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1. INTRODUCTION

The word photocatalysis is derived from Greek language in which the first word photo means

any light and catalysis means to tear apart. Photocatalysis in general sense means to enhance the

rate of photochemical process by using certain materials which are activated in the presence of

light called photocatalyst. Without light, it is not possible to carry out photocatalysis. The whole

photocatalytic process is depended on the creation of electrons and holes pairs which further

generate radicals like hydroxyl radicals. These radicals undergo further reaction to produce final

product. First maor application of Photocatalysis came into existence when it was found that

water can be electrolysed by using photocatalyst like titanium dioxide and !inc oxide. "uring

research it was found that semiconductors are best suited to be used as photocatalyst because

they can be easily excited and they have good light absorbing capacity. Pure semiconductors

have less conductivity. #o doping is done to increase their conductivity. #mall band gap exists in

case of doped semiconductors. The electron$hole recombination process is found to be extremely

low in case of semiconductors. #o they exchange charge with the substance present on its surface

and hence chemical reaction occur and we get final product%&'.

1. PHOTOCHEMISTRY

(ccording to Grotthuss, photochemistry is most important branch of chemistry which deals with

interactions between atoms, molecules and light )infrared, visible, ultraviolet, vacuum ultraviolet

light*. +ommon examples which we daily see related to photochemical reaction are

photosynthesis and plastic degradation. xcited species are created when light is absorbed by

photocatalytic material. They undergo different reactions like-%'.

&* /nimolecular reactions- "issociation, 0oni!ation and 0someri!ation

* 1imolecular reactions- a* 2eactions between two molecules

2.1 ADVANTAGES OF PHOTOCHEMISTRY

The main difference between photochemical and thermal reaction is that rate of

photochemical reaction increases very fast once it is initiated.

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#ome products are impossible to produce without photocatalysis .

They are eco friendly as they do not create pollution.

They are energy saving

0t re3uires least chemicals

2.2 LAWS OF PHOTOCHEMISTRY

&st law of photochemistry-

This law was proposed by two scientists. They are Grotthuss and "raper. The law statement in

general language is given as 4(bsorption of light is basic re3uirement for initiation of

photochemical process.5

nd law of photochemistry-

This law was also proposed by two scientists. They are #tarck and instein. The general

statement of this law is4When one photon is absorbed then only one molecule is excited

%6'.5

2.3 QUANTUM YIELD

0t is the ratio of number of moles of reactant disappearing or product formed to the insteins of

light absorbed. 7echanism of reactions can be easily studied with the help of 3uantum yield. 0t

gives an idea of effectiveness of a photochemical process occurs in a given conditions. "ue to

the complex nature of some photochemical reaction, their 3uantum yield is always known before

so that there is no problem during studying those reactions.

2.3.1 TYPES OF QUANTUM YIELD Primary 3uantum yield

Product 3uantum yield

8uantum yield of fluorescence

8uantum yield of Phosphorescence

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8uantum yield of "ecomposition

8uantum yield of 2earrangement

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0n general sense, 3uantum yield of primary reaction is always less than &.0n some reactions such

as chain reactions, rate of formation of product is higher because of production of free radicals

and hence they have 3uantum yield greater than one .#mall 3uantum yields indicate deactivation,

fluorescence or other processes that lead to a small chemical change. 9arge 3uantum yields is

the indication of the photochemical change forming the products.

2. BASIC CONCEPT OF PHOTOCATALYSIS

(n electronic property of different materials is given by band theory. We know that in solids

atoms are closely spaced. (s a result their energy levels are very near to each other which results

in the formation of band. ach band is characteri!ed by different energy and filling of electrons

takes place from lowest energy level to highest energy level. :alence band has highest energy of

all bands in which electrons are filled. (bove the valence band, there lies conduction band. 1and

gap separated valence band from conduction band. When electric field is applied to solids,

electrons will be excited from valence band to conduction band. 9arge band gap exists in case of

insulators so electrons cannot ump from valence to conduction band. 0n case of metals, due to

small band gap or absence of band gap, electrons can be easily excited from valence to

conduction band. ;ow in semiconductors, band gap is moderate which means it is in between

conductors and insulators. 1y irradiating it with light, electrons can easily be excited to

conduction band %


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Fig 6.& nergy band gap in &* insulator * semiconductor 6* conductor %='

2.1 MECHANISM OF PHOTOCATALYSIS

Pairs of electrons and holes are formed when titanium dioxide absorbs ultraviolet light . When

light is passed through titanium dioxide material, electrons present in valence band excites to

conduction band. "ue to this, holes are created and negative charge is developed in conduction

band. Titanium dioxide is then said to be photoexcited. 9ight source having wavelength around

6>? nm is re3uired for this purpose. @ydrogen gas is released when holes created by photo

excitation reacts with molecules of water. lectrons having negative charge react with

atmospheric oxygen to produce oxide anion. The whole process is continuously repeated until

light source is available.

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Fig 6. 7echanism of photocatalysis %A'

#+ B hv C #+D

#+D C e$ B hB

hB

B @E C@B

B E@⁰

# B E@⁰ C Products

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Fig 6.6 Photocatalysis mechanism in TiE %'

2.2 TYPES OF PHOTOCATALYSIS

Photocatalysis is generally divided into two main types. They are as follows-$

)&* @omogeneous photocatalysis

)* @eterogeneous photocatalysis

3.2.1 HOMOGENEOUS PHOTOCATALYSIS

0n this first type, reactants and products are in the same phase. Two most commonly used

homogeneous photocatalysts in todays modern world are photo$Fenton system and o!one.

@ydroxyl radical is the main reactant which carry out many functions. There are two paths which

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o!one follows for production of radicals of hydroxyl. They are-

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E6 B hv C E B E

E B @E C HE@ B H E@

E B @E C @E

@E B hv C HE@ B H E@

#imilar to above process, this process forms hydroxyl radical by another way which is given

below-

FeB

B @EC @EH B Fe6B

B E@I

Fe6B

B @EC FeB

B @EH B @B

FeB B @EH C Fe6B B E@I

0n this process, two additional means of producing hydroxyl ions are developed in presence of

/ltraviolet light. They are like- breakdown of @ydrogen Peroxide and reduction of Fe6B

ions %J'.

@E B hK C @EH B @EH

Fe6B

B @E B hK C FeB

B @EH B @B

+oncentration of hydrogen peroxide, intensity of /: and p@ are the parameters which

influences the overall efficiency of Fenton system.

3.2.2 HETEROGENEOUS PHOTOCATALYSIS

0n this type of photocatalysis, the catalyst and reactants are in different phase. The two maorly

used heterogeneous photocatalyst are transition metal oxides and semiconductors. 0n the

semiconductors, there is a band gap which is present between valence and conduction band.

When they absorb photon with energy greater than its band gap, excitation of electrons takes

place. (s a result, holes are generated in valence band. 1oth holes and electrons are important.

#o they must not recombine with each other. 0f they recombine then efficiency will be low.

lectrons which are excited reacts with an oxidi!ing agent to give reduced product and holes

produced react with reducing agent to give oxidi!ed product. "ue to the generation of electrons

and holes, oxidation and reduction reactions between substrate and excited species takes place at

the surface of semiconductors. 0n the oxidation reaction, holes interact with water to form

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3.3.2 INFLUENCE OF LIGHT WAVELENGTH

The rate of reaction is a function of wavelength which follows the absorption spectrum of the

catalyst, with a threshold value that corresponds to the energy of 1and$Gap.

3.3.3INFLUENCE OF INTIAL REACTANT CONCENTRATION

0n general sense, the kinetics of the process follows 9angmuir$ @inshelwood mechanism.

The rate of reaction changes proportionally with the degree L according to

r M kLM k)N+O&BN+*


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3.3.4 INFLUENCE OF REACTION TEMPERATURE

The rate of reaction maximi!es initially with increase in temperature upto a certain value. (fter

that reaction rate remains constant for a certain range of temperature and then finally rate

decreases with further increase in temperature.

3.3.5 INFLUENCE OF PHOTONIC FLU$

The reaction rate is directly proportional to photonic flux upto its threshold value. (fter the

threshold value the reaction rate becomes proportional to ?.=

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4. PHOTOCATALYST

0t is a substance which is used to alter the rate of photochemical reaction and without beingconsumed at the completion of reaction. +hlorophyll is a good example of natural photocatalyst

whereas nanotechnology prepared TiE is a man made photocatalyst. +hlorophyll make use of

sunlight to convert water and carbon dioxide into glucose and oxygen whereas TiE creates

powerful oxidi!ing agents and electronically positively charged holes which decomposes organic

substances to release carbon dioxide and water in the presence of light source . There are mainly

< types of photocatalyst used. They are-$

&* #emiconductors

* #pinel

6* Perovskite


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enhancing efficiency of photocatalytic process because they reduce the activation energy b

arriers for both oxidation and reduction reactions.

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Table


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gemstone which was mistaken as ruby due to its red colour. #pinels differ considerably from

ruby and sapphire as it has octahedral structure and single refraction. (lso spinel has low 7ohs

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hardness as compared to ruby and sapphire. Their luster is vitreous. The crystals are transparent

to translucent and sometimes nearly opa3ue. They possess isometric crystal system. Their

specific gravity ranges from 6.=$


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Fig


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4.2.1 LIST OF SPINELS USED AS PHOTOCATALYST

&* QnFe$x+r xE< nanoparticles are used for the removal of a!o dye from a3ueous solution.

* Qn(lE< is used for the photodegradation of methyl orange dye, Procion red dye, +hromium

(cidic black dye and 2hodamine 1 dye from its a3ueous solution.

6* Qn+r E. (lso it is used for the degradation of (F dye in a3ueous solution.

4.3 PEROVS#ITES

Perovskites are an important photocatalytic material which possesses general formula of (1E6.

@ere ( is the cation of larger molecular mass and 1 is the cat of smaller molecular mass. They

have wide range of applications. +ompounds like PbQrE6 are used as pie!oelectric compounds

while compounds like #rTiE6 show photocatalytic properties. 1 cations have strong interactions

with E while ( cations have weak interactions with E. Photocatalytic properties of perovskites is

due to its crystal structure.

Fig


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Perovskites can offer favourable band edge potential which might be very useful for carrying out

photoinduced reactions. When we compare perovskites with mixed oxides then we find that

perovskites has sufficient cathodic conduction band energy for the evolution of hydrogen. Ene of

the main advantage of using perovskites as a photocatalyst is that they are both visible light and

/: light active.

4.3.1 TITANATE PEROVS#ITES

7ost of the titanate perovskites produce mindblowing catalytic properties under /: radiation.

They have band gap value greater than three. 1ut by doping them with suitable materials we can

change their response from /: to visible light. #ome titanates have conduction band )+1*

energy more negative than TiE . (s a result they are used instead of them for hydrogen fuel

generation. Two important characteristics of titanates are that they offer good photostability and

they are also resistant to corrosion in a3ueous solution. #ome examples are-

1% S&T'O3

0t has a simple cubic structure with a band gap of 6.= e:. When we dope it with co$catalyst like

2h, it shows water splitting under /: light. When we dope Ti with some materials like 7n, 2u

and 0r, it induces mid gap states in the band gap which then allows it to absorb visible light. 0t

has been found that when we dope it with 7n and 2u, then it is useful for E evolution and when

we dope it with 2u and 2h it is found to be useful for @ evolution.

2% B(T'O3

0t has a band gap of 6.A e:. When we dope it with +u, it changes its response from /: to visible

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light absorption and shows photocatalytic water decomposition. Ether advantage of doping it

with +u is that it also enhances @ evolution under /: light when we use ;iEx is used as a co$

catalyst. (lkali metal titanates like +a, 1a, etc have enough conduction band potential for

hydrogen evolution. 1ut there are transition metals titanate which have enough +1 potential but

then also they do not show hydrogen evolution. #uch materials can be used for degradation of

organic compounds or other photocatalytic processes.

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Fig =.& TiE powder%&&'

1ut nowadays it is seen that TiE is widely used as a photocatalyst because it has high reactivity,

chemically inert and stable, low cost and less toxic. When we use it in nanoparticle form, we can

improve water splitting efficiently into hydrogen and oxygen and we can also generate electricity

from it. (s a result, it finds application in 9+" and 9". 0t can also be used as an oxygen sensor

because in the presence of reducing atmosphere, it will lose oxygen and will become

semiconductor. #o if oxygen content is low in atmosphere it will show high electrical resistance.

0n this way it can be used as an oxygen sensor.

5.1T'O2 *&+,-(/&('* /&,

&* 2utile- 0t is most stable and it is chemically inert. 0t can be used to absorb both /: as well as

visible light.

* (natase- (ctive under visible light and can be transformed to rutile form at high temperature.

6* 1rookite- 0nsensitive to /: light and the orthorhombic crystals system of it can be converted to

rutile form by giving heat.

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Fig =. +rystal structure of 2utile, (natase and 1rookite %&&'

Fig =.6 +rystal images of 2utile, (natase and 1rookite %&&'

5.2 SUPER HYDROPHILIC PROPERTY

When we expose the surface of thin film of photocatalyst to light source, there is a reduction in

the contact angle between surface of photocatalyst and water. (fter the light is exposed for

enough time, it becomes super hydrophilic. 0t means that the photocatalyst is no longer

hydrophobic. (s a result, water does not exist in form of drops but it starts spreading on the

surface of molecule of substrate.

Fig =.< #uper hydrophilic property of TiE%&'

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5.3EFFECTS OF T'O2

There are mainly < effects of TiE .They are-

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1% A-' 7(*-8&'( 88*-

Titanium dioxide is widely used as an anti bacterial agent. This is because it not only kills the

bacteria but it also decomposes the end toxin which is produced at the death of the cell. The

photocatalytic action of titanium dioxide works even when bacteria are present on the surface.

Titanium dioxide shows a long term anti bacterial effect as it not deteriorated easily. 0t has been

found that titanium dioxide is three times more effective as compared to chlorine and &.= times

more effective than o!one as an anti$bacterial agent.

2% A'& 9&'+' 88*-

This is one of the maor application of titanium dioxide. The photocatalytic activity of titanium

dioxide can be applied to remove or eliminate air pollutants like ;Ex, cigarette smoke and other

volatile compounds which arise from the construction sites. (s a result tents can be prevented

from becoming sooty and dark. @armful compounds like chlorofluorocarbons and its substitutes,

greenhouse gases, nitrogen and sulphur containing compounds undergo degradation either

directly or indirectly in the presence of sunlight. 0n this way titanium dioxide can be used to

purify air. (ir can be purified with >>.> purity.

3% D8/)/&':' 88*-

"ue to the photocatalytic activity of titanium dioxide, hydroxyl radicals are generated which

helps in the breakdown of volatile organic compound by breaking its molecular bonds. (s a

result, organic gases are generated which will combine to form a single molecule which will not

harm human beings and thus air will indirectly get purified. xamples of odor molecules include

gasoline, smoke, formaldehyde and other hydrocarbon molecules present in the air.

4% S8 *8(' 88*-

We know that the exterior walls of the house gets covered with automobile exhaust fumes whichmainly contains oily compounds. #o if we will coat the exterior walls with titanium dioxide , it

will decompose the oily components due to its photocatalytic activity and walls will get clean.

"irt present on the walls will be removed along with rainfall.

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Fig =.= #elf cleaning effect of TiE %&'

6. REVIEW PAPER ANALYSIS

The review paper is basically related to the applications of TiE photocatalyst. 0t also involves

case study of how wastewater from rice hull disinfection is treated in Uapan.

6.1 T&8(-8- / ;(,-8;(-8& 9,8) /& )','8*-' &'*8 9

2ice hull are generally infected with chemicals like pesticides. #o water is used to remove these

toxic chemicals from it. Wastewater is amounted to around A????? metric tons per year in

countries like Uapan and (merica. To handle this wastewater the technically minded Uapanese

researchers developed the glass wool mat with a large surface area which was deposited with

nanoparticles of TiE. The waste water is taken into the mat which spread over the ground.

+hemicals were degraded within few weeks. The initial total organic carbon values of thousand

ppm level decreased to !ero in around one week.

6.2 T&8(-8- / *8'*(+ )8,-&/+8) ,/'

+ompounds like trichloroethylene are more commonly used as solvents for dry cleaning agentfor clothes. They are polluting ground$water as well as soil . The main problem is that they are

highly toxic in nature. The solution to above problem is that we can actually supply heat so that

volatile matter can vapori!e and we get soil free from impurities. 0n doing this practically,

efficiency was found to be very less.

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V

Figure A.& Purification system for pollutant soil utili!ing photocatalytic sheet%&6'

(bove figure shows that soil is first dug and then it is covered with sheet of TiE containing

charcoal. #oil is heated and as a result gases are released which are taken up by adsorption on

charcoal on the sheet.

6.3 T&8(-8- / ;(,-8;(-8& */-('' ('& *//&(-,

Wastewater from bathroom was collected and treated with reactor system containing

photocatalytic material.. The result showed that there was a significant decrease in +E" and

toxicity of wastewater in the period of = hours. (fter treatment was over, flocs were observed.

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6.4 T&8(-8- / D+8,

Photocatalytic bed is prepared to treat textile waste water. The a!o dye causes environmental

damage. #hortage of water is maor problem in many countries. 2ecycling of water is highly

appreciated to meet water needs. To solve this problem, the reactor was added with short 3uart!

tube loaded with TiE which shows better catalytic activity. The rate of decolouri!ation of

methyl orange is around >J. under /: radiation for J hours.

6.5 P/-/*(-(+-'* D8*//,'-'/ / B8:88 D8&'


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=* #erpone, 7. and Peli!!etti, . )ditors*. Photocatalysis- Fundamentals and applications.

Uohn Wiley X #ons, 0nc.)&>J>*,&=$&.

A* 9insebigler, (my 9. 9u, Guang3uan. Sates, Uohn T. ZPhotocatalysis on TiE #urfaces-

Principles, 7echanisms, and #elected 2esultsZ. +hemical reviews )&>>=*.

* "aneshvar, ; #alari, " Nhataee, (.2 )??

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