- Email: [email protected]
Yb3+ codoped oxyapatite based glass ceramics
Journal Pre-proof Enhanced broadband excitable near-infrared luminescence in Ce oxyapatite based glass ceramics
3+ 3+ /Yb codoped
Weijuan Zhang, Pengbo Li, Xiaoxiao Duan, Shiquan Liu, Zhi Wang PII:
S0921-4526(19)30777-X
DOI:
https://doi.org/10.1016/j.physb.2019.411898
Reference:
PHYSB 411898
To appear in:
Physica B: Physics of Condensed Matter
Received Date: 17 August 2019 Revised Date:
13 November 2019
Accepted Date: 18 November 2019
Please cite this article as: W. Zhang, P. Li, X. Duan, S. Liu, Z. Wang, Enhanced broadband excitable 3+ 3+ near-infrared luminescence in Ce /Yb codoped oxyapatite based glass ceramics, Physica B: Physics of Condensed Matter (2020), doi: https://doi.org/10.1016/j.physb.2019.411898. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier B.V.
CRediT author statement Weijuan Zhang: Conceptualization, Methodology, Writing original draft, Pengbo Li: Data curation, Xiaoxiao Duan: Visualization, Investigation. Shiquan Liu: Resources, Project administration. Zhi Wang: Writing- Reviewing and Editing.
Enhanced broadband excitable near-infrared luminescence in Ce3+/Yb3+ codoped oxyapatite based glass ceramics Weijuan Zhanga, b,* Pengbo Lia, Xiaoxiao Duana, Shiquan Liua, Zhi Wanga a
School of Material Science and Engineering and Shandong Provincial Key Lab of Preparation and Measurement of Building Materials, University of Jinan, Jinan 250022, P. R. China b State Key Lab of Luminescent Materials and Devices and Institute of Optical Communication Materials, South China University of Technology, Guangzhou 510641, P.R. China Abstract Oxyapatite-containing glass ceramics coactivated with Ce3+and Yb3+ were prepared with reducing Al-powder. The precipitation of Ba4La6(SiO4)6O polycrystals exhibiting lattice contraction due to their preferential enrichment of rare earth ions was confirmed by crystallization and microstructural analysis. Upon 366 nm excitation, intense near infrared emission around 1µm due to Yb3+: 2F5/2 → 2F7/2 transition was achieved in glasses and glass ceramics, indicating the occurrence of energy transfer from Ce3+ to Yb3+. The underlying mechanisms could be described as a single-step indirect process via intermediate Ce4+─Yb2+ charge transfer state, most likely involving Ce3+ ions on La3+ neighbor sites. An optimal enhancement as much as three folds in near infrared emission was obtained after crystallization, which probably resulted from the partition of Ce3+ and Yb3+ into crystals. The results show that glass ceramic may be a promising candidate applicable for the development of high-efficiency c-Si solar cells.
Keywords: Glass ceramics; Oxyapatite; Energy transfer; Yb3+ ion spectroscopy *
Author to whom correspondence should be addressed. Email: [email protected] 1
1. Introduction The prevailing global energy crisis has triggered considerable efforts to explore clean and sustainable alternatives to decreasing fossil fuel. Solar energy represents a clean and inexhaustible energy source that can be converted into electricity by photovoltaic (PV) technology. For decades, the PV market has been dominated by crystalline Si (c-Si) solar cells, which has limiting efficiency of 30% estimated by Shockley and Queisser [1]. Nevertheless, the practical energy efficiency of c-Si solar cells is only about 15% [2, 3]. The main factor limiting energy conversion efficiency arises from the spectral mismatch between the incident solar spectrum and the spectral response of solar cell [4, 5]. The problem lies in the fact that a c-Si solar cell with the band gap (Eg) of 1.12 eV (λ = 1100 nm) exhibits optimal response only in the range of 900─1100 nm [6]. The long-wavelength photons with energy lower than Eg are transmitted rather than absorbed by solar cell thus constituting transmission losses while high-energy photons with energy greater than Eg lose the extra energy via lattice thermalization [7, 8]. A promising approach to reduce thermalization losses involves quantum cutting (QC), which refers to converting one incident high-energy photon into two or more low-energy photons with energy approaching the band gap of Si [3, 7─9]. The possibility of increasing theoretical efficiency limit up to 38.6% via QC was demonstrated by Trupke et al. and had ignited extensive research activities of exploring efficient quantum cutting materials [4]. Many efforts have been dedicated to RE3+-Yb3+ (RE = Tb3+, Tm3+, Pr3+, Ho3+, Er3+, and Nd3+) codoped phosphors, glasses, and glass ceramics (GCs) [9─12], which are capable of converting the UV─green portion of solar radiation into the near infrared (NIR) luminescence of Yb3+ at around 1000 nm. Utilization of Yb3+ ions as an acceptor is justified by the 2
fascinating properties such as high luminescence quantum efficiency approaching 100% and the simple energy level structure consisting of 2F7/2 and 2F5/2 manifolds separated by ~ 1.2 eV, which matches well with the band gap of Si. However, inherent line-like weak absorptions of these sensitizer ions owing to the parity forbidden 4f-4f transitions render them inefficient for absorbing the high energy part of solar spectrum. Ce3+ and Eu2+ ions may serve as good sensitizers because of their comparatively broad absorption bands with high efficiency due to the allowed 4f-5d transition, which could also be tuned by adjusting the crystal field strength and covalence of the host [5, 13]. Among the Ce3+-Yb3+ codoped various hosts, GCs have gained considerable interest because of their advantages including easy synthesis and robustness inherited from mother glass as well as strong ligand field and high quantum yield associated with the crystalline phase [6, 14]. Oxyapatite structured silicate compounds have been identified as efficient host for RE ions due to the diverse crystallographic sites, excellent chemical and thermal stability. Lately, there are several reports on the photoluminescence properties of rare earth ions doped oxyapatite phosphors [15, 16]. However, the possibility of QC in Ce3+/Yb3+ codoped oxyapatite based GCs has not been explored yet. In the present work, we report the preparation of Ce3+/Yb3+ codoped GCs containing Ba4La6(SiO4)6O polycrystals with reducing Al-powder. Thermal properties of as prepared glass have been investigated by DSC prior to the crystallization and microstructural analysis of GCs using XRD and TEM. Furthermore, efficient broad-band spectral conversion from UV to NIR as a result of energy transfer (ET) from Ce3+ to Yb3+ in glasses and GCs have also been discussed. 3
2. Material and methods 2.1 Sample preparation Glass
samples
with
nominal molar
composition
28.85SiO2–19.23SrO–28.85BaO–9.62B2O3–4.81La2O3–0.96Al2O3–7.69Al:
1P2O5,
of xCeO2,
yYb2O3 (x = 0.5, 1.0, and 2.0, y = 0; x = 1.0, y = 0.125, 0.25, 0.5, and 1) were fabricated using conventional melt quenching method. For each batch (15 g), analytical reagent-grade SiO2, SrCO3, BaCO3, H3BO3, La2O3, Al2O3, NH4H2PO4 as well as CeO2 and/or Yb2O3 with a purity of 99.99% were weighed accurately and mixed thoroughly in an agate mortar. For the sake of reducing Ce4+ to Ce3+, aluminum powder was incorporated as oxygen absorber. Then, homogeneously mixed starting materials were put into a 30-mL corundum crucible with a lid, which was subsequently transferred into an electric furnace at 1400 °C and kept for a period of 90 min in ambient air. The glass melt was poured into a preheated (200 oC) stainless steel mold and rapidly transferred into a preheated muffle furnace for annealing at 590 oC for 2 h. For the sake of convenience, Ce3+ singly doped and Ce3+/Yb3+ codoped glasses fabricated in this way were labeled as G1─3 and G5─8, respectively. For comparison, the sample with a nominal molar composition of 30SiO2–20SrO–30BaO–10B2O3–5La2O3–5Al2O3: 1P2O5, 2CeO2 was melted in an open crucible and referred to as G4. Subsequently, the bulk glasses were cut into small pieces and heat treated at 780 oC for 4 h to obtain GCs denoted as GC1─8, correspondingly. Finally, all the studied samples were polished to a thickness of 2 mm for optical measurements.
4
2.2 Characterization and measurements Differential scanning calorimetry (DSC) measurement was performed on a Netzsch STA 409EP calorimeter in an argon atmosphere at a heating rate of 10 oC/min. X-ray power diffraction (XRD) patterns were obtained using a Brucker D8-advance X-ray powder diffractometer (Cu Kα, 40 kV/40 mA) for crystalline phase identification. Microstructural characterization of the resultant glass ceramics was conducted by a transmission electron microscope (TEM, JEM-2100F) assembled with the selected area electron diffraction (SAED). TEM specimen was prepared by dipping a drop of sonicated suspension containing fine-ground sample on a carbon coated copper grid, followed by drying under an infrared lamp. Optical absorption spectra were recorded on a Perkin-Elmer Lambda 900 UV/VIS/NIR spectrophotometer with the resolution of 1 nm. Diffuse reflection spectra in the 200─800 nm range were measured with a Shimadzu UV-2600 spectrophotometer using BaSO4 as reference. X-ray photoelectron spectroscopy (XPS) for analyzing the valence of cerium ions was performed using Thermo ESCALAB 250XI spectrometer. The excitation and emission spectra along with decay curves of the fluorescence signals were recorded on an Edinburgh Instruments FLS920 spectrofluorometer equipped with a 450 W Xenon lamp (Xe900) as the excitation source. All the measurements were conducted at room temperature.
3. Results and discussion 3.1 Thermal properties, crystallization, and microstructure Fig. 1 reveals the DSC curves for G2 glass in comparison with the undoped blank glass (denoted as G0). The glass transition temperature Tg is determined to be 590 oC and 600 oC 5
from the DSC curves of G0 and G2, respectively. There is a slight increase in Tg with the CeO2 addition. It is indicative of the strengthening of glass network which can be accounted for by considering that Ce4+ exhibits high field strength and could enter glass network as intermediate [17]. The DSC thermograms of G0 and G2 show a faint exothermic peak at 840 o
C and 850 oC assigned to the crystallization of Ba4La6(SiO4)6O (JCPDS card No.27–0037) as
confirmed by XRD results (Fig.2). Similarly, a slight increase in crystallization peak temperature may be associated with of the network strengthening induced by CeO2.
Fig. 1 DSC traces of G0 and G2 glass samples recorded at a heating rate of 10 K/min. XRD patterns of glasses annealed at 750 oC and 780 oC for 4 h were taken and illustrated in Fig. 2 to identify the crystalline phase precipitated from glass. As noted in Fig. 2(a), G1 sample heat-treated at 750 oC is nearly amorphous. The XRD pattern exhibits only feeble diffraction peaks superimposed on dominant diffuse humps. After thermal treatment at 780 oC for 4 h, intense diffraction peaks assigned to hexagonal Ba4La6(SiO4)6O (JCPDS card No.27-0037) are clearly revealed in the XRD plots of GC samples labeled as GC0-780-4h, GC2-780-4h, GC4-780-4h, and GC8-780-4h. As presented in Fig. 2(b), a small shift of diffraction peaks toward higher angles is noticeable in GC2 sample with addition of 1 mol% 6
CeO2. It is indicative of the lattice contraction induced by the partial substitution of bigger La3+ ions by smaller Ce3+ ions as consistent with our previous research [18].
Fig. 2(a) XRD patterns of GC samples doped with or without cerium ions (labeled as GC0) obtained after heat-treatment at 750 oC and 780oC for 4 h, respectively, and (b) enlarged XRD patterns indicating red shift of diffraction peaks of GC2 compared with GC0. The microstructure of GC8 heat-treated at 780 °C for 4 h is characterized by the transmission electron microscopy. It can be clearly seen from Fig.3(a) that some dark particles are randomly dispersed on the bright background corresponding to glassy matrix. An observation of densely distributed spots forming concentric rings on the corresponding SAED 7
pattern (inset of Fig.3(a)) demonstrates the precipitation of randomly orientated polycrystals in the glass matrix. Furthermore, the HRTEM image of individual crystals (Fig.3(b, c)) exhibits well-resolved lattice fringe pattern, which indicates the high crystallinity of these crystals. The HRTEM image of an individual crystal in Fig.3(c) presents well-resolved lattice fringe with the measured spacing of 0.334 nm corresponding to the orientation [102] of oxyapatite (Ba4La6(SiO4)6O) crystal. This value is smaller than the one evaluated using Bragg equation. For GC0 and GC2 samples, the interplanar spacing determined from [102] diffraction peak is 0.338 nm and 0.337 nm. Thus, it is reasonable to infer that more rare earth (Ce3+ and/or Yb3+) ions are enriched into the precipitated crystals at high doping level, contributing to a steady decrease in interplanar spacing.
Fig.3(a) Bright field TEM image and corresponding selected area electron diffraction pattern (inset) of GC8 heat-treated at 780 °C for 4 h, (b) HRTEM image of dark particles in GC8, and (c) HRTEM image of an individual particle in GC8 showing well-defined (102) lattice fringes of Ba4La6(SiO4)6O crystal with a d spacing of 0.334 nm. 8
3.2 Optical absorption The absorption edge can be evaluated from the measured absorption spectrum of blank glass G0 using the following relation proposed by Mott and Davis [19, 20]:
α hν = B 2 (hν − Eg )r
(1)
where α is the absorption coefficient near the absorption edge derived from the measured absorbance A as a function of frequency, and hν is the incident photon energy. B is a constant called the band tailing parameter, and r is the index which can have values of 1/2 and 2 corresponding to the direct and indirect allowed electronic transition for the absorption. As shown in Fig.4, plotting (αhν)1/r versus photon energy (hν) reveals a linear region, which can be extrapolated to meet the hν axis at (αhν)1/r = 0, thus yielding the value of the optical band gap Eg = 3.3 and 3.94 eV for indirect and direct allowed transitions, respectively.
Fig.4 Variation of (αhν)1/2 (indirect) and (αhν)2 (direct) with (hν) for G0 glass sample. The absorption spectra of glasses (G7, G8) and GCs (GC7, GC8) codoped with cerium and ytterbium ions are presented in Fig.5(a). A broad absorption band peaked at 977 nm can be ascribed to Yb3+: 2F7/2 → 2F5/2 transition. However, the absorption bands of cerium ions 9
cannot be distinguished due to the convergence with inherent absorption cut-off edge of glass. This is especially true in GCs, which exhibit a red shift of the absorption edge due to the scattering from crystallites. As shown in the inset of Fig.5(a), all the obtained glasses appear transparent and pale yellow, while all the corresponding GCs except GC7 are translucent. GC7 sample appears more transparent than the other GCs probably due to the much smaller size of the precipitated crystals to avoid light scattering. This can be explained as follows: the gradual addition of Yb2O3 leads to a decrease in viscosity and an increase in diffusivity, thus favoring the crystallization of apatite. Since the apatite (Ba4La6(SiO4)6O) exhibits around 4.33 network modifiers charges per network former, which is larger than that of the glass matrix (~1.61), the crystallization of such apatite phase enriched in network modifiers would in turn increase the viscosity near the crystal. Hence, further crystal growth can be suppressed by diffusion barriers forming around crystallites [21, 22]. However, as the Yb2O3 content increased to 1 mol%, trace of undissolved particles were visible in GC8 due to its limited solubility. In the process of crystallization, these undissolved particles may act as heterogeneous nucleation site for crystal growth. Eventually, the light scattering by large particles results in the loss of transparency.
Fig.5(a) Absorption spectra of Ce3+/Yb3+ codoped glasses (G7 and G8) and corresponding 10
GCs. Apart from G7, all the absorption curves are shifted upward along the vertical axis for clarity. The inset shows the photograph of these samples. (b) Absorption spectra of G5–8 glasses and GC8 (compared in the inset) transformed from the diffuse reflection spectra. To explore the absorption caused by cerium ions, the diffuse reflection spectra of finely ground powder samples were measured and further transformed into absorption spectra in Fig.5(b) according to the relation A = (1-R)2/R, where R is the reflectivity. The resulting absorption spectra of G5─8 glasses exhibit a broad band in the range of 200─400 nm. It is reported that Ce4+ charge transfer (CT) absorption causes the band peaked at 240 nm with much greater absorption cross section than Ce3+ [23, 24]. Thus, the dominant band peaked at 278 nm may be associated with the absorption due to Ce3+ and small amounts of Ce4+. The shoulder at around 350 nm can be assigned to Ce3+: 4f → 5d transition. It is worthwhile to mention that a dip near 324 nm isolating the two regions is caused by lamp switching. It can be noticed from the inset of Fig.5(b) that the 4f-5d absorption band shifts toward the longer wavelength side with the precipitation of Ba4La6(SiO4)6O crystals in GC8 due to the stronger crystal field. Hence, the selection of Ce3+ as sensitizer ions is justified because its 4f-5d absorption band not only falls into the region of solar spectrum but also in the inefficient spectral response range of c-Si solar cell. 3.3 XPS spectra XPS spectrum was taken for G2 glass to study the valence state of cerium ions. As presented in Fig.6, the Ce 3d core level spectrum is composed of two main broad peaks around 901.5 eV and 890 eV corresponding to Ce 3d3/2 and Ce 3d5/2 multiplet, respectively. It is known that cerium ion can exist in two different oxidation states, i.e., trivalent and 11
tetravalent cerium. Both of these species are characterized by 3d5/2 and 3d3/2 multiplet splitting. The observation of faint peak at a BE of 915.4 eV ascribed to the initial state of the Ce4+ (called "f0" configuration) suggests that there still exists a tiny proportion of residual tetravalent Ce in the present glass [17]. As the same experimental conditions were kept for the reduction of Ce4+ during glass fabrication, the variation of Ce3+ concentration among G1─3 and G5─8 glasses may be assumed to be negligible. By referring to the number of peaks as well as their binding energy position reported in previous works [17, 25], Gaussian type deconvolution of this superimposed spectrum involving Ce3+ and Ce4+ was conducted, which gives eight components as depicted in Fig.6. In comparison with the XPS spectra of CeO2 and Ce2O3 [26], the fitted peaks at 882.3 and 886.4 eV corresponds to 3d5/2 of Ce3+, and those at 901 and 905.2 eV are associated with 3d3/2 of Ce3+. Meanwhile, the contribution from Ce4+ can account for the peaks at 882.3, 890.3, and 895.6 eV corresponding to 3d5/2 along with those at 901, 910, and 915.4 eV for 3d3/2.
Fig.6 XPS spectrum of G2 glass singly doped with 1 mol% cerium ions, curve fitted with eight components contributing to Ce 3d core level peaks. 12
Fig.7 XPS spectrum of G2 glass singly doped with 1 mol% cerium ions, curve fitted with three components contributing to O 1s core level peaks. Fig.7 shows the O 1s core level spectrum for G2, which exhibits an asymmetric band at an average binding energy around 531 eV. This peak can be well fitted using three sub-peaks at 530.2, 531.3, and 532.6 eV representing different types of oxygen sites. Specifically, the peak at 532.6 eV is attributed to the contribution of bridging oxygen (BO). Theoretically, the BO atoms could be expected in the Si–O–Si, Si–O–B, Si–O–Al, and B–O–B configurations for the present glass system. However, further deconvolution of the peak at 532.6 eV with the FWHM of 1.89 eV is of no significance due to the difficulty in evaluating its validity. Consistent with the previous reports [27, 28], the two components at the BE of 530.2 and 531.3 eV can be assigned to non-briding oxygen (NBO) bound to boron and silicon, correspondingly. For the alkaline earth oxides (SrO and BaO) enriched glass studied in this work, the fraction of NBO can be evaluated to be 86.2% using the area ratio of the peaks associated with NBO to the total area of the O 1s spectrum.
13
3.4 Luminescence properties To understand the luminescence properties of Ce3+, emission spectra (λex = 366 nm) of G1─3 glasses singly doped with increasing amount of cerium ions were taken and presented in Fig.8. It is noted that excitation G1 and G2 glasses at 366 nm yields a broad emission band with a peak at 486 nm and a shoulder at 453 nm. It is well-known that the 4f ground state of Ce3+ splits into doublet 2F5/2 and 2F7/2 levels separated by about 2000 cm-1 [29]. With an energy difference of ~1500 cm-1 between 453 and 486 nm, thus the two Ce3+ emission components can be ascribed to 5d→ 4f transition occurring from the excited state down to such multiplets. The mean lifetime of excited Ce3+ ions in G2 is evaluated as 33.8 ns from the luminescence decay curve monitored at 486 nm, which exhibits a double-exponential behavior (Fig.S1). The emission intensity decreases with increasing doping level predominantly due to the quenching effect of Ce4+. Moreover, this emission band is hardly seen in G4.This can be explained by considering the fact that the CT absorption band of Ce4+ overlaps Ce3+ 4f-5d absorption band in the UV region. In this case, some irradiation energy could be transferred to Ce4+, followed by non-radiative decay [23]. The presence of high level of Ce4+ in G3 glass can be deduced from its appearance (see inset of Fig. 8). It has been reported that the tail of CT absorption band of Ce4+ can extend to the visible region, resulting in the color of Ce4+ doped glasses [23]. G3 glass shows brownish color similar to G4 glass obtained in an ambient atmosphere without metallic aluminum powder. On the other hand, it is implied that the amount of Al powder is insufficient for complete reduction of CeO2 at higher doping concentrations. For comparative purposes, Ce3+/Yb3+ codoped G5─8 glasses were fabricated under the same experimental condition with G2. 14
Fig.8 Emission (λex = 366 nm) spectra of G1─4 glasses as a function of Ce3+ concentration. The inset shows the photograph of G0─4 glasses. Fig. 9 gives exemplary excitation and emission spectra of G6 glass. Excitation at 366 nm gives rise to the emission spectrum consisting of two broad emission bands at about 480 nm and 976 nm, assigned to Ce3+: 5d → 4f and Yb3+: 2F5/2 → 2F7/2 transitions, respectively. As Yb3+ posses only one excited (2F5/2) level lying around 10,000 cm−1, thus it is indicative of the occurrence of ET from Ce3+ to Yb3+. The excitation spectra monitored at 480 and 976 nm are compared on the left side of the figure. The 4f → 5d transition of Ce3+ is clearly revealed by a broad excitation band located within the range of 300─450 nm, which is in coincidence with the absorption spectra discussed previously. The excitation band for 480 nm emission exhibits asymmetric feature and can be deconvoluted into three Gaussian peaks with the dominant component at 368 nm (see inset).
15
Fig.9 Excitation spectra (left side) of G6 glass monitored at 480 and 976 nm and visible-to-NIR emission spectra under 366 nm excitation. The inset shows the Gaussian deconvolution of the excitation band monitored at 480 nm after baseline subtraction. It is well established that the position of 5d levels of Ce3+ is dependent on nephelauxetic (covalency) effect and crystal field splitting [30]. In the case of glass, the contribution of crystal field splitting is weak. The covalency between Ce3+ and oxygen ligands gives a dominant contribution to centroid shift. On the basis of the aforementioned XPS data regarding O 1s core level, it can be inferred that there exist a large amount of alkaline earth ions (Sr2+ and Ba2+) attached to the non-bridging oxygen. The smaller ionic radius of Sr2+ than Ba2+ implies stronger ability of attracting electron away from oxygen ions, which reduces the Ce─O bonding covalency and thus predicts a smaller centroid shift of the Ce3+ 5d configuration [31]. It is conceivable that the observed broad excitation band represents the contribution of Ce3+ ions on sites with different ligand field strength. As centroid shift tends to decrease with the decrease of Ce─O bonding covalency in the order of BaO→SrO→La2O3, the three deconvoluted components at 398, 368, and 340 nm could be assigned to the 4f → 5d 16
transitions of Ce3+ ions coordinated by oxygen atoms binding with Ba2+, Sr2+, and La3+, respectively. Similarly, a deconvolution of the excitation band monitoring at 976 nm with a Gaussian function was carried out, which gave three components with the dominant one at 340 nm. It is indicated that Ce3+ ions on La3+ neighbor sites play a dominant role in the Ce3+ → Yb3+ ET process. This is understandable since the similar ionic radii of Yb3+ (e.g., 1.07 Å at CN = 7) and La3+ ions (e.g., 1.24 Å at CN = 7 [32]) facilitate Yb3+ to occupy La3+ sites, possibly resulting in the formation of Ce─O─Yb bonds. For such a short interionic distance between Ce3+ and Yb3+ ions, the ET process from Ce3+ to adjacent Yb3+ ions would take place more efficiently. It is worthwhile to mention that the emission intensity of Ce3+ is severely quenched as a result of energy transfer from Ce3+ to Yb3+, thus it is difficult to track the emission intensity changes against time. The luminescence decay curves of Ce3+ in codoped system were not obtained in this work. The NIR emission spectra of Ce3+/Yb3+ codoped glasses and GCs were obtained with the excitation at 340 nm. As presented in Fig. 10(a), there exists a typical broad emission band centered at 1014 nm along with a sharp peak at 976 nm on the spectrum of each sample, which can be attributed to the 2F5/2 → 2F7/2 radiative transition of Yb3+ ions. It is demonstrated that Yb3+ is activated through efficient ET process from Ce3+ to Yb3+. Previous studies have proposed the cooperative energy transfer (CET) mechanism for such ET process [33, 34]. With the energy gap of Ce3+: 4f-5d1 approximately twice the energy of Yb3+: 2F5/2 → 2F7/2 transition, it is assumed that one excited Ce3+ ion transfers energy to two adjacent Yb3+ ions, followed by the generation of two NIR photons. However, the experimental quantum yield of Yb3+ emission below 100% failed to support the model [5]. This result can be understood by 17
considering that CET is a second order process with the probability of three orders of magnitude lower than a first-order one [35]. Additionally, other researchers consider that ET from Ce3+ to Yb3+ proceeds probably through Ce4+─Yb2+ charge transfer state (CTS) [5, 36]. It has been reported that Ce4+─Yb2+ CTS forms via the redox reaction: Ce3+ + Yb3+ → Ce4+ + Yb2+ with the participation of bridge O2- between Ce3+ and Yb3+ ions [37]. To further explore the ET mechanism, the decay curve of Yb3+emission at 1016 nm in GC7 were recorded with an excitation wavelength of 353 nm. As shown in Fig. 10(b), the dynamic photoluminescence of Yb3+ is well depicted by the initial rise and decay components, which is consistent with the previous study [5]. At early stage (almost within 150 µs after the irradiation), the Yb3+ emission is enhanced remarkably, and then fades with time. From the above discussion, it can be speculated that ET from Ce3+ to Yb3+ cannot be a direct process since the rise time for the NIR luminescence of Yb3+ is longer than the lifetime of Ce3+ (33.8 ns). The most probable ET pathway from Ce3+ to Yb3+ in this system can be considered as a single-step indirect process via intermediate Ce4+─Yb2+ CTS. The emission intensity is found to increase gradually with increasing Yb3+ concentration from 0.25 up to 1 mol %, and begins to fall at the concentration of 2 mol % due to concentration quenching effect [33]. The integral intensity of emission around 1000 nm in GC5─7 exhibits a significant enhancement relative to corresponding glasses. This can be associated with the incorporation of active centers into crystal phase as confirmed by XRD and HRTEM results, thus promoting the NIR luminescence by providing the preferred crystalline enviroment and shortening the interionic distance between Ce3+ and Yb3+. However, there exhibits a more severe quenching of the NIR luminescence in GC8, which is 18
possibily caused by rapid energy migration among neighboring Yb3+ ions at high doping concentration.
Fig.10(a) NIR emission spectra of Ce3+/Yb3+ codoped G6 glass in comparison to GC samples as a function of Yb3+ content under 340 nm excitation. Inset: the dependence of integrated NIR emission intensity on Yb3+ doping level for glasses and GC samples. (b) The decay curve of Yb3+emission in GC7 monitored at 1016 nm upon the excitation at 353 nm.
4. Conclusions In this paper, the GCs containing Ba4La6(SiO4)6O crystals were prepared from the glass system SiO2–SrO–BaO–B2O3–La2O3–Al2O3. XRD and TEM characterization of the GCs doped with cerium and/or ytterbium ions not only clearly confirms the precipitation of oxyapatite polycrystals in the glass matrix but also indicates the partition of rare earth ions 19
into crystals. Optical absorption spectrascopy identifies a broad band in the range of 200─400 nm contributed predominantly by the trivalent cerium ions (Ce3+) as evidenced by XPS results. Upon 366 nm excitation, the Ce3+/Yb3+ codoped glass exhibits a broad-band Ce3+ emission around 480 nm assigned to the 5d → 4f transition and intense NIR luminescence of Yb3+ as a result of ET from Ce3+ to Yb3+ occurring probably via intermediate Ce4+─Yb2+ CTS. Moreover, it is found that Ce3+ ions on La3+ neighbor sites play a dominant role in the Ce3+ → Yb3+ ET process. A significant enhancement (about threefold) in NIR luminescence was achieved in GC compared to the glasses with increasing Yb3+ concentration up to 1 mol%. Therefore, the oxyapatite based GCs can be considered as promising hosts for efficient broad-band spectral conversion from UV to NIR, which is potentially applicable in enhancing the efficiency of c-Si solar cells.
Acknowledgments This work was financially supported by NSFC (Grant No. 51602125), Fund of University of Jinan (Grant No. XBS1353 and XKY1403) and the Open Fund of the State Key Laboratory of Luminescent Materials and Devices (2016-skllmd-10).
20
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downconversion of NaYF4: Ho3+, Yb3+ phosphors, Physica B 406 (2011) 1248–1252. [11] V. D. Rodríguez, V. K. Tikhomirov, J. Méndez-Ramos, A.C. Yanes, V.V. Moshchalkov, Towards broad range and highly efficient down-conversion of solar spectrum by Er3+–Yb3+ co-doped nano-structured glass-ceramics, Sol. Energy Mater. Sol. Cells 94 (2010) 1612–1617. [12] F. B. Costa, K. Yukimitu, L. A. O. Nunes, M. S. Figueiredo, J. R. Silva, L. H. C. Andrade, S. M. Lima, J. C. S. Moraes, High Nd3+→Yb3+ energy transfer efficiency in tungsten-tellurite glass: A promising converter for solar cells, J. Am. Ceram. Soc. 100 (2017) 1956–1962. [13] H. Lin, D. Chen, Y. Yu, Z. Shan, P. Huang, A. Yang, Y. Wang, Broadband UV excitable near-infrared downconversion luminescence in Eu2+/Yb3+: CaF2 nanocrystals embedded glass ceramics, J. Alloy. Compd. 509 (2011) 3363–3366. [14] Z. Fang, R. Cao, F. Zhang, Z. Ma, G. Dong, J. Qiu, Efficient spectral conversion from visible to near-infrared in transparent glass ceramics containing Ce3+–Yb3+ codoped Y3Al5O12 nanocrystals, J. Mater. Chem. C 2 (2014) 2204–2211. [15] G. S. R. Raju, E. Pavitra, L. K. Bharat, G. M. Rao, T. J. Jeon, Y. S. Huh, Y. K. Han, Enhanced green upconversion luminescence properties of Er3+/Yb3+ co-doped strontium gadolinium silicate oxyapatite phosphor, Ceram. Int. 44 (2018) 13852–13857. [16] Z. Zhang, L. Wang, S. Yang, W. Chen, X. Chu, Synthesis and characterizations of novel Ba2La8 (SiO4)6O2: Eu3+ oxyapatite phosphors, Dyes Pigments 142 (2017) 272–276. [17] A. Mekki, X-ray photoelectron spectroscopy of CeO2–Na2O–SiO2 glasses, J. Electron Spectrosc. 142 (2005) 75–81. [18] W. J. Zhang, X. B. Li, L. J. Wu, Y. Y. Yu, X. Z. Wang, S. Q. Liu, Z. Wang, W. C. Wang, Y. Liu, Downshifting by energy transfer in Eu3+/Yb3+ codoped Ba4La6 (SiO4)6O glass ceramics, 22
Physica B 508 (2017) 22–26. [19] N.F. Mott, E. Davis, Electronic Processes in Non-crystalline Materials, second ed., Oxford university press, New York, 2012. [20] D. Souri, S. A. Salehizadeh, Effect of NiO content on the optical band gap, refractive index, and density of TeO2–V2O5–NiO glasses, J. Mater. Sci. 44 (2009) 5800–5805. [21] C. Rüssel, Nanocrystallization of CaF2 from Na2O/K2O/CaO/CaF2/Al2O3/SiO2 glasses, Chem. Mater., 17 (2005) 5843–5847. [22] S. Bhattacharyya., C. Bocker, T. Heil, J. R. Jinschek, T. Höche, C. Rüssel, H. Kohl,. Experimental evidence of self-limited growth of nanocrystals in glass, Nano lett., 9 (2009), 2493–2496. [23] A. Herrmann, H. A. Othman, A. A. Assadi, M. Tiegel, S. Kuhn, C. Rüssel, Spectroscopic properties of cerium-doped aluminosilicate glasses, Opt. Mater. Express 5 (2015) 720–732. [24] J. S. Stroud, Photoionization of Ce3+ in glass, J. Chem. Phys. 35 (1961) 844–850. [25] S. Gavarini, M. J. Guittet, P. Trocellier, M. Gautier-Soyer, F. Carrot, G. Matzen, Cerium oxidation during leaching of CeYSiAlO glass, J. Nucl. Mater. 322 (2003) 111–118. [26] D. A. Creaser, P. G. Harrison, M. A.Morris, B. A. Wolfindale, X-ray photoelectron spectroscopic study of the oxidation and reduction of a cerium (III) oxide/cerium foil substrate, Catal. Lett. 23 (1994) 13–24. [27] G. Speranza, M. Ferrari, M. Bettinelli, X-ray photoemission study of Pr3+ in zinc borate glasses, Philos. Mag. B 79 (1999) 2145–2155. [28] Y. Miura, H. Kusano, T. Nanba, S. Matsumoto, X-ray photoelectron spectroscopy of sodium borosilicate glasses, J. Non-Cryst. Solids 290 (2001) 1–14. 23
[29] G. Li, Y. Zhang, D. Geng, M. Shang, C. Peng, Z. Cheng, J. Lin, Single-composition trichromatic white-emitting Ca4Y6(SiO4)6O: Ce3+/Mn2+/Tb3+ phosphor: luminescence and energy transfer, ACS Appl. Mater. Interfaces 4 (2012) 296–305. [30] P. Dorenbos, 5d-level energies of Ce3+ and the crystalline environment. III. Oxides containing ionic complexes, Phys. Rev. B 64 (2001) 125117. [31] P. Dorenbos, 5d-level energies of Ce3+ and the crystalline environment. I. Fluoride compounds, Phys. Rev. B 62 (2000) 15640. [32] R. D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Cryst. 32 (1976) 751–767. [33] W. Zhou, J. Yang, J. Wang, Y. Li, X. Kuang, J. Tang, H. Liang, Study on the effects of 5d energy locations of Ce3+ ions on NIR quantum cutting process in Y2SiO5: Ce3+, Yb3+, Opt. Express 20 (2012) A510–A518. [34] Z. Liu, J. Li, L. Yang, Q. Chen, Y. Chu, N. Dai, Efficient near-infrared quantum cutting in Ce3+–Yb3+ codoped glass for solar photovoltaic, Sol. Energy Mater. Sol. Cells 122 (2014) 46–50. [35] C. Zhang, L. Wang, X. Ji, Y. Ping, P. Zhu, G. Li, Effect of annealing temperatures on luminescence properties in Ce3+-Yb3+ codoped oxyfluoride glass-ceramics for solar cells, Optik 171 (2018) 308–312. [36] D. C. Yu, F. T. Rabouw, W. Q. Boon, T. Kieboom, S. Ye, Q. Y. Zhang, A. Meijerink, Insights into the energy transfer mechanism in Ce3+−Yb3+ codoped YAG phosphors, Phys. Rev. B 90 (2014) 165126. [37] H. Lin, S. Zhou, H. Teng, Y. Li, W. Li, X. Hou, T. Jia, Near infrared quantum cutting in 24
heavy Yb doped Ce0.03Yb3xY(2.97−3x)Al5O12 transparent ceramics for crystalline silicon solar cells, J. Appl. Phys. 107 (2010) 043107.
25
Declaration of interests √☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
None
3+ 3+ /Yb codoped
Weijuan Zhang, Pengbo Li, Xiaoxiao Duan, Shiquan Liu, Zhi Wang PII:
S0921-4526(19)30777-X
DOI:
https://doi.org/10.1016/j.physb.2019.411898
Reference:
PHYSB 411898
To appear in:
Physica B: Physics of Condensed Matter
Received Date: 17 August 2019 Revised Date:
13 November 2019
Accepted Date: 18 November 2019
Please cite this article as: W. Zhang, P. Li, X. Duan, S. Liu, Z. Wang, Enhanced broadband excitable 3+ 3+ near-infrared luminescence in Ce /Yb codoped oxyapatite based glass ceramics, Physica B: Physics of Condensed Matter (2020), doi: https://doi.org/10.1016/j.physb.2019.411898. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier B.V.
CRediT author statement Weijuan Zhang: Conceptualization, Methodology, Writing original draft, Pengbo Li: Data curation, Xiaoxiao Duan: Visualization, Investigation. Shiquan Liu: Resources, Project administration. Zhi Wang: Writing- Reviewing and Editing.
Enhanced broadband excitable near-infrared luminescence in Ce3+/Yb3+ codoped oxyapatite based glass ceramics Weijuan Zhanga, b,* Pengbo Lia, Xiaoxiao Duana, Shiquan Liua, Zhi Wanga a
School of Material Science and Engineering and Shandong Provincial Key Lab of Preparation and Measurement of Building Materials, University of Jinan, Jinan 250022, P. R. China b State Key Lab of Luminescent Materials and Devices and Institute of Optical Communication Materials, South China University of Technology, Guangzhou 510641, P.R. China Abstract Oxyapatite-containing glass ceramics coactivated with Ce3+and Yb3+ were prepared with reducing Al-powder. The precipitation of Ba4La6(SiO4)6O polycrystals exhibiting lattice contraction due to their preferential enrichment of rare earth ions was confirmed by crystallization and microstructural analysis. Upon 366 nm excitation, intense near infrared emission around 1µm due to Yb3+: 2F5/2 → 2F7/2 transition was achieved in glasses and glass ceramics, indicating the occurrence of energy transfer from Ce3+ to Yb3+. The underlying mechanisms could be described as a single-step indirect process via intermediate Ce4+─Yb2+ charge transfer state, most likely involving Ce3+ ions on La3+ neighbor sites. An optimal enhancement as much as three folds in near infrared emission was obtained after crystallization, which probably resulted from the partition of Ce3+ and Yb3+ into crystals. The results show that glass ceramic may be a promising candidate applicable for the development of high-efficiency c-Si solar cells.
Keywords: Glass ceramics; Oxyapatite; Energy transfer; Yb3+ ion spectroscopy *
Author to whom correspondence should be addressed. Email: [email protected] 1
1. Introduction The prevailing global energy crisis has triggered considerable efforts to explore clean and sustainable alternatives to decreasing fossil fuel. Solar energy represents a clean and inexhaustible energy source that can be converted into electricity by photovoltaic (PV) technology. For decades, the PV market has been dominated by crystalline Si (c-Si) solar cells, which has limiting efficiency of 30% estimated by Shockley and Queisser [1]. Nevertheless, the practical energy efficiency of c-Si solar cells is only about 15% [2, 3]. The main factor limiting energy conversion efficiency arises from the spectral mismatch between the incident solar spectrum and the spectral response of solar cell [4, 5]. The problem lies in the fact that a c-Si solar cell with the band gap (Eg) of 1.12 eV (λ = 1100 nm) exhibits optimal response only in the range of 900─1100 nm [6]. The long-wavelength photons with energy lower than Eg are transmitted rather than absorbed by solar cell thus constituting transmission losses while high-energy photons with energy greater than Eg lose the extra energy via lattice thermalization [7, 8]. A promising approach to reduce thermalization losses involves quantum cutting (QC), which refers to converting one incident high-energy photon into two or more low-energy photons with energy approaching the band gap of Si [3, 7─9]. The possibility of increasing theoretical efficiency limit up to 38.6% via QC was demonstrated by Trupke et al. and had ignited extensive research activities of exploring efficient quantum cutting materials [4]. Many efforts have been dedicated to RE3+-Yb3+ (RE = Tb3+, Tm3+, Pr3+, Ho3+, Er3+, and Nd3+) codoped phosphors, glasses, and glass ceramics (GCs) [9─12], which are capable of converting the UV─green portion of solar radiation into the near infrared (NIR) luminescence of Yb3+ at around 1000 nm. Utilization of Yb3+ ions as an acceptor is justified by the 2
fascinating properties such as high luminescence quantum efficiency approaching 100% and the simple energy level structure consisting of 2F7/2 and 2F5/2 manifolds separated by ~ 1.2 eV, which matches well with the band gap of Si. However, inherent line-like weak absorptions of these sensitizer ions owing to the parity forbidden 4f-4f transitions render them inefficient for absorbing the high energy part of solar spectrum. Ce3+ and Eu2+ ions may serve as good sensitizers because of their comparatively broad absorption bands with high efficiency due to the allowed 4f-5d transition, which could also be tuned by adjusting the crystal field strength and covalence of the host [5, 13]. Among the Ce3+-Yb3+ codoped various hosts, GCs have gained considerable interest because of their advantages including easy synthesis and robustness inherited from mother glass as well as strong ligand field and high quantum yield associated with the crystalline phase [6, 14]. Oxyapatite structured silicate compounds have been identified as efficient host for RE ions due to the diverse crystallographic sites, excellent chemical and thermal stability. Lately, there are several reports on the photoluminescence properties of rare earth ions doped oxyapatite phosphors [15, 16]. However, the possibility of QC in Ce3+/Yb3+ codoped oxyapatite based GCs has not been explored yet. In the present work, we report the preparation of Ce3+/Yb3+ codoped GCs containing Ba4La6(SiO4)6O polycrystals with reducing Al-powder. Thermal properties of as prepared glass have been investigated by DSC prior to the crystallization and microstructural analysis of GCs using XRD and TEM. Furthermore, efficient broad-band spectral conversion from UV to NIR as a result of energy transfer (ET) from Ce3+ to Yb3+ in glasses and GCs have also been discussed. 3
2. Material and methods 2.1 Sample preparation Glass
samples
with
nominal molar
composition
28.85SiO2–19.23SrO–28.85BaO–9.62B2O3–4.81La2O3–0.96Al2O3–7.69Al:
1P2O5,
of xCeO2,
yYb2O3 (x = 0.5, 1.0, and 2.0, y = 0; x = 1.0, y = 0.125, 0.25, 0.5, and 1) were fabricated using conventional melt quenching method. For each batch (15 g), analytical reagent-grade SiO2, SrCO3, BaCO3, H3BO3, La2O3, Al2O3, NH4H2PO4 as well as CeO2 and/or Yb2O3 with a purity of 99.99% were weighed accurately and mixed thoroughly in an agate mortar. For the sake of reducing Ce4+ to Ce3+, aluminum powder was incorporated as oxygen absorber. Then, homogeneously mixed starting materials were put into a 30-mL corundum crucible with a lid, which was subsequently transferred into an electric furnace at 1400 °C and kept for a period of 90 min in ambient air. The glass melt was poured into a preheated (200 oC) stainless steel mold and rapidly transferred into a preheated muffle furnace for annealing at 590 oC for 2 h. For the sake of convenience, Ce3+ singly doped and Ce3+/Yb3+ codoped glasses fabricated in this way were labeled as G1─3 and G5─8, respectively. For comparison, the sample with a nominal molar composition of 30SiO2–20SrO–30BaO–10B2O3–5La2O3–5Al2O3: 1P2O5, 2CeO2 was melted in an open crucible and referred to as G4. Subsequently, the bulk glasses were cut into small pieces and heat treated at 780 oC for 4 h to obtain GCs denoted as GC1─8, correspondingly. Finally, all the studied samples were polished to a thickness of 2 mm for optical measurements.
4
2.2 Characterization and measurements Differential scanning calorimetry (DSC) measurement was performed on a Netzsch STA 409EP calorimeter in an argon atmosphere at a heating rate of 10 oC/min. X-ray power diffraction (XRD) patterns were obtained using a Brucker D8-advance X-ray powder diffractometer (Cu Kα, 40 kV/40 mA) for crystalline phase identification. Microstructural characterization of the resultant glass ceramics was conducted by a transmission electron microscope (TEM, JEM-2100F) assembled with the selected area electron diffraction (SAED). TEM specimen was prepared by dipping a drop of sonicated suspension containing fine-ground sample on a carbon coated copper grid, followed by drying under an infrared lamp. Optical absorption spectra were recorded on a Perkin-Elmer Lambda 900 UV/VIS/NIR spectrophotometer with the resolution of 1 nm. Diffuse reflection spectra in the 200─800 nm range were measured with a Shimadzu UV-2600 spectrophotometer using BaSO4 as reference. X-ray photoelectron spectroscopy (XPS) for analyzing the valence of cerium ions was performed using Thermo ESCALAB 250XI spectrometer. The excitation and emission spectra along with decay curves of the fluorescence signals were recorded on an Edinburgh Instruments FLS920 spectrofluorometer equipped with a 450 W Xenon lamp (Xe900) as the excitation source. All the measurements were conducted at room temperature.
3. Results and discussion 3.1 Thermal properties, crystallization, and microstructure Fig. 1 reveals the DSC curves for G2 glass in comparison with the undoped blank glass (denoted as G0). The glass transition temperature Tg is determined to be 590 oC and 600 oC 5
from the DSC curves of G0 and G2, respectively. There is a slight increase in Tg with the CeO2 addition. It is indicative of the strengthening of glass network which can be accounted for by considering that Ce4+ exhibits high field strength and could enter glass network as intermediate [17]. The DSC thermograms of G0 and G2 show a faint exothermic peak at 840 o
C and 850 oC assigned to the crystallization of Ba4La6(SiO4)6O (JCPDS card No.27–0037) as
confirmed by XRD results (Fig.2). Similarly, a slight increase in crystallization peak temperature may be associated with of the network strengthening induced by CeO2.
Fig. 1 DSC traces of G0 and G2 glass samples recorded at a heating rate of 10 K/min. XRD patterns of glasses annealed at 750 oC and 780 oC for 4 h were taken and illustrated in Fig. 2 to identify the crystalline phase precipitated from glass. As noted in Fig. 2(a), G1 sample heat-treated at 750 oC is nearly amorphous. The XRD pattern exhibits only feeble diffraction peaks superimposed on dominant diffuse humps. After thermal treatment at 780 oC for 4 h, intense diffraction peaks assigned to hexagonal Ba4La6(SiO4)6O (JCPDS card No.27-0037) are clearly revealed in the XRD plots of GC samples labeled as GC0-780-4h, GC2-780-4h, GC4-780-4h, and GC8-780-4h. As presented in Fig. 2(b), a small shift of diffraction peaks toward higher angles is noticeable in GC2 sample with addition of 1 mol% 6
CeO2. It is indicative of the lattice contraction induced by the partial substitution of bigger La3+ ions by smaller Ce3+ ions as consistent with our previous research [18].
Fig. 2(a) XRD patterns of GC samples doped with or without cerium ions (labeled as GC0) obtained after heat-treatment at 750 oC and 780oC for 4 h, respectively, and (b) enlarged XRD patterns indicating red shift of diffraction peaks of GC2 compared with GC0. The microstructure of GC8 heat-treated at 780 °C for 4 h is characterized by the transmission electron microscopy. It can be clearly seen from Fig.3(a) that some dark particles are randomly dispersed on the bright background corresponding to glassy matrix. An observation of densely distributed spots forming concentric rings on the corresponding SAED 7
pattern (inset of Fig.3(a)) demonstrates the precipitation of randomly orientated polycrystals in the glass matrix. Furthermore, the HRTEM image of individual crystals (Fig.3(b, c)) exhibits well-resolved lattice fringe pattern, which indicates the high crystallinity of these crystals. The HRTEM image of an individual crystal in Fig.3(c) presents well-resolved lattice fringe with the measured spacing of 0.334 nm corresponding to the orientation [102] of oxyapatite (Ba4La6(SiO4)6O) crystal. This value is smaller than the one evaluated using Bragg equation. For GC0 and GC2 samples, the interplanar spacing determined from [102] diffraction peak is 0.338 nm and 0.337 nm. Thus, it is reasonable to infer that more rare earth (Ce3+ and/or Yb3+) ions are enriched into the precipitated crystals at high doping level, contributing to a steady decrease in interplanar spacing.
Fig.3(a) Bright field TEM image and corresponding selected area electron diffraction pattern (inset) of GC8 heat-treated at 780 °C for 4 h, (b) HRTEM image of dark particles in GC8, and (c) HRTEM image of an individual particle in GC8 showing well-defined (102) lattice fringes of Ba4La6(SiO4)6O crystal with a d spacing of 0.334 nm. 8
3.2 Optical absorption The absorption edge can be evaluated from the measured absorption spectrum of blank glass G0 using the following relation proposed by Mott and Davis [19, 20]:
α hν = B 2 (hν − Eg )r
(1)
where α is the absorption coefficient near the absorption edge derived from the measured absorbance A as a function of frequency, and hν is the incident photon energy. B is a constant called the band tailing parameter, and r is the index which can have values of 1/2 and 2 corresponding to the direct and indirect allowed electronic transition for the absorption. As shown in Fig.4, plotting (αhν)1/r versus photon energy (hν) reveals a linear region, which can be extrapolated to meet the hν axis at (αhν)1/r = 0, thus yielding the value of the optical band gap Eg = 3.3 and 3.94 eV for indirect and direct allowed transitions, respectively.
Fig.4 Variation of (αhν)1/2 (indirect) and (αhν)2 (direct) with (hν) for G0 glass sample. The absorption spectra of glasses (G7, G8) and GCs (GC7, GC8) codoped with cerium and ytterbium ions are presented in Fig.5(a). A broad absorption band peaked at 977 nm can be ascribed to Yb3+: 2F7/2 → 2F5/2 transition. However, the absorption bands of cerium ions 9
cannot be distinguished due to the convergence with inherent absorption cut-off edge of glass. This is especially true in GCs, which exhibit a red shift of the absorption edge due to the scattering from crystallites. As shown in the inset of Fig.5(a), all the obtained glasses appear transparent and pale yellow, while all the corresponding GCs except GC7 are translucent. GC7 sample appears more transparent than the other GCs probably due to the much smaller size of the precipitated crystals to avoid light scattering. This can be explained as follows: the gradual addition of Yb2O3 leads to a decrease in viscosity and an increase in diffusivity, thus favoring the crystallization of apatite. Since the apatite (Ba4La6(SiO4)6O) exhibits around 4.33 network modifiers charges per network former, which is larger than that of the glass matrix (~1.61), the crystallization of such apatite phase enriched in network modifiers would in turn increase the viscosity near the crystal. Hence, further crystal growth can be suppressed by diffusion barriers forming around crystallites [21, 22]. However, as the Yb2O3 content increased to 1 mol%, trace of undissolved particles were visible in GC8 due to its limited solubility. In the process of crystallization, these undissolved particles may act as heterogeneous nucleation site for crystal growth. Eventually, the light scattering by large particles results in the loss of transparency.
Fig.5(a) Absorption spectra of Ce3+/Yb3+ codoped glasses (G7 and G8) and corresponding 10
GCs. Apart from G7, all the absorption curves are shifted upward along the vertical axis for clarity. The inset shows the photograph of these samples. (b) Absorption spectra of G5–8 glasses and GC8 (compared in the inset) transformed from the diffuse reflection spectra. To explore the absorption caused by cerium ions, the diffuse reflection spectra of finely ground powder samples were measured and further transformed into absorption spectra in Fig.5(b) according to the relation A = (1-R)2/R, where R is the reflectivity. The resulting absorption spectra of G5─8 glasses exhibit a broad band in the range of 200─400 nm. It is reported that Ce4+ charge transfer (CT) absorption causes the band peaked at 240 nm with much greater absorption cross section than Ce3+ [23, 24]. Thus, the dominant band peaked at 278 nm may be associated with the absorption due to Ce3+ and small amounts of Ce4+. The shoulder at around 350 nm can be assigned to Ce3+: 4f → 5d transition. It is worthwhile to mention that a dip near 324 nm isolating the two regions is caused by lamp switching. It can be noticed from the inset of Fig.5(b) that the 4f-5d absorption band shifts toward the longer wavelength side with the precipitation of Ba4La6(SiO4)6O crystals in GC8 due to the stronger crystal field. Hence, the selection of Ce3+ as sensitizer ions is justified because its 4f-5d absorption band not only falls into the region of solar spectrum but also in the inefficient spectral response range of c-Si solar cell. 3.3 XPS spectra XPS spectrum was taken for G2 glass to study the valence state of cerium ions. As presented in Fig.6, the Ce 3d core level spectrum is composed of two main broad peaks around 901.5 eV and 890 eV corresponding to Ce 3d3/2 and Ce 3d5/2 multiplet, respectively. It is known that cerium ion can exist in two different oxidation states, i.e., trivalent and 11
tetravalent cerium. Both of these species are characterized by 3d5/2 and 3d3/2 multiplet splitting. The observation of faint peak at a BE of 915.4 eV ascribed to the initial state of the Ce4+ (called "f0" configuration) suggests that there still exists a tiny proportion of residual tetravalent Ce in the present glass [17]. As the same experimental conditions were kept for the reduction of Ce4+ during glass fabrication, the variation of Ce3+ concentration among G1─3 and G5─8 glasses may be assumed to be negligible. By referring to the number of peaks as well as their binding energy position reported in previous works [17, 25], Gaussian type deconvolution of this superimposed spectrum involving Ce3+ and Ce4+ was conducted, which gives eight components as depicted in Fig.6. In comparison with the XPS spectra of CeO2 and Ce2O3 [26], the fitted peaks at 882.3 and 886.4 eV corresponds to 3d5/2 of Ce3+, and those at 901 and 905.2 eV are associated with 3d3/2 of Ce3+. Meanwhile, the contribution from Ce4+ can account for the peaks at 882.3, 890.3, and 895.6 eV corresponding to 3d5/2 along with those at 901, 910, and 915.4 eV for 3d3/2.
Fig.6 XPS spectrum of G2 glass singly doped with 1 mol% cerium ions, curve fitted with eight components contributing to Ce 3d core level peaks. 12
Fig.7 XPS spectrum of G2 glass singly doped with 1 mol% cerium ions, curve fitted with three components contributing to O 1s core level peaks. Fig.7 shows the O 1s core level spectrum for G2, which exhibits an asymmetric band at an average binding energy around 531 eV. This peak can be well fitted using three sub-peaks at 530.2, 531.3, and 532.6 eV representing different types of oxygen sites. Specifically, the peak at 532.6 eV is attributed to the contribution of bridging oxygen (BO). Theoretically, the BO atoms could be expected in the Si–O–Si, Si–O–B, Si–O–Al, and B–O–B configurations for the present glass system. However, further deconvolution of the peak at 532.6 eV with the FWHM of 1.89 eV is of no significance due to the difficulty in evaluating its validity. Consistent with the previous reports [27, 28], the two components at the BE of 530.2 and 531.3 eV can be assigned to non-briding oxygen (NBO) bound to boron and silicon, correspondingly. For the alkaline earth oxides (SrO and BaO) enriched glass studied in this work, the fraction of NBO can be evaluated to be 86.2% using the area ratio of the peaks associated with NBO to the total area of the O 1s spectrum.
13
3.4 Luminescence properties To understand the luminescence properties of Ce3+, emission spectra (λex = 366 nm) of G1─3 glasses singly doped with increasing amount of cerium ions were taken and presented in Fig.8. It is noted that excitation G1 and G2 glasses at 366 nm yields a broad emission band with a peak at 486 nm and a shoulder at 453 nm. It is well-known that the 4f ground state of Ce3+ splits into doublet 2F5/2 and 2F7/2 levels separated by about 2000 cm-1 [29]. With an energy difference of ~1500 cm-1 between 453 and 486 nm, thus the two Ce3+ emission components can be ascribed to 5d→ 4f transition occurring from the excited state down to such multiplets. The mean lifetime of excited Ce3+ ions in G2 is evaluated as 33.8 ns from the luminescence decay curve monitored at 486 nm, which exhibits a double-exponential behavior (Fig.S1). The emission intensity decreases with increasing doping level predominantly due to the quenching effect of Ce4+. Moreover, this emission band is hardly seen in G4.This can be explained by considering the fact that the CT absorption band of Ce4+ overlaps Ce3+ 4f-5d absorption band in the UV region. In this case, some irradiation energy could be transferred to Ce4+, followed by non-radiative decay [23]. The presence of high level of Ce4+ in G3 glass can be deduced from its appearance (see inset of Fig. 8). It has been reported that the tail of CT absorption band of Ce4+ can extend to the visible region, resulting in the color of Ce4+ doped glasses [23]. G3 glass shows brownish color similar to G4 glass obtained in an ambient atmosphere without metallic aluminum powder. On the other hand, it is implied that the amount of Al powder is insufficient for complete reduction of CeO2 at higher doping concentrations. For comparative purposes, Ce3+/Yb3+ codoped G5─8 glasses were fabricated under the same experimental condition with G2. 14
Fig.8 Emission (λex = 366 nm) spectra of G1─4 glasses as a function of Ce3+ concentration. The inset shows the photograph of G0─4 glasses. Fig. 9 gives exemplary excitation and emission spectra of G6 glass. Excitation at 366 nm gives rise to the emission spectrum consisting of two broad emission bands at about 480 nm and 976 nm, assigned to Ce3+: 5d → 4f and Yb3+: 2F5/2 → 2F7/2 transitions, respectively. As Yb3+ posses only one excited (2F5/2) level lying around 10,000 cm−1, thus it is indicative of the occurrence of ET from Ce3+ to Yb3+. The excitation spectra monitored at 480 and 976 nm are compared on the left side of the figure. The 4f → 5d transition of Ce3+ is clearly revealed by a broad excitation band located within the range of 300─450 nm, which is in coincidence with the absorption spectra discussed previously. The excitation band for 480 nm emission exhibits asymmetric feature and can be deconvoluted into three Gaussian peaks with the dominant component at 368 nm (see inset).
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Fig.9 Excitation spectra (left side) of G6 glass monitored at 480 and 976 nm and visible-to-NIR emission spectra under 366 nm excitation. The inset shows the Gaussian deconvolution of the excitation band monitored at 480 nm after baseline subtraction. It is well established that the position of 5d levels of Ce3+ is dependent on nephelauxetic (covalency) effect and crystal field splitting [30]. In the case of glass, the contribution of crystal field splitting is weak. The covalency between Ce3+ and oxygen ligands gives a dominant contribution to centroid shift. On the basis of the aforementioned XPS data regarding O 1s core level, it can be inferred that there exist a large amount of alkaline earth ions (Sr2+ and Ba2+) attached to the non-bridging oxygen. The smaller ionic radius of Sr2+ than Ba2+ implies stronger ability of attracting electron away from oxygen ions, which reduces the Ce─O bonding covalency and thus predicts a smaller centroid shift of the Ce3+ 5d configuration [31]. It is conceivable that the observed broad excitation band represents the contribution of Ce3+ ions on sites with different ligand field strength. As centroid shift tends to decrease with the decrease of Ce─O bonding covalency in the order of BaO→SrO→La2O3, the three deconvoluted components at 398, 368, and 340 nm could be assigned to the 4f → 5d 16
transitions of Ce3+ ions coordinated by oxygen atoms binding with Ba2+, Sr2+, and La3+, respectively. Similarly, a deconvolution of the excitation band monitoring at 976 nm with a Gaussian function was carried out, which gave three components with the dominant one at 340 nm. It is indicated that Ce3+ ions on La3+ neighbor sites play a dominant role in the Ce3+ → Yb3+ ET process. This is understandable since the similar ionic radii of Yb3+ (e.g., 1.07 Å at CN = 7) and La3+ ions (e.g., 1.24 Å at CN = 7 [32]) facilitate Yb3+ to occupy La3+ sites, possibly resulting in the formation of Ce─O─Yb bonds. For such a short interionic distance between Ce3+ and Yb3+ ions, the ET process from Ce3+ to adjacent Yb3+ ions would take place more efficiently. It is worthwhile to mention that the emission intensity of Ce3+ is severely quenched as a result of energy transfer from Ce3+ to Yb3+, thus it is difficult to track the emission intensity changes against time. The luminescence decay curves of Ce3+ in codoped system were not obtained in this work. The NIR emission spectra of Ce3+/Yb3+ codoped glasses and GCs were obtained with the excitation at 340 nm. As presented in Fig. 10(a), there exists a typical broad emission band centered at 1014 nm along with a sharp peak at 976 nm on the spectrum of each sample, which can be attributed to the 2F5/2 → 2F7/2 radiative transition of Yb3+ ions. It is demonstrated that Yb3+ is activated through efficient ET process from Ce3+ to Yb3+. Previous studies have proposed the cooperative energy transfer (CET) mechanism for such ET process [33, 34]. With the energy gap of Ce3+: 4f-5d1 approximately twice the energy of Yb3+: 2F5/2 → 2F7/2 transition, it is assumed that one excited Ce3+ ion transfers energy to two adjacent Yb3+ ions, followed by the generation of two NIR photons. However, the experimental quantum yield of Yb3+ emission below 100% failed to support the model [5]. This result can be understood by 17
considering that CET is a second order process with the probability of three orders of magnitude lower than a first-order one [35]. Additionally, other researchers consider that ET from Ce3+ to Yb3+ proceeds probably through Ce4+─Yb2+ charge transfer state (CTS) [5, 36]. It has been reported that Ce4+─Yb2+ CTS forms via the redox reaction: Ce3+ + Yb3+ → Ce4+ + Yb2+ with the participation of bridge O2- between Ce3+ and Yb3+ ions [37]. To further explore the ET mechanism, the decay curve of Yb3+emission at 1016 nm in GC7 were recorded with an excitation wavelength of 353 nm. As shown in Fig. 10(b), the dynamic photoluminescence of Yb3+ is well depicted by the initial rise and decay components, which is consistent with the previous study [5]. At early stage (almost within 150 µs after the irradiation), the Yb3+ emission is enhanced remarkably, and then fades with time. From the above discussion, it can be speculated that ET from Ce3+ to Yb3+ cannot be a direct process since the rise time for the NIR luminescence of Yb3+ is longer than the lifetime of Ce3+ (33.8 ns). The most probable ET pathway from Ce3+ to Yb3+ in this system can be considered as a single-step indirect process via intermediate Ce4+─Yb2+ CTS. The emission intensity is found to increase gradually with increasing Yb3+ concentration from 0.25 up to 1 mol %, and begins to fall at the concentration of 2 mol % due to concentration quenching effect [33]. The integral intensity of emission around 1000 nm in GC5─7 exhibits a significant enhancement relative to corresponding glasses. This can be associated with the incorporation of active centers into crystal phase as confirmed by XRD and HRTEM results, thus promoting the NIR luminescence by providing the preferred crystalline enviroment and shortening the interionic distance between Ce3+ and Yb3+. However, there exhibits a more severe quenching of the NIR luminescence in GC8, which is 18
possibily caused by rapid energy migration among neighboring Yb3+ ions at high doping concentration.
Fig.10(a) NIR emission spectra of Ce3+/Yb3+ codoped G6 glass in comparison to GC samples as a function of Yb3+ content under 340 nm excitation. Inset: the dependence of integrated NIR emission intensity on Yb3+ doping level for glasses and GC samples. (b) The decay curve of Yb3+emission in GC7 monitored at 1016 nm upon the excitation at 353 nm.
4. Conclusions In this paper, the GCs containing Ba4La6(SiO4)6O crystals were prepared from the glass system SiO2–SrO–BaO–B2O3–La2O3–Al2O3. XRD and TEM characterization of the GCs doped with cerium and/or ytterbium ions not only clearly confirms the precipitation of oxyapatite polycrystals in the glass matrix but also indicates the partition of rare earth ions 19
into crystals. Optical absorption spectrascopy identifies a broad band in the range of 200─400 nm contributed predominantly by the trivalent cerium ions (Ce3+) as evidenced by XPS results. Upon 366 nm excitation, the Ce3+/Yb3+ codoped glass exhibits a broad-band Ce3+ emission around 480 nm assigned to the 5d → 4f transition and intense NIR luminescence of Yb3+ as a result of ET from Ce3+ to Yb3+ occurring probably via intermediate Ce4+─Yb2+ CTS. Moreover, it is found that Ce3+ ions on La3+ neighbor sites play a dominant role in the Ce3+ → Yb3+ ET process. A significant enhancement (about threefold) in NIR luminescence was achieved in GC compared to the glasses with increasing Yb3+ concentration up to 1 mol%. Therefore, the oxyapatite based GCs can be considered as promising hosts for efficient broad-band spectral conversion from UV to NIR, which is potentially applicable in enhancing the efficiency of c-Si solar cells.
Acknowledgments This work was financially supported by NSFC (Grant No. 51602125), Fund of University of Jinan (Grant No. XBS1353 and XKY1403) and the Open Fund of the State Key Laboratory of Luminescent Materials and Devices (2016-skllmd-10).
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Declaration of interests √☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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