Weijiong Dai

Cassandre Kouvatas

Wenshu Tai

Guangjun Wu

Naijia Guan

Landong Li

Valentin Valtchev

Plate-like MFI Crystals with Controlled Crystal Faces Aspect Ratio

Keywords: Zeolite, Morphology, Ultra-thin crystals, Catalysis

Zeolite crystals offering a short diffusion pathway through the pore network are highly desired for a number of catalytic and molecule separation applications. Herein, we develop a simple synthetic strategy toward reducing the thickness along b-axis of MFI-type crystals, thus providing a short diffusion path along the straight channel. Our approach combines preliminary aging and a fluoride-assisted low-temperature crystallization. The synthesized MFI crystals are in the micron-size range along a-and c-axis, while the thickness along the b-axis is a few tens of nanometers. The synthesis parameters controlling the formation of plate-like zeolite are studied, and the factors controlling the zeolite growth identified. The synthesis strategy works equally well with all-silica MFI (silicalite-1) and its Al-and Ga-containing derivatives. The catalytic activity of plate-like ZSM-5 in the methanol-to-hydrocarbons (MTH) reaction is compared with a commercial nano-sized ZSM-5 sample, as the plate-like ZSM-5 exhibits a substantially extended lifetime. The synthesis of plate-like MFI crystals is successfully scaled up to a kilogram scale.

Introduction

Zeolites are initially discovered as minerals in sediment and pegmatite rocks. [START_REF] Barrer | Hydrothermal Chemistry of Zeolites[END_REF] Natural zeolites are mainly used in agriculture, water, and soil remediation, while their synthetic counterparts have diverse industrial applications, as heterogeneous catalysis and molecular sieving are the major industrial fields of use. [START_REF] Milton | Molecular Sieve Science and Technology, A Historical Perspective[END_REF][START_REF] Breck | Zeolite Molecular Sieves: Structure, Chemistry and Use[END_REF] More than 40% of acid-catalyzed reactions are currently based on synthetic zeolites. [START_REF] Na | Recent Advances in the Synthesis of Hierarchically Nanoporous Zeolites[END_REF] The great success of zeolites in catalysis is related to their unique structural features, i.e., the ordered channels of molecular dimensions and the presence of modulable active sites. [START_REF] Hartmann | Catalytic Test Reactions for the Evaluation of Hierarchical Zeolites[END_REF] However, the narrow channels of zeolite impose a significant restriction on the transport of molecules, causing an increase in the secondary reactions as well as the subsequent catalyst deactivation by coking. [START_REF] Hartmann | Catalytic Test Reactions for the Evaluation of Hierarchical Zeolites[END_REF][START_REF] Wei | Tailoring and Visualizing the Pore Architecture of Hierarchical Zeolites[END_REF][START_REF] Guisnet | Deactivation and Regeneration of Solid Catalysts[END_REF][START_REF] Choi | Stable Single-Unit-Cell Nanosheets of Zeolite MFI as Active and Long-Lived Catalysts[END_REF] The diffusional problems can be partially circumvented by reducing the path length through the zeolite crystal. The two strategies employed to reduce the diffusion path in zeolites are the decrease of the crystals to the nano-sized range [START_REF] Valtchev | Porous Nanosized Particles : Preparation, Properties, and Applications[END_REF][START_REF] Mintova | Nanosized Microporous Crystals: Emerging Applications[END_REF][START_REF] Mintova | Advances in Nanosized Zeolites[END_REF][START_REF] Tosheva | Synthesis, Crystallization Mechanism, and Applications[END_REF] , and the introduction of larger secondary (meso-/macro-) pores in the crystals. [START_REF] Möller | Mesoporositya New Dimension for Zeolites[END_REF][START_REF] Serrano | Synthesis Strategies in the Search for Hierarchical Zeolites[END_REF][START_REF] Tao | Mesopore-Modified Zeolites : Preparation, Characterization, and Applications[END_REF] Zeolites comprising a secondary pore system, which serves as highways for reactants and products, are often called hierarchical, and their preparation is usually complex. [START_REF] Serrano | Synthesis Strategies in the Search for Hierarchical Zeolites[END_REF][START_REF] Holm | Catalysis with Hierarchical Zeolites[END_REF] The synthesis of zeolite nanocrystals also has a downside as the specific synthesis conditions and complex product processing are required. [START_REF] Valtchev | Porous Nanosized Particles : Preparation, Properties, and Applications[END_REF] An alternative solution that could solve the problem with the diffusion limitations in zeolites without using additional treatments, as in the case of hierarchical and nano zeolites, is the synthesis of micron-sized zeolites with particular morphology. More precisely, the control of zeolite morphology could be used to limit the growth in the direction along the channel system of interest, thus shortening the diffusion pathway. Tuning zeolite morphology could also allow developing the face where the channel system of interest is presented on the zeolite surface, thus increasing the number of pore opening per unit crystals surface. This strategy is particularly important in preparing zeolites with a biased performance of different channel systems. For instance, MFI-type material, used in various catalytic and separation processes, contains straight and zig-zag channel systems running along b and a crystal axis, respectively (Figure S1). Shortening the diffusion path along the straight channel (b-axis) changes zeolite performance substantially. [START_REF] Meng | On the Role of Acidity in Bulk and Nanosheet [T]MFI (T=Al 3+ , Ga 3+ , Fe 3+ , B 3+ ) Zeolites in the Methanol-to-Hydrocarbons Reaction[END_REF][START_REF] Liu | Uniform Hierarchical MFI Nanosheets Prepared via Anisotropic Etching for Solution-Based Sub-100-nm Thick Oriented MFI Layer Fabrication[END_REF][START_REF] Xiao | Simultaneous Realization of High Catalytic Activity and Stability for Catalytic Cracking of: N -Heptane on Highly Exposed (010) Crystal Planes of Nanosheet ZSM-5 Zeolite[END_REF][START_REF] Jeon | Ultra-Selective High-Flux Membranes from Directly Synthesized Zeolite Nanosheets[END_REF] An ultimate example of this strategy was the preparation of zeolite nanosheets, first prepared by Ryoo and co-workers. [START_REF] Choi | Stable Single-Unit-Cell Nanosheets of Zeolite MFI as Active and Long-Lived Catalysts[END_REF] Ryoo's group decreased the thickness of MFI-type zeolite along the b-axis to one unit cell. Nanosheet-like zeolites focused zeolite community's attention with remarkable catalytic performance in several reactions, [START_REF] Hu | Selective Formation of Propylene from Methanol over High-Silica Nanosheets of MFI Zeolite[END_REF][START_REF] Ji | Synthesis of Isomorphous MFI Nanosheet Zeolites for Supercritical Catalytic Cracking of n-Dodecane[END_REF][START_REF] Seo | Characterization of the Surface Acidity of MFI Zeolite Nanosheets by 31 P NMR of Adsorbed Phosphine Oxides and Catalytic Cracking of Decalin[END_REF][START_REF] Xu | Deactivation of Multilayered MFI Nanosheet Zeolite during Upgrading of Biomass Pyrolysis Vapors[END_REF][START_REF] Kim | Surfactant-Directed Zeolite Nanosheets: A High-Performance Catalyst for Gas-Phase Beckmann Rearrangement[END_REF] and superior separation ability in membrane applications. [START_REF] Jeon | Ultra-Selective High-Flux Membranes from Directly Synthesized Zeolite Nanosheets[END_REF][START_REF] Kim | Para-Xylene Ultra-Selective Zeolite MFI Membranes Fabricated from Nanosheet Monolayers at the Air-Water Interface[END_REF][START_REF] Zhang | Open-Pore Two-Dimensional MFI Zeolite Nanosheets for the Fabrication of Hydrocarbon-Isomer-Selective Membranes on Porous Polymer Supports[END_REF] However, these zeolitic materials are synthesized by using the complex and expensive structure-directing agents (SDAs), [START_REF] Na | Disordered Assembly of MFI Zeolite Nanosheets with a Large Volume of Intersheet Mesopores[END_REF][START_REF] Na | Pillared MFI Zeolite Nanosheets of a Single-Unit-Cell Thickness[END_REF][START_REF] Shen | A Hierarchical MFI Zeolite with a Two-Dimensional Square Mesostructure[END_REF] or by tedious swelling, pillaring, and exfoliation procedures. [START_REF] Maheshwari | Layer Structure Preservation during Swelling, Pillaring, and Exfoliation of a Zeolite Precursor[END_REF] The size of the zeolite nanosheet is usually tiny (100-200 nm in length), and the handling is even more complex than the conventional nano-zeolites.

From a practical point of view, the synthesis of zeolite crystals with morphology that enhances the transport through a particular channel system without using an exotic SDA is highly desired. The aspect ratio between different crystal faces should be changed in a way to minimize the length of the channel.

Maximizing the face's size where the channel system of interest appears on the crystal surface is also important. Keeping the zeolite crystal size in the micrometer range and decreasing the size to a few tens of nanometers in a particular crystallographic direction has significant practical value. Such crystals can be processed employing the conventional post-synthetic protocols and do not require any additional treatments.

Previously studies have demonstrated that adjusting synthesis parameters or adding synthesis modifiers could substantially change zeolite crystal morphology. [START_REF] Itani | Factors That Control Zeolite L Crystal Size[END_REF][START_REF] Larlus | Crystal Morphology Control of LTL-Type Zeolite Crystals[END_REF][START_REF] Zhang | Amino Acid-Assisted Construction of Single-Crystalline Hierarchical Nanozeolites via Oriented-Aggregation and Intraparticle Ripening[END_REF][START_REF] Larlus | Control of the Morphology of All-Silica BEA-Type Zeolite Synthesized in Basic Media[END_REF][START_REF] Li | Strategies to Control Zeolite Particle Morphology[END_REF][START_REF] Zhang | High-Quality Single-Crystalline MFI-Type Nanozeolites: A Facile Synthetic Strategy and MTP Catalytic Studies[END_REF] For instance, long to short prismatic LTL-type crystals were synthesized by simple changing of water content in the synthesis gel. [START_REF] Larlus | Crystal Morphology Control of LTL-Type Zeolite Crystals[END_REF] The zeolite Beta morphology was controlled, varying the pH of the system. [START_REF] Larlus | Control of the Morphology of All-Silica BEA-Type Zeolite Synthesized in Basic Media[END_REF] Crystals with well-developed pyramidal (h0l) or pinacoidal (001) faces were obtained. For instance, at high pH plate-like zeolite Beta with a pinacoidal face dominating crystal morphology was obtained. The use of additives can also effectively tailor the aspect ratio of zeolite crystals. Different types of additives, including alcohol, [START_REF] Chen | Fabrication of Silicalite-1 Crystals with Tunable Aspect Ratios by Microwave-Assisted Solvothermal Synthesis[END_REF][START_REF] Lupulescu | A Facile Strategy to Design Zeolite L Crystals with Tunable Morphology and Surface Architecture[END_REF] tributylphosphine oxide, [START_REF] Lupulescu | Tailoring Silicalite-1 Crystal Morphology with Molecular Modifiers[END_REF] graphene oxide, [START_REF] Li | Graphene Oxide Facilitates Solvent-Free Synthesis of Well-Dispersed, Faceted Zeolite Crystals[END_REF] peptoids, [START_REF] Li | Designed Peptoids as Tunable Modifiers of Zeolite Crystallization[END_REF] and amino acid arginine, [START_REF] Qin | Molecular Modifiers Suppress Nonclassical Pathways of Zeolite Crystallization[END_REF] were employed to modify crystal morphology of different types of zeolites. Although some positive results were published, a dramatic change in the aspect ratio between different faces was not reported to the best of our knowledge. None of these studies guide how one can tune the crystal morphology to nano-size range in a particular crystallographic direction.

Fluorine ion (F -) is the alternative OH -as a mineralizing agent in zeolite synthesis. [START_REF] Kessler | The Opportunities of the Fluoride Route in the Synthesis of Microporous Materials[END_REF][START_REF] Caullet | The Fluoride Route: A Strategy to Crystalline Porous Materials[END_REF] It is usually used in the synthesis of high-or all-silica zeolites in a quasi-neutral medium. It was reported that fluoride could also be used to achieve the oriented growth of thin zeolite films. [START_REF] Zhou | A Uniformly Oriented MFI Membrane for Improved CO 2 Separation[END_REF] Hence, we selected the ammonium fluoride (NH 4 F) as a zeolite synthesis modifier to change the crystal face aspect ratio between the faces of the crystals.

The present study aims to obtain ultra-thin zeolite plates, keeping the other crystals dimension in the micron-size range. Fulfilling this goal would provide a relevant approach to zeolites with improved accessibility and decreased diffusion limitations, avoiding the use of complex organic structure-directing agents (SDA) and multi-step procedures. Our approach is exemplified by preparing MFI-type crystals with controllable aspect ratios between different crystal faces. We selected the MFI-type zeolite because it is an industrially relevant zeolite, [START_REF] Pereira | Biomass-Mediated ZSM-5 Zeolite Synthesis: When Self-Assembly Allows to Cross the Si/Al Lower Limit[END_REF] and exhibits two different channel systems, highlighting the effect of crystal morphology on zeolite accessibility and transport through the channels.

Results and discussion

Substantial preliminary work has been executed to adjust the ratio between different reactants and pre-synthesis aging conditions. The reference, NH 4 F-free synthesis, with a molar composition of 0.1 TPAOH: 1.0 SiO 2 : 30 H 2 O was performed at 90 °C for 72 h under static condition. A highly crystalline product was obtained (Figure S2f). The crystal size was close to 1 µm, as each crystal contained a 90° twin crystal, a morphological feature often observed for the MFI-type material (Figure S3a). The impact of NH 4 F on the MFI crystal morphology was studied with a gel of composition 0.1 TPAOH: 1.0 SiO 2 : 0.8 NH 4 F: 30 H 2 O (see Supporting Information for details). This system yielded well-shaped, very thin plate-like crystals. The XRD pattern, Ar physisorption analysis, and representative TEM and SEM images of the crystals are shown in Figure 1. The product was highly crystalline, according to the XRD study (Figure 1a). This result was confirmed by the type I physisorption isotherm, with fast uptake at low relative pressure, characteristic of a zeolite type material (Figure 1b). The micropore volume (0.14 cm 3 /g) and specific surface area (430 m 2 /g) of the plate-like MFI zeolite crystals are characteristic of highly crystalline MFI-type material (Table S1). The high crystallinity of the single-crystalline plate-like zeolite is also illustrated by the well-defined lattice fringes and the selected area electron diffraction (SAED) pattern in the HR-TEM image (Figure 1f and1g). Microscopy study (Figure 1c, 1d and 1e) reveals that crystals' length-width dimensions are in the micron scale. Meanwhile, the thickness of the obtained crystals was ~100 nm. Thus, the crystals are well-shaped with developed (100) and (0kl) faces, while the (010) face is very short. In the enlarged view HAADF-STEM image (Figure 1f), the zeolite channels along the b-axis can be clearly observed. A systematic study of the ammonium fluoride content on the MFI morphology was performed using the system: 0.1 TPAOH: 1.0 SiO 2 : y NH 4 F: 30 H 2 O, where y was varied between 0 and 1.6. The morphology of twin crystals obtained without NH 4 F (Figure S3a) changed gradually with the increase of NH 4 F content in the gel. The most noticeable change was the disappearance of 90° twin crystal (Figure S3d). The other significant change was the change of the aspect ratio between (010) and (100) faces.

This effect is a consequence of the decrease in the size of (100) face. The increase of y value to 1.2 and 1.6 further accentuated the morphological changes of MFI-type material, as crystal's thickness reached several tens of nanometers (Figure S3e and S3f). Noteworthy, the crystals synthesized in the NH 4 F-containing medium are larger than that of the reference sample. Thus, the silicalite-1 crystals grow from ~700 nm to ~3000 nm along the c-axis, at the same time decreasing the thickness from ~500 nm to ~100 nm along the b-axis (Figure 2a). These morphological changes are better presented via the aspect ratio between different faces. The ratio between c-and a-axis (L c /L a ) as well as between c-and b-axis (L c /L b ) was used to evaluate the changes in the (100) and (010) faces, where the pore mouths of zig-zag and straight channel appears on the surface, respectively. The L c /L b aspect ratio increased from ~2.5 to ~26, when NH 4 F content in the gel changed from 0 to 1.6 (Figure 2c). At the same time, as can be observed from Figure 2b, the change in the L c /L a was not dramatic (from ~1.3 to ~2.7), revealing that the growth in b direction was greatly affected by the NH 4 F addition. The zeolite crystallinity also depends on the NH 4 F content in the gel, as can be seen in Figure S2. At low NH 4 F content (y = 0.05 and 0.2), an amorphous phase containing a few MFI crystals was observed, even though the crystallization was extended to 168 h (Figure S3b and S3c). In contrast, a highly crystalline product was obtained within 72 h when y increased to 0.8. Further increasing y to 1.2 and 1.6 shortens the crystallization time to 48 h and 24 h, respectively. However, it did not bring substantial change in the crystalline product (Figure S3e and S3f), showing a threshold of NH 4 F concentration, after which the morphology is not impacted. The best result in terms of high L c /L b aspect ratio and short crystallization time was obtained with y = 0.8. In this case, the average crystal thickness along b-axis was ~100 nm, and the crystalline product was obtained within 72 h. The set of experimental results unambiguously shows that the NH 4 F concentration has a direct effect on the MFI crystal growth, including both the crystallization kinetics and the crystal morphology.

It should be noted the effect of the 90 °C aging prior to the addition of NH 4 F to the gel, which plays a crucial role in the formation of plate-like MFI zeolite crystals. Without this aging step, the gel with a similar molar composition (0.1 TPAOH: 1.0 SiO 2 : 0.8 NH 4 F: 30 H 2 O) did not yield crystalline material up to 168 h crystallization (Figure 2e). In order to get an insight into the 90 °C aging on gel structure, we performed IR analysis. The FTIR spectra of the samples collected after aging at 90 ℃ for 12 h are shown in Figure S4. Compared to the amorphous silica, the sample taken after 12 h exhibits a broad band at ~550-570 cm -1 , which reveals that elements of pentasil structure are already present in the solid. [START_REF] Kirschhock | Identification of Precursor Species in the Formation of MFI Zeolite in the TPAOH-TEOS-H 2 O System[END_REF] Increasing the aging time from 12 to 24 and 36 h changed the growth rate, and thus the crystallization time was reduced to 72 h, 36 h and 24 h, respectively. However, 90° twin crystals were yielded instead of single plate-like crystals when the aging time was extended to 24 and 36 h (Figure 2e).

It is known that the aging process impacts zeolite nucleation and thus crystal growth. [START_REF] Li | Aging Effects on the Nucleation and Crystallization Kinetics of Colloidal TPA-Silicalite-1[END_REF] In this work, the aging step was necessary to obtain MFI crystals in the NH 4 F containing gel as 12 h was the optimal aging time yielding very thin plate-like crystals. The crystal morphology is impacted by the species generated during the aging process. We anticipate that the plate-like morphology is a consequence of a high concentration of precursors species with similar development and a particular mechanism of oriented aggregation, which leads to the preferential growth along the a-and c-axis, and results in a highly developed (010) face. However, the intimate mechanism laying behind this particular growth is difficult to be elucidated.

The impact of the SDA content on the crystal morphology was studied using the system: x TPAOH:

1.0 SiO 2 : 0.8 NH 4 F: 30 H 2 O, where x was varied in the range of 0.08-0.35. As shown in Figure S5, with the increase of SDA content, MFI crystals' size decreased. This is the expected effect due to the more abundant nucleation in TPA-rich systems. [START_REF] Mintova | Advances in Nanosized Zeolites[END_REF] Meanwhile, a decrease from ~30 to ~20 of the L c /L b ratio can be observed when x is below 0.125. After that, no change of the L c /L b ratio was detected (Figure S6).

However, the L c /L a was not influenced too much by the SDA concentration. It remained in the range of 1.8-2.8 (Figure S6). The decrease of the overall crystal dimension resulted in much thinner along b-axis crystals. Thus crystallites with an impressive thickness of ~10 nm along the b-axis without using any organic modifier were obtained (Figure 2d andS5). These results show that the SDA content has a limited influence on the aspect ratio between different faces. However, the TPAOH content exhibits a noticeable impact on the crystallinity zeolite products, as shown in Figure 2d. For instance, at x = 0.08, the relative crystallinity of 72 h synthesized sample is ~63 %. It can be improved by extending the crystallization time to 120 h (Figure S7). Increasing x from 0.08 to 0.35 led to highly crystalline zeolite crystals, which was obtained within 72 h.

As MFI-type zeolites are often synthesized at elevated temperatures (T≥150 o C), we have studied the impact of synthesis temperature on the morphology of the plate-like crystals. Noteworthy, plate-like MFI crystals can be obtained over a temperature range of 90-180 o C using the system 0.1 TPAOH: 1.0 SiO 2 : 0.8 NH 4 F: 30 H 2 O (Figure S8). As shown in Figure S8 and S9, increasing the crystallization temperature from 90 to 180 o C resulted in a slight increase in the crystal size. Thus the length along the c-axis changed from ~2600 nm to ~3500 nm, while along the b-axis, the increase was only from ~100 nm to ~150 nm. Meanwhile, the length along a-axis did not show any obvious change. Consequently, the aspect ratios, L c /L a and L c /L b , remained nearly unchanged (Figure S9d). Thus the crystallization temperature shows a weak influence on the aspect ratio between different crystal faces. The corresponding relative crystallinity increased from ~85 to ~100%, with the temperature rising from 90 °C to 180 °C (Figure S9a).

Summarizing the set of experimental results, one can state that an aging step of 12 h generating nuclei in the system before the addition of NH 4 F is indispensable for the formation of plate-like MFI crystals.

Further, with precise control of NH 4 F content and tuning the synthesis parameters, single-crystalline plate-like MFI zeolite with adjustable thickness can be obtained (Figure 2).

Mechanism of plate-like MFI crystals formation

The mechanistic analysis of plate-like MFI formation was performed using an all-silica system with molar composition 0.1 TPAOH: 1.0 SiO 2 : 0.8 NH 4 F: 30 H 2 O. The intermediates formed in the course of zeolite synthesis were studied to shed light on the process. As pointed out above, the gel was aged at 90 °C for 12 h, then dissolved NH 4 F was added, and the resultant mixture stirred at 90°C for 12 h more.

Finally, the synthesis was performed under static conditions at 90 °C for 48 h, as a sample was taken after 3, 6, 9, 12, 16, 24, 36, and 48 h.

The XRD pattern of the 12 h aged gel showed that it is amorphous (Figure S10). After the addition of NH 4 F, the first MFI trace, indicated by the characteristic diffraction line at 23.1° Two Theta, appeared after 3 h crystallization. The crystalline phase developed rapidly, and a highly crystalline material was obtained after 48 h synthesis. Meanwhile, the amorphous phase gradually decreased and fully disappeared after 48 h crystallization (Figure S12).

The obtained intermediates, as well as the 12 h aged precursor, were inspected by transmission electron microscopy. After 3 h aging, only bulk, uniform in texture gel particles were observed (Figure S13). No substantial changes in the gel structure were found after 12 h aging (Figure 3a). Upon the addition of NH 4 F and 3 h hydrothermal treatment at 90 °C, the gel precursor structure changed dramatically. Bulk gel disintegrated, and small nanometric particles appeared in the system. We attribute this effect to the fluorine ions' mineralizing ability, which breaks the gel structure and reorganizes silica species. Small, 10-20 nm in size isometric particles with oval shape was the only phase observed in the product (Figure S14). The formation of nanoparticles certainly changes the exchange between the solid and liquid phase and contributes to the reaction kinetics in general. We anticipate the structure/chemistry of newly formed upon the fluorine addition nanoparticles contributes to the formation plate-like zeolite crystals. However, we do not have experimental prove supporting our suggestion. These tiny nanoparticles continue existing up to 12 h hydrothermal treatment. However, some changes in their textural organization occur. Namely, the particles agglomerate and built agglomerates with an elongated shape. The individual 10-20 nm nanoparticles are still well distinguishable in the aggregates (Figure 3b).

Plate-like particles appear in the solid after 12 h treatment at 90 °C (Figure 3c). According to the XRD study, the solid is still amorphous at this stage (Figure S11). Upon autoclave treatment, the number of plate-like particles increases, and they evolve into plate-like crystals with the MFI-type morphology.

The first XRD trace of MFI-type material is detected after 3 h hydrothermal treatment in the autoclave (Figure S12). With the crystallization time extension, the amount of crystals increases at the expense of the isometric 10-20 nm particles (Figure 3d-f). These data show that the small colloid species with oval shape, observed at the beginning of the crystallization process, serve as nutrients for the crystal growth process. In the last stages of crystals growth (24-48 h), substantial growth of zeolite crystals is observed (Figure 3g-i). At this later stage, there is almost no amorphous phase in the system. Therefore, the growth is a consequence of the Ostwald ripening of the crystals, which does not change crystal morphology. Thus plate-like with very high L c /L b aspect ratio MFI crystals are obtained (Figure 3i).

The combined XRD-TEM investigation revealed that the addition of NH 4 F and the treatment at 90 °C changed the gel structure substantially. Our interpretation is that the fluoride species break the gel structure, and thus, very small nanoparticles are formed, which form larger agglomerates. First zeolite crystallites are formed during this stage, most probably by the oriented aggregation of precursors particles. [START_REF] Grand | Mechanism of Zeolites Crystal Growth: New Findings and Open Questions[END_REF] We do not have molecular level information of the mechanism leading to the formation of extra-thin MFI-type crystals and the preferential growth along a-and c-axis leading to the well-developed (010) face. However, we clearly identify the formation of first crystallites during the 90 °C aging prior to the autoclave treatment, as indicated by the splitting of the pentasil vibration at 540~570 cm -1 in the FTIR spectra (Figure S4). [START_REF] Lesthaeghe | Fingerprint: How Pentasil-Induced IR Bands Shift during Zeolite Nanogrowth[END_REF] The autoclave treatment just accelerates the growth rate without altering the growth mechanism. Thus, the particular morphology is a result of the preliminary 90 °C aging of the gel in the presence of NH 4 F. On the ground of collected results, the system's evolution leading to plate-like MFI crystals is presented in Scheme 1. resulted in the formation of oval shape 90° twins with a size of about 1 µm (Figure S15). Since the fluoride content in the systems is similar, the observed effect raises the question about NH 4 + cation role.

Sodium and potassium exhibit a structure-directing effect, and many zeolites are synthesized solely using these two alkali metals. In contrast, ammonium itself cannot generate a zeolite type material due to the weak charge and, consequently, limited interactions with the negatively charged silica species.

Hence, we consider ammonium itself does not play an essential role in the growing process and thus does not disturb the formation of plate-like crystallites. Meanwhile, the NH 4 F is a buffer that controls the pH. After its addition, the pH of the gel changed from ~12.3 to ~8.5. The introduction of similar molar contents of NaF or KF, adjust the pH value to ~9.7 and ~10.1, respectively. The great decrease of pH value from ~12.3 to ~8.5 certainly decreases the solubility of silicate species, and thus macromolecular colloid species are generated. [START_REF] Lesthaeghe | Fingerprint: How Pentasil-Induced IR Bands Shift during Zeolite Nanogrowth[END_REF][START_REF] Fan | Crystallization Mechanism, and Catalytic Properties of Titanium-Rich TS-1 Free of Extraframework Titanium Species[END_REF] Therefore, the addition of NH 4 F has two well-established effects on the initial gel: i) breaking the gel structure and generation of nanoparticles that play an important role in forming plate-like crystals; and ii) decreasing the pH and thus changing the solubility of the silica species. The consequence of these two effects is the preferred growth in aand c-directions, owing to the face with the slowest growth rate is the largest, [START_REF] Sunagawa | Crystals: Growth, Morphology, and Perfection[END_REF] leading to substantial development of the (010) face. The ultimate result is the formation of ultra-thin crystals with a high aspect ratio between different crystal faces. It is worthy to note that the F -is vital for promoting the crystallization of MFI zeolite crystals. In a similar synthesis system, where the NH 4 Cl replaced NH 4 F, only an amorphous product was obtained after two weeks of crystallization (Figure S16).

Silicalite-1 synthesized, according to the present procedure in a mixed OH -/F -medium was compared with the counterparts synthesized in OH -and F -medium by means of solid-state NMR. The comparative study was based on [START_REF] Na | Pillared MFI Zeolite Nanosheets of a Single-Unit-Cell Thickness[END_REF] Si MAS, [START_REF] Na | Pillared MFI Zeolite Nanosheets of a Single-Unit-Cell Thickness[END_REF] Si{ 1 H} CPMAS NMR, [START_REF] Na | Pillared MFI Zeolite Nanosheets of a Single-Unit-Cell Thickness[END_REF] Si{ 19 F} CPMAS NMR, and [START_REF] Xiao | Simultaneous Realization of High Catalytic Activity and Stability for Catalytic Cracking of: N -Heptane on Highly Exposed (010) Crystal Planes of Nanosheet ZSM-5 Zeolite[END_REF] F MAS NMR analyses. All the samples display resonances in the [-107 ppm; -120 ppm] region corresponding to Q 4 species. However, the sample synthesized in pure F -medium (Figure S17a3) shows well-resolved peaks corresponding to Q 4 (Si(OSi) 4 ) silicon species, reflecting different crystallographic T positions in MFI-structure. The Q 4 resonances in samples synthesized in a mixed OH -/F -(Figure S17a1) and OH - (Figure S17a2) media are broader because of a less ordered three-dimensional Si(OSi) 4 network. This effect is due to local disordering, with a distribution of Si-O-Si angle and distance for each crystallographic silicon T site, leading to a broadening of the well-resolved peaks observed for a high-ordered 3d-network. [START_REF] Fyfe | Natural-Abundance Two-Dimensional 29 Si MAS NMR Investigation of the Three-Dimensional Bonding Connectivities in the Zeolite Catalyst ZSM-5[END_REF][START_REF] Cadars | Probing Local Structures of Siliceous Zeolite Frameworks by Solid-State NMR and First-Principles Calculations of 29 Si-O-29 Si Scalar Couplings[END_REF] It is even more obvious in the case of OH -medium synthesized silicalite-1 (Figure S17a2), where almost no discontinuity of the spectral line is observed. In addition, a broad resonance at -103 ppm corresponding to Q 3 (Si(OSi) 3 OH) is recorded for the silicalite-1 synthesized in mixed (OH -/F -) (Figure S17a1 and S17b1) and pure OH -medium (Figure S17a2 andS17b2). This resonance is a consequence of discontinuity between SiO 4 tetrahedron of the zeolite framework, leading to the formation of Si-OH bonds, and thus defects Q 3 . Nevertheless, the fitting of [START_REF] Na | Pillared MFI Zeolite Nanosheets of a Single-Unit-Cell Thickness[END_REF] Si MAS spectra pointed out a higher amount of defects for silicalite-1 synthesized in OH -medium (25 %) than for silicalite-1 synthesized in mixed medium (8 %). No Q 3 species are observed in the case of the sample synthesized in NH 4 F medium (Figure S17a3), confirmed by no Q 3 signals enhancement in the 29 Si{ 1 H} CPMAS spectrum (Figure S17b3), indicating a lack of Si-OH defects for this sample and reinforcing the idea of a well-ordered 3d-structure.

Furthermore, a broad peak appears at -125 ppm for the silicalite-1 sample synthesized in F -medium, which is not observed for the pure OH -medium synthesized counterpart. This peak is attributed to the fluoride ions, which have certain mobility between SiO 4/2 tetrahedron. The large signal is typical of framework silicon sites that undergo dynamic exchange between 4-and 5-coordinated environments due to fluoride ion mobility. [START_REF] Koller | Five-Coordinate Silicon in High-Silica Zeolites[END_REF] This signal is not observed in the sample synthesized in mixed (OH -/F -) medium (Figure S17a1 and S17b1). However, this broad resonance at -125 ppm appears and is enhanced by [START_REF] Na | Pillared MFI Zeolite Nanosheets of a Single-Unit-Cell Thickness[END_REF] Si{ [START_REF] Xiao | Simultaneous Realization of High Catalytic Activity and Stability for Catalytic Cracking of: N -Heptane on Highly Exposed (010) Crystal Planes of Nanosheet ZSM-5 Zeolite[END_REF] F} CP transfer (Figure S17c1), suggesting the mobility of F -ions between SiO 4/2

tetrahedron. The resonances at -109 and -117 ppm are also broadened slightly by [START_REF] Na | Pillared MFI Zeolite Nanosheets of a Single-Unit-Cell Thickness[END_REF] Si{ 19 F} CP process, pointing out that these silicon sites are affected by a disordering of silicon atoms at the vicinity of F -, or an influence of the mobility of the F -ion but to a lesser degree. Furthermore, the resonance attributed to Q 3 defects disappears in [START_REF] Na | Pillared MFI Zeolite Nanosheets of a Single-Unit-Cell Thickness[END_REF] Si{ [START_REF] Xiao | Simultaneous Realization of High Catalytic Activity and Stability for Catalytic Cracking of: N -Heptane on Highly Exposed (010) Crystal Planes of Nanosheet ZSM-5 Zeolite[END_REF] F} CPMAS spectrum, in agreement with this assignment.

Figure S18 shows the [START_REF] Xiao | Simultaneous Realization of High Catalytic Activity and Stability for Catalytic Cracking of: N -Heptane on Highly Exposed (010) Crystal Planes of Nanosheet ZSM-5 Zeolite[END_REF] F MAS NMR spectra of silicalite-1 synthesized in NH 4 F and mixed (OH -/F -) medium (Figure S18a and S18b). The spectrum area in the dotted line is emphasized in the inset. Both spectra exhibit two isotropic signals at -79 ppm and -64 ppm (with a higher intensity for the latter), indicating two environments for fluoride ions. Fits of these spectra show integrated intensities of 99.5 % and 89 % for the main signal for silicalite-1 synthesized in NH 4 F and mixed medium, respectively.

The signal at -64 ppm is typical of as-synthesized F-MFI. It is known to result from F -located in [4 1 5 2 6 2 ] cavity types, where F -ions are subjected to a dynamic exchange between penta-coordinated Si sites ([SiO 4/2 F] -). [START_REF] Koller | Five-Coordinate Silicon in High-Silica Zeolites[END_REF][START_REF] Fyfe | Location of the Fluoride Ion in Tetrapropylammonium Fluoride Silicalite-1 Determined by 1 H/ 19 F/ 29 Si Triple Resonance CP, REDOR, and TEDOR NMR Experiments[END_REF][START_REF] Dib | Preferential Orientations of Structure Directing Agents in Zeolites[END_REF] The second site at -79 ppm can be due to the SDA (TPA + ) in the straight channels, encountering F -ions in the cavities. This signal is also attributed to fluorine close to defects. However, the last assignment is in disagreement with the fact that the fluoride medium synthesized silicalite-1

does not contain framework defects, as illustrated in Figure S17b3 ( 29 Si{ 1 H} CPMAS spectrum). Thus, the most plausible interpretation of these two signals would be that there exist two [START_REF] Xiao | Simultaneous Realization of High Catalytic Activity and Stability for Catalytic Cracking of: N -Heptane on Highly Exposed (010) Crystal Planes of Nanosheet ZSM-5 Zeolite[END_REF] F environments, resulting from different interactions of [SiO 4/2 F] -with the TPA + branches with different conformations in straight and sinusoidal channels. [START_REF] Rojas | Zeolite Structure Direction by Simple Bis(Methylimidazolium) Cations: The Effect of the Spacer Length on Structure Direction and of the Imidazolium Ring Orientation on the 19 F NMR Resonances[END_REF] This assignment is also in agreement with other existing studies. [START_REF] Fyfe | Location of the Fluoride Ion in Tetrapropylammonium Fluoride Silicalite-1 Determined by 1 H/ 19 F/ 29 Si Triple Resonance CP, REDOR, and TEDOR NMR Experiments[END_REF][START_REF] Dib | 14 N Solid-State NMR: A Sensitive Probe of the Local Order in Zeolites[END_REF]

Large-scale synthesis of plate-like MFI zeolite

Upscaling synthesis is a challenging and often difficult exercise. A number of parameters are changed when upscale from laboratory to semi-industrial or large scale vessels. For instance, the contact surface between gel and autoclave, heat transfer, concentration gradients, vapor disengagement, and reagent grade. [START_REF] Hendershot | Safe Chemical Reaction Scale Up[END_REF] Thus, there exists a large gap between the laboratory-scale synthesis and their industrial implication. Consequently, scale-up is a key problem in zeolite production, which greatly limits the industrial application of zeolites with various topologies.

The ultra-thin plate-like silicalite-1 can be easily synthesized at low temperature and under ambient pressure, the conditions gracefully suitable to large-scale synthesis. The applicability of our approach to a large scale synthesis was verified by using a 20 L open glass reactor (Figure 4a). The transparent glass reactor allows monitoring the physical changes in the synthesis system, as shown in Figure 4b. Also, the crystallization process in the employed open synthesis system can be easily monitored by sampling at different times. The scale-up experiment was successful, and ~1320 g dry zeolite powder was obtained (Figure 4c, after calcination). The product yield is calculated to be ~97% based on silica conversion, which is impressively high for the large-scale zeolite synthesis. The XRD patterns (Figure 4d) and SEM images (Figure 4e) revealed that the crystallization in the 20 L glass reactor was completed within 96 h, noticeable longer than that in the 20 mL autoclave. This was the only negative effect of the scaled-up synthesis, which we attribute to the slightly lower temperature employed (88±2 versus 90 o C) and a slower rise of the temperature in the larger vessel. Nevertheless, the quality of the final product was just as good as the counterpart obtained with laboratory equipment (Figure 1 and4). The result of scaled-up synthesis clearly indicates that the synthesis strategy developed in this work is readily applicable to industrial scale.

Synthesis of heteroatom-containing plate-like MFI

The catalytic applications of zeolites require the presence of active sites. In this section, we report the synthesis of Al-MFI (ZSM-5) and Ga-MFI counterparts of silicalite-1. The procedure used for the synthesis of plate-like silicalite-1 was employed as a heteroatom was introduced in the initial gel (see Supporting Information for details). Thus a gel with Si/Al ratio 50 was prepared and subjected to hydrothermal treatment at 90 °C. All obtained materials were highly crystalline. The initial gel with Si/Al ratio of 50 yielded crystalline ZSM-5 crystals with Si/Al of 72 (Table S1). The framework Al content did not increase even when an initial gel with Si/Al ratio of 25 was employed. This result pointed out a threshold of the framework Al incorporation under our synthesis conditions. The most plausible reasons limiting the Al incorporation in the ZSM-5 crystals are: i) the presence of F -ions that compensate the positively charged TPA + ; ii) the absence of alkali cations; and iii) the low synthesis temperature. The synthesized ZSM-5 contained, namely, tetrahedral Al as shown by the dominant resonance at 55 ppm in the [START_REF] Zhang | Open-Pore Two-Dimensional MFI Zeolite Nanosheets for the Fabrication of Hydrocarbon-Isomer-Selective Membranes on Porous Polymer Supports[END_REF] Al MAS NMR spectrum (Figure S19). The as-obtained plate-like ZSM-5 has similar physicochemical characteristics with the industrial ZSM-5 nanocrystalline sample (Figure S20 and Table S1). Compared with plate-like silicalite-1 crystals, the ZSM-5 counterpart exhibits similar morphology and textural properties (Figure 1c, 5a, S21 and Table S1).

Ga-MFI counterpart of silicalite-1 was also successfully obtained (Figure S22). The materials showed Si/Ga ratio close to 100 (Si/Ga = 98), high crystallinity, and pore volume similar to silicalite-1 synthesized under the same conditions (Table S1). The crystals still exhibited plate-like morphology (Figure S23 and S24), however, compared to the crystals of silicalite-1, smaller crystals with increased thickness along the b-axis can be observed. The aspect ratio of L c /L b for Ga-MFI was ~10, which is much lower than silicalite-1 synthesized under the same conditions.

Catalytic performance of plate-like ZSM-5 in the MTH reaction

Methanol-to-hydrocarbons (MTH) is often used as a important model reaction for exploring the improved diffusion behaviors of morphology-controlled zeolite catalysts. [START_REF] Gallego | Simple Organic Structure Directing Agents for Synthesizing Nanocrystalline Zeolites[END_REF][START_REF] Dai | Finned Zeolite Catalysts[END_REF] Herein, the catalytic performance of plate-like HZSM-5 crystals, in comparison with commercial ZSM-5 nanocrystals with a similar number of Brønsted acid sites (Table S1), was evaluated in the MTH reaction. As shown in Figure 5b, plate-like HZSM-5 samples exhibit an ultra-long lifetime of 250 h (methanol conversion >99%), which is five times longer than that of commercial ZSM-5 nanocrystals under identical reaction conditions. The extended lifetime is attributed to the shorter diffusion path along the straight channel of ZSM-5 crystals. The plate-like crystals of thickness around 100 nm improve the mass transfer significantly and slow down the coke accumulation. According to the time-dependent product distribution in MTH reaction (Figure 5c-d), higher propylene selectivity (~30% versus ~26%) but lower BTX selectivity (~14% versus ~17%) was obtained on plate-like ZSM-5 samples in comparison with commercial ZSM-5 nanocrystals, as a result of the diffusion control. Therefore, the plate-like HZSM-5 is a high performing acid catalyst for the industrial MTH reaction, and it also holds promises for other catalytic applications, especially when dealing with bulky molecules.

Conclusions

A synthetic approach yielding ultra-thin plate-like MFI-type crystals without using special structure-directing reagents or organic growth-modifiers is developed. The crystal thickness along the straight channel (b-axis) can be effectively tuned to several tens of nm, thus shortening the diffusion path. This simple approach also allows tuning the aspect ratio between different crystal faces, thus developing the crystal face with the channel system of interest. Consequently, the number of pore openings per unit crystal surface is maximized, and the diffusion limitations' impact is minimized.

The synthetic approach combines preliminary aging in F-free medium with a fluoride-assisted low-temperature crystallization process. Thus the nucleation process is split from the crystallization, which takes place in F-containing medium. A suitable aging time (12 h) is indispensable for the formation of viable MFI nuclei, while the crystallization is controlled by the presence of fluorine mineralizer. The specific effect of F -on the precursor gel structure, the pH of the gel, and thus the solubility of the silica species, resulting in an alteration of the growing mechanism, leads to the formation of ultra-thin crystals with high L c /L b aspect ratio.

The above-described synthesis strategy is versatile, efficient, and applicable for the industrial production of plate-like zeolites. We have obtained a similar result, i.e., ultra-thin plate-like FER-type zeolite (Figure S25), which proves that a general strategy applicable to different zeolite types is developed. It can be scaled up for the synthesis of kilogram-scale siliceous MFI zeolites in an open glass reactor with an extremely high yield of ~97%, and it can also be extended to the synthesis of heteroatom containing MFI zeolites like Al-MFI and Ga-MFI.

Associated content

Supporting information

The supporting information is available for this paper, including Materials and Methods, Figures S1-S25 and Table S1.

Table of Contents Graphic

Figure 1 .

 1 Figure 1. General features of silicalite-1 obtained in the 0.1TPAOH:1.0SiO 2 :0.8NH 4 F:30H 2 O system: (a) XRD pattern of as-synthesized solid products; (b) Ar physisorption isotherm of the calcined product with pore size distribution shown as an inset; (c) SEM and (d, e) low-magnification HAADF-STEM images of silicalite-1 crystals; (f) high-resolution TEM images, (g) the selected area electron diffraction (SAED) pattern; (h) high-magnification HAADF-STEM image of silicalite-1 crystals after calcination.

Figure 2 .

 2 Figure 2. Synthesis parameters relevant to the formation of plate-like MFI zeolite crystals from the synthesis system x TPAOH: 1.0 SiO 2 : y NH 4 F: 30 H 2 O crystallized at 90°C: (a) the length along the three axes; (b) L c /L a and (c) L c /L b aspect ratios; (d) effects of TPAOH content on the relative crystallinity and b-axis thickness of MFI zeolite; (e) XRD patterns of solid products subjected to 0 h, 12 h, 24 h, and 36 h aging time (the crystallization time is shown in parentheses) from the system with x = 0.1, y = 0.8. Inset (e): representative SEM images of solid products.

Figure 3 . 1 .

 31 Figure 3. TEM analyses of products at different stages of zeolite synthesis: (a) After aging for 12 h at 90 °C. After the addition of NH 4 F and stirring at 90 °C for 6 (b) and 12 h (c). After hydrothermal treatment in the autoclave for 6 (d), 9 (e), 12 (f), 24 (g), 36 (h) and 48 h (i). Scale bar: 2 μm. Insets: views of single plate-like MFI zeolite crystal (marked with yellow) and the selected area electron diffraction patterns (marked with red).

Figure 4 .

 4 Figure 4. Large-scale syntheses of plate-like MFI zeolite from a gel with composition: 0.1 TPAOH: 1.0 SiO 2 : 0.8 NH 4 F: 30 H 2 O; T = 88±2 o C, crystallization time 96 h. (a) Picture of a 20 L glass reactor for large-scale synthesis; (b) Visualization of synthesis process from the initial synthesis gel (upper level) to crystallized product (lower level); (c) Silicalite-1 product after calcination; (d) XRD patterns of solid products obtained after different synthesis time; (e) SEM images of solid products obtained after different synthesis time.

8

 8

Figure 5 .

 5 Figure 5. Comparative study of the MTH performance of plate-like HZSM-5 crystals and commercial ZSM-5 nanocrystals: (a) XRD patterns of calcined plate-like HZSM-5 (top, Si/Al = 72) and commercial ZSM-5 nanocrystals (bottom, Si/Al = 74); Inset: representative SEM images of solid products; (b) Methanol conversion as a function of reaction time over two different ZSM-5 samples; Product

Acknowledgments

We acknowledge the National Natural Science Funds of China (21722303, 22025203), the Municipal Natural Science Funds of Tianjin (18JCJQJC47400), 111 Project (B18030), and the China Scholarship Council for supporting the work. V.V. acknowledges the financial support from the Industrial Chair ANR-TOTAL "NanoClean Energy" (ANR-17-CHIN-0005-01) and FEDER 18P01675.

Author information

Corresponding Author: *E-mail: wuguangjun@nankai.edu.cn (G.W.); *E-mail: valentin.valtchev@ensicaen.fr (V.V.).

ORCID:

Valentin Valtchev: 0000-0002-2341-6397

Notes

The authors declare no competing financial interests.


domain: phys chim spi

text:
A Belsky

K Lebbou

V Kononets

O Sidletskiy

A Gektin

E Auffray

D Spassky

A N Vasil'ev

Mechanisms of luminescence decay in YAG-Ce,Mg Fibers Excited by γand X-rays

Time-resolved luminescence of YAG-Ce (150 ppm) fiber crystal with Mg co-doping was studied under pulsed X-ray excitation and γ-rays (Cs 662 keV). The initial part of decay kinetics under X-ray excitation is faster than for direct cerium excitation (63 ns). Decay kinetics is also characterized by the presence of slow components with at least two characteristic times longer than Ce 3+ radiation time. The slowest one which dominates for t>500 ns in YAG-Ce without Mg co-doping practically vanishes for samples with 50 ppm co-doping. Decay kinetics under γ-rays are characterized by slower rise time than that under X-rays. These properties can be explained by competition of energy transfer and energy losses in track regions. The distribution of excitations in tracks produced by X-rays differs from that in tracks produced by γ-rays, since the energy of primary electron after γ-quantum conversion is much higher than after X-ray absorption. The stopping power for energetic electrons decreases with increase of electron energy, and therefore the density distribution after X-ray conversion is shifted to higher densities, Therefore, the acceleration of recombination and quenching of excitations is more prominent under X-ray excitation. Specific role of Ce 4+ induced by Mg co-doping is also discussed in the paper.

Introduction

Relationships between the structure of excited region (track) formed by high energy particle or photon in a scintillator, and mechanisms of energy transfer from intrinsic electronic excitations to luminescence centers is important for understanding the processes in scintillators. Electronic excitations are distributed nonhomogeneously within a track, which may affect the light yield, luminescence rise and decay times, and energy resolution of a scintillator. Basing on the example of YAG-Ce, this paper discusses some aspects of energy transfer in activated inorganic scintillators with account for inhomogeneous distribution of electronic excitation in tracks.

Influence of co-dopants on scintillation process has been extensively studied since 90 th and during recent years [1][2][3][4]. Co-doping is a way to enhance light yield and accelerate luminescence decay.

There are two hypotheses on the role of divalent co-dopant in the modification of optical and scintillation properties of Ce-doped complex oxides. The first one claims that introduction of Mg 2+ or Ca 2+ leads to a concentration decrease of traps responsible for afterglow, and creation of new types of traps spatially correlated with Ce 3+ , which makes carrier exchange between activator and host easier. The second hypothesis claims the relation of the ultraslow component weakening to the Се 4+ concentration increase at co-dopant addition [3,4,6]. It was suggested that Се 4+ ion is activated by the electron capture from the conductance band with subsequent photon emission by the formed (Се 3+ )* ion.

In this work, the effect of Mg co-doping of YAG-Ce scintillator has been reconsidered basing on competition between energy transfer channels in regions with different concentration of electronic excitations.

Experimental

Samples of YAG-Ce (150 ppm) with Mg co-doping were grown using the micro-pullingdown technique [START_REF] Kononets | Engineering of Scintillation Materials and Radiation Technologies[END_REF]. The dopant values refer to the concentrations over yttrium sites. Chunks of YAG and YAG:Ce crystals produced by CRYTUR and Institute for Scintillation Materials NASU were used as raw materials. Raw materials were melted in iridium crucible and crystallized onto a YAG:Ce seed. Thermal gradients in the crystallizer were controlled by the iridium afterheater and alumina ceramic heat insulation. The pulling rate was 0.3 mm/hour. After growth the fibers were annealed in air at 1200 °C during 24 hours. As admixture segregation back to the melt is very weak in the micro-PD method, the Ce and Mg concentrations in melt and fiber are similar.

The luminescence of samples was studied under pulsed X-ray excitation, γ-rays (Cs 662 keV). For X-ray excitation we use Hamamatsu X-ray tube with the operating voltage 30kV, W anode, and 500 μm thick Be output window. Photocathode of this tube was excited by diode laser with the 50 ps pulse length and 200 kHz repetition rate (5 µs repetition period). Emission spectrum of such tube has maximum at 10 keV and is limited by Be transmission from the low energy side, and by anode voltage from the high energy side. Decay kinetics for Ce 3+ emission was measured by fast PMT with spectral separation using interference filter with central wavelength of 560 nm and bandwidth of 20 nm. PMT signal together with synchronization signal from diode laser were inputs for PicoQuant Time-Correlated Single Photon Counting System. The time resolution of this system was about 200 ps. Decay signal of gamma scintillation was detected by 9814QB PMT from ET Enterprises and measured using an 2 GHz oscilloscope with 1 ns sampling interval. A small percentage of the pulses was originating from single photoelectron noise of the PMT since very low trigger threshold was used to record the pulse shapes. Such events have been discarded and only the pulses from scintillation were used to calculate the average scintillation waveform. Scintillation kinetics of the samples was averaged over 1000 pulses.

Experimental results and discussion

At pulse X-ray irradiation with 5 µs repetition time T a slow luminescence decay component is clearly manifested with the much longer decay time compared to the pulse length. The constant level due to cumulative contribution of previous pulses is manifested in the measured decay curve for sample without co-doping (Fig. 1, blue curve, 0 ppm Mg). In case of periodic excitation, one should take into account the signals from all previous pulses. Therefore, this slow tail may be fit by a sum of hyperbolic functions ( )

0 0 n t t nT α ∞ - = + +

∑

. The fitting gives α≈1.1. The corresponding curve is plotted as green dashed line. This component is the main contribution into afterglow. Decay curves integrated within the 5 µs repetition period of X-ray pulses for samples containing 0, 25, 50 ppm of Mg (gray hexahons -total integral), integral contribution from the ultraslow decay component (cumulative constant level) (green inverted triangles -Slow), scintillation component intensity, i.e. the difference between total and slow (magenta triangles -Fast). The samples were cut from fiber heads (1) and tails (2).

The quantitative estimations of the total YAG-Ce, Mg emission yield, ultraslow components and scintillation yield (integral of the decay over first 2 µs) are presented in Fig. 2 as a function on Mg concentration. With increasing Mg concentration, the contribution from the ultraslow component decreases by more than 10 times (green inverted triangles). In the Mg-free sample the latter constitutes about half of the overall luminescence yield. Meanwhile, it is different on heads and tails of the fiber. The scintillation component intensity (magenta triangles), which is the difference between total integral and integral of the cumulative constant level, remains about the same on both ends of the fiber at all Mg concentrations, and its contribution increases by about twice with Mg concentration increase from 0 to 50 ppm. One may conclude that defects are distributed nonuniformly along the fiber. Their concentration is higher near one of the fiber ends. The scintillation yield from both fiber sides is the same, it proves the efficiency of the µ-PD method for fabrication of scintillation elements. Study of about 10 samples with different Ce and Mg concentrations indicates that the slow component elimination occurs at the co-doping level about 50-100 ррm of Mg. As similar result was observed in other oxides doped with Ce or Pr, a mechanism of this phenomenon is, probably, the same.

The slow tail of emission is a negative factor both in fast-timing and scintillation applications. Even disregarding the hyperbolic tail, the decay kinetics under X-ray excitation differs significantly from single exponential law. Obviously, this decay curve shape is a result of competition between several mechanisms of Ce excitation. If we compare the decay curves under X-ray excitation with decay under direct excitation of Ce 3+ centers, we can see the difference both at short and long times. It is well-known that direct 410 nm excitation of 4f-5d1 Ce 3+ transition in YAG result in single exponential decay with intrinsic radiation time τR = 62 ns (see also Fig. 3 below). This time roughly corresponds to the middle part of the decay under X-ray excitation (see Fig. 1). The decay at short times show faster components, the appearance of which can be explained by the acceleration of kinetics after quenching of Ce 3+ emission due to interaction with surrounding excitations. The appearance of components with characteristic times longer than intrinsic radiation time τR is related to additional intermediate stages of carrier transport (including carrier trapping) prior to energy transfer to Ce 3+ ions.

Fitting the decay curve for the sample without co-doping by 3 exponentials within the initial 500 ns shows that the basic contribution comes from the components with the 80 ns (42%) and 300 ns (57%) decay times. The ultrafast component contribution with the decay time of about 10 ns is below 0.5 %. Doping with 50 ppm Mg results in significant decrease of the slow contribution, and simultaneously in the decay acceleration at the initial stages (Fig. 1, red curve, 50 ppm Mg). A contribution from the ultrafast component increases up to 5%. The rest is distributed between the 50 ns (66 %) and 190 ns (29%) components. Another noted effect is the difference in scintillation decay measured under excitation of the same sample by X-rays and γ-rays. To illustrate this effect, Fig. 3 shows the initial 250 ns of Ce 3+* luminescence decay curves of Mg-co-doped sample (the same as shown in Fig. 1) for three types of excitation: direct excitation of the 4f-5d1 transition in Ce 3+ (measured using picosecond laser diode with 455 nm wavelength), excitation by 137 Сs γ-quanta, and photons emitted by X-ray tube. The direct excitation results in single exponential decay with the 62 ns characteristic time. As mentioned above, the luminescence decay kinetics is much more complicated under excitation with ionizing particles than under direct photoexcitation, and differs significantly for γ-quanta-and X-ray excitation. The initial part of decay curve under X-rays is faster to that at photoexcitation and characterized by decay 9 ns component. On the contrary, the rise-time of 5 ns is observed under gamma rays. Further trends of the decay curves are different as well: X-ray excited decay slows down and show 50 ns and 190 ns decay times, while the decay under γ-excitation is composed of two components 69 ns and 200 ns. The time resolution for both experimental setups was better than 1 ns. Therefore, we have to state that the balance of different energy transfer and cerium excitation mechanisms is significantly changed when the energy of exciting photon changes from 10 keV to 662 keV. This effect is general, but varies with parameters of hot relaxation of electronic excitations. The observed effects can be explained if one takes into account inhomogeneous distribution of electronic excitations in the track region and the difference of these distributions for γ and X-ray excitations.

It is well known that the luminescence efficiency and decay kinetics efficiency in scintillators and luminophores strongly depends on the concentration of thermalized excitations which defines energy transfer processes and competition between different channels of relaxation of electronic excitations (see, e.g. [7]). The distribution of carriers is essentially non-uniform in the track region, and therefore the decay of scintillation ( ) ; I t E can be estimated if we know the distribution of excitations and the luminescence response as a function of concentration [START_REF] Vasil'ev | Engineering of Scintillation Materials and Radiation Technologies[END_REF]9]:

( ) ( ) ( ) ; ; ; log I t E w n E y t n n d n = ∫
, where ( ) ; y t n is the time-dependent yield (per one excitation) of luminescence from the crystal with initial uniform concentration of excitations n. This yield can be estimated using the method of rate equations (see, e.g. [10]). To do so, the rate equations are solved separately for each of the excitation densities in the distribution. The distribution ( )

; w n E of concentrations n over log n
shows the fraction of the regions with concentration between n and ( )

1

∫

, where

( )

N E is the total number of excitations created by X-ray or γ-photon with energy E.

The spatial distribution of thermalized electronic excitations in the track region ( ) ; w n E can be estimated in the following way. During the first stage of interaction of primary electron with media this electron creates hot excitations along the electron path, and the linear density of energy deposit is the stopping power d E d x -. This stopping power strongly increases with the slowing down of the primary electron, and therefore initial part of track has much lower linear density of electronic excitations. The higher the initial energy of primary electron, the longer the initial low-density part of track is. The range of 10 keV electron is about 1 µm, whereas the range of 662 keV electron 1 mm. So, the mean linear energy deposit along the track is about 20 times less for γ-photon than for X-ray photon. The cascade stage is finished faster than in 1 ps, and after this cascade all electrons and holes are distributed in energy in such a way that their kinetic energy is less than Eg. The thermalization of such electrons and holes due to phonon emission is accomplished by the shift of excitations from their birthplace. This shift is just 3D diffusion of the carriers along with thermalization. The spatial distribution is the convolution of Gaussians with the energy distribution of secondary excitations. This stage is described in details in [11]. When we convolve this 3D spread during thermalization with the 1D distribution of linear energy deposit along the track, we obtain spatial distribution of carriers in the track region. The distribution ( )

;
w n E can be calculated as the histogram of the concentrations using equidistant bins in log n . This procedure is described in [START_REF] Vasil'ev | Engineering of Scintillation Materials and Radiation Technologies[END_REF].

The results for calculation of excitation density distribution are presented in Fig. 4. A typical density distribution of created excitations spreads from 10 10 to 10 22 excitations per cm 3 . The distributions of created excitation densities in YAG after absorption of gamma 662 keV photon (magenta curve A) are shifted to the lower concentration region in comparison with that for X-ray photon of 10 keV (blue curve B). The shift roughly corresponds to 20 times lower concentration (in accordance with the presented above estimation of linear energy deposit along the tracks). This increase of the weight of higher concentrations significantly modify the scintillation properties. The vertical lines in Fig. 4 separates the regions when electron and hole concentrations less or higher than the concentrations of cerium ions and defects considered in our simulation and expressed in ppm (measured in units of yttrium concentration). Vertical line 0 in Fig. 4 corresponds to the distance between excitations equal to dipole-dipole interaction radius.

;
w n E for electron densities in the track in YAG at 300 K after absorption of γ-quantum (magenta curve A) and X ray photon of 10 keV (blue curve B). Vertical lines mark the concentrations of cerium ions and defects values considered in our simulation (1 -100 ppm, 2 -10 ppm, 3 -1 ppm, 4 -0.1 ppm) and 0 line corresponds to characteristic distance of dipoledipole interaction between excitons resulting in the destroying of excitations. Top axis gives the mean distances between excitations.

The simplified scheme of recombination and capture processes should include at least three main processes. Two of them result in creation of excited cerium ion (1) due to the energy transfer from coupled electron-hole pair (exciton) to cerium ion Ce 3+ , and (2) due to the capture of an isolated electron from the conduction band by Ce 4+ ion. We suppose that the concentration of Се 4+ ions in YAG crystal increases with the increase of concentration of co-activator Mg 2+ [3], therefore the second process becomes more important in crystals with co-doping. The third important process is the capture of an electron by the trap followed by its release. After the delayed release the electron can be either captured by a hole and then pass the energy of the created exciton to the Ce 3+ or can be captured by Ce 4+ ion. In addition, we have to take into account the radiationless decay of excitations and migration of the charge carriers out of the track region. The fraction of different channels of recombination depending on the concentration of excitations is presented in Fig. 5 for YAG-Ce (top panel, a) and YAG-Ce,Mg (bottom panel, b). The fraction of each channel can be estimated as the ratio of the rate of this channel to the total rate of channels. For instance, an electron can be (1)

III

proportional to concentration of traps h n . The latter process is preferable for high concentrations (it is bilinear in concentrations of electrons and holes), since initial concentration of holes is about concentration of electrons. Therefore, the fraction of electrons which are captured by, e.g. Ce 4+ ions, equals to w + . This fraction multiplied by the number of excitations corresponding to n and ( )

1 log n d n +
interval is presented by orange curve 4 in Fig. 5b. All other curves in Fig. 5b are estimated in the same way. This approach is only qualitative, in order to get result quantitively one should solve the rate equations for all concentrations with precise knowledge of coefficients, which is the weak point in usage of rate equations.

In case of YAG-Ce the concentration of Се 4+ is small (we take it as 10 -3 of the Ce 3+ concentration), and the most probable reactions are (1) the creation of excitons followed by the energy transfer to Ce 3+ [START_REF] Kirm | Physics and Chemistry of Luminescent Materials[END_REF] (region Ia, e+h → ex, ex+Ce 3+ → Ce 3+ *), and (2) capture of electrons by traps followed by afterglow (region II, e + trap → trap -→ e + trap,). The first channel is fast whereas the second one is slow due to the slow release of electrons from traps. The first channel is realized in the track regions where the concentration of electronic excitations is higher than the concentration of traps. On the contrary, the regions of lower concentration of electronic excitations is characterized by the predominantly capture of electrons by traps. In this consideration we suppose that concentration of traps is 1% of the concentration of Се 3+ . For Xray excitation about 20% of electron-hole pairs occur in the region II. If we compare the of distributions between cases of X-ray excitation and γ-excitation (see Fig. 4), we see that for the latter case about 50% of e-h pairs occur in region II. Moreover, high fraction of electronic excitations in the regions with their low concentration results in the observed long rise time for γexcitation. Excitons in these regions are created after relatively long migration of electrons over traps. Therefore this migration results in the delayed excitation of Ce 3+ ions and the increase of the emission time in the comparison with intrinsic radiation time of Ce 3+* centers. This rise time formation is similar to that observed under UV photoionization of cerium when the higher photon energy results in increase of the rise time due to increase of the thermalization length for electron with increase of its initial kinetic energy [13].

In the region of high concentration of electrons and holes (n>5×10 18 cm -3 ) excitons are created with high probability. But this region is characterized by high concentration of created excitons and therefore high probability of dipole-dipole interaction between excitons resulting in their death in Auger process (ex+ex→e+h, recombination of one exciton results in transformation of the second one into energetic electron-hole pair due to Förster or Dexter energy transfer) [14]. The threshold for this process is shown in Fig. 5 by vertical line 3. In this region III excitons can be destroyed also due to interaction with electrons and holes, e.g. ex+e→e (Auger process when the energy of exciton is passed to the electron) [15]. The radiationless death of excitons occur predominantly before the energy transfer to Ce 3+ . This quenching results in the limitation of the light yield of YAG-Ce in comparison with theoretical limit. Please note that mentioned above processes of exciton quenching in regions of high concentration of excitations result only on decrease of scintillation yield without influence on the kinetics of cerium emission, since this quenching occurs prior to energy transfer to cerium.

Not only excitons can be quenched in regions of high concentration of other excitations. The excited cerium ions also can be quenched by free electrons, holes and excitons in these regions. Schematic presentation of interaction of excited cerium with excitons and electrons are shown in Fig. 6. These processes result in radiationless deexcitation of cerium (quenching) and reveal as acceleration of decay with characteristic times shorter than cerium radiation time. Fig. 6. Auger-type interaction of excited Ce 3+* with exciton resulting in Ce 4+ and non-thermalized electron (left panel) and with electron resulting in deexcited Ce 3+ and non-thermalized electron (middle). The process of de-excitation by hole is presented in right panel. These processes result in cerium deexcitation and decrease of the total number of electronic excitations. D -donor excitation, A -acceptor transition.

When we add Mg 2+ co-doping, a part of Се 3+ is converted into Се 4+ . We consider the case when the resulting concentration of Ce 4+ ions reaches 10% of Се 3+ (Fig. 5b). There is no effect of Mg 2+ co-doping in the regions of high concentration of electronic excitations, whereas in regions of low and moderate concentration of excitations the role of Ce 4+ is important. The capture of electrons by Ce 4+ is more probable than the capture by traps since the concentration of Ce 4+ is 10 times higher than the concentration of traps. Moreover, the capture cross-section for e+Ce 4+ reaction is high due to Coulomb attraction. This results in the observed suppression of the long tails in kinetics, since the traps are not filled. Figure 5b shows that the region of concentrations for effective creation of excitons is narrower in comparison with Fig. 5a. Ce 4+ ions can be effective in the regions of low concentration since they are converted into excited Ce 3+* states without migration over traps and without intermediate stage of exciton creation. Such centers can emit photons with cerium intrinsic radiation time. We use this process for the explanation of the decrease of decay components with characteristic time about hundreds of nanoseconds and increase of the components with 60 ns lifetime for crystals with Mg 2+ co-doping. The presented discussion shows that about 10% of Ce 4+ ions converted from Ce3+ ions results in the recombination of 30-40% of electronic excitations through this channel. This effect exists also for γ-excitation, but the decay kinetics in this case is still slower than for X-ray excitation due to the longer migration of electrons to cerium centers.

Many experiments show that the concentration of Mg above 1000 ppm results in the strong quenching of cerium emission resulting in the decrease of the yield and accelerating of in comparison with cerium radiation time. We also observe this effect (Fig. 1) but in our experiment Mg concentration does not exceed 50 ppm. We propose the following mechanism of quenching of Се 3+* emission when Се 4+ concentration is above 5% of Ce 3+ concentration. After the capture of electrons by Ce 4+ the excess holes are still existing in the valence band. Such non-equilibrium concentration of holes can exist only in samples with the presence of Се 4+ . The band is rather broad in oxides, its width for YAG is about 6 eV [16]. Therefore free hole can effectively quench Ce 3+* state due to Auger process h+Ce 3+* →h+Ce 3+ when the cerium excitation passes to the hole with increase of its kinetic energy (thermalized hole from the top of the valence band goes deeper to the its bottom, see Fig. 6, right panel). This quenching process results in the appearance of the decay components with characteristic time shorter than Ce 3+* radiation time, similar to the processes presented in Fig. 6. The distribution of concentration of electronic excitations for X-ray track is shifted to higher concentrations in comparison with γexcitation, and holes are closer to Ce 3+* in the former case. Therefore the quenching is more for X-ray excitation in case of high Mg 2+ concentration. This result directly demonstrates the reasons of the quenching for high Ce 4+ concentrations.

Conclusions

Samples YAG-Ce (150 ppm) with and without Mg co-doping were grown using micro-pullingtechnique [1]. The samples were studied under pulsed X-ray excitation (repetition period 5 µs, anode voltage 30 kV), γ-rays (Cs 662 keV).

Decay kinetics under X-ray excitation differs significantly from single exponential law. It is characterized by long hyperbolic tail t -α , α≈1.1 at decay times much longer than cerium radiation time (63 ns). At short times the initial part of decay kinetics has decay faster than for direct excitation. At the intermediate interval 100-500 ns the decay becomes faster with increase of Mg concentration. Slow component (long hyperbolic tail) which dominates in YAG-Ce without Mg co-doping for t>500 ns practically vanishes for samples with 50 ppm co-doping.

Decay kinetics under γ-rays also becomes faster with increase of Mg concentration. However, their profiles significantly differ from that observed under X-ray excitation. They are characterized longer rise component of about 5 ns and slower decay in intermediate interval than under X-rays.

Using simulations described in [7][START_REF] Vasil'ev | Engineering of Scintillation Materials and Radiation Technologies[END_REF][9][10][11] based on convolution of 1D linear distribution of secondary excitations along the track with 3D spatial distribution of thermalized excitations, this difference can be explained by the difference of the density distribution of excitations in the track region. The distribution under X-rays is shifted to higher densities due to much shorter range of primary electron, and this induces the acceleration of recombination and quenching. The analysis of the decay curves under X-ray and γ-excitation shows that most of excitation mechanisms involve the capture of an electron by Ce 4+ ion, regardless of its nature (created during synthesis or created by ionization during excitation in the picosecond initial stages). The recombination is controlled by the distance between Ce 4+ and electron and therefore depends on the density of excitations and mobility of electrons, which is controlled by the concentration and depth distribution of traps.

Aknowledgements

This research is carried out in the frame of Crystal Clear Collaboration and is supported by a European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No 644260 (INTELUM) and COST ACTION TD1401 (FAST). Authors kindly acknowledge Dr. V. Makhov and Dr. T. Syreishikova for measuring of characteristic decay time under direct excitation of Ce 3+ in YAG-Ce,Mg fiber samples. Decay curves integrated within the 5 µs repetition period of X-ray pulses for samples containing 0, 25, 50 ppm of Mg (gray hexahons -total integral), integral contribution from the ultraslow decay component (cumulative constant level) (green inverted triangles -Slow), scintillation component intensity, i.e. the difference between total and slow (magenta triangles -Fast). The samples were cut from fiber heads (1) and tails (2). Fig. 3. YAG:Ce, Mg (50ppm) luminescence decay curves under gamma (blue), X-ray (magenta), and photo-excitation (green). Dashed curves are the result of fitting by three exponentials with characteristic times mentioned in the text.

Figure captions

;
w n E for electron densities in the track in YAG at 300 K after absorption of γ-quantum (magenta curve A) and X ray photon of 10 keV curve B). Vertical lines mark the concentration of cerium ions and defects values in our simulation (1 -100 ppm, 2 -10 ppm, 3 -1 ppm, 4 -0.1 ppm) and 0 line corresponds to characteristic distance of dipole-dipole interaction between excitons resulting in the destroying of excitations. Top axis gives the mean distances between excitations. Fig. 5. Schematic presentation of number of excitations which pass through different recombination channels as functions of carrier concentration. Top panel (a) corresponds to low concentration of Ce 4+ case of YAG-Ce), bottom panel (b) -corresponds to YAG-Ce, Mg, with 10% of Ce 4+ . Density distribution for electronic excitations in the 10 keV X-ray track in YAG (blue curve); fraction of electrons which are captured by a hole with exciton creation and transfer to Ce 3+ (red curve, 1); fraction of electrons captured by traps (black curve, 2); fraction of excitations which are destroyed due to interaction (violet curve, 3), fraction of electrons captured by Ce 4+ (orange curve, 4). Vertical lines: 1 -Ce 3+ concentration, 2concentration of traps, 3 -density of electronic excitations corresponding to quenching threshold, 4a,b -initial concentration of Ce 4+ . Regions I to IV limited by dashed lines are discussed in text. Fig. 6. Auger-type interaction of excited Ce 3+* with exciton resulting in Ce 4+ and non-thermalized electron (left panel) and with electron resulting in deexcited Ce 3+ and non-thermalized electron (right). Both processes result in cerium deexcitation and decrease of the total number of electronic excitations.

Fig. 1 .

 1 Fig. 1. Typical 560 nm luminescence decay curves of YAG-Ce (violet points,) and YAG-Ce,Mg (red points) under Х-ray excitation. Green dotted line is the slow decay tail fitted by a sum of hyperbolic functions (see text).

Fig. 3 .

 3 Fig.3. YAG:Ce, Mg (50ppm) luminescence decay curves under gamma (blue), X-ray (magenta), and photo-excitation (green). Dashed curves are the result of fitting by three exponentials with characteristic times mentioned in the text.

Fig. 4 .

 4 Fig.4. Excitation density distribution functions ( )

Fig. 5 .

 5 Fig. 5. Schematic presentation of number of excitations which pass through different recombination channels as functions of carrier concentration. Top panel (a) corresponds to low concentration of Ce 4+ (the case of YAG-Ce), bottom panel (b) -corresponds to YAG-Ce, Mg, with 10% of Ce 4+ . Density distribution for all electronic excitations created in the 10 keV X-ray track in YAG (blue curve); fraction of electrons which are captured by a hole with exciton creation and followed transfer to Ce 3+ (red curve, 1); fraction of electrons captured by traps (black curve, 2); fraction of excitations which are destroyed due to interaction (violet curve, 3), fraction of electrons captured by Ce 4+ (orange curve, 4). Vertical lines: 1 -Ce 3+ concentration, 2 -concentration of traps, 3 -density of electronic excitations corresponding to quenching threshold, 4a,b -initial concentration of Ce 4+ . Regions I to IV limited by dashed lines are discussed in text.

  increase of the concentration due to increase of e h

Fig. 1 .

 1 Fig. 1. Typical 560 nm luminescence decay curves of YAG-Ce (violet points,) and YAG-Ce,Mg (red points) under Х-ray excitation. Green dotted line is the slow decay tail fitted by a sum of hyperbolic functions (see text).

Fig 2 .

 2 Fig 2. Decay curves integrated within the 5 µs repetition period of X-ray pulses for samples containing 0, 25, 50 ppm of Mg (gray hexahons -total integral), integral contribution from the ultraslow decay component (cumulative constant level) (green inverted triangles -Slow), scintillation component intensity, i.e. the difference between total and slow (magenta triangles -Fast). The samples were cut from fiber heads (1) and tails(2).

Fig. 4 .

 4 Fig.4. Excitation density distribution functions ( )