In Chapter 4, the hierarchical structuring of biomaterials from the nanometer length scale up to hundreds of micrometers was addressed by synthesizing LC polymerLaponite composite materials structured on three hierarchical levels by one-step self-organization. The organic component was a polyoxazoline with pendant cholesteryl and carboxyl (N-Boc-protected amino acid) side chains, enabling the polymer to form a chiral nematic lyotropic phase, thus forming two hierarchical levels, and to bind to the positively charged rims of Laponite nanoplatelets. Long-range orientation of the polymeric LC phase was induced by shearing.
The Laponite clay nanoparticles, which are also able to form a liquid crystalline phase due to their anisotropic shape,134,135 assemble into a mesocrystalline arrangement within the LC polymer matrix and consequently form the third level of the hierarchical structuring.
5.2.1 Structure and properties of vanadium pentoxide
Vanadium pentoxide (V2O5) is known to exhibit charged anisotropic particles. It is thus a good candidate for the above-mentioned hierarchical structuring driven by liquid crystal formation. V2O5–H2O was already found in the 1920s by Zocher et al.177-180 to form tactoid sol phases and is chemically classified as a lyotropic, inorganic, nematic liquid crystal.181,182 The tactoids are composed of rod-shaped nanoparticles, which are mutually oriented in a nematic and anisotropic manner.183,184 To clarify the notions used in the following text, a model of the V2O5 structure (Scheme 5.2, adapted from literature) is illustrated.183,185-188
V2O5 is composed of V2O5 sheets (consisting each of two V2O5 layers, made of square pyramidal VO5 units) which are separated by intercalated water molecules with a defined intersheet distance. Stacking of these V2O5 sheets forms ribbons (rod-like primary ribbons), which can then arrange into fibers and layers (see Scheme 5.2).
Scheme 5.2: Description of vanadium pentoxide (V2O5) structure.
85 consisting of preoriented nanoparticle building units, V2O5 mesocrystals were obtained via three different ways: addition of salt, application of a strong external sedimentation force, and addition of a surface-active polymer. The approach of adding a surface-active polymer that adsorbs and stabilizes the oxygen terminated (020) faces of V2O5 was found to be superior to the other two approaches. This is due to the defining shape and alignment of primary V2O5
ribbons, reducing the repulsive forces of the superstructure building units. The concept of mesocrystal formation and non-classical crystallization is well established in the field of biomineralization53,189-196 as well as for the development of biomimetic composite structures in the context of improved material properties.61,197-200 In this regard, vanadium oxides, and vanadium pentoxide in particular, are of high interest due to their broad range of applications, e.g., in the field of electrochromic/-optical devices (see Section 5.2.2),201-204 electrochemistry,205-207 electrodes for lithium batteries,208-211 catalysis,212-217 gas sensors,218-221 and photocatalytic reactions.222-224 The importance of V2O5 nanoparticles acting as catalysts was only recently shown by their peroxidase-like activity225 as well as their intrinsic bromination activity, mimicking vanadium haloperoxidases that prevent marine biofouling.226 Due to its layered structuring, V2O5 is used as an intercalation material.227-237 Defined nano- and mesostructures of vanadium oxide materials considerably enhance the material’s capacities for sensing238 as well as for energy-storage devices239 in comparison to bulk vanadium oxide. For example, the mechanical deformation effect by intercalation of ions in vanadium oxide was used to mimic human skeletal muscle.240
5.2.2 Electrochromism of vanadium pentoxide
Electrochromic materials are able to change their optical properties induced by applying an electrical voltage.241 Prerequisite for technical applications is often a reversible electrochromic process.242
Electrochromic cells follow the principles of simple electrochemical cells, i.e. electrochemical cells consisting of two metal electrodes that are both in contact with a solution containing its own ions.242 By applying a potential unequal to the open circuit potential Ecell (which is defined as the electrical potential difference between the two half cells of an electrochemical
86 system) a charge will flow and, consequently, an electrochromic reaction may occur. The charge is measured per unit time as current I, being proportional to the rate at which electronic charge Q at an electrode is consumed or generated by the electroactive species. Assuming that the electroactive species is in solution, the electrochemical current depends on three rates at the electrode: the electron transport through the electrode material, the electron movement through the electrode-solution interface, and the movement of the electroactive species through the solution before the electron-transfer reaction.242
Electrochromic properties of materials can be investigated by means of cyclic voltammetry (CV). This technique requires a three-electrode system consisting of a working electrode, a counter electrode and a reference electrode. During the experiment, the current flow is recorded depending on the potential at the working electrode. The potential varies linearly with time in the course of the experiment and exhibits a cyclic change.242
A current flow varies the potentials at both electrodes, the working and the counter electrode.
In order to isolate the processes taking place at the working electrode, the processes occurring at the counter electrode are ignored. Consequently, only the potential between the working electrode and the reference electrode is measured. The potential of the reference electrode can be considered as constant, allowing to observe the varying potential at the working electrode.
Saturated calomel electrodes (Hg/Hg2Cl2/KCl) or Ag/AgCl electrodes are commonly used as a reference electrode. Quasi-reference electrodes are also applied, such as bare silver or platin wires. In order to render these electrodes to reference electrodes, an internal reference is approximately 200 mV anodic of the oxidation potential of the electroactive species (assuming a reversible, one-electron redox couple in solution). By acquiring several cycles, a saw-tooth-like potential change is obtained. This profile is provided by a potentiostat, which measures the current depending on the applied potential.242
A standard electrochromic cell for investigating the electrochromic behavior of thin films of metal oxides is schematically illustrated in Figure 5.1a. Glass substrates with electrically conducting transparent layers form the outer part of the cell. Materials used for the conducting
87 films include indium-tin oxide (ITO, In2O3 : Sn) and fluorine-doped tin oxide (FTO, SnO2 : F). The inner part of the cell is composed of the electrochromic film, which is able to conduct ions and electrons, and the ion conductor (electrolyte), which is in direct contact with the electrochromic film. A second electrochromic film can be part of the inner cell, which serves as an ion storage.241 An example electrochromic cell containing a mesoporous V2O5
film is shown in Figure 5.1b.201
Figure 5.1: (a) Schematic illustration of an electrochromic device. Arrows indicate the movement of positive ions by means of an electric field.241 (b) Assembly of an electrochromic cell, sandwiching a mesostructured V2O5
film.201
Electrochromism of thin films of transition-metal oxides, such as tungsten oxide (WO3) or V2O5, were previously studied. Different methods to produce appropriate electrochromic films were reported, such as sputtering,243 spin-coating,244-246 and electrodeposition.201,247,248
Upon cation intercalation (reduction) of the metal oxide, a change in absorbance in the visible region and, consequently, a change in colour of the material is observed.249 Depending on whether a metal oxide colors under charge insertion or extraction, the material is termed cathodic or anodic electrochromic material, respectively. V2O5 exhibits both cathodic and anodic coloration, depending on the observed wavelength region.241
The redox process of vanadium pentoxide is described in non-aqueous solutions as:242
In aqueous solution, an alternative reaction can take place:242
The redox reaction was reported to occur in two consecutive steps. In the first step, the V5+
ions are partially reduced to V4+. In the second step, the remaining V5+ ions are reduced to V4+
88 (which might not be quantitative in total).202,242,250,251
In line with the redox state, V2O5 undergoes a phase-transition from α-V2O5 to ε-V2O5, and from ε-V2O5 to δ-V2O5.202,252 α-V2O5 corresponds to x = 0.0, ε-V2O5 to x = 0.4, and δ-V2O5 to x = 1.0.253 Cyclic voltammetry of thin V2O5 films prepared by sputtering reveals two defined quasi-reversible redox couples with two anodic and two cathodic peaks.242 Several species contribute to the optical spectrum of the partially reduced species. Thus, spectral regions fully characterized by the Beer–Lambert law were not found.242,254
V2O5 films have a yellow-brown color, arising from the tail of an UV band that appears in the visible region of the spectrum.255 Upon reduction (i.e. Li insertion), the color changes due to a shifting of the band edge from ca. 450 nm to ca. 350 nm.254 Fujita at al. explained the bluish color of the reduced state as a result of the formation of blue VO2.256 Note that the reported color changes observed during the redox process vary and obviously strongly depend on the method used for film formation.
Electrochromism is of technical relevance for the realization of ‘smart’ windows due to their low operating potential and the memory effect, allowing to maintain the respective oxidation state with a small amount of or without any additional power.241,257 These windows combine two important characteristics: energy-efficiency and indoor comfort.241 A prominent example is the application of electrochromism in aircraft windows of the Boeing ‘Dreamliner’
787.242,258
In electrochromic devices, the switching rate is limited by the diffusion rate of the respective ions during the redox processes and the coloration contrast is restricted by the number of available intercalation sites for the ions.259 Recent developments of electrochromic materials leading to increased electrochromic performance in terms of higher coloration efficiencies and faster response times (switching times) include the use of ordered mesoporous and interconnected inorganic structures, giving rise to a good ion excess and high electric conductivities.201,202,247,260
An issue regarding technical applications is the development of mechanically resistant electrochromic films with low susceptibility to cracking under mechanical stress. These properties are hardly achievable with thin films of metal oxides, as purely inorganic structures are mechanically fragile.242 Consequently, mechanical elasticity as known for organic–inorganic biocomposites is an attractive feature for films in the context of electrochromic devices.
89 5.2.3 Aims and motivation
The aim of this part of the thesis was to synthesize hierarchically organized, multifunctional composite materials. To this end, the fabrication concept for hierarchically structured composite materials (see Chapter 4) is applied to the synthesis of biomimetic LC ‘gluing’
polymerV2O5 composites via a one-step self-organization on six hierarchical levels. The V2O5 sheets self-assemble into V2O5 ribbons (first level), to which the copolymer binds via the carboxylate gluing units to form a V2O5polymer hybrid material. The copolymer cholesteryl side chains form a chiral nematic LC phase (second and third hierarchical level).
The V2O5 ribbons may assemble with LC polymer in between, herein called fibers, laying flat, parallel to the surface (fourth level), finally building up a composite material with a layered structuring in the horizontal dimension (fifth level) and a textured substructure in the vertical dimension (sixth level). Aiming for composites combining advantageous mechanical and electrochromic properties, both the mechanical and the electrochromic performance of these composites were investigated. The combination of both features may lead to promising