- Email: [email protected]
Great enhancement of monodispersity and luminescent properties of Gd2O3:Eu and Gd2O3:[email protected] nanospheres
Materials Science & Engineering B 241 (2019) 1–8
Contents lists available at ScienceDirect
Materials Science & Engineering B journal homepage: www.elsevier.com/locate/mseb
Great enhancement of monodispersity and luminescent properties of Gd2O3:Eu and Gd2O3:Eu@Silica nanospheres
T
Tran Kim Anha, Nguyen Thanh Huongb,c, Pham Thi Lienb,c, Do Khanh Tungb,c, Vu Duc Tuc,e, ⁎ Nguyen Duc Vanb, Wieslaw Strekd, Le Quoc Minha,b,c, a
Duy Tan University, 182 Nguyen Van Linh, Da Nang, Viet Nam Institute of Materials Science, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Viet Nam c Graduate University of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Viet Nam d Institute of Low Temperature and Structure Research, Polish Academy of Science, Ul.Okolna 250-422, Wroclaw, Poland e National Chung Cheng University, Ming Hsiung, Chia Yi 621, Taiwan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Gd2O3:Eu Gd2O3:Eu@silica Monodisperse Nanospheres Photoluminescence
In this paper, the experimental parameters that control the monodisperse and high quality Gd2O3:Eu3+ and Gd2O3:Eu3+@silica nanospheres (NSPs) systematically investigated. Gd2O3:Eu3+ and Gd2O3:Eu3+@silica NSPs in size range from 100 to 200 nm with a very low standard size deviation of ± 5.00% have been successfully fabricated. The luminescence spectra of Gd2O3:X%Eu3+ and Gd2O3:X%Eu3+@silica NSPs were studied as a functions of Eu3+ ion concentration (X = 3.50; 5.00; 6.00; 7.00; 7.50 and 8.00 at.%). The strongest emission peak at 611 nm corresponding to 5D0–7F2 transition of Eu3+ ions in Gd2O3:Eu3+ NSPs was very sharp. Interestingly, for the silica coated Gd2O3:X%Eu3+ NSPs, a new emission peak at 621 nm was appeared in depend on the coating condition. The intensity ratio of the emission peaks at 611 nm and 621 nm was decreased with the increase of the silica thickness. The highly monodispersed, mesoscaled Gd2O3:Eu3+ and Gd2O3:Eu3+@silica NSPs synthesized in this work are high potential for application, especially in photonic crystals, bioimaging and drug delivery.
1. Introduction Rare Earth (RE) actived nanomaterials with various sizes and shapes exhibit high application potentials in electronics [1,2], photonic [3], chemistry [4–6] and biomedicine [7–9]. The RE contained nanospheres (NSPs) capable of constructing systematic 2D or 3D nano-frameworks, have attracted great attention in the fields of display, catalyst [10,11], photonic crystals and security printing [12,13]. Additionally, large NSPs inherently possess identical hydrodynamic properties in fluidic media, thus making them as a promising platform for cell imaging and drug delivery [14–16]. It is expected that the real-time monitoring of infiltration of RE-doped NSPs into tissues can be achieved, leading to controlled and targeted drug delivery. It is well-known that gadolinium oxide (Gd2O3) has been considered as an efficient host matrix for doping RE ions because of its valuable features such as thermal stability, low phonon energy and chemical durability [17,18]. One of the Gd2O3-based phosphors, Gd2O3:Eu3+, has been intensively studied in recent years for display, security printing and biomedicine [1,12,18,19]. The average grain size of Gd2O3 NSPs in the mesoscale
⁎
range from 100 to 500 nm is ideal for simultaneous cell imaging and drug delivery [8,17]. Recently, various synthetic methods have been developed for the fabrication of a considerable number of homogeneous nanoparticles, either amorphous or crystalline, in many shapes, including NSPs. To the best of our knowledge, for synthesizing oxide-based NSPs, several typical chemical methods, for instance, Stober sol-gel [20,23] or multistep chemical precipitation with urea [21,28], have been developed. The Stober method has been commonly used for the synthesis of silica NSP (SiO2–X(OH)X) with high monodispersity [3,22] meeting the requirement for fabrication of photonic crystal structure as well as potential use in biomedical application [16,24]. Besides, sol-gel process is also widely-used in shell coating, surface immobilization, functionalization of biocompatible nanomaterials [25–27]. On the other hand, the multistep precipitation with urea has successfully employed to synthesize several metaloxides (MeO) and lanthanide oxides (Ln2O3) [11,21,28]. Recently, this method has been used to synthesize some highly efficient luminescent RE-doped NSPs, which is able to provide multifunctional nanomedical platforms for diagnosis
Corresponding author at: Institute of Materials Science, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Viet Nam. E-mail address: [email protected] (L.Q. Minh).
https://doi.org/10.1016/j.mseb.2019.01.020 Received 11 June 2018; Received in revised form 3 December 2018; Accepted 31 January 2019 0921-5107/ © 2019 Published by Elsevier B.V.
Materials Science & Engineering B 241 (2019) 1–8
T.K. Anh et al.
Gd2O3:X%Eu3+NSPs with various doping concentrations (X = 3.50; 5.00; 6.00; 7.00; 7.50 and 8.00 at.%) were followed a similar process to that for preparing the Gd2O3:6%Eu3+ NSPs.
and therapy [19,29]. For the use of NSPs as a drug nanocarrier in treatment of cancer disease, the particle size of the few hundred nanometers is preferred for the targeted drug delivery into tumor cell [3,8,10]. The homogenous distribution of NSPs is very critical in the functionalization of bioimaging probes as well as drug carrier agents. The urea assisted preparation is a simple and effective route for fabrication of lanthanide hydroxycarbonate NSP that was first found by Matijevic [21]. Since then, the further studies [11,28] have been investigated to find the proper experimental conditions for obtaining spherical lanthanide and transition metal hydroxycarbonates. Until now, there has been a variety of researches based on urea precipitation method to synthesize the Ln(OH)CO3·H2O and Ln2O3 NSPs, however, the effect of synthesis parameters and especially annealing regime on the size and shape of NSPs have not been addressed. Particularly, there are only few studies using Gd2O3:Eu3+ NSPs for simultaneous bioimaging and drug delivery [15,18,30]. Until now, there is only one report using the urea precipitation method to fabricate Gd2O3: 5% Eu NSPs with an average size of 150 nm and a standard size deviation of ± 10.60 [16]. Herein, we systematically investigated the urea-based precipitation method for enhancing the monodispersity of Gd2O3:X%Eu3+ NSPs (X = 3.50; 5.00; 6.00; 70.0; 7.50 and 8.00 at.%). Subsequently, these NSPs were coated with a silica shell, followed by annealing process at different annealing temperatures. Thermogravimetric analyses (TGA) have been analyzed to find an optimized heating regime to remain homogeneous size and shape of Gd2O3:Eu3+ and silica coated Gd2O3:Eu3+ NSPs. The physical and chemical properties of as-synthesized samples have been characterized by X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), energy dispersion spectroscopy (EDS), and Fourier transform-infrared spectrometry (FT-IR). The luminescence properties of the uncoated and coated Gd2O3:Eu3+ NSPs have been studied, showing high potential for a wide-ranging application in photonic crystals, bioimaging and drug delivery.
2.1.2. Preparation of Gd(OH)CO3·H2O:X%Eu3+@silica and Gd2O3:X% Eu3+@silica The core/shell structured Gd(OH)CO3·H2O:X%Eu3+@silica nanospheres were synthesized by using a modified Stober process. In a typical procedure, 100 mg Gd(OH)CO3·H2O:X%Eu3+@silica particles were obtained after drying at 70 °C for 24 h, followed by re-dispersing in a mixture of 180 ml of ethanol and 100 ml of DI water by using ultrasonication for 45 min. After that, 150 µl of TEOS in 10 ml ethanol was slowly added dropwise into the reaction flash. Next, 1 ml of 25% NH4OH was added upon intensive stirring and the reaction solution was stirred for another 10 h. The final products were separated by centrifugation at 5000 rpm and washed with ethanol, DI water and then dried at 60 °C in air. Finally, the samples were heated at 105 °C, 200 °C and 650 °C to investigate the conversion of silica coating process from Gd(OH)CO3·H2O:Eu3+@silica to Gd2O3:Eu3+@silica samples. 2.2. Heating, measurement and characterization The subsequent heating processes have been implemented by using a high qualitative furnace Carbolite furnace (Serial No: 12/00/3200; Type: STF 15/75/450, Max temp (°C): 1500). The thermal properties of the samples Gd(OH)CO3·H2O:Eu3+ were analyzed with a equipment thermogravimetric analysis (TGA) – TG209F1 Libra Analyzer Netzsch, in range from 27 °C (at room temperature) to 1000 °C, with temperature increase rate of 10 °K/min, in N2 atmosphere. X-ray diffraction (XRD) measurements were performed to determine the phase of the as-prepared samples at room temperature with a Bruker D8-Advance diffractometer, using Cu Kα radiation (λ = 1.5405 Å), in the 2θ range from 10 to 70° with a scanning step of 0.02 °C/min. The energy dispersion spectroscopy (EDS) was measured with a F6400 JEOL. The morphology of the samples was examined with FESEM (Hitachi S-4800) and TEM (JEM 1010-JEOL). FT-IR spectra of all the samples were measured with a Nicolet spectrophotometer using KBr pellets in the range of 400–4000 cm−1. The UV–VIS absorption spectra were recorded with a SL159ELICO UV–VIS spectrophotometer. Photoluminescence excitation (PLE) and emission (PL) spectra were recorded at room temperature with a Hitachi F-4500 spectrophotometer equipped with a 150 W xenon lamp or a LED as the excitation sources.
2. Experimental section 2.1. Materials 2.1.1. Preparation of Gd2(OH)CO3H2O:X%Eu3+ and Gd2O3:X%Eu3+ Gadolinium(III) nitrate hexahydrate (Gd(NO3)3·6H2O, fine powder, 99.99%), europium(III) nitrate pentahydrate (Eu(NO3)3·5H2O, powder, 99.99%) and tetraethoxysilane (TEOS, liquid, 99%) from Sigma-Aldrich were purchased and used without further purification. Urea (CO(NH2)2, powder, 98%) and nitric acid (HNO3, solution, 65%) was purchased from Merck. The DI water was used for all the experimental processes. For fabricating the mixed oxide of Gd and Eu with various content ratios, the 0.004 M Eu(NO3)3, 0.3 M Gd(NO3)3 and 0.8 M urea stock solutions have been prepared. The stock solution of Eu(NO3)3 was prepared as follows: 0.211 g Eu2O3 was added in 50 ml of HNO3 25% in 100 ml bottom flask and warmed up at 70–80 °C under stirring to obtain a clear solution. After that, the above mixture was heated gently to obtain fine powder. The Gd2O3:6%Eu3+ NSPs have been prepared by a modified multistep co-precipitation with the presence of urea. In a typical synthesis process, 10 ml of 0.3 M Gd(NO3)3 solution was added into a 250 ml round-bottomed flask containing 47.87 ml of 0.004 M Eu(NO3)3 with magnetic stirring at room temperature for 60 min. Subsequently, 98.68 ml of 0.8 M urea solution was added along with 60 ml of DI water, followed by heating to 85 °C for 68–70 min in an oil bath. The reaction was then cooled by a water stream outside of the flask. The obtained suspension was separated by centrifugation at 5000 rpm and collected after washing with deionized water and ethanol for two times. The resulting NSPs was dried at 70 °C for 12 h in air and then annealed by stepwise heating process at 105 °C for 5 h; 200 °C for 2 h; 600 °C for 2 h and 650 °C for 3 h to obtain Gd2O3:6%Eu3+ as final product. The
3. Results and discussion 3.1. Controlling synthetic process and physicochemical features of Gd (OH)CO3·H2O:X%Eu3+ and Gd2O3:X%Eu3+ NSPs We first investigated the experimental parameters to find a proper condition to obtain the high monodispersity of Gd2O3:X%Eu3+ and Gd2O3:X%Eu3+@silica NSPs in size range from 100 to 200 nm with considerable modification compared to the pioneering work of Matijevic [21] and other reports [11,16,28]. In this investigation, the pH values from 4.90 to 5.10; reaction temperature at 85 °C ± 1 °C, Gd (NO3)3 concentration of 1.35.10-2 M and urea concentration from 4.10-1 to 8.10-1M were maintained throughout the synthetic process. In the meantime, the other experimental parameters were changed as follows: content rates of Ln3+ ions (i.e. Gd3+ and Eu3+) and urea from 1/20 to 1/40, aging reaction time from 50 to 80 min, and stepwise heating regime in temperature range from 70 to 650 °C. The TGA analysis in the range of room temperature to 1000 °C of the as-fabricated samples are shown in Fig. 1. The first weight loss of Gd (OH)CO3·H2O:6% Eu3+ NSPs was due to the release of water molecules adsorbed on the sample surface in the range from room temperature to 168.4 °C. The second weight loss found in the range from 168.4 to 2
Materials Science & Engineering B 241 (2019) 1–8
T.K. Anh et al.
Eu3+ NSPs and final products Gd2O3:6%Eu3+ reveal that a high homogeneous morphology was achieved for all these samples (Fig. 2a–d). Fig. 2a and b show the FESEM images of Gd(OH)CO3·H2O NSPs annealed at 200 °C and Gd2O3:6%Eu3+ NSPs annealed at 650 °C, respectively. The TEM images of the pure Gd2O3:6%Eu3+ NSPs and silica coated Gd2O3:6%Eu3+ are displayed in Fig. 2c and d, respectively. The results confirm that the monodispersed Gd2(OH)CO3·H2O:6%Eu3+ NSPs was successfully fabricated in the size range of 100–200 nm. The size change of the NSPs (see Fig. 3) before and after annealing at 650 °C was about 13.5% corresponding to total weight loss of nearly 37%, due to the conversion from Gd(OH)CO3·H2O:X%Eu3+ to Gd2O3:X%Eu3+. The result indicates that the nanospheres size increases with the ratio of urea and Ln3+ ions (e.g. Gd3+ and Eu3+). The ratio from 20/1 to 40/1 was chosen to fabricate NSPs with size from 100 to 200 nm, as shown in Fig. 3a. It clearly shows that the average size of NSPs rises with increasing aging reaction time in the range of 50–80 min. The proper combination of the urea/Ln3+ ratio and the aging reaction time is prerequisites in the synthetic process to obtain the Gd(OH)CO3·H2O:6%Eu3+ and Gd2O3:6%Eu3+ NSPs with high monodispersity. It is noteworthy that the monodispersed NSPs with identical hydrodynamic feature and high solubility in the solvent or aqueous media are very useful for further systematic construction of 2D or 3D structures [3,22] and particularly for surface modification, functionalization and conjugation to create a bioimaging probe or targeting drug delivery [32]. The size distributions of Gd(OH)CO3·H2O:6%Eu3+, Gd2O3:6%Eu3+ and Gd2O3:6%Eu3+@silica NSPs based on the FESEM and TEM results is described in Fig. 4. The mean diameter of Gd(OH)CO3·H2O:6%Eu3+ NSPs was estimated to be 190 nm with a standard deviation (SD) of ± 3.20%. The uncoated Gd2O3:6%Eu3+ NSPs have spherical shape with a uniform size of approximately 150 nm and SD of ± 4.85%. After silica coating, the average size of Gd2O3:6%Eu3+@silica NSPs is about 160 nm with a SD of ± 3.50% (Table 1). The result indicates that the NSPs were successfully synthesized with a significant improvement in quality in comparison to those obtained in Ref. [16].
Fig. 1. TGA curves of as-fabricated Gd(OH)CO3·H2O:Eu3+.
608 °C is related to the removal of water molecules via the dehydration of hydrated water and the self-condensation process of hydroxyl groups (OH). Another weight loss was observed in the range of 608–750 °C corresponding to the strong release of CO2 molecules to form Gd2O3:Eu3+ NSPs [31]. As indicated in Fig. 1, the highest weight loss of 15.33% was in the temperature range of 168.4–608 °C in TGA curves. The large difference at the end point of 608 °C observed in our study is the same as that observed in the Ref [16] for at 550 °C in air [16]. The dissimilarity in temperature might be attributed to the difference in measuring atmosphere. According to the TGA results, a proper heating regime to obtain the highly monodispersed Ln2O3 NSPs was addressed. The heating regime for silica coated Gd(OH)CO3·H2O:Eu3+ NSPs was similar to that for the uncoated products. By analyzing the thermoproperties of gadolinium hydroxycarbonate with and without silica coating, it could be found the stepwise subsequent heating regime in air. This stepwise regime with the heating at 105 °C for 5 h; 200 °C for 2 h; 600 °C for 2 h and 650 °C for 3 h enables to maintain and remain the high homogeneity of size and shape of nanospheres without any deforming or cleft. The FESEM and TEM images of the as-prepared Gd(OH)CO3·H2O:6%
Fig. 2. FESEM images of (a) Gd(OH)CO3·H2O:6%Eu3+ NSPs annealed at 200 °C, (b) Gd2O3:6%Eu3+ NSPs annealed at 650 °C, and TEM images of (c) Gd2O3:6%Eu3+ NSPs annealed at 650 °C and (d) silica coated Gd2O3:6%Eu3+ NSPs annealed at 650 °C. 3
Materials Science & Engineering B 241 (2019) 1–8
T.K. Anh et al.
Fig. 3. Average particle size of Gd(OH)CO3·H2O:6%Eu3+ and Gd2O3:6%Eu3+ NSPs as a function of (a) the content rates of urea and Ln3+ ions (e.g. Gd3+ and Eu3+), with reaction time of 70 min, (b). The aging reaction time, the ratio of urea and Ln3+ ions (e.g. Gd3+ and Eu3+) is 25/1.
(M2 curve); Gd(OH)CO3·H2O:6%Eu3+sample heated at 200 °C, coated with silica and annealed at 650 °C (M3 curve) and Gd(OH)CO3·H2O:6% Eu3+ heated at 650 °C, coated with silica and annealed at 200 °C (M4 curve). From the powder XRD analysis, all of samples have cubic phase structure of Gd2O3 crystal. However, there were extra peaks at 30.70 and 31.80° corresponding to unincoporated Eu2O3 phase (JCPDS file No. 817979) in the diagram of coated Gd(OH)CO3·H2O:6%Eu3+sample dried at 70 °C before silica shell coating (Fig. 5b, M2 curve). It can be explained that, with the selected coating procedure, the hydrated water in Gd(OH)CO3·H2O was not released but only the adsorbed water instead (Fig. 1). This might lead to the less incoporation of the doped Eu3+ ions into Gd(OH)CO3·H2O lattice than the other samples shown in M3 and M4 curves of Fig. 5b. In another context, for two latter samples (M3 and M4) that completely dehydrated, the freshly-produced anhydrous Gd(OH)CO3 is probably more active than Gd(OH)CO3·H2O for the total amount of the dopant to be incoporated into. We will return to discuss in more details this case in the next paragraph mentioning to luminescent characteristics of Gd2O3: Eu3+@silica NSPs fabricated by this sol-gel process.
Fig. 4. Size distributions of Gd2O3:6%Eu3+ (a), Gd2O3:6%Eu3+@silica (b) and Gd(OH)CO3·H2O:6%Eu3+ (c) NSPs. Table 1 The typical average size and standard deviation of Gd(OH)CO3·H2O:6%Eu3+; Gd2O3:6%Eu3+; Gd2O3:6%Eu3+@silica NSPs obtained from this work and Gd (OH)CO3·H2O:5%Eu3+ NSPs obtained from Ref. [16]. Item 1 2 3 4
Sample 3+
Gd(OH)CO3·H2O:6%Eu Gd(OH)CO3·H2O:5%Eu3+ Gd2O3:6%Eu3+ Gd2O3:6%Eu3+@silica
Average size (nm)
Standard deviation (%)
190 150 150 160
± 3.20 (this work) ± 10.60 [16] ± 4.85 (this work) ± 3.50 (this work)
3.2.2. EDS analysis The EDS results of the pure Gd2O3:6%Eu3+ and Gd2O3:6%Eu3+ @ silica samples are shown in Fig. 6. It confirms the presence of the basics elements, including Gd, Eu and O in both samples while the presence of Si is only observed in silica coated nanoparticles as expected. No other impurities were detected in the uncoated and silica coated samples from EDS analysis. Table 2 presents the weight and atomic percentage values of the uncoated Gd2O3:6%Eu3+ and Gd2O3:Eu3+@silica NSPs. The weight and atomic percentage value of each element are good in corresponding to the composition of nanospheres. It exhibits that the element composition of the uncoated and silica coated nanospheres was homogeneous distribution.
3.2. Structures, infrared spectra and luminescent properties 3.2.1. XRD analysis XRD patterns of Gd(OH)CO3·H2O:6%Eu3+ NSPs annealed at 200 and 650 °C are shown in Fig. 5a. We found that the sample annealed at 200 °C was almost amorphous (black curve). After being annealed at 650 °C, the diffraction peaks occurred (red curve). All these observable peaks at 2θ: 28.70; 33.10; 47.60; 56.40; and 59.10° are indexed according to the standard pattern (JCPDS file No. 110604), indicating that the Gd2O3:6%Eu3+ NSPs were pure cubic phase and no impurities were found. The phase identification of samples synthesized with the different heating treatment regimes for coating process was also investigated. Fig. 5b presentes the XRD patterns of the uncoated Gd2O3:6% Eu3+sample annealed at 650 °C (M1 curve); Gd(OH)CO3·H2O:6% Eu3+sample dried at 70 °C, coated with silica and annealed at 650 °C
3.2.3. FTIR spectra To investigate the chemical group, chemical bonding and composition of the fabricated sample, the FTIR spectra of the uncoated and coated Gd2O3:6%Eu3+ samples have been measured (see Fig. 7). For FTIR of the uncoated Gd2O3:6%Eu3+ sample (curve a), the presence of carbonate anion (CO3) in the structure of the final product gadolinium oxide annealed at 650 °C was almost defused, which is confirmed by an appearance of the absorption doublets in the region of 1350–1600 cm−1 and the multiabsorptions ranging from 500 to 1000 cm−1 [21]. The characteristic bands of Gd2O3 were found at 415.49, 449.66 and 542.9 cm−1 [33]. The band at 543 cm−1 can be assigned to the Gd-O stretching vibration, which also confirms the formation of Gd2O3 via the urea-based precipitation and the subsequent 4
Materials Science & Engineering B 241 (2019) 1–8
T.K. Anh et al.
Fig. 5. XRD pattern of (a) Gd(OH)CO3·H2O:6%Eu samples annealed at 200 °C and 650 °C, (b) M1, M2, M3 and M4 samples.
3.3. Silica coated Gd2O3: X%Eu3+
annealing process at high temperatures [15]. The strong absorption band at 3446 and 1634 cm−1 are the characteristic absorption of surface adsorbed water, OH groups and water of hydration [16]. The spectra of silica coated Gd2O3:6%Eu3+ are shown in curve b. The characteristic bands of OH and Si-O-Si group are observed in the ranges of 1050–1250 cm−1 and 650–800 cm−1, respectively. Together with EDS analysis, the FTIR spectra of the pure Gd2O3:6%Eu3+and silica coated Gd2O3:6%Eu3+ confirms the forming silica shell on surface of the NSPs.
In our case, the Gd2O3:Eu3+nanophosphores have central symmetric shape with high monodispersity with the size from 100 to 200 nm, showing the bright red emission corresponding to the sharp peak at 611 nm. These spherical nanophosphores enable a very promising platform to develop photonic system, bioimaging probe, and targeting drug carrier. To meet the requirement for these application, the surface modification of these NSPs is effective strategy not only to protect the surface but also to provide new functional group for subsequent bioconjugation with various biomolecules. Indeed, there are many investigations to coat the nanophosphors by sol-gel method, especially, to form core/shell nanostructures or nanocomposite. In our study, the typical nanophosphor of Gd2O3:6%Eu3+ was selected due to its physicochemistry and emission feature. Fig. 8b shows the photoluminescence spectra of Gd2O3:6%Eu3+coated with silica shell in different thickness. First, the photoluminescence intensity of Gd2O3:6% Eu3+@silica NSPs dried at 70 °C before being coated and annealed at 650 °C was stronger than that of pure core phosphor. After silica coating, the photoluminescence spectra of NSPs exhibit a new emission peak located at 621 nm. The emission intensity of 621 nm peak increases with the thickness of silica layer. It leads to the reduction of the ratio of the 611 and 621 nm emissions, which generates brighter red emission observed from the Gd2O3:Eu3+@silica NSPs. Next, we investigate the effect of silica coating conditions on the emission spectra of NSPs. Two samples, namely, Gd2O3:6%Eu3+sample annealed at 200 °C, being coated with silica shell before annealed at 650 °C and Gd2O3:6%Eu3+samples annealed at 650 °C, coated silica shell than heated up to 200 °C were subjected to this study. Fig. 8c demonstrated the luminescence spectra of these two experiments. It show that the peak at 621 nm in the both cases was almost disappeared and only as a shoulder. The emission spectra of Gd2O3:6%Eu3+@silica
3.2.4. Luminescent properties of Gd2O3:X%Eu and Gd2O3:X%Eu@Silica The photoluminescence spectra of Gd2O3:X%Eu NSPs (X = 3.50; 5.00; 6.00; 7.00 and 8.00 at.%) under 270 nm excitation are shown in Fig. 8a. The emission spectra exhibit the strongest emission peak at 611 nm corresponding to an electron dipole transition of Eu3+ ions (5D0 → 7F2). It is well-known that this transition is allowed when europium is occupied in low symmetries. The emission intensity of other peaks at 590, 625, 650 and 705 nm assigning to the 5D0 → 7F1; 5 D0 → 7F3; 5D0 → 7F4; 5D0 → 7F5 transitions, respectively are also observed. The emission peak at 590 nm is mainly a magnetic dipole transition and not dependent on the site symmetry of europium ions in host lattice. The ratio between the electron dipole transition (611 nm) and the magnetic dipole transition (590 nm) is the grade of the site symmetry at which europium ions is positioned. For Gd2O3:X%Eu (X = 3.50; 5.00; 6.00; 7.00; 7.50; and 8.00 at.%) NSPs, the electron dipole transition was much higher than magnetic dipole transition. The intensity ratio of the 611 and 590 nm emissions of Gd2O3:X%Eu samples (X = 3.50; 5.00; 6.00; 7.00 and 8.00 at.%) is over 15/1. In the concentration interval from 3.50% to 8.00% of Eu3+ the luminescence intensity was increased until 5.00 and 6.00%, then reduced near 8.00%, in which the concentration quenching is appeared and become stronger in the higher concentration.
Fig. 6. EDS spectra of (a) Gd2O3:6%Eu3+ and (b) Gd2O3:6%Eu3+@silica powder samples annealed at 650 °C. 5
Materials Science & Engineering B 241 (2019) 1–8
T.K. Anh et al.
Table 2 Element compositions by EDS data of the uncoated Gd2O3:6%Eu3+ andGd2O3:6%Eu3+@silica samples. Element
Gd (L)
3+
1
Gd2O3:6%Eu
2
Gd2O3:6%Eu3+@silica
Eu (L)
O (K)
Si (K)
Weight %
Atomic %
Weight %
Atomic %
Weight %
Atomic %
79.72 80.62 74.82
37.26 34.52 27.07
7.40 4.28 5.22
3.58 1.90 1.95
12.88 15.11 19.96
59.17 63.58 70.98
55.64 61.20 50.20
13.44 16.41 10.54
5.63 5.20 3.75
1.41 1.44 0.81
32.10 27.97 38.81
76.20 73.70 80.13
Weight %
Atomic %
6.63 5.63 7.24
8.96 8.45 8.52
spectrum of the uncoated Gd2O3:6%Eu3+ was 465 nm (see inset of Fig. 8d). The alternative of the strongest peaks in the photoluminescence excitation spectra of the uncoated and silica coated Gd2O3:Eu3+ was indicated also the occurrence of crystal reforming in Gd2O3 nanophosphor. Moreover, the doped Eu3+ions occupies the Gd3+ octahedral site based on their close ionic radii and the same valency state (+3). As consequence, the crystal reforming was caused to open a new transition channel in the energy levels 5D0 → 7F2. In other word, Eu3+ ions can be used as an optical probe for detection of structure change in solid state. In short, the silica shell coating strategy of nanomaterials by sol-gel technique with the aim to modify and enhance their quality, for instance, better dispersion, stronger emission intensity and good compatibility to environment is very effective. The silica coating result is depending on the design and fabrication technique. Thus the silica shell wrapping core was able the cause to decrease the luminescence intensity of Gd2O3:Eu3+@SiO2 in [23], as well as to enhance the luminescence intensity in this work. However, the sol-gel coating can be a key technique that possibly manipulates a nanomaterial for application in photonic, biology, medicine and also pharmacy.
Fig. 7. FTIR spectra of the pure Gd2O3:6%Eu3+ (curve a) and Gd2O3:6%Eu3+@ silica (curve b) samples.
were similar to those of the uncoated Gd2O3:6%Eu3+ samples. It indicated there is no crystal reforming. It is noted that if the core materials Gd(OH)CO3·H2O:6%Eu3+ dried only at low temperature 70 °C (see Fig. 8b) the occurrence of peak at 621 nm was strong. It can be proposed the silica shell coating affected on the conversion process from gadolinium hydroxyl carbonate to gadolinium oxide. As shown above in Fig. 7 the X-ray diffraction pattern of Gd(OH)CO3·H2O:6%Eu3+ dried at 70 °C, coated with silica and annealed at 650 °C (curve M2) and Gd (OH)CO3·H2O:6%Eu3+ heated at 200 °C, coated with silica and annealed at 650 °C (curve M3), appeared the extra peaks, which are not to belong to those of cubic crystal phase of Gd2O3:Eu3+but rather of Eu2O3 phase. It was able the reform in crystal structure which causes lower site symmetry of Gd2O3host matrix in which Eu3+ ions is situated [34]. Thus the new crystal form appeared maybe a reason for occurrence of luminescence peak at 621 nm. To this suppose is also confirmed the increased intensity of peak at 621 nm while the silica shell thicker (Fig. 8b). The doped Eu3+ occupies the Gd3+ site due to their close ionic radii. Maybe, under the silica coating conditions there was a part penetration of SiO2–X(OH)X molecules into the big Gd(OH)CO3·H2O NSPs, which was in soft amorphous state after being treated at very low temperature (70 °C). Under this coating conditions, SiO2-X(OH)X NSPs can easy move insight to lattice and reacted with host molecule to initiate the crystal reform with the annealing at 650 °C. To look more insight the impact of silica coating on the optical and structural properties of core-Gd2O3: Eu3+ it was measured the photoluminescence excitation (PLE) spectra. The PLE spectra of Gd2O3:6% Eu3+@silica samples are shown in Fig. 8d. The PLE spectra obtained was observed the emission peak at 621 nm. For comparison, the PLE emission peak at 611 nm of the uncoatedGd2O3: Eu3+was also presented in the inset of Fig. 8d. In the PLE spectra of Gd2O3:6%Eu3+@ silica, there are 4 peaks near 362, 380, 394 and 465 nm. The intensity of the peaks was increased if the silica thickness increased. In this case the strongest peak was at 394 nm. In contrast, the strongest peak in the PLE
4. Conclusion We demonstrate the optimal conditions to fabricate NSPs Gd2O3:Eu3+ and Gd2O3:Eu3+@silica with high monodispersity, low standard deviation ≤ ± 5% and in size range from 100 to 200 nm. The most important parameters are the selected ratio urea/(gadolinium + europium) ions, short aging reaction time at 85 °C and subsequent stepwise annealing regime from 70 to 650 °C. The spherical Gd2O3:Eu3+ and Gd2O3:Eu3+@silica NSPs show strong luminescence in the red color and very easy disperse in solvent and acquire media. The emission spectra of the nanophosphores are the energy transitions from 5 D0 to 7Fj of Eu3+ions. The ratio of the emitting intensity of 5D0 to 7F2 and 5D0 to 7F1 is over 15/1. The silica shell was almost wrapped the nanophosphore core, which is enabling effective protection, compatibility and enhancement of luminescence intensity. Moreover, the effect of silica coating on optical properties was enhanced the luminescence intensity of the main and strongest peak at 611 nm and also 621 nm. The impacts of silica coating on the structural properties of the core Gd2O3:X%Eu3+ nanophosphores (X = 3.50; 5.00; 6.00; 7.00; 7.55 and 8.00 at.%) depended on the fabrication condition. In the 5D0 to 7F2 energy transitions, the occurrence of a new peak at 621 nm in fluorescence spectra is caused by the crystal reform in part of cubic phase of Gd2O3:Eu3+@silica. The high monodispersity in size and shape of pure and silica coated Gd2O3: Eu3+ nanophosphores are very promising platform for application in photonic and also in biology-medicinepharmacy. Acknowledgments This paper is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED), Vietnam, under grant No. 103.03-2015.85. The authors are closely collaborated with research 6
Materials Science & Engineering B 241 (2019) 1–8
T.K. Anh et al.
a.
Photoluminescence
spectra
b. Photoluminescence emission and
of
Gd2O3:X%Eu samples (X=3.50; 5.00; 6.00;
excitation spectra of Gd2O3:6%Eu3+@silica
7.00 and 8.00 at.%), excitation at 270 nm.
samples as a function of silica shell thickness.
c.
d. Photoluminescence excitation (PLE)
Photoluminescence
spectra
of
Gd2O3:6%Eu3+@silica samples as a function
spectra
of the fabricated conditions.
Gd2O3:6%Eu3+@silica samples with different
of
Gd2O3:6%Eu3+
and
silica thickness. Fig. 8. Photoluminescence emission and excitation spectra of Gd2O3:X%Eu (X = 3.50; 5.00; 6.00; 7.00 and 8.00 at.%) and Gd2O3:6%Eu3+@silica samples. Fig. 8a. Photoluminescence spectra of Gd2O3:X%Eu samples (X = 3.50; 5.00; 6.00; 7.00 and 8.00 at.%), excitation at 270 nm. Fig. 8b. Photoluminescence emission and excitation spectra of Gd2O3:6%Eu3+@silica samples as a function of silica shell thickness. Fig. 8c. Photoluminescence spectra of Gd2O3:6%Eu3+@silica samples as a function of the fabricated conditions. Fig. 8d. Photoluminescence excitation (PLE) spectra of Gd2O3:6%Eu3+ and Gd2O3:6%Eu3+@silica samples with different silica thickness.
institutions and universities in country and abroad. The authors thank the Institute of Fundamental Science and Application, Duy Tan University, Institute of Materials Science, Vietnamese Academy of Sciences and Technology, Institute of Low Temperature and Structure Research, Poland, National Chung Cheng University, Taiwan for support. The authors are indebted Dr. Le Nguyen Bao, Prof. Le Van Vu, for discussion and support.
[2]
[3]
[4]
References [5] [1] J. Adam, W. Metzger, M. Koch, P. Rogin, T. Coenen, J.S. Atchison, P. Koenig, Light
7
emission intensities of luminescent Y2O3: Eu and Gd2O3: Eu particles of various sizes, Nanomaterials 7 (2017) 26. D. Warzynczyk, M. Nyk, A. Bernarkiewiecz, W. Strek, M. Sarnoc, Morphology- and size-dependent spectroscopic properties of Eu3+ doped Gd2O3 colloidal nanocrystals, J. Nanopart. Res. 16 (2014) 2690. L.D. Tuyen, J.H. Lin, C.Y. Wu, P.T. Tai, J. Tang, L.Q. Minh, H. ChihKan, C.C. Hsu, Pumping-power-dependent photoluminescence angular distribution from an opal photonic crystal composed of monodisperse Eu3+/SiO2 core/shell nanospheres, Opt. Express 20 (2012) 15418–15426. S. Zinatloo-Ajabshir, M. Salavati-Niasari, M. Hamadanian, Praseodymium oxide nanostructures: novel solvent-less preparation, characterization and investigation of their optical and photocatalytic properties, RSC Adv. 5 (2015) 33792. S. Zinatloo-Ajabshir, M. Salavati-Niasari, A. Sobhani, Z. Zinatloo-Ajabshir, Rare earth zirconate nanostructures: recent development on preparation and
Materials Science & Engineering B 241 (2019) 1–8
T.K. Anh et al.
the micronsize range, J. Colloid Interface Sci. 26 (1968) 62–69. [21] W.P. Hsu, L. Ronnquist, E. Matijevie, Preparation and properties of monodispersed colloidal particles of lanthanide compounds. 2. Cerium(IV), Langmuir 4 (1988) 31–37. [22] J. Wang, J. Zhu, Recent advances in spherical photonic crystals: generation and applications in optics, Eur. Polym. J. 49 (2013) 3420–3433. [23] K.-M. Lin, C.-C. Lin, Y.-Y. Li, Luminescent properties and characterization of Gd2O3:Eu3+@SiO2 and Gd2Ti2O7:Eu3+@SiO2 core–shell phosphors prepared by a sol–gel process, Nanotechnology 17 (2006) 1745–1751. [24] B. Sun, G. Zhou, H. Zhang, Synthesis, functionalization, and applications of morphology-controllable silica-based nanostructures: a review article, Progr. Solid State Chem. 44 (2016) 1–19. [25] Y. Cheng, K. Sun, Upconversion photoluminescence of core-shell structured SiO2@ YVO4:Yb3+, Er3+, Eu3+ nanospheres, Appl. Opt. 56 (2017) 4906–4910. [26] N.T. Huong, P.T. Lien, N.M. Hung, N.D. Van, T.T. Thuy, N.T. Binh, L.Q. Minh, Conjugation of TbPO4.H2O-based nanowires with immunoglobulin G for bioimaging, J. Electron. Mater. 45 (2016) 2463–2467. [27] A. Jain, G.A. Hirata, M.H. Farías, F.F. Castillón, Synthesis and characterization of (3-Aminopropyl)trimethoxy-silane(APTMS) functionalized Gd2O3:Eu3+ red phosphor with enhanced quantum yield, Nanotechnology 27 (2016) 12 065601. [28] H.A.I.Y. Tok, L.T. Su, F.Y.C. Boey, S.H. Ng, Homogeneous precipitation of Dy2O3 nanoparticles – effects of synthesis parameters, J. Nanosci. Nanotechnol. 7 (2007) 1–9. [29] D. Yang, P.Z. Ma, Z. Cheng, C. Li, J. Lin, Current advances in lanthanide ion (Ln3+)based upconversion nanomaterials for drug delivery, Chem. Soc. Rev. 44 (2015) 1416–1448. [30] Y.R. Lu, M.-Y. Gou, L.-Y. Zhang, L. Li, T.-T. Wang, C.-G. Wang, Facile one-pot synthesis of hollow mesoporous fluorescent Gd2O3:Eu/calcium phosphate nanospheres for simultaneous dual-modal imaging and pH-responsive drug delivery, Dyes Pigments 147 (2017) 514–522. [31] S. Lechevallier, P. Lecante, R. Mauricet, H. Dexpert, J. Dexpert-Ghys, H.K. Kong, G.I. Law, K.L. Wong, Gadolinium–europium carbonate particles: controlled precipitation for luminescent biolabeling, Chem. Mater. 22 (2010) 6153–6161. [32] Z. Xu, Y. Gao, S. Huang, P. Ma, J. Lin, J. Fang, A luminescent and mesoporous coreshell structured Gd2O3:Eu3+@nSiO2@mSiO2 nanocomposite as a drug carrier, Dalton Trans. 40 (2011) 4846–4854. [33] S.K. Ranian, A.K. Soni, V.K. Rai, Frequency upconversion and fluorescence intensity ration method in Yb3+-ion sensitized Gd2O3:Er3+-Eu3+ phosphors for display and temperature sensing, Methods Appl. Fluorsc. 5 035004 (2017) (9 pages). [34] T.K. Anh, P.T.M. Chau, N.T.Q. Hai, V.T.T. Ha, H.V. Tuyen, S. Bounyavong, N.T. Thanh, L.Q. Minh, Facile fabrication and properties of Gd2O3:Eu3+, Y2O3:Eu3+ nanophosphors and Gd2O3:Eu3+/silica, Y2O3:Eu3+/silica nanocomposites, J. Electron. Mater. 47 (2018) 585–593.
photocatalytic applications, J. Alloys Compounds 767 (2018) 1164e1185. [6] S. Zinatloo-Ajabshir, M.S. Morassaei, M. Salavati-Niasari, Nd2Sn2O7 nanostructures as highly efficient visible light photocatalyst: green synthesis using pomegranate juice and characterization, J. Cleaner Prod. 198 (2018) 11–18. [7] D. Peer, J.M. Karp, S. Hong, O.C. Farokhzad, R. Margalit, R. Langer, Nanocarriers as an emerging platform for cancer therapy, Nat. Nanotech. 2 (2007) 751–760. [8] P. Couvreur, Nanoparticles in drug delivery: past, present and future, Adv. Drug Deliv. Rev. 65 (2013) 21–23. [9] A. Escudero, A.I. Becerro, C. Carrillo-Carrión, N.O. Núñez, M.V. Zyuzin, M. Laguna, D. González-Mancebo, M. Ocana, W.J. Parak, Rare earth based nanostructured materials: synthesis, functionalization, properties and bioimaging and biosensing applications, Nanophotonics 6 (5) (2017) 881–921. [10] K.N. Orekhova, D.A. Eurov, D.A. Kurdyukov, V.G. Goolubev, D.A. Kirilenko, V.A. Kravets, M.V. Zamoryanskay, Structural and luminescent properties of Gd oxide doped with Eu3+ embedded in mesopores of SiO2 particles, J. Alloys Compd. 678 (2016) 434–438. [11] V. Stengl, J. Subrt, P. Bezdicka, M. Mankova, S. Bakardjiena, Homogeneous precipitation with urea-an easy process for making spherical hydrous metal oxides, Solid State Phen. 90–91 (2003) 121–126. [12] T.K. Anh, D.X. Loc, T.T. Huong, N. Vu, L.Q. Minh, Luminescent nanomaterials containing rare earth ions for security printing, Int. J. Nanotechnol. 8 (2011) 335–346. [13] L.D. Tuyen, A.C. Liu, C.C. Huang, P.C. Tsai, J.Hu. Lin, C.W. Wu, L.K. Chau, T.S. Yang, L.Q. Minh, H. ChihKan, C.C. Hsu, Doubly resonant surface-enhanced Raman scattering on gold nanorod decorated inverse opal photonic crystals, Opt. Express 20 (2012) 29266–29275. [14] J. Liu, X. Tian, N. Luo, C. Yang, J. Xiao, Y. Shao, X. Chen, G. Yang, D. Chen, L. Li, Sub-10 nm monoclinic Gd2O3:Eu3+ nanoparticles as dual-modal nanoprobes for magnetic resonance and fluorescence imaging, Langmuir 30 (2014) 13005–13013. [15] H. Liu, J. Liu, Hollow mesoporous Gd2O3:Eu3+ spheres with enhanced luminescence and their drug releasing behavior, RSC Adv. 6 (2016) 99158–99164. [16] W. Song, W. Di, W. Qin, Synthesis of mesoporous-silica-coated Gd2O3:Eu@silica particles as cell imaging and drug delivery agents, Dalton Trans. 45 (2016) 7443–7449. [17] D.A. Eurov, D.A. Kurdyukov, D.A. Kirilenko, J.A. Kukushkina, A.V. Nashchekin, A.N. Smirnov, V.G. Golubev, Core–shell monodisperse spherical mSiO2/ Gd2O3:Eu3+@mSiO2 particles as potential multifunctional theranostic agents, J. Nanopart. Res. 17 (82) (2015) 10. [18] X. Rena, P. Zhang, Y. Han, X. Yang, H. Yang, The studies of Gd2O3:Eu3+ hollow nanospheres with magnetic and luminescent properties, Mater. Res. Bull. 72 (2015) 280–285. [19] Z. Fan, P.P. Fu, H. Yu, P.C. Ray, Theranostic nanomedicine for cancer detection and treatment, J. Food Drug Anal. 22 (2014) 3–17. [20] W. Stoeber, A. Fink, E. Bohn, Controlled growth of monodisperse silica spheres in
8
Contents lists available at ScienceDirect
Materials Science & Engineering B journal homepage: www.elsevier.com/locate/mseb
Great enhancement of monodispersity and luminescent properties of Gd2O3:Eu and Gd2O3:Eu@Silica nanospheres
T
Tran Kim Anha, Nguyen Thanh Huongb,c, Pham Thi Lienb,c, Do Khanh Tungb,c, Vu Duc Tuc,e, ⁎ Nguyen Duc Vanb, Wieslaw Strekd, Le Quoc Minha,b,c, a
Duy Tan University, 182 Nguyen Van Linh, Da Nang, Viet Nam Institute of Materials Science, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Viet Nam c Graduate University of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Viet Nam d Institute of Low Temperature and Structure Research, Polish Academy of Science, Ul.Okolna 250-422, Wroclaw, Poland e National Chung Cheng University, Ming Hsiung, Chia Yi 621, Taiwan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Gd2O3:Eu Gd2O3:Eu@silica Monodisperse Nanospheres Photoluminescence
In this paper, the experimental parameters that control the monodisperse and high quality Gd2O3:Eu3+ and Gd2O3:Eu3+@silica nanospheres (NSPs) systematically investigated. Gd2O3:Eu3+ and Gd2O3:Eu3+@silica NSPs in size range from 100 to 200 nm with a very low standard size deviation of ± 5.00% have been successfully fabricated. The luminescence spectra of Gd2O3:X%Eu3+ and Gd2O3:X%Eu3+@silica NSPs were studied as a functions of Eu3+ ion concentration (X = 3.50; 5.00; 6.00; 7.00; 7.50 and 8.00 at.%). The strongest emission peak at 611 nm corresponding to 5D0–7F2 transition of Eu3+ ions in Gd2O3:Eu3+ NSPs was very sharp. Interestingly, for the silica coated Gd2O3:X%Eu3+ NSPs, a new emission peak at 621 nm was appeared in depend on the coating condition. The intensity ratio of the emission peaks at 611 nm and 621 nm was decreased with the increase of the silica thickness. The highly monodispersed, mesoscaled Gd2O3:Eu3+ and Gd2O3:Eu3+@silica NSPs synthesized in this work are high potential for application, especially in photonic crystals, bioimaging and drug delivery.
1. Introduction Rare Earth (RE) actived nanomaterials with various sizes and shapes exhibit high application potentials in electronics [1,2], photonic [3], chemistry [4–6] and biomedicine [7–9]. The RE contained nanospheres (NSPs) capable of constructing systematic 2D or 3D nano-frameworks, have attracted great attention in the fields of display, catalyst [10,11], photonic crystals and security printing [12,13]. Additionally, large NSPs inherently possess identical hydrodynamic properties in fluidic media, thus making them as a promising platform for cell imaging and drug delivery [14–16]. It is expected that the real-time monitoring of infiltration of RE-doped NSPs into tissues can be achieved, leading to controlled and targeted drug delivery. It is well-known that gadolinium oxide (Gd2O3) has been considered as an efficient host matrix for doping RE ions because of its valuable features such as thermal stability, low phonon energy and chemical durability [17,18]. One of the Gd2O3-based phosphors, Gd2O3:Eu3+, has been intensively studied in recent years for display, security printing and biomedicine [1,12,18,19]. The average grain size of Gd2O3 NSPs in the mesoscale
⁎
range from 100 to 500 nm is ideal for simultaneous cell imaging and drug delivery [8,17]. Recently, various synthetic methods have been developed for the fabrication of a considerable number of homogeneous nanoparticles, either amorphous or crystalline, in many shapes, including NSPs. To the best of our knowledge, for synthesizing oxide-based NSPs, several typical chemical methods, for instance, Stober sol-gel [20,23] or multistep chemical precipitation with urea [21,28], have been developed. The Stober method has been commonly used for the synthesis of silica NSP (SiO2–X(OH)X) with high monodispersity [3,22] meeting the requirement for fabrication of photonic crystal structure as well as potential use in biomedical application [16,24]. Besides, sol-gel process is also widely-used in shell coating, surface immobilization, functionalization of biocompatible nanomaterials [25–27]. On the other hand, the multistep precipitation with urea has successfully employed to synthesize several metaloxides (MeO) and lanthanide oxides (Ln2O3) [11,21,28]. Recently, this method has been used to synthesize some highly efficient luminescent RE-doped NSPs, which is able to provide multifunctional nanomedical platforms for diagnosis
Corresponding author at: Institute of Materials Science, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Viet Nam. E-mail address: [email protected] (L.Q. Minh).
https://doi.org/10.1016/j.mseb.2019.01.020 Received 11 June 2018; Received in revised form 3 December 2018; Accepted 31 January 2019 0921-5107/ © 2019 Published by Elsevier B.V.
Materials Science & Engineering B 241 (2019) 1–8
T.K. Anh et al.
Gd2O3:X%Eu3+NSPs with various doping concentrations (X = 3.50; 5.00; 6.00; 7.00; 7.50 and 8.00 at.%) were followed a similar process to that for preparing the Gd2O3:6%Eu3+ NSPs.
and therapy [19,29]. For the use of NSPs as a drug nanocarrier in treatment of cancer disease, the particle size of the few hundred nanometers is preferred for the targeted drug delivery into tumor cell [3,8,10]. The homogenous distribution of NSPs is very critical in the functionalization of bioimaging probes as well as drug carrier agents. The urea assisted preparation is a simple and effective route for fabrication of lanthanide hydroxycarbonate NSP that was first found by Matijevic [21]. Since then, the further studies [11,28] have been investigated to find the proper experimental conditions for obtaining spherical lanthanide and transition metal hydroxycarbonates. Until now, there has been a variety of researches based on urea precipitation method to synthesize the Ln(OH)CO3·H2O and Ln2O3 NSPs, however, the effect of synthesis parameters and especially annealing regime on the size and shape of NSPs have not been addressed. Particularly, there are only few studies using Gd2O3:Eu3+ NSPs for simultaneous bioimaging and drug delivery [15,18,30]. Until now, there is only one report using the urea precipitation method to fabricate Gd2O3: 5% Eu NSPs with an average size of 150 nm and a standard size deviation of ± 10.60 [16]. Herein, we systematically investigated the urea-based precipitation method for enhancing the monodispersity of Gd2O3:X%Eu3+ NSPs (X = 3.50; 5.00; 6.00; 70.0; 7.50 and 8.00 at.%). Subsequently, these NSPs were coated with a silica shell, followed by annealing process at different annealing temperatures. Thermogravimetric analyses (TGA) have been analyzed to find an optimized heating regime to remain homogeneous size and shape of Gd2O3:Eu3+ and silica coated Gd2O3:Eu3+ NSPs. The physical and chemical properties of as-synthesized samples have been characterized by X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), energy dispersion spectroscopy (EDS), and Fourier transform-infrared spectrometry (FT-IR). The luminescence properties of the uncoated and coated Gd2O3:Eu3+ NSPs have been studied, showing high potential for a wide-ranging application in photonic crystals, bioimaging and drug delivery.
2.1.2. Preparation of Gd(OH)CO3·H2O:X%Eu3+@silica and Gd2O3:X% Eu3+@silica The core/shell structured Gd(OH)CO3·H2O:X%Eu3+@silica nanospheres were synthesized by using a modified Stober process. In a typical procedure, 100 mg Gd(OH)CO3·H2O:X%Eu3+@silica particles were obtained after drying at 70 °C for 24 h, followed by re-dispersing in a mixture of 180 ml of ethanol and 100 ml of DI water by using ultrasonication for 45 min. After that, 150 µl of TEOS in 10 ml ethanol was slowly added dropwise into the reaction flash. Next, 1 ml of 25% NH4OH was added upon intensive stirring and the reaction solution was stirred for another 10 h. The final products were separated by centrifugation at 5000 rpm and washed with ethanol, DI water and then dried at 60 °C in air. Finally, the samples were heated at 105 °C, 200 °C and 650 °C to investigate the conversion of silica coating process from Gd(OH)CO3·H2O:Eu3+@silica to Gd2O3:Eu3+@silica samples. 2.2. Heating, measurement and characterization The subsequent heating processes have been implemented by using a high qualitative furnace Carbolite furnace (Serial No: 12/00/3200; Type: STF 15/75/450, Max temp (°C): 1500). The thermal properties of the samples Gd(OH)CO3·H2O:Eu3+ were analyzed with a equipment thermogravimetric analysis (TGA) – TG209F1 Libra Analyzer Netzsch, in range from 27 °C (at room temperature) to 1000 °C, with temperature increase rate of 10 °K/min, in N2 atmosphere. X-ray diffraction (XRD) measurements were performed to determine the phase of the as-prepared samples at room temperature with a Bruker D8-Advance diffractometer, using Cu Kα radiation (λ = 1.5405 Å), in the 2θ range from 10 to 70° with a scanning step of 0.02 °C/min. The energy dispersion spectroscopy (EDS) was measured with a F6400 JEOL. The morphology of the samples was examined with FESEM (Hitachi S-4800) and TEM (JEM 1010-JEOL). FT-IR spectra of all the samples were measured with a Nicolet spectrophotometer using KBr pellets in the range of 400–4000 cm−1. The UV–VIS absorption spectra were recorded with a SL159ELICO UV–VIS spectrophotometer. Photoluminescence excitation (PLE) and emission (PL) spectra were recorded at room temperature with a Hitachi F-4500 spectrophotometer equipped with a 150 W xenon lamp or a LED as the excitation sources.
2. Experimental section 2.1. Materials 2.1.1. Preparation of Gd2(OH)CO3H2O:X%Eu3+ and Gd2O3:X%Eu3+ Gadolinium(III) nitrate hexahydrate (Gd(NO3)3·6H2O, fine powder, 99.99%), europium(III) nitrate pentahydrate (Eu(NO3)3·5H2O, powder, 99.99%) and tetraethoxysilane (TEOS, liquid, 99%) from Sigma-Aldrich were purchased and used without further purification. Urea (CO(NH2)2, powder, 98%) and nitric acid (HNO3, solution, 65%) was purchased from Merck. The DI water was used for all the experimental processes. For fabricating the mixed oxide of Gd and Eu with various content ratios, the 0.004 M Eu(NO3)3, 0.3 M Gd(NO3)3 and 0.8 M urea stock solutions have been prepared. The stock solution of Eu(NO3)3 was prepared as follows: 0.211 g Eu2O3 was added in 50 ml of HNO3 25% in 100 ml bottom flask and warmed up at 70–80 °C under stirring to obtain a clear solution. After that, the above mixture was heated gently to obtain fine powder. The Gd2O3:6%Eu3+ NSPs have been prepared by a modified multistep co-precipitation with the presence of urea. In a typical synthesis process, 10 ml of 0.3 M Gd(NO3)3 solution was added into a 250 ml round-bottomed flask containing 47.87 ml of 0.004 M Eu(NO3)3 with magnetic stirring at room temperature for 60 min. Subsequently, 98.68 ml of 0.8 M urea solution was added along with 60 ml of DI water, followed by heating to 85 °C for 68–70 min in an oil bath. The reaction was then cooled by a water stream outside of the flask. The obtained suspension was separated by centrifugation at 5000 rpm and collected after washing with deionized water and ethanol for two times. The resulting NSPs was dried at 70 °C for 12 h in air and then annealed by stepwise heating process at 105 °C for 5 h; 200 °C for 2 h; 600 °C for 2 h and 650 °C for 3 h to obtain Gd2O3:6%Eu3+ as final product. The
3. Results and discussion 3.1. Controlling synthetic process and physicochemical features of Gd (OH)CO3·H2O:X%Eu3+ and Gd2O3:X%Eu3+ NSPs We first investigated the experimental parameters to find a proper condition to obtain the high monodispersity of Gd2O3:X%Eu3+ and Gd2O3:X%Eu3+@silica NSPs in size range from 100 to 200 nm with considerable modification compared to the pioneering work of Matijevic [21] and other reports [11,16,28]. In this investigation, the pH values from 4.90 to 5.10; reaction temperature at 85 °C ± 1 °C, Gd (NO3)3 concentration of 1.35.10-2 M and urea concentration from 4.10-1 to 8.10-1M were maintained throughout the synthetic process. In the meantime, the other experimental parameters were changed as follows: content rates of Ln3+ ions (i.e. Gd3+ and Eu3+) and urea from 1/20 to 1/40, aging reaction time from 50 to 80 min, and stepwise heating regime in temperature range from 70 to 650 °C. The TGA analysis in the range of room temperature to 1000 °C of the as-fabricated samples are shown in Fig. 1. The first weight loss of Gd (OH)CO3·H2O:6% Eu3+ NSPs was due to the release of water molecules adsorbed on the sample surface in the range from room temperature to 168.4 °C. The second weight loss found in the range from 168.4 to 2
Materials Science & Engineering B 241 (2019) 1–8
T.K. Anh et al.
Eu3+ NSPs and final products Gd2O3:6%Eu3+ reveal that a high homogeneous morphology was achieved for all these samples (Fig. 2a–d). Fig. 2a and b show the FESEM images of Gd(OH)CO3·H2O NSPs annealed at 200 °C and Gd2O3:6%Eu3+ NSPs annealed at 650 °C, respectively. The TEM images of the pure Gd2O3:6%Eu3+ NSPs and silica coated Gd2O3:6%Eu3+ are displayed in Fig. 2c and d, respectively. The results confirm that the monodispersed Gd2(OH)CO3·H2O:6%Eu3+ NSPs was successfully fabricated in the size range of 100–200 nm. The size change of the NSPs (see Fig. 3) before and after annealing at 650 °C was about 13.5% corresponding to total weight loss of nearly 37%, due to the conversion from Gd(OH)CO3·H2O:X%Eu3+ to Gd2O3:X%Eu3+. The result indicates that the nanospheres size increases with the ratio of urea and Ln3+ ions (e.g. Gd3+ and Eu3+). The ratio from 20/1 to 40/1 was chosen to fabricate NSPs with size from 100 to 200 nm, as shown in Fig. 3a. It clearly shows that the average size of NSPs rises with increasing aging reaction time in the range of 50–80 min. The proper combination of the urea/Ln3+ ratio and the aging reaction time is prerequisites in the synthetic process to obtain the Gd(OH)CO3·H2O:6%Eu3+ and Gd2O3:6%Eu3+ NSPs with high monodispersity. It is noteworthy that the monodispersed NSPs with identical hydrodynamic feature and high solubility in the solvent or aqueous media are very useful for further systematic construction of 2D or 3D structures [3,22] and particularly for surface modification, functionalization and conjugation to create a bioimaging probe or targeting drug delivery [32]. The size distributions of Gd(OH)CO3·H2O:6%Eu3+, Gd2O3:6%Eu3+ and Gd2O3:6%Eu3+@silica NSPs based on the FESEM and TEM results is described in Fig. 4. The mean diameter of Gd(OH)CO3·H2O:6%Eu3+ NSPs was estimated to be 190 nm with a standard deviation (SD) of ± 3.20%. The uncoated Gd2O3:6%Eu3+ NSPs have spherical shape with a uniform size of approximately 150 nm and SD of ± 4.85%. After silica coating, the average size of Gd2O3:6%Eu3+@silica NSPs is about 160 nm with a SD of ± 3.50% (Table 1). The result indicates that the NSPs were successfully synthesized with a significant improvement in quality in comparison to those obtained in Ref. [16].
Fig. 1. TGA curves of as-fabricated Gd(OH)CO3·H2O:Eu3+.
608 °C is related to the removal of water molecules via the dehydration of hydrated water and the self-condensation process of hydroxyl groups (OH). Another weight loss was observed in the range of 608–750 °C corresponding to the strong release of CO2 molecules to form Gd2O3:Eu3+ NSPs [31]. As indicated in Fig. 1, the highest weight loss of 15.33% was in the temperature range of 168.4–608 °C in TGA curves. The large difference at the end point of 608 °C observed in our study is the same as that observed in the Ref [16] for at 550 °C in air [16]. The dissimilarity in temperature might be attributed to the difference in measuring atmosphere. According to the TGA results, a proper heating regime to obtain the highly monodispersed Ln2O3 NSPs was addressed. The heating regime for silica coated Gd(OH)CO3·H2O:Eu3+ NSPs was similar to that for the uncoated products. By analyzing the thermoproperties of gadolinium hydroxycarbonate with and without silica coating, it could be found the stepwise subsequent heating regime in air. This stepwise regime with the heating at 105 °C for 5 h; 200 °C for 2 h; 600 °C for 2 h and 650 °C for 3 h enables to maintain and remain the high homogeneity of size and shape of nanospheres without any deforming or cleft. The FESEM and TEM images of the as-prepared Gd(OH)CO3·H2O:6%
Fig. 2. FESEM images of (a) Gd(OH)CO3·H2O:6%Eu3+ NSPs annealed at 200 °C, (b) Gd2O3:6%Eu3+ NSPs annealed at 650 °C, and TEM images of (c) Gd2O3:6%Eu3+ NSPs annealed at 650 °C and (d) silica coated Gd2O3:6%Eu3+ NSPs annealed at 650 °C. 3
Materials Science & Engineering B 241 (2019) 1–8
T.K. Anh et al.
Fig. 3. Average particle size of Gd(OH)CO3·H2O:6%Eu3+ and Gd2O3:6%Eu3+ NSPs as a function of (a) the content rates of urea and Ln3+ ions (e.g. Gd3+ and Eu3+), with reaction time of 70 min, (b). The aging reaction time, the ratio of urea and Ln3+ ions (e.g. Gd3+ and Eu3+) is 25/1.
(M2 curve); Gd(OH)CO3·H2O:6%Eu3+sample heated at 200 °C, coated with silica and annealed at 650 °C (M3 curve) and Gd(OH)CO3·H2O:6% Eu3+ heated at 650 °C, coated with silica and annealed at 200 °C (M4 curve). From the powder XRD analysis, all of samples have cubic phase structure of Gd2O3 crystal. However, there were extra peaks at 30.70 and 31.80° corresponding to unincoporated Eu2O3 phase (JCPDS file No. 817979) in the diagram of coated Gd(OH)CO3·H2O:6%Eu3+sample dried at 70 °C before silica shell coating (Fig. 5b, M2 curve). It can be explained that, with the selected coating procedure, the hydrated water in Gd(OH)CO3·H2O was not released but only the adsorbed water instead (Fig. 1). This might lead to the less incoporation of the doped Eu3+ ions into Gd(OH)CO3·H2O lattice than the other samples shown in M3 and M4 curves of Fig. 5b. In another context, for two latter samples (M3 and M4) that completely dehydrated, the freshly-produced anhydrous Gd(OH)CO3 is probably more active than Gd(OH)CO3·H2O for the total amount of the dopant to be incoporated into. We will return to discuss in more details this case in the next paragraph mentioning to luminescent characteristics of Gd2O3: Eu3+@silica NSPs fabricated by this sol-gel process.
Fig. 4. Size distributions of Gd2O3:6%Eu3+ (a), Gd2O3:6%Eu3+@silica (b) and Gd(OH)CO3·H2O:6%Eu3+ (c) NSPs. Table 1 The typical average size and standard deviation of Gd(OH)CO3·H2O:6%Eu3+; Gd2O3:6%Eu3+; Gd2O3:6%Eu3+@silica NSPs obtained from this work and Gd (OH)CO3·H2O:5%Eu3+ NSPs obtained from Ref. [16]. Item 1 2 3 4
Sample 3+
Gd(OH)CO3·H2O:6%Eu Gd(OH)CO3·H2O:5%Eu3+ Gd2O3:6%Eu3+ Gd2O3:6%Eu3+@silica
Average size (nm)
Standard deviation (%)
190 150 150 160
± 3.20 (this work) ± 10.60 [16] ± 4.85 (this work) ± 3.50 (this work)
3.2.2. EDS analysis The EDS results of the pure Gd2O3:6%Eu3+ and Gd2O3:6%Eu3+ @ silica samples are shown in Fig. 6. It confirms the presence of the basics elements, including Gd, Eu and O in both samples while the presence of Si is only observed in silica coated nanoparticles as expected. No other impurities were detected in the uncoated and silica coated samples from EDS analysis. Table 2 presents the weight and atomic percentage values of the uncoated Gd2O3:6%Eu3+ and Gd2O3:Eu3+@silica NSPs. The weight and atomic percentage value of each element are good in corresponding to the composition of nanospheres. It exhibits that the element composition of the uncoated and silica coated nanospheres was homogeneous distribution.
3.2. Structures, infrared spectra and luminescent properties 3.2.1. XRD analysis XRD patterns of Gd(OH)CO3·H2O:6%Eu3+ NSPs annealed at 200 and 650 °C are shown in Fig. 5a. We found that the sample annealed at 200 °C was almost amorphous (black curve). After being annealed at 650 °C, the diffraction peaks occurred (red curve). All these observable peaks at 2θ: 28.70; 33.10; 47.60; 56.40; and 59.10° are indexed according to the standard pattern (JCPDS file No. 110604), indicating that the Gd2O3:6%Eu3+ NSPs were pure cubic phase and no impurities were found. The phase identification of samples synthesized with the different heating treatment regimes for coating process was also investigated. Fig. 5b presentes the XRD patterns of the uncoated Gd2O3:6% Eu3+sample annealed at 650 °C (M1 curve); Gd(OH)CO3·H2O:6% Eu3+sample dried at 70 °C, coated with silica and annealed at 650 °C
3.2.3. FTIR spectra To investigate the chemical group, chemical bonding and composition of the fabricated sample, the FTIR spectra of the uncoated and coated Gd2O3:6%Eu3+ samples have been measured (see Fig. 7). For FTIR of the uncoated Gd2O3:6%Eu3+ sample (curve a), the presence of carbonate anion (CO3) in the structure of the final product gadolinium oxide annealed at 650 °C was almost defused, which is confirmed by an appearance of the absorption doublets in the region of 1350–1600 cm−1 and the multiabsorptions ranging from 500 to 1000 cm−1 [21]. The characteristic bands of Gd2O3 were found at 415.49, 449.66 and 542.9 cm−1 [33]. The band at 543 cm−1 can be assigned to the Gd-O stretching vibration, which also confirms the formation of Gd2O3 via the urea-based precipitation and the subsequent 4
Materials Science & Engineering B 241 (2019) 1–8
T.K. Anh et al.
Fig. 5. XRD pattern of (a) Gd(OH)CO3·H2O:6%Eu samples annealed at 200 °C and 650 °C, (b) M1, M2, M3 and M4 samples.
3.3. Silica coated Gd2O3: X%Eu3+
annealing process at high temperatures [15]. The strong absorption band at 3446 and 1634 cm−1 are the characteristic absorption of surface adsorbed water, OH groups and water of hydration [16]. The spectra of silica coated Gd2O3:6%Eu3+ are shown in curve b. The characteristic bands of OH and Si-O-Si group are observed in the ranges of 1050–1250 cm−1 and 650–800 cm−1, respectively. Together with EDS analysis, the FTIR spectra of the pure Gd2O3:6%Eu3+and silica coated Gd2O3:6%Eu3+ confirms the forming silica shell on surface of the NSPs.
In our case, the Gd2O3:Eu3+nanophosphores have central symmetric shape with high monodispersity with the size from 100 to 200 nm, showing the bright red emission corresponding to the sharp peak at 611 nm. These spherical nanophosphores enable a very promising platform to develop photonic system, bioimaging probe, and targeting drug carrier. To meet the requirement for these application, the surface modification of these NSPs is effective strategy not only to protect the surface but also to provide new functional group for subsequent bioconjugation with various biomolecules. Indeed, there are many investigations to coat the nanophosphors by sol-gel method, especially, to form core/shell nanostructures or nanocomposite. In our study, the typical nanophosphor of Gd2O3:6%Eu3+ was selected due to its physicochemistry and emission feature. Fig. 8b shows the photoluminescence spectra of Gd2O3:6%Eu3+coated with silica shell in different thickness. First, the photoluminescence intensity of Gd2O3:6% Eu3+@silica NSPs dried at 70 °C before being coated and annealed at 650 °C was stronger than that of pure core phosphor. After silica coating, the photoluminescence spectra of NSPs exhibit a new emission peak located at 621 nm. The emission intensity of 621 nm peak increases with the thickness of silica layer. It leads to the reduction of the ratio of the 611 and 621 nm emissions, which generates brighter red emission observed from the Gd2O3:Eu3+@silica NSPs. Next, we investigate the effect of silica coating conditions on the emission spectra of NSPs. Two samples, namely, Gd2O3:6%Eu3+sample annealed at 200 °C, being coated with silica shell before annealed at 650 °C and Gd2O3:6%Eu3+samples annealed at 650 °C, coated silica shell than heated up to 200 °C were subjected to this study. Fig. 8c demonstrated the luminescence spectra of these two experiments. It show that the peak at 621 nm in the both cases was almost disappeared and only as a shoulder. The emission spectra of Gd2O3:6%Eu3+@silica
3.2.4. Luminescent properties of Gd2O3:X%Eu and Gd2O3:X%Eu@Silica The photoluminescence spectra of Gd2O3:X%Eu NSPs (X = 3.50; 5.00; 6.00; 7.00 and 8.00 at.%) under 270 nm excitation are shown in Fig. 8a. The emission spectra exhibit the strongest emission peak at 611 nm corresponding to an electron dipole transition of Eu3+ ions (5D0 → 7F2). It is well-known that this transition is allowed when europium is occupied in low symmetries. The emission intensity of other peaks at 590, 625, 650 and 705 nm assigning to the 5D0 → 7F1; 5 D0 → 7F3; 5D0 → 7F4; 5D0 → 7F5 transitions, respectively are also observed. The emission peak at 590 nm is mainly a magnetic dipole transition and not dependent on the site symmetry of europium ions in host lattice. The ratio between the electron dipole transition (611 nm) and the magnetic dipole transition (590 nm) is the grade of the site symmetry at which europium ions is positioned. For Gd2O3:X%Eu (X = 3.50; 5.00; 6.00; 7.00; 7.50; and 8.00 at.%) NSPs, the electron dipole transition was much higher than magnetic dipole transition. The intensity ratio of the 611 and 590 nm emissions of Gd2O3:X%Eu samples (X = 3.50; 5.00; 6.00; 7.00 and 8.00 at.%) is over 15/1. In the concentration interval from 3.50% to 8.00% of Eu3+ the luminescence intensity was increased until 5.00 and 6.00%, then reduced near 8.00%, in which the concentration quenching is appeared and become stronger in the higher concentration.
Fig. 6. EDS spectra of (a) Gd2O3:6%Eu3+ and (b) Gd2O3:6%Eu3+@silica powder samples annealed at 650 °C. 5
Materials Science & Engineering B 241 (2019) 1–8
T.K. Anh et al.
Table 2 Element compositions by EDS data of the uncoated Gd2O3:6%Eu3+ andGd2O3:6%Eu3+@silica samples. Element
Gd (L)
3+
1
Gd2O3:6%Eu
2
Gd2O3:6%Eu3+@silica
Eu (L)
O (K)
Si (K)
Weight %
Atomic %
Weight %
Atomic %
Weight %
Atomic %
79.72 80.62 74.82
37.26 34.52 27.07
7.40 4.28 5.22
3.58 1.90 1.95
12.88 15.11 19.96
59.17 63.58 70.98
55.64 61.20 50.20
13.44 16.41 10.54
5.63 5.20 3.75
1.41 1.44 0.81
32.10 27.97 38.81
76.20 73.70 80.13
Weight %
Atomic %
6.63 5.63 7.24
8.96 8.45 8.52
spectrum of the uncoated Gd2O3:6%Eu3+ was 465 nm (see inset of Fig. 8d). The alternative of the strongest peaks in the photoluminescence excitation spectra of the uncoated and silica coated Gd2O3:Eu3+ was indicated also the occurrence of crystal reforming in Gd2O3 nanophosphor. Moreover, the doped Eu3+ions occupies the Gd3+ octahedral site based on their close ionic radii and the same valency state (+3). As consequence, the crystal reforming was caused to open a new transition channel in the energy levels 5D0 → 7F2. In other word, Eu3+ ions can be used as an optical probe for detection of structure change in solid state. In short, the silica shell coating strategy of nanomaterials by sol-gel technique with the aim to modify and enhance their quality, for instance, better dispersion, stronger emission intensity and good compatibility to environment is very effective. The silica coating result is depending on the design and fabrication technique. Thus the silica shell wrapping core was able the cause to decrease the luminescence intensity of Gd2O3:Eu3+@SiO2 in [23], as well as to enhance the luminescence intensity in this work. However, the sol-gel coating can be a key technique that possibly manipulates a nanomaterial for application in photonic, biology, medicine and also pharmacy.
Fig. 7. FTIR spectra of the pure Gd2O3:6%Eu3+ (curve a) and Gd2O3:6%Eu3+@ silica (curve b) samples.
were similar to those of the uncoated Gd2O3:6%Eu3+ samples. It indicated there is no crystal reforming. It is noted that if the core materials Gd(OH)CO3·H2O:6%Eu3+ dried only at low temperature 70 °C (see Fig. 8b) the occurrence of peak at 621 nm was strong. It can be proposed the silica shell coating affected on the conversion process from gadolinium hydroxyl carbonate to gadolinium oxide. As shown above in Fig. 7 the X-ray diffraction pattern of Gd(OH)CO3·H2O:6%Eu3+ dried at 70 °C, coated with silica and annealed at 650 °C (curve M2) and Gd (OH)CO3·H2O:6%Eu3+ heated at 200 °C, coated with silica and annealed at 650 °C (curve M3), appeared the extra peaks, which are not to belong to those of cubic crystal phase of Gd2O3:Eu3+but rather of Eu2O3 phase. It was able the reform in crystal structure which causes lower site symmetry of Gd2O3host matrix in which Eu3+ ions is situated [34]. Thus the new crystal form appeared maybe a reason for occurrence of luminescence peak at 621 nm. To this suppose is also confirmed the increased intensity of peak at 621 nm while the silica shell thicker (Fig. 8b). The doped Eu3+ occupies the Gd3+ site due to their close ionic radii. Maybe, under the silica coating conditions there was a part penetration of SiO2–X(OH)X molecules into the big Gd(OH)CO3·H2O NSPs, which was in soft amorphous state after being treated at very low temperature (70 °C). Under this coating conditions, SiO2-X(OH)X NSPs can easy move insight to lattice and reacted with host molecule to initiate the crystal reform with the annealing at 650 °C. To look more insight the impact of silica coating on the optical and structural properties of core-Gd2O3: Eu3+ it was measured the photoluminescence excitation (PLE) spectra. The PLE spectra of Gd2O3:6% Eu3+@silica samples are shown in Fig. 8d. The PLE spectra obtained was observed the emission peak at 621 nm. For comparison, the PLE emission peak at 611 nm of the uncoatedGd2O3: Eu3+was also presented in the inset of Fig. 8d. In the PLE spectra of Gd2O3:6%Eu3+@ silica, there are 4 peaks near 362, 380, 394 and 465 nm. The intensity of the peaks was increased if the silica thickness increased. In this case the strongest peak was at 394 nm. In contrast, the strongest peak in the PLE
4. Conclusion We demonstrate the optimal conditions to fabricate NSPs Gd2O3:Eu3+ and Gd2O3:Eu3+@silica with high monodispersity, low standard deviation ≤ ± 5% and in size range from 100 to 200 nm. The most important parameters are the selected ratio urea/(gadolinium + europium) ions, short aging reaction time at 85 °C and subsequent stepwise annealing regime from 70 to 650 °C. The spherical Gd2O3:Eu3+ and Gd2O3:Eu3+@silica NSPs show strong luminescence in the red color and very easy disperse in solvent and acquire media. The emission spectra of the nanophosphores are the energy transitions from 5 D0 to 7Fj of Eu3+ions. The ratio of the emitting intensity of 5D0 to 7F2 and 5D0 to 7F1 is over 15/1. The silica shell was almost wrapped the nanophosphore core, which is enabling effective protection, compatibility and enhancement of luminescence intensity. Moreover, the effect of silica coating on optical properties was enhanced the luminescence intensity of the main and strongest peak at 611 nm and also 621 nm. The impacts of silica coating on the structural properties of the core Gd2O3:X%Eu3+ nanophosphores (X = 3.50; 5.00; 6.00; 7.00; 7.55 and 8.00 at.%) depended on the fabrication condition. In the 5D0 to 7F2 energy transitions, the occurrence of a new peak at 621 nm in fluorescence spectra is caused by the crystal reform in part of cubic phase of Gd2O3:Eu3+@silica. The high monodispersity in size and shape of pure and silica coated Gd2O3: Eu3+ nanophosphores are very promising platform for application in photonic and also in biology-medicinepharmacy. Acknowledgments This paper is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED), Vietnam, under grant No. 103.03-2015.85. The authors are closely collaborated with research 6
Materials Science & Engineering B 241 (2019) 1–8
T.K. Anh et al.
a.
Photoluminescence
spectra
b. Photoluminescence emission and
of
Gd2O3:X%Eu samples (X=3.50; 5.00; 6.00;
excitation spectra of Gd2O3:6%Eu3+@silica
7.00 and 8.00 at.%), excitation at 270 nm.
samples as a function of silica shell thickness.
c.
d. Photoluminescence excitation (PLE)
Photoluminescence
spectra
of
Gd2O3:6%Eu3+@silica samples as a function
spectra
of the fabricated conditions.
Gd2O3:6%Eu3+@silica samples with different
of
Gd2O3:6%Eu3+
and
silica thickness. Fig. 8. Photoluminescence emission and excitation spectra of Gd2O3:X%Eu (X = 3.50; 5.00; 6.00; 7.00 and 8.00 at.%) and Gd2O3:6%Eu3+@silica samples. Fig. 8a. Photoluminescence spectra of Gd2O3:X%Eu samples (X = 3.50; 5.00; 6.00; 7.00 and 8.00 at.%), excitation at 270 nm. Fig. 8b. Photoluminescence emission and excitation spectra of Gd2O3:6%Eu3+@silica samples as a function of silica shell thickness. Fig. 8c. Photoluminescence spectra of Gd2O3:6%Eu3+@silica samples as a function of the fabricated conditions. Fig. 8d. Photoluminescence excitation (PLE) spectra of Gd2O3:6%Eu3+ and Gd2O3:6%Eu3+@silica samples with different silica thickness.
institutions and universities in country and abroad. The authors thank the Institute of Fundamental Science and Application, Duy Tan University, Institute of Materials Science, Vietnamese Academy of Sciences and Technology, Institute of Low Temperature and Structure Research, Poland, National Chung Cheng University, Taiwan for support. The authors are indebted Dr. Le Nguyen Bao, Prof. Le Van Vu, for discussion and support.
[2]
[3]
[4]
References [5] [1] J. Adam, W. Metzger, M. Koch, P. Rogin, T. Coenen, J.S. Atchison, P. Koenig, Light
7
emission intensities of luminescent Y2O3: Eu and Gd2O3: Eu particles of various sizes, Nanomaterials 7 (2017) 26. D. Warzynczyk, M. Nyk, A. Bernarkiewiecz, W. Strek, M. Sarnoc, Morphology- and size-dependent spectroscopic properties of Eu3+ doped Gd2O3 colloidal nanocrystals, J. Nanopart. Res. 16 (2014) 2690. L.D. Tuyen, J.H. Lin, C.Y. Wu, P.T. Tai, J. Tang, L.Q. Minh, H. ChihKan, C.C. Hsu, Pumping-power-dependent photoluminescence angular distribution from an opal photonic crystal composed of monodisperse Eu3+/SiO2 core/shell nanospheres, Opt. Express 20 (2012) 15418–15426. S. Zinatloo-Ajabshir, M. Salavati-Niasari, M. Hamadanian, Praseodymium oxide nanostructures: novel solvent-less preparation, characterization and investigation of their optical and photocatalytic properties, RSC Adv. 5 (2015) 33792. S. Zinatloo-Ajabshir, M. Salavati-Niasari, A. Sobhani, Z. Zinatloo-Ajabshir, Rare earth zirconate nanostructures: recent development on preparation and
Materials Science & Engineering B 241 (2019) 1–8
T.K. Anh et al.
the micronsize range, J. Colloid Interface Sci. 26 (1968) 62–69. [21] W.P. Hsu, L. Ronnquist, E. Matijevie, Preparation and properties of monodispersed colloidal particles of lanthanide compounds. 2. Cerium(IV), Langmuir 4 (1988) 31–37. [22] J. Wang, J. Zhu, Recent advances in spherical photonic crystals: generation and applications in optics, Eur. Polym. J. 49 (2013) 3420–3433. [23] K.-M. Lin, C.-C. Lin, Y.-Y. Li, Luminescent properties and characterization of Gd2O3:Eu3+@SiO2 and Gd2Ti2O7:Eu3+@SiO2 core–shell phosphors prepared by a sol–gel process, Nanotechnology 17 (2006) 1745–1751. [24] B. Sun, G. Zhou, H. Zhang, Synthesis, functionalization, and applications of morphology-controllable silica-based nanostructures: a review article, Progr. Solid State Chem. 44 (2016) 1–19. [25] Y. Cheng, K. Sun, Upconversion photoluminescence of core-shell structured SiO2@ YVO4:Yb3+, Er3+, Eu3+ nanospheres, Appl. Opt. 56 (2017) 4906–4910. [26] N.T. Huong, P.T. Lien, N.M. Hung, N.D. Van, T.T. Thuy, N.T. Binh, L.Q. Minh, Conjugation of TbPO4.H2O-based nanowires with immunoglobulin G for bioimaging, J. Electron. Mater. 45 (2016) 2463–2467. [27] A. Jain, G.A. Hirata, M.H. Farías, F.F. Castillón, Synthesis and characterization of (3-Aminopropyl)trimethoxy-silane(APTMS) functionalized Gd2O3:Eu3+ red phosphor with enhanced quantum yield, Nanotechnology 27 (2016) 12 065601. [28] H.A.I.Y. Tok, L.T. Su, F.Y.C. Boey, S.H. Ng, Homogeneous precipitation of Dy2O3 nanoparticles – effects of synthesis parameters, J. Nanosci. Nanotechnol. 7 (2007) 1–9. [29] D. Yang, P.Z. Ma, Z. Cheng, C. Li, J. Lin, Current advances in lanthanide ion (Ln3+)based upconversion nanomaterials for drug delivery, Chem. Soc. Rev. 44 (2015) 1416–1448. [30] Y.R. Lu, M.-Y. Gou, L.-Y. Zhang, L. Li, T.-T. Wang, C.-G. Wang, Facile one-pot synthesis of hollow mesoporous fluorescent Gd2O3:Eu/calcium phosphate nanospheres for simultaneous dual-modal imaging and pH-responsive drug delivery, Dyes Pigments 147 (2017) 514–522. [31] S. Lechevallier, P. Lecante, R. Mauricet, H. Dexpert, J. Dexpert-Ghys, H.K. Kong, G.I. Law, K.L. Wong, Gadolinium–europium carbonate particles: controlled precipitation for luminescent biolabeling, Chem. Mater. 22 (2010) 6153–6161. [32] Z. Xu, Y. Gao, S. Huang, P. Ma, J. Lin, J. Fang, A luminescent and mesoporous coreshell structured Gd2O3:Eu3+@nSiO2@mSiO2 nanocomposite as a drug carrier, Dalton Trans. 40 (2011) 4846–4854. [33] S.K. Ranian, A.K. Soni, V.K. Rai, Frequency upconversion and fluorescence intensity ration method in Yb3+-ion sensitized Gd2O3:Er3+-Eu3+ phosphors for display and temperature sensing, Methods Appl. Fluorsc. 5 035004 (2017) (9 pages). [34] T.K. Anh, P.T.M. Chau, N.T.Q. Hai, V.T.T. Ha, H.V. Tuyen, S. Bounyavong, N.T. Thanh, L.Q. Minh, Facile fabrication and properties of Gd2O3:Eu3+, Y2O3:Eu3+ nanophosphors and Gd2O3:Eu3+/silica, Y2O3:Eu3+/silica nanocomposites, J. Electron. Mater. 47 (2018) 585–593.
photocatalytic applications, J. Alloys Compounds 767 (2018) 1164e1185. [6] S. Zinatloo-Ajabshir, M.S. Morassaei, M. Salavati-Niasari, Nd2Sn2O7 nanostructures as highly efficient visible light photocatalyst: green synthesis using pomegranate juice and characterization, J. Cleaner Prod. 198 (2018) 11–18. [7] D. Peer, J.M. Karp, S. Hong, O.C. Farokhzad, R. Margalit, R. Langer, Nanocarriers as an emerging platform for cancer therapy, Nat. Nanotech. 2 (2007) 751–760. [8] P. Couvreur, Nanoparticles in drug delivery: past, present and future, Adv. Drug Deliv. Rev. 65 (2013) 21–23. [9] A. Escudero, A.I. Becerro, C. Carrillo-Carrión, N.O. Núñez, M.V. Zyuzin, M. Laguna, D. González-Mancebo, M. Ocana, W.J. Parak, Rare earth based nanostructured materials: synthesis, functionalization, properties and bioimaging and biosensing applications, Nanophotonics 6 (5) (2017) 881–921. [10] K.N. Orekhova, D.A. Eurov, D.A. Kurdyukov, V.G. Goolubev, D.A. Kirilenko, V.A. Kravets, M.V. Zamoryanskay, Structural and luminescent properties of Gd oxide doped with Eu3+ embedded in mesopores of SiO2 particles, J. Alloys Compd. 678 (2016) 434–438. [11] V. Stengl, J. Subrt, P. Bezdicka, M. Mankova, S. Bakardjiena, Homogeneous precipitation with urea-an easy process for making spherical hydrous metal oxides, Solid State Phen. 90–91 (2003) 121–126. [12] T.K. Anh, D.X. Loc, T.T. Huong, N. Vu, L.Q. Minh, Luminescent nanomaterials containing rare earth ions for security printing, Int. J. Nanotechnol. 8 (2011) 335–346. [13] L.D. Tuyen, A.C. Liu, C.C. Huang, P.C. Tsai, J.Hu. Lin, C.W. Wu, L.K. Chau, T.S. Yang, L.Q. Minh, H. ChihKan, C.C. Hsu, Doubly resonant surface-enhanced Raman scattering on gold nanorod decorated inverse opal photonic crystals, Opt. Express 20 (2012) 29266–29275. [14] J. Liu, X. Tian, N. Luo, C. Yang, J. Xiao, Y. Shao, X. Chen, G. Yang, D. Chen, L. Li, Sub-10 nm monoclinic Gd2O3:Eu3+ nanoparticles as dual-modal nanoprobes for magnetic resonance and fluorescence imaging, Langmuir 30 (2014) 13005–13013. [15] H. Liu, J. Liu, Hollow mesoporous Gd2O3:Eu3+ spheres with enhanced luminescence and their drug releasing behavior, RSC Adv. 6 (2016) 99158–99164. [16] W. Song, W. Di, W. Qin, Synthesis of mesoporous-silica-coated Gd2O3:Eu@silica particles as cell imaging and drug delivery agents, Dalton Trans. 45 (2016) 7443–7449. [17] D.A. Eurov, D.A. Kurdyukov, D.A. Kirilenko, J.A. Kukushkina, A.V. Nashchekin, A.N. Smirnov, V.G. Golubev, Core–shell monodisperse spherical mSiO2/ Gd2O3:Eu3+@mSiO2 particles as potential multifunctional theranostic agents, J. Nanopart. Res. 17 (82) (2015) 10. [18] X. Rena, P. Zhang, Y. Han, X. Yang, H. Yang, The studies of Gd2O3:Eu3+ hollow nanospheres with magnetic and luminescent properties, Mater. Res. Bull. 72 (2015) 280–285. [19] Z. Fan, P.P. Fu, H. Yu, P.C. Ray, Theranostic nanomedicine for cancer detection and treatment, J. Food Drug Anal. 22 (2014) 3–17. [20] W. Stoeber, A. Fink, E. Bohn, Controlled growth of monodisperse silica spheres in
8