Charge transfer luminescence of hafnates under synchrotron vacuum ultraviolet excitation

Abstract

Under synchrotron vacuum ultraviolet excitation, three different hafnates (BaHf(PO₄)₂, BaHf(BO₃)₂ and BaHfSi₃O₉) show self-activated luminescence with peaks ranging from 275–335 nm, which is attributed to the charge-transfer luminescence from the hafnate (HfO₆⁸⁻) group. Upon doping Eu²⁺ into BaHfSi₃O₉, host sensitization of Eu²⁺ emission via hafnate intrinsic emission is demonstrated.

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Oxide compounds consisting of transition-metal ions without d-electrons, such as titanates, vanadates, molybdates, niobates and tungstates, are known to luminesce.¹ The mechanism responsible for the observed luminescence is the ligand-to-metal charge transfer (LMCT).¹ The corresponding charge transfer absorptions in these compounds are found to be generally situated in the ultraviolet region.¹ Recent studies on LMCT from transition-metal ions have been extended to zirconates, in which Zr⁴⁺ with a d⁰ configuration plays an important role.^2,3 Unlike these transition-metal containing compounds mentioned above, the charge transfer (CT) absorptions of zirconates are found to locate at a much higher energy level,^2,3 and generally it requires the excitation light to have an energy in the vacuum ultraviolet region (VUV; E > 50 000 cm⁻¹; λ < 200 nm) so that zirconates can be excited into their charge transfer state.^2,3 Hf⁴⁺ belongs to the same fourth group element as that of Ti⁴⁺ and Zr⁴⁺, thus CT luminescence is expected from hafnates as well. However, after examining the existing literature, only a few works on the luminescence of hafnates were reported, among which most are focused on the X-ray excited luminescence,^4–7 partially for the purpose of developing phosphor for medical X-ray imaging applications. Examples are BaZr_1−xHfPO₄,⁴ Li₂HfO₃,⁵ HfP₂O₇,⁶ Hf_1−xZr_xGeO₄.⁷ From these works, we aware that hafnates can give luminescence upon X-ray excitation, but information regarding their absorption is still rare. The lack of studies for the charge transfer luminescence from hafnates is presumably due to the technique and method of conventional UV excitations that are unable to excite hafnates, as is the case for zirconates. With the advancement of synchrotron radiation, it is possible to investigate the photoluminescence properties of phosphors in VUV region. Investigation of spectroscopic properties of a material in the VUV region provides important and instructive information needed for engineering VUV phosphors used for PDPs and Hg-free lamp. In this paper, we investigated the CT luminescence of three different hafnates (BaHf(PO₄)₂, BaHf(BO₃)₂ and BaHfSi₃O₉) which have HfO₆⁸⁻ group in their crystal structure using synchrotron VUV excitation. In addition, Eu²⁺ has been widely used as the luminescent center for phosphor applied in lighting and displays,⁸ with an aim to develop new VUV phosphor, we also studied the VUV-excited luminescence of Eu²⁺-doped BaHfSi₃O₉ and demonstrated the unprecedented host sensitization of Eu²⁺ emission using hafnate intrinsic emission.

Samples of undoped BaHf(PO₄)₂, BaHf(BO₃)₂, BaHfSi₃O₉ and Eu²⁺-doped BaHfSi₃O₉ were prepared in powder form by using high temperature solid state reaction methods. The phase purity of all samples was checked by using powder X-ray diffraction (XRD) analysis. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra were measured at BL03A beamline of the National Synchrotron Radiation Research Center (NSRRC) in Hsinchu, Taiwan. Samples synthesis details and experimental setup for photoluminescence spectra are provided as Supplementary Information.

Fig. 1 shows the XRD patterns of BaHf(BO₃)₂, BaHf(PO₄)₂ and BaHfSi₃O₉ synthesized in present work. Since no reported standard XRD pattern is available for BaHf(BO₃)₂ and BaHfSi₃O₉, their phase purity was identified by using the XRD data of the corresponding zirconate as a reference. The XRD patterns of the three hafnates are in good agreement with the reference data reported in ICSD no. 254690 for BaHf(PO₄)₂, ICSD no. 95527 for BaZr(BO₃)₂ and ICSD no. 70105 for BaZrSi₃O₉, with the exception of an unknown phase marked by an asterisk (I% < 4%, 2θ = 21.1°, 28.3° and 32.8°) observed in BaHfSi₃O₉ (Fig. 1). This observation indicates that each hafnate retained the same crystal structure as that of its corresponding zirconate. Previous structural study on BaHf(PO₄)₂ (ref. 9), BaZr(BO₃)₂ (ref. 10) and BaZrSi₃O₉ compounds (ref. 11) has shown that a Zr–O octahedron unit exists in their crystal structures. Due to the similar XRD patterns between the zirconate and hafnate, we suggest the hafnium is six-coordinated in all of the three hafnates under investigation.

Fig. 1 XRD patterns of BaHf(BO₃)₂, BaHf(PO₄)₂ and BaHfSi₃O₉.

Fig. 2 shows the normalized PLE and PL spectra of BaHf(BO₃)₂, BaHf(PO₄)₂ and BaHfSi₃O₉. Under excitation at 172 nm, BaHf(PO₄)₂, BaHf(BO₃)₂ and BaHfSi₃O₉ show broad emission with maxima at 335 nm, 320 nm and 275 nm, respectively. We set the excitation light at 172 nm because in practice the available VUV light source (Xe/Ne excimer discharge) gives the main emission at this wavelength. This observation suggests that hafnates may serve as UV-emitting phosphors and find their new applications in transcription of repair enzymes and treatment of skin diseases such as psoriasis et al.¹² In analogy to the case of the VUV-excited luminescence of zirconates,^1–3 the observed broad emission is assigned to the CT luminescence from the hafnate (HfO₆⁸⁻) group, which can be explained as the recombination of the excited electron on Hf⁴⁺ ions with hole left on O²⁻ ligand during the CT absorption process. In reality, it is considered that the charge transfer transition doesn't involve in transferring one electron but does involve a considerable reorganization of the charge density distribution around the metal ion.¹³ The PLE spectra for these three hafnates obtained by monitoring the emission maximum of each compound are all composed of an intense band in the range of 125–200 nm with peaks at 185 nm, 184 nm and 176 nm for BaHf(PO₄)₂, BaHf(BO₃)₂ and BaHfSi₃O₉, respectively, which is attributed to the charge transfer transition from ligand O²⁻ to metal Hf⁴⁺, i.e., electron is transferred from the 2p orbital of the surrounding O²⁻ ions into the empty 5d orbital of Hf⁴⁺. After electron transfer to Hf⁴⁺, the hole appears to be distributed over ligands around the Hf³⁺ ion. In the reverse process, the radiative recombination of electron at Hf³⁺ (formed after electron transfer) with the hole localized on O²⁻ gives rise to the observed charge transfer luminescence. According to the Forster–Dexter energy transfer theory,¹⁴ resonant energy transfer from a sensitizer to an acceptor happens when certain conditions are met, i.e., (1) a considerable spectral overlap between the sensitizer's emission and acceptor's absorption, and (2) a reasonable distance between the sensitizer and acceptor.¹⁴ In our previous work, Eu²⁺ has been shown to have a broad 4f–5d absorption ranging from 280–400 nm in BaHfSi₃O₉–Eu²⁺,¹⁵ while in present work the hafnate group in BaHfSi₃O₉ gives broad UV emission with maximum at 275 nm under VUV excitation (see Fig. 2). This fact provides the beneficial conditions for energy transfer from hafnate group to Eu²⁺ based on the Forster–Dexter energy transfer theory, and implies that the CT emission from hafnates could be utilized to sensitize Eu²⁺ emission in BaHfSi₃O₉–Eu²⁺ upon VUV excitation. To see whether sensitization of Eu²⁺ emission by hafnates CT luminescence occurs, we measured the PL spectra of BaHfSi₃O₉ doped with various concentrations of Eu²⁺ (see Fig. 3) under 172 nm excitation. When 2% Eu²⁺ is introduced into BaHfSi₃O₉, it is found that the CT luminescence from hafnate is greatly reduced. With further increasing the Eu²⁺ doping concentration, accompanied by an increase of Eu²⁺ 5d–4f emission at 474 nm, the charge transfer luminescence from hafnate decreases further, demonstrating that energy transfer from hafnate group to Eu²⁺ indeed occurs via hafnate CT luminescence. The CT luminescence from hafnate group is almost quenched due to such energy transfer when the Eu²⁺ doping concentration is increased to 6%. With increasing Eu²⁺ doping concentration, more Eu²⁺ ions are distributed in the host matrix, thus the energy transfer probability between hafnate group and Eu²⁺ is increased, resulting in the observation that the CT luminescence decreases while the Eu²⁺ emission increases with increasing Eu²⁺ dopant concentration. When Eu²⁺ doping concentration is increased to 14%, Eu²⁺ emission starts to decrease due to concentration quenching.

Fig. 2 PL and PLE spectra of BaHf(BO₃)₂, BaHf(PO₄)₂ and BaHfSi₃O₉.

Fig. 3 PL spectra of BaHfSi₃O₉:xEu²⁺ (0 ≤ x ≤ 18%) under excitation at 172 nm.

Fig. 4 presents the PL excitation spectra of a 2% Eu²⁺ doped sample detected by monitoring the two emission bands located at 275 nm and 474 nm together with the PL spectrum of host matrix BaHfSi₃O₉. The PLE spectrum detecting Eu²⁺ emission and that detecting hafnate CT luminescence has a similar absorption band below 200 nm assigned to Hf⁴⁺–O²⁻ charge transfer absorption. The small shift for the Hf⁴⁺–O²⁻ charge transfer absorption band observed in the PL excitation spectra probably due to the emission grating blazes differently at 275 nm and 474 nm. The presence of the Hf⁴⁺–O²⁻ CT absorption band in the excitation spectrum monitored within the Eu²⁺ emission further indicates that the energy transfer from hafnate group to Eu²⁺ ions via Hf⁴⁺–O²⁻ CT luminescence has occurred. The two additional bands above 200 nm in the PLE spectrum monitoring the Eu²⁺ emission (that are not observed in the PL spectrum monitoring Hf⁴⁺–O²⁻ CT luminescence) are assigned to the 4f–5d absorption of Eu²⁺ due to crystal field splitting. The perfect spectral overlap between Hf–O CT emission and Eu²⁺ 4f–5d absorption provides favorable conditions for energy transfer from hafnate group to Eu²⁺. Process of Hf⁴⁺–O²⁻ CT luminescence and energy transfer from hafnate intrinsic emission to Eu²⁺ is schematically illustrated in Scheme S1 (ESI†).

Fig. 4 PLE spectra of BaHfSi₃O₉–2% Eu²⁺ monitoring Hf–O charge transfer emission at 275 nm (red solid line) and Eu²⁺ 5d–4f emission at 474 nm (black solid line) together with the PL spectrum of host BaHfSi₃O₉ (blue dot line), respectively.

In summary, photoluminescence of three different hafnium-containing compounds, BaHf(PO₄)₂, BaHf(BO₃)₂ and BaHfSi₃O₉, has been studied using synchrotron vacuum ultraviolet excitation. These three hafnates show absorption in the region of 125–200 nm with a maximum close to 180 nm, which results from charge transfer from O²⁻ ligand to Hf⁴⁺ metal. Under VUV excitation at 172 nm, BaHf(PO₄)₂, BaHf(BO₃)₂ and BaHfSi₃O₉ show self-activated luminescence peaking at 335 nm, 320 nm and 275 nm, which is attributed to the charge-transfer luminescence from the hafnate (HfO₆⁸⁻) group, and can be rationalized as the recombination of the excited electron on Hf⁴⁺ ions with the hole left on the O²⁻ ligand created during the charge transfer absorption. Upon doping Eu²⁺ into the host matrix of BaHfSi₃O₉, sensitization of Eu²⁺ emission via Hf⁴⁺–O²⁻ charge transfer luminescence occurs, which is evidenced by the significant decreasing of Hf⁴⁺–O²⁻ charge transfer luminescence and increasing of Eu²⁺ emission intensity with increasing Eu²⁺ doping concentration, along with the presence of similar excitation band for the Eu²⁺ doped and undoped BaHfSi₃O₉.

Acknowledgements

This work was supported by National Science Council of Taiwan through Contract nos. NSC 100-2811-M-009-068, NSC 101-2113-M-009-021-MY3 and in part by the Fundamental Research Funds for the Central Universities (LZUJBKY-2014-36) and Open Project of Lanzhou University Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education (LZUMMM2014012).

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Footnote

† Electronic supplementary information (ESI) available: Samples synthesis details and experimental setup for photoluminescence spectra, schematic process of Hf–O CT luminescence and energy transfer from hafnate instrince emission to Eu²⁺. See DOI: 10.1039/c4ra03481c

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