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Title:Exploiting polar interfaces for photocatalytic water splitting: application to the anatase-cuprous oxide heterojunction
Author(s):Shah, Rajiv
Advisor(s):Ertekin, Elif
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):polar catastrophe
density functional theory
Quantum Espresso
photocatalytic water splitting
binding energy
work function
defect engineering
water adsorption
charge carrier
electrochemical diode
surface energy
surface chemistry
electrostatic potential
dipole correction
electrostatic stability condition
charge transfer
Abstract:The conversion of solar energy to produce hydrogen by means of water splitting mechanism is a potentially transformative approach to achieving a clean and renewable energy society. Photo-produced hydrogen is a valuable fuel and energy carrier, which is easier to store than electricity or heat, and it is nonpolluting, inexhaustible, and flexible with respect to energy conversion in heat or electricity. However, despite its promise, the photocatalytic splitting of water into hydrogen and oxygen using sunlight still suffers from low energy conversion efficiencies. In this study, we explore the possibilities of obtaining enhanced photocatalytic conversion by exploiting the properties of polar interfaces coupled with finite size effects. By tuning the properties of a polar heterojunction, it may be possible to realize tailored surface chemistries that are optimized for oxidation and/or reduction. To demonstrate this concept, we computationally explore the intersection of finite size effects and surface polarity compensation mechanisms in the anatase (TiO2) / cuprous oxide (Cu2O) heterojunction, a system which has recently been realized in experiment. Due to its low valence band edge, anatase has a good oxidation potential; by contrast cuprous oxide has a good reduction potential due to its high conduction band edge. This makes the heterosystem an intriguing candidate for photocatalytic water splitting; however, the interface between the two materials is polar, unstable in the thermodynamic limit, and subject to polarity compensation mechanisms. Using the first-principles techniques based on density functional theory, we model the polar interface between TiO2 and Cu2O and analyze the polarity compensation mechanisms that take place for finite-sized systems. In the thermodynamic limit, such polar heterojunctions have diverging potential (polar catastrophe). However, for thin-film systems composed of only several atomic layers on either side of the heterointerface the intersection of this polarity with finite size effects results in novel compensation mechanisms that can result in tunable surface chemistries suitable for enhanced photocatalysis. Finally, we study the water adsorption mechanism on the TiO2/Cu2O system. We observe that the charge transfer takes place at the interface of H2O- TiO2/Cu2O where electrons from the water molecular are transferred to the heterostructure hence increasing the binding energy as well as the oxidation capability. We notice a variable behavior of binding energy for both compensated and uncompensated systems. We observe a range of 1-1.5 eV of binding energy as a tuning parameter fairly wide and maybe capable of capturing the photocatalytic activity peak. Thus, we successfully establish that polar thin-film heterointerfaces can be a useful system for catalysis, and demonstrate that computational modeling of these metal oxide systems can help guide experimental work in order to deterministically control surface orientation and chemistry for optimized photocatalysis.
Issue Date:2013-08-22
Rights Information:Copyright 2013 Rajiv Prasad Shah
Date Available in IDEALS:2013-08-22
Date Deposited:2013-08

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