Materials Research

EPA Grant Number: X83254101-1

Center: CEER at Alfred University

Investigators: Misture, Scott and Edwards, Doreen

Institution:  Alfred University

Project Period:  September 1, 2006 – February 28, 2008

Research Category: Congressionally Mandated Center

 

Description:  

The model system will be the layered Aurivillius phases, which exist in both centrosymmetric and ferroelectric forms and were recently shown to have high photocatalytic activity.  The Aurivillius phases are layered, with perovskite-like blocks sandwiched between [Bi₂O₂]²⁺ sheets.  Selective leaching using a simple acid solution removes the [Bi₂O₂]²⁺ sheets from the crystal, providing a means of creating a photocatalyst with grain sizes in the 3 nm range without expensive and difficult nanopowder synthesis methods.  The photocatalytic activity will be linked to the strains on the TiO₆ and NbO₆ octahedra in the nanocrystals by systematic chemical substitutions into the perovskite block.   An understanding of the links between the active Ti and Nb sites and their local environments presents the possibility to engineer the crystals to operate in the visible light range, providing a technologically useful catalyst.

 

Objectives/Hypotheses: 

The Aurivillius phases will host the active Ti and Nb cations and the local environment around these cations can be varied by substitutions onto the perovskite A sites.  Therefore, the objectives of the study are threefold.  Firstly, the systematic variation of Nb-O and Ti-O bond lengths in Bi₂Sr_2-xA_xNb₂TiO₁₂ (A = Ca, Ba, X = 0, 0.5, 1) and Bi₂Ln₂Ti₃O₁₂ (Ln = lanthanide) will be linked to catalytic activity, then the study will progress to a brief investigation of ferroelectric domain effects on charge recombination.  The latter is not yet understood, but holds great promise to improve activity by separating the electron-hole pair using the internal electric field of the ferroelectric domains.  Finally, the effects of dilute transition metal doping, which move the band gap into the visible region in TiO₂, will be linked to activity in the Aurivillius phases.

 

Approach:

Aurivillius phases will be synthesized over the composition ranges of interest, selectively leached to create the nanoscale catalysts, and evaluated for their performance in decomposition of model organic contaminants and for water splitting to form hydrogen.  Physical characterization will include X-ray diffraction to determine the bond lengths, as well as surface area and grain size characterization.  Band gaps will be measured using UV-visible spectroscopy and X-ray photoelectron spectroscopy.  The properties will be evaluated by tracking methylene blue decomposition and hydrogen production under visible and UV irradiation.

 

Expected Results:  

A thorough understanding of the local environment around the active cations, as well as the effects of ferroelectric domains, will be investigated for the first time in the model system.  The fundamental understanding of the system will be applicable to other photocatalyst materials, which will have a profound effect on the quality of our environment.  For example, the widespread use of photocatalytic air purifiers will dramatically improve indoor air quality by removing volatile organic compounds and biological contaminants.  The photocatalytic treatment of industrial waste streams will eliminate some pollution at its source.  Solar photocatalytic technologies will be able to clean contaminated soil and water economically.  The production of H from water could provide us with a virtually unending supply of clean-burning fuel, which could replace hydrocarbon fuels and dramatically decrease the amount of greenhouse emissions.

 

Supplemental Keywords: photocatalysis, titanates, titanium dioxide, hydrogen production, solar technologies