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Template-free hydrothermal synthesis of Gd2O2SO4:Eu3+ hollow spheres based on urea-ammonium sulfate (UAS) system
Accepted Manuscript Title: Template-free hydrothermal synthesis of Gd2 O2 SO4 :Eu3+ hollow spheres based on urea-ammonium sulfate (UAS) system Author: Jingbao Lian Fan Liu Jing Zhang Yanyu Yang Xuri Wang Zhaoren Zhang Feng Liu PII: DOI: Reference:
S0030-4026(16)30691-X http://dx.doi.org/doi:10.1016/j.ijleo.2016.06.069 IJLEO 57855
To appear in: Received date: Accepted date:
24-4-2016 17-6-2016
Please cite this article as: Jingbao Lian, Fan Liu, Jing Zhang, Yanyu Yang, Xuri Wang, Zhaoren Zhang, Feng Liu, Template-free hydrothermal synthesis of Gd2O2SO4:Eu3+ hollow spheres based on urea-ammonium sulfate (UAS) system, Optik - International Journal for Light and Electron Optics http://dx.doi.org/10.1016/j.ijleo.2016.06.069 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Template-free hydrothermal synthesis of Gd2O2SO4:Eu3+ hollow spheres based on urea-ammonium sulfate (UAS) system Jingbao Lian*, Fan Liu, Jing Zhang, Yanyu Yang, Xuri Wang, Zhaoren Zhang, Feng Liu School of Mechanical Engineering, Liaoning Shihua University, Fushun, 113001, PR China *Corresponding author E-mail address: [email protected]
Abstract Gd2O2SO4:Eu3+ hollow spheres were successfully synthesized through a template-free hydrothermal synthesis routine from commercially available Gd2O3, Eu2O3, HNO3, (NH4)2SO4 and CO(NH2)2 (urea) as the starting materials. The as-synthesized products were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric and differential scanning calorimetry (TG-DSC), scanning electron microscopy (SEM), field emission scanning electron microscopy (FE-SEM) and photoluminescence (PL) spectra. The XRD, FT-IR and TG-DSC results demonstrate that the precursor is composed of Gd2(OH)2CO3SO4·xH2O and could be converted into pure Gd2O2SO4 phase at 800 oC for 2 h in air. SEM and FESEM observations show that the obtained Gd2O2SO4 particles possess hollow sphere structure, which has a typical size of about 10 μm in diameter and about 1.5 μm in shell thickness. The formation mechanism of the Gd2O2SO4 hollow spheres is related to the Ostwald ripening process. PL spectroscopy reveals that the strongest red emission peak is located at 619 nm under 275 nm UV light excitation for the Gd2O2SO4:Eu3+ hollow spheres, which corresponds to the 5
D0→7F2 transition of Eu3+ ions. Decay study demonstrates that the 5D0→7F2
transition of Eu3+ ions has a single exponential decay behavior and its corresponding fluorescence lifetime is 1.50 ms according to the linear fitting result. Keywords: Gadolinium oxysulfate; Template-free hydrothermal synthesis; hollow sphere; Urea-ammonium sulfate (UAS) system; Photoluminescence
1. Introduction Currently, the synthesis of inorganic hollow spheres with remarkable interior space has become a hot topic of scientific research due to their unique chemical and physical properties, including high specific surface area, low density, high loading capacity, enhanced catalytic activity, good permeability, and controlled-release viability, giving them a wide range of potential applications in various fields, such as in drug delivery carriers, catalysis, absorbents, fillers and biotechnology, etc [1-6]. So a considerable effort has been devoted to the development of different methods for the design and fabrication of hollow structures. In general, synthetic methods toward hollow spheres can be sorted into two types: template strategies [7-9] and template-free strategies [10]. At present, most of the systems reported for the formation of hollow spheres were based on template-assisted processes, which often employ sacrificial template (hard or soft templates) to obtain hollow spheres. It should be mentioned that the removal of the sacrificial template may cause environmental problems as a result of the product of calcinations [11] or the etching agents [12]. Template-free strategies mainly rely on various well-known phenomena such as Ostwald
ripening
[13],
Kirkendall
effect
[14] and
chemically
induced
self-transformation [15], which all have been widely employed for fabricating hollow spheres due to its simple and economical characteristics. Recently, numerous studies have focused on synthesizing hollow spherical rare-earth (RE) compounds. For instance, as was reported [16], Gd2O3:Eu3+ hollow
nanospheres with excellent magnetic and luminescence properties were prepared through a self-template method using Gd(OH)CO3:Eu as a template to form hollow precursors. Zhigao Yi and co-workers reported a self-sacrificing route for fabrication of the Ce/Tb co-doped GdPO4 hollow spheres under hydrothermal conditions using the Gd(OH)CO3:Ce/Tb precursor as a template and NH4H2PO4 as a phosphorus source [17]. In particular, gadolinium oxysulfate (Gd2O2SO4) has attracted increasing attention in recent years due to its unique luminescent as well as unusual magnetic properties, which made it potential application in various fields such as optical/display devices, nanoscale reactors, magnetic devices, and oxygen storage materials [10]. In previous reports, a lot of effort has been devoted to the synthesis of Gd2O2SO4 nano/microstructures,
including thermal
decomposition
method
[18-19], co
precipitation method [20], homogeneous precipitation method [21-22], hydrothermal method [23], electrospinning method [24], complexation-thermal decomposition (CTD) method [25] and so on. Nevertheless, in order to extend the application area for Gd2O2SO4 materials, it is desirable to develop a facile, economical and green route for the synthesis of Gd2O2SO4 hollow sphere. Up to date, few reports touch upon the fabrication of Gd2O2SO4 hollow spheres. Xiaohe Liu et al [8] reported a hydrothermal synthesis procedure for preparation of Gd2O2SO4 hollow spheres via the calcination of corresponding gadolinium coordination compounds obtained by using L-cysteine as a biomolecular templates. Besides, to the best of our knowledge, there have been no reports on the preparation of the Gd2O2SO4 hollow spheres via the template-free strategy.
In this regard, we thus put forward a simple but effective hydrothermal route followed by an annealing process to synthesize Gd2O2SO4 hollow sphere without using any sacrificial templates based on a water-based urea-ammonium sulfate (UAS) system. Compared with the template synthesis, this method provides a number of merits such as environmental-friendly, simplicity, and convenience. Furthermore, the structure, morphology, formation process and photoluminescence (PL) properties of the Gd2O2SO4:(Eu3+) hollow spheres have been also investigated. 2. Experimental procedure 2.1 Materials and synthesis Gd2O3 (99.9% purity), Eu2O3(99.99% purity), HNO3(AR), (NH4)2SO4 (AR) and CO(NH2)2 (AR) were used as the starting materials without further purification. Gd2O3 and Eu2O3 powders were purchased from Shanghai New Materials Yuelong Co. Ltd and other reagents were purchased from Sinopharm Chemical Reagent Co. Ltd, China. Firstly, a Gd(NO3)3 solution with a Gd3+ ion concentration of 0.1 M, was prepared by dissolving Gd2O3 powder in diluted nitric acid, followed by adding (NH4)2SO4 and CO(NH2)2 to form mixed solutions with molar ratio of 2:1:1 for Gd3+:SO42-:CO(NH2)2. The mixed solution was then transferred into a 100 ml Teflon-lined stainless steel autoclave and some deionized water was added up to 75% of the total volume. The autoclave was sealed and kept in an electric blast drying oven for 24 h under 120 oC for hydrothermal synthesis. When the autoclave was cooled down to room temperature, the resultant white precipitation was collected after filtration, washed with distilled water and ethanol several times and dried at 80 °C for
8 h to obtain the precursor. Finally, the precursor was calcined at a designed temperature (800 °C and 1200 °C) for 2 h in air to form the target product. Furthermore, trivalent europium ion doped Gd2O2SO4 (the doping concentration of Eu3+ is 5 mol% of Gd3+ in Gd2O2SO4 host) was also obtained by the same procedure as Gd2O2SO4. 2.2 Characterization Phase analysis of the synthesized products was identified on a X-ray diffractometer (XRD, SHIMADZU-7000) operating at 40 kV and 30 mA, using Cu Kα (1.5406 Å) radiation. The particle morphology was observed by a VEGA3 TESCAN scanning electron microscope (SEM) and a HITACHI UHR SU8010 FESEM. Thermogravimetric and differential scanning calorimetry (TG-DSC) were carried out using a NETZSCH STA449F3C integrated thermal analyzer in a temperature range of 25-1400 °C at a heating rate of 10 °C min-1 under an air flow. Fourier transform infrared spectra (FT-IR) were recorded in the region of 4000-400 cm-1 using a Bruker Vertex 70 FT-IR spectrophotometer by KBr method. Photoluminescence (PL) spectra and decay time were obtained on a Hitachi F-4600 fluorescence spectrophotometer equipped with a 150 W xenon lamp as the excitation source. 3. Results and discussion 3.1 Formation mechanism of the precursor HSAB (hard-soft acid-base) principle is an extremely useful qualitative theory that enables predictions of what adducts will form in a complex mixture of potential
Lewis acids and bases. According to hard-soft acid-base principle (HSAB), hard acids prefer to bind to hard bases and soft acids prefer to bind to soft bases [26]. In the present hydrothermal system, urea is a weak brönsted base and releases CO32- and OH− groups at temperatures up to 120 oC. Moreover, CO32-, OH− and SO42- groups are hard bases and Gd3+ ions are hard acids. Then it is easy to bond each other to form gadolinium hydroxyl, carbonate and sulfate. The possible chemical reactions in the formation of the precursor can be formulated as follows [Eqs.(1)–(4)]: CO(NH2)2+H2OCO2↑+NH3↑
(1)
2NH3+2H2O2NH4++2OH-
(2)
CO2+H2O2H++CO32-
(3)
2Gd3++2OH-+CO32-+SO42-+xH2OGd2(OH)2CO3SO4·xH2O
(4)
3.2 Structural analysis of the precursor The crystallinity and phase structure of the as-synthesized precursor sample was examined by XRD. As shown in Fig.1a, the obvious diffraction peaks appear in the XRD pattern, indicating that the precursor has highly crystalline in nature. Unfortunately, there is no relevant crystal structure information in standard JCPD card database and the crystal structure of the precursor is yet to be unidentified. To further gain the composition and structure information of the precursor, the FT-IR investigation was performed and the result is presented in Fig.1b. The broad characteristic band appearing around 3430 cm-1 and the weak peak at 1645 cm-1 in the precursor are assigned to the characteristic –OH stretching vibration and bending vibration of water molecules, respectively. In addition, one peak at ~3530 cm−1 can be
attributed to structural hydroxyl group (OH−). The characteristic peaks appeared in the range of 1540-1440 cm-1 and 1000-740 cm-1 belong to the vibrations of the carbonate groups [27]. Especially, evidence of the presence of sulfate (SO42-) groups (near 1175 cm-1, 1095 cm−1 and 610 cm−1) in the precursor could be also obtained from FT-IR spectroscopy (Fig.1b). The FT-IR spectrum indicates that the precursor is mostly composed of gadolinium hydroxyl, carbonate and sulfate groups with some crystal water. 3.3 Thermal decomposition behavior of the precursor To understand the decomposition behavior and determine the appropriate calcination temperature for the precursor, DSC-TG of the precursor was conducted from room temperature to 1400 oC and the result is shown in Fig.2. The TG curve shows a continuous weight loss between room temperature and 1400 oC with an overall weight loss of approximately 31.90wt%. The total weight loss mainly consists of the following four steps in the whole temperature range. The weight loss in the temperature range from room temperature to ~300 oC seems to be related mostly to removal of water molecules from the precursor, including physically adsorbed water and crystal water. This weight loss corresponds to a weak endothermic peak at ~200 o
C in the DSC curve. The weight loss between ~300 oC and ~500 oC is associated
with the complete dehydroxylation of the precursor with a weak endothermic peak at ~460 oC in the DSC curve. This weight loss corresponds to the following reaction： Gd2(OH)2CO3SO4= Gd2OCO3SO4+H2O↑
(5)
The weight loss in the temperature range from ~500 oC to ~900 oC is associated with
the complete decomposition of the CO32- groups of the precursor according to the following reaction： Gd2OCO3SO4=Gd2O2SO4+CO2↑
(6)
The weak peak centered at ~620 oC on the DSC curve indicates that the above process is an exothermic reaction. The dramatic weight loss of about 15 wt % is from ~900 oC to ~1200 oC accompanied with a strong endothermic peak at 1160 oC in the DSC curve due to the decomposition of Gd2O2SO4 to form Gd2O3, corresponding to the following reaction： Gd2O2SO4=Gd2O3+SO3↑
(7)
Therefore, 800 oC was chosen as the final calcination temperature for the synthesis of pure Gd2O2SO4 phase in the present study. 3.4 Phase structure and morphology of the calcined products Fig.3 shows the XRD pattern and FTIR spectrum of the calcined products. Shown in Fig.3a, target Gd2O2SO4 phase (JCPD card No. 00-041-0683) was obtained as the main phase as labeled in the pattern. With continuously increasing the calcination temperature to 1200 oC, only the diffraction peaks of pure Gd2O3 phase (JCPD card No. 00-86-2477) could be observed from the XRD pattern (Fig.3b), further confirming the decomposition of Gd2O2SO4 to form Gd2O3 phase. Moreover, obvious change was observed in the FT-IR spectrum of the Gd2O2SO4 compared with its precursor. As shown in Fig.3c, the absorption peak of 3530 cm-1 disappears, suggesting the dehydroxylation of hydroxyl groups. The broad absorption peaks centered at 3430 cm-1 and 1645 cm-1 still exist because the Gd2O2SO4 is also easy to
adsorb water from the ambient atmosphere. The CO32- absorption bands (1460 cm-1 and 1400 cm-1) become negligible, indicating the decomposition of CO32- groups in the precursor. The broad SO42- absorption bands (1175 cm-1,1095 cm-1 and 610 cm-1) split into some sharper peaks, which were assigned to S-O asymmetric stretching vibration at 1200 cm-1, 1125 cm-1, and symmetric stretching vibration at 1060 cm-1, 1000 cm-1. In addition, the Gd-O bond peak at ~510 cm-1 becomes sharper. The above analysis shows that the precursor has been transformed into Gd2O2SO4 phase. When the precursor calcined at 1200 oC, it is obvious that the absorption peaks of adsorb water, CO32- groups and Gd-O bond still exist although small changes in peak position was observed in Fig.3d. However, the SO42- absorption bands become negligible, demonstrating the decomposition of SO42- anions in the precursor. Phase evolution upon calcination corresponding well with the thermal behavior analyzed before (Fig.2). Fig.4 shows SEM image and EDS of the Gd2O2SO4, which reveals that the Gd2O2SO4 particles are spherical in shape, well dispersed and have an average diameter of about 10 μm in size. Interestingly, the broken sphere as marked by an arrow in Fig. 4a clearly indicates that the as-synthesized Gd2O2SO4 particles are hollow sphere structure. Moreover, the elemental composition analyzed by EDS analysis are shown in Fig. 4b, which demonstrates that the hollow spheres consist of gadolinium (Gd), oxygen (O), and sulphur (S) elements. The elemental ratios of Gd to O and S contained in the Gd2O2SO4 hollow spheres are quantified as 23.7:65.5:10.76 and approximately equal to 2:6:1, which is the further evidence of the formation of
Gd2O2SO4 phase. To clarify the possible formation mechanism, the morphology of the Gd2O2SO4 hollow spheres was further characterized by FE-SEM. The low-magnification FE-SEM image shown in Fig.5a clearly reveals that a typical individual Gd2O2SO4 hollow sphere with a diameter of about 10 μm and a shell thickness of about 1.5 μm was achieved, which further confirms that the as-synthesized Gd2O2SO4 has a hollow structure. Fig. 5b is a higher magnification image of the corresponding hollow sphere in Fig. 5a and exhibits the detailed outer and interior structure information. Careful observation shows that the outer surface (right part) of the Gd2O2SO4 hollow sphere is composed of numerous nanosheets and these nanosheets assemble randomly to form cage-like morphology. The left part of Fig. 5b reveals the interior microstructure of the Gd2O2SO4 hollow sphere. It is clearly seen that the coarse interior is composed of a large quantity of tiny nanosheets. Based on the above experimental results, a possible mechanism for the formation of the Gd2O2SO4 hollow spheres was proposed as follows. First, numerous tiny nuclei was formed in the present hydrothermal reaction system and the growth of the nucleus lead to the formation of nanosheets due to its high-anisotropic crystal structure nature. Second, for the minimization of surface energy, a great deal of nanosheets tend to aggregate and assemble into a spherical structure during the reaction process. Finally, the hollow spheres were formed by Ostwald ripening process. It is well-known that Ostwald ripening involves “the growth of larger crystals from those of smaller size which have a higher solubility than the larger ones” [28]. Such a process involves dissolution–recrystallization and further growth processes. The smaller nanosheets in
the interior dissolved gradually and the nanosheets in the outer layers grew larger and faceted at the expense of the inner ones, which were accompanied by the formation of the interior space. Also, further work is underway to investigate the details of the self-assembly growth mechanism. 3.5 PL spectra of the Gd2O2SO4:Eu3+ hollow sphere Fig.6 gives the excitation and emission spectra of the Gd2O2SO4:Eu3+ hollow sphere. As can be seen from Fig.6a, a broad absorption band with a maximum at 275 nm exists, which is resulted from the charge-transfer (CT) transitions between O2− and Eu3+ ions. The weak narrow peaks in the near UV range of the excitation spectrum are assigned to intra-configurational 4f–4f transitions of Eu3+ ions in the oxysulfate host lattice, and the peaks at 395 nm and 465 nm are attributed to the 7F0→5L6 and 7
F0→5D2 transitions of Eu3+ ions. In addition, the intra-configurational 4f6 excitation
lines of the Eu3+ ions are very weak, indicating that the excitation of Eu3+ ions through the CT state for emission would be very efficient. The emission spectrum of the Gd2O2SO4:Eu3+ phosphor under 275 nm UV light excitation (Fig.6b) demonstrates the well-known 5D0→7Fj (j=0, 1, 2, 3, 4) transitions of Eu3+ ions. The strongest emission peak located at 619 nm corresponds to the forced electric dipole 5D0→7F2 transition of Eu3+ ions. These are typical emission peaks of Eu3+ ions in an oxysulfate host. Fig. 7 shows the decay curve of the 5D0→7F2 transition of Eu3+ ions under 275 nm UV light excitation for the Gd2O2SO4:Eu3+ hollow sphere. Shown in Fig.7a, the decay curve can be well fitted into single exponential function of I= I0+Aexp(-t/τ), in
which τ is lifetime of Eu3+ ions. In addition, logarithmic relationship between PL emission intensity (I) and decay time (t) is also given in Fig.7b. These data can be well fitted into linear function of Ln(I)=A-t/τ and which further confirms the 5D0→7F2 transition has a single exponential decay behavior. The fitting result shows the lifetime for the 5D0→7F2 transition of Eu3+ ions in the Gd2O2SO4 host is 1.50 ms. Conclusions Pure Gd2O2SO4:Eu3+ hollow spheres have been successfully achieved by a template-free hydrothermal synthesis routine. The present study shows that the Gd2(OH)2CO3SO4·xH2O precursor could be transformed into pure Gd2O2SO4 hollow spheres with a typical size of about 10 μm in diameter and about 1.5 μm in shell thickness at 800 oC for 2 h in air. The strongest red emission peak located at 619 nm under 275 nm UV light excitation was observed for the Gd2O2SO4:Eu3+ hollow spheres. The 5D0→7F2 transition of Eu3+ has a single exponential decay behavior and its corresponding lifetime is 1.50 ms. In summary, we have demonstrated a template-free hydrothermal route is feasible for synthesizing the Gd2O2SO4 hollow spheres. Although the mechanism of the formation of the hollow sphere structures is not very clear and is still under investigation, we expect that the Gd2O2SO4 hollow spheres could be employed in drug delivery carriers and biotechnology fields due to their large specific area, loading capacity and good stability. Acknowledgment This work was supported by Foundation of Liaoning Educational Committee (No. L2014149),
Liaoning
Provincial
Student's
Plaform
for
Innovation
and
Entrepreneurship Training Program (No. 201610148031) and the National Natural Science Foundation of China (No. 51175240).
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Figure Captions Fig.1 XRD pattern and FTIR spectrum of the precursor Fig.2 DSC-TG curves of the precursor with 10 oCmin−1 heating rate in air Fig.3 XRD patterns and FTIR spectra of the calcined products Fig.4 SEM image and EDS of the Gd2O2SO4 hollow spheres Fig.5 FE-SEM image of the Gd2O2SO4 hollow spheres Fig.6 Excitation (a) and emission (b) spectra of the Gd2O2SO4:Eu3+ hollow spheres Fig.7 The relationship between the Ln (I) and decay time for the Gd2O2SO4:Eu3+ hollow spheres
Fig.1
Fig.2
Fig.3
Fig.4
Fig.5
Fig.6
Fig.7
S0030-4026(16)30691-X http://dx.doi.org/doi:10.1016/j.ijleo.2016.06.069 IJLEO 57855
To appear in: Received date: Accepted date:
24-4-2016 17-6-2016
Please cite this article as: Jingbao Lian, Fan Liu, Jing Zhang, Yanyu Yang, Xuri Wang, Zhaoren Zhang, Feng Liu, Template-free hydrothermal synthesis of Gd2O2SO4:Eu3+ hollow spheres based on urea-ammonium sulfate (UAS) system, Optik - International Journal for Light and Electron Optics http://dx.doi.org/10.1016/j.ijleo.2016.06.069 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Template-free hydrothermal synthesis of Gd2O2SO4:Eu3+ hollow spheres based on urea-ammonium sulfate (UAS) system Jingbao Lian*, Fan Liu, Jing Zhang, Yanyu Yang, Xuri Wang, Zhaoren Zhang, Feng Liu School of Mechanical Engineering, Liaoning Shihua University, Fushun, 113001, PR China *Corresponding author E-mail address: [email protected]
Abstract Gd2O2SO4:Eu3+ hollow spheres were successfully synthesized through a template-free hydrothermal synthesis routine from commercially available Gd2O3, Eu2O3, HNO3, (NH4)2SO4 and CO(NH2)2 (urea) as the starting materials. The as-synthesized products were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric and differential scanning calorimetry (TG-DSC), scanning electron microscopy (SEM), field emission scanning electron microscopy (FE-SEM) and photoluminescence (PL) spectra. The XRD, FT-IR and TG-DSC results demonstrate that the precursor is composed of Gd2(OH)2CO3SO4·xH2O and could be converted into pure Gd2O2SO4 phase at 800 oC for 2 h in air. SEM and FESEM observations show that the obtained Gd2O2SO4 particles possess hollow sphere structure, which has a typical size of about 10 μm in diameter and about 1.5 μm in shell thickness. The formation mechanism of the Gd2O2SO4 hollow spheres is related to the Ostwald ripening process. PL spectroscopy reveals that the strongest red emission peak is located at 619 nm under 275 nm UV light excitation for the Gd2O2SO4:Eu3+ hollow spheres, which corresponds to the 5
D0→7F2 transition of Eu3+ ions. Decay study demonstrates that the 5D0→7F2
transition of Eu3+ ions has a single exponential decay behavior and its corresponding fluorescence lifetime is 1.50 ms according to the linear fitting result. Keywords: Gadolinium oxysulfate; Template-free hydrothermal synthesis; hollow sphere; Urea-ammonium sulfate (UAS) system; Photoluminescence
1. Introduction Currently, the synthesis of inorganic hollow spheres with remarkable interior space has become a hot topic of scientific research due to their unique chemical and physical properties, including high specific surface area, low density, high loading capacity, enhanced catalytic activity, good permeability, and controlled-release viability, giving them a wide range of potential applications in various fields, such as in drug delivery carriers, catalysis, absorbents, fillers and biotechnology, etc [1-6]. So a considerable effort has been devoted to the development of different methods for the design and fabrication of hollow structures. In general, synthetic methods toward hollow spheres can be sorted into two types: template strategies [7-9] and template-free strategies [10]. At present, most of the systems reported for the formation of hollow spheres were based on template-assisted processes, which often employ sacrificial template (hard or soft templates) to obtain hollow spheres. It should be mentioned that the removal of the sacrificial template may cause environmental problems as a result of the product of calcinations [11] or the etching agents [12]. Template-free strategies mainly rely on various well-known phenomena such as Ostwald
ripening
[13],
Kirkendall
effect
[14] and
chemically
induced
self-transformation [15], which all have been widely employed for fabricating hollow spheres due to its simple and economical characteristics. Recently, numerous studies have focused on synthesizing hollow spherical rare-earth (RE) compounds. For instance, as was reported [16], Gd2O3:Eu3+ hollow
nanospheres with excellent magnetic and luminescence properties were prepared through a self-template method using Gd(OH)CO3:Eu as a template to form hollow precursors. Zhigao Yi and co-workers reported a self-sacrificing route for fabrication of the Ce/Tb co-doped GdPO4 hollow spheres under hydrothermal conditions using the Gd(OH)CO3:Ce/Tb precursor as a template and NH4H2PO4 as a phosphorus source [17]. In particular, gadolinium oxysulfate (Gd2O2SO4) has attracted increasing attention in recent years due to its unique luminescent as well as unusual magnetic properties, which made it potential application in various fields such as optical/display devices, nanoscale reactors, magnetic devices, and oxygen storage materials [10]. In previous reports, a lot of effort has been devoted to the synthesis of Gd2O2SO4 nano/microstructures,
including thermal
decomposition
method
[18-19], co
precipitation method [20], homogeneous precipitation method [21-22], hydrothermal method [23], electrospinning method [24], complexation-thermal decomposition (CTD) method [25] and so on. Nevertheless, in order to extend the application area for Gd2O2SO4 materials, it is desirable to develop a facile, economical and green route for the synthesis of Gd2O2SO4 hollow sphere. Up to date, few reports touch upon the fabrication of Gd2O2SO4 hollow spheres. Xiaohe Liu et al [8] reported a hydrothermal synthesis procedure for preparation of Gd2O2SO4 hollow spheres via the calcination of corresponding gadolinium coordination compounds obtained by using L-cysteine as a biomolecular templates. Besides, to the best of our knowledge, there have been no reports on the preparation of the Gd2O2SO4 hollow spheres via the template-free strategy.
In this regard, we thus put forward a simple but effective hydrothermal route followed by an annealing process to synthesize Gd2O2SO4 hollow sphere without using any sacrificial templates based on a water-based urea-ammonium sulfate (UAS) system. Compared with the template synthesis, this method provides a number of merits such as environmental-friendly, simplicity, and convenience. Furthermore, the structure, morphology, formation process and photoluminescence (PL) properties of the Gd2O2SO4:(Eu3+) hollow spheres have been also investigated. 2. Experimental procedure 2.1 Materials and synthesis Gd2O3 (99.9% purity), Eu2O3(99.99% purity), HNO3(AR), (NH4)2SO4 (AR) and CO(NH2)2 (AR) were used as the starting materials without further purification. Gd2O3 and Eu2O3 powders were purchased from Shanghai New Materials Yuelong Co. Ltd and other reagents were purchased from Sinopharm Chemical Reagent Co. Ltd, China. Firstly, a Gd(NO3)3 solution with a Gd3+ ion concentration of 0.1 M, was prepared by dissolving Gd2O3 powder in diluted nitric acid, followed by adding (NH4)2SO4 and CO(NH2)2 to form mixed solutions with molar ratio of 2:1:1 for Gd3+:SO42-:CO(NH2)2. The mixed solution was then transferred into a 100 ml Teflon-lined stainless steel autoclave and some deionized water was added up to 75% of the total volume. The autoclave was sealed and kept in an electric blast drying oven for 24 h under 120 oC for hydrothermal synthesis. When the autoclave was cooled down to room temperature, the resultant white precipitation was collected after filtration, washed with distilled water and ethanol several times and dried at 80 °C for
8 h to obtain the precursor. Finally, the precursor was calcined at a designed temperature (800 °C and 1200 °C) for 2 h in air to form the target product. Furthermore, trivalent europium ion doped Gd2O2SO4 (the doping concentration of Eu3+ is 5 mol% of Gd3+ in Gd2O2SO4 host) was also obtained by the same procedure as Gd2O2SO4. 2.2 Characterization Phase analysis of the synthesized products was identified on a X-ray diffractometer (XRD, SHIMADZU-7000) operating at 40 kV and 30 mA, using Cu Kα (1.5406 Å) radiation. The particle morphology was observed by a VEGA3 TESCAN scanning electron microscope (SEM) and a HITACHI UHR SU8010 FESEM. Thermogravimetric and differential scanning calorimetry (TG-DSC) were carried out using a NETZSCH STA449F3C integrated thermal analyzer in a temperature range of 25-1400 °C at a heating rate of 10 °C min-1 under an air flow. Fourier transform infrared spectra (FT-IR) were recorded in the region of 4000-400 cm-1 using a Bruker Vertex 70 FT-IR spectrophotometer by KBr method. Photoluminescence (PL) spectra and decay time were obtained on a Hitachi F-4600 fluorescence spectrophotometer equipped with a 150 W xenon lamp as the excitation source. 3. Results and discussion 3.1 Formation mechanism of the precursor HSAB (hard-soft acid-base) principle is an extremely useful qualitative theory that enables predictions of what adducts will form in a complex mixture of potential
Lewis acids and bases. According to hard-soft acid-base principle (HSAB), hard acids prefer to bind to hard bases and soft acids prefer to bind to soft bases [26]. In the present hydrothermal system, urea is a weak brönsted base and releases CO32- and OH− groups at temperatures up to 120 oC. Moreover, CO32-, OH− and SO42- groups are hard bases and Gd3+ ions are hard acids. Then it is easy to bond each other to form gadolinium hydroxyl, carbonate and sulfate. The possible chemical reactions in the formation of the precursor can be formulated as follows [Eqs.(1)–(4)]: CO(NH2)2+H2OCO2↑+NH3↑
(1)
2NH3+2H2O2NH4++2OH-
(2)
CO2+H2O2H++CO32-
(3)
2Gd3++2OH-+CO32-+SO42-+xH2OGd2(OH)2CO3SO4·xH2O
(4)
3.2 Structural analysis of the precursor The crystallinity and phase structure of the as-synthesized precursor sample was examined by XRD. As shown in Fig.1a, the obvious diffraction peaks appear in the XRD pattern, indicating that the precursor has highly crystalline in nature. Unfortunately, there is no relevant crystal structure information in standard JCPD card database and the crystal structure of the precursor is yet to be unidentified. To further gain the composition and structure information of the precursor, the FT-IR investigation was performed and the result is presented in Fig.1b. The broad characteristic band appearing around 3430 cm-1 and the weak peak at 1645 cm-1 in the precursor are assigned to the characteristic –OH stretching vibration and bending vibration of water molecules, respectively. In addition, one peak at ~3530 cm−1 can be
attributed to structural hydroxyl group (OH−). The characteristic peaks appeared in the range of 1540-1440 cm-1 and 1000-740 cm-1 belong to the vibrations of the carbonate groups [27]. Especially, evidence of the presence of sulfate (SO42-) groups (near 1175 cm-1, 1095 cm−1 and 610 cm−1) in the precursor could be also obtained from FT-IR spectroscopy (Fig.1b). The FT-IR spectrum indicates that the precursor is mostly composed of gadolinium hydroxyl, carbonate and sulfate groups with some crystal water. 3.3 Thermal decomposition behavior of the precursor To understand the decomposition behavior and determine the appropriate calcination temperature for the precursor, DSC-TG of the precursor was conducted from room temperature to 1400 oC and the result is shown in Fig.2. The TG curve shows a continuous weight loss between room temperature and 1400 oC with an overall weight loss of approximately 31.90wt%. The total weight loss mainly consists of the following four steps in the whole temperature range. The weight loss in the temperature range from room temperature to ~300 oC seems to be related mostly to removal of water molecules from the precursor, including physically adsorbed water and crystal water. This weight loss corresponds to a weak endothermic peak at ~200 o
C in the DSC curve. The weight loss between ~300 oC and ~500 oC is associated
with the complete dehydroxylation of the precursor with a weak endothermic peak at ~460 oC in the DSC curve. This weight loss corresponds to the following reaction： Gd2(OH)2CO3SO4= Gd2OCO3SO4+H2O↑
(5)
The weight loss in the temperature range from ~500 oC to ~900 oC is associated with
the complete decomposition of the CO32- groups of the precursor according to the following reaction： Gd2OCO3SO4=Gd2O2SO4+CO2↑
(6)
The weak peak centered at ~620 oC on the DSC curve indicates that the above process is an exothermic reaction. The dramatic weight loss of about 15 wt % is from ~900 oC to ~1200 oC accompanied with a strong endothermic peak at 1160 oC in the DSC curve due to the decomposition of Gd2O2SO4 to form Gd2O3, corresponding to the following reaction： Gd2O2SO4=Gd2O3+SO3↑
(7)
Therefore, 800 oC was chosen as the final calcination temperature for the synthesis of pure Gd2O2SO4 phase in the present study. 3.4 Phase structure and morphology of the calcined products Fig.3 shows the XRD pattern and FTIR spectrum of the calcined products. Shown in Fig.3a, target Gd2O2SO4 phase (JCPD card No. 00-041-0683) was obtained as the main phase as labeled in the pattern. With continuously increasing the calcination temperature to 1200 oC, only the diffraction peaks of pure Gd2O3 phase (JCPD card No. 00-86-2477) could be observed from the XRD pattern (Fig.3b), further confirming the decomposition of Gd2O2SO4 to form Gd2O3 phase. Moreover, obvious change was observed in the FT-IR spectrum of the Gd2O2SO4 compared with its precursor. As shown in Fig.3c, the absorption peak of 3530 cm-1 disappears, suggesting the dehydroxylation of hydroxyl groups. The broad absorption peaks centered at 3430 cm-1 and 1645 cm-1 still exist because the Gd2O2SO4 is also easy to
adsorb water from the ambient atmosphere. The CO32- absorption bands (1460 cm-1 and 1400 cm-1) become negligible, indicating the decomposition of CO32- groups in the precursor. The broad SO42- absorption bands (1175 cm-1,1095 cm-1 and 610 cm-1) split into some sharper peaks, which were assigned to S-O asymmetric stretching vibration at 1200 cm-1, 1125 cm-1, and symmetric stretching vibration at 1060 cm-1, 1000 cm-1. In addition, the Gd-O bond peak at ~510 cm-1 becomes sharper. The above analysis shows that the precursor has been transformed into Gd2O2SO4 phase. When the precursor calcined at 1200 oC, it is obvious that the absorption peaks of adsorb water, CO32- groups and Gd-O bond still exist although small changes in peak position was observed in Fig.3d. However, the SO42- absorption bands become negligible, demonstrating the decomposition of SO42- anions in the precursor. Phase evolution upon calcination corresponding well with the thermal behavior analyzed before (Fig.2). Fig.4 shows SEM image and EDS of the Gd2O2SO4, which reveals that the Gd2O2SO4 particles are spherical in shape, well dispersed and have an average diameter of about 10 μm in size. Interestingly, the broken sphere as marked by an arrow in Fig. 4a clearly indicates that the as-synthesized Gd2O2SO4 particles are hollow sphere structure. Moreover, the elemental composition analyzed by EDS analysis are shown in Fig. 4b, which demonstrates that the hollow spheres consist of gadolinium (Gd), oxygen (O), and sulphur (S) elements. The elemental ratios of Gd to O and S contained in the Gd2O2SO4 hollow spheres are quantified as 23.7:65.5:10.76 and approximately equal to 2:6:1, which is the further evidence of the formation of
Gd2O2SO4 phase. To clarify the possible formation mechanism, the morphology of the Gd2O2SO4 hollow spheres was further characterized by FE-SEM. The low-magnification FE-SEM image shown in Fig.5a clearly reveals that a typical individual Gd2O2SO4 hollow sphere with a diameter of about 10 μm and a shell thickness of about 1.5 μm was achieved, which further confirms that the as-synthesized Gd2O2SO4 has a hollow structure. Fig. 5b is a higher magnification image of the corresponding hollow sphere in Fig. 5a and exhibits the detailed outer and interior structure information. Careful observation shows that the outer surface (right part) of the Gd2O2SO4 hollow sphere is composed of numerous nanosheets and these nanosheets assemble randomly to form cage-like morphology. The left part of Fig. 5b reveals the interior microstructure of the Gd2O2SO4 hollow sphere. It is clearly seen that the coarse interior is composed of a large quantity of tiny nanosheets. Based on the above experimental results, a possible mechanism for the formation of the Gd2O2SO4 hollow spheres was proposed as follows. First, numerous tiny nuclei was formed in the present hydrothermal reaction system and the growth of the nucleus lead to the formation of nanosheets due to its high-anisotropic crystal structure nature. Second, for the minimization of surface energy, a great deal of nanosheets tend to aggregate and assemble into a spherical structure during the reaction process. Finally, the hollow spheres were formed by Ostwald ripening process. It is well-known that Ostwald ripening involves “the growth of larger crystals from those of smaller size which have a higher solubility than the larger ones” [28]. Such a process involves dissolution–recrystallization and further growth processes. The smaller nanosheets in
the interior dissolved gradually and the nanosheets in the outer layers grew larger and faceted at the expense of the inner ones, which were accompanied by the formation of the interior space. Also, further work is underway to investigate the details of the self-assembly growth mechanism. 3.5 PL spectra of the Gd2O2SO4:Eu3+ hollow sphere Fig.6 gives the excitation and emission spectra of the Gd2O2SO4:Eu3+ hollow sphere. As can be seen from Fig.6a, a broad absorption band with a maximum at 275 nm exists, which is resulted from the charge-transfer (CT) transitions between O2− and Eu3+ ions. The weak narrow peaks in the near UV range of the excitation spectrum are assigned to intra-configurational 4f–4f transitions of Eu3+ ions in the oxysulfate host lattice, and the peaks at 395 nm and 465 nm are attributed to the 7F0→5L6 and 7
F0→5D2 transitions of Eu3+ ions. In addition, the intra-configurational 4f6 excitation
lines of the Eu3+ ions are very weak, indicating that the excitation of Eu3+ ions through the CT state for emission would be very efficient. The emission spectrum of the Gd2O2SO4:Eu3+ phosphor under 275 nm UV light excitation (Fig.6b) demonstrates the well-known 5D0→7Fj (j=0, 1, 2, 3, 4) transitions of Eu3+ ions. The strongest emission peak located at 619 nm corresponds to the forced electric dipole 5D0→7F2 transition of Eu3+ ions. These are typical emission peaks of Eu3+ ions in an oxysulfate host. Fig. 7 shows the decay curve of the 5D0→7F2 transition of Eu3+ ions under 275 nm UV light excitation for the Gd2O2SO4:Eu3+ hollow sphere. Shown in Fig.7a, the decay curve can be well fitted into single exponential function of I= I0+Aexp(-t/τ), in
which τ is lifetime of Eu3+ ions. In addition, logarithmic relationship between PL emission intensity (I) and decay time (t) is also given in Fig.7b. These data can be well fitted into linear function of Ln(I)=A-t/τ and which further confirms the 5D0→7F2 transition has a single exponential decay behavior. The fitting result shows the lifetime for the 5D0→7F2 transition of Eu3+ ions in the Gd2O2SO4 host is 1.50 ms. Conclusions Pure Gd2O2SO4:Eu3+ hollow spheres have been successfully achieved by a template-free hydrothermal synthesis routine. The present study shows that the Gd2(OH)2CO3SO4·xH2O precursor could be transformed into pure Gd2O2SO4 hollow spheres with a typical size of about 10 μm in diameter and about 1.5 μm in shell thickness at 800 oC for 2 h in air. The strongest red emission peak located at 619 nm under 275 nm UV light excitation was observed for the Gd2O2SO4:Eu3+ hollow spheres. The 5D0→7F2 transition of Eu3+ has a single exponential decay behavior and its corresponding lifetime is 1.50 ms. In summary, we have demonstrated a template-free hydrothermal route is feasible for synthesizing the Gd2O2SO4 hollow spheres. Although the mechanism of the formation of the hollow sphere structures is not very clear and is still under investigation, we expect that the Gd2O2SO4 hollow spheres could be employed in drug delivery carriers and biotechnology fields due to their large specific area, loading capacity and good stability. Acknowledgment This work was supported by Foundation of Liaoning Educational Committee (No. L2014149),
Liaoning
Provincial
Student's
Plaform
for
Innovation
and
Entrepreneurship Training Program (No. 201610148031) and the National Natural Science Foundation of China (No. 51175240).
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Figure Captions Fig.1 XRD pattern and FTIR spectrum of the precursor Fig.2 DSC-TG curves of the precursor with 10 oCmin−1 heating rate in air Fig.3 XRD patterns and FTIR spectra of the calcined products Fig.4 SEM image and EDS of the Gd2O2SO4 hollow spheres Fig.5 FE-SEM image of the Gd2O2SO4 hollow spheres Fig.6 Excitation (a) and emission (b) spectra of the Gd2O2SO4:Eu3+ hollow spheres Fig.7 The relationship between the Ln (I) and decay time for the Gd2O2SO4:Eu3+ hollow spheres
Fig.1
Fig.2
Fig.3
Fig.4
Fig.5
Fig.6
Fig.7