Using a dye found in raspberries to absorb sunlight,
a tin oxide electrode, a graphite electrode, and nanocrystalline titanium dioxide,
makes a solar cell that will produce a voltage.
Procedure
Wear eye protection
Chemical gloves recommended
Grind about 0.5 gram of nanocrystalline titanium dioxide (TiO2)
in a mortar and pestle with a few drops of very dilute acetic acid.
Alternate grinding and addition of a few drops of very dilute acetic
acid until you obtain a colloidal suspension with a smooth consistency,
somewhat like latex paint. (Very dilute acetic acid is prepared by adding 0.1 mL concentrated acetic
acid to 50 mL of water.)
Add a few drops of Triton X-100 surfactant or clear dishwashing detergent
and mix some more.
Identify the conducting side of a tin oxide-coated piece of glass
by using a multimeter to measure resistance. The conducting side will
have a resistance of 20-30 ohms.
With the conducting side up, tape the glass on three sides using one
thickness of tape. Wipe off any fingerprints or oils using a tissue wet
with ethanol.
Add some of the titanium dioxide suspension and quickly spread using
a glass rod. The tape serves as a 40-50 micrometer spacer to control
the thickness of the titanium dioxide layer. (If the layer dries out,
add more water.)
Carefully remove the tape without scratching the TiO2 coating.
Heat the glass on a hotplate in a hood for 10-20 minutes. The surface
turns brown as the organic solvent and surfactant dries and burns off
to produce a white or green titanium dioxide coating. (Note: this requires
a plate that gets quite hot.) Allow the glass to slowly cool by turning off
the hotplate.
Immerse the coating in a source of anthocyanins, such as raspberry
juice. The raspberry juice may be obtained most easily from frozen raspberries.
(Blackberries, pomegranate seeds, and Bing cherries can also be used.)
The white TiO2 will change color as the dye is absorbed and
complexed to the Ti(IV).
Rinse gently with water and then with ethanol. (The ethanol serves
to remove water from the porous TiO2.)
Pass a second piece of tin oxide glass, conducting side down, through
a candle flame to coat the conducting side with carbon (soot). For best
results, pass the glass piece quickly and repeatedly through the middle
part of the flame.
Wipe off the carbon along the perimeter of three sides of the carbon-coated
glass plate using a cotton swab.
Assemble the two glass plates with coated sides together, but offset
so that uncoated glass extends beyond the sandwich. Do not rub or slide
the plates. Clamp the plates together.
Add a few drops of a triiodide solution to the edge of the plate.
Capillary action will cause the KI3 solution to travel between
the two plates. (The KI3 electrolyte solution consists of 0.5 M KI and 0.05
M I2 in anhydrous ethylene glycol.)
Properties
Connect a multimeter using an alligator clip to each plate (the negative
electrode is the TiO2 coated glass and the positive electrode
is the carbon coated glass).
Test the current and voltage produced by solar illumination, or...
test the current and voltage produced by illumination from an overhead
projector.
Materials
A kit that contains the supplies (conductive glass, nanocrystalline TiO2,
binder clips, KI3 electrolyte, manual, etc.) to create five titanium
dioxide raspberry solar cells can be ordered from the Institute
for Chemical Education.
