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Metal Oxide and Oxynitride Semiconductors for Photoelectrochemical Water Splitting.

Record Type:	Electronic resources : Monograph/item
Title/Author:	Metal Oxide and Oxynitride Semiconductors for Photoelectrochemical Water Splitting./
Author:	Yew, Rowena Y.
Published:	Ann Arbor : ProQuest Dissertations & Theses, : 2021,
Description:	264 p.
Notes:	Source: Dissertations Abstracts International, Volume: 83-11, Section: B.
Contained By:	Dissertations Abstracts International83-11B.
Subject:	Nanostructured materials. -
Online resource:	http://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=29070872
ISBN:	9798426868274

Metal Oxide and Oxynitride Semiconductors for Photoelectrochemical Water Splitting.
Yew, Rowena Y.

Metal Oxide and Oxynitride Semiconductors for Photoelectrochemical Water Splitting. - Ann Arbor : ProQuest Dissertations & Theses, 2021 - 264 p.

Source: Dissertations Abstracts International, Volume: 83-11, Section: B.

Thesis (Ph.D.)--The Australian National University (Australia), 2021.

This item must not be sold to any third party vendors.

Photoelectrochemical (PEC) water splitting using semiconductor photoelectrodes to convert solar energy directly into hydrogen is an elegant approach towards realising storable and transportable, clean and renewable energy production. Although extensive research has been carried out since TiO2 was first used as a photocatalyst in 1972, achieving high efficiency, operational stability and low-cost remain the major obstacles for its practical implementation. In this dissertation, we investigated several strategies to address the challenges impeding the PEC performance of TiO2. First, we improve the optical absorption and charge transfer properties of TiO2 through morphological modification by preparing 3D ordered macroporous TiO2 inverse opal (IO) films. The underlying interconnected TiO2 porous network improves light trapping efficiency, provides enhanced catalytic active sites and allows direct charge transfer pathways. Light absorption of these IO films is enhanced in the visible and near IR region due to slow photon effects (a consequence of the pseudo-PBG of IOs) and light scattering within the pores. The photoresponse behaviour based on the direction of illumination shows that the macroporous IO films improve hole transport which is crucial for a photoanode to perform out efficient oxygen evolution reaction. Moreover, electrochemical-impedance spectroscopy analyses confirmed that charge transfer across the semiconductor-electrolyte interface (SEI) is enhanced due to electrolyte accessibility of photogenerated carriers in the space charge region. The greater surface area provides more active sites for photocatalysis. The current density of TiO2 is improved because of the above-mentioned effects from nanostructuring. Next, to improve the efficiency of TiO2, we investigate the effects of oxygen vacancies in these TiO2 IO films. Oxygen vacancies in TiO2 extend its light absorption, improve electrical conductivity, and reduce charge carrier recombination. Controlled surface defects in TiO2 have been shown to be a viable method to improve photon absorption and PEC water splitting performance; they can be generated easily and quickly by electrochemical reduction processes. TiO2 IO photoanodes were electrochemically reduced for the durations ranging from 300 to 500 seconds, with the photoanode reduced for 400 seconds producing the highest current density compared to pristine TiO2 IO photoanode. Raman spectroscopy and X-ray Photoelectron Spectroscopy (XPS) only detected oxygen vacancies and not Ti3+; since the latter is highly reactive with the atmosphere, making it difficult to detect using the above-mentioned techniques. The performance of the photoanodes is dependent on reduction times, with longer reduction time producing excessive oxygen vacancies that act as carrier traps for recombination. There is an increase in the photon absorption of the electrochemically reduced TiO2 IO photoanodes in the visible region and the photoconversion efficiency in the UV region increasing by almost three-fold. This improved performance ostensibly results from the creation of oxygen vacancies in the TiO2 IO photoanodes, resulting in a shift of the Fermi level towards the conduction band, which in turn leads to lower electron-hole recombination rates and improved charge carrier transport. By introducing oxygen vacancies, we can increase photogenerated carrier lifetime and thereby improving the conductivity of TiO2 to enhance PEC water splitting performance. However, the valence band edge does not change and the reduction of the band gap of TiO2 IO to extend its absorption into the visible light spectrum is not achieved, which will be addressed in the follow-up work. Band gap engineering is commonly used to develop visible light sensitivity of wide band gap materials for solar water splitting. Photocatalytic materials with narrow band gap are more susceptible to photo-corrosion compared to wider band gap materials, but the latter is not capable of harnessing visible light. Formation of a heterostructure by coupling different photocatalysts into a single photoelectrode can overcome the drawbacks of each individual photocatalyst and allowing favourable properties from each participating photocatalyst to be combined. Tantalum oxynitride (TaOxNy) is an ideal candidate to form a heterostructure with TiO2 because it has band edges which straddle the redox potential of water and a tuneable bandgap from 1.9 to 2.5 eV. However, the synthesis of TaOxNy requires the use of expensive Ta foil substrate to undergo nitridation process at very high temperatures. Deposition of TaOxNy on an affordable transparent substrate such as fluorine-doped tin oxide (FTO) coated glass is desirable, but FTO cannot withstand such high temperatures the from nitridation process. Therefore, to avoid nitridation, we use plasma-enhanced atomic layer deposition to deposit TaOxNy directly onto the FTO-coated glass substrates. We employ layered doping to control the oxygen and nitrogen stoichiometries by alternating the cycles of Ta-N and Ta-O. For the sample deposited using one super-cycle of fifty Ta-N to one Ta-O cycle ratio, we observed both photoanodic and photocathodic currents, known as photocurrent switching. While the exact mechanism behind the photocurrent switching behaviour needs to be explored further, the ability to control this behaviour by changing the composition through layering rather than homogenous doping is intriguing. This is a promising route to implement an unbiased PEC water splitting system with two photoelectrodes of the same material - a truly renewable method of energy production. Therefore, understanding the mechanism responsible for the photocurrent switching behaviour is crucial and should be explored in future studies. Finally, the narrow band gap TaOxNy is deposited on TiO2 IO to form a type II heterojunction, to improve the latter's overall PEC water splitting performance. The uniformity of the heterojunction throughout the IO film is crucial to its PEC performance, and the deposition temperature of TaOxNy plays an important role in achieving this. As a result of this heterostructure formation, the photon absorption of TiO2 is extended into the visible spectrum, a lower onset potential and improved stability in alkaline electrolyte are achieved for TaOxNy. The appropriate band alignment between TiO2 and TaOxNy creates a large built-in electric field for minority carriers which inhibits electron-hole recombination, extends photogenerated carrier lifetimes and promotes charge transport. Reduction in photogenerated hole accumulation at the SEI and longer photogenerated carrier lifetimes suppress self-oxidation during photo-irradiation, thus improving the photoanode stability in alkaline electrolyte. The formation of a type II heterojunction between TiO2 and TaOxNy in an IO structure leads to improved PEC performance compared to their standalone counterparts.

ISBN: 9798426868274Subjects--Topical Terms:

584856
Nanostructured materials.

Metal Oxide and Oxynitride Semiconductors for Photoelectrochemical Water Splitting.
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300 $a 264 p.
500 $a Source: Dissertations Abstracts International, Volume: 83-11, Section: B.
500 $a Advisor: Karuturi, Siva.
502 $a Thesis (Ph.D.)--The Australian National University (Australia), 2021.
506 $a This item must not be sold to any third party vendors.
520 $a Photoelectrochemical (PEC) water splitting using semiconductor photoelectrodes to convert solar energy directly into hydrogen is an elegant approach towards realising storable and transportable, clean and renewable energy production. Although extensive research has been carried out since TiO2 was first used as a photocatalyst in 1972, achieving high efficiency, operational stability and low-cost remain the major obstacles for its practical implementation. In this dissertation, we investigated several strategies to address the challenges impeding the PEC performance of TiO2. First, we improve the optical absorption and charge transfer properties of TiO2 through morphological modification by preparing 3D ordered macroporous TiO2 inverse opal (IO) films. The underlying interconnected TiO2 porous network improves light trapping efficiency, provides enhanced catalytic active sites and allows direct charge transfer pathways. Light absorption of these IO films is enhanced in the visible and near IR region due to slow photon effects (a consequence of the pseudo-PBG of IOs) and light scattering within the pores. The photoresponse behaviour based on the direction of illumination shows that the macroporous IO films improve hole transport which is crucial for a photoanode to perform out efficient oxygen evolution reaction. Moreover, electrochemical-impedance spectroscopy analyses confirmed that charge transfer across the semiconductor-electrolyte interface (SEI) is enhanced due to electrolyte accessibility of photogenerated carriers in the space charge region. The greater surface area provides more active sites for photocatalysis. The current density of TiO2 is improved because of the above-mentioned effects from nanostructuring. Next, to improve the efficiency of TiO2, we investigate the effects of oxygen vacancies in these TiO2 IO films. Oxygen vacancies in TiO2 extend its light absorption, improve electrical conductivity, and reduce charge carrier recombination. Controlled surface defects in TiO2 have been shown to be a viable method to improve photon absorption and PEC water splitting performance; they can be generated easily and quickly by electrochemical reduction processes. TiO2 IO photoanodes were electrochemically reduced for the durations ranging from 300 to 500 seconds, with the photoanode reduced for 400 seconds producing the highest current density compared to pristine TiO2 IO photoanode. Raman spectroscopy and X-ray Photoelectron Spectroscopy (XPS) only detected oxygen vacancies and not Ti3+; since the latter is highly reactive with the atmosphere, making it difficult to detect using the above-mentioned techniques. The performance of the photoanodes is dependent on reduction times, with longer reduction time producing excessive oxygen vacancies that act as carrier traps for recombination. There is an increase in the photon absorption of the electrochemically reduced TiO2 IO photoanodes in the visible region and the photoconversion efficiency in the UV region increasing by almost three-fold. This improved performance ostensibly results from the creation of oxygen vacancies in the TiO2 IO photoanodes, resulting in a shift of the Fermi level towards the conduction band, which in turn leads to lower electron-hole recombination rates and improved charge carrier transport. By introducing oxygen vacancies, we can increase photogenerated carrier lifetime and thereby improving the conductivity of TiO2 to enhance PEC water splitting performance. However, the valence band edge does not change and the reduction of the band gap of TiO2 IO to extend its absorption into the visible light spectrum is not achieved, which will be addressed in the follow-up work. Band gap engineering is commonly used to develop visible light sensitivity of wide band gap materials for solar water splitting. Photocatalytic materials with narrow band gap are more susceptible to photo-corrosion compared to wider band gap materials, but the latter is not capable of harnessing visible light. Formation of a heterostructure by coupling different photocatalysts into a single photoelectrode can overcome the drawbacks of each individual photocatalyst and allowing favourable properties from each participating photocatalyst to be combined. Tantalum oxynitride (TaOxNy) is an ideal candidate to form a heterostructure with TiO2 because it has band edges which straddle the redox potential of water and a tuneable bandgap from 1.9 to 2.5 eV. However, the synthesis of TaOxNy requires the use of expensive Ta foil substrate to undergo nitridation process at very high temperatures. Deposition of TaOxNy on an affordable transparent substrate such as fluorine-doped tin oxide (FTO) coated glass is desirable, but FTO cannot withstand such high temperatures the from nitridation process. Therefore, to avoid nitridation, we use plasma-enhanced atomic layer deposition to deposit TaOxNy directly onto the FTO-coated glass substrates. We employ layered doping to control the oxygen and nitrogen stoichiometries by alternating the cycles of Ta-N and Ta-O. For the sample deposited using one super-cycle of fifty Ta-N to one Ta-O cycle ratio, we observed both photoanodic and photocathodic currents, known as photocurrent switching. While the exact mechanism behind the photocurrent switching behaviour needs to be explored further, the ability to control this behaviour by changing the composition through layering rather than homogenous doping is intriguing. This is a promising route to implement an unbiased PEC water splitting system with two photoelectrodes of the same material - a truly renewable method of energy production. Therefore, understanding the mechanism responsible for the photocurrent switching behaviour is crucial and should be explored in future studies. Finally, the narrow band gap TaOxNy is deposited on TiO2 IO to form a type II heterojunction, to improve the latter's overall PEC water splitting performance. The uniformity of the heterojunction throughout the IO film is crucial to its PEC performance, and the deposition temperature of TaOxNy plays an important role in achieving this. As a result of this heterostructure formation, the photon absorption of TiO2 is extended into the visible spectrum, a lower onset potential and improved stability in alkaline electrolyte are achieved for TaOxNy. The appropriate band alignment between TiO2 and TaOxNy creates a large built-in electric field for minority carriers which inhibits electron-hole recombination, extends photogenerated carrier lifetimes and promotes charge transport. Reduction in photogenerated hole accumulation at the SEI and longer photogenerated carrier lifetimes suppress self-oxidation during photo-irradiation, thus improving the photoanode stability in alkaline electrolyte. The formation of a type II heterojunction between TiO2 and TaOxNy in an IO structure leads to improved PEC performance compared to their standalone counterparts.
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