Advanced One Dimensional TiO2 Nanomaterials Synthesized by Electrochemical Anodization: Electric and Catalytic Properties

Abstract

Since first discovery of the titanium element by William Gregor in 1791, the TiO2 has been regarded as one of the most fascinating materials in the various industrial fields due to its distinctive properties. With the advantages including proper electric band position, harmless to human, inertness to chemical, and superior photo-stability make TiO2 an important component in environmental and energy-related applications. These applications include photoanode in solar cells where solar energy is converted into electricity, photoelectrolysis where water is changed into hydrogen, and photocatalyst where organic pollutants are degraded into more eco-friendly chemical species. All these TiO2 applications require well contact with various electrolyte including solid, liquid, and gaseous phase, thus it is essential to develop suitable TiO2 architecture for optimal TiO2 applications. The ideal TiO2 architecture should be satisfied sever factors such as a large internal surface area, an efficient charge transport, low charge recombination and good contact between the semiconductor and the electrolyte. The most frequently employed TiO2 structure is three-dimensional networks of TiO2 nanoparticles (TNP). Although the nanoporous TiO2 networks of TNPs offer a large internal surface area with high surface to volume ratio, the disorderly connected TNPs induce randomly diffusive charge transport through the film leading high charge recombination chance and poor charge transport. Recently, highly-ordered TiO2 nanotubes (TNT) formed by electrochemical anodization provide an alternative electron transport material that reduces charge recombination and increases charge collection relative to conventional disordered nanoparticle channels due their intrinsic orthogonal structure. However, most previously reported TNT fabricated by anodization shows several problems such as only top-side opened (singly opened, SNT) structure, and insufficient contact between TNT and electrolyte, and bad charge collection in the closed bottom-side could be induced in this geometry. Recently, several reports have described the preparation of freestanding doubly open-ended TNT (DNT) exhibiting enhanced TiO2 performance in various applications compared to classic TiO2 SNT. The freestanding TiO2 DNT have been considered as most advanced NT structures up to now. DNT is usually produced by chemical etching or in-situ pore opening process, however, the TiO2 NT could be damaged during chemical etching or high voltage anodization leading inhomogeneous morphology of DNT. In addition, the curling or cracking problems of DNT membrane are obstacle of DNT applications. Therefore, novel DNT fabrication approach has been highly required to overcome aforementioned problems. In Chapter 2, we suggest a novel selective etching method for fabricating large scale and non-curling DNT. This method involves 4 steps including two-step anodization and annealing process (1st anodization → annealing → 2nd anodization → detaching & etching), and the key point of this method is the magnitude of applied temperature in annealing step maintaining amorphous phase of 1st anodized TiO2 NT layer to allow successive 2nd TiO2 NT layer formation by 2nd anodization step. The thin 2nd NT bottom layer was selectively removed by a 33% H2O2 solution from a physically stable, thick, amorphous 1st top layer, resulting in large-area noncurling, freestanding TNT arrays that did not crack, even in the amorphous state. The amorphous TiO2 DNTs were easily transferred to an FTO substrate for fabricating front-illuminated DSCs. The DNT-based DSCs exhibited a photoinduced electron lifetime 10 times longer than was observed in the TNP-based device, resulting in a 35 mV greater VOC. In addition, the DNT-based DSC yielded an cc exceeding 95%, 15% greater than that observed for TiO2 nanoparticles (TNP)-based DSCs, resulting in a power conversion efficiency (PCE) of 8.6% for a 14 m thick DNT layer. The simple method for fabricating DNTs may be useful in a wide range of application, including photocatalysis and chemical sensors. In Chapter 3, DNT arrays decorated with a few nm-sized TiO2 nanoparticles (sNP@NT hybrid structure) were prepared to increase surface area of DNT. The TiO2 nanoparticles (NPs) on the DNT array surfaces increased the dye loading on the sNP@NT-based DSCs by 9% compared to the undecorated DNT-based DSCs, thereby enhancing the light harvesting capabilities. The power conversion efficiency (PCE) of the sNP@DNT-based DSC prepared with 11 m thick DNT arrays was 10.0%, which constituted a 47% improvement over the corresponding DNT-based DSCs (which displayed a 6.8% PCE). Despite having a dye loading level that was 22% lower than the dye loading level in the conventional TiO2 NP-based DSCs, due to the limited internal surface area, the PCE of the sNP@DNT-based DSC was 28% greater than that of the conventional TiO2 NP-based DSC (8.1% PCE) prepared with a light scattering layer. The high charge collection efficiency and increased surface area of the sNP@DNT array and the good photovoltaic performance set a new record for efficiency among NT-based DSCs. In Chapter 4, we systematically investigated the charge transport properties of doubly or singly open-ended TiO2 nanotube arrays (DNT and SNT, respectively) for their utility as electrodes in dye-sensitized solar cells (DSCs). The SNT or DNT arrays were transferred in a bottom-up (B-up) or top-up (T-up) configuration onto a fluorine-doped tin oxide (FTO) substrate onto which had been deposited a 2 μm thick TiO2 nanoparticle (NP) interlayer. This process yielded four types of DSCs prepared with SNTs (B-up or T-up) or DNT (B-up or T-up). The photovoltaic performances of these DSCs were analyzed by measuring the dependence of the charge transport on the DSC geometry. High resolution scanning electron microscopy techniques were used to characterize the electrode cross-sections, and electrochemical impedance spectroscopy was used to characterize the electrical connection at the interface between the NT array and the TiO2 NP interlayer. We examined the effects of decorating the DNT or SNT arrays with small NPs (sNP@DNT and sNP@SNT, respectively) in an effort to increase the extent of dye loading. The DNT arrays decorated with small NPs performed better than the decorated SNT arrays, most likely because the Ti(OH)4 precursor solution flowed freely into the array through the open ends of the NTs in the DNT case but not in the SNT case. The sNP@DNT-based DSC exhibited a better PCE (10%) compared to the sNP@SNT-based DSCs (6.8%) because the electrolyte solution flow was not restricted, direct electron transport though the NT arrays was possible, the electrical connection at the interface between the NT array and the TiO2 NP interlayer was good, and the array provided efficient light harvesting. In Chapter 5, we prepared well-defined hierarchical structures comprising doubly open-ended TiO2 nanotube (DNT) arrays covered with various layers of few-nanometer-sized TiO2 nanoparticles (NPs) to investigate the electron collection mechanisms in homogeneous hybrid structures. We found that competitive electron transport pathways (direct transport through the NT and randomized transport through the NPs) are present in the homogeneous hybrid structures. Photoinduced electrons generated at the few-nanometer-sized TiO2 NPs directly connected with TiO2 DNTs (e.g., isolated and single-layer NPs on the surface of NTs) dominantly traveled to the NTs. With an increasing number of TiO2 NP layers, photoinduced electrons are randomly transported through the TiO2 NP layers. Enhanced light harvesting and efficient charge collection (∼95%) caused by the increased amounts of dye loading and the direct transport through the DNT, respectively, are achieved in a structure with ∼1.4 layers of few-nanometer-sized TiO2 NPs, resulting in a power conversion efficiency of 11.3% with a JSC value (22.9 mA/cm2) close to the theoretical value (∼26 mA/cm2) of a N719-based dye-sensitized solar cell. In Chapter 6, novel fabrication method of a well-defined and ultrathin nanostructured TiO2 electron transport layer (ETL) is introduced. An electron transporting layer ETL plays an important role in extracting electrons from a perovskite layer and blocking recombination between electrons in the fluorine-doped tin oxide (FTO) and holes in the perovskite layers, especially in planar perovskite solar cells. Dense TiO2 ETLs are used in many devices due to their ease of fabrication. A TiO2 precursor solution is directly spin-coated onto an FTO layer and then crystallized. No systematic studies have yet been conducted to evaluate the morphological defects in conventionally spin-coated dense TiO2 layers (S-TiO2) prepared on highly rough FTO surfaces and to examine the effects of the defects on the charge transport process. Significant efforts have been applied toward replacing conventionally spin-coated dense TiO2 layers (S-TiO2), yielding an improved power conversion efficiency (PCE). We found that fatal morphological defects at the S-TiO2 interface due to a rough FTO surface, including an irregular film thickness, discontinuous areas, and poor physical contact between the S-TiO2 and the FTO layers, were inevitable and lowered the charge transport properties through the planar perovskite solar cells. The effects of the morphological defects were mitigated here using a novel TiO2 ETL fabrication method based on sputtering and anodization. This method produced a highly connected rod-type single crystalline TiO2 ETL (A-TiO2) with a uniform film thickness, excellent transmittance through the rough FTO surface, and defect-free physical contact with the FTO. The power conversion efficiency (PCE) increased by 22% (from 12.5 to 15.2%), and the stabilized maximum power output efficiency increased by 44% (from 8.9 to 12.8%) compared with conventionally spin-coated TiO2 ETLs. A spectroscopic and rectifying analysis demonstrated outstanding electron extraction and hole blocking by the A-TiO2 layer due to its unique morphological characteristics. In Chapter 7, for investigation of photocatalytic ability of DNT, DNT arrays on a fluorine-doped tin oxide (FTO) substrate and freestanding hierarchical DNT membrane were prepared to apply in water splitting and degradation of gaseous pollutants, respectively. The unique through-hole structures of DNT-based TiO2 electrodes could lead increased TiO2 performance in various photocatalytic reactions, however, such investigations have been rarely reported due to difficulties of producing large area and non-curling DNT membrane. In previous discussions, we developed large-scale and non-curling DNT membrane and identified that the DNT-based electrodes led dramatically enhanced photovoltaic performance in the solar cells. In water splitting, flame and chemical reduction was performed on DNT substrate for inducing oxygen vacancies in DNT, and resulted DNT substrate exhibited significantly improve the saturation photocurrent density in the device. In degradation of gaseous pollutants, the hierarchical DNT membrane exhibits outstanding photocatalytic performance (fast degradation rate of gaseous pollutants) compare to conventional TiO2 electrodes including bare doubly or singly open-ended TiO2 nanotube (SNT) membrane and hierarchical SNT. We revealed that unique structures of hierarchical DNT membrane give a large photocatalytic reaction site and permit smooth mass diffusion for removing the intermediates generated by gaseous pollutants decomposition.