sup> primarily due to an increase in the surface area, which exposes more dopants ions on the surface and, therefore, enhances the non-radiative relaxations of the emitting and intermediate levels. Also, this increased surface area of smaller particles increases the number of surface defects which could create additional defect levels, therefore, leading to upconversion quenching. That justifies the use of bulk materials as an ideal material for quantitative measurement of the upconversion luminescence. But in many daily life applications such as biological imaging,
photodynamic therapy, etc.⁸ the use of bulk materials is highly limited. It is extremely desirable, therefore, to have highly bright and stable nanoparticles with nearly similar upconversion properties as those of bulk materials. As we mentioned before in this work, we emphasized our interest in creating a novel strategy for synthesizing sub-20 nm upconverting nanoparticles with a comparable emission performance to their bulk counterpart. To verify this, we compared the upconversion quantum yields of the nanoparticles prepared using anhydrous precursors with the bulk single crystal prepared using the modified Bridgman technique of identical dopant concentration. The NPs studied for quantum yield experiments were synthesized by decomposition of the anhydrous precursors at 290 °C for 1 h in a 1:1 mixture of ODE and OA (TEM image shown in Figure 8). The quantitative quantum yield analysis of upconversion emission was performed using the method described by Boyer and van Veggel.⁶⁴ From these measurements, as shown in Figure 11, we can see that from lower intensities (as low as 0.5 W/cm²) to higher intensities (up to 7 W/cm²) the QY of both the single crystals and the nanoparticles has an increasing trend and obvious plateau at higher intensities (due to saturation effect) for all the respective wavelengths. At low excitation, there are QY values for the nanoparticles that are only around one order lower than the values for a single crystal. At higher excitations (~10W/cm²), these QY values are quite comparable for identically doped single crystals and nanoparticles (Figure 11). Upconversion photoluminescence quantum yield (UC-QY) and energy conversion efficiencies (ECE) are reported in Table S10 and S11, respectively.

Considering that the quantum yield measurements were performed on 6 months-old samples and the average size of these nanoparticles (15 ± 5 nm) is much smaller than the bulk single crystals, the total quantum yield of our sample (about 1.3% at ~10W/cm², as compared to QY of 0.005 to 0.4% obtained previously on nanoparticles of similar size ⁶⁴) presents an improvement by a
factor of at least 3. Our QY also compares well with previously reported much bigger NPs of ~90 nm (Table S12),⁶⁵ thus justifying the use of anhydrous precursors as a synthetic strategy to avoid the risk of introducing surface and internal hydroxyl groups and, therefore, to produce small but highly bright UCNPs without necessitating a surface passivation coating of silica or undoped matrix.

<figure><img src="image_15.png" /><figcaption>**Figure 11.** Excitation power (P) dependent upconversion quantum yield (Φ<sub>uc</sub>) measurement of LiYF<sub>4</sub>: 16.5% Yb<sup>3+</sup>, 0.4% Tm<sup>3+</sup> single crystals (left) and nanoparticles (right) prepared by decomposition of the anhydrous precursors at 290 °C for 1 h in a 1:1 mixture of ODE and OA [as bulk v/s nano].</figcaption></figure>
4) UCNPs-embedded transparent TiO₂ metallogels for TiO₂-LiYF₄:Yb³⁺,Tm³⁺ nanocomposites with high surface area

In recent years, it has been shown that the nanocomposites of UCNPs with TiO₂ in the form of a core-shell structure could be a very promising approach for near infrared-induced photocatalytic applications.⁶⁶,⁶⁷ These strategies exploit the ability of the upconverting nanoparticles (UCNPs) to convert IR photons to UV-visible, which in turn could be used by titania for the photocatalysis. However, the obtained core-shell structures lack homogeneity and it is difficult to obtain a sufficiently thick layer of titania. Besides, reproducibility, stability and scalability of these structures are also challenging tasks. These issues can be tackled to an extent by alternate strategies such as composite thin films.⁶⁸ In this regard, embedding UCNPs in TiO₂-based metallogels could potentially be an excellent strategy to efficient composites with an intimate mixing of the two components. We describe here preliminary results on the preparation of transparent UCNPs-embedded TiO₂ metallogels as a precursor to UCNPs-TiO₂ xerogels having high surface area. The NPs used here were from the same lot prepared for the quantum yield studies in the above section.

To achieve robust gels, usually organic ligands with different functionalities are introduced
into gel networks. However, presence of these ligands might be detrimental for the
photocatalysis. Fortunately, monolithic TiO₂ metallogels can be obtained via sol-gel process in a
controlled acidic medium without using any organic ligand.⁶⁹,⁷⁰ In the present case, we prepared
well-defined transparent TiO₂ monolithic metallogel *via* controlled hydrolysis and
polycondensation reactions using optimized ratio of [Ti(OEt)₄]₃ and HCl in EtOH and
characterized the dried TiO₂ gel with and without heat treatment by powder XRD, TG-DTG and
N₂ adsorption and desorption isotherms (Figure S17). The incorporation of upconverting LiYF₄:
16.5% Yb³⁺, 0.4% Tm³⁺ nanoparticles in the solution of [Ti(OEt)₄]₃ in EtOH before its controlled hydrolysis leads to the formation of TiO₂ metallogel with uniform embedding of UCNPs. We chose different amount of UCNPs to be embedded in the TiO₂ metallogel. The uniform embedding of these UCNPs into the metallogels and their transparency, particularly for the low amounts of NPs, can be seen from the naked eyes (Figure 12a). Expectedly, the transparency of these TiO₂ metallogels decreases on increasing NPs contents. Figure 12b shows the digital images of these UCNPs-embedded metallogel being irradiated with the 973 nm laser excitation thus confirming further the even distribution of upconverting nanoparticles.

The powder XRD patterns of the as-dried and calcined xerogel samples containing maximum UCNPs i.e. 10% of LiYF₄: 16.5% Yb³⁺, 0.4% Tm³⁺ are given in Figure 12c. As expected, the as-dried sample shows the diffraction peaks only for LiYF₄: Yb³⁺, Tm³⁺ due to the amorphous nature of TiO₂, whereas the sample calcined at 400 °C /2 h exhibits XRD peaks characteristic of both LiYF₄ and TiO₂. Interestingly, there is a small percentage of YF₃ also in the sample, probably due to partial loss of volatile Lithium ions from the crystal sites. As compared to other alkali metals, lithium has more covalent character and, therefore, more volatile in nature. This is also one of the reasons why we took the lithium precursor in excess during the synthesis of LiYF₄: 16.5% Yb³⁺, 0.4% Tm³⁺ NPs at high temperature (see experimental section). It seems that the partial loss of Li⁺ ions is facilitated by the volatile contents of the metallogel because when the as-prepared LiYF₄: 16.5% Yb³⁺, 0.4% Tm³⁺ NPs were calcined alone at 400 °C, they did not yield any YF₃ impurity (Figure 12c). This underlines the importance of surrounding environment during synthesis and treatment of lithium-based upconverting nanoparticles.

The N₂ adsorption-desorption isotherms (Figure 12d) of these as-dried UCNPs-embedded TiO₂ xerogels are quite similar to the isotherm obtained for the undoped TiO₂ xerogel. Even though
the basic features of the obtained type IV isotherms remain unchanged, the BET surface area of
these UCNPs-embedded TiO2 xerogels, particularly for those with smaller % of NPs, are even
higher than that of the undoped TiO2 sample. Thus, as compared to a surface area of 489 m²/g
obtained for the as-dried undoped TiO2, the as-dried TiO2 xerogels with 1, 2 and 5% of UCNPs
show BET surface area of 581, 548 and 564 m²/g, respectively. Clearly, the incorporation of the
nanoparticles in the metallogel network provide additional surface and the total surface area of
the embedded metallogel has contribution from both the components. Because the gelation
happens after the incorporation of UCNPs, there is no significant change in the characteristics of
the NPs-embedded metallogels (as compared to undoped TiO2 metallogel). It also ensures
homogeneous distribution of the upconverting nanoparticles, even though the average particle
size of UCNPs is larger than the average pore size in the metallogel. A slightly less surface area
of 436 m²/g for the as-dried TiO2 xerogel with 10% UCNPs is probably due to partial blocking
of the pores of the network. The TEM images of a representative sample i.e. as-dried 10%
UCNPs-TiO2 xerogel show the intimate mixing of amorphous titania and well crystalline
upconverting NPs. While the HRTEM image confirms the presence of (112) planes
corresponding to tetragonal LiYF4, the presence of titanium is further confirmed by the EDX
spectrum (Figure S18).
<figure><img src="image_16.png" /><figcaption>**Figure 12.** (a) Blank and UCNPs-embedded TiO₂ metallogels containing varying amounts of LiYF₄: 16.5% Yb³⁺, 0.4% Tm³⁺ UCNPs. (b) Varied intensity of Tm³⁺ luminescence with different doping percentage of UCNPs in TiO₂ metallogels. (c) Powder XRD pattern of pure LiYF₄: 16.5% Yb³⁺, 0.4% Tm³⁺ NPs and its 10% UCNPs-TiO₂ metallogel composite with and without heat treatment, and (d) N₂ adsorption and desorption isotherms of undoped TiO₂ dried gel and TiO₂ gels embedded with different percentages of UCNPs.</figcaption></figure>

In the upconversion studies at different excitation power on the as-dried and the *in situ* calcined at 300 °C samples, a decrease in the UC emission intensity is observed after the calcination step (Figure S19). For instance, at higher power (20.8 W/cm²), the emission intensity
at 348 nm, representing ¹I₆ to ³F₄ transition of Tm³⁺ is reduced 2.9 times, whereas for the ¹D₂ to ³H₆ transition at 362 nm, the intensity is lowered by 1.4 times after calcination. This reduction in the upconversion emission intensity indicates that after the calcination step the interaction between the UCNPs and TiO₂ probably becomes more efficient for the non-radiative transfer of energy from UCNPs to TiO₂. However, lifetime data analysis is required to have a concrete conclusion. Figures 13 shows preliminary results on the photocatalytic activities of undoped and LiYF₄: 16.5% Yb³⁺, 0.4% Tm³⁺ UCNPs-doped TiO₂ for the degradation of methylene blue aqueous solution under Xe lamp with infrared irradiation band (800 nm -2500 nm) using HWB-800 filter. The experimental details are provided in the supporting information (Figure S20). The 5% UCNPs-TiO₂ and 10% UCNPs-TiO₂ catalysts show nearly 35% and 20% degradation of the dye in 12h (Figure 13, S21), the enhanced photocatalytic activity for the former sample is probably due to its higher surface area (564 m²/g for as-dried sample) than that of the 10% UCNPs-TiO₂ sample (436 m²/g). These preliminary results show that the metallogel-strategy to obtain UCNPs-TiO₂ composites with high surface area and the desired crystalline phases of the components is an efficient, easy-to-scale up and viable alternative to the core-shell approach to get IR-active photocatalysts.
<figure><img src="image_17.png" /><figcaption>**Figure 13.** Photodegradation of methylene blue by UCNPs-TiO₂ catalysts using Xe lamp under infrared irradiation (above 800 nm).</figcaption></figure>

**Conclusions**

Precursor-directed synthesis has an edge over other methods of synthesis of upconverting nanoparticles by having a very good control over many properties of the nanoparticles such as crystalline phase purity, size, morphology etc. In this article, we employed well-characterized novel anhydrous molecular precursors in a variety of different conditions for high-performance-oriented upconverting LiYF₄:Yb³+, Tm³+ nanoparticles. Many parameters were explored to have a better understanding of LiYF₄: Yb³+, Tm³+ upconverting system, which includes the understanding of the effect of i) varying the ratio of dopant ions, ii) ratio of solvents during thermal decomposition, iii) concentration of precursors, and iv) reaction time and temperature
over crystalline phase, size, shape, morphology and upconverting properties of nanoparticles. To
the best of our knowledge, this is the first detailed study of a morphology-selective synthesis of
LiYF₄ nanoparticles, leading to an isolation of morphologies other than the commonly obtained
diamond-shaped particles. Upconversion quantum yield of these nanoparticles was measured and
compared with their bulk counterpart of the same composition under the similar excitation
intensity and the collection geometry. Not only the quantum yield of these nanoparticles
compares well with the bulk single crystal, which is supposed to have far better performance
than nanoparticles due to fewer surface defects, but also with previously reported much bigger
NPs of ~90 nm, thus justifying the use of anhydrous precursors to obtain small but highly bright
UCNPs without necessitating a surface passivation coating of silica or undoped matrix.
Embedding these UCNPs in TiO₂-based metallogels provides an alternative strategy to efficient
composites with high surface area and an intimate mixing of the UCNPs and titania components.
This strategy could also be useful in introducing a suitable luminescent molecule in these
UCNPs-TiO₂ systems. These molecules, due to their antenna effect, may allow a broad
absorption and, therefore, further improve the optical properties of the composites. Further work
on these aspects are currently under progress.

Experimental

**Chemicals and general instrumentation procedures:** All the syntheses were carefully carried
out in an argon atmosphere using standard Schlenk lines and glovebox techniques. During the
synthesis of anhydrous precursors, the solvents used were dried with the help of MB SPS-800.
The chemicals (in bracket their supplier) such as Gadolinium oxide (Gd₂O₃; Alfa Aesar),
Ytterbium oxide (Yb₂O₃; Strem Chemicals), Yttrium oxide (Y₂O₃; Strem Chemicals), Thulium
oxide (Tm₂O₃; Strem Chemicals), trifluoroacetic acid (TFA-H; Sigma Aldrich), Lithium trifluoroacetate (LiTFA.H₂O; Strem Chemicals), Dimethoxyethane (anhydrous monoglyme; Sigma Aldrich), anhydrous ethanol (EtOH; Carlo Erba), oleic acid (C₁₈H₃₄O₂; 90%, Alfa Aesar) and 1-Octadecene (C₁₈H₃₆; 90%, Sigma Aldrich) were used without any further purification. ¹H NMR spectra were recorded in CDCl₃ on a Bruker AC-300 spectrometer. The infrared spectra were obtained as Nujol mulls on a Bruker Vector 22 FT-IR spectrometer at room temperature and registered from 4000 to 400 cm⁻¹. TEM experiments were performed using a JEM-2100HT at 200 kV. We used a Bruker D8 Advance A25 with Cu Kα1 + 2 (λ = 0.154184 nm) to measure the X-ray diffraction (XRD) patterns of the powdered crystals. During XRD, for the gadolinium sample, we used a special discrimination window to reduce the autofluorescence. Thermogravimetric analyses (TGA) were performed with a TGA/differential scanning calorimetry 1 STARe system from Mettler Toledo. For anhydrous precursors, around 5-10 mg of the sample was sealed in a 100 µL aluminium crucible in the glovebox and heated under an argon atmosphere at a heating rate of 5 °C/min, whereas for the powder samples, around 5-10 mg sample taken in an alumina crucible and heated in air or argon at a constant heating rate of 5°C/min.

**Upconversion measurement:** The upconversion studies were carried out using a homemade spectrofluorometer on dried and properly ground powders, which were placed on a sample holder with utmost care to ensure a flat and uniform deposition. The samples were irradiated with a continuous wave (CW) laser at 972 nm, which was focused to a square spot of 1 × 1 mm² on the sample with quasi-top-hat energy distribution. The emitted signal was then collected using Jobin-Yvon (TRIAX 320) monochromator equipped with automatic order
removing filters and coupled to a cooled photomultiplier (Hamamatsu R943-02). The signal from
the photomultiplier was sent to an SR400 counting module from Stanford Research Systems.

To measure the upconversion quantum yield, the chosen Ln³⁺-doped LiYF₄ samples were placed in an integrating sphere (IS200-4 from Thorlabs) and excited from above with one end of the sphere connected to the detection system. To ensure that the detection system obtains the absolute response at each wavelength, it was calibrated using an Oriel QT 63358 tungsten lamp traceable by NIST over the range 250–1000 nm. To determine the absorption of 972 nm photons by the above samples, an undoped LiYF₄ were prepared under identical conditions. The difference in laser scattering compared to doped and undoped samples gave us an estimate of the actual absorption of the laser by the sample. Thus, the actual number of absorbed photons (respective energy absorbed) and the actual number of emitted photons (respective energy emitted) were measured over the wavelength range from 250 nm to 1000 nm. The quantum upconversion efficiency (energy conversion efficiency) is the ratio of emitted photons (emitted power) to absorbed photons (absorbed power). Energy conversion efficiency (ECE) is the ratio between the total energy emitted over a spectral range and the total energy absorbed at the wavelength of excitation.

## Synthesis of anhydrous precursors

The anhydrous lithium and lanthanide trifluoroacetate precursors for lithium-based upconverting nanoparticles were synthesized using two different strategies. The first strategy involves ligand exchange of the water molecules in Li(TFA)(H₂O) and Ln(TFA)₃(H₂O)₃ with monoglyme ligand (method A). The second method of the synthesis involves refluxing of lanthanide oxides in
monoglyme (for Y) or a mixture of monoglyme + THF (for Gd, Tm and Yb), followed by crystallization by layering the concentrated mother solution with diethyl ether (method B).

## Method A

[Li(TFA)(monoglyme)] (1): 0.28 g (2 mmol) of commercial Li(TFA)(H₂O) was dissolved in 1 ml of monoglyme (9.3 mmol) and stirred the solution for 2 h. The obtained clear solution was concentrated to a minimum amount and carefully layered with diethylether (Et₂O) to obtain colourless crystals of 1. Yield, 0.36 g (86%). Anal. Calcd. for C₆H₁₀F₃LiO₄ (210): C, 34.28; H, 4.76; Found: C, 33.78; H, 4.61%. FT-IR (Nujol, cm⁻¹): 1704 (s) (vₐs CO₂), 1462 (s), 1372 (s), 1208 (w), 1147 (s) (v C–F, v C–O), 1072 (w), 1052 (w), 894 (w), 851 (m), 803 (s), 726 (s), 606 (w), 522 (w). ¹H NMR (acetone-d₆, ppm): δ 3.28 (s, 6 H, CH₃), 3.46 (t, 4 H, CH₂). ¹⁹F NMR (acetone-d₆): δ -76.2 ppm.

[Li(TFA)(THF)] (2): When Li(TFA)(H₂O) (0.577 g, 4.2 mmol) was dissolved in a mixture of THF and monoglyme (5 ml each), followed by crystallization by layering with Et₂O, it afforded colourless crystals of 2. Yield, 0.63 g (78%). Anal. Calcd. for C₆H₈F₃LiO₃ (192.1): C, 37.48; H, 4.16; Found: C, 37.0; H, 4.05%. FT-IR (Nujol, cm⁻¹): 1702 (s) (vₐs CO₂), 1459 (s), 1378 (s), 1207 (w), 1145 (s) (v C–F, v C–O), 1054 (s), 894 (w), 853 (m), 804 (s), 724 (s), 607 (w), 520 (w). ¹H NMR (CD₃CN, ppm): δ 1.80 (t, 4 H, β-H of THF), 3.65 (t, 4 H, α-H of THF). ¹⁹F NMR (CD₃CN): δ -77.2 ppm.

[Y(TFA)₃(monoglyme)] (3): Y(TFA)₃(H₂O)₃ (0.25 g, 0.52 mmol) was stirred in 1 ml monoglyme for 2 h followed by crystallization by carefully layering with Et₂O. Yield, 0.21 g (78%). Anal. Calcd. for C₁₀H₁₀F₉O₈Y (518): C, 23.17; H, 1.93; Found: C, 23.34; H, 2.05%. FT-IR (Nujol, cm⁻¹): 1756 (s), 1676 (m), 1648 (w) (vₐs CO₂), 1460 (s), 1377 (s), 1207 (s), 1150 (s) (v
C–F, v C–O), 1020 (m), 921 (w), 868 (m), 851 (m), 797 (s), 720 (s), 612 (s), 522 (s), 463 (m), 439 (m) (v Y–O). ¹H NMR (acetone-d6, ppm): δ 3.31 (s, 6 H, CH₃), 3.45 (t, 4 H, CH₂). ¹⁹F NMR (acetone-d6): δ -77.0 ppm.

By adopting the similar procedure as described for **3**, complexes
[**Gd(TFA)₃(monoglyme)**] (4), [**Tm(TFA)₃(monoglyme)**] (5) and [**Yb(TFA)₃(monoglyme)**] (6)
were also synthesized using appropriate quantity (given below in parentheses) of
Ln(TFA)₃(H₂O)₃ (Ln = Gd, Tm and Yb) in a mixture of monoglyme and THF and crystallized by
layering the concentrated mother solution with Et₂O.

[**Gd(TFA)₃(monoglyme)**] (4): Gd(TFA)₃(H₂O)₃ (1.16 g, 2.12 mmol) in monoglyme (0.5 ml, 4.8 mmol) and 5 ml THF). Yield, 1.1 g (86%). Anal. Calcd. for C₁₀H₁₀F₉GdO₈ (586.2): C, 20.47; H, 1.70; Found: C, 20.17; H, 1.61%. FT-IR (Nujol, cm⁻¹): 1712 (s), 1625 (w), (vₐs CO₂), 1459 (s), 1376 (s), 1209 (m), 1148 (m) (v C–F, v C–O), 1052 (m), 867 (w), 845 (m), 799 (s), 722 (s), 608 (s), 522 (m), 455 (m) (v Gd–O).

[**Tm(TFA)₃(monoglyme)**] (5): Tm(TFA)₃(H₂O)₃ (0.4 g, 0.7 mmol) in monoglyme (5 ml) and THF (5 ml). Yield, 0.35 g (82%). Anal. Calcd. for C₁₀H₁₀F₉O₈Tm (598): C, 20.1; H, 1.67; Found: C, 19.91; H, 1.61%. FT-IR (Nujol, cm⁻¹): 1742 (w), 1711 (s), 1674 (w), (vₐs CO₂), 1463 (s), 1376 (s), 1209 (w), 1150 (w), 1105 (w) (v C–F, v C–O), 1052(m), 1025 (w), 872 (s), 848 (s), 797 (s), 722 (s), 611 (s), 523 (s), 459 (m), 438 (m) (v Tm–O).

[**Yb(TFA)₃(monoglyme)**] (6): Yb(TFA)₃(H₂O)₃ (0.5 g, 0.88 mmol) in a mixture of monoglyme (0.35 ml, 3.3 mmol) and THF (5 ml). Yield, 0.42 g (80%). Anal. Calcd. for C₁₀H₁₀F₉O₈Yb (603): C, 19.90; H, 1.66; Found: C, 19.83; H, 1.61%. FT-IR (Nujol, cm⁻¹): 1743 (w), 1715 (s), 1676 (w), (vₐs CO₂), 1454 (s), 1377 (s), 1206 (s), 1149 (m), 1105 (w) (v C–F, v C–
O), 1050(w), 1025 (w), 873 (m), 848 (m), 798 (s), 722 (s), 610 (s), 522 (s), 459 (m), 438 (m) (ν Yb–O).

### Method B for the synthesis of (3)-(6)

Alternatively, the anhydrous lanthanide derivatives (3)-(6) could also be synthesized by a one-pot reaction of $Ln_2O_3$ with trifluoroacetic acid under reflux in a mixture of monoglyme and THF. As mentioned earlier, the coordinating solvent THF, which promotes the formation of $[Y(TFA)_3(THF)_2]$, needs to be avoided for the synthesis of the yttrium complex (3). For other complexes (4)-(6), the presence of THF has no adverse effects. Due to the similarity of the synthetic procedure, the synthesis of only yttrium complex (3) is described in detail below:

**[Y(TFA)₃(monoglyme)] (3):** A reaction mixture containing $Y_2O_3$ (0.5 g, 2.2 mmol), trifluoroacetic acid (1.1 ml, 14.4 mmol) and monoglyme (20 ml) was refluxed for 12 h at 85 °C. After cooling and filtering the reaction mixture, the filtrate was concentrated and layered carefully with diethyl ether to obtain colourless crystals in a 75% yield. The elemental analyses and the FT-IR spectra of (3)-(6) isolated by method B were consistent with the same products isolated by method A.

### Synthesis of Upconverting nanoparticles

**Synthesis of undoped LiYF₄ matrices:** In this section, we first present the general method of synthesis of lithium-based upconverting nanoparticles (UCNPs). We will describe the detailed synthesis methodology for undoped LiYF₄ nanoparticles, which could be extended to synthesize LiYF₄: Yb³⁺, Tm³⁺ NPs by mixing the appropriate amounts of Y, Yb and Tm precursors during the reaction. For this, we prepare two different solutions, A and B. Solution A was prepared by taking [Li(TFA)(monoglyme)] (0.81 g, 3.85 mmol) and [Y(TFA)₃(monoglyme)] (1.76 g, 3.40
mmol), in a round bottom Schlenk containing 2 ml each of oleic acid (OA) and 1-octadecene (ODE). The lithium precursor was taken in slight excess to compensate the loss of volatile lithium ions during decomposition process. This solution was maintained at 120 °C for 30 min under an inert atmosphere to obtain a clear homogeneous and transparent solution. Solution B was prepared in another round bottom Schlenk containing 3 ml each of OA and ODE. Unless specified otherwise, the ratio of OA and ODE was kept 1:1 for all the decompositions. This solution was first degassed with constant stirring under argon flow and then heated to 290 °C. Once the temperature became stable at 290 °C, we injected then pre-heated precursor solution A into the decomposition solution B all at once. This solution was kept under constant stirred at 290 °C for 1 h, after which it was gradually cooled down to room temperature under argon flow. The nanocrystals were then precipitated out by adding ethanol and were separated using centrifugation (4500 rpm, 5 min). The powder obtained was washed three times with ethanol to remove any organic impurities and then dried under air at room temperature for 24 hours. The nanocrystals obtained were phase-pure tetragonal (ICDD# 04-007-5501). A similar method was adopted to prepare undoped $LiYbF_4$ also (ICDD# 04-010-3625). To obtain the host matrix $LiYF_4$ NPs co-doped with 16.5 mol% $Yb^{3+}$ and x mol% $Tm^{3+}$ cations, the derivatives **1, 3, 5** and **6** were taken in appropriate amounts and then decomposed simultaneously. The details of each synthesis are summarized in the tables S2-S7. The synthesis of UCNPs-embedded $TiO_2$ metallogels and experimental details of photocatalytic degradation of dye are also described in supporting information.

## REFERENCES
(1) Zheng, K.; Loh, K. Y.; Wang, Y.; Chen, Q.; Fan, J.; Jung, T.; Nam, S. H.; Suh, Y. D.; Liu, X. *Recent Advances in Upconversion Nanocrystals: Expanding the Kaleidoscopic Toolbox for Emerging Applications.* *Nano Today* **2019**, *29*, 100797.
https://doi.org/10.1016/j.nantod.2019.100797.

(2) Zhang, Q.; Yang, F.; Xu, Z.; Chaker, M.; Ma, D. Are Lanthanide-Doped Upconversion Materials Good Candidates for Photocatalysis? *Nanoscale Horiz.* **2019**, *4*, 579-591.
https://doi.org/10.1039/C8NH00373D.

(3) Wen, S.; Zhou, J.; Zheng, K.; Bednarkiewicz, A.; Liu, X.; Jin, D. Advances in Highly Doped Upconversion Nanoparticles. *Nature Commun.* **2018**, *9*, 2415. https://doi.org/10.1038/s41467-018-04813-5.

(4) Li, X.; Zhang, F.; Zhao, D. Lab on Upconversion Nanoparticles: Optical Properties and Applications Engineering via Designed Nanostructure. *Chem. Soc. Rev.* **2015**, *44*, 1346-1378.
https://doi.org/10.1039/C4CS00163J.

(5) Yang, D.; Ma, P.; Hou, Z.; Cheng, Z.; Li, C.; Lin, J. Current Advances in Lanthanide Ion (Ln³⁺)-Based Upconversion Nanomaterials for Drug Delivery. *Chem. Soc. Rev.* **2015**, *44*, 1416-1448. https://doi.org/10.1039/C4CS00155A.

(6) Zhou, J.; Liu, Q.; Feng, W.; Sun, Y.; Li, F. Upconversion Luminescent Materials: Advances and Applications. *Chem. Rev.* **2015**, *115*, 395-465. https://doi.org/10.1021/cr400478f.

(7) Goesmann, H.; Feldmann, C. Nanoparticulate Functional Materials. *Angew. Chem. Int. Ed.* **2010**, *49*, 1362-1395. https://doi.org/10.1002/anie.200903053.

(8) Xue, X.; Uechi, S.; Tiwari, R. N.; Duan, Z.; Liao, M.; Yoshimura, M.; Suzuki, T.; Ohishi, Y. Size-Dependent Upconversion Luminescence and Quenching Mechanism of LiYF₄: Er³⁺/Yb³⁺
Nanocrystals with Oleate Ligand Adsorbed. *Opt. Mater. Express* **2013**, *3*, 989.
https://doi.org/10.1364/OME.3.000989.

(9) Tessitore, G.; Mandl, G. A.; Brik, M. G.; Park, W.; Capobianco, J. A. Recent Insights into Upconverting Nanoparticles: Spectroscopy, Modeling, and Routes to Improved Luminescence. *Nanoscale* **2019**, *11*, 12015–12029. https://doi.org/10.1039/C9NR02291K.

(10) Ledoux, G.; Joubert, M. F.; Mishra, S. Upconversion Phenomena in Nanofluorides. In *Photonic and Electronic Properties of Fluoride Materials*; Tressaud, A., Poeppelmeier, K., Eds.; Elsevier: Boston, 2016; pp 35–63. https://doi.org/10.1016/B978-0-12-801639-8.00003-9.

(11) Chen, X.; Peng, D.; Ju, Q.; Wang, F. Photon Upconversion in Core–Shell Nanoparticles. *Chem. Soc. Rev.* **2015**, *44*, 1318–1330. https://doi.org/10.1039/C4CS00151F.

(12) Vetrone, F.; Naccache, R.; Mahalingam, V.; Morgan, C. G.; Capobianco, J. A. The Active-Core/Active-Shell Approach: A Strategy to Enhance the Upconversion Luminescence in Lanthanide-Doped Nanoparticles. *Adv. Funct. Mater.* **2009**, *19*, 2924–2929.
https://doi.org/10.1002/adfm.200900234.

(13) 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.
https://doi.org/10.1021/cm3033498.

(14) Cao, C.; Liu, Q.; Shi, M.; Feng, W.; Li, F. Lanthanide-Doped Nanoparticles with Upconversion and Downshifting Near-Infrared Luminescence for Bioimaging. *Inorg. Chem.* **2019**, *58*, 9351–9357. https://doi.org/10.1021/acs.inorgchem.9b01071.

(15) Wang, Z.; Meijerink, A. Concentration Quenching in Upconversion Nanocrystals. *J. Phys. Chem. C* **2018**, *122*, 26298–26306. https://doi.org/10.1021/acs.jpcc.8b09371.
(16) Feng, Y.; Li, Z.; Li, Q.; Yuan, J.; Tu, L.; Ning, L.; Zhang, H. Internal OH⁻ induced cascade quenching of upconversion luminescence in NaYF₄:Yb/Er nanocrystals. *Light Sci. Appl.* **2021**, *10*, 105. https://doi.org/10.1038/s41377-021-00550-5.

(17) Chen, Y.; Mishra, S.; Ledoux, G.; Jeanneau, E.; Daniel, M.; Zhang, J.; Daniele, S. Direct Synthesis of Hexagonal NaGdF₄ Nanocrystals from a Single-Source Precursor: Upconverting NaGdF₄:Yb³⁺,Tm³⁺ and Its Composites with TiO₂ for Near-IR-Driven Photocatalysis. *Chem. Asian J.* **2014**, *9*, 2415-2421. https://doi.org/10.1002/asia.201402347.

(18) Mishra, S.; Daniele, S.; Ledoux, G.; Jeanneau, E.; Joubert, M.-F. Heterometallic Na–Y(Ln) Trifluoroacetate Diglyme Complexes as Novel Single-Source Precursors for Upconverting NaYF₄ Nanocrystals Co-Doped with Yb and Er/Tm Ions. *Chem. Commun.* **2010**, *46*, 3756–3758.
https://doi.org/10.1039/B921474G.

(19) Mishra, S.; Ledoux, G.; Jeanneau, E.; Daniele, S.; Joubert, M.-F. Novel Heterometallic Organic Complexes as First Single Source Precursors for up-Converting NaY(Ln)F₄ (Ln = Yb, Er, Tm) Nanomaterials. *Dalton Trans.* **2012**, *41*, 1490–1502.
https://doi.org/10.1039/C1DT11070E.

(20) Ayadi, H.; Fang, W.; Mishra, S.; Jeanneau, E.; Ledoux, G.; Zhang, J.; Daniele, S. Influence of Na⁺ Ion Doping on the Phase Change and Upconversion Emissions of the GdF₃: Yb³⁺, Tm³⁺ Nanocrystals Obtained from the Designed Molecular Precursors. *RSC Adv.* **2015**, *5*, 100535–100545. https://doi.org/10.1039/C5RA20781A.

(21) Purohit, B.; Jeanneau, E.; Cornier, T.; Ledoux, G.; Mishra, S. An anhydrous precursor approach to BaYF₅-based upconverting nanocrystals, *J. Ind. Chem. Soc.* **2022**, *99*, 100322.

(22) Purohit, B.; Amans, D.; Guyot, Y.; Mahler, B.; Joubert, M.-F.; Dujardin, C.; Daniele, S.; Ledoux, G.; Mishra, S. Quest to Enhance Up-Conversion Efficiency: A Comparison of
Anhydrous vs. Hydrous Synthesis of NaGdF₄: Yb³⁺ and Tm³⁺ Nanoparticles. *Mater. Today Chem.* **2020**, *17*, 100326. https://doi.org/10.1016/j.mtchem.2020.100326.

(23) Homann, C.; Krukewitt, L.; Frenzel, F.; Grauel, B.; Wgrth, C.; Resch-Genger, U.; Haase, M. NaYF₄:Yb,Er/NaYF₄ Core/Shell Nanocrystals with High Upconversion Luminescence Quantum Yield. *Angew. Chem. Int. Ed.* **2018**, *57*, 8765–8769.
https://doi.org/10.1002/anie.201803083.

(24) Naccache, R.; Yu, Q.; Capobianco, J. A. The Fluoride Host: Nucleation, Growth, and Upconversion of Lanthanide-Doped Nanoparticles. *Adv. Opt. Mater.* **2015**, *3*, 482–509.
https://doi.org/10.1002/adom.201400628.

(25) Mahalingam, V.; Vetrone, F.; Naccache, R.; Speghini, A.; Capobianco, J. A. Colloidal Tm³⁺/Yb³⁺-Doped LiYF₄ Nanocrystals: Multiple Luminescence Spanning the UV to NIR Regions via Low-Energy Excitation. *Adv. Mater.* **2009**, *21*, 4025–4028.
https://doi.org/10.1002/adma.200901174.

(26) Purohit, B.; Guyot, Y.; Amans, D.; Joubert, M.-F.; Mahler, B.; Mishra, S.; Daniele, S.; Dujardin, C.; Ledoux, G. Multicolor Solar Absorption as a Synergetic UV Upconversion Enhancement Mechanism in LiYF₄:Yb³⁺,Tm³⁺ Nanocrystals. *ACS Photonics* **2019**, *6*, 3126–3131. https://doi.org/10.1021/acsphotonics.9b01151.

(27) Sayevich, V.; Cai, B.; Benad, A.; Haubold, D.; Sonntag, L.; Gaponik, N.; Lesnyak, V.; Eychmüller, A. 3D Assembly of All-Inorganic Colloidal Nanocrystals into Gels and Aerogels. *Angew. Chem. Int. Ed.* **2016**, *55*, 6334–6338. https://doi.org/10.1002/anie.201600094.

(28) Niederberger, M. Multiscale Nanoparticle Assembly: From Particulate Precise Manufacturing to Colloidal Processing. *Adv. Funct. Mater.* **2017**, *27*, 1703647.
https://doi.org/10.1002/adfm.201703647.
(29) Felip-León, C.; Guzzetta, F.; Julián-López, B.; Galindo, F.; Miravet, J. F. Multimodal Light-Harvesting Soft Hybrid Materials: Assisted Energy Transfer upon Thermally Reversible Gelation. *J. Phys. Chem. C* **2017**, *121*, 21154–21159. https://doi.org/10.1021/acs.jpcc.7b06441.

(30) Wadhavane, P. D.; Galian, R. E.; Izquierdo, M. A.; Aguilera-Sigalat, J.; Galindo, F.; Schmidt, L.; Burguete, M. I.; Pérez-Prieto, J.; Luis, S. V. Photoluminescence Enhancement of CdSe Quantum Dots: A Case of Organogel–Nanoparticle Symbiosis. *J. Am. Chem. Soc.* **2012**, *134*, 20554–20563. https://doi.org/10.1021/ja310508r.

(31) Segarra-Maset, M. D.; Nebot, V. J.; Miravet, J. F.; Escuder, B. Control of Molecular Gelation by Chemical Stimuli. *Chem. Soc. Rev.* **2013**, *42*, 7086–7098. https://doi.org/10.1039/C2CS35436E.

(32) Cheng, T.; Marin, R.; Skripka, A.; Vetrone, F. Small and Bright Lithium-Based Upconverting Nanoparticles. *J. Am. Chem. Soc.* **2018**, *140*, 12890–12899. https://doi.org/10.1021/jacs.8b07086.

(33) Panov, N.; Marin, R.; Hemmer, E. Microwave-Assisted Solvothermal Synthesis of Upconverting and Downshifting Rare-Earth-Doped LiYF₄ Microparticles. *Inorg. Chem.* **2018**, *57*, 14920–14929. https://doi.org/10.1021/acs.inorgchem.8b02697.

(34) Liu, S.; Li, Y.; Zhang, C.; Yang, L.; Zhao, T.; Zhang, R.; Jiang, C. Upconversion Color Tuning in Ce³⁺-Doped LiYF₄:Yb³⁺/Ho³⁺@LiYF₄ Nanoparticles towards Ratiometric Fluorescence Detection of Chromium(III). *J. Colloid Interface Sci.* **2017**, *493*, 10–16. https://doi.org/10.1016/j.jcis.2017.01.022.

(35) Zhang, D.; De, G.; Zi, L.; Xu, Y.; Liu, S.; Liu, Y.; Wang, X. Dropwise Addition of Cation Solution: An Approach for Growing High-Quality Upconversion Nanoparticles. *J. Colloid Interface Sci.* **2018**, *512*, 141–150. https://doi.org/10.1016/j.jcis.2017.10.012.
(36) Maurizio, S. L.; Tessitore, G.; Mandl, G. A.; Capobianco, J. A. Luminescence Dynamics and Enhancement of the UV and Visible Emissions of Tm³⁺ in LiYF₄:Yb³⁺,Tm³⁺ Upconverting Nanoparticles. *Nanoscale Adv.* **2019**, *1*, 4492–4500. https://doi.org/10.1039/C9NA00556K.

(37) Liu, S.; Huang, J.; Yan, L.; Song, N.; Zhang, P.; He, J.; Zhou, B. Multiphoton ultraviolet upconversion through selectively controllable energy transfer in confined sensitizing sublattices towards improved solar photocatalysis. *J. Mater. Chem. A* **2021**, *9*, 4007–4017. https://doi.org/10.1039/D0TA11387E.

(38) Mishra, S.; Daniele, S. Molecular Engineering of Metal Alkoxides for Solution Phase Synthesis of High-Tech Metal Oxide Nanomaterials. *Chem. Eur. J.* **2020**, *26*, 9292–9303. https://doi.org/10.1002/chem.202000534.

(39) Brune, V.; Grosch, M.; Weißing, R.; Hartl, F.; Frank, M.; Mishra, S.; Mathur, S. Influence of the Choice of Precursors in the Synthesis of Two Dimensional Transition Metal Dichalcogenides, *Dalton Trans.* **2021**, *50*, 12365–12385. https://doi.org/10.1039/D1DT01397A.

(40) Mishra, S. Ultra-mild Synthesis of Nanometric Metal Chalcogenides using Organyl Chalcogenide Precursors, *Chem. Commun.* **2022**, *58*, 10136–10153. https://doi.org/10.1039/D2CC03458A.

(41) Mishra, S.; Daniele, S. Metal–Organic Derivatives with Fluorinated Ligands as Precursors for Inorganic Nanomaterials. *Chem. Rev.* **2015**, *115*, 8379–8448. https://doi.org/10.1021/cr400637c.

(42) Halimi, I.; Rodrigues, E. M.; Maurizio, S. L.; Sun, H.-Q. T.; Grewal, M.; Boase, E. M.; Liu, N.; Marin, R.; Hemmer, E. Pick Your Precursor! Tailoring the Size and Crystal Phase of Microwave-Synthesized Sub-10 Nm Upconverting Nanoparticles. *J. Mater. Chem. C* **2019**, *7*, 15364–15374. https://doi.org/10.1039/C9TC04817K.
(43) Mishra, S.; Hubert-Pfalzgraf, L. G.; Daniele, S.; Rolland, M.; Jeanneau, E.; Jouguet, B. Thermal Dehydration of Y(TFA)₃(H₂O)₃: Synthesis and Molecular Structures of [Y(µ,η¹:η¹-TFA)₃(THF)(H₂O)]₁∞·THF and [Y₄(µ₃-OH)₄(µ,η¹:η¹-TFA)₆(η¹-TFA)(η²-TFA)(THF)₃(DMSO)(H₂O)]·6THF (TFA = trifluoroacetate). *Inorg. Chem. Commun.* **2009**, *12*, 97–100. https://doi.org/10.1016/j.inoche.2008.11.021.

(44) Henderson, W. A. Glyme-Lithium Salt Phase Behavior. *J. Phys. Chem. B* **2006**, *110*, 13177-13183.

(45) Henderson, W. A.; Young, V. G.; Brooks, N. R.; Smyrl, W. H. Li⁺ Cation Coordination in [Li₂(CF₃SO₃)₂(Diglyme)] and [Li₃(C₂F₃O₂)₃(Diglyme)]. *Acta Crystallogr C Cryst Struct Commun* **2002**, *58*, m501–m503. https://doi.org/10.1107/S0108270102015135.

(46) Henderson, W. A.; Brooks, N. R.; Young, V. G. Tetraglyme–Li⁺ Cation Solvate Structures: Models for Amorphous Concentrated Liquid and Polymer Electrolytes (II). *Chem. Mater.* **2003**, *15*, 4685–4690. https://doi.org/10.1021/cm034352r.

(47) Na, H.; Jeong, J. S.; Chang, H. J.; Kim, H. Y.; Woo, K.; Lim, K.; Mkhoyan, K. A.; Jang, H. S. Facile Synthesis of Intense Green Light Emitting LiGdF₄:Yb,Er-Based Upconversion Bipyramidal Nanocrystals and Their Polymer Composites. *Nanoscale* **2014**, *6*, 7461–7468. https://doi.org/10.1039/C4NR00857J.

(48) Du, Y.-P.; Zhang, Y.-W.; Sun, L.-D.; Yan, C.-H. Optically Active Uniform Potassium and Lithium Rare Earth Fluoride Nanocrystals Derived from Metal Trifluroacetate Precursors. *Dalton Trans.* **2009**, 8574–8581. https://doi.org/10.1039/B909145A.

(49) Ma, C.; Xu, X.; Wang, F.; Zhou, Z.; Liu, D.; Zhao, J.; Guan, M.; Lang, C. I.; Jin, D. Optimal Sensitizer Concentration in Single Upconversion Nanocrystals. *Nano Lett.* **2017**, *17*, 2858–2864. https://doi.org/10.1021/acs.nanolett.6b05331.
(50) Rabouw, F. T.; Prins, P. T.; Villanueva-Delgado, P.; Castelijns, M.; Geitenbeek, R. G.; Meijerink, A. Quenching Pathways in NaYF₄:Er³⁺,Yb³⁺ Upconversion Nanocrystals. *ACS Nano* **2018**, *12*, 4812–4823. https://doi.org/10.1021/acsnano.8b01545.

(51) Thoma, R. E.; Brunton, G. D.; Penneman, R. A.; Keenan, T. K. Equilibrium Relations and Crystal Structure of Lithium Fluorolanthanate Phases. *Inorg. Chem.* **1970**, *9*, 1096–1101. https://doi.org/10.1021/ic50087a019.

(52) Sui, J.; Yan, J.; Wang, K.; Luo, G. Efficient Synthesis of Lithium Rare-Earth Tetrafluoride Nanocrystals via a Continuous Flow Method. *Nano Res.* **2020**, *13*, 2837–2846. https://doi.org/10.1007/s12274-020-2938-y.

(53) Zhang, X.; Wang, M.; Ding, J.; Gao, D.; Shi, Y.; Song, X. The Novel Upconversion Properties of LiYbF₄:Er Microcrystals Compared to the Na Counterpart. *CrystEngComm* **2012**, *14*, 8357–8360. https://doi.org/10.1039/C2CE26159F.

(54) Zhang, X.; Wang, M.; Ding, J. Shape-Selective Synthesis, Characterization and Upconversion Improvement of Yb³⁺/Er³⁺ Doped LiYF₄ Microphosphors through PH Tuning. *RSC Adv.* **2014**, *4*, 29165. https://doi.org/10.1039/C4RA02538E.

(55) Ye, S.; Hu, R.; Jiang, N.; Wang, H.; Wang, D. PH Value Manipulated Phase Transition, Microstructure Evolution and Tunable Upconversion Luminescence in Yb³⁺–Er³⁺ Codoped LiYF₄/YF₃ Nanoparticles. *Dalton Trans.* **2015**, *44*, 15583–15590. https://doi.org/10.1039/C5DT01552A.

(56) Bian, W.; Lin, Y.; Wang, T.; Yu, X.; Qiu, J.; Zhou, M.; Luo, H.; Yu, S. F.; Xu, X. Direct Identification of Surface Defects and Their Influence on the Optical Characteristics of Upconversion Nanoparticles. *ACS Nano* **2018**, *12*, 3623–3628. https://doi.org/10.1021/acsnano.8b00741.
(57) Wang, F.; Wang, J.; Liu, X. Direct Evidence of a Surface Quenching Effect on Size-Dependent Luminescence of Upconversion Nanoparticles. *Angew. Chem. Int. Ed.* **2010**, *49*, 7456–7460. https://doi.org/10.1002/anie.201003959.

(58) Mai, H.-X.; Zhang, Y.-W.; Si, R.; Yan, Z.-G.; Sun, L.; 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. https://doi.org/10.1021/ja060212h.

(59) 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. https://doi.org/10.1039/C4NR05954A.

(60) Shan, J.; Uddi, M.; Wei, R.; Yao, N.; Ju, Y. The Hidden Effects of Particle Shape and Criteria for Evaluating the Upconversion Luminescence of the Lanthanide Doped Nanophosphors. *J. Phys. Chem. C* **2010**, *114*, 2452–2461. https://doi.org/10.1021/jp908976n.

(61) Wang, W.; Li, J.; Duan, G.; Zhao, W.; Cao, B.; Liu, Z. Morphology/size effect on the luminescence properties of the [(YₓGd₁-x)₀.₉₈Dy₀.₀₂]₂O₃ phosphor with enhanced yellow emission. *J. Lumin.* **2017**, *192*, 1056–1064. https://doi.org/10.1016/j.jlumin.2017.07.046.

(62) Liu, N.; Marin, R.; Mazouzi, Y.; Cron, G. O.; Shuhendler, A.; Hemmer, E. Cubic versus Hexagonal–Effect of Host Crystallinity on the Tl Shortening Behaviour of NaGdF₄ Nanoparticles. *Nanoscale* **2019**, *11*, 6794–6801. https://doi.org/10.1039/C9NR00241C.

(63) Wang, J.; Wang, F.; Xu, J.; Wang, Y.; Liu, Y.; Chen, X.; Chen, H.; Liu, X. Lanthanide-doped LiYF₄ nanoparticles: Synthesis and multicolor upconversion tuning. *C. R. Chimie* **2010**, *13*, 731-736. doi:10.1016/j.crci.2010.03.021.
(64) Boyer, J.-C.; van Veggel, F. C. J. M. Absolute Quantum Yield Measurements of Colloidal NaYF₄: Er³⁺, Yb³⁺ Upconverting Nanoparticles. *Nanoscale* **2010**, *2*, 1417. https://doi.org/10.1039/c0nr00253d.

(65) Meijer, M. S.; Rojas-Gutierrez, P. A.; Busko, D.; Howard, I. A.; Frenzel, F.; Würth, C.; Resch-Genger, U.; Richards, B. S.; Turshatov, A.; Capobianco, J. A.; Bonnet, S. Absolute upconversion quantum yields of blue-emitting LiYF₄:Yb³⁺, Tm³⁺ upconverting nanoparticles. *Phys. Chem. Chem. Phys.* **2018**, *20*, 22556-22562.

(66) Zhang, Q.; Yang, F.; Xu, Z.; Chaker, M.; Ma, D. Are lanthanide-doped upconversion materials good candidates for photocatalysis? *Nanoscale Horiz.*, **2019**, *4*, 579-591.

(67) Ansari, A. A.; Sillanpaa, M. Advancement in upconversion nanoparticles based NIR-driven photocatalysts, *Renew. Sustain Energy Rev.* **2021**, *151*, 111631.

(68) Pennisi, F. M.; Pellegrino, A. L.; Licciardello, N.; Mezzalira, C.; Sgarzi, M.; Speghini, A.; Malandrino, G.; Cunibert, G. Synthesis, characterization and photocatalytic properties of nanostructured lanthanide doped β-NaYF₄/TiO₂ composite films. *Sci. Rep.* **2022**, *12*, 13748. https://doi.org/10.1038/s41598-022-17256-2.

(69) Konishi, J.; Fujita, K.; Nakanishi, K.; Hirao, K. Monolithic TiO₂ with Controlled Multiscale Porosity via a Template-Free Sol–Gel Process Accompanied by Phase Separation. *Chem. Mater.* **2006**, *18*, 6069–6074. https://doi.org/10.1021/cm0617485.

(70) El Mir, L.; Amlouk, A.; Elaloui, E.; Saadoun, M.; Pierre, A. C. Preparation and Optical Characterization of Transparent, Microporous TiO₂ Xerogel Monoliths. *Mater. Sci. Eng. B* **2008**, *146*, 69–73. https://doi.org/10.1016/j.mseb.2007.07.055.
**Supporting Information.** X-ray crystal structures of **2**, **4** and **6**, FT-IR, ¹H and ¹⁹F NMR spectra and DTG curves of **1-6**, TGA curves of hydrated Li(TFA) and Y(TFA)₃, powder XRD of GdF₃ + LiF obtained from the thermal decomposition of **1** and **4**, UC emission intensities of NPs and NPs-TiO₂ composites, additional TEM images and size distribution curves, characterization of TiO₂ (TG-DTG, PXRD, BET isotherms), TEM and EDX characterization, UC emission studies and photocatalytic studies of UCNPs-TiO₂ xerogels, additional synthetic and crystallographic data, Rietveld refinement parameters and UC quantum yield data in tabular forms. This material is available free of charge *via* the Internet at http://pubs.acs.org.

## Accession Codes

CCDC2087667, 2087668 and 2087669 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

## AUTHOR INFORMATION

### Corresponding Author

* S. M.: fax, (+33) 472445399; e-mail, shashank.mishra@ircelyon.univ-lyon1.fr.

### Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

### Notes

The authors declare no competing financial interest.
ACKNOWLEDGMENT

This work was funded by the Agence Nationale de la Recherche through grant number ANR-17-
CE09-0002. The authors thank Y. Aizac of IRCELYON for powder XRD data.

TABLE OF CONTENT SYNOPSIS

TOC Figure

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TOC text

This systematic study validates the strategy of using anhydrous precursor to get small and naked
but highly emitting upconverting NPs. The influence of key reaction parameters over size, shape,
morphology and upconverting properties of the Ln³⁺-doped LiYF₄ NPs are presented. Successful
incorporation of these UCNPs in transparent TiO₂ metallogels opens a route for many potential
applications.