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1. Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis method, antibacterial and photocatalytic activity of ZnO
nanoparticles for azo dyes in wastewater treatment: A review
Getu Kassegn Weldegebrieal
Department of Chemistry, College of Natural and Computational Sciences, Debre Berhan University, Ethiopia
G R A P H I C A L A B S T R A C T
A R T I C L E I N F O
Keywords:
Antibacterial activity
Azo dye
Biosynthesis
Photocatalysis
A B S T R A C T
These days, with the increasing problem of water contamination by effluents released from rising expansion of
textile, leather, paper and ink industries and problems related to disease causing pathogenic microorganisms, the
need for a substance with antimicrobial and photocatalytic activity is very high. Interesting properties of ZnO
nanoparticles (NPs) such as non-toxicity, low cost, biocompatibility, chemical stability, thermal and photo­
stability, high UV absorption ability have made it a promising photocatalyst for removing hazardous con­
taminants especially azo dyes in wastewater. However, the toxic nature of ZnO NPs for microbes, in particular,
bacteria coupled with the properties mentioned have made it a preferable antimicrobial agent in medicine and
food packaging for inhibiting microbial growth. In this review, the physical and chemical properties, synthesis
approaches, antibacterial activity, and photocatalytic activity of ZnO NPs are discussed. Since many properties of
ZnO NPs, and hence its application, depend on the synthesis method employed, focus is given on the biosynthesis
of ZnO NPs from plant extracts. Moreover, factors affecting photocatalytic activity in the photocatalytic de­
gradation of azo dyes in wastewater treatment as well as antibacterial activity of ZnO NPs are highlighted based
on recent reports in the literature.
1. Introduction
Nowadays, nanomaterials have received tremendous attention by
researchers owing to their multitude of application in various dis­
ciplines. The American physicist Richard Feynman’s talk in 1959
entitled “There is Plenty of Room at the Bottom” coupled with the in­
vention of the scanning tunneling microscope later in 1981 has opened
up the door for the development of nanoscience and nanotechnology,
which is concerned with the study of the property of materials in the
nanoscale and their vast array of application in various fields such as
https://doi.org/10.1016/j.inoche.2020.108140
Received 18 June 2020; Received in revised form 19 July 2020; Accepted 19 July 2020
E-mail address: gk21@dbu.edu.et.
Inorganic Chemistry Communications 120 (2020) 108140
Available online 21 July 2020
1387-7003/ © 2020 Elsevier B.V. All rights reserved.
T

2. chemistry, biology, physics, and materials science, etc. According to the
international organization for standardization (ISO) [1], a nanomaterial
is defined as a material with any external dimensions in the nanoscale
or having internal structure or surface structure in the nanoscale – the
length ranging from approximately 1 nm to 100 nm. Not only do ma­
terials in this size range have different properties from their bulk state
but also exhibit size and morphology dependent properties, making
them interesting to researchers because of their myriads of application
in different fields of study. Transition metal oxides are one among the
different class of nanomaterials which are heavily studied. These oxides
are of particular interest because of their varied metal to oxygen ratio
and large number of phases with different structures, properties, and
application. Zinc oxide (ZnO), which belongs to II-VI transition metal
oxides, is one of the most widely studied oxides as a result of its in­
teresting physicochemical, optoelectronic properties and hence its ap­
plication in many areas including in catalysis, sensors, medicine, per­
sonal care products, paints, batteries, solar cells, memory devices,
electronic and spintronics, and in agriculture to control crop disease
and as bio fertilizer [2-4]. A detailed overview of the different areas of
application of ZnO is given in reference [5]. In particular, by virtue of
its chemical stability, biocompatibility and biodegradability nature, it
has been extensively used as antibacterial agent in antifouling appli­
cation and also in the radiation–assisted degradation of organic con­
taminants in wastewater treatment. Nowadays, access to clean water is
becoming an increasing demand. However, due to increase of urbani­
zation and expansion of factories such as paper and ink, textile, leather
and others, the water body is becoming contaminated with residual
waste discharged from these factories causing harm to humans and
aquatic lives. Azo dyes are the most common water contaminants. They
are mainly used as colorants in the textile industry and though many of
them have good fixation capability, a large portion of the dyes ranging
from 10 − 15%, and even up to about 50%, in the case of a class of dyes
named reactive dyes, could remain unfixed by the fabrics during dyeing
and enter the waste stream [6,7]. Since a very small amount of dye (less
than 1 ppm) can produce highly vivid color and affect transparency,
water–gas solubility and thus adversely affect aquatic biota by de­
creasing sunlight penetration and hence preventing photosynthesis and
creating oxygen deficiency [8,9], it is important to treat the wastewater
carrying dyes from factories before it is discharged into the environ­
ment. There are several adopted methods already in place which are
used to treat such wastes such as chemical oxidation, reduction, ad­
sorption, precipitation, coagulation, flocculation, flotation, etc. How­
ever, the problem with these methods is that they either do degrade the
dyes only partially giving rise to secondary toxic metabolites or produce
huge sludge during treatment making them generally cost-ineffective
approaches [10-12]. In light of this, heterogeneous photocatalysis using
semiconductor catalysts is more advantageous over the mentioned
traditional methods for the following reasons: Firstly, it completely
mineralizes the dyes into CO2, H2O and simple mineral acids [13].
Secondly, it is cost effective for small amount of catalyst can be used
and reused and also nature sunlight can be used as energy source for
causing the degradation. Finally, it is eco-friendly and does not involve
formation of huge sludge during treatment.
Additionally, pathogenic microorganisms are the causes for many
diseases and deterioration or fouling of vegetables and food. Therefore,
an effective substance as antibacterial and antifungal agent is of im­
mense importance to prevent the diseases and prolong the shelf lives of
food and vegetables. ZnO NPs come into play in this regard as well.
As a result of its multifunctionality, ZnO NPs has been synthesized
using physical, chemical, biological or a hybrid of those synthetic ap­
proaches. The biosynthesis method using extracts of medicinal plants
has recently gained researchers’ interest over the more common che­
mical method because of its advantages including the nanomaterials
can be synthesized relatively fast, using simple lab equipment, synthesis
procedure, low cost of precursors and energy expenditure, while
maintaining high purity product, and that no toxicity is introduced
during handling procedure [14], making it preferable for application as
antibacterial agent for inhibiting bacterial growth and in the treatment
of wastewater carrying azo dyes.
In this review article, special focus has been given to the bio­
synthesis of ZnO NPs using extracts of plants. In doing so, extract pre­
paration method is briefly mentioned, for it has its own impact on the
particle size and morphology of the synthesized ZnO NPs, and also
phytochemical composition of the employed plant extracts extracted
using appropriate solvent, mainly water, as reported using standard
testing procedure listed down including some list of structural formula
of isolated compounds from these plant extracts so as to show how the
types of biomolecules in the plant extract play a role in the synthesis of
ZnO NPs, as capping and reducing agents, schematically illustrate the
likely synthesis mechanism as proposed by different authors, and ex­
plain why biosynthesized ZnO NPs outperform that of chemically syn­
thesized ZnO NPs as antibacterial agent. Moreover, toxicity of azo dyes
in wastewater is correlated to their chemical structure and that their
degradation efficiency using ZnO NPs as photocatalysts is affected by
several factors which are overviewed in detail based on reports in the
recent literature, and to my knowledge such a review has not been
made before.
2. Property and synthesis of ZnO NPs
2.1. Property of ZnO NPs
Bulk ZnO has physicochemical and optoelectronic properties dif­
ferent from ZnO NPs. ZnO occurs in three different phases. Namely, the
hexagonal wurtzite, zincblende, and rock salt both of which are cubic in
structure. The rock salt and zincblende phases are not stable at room
temperature (RT) and that while the former is synthesized at relatively
high pressures the later may be stabilized only on cubic substrates [15].
As its properties are listed in Table 1, wurtzite is the most thermo­
dynamically stable phase at RT. It has tetrahedral geometry, where
each Zn atom is tetrahedrally bonded to four oxygen atoms and vice-
versa. Unlike that of rock salt phase, where each atom has six nearest
neighbors, wurtzite and zincblende have each atom four neighbors in
their crystal structure, Fig. 1. In terms of atomic arrangement, both
wurtzite and zincblende have same tetrahedral geometry, the difference
being only in the angle between adjacent tetrahedral units, 60° in
zincblende and zero in wurtzite. Moreover, the two phases are also
different in terms of bond ionicity. While zincblende has absolutely
covalent character, wurtzite has bond ionicity that lies between that of
covalent and ionic, giving rise to its thermodynamic stability [16]. All
ZnO in this review refers to the wurtzite phase. Additionally, wurtzite
has high piezoelectric property which is associated with its lack of in­
version symmetry, being a phase with space group of P63mc. Com­
mercial ZnO is white in color whereas synthesized powder may have
different color depending on the synthesis method employed. For ex­
ample, ZnO nano powder synthesized using plant extract may assume
the color of the extract. Additionally, in nature it may exist in yellow or
red color because of the presence of impurities such as manganese [17].
It is thermochromic compound, turning to yellow when heated and
white again when cooled. Owing to oxygen vacancies or Zn interstitials,
ZnO is naturally n-type direct band gap semiconductor with bulk band
gap value of 3.37 eV at RT [18]. However, this value may be different
when its size is reduced to the nano-scale and when doped with other
elements. Reduction in particles size widens its band gap as a result of
quantum confinement effect. When it is doped with other elements, its
band gap may be widened or narrowed. For example, while for Mg-
doped ZnO NPs, a linear increase of band gap from 3.30 to 3.66 eV with
increase in Mg concentration
from 0 to 15% was observed, for Cd-doped ZnO the gap decreased
from 3.30 to 2.92 with increase of Cd doping from 0 to 10% [20].
Therefore, being able to control the band gap is one essential skill in
nano ZnO synthesis for many of its optical, photocatalytic and
G.K. Weldegebrieal Inorganic Chemistry Communications 120 (2020) 108140
2

3. antibacterial properties are associated with it.
2.2. Synthesis of ZnO NPs
Generally, nanomaterials can be synthesized by physical, chemical,
biological or hybrid method of syntheses, Fig. 2. In the physical
method, one starts with bulk matter and then goes breaking down to
smaller particles till nano-scale size is achieved, a process called top-
down approach. Particular examples include laser ablation, where
atoms are removed from a solid through thermal or nonthermal process
with intense laser beam, physical vapor deposition, in which a material
in the form of vapor particles is transferred from a source material
(target) to the substrate [21], and so on.
In the chemical synthesis method, nanomaterials are produced by
combining substances in a wet chemistry and adjusting reaction para­
meters. In this method, atoms, molecules or ions in solution first form
nucleation and then aggregates of those species that finally end up in
particles of nano-size regime, a process called bottom-up approach.
Nowadays, for synthesis convenience reasons, this is the most widely
used method of producing nanomaterials in industry [22].
So, there are various methods categorized under this approach in­
cluding sol–gel, solvothermal, microwave irradiation, pyrolysis, che­
mical precipitation, microemulsion, thermal decomposition of pre­
cursors, etc.
The biosynthesis method uses microorganisms or extracts of plants
along with a precursor to form the desired nanomaterial. ZnO NPs has
been synthesized by all three methods mentioned. For example, phy­
sical methods such as ball milling [23], physical vapor deposition [24],
and laser ablation [25], chemical methods such as hydrothermal [26],
sol–gel [27], microemulsion [28], and biological methods from various
extracts of plants as listed down in Table 2 are reported in the literature.
The biosynthesis method using plant extracts is more advantageous
than the chemical method in that the nanomaterials can be produced
more quickly with low cost of precursors and energy expenditure while
ensuring high purity product with simpler synthesis procedure and lab
equipment [14]. However, biosynthesis method using extracts of plants
has the disadvantage that control over particle size and shape is more
difficult compared to chemical method, and the purification of the
synthesized NPs from plant biomolecules could also be a challenge
[29].
A plant extract may be defined as a multicomponent mixture ob­
tained using an appropriate solvent in an extraction procedure.
Generally, plant synthesized compounds can be categorized into pri­
mary and secondary metabolites. While metabolites that are essential in
the growth, development and reproduction of the plant such as nucleic
acids, carbohydrates, chlorophyll, etc are regarded as primary meta­
bolites, those such as alkaloids, terpenoids, phenolics, etc which are not
involved in such roles are called secondary metabolites [30]. So, ex­
traction involves the separation of soluble metabolites from the in­
soluble plant matrix using an appropriate extraction solvent also named
menstruum. There are several plant extraction techniques that can be
used like the conventional techniques such as maceration, decoction,
Soxhlet extraction, and non-conventional techniques such as micro­
wave assisted extraction, ultrasound-assisted extraction, super critical
fluid extraction, etc which are reviewed in reference [31].
For the purpose of nanomaterials synthesis, crude extracts prepared
by conventional techniques could and have been used. Since it is the
secondary metabolites in the extract that play as reducing and capping
agents in the nanomaterial synthesis and that the composition and
concentration of the molecules is affected by the extract preparation
method, the method of extraction, as reported in the indicated refer­
ences, is briefly mentioned during the synthesis of ZnO NPs from ex­
tracts of the plants in Table 2.
It is the phytochemical compounds in the plant which are re­
sponsible for not only their reducing and capping ability but also for
their antioxidant, antimicrobial, and many medicinal properties such as
Table
1
Physicochemical
properties
of
wurtzite
ZnO.
Property
Value
Property
Value
Property
Value
Name
Zinc
oxide
Melting
point
1975
°C
(decomposes)
Exciton
binding
energy
(meV)
60
Formula
ZnO
Flash
point
1436
°C
Coordination
geometry
Tetrahedral
Appearance
White
solid
Toxicity
hazards
None
Crystal
structure
Hexagonal
wurtzite
(Lattice
parameters:
a
=
3.25
Å,
c
=
5.205
Å,
c/a
=
1.601;space
group:
P6
3
mc)
Odor
Odorless
Refractive
index
2.0041
Mechanical
property
Soft
material,
high
heat
capacity
and
conductivity
and
low
thermal
expansion
&
has
high
piezoelectric
century
Molar
mass
(g/mol)
81.4
Magnetic
susceptibility,
(cm
3
/mol)
x
27.2
10
6
Luminescence
Luminescent
in
the
UV
and
visible
light
Density(g/cm
3
)
5.6
Electricalconductivity
Semiconductor
Electron
Hall
mobility
at
300
K
(cm
2
/
Vs)
200
Solubility
in
water
Insoluble
Band
gap
3.37
eV
(Direct
at
RT)
Hole
mobility
at
300
K
(cm
2
/Vs)
5
–
50
G.K. Weldegebrieal Inorganic Chemistry Communications 120 (2020) 108140
3

4. delaying aging, reducing inflammation, and preventing certain cancers
[32,33]. Therefore, during the synthesis of ZnO NPs, or any other metal
oxide for that matter, using plant extracts, the phytochemical compo­
sition of the selected plant, method of extract preparation, and method
of ZnO NPs synthesis should be considered beforehand.
2.2.1. Selection of plant
Not extract of every plant available on the surrounding may be used
for ZnO NPs synthesis. The selected plant needs to have phytochemical
compounds capable of serving as reducing and capping agents in its
extract extracted using an appropriate extraction solvent. In general,
extracts containing polyphenols, flavonoids, alkaloids, terpenoids, gly­
cosides, reducing sugars, vitamins via their hydroxyl, carboxyl, and
amine functional groups in their molecular structure are reported to
play such roles [34-37]. Table 2 also lists phytochemical composition of
aqueous extracts of plants used for ZnO NPs synthesis using standard
testing procedure or using spectroscopic techniques, as reported in the
indicated references, with structural formula of specific biomolecules
isolated from some of these plant extracts shown in Fig. 4.
Thus, if a plant is desired to be applied for the successful synthesis of
ZnO NPs synthesis, it is important to in advance refer studies conducted
on its phytochemical make up in the literature or undergo preliminary
study on its possible composition using standard testing procedure if no
such data is already available before possibly running the risk of costly
chemicals. Moreover, it is important to decide which part of the plant,
leaves, flowers, roots, barks, or seeds to use to get extract of higher
phytochemical concentration possible per given dry weight of plant
part. This is because extracts of different parts of the same plant may
have different concentration of phytochemicals using same extraction
solvent. A case in point is methanol, chloroform, petroleum benzene,
and acetone extracts of stem and root parts of Micrococca mercurials
plant were higher in quantity than those obtained from its leaves [38].
Fig. 1. Crystal structures of (a) wurtzite, (b) zincblende, and (c) rock salt phases of ZnO (Reprinted with permission from [19]) Copyright 2005, American Institute of
Physics.
Nanomaterials Synthesis Methods
Physical Chemical Hybrid
Biological
* Physical vapor
deposition
* Laser ablation
* Ball milling
* Etching
Microorganisms Plants extracts
* Bacteria
* Fungi
* Yeast
* Actinomycets
* Algae
* Leaves
* Flowers
* Roots
* Barks
* Seeds
* Solvothermal
* Sol-gel
* Microemulsion
* Precipitation
* Pyrolysis
* Chemical reduction
* Biohydrothermal
* Electrochemical
* Chemical Vapor
Deposition
* Particle Arresting
in glass, polymers or
zeolites
Fig. 2. Outline of general synthesis method of nanomaterials.
G.K. Weldegebrieal Inorganic Chemistry Communications 120 (2020) 108140
4

5. Table
2
List
of
plants
including
extract
preparation
method
and
their
phytochemical
constituents
used
for
synthesizing
ZnO
NPs
(key
to
abbreviations:
M
t
OH
=
methanol,
E
t
OH
=
ethanol,
DW
=
distilled
or
deionized
water,
DDW
=
double
distilled
water,
MQW
=
milli
Q
water,
AGW
=
analytical
grade
water,
UPW
=
ultra-pure
water;
—
means
data
not
found).
S/N
Name
of
plant
Extraction
method
Phytochemical
constituents
tested
to
be
present
Ref.
Common
Name
Scientific
name
1
Olive
Olea
europaea
L.
10
g
in
100
mL
of
DW
and
heating
while
stirring
at
60
°C
for
30′
and
then
filtered
Phenol,
flavonoids,
protein,
alkaloids
[61]
2
Green
tea
Camellia
sinensis
5
g
in
100
mL
of
DW,
heating
at
70
°C
for
1
h
Alkaloids
(caffeine),
polyphenols
(catechins)
[62]
3
Thyme
Thymus
vulgaris
L.
6
g
in
100
mL
DW,
heating
at
80
°C
for
1
h,
and
then
filtered
Thymol,
carvacrol
[63,64]
4
Moringa
Moringa
oleifera
30
g
in
300
mL
DW,
heating
while
stirring
at
50
°C
for
1
h
45′
and
then
filtered
Vitamins,
flavonoidsvitamins,
phenolic
acids
[65]
5
Aloe
vera
Aloe
barbadensis
miller
250
g
finely
cut
and
boiled
with
DW
in
medium
flame
and
ground
to
get
the
extract
Tannins,
saponins,flavonoids,
[66]
6
Eucalyptus
Eucalyptus
globulus
20
g
in
100
mL
DW,
heating
at
80
°C
for
1
h.
Phenols,
saponins,
tannins,
flavonoids
(M
t
OH
extract)
[67]
7
Neem
Azadirachta
indica
25
g
of
fresh
leaves
in
100
mL
of
DDW,
heating
while
stirring
at
60
°C
for
20′
and
then
filtered
Alkaloids,
flavonoidssaponins,
reducing
sugars
[68,69]
8
Rosemary
Rosmarinus
officinalis
Finely
chopping
fresh
leaves
and
heating
at
70–80
°C
for
20′
and
filtering
and
final
volume
adjusted
to
100
mL
with
UPW
Flavonoids,
phenolic
acids,
terpenoids,
lignans
[70,71]
9
Basil
Ocimum
basilicum
25
g
powder
in
250
mL
methanol
solvent
using
Soxhlet
apparatus
Alkaloids,
flavonoidsphenols,
terpenoids
[72]
10
Imperata
Imperata
cylindrical
L.
5
g
powder
in
100
mL
of
DDW,
heating
while
stirring
at
70
°C
for
1
h
and
then
filtered
Saponins,
flavonoidsphenols,
terpenoids,
steroids
[73,74]
11
Crown
flower
Calotropis
gigantea
5
g
of
powder
in
100
mL
MQW
and
heating
at
60
°C
for
15′
and
then
filtered
Flavonoids,
alkaloidsterpenoids,
steroids
[3,75]
12
Roselle
Hibiscus
sabdariffa
Different
powdered
flower
to
volume
ratio
of
DW
was
prepared
(1,
4,
and
8%)
and
stirred
for
2
h
and
mashes
obtained
put
in
water
bath
at
60
°C
for
1
h
and
then
filtered
Alkaloids,
polyphenols,
flavonoids,
tannins,
saponins
(acidified
M
t
OH
extract)
[56,76]
13
Dead
sea
apple
Calotropis
procera
2
g
of
dried
and
crushed
leaves
boiled
in
100
mL
DW
for
10′
and
then
filtering
Alkaloids,
flavonoids,
saponin
glycosides,
steroids,
terpenoids
[14,77]
14
Blue
passionflower
Passifloraceae
caerulea
L.
5
g
of
fresh
leaves
in
100
mL
of
MQW
boiled
at
70
°C
for
8′
and
then
filtered
Polyphenols,
flavonoids,
alkaloids,
terpenoids
[78]
15
Bay
tree
Lauruas
nobilis
L.
20
g
of
powdered
leaves
in
200
mL
DW
heated
at
80
°C
for
10′
and
then
filtered
and
centrifuged
Polyphenols,
tannins,
flavonoids,
fatty
acids
[79,80]
16
Badam
Micrococca
mercurialis
(L.)
Benth.
5
g
of
dried
plant
parts
in
50
mL
of
DW
boiled
at
50
°C
in
water
bath
for
30′
and
then
filtered
Tannins,
saponins,
flavonoids,
glycosides,
terpenoids,
phenols
[81,82]
17
Padang
Pandanus
odorifer
5
g
of
freshly
collected
leaves
were
dried
and
powdered
and
extract
obtained
via
speed
extractor
at
50
oC
for
40′
Flavonoids,
phenols
[83]
18
Pennyroyal
Mentha
pulegium
L.
5
g
of
powdered
leaves
in
100
mL
of
DW
heated
at
60
oC
for
10′
and
then
filtering
Terpenoids,
flavonoids,
phenolics
[84]
19
Chinaberry
Melia
azedarach
L.
20
g
of
leaves
in
125
mL
of
DDW
subjected
to
Soxhlet
extraction
for
72
h
and
then
filtered
Terpenoids,
flavonoids,
steroids,
alkaloids,
saponins,
tannins
[85,86]
20
Wild
tomato
Solanum
torvum
Ground
and
powered
leaves
were
boiled
at
100
oC
for
45′
using
AGW
solvent
and
then
filtered
Alkaloids,
flavonoids,
glycosides,
phenols,
saponins,
sterols
[87,88]
21
Buxton’s
blue
Geranium
wallichianum
10
g
of
powdered
leaves
in
250
DW
was
heated
with
stirring
at
80
°C
for
2
h
and
then
filtered
Triterpenes,
steroids,
coumarins,
benzoic
acid
derivative
(EtOH
extract)
[89,90]
22
Garlic
Allium
Sativum
Fresh
and
finely
sliced
bulbs
were
boiled
at
70–80
°C
for
20′
and
then
filtered
Flavonoids,
anthocyanins,
vitamins
(B1,
B2,
B6,
etc.)
[70]
23
Tomato
Lycopersicon
esculentum
Fresh
tomatoes
were
washed,
squeezed
to
get
juice,
dissolved
in
DDW
by
stirring
at
30
°C
for
30′
and
then
filtering
Flavonoids,
phenolics,
carotenoids,
alkaloids
[91,92]
24
Papaya
Carica
papaya
10
g
of
papaya
leaf
in
100
mL
DW
was
refluxed
at
100
°C
for
30′
and
then
filtered
Saponins,
terpenoids,
coumarins,
steroids,
proteins
[93,94]
25
Black
myrobalan
Terminalia
chebula
10
g
of
fruits
incised
into
smaller
pieces
in
100
mL
of
DW
and
boiled
at
80
°C
while
stirring
Saponins,
steroids,
flavonoids,
tannins,
polyphenols
[95,96]
26
Italian
broccoli
Brassica
oleracea
L.
var.
italica
8
g
of
dried
florets
in
80
mL
of
DW
heated
at
70
°C
for
20′
and
then
filtered
phenolic
acids,
polyphenols,
flavonoids,
alkaloids,
steroids,
phenols,
tannins,
saponins,
terpenoids,
vitamin
C
[97,98]
27
Okra
Abelmoschus
esculentus
10
g
of
powdered
aerial
part
of
the
plant
were
extracted
via
60:40
ethanol
to
water
solvent
or
95%
ethanol
alone
Tannins,
steroids,
flavonoids,
phenols,
terpenoids,
saponins
[99,100]
28
Sweet
orange
Citrus
sinensis
Orange
peels
dried,
ground
into
fine
powder,
suspended
in
DW
in
agitation
for
3
h,
put
in
water
bath
at
90
°C
for
1
h
and
then
filtered
Flavonoids,
saponins,
tannins,
reducing
sugars,
amino
acids,
phenolics
[101,102]
29
Lime
Citrus
aurantifolia
50
g
of
dried
and
finely
cut
fruits
in
100
mL
DW
was
heated
at
60
°C
till
color
changes
to
pale
yellow
and
then
filtered
Saponins,
phenolics,
anthraquinones
[103,104]
30
Rambutan
Nephelium
lappaceum
L.
Fresh
peels
washed,
dried
at
50
°C
in
oven,
then
3
g
in
40
mL
DDW:
20
mL
E
t
OH
solvent
was
heated
at
80
°C
for
10′
and
then
filtered
Polyphenols,
flavonoids,
alkaloids,
tannins,
saponins
(E
t
OH
extract)
[105,106]
31
Mangosteen
Garcinia
mangostana
8
g
of
fruit
pericarps
in
100
mL
DW,
heated
at
70–80
°C
for
20′
and
then
filtered
Phenolic
acids,
flavonoids,
alkaloids,
triterpenoids
[107,108]
32
Pomegranate
Punica
granatum
F.
Fresh
peels
were
dried
in
oven
at
45
°C
and
powdered
with
blender
and
extract
prepared
by
heating
1:10
ratio
of
powder
to
DW
extract
collected
via
centrifugation
Alkaloids,
flavonoids,
tannins,
triterpenes,
organic
acids
[59,109]
33
Red
clover
Trifolium
pratense
L.
4.5
g
of
powdered
flowers
in
200
mL
of
DDW,
heated
while
stirring
at
80
°C
for
45′
Phenolic
acids,
flavonoids,
coumarins,
lignans
[110,111]
34
Rose
Rosa
indica
L.
20
g
of
powdered
leaves
in
100
mL
DW
boiled
at
70
°C
for
1
h
and
then
filtered
and
centrifuged
Phenolics,
flavonoids,
amines
[112]
35
Lebbeck
Albizia
lebbeck
stem
bark
20
g
of
bark
powder
in
100
mL
DW
heated
at
45
°C
for
24
h
and
filtered
Tannins,
flavonoids,
saponins,
carbohydrates
[113,114]
36
Pelargonium
Pelargonium
zonale
1
g
of
dried
powdered
leaves
in
100
mL
was
boiled
for
5′
and
then
filtered
Tannins,
phenolics,
proteins
[115]
37
Balloonvine
Cardiospermum
halicacab
10
g
of
finely
cut
fresh
leaves
in
100
mL
DW
was
heated
at
85
°C
with
microwave
irradiation
for
25′
while
stirring
and
then
filtered
Saponins,
alkaloids,
phytosterols
[116]
(continued
on
next
page)
G.K. Weldegebrieal Inorganic Chemistry Communications 120 (2020) 108140
5

6. Table
2
(continued)
S/N
Name
of
plant
Extraction
method
Phytochemical
constituents
tested
to
be
present
Ref.
Common
Name
Scientific
name
38
Bird’s
eye
Veronica
multifida
5
g
of
powdered
leaves
in
250
mL
of
DW
was
subjected
to
Soxhlet
extraction
for
3
days
and
then
filtered
—
[117]
39
Assyrian-plum
Cordia
myxa
L.
20
g
of
powdered
leaves
in
200
mL
of
DW
was
macerated
at
30–33
°C
with
occasional
shaking
and
process
was
repeated
till
extraction
was
exhausted
Saponins,
alkaloids,
terpenes,
flavonoids,
sterols
[118]
40
Desert
dill
Deverra
tortuosa
50
g
of
powdered
aerial
parts
of
the
plant
in
1
L
DW
was
kept
for
24
h
at
20
°C
while
shaking
at
100
rpm
and
then
filtered
Alkaloids,
coumarins,
glycosides,
steroids,
tannins,
terpenoids
[119]
41
Japanese
weed
Sargassum
muticum
2
g
of
powdered
Sargassum
muticum
algae
in
100
mL
DW
was
heated
at
100
°C
and
then
filtered.
Phenolics,
carotenoids,
sterols,
proteins
[120]
42
Castor
oil
plant
Ricinus
communis
L.
20
g
of
washed
leaves
were
taken
in
flask
and
heated
at
60
°C
and
then
filtered
Flavonoids,
tannins,
terpenoids,
saponins,
alkaloids,
glycosides
[121,122]
43
Indian
head
ginger
Costus
woodsonii
Different
masses
of
powdered
leaves
(3,
6,
and
9
g)
in
100
mL
of
DW
was
separately
boiled
at
60
°C
for
1
h
while
stirring
and
then
filtered
Terpenoids,
phenolic
acid,
flavonoids,
proteins
[123]
44
Paradise
tree
Simarouba
glauca
50
g
of
leaves
was
extracted
with
100
mL
DW
and
then
filtered
Alkaloids,
saponins,
glycosides,
flavonoids
[124,125]
45
Cammomile
flower
Matricaria
chamomilla
L.
2
g
of
dried
flowers
in
200
mL
DW
was
heated
in
water
bath
at
60–70
°C
for
4
h
and
then
filtered
Terpenes,
flavonoids,
tannins,
glycosides,
alkaloids
[2]
46
Bush
cancer
tea
Sutherlandia
frutescens
10
g
of
plant
was
added
in
some
amount
of
boiled
water
and
further
heated
at
80
°C
for
15′
and
then
filtered
Terpenoids,
glycosides,
flavonoids,
alkaloids,
phenols,
tannins,
steroids
[33]
47
Euphorbia
Euphorbia
petiolate
banks
50
g
of
powdered
leaves
in
500
mL
DDW
was
heated
at
80
°C
for
30′
and
then
filtered
Phenolics,
glycosides
[126]
48
Gaudichaudi
Tecoma
castanifolia
5
g
of
powdered
leaves
in
50
mL
DDW
was
kept
in
water
bath
at
60
°C
for
15′,
stirred
for
30′
and
then
filtered
Flavonoids,
terpenes,
alkaloids,
hydrocarbons
[127]
49
Henna
Lawsonia
inermis
1
g
of
powdered
leaves
in
10
mL
of
DW
was
heated
at
100
°C
for
10′
in
water
bath
and
then
filtered
Phenols,
terpenoids,
quinones
carbohydrates,
tannins
[128,129]
50
Milk
thistle
Silybum
marianum
15
g
of
powdered
leaves
in
150
mL
of
DW
was
heated
at
80
°C
for
1
h
and
then
filtered
Flavonoids,
polyphenols
[130]
51
Konjac
tuber
Amorphophallus
konjac
tuber
10
g
of
chopped
tubers
in
200
mL
of
DDW
was
heated
at
70
°C
for
30′
till
milky
solution
is
formed
and
extract
collected
via
centrifugation
Phenolics,
flavonoids,
proteins
[131]
52
Pongam
tree
Pongamiapinnata
10
g
of
powdered
leaves
in
50
mL
of
DW
were
refluxed
at
100
°C
for
2
h
and
then
filtered
Carbohydrates,
flavonoids
[132,133]
53
Mango
Mangifera
Indica
100
g
of
cleaned
leaves
in
100
mL
DW
was
heated
while
stirring
at
80
°C
for
30′
and
then
filtered
Polyphenols,
flavonoids,
tannins,
proanthocyanidins
[134,135]
54
Ringworm
senna
Cassia
alata
L.
10
g
of
fresh
leaves
in
100
mL
MQW
was
heated
at
60
°C
for
30′
and
then
filtered
Saponins,
terpenoids,
tannins,
flavonoids,
steroids
[136,137]
55
Bael
tree
Aegle
marmelos
5
g
of
powdered
leaves
in
100
mL
DW
was
heated
at
60
°C
in
water
bath
and
then
filtered
Alkaloids,
glycosides,
terpenoids,
saponins,
steroids,
flavonoids
[138,139]
56
Watercress
Nasturtium
officinale
100
g
of
powdered
leaves
in
200
mL
DDW
was
ultrasonicated
for
25′
and
then
the
resultant
mixture
was
put
on
hot
plate
stirrer
and
heated
while
stirring
at
60
°C
for
1
h
and
then
centrifuged
and
filtered
Phenols,
flavonoids,
alkaloids,
carbohydrates
[140,141]
57
Cowplant
Gymnema
sylvestre
15
g
of
leaves
in
100
mL
of
DW
was
heated
at
80
°C
for
1
h
and
then
filtered
Flavonoids,
tannins,
alkaloids,
phenols
[142,143]
58
Stinging
nettle
Urtica
dioica
L.
2
g
of
the
powdered
leaves
was
soaked
in
70
mL
DDW
for
72
h
and
then
filtered
Flavonoids,
tannins,
sterols,
volatile
compounds
[144,145]
59
Country
borage
Plectranthus
amboinicus
5
g
of
leaves
in
30
mL
of
DW
was
boiled
at
100
°C
for
15′
and
then
filtered
Alkaloids,
flavonoids,
tannins,
proteins,
terpenoids,
glycosides
[146,147]
60
Bamboo
Bambusa
vulgaris
Dried
leaves
were
soaked
in
100
mL
DW
and
stirred
for
24
h
till
color
change
colorless
to
brownish
green
is
achieved
and
then
filtered
Alkaloids,
glycosides,
phenols,
tannins,
saponins,
flavonoids
anthraquinones,
[148,149]
61
Tail
grape
Artabotrys
Hexapetalu
Dried
leaves
were
soaked
in
100
mL
DW
and
stirred
for
24
h
till
color
change
colorless
to
brownish
green
is
achieved
and
then
filtered
Alkaloids,
flavonoids,
tannins,
glycosides,
saponins,
phenolics,
steroids
62
Beach
morning
glory
Ipomoea
pes-caprae
10
g
of
dried
fine
cut
leaves
in
500
mL
DW
was
boiled
for
15′
till
color
changes
to
yellow
and
then
filtered
Flavonoids,
phenolics,
tannins,
proteins,
terpenoids
[150]
63
Black
cumin
Nigella
sativa
500
g
of
powdered
seeds
in
96%
E
t
OH
and
water
solvent
and
put
in
an
incubator
for
72
h
and
then
solvent
removed
in
vacuum
and
extract
dried
in
oven
Alkaloids,
saponins,
terpenoids
[151]
64
Astragalus
Astragalus
membranaceus
Different
grams
(0.5,
1.0,
and
1.5)
of
roots
of
the
plant
in
100
mL
DW
was
heated
at
60
°C
for
30′
and
then
filtered
Flavonoids,
amino
acids,
saponins,
polysaccharides
[152,153]
G.K. Weldegebrieal Inorganic Chemistry Communications 120 (2020) 108140
6

7. Similarly, branches of Isatis tinctorial gave highest quantity of crude
extract followed by leaves, and next flowers, and least quantity from its
roots using different solvents [39].
2.2.2. Selection of extraction solvent
Once selection of plant is over, the next step will be to select an
appropriate solvent. During extract preparation, the extraction solvent
employed is one of the most important parameters that plays crucial
role in determining the composition and concentration of secondary
metabolites that will be obtained [40]. Viscosity is one physical prop­
erty of solvent that could affect extraction. A solvent of low viscosity by
virtue of its ability to diffuse through the pores of the plant matrices
will achieve higher extraction yield than more viscous one [41]. An­
other solvent property that could affect extraction is its polarity. Based
on the principle of ‘like dissolves like’, a solvent with a polarity similar
to that of the solute is more likely to extract it better. Polar biomole­
cules such as phenolics, flavonoids, alkaloids, and terpenoids, which
play a role in the synthesis of nanomaterials can be extracted using the
most common polar solvents used in extraction, water, methanol,
ethanol, acetone, or their aqueous mixtures [42]. In terms of their po­
larity index, these are arranged from more polar to least one as
water > methanol > ethanol > acetone. More polar solvents tend to
dissolve polar solutes more. Though water is more polar than methanol,
higher concentration of phenolics, flavonoids, alkaloids, and terpenoids
were obtained from Severinia buxifolia plant using methanol than water
[43]. Moreover, methanolic extracts of leaves, stem, and rhizome parts
of Costus pictus exhibited higher yield of phytochemicals than the cor­
responding aqueous extract [44]. Though methanol could give higher
quantity of phytochemicals in an extraction, the disadvantage with it is
that it is toxic, flammable, and more costly solvent than water.
In general, an ideal solvent for extraction is one having the fol­
lowing properties. It has low toxicity, is capable of evaporating at lower
temperatures, and does not react with the extracted biomolecules on
standing, that is association or dissociation reactions should not exist
[45].
2.2.3. Preparation of plant extract
After an appropriate plant and solvent selection, extract is prepared
by adjusting extraction conditions. Extraction temperature is one main
parameter that can affect extraction efficiency. Generally, increase in
temperature results in improved extraction yield via enhancing the
diffusion rate and solubility of soluble phytochemicals in the extraction
solvent and facilitating easy penetration of solvent into the pores of the
plant matrices by reducing solvent viscosity and surface tension acting
on the solvent, solutes, and plant matrices [41,46]. Though the op­
timum extraction temperature depends on the type of plant and solvent
used and the types of targeted phytocompounds from the plant, higher
temperature than a specific optimal value may degrade thermolabile
biomolecules and also reduce solvent by evaporation and hence mini­
mize extraction efficiency [47].
For example, Akowuah and Zhari [48] reported high yield of major
polyphenolic compounds rosmarinic acid and sinensetin using 80%
methanol from Orthosiphon stamineus leaf extract at 40 °C followed at
60 °C for 4 h. However, further increase in temperature above 60 °C
resulted in significant reduction of the compounds due to their de­
gradation. A similar reduction in anthocyanins, type of phenolic com­
pounds, extracted from milled berries using ethanol or sulfured water at
temperature higher than 45 °C was also observed [49]. Sluiman et al
[50], using response surface methodology, obtained high extraction
yield of phenolic compounds from Clinacanthus nutans Lindau leaves at
Plant parts
* Flowers
* Roots
* Barks
* Seeds
* Leaves
Powdered
plant
Heating with stirring
Maceration
Soxhlet
Others
Extract Zn precursor
+
Application
Characterization
* XRD
* UV-vis
* FTIR
* SEM
* TEM
* Others
* Photocatalytis
* Antibacterial
* Chemical sensors
* Others
Dried powder
Calcined powder Heating while stirring
Drying
in oven
Calcining
in furnace
Fig. 3. Schematics of general biosynthesis of ZnO NPs using extracts of plants.
G.K. Weldegebrieal Inorganic Chemistry Communications 120 (2020) 108140
7

8. temperature of 80 °C and extraction time of 80 min or temperature of
60 °C and extraction time of 120 min employing extraction solvent of
80% aqueous ethanol. Nonetheless, further increase of extraction time
at 80 °C decreased yield of extraction due to oxidation and degradation
of compounds. So, in addition to temperature other factors such as
extraction time, the ratio of solvent to solid mixing, solvent types and
their ratio of mixing, particle size, stirring rate, and of course the
method of extraction employed can affect extraction yield and need to
be taken into consideration during extraction process [51].
At the end of the extraction process, the extract is separated from
the solid matter called marc, suspended impurities filtered off with
filter paper or separated via centrifugation and finally stored in fridge
(usually at a temperature of 4 °C). As a final note, it is also advisable not
to store the extract at RT for a long time or keep it in sunlight to avoid
possible loss of its initial composition due to formation of artifacts [52].
2.2.4. Biosynthesis of ZnO NPs using plant extract
After extract is prepared, the next step will be to prepare ZnO NPs
by combining with zinc precursor and adjusting reaction parameters.
Fig. 3 outlines the overall steps involved during the synthesis of ZnO
NPs starting from extract preparation step from powdered plant part,
and then mixing of extract with Zn precursor in a specific molar ratio
Phenolics
O
O
O
O
HO
HO OH
OH
Ellagic acid (thyme, rambutan)
(a) (b)
Punicalin (pomegranate)
O OH
OCH3
HO
OH O
Mangostin (mangosteen)
O
HO
OH
OH
OH
OH
H
Epicatechin (green tea)
(d)
(c)
O
OH
OH
OH
Emodin (Aloe vera)
O
H3C
O
OH
OH
HO
OH
O
O OH
Rosmarinic acid
(rosemary, basil)
Flavonoids
)
f
(
)
e
(
O
OH
HO
OH O
Apigenin (moringa oleifera,
Aloe vera, rambutan)
(a)
O
HO
OH
OH
OH
OH
OH
Gallocatechol (green tea)
(b)
OH
OH
O
Mangiferin (mango)
O
OH
HO
O
HO
OH OH
OH
(c)
Vitamins
Folic acid (cowplant)
(a)
Ascorbic acid (broccoli)
(b)
Querectin (desert dill)
Ferulic acid (desert dill)
Gallic acid (desert dill)
)
f
(
)
e
(
)
d
(
O
O
OH OH
OH
OH
HO
OH OH
HO
HO HO
O
O O
HO
OH
OH
O
O
O
O
OH
OH
HO
O OH O
OH
HO
H3CO
HN
N N
N
H2N
O
N
H
N
H
O
CO2H
CO2H
O
HO
OH
OH
OH
OH
O
O
O
OH
HO
HO
HO
H
Fig. 4. Structural formula of biomolecules isolated from various extracts of plants indicated in parentheses.
G.K. Weldegebrieal Inorganic Chemistry Communications 120 (2020) 108140
8

9. with gentle heating while stirring simultaneously, and next drying the
collected precipitate in an oven or further calcining it, and finally
characterizing and possibly using for specific application. Synthesis
conditions such as the molar ratio of extract to precursor mixing, pH,
temperature (both reaction temperature and calcination temperature)
can affect characteristics of the synthesized ZnO NPs including particle
size and distribution, shape, band gap, quantity of defect structures,
crystallinity, and concentration of surface Zn and oxygen vacancies,
which are related with its antibacterial and photocatalytic activity [53-
55].
One essential synthesis parameter is the extract to precursor ratio.
Soto-Robles et al [56] studied the effect of increasing Hibiscus sabdariffa
flower extract to a fixed amount of Zn precursor on particle size of ZnO
NPs. They added 2 g of Zn (NO3)2·6H2O separately into 42 mL of dif­
ferent concentration 1%, 4%, and 8% of extract, dissolved by agitation,
and then placed in water bath at 60 °C till a pasty consistency is
achieved, and finally calcined at 400 °C. The synthesized ZnO NPs ex­
hibited more homogenous particle morphology and size and that par­
ticle size decreased from 38.63 to 9.05 and 8.71 nm with increase of
extract concentration. In another study, Nava et al [57] reported a si­
milar trend of more uniform particle size distribution, and reduced
particle size from 17.47 nm to 9.04 nm as Camellia sinensis extract
Alkaloids
Allicin (garlic)
H2C
S
O
S
CH2
(a)
Terpenoids
Carvacrol (thyme)
(a)
O
O
O
O
CH3
CH3
H3C
CH3
O
O
H3C
O
O
O
CH3
CH3
(b)
O
CH3
H3C
CH3
(c)
Pulegone
(pennyroyal)
H3C CH3
CH3
O
Eucalyptol
(eucalyptus leaves)
(d)
HN
O
O
NH
O
CH3
H3C
O
Carpaine (papaya)
Tannins
Cinnamontannin B1
(Laurus Nobilis)
Steroids
(b) (c)
α-Tomatine (tomato)
CH3
H3C CH3
OH
Carotenoids
Zeaxantin (broccoli)
Glycosides
Saponins
Rutin (desert dill)
α-Hederin (black cumin)
Isoquinone
(black cumin)
(f)
Lupeol
(castor oil plant)
Theaflavin (tea)
(a)
(b)
(e)
H3C CH3
OH
H3C
CH3
H3C
HO CH3
CH3
CH3
CH3
CH3
O
O
O
OH
HO
OH
OH
O
O
O
HO
OH
OH
OH
HO
HO
H3C
O
O
O O
HO
OH
OH
OH
OH
OH
OH
OH
HO
HO
HO
OH
OH
OH
CH3
CH3
H3C
O
O
H3C CH3
CH3 CH3 CH3
H2C
CH3
CH3
HO
O
O
HO
OH
OH
OH
HO
OH
OH
OH
O
OH
O
O
OH
OH
HO
H3C
HO
CH3
CH3
H3C
O
OH
CH3
CH3
CH3
OH
O
HO O
NH
O
O
OH
HO
HO
O
OH
O O
OH
OH
HO
O
HO
HO
HO
OH
OH
H3C
H3C
(a)
O O
H3CO
Herniarin (buxton's blue)
(a)
(a)
(a)
Nimbin (neem)
Fig. 4. (continued)
G.K. Weldegebrieal Inorganic Chemistry Communications 120 (2020) 108140
9

10. concentration increased from 10 mL to 40 mL combined with a fixed
amount of 2 g of Zn (NO3)2·6H2O while keeping other parameters as
such. They ascribed the more uniform size distribution and reduced
crystallite size to the higher concentration of biomolecules playing as
capping agent, preventing agglomeration of particles. Moreover, the
influence of extract to precursor concentration, pH, and reaction tem­
perature on particle size and shape of ZnO NPs synthesized using Cherry
extract was studied by Mohammadi et al [54]. ZnO NPs was synthesized
by mixing 10 mL of extract with 30 mL of 0.005, 0.02, 0.05, and 0.3 M
Zn(NO3)2·6H2O solutions placed in separate beakers, stirred at 150 rpm
for 30 min at 25 °C and then aged in dark for 12 h, collected via cen­
trifugation, dried and finally calcined. They found the particle size to
increase from 20.7 to 96.5 nm with increase precursor concentration
from 0.005 to 0.3 M without varying extract concentration. The authors
have attributed this phenomenon to the higher rate of metal ion re­
duction with higher metal precursor and smaller concentration of bio­
molecules to stabilize particles. They have also studied the effect of pH
and reaction temperature on particle size by fixing other parameters
constant and reported optimum values to be 8 and 25 °C, respectively.
On increasing the temperature from RT to 90 °C without changing other
parameters, particle size increased from 87.5 nm to 116 nm and ex­
plained this observed increase to be due likely to the fast kinetics of
reaction at higher temperature and hence uncontrolled particle size or
to be due to the uncontrolled growth and fast particle aggregation as a
result of poorer reducing and capping ability of phytochemicals (as­
corbic acid) found in the extract owing to its instability at higher
temperature. Calcination temperature also has significant impact on
particle size and morphology, band gap energy, and crystallinity of ZnO
NPs. For example, Aziz et al [58] synthesized ZnO NPs by combining
200 mL of 0.02 M zinc acetate dihydrate with 2 g of Anchusa italica
flower extract – prepared by placing 50 g of flower in 200 mL distilled
water, magnetically stirring at 100 °C for 20 min and concentrating and
drying using roto evaporator, and next heating at 70 °C for 6 h, in water
bath, precipitate obtained collected via centrifugation and calcined at
100 and 200 °C for 2 h. They found that, on increasing the calcination
temperature from 100 to 200 °C, band gap decreased from 3.30 to
3.27 eV, crystallinity was improved and crystallite size increased from
around 10.8 to 16.2 nm. In another study conducted by Sukri et al [59],
ZnO NPs was synthesized by adding Zn(NO3)2·6H2O to Punica granatum
fruit peel extract in 1:10 ratio by stirring vigorously at 90 °C till aqueous
solvent is removed and formed a gel-like product, and finally calcined
at 400, 500, 600, and 700 °C. Results indicated with increase in tem­
perature, particle size increased from 22.39 to 57.36 nm with reduced
consistency of size distribution, crystallinity was improved, which ac­
cording to the authors was due to the migration of ZnO NPs to their
correct lattice positions leading to recrystallization assisted by the
higher kinetic energy supplied to the particles at higher temperature,
reducing defects. Variation in particle morphology was also observed,
while samples calcined at 600 °C were spherical, hexagonal structures
were also observed for the sample calcined at 700 °C, indicating clearly
that calcination temperature can have an impact on particle mor­
phology as well. Elavarasan et al [60] prepared ZnO NPs by adding
dropwise 20 mL of aqueous leaf extract of Sechium edule to 80 mL of
distilled water containing 2 g of zinc acetate dihydrate dissolved, next
stirred vigorously at a temperature of 60 °C for 2 h, obtained precipitate
collected via centrifugation, and dried in hot air oven. After that
powder was incubated at 100, 200, and 400 °C for 24 h. Results showed
while calcination at 100 and 200 °C did not produce ZnO NPs, sample
calcined at 400 °C produced ZnO NPs.
2.2.5. Synthesis mechanism of ZnO NPs
Extracts of plants contain various biomolecules which may vary in
composition and concentration depending on the type of the plant. As
such, the exact mechanism leading to the formation of ZnO NPs using
plant extracts is not well understood due to the possible simultaneous
involvement of many bioactive molecules in the synthesis [55,65].
However, there are two general proposed mechanisms. The first one
involves first the reduction of Zn2+
into elemental Zn by the active
phytochemical compounds (PC) in the extract which is then oxidized in
air at high temperature to give ZnO NPs, Eq. (1–2) [154].
+ +
+
Zn PC Zn PC(oxidized)
2
(1)
+
Zn O (air) ZnO(NPs)
2 (2)
The second one which is even more plausible mechanism suggested
by many researchers involves the following steps. First, rather than
forming elemental Zn by reduction, Zn2+
forms complex structure with
the active PC via their polar functional groups such as hydroxyl (—OH),
carboxyl (—COOH), carbonyl (—C = O), and amino (—NH2). Next, the
complex gets thermally decomposed by calcination, releasing ZnO NPs.
Finally, particle growth follows which ultimately gives the observed
capped and stabilized ZnO NPs, Eq. (3–5) [155,156].
+
+
Zn PC Zn PC(complex)
2
(3)
Zn PC(complex) ZnO (4)
ZnO ZnO(NPs)
Growth
(5)
For example, Gawade et al [14] reported the biogenic synthesis
mechanism of ZnO NPs using extract of Calotropis procera leaves. They
proposed that ZnO is formed first by the complex formation of Zn2+
with the active hydroxyl sites of polyphenolic compounds in the extract
followed by the thermal decomposition of the complex.
Similarly, Karnan and Selvakumar [105] forwarded the possible
formation mechanism of ZnO NPs using ellagic acid as one of the main
components of rambutan peel extract. First complexation is formed
between Zn2+
with the hydroxyl groups of the compound which finally
gives ZnO NPs upon calcination at 450 °C, Scheme 1. Moreover, Abbasi
et al [157] hypothesized a two-step formation mechanism of ZnO NPs
synthesized using flax (Linum usitatissimum L.) in vitro callus and ad­
ventitious root cultures extracts with the help of the main lignan
O
O
O
O
OH
OH
HO
HO
+ Zn(NO3)2
O
O
O
O
O
O
O
O O
O
O
O
O
O
O
O
Zn2+
OH2
OH2
Zn2+
Zn2+
OH2
OH2
OH2
OH2
Calcination
O
O
O
O
OH
OH
HO
HO
+ NO2 + O2
+
ZnO
Ellagic acid
O
O
O
O 2NO3
-
2NO3
-
2NO3
-
Scheme 1. Possible synthesis mechanism of ZnO NPs using ellagic acid as a
ligating agent. Reprinted with permission from [105]. Copyright 2016 Elsevier.
G.K. Weldegebrieal Inorganic Chemistry Communications 120 (2020) 108140
10

11. molecule, secoisolariciresinol diglucoside without calcination step in
the synthesis.
In their proposed mechanism, the first step involves complex for­
mation of Zn2+
with the molecule via its hydroxyl groups followed by
decomposition of complex to ZnO NPs when heated and lastly ag­
gregation of particles to form ZnO NPs which are stabilized by active
biomolecules in the extract. Suresh et al [158] predicted a slightly
different likely formation mechanism of ZnO NPs using insulin plant
(Costus pictus D. Don) extract with the help of the active biomolecule
diosgenin. In their mechanism, diosgenin forms a complex with Zn2+
which gives Zn(OH)2 when heated, and finally the hydroxide is con­
verted into ZnO NPs by calcination, Scheme 2. The idea of Zn(OH)2
formation from extract biomolecules and Zn2+
and then its thermal
decomposition upon calcination to give ZnO NPs was also initiated by
few other researchers in their proposed mechanism [159,160]. Król et al
[161] gave another mechanism of synthesis of ZnO NPs when proteins
are present in excess in the extract. According to the authors, the nu­
cleophilic attack of the electron from the deprotonated carboxyl groups
in the proteins abstracts hydrogen from the coordinated H2O molecule
in the zinc aqua complex [Zn(H2O)6]2+
to produce the zinc aqua-hy­
droxo complex [Zn(OH)(H2O)5]2+
in the first step, which is then finally
converted into ZnO NPs.
3. Photocatalytic activity of ZnO NPs for azo dyes
3.1. Azo dyes and their toxicity mechanism
Dyes are colored unsaturated organic substances that have the af­
finity to which they are applied. They are made of two main structural
elements – chromophores and auxochromes. While chromophores are
electron withdrawing groups in the structure of the molecule such as
azo (–N = N–), carbonyl (–C = O), nitro (–NO2), etc whose presence is
responsible for the color of the dye upon absorption of visible light,
auxochromes are electron donating groups such as amine (–NH2), car­
boxyl (–COOH), hydroxyl (–OH), etc and make the molecule soluble in
water as well as develop a strong attachment with fibers [162,163].
Though complete classification of dyes with respect to one parameter is
difficult due to their many structural varieties, generally dyes can be
divided into different groups and classes depending on dyes source,
structure, and fiber type with which they are most compatible [164].
According to their chemical structure, organic dyes are classified into
such groups as azo compounds, anthraquinones, triarylmethanes,
phthalocyanines, etc [165].
Azo dyes are a class of synthetic dyes that have the azo functional
group (–N = N–), flanked by benzene, naphthalene, or heterocyclic
groups, which can contain different substituents [166]. Depending on
the number of azo linkages available in a single molecule of the dye,
they can be classified as monoazo, bisazo, trisazo, and polyazo corre­
sponding for one, two, three, and more than three groups. Azo dyes can
be cationic, anionic or nonionic based on the charge of ionized dye in
aqueous medium. Moreover, based on method of application to textile
fibers for coloring purpose and usage, azo dyes are grouped into such
classes as reactive dyes, disperse dyes, acidic dyes, basic dyes, direct
dyes, vat dyes, and sulfur dyes, and solvent dyes [167]. Naming of azo
dyes using IUPAC (International Union of Pure and Applied Chemistry)
or CAS (Chemical Abstracts Service) nomenclature is inconvenient due
to complexity and long names. Using the color index (CI) name de­
veloped by the society of dyers and colorists is one convenient ap­
proach. In this method, each dye is assigned a CI name consisting of
application type, color or hue of the dye and sequential number [168],
Scheme 3.
Azo dyes account for up to 70% of all organic dyes produced
commercially and are commonly used as colorants in textile, food,
leather, paper, cosmetics, and pharmaceutical industries [169]. Though
generally the toxicity of most azo dyes is not as dire with LD50 values
between 250 and 2000 mg/kg body weight [170], the carcinogenic or
mutagenic property of some of them confirmed in animals is mainly due
to their metabolized products though there are few examples such as
methyl yellow, aniline yellow, and solvent yellow 3 which are very
toxic in their molecular form and therefore have been banned from
commercial use in many countries [171]. The toxicity of azo dyes is due
to the formation of aromatic amines or benzidine molecule under re­
ductive conditions. After toxic aromatic amine intermediates are re­
leased into water via reduction, they can damage the vital organs such
as liver, kidney, central nervous system, and reproductive system in
humans and prevent photosynthesis in plants [172]. Additionally,
Benzidine is a well-established bladder carcinogen in humans as a result
of reductive cleavage of the azo linkage of the dye via azo reductase
enzyme secreted by microorganisms [173]. In addition to urinary
bladder, benzidine is also reported to be carcinogenic to other organs of
the body such as pancreas, liver, lung, stomach, gallbladder, and large
intestine and that dye molecules that contain benzidine moiety in their
structure such as Congo red, direct blue 6, direct brown 95, and direct
black 38 etc. could metabolically be reduced to benzidine and are po­
tentially carcinogenic [171]. Reductive cleavage of the azo linkage to
give aromatic amines is facilitated by azo reductase enzymes in the
HO
H
O
H
O
H
H
H
Diosgenin
+ Zn(NO3)2.6H2O
O
H
O
H
O
H
H
H
O
H
O
H
O
H
H
H
Zn2+
OH2
OH2
OH2
OH2
HO
H
O
H
O
H
H
H
+ Zn(OH)2 + 2HNO3 + 4H2O
Calcination
450o
C
80o
C
2h magnetic stirring
40o
C
8h in oven
2H+ 2NO3
-
ZnO
Nanoparticles
Scheme 2. Synthesis mechanism of ZnO NPs using diosgenin as a chelating
agent [158]
Application type Color Identifying number
CI Acid yellow 36
Scheme 3. Generic name of CI Acid yellow 36 dye in the CI classification
method.
G.K. Weldegebrieal Inorganic Chemistry Communications 120 (2020) 108140
11

12. Table 3
List of some water contaminant azo dyes and their toxicity information as obtained from the indicated references (key to abbreviations: C = common, C.I. = color
index).
S/N C.I. or C Name Structural formula Molecular formula (Mw
(g/mol))
Dye ionicity Toxicity Ref.
1 Acid red 88 C20H13N2NaO4S (400.4) Anionic Carcinogen [177]
2 Acid violet 7 C20H16N4Na2O9S2
(566.5)
Anionic Chromosomal aberration,
lipid peroxidation
[178]
3 Acid yellow
36
C18H14N3NaO3S (375.4) Anionic Carcinogen [179]
4 Allura red C19H17ClN4S (369.0) Anionic Genotoxic in vivo in mice and
rats, hypersensitivity
[180]
5 Amaranth C20H11N2Na3O10S3
(604.5)
Anionic Allergy, tumor, birth defects,
respiratory problems
[181,182]
6 Aniline
Yellow
C12H11N3 (197.2) Nonionic Carcinogen [171]
7 Bismarck
brown R
C21H24N8·2HCl (461.4) Nonionic Carcinogen [183]
8 Bismarck
Brown Y
C18H20Cl2N8 (419.3) Cationic Carcinogen, induces
endocrine disruption in
aquatic microorganims
[184]
9 Direct blue 6 C32H20N6Na4O14S4
(932.8)
Anionic Carcinogen [185]
(continued on next page)
G.K. Weldegebrieal Inorganic Chemistry Communications 120 (2020) 108140
12

13. Table 3 (continued)
S/N C.I. or C Name Structural formula Molecular formula (Mw
(g/mol))
Dye ionicity Toxicity Ref.
10 Direct blue 15 C34H24N6Na4O16S4
(992.8)
Anionic Mutagenic in reduction, and
carcinogenic
[8]
11 Direct brown
95
C31H20N6SO9Na2
(762.1)
Anionic Carcinogen [185]
12 Direct red 28
(Congo red)
C32H22N6Na2O6S2
(696.7)
Anionic Carcinogenic and mutagenic [186]
13 Direct orange
26
C33H22N6Na2O9S2
(756.7)
Anionic Retards growth of some
plants
[187]
14 Disperse
orange 3
C12H10N4O2 (242.2) Nonionic Allergenic [188]
15 Disperse blue
291
C19H21BrN6O6 (509.3) Cationic Mutagenic, cytotoxic [189]
16 Disperse red 1 C16H18N4O3 (314.3) Nonionic Mutagenic, cyanosis, bladder
cancer
[190]
17 Disperse
yellow 3
C15H15N3O2 (269.3) Nonionic Allergenic, potential
carcinogenic
[190]
18 Methyl orange C14H14N3NaO3S (327.3) Anionic Mutagenic [191]
19 Methyl yellow C14H15N3 (225.2) Nonionic Carcinogenic [171]
(continued on next page)
G.K. Weldegebrieal Inorganic Chemistry Communications 120 (2020) 108140
13

14. mammalian liver or mainly by intestinal microbial azoreductase and
that genotoxicity, mutagenicity, and carcinogenicity of azo dyes is a
function of substituents to the azo linkage [174,175]. Table 3 lists some
azo dyes including their structural and molecular formula with poten­
tial or confirmed toxicity evidence reported in the literature. Scheme 4
gives the possible formation mechanism of N, N-dimethyl-1,4-diami­
nobenzene and benzidine toxic cleaved molecules from methyl orange
and Congo red dyes with the help of bacteria in human colon or liver.
The azo linkage –N = N– in the molecules is the most susceptible part of
the azo dyes for breakage via azo reductase enzyme secreted by mi­
croorganisms found in all tested mammals including humans [170].
Another mechanism for the genotoxicity of azo dyes is due to the for­
mation of aromatic amines by the cleavage of azo bond via intestinal
anaerobic bacteria or liver azo reductase which are metabolically oxi­
dized to reactive electrophilic species that bind with DNA or to geno­
toxic compounds by microsomal enzymes in to DNA reactive nitrenium
ion (C6H5NH+
) [176].
3.2. Photocatalytic degradation of azo dyes using ZnO NPs
Conventional methods for the removal of dyes from wastewater
including precipitation, coagulation, flotation, adsorption on activated
carbons, etc have inherent drawbacks such as use of consumable che­
micals, incomplete removal of azo dyes or their metabolites, generation
of huge sludge which requires secondary treatment or disposal and
hence increasing the overall cost of operation [166,194]. Advanced
oxidation processes (AOP), which are based on the use of highly re­
active radicals such as the hydroxyl radical, have the advantage that the
reactive radicals can react rapidly and non-selectively with a broad
range of dyes in the wastewater and does not involve sludge formation
[195].
Heterogeneous photocatalysis, a class of AOP, is a promising tech­
nology for the treatment of wastewater including azo dyes. It has such
merits as environmental friendliness, low cost for low amount of cata­
lyst and sunlight can be used in the reaction, can proceed at RT, and
completely degrade the dyes in to CO2, H2O, and simple mineral acids
[13]. Heterogeneous photocatalysis may be defined as photo induced
acceleration of the rate of a chemical reaction in the presence of a
semiconductor photocatalyst. The definition also includes photo­
sensitization, which involves the initiation of a reaction via the ab­
sorption of light by coexisting substance named photosensitizer and
transferring the energy to the photocatalyst. A convenient photocatalyst
possesses such features as high photocatalytic efficiency, large specific
surface area, full utilization of sunlight, and high recyclability [196].
Moreover, for efficient and successful application of photodegradation
reaction, the semiconductor photocatalyst, the pollutant, and radiation
source should be in contact with each other [197].
ZnO is one of the promising photocatalysts used for the degradation
of azo dyes in waste water. It is low cost, photo-stable, biologically and
chemically inert, and highly photoactive in the UV region which made
it a suitable photocatalyst in the degradation of azo dyes [198]. How­
ever, one of the factors that limits its efficiency as photocatalyst is its
wide band gap value, making it insensitive to visible light. The type of
plant extract used and, in general, the synthesis method employed
could also affect physical features such as particle size and morphology,
concentration of oxygen vacancies, and surface defects (edges and
corners) which in turn affect the photocatalytic efficiency of ZnO NPs
[199,200].
Table 3 (continued)
S/N C.I. or C Name Structural formula Molecular formula (Mw
(g/mol))
Dye ionicity Toxicity Ref.
20 Reactive black
5
C16H2N5Na4O19S6
(991.8)
Anionic Carcinogenic, decreases
urease activity
[178]
21 Reactive
brilliant red
C19H10N6Na2O7S2
(615.3)
Anionic Inhibits functioning of
human serum albumen
[170]
22 Sudan I C16H12N2O (248.3) Nonionic Potential carcinogen [192]
23 Sudan II C18H16N2O (276.3) Nonionic Mutagen [193]
24 Sudan III C22H16N4O (352.4) Nonionic Potential carcinogen [173]
25 Sudan IV C24H20N4O (380.4) Nonionic Potential carcinogen
G.K. Weldegebrieal Inorganic Chemistry Communications 120 (2020) 108140
14

15. 3.3. Degradation mechanism of azo dyes using ZnO NPs
When photons of a light source with energy (hν) greater than or
equal to the band gap energy (Eg) of ZnO catalyst shine on it, electrons
get excited from the valence band (VB) to the conduction band (CB)
region, leaving holes in the VB. This creates electron – hole ( +
e /h )
pairs, Eq. (6). Then, the photo excited electrons diffuse on to the surface
of the catalyst and react with adsorbed oxygen, reducing it to super­
oxide radical ion (O2
.
), Eq. (8) while the holes move to the surface and
react with neighboring adsorbed H2O or dye molecules, oxidizing them
to hydroxyl radical (OH.
), Eq. (10) and oxidized dye radical cation
( +
dye )
. , Eq. (11), respectively. Fig. 5 illustrates the overall degradation
process. It is mainly due to the formation of the reactive hydroxyl and
superoxide radicals in the solution that set in a chain of catalytic
reactions leading to the eventual degradation of the dye in to CO2, H2O,
and few mineral acids [201]. The redox reactions of the +
e /h pairs on
the surface of ZnO catalyst play an essential role in their separation and
+ + +
ZnO h ZnO(e h ) (6)
+
Dye h Dye (7)
+
e O O
2 2
.
(8)
+ + +
Dye O O Dye
2 2
. .
(9)
+ +
+ +
H O h OH H
2
. (10)
+ + +
Dye h Dye .
(11)
+
Dye OH or O Degradedproducts
.
2
.
(12)
hence enhancing the rate of degradation. However, fast re­
combination lowers formation of radicals and decreases rate of de­
gradation. The presence of oxygen in the solution as a scavenger for the
excited VB electrons and surface defects on ZnO catalysts, acting as
trapping centers for holes and electrons prevent pair recombination and
can maximize rate of degradation [202]. Additionally, for photo excited
electrons to be able to reduce O2 into O2
.
, the CB potential of ZnO must
be more negative than the reduction potential of electron acceptor O2.
Similarly, VB potential of ZnO must be more positive than the oxidation
potential of electron donors, H2O or azo dyes for photo generated holes
to be able to oxidize them [203].
The photocatalytic reactions are initiated due to not only the pho­
toexcitation of electrons from the VB of catalyst ZnO particles but also
can be sensitized by the dye molecules themselves. Dye molecules ab­
sorb photons and get energetically excited from their highest occupied
molecular orbital (HOMO) to lowest unoccupied molecular orbital
(LUMO), Eq. (7) which are then oxidized to cationic radicals +
Dye
.
by
injecting their LUMO electrons to the CB of the semiconductor or to
oxygen dissolved in solution, producing O2
.
, Eq. (9).
The overall photocatalytic degradation pathway has been sum­
marized into the following 5 independent steps [204]: (1) diffusion of
azo dye molecules to the catalyst surface, (2) adsorption of the mole­
cules onto the catalyst surface, (3) generation of reactive radicals fol­
lowed by the degradation of the dyes, (4) desorption of intermediates or
final products from the surface, and (5) diffusion of intermediates or
final products in to the bulk of the liquid.
3.4. Factors affecting the degree of photodegradation of azo dyes using ZnO
NPs
Degree of photodegradation or simply degradation percentage (D
(%)) of a dye indicates the extent of its decomposition at any time t
after the start of the reaction. It can be followed by measuring the
absorbance of the dye at its max at specified time intervals and is given
by Eq. (13)
= ×
D(%)
A A
A
100
0 t
0 (13)
where A0 and At are absorbance before exposure to radiation and after
exposure time t, respectively. Photocatalytic degradation is a complex
process that the degree of degradation depends on several factors. Here
under, the effect of factors including presence of dopants and other
substances (composites) with ZnO NPs, particle size and morphology,
the presence of supports, initial pH of solution, initial dye concentration
and catalyst dosage, aeration and stirring rate of the reaction mixture,
temperature, energy and intensity of the radiation source, presence of
oxidants, and presence of inorganic ions in the reaction mixture will be
discussed individually.
3.4.1. Effect of dopants and composites of ZnO NPs
Although pure ZnO is an efficient photocatalyst for the
N
N N(Me)2
NaO3S
NH2
(Me)2N NH2
NaO3S
N, N-dimethyl-1,4-diamonobenzene
(mutagenic)
Reductase
Methyl orange
+
Azoreductase enzyme
H2N NH2
NH2
SO3Na
H2N
NH2
SO3Na
NH2
+ +
Benzidine
(carcinogenic)
NH2
SO3Na
NH2
SO3Na
N N N N
Congo red
a)
b)
Scheme 4. Enzyme catalyzed reductive cleavage along the azo linkage (broken
lines) of methyl orange (a) and Congo red (b) azo dyes into their toxic muta­
genic and carcinogenic products.
Fig. 5. General photocatalytic degradation mechanism of ZnO NPs for hy­
pothetical azo dye.
G.K. Weldegebrieal Inorganic Chemistry Communications 120 (2020) 108140
15

16. decomposition of many azo dyes due to its photosensitivity and cata­
lytic properties, its catalytic efficiency is limited by such factors as fast
photo generated +
e /h pairs recombination, poor accessibility to visible
light owing to its high band gap energy, poor charge carrier transport to
surface reaction sites, and photo induced dissolution in aqueous solu­
tion in the presence of UV light [205]. As such, researchers have de­
vised strategies to counter act these problems and enhance its catalytic
activity via doping and coupling with other semiconductors (forming
composites). While doping refers to the intentional introduction of
impurities into an intrinsic semiconductor to change its properties,
composites may be defined as multicomponent systems made of two or
more chemically and physically different phases separated by distinct
interface which are deliberately combined and have structural and
functional properties different from any of the individual components
mixed [206].
When ZnO is doped or co-doped with appropriate amount of atoms
of dopant elements, its catalytic efficiency is enhanced due likely to the
following reasons. Firstly, the impurity atoms hinder +
e /h re­
combination rate by acting as trapping centers for photo-generated
electrons and holes. Secondly, the dopants also serve as carrier agents,
facilitating the migration of electrons and holes to the reaction sites in
ZnO. Finally, the band gap energy is narrowed due to energy levels
introduced near the top of the VB or the bottom of the CB potential.
This increases photonic efficiency by enabling a broader portion of the
UV–visible region to be harvested thereby maximizing the number of
reactive radical species such as hydroxyl and superoxide radicals, re­
sponsible for the degradation of the dye.
In a similar fashion, the enhanced catalytic efficiency for composites
of ZnO is due to the narrowing of the band gap which results in high
photonic efficiency and more effective separation of +
e /h pairs than in
pure ZnO. The separation is achieved due to the interfacial transfer of
photo-generated electrons from the CB of one semiconductor to the CB
of the other and diffusion of VB holes in the reverse direction, in a
staggered band gap structure, and build-up of inner electric field, in p-n
heterojunction structures [204].
There are numerous reports in the literature emphasizing on the
enhancement of photocatalytic degradation efficiency of ZnO for azo
dyes through doping or forming composites. Mohammadzadeh et al
[207] studied the effect of Ag-doped ZnO on the degradation of acid
blue 13 dye. They obtained that when Ag concentration was increased
from 0.04 mol% to 3.5 mol%, the degradation (%) dramatically in­
creased from just above 60% to around 95% employing initial dye
concentration of 20 ppm, catalyst dosage of 0.15 g/L in a duration of
75 min. In addition, the Ag-doped ZnO exhibited better recyclability
than undoped ZnO NPs. They concluded that the enhanced activity at
the optimum concentration of 3.5 mol% is due to prevention of +
e /h
recombination, with Ag acting as CB electron trapper, and formation of
oxygen vacancy owing to charge imbalance between Ag+
and Zn2+
.
However, further increase in Ag mol% to 4.2 and 4.9 brought the de­
gradation (%) down to the range of sixties and reasoned this to be
caused by the appearance of Ag agglomerates which can act as re­
combination centers and block light from reaching ZnO surfaces, re­
sulting in lowering rate of degradation. Zhang et al [208] provided si­
milar explanation to the enhancement of the degradation efficiency of
S/ZnO when co-doped with optimum Ag concentration of 10 wt% (Ag/
S/ZnO) to methyl orange dye from 62% to complete degradation using
25 mg of catalyst in 25 mL of 10 ppm methyl orange over a period of
90 min. Moreover, Raza et al [209] studied the effect of varying the
concentration of the rare earth metal Er dopant atoms of Er-doped ZnO
for the degradation of reactive red 241 dye. They reported maximum
efficiency at the optimum Er concentration of 5% and that further in­
crement of dopant concentration lowered efficiency. Superior photo­
catalytic performance of NiO/ZnO composite as compared to either
pure phase alone or the physical mixture of them for methyl orange dye
was reported by Liu et al [210]. Methyl orange was completely removed
in just 25 min using 0.1 g of catalyst loading and 200 mL of 10 ppm dye
concentration under UV light irradiation. They ascribed the outstanding
performance of the composite to the formation of p-NiO and n-ZnO
forming p-n heterojunction, which facilitates the effective separation of
charge carriers. That is, internal electric field is generated at the in­
terface between them while there is movement of electrons from ZnO to
NiO and holes from NiO to ZnO till equilibrium is established. Then,
when radiation of energy greater than or equal to their band gap energy
shines on them, photogenerated electrons will move from the CB of NiO
to the CB of ZnO while holes move in the reverse direction, thereby
effectively separating the pairs. Then after, they can undergo further
oxidation and reduction reactions with water and oxygen to give ra­
dicals which are responsible for the degradation of the dye, Fig. 6. Si­
milarly, Xu et al [211] attributed the maximum degradation efficiency
of 1:1 wt ratio of ZnO/Ag2O composite compared to either pure phase
alone for the degradation of the same dye to the effective carrier se­
paration at the interface between n-ZnO and p-Ag2O.
Nodehi et al [212] reported 100% removal of reactive blue 81 dye
using the catalyst NiO/ZnO/ZrO2 ternary composite in 1:2:0.3 ratio
under optimized conditions of catalyst dosage = 15 mg/L, pH = 3, dye
concentration = 5 ppm, UVC light source for a period of 3 h. While this
was greater by 15% than achieved by pure ZrO2, the improvement was
insignificant compared to that achieved by ZnO and ZnO/ZrO2 catalysts
separately, both being > 96%, over same time duration. They rea­
soned that the improvement of the ternary composite can be due to
higher photon absorption intensity in the visible spectrum and better
inhibition of carrier recombination brought about by the presence of
NiO. A study conducted by Peng et al [213] revealed that acid red 243
dye under experimental conditions of 50 mL of 50 ppm dye con­
centration and 50 mg catalyst loading using UV light source was 93.3%
degraded by 2% reduced graphene oxide (rGO)/ZnO composite in 2 h
time. Not only did addition of appropriate amount of GO (2%) im­
proved the degradation efficiency of ZnO by about 15% but also its
reusability. While bare ZnO suffered serious photo corrosion problem
due to reaction of photo-induced holes with surface oxygen atoms, the
addition of 2% rGO inhibited that and maximized its recyclability,
where 94.8% of first run made was retained in the 6th cycle. Another
recent report by Li et al [214] indicated that photocatalytic activity and
photo stability of cucurbit[8]uril (CB[8])/ZnO composite for Reactive
yellow X-RG dye was higher than bare ZnO and this has been attributed
to be due to the transfer and entrapment of photo generated holes to the
CB[8] leading to the effective separation of charge carriers. Moreover,
from trapping experiment, it was verified that superoxide ion radicals
and holes played dominant role in the photodegradation process of the
dye. Table 4 gives additional examples of the effect of dopants and
composites on the degradation efficiency of some azo dyes.
3.4.2. Effect of particle size and shape
Particle size and shape of a photocatalyst can influence its photo­
catalytic activity towards dyes. As particle size decreases, photo­
catalytic activity of a photocatalyst is expected to increase because of
Fig. 6. Schematics of +
e h
/ separation in ZnO/NiO n-p heterojunction compo­
site. Reprinted with permission from [210]. Copyright 2014. Elsevier.
G.K. Weldegebrieal Inorganic Chemistry Communications 120 (2020) 108140
16

17. the increase in the charge carrier transfer rate and number of active
sites available for reaction, following the resulting increase in specific
surface area. However, if particle size gets sufficiently small, charge
carrier recombination rate can counteract activity as a result of the
increased activity arising from higher specific surface area [230]. The
impact of average particle size of ZnO NPs photocatalyst on the de­
gradation efficiency of azo dye Congo red was reported by Divya et al
[231]. Additionally, particle shape plays an important role, often more
overriding role than particle size, in the photocatalytic activity. For
example, Mclaren et al [53] found hexagonal plate like ZnO NPs to
exhibit superior performance (by more than five-fold) compared to
hexagonal rod like particles for methylene blue dye. This was explained
to be due to the highest activity of the (001) and (00–1) polar planes of
the hexagonal plate like particles which adsorb more hydroxyl ions,
converting OH into OH.
, and high oxygen vacancy which can act as
electron traps and help in carrier separation and enhancing the rate of
degradation. Zhang et al [232] synthesized three samples of ZnO NPs
with different particle shapes using hydrothermal method and tested
their efficiency for the degradation of methyl orange under similar
photocatalytic experiment. The result they obtained in increasing order
of activity is short-and-fat-microrods < nanorods with a cone and
large aspect ratio < nanorods with a cone and small aspect ratio. Since
that was also the order in terms of the BET (Brunauer-Emmett-Teller)
specific surface area of the samples, they related the highest perfor­
mance of nanorods with a cone and small aspect ratio to their highest
specific surface area, providing them with more reactive adsorption/
desorption sites for photocatalytic reactions. Similar relationship be­
tween photocatalytic activity and particle size of ZnO NPs synthesized
via simple solution route for the same dye was observed [233].
However, often times, smaller particle size and higher specific sur­
face area doesn’t guarantee greater photocatalytic activity. For ex­
ample, investigation of the photocatalytic activity of commercial grade
microsized ZnO and needle like ZnO nanostructures synthesized by
thermal evaporation technique for methyl orange dye indicated that the
commercial grade microsized ZnO showed better performance despite
its smaller surface area and larger particle size [234]. In this case, the
presence of native defect points on the surface of ZnO due to oxygen
vacancy are believed to play more important role than crystallite size
and surface area in the kinetics of photocatalysis. Similarly, synthesized
ZnO tetrapods outperformed commercial ZnO nanoparticles with
higher BET surface area for the catalysis of methyl orange and acid
orange 7 azo dyes and this was attributed to the low concentration of
non-radiative defects in the tetrapods compared to the other nano­
particles [235]. Additionally, authors observed that the dominant re­
action mechanism occurs via direct reaction with photogenerated
charge carriers rather than with reductive oxygen species. Native de­
fects on catalyst surfaces can serve as either recombination or active
sites. The photogenerated holes may react with surface oxygen atoms
giving rise to the dissolution of ZnO, which reduces its activity. Liu et al
[236] found resistance to photocorrosion of ZnO NPs with different
morphologies to play more important role than surface area or surface
defect concentration for achieving higher rate of degradation. In gen­
eral, photocatalytic reaction involves complicated photochemistry and
can be influenced by not only catalyst crystallite size or specific surface
area but also by type and location of native defects and active facets
[235]. Table 5 gives examples of degradation efficiency achieved by
different morphology ZnO catalysts for different azo dyes.
3.4.3. Effect of the presence of supports
One problem that influences photocatalytic activity of ZnO NPs is
particle agglomeration when applied in slurry form, especially at high
loadings, during the catalytic study which leads to serious decrease of
photocatalytic performance [244]. This has been overcome by in­
troducing highly adsorbent support compounds such as silica (SiO2),
alumina (Al2O3), zeolite, activated carbon (AC), etc into ZnO forming
ZnO/support composites. The supports used may involve in the pho­
tocatalysis process or may not have any photoactivity for degradation
and serve only as adsorbents. The photocatalytic activity of ZnO in the
presence of supports is enhanced due to the following advantages
rendered to it by the support structures [245]. (i) more pollutant
compounds are attracted to catalyst surface due to increase in specific
surface area, (ii) intermediates may be adsorbed and retained, (iii)
catalyst life time and reusability is extended, and (iv) recombination of
photogenerated carriers is inhibited. Mohaghegh et al. [245] studied
the photocatalytic degradation of acid blue 92 azo dye using ZnO/
mordenite zeolite, ZnO/AC, and ZnO/Al2O3 composites under similar
Table 4
Photocatalytic degradation (%) of azo dyes using doped and composites of ZnO NPs.
S/N Doped or composite of ZnO NPs Azo dye Dye initial conc. (ppm), volume
(mL)
Catalyst loading, (conditions) D (%) Recyclability Ref.
1 4%Nd/ZnO Congo red 25, 50 0.02 g, (visible light,t = 30 min.) 93.7 — [215]
2 Ag/ZnO Methyl orange 10 μM, — 5 mg, (sunlight, t = 1 h) ∼100 — [216]
3 Graphene/ZnO(1:5) Methyl orange 0.02 mM, 100 50 mg, (8 W UV light,t = 6 h) 97.1 97.3% 5th
cycle [217]
4 GO/ZnO Methyl orange 10, 100 1 g/L, (UV light, t = 2 h, pH = 7) 97.7 ∼ 91.7% 4th
cycle [218]
5 α-Fe2O3/ZnO Methyl orange 20, 100 0.1 g, (500 W xenon lamp) ∼ 100 — [219]
6 ZnO/γ-Mn2O3 (90:10) Methyl orange 3x10-5
M, 500 0.5 g, (visible light, t = 210 min.) 90 — [220]
7 ZnO/Fe3O4/g-C3N4-50% Methyl orange 30, 50 10 mg, (500 W visible light) 97.87 95.5% 5th
cycle [221]
8 ZnO/Fe3O4/g-C3N4-50% Alizarin Yellow R 30, 50 10 mg, (500 W visible light) 98.05 — [221]
8 2% ZnO/WO3 Methyl orange 10, 100 0.1 g, (1 kW halogen lamp, t = 3 h) 100 — [222]
9 3% Mn/ZnO Orange II 10, 30 0.06 g, (sunlight, t = 210 min., pH = 9) 100 85% 10th
cycle [223]
10 11.4 wt% ZrS2/ZnO Naphthol blue black 30, 50 0.01 g, (UV light, t = 45 min.,
pH = 6.5)
100 98.5% 4th
cycle [224]
11 ZnO/PPy (polypyrrole) Acid violet 7 5, 250 0.05 g, (15 W UV light, t = 6 h) 66 — [225]
12 Koalinite/TiO2/ZnO
(50%:45%:5%)
Remazol red 100, — —, (sunlight, t = 2 h, pH = 2.5) 98 ∼ 80% 4th
cycle [226]
13 2% rGO/ZnO Acid red 249 50, 50 0.05 g, (15 W UV light, t = 2 h) 93.3 88.45% 6th
cycle [213]
15 TiO2/Zn2TiO4/ZnO/C Orange G 20, 50 0.05 g, (sunlight, t = 50 min.) 100 ∼95% 5th
cycle [227]
16 NiO/ZnO Methyl orange 10,200 0.1 g, (500 W UV light = 25 min.) 100 — [210]
18 CB [8]/ZnO Reactive Yellow X-
RG
400, 50 50 mg, (UV–visible light, t = 28 min.) 90.6 58% 3rd
cycle [214]
18 Ni/ZnO Reactive black 5 20, 1000 0.2 g, (125 W UVC light, t = 2 h) 91.35 91.35% 5th
cycle [228]
19 1 mol% Eu/ZnO Methyl orange 10, 100 0.1 g, (100 W UV light, t = 3 h,
pH = 6.2)
95.3 ∼95.3% 4th
cycle [229]
20 ZnO/ZrO2/NiO (1:2:0.3) Reactive blue 81 5, — 15 mg/L, (15 W UVC light, t = 3 h,
pH = 3)
100 80+% 3rd
cycle [212]
G.K. Weldegebrieal Inorganic Chemistry Communications 120 (2020) 108140
17

18. conditions of operational parameters. They obtained the highest pho­
tocatalytic activity of the first composite to be 25% ZnO/mordenite
zeolite and this was also higher than pure ZnO and the other composites
with similar mass ratio of catalyst to support material and the superior
performance was attributed to be due to the greater BET surface area
and more homogenous distribution of ZnO particles on the zeolites
surface due to its unique structure. Using different support materials
such as natural Tunisian clay [246], biomass activated carbon [247],
the photocatalytic activity of ZnO NPs for Congo red and orange G azo
dyes, respectively exhibited significant improvement over unsupported
ZnO powders.
In general, the performance of ZnO/support composites are ex­
pected to exhibit higher catalytic activity for azo dye degradation
compared to unsupported ZnO due to the enhanced charge carrier se­
paration efficiency and high adsorption capability of the composite
system.
3.4.4. Effect of initial pH of solution
The pH of solution is one parameter that plays crucial role in the
photocatalytic degradation of azo dyes. It affects the surface charge
properties of ZnO, the charge of azo dye molecules, the adsorption of
dyes onto the ZnO surface, the size of aggregates formed, the conduc­
tion and valence band edge potentials of ZnO, and the concentration of
hydroxyl radicals in solution [248,249]. Being an amphoteric oxide,
ZnO dissolves in strongly acidic (pH less than 3) and strongly alkaline
(pH > 11) environment, due to reactions (14) and (15) [250].
+ +
+ +
ZnO 2H Zn H O
2
2 (14)
+ +
ZnO H O 2OH Zn(OH)
2 4
2
(15)
So, this can be one possible reason for reduction in catalytic effi­
ciency of ZnO at extreme pH media. In addition, the amount of dye
adsorption on catalyst surface is affected by the coulombic interaction
force between them and catalytic activity is enhanced if attractive
forces prevail repulsive forces [251]. The point of zero charge (pzc),
defined as the pH where there is equal concentration of protonated and
deprotonated surface groups [252], of ZnO is 9.0 ± 0.3 [245]. So, the
surface charge of ZnO will be positive below this pH and negative above
it due to Eq. (16) and (17) [253].
+ + +
ZnOH H ZnOH2 (16)
+ +
ZnOH OH ZnO H O
2 (17)
Therefore, anionic dyes will be more strongly adsorbed at acidic pH
and cationic dyes at alkaline pH. The charge of the dye molecules is also
affected by solution pH. An anionic dye exists predominantly in its
anionic form when pH > pKa of the dye and optimum pH of
degradation is expected to occur at pKa < pH < pHpzc because in this
pH range surface charge of ZnO is positive and dye molecules are ne­
gatively charged and attractive force of interaction between them fa­
cilitate dye adsorption and enhance its rate of degradation [245]. The
photocatalytic behavior as a function of pH cannot be completely ac­
counted for using the concepts of coulombic interaction and adsorption
alone. Other factors such as dissolution in highly alkaline and acidic
media as given by Eq. (14) and (15), photochemical corrosion arising
from oxidation of ZnO by photogenerated holes, possible formation of
photocatalytically inert Zn(OH)2 layer on the surface of ZnO upon UV
irradiation, and competitive events whose relative dominance either
enhance or hinder the overall efficiency of catalysis need to be taken
into consideration as well [229,251].
The optimum solution pH that maximizes degradation efficiency
may vary depending on the synthesis method employed, whether pure
or modified ZnO NPs is used, and the nature of the azo dyes. For ex­
ample, Sanna et al [254] reported highest degradation efficiency at an
optimum pH of 9 for methyl orange using 10 ppm dye concentration
and 0.1 g/L ZnO loading over a period of 1 h. Whereas, an optimum
value of 7 was reported by Ghule et al [255] in another study using dye
concentration of 20 ppm and catalyst loading of 0.2 g/L during 80 min
of illumination. They ascribed the lower efficiency observed at low pH
of 3 to chemical or photo-induced dissolution of ZnO and that at higher
pH 10 to the reduced adsorption of dye molecules due to competition of
OH for adsorption. They suggested that the highest efficiency of cat­
alysis observed at neutral pH is due probably to the more dominant role
played by adsorption over formation of OH.
with rise in concentration
of OH at high pH, which should have resulted in enhancement of de­
gradation.
Chen et al [238] studied the removal efficiency for three azo dyes
methyl orange, Congo red, and direct blue 38 using 30 ppm dye con­
centration and 0.2 g/L of ZnO catalyst load during the first 10 min of
irradiation time and found maximum value at optimum pH of 2. They
provided two main reasons for highest efficiency observed at such
acidic pH. Firstly, all three azo dyes being anionic, their adsorption on
to ZnO surface is enhanced. Secondly, dye molecules predominantly
exist in their quinonoid forms in strongly acidic media, which are un­
stable and easier to degrade than when they exist in their azo form in
alkaline media. In another study, catalytic activity of Ce/ZnO catalyst
for Acid red 27 was studied at different pH values ranging from 3 to 12
[256]. Maximum value was found at pH 12 using 0.5 mM dye con­
centration and 4000 ppm catalyst loading during irradiation time of 1 h.
According to the authors, the lowered performance of catalyst at lower
pH is due to its acidic dissolution whereas the enhanced performance at
high pH is due to the excess concentration of OH both in solution and
catalyst surfaces which favor the formation of OH.
. Similar optimum pH
Table 5
Degradation efficiency achieved by varied morphology ZnO catalysts for azo dyes under the indicated operational parameters (— signifies no data given).
S/N ZnO shape (average particle size (nm)) Azo dyes Dye volume (mL)/Conc. (ppm) Catalyst loading (mg)/conditions D (%)/time (min.) Ref.
1 Spherical (25.67) Methyl orange 100/10 100/pH = 7.01, 8 W Hg lamp 83.99/120 [105]
2 Hexagonal (45) Methyl orange 30/20 30/UV lamp 94.67/75 [237]
Hexagonal (45) Methyl red 30/20 30/UV lamp 94.37/75
3 Rod-like (22.56) Methyl orange 50/30 0.8 g/L/1000 W UV lamp ( =365 nm), pH = 6.8 99.70/10 [238]
Rod-like (22.56) Congo red 50/30 0.8 g/L/1000 W UV lamp ( =365 nm), pH = 6.8 99.21/10
Rod-like (22.56) Direct blue 38 50/30 0.8 g/L/1000 W UV lamp ( =365 nm), pH = 6.8 99.45/10
4 Nanoflowers (35) Methyl orange —/15 20/UV light 99.46/50 [239]
Nanoflowers (35) Congo red —/15 20/UV light 99.37/80
Nanoflowers (35) Chicago sky blue —/15 20/UV light 96.68/80
5 Commercial Methyl orange —/15 20/UV light 76.86
6 Spherical and hexagonal (11.6) Methyl orange 50/10 M
4 30/8 W UV light ( = 254 nm) 97.3/60 [240]
7 Spherical (52) Titan yellow 100/1.2x10-4
M 100/Sunlight, pH = 2–12 96/60 [241]
8 Nano mushrooms Methyl orange 100/1.5x10-5
M 200/UV light 92/210 [242]
9 Rugby-like Methyl orange 100/20 100/ 125 W Hg lamp ( = 365 nm) 100/140 [233]
Flower-like Methyl orange 100/20 100/ 125 W Hg lamp ( = 365 nm) 90/140
10 Fern-like nano leaves Methyl orange 150/1.0x10-5
M 660 mg/L/50 W UV lamp ( = 365 nm) 100/180 [243]
G.K. Weldegebrieal Inorganic Chemistry Communications 120 (2020) 108140
18

19. for the efficient removal of Acid violet 7 using AgBr/ZnO catalyst was
obtained and observed phenomena explained in similar fashion [257].
Table 6 gives some additional examples of optimum pH values where
maximum removal efficiency was obtained using ZnO and ZnO based
NPs for different azo dyes.
3.4.5. Effect of initial dye concentration and catalyst dosage
Photocatalytic removal efficiency of an azo dye is also affected by
dye initial concentration and the amount of ZnO catalyst loading.
Increasing catalyst loading may enhance the rate of dye removal be­
cause of the corresponding increase of dye adsorption capacity and the
number of available surface active sites, which facilitate the generation
of more OH.
and O2
.
radicals [263]. However, adding more catalyst than
optimum value can hinder rate of catalysis owing to formation of par­
ticle agglomeration, and scattering or screening of light due to solution
turbidity [264]. Thus, the drive to using an optimum amount of catalyst
is not only for achieving maximum degradation efficiency but also
avoiding excessive usage of catalyst. Similar to catalyst dosage, in­
creasing the initial concentration of azo dye more than optimum lowers
its removal rate for the following reasons [248,265]. Firstly, keeping
other variables unchanged, the ratio of photogenerated reactive radi­
cals to dye molecules is decreasing and so is degradation efficiency.
Secondly, with increase in dye concentration, light absorption increases
in accordance to Beer-Lambert’s law and more photons get precluded
from reaching catalyst surface, especially in the deeper part of the so­
lution. This would abate charge carrier generation and efficiency of
degradation. Finally, OH ions may compete with anionic dye mole­
cules for adsorption resulting in reduced generation of OH.
and lower
removal efficiency. Liu et al. [266] studied the effect of dye initial
concentration and catalyst dose on removal efficiency using catalyst
ZnO and Reactive Brilliant red K-2BP dye. When catalyst dose was
raised from 0 to 0.5 g/L at a fixed dye concentration of 50 ppm and
fixed irradiation time and radiation intensity, efficiency increased
sharply from 11.5 to 99.5% but this reduced slightly to 97.9% when
catalyst amount was further increased to 0.7 g/L. In addition, when dye
concentration was varied from 30 to 100 ppm at constant catalyst
amount of 0.5 g/L, degradation decreased. They attributed the de­
creasing tendency of efficiency beyond the optimum value of 0.5 g/L to
particle aggregation and light scattering effects and that above 30 ppm
of dye concentration to reduced path length of photons and formation
of OH.
radicals. Jiang et al. [267], in an earlier study, found efficiency
sharply increased from 58.98 to 94.72% when the catalyst TiO2/ZnO
was increased from 1 to 6 g/L at constant Basic Blue 41 dye con­
centration of 20 ppm, and remained almost constant on increasing from
6 to 10 g/L but decreased slowly when increased beyond 10 g/L.
Moreover, when the dye concentration was varied from 20 to 100 ppm
at constant catalyst dose of 10 g/L, degradation decreased and similar
explanation as above was forwarded for the observed phenomena. In
another recent study, Ebrahimi et al. [268] studied the effect of dye
initial concentration and catalyst dose on degradation efficiency using
2%WO3/ZnO catalyst immobilized on glass substrate and Direct Blue
15 dye. When catalyst dosage was increased from 0.5 to 3 mg/cm2
at
constant dye concentration of 40 ppm over an irradiation time of 1 h,
efficiency increased to 72.8%, and shifted up only slightly when dose
was raised to 5 mg/cm2
. The authors explained the reason for the en­
hanced removal efficiency with increase in catalyst load to be due to the
increase in total active surface area and stated that catalysts im­
mobilized on fixed substrates have an advantage over suspended cata­
lysts that efficiency using such catalysts does not suffer from the pro­
blem of light scattering due to solution turbidity. However, on
increasing dye concentration from 20 to 100 ppm at fixed catalyst do­
sage of 3 mg/cm2
, efficiency decreased as expected by 49.4%. They
ascribed this behavior to surface active sites passivation by dye mole­
cules adsorption and reduced rate of reactive radical formation.
In general, one can see from these discussion that the efficiency of
degradation depends on factors such as the nature of catalyst and dye,
the form of the catalyst used, in slurry or fixed bed form and that initial
dye concentration and catalyst dose need to be optimized to ensure
maximum degradation efficiency and avoid catalyst overuse.
3.4.6. Effect of aeration and stirring of reaction mixture
Bubbling oxygen gas or air (aeration) in to the reaction mixture and
stirring are two other factors that can influence degradation efficiency.
The presence of dissolved oxygen in solution enhances efficiency by
separating photo-produced +
e /h pairs via capturing CB electrons to
produce O2
.
and subsequently HOO.
, and HO.
radicals, Eq. (18)–(23)
[269].
+
O e O
2 2
.
(18)
+ +
O H HOO
2
. .
(19)
+ +
HOO HOO H O O
. .
2 2 2 (20)
+ +
H O e HO HO
2 2
. (21)
+ + +
H O O HO HO O
2 2 2
. .
2 (22)
H O 2HO
2 2
.
(23)
Photoexcited electron in the CB of ZnO is able to reduce dissolved
oxygen in to O2
.
, Eq. (18) because it satisfies the requirement that the
CB level of ZnO (-0.51 eV vs. NHE) should be more negative than the
reduction potential of oxygen (-0.33 eV vs. NHE) [270]. Therefore, once
the O2
.
is formed, other radicals are also formed via charge transfer
mechanism to neighboring adsorbed species, Eq (19) to (23), resulting
in the overall enhancement of the rate of catalysis and degradation
efficiency. The effect of flowing oxygen or air into reaction mixture on
degradation efficiency is rarely reported in the literature. Nasrollah­
zadeh et al [271] studied the removal efficiency of methyl orange under
similar conditions but in air and nitrogen atmosphere, by initially
Table 6
Optimum pH values for maximum removal efficiency of ZnO or ZnO based catalyst for different azo dyes.
S/N Catalyst Azo dye Catalyst load (mg/L) Dye conc. (ppm) Irradiation time (min.) Optimum pH Ref.
1 ZnO Acid blue 92 100 20 120 6 [245]
2 Ce/ZnO Acid red 27 4000 0.5 mM 60 12 [256]
3 ZnO Cibacron brilliant yellow 3G-P 400 50 60 5.1 [249]
4 Ag/ZnO Direct blue 71 2000 100 210 1
3
5 Cu/ZnO Direct blue 71 2000 100 2102
3 [258]
6 Cu/ZnO Direct blue 71 3000 10 90 6.8 [259]
7 Eu/ZnO Methyl orange 1000 10 180 6.2 [229]
8 Mn/ZnO Orange II 1500 20 180 9 [223]
9 ZnO Reactive orange 4 4000 0.5 mM 20 7 [260]
10 ZnO Reactive red 120 4000 0.2 mM 30 5 [261]
11 ZnO/MWCNT Reactive blue 203 5 20 20 10 [262]
1
Using UV light
2
Using visible light
G.K. Weldegebrieal Inorganic Chemistry Communications 120 (2020) 108140
19