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Title:Electronic band engineering: Titanium dioxide particulate layers for photocatalysis
Author(s):Huang, Qilong
Director of Research:Seebauer, Edmund G.
Doctoral Committee Chair(s):Seebauer, Edmund G.
Doctoral Committee Member(s):Yang, Hong; Kenis, Paul J.A.; Ertekin, Elif
Department / Program:Chemical & Biomolecular Engr
Discipline:Chemical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Electronic Band Engineering
Titanium Dioxide
Abstract:Titanium dioxide (TiO2) is a wide bandgap semiconductor with many application advantages for photocatalysis. However, in the porous films that typify applications, photogenerated charge carriers typically drive reactions inefficiently due to fast recombination. To mitigate this problem, this work employs electronic band engineering principles drawn from integrated circuit design. In particular, the electric field normal to the nominal surface is extended spatially to sweep a larger fraction of photoholes toward the nominal surface to react before they recombine. This work demonstrates that the donor concentration (N_d) and surface potential (V_s) both control the spatial extent of the field in a predictable way, and enhance the reaction rate independently and in combination. Although a version of this concept has been demonstrated previously for nonporous polycrystalline films, such photocatalysts have limited practical value due to the small surface area per unit mass. The present work demonstrates that the concept can be extended to porous particulate films, which have much higher specific surface areas and therefore offer a much broader range of possible applications. The particulate films were synthesized by sintering commercially available nanoparticles together to provide an electrically continuous porous structure. N_d and V_s were measured by electrochemical impedance spectroscopy(EIS) and X-ray photoelectron spectroscopy(XPS), respectively. Apparent reaction rate constants were assessed using photocatalytic test reaction of methylene blue oxidation, which has well-known pseudo first order kinetics. The effective fluid diffusion coefficient of reactant D_(MB-pore) was controlled by adding polyethylene glycol of varied mass fraction to demonstrate the effects of reactant transport in the pores. The width(w) of the space charge layer(SCL) within the film was manipulated through N_d and V_s. N_dwas manipulated through hydrogen annealing(0~20mtorr) enabled variation of N_d from 1.27×〖10〗^17 〖cm〗^(-3) to 5.26×〖10〗^19 〖cm〗^(-3). V_s was manipulated using a deuterium plasma from 0.31 eV to 0.62 eV. These techniques permitted experimental variation of w from 4.6nm~172.3nm. A physics-based model was developed to rationalize the results. The model adequately fits the rate data with only two adjustable parameters: the intrinsic rate constant and fluid diffusion coefficient in the pores. Based upon this success, the model predicts how further improvements can be done through variations of V_s, N_d, film thickness, liquid diffusion coefficient, and light intensity and wavelength. Electronic band engineering through w (controlled by N_d and V_s) should increase reaction rates by up to an order of magnitude over the rate that would be observed if only hole diffusion influences the rate. The optimal film thickness is approximately 2~2.5 times the value of w. Higher fluid diffusion coefficients benefit the rate constant and effectiveness factor, but only to a certain point. For example, gas phase photoreactions do not benefit from band engineering. The intrinsic rate constant linearly influences the overall reaction rate. When the penetration depth of light is smaller than w, the advantage of band engineering declines steadily as the penetration depth decreases.
Issue Date:2018-08-16
Rights Information:Copyright 2018, Qilong Huang
Date Available in IDEALS:2019-02-06
Date Deposited:2018-12

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