Faculteit Wetenschappen

Departement Chemie

In-depth study of organophosphonic acid modification on titanium dioxide

Proefschrift voorgelegd tot het behalen van de graad van

Doctor in de Wetenschappen: Chemie

aan de Universiteit Antwerpen, te verdedigen door

Annelore Roevens

Promotoren: Prof. dr. Vera Meynen

Prof. dr. Frank Blockhuys Antwerpen, 2017 Table of contents

List of Abbreviations ...... 1

General introduction ...... 3

Chapter 1 ...... 7

1.1 Introduction ...... 9

1.2 Organosilanes ...... 14

1.2.1 Synthesis and characteristics ...... 14 1.2.2 Material properties ...... 20 1.2.3 Various supports ...... 21 1.2.4 Stability...... 22 1.2.5 Applications...... 23 1.3 Organophosphonic acids ...... 25

1.3.1 Synthesis and characteristics ...... 25 1.3.2 Material properties ...... 33 1.3.3 Various supports ...... 35 1.3.4 Stability...... 36 1.3.5 Applications...... 38 1.4 Grignard reagents ...... 41

1.4.1 Synthesis and characteristics ...... 41 1.4.2 Applications...... 43 1.5 Aim of this Ph.D...... 46

1.6 References ...... 48

Chapter 2 ...... 77

2.1 Experimental ...... 79

2.1.1 Materials ...... 79 2.1.2 Standard synthesis ...... 79 2.1.3 Adsorption experiments ...... 80 2.1.4 Structural names ...... 80 2.2 Characterization techniques ...... 82

2.3 Quantum chemical calculations ...... 87

2.4 References ...... 88

Chapter 3 ...... 91

3.1 Introduction ...... 93

3.2 Results and discussion ...... 96

3.2.1 Modification in water...... 96 3.2.2 Modification with toluene as the solvent ...... 113 3.3 Conclusions ...... 125

3.4 References ...... 127

Chapter 4 ...... 131

4.1 Introduction ...... 133

4.2 Results and discussion ...... 136

4.2.1 Solute–solvent interaction: modification with 3PA and PhPA in toluene ...... 136 4.2.2 Solvent–surface interaction: modification with 3PA and PhPA in water ...... 141 4.2.3 Effect of water and toluene on the reaction mechanism ...... 145 4.2.4 The impact of solvent on the obtained surface properties ...... 147 4.3 Conclusion ...... 154

4.4 References ...... 155

Chapter 5 ...... 159

5.1 Introduction ...... 161

5.2 Results and discussion ...... 163

5.2.1 Isothermal titration calorimetry...... 163 5.3.2 Influence of the concentration ...... 169 5.3.3 Influence of the type of organophosphonic acid ...... 174 5.3.4 Correlating the sorption experiments with synthesis ...... 178 5.3 Conclusion ...... 181

5.4 References ...... 182

General conclusions and suggestions for future research ...... 187

General conclusion ...... 187 Suggestions for future research ...... 191 Reference ...... 192

Summary ...... 193

Samenvatting ...... 199

List of publications ...... 205

Oral presentations at conferences...... 206

Poster presentations at conferences ...... 208

Received award ...... 209

Dissertation supervision ...... 209

List of Abbreviations

1G Metyl Magnesium Bromide

1PA Methylphosphonic Acid

12PA Dodecylphosphonic acid

16PA Hexadecylphosphonic acid

18PA Octadecylphosphonic Acid

3PA Propylphosphonic Acid

8PA Octylphosphonic Acid

BET Brunauer-Emmett-Teller

CaHAP Calcium Hydroxyapaptite

DFT Density Functional Theory

DRIFT Diffuse Reflectance Infrared Fourier Transform

IC Total Amount of Inorganic Carbon

IR Infrared

ITC Isothermal Titration Calorimetry

ITO Indium Tin Oxide

PA Organophosphonic Acid

PAW Projector Augmented Wave

PBC Periodic Boundary Conditions

PEG Polyethylene Glycols

List of Abbreviations | 1 PhG Phenyl Magnesium Bromide

PhPA Phenylphosphonic Acid

PS Polystyrene

31P- CP/MAS NMR Phosphorus-31 Solid-State Cross-Polarization Magic Angle Spinning Nuclear Magnetic Resonance

QE Quantum Espresso

SAM Self-Assembled Monolayer

SEM EDX Scanning Electron Microscope with Energy Dispersive X-ray Spectrometry

T-BAG Tethering by Aggregation and Growth

TC Total Amount of Carbon

TGA Thermogravimetric Analysis

TiP Titanium Organophosphonate

TiPhP Titanium Phenylphosphonate

TiPP Titanium Propylphosphonate

TOC Total Organic Carbon

2

General introduction

The use of materials in a specific application is strongly dependent of the physico-chemical properties of the materials. Metal oxides generally exhibit a high chemical stability. Furthermore, their properties can be steered depending on the synthesis conditions and post-synthesis treatments to obtain amorphous and/or crystalline and dense or porous structures with controllable surface area and pore size distributions. Metal oxides have hydroxyl groups at their surface, inducing hydrophilicity. The lack of diversity in surface functional groups (i.e. metal oxide or hydroxyl bonds) often limits their use in applications such as e.g. separation in nonpolar solvents or their specific interaction with selected molecules.

Hybrid materials, composed of inorganic and organic components, have been developed to introduce the required versatility. These materials can be divided into different types. On the one hand, there are polymers with inorganic particles. On the other, there are ceramic materials with organic functional groups. The latter can be further subdivided into materials created by in-situ methods and post-modification. Co-condensation is a common example of an in-situ method. With this method complete inorganic precursors react with precursors possessing an organic functional group during the formation of the hybrid metal oxide from solution, leading to a homogenous distribution of organic groups in the matrix and at the surface. Next to co-condensation, other methods exist in which the organic functional group is built into the full matrix of the inorganic material. With post-modification the organic groups are bonded afterwards, on an existing inorganic material. The most commonly used technique is surface modification of silica by organosilylation. However, modification of metal oxides is gaining more and more interest due to the high chemical stability in solvents and a wide pH range. These metal oxides can be

General Introduction | 3 modified with e.g. organosilanes, organophosphonic acids and Grignard reagents.

In this Ph.D., titanium dioxide (titania), a metal oxide, is modified with organophosphonic acids. Although this method has already been described in literature, important knowledge about the impact of synthesis conditions on the obtained physico-chemical properties is lacking. Therefore, the aim of this Ph.D. is to reveal the influence of the synthesis conditions on the surface properties of the modified material and its correlation to the interaction between the modified material and probe molecules.

The Ph.D. is divided in five chapters, each covering specific aspects that are of importance when modifying titania with organophosphonic acids.

1. Introduction to the modification of inorganic materials with organosilanes, organophosphonic acids and Grignard reagents: syntheses and applications.

In this first chapter, an overview of post-synthesis modification of metal oxides with organosilanes, organophosphonic acids and Grignard reagents is given. Emphasis is put on the synthesis and material properties. Moreover, the stability of the obtained hybrid materials is discussed and a brief summery is given of the possible applications for the different types of modifications.

2. Experimental conditions

An essential part of all research are the experiments. In this part the detailed methodology followed in this work as well as the applied characterization techniques are provided.

4 3. Revealing the influence of the solvent in combination with temperature, concentration and pH on the modification of titanium dioxide with propylphosphonic acid.

In this chapter, the influence of the synthesis conditions on the modification of titanium dioxide with propylphosphonic acid (3PA) is discussed. More specifically, the impact of concentration, temperature and pH in water and toluene on the properties of 3PA modified TiO2 is studied. Important characteristics of the obtained materials are found by combining and correlating complementary techniques (DRIFT, TGA and 31P-CP/MAS NMR).

4. Aqueous or solvent based surface modification: the influence of the combination solvent-organic functional group on the surface characteristics of titanium dioxide grafted with organophosphonic acids.

This part reports on the effect of using water, toluene and its mixtures when performing the modification of TiO2 with organophosphonic acids. Sorption and calorimetric measurements of surface interactions with various probing species on the final materials are used to determine the distribution of organic groups on the surface, identifying differences in surface energy and sorption capacity of the obtained materials. Furthermore, the impact of the type of organophosphonic acid functional group (linear of aromatic) is investigated in relation to the applied solvent.

General Introduction | 5 5. In-situ study of the initial adsorption steps in the surface modification of titanium dioxide with organophosphonic acid via isothermal titration calorimetry.

In the last chapter, isothermal titration calorimetry (ITC) is applied to study the initial adsorption steps in the modification mechanism of TiO2 with organophosphonic acids. Through the use of ITC during modification in combination with adsorption isotherms, information on the initial steps of the organophosphonic acid modification is obtained. These insights are further completed by their correlation with the final material’s physico-chemical surface properties via 31P-CP/MAS NMR, DRIFT, and TOC to determine the modification degree and binding type.

6 Chapter 1

Introduction to the modification of inorganic materials with organosilanes, organophosphonic acids and Grignard reagents: syntheses and applications.

1.1 Introduction

Surface modification has been applied for many decades to adjust the surface properties of a variety of materials. This way, a monolayer on the nanometer scale or in some cases even a submonolayer is obtained, tuned in its specific properties to change the material properties at the surface.1 The best known method used for the functionalization materials are modification with organosilanes on SiO2.2 These materials are used for a variety of applications such as adsorbents and chromatography. But, for applications such as membranes, the stability of SiO2 and therefore its use is limited. Consequently, more recently modification methods for metal oxides with different types of precursors have been developed.3

These metal oxides have a high mechanical and chemical stability. But, their surface is lacking flexible functionality. It is mainly composed of metal oxide bonds and hydrophilic surface hydroxyl groups that are left after synthesis and heat treatment. These hydroxyl groups are good anchoring points for the addition of organic and/or inorganic functionality to adapt the surface properties. Surface modification has the advantage that it is a post-synthetic method, so the original structural features of the material are preserved. Furthermore, a large variety of functional groups can be used. The number of functional groups can be controlled by selecting the appropriate method and synthesis conditions. As an organic group is grafted on the surface, these surface modified materials are denoted as organic-inorganic hybrid materials.

Alternatively, hybrid materials can be made in-situ by various methods such as co-condensation.4 With this method, inorganic precursors and precursors with an organic group condensate during the synthesis of the material. This leads to a homogeneous distribution of the organic groups. Furthermore, the organic groups are distributed both on the surface as well as in the matrix of the material.

Chapter 1 | 9 The drawback of in-situ synthesis is the limitation of the amount and type of functional groups that can be applied, as it should remain compatible with the formation mechanism of the porous material.4 In table 1 an overview of the in- situ (co-condensation) and the post-synthetic (surface modification - grafting) method is presented.

Table 1: Comparison between grafting and co-condensation.

Grafting Co-condensation

Organic groups at the surface Yes Yes Organic groups in the matrix No Yes Diversity of organic groups High Lower Number of groups on the Partial to full Only partial surface monolayer coverage

In this thesis, only surface modification is applied, as it is important that the method is applicable for a large diversity of support materials and functional groups with the aim of steering and controlling the surface hydrophobicity/hydrophilicity.

10

A

F B

E C

D

Figure 1: An illustrative example of modification with different reagent types applied for surface grafting: (A) organosilane, (B) organophosphonic acid, (C) alkyne and alkene, (D) Grignard reagent, (E) alcohol, (F) carboxylic acid.

Chapter 1 | 11 The last decades, a large number of surface modification techniques have been developed.3,5 The best known technique is the organosilylation of silica.2 It is known that organosilylation on TiO2 is vulnerable to hydrolysis.6 Although it is used in a variety of applications, this vulnerability puts specific constraints on the material such as the necessity to graft full, dense monolayers and capping of any remaining surface hydrophilicity to protect the Si-O-M bonds. Furthermore, as the organoalkoxysilanes and organohalosilanes polymerize easily, a careful control of the water content during modification is required to produce high quality monolayers, preventing vertical polymerization away from the surface and stimulating horizontal polymerization. These drawbacks have led to the development of a large variety of alternative modification methods:5 organophosphonic acids,7–9 carboxylates,10–12 alkynes,13–15 alkenes,14,16,17 alcohols18,19 and Grignard reagents20–22 (Figure 1). The different methods of modification can be divided into direct and indirect bonding types (Scheme 1). For the indirect modification, a linker is needed to attach the organic group to the metal oxide, such as when an M-O-R or an M-O-X-R bond is formed (X = a heteroatom e.g. P or Si). Conversely, the direct method results in a direct covalent M-R bond between the organic group and the metal oxide.

The modification types forming M-O-R bonds are not further discussed in this thesis, due to the lower hydrolytic stability of the monolayer.12,23,24 These methods are often used for controlled drug release,25 but are not considered for applications where a long lifetime is required (i.e. separation and catalysis). Modification of metal oxides with organosilanes, organophosphonic acids and Grignard reagents have a better hydrolytic stability. Furthermore, the modification degree can be controlled in the case of organosilanes and organophosphonic acids.

12 In this chapter, the state of the art on the influence of the synthesis conditions on the modification of metal oxides with organosilanes, organophosphonic acids and Grignard reagents is discussed. Furthermore, a summary of the main applications of these types of modified metal oxides is given. Finally, the aim of this Ph.D. for further progress organophosphonic acid modification is discussed.

direct M-R Grignard reagents

alcohol

alkyne M-O-R alkene

Surface modification Surface indirect carboxylic acid

organosilane M-O-X-R organoposphonic acid

Scheme 1: Overview of different types of surface modification methods.

Chapter 1 | 13 1.2 Organosilanes

1.2.1 Synthesis and characteristics

Organosilanes contain at least one group that is able to hydrolyse and/or condensate with the surface OH groups forming covalent bond between the organosilane and surface.2,26,27 When a reactive functional group is chosen in combination with well-selected synthesis conditions, it allows further modification of the functional group via organic synthesis approaches without changing the grafted layer.1 Two different approaches exist to modify surfaces with organosilanes: vapor phase28 and liquid phase methods.2 The main difference in both methodologies relies in the control over the obtained layer. Of course, vapor phase modification is only applicable for those precursors that can be evaporated at reasonable temperatures under high vacuum without decomposing.

1.2.1.1 Type of organosilanes

A B C

Figure 2: Type of organosilanes: (A) organoalkoxysilane, (B) organochlorosilane, (C) organohydrosilane.

14

Figure 3: Possible attachments of organosilanes with one, two or three reactive groups on SiO2.38 Combinations of the above possibilities are also possible as well as weaker types of interactions such as via hydrogen bonding.

Chapter 1 | 15 Three different types of organosilanes have been used for surface modification: organoalkoxysilanes,29–32 organochlorosilanes33–35 and organohydrosilane33,36,37 (Figure 2). Organochlorosilanes and organoalkoxysilanes can have n (with n = 1, 2 or 3) reactive groups, being a chlorine atom or alkoxy group and 4-n functional organic groups.38 The number of reactive groups determines the number of bonds it can enter into with the surface (Figure 3). Moreover, when more than one reactive group is present, the organosilane can polymerize vertical and/or horizontal to the surface (Figure 3).

Also, the type of reactive group is important: a chlorine-silicon bond hydrolyses faster than the bond to an alkoxy group, and the latter’s hydrolysis is further determined by the length of the alkyl chain.44 The hydrides undergo the slowest hydrolyses.45 The main drawback of using organochlorosilanes is the production of HCl as side product, which might adsorb or even react at the surface, corroding the support material. The modification with organoalkoxysilane produces the alkyl alcohol as side product.46 This alcohol is also reactive and might adsorb strongly or even bind covalently with the support material.44 Therefore, it slows down the modification process of the silylation and leads to a uniformity decrease. Surface grafting with organohydrosilane has only water and hydrogen as byproducts and simplifies the reaction.33,46 A drawback of this method is the lower modification degree compared to the other silylation methods.33

1.2.1.2 Synthesis methodologies

The modification with organosilanes is influenced by different parameters (i.e. reaction temperature, solvent, functional group etc.). The liquid modification reaction of organochlorosilanes and organoalkoxysilanes needs to be performed in solvents with the addition of small amounts of water to form a controlled full monolayer.1,26,38,40–43 In the first step of the reaction mechanism, the organosilanes are hydrolyzed by water,5,33 resulting in an

16 organohydroxysilane. This organohydroxysilane then reacts with hydroxyl groups, which might be from the support or from other organohydroxysilanes. Before a covalent bond is made, aggregates are formed.5 These aggregates are based on hydrogen bonds between the organohydroxysilanes and the support material. The aggregation is driven by the van der Waals forces between the alkyl chains. Due to this aggregation, the organohydroxysilanes become less mobile and will condensate with support material or neighboring silanol groups. This leads to a self-assembled layer. Therefore, this step is the most critical and indicates the importance of water molecules (Figure 4).

Figure 4: A schematic overview of the influence of water on the modification of metal oxides with organoalkoxysilane.9

Also the reaction temperature has an influence on the formation of the monolayer.41–43,47 If the reaction temperature increases, more polymerization between the organosilanes occurs.42 This results in a loosely packed, disordered surface layer.43 Low reaction temperatures slow down the kinetics of the

Chapter 1 | 17 modification, producing uniform and ordered monolayers. In such cases, IR measurements indicate an all-trans conformationI of the alkyl groups, associated with a dense packing and a crystalline state.43 Layers produced at higher reaction temperatures contain alkyl groups in a gauche conformation.II Nevertheless, the state of the monolayer is neither crystalline nor pure liquid phase. Moreover, the reaction temperature required to form a completely ordered monolayer depends on the chain length of the organosilanes. For shorter chains, lower reaction temperatures should be used. Important to note is that a complete closed-packed and ordered monolayer corresponds to a theoretical modification degree of 5 groups/nm² for the modification with organohydrosilanes.44 This has been calculated for the modification with organohydrosilanes via the cross-sectional area of an all-trans alkyl chain (0.2 nm²). If the modification is performed with organosilanes with larger headgroups, the modification degree is determined by the cross section of the headgroup. Fadeev et al.44,46 found for the modification of TiO2, SiO2, HfO2 and

ZrO2 with octadecylsilane a grafting density of 4.4 – 4.8 groups/nm². Of course, also the number of reactive groups on the surface is important, although in case of organosilanes, polymerization reactions between different organosilanes are also possible.

Furthermore, the solvent has an important role in the self-assembly process of the organosilylation. Different solvents27,41,47 have been investigated: toluene,32,38,41,47 cyclohexane,41 benzene,41 hexadecane,41 perfluorohexane,47 n-hexane,47 tetrachloromethane,41,47 chlorobenzene,47 tribromomethane,47 cyclooctane,41 octane,41 pentane,41 dioxane41 and dichloromethane.41 If aliphatic solvents with long chains like octane and hexadecane are used, a higher

I The alkane has an all-trans conformation when it has a fully stretched conformation and all carbon-carbon torsional angles assume a value of 180 °.220 II For a gauche conformation the alkane has one or more torsional angles of ± 65 °.220

18 modification degree is obtained compared to when solvents with a short aliphatic chain are used.41 Furthermore, aromatic solvents increase the modification degree even more.41 This is explained by the ability of aromatic solvents to extract water more easily from the surface than linear hydrocarbons. Moreover, a hydrocarbon solvent such as n-hexane can have a specific stabilization of the organosilane (strong solvent-solute interaction), which will slow down the adsorption process on the surface.47 When perfluorohexane is used, the adsorption is a factor 50 higher, because there is no interaction between the organosilane and the solvent.

Finally, the functional group itself has an impact on the modification. Amines are well known for their catalytic effect on the modification of organosilanes.2,48 Different types of amines are used: ammonia, trimethylamine and amino-organosilane. In the latter, the organosilane has a peripheral amine function (Figure 5). This results in an improved reactivity of the amino-organosilane, caused by its enhanced adsorption that facilitates covalent bonding with the adjacent surface hydroxyl group. This mechanism is known as the flip mechanism.49

Figure 5: Flip mechanism of aminoorganosilane: (A) hydrogen bonding, (B) proton transfer, (C) covalent attachment.49

Chapter 1 | 19 1.2.2 Material properties

1.2.2.1 Chain length of alkyl functional groups and aromatic groups

A large variety of functional groups can be used by varying the R group of the organosilanes.50 The choice of the chain length has a vast influence on the order of the alkyl group on the surface. The degree of order of the alkyl chains rises with increasing chain length39,47 and modification degree.44 Aromatic silanes (e.g. phenyl-, benzyl- and phenylethyltrichlorosilane) are also used to modify silica.39 The most densely packed modification is found for phenyl silane. The packing is less dense for the other organosilanes. It is assumed that for these organosilanes the phenyl rings are forced in a strong tilt to the surface, which prevents close packing and ordering.

1.2.2.2 Functional groups with a heteroatom

Typically, surface functionalization has been described for linear organosilanes. Although, for other applications other functional groups are interesting as well.30,51 Free carboxylic acids have been used for immobilization with biomolecules.52 Grafting silica with organosilanes with carboxylic functional groups is not possible due to the reactivity of the carboxylic groups with silanes. Therefore, the carboxylic groups need to be protected as an ester.53 The best protection groups are tert-butyl esters. The reason lies in the deprotection method. The tert-butyl group is removed by thermal deprotection before the thermal degradation of the entire organosilane starts. If a methyl ester is used as protecting group, only one weight loss is observed in TGA, meaning that the methyl ester is not selectively burnt from the hydrophobic chain of the organosilane. For an iso-propyl protecting group, the temperature for the deprotection is close to the temperature of the degradation of the organosilanes.

Click chemistry is used to add specific functionality to the grafted organosilanes. In a first step, silica is modified with organosilanes containing a reactive

20 functional group. In a following step, extra functionality is added on the organosilanes. Via this two-step modification, functional groups or polymers can be anchored to a modified organosilane layer.3,54 This approach ensures a wide diversity of applications. The organosilane act as a linker between the functionality and the support material. Foschiera et al.55 modified silica with 3-chloropropyltrimethoxysilane and 3-aminopropyltrimethoxysilane, followed by a SN2 reaction resulting in a covalent bond between an aromatic group (e.g. aniline, p-anisidine, benzylamine and 3-phenylchlorpropyl) and the organosilanes. Another example is given by Yadavi et al.56 In that case, a first functionalization is done with 3-aminopropyltrietoxysilane and in a second step with 9-bromofluorene to produce fluorene functionalized silica. These fluorene functionalized silicas can be used in sensors. Fadeev et al.44 modified surfaces with organosilanes that contain a reactive end-functional group being for example a double bond, organohydrosilane or bromoalkyl. With a second modification step, these reactive end-functional groups can be functionalized according the application. The modification with these silanes results in high modification degrees, comparable to the modification of alkylsilanes. Zhou et al.57,58 grafted γ-methacryloxypropyltrimethoxysilane on ZrO2 membranes. In the next step, poly-(N-isopropylacrylamide) was polymerized onto the γ-methacryloxypropyltrimethoxysilanes.

1.2.3 Various supports

The modification degree is dependent on the amount of hydroxyl groups on the surface.33,59 ZrO2 has the highest number of hydroxyl groups compared to HfO2,

TiO2 and Al2O3. This leads to the highest organic surface loading on ZrO2. This trend is independent from the used organosilanes (hydride or chloride as leaving group). The electronegativity of titanium is lower than that of silicon and

Chapter 1 | 21 therefore the reaction of octadecyltrichlorosilane is faster on TiO2 than on SiO2.60 Also, quartz crystals are successfully modified with dimethyl and trimethylchloridesilanes.28 Zhao et al.61 functionalized MCM-41 with trimethylchlorosilane and found that only free and geminal Si-OH groups react with the organosilane.

Helmy et al.46 found that organohydrosilane do not react with carbon black and organic polymers [poly(ethylene), poly(ethyleneterephthalate) and others]. But, its reaction with other supports (i.e. MgO, NiO, ZnO, γ-Al2O3, ZrO2, FeO3, TiO2,

MoO3, WO3) has been successful. The obtained grafting densities are similar for all these substrates. Therefore, it is concluded that not the iso-electric point of the metal oxides is important for the modification, but that the ionic character of the metal oxides is the driving force. The presence of strong acid-base centers (Lewis and Brönsted) on the surface might be caused by the presence of unsaturated metal ions, hydroxyl groups and oxygen anions. The water adsorbed on the acid and basic centers is used to hydrolyze the organosilane. So, the acid and basic centers of the solid acts as catalysts for the grafting of the organohydrosilane. For organohydrosilane, the solid does not have a role in the modification degree; the solid only needs hydroxyl groups.

1.2.4 Stability

Silica modified with organosilanes has a thermal stability lower than 400 °C in nitrogen.62,63 The thermal stability is similar in air, but TMAX decreases 50-100 °C in air compared to in nitrogen. For trimethylsilane the modified layer has an increased thermal stability, the maximum weight loss occurs at 520 °C in nitrogen. For the modification with RSi(CH3)2Cl, where the R chain is longer as a methyl group, two weight losses are observed, a first at 250 °C, which varies with the R group, and a second above 450 °C which is due to methyl groups.

22 The modified layers also have a good resistance to organic solvents, hot water and detergent solutions.1 The hydrolytic stability of TiO2 and ZrO2 functionalized with monofunctional silanes [RSi(CH3)2Cl] is lower than for the modification with organohydrosilanes (RSiH3).6 The stability of functionalization of ZrO2 and TiO2 with RSi(CH3)2Cl is low at room temperature, while for organohydrosilanes, the stability of the modification is good in the pH region between 1 – 10 at room temperature. If the temperature increases to 65 °C, the stability in the low pH region will decrease. Furthermore, between pH 4-10, hydrolysis is more pronounced at 65 °C than at room temperature. Moreover, the resistance SiO2 grafted with organosilanes is high in 0.1 M HCl. However, the layer degrades immediately in basic media (i.e. 0.1 M NaOH). In basic media the low stability is due to the hydrolysis of Si-O bonds.

1.2.5 Applications

Titanized silica functionalized with octadecylsilane is used for chromatography.64 Silica is first grafted with titanium to achieve a higher stability in a basic medium. By packing a column with this material, uracil, acetophenone, benzene, toluene and naphthalene were separated using acetonitrile-water mixture (60:40, v/v) as mobile phase.65 In addition, the hexadecylsilane modified silicas are used for gas chromatography. Different esters and ethers are separated with this type of column.66

Ceramic membranes prepared from metal oxides are modified with fluoroalkylsilanes to avoid the high hydrophilic behavior of the native membrane.67–71 These membranes are selective for water in water/salt mixtures.68 Although, by raising the salt concentration, the flux will decrease. Also, water-organic solvent mixtures are separated. The selectivity is larger for the water/methyl tert-butyl ether mixture than for the water/ethanol mixture. Moreover, it has been found that titania-grafted membranes show a better

Chapter 1 | 23 selectivity than alumina-modified membranes. Membranes are also coated with alkylsilanes with different chain lengths.72,73 The longer alkylsilanes are shielding the surface more than the short chains. Furthermore, by increasing the number of the surface hydroxyl groups before the modification, a more hydrophobic membrane has been formed.

Silica is also modified with organosilanes for gas adsorption.31,74 Alkylsilanes are used as modifying agent for the adsorption of N2,31 while amino-terminated alkylsilanes are used for the capture of CO2.74 Moreover, terminal amino or carboxyl groups are needed for the adsorption of biomolecules.52,75,76 Grafting with organosilanes on silica is also used for the immobilization of fluorescent sensor systems.77

As organosilanation is a widely applied method, several other examples can be found.

24 1.3 Organophosphonic acids

1.3.1 Synthesis and characteristics

Ramsier et al.78 were the first researchers who modified Al2O3 with phosphinic acid and different types of phosphonic acids: methyl-, aminomethyl-, hydroxymethylphosphonic acid. They concluded from infrared (IR) analysis that the phosphorus acids adsorb via a condensation reaction on Al2O3. Due to the disappearance of the P=O and P-OH stretch vibrations, they assumed that the modification of the organophosphonic acid (PA) to the surface is tridentate, whereas for the phosphinic acid a bidentate binding with Al2O3 was found. In the case of a tridentate binding, the PA is attached via three bonds with the surface. Mono- and bidentate bonded PA form one or two bonds with the metal oxide, respectively. In 1995, Randon et al.7 modified TiO2 and ZrO2 membranes with methyl-, ethyl-, butyl- and phenylphosphonic acid. The modification was performed in water, ethanol and toluene. Although different solvents were used, the explanation of the influence of the solvent on the reaction mechanism was limited. Only a strong increase (5 times higher) of the modification degree was found for the modification of TiO2 with butylphosphonic acid in toluene compared to in water.

Beside Ramsier et al.78 and Randon et al.7, also Gao et al.8 described the functionalization of metal oxides with octadecylphosphonic acid (18PA). They concluded that ZrO2 is the best substrate for modification with 18PA, while Al2O3 is less promising as support due to the formation of bulk aluminoalkylphosphonates. These aluminoalkylphosphonates are also formed when the modification is performed under milder reaction conditions (lower temperature and shorter reaction time). Even though PA modification is thus known for several years, the mechanism of this modification and control of the

Chapter 1 | 25 final material properties obtained is not yet fully understood. One of the reasons is the difficulty in characterization of the obtained materials. For the modification of ZrO2, it is possible to use IR spectroscopy to investigate the obtained

P-O-Zr bond, while for TiO2 it is not, due to the strong adsorption of TiO2 in the same wavenumber region. An alternative method is solid state NMR.

Powerful experimental techniques to characterize grafting of PA on metal oxides are 31P NMR and IR spectroscopy.79 However, mono-, bi-, and tridentate bindings modes are not easily distinguished via the 31P chemical shift, because the binding mode is not the only parameter influencing the 31P chemical shift. IR can indicate the binding of the PA to the metal oxide via the presence of the P-O-Ti bands and the disappearance of P-OH and P=O bands. But, the assignment of the different binding modes is hampered by the overlap of

P-O stretching region and the bands in the native TiO2 spectrum. Computational studies were used to complement experimental measurements. From our own work, it was concluded that for the modification of methylphosphonic acid (1PA) on anatase (101) monodentate and bidentate binding modes are stable.79 The monodentate binding has two hydrogen bonds and the bidentate binding mode has one hydrogen bond and one dissociated hydrogen atom. On this surface, tridentate binding mode is geometrically not possible. On the (001) surface, mono-, bi- and tridentate binding modes are possible. However, optimization of a monodentate binding mode always results in relaxation towards a bidentate binding. A fully dissociated bidentate binding mode and tridentate binding mode are the most stable on this surface. The binding of PAs on the (001) surface are more stable than the binding of it on the (101) surface. Furthermore, a comparison with other organic groups (phenyl and butyl) on the PA was made leading to the conclusion that the type of organic group does not influence the binding type.

26 1.3.1.1 Binding types

PAs do not polymerize like the organosilanes do, because the phosphorus atom is not susceptible to a nucleophilic substitution.9 So, no P-O-P bindings are formed.80 Hence, also the reaction and synthesis/structure control correlation is different from organosilanes and even synthesis in water is possible. The PA binds via P-O-M bonds with the metal oxide, due to the condensation of P-OH and surface hydroxyl groups.81 In addition, coordination of the phosphoryl oxygen to a surface metal atom can occur. As indicated above, the PA can bind via mono-, bi- and tridentate binding (Figure 6). Monodentate adsorption is possible in a molecular or a dissociative fashion.79 Bidentate adsorption is only possible when one or both hydrogen atoms are dissociated to the surface and tridentate adsorption is possible when both protons are dissociated to the surface. The remaining hydroxyl groups or phosphoryl group, in case of monodentate and bidentate binding, might form hydrogen bonds with the surface (Figure 7). These types of binding with the surface are called surface grafting. But, also metal organophosphonates can be formed: in that case the metal oxide locally dissolves and precipitates as a layered structure (see below).8 With 31P-CP/MAS NMR, it has been found that if aggressive synthesis conditions are used, bulk metal alkylphosphonates are formed.

Chapter 1 | 27 A

B

C

Figure 6: Possible binding types of organophosphonic acids: (A) monodentate on the (101), (B) bidentate on the (101) and (C) tridentate binding on the (001) anatase surface.79

28

Figure 7: Schematical representation of the different binding types of PAs.81,82

1.3.1.2 Phosphonates

PAs can dissolve the support material and precipitate as metal organophosphonates. This has been described for various support materials, such as Al2O3, ZrO2 and TiO2.8,9,83 Alberti et al. 84,85 were the first authors who described the formation of layered metal organophosphonates (Figure 8), but on purpose instead of as a side reaction of surface grafting. Metal organophosphonates are formed by heating an aqueous mixture of TiCl3 or TiCl4 with PA, HF and HCl.86 Depending on the synthesis conditions, the physical properties (surface area and crystallinity) can be varied. For example, when Ti4+ is used instead of Ti3+ as precursor, a material with a lower surface area and higher crystallinity is obtained. Alternatively, these metal phosphonates can be prepared by a reaction of Ti(SO4)2 with hexyl, octyl, and dodecyl phosphates in aqueous media.87 The formed titanium alkylphosphonates have a layered

Chapter 1 | 29 structure and crystallize above 900 °C into porous TiP2O7. Yet another method to prepare organophosphonates is via the sol-gel method, depending on the sol-gel synthesis conditions the metal/phosphorous ratio can be varied.9,88,89

Figure 8: Schematic representation of a metal organophosphonate.87

Periodic porous metal phosphonates with micro-, meso and macropores can be prepared.90–94 In that case organobiphosphonic acids are used. They will form an organic pillar between two inorganic layers (Figure 9). The pillar can contain functional groups (pendant groups). Furthermore, PAs are also used as non-pillaring groups; these groups are added to improve the porosity. These materials find applications in catalysis,95–97 adsorption,96,98,99 solar light utilisation96 and separation100.

30

Figure 9: Porous metal phosphonates. 91

1.3.1.3 Synthesis methods

For the modification with PAs different solvents are used: water,7,101–107 methanol-water mixtures,8,108–110 toluene,6,7,62,110,111 trimethylamine,107 ethyl ether,107 pyridine,107 acetonitrile,107 dimethyl sulfoxide (DMSO),107 tetrahydrofuran (THF),12,107,110–115 acetone,12,107 trichloroethylene,116,117 dichloromethane,118 methanol,107 ethanol,7,111,114,119 butanol,111 n-heptane-isopropanol mixtures120 and ethanol-octanol mixtures.104 Although different types of solvents are used, little has been reported about the influence of the solvent on the modification. Nevertheless, the solvent-solute and solvent-surface interactions are important for the modification.121 D’Andrea et al.111 modified calcium hydroxyapatite (CaHAP) with 18PA and found that the reaction is faster in toluene and ethanol than butanol and THF. The solubility of the solute is in the reverse order. When the solute-solvent interaction is strong, less solute molecules will adsorb at the surface and the reaction slows down. In a poor solvent for the solute, the adsorption of the solute molecules at the surface is stronger and leads to fast reaction kinetics. Chen et al.107 found that polar

Chapter 1 | 31 solvents interact strongly with the surface of indium tin oxide (ITO) and this leads to poorly packed self-assembled monolayers (SAMs). They concluded that if there is a strong interaction between the solvent and the surface, it is difficult for the solute (PA) to displace the adsorbed solvent molecules. Furthermore, successful modification of organophosphonic esters under microwave irradiation has been achieved.122

Alternative wet chemical grafting methods, have been followed to create hybrid materials starting from organophosphonic acid and organophosphonic ester precursors. One of these methods is to modify surfaces with PA is the T-BAG method (tethering by aggregation and growth).82,123–126 With this method, the substrate is submerged into a solution of PA in which the concentration needs to be lower than the critical micel concentration. The solvent is evaporated slowly leading to the adsorption of the PAs to the surface and their bonding with the surface. This method allows the modification of substrates such as electrodes etc.127 Furthermore, oxide nanoparticles have been modified with PA: oxide nanoparticles were synthesized by hydrolysis in aqueous medium, followed by precipitation of the nanoparticles, solvent exchange, surface modification, drying, washing and redispersion. Schmitt et al.128 used organosols for the modification. In this work, metal oxides are modified with PAs and simultaneously the modified particles are transferred from the aqueous to the organic phase. This method avoids the time-consuming procedures. Solvent-free and environmentally friendly alternatives to synthetic strategies are reactive milling129,130 and modification in vacuum.131–134 Both methods are successful to modify metal oxides with PAs.

1.3.1.4 Type of organophosphorous compound

Surface functionalization is performed with PAs, their diethyl esters ((OEt)2) and their trimethylsilyl esters ((OSiMe3)2) (Figure 10).135,136 Guerrero et al.102 found that all types of organophosphorous compounds react with the surface. But the

32 modification degree is higher for the modification with acids and trimethylsilyl esters than with diethyl esters. Furthermore, the reaction is easier to control in water for PAs than for their ester derivatives, due to the higher solubility of PA in water. However, titanium organophosphonates (TiPs) are formed when using these phosphorus compounds, while no TiPs are observed by using organophosphonic esters.

A B C

Figure 10: Different types of organophosphorous compounds: (A) organophosphonic acid, (B) dialkyl organophosphonate, (C) bis(trimethylsilyl) organophosphonate.

1.3.2 Material properties

1.3.2.1 Chain length of alkyl group and aromatic groups

For the functionalization with long aliphatic chains (longer than C16), a highly-ordered layer is formed. In this case, the chains have all-trans conformation, like crystalline alkyl chains.108,120,137,138 By decreasing the chain length of the aliphatic group, the disorder of the layer increases and gauche conformations are observed. This is due to the decrease of the van der Waals forces.

Chapter 1 | 33 In addition, the modification degree seems to be determined by the type of functional group. Feichtenschlager et al.137 found higher modification degrees for the modification with dodecylphosphonic acid than octadecylphosphonic acid. El Malti et al.114 achieved a higher modification degree for the modification of calcium carbonate with phenylphosphonic acid (4.8 groups/nm²) than dodecylphosphonic acid (3.5 groups/nm²) performed in ethanol and THF. But, in the case of the modification of phenylphosphonic acid in THF also calcium phenylphosphonates have been formed. Furthermore, mixtures of different functional groups have been used to create unique surface properties. Mixtures of phenylphosphonic acid with octadecyl- and dodecylphosphonic acid have been used for grafting.137 It is found that by increasing the amount of phenylphosphonic acid, methylene stretch vibration shifts to higher wavenumbers. So, a less ordered system has been formed. Moreover, co-modification has been performed with methylphosphonic acid and 11-methacryloyloxyundecylphosphonic acid.138 Methylphosphonic acid appeared too small to have structure-directing properties. But, it can bind with unreacted hydroxyl groups on the surface and increases the hydrophobicity. This technique is known as end-capping.

1.3.2.2 Functional groups with heteroatoms

Functional groups with heteroelements are being applied to introduce more versatile surface properties. Amino-terminated PAs107,139–142 and PA with carboxyl end groups are used for surface grafting.10,121,143–145 In the latter case, the synthesis is more complex as the PA can bind via the phosphorous group or via its carboxylic end group. However, for nanoscaled ZnO surfaces selective modification with the organophosphonic acid group occurs.143

Charged end groups are also used, e.g. pyridinium alkylphophonic acid is applied to introduce a positively charged end group, while alkylphopshonic acids with a sulfonic end group are applied to include a negative charged end

34 group.146,147 Moreover, PAs containing an ether group in their functional group are successfully used to modify metal oxides; this type of functional group is useful for organic thin film transistors.148 Also, functionalization with PA containing a thiophene is possible.149 Thiophenes have always occupied a central position for applications in the field of electrically conductive materials.150 Modification with fluoroalkylphosphonic acids111 is useful for the stationary phase in HPLC.66

Instead of grafting a PA that already contains the desired functional group, another method to add functionalities is via click chemistry.151 Here, an azide or alkyne end group is needed. By making use of a copper(I) catalyzed azide-alkyne cycloaddition, different functional groups have been added, such as 5-chloropentyne and benzyl azide.

1.3.3 Various supports

The modification of PAs has been performed on a large variety of supports. SiO2 is less promising for the modification with PAs, because the P-O-Si bond has a low hydrolytic stablility103,152 and the affinity of the PA to SiO2 is low.105 Therefore, only physically adsorbed PAs are observed.117 A solution to modify

SiO2 with PAs is to add an Al2O3 layer on the surface of the SiO2.103 Hofer et al.105 compared the impact of the isoelectric point on the modification degree by comparing grafting on SiO2 (1.8 – 2.2) and various metal oxides: TiO2 (4.7 – 6.2),

ZrO2 (4.0), Al2O3 (7.5 – 8.0), Ta2O5 (2.7 – 3.0) and Nb2O5 (3.4 – 3.6). They found that the isoelectric point is not important for the modification degree when grafting PAs. But, SiO2 has a lower affinity for the phosphorus group than the other metals.

117,153–157 Next to SiO2 also Al2O3 is modified with PAs. The strongly acidic sites on its surface are correlated to an enhanced interaction of the phosphoryl group

Chapter 1 | 35 with the surface.158 Giza et al.159 found that with an increasing number of surface hydroxyl groups, the reaction kinetics are accelerated. Furthermore, a variety of modification degrees are possible. The highest obtained modification degree is

4.7 groups/nm².102 TiO2 has a higher chemical stability than silica65 and is a commonly used substrate for surface functionalization with PAs.12,81,118,160,161 A modification degree of 3.5 – 4.2 groups/nm² is reported for this type of modification.6,45 The large differences in surface properties and their impact on the grafting efficiency, provide the opportunity to create selectively grafted surfaces. Mutin et al.110 grafted a TiO2-SiO2 substrate selectively on the TiO2, caused by the low stability of PAs on SiO2 and their strong interaction with TiO2.

Also ZrO2 has been used as substrate for the functionalization with PAs.6,12,104,137

The surface properties of ZrO2 are similar to those of TiO2158 but the concentration of hydroxyl groups is a factor 1.5 larger than on TiO2 (i.e. 8 OH groups/nm² for

TiO2 and 12 OH groups/nm² for ZrO2).6,158 Moreover, the ZrO2 surface has a higher basicity than the amphoteric TiO2. However, similar modification degrees have been reported (i.e. 3.7 groups/nm²).45 Other support materials that have been grafted with PAs are ZnO,125 Cu2O,162 CuO,162 Fe2O3,163 Ta2O5,105,106 nickel oxide,164 iron,165 ITO,140,166 stainless steel,167 calcium hydroxyapatite,111 calcium carbonate114 and nanodiamonds.168–170

1.3.4 Stability

The thermal stability of surfaces modified with long alkylphosphonic acids is similar to that of the modification with alkylsilanes and is not dependent on the support material.62,171 The thermal stability is lower than 400 °C for long alkyl groups in nitrogen and shifts 50-100 °C down in air. During the heating process the organic groups are burnt from the surface, while the inorganic part remains at the surface, with the formation of a phosphate.62 Ilia et al.172 found that the

36 thermal stability of TiO2 modified with styrylphosphonic acid is higher than with benzylphosphonic acid, because of the presence of the longer conjugated system. However, the organic group completely degrades under ultraviolet light.113,173 Also, in this case a phosphate is formed at the surface. Moreover, alkyl groups degrade faster under ultraviolet light than aromatic groups.173 Furthermore, the modified layers are not stable in an air plasma: a reduction of the carbon amount is observed after 1 s in the plasma.113

Surfaces functionalized with a full monolayer of PAs have a higher hydrolytic stability (pH 1-10) than those modified with organosilanes. This is due to the strong specific interaction between the PA and the surface.6 The hydrolytic stability is higher for the functionalization of ZrO2 than TiO2 and is attributed to its isoelectric point. ZrO2 (IEP 7-11) is more basic than TiO2 (IEP 4-7) and consequently generates a more stable binding with the PA.6 Moreover, also Al2O3 and ITO show a good hydrolytic stability between pH 3-7.171 In alkaline conditions the stability is lower compared to the stability in acidic conditions.

The stability is higher for functionalized Al2O3 than ITO, attributed to the more basic nature of the Al2O3 surface compared to ITO. Also, Cichomski et al.156 found a higher stability for the modification of Al2O3 with PAs in acid medium than in alkaline conditions.

Leaching tests with water and THF showed a high stability for the PA functionalized TiO2 than after functionalization with carboxylic acids.12 Kanta et al.113 found a good stability of the modified layer with long alkyl groups in cyclohexane, toluene and acetone. But, they reported a low stability in water, methanol and ethanol, which is opposite to the results of Marcinko et al.6 and Bhairamadgi et al.171

Chapter 1 | 37 1.3.5 Applications

Materials functionalized with PA can be used in a wide range of applications. For example surface modification with PAs is used to improve the selectivity of membranes.3,174 Randon et al.7,175 modified membranes with butylphosphonic acid to change the surface properties from hydrophilic to hydrophobic. These modified membranes enhance the separation of proteins. Moreover, the hydrophobic/hydrophilic behaviour of the surface can be tuned. In a recent study Mustafa et al.21 investigated the anti-fouling effect of TiO2 membranes modified with PA. The membranes modified with 1PA showed better anti-fouling properties for humic acid, laminarin and peptone than the bare membrane (see below). Also, modified membranes are being used in gas separation.72,176 The pores of Al2O3 membranes functionalized with alkyl and arylphosphonic acids. The separation principle is based on the hydrophobic properties of the surface. The modification with arylphosphonic acids leads to a membrane with a thermal stability until a temperature of 400 °C. The stability of these membranes is higher than membranes modified with alkylphosphonic acids. Al2O3 membranes grafted with butyl- and dodecylphosphonic acid improve the separation of propane/nitrogen mixtures.7,175 These membranes have a higher permeability and selectivity for the separation.

Chromatography is performed with ZrO2 modified with ethylenediamine-N,N′- tetramethylphosphonic acid. With this method proteins, globines and antibodies are separated.177-180 Furthermore, the modification with PAs is applied to different types of adsorbents. To obtain good adsorbents for CO2 from flue gas of coal power plants, TiO2 is modified with PAs with an amino-function at the end of the alkyl chain.139 Furthermore, the adsorbents for radiotracer 153Ga are prepared by surface grafting.181 Beside adsorbents and chromatography grafting with PAs is applied to catalysis.182–185 For example, for the asymmetric

38 hydrogenation of ketones a 4,4’-bisphosphonic acid-substituted BINAP-Ru-DPEN [2,2-bis(diphenylphosphino)-1,1’-binaphthylruthenium- diphenylethylenediamine] is grafted on iron particles.185 Another type of catalyst is used for the oxidation of alcohols to their carbonyl derivatives.184 In this case, 3-azidopropylphosphonic acid is attached to iron oxide particles. Via click chemistry a TEMPO (2,2,4,4-tetramethylpiperidine-1-oxyl) catalyst is bound to the 3-azidopropylphosphonic acid. The 3-azidopropylphosphosphonic acid modified iron oxide particles are also used for the Cu(I) catalyzed arylation of nitrogen nucleophiles.183 In this case proline ligands are immobilized via click chemistry.

Functionalized materials are also used to steer interaction with bioactive molecules.126,136,186–190 Different types of functional groups are used often containing an alcohol or carboxyl end group.186 Afterwards, the terminal functional group can be activated with a linker (carbonyldiimidazole for the hydroxyl groups and N-hydroxysuccinimide for the carboxyl groups). The linker can bind with molecules that possess an amine function (i.e. proteins). The possibility to link biomolecules on a functionalized surface is used for biosensor applications.143,191,192 Also in medical applications, surface grafting is often applied to improve interaction of materials with biomolecules. For example, the modification of titanium is useful to enhance the chemical interaction between implants and bone tissue for dental applications.193,194 Furthermore, the modification with PA is interesting for tissue engineering.195,196 Zeller et al.196 described a method based on the copolymerisation between styrene and vinyl- or vinylbenzylphosphonic acid. Afterwards the copolymer is grafted on TiO2. With this method nucleation sites are made for hydroxyapatite, which has a high potential for implants. Han et al.187 attached a full monolayer of mercapto-ending PA to the surface for the same purpose. A linker will bind with the thiol groups and in the last step proteins are bound to the linker. This is used to design

Chapter 1 | 39 biocompatible implants. Modified surfaces with PAs are also used for organic thin film transistors,148,182,197–201 organic electronics,82 dye-sensitized solar cells,182,202–206 transparent nanocomposites207 and protection of the surface against corrosion.208–212

40 1.4 Grignard reagents

1.4.1 Synthesis and characteristics

An alternative to modify metal oxides is via Grignard reagents. With this method, a direct bond between the metal atom and carbon group is established (see Scheme 1). This direct M-C bond distinguishes the method from the indirect methods. For the indirect methods, a heteroatom is used to attach a carbon group to a metal oxide. Yamamoto et al.213,214 described a new grafting procedure to form a direct Si-C bond. To achieve this, the surface of MCM-41 has been reacted with Grignard reagents (Figure 11). This Si-C bond has an improved hydrothermal stability, which is higher than the stability of alcohol modified MCM-41.213,214 The Si-OR groups are easily hydrolyzed and turned back into silanol groups during a hydrothermal treatment. The grafted MCM-41 obtained via the Grignard modification method maintained the alkyl groups, due to the strong Si-C bond. Moreover, the functionalization via the Grignard method leads to a higher hydrophobicity. This hydrophobicity is demonstrated by H2O adsorption measurements.214

A B

Figure 11: Grignard reagents: (A) organomagnesium bromide, (B) organomagnesium chloride.

The Grignard method is a new and promising technique to functionalize TiO2 as well.20,215 For this type of functionalization, a direct Ti-C bond is formed on the

TiO2 support, which is a less-hydrolysable covalent bonding. The direct Ti-C bond makes this modification technique unique, omitting the presence of

Chapter 1 | 41 heteroelements (such as Si and P). Furthermore, only one reactive group is present, avoiding polymerization between the grafting molecules. Moreover, no remaining reactive OH or P=O groups remain after grafting resulting in a more controlled interaction with surrounding molecules. A drawback of this method is the high reactivity of the Grignard reagents with moisture, leading to the necessity of working in ethers and water-free atmosphere. Furthermore, this technique requires a careful appropriate washing step to remove remaining magnesium salts, a side product of the reaction.

Due to the novelty of this method, little is known about the reaction mechanism and characteristics. In a communication, the first characteristics have been reported.20 A typical chemical shift below 50 ppm in the 13C-CP/MAS NMR spectra have been assigned to Ti-13C bond. The presence of the alkyl groups has been confirmed with FTIR. A commonly used technique to study functionalized materials is TGA and the weight loss between 200-400 °C is correlated to the thermo-oxidation of the alkyl groups. This is confirmed with TGA-MS, where a large amount of CH3 is measured.

A clear influence of grafting on the surface properties was observed. The results of N2 sorption measurements show a decrease of the C value, indicating an increase of hydrophobicity. This is confirmed by suspending the modified powder in hexane-water mixtures. More specifically, the hydrophobicity of the modified samples is dependent on the chain length of the alkyl group. If the modification is done with a pentyl chain or shorter, the modified material will still suspend in the water phase, while when performing the modification with an octyl chain or longer, the modified material is suspended only in the hexane phase.

Although some of the characteristic signals correlated to the surface properties have been reported, the reaction mechanism has not yet been fully revealed. The

42 reaction mechanism seems to be more complex than a nucleophilic attack on the surface followed by the Ti-O bond cleavage, indicated by a color change of the

TiO2 (from white to dark blue) observed during reaction. It is believed that this color change is induced by the transition of Ti4+ to Ti3+. Current research is ongoing to elucidate this reaction mechanism.

1.4.2 Applications

Grignard grafting has been used to modify ceramic nanofiltration membranes.21

TiO2 nanofiltration membranes have been modified with PAs and Grignard reagents for solvent filtration and fouling reduction. The grafting of the organic functional group induces changes in the hydrophobicity depending on the type of functional group. However, in case of grafting via the Grignard method, unique amphilic surfaces are always obtained.216,217

In case of anti-fouling membranes for water purification, both PA and Grignard modification have been applied. The surface properties differ depending on the type of functional group and grafting method applied. For the membranes modified with phenylphosphonic acid (PhPA) and hexadecylphosphonic acid (16PA) the permeability of water is low, therefore the fouling effect of these membranes has not yet been studied. For methyl Grignard (1G), phenyl Grignard (PhG) and methylphosphonic acid (1PA) modified membranes, the anti-fouling effect has been studied with different model foulants (e.g. humic acid, laminarin, meat peptone, dissolved organic matter etc.) in the presence and absence of ions such as Ca2+, which are often found in water.

The 1G grafted membrane showed an extremely low irreversible fouling in all studied cases. The PhG grafted membrane had less fouling with meat peptone than humic acid. For this membrane, no fouling is observed with laminarin. The anti-fouling behaviour of these membranes can be explained by the affinity of

Chapter 1 | 43 the foulants to the membrane. Those foulants that have an aromatic system, can form π-π interactions with phenyl grafted groups at the surface, leading to fouling on the PhG grafted membranes. The difference in fouling behaviour between non-modified, 1PA and 1G modified membrane is explained via the ability to have polar interactions. The 1PA modified membrane still allows polar interactions with the residual OH or phosphoryl groups of the PA bonded to the surface and the OH and COOH groups of the model foulants. But, these polar interactions are more sterically hindered by the organic group of the PA compared to non-modified membranes. These residual OH interactions seem to be much less present in case of 1G modified membranes, resulting in their uniquely high anti-fouling capacity.

The stability of the modified membranes has been investigated by cleaning the membrane. For native membranes harsh chemical cleaning (pH 12) is needed to remove all the foulants, while milder cleaning conditions (pH 10) can be used to clean the modified membrane. From these observations; it was concluded that the interaction with the foulants is less strong in the case of the modified membranes than from the native membranes.

Hosseinabadi et al.22 modified 1 nm TiO2 membranes with Grignard reagents. The Grignard reagents had different chain lengths: methyl, pentyl, octyl and dodecyl. The unmodified membranes are hydrophilic and have a high permeability for polar solvents. The Grignard modified membrane obtain an amphiphilic surface, which has a good permeability for both nonpolar and polar solvents. They show the potential of these modified membranes for affinity separation via retention measurements of polystyrene (PS) and polyethylene glycols (PEG) in several solvents with different polarities.22,216 The retention is determined by the strength of the interactions between the solvent, solute and membrane surface. By changing these interactions via the modification, the separation behaviour can be steered. It is found that the retention of PEG and PS

44 is low in nonpolar solvents and the retention increases with the polarity. For polar solvents, high retentions are observed. Furthermore, the influence of the pressure is described. For solvents with a low polarity, the retention increases with the pressure while this dependency is absent in case of solvents with a high polarity. This means that for solvents with a high polarity convection and size exclusion are the most dominant effects. The diffusion transport mechanism is the most important for the solvents with low polarity.

Furthermore, the 1 nm TiO2 membranes modified with Grignards reagents can be used for separation of biomass.218 In this case methyl or phenyl groups are attached to the surface. The amphiphilic character of the surface raises the possibility to permeate polar and nonpolar solvents, which results in a higher flux. By comparing the flux of these membranes with polymeric membranes, it is found that polymeric membranes show a lower flux. The lower flux is due to the lower porosity of the polymeric membrane. Another application of modified membranes with Grignard reagents is the separation of Pd-catalysts.219 Here,

1 nm TiO2 membranes are modified with pentyl or octyl groups. In this case a high rejection of the catalysts and a low rejection of the reaction products is achieved.

Chapter 1 | 45 1.5 Aim of this Ph.D.

In this chapter the impact of surface modification with organosilanes, organophosphonic acids and Grignard reagents has been discussed. The modified surfaces have been used in a wide range of applications, due to the ability of the modifying agent to change the surface properties. These surface properties are the driving force in their performance in these diverse applications. Apart from the type of functional group and modification method, also the applied synthesis conditions have a determining role on the modification as they tune the final surface properties. The influence of these synthesis conditions is already well known for the functionalization of silica with organosilanes. But, the interest in modifying transition metal oxides is growing, due to their high chemical stability, and thus other methods such as PA modifications attract increasing attention. Therefore, in this Ph.D., TiO2 is modified due to its high stability in a wide range of pH values and solvents and its wide variety of applications (e.g. membranes, sensors, photocatalysts etc.). In this thesis, only the modification with PAs is used, because of its advantages over organosilylation with respect to the absence of polymerization between the PAs and the higher stability of the modified layer. Moreover, the modification can be performed in water, which gives it an advantage over the Grignard modification method. However, the necessary knowledge to tune the surface properties of PA modified materials, is not yet fully available. Therefore, a fundamental study of the impact of the modification conditions on the physico-chemical properties is necessary.

During the modification process interactions between solute, solvent and surface are driving the functionalization (Scheme 2). In this Ph.D., the influence of the concentration, solvent, time, temperature, pH and type of functional group

(aliphatic or aromatic) on the binding type and number of PAs grafted on TiO2

46 are investigated. Several complementary techniques such as DRIFT, TGA, 31P-CP/MAS NMR and quantum chemical calculations are applied to deduce the properties of the obtained modified surfaces. Moreover, the contribution of each type of interaction between solute, surface and solvent is revealed. Furthermore, the surface properties of the obtained materials are studied with sorption experiments and flow calorimetry. According to the selected synthesis conditions, the surface properties can be altered in a controlled way. In chapter 2 the experiments and characterization techniques are briefly described. In chapters 3 and 4, first the modifications are performed and afterwards the samples are characterized. In chapter 5, the modification process is followed in-situ with isothermal titration calorimetry (ITC) measurements and complemented with adsorption isotherms and ex-situ characterization techniques.

Scheme 2: Different interactions during the surface modification process.

Chapter 1 | 47 1.6 References

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74 (218) Dubreuil, M. F. S.; Servaes, K.; Ormerod, D.; Van Houtven, D.; Porto- Carrero, W.; Vandezande, P.; Vanermen, G.; Buekenhoudt, A. Selective Membrane Separation Technology for Biomass Valorization towards Bio- Aromatics. Sep. Purif. Technol. 2016, 178, 56-65. (219) Ormerod, D.; Lefevre, N.; Dorbec, M.; Eyskens, I.; Vloemans, P.; Duyssens, K.; Diez De La Torre, V.; Kaval, N.; Merkul, E.; Sergeyev, S.; et al. Potential of Homogeneous Pd Catalyst Separation by Ceramic Membranes. Application to Downstream and Continuous Flow Processes. Org. Process Res. Dev. 2016, 20 (5), 911–920. (220) Lüttschwager, N. O. B. Raman Spectroscopy of Conformational Rearrangements at Low Temperatures; Springer: Heidelberg, 2014.

Chapter 1 | 75

Chapter 2

Experimental conditions

2.1 Experimental

2.1.1 Materials

Propylphosphonic acid (3PA) and phenylphosphonic acid (PhPA) were purchased from Sigma-Aldrich. TiO2 P25 (80 % anatase/20 % rutile, BET surface of 50 m²/g, average particle size 21 nm) was purchased from Degussa Hulls and used without any pre-treatment.

2.1.2 Standard synthesis

The non-porous TiO2 P25 from Degussa Hulls was used as the TiO2 support. 1.0 g of TiO2 P25 was stirred for 4 h in a heated solution of PA. The concentration of this solution was varied between 0.003 and 0.150 M and also the oil bath temperature was varied between 90 and 130 °C. Furthermore, two solvents were used: water or toluene. The samples were washed on a filter with 3 × 30 mL of the solvent used during the modification. For the samples made with PhPA in toluene an extra washing step was needed because TGA and DRIFT measurements indicated that physisorbed PhPA was still present. Therefore, the samples were additionally washed with 3 × 30 mL of water. After the washing procedure, the samples were dried overnight in an oven at 60 °C.

For the samples made in solvent mixtures, the same procedure (0.100 M and 130 °C) was applied as described above. The solvent mixtures contained 5, 10, 25 or 50 % of water in toluene. After the modifications, the samples underwent a first washing step with toluene, followed by a drying step overnight at 60 °C. The next day, these samples underwent an additional washing step with water and were again dried overnight at 60 °C.

To verify some of the hypotheses in chapter 5, 1.0 g of TiO2 P25 was modified in water by stirring it for 4 h at room temperature in a 20 mL PA 0.100 M solution.

Chapter 2 | 79 After modification, the sample was washed on a filter with 3 × 30 mL of water, followed by drying overnight in an oven at 60 °C.

2.1.3 Adsorption experiments

For the adsorption process described in chapter 5, the same conditions were used as for the ITC measurement, with the exception that all quantities were multiplied by a factor of 30, to have enough powder for analysis. 0.150 g of TiO2

P25 was dispersed in 30.00 g of H2O. To evaluate the adsorbed amount during the ITC measurements, 15 test tubes containing TiO2 P25 in Milli-Q water were used in parallel. At t=0, 300 µL of a PA stock solution (5 - 30 mM for the various set of experiments) was injected in all tubes under stirring and at room temperature. After 45 minutes, the first sample was centrifuged. The obtained powder was dried without washing in an oven at 60 °C and the solution was analyzed with TOC. For the other samples, every 45 minutes an extra injection (300 µL of the PA solution (5 mM – 30 mM)) was repeated and the material (powder and supernatant) was analyzed. The number of successive injections varied between 1 and 15 injections, each representing a data point in the adsorption isotherm.

2.1.4 Structural names

Each sample received a structural name, e.g., PhPA0.100M90W denotes TiO2 modified with a 0.100 M solution of PhPA in water (W) at 90 °C, and

3PA0.100M130T refers to TiO2 modified with a 0.100 M solution of 3PA in toluene (T) at 130 °C. For the name of samples made in the solvent mixtures the percentage of water in toluene is given in their name, e.g., “3PA10%” is TiO2 modified in a mixture of 10 % water in toluene with 0.100 M 3PA at 130 °C.

Furthermore, in chapter 5, short names according to the conditions of the adsorption experiment are given to these samples. 3PA5mM1 represents the first

80 point of the adsorption isotherm with a stock solution of 5 mM propylphosphonic acid (3PA) on TiO2 P25, while PhPA30mM15 indicates the 15th adsorption point of the isotherm with 15 successive injection with 300 µL of

30 mM phenylphosphonic acid (PhPA) on TiO2 P25.

Chapter 2 | 81 2.2 Characterization techniques

Diffuse Reflectance Infrared Fourier Transform (DRIFT) measurements were performed on a Nicolet 6700 Fourier Transform IR spectrometer, equipped with an electromagnetic source in the mid-IR region (4000-400 cm-1). The detector was a liquid N2 cooled MCT-B detector. The resolution was set to 4 cm-1 and for each spectrum 200 scans were accumulated. The sample holder contained a 2 wt% diluted sample in KBr and was measured under a constant flow of dry air. All samples were measured with two backgrounds: a 100 % KBr background and a background of 2 wt% of unmodified TiO2 in KBr to subtract the TiO2 background from the spectrum of the modified materials. The latter background was used to get a clearer view on the peaks originating from the binding between TiO2 and the PA.1 In these spectra a positive peak was assigned to a newly formed bond upon functionalization with the PA, while a negative peak indicated the disappearing of a specific bond. The combination of a positive and negative peak indicated a shift in the peak position.

Thermogravimetric analyses (TGA) were recorded on a Mettler Toledo TGA/SDTA851e. The measurements were performed in a continuous flow of oxygen and the samples were heated from 30-600 °C with a heating rate of 10 °C/min. During the heating process the hydrocarbon part of the PA will undergo a thermo-oxidation.1 In the region 270-520 °C the carbon group is burnt for 3PA (300-550 °C for PhPA), but the phosphorus group remains at the surface.2 Therefore, the modification degree can be calculated from this weight loss by using the following formula:

# 푤푡%(푅) ∗ 푁퐴 푚표푑. 푑𝑔. ( 2) = 푛푚 푀푀(푅) ∗ 푆퐵퐸푇 in which wt%(R) is the weight loss of the organic group measured in TGA,

MM(R) is the molar mass of the carbon group, SBET is the surface area of the

82 unmodified TiO2 and NA is Avogadro’s constant. The number of organic groups is equal to the amount of PA as each PA only has one functional organic R group. With respect to the formation of the titanium propylphosphonate (TiPP) phase in which the number of phosphate groups/TiO2 is higher than in the case of surface grafting and TiO2 has been dissolved, it is useful to calculate the organic loading also in function of mass (PA)/mass(TiO2). This is calculated with the following formulas in the temperature range T1 to T2:

(푚푇1 − 푚푇2)(푅) ∗ 푀푀(푃퐴) 푚(푃퐴) = 푀푀(푅)

푚𝑔(푃퐴) 푚(푃퐴) 푂푟𝑔푎푛표푝ℎ표푠푝ℎ표푛𝑖푐 푎푐𝑖푑 푙표푎푑𝑖푛𝑔 ( ) = 𝑔(푇𝑖푂2) 푚푇1 − 푚(푃퐴)

(mT1-mT2)(R) corresponds with the weight loss of the R-group in TGA, MM(PA) is the molar mass of 3PA and MM(R) the molar mass of the R group. mT1 is the mass measured of the sample before the weight loss.

SEM EDX analyses were performed on a Quanta 250 FEG microscope. The powder was attached to a carbon tape and for each sample 3 points were measured. From the average values the modification degree was calculated. The formula is similar to the one of TGA, but, in this case, the %P and MM of P are used.

Phosphorus-31 solid-state CP/MAS (cross-polarization magic angle spinning) NMR spectra were acquired at ambient temperature on an Agilent VNMRS DirectDrive 400 MHz spectrometer (9.4 T wide bore magnet) equipped with a T3HX 3.2 mm VT probe dedicated for small sample volumes and high decoupling powers. Magic angle spinning (MAS) was performed at 10 kHz using ceramic zirconia rotors of 3.2 mm in diameter (22 μL rotors). The phosphorus chemical shift scale was calibrated with orthophosphoric acid (H3PO4) at 0 ppm. Other acquisition parameters used were: a spectral width of 50 kHz, a 90° pulse

Chapter 2 | 83 length of 2.5 s, a spin-lock field for CP of 100 kHz, a contact time for CP of 1.7 ms, an acquisition time of 20 ms, a recycle delay time of 20 s and 200-3000 accumulations. High power proton dipolar decoupling during the acquisition time was set at 100 kHz. The Hartmann-Hahn condition for CP was calibrated accurately on the samples themselves.

Nitrogen sorption measurements were recorded at -196 °C using a Quantachrome Quadrasorb SI automated gas sorption system. Before the measurements, the samples were degassed for 16 hours under high vacuum at a temperature of 60 °C. From the Brunauer-Emmet-Teller (BET) method, the BET area and C-constant were calculated in the P/P0 region between 0.05 – 0.30.

Water sorption measurements were performed using a Quantachrome iQ-c automated gas sorption system. The samples were degassed under high vacuum conditions at 150 °C for 16 hours before measuring the water isotherm at 22 °C.

Flow adsorption microcalorimetry measurements were carried out with the aid of a Microscal flow microcalorimeter at room temperature. In a typical experiment, the cell was filled with 80-100 mg of powder and evacuated overnight down to a pressure lower than 0.1 mmHg without heating. Afterwards, the sample was flushed with n-heptane flowing through the cell at a constant flow of 4.2 mL/h in order to attain the thermal equilibrium. After 2 h, the flow of the solvent was replaced by a stream of a 2 g/L solution of 1-butanol in n-heptane. The concentration of the stock solution had been chosen in such a way to prevent the formation of alcohol dimers and to ensure the displacement of n-heptane molecules from the hydrophilic surface domains by the adsorbing alcohol species building up a monolayer on the surface. The displacement phenomenon was monitored by the evolution of heat measured by thermistors. The so-obtained thermal peak was integrated to determine the integral enthalpy of displacement. Calibration was carried out by Joule heating using a calibration

84 probe integrated in the calorimetric cell.3,4 By using n-heptane as a solvent interacting with the surface sites only through van der Waals forces, the integral enthalpy of displacement could be attributed directly to the polar (Lewis acid- base) contribution to the surface enthalpy of the adsorbent. Therefore, the enthalpy value divided by the BET specific surface area was considered as an adequate measure of the polar character of the solid surface studied.

Isothermal titration calorimetry (ITC) measurements were performed on a differential TAM III microcalorimeter (precision ± 0.0001 °C) equipped with a titration unit. The enthalpy changes were measured in a heat flow mode. Before each measurement 5 mg of TiO2 P25 was placed in a reaction cell to which 1.00 g

Milli-Q H2O was added. Successive injections of aliquots of a given PA solution (10 µL during 10 s) resulted in thermal peaks. An equilibration time of 45 min was applied between two injections. A gold stirrer was rotating at 150 rpm to maintain the homogeneity of the solution in the cell. The reference cell was filled with the same volume of water. The enthalpy values were calculated from the peak area. This enthalpy is related to the contribution of adsorption of PA on

TiO2 and the dilution effect of PA in the solvent in the cell.5 Different concentrations of PA were used: 5, 10, 15 and 30 mM. 3PA and PhPA were used as solutes. Similar procedures, without the addition of TiO2, were applied to evaluate the dilution effect and correct the enthalpy values accordingly to obtain the value of adsorption of PA on TiO2.5 The repeatability of the measurements was checked thoroughly by carrying out multiple experiments, working with three different nanocalorimeters under the same conditions.

Total organic carbon (TOC) was analysed on a Shimadzu TOC-VCPH equipped with a palladium normal sense catalyst. Combustion of the injected samples to

CO2 takes place at 680 °C. The amount of produced CO2 was measured with a non-dispersive infrared detector. Next to the total amount of carbon (TC), the total amount of inorganic carbon (IC) was determined by injection of the sample

Chapter 2 | 85 in the vessel containing 25% H3PO4. Three sequential measurements were performed for both the TC and IC measurement. The TOC-value was calculated by subtracting the IC from the TC measurement. In chapter 5, after each injection (adsorption point in the isotherm), the amount of 3PA in the supernatant was measured by TOC and the adsorbed amount of PA was obtained as the difference between the initial (injected) and the equilibrium (remaining) concentration in solution.

86 2.3 Quantum chemical calculations

Calculations were performed under Periodic Boundary Conditions (PBC) with the Quantum Espresso (QE) software package6 using plane waves as basis sets and the Wu and Cohen (WC) modification of the PBE functional.7 Treatment of the core electrons was based on the projector augmented wave (PAW) method8 using the Troullier-Martins form.9 The 1s electrons were treated as core electrons for C and O atoms, whereas P and Ti atoms were treated with the valence configurations 3s23p3 and 3s23p64s23d2, respectively. A cutoff of 600 eV and a 3 × 3 × 1 k-grid were used.

A 3-layer anatase (101) slab with a 15 Å vacuum width was constructed using the cif2cell program10 and functionalized with two benzylphosphonic acid molecules, both in monodentate binding modes. Geometries were optimized until the residual forces were below 0.001 Ry/au; the atoms in the lowest layer of the slab were constrained to their initial bulk positions, while all other layers were allowed to relax.

Chapter 2 | 87 2.4 References

(1) Roevens, A.; Van Dijck, J. G.; Tassi, M.; D’Haen, J.; Carleer, R.; Adriaensens, P.; Blockhuys, F.; Meynen, V. Revealing the Influence of the Solvent in Combination with Temperature, Concentration and pH on the

Modification of TiO2 with 3PA. Mater. Chem. Phys. 2016, 184, 324–334. (2) McElwee, J.; Helmy, R.; Fadeev, A. Y. Thermal Stability of Organic Monolayers Chemically Grafted to Minerals. J. Colloid Interface Sci. 2005, 285 (2), 551–556. (3) Groszek, A. J.; Partyka, S. Measurements of Hydrophobic and Hydrophilic Surface Sites by Flow Microcalorimetry. Langmuir 1993, 9 (10), 2721–2725. (4) Zajac, J.; Groszek, A. J. Adsorption of C60 Fullerene from Its Toluene Solutions on Active Carbons: Application of Flow Microcalorimetry. Carbon N. Y. 1997, 35 (8), 1053–1060. (5) Prelot, B.; Ayed, I.; Marchandeau, F.; Zajac, J. On the Real Performance of Cation Exchange Resins in Wastewater Treatment under Conditions of Cation Competition: The Case of Heavy Metal Pollution. Environ. Sci. Pollut. Res. 2014, 21 (15), 9334–9343. (6) Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I.; et al. QUANTUM ESPRESSO: A Modular and Open-Source Software Project for Quantum Simulations of Materials. J. Phys. Condens. Matter 2009, 21 (39), 395502. (7) Wu, Z.; Cohen, R. E. More Accurate Generalized Gradient Approximation for Solids. Phys. Rev. B - Condens. Matter Mater. Phys. 2006, 73 (23), 2–7. (8) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50 (24), 17953–17979.

88 (9) Troullier, N.; Martins, J. L. Efficient Pseudopotentials for Plane-Wave Calculations. Phys. Rev. B 1991, 43 (3), 1993–2006. (10) Björkman, T. CIF2Cell: Generating Geometries for Electronic Structure Programs. Comput. Phys. Commun. 2011, 182 (5), 1183–1186.

Chapter 2 | 89

Chapter 3

Revealing the influence of the solvent in combination with temperature, concentration and pH on the modification of titanium dioxide with propylphosphonic acid.

TiO2 + 3PA

Materials chemistry and physics, 2016, 184, 324-334

3.1 Introduction

Organic-inorganic hybrid materials are of specific interest for applications that need the robustness (mechanical and chemical stability) of inorganic materials in combination with the diversity of interaction typical for organic molecules.1–5 The surface of metal oxides consists of M-OH groups, making it hydrophilic and limiting the diversity of possible interactions. Therefore, to enhance its applicability, organic functional groups are being grafted to its surface.

Various studies4,6–14 have shown that organophosphonic acids (PAs) have an excellent affinity for different transition metal oxides, in particular Al2O3, ZrO2 and TiO2. On the one hand, for surface grafting, three different binding modes (mono-, bi- and tridentate) are described for modification with PA. On the other, Guerrero et al.15 described a dissolution-precipitation process, forming a titanium phenylphosphonate (TiPhP) layer, when forced conditions were used, i.e. modification of TiO2 with a solution (water or toluene) of 0.11 M phenylphosphonic acid (PhPA) at 120 °C for one day. These metal phosphonates could also be prepared by heating tetravalent metals, forming a complex with HF, above 60 °C. The metals then slowly precipitated by the formation of a complex with the PAs.16,17 Alberti et al.17 published the crystal structures of such metal organophosphonates. Randon et al.7 modified TiO2 with a 0.1 M solution of PA in water at reflux temperature, suggesting the formation of a tridentate binding mode. However, other binding modes could not be excluded, due to the strong absorption band of TiO2 in the P-O stretch region. Following Gao et al.8, it was found that modifying ZrO2 with a low concentration (i.e. a 5-fold excess of PA relative to the number of moles needed for full surface coverage of the metal oxide) of octadecylphosphonic acid (18PA) in a boiling methanol/water mixture resulted in a single broad asymmetric peak in the 31P MAS NMR spectrum, due to a distribution of different binding types and/or binding sites.

Chapter 3 | 93 By assuming a similar trend for the reaction of 18PA with ZrO2 as with SiO2, they expected the binding to be monodentate rather than tridentate. They stated that despite the fact that peaks for P=O and P-OH were absent in the IR spectrum, a definite assignment of those bands is speculative due to the overlap of the P-O stretching bands with the metal oxide bands and the dependence on the degree of hydrogen bonding or metal binding. They found only a slight shift of the 31P resonance signal in the 31P MAS NMR spectrum for the modification of

TiO2 with 18PA, compared to the shift of the pure compound, indicating only a weak interaction between PA and TiO2. For the modification of Al2O3, Lassiaz et al.18 found that the binding mode changed with increasing concentration of octylphosphonic acid (8PA) when the reaction was performed in water at room temperature. However, the binding mode itself was not identified. Next to the modifications in polar solvents, Fadeev et al.11,13 worked with an apolar solvent

(toluene) and investigated the thermal degradation of 18PA modified TiO2 using TGA. Although FTIR was used to describe the order of the alkyl groups, no explanation was given concerning the Ti-O-P region in IR. When modifying Indium Tin Oxide (ITO) with PAs, the solvent has a significant influence. Chen et al.19 found that grafting of 18PA and PhPA on ITO in a nonpolar solvent resulted in the formation of a well-ordered SAM, while in polar solvents only a poorly packed SAM was formed. They attributed this to a diminishing of the intermolecular chain-chain interactions in the polar solvent, affecting the SAM formation. Unfortunately, they did not study the impact of the solvent on the binding type. Furthermore, no influence of reaction temperature and PA concentration was described. Taking the above into account, changes in the binding type and the organic surface loading may be expected upon changing the reaction conditions and environment.

Thus, despite the various studies on PA-modified TiO2, describing the modification reaction under different reaction conditions (solvent, temperature

94 or concentration) a systematic comparative study on the same material and with the same PA is lacking. This hampers the ability to choose reaction conditions that will lead to the envisaged surface properties (density, number of functional groups, presence or absence of phases such as phosphonates etc.) required for a specific application. As the appropriate synthesis conditions may differ for each application, a study is needed comparing different synthesis methods (solvent, temperature and concentration) and rationalizing their impact on the binding type and modification degree.

This chapter reports the results of such a systematic comparative study to elucidate the influence of solvent, temperature, concentration and pH on the surface characteristics obtained. More specifically, the aim in this chapter is a detailed investigation of the effect of the reaction conditions on the types of functionalization (surface grafting or phosphonate formation) and density of the organic loading. Moreover, it will show differences in impact of changes in concentration and temperature dependent on the applied solvent. The modified

TiO2 is characterized by TGA, DRIFT, DRIFT in function of temperature (ex-situ), 31P-CP/MAS NMR and SEM EDX. Finally, a correlation is shown between characteristic signals and signal patterns, visualized by a combination of various characterization methods upon changing the surface characteristics of the TiO2. Only by understanding the impact of the synthesis conditions on the surface properties of 3PA-modified TiO2, surface properties fitting the application can be designed in a controlled way.

Chapter 3 | 95 3.2 Results and discussion

In the first part, the effect of concentration, temperature and pH is discussed for the modification of TiO2 with 3PA in water. In the second part, we compare the impact of the solvent (water and toluene) on the functionalization of TiO2 with 3PA. Although from a practical point of view the use of water is cheaper and more environmentally friendly, it also creates important differences compared to toluene that might influence the modification. Water can interact with the surface via hydrogen bonding, possibly creating strong competitive adsorption. Moreover, the PAs are acidic and thus change the pH of the solution. In contrast, toluene can only result in weak physical adsorption and there is no effect of the pH. Of course, also an influence of differences in the interaction of the solvent with the organic group can be expected. Furthermore, we specifically chose 3PA as it has a much lower tendency to create self-assembled layers.

3.2.1 Modification in water

The concentration of 3PA was varied between 0.025 and 0.150 M for the modification in water. Two different synthesis temperatures were applied by heating the solution in an oil bath at 90 or at 130 °C. After washing and drying, the samples were extensively characterized with TGA, 31P-CP/MAS NMR and (ex-situ) DRIFT.

96 3.2.1.1 TGA

Figure 1: TGA curves (dotted lines) and DTG curves (full lines) of TiO2 modified with different concentrations of 3PA at (A) 90 and (B) 130 °C in water as measured under an O2-flow and heating rate of 10 °C/min.

Chapter 3 | 97 Table 1: 3PA loading as measured with TGA for TiO2 modified with different concentrations of 3PA in water at 90 and 130 °C. The loading is calculated in mass 3PA/TiO2 (mg/g) and in number of 3PA/area (#/nm²). Also the pH has been measured after the functionalization.

Name 270 – 400 °C 400 – 520 °C 270 – 520 °C pH

3PA/TiO2 3PA/TiO2 3PA/area (mg/g) (mg/g) (#/nm²)

3PA0.025M90W 8 0 1.0 2.5 3PA0.050M90W 10 0 1.3 2.3 3PA0.075M90W 10 2 1.4 2.1 3PA0.100M90W 12 2 1.7 2.0 3PA0.150M90W 13 4 2.1 2.1 3PA0.025M130W 11 0 1.5 2.5 3PA0.050M130W 13 0 1.6 2.2 3PA0.075M130W 14 5 2.4 2.1 3PA0.100M130W 15 6 2.6 1.9 3PA0.150M130W 16 9 3.1 1.8

Figure 1A shows the TGA (dotted lines) and DTG curves (full lines) of the modification at 90 °C in water for different 3PA concentrations measured in an oxygen atmosphere. All the curves show a similar progression with a weight loss between 270-400 °C, increasing with the concentration of 3PA used. In this region pure TiO2 P25 does not display a weight loss (Figure 2). When the sample is modified in a solution of 0.075 M 3PA, additional weight losses start occurring at 450 °C and in case of 0.150 M also at 490 °C. The total number of organic groups grafted onto the TiO2 support, i.e., the modification degree or the organic loading, is calculated from the weight losses above 270 °C in the TGA (Table 1).

98 The 3PA loading is calculated in mg 3PA/g TiO2 and in number of groups of 3PA/nm². The organic surface loading is split into two parts. The first part of the organic surface loading is calculated between 270-400 °C. The second part is calculated from the weight loss between 400-520 °C, only occurring for the higher concentrations. The surface loading increases in both temperature ranges with enhancing concentration of 3PA. Similar results are observed for a higher modification temperature (130 °C), but all organic loadings are higher at the same concentration and the weight loss observed at 450 °C is relatively more important (Figure 1B and Table 1). Also, the weight loss at 490 °C becomes visible at lower concentrations (3PA0.100M130W versus 3PA0.150M90W) and even becomes larger than the weight loss at 450 °C in the case of the 3PA0.150M90W. Moreover, the contribution of the weight losses at 450 and 490 °C increases for the highest concentrations, showing a clear impact of temperature besides concentration on the binding strengths/interactions upon functionalization of

TiO2.

Figure 2: TGA curve (dotted line) and DTG curve (full line) of pure TiO2 measured under an O2-flow and heating rate 10 °C/min.

Chapter 3 | 99 3.2.1.2 DRIFT

Figure 3: DRIFT spectra of the P-O region of TiO2 modified with different concentrations of 3PA at 90 °C in water, measured against a 2wt% TiO2 background. An offset has been used to visualize all spectra more clearly.

The P-O region in IR (1300-900 cm-1) is interesting to correlate changes in thermal stability as observed in TGA with differences in binding types. Figure 3 compares the samples prepared with different concentrations of 3PA. For the lowest concentrations at 90 °C one broad peak (1076 cm-1) is visible, which is composed of a superposition of at least four underlying peaks at 1101, 1076, 1041 and 1017 cm-1. But, for sample 3PA0.150M90W the intensity ratios of the peaks clearly change with more pronounced signals at 1041 and 1017 cm-1 and the appearance of a small extra resolved peak at 1155 cm-1. These changes in the DRIFT spectrum coincide with the additional weight loss observed in TGA at 490 °C. For the modification at 130 °C a similar trend is observed (Figure 4) but the differences are much clearer and more distinct. This follows the trend of the higher relative contribution of the weight loss at 490 °C in TGA for these samples. Note that the sample modified with the highest concentration at 130 °C does not show an increase of the peak at 1155 cm-1, but in that case the peak at 1101 cm-1 is becoming more intense. The increase of the peaks at 1155 and 1101 cm-1 seems to be correlated with the weight loss at 450 and 490 °C, respectively. This

100 suggests that the weight losses between 400 and 520 °C and the changes of peak intensities in the P-O region in DRIFT are correlated to each other. Additional assignments will be possible from the 31P-CP/MAS NMR (see 3.2.1.4).

Figure 4: DRIFT spectra of the P-O region of TiO2 modified with different concentrations of 3PA at 130 °C in water, measured against 2wt% TiO2 background. An offset has been used to visualize all spectra more clearly.

Figure 5: DRIFT spectra of the alkyl and Ti-OH region for TiO2 modified with different concentrations of 3PA at 90 °C in water and measured with 2wt%

TiO2 background in KBr.

Chapter 3 | 101 Furthermore, all samples display alkyl stretching vibrations between 3000-2800 cm-1. By raising the concentration of 3PA during the modification, those peaks logically increase in intensity with increasing loading amounts, while the quantity of free Ti-OH groups diminishes (Figure 5). Also, the amount of physically adsorbed water decreases. This means that the sample becomes more hydrophobic with an expanding organic surface loading. The same observations are made for the grafting of 3PA at 130 °C. Peculiarly, for sample 3PA0.150M130W the peaks associated to the C-H stretching vibrations seem to split into two peaks for every type of C-H stretching vibration (Figure 6A, sample (RT)). For a small percentage of the alkyl groups, the whole C-H vibration region seems to be shifted to lower wavenumbers: from 2964, 2940, 2905 and 2879 cm-1 to 2954, 2927, 2900 and 2867 cm-1. These divided peaks are not visible when a solution of 0.075 M (Figure 7A) or a lower temperature is used. From the literature it is known that if the wavenumbers are shifted to lower values, the groups are more ordered at the surface, indicating some sort of self-assembly/enhanced interaction even though the chain length is only three carbon atoms long.6,9,11,20–22

3.2.1.3 Ex-situ DRIFT

To clarify the TGA results, ex-situ DRIFT measurements have been performed. The DRIFT measurements were made after treatment of the samples at different temperatures. The TGA process is simulated by heating the samples at 10 °C/min in an oven. Samples 3PA0.075M130W and 3PA0.150M130W were measured (Figures 6 and 7). The samples were heated to the temperature coinciding with the beginning, the maximum and the end of every weight loss in the DTG curve. Sample 3PA0.075M130W was heated to the following temperatures: 250, 310, 400, 450 and 500 °C. For sample 3PA0.150M130W the lowest temperatures were the same (250, 310, 400, 450 °C), but due to the extra

102 weight loss in TGA, this sample was also heated to higher temperatures: 470, 490 and 520 °C.

Figure 6: Ex-situ DRIFT spectra of (A) the alkyl region and (B) the P-O region of sample 3PA0.150M130W measured against a 2wt% TiO2 background. The samples were measured after heat treatment at different temperatures. An offset has been used to visualize all spectra more clearly.

When sample 3PA0.075M130W is heated below 250 °C, it does not undergo any changes (Figures 7A and 7B). By heating the sample further, the peaks of the propyl groups start to become smaller (Figure 7A). At 400 °C these peaks remain only slightly visible. As long as these alkyl groups are present, the P-O region remains unaltered (Figure 7B). When the peaks of the propyl group are almost

Chapter 3 | 103 absent, the spectrum gradually changes in the P-O region up to 500 °C. This can be explained by the release of water due to the condensation of the phosphate group with the TiO2 surface.23 The correlation with the TGA results indicates that the largest part of the modified layer thermally oxidizes between 250 and 400 °C. The small number of organic groups that are still present at 400 °C together with the loss of water during phosphate condensation probably causes the extra weight loss in TGA at 450 °C.

Figure 7: Ex-situ DRIFT spectra of (A) the alkyl region and (B) the P-O region of the sample 3PA0.075M130W in function of the temperature, simulating the

TGA process (measured against a 2wt% TiO2 background). An offset has been used to visualize all spectra more clearly.

104 This measurement demonstrates that the phosphorus atom remains bound to the surface, confirming previous literature reports.13 Sample 3PA0.150M130W has two extra weight losses in TGA, one at 450 °C and a second at 490 °C. With ex-situ DRIFT (Figure 6) the same trend is observed in the P-O region as for sample 3PA0.075M130W (Figure 7). However, the organic groups clearly remain at the surface even at higher temperatures up to 470 °C. When looking more closely, it becomes clear that the red-shifted peaks at 2954, 2927, 2900 and 2867 cm-1 are still present at 400 °C, but are not found at 490 °C, while the peaks at 2964, 2940, 2905 and 2879 cm-1 have almost completely vanished at 400 °C and are thus selectively burned off, similar to sample 3PA0.075M130W. For sample 3PA0.150M130W the oxidation of the phosphonate starts at 450 °C (Figure 6B). Similar to the behavior of 3PA0.075M130W, the changes in the phosphate region are only initiated when the majority of the organic groups are removed. The removal of the more stable organic part seems to coincide with the weight loss that starts at 470 °C and postpones the weight loss associated with the phosphate formation (broad signal between 1200-1100 cm-1) to higher temperatures (490 °C). Thus, in the case of

3PA0.150M130W, two binding types of 3PAs with TiO2 are present with different thermal stabilities, of which one type is stable up to 470 °C, which is 70 °C higher than the other type of 3PA modification. The thermally less stable modification, is similar to those present in the sample 3PA0.075M130W.

3.2.1.4 31P-CP/MAS NMR

Souma et al.24 found that changing the reaction time also had an influence on the thermal stability. Unfortunately, although 31P-CP/MAS NMR was used to assign the different binding modes, no correlation was made with DRIFT. Beside Souma et al.24, also Guerrero et al.25 and Gao et al.8 characterized the modified materials with 31P MAS NMR and were able to assign different types of functionalization to different shifts in 31P MAS NMR. This makes 31P MAS NMR a powerful technique to combine with the TGA and DRIFT results.

Chapter 3 | 105

Figure 8: 31P-CP/MAS NMR spectra of TiO2 modified in water at 130 °C with 3PA in concentrations of (A) 0.025, (B) 0.075, (C) 0.100 and (D) 0.150 M.

106 The samples prepared at 130 °C in water with concentrations of 0.025, 0.075, 0.100 and 0.150 M have been analyzed with 31P-CP/MAS NMR to correlate differences in the phosphorus environment to the differences observed in DRIFT and TGA (Figure 8). Pure 3PA has a sharp resonance signal at 37 ppm (Figure 9). Upon binding to the surface, an upfield shift is observed for all concentrations resulting in a broad asymmetric peak around 28 ppm with a small shoulder around 33 ppm. It is clear that this broad signal originates from a non-uniform binding type with minor differences in binding state. This upfield shift indicates that the PA is grafted to the surface. The signals interfere with each other, which prevents unambiguous quantification. Similar shifts were found in literature for surface modification of dodecylphosphonic acid (12PA) on TiO2.25 Furthermore, Geldof et al.26 calculated 31P chemical shift of different alkyl-substituted PAs on anatase (101) surfaces and found two partly overlapping broad signals, i.e., a main resonance centered around 28 ppm and a downfield shoulder at about 33 ppm. The peak at 33 ppm is assigned to bidentate binding mode and the peak at 28 ppm corresponds to monodentate bond between the PA and the (101) facet of anatase. The simultaneous presence of the two binding modes is in line with the small differences in adsorption energy obtained from the energy calculations.

Figure 9: 31P-CP/MAS NMR spectrum of pure 3PA.

Chapter 3 | 107 In addition, with increasing concentration, a second chemical shift becomes visible much more upfield, at 8 ppm. The intensity of this second signal rises with increasing concentration of 3PA. A 31P chemical shift around 8 ppm has not been reported for mono-, bi- or tridentate bonded PAs on TiO2.26 The 31P chemical shift for surface grafting of alkyl-substituted PAs on anatase (101) and (001) surface are found between 26-37 ppm. The large difference in chemical shift confirms the presence of two binding types and two clearly different coordination environments for the phosphorus atom of 3PA in the material coinciding with the TGA and DRIFT measurements. It has to be noted that each binding type still can consist of different binding modes to the surface, all with a similar thermal stability and P environment. For the modification of TiO2 with PhPA, it has been reported that the upfield shift in 31P-CP/MAS NMR spectra is due to the formation of titanium phenylphosphonate.15,27 Furthermore, Gao et al.8 assigned a large upfield shift to the formation of octadecylphosphonates with

TiO2, Al2O3 and ZrO2. The formation of these phosphonates is due to a dissolution-precipitation process. When these phosphonates are present, thin plate-like structures of titanium propylphosphonates (TiPPs) can be observed in TEM images (Figure 10). These thin plate-like structures are typically larger than

TiO2 particles grafted with 3PA and contain a higher P/Ti ratio as measured via EDX. In Figure 11 a schematic representation of a metal organophosphonate is displayed.

108

Figure 10: TEM images of TiO2 modified with 3PA: (A) 3PA0.025M130W, (B) 3PA0.150M130W, (C) 3PA0.100M90T (with a scale of 50 nm).

By comparing the different characterization techniques, it is observed that the extra weight loss in TGA above 450 °C and changes in DRIFT seem to coincide with a strong contribution of the peak at 8 ppm in 31P-CP/MAS NMR. The shift in wavenumber of the alkyl groups to lower values, the changes in the peaks in the P-O-Ti region and the higher temperature weight loss in TGA can thus be correlated to the formation of TiPP. The increased thermal stability of the TiPP is correlated to its layered structure. This means that the TiPP has a higher thermal stability than the TiO2 surface grafted with 3PA. Nevertheless, the 31P-CP/MAS NMR is not able to distinguish differences between the weight loss observed at 450 and at 490 °C as, except for a difference in peak ratio, no other 31P environment can be observed in the 3PA0.150M130W sample compared to the 3PA0.100M130W material.

Chapter 3 | 109

Figure 11: Schematic representation of a metal organophosphonate.28

In contrast to TiPP formation via dissolution-precipitiation, when 3PA grafting occurs only a weight loss between 270-400 °C is observed in TGA and a broad peak around 28 ppm is present in 31P-CP/MAS NMR. Furthermore, in IR there is no shift of the wavenumber of the alkyl groups and in the P-O-Ti region, the peak at 1076 cm-1 is the dominant one. Thus, raising the concentration of 3PA in combination with a higher reaction temperature (130 °C) promotes the conversion of the grafted PA groups to TiPP. Nevertheless, a contribution of the thermally less stable binding type (270-400 °C) also remains present (about 64% of the bonded groups; Table 1) under these severe thermal conditions and high concentration, and can be assigned to the surface grafting of TiO2 with 3PA.

110 3.2.1.5 Influence of the pH

Thus, at low temperatures and concentrations, the TiO2 surface is only functionalized via grafting. Only under more severe reaction conditions (high temperatures and enhanced 3PA concentrations) the formation of TiPP will take place. Gao et al.8 assigned this dissolution-precipitation process to the low pH in a water/methanol mixture, high concentration, high temperature and long reaction times (1 week). To study the effect of the pH, an experiment was set up by adding HCl to the modification with the lowest concentration (denoted as 3PA0.025M90WpH2.1), to obtain the same pH as the modification with the highest concentration (3PA0.150M90W). In this case, also TiPPs (3 mg/g

PA/TiO2) are found but less than when using 3PA0.150M90W (4 mg/g

PA/TiO2), although in 3PA0.025M90W 0 mg/g PA/TiO2 is found (Figure 12 and Table 2). Furthermore, the surface grafting itself has been enhanced compared to 3PA0.025M90W and 3PA0.150M90W. Similar results are found for the experiment at 130 °C. Therefore, a low pH not only initiates in some part a dissolution-precipitation process, but it also induces a higher amount of surface grafting, as more surface hydroxyl groups are protonated, favoring surface grafting. Nevertheless, a high concentration seems to be additionally needed to obtain the high amount of TiPP observed in 3PA0.150M130W. On the other hand, NaOH was added in the synthesis with the highest concentration being 3PA0.150M90WpH2.5 to obtain the same pH as 3PA0.025M90W resulting in a decrease of TiPPs. The amount of surface grafting is, however, similar to the modification without NaOH. From the results it is clear that a combination of pH with a high temperature and concentration is required to obtain high TiPP phase formation although the pH itself can induce already a partial control.

Chapter 3 | 111

Figure 12: TGA curves (dotted lines) and DTG curves (full lines) of TiO2 modified with 3PA in water at different adjusted pH values, measured under an O2-flow and heating rate 10 °C/min.

Table 2: 3PA loading as measured with TGA for TiO2 modified with a concentration of 0.025 M 3PA in water at 90 and 130 °C; the pH is adjusted to the pH of the modification with a concentration of 0.150 M by adding HCl. On the other hand, the pH of the samples modified with a concentration of 0.150 M is adjusted by adding NaOH to the pH of the modification with a concentration of 0.025 M. The loading is calculated in mass 3PA/TiO2 (mg/g) and in number of 3PA/area (#/nm²).

Name Conc T pH 250-400 °C 400-520 °C 250-520 °C (M) (°C) PA/TiO2 PA/TiO2 PA/area (mg/g) (mg/g) (#/nm²)

3PA0.025M90WpH2.1 0.025 90 2.1 16 3 2.4

3PA0.150M90WpH2.5 0.150 90 2.5 14 1 1.8

3PA0.025M130WpH1.8 0.025 130 1.8 18 3 2.6

3PA0.150M130WpH2.5 0.150 130 2.5 16 4 2.5

112 3.2.2 Modification with toluene as the solvent

3.2.2.1 TGA

Besides modification in water, toluene is often described in literature as a solvent for the functionalization of TiO2 with PAs. To achieve this, low (0.0025 M)11 as well as high (0.1 M)7,15 concentrations of PAs were used. For the grafting of 3PA onto TiO2 in toluene, the concentration has been varied between 0.003 M and 0.100 M. Different concentrations have been used in toluene from those in water because similar surface loadings as compared to the reaction in water are already obtained when using much lower 3PA concentrations. This can be attributed to the absence of competing interactions of the surface with toluene, in contrast to the situation in water.

Figure 13: DTG curves of TiO2 modified at 90 °C in toluene with different concentrations of 3PA as measured under an O2-flow and heating rate of 10 °C/min.

Chapter 3 | 113 Table 3: Modification degree as measured with TGA for TiO2 modified with different concentrations of 3PA in toluene at 90 °C. The loading is calculated in mass 3PA/TiO2 (mg/g) and in number of 3PA/area (#/nm²). The last column indicates the wavenumber of the asymmetric CH2 stretch in DRIFT. (/ is not measured)

-1 Name T 3PA/TiO2 3PA/area 3PA/area ν(cm ) CH2 (°C) (mg/g) (#/nm²) (#/nm²) TGA SEM EDX

3PA0.003M90T 90 6 0.7 0.9±0.5 2942.42

3PA0.005M90T 90 9 1.1 0.8±0.8 2941.26

3PA0.008M90T 90 11 1.4 2.0±0.5 2940.25

3PA0.010M90T 90 14 1.8 / 2940.09

3PA0.011M90T 90 14 1.8 2.4±0.5 2939.97

3PA0.012M90T 90 16 2.0 2.7±0.6 2939.73

3PA0.013M90T 90 17 2.2 2.7±0.8 2939.51

3PA0.020M90T 90 22 2.8 4.0±0.5 2939.28

3PA0.023M90T 90 22 2.9 4.0±0.5 2939.25

3PA0.026M90T 90 24 3.0 4.7±0.5 2939.13

3PA0.050M90T 90 22 2.9 4.0±1.1 2939.32

3PA0.099M90T 90 26 3.2 4.3±0.6 2939.08

The DTG curves of the samples modified at 90 °C in toluene are presented in Figure 13 and the PA loading is listed in Table 3. In addition, the modification degree is also calculated from the amount of P measured with SEM EDX (Table 3). A difference of 0.1% P (detection accuracy) corresponds with a difference in modification degree of 0.4 groups/nm². Therefore, this technique is only used to validate the method of calculating the organic loading with TGA. The sample with the lowest concentration (3PA0.003M90T) has a single weight loss with a maximum at 370 °C. The weight loss increases with increasing

114 concentration of 3PA. At a concentration of 0.02-0.03 M, the maximum organic loading (modification degree of 3.2 PA groups/nm² as measured with TGA) is achieved (Table 3 and Figure 14); for the modification at 130 °C, a similar observation is made (Figure 15 and Table 4). No TiPP phases are observed under any of the conditions. Therefore, to exclude kinetics, the reaction time has been varied between 15 min and 24 h (Figure 16 and Figure 17). Even after 24 h there is no formation of the TiPP. Moreover, the modification is already completed after 15 min.

Figure 14: Modification degree (deduced from TGA) and wavenumber of the asymmetric CH2 stretch plotted versus the 3PA concentration for the modification reaction in toluene at 90 °C.

Chapter 3 | 115 Table 4: Modification degree measured with TGA for TiO2 modified with different concentrations of 3PA in toluene at 130 °C. The loading is calculated in mass PA/TiO2 (mg/g) and in number of 3PA/area (#/nm²).

Name Conc (M) T (°C) PA/TiO2 PA/area (mg/g) (#/nm²)

3PA0.003M130T 0.003 130 6 0.8

3PA0.005M130T 0.005 130 8 1.0

3PA0.013M130T 0.013 130 18 2.3

3PA0.026M130T 0.026 130 26 3.3

3PA0.100M130T 0.100 130 26 3.3

Figure 15: TGA curves (dotted lines) and DTG curves (full lines) of TiO2 modified at 130 °C in toluene with different concentrations of 3PA measured under an O2-flow and heating rate 10 °C/min.

116

Figure 16: DTG curves of TiO2 modified at 90 °C in toluene with a concentration of 0.02 M 3PA with different reaction times (15 min – 24 h) measured under O2-flow and heating rate 10 °C/min.

Figure 17: Modification degree measured with TGA for TiO2 modified with different reaction times in toluene at 90 °C with a concentration of 0.02 M. The modification degree is calculated in groups/nm².

Chapter 3 | 117 Peculiarly, in the case of toluene at both temperatures (90 and 130 °C), the maximum of the weight loss in TGA shifts to lower temperatures, opposite to what was observed with increasing 3PA concentrations in water. This shift occurs gradually until a concentration of 0.02-0.03 M 3PA is reached. For the lowest 3PA concentration in water, the maximum weight loss was already observed at 310 °C (Figure 1); in toluene, the weight loss starts at 370 °C and decreases to 310 °C with increasing 3PA concentration (Figure 13). The shift to lower temperatures of the weight loss between 250-400 °C seems to be originating from the burn-off of surface grafting of 3PA on TiO2. It has been stated that the modification of TiO2 with PA in toluene was random, but became more ordered with an increase in the number of surface groups in the organic layer.11 Hence, this interaction between groups on the surface might be at the origin. Chen et al.19 described that domains of SAMs were formed for the modification of PA on ITO in water. This would suggest that in the presence of water local ordering in the modification would take place even at low modification degrees explaining a uniform thermal oxidation at 310 °C at lower loadings of TiO2 in case of water as a solvent. The totally different thermal behavior for modification in water and toluene could thus reflect the difference in surface properties and binding types as the result of the solvent effect (see Chapter 4).

118 3.2.2.2 DRIFT

Figure 18: DRIFT spectra of the alkyl region for TiO2 modified with different concentration of 3PA at 90 °C in toluene and measured against a 2wt% TiO2 background in KBr. An offset has been used to visualize all spectra more clearly.

The peak intensities of the CH2 and CH3 stretching vibrations logically increase with the loading of organic groups (Figure 18), so more 3PA molecules are attached to TiO2 leading to a more hydrophobic sample. When looking more closely at the wavenumbers of the CH2 groups (Table 3), it can be concluded that the wavenumbers slightly decrease with increasing loading, revealing that the organic groups are indeed becoming more ordered.6,9,11,20–22 Plotting these values in the same graph as the modification degree (Figure 14), reveals that the curves of surface loading and wavenumber of CH2 stretching vibration are strongly correlated.

Chapter 3 | 119

Figure 19: DRIFT spectra (measured with TiO2 background) of the P-O region of TiO2 modified with different concentrations of 3PA at 90 °C in toluene. An offset has been used to visualize all spectra more clearly.

When looking at the P-O spectral region of sample 3PA0.003M90T, a peak at 1245 cm-1 is observed (Figure 19). This peak shifts to higher wavenumbers with increasing concentration. This shift is not observed when working in water. Furthermore, for the lowest concentrations, a broad band is seen composed of a superposition of at least four signals, at 1110, 1045, 1077 and 1014 cm-1. These peaks become more intense with increasing 3PA concentration. Additionally, the intensity ratio between these peaks is changing; the peak intensity at 1077 cm-1 is the highest for concentrations between 0.005 M and 0.012 M 3PA. A further increase in concentration will result in supplementary changes in the intensity ratio of the peaks to a pattern similar to the one observed at high temperature and 3PA concentrations in water. In the case of toluene, the temperature has no influence on the DRIFT spectra (Figure 19 and 20). Moreover, in the case of toluene, the observed changes in the IR spectra do not coincide with a weight loss at higher temperatures in TGA nor with a clear red shift in the CH2 and CH3 signals (Figure 18), as found for water (Figure 6). The reason behind these

120 findings is still unclear. Therefore, it can be concluded that in the case of toluene as a solvent, only the concentration of 3PA has an influence on the modification degree and the binding type, while the increase in temperature does not induce any changes in the binding type or surface loading. This is an indication for a difference in binding mechanism, which could be due to several reasons, such as competitive adsorption of water, proton release of the acid, solubility of the 3PA, dielectric constant etc. in the case of the different solvents. Moreover, it is clear that the obtained modified surfaces have a totally different thermal stability, irrespective of similarities in the IR patterns.

Figure 20: DRIFT spectra of the P-O region for TiO2 modified with different concentrations of 3PA at 130 °C in toluene and measured against a 2wt% TiO2 background in KBr (offset).

3.2.2.3 31P/CP MAS NMR

To study the discrepancy between observations in modification in toluene with respect to DRIFT (with similar peak patterns as in water) and TGA (with strongly divergent behavior), 31P-CP/MAS NMR was performed to clarify the binding

Chapter 3 | 121 types. The samples prepared at 90 °C are characterized via 31P/CP MAS NMR to evaluate the changes in phosphorus environment induced by elevating the 3PA concentration (Figure 21). For the lowest 3PA concentration only a broad signal is observed around 28 ppm. The broadening can be explained by the presence of a distribution of slightly different chemical environments for the grafted PAs. With increasing 3PA concentration, the resonance signal at 28 ppm becomes sharper, indicating a progression towards a more uniform way of binding, in good agreement with the enhanced ordering observed from TGA and DRIFT data and different from what can be observed in case of water (Figure 8) where the 28 ppm signal stays broad. For the sample with the highest loading, the 28 ppm peak changes further and a small peak appears at 8 ppm. This is in agreement with DRIFT, showing (minor) changes in the P-O-Ti spectral region, which were also correlated with the 8 ppm chemical shift in the case of water. Nevertheless, this does not coincide with an additional weight loss in TGA.

It was previously shown that high concentrations of 3PA in water as solvent are needed to form TiPP. Of course, in the case of toluene, small amounts of water will be released in the condensation reaction between 3PA and the TiO2 surface hydroxyl groups, and, as the reactions were not performed under anhydrous conditions, trace amounts of water were always present. Therefore, these traces of water could be responsible for the small signal at 8 ppm. These observations indicate that the presence of water, low pH and high concentration of PA is essential in the formation of TiPP.

Moreover, comparing toluene as solvent with water, it is clear from 31P-CP/MAS NMR, TGA and even DRIFT in the C-H vibration region, that the local binding types are different in both solvents. Moreover, in the case of toluene, a maximum surface coverage can be reached with much lower concentrations of PA. A summary of the most important influences of solvent, temperature and concentration is presented in Scheme 1.

122

31 Figure 21: P-CP/MAS NMR spectra of TiO2 modified with 3PA at 90 °C in toluene at concentrations of (A) 0.005, (B) 0.012 and (C) 0.100 M.

Chapter 3 | 123

Scheme 1: A summary of the influence of solvent, temperature and

concentration on the modification of TiO2 with 3PA.

124 3.3 Conclusions

In this chapter, it is demonstrated that solvent, temperature and 3PA concentration have a substantial impact on the degree and type of functionalization. Furthermore, a correlation was made between the characteristic signals in different analytical methods showing coinciding changes as well as important discrepancies that might induce misinterpretations if only one of the techniques would be applied for analysis.

Regarding the reaction in water, an increase of concentration in combination with temperature induces the formation of TiPPs with a higher thermal stability. However, surface grafting always remains. The pH of the solution is a driving force to dissolve the TiO2 and precipitate it as TiPPs. Nevertheless, by working at similar pH with low concentrations of 3PA, less TiPP is formed and more surface grafting is achieved. Therefore, both pH and concentrations are important in the TiPP formation. In comparison, in toluene, organic ordered layers with a high surface loading are already formed at much lower 3PA concentrations than in water. Moreover, raising the reaction temperature during modification does not influence the surface properties of the grafted layer, while it has a considerable influence when water is used as a solvent both in grafting and in TiPP formation. In contrast to the modification in water, increasing the 3PA concentration in toluene results in a decrease in thermal stability coinciding with an indication of an increase in uniformity of the binding. Furthermore, in the case of toluene, a peak with a chemical shift at 8 ppm in 31P-CP/MAS NMR is only present in small amounts, which proves the need of water and high concentrations of PA for the formation of TiPP. Both solvents (water and toluene) clearly result in a different binding mechanisms and significant differences in surface properties. By revealing these influences of 3PA concentration in different solvents, concentrations and temperature, it becomes clear that

Chapter 3 | 125 materials with different surface properties can be prepared in a controlled way by choosing the most appropriate solvent, concentration, pH and temperature to adjust it to a specific application.

126 3.4 References

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C18H37PO(OH)2. Langmuir 2002, 18 (23), 8924–8928. (12) Mutin, P. H.; Guerrero, G.; Vioux, A. Organic-Inorganic Hybrid Materials Based on Organophosphorus Coupling Molecules: From Metal Phosphonates to Surface Modification of Oxides. Comptes Rendus Chim. 2003, 6 (8–10), 1153–1164. (13) McElwee, J.; Helmy, R.; Fadeev, A. Y. Thermal Stability of Organic Monolayers Chemically Grafted to Minerals. J. Colloid Interface Sci. 2005, 285 (2), 551–556. (14) Guerrero, G.; Alauzun, J. G.; Granier, M.; Laurencin, D.; Mutin, P. H. Phosphonate Coupling Molecules for the Control of Surface/interface Properties and the Synthesis of Nanomaterials. Dalton Trans. 2013, 42 (35), 12569–12585. (15) Guerrero, G.; Mutin, P. H.; Vioux, A. Anchoring of Phosphonate and Phosphinate Coupling Molecules on Titania Particles. Chem. Mater. 2001, 13 (11), 4367–4373. (16) Clearfield, A. Recent Advances in Metal Phosphonate Chemistry. Curr. Opin. Solid State Mater. Sci. 1996, 1 (2), 268–278. (17) Alberti, G.; Costantino, U.; Allulli, S.; Tomassini, N. Crystalline

Zr(R-PO3)2 and Zr(R-OPO3)2 Compounds (R = Organic Radical). J. Inorg. Nucl. Chem. 1978, 40 (6), 1113–1117.

128 (18) Lassiaz, S.; Galarneau, A.; Trens, P.; Labarre, D.; Mutin, H.; Brunel, D. Organo-Lined Alumina Surface from Covalent Attachment of Alkylphosphonate Chains in Aqueous Solution. New J. Chem. 2010, 34 (7), 1424. (19) Chen, X.; Luais, E.; Darwish, N.; Ciampi, S.; Thordarson, P.; Gooding, J. J. Studies on the Effect of Solvents on Self-Assembled Monolayers Formed from Organophosphonic Acids on Indium Tin Oxide. Langmuir 2012, 28 (25), 9487–9495. (20) Cohen, S. R.; Naaman, R.; Sagiv, J. Thermally Induced Disorder in Organized Organic Monolayers on Solid Substrates. J. Phys. Chem. 1986, 90 (14), 3054–3056. (21) Parikh, A. N.; Allara, D. L.; Azouz, I. Ben; Rondelez, F. An Intrinsic Relationship between Molecular Structure in Self-Assembled N- Alkylsiloxane Monolayers and Deposition Temperature. J. Phys. Chem. 1994, 98 (31), 7577–7590. (22) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. Carbon-Hydrogen Stretching Modes and the Structure of N-Alkyl Chains. 1. Long, Disordered Chains. J. Phys. Chem. 1982, 86 (26), 5145–5150. (23) Kor̋ osi,̈ L.; Papp, S.; Dékány, I. A Layered Titanium Phosphate

Ti2O3(H2PO4)2·2H2O with Rectangular Morphology: Synthesis, Structure, and Cysteamine Intercalation. Chem. Mater. 2010, 22 (15), 4356–4363. (24) Souma, H.; Chiba, R.; Hayashi, S. Solid-State NMR Study of Titanium Dioxide Nanoparticles Surface-Modified by Alkylphosphonic Acids. Bull. Chem. Soc. Jpn. 2011, 84 (11), 1267–1275. (25) Brodard-Severac, F.; Guerrero, G.; Maquet, J.; Florian, P.; Gervais, C.; Mutin, P. H. High-Field 17O MAS NMR Investigation of Phosphonic Acid Monolayers on Titania. Chem. Mater. 2008, 20 (16), 5191–5196.

Chapter 3 | 129 (26) Geldof, D.; Tassi, M.; Carleer, R.; Adriaensens, P.; Roevens, A.; Meynen, V.; Blockhuys, F. Binding Modes of Phosphonic Acid Derivatives

Adsorbed on TiO2 Surfaces: Assignments of Experimental IR and NMR Spectra Based on DFT/PBC Calculations. Surf. Sci. 2017, 655, 31–38. (27) Tassi, M.; Roevens, A.; Reekmans, G.; Vanhamel, M.; Meynen, V.; D’Haen, J.; Adriaensens, P.; Carleer, R. A Detailed Investigation of the

Microwave Assisted Phenylphosphonic Acid Modification of P25 TiO2. Adv. Powder Technol. 2017, 28 (1), 236–243. (28) Tanaka, H.; Masuda, K.; Hino, R. Formation and Structure of Titanium Alkyl Phosphates. 2002, 337, 331–337.

130 Chapter 4

Aqueous or solvent based surface modification: the influence of the combination solvent - organic functional group on the surface characteristics of titanium dioxide grafted with organophosphonic acids.

Applied Surface Science, 2017, 416, 716–724

4.1 Introduction

It is possible to carry out surface modification with PAs in both aqueous1–4 and organic environment (e.g. toluene,2,5 THF,4,6–8 methanol/water mixtures,9,10 butanol,11 ethanol,11 dichloromethane12). From an application point of view, the use of water based methods has an economical, health and ecological benefit. However, it is important to know its impact on structural control as the interaction of water with the surface is clearly different from that of any other solvent.

When grafting with PAs, it is important to create the desired surface properties and thus find the balance and interplay between surface, solute, and solvent. Ferreira et al.6 used equation (1) describing the enthalpy change for self-assembly of aliphatic PAs on TiO2/ZrO2 in THF to analyze the various contributions. In the equation ΔHr is the reaction enthalpy which is equal to the heat produced during the reaction. D is the energy of the bond between the solid and the SAM molecules, ΔHsol is the enthalpy of dissolution, ΔHdil is the enthalpy of dilution,

ES is the surface energy of the bare solid and ESAM is the surface energy of the SAM. The first contribution is related to the binding between self-assembled molecules and the surface. A second contribution is due to the solvent–solute interaction. The last contribution is related to the change in surface energy. They reported a changing reaction enthalpy from exothermic to endothermic by increasing the chain length (C1 to C18) of the PA. But, Ferreira et al.6 did not describe the influence of the type of organic group changing from aliphatic to aromatic, nor the influence of the use of aqueous versus organic solvents.

∆퐻푟 = −퐷 − (∆퐸푠표푙 + ∆퐻푑푖푙) − (퐸푆 − 퐸푆퐴푀) (1)

In chapter 3, the impact of the synthesis conditions on the solute-surface interaction was investigated by altering the propylphosphonic acid (3PA)

Chapter 4 | 133 concentration, pH and reaction temperature in water and toluene.13 It was concluded that the temperature and PA concentration in water have an influence on the modification type. In toluene, lower concentrations were needed to obtain high modification degrees. Due to these vast differences between both solvents, a clear interest in the interactions of the solvent with both solute and surface originated. Furthermore, the modified surfaces were studied with 31P-CP/MAS NMR, TGA and DRIFT, and a correlation between the characteristic signals was made.

The influence of solvents on the modification of Indium Tin Oxide (ITO) with PA (PhPA and 18PA) has already been described.4 Chen et al.4 observed that in polar solvents poorly packed self-assembled monolayers (SAMs) are formed, because the solvent adsorbs at the surface occupying binding sites for the PA. Displacement of solvent by PA gets harder for solvents with a higher polarity. However, although a thorough description of the solvent–surface interaction has been made, the role of the interaction between solvent and solute has not been investigated yet. Moreover, the impact on the properties of the resulting monolayer formation and hence their sorption behavior, were not described. The effect of the solvent on the grafting of 18PA on Calcium Hydroxyapatite (CaHAP) was also discussed.11 D’Andrea et al.11 modified CaHAP with aliphatic PAs in toluene, ethanol, 1-butanol and THF at two temperatures. The grafting reaction is the fastest in toluene and ethanol. It was argued that the different interactions of the solvent (solvent–solute and solvent–surface) caused changes in the kinetics of the formation of the grafted layer. In a good solvent, there were strong solvent–solute interactions, which diminished the solute adsorption and the binding onto the surface. In a poor solvent, the solute–surface interactions were stronger. Nevertheless, as only one type of organic functional group was applied, the impact of the organic functional group on the surface-solvent-solute interactions could not be assessed. Furthermore, also in this case, the influence

134 of the solvent on the final surface properties was not studied. Chen et al.4 and D’Andrea et al.11, both used PAs with long aliphatic chains to create hydrophobicity and assure self-assembly. However, many applications can also benefit from short alkyl groups, as e.g. membranes functionalized with small organic groups show interesting properties for water filtration.14

To study the benefit of replacing solvent based surface grafting by aqueous methods, a thorough analysis of solvent–solute–surface interactions is needed. These interactions may be -bonding, hydrogen bonding or/and van der Waals forces. In this chapter, the influence of water versus solvent (toluene) and its mixtures on the functionalization of TiO2 with PAs possessing different functional groups (propyl- and phenylphosphonic acid) is investigated. The impact of the combination of solvent and solute on the loading capacity and distribution of functional groups on the surface is unraveled. Via sorption measurements and flow calorimetry (heat of interaction) the effect of the solvent on the distribution of grafted groups is revealed.

Chapter 4 | 135 4.2 Results and discussion

4.2.1 Solute–solvent interaction: modification with 3PA and PhPA in toluene

In the DRIFT spectra negative peaks between 3600 and 3100 cm-1 and at 1630 cm-1 appear, indicating a decrease in hydrogen bonding and the amount of adsorbed water for TiO2 modified with PhPA in toluene (T) (Figure 1). Furthermore, negative peaks are also observed between 3800 and 3600 cm-1, due to a decrease of free hydroxyl groups. This implies the binding between the PhPA and the

TiO2. Typical peaks for PhPA are present at 3064, 1597, 1486, 1438 and 1147 cm-1, and at 1060 and 1040 cm-1 signals respectively related to the stretching vibrations of P-O-Ti and P-O-Hasym are visible (Table 1). The peaks related to the stretching

-1 vibration of PO3 are visible at 1023 and 998 cm . These value correlate well with calculated vibrational frequencies.15 Geldof et al.15 found the P-C stretch vibration of modified PhPA on an anatase (101) surface around 1120 cm-1. Furthermore, they reported the P-O-Ti stretch vibration at 1061 cm-1 and the asymmetric and symmetric P-O-H stretch vibration respectively at 1046 and 934 cm-1 for monodentate bonded PhPA on an anatase (101) surface. For bidentate bonded

PhPA on that surface they found the stretch vibrations of PO3 at 1033, 1001 and 945 cm-1. The typical P=O stretch vibration around 1260 cm-1 is only observed for isolated molecules: irrespective of the binding mode, adsorption on the TiO2 surface leads to a considerable shift of the P=O stretch to a lower wavenumber, and, consequently, the disappearance of this band cannot be used to assigning a particular binding mode. However, much attention should be paid when trying to assign the different binding modes from experimental IR spectra, because all bands of a solid sample are broadened. Furthermore, assignment of the binding modes is hampered due to the overlap of the P-O stretching region and the bands in the native TiO2 spectrum.

136 Table 1: Calculated and experimental vibrational frequencies (in cm-1) of isolated PPA and its adsorption complexes on the anatase (101) surface in the different binding modes.

Calculated15 Experimental PPA 101-Ma 101-Bb 001-Bc 001-Td This work ν (P-C) 1123 1124 1117 1136 1100 1147 ν (P=O) 1264 1101 ν (P-O-Ti) 1061 1060 ν (P-O-Hasym) 870 1046 1040 ν (P-O-Hsym) 821 934 ν (PO3) 1033 971 1023 1001 955 998 945 915 ν (P-O-Tisym) 928 ν (P-O-Tiasym) 902 a monodentate binding mode on anatase (101) surface b bidentate binding mode on anatase (101) surface c bidentate binding mode on anatase (001) surface d tridentate binding mode on anatase (001) surface

Figure 1: DRIFT spectra of TiO2 modified with 0.100 M PhPA in toluene (T) or water (W) measured with a 2 wt% TiO2 in KBr background. All spectra have an equal offset.

Chapter 4 | 137 In analogy with the modification with PhPA, TiO2 was also grafted with 3PA in toluene. A similar decrease in the signals originating from OH-groups, hydrogen bonding interactions and water is visible. The typical alkyl stretching vibrations of 3PA are assigned in the DRIFT spectra (Figure 2) to the peaks at 2967, 2939, 2911, 2879 cm-1 and the bending vibrations are found in the region between 1450 and 1340 cm-1.16 The P-C bending vibration is visible at 1461 cm-1 and the P=O stretch vibration might appear at 1246 cm-1. However, calculated results show that the P=O stretch shifts to lower wavenumbers.15 In the P-OTi region a superposition of four peaks is observed: 1110, 1082, 1045 and 1012 cm-1. These indicate different binding types onto the surface.13 By increasing the temperature from 90 to 120 °C the peak at 1012 cm-1 becomes more intense, while the peak at 1082 cm-1 diminishes. In the study of Geldof et al.15 the wavenumbers of 1PA and PhPA grafted on the anatase (101) and (001) surface are described. They reported only small shifts of wavenumbers for P-O stretch vibrations for different PAs, as visible in our experimental results.

Figure 2: DRIFT spectra of TiO2 modified with 0.100 M 3PA in toluene (T) or water (W) measured with a 2 wt% TiO2 in KBr background. All spectra have an equal offset.

138

Figure 3: TGA (dotted lines) and DTG (full lines) curves of TiO2 modified with PhPA in water and toluene at different temperatures measured under an

O2-flow and 10 °C/min heating rate.

In TGA/DTG a single, broad mass loss is visible between 250 and 450 °C for the modification of TiO2 with 3PA and two not fully resolved mass losses between 300 and 500 °C when PhPA was used (Figures 3 and 4). The modification degree was calculated from these mass losses (250-450 °C for 3PA and 300-500 °C for PhPA). It is known that the hydrocarbon part of the PA will undergo a thermo-oxidation while the phosphorus atom remains bonded to the surface.5 For both functional groups, no impact of temperature was observed. The loading of organic grafting is much higher for the modification of TiO2 with 3PA (3.2 groups/nm²) than with PhPA (1.3 groups/nm²) in toluene (Table 2).

Chapter 4 | 139

Figure 4: TGA (dotted lines) and DTG (full lines) curves of TiO2 modified with 3PA in water and toluene at different temperatures, measured under an

O2-flow and heated 10 °C/min.

Table 2: Overview of modification degrees for the modification of TiO2 with a 0.100 M concentration of 3PA or PhPA at different temperatures in water or toluene (modification degree expressed in groups/nm²).

PhPA 3PA

Water PhPA90W PhPA120W PhPA130W 3PA90W 3PA120W 3PA130W

2.0 2.8 3.1 1.7 1.7 2.5

Toluene PhPA90T PhPA120T PhPA130T 3PA90T 3PA120T 3PA130T

1.3 1.4 1.3 3.2 3.3 3.3

This suggests an interaction between solute and solvent, as these are the only variables. PhPA and toluene contain both an aromatic ring, which may provide an additional  type contribution to the van der Waals one to the solute– solvent interaction. This likely induces a competition between binding PhPA to the surface and its behavior in the solvent, thereby reducing its tendency to adsorb on the surface. On the contrary, 3PA contains an aliphatic chain and thus

140 interacts less strongly with the solvent. Consequently, much less hindrance for the deposition of 3PA units in toluene is observed, which results in a modification degree about 2.5 times higher. Important to note, the cross section of a PA group is 0.24 nm², so a theoretically full monolayer has a modification degree of 4.2 groups/nm².11 However, also the number of hydroxyl groups is a determining factor for the grafting density. A PA can bind to multiple hydroxyl groups, depending on the binding modes of the PA (mono, bi and tridentate).15 Furthermore, not all hydroxyl groups are necessarily available due to steric effects. Therefore, it is very unlikely to achieve 4.2 groups/nm². Even when longer reaction times (24 h) are used, no increase in modification degree has been observed (chapter 3).13 In the case of a grafting density of 3.2 groups/nm², the surface coverage does not exceed 75 % of that of theoretical a full monolayer.

4.2.2 Solvent–surface interaction: modification with 3PA and PhPA in water

When the modification of TiO2 with PhPA is performed in water, typical signals for PhPA appear in the DRIFT spectra (Figure 1).9,17 Each spectrum has clear peaks in the P-OTi stretch region as well as at 1085 and 1030 cm-1, and a small shoulder observed at 1045 cm-1. The presence of these bands (1085 and 1030 cm-1) indicates the presence of titanium phenylphosphonate (TiPhP), next to surface grafted groups. By elevating the reaction temperature in water, the TiPhP signal increases further. These changes coincide with the appearance of an extra mass loss in DTG at 500 °C (Figure 3). This mass loss at 500 °C rises with increasing reaction temperature. When grafting TiO2 with 3PA in water, the typical vibrations of 3PA are present in DRIFT (Figure 2). Also here, differences in the P-OTi region are visible. The sample modified at 90 °C has only one broad peak at 1077 cm-1 with small shoulders. By raising the temperature to 130 °C, these

Chapter 4 | 141 small peaks grow and the ratio between the peaks changes. In DTG, an extra mass loss is found between 400 and 520 °C next to the broad asymmetric mass loss with a maximum at 310 °C (Figure 4), indicating titanium propylphosphonate (TiPP) formation (chapter 3).13 For this sample, a modification degree of 1.7 up to 2.5 groups/nm² is calculated from the mass losses between 250-520 °C. The modification degree of the PhPA in water is calculated between 300-550 °C. The grafting density increases with the temperature for both PA modifications (Table 2), irrespective of the organic group present.

When comparing results in water versus toluene it is clear that the modification degree obtained for 3PA in toluene (3.2 groups/nm²) is higher than in water (1.7 groups/nm²) (Table 2). Due to the ability of water to form hydrogen bonds with the surface, PAs compete against water molecules to adsorb on the TiO2 surface. Before a PA can bind to the surface, it should first displace water and form hydrogen bonds with the surface hydroxyl groups. By elevating the reaction temperature, the solvent can desorb more easily from the surface, enhancing grafting. In toluene this competition at the surface is not present or not so strong and no such effects are observed. Moreover, in water the dissolution-precipitation mechanism can take place due to the presence of acidic protons of PA, which is much less likely in toluene (although an adsorbed layer of water might be present, but this seems insufficient). This dissolution of TiO2 is enhanced at elevated temperatures, increasing the presence of the titanium organophosphonate phases. Both effects can, therefore, influence surface grafted amounts.

In contrast to 3PA, modification with PhPA in water results in a higher surface loading of 2.0 up to 3.1 groups/nm². The higher modification degree in water for PhPA may be caused by the interaction between toluene and PhPA. Furthermore, the phenyl-phenyl interactions between the phenyl groups

142 adsorbed on the surface may induce a higher modification degree in water than in toluene. These interactions should be of the …H2O… type18 rather than direct … interactions considering the large inter-ring distances between the aromatic rings on the surface. To quantify these ideas, density functional theory (DFT) calculations were performed in which the stability of a pair of monodentate adsorbed PAs containing aromatic rings was evaluated when a single benzene ring or one or more water molecules was/were placed in between these rings. Figure 5A shows the resulting geometry in the former case: introduction of the benzene ring leads to a stabilization of 36.73 kJ/mol due to - interactions. Figure 5B shows the resulting geometry when a single water molecule is placed between the rings: a stabilizing OH… interaction is formed with the benzene ring on the right with an associated stabilization of 22.88 kJ/mol. Even though the latter number is smaller than the one obtained with the benzene molecule, it is clear that more water molecules can be accommodated between the rings, leading to further stabilization. Indeed, as can be seen in Figure 5C, a second water molecule hydrogen bonds to the first one but does not break the existing OH… interaction; this second interaction raises the stabilization to 70.28 kJ/mol, which is considerably greater than the value obtained with benzene. A third water molecule (Figure 5D) hydrogen bonds to the second one and forms an OH…O interaction with one of the free hydroxyl groups of the acid; the total stabilization has now become 119.76 kJ/mol. These three water molecules form a bridge between the two PAs. A fourth and a fifth water molecules were added but since these position themselves outside of the space between the benzene rings, they were not considered. The van der Waals forces between the aliphatic chains of 3PA and the solvent are less strong than the OH… interaction; therefore, higher organic loadings are found for PhPA in water than for 3PA.

Chapter 4 | 143

Figure 5: Optimized calculated geometries of the system with (A) one benzene ring, and (B) one, (C) two and (D) three water molecules.

144 4.2.3 Effect of water and toluene on the reaction mechanism

Figure 6: DRIFT spectra of TiO2 modified with 0.100 M 3PA in mixtures with different water percentages in toluene at 130 °C (measured with a 2 wt% TiO2 in KBr background). All spectra have an equal offset.

For the modification with 3PA, the formation of TiPP occurs by using water. But, the highest modification degree is reached when the reaction is performed in toluene. However, even when using toluene, a small amount of water will be present as water is set free by the condensation of the PA with the surface. Moreover, traces of adsorbed surface water might be present as well. Therefore, also the impact of mixtures of water in toluene were studied for the 3PA modification. When more than 25 % water is used, a two-layer system is formed and a similar DRIFT spectrum is found as when pure toluene is used (Figure 6). This is due to the low solubility of water in toluene which causes a two-layer system even when the solution has been heated and stirred. When the solvent mixture contains less than 25 % water (homogeneous aqueous toluene mixture), the formation of TiPP is observed in DRIFT and DTG (Figure 6 and 7). As water is able to adsorb on the TiO2 surface, local acid hydroxyl groups are present,

Chapter 4 | 145 leading to the dissolution-precipitation mechanism and the presence of phosphonate phases (chapter 3).13 The grafting densities are calculated for both grafting (250-400 °C) and TiPP formation (400-520 °C) (Figure 8). The amount of surface grafting is similar for most of the samples (~3.2 groups/nm2). Only in the sample prepared with 10 % of water the grafting density has increased (~3.6 groups/nm2). On the other hand, the amount of TiPP formation is clearly increasing with the addition of water. When a two-layer system is formed, the TiPP formation decreases as toluene seems to predominately interact with the surface.13 Clearly, as the grafted amount does not change, the increased loading capacity is caused by an additional dissolution-precipitation mechanism without affecting the grafting itself. Moreover, by working in a mixture of water and toluene, the organic loading exceeds that of modification in only water or toluene separately. Furthermore, the water traces present when working in pure toluene (3PA0%), do not seem high enough to initiate TiPP formation.

Figure 7: TGA (dotted lines) and DTG (full lines) curve of TiO2 modified with 0.100 M 3PA in mixtures with different water percentages in toluene at 130 °C measured under an O2-flow and 10 °C/min heating rate.

146

Figure 8: Modification degree (expressed in groups/nm²) calculated from TGA for modification in solvent mixtures of water/toluene of 3PA at 130 °C. The modification degree is calculated for grafting (250-400 °C) and TiPP formation

(400-520 °C). The C value is calculated from N2 sorption. In the figure the % of water in toluene is given.

4.2.4 The impact of solvent on the obtained surface properties

4.2.4.1 Sorption based on nitrogen and water as probe molecules

N2 sorption is used to calculate the C value from the BET equation.19 This parameter is frequently viewed as a measure of the interaction energy between the solid surface and the probing molecules. It is important to note that one

N2 molecule possesses an electric quadrupole moment (-4.65 x 10-40 C m2)20 and may also act as a borderline Lewis base. If one agrees that the polar contribution to the total adsorption energy is significantly lowered at -196 °C in comparison with the apolar one (due to dispersive forces), and when it is possible to neglect

Chapter 4 | 147 the confinement effects due to the adsorbent porosity (in our case: P25 is non-porous), a downward trend in the C value observed for a series of materials exhibiting similar surface chemistry can be used to monitor increasing hydrophobicity of the surface. Although such an analysis has already been used to demonstrate the increased hydrophobic character of organic layers supported on mineral oxides,3,9 much attention should be paid when trying to go too far into the detailed interpretation of the results obtained. For example, there will be higher probability of multisite attachment of nitrogen molecules to downwardly oriented organic species grafted on the oxide surface, which is at variance with the basic assumption of the BET model. This procedure has been tested by determining the C values for all materials studied in the present paper.

Table 3: Modification degree calculated from TGA for modification at different concentrations of 3PA at 90 °C in toluene and water. The C value is calculated from N2 sorption measurements. The heat of butanol adsorption is measured with flow calorimetry.

Toluene Water

Material Mod. C Q Material Mod. C Q deg. value (J/m²) deg. value (J/m²) (#/nm²) (#/nm²)

3PA0.003M90T 0.7 54 0.31 3PA0.025M90W 1.0 39 0.21

3PA0.005M90T 1.1 43 0.24 3PA0.050M90W 1.3 37 0.19

3PA0.013M90T 2.2 30 0.17 3PA0.075M90W 1.4 38 0.19

3PA0.026M90T 3.0 23 nd 3PA0.100M90W 1.7 37 0.20

3PA0.100M90T 3.2 24 0.07 3PA0.150M90W 2.1 39 0.20 nd: not determined

148 The C constant of the pristine TiO2 sample was determined to be equal to 108.

For the 3PA modified TiO2 in toluene with increasing concentrations, the surface loading increases and the C value decreases from 54 to 24 (Table 3). Since there are more groups attached to the surface, this likely means that the surface becomes more hydrophobic.3,9 For the samples prepared in the solvent mixtures (Figure 8) the C value is stable at around 22 and independent of the amount of water used during the reaction. This is similar to samples made in toluene with high modification degrees as little hydroxyl groups are attainable. In contrast, for the 3PA modification of TiO2 in water, the C value remains constant (38) regardless of the modification degree (Table 3). Moreover, for the same modification degree the values are different in water compared to toluene. This difference can be obviously rationalized by referring to the binding mechanism and different topology of both types of modified surfaces. Water is competing with functionalization. But, once a PA is bound to the surface the water molecules will have fewer tendencies to adsorb among the hydrophobic propyl chains. In that case, those chains most likely interact between themselves to form patches of grafted PAs at the surface, causing local SAM formation at the surface next to patches of hydrophilic nests. In contrast to water, it has been reported that the modification of PA in toluene is driven by the strong covalent interaction between the PA and TiO2, leading to a random distribution of grafted PAs among the remaining hydroxyl groups.16 With such random topology, the mean energy of adsorbate-adsorbent interactions will be much more sensitive to the energetic heterogeneity of surface sites and thus any increase in the surface hydrophobic character will show up clearly in the C constant. It should be mentioned that in the case of patchwise arrangement of homogeneous surface sites, the lateral interactions among the adsorbed nitrogen molecules can no longer be neglected even though the overall surface coverage remains relatively low. As a result, the

Chapter 4 | 149 differences in the interaction energy over the surface will be smoothed out by such a lateral contribution.

Further arguments were found by carrying out sorption measurements with water molecules probing more specifically for surface hydroxyl groups (Figure 9). For both types of systems, the surface coverage by water molecules at low P/P0 values decreases constantly when more and more 3PA units are grafted on the TiO2 surface. This is certainly a strong indication for the decreasing hydrophilic character of the modified surface. The formation of a fully hydrophobic layer, covering the whole surface has been obtained neither in water nor in toluene, though the 3PA0.100M90T system characterized by the highest modification degree in toluene provides a surface adsorbing very low amounts of water vapor. The main difference between the two types of systems lies in the trend observed for water sorption behavior with increasing modification degree. For similar values of the latter, a more pronounced decrease in the amount of water adsorbed is monitored for surfaces modified in water at low modification degrees [about 1 group/nm² (3PA0.005M90T and 3PA0.025M90W in Figure 9)], while at higher modification degrees [at ~2 groups/nm2 (3PA0.013M90T and 3PA0.150M90W)] a more pronounced decrease is visible for modification in toluene. Since the linear isotherm portions at higher partial pressures (matching multilayer formation) have similar slopes, the differences seem to appear in the initial sorption mechanism. Here, the observed decrease in the amount of water vapor retained on surfaces previously grafted in water is to be attributed to repulsive forces operating among water molecules when they are adsorbed on the extended hydrophilic patches, namely for lower grafting degrees.

150

Figure 9: Adsorption isotherm of water vapour at 22 °C on TiO2 and TiO2 modified with different concentrations of 3PA (A) in toluene and (B) in water at 90 °C. Degassing was done at 150 °C for 16 h.

Water sorption is also performed for modification of TiO2 with PhPA (Table 4 and Figure 10). For the samples made in water the monolayer formation is decreasing and is approximately constant for samples with a modification degree higher than 1.3 groups/nm² (Figure 10). This is similar for the modification with 3PA. Therefore, it is reasonable that also the modification of PhPA in water results in SAMs. However, the monolayer formation for the water probe molecule is always higher in the case of TiO2 modified with PhPA than with 3PA, which might coincide with the mechanism of …H2O… interactions present during modification in water suggesting a more directed packing of the PhPA on the surface. For the modification in toluene, no modification degrees higher than 1.3 groups/nm² are achieved (Table 4). Therefore, no conclusions about random adsorption can be drawn from the water adsorption isotherms of PhPA in toluene.

Chapter 4 | 151

Figure 10: Adsorption isotherms of water vapour at 22 °C on TiO2 and TiO2 modified with different concentrations of PhPA (A) in toluene and (B) in water at 90 °C. Degassing was done at 150 °C for 16 h.

Table 4: Modification degrees measured with TGA for different concentrations of PhPA at 90 °C in toluene and water.

Toluene Water Material Mod. deg. (#/nm²) Material Mod. deg. (#/nm²)

PhPA0.003M90T 0.7 PhPA0.008M90W 0.9 PhPA0.005M90T 0.8 PhPA0.050M90W 1.5 PhPA0.030M90T 1.1 PhPA0.075M90W 1.7 PhPA0.090M90T 1.2 PhPA0.100M90W 2.1 PhPA0.100M90T 1.3

152 4.2.4.2 Flow adsorption microcalorimetry

Liquid flow calorimetry allows the measurement of enthalpy of competitive adsorption between butanol and heptane. Taking into account the amphiphilic molecular structure of butanol molecules, this solute is capable of entering into competition with hydrophobic heptane to adsorb preferentially only on surface hydroxyl groups.21,22 The enthalpy change measured upon this displacement process on a per m² basis is further regarded to monitor changes in the hydrophilic character of the modified surfaces. The resulting enthalpy values are given in Table 3.

For the samples modified using toluene as a solvent, there is a nearly linear decrease of the heat effect with increasing modification degree. This indicates a steady decrease in the number of OH groups accessible to adsorbing butanol units. The nearly linear relationship between the integral heat of displacement and the modification degree points out that the affinity difference undergoes little change when passing from one hydroxyl group to another. Both observations again argue in favor of almost fully random distribution of the remaining OH groups on the modified surfaces. On the contrary, the heat effect for the samples prepared in water appears almost constant, irrespective of the PA grafting density. This can be explained by the lateral interaction among

C4-chains of the neighboring alcohol species retained on hydrophilic OH patches. This additional contribution to the total interaction energy is negligible within the bulk solution where the solute molecules are dispersed at random. On a surface with a patchwise distribution of active sites, such a contribution cannot be neglected any longer and it is included in the total enthalpy change recorded upon adsorption from solution. The overall thermal effect will be smoothed out by all such interaction modes involved in the displacement process.

Chapter 4 | 153 4.3 Conclusion

The solute-solvent–surface interactions involved in the modification of TiO2 with PA have been investigated. A clear impact of the solvent is visible and a number of aspects are dependent on the type of functional group, while others are only determined by the competing interaction of the solvent at the surface or the propensity to create acidity. In water, surface grafting and formation of TiP occurs, while, in toluene, surface grafting is the most important result. By adding small amounts of H2O to the reaction, a high modification degree next to additional TiP formation are found. Furthermore, the solvent has an influence on the surface distribution of grafted groups and interacts with specific functional groups (solute) leading to a lower modification degree. From this work, clear differences in structural properties can be concluded and a vast impact on coverage are inherent to the applied solvent and solute.

This work has demonstrated the importance of the choice of solvent with respect to the organic functional group with the purpose of tuning the surface properties. It has a significant impact on applications involving surface interactions such as sorption, separation, catalysis, controlled release, immobilization etc. of which the performance is directly correlated to/influenced by surface properties. In this chapter, the different interactions of the solvent with the surface and solute are described, which makes it possible to choose the reaction conditions that will result in a controlled surface with the appropriate characteristics for a specific application.

154 4.4 References

(1) Djafer, L.; Ayral, A.; Boury, B.; Laine, R. M. Surface Modification of Titania Powder P25 with Phosphate and Phosphonic Acids--Effect on Thermal Stability and Photocatalytic Activity. J. Colloid Interface Sci. 2013, 393, 335–339. (2) Randon, J.; Blanc, P.; Paterson, R. Modification of Ceramic Membrane Surfaces Using Phosphoric Acid and Alkyl Phosphonic Acids and Its Effects on Ultrafiltration of BSA Protein. J. Memb. Sci. 1995, 98 (1–2), 119– 129. (3) Lassiaz, S.; Galarneau, A.; Trens, P.; Labarre, D.; Mutin, H.; Brunel, D. Organo-Lined Alumina Surface from Covalent Attachment of Alkylphosphonate Chains in Aqueous Solution. New J. Chem. 2010, 34 (7), 1424-1435. (4) Chen, X.; Luais, E.; Darwish, N.; Ciampi, S.; Thordarson, P.; Gooding, J. J. Studies on the Effect of Solvents on Self-Assembled Monolayers Formed from Organophosphonic Acids on Indium Tin Oxide. Langmuir 2012, 28 (25), 9487–9495. (5) McElwee, J.; Helmy, R.; Fadeev, A. Y. Thermal Stability of Organic Monolayers Chemically Grafted to Minerals. J. Colloid Interface Sci. 2005, 285 (2), 551–556. (6) Ferreira, J. M.; Marcinko, S.; Sheardy, R.; Fadeev, A. Y. Calorimetric Study of the Reactions of N-Alkylphosphonic Acids with Metal Oxide Surfaces. J. Colloid Interface Sci. 2005, 286 (1), 258–262. (7) Angelomé, P. C.; Aldabe-Bilmes, S.; Calvo, M. E.; Crepaldi, E. L.; Grosso, D.; Sanchez, C.; Soler-Illia, G. J. A. A. Hybrid Non-Silica Mesoporous Thin Films. New J. Chem. 2005, 29 (1), 59-63.

Chapter 4 | 155 (8) Mutin, P.; Lafond, V.; Popa, A. Selective Surface Modification of SiO2-

TiO2 Supports with Phosphonic Acids. Chem. Mater. 2004, 16 (26), 5670– 5675. (9) Guerrero, G.; Mutin, P. H.; Vioux, A. Anchoring of Phosphonate and Phosphinate Coupling Molecules on Titania Particles. Chem. Mater. 2001, 13 (11), 4367–4373. (10) Gao, W.; Dickinson, L.; Grozinger, C.; Morin, F. G.; Reven, L. Self- Assembled Monolayers of Alkylphosphonic Acids on Metal Oxides. Langmuir 1996, 12 (26), 6429–6435. (11) D’Andrea, S. C.; Fadeev, A. Y. Covalent Surface Modification of Calcium Hydroxyapatite Using N-Alkyl- and N-Fluoroalkylphosphonic Acids. Langmuir 2003, 19 (19), 7904–7910. (12) Lafont, U.; Simonin, L.; Gaberscek, M.; Kelder, E. M. Carbon Coating via

Alkyl Phosphonic Acid Grafting Route: Application on TiO2. J. Power Sources 2007, 174, 1104–1108. (13) Roevens, A.; Van Dijck, J. G.; Tassi, M.; D’Haen, J.; Carleer, R.; Adriaensens, P.; Blockhuys, F.; Meynen, V. Revealing the Influence of the Solvent in Combination with Temperature, Concentration and pH on the

Modification of TiO2 with 3PA. Mater. Chem. Phys. 2016, 184, 324–334. (14) Mustafa, G.; Wyns, K.; Vandezande, P.; Buekenhoudt, A.; Meynen, V. Novel Grafting Method Efficiently Decreases Irreversible Fouling of Ceramic Nanofiltration Membranes. J. Memb. Sci. 2014, 470, 369–377. (15) Geldof, D.; Tassi, M.; Carleer, R.; Adriaensens, P.; Roevens, A.; Meynen, V.; Blockhuys, F. Binding Modes of Phosphonic Acid Derivatives

Adsorbed on TiO2 Surfaces: Assignments of Experimental IR and NMR Spectra Based on DFT/PBC Calculations. Surf. Sci. 2017, 655, 31–38.

156 (16) Helmy, R.; Fadeev, A. Y. Self-Assembled Monolayers Supported on TiO2:

Comparison of C18H37SiX3 (X = H, Cl, OCH3), C18H37Si(CH3)2Cl, and

C18H37PO(OH)2. Langmuir 2002, 18 (23), 8924–8928. (17) Socrates, G. Infrared and Raman Characteristic Group Frequencies, 3th ed.; John Wiley & Sons Ltd: Chichester (Engeland), 2000. (18) Tarakeshwar, P.; Choi, H. S.; Lee, S. J.; Lee, J. Y.; Kim, K. S.; Ha, T.-K.; Jang, J. H.; Lee, J. G.; Lee, H. A Theoretical Investigation of the Nature of

the π-H Interaction in ethene–H2O, benzene–H2O, and benzene–(H2O)2. J. Chem. Phys. 1999, 111 (13), 5838-5850. (19) Brunauer, S.; Emmett, P. H.; Teller, E. Adsorption of Gases in Multimolecular Layers. J. Am. Chem. Soc. 1938, 60 (2), 309–319. (20) Graham, C.; Imrie, D. A.; Raab, R. E. Measurement of the Electric

Quadrupole Moments of CO2, CO, N2, Cl2 and BF3. Mol. Phys. 1998, 93 (1), 49–56. (21) Groszek, A. J.; Partyka, S. Measurements of Hydrophobic and Hydrophilic Surface Sites by Flow Microcalorimetry. Langmuir 1993, 9 (10), 2721–2725. (22) Zajac, J.; Groszek, A. J. Adsorption of C60 Fullerene from Its Toluene Solutions on Active Carbons: Application of Flow Microcalorimetry. Carbon N. Y. 1997, 35 (8), 1053–1060.

Chapter 4 | 157

Chapter 5

In-situ study of the initial adsorption steps in the surface modification of titanium dioxide with organophosphonic acids with isothermal titration calorimetry.

In Preparation for Colloids and Surface A: Physicochemical and engineering aspects

5.1 Introduction

The impact of synthesis conditions on the type, amount and distribution of grafted groups on the surface (e.g. patch-wise or random distribution) has been studied (chapter 3 and 4)1,2 as well as the resulting interaction (energy) of probe molecules with the modified surface (chapter 4).1,3 In these studies, mainly ex-situ characterization has been applied, meaning that the synthesis conditions have been varied and the surface properties characterized on the final materials. The characteristics of the functionalized surfaces have been unravelled with a wide variety of techniques such as IR,1,2,4,5 NMR (1H, 13C, 17O, 31P),5–9 TGA,1,2,10 sorption experiments,1,3 TEM,11 XPS,12 XRD,13 EDX13 and flow calorimetric measurements.1 However, to understand the surface grafting process in more detail, it is important to investigate the formation of the monolayer during the modification itself and especially during the first steps, when the interaction with the surface occurs.

Isothermal titration calorimetry (ITC) is a powerful tool from a thermodynamic and kinetic point of view and is sensitive to study interactions between species in solution or adsorbed onto solids.14 ITC has been used to study the adsorption behavior of proteins on nanoparticles,15 biomolecular interactions,16 the binding of amino acids to nanoparticles14 and phosphate sorption.17 Ferreira et al.18 used ITC to investigate the formation and the stability of self-assembled monolayers

(SAM) of PAs on TiO2 and ZrO2. Methyl-, octyl-, and octadecylphosphonic acid were used as solute and THF as solvent. They proposed an equation decoupling the energetic contributions of the solid-solute, solute-solvent and solvent-solid interactions to the net enthalpy of formation of SAMs at the solid-liquid interface. They found that the enthalpy for modification with long aliphatic hydrocarbons (e.g. 18PA) is endothermic, but exothermic for shorter chains.

Chapter 5 | 161 The study of Ferreira et al.18 shows the potential of in-situ studies with ITC. Nevertheless, the effect of the concentration of PAs and the impact of the type of functional group (aliphatic or aromatic) has not been investigated, even though, they cause significant differences in the obtained surface properties.1,2,19 Interestingly, various properties are expected to originate from differences in molecular interaction energies. Therefore, this chapter focuses on the impact of the concentration of two types of PAs (3PA and PhPA) in water. To correlate the data on the interaction energy with surface properties (binding type and modification degree), adsorption experiments under the same reaction conditions are performed, revealing detailed information on the grafting mechanism.

162 5.2 Results and discussion

5.2.1 Isothermal titration calorimetry

Isothermal titration calorimetry was performed to study the energy of interaction produced when 3PA is added to the synthesis mixture containing TiO2 P25. Figure 1 shows the results of the ITC experiment using a 5 mM 3PA solution on

TiO2 together with a comparison of the dilution effect in the absence of powder in the cell. The measurements are performed at room temperature. The heat flow is similar for the first four injections, and then gradually decreases with increasing number of injections. This indicates a regular decrease of the exothermic behaviour until only heat of dilution remains. The enthalpy (ΔH) has been calculated from the integration of the heat response obtained from the raw ITC data. Enthalpy may be plotted in function of the molar ratio of adsorbents and adsorbate.20 In the present study, grafting PA on TiO2 occurs with different binding modes (mono-, bi- and tridentate), each binding type reacting with a different number of TiO2 hydroxyls (respectively one, two or three). This precludes plotting the interaction enthalpy versus the molar ratio. In order to follow the sorption process, the enthalpy is here plotted against the amount (mmol/g) of adsorbed 3PA (see further).

Chapter 5 | 163

Figure 1: Thermogram of the ITC measurements of the sorption of 3PA on TiO2 in water performed with an initial concentration of 5 mM for the stock solution of 3PA (grey curve measurement with TiO2) and black curve represents the dilution of 3PA (measurement without TiO2).

To that purpose, adsorption measurements are performed under similar conditions to determine the adsorbed amount. In Figure 2, the adsorbed amount is plotted versus the equilibrium concentration in the solution. The sorption first increases with the equilibrium concentration, and then reaches a pseudo-plateau at Ceq around 1.5 mM, with the maximum amount adsorbed about 0.13 mmol/g. Since each experimental data point is correlated to an injection in the ITC measurement, this means that the amount adsorbed decreases as the number of injections increases.

164

Figure 2: Adsorption isotherm of 3PA on TiO2 in water for different initial concentrations of the stock solution. The adsorption isotherm of a stock solution of 5 mM 3PA on TiO2 is shown as inset to make it more clearly visible as it overlaps with the other isotherms.

The powders obtained for each adsorption point with increasing concentration and thus at various loading were analysed using DRIFT (Figure 3). For low surface coverage (from samples 3PA5mM1 to 3PA5mM8), three peaks at 1107, 1046 and 1016 cm-1 are observed in the P-O-Ti region. This P-O-Ti region changes gradually for higher concentrations, indicating alterations in the binding of PA to the surface. This may be correlated to the regular increase of the loading, and the high heat flow in ITC. Starting from sample 3PA5mM8 the spectrum has clearly changed, with the appearance of a peak at 1070 cm-1 that becomes gradually more intense with increasing concentration. This peak is typically observed in surface modification of TiO2 with 3PA.2

Chapter 5 | 165

Figure 3: DRIFT spectra of the P-O region of the powder of each experimental data point of the adsorption of 3PA5mM on TiO2 measured against a 2 wt%

TiO2 background. An offset has been used to visualize all spectra more clearly.

The differences in binding type were characterized using 31P-CP/MAS NMR (Figure 4). One observes that the chemical environment of the phosphate group changes depending on the surface coverage with increasing concentration. Clear changes in 31P-CP/MAS NMR spectra are visible between the 1st and the 5th adsorption point of the isotherm (Figures 4A and 4B). Fewer differences are visible in Figures 4C and 4D representing points #10 and #13, respectively. The chemical shifts between 38 and 10 ppm are attributed to surface grafting of 3PA on TiO2 and are a superposition of different underlying chemical shifts indicating the presence of monodentate and bidentate binding types.5 Each of these binding types have different interactions with the surface, causing the presence of diverse signals and the broadening of the signals. At higher surface loading, the peak at 26 ppm is increasing and becomes dominant. This 31P chemical shift is via calculations assigned to monodentate binding of 3PA on an anatase (101) surface.21 Next to the large signal at 26 ppm, two chemical shifts can be observed at 10 ppm and -2 ppm. The first one can be assigned to the formation of titanium propylphosphonate (TiPPs),2 while the chemical shift at -2 ppm might be

166 correlated to pyrophosphates.22 The presence of pyrophosphates has not been observed in previous reports on the modification of TiO2 with PAs. In paragraph 5.3.4 possible reasons for the extra peak are discussed.

Figure 4: 31P-CP/MAS NMR spectra of different experimental data points of the adsorption of 3PA5mM on TiO2: (A) 3PA5mM1, (B) 3PA5mM5, (C) 3PA5mM10 and (D) 3PA5mM13.

Based on the determined adsorbed amount at each point of the adsorption isotherm, the enthalpy of the real dilution effect is now subtracted from the enthalpy measured during modification, resulting in the cumulative displacement enthalpy (ΔHdpl). Following procedures for data processing

Chapter 5 | 167 described previously,23 the raw heat flow signals measured from ITC were corrected for dilution effects. The ΔHdpl is then plotted as function of the amount adsorbed as shown in Figure 5. The ΔHdpl is negative (exothermic reaction) and increases linearly with the adsorbed amount. When the loading increases and reaches the pseudo-plateau of the sorption isotherm, the intensity gradually levels off. The ΔHdpl or sorption enthalpy describes the evolution of the interactions during the grafting process. For low coverage and before surface saturation, the ITC shows a linear evolution of the binding process. Then, when the maximum capacity is almost reached, the enthalpy levels off.

Figure 5: Cumulative displacement enthalpy versus the adsorbed amount for adsorption of 3PA on TiO2 with various initial concentrations of the stock solution (5, 10, 15 and 30 mM 3PA).

168 5.3.2 Influence of the concentration

Different initial concentrations of the 3PA stock solution (5, 10, 15 and 30 mM) have been used in the ITC measurements. When the concentration rises, the heat flow is higher since the amount of added and grafted PA is higher (Figure 6). The

ΔHdpl obtained after appropriate calculation and with different initial concentrations is displayed in Figure 5. For a given adsorbed amount, the ΔHdpl is similar and does not depend on the used concentration. When the powders are analysed (DRIFT in Figure 7), observations and conclusions are similar to those obtained for experiments with 5 mM. On the first spectra, peaks at 1107, 1046 and 1016 cm-1 are observed in the P-O-Ti region. For an initial concentration of 10 mM appears the peak at 1070 cm-1 from the forth adsorption point. For the various initial concentrations (10, 15 and 30mM), the stock solution is 2, 3 or 6 times more concentrated compared to 5 mM, and the 1070 cm-1 contribution emerges 2, 3 or 6 times earlier. These alterations in the binding type seem to coincide with the ITC heat flow and the surface loading (Figures 1 and 2) when saturation is starting.

Figure 6: Thermogram of the ITC measurements of the sorption of 3PA on TiO2 in water performed with different concentrations of the stock solution: 10, 15 and 30 mM (an offset of 10 µW is used).

Chapter 5 | 169 Also from the 31P-CP/MAS NMR (Figure 8) the effect of the concentration can be observed. The changes are similar to the ones observed with the initial concentration at 5 mM. Since the concentrations are higher, the modifications are seen earlier after a lower number of data points in the adsorption isotherm. Furthermore, samples with a similar cumulative amount injected can be compared. It can be deduced that the amount of TiPPs and pyrophosphates are higher for sample 3PA5mM10 (Figure 4) than for sample 3PA10mM5 (Figure 8). This indicates that the stepwise addition of PAs probably induces the formation of TiPPs and pyrophosphates.

Similar observations are found in the adsorption measurements, i.e. the number of adsorption points needed to reach the saturation level decreases when higher initial concentrations in the stock solutions are used (Figure 2). The amount adsorbed is irrespective from the injected concentration. This means there is no forced adsorption when higher concentrations are injected. The maximal adsorbed amount is 1.6 groups/nm². A full monolayer (3.2 groups/nm²) is not achieved under these experimental conditions.1 Those high modification degrees have been reported when working in aprotic solvents at higher temperatures and/or when TiPs are formed.2 Therefore, we assume that an activation barrier might be present under the here applied conditions.

170

Figure 7: DRIFT spectra of the P-O region of the powder of each experimental data point of the adsorption of (A) 3PA 10 mM, (B) 3PA 15 mM and

(C) 3PA 30 mM on TiO2 measured against a 2wt% TiO2 background. An offset has been used to visualize all spectra more clearly.

Chapter 5 | 171 C

B

A

Figure 8: 31P-CP/MAS NMR spectra of different experimental data points of the adsorption of 3PA10mM on TiO2: (A) 3PA10mM1, (B) 3PA10mM5 and (C) 3PA10mM13.

To confirm that previous adsorbed molecules do not induce forced adsorption upon the following adsorption point and no further adsorption can take place ones the saturated value of 1.6 groups/nm² has been reached, we investigated the sorption of PA on PA pre-grafted materials. We measured the ITC heat flow and binding of additional 3PA in a subsequent experiment on functionalized

TiO2 instead of pure TiO2 as done previously. Synthesised samples with modification degrees of 1.4 and 3.2 groups/nm² (prepared in hot toluene to achieve these high loadings) have been used. As can be seen in Figure 9, no changes could be observed on the thermogram. The heat flow is similar for the sorption and the dilution experiments, for both materials. Furthermore, the

172 intensity of the signal is quite small compared to the one obtained on neat TiO2. This indicates that no further adsorption of 3PA occurs after the saturation has been reached even when a complete monolayer has not yet been formed. Moreover, it is conceivable that the organization of grafted groups on the pre- adsorbed materials make the surface less accessible. This might explain why, higher reaction temperatures should be used to obtain the full monolayer (see paragraph 5.3.4).2

Figure 9: Thermogram of the ITC measurements of the sorption of 3PA on modified TiO2 (A) 3PA 1.4 groups/nm² and (B) 3PA 3.2 groups/nm² in water performed with initial concentration 10 mM for the stock solution of 3PA (grey curve measurement with modified TiO2 and black curve measurement without TiO2).

Chapter 5 | 173 5.3.3 Influence of the type of organophosphonic acid

The surface properties of the final material depend on the grafted groups.1 Therefore, sorption studies similar to the ones reported on above for 3PA have been performed on PhPA. The adsorption isotherms in Figure 10 are comparable with the sorption of 3PA and a similar modification degree of 1.6 groups/nm² has been found. The DRIFT spectra (Figure 11) for all samples, show typical peaks for surface grafting of PhPA on TiO2 in the P-O-Ti region: 1058, 1040 and 1023 cm-1. Starting from PhPA5mM6 an extra peak at 1080 cm-1 appears, which is typical for titanium phenylphosphonate (TiPhP).5 When higher concentrations are used (data not shown), the peak of the TiPhP is visible when fewer injections have been done. The TiPhPs are also found in the 31P-CP/MAS NMR spectra with a peak around -2 ppm for sample PhPA5mM5 (Figure 12). Furthermore, a large peak between 4-26 ppm is present, composed of multiple contributing signals assigned to surface grafting with a variety of binding modes. For the modification of TiO2 with PhPA no pyrophosphates are observed, but the peak of the pyrophosphates and TiPhP might overlap.

Figure 10: Adsorption isotherm of PhPA on TiO2 in water for different initial concentrations of the stock solution. The adsorption isotherm of a stock solution of 5 mM PhPA on TiO2 is shown as inset to make it more clearly visible as it overlaps with the other isotherms.

174

Figure 11: DRIFT spectra of the P-O region of the powder of each experimental data point of the adsorption of PhPA 5 mM on TiO2 measured against a 2wt%

TiO2 background. An offset has been used to visualize all spectra more clearly.

B

A

Figure 12: 31P-CP/MAS NMR spectra of different experimental data points of the adsorption PhPA5mM on TiO2 (A) PhPA5mM5 and (B) PhPA5mM10.

Chapter 5 | 175 In the ITC measurements, the saturation and the return to smaller signals are observed after the same number of injections for both PhPA and 3PA using the same concentration (Figures 1, 6, 13 and 14). This is related to the similar modification degree as observed in Figures 2 and 10. Small variations are found in peak heights, but the integrated peak area is similar for the modifications with both PAs. This means that the total heat effect is comparable. Also, the ΔHdpl as a function of surface coverage (Figures 5 and 15) are quite similar for both PAs. All these results show that the sorption process is not influenced by the nature of the PA, nor the concentration of PA at room temperature.

Figure 13: Thermogram of the ITC measurements of the sorption of PhPA on

TiO2 in water performed with initial a concentration of 5 mM for the stock solution of PhPA (grey curve measurement with TiO2) and black curve represents the dilution of PhPA (measurement without TiO2).

176

Figure 14: Thermogram of the ITC measurements of the sorption of PhPA on

TiO2 in water performed with different concentrations of the stock solution: 10, 15 and 30 mM (an offset of 10 µW is used).

Figure 15: Cumulative displacement enthalpy versus the adsorbed amount for adsorption of PhPA on TiO2 with various initial concentrations of the stock solution (5, 10, 15 and 30 mM PA).

Chapter 5 | 177 5.3.4 Correlating the sorption experiments with synthesis

In previous studies the surface modification of TiO2 has been performed in a one- step addition of the PA. 1 g of TiO2 was modified with PA and the applied PA concentration was 100 mM. Therefore, the correlation between sorption experiments of the present work and one-pot synthesis is not straightforward.

When 3PA at 100 mM is grafted via a one-step process on TiO2 in water at room temperature, the modification degree is 1.6 groups/nm². When comparing the DRIFT spectra (Figure 16), it is clear that the main peak at 1077 cm-1 is present for both solids prepared with 100 mM in one step, and those prepared after 15 successive additions of 5mM, but the pattern (relative intensity of the peaks present) and, consequently, the way of binding are different. The main difference in the experimental conditions between sorption and synthesis is probably the formation of titanium phosphonates and pyrophosphates.24

Figure 16: DRIFT spectra of the P-O region of TiO2 modified with 3PA at 20 °C in water via a one-step process or successive steps of 3PA addition, measured against a 2wt% TiO2 background. An offset has been used to visualize all spectra more clearly.

178 To unravel the origin of these differences the influence of the washing procedure was investigated. Indeed, the samples obtained after adsorption were centrifuged, while for the one-step synthesis method the samples were washed on a filter. New materials were prepared and compared using 31P-CP/MAS NMR. A first sample was prepared using the procedure of the adsorption isotherms, but followed by washing on a filter. Furthermore, a material obtained using the standard synthesis conditions was centrifuged without washing to mimic the adsorption experiments. The 31P-CP/MAS NMR spectra (Figure 17) were found similar for both experiments, meaning that the washing procedure does not influence the formation of TiPPs and pyrophosphates.

Also, when using a very low concentration of 3PA no peaks of TiPPs or pyrophosphates are observed. Hence, the reason for the formation of these two phases when subsequent injections are applied, is still unclear. The presence of pre-adsorbed PA seems to be the reason for TiPP formation, which could initiate some kind of assisted self-assembly enhancing dissolution and TiPP precipitated at the surface.

Chapter 5 | 179 D A

3

C1 P 3- 1C B P

-3/ A C1M PA P /3S - M1N C Figure 17: 31P-CP/MAS NMR spectra of 3PA modified TiOAPM2 with different P washing procedures: (A) 3PA0.100M filter washed, (B) 3PA0.100MS-R centrifuged, / (C) 3PA5mM10 filter washed and (D)3PA5mM10 centrifugedNCs . M MPp A R/e S sMc N pAt M eSr R cNa s tMo p rRf e as c opd t fes 180 r aco a dtr o 5.3 Conclusion

The combination of ITC with adsorption measurements provides a novel method to analyze surface modification of TiO2 with PAs. For an initial concentrations of 5 mM 3PA, six injections of 3PA are necessary to achieve the saturation of 3PA at the surface. In DRIFT and 31P-CP/MAS NMR small changes in binding type are observed for the first grafted groups. Furthermore, next to surface grafting also titanium propylphosphonates and pyrophosphates are formed in the case of subsequent injections.

The amount adsorbed is similar, irrespective of the concentration of the PA solution used during adsorption experiments but fewer injections are required to reach saturation when higher concentrations are used, which makes senses. The maximum amount adsorbed is 1.6 groups/nm². This is half the number of groups to obtain a full monolayer and is the highest amount achievable at room temperature. Moreover, the type of the PA functional group does not influence the modification degree. It is important to note that with the classical one-step synthesis method only surface grafting occurs, while under sorption conditions based on a step-by-step addition, TiPs and pyrophosphates are also observed.

The combination of ITC with adsorption experiments and correlated to the final materials properties via DRIFT and 31P-CP/MAS NMR is a powerful methodology to study surface grafting mechanism. This might also be useful for other types of grafting, e.g., organosilanes, organocarbonic acids etc.

Chapter 5 | 181 5.4 References

(1) Roevens, A.; Van Dijck, J. G.; Geldof, D.; Blockhuys, F.; Prelot, B.; Zajac, J.; Meynen, V. Aqueous or Solvent Based Surface Modification: The Influence of the Combination Solvent – Organic Functional Group on the Surface Characteristics of Titanium Dioxide Grafted with Organophosphonic Acids. Appl. Surf. Sci. 2017, 416, 716–724. (2) Roevens, A.; Van Dijck, J. G.; Tassi, M.; D’Haen, J.; Carleer, R.; Adriaensens, P.; Blockhuys, F.; Meynen, V. Revealing the Influence of the Solvent in Combination with Temperature, Concentration and pH on the

Modification of TiO2 with 3PA. Mater. Chem. Phys. 2016, 184, 324–334. (3) Lassiaz, S.; Galarneau, A.; Trens, P.; Labarre, D.; Mutin, H.; Brunel, D. Organo-Lined Alumina Surface from Covalent Attachment of Alkylphosphonate Chains in Aqueous Solution. New J. Chem. 2010, 34 (7), 1424-1435.

(4) Helmy, R.; Fadeev, A. Y. Self-Assembled Monolayers Supported on TiO2:

Comparison of C18H37SiX3 (X = H, Cl, OCH3), C18H37Si(CH3)2Cl, and

C18H37PO(OH)2. Langmuir 2002, 18 (23), 8924–8928. (5) Tassi, M.; Roevens, A.; Reekmans, G.; Vanhamel, M.; Meynen, V.; D’Haen, J.; Adriaensens, P.; Carleer, R. A Detailed Investigation of the

Microwave Assisted Phenylphosphonic Acid Modification of P25 TiO2. Adv. Powder Technol. 2017, 28 (1), 236–243. (6) Gao, W.; Dickinson, L.; Grozinger, C.; Morin, F. G.; Reven, L. Monolayers: A 13C Solid-State NMR Study. Langmuir 1997, 13, 115–118. (7) Brodard-Severac, F.; Guerrero, G.; Maquet, J.; Florian, P.; Gervais, C.; Mutin, P. H. High-Field 17O MAS NMR Investigation of Phosphonic Acid Monolayers on Titania. Chem. Mater. 2008, 20 (16), 5191–5196.

182 (8) Lafond, V.; Gervais, C.; Maquet, J.; Prochnow, D.; Babonneau, F.; Mutin, P. H. 17O MAS NMR Study of the Bonding Mode of Phosphonate Coupling Molecules in a Titanium Oxo-Alkoxo-Phosphonate and in Titania-Based Hybrid Materials. Chem. Mater. 2003, 15 (21), 4098–4103. (9) Tassi, M.; Reekmans, G.; Carleer, R.; Adriaensens P. Fully Quantitative

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Chapter 5 | 185

General conclusions and suggestions for future research

General conclusion

In this thesis, the influence of the synthesis conditions on the modification of TiO2 with organophosphonic acids (PA) is revealed. Furthermore, attention is paid to the solute-solvent-surface interactions during the modification. A combination of different characterization techniques (DRIFT, TGA and 31P-CP/MAS NMR) is used to study the modification. Based on this, it is found that misinterpretations might be made if only one technique is used. Furthermore, sorption and calorimetric measurements are used to investigate the interaction with probe molecules, providing information on the distribution of the grafted molecules on the surface.

The synthesis method is executed at elevated temperatures. When this method is applied, the amount of PA influences the modification degree and binding type. The modification degree rises with increasing PA concentration. Furthermore, next to surface grafting also titanium organophosphonates (TiPs) are formed if high PA concentrations are used in combination with water as solvent. The pH of the reaction mixture also influences the modification degree and binding type. When the pH of the reaction mixture is lowered, higher organic loadings are achieved. In this case, TiPs can be found at lower PA concentrations. But, also the amount of surface grafting rises. The type of solvent has an effect on the modification degree and binding type. But, the type of solvent changes as well the modification mechanism due to differences in the solvent-surface interaction. For the modification in water, the competitive adsorption of water leads to the formation of patches grafted with PA and

General conclusions and suggestions for future research | 187 patches of surface hydroxyl groups. In contrast, the grafting mechanism is random when the reaction is performed in toluene.

Next to the influence of the solvent-surface interaction also the interaction between solute-solvent influences the modification. For the modification of TiO2 with phenylphosphonic acid (PhPA) in toluene a lower modification degree is observed in toluene than when the reaction is performed with 3PA. This is due to the interaction between PhPA and toluene, causing a competition for modification. For propylphosphonic acid (3PA), closed packed monolayers are formed at a lower concentration in toluene than in water. However, when the reaction is done in water TiPs are more easily formed than when toluene is used as solvent. By applying the modification in solvent mixtures (water-toluene), a high modification degree and TiPs formation can be achieved. However, the amount of water must be lower than 25 %. Furthermore, the reaction temperature influences the modification in water, but it does not when the reaction is executed in toluene. In scheme 1, the different parameters affecting the modification are summarized.

The initial adsorption step of modification is investigated by combining adsorption experiments and isothermal titration calorimetry (ITC). In this case the reaction is performed at room temperature. Small changes in binding type are observed during the first PA injections. With subsequent injections TiPs and pyrophosphates are formed next to surface grafting. After several injections, the saturation level is achieved, which corresponds to a modification degree of 1.6 groups/nm². Furthermore, in this case, the initial concentration of the stock solution and type of PA does not influence the modification degree, nor the type of binding.

188 •Modification degree Amount of PA •Type of binding

•Modification degree •Type of binding Solvent •Competitive adsorption or not •Modification mechanism: islands or random

•Type of binding pH •Modification degree •Modification degree in water Temperature •Little influence in toluene Type of PA •Interaction with solvent

Scheme 1: summery of different parameters influencing the modification of

TiO2 with PA.

An important point is that when the modification is executed via the classical one-step synthesis method at room temperature, only surface grafting occurs. But, when modification is done with step-by-step addition of PA, surface grafting, TiPs and pyrophosphates are formed. This points to an influence of pre-adsorbed PAs on the obtained structural properties.

By comparing the results of the synthesis method with the adsorption method, it can be concluded that in the case of the classical one-pot synthesis method the type of PA influences the modification degree, while it does not when the adsorption method is applied (scheme 2). Moreover, when the adsorption method is applied, which is done at room temperature, next to surface grafting also TiPs and pyrophosphates are formed. In the case of the synthesis method surface grafting always occurs and TiPs might be formed depending on the applied conditions, i.e. to form TiPs a high PA concentration or elevated reaction temperatures should be used. Another difference is that the concentration has an

General conclusions and suggestions for future research | 189 influence on the modification degree in the synthesis method, while the initial concentration of the stock solution does not influence the modification degree achieved via the adsorption method. Only less adsorption points are needed to reach the saturation level when a higher Ci is used.

Synthesis method Adsorption method

Type of PA Influence on the No influence on the modification degree modification degree

Binding type Surface grafting Surface grafting

TiPs only when high TiPs concentrations of PA and Pyrophosphates elevated temperatures are used

No pyrophosphates

Concentration Influences the modification No change in modification degree degree

Less injections needed to reach the saturation level

Scheme 2: comparison between synthesis – and adsorption method.

This Ph.D. demonstrated that the choice of synthesis conditions has a large impact on the final surface properties and their behavior in application as shown here by adsorption and displacement measurements with probe molecules. Additionally, combining ITC with adsorption experiments is a powerful methodology to study surface grafting.

190 Suggestions for future research

This Ph.D. dissertation reveals the influence of reaction conditions on the modification of TiO2 P25 with PAs. More specifically the solute-solvent-surface interactions are studied. TiO2 P25 consists of 80 % anatase, 20 % of rutile and a small amount of amorphous TiO2. From calculations, it is found that the type of

TiO2 has an influence on the binding type which is the most stable. 1 Therefore, if another type of TiO2 is being used, this might have an influence on the modification type. For example, on the anatase (101) surface only mono- and bidentate binding modes can be formed, while on the anatase (001) surface also tridentate binding modes are possible. On the latter surface, the optimization of a monodentate binding mode always results in a bidentate binding.1

Furthermore, not only the type of TiO2 will have an influence on the modification, also the amount of hydroxyl groups at the surface will have an influence on the modification type and degree. In future research, the influence of the crystal type, amount of hydroxyl groups and other types of metal oxides will be interesting to investigate. Therefore, attention must be paid by trying to generalize the obtained results in this Ph.D. and distinguish the surface specific aspects from the general aspects. However, the solute-solvent interactions will be general for PA modifications on different types of metal oxides. For example, also for the modification of other types of metal oxides, PhPA will interact with toluene, expecting to lead to a lower modification degree than when a PA without aromatic group is used. Furthermore, because it is necessary for the modification with PA to have a support material with hydroxyl groups at the surface, water will compete with the grafting reaction when water is used as solvent. This leads to different distributions of the organic groups on the support material. The size of the solvent-surface competition depends on the hydrophilic character of the surface and the interaction strength between the metal oxide and

General conclusions and suggestions for future research | 191 PA. Thus, only the presence of the solvent-surface competition can be predicted, but not the size of it.

Porous materials are often used in applications because of their large surface area. Therefore, it will be also intriguing to investigate the modification of porous materials. In the case of porous materials, diffusion limitation might occur. This will lead to a non-homogenous distribution of grafted groups. Hence, attention should be paid how to form a uniform distribution of grafted groups in the pores, while avoiding pore blocking. Furthermore, also the effect of the formation of TiPs should be studied. For example: these TiPs might cause pore blocking, hindering the modification in the pores and resulting in a lower accessible surface area. Sorption studies will be a useful technique to investigated the influence of pores on the modification.

In chapter 5, ITC measurements are combined with adsorption experiments. In that chapter, water is used as solvent. It would be useful to perform a similar study with other solvent types. During this Ph.D., also ITC measurements with toluene and cyclohexanone were tried, but unfortunately they were unsuccessful. In the case of toluene, no stable baseline could be achieved with ITC. Cyclohexanone adsorbs strongly or even binds at the surface leading to a competition of the modification with PA. Furthermore, it might be intriguing to use the ITC method with other modification types.

Reference

(1) Geldof, D.; Tassi, M.; Carleer, R.; Adriaensens, P.; Roevens, A.; Meynen, V.; Blockhuys, F. Binding Modes of Phosphonic Acid Derivatives

Adsorbed on TiO2 Surfaces: Assignments of Experimental IR and NMR Spectra Based on DFT/PBC Calculations. Surf. Sci. 2017, 655, 31–38.

192 Summary

Hybrid materials are being developed to add organic functionalities to inorganic materials. The hybrid materials can be synthesised using an in-situ process or via post-modification. In the case of in-situ formation, a precursor containing the organic group is added to the inorganic precursor material in the synthesis mixture. Post-modification is based on surface modification, introducing the organic functional group on an existing inorganic material. Via surface modification a (sub)monolayer is grafted on the surface. This layer alters the surface properties without changing the main structural material properties. Although this has to be somewhat nuanced as dissolution-precipitation process can locally change the structural properties.

In several applications, the surface properties of the materials play a key role in their performance (e.g., separation, sorption, catalysis, sensors). Hence, the surface properties of these materials need to be specifically adjusted to meet the requirements of each application. Even though many applications have been described for these hybrid materials in literature, a lack of knowledge of their physico-chemical properties and the correlated activity/performance remains.

The best known method for surface modification is organosilylation of silica. Irrespective of the wide use of silica, grafting on metal oxides has acquired interest due to their stability in a wide variety of solvents and pH ranges. Metal oxides have been modified with different organic coupling agents like organosilanes, Grignard reagents and organophosphonic acids. A drawback of modification with organosilanes is that they polymerize, leading to a lower hydrolytic stability. Furthermore, the amount of water added to the solvent needs to be controlled to obtain a stable monolayer. The modification with Grignard reagents is a multiple step synthesis, which needs to be performed in

Summary | 193 dry conditions in accordance with the high reactivity of Grignard reagents with water. Moreover, only partial monolayers are obtained. Organophosphonic acid grafting, applied in this work, can be performed in water and solvents, and they do not polymerize. Furthermore, high modification degrees can be achieved.

To design materials with suitable properties adjusted to the applications, thorough understanding of the influence of the modification method and conditions on the surface characteristics is necessary. The research described in this doctoral thesis is focused on the modification of TiO2 with organophosphonic acids (PA); more specifically attention is paid to the impact of the synthesis conditions on the final material properties.

In first instance, titanium dioxide is modified with propylphosphonic acid (3PA) under different synthesis conditions. The samples are characterised with DRIFT, TGA and 31P-CP/MAS NMR and a correlation between the characteristic signals is made. Combining these techniques, it is revealed that misinterpretations might be made if only one technique is applied. By a detailed investigation of the synthesis conditions, clear differences induced by the combination of solvent, temperature, pH and 3PA concentration are found. All these aspects have an impact on the modification type and degree.

In toluene, a lower concentration of 3PA is required to obtain closed packed monolayers than in water. Furthermore, elevated reaction temperatures do not influence the modification degree when the reaction is performed in toluene, while they do when water is used. In water, next to surface modification, titanium propylphosphonates (TiPPs) are formed. These TiPPs have a higher thermal stability of 70 °C as compared to surface grafted groups. Furthermore, by raising the 3PA concentration in water, more of these TiPPs are formed. Another driving force to form these TiPPs is the pH. It was also found that the factors that lead to the TiPPs formation such as pH, concentration and

194 temperature influence each other. Indeed, working with low 3PA concentrations and a similar pH leads to less TiPPs formation than when higher 3PA concentrations are used. Irrespective of TiPP formation, surface modification always occurs when the modification is performed in water. It can be concluded that modification in water and toluene result in different binding mechanisms, leading to distinct differences in surface properties.

In the next part of the research further attention was paid to the impact of the solute-solvent-surface interactions during the PA modification of TiO2. For this,

TiO2 was grafted with PhPA and 3PA in water, toluene and their mixtures. A clear influence of the solvent and the type of functional groups is found. Competing interactions between solute-solvent-surface clearly influence the modification.

Modification in toluene results in a higher modification degree for 3PA than for PhPA, as the solvent-solute interaction might hinder the grafting with PhPA in toluene. Water is preferred as solvent for PhPA modification at least at elevated temperatures as stabilizing OH... interactions enhance surface grafting, overcoming the competitive interaction of water at the surface as observed with 3PA. By working at room temperature, the same modification degree is found for the modification of TiO2 with 3PA and PhPA in water. Furthermore, calculated results confirm this. By using water/toluene mixtures for the functionalization of TiO2 with 3PA, higher modification degrees can be obtained than when using only water or toluene. Furthermore, adding small amounts of water leads to the formation of TiPPs, next to surface grafting.

Via sorption and calorimetric measurements after modification, it was found that the solvent has a clear impact on the distribution of the grafted groups on the surface. In toluene, which weakly interacts with the surface hydroxyl groups, random grafting of the PAs occurs. In this case, the surface energy and sorption

Summary | 195 capacity for probe molecules gradually changes with the number of functional groups. In contrast, when water is used as solvent, it will compete with surface functionalization due to its interaction with the surface. This leads to patches of grafted PA groups and patches of hydroxyl groups. The obtained surface energy and sorption capacity is the same, irrespective of the modification degree. In this case, a relatively stable average surface energy is measured and much less changes in sorption capacity of water upon increasing number of functional groups is observed. It can be concluded that the applied solvent and solute have a clear influence on the modification degree, the distribution of the grafted groups and consequently their interaction behavior.

In the last chapter of this Ph.D., attention has been paid to the initial steps of the modification mechanism. Furthermore, the energetic aspects related to the modification are revealed. The initial adsorption is one of the key steps of the binding of the PA to the surface and the conditions by which this is affected. They form the fundamental basis that determines the final material properties obtained and thus can identify means for further structural control. Isothermal titration calorimetry (ITC), a sensitive method to measure the heat released by molecular interactions, is applied to study this particular aspect of the modification mechanism. Using the combination of adsorption experiments and ITC results, differences in binding modes induced during the initial adsorption step of modification, are investigated.

After the first adsorption points of 3PA, small changes in binding types are observed. The saturation level of surface modification (1.6 groups/nm²) is achieved at an equilibrium concentration of 1.5 mM. The concentration of the PA added during the sorption measurement does not influence the modification degree. If higher concentrations are used, fewer adsorption points are needed to achieve this saturation level. Also the type of functional group does not influence

196 the sorption process, i.e., at the same Ceq the saturation level is reached. The saturation level found is actually only a half monolayer (a full monolayer contains 3.2 groups/nm² as determined in chapter 3), which seems to indicate that at the here applied conditions of room temperature, an energy barrier seems to exist. Pyrophosphates and titanium organophosphonates are observed when analyzing the materials obtained via the stepwise adsorption, while only surface grafting occurs when the synthesis is executed in one step, pointing to an influence of pre-adsorbed species (e.g., assembly effects) on the obtained structural properties.

In the framework of this Ph.D., it has been demonstrated that the choice of synthesis conditions (concentration, solvent, temperature, type of PA, pH and method of addition) have a large impact on the final surface properties. By paying attention to the surface-solvent-solute interactions, materials with specific surface properties can be prepared in a controlled way. This is expected to have a significant influence on applications involving surface interactions such as sorption, separation, catalysis, controlled release, immobilization, sensors etc. of which the performance is directly correlated to the surface properties. Furthermore, a powerful methodology to study surface grafting by the combination of ITC with adsorption experiments is demonstrated.

Summary | 197

Samenvatting

Hybride materialen worden ontwikkeld om organische groepen toe te voegen aan anorganische materialen. Deze hybride materialen kunnen op verschillende manieren gesynthetiseerd worden: in-situ of via post-modificatie. Als de in-situ methode gebruikt wordt, wordt er tijdens de synthese van het materiaal een precursor met een organische groep toegevoegd aan de anorganisch precursoren. Dit zorgt voor een homogene verdeling van de organische groepen in de matrix en aan het oppervlak. Bij post-modificatie wordt enkel het oppervlak gemodificeerd: hierbij zullen er organische functionele groepen gebonden worden aan de hydroxylgroepen op het oppervlak. Bij deze methode worden de oppervlakte-eigenschappen veranderd, zonder dat de structurele eigenschappen van het dragermateriaal aangepast worden. In het geval dat er een dissolutie-precipitatieproces optreedt, kunnen de structurele eigenschappen van het dragermateriaal veranderen.

De oppervlakte-eigenschappen van materialen zijn bepalend voor de goede werking van de materialen in toepassingen (bv. scheiding, sorptie, katalyse of sensoren). Daardoor moeten de oppervlakte-eigenschappen geoptimaliseerd worden voor iedere toepassing. Hoewel oppervlaktemodificatie al voor veel verschillende toepassingen gebruikt wordt, is er een gebrek aan kennis van de fysico-chemische eigenschappen en de gecorreleerde activiteit.

De best gekende methode voor oppervlakmodificatie is organosilanering van silica. Ondanks dat silica veel gebruikt wordt, stijgt de interesse naar de modificatie van andere metaaloxiden. Dit is te wijten aan hun hoge stabiliteit in vele solventen en een breed pH-gebied. Metaaloxiden worden gemodificeerd met organosilanen, organofosfonzuren en Grignard-reagentia. Nadelig aan de modificatie met organosilanen is dat ze gemakkelijk polymeriseren: dit geeft aanleiding tot een modificatie met een lagere hydrolytische stabiliteit. Om een

Samenvatting | 199 volledige monolaag te krijgen, dient er een precieze hoeveelheid water toegevoegd te worden aan het solvent. De modificatie met Grignard-reagentia is dan weer een meerstapssynthese die moet uitgevoerd worden in droge condities, daar Grignard-reagentia een hoge reactiviteit hebben met water. Verder geeft de modificatie met deze reagentia aanleiding tot gedeeltelijke monolagen. De modificatie met organofosfonzuren kan uitgevoerd worden zowel in water als in organische solventen. Daarnaast reageren de organofosfonzuren niet met elkaar, waardoor een polymerisatiereactie vermeden wordt. Bovendien kunnen hoge modificatiegraden bereikt worden met deze methode.

Om materialen te ontwerpen met aangepaste eigenschappen voor iedere toepassing, is een doorgedreven kennis van de invloed van de modificatiemethode en condities op de oppervlaktekarakteristieken noodzakelijk. Het onderzoek beschreven in deze doctoraatsthesis is gefocust op de modificatie van TiO2 met organofosfonzuren (PA); meer specifiek is de nadruk gelegd op de invloed van de synthesecondities op de finale materiaaleigenschappen.

In eerste instantie wordt TiO2 gemodificeerd met propylfosfonzuur (3PA) onder verschillende synthesecondities. De gemodificeerde stalen worden gekarakteriseerd met DRIFT, TGA en 31P-CP/MAS NMR en een correlatie tussen de verschillende karakteristieke signalen is gemaakt. Misinterpretaties zullen gemaakt worden indien één enkele techniek gebruikt wordt. Door een gedetailleerd onderzoek van de synthesecondities, werd een duidelijke invloed van de combinatie van solvent, temperatuur, pH en 3PA concentratie gevonden. Al deze aspecten hebben een invloed op het modificatietype en de modificatiegraad.

In tolueen kan er met een lagere 3PA-concentratie gewerkt worden om een dense monolaag te bekomen dan wanneer er in water gewerkt wordt. Een hogere

200 reactietemperatuur heeft geen invloed op de modificatiegraad als er gewerkt wordt in tolueen; in water heeft dit wel een invloed. Als de modificatie in water bij een hogere reactietemperatuur wordt uitgevoerd, worden er ook titanium propylfosfonaten (TiPPs) gevormd. De thermische stabiliteit van TiPPs is 70 °C hoger dan voor oppervlakte-gemodificeerde groepen. Daarnaast speelt ook de 3PA-concentratie een rol voor de vorming van de TiPPs: als de concentratie stijgt, stijgt ook de hoeveelheid TiPPs. Een andere factor die een invloed heeft op de vorming van TiPPs is de pH. In feite is het de combinatie van deze drie factoren die bepaalt hoeveel TiPPs er worden gevormd. Zo worden er bv. minder TiPPs gevormd als er bij een lage concentratie gewerkt wordt maar wel met dezelfde pH dan als er bij een hoge concentratie gewerkt wordt. Onafhankelijk van de vorming van de TiPPs, is er ook steeds oppervlaktemodificatie. Er kan geconcludeerd worden dat modificaties in water en tolueen resulteren in verschillende bindingsmechanismen, die dan weer aanleiding geven tot duidelijke verschillen in oppervlakte-eigenschappen.

In een vervolgonderzoek werd er aandacht besteed aan de invloed van de opgeloste stof-solvent-oppervlak interacties tijdens de modificatie van PA op

TiO2. Om dit uit te pluizen werd TiO2 gemodificeerd met PhPA en 3PA in water, tolueen en een mengsel van beide. Een duidelijke invloed van het solvent en type van functionele groep is gevonden. De competitieve interacties tussen opgeloste stof, solvent, en oppervlak hebben een duidelijke invloed op de modificatie.

Modificatie in tolueen resulteert in een hogere modificatiegraad voor 3PA dan PhPA. Dit kan verklaard worden door de solvent-opgeloste stof interactie die grafting van PhPA hindert in tolueen. Water is dan weer een beter solvent voor de modificatie van PhPA omdat de OH... interacties zorgen voor een verhoogde hoeveelheid grafting als er gewerkt wordt bij hoge temperaturen en de competitieve interactie van water aan het oppervlak dat geobserveerd wordt bij 3PA voorkomen wordt. Als er bij kamertemperatuur gewerkt wordt, wordt

Samenvatting | 201 dezelfde modificatiegraad bekomen voor de modificatie van TiO2 met 3PA en PhPA in water. Door gebruik te maken van een solventmengsel (i.e. water in tolueen) voor de modificatie van TiO2 met 3PA, worden hogere modificatiegraden bereikt dan wanneer enkel tolueen of water wordt gebruikt. Daarnaast zorgt de toevoeging van kleine hoeveelheden water voor de vorming van TiPPs, naast oppervlakmodificatie.

Via sorptie en flowcalorimetrie, die uitgevoerd werden na de modificatie, werd er aangetoond dat het solvent een duidelijke impact heeft op de verdeling van gegrafte groepen aan het oppervlak. In tolueen dat slechts een zwakke interactie vertoont met de oppervlaktehydroxylgroepen, treedt er een random verdeling van gemodificeerde groepen op. In dat geval verandert de oppervlakte-energie en sorptiecapaciteit voor de adsorptiemoleculen gradueel met het aantal functionele groepen. Als er in water wordt gewerkt, treedt er competitie op tussen de adsorptie van PA en water aan de oppervlaktehydroxylgroepen. Dit geeft aanleiding tot eilandmodificatie. Hierbij wordt oppervlakte-energie en sorptiecapaciteit uitgemiddeld, onafhankelijk van de modificatiegraad. In dit geval wordt er bij een toename van het aantal functionele groepen een relatief stabiele gemiddelde oppervlakte-energie gemeten en weinig veranderingen in sorptiecapaciteit van water. Er kan geconcludeerd worden dat de gebruikte hoeveelheid PA, solvent en type PA een invloed hebben op de modificatiegraad, de verdeling van gemodificeerde groepen en het interactiegedrag.

In het laatste hoofdstuk van dit doctoraat werd er aandacht besteed aan de initiële stappen van het modificatiemechanisme. De energetische aspecten gerelateerd aan de modificatie werden onderzocht. De initiële adsorptie is een van de sleutelstappen in de binding van PA aan het oppervlak en de condities waardoor het beïnvloed wordt. Deze initiële adsorptie vormt de fundamentele basis die bepalend is voor de uiteindelijke materiaaleigenschappen. Isotherme titratiecalorimetrie (ITC) is een gevoelige methode om de warmte geproduceerd

202 door de moleculaire interacties te meten. ITC wordt gebruikt om dit aspect te bestuderen. Door ITC te combineren met adsorptie-experimenten worden verschillen in bindingsmodi tijdens de initiële adsorptie bestudeerd.

Na de eerste adsorptiepunten van PA worden kleine verschillen in bindingsvorm geobserveerd. Bij een concentratie van 5 mM 3PA wordt bij een

Ceq van 1,5 mM het saturatieniveau bereikt (1,6 groepen/nm²). Als hogere concentraties gebruikt worden, zijn minder adsorptiepunten nodig om het saturatieniveau te bereiken. Ook het type functionele groep heeft geen invloed op het Ceq dat nodig is om het saturatieniveau te bereiken. Dit saturatieniveau is slechts een halve monolaag (een volledige monolaag bevat 3,2 groepen/nm² zoals bepaald werd in hoofdstuk 3), dit wijst op een te hoge energiebarrière voor een complete monolaag te bekomen bij kamertemperatuur. De concentratie 3PA gebruikt tijdens het adsorptie-experiment heeft geen invloed op de modificatiegraad. Pyrofosfaten en titanium organofosfonaten worden gevormd als er stapsgewijze toevoeging plaatsvindt van de PAs, terwijl enkel oppervlaktemodificatie optreedt als de synthese wordt uitgevoerd in één stap. Dit toont de invloed van geadsorbeerde species (i.e. assembly-effecten) aan op de structurele eigenschappen.

In het kader van dit doctoraat werd er aangetoond dat de keuze van de synthesecondities (bv. concentratie, solvent, temperatuur, type PA, pH en methode van toevoeging) een grote invloed heeft op de uiteindelijke materiaaleigenschappen en de interactie met adsorptiemoleculen. Door aandacht te schenken aan de opgeloste stof-solvent-oppervlak interacties kunnen materialen met specifieke oppervlakte-eigenschappen op een gecontroleerde manier gemaakt worden. Hiervan wordt verwacht dat ze een significante invloed zullen hebben op toepassingen die gebruik maken van oppervlakinteracties zoals sorptie, scheiding, katalyse, gecontroleerde vrijgave, immobilisatie, sensoren etc. waarbij prestatie gecorreleerd is aan de oppervlakte-

Samenvatting | 203 eigenschappen. Verder werd er een interessante methodologie aangetoond om oppervlaktemodificatie te bestuderen, namelijk door ITC-resultaten met adsorptie-experimenten te combineren.

204 List of publications

Revealing the influence of the solvent in combination with temperature, concentration and pH on the modification of TiO2 with 3PA. Annelore Roevens, Jeroen G. Van Dijck, Marco Tassi, Jan D’Haen, Robert Carleer, Peter Adriaensens, Frank Blockhuys and Vera Meynen Materials chemistry and physics, 2016, 184, 324-334

Aqueous or solvent based surface modification: The influence of the combination solvent - organic functional group on the surface characteristics of titanium dioxide grafted with organophosphonic acids. Annelore Roevens, Jeroen G. Van Dijck, Davy Geldof, Frank Blockhuys, Benedicte Prelot, Jerzy Zajac and Vera Meynen Applied Surface Science, 2017, 416, 716-724

In-situ study of the initial adsorption steps in the surface modification of titanium dioxide with organophosphonic acids with isothermal titration calorimetry. Annelore Roevens, Benedicte Prelot, Amine Geneste, Marco Tassi, Gunter Reekmans, Robert Carleer, Peter Adriaensens, Frank Blockhuys and Vera Meynen In Preparation

Binding Modes of Phosphonic Acid Derivatives Adsorbed on TiO2 Sufaces: Assignment of Experimental IR and NMR Spectra Based on DFT/PBC Calculations. Davy Geldof, Marco Tassi, Robert Carleer, Peter Adriaensens, Annelore Roevens, Vera Meynen, Frank Blockhuys Surface Science, 2017, 655, 31-38

List of publications | 205 A detailed investigation of the microwave assisted phenylphosphonic acid modification of P25 TiO2. Marco Tassi, Annelore Roevens, Gunter Reekmans, Martine Van Hamel, Jan D’Haen, Vera Meynen, Peter Adriaensens, Robert Carleer Advanced Powder Technology, 2017, 28, 1, 236-243

Characterization and analysis of the adsorption immobilization mechanism of β-galactosidase on metal oxide powders. Yamini Satyawali, Sandra Van Roy, Annelore Roevens, Vera Meynen, Steven Mullens, Peter Jochems, Wim Doyen, Lieve Cauwenberghs, Winnie Dejonghe RSC Adv., 2013, 3, 24054-24062

Oral presentations at conferences

The impact and diversity of surface chemistry and functionalization on membrane separation technology. A. Roevens, J. Van Dijck, G. Mustafa, M. Dorbec, D. Ormerod, A. Buekenhoudt, V. Meynen 41st International Conference and Expo on Advanced Ceramics and Composites (ICACC’17) 22-27 January 2017, Daytona, USA

Understanding the influence of synthesis conditions on the properties of functionalised TiO2. A. Roevens, J. Van Dijck, M. Tassi, P. Adriaensens, V. Meynen Euromat 2015, 20-24 September 2015, Warsaw, Poland

Revealing the impact of synthesis conditions on the properties of modified TiO2. A. Roevens, J. Van Dijck, M. Tassi, P. Adriaensens, V. Meynen

206 6th Advanced Micro- and Mesoporous Materials (AMMM), 6-9 September 2015, Black sunset, Bulgari

Surface sensitive techniques for static and dynamic characterisation of organic grafted molecular monolayers on TiO2. P. Van Heetvelde, A. Roevens, S. Maurelli, P. Adriaensens, S. Van Doorslaer, V. Meynen 10th International Symposium on the Characterisation of Porous Solids (COPS-X 2014), 11-14 May 2014, Granada, Spain

Influence of the synthesis conditions of organophosphonic acid modified TiO2 on sorption behavior. A. Roevens, V. Meynen Chemistry Conference for Young Scientists 2014 (Chemcys 2014), 27-28 February 2014, Blankenberge, Belgium

Exploring a new method via organometallic chemistry to obtain grafted molecular monolayers on a TiO2 supports. P. Van Heetvelde, A. Roevens, S. Maurelli, A. Buekenhoudt, P., Adriaensens, S. Van Doorslaer, V. Meynen Chemistry Conference for Young Scientists 2014 (Chemcys 2014), 27-28 February 2014, Blankenberge, Belgium

The benefit of in-situ IR accessories for materials research. V. Meynen, P. Van Heetvelde, A. Roevens Spectroscopy solutions seminar (Thermo Scientific), 24 April 2013, Neder-Over-Heembeek, Belgium

List of publications | 207 Poster presentations at conferences

Influence of synthesis conditions on organophosphonic acid surface modification. J. Van Dijck, A. Roevens, V. Meynen Balard Chemistry Conferences 2016, 5-8 April 2016, Montpellier, Frankrijk

Revealing the impact of synthesis conditions on the properties of modified TiO2. A. Roevens, J. Van Dijck, M.Tassi, P. Adriaensens, V. Meynen 6th Advanced Micro- and Mesoporous Materials (AMMM), 6-9 September 2015, Black sunset, Bulgarije

Revealing the impact of synthesis conditions on the properties of organic modified surfaces. A. Roevens, J. Van Dijck, M.Tassi, P. Adriaensens, V. Meynen Advanced Complex Inorganic Nanomaterials (ACIN), 13-17 July 2015, Namen, België

Surface characterisation of organophosphonic acid modified TiO2 made under different synthesis conditions. A. Roevens, J. Van Dijck, M. Tassi, P. Adriaensens, V. Meynen DZA14, 7 October 2014, Gent, Belgium

Sorption behaviour of organic modified titanium dioxide. A. Roevens, J. Van Dijck, P. Van Heetvelde, V. Meynen 10th International Symposium on the Characterisation of Porous Solids (COPS-X 2014), 11-14 May 2014, Granada, Spain

208 Received award

Travel grant: OJO (“omkadering jonge onderzoekers”) to go abroad (Montpellier France) with reference number (OJO2015/4/004)

Dissertation supervision

“Oppervlakmodificatie van TiO2 met organo-fosfonzuren.” Dean Van Riel, Masterthesis ingenieurswetenschappen, Universiteit Antwerpen, 2016

“Invloed van de omgevingsomstandigheden op de oppervlaktemodificatie van titania met fosfonzuren en omgekeerd.” Jeroen Van Dijck, Masterthesis Chemie, Universiteit Antwerpen, 2014

“Chemische stabiliteit van organofosfonzuur gemodificeerde titania oppervlakken.” Nicolas Van Oeteren, Masterthesis ingenieurswetenschappen, Artesis, 2013

“Chemische stabiliteit van organofosfonzuur gemodificeerde titania oppervlakken.” Ellen Meyntjens, Bachelorthesis ingenieurswetenschappen, Artesis, 2013

List of publications | 209