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Artificial Photosynthesis

Emerging Technology

Bahinting, Sandra Enn Dimo, Andro Van Magcanta, Sheila Mae

January 27, 2011

Introduction

If the smartest energy source is one that's abundant, cheap and clean, then plants are a lot

smarter than humans. Over billions of years, they developed perhaps the most efficient power supply in

the world: photosynthesis, or the conversion of sunlight, carbon dioxide and water into usable fuel,

emitting useful oxygen in the process.

In the case of plants (as well as algae and some bacteria), "usable fuel" is carbohydrates,

proteins and fats. Humans, on the other hand, are looking for liquid fuel to power cars and electricity to

run refrigerators. But that doesn't mean we can't look to photosynthesis to solve our dirty-, expensive-,

dwindling-energy woes. For years, scientists have been trying to come up with a way to use the same

energy system that plants do but with an altered end output.

Using nothing but sunlight as the energy input, plants perform massive energy conversions,

turning 1,102 billion tons (1,000 billion metric tons) of CO2 into organic matter, i.e., energy for animals

in the form of food, every year [source: The promise of artificial photosynthesis by Philip Hunter]. And

that's only using 3 percent of the sunlight that reaches Earth [source: Scientists seek to make energy as

plants do By Robert S. Boyd].

The energy available in sunlight is an untapped resource we've only begun to really get a handle

on. Current photovoltaic-cell technology, typically a semiconductor-based system, is expensive, not

terribly efficient, and only does instant conversions from sunlight to electricity -- the energy output isn't

stored for a rainy day (although that could be changing: See "Is there a way to get solar energy at

night?"). But an artificial photosynthesis system or a photo electrochemical cell that mimics what

happens in plants could potentially create an endless, relatively inexpensive supply of all the clean "gas"

and electricity we need to power our lives -- and in a storable form, too. [1]

[1] How Artificial Photosynthesis Works by Julia Layto. Retrieved January 11, 2010 from:

http://science.howstuffworks.com/environmental/green-tech/energy-production/artificial- photosynthesis.htm

Artificial Photosynthesis Approaches and Mechanisms

To understand fully how artificial photosynthesis works, let’s have a short review of what

photosynthesis is.

What is Photosynthesis?

Photosynthesis is the process by which plants, some bacteria, and some protistans use the

energy from sunlight to produce sugar, which cellular respiration converts into ATP, the "fuel" used by

all living things. The conversion of unusable sunlight energy into usable chemical energy is associated

with the actions of the green pigment chlorophyll. And most of the time, the photosynthetic process

uses water and at the same time release the oxygen that we absolutely must have to stay alive. Oh yes,

we need the food as well!

We can write the overall reaction of this process as:

6H2O + 6CO2 ----------> C6H12O6+ 6O2

Artificial photosynthesis is a research field that attempts to replicate the natural process

of photosynthesis, converting sunlight, water, and carbon dioxide into carbohydrates and oxygen.

Sometimes, splitting water into hydrogen and oxygen by using sunlight energy is also referred to

as artificial photosynthesis. The actual process that allows half of the overall photosynthetic reaction to

take place is photo-oxidation. [2]

To recreate the photosynthesis that plants have perfected, an energy conversion system has to

be able to do two crucial things (probably inside of some type of nano tube that acts as the structural

"leaf"): harvest sunlight and split water molecules.

Plants accomplish these tasks using chlorophyll, which captures sunlight, and a collection of

proteins and enzymes that use that sunlight to break down H2O molecules into hydrogen, electrons and

oxygen (protons). The electrons and hydrogen are then used to turn CO2 into carbohydrates, and the

oxygen is expelled.

[2]Artificial Photosynthesis: Turning Sunlight Into Liquid Fuels Berkley Laboratory by Lynn Yarris (10 Mar 2009)

For an artificial system to work for human needs, the output has to change. Instead of releasing

only oxygen at the end of the reaction, it would have to release liquid hydrogen (or perhaps methanol)

as well. That hydrogen could be used directly as liquid fuel or channeled into a fuel cell. Getting the

process to produce hydrogen is not a problem, since it's already there in the water molecules. And

capturing sunlight is not a problem -- current solar-power systems do that.

The hard part is splitting the water molecules to get the electrons necessary to facilitate the

chemical process that produces the hydrogen. Splitting water requires an energy input of about 2.5 volts

. This means the process requires a catalyst -- something to get the whole thing moving. The catalyst

reacts with the sun's photons to initiate a chemical reaction.

There have been important advances in this area in the last five or 10 years. A few of the more

successful catalysts include:

Manganese: Manganese is the catalyst found in the photosynthetic core of plants. A single

atom of manganese triggers the natural process that uses sunlight to split water. Using

manganese in an artificial system is a biomimetric approach -- it directly mimics the

biology found in plants.

Dye-sensitized titanium dioxide: Titanium dioxide (TiO2) is a stable metal that can act as an

efficient catalyst. It's used in a dye-sensitized solar cell, also known as a Graetzel cell, which

has been around since the 1990s. In a Graetzel cell, the TiO2 is suspended in a layer of dye

particles that capture the sunlight and then expose it to the TiO2 to start the reaction.

Cobalt oxide: One of the more recently discovered catalysts, clusters of nano-sized cobalt-

oxide molecules (CoO) have been found to be stable and highly efficient triggers in an

artificial photosynthesis system. Cobalt oxide is also a very abundant molecule -- it's

currently a popular industrial catalyst.[3]

[3] How Artificial Photosynthesis Works by Julia Layto. Retrieved January 11, 2010 from:

http://science.howstuffworks.com/environmental/green-tech/energy-production/artificial- photosynthesis1.htm

To further emphasize the mechanisms, the research being done can be split up into a series of

approaches:

The approach of the photoelectrochemical cell

The approach of the dye-sensitized solar cell

The approach of the NADP+/NADPH Coenzyme

The approach of the Photocatalytic Water Splitting Under Solar Light

Photoelectrochemical cell

PEC cells utilize light energy (photons) to perform a chemical reaction, in this case the splitting

of water into hydrogen (H2) and oxygen (O2) gases. They consist of an anode and a cathode immersed in

an electrolyte and connected in an external circuit. Typically, the anode or the cathode consists of a

semiconductor that absorbs sunlight, and the other electrode is typically a metal.

Water is oxidized at the anode according to the reaction:

2h+ + H2O (l) -> ½O2 (g) + 2H+,

At the cathode, H+ ions are reduced to form hydrogen gas via the reaction:

2H+ + 2e- -> 2H2 (g),

Research is being done into finding catalysts that can convert water, carbon dioxide, and

sunlight to carbohydrates. For the first type of catalysts, nature usually uses the oxygen evolving

complex. Having studied this complex, researchers have made catalysts such as blue dimer to mimic its

function, but these catalysts were very inefficient. Another catalyst was engineered by Paul

Kögerler,[4] which uses four ruthenium atoms.

The carbohydrate-converting catalysts used in nature are the hydrogenases. Catalysts invented

by engineers to mimic the hydrogenases include a catalyst by Cédric Tard,[5] the rhodium atom catalyst

from MIT,[6] and the cobalt catalyst from MIT.[7] Dr. Nocera of MIT is receiving funding from the Air Force

Office of Scientific Research to help conduct the necessary experiments to push forward

in catalyst research. The government funding has helped Nocera make the research possible and in turn

he is providing them with strong results.[8]

Advantages

Dye-sensitized cells can be made at one-fifth of the price of silicium cells.[9]

The solar energy can be immediately converted and stored, unlike in PV cells, for example, which

need to convert the energy and then store it into a battery (both operations implying energy losses).

Furthermore, hydrogen as well as carbon-based storage options are quite environmentally friendly.

Renewable, carbon-neutral source of energy, whether it is used for transportation or homes. Also

the CO2 emissions that have been distributed from fossil fuels will begin to diminish because of the

photosynthetic properties of the reactions.[2][10]

[4] Photoelectrochemical cell by Paul Kögerler. Retrieved January 11, 2010 from: www.wikipedia.com

[5] Photoelectrochemical cel:l Cédric Tard, Xiaoming Liu, Saad K. Ibrahim, Maurizio Bruschi, Luca De Gioia, Siân C. Davies, Xin

Yang, Lai-Sheng Wang, Gary Sawers and Christopher J. Pickett Nature (10 Feb 2005) 433, 610 - 613. Retrieved January 11,

2010 from: www.wikipedia.com

[6] Photoelectrochemical cell :Science, 31 August 2001. Retrieved January 11, 2010 from: www.wikipedia.com

[7] Photoelectrochemical cell:Hu, Xile; Cossairt, Brandi M.; Brunschwig, Bruce S.; Lewis, Nathan S.; Peters, Jonas C. Chem.

Commun., 2005 37, 4723-4725. Retrieved January 11, 2010 from: www.wikipedia.com

[8] Advantages Photoelectrochemical cell:Molly Lachance 26 Oct 2008 AF Funding Enables Artificial Photosynthesis. Retrieved

January 11, 2010 from: www.wikipedia.com

[2]Artificial Photosynthesis: Turning Sunlight Into Liquid Fuels Berkley Laboratory by Lynn Yarris (10 Mar 2009)

[9] Advantages Photoelectrochemical cell: G24innovations claiming 1/5th of the price of silicium cells. Retrieved January 11,

2010 from: www.wikipedia.com

[10] Advantages Photoelectrochemical cell: Anne Trafton (30 July 2008) Major Discovery from MIT Primed to Unleash Solar

Revolution.Retrieved January 11, 2010 from: www.wikipedia.com

Disadvantages

Artificial photosynthesis cells (currently) last no longer than a few years (unlike PV and passive solar

panels, for example, which last twenty years or longer).

The cost for alteration right now is not advantageous enough to compete with fossil

fuels and natural gas as a viable source of mainstream energy.

Dye-sensitized solar cell

This form of cell uses a dye in the second part of the energy creation to separate the charges

and create a current. Silicon is only used as a photoelectron source. The main advantages are lower

production costs and a higher energy/cost ratio.

A dye-sensitized solar cell (DSSC, DSC or DYSC) is a class of low-cost solar cell belonging to the

group of thin film solar cells. It is based on a semiconductor formed between a photo-sensitized anode

and an electrolyte; a photoelectrochemical system. This cell was invented by Michael Grätzel and Brian

O'Regan at the École Polytechnique Fédérale de Lausanne in 1991[3] and are also known as Grätzel

cells.Michael Grätzel won the 2010 Millennium Technology Prize for the invention of the Grätzel cell.

Because it is made of low-cost materials and does not require elaborate apparatus to

manufacture, this cell is technically attractive. Likewise, manufacture can be significantly less expensive

than older solid-state cell designs. It can also be engineered into flexible sheets and is mechanically

robust, requiring no protection from minor events like hail or tree strikes. Although its conversion

efficiency is less than the best thin-film cells, in theory its price/performance ratio

(kWh/(m2·annum·dollar)) should be high enough to allow them to compete with fossil fuel electrical

generation by achieving grid parity. Commercial applications, which were held up due to chemical

stability problems, are now forecast in the European Union Photovoltaic Roadmap to significantly

contribute to renewable electricity generation by 2020.

Grätzel’s cell is composed of a porous layer of titanium dioxide nanoparticles, covered with a

molecular dye that absorbs sunlight, like the chlorophyll in green leaves. The titanium dioxide is

immersed under an electrolyte solution, above which is a platinum-based catalyst. As in a conventional

alkaline battery, an anode (the titanium dioxide) and a cathode (the platinum) are placed on either side

of a liquid conductor (the electrolyte).

Sunlight passes through the transparent electrode into the dye layer where it can excite

electrons that then flow into the titanium dioxide. The electrons flow toward the transparent electrode

where they are collected for powering a load. After flowing through the external circuit, they are re-

introduced into the cell on a metal electrode on the back, flowing into the electrolyte. The electrolyte

then transports the electrons back to the dye molecules.

Dye-sensitized solar cells separate the two functions provided by silicon in a traditional cell

design. Normally the silicon acts as both the source of photoelectrons, as well as providing the electric

field to separate the charges and create a current. In the dye-sensitized solar cell, the bulk of the

semiconductor is used solely for charge transport, the photoelectrons are provided from a separate

photosensitive dye. Charge separation occurs at the surfaces between the dye, semiconductor and

electrolyte. The dye molecules are quite small (nanometer sized), so in order to capture a reasonable

amount of the incoming light the layer of dye molecules needs to be made fairly thick, much thicker

than the molecules themselves. To address this problem, a nanomaterial is used as a scaffold to hold

large numbers of the dye molecules in a 3-D matrix, increasing the number of molecules for any given

surface area of cell. In existing designs, this scaffolding is provided by the semiconductor material, which

serves double-duty.

Research is also being done into a streamlined form of photosynthesis that breaks water into

oxygen and hydrogen.[11] This process is called photoelectrolysis. This process is the first stage of plant

photosynthesis (the light-dependent reaction). Carbon dioxide is not consumed in this process. The

hydrogen released by the eletrolysis could be used immediately to generate electricity or could be

stored and used as a fuel.

[11]Dye- Synthesized cell: Penn State Research. Retrieved January 11, 2010 from: www.wikipedia.com

NADP+/NADPH Coenzyme

The coenzyme, behaving in a cyclic manner, goes between picking up a proton and

two electrons. It then delivers the hydride to an area where they await the production of carbohydrates.

The coenzyme in a natural photosynthetic cycle is recyclable, however the problem is this process

cannot yet be replicated in a laboratory.

Right now the main aspiration for scientists is to obtain an NADPH-inspired catalyst capable of

recreating the natural cyclic process. Utilizing light, hydride donors will be regenerated as well as

produced where the molecules are continuously used. Brookhaven chemists are now using a ruthenium-

based complex to serve as the acting model. The complex is proven to perform correspondingly with

NADP+/NADPH, behaving as the foundation for the proton and two electrons needed to

convert acetone to isopropanol.

Currently the Brookhaven researchers are aiming to find ways to have light generate the hydride

donors. The general idea is to use this theory to produce fuels from carbon dioxide.[12]

Photocatalytic Water Splitting Under Solar Light

In the overall reaction of photosynthesis, plants transform water and carbon dioxide in the

presence of light into oxygen and carbohydrates. In effect then, H2O is split into O2 and H2, where the

hydrogen is not in the gaseous form but bound by carbon.

6CO2 + 6H2O Light C6H12O6 + 6O2

The aim of artificial photosynthesis is the light-driven splitting of water into H2 and O2, which has been

called a ‘holy grail’ in chemistry. Water represent a plentiful energy resource, which, in a

thermodynamically uphill reaction (ΔG ≈237.2 kJ/mol), is converted into a clean and storable fuel (H2)

with sunlight.2H2O Light O2 + 2H2.The photo electrochemical (PEC) path to water splitting involves

separating the oxidation and reduction processes into half-cell reactions.

[12] NADP+/NADPH Coenzyme: Karen Walsh 27 Mar 2007 Building a Bio-inspired Catalytic Cycle for Fuel Production. Retrieved

January 11, 2010 from: www.wikipedia.com

In the half-cell reactions with their corresponding standard reduction potential E° with respect to the

standard hydrogen electrode (SHE) are shown. The equation below shows the overall reaction and the

corresponding ΔE°. The negative ΔE° indicates that water splitting is not a thermodynamically

spontaneous process.

For the reaction to proceed 1.23 V must be provided externally.

Oxidation: 2H2O Light O2 + 4H+ + 4e– E° = 1.23 V vs. SHE

Reduction: 2H+ + 2e– Light H2 E° = 0.00 V vs. SHE

Overall: 2H2O Light O2 + 2H2 ΔE° = −1.23 V

For that purpose, materials are necessary which upon light absorption can drive the water

splitting reaction. Three fundamental requirements should be met by any system harvesting and

converting solar energy into chemical energy:

i) The photo response of the system must optimally match the solar spectrum;

ii) Photo excited charges must be separated efficiently to prevent recombination;

iii) The charges must have sufficient energy to carry out the desired chemical reactions such as

water splitting.

Sustainable hydrogen production is a key target in the development of alternative energy

systems of the future for providing a clean and affordable energy supply. The conversion of solar energy

into hydrogen via a water-splitting process assisted by photosemiconductor catalysts is one of the most

promising technologies for the future because large quantities of hydrogen can potentially be generated

in a clean and sustainable manner. The conversion of solar energy into a clean fuel (H2) under ambient

conditions is one of the greatest challenges facing scientists in the twenty-first century. This process is

assisted by photocatalysts suspended directly in water instead of using photovoltaic and an electrolytic

system, therefore the reaction is in just one step and it can be more efficient than Photoelectrochemical

water splitting[13[14]

[13] Photocatalytic Water Splitting Under Solar Light: del Valle, F. et al (Jun 2009). "Water Splitting on Semiconductor Catalysts

under Visible- Light Irradiation". CHEMSUSCHEM(CHEMSUSCHEM) 2 (6): 471–485. doi:10.1002/cssc.200900018. Retrieved

January 11, 2010 from: www.wikipedia.com

[14] Photocatalytic Water Splitting Under Solar Light: del Valle, F. et al (2009). "“Photocatalytic water splitting under visible

Light: concept and materials requirements”". ADVANCES IN CHEMICAL ENGINEERING (ScienceDirect) 36: 111–

143.doi:10.1016/S0065-2377(09)00404-9. Retrieved January 11, 2010 from: www.wikipedia.com

Potential Global Impact

Artificial photosynthesis is a renewable, carbon-neutral source of fuel, producing either

hydrogen, or carbohydrates. This sets it apart from the other popular renewable energy sources —

hydroelectric, solar photovoltaic, geothermal, and wind — which produce electricity directly, with no

fuel intermediate. [15]

Artificial photosynthesis could offer a new, possibly ideal way out of our energy predicament.

For one thing, it has benefits over photovoltaic cells, found in today's solar panels. The direct conversion

of sunlight to electricity in photovoltaic cells makes solar power a weather- and time-dependent energy,

which decreases its utility and increases its price. Artificial photosynthesis, on the other hand, could

produce a storable fuel.

And unlike most methods of generating alternative energy, artificial photosynthesis has the

potential to produce more than one type of fuel. The photosynthetic process could be tweaked so the

reactions between light, CO2 and H2O ultimately produce liquid hydrogen. Liquid hydrogen can be used

like gasoline in hydrogen-powered engines. It could also be funnelled into a fuel-cell setup, which would

effectively reverse the photosynthesis process, creating electricity by combining hydrogen and oxygen

into water. Hydrogen fuel cells can generate electricity like the stuff we get from the grid, so we'd use it

to run our air conditioning and water heaters.

Methanol is another possible output. Instead of emitting pure hydrogen in the photosynthesis

process, the photo electrochemical cell could generate methanol fuel (CH3OH). Methanol, or methyl

alcohol, is typically derived from the methane in natural gas, and it's often added to commercial gasoline

to make it burn more cleanly.

As such, artificial photosynthesis may become a very important source of fuel for transportation.

Moreover,unlike biomass energy, it does not require arable land, and so it need not compete with the

food supply. Since the light-independent phase of photosynthesis fixes carbon dioxide from the

atmosphere, artificial photosynthesis may provide an economical mechanism for carbon sequestration,

reducing the pool of CO2 in the atmosphere, and thus mitigating its effect on global warming.

Specifically, net reduction of CO2 will occur when artificial photosynthesis is used to produce carbon-

based fuel which is stored indefinitely.

[15] Helmholtz Association of German Research Centres (2008, March 26) Artificial Photosynthesis Moves A Step Closer.

Retrieved January 11, 2010 from: www.wikipedia.com

Thus, the ability to produce a clean fuel without generating any harmful by-products, like

greenhouse gasses, makes artificial photosynthesis an ideal energy source for the environment. It

wouldn't require mining, growing or drilling. And since neither water nor carbon dioxide is currently in

short supply, it could also be a limitless source, potentially less expensive than other energy forms in the

long run. In fact, this type of photo electrochemical reaction could even remove large amounts of

harmful CO2 from the air in the process of producing fuel. [16] [17]

Challenges in Creating Artificial Photosynthesis

While artificial photosynthesis works in the lab, it's not ready for mass consumption. Replicating

what happens naturally in green plants is not a simple task.

Efficiency is crucial in energy production. Plants took billions of years to develop the

photosynthesis process that works efficiently for them; replicating that in a synthetic system takes a lot

of trial and error.

The manganese that acts as a catalyst in plants doesn't work as well in a man-made setup,

mostly because manganese is somewhat unstable. It doesn't last particularly long, and it won't dissolve

in water, making a manganese-based system somewhat inefficient and impractical. The other big

obstacle is that the molecular geometry in plants is extraordinarily complex and exact -- most man-made

setups can't replicate that level of intricacy.

Stability is an issue in many potential photosynthesis systems. Organic catalysts often degrade,

or they trigger additional reactions that can damage the workings of the cell. Inorganic metal-oxide

catalysts are a good possibility, but they have to work fast enough to make efficient use of the photons

pouring into the system. That type of catalytic speed is hard to come by. And some metal oxides that

have the speed are lacking in another area -- abundance.

[16] Potential Global Impact: Celia Clark 18 May 2007 Interview with Pill-Soon Song. Retrieved January 11, 2010 from:

www.wikipedia.com

[17] How Artificial Photosynthesis Works by Julia Layto. Retrieved January 11, 2010

from:http://science.howstuffworks.com/environmental/green-tech/energy-production/artificial- photosynthesis2.htm

In the current state-of-the-art dye-sensitized cells, the problem is not the catalyst; instead, it's

the electrolyte solution that absorbs the protons from the split water molecules. It's an essential part of

the cell, but it's made of volatile solvents that can erode other components in the system.

Advances in the last few years are starting to address these issues. Cobalt oxide is a stable, fast

and abundant metal oxide. Researchers in dye-sensitized cells have come up with a non-solvent-based

solution to replace the corrosive stuff. Research in artificial photosynthesis is picking up steam, but it

won't be leaving the lab any time soon. It'll be at least 10 years before this type of system is a reality.

And that's a pretty hopeful estimate.[18]

[18] How Artificial Photosynthesis Works by Julia Layto. Retrieved January 11, 2010 from:http://science.howstuffworks.com/environmental/green-tech/energy-production/artificial- photosynthesis3.htm

References

[1] How Artificial Photosynthesis Works by Julia Layto. Retrieved January 11, 2010 from:

http://science.howstuffworks.com/environmental/green-tech/energy-production/artificial-

photosynthesis.htm

[2]Artificial Photosynthesis: Turning Sunlight Into Liquid Fuels Berkley Laboratory by Lynn Yarris (10 Mar

2009)

[3] How Artificial Photosynthesis Works by Julia Layto. Retrieved January 11, 2010 from:

http://science.howstuffworks.com/environmental/green-tech/energy-production/artificial-

photosynthesis1.htm

[4] Photoelectrochemical cell by Paul Kögerler. Retrieved January 11, 2010 from: www.wikipedia.com

[5] Photoelectrochemical cel:l Cédric Tard, Xiaoming Liu, Saad K. Ibrahim, Maurizio Bruschi, Luca De

Gioia, Siân C. Davies, Xin Yang, Lai-Sheng Wang, Gary Sawers and Christopher J. Pickett Nature (10

Feb 2005) 433, 610 - 613. Retrieved January 11, 2010 from: www.wikipedia.com

[6] Photoelectrochemical cell :Science, 31 August 2001. Retrieved January 11, 2010 from:

www.wikipedia.com

[7] Photoelectrochemical cell:Hu, Xile; Cossairt, Brandi M.; Brunschwig, Bruce S.; Lewis, Nathan S.;

Peters, Jonas C. Chem. Commun., 2005 37, 4723-4725. Retrieved January 11, 2010 from:

www.wikipedia.com

[8] Advantages Photoelectrochemical cell:Molly Lachance 26 Oct 2008 AF Funding Enables Artificial

Photosynthesis. Retrieved January 11, 2010 from: www.wikipedia.com

[9] Advantages Photoelectrochemical cell: G24innovations claiming 1/5th of the price of silicium cells.

Retrieved January 11, 2010 from: www.wikipedia.com

[10] Advantages Photoelectrochemical cell: Anne Trafton (30 July 2008) Major Discovery from MIT

Primed to Unleash Solar Revolution. Retrieved January 11, 2010 from: www.wikipedia.com

[11]Dye- Synthesized cell: Penn State Research. Retrieved January 11, 2010 from: www.wikipedia.com

[12] NADP+/NADPH Coenzyme: Karen Walsh 27 Mar 2007 Building a Bio-inspired Catalytic Cycle for Fuel

Production. Retrieved January 11, 2010 from: www.wikipedia.com

[13] Photocatalytic Water Splitting Under Solar Light: del Valle, F. et al (Jun 2009). "Water Splitting on

Semiconductor Catalysts under Visible- Light Irradiation". CHEMSUSCHEM(CHEMSUSCHEM) 2 (6):

471–485. doi:10.1002/cssc.200900018. Retrieved January 11, 2010 from: www.wikipedia.com

[14] Photocatalytic Water Splitting Under Solar Light: del Valle, F. et al (2009). "“Photocatalytic water

splitting under visible Light: concept and materials requirements”". ADVANCES IN CHEMICAL

ENGINEERING (ScienceDirect) 36: 111–143.doi:10.1016/S0065-2377(09)00404-9. Retrieved January

11, 2010 from: www.wikipedia.com

[15] Helmholtz Association of German Research Centres (2008, March 26) Artificial Photosynthesis

Moves A Step Closer. Retrieved January 11, 2010 from: www.wikipedia.com

[16] Potential Global Impact: Celia Clark 18 May 2007 Interview with Pill-Soon Song. Retrieved January

11, 2010 from: www.wikipedia.com

[17] How Artificial Photosynthesis Works by Julia Layto. Retrieved January 11, 2010

from:http://science.howstuffworks.com/environmental/green-tech/energy-production/artificial-

photosynthesis2.htm

[18] How Artificial Photosynthesis Works by Julia Layto. Retrieved January 11, 2010

from:http://science.howstuffworks.com/environmental/green-tech/energy-production/artificial-

photosynthesis3.htm