金属粒子を担持した酸化亜鉛粒子の生成と有機染料
分解機能の評価
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言語: English
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公開日: 2021-12-09
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キーワード (En):作成者: 李, 永俊
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https://doi.org/10.15118/00010424

Generation of metal-supported ZnO-based particles and evaluationof photocatalytic properties for organic dye treatment

金属粒子を担持した酸化亜鉛粒子の生成と有機染料
分解機能の評価

A Thesis Presented to the Faculty of Engineering
Muroran Institute of Technology

byYoung-Jun LEE

September 2021

Abstract
Synthetic dyes are used in industries such as petrochemical, dyeing, paper, and pulp, but containharmful substances that can damage the natural environment. The main consumption industry ofsynthetic dyes is the textile industry, and a large amount of dye is lost to wastewater during thecoloring process of the textile. As the global population increases and the demand for textiles
increases, the amount of wastewater discharged from synthetic dyes is also increasing, which is amajor cause of the recent pollution problem. Because synthetic dyes have various chemical properties,
it is not easy to treat with conventional wastewater treatment methods such as adsorption, filtration,
coagulation, membranes, etc. Therefore, the development of eco-friendly, high-efficiency, low-costorganic pollutant treatment methods has become an important task for researchers.
Recently, photocatalytic oxidation of organic pollutants has been proposed as a promisingalternative. Photocatalytic oxidation treatment can photodegrade organic pollutants into H2O, CO2,and inorganic substances without generating toxic secondary substances, as well as remove odors anddisinfect them. However, the main drawback of semiconductor photocatalysts is the reduced
efficiency due to the rapid recombination of excited electron-hole pairs. When a noble metal is
supported on a photocatalyst, a noble metal acts as an electron sink and delays the recombination of
the excited electrons and holes, so the efficiency of the photocatalyst can be improved. Moreover,another drawback of photocatalytic treatment is that it is very difficult to recover photocatalysts after
removing organic pollutants from wastewater, and reuse is greatly limited. Therefore, in this study,
an experiment was performed to improve the photocatalytic properties by supporting Ag or Au on theZnO photocatalyst, and the magnetic properties of the particles were also observed through Fe3O4
support.
Nanoparticles show good photocatalytic efficiency because of their large specific surface area buthave the disadvantage of being difficult to recover after use. In addition, the wet method requiring
post-treatment may reduce the photocatalytic efficiency due to agglomeration between particles.
Therefore, there was a need for a method that could produce submicron-level particles without
agglomeration in a single process. In addition, there was a need for a generation method that could
increase the efficiency of the photocatalyst by supporting the noble metal as a single crystal. The
ultrasonic spray pyrolysis method is a method that meets these requirements. Therefore, in Chapter
2, Au/ZnO particles were generated using the ultrasonic spray pyrolysis method. The Au/ZnOaparticles produced using the USP method showed structural patterns of ZnO and Au. TEM images
showed dark spots dispersed throughout the Au/ZnO particles. EDX mapping and STEM images
confirmed that the dark spots corresponded to nano-sized Au particles dispersed throughout the ZnO
particles. The photolysis of organic Rhodamin B (RhB) aqueous solution was investigated under UVlight irradiation. Under all conditions, Au/ZnO particles showed better photocatalytic activity thanZnO particles. The best photocatalytic activity was observed in particles with a 0.1 mass% Au
supported on ZnO particles. Moreover, the USP could simply generate particles in one step. Althoughdecomposition experiments on actual wastewater were beyond the scope of this study, the potentialof purifying organic pollutants using this material was confirmed.
Precious metals such as Au are expensive, so industrial applications can have significant limitations.
Therefore, relatively inexpensive Ag carrying research is attracting attention. Therefore, in Chapter
3, Ag/ZnO particles were generated using the ultrasonic spray pyrolysis method. The resulting
Ag/ZnO particles were spherical and had a size of 0.5 to 1.0 μm, and there was no change in shapeby the Ag support. ZnO and Ag crystal peaks were observed in all Ag/ZnO particles. In the TEM
image, dark spots dispersed in Ag/ZnO particles were confirmed, and the size of the dark spots varied
according to the concentration of added silver nitrate. EDX mapping and STEM images confirmedthat these dark spots correspond to Ag particle nanoparticles dispersed throughout the ZnO
particles.The photocatalytic activity of the generated particles was investigated by photolysis of the
RhB aqueous solution under UV irradiation. All Ag/ZnO particles showed better photocatalytic
activity than ZnO particles. Particularly, the particles with 0.2 mass% silver nitrate supported on ZnO
particles showed 2 times better photolysis activity than P25. It was observed that the particles
generated could not only be recovered simply by gravity sedimentation after the photolysis
experiment but also had a slow deterioration rate. Using the USP method, we were able to easily
generate submicron-level complex particles and confirmed the possibility of purifying and reusingorganic pollutants.
The application of photocatalyst to organic dye treatment is that it is difficult to recover fine
photocatalyst particles from photolysis treated wastewater. Methods such as filtration or
centrifugation for particle recovery can cause destruction and loss of particles, resulting in highoperating costs and secondary pollution. Therefore, there is a need for a study that can simply recover
the particles. The combination of photocatalyst and magnetic particles can be a reliable solution.
When the photocatalyst and magnetic particles such as magnetite (Fe3O4) are combined, not only canbthe particles be easily recovered through the magnetic separation process, but also can be recycled.
Several methods are used for the synthesis of ZnO/Fe3O4. In Chapter 4, the synthesis of ZnO/Fe3O4
and ZnO/Fe3O4/Ag composite materials was attempted through ultrasonic spray pyrolysis and coprecipitation methods, and the physical properties of the resulting particles were confirmed.
When the ultrasonic spray pyrolysis method was used as a single method, crystal peaks of the target
products ZnO and Fe3O4 were not observed. However, under certain conditions, a single crystal peakof ZnFe2O4 was identified. Because the metal reactivity of Zn and Fe is similar, it precipitated as
ZnFe2O4 single crystal during pyrolysis process. Studies on ZnFe2O4 particles have been reported,but due to the low bandgap, it is not suitable for organic matter treatment. When the ultrasonic spraypyrolysis and co-precipitation methods were used together to generate particles, crystals of the target
products, ZnO and Fe3O4, were observed. Although the magnetic properties of the particles wereobserved, uncoated ZnO particles were identified. Therefore, it was confirmed that the surface
modification using sodium citrate is not suitable for the formation of ZnO/Fe3O4 composite particles.
When co-precipitation was used as a single method, crystals of the target products ZnO, Ag, andFe3O4 were observed. In the photolysis experiment, it was confirmed that the photolysis properties of
the ZnO/Fe3O4 composite particles were not good compared to the pure ZnO particles. However,
although the ZnO/Fe3O4/Ag composite particles contained Fe3O4, the photodecomposition properties
were superior to those of pure ZnO particles. The optimal condition was when 0.2 mass% silver
nitrate was added, which is consistent with the results of Chapter 3. The co-precipitation single
method confirmed some agglomeration of particles, but it was judged to be most suitable for thegeneration of ZnO/Fe3O4 and ZnO/Fe3O4/Ag composite particles.

cGeneration of metal-supported ZnO-based particles andevaluation of photocatalytic properties for organic dye treatment
by Young-Jun LEE

Chapter 1

Introduction.................................................................... 1

1.1. Background ................................................................................................................. 1
1.2. Type of organic dye ..................................................................................................... 2
1.2.1. Congo red (CR) .................................................................................................... 2
1.2.2. Crystal violet (CV) ............................................................................................... 2
1.2.3. O-Phenylenediamine (OPD) ................................................................................ 3
1.2.4. Rhodamine 6G (Rh6G) ........................................................................................ 3
1.2.5. Rhodamine B (RhB)............................................................................................. 3
1.2.6. Phenol red (PR) .................................................................................................... 4
1.2.7. Methylene blue (MB) ........................................................................................... 4
1.2.8. Malachite green (MG) .......................................................................................... 4
1.2.9. Other dyes ............................................................................................................ 4
1.3. Conventional removal method for synthetic dyes....................................................... 6
1.3.1. Physical and chemical removal method ............................................................... 6
1.3.1.1. Absorption ..................................................................................................... 6
1.3.1.2. Filtration process ........................................................................................... 7
1.3.1.3. Oxidation with sodium hypochlorite............................................................. 8
1.3.1.4. Oxidation by Ozonation ................................................................................ 8
1.3.1.5. Coagulation ................................................................................................... 9
1.3.1.6. Electrocoagulation (EC) ................................................................................ 9
1.3.1.7. Ionic exchange ............................................................................................ 10
1.3.2. Biological treatment method .............................................................................. 10
1.3.2.1. Aerobic biological treatment ....................................................................... 10
1.3.2.2. Anaerobic biological treatment ................................................................... 11
1.4. Semiconductor photocatalyst .................................................................................... 12
Absorption ................................................................................................................ 12
Oxidation with sodium hypochlorite........................................................................ 12d

Oxidation by Ozonation ........................................................................................... 12
Coagulation .............................................................................................................. 12
Electrocoagulation ................................................................................................... 12
Ionic exchange ......................................................................................................... 12
Aerobic biological treatment .................................................................................... 12
Anaerobic biological treatment ................................................................................ 12
1.4.1. Zinc oxide (ZnO) ............................................................................................... 16
1.5. Summary of this thesis .............................................................................................. 16
1.6. References ................................................................................................................. 18

Chapter 2

Evaluation of Photocatalysis of Au Supported ZnO

Prepared by the Spray Pyrolysis Method ....................................... 25
2.1. Introduction ............................................................................................................... 25
2.2. Experimental equipment and theory ......................................................................... 27
2.2.1. Ultrasonic Spray Pyrolysis ................................................................................. 27
2.2.1.1. Atomization ................................................................................................. 31
2.2.1.2. Electrostatic collector .................................................................................. 33
2.3. Experimental setup and Method ............................................................................... 35
2.3.1. Materials............................................................................................................. 35
2.3.2. Characterization ................................................................................................. 35
2.3.3. Photocatalytic activity ........................................................................................ 36
2.3. Experimental Results and Discussion ....................................................................... 36
2.3.1. Characteristics of ZnO particles ......................................................................... 36
2.3.2. Characteristics of Au/ZnO particles ................................................................... 39
2.3.3. Photocatalytic activity ........................................................................................ 42
2.4 Conclusion ................................................................................................................. 47
2.5. References ................................................................................................................. 48

Chapter 3

Evaluation of Photocatalysis of Ag Supported ZnO

Prepared by the Spray Pyrolysis Method ....................................... 53
3.1. Introduction ............................................................................................................... 53
3.2. Experimental setup and Method ............................................................................... 54
3.2.1. Materials............................................................................................................. 54
3.2.2. Synthesis of Ag/ZnO composite particles .......................................................... 54e3.2.3. Characterization ................................................................................................. 55
3.3. Experimental Results and Discussion ....................................................................... 56
3.3.1. Characteristics of Ag/ZnO particles ................................................................... 56
3.3.2. Photocatalytic activity ........................................................................................ 60
3.4. Conclusion ................................................................................................................ 67
3.5. Reference .................................................................................................................. 67

Chapter 4

ZnO/Fe3O4 and ZnO/Fe3O4/Ag composite particle

generation process ............................................................................. 70
4.1. Introduction ............................................................................................................... 70
4.2. Experimental setup and Method ............................................................................... 71
4.2.1. Materials............................................................................................................. 71
4.2.2. Generation of ZnO/Fe3O4 particles using USP method ..................................... 71
4.2.3. Generation of ZnO/Fe3O4 composite particles using USP and coprecipitation method .................................................................................... 72
4.2.4. Generation of ZnO/Fe3O4 and ZnO/Fe3O4/Ag composite particles using
co-precipitation method ............................................................................... 74
4.3. Experimental Results and Discussion ....................................................................... 76
4.3.1. Characteristics of ZnO/Fe3O4 composite particles generated using USP
method .......................................................................................................... 76
4.3.2. Characteristics of ZnO/Fe3O4 composite particles generated using USP
and co-precipitation method......................................................................... 78
4.3.3. Characteristics of ZnO/Fe3O4 and ZnO/Fe3O4/Ag particles generatedusing co-precipitation method ...................................................................... 82
Table 4-3 Comparison of treatment rates by photolysis and recovery rates of
Ag/ZnO (USP method) and ZnO/Fe3O4/Ag (co-precipitation method)
particles; initial concentration of RhB: 5 mg/L; photocatalyst dose:
15 mg; solution volume: 100 mL, dope amount of silver nitrate: 0.2
mass%. ......................................................................................................... 87
4.4. Conclusion ................................................................................................................ 87
4.5. Reference .................................................................................................................. 88

Chapter 5Conclusions ................................................................... 92

APPENDIX ........................................................................................ 94f

Nomenclature ................................................................................................................... 94
List of Figures .................................................................................................................. 95
List of Tables .................................................................................................................... 99

Acknowledgment ............................................................................. 101

gChapter 1 Introduction

Chapter 1

Introduction

1.1. Background
Natural dyes extracted from fruits, vegetables, flowers, insects, etc. have been used since 3500 BC
and are also found in historical records. However, natural dyes could not meet the required demand
due to the limited color spectrum and production. Thus, in 1856, W. H. Perkins developed syntheticdyes with a broader spectrum of colors and providing lighter shades to replace natural dyes and arestill in use today [1]. These synthetic dyes are used in industries such as petrochemical, dyeing, paper,and pulp, but contain harmful substances that can damage the natural environment [2-3]. For example,
wastewater containing synthetic dyes mainly contains carcinogens such as nitro, sulfo, chloro,
naphthalene, benzamine, and other aromatic compounds, which adversely affect human health, theenvironment, and aquatic ecosystems [2, 4-6]. The main consumption industry of synthetic dyes is the
textile industry, and a large amount of dye is lost to wastewater during the coloring process of the
textile. It is estimated that more than 15,000 dyes and pigments are produced worldwide, and morethan 900,000 tons are used [7]. Due to the inefficiency of the dyeing process, 200,000 tons of dye,
which is about 1-20% of the world's dye production, are discharged into wastewater [2, 7, 8]. Therefore,
the International Organizations for Environmental Protection has set an acceptable standard forenvironmental emission of organic dyes at a level of a few micrograms per liter [9].
As the global population increases and the demand for textiles increases, the amount of wastewater
discharged from synthetic dyes is also increasing, which is a major cause of the recent pollution
problem. This is because conventional wastewater treatment methods such as adsorption, filtration,
coagulation, membranes, etc. cannot decompose pollutants, but also cause secondary pollution bysimply moving organic pollutant molecules to another medium [3, 8, 10]. It is also very difficult totreatment synthetic dyes with high thermal and light stability [2, 8]. Therefore, the development of ecofriendly, high-efficiency, low-cost organic pollutant treatment methods has become an important task
for researchers.
About 72 toxic chemicals have been identified in wastewater from the textile industry, of which 30
cannot be removed [1]. If the wastewater contains a high concentration of dye, the oxygen saturationof the water decreases and sunlight is blocked, which interferes with photosynthesis of aquatic plants
such as algae [1, 10]. In particular, because these dyes are not biodegradable, they accumulate in– 1 –Chapter 1 Introduction

ecosystems and aquatic organisms. So it could potentially be a problem for human health as well [11].
Because of these hazards, it is necessary to separate and remove residual dyes from wastewater andactively work in industrial sites. However, because synthetic dyes have various chemical properties,
it is not easy to treat with conventional treatment methods. In particular, toxic substances can beproduced by weathering organic dyes in wastewater through oxidation, hydrolysis or other chemical
reactions [8].

1.2. Type of organic dye
In general, dyes can be divided into three categories: anionic (acidic and reactive dyes), cationic(alkaline dyes) and nonionic (disperse dyes). A large number of chemically different dye groups suchas azo, disperse, acidic, alkaline and triphenylmethane were used for textile dyeing. Among them, azodyes (anionic) with nitrogen-nitrogen double bonds (N = N) are considered the most used organicdyes, but their treatment is complicated due to their aromatic structure and difficult decomposition
properties. Examples of some of the dyes mainly used commercially are shown in Table 1-1.
1.2.1. Congo red (CR)
These are the most used azo dyes worldwide. The CR is a benzidine-based anionic azo dye
containing nitrogen-nitrogen double bonds (-N = N-). The complex aromatic molecular structure of
CR dyes provides optical, thermal, and physicochemical stability, so their main applications are in
the textile, printing, dyeing, paper, rubber, and plastics industries. The toxic of CR benzidine could
cause mutagenicity and carcinogenicity in humans. Also, frequent touch of this dye can cause anallergic reaction. Therefore, it is very important to remove the remnants of the CR before mixing itwith the water source. Although CR dyes have been banned in many countries due to health concerns,
they are still widely used in many countries [12].
1.2.2. Crystal violet (CV)Crystal Violet, a triphenylmethane-based dye, is widely used in the textile processing industry andin biological dyeing in human medicine. However, since CV is a recalcitrant dye, it remains in theenvironment for a long time and is therefore toxic to terrestrial and aquatic organisms. The CV has
been reported to cause problems in the reproductive system of rabbits and fish, and is known as a
powerful carcinogen that promotes tumor growth in biological cells. Inhalation of CV can cause– 2 –Chapter 1 Introduction

irritation of the respiratory tract, and ingestion can cause gastrointestinal problems. It can be seenfrom the chemical structure of the dye that there is an aromatic ring in three directions attached to thecentral carbon atom, and it has a maximum absorption peak of 584 nm, so it is purple. The CV is
widely used in analytical chemistry laboratories as a pH indicator for testing the pH range from 0 to
1.6. At low pH, the CV turns yellow and becomes blue-violet at the alkaline end [13-14].
1.2.3. O-Phenylenediamine (OPD)
O-Phenylenediamine (OPD) called 1,2-diaminobenzene or 1,2 phenylenediamine, is an aromatic
amine used as a dyeing material for polymers, drugs, and textiles. However, OPD has been shown tobe harmful by ingestion, inhalation, and eye contact and can cause cancer cells. Therefore, excessive
exposure to OPD can damage the respiratory, digestive and liver and lead to cancer. These OPD are
considered one of the serious environmental pollutants due to their toxicity and carcinogenicity.
Because OPD is water soluble, it can easily contaminate ground and surface water, and it is very
important to monitor OPD in drinking and environmental waters [15-16].
1.2.4. Rhodamine 6G (Rh6G)
Rhodamine-based dye, a highly recalcitrant textile dye, commonly used for coloring cotton, wool,
silk, and paper. In addition, Rh6G is also used as a diagnostic tool to detect antigens and has excellent
staining properties in dark reddish purple. The Rh6G is also used in fluorescence microscopy andfluorescence correlation spectroscopy in biotechnology. However, water with rhodamine dye causes
irritation to human skin, eyes, and respiratory system. And, unfortunately, Rh-6G is toxic andcarcinogenic to cells and tissues in organisms, and is associated with mutagenicity and teratogenicity.
Some studies have demonstrated that drinking water contaminated with R6G causes serious damage
to retinal ganglion cells [17-18].
1.2.5. Rhodamine B (RhB)
It is a reddish-purple powder of xanthene class with high water solubility used under the brandname D & C Red No. 19. The RhB is widely used as a colorant. However, if swallowed by humans
and animals, it is harmful and causes irritation to the skin, eyes and respiratory system. It has been
medically proven that drinking water contaminated with RhB dye can cause highly carcinogenic
tumors. In recent years, carcinogenicity, reproduction, development, neurological and chronic toxicity– 3 –Chapter 1 Introduction

to humans and animals have been intensively investigated and proven [19-21].
1.2.6. Phenol red (PR)It exists as red crystals and is a textile dye that belongs to the triphenylmethane dye class. A
commonly known synonym for phenol red is phenol sulfonephthalein. Since it is stable in water andair and has a pH range of 6.8 (yellow) to 8.2 (bright pink), it is often used as a reagent for measuring
the pH of water and cell biology laboratories. It has been reported that PR inhibits the growth of renal
epithelial cells and has a lethal toxicity that causes mutations in muscle fibers. In addition, direct orindirect contact may cause skin, eye, and respiratory irritation [22-23].
1.2.7. Methylene blue (MB)It is a dark green cationic dye, and the aqueous solution has a blue color. It is widely used fordyeing paper, coloring, wool, silk leather, and paper coatings. Although MB is considered harmlesscompared to other synthetic dyes, it can also cause some detrimental effects such as cyanosis,
vomiting, increased heart rate, limb paralysis, shock, jaundice and human tissue necrosis [24].
1.2.8. Malachite green (MG)In addition to being used in the dyeing process, it has also been used as an antibacterial agent toprevent fungal infections in fish farms. However, it can persist for a long time in edible fish tissues
and bioaccumulate in terrestrial and aquatic ecosystems, which has a detrimental effect on theenvironment and human health. Also, due to its toxicity and carcinogenic effects, the U.S. Food andDrug Administration (FDA) designated MG as the preferred chemical for carcinogenicity testing by
the National Toxicology Program in 1993. The use of MG has been banned worldwide since 2002,
but it is still used in some environments [24-26].
1.2.9. Other dyes
In addition, Janus Green B (JGB), Janus Green B (JGB), Alizarin Red S (ARS), methyl orange(MO), methyl red (MR), methyl red (MR), indigo carmine (IC), eriochrome black T (EBT),
eriochrome blue black T (EBBT), alizarin cyanin green (ACG), victoria blue B (VBB), methyl violet(MV), reactive red 195 (RR-195), etc. Likewise, toxicity has been reported.
– 4 –Chapter 1 Introduction

Table 1-1 Some Examples of Commercial Dyes.
Dyes

Structure

Chemical formulaMw

CR

C32H22N6Na2O6S2

696

CV

C25H30ClN3

408

OPD

C6H4(NH2)2

108

Rh6G

C28H31N2O3Cl

497

RhB

C28H31ClN2O3

497

PR

C19H14O5S

354

MB

C16H18N3SCl

320

MG

C23H25ClN2

365

– 5 –

Chapter 1 Introduction

1.3. Conventional removal method for synthetic dyes
Wastewater treatment methods can be classified into physical, chemical, and biological methods
as shown in Table 1-2. Proprietary treatment by one of these three methods has proven insufficient toremove organic dyes from textile wastewater. The same process may not apply to other types ofcolored wastewater, even if some processes have reported successful decolorization of certain
wastewater [1]. Some processes, such as electrochemical technology and ion pair extraction, arerelatively new treatments for textile wastewater treatment, while others have long been used in theindustry. Each process has its own limitations in terms of cost, feasibility, reliability, stability,
environmental impact, sludge treatment, operational difficulties, pretreatment requirements, organics
removal range and potential toxic by-products.
Table 1-2 Wastewater treatment method.
PhysicalAbsorption
Filtration
Reverse Osmosis
Membrane Treatment

Chemical
Oxidation
Reduction
Catalysis
Ion ExchangeElectrocoagulation
Coagulation
Chlorination
Ozonation

Biological
Aerobic
Anaerobic

1.3.1. Physical and chemical removal method
1.3.1.1. Absorption
Removal of organic dyes by adsorption has been regarded as one of the best existing methods. The
most used activated carbon is known for its excellent adsorption capacity for a variety of aqueous andgaseous pollutants. In addition, porous materials such as ceramic, fly ash, sepiolite, hydrogel, clay
minerals, hydrotalcite, perlite, biopolymers such as chitosan, and agricultural by-products can be used
to move pollutants to the surface of the adsorbent. The adsorption capacity and cost of adsorbents
differ for each material, and the adsorption capacity differs depending on the surface area, PH, andtemperature. In addition to the most widely used activated carbon, agricultural waste such as sugar
cane, rice husk, and wood sawdust can be used and has an adsorption efficiency of 40-90%. Thesematerials can be used extensively because of their low cost, but they are less efficient than activated– 6 –Chapter 1 Introduction

carbon and require large amounts [22, 24, 27-28].
The use of activated carbon systems, including the regeneration of adsorbents, is too expensive.
Particularly, large dye molecules adsorbed on the surface of the adsorbent are difficult to desorb due
to the strong chemical bond between the adsorbent and the dye, so regeneration cost is high andimpractical. Therefore, disposal of used adsorbent creates additional problems. Also, certain
adsorbents can only be applied to certain types of dyes. For example, negatively charged adsorbents
have a high adsorption capacity for cationic dyes. Therefore, using only one type of adsorbent isdifficult to deal with the complex composition of dye waste.
1.3.1.2. Filtration process
Ultra-filtration (UF) [28-30]
Ultra-filtration can remove macromolecules and large particles, but does not completely remove
contaminants such as dyes. Even in the best case, the quality of the treated wastewater does not allow
reuse for sensitive processes such as fabric dyeing. The Ultra-filtration can be used for pretreatment
for reverse osmosis or in conjunction with biological reactors. The UF membranes are made ofpolymeric materials such as polysulfone, polypropylene, nylon-6, polytetrafluoroethylene (PTFE),
polyvinyl chloride (PVC), acrylic copolymer, etc.
Micro-filtration (MF) [28-30]
Micro-filtration can also be used as a pretreatment for nanofiltration or reverse osmosis. The
suspended solids, macromolecules and colloids floating in wastewater can be separated. The MF
membrane is made of specific polymers such as Poly (Ether Sulfone), Poly (Vinylidiene Fluoride),
Poly(Sulfone),

Poly(Vinylidene

Difluoride),

Polycarbonate,

Polypropylene,

Poly

Tetrafluoroethylene (PTFE), etc. Ceramic, glass, carbon, zirconia coated carbon, alumina and sinteredmetal membranes with excellent chemical resistance have been used where high temperature
operation is required.
Nano-Filtration (NF) [28-30]
Nano-Filtration membranes are usually made of cellulose acetate and aromatic polyamide, and– 7 –Chapter 1 Introduction

inorganic materials such as ceramics, carbon-based membranes, and zirconia are also used to makeNF and RO membranes. Dyeing wastewater treatment by NF is one of the rare applications that canhandle highly concentrated and complex solutions. However, since it is very sensitive tocontamination by macromolecules, the influent water needs pretreatment.
Reverse Osmosis (RO) [28-30]
Reverse Osmosis can remove most types of ionic compounds, reactive dyes, and chemical agentsin a single step. The osmotic pressure becomes more important the higher the concentration ofdissolved dye. Therefore, more energy is required for the separation process. The RO, like NF, is also
very sensitive to contamination and the influent must be carefully pretreated. The RO membranes areusually made of cellulose acetate and aromatic polyamides, as well as inorganic materials.
1.3.1.3. Oxidation with sodium hypochlorite
Sodium hypochlorite discharges chlorine gas in aqueous solution, and dyes are decolorized andremoved by the discharged chlorine gas. Increasing the chlorine concentration favors the dye removal
and decolorization process and also reduces the pH. Dyes containing amino or substituted aminogroups in the naphthalene ring are most susceptible to chlorine bleaching. This treatment is noteffective for discoloration of disperse dyes, and the frequency of use is decreasing due to the negative
effect of the emission of organic halogens during the treatment [28].
1.3.1.4. Oxidation by Ozonation
As is well known, ozone is a powerful oxidizing agent and can decompose synthetic dyes. Ozone
molecules act as electron acceptors and can convert hydroxide ions into hydroxyl free radicals (·OH)
that react with organic and inorganic pollutants. Therefore, at a pH with a high dissolution amount ofhydroxyl ions (OH-), the formation of hydroxyl radicals (·OH) may be promoted, and the oxidationof organic dyes may become active. The input of H2O2 or UV can produce more ozone, and as a result,more OH radicals can be produced, which can improve the rate of decolorization. The advantage ofozonation is that it directly decomposes organic pollutants, so no other secondary pollution occurs.
However, instability due to temperature and PH and high facility installation cost remain
disadvantages. The oxidation process of many dyes and other organic pollutants (RH) by ozone is asfollows [29,31-32]:– 8 –Chapter 1 Introduction

H2 O+O3 → HO3 + + OH−(1-1)

HO3 + + OH− → 2HO2

(1-2)

O3 + HO2 → HO ∙ +2O2

(1-3)

HO∙ + HO2 → +H2 O + O2

(1-4)

RH + HO ∙→ R ∙ +H2 O(1-5)

1.3.1.5. Coagulation
The addition of chemicals or coagulants is required to speed up the agglomeration or particle
growth of organic matter in wastewater. Coagulants include inorganic coagulants such as lime, ferrous
sulfate, ferric sulfate, ferric chloride, aluminum sulfate, aluminum chloride, and organic coagulants
such as polyelectrolytes. Organic coagulants are cationic, anionic, and nonionic, all three being used
extensively in wastewater treatment. When a coagulant is added, ions of organic matter can be
neutralized and stable colloids can be produced. Organic coagulants such as polyelectrolytes canprovide adsorption and crosslinking between particles. As a result, particle clusters are formed andcan be precipitated or filtered under gravity. However, using it alone for dye wastewater treatment isrelatively inefficient. For example, only 50% can be removed by using alum or ferrous sulfate for an
azo active yellow dye. However, the downside to this treatment is that the sludge is finally produced
and there is an additional cost for the treatment of the sludge [33].
1.3.1.6. Electrocoagulation (EC)EC generate metal ions in wastewater by using electric current and sacrificial anodes (such as Fe
and Al). When metal ions with opposite charges are provided to wastewater, organic matter can be
neutralized to form colloids. In other words, since the metal hydroxide released from the anode acts
– 9 –Chapter 1 Introduction

as a coagulant, colloids are formed and easily rise to the surface by H2 generated at the cathode. And
it has been found that the external addition of coagulant improves the processing efficiency. However,since iron hydroxide sludge is generated on the electrode surface, its use is limited. In general, in EC
treatment, the decolorize efficiency was found to be 90-95%, and COD removal was found to be 3036% under optimal conditions. In addition, factors that affect electrochemical performance include
current strength, number and spacing of electrodes used, pH, temperature, properties of electrolytes,
surface tension and flow rates of wastewater, and properties of dyes. The mechanism ofelectrocoagulation is as follows [29, 34]:

Anode4Fe ↔ 4Fe2+ + 8e−(1-6)

Solution

4Fe+ + 10H2 O + O2 ↔ 4Fe(OH)3 + 8H+(1-7)

Cathode8H+ + 8e− → 4H2(1-8)

1.3.1.7. Ionic exchange
Ion exchange has not been widely used in wastewater treatment containing dyes. This is because
ion exchangers cannot accommodate a wide range of dyes. The colored wastewater is passed throughthe ion exchange resin until the available exchange sites are saturated. Using this method, cationicand anionic dyes can be successfully removed from wastewater. One major drawback is the highoperating cost [28, 31].
1.3.2. Biological treatment method
1.3.2.1. Aerobic biological treatment
Currently, the most used biodegradation method involves aerobic microorganisms, which use
molecular oxygen as a reducing equivalent receptor in the respiratory process. The microorganisms
used depend on the dye, and fungi such as lignolytic fungi, Phanerochaete chrysosporium,– 10 –

Chapter 1 Introduction

Geotrichum candidum, Trametes versicolor, Bjerkandera adusta, Penicillium sp., Pleurotus ostreatus,
Pycnoporus cinnabarinus, and Pyricularia oryzae are used for aerobic decomposition. In addition,
bacterial strains of Pseudomonas stutzeri, Acetobacter liquefaciens and Klebsiella pneumonia are also
used. Wastewater treatment plants are not a natural habitat for microorganisms, so special care must
be taken to establish these fungi in wastewater treatment systems.
In general, most azo dyes in conventional aerobic sewage treatment plants are not decomposed bybacteria. Therefore, conventional aerobic sewage treatment systems are not useful for decolorizationof wastewater containing azo dyes. In addition, about 50 to 150 mg of each dye/l has the disadvantage
of taking a little time because it takes 5 to 10 days to decolorize [11, 28, 35-36].
1.3.2.2. Anaerobic biological treatment
Azo dyes make up 60 ± 70% of all textile dyes, but in aerobic sewage treatment plants, most azo
dyes are not easy to degrade. On the other hand, anaerobic biological purification can decolorize azo
and other water-soluble dyes. Anaerobic microorganisms capable of decolorizing include
Pseudomonas, Bacillus, and Clostridium, and the degree of decolorizing depends on the type of dye,
substituent, and molecular weight. Decolorization is reported to involve oxidation-reduction reactions
with hydrogen rather than oxygen, as in aerobic systems, and to produce methane, carbon dioxide,hydrogen sulfide, and other gaseous compounds. In anaerobic systems, azo dyes break azo bonds anddecolorize, but toxic amines are formed. Anaerobic decomposition only results in azo reduction andno mineralization occurs. While many microbes can break the chromophores of some dyes, few
microbes can mineralize dyes into CO2 and H2O. Obviously, this is due to the complex structure of
the dye. The amines produced by reduction of azo dyes are colorless, but very resistant to further
degradation under anaerobic conditions. Therefore, careful monitoring is required before the treatedwastewater is discharged into the waterway. It also has the disadvantage of storing wastewater in largetanks because the biological treatment process requires a long time [8, 11, 28, 35].

– 11 –

Chapter 1 Introduction

1.4. Semiconductor photocatalyst
As shown in Table 1-3, the existing wastewater treatment technology has disadvantages such as
cost, stability, slurry, pretreatment, and slow decomposition rate. [3, 8, 10].
Table 1-3 Disadvantages of the existing wastewater treatment.
Processes

Disadvantages

Absorption

Cost of activated carbon
Difficult to recycle

Ultra-filtration
Micro-filtration

Insufficient quality of the treatedwastewater
Formation of sludge
Insufficient quality of the treatedwastewater
Formation of sludge

Nano-Filtration

Pretreatment required for removal ofmacromolecules

Reverse Osmosis

Pretreatment required for removal ofmacromolecules
High pressure

Oxidation with sodium hypochlorite

Release of aromatic amines

Oxidation by Ozonation

Short half-life (20 min)High equipment cost

Coagulation

Formation of sludge
Production of sludge blocking filter

Electrocoagulation

Formation of sludge

Ionic exchange

Not effective for all dyes
High operating cost

Aerobic biological treatment

Low biodegradability of dyes
Microorganisms used depend on the dye

Anaerobic biological treatment

Low biodegradability of dyes
Microorganisms used depend on the dye– 12 –

Chapter 1 Introduction

Therefore, the development of an eco-friendly, high-efficiency, low-cost organic pollutant
treatment method has become an important task for researchers. Recently, photocatalytic oxidationof organic pollutants has been proposed as a promising alternative [4]. Because photocatalyticoxidation treatment can photodecompose organic pollutants into H2O, CO2, inorganic substances
without generating secondary substances, no slurry is created. In addition, it does not require
pretreatment and has excellent characteristics in terms of cost. The process of decomposing organic
pollutants using inexpensive eco-friendly metal oxide photocatalysts such as WO3 [37], Fe2O3 [38], ZnO[39-40], TiO2 [41-42], and SnO2 [43] has been extensively studied. In addition, because the photocatalytic

reaction occurs on the surface of the photocatalyst, nano-sized particles with a large specific surface
area are attracting attention [44-45].
As shown in Figure 1-1, it is well known that when ZnO is irradiated with photon energy equal toor greater than the bandgap energy, ZnO absorbs the photon energy so that electrons in the valenceband (VB) are excited to the conduction band (CB):ZnO + ℎ𝑣 → ZnO(e− CB + h+ )Figure 1-1 Photocatalytic reaction mechanism of ZnO particle.
– 13 –(1-9)

Chapter 1 Introduction

When the separation of the holes of the VB and electrons of the CB is maintained, the holes andelectrons can be moved to the surface of the catalyst and used for the oxidation/reduction reaction.
Holes can form hydroxyl radicals (•OH) which react with H2O or OH¯ on the surface of the catalyst
and contribute to photodecomposition (Eqs. 1-10 and 11). Free electrons can form superoxide radical
anion (O2•¯) which is an active species by oxygen. The superoxide radical anion (O2•¯) reacts with
H+ to form hydrogen peroxide (H2O2), and the generated hydrogen peroxide (H2O2) is converted intohydroxyl radicals (•OH) by ultraviolet light (Eqs. 1-12,13,14 and 15). The resulting ∙OH radical acts
as a strong oxidizing agent and converts the organic material or polymer into carbon dioxide (CO2),
water (H2O) and other less hazardous molecules than the original material (Eq. 1-16) [3, 46-47]: Manyorganic dyes have benzene rings with high chemical stability as shown in Table 1-1. Therefore, since
RhB is often used as a model material for organic dyes, it was also adopted in this study.

OH− + h+ →∙ OH(1-10)

H2 O + h+ →∙ OH + H+

(1-11)

O2 + e− → O ∙2 −(1-12)O ∙2 − + H+ →∙ OOH(1-13)

2 ∙ OOH → O2 + H2 O2(1-14)H2 O2 + ℎ𝑣 → 2 ∙ OH

(1-15)

Pollutant +∙ OH → CO2 + H2 O + degradation product(1-16)The main drawback of semiconductor photocatalysts is the reduced efficiency due to the rapid– 14 –

Chapter 1 Introduction

recombination of excited electron-hole pairs. [3, 10]. For example, semiconductor photocatalysts such
as ZnO and TiO2 have limited photocatalytic activity due to the fast electron-hole recombination rate.
To control the electron-hole recombination of a photocatalyst, doping of nanotubes [42], graphene [41],base metals [48], metal oxides [49], metal sulfides [50], polymers [51], and noble metals [52-53] has beenstudied. In particular, the combination of a noble metal with a semiconductor such as ZnO provides
an improved photocatalytic activity by trapping excitation electrons. Figure 1-2 shows the reaction

Figure 1-2 Photocatalytic reaction mechanism of Au/ZnO composite particles.
mechanism of a noble metal-supported photocatalyst. When a noble metal is supported on a
photocatalyst, a noble metal acts as an electron sink and delays the recombination of the excitedelectrons and holes, so the efficiency of the photocatalyst can be improved [10]. Among a noble metal,
research on supporting Au [54-55], Pt [54, 56], Pd [57], and Ag [3,58-63] is actively underway. Various methods
have been developed to synthesize noble metal doped photocatalysts. These include chemical bathdeposition [58], photochemical [59], sol-gel [60], solvothermal [61], precipitation-decomposition [62],hydrothermal [63], and flame spray pyrolysis [3].
Moreover, another disadvantage of photocatalytic treatment is that it is very difficult to recovernano-sized photocatalysts after removing organic pollutants from wastewater, and reuse is greatly– 15 –

Chapter 1 Introduction

limited [8]. The inability to recycle photocatalytic particles not only increases the cost but can lead tonew pollution [44]. Therefore, research is being conducted to generate particles with relatively large
particle size and high activity or to support magnetic particles.
1.4.1. Zinc oxide (ZnO)
In the semiconductor metal oxide series, II-VI group semiconductors are recognized as applications
in solar cells, solar cells, field effect transistors, optoelectronic devices, and photoluminescenceappliances [64-66]. Among these semiconductors, ZnO is a direct bandgap semiconductor (3.4eV) witha huge exciton binding energy of 60meV, and has been regarded as a semiconductor having numerous
applications in the scientific community since the 1930s [67]. Therefore, in recent years, it is a versatilematerial that has been used in various applications due to its excellent biocompatibility and low
toxicity. Examples include solar cells [68], gas detectors [69], photocatalysts, optoelectronic [70], etc. In
addition to this, ZnO is used as an additive in industrial products such as pigments, adhesives, cement,
lubricants, glass, paint, rubber, cosmetics, and coatings [71]. Research on composite materials is also
reported, and transition metal/rare earth (RE) doped ZnO structure shows superior structural andoptical properties such as photoluminescence, transmittance, diffraction, and magnetic behavior thanpure ZnO.
In this study, we adopted the spray pyrolysis method to generate submicron particles and support
other metals. Along with ZnO, TiO2 is also widely used as a photocatalyst, but since a water-soluble
raw material is used in the spray pyrolysis method, zinc oxide was produced using zinc nitrate, which
can be used as a source at a relatively low cost.

1.5. Summary of this thesis
In this paper, two experiments were presented to confirm the photolysis of organic pollutants usinga photocatalyst and the reusability of the photocatalyst. First, photocatalytic nanoparticles have a large
specific surface area, so they have good photodegradation properties, but have a disadvantage in thatthey are expensive because they cannot be recovered and recycled. Therefore, relatively large submicro-sized particles were simply one-pot synthesized using the spray pyrolysis method, and singlecrystal metal particles were doped inside the photocatalyst to enhance the photocatalytic activity. And
the possibility of recovery and recycling through gravity sedimentation was confirmed. Second, in– 16 –

Chapter 1 Introduction

order to simplify the recovery of the photocatalyst particles, a magnetic recovery experiment throughmagnetite doping was conducted. Micro-sized ZnO/Fe3O4 particles were generated using the coprecipitation method, and Ag doping was performed for higher photocatalytic properties. And thepossibility of recovery and reuse of particles using magnetism was confirmed.
In Chapter 2, the electron-hole recombination control of the photocatalyst was investigated byloading Au in ZnO (Au/ZnO). Using the Ultrasonic Spray Pyrolysis (USP) method, Au/ZnO particle
generation is easily achieved under various conditions. XRD analysis confirms the crystal peaks of
ZnO and Au. The EDX mapping and STEM images of the particles show that the Au crystals are welldispersed in the inner and outer portions of ZnO. The photocatalytic decomposition rate of organicdye (Rhodamine-B) is faster than that of ZnO in all Au/ZnO particles, and the best photocatalytic
activity occurs in particles with 0.1 mass% Au supported on ZnO particles. In addition, optimal
photolysis activity occurs in 100 ml of 5 mg/L RhB aqueous solution and 10 mg dose of Au/ZnO
particles.
In Chapter 3, the inhibition of electron-hole recombination was investigated by supporting Ag
nanoparticles on a ZnO photocatalyst. Ag/ZnO particles were generated under various conditions
using the Ultrasonic Spray Pyrolysis method. Through XRD analysis, the crystal peaks of Ag and
ZnO were confirmed in the generated particles. EDX mapping and STEM images show that Ag
nanoparticles are well dispersed in ZnO. The photolysis rate of organic dye (Rhodamine-B) was fasterthan that of ZnO in all Ag/ZnO particles, and particles with 0.2 mass% silver nitrate supported on
ZnO particles showed 2 times better photolysis activity than P25. In addition, the optimal photolysis
activity was confirmed in 100ml of 5mg/L RhB aqueous solution and 10mg of Ag/ZnO particles, andreusability was also confirmed.
In Chapter 4, synthesis of ZnO/Fe3O4 composite particles was attempted using the USP and coprecipitation method under various conditions. When the USP method was used as a single method,
the crystal peaks of the target products ZnO and Fe3O4 were not observed, and a single crystal peakof ZnFe2O4 was confirmed under certain conditions. When particles were generated using the USP
and co-precipitation method together, the crystal peaks of the target products, ZnO and Fe3O4 wereobserved, and the magnetism of the generated particles was confirmed. However, since theaggregation of particles and uncoated ZnO particles were confirmed, it was confirmed that the surface
modification using sodium citrate was not suitable for the generation of ZnO/Fe3O4 composite– 17 –

Chapter 1 Introduction

particles. When co-precipitation was used as a single method, crystal peaks of the target products
ZnO, Ag, and Fe3O4 were observed, and the magnetism of the generated particles was also observed.
In addition, the Ag particles dispersed in the particles could be confirmed with backscattered electron
images. In the photolysis experiment, excellent photolysis properties were confirmed in ZnO/Fe3O4
composite particles generated by adding 0.2 mass% of silver nitrate.
Final conclusions are presented in Chapter 5.

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K. Nomura, H. Ohta, K. Ueda, T. Kamiya, M. Hirano, H. Hosono, Thin-Film Transistor
Fabricated in Single-Crystalline Transparent Oxide Semiconductor, Science, 300 (2003)
1269-1271.

67.

Ü. Özgür, Ya.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Doğan, V. Avrutin, S.J. Cho,
H. Morkoç, A comprehensive review of ZnO materials and devices, Journal of Applied
Physics, 98 (2005).

68.

D.T. Wang, X.H. Zhu, Y.Z. Fang, J.H. Sun, C. Zhang, X.X. Zhang, Simultaneously
composition and interface control for ZnO-based dye-sensitized solar cells with highly
enhanced efficiency, Nano-Structures & Nano-Objects, 10 (2017) 1-8.

69.

A.A. Tomchenko, G.P. Harmer, B.T. Marquis, J.W. Allen, Semiconducting metal oxide
sensor array for the selective detection of combustion gases, Sensors and Actuators B, 93(2003) 126-134.

70.

S. Singh, M. Joshi, P. Panthari, B. Malhotra, A.C. Kharkwal, Citrulline rich structurally
stable zinc oxide nanostructures for superior photo catalytic and optoelectronic applications:
A green synthesis approach, Nano-Structures & Nano-Objects, 11 (2017) 1-6.

71.

N. Serpone, D. Dondi, A. Albini, Inorganic and organic UV filters: Their role and efficacy
in sunscreens and suncare products, Inorganica Chimica Acta, 360 (2007) 794-802.

– 24 –

Chapter 2

Evaluation of Photocatalysis of Au Supported ZnO Prepared by the Spray Pyrolysis Method

Chapter 2

Evaluation of Photocatalysis of Au Supported ZnO
Prepared by the Spray Pyrolysis Method

2.1. Introduction
Environmental pollution by residual dyes in the paper, textiles, and other industries is a very serious
problem in developing countries as described in Chapter 1. Typically, wastewater is treated using
adsorption or chemical coagulation methods. However, these methods only transfer dyes from water
to solids, and require further treatment to decompose. Therefore, a better wastewater treatment
method is necessary. Photocatalytic oxidation treatment of organic pollutants has recently been
proposed as a promising alternative [1–3].
Photocatalytic applications of ZnO and TiO2 have grown significantly due to their chemical andphysical stabilities and low cost. ZnO is a semiconductor material with a wide bandgap energy (3.37
eV) and a large exciton binding energy of 60 meV [4]. Although TiO2 is a more common photocatalyst
than ZnO [5], ZnO can be used as a substitute for TiO2 because they have similar bandgap energies.
In addition, ZnO has a higher reported quantum efficiency than TiO2 and has attracted attention forthe photolysis of pollutants due to its high stability and wide bandgap energy [6-7]. This wide bandgap
and large exciton binding energy are not only useful for the photolysis of organic pollutants but canalso disinfect and eliminate odors. In addition, ZnO has several potential applications such as gas
sensors, solar cells, varistors, and optoelectronic devices [8].
However, as previously explained, semiconductor photocatalysts such as ZnO and TiO2 have
limited photocatalytic activity due to the high electron-hole recombination rate [4, 7]. Therefore, inorder to prevent the recombination of the excited electron-hole pairs and improve the photocatalytic
properties, a composite material that supports nanotubes [9], graphene [10], base metals [11], metaloxides [12], metal sulfides [13], polymers [14] and noble metals [15-16] on the photocatalyst is being studied.
In particular, since there are many applications depending on the type of supported noble metal andthe method of synthesizing the particles, various types of supporting studies have been reported.
Therefore, various methods are being developed to synthesize photocatalysts supporting noble metals.
Methods of generating particles include combustion, flame spray pyrolysis, sonochemical reaction,and hydrothermal, and the results of generating particles by each method have been reported. Pathak– 25 –

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Evaluation of Photocatalysis of Au Supported ZnO Prepared by the Spray Pyrolysis Method

et al. produced Au/ZnO and Ag/ZnO nanoparticles using the combustion method and evaluated thephotocatalytic efficiency using methylene blue dye [17]. Pawinrat et al. produced Au/ZnO and Pt/ZnO
nanoparticles using flame spray pyrolysis and evaluated the photocatalytic efficiency using methylene
blue [18]. Kim et al. carried out Au nanoparticles on ZnO nanorods using the hydrothermal method
and sonochemical method, and evaluated the photocatalytic efficiency using rhodamine B [19]. These
nanoparticles show good photocatalytic efficiency because of their large specific surface area, buthave the disadvantage of being difficult to recover after use. In addition, the wet method requiring
post-treatment may reduce the photocatalytic efficiency due to agglomeration between particles.
Haugen et al. confirmed that the size of the doped Au or Ag crystals increased and the photocatalytic
activity decreased after calcination of the particles [20]. Therefore, there was a need for a method that
could produce submicron-level particles without agglomeration in a single process. In addition, there
was a need for a generation method that could increase the efficiency of the photocatalyst by
supporting the noble metal as a single crystal.
Recently, Ebins et al. confirmed the crystallite size and catalyst characteristics after generating zincoxide by temperature changes using ultrasonic spray pyrolysis (USP) [21]. The USP method cangenerate continuous particles in a single process, and it is possible to adjust the particle shape bytemperature control. In addition, it was possible to control the crystallite size by the change in theconcentration of the raw material solution, and it was possible to dope other substances by adding thetarget precursor to the aqueous solution of the raw material [22-23]. The USP method consists of fourmain steps: droplet generation by ultrasonic nebulizer, precipitation by evaporation of solution,
chemical reaction of precursor at high temperature, and densification of initial particles [24-25].
In this study, we employ the USP method to generate functional particles with high oxidation
characteristics that can be used in the oxidation treatment of organic pollutants. First, we
experimentally examine the conditions in which ZnO particles are generated. Then, we prepare Audoped ZnO (Au/ZnO) particles using chloroauric acid as the noble metal source. Finally, we analyze
the structural, morphological, and photolysis characteristics of the obtained particles.

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Evaluation of Photocatalysis of Au Supported ZnO Prepared by the Spray Pyrolysis Method

2.2. Experimental equipment and theory
2.2.1. Ultrasonic Spray Pyrolysis

Figure 2-1 Schematic diagram of ultrasonic spray pyrolysis.
Application research on particles with a size of several μm or less is being conducted in electronicmaterials, catalysts, and analytical chemistry. Particles change their properties by their shape, size,and composition, so a controllable particle generation process is important. In addition, from the pointof view of industrial application, the equipment must be simple and continuous operation, highproduction speed and low cost. Aerosol process, a method for producing particulate materials thatcombines both liquid and gas phase aerosol processes, may be such a process. Aerosol is a type of
colloid in which solid or liquid particles are suspended in the atmosphere. The term "aerosol" is usedin the fields of air, health and air pollutants and materials processing.

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Evaluation of Photocatalysis of Au Supported ZnO Prepared by the Spray Pyrolysis Method

There are two main routes for particle production by the aerosol process. It is a method of
converting from gas to particles and a method of converting from liquid to particles. In gas-particle
conversion, particles are synthesized by cooling supersaturated vapors using a method called physicalvapor deposition (PVD) or chemical vapor deposition (CVD) [25]. In the PVD method, evaporation ofa solid or liquid is a source of vapor, and solid particles are formed via nucleation and condensationof saturated vapor in the cooling step. Many types of reactors, such as flame, furnace, plasma or laser,
have been used to generate the vapor of the precursor. In the CVD method, vapors evaporated froma precursor are reacted with other precursor vapors or surrounding gases or pyrolyzed. Target particles
are produced by chemical reaction or pyrolysis in the air and undergo condensation and agglomeration.
The main advantages of the gas-particle conversion method are the narrow size distribution, small
particle size and high purity of the synthesized particles. However, it is difficult to prepare particulatematerials due to the disadvantage that large agglomerates are formed during agglomeration andcondensation in the gas phase. Differences in chemical reaction rates, vapor pressures, nucleation andgrowth rates during particle generation can lead to non-uniform particle synthesis results.
Liquid-to-particle conversion is typically a spray pyrolysis method. Because a solution or sol isused, the spray method is often classified as a liquid method. This method can be used to synthesize
functional particles such as multi-component materials. Compared to the gas-to-particle conversion
path, the spray method is a simple and inexpensive process, but little is known about the process itself[22].

Evonik’s TiO2 benchmark P25 is a good example of a photocatalyst produced on an industrial scaleby vapor-fed aerosol flame synthesis (VAFS). This flame-assisted process involves burning in a flameby supplying a volatile precursor such as titanium tetrachloride (TiCl4) in the gaseous phase. However,finding suitable volatile metal precursors can be limiting. Alternatively, techniques such as ultrasonicspray pyrolysis (USP) can solve this problem. USP can use a water-soluble precursor to generate
aerosols using an ultrasonic atomizer and transport aerosol particles into the reactor using a carriergas. In the USP method, a tube furnace may be used for the combustion or pyrolysis of the precursor[26].

The spray pyrolysis method is used not only in the synthesis of particles, but also in experiments
such as thin film deposition and in industrial sites. In general, in the spray pyrolysis method, the– 28 –

Chapter 2

Evaluation of Photocatalysis of Au Supported ZnO Prepared by the Spray Pyrolysis Method

aerosol generated by the spray device is transported to the reactor by the carrier gas, and particles are
generated through pyrolysis. Fig. 2-1 shows an experimental apparatus used in this research. The USP
apparatus was composed of a nebulizer, a quartz glass reaction tube, electric furnaces, a diffusion
dryer, and an electrostatic precipitator. In the USP device, the precursor solution is placed inside theultrasonic nebulizer, which is a device containing a high-frequency ultrasonic vibrator. The carrier
gas is introduced into the ultrasonic nebulizer and the aerosol is transported to the tubular reactor by
the gas flow. Particles generated by pyrolysis in the tubular reactor move to the collector (filter,electrostatic precipitator, etc.).
The spray pyrolysis method has the following advantages. (i) The resulting particles are spherical.
(ii) The diameter distribution is uniform and controllable from micrometer to submicron. (iii) The
product is of high purity. (iv) The process is continuous. Since each sprayed droplet has the same
composition, properties can be controlled through chemical control of the precursor solution. It isvery promising to apply the spray pyrolysis process to industry because the speed of particle
generation is fast and particles can be generated within seconds. Conversely, using conventional solid
or liquid phase methods requires post-treatments such as calcination and milling to obtain the desiredparticle size, which often introduces impurities into the particles.
Figure 2-2 shows the particle formation process in the spray pyrolysis method. When the raw
material droplets are heated, the solvent evaporates from the droplet surface, and the soluteconcentration on the droplet surface increases. Therefore, the solute diffuses toward the center of thedroplet where the solute concentration is low, but when heating is performed rapidly, the solvent
evaporates rapidly and the solute concentration on the droplet surface rapidly increases, eventually
becoming supersaturated and precipitation of salt occurs. Then, the solvent inside the shell evaporates,
and hollow particles and ruptured particles are generated. On the contrary, when gentle heating is
performed, the evaporation rate of the solvent slows down and the solute diffuses toward the centerof the droplet, so that the surface of the droplet does not become supersaturated, and the droplet
solvent evaporates only from the surface of the droplet. And solid particles are formed [27]. As
precursor solutions, inexpensive materials such as nitrates, chlorides and acetates are commonly used.
In general, water or alcohol is used as the solvent.

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Evaluation of Photocatalysis of Au Supported ZnO Prepared by the Spray Pyrolysis Method

The properties of the precursor, the carrier gas flow rate and temperature of furnaces are the main
parameters that affect the morphology of the particles produced by spray pyrolysis. Hollow or
ruptured particles often produced by spray pyrolysis are undesirable for applications. Depending on
the production conditions, single crystal or polycrystalline particles are also formed. The average sizeand size distribution of the final particles can be roughly determined from the droplet size and theinitial concentration of the starting solution. Therefore, the size or shape of the final particles
produced is determined by the concentration and movement speed of the droplets produced by thenebulizer. Therefore, various spraying methods have been used in spray pyrolysis studies.

Figure 2-2 Particle formation process in USP method.
– 30 –

Chapter 2

Evaluation of Photocatalysis of Au Supported ZnO Prepared by the Spray Pyrolysis Method

2.2.1.1. Atomization
Atomization is a key parameter to determine particle size. In the early stages of particle generation,precursors are supplied to specific atomization equipment to convert them into droplets. Certain skills
are required to perform this droplet formation operation. In traditional methods, aerosol generation
through temperature assisted evaporation treatment is commonly used. The principle of this method
is to vaporize the precursor at a specific temperature to create droplets [28]. This method can produce
single and uniform droplets, but cost becomes an issue. To solve this drawback, the use of certain
momentum can be used as an alternative technique to traditional methods. Several types of driving
force can be used to support the atomization process. For example, pressure, centrifugation,
electrostatic and ultrasonic energy are selected depending on the required droplet size [29]. In addition
to selecting the sprayer configuration, surface tension, viscosity, density, etc., should be considered
in order to control the droplet size and properties of the spray precursor [30].

Table 2-1 Typical droplet sizes of depending on spray method.

Atomizer Type

Average Initial Droplet Size[μm]

Ultrasonic (submerged)

1 to 10 [33]Ultrasonic (nozzle)

10 to 1000 [33]Electrospray

15 to 25 [34]RotaryAir-blast orchard sprayer

10 to 1000 [33]110 to 200 [34]120 to 250 [35]130 to 200 [34]Two-fluid

5 to 100 [35]Cold–fogger

8 to 22 [34]Grooved spinning disc

70 to 150 [34]

Smooth spinning disc

90 to 150 [34]Knapsack mistblower

100 to 200 [34]Pressure

Among the many atomization methods (Table 2-1), ultrasonic atomization devices are widely usedbecause of the high energy efficiency of aerosol generation, ease of use, and fast aerosol generation
speed. Therefore, the USP method has been found to be effective in synthesizing various types of– 31 –

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Evaluation of Photocatalysis of Au Supported ZnO Prepared by the Spray Pyrolysis Method

materials and has the advantage of easy scalability [31-32]. In the USP method, when high-frequency
ultrasonic waves pass through a liquid precursor solution and collide with the liquid-gas interface, adroplet aerosol of micrometer size is created.
The formation of droplets by ultrasound was first reported by R. W. Wood and A. L. Loomis in
1927, and Lang experimentally confirmed the relationship between ultrasound frequency and average
droplet size in 1962. If a single particle is generated without the splitting and combining of dropletsof the sprayed raw material solution, the generated droplet size can be estimated as [36]8𝜋𝛾 1/3
𝐷droplet = 0.34 ( 2 )
𝜌𝑓

(2-1)

where f is the frequency of the ultrasonic transducer,  is the density of the solution, and  is thesurface tension of the solution. Since the experiments are performed in very dilute solutions, theestimated  and  values are equal to those of pure water. Assuming f = 1.7 MHz,  = 0.0729 N/m,and  = 1000 kg/m3, the droplet size is calculated as 𝐷droplet = 2.92 m from equation (2-1).

Figure 2-3 Droplet size by ultrasonic frequency intensity.

– 32 –

Chapter 2

Evaluation of Photocatalysis of Au Supported ZnO Prepared by the Spray Pyrolysis Method

Laboratory-grade ultrasonic spray pyrolysis using high frequency (approximately 2 MHz) is very
useful for fast hypothesis validation and facilitates small-scale synthesis of new materials. However,
the intensity of ultrasonic waves on the surface of a high-frequency oscillator is not sufficient to spraysolvents or slurries with high viscosity, density, and boiling point. Therefore, commercial devices thatgenerate large amounts of droplets using high-intensity ultrasonic waves are on the market. Theseultrasonic nozzles operate at frequencies of 20 to 120 kHz and are capable of producing droplets ofdifferent sizes (up to about 10 μm). Even liquids or slurries with high viscosity and low vapor pressure
can be sprayed up to about 100 mL/min [33]. The yield and quality of the final product depends on theflow rate of gases and liquids through the reactor, the temperature in the reactor, the ultrasonic output
solvent used, and the chemical composition of the precursor mixture. Very high yields can be obtainedby changing the experimental parameters. Silica, titania, iron oxide, zinc oxide and various
binary/ternary metal oxides can be prepared by all USP [37-41]. Also, Figure 2-3 shows the droplet size
as a function of the frequency of the ultrasonic transducer.
2.2.1.2. Electrostatic collector
In the USP method, cyclones, electrostatics, and filters are generally used to collect the generated
particles. In this study, an electrostatic collector was used and its schematic diagram is shown in
Figure 2-4. Insect specimen stainless steel micro-needle (Shiga Konchu Co., Ltd., manufactured by
0.18 mm) was soldered to the terminal crimp using a special jig, and fixed with screws to be used asdischarge electrodes. An aluminum plate was used for the particle collecting electrode of the flat plate.
5.00 kV DC power was applied using a high voltage power supply. Since the tip of the stainless-steel
needles were worn by the discharge, the six discharge electrodes in the collector were wired three by
three, and the discharge electrodes that applied voltage at 3 h were replaced. This enabled continuous
collection for 6 hours. In order to prevent dew condensation inside the collector due to rapid cooling,
the electrostatic collector was wrapped by a ribbon heater and the temperature of the collector was
kept at 50 °C using a temperature controller.

– 33 –

Chapter 2

Evaluation of Photocatalysis of Au Supported ZnO Prepared by the Spray Pyrolysis Method

Figure 2-4 Schematic diagram ofelectrostatic collector.

– 34 –

Chapter 2

Evaluation of Photocatalysis of Au Supported ZnO Prepared by the Spray Pyrolysis Method

2.3. Experimental setup and Method
ZnO particles were generated using the USP apparatus in Figure 2-1. Zn(NO3)2 6H2O was used as
the zinc precursor. The precursor was dissolved to 100 ml distilled water with 2.5 to 10 mass% andthe precursor solution was introduced into the nebulizer at a rate of 15 mL/h. An ultrasonic nebulizer(OMRON, NE-U17) with a resonance frequency of 1.7 MHz was used to spray the precursor solution.
Droplets were introduced into the quartz glass reaction tube at a rate of 1.0 SLM by the carrier gas.
The flow rate of sheath gas was fixed at 0.5 SLM to prevent droplet precipitation. The quartz glass
reaction tube was installed inside four electric furnaces with a temperature controller of ±1 °C. And
the temperature of the electric furnace was measured at between the inner wall of the furnace and theouter wall of a quartz glass reaction tube by thermocouples. The inner diameter, outer diameter, andlength of the quartz glass reaction tube were 15.25, 18.25, and 1208.50 mm, respectively. Theseelectric furnaces were used to heat the quartz reaction tube: two 80-mm preheating furnaces of No. 1(40 °C) and No. 2 (70 °C), and two 290-nm long heating furnaces of No. 3 (150 °C or 1000 °C) andNo. 4 (1000 °C). The particles obtained by decomposition of precursor solution droplets were thendried by a diffusion drier. Finally, particles were collected in the electrostatic precipitator.
2.3.1. Materials
Zinc nitrate hexahydrate (Zn(NO3)2 6H2O, ≥99.0%) and chloroauric acid (HAuCl4, ≥99.9%) werepurchased from FUJIFILM Wako Pure Chemical Industries, and Rhodamine-B (C28H31ClN2O3,>95.0%) was purchased from Tokyo Chemical Industries. Titanium dioxide nanopowder as a standard
photocatalyst (Evonik AEROXIDE○R P25, formerly Degussa) was purchased from Sigma–Aldrich.
All reagents were used as received without further purification. Distilled water was used in all
experiments.
2.3.2. Characterization
The size distribution of ZnO particles was measured by laser diffraction spectrometry (Microtrac,
MT3000EX). The size and shape of the ZnO particles were observed using a scanning electron
microscope (SEM, JEOL JSM-6380A). X-ray diffraction (Rigaku, Multiflex) using Cu K radiation
was used to examine the crystallite size and crystalline phase of the generated particles. The mean
crystallite size of Au/ZnO composite particles was calculated using Scherrer's equation. The shape,– 35 –

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Evaluation of Photocatalysis of Au Supported ZnO Prepared by the Spray Pyrolysis Method

size, and elemental of Au/ZnO composite particles were observed by a transmission electron
microscope (TEM, JEOL JEM-2100F) combined with energy dispersive X-ray spectroscopy (EDXS).
The generated Au/ZnO composite particles were dispersed in water, and the UV-vis spectrum wasanalyzed using a spectrometer with an absorption wavelength of 200–850 nm (SIMAZU, UV-1800).
2.3.3. Photocatalytic activity
The photocatalytic activities of the synthesized powders were evaluated by photolysis of
Rhodamine-B (RhB, Tokyo Chemical Industries) dyes. For each condition, 2–15 mg of particles and
100 ml of 5 mg/L RhB aqueous solution were mixed in a 250 ml beaker. The photocatalyst was
ultrasonication for 10 minutes in an RhB aqueous solution and left in the dark for 5 minutes toestablish an adsorption-desorption equilibrium between the photocatalyst and the dye before
photocatalytic decomposition. And during UV light irradiation, the mixture was continuously stirred
to maintain full suspension of the particles in the experiments. The photolysis reaction was performed
under UV light irradiation from a single 15W lamp of 315–400 nm (TOSHIBA, FL15BL) locatedhorizontally 65-mm above the liquid surface. A lux meter maintained the light intensity of the lamp.
A 4-ml aliquot sample was extracted every 30 minutes. The residual dye concentration was measured
after removal of the photocatalyst by precipitation. The photocatalyst precipitation process did not
involve light irradiation. The change in the RhB concentration was analyzed by measuring theabsorbance of the sample aliquots using a UV-Vis spectrophotometer. The concentration of RhB was
analyzed by recording the change in the maximum absorption peak of RhB (555 nm).

2.3. Experimental Results and Discussion
2.3.1. Characteristics of ZnO particles
The estimated diameter of the generated ZnO particles can be calculated as

3

𝐷ZnO = √3
𝐷droplet𝐶Zn(NO3)2 aq. 𝑀𝑊ZnO

𝑀𝑊Zn(NO3)2 𝜌ZnO

– 36 –(2-2)

Chapter 2

Evaluation of Photocatalysis of Au Supported ZnO Prepared by the Spray Pyrolysis Method

where 𝜌ZnO is the density of ZnO (5.606 × 10³ kg/m³), 𝑀𝑊Zn(NO3)2 is the molecular weight of zinc
nitrate (297.49 kg/kmol), 𝑀𝑊ZnO is the molecular weight of ZnO (81.37 kg/kmol), 𝐶Zn(NO3)2 aq. is
the raw material concentration [g/L], and 𝐷ZnO is the ZnO particle diameter [m]. The estimateddiameter of the particles is calculated as 0.363–0.573 μm from the raw material concentration of 10–
2.5 mass%.
The USP method often generates hollow or burst particles [24]. These not only hinder a uniform
particle size distribution but are also susceptible to corrosion. Lenggoro et al. (2000) suggested that
controlling the concentration of the raw material solution and the temperature of the furnace may
realize dense particle generation [22]. Therefore, we attempted to generate dense particles bycontrolling these parameters. Table 2-2 lists the experimental conditions and the particle size
distribution. Figure 2-5 shows the SEM observation result of the obtained particles.

Table 2-2 Experimental conditions and particle size distribution of ZnO particles.

SampleZinc Nitrate[mass%]Temp.
No.3[ oC ]

Dp10
[μm]

Dp50
[μm]

Dp90
[μm]

Dp90/Dp10

Estimated particle
diameter
[μm]

1

10

1000

0.415

0.608

1.332

3.21

0.573

2

5

1000

0.570

0.738

1.219

2.24

0.457

3

5

150

0.600

0.747

1.178

1.96

0.457

4

2.5

150

0.574

0.693

1.041

1.81

0.363

Sample 1 has a large particle size distribution (=Dp90/Dp10) of 3.21. This is consistent with the SEM
image, which shows a mixture of dense and hollow particles. Samples 2, 3, and 4, which have a lower
concentration of the raw material solution, show a narrower particle size distribution of 2.24, 1.96,
1.81, respectively. The SEM image of sample 4 indicates a distribution of small, dense, and narrow
sized particles. Despite the small particle size and narrow size distribution, the particle size obtained
under condition 4 is slightly larger than the estimated diameter obtained from equation (2). Thisslightly larger size is attributed to the generation of some larger particles by the combination of theinitial spray droplets or the presence of a small number of hollow particles.

– 37 –

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Evaluation of Photocatalysis of Au Supported ZnO Prepared by the Spray Pyrolysis Method

Figure 2-6 shows the XRD pattern of sample 4. All peaks of ZnO [(100), (002), (101), (102), (110),(103), (200) and (112)] are characteristic of the wurtzite (hexagonal) structure (JCPDS Card No:361451). Samples 1, 2, and 3 have similar XRD pattern to that shown in Figure 2-6. Additionally, the
XRD peak did not change with furnace temperature or concentration.

Figure 2-5 SEM images of ZnO particles obtained at different conditions.

Figure 2-6 XRD pattern of ZnO particle (sample 4).

– 38 –

Chapter 2

Evaluation of Photocatalysis of Au Supported ZnO Prepared by the Spray Pyrolysis Method

2.3.2. Characteristics of Au/ZnO particles
The Au/ZnO composite particles were generated using the same procedure as above, except
chloroauric acid was added to the precursor solution. Because ZnO particles of sample 4 generated
in the previous section were small and dense particles with a narrow size distribution, Au/ZnO
particles were prepared using the same conditions as sample 4. Table 2-3 shows the experimental
conditions to generate Au/ZnO composite particles and the Au crystallite size.

Table 2-3 Experimental concentrations of Au/ZnO composite particles andcrystallite size of Au calculated by Scherrer's equation.

SampleConcentration [mass%]
Zn(NO3)2

a
b
c
d

2.5

HAuCl4

Crystallite size of Au
(nm)

0.025

7.2

0.05

10.2

0.1

11.7

0.2

13.5

Figure 2-7 shows the XRD pattern of the obtained Au/ZnO particles. Similar to the XRD results of
the ZnO particles shown in Figure 2-6, all samples display ZnO hexagonal peaks. Regardless of theZn/Au molar ratio, the face-centered cubic structure of Au (JCPDS Card No:04-0784) appears in all
Au/ZnO composite particles, suggesting that the metal form of Au is conserved in the composite
particles. The peaks (111), (200), and (220), which are characteristic of the fcc structure of Au,
decrease as the addition ratio of chloroauric acid decreases. Based on Scherrer's equation, thecrystallite size of Au was calculated using (111) diffraction peak. The crystallite sizes of Au in
samples a, b, c, and d are 7.2, 10.2, 11.7, and 13.5 nm, respectively. The small peak at 2θ = 11 ° isthought to be the result of the reaction of chlorine, which is separated when chloroauric aciddecomposes, with intermediate products during pyrolysis. These results approximately correspondwith the results of our previous study, which employed compressed air as the carrier gas [42].

– 39 –

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Evaluation of Photocatalysis of Au Supported ZnO Prepared by the Spray Pyrolysis Method

Figure 2-7 XRD pattern of Au/ZnO composite particles.
Figure 2-8 shows TEM images of samples a–d, which were generated at various concentrations ofchloroauric acid. Dark spots are dispersed throughout the particle. The size of dark spots depends on
the addition ratio of chloroauric acid. Therefore, we used EDX mapping to analyze the particle
components.
Figure 2-9 shows the EDX mapping images and STEM image of sample d particle. The mapping
image of the Au element and STEM image show that the dark spots on the particle correspond to Au.
The presence of Zn and O throughout the particle means that small Au particles are dispersed
throughout the ZnO particle. In other words, Figs. 2-8 and 2-9 complement the XRD results andindicate that the Au particles are preserved in the form of metal in the Au/ZnO composite. The Au
particles identified by the EDX mapping imades of Sample d were measured through the STEM
image. Sizes of about 60 Au particles were measured using a scale ruler in the STEM image, and thearithmetric mean size was 13.2 nm. This was in close agreement with the crystallite size obtained byScheller's equation in the XRD pattern. This means that Au particles are dispersed as single crystalsof metal in Au/ZnO composite.
– 40 –

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Evaluation of Photocatalysis of Au Supported ZnO Prepared by the Spray Pyrolysis Method

Figure 2-8 TEM images of Au/ZnO composite particles obtained by USP synthesis.

Figure 2-9 STEM image of sample d particle and the corresponding EDX mapping images of
the Au, Zn, and O, respectively.
– 41 –

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Evaluation of Photocatalysis of Au Supported ZnO Prepared by the Spray Pyrolysis Method

2.3.3. Photocatalytic activity
Assuming that the photolysis reaction of RhB is a first order reaction, the change in RhB
concentration 𝑐 can be expressed as

−ln

𝐶
= 𝑘𝑡
𝐶0

(2-3)

where 𝐶0 , 𝑘, 𝑡 are the initial concentration of the RhB, rate constant, and time, respectively.
Photocatalysis was performed by adding 5 mg of the photocatalyst in aqueous RhB. Figure 2-10
shows a semilogarithmic plot of the changes in the RhB concentration of ZnO (sample 4) particles
and the Au/ZnO composite particles (samples a–d) as a function of UV light irradiation time. P25 wasincluded here as a standard photocatalyst. The negligible concentration change of the RhB aqueous
solution without a photocatalyst under UV irradiation for 180 minutes demonstrates that RhB is
sufficiently stable in the photocatalytic activity analysis. All Au/ZnO composite particles show ahigher photocatalytic activity than pure ZnO particles. However, sample d, which had 0.2 mass% ofadded chloroauric acid, shows a decreased photocatalytic activity. The removal of organic matter

Figure 2-10 Semilogarithmic plot of changes in RhB concentration by UV light irradiation
time at ZnO (Sample 4), samples a-d, and P25; initial concentration of RhB:
5mg/L; photocatalyst dose: 5mg; solution volume: 100ml.
– 42 –

Chapter 2

Evaluation of Photocatalysis of Au Supported ZnO Prepared by the Spray Pyrolysis Method

using a photocatalyst is an issue in the field of water purification because photocatalyst performance
deteriorates over time and toxicity of photocatalysts may be released. Although the decomposition
activity of our particles is similar to that of P25, they can be easily removed from water by means ofgravitational sedimentation after the decomposition of organic pollutants because the particles
produced by the USP method have a relatively large particle size.

Table 2-4 First-order reaction rate constant k and the coefficient of determination R2
corresponding to the slope of the line.

Reaction rate constant, k [min-1]

Coefficient of determination, R2

a0.004790.987

b0.005270.990

c0.008350.994

d0.00544

0.996

ZnO

0.00234

0.993

P25

0.00855

0.985

SamplesFigure 2-11 First-order reaction rate constant for RhB photolysis using ZnO (Sample 4),
samples a-d, and P25 under UV light irradiation; initial concentration of RhB: 5
mg/L; photocatalyst dose: 5 mg; solution volume: 100 ml.
– 43 –

Chapter 2

Evaluation of Photocatalysis of Au Supported ZnO Prepared by the Spray Pyrolysis Method

Table 2-4 shows the first-order reaction rate constant k and the coefficient of determination R2
corresponding to the slope of the line. And the first-order reaction rate constant k is graphically plottedin Figure 2-11. From the result of the coefficient of determination shown in Table 2-4 is close to 1,
the photolysis reaction can be considered to be a first-order reaction. Compared to pure ZnO particles,
samples a, b, and c have about 2.5 times higher photolysis rate constants, while that for sample c isabout 4 times higher. Although sample d had a higher amount of Au than that of sample c, thephotolysis rate constant is about half that of sample c. From the TEM analysis of Figure 2-8, thenumber and particle size of Au nanoparticles deposited on ZnO particles increases as the amount of
supported Au increases. However, when an excessive amount of Au nanoparticles is supported onto
ZnO particles, Au nanoparticles act as recombination centers for excitation electrons. This leads to adecrease in photocatalytic activity [15, 43].

Figure 2-12 (a) Fermi level of ZnO and Au particles before compounding, (b) Fermi level of
ZnO and Au particles after compounding. E0:vacuumlevel; (Ef)Au, (Ef)ZnO, Ef:
Fermi levels of Au, ZnO, and Au/ZnO; CV and VB : conduction band andvalence band of ZnO.
This change in photocatalytic activity can be explained by the change in the Fermi level of ZnO
and Au nanoparticles. Figure 2-12 shows the change in Fermi level due to contact between ZnO andAu particles. As shown in Fig. 2-12(a), the Fermi levels of ZnO and Au particles before contact
differed by -5.3 and -4.99 eV, respectively. When ZnO and Au particles come into contact, it becomes
Fermi level as shown in Fig. 2-12 (b), and the Schottky barrier is formed because the Fermi level of
ZnO is relatively high. And since the voltage of the excited electrons is higher than the Schottky
barrier, they can migrate to the Au particles, and the Au particles can act as a sink for the electrons.
However, as the Au particles grow, the Fermi level decreases due to the quantum size effect [44].
– 44 –

Chapter 2

Evaluation of Photocatalysis of Au Supported ZnO Prepared by the Spray Pyrolysis Method

Therefore, the larger the Au particle, the higher the Schottky barrier and may decrease the number of
excited electrons migrate to the Au particle. That is, when ZnO and Au particles come into contact, aSchottky barrier is created, and if the voltage of the excited electrons is sufficiently higher than the
Schottky barrier, the photolytic activity increases with the increase of Au support. However, when an
excessive amount of Au nanoparticles is supported onto ZnO particles, the excited electrons cannot
pass through the increased Schottky barrier and can recombine. Therefore, the number of excitons
decreases, resulting in a decrease in photocatalytic activity.

Figure 2-13 Semilogarithmic plot of changes in RhB concentration by UV light irradiation
time at different photocatalyst dose of Sample c; initial concentration of RhB: 5
mg/L; solution volume: 100 ml.
To confirm the photolysis rate by the sample c dose, 2–15 mg of the photocatalyst was added to
RhB aqueous solution. Figure 2-13 shows the semilogarithmic plot of the changes in the RhB
concentration as a function of UV light irradiation time. The first-order reaction rate constant depends
on the photocatalyst dose. The first-order reaction rate constant k and the coefficient of determination
R2 are shown in Figures 2-14 and 2-5.
The photocatalytic activity gradually improves with the increased sample c dose. The activity ofthe photocatalyst increases from 2–10 mg in proportion to the amount of the sample. However, doses
above 10 mg (e.g., 15 mg) do not significantly enhance the activity. These results indicate that the
turbidity of the suspension affects the photocatalytic activity. The turbidity of the suspension cancutoff the UV light, reducing the excitons, which are important for photocatalytic activity [45–46].
– 45 –

Chapter 2

Evaluation of Photocatalysis of Au Supported ZnO Prepared by the Spray Pyrolysis Method

Our study results were compared with the recently reported Au/ZnO particle study in Table 2-6,
and it indicates that our particles have a competitive performance compared to other studies. Because
the USP method employed in this study is possible to generate particles in one step without posttreatment, it has the advantage of less agglomeration between particles compared to other methods
such as the wet method. We succeeded in producing Au/ZnO particles with little aggregation using
the USP method and proved that Au nanoparticles exist as single crystals in ZnO particles. Haugen etal. confirmed that the size of the doped Au or Ag crystals increased and the photocatalytic activity
decreased after calcination of the particles. It was suggested that the photocatalytic activity of large
sized Au or Ag crystals was decreased because they acted as the center of hole-electron recombination[20]. That is, through this study, competitive Au/ZnO particles were produced, and it is considered thatthe approach in the field of organic wastewater and application to other fields are possible.

Table 2-5 First-order reaction rate constant k and the coefficient of determination R2 for dose.
Dose [mg]Reaction rate constant, k [min-1]

Coefficient of determination, R2

20.004200.992

50.008350.994

100.015020.981

15

0.01629

0.989

Figure 2-14 First-order reaction rate constant for RhB photolysis using different photocatalyst
dose of Sample c under UV light irradiation; initial concentration of RhB: 5mg/L; solution
volume: 100ml.
– 46 –

Chapter 2

Evaluation of Photocatalysis of Au Supported ZnO Prepared by the Spray Pyrolysis Method

Table 2-6 Photocatalytic activity of Au/ZnO system.

Dye

UV
Light

Initial dye
conc.
[M]

Catalyst
conc.
[mg/ml]k[min-1]

Au/Zn
O
[mol)

MB

8W×2

2×10−51.50

0.00190.0204MB

15W×2

3×10−5

0.10

0.019

0.0306

P. Pawinrat et al. [18]

RhB

30W

3×10−50.05

0.00206

0.0163

J. Kim et al. [19]Orange II

8W

2.85×10−51.50

0.01190.0096

P.K. Chen et al. [47]

RhB

15W

1.04×10−50.10

0.015020.0222This study

Ref.
T.K. Pathak et al.
[17]

2.4 Conclusion
In this study, the USP method was used to generate ZnO and Au/ZnO particles under various
conditions. All ZnO exhibited a hexagonal structure. Under the proper conditions, small, dense, andnarrow sized ZnO particles were generated. XRD patterns of Au/ZnO particles revealed the structural
patterns of ZnO and Au. The peak height of Au changed with the amount of supported Au. The TEM
images showed dark spots dispersed throughout the Au/ZnO particles. The size of the dark spots
varied with the concentration of chloroauric acid in the precursor solution. EDX mapping and STEM
images confirmed that the dark spots corresponded to nano-sized Au particles dispersed throughout
the ZnO particles.
The photolysis of the RhB aqueous solution was investigated under UV light irradiation. Under all
conditions, Au/ZnO particles showed a better photocatalytic activity than ZnO particles. The
photolysis rate constant increased as the Au loading increased from 0.025 mass% to 0.1 mass%.
However, the photolysis rate constant decreased for 0.2 mass% Au. The excess Au loading may
accumulate excess exciton electrons, attract holes, and promote the recombination of electron-holedue to the large contact surface with ZnO particles.
Hence, Therefore, the photolysis activity by the dose of the photocatalyst was investigated. The
photocatalytic activity increased in proportion to the dose from 2 mg to 10 mg, but it did not change
significantly at higher doses. In other words, UV light, which is important for electron excitation, was– 47 –

Chapter 2

Evaluation of Photocatalysis of Au Supported ZnO Prepared by the Spray Pyrolysis Method

blocked in the turbid suspension. Lastly, the best photocatalytic activity was observed in particles
with a 0.1 mass% Au supported on ZnO particles. Moreover, the USP could simply generate particles
in one step. Although decomposition experiments on actual wastewater were beyond the scope of thisstudy, the potential of purifying organic pollutants using this material was confirmed.

2.5. References
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H. Wang, H.L. Wang, W.F. Jiang, Z.O. Li, Photocatalytic degradation of 2,4-dinitrophenol(DNP) by multi-walled carbon nanotubes (MWCNTs)/TiO2 composite in aqueous solution
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Q. Zhou, Y.H. Zhong, X. Chen, J.H. Liu, X.J. Huang, Y.C. Wu, Adsorption andphotocatalysis removal of fulvic acid by TiO2–graphene composites, Journal of Materials
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nanophotocatalyst for visible light assisted degradation of a textile dye in aqueous solution,
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A. Franco, M.C. Neves, R. M.M.L. Ribeiro Carrott, M.H. Mendonça, M.I. Pereira, O.C.
Monteiro, Photocatalytic decolorization of methylene blue in the presence of TiO2/ZnS
nanocomposites, Journal of Hazardous Materials, 161 (2009) 545-550.

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S. Ameen, M. Shaheer Akhtar, Y.S. Kim, O.B. Yang, H.S. Shin, An effective nanocomposite
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V. Vaiano, M. Matarangolo, J.J. Murcia, H. Rojas, J.A. Navío, M.C. Hidalgo, Enhanced
photocatalytic removal of phenol from aqueous solutions using ZnO modified with Ag,
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Y.H. Lu, M. Xu, L.X. Xu, C.L. Zhang, Q.P. Zhang, X.N. Xu, S. Xu, K. Ostrikov, Enhanced
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P. Pawinrat, O. Mekasuwandumrong, J. Panpranot, Synthesis of Au–ZnO and Pt–ZnO
nanocomposites by one-step flame spray pyrolysis and its application for photocatalytic
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J. Kim, K. Yong, A facile, coverage controlled deposition of Au nanoparticles on ZnO
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A.B. Haugen, I. Kumakiri, C. Simon, M.A. Einarsrud, TiO2, TiO2/Ag and TiO2/Au
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nanoparticles and porous particles by ultrasonic spray pyrolysis using a zinc nitrateprecursor, International Journal of Minerals, Metallurgy and Materials, 19 (2012) 651-655.

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the spray pyrolysis method using ultrasonic atomization techniques, Journal of Materials
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nucleation in the preparation of SiO2 thin film from SiCl2 and O2 gases using CVD process,Chemical Engineering Science, 46 (1991) 1545–1560.

26.

K. Kaneko, W.J. Moon, K. Inoke, Z. Horita, S. Ohara, T. Adschiri, H. Abe, M. Naito,
Characterization of TiO2–Ag nanocomposite particles prepared by spray pyrolysis processusing transmission electron microscopy and three-dimensional electron tomography,
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O. Milošević, D. Uskoković, L.J. Karanović, M. Tomašević-Čanović, M. Trontelj,
Synthesis of ZnO-based varistor precursor powders by means of the reaction spray process,
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K. Leong, Morphological control of particles generated from the evaporation of solution
droplets: experiment, Journal of Aerosol Science 18 (1987) 525–552.

29.

K. Masters, Spray drying handbook, Longman Scientific & Technical, London, 1994.

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A.J. Hewitt, Droplet size spectra produced by air-assisted atomizers, Journal of Aerosol
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production of controlled morphology particles: From the nanometer to submicrometer size
ranges, Advanced Powder Technology, 22 (2011) 1-19.
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J.H. Bang, K.S. Suslick, Applications of Ultrasound to the Synthesis of Nanostructured
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Biopesticides — an Overview, Biosystems Engineering, 84 (2003) 119-125.

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M.B. Adali, A.A. Barresi, G. Boccardo, R. Pisano, Spray Freeze-Drying as a Solution toContinuous Manufacturing of Pharmaceutical Products in Bulk, Processes, 8 (2020) 1-27.

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R.J. Lang, Ultrasonic atomization of liquids, Journal of the Acoustical Society of America,
34 (1962) 6-8.

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G.L. Messing, S.C. Zhang, G.V. Jayanthi, Ceramic Powder Synthesis by Spray Pyrolysis,
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38.

W.H. Suh, K.S. Suslick, Magnetic and Porous Nanospheres from Ultrasonic Spray
Pyrolysis, Journal of the American Chemical Society, 127 (2005) 12007-12010.

39.

W.H. Suh, A.R. Jang, Y.H. Suh, K.S. Suslick, Porous, Hollow, and Ball-in-Ball Metal Oxide
Microspheres: Preparation, Endocytosis, and Cytotoxicity, Advanced Materials, 18 (2006)
1832-1837.

40.

W.H. Suh, K.S Suslick, Y.H. Suh, Therapeutic Agents for Alzheimer's Disease, Current
Medicinal Chemistry - Central Nervous System Agents, 5 (2005) 259-269.

41.

F. Iskandar, Mikrajuddin, K. Okuyama, Controllability of Pore Size and Porosity on SelfOrganized Porous Silica Particles, Nano Letters, 2 (2002) 389-392.

42.

Y.J. Lee, T. Fujimoto, S. Yamanaka, Y. Kuga, Formation and Evaluation of Au/ZnO
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W. Lu, G. Liu, S. Gao, S. Xiong, J. Wang, Tyrosine-Assisted Preparation of Ag/ZnO
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S. Shuang, Z. Zhang, The Effect of Annealing Treatment and Atom Layer Deposition to
Au/Pt Nanoparticles-Decorated TiO2 Nanorods as Photocatalysts, Molecules, 23 (2018) 1-– 51 –

Chapter 2

Evaluation of Photocatalysis of Au Supported ZnO Prepared by the Spray Pyrolysis Method

11.
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C. Lu, Y. Wu, F. Mai, W. Chung, C. Wu, W. Lin, C. Chen, Degradation efficiencies andmechanisms of the ZnO-mediated photocatalytic degradation of Basic Blue 11 under visible
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P.K. Chen, G.J. Lee, S.H. Davies, S.J. Masten, R. Amutha, J.J. Wu, Hydrothermal synthesisof coral-like Au/ZnO catalyst and photocatalytic degradation of Orange II dye, Materials
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– 52 –

Chapter 3

Evaluation of Photocatalysis of Ag Supported ZnO Prepared by the Spray Pyrolysis Method

Chapter 3

Evaluation of Photocatalysis of Ag Supported ZnO
Prepared by the Spray Pyrolysis Method

3.1. Introduction
In Chapter 2, it was confirmed that the Au nanoparticles doped on the ZnO particles act as electron
sinks to delay the recombination of excited electrons and holes, thereby they increased the efficiencyof the photocatalyst. Noble metals include Pt [1-2], Pd [3], and Ag [4-10], and are reported to act aselectron sinks like Au. However, because the noble metals are expensive, industrial applications may
have significant limitations. Therefore, relatively inexpensive Ag-supported research is attracting
attention. In addition, several studies have shown that Ag/ZnO photocatalysts have higher photolysis
efficiency than Au/ZnO. For example, Senthilraja et al. generated Au/ZnO, Ag/ZnO, and Au/Ag/ZnO
particles by a precipitation–decomposition method, and as a result of Methylene blue (MB) dye
photolysis experiments, Ag/ZnO particles showed higher photolysis efficiency than Au/ZnO particles[11]. Pathak et al. generated Au/ZnO and Ag/ZnO particles doped at a concentration of 2 mol% Ag andAu using a combustion method. In addition, by MB dye photolysis experiments, it was confirmedthat Ag/ZnO particles have better photocatalytic efficiency than Au/ZnO particles [12]. Interestingly,
Zhai et al. generated Au/ZnO, Ag/ZnO, Au/Ag/ZnO particles by photo-deposition procedure and lowtemperature calcination method, and performed ethylene photolysis experiments for the applicationof preservation of fruits and vegetables. It has been proven that Ag/ZnO particles have better ethylene
decomposition efficiency than Au/ZnO [13]. In addition, a study on the generation and characteristiccomparison of Ag/ZnO and Au/ZnO particles using the microwave and ultrasonic combined
technique [14] and the borohydride reduction method [15] have been reported.
However, as previously explained, the nanoparticles generated by those kinds of methods have alarge specific surface area, so the photocatalytic efficiency is good, but it is difficult to be collected
after use. Hence, the inability to recycle photocatalytic particles not only increases the cost but canlead to new pollution [16-17]. In Chapter 2, it was confirmed that the Ultrasonic Spray Pyrolysis (USP)
method was used to generate submicron-level Au/ZnO particles with photocatalytic activity similar
to P25 and that they could be collected simply by a gravity sedimentation. In addition, since the USP
method is a continuous one-step process, the high-purity powder can be produced simply, and since
there is no post-treatment, agglomeration between particles can be suppressed as much as possible[18]. Therefore, in this chapter, it is reported generation of submicron-level Ag/ZnO particle by using– 53 –

Chapter 3

Evaluation of Photocatalysis of Ag Supported ZnO Prepared by the Spray Pyrolysis Method

the USP method and evaluations of photocatalytic efficiency. In addition, the morphology andstructural properties of the obtained particles were investigated, and the reusability of particles
through gravity sedimentation was investigated.

3.2. Experimental setup and Method
3.2.1. Materials
Zinc nitrate hexahydrate (Zn(NO3)2 6H2O, ≥99.0%) and silver nitrate (AgNO3, ≥99.8%) werepurchased from FUJIFILM Wako Pure Chemical Industries, and Rhodamine-B (RhB, C28H31ClN2O3,>95.0%) was purchased from Tokyo Chemical Industries. Titanium dioxide nanopowder as standardRphotocatalyst (Evonik AEROXIDE○
P25, formerly Degussa) was purchased from Sigma–Aldrich.

All reagents were used as received without further purification. Distilled water was used in all
experiments.
3.2.2. Synthesis of Ag/ZnO composite particles
Ag/ZnO composite particles were synthesized using the USP method as described in Chapter 2.
Therefore, the device and its operation method are briefly described here. The USP apparatus wascomposed of a nebulizer with a resonance frequency of 1.7 MHz, a quartz glass reaction tube, fourelectric furnaces with a temperature controller of ±1 °C, a diffusion dryer, and an electrostatic
precipitator. The Zinc nitrate hexahydrate were dissolved in 100 mL of distilled water withconcentration maintained at 2.5 mass%. And silver nitrate concentration ranged between 0 and 0.4
mass% was added into the precursor solution. The rate introduced into the nebulizer of precursor
solution was 15 mL/h. The generated droplets by nebulizer were introduced into the quartz glass
reaction tube using a nitrogen flow at a rate of 1.0 SLM. And flow rate of sheath gas to prevent
precipitation was fixed at a rate of 0.5 SLM. Four electric furnaces for heating the quartz reaction
tube were used, two 80 mm preheating furnaces from the bottom were set to 40 °C and 70 °C,
respectively, and two 290 nm heating furnaces were set to 150 °C and 1000 °C. The length, outer
diameter, and inner diameter of the quartz glass reaction tube were 1208.50 mm, 18.25, and 15.25,
respectively, and the temperature was measured between the outer wall of the quartz glass reaction
tube and the inner wall of the furnace. The synthesized Ag/ZnO composite particles by thedecomposition of droplets were then dried by a diffusion drier. Afterward the Ag/ZnO composite– 54 –

Chapter 3

Evaluation of Photocatalysis of Ag Supported ZnO Prepared by the Spray Pyrolysis Method

particles were collected in the electrostatic precipitator.
3.2.3. Characterization
The size and morphology of the particles were verified using a scanning electron microscope (SEM,
JEOL JSM-6380A). Crystallite size and crystalline phase of the generated particles were studied by
X-ray diffraction (Rigaku, Multiflex) using Cu K radiation. The crystallite size of the Ag and ZnO
particles was estimated using Scherrer's equation. The particle size, morphology, and elemental wererecorded by a transmission electron microscope (TEM, JEOL JEM-2100F) equipped with energydispersive X-ray spectroscopy (EDXS). To study the optical properties, the change in concentrationof the RhB solution decomposed by the Ag/ZnO composite particles was measured using aspectrometer with an absorption wavelength of 200–850 nm SIMAZU, UV-1800).

Table 3-1 Experimental concentrations of ZnO and Ag/ZnO particles and crystallite size
calculated by Scherrer's equation.

SampleConcentration [mass%]Zn (NO3)2

AgNO3

Crystallite size of Ag Crystallite size of ZnO
(nm)
(nm)

a

-

-

35.6

b

0.05

15.7

34.2

0.1

16.1

31.5

d

0.2

16.3

30.6

e

0.4

22.9

31.2

c

2.5

– 55 –

Chapter 3

Evaluation of Photocatalysis of Ag Supported ZnO Prepared by the Spray Pyrolysis Method

3.3. Experimental Results and Discussion
3.3.1. Characteristics of Ag/ZnO particles

Figure 3-1 SEM images of samples a-e obtained at different conditions.
In our previous study, small and dense particles were generated using the USP method, and Au/ZnO
particles were successfully generated. Figure 3-1 shows the SEM images of particles (sample a) and
Ag/ZnO particles (sample b-d) generated under various conditions. The SEM images show that thegenerated Ag/ZnO particles are small and dense particles like the Au/ZnO particles of the previous
study, and there was no change in the size and shape of the particles by the concentration of silver
nitrate. In addition, the particle size has a distribution of 0.5 to 1.0 μm and has a spherical shape.
Figure 3-2 shows the XRD pattern of the obtained ZnO and Ag/ZnO particles by the USP method.
Diffraction peaks of all samples [(100), (002), (101), (102), (110), (103), (200), and (112)] arecharacteristic of the wurtzite structure of ZnO (JCPDS Card No:36-1451). And diffraction peaks (111),(200), and (220), which are characteristic of the fcc structure of Ag (JCPDS Card No:04-0783), wereidentified in all samples generated with silver nitrate, and the height of the peaks increased as the
addition ratio of silver nitrate increased. The peaks of Ag appear regardless of the Zn/Ag molar ratio,
and the peaks of other intermediate products were not observed. Thus, it suggests that Ag exists as a

– 56 –

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Evaluation of Photocatalysis of Ag Supported ZnO Prepared by the Spray Pyrolysis Method

Figure 3-2 XRD pattern of ZnO and Ag/ZnO particles.
metallic form in all Ag/ZnO composite particles.
The ZnO crystallite size and Ag crystallite size of the sample were calculated using the Scherrer
equation and are shown in Table 3-1. The (101) diffraction peak of ZnO and the (111) diffraction peakof Ag were used for calculation. The Ag crystallite size of the Ag/ZnO sample increased from 15.7 to22.9 nm as the addition rate of silver nitrate increased. Conversely, the ZnO crystallite size did notchange significantly with the addition of silver nitrate.
Figure 3-3 shows TEM images of pure ZnO and Ag/ZnO particles prepared by adding silver nitrate
at various concentrations to the raw material solution. When ZnO is doped with Ag (silver nitrate:
0.05-0.4 mass%), dark spots were found on inside and outside of Ag/ZnO particles as shown in Fig.
3-3(b-e). The dark spots were not observed in Fig. 3-3(a), which is a TEM image of ZnO particles(Sample a) and tends to increase from Fig. 3-3 (b) to (e) due to the increase in the concentration ofsilver nitrate. This is a similar result to our previous study in which ZnO was doped with Au, and it
could be assumed that the dark spots were a metal particle. Therefore, the particle composition was
analyzed using EDX mapping to confirm whether the dark spot was an Ag nanoparticle or not.

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Evaluation of Photocatalysis of Ag Supported ZnO Prepared by the Spray Pyrolysis Method

Figure 3-3 TEM images of samples a-e obtained by USP synthesis.
Figure 3-4 shows the STEM images of sample d and the elemental mapping images of Zn, O and
Ag. The mapping image of the Ag element shown that the dark spots on ZnO correspond to Ag. It isclearly evident that both Zn and Ag elements were distributed uniformly at the surface, which will bebeneficial for charge carrier transform between ZnO and Ag. That is, Ag nanoparticles in the metal
form are dispersed throughout the ZnO particles, which suggests that excited electrons can carriertransform to Ag nanoparticles.

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Evaluation of Photocatalysis of Ag Supported ZnO Prepared by the Spray Pyrolysis Method

Figure 3-4 STEM image of sample d particle and the corresponding EDX mapping images of the Ag,
Zn, and O, respectively.

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Evaluation of Photocatalysis of Ag Supported ZnO Prepared by the Spray Pyrolysis Method

3.3.2. Photocatalytic activity

Figure 3-5 Semilogarithmic plot of changes in RhB concentration by UV light irradiation timeat ZnO (Sample a), samples b-e, and P25; initial concentration of RhB: 5 mg/L;
photocatalyst dose: 5 mg; solution volume: 100 mL.

Figure 3-5 shows a semilogarithmic plot of the changes in the RhB concentration of samples a-e
as a function of UV light irradiation time. Each 5 mg of photocatalyst was added to RhB aqueous
solution of 5 mg/L, and photocatalysis was performed under UV light irradiation for 180 minutes.
P25 was included here as reference sample. Compared to pure ZnO particles, all Ag/ZnO particles
have higher photocatalytic efficiency, and as the amount of doped Ag increases, the photocatalytic
efficiency tends to increase. Optimal decomposition activity exhibit in sample d with 0.2 mass% Ag
doped. However, in the case of sample e, which doped the silver nitrate excessively than the sample
d, the photocatalytic efficiency decreased. Interestingly, samples c and d showed better results thancommercial titania powder, and our submicron-sized particles showed better results than nano-sizedtitania.
When decomposing organic dyes using a photocatalyst, the surface-to-volume ratio of thephotocatalyst is very important, and the separation of electrons and holes can be greatly improved by
reducing the ZnO to nano size. However, it is difficult to retrieve the finely powdered photocatalyst
from the mixed solution, which can be limited in commercial applications. In addition, since thephotocatalyst may deteriorate in performance and release toxicity over time, retrieval after use of the– 60 –

Chapter 3

Evaluation of Photocatalysis of Ag Supported ZnO Prepared by the Spray Pyrolysis Methodphotocatalyst is a very important part of water purification applications [16-17]. Our Ag/ZnO particles(such as samples c-d) produced by the USP method not only showed better activity than P25 but also
had a relatively large particle size, so they could be easily removed from the mixed solution by
gravitational sedimentation.

Table 3-2 First-order reaction rate constant k and the coefficient of determination R2
corresponding to the slope of the line.

Samples

Reaction rate constant, k [min-1]

Coefficient of determination, R2

ZnO (a)0.002340.993

b0.008100.997

c0.011490.993

d0.015390.978

e0.00709

0.998

P25

0.00855

0.985

Figure 3-6 First-order reaction rate constant for RhB photolysis using ZnO (Sample a),samples b-e, and P25 under UV light irradiation; initial concentration of RhB: 5
mg/L; photocatalyst dose: 5 mg; solution volume: 100 mL.

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Evaluation of Photocatalysis of Ag Supported ZnO Prepared by the Spray Pyrolysis Method

Table 3-2 shows the first-order reaction rate constant k and the coefficient of determination R2
corresponding to the slope of the line. And the first-order reaction rate constant k is graphically plottedin Figure 3-6. Compared to pure ZnO particles (sample a), the first order reaction rate constant k was
about 3 times higher for sample b and about 4.5 times for sample c and about 6 times for sample d,and about 4 times for sample e. Sample d, which exhibited optimal decomposition activity, had areaction rate constant of about 2 times higher than that of P25. Although more Ag particles were
doped in sample e, the photolysis rate constant was lower than that of samples c and d. As a result of
the analysis in Figs. 2 and 3, it was confirmed that as the amount of silver nitrate added to the raw
material solution increased, the crystallite size of the Ag and the amount of Ag particles in the ZnO
particles increased. This result is consistent with our previous experiments with Au doped [25], and it
was found that the excessive noble metal doped act as the center of recombination of the excitationelectrons. In the other words, when Ag is doped on ZnO particles, Ag receives excited electrons fromZnO particles and contributes to the formation of hydroxyl radicals, but when Ag is excessively doped,
excessive excitation electrons are accumulated in Ag, which can promote electron-hole recombination.
This change in photocatalytic activity can be explained by the change in the Fermi level of ZnO
and Ag nanoparticles. Figure 3-7 shows the change in Fermi level due to contact between ZnO andAg particles. As shown in Fig. 3-7(a), the Fermi levels of ZnO and Ag particles before contact differedby -4.26 and -4.99 eV, respectively. When ZnO and Ag particles come into contact, the Fermi level is

Figure 3-7 (a) Fermi level of ZnO and Ag particles before compounding, (b) Fermi level of
ZnO and Ag particles after compounding. E0:vacuum level; (Ef)Ag, (Ef)ZnO, Ef:
Fermi levels of Ag, ZnO, and Ag/ZnO; CV and VB : conduction band andvalence band of ZnO.
– 62 –

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Evaluation of Photocatalysis of Ag Supported ZnO Prepared by the Spray Pyrolysis Method

reached as shown in Fig. 3-7 (b), and Ohmic contact is formed because the Fermi level of ZnO isrelatively lower than the Fermi level of Ag. And since excited electrons can easily move to Ag
particles due to Ohmic contact, Ag particles can act as a sink of electrons. And the photolytic activity
is increased because the large loading of Ag particles causes more excitation electron transfer to theAg particles. However, when Ag particles become larger with excessive loading, the Fermi level may
decrease due to the quantum size effect and become similar to that of ZnO. Therefore, excessive
loading causes the Fermi level of Ag particles to decrease and the movement of excitation electrons
can be suppressed. As a result, the recombination of the excited electrons increases and the activityof the photocatalyst decreases.

Figure 3-8 Semilogarithmic plot of changes in RhB concentration by UV light irradiation time
at different photocatalyst dose of Sample d; initial concentration of RhB: 5mg/L;
solution volume: 100mL.
In order to know the optimal dose of sample d, which showed the most efficient photolysis, aphotolysis experiment was performed by dispersing 2–15 mg of sample d in an aqueous RhB solution.
Figure 3-8 shows the semilogarithmic plot of the changes in the RhB concentration as a function of
UV light irradiation time. The first-order reaction rate constant depends on the photocatalyst dose.
The first-order reaction rate constant k and the coefficient of determination R2 are shown in Figures
3-9 and 3-3.
The photocatalytic activity improved as the dose of sample d increased and increased from 2-10
mg in proportion to the amount of sample. However, when 15 mg was dose, the increase in thephotolysis rate was insignificant compared to the photolysis rate of 10 mg. It means that an excessive– 63 –

Chapter 3

Evaluation of Photocatalysis of Ag Supported ZnO Prepared by the Spray Pyrolysis Methoddose of Ag to particles increases the turbidity of the suspension and affects the photocatalytic activity.
That is, the increase in turbidity of the suspension due to excessive dose of the particles may block
UV light and hinder the UV light absorption of the photocatalyst [19-20].

Table 3-3 First-order reaction rate constant k and the coefficient of determination R2 for dose.

Dose [mg]Reaction rate constant, k [min-1]

Coefficient of determination, R22mg

0.00703

0.999

5mg

0.01376

0.993

10mg

0.02592

0.961

15mg

0.02973

0.965

Figure 3-9 First-order reaction rate constant for RhB photolysis using different photocatalyst
dose of Sample d under UV light irradiation; initial concentration of RhB: 5mg/L;
solution volume: 100mL.

– 64 –

Chapter 3

Evaluation of Photocatalysis of Ag Supported ZnO Prepared by the Spray Pyrolysis Method

Figure 3-10 Reusability of sample d photocatalyst under UV light irradiation; initialconcentration of RhB: 5mg/L; photocatalyst dose: 10mg; solution volume:
100mL; UV light irradiation time: 120min.

Figure 3-11 XRD peaks before and after the photolysis experiment of sample e.

– 65 –

Chapter 3

Evaluation of Photocatalysis of Ag Supported ZnO Prepared by the Spray Pyrolysis Method

In order to confirm the possibility of reusability of our particles, after the photolysis experiment,
the particles were separated from the mixed solution by gravity sedimentation. And a photolysis
experiment was performed by adding a new RhB solution, and the process was repeated. Sample d
was added with 10 mg identified as the optimal dose in Figure 3-8. In order to observe thedeterioration of the photocatalyst, a photolysis experiment was performed for 120 minutes which thedye degradation did not reach 100%. Figure 3-10 shows the reusability of photocatalyst sample d for
RhB dye degradation analyzed over 7 cycles under UV irradiation. In the first cycle without reuse, it
showed 93.2% of dye degradation. After that, compared to the photolysis efficiency of the first cycle,
there was no significant change with an error of ± 1% for 7 cycles, suggesting that the catalyst has
excellent persistence and stability.
Figure 3-11 shows the XRD peaks before and after the photolysis experiment, which can further
explain the stability of the photocatalyst. There was no significant change in the XRD peaks before
and after the photolysis experiment of the particles, and no Ag oxidation was observed. In other words,
the Ag/ZnO particles generated in this study are very stable against oxidation and have resistance toparticle deterioration because Ag particles are trapped in the ZnO particles.

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Evaluation of Photocatalysis of Ag Supported ZnO Prepared by the Spray Pyrolysis Method

3.4. Conclusion
It was possible to generate ZnO and Ag/ZnO particles using the USP method under various
conditions. All particles were spherical and 0.5 to 1.0 μm in size, and there was no change inmorphology by Ag support. The crystal peaks of ZnO were observed in all of the generated particles,
and the Ag crystal peaks were observed in all Ag/ZnO particles. The crystal peak of Ag crystal peak
was higher as the amount of silver nitrate added to the raw material solution increased, and the
crystallite size of Ag also increased. In the TEM image, dark spots dispersed in Ag/ZnO particles
were identified, and the size of the dark spots was different depending on the concentration of added
silver nitrate. It was confirmed from EDX mapping and STEM images that this dark spot corresponds
to the Ag particle nano dispersed throughout the ZnO particles.
The photocatalytic activity of the generated particles was investigated by photolysis of the RhB
aqueous solution under UV irradiation. All Ag/ZnO particles showed better photocatalytic activity
than ZnO particles. Particularly, the particles with 0.2 mass% silver nitrate supported on ZnO particles
showed 2 times better photolysis activity than P25. As the dope amount increased, the photolysis rate
constant increased, but it tended to decrease above a specific concentration. This is because excessive
Ag dope accumulates excessive exciton electrons, attracts holes, and promotes electron-hole
recombination due to the large contact surface with ZnO particles.
In order to investigate the optimal dose required for photolysis, photolysis experiments were
performed at various doses. The photocatalytic activity increased proportionally as the dose increased
from 2 mg to 10 mg, but no significant change was observed above 10 mg. This is because theultraviolet rays, which are important for electronic excitation, have been blocked by the cloudy
suspension. It also confirmed the reusability of the particles for commercial applications. It wasobserved that the generated particles not only can be recovered by gravity precipitation after thephotolysis experiment, but also have a slow deterioration rate. In this chapter, the USP method wasused to easily generate submicron complex particles and confirmed the possibility of purification andreuse of organic pollutants.

3.5. Reference
1.

P. Pawinrat, O. Mekasuwandumrong, J. Panpranot, Synthesis of Au–ZnO and Pt–ZnO– 67 –

Chapter 3

Evaluation of Photocatalysis of Ag Supported ZnO Prepared by the Spray Pyrolysis Method

nanocomposites by one-step flame spray pyrolysis and its application for photocatalytic
degradation of dyes, Catalysis Communications, 10 (2009) 1380–1385.
2.

H.B. Zeng, P.S. Liu, W.P. Cai, S.K. Yang, X.X. Xu, Controllable Pt/ZnO porous nanocages withimproved photocatalytic activity, The Journal of Physical Chemistry C, 112 (2008) 19620–19624.

3.

Y.G. Chang, J. Xu, Y.Y. Zhang, S. Ma, L.H. Xin, L. Zhu, C.T. Xu, Optical Properties andPhotocatalytic Performances of Pd Modified ZnO Samples, The Journal of Physical ChemistryC, 113 (2009) 18761–18767.

4.

M.J. Height, S.E. Pratsinis, O. Mekasuwandumrong, P. Praserthdam, Ag-ZnO catalysts for UVphotodegradation of methylene blue, Applied Catalysis B: Environmental, 63 (2006) 305–312.

5.

Y.F. Wang, J.H. Yao, G. Jia, H. Lei, Optical Prosperities of Ag–ZnO Composition Nanofilm
Synthesized by Chemical Bath Deposition, Acta Physica Polonica A, 119 (2011) 451–454.

6.

W. Xie, Y.Z. Li, W. Sun, J.C. Huang, H. Xie, X.J. Zhao, Surface modification of ZnO with Ag
improves its photocatalytic efficiency and photostability. Journal of Photochemistry andPhotobiology A: Chemistry, 216 (2010), 149–155.

7.

Y.C. Lu, Y.H. Lin, D.J. Wang, T.F. Xie, T.F. Jiang, Surface charge transfer properties of highperformance Ag-decorated ZnO photocatalysts, Journal of Physics D: Applied Physics, 44 (2011)
1-7.

8.

Y.H. Zheng, L.R. Zheng, Y.Y. Zhan, X.Y. Lin, Q. Zheng, K.M. Wei, Ag/ZnO heterostructure
nanocrystals: synthesis, characterization, and photocatalysis, Inorganic Chemistry, 46 (2007),
6980–6986.

9.

B. Subash, B. Krishnakumar, M. Swaminathan, M. Shanthi, Synthesis and characterization of
cerium–silver co-doped zinc oxide as a novel sunlight-driven photocatalyst for effective
degradation of Reactive Red 120 dye. Materials Science in Semiconductor Processing, 16 (2013),
1070–1078.

10. W.W. Lu, S.Y. Gao, J.J. Wang, One-Pot Synthesis of Ag/ZnO Self-Assembled 3D Hollow
Microspheres with Enhanced Photocatalytic Performance, The Journal of Physical Chemistry C,
112 (2008) 16792–16800.
11. A. Senthilraja, B. Subash, B. Krishnakumar, D. Rajamanickam, Synthesis, characterization andcatalytic activity of co-doped Ag–Au–ZnO for MB dye degradation under UV-A light, Materials
Science in Semiconductor Processing, 22 (2014) 83-91.

– 68 –

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Evaluation of Photocatalysis of Ag Supported ZnO Prepared by the Spray Pyrolysis Method

12. T.K. Pathak, R.E. Kroon, H.C. Swart, Photocatalytic and biological applications of Ag and Au
doped ZnO nanomaterial synthesized by combustion, Vacuum, 157 (2018) 508-513.
13. H.S.Zhai, X.L. Liu, Z. Wang, Y.Y. Liu, Z.K. Zheng, X.Y. Qin, X.Y. Zhang, P. Wang, B.B. Huang,ZnO nanorod decorated by Au-Ag alloy with greatly increased activity for photocatalytic
ethylene oxidation, Chinese Journal of Catalysis, 41 (2020) 1613-1621.
14. H. Li, E.T. Liu, F.Y.F. Chan, Z. Lu, R. Chen, Fabrication of ordered flower-like ZnOnanostructures by a microwave and ultrasonic combined technique and their enhanced
photocatalytic activity, Materials Letters, 65 (2011) 3440-3443.
15. N. Güy, M. Özacar, The influence of noble metals on photocatalytic activity of ZnO for Congo
red degradation, International Journal of Hydrogen Energy, 41 (2016) 20100-20112.
16. X.F. Bian, K.Q. Hong, X. Ge, R. Song, L.Q. Liu, M.X. Xu, Functional HierarchicalNanocomposites Based on ZnO Nanowire and Magnetic Nanoparticle as Highly Active
Recyclable Photocatalysts, The Journal of Physical Chemistry C, 119 (2015) 1700–1705.
17. X. Zhang, S. Xu, G.R Han, Fabrication and photocatalytic activity of TiO2 nanofiber membrane,
Materials Letters, 63 (2009) 1761-1763.
18. A.B.D. Nandiyanto, K. Okuyama, Progress in developing spray-drying methods for the
production of controlled morphology particles: From the nanometer to submicrometer size ranges,
Advanced Powder Technology, 22 (2011) 1-9.
19. C.S. Lu, Y.T. Wu, F. Mai, W.S. Chung, C.W. Wu, Degradation efficiencies and mechanisms of
the ZnO-mediated photocatalytic degradation of Basic Blue 11 under visible light irradiation,
Journal of Molecular Catalysis A, 310 (2009) 159-165.
20. B. Neppolian, H. C. Choi, S. Sakthivel, B. Arabindoo, V. Murugesan, Solar light induced andTiO2 assisted degradation of textile dye reactive blue 4, Chemosphere, 46 (2002) 1173-1181.

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Chapter 4

Chapter 4

ZnO/Fe3O4 and ZnO/Fe3O4/Ag composite particle generation process

ZnO/Fe3O4 and ZnO/Fe3O4/Ag composite particle
generation process

4.1. Introduction
An issue in the application of ZnO photocatalyst to organic dye treatment is that it is difficult torecover fine photocatalyst particles from photolysis treated wastewater. Methods such as filtration or
centrifugation for particle recovery can cause destruction and loss of particles, resulting in highoperating costs and secondary pollution [1]. Therefore, there is a need for a study that can simply
recover the particles. In Chapters 2-3, submicron-level ZnO particles carrying Au and Ag weregenerated using the spray pyrolysis method, and recovery and reusability were confirmed by gravity
sedimentation. However, the combination of photocatalyst and magnetic particles may be a morereliable solution. By combining the photocatalyst and magnetic particles, not only can the particles
be easily recovered through magnetic separation processes, but they can also be recycled [2-4].
Among the many types of iron oxide, magnetite (Fe3O4), maghemite (γ-Fe2O3), and hematite (αFe2O3) are the most common in nature [5]. Since hematite (α-Fe2O3) is stable and exists most often inrocks and soils, there are many applications [6]. And due to its semiconductor properties, it can also
be used in photocatalytic applications [7]. Therefore, hematite (α-Fe2O3) is attracting attention in thefields of advanced magnetic materials, catalysts, colored pigments, high-density magnetic recording
media, and medical diagnosis because of its stability, low cost, and bandgap (2.1eV) [8- 13]. However,since the magnetic strength is in the order of Fe3O4 > γ-Fe2O3 > α-Fe2O3 [14-15], recovery of thephotocatalyst through hematite complexation may be very difficult. In particular, magnetite (Fe3O4)
has attracted more attention over the past 10 years due to its strong superparamagnetic properties, low
toxicity, and easy production method [16]. Magnetite forms a central cubic (FCC) lattice and has aspinel structure with the same number of Fe2+ and Fe3+. However, due to the disadvantage thatmagnetite is easily oxidized and agglomerated under acidic conditions, the application of organicdyes may be limited. Therefore, the combination of ZnO photocatalyst and magnetite can be a good
way to overcome the shortcomings.
For the synthesis of ZnO/Fe3O4 composites, precipitation [17], co-precipitation [18], deposition [19],wet milling [20], solvothermal [21], and wet chemical route [22] are used. Since co-precipitation is thecheapest and easiest method, it is well suited for commercial applications. Therefore, in this chapter,– 70 –

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ZnO/Fe3O4 and ZnO/Fe3O4/Ag composite particle generation process

the synthesis conditions of ZnO/Fe3O4 and ZnO/Fe3O4/Ag composite materials were investigatedusing the gas-phase ultrasonic spray pyrolysis method and the liquid phase co-precipitation method,
and the characteristics of the generated particles were confirmed.

4.2. Experimental setup and Method
4.2.1. Materials
Zinc nitrate hexahydrate (Zn (NO3)2 6H2O, ≥99.0%), and Silver nitrate (AgNO3, ≥99.8%), Iron (Ⅱ)
sulfate heptahydrate (FeSO4 7H2O, ≥99.0%), Iron (Ⅲ) chloride hexahydrate (FeCl3 6H2O, ≥99.0%),
and Trisodium citrate (C6H5Na3O7) were purchased from FUJIFILM Wako Pure Chemical Industries.
And Rhodamine-B (C28H31ClN2O3, >95.0%) was purchased from Tokyo Chemical Industries.
Sodium hydroxide (NaOH, >95.0%) was purchased from Kanto Chemical Industries. TitaniumRdioxide nanopowder as standard photocatalyst (Evonik AEROXIDE○
P25, formerly Degussa) was

purchased from Sigma–Aldrich. All reagents were used as received without further purification.
Distilled water was used in all experiments.
4.2.2. Generation of ZnO/Fe3O4 particles using USP method
The generation of ZnO/Fe3O4 composite particles was attempted using the USP method describedin Chapter 2. Therefore, the device and its operation method are briefly described here. The USP
apparatus was composed of a nebulizer with a resonance frequency of 1.7 MHz, a quartz glass
reaction tube, four electric furnaces with a temperature controller of ±1 °C, a diffusion dryer, and anelectrostatic precipitator. The Zinc nitrate hexahydrate was dissolved in 100 mL of distilled waterwith concentration maintained at 10 mass%. And Iron (Ⅲ) chloride hexahydrate concentration 2.5
mass% was added into the precursor solution. The rate introduced into the nebulizer of precursor
solution was 15mL/h. The generated droplets by nebulizer were introduced into the quartz glass
reaction tube using a nitrogen or air flow at a rate of 1.0 or 0.65 SLM. And flow rate of sheath gas toprevent gravity sedimentation was fixed at a rate of 0.5 or 0.15 SLM. Four electric furnaces forheating the quartz reaction tube were used, two 80 mm preheating furnaces from the bottom were set
to 40 °C and 70 °C, respectively, and two 290 nm heating furnaces were set to 1000 °C. The composite
particles synthesized by droplet decomposition were dried with a diffusion dryer. After that, thecomposite particles were collected in the electrostatic precipitator. The experimental parameters for– 71 –

Chapter 4

ZnO/Fe3O4 and ZnO/Fe3O4/Ag composite particle generation process

the synthesis of ZnO/Fe3O4 composite particles are listed in Table 4-1.

Table 4-1 Experimental parameters of the ZnO/Fe3O4 composite particles by USP method.

SampleConcentration
Zn (NO3)2

Rate of flow (SLM)

FeCl3Carrier gas

Sheath gas

1.00 (N2)0.50 (N2)0.65 (N2)0.15 (N2)

1.00 (N2)0.50 (Air)0.65 (Air)0.15 (Air)0.65 (Air)0.15 (Air)

a
b
c

10
(mass%)

2.5(mass%)de

Zn: 1(Molar ratio)

Fe: 2(Molar ratio)

4.2.3. Generation of ZnO/Fe3O4 composite particles using USP and co-precipitation method
Many studies have been reported on coating ZnO on Fe3O4 using a co-precipitation method. In
particular, sodium citrate is used as a surface modification material for the complexation of Fe3O4
and ZnO [18,23-24]. For example, Hong et al. generated Fe3O4 by co-precipitation method andsynthesized ZnO/Fe3O4 nanoparticles by coating ZnO after surface modification of Fe3O4 withsodium citrate [18]. However, ZnO generated by the co-precipitation method requires post-processingof calcination and sintering, which has the disadvantage of causing particle agglomeration. Therefore,
in this study, Fe3O4 was generated by the co-precipitation method, the surface was modified withsodium citrate, and the ZnO produced by the USP method was attempted to form a complex.
Figure 4-1 shows the procedure of formation of Fe3O4 by co-precipitation method. The Fe3O4
particles were synthesized by the co-precipitation of Fe+2 and Fe+3 ions with molar ratio 1:2 in thepresence of NaOH as a precipitate agent according to the following chemical reaction:

FeSO4 +2FeCl3 +8NaOH → Fe3 O4 + Na2 SO4 + 6NaCl– 72 –(4-1)

Chapter 4

ZnO/Fe3O4 and ZnO/Fe3O4/Ag composite particle generation process

FeSO4·7H2O and FeCl3·6H2O were dissolved in distilled water, and heated and stirred at 80°C.
Thereafter, the NaOH solution was added dropwise while stirring, and the resulting particles were
washed three times with distilled water. The particles were dried in a dryer, and the obtained powder
was pulverized using a mortar.

Figure 4-1 Procedure of formation of Fe3O4 by co-precipitation method.
Figure 4-2 shows the procedure of formation of ZnO/Fe3O4 composite particles. First, 0.4 g of
Fe3O4 particles obtained by the co-precipitation method were added to 20 g of a 0.5 M aqueous
solution of trisodium citrate and stirred for 3 hours. Then, it washed twice with distilled water, andadjusted by adding distilled water. In addition, 0.4 g, 0.8 g, and 1.2 g of ZnO particles produced by
the spray pyrolysis method were added, respectively, so that the mass ratio of ZnO and Fe3O4 was
1:1, 2:1, 4:1, and stirred for 24 hours. The obtained particles were washed three times with distilledwater, and dried with a dryer at 100°C. The obtained composite powder was pulverized using a mortar,and finally ZnO/Fe3O4 particles were obtained.

– 73 –

Chapter 4

ZnO/Fe3O4 and ZnO/Fe3O4/Ag composite particle generation process

Figure 4-2 Procedure of formation of ZnO/Fe3O4 composite particles.
4.2.4. Generation of ZnO/Fe3O4 and ZnO/Fe3O4/Ag composite particles using co-precipitation
methodSynthesis of ZnO/Fe3O4 and ZnO/Fe3O4/Ag was attempted using the co-precipitation method. The
Fe3O4 particles were generated by the method described previously and coated with ZnO and Ag
using the co-precipitation method. Figure 4-3 shows the procedure of formation of ZnO/Fe3O4 andZnO/Fe3O4/Ag composite particles. Zinc nitrate was dissolved in 300 ml of distilled water and heatedto 90 °C. Fe3O4 particles generated by the co-precipitation method were added, and NaOH solution
was added dropwise to PH8.5 while stirring. Here, the mass ratio of ZnO and Fe3O4 is set to be 2:1.
The chemical reaction formula for ZnO is as follows:

Zn(NO3 )2 +2NaOH → Zn(OH)2 ↓ +2NaNO3(4-2)

Zn(OH)2 → ZnO + H2 O

(4-3)

Silver nitrate and NaOH were added to support Ag. Silver nitrate was added 0.05, 0.1, 0.2, and 0.4
mass% compared to zinc oxide, and the reaction formula is as follows:– 74 –

Chapter 4

ZnO/Fe3O4 and ZnO/Fe3O4/Ag composite particle generation process

2AgNO3 +2NaOH → Ag2 O ↓ +H2 O + 2NaNO3(4-4)

2Ag2 O → 4Ag + O2

(4-5)

Finally, the product was dried and calcined at 250 °C after washing with distilled water, and theresulting product was pulverized with a mortar to obtain the target powder. The experimental
parameters of the ZnO/Fe3O4 and ZnO/Fe3O4/Ag composite particles are listed in Table 4-2.

Figure 4-3 Procedure of formation of ZnO/Fe3O4 and ZnO/Fe3O4/Ag composite particles.

Table 4-2 Experimental parameters of the ZnO/Fe3O4 andZnO/Fe3O4/Ag composite particles by co-precipitation
method.

SampleMass ratio [g](ZnO:Fe3O4)

A
B
C
D
E

2:1

– 75 –

AgNO3[mass%]
0.05
0.1
0.2
0.4

Chapter 4

ZnO/Fe3O4 and ZnO/Fe3O4/Ag composite particle generation process

4.3. Experimental Results and Discussion
4.3.1. Characteristics of ZnO/Fe3O4 composite particles generated using USP method
Particles generated using the USP method under different conditions were analyzed using XRD,
and the results are shown in Figure 4-4. In sample a generated under nitrogen flow conditions, peaksof the target products, ZnO and Fe3O4, were not observed. Sample b was generated by reducing theamount of nitrogen introduced for sufficient pyrolysis. Although the amount of nitrogen introduced
was reduced to increase the residence time of the droplets in the reaction tube, the target productcould not be obtained. However, compared to sample a, the peak of Fe was large in sample b.
Therefore, samples c and d were generated under oxygen flow for sufficient oxidation. Althoughsufficient reaction time was given under oxygen flow, the target product was not obtained in samples
c and d. In addition, in the sample e generated with a Zn:Fe molar ratio of 1:2, only the peak of
ZnFe2O4 was observed.
Certainly, the generation of iron oxide (Fe3O4, γ-Fe2O3, α-Fe2O3) [25-27] or ZnO single unit by spraypyrolysis has been reported. However, it was confirmed that the generation of the ZnO/Fe3O4
composite using the spray pyrolysis method was difficult. In a similar way, Nunome et al. produced
n- and p-type ZnFe2O4 and evaluated their properties [28]. ZnFe2O4 is a paramagnetic material and is
known to have a band gap of 1.9 eV [29]. However, its low magnetism and bandgap are not suitablefor organic matter treatment. The generation of ZnFe2O4 is thought to be related to metal reactivity.
Since the metal reactivity of Zn and Fe is similar, it can be precipitated as one crystal during thepyrolysis process. On the contrary, since Au and Ag have a large metal reactivity difference and small
reactivity compared to Zn, they can be precipitated as different crystals during the pyrolysis process.

– 76 –

Chapter 4

ZnO/Fe3O4 and ZnO/Fe3O4/Ag composite particle generation process

Figure 4-4 XRD result of composite particles
generated using USP method.

– 77 –

Chapter 4

ZnO/Fe3O4 and ZnO/Fe3O4/Ag composite particle generation process

4.3.2. Characteristics of ZnO/Fe3O4 composite particles generated using USP and co-precipitation
methodMass ratio [g](ZnO:Fe3O4)

Distributed

No magnet(1 minute later)

Magnet(1 minute later)

1:1

2:1

4:1

Figure 4-5 Magnetism of ZnO/Fe3O4 composite particles generated using USP and coprecipitation method.
The observation result of the presence or absence of magnetism of the generated particles is shownin Figs. 4-5. 2 mg of the generated particles were dispersed in 5 ml of water, and the sedimentation
rate of the generated particles was observed with a magnet. In any condition, there was a differencein the sedimentation rate of the generated particles depending on the presence or absence of a magnet.
In addition, the higher the ratio of ZnO, the slower the particle sedimentation rate. Therefore, the caseof the mass ratio (ZnO:Fe3O4) 1:1 is considered to have the greatest magnetism.

– 78 –

Chapter 4

ZnO/Fe3O4 and ZnO/Fe3O4/Ag composite particle generation process

Figure 4-6 XRD result of ZnO/Fe3O4 composite particles generated using USP and coprecipitation method.
Figure 4-6 shows the XRD measurement results of the generated particles. Under any conditions,the peaks of ZnO and Fe3O4 were confirmed together. The peak of ZnO increased as the mass ratioof ZnO increased. In addition, a small peak was detected in the vicinity of 2 = 22 °, and neither ZnO
nor Fe3O4. A similar peak was not detected in the XRD measurement of ZnO produced by spraypyrolysis and Fe3O4 produced by co-precipitation method, so it is presumed to be a by-productproduced in the surface modification process.
Figure 4-7 shows the SEM images of the generated particles. In all conditions, the ZnO/Fe3O4
composite material was confirmed through SEM images. However, large-scale aggregates wereformed due to the agglomeration of the particles, and the majority of uncoated ZnO particles werealso identified. In particular, the formation of such large-scale aggregates can degrade photolysis
properties. In addition, uncoated ZnO particles cannot be recovered after photolysis treatment andmay cause secondary contamination. Therefore, it was confirmed that the surface modification using
sodium citrate was unsuitable for coating ZnO particles produced with USP.

– 79 –

Chapter 4

ZnO/Fe3O4 and ZnO/Fe3O4/Ag composite particle generation process

Figure 4-7 SEM images of ZnO/Fe3O4 composite particles generated using USP and coprecipitation method.
The photolysis experiment was carried out in the same manner as described in Chapter 2, theconcentration of RhB aqueous solution was set to 2 mg/L, and 20 mg of the resulting particles wereadded. Figure 4-8 shows the photolysis test results of the obtained ZnO/Fe3O4 composite material.
The decomposition of the RhB solution by ultraviolet rays only decomposed 4% for 300 minutes,
whereas the addition of the ZnO/Fe3O4 composite material showed remarkable photolysis. In thephotolysis experiment for 300 minutes, about 90% in the case of ZnO, about 24% in the mass ratio
ZnO: Fe3O4 = 4: 1, about 34% in the mass ratio ZnO: Fe3O4 = 2: 1, and about 77% in the mass ratio
ZnO: Fe3O4 = 1: 1 photolysis was observed. That is, as the mass ratio of ZnO increases, the– 80 –

Chapter 4

ZnO/Fe3O4 and ZnO/Fe3O4/Ag composite particle generation process

Figure 4-8 Changes in RhB concentration by UV light irradiation time at ZnO/Fe3O4 andZnO generated using USP and co-precipitation method; initial concentration of
RhB: 2 mg/L; photocatalyst dose: 20 mg; solution volume: 100 mL.
decomposition rate of RhB also increases. When ZnO and Fe3O4 are combined, recovery is possible
due to magnetism, but the photocatalytic surface area decreases, resulting in a decrease in photolysis
efficiency. Therefore, there is a need for a new synthetic method capable of showing highphotocatalytic activity even after complexation.

– 81 –

Chapter 4

ZnO/Fe3O4 and ZnO/Fe3O4/Ag composite particle generation process

4.3.3. Characteristics of ZnO/Fe3O4 and ZnO/Fe3O4/Ag particles generated using co-precipitation
methodFigure 4-9 XRD result ZnO/Fe3O4 and ZnO/Fe3O4/Ag particles generated using coprecipitation method.
The XRD pattern of the composite material obtained by this experiment is shown in Figure 4-9. In
the XRD pattern, a hexagonal peak of ZnO was observed in all samples. In addition, the crystal peaksof Ag were confirmed at 2 = 38, 44, and 65 °, and it can be seen that ZnO and metallic Ag exist in
the obtained composite material.

It was confirmed that the Ag peak increased as the ratio of silver

nitrate added to the raw material solution increased. However, the peak of Fe3O4 overlapped with thepeak of ZnO, making it difficult to confirm. Figure 4-10 shows the crystal peaks of Fe3O4 andZnO/Fe3O4 produced by the co-precipitation method. Before the complexation, the Fe3O4 crystal
peak was observed in the Fe3O4 single unit, and it was confirmed that the peaks of ZnO overlap.
Therefore, the presence or absence of Fe3O4 in the product was investigated by observing themagnetic reaction of the product. 10 mg of the product was dispersed in distilled water and a magnet
was placed at the bottom of the sample bottle. And for comparison, a comparison sample without– 82 –

Chapter 4

ZnO/Fe3O4 and ZnO/Fe3O4/Ag composite particle generation process

magnets was prepared. Figure 4-11 shows the appearance of dispersion solution after 1 minute withor without magnets. Certainly, it was observed that the settling speed of the product was high whenthe magnet was placed on the bottom of the sample bottle. It was confirmed that the resulting
composite material contained Fe3O4 and had magnetic properties.

Figure 4-10 Crystal peaks of Fe3O4 and ZnO/Fe3O4 produced using co-precipitation method.

Figure 4-11 Magnetism of ZnO/Fe3O4 composite particles generated using co-precipitation
method: (Left) before experiment and (Right) 1 minute later– 83 –

Chapter 4

ZnO/Fe3O4 and ZnO/Fe3O4/Ag composite particle generation process

Fe₃O₄

ZnO

ZnO/Fe₃O₄

Figure 4-12 SEM images of ZnO, Fe3O4, and ZnO/Fe3O4 composite particles generated using
co-precipitation method.
Figure 4-12 is an SEM picture of the obtained composite. Large and small ZnO and Fe3O4 particlesof less than 5 μm were identified, and large and small particles of less than 20 μm in the ZnO/Fe3O4
composite were identified. Therefore, it is thought that large ZnO/Fe3O4 particles are formed bychemical bonding between large and small ZnO and Fe3O4 particles. Figure 4-13 shows thebackscattered electron images of ZnO/Fe3O4 and ZnO/Fe3O4/Ag composite particles. The
backscattered electron image (b) of the composite obtained by adding silver nitrate is brighter than
the image (a) of the composite obtained without adding silver nitrate. Since the reflective electronsof Ag, which have an atomic number larger than that of O, Zn, and Fe, are strong, a bright image
appears. Therefore, it was confirmed that Ag in a metallic state was dispersed on the surface of thecomposite with the measurement result of XRD.

– 84 –

Chapter 4

ZnO/Fe3O4 and ZnO/Fe3O4/Ag composite particle generation process

Figure 4-13 Backscattered electron images of (a) ZnO/Fe3O4 and (b) ZnO/Fe3O4/Ag composite
particles generated using co-precipitation method.
The photolysis experiment was carried out in the same manner as described in Chapter 2, theconcentration of RhB aqueous solution was set to 5 mg/L, and 15 mg of the resulting particles wereadded. Figure 4-14 shows the changes in the RhB concentration of obtained composite as UV lightirradiation time. As a result of photolysis, it was confirmed that the dye was decomposed by about
21% for ZnO and about 44% for the ZnO/Fe3O4 composite material. In addition, thephotodecomposition rate of the Ag-supported ZnO/Fe3O4/Ag composite was increased compared tothat of the ZnO/Fe3O4 particles. All ZnO/Fe3O4/Ag composite particles show higher photocatalytic

Figure 4-14 Changes in RhB concentration by UV light irradiation time at ZnO, ZnO/Fe3O4,
and ZnO/Fe3O4/Ag generated using co-precipitation method; initial concentrationof RhB: 5 mg/L; photocatalyst dose: 15 mg; solution volume: 100 mL.
– 85 –

Chapter 4

ZnO/Fe3O4 and ZnO/Fe3O4/Ag composite particle generation processactivity than pure ZnO and ZnO/Fe3O4 particles. However, 0.4 mass% of added silver nitrate, showsa decreased photocatalytic activity. Paradoxically, an increase in the amount of supported Ag particles
may decrease the photocatalytic activity. Increasing the amount of supported Ag promotes both theseparation of electron-hole and the transfer of excited electrons to Ag nanoparticles. However, when
excess Ag is supported on ZnO particles, the excess excitons in the Ag nanoparticles accumulate andattract holes. In particular, the excessive supported Ag can promote electron-hole recombination due
to the large contact area with ZnO particles. Thus, the number of excitons decreases, leading to adecrease in photocatalytic activity.
Figure 4-15 show the changes in RhB concentration by UV light irradiation time as the sample D
dose. In order to know the optimal dose of sample D, which showed the most efficient photolysis, aphotolysis experiment was performed by dispersing 5–20 mg of sample D in an aqueous RhB solution.
The photocatalytic activity improved as the dose of sample D increased and increased from 5-15 mg
in proportion to the amount of sample. However, when 20 mg was dose, the increase in the photolysis
rate was insignificant compared to the photolysis rate of 15 mg. It means that an excessive dose ofparticles increases the turbidity of the suspension and affects photocatalytic activity. That is, theincrease in turbidity of the suspension due to excessive dose of the particles may block UV light andhinder the UV light absorption of the photocatalyst.

Figure 4-15 Changes in RhB concentration by UV light irradiation time as the sample D dose.

– 86 –

Chapter 4

ZnO/Fe3O4 and ZnO/Fe3O4/Ag composite particle generation process

Table 4-3 Comparison of treatment rates by photolysis and recovery rates of
Ag/ZnO (USP method) and ZnO/Fe3O4/Ag (co-precipitation method) particles;
initial concentration of RhB: 5 mg/L; photocatalyst dose: 15 mg; solution
volume: 100 mL, dope amount of silver nitrate: 0.2 mass%.

Photolysis time
Particle recovery time
Number of photolysis cycles
Total time

Ag/ZnO2h 30min10h5
62h 30min

Ag/ZnO/Fe3O4
4h 30min3h 30min540h

Table 4-3 compares the treatment rates of Ag/ZnO and Ag/ZnO/Fe3O4 particles generated by the
USP and co-precipitation method. Compared with Ag/ZnO/Fe3O4 particles generated by the coprecipitation method, the Ag/ZnO particles generated by the USP method had a faster photolysis rate
but a slower recovery rate. In particular, the recovery process using magnetism showed a very fast
speed compared to gravity sedimentation, and it was found that it was an important factor in theoverall treatment rates. As a result, the total photolysis time of Ag/ZnO/Fe3O4 particles was fasterthan that of Ag/ZnO particles in the 5th recycling experiment.

4.4. Conclusion
This experiment tried to synthesize ZnO/Fe3O4 composite particles using the USP and coprecipitation method under various conditions. When the USP method was used as a single method,
the crystal peaks of the target products ZnO and Fe3O4 were not observed. The target product was notobserved even under oxygen conditions and changes in gas flow rate to establish sufficient thermal
decomposition and oxidation conditions. However, under certain conditions, a single crystal peak of
ZnFe2O4 was confirmed. Research on ZnFe2O4 particles has been reported, but due to the low bandgap, it is not suitable for organic matter treatment. The generation of ZnFe2O4 is related to metal
reactivity, and since the metal reactivity of Zn and Fe is similar, it can be precipitated as a single
crystal during the pyrolysis process.
When particles were generated using the USP and co-precipitation method together, crystals of thetarget products, ZnO and Fe3O4, were observed. In addition, it was possible to confirm the magnetism– 87 –

Chapter 4

ZnO/Fe3O4 and ZnO/Fe3O4/Ag composite particle generation processof the generated composite particles. However, large-scale agglomerates were formed due to theagglomeration of the particles, and most of the uncoated ZnO particles were also identified. Therefore,
problems of photolysis and secondary pollution may occur. In the photolysis experiment, as theamount of Fe3O4 added increased, it was observed that the photolysis properties decreased. Therefore,
it was confirmed that the surface modification using sodium citrate is not suitable for the formationof ZnO/Fe3O4 composite particles.
When co-precipitation was used as a single method, crystals of the target products ZnO, Ag, andFe3O4 were observed. In the XRD crystal peak, it was difficult to observe the Fe3O4 crystal peak due
to the overlapping of the ZnO and Fe3O4 peaks, but the magnetic experiment showed Fe3O4 in thecomposite particles. Ag particles dispersed in the particles could be confirmed with backscatteredelectron images. In the photolysis experiment, it was confirmed that the photolysis properties of theZnO/Fe3O4 composite particles were not good compared to the pure ZnO particles. However,
although the ZnO/Fe3O4/Ag composite particles contained Fe3O4, the photodecomposition properties
were superior to those of pure ZnO particles. The optimal condition was when 0.2 mass% silver
nitrate was added, which is consistent with the results of Chapter 3. The co-precipitation single
method confirmed some agglomeration of particles, but it was judged to be most suitable for thegeneration of ZnO/Fe3O4 and ZnO/Fe3O4/Ag composite particles.

4.5. Reference
1.

J.F. Guo, B. Ma, A. Yin, K. Fan, W.L. Dai, Photodegradation of rhodamine B and 4-chlorophenol
using plasmonicphotocatalyst of Ag–AgI/Fe3O4@SiO2magnetic nanoparticle under visible lightirradiation, Applied Catalysis B: Environmental, 101 (2011) 580–586.

2.

P.G. Tratnyek, R.L. Johnson, Nanotechnologies for environmental cleanup, Nano Today, 1 (2006)
44–48.

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S.C.N. Tang, I.M.C. Lo, Magnetic nanoparticles: essential factors for sustainable environmental
applications, Water Research, 47 (2013) 2613–2632.

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Y.F. Shen, J. Tang, Z.H. Nie, Y.D. Wang, Y. Ren, L. Zuo, Tailoring size and structural distortionof Fe3O4 nanoparticles for the purification of contaminated water, Bioresource Technology, 100(2009) 4139–4146.

5.

R.M. Cornell, U. Schwertmann, The Iron Oxides: Structure, Properties, Reactions, Occurrences– 88 –

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and Uses, 2nd Edition (Wiley-VCH, Weinheim, 2003).
6.

R. Dieckmann, Point defects and transport in haematite (Fe2O3-ε), Philosophical Magazine A, 68(1993) 725-745.

7.

H. H. Kung, Transition Metal Oxides: Surface Chemistry and Catalysis (Elsevier, New York,1989).

8.

F. Caruso, M. Spasova, A. Susha, M. Giersig, R.A. Caruso, Magnetic Nanocomposite Particles
and Hollow Spheres Constructed by a Sequential Layering Approach, Chemistry of Materials,
13 (2001) 109-116.

9.

T.G. Hyeon, S.S. Lee, J.N. Park, Y.H. Chung, H B. Na, Synthesis of Highly Crystalline andMonodisperse Maghemite Nanocrystallites without a Size-Selection Process, Journal of theAmerican Chemical Society, 123 (2001) 12798-12801.

10. Y. Xiong, Y. Xie, S. Chen, Z. Li, Fabrication of Self‐Supported Patterns of Aligned β‐FeOOH
Nanowires by a Low‐Temperature Solution Reaction, Chemistry—European Journal, 9 (2003)
4991-4996.
11. K. Woo, H.J. Lee, J.P. Ahn, Y.S. Park, Sol–Gel Mediated Synthesis of Fe2O3 Nanorods, Advanced
Materials. 15 (2003) 1761-1764.
12. Y. Wang, X. Teng, J. Wang, H. Yang, Solvent-Free Atom Transfer Radical Polymerization in theSynthesis of Fe2O3@Polystyrene Core−Shell Nanoparticles, Nano Letters, 3 (2003) 789-793.
13. Y.F. Chong, K.L. Pey, A.T.S. Wee, M.O. Thompson, C.H. Tung, A. See, Laser-induced
amorphizationofsiliconduringpulsed-laser

irradiation

ofTiN/Ti/polycrystalline

silicon/SiO2/silicon, Applied Physics Letters, 81 (2002) 3768-3788.
14. P. Kumar, H.N. Lee, R. Kumar, Synthesis of phase pure iron oxide polymorphs thin films andtheir enhanced magnetic properties, Journal of Materials Science: Materials in Electronics, 25(2014) 4553-4561.
15. M.S. Isiam, J. Kurawaki, Y. Kusumoto, M. Abdulla-Al-Mamun, M.Z. Bin Mukhlish,
Hydrothermal Novel Synthesis of Neck-structured Hyperthermia-suitable Magnetic (Fe3O4, γFe2O3 and α-Fe2O3) Nanoparticles, Journal of Scientific Research, 4 (2012) 99-107.
16. M. Helmi-Rashid-Farimani, N. Shahtahmasebi, M. Rezaee Roknabadi, N. Ghows, A. Kazemi,
Study of structural and magnetic properties of superparamagnetic Fe3O4/SiO2 core–shell
nanocomposites synthesized with hydrophilic citrate-modified Fe3O4 seeds via a sol–gel– 89 –

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approach, Physica E, 53 (2013) 207–216.
17. A. Bahari, M. Roeinfard, A. Ramzannezhad, Characteristics of Fe3O4/ZnO nanocomposite as apossible gate dielectric of nanoscale transistors in the field of cyborg, Journal of Materials
Science: Materials in Electronics, 27 (2016) 9363–9369.
18. R.Y. Hong, S.Z. Zhang, G.Q. Di, H.Z. Li, Y. Zheng, J. Ding, D.G. Wei, Preparation,
characterization and application of Fe3O4/ZnO core/shell magnetic nanoparticles, Materials
Research Bulletin, 43 (2008) 2457–2468.
19. M. Paul, D. Kufer, A. Muller, S. Brück, E. Goering, M. Kamp, J. Verbeeck, H. Tian, G.V.
Tendeloo, N.J.C. Ingle, M. Sing, R. Claessen, Fe3O4/ZnO : A high-quality magnetic oxidesemiconductor heterostructure by reactive deposition, Applied Physics Letters, 98 (2011) 1–3.
20. D.S. Winatapura, S.H. Dewi, W.A. Adi, Synthesis, Characterization, and Photocatalytic Activityof Fe3O4@ZnO Nanocomposite, International Journal of Technology, 3 (2016) 78-85.
21. M. Machovsky, I. Kuritka, Z. Kozakova, Microwave assisted synthesis of nanostructured
Fe3O4/ZnO microparticles, Materials Letters,86 (2012) 136-138.
22. A. Roychowdhury, S.P. Pati, A.K. Mishra, S. Kumar, D. Das, Journal of Physics and Chemistryof Solids Magnetically addressable fluorescent Fe3O4/ZnO nanocomposites : Structural , opticaland magnetization studies, Journal of Physics and Chemistry of Solids, 74 (2013) 811-818.
23. A.A. Shal, A. Jafari, Study of Structural and Magnetic Properties of Superparamagnetic Fe3O4–
ZnO Core–Shell Nanoparticles, J Supercond Nov Magn, 27 (2014) 1531-1538.
24. M. Safari, M.H. Rostami, M. Alizadeh, A. Alizadehbirjandi, S.A.A. Nakhli, R. Aminzadeh,
Response surface analysis of photocatalytic degradation of methyl tert-butyl ether by core/shell
Fe3O4/ZnO nanoparticles, Journal of Environmental Health Sciences & Engineering, 12 (2014)
1-10.
25. R. Strobel, S.E. Pratsinis, Direct synthesis of maghemite, magnetite and wustite nanoparticles by
flame spray pyrolysis, Advanced Powder Technology, 20 (2009) 190-194.
26. J.W. Zheng, K.H. Liu, W. Cai, L. Qiao, Y. Ying, W.C. Li, J. Yu, M. Lin, S.L. Che, Effect ofchloride ion on crystalline phase transition of iron oxide produced by ultrasonic spray pyrolysis,
Advanced Powder Technology, 29 (2018) 1953-1959.
27. A. Kato, F. Tokunaga, Fine Powders by Spray-pyrolysis Technique II. Iron Oxide Powders,
Journal of Japanese Society of Powder and Powder Metallurgy, 24 (1977) 219 (in Japanese)– 90 –

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ZnO/Fe3O4 and ZnO/Fe3O4/Ag composite particle generation process

28. T. Nunome, H. Irie, N. Sakamoto, O. Sakurai, K. Shinozaki, H. Suzuki, N. Wakiya, Magnetic
and photocatalytic properties of n- and p-type ZnFe2O4 particles synthesized using ultrasonicspray pyrolysis, Journal of the Ceramic Society of Japan, 121 (2013) 26-30.
29. M.A. Valenzuela, P. Bosch, J. Jiménez-Becerrill, O. Quiroz, A.I. Páez, Preparation,
characterization and photocatalytic activity of ZnO, Fe2O3 and ZnFe2O4, Journal of
Photochemistry and Photobiology A: Chemistry, 148 (2002) 177-182.

– 91 –

Chapter 5 Conclusions

Chapter 5 Conclusions
Existing wastewater treatment methods such as adsorption, filtration, coagulation, membranes, etc.
have a problem in that pollutants cannot be decomposed, and organic pollutant molecules are simply
transferred to another medium to cause secondary pollution. Therefore, in this study, the photocatalyst
ZnO was studied for the oxidation treatment of organic dyes. However, the main drawbacks of ZnO
are that the efficiency decreases due to the rapid recombination of the excited electron-hole pairs, andthe recovery of the photocatalyst after photolysis treatment is very difficult and reuse is greatly limited.
In order to compensate for the shortcomings of ZnO, a study on the loading of Au and Ag, which acts
as an electron sink, and the complexation of Fe3O4 were conducted.
In Chapter 2-3, Ag/ZnO and Au/ZnO particles were generated using the USP method. All particles
were spherical and the size of between 0.5 μm and 1.0 μm, and there was no change in shape due tothe support of Ag and Au. In all of the generated particles, crystal peaks of Ag or Au were observed
along with a crystal peak of ZnO. TEM images showed dark spots scattered throughout the Au/ZnO
and Ag/ZnO particles. EDX mapping and STEM images confirmed that these dark spots correspondto Ag or Au nanoparticles dispersed throughout the ZnO particles.
The photocatalytic activity of the resulting particles was investigated by photolysis of the aqueous
RhB solution under UV irradiation. All Au/ZnO and Ag/ZnO particles showed higher photocatalytic
activity than pure ZnO particles. In addition, Au/ZnO particles supporting 0.1 mass% HAuCl4 had aphotolysis rate constant about 4 times higher than that of pure ZnO particles, and Au/ZnO particles
supporting 0.2 mass% AgNO3 were about 8 times higher than that of pure ZnO particles. And
interestingly, some particles showed better photolysis activity than P25. However, when HAuCl4 andAgNO3 were supported by 0.2 mass% and 0.4 mass %, respectively, the photocatalytic activity was
greatly reduced. That is, as the amount of Au and Ag increased, the photodecomposition activity
increased, but when the noble metal nanoparticles were excessively supported, the photocatalytic
activity decreased. It is thought that this is because if excess Au or Ag nanoparticles are supported on
ZnO, excessive excitation electrons are accumulated and recombination of electron-holes is promoted
by electrostatic attraction.
In order to investigate the optimal dose required for photolysis, photolysis experiments were
performed with various doses. The photocatalytic activity of both Au/ZnO and Ag/ZnO particles– 92 –

Chapter 5 Conclusions

increased in proportion to the dose from 2 mg to 10 mg, but did not change significantly at higher
doses. This is because the ultraviolet rays, which are important for electronic excitation, have been
blocked by the cloudy suspension. It also confirmed the reusability of Ag/ZnO particles forcommercial applications. It was observed that the generated particles could not only be recovered bygravity precipitation after photolysis experiments, but also had a slow deterioration rate.
Decomposition experiments on actual wastewater were outside the scope of this study, but thepossibility of using this material to purify organic pollutants was confirmed.
In Chapter 4, the synthesis of ZnO/Fe3O4 composite particles was attempted by using USP andcoprecipitation method for simple recovery of magnetic particles. When the USP method was used
as a single method, crystal peaks of the target products ZnO and Fe3O4 were not observed, and only
crystal peaks of ZnFe2O4 were observed. When particles were generated using the USP andcoprecipitation method along with surface modification using sodium citrate, crystals of the target
products, ZnO and Fe3O4, were observed, and the magnetic properties of the particles were confirmed.
However, agglomeration of most of the uncoated ZnO particles and particles was observed. When the
coprecipitation method was used as a single method, crystals of the target products ZnO, Ag, andFe3O4 were observed. In the photolysis experiment, it was confirmed that the photolysis propertiesof the ZnO/Fe3O4 composite particles were not as good as those of pure ZnO particles. However, theZnO/Fe3O4/Ag composite particles contain Fe3O4, but the photolysis properties were superior to thoseof pure ZnO particles. Therefore, the ZnO/Fe3O4/Ag composite particles produced by the coprecipitation method can be considered as a promising material for organic dye treatment.

– 93 –

Appendix

APPENDIX
Nomenclature
initial concentration of the RhB[mg/L]

raw material concentration[g/L]

𝐷dropletdroplet size

[m]

𝐷ZnO

ZnO particle diameter[m]𝑓frequency of the ultrasonic transducer

[MHz]

𝑘

rate constant[min-1]molecular weight of zinc nitrate[kg/kmol]

𝑀𝑊ZnO

molecular weight of ZnO[kg/kmol]O2 • ¯superoxide radical anions

[-]

• OH

hydroxyl radicals

[-]

𝜌ZnO

density of ZnO[kg/m³]

𝑡

time

[min]

𝜌

density of the solution[kg/m3]𝛾

surface tension of the solution

[N/m]

R2

Coefficient of determination[-]

𝐶0
𝐶Zn(NO3)2 aq.

𝑀𝑊Zn(NO3)2– 94 –

Appendix

List of Figures
Chapter 1
Figure 1-1 Photocatalytic reaction mechanism of ZnO particle.
Figure 1-2 Photocatalytic reaction mechanism of Au/ZnO composite particles.
Chapter 2
Figure 2-1 Schematic diagram of ultrasonic spray pyrolysis.
Figure 2-2 Particle formation process in USP method.
Figure 2-3 Droplet size by ultrasonic frequency intensity.
Figure 2-4 Schematic diagram of electrostatic collector.
Figure 2-5 SEM images of ZnO particles obtained at different conditions.
Figure 2-6 XRD pattern of ZnO particle (sample 4).
Figure 2-7 XRD pattern of Au/ZnO composite particles.
Figure 2-8 TEM images of Au/ZnO composite particles obtained by USP synthesis.
Figure 2-9 STEM image of sample d particle and the corresponding EDX mapping images of theAu, Zn, and O, respectively.
Figure 2-10 Semilogarithmic plot of changes in RhB concentration by UV light irradiation time atZnO (Sample 4), samples a-d, and P25; initial concentration of RhB: 5mg/L;
photocatalyst dose: 5mg; solution volume: 100ml.
Figure 2-11 First-order reaction rate constant for RhB photolysis using ZnO (Sample 4), samples
a-d, and P25 under UV light irradiation; initial concentration of RhB: 5 mg/L;
photocatalyst dose: 5 mg; solution volume: 100 ml.
Figure 2-12 (a) Fermi level of ZnO and Au particles before compounding, (b) Fermi level of ZnO
and Au particles after compounding. E0:vacuumlevel; (Ef)Au, (Ef)ZnO, Ef: Fermi levelsof Au, ZnO, and Au/ZnO; CV and VB : conduction band and valence band of ZnO.

– 95 –

Appendix
Figure 2-13 Semilogarithmic plot of changes in RhB concentration by UV light irradiation time atdifferent photocatalyst dose of Sample c; initial concentration of RhB: 5 mg/L;
solution volume: 100 ml.
Figure 2-14 First-order reaction rate constant for RhB photolysis using different photocatalyst doseof Sample c under UV light irradiation; initial concentration of RhB: 5mg/L;
solution volume: 100ml.
Chapter 3
Figure 3-1 SEM images of samples a-e obtained at different conditions.
Figure 3-2 XRD pattern of ZnO and Ag/ZnO particles.
Figure 3-3 TEM images of samples a-e obtained by USP synthesis.
Figure 3-4 STEM image of sample d particle and the corresponding EDX mapping images of theAg, Zn, and O, respectively.
Figure 3-5 Semilogarithmic plot of changes in RhB concentration by UV light irradiation time atZnO (Sample a), samples b-e, and P25; initial concentration of RhB: 5 mg/L;
photocatalyst dose: 5 mg; solution volume: 100 mL.
Figure 3-6 First-order reaction rate constant for RhB photolysis using ZnO (Sample a), samples be, and P25 under UV light irradiation; initial concentration of RhB: 5 mg/L;
photocatalyst dose: 5 mg; solution volume: 100 mL.
Figure 3-7 (a) Fermi level of ZnO and Ag particles before compounding, (b) Fermi level of ZnO
and Ag particles after compounding. E0:vacuum level; (Ef)Ag, (Ef)ZnO, Ef:: Fermi levelsof Ag, ZnO, and Ag/ZnO; CV and VB : conduction band and valence band of ZnO.
Figure 3-8 Semilogarithmic plot of changes in RhB concentration by UV light irradiation time atdifferent photocatalyst dose of Sample d; initial concentration of RhB: 5mg/L; solution
volume: 100mL.
Figure 3-9 First-order reaction rate constant for RhB photolysis using different photocatalyst doseof Sample d under UV light irradiation; initial concentration of RhB: 5mg/L; solution
volume: 100mL.

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Appendix
Figure 3-10 Reusability of sample d photocatalyst under UV light irradiation; initial concentrationof RhB: 5mg/L; photocatalyst dose: 10mg; solution volume: 100mL; UV lightirradiation time: 120min.
Figure 3-11 XRD peaks before and after the photolysis experiment of sample e.
Chapter 4
Figure 4-1 Procedure of formation of Fe3O4 by co-precipitation method.
Figure 4-2 Procedure of formation of ZnO/Fe3O4 composite particles.
Figure 4-3 Procedure of formation of ZnO/Fe3O4 and ZnO/Fe3O4/Ag composite particles.
Figure 4-4 XRD result of composite particles generated using USP method.
Figure 4-5 Magnetism of ZnO/Fe3O4 composite particles generated using USP and co-precipitation
method.
Figure 4-6 XRD result of ZnO/Fe3O4 composite particles generated using USP and coprecipitation method.
Figure 4-7 SEM images of ZnO/Fe3O4 composite particles generated using USP and coprecipitation method.
Figure 4-8 Changes in RhB concentration by UV light irradiation time at ZnO/Fe3O4 and ZnO
generated using USP and co-precipitation method; initial concentration of RhB: 2
mg/L; photocatalyst dose: 20 mg; solution volume: 100 mL.
Figure 4-9 XRD result ZnO/Fe3O4 and ZnO/Fe3O4/Ag particles generated using co-precipitation
method.
Figure 4-10 Crystal peaks of Fe3O4 and ZnO/Fe3O4 produced sing co-precipitation method.
Figure 4-11 Magnetism of ZnO/Fe3O4 composite particles generated using co-precipitation
method: (Left) Before experiment and (Right) 1 minute later
Figure 4-12 SEM images of ZnO, Fe3O4, and ZnO/Fe3O4 composite particles generated using coprecipitation method.
Figure 4-13 Backscattered electron images of (a) ZnO/Fe3O4 and (b) ZnO/Fe3O4/Ag composite– 97 –

Appendix
particles generated using co-precipitation method.
Figure 4-14 Changes in RhB concentration by UV light irradiation time at ZnO, ZnO/Fe3O4, and
ZnO/Fe3O4/Ag generated using co-precipitation method; initial concentration of
RhB: 5 mg/L; photocatalyst dose: 15 mg; solution volume: 100 mL.
Figure 4-15 Changes in RhB concentration by UV light irradiation time as the sample D dose.

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Appendix

List of Tables
Chapter 1
Table 1-1 Some Examples of Commercial Dyes.
Table 1-2 Wastewater treatment method.
Table 1-3 Disadvantages of the existing wastewater treatment.
Chapter 2
Table 2-1 Typical droplet sizes of depending on spray method.
Table 2-2 Experimental conditions and particle size distribution of ZnO particles.
Table 2-3 Experimental concentrations of Au/ZnO composite particles and crystallite size of Au
calculated by Scherrer's equation.
Table 2-4 First-order reaction rate constant k and the coefficient of determination R2 corresponding
to the slope of the line.
Table 2-5 First-order reaction rate constant k and the coefficient of determination R2 for dose.
Table 2-6 Photocatalytic activity of Au/ZnO system.
Chapter 3
Table 3-1 Experimental concentrations of ZnO and Ag/ZnO particles and crystallite size calculatedby Scherrer's equation.
Table 3-2 First-order reaction rate constant k and the coefficient of determination R2 corresponding
to the slope of the line.
Table 3-3 First-order reaction rate constant k and the coefficient of determination R2 for dose.
Chapter 4
Table 4-1 Experimental parameters of the ZnO/Fe3O4 composite particles by USP method.

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Appendix
Table 4-2 Experimental parameters of the ZnO/Fe3O4 and ZnO/Fe3O4/Ag composite particles byco-precipitation method.
Table 4-3 Comparison of treatment rates by photolysis and recovery rates of Ag/ZnO (USP
method) and ZnO/Fe3O4/Ag (co-precipitation method) particles; initial concentrationof RhB: 5 mg/L; photocatalyst dose: 15 mg; solution volume: 100 mL, dope amountof silver nitrate: 0.2 mass%.

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Acknowledgment
I would like to express my sincere gratitude to my supervisor, Associate Professor Toshiyuki
Fujimoto, for continuing to support my doctorate with his research knowledge and advice. In addition
to his extensive research knowledge, I was able to learn persistence, perseverance, Japanese culture,
insight, etc., which made my Ph.D. study in his laboratory unforgettable and very meaningful. Above
all, his guidance through seminars and discussions in the laboratory helped me a lot in researchingand solving this thesis.
Sincere thanks to my thesis committee members, Professor Hideyuki Nakano and Associate
Professor Shinya Yamanaka. Their advice and insightful questions allowed me to think about myresearch from more diverse perspectives and improved the quality of my research. And I would liketo thank Professor Takafumi Seto, who gave me a lot of advice during my internship period at
Kanazawa University. Without his consideration, I would not have been able to do my internshipduring these very difficult times due to COVID-19. Also, I would like to express my sincere gratitude
to the kindness and courtesy of the people involved in the research lab at Kanazawa University.
I would also like to thank all members of the Aerosol Chemical Engineering Lab. First of all, I
would like to thank Watanuki, who was my tutor and classmate. Her kindness helped me adjust to thelab and sometimes taught me Japanese. I also would would like to express my gratitude to my lab
juniors, Hashimoto, Takahashi, and Kawashima. Takoyaki, nabe, curry, and Christmas parties in thelaboratory became great memories during my life of studying abroad.
I would like to sincerely thank my seniors Wonsig Jin and Juhyeon Yu for recommending me to
study abroad and for always giving me advice. They always gave me a lot of advice through my study
abroad experience and helped me to finish my study abroad life successfully. And I am also grateful
to my graduate school classmate and friend Jongil Kim. He is my drinking buddy, and because ofhim, we were able to finish our difficult study abroad life together.
Finally, I am especially grateful to my family for supporting me during this long and sometimes
difficult journey. Although there were many difficult times during my study abroad life, graduation
would not have been possible without their encouragement and love. And I am really grateful to mylate grandmother who loved me and always supported me in my study abroad life. I am really grateful– 101 –

to everyone who supported me during my study abroad life.
Muroran, September 2021
Youngjun Lee– 102 –