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photocatalytic material (photocatalyst), activate by absorbing…
photocatalytic material (photocatalyst)
Substance does not take part in reaction
it is recoverable & reusable after reaction
no permanent change
they are semiconductor
contain conduction band and valence band
Conduction band
It has to be located at more negative potential than 0V
Holes
valence band
It has to be located at more positive potential than 1.23V
electrons
the space between them is called band gap
determine by wavelength of light
minimum bandgap of photocatalytic materials for water splitting: 1.23eV (equivalent to 1.97046 x 10^(-19) J)
( 1 eV = 1.602 x 10^(-19) J )
one of the factor to determine whether water splitting can happen
other factors: can also put it into other testings to see if it is a suitable photocatalyst
Water Splitting Reaction
Electronvolt (eV) = Voltage (V) x Elementary Charge (C)
want electrons and holes to reach surface quickly to prevent recombination
decrease size of photocatalyst
more surface area
more conatct with water
faster the reaction
Have smaller particles
shorter distance of travel
Synthesize perfect crystals with no boundaries/defects/impurities
electrons and holes will not get trap easily
more mobility
reaction take place on surface of the photocatalyst
Hydrogen Evolution Reaction
2H+ + 2e- --> H2 (Eo = 0.00 V)
Oxygen Evolution Reaction
2H2O --> O2 + 4H+ + 4e- (Eo = -1.23 V)
Overall Reaction
2H2O --> 2H2 + O2
Eo reaction - Eo red (Cathode) - Eo red (Anode)
= 0 V + (-1.23 V)
= -1.23 V
wavelength of light within the solar spectrum would produce enough energy for water splitting.
formula: E = hc/λ
1.23eV = [(4.14 x 10^-15)(3 x 10^8)]/wavelength
wavelength = 1009.75nm
help to find wavelength
h = 6.63 x 10^-34 J.s = 4.14 x 10^-15 eV
c = 3 x 10^8 m/s
E = energy of photon
Factors affecting yield of light
electric potential difference
A different material with suitable energy levels for oxidation and reduction that encourage electron hole separation. Can be selected
There is not enough electric potential difference to separate electrons and holes in different directions.
crystal boundaries
Crystal boundary and defects trap electrons and holes
perfect crystals can be synthesized to have no boundaries or defects resulting in higher mobility and yield
particle size
Light will penetrate into the particle which will allow a bigger percentage of electrons to jump onto the valence band.
Smaller particles would then mean a shorter distance for the particle to travel
Photocatalytic materials are able to yield in light
activate by absorbing light/photon to increase rate of reaction
produces bubbles (form from hydrogen and oxygen gas)
no bubbles/gases produced = not a suitable photocatalyst