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Intragap States Induced Visible Light Absorption of TiO2 Nanoparticles: En Route to Solar Fuel Production

Published online by Cambridge University Press:  01 July 2015

Houman Yaghoubi
Affiliation:
Department of Electrical Engineering, University of South Florida, Tampa, FL 33620, U.S.A.
Thomas M. Casey
Affiliation:
Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200, U.S.A.
Umar T Twahir
Affiliation:
Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200, U.S.A.
Alexander Angerhofer
Affiliation:
Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200, U.S.A.
Gail E. Fanucci
Affiliation:
Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200, U.S.A.
Zhi Li
Affiliation:
Department of Electrical Engineering, University of South Florida, Tampa, FL 33620, U.S.A.
Rudy Schlaf
Affiliation:
Department of Electrical Engineering, University of South Florida, Tampa, FL 33620, U.S.A.
Arash Takshi
Affiliation:
Department of Electrical Engineering, University of South Florida, Tampa, FL 33620, U.S.A.
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Abstract

Carbon dioxide is the major greenhouse gas that is a bi-product of industrial approaches to energy production. Forests and nonagricultural lands act as a natural sink for CO2 removal from the atmosphere; however, the amount of emitted CO2 is significantly larger than the capacity of these natural sinks. This is particularly problematic as two cornerstones of our modern world, electricity generation and transportation, hold the largest share in greenhouse gas (such as CO2) emission. This leads to malignant impacts on the natural environment and human life, such as global warming. The obvious approach to reduce the amount of generated CO2 is to limit the use of fossil fuels. However, coal-fired power plants remain the largest source of electricity generation in 2014 and an equally potent and financially reasonable source is yet to be fully developed. Hence, new systems and strategies are crucial for the remediation of CO2. In this work, we present novel TiO2 nanoparticles, synthesized via a facile solution-phase method, which show a significant visible light absorption. The synthesized nanoparticles can be applied towards photoreduction of CO2 for hydrocarbon solar fuels production. A thorough photoemission spectroscopy analysis outlined the energy structure of the materials which uncovered a sub-bandgap absorption in the visible range due to the presence of intragap states. The origins of intragap states were investigated in greater detail using various characterization techniques. An in-depth chemical composition study of the developed material using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) indicated that the synthesized material is considerably un-doped. Further structural analysis using transmission electron microscopy (TEM) showed that distances between visible lattice fringes are matched with ordered crystalline phases of TiO2. The core emission study using XPS revealed that the oxygen vacancy defects in the structure--i.e. likely due the synthesis--are responsible for intragap states formation. Charge dynamics were investigated using Electron Paramagnetic Resonance (EPR) spectroscopy. EPR spectra were dominated by signals from oxygen-centered surface hole trapping sites with principle g values [2.003, 2.010, 2.023]--i.e. Ti4+ ˗ O2- on anatase. A faint signal was also observed as a function of visible light illumination at 5 K with principle g value of 1.975 that is suggestive of Ti3+ in rutile, a typical product of UV light exposure. In general, this study demonstrates the potential of a relatively inexpensive material for photoreduction of CO2 and generation of solar fuels.

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Articles
Copyright
Copyright © Materials Research Society 2015 

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