Composition and Fluorescence of Gadolinium (III) Acetyl- acetonate Derivatives by Solvothermal Method

Citation: Lv F, Zhang Y, Chen X, Ma Y (2017) Composition and Fluorescence of Gadolinium (III) Acetylacetonate Derivatives by Solvothermal Method. Int J Opt Photonic Eng 2:005 Copyright: © 2017 Lv F, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. *Corresponding author: Fengzhu Lv and Yihe Zhang, Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, No. 29 Xueyuan Road, Haidian District, Beijing 100083, China, Tel: +86-1082322759, Fax: +86-10-82322345, E-mail: lfz619@cugb.edu.cn; zyh@cugb.edu.cn VIBGYOR ISSN: 2631-5092


Introduction
Generally, rare earth complexes have many potential applications in novel lighting devices, optical fibers and luminescent sensors benefiting from their long emission lifetime, narrow bandwidth and large Stokes shift in visible and near-infrared regions under Ultraviolet (UV) excitation [1][2][3]. The luminescence intensity of rare earth complexes related to the absorption efficiency of ligands, which is dependent on the ligand molecule with large π-conjugated system. Good solubility of substituted rare earth β-diketonate complexes in common organic solvents highly enhances their applications in the solution processable organic semiconductors and also provides more efficient approach to fabricate OLED devices by solution techniques [4,5]. So the fluorescence of some rare earth β-diketonate complexes were studied and modified to widen their application [6,7]. But preparation of multifunctional materials, such as materials with fluorescence and magnetic properties, by simple method is the project of researchers [8,9].
Thermal decomposition of metal acetylacetonates has recently attracted much attention to fabricate thin films and nanoparticles of metals, metal alloys and metal oxides for microelectronic, catalytic and biomedical applications [10][11][12][13]. Colloidal cubic Gd 2 O 3 nanorings and nanoplates were selectively prepared from low-temperature (90 °C) hydrolysis of Gd-(AcAc) 3 and subsequent in situ thermal dehydration of the hydrolyzed precursor-surfactant aggregates at 320 o C [14]. But seldom reports focus on the partial decomposed samples as which is preferred possessing solubility in organic solvent and changed fluorescence compared to rare earth acetylacetonate.
In the present work Gadolinium (III) acetylacetonate hydrate was treated by solvothermal method in absolute diphenyl ether which is a water lack system where the hydrolysis of acetylacetonate is limited. The products different temperature for 2.0 h. After that the autoclave was cooled to room temperature, the as-obtained yellow suspension was mixed with an equal volume of acetone, and the formed precipitate was separated by centrifugation and washed with acetone. The final samples were assigned as Gd-S-T, where T indicated the formed temperature by solvothermal. Gd-S-160 was grounded and calcinated at 450 °C for 10 h in air to give Gd-C-160. Afterwards, Gd-C-160 was furnace-cooled to room temperature, and grounded again into powder for measurement.

Characterization
Thermal Gravimetric Analysis (TGA) was measured on Perkin-Elmer TGA 7 with a heating rate of 20 °C/min ranging from 45 °C to 800 °C in air, on the same time a NETZSCH STA 449F3 Thermo Gravimetric Analyser was coupled, using a heated capillary at 190 °C, to a QMS 403 Mass Spectrometer (TGA-Ms) to identify the released gases during the thermal degradation of a material. The Fourier Transform Infrared Spectra (FTIR) of the solid samples was obtained with a Perkin Elmer Spectrum 100 FT-IR spectrometer. KBr pellet was used as background. The structure of the products was studied by powder X-ray diffraction (Rigaku D/max-rA) equipped with Cu Kα (λ = 0.1542 nm) radiation at 40 kV and 30 mA. The surface morphology and the elemental distribution of the samples were probed with Scanning were preferred possessing solubility in organic solvent and changed fluorescence compared to rare each acetylacetonate as well as magnetic properties. So different kinds of Gadolinium (III) acetylacetonate derivatives were produced. The composition, fluorescence and magnetic properties were studied in detail. Under appropriate preparation conditions, Gadolinium (III) acetylacetonate derivatives with higher fluorescent intensity and magnetic properties were produced.

Experimental Materials
The raw material for synthesis of Gadolinium (III) acetylacetonate derivatives was gadolinium acetylacetonate trihydrate (Gd(AcAc) 3 ·3H 2 O, MW = 508.62 g/ mol), which was purchased from Beijing HWRK Chemical Co., Ltd.. Diphenyl ether was applied by Sinopharm Chemical Reagent Beijing Co., Ltd. Other solvents were from Beijing Chemical factory.

Preparation of Gadolinium (III) acetylacetonate derivatives
In a typical synthesis for the derivatives of Gd(A-cAc) 3 ·3H 2 O, 1.5787 g (0.003 mol) of Gd(AcAc) 3 ·3H 2 O was dispersed into 10.5 mL of absolute diphenyl ether to form a solution under magnetic stirring, which was poured into 25 ml teflon-lined autoclave and heated at Electron Microscope (SEM, HITACHI S-4300) operated at an acceleration voltage of 5 kV. The zeta potential and size distribution of the hybrid particles were measured on Malvern Instruments Zetasizer Nano-ZS90. The Ultraviolet-Visible (UV-vis) spectra were measured with UV6100 double beam spectrophotometer. Room temperature excitation and emission spectra were characterized by F97 spectrofluorometer (Shanghai Lengguang Technology Co., China) with a 150 W Xe lamp as the excitation source. The temperature-dependent luminescence properties were measured on the fluorescence spectrophotometer (F-4600, HITACHI, Japan), which was equipped with a self-made heating attachment and a computer controlled electric furnace. The luminescence decay curve was obtained from a spectrofluorometer (HORIBA, JOBIN YVON FL3-21) with a 335 nm pulse laser radiation (nano-LED) as the excitation source, and the pulse width of the laser was 12 ns.

Results and Discussion
To prepare derivatives of Gd(AcAc) 3 ·3H 2 O by solvothermal method, TGA together with Differential Thermal Analysis (DTA) were performed to determine the decomposition temperature of Gd(AcAc) 3 ·3H 2 O. As shown in Figure 1a, the weight losses for all the samples below 150 °C are assigned to the departure of adsorbed water and comlexed water with gadolinium [15]. Gd(A-cAc) 3 ·3H 2 O exhibits complicated decomposition behavior due to more ligands in it. It shows one little and two broad DTA peaks with shoulders between 190-350 °C and 350-500 °C (Figure 1b). So it is preferred, Gd(A-cAc) 3 ·3H 2 O derivatives may form around 200 °C where Gd(AcAc) 3 ·3H 2 O decomposes partially. Figure 2 shows the FT-IR spectra of the products prepared at around 200 °C. In the spectrum of Gd(A-cAc) 3 ·3H 2 O (Figure 2a), the peak at 1602 cm -1 is associated with the enol-keto tautomers of the β-diketone corresponding to the C = O stretching vibration of keto. The peak at 1523 cm -1 is the C = C stretching vibration of enol. The peaks at 1391 cm -1 and 1016 cm -1 are the vibrations of -CH 3 [15,16]. The wide band at about 3300 cm -1 is the vibration of crystallined water of Gd(AcAc) 3 ·3H 2 O. The peak at 536 cm -1 is the vibration of a Gd-O stretching absorption. While for the solvothermal samples ( Figure  2b-2h), the still appearing peak at 1016 cm -1 relating to the vibration of -CH 3 confirms that some acetylacetone groups are still in the final samples although the solvothermal temperature is up to 260 °C (Figure 2h). The peaks at 1548 and 1441 cm -1 are the C = O and C = C stretching vibration of β-diketone which left shift due to the substitution of one or two acetylacetone groups by OH in the solvothermal process [16]. The peak at 495 cm -1 is the vibration of Gd-O-Gd. The broad peak at ~3415 cm -1 is considered as structural hydroxyl groups connecting Gd. As long then the reaction time from 2 h to 12 h, the FT-IR spectra of Gd-S-160 ( Figure 3) have no obvious change indicating that acetylacetone groups are stable in relative severe solvothermal process. In order to verify that acetylacetone groups are still in the formed samples, Gd-S-160 is calcinated at 450 °C for 2 h to give Gd-C-160. Its FTIR spectrum (Figure 3e) shown the peaks at 1548 cm -1 and 1441 cm -1 disappear and bands at 1504 cm -1 , 1398 cm -1 , 1096 and 863 cm -1 due to ν as OC-O, ν s C-O, p-CO 3 2appear, confirming the calcinated sample is Gd(OH)CO 3 2or (GdO 2 )CO 3 [17]. So it is referred that the solvothermal samples are Gd(OH) x (AcAc) 3-x , where x is between 0-3 and AcAc is the acetylacetonate group.
In order to clearly determine the composition of the prepared samples, TGA and DTA of Gd-S-160 and Gd-S-220 were measured (Figure 1). Gd-S-160 and Gd-S-220 all show one obvious degradation DTA peak be- tween 250-400 °C which corresponds to ~44% and 32% weight loss, respectively. If considering the sample is Gd(OH) (AcAc) 2 or Gd(OH) 2 (AcAc), its corresponding degradation weight loss would be 45% or 32%. That is the weight losses of Gd-S-160 and Gd-S-220 can well match that of Gd(OH)(AcAc) 2 and Gd(OH) 2 (AcAc). So the formula of Gd-S-160 and Gd-S-220 are Gd(OH)(AcAc) 2 and Gd(OH) 2 (AcAc), respectively. Meanwhile the degradation products of Gd-S-160 are measured by TGA-MS (Figure 1c). Between 300-400 °C, small molecules including CO, CO 2 , H 2 O are produced confirming the organic groups are preserved in Gd-S-160. In the DTA curve of Gd-S-160 between 450-600 °C, one obvious broad band appear while no degradation occur showing a phase change takes place which may be indexed as the transition of Gd with 3 complex number to Gd 2 O 3 .  Figure 4 shows the XRD patterns of Gd-S-T and Gd-C-160. New peaks appear between 10-15 o compared to Gd(AcAc) 3 ·3H 2 O with the increase of reaction temperature and reaction time. This is due to the substitution of acetylacetone by -OH with the hydrolysis. Similar phenomenon was reported by hydrolysis of Gd(A-cAc) 3 ·3H 2 O in DMF [18]. The characteristic diffraction peaks of Gd-C-160 can well match that of (GdO 2 )CO 3 (JPCDS-47-0579). These result confirms that in diphenyl ether the trace water from structural water of Gd(A-cAc)·3H 2 O can cause Gd(AcAc)·3H 2 O hydrolyze to form Gd(AcAc) x (OH) 3-x where x is between 0 and 3.
The SEM and the element dispersions of the prepared Gd-S-160 are shown in Figure 5. In the magnetified photograph (Figure 5b), it can be seen that Gd-S-160 is in rod shape with some irregular particles, the scanning photos show that C, Gd and O uniformly dispersed on the particle confirming the formation of Gd(AcAc) x (OH) 3-x .
The particle sizes of Gd-S-160 and Gd-C-160 were measured by Z potential and shown in Figure 6. The particle size of most of Gd-S-160 is 200-400 nm with little part being about 800 nm. While about 40% of Gd-C-450 is in the particle size of 200-500 nm, about 50% in 700-1200 nm, and about 10% in 1300-1600 nm. These data show that under suitable solvothermal conditions, the prepared Gd-S-T not only contain organic groups but also have relative small particle size compared to general solvothermal-calcination process. The interesting thing is that no matter Gd-S-T or its calcinated products show positive surface charges. The Z potential of Gd-S-160 and Gd-C-160 are 42.1 mV and 34.9 mV. Figure 7 shows the UV-v is absorbance of Gd(A-cAc) 3 ·3H 2 O and Gd-S-T. For Gd(AcAc) 3 ·3H 2 O, only one absorbant peak centered at 294 nm is observed which is attributed to the π-π * transition of β-diketonate ligand [5]. Due to the forbidden of electron transition between ligands and Gd 3+ which has stable electron arrangement, still only one peak is observed for Gd-S-140, but right shifts to 308 nm indicating the π-π * transition of β-diketonate ligand becomes more easier and the conjugation degree Gd with acetylacetone groups is weakened. For Gd-S-160 and Gd-S-200, two absorbent peaks appear. Except the absorbance at 308 nm, a peak at 275 nm due to the π-π * transition of β-diketonate due to the energy match of ligands after substitution of acetylacetone by OH groups.
Generally speaking, rare earth organic complexes produce intense emission peaks in visible and near-infrared regions under Ultraviolet (UV) excitation. For obtaining optimum emission spectra, the excitation wavelength for emission measurement is determined by excitation spectrum measurement. Figure 8a is the excitation fluorescence of Gd-S-T in ultraviolet region under emission of 490 nm. Gd(AcAc) 3 ·3H 2 O shows two absorbance, one is between 320-370 nm, the other is between 370-450 nm attributed to π-π * and n-π * transitions of β-diketonate ligands [5]. While for Gd-S-140 and Gd-S-160, three peaks are observed which are at 310-350, 350-375 and 375-450 nm. As emission exited under UV irradiation is aimed in the present work, so emission fluorescence is excited by 365 nm and the spectra are shown in Figure 8b. Gd(A-cAc) 3 ·3H 2 O shows one strong emission peak at 520 nm. While the emission of Gd-S-140 and Gd-S-160 left shift indicating that high energy luminescence is produced. Although the acetylacetone groups of Gd(AcAc) 3 ·3H 2 O is more than Gd-S-160, but the emission peak of Gd-S-160 is the highest. Just as shown in Figure 9 [19], as the bound ability of -OH to electron is stronger than acetylacetone group, substitution of some of the acetylacetone group by -OH will result in weak interaction of Gd 3+ with the left acetylacetone groups of the prepared samples, and thus the controlling ability of acetylacetone group to electron is increased. So the energy between the ground state and exited state in the prepared samples is highered which results in emission of fluorescence with shorter wavelength compared to Gd-acetylacetonate [7,20]. As  Figure 8c shows the temperature dependence of Gd-S-160. Generally, the fluorescence of organic complex decreases with temperature. At higher temperature, the vibration of electron in acetylacetone increases, so more energy is     interaction strength between acetylacetone and Gd with temperature and the controlling ability of acetylacetonate to electrons is increased. Therefore the obtained samples are not temperature stable enough. Figure 10 shows M-H curve of Gd-S-160 measured at 300 K. The magnetization of Gd-S-160 increases linearly with the testing magnetic strength increase. This phenomenon is due to the magnetizing current for Ms Measurement of Gd-S-160 is too high. In other words, Gd-S-160 is relatively easy magnetization. As the applied magnetic field is 8000 Oe, the magnetization of Gd-S-160 is 0.72797 emµ/g confirming the produced samples can response to external magnetic field.

Conclusions
Gadolinium (III) acetylacetonate was treated by solvothermal method to produce Gadolinium (III) acetylacetonate derivatives. The composition, fluorescence, and magnetic properties of the products are studied in detail. The fluorescence measurement showed the products possessed higher energy emission and excitation fluorescence compared to Gadolinium (III) acetylacetonate. Also the product produced at 160 °C possessed higher fluorescent intensity. Based on the fluorescent study the fluorescent emission mechanism is proposed. The magnetic measurement indicated the products were paramagnetic materials. So the products were hydrophobic materials with magnetic and fluorescent properties.