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Artificial Photosynthesis on Titanium Oxide Passivated III-V Semiconductors.
紀錄類型:
書目-電子資源 : Monograph/item
正題名/作者:
Artificial Photosynthesis on Titanium Oxide Passivated III-V Semiconductors./
作者:
Zeng, Guangtong.
出版者:
Ann Arbor : ProQuest Dissertations & Theses, : 2016,
面頁冊數:
110 p.
附註:
Source: Dissertations Abstracts International, Volume: 80-01, Section: B.
Contained By:
Dissertations Abstracts International80-01B.
標題:
Chemistry. -
電子資源:
http://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=10820728
ISBN:
9798209707608
Artificial Photosynthesis on Titanium Oxide Passivated III-V Semiconductors.
Zeng, Guangtong.
Artificial Photosynthesis on Titanium Oxide Passivated III-V Semiconductors.
- Ann Arbor : ProQuest Dissertations & Theses, 2016 - 110 p.
Source: Dissertations Abstracts International, Volume: 80-01, Section: B.
Thesis (Ph.D.)--University of Southern California, 2016.
This item must not be sold to any third party vendors.
Artificial photosynthesis has become a hot research area since first demonstration of photocatalytic water splitting using TiO2 in 1972. For photocatalytic materials, III-V semiconductors such as GaP, InP, and GaAs are promising candidates with theoretical rational band gap energies and high maximum photocurrent densities of 9mA/cm², 35mA/cm², and 32mA/cm², respectively. However, photocatlytic corrosion of III-V semiconductors prevents them from being utilized as reliable photocatalysts. During my PhD program, our research group has successfully developed a strategy of using the atomic layer deposition (ALD) passivates the surface of III-V semiconductors with a thin layer of TiO2 (less than 10nm), which protects them from corrosion. What's more, this thin layer of TiO2 enhance overall photoconversion efficiency substantially. This dissertation will begin with an introduction of the fundamentals of photocatalytical semiconductors, followed by the application of CO2 reduction. After that, we also discuss about the factors limiting the photocatalytic conversion efficiency. And we discuss the advantages of III-V semiconductors and TiO2 passivation. In the following chapters, we will report the achievements we have made on the enhanced photocatalytic CO2 reduction processes. In Chapter 2, we report photocatalytic CO2 reduction with water to produce methanol using TiO2-passivated GaP photocathodes under 532nm wavelength illumination. The TiO2 layer prevents corrosion of the GaP, as evidenced by atomic force microscopy and photoelectrochemical measurements. Here, the GaP surface is passivated using a thin film of TiO2 deposited by atomic layer deposition (ALD), which provides a viable, stable photocatalyst without sacrificing photocatalytic efficiency. In addition to providing a stable photocatalytic surface, the TiO22passivation provides substantial enhancement in the photoconversion efficiency through passivation of surface states, which cause non2radiative carrier recombination. In addition to passivation effects, the TiO2 deposited by ALD is n2type due to oxygen vacancies, and forms a pn2junction with the underlying p2type GaP photocathode. This creates a built2in field that assists in the separation of photogenerated electron2hole pairs, further reducing recombination. This reduction in the surface recombination velocity (SRV) corresponds to a shift in the overpotential of almost 0.5V. No enhancement is observed for TiO2 thicknesses above 10nm, due to the insulating nature of the TiO2, which eventually outweighs the benefits of passivation. In Chapter 3, photocatalytic CO2 reduction with water to produce methanol is demonstrated using TiO22passivated InP nanopillar photocathodes under 532nm wavelength illumination. In addition to providing a stable photocatalytic surface, the TiO22passivation layer provides substantial enhancement in the photoconversion efficiency through the introduction of O vacancies associated with the non2stoichiometric growth of TiO2 by atomic layer deposition. Plane wave-density functional theory (PW2DFT) calculations confirm the role of oxygen vacancies in the TiO2 surface, which serve as catalytically active sites in the CO2 reduction process. PW2DFT shows that CO2 binds stably to these oxygen vacancies and CO2 gains an electron (-0.897e) spontaneously from the TiO2 support. This calculation indicates that the O vacancies provide active sites for CO2 absorption, and no overpotential is required to form the CO22 intermediate. In Chapter 4, we present a robust and reliable method for improving the photocatalytic performance of InP, which is one of the best known materials for solar photoconversion (i.e., solar cells). In this article, we report substantial improvements (up to 18X) in the photocatalytic yields for CO2 reduction to CO through the surface passivation of InP with TiO2 deposited by atomic layer deposition (ALD). Here, the main mechanisms of enhancement are the introduction of catalytically active sites and the formation of a pn2junction. Photoelectrochemical reactions were carried out in a non2aqueous solution consisting of ionic liquid (12ethyl232methylimidazolium tetrafluoroborate ([EMIM]BF2)) dissolved in acetontrile, which enables CO2 reduction with a Faradaic efficiency of 99% at an underpotential of +0.78V. While the photocatalytic yield increases with the addition of the TiO2 layer, a corresponding drop in the photoluminescence intensity indicates the presence of catalytically active sites, which cause in increase in the electron2hole pair recombination rate. NMR spectra show that the [EMIM]2 ions in solution form an intermediate complex with CO22, thus lowering the energy barrier of this reaction. In Chapter 5, we demonstrate that a thin layer of n2type TiO2 using atomic layer deposition (ALD) prevents corrosion of p2type GaP, as evidenced by atomic force microscopy and photoelectrochemical measurements. In addition, the TiO2 passivation layer provides an enhancement in photoconversion efficiency through the formation of a charge separating pn2region. Plasmonic Au nanoparticles deposited on top of the TiO22passivated GaP further increases the photoconversion efficiency through local field enhancement. Finite difference time domain (FDTD) simulations of the electric field profiles in this photocatalytic heterostructure corroborate the experimental results. In Chapter 6, in order to separate the various mechanisms of the catalysis and enhancement, we will use the use vibrational sum frequency generation (vSFG) spectroscopy to identify the reactant and intermediate species adsorbed at the active surface sites on the photocatalytic substrate. Also, we will use the reflection and total internal reflection Fourier transform infrared spectroscopy (FTIR) measurements to analyze the vibrational frequency range of the OH, CH and CO stretches. Those two measurements will provide a more complete understanding of the surface bound intermediates in this photocatalytic reaction system.
ISBN: 9798209707608Subjects--Topical Terms:
516420
Chemistry.
Subjects--Index Terms:
Rational band gap energies
Artificial Photosynthesis on Titanium Oxide Passivated III-V Semiconductors.
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Artificial photosynthesis has become a hot research area since first demonstration of photocatalytic water splitting using TiO2 in 1972. For photocatalytic materials, III-V semiconductors such as GaP, InP, and GaAs are promising candidates with theoretical rational band gap energies and high maximum photocurrent densities of 9mA/cm², 35mA/cm², and 32mA/cm², respectively. However, photocatlytic corrosion of III-V semiconductors prevents them from being utilized as reliable photocatalysts. During my PhD program, our research group has successfully developed a strategy of using the atomic layer deposition (ALD) passivates the surface of III-V semiconductors with a thin layer of TiO2 (less than 10nm), which protects them from corrosion. What's more, this thin layer of TiO2 enhance overall photoconversion efficiency substantially. This dissertation will begin with an introduction of the fundamentals of photocatalytical semiconductors, followed by the application of CO2 reduction. After that, we also discuss about the factors limiting the photocatalytic conversion efficiency. And we discuss the advantages of III-V semiconductors and TiO2 passivation. In the following chapters, we will report the achievements we have made on the enhanced photocatalytic CO2 reduction processes. In Chapter 2, we report photocatalytic CO2 reduction with water to produce methanol using TiO2-passivated GaP photocathodes under 532nm wavelength illumination. The TiO2 layer prevents corrosion of the GaP, as evidenced by atomic force microscopy and photoelectrochemical measurements. Here, the GaP surface is passivated using a thin film of TiO2 deposited by atomic layer deposition (ALD), which provides a viable, stable photocatalyst without sacrificing photocatalytic efficiency. In addition to providing a stable photocatalytic surface, the TiO22passivation provides substantial enhancement in the photoconversion efficiency through passivation of surface states, which cause non2radiative carrier recombination. In addition to passivation effects, the TiO2 deposited by ALD is n2type due to oxygen vacancies, and forms a pn2junction with the underlying p2type GaP photocathode. This creates a built2in field that assists in the separation of photogenerated electron2hole pairs, further reducing recombination. This reduction in the surface recombination velocity (SRV) corresponds to a shift in the overpotential of almost 0.5V. No enhancement is observed for TiO2 thicknesses above 10nm, due to the insulating nature of the TiO2, which eventually outweighs the benefits of passivation. In Chapter 3, photocatalytic CO2 reduction with water to produce methanol is demonstrated using TiO22passivated InP nanopillar photocathodes under 532nm wavelength illumination. In addition to providing a stable photocatalytic surface, the TiO22passivation layer provides substantial enhancement in the photoconversion efficiency through the introduction of O vacancies associated with the non2stoichiometric growth of TiO2 by atomic layer deposition. Plane wave-density functional theory (PW2DFT) calculations confirm the role of oxygen vacancies in the TiO2 surface, which serve as catalytically active sites in the CO2 reduction process. PW2DFT shows that CO2 binds stably to these oxygen vacancies and CO2 gains an electron (-0.897e) spontaneously from the TiO2 support. This calculation indicates that the O vacancies provide active sites for CO2 absorption, and no overpotential is required to form the CO22 intermediate. In Chapter 4, we present a robust and reliable method for improving the photocatalytic performance of InP, which is one of the best known materials for solar photoconversion (i.e., solar cells). In this article, we report substantial improvements (up to 18X) in the photocatalytic yields for CO2 reduction to CO through the surface passivation of InP with TiO2 deposited by atomic layer deposition (ALD). Here, the main mechanisms of enhancement are the introduction of catalytically active sites and the formation of a pn2junction. Photoelectrochemical reactions were carried out in a non2aqueous solution consisting of ionic liquid (12ethyl232methylimidazolium tetrafluoroborate ([EMIM]BF2)) dissolved in acetontrile, which enables CO2 reduction with a Faradaic efficiency of 99% at an underpotential of +0.78V. While the photocatalytic yield increases with the addition of the TiO2 layer, a corresponding drop in the photoluminescence intensity indicates the presence of catalytically active sites, which cause in increase in the electron2hole pair recombination rate. NMR spectra show that the [EMIM]2 ions in solution form an intermediate complex with CO22, thus lowering the energy barrier of this reaction. In Chapter 5, we demonstrate that a thin layer of n2type TiO2 using atomic layer deposition (ALD) prevents corrosion of p2type GaP, as evidenced by atomic force microscopy and photoelectrochemical measurements. In addition, the TiO2 passivation layer provides an enhancement in photoconversion efficiency through the formation of a charge separating pn2region. Plasmonic Au nanoparticles deposited on top of the TiO22passivated GaP further increases the photoconversion efficiency through local field enhancement. Finite difference time domain (FDTD) simulations of the electric field profiles in this photocatalytic heterostructure corroborate the experimental results. In Chapter 6, in order to separate the various mechanisms of the catalysis and enhancement, we will use the use vibrational sum frequency generation (vSFG) spectroscopy to identify the reactant and intermediate species adsorbed at the active surface sites on the photocatalytic substrate. Also, we will use the reflection and total internal reflection Fourier transform infrared spectroscopy (FTIR) measurements to analyze the vibrational frequency range of the OH, CH and CO stretches. Those two measurements will provide a more complete understanding of the surface bound intermediates in this photocatalytic reaction system.
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