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Hong Kong Chemistry Olympiad for Secondary School
Group Members:
Lam Ho Tin Tovi Wu Ming Hin Benny Kuk Man Hin Terry Ng Ka Fai Calvin Lam Choi Yat
UV-visible light-induced hydrogen productionusing natural chlorophyll sensitized Cu2+-doped TiO2
Content
A. Abstract B. Introduction C. Principle
C1. Band Gap theory C2. Ultra-violet light photoexcitation C3. Photocatalytic hydrogen production through glycerol and watersplitting C4. Metal ion-doping C5. Natural dye sensitization
D. Methodology D1. Alkaline hydrolysis of gutter oil D2. Synthesis of TiO2 D3. Metal ion-doped TiO2 D3.1. Preparation of metal ion-doping D3.2. Doping procedures of M2+ ion-doped TiO2 D4. Extraction of natural dyes D4.1. Extraction of chlorophyll from spinach D4.2. Extraction of anthocyanin from red wave lettuce D4.3. Extraction of 𝛽-carotene from carrots D5. Preparation of pH buffers D6. Procedures of general set-up
E. Experiments E1 Different photocatalysts E2 Volume ratio of water to glycerol E3 Metal ion-doping E3.1 Concentration effect and screening effect E3.2 Extent of metal ion-doping E4 Natural dye sensitization
E4.1 Comparison between natural dye sensitized and non-sensitized TiO2
E4.2 Comparison among chlorophyll, 𝛽-carotene and anthocyanin on the effect of hydrogen production
E5 pH effect E5.1 Effect of pH on Cu2+-ion doped TiO2 in hydrogen production E5.2 Effect of pH on chlorophyll sensitized Cu2+-ion doped TiO2 in hydrogen production
F. Conclusion
G. Comparison Table
H. Significance
I. Limitation
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 2
A. Abstract
Hydrogen is the ideal fuel for the future because it is clean and energy efficient while a wide
range of technologies can be used to generate hydrogen, but only some of them are considered
environmentally friendly.
TiO2 photocatalytic watersplitting technology has great potential for low-cost, environmental
friendly solar-hydrogen production to support the future hydrogen economy. However, the
solar-to-hydrogen energy conversion efficiency is too low for the technology to be economically
sound.
In this report, our team first studied a plethora of photocatalysts including TiO2, ZnO, ZnS and
Fe2O3. Among the 4 photocatalysts, TiO2 was found to exhibit highest photocatalytic
watersplitting activities under ultraviolet light. Moreover, glycerol extracted from gutter oil or
oil from food waste through alkaline hydrolysis is used as a sacrificial reagent to prevent
recombination of photo-generated electron-hole (e-/h+) pair and it was founded that the
optimum volume ratio of water to glycerol is 9:1.
However, TiO2 has a wide band gap of 3.2 eV in which only light of wavelength lower than 400
nm are utilized for photocatalytic watersplitting reaction and average amount of total solar
radiation reaching the earth's surface is about 100 mW per square cm, so UV radiations only
account about 3% of total solar radiations reaching the ground. Hence, the efficiency on the
production of hydrogen is low. In response to the adversity mentioned, investigation of the
effect of metal-ion doping, natural dye sensitization and optimum working pH were
investigated and it was founded that 0.4g Cu2+ ion-doped TiO2 (anatase) calcined at 400oC and
sensitized by 9cm3 3M chlorophyll extraction in a medium of pH 10 exhibits the highest
photocatalytic hydrogen production efficiency. Finally, the advantages of using our hydrogen
production method and the comparison between the differences in hydrogen production
through traditional steam-methane reforming and our method were discussed. Our team
strongly believes that our investigation can provide insight for the further development of
photocatalytic watersplitting using TiO2.
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 3
B. Introduction
Hydrogen is considered as an ideal fuel in the future. With a vast array of application of
hydrogen in various industries, the demand for hydrogen gas is expected to skyrocket. For
instance, hydrogen has a high thermal conductivity which enables it to serve as a coolant in
electrical generators at power stations. Hydrogen is seen with a broad spectrum of application,
ranging from chemical synthesis to carbon-free electricity generation. Therefore, hydrogen
holds a very significant position in the energy economy of the modern world.
Currently, about 95% of hydrogen produced is synthesized by steam-methane reforming
reaction, in which methane gas is converted to CO and H2 at high temperatures (700 – 1100°C)
and pressure (300-2500 kPa) with the presence of a metal-based catalyst (e.g. nickel), steam
reacts with methane to yield CO and H2 as the following equation:
CH4 + H2O ⇌ CO + 3 H2 (1)
Additional hydrogen can be recovered by a lower-temperature water-gas-shift reaction with
the CO produced. The reaction is summarized by:
CO + H2O ⇌ CO2 + H2 (2)
However, the above processes are energy negative, meaning that more energy is spent for
generating hydrogen than that could be generated by simply burning methane. Obviously, this
is not a sustainable way to produce hydrogen. Therefore, it is desirable to produce hydrogen
from a renewable resource like solar power.
Using TiO2 as the photocatalyst in photocatalytic watersplitting technology received much
attention for production of renewable hydrogen from water on a large scale and it offers a
promising way for clean, low-cost, environmentally friendly production of hydrogen by solar
energy. However, photocatalytic watersplitting hydrogen production is not popular yet due to
the cost is still high and the efficiency of this method is too low for the technology to be
economically sound. The main barriers are the rapid recombination of photo-generated
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 4
electron and its hole (e-/h+) pairs as well as the poor activation of photocatalysts by visible light
due to their large band gap.
The aim of our project is to increase the efficiency of photocatalysis by using materials which
are abundant and easily to be extracted so as to minimize the cost and enhance the efficiency
of the technology.
Therefore, glycerol is chosen as a sacrificial reagent to prevent the recombination of electrons
and its hole pairs to prevent the backward reaction of e-/h+ pair recombination. Glycerol is one
of the biomass derivatives and a side product in biodiesel production. As the production of
biodiesel is increasing, the amount of glycerol production is also increasing to the point of
wasteful because of the lack of supporting capacity in glycerol-utilize industries. Besides,
following the year of 2013, the outbreak of the adulteration of olive oil scandal in Taiwan, the
"gutter oil" has become a dreadful social focus. For the oil itself, and the main ingredients are
triglycerides, by a molecule of glycerol with three fatty acid molecules. Before we can use the
waste oil, alkaline hydrolysis was carried out for the extraction of glycerol from gutter oil by a
buffer of pH 10 to ensure a stable hydrolysis process.
Accordingly, in response to deficiencies of photocatalytic watersplitting, our group is going to
further investigate different methods to extend the activating spectrum to the visible range.
The effects of using four different but abundant metal ion-doping including Cu2+, Zn2+ , Co2+ and
Ni2+ ions and are calcined at different temperatures. Also, 3 natural dyes sensitization, including
chlorophyll, 𝛽-carotene and anthocyanin are being studied and the optimum working pH
medium of Cu2+ ion-doped TiO2 and chlorophyll are investigated in this report.
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
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C. Principle
C1. Band Gap theory
Band Gap theory states that when infinite number of atoms are composed to give an array,
their discrete energy levels will coincide with each other to form two bands, namely conduction
band (CB) and valence band (VB). Valance band is formed by the orbitals lower in energy, while
conduction band is formed by those higher in energy. Band Gap theory can be used to explicate
the characteristics of conductor, semiconductor and insulator.
In electrical conductors, it is likely that the
conduction band and valance band coincide with
each other. There is no energy gaps between two
bands (Fig.1), so electrons from valence band can
flow freely to and from conduction band. This
explains that why metals are able to conduct
electricity.
In semiconductors, the conduction band and
valence band are separate, but in a relatively
smaller energy gaps. External work done, like heat
and light, can trigger photoexcitation (Fig.2), which
causes electrons from the valence band to be
excited. If the frequency of the light is high enough,
valence band electrons will flow to conduction
band. This correlates why semiconductors conduct
electricity only under certain circumstances. TiO2
falls into the league of semiconductors, with its
band gap at 3.2 eV.
Fig. 2 Energy diagram showing conduction band and
valence band separated by the band gap.
Photoexcitation promotes electrons from VB to CB.
Fig. 1 Energy diagram showing no separation
in conduction band and valence band in
conductors.
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 6
In insulators, the separations between conduction
bands and valence bands are too large that hardly
will there be any methods provide enough energy for
electrons to be excited from valence band towards
conduction band (Fig.3). Electrons are localized in
valence band and are restricted to flow to the
conduction band. This corresponds to the electrical
insulation characteristic of such materials.
C2. Ultra-violet light photoexcitation
Ultra-violet light possesses high energy content, which can provide enough energy to trigger
photoexcitation in TiO2. According to the Planck-Einstein relation, photons from a UV source
with comparable wavelength to 370 nm fall onto the valence band:
𝐸 =ℎ𝑐
𝜆 (3)
The photocatalytic mechanism is initiated by the absorption of the photon with energy equal to
or greater than the band gap of TiO2 (3.2 eV for the anatase phase) producing an e-/h+ pair on
the surface of TiO2 nanoparticle. An excited electron is promoted to the conduction band while
a positive hole is formed in the valence band as the following equation:
TiO2 +ℎ𝑣 → e-cb + hvb
+ (4)
Excited-state electrons and holes can recombine and dissipate the input energy as heat, get
trapped in metastable surface states, or react with electron donors and electron acceptors
adsorbed on the semiconductor surface. After reaction with water, these holes can produce
Fig. 3 Energy diagram showing too far
separation between VB and CB. No
transfer of electrons occurs.
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 7
hydroxyl radicals1 with high redox oxidizing potential from water and the electrons act as strong
reducing agents to reduce hydrogen ions to hydrogen gas at the same time.
TiO2 has been a widely used photocatalyst for
photocatalytic watersplitting because its energy levels
are appropriate to initiate the watersplitting reaction
(Fig.4). In other words, the conduction band of TiO2 is
more negative than the reduction energy level of
water, while the valence band is more positive than the
oxidation energy level of water.
C3. Photocatalytic hydrogen production through
glycerol and watersplitting
Generated by high energy photons, holes in valence
band of the photocatalysts have a strong tendency to
gain electrons from the environment, which shows a
strong oxidizing property to oxidize organic compounds
found in food waste, for example, hydrogen gas will be
given off from glycerol. Meanwhile, electrons in the
conduction band act as reducing agents which reduce
the hydrogen ions in water to hydrogen gas according
to the following equation:
2H+ + 2e- → H2 (5)
Hence, both reactions in valence band and conduction band produce hydrogen.
1 Ken-ichi Ishibashi; Akira Fujishima; Toshiya Watanabe; Kazuhito Hashimoto, Quantum yields of active oxidative species
formed on TiO2 photocatalyst, Journal of Photochemistry and Photobiology A: Chemistry, Volume 134, Issues 1–2, 2000, 139-
142
Fig. 4 Energy diagram showing band
gap of water and TiO2
Fig. 5 Energy diagram showing reduction
and oxidation at CB and VB
respectively. Photoexcitation leads
to formation of e-/h+
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 8
With such photocatalytic redox reaction, organic compounds like glycerol can be converted
back to hydrogen and water (by-product) through 3 times of oxidation and decarboxylation
respectively (Fig. 6). The pure hydrogen gas produced in those reactions can be further used as
the fuel in hydrogen fuel cells for energy generation while the water can be used for
regeneration of hydrogen.
Fig. 6 Proposed fate of glycerol at h+vb of TiO2. Through 3 times of oxidation and decarboxylation
respectively, H2O and H2 are produced at the end of the reaction.
C4. Metal ion-doping
Transitional metal ion-dopings have been extensively investigated for enhancing the TiO2
photocatalytic activities. Metal ion-doping is known to show ability to expand the photo-
response range of TiO2 into the visible light spectrum through charge pair regeneration and
reduce recombination of e-/h+ pair through charge and hole trapping, both enhance its
efficiency of hydrogen production. The mechanism is explained below.
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 9
Electron pair regeneration:
As shown in the above diagrams, the original electron pair regeneration has only one route
(illustrated by equation 4). However, when metal ion-doping was carried out and the metal ions
TiO2 + ℎ𝑣 →e-cb + h+
vb (4) M2++ ℎ𝑣 →M3+ + e-
cb (6)
Fig. 7 Energy diagram showing transfer of electrons from VB to CB in TiO2; formation of hole in CB (h+) under illumination of UV light.
Fig. 8 Energy diagram showing transfer of electrons from VB of metal ion (M+) to CB in TiO2; formation of hole in CB of metal ion (h+) under illumination of visible light.
M2+ + ℎ𝑣 →M+ + h+vb (7) M2+ + ℎ𝜈 → M2+
h+vb + M2+
e-cb (8)
Fig. 9 Energy diagram showing transfer of electrons from VB of TiO2 to CB of metal ion(M+); formation of hole in CB of metal ion (h+) under illumination of visible light.
Fig. 10 Energy diagram showing transfer of electrons from VB of metal ion (M2+) to CB of metal ion (M2+); formation of hole in CB of metal ion (h+) under illumination of visible light.
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
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are incorporated into the lattice of TiO2, 3 more extra routes (equation 6, 7 & 8/Fig. 8, 9 & 10)
are available for electron pair regeneration. Hence, metal-ion doping not only provides more
alternative ways for e-/h+ pair generation, but also expands the photo-response range of TiO2 to
the visible light spectrum as the band gap of metal ions is narrower, hence visible light with a
longer wavelength can trigger photoexcitation of electrons, which in turn induces a red shift of
the absorption spectrum of TiO2.
2a. Electron trapping
Originally, e-/h+ of TiO2 has only one route (equation 9/Fig. 11), this type of recombination
causes a decline in the amount of hydrogen gas produced. As the hole and electron recombine,
there is no oxidizing agent and reducing agent for the redox reaction to occur (mentioned in
C2), hence, the amount of hydrogen gas produced becomes less. When metal ions are doped
into the TiO2 lattice structure, it provides an alternative way for electron trapping as the
valence band of the metal ion traps the electron from the conduction band of TiO2 (equation
10/Fig. 12). Therefore, the holes at the valence band of the TiO2 have a lower chance of
recombination with electrons, so more holes are available for oxidation of glycerol to hydrogen.
Ti4+ + e-cb → Ti3+ (9) M2+ + e-
cb → M+ (10)
Fig. 11 Energy diagram showing electron in CB of a TiO2 trapped by VB of another TiO2.
Fig. 12 Energy diagram showing electron in CB of TiO2 trapped by the VB of the metal ion (M2+) doped into the lattice of TiO2.
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 11
2b. Hole trapping:
M2+ + hvb+ → M3+ (11) OH- + hvb
+ → OH· (12)
Fig. 13 Electrons from the VB of the metal ion-dopant are transferred to the h+VB of TiO2. Thus h+VB of the metal ion-dopant is continuously generated, providing a site for oxidation of glycerol.
Originally, only one routine is available for the regeneration and recombination of e-/h+ in
undoped TiO2 (illustrated in Fig. 5). However, when metal ions are doped into the TiO2 lattice,
an alternative routine is available for hole trapping (equation 11/Fig. 13), hence increase the
number of holes available for oxidation of glycerol and hydroxide ions to hydrogen gas and
hydroxyl radicals (equation 12/Fig. 14) respectively.
Furthermore, carrier trapping is as important as carrier transferring in photocatalytic reactions,
hence, the extent of doping should be taken into consideration. Photocatalytic reactions can be
occurred only if the trapped electrons and holes are transferred to the surface of TiO2.
Therefore, metal ions should be doped near the surface of TiO2 particles for a better charge
transferring. In case of deep doping, metal ions are likely to behave as recombination centers,
since electron and hole transferring to the interface is more difficult. (For details please refer to
E3.2)
In this report, 4 metal ions including Zn2+, Cu2+, Ni2+ and Co2+ ions were being investigated. The
metal ion-doped TiO2 (anatase) was prepared by sol-gel method mentioned in D3 and were
calcined at 200, 400 and 600oC for each metal ion-doped TiO2.
Fig. 14 Hydroxide ions ionized from
water are oxidized to hydroxyl
radicals.
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 12
C5. Natural dye sensitization
Dye sensitization is widely used to utilize visible light for energy conversion. Some dyes having
redox property and visible light sensitivity can be used in solar cells2 as well as photocatalytic
systems. Illuminated by visible light, the excited dyes can inject photoexcited electrons to the
conduction band of semiconductors like TiO2 to initiate the catalytic reactions according to the
following equations:
dye + ℎ𝑣 → dye* (13)
dye* TiO2
→ dye+ + e- (14)
Even without semiconductors, some dyes are able to absorb visible light and produce electrons,
as reducing agents are strong enough to produce hydrogen. Nevertheless, without
semiconductors acting as efficient charge separators, the rate of hydrogen production by dyes
is very low. High hydrogen production rate can be obtained by efficient absorption of visible
light and efficient transfer of electrons from excited dyes to the conduction band of TiO2.
To achieve a higher efficiency in converting absorbed light into hydrogen energy, fast electron
injection and slow backward reaction are required. The fast electron injection and slow
backward reaction make dye-sensitized semiconductors feasible for energy conversion.
When illuminated by light, electrons in the conjugated systems of the natural dyes are excited
and are transferred and injected into the conduction band of the TiO2 (Fig. 15). The electrons in
the conduction of TiO2 act as a reducing agent. Thus the hydrogen ions in water are reduced to
produce hydrogen.
2 Hubert Hug; Michael Bader; Peter Mair; Thilo Glatze, Biophotovoltaics: Natural pigments in dye-sensitized solar cells, Applied
Energy, Volume 115, 15 February 2014, 216-225
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 13
Fig. 15 (a) Energy diagram illustrating the photoexcitation of dye when illuminated by visible light (b)
transfer of electrons from CB of dye to CB of TiO2 (c) electron successfully transferred
In this report, 3 different natural dyes including chlorophyll, 𝛽-carotene and anthocyanin were
being investigated.
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
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D. Methodology
D1. Alkaline hydrolysis of gutter oil
1. Add 30 cm3 of gutter oil in a 100 cm3 beaker.
2. Add 50 cm3 of pH 10 buffer into the mixture, the
hydrolysis process will occur as follows (Fig. 16):
3. Glycerol is obtained.
D2. Synthesis of TiO2
1. Add 20 cm3 of TiCl4 into a test tube (Fig. 17).
2. Boil it in a water bath for 2 hours. TiCl4 reacts
with oxygen in the air to form TiO2 and Cl2.
TiCl4 + O2→TiO2 + 2Cl2 (16)
3. The white precipitate obtained is TiO2.
D3.1 Metal ion-doped TiO2
Sol-gel method was chosen to prepare Ni2+, Cu2+,
Zn2+ and Co2+ ion-doped TiO23. We first dissolve
the metal ion dopants into ethanol in a 500 cm3
conical flask on a magnetic stirrer hot plate with
3 Adriana Zaleska, Doped-TiO2: A Review, Department of Chemical Technology, Gdansk University of Technology, 80-952-
Gdansk, Poland, 2008, 2, 157-164
Fig. 16 Alkaline hydrolysis of gutter oil
Fig. 17 Synthesis of TiO2 using TiCl4 as the
precursor
Fig. 18 Metal ion dopants (a) NiSO4 (b) CuSO4 (c)
ZnSO4 (d) CoSO4
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 15
stir bar in it, we used CuSO4 -5H2O, NiSO4-6H2O, CoSO4 and ZnSO4 (Fig. 18) because the dopants
can provide Cu2+, Ni2+, Co2+ and Zn2+ ions and the spectator SO42−ions, which have no reactions
with the substances in reaction mixture as it is stable. After the metal ions dissolve in ethanol,
and the mixture was mixed with the titanium precursor tetrabutyl titanate C16H36O4Ti. Absolute
ethanol was used for slowing down the hydrolysing process of tetrabutyl titanate as it will be
hydrolysed and become titanium hydroxide Ti(OH)4 instantaneously. Upon mixing, water is
added into the mixture in order to hydrolyse titanium precursor to titanium hydroxide at a
moderate rate. Then, the mixture was heated at 345oC and followed by filtration to remove
undoped metal ions and pulverization. Finally the powder obtained undergoes calcinations at
temperature ranges from 200oC to 600oC for 3 hours.
D3.2 Preparation procedure of metal ion-doped TiO2:
1. Calculate 1 mole of CuSO4 -5H2O, NiSO4-6H2O, CoSO4 and ZnSO4
by measuring the mass of the chemicals in a beaker with an
electronic balance.
2. Measure 34 cm3 of tetrabutyl titanate with a 50 cm3 measuring
cylinder (Fig. 19).
3. Put a conical flask containing a 9 cm long stir bar into a 500 cm3
conical flask on a magnetic stirrer hot plate and add about 50 cm3
of ethanol into it, start the stirrer at about 500 rpm.
4. Add the beaker of metal sulphate crystals into the flask followed
by tetrabutyl titanate (Fig. 20).
5. Measure 100 cm3 of ethanol and deionized water with two
100 cm3 measuring cylinders respectively.
6. Add deionized water into the flask at a moderate rate, followed
by adding ethanol.
7. The reaction mixture was stirred for about 2 hours until both
Fig. 19 34 cm3 tetrabutyl
titanate
Fig. 20 Sol-gel method -
synthesis of Cu2+-TiO2
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 16
dopant and precursor dissolve.
8. Heat the reaction mixture for about 345oC to boil the remaining water and ethanol as well
as butan-1-ol, which produced as a by-product of hydrolysis of tetrabutyl titanate.
9. After removing the substances mentioned above, obtain the remained solid and pulverize
them with mortar and pestle until they are powderized.
10. The powder obtained was transferred to a funnel with a filter paper in it, and filtration was
carried out by using deionized water in order to remove the unreacted metal sulphate
crystals.
11. Repeat step 10 for 5-6 times until the
filtrate become colourless in order to obtain
a more pure metal ion-doped TiO2 (Fig. 21).
12. The residue remained on the filter paper was transferred to oven and dried at about 80oC.
13. The obtained powder was calcined at 200oC, 400oC and 600oC by a magnetic stirrer hot
plate for 3 hours.
D4. Extraction of natural dyes
Chlorophyll, β-carotene and anthocyanin were extracted for investigation.
D4.1. Extraction of chlorophyll from spinach
1. Peel off the leaves from spinach.
2. Completely dry the spinach leaves in a 90oC
oven for 3 hours.
3. Grind the dried leaves with mortar and pestle.
Fig. 21 (a) Zn2+-TiO2 (b) Co2+-TiO2 (c) Ni2+-TiO2 (d)
Cu2+-TiO2
Fig. 22 Grinding dried spinach leaves using a
mortar and pestle
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 17
4. Transfer the obtained leaf powder into a beaker.
5. Add 200 cm3 of hexane into the spinach powder as the chlorophyll is soluble in it.
6. Cover the beaker with aluminium foil and stir the solution for 24 hours.
7. Decant the chlorophyll extract to remove the spinach leaves.
8. Chlorophyll solution is obtained.
D4.2. Extraction of anthocyanin from red wave lettuce
1. Peel off the leaves from a red wave lettuce.
2. Use a blender to blend the leaves of red wave lettuce and adding
water into the blender to make it easier to be blended (Fig. 23).
3. Dry the anthocyanin mixture completely by an oven at 90oC (Fig. 24).
4. Pour the anthocyanin mixture into a 500 cm3 beaker.
5. Add 100 cm3 ethanol into the anthocyanin mixture (Fig.25).
6. Add 50 cm3 of HCl (0.5M) into the anthocyanin mixture.
7. Stir the anthocyanin mixture for 24 hours.
8. Filtrate the anthocyanin mixture with a suction filtration set up for
3 times.
9. Add 50 cm3 of NaOH (0.5M) into the mixture to neutralize the pH
of the anthocyanin extract.
Fig. 23 Blending
red wave
lettuce
Fig. 24 Drying red
wave lettuce by
an oven at 90oC
Fig. 25 Anthocyanin
dissolves in ethanol
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 18
D4.3. Extraction of β-carotene from carrots
1. Peel off two carrots and cut them into pieces.
2. Add 200 cm3 of trichloromethane into a blender.
3. Blend the carrots in the blender.
4. β-carotene is extracted as it is soluble in trichloromethane.
5. Filter the solution for a few times by suction filtration to
remove residue.
6. The obtained filtrate is β-carotene dissolved in
trichloromethane.
D5. Preparation of pH buffers
The pH effect on both Cu2+-ion doped TiO2 and chlorophyll in hydrogen production were
investigated. Different colourless pH buffers were prepared for the investigation.
pH 0: 1M sulphuric acid
pH 2: Walpole’s sodium acetate hydrochloric acid buffer
1. Dissolve 20.6g of anhydrous sodium acetate in 250 cm3 of distilled
water.
2. Mix 20 cm3 of anhydrous sodium acetate solution with 21 cm3 of 1.0M
hydrochloric acid.
3. Make up to a final volume of 100 cm3 with distilled water.
pH 4: Citrate-phosphate buffer
Mix 61.4 cm3 of 0.1 M citric acid with 38.6 cm3 of 0.2 M disodium
hydrogenphosphate.
pH 6: Citrate-phosphate buffer
Mix 36.8 cm3 of 0.1 M citric acid with 63.2 cm3 of 0.2 M disodium
hydrogenphosphate.
Fig. 26 Blending carrots in
trichloromethane
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 19
pH 8: Citrate-phosphate buffer
Mix 2.8 cm3 of 0.1 M citric acid with 97.2 cm3 of 0.2 M disodium
hydrogenphosphate.
pH 10: Glycine-sodium hydroxide buffer
1. Dissolve 3.75g of glycine in 250 cm3 of distilled water.
2. Mix 60.98 cm3 of glycine solution with 39 cm3 of 0.2M sodium hydroxide
solution.
pH 12: Potassium chloride-sodium hydroxide buffer
1. Dissolve 3.725g of potassium chloride in 250 cm3 of distilled water.
2. Mix 80.65 cm3 of potassium chloride solution with 19.35 cm3 of 0.2M
sodium hydroxide solution.
pH 14: 1 M sodium hydroxide solution
D6. Procedures for general set-up
In general, 54 cm3 of water and 6 cm3 of different glycerol and parameters were added to the
reaction mixture in the conical flask and irradiated under ultraviolet light4 or visible light5. The
effectiveness of the photocatalytic reaction can be determined by measuring the increased in
4 8W, 280-315 nm 5 8W, 310-700 nm
Fig. 27 Preparation of pH 0-14 buffers
(universal indicator is added to confirm pH value)
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 20
pressure in the reaction mixture by using a pressure sensor shown in Fig. 28.
Fig. 28 Cross-sectional diagram showing general set-up
Fig. 29 General set-up for investigation on hydrogen production by photocatalytic watersplitting
The set-up for procedures measuring the volume of hydrogen gas generated in the reaction
mixture by detection of pressure change:
1. 54.0 cm3 of distilled water, 6.0 cm3 of glycerol and different parameters were transferred
into a 100.0 cm3 conical flask by a 100 cm3 and a 10 cm3 measuring cylinder respectively.
2. The pressure sensor’s tube was inserted through a stopper, and it was linked to the
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 21
pressure sensor.
3. The temperature sensor was connected to the bottom of the conical flask and linked to
the pressure sensor to detect temperature fluctuations in order to minimize error.
4. An O-ring ultraviolet light / Fluorescent light tube was fit onto the conical flask.
5. A magnetic stirrer was added to the mixture with its stirring speed set to 1000 rpm.
6. The conical flask was stoppered with the tube of pressure sensor connection to it.
7. Parafilms was used to seal the gaps between the stopper and the conical flask to make
sure no gas leakage in the experiment.
8. The set up was covered with a box wrapped with aluminum foil.
9. The initial temperature and reading of the hydrogen were recorded. After 12 hours,
recordings were stopped and the difference in pressure reading is calculated.
10. Steps 1 – 10 were repeated with different parameters.
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 22
E. Experiments
E1. Different photocatalysts
Experimental Procedures:
Please refer to D8 for experimental procedures. In
this experiment, 30 cm3 of distilled water and
glycerol were used respectively. Different masses
of different photocatalysts (Fig.30) illuminated by
UV light were being investigated.
Result:
Fig. 31 Result of E1. Different photocatalysts
Fig. 30 Different photocatalysts being investigated, including
ZnO, ZnS, TiO2 and Fe2O3
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 23
Analysis:
For TiO2, ZnO and Fe2O3, the increase of photocatalysts from 0.100 to 0.400g leads to an
increase of pressure of 48.720 kPa, 35.196 kPa and 30.504 kPa respectively. However, the
increase in pressure decrease with further increase in mass of the photocatalyst employed after
attaining the optimum level. For ZnS, the increase of photocatalyst from 0.100 to 0.900g leads
to a continuous increase of pressure of 13.104 kPa. The above results can be explained by the
following aspects.
We reason that the increase in pressure reading is mainly due to the redox reaction at the
conduction band and valence band. When photons are emitted from the O-ring UV lamp, the
electrons are then excited and have enough kinetic energy that allows them to jump from the
valence band to the conduction band. The photoexcited electrons at the conduction band act as
a strong reducing agent which reduces hydrogen ions in water to hydrogen gas while the holes
on the valence band with strong oxidizing potentials tend to oxidize glycerol and water to
hydrogen gas and hydroxyl radical respectively (Fig. 5). Both redox reactions at the valence
band and conduction band produce hydrogen and hence the higher the concentration of the
photocatalyst is, the more the number of photocatalytic reactions is, thus producing more
hydrogen and giving a higher pressure reading.
However, the curves tend to fall after the optimum pressure reading. This trend can be
explained by the intensive competition among photocatalysts and screening effect. Although
there are more sites for undergoing photocatalytic watersplitting reactions as more and more
photocatalysts are used, but the competition among the TiO2 for photons, which contain
energy for photoexcitation, become far much intensive. Furthermore, one may block the UV
light passing through and reduce the light available for other catalysts, especially for the TiO2
with high ultraviolet-blocking power. Hence, the more the photocatalysts, the stronger the
screening effect and hence less hydrogen is produced, thus giving a lower pressure reading.
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 24
For the comparison of the optimal hydrogen production among the 4 photocatalysts, the
optimal pressure readings are arranged in ascending order:
ZnS < Fe2O3 < ZnO < TiO2
The result can be explained by the Band Gap theory (C1) and their respective absorption
spectrum.
For the 4 semiconductors, there is a valence band and a conduction band separated namely by
the band gap. When photons with energy higher or equal to the band gap of the
semiconductor, an electron from the valence band is excited and has enough energy to flow to
the conduction band. According to the Band Gap theory, the wider the gap is, the energy
required increases as well. By rearranging equation (3), we have
𝜆 =𝑐ℎ
𝐸
the band gap and absorption spectrum calculated of the 4 photocatalysts are shown below:
Table 1 Band gap and absorption spectrum of respective photocatalysts
Types of photocatalysts Band gap (eV) Absorption spectrum (nm)
TiO2 3.2 200-400
ZnO 3.3 <365
Fe2O3 2.3 <585
ZnS 3.6 <400
Seemingly, Fe2O3 has the lowest band gap of 2.3 eV and the largest absorption spectrum (Fig.
32). However, it gives a lower pressure than ZnO and TiO2 increase due to small optical
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 25
absorption coefficient, and rapid e-/h+ recombination resulting in short carrier diffusion lengths
and slow surface reaction kinetics6.
TiO2 gives the highest pressure reading as it has a narrower band gap and a larger absorption
spectrum than ZnS and ZnO, thus its photocatalytic watersplitting activity is the highest among
all. ZnS has the widest band gap thus gives the lowest pressure reading as more energy is
needed to overcome the wide band gap.
6 Flavio Leandro Souza; Kirian Pimenta Lopes; Elson Longo; Edson Roberto Leite, The influence of the film thickness of
nanostructured a-Fe2O3 on water photooxidation, 2008, 1215-1219
Fig. 32 Band gap of ZnO, ZnS, TiO2 and Fe2O3
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 26
E2. Volume ratio of water to glycerol
Experimental Procedures:
Please refer to D8 for experimental procedures. In this experiment, different volumes of
distilled water and glycerol were used to investigate its effect on hydrogen production.
Result:
Fig. 33 Result of E2. Volume ratio of water to glycerol
As the water to glycerol volume ratio changes from 0:60 to 54:9, results in an increase of
pressure from 27.312 to 73.284 kPa. However, the increase in pressure falls after attaining the
highest point from 73.284 to 65.568 kPa. The volume ratio of 54:9 shows the optimum
photocatalytic watersplitting activity. We believe that the above result can be explained by the
rapid recombination of e-/h+ pairs.
27.312
32.484
36.228
41.004
45.144
48.898
60.445
69.18071.500
73.284
65.568
20.0
30.0
40.0
50.0
60.0
70.0
00--60 06--54 12--48 18--42 24--36 30--30 36--24 42--18 48--12 54--06 60--00
Pre
ssu
re In
cre
ase
d (k
Pa)
Volume ratio (Water : Glycerol)
The effect of volume ratio of water to glycerol on hydrogen production
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 27
The persistent increase can be explained by
the increase in hydrogen ions in the reaction
mixture. Under the illumination of ultraviolet
light, there is rapid recombination of electrons
and its holes pairs in TiO2. As a result, the
holes at the valence band of TiO2 are not likely
available for oxidation of glycerol. Therefore,
the main site for hydrogen production is the
conduction band. Hence, the main reaction is
reduction of hydrogen ions from water in the
conduction band oxidation of glycerol at the
valence band instead of glycerol. Therefore,
when the volume of water increases, the
number for hydrogen ions available for reduction to hydrogen also increases. Thus, the
pressure also increases as the volume ratio of water to glycerol increases.
However, the slight decrease in the pressure reading when the volume ratio of water to
glycerol is at 60:0. We reason that the result was due to the absence of glycerol, so no
hydrogen was able to be produced at the conduction band, even though glycerol account for
only a small proportion of hydrogen produced. Therefore, less hydrogen is produced and give a
lower pressure reading.
Fig. 34 Energy diagram showing recombination of e-
/h+ (a) e-/h+ pair recombination after
reaching CB (b) 𝐸photons< 3.2 eV leading
to recombination of e-/h+ before reaching
CB
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 28
E3. Metal ion-doping
Experimental Procedures:
Please refer to D8 for experimental procedures. In this experiment, different masses of the 4
types of metal-ion doped TiO2 calcined at different temperatures are being investigated.
Result:
Fig. 35 Result of E3. Metal-ion doped TiO2 compared to undoped TiO2 under illumination of visible light
122.79
114.232
103.203
90.156
1.0320
20
40
60
80
100
120
copper(II) ion nickel(II) ion Zinc ion Cobalt(II) ion Pure titanium dioxide
Ad
just
ed
dis
pla
cem
en
t(k
Pa)
Species
Optimum hydrogen production level of different types of metal-ion doped TiO2
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 29
Fig. 36 Result of E3. Cu2+ ion-doped TiO2 (dc denoted as oC)
Fig. 37 Result of E3. Ni2+ ion-doped TiO2 (dc denoted as oC)
118.425 118.555
119.737 120.54
120.277 119.838
118.542
120.343
120.78
121.822
122.79
122.23
121.542
120.889
118.1
118.349
119.345
120.782
120.552
119.445
118.324
117
118
119
120
121
122
123
124
0.1 0.2 0.3 0.4 0.5 0.6 0.7
Pre
ssu
re In
cre
ased
(kP
a)
Mass (g)
Effect of Cu2+ ion-doped TiO2 calcined at different temperatures on hydrogen production
200dc
400dc
600dc
110.324
110.785
112.146112.146 111.645
110.865
110.586
112.787112.993
113.737
114.232
113.903
112.764
112.239
110.468110.893
111.565
112.193
111.589
110.767
110.665
110
111
112
113
114
115
0.1 0.2 0.3 0.4 0.5 0.6 0.7
Pre
ssu
re In
cre
ase
d (k
Pa)
Mass (g)
Effect of Cu2+ ion-doped TiO2 calcined at different temperatures on hydrogen production200dc
400dc
600dc
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 30
Fig. 38 Result of E3. Co2+ ion-doped TiO2 (dc denoted as oC)
85.313
86.178
86.828
87.213
86.899
86.373
85.328
88.233
89.078
89.875 90.156 90.021
89.353
88.543
84.562
85.379
86.433 86.798 86.433
85.723 85.303
84
85
86
87
88
89
90
91
0.1 0.2 0.3 0.4 0.5 0.6 0.7
Pre
ssu
re In
cre
ased
(kP
a)
Mass(g)
Effect of Co2+ ion-doped TiO2 calcined at different temperatures on hydrogen production
200dc
400dc
600dc
Fig. 39 Result of E3. Zn2+ ion-doped TiO2 (dc denoted as oC)
99.408
99.76
100.798
101.535
100.973
100.246
99.78
101.652101.798
102.678103.203
102.674
102.002101.782
100.056100.354
100.935 101.237
101.037
100.559
99.896
99
100
101
102
103
104
0.1 0.2 0.3 0.4 0.5 0.6 0.7
Pre
ssu
re In
cre
ase
d (k
Pa)
Mass (g)
Effect of Zn2+ ion-doped TiO2 calcined at different temperatures on hydrogen production
200dc
400dc
600dc
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 31
Analysis:
Table 2 Optimum increase in pressure of different metal ion-doped TiO2 calcined at different temperatures
Type of M2+-TiO2 200oC 400oC 600oC
Cu2+-TiO2 120.540 kPa 122.790 kPa 120.782 kPa
Ni2+ -TiO2 112.146 kPa 114.232 kPa 112.193 kPa
Co2+-TiO2 87.213 kPa 90.156 kPa 86.789 kPa
Zn2+-TiO2 101.535 kPa 103.203 kPa 101.237 kPa
For 200, 400 and 600oC calcined Cu2+ ion-doped TiO2, the increase of catalyst leads to an
optimum change in pressure of 120.54, 122.790 and 120.782 kPa respectively. For 200, 400,
and 600oC calcined Ni2+ ion doped TiO2, the increase in mass of the catalyst leads to an increase
in pressure of 112.146, 114.232 and 112.193 kPa respectively. For 200, 400, and 600oC calcined
Co2+ ion doped TiO2, the increase of catalyst leads to an increase in pressure of 87.213, 90.156
and 86.789 kPa respectively. For 200, 400, and 600oC calcined Zn2+ ion doped TiO2, the increase
in mass of the catalyst leads to an increase in pressure of 101.535, 103.203 and 101.237 kPa
respectively. However, the increase in pressure decreases with further increase in mass of the
metal ion-doped TiO2 after attaining its optimum level.
Fig.35 shows the optimum hydrogen production of the 4 different types of metal-ion doped
TiO2 calcined at 400oC illuminated by visible light was compared to that of undoped pure TiO2.
All metal-ion doped TiO2 were able to increase the output of hydrogen when compared with
the undoped TiO2 reaction mixture under visible light. The ability of enhancing hydrogen gas
out by metal ion-doping can be explained by the holes and electron trapping ability of metal
ion-doped TiO2.
The presence of metal ion dopants influence the photoreactivity of TiO2 by providing addition
charge pair regeneration pathways and hence increase the chance of successful
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 32
photoexcitation and generation of hole pair electrons as the following equations (mentioned in
C4):
M2++ ℎ𝑣 →M3+ + e-cb (6)
M2+ + ℎ𝑣 →M+ + hvb+ (7)
M2+ + ℎ𝜈 → M2+h+
vb + M2+e-
cb (8)
Also, visible light can be utilized for photoexcitation as the energy level for M2+ ion lies below
the conduction band edge (Ecb) and the energy level for M2+ ion above the valence band edge
(Evb). Introduction of such energy levels in the energy profile narrow down the band gap and
induce the red shift in the band gap transition and the visible light absorption through a charge
transfer between a dopant and conduction band or valence band. Furthermore, the metal ions
act as electron and hole traps, thus reducing the possibility of electron-hole recombination
(mentioned in C4 2ab). Electrons are promoted to valence band more rapidly and the
availability of holes increases. Therefore, the mean lifetime of a single electron tends to be
longer compared to the undoped TiO27.
The obtained result in the 4 graphs (Fig 36-39) can be explained by the (E3.1) concentration
effect and the screening effect, (E3.2) extent of metal ion-doping, and (E3.3) the electron and
hole trapping ability of the metal ion-doped TiO2.
E3.1. The concentration effect and the screening effect
The upward sloping of curves on the graphs from 0.100 g to their highest point on the curve is
mainly due to the increase in the concentration of the metal-ion doped TiO2. As the number of
TiO2 increases, the sites for photocatalytic watersplitting reactions to take place also increase.
Moreover, as the concentration of the metal-ion doped TiO2 increase, the ability to capture
photons emitted from the light source (i.e the O-ring fluorescent lamp), which can be utilized
7 Wonyong Choi; Andreas Termin; Michael R. Hoffmann, The Role of Metal Ion Dopants in Quantum-Sized TiO2: Correlation
between Photoreactivity and Charge Carrier Recombination Dynamics, W. M. Keck Laboratories, Califomia Institute of
Technology, Pasadena, California 91125,1994, 13669-13679
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 33
for overcoming the band gap, also increases. Hence, more electrons can be transferred from
the valence band to the conduction band and at the same time, the number of holes on the
valence band also increases. Therefore, the wastage of energy from photons is minimized and
more energy is utilized for photoexcitation. Consequently, both the number of oxidation and
reduction reactions for producing hydrogen increases, which leads to a higher increase in
pressure reading.
On the other hand, the downward sloping of the curves on the graphs indicates the hydrogen
production decrease with further increase in the mass of metal ion-doped TiO2. This can be
explained by the increasing competition of photons among the TiO2. Although the sites for
photocatalytic reactions increase with increasing number of metal-ion doped TiO2, but the
mixture becomes more crowded and one may block the photons available for others, hence
less photons can be received by the metal-ion doped TiO2 and less energy can be utilized for
overcoming the band gap, decreasing the number of photocatalytic watersplitting reactions.
Consequently, less hydrogen is produced and the increase in pressure drops.
E3.2. Extent of metal ion-doping
The graphs also showed that the 400oC calcined metal ion-doped
TiO2 shows best performance in the production of hydrogen as the
curves of 400oC calcined metal ion-doped TiO2 lies above the
curves of 200 and 600oC. The results can be explained by the
extent of doping at various temperatures.
When the metal ion-doped TiO2 is heated at 200oC, the metal ion is
doped slightly on the surface of TiO2. Thus, the band gap between
the conduction band and the valence band is reduced but with a
very small extent, i.e. the new energy difference in the band gap is
not small enough (Fig. 41b). As a result, the visible light source cannot provide enough energy
for valence band electrons to promote to excited state, because of the photons from the visible
Fig. 40 Extent of metal-ion
(M2+) doping at 200,
400 and 600oC
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 34
light have a relatively lower energy than that needed to overcome the band gap and eventually
lead to poorer photocatalytic activity and hence less hydrogen is produced, indicated by a lower
increase in pressure.
For metal ion-doped TiO2 calcined at 600oC, the hydrogen production is similar to those
heated at 200oC, as calcined at relatively high temperature will result in deep doping of
metal ions. The result of deep doping is that doped metal ion will behave as recombination
centers (Fig. 41d), so electron and hole transferring to the interface of photocatalyst
becomes more difficult. Consequently, photocatalytic activity will decrease as the valence
band receives less excited electrons and leads to poorer hydrogen production.
So, metal ion-doped TiO2 calcined using at 400oC, shows the highest hydrogen production,
because when heated at 400oC is neither slightly doped nor deeply doped. As a result, the
energy difference in the band gap is reduced successfully and significantly (Fig. 41c).
Moreover, H2O, H+ ions and glycerol on the surface of photocatalysts can be reduced or
oxidized effectively as the metal ion is able to exert the effect of preventing the
recombination of e-/h+ pairs, which in turn lead to highest hydrogen production among
metal ion-doped TiO2 calcined at three different temperatures.
Fig. 41 Extent of doping at 200,400 and 600oC (a)
Original band gap of undoped TiO2 (b) Slight
doping (c) Efficient doping, shows ability in
reducing the band gap (d) Deep doping, metal ion
acts as recombination center
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 35
E3.3. Distinctive features and ability of the metal ion-doped TiO2
Among the four types of metal ion-doped TiO2 used, Cu2+ ion-doped TiO2 shows the highest
hydrogen production in watersplitting, while Ni2+ion-doped TiO2 and Co2+ion-doped TiO2 shows
lower hydrogen production and Zn2+ion-doped TiO2 show the lowest hydrogen production. The
order of the increase in pressure of the four metal ion-doped TiO2 is arranged in descending
order of:
Cu2+> Ni2+>Zn2+> Co2+.
The reason for the excellence in performance of Cu2+ ion-doped TiO2 in hydrogen production is
that the capability of Cu2+ ion to trap both electrons and holes8, using Cu2+ion-doped TiO2 can
maximize the efficiency of photocatalytic reaction as both electron and hole can be transferred
to the surface. When compared to Ni2+, Co2+ and Zn2+, which can only trap one type of charge
carrier, their efficiency in photocatalytic watersplitting reaction is lower than using Cu2+.
Ni2+ ion-doped TiO2 shows higher hydrogen production than Zn2+ion-doped TiO2 due to the fact
that Ni-doped TiO2 leads to a significant lattice deformation, which changes the charge
distribution pattern, making the separation of photoexcited e-/h+ pair easier9.
Moreover, the result of Co2+ ion doped TiO2 shows a higher photocatalytic activity than Zn2+ion.
We reason that the quantum yield of Co2+ is higher than that of the Zn2+, such that more
photons can be harvested by the Co2+ ions compared to that of the Zn2+ as recent researches
showed that the fluorescent quantum yield of Zn2+ ion-doped TiO2 is 0.20, which is higher than
that of the Co2+ion-doped TiO2, which is 0.08. As Zn2+ ion doped TiO2 has a higher quantum
yield, more energy from the photons can be utilized for photoexcitation and overcoming of the
band gap, thus producing more hydrogen and causes a higher increase in pressure.
8 G. Colón; M. Maicu; M.C. Hidalgo; J.A. Navío, Cu-doped TiO2 systems with improved photocatalytic activity, Applied
Catalysis B: Environmental, Volume 67, Issues 1–2, 2006, 41-51 9 Yan-Ming Lin; Zhen-Yi Jiang; Chao-Yuan Zhu; Xiao-Yun Hu; Xiao-Dong Zhang; Jun Fan, Visible-light photocatalytic activity
of Ni-doped TiO2 from ab initio calculations, Materials Chemistry and Physics, Volume 133, Issues 2–3, 2012, 746-750
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 36
E4. Natural dye sensitization
Experimental Procedures:
Please refer to D8 for experimental procedures. In this
experiment, different volumes of natural dyes were
added to the reaction mixture to investigate their
sensitization effect of TiO2 on hydrogen production
under illumination of visible light.
Result:
Fig. 43 Result in E4. Effect of different natural dyes sensitization
Fig. 42 Natural dyes including anthocyanin, β-carotene
and chlorophyll being investigated
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 37
Fig. 44 Result in E4. Effect of different natural dyes sensitization compared to pure TiO2
87.353 87.374
92.032
Anthocyanin β-Carotene Chlorophyll a Pure TiO2
Pre
ssu
re In
cre
ase
d (
kPa)
Species
1.032
Comparison between sensitized and pure TiO2 illuminated by visible light
Fig. 45 Absorbance and absoprtion spectrum of natural dyes through spectrometry analysis
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 38
Analysis:
3 different natural dyes including chlorophyll, β-carotene and anthocyanin are being extracted
through various methods mention in (D4, D5 & D6) and their sensitization performances in the
hydrogen production by photocatalytic watersplitting of TiO2 are being investigated.
For chlorophyll sensitization, the increase of chlorophyll extract from 1.00 cm3 to 8.00 cm3 leads
to an increase in pressure reading from 90.878 to 92.032 kPa, which is the highest point of the
curve. For β-carotene sensitization, the increase of β-carotene extract from 1.00 cm3 to 8.00
cm3 leads to an increase in pressure reading from 85.232 to 87.353 kPa. For anthocyanin
sensitization, the increase of β-carotene extract from 1.00 cm3 to 8.00 cm3 leads to an increase
in pressure reading from 85.383 to 87.374 kPa, which is the optimum of the curve. However,
the pressure change decrease with further increase in the volume of natural dyes from 9.00 to
10.00 cm3 after attaining their optimum levels.
The above results can be explained in two aspects: (1) Comparison between natural dye
sensitized and pure TiO2 and (2) Comparison of chlorophyll, anthocyanin and β-carotene
sensitized TiO2 on the effect of hydrogen production among.
E4.1. Comparison between natural dye sensitized and non-sensitized TiO2
By comparing the effect of natural dye sensitized TiO2 illuminated by visible light on the
hydrogen produced with that of non-sensitized TiO2, it shows that all sensitized TiO2 by natural
dyes were able to increase the output of hydrogen with a surprisingly large extent. The ability
of enhancing hydrogen gas through sensitization by natural dyes can be explained by the
following aspects.
Originally, TiO2 was only able to utilize 3% of UV light in the solar spectrum as it has a wide
band gap of 3.2 eV. So, the chart on the graph shows slight increase in the pressure increased
under the illumination of the O-ring fluorescent lamp. However, the natural dyes sensitized TiO2
shows an unexpectedly high increase in pressure reading, which indicated the success of
hydrogen production by TiO2 through photocatalytic watersplitting illuminated by visible light.
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 39
The reason for the success sensitization is explained by the presence of conjugated double
bonds.
All of the above structural formula of anthocyanin, chlorophyll and β-carotene shows the
presence of conjugated double bonds, we reason that the conjugated systems are responsible
for providing electrons for the conduction band of the TiO2. When illuminated under visible
light, all 3 natural dyes show the ability to absorb and utilize the light in the visible spectrum, as
they have a highly delocalized electron density, making them a highly plausible light sensitizer.
The abundant conjugated double bonds in all dyes shorten the energy gap between the valence
band and the conduction band. As a result, photons from visible light, which has a relatively
lower energy than those from the ultraviolet light, and are able to overcome the band gap in
the 3 natural dyes. Hence, electrons in the conjugated double bonds are ‘excited’ and become
highly delocalized. The electrons are then transferred from the conjugated system to the
conduction band of the TiO2. Therefore, more electrons with high reducing potentials are
Fig. 46 Location of conjugated double bonds of natural dyes and
excited electrons transfer
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 40
available at the conduction band of the TiO2. Consequently, more hydrogen ions are being
reduced, thus producing more hydrogen gas and lead to a higher increase in pressure readings.
E4.2. Comparison of chlorophyll, anthocyanin and β-carotene sensitized TiO2 on the effect of
hydrogen production
By comparing the effect of sensitization by chlorophyll, anthocyanin and β-carotene in
hydrogen production, according to the above graph, chlorophyll sensitization of TiO2 shows
unexpectedly high increase in pressure reading, followed by anthocyanin and β-carotene. The
above results can be explained by their respective absorption spectrum and quantum yield.
In Fig. 45, chlorophyll absorbs light most strongly in the red and violet parts of the spectrum
ranging from 710 nm to 300 nm, and it best absorbs light in the 330 nm (violet-blue) and 650
nm (red) area of the visible light spectrum. β-carotene absorbs light most strongly in the red
and yellow parts of the visible light spectrum ranging from 690 nm to 455 nm, and it shows the
highest absorption at 588 nm (orange) area of the visible light spectrum. Anthocyanin absorbs
light mostly in the red and yellow part of the visible light spectrum ranging from 700 nm to 300
nm, and it shows the highest absorption at 680 nm (red) area of the visible light spectrum.
Chlorophyll shows the largest absorption range as both violet-blue and red lights can be utilized
for photoexcitation. Also, it exhibits a high quantum yield of approximately 30%10. Hence, the
number of times of photoexcitation occur per photon absorbed by the system is the highest
among the 3 natural dyes. In addition, its abundant conjugated double bonds system allows
numerous photoexcited electrons transfer from the chlorophyll to the valence band of TiO2.
Furthermore, electrons in chlorophyll are excited far much easier than those in TiO2 as the band
gap of chlorophyll is relatively smaller in the presence of abundant conjugated double bonds.
Consequently, it gives the highest pressure increased among the 3 natural dyes used.
Although β-carotene appears to have the highest absorbance at its peak among the 3 natural
dyes employed and has relatively more conjugated double bonds and hence the number of
10 Amarendra Narayan Misra; Meena Misra; Ranjeet Singh, Chlorophyll Fluorescence in Plant Biology, Post-Graduate
Department of Biosciences & Biotechnology, India, 2012, 7, 171-192
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 41
electrons for photoexcitation than chlorophyll and anthocyanin, but the quantum yield of β-
carotene was found to be (0.034 ± 0.002) %11, which is far much lower than the quantum yield
of anthocyanin and chlorophyll. Hence, number of times of photoexcitation occur per photon
absorbed by the system is the lowest among the 3 natural dyes, resulting in the lowest
photocatalytic activity.
Even though anthocyanin tends to be much lower than that of chlorophyll and β-carotene, the
maximum quantum yield of anthocyanin is 0.83%12. Even though the absorbance of
anthocyanin appears to be below that of β-carotene, its relatively higher quantum yield and the
ability of chelating to TiO2 in the presence of carboxylic anchoring groups (Fig. 47) offset the
effect of its low absorbance in the visible light spectrum, hence both dyes give a similar result in
the sensitization of TiO2 for hydrogen production through photocatalytic watersplitting.
11 Krzysztof Pawlak; Andrzej Skrzypczak; Grazyna E. Bialek-Bylka, Inner Filter Effect in the Fluorescence Emission
Spectra of Room Temperature Ionic Liquids
with-β-Carotene, Institute of Physics, Faculty of Technical Physics, Poland, 2011, 19, 401-420 12 Kevin Gould; Kevin M Davies; Chris Winefield, Anthocyanins: Biosynthesis, Functions, and Applications 2008
Fig. 47 Ability of anthocyanin
chelating to TiO2 in the
presence of carboxylic
anchoring groups
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 42
E5. pH effect
Experimental Procedures:
Please refer to D8 for experimental procedures.
In this experiment, different pH buffers (Fig. 48)
of same volume were added to the reaction
mixture to investigate their effect on Cu2+ ion-
doped TiO2 and chlorophyll sensitized on Cu2+
ion-doped TiO2 hydrogen production under
illumination of visible light.
E5.1 Effect of pH on Cu2+ ion-doped TiO2 in hydrogen production
Result:
Fig. 49 Result on E5.1 Effect of pH on Cu2+ ion-doped TiO2 in hydrogen production
Different colourless pH buffers were added to the reaction mixture to examine the effect of pH
on both Cu2+ ion-doped TiO2 for hydrogen production by photocatalytic watersplitting. The
121.689
123.976
124.846
126.384
129.759
133.834
137.746 138.374
120
122
124
126
128
130
132
134
136
138
140
0 2 4 6 8 10 12 14
Pre
ssu
re In
cre
ase
d (k
Pa)
pH
Effect of pH on Cu2+ ion-doped TiO2 in hydrogen production
Fig. 48 pH buffers ranging from pH 0-14
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 43
increase in pH from 0.00 to 14.00 leads to an increase in pressure reading from 121.689 to
138.374 kPa. Among all, pH 14.00 shows the highest pressure increased.
The above results can be explained by the Point of Zero Charge (PZC) on the surface of the Cu2+
ion-doped TiO2.
PZC describes the condition when the electrical charge density on a surface is zero. It is usually
determined in the relation to an electrolyte's pH, and the PZC value is assigned to a given
substrate or colloidal particle. In other words, PZC is the pH value at which a solid submerged in
an electrolyte exhibits zero net electrical charge on the surface. The value of pH is used to
describe PZC in this experiment.
As pH 14 shows the highest pressure reading, which in turn indicates the highest hydrogen
production and hence the optimum activity of photocatalytic watersplitting reaction. To
simplify, we assume that the Ti4+ ion sites can be divided into two kinds at pHpzc. One kind is
sites absorbed with OH- ions, the other
being ones absorbed with associated
H2 molecules. The pHpzc value was
reported to be 6±0.213 (Fig. 50).
13 Cao jiang-lin; lengwen-hua; zhang jian-qing; Cao chu-nan, Adsorption Behavior and Photooxidation Kinetics of OH at TiO2
Thin Film Electrodes, Department of chemistry, Zhejiang university, Hangzhou, 2004, 20(7), 735-739
Fig. 50 pHpzc= 6±0.2, 0 net electrical charge on the surface
of TiO2 and able to attract hydrogen ions for
reduction
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 44
When the pH is lower than the pHpzc value
(<6), the system is considered to be below
the pHpzc, in which the acidic water
donates more protons than hydroxide
ions in water to the metal ion-doped TiO2
and hence the surface of metal ion-doped
TiO2 is positively charged and has higher
potential in attracting OH- ions (anions). As a result, less hydrogen ions are attracted when the
reaction mixture is below pH 6, hence less hydrogen ions are reduced to produce hydrogen gas,
leading to a smaller increase in pressure as pH decrease (Fig. 51)
Conversely, above pHpzc value >6, the surface is negatively charged and has high hydrogen ions
(cations) attracting potentials and thus repelling hydroxide ions (anions). Consequently, more
hydrogen ions are attracted towards the metal ion-doped TiO2 and there is a higher chance of
reduction of hydrogen ions as the pH increases, thus producing more hydrogen and leading to a
greater increase in pressure readings
(Fig. 52).
Fig. 51 pHpzc< 6±0.2, positive net electrical charge on the
surface of TiO2 and repelling hydrogen ions for
reduction
Fig. 52 pHpzc> 6±0.2, negative net electrical charge on the
surface of TiO2 and able to attract more hydrogen ions
for reduction
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 45
E5.2 Effect of pH on chlorophyll sensitized Cu2+ ion-doped TiO2 in hydrogen production
Result:
Fig. 53 Result of E5.2 Effect of pH on chlorophyll sensitized Cu2+-TiO2 in hydrogen production
Different colourless pH buffers were added to the reaction mixture to examine the effect of pH
on both Cu2+ ion-doped TiO2 and chlorophyll sensitization of hydrogen production by
photocatalytic watersplitting. The increase in pH from 0.00 to 10.00 leads to an increase in
pressure reading from 160.889 to 174.465 kPa. However, the pressure readings tend to
decrease from 174.456 to 170.378 kPa as the pH increase from 10.00 to 14.00. In the graph, pH
10.00 shows the highest pressure increased.
We believed that the above result can be explained by the shifting of absorption spectrum and
absorbance of chlorophyll and its structural change at different pH medium.
As mentioned in experiment 4, chlorophyll absorbs light most strongly in the red and violet
parts of the visible light spectrum ranging from 710 nm to 300 nm, and are able to best absorbs
light in the 330 nm (violet-blue) and 650 nm (red) area of the light spectrum and show highest
absorbance of 2.48 at its peak. However, the absorbance and absorption spectrum is highly
affected by the pH, which affects the sensitization efficiency and the photocatalytic rate of TiO2.
160.889
163.103
166.246
170.338
173.676
174.465
172.879
170.378
160
162
164
166
168
170
172
174
176
0 2 4 6 8 10 12 14
Pre
ssu
re In
cre
ased
(kP
a)
pH
Effect of pH on chlorophyll sensitized Cu2+ ion-doped TiO2 in hydrogen production
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 46
For pH <7, the absorbance of chlorophyll tends to decrease with decreasing pH and it shows a
blue shift of the absorption spectrum (Fig. 54). Hence, the pressure reading tends to decrease
with decreasing pH because as less light energy is absorbed by chlorophyll, so less electrons will
be excited. Furthermore, when under acidic conditions, the magnesium atom Mg2+ is lost and
the colour changes to the characteristic pheophytin olive green colour (Fig. 55), so the
sensitization effect is removed as the chlorophyll is degraded in acidic condition. Both factors
lead to less excited electrons being donated to the conduction band for reduction of hydrogen,
leading to a lower increase in pressure as pH decreases.
N
N
N
N
CH3
O
OO
O O
CH3
CH3
CH2
CH3CH3
H
H H
CH3
R R = phytyl
Abs
orba
nce
Fig. 54 Fluorescence of chlorophyll: excitation at 655 nm,
influence of pH.
Fig. 55 Mg2+ ion is lost under very acidic
medium indicated by the red
circle
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 47
For pH>7, the absorbance of chlorophyll tends to
increase rapidly with increase pH and it shows a red shift
of the absorption spectrum (Fig. 54). As a result, more
photoexcited electrons can be transferred to the
conduction band for reducing hydrogen ions to
hydrogen gas thus giving a higher pressure reading as
pH increase. However, when under very alkaline
conditions pH>10, the methyl and phytyl esters are
removed (Fig. 56), producing chlorophyllin which is in
bright green color, hence, chlorophyll tends to degrade,
causing the less excited electrons transfer to the
conduction band for the reduction of hydrogen. This is
also one of the factors to show a lower pressure reading.
To conclude, pH 10 is the optimum working medium for chlorophyll sensitized metal ion-doped
TiO2. Even though the optimum work condition for Cu2+ ion-doped is pH 14, its efficiency is
sacrificed for boosting the absorbance of chlorophyll and to prevent its degradation under very
alkaline condition.
N
N
N
N
CH3
O
O-
O
O O-
CH3
CH3
CH2
CH3CH3
H
H HMg+2
No ester groups
Chlorophyllin
Fig. 56 Methyl and phytyl esters removed under very
alkaline condition
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 48
F. Conclusion
A series of experiments were carried out to investigate the factor affecting the photocatalytic
performances of TiO2 and thus improving its efficiency.
Parameters like types and amount of catalysts used, volume ratio of glycerol to water were
being studied. Moreover, the problems of rapid recombination of e-/h+ pairs and inability to
utilize visible light of TiO2 were alleviated through investigations of metal ion-doping, natural
dye sensitization and working pH medium of Cu2+ ion-doped TiO2.
The optimized and recommended catalyst for hydrogen production through photocatalytic
redox reaction of glycerol and water is 9 cm3 3M chlorophyll sensitized 0.400g Cu2+ ion-doped
TiO2 calcined at 400oC, and at a working medium of pH 10.00.
The optimum hydrogen production was calculated by the Ideal Gas Law14:
𝑃𝑉 = 𝑛𝑅𝑇 (18)
By arranging the constants, we have 𝑛 =174.465×1000×4×10−5
8.314472×304.15= 2.76 × 10−3
Volume of hydrogen produced = 2.76 × 10−3 × 24 × 1000 = 66.23 𝑐𝑚3
Thus, the average rate of hydrogen production through photocatalytic watersplitting is
66.230 ÷ 12 = 5.519 𝑐𝑚3ℎ−1.
14 P is the partial pressure of hydrogen, V is the volume of hydrogen, R is the universal constant, T is temperature in Kelvin, n is
number of moles of hydrogen
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 49
G. Comparison table Factors Steam-methane reforming Our photocatalytic hydrogen production
1 Pressure 300-2500 kPa 101.325 kPa
2 Temperature 700oC – 1100oC 31.00oC
3 Availability of
reactants
Reactant: Natural gas
Availability: low
Have a limited supply because it cannot be
replenished on a human time frame
Reactants: Food waste, glycerol
Availability: high
According to the British Institution of Mechanical Engineers (IME) ,half of
the food is wasted worldwide in 2013
Glycerol is formed as a by-product in the biodiesel production or alkaline
hydrolysis of gutter oil
Approximately 950,000 tons per year of glycerin are produced in the United
States and Europe
4 Cost of Reactant Higher
Nickel catalyst used is a scarce metal and is
expensive
Lower
Food wastes oil are abundant and cheap
TiO2 is cheap, abundant in nature and can be reused
5 Production cost HKD$36.81~HKD$52.31 based on a financial
report of a firm
HKD$2.8~HKD$3.0 based on our calculation
6 Risk High risk
Sulphur is used during the production as a
catalyst, such an unstable element can
easily cause explosion when ignited
accidentally
No risks
Almost no risks is involved during its H2 production due to the condition for
the production take place does not require a high pressure environment,
hence has no chance of explosion
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 50
7 Problem of food
waste solved?
No
Its production method is irrelevant to the
food waste problem
Yes
Food waste is being consumed in the production, so this method is able to
reduce the amount of food waste
8 Use of renewable
resources
Non-renewable resource
Natural gas is a kind of non-renewable
resource, is being consumed for the
production
Renewable resource
Sunlight, one of the renewable resources is used
Glycerol has been a well-known renewable chemical for centuries, as a by-
product of biodiesel production
9 Environmentally
friendly?
No
Greenhouse gases like carbon dioxide and
carbon monoxide is produced
Yes
No harmful substance is formed or produced during the production, it is
considered as an environmentally friendly H2 production method
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 51
H. Significance
Waste is a very common problem for every affluent society. In Hong Kong, like many other
developed cities, its wasteloads have kept raising as its economy grows. According to the
Environmental Protection Department in Hong Kong, wasteloads of Hong Kong have generally
been increasing since 1986. This consistent growth in wasteloads is alerting us that the landfills
in Hong Kong is running out of space far earlier than it had been expected, and it is estimated
that if the waste level continues to increase at current rate, the existing landfills will be
exhausted one by one by 2020, which is 5 years later. Furthermore, the outbreak of the
adulteration of olive oil scandal in Taiwan, the "gutter oil" has become a dreadful social focus,
as it will pollute water if it is disposed into the sea, and more importantly, it harms our health
as these gutter oil contains lots of bacteria and viruses, which will cause infection and threaten
our health.
Among all the waste produced, food waste is found to be the major constituent of the
municipal waste in Hong Kong. For the reason of making good use of the food waste, we intend
to utilize the waste oil in our hydrogen production. Before we can use the waste oil, purification
of glycerol from the hydrolysis products of waste oil is needed. One mean of purification of
glycerol is by using the combined approach of chemical treatment and vacuum distillation, so
that the impurities, such as ash and water, are removed successfully15.
Apart from the utilization of food waste oil, the hydrogen produced can be used as fuel in fuel
cells to alleviate the problem of energy crisis. As a matter of fact, fossil fuels will be used up
within years according to the prediction of the scientists. The reason behind this is that the
fossil fuels are non-renewable, meaning that it is impossible to be replenished before they are
used up. Worse, the demand for the fossil fuels nowadays is still increasing rapidly and
continuously. Therefore, it is very important to alleviate the energy crisis problem as soon as
15 Cai, T.F.; Li, H.P.; Zhao H.; Liao K.J., Purification of Crude Glycerol from Waste Cooking Oil
Based Biodiesel Production by Orthogonal Test Method. China Petroleum Processing and Petrochemical
Technology, 2013, 15, 48-53
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 52
possible before it is too late. The maximum life span of oil is 43 years, in order to achieve a
sustainable development, producing hydrogen by our method not only provides clean and high
purity hydrogen using renewable solar energy, but also produces clean energy as hydrogen gas
produced is used for hydrogen fuel cell and the only waste is water, which is harmless to the
environment. Thus it is a clean energy source.
Furthermore, our way of producing hydrogen through photocatalytic watersplitting provides
insight for future development of hydrogen production and it is far much efficient than that of
the traditional method. Originally, TiO2 can only absorbs UV light to initiate the watersplitting
reaction, which utilize only 3% of the sunlight. It is uneconomical and ineffective as the solar
energy cannot be fully utilized. However, about 43% of visible light can be used to initiate the
reaction by using our method (Fig. 56), which is far much effective and efficient.
In conclusion, we can see that our new way of hydrogen production is extremely clean,
renewable and cost effective, which helps to achieve a sustainable development of our future
energy economy.
Fig. 57 Mechanism of our way to produce hydrogen:
Chlorophyll sensitized Cu2+ -ion doped TiO2
UV-visible light-induced hydrogen production using natural chlorophyll sensitized Cu2+-doped TiO2
Hong Kong Chemistry Olympiad for Secondary School 53
I. Limitations
Difference in types of nano-TiO2
The type of TiO2 used in hydrogen production by photocatalytic watersplitting is the anatase
form. However, another type of nano-TiO2 called rutile, which has not been used in the
production of hydrogen. Therefore, we are not able to know which type of nano-TiO2 can
facilitate operation of the cell to the best. However, it was reported that anatase has 98% of
TiO2 in content while rutile has a maximum of 93% only, thus anatase should have a better
performance than rutile in hydrogen production in watersplitting reaction.
Comparison between synthetic dye and natural dye
Due to the Occupy Central Movement in Hong Kong, there is an embargo on transportation of
chemicals, therefore synthetic dyes are not available. Natural dyes are perfect substitutes that
can be easily extracted from nature. However, there is a surprisingly impressive result when we
use chlorophyll, 𝛽-carotene and anthocyanin.
Sunlight
Altering sunlight condition hinders the experiment to be carried out outdoors. Instead, the O-
ring ultraviolet and fluorescent lamp were used to imitate the sunlight effect. Furthermore, the
temperature emitted from the lamp is constant, which leads to a less fluctuated pressure
reading and hence gives a more accurate result.
Gas leakage
Gas leakage occurs easily. This is because the connection of the stopper to the conical flask was
insufficiently firm, which leads to an underestimation of the volume of hydrogen gas evolved.
However, parafilm is used to seal the gaps between and stopper and the opening of the conical
flask in order to minimize the gas leakage.