Master thesis - Nanologica AB

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Master thesis: Towards Mesoporous Titania Microspheres by
Supramolecular Self-assembly

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Master thesis - Nanologica AB

  1. 1. Towards Mesoporous Titania Microspheres by Supramolecular Self-assembly Master’s Thesis in Engineering Nanoscience Martin A. Olsson Lund 2009 Department of Materials Chemistry Lunds universitet SE-581 83 Lund, Sweden Lunds tekniska högskola Lunds universitet 581 83 Lund
  2. 2. Towards Mesoporous Titania Microspheres by Supramolecular Self-assembly Master’s Degree Project in Materials Chemistry at the Ångström Laboratory Martin A. Olsson Supervisors: Dr. Alfonso E. Garcia-Bennett Nanotechnology and Functional Materials, dept. Engineering Sciences, Uppsala University. Nanologica AB, Uppsala Science Park Prof. Reine Wallenberg Dept. Materials Chemistry, Lund Institute of Technology Examiner: Prof. Staffan Hansen Dept. Materials Chemistry, Lund Institute of Technology Lund, August 26th, 2009
  3. 3. Avdelning, Institution Division, Department Datum Date Division for Chemical Engineering Department of Materials Chemistry Lund University SE-581 83 Lund, Sweden Språk Language Rapporttyp Report category ISBN 2 Svenska/Swedish 2 Licentiatavhandling ISRN 2 Engelska/English 2009-08-26 2 Examensarbete 2 C-uppsats 2 D-uppsats 2 2 Övrig rapport — Serietitel och serienummer ISSN Title of series, numbering — 2 URL för elektronisk version http://theses.lub.lu.se/undergrad/ Titel Title Porös Titandioxid genom Supramolekylär Självorganisering Towards Mesoporous Titania Microspheres by Supramolecular Self-assembly Författare Martin A. Olsson Author Sammanfattning Abstract The increasing demand for electricity with a larger population of our planet forces solutions to ensure a future energy source for humanity. The climate change demands that this energy source is renewable. Such an energy source is the sunlight that hits our planet every day, which exceeds the present world consumption of energy per year. Dye-sensitized solar cells (DSSCs) are attractive light-harvesting devices since their design is suitable for large-scale mass production compared to all other present solar cell devices. DSSCs would be highly suitable as rooftop solar collectors if their conversion efficiency was increased. However, there is room for improvement of the power conversion efficiency of dye-sensitized solar cells. The high accessible surface area to sensitize titania is a viable route to improvement in the conversion efficiency of DSSCs. Employing the sol-gel method of producing titanium dioxide causes an inexpensive method for producing nanostructured titania nanomaterials with high surface area. The results in this thesis show that the sol-gel method with supramolecular self-assembly templating is a plausible approach of generating high surface area TiO2 , since there is an evident templating effect in titania. Decreasing the pore size has been investigated for the self-assembly of folate molecules in solution to achieve mesoporous titania by lower reaction rates. The results from this thesis show that formation of microspheres of titania by spray-drying increases the surface area by a factor of two. In this thesis the titania does not reach the goals for application in dye-sensitized solar cells due to constraints on a pH > 7 which causes too high reaction rates and low crystallinity. The main conclusion is that acidic conditions for titania synthesis is essential to the anatase crystallization and therefore the methods employed except chelation will improve the titanium dioxide and hence the efficiency of dye-sensitized solar cells. Materials prepared towards the end of this thesis, show that the relationship between surface area, crystallinity and dye adsorption capacity is not evident. High dye adsorption capacities were achieved by materials prepared by an atrane route which shows 28% of the dye adsorption capacity of the commercial titania P-25. Nyckelord Keywords Sol-Gel, Mesoporous, Titania, Chelation, Atrane, Acetylacetone, Folic acid, DSSC
  4. 4. Abstract The increasing demand for electricity with a larger population of our planet forces solutions to ensure a future energy source for humanity. The climate change demands that this energy source is renewable. Such an energy source is the sunlight that hits our planet every day, which exceeds the present world consumption of energy per year. Dye-sensitized solar cells (DSSCs) are attractive light-harvesting devices since their design is suitable for large-scale mass production compared to all other present solar cell devices. DSSCs would be highly suitable as roof-top solar collectors if their conversion efficiency was increased. However, there is room for improvement of the power conversion efficiency of dye-sensitized solar cells. The high accessible surface area to sensitize titania is a viable route to improvement in the conversion efficiency of DSSCs. Employing the sol-gel method of producing titanium dioxide causes an inexpensive method for producing nanostructured titania nanomaterials with high surface area. The results in this thesis show that the sol-gel method with supramolecular self-assembly templating is a plausible approach of generating high surface area TiO2 , since there is an evident templating effect in titania. Decreasing the pore size has been investigated for the self-assembly of folate molecules in solution to achieve mesoporous titania by lower reaction rates. The results from this thesis show that formation of microspheres of titania by spray-drying increases the surface area by a factor of two. In this thesis the titania does not reach the goals for application in dye-sensitized solar cells due to constraints on a pH > 7 which causes too high reaction rates and low crystallinity. The main conclusion is that acidic conditions for titania synthesis is essential to the anatase crystallization and therefore the methods employed except chelation will improve the titanium dioxide and hence the efficiency of dye-sensitized solar cells. Materials prepared towards the end of this thesis, show that the relationship between surface area, crystallinity and dye adsorption capacity is not evident. High dye adsorption capacities were achieved by materials prepared by an atrane route which shows 28% of the dye adsorption capacity of the commercial titania P-25. v
  5. 5. Acknowledgments This master’s thesis project has been financed by Nanologica AB. I want to thank Dr. Alfonso Garcia-Bennett in particular for giving me this opportunity to work in a state of the art research environment in the field of mesoporous materials and for for his advice. I want to thank Prof. Reine Wallenberg for his supervision. I want to acknowledge the generous advice for UV-vis spectroscopy by the Nanologica employee Dr. Roberto Hanoi. I want to thank Ph.D.-student Rambabu Atluri for sharing his office with me at the division for Nanotechnology and Functional Materials at the Ångström Laboratory. vii
  6. 6. Contents 1 Introduction 1.1 Aims of the thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Related work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Background 2.1 Dye-Sensitized Solar Cells . . . . . . . . . 2.2 Crystal structure of titania . . . . . . . . 2.3 Sol-gel chemistry . . . . . . . . . . . . . . 2.3.1 The effect of synthesis parameters 2.4 Mesoporosity . . . . . . . . . . . . . . . . 2.5 Self-assembly . . . . . . . . . . . . . . . . 2.6 Hydrolysis retardation . . . . . . . . . . . 2.7 Industrial drying of powders . . . . . . . . 2.8 Characterization techniques . . . . . . . . 2.8.1 Scanning electron microscopy . . . 2.8.2 X-ray diffraction (XRD) . . . . . . 2.8.3 N2 −sorption . . . . . . . . . . . . 2.8.4 Thermogravimetric measurements 2.8.5 Ultraviolet-visible spectroscopy . . 2.9 Methods . . . . . . . . . . . . . . . . . . . 3 4 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 5 6 7 10 11 12 15 15 17 17 18 19 19 19 20 3 Experimental 3.1 Synthesis of TiO2 by folate route (NFM-2) 3.2 Slowing down the hydrolysis rate . . . . . . 3.2.1 Acetylacetone route (NFM-2-AC) . . 3.2.2 Atrane route (NFM-2-AT) . . . . . . 3.3 Calcination . . . . . . . . . . . . . . . . . . 3.4 Spray-drying (NFM-2-SD) . . . . . . . . . . 3.5 Characterization . . . . . . . . . . . . . . . 3.5.1 XRD . . . . . . . . . . . . . . . . . . 3.5.2 SEM . . . . . . . . . . . . . . . . . . 3.5.3 N2 -sorption . . . . . . . . . . . . . . 3.5.4 Thermogravimetric measurements . 3.5.5 UV-vis absorbance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 23 23 23 24 25 25 26 26 26 27 27 27 ix
  7. 7. x Contents 4 Results and discussion 29 4.1 Designed materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 5 Conclusions 39 6 Future work 41
  8. 8. List of Abbreviations DSSC : Dye Sensitized Solar Cell SEM : Scanning Electron Microscopy FA : Folic acid APES : (3-aminopropyl)triethoxysilane TTIP : Titanium(IV) isopropoxide CSDA : Co-Structuring Directing Agent FWHM : Full Width at Half Maximum XEDS : X-ray energy dispersive spectrometry TG : Thermogravimetric measurements TEAH3 : triethanolamine (2,2’,2”-nitriloethanol) NFM-2 : Titania prepared by folate route TG : Thermogravimetric measurement SE : Secondary electron Material-suffix AT : Atrane route AC : Acetylacetone route SD : Spray-drying route
  9. 9. Chapter 1 Introduction There has been a long search for a low-cost solar cell since the early 90s [1–5] that can replace and compete with solid state solar cell devices based on silicon. Plant cells and bacteria uses photosynthesis to harvest light energy. Using light-harvesting molecules for making solar cells was firstly developed by Michael Grätzel et al. [1] . A DSSC uses light-harvesting molecules to make a semiconductor sensitive towards light i.e. dye-sensitization of the semiconductor. Titanium dioxide is one of the most studied materials for DSSCs as the sensitized semiconductor. [3] The high amount of research on titania as a suitable electrode material is due to its wide band gap and electron transfer properties of one of its crystal structures, namely anatase [6] . A detailed description of a DSSC will be given in the following chapter. DSSCs has the advantage of low-cost processing compared to other solar cell devices [7] . The demand for electrical grade silicon for the electronics industry is undoubtedly high. The largest cost of the conventional pn-junction solar cells comes from processing. For the DSSCs, the largest part of the cost comes from the dyes which roughly costs about $750 per gram. The drawback of DSSCs is that the manufacturing process has not yet really been improved much since the original design. The highest power conversion efficiency that has been achieved is 11% for non-transparent DSSCs. [8] The conversion efficiency is determined by several parameters e.g. the thickness of the titania electrode [9] , surface area of the titania [2] and the dye absorbance spectra amongst others [10] , the surface morphology [11] and the amount of inter-particle connections [7] that causes increased conductivity. Synthesis of titania is achieved at low temperature with weak dissociative acids or bases. Sol-gel chemistry occurs via nucleophilic attack during acidic, neutral and basic [12] conditions for salts and transition metal alkoxides. [13;14] Titania has been recognized for its potential as an electrode material due to its photoinduced electronic transfer properties associated with the anatase phase. [1] Other applications of the anatase are in catalysis [15;16] , lithium-ion batteries [17] and cosmetics [18] . Therefore the titania has been intensely studied the past years for applications such as: DSSCs, self-cleaning coatings [19] , antibacterial tooth implants [19] , sensors [20] and depolluting layers [20] . This master’s thesis is concerned with the design of titania for DSSCs with the use of folic acid as nanoscale pore forming agents. Previously, mesoporous titania has been prepared 3
  10. 10. 4 Introduction for DSSCs giving mixed results in efficiency in similar cell designs e.g. mesoporous titania by surfactant routes has given 5% [10] and 8% [21] . When mesoporous titania is sintered during processing the mesopores are blocked and the surface area decreases. This thesis will describe a novel route to slow down hydrolysis under basic conditions for titanium alkoxides. The thesis elaborates upon titania materials obtained during this thesis work. The thesis starts off in the application of titanium dioxide in DSSCs as a motivation for an in-depth sol-gel chemistry review. This is followed by a brief general extension of sol-gel chemistry to mesoporous materials. Self-assembly and its application for synthesis of mesoporous materials is described. The literature review of the thesis covers mostly synthesis but also processing of the material. The characterization with the procedures used are presented as well, since some of the techniques are not conventional in nanoscience. Detailed procedures are not intended to be exhaustive but merely increase the reproducibility of the work. 1.1 Aims of the thesis • To investigate a novel self-assembly synthesis route for making a new titania based material. • Investigation of the pore size control in titania by supramolecular self-assembly. • To investigate an industrial drying processing method of the titania material by laboratory bench-scale method for obtaining solid spherical microparticles of titania for use as electrode material in DSSCs. • To compare the dye adsorption of standard commercial available titania powder used for DSSCs (Degussa P-25) [22;23] and the synthesized titania material. 1.2 Related work There are several scientific reports available on the sol-gel synthesis of mesoporous titania. [19] However, since there are a lot of patents on the use of surfactant routes it has become difficult to commercialize those routes further [24;25] . Since sol-gel chemistry is a broad field with a lot of different precursors, a general treatment is not the scope for a master’s thesis project. A full treatment of sol-gel chemistry is covered by Brinker (1990) [13] . Transition metal oxides are covered by Livage et al. (1988) [14] . Since the synthesis conditions are very important in the case of sol-gel chemistry, two different materials are not often comparable with the same synthesis conditions. The work in this thesis is based on the recent synthetic results of self-assembly templating by Garcia-Bennett et al. (2009). The following chapter is a brief review based on and around this literature.
  11. 11. Chapter 2 Background 2.1 Dye-Sensitized Solar Cells A DSSC is an electrochemical cell. The cell design is depicted in Figure 2.1(a). The principle of a DSSC is a sandwich structure that consists of two transparent glass layers each coated with an electrode material. The whole cell is denoted as F-doped ITO//nc-TiO2 //N719//electrolyte//Pt//F-doped ITO, where nc-TiO2 is referred to as nanocrystalline titania. One of the electrodes is a titania layer and the other is a transparent conducting oxide (TCO), usually Indium Tin Oxide, ITO. The titania electrode is soaked in a dye (Figure 2.1(b)) during processing causing the dye to chemisorb to the surface of the titania by carboxylate linkages. Between the – two electrodes there is an electrolyte usually I – /I3 which serves as a redox pair. The dye adsorbs UV-light and causes a charge separation [26] of electrons and holes in the ground and excited redox states. The electrons are injected to the titania conduction band through the carboxylate-titanium linkage. The injected electrons from the dye is replaced by electrons from the oxidation of the electrolyte redox molecule iodide. The electrons are transported through the titania network to a back contact and performs work in an external circuit. The electrons return to the counter electrode and take place in the reduction of triiodide. There is a decreasing energy path through the cell, hence it is thermodynamically possible. In general the efficiency reaches 10% when sintered nanocrystalline TiO2 is used. Novel designs for the electrode material are TiO2 -nanotubes [27] , ZnO nanowires [28;29] with conversion efficiency 0.5-1.5 % [30] and microspheres of nanocrystalline ZnO [30] with conversion efficiency 5.4% [30] . The latter design will be further elaborated upon for titania in this master’s thesis. 5
  12. 12. 6 Background Figure 2.1: (a) The principle of a dye-sensitized solar cell. (b) The most widely used dye for DSSCs, namely N719. The oxidation and reduction reactions taking place in the DSSC are Oxidation: 3 I− − 2 e− → I− 3 Reduction: I− + 2 e− → 3 I− 3 2.2 Crystal structure of titania Titania is a wide band gap semiconductor with three polymorphs [31] . The three crystal structures are rutile, brookite and anatase. Since brookite is not relevant to this master’s thesis it is excluded from this description. The rutile crystal structure is shown in 2.2(a)-(b) whereas the anatase crystal structure is depicted in Figures 2.2(c)-(d). Both rutile and anatase are tetragonal minerals. Rutile can be viewed as removing every other row of octahedra from an NiAs crystal structure. Compared to the NiAs crystal structure which is ionic, the TiO2 is covalently bonded. The structure has a Ti:O coordination of 6:3 and is nearly close-packed. [32] The titanium atom is coordinated by six oxygen in a slightly distorted octahedron. Viewing the structure from the perspective of the oxygen, one oxygen atom is surrounded by three planar titanium atoms at the corners of a triangle. The structure can also be viewed in terms of chains of octahedra sharing corners and opposite edges [32] . Anatase is body centred. The crystal structure also has a glide plane along the c-axis in the ab-plane. The TiO6 octahedra are corner sharing in the plane in an alternating pattern. The space group for rutile is the tetragonal P 42 /mnm where P means a primitive lattice, 42 describes how the elements are screwed along a four-fold axis 90◦ with translation half of the lattice vector, the m and n indices are mirror plane and a n-glide plane , respectively. An n-glide plane is a translation half the lattice vector and mirroring of a diagonal of a face for the unit cell. For anatase the space group is I41 /amd, where I means a body centred lattice, 41
  13. 13. 2.3 Sol-gel chemistry 7 means a screw axis where a rotational symmetry occurs by 90◦ with four-fold symmetry and translation one fourth of the lattice vector. [6;32] The letters a and d are glide planes where a is a translation 1/2 of the lattice vector along only the a-axis and d is a glide plane along a fourth of a space diagonal or face of the unit cell. Figure 2.2: (a) The rutile structure (b) The rutile structure in the [110]-projection. (c) The anatase crystal structure viewed from the [001]-projection. (d) The anatase crystal structure viewed from the [100]-projection. The titanium atoms are illustrated with the larger spheres and the polyhedra. Oxygen atoms are represented with the smaller spheres. [33;34] 2.3 Sol-gel chemistry Preparing titanium dioxide in the simplest possible manner by the sol-gel method is straightforward and comprises adding a highly reactive titanium alkoxide to water, which causes a white precipitate. Templating titanium dioxide on the nanoscale by supramolecular self-assembly is however not as straightforward. In order to control the reactions occurring, a deeper understanding is appropriate. The two types of reactions that take place for sol-gel chemistry are hydrolysis and condensation. Polycondensation causes formation of colloids which readily polymerize to precipitates in solution if the hydrolysis rate is too fast. [13] It is the initial hydrolysis step that is the rate determining step in gel formation and the gelation rates is increased by the addition of spectator solvent. [35] Sol-gel chemistry is a subfield of inorganic and materials chemistry. Sol-gel chemistry has been recently re-visited due to the discovery of mesoporous materials. The sol-gel chemistry uses precursor molecules that usually are in the form of alkoxides but halogen derivatives are also common but even more reactive [13] . Some typical precursor alkoxides are depicted in Figure 2.3. Precipitation occurs by the same theory as proposed by Flory [13;14] on the general polymerization, extending the definition from organic polymers to inorganic. Even if the inorganic framework is polymerizing it is not this
  14. 14. 8 Background fact that causes gelling, but the polymerization of colloids. The meaning of a colloid in the sense of sol-gel chemistry is strictly an oligomeric entity. The transition metal alkoxides are very reactive due to the electronegative alkoxide groups. This makes the titanium alkoxides highly unstable to nucleophilic attack which is the main reaction for the alkoxide chemistry. Therefore titanium dioxide readily precipitates upon contact with merely water. [14] The general differences for silica precursors and titania precursors can be summarized as: • Silica has been the most studied material system, but the sol-gel chemistry of silica is somewhat different from titania. The titanium atom, in the precursor alkoxides, is more electropositive than the silicon atom. [13] • Titanium alkoxides exhibit various stable coordinations. When the alkoxide is coordination unsaturated i.e. has a stable coordination different from the oxidation number, it is able to undergo coordination expansion by olation and alkoxy-bridging. [13] Figure 2.3: (a) Tetraethylorthosilicate (b) Titanium(IV) ethoxide (c) Titanium(IV) isopropoxide. The alkoxy group OR where R is alkyl or an aryl-group, is an electron donating group and stabilizes the highest oxidation state of the titanium atom in the molecule. A silicon atom (14 Si) has the orbital structure 1s2 2s2 2p6 3s2 3p2 whereas a titanium atom (22 Ti) is described by an orbital structure 1s2 2s2 2p6 3s2 3p6 4s2 3d2 . The lone d-electron pair of the titanium causes a very high reactivity in comparison to the p-electrons of silicon. The consequence is that titanium alkoxides are much more reactive than its corresponding silicon alkoxides. Titania therefore precipitates by hydrolysis and condensation of titanium alkoxides. The mechanism for hydrolysis and condensation is nucleophilic substitution. The hydrolysis takes place by nucleophilic addition of a water molecule to the titanium alkoxide at neutral conditions. This leads to a transition state with an intermediate with coordination number five [36] , and produces a good leaving group shown in Figure 2.4. The leaving group can be deduced by a partial charge model. In the non-catalyzed reaction a proton transfer is essential to take place in the transition state. A proton is transferred from the incoming water molecule to the leaving alkoxy-group producing the parent alcohol as the leaving group in the neutral reaction mechanism shown in Figure 2.4. [14] The meaning of ’parent’ in this sense is the corresponding alcohol to the alkoxy-groups. The molecular species with the largest negative partial charge is the nucleophile. The partial charge of a molecular species is
  15. 15. 2.3 Sol-gel chemistry 9 Figure 2.4: The reaction formulas under neutral conditions pH=7. [13;14;36] . related to its reactivity. When two atoms combine in a reaction a charge transfer occurs. The electronegativity is linear with the atom charge. The charge transfer stops when the electronegativities of the moleuclar species becomes equal to the mean electronegativity. This is the principle of electronegativity equalization and corresponds to the electronic corresponding principle of chemical equilibrium in thermodynamics. [14] . In the absence of a catalyst, hydrolysis and condensation is taking place by a SN -mechanism according to nucleophilic addition and a consecutive proton transfer in the transition state and the production of a leaving group which receives the proton as depicted in Figure 2.4. The reactions depend on the strength of the nucleophile, the electrophilicity of the metal atom and the partial charge of of the leaving group, δ(molecule) . The general reaction is that an alkoxy-group where the partial charge is negative, attacks the metal atom where the partial charge is positive. Silicon alkoxides are not very reactive with water (pH=7) compared to the transition metal alkoxides which causes highly exothermic reactions. The partial charge of the titanium metal atom in Ti(OEt)4 is +0.63 and for Si in Si(OEt)4 it is +0.32. A minimum value of the rate of hydrolysis can be estimated to k = 1 mmoldm−3 s−1 at pH=7 for Si(OR)4 which is five orders of magnitudes less than for Ti(OR)4 . [13] Acid or base catalysts can influence both the hydrolysis and condensation rates, but also the structure of the condensed product. [14] Acids serve to protonate nucleophilic alkoxide groups in the reaction mechanism, enhancing the reaction kinetics by producing good leaving groups and eliminate the need for proton transfer in the transition state. Therefore hydrolysis goes to completion if sufficient water is added. [13] Under basic conditions the hydroxyl ion acts as a nucleophile towards titanium alkoxides and no proton transfer is needed in the transition states [37] . For example
  16. 16. 10 Background Figure 2.5: The reaction mechanisms under acidic conditions. The acidic hydrolysis is related to a SN 2-reaction and the condensation is a SN 1-type of reaction. the hydrolysis of titanium isopropoxide is not generating any isopropanol leaving molecules under basic conditions compared to acidic conditions. Increasing the synthesis temperature causes a faster hydrolysis and condensation, in general, due to lowering of the energy barrier for the reaction. In general under a large pH range for acidic conditions the hydrolysis is fast and the condensation is slow. Under basic conditions the hydrolysis is slow and condensation is fast. By dissolving the transition metal alkoxide in different media, different nucleophilic interactions occurs due to the polarity of the solvent. For example, the titanium(IV) ethoxide takes the form of an oligomer in non-polar solvents. This is due to the coordination number of 5-6. This type of alkoxy bridges are is stable to hydrolysis and therefore slows down the hydrolysis rate, but causes the formation of unwanted gels instead. When dissolving titanium ethoxide in ethanol the alkoxide is instead monomeric due to solvent interaction instead. Metal alkoxides are often dissolved in organic solvent before hydrolysis is performed. As a general rule, dilution should lead to lower alkoxy association but the polarity of the solvent has to be taken into account e.g. Ti(OEt)4 remains trimeric in non-polar solvent such as hexane, but the same is not true for using polar solvent such as ethanol. This is due to the nucleophilicity of the polar solvent causing solvation of the oligomer. The polar solvent is thus expected to be associated with the alkoxide instead. 2.3.1 The effect of synthesis parameters The changes upon pH of the sol-gel chemistry of titania under acidic and basic conditions is explained by adsorption and desorption of hydronium ions or hydroxide ions respectively at titanium and oxygen-sites. [38] The time for reaching the equilibrium of adsorption of hydroxide ions to Ti(OH)4 is a few hours at room temperature
  17. 17. 2.4 Mesoporosity 11 and is accelerated at higher temperatures. [38] Neutral electrolytes also can enhance the anatase transformation due to shielding the effect of the attractive forces from titanium-atoms to hydroxide ions under basic conditions. Increasing the adsorption of hydroxide ions to TiO2 nuclei is the reason for the declining nucleation rate of anatase TiO2 with basic pH. [38] The synthesis temperature determines the phase transformation temperature during calcination. For a lower water-to-alkoxide ratio, organic components are easier to eliminate and the grain size is smaller explained by less degree of polymerization. [39] Figure 2.6: The reaction mechanisms under basic conditions. 2.4 Mesoporosity The discovery of the first synthetic zeolite ZSM-5 was done by what today is the Exxon Mobil Corporation. But it was not until the discovery of the first mesoporous silica MCM-41 that the wide research into the mesoscale porosity (2-50 nm) begun in 1992 [40] . If the pores are below 2 nanometres they are called micropores and above 50 nanometres they are called macropores. It was mesoporous silica with the use of cationic surfactants that was firstly discovered in [40] . An alkaline route renders the silica negatively charged, whereas the surfactant is positively charged. This leads to a charge interaction between a micellar phase above a critical micelle concentration. It is owing to this charge interaction that there is a pore formation. The framework condenses around an already formed surfactant liquid crystalline phase. Several different mesoporous titania routes have been reported since then such as the use of the anionic surfactant Dodecyl sulphate [41] , the neutral surfactant Dodecylamine [42] , the cationic surfactant Cetyltrimethylammonium bromide [43] , the neutral poly-
  18. 18. 12 Background meric surfactant P123 [44] and a non-surfactant β-cyclodextrin/urea-mixture [45] . The use of derivatives of nucleotides as templates has only been reported in silica [46] . Figure 2.7: (a) Folic acid molecule. (b) Tetrameric self-assembly unit of folate molecules [46] . Figure 2.8: The sequence of self-assembly with stacking into columns and into an hexagonal phase with p6mm symmetry. 2.5 Self-assembly Supramolecular self-assembly is defined as the process by which a molecular species forms spontaneously from its monomer molecules. Common interactions are aromatic interactions, van der Waals forces and hydrogen bonding. It is possible to analyze the interactions based on combined perpendicular T-geometry, face-to-face interactions and offset interactions of π − π-stacked systems [47;48] . Net favourable π − π interactions are not due to attractive π − π interactions but occur due to attractive interactions between π-electrons and the σ-framework i.e. π − σ interactions that are larger than the repulsion of the π − π interactions. [49] . Garcia-Bennett et al. showed that folic acid, see Figure 2.7(a), self-assemble and can be utilized for pore formation with a silica framework, using (3-aminopropyl)triethoxysilane for inducing the self-assembly [46] . The fact that folic acid derivatives self-assemble in salt solution has been reported previously [50;51] . The self-assembly with ions are best with potassium ions for guanosine derivatives and sodium ions for folic
  19. 19. 2.5 Self-assembly 13 acid which utilizes cation-pi interactions [52–54] . The self-assembly used in sol-gel by Garcia et al. [46] of folate is somewhat different from the surfactant self-assembly since no hexagonal phase exists but is formed in-situ. The self-assembly rely on the hydrogen-bonding of one of the tautomers of the pterin-group. The pterin-group is the nucleotide-derivative of guanine. Four folate pterin-groups with two hydrogen bonds per group arrange the folate molecules into tetramers, shown in Figure 2.7(b). The tetramers stack themselves as face-to-face [46;55] in a beautiful way as chiral columns with the glutamate groups of the folic acid sticking out from the columns, see Figure 2.8. The distance between tetramers in the stacks is 0.33-0.34 nm for sodium folates. The sodium is thought to be present between the tetramers stabilizing the structure. Both carboxylates of folate molecules interact with APES. The co-structure directing agent (3-aminopropyl)triethoxysilane is depicted in Figure 2.11. It acts as a base causing production of OH – catalysing hydrolysis of the alkoxide. But it also associates with its protonated positive amine-group to the negative deprotonated carboxylic group of the folate-stacks. The alkoxy groups of (3-aminopropyl)triethoxysilane can then contribute in hydrolysis and condensation of an inorganic network. The tetrameric stacks produce a hexagonal phase within a framework. The stacks formed by the discoid tetramers are depicted in Figure 2.8. The APES is critical to the pore condensation as it will cause condensation to occur at the surface of the folate-stacks. The pore condensation is depicted for a single folate molecule of a tetramer in Figure 2.9(a) and a corresponding pore is sketched in Figure 2.9(b). Figure 2.9: (a) Condensation of a mesopore under basic conditions. The titania has a negatively charged surface.(b) The Figure is a sketch of a plausible pore caused by one tetramer of a folate-stack. When the as-synthesized titania is calcined the organic is burned.
  20. 20. 14 Background Figure 2.10: The relationship of the pH to the self-assembly process at room temperature. Note the neutral amine-group of the folate above pKa = 8.3. Below the pH of 3.5 the amine-group of the folic acid is positively charged and both carboxylic groups of the glutamic acid moiety are not deprotonated. Below the pH of 5 the α-carboxylate is formed. Below the pH of 8.3 the γ-carboxylic group is deprotonated. Above the pH of 8.3 the amine of the pterin-group is deprotonated. The neutral amine group of the pterin moiety is critical to the Hoogsten-type hydrogen bonding for the formation of tetramers. Folate forms at a pH above 3.5. [56] Above the pH of the pKa = 8.3, the amine on the pterin-group of folate is deprotonated. [46;57] The amine takes part in Hoogstentype hydrogen bonding [49] causing the formation of the tetrameric unit. Below the pKa = 9.8 of the base APES the amine of (3-aminopropyl)triethoxysilane is protonated [46] . The synthesis conditions are therefore in the approximate pH range of 8.3 < pH < 9.8. The reason for using a CSDA is that the titania is negatively charged under basic conditions. The folate is also negatively charged hence an electrostatic interaction causes the possibility to condense a framework surrounding the folate stacks. The relationship with pH of the folate is similar to the titration curve of glutamic acid. The scheme with relation to the pKa -values is shown in Figure 2.10. Figure 2.11: (3-aminopropyl)triethoxysilane. The co-structuring agent (CSDA).
  21. 21. 2.6 Hydrolysis retardation 15 The crystal structure of titania that is obtained by synthesis is determined primarily by the preparation. The crystallite size of anatase can be controlled by adjusting the aging time of the dry gel. The rutile content can be controlled by the wet gel aging time. Moreover, the surface structure also contributes to the nucleation of rutile. [58] Figure 2.12: Organic reaction mechanism of 1-isopropoxytitanatrane. 2.6 Hydrolysis retardation Selecting the proper solvent for the reaction can determine the precipitation of a stable colloid. [13;14;20] Sol-gel reactions of TTIP has all led to immediate precipitation in water and organic solvent. This is not the case if a chelating agent is used. [59] As was stated in section 2.3 the hydrolysis rate of titanium alkoxides are very high. The reaction rate can be decreased by lowering the functionality i.e. the amount of available alkoxy-groups that can be hydrolysed can be decreased by addition of a chelating agent. [14] Increasing the coordination number, causes a higher stability to nucleophilic substitution. [13;14] Displacing the equilibrium can be made with the parent alcohol in excess in the synthesis solution under acidic conditions. [13] In terms of the available theory on sol-gel chemistry the most straightforward way to obtain a lower hydrolysis rate would be to decrease the partial charge of the titanium atom in the precursor. [13] The partial charge of titanium isopropoxide was emphasized in section 2.3 where it was compared to the silica precursor tetraethylorthosilicate. The decrease of only the functionality can be obtained with acetylacetone. Acetylacetone has been shown to stabilize the anatase crystal structure and prevent rutile formation up to 1000◦ C. [60] The replacing ligands are however not larger than e.g. isopropoxide groups, hence not blocking coordination sites. [61] The sol-gel reaction of acetylacetone modified titanium alkoxides causes acetylacetone to remain bound to titania even when hydrolysis is performed with excess of water. [62] Lowering the partial charge and the functionality can be obtained by the use of an atrane complex. Triethanolamine has been used to slow down the hydrolysis rate of titanium dioxide successfully. [38] 2.7 Industrial drying of powders This section will deal with an industrial method for large-scale drying of titania powders. Spray-drying is a drying method that has been considered as a bottom-
  22. 22. 16 Background Figure 2.13: Spray-dryer setup. The droplets produced is in the micron range. (a) feed solution (b) peristaltic pump (c) nozzle column (d) nozzle (e) spray cylinder (f) outlet (g) cyclone (h) vessel (i) filter up approach since it causes the possibility of designing functional materials with nanoscale properties of architectures in the size of microns from nanoparticles. [63] Spray-drying is an already used method of preparing large amounts of dried powders for the food industry [63;64] e.g. as dairy powders [63;64] , for the chemical industry [64] and the pharmaceutical industry [64–68] . The method has been increasingly used for preparation of materials for controlled release of drugs in drug-delivery. [69;70] This is even the method for making instant coffee. [71] Spray-drying has shown to produce micron-sized particles which are collapsed [72] , donought-shaped, dispersed spherical shells [63;72;73] , dispersed [73;74] and agglomerated [73] spherical solids in silica. Spray-drying has also been used to spray-dry mesoporous silica successfully into microspheres using surfactant [73] . Moreover, collapsed [75] , spherical shells [75;76] and solid spheres [77] have been reported for titanium dioxide. Nanocrystalline titania microspheres of an approximate size of 10 µm has shown to have a surface area of 560 m2 /g even without any template for mesoporosity. [73] Even mesoporous titania microspheres of size 8.3 µm that are solid have been reported but with a low surface area of 50.1 m2 /g and smooth surface texture [78] . It is therefore desirable to investigate spray-drying of nanocrystalline titania but also to investigate if mesoporosity can give higher surface area. The synthetic conditions contributes to high temperatures due to the water content i.e. outlet temperatures needed is at least 100◦ C at (d) in Figure 2.13. A lower temperature is expected to improve the morphology of the particles. A donought shape form due to higher evaporation rates in the centre of the droplet, which is due to the higher temperature in the droplet centre. The formation of a crust in the beginning of the drying and the migration of dispersant to the surface causes the crust formation that results in particle shells. The spray-dryer used is depicted in Figure 2.13 with letters corresponding to the different parts of the equipment. The gas supply is attached to the N2 -inlet and is heated up by an electric heater. The feed solution (a) is pumped with a simple
  23. 23. 2.8 Characterization techniques 17 peristaltic pump (b). The feed solution is heated up in the nozzle tube (c) by the hot gas. The sprayed solution leaves the nozzle at (d) and travels through the spray cylinder (e). The aerosols travel to a cyclone (g) to separate from the gas stream and the product is obtained in a vessel(h). The outgoing air is filtered and an aspirator pumps air through the system. [79] Solvent circulates in a closed loop. The inert gas with solvent vapour is cooled hence the solvent condenses into a bottle. [79] The cleaned gas stream is pre-heated and flows back into the spray dryer. [79] In general, hollow microspheres are produced from well-dispersed sols whereas if the sols are partially aggregated they produce solid particles [80] . 2.8 2.8.1 Characterization techniques Scanning electron microscopy Characterization of the material morphology is performed with scanning electron microscopy (SEM). A scanning electron microscope uses electrons instead of photons to produce an image. To produce electrons an electron gun is used. For high resolution SEM field emission sources are used exclusionary. The lenses used to focus the electron beam are magnetic and therefore has hysteresis built in which causes astigmatism. Bombarding a conducting sample with high energy electrons, can be used to produce signals that contain morphological information. Secondary electrons (SEs) that are knocked out from core electron shells have a low kinetic energy and can be collected with a detector that attracts the electrons with a positive potential. This way of detecting the SEs causes an artificial light in the images. Also due to the bombarding of electrons, X-rays are produced from the excited volume when electrons fall back from their respective excited states. X-ray energy dispersive spectroscopy can be used to analyse the characteristic X-rays produced. Figure 2.14: The image shows a LEO1550 scanning electron microscope equipped with a high resolution SE-detector called InLens within the pole piece.(a) N2 -liquid dewar (b) Secondary electron detector (c) Column of lenses (d) IR-camera (e) XEDS detector that can be inserted in to the chamber.
  24. 24. 18 2.8.2 Background X-ray diffraction (XRD) Characterization of the crystal structure is performed with the use of X-ray radiation. The X-rays have wavelengths in the order of atomic distances in crystal structures i.e. in the order of 0.1 nm. X-ray diffraction is described by the Bragg law nλ = dhkl sin θhkl (2.1) Figure 2.15: Image of a Bragg-Bretano Siemens D5000 X-ray diffractometer geometry in locked coupled mode; the x-ray tube and detector are aligned at the same angle. The order of the X-ray pathway is (a) X-ray tube (b) divergence slit (c) illuminated spot (d) anti-scattering slit (e) filter slit for calibration of the equipment (f) detector slit (g) detector. In (h) a sample holder is shown and in (i) a sample holder lid. In (j) a titania specimen prepared by capillarity effects is shown. where dhkl is the spacing between sets of crystal planes with indices hkl, θhkl is half the angle between the incoming light beam and the outgoing ray. This law describes the constructive and destructive interference of the radiation when incident on the set of crystal planes. The crystallite size is described by the Scherer equation which is equation 2.2. This equation states qualitatively that the crystallite size decreases with a broad Full-Width-at-Half-Maximum (FWHM). The larger FWHM, the smaller crystallite size. The opposite is also true i.e. that for a high intensity with a small FWHM has a large crystallite size. Dhkl = Kλ βhkl cos θhkl (2.2) The value n in equation 2.1 is an integer and λ is the wavelength of incoming rays. The Dhkl is the crystallite thickness, the K-factor takes a value between 0.94 and 0.98 for perfect cubic and perfect spherical crystallites, respectively. All angles are in radians as well as the peak width βhkl and in θ(◦ ). For anatase the crystallite size is calculated from the 101 peak since it has the highest abundance in the crystallites. Reflections at low-angles < 6◦ are equivalent to order at distances corresponding to mesoscale order. Low-angle XRD cannot measure mesoporosity, only the quality of the mesoporosity in terms of the orderity. The difficulties with measurements at low angles is that the background noise from the sample holder increases, it is also possible that artefacts from the detector appear. Zero noise silicon wafer sample holders are preferred for these measurements.
  25. 25. 2.8 Characterization techniques 2.8.3 19 N2 −sorption A typical equipment for measuring surface area and pore size is shown in Figure 2.16. For a mesoporous material, filling the pores with N2 -gas will primarily lead to monolayer coverage. The next layer will then be physisorbed to the first layer at a higher pressure. At a high enough pressure the pores will be filled with gaseous N2 that condenses to liquid N2 . When lowering the pressure isothermally, it takes more energy to fill the pore than to evaporate the liquid N2 . [81] This can be seen as a relative pressure drop in desorption and hence hysteresis in the sorption data. Figure 2.16: A Micromeritics ASAP 2020 equipment for N2 -sorption measurements.(a) degas stations (b) reference tubes with thermos filled with liquid nitrogen (c) analysis station with thermos filled with nitrogen (d) glass balloon for N2 -sorption measurements (e) isothermal jacket. 2.8.4 Thermogravimetric measurements A thermogravimetric analyser is depicted in Figure 2.17(a). The thermogravimetric analysis (TG) is used to measure weight-loss of a sample by heating at a constant rate inside a furnace (b). The furnace contains a delicate balance (c) which is able to record the weight of the sample. For porous materials the weight-loss from burning the organic template can be deduced from the change in weight within the corresponding temperature interval where the organic phase is burned. 2.8.5 Ultraviolet-visible spectroscopy Chromophores have the ability to absorb light. This is owing to the potential wells formed by the delocalization of electrons. The electrons can therefore be excited to higher energy levels. The electronic states can be divided into singlet states and triplet states. When electrons fall back from excited singlet states it causes fluorescence for singlet state transitions and phosphorescence for electrons falling back to triplet states. Ultraviolet-visible spectroscopy is the opposite of measuring the fluorescence i.e. measuring the transmittance of light and hence the absorption by excitation in the ultraviolet-visible region. The UV-vis spectroscopy has been used to measure the light absorbance as a function of adsorption of dye to titanium dioxide.
  26. 26. 20 Background Figure 2.17: The figure shows a thermogravimetric analyser to measure weight loss upon heating a sample. (a) A thermogravimetric analyser (b) a furnace (in cavity) (c) a delicate balance inside the furnace (d) air flow meter (e) N2 -flow meter. Figure 2.18: The figure shows the box that is a UV-vis spectrometer. 2.9 Methods The method to achieve the aims of this thesis have been synthesis of titanium dioxide by the sol-gel method and characterization thereof. There are several different available pathways including chemical vapour deposition, inert gas condensation [82] , oxidation-hydrothermal synthesis of metallic titanium, non-hydrolytic [83] sol-gel synthesis of anatase at low temperature and hydrolytic [84] synthesis of titania from titanium alkoxides and titanium chlorides [85] . The reason for choosing the hydrolytic synthesis lies in the affordability [86] of the method for up-scaling with widely researched titania precursors. The fact that this master’s thesis has the intention of investigating an extension of a silica route for Nanologica AB to the synthesis of mesoporous titania has come with certain challenges. The largest challenges is the basic conditions of the folate route [46] that comes from the use of the folic acid as a template. This comprises a narrow pH range that is essential to have a templating effect. [46] Moreover, the reasons for using π − πstacking for preparing mesoporous titania is that a hierarchical supramolecular self-assembly route has not yet been reported for titania in the literature. In addition, folic acid have interesting electronic properties [87] . The semiconducting properties of guanine-based nanowires has been reported to conduct charge [88] . A general method can be extended to a range of transition metal oxides that are interesting as supports
  27. 27. 2.9 Methods 21 in heterogeneous catalysis. A novel folate route [46] with potential is investigated for obtaining control of mesoporosity. Characterization has been made of crystallinity, morphology, organic weight-loss, surface area and adsorption. In chapter 2, the reasons for synthesizing titania was stated for the use as a dye-sensitized electrode. Several precursors can be used for the preparation of titanium dioxide. Common precursors used are Ti(OR)4 where R=ethyl [38] , isopropyl [12;38;89;90] and butyl [39;91] . In this thesis, titanium isopropoxide was chosen because it is a fluid at room temperature compared e.g. titanium ethoxide. Titanium ethoxide is decisive because one might think that this precursor will behave as tetraethylorthosilicate (TEOS), which has become a standard in the literature for synthesis of mesoporous silica [46] . As was stated in section 2.3 the titanium isopropoxide has more chemical similarities with TEOS owing to the fact that the coordination number equals the oxidation state. APES can be used as a co-structuring agent and as the base. [46] The basic conditions are needed for the charge-interaction of APES and folate. The recipes used is similar to and modifications of a folate route proposed by Garcia-Bennett et al. [46] using TEOS as precursor. In the synthesis, the reacted solution is unstirred overnight. Keeping the solution unstirred was a suitable way for storage in oven, at synthesis temperature, decreasing the batch time in oil-bath. That means a higher quantity of batches can be made consecutively. In the following chapter the synthesis of titanium dioxide will be described. There are a couple of ways to consider for lowering the hydrolysis rate in the synthesis. By capping the titanium with a nitrogen atom the nucleophilicity is lowered under basic conditions. [13] Since titanatranes have a high viscosity, an 1-isoporpoxytitanatrane was diluted in polar solvent to make it possible to pipette from a stock solution according with section 2.3. The following folate route was prepared at room temperature with the use of the 1-isopropoxytitanatrane in ethanolic solution as precursor. Addition of solvent causes dilution of the alkoxide prior to mixing which causes a slower reaction [84] . A lower water content also lowers the hydrolysis rate of titanium alkoxides. [84] In order to commercialize a mesoporous titania powder a spray-drying technique that causes smaller monodispersed particles is needed for use in DSSCs. Spray-drying has been reported to be a viable route to obtaining spherical shaped particles from slurry and was described in section 2.7.
  28. 28. Chapter 3 Experimental 3.1 Synthesis of TiO2 by folate route (NFM-2) All synthesis of NFM-2 materials was carried out by the author. Titanium(IV) isopropoxide (TTIP) was used as a precursor. The (3-aminopropyl)triethoxysilane (APES) was used as the base. The templating agent was folic acid (FA). All chemicals were purchased from Sigma-Aldrich and used as received. Safety precautions were taken due to the high reactivity of the base and the TTIP. The chemicals were prepared in the molar ratio of 0.13 FA/ 1 TTIP/0.33 APES/ 230.5 H2 O [92] . An amount of 1.21 g of folic acid was added to 87.6 ml distilled water in a plastic bottle. The aqueous folic acid yellow solution was stirred in a closed flask vigorously at 60◦ C on a Heidolph MR3002 heater/stirrer. The base APES was added at once using a pipette. The colour change upon addition of APES was from dark yellow to bright yellow. The precursor TTIP was added in a shot of 6.0 mL. The (3-aminopropyl)triethoxysilane was added firstly and then the titanium(IV) isopropoxide within 15 seconds. A rule of thumb was to add the precursor when the solution changed colour. The synthesis was heated and stirred for about 20 min and was then left unstirred for 18 h. The reacted solution was vacuum filtered and washed with about 30 ml of distilled water. 3.2 Slowing down the hydrolysis rate Slowing down the hydrolysis rate was investigated through preparing chelated titanium(IV) isopropoxide with acetylacetone and triethanolamine for use in the synthesis of NFM-2-AC and NFM-2-AT, respectively. 3.2.1 Acetylacetone route (NFM-2-AC) The recipe was only changed by increasing the complexity by preparing the precursor. Instead of adding the TTIP precursor directly to the synthesis it was chelated with acetylacetone. An amount of 6 mL acetylacetone was stirred together with TTIP 23
  29. 29. 24 Experimental for 10 minutes. The solution turned colour from being transparent to being bright yellow. 3.2.2 Atrane route (NFM-2-AT) Folic acid was added to a plastic bottle and was dissolved in distilled water. The prepared titanatrane was added in a shot. The solution was stirred at room temperature for about 5 minutes. The solution was clear yellow with no precipitation occurring. The flask with the containing solution was placed in the oven at the synthesis temperature 60 ◦ C. A phase separation of solvent and the wet powder could be seen after aging in the oven. The powder was a paste and the water content was high even after filtering. The paste was dried in the oven at 40◦ C. An 1-isopropoxytitanatrane was prepared by organic synthesis with the reaction depicted in Figure 3.1. Preparation was firstly investigated by rotoevaporation at 40◦ C to collect the isopropanol formed. It was found that this is an expensive way, since a large amount is evaporated in the vacuum line. Instead isopropanol was smelled during evaporation without vacuum in fume cupboard and then a vacuum line attached. The preparation of the titanatrane used 30 mL TTIP added to a round-bottom flask. Figure 3.1: The reaction scheme for the synthesis of 1-isopropoxytitanatrane. Figure 3.2: Setup for organic synthesis of 1-isopropoxytitanatrane. A vacuum line is used for evaporation of isopropanol.
  30. 30. 3.3 Calcination 25 Figure 3.3: A Nabertherm B150 oven used for calcination. The oven can heat up to 2000◦ C. 3.3 Calcination The procedure used for calcination was cleaning ceramic boats with ethanol and which were dried in oven at 100◦ C. A temperature ramp of 10 h for reaching 550◦ C was used. Samples were heated isothermally for 6 h. The calcination was performed with a Nabertherm B150 oven. The synthesis temperature was chosen to 60◦ C to enhance anatase transformation during calcination. 3.4 Spray-drying (NFM-2-SD) A Büchi Mini Spray-Dryer B-290 was used. The glassware of the spray-dryer was cleaned thoroughly for prevention of possible contamination of processed powder. A feed solution was prepared by normal synthesis as described in section 3.1. The spray-dryer was started-up and the aspirator was set to 50 %. The N2 -carrier gas was switched on and the oxygen sensor was controlled so that the apparatus was evacuated with air and filled with N2 -flow. The flow was controlled with respect to a pressure of 20 bar. The inlet temperature was typically set to around 200◦ C to reach an outlet temperature of 100◦ C. During spray-drying with the pumping rate set too high the outlet temperature dropped. Sample NFM-2-SD1 NFM-2-SD2 NFM-2-SD3 NFM-2-SD4 N2 -flow (%) feed (wt%) pump (%) inlet (◦ C) outlet (◦ C) 40 55 35 35 5 5 20 20 40 20 20 20 196 204 205 180 70 77 70 82 Table 3.1: Spray-drying conditions as described in section 2.7. The specific conditions are only relevant to the Büchi Mini Spray-Dryer B-290 and the corresponding industrial spray-dryer.
  31. 31. 26 3.5 Experimental Characterization The characterization techniques used were briefly described in chapter 2. These are low-angle XRD, TG, high-angle XRD, SEM, XEDS, N2-sorption, SEM and UV-vis spectroscopy. All characterization has been performed by the author. 3.5.1 XRD For a typical specimen preparation the powder sample was crushed to a finer powder by using a mortar and pestle. The sample was placed in a plastic holder with a cavity for the sample. The amount of sample was approx 1 g. Powder samples were prepared by making the powder smooth in the plastic sample holder by pressing with a glass piece. A Bragg-Bretano Siemens D5000 diffractometer was used for both high-angle and low-angle investigations. The scanning-routine used was locked coupled. The diffractometer was set up by increasing the voltage manually to 45 kV working voltage and the working current to 40 mA. For low-angle XRD, the divergence and receiving slits were set to 1.0 mm and the detector slit used was 0.2 mm. For high-angle XRD the divergence and receiving slits was 1.0 mm whereas the detector slit used was 0.6 mm. The samples were typically scanned with 2.5 sec/step with an increment of 0.03◦ (2θ) for 1 h runs, with 3.4 sec/step with an increment of 0.02◦ (2θ) for 2 h runs and with step size 0.014◦ (2θ) and 10 sec/step for night runs. The magnetic specimen holder was inserted into the diffractometer. 3.5.2 SEM A LEO1550 was used. Samples were prepared with low amount of powder for the SEM by mounting conducting carbon films on the specimen holders. The SEM was set up to start at an intermediate voltage typically 7-10 kV. Depending on the sample, typically a lower accelerating voltage of 2 kV was later required. An improved secondary electron detector was typically used called InLens. The InLens detector was used for higher resolutions at a low working distance about 2-3 mm and the normal secondary electron detector was used higher resolution than the InLens detector at higher working distance. A typical alignment of the column was made by first choosing the voltage and aperture to use. A standard 30 µm aperture was used. The aperture alignment was followed by stigmation at high magnifications. This was done repeatedly several times at a higher magnifications for improvement of the resolution. An Oxford Instruments INCA system was used for XEDS. XEDS measurements were performed with a voltage higher than the electronic transitions between the electron shells in titanium, silicon and oxygen where the highest voltage needed is above the Kβ peak of titanium. The working distance was set to 15 mm. The XEDS detector with ultra-thin window was inserted into the chamber pointing towards the specimen. The conditions used in the INCA software was an acquisition rate of 0.5 kcps, dead time of 7% and acquisition time of 100 seconds.
  32. 32. 3.5 Characterization 3.5.3 27 N2 -sorption The isotherm-equipment, Micromeritics ASAP 2020, was filled up with liquid nitrogen. Glass balloons were flushed with acetone which was evaporated in an oven at 60◦ C. The glass balloons were weighed firstly without sample, secondly with sample and thirdly with frit. Samples were degassed for 18 h at 120◦ C to remove any residual moisture. The degassed samples were weighed again to obtain the dry weight. After the dry weight measurement the sample was inserted in the instrument again to obtain the isotherm with the sample weight as input in the software routine. 3.5.4 Thermogravimetric measurements Thermogravimetric measurements were performed on a Mettler-Toledo TGA /SDTA851e . A small crucible sample holder was used and cleaned in a prepared HCl bath. The crucibles were handled with pincette. The crucible was dried in an oven at 60◦ C for a couple of minutes and the apparatus was tared with the sample holder inside. A small amount of sample was placed in the crucible which was placed in the furnace. The software was set up to run the measurements. 3.5.5 UV-vis absorbance Absorbance tests were done using a SHIMADZU UV-1650PC spectrometer and the software UVProbe 2.21 from SHIMADZU. A dilute dye stock solution was prepared with 1·10−4 M of the dye N719. The dye was bought from DyeSol, Australia and used as received. An amount of 0.1 g titania was added directly to prepared solution of 20 mL stock-solution in glass-vials. Samples were tested normally after calcination. The stock-solution was prepared with tertiary butanol/acetonitrile in 1:1 volume ratio. The tertiary butanol/acetonitrile was also used as a blank. Solutions with analyte were heated at 40◦ C for 1 h before filtering and measurements. Scans were taken from 190 - 450 nm with 0.2 nm sampling interval. The analyte was prepared by stirring the titania in stock solution for 24 hours before filtration and measurements. The following chapter describes the results obtained from the characterization of titanium dioxide obtained from the synthetic routes and processing.
  33. 33. Chapter 4 Results and discussion 4.1 Designed materials The yellow powder obtained from NFM-2 synthesis varied in colour very much when changing the synthetic conditions, such as temperature and pH. Below room temperature and at 45◦ C, large precipitates formed whereas at 80◦ C a paste was obtained. SEM micrographs of NFM-2 are shown in Figures 4.4(a)-(d). From Figure 4.4(a) it can be seen that NFM-2 powders consists of large particles, which have large-scale porosity and smaller porosity like channels and random large scale pores in the micron range. The Figure 4.4(b) shows that the porosity is macroporous. The largest pores are approximately 2-3 µm. The Figure 4.4(c) shows pore walls have small particles on the surface. The Figure 4.4(d) shows that the nanoparticles on the pore walls have a surface texture below 50 nm that is not attributed to nanocrystallinity due to the amorphous nature of the samples. The composition of NFM-2 was determined from XEDS-spectrometry and the average composition was 63.7 at% oxygen, 32.2% titanium and 4.1 at% silicon. The XEDS-spectra is shown in Figure 4.1. The composition was therefore concluded to be 88.6 % TiO2 and 11.4% SiO2 . Figure 4.2 shows low-angle XRD patterns of the different NFM-2 derivatives. The curve (a) is a reference curve for NFM-1 which is the mesoporous silica by the folate route. The curve (b) shows only a broad peak. Curve (c) shows a typical peak of no order at lower angles obtained for the materials NFM-2, NFM-2-AT and NFM-2-SD. The Figure 4.3 shows six X-ray diffraction patterns that are shifted in relative intensity. The curve (a) in Figure 4.3 shows the high crystallinity of P-25. The curve (b) in Figure 4.3 shows a high intensity 101 reflection at 25.4◦ . A superposition of 103, 004 and 112 reflections can be seen at 37.7◦ . At 48.1◦ the 200 reflection is present as a single peak and the peak 54.7◦ is a superposition of the 105 and the 211 peaks. The 101 reflection has the highest intensity and therefore has the highest abundance in the crystallites. Curve (e) has only a large broad peak which 29
  34. 34. 30 Results and discussion Figure 4.1: XEDS measurement on NFM-2-AC. Inset shows the site of interest for the measurement using a SE-detector at a working distance of 15 mm. Figure 4.2: Low-angle XRD patterns. (a) NFM-1 (b) NFM-2-AC (c) NFM-2-AT and NFM-2
  35. 35. 4.1 Designed materials 31 Figure 4.3: XRD patterns of calcined NFM-2 with synthesis temperature at 60◦ C (a) P-25 (b) NFM-2-d, pH=8.3 (c) NFM-2-a, pH=8.6 (d) NFM-2-c, pH=9.0 (e) NFM-2-b, pH=9.6 (f) NFM-2-AT means that the powder is completely amorphous. Since 4.3(b) shows low degree of order, the crystallinity is indeed low. In Figure 4.3 diffraction patterns are shown of NFM-2 with curves (b)-(e) in the order of increasing pH. It can be seen that the crystallinity increases in the reverse order (e),(d),(c) and (b). Therefore it can be deduced that the crystallinity increases with decreasing pH. This can be explained by the adsorption and desorption of hydroxide ions to titanium-sites at basic pH that blocks condensation and prevents the formation of anatase with agreement with Sugimoto et al. [38] described in section 2.3. Therefore a low pH in the available range for pi-pi-stacking to occur, is highly favourable. The curve (f) shows the diffraction pattern from the NFM-2-AT material which shows no large difference in crystallinity compared to NFM-2. Channels were found in most samples to be pores in a random pattern. Large scale aggregation seems to occur upon drying. Based on the fact that the size of a folate-stack is about 3 nm, the stacks should self-assemble into larger columns. A hypothesis was established that pore control may be achieved with hydrolysis rate. Therefore the acetylacetone route and atrane routes were designed by means of using a chelating agent. The Figure 4.8 shows SEM micrographs of NMF-2-AC samples. The Figures 4.5(a)-(d) shows SEM micrographs of NFM-AT samples. From Figure 4.5(a)-(b) it can be seen that the particle morphology is monolithic.
  36. 36. 32 Results and discussion Figure 4.4: Representative SEM micrographs of NFM-2 obtained from the folate route. From 4.5(c)-(d) it can be deduced that there is no obvious templating effect. The Figures 4.6(a)-(d) show images from SEM imaging of NFM-SD with in-situ spray-dried titania. The Figure 4.6(a) shows almost spherical shaped particles in a range of approximate sizes of 1-15 µm. The Figure 4.6(a) also shows cracks in the particles that makes it possible to conclude that the spherical particles are hollow [75] and not solid throughout. The Figure 4.6(b) shows that the spherical particles are dispersed and are somewhat donought shaped, which can be spotted with the change in contrast on the large particles. In Figure 4.6(c) the donought shape of the particles has increased in comparison to 4.6(a). This fact is even more evident at higher magnification as shown in Figure 4.6(d). The spray-dried samples depicted in Figure 4.6(a)-(d) are thought to have the same particle size distribution owing to the droplet sizes produced by the spray-dryer nozzle. The Figure 4.7(a)-(d) shows images from the NFM-2-SD material obtained from as-synthesized slurry compared to the NFM-2-SD in-situ spray-dried material depicted in Figure 4.6. In Figure 4.7(a) several spherical titania particles can be seen together with some non-spherical particles. The particles have a large distribution in sizes. In Figures 4.7(b)-(d) images at high magnification are shown on three different spherical particles. The Figures 4.7(b) and 4.7(c) shows particles of similar size of 4 µm. The Figure 4.7(d) shows a spherical particle of smaller size of 1.7 µm in diameter.
  37. 37. 4.1 Designed materials 33 Figure 4.5: SEM micrographs of the NFM-2-AT material obtained from the atrane route. Figure 4.6: SEM imaging of morphology obtained from spray-drying. (a) NFM-2-SDA1 (b) NFM-2-SDA1 (c) NFM-2-SDA2 (d) NFM-2-SDA2 .
  38. 38. 34 Results and discussion Figure 4.7: The spherical titania aggregates of NFM-2-SD obtained from spray-drying.(a) NFM-2-SD1 (b) NFM-2-SD2 (c) NFM-2-SD3 (d) NFM-2-SD4. In Figure 4.8(a)-(d) SEM micrographs from the NFM-2-AC material obtained from the acetylacetone route is depicted. The Figure 4.8(a) shows a monolithic particle without macroporosity. The particles have a rough surface. The Figure 4.8(b) shows that some particles are smaller and that the monolithic particles are in fact aggregated from smaller entities which also is verified from Figure 4.8(a) that shows a range of aggregate sizes 1-10 µm that are aggregated to a monolith. The Figure 4.8(d) shows smaller entities of approximately 1 µm and a flake that has a texture perpendicular to the plane of the flake. The flake surface has at least one smooth surface compared to the rough entities in close proximity. The channelled texture is thought to origin from folate-stack templating. To obtain further information about the folic acid inside the the as-synthesized titania material, thermogravimetric measurements were performed. The Table 4.2 shows no trend in the weight loss of folic acid and the surface area or pH. It was realized that the pKa of the alcohol matters in the sense of retarding hydrolysis and condensation. The alkaline mechanism was described in section 2.3. The formation of RO – does not promote hydrolysis since the reaction is catalysed by OH – . The base that catalyses the condensation is also OH – . Thus, if the amount of RO – increases in the solution, the hydrolysis rate will not increase. Instead it will decrease due to displacement of the equilibrium of hydrolysis and condensation. The needed displacement is thought to be 5 orders of magnitudes in the reaction rate.
  39. 39. 4.1 Designed materials 35 Figure 4.8: SEM imaging from acetylacetone route prepared NFM-2-AC materials. Figure 4.9: Thermogravimetric measurements of percentage weight-loss of NFM-2 and its derivatives.
  40. 40. 36 Results and discussion Figure 4.10: The first derivative of weight-loss obtained from NFM-2 and its derivatives. Figure 4.11: N2 -sorption curves of the different NFM-2 materials.
  41. 41. 4.1 Designed materials 37 Sample pH BET (m2 /g) Org. w.-l. (%) D101 (nm) NFM-2-d NFM-2-a NFM-2-c NFM-2-b NFM-2-AT NFM-2-AC NFM-2-SD3 P-25 8.3 8.6 9.0 9.6 9.1 7.9 8.3 - 41.5 42.9 86.2 9.5 81.1 49.8 24.9 28.4 17.3 21.2 14.9 50.2 24.9 - 8.0 4.0 amorphous amorphous 6.5 amorphous 6.6 25 Table 4.1: Summary of characterization of all the materials. Figure 4.12: Absorbance of light with concentration. Adsorption of dye to titanium dioxide. (a) N719 dye stock solution (b) NFM-2 (c) NFM-2-AT (d) P-25. The P-25 titania has a good dye-adsorption attributed to super-hydrophilicity with wetting angles lower than 3◦ . Since it is the carboxylic groups of the dye that links to the titania a hydrophilic interaction is critical to the adsorption of dye. [93] The data in Figure 4.12 shows flattened peaks that are due to the filtering of titania in the preparation but has no direct influence on the evaluation. The Figure 4.12 shows absorbance curves (b) and (c) corresponding to the developed materials with the dye stock solution in curve (a) as the absolute reference. The absorbance for P-25 is also used for comparison depicted as curve (d). The higher amount of adsorption to titania, the lower is the absorbance with respect to the dye stock
  42. 42. 38 Results and discussion Sample BET (m2 /g) N719-adsorption (%) Solution colour NFM-2-b NFM-2-AT P-25 86.2 9.5 49.8 19 28 100 purple pale pink transparent Table 4.2: Summary of adsorption results. solution. The curve (c) describes the absorbance with after exposure to NFM-1-AT which shows a considerable drop in absorbance at a wavelength of 313 nm. The peak at 313 nm corresponds to 4d − π ∗ metal-to-ligand absorbance. Both the peaks at 222 nm and 266 nm correspond to π − π ∗ ligand-centred absorbance. In comparison to the 4d − π ∗ of the curves (a)-(d) the P-25 samples have the same peak inverted. Note that NFM-2-AC was not tested due to the time frame.The only NFM-2 material that compares to P-25 is the NFM-2-AT which has an adsorption of 28%, with reference to the used stock solution.
  43. 43. Chapter 5 Conclusions From the results in this thesis it can be deduced that the synthesis of titanium dioxide with folate-stacks as template has inherent limitations. The thesis has been carried out with respect to the aims of the project description from Nanologica AB. Using alkaline conditions is not the way for obtaining highly nanocrystalline anatase compared to acidic conditions. The use of alkaline conditions for synthesis of titanium dioxide should be avoided, since those conditions will render the material nearly amorphous. Spray-drying was shown to produce spherical particles. The largest impact on smaller spheres was using a low weight-fraction of as-synthesized titania powder in the feed solution. 39
  44. 44. Chapter 6 Future work Future research should focus on the advantages of the acidic conditions of the sol-gel chemistry of titanium alkoxides that would cause nanocrystalline anatase at low calcination temperatures. Guanosine-monophosfate is the first choice of a π − π-stacked template for a similar approach as in this thesis because it will stack under acidic conditions. [53] More research on controlling the mechanism of forming π − π-stacks of aromatic molecules with respect to sol-gel chemistry is needed. Also research on the in-situ mechanism of phase formation for π − π-stacks in comparison to surfactant self-assembly is needed. 41
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