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(1)AGH University of Science and Technology Faculty of Geology, Geophysics and Environmental Protection Department of Mineralogy, Petrography and Geochemistry. PhD Thesis. PHOTOACTIVE HYBRID NANOMATERIALS DERIVED FROM LAYERED MINERALS. Anna Koteja. Supervisor: dr hab. inż. Jakub Matusik. Kraków 2019.

(2) Podziękowania. Przede wszystkim dziękuję mojemu promotorowi Jakubowi Matusikowi za możliwość współpracy, za zaproponowanie tematu badań i co najważniejsze – za wciągnięcie w świat nauki. Ponadto, za aktywne uczestniczenie i zaangażowanie na wszystkich etapach tworzenia pracy doktorskiej. Serdecznie dziękuję moim współautorom: Markowi Szczerbie i Katarzynie Luberdzie-Durnaś za wykonanie niezwykle cennych analiz, które zdecydowanie podniosły poziom naukowy mojej pracy, jak również za wspólne dyskusje, w tym cenne uwagi i komentarze. Dziękuję pracownikom Wydziałowego Laboratorium za wykonanie analiz strukturalnych i teksturalnych próbek, a w szczególności panu Adamowi Gawłowi za zaangażowanie w realizację pracy, nieocenioną pomoc w przeprowadzaniu eksperymentów oraz konstruktywną krytykę i wartościowe komentarze do prowadzonych przeze mnie badań. Dziękuję profesorowi Lucjanowi Chmielarzowi z Wydziału Chemii UJ, za udostępnienie aparatury do badań spektrofotometrycznych UV-Vis oraz Marcelinie Radko za pomoc przy wykonaniu analiz. Profesorowi Maciejowi Maneckiemu za zaszczepienie naukowej pasji i ciekawości świata już na studiach magisterskich, za wskazywanie tego, co w nauce jest naprawdę ważne oraz za zarażanie pozytywnym myśleniem i przypominanie o tym że „sky is the limit”. W końcu dziękuję mojemu mężowi Potrkowi za ogromne wsparcie psychiczne oraz bezgraniczną cierpliwość. Rodzicom, u których zawsze mogłam szukać dobrej rady. Wszystkim koleżankom i kolegom zarówno z Katedry jak i spoza niej: Asi, Bartkowi, Chrisowi, Gabrysi, Grześkowi, Karolinie R., Karolinie K., Kasi, Paulinie, Piotrkowi, Marysi, Michałowi, a także Oldze, Konradowi i Maćkowi – za sprawienie że czas doktoratu był niezapomnianą przygodą, za wszystkie spędzone wspólnie chwile na dyskusjach naukowych, życiowych, poważnych i niepoważnych. Praca była finansowana z Narodowego Centrum Nauki w ramach projektu OPUS o numerze 2014/13/B/ST10/01326.. Anna Koteja.

(3) LIST OF CONTENTS ABSTRACT.................................................................................................................. 1 STRESZCZENIE .......................................................................................................... 2 1. INTRODUCTION ................................................................................................. 3 1.1. Layered minerals ............................................................................................... 3 1.1.1.. Clay minerals: smectites and kaolinite .................................................... 3. 1.1.2.. Synthetic α–zirconium phosphate ........................................................... 4. 1.2. Modification of layered minerals ....................................................................... 5 1.2.1.. Organic modification of smectites .......................................................... 6. 1.2.2.. Organic modification of kaolinites.......................................................... 7. 1.2.3.. Organic modification of α–zirconium phosphate .................................... 9. 1.3. Photoactive chemical compounds and photoactive materials ............................ 10 1.3.1.. Azobenzenes – basic information and the idea of photoactive materials 10. 1.3.2.. Photoactive layered materials ............................................................... 11. 1.4. Research goal .................................................................................................. 13 2. EXPERIMENTAL PROCEDURES AND METHODS ..................................... 14 2.1. Starting minerals .............................................................................................. 14 2.1.1.. Natural clay minerals: montmorillonite, beidellite, kaolinite ................. 14. 2.1.2.. Synthesis of the α–zirconium phosphate ............................................... 14. 2.2. Modification procedures .................................................................................. 15 2.2.1.. Smectite group minerals modification .................................................. 15. 2.2.1.1. Intercalation with quaternary alkylammonium salts ..................... 15 2.2.1.2. Intercalation with azobenzene and p-aminoazobenzene................ 16 2.2.2.. Kaolinite modification.......................................................................... 16. 2.2.2.1. Preparation of the metoxy–kaolinite precursor ............................. 17 2.2.2.2. Intercalation with quaternary alkylammonium salts ..................... 17 2.2.2.3. Intercalation with azobenzene ...................................................... 17 2.2.3.. The modification of α–zirconium phosphate ......................................... 18. 2.2.3.1. Direct intercalation with p–aminoazobenzene .............................. 18 2.2.3.2. Co–intercalation of quaternary alkylammonium salts and azobenzene ................................................................................................ 18 2.3. Characterization methods ................................................................................. 21 2.3.1.. Analytical methods for the materials characterization ........................... 21. 2.3.2.. Structure determination of the ZpA sample........................................... 22.

(4) 2.4. Photoinduced reactions .................................................................................... 23 2.5. Molecular dynamics simulations ...................................................................... 23 2.5.1.. Molecules arrangement in smectite’s interlayer and investigation of. structures containing trans– and cis–azobenzene................................................ 23 2.5.2.. Mechanism of intercalation and photoinduced behavior of p–. aminoazobenzene intercalatated ZrP.................................................................. 24 3. RESULTS: Characterization of the materials.................................................... 26 3.1. Smectite-based materials.................................................................................. 26 3.1.1.. Starting minerals characterization......................................................... 26. 3.1.2.. Characterization of the alkylammonium intercalated smectites ............. 31. 3.1.3.. Characterization of the azobenzene and p–aminoazobenzene intercalated. smectites ........................................................................................................... 36 3.2. Kaolinite-based materials ................................................................................. 40 3.2.1.. Starting kaolinite mineral and the DMSO and methoxy precursors ....... 40. 3.2.2.. Characterization of the benzylalkylammonium intercalated kaolinite.... 42. 3.2.3.. Characterization of the azobenzene intercalated kaolinite ..................... 45. 3.3. Materials based on the α-zirconium phosphate ................................................. 47 3.3.1.. Characterization of the synthesized α–zirconium phosphate ................. 47. 3.3.2.. Characterization of the BCn and Az intercalated ZrP ............................ 49. 3.3.3.. Characterization of the p–aminoazobenzene intercalated ZrP ............... 54. 4. RESULTS: Photoactive behavior of the intercalation compounds.................... 57 4.1. Monitoring the azobenzene and p–aminoazobenzene isomerization phenomena ............................................................................................................... 59 4.1.1.. UV–Vis analyses of azobenzene intercalates ........................................ 59. 4.1.2.. FTIR analyses of azobenzene intercalates ............................................. 61. 4.1.3.. The study of p–aminoazobenzene intercalates ...................................... 68. 4.2. Structural changes of the host mineral upon azobenzene isomerization ............ 71 4.2.1.. The basal spacing shifts in smectite–based materials ............................ 71. 4.2.1.1. Molecular dynamics simulations .................................................. 73 4.2.2.. Structural changes of kaolinite and ZrP materials ................................. 77. 5. CONCLUDING REMARKS............................................................................... 80 References................................................................................................................... 82 List of figures .............................................................................................................. 94 List of tables................................................................................................................ 97.

(5) ABSTRACT The research goal was to synthesize and characterize photoactive materials based on layered minerals and then explain the processes and interactions occurring within their structure. Azobenzenes (Az) are molecules which exhibit trans–cis isomerization under UV and visible (Vis) light. Photoactive materials can be prepared through intercalation of Az into the layered crystalline phase, where the UV–induced isomerization influences the properties of the organo–mineral complex. The Az isomerization was previously observed for selected clay minerals, i.a. mica and montmorillonite, and in some cases it led to the basal spacing shifts. This work enhances the number of host–guest combinations: two smectites (montmorillonite and beidellite), kaolinite and a synthetic α–zirconium phosphate were modified with azobenzenes. The performed modifications allowed the intercalation of Az into the interlayer space of the minerals. Azobenzene isomerized freely and reversibly under the UV/Vis irradiation. Efficient isomerization was pronounced for samples with low packing density of the molecules. The basal spacing shifts were particularly visible for beidellite, which was due to the presence of low layer charge. The careful selection of the host material, guest compound and modification procedure leads to the synthesis of a photoactive material with desired properties.. 1.

(6) STRESZCZENIE Celem pracy była synteza materiałów fotoaktywnych na bazie minerałów warstwowych, ich charakterystyka oraz wyjaśnienie procesów i interakcji zachodzących w ich strukturach. Pochodne azobenzenu (Az) to związki, które wykazują izomeryzację trans–cis pod wpływem światła UV i widzialnego (Vis). Materiały fotoaktywne mogą być otrzymane przez interkalację Az do struktury faz warstwowych, w których izomeryzacja Az wpływa na właściwości kompleksu organo–mineralnego. Izomeryzacja Az była wcześniej obserwowana w minerałach ilastych, m.in. w mikach i montmorillonicie, w niektórych przypadkach prowadziła do zmian odległości międzypakietowej. W prezentowanej pracy rozszerzono liczbę kombinacji organiczno–mineralnych: dwa smektyty (montmorillonit i beidellit),. kaolinit. oraz syntetyczny α–fosforan. cyrkonu,. były. modyfikowane. azobenzenem. Przeprowadzone modyfikacje umożliwiły interkalację Az do przestrzeni międzywarstwowej minerałów, gdzie Az swobodnie izomeryzował pod wpływem UV i Vis, a reakcja była odwracalna. Izomeryzacja była bardziej efektywna w próbkach o niskim stopniu upakowania cząsteczek. Zmiany odległości międzypakietowej były szczególnie widoczne dla beidellitu, dzięki obecności niskiego ładunku pakietu. Odpowiedni dobór wyjściowego minerału, wprowadzanej cząsteczki oraz procedury modyfikacji prowadzi do otrzymania materiału fotoaktywnego o żądanych właściwościach.. 2.

(7) 1. INTRODUCTION 1.1. Layered minerals 1.1.1. Clay minerals: smectites and kaolinite Clay minerals were defined by the joint nomenclature committee (JNC – AIPEA and CMS) as: ‘phyllosilicate minerals and minerals which impart plasticity to clay and which harden upon drying or frying’ (Guggenheim & Martin 1995). The uniqueness of clay minerals is that they fall in the interest of many disciplines, both fundamental and applied. The term clay minerals includes both natural and synthetic phases and does not limit the crystallite size (e.g. macroscopic mica, vermiculite, chlorite also belong to clay minerals). Additionally, clay minerals exhibit unique properties such as: layered structure, where one dimension is in nanometer range, anisotropic layers or particles, different types of surfaces (external, edge, and interlayer surfaces) which are susceptible to modification (by adsorption, ion–exchange, grafting etc.) (Bergaya & Lagaly 2013). The basic structural unit of clay minerals is a layer built of a tetrahedral sheet (T) and octahedral sheet (O), which are joint together to form a layer of a 1:1 type (TO) or 2:1 type (TOT). Tetrahedrons are composed of a central cation (usually Si4+, Al3+ or Fe3+) coordinated by four oxygen atoms. Three of them are joint with the neighboring tetrahedrons and this way they form a two– dimensional hexagonal mesh–patterned sheet. The fourth oxygen (free corner of the tetrahedron) is joint with an octahedron, through a Si–O–Al bond. Central cations in the octahedrons (usually Al3+, Fe3+, Mg2+ or Fe2+) are coordinated by six oxygen atoms. The octahedrons share edges and form a hexagonal or pseudo–hexagonal sheet. The unit cell consists of six octahedrons and four (1:1 layered structures) or eight (2:1 layered structures) tetrahedrons. The octahedral sites may be all occupied, and then the structure is called trioctahedral, or only four out of six sites are occupied, and then the structure is called dioctahedral. Due to the heterovalent isomorphous substitutions within the tetrahedral or octahedral sheet the layers may be negatively charged. The charge density, distribution as well as the type of charge–balancing ion determine many properties of the clay minerals and influence the possibility of surface modification. Smectites belong to the class of 2:1 clay minerals (Fig.1.1), they possess a negative layer charge in the range of 0.2 to 0.6 per half unit cell (phuc) and may be both dioctahedral or trioctahedral. The layer charge is balanced by the hydrated, exchangeable cations, which. 3.

(8) occupy the interlayer. Montmorillonite and beidellite, which are the subject of the dissertation, belong to the dioctahedral smectites with the general formula: (M+x+y·nH2O)(R3+2-yR2+y)(Si4-xAlx)O10(OH)2 where: x, y is the layer charge from tetrahedral and octahedral sites, respectively; R2+, R3+ – generic divalent and trivalent octahedral cation; M+ – generic interlayer cation. In case of the montmorillonite mineral the majority of isomorphous substitutions take place within the internal octahedral sheet and thus the layer charge is condensed in this part of the structure. Whereas for the beidellite, the substitutions take place mostly in the outer tetrahedral sheets. Properties of the smectite minerals which make them special among the rest of clay minerals are their swelling abilities and the ease for interlayer cation exchange. Smectites are able to absorb into their interlayer spaces a large amount of water molecules as well as many organic and inorganic molecules, which may lead to the delamination and/or exfoliation of the layers. The interlayer cations can be easily exchanged with other cations both of inorganic and organic character. The amount of cations that can be exchanged is limited by the cation exchange capacity (CEC) of the mineral, which is related to the layer charge. These two properties are crucial for the smectites’ usefulness in the applied sciences and industry. Kaolinite is the main member of the kaolin group minerals, which are built of a 1:1 dioctahedral layer type (Fig.1.1). The basic structural element is an asymmetric layer with a chemical composition of Al2Si2O5(OH)4. The OH groups of the octahedral sheet are exposed and point towards the interlayer space of kaolinite. The stacked layers are joint via hydrogen bonds which control the interlayer distance (d001 = 7.2 Å). Raw kaolinites exhibit relatively low sorption capacity due to the inaccessible interlayer space, which prevents the migration of ions and molecules. Moreover, the low charge of layers limits the surface ion–exchange processes. This results in a very low cation exchange capacity (up to 4 meq/100 g), as compared to smectites (up to 120 meq/100 g) (Borden & Giese 2001). 1.1.2. Synthetic α–zirconium phosphate An α–zirconium phosphate (ZrP) is a synthetic phase. Its first crystalline form was discovered and obtained by Clearfield and Stynes (Clearfield & Stynes 1964), who performed a reflux synthesis from an amorphous ZrP and phosphoric acid. The ZrP is a layered two–dimensional phase (Fig.1.1), with layers built of zirconium atoms, octahedrally coordinated with oxygens and each oxygen is linked with a phosphorus atom. 4.

(9) Phosphorus is tetrahedrally coordinated, where three oxygens are bonded with three different zirconium atoms, while the fourth one forms an OH group and is pointed towards the interlayer space, perpendicularly to the layer surface. One water molecule can be attributed to one Zr atom and is located in a gap on the layer surface, between HPO3 tetrahedras. The chemical formula of the ZrP compound is the following Zr(HPO4)2 · H2O (Troup & Clearfield 1977).. Fig.1.1. Schematic representation of the layered structures of smectite group minerals, kaolinite and α–zirconium phosphate.. 1.2. Modification of layered minerals Layered structures, both of natural and synthetic origin, often serve as supports for the production of functional materials applied in the industry and environmental protection. Several physical and chemical methods for clay minerals modification were developed, and they include mainly: acid activation, thermal treatment, interaction with organic molecules or pillaring reactions. Acid activation is usually done through the treatment of the clay with a inorganic acid (e.g. HCl, H2SO4), in order to partly dissolve the mineral, which leads to the enhanced specific surface area, porosity and acidity (Komadel 2003; Komadel & Madejová 2013). Thermal treatment includes a wide spectrum of processes depending on the treated mineral and the temperature, this includes: freezing of the mineral, dehydration, dehydroxylation, transformation into high–temperature phases (Heller-Kallai 2013). Clay pillaring is a process which involves incorporation of large oligocations between the mineral layers and subsequent thermal treatment, which leads to formation of permanent ‘pillars’ between the layers, that provide a porous structure (Vicente et al. 2013; Georgescu et al. 2015). In this work the layered minerals were functionalized through the organic modification. The organic modification of clays includes surface adsorption of organic 5.

(10) molecules as well as incorporation of the molecules between the layers, which both may occur through an intercalation, grafting, and ion–exchange reactions, among others (de Paiva et al. 2008; Lagaly et al. 2013). In general, the intercalation reactions include all the processes where the organic molecule is transferred into the interlayer space and is held there through weak interactions and no chemical bonding is involved. These might be: the ion–exchange processes, electrostatic attraction, hydrophobic interactions etc. In turn, the grafting term concerns only the reactions, where the introduced organic molecule is covalently bonded with the surface of the layered mineral. In contrast to the intercalation compounds, the grafted organo–mineral complexes are resistant to extensive washing with water (Letaief & Detellier 2007). In some organo–mineral hybrids the mineral component serves only as a support for a functional organic molecule. In this case, the immobilization on a solid support enhances the functionality of the organic compound or even activates its valuable properties. In other materials, the organic modification is done in order to enhance the properties of the mineral component. In all cases, the functionalities of the final hybrid nanomaterials depend on both the type of inorganic host and the properties of the introduced guest molecules. The experimental approach for organic modifications differs significantly, depending on the mineral type, reactivity of introduced molecules as well as the modification purpose. 1.2.1. Organic modification of smectites The smectite group minerals, with their expandable interlayer space are perfect candidates for organic modifications (de Paiva et al. 2008). Smectites easily uptake a broad variety of organic molecules through many types of mechanisms, thus the diversity of possible smectite–organics reactions and possible applications of these materials is huge. Neutral molecules may undergo complexation with the interlayer cations or can interact with clays via H–bonds, ion–dipole interactions, coordination bonds, acid–base reactions, charge transfer or van der Waals forces. Polar compounds can exchange the water molecules in the interlayer (Lagaly et al. 2013). Larger molecules, which cannot be directly intercalated into the interlayer space can be inserted through a multi–step procedures. Intercalation of polymers into the interlayer space of smectites and formation of clay–polymer nanocomposites (CPN) was also widely studied (Liu & Wu 2001; Zhang et al. 2006). Finally, due to the negative layer charge of smectites, in most studies the modification mechanism is based on the ion–exchange process. A countless clay–organic functional materials, widely used in the industry, are prepared by intercalation of cationic surfactants, such as quaternary 6.

(11) ammonium salts. These surfactants are easily introduced into the interlayer space of a smectite and they change significantly the surface chemistry of the mineral (Lagaly 1981; He et al. 2006b). First of all, this increases the interlayer distance and thus opens the possibility to perform more challenging modifications. Secondly, the presence of long alkyl chains of surfactants creates a hydrophobic environment in the interlayer, which is usually more compatible for organic molecules as well as creates advantageous rheological properties. Therefore, the surfactant intercalated smectites often are directly applied in various processes or serve as precursors for further modifications with other functional organic compounds. Organically modified smectites were tested for wide variety of applications. Numerous papers were devoted to adsorption properties of organo–smectites modified with alkylammonium surfactants, most often the hexadecyltrimethylammonium cation (abbreviated as HDTMA or CTAB). The hydrophobicity of these materials made them suitable for adsorption of organic pollutants (Yilmaz & Yapar 2004; Chen et al. 2014). On the other hand, when the amount of intercalated cationic surfactant exceeds the cation exchange capacity of smectite, then the surface of the obtained material becomes positively charged. In that case, it is possible to adsorb anionic pollutants (Chitrakar et al. 2011; Luo et al. 2017). Simultaneous adsorption of inorganic anions and organic pollutants (dual adsorption properties) was also explored (Gładysz-Płaska et al. 2012). The HDTMA modified bentonite was used for retention of gaseous products (CO, CH4, SO2) (Volzone et al. 2006). A tetramethylammonium modified montmorillonite after acid activation exhibited catalytic properties (Moronta et al. 2002). Another class of clay based catalytic materials are smectites intercalated with heavy metal complexes (Pinnavaia 1983). Organo– smectites are studied and commercially used as fillers in polymers and rubbers (Hasegawa et al. 2000; Liu & Wu 2001; Arroyo et al. 2003; Zhang et al. 2006; Mansa et al. 2016). According to Markarian (2005), nearly 70% of materials used for polymer nanocomposites preparation are organoclays. Other intensively developed applications of organo–smectites include: advanced pigments and color sensors (Theng 1971; Ogawa et al. 2017), ion– selective materials and potentiometric sensors (Darder et al. 2005; Jović-Jovičić et al. 2016; Ruiz-Hitzky et al. 2018), or drug delivery carriers (Rebitski et al. 2018). 1.2.2. Organic modification of kaolinites Kaolinite minerals possess negligible layer charge, which limits the possibility of ion–exchange reactions. Moreover, the stacked layers are joint through relatively strong 7.

(12) hydrogen bonds, and the interlayer space is inaccessible for most of the organic molecules. Therefore, the organic modification of kaolinites is much more challenging than modification of smectites. There is a limited number of organic compounds that, in appropriate conditions, may access the interlayer space of kaolinite. These compounds were divided into three groups (Lagaly et al. 2013): (1) compounds that form hydrogen bonds, e.g. urea or formamide (Olejnik et al. 1971; Makó et al. 2009), (2) highly polar compounds like dimethyl sulfoxide – DMSO (Olejnik et al. 1968; Scholtzová & Smrčok 2009), and (3) potassium, rubidium, cesium, ammonium salts of short chain acids, like potassium acetate (Wada 1961). As these compounds are only intercalated in the interlayer space of kaolinite, they can be easily removed with water, but also they might be exchanged with other molecules. Therefore, they serve as excellent precursors for further modifications with more complex molecules, also through grafting reactions. Covalent bonding of molecules is possible due to the 1:1 layer structure, which exhibits an asymmetrical interlayer environment: on one side, surface oxygens of the tetrahedral sheet are present, while the other side contains exposed inner surface hydroxyls, which are reactive and enable the grafting reactions. This in case of smectites is possible exclusively at the layer edges. Some of the compounds successfully grafted into kaolinite were e.g. alcohols (Matusik et al. 2012; Cheng et al. 2015), diols (Gardolinski & Lagaly 2005), aminoalcohols (Letaief & Detellier 2007; Koteja & Matusik 2015), and organic acids (da Silva et al. 2016). Moreover, earlier performed research indicated that the use of hydrophobic methoxy derivative of kaolinite enables preparation of intercalates with alkylammonium surfactants in analogy to smectites (Kuroda et al. 2011; Matusik & Kłapyta 2013; Matusik et al. 2013; Cheng et al. 2016). Subsequently, the obtained hydrophobic interlayer environment opens the possibility to intercalate other non–polar organic species into kaolinite. Due to the thermal stability and resistance to hydrolysis, the grafted kaolinites were studied for wide variety of applications. Kaolin group minerals grafted with aminoalcohols exhibited enhanced sorption capacity towards heavy metal cations (Matusik & Wścisło 2014; Koteja & Matusik 2015) as well as toxic oxyanions (Matusik & Bajda 2013; Matusik 2014). A picolinate grafted kaolinite was employed as a precursor to prepare heterogeneous catalysts (de Faria et al. 2012) or luminescent materials (de Faria et al. 2011). Electrochemical sensors were prepared by kaolinite grafting with ionic liquids (Dedzo et al. 2012; Dedzo & Detellier 2013; Nguelo et al. 2018). Raw platy and tubular kaolinite as well as pre–intercalated kaolinite–DMSO compounds were used to produced polymer nanocomposites (Matusik et al. 2011a; Fafard & Detellier 2015). A comprehensive review 8.

(13) on kaolinite modifications and applications was recently published by Dedzo and Detellier (2016). 1.2.3. Organic modification of α–zirconium phosphate The interlayer space of the layered α–zirconium phosphate is relatively easily accessible for organic molecules. Therefore, the ZrP structure may be modified analogously to some clay minerals. Moreover, the P–OH groups exposed in the interlayer are prone to react with the introduced molecules and form strongly bonded complexes. The layered ZrP is a good host for cationic and polar compounds. A great advantage of the synthetic ZrP material over natural minerals is the controllable chemical composition and structure. Therefore, immobilization of organic units in precisely selected sites of the ZrP surface is possible, and this leads to an organic–inorganic complex with desired properties and functionalities (Mosby et al. 2014; Tang et al. 2015). The intercalation mechanism of strong Lewis bases, containing an amine group is based on the interaction between the amine group and P–OH group at the ZrP surface. This mechanism was revealed for toluidines intercalated into an α–ZrP, where a hydrogen bond was formed between the amine group of toluidine and P–OH group, according to the FTIR results (Beneš et al. 2006; Beneš et al. 2007). Another method for preparation of organic derivatives of ZrP was a direct introduction of the organic molecules simultaneously with the synthesis of ZrP. In that case the phosphoric acid, used for the ZrP synthesis, was substituted with phosphonic acid or phosphoric acid esters. The obtained materials possessed the inorganic layered structure, but in the place of OH groups the alkyl or aromatic groups were present (Alberti et al. 1978; Poojary et al. 1994). In these derivatives the organic molecules were covalently bonded to the inorganic layers. An analogous DMSO derivative of ZrP was also prepared in a twostep procedure: the pristine crystalline zirconium phosphonate was reacted with DMSO (Alberti et al. 1997). The DMSO derivatives of ZrP were used as versatile exchangers for anionic ligands (Alberti et al. 1997; Alberti et al. 2002). Recent papers reported examples of the organo–ZrP materials employed in medical applications. A simultaneous synthesis of ZrP and its modification with HDTMA was performed in order to produce a drug delivery carrier (Kalita et al. 2016). In other works a drug compound was directly intercalated into the ZrP material and then also tested for the drug delivery and release (Díaz et al. 2013; Saxena et al. 2013).. 9.

(14) 1.3. Photoactive chemical compounds and photoactive materials 1.3.1. Azobenzenes – basic information and the idea of photoactive materials Among the chemical compounds that react to UV or Vis radiation, azobenzenes are widely studied and applied in technology. Azobenzene (Az) as well as p– aminoazobenzene (pAz) are triggered with the UV light (≈365 nm) (Fig. 1.2). Normally, the molecules occur in the form of a stable trans–isomer. The UV irradiation excites the molecules and thus induces their isomerization to the meta–stable cis–isomer. The reverse reaction, which is the transfer of Az molecules back to the ground state, occurs spontaneously. Comparing to the trans to cis reaction, the reverse re–isomerization is relatively slow, yet heating or exposure to visible light (Vis) accelerates the reaction. The cis and trans isomers differ in electronic properties and molecular geometry. The distance between 4 and 4’ carbon atoms in the fully stretched trans isomer is ~9 Å and the molecule has no dipole moment, while for the cis isomer the distance is ~6.5 Å and it possesses a dipole moment equal to 3 D (Russew & Hecht 2010).. Fig. 1.2. The structure and isomerization scheme of azobenzene and p–aminoazobenzene.. The immobilization of molecular switches, such as azobenzenes, on the surface of a solid support leads to the formation of novel hybrid materials, which are responsive to radiation of specific energy (Klajn 2010; Mahimwalla et al. 2012; Yan & Wei 2015). Most importantly, the crystalline form of azobenzene reacts to the UV light inefficiently, due to the steric hindrance of densely packed molecules within the well ordered Az crystals. The Az isomerization was observed when the molecules where dispersed in solutions or in solid/polymer matrix (Webb et al. 1986; Lednev et al. 1998). Therefore, the phenomena of UV induced Az switching can be effectively applied only if the molecule is combined with a solid support or dispersed in a solution. In some of the hybrid systems the motion 10.

(15) of organic molecules in the atomic scale may affect the macroscopic properties of the whole material. Combination of the azobenzene photo–switching properties with polymer matrix resulted in synthesis of smart materials controlled remotely with light. An azobenzene bearing polymer film was synthesized by Yu et al. (2003), and the obtained material bended in a precisely chosen direction upon linearly polarized light. Basing on this founding, Yamada et al. (2008) constructed a simple device of few millimeters in size, where a polymer–azobenzene belt propelled the device upon the UV light, and this way a light– driven plastic motor was produced. In other works, the azobenzene derivatives were attached to the surface of nanoparticles. In this case the photo–switching reaction modulated the nanoparticles properties, such as optical, electrical, luminescent, or magnetic properties (Klajn et al. 2010). Also, the aggregation and disaggregation of the nanoparticles could be forced with radiation and this way the sedimentation behavior of particles was controlled (Ueda et al. 1994). Azobenzene hybrid materials were produced for the environmental applications as well. Metal–organic frameworks (MOF) with introduced azobenzene–based molecules served as a CO2 adsorbent. The adsorption process was controlled with UV or heat treatment (Park et al. 2012; Knebel et al. 2017; Müller et al. 2017). Similar process was studied in the azobenzene modified montmorillonite, where the adsorption and desorption of phenol was controlled with UV and Vis light (Okada et al. 2008). 1.3.2. Photoactive layered materials Selected layered minerals and synthetic phases were reported as hosts for photoactive species (Ogawa & Kuroda 1995). Among others, these include clay minerals, synthetic micas, transition metal oxides, zirconium phosphates, perovskites, and layered double hydroxides (Sasai & Shinomura, 2013; Yan & Wei, 2015). The isomerization of azobenzene–type species, intercalated in the layered inorganic structures, may lead to changes of the interlayer distance of the host. Subsequently, the interplanar distances in the structure of intercalation compounds can be remotely controlled at the molecular level. The clay minerals were previously studied as hosts for photoactive materials, yet the studies were limited to only few host–guest combinations. One of the examined modification approaches involved a direct intercalation of cationic azobenzene derivatives into the interlayer space of micas or montmorillonites (Ogawa 1996; Ogawa et al. 2000). This was done via a simple ion–exchange reaction. The arrangement and photoresponse of intercalated molecules was analyzed mainly with use of the UV–Vis spectroscopy. 11.

(16) The UV and Vis radiation forced the reversible trans–cis isomerization of cationic Az molecules. The authors pointed out that the efficiency of isomerization depended on the rigidity of the interlayer environment which in turn was affected by the type and density of intercalated molecules. Despite the fact that the molecules movement occurred within the interlayer space, no accompanying shifts of basal spacing (d001) were observed. This was explained as the effect of the organic chain flexibility, so the molecules could rearrange and minimize the change of basal basing (Ogawa & Ishikawa 1998). Another approach was a two–step procedure applied for modification of micas. This involved pre– intercalation of a cationic surfactant and after that co–intercalation of azobenzene (Ogawa et al. 1996; 1999). It was revealed that using the gas–form of azobenzene led to a particularly high intercalation efficiency. The highly mobile Az molecules in the gaseous from readily penetrated the hydrophobic interlayer spaces of the modified micas. These materials were examined in terms of the interlayer distance changes induced with UV light. In several of these mica–azobenzene complexes, the basal spacing visibly shifted along with the azobenzene isomerization (Fujita et al. 1998). The authors performed numerous experiments involving the synthesis of organo–micas with different types of surfactants and the different loading of surfactant and azobenzene. They revealed that the presence and magnitude of the UV induced d001 shifts depended heavily on the type and density of pre– intercalated surfactant as well as on the amount of intercalated azobenzene molecules (Fujita et al. 2001; 2003). More recently, a detailed analysis of the photoresponse mechanisms in different experimental conditions, including humidity, was performed for the azo– functionalized layered silicate – Na-magadiite (Okada et al. 2017). The mineral was directly ion–exchanged with two cationic azo dyes and then treated with UV light in low and high humidity conditions. For one of the samples, an increase of basal spacing was observed after UV irradiation, but only in high humidity conditions. Thus the mechanism of basal spacing shift was ascribed to the hydration of the cis–Az isomer. In the other sample, the basal spacing shift was observed regardless of the humidity conditions. In this case the shifts were induced directly by the trans–cis isomerization of the azo dye. In the work of Gentili et al. (2004) a layered zirconium phosphonate was grafted with an alkylamino group and subsequently intercalated with azobenzene. Thanks to the separation of azobenzene molecules with alkyl chains the UV induced isomerization occurred. On the other hand, the trans–cis conversion of azobenzene in this material lead to destruction of the original structure, probably because of the azobenzene evaporation. Thus the photo–activity of the intercalate was not durable. 12.

(17) 1.4. Research goal The main research goal was to synthesize layered nanomaterials of desired structure and photoactive properties, controllable at the molecular level using UV and Vis light. This was done through realization of the following tasks: . Modification of selected layered minerals, which enables the incorporation of photoactive molecules – azobenzene and p–aminoazobenzene – into the interlayer space.. . Chemical and structural characterization of the obtained organo–mineral hybrids, in order to resolve their interlayer nanostructure and to understand the interactions between the organic molecules as well as the interactions between the organic and mineral component.. . Investigation of the photoresponsive behavior of the azo–functionalized materials upon UV and Vis treatment, regarding the occurrence of the trans–cis isomerization and the related structural changes.. . Identification of the structural and chemical features of the obtained materials, which have the greatest impact on their photoresponsive behavior.. 13.

(18) 2. EXPERIMENTAL PROCEDURES AND METHODS 2.1. Starting minerals Both natural clay minerals and a synthetic layered phase were used as host structures for the photoactive materials synthesis. Among the natural minerals, two smectites were selected – montmorillonite and beidellite, and one sample from the kaolin group minerals – kaolinite. The synthetic layered phase was an α–zirconium phosphate, prepared by the author. 2.1.1. Natural clay minerals: montmorillonite, beidellite, kaolinite The Na–montmorillonite originates from the Wyoming deposit (USA) and was purchased from the Clay Minerals Society repository under the name SWy-2 (in this work abbreviated as SWy). Detailed studies on the origin, chemistry and structure of the SWy-2 source clay were done in previous years and the results can be found in the literature (Chipera & Bish 2001; Guggenheim & van Groos 2001; Kogel & Lewis 2001; Madejová & Komadel 2001; Mermut & Cano 2001; Mermut & Lagaly 2001; Moll 2001). Beidellite is from the Idaho deposit (USA), also purchased from the Clay Minerals Society repository, under the name SBId-1 (in this work abbreviated as BId). Beidellite and associated clays from the Idaho deposit were investigated and described by Post el al. (1997). Both SWy and BId samples were used ‘as received’, only grinding was performed before the experiments. Kaolinite was purchased from the Surmin Kaolin company and it originates from the Maria III deposit in Nowogrodziec (Poland) (abbreviated as M). According to previous works, the mineral sample contains about 80% of kaolinite of high structural order (Hinckley index = 1.31) with additional admixtures of quartz and illite. The separated fraction was below 40 µm, with an average particle size equal to 7 µm. The standard clay fraction below 2 µm was not separated as preliminary experiments showed much lower intercalation efficiency in the case of finer fractions (Matusik et al. 2009; Matusik et al. 2011b). 2.1.2. Synthesis of the α–zirconium phosphate The α–zirconium phosphate was synthesized in a reflux system according to a procedure reported in earlier studies (Sun et al. 2007). A 10 g portion of zirconyl chloride (ZrOCl2 · 8H2O) was mixed with 6 M aqueous solution of phosphoric acid at 100°C for 24 h. After that, the precipitate was washed twice with redistilled water and dried. The sample was abbreviated as ZrP. 14.

(19) 2.2. Modification procedures Reagents used in the syntheses and the modification procedures are listed in the Table 2.1. All the obtained samples are summarized at the end of the 2.2 section in the Table 2.2. The schematic representation of the performed modification routes is shown in the Fig. 2.1. Tab. 2.1. Reagents used in the syntheses and in the modification procedures. Reagent name. Chemical formula a. Zirconyl chloride. ZrOCl2 · H2O. Phosphoric acidb. H3PO4. Dodecyl trimethylammonium chloride (C12). c. CH3(CH2)11N+(CH3)3 Cl-. Tetradecyl trimethylammonium chloride (C 14)a. CH3(CH2)13N+(CH3)3 Cl-. Hexadecyl trimethylammonium chloride (C 16)a. CH3(CH2)15N+(CH3)3 Cl-. Dodecyl benzyldimethylammonium chloride (BC 12)a. CH3(CH2)11N+(CH3)2C7H5 Cl-. Tetradecyl benzyldimethylammonium chloride (BC14)d. CH3(CH2)13N+(CH3)2C7H5 Cl-. Hexadecyl benzyldimethylammonium bromide (BC16)a. CH3(CH2)15N+(CH3)2C7H5 Br-. Azobenzenea. C6H5–N=N–C6H5. p–aminoazobenzenee. C6H5–N=N–C6H4–NH2 b. Dimethyl sulfoxide (DMSO). (CH3)2SO. b. Methanol. CH3OH. b. Ethanol a. C2H5OH b. c. d. e. Sigma-Aldrich; Avantor; Fluka; Alfa Aesor; TCI (Tokyo Chemical Industry). 2.2.1. Smectite group minerals modification Direct intercalation of photoactive molecules into the smectites’ interlayer space was not possible, thus a two–step modification procedure was performed. Firstly, the selected cationic surfactants (quaternary alkylammonium salts) were introduced, which provided a hydrophobic environment in the interlayer space and enabled performing the second step – intercalation of the photoactive azobenzene or p–aminoazobenzene. 2.2.1.1. Intercalation with quaternary alkylammonium salts The. smectite. samples. were. intercalated. with. six. surfactants:. alkyltrimethylammonium Cn and alkylbenzyldimethylammonium BCn bromides or chlorides, where n stands for the number of carbon atoms in the alkyl chain and is equal to: 12, 14 and 16 (Tab. 2.1). The intercalation of alkylammonium salts was performed via the ion–exchange process. Firstly, a water solution of each alkylammonium salt was prepared, 15.

(20) where the concentration of organic molecules corresponded to 2.0 CEC of each smectite (35.56 mmol/L for SWy and 17.44 mmol/L for BId). In order to facilitate the surfactant dissolution, the prepared solutions were stirred for 2 h at 60°C. Separately, the smectite minerals were suspended in water, and stirred for 2 h at 60°C in order to obtain a homogenous dispersion. The surfactant solutions were poured into the appropriate mineral suspensions, and the mixtures were further stirred for 24 h at 60°C. The resulting solid– solution ratio was equal to 20 g/L. Afterwards the intercalated materials were washed several timed with deionized water until the negative AgNO3 test proved the lack of Cl- or Br- ions in the solution. The centrifuged samples were dried at 60°C for 24 h. The resulting intercalates were abbreviated as SWy–Cn, SWy–BCn, BId–Cn, and BId–BCn. 2.2.1.2. Intercalation with azobenzene and p-aminoazobenzene The reaction with azobenzene was performed basing on procedures proposed in previous studies (Fujita et al. 2003). The organo–smectites described in section 2.2.1.1 were placed in glass cuvettes inside a teflon vessel, together with a crystalline azobenzene (Az). The direct contact of the mineral with Az was avoided. The Az to organo–smectite weight ratio was equal to 0.2. The hermetically closed vessel was heated for 24 h at 120°C, which is above the evaporation temperature of azobenzene (60°C). In such conditions the gaseous form of Az readily penetrates the interlayer spaces of smectites. After cooling and grinding, the samples were ready for further analyses. The p–aminoazobenzene (pAz) was intercalated using a solid–solid reaction, according to Ogawa et al. [24]. The pAz was ground with smectites in an agate mortar for ~2 minutes. The pAz to organo–smectite mass ratio was equal to 0.2. 2.2.2. Kaolinite modification Organic modification of kaolin group minerals is more challenging than smectite minerals. This is because kaolinite layers do not possess charge and at the same time are strongly H–bonded to each other, which hampers the introduction of organic molecules into the interlayer space. It is known that selected small and highly polar compounds can be intercalated into kaolinite, which opens the possibility for further modifications (see paragraph 1.2.2). This approach was employed for kaolinite modification with photoactive molecules, and a multi–step procedure was performed. Firstly, the mineral was intercalated with dimethyl sulfoxide, than grafted with methanol, the obtained methoxy– 16.

(21) kaolinite precursor was intercalated with alkylammonium surfactants, and finally with the azobenzene compound. 2.2.2.1. Preparation of the metoxy–kaolinite precursor The procedure for obtaining the methoxy–kaolinite grafted compound is well known and was described in numerous papers (Tunney & Detellier 1996; Matusik et al. 2012). In the first step, the mineral was pre–intercalated using dimethyl sulphoxide (DMSO), which is a small and highly polar compound that easily penetrates the interlayer spaces of kaolinite. This was done by mixing 3 g of the mineral with 50 ml of DMSO containing 3% of re– distilled water. The addition of water reduces the attraction forces between highly polar DMSO molecules, and thus increases the intercalation degree (Olejnik et al. 1968). The formed suspension was stirred for three days at room temperature and additionally, after first day of stirring, sonicated for 1 minute. The formed intercalation compound (MDS) was centrifuged, dried at 70ºC for 48 h and ground for further use. In the second step, the MDS intercalate was mixed with 50 ml of methanol for 24 h at room temperature. After that, the suspension was centrifuged and the solid was washed with a fresh 50 ml portion of methanol. The washing was repeated until the vibrational bands attributed to the DMSO molecules were not detected by FTIR spectroscopy. The final methoxy–kaolinite grafted compound (MM) was dried at 70ºC for 24 h and powdered. 2.2.2.2. Intercalation with quaternary alkylammonium salts The MM sample was modified with alkylbenzyldimethylammonium chlorides (BCn, n = 12, 14 or 16). The organic salts were dissolved in methanol in a concentration equal to 0.5 M. The high surfactant loading was necessary to induce the intercalation, as reported in earlier studies (Matusik & Kłapyta 2013). The MM to BCn solution ratio was equal to 200 mg/5 ml. The suspensions were stirred for 72 h at room temperature, then centrifuged, dried at 70ºC for 24 h, and the obtained solids (MBCn) were powdered. The attempt to intercalate the alkyltrimethylammonium (Cn) surfactants was also conducted, however the intercalation was inefficient, thus this part was not included and discussed in the PhD thesis. 2.2.2.3. Intercalation with azobenzene Azobenzene was reacted with the MBCn samples in an analogues procedure as the smectite samples. The Az together with the MBCn were placed in a closed teflon vessel 17.

(22) and heated at 120°C. The intercalation of azobenzene was not that efficient and clear as in the case of smectites, therefore the experimental conditions were expanded as compared to the smectites modification. In this case, the Az:MBCn wt. ratio was set to 0.2, 0.5, and 1.0, and the reaction was carried out for 1, 3 and 7 days. To distinguish the samples obtained in different conditions, an additional description was added in brackets after the sample symbol, e.g. MBCnA(0.2-3d) refers to a sample where Az to MBCn ratio was equal to 0.2, and the reaction time was 3 days. 2.2.3. The modification of α–zirconium phosphate 2.2.3.1. Direct intercalation with p–aminoazobenzene The α–zirconium phosphate intercalation with p–aminoazobenzene compound was done directly, without any previous pretreatments. A 0.3 M ethanol solution of pAz was prepared, and then the powdered ZrP was added, using solid to solution ratio equal to 20 g/L. The dispersion was stirred for 24 h at room temperature and afterwards centrifuged and dried. The obtained product was abbreviated as ZpA. 2.2.3.2. Co–intercalation of quaternary alkylammonium salts and azobenzene Direct intercalation of azobenzene (Az) was not possible, thus a two steps procedure was performed, analogues to the procedures used for smectites and kaolinite. Firstly, the ZrP sample was pre–intercalated with alkyldimethylbenzylammonium surfactants: BCn (n = 12, 14, and 16). A 0.5 M methanol solutions of BCn surfactants was prepared and then the powdered ZrP sample was added maintaining the solid/solution ratio equal to 20 g/L. The suspensions were stirred for 72 h at room temperature. Additionally, the BC16 intercalation was done with the 0.05, 0.1, 0.2 M methanol solutions. Secondly, the dried and ground ZBCn intercalates were reacted with azobenzene in a closed teflon vessel at 100ºC. The reaction was conducted for 3 days and the Az/ZrP ratio was set to 0.2. The resultant materials were abbreviated as ZBCnA.. 18.

(23) Fig. 2.1. Scheme presenting the performed modification routes.. 19.

(24) Tab. 2.2. Summary of samples’ description and symbols. Symbol. Material description. SWy. Na–montmorillonite from Wyoming deposit, USA. SWy-Cn. SWy ion–exchanged with alkyltrimethylammonium chlorides or bromides (Cn). SWy-BCn. SWy ion–exchanged with alkylbenzyldimethylammonium chlorides (BCn). SWy-CnA. SWy–Cn modified with azobenzene. SWy-BCnA. SWy–BCn modified with azobenzene. SWy-CnpA. SWy–Cn modified with p-aminoazobenzene. SWy-BCnpA. SWy–BCn modified with p-aminoazobenzene. BId. Beidellite from Idaho deposit, USA. BId-Cn. BId ion–exchanged with alkyltrimethylammonium chlorides or bromides (Cn). BId-BCn. BId ion–exchanged with alkylbenzyldimethylammonium chlorides (BC n). BId-CnA. BId–Cn modified with azobenzene. BId-BCnA. BId–BCn modified with azobenzene. BId-CnpA. BId–Cn modified with p-aminoazobenzene. BId-BCnpA. BId–BCn modified with p-aminoazobenzene. M. Kaolinite from Maria III deposit, Poland. MDS. M kaolinite intercalated with DMSO. MM. Methoxy–kaolinite – M kaolinite grafted with methanol. MBCn. MM sample intercalated with alkylbenzyldimethylammonium chlorides (BCn). MBCnA. MBCn modified with azobenzene. ZrP. α–zirconium phosphate. ZpA. ZrP modified with p-aminoazobenzene. ZBCn. ZrP modified with alkylbenzyldimethylammonium chlorides (BCn). ZBCnA. ZBCn modified with azobenzene. 20.

(25) 2.3. Characterization methods 2.3.1. Analytical methods for the materials characterization The XRD analysis was performed at the Laboratory of Phase, Structure, Texture and Geochemical Analyses at the Faculty of Geology, Geophysics and Environmental Protection, AGH (WLBFSTiG), by Adam Gaweł. The patterns were recorded for powdered and non–oriented samples with use of the Rigaku MiniFlex diffractometer (CuKα radiation) in the 1–30°2θ range with 0.05°2θ step. The FTIR analysis was done at the WLBSTiG laboratory, using two techniques: the DRIFT technique (3% wt. sample/KBr, powder sample) was used for the characterization of smectite samples while the transmission technique (1% wt. sample/KBr, pressed disks) was used for kaolinite and ZrP samples. The measurements were done in the 4000–400 cm-1 spectral range at the 4 cm-1 resolution with 64 scans (Thermo Scientific Nicolet 6700 spectrometer equipped with Harrick Praying Mantis device for the DRIFT technique). The UV-Vis spectroscopy was done at the Department of Chemical Technology (Faculty of Chemistry, Jagiellonian University), with the help of prof. Lucjan Chmielarz and Marcelina Radko. The spectra were recorded for powdered samples with the use of ThermoEvolution 600 spectrophotometer in the 200– 700 nm range with 2 nm data interval. The CHN elemental analysis was done at the Department of Pharmaceutical Chemistry (Faculty of Pharmacy, Jagiellonian University), by dr Paweł Żmudzki. The analysis was performed through the combustion of samples and the measurement of purified and separated gaseous products (VarioEL III Elementar analyzer). Thermogravimetric (TG, DTG) and differential thermal analysis (DTA) was carried out at the WLBSTiG laboratory by Adam Gaweł. The temperature range was 20– 1000°C with the 10°C/min heating rate in air atmosphere (Netzsch STA 449F3 Jupiter instrument). Simultaneously the evolved gaseous products were analyzed with the mass spectrometer QMS 403 C Aeolos. The chemical composition of pure smectites and kaolinite was determined with the X–Ray fluorescence spectroscopy with use of the Rigaku ZSX Primus II apparatus equipped with the Rh lamp (WLBSTiG laboratory, by Adam Gaweł). The cation exchange capacity (CEC) was measured for smectite minerals by the methylene blue adsorption method, and the location of the layer charge was determined with the Greene–Kelly test (Greene-Kelly 1953). The specific surface area was calculated on the basis of a low– temperature N2 adsorption/desorption measurements (77 K) according to the BET equation (ASAP 2020 instrument). Prior to analysis the samples were outgassed at 423 K. 21.

(26) The particle–size distribution was determined with the Saturn DigiSizer II 5205 instrument (WLBSTiG laboratory, by Anna Tomczyk). The scanning electron microscope analysis was performed in order to observe the morphology of the synthesized ZrP sample. This was done using a JEOL JSM 7500F microscope at the Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences (Kraków) by dr Małgorzata Zimowska. 2.3.2. Structure determination of the ZpA sample The ZpA sample showed a well defined crystal structure thus it was subjected to a more detailed analysis, performed by dr Katarzyna Luberda–Durnaś, Institute of Geological Sciences, Polish Academy of Sciences. For this purpose the capillary measurements (sodium glass capillary with 0.3 mm dimension) were performed using a Bruker D8 Advanced diffractometer (Karlsruhe, Germany) working in Debye–Scherrer geometry with CoKα X–ray tube (35 kV, 40 mA). The primary X–ray beam was monochromatised and formed using the Göbel mirror, 0.2 mm fixed divergence slit, 2.5˚ Soller slits, and a beam knife. On the secondary beam, VANTEC detector with a Radial Soller and 2.5˚ Soller slits was used. The scan range was set from 2.5 to 110°2θ with the step size 0.014°2θ. In order to improve the structure determination, the synchrotron measurements were performed (ESRF synchrotron in Grenoble); the polarized radiation with the wavelength λ=0.35433 Å was used. Cell parameters and space group were obtained using TOPAS ver 5. The positions of heavy atoms as well as atoms of organic parts were found using the direct space optimisation method implemented in FOX (Favre-Nicolin & Černý 2002), working on both, synchrotron and laboratory diffraction patterns. The structure model was refined with the JANA2006 program (Petříček et al. 2014) using the Rietveld method, in this case however, based only on synchrotron data. Since in the sample 30% of substrate Zr(HPO4)2H2O was found, the two–phase optimization procedure was applied. In the first step, the La Bail fitting was performed for both phases; the background, profile factors, cell parameters and asymmetry were fitted one by one. For the main phase, the March–Dollase model (Dollase 1986) of texture in the [100] direction was assumed, while for Zr(HPO4)2H2O in [001] direction. Due to the strong anisotropic particle broadening, for Zr(HPO4)H2O intercalated with p–aminoazobenzene, the adjustment along [100] direction was used. The position of all atoms was refined and isotropic displacement parameters were applied for all of the non–hydrogen atoms. Geometric restraints on bond lengths and angles as well as constraints on atomic displacement parameters were used 22.

(27) to improve the stability of the refinement. The positions of all hydrogen atoms were added and their impact was took into account for the structure factor calculations. 2.4. Photoinduced reactions The photoinduced activity of the obtained Az and pAz intercalated materials was tested with the use of two UV sources. The first one was a stationary custom–made UV lamp produced by Optister company (Krakow, Poland), where the UV light source emitted 365 nm radiation wavelength. The source was equipped with an IR cut–off filter to prevent the sample from heating. The second source was a UV LED flashlight (MR® 96 B Minilight), emitting 365 nm radiation wavelength, which was used for quick in–situ measurements. In both of the sources the radiation power could be set in the range of 0.5–50 mW/cm2 and in most tests the 5–15 mW/cm2 range was used. Before the procedure and after irradiation, the UV–Vis spectra, FTIR spectra and the XRD patterns were recorded to monitor the isomerization of photoactive molecules and the d001 value shifts. For the UV–Vis analysis, powdered samples were irradiated directly in the UV–Vis dedicated holder. In the case of FTIR analysis the transmission technique was used, and the KBr pressed disks were directly used for the UV exposure. For the XRD analysis the powdered samples were placed on glass slide holders for the UV exposure. 2.5. Molecular dynamics simulations The molecular dynamics (MD) simulations were performed in order to resolve some uncertain issues in terms of phenomena taking place during the sample irradiation. The method was employed for smectite intercalates, where evident structural changes were visible upon UV irradiation, in order to understand the relationship between the arrangement of the molecules and the observed changes. Moreover, the possible influence of water adsorption/desorption on the observed changes was considered. The MD simulation was also done for the ZpA sample to explain the reason for lack of pAz isomerization. All the MD simulations were performed in cooperation with dr Marek Szczerba at the Institute of Geological Sciences, Polish Academy of Sciences (Kraków). 2.5.1. Molecules arrangement in smectite’s interlayer and investigation of structures containing trans– and cis–azobenzene The simulations were conducted in order to provide the most probable model of molecules arrangement in the interlayer space of smectites and also to give further insights 23.

(28) into differences between structures containing cis– and trans–azobenzene. All the MD simulations were performed for simplified montmorillonite (model of SWy-1 smectite) and beidellite with charge equal to 0.33 phuc. The charge of this model was higher than for experimentally used Bld beidellite, which was done to directly compare results of the simulations. The smectite models were intercalated with C16 and BC16 in the amount equal to 1.0 CEC. The ratio between co–intercalated Az and alkylammonium ion molecules was set to 1.5. All the simulations were performed using CLAYFF (Cygan et al. 2004) and GAFF (Wang et al. 2004) force fields for smectite and organic molecules, respectively. In the first set of simulations constant temperature and pressure MD simulations (NPT) were performed. A wide range of different initial starting structures was tested in three different temperature regimes for ~2% wt. H2O in order to find the most probable structure with the lowest potential energy. In the first attempt the temperature was kept constant at 298 K and the simulation was run for 2 ns. In the second approach the simulation was run for 0.5 ns at 298 K, then for 0.5 ns at 398 K and then temperature was dropping in steps of 10 K per 0.05 ns to 298 K, where it was finally kept for 0.8 ns. In the third simulation the procedure was the same as the second one but the highest temperature was 498 K. The simulation supercell was 8 x 4 x 1 unit cells in the a, b, and c crystallographic directions, respectively (~41.6 Å x 36.1 Å x 33.5–34.0 Å). In the second set of simulations, energy of water molecules adsorption on smectite with C16 and BC16 and Az was calculated for different hydration levels: 0, 0.625, 1.25, 2.5, 5.0 and 7.5 water molecules phuc (per half unit cell). For each hydration level the energy of water molecules adsorption on smectite with alkylammonium ions and Az was calculated from the equation: ΔU(N)=(U(N)–U(0))/N where: U(N) is the average potential energy of an equilibrium system with N water molecules in the interlayer and U(0) is the average potential energy of equilibrated system not containing water molecules, used as a reference state. 2.5.2. Mechanism of intercalation and photoinduced behavior of p–aminoazobenzene intercalatated ZrP In order to explain mechanism of intercalation and photo–induced behavior of p-aminoazobenzene intercalated in ZpA, two series of molecular simulations were performed. In the first one, theoretical IR spectra were calculated for neutral and protonated p-aminozaobenzene. Molecular geometry optimization was performed with Density 24.

(29) Functional Theory at the B3LYP/DGDZVP level of theory (Miertuš et al. 1981; Lee et al. 1988; Becke 1993), using Gaussian Inc. software (Frisch et al. 2004). The vibrational frequencies, and intensities were then computed. Based on the structure refinement of ZpA, a corresponding molecular model was built. The simulation supercell was 2 x 4 x 1 unit cells in the a, b, and c crystallographic directions, respectively (~18.1 Å x 21.1 Å x 31.8 Å). It consisted of one Zr(PO4-) layer and protonated p-aminoazobenzene molecules connected through very strong hydrogen bonds to the deprotonated surface oxygens: ~20 kcal/mol (harmonic potential). Coverage of the surface with pAz molecules was 100%. The interatomic interactions were described with the Universal Force Field (UFF) (Rappé et al. 1992), which has been used for molecular studies of metal–organic frameworks (Addicoat et al. 2014). Close agreement between optimized supercell sizes and results of structure refinement confirmed that the parameters were chosen properly. The aim of these simulations was to compare calculated potential energy of C–N=N–C rotation for single p–aminoazobenzene molecule in vacuum, single molecule in water box and a molecule constituting a layer of molecules on the Zr(PO4-) surface. All these simulations were performed using the LAMMPS computer program (Plimpton 1995; lamps.sandia.gov, 2018).. 25.

(30) 3. RESULTS: Characterization of the materials 3.1. Smectite-based materials 3.1.1. Starting minerals characterization The chemical composition, obtained from the XRF analysis (Tab. 3.1), revealed that the Al/Si ratio is lower for SWy than for BId sample while the (Fe+Mg)/Si ratio is higher for SWy than for BId sample. This stays in agreement with the fact that for montmorillonites the substitutions take place mainly within the octahedral sheet (where Fe(II) and Mg substitutes the Al), while for beidellites in the tetrahedral sheet (where Si is substituted with Al). The Greene-Kelly test involved: (1) saturation of samples with Li, (2) heating at 270°C, and (3) glycolation. The first step resulted in the d001 increase to ~15 Å, both in the SWy and BId samples. Heating the SWy sample caused a collapse of the interlayer gallery (d001=9.6 Å), and the subsequent saturation with glycol did not lead to further increase of the interlayer distance. In the case of BId, the heated sample was able to incorporate glycol, and the d001 increased to 17.2 Å (Fig. 3.1). This confirmed that the layer charge was mostly located within the octahedral sheet in the SWy sample and in the tetrahedral sheet in the BId sample (Greene-Kelly 1953). The cation exchange capacity (CEC) determined with the methylene blue adsorption method was equal to 88.9 meq/100 g and 43.5 meq/100 g, for SWy and BId, respectively (Tab. 3.1). In the case of SWy sample the examined CEC stayed relatively close to the sum of exchangeable cations (Ca2+, Na+ and K+). In the case of BId sample there was a difference between the CEC value and the sum of exchangeable cations, which might result from the presence of admixtures of other minerals that are a source of silica and aluminum, and thus the relative amount of exchangeable cations is lowered. The particle size distribution analysis revealed that in the SWy sample 80% of particles were below 10 μm, while in the BId sample the particle diameters were visibly larger – 80% of particles fall below 70 μm (Fig. 3.2). The difference in particle size between SWy and BId samples is visible also in the SEM images (Fig. 3.3 and Fig. 3.4). The images revealed the flake–like morphology of the samples, which is typical for smectite minerals. The textural parameters are similar for the two minerals. Specific surface area is equal to 28 m2/g and 26 m2/g for SWy and BId, respectively. The total pore volume (Vtot) is ~0.07 cm3/g for both smectites and the mesopores (2–50 nm) dominate, representing ~50% of the Vtot (Tab. 3.1).. 26.

(31) Tab. 3.1. The chemical composition, cation exchange capacity and textural parameters of starting minerals: SWy and BId.. Element/parameter. SWy. BId. Si [wt.%] Al [wt.%] Fe [wt.%] Mg [wt.%] Ca [wt.%] Na [wt.%] K [wt.%] Ti [wt.%]. 32.02 5.28 1.64 1.63 1.02 0.60 0.27 0.11. 28.89 7.09 0.71 0.55 0.29 0.04 0.44 0.51. CEC [meq/100 g] SBET [m2/g] Vtot [cm3/g] Vmic/Vtot Vmez/Vtot Vmac/Vtot. 88.9 28 0.070 0.186 0.500 0.314. 43.6 26 0.067 0.179 0.582 0.239. CEC – cation exchange capacity; SBET – specific surface area; Vtot, Vmic, Vmez, Vmac – pore volumes: total, micropores, mesopores and macropores.. According to the XRD patterns, the basal spacing values (d001) of the SWy and BId samples were equal to 11.9 Å and 13.7 Å, respectively (Fig. 3.5). An admixture of kaolinite was detected in the BId sample: the XRD pattern showed a reflection at 7.2 Å, which is a characteristic d001 value of kaolinite. In addition in the FTIR spectra 3698 cm-1 and 3621 cm-1 bands were present, that correspond to the kaolinites inner surface and inner hydroxyls, respectively (Fig. 3.6). The characterization revealed that the main difference between the two starting minerals is the magnitude and location of the negatively charged sites as well as the particle size.. 27.

(32) Fig. 3.1. The Greene-Kelly test results – XRD patterns of SWy and BId samples: Li saturated, heated at 270°C, and glycolated.. Fig. 3.2. The particle size distribution in raw SWy and BId samples – incremental volume percent vs. particle diameter.. 28.

(33) Fig. 3.3. The SEM images of pure SWy sample.. 29.

(34) Fig. 3.4. The SEM images of pure BId sample.. 30.

(35) 3.1.2. Characterization of the alkylammonium intercalated smectites The increase of the d001 basal spacing of the smectites (Fig. 3.5) indicated that the alkylammonium surfactants were intercalated between the smectite layers. There was a clear correlation between the alkyl chain length of the intercalated salt and the resultant d001 value. The alkyl chain length controlled the arrangement of the molecules in the interlayer, which adopted either a monolayer, bilayer or a pseudotrimolecular structure (Lagaly 1986; Lagaly et al. 2013). Intercalation with C12, C14, C16, BC12, and BC14 molecules in all samples resulted in one well resolved (001) reflection corresponding to the d001 basal spacing values, indicating a homogeneous and unambiguous distribution of organic molecules within the clay mineral layers. The values were equal to 16.4 Å, 18.2 Å, 20.5 Å, 18.0 Å, and 19.6 Å, respectively in case of the SWy samples, and to 18.0 Å, 18.5 Å, 19.5 Å, 18.9 Å, and 21.3 Å, respectively in case of the BId samples. The BC16 intercalates showed additional (001) reflections: 18.0 Å, 26.5 Å, and 47.5 Å for SWy and 17.2 Å and 25.5 Å for BId. Most probably a mixture of at least two types of organo–smectite layers was formed in terms of surfactant arrangement: the bilayer and the pseudotrimolecular (He et al. 2006a).. Fig. 3.5. The XRD patterns of SWy and BId samples intercalated with alkylammonium salts.. 31.

(36) Fig. 3.6. The FTIR spectra of SWy and BId samples intercalated with alkylammonium salts. (a) 4000–900 cm-1 range; (b) 3100–2700 cm-1 range (aromatic and aliphatic C–H stretching region).. The presence of surfactants in the intercalated smectites was confirmed with the infrared spectra (Fig. 3.6). For the Cn modified clay minerals bands at 3000–2800 cm-1 and 1480–1460 cm-1 regions corresponded to the stretching νCH2 and bending δCH2 vibrations in the alkyl chain, respectively (Yariv & Cross 2001). While for the BCn modified samples additional bands in the 3100–3000 cm-1 region were due to the vibrations of aromatic C–C and C–H bonds within the benzyl group (Coates 2000). Along with the increasing alkyl chain length, the CH2 stretching bands shifted towards lower energies (Fig. 3.6.b), which is correlated with a change of the alkyl chains conformation. The disordered gauche conformers show the νasCH2 vibration band at ~2928 cm-1, while the highly ordered all-trans conformers (fully stretched alkyl chains) at ~2917 cm-1 (Vaia et al. 1994; He et al. 2004; Madejová et al. 2016). The vibrational bands of the structural OH groups (3700–3620 cm-1) as well as the aluminosilicate framework vibrations (1200– 900 cm-1) (Farmer & Russel 1966) were not changed during the organic modification, thus 32.

(37) the structure of layers remained undisturbed (Fig. 3.6.a). The intensity of H2O vibration bands at 1636 cm-1 and in the 3500–3000 cm-1 region decreased, indicating a decrease of water content, due to elevated hydrophobicity of the organo–smectites as compared to the pure minerals. The elemental analysis of carbon, nitrogen and hydrogen (CHN) allowed to calculate the content of intercalated organic molecules (Tab. 3.2 and Tab. 3.3). Firstly, the C/N molar ratio was calculated and it was nearly equal to the theoretical C/N ratio resulting from the chemical formula of each molecule. Therefore, no other organic impurities were present in the samples and the molar content of alkylammonium salts could be calculated directly from the CHN analysis. The molar content of surfactants varied between 70 and 130 mmol/100 g for the SWy sample and between 50 and 70 mmol/100 g for the BId sample (Fig. 3.7a). This is equivalent to ~0.25 mole up to ~0.50 mole of salt per half–unit cell (phuc: Si4O10) for the SWy and 0.17 mole to 0.25 mole phuc for the BId. The content of organic cations stayed relatively close or was slightly larger than the CEC value of each clay mineral. Thus the exchangeable sites were fully saturated with the ammonium organic cations. The increasing number of intercalated molecules along with the increasing alkyl chain length was observed. This could result from the fact that the larger molecules are less soluble in water and thus they were not completely washed out, in the applied experimental conditions. Tab. 3.2. The results of CHN elemental analysis of SWy derivatives. The nitrogen content was used to calculate the content of alkylammonium surfactants and azobenzene. N. C. H. [wt. %] SWy -C12 -C14 -C16 -BC12 -BC14 -BC16. 0.00 0.88 1.05 1.40 0.95 1.09 1.15. 0.26 11.86 15.92 23.08 16.99 21.81 24.87. 1.25 2.64 3.44 4.67 2.92 3.66 4.26. Alkylammonium content [mmol/100 g]. [wt. %]. [mol/phuc]. -. -. -. 72.77 91.75 137.64 83.34 103.17 114.88. 19.32 25.03 36.30 22.85 28.46 32.41. 0.26 0.32 0.49 0.29 0.36 0.41. Azobenzene content -C12A -C14A -C16A -BC12A -BC14A -BC16A. 2.58 3.25 3.24 2.44 2.82 2.68. 20.17 26.34 31.02 23.54 28.35 30.51. 2.56 3.81 4.81 2.83 3.23 3.49. 83.55 123.43 114.91 77.27 97.60 89.01. 10.94 14.23 11.83 9.63 11.19 9.84. 0.29 0.43 0.40 0.27 0.34 0.31. Az/surfactant molar ratio 1.15 1.35 0.83 0.93 0.95 0.77. 33.

(38) Tab. 3.3. The results of CHN elemental analysis of BId derivatives. The nitrogen content was used to calculate the content of alkylammonium surfactants and azobenzene. N. BId -C12 -C14 -C16 -BC12 -BC14 -BC16. C [wt. %] 0.00 0.06. H. Alkylammonium content [wt. %] [mol/phuc] -. 1.47. [mmol/100 g] -. 0.63. 8.61. 2.43. 50.05. 13.87. 0.63 0.82. 9.29 13.42. 2.41 2.78. 50.34 68.58. 15.10 21.12. 0.63 0.70 0.78. 11.42 14.01 17.30. 2.29 2.55 2.83. 50.97 58.58 68.99. 15.11. 0.24 0.18. 18.17. 0.21. 22.03. 0.24. 0.18 0.18. Azobenzene content -C12A -C14A -C16A -BC12A -BC14A -BC16A. 2.57 2.88 2.53 2.17 2.49 2.05. 17.92 20.57 21.27 18.43 21.99 22.05. 2.48 2.68 2.94 2.39 2.70 2.91. 91.00 110.38 85.58 72.67 89.44 62.83. 12.51 14.53 11.05 9.99 11.58 8.15. 0.32 0.39 0.30 0.25 0.31 0.22. Az/surfactant molar ratio 1.82 2.19 1.25 1.43 1.53 0.91. Fig. 3.7. The Molar content of ammonium salts (a) and azobenzene (b) in the SWy and BId samples, plotted against the number of carbon atoms in the alkyl chains of the intercalated salts. Samples denoted as 2xUV relate to the BId-BCnA samples after two cycles of alternating UV/Vis irradiation.. Thermal analysis was performed for selected smectite samples: the SWy sample – pure and modified with C16 cation, as representatives of all the smectite intercalates. The pristine C16 compound was also analyzed (Fig. 3.8). The first effect visible in the TG curve at ~100°C was attributed to the evaporation of the interlayer water. Its amount decreased from 11 wt.% for the pure smectite to 2.5 wt.% for the C16 modified sample, due to the hydrophobic nature of the intercalated surfactants. The dehydroxylation of framework OH groups in the SWy-C16 sample occurred at 615°C (endothermic DTA peak), which 34.