Eu-Tb-Doped Y-BTC MOF: A Step Towards Optimization of an Energy Conversion System

Abstract

Lanthanide-based metal–organic frameworks (Ln-MOFs) represent a key material in various optical applications. Thus, they offer the possibility of fine-tuning their functional properties by adjusting the composition, stoichiometry, and ligand nature. This work reports for the first time the environmentally friendly one-pot synthesis of Eu-Tb-doped yttrium-1,3,5-benzenetricarboxylate MOF, i.e., Y-BTC: Eu (10%), Tb (10%), under mild conditions of temperature and pressure. Structural and morphological investigations were conducted through ATR-IR, XRD, and FE-SEM characterization. The doping percentage was analyzed by EDX spectroscopy. The luminescence properties confirm the down-shifting behavior of the MOF, paving the way for using this Eu-Tb-doped Y-BTC system in photovoltaic technology.

1. Introduction

Due to their unique structural and functional characteristics, lanthanide-based metal–organic frameworks (Ln-MOFs) have attracted much attention. These properties make them suitable for a variety of applications, including drug delivery [1], gas storage [2,3], catalysis [4,5], sensing [6,7,8,9,10], and photovoltaics [11,12,13]. Fine-tuning metal centers, organic linkers, and guest molecules can readily modify their structure to optimize optical properties.

The physicochemical properties of lanthanide elements are constant across the series and include similar ionic radii, redox potentials and a common trivalent oxidation state [14]. Their luminescence is usually modest in simple salts due to electronic transitions being mostly unaffected by the surrounding chemical environment [14,15]. However, when the element is coordinated in more complex systems, such as three-dimensional networks as in MOFs, where a direct metal–carbon interaction is present, the “antenna effect” can occur [16,17]. In this case, organic ligands absorb radiation and transfer energy to the metal electronic states, enhancing the luminescence and increasing emission in the visible or near-IR region. This phenomenon has led to numerous practical applications, including optical sensing [18,19], temperature sensing [17,20,21,22,23,24], bioimaging [8,25], anticounterfeiting [26,27], and lighting [28,29,30].

In the context of photovoltaics, one of the key challenges remains increasing the energy conversion efficiency, which is limited by the incomplete exploitation of the solar spectrum, particularly photons whose energies do not match the bandgap of the active layer (typically silicon) [31]. To address this limitation, energy conversion approaches such as up-conversion (UC), down-conversion (DC), and down-shifting (DS) have been explored [32,33]. These processes rely on the possibility of integrating materials capable of absorbing solar radiation and converting them into a spectral range more suitable for a photovoltaic device. Lanthanide-based MOFs, thanks to their peculiar structure and tunability together with efficient luminescent properties, represent an attractive class of hybrid materials for implementing these mechanisms. Specifically, the presence of organic ligands within the supramolecular structure that sensitize lanthanide metal center emission can mimic DS behavior by converting UV radiation into visible light with high transfer efficiency [12]. Furthermore, co-doping multiple lanthanide ions within the MOF framework can promote intermetallic energy transfer, potentially enabling DC processes [12]. Despite these promising features, only one study has investigated the direct integration of luminescent MOFs into photovoltaic devices specifically for down-shifting and down-conversion applications [12].

Most Ln-MOFs are synthesized using solvothermal or hydrothermal methods, often in toxic solvents like DMF [20,34,35,36,37,38], at high temperatures and pressures over extended periods. In contrast, greener synthesis approaches involve using water–ethanol mixtures, low-temperature synthesis [29,39,40], ultrasonic-assisted processes, and mechanical synthetic techniques that do not require solvents [23,41].

Our work presents a green synthetic method for producing Y-BTC: Eu (10%), Tb (10%) (H₃-BTC = 1,3,5-benzenetricarboxylic acid) in a mild, environmentally friendly one-pot process. It involves a simple approach for synthesizing Ln-doped MOFs without using toxic solvents, in contrast to other works that mainly depend on conventional solvothermal techniques. The present Ln-MOF was characterized using Fourier transform infrared spectroscopy (FTIR) and powder X-ray diffraction (PXRD) to evaluate its structural and vibrational properties. Furthermore, the morphology was examined using field emission scanning electron microscopy (FE-SEM), and the compositional study was conducted through EDX microanalysis. The luminescence properties in the visible region were also investigated to test the MOF’s properties as a DS system.

2. Materials and Methods

2.1. Starting Reagents

Yttrium(III) acetate hydrate, europium(III) acetate hydrate, and terbium carbonate hydrate, Tb₂(CO₃)₃(H₂O)_n, were supplied by STREM Chemicals. Glacial acetic acid was obtained from Panreac AppliChem ITW Reagents, while 1,3,5-benzenetricarboxylic acid was obtained from Sigma-Aldrich. Every chemical was used straightaway, without any further purifying procedures.

2.2. Y-BTC: Eu (10%), Tb (10%) Synthesis

Fifty milliliters of deionized water were mixed with 3.2 mmol Y(CH₃COO)₃⋅xH₂O in a ground-neck glass balloon, along with 0.4 Eu(CH₃COO)₃⋅xH₂O and 0.4 mmol of Tb₂(CO₃)₃(H₂O)_n. Then, 2.4 mmol of glacial acetic acid was added to remove all carbonates and bicarbonates found in the salts. In a separate beaker, 4 mmol of 1,3,5-benzenetricarboxylic acid was dissolved in 50 mL of 96% ethanol. The two solutions were then mixed together in a ground-neck glass balloon. After that, the reaction mixture was refluxed for 24 h at 80 °C. The reaction was followed by filtering, rinsing with a water/ethanol combination, and letting the product air-dry. Details of the synthetic procedure are reported in ref. [40].

2.3. Characterizations

To assess the morphological and structural characteristics, the synthesized material underwent extensive characterization. The crystalline structure of the samples was determined by X-ray diffraction (XRD) using a Smartlab Rigaku diffractometer (Rigaku corporation, Akishima-shi, Tokyo, Japan) with a rotating Cu K_α anode at 45 kV and 200 mA. With a 0.02° step increase, XRD patterns were acquired in Bragg–Brentano geometry. Fourier transform infrared (FTIR) spectra in ATR mode of powder samples were recorded on a Thermo Scientific Nicolet iS50 FTIR spectrometer (Thermo-Fisher Scientific, Madison, WI, USA).

A ZEISS SUPRA 55VP field-emission scanning electron microscope was employed for morphological characterization. The samples were prepared by mounting them onto aluminum stubs with double-sided adhesive graphite tapes. Using an INCA Oxford windowless detector with a resolution of 127 eV based on the full-width half-maximum (FWHM) of the Mn K_α peak, EDX analysis was used to ascertain the atomic composition of the samples.

The excitation spectra were collected with a spectrofluorimeter (Fluorolog 3, Horiba-Jobin Yvon, Horiba Ltd, Kyoto, Japan) with a Xe lamp and a single emission monochromator (mod. HR320), detected using a photomultiplier in photon counting mode (spectral resolution of 1 nm). The same experimental setup was used for the decay curve with an Xe microsecond pulsed lamp as the excitation source.

The emission spectra of the Y-BTC: Eu (10%), Tb (10%) MOF were excited with an LED flashlight centered at 385 nm coupled with a bandpass filter (centered at 390 nm), a 488 nm diode laser, and a UV lamp emitting around 300 nm. The emission spectra were recorded at 90° geometry with a 4x microscope objective using a half-meter Czerny–Turner monochromator (Shamrock 500i, Andor, Oxford Instruments plc, Abingdon, United Kingdom, grating 1200 lines/mm) with a spectral resolution of 0.07 nm. A CCD camera (iDus 420, Andor, Oxford Instruments) cooled to −80 °C was used to collect the signal.

3. Results

3.1. Y-BTC: Eu (10%), Tb (10%)

Following a previously described protocol for the synthesis of Y-BTC [40], the lanthanide-doped MOF was produced using mild conditions of temperature and pressure and reduced time using commercially available chemicals. With the H₃-BTC organic linker, the synthesis was produced in a 1:1 mixture of Et-OH/H₂O as solvents using the luminescent element concentration according to the following equation:

$$\begin{gathered} 0.8 \mathrm{Y}({\mathrm{CH}}_3\mathrm{COO}{)}_3·{\mathrm{xH}}_2\mathrm{O}+0.1 \mathrm{Eu}({\mathrm{CH}}_3\mathrm{COO}{)}_3·{\mathrm{xH}}_2\mathrm{O}+0.1 {\mathrm{Tb}}_2\left({\mathrm{CO}}_3{)}_3\right({\mathrm{H}}_2\mathrm{O}{)}_{\mathrm{n}}+{\mathrm{H}}_3\text{-} \\ \mathrm{BTC} \to \left[{\mathrm{Y}}_{0.8}{\mathrm{Eu}}_{0.1}{\mathrm{Tb}}_{0.1}\right(\mathrm{BTC}{)}_3]+3{\mathrm{CH}}_3\mathrm{COOH}+{\mathrm{CO}}_2+\mathrm{y} {\mathrm{H}}_2\mathrm{O} \end{gathered}$$

FTIR and PXRD analyses were performed to investigate the vibrational and structural characteristics of Y-BTC: Eu (10%), Tb (10%) MOFs.

The FTIR spectrum of the synthesized MOF (Figure 1a) revealed substantial changes compared to that of the free ligand H₃-BTC. Specifically, the disappearance of the characteristic carbonyl ν(C=O) band at 1700 cm⁻¹ and of the broad hydroxyl ν(O–H) band between 3300–3500 cm⁻¹ suggests the deprotonation and coordination of carboxylic groups. New vibrational bands instead appeared at 1612–1555 cm⁻¹ and 1442–1383 cm⁻¹, corresponding to the asymmetric and symmetric stretching vibrations of coordinated carboxylates (COO⁻), confirming the successful coordination of Ln³⁺ with BTC, as reported in ref. [42]. Additional peaks in the 1000–700 cm⁻¹ fingerprint region can be attributed to the vibrational modes of the benzene rings of the BTC organic linker. No bands around 3500 cm⁻¹ due to -OH stretching were visible, indicating that no coordinated or crystallized water molecules were present in our sample.

Figure 1b reports the PXRD pattern of the Y-BTC: Eu (10%), Tb (10%) sample, which closely matches the data reported in ref. [43] for a MIL78(Y, Eu) MOF (red line) with the chemical formula Y_1.952Eu_0.048O₁₂C₁₈H₆. Several peaks of the Y-BTC: Eu (10%), Tb (10%) sample perfectly match the ones reported in [43] related to the Rietveld refinement PXRD of MIL78(Y, Eu). The most intense peaks at 10.83°, 11.96°, 16.23°, and 25.68° are related to the 001, 020, 021, and 20-1 reflection peaks, respectively.

This kind of MOF does not contain coordinated or crystallization water, corroborating the findings from the measured FTIR spectrum. The crystalline structure is consistent with that found by Serre et al. [43], where eight oxygen atoms from the BTC groups surround each metal ion (Y, Eu, and Tb). Then, by analogy with ref. [43], we can deduce that, in our sample, the Eu³⁺ and Tb³⁺ ions substitute the Y³⁺ ones in the crystalline lattice. Moreover, such an assumption is also supported by the fact that Y³⁺, Eu³⁺, and Tb³⁺ in the square antiprismatic coordination have similar ionic radii of 1.019 Å, 1.066 Å, and 1.040 Å, respectively [44].

The characterization through EDX microanalysis confirmed the presence of all the lanthanide metals (Y, Tb, and Eu) in the structure, along with C and O from the organic linker. The EDX spectrum for the Y-BTC: Eu (10%), Tb (10%) MOF is shown in Figure 2. The X-ray L lines of Eu, Tb, and Y were observed in the range of 5.8–7.0 keV and at 1.92 keV, in addition to the C K peak at 0.28 keV and the O K peak at 0.52 keV. Quantitative analysis indicated doping levels of approximately 9% for Eu and 10% for Tb, consistent with the nominal composition.

The FE-SEM images of Y-BTC: Eu (10%), Tb (10%) are shown in Figure 3. The sample exhibited elongated microplate structures of varying sizes and sub-micron widths. The predominant morphology consisted of needle-like structures with lengths ranging from 2 to 4 µm. At higher magnifications, these needle-like structures revealed a circular structure rather than a plate-like one, indicating the formation of parallelepipeds with dimensions not exceeding 500 nm but with considerable length.

3.2. Luminescence Properties

Excitation spectra (Figure 4) were acquired at room temperature, monitoring the emission at 702 nm (red line) assigned to the ⁵D₀→⁷F₄ transition of Eu³⁺ ions and the one at 543 nm (green line) due to the ⁵D₄ → ⁷F₅ transition of Tb³⁺ ions. The broad band centered at 294 nm is attributed to BTC’s π-π* electron transition, while much weaker bands in the visible region are assigned to the 4f-4f transitions of Eu³⁺ and Tb³⁺ ions [45]. An intense band for Tb³⁺ was observed around 488 nm due to the ⁷F₆→ ⁵D₄ transition. Characteristic narrow bands due to transitions for the Eu³⁺ ions, i.e., ⁷F₀→ ⁵L₆ centered at 392 nm, ⁷F₀→ ⁵D₂ centered at 465 nm, and ⁷F_J→ ⁵D₁ (J = 0,1) centered around 530 nm, were identified.

The broad band observed in the Eu³⁺ excitation spectrum (Figure 4, red line) around 488 nm indicates an energy transfer from Tb³⁺ to Eu³⁺ ions. Emission spectra for the Y-BTC: Eu (10%), Tb (10%) sample were monitored at room temperature (Figure 5) by exciting into the π-π* electron transition of the BTC ligand at 300 nm (black line), into the ⁷F₀→ ⁵L₆ excitation band of Eu³⁺ at 390 nm (red line), and into the ⁷F₆→ ⁵D₄ excitation band of Tb³⁺ at 488 nm (green line). By exciting into the Eu³⁺ absorption band at 390 nm, the emission spectrum showed several bands assigned to the ⁵D₀→⁷F_J (J = 0, 1, 2, 3, 4) transitions typical of Eu³⁺ ions (see Figure 5, red line). The Eu³⁺ emission spectrum was very similar to that observed by Serre et al. for a MIL78(Y, Eu) sample [43], confirming the crystal structure indicated by the PXRD pattern and the FTIR results. In particular, the Lorentzian-shaped band peaking at 579.80 ± 0.05 nm can be assigned to the ⁵D₀→⁷F₀ transition (see inset of Figure 5a), showing that the Eu³⁺ ions are accommodated in one main site, as also found by Serre et al. [43]. It is worth noting that by exciting the ⁷F₆→ ⁵D₄ transition of Tb³⁺ ions using a diode laser at 488 nm (see Figure 5a, green line), emissions from both Eu³⁺ and Tb³⁺ ions were observed. In particular, the bands around 550 and 585 nm are due to Tb³⁺ ions, which correspond to the ⁵D₀ → ⁷F₅ and ⁵D₀ → ⁷F₄ transitions, respectively. These results indicate that the Tb³⁺ ions sensitize the emission of Eu³⁺ ions due to energy transfer processes (i.e., dipole–dipole, exchange mechanisms, and so on), as reported by many authors [46,47]. An energy level scheme of the lanthanide ions and energy transfer is shown in Figure 5b.

To obtain further insight into the dynamics of the spectroscopic processes, we measured the decay curves for the Y-BTC: Eu (10%), Tb (10%) MOF, considering excitation into Eu³⁺ and Tb³⁺ energy levels (at 390 nm and 488 nm, respectively), as shown in Figure 6. The decay curves can be well fitted using a single-exponential function. The values of the lifetimes τ (ms) are reported in the figure. The Eu³⁺ emission in this MOF has a relatively long lifetime (3.31 ± 0.01 ms), which agrees with the absence of water molecules in the Eu³⁺ coordination sphere that would act as emission quenchers, favoring nonradiative decays [48].

Meng et al. [49] calculated the radiative lifetime τ_R of Eu³⁺ in a MOF using the following equation:

$${\displaystyle \frac{1}{{\tau }_R}}=A_{MD,0}{n}^3 {\displaystyle \frac{I_{tot}}{I_{MD}}}$$

where n is the refractive index of the medium, A_MD,0 is the spontaneous emission probability for the ⁵D₀→⁷F₁ transition in vacuo (value of 14.65 s⁻¹), and I_Tot/I_MD is the ratio of the integrated emission intensity of the Eu³⁺ emission (I_Tot) to the area of the emission due to the ⁵D₀→⁷F₁ transition [50]. According to Meng et al. [49] and Chong et al. [51], we assumed the refractive index of our Y-BTC MOF to be 1.4. From the calculated I_Tot/I_MD ratio, the radiative lifetime τ_R of Eu³⁺ is estimated as 7.0 ms. Considering the measured lifetime for the Eu³⁺ ions, around 3.4 ms (see Figure 6), the intrinsic quantum yield for Eu³⁺ can be evaluated by the following formula:

$${\mathsf{\Phi }}_{int-Eu}={\displaystyle \frac{\tau }{{\tau }_r}}$$

The calculated value ${\mathsf{\Phi }}_{int-Eu}$ was around 49%, which is an excellent value compared to the one found for MOF-76 doped with Eu³⁺ ions (26%, [49]). This value also aligns with the absence of water molecules as quenchers around the Eu³⁺ ions.

4. Discussion

The current work produced and studied luminescent metal–organic frameworks (MOFs) based on yttrium-1,3,5-benzenetricarboxylate (Y-BTC) doped with lanthanide ions, namely, terbium (Tb³⁺) and europium (Eu³⁺). Specifically, the goal was the fabrication of MOFs with controlled luminescent characteristics and the examination of their structural and morphological peculiarities.

The main advantages of the present approach are related to the possibility of obtaining a highly crystalline arrangement of doped MOF using a green approach under mild conditions of temperature and pressure starting from commercially available reagents. The added value of this material compared to the Y-BTC counterpart is related to the high luminescent properties arising from the Eu and Tb doping ions while maintaining the same crystalline structure, as confirmed by literature comparison of the pattern. Furthermore, an important advantage of this MOF is related to the high photostability due to the highly stable lanthanide luminescent active centers [21,52].

The morphology analyzed by FE-SEM displayed a needle-like structure quite different from the one found in the Y-BTC MOF obtained under similar synthetic conditions [40], in which a plate-like structure was formed. This difference can be likely attributed to the doping component’s effect in the synthesis’s starting mixture.

A key aspect of this study was the investigation of luminescent properties of this luminescent MOF in solid-state for energy conversion applications. Compared to conventional materials used for spectral conversion—such as quantum dots, organic dyes, and inorganic phosphors—MOFs offer advantages in terms of stability, modular synthesis, and reduced toxicity, making them compelling candidates for enhancing solar energy harvesting technologies. To examine the materials’ optical performance, emission spectra were acquired, emphasizing the energy transfer between Eu³⁺ and Tb³⁺ ions and the effect of the BTC ligand as an antenna. The possibility to use these lanthanide-doped MOFs in cutting-edge luminous technologies is further highlighted by the observed sensitization of Eu³⁺ emission by Tb³⁺ ions, as demonstrated by energy transfer processes. Interestingly, the long decay lifetime of Eu³⁺, together with information from the ATR-IR spectrum, excludes the presence of water molecules in the Eu³⁺ coordination sphere that otherwise would have acted as quenching channels, favoring nonradiative decays.

Eu³⁺ and Tb³⁺’s distinctive emission bands were visible in the data, indicating that the MOFs were successfully doped with both ions to produce a multi-color emission profile. Relatively narrow emission bands and variations in intensity ratios imply that the materials’ emission properties may be adjusted, paving the way to customize luminescence in a wide range of sensing and imaging applications in the optical region.

5. Conclusions

This work reports for the first time the fabrication of Eu–Tb-doped yttrium-1,3,5-benzenetricarboxylate MOF, i.e., Y-BTC: Eu (10%), Tb (10%). A deep investigation assessed the material’s structural and morphological aspects. In particular, ATR-IR and XRD measurements confirmed the crystalline arrangement of the MOF, which is isostructural with the MIL-78(Y, Eu) MOF, where eight oxygen atoms from the BTC³⁻ groups surround each Y³⁺, Eu³⁺, or Tb³⁺ ion. The FE-SEM analyses showed the formation of needle-like structures with lengths ranging from 2 to 4 µm, while the EDX spectrum confirmed the stoichiometry of the doping ions. Interestingly, the luminescence measurement demonstrated the functional properties of the Y-BTC: Eu (10%), Tb (10%), showing an intense emission of the Eu and Tb moieties and the effect of the BTC ligand.

The co-doping within the MOF framework promotes energy transfer, potentially enabling DC processes. In this context, the present study represents a first step towards a very attractive direct integration of luminescent MOFs into photovoltaic devices for down-shifting and down-conversion applications.