Department of Environmental SciencesFaculty of Metrology, Environment and Arid Land Agriculture

King Abdulaziz University, Jeddah, Saudi Arabia

Topic: Photocatalysis

ADVANCE METHODS FOR WASTEWATER

TREATMENT

Submitted by:

Submitted to: Dr. Mohamed A. Barakat

Final ReportENS-762

(1503421)

Photocatalysis

1

ENS-795 ID: 1503421)

CONTENTS

1. INTRODUCTION TO PHOTOCATALYSIS..................................................................32. PRINCIPLE AND MECHANISM.....................................................................................3

2.1. Pollutant Degradation..................................................................................................42.2. Role of Oxidation Potential.........................................................................................4

3. TYPES OF PHOTOCATALYSIS......................................................................................53.1. Homogeneous Photocatalysis......................................................................................53.2. Heterogeneous Photocatalysis.....................................................................................5

4. APPLICATION OF PHOTOCATALYSIS........................................................................64.1. Environmental Applications........................................................................................64.2. Chemical synthesis......................................................................................................84.3. Energy Production.......................................................................................................8

5. MODIFICATION OF PHOTOCATALYST......................................................................95.1. Doping of Photocatalyst..............................................................................................9

5.1.1.Cation Doping...................................................................................................105.1.2.Anion Doping....................................................................................................11

5.2. Sensitization..............................................................................................................115.2.1.Dye sensitization...............................................................................................115.2.2.Composite Semiconductor................................................................................12

5.3. Noble metal loading..................................................................................................135.4. Metal ion-implantation..............................................................................................13

6. SELECTION OF THE DOPENT.....................................................................................147. METHODS OF DOPING.................................................................................................15

7.1. Methods for Metal Ion DopingMicro-emulsion method...........................................157.2. Non metal doping Methods.......................................................................................167.3. Sol-Gel Methods........................................................................................................17

7.3.1. Advantages of Sol-Gel Method.......................................................................198. ADVANTAGES AND DISADVANTAGES OF PHOTOCATALYSIS......................19

8.1. Advantages.................................................................................................................198.2. Disadvantages............................................................................................................19

9. CONCLUSION.................................................................................................................2010. REFERENCES..................................................................................................................20

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1. INTRODUCTION

The word photocatalysis is a composite word which is composed of two parts, “photo”

and “catalysis”. Catalysis is the process where a substance participates in modifying the rate

of a chemical transformation of the reactants without being altered or consumed in the end.

This substance is known as the catalyst which increases the rate of a reaction by reducing the

activation energy. In catalysed photolysis, light is absorbed by an adsorbed substrate. In

photogenerated catalysis, the photocatalytic activity (PCA) depends on the ability of the

catalyst to create electron–hole pairs, which generate free radicals (e.g. hydroxyl radicals:

•OH) able to undergo secondary reactions.

The field of photocatalysis has expanded rapidly within the last four decades, having

undergone various developments especially in relation to energy and the environment. It can

be defined as the acceleration of photoreaction in the presence of a catalyst. The two most

significant applications of photocatalysis have been in solar water splitting and the

purification of air and water containing low concentrations of pollutants. The

multidisciplinary nature of the field has also increased significantly and includes

semiconductor physics, surface sciences, photo and physical chemistry, materials science

and chemical engineering. Heterogeneous photocatalysis can be described as the acceleration

of photoreaction in the presence of a catalyst. In the contexts of history and research, interest

in heterogeneous photocatalysis can be traced back to many decades when Fujishima and

Honda discovered in 1972 the photochemical splitting of water in to hydrogen and oxygen in

the presence of TiO2. From this time, extensive research, much of it published, has been

carried out to produce hydrogen from water in oxidation reduction reactions using a variety

of semiconductor catalyst materials.

2. PRINCIPLE AND MECHANISM

The acceleration of a chemical transformation by the presence of a catalyst with light is

called photocatalysis. The catalyst may accelerate the photoreaction by interaction with the

substrate in its ground or excited state and/or with a primary photoproduct, depending upon

the mechanism of the photoreaction and itself remaining unaltered at the end of each catalytic

cycle. Heterogeneous photocatalysis is a process in which two active phases solid and liquid

are present. The solid phase is a catalyst, usually a semiconductor. The molecular orbital of

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semiconductors has a band structure.The bands of interest in photocatalysis are the populated

valence band (VB) and it’s largely vacant conduction band (CB), which is commonly

characterized by band gap energy (Ebg). The semiconductors may be photoexcited to form

electron-donor sites (reducing sites) and electron-acceptor sites (oxidising sites), providing

great scope for redox reaction. When the semiconductor is illuminated with light (hǎ) of

greater energy than that of the band gap, an electron is promoted from the VB to the CB

leaving a positive hole in the valence band and an electron in the conduction band.

If charge separation is maintained, the electron and hole may migrate to the catalyst

surface where they participate in redox reactions with sorbed species. Specially, h+vb may

react with surface-bound H2O or OH -to produce the hydroxyl radical and e cb is picked up

by oxygen to generate superoxide radical anion (O2 ), as indicated in the following equations

2.1. Pollutant Degradation

It has been suggested that the hydroxyl radical (OH o) and superoxide radical anions (O2

o-) are the primary oxidizing species in the photocatalytic oxidation processes. These

oxidative reactions would results in the degradation of the pollutants.

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2.2. Role of Oxidation Potential

For oxidation reactions to occur, the VB must have a higher oxidation potential than the

material under consideration. The redox potential of the VB and the CB for different

semiconductors varies between +4.0 and -1.5 volts versus Normal Hydrogen Electrode

(NHE) respectively. The VB and CB energies of the TiO2 are estimated to be +3.1 and -0.1

volts, respectively, which means that its band gap energy is 3.2 eV and therefore absorbs in

the near UV light (nj<387 nm). Many organic compounds have a potential above that of the

TiO2 valence band and therefore can be oxidized. In contrast, fewer organic compounds can

be reduced since a smaller number of them have a potential below that of the TiO2

conduction band.

3. TYPES OF PHOTOCATALYSIS

Photcatalysis process can be carried out by two ways, therefore categorized in to two

types:

o Homogeneous photoctalysis

o Heterogeneous photocatalysis

3.1. Homogeneous Photocatalysis

In homogeneous photocatalysis, the reactants and the photocatalysts exist in the same

phase. The most commonly used homogeneous photocatalysts include ozone and photo-

Fenton systems (Fe+ and Fe+/H2O2). The reactive species is the •OH which is used for

different purposes. The mechanism of hydroxyl radical production by ozone can follow two

paths.

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3.2. Heterogeneous Photocatalysis

Heterogeneous catalysis has the catalyst in a different phase from the reactants. In

heterogeneous photocatalysis two or more phases are used in the photocatalytic reaction. The

photo-generated holes at the surface of the irradiated semiconductor can oxidize a variety of

hazardous species in water. Most common heterogeneous photocatalyts are transition metal

oxides and semiconductors, which have unique characteristics. Unlike the metals which have

a continuum of electronic states, semiconductors possess a void energy region where no

energy levels are available to promote recombination of an electron and hole produced by

photoactivation in the solid. The void region, which extends from the top of the filled valence

band to the bottom of the vacant conduction band, is called the band gap. When a photon

with energy equal to or greater than the materials band gap is absorbed by the semiconductor,

an electron is excited from the valence band to the conduction band, generating a positive

hole in the valence band. The excited electron and hole can recombine and release the energy

gained from the excitation of the electron as heat. Recombination is undesirable and leads to

an inefficient photocatalyst. The ultimate goal of the process is to have a reaction between the

excited electrons with an oxidant to produce a reduced product, and also a reaction between

the generated holes with a reductant to produce an oxidized product. Due to the generation of

positive holes and electrons, oxidation-reduction reactions take place at the surface of

semiconductors. In the oxidative reaction, the positive holes react with the moisture present

on the surface and produce a hydroxyl radical.

Oxidative reactions due to photocatalytic effect is given as follows:

4. APPLICATION OF PHOTOCATALYSIS

4.1. Environmental Applications

Environment pollution, including water air and soil is becoming an increasingly serious

problem today. The environmental applications of photocatalysis is enlisted below.

Wastewater treatment for degradation of pesticides, dyes, organic pollutants in

pharmaceuticals, drugs, cosmetics, petrochemical and various industrial pollutant.

Production of value added products such as bio solvents, solid fuel, liquid fuels,

syngas, pharmaceutical intermediates and etc.

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Disinfection of surfaces is in order to reduce the concentration of bacteria and to

prevent bacterial transmission and disinfection of drinking water.

Self cleaning materials: Antibacterial materials (glass and ceramics containing TiO2,

for hospitals and buildings.

Figure: Photocatalytic glass and mirrors.

4.1.1. Case Study I: Detoxification of wastewater used for rice hull disinfection

Disinfection solutions for rice hulls (to stop the spread of plant diseases) contain highly

concentrated agricultural chemicals. The corresponding wastewater is mostly disposed of by

pouring onto the ground, which causes soil pollution. The system composed of a glass wool

mat with a large surface area deposited with very photoactive TiO2 nanoparticles; the

wastewater solution is easily purified using the photocatalytic mat under only solar light. The

treatment method is very simple, i.e., the wastewater is poured onto mats that are spread over

a wide area on the ground. The agricultural chemicals were completely decomposed under

sunlight in a few days.

4.1.2. Case Study II: Water treatment of hydroponic

A recycling system in a hydroponic culture system using the TiO2 photocatalyst and

solar light has been developed. The wastewater from the hydroponic culture with a planting

area of about 80 m2 was introduced into a shallow vessel with a bottom area of 4m2 and a

depth of 10 cm in which porous ceramic plates coated with TiO2 photocatalyst nanoparticles

were placed.

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The organic contaminants in the wastewater were easily decomposed under solar light,

but the nutrient compounds contain-ing nitrogen, phosphorous and potassium were not,

because these components ex-isted in their most oxidized forms, NO3–, PO4

3– and K+, in the

nutrient solution. Therefore, this system is also expected to serve as a nutrient-saving

technology.

4.1.3. Case Study III: Treatment of VOC-polluted soil

Volatile chlorinated organic compounds (VOCs), such as trichloroethylene and

tetrachloroethylene, have been widely used as solvents for the dry cleaning of clothes and

washing of semiconductors. The photocatalytic sheets to purify the polluted soil on the

ground using sunlight were developed. The polluted soil is dug up and covered with the

sheet, which is made of corrugated paper containing both TiO2 powder adsorbed on activated

carbon powders. Then the covered soil is heated (for example, by mixing with calcium

oxide), volatilizing the pollutant gases captured by adsorption on activated carbon

incorporated in the sheet material. The sheet is allowed to remain undisturbed under sunlight,

while TiO2 in the sheet decomposes the pollutants completely by photocatalytic reaction.

Figure: Application of TiO2 photocatalysis in various fields.

4.2. Chemical synthesis

The photcatalysis process has wide application in synthesis variety of chemicals. Some

examples of synthetic products using different types of catalysis are summarized in the table.

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4.3. Energy Production

Hydrogen fuel production has gained increased attention as oil and other non-renewable

fuels become increasingly depleted and expensive. Methods such as photocatalytic water

splitting are being investigated to produce hydrogen fuel, which burns cleanly and can be

used in a hydrogen fuel cell. Water splitting holds particular interest since it utilizes water, an

Table: Chemical of synthesis of various compounds through photocatalysis.

Reactants Cataysts Products

Olephins (Prophilene) TiO2, MoO3 Alcohols, aldehydes, and ketones

Toluene TiO2 Benzaldehyde

Cumene TiO2 Acetophenone

Butadienes TiO2 Acetaldehyde

Propene TiO2 SnO2, Sb2O4 Acetaldehyde, acetone, acroelin

Cyclohexane V2O5/ZrO2 cyclohexanol, cyclohexanone

Cyclohexane MoO3/TiO2 Benzene, cyclohexene

Ethylbenzene MoO3/g-Al2O3 Styrene

inexpensive renewable resource. Photocatalytic water splitting has the simplicity of using a

powder in solution and sunlight to produce H2 and O2 from water and can provide a clean,

renewable energy, without producing greenhouse gases or having many adverse effects on

the atmosphere. A general reaction of water splitting is given as follows.

5. MODIFICATION OF PHOTOCATALYST

5.1. Doping of Photocatalyst

Research into photocatalyst doping has spanned several decades. Usually doping

involves the use of metals or non-metals and is designed to extend the photocatalytic activity

of a semiconductor lower energy excitation. Technically, doping is the introduction of foreign

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elements into the parent photocatalyst without giving rise to a new crystallographic forms,

phases or structures and the aims are to enhance the net separation of photo generated

charges and thereby efficiently harness the wide visible-light component of about 43% in the

solar spectrum as opposed to the narrow ultraviolet component of 5%. It is thus an area of

increasing research activity in photocatalysis. For modification of photocatalyst two types of

doping are applied. The doping is of two types:

1. Cation Doping

2. Anion Doping

5.1.1. Cation Doping

Transitional metal ion doping and rare earth metal ion doping have been extensively

investigated for enhancing the TiO2 photocatalytic activities. A study carried out a systematic

investigation to study the photoreactivity of 21 metal ions doped into TiO2. It was found that

doping of metal ions could expand the photo-response of TiO2 into visible spectrum. As

metal ions are incorporated into the TiO2 lattice, impurity energy levels in the band gap of

TiO2 are formed, as indicated below:

where M and Mn+ represent metal and the metal ion dopant, respectively. Furthermore,

electron (hole) transfer between metal ions and TiO2 can alter electron-hole recombination

as:

The energy level of Mn+/M(n-1)+should be less negative than that of the CB edge of TiO2,

while the energy level of Mn+/M(n-1)+ should be less positive than that of the VB edge of TiO2.

For photocatalytic reactions, carrier transferring is as important as carrier trapping. Only if

the trapped electron and hole are transferred to the surface, photocatalytic reactions can

occur. Therefore, metal ions should be doped near the surface of TiO2 particles for a better

charge transferring. In case of deep doping, metal ions likely behave as recombination

centres, since electron/hole transferring to the interface is more difficult. Furthermore, there

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exists an optimum concentration of doped metal ion, above which the photocatalytic activity

decreases due to the increase in recombination. Among the 21 metal ions studied, Fe, Mo,

Ru, Os, Re, V, and Rh ions can increase photocatalytic activity, while dopants Co and Al ions

cause detrimental effects. The different effects of metal ions result from their abilities to trap

and transfer electrons/holes. For example, Cu and Fe ions can trap not only electrons but also

holes, and the impurity energy levels introduced are near to CB as well as VB edges of TiO 2.

Therefore, doping of either Cu or Fe ions could be recommended for enhancement of

photocatalytic activity.

5.1.2. Anion Doping

The use of anion doping to improve hydrogen production under visible light is rather a

new method with few investigations reported in open literature. Doping of anions (N, F, C, S

etc.) in TiO2 crystalline could shift its photo-response into visible spectrum. Unlike metal

ions (cations), anions less likely form recombination centers and, therefore, are more

effective to enhance the photocatalytic activity. The substitutional doping contents of C, N, F,

P and S for O in anatase TiO2 is also determined.

Dopants such as C and P are less effective as the introduced states were so deep that

photo-generated charge carriers were difficult to be transferred to the surface of the catalyst.

The nitrogen doped TiO2 thin film was prepared by sputtering TiO2 in an N2 (40%)/Ar gas

mixture, followed by annealing at 550 oC in N2 for about 4h. Nitrogen doped TiO2powder

was also prepared by treating TiO2 in NH3 (67%) = Ar at 600 oC for 3h. The N-doped TiO2

was reported to be effective for methylene blue decomposition under visible light (4400nm).

5.2. Sensitization

Sensitization often is characterized by an enhancement of response to a whole class of

stimuli in addition to the one that is repeated. Sansitization process of modification of the

catalyst may be done by two way:

1. Dye sensitization

2. Composite Semiconductor

5.2.1. 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 cells as well as

photocatalytic systems. Under illumination by visible light, the excited dyes can inject

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electrons to CB of semiconductors to initiate the catalytic reactions. Even without

semiconductors, some dyes, for example safranine O/EDTA and T/EDTA, are able to absorb

visible light and produce electrons as reducing agents strong enough to produce hydrogen.

The excitation, electron injection and dye regeneration can be expressed as follows:

Figure: Mechanism of dye-sensitized photocatalytic hydrogen production under visible light

irradiation

5.2.2. Composite Semiconductor

Semiconductor composition (coupling) is another method to utilize visible light for

photocatalysis reaction. When a large band gap semiconductor is coupled with a small band

gap semiconductor with a more negative CB level, CB electrons can be injected from the

small band gap semiconductor to the large band gap semiconductor. Thus, a wide electron-

hole separation is achieved.

The process is similar to dye sensitization. The difference is that electrons are injected from

one semiconductor to another semiconductor, rather than from excited dye to semiconductor.

Successful coupling of the two semiconductors for photocatalytic water-splitting hydrogen

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production under visible light irradiation can be achieved when the following conditions are

met:

Semiconductors should be photocorrosion free

The small band gap semiconductor should be able to be excited by visible light

The CB of the large band gap semiconductor should be more negative than EH2=H2O

Electron injection should be fast as well as efficient

The CB of the small band gap semiconductor should be more negative than that of the

large band gap semiconductor

Figure: Electron Injuction in composite semiconducton.

5.3. Noble metal loading

Noble metals, including Pt, Au, Pd, Rh, Ni, Cu and Ag, have been reported to be very

effective for enhancement of TiO2 photocatalysis. As the Fermi levels of these noble metals

are lower than that of TiO2, photo-excited electrons can be transferred from CB to metal

particles deposited on the surface of TiO2, while photo-generated VB holes remain on the

TiO2. These activities greatly reduce the possibility of electron-hole recombination, resulting

in efficient separation and stronger photocatalytic reactions.

5.4. Metal ion-implantation

Metal ion-implantation was recently reported to be an effective method to modify

semiconductor electronic structures to improve visible light response. When TiO2 is

bombarded with high-energy transitional metal ions (accelerated by high voltage), these high-

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energy ions are injected into the lattice and interact with TiO2. This process modifies TiO2

electronic structure and shifts its photo-response to the visible region (up to 600nm).

Presently metal ion implanted TiO2 is believed to be the most effective photocatalyst for solar

energy utilization and is in general referred as the ‘second generation photocatalyst.’

Takeuchi et al. (2000) implanted Cr-ion into TiO2 thin film for NO decomposition under

visible light irradiation. The TiO2 thin film was prepared by ionized cluster beam (ICB)

method. In this method, metal Ti was heated up to 2000 K and the Ti vapor was introduced

into a high vacuum chamber in the presence of O2 to produce TiO2 thin film.

The main advantages of ICB method are:

(i) High crystalline TiO2 can be obtained as calcination is avoided

(ii) The properties of thin film, such as thickness of the coating, can be controlled

(iii) Contamination with impurities can be prevented as this process is usually carried out in a

high vacuum chamber

6. SELECTION OF THE DOPENT

The slection of the dopent for doping of the catalyste, following important factors should

be considered.

1. Electron/Hole Trap Efficiency: The dopant should have higher efficiency to trap exited

electrons and holes. As Cu, Mn and Fe ions can trap both electrons and holes, doping of

these metal ions may work better than doping of Cr, Co and Ni ions. The latter metal

ions can only trap one type of charge carrier.

2. Dopant Content: Enhanced photocatalytic activities and red shift of photo-response

were observed at certain doping content. Suitable amounts of dopants helped to control

the crystallite size of nano-doped-TiO2 while producing a high specific surface area of

nano-doped-TiO2. For example the TiO2 thin film doped by Mn non-uniformly at the

optimal dopant concentration (0.7 at %) is of the highest activity.

3. Least Structural Defects: Several studies have indicated that doping creates structural

defects that could be sources for charge recombination and in this sense are potentially

negative in their effects. Doping using high-energy sputtering, which provide the

uncommon existence of tetravalent dopants such as Fe4+ and Cr4+ that match the valency

of Ti4+ in TiO2.

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4. Surface Doping: Doping the metal ions near the surface was beneficial for charge

carrier transferring, while deep doping led to poor performance. For instance; Be ion

doped TiO2 on photocatalytic hydrogen production in the presence of electron donors

(ethanol). Dopants C and P were found to be less effective as the introduced states were

so deep that photo-generated charge carriers were difficult to be transferred to the surface

of the catalyst

5. Dopant Methods: Solution-based method such as hydrothermal, sol–gel, and

precipitation method is the most widely used method for doping nonmetal into TiO2.

(1) Heat treatment of TiO2 at high temperatures: The method is generally used to prepare

N- or C-doped TiO2. (2) Physical Method: Sputtering TiO2 target in an atmosphere

containing doping species such as N2–Ar will incorporate N in the TiO2 target. (3) Sol-

gel: Cu-, Fe-, and Al-doped TiO2 prepared by the sol-gel have least effect on crysatlinity.

6. Narrow Band Gap: TiO2 was doped with S, the mixing of S 3p states with the VB of

TiO2 increased the width of VB, resulting in band gap narrowing. The efficient

photocatalytic activity of S-doped TiO2 was evaluated by photodecomposition of 2-

propanol and methylene blue.

7. Reproducibility: Doping with metallic or non-metallic elements, have in common the

difficulty in the reproducibility of the photocatalytic activity from batch to batch of

doped semiconductor and from one laboratory to other.

8. Crystallinity: The difficulty in reproducibility may arise from the different crystallinity

of the materials that makes it generally advisable to anneal the solid at high temperatures

for some time to increase crystallinity. The process of annealing: promote relocation of

the dopant and, eventually, its expelling from the anatase lattice.

9. Recombination centres: The dopant can act as a charge recombination centre that acts

against the separation of excited electron and hole. These problems can be countered by

controlling the dosages of the dopants. Doping of anions (N, F, C, S etc.) in TiO2

crystalline could shift its photo-response into visible spectrum.

7. METHODS OF DOPING

Doping has been proven to be a great method to enhance the photocatalytic activity. In recent

years, nonmetal-doping has demonstrated great potential to be applied in the photocatalytic

field. There of number of methods are available for doping of photocatalystes. Some of these

are defined below.

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7.1. Methods for Metal Ion Doping

1. Micro-emulsion method: Micro-emulsions are transparent, optically isotropic and

thermodynamically stable colloidal dispersions In which two liquids Initially immiscible

typically water and oil coexist.

2. Ambient chemical method: There is disclosed a method of controlling surface dopant

concentration in a semiconductor material in which the dopant is diffused from a doped

oxide source. The method involves the use of an oxidizing ambient during the doping

operation which creates a growing interface oxide barrier to moderate the doping of the

substrate.

3. Sol gel Method: Include synthesis of nanosized crystallized powder of high purity at

relatively low temperature, possibility of stoichiometry controlling process.

4. Solution combustion method: Combines features of chemical reaction and high

temperature routes, has become recently of increasing interest as easy and cheap

technique for the synthesis of oxide nanomaterials.

Table: Preparation of photocatalystes using various methods of metal ion doping.

S. No. Method Doped catalyste

1. Microemulsion method Fe3+ cations with TiO2

2. Ambient chemical method V-doped TiO2photocatalyst

3. Sol gel Method Co3+, Cr3+, Ce3+,Mn2+, Al3+, and Fe3+

4. Solution combustion method W, V, Ce, and Cu metal-doped anatase TiO2

7.2. Non metal doping Methods

1. Vapour deposition method: The substrate is exposed to one or more volatile precursors,

which react and/or decompose on the substrate surface to produce the desired deposit.

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Frequently, volatile by-products are also produced, which are removed by gas flow

through the reaction chamber.

2. Ion Implantation Technique: Dopant ions such as boron, phosphorus or arsenic are

generally created from a gas source, so that the purity of the source can be very high.

When implanted in a semiconductor, each dopant atom can create a charge carrier in the

semiconductor after annealing.

3. Spray pyrolysis technique: The solution is sprayed by means of glass nozzle on the

hotter substrate with a spray rate of 2ml/min.

Table: Preparation of photocatalystes using various methods of non-metal ion doping.

S. No. Method Doped Catalystes

1. Vapour deposition method C-doped TiO2 nanotubes

2. Ion implantation technique F-doped TiO2

3. Direct hydrolysis method I-doped TiO2

4. Spray pyrolysis technique N, F codoped TiO2

7.3. Sol-Gel Methods

The sol-gel process is a wet-chemical technique widely employed recently in the fields

of materials science and ceramic engineering. Such methods are utilized primarily for the

fabrication of materials (typically a metal oxide) starting from a chemical solution which acts

as the precursor for an integrated network (or gel) of either discrete particles or network

polymers.

Typical precursors are metal alkoxides and metal chlorides, which undergo various forms

of hydrolysis and polycondensation reactions. The formation of a metal oxide involves

connecting the metal centers with either oxo (M–O–M) or hydroxo (M–OH–M) bridges, and

generating metal-oxo or metal-hydroxo polymers in solution. Thus, the sol evolves towards

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the formation of a gel-like diphasic system containing both liquid and solid phases whose

morphologies range from discrete particles to continuous polymer networks.

In the case of the colloid, the volume fraction of particles (or particle density) may be so

low that a significant amount of fluid may need to be removed initially for the gel-like

properties to be recognized. This can be accomplished through plenty of ways. The simplest

method is to allow time for sedimentation to occur, and then pour off the remaining liquid.

Centrifugation can also be used to accelerate the process of phase separation.

Removal of the remaining liquid (solvent) phase requires a drying process, which is

typically accompanied by a significant amount of shrinkage and densification. The rate

atwhich the solvent can be removed is ultimately determined by the distribution of porosity in

the gel. The ultimate microstructure of the final component will be strongly influenced

bychanges imposed upon the structural template during this phaseof processing. Afterwards,

a thermal treatment, or a firing process, is also necessary in order to favor further poly-

condensation and to enhance the mechanical properties and structural stability via final

sintering, densification, and grain growth. One of the distinct advantages of this

methodology, as opposed to the more traditional processing techniques, is that densification

is often achieved at much lower temperatures. The general procedure of sol-gel method is

describe in the figure.

Figure: General procedure of Sol-Gel

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The precursor sol can be either deposited on a substrate to form a film (e.g., by dip

coating or spin coating), casted into a suitable container with the desired shape (e.g., to obtain

monolithic ceramics, glasses, fibers, membranes, and aerogels), or used to synthesize

powders (e.g., microspheres, nanospheres)

7.3.1. Advantages of Sol-Gel Method

The advantages of sol-gel method are:

1. No special apparatuses are required, easy to operate

2. Uniform films can be easily prepared, the purity of the films are high

3. Phase structure of films can be controlled and

4. Applied to the industrial production.

8. ADVANTAGES AND DISADVANTAGES OF PHOTOCATALYSIS

8.1. Advantages

The photocatalytic process gradually breaks down the contaminant molecules, no

residue of the original material remains and therefore no sludge requiring disposal to landfill

is produced. The catalyst itself is unchanged during the process and no consumable chemicals

are required. This results in considerable savings and a simpler operation of the equipment

involved. Additionally, because the contaminant is attracted strongly to the surface of the

catalyst, the process will continue to work at very low concentrations. Taken together, these

advantages mean that the process results in considerable savings in water production cost and

keeping the environment clean.

8.2. Disadvantages

Althourgh there are number of applications of the photocatalysis, however, there are

some challenges that limits the use this process. Nanoparticle TiO2 does not only destroy all

organic materials but also the organic matrix in which the nanoparticles are embedded. This

limits its application to inorganic environments. Nanoparticle can accumulate if its use is

widespread and could potentially have health impacts for workers exposed to nanoparticle

TiO2 dust. Another challagne is associated with its limited application in industries and

integration with existing wastewater treatment plant. Similarly, the high cost of some

photocatalyste make the process uneconomical. There are currently no regulations related to

the use of nanoparticle TiO2 for water treatment, but standards on test methods for

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photocatalytic water purification are under development. Stricter water treatments standards

would require new treatment methods and could thus further the application of photocatalytic

systems with nanomaterials.

9. CONCLUSION

The widespread application of photocatlysis have beneficial impacts on the health of the

general public and thus on the quality of life. Since nanoparticle TiO2 particles are

inexpensive and may be integrated into different materials, photocatalytic systems and

surfaces, they are not limited to large-scale applications in water treatment facilities. They

may also be applied in homes, hospitals, or offices for disinfection or the degradation of

water and air pollutants. The use of nanoparticle TiO2 for water treatment and disinfection is

expected to have a positive effect on the environment, as it can replace more toxic substances

such as organic biocides. This has a significant implication for developing nations. It can also

improve the quality of the water released from water treatment plantsby assisting traditional

treatment methods to target more substances and thereby obtain a higher efficiency of the

whole process.

10. REFERENCES

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photocatalysis-ppt#download

Choi W.Y., Termin A., Hoffmann M.R. (1994). The role of metal ion dopants in quantum-

sized TiO2: correlation between photoreactivity and charge carrier recombination

dynamics. J Phys Chem, 84:13669–79

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