Mechanisms of Tenebrescence and Persistent Luminescence in

Apr 18, 2016 - ABSTRACT: Synthetic hackmanites, Na8Al6Si6O24(Cl,S)2, showing efficient purple tenebrescence and blue/white persistent luminescence ...
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Mechanisms of Tenebrescence and Persistent Luminescence in Synthetic Hackmanite Na8Al6Si6O24(Cl,S)2 Isabella Norrbo,† Pawel Gluchowski,‡ Iko Hyppan̈ en,† Tero Laihinen,†,§ Pekka Laukkanen,⊥ Jaakko Mak̈ ela,̈ §,⊥ Fikret Mamedov,∥ Hellen S. Santos,†,§ Jari Sinkkonen,† Minnea Tuomisto,†,§ Antti Viinikanoja,† and Mika Lastusaari*,†,¶ †

Department of Chemistry, ⊥Department of Physics and Astronomy, and ¶Turku University Centre for Materials and Surfaces, University of Turku, FI-20014 Turku, Finland ‡ Institute of Low Temperature and Structure Research Polish Academy of Sciences, PL-50422 Wroclaw, Poland § Doctoral Programme in Physical and Chemical Sciences, University of Turku Graduate School (UTUGS), FI-20014 Turku, Finland ∥ Department of Chemistry, Molecular Biomimetics, Uppsala University, SE-75120 Uppsala, Sweden S Supporting Information *

ABSTRACT: Synthetic hackmanites, Na8Al6Si6O24(Cl,S)2, showing efficient purple tenebrescence and blue/white persistent luminescence were studied using different spectroscopic techniques to obtain a quantified view on the storage and release of optical energy in these materials. The persistent luminescence emitter was identified as impurity Ti3+ originating from the precursor materials used in the synthesis, and the energy storage for persistent luminescence was postulated to take place in oxygen vacancies within the aluminosilicate framework. Tenebrescence, on the other hand, was observed to function within the Na4(Cl,S) entities located in the cavities of the aluminosilicate framework. The mechanism of persistent luminescence and tenebrescence in hackmanite is presented for the first time. KEYWORDS: markers, persistent luminescence, tenebresence, photochromism, hackmanite, sodalite, XPS, NMR, EPR back to white with intense enough visible light.8 Such reversible photochromism is called tenebrescence, and it is used, e.g., to color minerals such as diamonds,9 beryl, corundum, quartz, topaz, zircon, hackmanite,10 and scapolite11 for use as gem stones. Another well-established use for inorganic photochromic materials is in glasses sensitive to solar radiation.12 Organic photochromic materials are used in, e.g., optical data storage,13 molecular switches,14 and sensing and bioimaging.15 Currently, it is well agreed that the coloring in hackmanites is due to chlorine vacancies that capture electrons originating from the (di)sulfide ions.16−18 Irradiation with UV or higher energy is required to remove the electrons from sulfur. These electrons are trapped in chloride vacancies, thus forming color centers, until optical stimulation with visible light gradually removes them from the traps and the color fades. Here, we construct for the first time a quantitative mechanism for tenebrescence in hackmanites. To our knowledge, this is also the first time that a quantitative tenebrescence mechanism has been presented for any material. Like many minerals, also hackmanite shows photoluminescence under UV excitation,16,19 but there is only one report on the persistent luminescence of a synthetic hackmanite7 and one

1. INTRODUCTION Optical multiplexing is a technology for acquiring multiple information from one optical signal. It has been employed in, e.g., optical data storage,1 security marking,2 and medical diagnostics.3 In diagnostics, much attention has been given for multiplexing with up-converting luminescent nanoparticles. The signal multiplicity is then achieved using materials with either different lanthanide dopants or their different concentrations, for example, with NaYF4:Yb,Er/Tm.2,4 Also, differences in luminescence emission lifetimes can be employed, as has been reported for KYb2F7:Ho5 or NaYF4:Yb,Tm.6 Hackmanites, Na8Al6Si6O24(Cl,S)2, seem to be ideal materials for multiplexing because they can be tailored to show different luminescence colors and lifetimes.7 They have the advantage of not requiring any expensive lanthanide dopants. Moreover, they can be tuned to show also persistent luminescence and tenebrescence by using only a regular hand-held UV lamp with wavelengths of 254 and 365 nm.7 Thus, they show great potential for future complex multiplexing, but they have not been studied for such applications. In the present report, we elucidate the mechanisms behind the multiple optical phenomena obtainable from the hackmanites. This information can be used to facilitate the tailoring of their optical properties. Hackmanite, Na8Al6Si6O24(Cl,S)2, is the photochromic variety of sodalite, Na8Al6Si6O24Cl2. The color of this material changes under UV excitation from white to blue/purple and © 2016 American Chemical Society

Received: February 16, 2016 Accepted: April 18, 2016 Published: April 18, 2016 11592

DOI: 10.1021/acsami.6b01959 ACS Appl. Mater. Interfaces 2016, 8, 11592−11602

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ACS Applied Materials & Interfaces on natural hackmanite.20 However, for the natural case, no detailed characterization has been reported in addition to the statement of it showing “a prolonged after-glow of white light”.20 Persistent luminescence means that a material will continue to emit light for minutes or hours after the excitation source has been removed. Materials with this property such as ZnS:Cu and SrAl2O4:Eu2+,Dy3+ have been used for decades in self-lit clock arms and exit signs.21 They are also studied for application in sensors22 and bioimaging.23 The general mechanism of persistent luminescence21 is such that upon excitation electrons from the emitting ion, e.g., Eu2+, transfer via the excited levels of the emitter to the conduction band and from there to trap levels, such as impurities and vacancies, which are located close below the bottom of the conduction band. Thermal energy raises the electrons back from the traps to the conduction band. There they migrate back to the excited levels of the emitter and relax as persistent luminescence. In the present paper, we present for the first time a quantitative persistent luminescence mechanism for synthetic hackmanites. Previously, such mechanisms have been presented for, e.g., Eu2+-,21 Tb3+-,24 and Ti3+-doped25 persistent luminescence materials. The crystal structure of hackmanite is similar to that of sodalite. The structure is cubic [space group P4̅3n (No. 218), Z = 1] with a = 8.88696(5) Å.26 It is composed of a network of corner-sharing AlO4 and SiO4 tetrahedra. The aluminum and silicon atoms reside on the faces of the unit cell, and the respective 24 tetrahedra form the so-called sodalite cage (Figure 1). A ClNa4 tetrahedron with the chlorine atom

the details of optical functionality in synthetic hackmanite Na8Al6Si6O24(Cl,S)2. The results are discussed and combined to give, for the first time, a quantitative view of the mechanisms of tenebrescence and persistent luminescence in these materials.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. The hackmanite samples were prepared with a solid-state reaction using stoichiometric amounts of Zeolite A (Sigma-Aldrich, product no. 96096), NaCl (J. T. Barker, 99.5%), and Na2SO4 (E. Merck, 99%) as the starting materials. The materials were prepared with S/Cl molar ratios of 0.06 and 0.13, as shown previously.7 Titanium- and manganese-doped materials were obtained by adding 2 mol % (with respect to silicon and aluminum) of TiO2 (Merck 808) or MnO2 (E. Merck, zur Analyze) to the starting material mixture. The addition will result in a slight nonstoichiometry with excess Al/Si, which we expect to form separate oxide phases. However, this nonstoichiometry should not have any effect on the fact that the intentionally doped materials can be used to identify the luminescence centers in the nondoped materials. Zeolite A was first dried at 500 °C for 1 h. The initial mixture was then heated at 850 °C in air for 48 h. The product was freely cooled to room temperature and ground. Finally, the product was reheated at 850 °C for 2 h under a flowing 12% H2 + 88% N2 atmosphere. The asprepared materials were washed with water to remove any excess NaCl impurity. 2.2. Characterization. The purity of the materials was checked by routine powder X-ray diffraction measurements using a Huber G670 detector and Cu Kα1 radiation (λ = 1.54060 Å). No impurities were detected (Figure S1). The overall composition of the samples was analyzed with X-ray fluorescence (XRF) spectroscopy using a PANalytical Epsilon 1 apparatus with its internal Omnian calibration. The parts per million impurity levels were further investigated with ICP-MS measurements using a PerkinElmer 6100 DRC Plus apparatus. Elemental distributions in the hackmanite grains were studied with SEM−EDS analyses using a Leo 1530 Gemini microscope equipped with a Thermo Scientific UltraDry SDD EDS system. Photoluminescence emission and excitation spectra as well as decay curves at room temperature were measured with a Varian Cary Eclipse fluorescence spectrophotometer equipped with a Hamamatsu R928 photomultiplier tube (PMT) and a 15 W xenon lamp. Lowtemperature measurements were carried out on the MAX-lab synchrotron (Lund, Sweden) beamline I3 consisting of a 6.65 m normal incidence primary monochromator as well as an ARC SP2300 CCD and a PMT for detection of luminescence. The samples were attached to the coldfinger of a liquid helium flow-type cryostat. The persistent luminescence spectra were measured with an Avantes Avaspec HS-TEC spectrometer after 5 min of irradiation with a hand-held 4 W UV lamp (UVP UVGL-25) at 365 nm. The persistent luminescence excitation spectra were measured with the Varian Cary Eclipse fluorescence spectrophotometer with the following procedure: First, the sample was irradiated for 1 min with the chosen wavelength. Then, the emission spectrum was measured 1 min after irradiation was stopped. The spectrum was measured in 1 min intervals until no emission was observed. After this, the irradiation wavelength was changed ,and the procedure was repeated. The intensity values obtained by integration over all visible wavelengths in the first spectrum of each irradiation wavelength were plotted as a function of the irradiation wavelength to give the persistent luminescence excitation spectrum. The TL measurements were carried out with a MikroLab Thermoluminescent Materials Laboratory Reader RA’04 using a heating rate of 10 °C s−1. Prior to the measurements, the samples were irradiated at room temperature with a 4 W UVGL-25 UV lamp at 254 or 365 nm. The thus-obtained glow curves were corrected for thermal quenching of luminescence in the materials. Data analysis was done with the initial rise method.29

Figure 1. Unit cell of sodalite drawn based on the data from ref 26.

located in the middle of the cage resides in each cage, and the sodium atom is coordinated in a distorted tetrahedral arrangement to form NaO3Cl units. In hackmanite, the chlorine atoms are partly replaced by sulfur species commonly assumed to be either S22− or S2−.27,28 In natural specimens, ca. 2−10% of the chlorine atoms have been replaced by sulfur atoms, and also approximately the same amount of chlorine sites are vacant.22 At the moment, the lack of detailed knowledge on the mechanisms of tenebrescence and persistent luminescence in these materials is a factor limiting their development to full potential use. In the present work, we aim to elucidate these mechanisms. We employ luminescence, reflectance, 35Cl and 27 Al magic angle spinning nuclear magnetic resonance (MAS NMR), electron paramagnetic resonance (EPR) and Fourier transform infrared (FTIR) spectroscopies, and thermoluminescence (TL) and inductively coupled plasma−mass spectrometry (ICP-MS) measurements to obtain information on 11593

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ACS Applied Materials & Interfaces The coloring and discoloring of the samples were measured with reflectance measurements under illumination from a 60 W incandescent light bulb located 20 cm above the sample. The coloring was induced with a 150 W xenon short-arc lamp (Osram XBO 150W/ 4) connected to an Optometrics DMC1-01 monochromator (fwhm = 3 nm) using a time of 30 min for each wavelength. The discoloring was studied with the same setup after first coloring the sample with 254 nm for 5 min. The spectra were collected using an Avantes AvaSpec2084x14 spectrometer, and the data were corrected with the intensity of the light passing the monochromator. The valence of titanium was studied with X-ray photoelectron spectroscopy (XPS) measurements that were carried out using a PerkinElmer PHI 5400 spectrometer with a Mg Kα X-ray source and a hemispherical electron energy analyzer. A neutralizer with a constant electron flux was used during the measurements to avoid electrical charging of the insulating powder samples. The fitting was carried out using CasaXPS software, version 2.3.16. The presence of unpaired electrons was probed by EPR spectroscopy at room temperature using a Bruker ELEXSYS E500 X band EPR spectrometer equipped with a SuperX EPR049 microwave bridge and a SHQ4122 resonator at a frequency of 9.8 GHz. A microwave power of 5 mW and a modulation amplitude of 3 G were used. Spectra were measured from nonirradiated materials as well as those irradiated 4 min in situ with a 355 nm Nd:YAG laser (Spectra Physics, USA) and 30 min ex situ with a 4 W UVGL-25 UV lamp at 254 nm. The local bonding structure was investigated by FTIR spectroscopy. The spectra were measured between 400 and 4000 cm−1 with a Nicolet Nexus 807 FTIR ESP spectrometer with a 4 cm−1 resolution. The materials were mixed with KBr and then pressed to transparent disks. The local structure around the Cl− ions was studied by solid-state 35 Cl and 27Al MAS NMR spectroscopy. The data were recorded for 30 min at room temperature with a Bruker AV400 apparatus using a 10000 Hz spinning rate and a 0.1 s relaxation time. The parts per million scale was calibrated against 1 M aqueous NaCl (0.0 ppm) and 1.1 M aqueous Al(NO3)3 (0.0 ppm).

Nevertheless, it could be a possible emitter because luminescence of Mn2+ has been reported at 540 nm for synthetic sodalite.30 This could contribute to the wide emission band of synthetic Na8Al6Si6O24(Cl,S)2. Finally, there are only a very few publications on the luminescence of Tin+ species in tetrahedral sites, but an emission spectrum very similar to that in the present materials has been reported for Ti3+ in topaz (Al2SiO4F2)31 and ZrO2.25 In light of these reports, we prepared hackmanites doped with titanium and manganese to investigate their effect on the luminescence. First, we compared the emission and excitation spectra of the titanium- and manganese-doped materials to those of the nondoped one (Figure 2). It is evident that titanium doping

3. RESULTS AND DISCUSSION 3.1. Determination of the Luminescence Center. As the first step in unveiling the details of the optical functionality of the synthetic Na8Al6Si6O24(Cl,S)2 materials, we set out to determine the luminescence center. First, we will discuss the nondoped materials, for which we reported previously that this center, emitting with a maximum at 460 nm, is responsible for both photoluminescence and persistent luminescence, but its exact nature could not be determined.7 The starting materials used in the synthesis contained some 100 ppm of iron, titanium, and manganese, as suggested by XRF measurements. These elements are all known to act as possible luminescent ions. Thus, we now carried out ICP-MS measurements to quantify their amounts in one material, i.e., that with n(S)/ n(Cl) = 0.06. The results revealed that titanium (74 ± 50 ppm) and iron (136 ± 4 ppm) were present most abundantly, whereas there was much less manganese (2 ± 0.4 ppm). Moreover, some chromium (11 ± 1 ppm) was detected as well, which is another possible luminescence center. Luminescence of Fe3+ has been reported for aluminosilicates, where it has replaced Al3+ and Si4+ in tetrahedral sites.30 In synthetic sodalite (Na8Al6Si6O24Cl2), the Fe3+ emission is in red at 680 nm.23 Similarly, Cr3+ readily substitutes for Al3+, as well as sometimes also Si4+, and emits in red. Cr4+ and Cr5+ emit in the IR range.11 Therefore, iron and chromium were not considered as the possible main emitters. The content of manganese in the present materials is very low, suggesting that it would not luminesce under a low-power UV lamp.

Figure 2. Normalized room-temperature UV-excited photoluminescence emission (top) and excitation (bottom) spectra of nondoped as well as titanium-doped (2%) and manganese-doped (2%) Na8Al6Si6O24(Cl,S)2 materials. The bottom figure also shows the difference between the excitation spectra of the nondoped and titanium-doped materials (dotted curve) as well as the persistent luminescence excitation spectrum of the nondoped material.

does not considerably change the shape or position of the spectra, whereas manganese does. With titanium doping, the shape of the emission spectrum changes minutely; i.e., the shoulder on the high-energy side weakens. The shoulder may be due to some other luminescence center, whose contribution weakens with the addition of extra titanium. This is also observable from the respective excitation spectra: there is a shoulder on the high-energy side that weakens with the addition of extra titanium. For the manganese-doped material, the emission spectrum contains an additional shoulder on the green side, if excited with 310 nm, which increases if excitation is shifted to 254 nm (Figure 2). Such green emission could not be observed for the nondoped7 or titanium-doped materials. In the excitation spectrum, one can observe that this emission can 11594

DOI: 10.1021/acsami.6b01959 ACS Appl. Mater. Interfaces 2016, 8, 11592−11602

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nm). For Ti2O3, the band gap is so small (0.06 eV33) that it would not be possible to observe any photoluminescence from it. To be sure that titanium is not present as luminescent oxides that are not observed due to their low concentration, we recorded the emission spectrum of the TiO2 starting material as well as TiO2 subjected to the same heating protocol that is used for the preparation of hackmanites, but no luminescence was observed from the oxides with 310 nm excitation (Figure S2). It must also be noted that high-intensity laser excitation is required to obtain luminescence from TiO2,34 while in this work, we used only a low-power UV lamp. To finally confirm the incorporation of titanium in the hackmanite lattice, we carried out SEM−EDS analyses. The results show that, for the 2% titanium-doped material, titanium is evenly distributed in the hackmanite grains (Figure S3). Finally, we tested the possibility of emission from O2−. This molecule has been reported to show blue luminescence in minerals.31 It can be identified from the vibrational bands appearing at ca. 1000 cm−1 (ca. 20 nm) intervals. Such emission bands superimposed on a band peaking at 450 nm have also been reported for synthetic sodalite.30 We carried out our measurements at 20 K using synchrotron radiation excitation at 155 nm and found weak bands with 1000 cm−1 intervals on the high-energy side of the main emission band (Figure 4, inset).

be excited best with 240 nm. It is evident that this emission at 535 nm is due to the emission of Mn2+, which has been reported earlier at 542 nm for Mn2+-doped sodalite.30 Because excitation of the nondoped material with 254 nm does not give such green emission, we excluded Mn2+ as a possible emitter in the nondoped materials. Next, we compare the persistent luminescence spectra (Figure 3, top) of the nondoped and titanium-doped materials.

Figure 3. Normalized persistent luminescence spectra (top) with the baseline of the former shifted for clarity and persistent luminescence decay curves (bottom) of nondoped and titanium-doped (2%) Na8Si6Al6O24(Cl,S)2 materials.

Figure 4. Synchrotron-radiation-excited photoluminescence excitation and emission spectra of a nondoped material. The two insets show weak vibrational fine structure around 400 and 650 nm.

The two spectra are very similar in shape; the only difference is that with titanium doping the persistent emission gets more intense and its duration increases (Figure 3, bottom). However, the lifetimes associated with the emission fading do not change (Table S1). This strongly suggests that titanium is the persistent emitter in the nondoped hackmanite. If one compares the persistent spectrum (Figure 3) with the photoluminescence one (Figure 2), it is clear that the highenergy-side shoulder is not present at all in the former. This suggests that photoluminescence is caused by two emitting centers, whereas the persistent one is from one center only. There may be some doubt about whether titanium has actually entered the hackmanite lattice at such low temperatures as the 850 °C used in the present work. One could assume that the TiO2 starting material stays unreacted or is reduced to Ti2O3 during the N2/H2 treatment. However, the present luminescence results strongly suggest that titanium has entered the hackmanite structure. For example, while the bandgap energies of rutile- and anatase-type TiO2 are 3.0 and 3.2 eV32 (413 and 387 nm), respectively, we do not observe these in the excitation spectra (Figure 2, bottom). Instead, the excitation maximum is at a much higher energy (4.0 eV, 310

These bands are much weaker than those reported previously.30 This confirms that the O2− entity takes part in photoluminescence, but its contribution is much less than that of Ti3+. The dioxide emission can be assigned to the 2Σg → 2Π3/2 transition.31 It must be noted that the synchrotron emission spectrum shows also a band at 690 nm. It does not participate in persistent luminescence, and thus it is not discussed here. However, we will return to this emission later on in the context of tenebrescence. An additional feature of the synchrotron-radiation-excited spectrum is that the emission maximum is blue-shifted to ca. 400 nm. This is because the measurement was carried out during excitation, while those shown above (Figure 2) were measured after a 0.1 ms delay. In fact, the emission maximum shifts toward lower energy with increasing delay time between stopping excitation and carrying out the emission measurement (Figure 5, top). When regular photoluminescence has completely faded, i.e., only persistent luminescence remains, the spectral shape and position no longer change. These 11595

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Figure 6. Ti 2p XPS spectra of Na8Al6Si6O24(Cl,S)2:Ti (2%), heattreated TiO 2, and TiO2 starting material fitted with Ti 2p3/2 components for Ti4+ and Ti3+ states.

indicating the presence of a considerable amount of Ti3+, which can also be observed in the XPS spectrum. This indicates that there is enough reducing power available to convert Ti4+ to Ti3+, but even if the structure of TiO2 changes from anatase to rutile type (Figure S4), there is not enough energy to break down the whole TiO2 matrix to form Ti2O3. Similarly, we can assume that, also with the hackmanite matrix, the reduction of Ti4+ does not alter the overall structure, but it results in the creation of charge-compensation defects. Such defects are probably oxygen vacancies, which will play an important role in persistent luminescence, as will be disussed later in the text. The XPS data for the hackmanite sample confirms the presence of Ti3+. Considering these XPS results and the fact that the luminescence observed from hackmanite is very weak before reduction (Figure S5), we conclude that Ti3+ is the persistent emitter in hackmanite. The origin of the blue luminescence of titanium (at ca. 420 nm) is not fully agreed on in the literature. One model states that the emission is due to the charge-transfer transition between Ti4+ and O2−.38 Another model involves photoionization of Ti3+, i.e., the transfer of an electron to the conduction band, and the subsequent capture of the electron by an F+ center (i.e., color center, VO•). This creates an excited color center (VO••) that relaxes, emitting in the blue region.39 The latter model has also been named as Ti4+−F center emission due to the necessity of the Ti3+ ions in the process.39 The emission of titanium in red, on the other hand, is explained with the simple 2E → 2T2 transition (in octahedral symmetry) of Ti3+.31,40 In the present case, the second model seems more appropriate because Ti3+ was identified as the emitter. However, because persistent luminescence cannot be obtained at decreased temperatures (it is defined as thermally stimulated luminescence at room temperature), it must involve a step requiring thermal energy. Such a step is the transfer of electrons via the conduction band from a trap to the luminescence center. This is generally agreed on for persistent luminescence.21,41 Thus, we suggest that the emitting center is a Ti3+− VO pair (TiAlX−VO•• and/or TiSi′−VO••) and that the persistent emission is caused by the entrapment of an electron to VO and its subsequent release via the conduction band back to the Ti3+−VO pair:

Figure 5. Effect of the measurement delay time on the emission of Na8Al6Si6O24(Cl,S)2 with n(S)/n(Cl) = 0.06 excited at 310 nm at room temperature (top). Note that the high-energy side of the measurement with no delay is cut by the filter used for blocking the excitation radiation. The bottom curves show the emission decays recorded at different wavelengths.

observations confirm the assumption that there would be two emitting centers: the emission of O2− at ca. 400 nm fades faster than the titanium one at ca. 515 nm. Further support is given by normalized luminescence decay curves, which indicate that the higher the emission wavelength monitored, the slower the initial fading (Figure 5, bottom). On the basis of the data given above, it can be assumed that titanium is the persistent emitter. Taking into account the ionic radii35 of four-coordinated Na+ (0.99 Å), Al3+ (0.39 Å), Si4+ (0.26 Å), and Ti4+ (0.42 Å), we can assume that titanium occupies preferably the Al3+ site as both an impurity in the Zeolite A starting material and the hackmanite annealed 48 h in air but not yet reduced. One can, however, not rule out that there may also be small amounts of titanium in the Na+ and Si4+ sites. The latter should be the more preferred one of these two, however, because of the better match in size and, especially, in charge. We can also expect that before the reduction step of the synthesis titanium will be present as Ti4+ because of the oxidizing conditions. The reduction step of the preparation can be assumed to result in Ti3+, which still resides predominantly in the Al3+ sites. To confirm the valence of the titanium emitter, we carried out XPS measurements for the TiO2 starting material, TiO2 subjected to the same heating protocol that is used for the preparation of hackmanites and hackmanite doped with 2% titanium. The results (Figure 6) show, as expected, that no observable amount of Ti3+ is present in the TiO2 starting material when fitted to a typical binding energy of 457.2 eV for Ti3+, which is about 1.4−2.0 eV lower than that for Ti4+.36,37 The heat-treated titanium oxide has a dark-gray body color, 11596

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ACS Applied Materials & Interfaces TiAl X−VO•• + hν → [TiAl X−VO••]* → TiAl •−VO• → [TiAl X−VO••]* → TiAl X−VO•• + hν′

and/or

TiSi′−VO••

X

+ hν →

[TiSi′−VO••]*

→ TiSi −VO•

→ [TiSi′−VO••]* → TiSi′−VO•• + hν′

The existence of the oxygen vacancies is expected because of the reducing preparation conditions, causing the evaporation of oxygen [2O2−(s) → O2(g)] and the need of charge balance due to the reduction Ti4+ → Ti3+. One must note that, in the sodalite structure, the Ti3+ ions will occupy tetrahedral sites because they substitute for Al3+ and/or Si4+. Thus, the crystal field will be weaker than that in the octahedral symmetry, and the emission is observed in green rather than blue. 3.2. Energetics of Persistent Luminescence. Now that the persistent emitter has been identified as the Ti3+−VO pair, we quantify the energetics associated with both photoluminescence and persistent luminescence. Previously,7 we reported that both phenomena show a similar emission extending from 400 to 800 nm (1.6−3.1 eV). The maximum for persistent luminescence is at 515 nm (2.4 eV; Figure 3). In order to investigate how this emission can be excited and the excitation energy stored, the corresponding excitation spectra were analyzed. For photoluminescence, excitation can be achieved with wavelengths in the range 230−360 nm (3.4−5.4 eV) with the maximum at 310 nm (4.0 eV; Figure 2, bottom). This indicates that the Stokes shift for the emission is 1.6 eV. For persistent luminescence, the range is somewhat wider, extending to 370 nm (3.4 eV) in the low-energy side (Figure 2, bottom). The fact that persistent luminescence can be excited with lower energy than photoluminescence can be explained with the role of thermal energy: at room temperature, there is enough energy to lift the excitation to the conduction band via the excited levels of Ti3+−VO even if the energy of these levels is not reached by the excitation itself. The fact that luminescence can be observed indicates that the emitting level must be lower in energy than the conduction band, and on the basis of the persistent luminescence excitation spectrum, the ground level of Ti3+ can be placed to ca. 3.4 eV below the bottom of the conduction band. Furthermore, a comparison of the two excitation spectra shows that, as the excitation wavelength goes below 310 nm (E > 4.0 eV), the photoluminescence intensity falls steeply, whereas that of persistent luminescence does not. This indicates that once the energy exceeds that of the excited level of Ti3+−VO (4.0 eV), it is free to migrate away from the luminescence center to the conduction band. From there, it can transfer to trap (defect) energy levels responsible for storing the energy for persistent luminescence. The energy characteristics of the trapping sites were then analyzed by using TL measurements (Figure 7). The samples were irradiated with both 254 and 365 nm because our previous findings7 showed a clearly different persistent luminescence duration depending on the irradiation energy. With 365 nm irradiation, a single peak at 145 °C is obtained in the glow curve, whereas with 254 nm, an additional weaker signal is observed at ca. 300 °C. Initial rise calculations give 0.5 eV as the activation energy for the signal at 145 °C, i.e., the shallow trap. The activation energy of the second signal, the deep trap, could be obtained by isolating the peak with preheating to 250 °C. The initial rise calculation then yielded 1.2 eV as the energy. These energy values obtained with TL indicate the energy

Figure 7. TL glow curves of Na8Al6Si6O24(Cl,S)2 with n(S)/n(Cl) = 0.06 irradiated with 254 and 365 nm.

difference between the trap level and the bottom of the conduction band. This means that the material possesses two trap levels for persistent luminescence: one at 0.5 eV and the other 1.2 eV below the bottom of the conduction band. The above discussion has assumed that it is the electrons rather than the holes that act as the charge carriers in the system. This requires that the ground level of the emitting center would be located in the band gap rather than the valence band. To verify this assumption, we recorded the excitation spectrum of the 460 nm emission by using synchrotron radiation because it can reach wavelengths below the 200 nm conventionally available in laboratory devices. As a result, we were able to observe the valence-to-conduction band transition at 175 nm (7.1 eV; Figure 4). This signal is, however, masked by the presence of excitons. Thus, it is conventional to take the value of the energy gap (Eg) 8% higher.42 The resulting bandgap energy is then 7.7 eV. This value is in good agreement with those of other luminescent silicates and aluminates (typically between ca. 7 and 8 eV43) even if a value as low as 5.0 eV has been reported for sodalite based on first-principles density functional theory (DFT) calculations. 44 However, DFT calculations are known to give underestimated band-gap energies. Considering that we placed the ground level of Ti3+ 3.4 eV from the conduction band, it can be said to be located well in the band gap, and thus the mechanism can be considered based on electrons as the charge carriers. One must also note that the fact that it was possible to observe the valence-to-conduction band transition from the excitation spectrum of Ti3+ serves as an additional confirmation that titanium has entered the hackmanite lattice. 3.3. Energetics of Tenebrescence. Previous literature is in agreement that tenebrescence in hackmanites is due to electrons leaving from sulfur to chloride vacancies, thus forming color centers.18,27,28 Our previous results also indicated that the color change in Na8Al6Si6O24(Cl,S)2 was observed only if sulfur was present,7 confirming the essential role of sulfur. However, it is not clear whether the sulfur species acting as the electron donor is S2−,18 S22−,27 or S2−.18 Goettlicher et al.28 suggested based on X-ray absorption near-edge structure (XANES) studies that sulfur is present as either the sulfide S2− or disulfide S22− in hackmanite and, during coloration, these would change to their corresponding radicals S− and S2−, but it was not possible to differentiate between these two possibilities. On the basis of the report of Gambardella et al.,45 who studied lazurite [(Na,Ca)8(Al6Si6O24)(SO4,S,Cl)2] with XANES, also other polysulfide species such as S3−, S32−, S4−, and S42− could 11597

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ACS Applied Materials & Interfaces give signals at energies similar to those of S22−. The radical species themselves can act as absorbers of visible light. For example, S2− and S3− absorbing at 400 and 595 nm, respectively, are responsible for the color of ultramarine-type sodalites.46 On the basis of these reports, we assume that, because no color is present in our nonirradiated hackmanites, no radical sulfur species would be present before irradiation. Next, we consider whether sulfur is present as sulfide or disulfide. If we assume, as suggested by Goettlicher et al.,28 that the coloration of hackmanite during tenebrescence results in the creation of radicals, we should be able to observe luminescence of S2−, which is well established in different minerals.19,31 In the synchrotron-radiation-excited emission spectrum (λexc = 155 nm, which also results in coloring of the material) presented above (Figure 4), there is a band in red (685 nm, 1.8 eV), which is typical of the disulfide luminescence in minerals. However, also the red Ti3+ emission may be observed in this region.47 Like luminescence of the dioxide molecule, the S2− one can be identified from equidistant vibration peaks with a ca. 500 cm−1 separation.31 This has been reported for natural hackmanite,19 and in the present work, such a vibrational structure was present, as well, albeit weak (Figure 4). This confirmed the presence of S2− and that the lowest excited (emitting) level of the molecule lies in the band gap. According to earlier reports,47 the S2− emission can be excited with wavelengths below 400 nm (E > 3.1 eV). Our results also show that the red emission was obtainable with 400 nm excitation, but it was not clear whether this emission was from S2− or Ti3+. This is because at the synchrotron such a low excitation energy could not be used, and in the laboratory data, we could not observe any vibronic peaks regardless of the temperature used. However, the presence of the weak vibrational peaks in the synchrotron-excited luminescence spectrum indicates that sulfide would be present as diatomic species. Considering the XANES results of Goettlicher et al.,28 we conclude that the process involved in tenebrescence is as follows:

Figure 8. Tenebrescence excitation (triangles) and bleaching (squares) spectra of Na8Al6Si6O24(Cl,S)2 with n(S)/n(Cl) = 0.13. The intensity of the color refers to the decrease in the overall reflectance with respect to that of the white material.

The results indicate that the color can be clearly removed with wavelengths of 500−630 nm (2.0−2.5 eV) with the maximum effect at 625 nm (2.0 eV; Figure 9). This information indicates that the ground level of the color center is located 2.0 eV below the conduction band. When the energy is raised above 2.5 eV, S22− starts to absorb, decreasing the effective dose absorbed by

(S2 )Cl ′ + VCl • ↔ (S2 )Cl X + VCl X

To place the ground level of S22− in the band gap, we performed tenebrescence excitation measurements; i.e., we recorded the reflection spectrum of the material as a function of the irradiation wavelength to monitor the coloring of the material. The results indicate that coloration is achieved between 225 and 290 nm (4.3−5.5 eV; Figure 8). Because coloration requires that electrons escape from S22− to the conduction band, the present results suggest that the ground level of S22− is located 4.3 eV below the bottom of the conduction band. The next step was then to consider the energetics of the color center. From our previous results, we know that Na8Al6Si6O24(Cl,S)2 absorbs a wide band between 425 and 700 nm (1.8−2.9 eV), with the maximum at 550 nm (2.3 eV) giving it a purple appearance.7 We know that the color fades at room temperature if the lights are on but not in the dark.27 This means that the excited levels of the color center overlap with the conduction band so that always a part of the absorbed visible-light energy escapes to the conduction band. This causes the gradual fading of the color with time. Thus, in order to investigate how far the ground level of the color center is from the conduction band, we measured color bleaching spectra; i.e., we first colored the sample with 254 nm irradiation and thereafter recorded the effect of irradiation at different energies.

Figure 9. Room temperature EPR spectrum of Na8Al6Si6O24(Cl,S)2 with n(S)/n(Cl) = 0.06 before and after 254 and 355 nm irradiation. The arrows show the features observed only after irradiation, and the bottom figure gives a close up on those features. 11598

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different parts of the structure. Previously, e.g., Annersten and Hassib51 and Cano et al.52 reported that the coloring of sodalite is manifested as a single EPR signal at g = 2.011, which is in rather good agreement with our results. On the other hand, McLaughlan and Marshall53 observed that the signal was split into 13 components with 31 G hyperfine spacing centered at g = 2.002. We did not observe such hyperfine lines. This may be because of a too low concentration of sulfur and/or oxygen vacancies in the sample. We did not observe any separate signals from the S2− radical, but according to Hodgson et al.,54 this could be due to its very large g anisotropy even at 77 K. To our knowledge, there is no literature about EPR of persistent luminescence materials measured during persistent luminescence. However, Takeyama et al.55 have reported that the signal of an electron trapped to an AlO4− entity is at g = 1.999 in SrAl2O4:Dy. This is a persistent luminescence matrix if doped with both Eu2+ and Dy3+, but the latter alone does not give persistent luminescence. Moreover, the EPR spectrum of nondoped CaAl2O4 has been reported to show an EPR signal at g = 2.0,56 but again there is no luminescence center present. These values cannot be used very well for comparison because the general understanding of persistent luminescence is that the trapped electron originates from the luminescence center. Next, we carried out similar measurements with FTIR spectroscopy (Figure S6). The sample shows the typical sodalite frame bending (at ca. 450 cm−1) as well as symmetric (ca. 700 cm−1) and asymmetric (ca. 1000 cm−1) T−O−T signals (T = Al and Si).57 The FTIR spectrum does not change with 365 nm irradiation, but 254 nm irradiation induces small but observable changes, especially in the asymmetric T−O−T signals. We propose that, because persistent luminescence (365 nm irradiation) involves electron transfer within the covalent AlO4/SiO4 units, the resulting structural change will be much less than that when the electron escapes from S22− to a chlorine vacancy because of tenebrescence (254 nm irradiation). Considering that the size difference between O22− and O2− is ca. 10%,58 we can assume a similar significant size difference for the corresponding sulfide species and that it would cause observable local structural changes. Previously, Armstrong and Weller27 reported, based on neutron powder diffraction data, a very small but reproducible increase in the Na−O distances (ca. 0.0004 Å) upon the coloring of hackmanite. In the present work, we observe the effect of this distance change on the AlO4/SiO4 units. All in all, also the FTIR data support the assumption that persistent luminescence and tenebrescence take place in different sites of the hackmanite structure. Finally, the structural effects were probed by solid-state 35Cl and 27Al MAS NMR spectroscopy before and after irradiation (Figure S7). The chlorine spectrum of a nonirradiated sample shows a signal at −131 ppm due to the Cl− ion in the tetrahedral (Na4Cl)3+ entities of the sodalite structure without nearby color centers.59 A trace of nonreacted NaCl can be observed from the signal at −51 ppm.7 Similarly, for aluminum only, one signal, at 65 ppm, is observed for the nonirradiated sample corresponding to tetrahedral aluminum species with no nearby color centers.60 Irradiation causes no change to either spectrum. This is most probably due to the rather low sensitivity of NMR and the low concentration of charge carriers trapped during tenebrescence and persistent luminescence.

the color center, and at even higher energies, the electrons originating from S22− flow toward the color center, canceling the bleaching effect. This is in agreement with the fact that natural hackmanite can be colored by exposure to direct sunlight. Finally, one must also note that the tenebrescence color observed is probably partly due to absorption of the S2− radical (considered as a yellow chromophore46) formed in the process. This is because we reported earlier that the synthetic hackmanites also have an absorption band around 400 nm when in the colored form.7 The presence of this band also supports the assumption of the presence of sulfur as disulfide ions. However, the main absorption (with the maximum at 550 nm7) in the visible range is due to the color center discussed above. 3.4. Structural Effects of Energy Storage. Above we have clarified the origin of the energy carriers being stored in the synthetic Na8Al6Si6O24(Cl,S)2 materials as well as the energies connected with the processes in persistent luminescence and tenebrescence. At this point, it is still unclear where the energy is stored. This information is undoubtedly always difficult, if not impossible, to obtain for the optical energy storage material, even if, e.g., those employed in dosimetry have now been studied extensively for decades. Here, we used EPR, FTIR, and solid-state NMR spectroscopies to obtain more information on the structural effects of energy storage. The elements in the basic hackmanite composition Na8Al6Si6O24(Cl,S)2 contain no unpaired electrons, and thus there should not be any EPR signals. However, the EPR spectrum of a nonirradiated sample shows multiple signals between 3150 and 3800 G (Figure 9). The six strong hyperfine lines with the separation increasing from 88 to 98 G with increasing field are typical of the allowed transitions (Δm = 0) of Mn2+,48 confirming the presence of a Mn2+ impurity as suggested by ICP-MS. This set of lines is centered at g = 2.003. Each of these strong lines also shows a split component, whose separation from the strong peak increases from 4 to 12 G with increasing field. We propose that these paired signals are due to Mn2+ in the Al3+ and Si4+ sites, i.e., the MnAl′ and MnSi″ species. Between each pair of strong lines is a pair of weaker lines with a separation of ca. 20 G. These are due to the forbidden transitions (Δm = ±1) of Mn2+.48 Also, these forbidden lines show a splitting due to the two sites that Mn2+ occupies. All of the remaining signals in the EPR spectrum are weak. Thus, it is difficult to judge whether there are signals from the other d transition-metal impurities or not. The EPR signal of Ti3+ has been reported at g = 1.94 (corresponding here to 3627 G) in mullite 3Al2O32SiO249 and at g = 1.952 (here 3605 G) in Zeolite A.50 Because of the strong contribution of Mn2+, it is difficult to say whether such signals are present in the present data. Next, we irradiated the sample to study its effect on the EPR spectrum. After excitation with 355 nm, the hackmanites show persistent luminescence but no tenebrescence. In the EPR spectrum, the effect of persistent luminescence is observed as a single signal at g = 2.005 due to the formation of a radical (Figure 9). With 254 nm irradiation, both tenebrescence and persistent luminescence are obtained, but here we waited for 1 week before measuring the EPR spectrum of a colored material so that only the effect of tenebrescence would be present. The resulting EPR spectrum lacks a line at g = 2.005, but it shows a line at g = 2.001 instead. This confirms that the two phenomena create different radicals, in agreement with our above hypothesis that the two phenomena would take place in 11599

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ACS Applied Materials & Interfaces Author Contributions

4. CONCLUSIONS On the basis of the data presented above, we present for the first time the mechanisms for tenebrescence and persistent luminescence in the Na8Al6Si6O24(Cl,S)2 materials (Figure 10):

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

Research leading to the present results was funded by The Academy of Finland, Nordic Energy Research, Swedish Energy Agency, Knut and Alice Wallenberg Foundation, Brazilian funding agency CNPq (Ciência sem Fronteiras scholarship), and the UTUGS (Turku, Finland). The research leading to these results has also received funding from the European Community’s Seventh Framework Programme (FP7/2007− 2013) CALIPSO under Grant 312284. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Lic. Tech. Paul Ek (Åbo Akademi University, Turku, Finland) is thanked for the ICP measurements and MSc Linus Silvander (Åbo Akademi University, Turku, Finland) is thanked for the SEM−EDS analyses. Dr. Sergey Omelkov and Dr. Sebastian Vielhauer (University of Tartu, Tartu, Estonia) are thanked for their help during the synchrotron measurements. Dr. Joachim Lindblom (Morris Linblom & Co., Turku, Finland) is thanked for helpful discussions concerning the coloration and luminescence of minerals.

Figure 10. Mechanism for tenebrescence and persistent luminescence in the Na8Al6Si6O24(Cl,S)2 materials. The numbers correspond to energies in electronvolts.

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Persistent luminescence takes place within the SiO4 and AlO4 tetrahedra, and the emission is due to close pairs of Ti3+ impurity ions and oxygen vacancies. In the charging stage, the Ti3+−VO pair absorbs energies of 3.4 eV and above, transferring electrons from its excited levels via the conduction band to nearby oxygen vacancies acting as traps for energy storage. In the decharging stage, the thermal energy available at room temperature (E > 0.5 eV) raises the electrons gradually from the traps back to the conduction band and the excited levels of the Ti3+−VO pair. As a result, the pair then emits blue/green persistent luminescence at 2.4 eV. Tenebrescence takes place within the [Na4(□,S)] units. In the charging stage, S22− ions absorb energies 4.3 eV and above, transferring electrons from its excited levels to the conduction band. The electrons transfer via the conduction band to chloride vacancies, forming color centers. The color center then absorbs visible light between 1.8 and 2.9 eV. This results in the purple color of the material. In the decharging stage, the absorbed light gradually lifts electrons back to the conduction band where they can transfer back to the excited levels of the S 2 2− ion. The excitation can relax radiatively as red luminescence at 1.8 eV, but the emptying of the color centers is a very slow process, and thus the emission intensity will be very low.

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(1) Zijlstra, P.; Chon, J. W. M.; Gu, M. Five-Dimensional Optical Recording Mediated by Surface Plasmons in Gold Nanorods. Nature 2009, 459, 410−413. (2) Meruga, J. M.; Cross, W. M.; May, P. S.; Luu, Q.; Crawford, G. A.; Kellar, J. J. Security Printing of Covert Quick Response Codes Using Upconverting Nanoparticle Inks. Nanotechnology 2012, 23, 395201−1−9. (3) Zhang, F.; Haushalter, R. C.; Haushalter, R. W.; Shi, Y.; Zhang, Y.; Ding, K.; Zhao, D.; Stucky, G. D. Rare-Earth Upconverting Nanobarcodes for Multiplexed Biological Detection. Small 2011, 7, 1972−1976. (4) Wang, F.; Liu, X. Upconversion Multicolor Fine-Tuning: Visible to Near-Infrared Emission from Lanthanide Doped NaYF4 Nanoparticles. J. Am. Chem. Soc. 2008, 130, 5642−5643. (5) Ding, M.; Chen, D.; Ma, D.; Liu, P.; Song, K.; Lu, H.; Ji, Z. Tuning the Upconversion Luminescence Lifetimes of KYb2F7:Ho3+ Nanocrystals for Optical Multiplexing. ChemPhysChem 2015, 16, 3784−3789. (6) Deng, R.; Liu, X. Tunable Lifetime Nanocrystals. Nat. Photonics 2013, 8, 10−12. (7) Norrbo, I.; Głuchowski, P.; Paturi, P.; Sinkkonen, J.; Lastusaari, M. Persistent Luminescence of Tenebrescent Na8Al6Si6O24(Cl,S)2: Multifunctional Optical Markers. Inorg. Chem. 2015, 54, 7717−7724. (8) Lee, O. I. A New Property of Matter: Reversible Photosensitivity in Hackmanite from Bancroft, Ontario. Am. Mineral. 1936, 21, 764− 776. (9) Byrne, K. S.; Anstie, J. D.; Chapman, J. G.; Luiten, A. N. Optically Reversible Photochromism in Natural Pink Diamond. Diamond Relat. Mater. 2012, 30, 31−36. (10) Pough, R. H.; Rogers, T. H. Experiments in X-ray Irradiation of Gem stones. Am. Mineral. 1947, 32, 31−43. (11) Allen, T.; Renfro, N.; Nelson, D. Tenebrescent Irradiated Scapolite. Gems & Gemology 2014, 50, 91−92. (12) Kanu, S. S.; Binions, R. Thin Films for Solar Control Applications. Proc. R. Soc. London, Ser. A 2010, 466, 19−44. (13) Kawata, S.; Kawata, Y. Three-Dimensional Optical Data Storage Using Photochromic Materials. Chem. Rev. 2000, 100, 1777−1788.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b01959. Powder X-ray diffraction data, additional luminescence results, and SEM−EDS, FTIR, and NMR data (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: miklas@utu.fi.

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ACS Applied Materials & Interfaces (14) Feringa, B. L.; van Delden, R. A.; Koumura, N.; Geertsema, E. M. Chiroptical Molecular Switches. Chem. Rev. 2000, 100, 1789−1816. (15) Zhang, J.; Zou, Q.; Tian, H. Photochromic Materials: More Than Meets the Eye. Adv. Mater. 2013, 25, 378−399. (16) Kirk, R. D. The Luminescence and Tenebrescence of Natural and Synthetic Sodalite. Am. Mineral. 1955, 40, 22−31. (17) Williams, E. R.; Simmonds, A.; Armstrong, J. A.; Weller, M. T. Compositional and Structural Control of Tenebrescence. J. Mater. Chem. 2010, 20, 10883−10887. (18) Warner, T. E.; Hutzen Andersen, J. The Effects of Sulfur Intercalation on the Optical Properties of Artificial “Hackmanite”, Na8[Al6Si6O24]Cl1.8S0.1; “Sulfosodalite”, Na8[Al6Si6O24]S; and Natural Tugtupite, Na8[Be2Al6Si6O24](Cl,S)2‑δ. Phys. Chem. Miner. 2012, 39, 163−168. (19) Gaft, M.; Panczer, G.; Nagli, L.; Yeates, H. Laser-Induced TimeResolved Luminescence of Tugtupite, Sodalite and Hackmanite. Phys. Chem. Miner. 2009, 36, 127−141. (20) Warner, T. E. Synthesis, Properties and Mineralogy of Important Inorganic Minerals; John Wiley & Sons: Somerset, U.K., 2011. (21) Lastusaari, M.; Jungner, H.; Kotlov, A.; Laamanen, T.; Rodrigues, L. C. V.; Brito, H. F.; Hölsä, J. Understanding Persistent Luminescence: Rare-Earth- and Eu2+-Doped Sr2MgSi2O7. Z. Naturforsch., B: J. Chem. Sci. 2014, 69, 171−182. (22) Xu, C.-N.; Watanabe, T.; Akiyama, M.; Zheng, X.-G. Artificial Skin to Sense Mechanical Stress by Visible Light Emission. Appl. Phys. Lett. 1999, 74, 1236−1238. (23) Maldiney, T.; Bessière, A.; Seguin, J.; Teston, E.; Sharma, S. K.; Viana, B.; Bos, A. J. J.; Dorenbos, P.; Bessodes, M.; Gourier, D.; Scherman, D.; Richard, C. The in Vivo Activation of Persistent Nanophosphors for Optical Imaging of Vascularization, Tumours and Grafted Cells. Nat. Mater. 2014, 13, 418−426. (24) Rodrigues, L. C. V.; Brito, H. F.; Hölsä, J.; Stefani, R.; Felinto, M. C. F. C.; Lastusaari, M.; Laamanen, T.; Nunes, L. A. O. Discovery of the Persistent Luminescence Mechanism of CdSiO3:Tb3+. J. Phys. Chem. C 2012, 116, 11232−11240. (25) Carvalho, J. M.; Rodrigues, L. C. V.; Hölsä, J.; Lastusaari, M.; Nunes, L. A. O.; Felinto, M. C. F. C.; Malta, O. L.; Brito, H. F. Influence of Titanium and Lutetium on the Persistent Luminescence of ZrO2. Opt. Mater. Express 2012, 2, 331−340. (26) Hassan, I.; Antao, A. M.; Parise, J. B. Sodalite: HighTemperature Structures Obtained from Synchrotron Radiation and Rietveld Refinements. Am. Mineral. 2004, 89, 359−364. (27) Armstrong, J. A.; Weller, J. A. Structural Observation of Photochromism. Chem. Commun. 2006, 1094−1096. (28) Goettlicher, J.; Kotelnikov, A.; Suk, N.; Kovalski, A.; Vitova, T.; Steininger, R. Sulfur K X-Ray Absorption Near Edge Structure Spectroscopy on the Photochrome Sodalite Variety Hackmanite. Z. Kristallogr. - Cryst. Mater. 2013, 228, 157−171. (29) Chen, R.; McKeever, S. W. S. Theory of Thermoluminescence and Related Phenomena; World Scientific: Singapore, 1997. (30) van Doorn, C. Z.; Schipper, J. Luminescence of O2−, Mn2+ and Fe3+ in Sodalite. Phys. Lett. A 1971, 34, 139−140. (31) Gaft, M.; Reisfeld, R.; Panczer, G. Luminescence Spectroscopy of Minerals and Materials; Springer: Heidelberg, Germany, 2005. (32) Li, L.; Yan, J.; Wang, T.; Zhao, Z.-J.; Zhang, J.; Gong, J.; Guan, N. Sub-10 nm Rutile Titanium Dioxide Nanoparticles for Efficient Visible-Light-Driven Photocatalytic Hydrogen Production. Nat. Commun. 2015, 6, 5881−1−10. (33) Adler, D. Antiferromagnetism in Ti2O3. Phys. Rev. Lett. 1966, 17, 139−141. (34) Knorr, F. J.; McHale, J. L. Spectroelectrochemical Photoluminescence of Trap States of Nanocrystalline TiO2 in Aqueous Media. J. Phys. Chem. C 2013, 117, 13654−13662. (35) Shannon, R. D. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1976, 32, 751−767.

(36) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Physical Electronics Division, Perkin-Elmer Corp.: Eden Prairie, MN, 1992. (37) Xiong, L. B.; Li, J. L.; Yang, B.; Yu, Y. Ti3+ in the Surface of Titanium Dioxide: Generation, Properties and Photocatalytic Application. J. Nanomater. 2012, 2012, 1−13. (38) Wong, W. C.; McClure, D. S.; Basun, S. A.; Kokta, M. R. Charge-Exchange Processes in Titanium-Doped Sapphire Crystals. I. Charge-Exchange Energies and Titanium-Bound Excitons. Phys. Rev. B: Condens. Matter Mater. Phys. 1995, 51, 5682−5692. (39) Mikhailik, V. B.; Kraus, H.; Wahl, D.; Mykhaylyk, M. S. Luminescence Studies of Ti-doped Al2O3 Using Vacuum Ultraviolet Synchrotron Radiation. Appl. Phys. Lett. 2005, 86, 101909−1−3. (40) Mikhailik, V. B.; Di Stefano, P. C. F.; Henry, S.; Kraus, H.; Lynch, A.; Tsybulskyi, V.; Verdier, M. A. Studies of Concetration Dependencies in the Luminescence of Ti-doped Al2O3. J. Appl. Phys. 2011, 109, 053116−1−6. (41) Dorenbos, P. Mechanism of Persistent Luminescence in Sr2MgSi2O7:Eu2+,Dy3+. Phys. Status Solidi B 2005, 242, R7−R9. (42) Dorenbos, P. The Electronic Level Structure of Lanthanide Impurities in REPO4, REBO3, REAlO3, and RE2O3 (RE = La, Gd,Y, Lu, Sc) Compounds. J. Phys.: Condens. Matter 2013, 25, 225501−1−7. (43) Dorenbos, P. Lanthanide 4f-Electron Binding Energies and the Nephelauxetic Effect in Wide Band Gap Compounds. J. Lumin. 2013, 136, 122−129. (44) Cano, N. F.; Ayta, W. E. F.; Watanabe, S. The Electronic and Optical Properties of Sodalite (Na8Al6Si6O24Cl2) From First Principles. Solid State Commun. 2010, 150, 195−197. (45) Gambardella, A. A.; Schmidt Patterson, C. M.; Webb, S. M.; Walton, M. S. Sulfur K-Edge XANES of Lazurite: Toward Determining the Provenance of Lapis Lazuli. Microchem. J. 2016, 125, 299−307. (46) Reinen, D.; Lindner, G.-G. The Nature of the Chalcogen Colour Centres in Ultramarine-Type Solids. Chem. Soc. Rev. 1999, 28, 75−84. (47) Aierken, S.; Kusachi, I.; Kobayashi, S.; Atobe, K.; Yamashita, N. Photoluminescence Spectra of S2− Center in Natural and HeatTreated Scapolites. Phys. Chem. Miner. 2008, 35, 137−145. (48) Hassib, A. Electron Spin Resonance of Mn2+ in Sodalite. Phys. Chem. Miner. 1980, 6, 31−36. (49) Schneider, H.; Rager, H. Occurrence of Ti3+ and Fe2+ in Mullite. J. Am. Ceram. Soc. 1984, 67, c248−c250. (50) Kuznicki, S. M.; DeVries, K. L.; Eyring, E. M. An ESR Study of Titanium(III)-Exchanged Zeolite A. J. Phys. Chem. 1980, 84, 535−537. (51) Annersten, H.; Hassib, A. Blue Sodalite. Can. Miner. 1979, 17, 39−46. (52) Cano, N. F.; Blak, A. R.; Watanabe, S. Correlation Between Electron Paramagnetic Resonance and Thermoluminescence in Natural Sodalite. Phys. Chem. Miner. 2010, 37, 57−64. (53) McLaughlan, S. D.; Marshall, D. J. Paramagnetic Resonance of F-Type Centres in Photochromic Sodalites. Phys. Lett. A 1970, 32, 343−344. (54) Hodgson, W. G.; Brinen, J. S.; Williams, E. F. Electron Spin Resonance Investigation of Photochromic Sodalites. J. Chem. Phys. 1967, 47, 3719−3723. (55) Takeyama, T.; Nakamura, T.; Takahashi, N.; Ohta, M. Electron Paramagnetic Resonance Studies on the Defects Formed in the Dy(III)-Doped SrAl2O4. Solid State Sci. 2004, 6, 345−348. (56) Hölsä, J.; Aitasalo, T.; Jungner, H.; Lastusaari, M.; Niittykoski, J.; Spano, G. Role of Defect States in Persistent Luminescence Materials. J. Alloys Compd. 2004, 374, 56−59. (57) Buhl, J.-C.; Schuster, K.; Robben, L. Nanocrystalline Sodalite Grown from Superalkaline NaCl Bearing Gels at Low Temperature (333 K) and the Influence of TEA on Crystallization Process. Microporous Mesoporous Mater. 2011, 142, 666−671. (58) Jenkins, H. D. B.; Thakur, K. P. Reappraisal of Thermochemical Radii for Complex Ions. J. Chem. Educ. 1979, 56, 576−577. (59) Trill, H.; Eckert, H.; Srdanov, V. NMR Study of Interacting F Centers in Na8[Al6Si6O24]Cl2 Sodalite. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 71, 014412. 11601

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Research Article

ACS Applied Materials & Interfaces (60) Heinmaa, I.; Vija, S.; Lippmaa, E. NMR Study of Antiferromagnetic Black Sodalite Na8(AlSiO4)6. Chem. Phys. Lett. 2000, 327, 131−136.

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NOTE ADDED AFTER ASAP PUBLICATION This paper published ASAP on 5/2/16. A change was made in section 3.4 and the revised version was reposted on 5/11/16.

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