Challenge of TiO 2 nanotubes arrays on DSSCs

From the literature, it has known crystalline nanotube or nanowire based on TiO2,

in contrast with the random transport path in nanoparticle, have been investigated to improve electron collection. Then, there are challenges existing and need to overcome.

One of challenges is that 1-D nanostructures have a lower internal surface area than mesoporous films. In DSSC application, the reported efficiency of TiO2 nanotube based DSSC is generally much lower than DSSCs based on nanoparticles and amounted to 0.61%-2.9% [85,86,73,87]. The possible reason is that the internal surface area of nanotube based photoanode is much smaller than that of nanoparticles due to a lower dye loading and sunlight adsorption.

2.3.6 TiO

2

hybrid structure (TNWs/TNAs)

There are various methods fabricating TiO2 which can be organized according the templates used during the experiment. Sol-gel processing which transcription used organo-gelators as templates [88,89], seed growth [90], deposition into a nanoporous alumina templates, and hydrothermal processes [91].

Among them, anodizing oxidation method is better due to the highly order arrays.

Anodizing oxidation carries on producing a long TiO₂ nanotube arrays in viscous

electrolyte, and the TiO2 nanowires are found [92]. In 2012, Haijun and the coworkers [93] found the TiO₂ nanobelts exist between TiO₂ nanowires as shown in Figure 2.11

2.3.7 Anneal treatment for TiO

₂

material

Titanium dioxide (TiO2) can exist in three distinct crystalline polymorphs: anatase, rutile, or brookite crystalline phase, respectively shown in Figure 2.12 [94]. From Figure 2.12, all three crystal structures are made up of distorted octahedra, each one representing a TiO₆ unit, where each Ti⁴⁺ is at the center of the unit and coordinates six O2- ions. The manner in which the octahedra assemble to form a TiO6 based chain is different and characteristic of each polymorph. In these three phases, rutile and anatase are the most commonly synthesized phase. Anatase and brookite are metastable phases and convert into rutile at high temperature, usually above 600 °C [95]. Table2.2 lists some of the key properties [90,96] of the three TiO2 polymorphs. Both rutile and anatase have a tetragonal crystal structure, where brookite has an orthorhombic symmetry. Rutile is the densest phase and has the highest refractive index, while anatase is characterized by the widest band-gap (~3.2 eV) [92]. The properties (density, band-gap, and refractive index) of brookite fall between those of rutile and anatase.

Figure 2.1 Schematic of DSSC operation [1]

Figure 2.2 (a) Spectral response curves of the photocurrent for the DSSC sensitized by N3 and the black dye. (b) The chemical structures of N3 dye and black dye. [97]

Figure 2.3 Chemical structures of N719 dye and N749 (black dye) [39]

Figure 2.4 Structures of the ruthenium sensitizers RuL3 (yellow) cis-RuL2(NCS)2 (red) and RuL’(NCS)₃ (green), where L=2,2’-bipyridyl-4,4’-dicarboxylic acid and L’=) 2,2’,2”-terpyridyl -4,4’,4”-tricarboxylic acid. The lower part of the picture shows nanocrystalline TiO₂ films loaded with a monolayer of the respective sensitizer.[98]

Figure 2.5 Lateral view of the nanotubes formed in 0.1 M KF, and 1 M H₂SO₄, and 0.2 M citric acid solution (25 V, 20 h)[72]

Figure 2.6 A comparison between SEM cross-sectional images of nanotubes in (a) an aqueous based and (b) organic electrolyte [74]

Figure 2.7 Schematic diagrams illustrating the formation mechanism of TiO2 nanotubes

structures (a) oxide layer formation, (b) semicircle pores formation on the oxide film, (c) growth of the semicircle pores into scallop shaped pores, and (d) fully developed nanotube arrays.

Figure 2.8 Schematics of the pH profile developing within the tubes during the anodization process according to Macak et al. [70]

Figure 2.9 Schematic diagrams of the oscillation mechanism: formation of tube spatial periodicity and corresponding current behavior under different conditions: (a) without stirring; (b) at medium stirring rate; (c) at high stirring rate or with periodic modulated voltage. [80]

Figure 2.10 FE-SEM images of nanotube arrays anodized under 10V at: (a) 5°C with an average wall thickness of 34 nm, and (b) 50°C with an average wall thickness of 9 nm.

The pore size is 22 nm for all samples. [81]

Figure 2.11 Schematic illustration of TiO2 nanowires/ nanobelts standing on TiO2

(Right), nanotube arrays section drawing of given regions (Middle), and FE-SEM images corresponding to section drawings (Right) [92]

Figure 2.12 Different TiO2 crystal structures: (a)rutile, (b)anatase, and (c) brookite.

Images courtesy of Joseph R. Smith, University of Colorado [90,92]

Table 2.1 Average wall-thickness and tube-length of 10V titanium nanotube arrays anodized at different bath temperatures [81].

Anodization temperature(°C) Wall thickness (nm) Tube length (nm)

5°C

34 224

25°C

24 176

35°C

13.5 156

50°C

9 120

Table 2.2 Different TiO2 polymorphs and some of their physical properties[90,92]

Rutile Anatase Brookite

Crystal System

tetragonal tetragonal Orthorhombic

Density (g/cm3)[90,92]

4.13-4.26 3.79-3.84 3.99-4.11

Band-Gap (eV)[92]

3.0 3.2 3.11

Refractive Index[92]

2.72 2.52 2.63

Melting Point(°C)

1855 Converts to rutile

Chapter 3 Experimental Section

3.1 Materials

Materials for fabricating TiO

₂

nanotube arrays structure (TNAs) and hybrid structure (TNWs/TNAs)

1. Titanium foils 99.9 % purity, 0.5mm thickness and 0.127μm thickness. Sample size 0.28 cm²

2. NH4F from SHOWA for contributing F^-.

3. Ethylene glycol (EG) from SHOWA for a high viscosity electrolyte. The structure is :

Materials for DSSCs

1. Titanium tetrachloride (TiCl₄) (from SHOWA) is for post-treatment of TiO₂ film 2. Titanium foil 99.9 % purity, 0.5mm thickness and 0.127μm thickness.

3. Ethanol (C₂H₅OH) (from ECHO) is used as a solvent for dye solution and the chemical structure is:

4. Surlyn® (SX1170-60) (from SOLARONIX) is used as the spacer and sealing material

5. N719 dye from UniRegion Bio-Tech and the chemical structure is:

Materials for electrolyte

1. Lithium iodide (LiI) from MERCK 2. Iodine (I2) from SHOWA

3. 1-methylbenzimidazole from Alfa Aesar and the chemical structure is:

4. Guanidine thiocyanate from Alfa Aesar and the chemical structure is:

5. 1-Methoxypropionitrile (from Alfa Aesar) is a solvent for the liquid electrolyte and the chemical structure is:

3.2 TiO

2

films Preparation

3.2.1 TiO

₂

nanotube arrays film (TNAs)

Titanium foils (99.9% purity) of 0.5 mm or 0.127 mm thickness with a sample size of 0.28cm², were used as the substrate for forming TiO₂ layer by anodic oxidation. Prior to anodization, Ti foil was ultrasonically cleaned by distill water, rinsed by acetone, and then dried by a purging N₂ gas. The schematic diagram of anodizing reaction system is illustrated in Figure 3.1.

All anodizing experiments were carried out at room temperature using a two-electrode electrochemical cell consisting of a stainless steel foil (SS304) as the cathode and a Ti foil as the anode, at a constant DC potential. The electrolyte: 0.5 wt%

NH4F dissolved in the ethylene glycol (EG) solution with 1 wt.% H2O. Anodizing conditions are (1) 40 V for 30-40 min and (2) 30 V for 4h under stirring at 300rpm.

The working layer of DSSCs in the study was fixed at 12µm, unless stated otherwise.

3.2.2 TiO

2

hybrid film (TNWs/TNAs)

A hybrid structure composite of TiO2 nanotube and nanowire were fabricated by using electrolyte consisting of EG and water (99:1 in wt.%) with 0.5 wt.% NH₄F. The conditions of anodizing voltage and processing time were designed to elucidate the formation mechanism of TNWs/TNAs and also fixed in the same thickness to compare the efficiency performance. The electrolyte: 0.5 wt % NH4F dissolved in the ethylene glycol (EG) solution with 1 wt.% H₂O. Anodizing conditions are (1) 40 V for 2h and (2) 30 V for 8h. The working layer of DSSC was fixed at 12µm. Otherwise, we increase the processing time at anodizing voltage of 40 V from 2 h to 5 h in order to examine the transition from nanobelts to nanowires and thickness reaches 15 µm.

3.2.3 Post-treatment for TiO

₂

films

TiCl

₄

treatment

The post-treatment was done by immersing the TiO₂ anode into the 0.1 M TiCl₄ water solution for 30 min in ice bath to form small TiO2 particles on the wall of TNAs and TNWs/TNAs for increasing the surface area. This can improve the charge transfer between TiO2 films and dye adsorption.

Annealing for TiO

2

films

After TiCl4 treatment, TiO2 anode was sintered at 400℃ for one hour to transform the TiO₂ film into anatase phase. For TNAs and hybrid structures, the thermal annealing room temperature for 24 hours for dye adsorption. After sensitized, the TiO2 photoanode was dipped into ethanol to remove extra dye which did not adsorb on the TiO2 surface.

DSSC was fabricated by sealing the dye-sensitized TiO2 photoanode and Pt-sputtered counter electrode around 100℃ with a hot-melt sealing foil. The hot-melt sealing foil was also served as a 60 μm spacer. There are two tiny holes on the Pt-sputtered counter electrode for injection of electrolyte.

The electrolyte composition was 0.5M LiI, 0.05M I2, 0.2M 1-methylbenzlmidazole, and 0.5M guanidine thiocyanate in 1-methoxypropionitrile solvent. The electrolyte was injected into the cell through two tiny holes on the counter electrode. After the extra electrolyte was removed, the two tiny holes were also sealed by the hot melt sealing foil

with a normal glass. Figure 3.2 illustrates the schematic diagram of DSSC fabrication observed by a field emission scanning electron microscope (FESEM) (HITACHI-S2500 JSM-6500F). FESEM is a powerful analytic tool for characterizing the microstructure down to several ten nanometers. The detector gathers secondary electrons signal, and transfers into a SEM photo through an amplifier. In practice, a metal layer (ex. Au, Pt) is coated on the samples to alleviate charging effect. FESEM was operated at an accelerating voltage at 15.0 KV. The thickness of TiO₂ films were examined by focus ion beam microscope (FIBSEM), which was carried out at an accelerating voltage at 5.0 KV. adsorption, which is a commonly used method in DSSC applications [99]. Specifically, the amount of dye adsorption was determined by desorbing the dye from the TiO2 films into 5mM NaOH aqueous solution. The quantification was based on the dye’s maximum absorption values at 505 nm in the dye-desorbed NaOH solutions as measured by an UV-visible light spectrometer (Evolution300), using a dye solution of concentration 5×10^-3 mM as a reference.

3.4.3 Conversion Efficiency

The AM1.5 solar simulator (Newport 3A) was used as the light source, and the incident light spectrum was AM1.5, 1 sun (100mW/cm²) calibrated with standard Si solar cell (ORIEL). I-V curve was recorded with Keithley by scanning DSSC from -0.05V to 0.85V, and the photoelectrochemical characterizations of DSSCs were carried out by computer calculation with the active area 0.28cm².

3.5 Experimental flow

The experiment design and flow chart are schematically illustrated in Figure 3.3.

The working layer of TiO2 films were fabricated by anodic oxidation technique described in Section 3.2. Subsequently, TiO₂ films were treated with TiCl₄, followed by annealing, then immersing into a N719 dye solution. Finally, the DSSCs were assembled and packaged as described in Figure 3.2. The DSSC performance such as I-V curve was characterized.

For the film properties such as surface area and morphology, TiO2 films were immersed into the dye solution for dye adsorption. The dye was then desorbed into NaOH water solution for the dye adsorption measurement using an UV-visible spectrometer. The morphology of TiO2 films under various voltages and processing time was observed by SEM.

By UV-visible spectrum, the dye adsorption amount could be quantified. The TiO2

electrodes with light-scattering layer after TiCl4 post-treatment were dye-sensitized and fabricated into DSSCs for efficiency measurement. The relationship between morphology and DSSC performance will be examined and discussed.

Figure 3.1 Schematic diagram of anodization reaction system.

Figure 3.2 The DSSC fabrication process.

(a) Sealing the dye-sensitized photoanode and Pt-coated counter electrode.

(b) Injecting electrolyte injection through the injection holes, (c) Sealing the injection holes on the counter electrode.

Figure 3.3 Diagram of Experimental Design and Flow: sample preparation and characterization

Chapter 4 Results and Discussion

Two types of TiO2 nanostructure, i.e. TiO2 nanotubes (TNAs) and TiO2 nanowires on TiO₂ nanotubes (TNAs) (TNWs/TNAs hybrid structure) will be fabricated by anodizing titanium metal foil in this study. The parameters of anodization include anodic voltage, processing time, and mechanical stirring, which affect the diffusion of ionic species and electric field, leading to different morphologies. The DSSC performance of TNWs/TNAs hybrid structure will be measured and compared with TNAs and conventional TiO2 nanoparticle film.

4.1 Influence of anodization parameters

4.1.1 Influence of anodizing voltage and processing time without mechanical stirring

Figures 4.1(a) to (c) illustrate the SEM images of TiO2 films prepared by various anodizing voltages from 30 V to 50 V without stirring, in a 0.5 wt.% NH₄F solution under a constant anodizing time of 0.5 h. For the case of 30 V as shown in Figure 4.1(a), the surface morphology shows the highly ordered TiO₂ nanotube arrays with a wall thickness of 56 nm. When the anodizing voltage was increased to 40 V, the wall thickness was reduced to 32 nm. For applying voltage of 50 V, the wall thickness was 26 nm as illustrated in Figure 4.1(c).

In the parallel electrode, the increases voltage induces a stronger electron field. The ions would gather due to the electron field. It can be related to the capacitor, the C=q/V= ε*A/d, the ions concentrations is the same as the charge number; the capacitor value is the same. Thus, the charge will increase with the applied voltage. Moreover, fluorine concentration originated from NH₄F induces the electrochemical etching. The increased voltage influences the concentration F^- ions on electrode surface. Therefore, the higher anodizing voltage, the thinner wall thickness of the nanotube become due to a

higher etching rate in the top section. [79]

Recently, Hsu et al. [100] fabricated TNWs/TNAs hybrid structures by using a one-step anodization method without mechanical stirring. At an anodizing time of 30 min without stirring, it can be observed the pure TiO₂ nanotube arrays (TNAs) and the steady-state growth of TNAs length is approximately ~0.4 µm/min at 40 V before the emergence of nanowires. This suggests that the H⁺ concentration is maintained at the bottom during the chemical drilling [75] because the high-viscosity EG electrolyte limits the ionic diffusion of the electrolyte with a protective environment maintained the pore walls and at the pore mouth during the chemical drilling. With the processing time increased, the mouth of nanotube became fragile. Finally, the wall thickness was too thin to hold and collapse on the tube mouth as illustrate in Figure 4.2.

In this study, we further examine how the tube is changed to nanobelts, then nanowires under the anodizing processing conditions. Figure 4.3 illustrated the condition of required anodizing voltage and processing time (shaded zone) for forming forming TNWs/TNAs (white zone) and the excluded shaded zone for forming TNAs only.

In order to observe the evolution of TiO2 film, we fixed the experiment at a constant anodizing voltage of 40 V and enlarged the processing time. Figure 4.4(a)-(c) show the SEM images for surface morphology of TiO2 films prepared under 40 V using an anodizing time of 0.5h, 2h, and 4h respectively. The length of TNWs/TNAs reach in 10-11µm when anodizing time is 0.5h. Further, it reaches 12µm at 2 h, 15 µm for 4 h and 16µm for 5 h.

The nanowire still etched under the anodizing, the width of nanowire was changed in a period. The width of nanowire is ~70 nm at the voltage 40 V for 2h. As the treatment time was further increased 3 h, 4 h, and further increased to 5 h, the width of

nanowire became ~45 nm, 70nm, and ~45 nm as show in Figure 4.5. The evolution shows the nanowire would be etched until vanish. Meanwhile the tube wall collapsed and inner diameter increased from ~80nm to ~100nm. The Figures 4.6(a)-(b) illustrate how the nanobelt divide into nanowire, and the growth and decline affect the growth rate of TNWs/TNAs under 40 V.

Both the applied electric field and processing time play important roles in the formation of TNWs/TNAs. The emergence of nanowire was controlled by processing time and voltage. The flux of ions in the presence of electric field can be expressed as:

(4.1)

Where is the flux of species i of concentration in direction x, is the concentration gradient, is the diffusion coefficient, is the mobility of species i, and E is the electric field strength. According to the formula, we can understand the ion transport in the electrolyte is significantly influenced by electric field. The ion migration is contributed by the applied electric field and the process of ion diffusion under a concentration gradient.

As a result, the TNWs/TNAs hybrid structure was formed only under anodizing voltage of < 60 V as shown in Figure 4.2, with longer treatment time for lower voltages.

At voltages higher than 60V, no formation of nanowires was observed. In this study, we further identify the required processing time for applied voltage down to 25V.

The formation mechanism of TNWs/TNAs without mechanical stirring has been proposed by Hsu et al. [100]. TiO2 nanowires are found to evolve and form on the top of the TNAs through several stages. Figure 4.2 (a) to (d) [100] show the schematic diagrams along with their corresponding surface morphology images for four key stages in the TNWs/TNAs formation mechanism. First, as the anodic titanium oxide reaction began, the order TNAs was formed, resulting from the field-enhanced chemical drilling

by high H⁺ concentration at the pore bottom of the tubes, in conjunction with a protective environment maintained along the pore walls by the highly viscous EG solution.

As the anodic oxidation reaction proceeded, field-enhanced dissolution in the tube bottom still prevailed to further increase the aspect ratio (highest/diameter) of the TNA at this stage, as Figure 4.2 (a). However, the wall thickness near the tube mouth shown in Figure 4.2 (a) became smaller due to enhanced dissolution of TiO2.

In the electrolyte, the migration of F^- toward the electric field of the bottom electrode is inhibited by highly viscous solution. This results in F^- concentration much higher at the tube bottom [101].With the presence of water in our case, the hydrogen ions further boot the chemical dissolution reaction of forming TiO2 nanotube [102]:

TiO₂+6F^-+4H+→TiF6

2-+2H₂O (4.2) The distribution of the hydrogen ions also result in the wall thickness near the tube mouth was thinner than the foundation as illustrated in Figure 4.2(b)

Meanwhile, the inner surface of the tubes was rough without mechanical stirring in the electrolyte bath as reported by Liu, Shen, and Tao.[103] Thus, the inner tube diameter of the TNAs was not uniform, as schematically illustrated in the inset 1 of Figure 4.2(b) and marked by arrows.

Under an anodizing condition including the specific voltage and processing time, in which the wall thickness at the tube mouth is < 10nm, the area of the inner wall thickness near the top of the TNAs would be etched through by the increased TiO2

dissolution reaction as illustrated in inset 2. As the processing goes by, the strings of through holes on the tubes in the top TNAs, were formed from top to bottom, along the F^- ions migration direction under electric field, as shown in Figure 4.2(b)

After the strings of through holes on the tube form, the tube wall would initial and

propagate downward. As illustrated in Figure 4.2(c), the holes near the top expand and became connected to split into nanowire. With the nanowire further electrochemically etched, resulting in the smooth wire edge and nanowire wire width of nanowire as illustrated in Figure 4.2(d). In addition, the collapsed nanowire on the TNAs, its length is ~5-10 µm.

In short a strings-of-through-holes model [100] is based on the enhanced TiO2

dissolution reaction near the top section in conjunction with a threshold wall thickness of ~10 nm for forming nanowire and high thickness non-uniformity without a mechanical stirring. Four key stages in the TNWs/TNAs formation mechanism are: (a) thinning of the tube wall thickness with high roughness near TNAs mouths, (b) forming strings of through holes in the top section of TNAs, (c) splitting into nanowires, and (d) collapsing and further thinning of nanowire.

4.1.2 Influence of anodizing voltage and processing time under mechanical stirring

Figure 4.7 shows the inner diameter and wall thickness of TNAs near the top section prior to the emergence of nanowires, as a function of voltage and mechanical stirring. In these cases, a processing time of 0.5 h was used to ensure no formation of TNWs.

Regardless the stirring, the wall thickness decreases from 20 nm to 8 nm with increasing voltage, while the tube diameter increases from 15-30 nm to 80-100 nm with increasing voltage. In specific, the wall thickness of nanotube top section without mechanical stirring is larger than that with mechanical stirring. Conversely, the tube inner diameter is larger in the cases with mechanical stirring as compared to those without mechanical stirring.

The stirring was driven by a stirring bar, which rotated the electrolyte between the electrodes, with a constant rate by controller. This resulted in uniform tube diameter and

wall thickness. In contrast, thinner wall thickness near the top section was expected due to high F^- concentration when no mechanical stirring was used. Based on this study and previous work in this research group [100], the criteria of wall thickness for forming TNWs/TNAs structure is ~9-10 nm. This implies that the required time for forming TNWs/TNAs structure will be extended if mechanical stirring is used.

Figures 4.8 (a)-(d) illustrate the SEM images of the TiO₂ films prepared at 30 V in a 0.5wt% NH4F solution for 4 h and 6h with mechanical stirring [(a) and (b)] and without stirring [(c) and (d)], respectively. The surface morphology in Figure 4.8 (a) shows

Figures 4.8 (a)-(d) illustrate the SEM images of the TiO₂ films prepared at 30 V in a 0.5wt% NH4F solution for 4 h and 6h with mechanical stirring [(a) and (b)] and without stirring [(c) and (d)], respectively. The surface morphology in Figure 4.8 (a) shows

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