Ions in Î²-Ca2SiO - ACS Publications - American

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Role of Synthesis Method on Luminescence Properties of Europium(II, III) Ions in β‑Ca2SiO4: Probing Local Site and Structure Rajaboopathi Mani,† Huaidong Jiang,*,†,‡ Santosh Kumar Gupta,§ Ziqing Li,† and Xiulan Duan† †

State Key Laboratory of Crystal Materials, Shandong University, Jinan, Shandong 250100, China School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China § Radiochemistry Division, Bhabha Atomic Research Centre Trombay, Mumbai 400085, India ‡

S Supporting Information *

ABSTRACT: The europium ion probes the symmetry disorder in the crystal structure, although the distortion due to charge compensation in the case of aliovalent dopant remains interesting, especially preparation involves low and high temperatures. This work studies the preparation of the βCa2SiO4 (from here on C2S) particle from Pechini (C2SP) and hydrothermal (C2SH) methods, and its luminescence variance upon doping with Eu2+ and Eu3+ ions. The blue shift of the charge-transfer band (CTB) in the excitation spectra indicates a larger Eu3+−O2− distance in Eu3+ doped C2SH. The changes in vibrational frequencies due to stretching and bending vibrations in the FTIR and the Raman spectra and binding energy shift in the XPS analysis confirmed the distorted SiO44− tetrahedra in C2SH. The high hydrothermal temperature and pressure produce distortion, which leads to symmetry lowering although doping of aliovalent ion may slightly change the position of the Ca atoms. The increasing asymmetry ratio value from C2SP to C2SH clearly indicates that the europium ion stabilized in a more distorted geometry. It is also supported by Judd−Ofelt analysis. The concentration quenching and site-occupancy of Eu3+ ions in two nonequivalent sites of C2S were discussed. The charge state and concentration of europium ions in C2SP and C2SH were determined using X-ray photoelectron spectroscopy measurements. The C2S particles were studied by X-ray powder diffraction, FTIR, Raman, BET surface area, TGA/DTA, electron microscopy, XPS, and luminescence spectroscopy. The impact of citrate ion on the morphology and particle size of C2SH has been hypothesized on the basis of the microscopy images. This study provides insights that are needed for further understanding the structure of C2S and thereby improves the applications in optical and biomedical areas and cement hydration.

1. INTRODUCTION The β-dicalcium silicate or β-Ca2SiO4 (from here on C2S) is an important material that has been focused on the applications of ordinary Portland cement,1,2 synthetic bone and dental tissue,3 and phosphors.4,5 Different polymorphs of C2S such as α′H and α′L are derived from the α form by lowering of the symmetry. Disorder of SiO44− tetrahedra in the C2S can decrease the symmetry which changes slightly the position of Ca atoms.6,7 The disorder can also occur when a guest ion is introduced into the C2S, for example, being aliovalent because of charge imbalance, misfit of the ion size, and influence of different growth methods.8−10 In the case of tricalcium silicate (C3S), experimentally an orientational disorder of SiO44− and its effect on the Ca ion coordinations11,12 have been observed whereas there is no experimental evidence for C2S. Upon consideration of the many uses of C2S, the different preparation methods have received significant attention to avoid undesirable impurities13,14 and unstable phases,15 for example, solid-state reaction,13,16 sol−gel,13,17 spray-drying,13 gel combustion,18 solution-PVA polymerization,19 Pechini,20,21 and hydrothermal2,22,23 methods. Of these, preparation of C2S by Pechini and © XXXX American Chemical Society

hydrothermal methods produces highly pure particles at low temperature21 and small particle size with high surface area (∼13 m2/g).2,13,19,23 The Pechini method is a polymeric precursor route, which involves the formation of the metal complex followed by polymerization. Certain α-hydroxycarboxyl acids such as citric acid (CA), tartaric acid, and ethylenediaminetetraacetic acid (EDTA) form a chelate with the metal precursor which undergoes polyesterification when the solution is heated with a polyhydroxy alcohol, such as ethylene glycol and polyethylene glycol.21,24 The use of nitrate precursor in pilot scale synthesis produces a large amount of NO2 gas, a serious gas pollutant.25 Despite the Pechini method which produces fine particles, the high surface area with different sized particles has been produced by a modified Pechini method.24 The influence of different chelating agents on the thermal and structural characteristics of nanoparticles has been reported in ref 26. To our knowledge, hitherto there is no organic additive or Received: July 25, 2017

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DOI: 10.1021/acs.inorgchem.7b01878 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry shape modifier, such as trisodium citrate,27 trisodium phosphate,28 or citric acid,29 used for controlling the morphology and microstructure of C2S particles. The citrate ion adsorbs strongly on the metal ions which significantly alters the surface properties of the particles.30−32 The use of different surfactants (citric acid, poly(acrylic acid), EDTA, and sodium citrate) on the emission properties of rare earth doped nanoparticles has been investigated.33 The binding affinity of surfactant at specific planes leads to preferential growth or control of some planes at the expense of others.34 Our focus in this report is to prepare the C2S particles in the Pechini and hydrothermal methods to determine the structural influences on the fluorescence properties using europium (Eu2+ and Eu3+) dopants. The citric acid is used to investigate the morphological variation at different reaction temperatures in the hydrothermal method. The Eu dopant is being used as a probe to identify the structural changes in the host materials.10,35−37 The sharp-line spectra of the Eu3+ ion and nondegenerate 5D0 emissive state wherein the spectrum displays a distinct electric (5D0−7F2) and magnetic (5D0−7F1) dipole moment give detailed information about the surroundings of Eu3+ ions in the host lattice.38 The unique 5D0−7F0 transition, which is magnetically as well as electrically forbidden gave an idea about the point group symmetry, and the splitting in it establishes information on the number of metal ion sites. Extensive work on the exploration of Eu luminescence for probing structural phase transition,39 local site and energy-transfer dynamics,40,41 and point group symmetry,42 etc., has been investigated. The signature of emission peaks also proves that there are different phases of phosphors.43,44 The Ca2+ in C2S has two different coordination sites, i.e., Ca(1) and Ca(2) atoms surrounded by eight and seven oxygen coordinates, respectively. Doping of Eu2+ or Eu3+ occupies both sites.45 The priority in the site selection of either Ca(1) or Ca(2) or both has already been discussed in refs 4 and 46. DFT study investigates the charge compensation when Eu3+ is doped at the Ca2+ site and vacancies are created.47 Recently, combined force field and DFT calculations investigate the impact of doping minor ions such as Mg2+, Al3+, and Fe3+ in the C2S and structural changes that take place to accommodate these ions. The oxygen atoms are responsible for the charge imbalance in the form of creating vacancies.8,48 The study concludes that there is a rearrangement in the silicate group orientation and calcium coordination polyhedra to accommodate the dopants. Furthermore, the systematic changes in the cell dimension of the C2S with Sr doping have been investigated in ref 49. Other theoretical studies also discuss the structural changes and reactivity with the water when the C2S accommodates foreign particles.50 The charge compensation process lowers the symmetry of the occupied site, which can be clearly identified by the emission spectrum of the Eu3+ doped host.43,51 Our investigation in this report is composed of two parts. First, we describe the preparation of β-dicalcium silicate (C2S) via Pechini and hydrothermal methods to study the impact of different preparation methods on the particle morphology and microstructural changes. Second, the europium ions (Eu2+ and Eu3+) are used as a probe to investigate the structural distortions in the C2S particles especially due to hydrothermal condition and an aliovalent dopant. The structure−property changes have been investigated using powder X-ray diffraction; Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy; Brunauer, Emmett, and Teller (BET) surface

area analysis; TGA/DTA; scanning electron microscopy (SEM); transmission electron microscopy (TEM); X-ray photoelectron spectroscopy (XPS); and fluorescence spectroscopy.

2. EXPERIMENTAL SECTION 2.1. Materials. In our experiments, all the reagents were of analytical grade and were used as purchased. The chemicals calcium nitrate tetrahydrate (CaN2O6·4H2O, Sigma-Aldrich, ≥ 99.0%), colloidal silica (40% suspension in H2O, Sigma-Aldrich, lot# MKBS7803V), ethylene glycol (C6H6O2, Sinopharm, 99%), citric acid monohydrate (C6H8O7·H2O, Sinopharm, 99.5%), and europium nitrate hexahydrate (Eu(NO3)3·6H2O, Aladdin, 99.9%) were used for the preparation of pure and doped C2S. 2.2. Preparation of C2S and Europium Doped C2S in Pechini and Hydrothermal Methods. 2.2.1. Preparation of C2S in Pechini Method. The C2S was prepared as described in the literature.25 The calcium nitrate (0.8 M) and colloidal silica (0.4 M) with a molar ratio of 2 were dissolved separately in 12 mL of water. The solutions were mixed together using a magnetic stirrer for 30 min followed by the addition of citric acid (CA), which was previously prepared in 3 mL of water with a molar ratio of CA to total cation equal to 1. The ethylene glycol (EG) was further added with a molar ratio of EG to CA equal to 2. The final solution was stirred for an hour and then heated at 70 °C under a fume hood until there is complete evaporation of excess liquid. The obtained resin-like product was dried at 140 °C for 12 h in an oven and then calcinated under air flow atmosphere in a box furnace at 800 °C for 3 h to obtain the C2S (from here on, C2SP where “P” denotes particles prepared from Pechini method). 2.2.2. Preparation of C2S in Hydrothermal Method. The solution for the hydrothermal treatment was prepared as described in Section 2.2.1. The final solution was moved into a 40 mL Teflon lined autoclave after it stirred for an hour. Three different solutions were prepared and kept at the temperatures 140, 180, and 200 °C for 20 h at an autogenous pressure in the vessel. The pH of the solution was 1 and increased to 3−4 pH in the hydrothermal treatment at 180 and 200 °C. The hydrothermally treated solution at 140 °C was clear as observed before the treatment. The solutions treated at 180 and 200 °C were colloidal gel-like (see Figure S1) so that they were then centrifuged and then washed with water followed by ethanol. Nonetheless, small sized single crystals were observed along with the washed products. The crystals were sorted out using a microscope, and single crystal X-ray diffraction measurements were performed. The result shows that the crystals were calcium citrate. The effect of calcium citrate on the morphology is discussed below (see Section 5). All the products were dried separately at 140 °C for 12 h in the oven and then calcinated under air flow atmosphere in a box furnace at 800 °C for 3 h to obtain the C2S (from here on, these are identified as C2S:140H, C2S:180H, and C2S:200H where “H” denotes particles prepared from the hydrothermal method). 2.2.3. Preparation of Europium(III, II) Ion Doped C2S from Pechini and Hydrothermal Methods. A 0.8 M solution of Eu(NO3)3·6H2O was dissolved separately in 0.45 and 0.6 mL of water to obtain 3% and 4% Eu3+ doped C2S. The solution was mixed with 0.8 M calcium nitrate solution, and then, the calculated amount of colloidal silica was added. The remaining procedure follows the same approach as described in Sections 2.2.1 and 2.2.2 for the preparation of 3% and 4% Eu3+ doped C2SP, C2S:140H, C2S:180H, and C2S:200H. For Eu2+ doped C2S, the final products were gently ground and recalcinated under 5% H2 and 95% N2 mixed gas flow in a closed furnace at 800 °C for 3 h. The flow diagram of all the particles (pure and doped) prepared from the Pechini and hydrothermal methods is presented in Figure S2. It is notable that the presence of nanometer size free lime in the C2S is very sensitive to its storage environment, which readily reacts with the moisture to form a calcium hydroxide and then form a calcite by subsequent reaction with CO2. Therefore, all the samples were hermetically sealed in a glass container separately and stored at 60 °C until the time of measurement. B


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Inorganic Chemistry

3. CHARACTERIZATION TECHNIQUES The X-ray powder diffraction patterns were recorded on a Bruker AXS-D8 Advance diffractometer using Cu Kα radiation (λ = 1.5406 Å) at 40 kV over the 2θ range 10−70° and a scan rate at 0.1 s/steps with 0.02 step size. The profile fitting was performed using Jade 6 software.52 A pseudo-Voigt peak profile function resolves the overlapping peaks of different phases.53 The presence of the impurity in C2S was quantified using a reference intensity ratio value (RIR) of 0.76 and 4.53 for C2S and free lime, respectively. The FTIR spectra were recorded on a Bruker ALPHA-T spectrometer with a resolution of 4 cm−1. About 1.5 mg of sample was ground with 250 mg of KBr using a mortar and pestle and pressed with a metal die to prepare the pellet. The Raman spectra were recorded with a Renishaw Raman Microprobe equipped with a Physics model 127 He−Ne laser source at a wavelength of 632.8 nm and a diode laser at a wavelength of 780 nm, and a cooled CCD detector from Renishaw. The Olympus BHZUHA microscope fitted with objective lenses (×10, ×20, ×50) was used to focus the samples. The spot size was about 5 μm. A small amount of solid sample was placed on a clean stainless steel plate for the measurement. The surface area and pore size distribution were measured using a Micromeritics ASAP 2020 adsorption and desorption analyzer. The samples were degassed at 300 °C for 5 h before the measurement. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were performed on a PerkinElmer thermal analyzer over the temperature range from 25 to 1000 °C at a heating rate of 10 K/min in a sealed aluminum pan under a flow of dry nitrogen gas at the rate of 70 mL/min. Air was used as a purge gas for precursor powders obtained in Pechini and hydrothermal methods. The morphologies of the samples were examined using a Hitachi-S4800 scanning electron microscope (SEM). The samples were fixed on a metal stud using a carbon tape and then dried under vacuum, covered by about 10 nm of gold coating. The particle size and SAED pattern were obtained using JEOL JEM-2100 TEM microscopy. The samples were prepared by drop-casting two to three drops of sample on to carbon-coated copper grid and allowed to dry under a mercury lamp. Acetone was used to disperse the samples. The X-ray photoelectron spectra were recorded on a Thermo Scientific ESCALAB 250 instrument equipped with Al Kα radiation at 1486.6 eV. The spectral peaks were fitted with CasaXPS software using Gaussian−Lorentzian (30% Lorentzian) line shapes. The background was subtracted using the Shirley method. The charging effect and corresponding binding energy shift for all the elements have been calibrated for the charging adventitious carbon 1s peak at 284.6 eV. The room temperature solid-state photoluminescence spectra and decay curves were measured with a high-resolution spectrofluorometer (UK, Edinburg instrument FLS920) equipped with a xenon lamp as an excitation source for the emission and excitation spectra and 375 nm pulsed laser source for decay curve measurement.

Figure 1. X-ray powder diffraction pattern of C2S prepared from Pechini (C2SP) and hydrothermal (C2SH) methods. The symbols β and * indicate characteristic peaks of C2S and CaO, respectively. Y-axis is in log scale to show all the characteristic peaks.

C2SP or C2S:140H as presented in Figure S3 (see Supporting Information). To quantify the free lime in C2S, profile fitting was performed using Jade 6 software.52 The amount is 2.2%, 9.1%, 10.4%, and 17.8%, respectively, for C2SP, C2S:140H, C2S:180H, and C2S:200H (see Figure S4). The residual error of fit is 3.42%, 4.40%, 4.25%, and 4.81% for C2SP, C2S:140H, C2S:180H, and C2S:200H, respectively. No peak is observed at 26.6° for quartz55 and at 22° for amorphous SiO256 in C2SH. 4.2. Vibrational Spectroscopy. The FTIR and Raman spectroscopy are sensitive to vibrational modes of the local atomic structure. The spectral analyses give information about the internal structure and interactions. Silicate vibrations in the FTIR spectra can be described as follows. The FTIR spectra of C2S nanoparticles are presented in Figure 2. The Si−O bond character of calcium silicate can be identified by vibrational spectra. The internal and external (lattice) vibrations of silicate are clear when it combines with alkaline earth metal like Ca.57 For instance, alkaline earth metal orthosilicate exhibits four normal modes of vibrations at ∼450 (ν2), 530 (ν4), 830 (ν1), and 1000 cm−1(ν3) in the region 400−1000 cm−1.54,57 The Si− O stretching (ν1, ν3) and O−Si−O bending modes (ν2, ν4) are, respectively, observed in the bands at 812−994 and 400−562 cm−1. The Si−O stretching vibrations are observed at 848, 923, and 999 cm−1 for C2SP. Similar vibrational peaks are observed for C2S:140H except for the absence of a sharp peak at 848 cm−1, but also intense peaks for the latter wavenumbers. The O−Si−O bending modes are observed around 520 cm−1 for C2SP and C2S:140H and 475 cm−1 for C2S:180H and C2S:200H. The typical silicate absorption peak at 472 cm−1 has been reported in ref 58. The Si−O stretching vibration is shifted to 1115 cm−1 for C2S:180H and C2S:200H. The deconvolution of this peak shows three vibrations at 1015, 1115, and 1224 cm−1 (see Figure S5), attributed to the Si−O vibration.59,60 The peak traced at 810 cm−1 is assumed to be the bending vibration of free SiO2.56 It is clear from Si−O

4. RESULTS AND DISCUSSION 4.1. X-ray Powder Diffraction. The X-ray powder diffraction patterns of C2SP, C2S:140H, C2S:180H, and C2S:200H are presented in Figure 1. The XRD pattern is in good agreement with the pattern reported in ref 21 and PDF 360-642 for C2SP. The principal diffraction peaks of C2S are identified at 2θ values of 31.77°, 32.05°, 32.14°, 32.59°, 32.93°, 34.33°, 41.21°, and 45.74°. The C2S can be quantitatively determined using a resolvable peak observed at 31.2°. The XRD patterns of C2SH are matched with the C2SP while some of the characteristic peaks are overlapped with free lime (free lime PDF 371497). Similar free lime has been observed in hydrothermally prepared C2S.54 The line shape and relative intensity of the (200) plane in the C2SH are only a little different from those of C2SP, which may be due to a preferential growth effect caused by the formation of calcium citrate (see Section 5). Furthermore, the intensities of C2S:180H and C2S:200H have diminished compared with C


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Inorganic Chemistry

Figure 2. FTIR spectra of C2SP and C2SH particles. The horizontal lines in C2SP indicate a range of Si−O−Si and Si−O vibrations.

Figure 3. TGA/DTA curve of C2S particles before and after calcination. Precursor powders obtained from Pechini method (a) and hydrothermal method at 180 °C (b). C2S particle obtained after calcination: C2SP (c), C2S:140H (d), C2S:180H (e), and comparison of TGA curves (f).

4.3. Thermal Analysis. 4.3.1. TGA/DTA Analysis of Precursor Powders Obtained from Pechini and Hydrothermal Methods. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) of precursor powder obtained before the calcination of C2SP and C2S:180H (precursor powder heat-treated after 140 °C, see Figure S2) are presented in Figure 3a,b. In Figure 3a, the mass loss about 3−5% around 100 °C is attributed to impurities or water molecules adsorbed from the atmosphere after the heat treatment, because the samples were not preheated before this measurement. The steep mass loss of about 40% with a small endothermic peak at 335 °C is due to the breakdown of the polymers which occurs by fragmentation and release of

stretching and O−Si−O bending vibrations that there is a change in the silicate structure of C2SH (hydrothermal temperature at 180 and 200 °C) by considering peak shift and intensity variations compared to those for C2SP. The silicate vibration of C2S in the Raman spectra (Figure S6) is discussed in Section S1. CaO vibrations in the FTIR spectra can be described as follows. The peaks observed around 1425 and 3650 cm−1 in C2SP are assigned to CaO. The former peak is shifted and intense around 1418 cm−1 which may be due to the presence of free CaO in C2SH. Additionally, a peak traced around 875 cm−1 is assigned to the presence of free CaO in C2S:180H and C2S:200H.56 D


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Inorganic Chemistry

Figure 4. SEM micrographs of C2SP (a, b) and C2S:140H (c, d). The aggregate of particles (a) is magnified in part b. The arrow in part c indicates a spherical plate-like particle presented among the rectangular plate-like particles.

Figure 5. SEM microscopy image of C2S:180H (a, b) and C2S:200H (c, d). The different sized particles in part c are magnified in part d. The insets in parts b and d represent CaO presented in C2S:180H and C2S:200H.

water and carbonic gas.26,61 An endothermic peak at 490 °C with mass loss around 23% is assigned to the elimination of organic materials (EG and CA), in other words, decomposition of carboxylic groups, carbonates, and nitrates,26 which originated from the polymer and nitrate salt. The mass loss remains constant after 715 °C indicating that the suitable temperature begins for the phase formation of C2SP. The residual mass is 20% at 1000 °C. As mentioned in Section 2.2.2, the effect of calcium citrate presented in the precursor powder

is observed in Figure 3b. The mass loss of C2S:180H is similar to that of C2SP except there is a steep mass loss instead of a gradual reduction, especially at 700 °C. The first mass loss around 100 °C is due to release of water molecules. It indicates some hydrates of C2S might have already formed during the hydrothermal treatment or from surface adsorbed water.62 The second mass loss in the range 395−495 °C with few stages of mass loss is associated with decomposition of citrate ion. The final stage of mass loss of about 22% can be assigned to calcium E


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Inorganic Chemistry

Figure 6. TEM microscopy image of C2SP (a, b), C2S:180H (c, d), and C2S:200H (e, f). The diffracting planes are indicated in the SAED pattern. The two insets in part e represent spherical particles, which are observed among the anchor-like morphology.

compared with C2SP and C2S:140H. The thermal behavior remains the same for all the samples after the decomposition of CaCO3 at 650 °C (see Figure 3f). 4.4. SEM Microscopy. The particle morphology and size are observed by SEM images of C2SP (Figure 4a,b), C2S:140H (Figure 4c,d), C2S:180H (Figure 5a,b), and C2S:200H (Figure 5c,d) as presented in Figures 4 and 5. From the BET surface area measurement results in Table S1 (see Supporting Information Section S2 and Table S1), it is worth noting that the surface area of C2S:180H and C2S:200H has increased in correspondence with reduced average particle size compared with those of C2SP and C2S:140H. This result is in accordance with the following micrograph results. The SEM image of C2SP shows large particles resulting from the aggregation of small particles (Figure 4a). The magnified view of C2SP in Figure 4a is presented in Figure 4b. The particles are of spherical platelike structure or are interlocking tile-like, having many pores. The boundaries of the plates are uneven, and an anchor-like structure may be feasible for interlocking with other particles. The pores created in the plates are due to the removal of organic matter during calcination. The typical morphology has been observed for C2S particles prepared from the polymeric route.19 Since it is a plate-like structure, the average size of particles is observed to be about ≥100 nm. For the hydrothermally prepared C2S, the morphology of C2S:140H is similar as is observed in C2SP. In the case of C2S:180H, along with a few spherical plate-like particles, there is a rectangular plate-like structure with the size of ≤100 nm observed. The width of the particles is ≥50 nm. This indicates that the average particle size has been reduced compared with those of C2SP and C2S:140H. Despite this, we realized the reduction of particle size in C2S:180H; the low- and high-magnification SEM micrographs of C2S:200H clearly show the different sized particles (Figure 5c,d). We could also see uniformly sized spherical particles (25 nm) among C2S:180H and C2S:200H indicating the presence of CaO impurity (see insets in Figure 5).

carbonate, which decomposes to CaO by emitting CO2 around 700 °C.63 Besides the hydrothermal method, the main source of the CaO phase observed in C2S:180H (also C2S:200H, not shown here) is due to the presence of calcium citrate according to the conversion of calcium citrate → CaCO3 → CaO + CO2. The residual mass is 30% at 1000 °C. Though the thermal process of both precursor powders looks similar because of the same organic materials involved in the preparation, the polymer breakdown which occurs around 335 °C in C2SP has not been observed in C2S:180H, indicating the absence of polyesterification during hydrothermal treatment. However, in the case of C2S:140H, since the treated solution has not been washed, the polyesterification may occur during the drying process at 140 °C, similar to polyesterification at 70 °C in the Pechini method (see Figure S2). Needless to say, it has been confirmed while grinding the hydrothermal product of C2S:140H that the ceramic-like product is hard, similar to C2SP. It can be further confirmed by a similar XRD pattern for C2SP and C2S:140H, especially the (200) plane in Figure 1. 4.3.2. TGA/DTA of C2SP and C2SH Particles. The TGA/ DTA plots of C2SP, C2S:140H, and C2S:180H are presented in Figure 3c−e. All the samples were preheated at 60 °C before the measurement. The mass loss is about 3−5% for C2SP and C2S:140H and 12% for C2S:180H in the range 500−600 °C. It is attributed to the decomposition of CaCO3 into free lime (CaO) by emitting CO2.62 The C2S:180H shows an additional mass loss of 8% around 370−400 °C associated with the dehydroxylation of portlandite, accordingly, Ca(OH)2 → CaO +H2O.64 The similar mass loss and exothermic peak for the decompositions of Ca(OH)2 and CaCO3 have been observed, respectively, in the temperature ranges 410−460 and 520−730 °C for the CaCO3 blended Portland cement reported in ref 62. The presence of CaO impurity in the C2S interacts with atmospheric moisture and CO2, forming Ca(OH)2 and CaCO3, accordingly, CaO + H2O → Ca(OH)2 + CO2 → CaCO3 + H2O. This CaCO3 may be the reason for the higher percentage of mass loss observed around 400−610 °C in C2S:180H F


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Inorganic Chemistry

Figure 7. XPS survey spectra of pure and Eu doped C2S (a). Valence band spectra of C2S (b). Eu 3d5/2 core level spectra of C2S (c−f). The insets show the as-recorded Eu 3d5/2 spectra.

Figure 8. Ca2p, O1s, and Si2p core level spectra of C2SP (a, c, e) and C2S:180H (b, d, f).

4.5. TEM Microscopy. The TEM images of the C2SP (left), C2S:180H (middle), and C2S:200H (right) and the corresponding selected area electron diffraction (SAED) patterns are presented in Figure 6. The micrographs confirm the crystallinity, particle size, and morphology. The outline of C2SP particles is not distinctive exactly, but we observe agglomerated particles, mostly spherical plate-like or anchorlike in shape as discussed in the SEM images (see Figure 4a,b). The size of the particles is not observed to be uniform in the range 60−90 nm. The distinctive bright spots in the SAED pattern clearly represent the crystalline nature of C2SP. It was very difficult to exactly distinguish the crystallinity and particle sizes of C2S:180H and C2S:200H from CaO because of the coexistence of C2S and CaO phases. Furthermore, the particle size is not uniform in both hydrothermally prepared particles as seen in the SEM images (see Figure 5c,d). However, we had

tried different regions in the particles and recorded the SAED pattern. The diffraction ring is made up of many dots and individual bright spots (Figure 6f), which are clearly less bright and smaller than the diffraction spots obtained in C2SP. It can be taken as evidence for the polycrystalline nature of C2S:180H and C2S:200H. Furthermore, the BET surface area (see Table S1) and the SEM results can be taken into account to confirm the reduced particle size. Needless to say, while grinding C2SP we observe more granules whereas C2S:180H and C2S:200H are very fine particles. The inset in Figure 6e is the magnified view to represent the anchor-like and spherical morphologies. The green boundary lines differentiate the particles. 4.6. XPS Analysis. The survey and Ca2p, O1s, and Si2p core level spectra of pure and Eu doped C2SP and C2S:180H are presented in Figures 7a and 8, and binding energy and fwhm are presented in Table 1. The Eu doped C2S results presented G


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Inorganic Chemistry Table 1. Peak Position and fwhm of Core Level Spectra of Ca2p, O1s, and Si2p Ca2p C2SP C2S:180H O1s

position (Ca 2p3/2)

fwhm

position (Ca 2p1/2)

346.7590 346.7585 position (Si−O−Si)

1.67515 350.2782 1.66971 350.2651 position (Si−O−Ca)

fwhm

C2SP C2S:180H Si2p

533.9166 532.3102 position (Si 2p1/2)

2.47324 2.81048 fwhm

532.1997 531.2765 position (Si 2p3/2)

C2SP C2S:180H

102.3652 105.3154

2.37209 2.45598

100.8184 103.3866

fwhm

fwhm

1.62718 1.67136 position (Ca−O−Ca)

fwhm

1.96744 1.56545 fwhm

530.6129 530.8370 position (Ca−O−Ca)

2.15863 1.85383 fwhm

100.8580

2.25281

1.90056 2.30819

energy is moved to a lower value than those for the above two.69 These peaks are presented in Figure 8c,d for C2SP and C2S:180H, and peak position and fwhm are presented in Table 1. The relative binding energy difference of BO and NBO is 1.5−2 eV70 which is close to ∼1.4 eV in our case. The Si−O bond length of BO is about 0.025 Å longer than the NBO.71 The atomic ratio of calcium to silicate and its influence on the bonding state of BO and NBO in calcium−silicate−hydrate have been thoroughly investigated.72 The BO, NBO, and MBO ratio is 3:2:∼273 so that, in our XPS fitting, the area of the NBO and MBO is constrained to be equal to 1. The binding energy of both NBO and BO of C2S:180H are shifted to a lower side compared with C2SP. However, there is no significant change in the position of MBO between C2S:180H and C2SP. It is clear from Figure 8d that the contribution or intensity of BO in the C2S:180H is higher than that of the C2SP. It is noteworthy that the narrowing peak in the C2S:180H indicates the presence of fewer environments in the O 1s surroundings compared with that of C2SP. This can be taken as further evidence to confirm the changes in silicate structure of C2S:180H or distorted silicate structure. 4.6.4. Si 2p Core Level Spectra. The Si 2p has 2p1/2 and 2p3/2 doublet peaks. These peaks are fitted on the basis of the intensity ratio of two peaks. The Si 2p1/2 (higher energy side) peak is assigned to precisely half of the Si 2p3/2 intensity.74 Generally, the energy separation between the two peaks is 0.6.66 The Si 2p peak in ZrSiO4 has been observed at 101.8 eV in ref 75. For the C2SP, two component peaks fit the Si 2p (Figure 8e,f). The peaks may be assigned to Si−O and Si−O− Si bonding. The peak position at lower binding energy (100.82 eV) is assigned to Si 2p3/2 while the higher binding energy peak (102.36 eV) is assigned to Si 2p1/2. We follow the same parameters to fit the Si 2p peak of C2S:180H. Instead of two component peaks, three component peaks fit the Si 2p in C2S:180H. Besides the shift to higher binding energy side and peak broadening, the additional peak or small tail on the lower energy side is observed at 100 eV for C2S:180H (see Table 1). This may be assigned to the presence of other phases of Si or free or amorphous SiO2 due to the hydrothermal synthesis. The higher binding energy shift is consistent with the increased BO contribution in the O 1s spectrum.69 4.7. Fluorescence and Decay Time of Europium Doped C2S. 4.7.1. Excitation, Emission, and Decay Time of Eu3+ Doped C2S Particles. The excitation and emission spectra of 3% and 4% of Eu3+ doped C2SP and C2S:180H are presented in Figure S7. The emission quenching occurs at 4%, similar to Eu2+ doped C2S discussed in Section S3 (see Supporting Information). The emission of Eu3+ corresponds to the transition from the excited 5D0 to the 7FJ (J = 0, 1, 2, 3, 4, 5, 6) levels of intra-4f−4f transition. The emission spectrum is obtained at the excitation wavelength of 393 nm, and the

in Figure 7c−f will be discussed in Section 4.7.3. The changes in chemical bonding configuration between C2SP and C2S:180H are investigated using XPS core level spectra. The XPS results are more important for the structural investigation when there is a firm basis to correlate the binding energy shift. Further, it is feasible to analyze the results when comparing the binding energy shift with the binding energy of the pure sample. By accounting for the crystal structure of C2S, the chemical bonding between the elements is assumed to be Si− O−Si (bridging, BO, where oxygen atom that bonds two Si atoms together), Si−O−Ca (nonbridging, NBO, where oxygen atom bonds Si atom to a metal cation), and Ca−O−Ca (metal bridging, MBO, where oxygen atom bonds two metal atoms together) associated with oxygen.4,65 4.6.1. XPS Survey and Valence Band Spectra of C2SP and C2S:180H. From Figure 7a, the stoichiometries of elements presented in the C2SP and C2S:180H are 2.01 Ca, 1.00 Si, and 4.10 O, and 2.03 Ca, 1.00 Si, and 5.58 O, respectively. The intensities of Si 2p and Ca 2p are diminished in the C2S:180H compared with the C2SP. Though it is not very clear regarding the changes in the tail of the O2s and O2p respectively in the valence band spectra near 23 and 5−10 eV (Figure 7b), it can be assumed that these are due to either the changes in particle size or the distorted SiO44− unit. 4.6.2. Ca2p Core Level Spectra. The Ca has two different coordination environments that are strong and weak interactions with oxygen. No significant changes in the binding energy of Ca 2p (Figure 8a,b) between the C2SP and C2S:180H are present. There are two peaks such as Ca 2p1/2 and Ca 2p3/2 that are due to orbital splitting. No constraints were used to fit the peaks. The energy separation between these two peaks is 3.55 eV66 which is consistent with our results (∼3.51 eV). 4.6.3. O1s Core Level Spectra. The binding energies of the core level O 1s electron are 530 and 531 eV, respectively, for C2SP and C2S:180H, located between the binding energies of pure CaO (529.4 eV) 67 and SiO 2 (533 eV). 68 The deconvoluted O 1s peak is presented in Figure 8c,d. The higher binding energy side O 1s peak represents the BO signal; the middle peak represents NBO, and the low binding energy peak is related to MBO. The changes in binding energy associated with these peaks can be described on the basis of the electron density and electronegativities of Ca, Si, and O atoms. In other words, high electron density screens the core electron, leading to low binding energy. The O (3.44) has the highest electronegativity; Si (1.90) has an intermediate value, and Ca (1) has the lowest value. In BO (Si−O−Si), the electron density should have diminished, and the greatest amount of energy consequently would be required to eject a photoelectron from its O 1s orbital. In NBO (Si−O−Ca), the oxygen is bonded with Si, and thus electron density has increased. This is even higher for MBO (Ca−O−Ca); eventually, the binding H


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Figure 9. (a) Excitation spectrum of Eu3+ doped C2SP and C2SH. CTB denotes the charge-transfer band which occurs due to O2−−Eu3+. The dotted lines presented to show the CTB of C2SH (∼295 nm) shifted to lower wavelength compared to that of C2SP (305 nm) whereas 7F0−5H6 remains the same. Excitation from 7F0 to different levels is denoted. The data are recorded for C2S doped with 3% of Eu3+. (b) The emission spectrum of Eu3+ doped C2SP and C2SH. Different transitions to nondegenerate levels are indicated near the emission peaks. The yellow shaded region indicates that crystal field splitting is blunt for C2SH. Further, the emission intensity of C2S:180H and C2S:200H is higher than that of C2S:140H.

Table 2. Photophysical Properties of Eu3+ Doped C2Sa

a

sample

ARAD (s−1)

ANRAD (s−1)

η (%)

Ω2 (10−20 cm2)

Ω4 (10−20 cm2)

Ω2/Ω4

C2SP C2S:140H C2S:180H C2S:200H

257 263 284 299

92 621 269 60

73.6 29.8 51.4 83.1

2.58 2.92 3.40 3.70

3.43 3.05 2.94 2.93

0.75 0.95 1.15 1.26

The following abbreviations apply: ARAD radiative, ANRAD nonradiative, η quantum efficiency, Ω2 short-range ordering, Ω4 long-range ordering.

CTB band is shifted to lower wavelength and got broader for C2SH compared to C2SP. In general, the higher energy of CTB indicates a larger O2−−Eu3+ distance corresponding to that for the Ca2+ ion.78 Therefore, the blue shift of CTB indicates that the atom surrounding Eu3+ ions is distant for C2SH compared to C2SP. This can be attributed to different methods adopted for the synthesis of the C2S:Eu3+ phosphor. The different methodology may lead to different size and shape for the C2S:Eu3+ phosphor which resulted in such a shift. The reduced particle size can change the crystal field strength.10 There is no change in the line shape and position of the 4f−4f lines, which are in good agreement with reports in refs 46 and 79. The CTB band has been observed at 250 nm for Eu3+ doped CaO.80 In our case, it was observed around 300 nm, which gives strong evidence that the influence of the CaO impurity in the Eu3+ doped C2S:180H and C2S:200H is absent. The emission spectra of Eu3+ doped C2S display the typical transitions 5D0−7FJ (J = 0−4) as shown in Figure 9b. Among these, 5D0−7F0 and 5D0−7F3 are allowed neither by magnetic dipole transition (MDT) nor by electrical dipole transition (EDT). However, their presence, particularly the 5D0−7F0 transition, establishes the information about the local symmetry around the Eu3+ ion. The 5D0−7F1 is allowed by MDT and is not sensitive to the local crystal field whereas 5D0−7F2 is allowed by EDT and is highly sensitive to the local environment. In fact, 5D0−7F2 (ΔJ = ±2) is hypersensitive to a change in the local surrounding of the europium ion. The symmetry of the Eu3+ site can be identified by the intensity radio of 5D0 → 7F1 and 5D0 → 7F2 transitions; i.e., R = I(5D0 → 7 F2)/I(5D0 → 7F1). This asymmetry ratio value (R) indicates how far the local environment of the Eu 3+ ions is centrosymmetric. The R-value depends on the 5D0 → 7F2 transition because of sensitivity to the crystal field effect; in other words, it strongly depends on the asymmetry of the Eu3+ site. The increase of the R-value indicates the strong covalent

excitation spectrum is obtained by monitoring the emission wavelength at 620 nm. To design a high-efficiency red phosphor, it is imperative to know the mechanism of concentration quenching. Considering the structural characteristics of the host, one can approximately determine the critical transfer distance (Rc) for nonradiative energy transfer between the dopant ions using the Blasse equation76 ⎛ 3V ⎞1/3 R c = 2⎜ ⎟ ⎝ 4πNXC ⎠

(1)

where V is the volume of the unit cell, XC is the critical concentration at which the quenching occurs, and N is the number of available crystallographic cationic sites to occupy the activator ions in the unit cell. In C2S, the unit cell volume is V = 1036.31 Å3 containing Z = 4 formula units. There are two calcium ions in one formula unit. This makes N = 8 calcium sites in a unit cell which can be occupied by a europium ion. Considering XC = 4.0%, the critical energy-transfer distance Rc in the Eu3+ doped C2S was calculated to be 18.35 Å. Therefore, the electric multipolar interaction is responsible for the nonradiative energy transfer among the Eu 3+ ions in C2S:Eu3+. A similar critical distance has been reported for XC = 3.5% in the Ca1.65Sr0.35SiO4 host.77 In our case, the concentration quenching by reabsorption mechanism is ruled out. The excitation and emission spectra of 3% of Eu3+ doped C2SP and C2SH are presented in Figure 9a,b. The excitation spectrum contains a broad band centered at 305 nm with fwhm = 56 nm for C2SP and at ∼295 nm with fwhm = 62 nm for C2SH. The sharp lines are observed in the range 325−545 nm. The broad band is related to the O2−−Eu3+ charge-transfer band (CTB), caused by the electron transfer from the filled 2p orbital of O2− ions to the empty 4f orbital of Eu3+ ions. Upon consideration of the values of peak position and fwhm, the I


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Figure 10. Stark splitting pattern for Eu3+ doped C2SP sample.

bonding neighbors and asymmetry of the dopant site.78,81 The R-value is calculated using the integral intensity of peaks (5D0 → 7F2) at 613 nm and (5D0 → 7F1) at 593 nm. It is 1.5088, 2.0565, 2.7765, and 3.2791, respectively, for C2SP, C2S:140H, C2S:180H, and C2S:200H. The increasing R-value clearly indicates when the synthesis methodology changed from the Pechini method to the hydrothermal method; the europium ion stabilized in a more distorted geometry. This can be attributed to the force exerted by temperature and pressure in the hydrothermal method. This is also explained in terms of the Judd−Ofelt (JO) parameter calculated for all the four samples and tabulated in Table 2. The Judd−Ofelt analysis is an extremely powerful technique for elucidating photophysical properties of europium ion and its local structure in the doped sample using the corrected emission profile with regard to the source, monochromator, and detector. The details of all the calculations used are explained extensively elsewhere.82 It is known that Ω2 reflects the short-range order, indicating covalency and polarizability around the europium ion. On the other hand, Ω4 is a reflection of long-range ordering, which gives an idea about rigidity, viscosity, etc. It is seen from Table 2 that Ω4 is greater than Ω2 for C2SP whereas the reverse is true for C2SH samples. This is indicative of the relatively symmetric environment around the Eu3+ ion in C2SP compared to C2SH. Moreover, it can also be seen from Table 2 that the Ω2 value in the C2SH sample increases as a function of temperature which clearly indicated increased asymmetry and polarizability around the europium ion in C2SH samples as the hydrothermal temperature is raised from 140 to 200 °C. The high temperature reduces the surface

to volume ratio; particles are more agglomerated and assume a more distorted environment. However, in terms of photophysical characteristics, the sample C2SP is much more efficient than C2S:140H as it has a very high probability for radiative transition and low nonradiative transition rate which are finally reflected in the high quantum yield. However, the C2S:140H sample has more defects and a highly distorted lattice which increase the probability of nonradiative transition rate (621 s−1) and reduce the quantum efficiency, but the high-temperature hydrothermally synthesized sample shows a drastic improvement in quantum efficiency because surface defects are annealed out, providing fewer chances for the europium ion to decay nonradiatively which is reflected in its very high quantum yield. 4.7.2. Exploring Europium Luminescence as Structural Probe. The peculiarity of europium ion was explained earlier, as it is highly sensitive to the local crystal field induced by the host lattice. In fact, it gets reflected in its emission profile in terms of Stark splitting. 4.7.2a. Number of Available Metal Ion Crystallographic Site: Nm. High resolution of the Eu (5D0 → 7F0) transition, which is unique for a given chemical environment associated with spectral decomposition and with Lorentzian−Gaussian shape functions, gives direct access to Nm.83 In the energy level diagram of the europium ion, both 7F0 and 5D0 states are nondegenerate. Regarding the fact that both the emitting and end states are nondegenerate, its Stark component indicates the number of available metal ion crystallographic Site (Nm).84 The splitting in 5D0 → 7F0 in the emission spectra (Figures 10 and 11) for C2SP and C2S:140H indicates that a Eu3+ ion is J


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Inorganic Chemistry

Figure 11. Stark splitting pattern for Eu3+ doped C2S:140H sample.

structural and lattice distortion and further reduces the original symmetry of the calcium ion. Due to aliovalent substitution, i.e., trivalent Eu3+ at the divalent Ca2+ site, cation vacancies are formed. The Kroger−Vink notation for such substitution is mentioned below:

localized in two nonequivalent sites, i.e., CaO7 and CaO8 sites. Europium in CaO8 leads to MDT and in CaO7 leads to EDT. 4.7.2b. Point Group Symmetry of the Europium Ion in C2SP and C2S:140H. When the Eu3+ ion is stabilized in a certain local environment, the (2J + 1)-degenerate J-levels get split by the field induced by the ligand known as Stark sublevels; the whole number depends on the site symmetry around the europium ion. As discussed earlier, radiative transitions from 5 D0 to levels with J = 0, 3, or 5 are forbidden by both EDT as well as MDT, and only feeble transitions from 5D0 to these levels could be seen due to the crystal-field-induced mixing of Jlevels.85 In fact, it is reported that the 5D0 → 7F0 transition can be seen only in 10 site symmetries: Cs, C1, C2, C3, C4, C6, C2v, C3v, C4v, and C6v, according to the ED selection rule.86 When Eu3+ is doped in C2S, on the basis of ionic radii, it should occupy the Ca2+ site. However, due to ionic size and ionic charge mismatch, the Eu3+ doping induces significant

• 2Eu••• + 3Ca•• Ca ↔ 2Eu Ca + V″ Ca

(2)

The fact that the 5D0 → 7F2 peak at 613 nm (EDT) is very intense in comparison to the 5D0 → 7F1 peak at 593 nm (MDT) indicates that the majority of Eu3+ ions occupy relatively asymmetric environments without inversion symmetry (Ci). The 5D0 → 7F0 transition, which is allowed only when the site symmetry around the dopant ion is having 10-point group symmetries aforementioned, is observed in the emission spectrum. According to the branching rules of various point groups,87,88 one can deduce the point group symmetry around europium ion in Eu3+ doped C2S. K


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symmetry and high nonradiative losses. The excited state of the Eu3+ ion is a charge-transfer state. The high rate of radiation-less losses occurs with the expansion of the Eu3+ surrounding ions in the excited state, i.e., larger Eu3+−O2− distance corresponding to the Ca2+ ion.38 On the other hand, in the cases of C2S:180H and C2S:200H, lifetime value increases due to decreases in surface defects at the higher temperature. Surface defects are known to aid in the nonradiative transition and thereby decrease the fluorescence lifetime. 4.7.3. Distribution of Eu2+ and Eu3+ in C2SP and C2SH by XPS Spectra. The XPS spectra of Eu ion doped C2SP and C2S:180H are presented in Figure 7c−f. The deconvoluted peaks centered at 1136 and 1127 eV are associated with Eu3+ 3d5/2 and Eu2+ 3d5/2, respectively, though the amount of dopant is less (3%) in our case. The surface distribution values of Eu2+ can be obtained by the area ratio of the peaks Eu2+ 3d5/2 and Eu3+ 3d5/2.89 From Table 4, it can be seen that the surface distribution of Eu2+ in C2S180H:Eu2+ is higher than that in C2SPEu2+. As expected, the presence of Eu2+ in C2SPEu3+ is almost absent. The line spectra (Figure 9) and lifetime value in milliseconds (Figure S9) further support the presence of Eu3+ dopant. Compared to C2SPEu3+, the presence of Eu2+ is high in C2S180H:Eu3+ and can be clearly seen by the intensity variation of the peak at 1127 eV in Figure 7c,e (also see insets in Figure 7c−f). This could be the reason for reduced emission intensity of C2S180H:Eu3+ compared to that of C2SPEu3+ in Figure 9. Furthermore, the presence of Eu2+ can be associated with the creation of oxygen vacancy (adding two electrons) to accommodate Eu3+ at Ca2+ sites in C2SPEu3+ whereas for C2S180H:Eu3+, in addition to that, the effect of the hydrothermal method or distorted symmetry may be the reason for the reduced valence state. Furthermore, it is to be noted that the intensity of the peak at 1127 eV in Figure 7d is higher than that in Figure 7f. A similar observation of Eu2+ has also been reported for the Au@Y2O3:Eu3+ sample.90 The high surface distribution of Eu2+ in C2S180H:Eu2+ provided further evidence for its increased emission intensity due to reduced particle size where the packing density is high (see Figure S8b).

The pure magnetic dipole and electric dipole transitions are taken into consideration for the calculation of the same because others have mixed character although the Stark splitting pattern is given for all the transitions. Furthermore, the splitting in 5 D0−7F2 and 5D0−7F4 is different in Pechini and hydrothermal synthesis as can be seen pictorially from Figures 10 and 11. It is 3 and 4 in Pechini and 3 and 6 in hydrothermal which indicates that the symmetry of the europium ion has further been degraded by hydrothermal synthesis. From the Stark component observed to local field splitting shown in Figure 10, three and four peaks for the 5D0−7F1 (ΔJ = ±1) and 5 D0−7F2 (hypersensitive, ΔJ = ±2) transitions of Eu3+ are resolved for C2SP:Eu3+. According to the branching rules of various point groups,87,88 we infer that the actual site symmetry of Eu3+ in C2S is C2v. On the other hand, the point group symmetry of the europium ion in C2S:140H is less than C1/Cs on the basis of the Stark splitting of 3 and 6 in MDT and EDT. Furthermore, it is noteworthy that the splitting is very sharp for C2SP whereas it is blunt for C2SH, indicating that the doping of Eu3+ in the latter case weakens the crystal field splitting (yellow shaded in Figure 9b). Since the doping concentration is the same for all the samples, the change of strain in the crystal (change in interatomic distance) or blunt splitting due to change of the E3+−O2− distance in C2SH is solely due to the synthesis method. The emission intensity of C 2S:180H and C2 S:200H is increased compared with C2S:140H. It is due to the reduced particle size of the former compared to the latter (see BET in Table S1). In small crystallites, a large percentage of the dopant ions would reside on or toward the surface of the nanocrystals resulting in increased emission intensity.81 The decay time of Eu3+ doped C2SP and C2SH is presented in Figure S9. It has two lifetime values (T1, long; T2 short) as shown in Table 3 similar to Eu2+ doped C2S presented in Table Table 3. Lifetime Values of Eu3+ Doped C2SP and C2SH Particles lifetime/Eu3+

C2SP

C2S:140H

C2S:180H

C2S:200H

T1 T2

3280 555

2193 59

2495 78

3256 92

5. MECHANISM: EFFECT OF GROWTH METHODS AND FLUORESCENCE INTENSITY VARIATIONS This hypothesis focuses on the particle growth of C2SH and its fluorescence variation in Eu ion doped C2SP and C2SH. The C2SP is considered as a reference to compare the results of C2SH. As mentioned in the Experimental Section, transparent single crystals of calcium citrate were observed after the hydrothermal treatment at 180 and 200 °C but not at 140 °C. The crystal image is presented in Figure S10. The single crystal XRD result shows that the crystals are tricalcium dicitrate tetrahydrate, simply put, calcium citrate with cell parameter values a = 5.95 Å, b = 10.24 Å, c = 16.61 Å, α = 72.27°, β = 79.90°, γ = 89.96°, and cell volume V = 947 Å3. These values coincide well with the structure reported recently.63 Since citrates are being used as a modifier in the nanoparticle growth,

S2. This is indicative of the fact that europium stabilizes in CaO7 and CaO8 sites which is in line with our emission data and Stark splitting pattern of 5D0−7F0. With the assumption of the concept of phonon energy, the T2 is attributed to Eu3+ being localized in an asymmetric site as the f−f rule relaxes in this case whereas T1 is attributed to Eu3+ sitting at a relatively more symmetric site as f−f transitions are more forbidden in this case. Therefore, T2 is because of Eu3+ at CaO7 polyhedra whereas T1 is in symmetric CaO8 polyhedra. Moreover, the lifetime value of Eu3+ in C2SH is less than C2SP because of more defects and a large extent of lattice distortion. The reduced decay time is a direct consequence of reduced

Table 4. Distribution of Eu2+ in Europium Doped C2SP and C2SH Obtained from XPS Measurement C2SP:Eu2+

C2SPEu3+

C2S180H:Eu2+

sample

Eu2+5/2

Eu3+5/2

Eu2+5/2

Eu3+5/2

Eu

peak position/eV % conc area ratio = Eu2+/Eu3+

1127.1 12.23 0.14

1136.2 87.77

1127.6 5.76 0.06

1135.9 94.24

1126.7 15.16 0.18

L

2+

5/2

C2S180H:Eu3+ 3+

Eu

5/2

1136.0 84.84

2+

Eu

5/2

1126.9 9.36 0.11

Eu3+5/2 1135.7 90.64


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ions in C2SP:Eu3+. The charge-transfer band (CTB) in the excitation spectra of Eu3+ doped C2SH was blue-shifted about 10 nm compared to that of C2SP. The actual site symmetry of Eu3+ in C2SP is C2v whereas it is less than C1/Cs in C2SH samples. The aliovalent dopant and high temperature and pressure in the hydrothermal method altogether lowered the symmetry of C2SH which resulted in a shift of the CTB. From the decay profile, it was attributed that the Eu ion stabilizes in two different crystallographic sites (CaO7 and CaO8), which are in line with our emission data and Stark splitting pattern of 5 D0−7F0. The shorter lifetime is attributed to the Eu ion localized at CaO7 whereas the longer one stabilizes at CaO8. From the XPS measurement, the area ratio (Eu2+/Eu3+) values 0.14 and 0.18 were calculated, respectively, for C2SP:Eu2+ and C2S180H:Eu2+, whereas they are 0.06 and 0.11 for C2SP:Eu3+ and C2S180H:Eu3+, respectively. The distortion of the silicate unit due to aliovalent dopant and hydrothermal treatment and its effect on the fluorescence intensity were hypothesized.

the effect of calcium citrate on the formation of C2SH is important. For example, hydroxyapatite was prepared by hydrothermal method at 180 °C using trisodium citrate as a modifier, in which the citrate forms via chelating of the calcium citrate complex [Ca(Cit)]− in solution. Under the hydrothermal conditions, the complex would be weakened, and thus, Ca2+ would be released gradually. The Ca2+ ion later involves hydroxyapatite formation. The citrate ion facilitates this way to slow down the particle growth.27,91 Similarly, when sodium citrate or citric acid in our case is added, the citrate ion can chelate with calcium forming the calcium citrate complex.92,93 It acts as a protecting agent against the particle growth at the particular direction where it adsorbed in C2S. Interestingly, the particle size of C2S at 180 and 200 °C has greatly reduced compared to the hydrothermal temperature at 140 °C (see BET, Table S1). Further, the morphologies of the former are rectangular rod-like plates whereas a spherical plate-like morphology with more pores is observed at 140 °C. Though changes in the morphologies are observed, the effect is not high because of the less asymmetric nature of calcium silicate crystals.94 Furthermore, not only the [Ca(Cit)]− complex, but also hydrothermal temperature and pH of the solution may be reasons for the reduced particle size. The variance of fluorescence emission intensity and shift of the charge-transfer band have been observed for europium doped C2S. The hydrothermal method produces a distorted silicate structure, which has been confirmed through the FTIR, Raman, XPS analysis, and fluorescence studies. Furthermore, the aliovalent dopant can create the vacancy to compensate for the charge difference or distortion while occupying the Ca2+ sites. These can altogether lower the symmetry of europium doped C2SH. This has clearly been probed through the increased asymmetry ratio value (R) and the Judd−Ofelt (JO) parameter and Stark splitting pattern in 5D0−7F2 and 5D0−7F4. The distorted structure influences the distance between the O2− and Eu3+ ions which can be clearly identified through the difference between the CTB band of C2SP and C2SH (10 nm) in the excitation spectra of Eu3+.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01878. Images of solutions and precursors obtained during C2S preparation, details of experimental steps involved in C2S preparation, as-recorded X-ray power diffraction patterns, quantitative representation of free lime and C2S, deconvoluted FTIR spectrum, Raman spectra and its discussion, BET surface area, excitation and emission spectra of Eu3+ doped C2S180H, decay curve of Eu3+ doped C2S, as-recorded XPS spectra of europium doped C2SP and C2SH, and crystal image of calcium citrate (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

6. CONCLUSIONS The pure and europium (Eu2+ and Eu3+) doped β-dicalcium silicate (C2S) nanoparticles were prepared by Pechini (C2SP) and hydrothermal (C2S:140H, C2S:180H, C2S:200H) methods. The influence of calcium citrate on the particle growth was identified through changes in line shape and relative intensity of the (200) plane in the powder XRD pattern of C2SH compared to that of C2SP. From the FTIR spectra, the distortion of the silicate group in the C2SH was confirmed by higher and lower wavenumber side shifts of Si−O stretching (1000−1100 cm−1) and bending (521−475 cm−1) vibrations, respectively. The BET results showed that the surface area of the C2SP and C2S:140H (110−130 nm) was around 20 m2/g, whereas it is 50 m2/g for C2S:180H and C2S:200H (∼50 nm). From the SEM and TEM micrographs, the morphology of C2SP and C2S:140H was spherical plate-like. The morphology has been controlled by the formation of calcium citrate during hydrothermal treatment at 180 and 200 °C, observed as rectangular plate-like for the C2S:180H and C2S:200H. The emission intensity was increased for Eu2+ doped C2SH, whereas it was diminished for Eu3+ doped C2SH. The maximum concentration of the Eu ion was optimized to be 3.0 mol %, and beyond that fluorescence quenching takes place. The electric multipolar interaction is responsible for the nonradiative energy transfer among the Eu3+

ORCID

Rajaboopathi Mani: 0000-0002-2535-8987 Huaidong Jiang: 0000-0002-0895-1690 Santosh Kumar Gupta: 0000-0002-1178-0159 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors R.M. and H.J. are grateful to Shandong University, China, for the financial support under the Shandong University International Postdoctoral Exchange Programme to carry out this research work. The author R.M. thanks Dr. Patricia HaroGonzalez, Fluorescence Imaging Group, Departamento de ́ Fisica de Materiales, Universidad Autonoma de Madrid, Spain, for her opinion on fluorescence studies.

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REFERENCES

(1) Allen, A. J.; Thomas, J. J.; Jennings, H. M. Composition and Density of Nanoscale Calcium-Silicate-Hydrate in Cement. Nat. Mater. 2007, 6, 311−316. (2) Georgescu, M.; Tipan, J.; Badanoiu, A.; Crisan, D.; Dragan, I. Highly Reactive Dicalcium Silicate Synthesized by Hydrothermal Processing. Cem. Concr. Compos. 2000, 22, 315−319.

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