ΦΩΤΟΧΗΜΕΙΑ

Heat Electricity Electromagnetic

irradiation (light)

ENERGY

Introduction

Chemical bond energies:

from 100 – 1000 kJ/mol

Light energies:

604 kJ/mol-1 302 151

200 nm 400 nm 800 nm

ULTRAVIOLET VISIBLE INFRARED

So UV – and VIS region is expected to induce

chemical reactions.

Chemical reactions accompanied with light

1. Action of light → chemical change

(light induced reactions)

2. Chemical reaction → light emission

(chemiluminescence)

- Chemiluminescence:

P4 (g) + O2 (g)+H2O (g) P4 O10 + hn

green

- Bioluminescence: - mushrooms

- insects

- fish

Luminescence:

Light: electromagnetic field vibration

spreading in quanta

(photons)

Photon: the smallest amount of light

carrying energy

Definitions and terms

h = Planck’s constant (6.6 · 10-34 Js)

c = speed of light (3 · 108 ms-1)

l = wavelength

n = frequency

E = c

lhn h=

Deviations from the Beer-Lambert Law

The Beer-Lambert law assumes that all

molecules contribute to the absorption and

that no absorbing molecule is in the shadow

of another

Low

c

High

c

Energy level diagram

3210

S0

0123

S1

0123

S2

0123

T2

0123

T1

Absorption & Emission

l E = hc / l

Laws of Photochemistry Grotthus-Draper Law:

Μόνο το φως που απορροφάται από ένα μόριο είναι αποτελεσματικό στο να δημιουργεί φωτοχημικές αλλαγές στο

μόριο.

Stark-Einstein’s Law ( Second Law of Photochemistry):

Αναφέρει ότι για κάθε φωτόνιο που απορροφάται από ένα χημικόσύστημα, μόνο ένα μόριο ενεργοποιείται για φωτοχημική αντίδραση.Η ενέργεια που απορροφάται από ένα γραμμομόριο αντιδρώντων

μορίων δίνεται από E=Nhv. Αυτή η ενέργεια αντιστοιχεί σε ένα (1)einstein.

Mechanisms of Light Absorption

Excitation:

X2hn *X2

A bonding electron is lifted to a higher energy level

(higher orbital)

INTERACTION OF LIGHT AND MATERIALS:

a) X2* → X2 + M* (excess energy transferred to

the surrounding)

b) X2* → X2 + hn (fluorescence or phosphorescence)

c) X2* + Y → chemical reaction (excess energy

supplies the activation energy of the

reaction)

How to Utilize the Energy Content?

• If excited states channel their energy into specific bonds, then

photochemistry can occur.

• If scavengers or quenchers can find the excited state or free

radical in time, then the electronic or chemical energy can be

captured by the, ordinarily, stable scavenger or quencher.

Intermolecular Excited-State Reactions

• Energy Transfer

D* + Q D + Q*

• Electron Transfer

D* + A D+ + A

D + A* D+ + A-

• Hydrogen Abstractions

Note:

Have to have excited states that live long enough to find quenching partner by diffusion

A*(S1) + Q A(S0) + Q*

A(S1)* + Q ( A+...Q) A(S0) + Q

( A...Q+) A(S0) + Q

Processes from S1 state:

- fluorescence (F)

- internal conversion (IC)

- intersystem crossing (ISC) S1 T1

- chemical reaction (RS)

- quenching (+Q):

- phosphorescence (P)

- intersystem crossing (ISC) T1 S0

- chemical reaction (RT)

- quenching (+Q)

Processes from T1 state:

Physical and chemical properties of molecules

in the excited states

Συμπέρασμα: Μόρια στις διηγερμένες καταστάσεις χαρακτηρίζονται από

διαφορετικές φυσικές και χημικές ιδιότητες, σε σχέση με αυτές στη βασική

κατάσταση. Δρουν ως ξεχωριστά χημικά είδη

1. Energy (80 400 kJ/mol)

2. Lifetimes (1012 100 s)

3.Geometry of excited molecules ( bond lengths, angles)

4. Dipole moments (redistributions of electron densities)

5. Chemical properties (photochemical reactions)

Jablonski diagram

Lifetimes:

0.0

0.5

1.0

tS

1/e

SeS[S 11

t

0][]0][

]

1

1

S

[S

Lifetimes:

Lifetime of a molecular entity, which decays by first-order kinetics, is the time needed for a

concentration of the entity to decrease to 1/e of its original value, i.e., c(t =) = c(t = 0)/e.

It is equal to the reciprocal of the sum of the first-order rate constants of all processes causing

the decay of the molecular entity.

][

11

QSS

qrISCICf

i

i kkkkkk

][

11''' QTT

qrISCp

i

i kkkkk

Excited Singlet - state Lifetime

-dS/dt = kολ [S1]

The competing intramolecular photophysical processes that can

occur from S1 are fluorescence, intersystem crossing and internal

conversion, with first - order rate constants of kf , kisc and kic

SeS[S 11

t

0][]rate of disappearance

Το «μείον» σημαίνει ότι έχουμε κατανάλωση της απλής κατάστασης

SeS[S 11

t

0][]

Για χρόνο t = τs, τότε [S1] = [S1]0 / e

The excited singlet - state radiative lifetime, 1τ0 , of S1 is the lifetime of S1 in

the absence of any radiationless transitions; that is, the only deactivation

process is fluorescence. 1τ0 is the reciprocal of the rate constant for fluorescence,

kf :

Excited Singlet - state Radiative Lifetime

1τ0 = 1 / kf

Since 1ktotal is greater than kf , the observed excited singlet - state lifetime

is less than the excited singlet - state radiative lifetime.

1τ only approaches 1τ0 as intersystem crossing and internal conversion from

S1 become much slower processes than fluorescence.

Quantum yields :

Number of defined events occurring per photon absorbed by the system.

Integral quantum yield:

For a photochemical reaction A B :hv

R amount of reactant consumed or product formed

amount of photons absorbed

number of events

number of photons absorbed

Κινητικός ανταγωνισμός των διαδικασιών απενεργοποίησης

Κβαντική απόδοση μιας διαδικασίας: M + hv P

αριθμός των μορίων που παράγονται (ανά μονάδα χρόνου ...)

αριθμός των κβάντα φωτός που απορροφούνται (ανά μονάδα χρόνου,...)

Κβαντική απόδοση φθορισμού: M + hv Μ* Μ + hv’

αριθμός των φωτονίων hν που απορροφούνται (ανά μονάδα χρόνου ...)

αριθμός των φωτονίων hν’ που εκπέμπονται (ανά μονάδα χρόνου…)

So, the fluorescence quantum yield , φf , is the fraction of excited molecules that

fluoresce. This is given by the rate of fluorescence, Jf , divided by the rate of

absorption, Jabs

Under conditions of steady illumination, a steady state will be reached, where the

rate of formation of excited molecules is equal to the rate of deactivation by the

intramolecular processes:

Φf = Jf / Jabs = kf [S1] / ktotal [S1] Φf = kf / ktotal

Φf = τ / τ0

Jabs = 1Jtotal

The intramolecular processes responsible for radiative and

radiationless deactivation of excited states we have considered so far

have been unimolecular processes; that is, the processes involve only

one molecule and hence follow first - order kinetics.

If the excited molecules are deactivated and the fluorescence stops,thephenomenon is called ‘Quenching’.

When the activated molecules undergo a change from a singlet excited state to triplet excited state.This is called ‘internal quenching’.

When the activated molecules collide with the other molecules/quenchers which are the externally added species and transfer their energy to those molecules.This is called ‘external quenching’.

Διαμοριακές αποδιεγέρσεις

QUENCHING PROCESSES

Molecular oxygen is a very efficient quencher, such that in any quantitative

work it is necessary to exclude oxygen either by bubbling or freeze-pump-thaw

Quenching is a bimolecular process; that is, it involves the collision of both S1

and Q molecules. Thus, in the presence of the quencher, where the rate

constant is kQ and the rate of deactivation by quenching is QJ:

QJ = kQ [S1] [Q]

QJtotal = QJ + 1Jtotal =kQ [S1] [Q] + ktotal [S1]

Συνολικός ρυθμός αποδιέγερσης της S1 (μονομοριακές/διμοριακές)

The fluorescence quantum yields in the presence and absence of a quencher,

Qφf and φf respectively, are:

QΦf = Jf / QJtotal = kf [S1] / ktotal [S1] + kQ[S1][Q]) = kf / ktotal + kQ[Q])

Φf = kf / ktotal

The ratio of the two quantum yields leads to the Stern – Volmer equation :

Φf / QΦf = (ktotal + kQ[Q]) / ktotal = 1 + kQ[Q] / ktotal = 1 + kQ1τ[Q]

or

Φf / QΦf = 1 + KQ[Q]

KQ is the Stern – Volmer quenching constant

hnX2 X + X (photodissociation)

2.

(energy of the photon supplies the „dissociation heat”)

Types of photochemical reactions:

a) Photodissociation

b) Photosynthesis: when a larger molecule is

formed from simple ones

c) Photosensitized reactions: when an excited

molecule supplies activation energy for the

reactants

Photochemical transformation reactions

• Direct photolysis = transformation of a compound due to its

absorption of UV light

• Indirect photolysis = transformation of a compound due to

its interaction with a reactant generated by the influence of

UV light (photosensitizer or reactive oxygen species)

Photodissociation

Photolysis of hydrogen bromide

HBrnh

H + Br (photochemical reaction)

H + HBr H2 + Br

Br + Br Br2

(dark reactions)

Overall:

2HBrhn H2 + Br2

Note:

1 photon absorbed, 2 molecules of HBr dissociated:

QUANTUM YIELD = 21

= 2

number of molecules undergoing the processnumber of quanta absorbed

=

Ozone formation in the atmosphere

(at about 25 km altitude)

O2 O + O (l<240 nm)nh

2O2 + 2O (+M) 2O3 (+M*)

Notes: M absorbs energy released in the reaction

QUANTUM YIELD = 21

= 2

Ozone formed in the reaction above absorbs UV

light as well:

O3 O2 + O (l<340 nm)nh

O + O3 2O2

Notes:

1.Ozone shield protects the Earth surface from

high energy UV radiation (of the Sun)

2.Air pollution (freons: fully halogenated hydrocarbons;

nitrogen oxides emitted by aeroplanes etc.) may

accelerate the decomposition of ozone ozone hole

Photosynthesis

The photosynthesis of hydrogen chloride

Overall reaction:

Cl2 + H2 2HCl [no reaction in darkness]

Mechanism:

hnCl2

< 500 nm2Cl Photochem. initiation

Cl + H2 HCl + H Dark reactions

H + Cl2 HCl + Cl Chain reaction

H + H + M

H2 + M*

Cl2 + M* Cl + Cl + M

Recombination reactions (chainis terminated)

Note:

Quantum yield is about 106 (explosion)

DELAYED FLUORESCENCE

In certain compounds a weak emission has been observed with the same

spectral characteristics (wavelengths and relative intensities) as fluorescence,

but with a lifetime more characteristic of phosphorescence.

Two mechanisms are used to account for delayed fluorescence.

P - type delayed fluorescence is so called because it was first observed in

pyrene. The fluorescence emission from a number of aromatic hydrocarbons

shows two components with identical emission spectra.

One component decays at the rate of normal fluorescence and the other has

a lifetime approximately half that of phosphorescence.

The implication of triplet species in the mechanism is given by the fact that the

delayed emission can be induced by triplet sensitisers.

1. απορρόφηση: S0 + h ν → S1

2. διασυστημική διασταύρωση: S1 → T1

3. εκμηδένιση τριπλής-τριπλής: T1 + T1 → X → S1 + S0

4. ύστερος φθορισμός: S1 → S0 + h ν

Προτεινόμενος μηχανισμός

It is the S1 state produced by the triplet – triplet annihilation process that

is responsible for the delayed fluorescence.

Although it is emitted at the same rate as normal fluorescence, its decay is

inhibited because it continues to be regenerated via step 3.

E - type delayed fluorescence is so called because it was first observed in eosin.

Thermal activation from T1 to S1

The intensity of the delayed fluorescence emission from eosin decreases as the

temperature is lowered and this indicates that an energy barrier is involved.

Since the delayed fluorescence is spectrally identical to normal fluorescence,

emission must occur from the lowest vibrational level of S1 .

However, the fact that the lifetime is characteristic of phosphorescence implies

that the excitation originates from T1 .

The explanation of this requires a small S1 – T1 energy gap, where T1 is initially

populated by intersystem crossing from S1 .

T1 to S1 intersystem crossing then occurs by thermal activation.

ΔΙΑΜΟΡΙΑΚΗ ΜΕΤΑΦΟΡΑ ΕΝΕΡΓΕΙΑΣ

Η ενέργεια διέγερσης ενός μορίου D (δότης ενέργειας) μπορεί να μεταφερθεί σε ένα

άλλο μόριο Α (δέκτης ενέργειας) είτε μέσω μιας ακτινοβόλου ή μη ακτινοβόλου

μηχανιστικής πορείας.

Η ακτινοβόλος μεταφορά ενέργειας προφανώς μπορεί να συμβεί μόνο εάν

ικανοποιείται η αρχή διατήρησης της ενέργειας. Η προϋπόθεση αυτή είναι αληθινή

εάν το ολοκλήρωμα επικάλυψης μεταξύ των φασμάτων φθορισμού του δότη και της

απορρόφησης του δέκτη να είναι μη μηδενική: J ≠ 0

Η ακτινοβόλος μεταφορά ενέργειας συνεπάγεται την απενεργοποίηση της

ακτινοβολίας του μορίου του δότη (φθορισμός) και την επακόλουθη

επαναπορρόφηση της εκπεμπόμενης ακτινοβολίας από το μόριο του δέκτη. Η

πιθανότητα της μεταφοράς ενέργειας (σταθερά ταχύτητας) δίνεται από τη σχέση: kET

[Α].ℓ.J, όπου J είναι το ολοκλήρωμα της φασματικής επικάλυψης, [Α] είναι η

συγκέντρωση του δέκτη, και ℓ είναι το πάχος του δείγματος

Μηχανισμός με εκπομπή-απορρόφηση ενέργειας. Η διέγερση του δέκτη γίνεται χάρη στην

απορρόφηση της ακτινοβολίας που εξέπεμψε ο δότης, κατά την αποδιέγερση του τελευταίου

με φωσφορισμό ή φθορισμό.

Ο μηχανισμός αυτός θεωρείται μικρής σημασίας.

Χαρακτηριστικό παράδειγμα είναι η εκπομπή ακτινοβολίας από τον ήλιο και η απορρόφησή

της από τη γη.

D*→D+ hνhν + A→ A*

Διαμοριακή μεταφορά ηλεκτρονιακής ενέργειας

Δύο μηχανισμοί της μη ακτινοβολούσαςμεταφοράς ενέργειας είναι γνωστοί:α) Ο μηχανισμός ανταλλαγής ηλεκτρονίων (Dexter)που συμβαίνει μέσω ενός διηγερμένου συμπλόκου(exciplex) και συμβαίνει όταν οι αποστάσεις μεταξύδότη-δέκτη είναι μικρές, ~ 10 Å.β) Ο μηχανισμός Förster (FRET) που είναι έναςμηχανισμός μεταφοράς ενέργειας μέσω συντονισμούπου συμβαίνει μέσω αλληλεπίδρασεων διπόλου -διπόλου.

Και οι δύο μηχανισμοί έχουν ως αποτέλεσμα τηναπόσβεση του φθορισμού της διεγερμένηςκατάστασης του δότη. Το μόριο του αποδέκτημπορεί να θεωρηθεί ως ένας φυσικόςαποσβέστης.

INTERMOLECULAR ELECTRONIC ENERGY TRANSFERSHORT - RANGE ELECTRON – EXCHANGE ENERGY TRANSFER (DEXTER)

Ο μηχανισμός ανταλλαγής (exchange mechanism) προϋποθέτει μοριακή σύγκρουση έτσι ώστε τα

ηλεκτρονιακά νέφη του δότη και του δέκτη να επικαλυφθούν και το διηγερμένο ηλεκτρόνιο να μεταφερθεί στο

μόριο του δέκτη. Αυτό που συμβαίνει είναι ταυτόχρονη ή και σε δύο στάδια ανταλλαγή ηλεκτρονίων. Το

ενδιάμεσο σύμπλοκο που σχηματίζεται, ονομάζεται διηγερμένο σύμπλοκο exciplex (excited complex).

Η ταχύτητα μεταφοράς ενέργειας με αυτό το μηχανισμό δίνεται από την ακόλουθη σχέση

(Dexter theory)

kexchange=k J exp.(-2R/L)

όπουk σταθερά αλληλεπίδρασης των τροχιακών,j ολοκλήρωμα της επικάλυψης των φασμάτων εκπομπής και απορρόφησης του δότη και δέκτη αντίστοιχα

R απόσταση μεταξύ δότη και δέκτηL ακτίνα van der Waals

Ο μηχανισμός ανταλλαγής είναι υπολογίσιμος για αποστάσεις μικρότερες των 10A.

Μηχανισμός μεταφοράς ενέργειας με συντονισμό. Ένα διηγερμένο μόριο λόγω της κίνησης

του διηγερμένου ηλεκτρονίου του, δημιουργεί γύρω του ηλεκτρικό πεδίο ανάλογο με αυτό που

δημιουργεί ένα ταλαντούμενο δίπολο.

Το πεδίο αυτό ασκεί στα ηλεκτρόνια γειτονικών μορίων ηλεκτροστατικές δυνάμεις Coulomb.

Η αλληλεπίδραση μέσω αυτών των ηλεκτροστατικών δυνάμεων είναι υπεύθυνη για τη διέγερση

ενός γειτονικού μορίου. mutual electrostatic repulsion between the electrons of the donor andacceptor molecules

LONG - RANGE DIPOLE – DIPOLE (COULOMBIC) ENERGY TRANSFER

Διαμοριακή μεταφορά ηλεκτρονιακής ενέργειαςΟ μηχανισμός της μεταφοράς ενέργειας λόγω συντονισμού Förster (διπόλου-διπόλου) (FRET) συμβαίνειμεταξύ μορίων που διαχωρίζονται από αποστάσεις που υπερβαίνουν σημαντικά το άθροισμα των ακτίνων vander Waals. Περιγράφεται από την σκοπιά των αλληλεπιδράσεων μεταξύ διπόλου - διπόλου. Η ταχύτηταμεταφοράς σταθερά kET δίνεται από την σχέση:

όπου Κ είναι ένας παράγοντας προσανατολισμού, n ο δείκτης διάθλασης του μέσου, τr0 ο χρόνος ζωής της

ακτινοβολίας του δότη, r η απόσταση [cm] μεταξύ του δότη (D) και δέκτη (Α), και το J είναι η φασματικήεπικάλυψη (σε συνεκτικές μονάδες cm6·mol-1) μεταξύ του φάσματος απορρόφησης του δέκτη και του φάσματοςφθορισμού του δότη. Η κρίσιμη ακτίνα απόσβεσης r0 είναι η απόσταση στην οποία kET = kr

0= 1 /τr0

Διαμοριακή μεταφορά ηλεκτρονιακής ενέργειας

ΜηχανισμόςFörster (διπόλου-διπόλου) (FRET)

Note that the electrons initially on D * remain on D and electrons initially on

A remain on A * .

This energy transfer does not require physical contact between the

donor and acceptor

Ο μηχανισμός μεταφοράς ενέργειας με συντονισμό συμβαίνει όταν το φάσμα απορρόφησης του δέκτη

επικαλύπτεται με το φάσμα εκπομπής του δότη

Ο μηχανισμός αλληλεπίδρασης διπόλων είναι υπολογίσιμος για μοριακές αποστάσεις μέχρι και 30A. Ένα

ενδιαφέρον παράδειγμα του μηχανισμού αυτού είναι η μεταφορά ενέργειας μεταξύ μορίων χλωροφύλλης στα

φωτοσυνθετικά κέντρα.

OZON LAYER

The presence of a high-altitude ozone layer in the atmosphere was first determined in the 1920s from observations of the

solar UV spectrum. A theory for the origin of this ozone layer was proposed in 1930 by a British scientist, Sydney Chapman,

and is known as the Chapman mechanism. It lays the foundation for current understanding of stratospheric ozone.

Chapman proposed that the ozone layer originates from the photolysis of atmospheric O2. The bond energy of the O2

molecule (498 kJ mol-1) corresponds to the energy of a 240 nm UV photon; only photons of wavelengths less than 240 nm

can photolyze the O2 molecule. Such high-energy photons are present in the solar spectrum at high altitude

Chapman oxygen - only mechanism

(slow photolysis)

(fast exothermic)

(fast photolysis)

slow exothermic

λ 240-320 nm

HOST – GUEST SUPRAMOLECULAR PHOTOCHEMISTRY

The Norrish type 1 photolysis of an asymmetrical ketone A.CO.B normally gives

a product ratio of AA : AB : BB as 25 : 50 : 25 %.

In micelles of the surfactant cetyltrimethylammonium chloride (CTAC), the ratio

of AA : AB : BB is < 1 : > 98 : < 1 %. The CTAC micelles provide a cage effect,

which greatly enhances the joining of the A and B radicals produced by the

photolysis

Photosynthesis in plants

Overall reaction:

6CO2 + 6H2O C6H12O6+6O2

carbohydrate

hn; chlorophyll

several steps

Photosynthesis occurs in two stages:

• Light reactions : light energy is converted into short - term chemical energy, producing oxygen as a by - product.

• Dark reactions : the short - term chemical energy is used to convert carbon dioxide into carbohydrate

Photosynthesis occurs in two stages. In the first stage, light-dependent reactions or light

reactions capture the energy of light and use it to make the energy-storage molecules ATP and

NADPH.

During the second stage, the light-independent reactions use these products to capture and

reduce carbon dioxide.

Notes:

1. Chlorophyll acts as a catalyst absorbing and

transferring the photon energy for reduction of

carbon dioxide to carbohydrate

2. This reaction maintains the life on the Earth:

sunlight carbohydrateCO2; H2O

Fossile energy

(coal, oil, natural gas)Food

The leaves of plants are green because they contain the primary light -absorbing pigments

called the chlorophylls , which absorb strongly in the blue and red regions of the visible

spectrum (400-450 and 650-700 nm), leaving the intermediate green wavelengths to be

reflected to our eyes

The chlorophyll molecule consists of:• A rigid, planar, conjugated porphyrin ring , which functions as anefficient absorber of light.• A hydrophobic phytol chain , which keeps the chlorophyll moleculeembedded in the photosynthetic membrane.

Accessory pigments called carotenoids , such as β - carotene, are also found in

plants. These contain an extended conjugated system and absorb principally

in the blue and green regions of the visible spectrum, while reflecting the red,

orange and yellow wavelengths.

Carotenoids complement chlorophylls in the light – harvesting process.

Χλωροφύλλη και βοηθητικές χρωστικές

Light harvesting : several hundred pigment molecules act together as the photosynthetic unit, which is made up

from the light - harvesting antenna and the reaction centre , consisting of a chlorophyll dimer.

The complex arrangement of pigment molecules held together in protein complexes by intermolecular forces

acts as the light -harvesting antenna that allows the absorption of a broad range of wavelengths.

The antenna also provides the means whereby rapid stepwise transfer of the excitation energy to the reaction

centre is achieved (Coulombic long -range mechanism). Energy is repeatedly passed between adjacent pigment

molecules such that each successive step involves energy transfer to a pigment molecule that absorbs light of

equal or longer wavelength (lower energy).

The light-dependent reaction of plants uses two photosystems. In Photosystem II, the captured

energy that reaches the reaction center chlorophyll boosts an electron to a higher orbital.

That high-energy electron is picked up by an electron acceptor in the thylakoid membrane and

transported through a series of proteins that make up an electron transport system.

As the electron moves through these proteins, it loses energy.

It then reaches a second photosystem (called Photosystem I).

The energy from the sun is absorbed by the pigments in this photosystem and again used to raise

the energy level of the electron.

Once again the electron is passed through an electron transport system giving up energy.

Ultimately, this energy is converted to ATP and NADPH which will be used in the light-independent

reactions.

Φωτοηλεκτροκαταλυτικά κελιά The nanostructured photocatalyst is a semiconductor S in contact with the

electrolyte.

• Absorption of photons generates electrons and holes S + h → S(e− + h+)

• Most photogenerated electrons, which escape recombination, flow in the

external circuit. Some may interact with O2, if present, producing superoxide

and finally hydroxyl radicals.

• Those photogenerated holes, which escape recombination, interact with

the fuel or water by the following principal reaction schemes. The fuel is

represented by CxHyOz-type compounds (alcohols, organic acids,

carbohydrates), since they are the best fuel both for electricity and hydrogen

production:

Oxidation taking place mainly at low pH

CxHyOz + (2x − z)H2O + (4x + y − 2z)h+ → xCO2 + (4x + y − 2z)H+

Oxidation taking place mainly at high pH

OH− + h+ → •OH

CxHyOz + (4x + y − 2z)•OH → xCO2 + (2x + y − z)H2O

In the absence of fuel, water itself is oxidized. The following two reactions correspond to two pH extremes.

2H2O + 4h+ →O2 + 4H+ [+1.23V vs NHE at pH = 0]

4OH− + 4h+ →O2 + 2H2O [+0.40V vs NHE at pH = 14]

Αντιδράσεις οξείδωσης στη φωτοάνοδο

ethanol→acetaldehyde→acetic acid→CO2 + H2O

Inert environment (no O2 present)

• Low pH (0.00V vs NHE at pH = 0) 2H+ + 2e− →H2

• Alkaline pH (−0.83V vs NHE at pH = 14) 2H2O + 2e− →H2 + 2OH−

Aerated electrolyte or cathode exposed to ambient air

• Low pH (1.23V at pH = 0) 2H+ + ½ O2 + 2e− →H2O

• Alkaline pH (0.40V at pH = 14) H2O + ½ O2 + 2e− →2OH−

Αντιδράσεις αναγωγής στην κάθοδο (εξάρτηση από pH και ατμόσφαιρα)

Οι ημι-αντιδράσεις οξείδωσης στη φωτοάνοδο εξισορροπούνται από τις ημι-αντιδράσεις αναγωγής στην κάθοδο

Οπότε η αντίδραση οξείδωσης στην άνοδο CxHyOz + (4x + y − 2z)•OH → xCO2 + (2x + y − z)H2O

εξισορροπείται από την αντίστοιχη στη κάθοδο 2H2O + 2e− →H2 + 2OH−

ή από την H2O + ½ O2 + 2e− →2OH−

ανάλογα αν η ατμόσφαιρα στο διάλυμα είναι αδρανής ή κορεσμένη με οξυγόνο

Σε αδρανή ατμόσφαιρα

CxHyOz + (2x − z)H2O → xCO2 + (2x + y/2 − z)H2

Παρουσία οξυγόνου

CxHyOz + (x + y/4 − z/2)O2 → xCO2 + y/2H2O

ΣΥΝΟΛΙΚΑ…

Oxidation then of an organic fuel or water itself leads to current flow

and hydrogen production in an inert environment while in the presence

of oxygen only electricity is produced. As it will become clear below, since

in most studied cases hydrogen is produced under applied external electric

bias, it makes no practical sense to talk about simultaneous production of

hydrogen and electricity. Electricity is practically produced only in the

presence of oxygen.

Εκκινούσα δύναμη

For semiconductor photocatalysts, the photoanode

potential is close to the potential of the conduction

band (CB) of the semiconductor. The cathode

electrode potential should be approximately equal to

the redox potential of the reduction reaction.

Therefore, the driving force is analogous to the

difference between the CB of the photocatalyst and

the redox potential of the reduction reaction.

CB and VB levels of a few popular semiconductor photocatalysts

in contrast with reduction potentials in aqueous environment.

The shown photocatalysts have a CB which is slightly more

negative (TiO2), equal (BiVO4) or even more positive (WO3 or

Fe2O3) than hydrogen production level.

Even in the case of titania, the favorable difference is eliminated

by inevitable losses.

Therefore, electrons can hardly flow from the photoanode

to cathode electrode unless an external bias is applied.

Photoelectrocatalytic hydrogen production is realized only under external bias. On the contrary, if the cathode

electrode is aerated, then oxygen may be reduced at a potential which is as positive as 1.23 V vs SHE.

Photoelectrocatalysis cell operating under external electric bias and producing hydrogen (a) and operating without bias, producing electricity (b)

Among the four photocatalysts, TiO2 and WO3

can effectively function in a PFC and produce electricity.

TiO2 and WO3 have a CB which is more negative than the

oxygen reduction level, therefore, they can function

without bias, and have a VB which is more positive than

the hydroxyl radical formation level, and therefore, they

can efficiently oxidize any organic fuel.

Oxidation reactions are facilitated by hydroxyl radicals since the target molecules are in the bulk solution and only a limitednumber may be attached on the photocatalyst surface

The first step is the absorption of a photon by the sensitizer S, leading to the excited sensitizer S which

injects an electron into the conduction band of the semiconductor, leaving the sensitizer in the oxidized state

S+.

The injected electron flows through the semiconductor network to arrive at the back contact and then

through the external load to the counter electrode to reduce the redox mediator which in turn regenerates

the sensitizer.

The regeneration of the sensitizer by iodide intercepts

the recapture of the conduction band electron by the

oxidized dye.

The voltage generated under illumination corresponds

to the difference between the Fermi level of the electron

in the solid and the redox potential of the electrolyte.

Overall the device generates electric power from light

without suffering any permanent chemical transformation.