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Physical deposition methods
Physical deposition methods have been extensively investigated as a way to deposit porous TCO films because of innate advantages, like the relative simplicity of the protocol and high reproducibility owing to tight control. Their main drawback is the cost of the specialized equipment that is required. One of the first methods used by Naghavi et al. was the pulsed laser deposition which could be used to deposit disordered mesoporous ATO with electrochromic properties.82 The same method was applied for the deposition of AZO83–85 and ITO films.86 It was also found that the morphology could be adjusted to obtain different levels of porosity by manipulating the pressure in the chamber (Figure 1.7). The ratio between the amount of adsorbed dye and the amount of dye that can be detected electrochemically was determined with films prepared in different conditions. Almost 80 % of the dye was detectable by electrochemistry when the layer was deposited at the pressure of 200 mTorr. The most popular physical deposition method is vapor deposition which could be realized in different configurations. Joanni et al. reported first protocol that involved radio frequency (RF) sputtering of ITO which yielded a structure of branching nanowires.87 RF sputtering also proved to be useful in deposition of flexible layers of ITO with small degree of porosity achieved by pretreatment of the polymer substrate. The layers were transparent with sheet resistance of 230 Ω sq-1.88,89 AZO films with small levels of porosity were also successfully deposited using RF sputtering.90 Regular physical vapor deposition of ITO resulted in a simple nanowire morphology as shown by Noh et al. with the aim to challenge the major problem of solar cells that is charge recombination. It was observed that the 3D architecture lowered the charge collection time significantly by 4 to 10 times (depending on the current density) compared to a planar electrode.91 Similar ITO nanowire electrode was also reported with an average wire length of 12 μm.
Energy generation and storage (photovoltaics, water splitting)
Photovoltaics became an important field for the development of transparent and porous electrodes as there is an intense ongoing research for efficient ways of utilizing sunlight as power source to reduce the reliance on fossil fuels due to climate and economic issues. Most popular realization of this idea are dye-sensitized solar cells (DSSCs). A work by Chappel et al. could be considered as a precursor in the field where SnO2 mesoporous electrode was used as a base for the preparation of a DSSC with photocurrent conversion efficiency (PCE) reaching 1.125 %.31 First application of TCOs came in 2007 with the report from Joanni et al. with branched ITO nanowires that were generating current density of 0.066 mA cm-1 and open circuit voltage (OCV) at 523 mV at illumination of 100 mW cm-1. However, the PCE was only 0.15%.87 Since then, a number of DSSCs based on porous electrodes were developed.46,49,52,72,105,107,117 Most notable examples are AZO macroporous films achieving 4.9 % PCE101 or FTO-nanoparticle based electrode developed by Yang et al. reaching 4.6 % PCE, OCV at 760 mV and current density of 13 mA cm-2 under illumination of AM 1.5 G. In their research the importance of using more conductive, fluorine-doped tin oxide was highlighted (Figure 1.11).50 In another example, ITO nanowires were shown to reduce the charge collection time 4 to 10 times over nanoparticle-based ones, achieving 3.8 % PCE.91 Following the DSSCs in concept, semiconductor-sensitized solar cells have also been prepared47,48 with highest achieved efficiency on porous electrodes of 1.66%27 as well as organic solar cells with 6.74 % PCE.89 Moreover, by simple dye sensitization of mesoporous ITO a photocurrent generation 150-fold higher than on flat electrode was made possible.59 In another application an improvement of PCE of tandem solar cells from 12.27 % to 13.6 % based on GLAD-ITO was reported.16 Finally, several biophotovoltaic applications have recently appeared. An electrode based on ATO nanoparticles was applied in generating photocurrents from deposited bacterial centers and was capable of generating 10 mA*cm-2 current density.68 A different approach for generating photocurrents was demonstrated using deposited photosystem I on meso- and macroporous ATO.71 It was found out that bigger pores are a better host for the protein, enhancing the generated currents 11-fold over planar electrode. Templated ITO electrode was also used to accommodate photosystem I protein, generating current density of 150 μA*cm-2.
Another application of porous electrodes in energy production is the photoelectrochemical water splitting. It shares the principle of capturing the solar energy with photovoltaics but instead of electrical current the energy is transformed into the breaking of O-H bonds in water molecule and in some cases to generate hydrogen that could be later stored and used as a clean fuel. In 2012, the first application of mesoporous ITO in water splitting was shown by Kato et al. where Photosystem II (PSII) catalyzed oxygen generation from water, leading to a current density of 1.6 μA*cm-2 during a red light irradiation of 8 mW*cm-2.29 Later, it was found out that immobilization of PSII through covalent bond is also possible.44 It is also feasible to couple the PSII-covered electrode with one electrode functionalized with a hydrogenase to achieve final light-to-hydrogen conversion efficiency of 5.4 %.79 In the latest application of this electrode design, Os-based redox polymer (Figure 1.12) light-to-current efficiency of 6.9 % for oxygen evolution with turnover frequency (TOF) of 3.6 mol O2*(mol PSII)-1*s-1 with no mediator addition.81 Two templated electrodes deposited by ALD were also used for water splitting after deposition of hematite. In the first one, NTO was used, reaching a current density of 2.35 mA cm-2 thanks to the host-guest architecture.103 In the second, a porous ITO allowed for achieving 1.53 mA cm-2.102 Another approach involves drilled FTO electrodes. It has been applied in water photoelectrolysis to improve certain parameters of the reaction like lowering of the onset potential and improving the charge transfer.104 Finally, ITO nanowire electrode has been applied in hydrogen generation and achieved 2.4-fold increase over planar TiO2 system with current density of 16.2 mA cm-2 at 1.0 VRHE.
Porous and transparent organic electrode materials
Compared to inorganic transparent and conductive materials organic materials like polymers or carbon presents a different range of useful qualities like flexibility118 or lower cost of preparation due to lack of need for expensive metals. In the literature many examples of transparent carbon materials21 have been presented but this introductory part will discuss only examples with porous structures on meso- and macroscale. In 2010 Malavé Osuna et al. reported the deposition of PEDOT on carbon nanotube films that resulted in the formation of a macroporous transparent electrode.119 Another interesting protocol involved using a mesoporous silica film (SBA-15) as template for the deposition of a thin organic layer that was later carbonized to obtain conductive mesoporous carbon electrode (Figure 11).120 A different way to prepare transparent macroporous carbon electrodes relies on chemical vapor deposition on an anodized alumina template with channels 80 nm in diameter and 140 nm in length. Additionally, an array of nanotubes that were 25 nm in diameter and 330 nm in length could be added on top of those films to increase the surface area.121 Furthermore, deposition of a composite polyaniline – reduced graphene oxide (PANI-RGO) on FTO substrate was demonstrated which resulted in a worm-like morphology.122 Finally, two protocols of porous graphene were published. In the first one, a micro- and mesoporous electrode was prepared using prismlike graphene blocks with surface area of 909 m2*g-1.123 In the second, one graphene was deposited using MOCVD with NaCl crystals as template achieving surface area of 1105.6 m2*g-1.124 Most popular application of those organic electrodes is in flexible transparent supercapacitors. They are capable of reaching the power densities of 19 mW*cm-3 121, 190 mW*cm-3 123 and 562 mW*cm-3.124 In another report WO3 on macroporous PEDOT/CNT composite is used for potential display applications.119 Finally, a mesoporous PANI-RGO composite electrode was tested as a counter electrode in bifacial DSSC.
Cyclic voltamperometry was performed in a solution containing 5 mM of dimethanolferrocene and 0.1 M KCl. Ascorbic acid electroanalysis was evaluated in 0.1 M KCl as well. Palmsense Emstat potentiostat was used for those measurements with standard three-electrode setup. Reference electrode was Ag/AgCl/KCl 3M. Counter electrode was made of stainless steel and working electrodes had geometric surface area about 12.6 mm2. For deposition of Prussian blue on nanofibers solution of 5 mM FeCl3, 5 mM K3[Fe(CN)6], 0.01 M HCl and 0.1 M KCl was used and the deposition was performed for 60 s in 0.5 V.
Spectro electrochemical measurements
Agilent Technologies Cary 60 UV-Vis spectrometer was used for spectroelectrochemical measurements. The same potentiostat as mentioned before were used and the experiments were conducted in 0.1 M KCl, 0.01 M HCl solution. Three-electrode setup was used with Ag/AgCl pseudoreference electrode, platinum wire as counter electrode and ITO nanofiber mats as working electrodes.
Preparation of indium tin oxide nanofilament layers
Fused silica plates used as substrates were polished with alumina (0.1 and 0.05 μm) and afterwards rinsed with water and ethanol. Then the plates, dimensions 20x20x1 or 30x10x1 mm, were covered by polystyrene solution in toluene 10% w/w by drop coating and left for solvent evaporation under the hood for around 1h. The volume used for the thick film formation was 300 and 225 μl respectively (0.75 μl mm-2).
Electrospinning experiment and post-treatment
The electrospinning procedure and equipment is similar to one used for obtaining ITO free-standing mats (section 2.3.1) including the flow rate of 2 μl min-1 and the size of the collector plate of 28 x 28 cm. The only changes to the protocol were as follows: distance between the needle tip and the collector plate was kept at 10 cm, voltage applied was 9 kV and in the middle of collector plate the fused substrates covered with PS were immobilized with double-sided carbon tape. Time of deposition was 20 minutes and relative humidity was around 50% at room temperature. After the electrospinning the fused silica plates covered with nanofibers were transferred to the oven to undergo calcination in 500 ˚C in air for 1 h (with temperature ramp of 100 ˚C h-1). Afterwards, they were treated at 1000 ˚C for 1h under nitrogen atmosphere (with temperature ramp of 100 ˚C h-1) unless stated otherwise in the results section. Nitrogen gas was treated with help of two adsorbent cartridges (oxygen adsorbent and water adsorbent) provided from ―Spectron Gas Control Systems GmbH, Germany‖. Then, the samples have been left to cool down in the oven. Finally, after heat treatment the samples were rinsed with water and then with ethanol. The whole procedure of preparing the samples for further experiments is presented in graphic form on Figure 2.2.
Deposition of click-functionalized mesoporous silica films
The functionalization of ITO plates or ITO nanofilament layers on fused silica with methylated mesostructured silica was achieved by applying -1.3 V for 20 s or longer if specified in deposition solution according to previously published procedure.125 The composition of the sol was prepared as follows: 200 mM of TEOS and 50 mM of AzPTMS as the silica precursor, 64 mM CTAB in 0.1 M NaNO3 as supporting electrolyte. The sol pH was adjusted to 3 by the addition of 0.1 M HCl and stirred for 2.5 hours. Afterwards the plates were put in oven at 130 °C overnight. Finally, the surfactant was removed in 0.1M HCl in ethanol under stirring for 15 minutes.
Functionalization with Ru complex
The [Ru(bpy)3(bpy‘)]2+-functionalized silica films have been prepared on flat and porous ITO electrodes by by functionalizing vertically-aligned mesoporous films described in Section 18.104.22.168, using Huisgen click chemistry with ethynyl-Ru(bpy)2(bpy‘). The click reaction was conducted for 18-24 hours, where the surfactant-extracted azide-functionalized films were immersed in the dark in a mixture solution of ethynyl-Ru(bpy)2(bpy‘) (20.0 mg) in DMF/H2O (10/10 mL), Cu(II)acetate 1 M (20 μL) and Ascorbic Acid 1 M (50 μL).
Electrochemical behavior of calcined ITO nanofibers
A common drawback of nanofibers is related to the polycrystallinity observed for low calcination temperatures that results in a relatively lower conductivity than in thin films.27 However, one requirement for application of ITO nanofibers in electrochemistry is that the electrode provides a suitable conductivity. The optimal material prepared by electrospinning at 10 kV for 20 min and calcined at 500 °C in air (so-called 500Air) was immobilized on a scotch tape for characterization. In these conditions, a sheet resistance of 4.5 ± 0.05 kΩ sq-1 was measured, which is about 100 times higher than the conductivity reported for ITO electrodes (10-50 Ω sq-1).
The electrochemical properties of ITO materials were here evaluated by cyclic voltamperometry (CV) of dimethanolferrocene (Figure 3.2). In this experiment the potential of the working electrode was scanned at 50 mV s-1 between -0.2 V to 1.2 V vs Ag/AgCl 3 M reference electrode. When the CV was performed with a commercial ITO film electrode with low resistivity (22 Ω sq-1 using the same method of measurement as for nanofiber electrodes), well defined peaks of current were observed at 0.342 V and 0.183 V for oxidation and reduction reactions, respectively (Figure 3.2A). The difference of potential between these two peaks was 0.159 V which correspond to a pseudo reversible electrochemical signal and the redox potential was 0.26 V vs Ag/AgCl. Peak currents were in the range of 120 μA.
The same experiment on the ITO nanofiber sample 500Air led to much worse electrochemical response. The high resistivity of the electrode material did not prevent the electrochemical experiment to be performed, however the oxidation was observed at more positive potential, around 1V, was poorly defined and the current was at least 5 times lower on the nanofibers than on ITO film electrode. Moreover no reduction signal could be measured. The electrochemistry of dimethanolferrocene on ITO nanofiber calcined at 500 °C was thus irreversible. Such electrode is basically unsuitable for any electrochemical experiment, no matter the application. Therefore, different post-treatments had to be explored in order to increase the conductivity and the reactivity of the electrospun ITO nanofibers.
Table of contents :
Chapter 1. Literature survey of transparent and porous electrodes and their applications
1.2. Preparation methods of the porous transparent conductive oxides
1.2.1. Non-templated particle deposition
1.2.2. Templated deposition
1.2.3. Physical deposition methods
1.2.4. Top-down methods
1.3. Applications of the porous transparent conductive oxides
1.3.1. Energy generation and storage (photovoltaics, water splitting)
1.3.3. Other applications
1.4. Porous and transparent organic electrode materials
Chapter 2. Experimental
2.2. Preparation of the suspension
2.3. Preparation of indium tin oxide free-standing nanofiber mats
2.3.1. Electrospinning experiment
2.3.3. Preparation of mats for measurements
2.4. Measurement procedures for indium tin oxide free-standing nanofiber mats
2.4.1. Electrochemical measurements
2.4.2. Spectroelectrochemical measurements
2.5. Preparation of indium tin oxide nanofilament layers
2.5.1. Substrate preparation
2.5.2. Electrospinning experiment and post-treatment
2.5.3. Mesostructured methylated silica deposition
2.5.4. Click-functionalized silica deposition and Ru complex functionalization
2.6. Measurement procedures for indium tin oxide nanofilament layers
2.6.1. Electrochemical measurements
2.6.2. Spectral measurements
2.6.3. Electrochemiluminescence measurements
Chapter 3. Indium tin oxide free-standing nanofiber mats and their potential applications in electrochemistry and spectroelectrochemistry
3.2. Results and discussion
3.2.1 Optimization of electrospinning
3.2.2. Electrochemical behavior of calcined ITO nanofibers
3.2.3. Thermal treatment of ITO nanofibers
3.2.4. Immobilization of the electrospun nanofibers with SnCl4
3.2.5. Application to the electrochemical detection of ascorbic acid
3.2.6. Application to spectroelectrochemistry
Chapter 4. Indium tin oxide nanofilament layers and their potential applications in electrochemistry and spectroelectrochemistry
4.2. Results and discussion
4.2.1. Electrospun material characterization
4.2.2. Electrochemical properties of ITO nanofilaments
4.2.3. Electrochemical deposition of methylated silica
4.2.4. Electrochemical and spectral detection of industrial dyes
Chapter 5. Application of indium tin oxide nanofilaments in electrochemiluminescence generation
5.2. Results and discussion
5.2.1. Electrochemiluminescence of dissolved Ru(bpy)32+
5.2.2. Electrochemiluminescence of adsorbed Ru(bpy)32+
5.2.3. Electrochemiluminescence of click-immobilized Ru(bpy)32+
5.2.4. Stability comparison between adsorbed and covalently bound luminophore
5.2.5. Possible explanations of the surprising phenomena
General conclusions and outlook
Conclusions générales et perspectives