AN INVESTIGATION INTO THE DOPING AND CRYSTALLINITY OF ANODICALLY FABRICATED TiO2 NANOTUBE ARRAYS: TOWARDS AN EFFICIENT MATERIAL FOR SOLAR ENERGY APPLICATIONS

Open Access
Author:
Abdel-Motalib, Nageh Khalaf Allam
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
February 28, 2009
Committee Members:
  • Craig Grimes, Dissertation Advisor
  • Craig A Grimes, Committee Chair
  • Susan E Trolier Mckinstry, Committee Member
  • Thomas E Mallouk, Committee Member
  • James Bernhard Anderson, Committee Member
  • Joan Marie Redwing, Committee Member
Keywords:
  • Fluoride-free
  • Photocurrent
  • Nanorode
  • Tantala Nanotubes
  • Crystallinity
  • Doping
  • Hydrogen Production
  • Solar Energy Conversion
  • Titania nanotubes
  • Anodization
  • Solar Cells
Abstract:
Anodically fabricated TiO2 nanotube arrays have attracted significant attention in the scientific community because it has proven to be a robust and cost-effective functional material, widely investigated in many applications especially those related to energy conversion such as photoelectrochemical water splitting and solar cells. However, the properties of this material must be modified in order to increase its energy conversion efficiency. For example, the wide bandgap of TiO2 (~ 3.0 eV for rutile and 3.2 eV for anatase) limits its absorption to UV radiation, which accounts for only ≈ 5% of the solar spectrum energy. Also, despite many efforts, it remains a major challenge to successfully convert the amorphous walls of the as-fabricated TiO2 nanotubes to crystalline walls while simultaneously retaining the barrier layer at a minimal thickness. The barrier layer formed during high temperature crystallization acts to hinder electron transfer in applications such as water photoelectrolysis, which in turn leads to reduction in the overall water splitting efficiency. The oxide growth from the underlying Ti foil layer can percolate up and destroy the tube morphology with very high temperature anneals. These properties are believed to be among the critical factors limiting the efficient use of this material especially in photoelectrochemical applications. The important question is; how can TiO2 be modified so as to achieve the requisite performance as a photoelectrode? The primary focus of this dissertation was to improve the properties of the anodically fabricated TiO2 nanotube arrays; notably its band gap and crystallinity while retaining its tubular structure unaffected. The underlying hypothesis was that controlling the crystallinity and band gap while retaining the tubular structure will result in an enormous enhancement of the photoconversion capability of the material. To this end, a direct one-step facile approach for the in-situ doping of TiO2 nanotube arrays during their electrochemical fabrication in both aqueous and non-aqueous electrolytes has been investigated. The effect of doping on the morphology, optical and photoelectrochemical properties of the fabricated nanotube arrays is discussed. Upon the use of different cathode materials for the anodization of titanium, it was possible to dope TiO2 with various metal ions which were found to enhance the optical and photoelectrochemical properties of the material. The highest photoconversion efficiency of 6.9% under UV illumination (320-400 nm) was recorded for a ~2.5μm-sample prepared using a Fe cathode. In another set of experiments, mixtures of H2O2 and HCl in ethylene glycol electrolytes were found to drive modest amounts of carbon within the resulting TiO2 structures which in turn led to an enhancement of the optical and photoelectrochemical properties of the resulted material as well. The highest Air Mass (AM) 1.5 spectrum efficiency of 0.42% was recorded for ~ 6 μm-sample prepared in EG-containing electrolyte in the presence of 0.5M HCl and 0.4M H2O2. This is ~15% more efficient than that reported for 30 μm long undoped TiO2 nanowires under AM 1.5 (5M KOH, 0.61VSCE) although our sample is 5 times shorter and tested in less concentrated KOH electrolyte. The incorporation of these ions into the TiO2 nanotube arrays during their synthesis gives rise to the possibility of in-situ bandgap engineering of the material during its fabrication without any structural collapse. These results showed the validity of the hypothesis that doping the material while retaining its tubular structure enhances its photoactivity. In an effort to improve the crystallinity of the anodically fabricated TiO2 nanotube arrays while retaining the tubular morphology, novel processing routes have been investigated to fabricate crystalline TiO2 nanotube array electrodes. For the sake of comparison, the nanotubes were annealed at high temperature using the conventionally used procedure. The samples were found to be stable up to temperatures around 580 °C, however, higher temperatures resulted in crystallization of the titanium support which disturbed the nanotube architecture, causing it to partially and gradually collapse and densify. The maximum photoconversion efficiency for water splitting using 7 μm-TiO2 nanotube arrays electrodes annealed at 580 °C was measured to be about 10% under UV illumination. Could we enhance the photoelectrochemical properties of the material via any subsequent treatment? To this end, we investigated the effect of subsequent low temperature crystallization step. Combining high temperature furnace annealing with a 140°C (≈ 50 psi) ethanol vapor treatment successfully enhanced the photoconversion efficiency by about 30% under UV illumination and 40% under full AM 1.5 illumination. Despite this improvement, the barrier layer thickness was still the same as that of the thermally annealed sample. Consequently, it was believed that finding a technique which could be used to crystallize the nanotubes either at short time intervals and/or low temperature would result in better crystallinity of the tube walls as well as in a minimal barrier layer thickness which in turn would result in an enhancement of the photoconversion efficiency. Consequently, the possibility of both avenues has been investigated, i.e. the possibility of crystallizing the nanotubes at low temperature and/or at short time. Rapid infrared (IR) annealing was found to be an efficient technique for crystallizing the nanotube array films within a few minutes. The IR-annealed 7μm-nanotube array films showed significant photoconversion efficiencies (=13.13%) upon their use as photoanodes to photoelectrochemically split water under UV illumination. This was related, in part, to the reduction in the barrier layer thickness from 1100 nm for the thermally annealed sample down to 200 nm for the IR-annealed sample under same conditions. These results support the hypothesis that reducing the barrier layer thickness would result in better performance of the material. Could the fabrication temperature and in turn the barrier layer thickness be reduced any more? With this in mind, the possibility of low temperature crystallization was investigated with the hypothesis that this route might result in even thinner barrier layer. Regarding the possibility of low temperature crystallization, this dissertation encompasses the first report on low-temperature synthesis of crystalline TiO2 nanotube arrays. Nanotube arrays of up to 1.4 μm length using a two-step process have been demonstrated. The two-step process consists of initial treatment of the Ti foil in an oxidizing agent (H2O2 or (NH4)2S2O8)-containing electrolytes, followed by potentiostatic anodization of the resulting foil in NH4F-containing electrolytes. The as-synthesized crystalline nanotube arrays were successfully tested as anode electrodes for water photoelectrolysis, with performances comparable to samples annealed at high temperatures, and for liquid junction dye (N 719 dye)-sensitized solar cells. The photoconversion efficiencies for the as-synthesized nanotube arrays, under 320 nm – 400 nm illumination, were approximately 3% for the 800nm-H2O2-fabricated nanotubes and 4.4% for the 1.4 μm-(NH4)2S2O8-fabricated nanotubes upon their use to split water photoelectrochemically. Although this method showed that low temperature crystallization drastically improved the photoelectrochemical properties of the material, the length of the resulted nanotube arrays was limited to 1.4 μm, largely determined by the thickness of the oxide layer formed in the first step of fabrication. The obtained improvement in the photoconversion efficiency despite the limitation in the thickness of the fabricated nanotube arrays (η=4.4% for a 1.4μm nanotube sample) motivated me to investigate the possibility of achieving crystalline nanotubes directly from the anodization bath. This avenue would have the advantage of overcoming the limitation in tube length and controlling the barrier layer thickness since no high temperature would be employed. With the ultimate goal to achieve room or low temperature, crystalline TiO2 nanotube arrays, the effect of electrolyte composition on the possibility of inducing crystallinity in the vertically oriented TiO2 nanotube array films during their synthesis via potentiostatic anodization was investigated. Is there any electrolyte composition that might enable in-situ crystallization? With the use of 3M HCl electrolytes, partially crystalline, vertically oriented TiO2 nanotubes with thicknesses up to 300 nm were achieved between 10 V and 13 V. The addition of H2O2 to HCl-containing electrolytes was shown to extend the tube length up to 870 nm and to improve the crystallinity of the formed architectures. In regard to water splitting, the 870nm-partially crystalline as-anodized sample showed photoconversion efficiency of 0.025% under AM 1.5 illumination. Note that regularly fabricated TiO2 nanotube arrays in fluoride-containing electrolytes show almost zero photoconversion efficiency. However, annealing the partially crystalline samples at 5000C resulted in an improvement in the photoconversion efficiency (0.14%). This is the first report on the fabrication of partially crystalline TiO2 nanotubes and in fluoride-free HCl aqueous electrolytes. With the motivation of finding an electrolyte composition that might yield better crystalline nanotubes than that obtained in the HCl-containing electrolytes, the effect of using some polyol electrolytes (diethylene, triethylene, tetraethylene and polyethylene glycols) on the crystallinity and morphology of the fabricated TiO2 nanotube arrays was investigated. The study showed that the use of these electrolytes helped to induce partial crystallinity in the formed nanotube arrays with the intensity of anatase (101) peak was found to increase with increasing the molecular weight of the polyol electrolyte. Water content in the electrolyte was found to be a critical factor in obtaining such partial crystallinity. The as-anodized nanotube arrays showed low photoconversion efficiency upon their use as photoanodes to split water photoelectrochemically indicating only partial crystallinity of the tubes. Annealing the tubes at only 3000C increased the photocoversion effciency to values comparable to those usually seen when nanotubes annealed at high temperature (above 5000C) were used. There is a possibility that the pre-existing anatase crystallites acted as nucleation sites (seed layer) facilitating the nucleation and growth of more crystallites at 3000C resulting in comparable crystallization to that obtained for totally amorphous nanotubes annealed at higher temperature (above 5000C). Building upon my acquired expertise in synthesis of TiO2 nanotube arrays, I have investigated the possibility of formation of Ta2O5 nanotube arrays. Ta2O5 is the starting material to fabricate TaON which was shown to split water efficiently and to be highly responsive to excitation wavelengths up to about 600nm with IPCE values up to 34%. The underlying hypothesis was that fabricating this material in the nanotubular structure would enhance its efficiency to split water due to the high surface area and better charge transfer. This thesis reports, for the first time, synthesis of high-aspect-ratio tantalum oxide nanotube arrays via one-step anodization of Ta foil. The use of aqueous electrolytes containing HF:H2SO4 in the volumetric ratios 1:9 and 2:8 results in formation of ordered nanodimpled surfaces with 40-55 nm pore diameters over the potential range 10-20 V. The addition of 5-10% of either ethylene glycol (EG) or dimethyl sulfoxide (DMSO) to the HF and H2SO4 aqueous electrolytes resulted in the formation of Ta oxide nanotube arrays up to 19 µm thick, either securely anchored to the underlying Ta film or as robust free-standing membranes, as dependent upon the anodization time and applied voltage.