Towards efficient photoanodes for solar fuel production

The solar energy conversion efficiency is a materials-limited process as there is always a trade-off between the light absorption capability of the material and its stability. For solar hydrogen production, for example, wide-bandgap semiconductors are stable but only absorb in the UV region of the light spectrum. Small-bandgap semiconductors, on the other hand, are not stable in aqueous electrolytes. In this thesis, two metal oxide-based photoanode systems were studied in an attempt to find a balance between their optical and photocatalytic properties as well as their stability. In the first part of the thesis, one-dimensional TiO2 nanotubes/ZnO core-shell nanostructured electrodes were investigated. Increasing the ZnO shell thickness resulted in different morphological, structural and optical characteristics. The crystallinity of the core nanotubes was found to be a determinant factor in the formation of the TiO2/ZnO heterojunctions as revealed by the FESEM, GAXRD, XPS and Raman analyses. The TiO2/ZnO heterojunction showed almost 80% increase in the photoconversion efficiency (7.3%) compared to pure TiO2 (4.1%) under UV illumination (320-400 nm, 100 mW/ cm2, 0.5 M Na2SO4). The main reasons responsible for the observed enhancement in the photoactivity were discussed. In the second part of the thesis, Nb2O5 based photoanodes were investigated. The fabrication of Nb2O5 ordered structures (nanopores, nanorods, nanochannels and microcones) is achieved by a simple electrochemical method. The microcone structure was the most stable morphology and showed higher absorption ( 450 nm) compared to other structures (380 nm). An in-situ approach for the direct synthesis of crystalline Nb2O5 microcones at room temperature is demonstrated for the first time. Also, the successful formation of niobium oxynitride microcones is achieved for the first time as confirmed via XPS, XRD and Raman spectroscopy measurements. The fabricated NbON microcones showed exceptional optical properties with an absorption profile extending to 770 nm

Correlation between geometrical and structural properties of mixed oxide ultrathin nanotubes and their solar water splitting performance

The objective of this study was to study the effect of Nb alloying with Ti on the photoelectrochemical performance of the resulted oxide upon anodization. In this regard, nanotubes were grown on Ti-Nb alloy via electrochemical anodization and their corresponding photocatalytic behavior was investigated and compared with those grown on an ordinary Ti substrate. After preparing and optimizing the nanotubes dimensions for the required geometrical structure, the as formed tubes were annealed at different temperatures and in air), then characterized with respect to their morphological, structural, and photoelectrochemical properties. From the morphological and structural point of view, optimized and well aligned ultra-thin wall nanotubes were successfully synthesized on the surface of Ti-Nb alloy. To the best of our knowledge, these dimensions have not been reported before. One of the challenges was that the oxide layer formed on the surface of the alloy was not precisely identified in literature, where some authors reported the formation of combination of individual oxides (TiO2 and Nb2O5), whereas, others claimed it was a mixed oxide TiNbOx. Raman and X-ray diffraction test results confirmed the formation of individual anatase and monoclinic Nb2O5 phases. Detailed XRD analysis was performed and the crystallite size as well as microstrain were calculated and found to be minimal indicating negligible effect of lattice induced tension or compression. It is worth mentioning that insignificant structural changes are favorable to maintain good electron mobility. Hence, point defect equations were deduced and it was found that that oxygen vacancies were the prevailing ionic defects rather than electronic Nb compensation. From the aforementioned results, ultrathin wall nanotubes formed on TiNb alloy were achieved, for the first time, with clear representation of the oxide layer composition. Such oxide layer showed better stability upon annealing at high temperatures. Although, UV-Vis test results showed small or negligible enhancement in the absorption, profile the photo-electrochemical measurements showed much higher photocurrent for Ti-Nb oxide alloy than bare TiO2 prepared at the same conditions for the sake of comparison. In conclusion, the Ti-Nb NTs showed enhanced stability over a wide range of temperatures, where the transition from anatase to rutile was shifted to higher temperature in addition to an increase in the photoconversion capability, resulting in a more efficient water splitting process

Fabrication and Modification of Titania Nanotube Arrays for Harvesting Solar Energy and Drug Delivery Applications

The fast diminishing of fossil fuels in the near future, as well as the global warming caused by increasing greenhouse gases have motivated the urgent quest to develop advanced materials as cost-effective photoanodes for solar light harvesting and many other photocatalytic applications. Recently, titania nanotube arrays (TNTAs) fabricated by anodization process has attracted great interest due to their excellent properties such as: high surface area, vertically oriented, highly organized, one-dimensional, nanotubular structure, photoactivity, chemical stability and biocompatibility. This unique combination of excellent properties makes TNTAs an excellent photoanode for solar light harvesting. However, the relatively wide band gap energy of titania limits its photoactivity to the UV spectra which accounts only for 5 % of solar light spectra. The specific objectives of this thesis are to: First, fabricate reproducible well-organized, vertically-oriented TNTAs in different viscous electrolytes and optimize the fabrication parameters. Second, modify the TNTAs by doping nitrogen and carbon and study the effect of modification on optical properties and photoelectrochemical performance. And third, functionalizing the TNTAs surface by monodispersed magnetic ferrite nanoparticles for improved solar light harvesting and drug delivery application in cancer treatment. The effect of each fabrication parameter such as electric potential, pH, water content, anodization time and electrolyte composition was discussed. TNTAs were successfully fabricated in an inexpensive viscous electrolyte composed of 2 wt.% sodium carboxy methylcellulose (CMC). TNTAs were successfully fabricated on both sides of a Ti disc with total tube length of 9.5 µm with a unique structure composed of conducting Ti metal sandwiched between two semiconducting layers of TNTAs on each side with a new potential electronic and photocatalytic applications. A new, facile, low cost, environment-friendly and nanoarchitecture-safe method was introduced to fabricate N- and C-modified TiO2 nanotube arrays. Modified optical properties with narrow band gap energy, Eg, of 2.65 eV was obtained after annealing the modified TNTAs at 550°C. Modified TNTAs showed enhanced photoelectrochemical performance. Photoconversion efficiency (PCE) was increased from 4.35% for pristine (unmodified) TNTAs to 5.18% for modified TNTAs, an increase of 19%. Effect of nanotubes length of modified TNTAs on photoelectrochemical performance was also studied. Photocurrent density and PCE were increased by increasing nanotube length with a maximum PCE of 6.38% for nanotube length of 55 µm. This implies an excellent light penetration up to 55 µm depth into photoanode which is about 3.6 times higher than the maximum penetration depth (15 µm) in the nanoparticulate photoanode. This increasing pattern of photoconversion efficiency with increasing nanotubes length also implied a high charge separation rate and lower charge recombination rate. This high PCE value was attributed to: band gap reduction due to N- and C-modification of TNTAs surface, increased surface area of long TNTAs compared with short TNTAs, investigated in previous studies, and the excellent light penetration and harvesting properties. Ferrite NPs-encapsulated TNTAs were fabricated for the first time using a facile and efficient method. Ferrite nanoparticles of 13 ± 3 nm diameters were successfully distributed all over the top and inner surface of the nanotubes. UV-Vis reflectance spectra showed excellent visible light absorbance up to wave length of 660 nm (Eg = 1.88 eV). The prepared magnetic nanocomposite showed their potential capability to controlling the drug release of an anti-cancer drug (5-fluorouracil). The drug release of 5-fluorouracil by diffusion was sustained with controlled initial burst effect. The suitability of magnetic nanocomposite for cancer drug delivery was confirmed by in vitro cytotoxicity study

Photoelectrochemical Systems for Hydrogen Production.

The goal of this research is to advance the understanding and development of efficient and stable photoelectrochemical cells for renewable hydrogen production. To this end, three main strategies were investigated for improving the photoanode performance: producing the semiconducting oxide (i.e. titanium dioxide, TiO2) in the form of long nanotube arrays, incorporating gold nanoparticles onto the surface, and combining this photocatalyst with a solar cell. Highly ordered TiO2 nanotube (TiNT) arrays were fabricated using an anodization process. By varying the anodization conditions, TiNTs with different dimensions were fabricated. Increasing the nanotube length resulted in increased photocurrents up to lengths that exceeded the diffusion length of electrons in TiO2 (~20 µm). Gold nanoparticles with average diameter ranging from 3-12 nm were deposited onto selected TiNTs using a modified deposition precipitation method. The pH of the solution used during the Au loading is the crucial parameter determining the gold particle size and metal loading. Furthermore, small gold nanoparticles (less than 5 nm) significantly improved the electrocatalytic properties of TiO2 by adding active sites for water oxidation. Studies relating Au particle size and hydrogen rate per active Au species suggested that for Au particles bigger than 5 nm the most active sites were located on the surface of the metal, and for Au particles smaller than 5 nm the most active sites seemed to be at the perimeter in contact with the oxide support. Efficiencies for PEC cells were calculated and the Au/TiNT photoelectrodes shown efficiencies in excess of 1.2 %, which are one order of magnitude higher than the efficiencies reported for TiO2 powder photoelectrodes. In addition, this efficiency is about 100% higher than the efficiencies reported in the literature for photoanodes made similar nanotube arrays. The novel Au/TiNT photocatalyst was combined with Si solar cells in a hybrid arrangement. In this tandem cell the photocatalyst film and the solar cell were connected in series (adding the voltage produced by each component) and gave a conversion efficiency of 1.6 %.Ph.D.Mechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/78865/1/paurora_1.pd

Doctor of Philosophy

dissertationHydrogen is envisioned as a viable fu e l o f the future. Photoelectrochemical (PEC) hydrogen generation by water splitting reaction is the most promising method to obtain renewable hydrogen. The U.S. Department of Energy has determined that for PEC hydrogen to be economically feasible, and competitive with steam reforming hydrogen production, a solar-to-hydrogen efficiency of 10% maintained for 1 ,0 0 0 hours of operation is required. Selection of durable photo-electrodes capable of withstanding the harsh aqueous environment in PEC hydrogen generation is an important factor. Semiconductor nanostructured metal oxides, such as titanium dioxide, are generally more stable in such environments, making them suitable candidate materials. In the present investigation, self-organizing nanotubular titanium dioxide synthesized by electrochemical anodization and heterostructures thereof were examined for PEC hydrogen generation. In the first part, new synthesis methods were explored such as light-assisted anodization, surface treatment prior to anodization to achieve hierarchical nanotubular titanium dioxide, and binary acid anodization for in situ metal doping. A mechanism for pore nucleation and nanotube wall separation has also been proposed. In the second part, titania nanotubes were sensitized with nanocrystalline CdO, CdS, and Mn2+ or Co2+ doped CdS as visible light absorber layers. The material properties were examined using different characterization techniques such as scanning electron microscopy (SEM), x-ray diffraction (XRD), ultra violet-visible (UV-vis) photospectroscopy, x-ray photon spectroscopy (XPS), and Raman spectroscopy. The PEC activity of the photoanodes was examined under simulated air mass (AM) 1.5 irradiation. Electrochemical impedance spectroscopy and Mott-Schotty analysis were also used to ascertain the PEC results and correlate with material properties

Fabrication and characterization of semiconductor based photo-catalysis for light-Driven water splitting

The straightforward, low-priced and hence extensive conversion of sun light utilizing photocatalysis in a water splitting process is the main source to provide a clean and renwable hydrogen supply. Principally, photocatalysts are semiconductor materials with a suitable band gap that can absorb incident photons to produce photogeneated charges which consequently initiate the water splitting reaction to generate oxygen and hydrogen. The process itself is typically influenced by the material properties of the semiconductor (band gap, redox potentials and crystallinity) thus, altering the band structure of the semiconductor would help build up a photocatalyst that is appropriate for susbtaintial hydrogen generation. This thesis exemplifies a detailed study of high performance yet affordable photo-electrodes for solar-driven hydrogen production using Titanium (II) oxide (TiO2). TiO2 is considered to be a favorable photocatalyst that can be used as a photoanode in the photoelectrochemical cell due to its unique properties. In particular it\u27s high physical and chemical stability, high oxidizing power of the photogenerated holes, low-cost and non-toxicity. However, TiO2 is ideal for water splitting only under ultraviolet (UV) light due to its band gap that reaches 3.2 eV which makes its photocatalytic activity only restricted to the UV range that comprises only about 3% of the whole solar spectrum. In this study, two titania based photoanode systems were investigated in an effort to optimize the trade-off between the low external bias needed (electrical energy input) and the high photocurrent spectral response (H2 output). In the first part, Na-modified TiO2 nanostructured electrodes were studied. Varying the Na content showed a noticeable impact on the optical as well as the photoelectrochemical characteristics. The morphological characterization affirmed the presence of a discontinuous layer adsorbed over the surface of the TiO2 nanotubes where the tublar structure is kept preserved after treatment. Chemical analysis revealed no significant change in the structural properties of TiO2 upon modification which proves that the alkali ions were just dispersed within the TiO2 network. Optical properties illustrate the inclusion of conduction band tail states attributed to the disordered structure where the absorption edge is slightly shifted towards higher wavelength regions. The modified electrodes maintained nearly 81 % enhancement in the photoconductivity (0.9928 mA cm-2) in comparison with that of bare TiO2 (0.1821 mA cm-2) under AM 1.5G illumination (100 mW cm-2, 0.05 M Ba (OH) 2). Also, improved carriers\u27 separation and mobility has been accomplished which was asserted by the electrochemical impedance spectroscopy that revealed less charge transfer resistance as well as space charge capacitance for the surface modified electrodes. Further, the Mott-Schottky analysis affirmed the observed Voc enhancement by demonstrating a negative shift in the flat band potential for all the Na+-modified electrodes with respect to that of the pristine TiO2 implying less band bending requirements. Finally, DFT calculations were implemented to add further details on the electronic structure of the disordered titania confirming the empirical findings obtained upon surface modification. In the second part of this work, hybrid PEDOT/TiO2 photoelectrodes were analyzed. The development of such nanocomposites was accomplished by controlled electrochemical anodization of Ti foil, followed by a simple and fast spin coating of PEDOT. The heterojunctions maintained superior optical sensitivity where the absorption band edge reaches nearly 694 nm with respect to that of the unsensitized (TiO2 382 nm). This clearly indicates the ability to promote water splitting under visible irradiation. Likewise, superior photoelectrochemical performance concerning the photoconductivity, and the charge transfer kinetics were recognized mainly due to the fact that the highest occupied molecular orbit (HOMO) and lowest unoccupied molecular orbit (LUMO) of PEDOT are more negative than the conduction band (CB) and the valence band (VB) of TiO2. This in return, not only narrows down the band gap but also facilitates the separation of photo-induced charges and accordingly improves the photocatalytic activity

Exploiting the synergistic catalytic effects of CoPi nanostructures on Zr-doped highly ordered TiO2 nanotubes for efficient solar water oxidation

Photoelectrochemical (PEC) catalysis offers promising strategies for sustainable development. This study demonstrated the synergistic catalytic behavior of ZrO2 and a cobalt phosphate on anodized TiO2 nanotubes (TNTs), which significantly enhanced the PEC performance for visible-light-driven water splitting reactions. The sequential addition of ZrO2/CoPi-decorated TNTs was performed via electrodeposition and photoassisted electrodeposition. The substitution of Zr4+ by Ti4 can lead to the creation of oxygen vacancies, enabling electron trapping, reducing charge recombination, and thereby enhancing the charge transfer efficiency. Further, in the case of TNTs/ZrO2/CoPi photoanode, the CoPi WOC functioned as a hole-transfer relay to promote the water-splitting reaction. Specifically, incorporating ZrO2/CoPi rushes the surface reaction kinetics of TNTs and considerably improves charge transfer efficiency (ηCT = 90%), photocurrent density (0.86 mA/cm2 at 1.23 VRHE) and durability were obtained. Further, the mechanistic examination by impedance measurements showed the enhanced charge transfer, and surface conductivity for prepared materials. The proposed method can be widely used to develop electrodes made of other materials to produce solar fuels

Functional nanostructred photoanodes for solar fuel production

Improving the efficiency of water splitting process is one of the main obstacles that are facing the generation of renewable energy. Charge carriers separation is always coupled with low visible light absorption and stability of the materials used. Various efforts have been done in order to construct a full system of different materials that can absorb visible light efficiently, with an enhanced electron hole separation process for an efficient water splitting. However, most of the reported systems suffer either from crystal mismatch between the multiple materials the use of long linkers that promote the recombination of the carriers. In this thesis, we are introduce a new system of titania nanotubes that are functionalized with graphene quantum dot, as a photosensitizer and an efficient charge carrier collector and transporter. In the first part of the thesis, one-dimensional TiO2 nanotubes photoanodes were investigated. We are able to produce ultra thin walled titania nanotubes, for the first time. Thin walled titania nanotubes showed higher quantum efficiency; about 50% compared to 15 % for conventional thick-walled nanotubes, with a 50% enhancement in the photocurrent. This enhancement is mainly attributed to the very small wall thickness (3 nm), allowing the diffusion of the charge carriers across the wall, regardless the potential across the region. In the second part, the effect of hydrogen annealing on the optical and electrical properties of the thin-walled nanotubes was investigated. It was found hydrogen annealing for 4 hours passivate the trap states on the surface of titania, while annealing for longer times acts to create more defect states, larger carrier concentration, larger dark current, higher resistance. In addition, we introduced a new concept by adding KOH and hydrogen annealing resulted in lower resistance and higher charge carrier concentration and photocurrent. In the third part, we produced graphene quantum dots, and for the first time, we were able to functionalize graphene quantum dots with different groups (mercapto propanoic acid, and malonyl group). We also introduced a new anchoring method for graphene quantum dots on the surface of titania. The photocurrent was enhanced by 50%, and the reasons were discussed in details. Finally, we showed the possible new applications for titania nanotubes that are functionalized with our graphene quantum dots

Enhancing the photoelectrochemical water splitting characteristics of titanium and tungsten oxide based materials via doping and sensitization

To better utilize solar energy for clean energy production, efforts are needed to overcome the natural diurnal variation and the diffuse nature of sunlight. Photoelectrochemical (PEC) hydrogen generation by water splitting is a promising approach to harvest solar energy. Hydrogen gas is a clean and high energy capacity fuel. However, the solar-to-hydrogen conversion efficiency is determined mainly by the properties of the materials employed as photoanodes. Improving the power-conversion efficiency of PEC water splitting requires the design of inexpensive and efficient photoanodes that have strong visible light absorption, fast charge separation, and lower charge recombination rate. In the present study, PEC characteristics of various semiconducting photoelectrodes such as TiO2, WO3 and CuWO4 were investigated. Due to the inherent wide gap, such metal oxides absorb only ultraviolet radiation. Since ultraviolet radiation only composes of 4% of the sun's spectrum, the wide band gap results in lower charge collection and efficiency. Thusto improve optical absorption and charge separation, it is necessary to modify the band gap with low band gap materials.The two approaches followed for modification of band gap are doping and sensitization. Here, TiO2 and WO3 based photoanodes were sensitized with ternary quatum dots, while doping was the primary method utilized to investigate the modification of the band gap of CuWO4.The first part of this dissertation reports the synthesis of ternary quantum dot - sensitized titania nanotube array photoelectrodes. Ternary quantum dots with varying band gaps and composition (MnCdSe, ZnCdSe and CdSSe) were tethered to the surface of TiO2 nanotubes using succcessive ionic layer adsorption and reaction (SILAR) technique. The stoichiometry of ternary quantum dots was estimated to beMn0.095Cd0.95Se, Zn0.16Cd0.84Se and CdS0.54Se0.46. The effect of varying number of sensitization cycles and annealing temperature on optical and photoelectrochemical properties of prepared photoanodes were studied. The absorption properties and surface morphology of the sensitized tubes was analyzed using UV-visible spectroscopy and scanning electron microscopy. The phase composition was determined using X-Ray diffraction and X-ray photoelectron spectroscopy techniques. Electrodes were also evaluated for their stability using inductively coupled plasma optical emission spectrometry. Results show that the sensitization of TiO2 nanotubes with MnCdSe (8.79 mA/cm2), ZnCdSe (12.70 mA/cm2) and CdSSe (15.58 mA/cm2) resulted in up to a 30 fold increase in photocurrent compared to unsensitized nanotubes (0.4 mA/cm2).In the second part, the application of WO3 as photoanode for water splitting was explored. The porous thin films of WO3 films were sensitized with ternary quantum dots (ZnCdSe) using the SILAR technique. The structural, surface morphological and optical properties of the sensitized WO3 thin films were studied. PEC characteristics of the sensitized films were found to be 120 fold increase (8.53 mA/cm2) in comparisonto that of unmodified WO3 films (0.07 mA/cm2).In the last part of this dissertation, CuWO4 was investigated as the potential photoanode material. The band gap of CuWO4 was estimated using density functional theory (DFT) calculations. The band structure was obtained using the first-principles plane wave self-consistent field (pwscf) method and the effect of nickel dopant on the band gap and optical properties of CuWO4 was evaluated. Theoretical calculations showed that doping led to a decrease in band gap. The validity of the theoretical approach was evaluated by experimentally synthesizing Ni-doped CuWO4 electrodes. Experimental results showed that the bandgap indeed decreases when CuWO4 was doped with Ni, and thus validated the DFT approach. Ternary quantum dots were found to increase the PEC activity of TiO2 and WO3 based photoelectrodes by 120 fold. In addition, a method of computing band gap of semiconductor using DFT modeling was developed and validated with experimental results