Review on oxide nano-photocatalysts

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?


SEMICONDUCTOR


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


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 ?


Contd.: Motivation

Self-cleaning windows: coated with TiO2 photocatalytic material

Biocompatible Coatings for medical applications

TiO2 is a versatile biocompatible coating material!

http://csmres.co.uk/cs.public.upd/article-images/Fig-2-24322.jpg




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.


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.


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.


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

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]][[][d][d

ghekhkt

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]][[][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


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


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.


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


What happens to the bacteria?


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


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


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.


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!


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.


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


Computational methods for materials discovery and design:

Source: http://theory.mse.cornell.edu/research.html


Elements constructing heterogeneous photocatalysts:

A. Kudo and Y. Miseki, Chemical Society Reviews, 2009, 38, 253-278.

Cr doped TiO2

BiVO4

Science paper


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+


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


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.


Literatures Summary: Photocatalyst


Contd.: Heterogeneous Photocatalyst


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


✘ 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:


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


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.


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.


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:


CoOx doped Hematite:


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

Review on new photocatalytic materials 10/14/22

Review on oxide nano-photocatalysts

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