applied sciences

Article Synthesis and Features of Luminescent Bromo- and Iodohectorite Nanoclay Materials

Hellen Silva Santos 1,2,*, Isabella Norrbo 1,2 ID , Tero Laihinen 1, Jari Sinkkonen 1, Ermei Mäkilä 2,3, José M. Carvalho 4, Pia Damlin 1,5, Hermi F. Brito 6, Jorma Hölsä 7 and Mika Lastusaari 1,5

1 Department of Chemistry, University of Turku, FI-20014 Turku, Finland; tijnor@utu.fi (I.N.); tero.laihinen@utu.fi (T.L.); jari.sinkkonen@utu.fi (J.S.); pia.damlin@utu.fi (P.D.); mika.lastusaari@utu.fi (M.L.) 2 Doctoral Programme in Physical and Chemical Sciences, University of Turku Graduate School (UTUGS), FI-20014 Turku, Finland; ermei.makila@utu.fi 3 Department of Physics and Astronomy, University of Turku, FI-20014 Turku, Finland 4 Department of Applied Physics, Institute of Physics, University of São Paulo, São Paulo BR-05508090, Brazil; [email protected] 5 Turku University Centre for Materials and Surfaces (MatSurf), FI-20014 Turku, Finland 6 Department of Fundamental Chemistry, Institute of Chemistry, University of São Paulo, São Paulo BR-05508000, Brazil; [email protected] 7 Department of Physics, University of the Free State, Bloemfontein ZA-9300, South Africa; jorma.holsa@utu.fi * Correspondence: hellen.santos@utu.fi; Tel.: +358-29-450-3202

Received: 1 November 2017; Accepted: 27 November 2017; Published: 30 November 2017

Abstract: The smectites represent a versatile class of clay minerals with broad usage in industrial applications, e.g., cosmetics, drug delivery, bioimaging, etc. Synthetic hectorite Na0.7(Mg5.5Li0.3)[Si8O20](OH)4 is a distinct material from this class due to its low-cost production method that allows to design its structure to match better the applications. In the current work, we have synthesized for the first time ever nanoclay materials based on the hectorite structure but with the hydroxyl groups (OH−) replaced by Br− or I−, yielding bromohectorite (Br-Hec) and iodohectorite (I-Hec). It was aimed that these materials would be used as phosphors. Thus, OH− replacement was done to avoid luminescence quenching by multiphonon de-excitation. The crystal structure is similar to nanocrystalline fluorohectorite, having the d001 spacing of 14.30 Å and 3 nm crystallite size along the 00l direction. The synthetic materials studied here show strong potential to act as host lattices for optically active species, possessing mesoporous structure with high specific surface area (385 and 363 m2 g−1 for Br-Hec and I-Hec, respectively) and good thermal stability up to 800 ◦C. Both materials also present strong blue-green emission under UV radiation and short persistent luminescence (ca. 5 s). The luminescence features are attributed to Ti3+/TiIV impurities acting as the emitting center in these materials.

Keywords: bromohectorite; iodohectorite; clay materials; host lattice; titanium luminescence

1. Introduction Clay minerals have been known since antiquity, when their applications were first explored to obtain products ranging from ceramics to health-related materials [1,2]. Nowadays, these materials play important role in the research of diversified fields such as solar cell production [3,4], environmentally friendly heterogeneous catalysts [5,6], photo-catalysts [7], coatings [8,9], cosmetics products [10] and biomedical applications (e.g., drug delivery, bioimaging, tissue engineering, regenerative medicine) [1]. This wide variety of applications is due to their convenient physicochemical features such as plasticity,

Appl. Sci. 2017, 7, 1243; doi:10.3390/app7121243 www.mdpi.com/journal/applsci Appl. Sci. 2017, 7, 1243 2 of 20 swelling capacity, anisotropy of the layers, ion-exchange properties as well as the associated low cost of production [1,2,11]. Clay minerals are composed of two types of polymeric sheets. One type has tetrahedral (T) units as building blocks, and the second is formed from octahedral (O) units. The different types of clay minerals are classified into two groups based on the way these sheets connect to each other, i.e., T:O and T:O:T. The T:O type of clay minerals have rigid structures with the layers sequentially stacked to each other with a very small space between them. These types of clay minerals are thus used, e.g., as scaffolds for the production of ceramic materials [2], immobilization of proteins [12] and generation of nano-inorganic micelles to capture hydrocarbon and aromatic oils in vapor and liquid states [13]. On the other hand, T:O:T type materials have a much more flexible structure because the layers are connected to each other through a large interlayer space filled with hydrated cations in order to compensate for the negative charges originating from the aliovalent substitutions. Such negatively charged layers are regarded as one of the most important characteristics of the T:O:T type clay minerals due to the easy cation exchange and high absorption potential of the interlayer space. In fact, it is in the interlayer space where the most important technological functionalities of the T:O:T type clay minerals originate [2]. Different types of T:O:T materials have been employed in a broad range of applications. For instance, many smectite clays have been used for producing bionanocomposites for scaffolds and regenerative medicine [14]. Mechanochemically activated saponite can be used for the selective removal of Cu2+ and Ni2+ from aqueous solutions [15]. Fluorohectorite clay has proven to be an effective material for drug delivery of ciprofloxacin with a controlled release rate [16], and hectorite has been used to improve the abrasion resistance, water vapor barrier and anticorrosion properties of nanocomposite coatings [17]. Among this large group of T:O:T type clay minerals, the smectites represent the largest and most important class [11] because of their distinguished features such as high specific surface area, easily tunable structure, moderate layer charge and propensity for intercalating extraneous species of variable sizes [2]. Moreover, the ability of intercalating additional activator species into the interlayer space of smectites can produce a variety of optically active materials [18]. Although natural smectites can be found in abundance, they usually contain high concentrations of impurities (e.g., quartz, K, Na, Ca, Mg, Fe, and diverse organic impurities, depending on the sedimentation characteristic of the extracting mine) and structural defectAs (e.g., cis- or trans-vacancies, orientation of tetrahedra, localization of isomorphous substitutions, and stacking faults) [11,19], which may deteriorate the performance of luminescent materials. Therefore, the current work is focused on the study of synthetic hectorite-like types of smectites, aiming for their application as host matrices for optically active species. Synthetic hectorite Na0.7(Mg5.5Li0.3)[Si8O20](OH)4 is easily obtained at low temperatures and pressures [11,20], which enables its production in high quantities, at low prices and with controllable composition, purity and crystal dimensions [1]. Hectorite is a T:O:T (or 2:1) type of trioctahedral smectite, meaning it is composed of two sheets of corner-sharing SiO4 tetrahedra (T) intercalated 2+ + with one sheet of edge-sharing M(O,OH)6 octahedra (O), where M is Mg or Li . These T:O:T units are negatively charged, and they are electrostatically connected to each other across the interlayer space composed of hydrated Na+ species [2,11]. This composition may vary, for instance when OH− groups are partially replaced by F−, creating fluorohectorite [11,21]. When aiming for luminescence applications, the OH− groups of the hectorite structure must be avoided since the high energy phonons of these ions (3500 cm−1) can cause luminescence quenching through multiphonon de-excitation [22]. The primary goals of the current work were to design and synthesize bromohectorite (Br-Hec) and iodohectorite (I-Hec) nanoclay materials based on the hectorite structure, but with the hydroxyl groups replaced by Br− and I−. The second intention was to maintain the essential features characteristic of hectorites that are needed for future applications, i.e., high surface area and good thermal stability. Finally, it was intended that the materials would show the inherent broadband luminescence emission observed earlier for fluoro- [23] and chlorohectorite [24]. To the best of the authors’ knowledge, Appl. Sci. 2017, 7, 1243 3 of 20 the synthesis and properties of bromohectorite and iodohectorite are now being reported for the first time.

2. Materials and Methods

2.1. Synthesis

The Br-Hec and I-Hec Na0.7(Mg5.5Li0.3)[Si8O20](X)4, X = Br, I) were synthesized using a hydrothermal crystallization method [20] in a reflux system at 100 ◦C. The Br-Hec was made by preparing a colloidal mixture with stoichiometric amounts of LiBr (≥99%, Sigma-Aldrich, St. Louis, MO, USA), MgO (≥97%, Merck, Darmstadt, Germany), SiO2 (99.8%, Sigma-Aldrich, St. Louis, MO, USA) and a threefold excess of NaBr (≥99%, J.T. Baker, Center Valley, PA, USA) in a neutral aqueous solution, keeping the mixture in a reflux system for 24 h. I-Hec was otherwise synthesized similarly but with LiBr replaced by LiI (≥99.9%, Fluka, Buchs, Switzerland) and NaBr replaced by NaI (≥99.5%, Merck, Darmstadt, Germany). After ceasing the reflux, both materials were vacuum filtered and washed with deionized water six consecutive times to remove the excess halogen ions from the filtrate liquids, which were checked with the silver nitrate test. Thereafter, both materials were dried for 12 h at 200 ◦C. The X-Hec materials were doped with Ti3+ in three steps, through a cation exchange reaction described previously [24]. First, homogeneous colloidal suspensions were generated by adding 0.1 g of neutral X-Hec into 10 cm3 of deionized water, while continuously stirring and heating (30 min at ◦ −3 50 C). Then, stoichiometric amounts of a 0.1 mol dm aqueous solution of TiCl3 (about 15% of TiCl3 in about 10% of hydrochloric acid, Merck, Darmstadt, Germany) were added to these suspensions to obtain 0.1 mol % of Ti3+ (relative to the Na+ amount in the interlayer space). The suspensions were stirred and heated (3 h at 200 ◦C) and then vacuum filtered and dried (12 h at 200 ◦C).

2.2. Characterization The crystal structure and purity of the Br-Hec and I-Hec were investigated by X-ray powder diffraction (XPD) measurements at room temperature (RT) with a Huber G670 Guinier camera (Huber, Rimsting, Germany), using an image plate detector and Cu Kα1 radiation. The crystallite size was calculated with the Scherrer formula [23–25]. The chemical bonds formed at the molecular level of both nanoclays were studied with FTIR measurements. The spectra were determined at RT in the range of 400–4000 cm−1 using a Bruker Vertex 70 spectrometer (Bruker Optics Scandinavia AB, Solna, Sweden) equipped with a RT-DLaTGS detector and a VideoMVPTM Diamond ATR accessory. For each spectrum, a total of 320 interferograms were co-added using 4 cm−1 as resolution. For the diamond as internal reflection element in the attenuated total reflection (ATR) method, the limitation is as follows: intrinsic absorption from approximately 2300 to 1800 cm−1 limits its usefulness in this region (gives in this range only 5% transmission). The qualitative compositions of the Br- and I-Hec were analysed through X-ray fluorescence spectroscopy (XRF) with a Panalytical Epsilon 1 apparatus (Panalytical, Almelo, The Netherlands), using Ag Kα radiation (22.16 keV), and a high-resolution Si drift detector (with a typical energy resolution of 135 eV at 5.9 keV/1000 cps). The chemical environment around the SiIV cations of the Br-Hec and I-Hec was studied with solid-state 29Si MAS NMR spectroscopy. The spectra were recorded for 24 h at room temperature with a Bruker AV400 spectrometer (Bruker, Fällanden, Switzerland) using a 10,000 Hz spinning rate and a 60 s relaxation time. The parts per million scale was calibrated against tetramethylsilane (TMS) at 0.00 ppm, designated by the equipment. The surface area and porosity of both nanoclays were determined by nitrogen sorption at 77 K, measured through a TriStar 3000 apparatus (Micromeritics, Norcross, GA, USA). The surface area was calculated with the Brunauer–Emmett–Teller (BET) method [26]. The volume of the mesopores was estimated using the total adsorbed amount at a relative pressure p/p0 = 0.97 [27]. Appl. Sci. 2017, 7, 1243 4 of 20

The thermal stability of the Br-Hec and I-Hec was studied with a TA Instruments SDT Q600 simultaneousAppl. Sci. 2017 TGA-DSC, 7, 1243 apparatus (TA Instruments, New Castle, DE, USA). Approximately4 20of 20 mg of ◦ 3 −1 each material was heated from 25 to 1000 C in flowing N2 (rate flow: 100 cm min , heating rate: 10 ◦C minThe−1). thermal An empty stability aluminum of the Br-Hec oxide and crucible I-Hec was studied used as with the a reference TA Instruments material, SDT and Q600 similar cruciblessimultaneous were used TGA-DSC as sample apparatus pans. (TA Instruments, New Castle, DE, USA). Approximately 20 mg of each material was heated from 25 to 1000 °C in flowing N2 (rate flow: 100 cm3 min−1, heating rate: The visual appearance of both materials was recorded with photos taken with a Canon EOS 10 °C min−1). An empty aluminum oxide crucible was used as the reference material, and similar 60 Dcrucibles camera (Canon,were used Tokyo, as sample Japan), pans. which has 18 megapixel resolution and 5.3 fps shooting speed. The emissionThe visual and excitation appearance spectra of both as materials well as was luminescence recorded with decay photos curves taken were with measured a Canon EOS at RT 60 with an AgilentD camera Varian (Canon, Cary Tokyo, Eclipse Japan), spectrophotometer which has 18 megapixel (Agilent, Santaresolution Clara, and CA, 5.3 USA),fps shooting using aspeed. 150 W Xe lampThe as theemission excitation and source.excitation spectra as well as luminescence decay curves were measured at RT with an Agilent Varian Cary Eclipse spectrophotometer (Agilent, Santa Clara, CA, USA), using a 3. Results150 W Xe lamp as the excitation source.

3.1. Structural3. Results Characterization and Purity

The3.1. Structural XPD patterns Charac ofterization the bromohectorite and Purity and iodohectorite (Figure1) show that both materials have a hexagonal crystal structure similar to nanocrystalline fluorohectorite, indicating that no crystalline impuritiesThe were XPD detected patterns withinof the bromohectorite the detection and limit iodohectorite of the equipment. (Figure 1) The show Br-Hec that both and materials I-Hec have a have a hexagonal crystal structure similar to nanocrystalline fluorohectorite, indicating that no d spacing of 14.30 Å (equivalent to 6.2◦ in 2θ), indicating two hydrate layers in their interlayers 001 crystalline impurities were detected within the detection limit of the equipment. The Br-Hec and when recorded at room temperature [28]. The spacing falls in the middle of the range of 10–20 Å I-Hec have a d001 spacing of 14.30 Å (equivalent to 6.2° in 2θ), indicating two hydrate layers in their predictedinterlayers for the when basal recorded spacing at of room smectites temperature [29]. Due [28]. toThe the spacing nanosize falls of in thethe crystallites,middle of the the range respective of reflections10–20 Å are predicted regularly for broad,the basal generating spacing of ansmectites extensive [29]. overlapDue to the that nanosize does not of the allow crystallites, for anaccurate the determinationrespective reflections of their FWHMs. are regularly However, broad, generating the single an (001)extensive reflection overlap at that 6.2 does◦ (in not 2θ allow) is suitable for an for carryingaccurate out thedetermination Scherrer calculations of their FWHMs. [25] to However, determine the the single diffracting (001) reflection domain size,at 6.2° which (in 2θ indicates) is the sizesuitable of approximately for carrying out 3 nmthe alongScherrer the calculations 001 direction [25] forto determine both materials. the diffracting The XPD domain patterns size, of the materialswhich also indicates indicate the the size formation of approximately of a plate-like 3 nm along morphology, the 001 direction because for the both (00l) materials. reflections The areXPD wider than thepatterns others, of the which materials means also that indicate the crystallites the formation are largerof a plate-like in the other morphology, directions, because i.e., thethe (h00)(00l) and reflections are wider than the others, which means that the crystallites are larger in the other (0k0) directions. directions, i.e., the (h00) and (0k0) directions.

3 Na (Mg Li )[Si O ]X 0.7 5.5 0.3 8 20 4 001 Cu Kα1(1.5406 Å) 293 K

2 # Sample holder # X: I, Iodohectorite

1 Br, Bromohectorite Fluorohectorite Intensity/ Arb. Units Nanocrystalline * * (00l) reflections * * Microcrystalline * * 0 * 10 20 30 40 50 60

θ 2 / Degrees FigureFigure 1. The 1. The XPD XPD patterns patterns of of the the bromohectorite bromohectorite and io iodohectorite.dohectorite. The The reference reference patterns patterns for nano- for nano- and microcrystallineand microcrystalline fluorohectorite fluorohectorite were were calculatedcalculated with with PowderCell PowderCell v 2.4 v 2.4program program (W. (W.Kraus Kraus and and G. Nolze,G. Nolze, BAM, BAM, Berlin, Berlin, Germany, Germany, 2000) 2000) using using structural structural data from from [30]. [30 ].

The XRF spectra of Br-Hec and I-Hec (Figure 2) confirmed the expected chemical composition Theof the XRF materials. spectra Both of Br-Hec spectra and show I-Hec the lines (Figure related2) confirmed to the Mg theKα1, expected Si Kα1, and chemical Ca Kα2 emissions composition at of the materials.1.25, 1.74, Bothand 3.69 spectra keV, show respectively the lines [31]. related Here to the the presence Mg Kα 1of, Si calcium Kα1, and may Ca be K αassociated2 emissions to its at 1.25, 1.74, andcommon 3.69 presence keV, respectively as impurity [ 31in ].the Here sodium the halide presence precursors of calcium used mayin the be synthesis. associated Therefore, to its common it is presence as impurity in the sodium halide precursors used in the synthesis. Therefore, it is expected Appl. Sci. 2017, 7, 1243 5 of 20

Appl. Sci. 2017, 7, 1243 5 of 20 that the calcium cations share the interlayer space of the nanoclay materials together with the Na+ ions. Also, theexpected spectra that show the thecalcium lines cations of Ag Lshare1,Lα the1,2,L interlayβ1,Lβer2 andspace L γof1 ,the from nanoclay 2.63 to materials 3.52 keV together [31], which with are due tothe Na silver+ ions. excitation Also, the sourcespectra ofshow the the equipment. lines of AgThe L1, L XRFα1,2, L spectrumβ1, Lβ2 and L ofγ1, the from Br-Hec 2.63 to exhibits 3.52 keV the [31], which are due to the silver excitation source of the equipment. The XRF spectrum of the Br-Hec line of Br Lα1,2 emission at 1.48 keV, confirming the presence of bromine. Similarly, the iodine in the exhibits the line of Br Lα1,2 emission at 1.48 keV, confirming the presence of bromine. Similarly, the composition of the I-Hec is observed as the emission lines of I Lα1,Lβ1 and Lβ2 at 3.94, 4.22 and iodine in the composition of the I-Hec is observed as the emission lines of I Lα1, Lβ1 and Lβ2 at 3.94, 4.51 keV, respectively. Moreover, the spectra of both materials indicate the presence of Ti as an impurity 4.22 and 4.51 keV, respectively. Moreover, the spectra of both materials indicate the presence of Ti as witnessed by the weak Kβ line at 4.9 keV. an impurity witnessed1 by the weak Kβ1 line at 4.9 keV.

0.3 Na (Mg Li )[Si O ]Br 16 0.7 5.5 0.3 8 20 4 # XRF: 293 K # Ti Ag Kα1: 22.16 keV 0.2 K : 4.9 β1 12 Si K : 1.74 3.9 4.2 4.5 4.8 α1 ) / Counts ) / 6 *Ag L : 2.63 8 1 L : 2.98 α2

*Mg Lβ1: 3.15 K : 1.25 α1 Lβ2: 3.35 4 * Br Lγ1: 3.52

Intensity (x 10 (x Intensity L : 1.48 *Ca α1,2 * K : 3.69 α2 * * * * # Ti 0 * * 0123456 Energy / keV

3.0 #Ti Na0.7(Mg5.5Li0.3)[Si8O20]I4 K : 4.50 16 2.5 α1,2 XRF: 293 K K : 4.9 # β1 Ag Kα1: 22.16 keV 2.0 * * # * Si 1.5 12 Kα : 1.74 1 3.9 4.2 4.5 4.8 *Ag

) / Counts *I 6 L1: 2.63 L : 3.94 α1 8 Lα : 2.98 2 L : 4.22 β1 Lβ1: 3.15 L : 4.51 Lβ2: 3.35 β2 L : 3.52 4 * γ1 *Ca Mg Intensity (x 10 (x Intensity K : 3.69 * α2 Kα : 1.25 1 * * # Ti 0 * * * * * 0123456 Energy / keV

Figure 2.FigureThe 2. XRF The spectra XRF spectra of bromohectorite of bromohectorite (top (top) and) and iodohectorite iodohectorite ((bottombottom).). The The upper upper insertions insertions show theshow presence the presence of Ti impuritiesof Ti impurities in both in both materials materials (• :(•: instrumental instrumental signal). signal).

The FTIR spectra of Br-Hec and I-Hec (Figure 3) show a strong band at 460 cm−1 related to the The FTIR spectra of Br-Hec and I-Hec (Figure3) show a strong band at 460 cm −1 related to the symmetric stretching of Mg–O in the octahedral units of hectorite type clay minerals [32]. In symmetricaddition, stretching both spectra of Mg–O present in the the octahedral Si–O–Si bands units of ofthe hectorite tetrahedral type layers clay of minerals ordered hectorite [32]. In addition,at 660, both spectra800, and present 1010–1090 the cm Si–O–Si−1 related bands to the of bending the tetrahedral as well as layers symmetric of ordered and asymmetric hectorite stretching at 660, 800, − and 1010–1090modes of cm Si–O–Si,1 related respectively to the bending [32,33]. None as well of asthe symmetric synthesized and materials asymmetric show indication stretching of modes an of Si–O–Si,amorphous respectively silica phase [32,33 that]. None would of be the observed synthesized as a weak materials band at show1200 cm indication−1 [33]. The of weak an amorphous bands at silica phaseca. 1625 that and would 3300 becm observed−1 are assigned as a weakto the bandH–O–H at 1200bending cm −an1 d[33 stretching,]. The weak respectively, bands at ca.indicating 1625 and 3300 cm−1 are assigned to the H–O–H bending and stretching, respectively, indicating low amounts of adsorbed water on the clays and/or water present in the hydration shell of the Na+ cations in the Appl. Sci. 2017, 7, 1243 6 of 20

Appl. Sci. 2017, 7, 1243 6 of 20 low amounts of adsorbed water on the clays and/or water present in the hydration shell of the Na+ Appl. Sci. 2017, 7, 1243 6 of 20 cations in the interlayer space of both clay materials [34]. Moreover, in the FTIR spectra of Br-Hec or interlayerI-Hec therelow space amounts are no of bands bothof adsorbed clay related materials water to the on [vibrationthe34]. clays Moreover, and/or of hydroxyl water in the present groups FTIR spectrain in the the hydration ofoctahedral Br-Hec shell or layers of I-Hec the ofNa there clay+ areminerals. no bandscations In common relatedin the interlayer tohectorite the vibration space clays of both these of hydroxylclay are material observ groupss [34].ed as inMoreover, strong the octahedral sharp in the FTIRbands layers spectra in ofthe clayof region Br-Hec minerals. fromor In3600 common toI-Hec 3700 hectoritethere cm −are1 [32,34]. no clays bands theseTherefore, related are to observed thethe vibration absence as of strong of hydroxyl these sharp groupshydroxyl bands in the inbands octahedral the regionindicates layers from that of 3600 clay both to 3700materials, cmminerals.−1 [Br-Hec32,34 In]. Therefore,commonand I-Hec, hectorite the have absence claystheir ofthese octahedr these are hydroxyl observal unitsed bands as formed strong indicates sharpwith thatbandsthe bothrespective in the materials, region halogens, from Br-Hec 3600 to 3700 cm−1 [32,34]. Therefore, the absence of these hydroxyl bands indicates that both andinstead I-Hec, of the have common their octahedral OH− groups units of hectorite. formed with the respective halogens, instead of the common materials, Br-Hec and I-Hec, have their octahedral units formed with the respective halogens, OH− groups of hectorite. instead of the common OH− groups of hectorite. Na (Mg Li )[Si O ]X 0.7 5.5 0.3 8 20 4 1.2 * δ (Si-O-Si): 660 Na (Mg Li )[Si O ]X 0.7 5.5 0.3 8 20 4 #1.2 υ (Si-O-Si): 800 s * δ (Si-O-Si): 660 # υ (Si-O-Si): 800 s δ υ 0.8 (H-O-H): 1625 (H-O-H) X: Br # δ 3200-3400 υ 0.8 (H-O-H): 1625 (H-O-H) X: BrI # * 3200-3400 I * 0.4 0.4

υ (Si-O-Si): 1010-1090 a υ a (Si-O-Si): 1010-1090 Normalized Transmittance

0.0Normalized Transmittance -1 0.0υ (Mg-O):υ 460 cm -1 s s (Mg-O): 460 cm

10001000 2000 2000 3000 3000 4000 4000 Wavenumber / cm-1-1 Wavenumber / cm Figure 3. The FTIR spectra of the bromohectorite and iodohectorite. FigureFigure 3. The 3. FTIRThe FTIR spectra spectra of the of the brom bromohectoriteohectorite and and iodohectorite.iodohectorite.

The 29Si NMR spectra of Br-Hec and I-Hec (Figure 4) indicate that the fractions of silicon present The 29SiSi NMR NMR spectra spectra of of Br-Hec Br-Hec and I-Hec (Figure 44)) indicateindicate thatthat thethe fractionsfractions ofof siliconsilicon presentpresent in the tetrahedral sheets have the same chemical environment that is predicted for the tetrahedral in the tetrahedral sheets have the same chemical environment that is predicted for the tetrahedral in the tetrahedrallayers of synthetic sheets hectorite. have the The same most chemical intense resonance environment signals that at − 98is (Br-Hec)predicted and for −96 the ppm tetrahedral (I-Hec) layers of synthetic hectorite. The most intense resonance signals at −98 (Br-Hec) and −96 ppm (I-Hec) layers ofare synthetic related to hectorite. the Q3 sites The of most the intenseSi(OMg)(OSi) resonance3 type, signals as anticipated at −98 (Br-Hec) for the hectorite and −96 framework ppm (I-Hec) 3 are relatedrelated[35,36]. to to the Thethe Q signalQsites3 sites observed of theof the Si(OMg)(OSi) for Si(OMg)(OSi) both clays3 attype, ca.3 type, − as87 anticipatedppm as anticipatedis associated for the forwith hectorite the the hectoriteQ2 frameworkchemical framework shift [35 of,36 ]. 2 2 The[35,36]. signal SiOThe4 observed tetrahedrasignal observed forlinked both to for claysother both two at clays ca. tetrahedral− at87 ca. ppm − unit87 is ppms associatedby oxygenis associated bonds with the[37],with Q which thechemical Q in chemicalthe Br-Hec shift ofshift and SiO of4 tetrahedraSiO4 tetrahedraI-Hec linked structures linked to other may to other be two represented two tetrahedral tetrahedral by Si(OMg)(OSi) units unit bys by oxygen2 Xoxygen (X = Br bonds −bonds or I−) [ groups,37 [37],], which which having in in the the the halogens Br-HecBr-Hec at andand − − I-Hec structuresthe apical maymay(trans) bebe positions represented in the byby Si(OMg)(OSi)Si(OMg)(OSi)octahedral layers22X (X [35]. = Br BrAdditionally− oror I− I) groups,) groups, to the having havingexpected the the halogensprevious halogens at atthe the apical apicalsignals, (trans) (trans) both positionsmaterials positions alsoin in the present the octahedral octahedral a weaker layers layerssignal [35]. [at35 ca.]. Additionally Additionally−111 ppm indicating to to the the the expected presence previousprevious of Q4 Si(OSi)4 centers, which may be due to small amounts of an amorphous silica framework (amorphous 4 signals, both materialsmaterials alsoalso presentpresent aa weakerweaker signalsignal atat ca.ca. −−111 ppm indicating the presencepresence ofof QQ4 silica) [35,37]. Si(OSi)44 centers, which may be due to small amounts of an amorphous silica framework (amorphous silica) [[35,37].35,37]. 5

Na0.7(Mg5.5Li0.3)[Si8O20]X4 5 4 29Si MAS NMR 3 24 h @ 293 K Q Na0.7(Mg100005.5 HzLi0.3)[Si8O20]X4 29Si3 MAS NMR 4 ) / Counts 8 3 24 h @ 293 K Q 10000 Hz 2 2 3 Q ) / Counts 8 X: I 4 1 Q Br 2 2 Intensity (x 10 Q 0 X: I 4 1 Q Br0 -40 -80 -120 -160 -200 Intensity (x 10 Chemical shift / ppm 0 Figure 4. The 29Si MAS0 NMR -40spectra of -80 bromohectorite -120 -160and iodohectorite -200 clay materials. Chemical shift / ppm

29 FigureFigure 4.4. The 29SiSi MAS MAS NMR NMR spectra spectra of bromohectorite and iodohectorite clay materials. Appl. Sci. 2017, 7, 1243 7 of 20 Appl. Sci. 2017, 7, 1243 7 of 20

The degree of branching relative to the chain (of the tetrahedral layers) was estimated by the ratioThe between degree ofthe branching intensities relative of the toQ2the and chain Q3 signals (of the (Q tetrahedral2/Q3) [37]. layers)The calculated was estimated ratios were by the 0.37 ratio and between0.41 for the the intensities Br-Hec and of the I-Hec, Q2 and respectively, Q3 signals (Qwhic2/Qh3 )[indicates37]. The a calculated higher degree ratios of were branching 0.37 and for 0.41 the forpolymeric the Br-Hec sheets and I-Hec, for I-Hec respectively, than Br-Hec. which indicates a higher degree of branching for the polymeric sheets for I-Hec than Br-Hec. 3.2. Porosity, Packing, and Thermal Stability 3.2. Porosity, Packing, and Thermal Stability The porosity and packing properties of Br-Hec and I-Hec were evaluated through nitrogen sorptionThe porosity isotherms, and recorded packing in properties triplicate ofat 77 Br-Hec K for andboth I-Hecmaterials were (Figure evaluated 5). Both through materials nitrogen display sorptionadsorption isotherms, isotherms recorded with inthe triplicate shape typical at 77 K of for type both V, materials according (Figure to the5). IUPAC Both materials classification display [38], adsorptionwhich is isothermscharacteristic with of the mesoporous shape typical materials. of type Since V, according Br-Hec toand the I-Hec IUPAC behave classification as mesoporous [38], whichmaterials, is characteristic the hysteresis of mesoporous loop of their materials. sorption isotherms Since Br-Hec can andbe correlated I-Hec behave with astheir mesoporous texture. The materials,sorption theisotherms hysteresis of both loop clay of theirmaterials sorption show isotherms the H3 hysteresis can be correlated loop, according with their to the texture. IUPAC Theclassification sorption isotherms [38]. Such of botha type clay of materialshysteresis show loop thepoints H3 hysteresisout that these loop, mesoporous according to materials the IUPAC have classificationslit-shaped [pores,38]. Suchwhich a indicates type of hysteresisthat there are loop no pointslimitations out thatfor adsorption these mesoporous at high p/p materials0. This is a havetypical slit-shaped feature pores,of non-rigid which aggregates indicates thatof plate-like there are particles no limitations [38]. This for plate-like adsorption structure at high was p/p also0. Thisevident is a typical in the feature XPD patterns of non-rigid (discussed aggregates above), of plate-likeand is confirmed particles with [38]. the This respective plate-like N structure2 sorption wasisotherms. also evident in the XPD patterns (discussed above), and is confirmed with the respective N2 sorptionThe isotherms. physisorption phenomena were analysed using two data reduction methods, in order to calculateThe physisorption the surface area phenomena and the werepore volume analysed of using each twomaterial data (Figure reduction 5). methods,The surface in areas order were to calculatecalculated the using surface the area BET and method the pore [26], volume considering of each the material model of (Figure a multilayer5). The surfacecoverage. areas The were Br-Hec calculatedhas a surface using area the BET of 385 method m2 g− [126, while], considering the I-Hec the gives model the of area a multilayer of 363 m2 coverage. g−1, which The are Br-Hec higher has than − − a surfacethose described area of 385 in m the2 g literature1, while the for I-Hec synthetic gives hectorites the area of (5 363 to m 3002 g m1,2 whichg−1) [39], are but higher similar than to those those − describedreported in for the Laponite literature® (between for synthetic 279 hectoritesand 375 m (52 g to−1) 300[39,40]. m2 g 1)[39], but similar to those reported − for LaponiteThe volume® (between of the 279 mesopores and 375 m 2wasg 1calculated)[39,40]. using the method described by Rouquerol et al. [27],The indicating volume of the the value mesopores of 0.48 was and calculated 0.50 cm3 usingg−1 for the the method Br-Hec described and I-Hec, by Rouquerolrespectively. et al.The [27 pore], − indicatingvolumes the of valuethe materials of 0.48 and did 0.50 not cm change3 g 1 forfrom the the Br-Hec adsorption and I-Hec, to the respectively. desorption The curve. pore Thus, volumes these ofmaterials the materials have did the not reversible change adsorption from the adsorption process expected to the desorption for mesoporous curve. materials Thus, these and materials needed for havecatalysts. the reversible adsorption process expected for mesoporous materials and needed for catalysts.

350

Na0.7(Mg5.5Li0.3)[Si8O20]X4 -1 300

g Nitrogen sorption isotherms 3 250 77 K

200

150

100 BET area Pore Volume X: m2g-1: cm3g-1: I 363 ± 4 0.50 ± 0.01 Volume adsorbed / cm adsorbedVolume 50 Br 385 ± 5 0.48 ± 0.01 0 0.0 0.2 0.4 0.6 0.8 1.0 Relative pressure / pp -1 0

FigureFigure 5. The5. The nitrogen nitrogen sorption sorption isotherms isotherms of bromohectoriteof bromohectorite and and iodohectorite. iodohectorite.

The TGA-DSC curves of Br-Hec and I-Hec (Figure 6) show that the mineral structure is stable The TGA-DSC curves of Br-Hec and I-Hec (Figure6) show that the mineral structure is stable up to 800 °C. The materials show the three main events expected for hectorite type of clay minerals. up to 800 ◦C. The materials show the three main events expected for hectorite type of clay minerals. The first asymmetric endothermic signal is related to the loss of adsorbed water molecules [35,41]. The first asymmetric endothermic signal is related to the loss of adsorbed water molecules [35,41]. This is observed in the DSC curves as a single event at 89 °C for the Br-Hec and at 114 °C for the This is observed in the DSC curves as a single event at 89 ◦C for the Br-Hec and at 114 ◦C for the I-Hec. I-Hec. The temperature ranges related to these occurrences are better observed in the first The temperature ranges related to these occurrences are better observed in the first derivatives of the Appl.Appl. Sci. Sci. 20172017,, 77,, 1243 1243 88 of of 20 20 derivatives of the TGA curves (Figure A1), which indicate that these first events are accompanied withTGA mass curves losses (Figure of 3%A1 ),(30–135 which °C) indicate and 1.8% that these(30–191 first °C) events for Br-Hec are accompanied and I-Hec, withrespectively. mass losses of 3% ◦ ◦ (30–135The secondC) and 1.8%event (30–191 representsC) forthe Br-Hecloss of andthe coordinated I-Hec, respectively. water from the interlayer space of the smectitesThe second(in the eventinner hydration represents shell the loss of Na of+ the) and coordinated locked water water inside from possible the interlayer voids in space the clays’ of the + structuresmectites [35,41]. (in the Although inner hydration the TGA shell and of DSC Na )curves and locked do not water show inside this event possible clearly, voids it can in the be clays’well identifiedstructure [from35,41 the]. Although first derivatives the TGA (Figure and DSC A1). curves This loss do of not coordinated show this eventwater clearly,involves it a can mass be wellloss ofidentified 7.2% and from 6.5% the for first Br-Hec derivatives and I-Hec, (Figure respectively. A1). This The loss total of coordinated water losses water were involves 10.2% and a mass 8.3% lossfor Br-Hecof 7.2% and and I-Hec, 6.5% forrespectively, Br-Hec and which I-Hec, is respectively.consistent with The the total higher water surface losses area were observed 10.2% and for 8.3%Br-Hec. for Br-HecThe and strong I-Hec, exothermic respectively, signal which in isthe consistent DSC curves with shows the higher the collapse surface areaof the observed mineral for structure Br-Hec. (FigureThe 6). strong This was exothermic observed signal at 828 in and the 813 DSC °C curves for the shows Br-Hec the and collapse I-Hec, ofrespectively, the mineral showing structure a ◦ mass(Figure loss6). of This ca. was 2.5% observed for both at materials. 828 and 813 The Ctota forl themass Br-Hec losses and (from I-Hec, 30 to respectively, 1000 °C) were showing 12.7% a massand ◦ 10.8%loss of for ca. the 2.5% bromo for both and materials. iodo-hectorites, The total respective mass lossesly. (fromThe collapse 30 to 1000 resultsC) weremainly 12.7% in the and formation 10.8% for ofthe two bromo other and minera iodo-hectorites,ls: enstatite respectively. (MgSiO3) and The collapsecristobalite results (SiO mainly2), which in the was formation confirmed of twoby otherXPD measurementsminerals: enstatite of the (MgSiO decomposed3) and cristobalite materials (SiOafter2 ),the which thermal was confirmedanalysis, where by XPD the measurements materials were of ◦ heatedthe decomposed up to 1000 materials °C (Figure after A2). the Because thermal the analysis, XRF data where showed the materials a similar were Br heatedand I content up to 1000 in theC materials(Figure A2 before). Because and after the XRFheating data to showed 1000 °C a (F similarigure BrA3), and it may I content be that in themagnesium materials bromide before and or ◦ iodideafter heating is also present. to 1000 However,C (Figure itA3 is), difficult it may beto confirm that magnesium this from bromide the XPD orpatterns, iodide isbecause also present. of the broadHowever, features it is of difficult the reflections to confirm between this from 26° and the XPD32°, where patterns, the becausemost intense of the reflections broad features of MgBr of2the or ◦ ◦ MgIreflections2 should between be observed 26 and (Figure 32 , whereA2). The the formatio most intensen of lithium reflections oxide, of MgBrsodium2 or oxide, MgI2 andshould other be crystallineobserved (Figure phases A2 with). The the formation respective of lithiumhalogens oxide, are also sodium possible. oxide, They and other were crystalline not identified phases in withthe XPDthe respective analysis, but halogens this may are be also due possible. to the overlappin They wereg not of identifiedreflections inin thethe XPD diffractograms analysis, but or thisthe maylow crystallinitybe due to the of overlapping those species. of reflections in the diffractograms or the low crystallinity of those species.

110 exo 20 Na0.7(Mg5.5Li0.3)[Si8O20]Br4 ° ° TGA-DSC (N2, 10 C/min) 828 C collapse of mineral 100 structure endo 0

89 °C loss of adsorbed water 380 to 640 °C -20 90 loss of coordinated Heat Flow / mW Flow Heat water

-40

80 30 Na0.7(Mg5.5Li0.3)[Si8O20]I4

Weight / % ° TGA-DSC (N2, 10 C/min) 813 °C collapse of 100 mineral structure 0 114 °C loss of adsorbed water 340 to 625 °C loss of coordinated 90 water -30

0 200 400 600 800 1000

0 Temperature / C

FigureFigure 6. 6. TheThe heating heating TGA-DSC TGA-DSC curves curves of of the the bromohectorite bromohectorite ( (toptop)) and and iodohectorite iodohectorite ( (bottombottom).).

3.3. Luminescence Properties The Br-Hec and I-Hec materials display a yellowish body color under white light; however, when the materials are exposed to UV radiation, they show blue emission (Figure 7). The emission Appl. Sci. 2017, 7, 1243 9 of 20

3.3. Luminescence Properties The Br-Hec and I-Hec materials display a yellowish body color under white light; however, Appl. Sci. 2017, 7, 1243 9 of 20 when the materials are exposed to UV radiation, they show blue emission (Figure7). The emission spectraspectra ( λ(excλexc:: 255255 nm)nm) showshow aa mainmain broadbroad blue-green blue-green emission emission band band for for both both materials materials (Figure (Figure7 ,7, upper).upper). This This main main emission emission is is centered centered at at 470 470 nm nm for for the the Br-Hec Br-Hec material, material, while while the the I-Hec I-Hec material material hashas the the center center of of the the band band red red shifted shifted to to 490 490 nm. nm. Additionally, Additionally, both both materials materials have have a a weak weak red red emissionemission at at ca. ca. 710 710 nm, nm, which which is is more more prominent prominent for for Br-Hec Br-Hec but but probably probably overlapping overlapping with with the the main main emissionemission for for the the I-Hec I-Hec material, material, where where it it is is seen seen as as a a weak weak shoulder. shoulder. When When the the materials materials are are excited excited atat 365 365 nm, nm, their their main main emission emission band band appears appears as as a a broad broad green-yellow green-yellow emission emission (Figure (Figure7, 7, lower), lower), havinghaving its its barycenter barycenter at at ca. ca. 530 530 and and 562 562 nm nm for for Br-Hec Br-Hec and and I-Hec, I-Hec, respectively, respectively, while while the the weak weak red red emissionemission remains remains at at ca. ca. 710 710 nm. nm. When When the the materials materials are are excited excited at at different different wavelengths, wavelengths, there there is is a a tendencytendency that that the the main main emission emission band band is is shifted shifted towards towards red red with with increasing increasing excitation excitation wavelength wavelength (Figures(Figures A4 A4 and and A5 A5).). This This change change in in the the barycenter barycenter of of the the emission emission bands bands with with the the variation variation of of the the excitationexcitation wavelength wavelength is partiallyis partially due due to the to cutoff the cuto causedff caused by the filtersby the used filters to eliminate used to theeliminate excitation the signalexcitation from thesignal emission from the spectrum. emission However, spectrum. the shiftHowever, may indicatethe shift either may thatindicate the activator either that center the isactivator occupying center multiple is occupying sites with multiple different sites crystal with field different strengths crystal in field the host strengths lattice, in or the the host presence lattice, of or defectsthe presence in thestructure of defects of in the the X-Hec structure materials. of the The X-He multisitec materials. origin The for multisite emission origin is also for witnessed emission in is thealso decrease witnessed of emission in the decrease band width of emission [42] with band increasing width [42] excitation with increasing wavelength. excitation wavelength.

FigureFigure 7. 7.The The emission emission spectra spectra of theof the bromohectorite bromohectorite and and iodohectorite iodohectorite under under 255 nm255( topnm) ( andtop) 365and nm 365 (bottomnm (bottom) excitation.) excitation. The inserted The inserted photos photos illustrate illustrate the visual the visual aspect aspect of the materialsof the materials under whiteunder lightwhite andlight when and irradiatedwhen irradiated with 255 with nm 255 or 365nm nm.or 365 nm.

Similar blue-green emission has previously been observed for other silicates such as fluorohectorite [23], chlorohectorite [24], montmorillonite [43], kaolinite [44], pyrophyllite [45], topaz (Al2SiO4(F,OH)2) [46], synthetic hackmanite (Na8Al6Si6O24(Cl,S)2) [47,48], benitoite (BaTiSi3O9) [49], and SiO2 [50,51]. The literature presents mainly two different explanations for the origin of this blue-green emission. One proposition categorizes this emission as photoelectric response as well as inner center and recombination type of luminescence [52], attributing the blue emission to Appl. Sci. 2017, 7, 1243 10 of 20

Similar blue-green emission has previously been observed for other silicates such as fluorohectorite [23], chlorohectorite [24], montmorillonite [43], kaolinite [44], pyrophyllite [45], topaz (Al2SiO4(F,OH)2)[46], synthetic hackmanite (Na8Al6Si6O24(Cl,S)2)[47,48], benitoite (BaTiSi3O9)[49], and SiO2 [50,51]. The literature presents mainly two different explanations for the origin of this blue-green emission. One proposition categorizes this emission as photoelectric response as well as inner center and recombination type of luminescence [52], attributing the blue emission to triplet-singlet transition in twofold coordinated Si centers [53], due to oxygen deficit and irradiation effects [50]. The second proposition associates the emission with luminescent centers containing trivalent titanium paired with an oxide vacancy, which may exist either as matrix element or as an impurity [23,24,46–49]. It was shown previously for synthetic chlorohectorite [24] that the intensity of the blue emission band increases if titanium is deliberately doped to the material. Therefore, the band was assigned to Ti3+. It has also been verified that although natural hectorite presents a similar blue emission often 2− attributed to [SiO2] centers, it has different luminescence features than have been observed for halogen-hectorites [24]. Also, previous reports have shown that Ti species can occur as impurities in silicates, generating similar emission features as reported for silicate glasses [54], fluorohectorites [23] and chlorohectorites [24]. The materials studied in the current work have low contents of Ti, confirmed from their respective XRF spectra (Figure2). It has been proposed previously that, in the case of synthetic halogen-hectorites, the titanium impurities originate from the SiO2 used as a precursor to prepare the materials, where SiIV has been replaced by TiIV. Since the octahedral coordination is the preferred geometry for titanium compounds, the initial impurities have been suggested to migrate to the octahedral layers substituting isomorphically either Mg2+ or Li+. The low reduction potential between TiIV and Ti3+ allows the easy interconversion of these species; however, the trivalent form of titanium should be the majority species because it better matches the size and charge of the octahedral units [23,24]. It can thus be assumed that a titanium-related center is responsible for the blue emission in Br-Hec and I-Hec. According to previous literature, this blue emission can be attributed to TiO6 3+ entities as reported for Benitoite (BaTiSi3O9)[43] or to Ti -VO (oxygen vacancy) pairs [23,24,47,48]. 3+ Moreover, the doping of the X-Hec materials with Ti (xTi = 0.1 mol %) increases the intensity of the main emission (Figure A6), without significant changes to its band position confirming titanium as the luminescent center. In addition to the blue emission band, there are also luminescence signals at ca. 720 and 830 nm. 3+ 2 2 As is known from the Ti:sapphire laser [22], Ti can also emit in the red and NIR due to the E→ T2 transition. This is present as a wide band also in natural Benitoite [55]. For Br-Hec and I-Hec these red and NIR emissions are observed as bands or lines depending on the excitation wavelength (Figures7, A4 and A5). The line-like shape suggests the presence of other emitters than Ti3+. Thus, impurity 3 3+ 4+ 2 2 4 3d ions Cr and Mn which show the characteristic emission R lines ( E, T2→ A2 transition) in the red-NIR range seem to be probable candidates. These ions also show emission bands in the same spectral range at appropriate crystal field strength [22]. The contents of Mn and Cr in the present materials were determined with ICP-MS to be around 4 (Mn) and 0.5 ppm (Cr). Therefore, both Cr3+ and Mn4+ can be considered as possible emitters along with Ti3+, as was previously reported for natural benitoite [55]. The main blue-green emission band has a red shift trend: the center of this blue emission has been identified at 460, 465, 470 and 490 nm for fluorohectorite [23], chlorohectorite [24], bromohectorite and iodohectorite, respectively. This observed emission red shift can be explained using the molecular orbital theory: the halides are π-donor ligands (π-donor stengths: F− < Cl− < Br− < I−), which have full π orbitals lower in energy than the 3d1 orbitals of Ti3+. When they form molecular orbitals with 3+ 3+ the t2g orbitals of Ti , the hitherto nonbonding Ti t2g orbitals split into bonding and antibonding MOs. The energy is increased, bringing them closer in energy to the antibonding eg orbitals [56]. The energy difference between the antibonding t2g and eg MOs may be considered as the ligand field splitting responsible for the transition in visible. The higher is the π-donor strength, the lower is the energy difference between t2g and eg generating the noticed red shift. Similar observations have been Appl. Sci. 2017, 7, 1243 11 of 20 reported, for example, in perovskites showing the red shift when bromine was replaced by iodine in the inorganic octahedral PbX6 (X = Br or I) [57]. The excitation spectra of the bromohectorite and iodohectorite (Figure8) confirm the role of Ti 3+ as an activator center in these materials. The broad bands observed at ca. 400 and 410 nm for Br-Hec and 3+ I-Hec, respectively, are assigned to electronic transitions from the t2g to eg energy levels of Ti , which 3+ agreesAppl. Sci. with 2017 previous, 7, 1243 reports for Ti in fluorohectorite (380 nm) [23], chlorohectorite (370 nm)11 [24 of] 20 and monoclinic ZrO2 (314 nm) [58], considering that d-d transitions are sensitive to the crystal field − − strength.field strength. The wide The bands wide at ca.bands 300 at nm ca. are 300 related nm toare the related e (O2 to(2p)) the→ e−Ti(OIV2−charge(2p))→ transfer,TiIV charge which transfer, can b occurwhich through can occur two through possible two molecular possible transitions: molecular t 1utransitions:(π )→t2g( πt1u*)(π orb) t→2ut(2gπ()π→*)t 2gor( πt2u*),(π being)→t2g( typicallyπ*), being observedtypically at observed ca. 280 nm at [ 23ca., 24280,42 nm,48 ,[23,24,42,48,59].59]. Lastly, the energy Lastly, gaps the betweenenergy gaps the valencebetween and the the valence conduction and the bandsconduction are identified bands are at ca. identified 256 nm (4.84at ca. eV)256 andnm 248(4.84 nm eV) (5.00 and eV) 248 for nm Br-Hec (5.00 eV) and for I-Hec, Br-Hec respectively, and I-Hec, whichrespectively, agree with which the agree characteristic with the bandcharacteristic gap energy band 4.7 gap eV energy (260 nm) 4.7 ofeV hectorites(260 nm) of [60 hectorites]. Since the [60]. excitationSince the spectra excitation were spectra recorded were by recorded monitoring by themonitoring blue emission, the blue the emission, observation the ofobservation the valence of tothe conductionvalence to band conduction transition band confirms transition that confirms titanium isth inat thetitanium hectorite is in structure the hectorite and not structure present and as an not isolatedpresent impurity as an isolated phase. impurity phase.

800 Na (Mg Li )[Si O ]X 0.7 5.5 0.3 8 20 4 E + exciton levels Excitation Spectra, 293 K g X: I, λ : 490 nm 600 em Br, λ : 470 nm em

400 CT: e-(O2-(2p)) TiIV

Intensity / Arb. Units 200 Ti3+

0 200 300 400 500 Wavelength / nm

Figure 8. The excitation spectra of the bromohectorite (λem: 470 nm) and iodohectorite (λem: 490 nm). Figure 8. The excitation spectra of the bromohectorite (λem: 470 nm) and iodohectorite (λem: 490 nm).

When the excitation spectra are recorded at different emission wavelengths (Figures A7 and When the excitation spectra are recorded at different emission wavelengths (Figures A7 and A8), A8), these three bands remain mainly in the same spectral position, which proves that the same these three bands remain mainly in the same spectral position, which proves that the same activator activator center (Ti3+) is responsible for the observed blue-green emission. When the excitation center (Ti3+) is responsible for the observed blue-green emission. When the excitation spectrum spectrum of Br-Hec is recorded at 710 nm emission (Figure A7), it shows two bands at ca. 400 and of Br-Hec is recorded at 710 nm emission (Figure A7), it shows two bands at ca. 400 and 577 nm. 577 nm. In minerals, the 4A2→4T1 and 4A2→4T2 transitions that excite the red/NIR emission are In minerals, the 4A →4T and 4A →4T transitions that excite the red/NIR emission are commonly commonly observed2 at1 410 and2 5502 nm for Cr3+ as well as 420 and 530 nm for Mn4+ [46]. This observed at 410 and 550 nm for Cr3+ as well as 420 and 530 nm for Mn4+ [46]. This confirms that one confirms that one or both of these ions is the origin of this line emission at 710 nm. or both of these ions is the origin of this line emission at 710 nm. The luminescence decay curves of Br-Hec and I-Hec (Figure 9) indicated that the materials The luminescence decay curves of Br-Hec and I-Hec (Figure9) indicated that the materials present present a short persistent luminescence, with a duration of ca. 5 s, at room temperature. Both a short persistent luminescence, with a duration of ca. 5 s, at room temperature. Both materials have materials have at least three lifetimes obtained from a multiexponential fitting function. However, at least three lifetimes obtained from a multiexponential fitting function. However, even with three even with three components, only a part of each decay curve could be fitted reasonably. This components, only a part of each decay curve could be fitted reasonably. This indicates that several indicates that several contributing processes are involved in the overall decay. It has been reported contributing processes are involved in the overall decay. It has been reported that Ti3+-doped materials that Ti3+-doped materials have a typical emission lifetime lasting microseconds [61], but have a typical emission lifetime lasting microseconds [61], but halogen-hectorites have shown much halogen-hectorites have shown much longer lifetimes, on the order of hundreds of milliseconds to longer lifetimes, on the order of hundreds of milliseconds to seconds [23,24], which agrees with the seconds [23,24], which agrees with the lifetimes obtained from the fits to the decay curves of Br-Hec lifetimes obtained from the fits to the decay curves of Br-Hec and I-Hec in this work (Table1). This short and I-Hec in this work (Table 1). This short persistent luminescence is explained through the persistent luminescence is explained through the existence of defects in the layered structure of these existence of defects in the layered structure of these materials acting as shallow electron traps for materials acting as shallow electron traps for short-time energy storage. Even though the bromo- and short-time energy storage. Even though the bromo- and iodohectorites can store optical energy, they possess shallow traps that do not allow for any significant persistent luminescence at room temperature. Appl. Sci. 2017, 7, 1243 12 of 20 iodohectorites can store optical energy, they possess shallow traps that do not allow for any significant persistentAppl. Sci. luminescence2017, 7, 1243 at room temperature. 12 of 20

FigureFigure 9. The 9. The luminescence luminescence decay decay (λexc (:λ 255exc: 255 nm) nm) of bromohectorite of bromohectorite (λem (:λ 470em: 470 nm) nm) and and iodohectorite iodohectorite

(λem(λ: 490em: 490 nm). nm).

TableTable 1. Emission 1. Emission lifetimes lifetimes calculated calculated for bromohectorite for bromohectorite and iodohectorite and byiodohectorite fitting theluminescence by fitting the intensity (I) to the function: I = I + A e−t/τ1 + A e−t/τ2 + A e−⁄t /τ3. Here,⁄ A = amplitude,⁄ t = time luminescence intensity (I) to0 the 1 function: 2 = +3 + + . Here, A = andamplitude,τ = lifetime. t The= time values and inτ parentheses= lifetime. The show values the SDin (standardparenthese deviation)s show the calculated. SD (standard deviation) calculated. Parameters τ1/s A1/% τ2/s A2/% τ3/s A3/% BromohectoriteParameters0.04(6) τ1/sA 611/% 0.20(4)τ2/sA 292/% 0.87(11)τ3/sA103/% IodohectoriteBromohectorite 0.06(6) 0.04(6) 53 61 0.30(3)0.20(4) 3529 1.07(8)0.87(11) 1210 Iodohectorite 0.06(6) 53 0.30(3) 35 1.07(8) 12 4. Discussion 4. Discussion Br-Hec and I-Hec have been successfully synthesized and, to the best of the authors’ knowledge, their structuralBr-Hec andand spectroscopic I-Hec have featuresbeen successfully are here reported synthesized for the firstand, time. to the They best present of the the sameauthors’ hexagonalknowledge, crystal their structure structural as nanocrystallineand spectroscopic fluorohectorite features are withouthere reported impurity for phasesthe first detectable time. They withpresent XPD. Boththe same materials’ hexagonal crystallites crystal are structure approximately as nanocrystalline 3 nm along the fluorohectorite 001 direction andwithout have impurity a d001 spacingphases of 14.30detectable Å, which with is suitableXPD. Both for intercalationmaterials’ crystallites of two monolayers are approximately of water in their3 nm interlayers along the at 001 roomdirection temperature. and have The a XRFd001 spacing spectra confirmedof 14.30 Å, thewhich chemical is suitable composition for intercalation expected of for two the monolayers materials, of showingwater thein their signals interlayers related toat theroom Mg temperature. Kα1 and Si KTheα1 emissionsXRF spectra for confirmed both and thethe respectivechemical composition halogen α1 α1 emissionexpected of Brfor L αthe1,2 materials,or I Lα1,IL showinβ1 andg Ithe Lβ 2signals. Additionally, related to the the spectra Mg K of bothand Si materials K emissions indicate for the both α1,2 α1 β1 β2 presenceand the of Tirespective as an impurity, halogen seen emission mainly of through Br L the or weakI L , lineI L of and its relatedI L . Additionally, Kβ1 emission the at 4.9spectra keV. of bothThe materials FTIR spectra indicate of Br-Hec the presence and I-Hec of Ti indicate as an impurity, the total replacement seen mainlyof through the hydroxyl the weak groups line of its hectoriterelated by Kβ the1 emission respective at 4.9 halides, keV. since bands related to the vibrations of hydroxyl groups in the octahedralThe layers FTIR ofspectra clay minerals of Br-Hec were and not I-Hec observed. indicate The th29eSi total NMR replacement spectra of bothof the materials hydroxyl show groups the of samehectorite chemical by environmentthe respective predicted halides, forsince the bands silicon rela tetrahedralted to the layers vibrations of synthetic of hydroxyl hectorite, groups giving in the octahedral3 layers of clay2 minerals were not observed. The 29Si NMR spectra of both− materials− show the Q (ca. −97 ppm) and Q (−87) signals of Si(OMg)(OSi)3 and Si(OMg)(OSi)2X (X = Br or I ) sites, respectively.the same Thechemical Q2/Q environment3 ratio indicates predicted a higher for degree the si oflicon branching tetrahedral for the layers polymeric of synthetic sheets hectorite, of the I-Hecgiving (0.41) the than Q3 for(ca. the−97 Br-Hec ppm) and (0.37) Q2 materials. (−87) signals of Si(OMg)(OSi)3 and Si(OMg)(OSi)2X (X = Br− or I−) sites,The respectively. nitrogen sorption The Q isotherms2/Q3 ratio of indicates Br-Hec and a higher I-Hec degree indicated of thatbranching both are for mesoporous the polymeric materials sheets of withthe plate-like I-Hec (0.41) packing. than Br-Hecfor the Br-Hec has a surface (0.37) areamaterials. of 385 m2 g−1, while I-Hec shows an area of 363 m2 g−1, consideringThe nitrogen the modelsorption of aisotherms multilayer of coverage. Br-Hec an Thed materialsI-Hec indicated have a that mesopore both volumeare mesoporous of ca. 0.50materials cm3 g−1 ,with constant plate-like for the packing. adsorption Br-Hec and has desorption a surface area curves, of 385 suggesting m2 g−1, while a potential I-Hec shows feature an for area catalystof 363 application. m2 g−1, considering Both materials the showmodel thermal of a multilayer stabilityup coverage. to 800 ◦ C.The materials have a mesopore volume of ca. 0.50 cm3 g−1, constant for the adsorption and desorption curves, suggesting a potential feature for catalyst application. Both materials show thermal stability up to 800 °C. Br-Hec and I-Hec have blue-green emission under UV radiation. The luminescence features are attributed to a Ti3+ impurity acting as the luminescent center in these materials. When irradiated at 255 nm, Br-Hec has this main emission band at 470 nm, while observed at 490 nm for the I-Hec. This Appl. Sci. 2017, 7, 1243 13 of 20

Br-Hec and I-Hec have blue-green emission under UV radiation. The luminescence features are attributed to a Ti3+ impurity acting as the luminescent center in these materials. When irradiated at 255 nm, Br-Hec has this main emission band at 470 nm, while observed at 490 nm for the I-Hec. This red shift was explained using the MO theory to occur due to the differing π-donor strengths of the halogens. The center of the main emission band varies with the excitation wavelength, indicating that the activator center is occupying multiple sites in the host lattice, which was also observed from the emission band widths. The excitation spectra of Br-Hec and I-Hec confirm Ti3+ as the activator center in these materials, showing the bands related to electronic transitions from the t2g to eg energy levels of Ti3+ (ca. 400 nm), e−(O2−(2p))→TiIV charge transfer (ca. 300 nm), and the band gap energy (ca. 250 nm). Both materials have persistent luminescence on the order of 5 s, indicating the existence of electron traps that can store excitation energy for short periods of time only.

5. Conclusions The synthesis and properties of the Br-Hec and I-Hec materials have been presented for the first time in the current work. The materials adopt the same crystal structure and chemical environment as hectorite clays. However, replacement with halogen ions gives these materials broader possibilities for application in the production of luminescent materials, thus making them potential candidates for a wide range of follow-up research. The X-Hec materials are candidates for generating luminescent materials by doping with rare earth ions. These materials could be used in the quantitative determination of bio-molecules, and bio-imaging in vitro, due to the high bio-compatibility of clay minerals. Also, up-convertor materials may be obtained through the co-doping of X-Hec materials with rare earth ions, e.g., Er3+-Tm3+-Yb3+, Yb3+-Tm3+, Tm3+-Er3+, etc. Such materials could be used as light-converting elements, enhancing the efficiency of commercially available solar cell devices. The high specific surface area found in both materials could enable their further application in gas sensors or in electrochemical devices for charge storage. Br-Hec and I-Hec materials could also be used as heterogeneous catalysts in polymeric reactions: since they possess moderated thermal stability, easy structural modification, and nanosize along the 001 direction, they could generate a functionalized modifier of polymeric structures, thus enhancing the physical properties of the final product. Moreover, the production of the p-n type of solar cells having X-Hec materials as the n-type element can be studied because of the X-Hec materials’ abundance in negative free-carriers in their negative layers.

Acknowledgments: H.S.S. and J.H. acknowledge the Brazilian funding agency CNPq for the Ciência sem Fronteiras scholarships and the UTUGS for further funding. H.S.S. and J.M.C. acknowledge the Academy of Finland—CNPq Finland-Brazil bilateral project (#490242/2012-0). H.S.S. also acknowledges Lucas C. V. Rodrigues of the Universidade de São Paulo-SP-Brazil for his collaboration on the application and extension of the research grant. Author Contributions: H.S.S. conceived, designed and performed most of the experiments, analyzed the data and wrote the paper; I.N. and J.S. performed the 29Si MAS NMR measurements and revised the interpretation of the data; T.L. helped with the interpretation of the data and revised the manuscript; E.M. measured the nitrogen sorption isotherms and revised the interpretation of the data; J.M.C. assisted in the interpretation of the titanium luminescence and revised the manuscript; P.D. helped with the FTIR measurements and revised the interpretation of the data; H.F.B. contributed on the revision of the manuscript; J.H. contributed on the elaboration of the project, interpretation of the data, and revision of the manuscript; M.L. supervised the research project, elaborated the experiments, provided the materials and apparatus needed, and co-wrote and revised the manuscript. Conflicts of Interest: The authors declare no conflict of interest. Appl. Sci. 2017, 7, 1243 14 of 20

Appl. Sci. 2017, 7, 1243 14 of 20 AppendixAppl. Sci. 2017, 7, 1243 14 of 20 Appendix A Appendix A 0.02 Na (Mg Li )[Si O ]X 0.02 0.7 5.5 0.3 8 20 4

-1 Na (Mg Li0 )[Si O ]X TGA0.7 (N , 105.5 C/min)0.3 8 20 4 C 2 -1 o TGA (N , 10 0C/min) C 2 o 0.00 X: Br, bromohectorite 0.00 X:I, idodohectorite Br, bromohectorite I, idodohectorite

__-0.02 __-0.02 loss of coordinated colapse of mineral colapse of mineral _ loss water of coordinated structure -0.04_ water structure Derivative Weight / %. -0.04

Derivative Weight / %. loss of adsorbed losswater of adsorbed _-0.06 water _-0.06 200 400 600 800 200 400 600 800 o Temperature / Co Temperature / C Figure A1. The first derivative of the TGA curves of bromohectorite and iodohectorite. Figure A1. The first derivative of the TGA curves of bromohectorite and iodohectorite. Figure A1. The first derivative of the TGA curves of bromohectorite and iodohectorite.

40 Na (Mg Li )[Si O ]X 0.7 5.5 0.3 8 20 4 40 Na (Mg Li )[Si O ]X After TGA-DSC0.7 5.5 analysis0.3 8 20 4 (1000After °C) TGA-DSC analysis 35 (1000 °C) 35 Cu Kα1(1.5406 Å), RT Cu Kα1(1.5406 Å), RT

30 X: I 30 IodohectoriteX: I Iodohectorite 25 25

Br Bromohectorite

Br 20 Bromohectorite 20 Enstatite Intensity / Arb. Units / Arb. Intensity MgSiO Enstatite3 Intensity / Arb. Units / Arb. Intensity MgSiO 15 3 15Cristobalite SiO Cristobalite2 SiO MgI 2 10 2 MgI 10 2 MgBr 2 MgBr 5 2 5 20 40 θ 202 / Degrees 40 2θ / Degrees Figure A2. The XPD measurements of bromohectorite and iodohectorite after heating to 1000 °C. Figure A2. The XPD measurements of bromohectorite and iodohectorite after heating to 1000 °C. Figure A2. The XPD measurements of bromohectorite and iodohectorite after heating to 1000 ◦C. Appl. Sci. 2017, 7, 1243 15 of 20 Appl. Sci. 2017, 7, 1243 15 of 20 Appl. Sci. 2017, 7, 1243 15 of 20

*Mg 0.12 * Na (Mg Li )[Si O ]Br K α*Mg: 1.25 16 0.7 5.5 0.3 8 20 4 0.12 1 Na (Mg Li )[Si O ]Br 0.09 * K *Br: 1.25 16 After0.7 TGA-DSC5.5 analysis0.3 8 20 4 α1 (1000 °C) 0.09 L : 1.48 After TGA-DSC analysis 0.06 α 1,2*Br (XRF:1000 293°C) K L : 1.48 0.06 α1,2* : 0.03 12 XRF:Ag K α1293 22.16 K keV * : 0.03 12 Ag Kα1 22.16 keV 1.21.31.41.51.6 ) / Counts

6 1.21.31.41.51.6

) / Counts *Ag 6

*Mg *Ag L1 : 2.63

K : 1.25 8 α *Mg1 L1: 2.63 K : 1.25 Lα : 2.98 8 α1 2 *Br * LL :: 2.983.15 L *Br: 1.48 αβ12 α1,2 * LL :: 3.153.35 L : 1.48 β1β2 4 α 1,2 *Si L : 3.35 Lβ2γ1: 3.52 K α *Si: 1.74 Intensity (x 10 Intensity (x 4 1 Lγ1: 3.52 Kα : 1.74 Intensity (x 10 Intensity (x 1 * * * * * * * * 0 * * * * * 0 0123456* 0123456 Energy / keV Energy / keV Na (Mg Li )[Si O ]I *I 0.7 5.5 0.3 8 20 4 L : 4.22 16 β 1*I NaAfter TGA-DSC(Mg Li analysis)[Si O ]I 0.8 0.7 5.5 0.3 8 20 4 LL :: 4.224.51 16 ° ββ12 After(1000 TGA-DSC C) analysis 0.8 * *L : 4.51 (XRF:1000 293°C) K * β2 * XRF:Ag K 293: 22.16 K keV 12 α1 Ag Kα1: 22.16 keV 12 3.9 4.2 4.5 4.8 3.9 4.2 4.5 4.8 ) / Counts

6 *Ag

) / Counts

6 *Ag L1: 2.63 8 L : 2.63 8 L1 : 2.98 * α2 LL :: 2.983.15 * αβ12 LL :: 3.153.35 β1β2 *I 4 L : 3.35 *Mg *Si Lβ2γ1: 3.52 L *I: 4.22 4 β1 K *Mg: 1.25 K *Si: 1.74 Lγ1: 3.52 L : 4.22

Intensity (x 10 Intensity (x α α β 1 1 Lβ1 : 4.51 K : 1.25 K : 1.74 * 2 Intensity (x 10 Intensity (x α α L : 4.51 1 *1 * * β2 0 * * * * * * * * * 0 0123456* * * 0123456 Energy / keV Energy / keV Figure A3. The XRF measurements of bromohectorite (top) and iodohectorite (bottom) after heating FigureFigure A3. A3.The The XRF XRF measurements measurements of bromohectoriteof bromohectorite (top (top) and) and iodohectorite iodohectorite (bottom (bottom) after) after heating heating to to 1000◦ °C. 1000to 1000C. °C.

Na (Mg Li )[Si O ]Br 0.7 5.5 0.3 8 20 4 800 Ti3+-V Na (Mg Li )[Si O ]Br O 0.7 5.5 0.3293 8K 20 4 800 Ti3+-V O 293Delay K 0.1 ms DelayGate 100.1 ms ms 600 Gateλ = 10255 ms nm exc 600 λ = 255 nm exc 3+ 3+ 300 nm Ti or Cr 300365 nmnm 3+ IV3+ Ti or or Mn Cr 365400 nmnm 400 IV 400 or Mn 400470 nmnm 470575 nmnm 575 nm Intensity / Arb. Units 200 Intensity / Arb. Units 200

0 0300 400 500 600 700 800 900 1000 300 400 500 600 700 800 900 1000 Wavelength / nm Wavelength / nm Figure A4. The emission spectra of bromohectorite recorded with several excitation wavelengths. FigureFigureThe data A4. A4. areThe The not emission emission corrected spectra spectra for the of excitationof bromohectorite bromohectorite source intensity recorded recorded at with withdifferent several several wavelengths. excitation excitation wavelengths. wavelengths. TheThe data data are are not not corrected corrected for for the the excitation excitation source source intensity intensity at at different different wavelengths. wavelengths. Appl. Sci. 2017, 7, 1243 16 of 20 Appl. Sci. 2017, 7, 1243 16 of 20 Appl. Sci. 2017, 7, 1243 16 of 20 1000 1000 3+ Na (Mg Li )[Si O ]I Ti -V 0.7 5.5 0.3 8 20 4 3+ O Na (Mg Li )[Si O ]I Ti -V 0.7 5.5 0.3 8 20 4 O 293 K 800 293Delay K 0.1 ms 800 DelayGate 100.1 ms ms Gate 10 ms λ = 255 nm 600 λ exc = 255 nm 600 exc 282 nm 282 300 nm nm 300 330 nm nm 400 330 365 nm nm 400 365 425 nm nm Ti3+ or Cr3+ 425 nm Intensity / Arb. Units Intensity / Arb. 3+ 3+ Ti or CrIV Intensity / Arb. Units Intensity / Arb. 200 or Mn 200 or MnIV

0 0300 400 500 600 700 800 900 1000 300 400 500 600 700 800 900 1000 Wavelength / nm Wavelength / nm Figure A5. Emission spectra of iodohectorite recorded at several excitation wavelengths. The data FigureFigure A5. A5.Emission Emission spectra spectra of iodohectoriteof iodohectorite recorded recorded at several at several excitation excitation wavelengths. wavelengths. The dataThe aredata are not corrected for the excitation source intensity at different wavelengths. notare corrected not corrected for the for excitation the excitation source source intensity intensity at different at different wavelengths. wavelengths. 1000 1000 3+ Na (Mg Li )[Si O ]Br :Ti Na 0.7(Mg 5.5Li 0.3)[Si 8O 20]Br 4:Ti3+ 0.7 5.5 0.3λ : 2558 20 nm 4 3+ exc 800 Ti -V λ : 255 nm 800 Ti3+-V O 293exc K, PMT detector O 293Delay K, PMT0.1 ms detector DelayGate 100.1 ms ms 600 Gate 10 ms 600 x mol-%= 0 Ti x mol-%= 0 400 Ti 0.1 400 0.1

Intensity / Arb. Units Intensity / Arb.

Intensity / Arb. Units Intensity / Arb. 200 200 Ti3+ or Cr3+ or MnIV Ti3+ or Cr3+ or MnIV 0 0300 400 500 600 700 800 900 1000 300 400 500 600 700 800 900 1000 Wavelength / nm Wavelength / nm 3+ 1000 Na (Mg Li )[Si O ]I :Ti3+ Ti3+-V Na 0.7(Mg 5.5Li 0.3)[Si 8O 20]I 4:Ti 1000 3+ O 0.7 5.5 0.3λ : 2558 nm20 4 Ti -V exc O λ : 255 nm 293exc K, PMT detector 293Delay K, PMT0.1 ms detector 750 DelayGate 100.1 ms ms 750 Gate 10 ms

x mol-%= 0 x Ti mol-%= 0 500 Ti 0.1 500 0.1

Intensity / Arb. Units Intensity / Arb. 250

Intensity / Arb. Units Intensity / Arb. 250 Ti3+ or Cr3+ or MnIV Ti3+ or Cr3+ or MnIV 0 0300 400 500 600 700 800 900 1000 300 400 500 600 700 800 900 1000 Wavelength / nm Wavelength / nm Figure A6. Emission spectra (λexc: 255 nm) of Br-Hec:Ti3+ (top) and I-Hec:Ti3+ (bottom), xTi = 0.1 mol %. Figure A6. Emission spectra (λexc: 255 nm) of Br-Hec:Ti3+3+ (top) and I-Hec:Ti3+3+ (bottom), xTi = 0.1 mol %. Figure A6. Emission spectra (λexc: 255 nm) of Br-Hec:Ti (top) and I-Hec:Ti (bottom), xTi = 0.1 mol %. Appl. Sci. 2017, 7, 1243 17 of 20 Appl. Sci. 2017, 7, 1243 17 of 20 Appl. Sci. 2017, 7, 1243 17 of 20

Na (Mg Li )[Si O ]Br Na0.7(Mg5.5Li0.3)[Si8O20]Br4 0.7 5.5 0.3 8 20 4 Excitation Spectra, 293 K E + exciton levels Excitation Spectra, 293 K g λ : 470 nm E + exciton levels λem g em: 470 nm 540 nm 540 nm 710 nm CT: 710 nm CT:- 2- IV 200 e-(O2-(2p)) TiIV 200 e (O (2p)) Ti

3+ IV Cr 3+ or Mn IV Cr or Mn

3+ 3+ Ti3+ or Cr3+ Intensity / Arb. Units Intensity / Arb. Ti or CrIV Intensity / Arb. Units Intensity / Arb. or MnIV or Mn

0 200 300 400 500 600 700 Wavelength / nm Wavelength / nm FigureFigure A7. A7.Excitation ExcitationExcitation spectra spectraspectra of ofof bromohectorite bromohectoritebromohectorite recorded recordedrecorded at atat several severalseveral emission emissionemission wavelengths. wavelengths.wavelengths. The TheThe data datadata areare not not corrected corrected for for the the detector detector sensitivity sensitivity at at different different emission emission wavelengths. wavelengths.

E + exciton levels Na (Mg Li )[Si O ]I Eg + exciton levels Na0.7(Mg5.5Li0.3)[Si8O20]I4 g 0.7 5.5 0.3 8 20 4 600 Excitation Spectra, 293 K 600 Excitation Spectra, 293 K λ : 490 nm λem em: 490 nm 560 nm 560 nm 710 nm 710 nm 400 CT: CT:- 2- IV e-(O2-(2p)) TiIV e (O (2p)) Ti

200

Intensity / Arb. Units Intensity / Arb. 200 Intensity / Arb. Units Intensity / Arb. 3+ Ti3+ Ti

0 200 300 400 500 600 700 Wavelength / nm Wavelength / nm Figure A8. Excitation spectra of iodohectorite recorded at several emission wavelengths. The data FigureFigure A8. A8.Excitation Excitation spectra spectra of iodohectoriteof iodohectorite recorded recorded at several at several emission emission wavelengths. wavelengths. The dataThe aredata are not corrected for the detector sensitivity at different emission wavelengths. notare corrected not corrected for the for detector the detector sensitivity sensitivity at different at different emission emission wavelengths. wavelengths.

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