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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.



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



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.



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.



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.



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+



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.



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.



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.



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.



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



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.



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



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



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



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



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.



hydrogenphosphate.

Fig. 26 Blending carrots in

trichloromethane





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)



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



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.



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



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.



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



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



E2. Volume ratio of water to glycerol


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



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



E3. Metal ion-doping


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



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



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



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



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



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



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



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



E4. Natural dye sensitization


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



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



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.



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



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



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



E5. pH effect


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



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



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



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



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



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



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



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



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



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



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



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.

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