Post on 21-Jan-2023
Review on new photocatalytic
materialsBy:
Pradip Basnet03-04-2014
Department of Physics & Astronomy
The University of Georgia
10/14/22Review on new photocatalytic materials
PHOTOCATALYSIS
- is a process of accelerating the photoreaction in the presence of a catalyst (called a photocatalyst; and usually it’s a semiconductor particle or combination).
WHAT IS IT?
10/14/22Review on new photocatalytic materials
Outline: Photocatalyst/ Photocatalysis: - Motivation & History - Fundamental Aspects of Photocatlyst(s)
Theory: Photocatalysis - Photocatalytic Activity, Kinetics & Theoretical Considerations
- How Do We Design New Photocatalytic Materials?
Summary: New Photocatalytic Materials - Photocatalysts & Challenges - Possible Strategies to Improve the Photocatalytic Efficiency
Purify via adsorption
10/14/22Review on new photocatalytic materials
The world (we live) is composed of ~ 70 % of waterONLY ~ 2.5 % is fresh water
Salt water
Cont
amin
ated
wa
ter
Germicidal UV radiation
Photocatalyst/ Photocatalysis: Motivation
NEED AN EFFICIENT… SAFER METHOD ?
10/14/22Review on new photocatalytic materials
Contd.: Motivation
Self-cleaning windows: coated with TiO2 photocatalytic material
Biocompatible Coatings for medical applications
TiO2 is a versatile biocompatible coating material!
10/14/22Review on new photocatalytic materials
In 1967, Prof. Fujishima of Tokyo University, Japan, accidently discovered the Photocatalysis: evolution of Oxygen by splitting water and by using TiO2 and water without electricity BUT irradiating light!
In 1972, the discovery of photocatalysis was published in the British Science journal Nature.
In 1980’s, various effects of photocatalysis were discovered and applied to the industrial technology.
In 1992, the technology to apply a thin layer of TiO2 was developed in Japan.
Photocatalyst/ Photocatalysis: History
The late 1990s: Pilkington, PPG, SSG patent SELF-CLEANING windows.
10/14/22Review on new photocatalytic materials
Number of publications regarding TiO2/TiO2-photocatalysis per year (ISI-CD source)
Cont.: The most used photocatalyst
O. Carp, C. L. Huisman and A. Reller, Progress in Solid State Chemistry, 2004, 32, 33-177.
10/14/22Review on new photocatalytic materials
Photocatalyst/ Photocatalysis: Applications
Photocatalysis
Degradation ofOrg. Pollutants
Disinfection: destruction of Biological materials
Fig. Photo-induced processes on TiO2:
O. Carp, C. L. Huisman and A. Reller, Progress in Solid State Chemistry, 2004, 32, 33-177.
10/14/22Review on new photocatalytic materials
Fundamental Aspects of Photocatalysts:1. Absorption of light (of suitable wavelength; photon
energy, hν Eg)2. Transfer of an electron (e-) and a hole (h+) to the surface3. Recombination of e- -h+ pairs during the reaction process 4. Stabilization of the e- and h+ at the surface to form a
trapped electron and a hole5. Reduction and oxidation of molecule(s) at the surface6. Exchange of a product at the surface with a reactant at a
medium
Note: Absorption of light in the bulk (step 1) & subsequent redox reactions at the surface (step 5) are the key processes in the photocatalysis. Step 2 and/or 4 sometimes occur too fast to be included in the reaction steps.
)!(exchange
ionrecombinatTiO2
productsoxidation
productsreduction
h TiO2
'
DADA
he
DDh
AAe
he
r
r
h
e
k
k
k
k
g
ghekek
te
re
]][[][d][d
ghekhkt
hrh
]][[][d][d
)s(cm pairs edphotoinduc of rate therepresents tors.semiconduc in the holes, band valanceand electrons band
conduction of)(cm densities are][ and][
1-3-
-3
heg
he
10/14/22Review on new photocatalytic materials
TiO2 Photocatalyst: Redox Reaction
e-
h+
CB
VB
Reduction
Oxidation~3.2 ev (UV light)
Electron-Hole recombination is on the order of < 30 ps, hence efficiency is low (< 5%).
Efficiency of photocatalysis depends on how well one can prevent this charge recombination
10/14/22Review on new photocatalytic materials
The reaction continues until the organic molecules become harmless; and the photocatalysis is almost a permanent process!
TiO2 Phototcatalysis
TiO2
hν Eg
Orders
Bacteria
Organic Molecules
Stains
Surface
Decompose &
DetoxifyDyesetc.
10/14/22Review on new photocatalytic materials
Photocatalytic coating
O2 O2
O2
O2
O2- O2- O2- O2-
Principle of Phototcatalysis
H2OH2O H2O H2O
OH OH OH OH
Hydroxy Radical
Bacteria
Org. Mol.
H2O CO2
10/14/22Review on new photocatalytic materials
Theory: Photocatalysis
HOW DOES PHOTOCATALYTIC SEMICONDUCTOR WORK? WHAT DOES ITS ACTIVITY MEAN?
t = 4 ht = 0 min
Methyl Orange (MO): C0 = 10 mg/mlCu2O NRs (glass)
MO degradation under Vis light irradiation Intensity: 65 ± 5 mW/cm2
10/14/22Review on new photocatalytic materials
nmX
11
nmX
nmX 1
11
nmX1n
mX
1nmX
11
nmX
nmX 1
11
nmX
# holes in a particle
# el
ectr
ons
in a
par
ticle
Fig. Two-dimensional (2D) ladder model for the photocatalytic reactions at a small semiconductor particle.
Ref: Photocatalyis Science and Technology (Springer), by: Kodansha 2002
represents the distribution of particles containing n electrons and m holes at some instant.
nmX
00X 0
1X
10X 1
1X
0mX
nX 0
Contd.: Photocatalytic reaction, 2D model
10/14/22Review on new photocatalytic materials
Photocatalytic Activity: Gas-phase
Palmisano et. al., Chemical Communications, 2007, 3425-3437
Representation of Photocatalytic CO2 conversion (to CO), in which Ru and Co complexes act as photosensetizer and catalyst , respectively.
Schematic drawing of the exerimental setup applied in the reduction of CO2 in the gas phase.
10/14/22Review on new photocatalytic materials
Kinetics & Theoretical ConsiderationsI. Oxidation and/or reduction of Org. liquid (e.g. dyes) and/or inorganic
gas (e.g. CO2 conversion)
II. Degradation or change in concentration Color change of
visible org. liquidII. Molecular weight of gas
changeIII. Time dependent UV-Vis spectroscopy
(also in-situ) III. Mass spectroscopy
Results from Step III can be used to study the kinetics!
10/14/22Review on new photocatalytic materials
http://energy.lbl.gov/coolroof/intro.htm
Why new photocatalytic materials?
TiO2 is NOT visible light active material!
Towards Indoor Applications
UV intesnity of indoor : ~ 1 μW/ cm2; and outdoor : ~ 1 mW/ cm2.
10/14/22Review on new photocatalytic materials
How do we design new photocatalytic materials?Experimentally (I do have some experience!)Computational methods (?)
Requirements for the new materials?
Stable
Low Cost
Recyclable
etc.
Non-toxic
Depends on Applications
10/14/22Review on new photocatalytic materials
Computational methods for materials discovery and design:
Source: http://theory.mse.cornell.edu/research.html
10/14/22Review on new photocatalytic materials
Elements constructing heterogeneous photocatalysts:
A. Kudo and Y. Miseki, Chemical Society Reviews, 2009, 38, 253-278.
Cr doped TiO2
BiVO4
Science paper
10/14/22Review on new photocatalytic materials
cb
vbFe3+/4+
Fe2+/3+
Eg.-Metal Ion Doped-Titania Photocatalyst
e-
h+ Oxidation
Reduction
Dopants influences intrinsic properties of TiO2 resulting in lowering the band gap and shifting light absorption into visible spectral rangeDopants should be both good electron and hole traps
Efficiency of photocatalysis depends on various charge transfer events and migration of charges to the surface
(Visible light)
Migrate to the surface
e-
h+
10/14/22Review on new photocatalytic materials
Role of metal in metal/titania nanocomposites
P. V. Kamat. J. Phys.Chem. B., 2002, 106, 7729-7744
Metal nanoparticles act as an electron sink, promoting interfacial charge transfer reducing charge recombination
e-
h+
cb
vb~3.2ev (UV light)
Oxidation
O2
O2-
e-
metal
10/14/22Review on new photocatalytic materials
Band positions of semiconductors in contact with aq. Electrolyte at pH 1. On the right side the standard potentials of several redox couples are presented against the standard hydrogen electrode potential.
Eg.- Heterogeneous photocatalysts: bi-layered, multi- layered oxide semiconductors The first property relevant to the photocatalytic
activity of a semiconductor is its energy band configuration, which determines the absorption of incident photons, the photoexicitation of electron-hole pairs, the migration of carriers, and the redox capabilities of excited-state electrons and holes.
10/14/22Review on new photocatalytic materials
Ohtani, Journal of Photochemistry and Photobiology C: Photochemistry Reviews, 2010, 11, 157-178.
Photocatalysis A to Z—What we know and what we do not know in a scientific sense
10/14/22Review on new photocatalytic materials
✘ TiO2 does NOT work as a Photocatalyst in visible light!
✘ Some of visible active materials relatively have NOT enough oxidation and/or reduction power (for some application; difficult to use them)!
✘ Non-optimal alignment of conduction band
✘ Stability versus designing the new visible light active materials
✘ Toxicity of visible light active materials (for e.g. for food application it needs to be approved by USDA)
Challenges on photocatalytic materials:
10/14/22Review on new photocatalytic materials
Solar Spectrum is from J.H.Seinfeld and S.N.Pandis “Atmospheric Chemistry and Physics” John Wiley &Sons, Inc. New York, Chichester, Brisbane, Singapore, Toronto (1998)
200 300 400 500 600 700 800Wavelength (nm)
2.5
0.5
1.0
1.5
Solar Spectral Irradiance (W / m nm)
2
2.5
0.5
1.0
1.5
Absorbance
200 300 400 500 600 700 800Wavelength (nm)
2.5
0.5
1.0
1.5
Solar Spectral Irradiance (W / m nm)
2
Ag Nanoparticles as Efficient Antennae for Capturing of Solar Energy
2.5
0.5
1.0
1.5
Absorbance
10/14/22Review on new photocatalytic materials
Semiconductor Metal Oxide Nanoparticles for Visible Light Photocatalysis
Honda–Fujishima effect-water splitting using a TiO2 photoelectrode
Solar hydrogen production from water using a powdered photocatalyst.
A. Kudo and Y. Miseki, Chemical Society Reviews, 2009, 38, 253-278.
10/14/22Review on new photocatalytic materials
For Hematite (crystalline Fe2O3): absorbs visible light (Eg around
2.2 eV) Indirect band-gap → low photon absorption low conductivity, short hole
diffusion length (4-5 nm) → low photon transport
Physically thin, optically thick nanostructure • Enhance light absorption• Metallic nanostructures supporting localized plasmonic resonancesE.g. Hematite –LSPR with Au may improve light collection and charge separation
Possible strategies to improve the photocatalytic efficiency
B. Iandolo, T. J. Antosiewicz, A. Hellman and I. Zoric, Physical chemistry chemical physics : PCCP, 2013, 15, 4947-4954.
10/14/22Review on new photocatalytic materials
Zorić et.al., ACS Nano 2011, DOI: 10.1021/nn102166t
ELSPR depends on :• metal• shape, size• refractive index of the nanoenvironment
E, eV
Electric field enhancement
Hematite –LSPR with Au:
10/14/22Review on new photocatalytic materials
Fig. showing voltage gap between the two crucial processes of oxidation and reduction with semiconductor bi-layers (with Fe2O3).
Hematite-Based Water Splitting with Low Turn-On Voltages
C. Du, X. Yang, M. T. Mayer, H. Hoyt, J. Xie, G. McMahon, G. Bischoping and D. Wang, Angewandte Chemie International Edition, 2013, 52, 12692-12695.
Some Selected References:
1) S. Hotchandani and P. V. Kamat, Journal of Physical Chemistry, 1992, 96, 6834-6839.
2) V. Subramanian, E. E. Wolf and P. V. Kamat, Journal of the American Chemical Society, 2004, 126, 4943-4950.
3) K. Vinodgopal, D. E. Wynkoop and P. V. Kamat, Environmental Science & Technology, 1996, 30, 1660-1666.
4) A. Kudo and Y. Miseki, Chemical Society Reviews, 2009, 38, 253-278.5) H. Tong, S. X. Ouyang, Y. P. Bi, N. Umezawa, M. Oshikiri
and J. H. Ye, Advanced Materials, 2012, 24, 229-251.6) M. Barroso, A. J. Cowan, S. R. Pendlebury, M. Grätzel, D.
R. Klug and J. R. Durrant, Journal of the American Chemical Society, 2011, 133, 14868-14871.
7) P. V. Kamat, Chemical Reviews, 1993, 93, 267-300.8) I. Cesar, A. Kay, J. A. Gonzalez Martinez and M. Grätzel,
Journal of the American Chemical Society, 2006, 128, 4582-4583.9) A. Duret and M. Grätzel, The Journal of Physical Chemistry B, 2005,
109, 17184-17191.10) M. Gratzel, Nature, 2001, 414, 338-344.11) K. Sivula, F. Le Formal and M. Gratzel, Chemsuschem, 2011, 4,
432-449.12) M. Barroso et. al., Proceedings of the National Academy of Sciences of
the United States of America, 2012, 109, 15640-15645.Review on new photocatalytic materials 10/14/22