CRITICAT CDT
Dr Kevin Jones
An Introduction to Photocatalysis
Disclaimer for Experimental Work
The procedure described within must be conducted at one's own risk. St Andrews University and
affiliates do not warrant or guarantee the safety of individuals using these procedures and hereby
disclaim any liability for any injuries or damages claimed to have resulted from or related in any way
to the procedures herein.
An appropriate risk assessment must be conducted before hazardous materials are used or
hazardous procedures undertaken. This is the responsibility of the employer and users of resources
involving a practical activity to review and comply with the appropriate risk assessment supplied by
the employer.
CRITICAT CDT
Dr Kevin Jones
An Introduction to Photocatalysis
Background and Introduction: TiO2 is an inorganic material found in a variety of household products
including toothpaste, chewing gum, tablet casings and cosmetics.
TiO2 is also found in sunscreen in which it absorbs potentially harmful UV radiation and dissipates it
as heat. Without appropriate protection, UV radiation can lead to direct damage of our DNA
(dimerization of thymine bases) and formation of reactive free radicals, both of which can be
detrimental to our health.
When TiO2 absorbs UV radiation, it promotes an electron from the valance to the conductance band.
This results in the formation of a “free electron” and a “positive hole” (Figure 1). Free electrons can
be donated to an appropriate acceptor and the “positive holes” can accept electrons from an
appropriate donor.
Figure 1 - Generalised mechanism for activation of TiO2
Exposing TiO2 to UV radiation in an aqueous environment can lead to the formation of highly
oxidising species derived from oxygen and water (Figure 2; highlighted in red).
.
The hydroxyl radical (OH ) is a particularly powerful oxidising agent capable of converting organic
material (e.g. bacteria) to benign inorganic by-products. For these reasons several researchers are
examining the use of TiO2 in air/water purification.
Figure 2 – a) TiO2 catalysed formation of superoxides and hydroxyl radicals in water.
b) Conversion of organic material to benign by-products as a method for water purification.
CRITICAT CDT
Dr Kevin Jones
Aim: In this experiment we will examine the use of TiO2 as a photocatalyst. We will excite TiO2 with
UV radiation from sunlight (or another appropriate light source such as a UV light) in the presence of
the organic dye 2,6-dichlorophenolindophenol (DCIP). The dye accepts an electron from TiO2 and
leads to a colour change.
Figure 3 – Reduction of DCIP
Glycerol is also present in the reaction mixture and acts as a “sacrificial electron donor”. Glycerol is
ultimately oxidised through a complex pathway to carbon dioxide and water.
This was originally reported by M. W. Pitcher, S. M. Emin and M. Valant;
J. Chem. Educ. 2012, 89, 1439-1441.
CRITICAT CDT
Dr Kevin Jones
Experimental procedure:
Preparation of 2,6-dichlorophenolindophenol (DCIP) stock solution:
DCIP is used as a 0.5mg/mL of distilled water. 5mg/10mL of water provides enough solution to run
~10 experiments.
The pictures below are designed to provide a rough estimate of how much DCIP dye constitutes 5mg
if an analytical balance is not available.
We will perform three (or four) experiments simultaneously. Each experiment should be performed
in a separate test tube.
Reaction 1:



0.3g glycerol (~8 drops from a Pasteur pipette)
1mL of DCIP stock solution
5mL distilled water
Reactions 2 & 3: Reactions 2 and 3 are identical, but one reaction vessel is wrapped in tin foil




0.3g glycerol (~8 drops from a Pasteur pipette)
1mL of DCIP stock solution
5mL distilled water
10mg TiO2
Experiment #4 (optional):




0.3g glycerol (~8 drops from a Pasteur pipette)
1mL of DCIP stock solution
5mL distilled water
10mg SiO2 (or any other white, insoluble powder such as baking flour)
Once each experiment has been prepared, they are placed in the sun (or in front of an appropriate
light source) and a timer started. The solutions can be stirred/shaken in 30 second intervals although
this is not required for the colour change to occur. Monitor the reaction over time and make a note
of any colour changes.
CRITICAT CDT
Dr Kevin Jones
Monitoring reaction with a “colour picker” application:
Able Opus Colour Extractor is a freely available programme that allows “picking” of colours from
loaded or screen captured image. One piece of information that is particularly useful in the context
of the photocatalysis demonstration is the HSB value(s). HSB stands for hue, saturation and
brightness (you may also hear this referred to as HSL). Three values will be presented, the last of
which refers to brightness. This is a value between 0 and 100 (0=black, 100=white). In this manner,
we can take a series of screenshots from a video and monitor the colour change (or increasing
brightness) over time.
The programme can be obtained free of charge at: https://sourceforge.net/projects/ableopuscolorex/
Time 1
Time 2
HSB Value
It should be noted that it is important to click on roughly the same area of each image. Even with
this caveat, brightness values may fluctuate. Although not ideal it introduces students to sampling
error, repeat measurements and handling “real” data.
CRITICAT CDT
Dr Kevin Jones
The collected data can be processed and plotted to introduce an analytical component to the
experiment.
Photocatalytic Decomposition of Organic Dye
Brightness (a.u.)
40
30
20
10
0
0
20
40
60
80
100
120
Time (s)
This can potentially be used as part of an independent project in which various conditions are
altered and rate of “increasing brightness” is monitored.
CRITICAT CDT
Dr Kevin Jones
Questions:
1.
2.
3.
4.
What did you observe experimentally?
What was the purpose of experiments 1, 3 and 4?
TiO2 is said to be a semi-conductor. What is a semiconductor?
During the experiment we observed the conversion of A into B. Is this an oxidation or a
reduction?
5. Is the conversion of C to D an oxidation or reduction?
6. What is the difference between a heterogeneous catalyst and a homogeneous catalyst?
Which category does TiO2 fall into?