Thesis advisor: Dunwei Wang / Due to the increasing energy demand from human activities, efficient utilization of renewable energy, such as wind, solar and geothermal energies, becomes necessary and urgent. Photoelectrochemical water splitting offers a great example to convert solar energy and storage it in the term of chemical bond. Seeking suitable photoanode materials becomes the research focus of my study, because the development of photoanode materials significantly lags that of robust photocathode (such as Si). The main challenge is to fabricate an efficient and stable photoanode material which can deliver high photocurrent and sufficient photovoltage which can match well with those of photocathode when made into tandem cell configuration. Hematite (α-Fe2O3) represents a promising metal oxide photoanode material, with a suitable band gap (2.1 eV), low cost and toxicity. Applying nanostructures and appropriate surface modification layers help address existing research challenges. As a result, a much lower turn on potential and greater photocurrent density is achieved. Another photoanode material attracts our attention is tantalum nitride (Ta3N5), with a similar band gap to hematite but much better light absorption properties, shows a poor stability in aqueous electrolyte. For both photoanode materials, thermodynamic and kinetic aspects are studied in details when tested in water splitting devices. These works provide new ideas and insights on the future studies. / Thesis (PhD) — Boston College, 2015. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

The energy requirements of our planet will continue to grow with increasing world population and the modernization of currently underdeveloped countries. This will force us to search for environmental friendly alternative energy resources. Solar energy by far provides the largest of all renewable energy resources with an average power of 120 000 TW irradiated from the sun which can be exploited through solar electricity, solar fuel, and biomass. Nanostructured materials have been the subject of extensive research as the building block for construction of solar energy conversion devices for the past decades. The nanostructured materials sometimes have peculiar electrical and optical properties that are often shape and size dependent and are not expected in the bulk phase. Recent research has focused on new strategies to control nanostructured morphologies and compositions of semiconductor materials to optimize their solar conversion efficiency. In this dissertation, we discuss the synthesis and characterizations of one dimensional nanostructured TiO₂ based materials and their solar energy conversion applications. We have developed a solvothermal synthesis method for growing densely packed, vertical, single crystalline TiO₂ rutile nanowire arrays with unprecedented small feature sizes of 5 nm and lengths up to 4.4 [mu]m. Because of TiO₂'s large band gap, the working spectrum of TiO₂ is limited to the ultra violet region with photons shorter than 420 nm. We demonstrate that the active spectrum of TiO₂ can be shifted to ~ 520 nm with incorporation of N via nitridation of TiO₂ nanowires in NH₃ flow. In addition, we demonstrate a synergistic effect involving hydrogenation and nitridation cotreatment of TiO₂ nanowires that further redshift the active spectrum of TiO₂ to 570 nm. The Ta and N co-incorporated TiO₂ nanowires were also prepared and showed significant enhancement in photoelectrochemical performance compared to mono-incorporation of Ta or N. This enhancement is due to fewer recombination centers from charge compensation effects and suppression of the formation of an amorphous layer on the nanowires during the nitridation process. Finally, we have developed hydrothermal synthesis of single crystalline TiO₂ nanoplatelet arrays on virtually all substrates and demonstrated their applications in water photo-oxidation and dye sensitized solar cells. / text

The goal of this research was to develop stereochemically defined multimetallic systems for use as light absorbers and electron donor / light absorber dyads in photoinitiated electron transfer and electron collection. The basis for the stereochemical control was provided by the symmetric bridging ligands 2,3,5,6-tetra(pyridyl)pyrazine (tpp) and 2,2'-bipyrimidine (bpm). The symmetric bidentate ligand 4N-perylene was designed and the majority of the synthesis was completed. The bimetallic complexes [(tpy)M(tpp)Ru(LLL)]²⁺, where M = Ru or Os and LLL = Cl₃, (CH₃CN)₃, tpp, or (dpq)Cl, and the model monometallic complexes [(tpy)M(tpp)]²⁺, where M = Ru or Os, and [(tpy)Ru(CH₃CN)₃]²⁺ were synthesized and characterized using electrochemistry, UV-vis spectroscopy, and UV-vis spectroelectrochemistry. The bimetallic complexes were investigated as potential light-absorber / electron-donor complexes to be used in larger supramolecular devices for photoinitiated electron collection and electron transfer. The electrochemistry showed that the relative energy of the metal orbitals is suitable for the metal in the (tpy)M(tpp) coordination environment to act as an electron donor. These bimetallic complexes possess extremely complicated UV-vis spectroscopy due to the number of possible transitions. The assignment of the UV-vis spectroscopy and the electrochemistry of these complexes was greatly facilitated by the UV-vis spectroelectrochemistry. The metal-to-metal charge transfer spectra of the mixed-valence species of the bimetallic complexes were obtained using NIR spectroelectrochemistry and indicate a significant degree of metal-metal communication through the bridging tpp. The bimetallic complexes [(tpy)Ru(tpp)Ru(tpy)]⁴⁺, [(tpy)Ru(tpp)Ru(tpp)]⁴⁺ [(tpy)Os(tpp)Ru(tpp)]⁴⁺, and [(tpy)Ru(tpp)Ru(CH₃CN)₃]⁴⁺ were found to have emission lifetimes on the order of 100 ns. The complexes [(bpy)₂Ru(bpm)]²⁺, [(bpy)₂Ru(bpm)Ru(bpy)₂]⁴⁺, [(bpm)₂IrCl₂]⁺, and {[(bpy)₂Ru(bpm)]₂IrCl₂}⁵⁺ were synthesized and characterized using electrochemistry, UV-vis spectroscopy, and UV-vis spectroelectrochemistry. The complex {[(bpy)₂Ru(bpm)]₂IrCl₂}⁵⁺ is a LA-EC-LA device for photoinitiated electron collection. The UV-vis spectroelectrochemistry of these complexes facilitated the assignment of the UV-vis spectroscopy as well as the electrochemistry. The UV-vis spectrum of the electrochemically generated two electron reduced form of {[(bpy)₂Ru(bpm)]₂IrCl₂}⁵⁺ was obtained. This spectrum is critical in the understanding of future studies of the photochemically generated two electron reduced species. The symmetric, planar, bidentate bridging ligand 4N-perylene was designed. This ligand would eliminate some of the isomers associated with multimetallic complexes bridged by unsymmetric bidentate bridging ligands. The large π system of 4N-perylene would likely result in a low energy π* orbital compared to dpp, dpq, or bpm. The ligand 4N-perylene would hold bridged metals at a greater distance than 2,2'-bipyrimidine and should facilitate the formation of multimetallic complexes. The synthesis of 1,8-dichloro-2,7-naphthyridine has been completed. 1,8-dichloro-2,7-naphthyridine is a possible reactant in the homo-coupling reaction of a substituted 2,7-naphthyridine to form 4N-perylene. The stereochemically defined molecular systems developed in this work show great promise for use in larger supramolecular complexes designed for photoinitiated electron transfer and electron collection. / Ph. D.

A Metal-Insulator-Metal (MIM) diode is a high frequency device used for energy harvesting purpose in the RECTENNA. The main objective of this thesis work is to design, fabricate and characterize a thin-film MIM diode. A key issue associated in this research work is the development MIM diode with nanometer thin insulator region. The reason for the development of MIM diode is to rectify a wide spectrum of AC signal to usable DC power. In this thesis work, a planar MIM diode with Aluminum/Aluminum-Oxide/Gold has been fabricated. The thickness of the insulator region obtained was about 3nm. The Metal and insulator depositions were done by sputtering and plasma oxidation, respectively. I-V Characteristics of the diode was measured by making use of in-house set-up and 70% of the devices on a single wafer yielded with better result. Most of the I-V curves obtained were highly non-linear and asymmetric. Based on the I-V measurement, the logarithmic derivative of I vs. V was plotted and the tunneling behavior was also observed.

abstract: As sunlight is an ideal source of energy on a global scale, there are several approaches being developed to harvest it and convert it to a form that can be used. One of these is though mimicking the processes in natural photosynthesis. Artificial photosynthetic systems include dye sensitized solar cells for the conversion of sunlight to electricity, and photoelectrosynthetic cells which use sunlight to drive water oxidation and hydrogen production to convert sunlight to energy stored in fuel. Both of these approaches include the process of the conversion of light energy into chemical potential in the form of a charge-separated state via molecular compounds. Porphyrins are commonly used as sensitizers as they have well suited properties for these applications. A high potential porphyrin with four nitrile groups at the beta positions, a β-cyanoporphyrin (CyP), was investigated and found to be an excellent electron acceptor, as well as have the necessary properties to be used as a sensitizer for photoelectrosynthetic cells for water oxidation. A new synthetic method was developed which allowed for the CyP to be used in a number of studies in artificial photosynthetic systems. This dissertation reports the theories behind, and the results of four studies utilizing a CyP for the first time; as a sensitizer in a DSSC for an investigation of its use in light driven water oxidation photoelectrosynthetic cells, as an electron acceptor in a proton coupled electron transfer system, in a carotene-CyP dyad to study energy and electron transfer processes between these moieties, and in a molecular triad to study a unique electron transfer process from a C60 radical anion to the CyP. It has been found that CyPs can be used as powerful electron acceptors in molecular systems to provide a large driving force for electron transfer that can aid in the process of the conversion of light to electrochemical potential. The results from these studies have led to a better understanding of the properties of CyPs, and have provided new insight into several electron transfer reactions. / Dissertation/Thesis / Doctoral Dissertation Chemistry 2015

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In this thesis, we have developed a reproducible, safe, and scalable sequential thermal vapour deposition (STVD) method for the growth of quality 3D halide perovskite (HaP) thin films. High-quality methylammonium lead tri-bromide (MAPbBr3), methylammonium lead tri-iodide (MAPbI3), and methylammonium lead bromide-iodide (MAPb(I1-xBrx)3) thin films have been optimised using the STVD technique. The structural, optical, morphological, and electrical properties were tuned by varying the thicknesses of the organic (MAI, MABr) and inorganic (PbI2, PbBr2) precursor thin films and post-annealing times of the HaP. X-ray diffractograms confirmed the cubic MAPbBr3 structure with the Pm¯3 m space group, tetragonal MAPbI3 crystal structure with I4/mcm space group, and the tetragonal MAPbI3 structure being transformed to cubic MAPbBr3 system as MAPb(I1-xBrx)3 (x=0.89-0.95) forms. UV-Vis spectra revealed broad absorption bands with a redshift in absorption onset from 540 to 550 nm for MAPbBr3 and 750 to 780 nm for MAPbI3 as the thickness of respective organic precursors increased from 300 to 500 nm. The bandgap of MAPb(I1-xBrx)3 decreased from 2.21 to 2.14 eV as the thicknesses of MABr precursors increased from 300 to 500 nm. The crystallisation of the HaP started within the chamber, and prolonged post-annealing times exceeding 10 min caused the transformation of MAPbI3 to PbI2. Scanning Electron Micrographs show pin-hole-free and densely packed grains with an average size that increases as thicknesses increase. The charge carrier mobility increases while trap density decreases as the thickness of organic precursors increased. Besides, the thesis investigated the growth and stability of thin MAPbBr3 films at metal/MAPbBr3 interfaces. We studied the structure, morphology, and stability of the optimised MAPbBr3 perovskite on aluminium (Al), tin (Sn), silver (Ag), gold-zinc (Au-Zn) and gold (Au) electrodes, immediately and 60 days later. FE-SEM images show an average grain size that increased linearly with the work function from 294 nm for Al to 850 nm for Au. The MAPbBr3 grains remain glued to Sn, Ag, Au-Zn but delaminate quickly on Al. X-ray analysis of MAPbBr3 reveals variable crystallographic texturing for various metals and loss in intensity of prominent peaks at different rates over time. We found that the best thicknesses of 100 nm PbI2 and 500 nm MAI, and 100 nm PbBr2 and 500 nm MABr are needed for the preparation of quality MAPbI3 and MAPbBr3 thin films for solar cells, respectively. Quality thin MAPb(I0.11Br0.89)3 film is formed by inter-diffusion and halide exchange processes when optimised MAPbBr3 is grown on optimised MAPbI3 as a bottom layer. Al speeds up the degradation of MAPbBr3 at Al/MAPbBr3 while MAPbBr3 is relatively stable at Au-Zn/MAPbBr3 interfaces. / Thesis (PhD (Physics))--University of Pretoria, 2020. / University of Pretoria, the National Research Foundation/The World Academy of Sciences (NRF-TWAS), and NRF grant no N0115/115463 of the SARChI / Physics / PhD (Physics) / Restricted

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<p> The MIS tunnel solar cell has recently attracted most of the attention in the solar energy conversion field. Construction is very simple and eliminates the costly diffusion of dopants. As in the Schottky type, a metal of proper work function is chosen to induce an inversion layer at the surface of the semiconductor (Al in the case of p type Si). An ultra thin (< 1.5 nm) oxide between the semiconductor and the metal passivates the surface by reducing surface states while permitting tunneling from the semiconductor to the metal.</p> <p> Good fill factors (> .7) have been obtained but high reflectivity of the Al has reduced the current output. Open circuit voltages greater than .61 volts and short circuit current density of 21 ma/cm^2 have been measured. Experimental evidence of the presence of an oxide different from SiO2 within 1.4 nm of the surface will be given and related to the thickness variation of the open circuit voltage. A maximum in VOC around 1.4 nm was found. A maximum efficiency of 7% was achieved without anti reflexion coating and a curve factor of .81 was observed in one of the cells. A slight variation in efficiency with the cell area was also observed.</p> / Thesis / Master of Engineering (MEngr)