Luminescence properties, centroid shift and energy transfer of Ce3+ in aqueous chloride solutions

Journal of Luminescence 146 (2014) 440–444

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Luminescence properties, centroid shift and energy transfer of Ce3 þ in aqueous chloride solutions Jiwei Wang a, Yong Mei a, Peter A. Tanner b,n a

Faculty of Physics, Liaoning University, Shenyang 110036, PR China Department of Science and Environmental Studies, The Hong Kong Institute of Education, 10 Lo Ping Road, Tai Po, New Territories, Hong Kong S. A. R., PR China

b

art ic l e i nf o

a b s t r a c t

Article history: Received 4 August 2013 Received in revised form 9 October 2013 Accepted 12 October 2013 Available online 19 October 2013

This paper focuses upon three themes: all related to aqueous cerium chloride solutions. First, the features in the absorption spectra of CeCl3 solutions do not shift noticeably with concentration and are at similar energies to bands in the solid-state absorption spectrum of CeðH2 OÞ39 þ , with the exception ofhthe weak band at 297 nm which is due to CeðH2 OÞ38 þ . The broad emission band in solution is i only due to CeðH2 OÞ38 þ n and the emission quenches at concentrations 40.06 M. Bands in the excitation spectra of aqueous CeCl3 solutions apparently change position with increasing concentration, due to absorption by CeðH2 OÞ39 þ which does not contribute to emission. At concentrations above 1 M, there is total extinction of incident radiation for wavelengths shorter than 310 nm. Second, this system is chosen to illustrate the revised calculation of centroid shift, by taking into account the vibronic nature of spectral features, in contrast with the pure electronic transition of the free ion. Similar calculations are applicable to other Ce3 þ systems. Thirdly, excitation spectra are employed to demonstrate the energy transfer occurring from Ce3 þ to Tb3 þ and Eu3 þ in aqueous chloride solutions, which is an unusual energy transfer, occurring from 5d to 4f states and between hydrated lanthanide ions in solution. & 2013 Elsevier B.V. All rights reserved.

Keywords: Energy transfer Hydrated lanthanide ions Centroid shift Photodissociation 5d–4f transition

1. Introduction The emission intensities of lanthanide ions (Ln3 þ ) in aqueous solutions are usually very weak due to strong nonradiative quenching by O–H vibrations. However Gd3 þ , Tb3 þ , Eu3 þ and Ce3 þ ions do emit characteristic luminescence. In these cases the multiphonon nonradiative processes are slower due to the large energy gaps below the luminescent terms. The emission of Ce3 þ ions is much stronger than that of other Ln3 þ because it corresponds to a first-order allowed 5d1–4f1 transition, so that it is visible even in solutions of very low concentration. The role that Ce3 þ plays in solution has been especially studied in this work via room temperature and liquid nitrogen temperature excitation and emission spectra of CeCl3 solutions. The energy transfer studies of Ln3 þ in solution have been largely confined to ligand – Ln3 þ energy transfer by the antenna effect [1–4]. There are several papers about energy transfer between Ln3 þ in the environments of micelles [5,6], where lanthanide complex anions are associated with a cation micellar surface. Energy transfer from Tb3 þ to other Ln3 þ was measured in solution from luminescence quenching, with comparisons of the nature of anion and solvent [7], and we

have previously reported the energy transfer of Gd3 þ to other Ln3 þ ions in solution [8,9]. In this work, we demonstrate the energy transfer from Ce3 þ to Tb3 þ and Eu3 þ in aqueous chloride solutions. This transfer is unique because it corresponds to that from the 5d state to 4f states, and also because it is between hydrated lanthanide ions in solution. 2. Experimental Rare earth oxide powders: CeCl3 99.9% (Strem Chemicals), Tb4O7, Eu2O3 99.99% (International Laboratory, USA), were dissolved in aqueous hydrochloric acid (Scharlau Chemie, Germany) in order to obtain solutions of high concentration. The solutions were poured into quartz cells with 1 cm path length for optical measurements. Absorption spectra were measured by a PerkinElmer Lambda 19 UV/VIS/NIR spectrometer using a resolution of 2 nm. Emission and excitation spectra were acquired by a JobinYvon Horiba Fluorolog spectrometer with resolution of up to 1 nm. The experiments were also performed by housing the quartz cell in a liquid nitrogen dewar flask. 3. Results and discussion

n

Corresponding author. Tel.: þ 852 902 906 10. E-mail address: [email protected] (P.A. Tanner).

0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.10.030

The absorption spectrum of Ce3 þ doped into solid La (C2H5SO4)3  9H2O, which has a tricapped trigonal pyramidal

J. Wang et al. / Journal of Luminescence 146 (2014) 440–444

arrangement of aqua ligands, exhibits bands at 256, 238, 224, 211 and 200 nm [10]. This is consistent with D3h site symmetry of þ Ce3 þ , with transitions from a Γ7 ground state to D3=2 (Γ7 þ Γ9) and þ D5=2 (Γ7 þΓ8 þΓ9) crystal field states. Fig. 1 shows the absorption spectra of Ce3 þ at various concentrations in aqueous chloride solutions. The bands at 252, 238, 221 and 211 nm are at similar energy to those above and are assigned to the lowest four of the five 4f1-5d1 electronic transitions of the ion CeðH2 OÞ39 þ , since also the spectrum is independent of the anion present in the solution [11]. From Raman and X-ray diffraction results [12] it has been concluded that the first coordination shell of the cations comprises aqua ligands and that the cations are arranged highly symmetrically. The structure of the absorption bands does not change or shift noticeably with increasing Ce3 þ concentration or changing pH. The fifth absorption band has been assigned at 200 71 nm [13,14]. The peak maxima of the above absorption bands have been taken by Dorenbos in order to calculate the centroid shift of 5d1, i.e. the shift in the 5d1 barycentre from the free ion value of 51,213 cm  1 (6.35 eV) [15]. The values obtained were 0.800 eV for Ce3 þ in La (C2H5SO4)3  9H2O, and 0.796 eV for CeðH2 OÞ39 þ . These nephelauxetic shifts appear to be too small, and since they are derived from the comparison of vibronic energies in the condensed phases with the zero phonon line energy in the gaseous state, an alternative logic is now provided in the derivation of these quantities. The 4fN4fN 15d1 absorption spectra of Ce3 þ comprise vibrational progressions based upon zero phonon lines, with the strongest ones being in totally-symmetric vibrational modes, so that peak maxima represent the transition to a particular vibrational level of the excited 5d1 state. We have observed that this corresponds to v′¼ 1 in a case where the vibrational progressions are well-resolved for Ce3 þ [16]. The OH2 vibration in the Raman spectrum of an aqueous solution of Ce3 þ has a maximum intensity at 3405 cm  1 [17]. Hence the lowest 4 f1–5d1 zero phonon line is estimated to be lower by roughly this energy from the vibronic peak, at  35,660 cm  1 for CeðH2 OÞ39 þ (aq). An alternative estimation can be made by using the energies of the 5d1-4f1 emission bands of CeðH2 OÞ39 þ in La (C2H5SO4)3  9H2O, at 30,000 and 32,000 cm  1. These bands represent seven unresolved transitions to the spin-orbit split 4f1 2F5/2, 2F7/ 3þ . From the comparison of the emission and 2 multiplets of Ce absorption spectra of La(C2H5SO4)3  9H2O:Ce3 þ in Fig. 2 of [14], the zero phonon line is estimated from extrapolation at  35,680 cm  1. The first absorption maximum is then at  3383 cm  1 to higher energy. Although these are estimated values, note that the local coordinated-OH2 excited state frequency for the first coordination

441

sphere of Ce3 þ is expected to be slightly lower than the bulk OH2 ground state frequency probed by Raman spectroscopy, where the O–H bond is slightly stronger. A similar conversion of the other absorption peak maxima into zero phonon line energies leads to the revised centroid shift of  1.2 eV. A similar argument can be applied to other Ce3 þ systems when determining the centroid shift, taking into account their vibrational behaviour. An additional weak band at 297 nm is present in Fig. 1. This additional band has been associated with a ligand dissociated species, and not with a proton-dissociated species since it is independent of HClO4 acid concentration [18]. In the case of aqueous solutions of cerium(III) perchlorate, the presence of an isosbestic point in the absorption spectra of solutions with different concentrations indicated the presence of only two species [18]. The additional absorption band has therefore been associated with the equilibrium between nine and eight aqua-coordinated Ce3 þ species, with the former dominant [10,11,14]. The weak band at 297 nm becomes more evident at high Ce3 þ concentrations and is assigned to the lowest 4f1-5d1 absorption band of ½CeðH2 OÞ38 þ  [14]. In the presence of other anions, the formation of other 8-coordinated species has however been argued in the literature, also in the form of second sphere complexes ½CeðH2 OÞ38 þ X  [11,17,19–21]. The emission bands of CeðH2 OÞ39 þ in La(C2H5SO4)3  9H2O:Ce3 þ at 312 nm and 333 nm [14] are not apparent in aqueous solution. It is generally accepted that the excited state ion ½CeðH2 OÞ39 þ n rapidly dissociates to give ½CeðH2 OÞ38 þ n [10,14]. The change in coordination number may be anticipated due to the excitation into the 5d1(eg) orbital [10,22]. Excitation into the Ce3 þ absorption bands in Fig. 1 gives a 5d1-4f1 emission spectrum, which consists of a broad feature between 310 and 440 nm with maximum intensity at 360 nm as shown in Fig. 2. This broad band is assigned to unresolved vibronic emission from ½CeðH2 OÞ38 þ n to the 4f1 2F5/2,7/2 J-multiplet terms. The emission is strong at very low concentration of Ce3 þ , and this contrasts with the weak intraconfigurational emission transitions of other Ln3 þ in solution. The longer wavelength emission of ½CeðH2 OÞ38 þ , compared with ½CeðH2 OÞ39 þ , results from the increased 5d orbital splitting associated with a smaller Ce–ligand distance, so that the lowest 5d state is depressed. The emission band is unstructured, unlike those for most Ce3 þ systems, where two partially-resolved features are observed corresponding to transitions to the distinct ground state J-multiplets. One or two contributing reasons could account for the large breadth and the unstructured character of the emission band. First, the presence of species with different Ce3 þ –Cl  distances in ½CeðH2 OÞ38 þ 3Cl  (aq) would lead to different transition energies. Second, the OH2 progression interval is

3

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Absorbance

0.05 M 0.2 M 0.75 M 3 M 6 M

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0.001 M 0.004 M 0.015 M 0.06 M 0.25 M 1M

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300

0.00025 M 0.001 M 0.004 M 0.015 M 0.06 M 0.25 M 1M

400

Fig. 1. 300 K absorption spectra of different concentrations of aqueous CeCl3 solutions, from 0.00025 M (bottom) to 1 M (top). The inner figure represents the absorption spectra of different concentrations of HCl solution, with red-shift from 0.05 M to 6 M.

Intensity (arb. units)

3

80

40

0 300

350 400 Wavelength (nm)

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Fig. 2. Emission spectra of different concentrations of CeCl3 solution excited at 300 nm. The concentrations (M) from the bottom of the figure upwards are 1, 0.25, 0.001, 0.004, 0.015, 0.06, respectively.

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J. Wang et al. / Journal of Luminescence 146 (2014) 440–444

greater than the J-splitting so that multiplet structure will be washed out. Inspection of Fig. 2 shows that the emission at first increases in intensity with concentration, but is then quenched above 0.06 M. Azhena et al. [19] discerned shorter- and longer-lived species in their study of cerium(III) acetate complexes in aqueous solution and concluded that the shorter-lived species was more emissive and that although the addition of acetate did not change the type of complex formed, it preferentially quenched the longerlived species so that the overall emission intensity decreased. Kaizu et al. [10] attributed the longer-lived excited species and longerwavelength emission in aqueous solutions of cerium(III) chloride to ½CeðH2 OÞ38 þ , whereas a faster (ps) decay and shorter wavelength emission was associated with ½CeðH2 OÞ39 þ n. The rate of interconversion from the 9- to 8-coordinate species was much faster than the individual decay rates. Fig. 3 shows the excitation spectra of the Ce3 þ 5d1-4f1 emission at various concentrations of Ce3 þ . Three phenomena occur with increasing Ce3 þ concentration. Two bands at 260 nm and 300 nm are observed for 0.00025 M Ce3 þ . On increasing the concentration, the bands sharpen; the intensity of the lower energy component dominates over the higher energy one; and the bands shift to lower energy. For concentrations of Ce3 þ above 1 M, there is total extinction of incident radiation for wavelengths shorter than 310 nm. The first spectrum, at concentration of 0.00025 M, shows two bands at 259 nm and 298 nm. The latter corresponds to the lowest 5d1 state CeðH2 OÞ38 þ . The former is just to longer wavelength than the absorption maximum (at 252 nm) of the lowest CeðH2 OÞ39 þ band. The emission therefore does not include the participation of higher 5d1 states of CeðH2 OÞ39 þ . This could be an indication that the nonradiative relaxation rates from the upper CeðH2 OÞ39 þ states to the lowest one is slower than the rate of coordination number change. Alternatively, the absorption of incident radiation is dominated by CeðH2 OÞ39 þ , but excitation transfer to the 8-coordinate species does not occur. The final plot, at 1.0 M concentration, shows

a band at the absorption range of CeðH2 OÞ39 þ , at 318 nm. Clearly, the sensitization of emission by the 9-coordinate species by CeðH2 OÞ38 þ does not therefore occur at this concentration. It appears from the spectra at intermediate concentrations that the extent of selfabsorption and photoextinction by CeðH2 OÞ39 þ increases with concentration. This type of behaviour is typical of inner filter effects [23]. Fig. 4 exhibits the contrast between the excitation spectra of the room temperature solution and the frozen solid for both low and high concentration CeCl3 samples. The emission was monitored at 370 nm, which is the same peak maximum as that from CeðH2 OÞ38 þ in solution. As above, in the solution spectra there are bands at 267, 298 nm: Fig. 4(a), and 329 nm: Fig. 4(b). In Fig. 4(a), for the low concentration sample, the band at 300 nm changes to a very weak absorption band at 322 nm in the frozen solid, showing that the proportion of CeðH2 OÞ38 þ in the frozen solid is smaller. One feature remains, at 256 nm, similar to the wavelength in solid La (C2H5SO4)3  9H2O:Ce3 þ . When monitoring different emission wavelengths at 390, 360 and 347 nm, the location of this band in the frozen solid is unchanged. For the 2.8 M Ce3 þ sample shown in Fig. 4(b), the narrow band at 329 nm in solution at high concentration vanishes completely in the frozen solid. There are two bands at 272 nm and 303 nm in the excitation spectrum of the solid, indicating that CeðH2 OÞ38 þ is present in the frozen solid for the high concentration sample. We also report herein the energy transfer from Ce3 þ to Eu3 þ and Tb3 þ in solution. Fig. 5 shows the excitation spectrum of 0.03 M EuCl3 solution, and also of a solution with the same Eu3 þ concentration mixed in 3 M CeCl3 solution. In both cases the 5D0-7F1 emission transition of Eu3 þ is monitored. The comparison with the excitation spectrum of 3 M CeCl3 (in the inset of the figure) shows that the feature at 333 nm, corresponding to an absorption band of Ce3 þ , is present in the excitation spectrum of (Ce, Eu)Cl3 solution (where the peak is marked). This demonstrates that energy transfer occurs from Ce3 þ to Eu3 þ in solution. Energy transfer can also occur from Ce3 þ to Tb3 þ as shown from the excitation spectrum of

0.001 M

0.00025 M

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240

Wavelength (nm) Fig. 3. Excitation spectra of different concentrations of CeCl3 solution by monitoring the 370 nm emission band.

J. Wang et al. / Journal of Luminescence 146 (2014) 440–444

200

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0.03 M TbCl3 solution

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Fig. 6. Excitation spectrum of 0.03 M TbCl3 solution (red: thick line), and of 0.03 M Tb3+ in 2.5 M CeCl3 chloride solution (black: thin line), monitoring 547 nm Tb3+ emission. The inner figure is the excitation spectrum of 2.5 M CeCl3 solution monitoring 370 nm Ce3+ emission.

Solution Frozen

transfer mechanism from the Ce3þ donor to Ln3þ acceptor is most likely dipole–dipole between aquated lanthanide ions.

4. Conclusions

250

350

300

Wavelength (nm) Fig. 4. Excitation spectra of CeCl3 solutions (dashed line) at room temperature (RT) and frozen solutions (solid line) at liquid nitrogen temperature (N2) by monitoring the emission at 370 nm. (a) 0.015 M, (b) 2.8 M.

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absorbed

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½CeðH2 OÞ38 þ n.

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The 5d1–4f1 transition of Ce3 þ exhibits strong luminescence in aqueous solution at very low concentrations. As the concentration of Ce3 þ increases, the bands present in the 5d1–4f1 excitation spectrum shift to low energy and consequently form a condensed narrow feature peaking at between 320 nm to 340 nm for Ce3 þ concentrations above 1 M. The unusual behaviour is attributed to an excited state dissociation of ½CeðH2 OÞ39 þ  to form ½CeðH2 OÞ38 þ n which subsequently luminesces. The exciting radiation is strongly

0.03 M EuCl3 solution 45

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Fig. 5. Excitation spectra of 0.03 M EuCl3 solution (red: thick line), and 0.03 M Eu3+ in 3 M CeCl3 solution (black: thin line), monitoring 592 nm Eu3+ emission. The inner figure is the excitation spectrum of 3 M CeCl3 solution by monitoring 370 nm Ce3+ emission.

the Tb3 þ 5D4-7F5 emission in Fig. 6 where the band due to Ce3 þ is marked. In both cases, the high concentration of CeCl3 in solution produces an extinction of spectral bands at shorter wavelengths than the 330 nm CeðH2 OÞ39 þ band, irrespective of whether they are due to Ce3 þ , Eu3 þ or Tb3 þ . The energy transfer processes are not radiative, i.e., not processes where Ce3þ emits 5d1-4f1 and the photons are absorbed by Tb3 þ or Eu3 þ leading to their emissions. The energy transfer from Ce3þ to Ln3 þ is a nonradiative energy transfer process across the H2O layers around Ce3 þ as well as Tb3þ (or Eu3þ ), and this long-range dynamical

½CeðH2 OÞ39 þ 

but

emission

only

occurs

from

The inner coordination sphere of Ce3 þ has been investigated by EXAFS [24], where the accuracy largely depends upon the modelling approach used. It was concluded that Ce3 þ in a solution of 0.25 M HCl has an aqua coordination number of 9.3 70.4. However for 14 M Cl  concentration, evidence showed that between one and two Cl  ions enter the inner coordination sphere. In the present study, the Cl  concentration has been at rather lower levels and there is no striking evidence for a second emissive species. This conclusion is in line with a recent study of luminescence quenching in aqueous cerium(III) chloride solutions [25]. The addition of other components to the aqueous solution, such as alcohols, produces changes in the inner coordination sphere, also with spectral changes involving longer-wavelength absorption band(s) [20,26]. Our study of frozen solutions shows the presence of a band at  300 nm which is characteristic of the 9-coordinate species. The only other study of frozen Ce3 þ aqueous solutions that we are aware of has utilised electron paramagnetic resonance to demonstrate inhomogeneity in the ground state coordination when an additional component such as ethanol is present [27]. We have employed the CeðH2 OÞ39 þ complex to illustrate the problems associated with the derivation of the 5d1 centroid shift. Basically, these result from the comparison of the pure electronic spectrum of the free ion with vibronic spectra in condensed phases. The error in zero phonon line location in the condensed phase will be largest for Ce3 þ systems with high frequency totallysymmetric vibrations, whereas the error will be much smaller, for example, in bromo- or iodo-complexes with low frequency modes. The location of the zero phonon line in the condensed phase can

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be accomplished by one of the two methods employed herein. However, in many cases the vibronic progressions will involve the participation of many modes and the situation is more complex than for the one high frequency OH2 mode herein. Energy transfer from Ce3 þ to Tb3 þ and Eu3 þ is clearly demonstrated by the excitation spectra of aqueous solutions, and the use of the 5d1 ion Ce3 þ as donor in solution is reported for the first time. This transfer process occurs between hydrated Ce3 þ and Eu3 þ (or Tb3 þ ) in conditions of high donor concentration.

Acknowledgements This work is supported by the National Natural Science Foundation of China (project 11274150), and Program for Liaoning Excellent Talents in University (LNET, LR2013001). References [1] N. Sabbatini, M. Guardigli, I. Manet, Handbook on the Physics and Chemistry of Rare Earths, 23, 1996; 69. [2] P.R. Selvin, IEEE J. Sel. Top. in Quantum Electron. 2 (1996) 1077. [3] B.S. Panigrahi, J. Lumin. 82 (1999) 121. [4] S. Quici, M. Cavazzini, G. Marzanni, G. Accorsi, N. Armaroli, B. Ventura, F. Barigelletti, Inorg. Chem. 44 (2005) 529.

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