Graduation Year

2015

Document Type

Thesis

Degree

M.S.M.S.E.

Degree Name

MS in Materials Science and Engineering (M.S.M.S.E)

Department

Chemical Engineering

Degree Granting Department

Chemical Engineering

Major Professor

Venkat R. Bhethanabotla, Ph.D.

Co-Major Professor

John N. Kuhn, Ph.D.

Committee Member

Alberto A. Sagüés, Ph.D.

Keywords

Band Gap, Flat Band Potential, Mid Gap States, Photocatalyst Design, Tauc Plot

Abstract

For the past few decades, a frenzy of attention has been given towards the

development of an assortment of photocatalysts as a solution for various environmental

problems. TiO2 is the most widely used photocatalyst. TiO2 is biocompatible, chemically

and thermally stable but TiO2 and a vast majority other photocatalysts have large band

gaps, and hence they find applicability only in the UV region of the solar spectrum.

These large band gap photocatalysts suffer a severe limitation with regard to their

overall process efficiency as the UV region contributes to about 3 to 4 % of the solar

spectrum in terms of energy.

This thesis concentrates on the progress towards the generation of visible light

active photocatalysts. Lanthanum transition metal perovskites were synthesized to

incorporate B site doping in the following fashion; LaCrxFe1-xO3, LaMnxFe1-xO3 and

LaCrxMn1-xO3 (x= {1, 0.25, 0.5, 0.75}). These perovskites configurations were selected

as LaCrO3 has a conduction band edge suitable to activate most photocatalytic

reactions, but LaCrO3’s large band gap energetically hinders the photocatalysis. Doping

LaCrO3 with Fe and Mn allowed for tuning their band gaps and made various

photocatalytic reactions feasible, namely CO2 reduction and photoassisted dye

degradation.

Validation of the perovskite's crystal structure was established through the study

of their XRD patterns. The perovskite exhibited crystallinity throughout all doping

concentrations. At some doping concentrations, due to low or high degree of tolerance

factor, the presence of hexagonal and rhombohedral crystal phases was seen.

Analysis of the electronic structure of these perovskites was conducted through

diffuse reflectance spectroscopy measurements and electrochemical impedance

spectroscopy. Doping transition metals in B site of the perovskite led to the narrowing of

band gap energy with the increase in the concentration of the higher atomic number

transition metal. About 38% reduction in band gap was achieved in LaCrxFe1-xO3. The

band gap constituted of Mott- Hubbard gap and charge transfer gap.

For the species LaCrxFe1-xO3, interband states exist with an energy gap as large

as 1.3 eV for X=1 and 0.75. These states manifested as Urbach tails and are clearly

documented in the absorption spectrum data. At x=0.5 and below, evidence of mixing is

seen in LaCrxFe1-xO3, leading to the diminution of these interband states, although not

to full extent, their energy was reduced by about 0.5 eV. In LaCrxMn1-xO3 and LaMnxFe1-

XO3, the absence of Urbach tail and absorption edge is observed.

The band edge positions of most of these perovskites provided a large enough

over potential to cause the reduction of CO2. Future efforts on the photocatalytic activity

study of these perovskites through dye degradation and CO2 reduction are in progress.

Preliminary results of photoassisted dye degradation are shared in this thesis.

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