![Chemical structures of polyaniline emeraldine salt form (PANI-ES) and polyaniline emeraldine base form (PANI-EB), where A denotes an anion such as Cl -. Adapted from Stejskal, 2020 [22].](https://www.researchgate.net/publication/350765352/figure/fig1/AS:1010680962490368@1617976527203/Chemical-structures-of-polyaniline-emeraldine-salt-form-PANI-ES-and-polyaniline_Q320.jpg)
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Water 2021, 13, 1025. https://doi.org/10.3390/w13081025 www.mdpi.com/journal/water
Article
Preparation of PANI Modified ZnO Composites via Different
Methods: Structural, Morphological and
Photocatalytic Properties
Nazli Turkten 1, Yunus Karatas 1 and Miray Bekbolet 2,*
1 Department of Chemistry, Faculty of Arts and Sciences, Kirsehir Ahi Evran University,
Kirsehir 40100, Turkey; nazli.turkten@ahievran.edu.tr (N.T.); ykaratas@ahievran.edu.tr (Y.K.)
2 Institute of Environmental Sciences, Bogazici University, Bebek, Istanbul 34342, Turkey
* Correspondence: bekbolet@boun.edu.tr
Abstract: Polyaniline modified zinc oxide (PANI-ZnO) photocatalyst composites were synthesized
by focusing on dissolution disadvantage of ZnO. In-situ chemical oxidation polymerization method
was performed under neutral conditions (PANI-ES) whereas in hybridization method physical
blending was applied using emeraldine base of polyaniline (PANI-EB). PANI-ZnO composites were
prepared in various ratios of aniline (ANI) to ZnO as 1%, 3%, 6% and 9%. The alterations on the
structural and morphological properties of PANI-ZnO composites were compared by Fourier
Transform Infrared (FT-IR), Raman Spectroscopy, X-Ray Diffraction (XRD) and Scanning Electron
Microscopy-Energy Dispersive X-ray Analysis Unit (SEM-EDAX) techniques. FT-IR and Raman
spectroscopy confirmed the presence of PANI in all composites. SEM images revealed the morpho-
logical differences of PANI-ZnO composites based on PANI presence and preparation methods.
Photocatalytic performances of PANI-ZnO specimens were investigated by following the degrada-
tion of methylene blue (MB) in aqueous medium under UVA irradiation. The effects of catalyst dose
and initial dye concentration were also studied. MB degradation was followed by both decoloriza-
tion extents and removal of aromatic fractions. PANI-ZnO composites expressed enhanced photo-
catalytic performance (~95% for both methods) as compared to sole ZnO (~87%). The hybridization
method was found to be more efficient than the in-situ chemical oxidation polymerization method
emphasizing the significance of the neutral medium.
Keywords: hybridization method; in-situ chemical oxidation polymerization method; methylene
blue; PANI-ZnO; photocatalysis; polyaniline
1. Introduction
Due to the continuous release of contaminants to the environment, application of
photocatalysis as an alternative advanced oxidation treatment process is attracting the
main focus of researchers. Besides TiO2, ZnO is the second most extensively used photo-
catalyst exhibiting excellent photocatalytic activity, stability and almost similar band gap
energy (Ebg 3.2 eV and 3.0 eV, respectively) [1–4]. The major advantage of ZnO is its broad
band in ultraviolet-visible (UV-vis) region in the wavelength range of 350–470 nm ena-
bling charge transfer from oxide 2p orbitals (valence band) to 3d orbitals of Zn2+ cations
(conduction band) [5]. ZnO has more negative conduction band potential and has a higher
exciton binding energy compared to TiO2. Comparative studies on the use of TiO2, ZnO
and TiO2/ZnO photocatalysts for pollutant abatement were also reported in the literature
[6–12].
To overcome the disadvantages of n-type semiconductors expressing large band
gaps, coupling with conducting polymers exhibiting small bandgaps and extended π–e−
Citation: Turkten, N.; Karatas, Y.;
Bekbolet, M. Preparation of PANI
Modified ZnO Composites via
Different Methods: Structural,
Morphological, and Photocatalytic
Properties. Water 2021, 13, 1025.
https://doi.org/10.3390/w13081025
Academic Editor: Pei Xu
Received: 10 March 2021
Accepted: 6 April 2021
Published: 8 April 2021
Publisher’s Note: MDPI stays neu-
tral with regard to jurisdictional
claims in published maps and institu-
tional affiliations.
Copyright: © 2021 by the authors. Li-
censee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and con-
ditions of the Creative Commons At-
tribution (CC BY) license (http://crea-
tivecommons.org/licenses/by/4.0/).
Water 2021, 13, 1025 2 of 31
systems drew current interest. With respect to attained beneficial effect via “sensitiza-
tion”, coupling of p-type conducting polymers with an n-type semiconductor opened a
new area of research. In recent years, the synthesis of p-conjugated polymer modified ZnO
materials gained great attention for photocatalytic applications [13–17]. PANIis the most
popular conducting polymer containing structures either benzenoid, quinoid or even both
of them. The combination of a p-type polymer as PANI with an n-type semiconductor e.g.,
ZnO results in p-n junction thereby reducing the rate of electron-hole recombination and
inhibiting photocorrosion [18]. Moreover, PANI could also act as e- donor and h+ acceptor
[19]. Besides being a low-cost material, PANI could be synthesized easily and express high
environmental stability. Accordingly, PANI is regarded as a good polymer candidate in
preparation of PANI-ZnO composites suitable for photocatalytic systems [16,17,20,21].
The main challenge of synthesis of PANI-ZnO composites is to prevent the dissolu-
tion of ZnO both in highly acidic (release of Zn2+ ions) and alkaline aqueous conditions
(formation of Zn(OH)42− due to the reactivity of amphoteric ZnO as expressed by the fol-
lowing reactions 1 and 2:
In extreme acidic aqueous environment,
ZnO + 2H+
→
Zn2+ + H2O (1)
In extreme alkaline aqueous environment,
ZnO + H2O + 2OH−
→
Zn(OH)42− (2)
On the other hand, photocorrosion of ZnO under UVA irradiation could also be elim-
inated by coupling with a conducting polymer i.e., PANI (reactions 3–5).
ZnO + h
ν
(E > Ebg = 3.2 eV)
→
ZnO*(hVB+ + eCB−) (3)
(hVB+ + eCB−) hVB+ + eCB- (4)
ZnO + h+ + nH2O
→
Zn(OH)n (2−n)+ + ½ O2 + nH+ (5)
PANI synthesis starts from the combined usage of monomers such as aniline ANI or
its derivatives and free radical initiators such as ammonium persulfate (APS), potassium
persulfate, potassium dichromate etc. acting as effective oxidants. The mostly used do-
pants are hydrochloric acid, sulfuric acid and dodecylbenzenesulfonic acid [20]. PANI un-
der acidic conditions (pH < 4) exists as polyaniline emeraldine salt form (PANI-ES) and
converts to PANI-EB above pH 4–6.
The standard synthesis of PANI is performed by using a strong acid such as HCl at
pH < 2.5 [22]. There have also been studies on modified composites of PANI e.g., most
commonly investigated with titanium dioxide (PANI-TiO2) by in-situ chemical oxidation
polymerization at lower pH values [23–27]. The dissolution problem of ZnO could only
be avoided by achieving the synthesis of PANI-ZnO composites under neutral pH condi-
tions. Thus, Gilja and colleagues have successfully synthesized PANI-ZnO composites in
neutral medium without using acidic dopants via in-situ chemical oxidation polymeriza-
tion mechanism preventing the dissolution issue of ZnO [14].
As an alternative approach, PANI-ZnO composites could also be prepared via the
hybridization method that was based on conversion of PANI-ES to emeraldine base
PANI-EB and physically mixed with ZnO [15,28,29]. Performance studies of thus pre-
pared PANI-ZnO composites mainly focused on degradation of model substrates (dyes
such as acid blue, methylene blue and malachite green) and organic pollutants (antibiotics
such as ampicillin) [13–15,29].
Therefore, the preparation of the PANI-ZnO composites under neutral conditions
expressed prime importance to be investigated. To fulfill this gap, the present study was
designed to synthesize PANI-ZnO composites safely by two main pathways as in-situ
chemical oxidation polymerization and hybridization methods. Structural and morpho-
logical properties of the composites were elucidated by FT-IR and Raman spectroscopy,
XRD and SEM-EDAX analysis. Photocatalytic activities of thus prepared photocatalysts
Water 2021, 13, 1025 3 of 31
were assessed by following the degradation of a model substrate in neutral aqueous me-
dium under UV light irradiation in a comparative manner. Methylene blue (MB) as a cat-
ionic dye was selected due to its resistance to biodegradation thereby being widely em-
ployed almost as a standard dye for photocatalytic activity tests [30]. In addition, photo-
catalyst dose and initial dye concentration effects on the removal efficiencies of MB were
also examined.
2. Materials and Methods
2.1. Materials
Zinc oxide (99%) was used as the primary photocatalyst. ANI, ammonium persulfate
(APS), hydrochloric acid (37%), tetrahydrofuran and ammonia solution (25%) were used
in various preparation steps. ZnO, MB (C16H18ClN3S⋅2H2O) and all other chemicals were
provided from Merck and used without further purification. All aqueous solutions were
prepared with distilled water (conductivity 2 µS/cm at 25 °C).
2.2. Preparation of PANI-ZnO Composites
Prior to synthesis of composites, PANI-ES was synthesized by in situ-chemical oxi-
dation polymerization method, then converted to PANI-EB by adding alkaline solution.
PANI-ZnO composites were prepared by in situ chemical oxidation polymerization and
hybridization methods.
2.2.1. Preparation of PANI-ES and PANI-EB
PANI-ES was synthesized by in-situ chemical oxidation polymerization method with
reference to the procedure reported by Li and colleagues [24]. Solution A was prepared
by dissolving 17.13 g APS in 240 mL of 1 M HCl solution and placed in a dropping funnel.
Solution B was prepared by adding ANI (6.99 g) slowly into 240 mL of 1 M HCl solution
in a flat-bottomed flask placed in an ice bath with vigorous stirring by magnetic stirrer.
Afterwards, solution A was added dropwise (approximately one drop/second) into the
solution B in the ice bath. The stirring was continued for another 24 h at room temperature
to achieve polymerization. PANI-ES was filtered through Gooch funnel and extensively
washed with distilled water and ethanol and dried in an air oven at 80 °C for 24 h.
The obtained PANI-ES was further converted to PANI-EB. The base treatment was
applied with adding 12.1 mL ammonia solution (10%) to 2 g PANI-ES and the solution
stirred for 2 h. The filtered PANI-EB was then washed in turn with distilled water and
ethanol and finally dried at 80 °C until reaching constant weight. The synthesized PANI-
ES and PANI-EB samples were kept in glass bottles at room temperature.
2.2.2. In-situ Chemical Oxidation Polymerization Method
PANI-ZnO composites (PZI) using in-situ chemical oxidation polymerization
method were synthesized by following the procedure reported by Gilja and colleagues
[14]. The mole ratio of ANI to APS was kept constant as 1:0.25 in all samples. Weight
percentages (wt%) of ANI/ZnO were adjusted to 1%, 3%, 6% and 9%, and these compo-
sites were denoted as PZI-1, PZI-3, PZI-6 and PZI-9, respectively. In a typical synthesis of
PZI-1, solution A was prepared by adding 0.031 g APS to 240 mL of distilled water and
placed in a dropping funnel. Solution B was prepared by dissolving 5 g ZnO in 240 mL
distilled water via sonication in ultrasonic bath for 15 min to obtain a uniform and stable
solution. 0.05 g ANI was added slowly into Solution B. Solution A placed in a dropping
funnel was dropwise added (approximately one drop/second) into Solution B in a flask
placed in an ice bath with vigorous stirring to obtain oxidative polymerization. The reac-
tion mixture was further stirred for 24 h at room temperature. The obtained composite
was filtered, washed with distilled water and dried in an air oven at 80 °C for 24 h. PZI-3,
PZI-6 and PZI-9 were prepared accordingly.
Water 2021, 13, 1025 4 of 31
2.2.3. Hybridization Method
PANI-ZnO composites (PZS) by hybridization method were synthesized by simple
physical mixing of PANI-EB and ZnO. Four different PANI-hybridized ZnO composites
with different mass ratios containing 1%, 3%, 6% and 9% were prepared and the compo-
sites were labelled as PZS-1, PZS-3, PZS-6 and PZS-9, respectively. In a typical synthesis
of PZS-1, the mixture of 5 g ZnO and 0.05 g PANI-EB in 50 mL tetrahydrofuran was soni-
cated for 15 min in an ultrasonic bath and continuously stirred for 24 h at room tempera-
ture. The formed composite was filtered, washed with distilled water and followed by
drying in an air oven at 80°C for 24 h. PZS-3, PZS-6 and PZS-9 were prepared accordingly.
A schematic of PANI-ZnO preparation method including the ways preventing ZnO dis-
solution is presented as Figure S1, in Supplementary information (SI Part 1, Figure S1).
2.3. Characterization Techniques
FT-IR spectra were recorded in the range between 4000–400 cm−1 using Thermo Sci-
entific Nicolet 6700 Spectrometer. All samples were prepared as KBr pellets. Raman spec-
troscopy was acquired by Thermo Scientific DXR Raman Microscope using Ar+ laser exci-
tation at λ = 532 nm. The laser power and spectral resolution were 10 mW and 2 cm−1,
respectively. XRD patterns were recorded on a Rigaku-D/MAX-Ultima diffractometer us-
ing Cu Kα radiation (λ = 1.54 Å). The scan varied between 5 to 80 (2θ) with a scan rate of
2° min−1. The operating voltage and current were 40 kV and 40 mA, respectively. SEM
analysis was employed on a FEI-Philips XL30 Scanning Electron Microscope equipped
with an EDAX unit operating at 10 kV using catalyst powders supported on carbon tape.
2.4. Photocatalytic Activity
Photocatalytic activity testing was performed in a cylindrical Pyrex reaction vessel
containing 50 mL of 10 mg/L MB solution and 0.25 g/L photocatalyst without pH adjust-
ment. The reaction vessel was illuminated from the top with a 125 W black light fluores-
cent lamp (BLF). BLF light source emitted radiation in a wavelength range between 300
nm and 420 nm with a maximum at λ = 365 nm. The light intensity reaching the reaction
medium was quantified by solution-phase potassium ferrioxalate actinometer [31].
Irradiation time-based (0 min-150 min) changes in the concentration of MB were
monitored by examining the variations in maximal absorption in UV-visible spectra at 664
nm mainly attributed to decolorization, and two minor peaks located at λ = 292 nm and λ
= 246 nm. Accordingly, MB was characterized by UV-vis parameters designated as A664,
A292 and A246 with reference to the corresponding absorption wavelengths. Respective mo-
lar absorption coefficients were calculated using concentration vs. absorbance values and
reported as MB concentration. Thermo Scientific Genesys 10S double beam UV-vis spec-
trophotometer was employed using 1 cm quartz cells.
Following each treatment period, the photocatalysts were immediately removed
from the reaction medium via double filtration through 0.45 µm and 0.20 µm membrane
filters immediately to avoid post-adsorption of the organics onto photocatalyst specimens.
The reproducibility of the results was determined as <5% via preliminary experiments
performed in triplicate prior to the main experimental part.
Preliminary experiments were performed (i), under irradiation in the absence of
light, and (ii),under dark conditions in the absence of photocatalyst. Complementary ex-
periments under similar conditions were also carried out to elucidate the effect of initial
MB concentration (5 mg/L, 10 mg/L, 20 mg/L and 30 mg/L) by using constant 0.25 g/L
photocatalyst dose and the effect of photocatalyst loading (0.125 g/L, 0.25 g/L and 0.50
g/L) in 10 mg/L MB dye solution.
Water 2021, 13, 1025 5 of 31
3. Results
The yields of PANI-ES and PANI-EB were found as 95% and 77%, respectively, and
all PANI-ZnO composites were obtained in high yields as 95% ± 2.8 irrespective of the
preparation methodology.
3.1. Characterization of the PANI-ZnO Composites
3.1.1. FT-IR Spectroscopy
FT-IR spectroscopy was used to monitor possible structural changes due to interac-
tions between PANI and the prepared composites. PANI-ES is one of the oxidative states
of PANI containing both benzenoid and quinoid structure units. The partially reduced
unit in PANI-ES consists of benzenoid structure where hydrogen atoms are attached to
nitrogen atoms, while the partially oxidized unit is comprised quinoid structure where no
hydrogen atom is bonded to nitrogen [20]. PANI-EB is the deprotonated form of PANI-
ES in alkaline medium [32]. The structures of these oxidized forms of PANI are presented
in Figure 1.
Quinoid Structure
NH
NH
N H
+
N H
+
n
NH
NH
NN
n
A
-
A
-
OH
-
H
+
Polyaniline Emeraldine Salt
Polyaniline Emeraldine Base
Benzenoid Structure
1-y y
Figure 1. Chemical structures of polyaniline emeraldine salt form (PANI-ES) and polyaniline em-
eraldine base form (PANI-EB), where A denotes an anion such as Cl-. Adapted from Stejskal, 2020
[22].
FT-IR spectral variations of PANI-ES and PANI-EB could be visualized by the pres-
ence of strong vibrations of PANI-EB in comparison to PANI-ES (Figure 2a). The main
characteristic peaks in PANI-ES recorded at 1450 cm−1 and 1557 cm−1 were attributed to
the stretching vibrations of benzenoid unit and quinoid unit groups, respectively [20,33].
The broad shoulder located around 3400 cm−1 could be assigned to N-H stretching [14].
The peaks at 1290 cm−1 and 788 cm−1 corresponded to secondary aromatic amine C–N
stretching and aromatic C–H out of plane bending, respectively [33]. These peaks were
moved to higher wavenumbers, which were located at 1298 cm−1 and 820 cm−1 in PANI-
EB. The peaks related to benzenoid and quinoid shifted to 1493 and 1587 cm−1. Further-
more, 1218 cm−1 was related to C–N+ stretching vibration in PANI-ES and this peak dis-
appeared in the FT-IR spectrum of PANI-EB [34,35]. The peaks were observed at 1069 cm−1
and 581 cm−1 indicating residual sulfate anions due to the usage of the oxidant source of
Water 2021, 13, 1025 6 of 31
ammonium peroxydisulfate. In PANI-EB, the peaks corresponding to the presence of sul-
fate anions disappeared while a new peak appeared at 1160 cm−1 that was ascribed to the
aromatic C–H in plane bending vibration [34].
FT-IR spectra of ZnO and PZI composites along with PANI-ES were displayed in
Figure 2b. A broad peak centered a maximum at 3375 cm−1 represented the stretching of
water molecules in ZnO spectrum, while the maxima was located in the range of 3355–
3379 cm−1 in PZI composites [36]. Another peak in the FT-IR spectrum of ZnO at 1699 cm−1
corresponded to H–O–H bending due to the presence of H2O in the ZnO nanoparticles
and PZI composites [37]. The peaks at ~2980 cm−1 and ~2897 cm−1 were formed due to C–
H stretching modes of alkyl groups [38,39]. The asymmetric and symmetric C=O stretch-
ing modes of zinc acetate residue on the surface of the particles were manifested by the
peaks at 1562 cm−1 and 1382 cm−1, respectively [40]. In PZI samples, the intensity of these
alkyl modes was much higher compared to ZnO. The peaks at ~1380 cm−1, ~1500 cm−1 and
~1550 cm−1 corresponded to C-N stretching, C=C stretching of benzenoid rings and C=C
stretching of quinoid rings, respectively [41]. These peaks were slightly shifted compared
to PANI. The reason could be attributed to the existing of interactions such as the for-
mation of hydrogen bonding between NH group of PANI and ZnO surface [15,36]. More-
over, a peak overlap was observed at ~1560 cm−1 for quinoid and symmetric C=O stretch-
ing peak. The peak related to benzenoid unit in PANI-ES shifted to ~1500 cm−1 in PZI com-
posites and confirmed the presence of PANI in composites. It could be clearly seen that
the intensity of this peak increased in composites containing higher PANI amounts. The
residual sulfate peaks were slightly shifted to lower wavenumbers located at ~1050 cm−1
and ~560 cm−1 in PZI spectra. Furthermore, the peaks were positioned in the region of 430–
870 cm−1 were assigned to Zn–O–Zn and Zn–O stretching vibrations and presented in ZnO
and PZI composites [14].
4000 3600 3200 2800 2400 2000 1600 1200 800
400
(a)
826
1493
581
788
1069
1557
PANI-EB
Transm ittance, %
Wavenumbers, cm
-1
PANI-ES
3215
1450
1298
1218
1160
1587
Water 2021, 13, 1025 7 of 31
4000 3600 3200 2800 2400 2000 1600 1200 800 400
(b)
558
1053
1699
1382
1500
1557
1382
1562
2897
2980
3369
3380
3365
3355
Transmittance, %
Wavenumbers, cm
-1
PANI-ES
ZnO
PZI-3
PZI-1
PZI-6
PZI-9
3375
4000 3600 3200 2800 2400 2000 1600 1200 800
400
(c)
PZS-9
PZS-6
PZS-3
PZS-1
ZnO
PANI-EB
Transmittance, %
Wavenumbers, cm
-1
3375
1699
1562 1382
1382
1499
1579
2897
2980
3355
Figure 2. Stacked Fourier Transform Infrared (FT-IR) spectra of (a) PANI-ES and PANI-EB, (b)
PANI-ES, zinc oxide (ZnO) and PANI-ZnO composites using in-situ chemical oxidation polymeri-
zation method (PZI), (c) PANI-EB, ZnO and PANI-ZnO composites by hybridization method
(PZS).
All peaks detected in PZI composites were also recorded in the spectra of PZS com-
posites (Figure 2c). The strong peaks at ~1500 cm−1 and ~1585 cm−1 corresponded to ben-
zenoid and quinoid units, respectively. Besides, the peak represented quinoid unit was
clearly evident as compared to PZI composites.
3.1.2. Raman Spectroscopy
Raman spectrum of PANI-ES displayed two main bands at 1567 cm−1 and 1345 cm−1
corresponded to C=C stretching of quinoid units and C–N+• stretching (Figure 3a). The
quinoid unit band was shifted to a slightly higher frequency (1580 cm−1) and the radical
cation band C–N+• was vanished in PANI-EB. The vibration bands observed at 1161 cm−1,
1217 cm−1 and 1490 cm−1 were assigned to C–H bending in-plane of benzenoid or quinoid
unit, C–N bending in-plane of benzenoid unit and N=C=N stretching in quinoid units,
respectively [41–44].
Water 2021, 13, 1025 8 of 31
200 400 600 800 1000 1200 1400 1600 1800
2000
1217
1161
1490 1580
1345
PANI-ES
PANI-EB
(a)
Intensity (a.u.)
Raman shift (cm
-1
)
1567
200 400 600 800 1000 1200 1400 1600 1800 2000
1518
1519
1527
1151
1546
663
578
384
329
435
(b)
ZnO
PZI-1
PZI-3
PZI-6
PZI-9
Intensity (a.u.)
Raman shift (cm-1)
1148
579
379
328
434
200 400 600 800 1000 1200 1400 1600 1800 2000
1215
1209 1586
1480
(c)
Raman shift (cm
-1
)
Intensity (a.u.)
ZnO
PZS-1
PZS-3
PZS-6
PZS-9
1151
663
578
384
329
435
1158
579
376
327 433
1215 1585
1483
14801586
1215 14831585
1300 1350 1400 1450 1500 1550 1600 1650 1700
(d)
PZI-1
PZI-3
PZI-6
PZI-9
Intensity (a.u.)
Raman shift (cm
-1
)
1300 1350 1400 1450 1500 1550 1600 1650 1700
(e)
PZS-1
PZS-3
PZS-6
PZS-9
Intensity (a.u.)
Raman shift (cm
-1
)
Figure 3. Raman spectra of (a) PANI-ES and PANI-EB, (b) ZnO and PZI composites, (c) ZnO and
PZS composites, (d) regional enlarged spectra of PZI composites and (e) regional enlarged spectra
of PZS composites.
Raman spectrum of ZnO displayed a main and sharp band located at 435 cm−1 repre-
senting E2 (high) mode as expected (Figure 3b). The source of this band was the oxygen
atoms present in ZnO. The other bands at 329 cm−1, 384 cm−1, 578 cm−1, 663 cm−1 and 1151
cm−1 could be attributed to 2E2 mode, A1 (TO) mode, A1 (LO) modes, TA+LO, contributions
of 2A1 (LO) and 2E1 (LO) modes, respectively [42,45]. The bands related to ZnO modes
slightly shifted towards a lower frequency in PZI-1 spectrum, while quinoid unit band of
PANI-ES was observed as a broad band at 1546 cm−1. This shift in quinoid unit band of
PANI-ES suggested a decrease in the conjugated structure of PANI-ES and interaction
between PANI and ZnO [43]. Incremental increase of PANI ratio in PZI composites am-
plified the intensities of the characteristic bands of PANI-ES along with decreases in the
Water 2021, 13, 1025 9 of 31
intensities of ZnO bands. This situation had also been observed especially in PZI-9 spec-
trum and the quinoid unit band of PANI-ES especially shifted to 1518 cm−1. On the other
hand, PANI-EB bands at 1214 cm−1, 1487 cm−1 and 1584 cm−1 shifted to lower frequencies
and the band at 1161 cm−1 disappeared in the Raman spectrum of PZS-1 (Figure 3c). In
Raman spectrum of ZnO, the bands of 2E2 mode, A1 (TO) mode, E2 (high), A1 (LO) mode,
TA+LO and 2E1 (LO) modes shifted to 327 cm−1, 376 cm−1, 433 cm−1, 579 cm−1 and 1158 cm−1,
respectively. Moreover, the observed band intensity increases in PANI-EB could be cor-
related with the increasing PANI amount in composites.
From a general perspective, the intensities of the characteristic bands of PANI-ES and
PANI-EB increased with increasing PANI amount in PZI and PZS composites (Figure
3d,e). The intensity of the characteristic ZnO Raman bands (especially E2 (high) band at
433 cm−1) was reduced in both PZI and PZS composites compared to bare ZnO the reason
of which could be explained by the formation of a strong interaction between PANI and
ZnO. Furthermore, the band related to the oxygen deficiency in ZnO at ~580 cm−1 was
present in all PZI and PZS composites indicating surface defects [41]. This interaction was
supported by FT-IR spectral features confirming the presence of PANI in PANI-ZnO com-
posites.
3.1.3. XRD Spectroscopy and Crystal Structure
The XRD diffractograms of PANI-ES and PANI-EB showed several weak peaks with
reasonable intensities (Figure 4a). The peak diffracted at an angle of 2θ = 20.34° and 2θ =
5.30° (d spacing of 4.366 Å and 3.520 Å, respectively) showed low crystallinity of the con-
ductive polymers due to the repetition of benzenoid and quinoid rings in PANI chains
[46]. PANI-ES and PANI-EB expressed a semi-crystalline phase. The characteristic peaks
of PANI-ES were present in the region of 2θ = 5°–30°. These peaks located at 2θ = 8.99°,
15.42°, 20.34°, 25.30° and 27.18° corresponded to (0 0 1), (0 1 1), (0 2 0), (2 0 0) and (1 2 1)
reflections in PANI-ES which were slightly shifted and broadened in PANI-EB [47]. XRD
diffractogram of PANI-EB displayed the peaks at 2θ = 9.82°, 15.82°, 20.31° and 24.38°
whereas the peak at 2θ = 27.18° could be related to the acidity of the medium [47]. The
diffractogram of ZnO (JCPDF 36-1451) exhibited the characteristic peaks at 2θ = 31.92°,
34.58°, 36.40°, 47.68°, 56.72°, 62.99°, 66.52°, 68.08° and 69.20° corresponding to (1 0 0), (0 0
2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2) and (2 0 1) planes of zincite, respectively.
The XRD diffractograms of PZI and PZS composites displayed the presence of ZnO
peaks with no remarkable shifts (Figure 4b,c). The reason of unobservable PANI peaks
could be attributed to the suppression exerted by high intensity of ZnO peaks related to
the interaction of ZnO and PANI via hydrogen bonding between H–N and oxygen of ZnO
and the low amount of PANI concentration in the composites [46]. The intensities of the
peaks related to ZnO were gradually decreased in composites due to the dilution by PANI
as expected. The interplanar spacing for ZnO and PANI-ZnO composites were almost the
same (~2.472 Å) that corresponded to the spacing between (1 0 0) planes. Based on the
diffraction plane (1 0 0) of zincite, the crystallite size (D, nm) of the composites could be
calculated by using Scherrer Equation (6) as expressed below:
K
D
cos
(6)
where K = 0.9, λ = 1.5418 Å for Cu Kα, θ = Bragg angle and β = full width at half maximum
intensity (FWHM, radians) [48]. The crystallite size of ZnO was calculated as 46 nm and a
very slight decrease was acquired for PANI-ZnO composites. Average 2θ were found to
be insignificantly different from each other with an average of 36.32° ± 0.0096 resulting in
primary particle size (crystal size) as 45.78 ± 0.44 nm. Therefore, it could be deduced that
PANI coating of ZnO surface did not significantly affect the crystallite sizes of the com-
posites mainly existing as agglomerates or forming bridges rather than complete encap-
sulation [28,42,43].
Water 2021, 13, 1025 10 of 31
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
(a)
(
1 2 1
)
(
2 0 0
)
(
0 2 0
)
(
0 1 1
)
(
0 0 1
)
Intensity (a.u)
2
(
Degree
)
PANI-ES
PANI-EB
(
0 2 0
)
(
0 1 1
)
(
0 0 1
)
(
2 0 0
)
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
JCPDF 36-1451
(b)
Intensity (a.u)
2(Degree)
ZnO
PZI-1
PZI-3
PZI-6
PZI-9
(2 0 0)
(0 0 2)
(1 0 0)
(1 0 1)
(1 0 2)
(1 1 0)
(1 0 3)
(1 1 2)
(2 0 1)
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75
80
JCPDF 36-1451
(c)
(2 0 0)
Intensity (a.u)
2
(
Degree
)
ZnO
PZS-1
PZS-3
PZS-6
PZS-9
(0 0 2)
(1 0 0)
(1 0 1)
(1 0 2)
(1 1 0)
(1 0 3)
(1 1 2)
(2 0 1)
Figure 4. X-Ray Diffraction (XRD) spectra of (a) PANI-ES and PANI-EB, (b) ZnO and PZI compo-
sites and (c) ZnO and PZS composites.
3.1.4. SEM-EDAX Analysis and Morphological Structure
The morphologies of PANI-ES, PANI-EB, ZnO and PANI-ZnO composites were ex-
amined at two different magnifications (×50,000 and ×100,000) by SEM analysis accompa-
nied by EDAX analysis to express both quantitative and qualitative elemental data about
chemical structure (Figure 5a–f and 6a–f). SEM image of PANI-ES displayed the presence
of slightly agglomerated particles in almost uniform and homogeneous distribution (Fig-
ure 5a). The flower like morphology of PANI-ES particles expressed a dandelion like
Water 2021, 13, 1025 11 of 31
shape the diameter of which could be envisioned as 300–500 nm [49]. The EDAX analysis
of PANI-ES indicated the presence of C, N, Cl and S (Figure 5a).
SEM image of ZnO consisted of various polyhedral shapes with particle sizes chang-
ing in the range from 100 to 500 nm (presented both in Figure 5b and 6b for easiness of
comparison). It should also be indicated that ZnO could display morphological variations
with respect to source and surface area properties as shown by SEM images [11].
PZI-1 composite morphology was rather similar to ZnO with a slightly distorted dan-
delion like shape of polyaniline particles (Figure 5c). The observed change in ZnO mor-
phology was reflected as an enhancement in size of aggregates due to the increase in PANI
concentration especially in PZI-6 and PZI-9 where the flower like shape of PANI was
slightly distorted. The EDAX spectra of PZI composites indicated the major components
as Zn, O and C with low levels of N, and Cl elements. This finding confirmed the existence
of both PANI and ZnO in PZI samples. The EDAX results also implied that PANI amount
was effective on the weight percentage of carbon in PZI-6 and PZI-9. Even though no im-
purities were observed in all EDAX spectra it should also be emphasized that the compo-
sitional differences mostly relied on the sampling point rather than the actual variations.
Figure 5. SEM images (left) ×50,000, (right) ×100,000 and Energy Dispersive X-ray Analysis Unit
(EDAX) elemental analysis (a) PANI-ES, (b) ZnO, (c) PZI-1, (d) PZI-3, (e) PZI-6 and (f) PZI-9.
Water 2021, 13, 1025 12 of 31
SEM image of PANI-EB expressed the presence of dandelion-like shaped particles re-
vealing a more condensed distribution thereby higher agglomeration compared to
PANI-ES specimen (Figure 6a). The dense structure of PANI-EB could be attributed to
the change in crystallinity of the polymer after conversion as compared to PANI-ES [50].
The average diameter size of the dense-shaped dandelions was observed at ~300 nm.
The EDAX analysis of PANI-EB also indicated the presence of C, N and Cl. The weight
percent of Cl remarkably decreased and reached to a nearly negligible value in PANI-
EB. As well, sulfur completely vanished in PANI-EB and this finding also confirmed FT-
IR results as explained above.
Figure 6. Scanning Electron Microscopy (SEM) images (left) ×50,000, (right) ×100,000 and EDAX
elemental analysis (a) PANI-EB, (b) ZnO, (c) PZS-1, (d) PZS-3, (e) PZS-6 and (f) PZS-9.
PZS-1 showed polyhedron particles due to slight surface modification of ZnO with
PANI-EB resulting in deformation of flower like shape of PANI-EB that could hardly be
detected in the image of PZS-1 although the existence of PANI-EB could be clearly verified
in EDAX spectra (Figure 6b). Agglomeration phenomena of PZS composites increased
Water 2021, 13, 1025 13 of 31
with respect to increase in PANI-EB concentration due to prevailing interactions between
PANI-EB and ZnO [28].
SEM images of composites displayed the presence of PANI polymer chains in be-
tween ZnO particles either as partial encapsulation form or extending in the periphery
domain of ZnO particles. Furthermore, a major outcome could be that low PANI coating
of PZS was more evident compared to PZI composites. The reason could be attributed to
the poor hydrogen bonding on the surface between PANI molecules and ZnO nanoparti-
cles.
3.2. Photocatalytic Activity Experiments
Due to the possibility of various reactions prevailing between components, mono-
and binary systems were defined as follows:
Interactions under irradiation; (i), light and sole MB revealing direct photolysis of
MB to be described by UV-vis spectral features, (ii), light and sole PANI specimens ex-
pressing stability under irradiation followed by UV-vis spectral analysis, (iii), light and
“MB and photocatalyst specimens” indicating photocatalysis described by UV-vis spec-
tral analysis, parameters as well as kinetic modeling.
Interactions under dark conditions: (i), sole PANI specimens expressing stability un-
der dark conditions followed by UV-vis spectral analysis, (ii), MB adsorption onto all pho-
tocatalyst specimens revealing surface interactions followed by UV-vis spectral features.
3.2.1. Direct Photolysis of MB
UV-vis spectrum of MB displayed one main absorption band with a maximum at λ =
664 nm (A664) with a shoulder at λ = 613 nm in visible region, while two minor bands were
located at λ = 246 nm (A246) and λ = 292 nm (A292) in the UV region. The conjugation system
between the dimethylamines through sulfur and nitrogen atoms in substituted aromatic
rings were responsible for the chromophores and auxochromes present in MB structure
(SI Part 1, Figure S2). The small shoulder at λ = 613 nm is the well-known effect of dye
dimmer [51,52]. The absorption peaks located in the UV region indicated the conjugated
system of phenothiazine species and benzene ring structures. Upon irradiation (Io = 1.65 ×
1016 quanta/sec), direct photodegradation of sole MB (10 mg/L) was not expected due to
the non-overlapping trend of BLF lamp output spectrum and MB absorption spectral fea-
tures. UV-vis spectral features were monitored for 60 min and no change was attained as
expected irradiation (SI Part 1, Figure S2).
3.2.2. Adsorptive Interactions in the Absence of Light
Since photocatalytic degradation of substrates occurs predominantly on the photo-
catalyst surface, adsorptive interactions of the components hold prime importance. Pri-
marily, these interactions prevailing between MB and either PANI-ES or PANI-EB were
investigated under dark conditions.
Insignificant amount of MB (A664: ~1%) was retained on the surface of both PANI
specimens. The pH of zero-point charge (pHpzc) of PANI-ES was 8.25 while PANI-EB ex-
hibited negative zeta potential at all pH values due to dedoping process via alkali treat-
ment [53]. Since MB as a cationic dye was positively charged, both repulsive (PANI-ES
and MB) and attractive (PANI-EB and MB) electrostatic interactions should be considered
under working pH ~4.5 conditions. However, insignificant initial adsorption extents could
simply imply that the binary system was far beyond equilibrium conditions as expected
[54]. Due to structural properties of PANI specimens (amine and imine groups) and MB,
N to N hydrogen bonding and π-π attractions should also be considered [53,55]. How-
ever, upon use of sole ZnO (pHzpc = 9), initial adsorption extent of MB (A664) was 15%
although both counterparts were positively charged most probably due to dominating
hydrophobic and van der Waals forces. The possible role of very low surface area of ZnO
(BET surface area = ~5 m2/g) should also be encountered [5].
Water 2021, 13, 1025 14 of 31
In the presence of PANI/ZnO composites the extent of surface interactions did not
display any consistent trend with respect to both the preparation method and PANI per-
centage (SI Part 1, Figure S3). All UV-vis absorbing groups of MB displayed almost similar
adsorptive behavior towards the surfaces of the photocatalyst specimens. Since MB is
known as a cationic dye expressing a positive charge, all of the surface interactions were
simplified by excluding pH dependent variations in electrostatic attractions. However,
one major difference was attained upon use of PZI-1 in comparison to the other specimens.
Highest adsorption was attained through attractions between the color forming groups
and PZI-1 surface followed by a decreasing order of A292 and A246 reflecting the binding
sites of MB. Moreover, PZI3-PZI-9 samples displayed almost similar adsorption extents
with ZnO whereas PZS specimens expressed sample specific behavior in all UV-vis pa-
rameters. The most remarkable difference was observed for PZS-6 and PZS-9 specimens
implying all counterparts present in the structure of the composites expressed diverse
tendencies towards MB. From a general perspective, PZS specimens were quite different
than PZI specimen indicating that the preparation method played the major role in surface
interactions that was the prerequisite step for surface-oriented nature of photocatalysis.
3.2.3. Photocatalytic Degradation of MB using PANI-ZnO Specimens
Fundamentally, upon irradiation with UV/visible light, semiconductors catalyze re-
dox reactions in presence of air/O2 and water. Through the formation of reactive oxygen
and radical species degradation/decomposition reactions take place. Under the specified
reaction conditions, photocatalytic progression was monitored by UV-vis spectral fea-
tures of MB during exposure of 0–150 min in 10 min intervals using all photocatalyst spec-
imens i.e., PANI-ES, PANI-EB, ZnO and PANI-ZnO composites (Figures 7 and 8).
200 300 400 500 600 700 800
900
0.0
0.5
1.0
1.5
2.0
PANI-ES
Initial
0 min
5 min
10 min
15 min
20 min
30 min
40 min
50 min
60 min
90 min
120 min
150 min
Absorbance
Wavelength, nm
200 300 400 500 600 700 800 900
0.0
0.5
1.0
1.5
2.0
Absorbance
Wavelength, nm
Initial
0 min
5 min
10 min
15 min
20 min
30 min
40 min
50 min
60 min
90 min
120 min
150 min
ZnO
200 300 400 500 600 700 800 900
0.0
0.5
1.0
1.5
2.0
PZI-1
Initial
0 min
5 min
10 min
15 min
20 min
30 min
40 min
50 min
60 min
90 min
120 min
150 min
Absorbance
Wavelength, nm
200 300 400 500 600 700 800 900
0.0
0.5
1.0
1.5
2.0
PZI-3
Initial
0 min
5 min
10 min
15 min
20 min
30 min
40 min
50 min
60 min
90 min
120 min
150 min
Absorbance
Wavelength, nm
Water 2021, 13, 1025 15 of 31
200 300 400 500 600 700 800 900
0.0
0.5
1.0
1.5
2.0
PZI-6
Initial
0 min
5 min
10 min
15 min
20 min
30 min
40 min
50 min
60 min
90 min
120 min
150 min
Absorbance
Wavelength, nm
200 300 400 500 600 700 800
900
0.0
0.5
1.0
1.5
2.0
PZI-9
Initial
0 min
5 min
10 min
15 min
20 min
30 min
40 min
50 min
60 min
90 min
120 min
150 min
Absorbance
Wavelength, nm
Figure 7. Ultraviolet-visible (UV-vis) absorption spectra of methylene blue (MB) using PANI-ES,
ZnO and PZI composites.
Water 2021, 13, 1025 16 of 31
200 300 400 500 600 700 800 900
0.0
0.5
1.0
1.5
2.0
PANI-EB
Initial
0 min
5 min
10 min
15 min
20 min
30 min
40 min
50 min
60 min
90 min
120 min
150 min
Absorbance
Wavelength, nm
200 300 400 500 600 700 800 900
0.0
0.5
1.0
1.5
2.0
Absorbance
Wavelength, nm
Initial
0 min
5 min
10 min
15 min
20 min
30 min
40 min
50 min
60 min
90 min
120 min
150 min
ZnO
200 300 400 500 600 700 800
900
0.0
0.5
1.0
1.5
2.0
PZS-1
Initial
0 min
5 min
10 min
15 min
20 min
30 min
40 min
50 min
60 min
90 min
120 min
150 min
Absorbance
Wavelength, nm
200 300 400 500 600 700 800
900
0.0
0.5
1.0
1.5
2.0
PZS-3
Initial
0 min
5 min
10 min
15 min
20 min
30 min
40 min
50 min
60 min
90 min
120 min
150 min
Wavelength, nm
Absorbance
200 300 400 500 600 700 800 900
0.0
0.5
1.0
1.5
2.0
PZS-6
Initial
0 min
5 min
10 min
15 min
20 min
30 min
40 min
50 min
60 min
90 min
120 min
150 min
Wavelength, nm
Absorbance
200 300 400 500 600 700 800
900
0.0
0.5
1.0
1.5
2.0
PZS-9
Initial
0 min
5 min
10 min
15 min
20 min
30 min
40 min
50 min
60 min
90 min
120 min
150 min
Absorbance
Wavelength, nm
Figure 8. UV-vis absorption spectra of MB using PANI-EB, ZnO and PZS composites.
From a general perspective, A664 gradually decreased indicating the degradation of
auxochrome group irrespective of the type of the photocatalyst specimens. Even though
A292 and A246 peaks were also gradually diminished revealing the degradation of the con-
jugated system of phenothiazine species and the benzene ring structures, no new bands
were recorded indicating that no further transformation products or intermediates were
formed.
During photocatalysis, the color of MB solution became less intensive due to hypo-
chromic effect degrading all or part of the auxochrome groups related to methyl or me-
thylamine under UV light irradiation [56]. The reason of the blue-shift could be explained
by N-demethylation of MB via oxidative degradation (SI Part 1, Figure S4). These absorp-
tion bands recorded at wavelengths as 655–648 nm (tirr = 20 min), 634–620 nm (tirr = 40
min), 606–614 nm (tirr = 60 min) and 604 nm (tirr = 90 min) were attributed to Azure B,
Azure A, Azure C and thionine (N-demethylated intermediates of MB), respectively [56–
59].
In a similar fashion, A246 and A292 also decreased significantly and vanished com-
pletely upon prolonged irradiation periods i.e., in 150 min. There were no new absorption
Water 2021, 13, 1025 17 of 31
bands in UV spectra indicating complete oxidative degradation of the phenothiazine spe-
cies has occurred and any intermediate containing a phenothiazine moiety was not
formed [56]. The maximum intensity diminished slightly, without revealing any blue-shift
in the UV-vis spectrum of both PANI-ES and PANI-EB specimens.
The underlying mechanism of photocatalytic degradation of MB was presented by
Lin and colleagues revealing various intermediates and final products initiated by sole
attack of hydroxyl radical directly to the N-S- conjugated linkage of the aromatic sub-
groups. The plausible pathway was explained by irradiation time dependent formation
and successive removal of substances defined by ESI-MS m/z values primarily as
C8H11N2SO3 and C8H12N2. Upon further irradiation periods, C6H4NSO5, C6H6SO3, C6H10O2,
C8H11NO and C6H6O2 were formed which were also degraded to C3H6O2 and C3H4O4 [60].
Upon use of the specified specimens, considering the hydroxyl radical as the main oxida-
tive species a similar mechanism could be expected. Each of these compounds could also
contribute to the UV-vis spectral features under continuously and simultaneously operat-
ing formation and removal pathways during photocatalysis.
Prior to treatment of MB using either PANI-ES or PANI-EB, stability tests of these
specimens were performed both under dark and irradiation conditions. Similar reaction
settings (reactor configuration) were applied using a constant dose of 0.25 mg/mL in aque-
ous solution for a period of 60 min. Possible release of any organics were monitored by
UV-vis spectra following double filtration through 0.45 µm and 0.20 µm membrane filters.
No detectable absorbance was recorded indicating that both PANI-ES and PANI-EB were
stable under the specified experimental conditions.
3.2.4. Kinetics of Photocatalytic Degradation
As evidenced by spectral features (Figures 7 and 8), all UV-vis parameters (A664, A292
and A246) displayed logarithmic decay profiles (SI, Part II, Figures S5–S9) following pseudo
first-order kinetic model expressed by Equation (7):
Rate (R)= − dA
dt = kA
(7)
where,
R: pseudo first order rate (cm−1min−1),
Ao: initial absorbance of MB expressed as A664, o, A292,o and A246,o,
A: absorbance of MB expressed as A664, A292 and A246 at time t,
t: irradiation time, min,
k: pseudo first order reaction rate constant, min−1.
Half-life (t1/2, min) could easily be calculated by the following equation, t1/2 = 0.692/k.
Kinetic model parameters (R2 > 0.85) were presented in Table 1.
Upon use of ZnO, the decolorization rate constant of MB (A664) was calculated as k =
5.39 × 10−2 min−1 and rate as R = 9.70 × 10−2 cm−1 min−1. Degradation of aromatic domains
revealed rate constants as k = 3.32 × 10−2 min−1 and 2.36 × 10−2 min−1 for A292 and A246, re-
spectively. Respective half-life (t1/2, min) values for A664 were in the range of 9.73–12.9 and
in the following ranges as 17.9–22.2 and 27.9–35.9 for A292 and A246, respectively.
PANI-ZnO composites expressed enhanced photocatalytic activities in an incon-
sistent trend irrespective of the photocatalyst specimen types. PANI acts as a photosensi-
tizer with a narrow band gap, Ebg = 2.8 eV since the excitation between HOMO and LUMO
levels is low [24,26]. The reason could also be explained by an electron transfer from the
valence band of PANI to the conduction band of ZnO under irradiation because of their
excellent matching of energy levels [43]. Thus, hydroxyl radicals were synchronously
formed which were the most important reactive oxygen species in photocatalytic degra-
dation processes [14]. Comparison of two methods revealed that the composites prepared
by hybridization method (PZS’s) exhibited better photocatalytic activity compared to the
photocatalyst specimens prepared by in-situ polymerization method (PZI’s). A plausible
Water 2021, 13, 1025 18 of 31
explanation could be related to the SEM images of PZI composites displaying more ag-
gregated particles than PZS composites. The fastest decolorization rate constant was
achieved upon use of PZS-3 among PZS composites and PZI-6 among PZI composites.
Upon use of all specimens the trend in photocatalytic degradation rate constants
could be presented in a decreasing order as:
A664: PZS-3 > PZS-6 > PZS-1 > PZI-6 > PZI-3 > PZI-1 = PZS-9 > PZI-9 > ZnO.
A292: PZS-3 > PZS-6 > PZS-1 > PZI-6 > ZnO > PZS-9 > PZI-3 > PZI-1 > PZI-9.
A246: PZS-6 > PZS-3 > ZnO > PZS-1> PZS-9 > PZI-6 > PZI-3 > PZI-9 > PZI-1.
It could also be plausible that through “breakage” of MB structure via N- and S- link-
ages, formation of new aromatic structures possibly depressed the removal kinetics of A292
and A246.
Table 1. Photocatalytic degradation kinetics of MB expressed by A664, A292 and A246.
First Order Kinetic Parameters
A664 k × 10−2, min−1 t1/2, min Rate, cm−1 min−1
ZnO 5.39 12.9 0.0970
PZI-1 5.73 12.1 0.105
PZI-3 5.85 11.8 0.107
PZI-6 6.14 11.3 0.112
PZI-9 5.55 12.4 0.102
PZS-1 6.47 10.7 0.118
PZS-3 7.13 9.73 0.130
PZS-6 6.82 10.2 0.125
PZS-9 5.73 12.1 0.105
A292
ZnO 3.32 20.9 0.0359
PZI-1 3.18 21.8 0.0343
PZI-3 3.22 21.5 0.0348
PZI-6 3.34 20.7 0.0361
PZI-9 3.12 22.2 0.0337
PZS-1 3.51 19.8 0.0379
PZS-3 3.87 17.9 0.0418
PZS-6 3.82 18.1 0.0413
PZS-9 3.28 21.1 0.0355
A246
ZnO 2.36 29.4 0.0120
PZI-1 1.93 35.9 0.00983
PZI-3 2.07 33.6 0.0105
PZI-6 2.10 33.0 0.0107
PZI-9 2.00 34.7 0.0102
PZS-1 2.24 30.9 0.0114
PZS-3 2.43 28.5 0.0124
PZS-6 2.48 27.9 0.0126
PZS-9 2.19 31.7 0.0111
Photocatalytic activity results showed that PANI amount played an important role
on the photocatalytic decolorization process. The lowest photocatalytic rate constants
were attained in the composites containing the highest PANI concentration (PZI-9 and
PZS-9) prepared by both methods due to aggregation issue observed in SEM images. The
decolorization rate constants attained upon use of PZS-9 and PZI-1 composites were the
same as k = 5.73 × 10−2 min−1 (Table 1). In addition, the surface coverage of these composites
was also the same (~17%) and slightly lower than the other composites and ZnO. The
Water 2021, 13, 1025 19 of 31
PANI amount in composites influenced the adsorption extents of MB due to the different
preparation methods. The composite containing the highest PANI concentration by hy-
bridization method while the one containing the lowest PANI amount by in-situ oxidation
polymerization method exhibited the lowest initial adsorption. In PZS method, PANI-EB
was used to prepare PZS composite while protonated form of PANI (PANI-ES) was used
in PZI method. The reason could be explained by the fact that PANI-EB was more efficient
in adsorption of anionic dyes than PANI-ES [22]. This implied that the lower molar ratio
of PZS sample containing PANI-ES exhibited the same surface coverage with PZS-9. Be-
sides pre-adsorption conditions, under irradiation, surface interactions via adsorption
and desorption processes would take place continuously within the species present in the
reaction medium.
Furthermore, the degree of MB decolorization by using PANI-ES, PANI-EB, sole ZnO
and PANI-ZnO composites (Figures 7–8) was also calculated by the following Equation
(8):
Decolorization, %= ((A664,o − A664)/ A664,o)) x 100 (8)
where A664,o = initial absorbance of MB and A664 = absorbance of MB at irradiation time t.
A slight decolorization percentage of MB was observed in the presence of PANI-ES,
ca 14% under 60 min UVA irradiation. This result proved that PANI-ES had a minor abil-
ity to decolorize MB under UV light. The removal percentage of PANI-EB was two times
higher (ca 28%) than PANI-ES. As the irradiation time increased the decolorization of MB
also increased slightly.
Decolorization percentages attained upon use of sole ZnO and PANI-ZnO compo-
sites were found to be directly proportional to irradiation time. A prolonged period up to
120 min resulted in an almost complete decolorization of MB upon use of sole ZnO and
PANI-ZnO composites. After 150 min irradiation time, complete decolorization was
achieved due the cleavage of peaks that belong to the auxochrome and chromophore
groups. From a general perspective, decolorization percentage of MB from aqueous solu-
tion upon use of PANI-ZnO composites was slightly higher as compared to sole ZnO fol-
lowing 60 min of irradiation.
Furthermore, the decolorization percentage of MB in the presence of PZI-1 exhibited
the highest performance following 60 min irradiation upon use of all PZI samples, while
PZS-3 and PZS-6 composites revealed almost highest decolorization percentages. Con-
versely, the decolorization percentage of MB in the presence of both PZI-9 and PZS-9 com-
posites was considerably lower than other PANI-ZnO composites. The reason could be
attributed to the fact that agglomeration expressed a negative effect on photocatalysis by
diminishing light absorption thereby influencing the photocatalytic activity [61,62]. This
implied that the formation agglomerated particles resulted in a decrease on photocatalytic
degradation in agreement with SEM results. Besides, it was also demonstrated that the
photocatalytic activity decreased with an enhancement of polymer content in synthesized
photocatalysts [29].
Photocatalytic degradation of MB was also related to concentration through using
calibration curves mainly revealing molar absorption coefficients. Since MB solution
could be considered primarily composed of a monomer structure with a distinct absorp-
tion maximum at 664 nm (A664) and absorption coefficient as ε = 39,440 cm−1M−1. In a sim-
ilar manner, absorption coefficients were calculated as ε = 30,749 cm−1M−1 and as ε = 15,256
cm−1M−1 for A292 and A246, respectively. Degradation efficiencies were found to be ~20%
lower than the removal efficiencies expressed by A664, A2924 and A264. The reason could be
attributed to the minor blue shifts in spectral profiles especially observed in higher con-
centrations. Therefore, no explicit correlation could be demonstrated between the removal
of UV-vis parameters and MB concentration.
Most of the studies using MB as the model substrate excluded UV spectral features
(A292 and A264) and considered only decolorization (A664) as the main reaction pathway to
explain decomposition and or degradation. It is highly recommended that decolorization
Water 2021, 13, 1025 20 of 31
should also be supported by mineralization extents expressed by DOC contents. Moreo-
ver, besides Cl−, release of oxyanions of N (NO3−) and S (SO42−) could also be considered as
by-products of decolorization. However, under the specified experimental conditions, all
of these ions could not be detected most probably due to the adsorptive removal by se-
quential filtration thereby being below the detection limits of the applied methods.
It should also be mentioned that the main interest in this study was directed to the
preparation of the PANI modified ZnO specimens using two different methodologies ra-
ther than focusing on the degradation pathways of MB under the specified experimental
conditions.
3.2.5. Photocatalyst Dose Effect
Photocatalyst dose effect was investigated using 0.125 g/L, 0.25 g/L and 0.50 g/L of
each specimen as PANI-ES, PANI-EB (Figure 9), ZnO (Figure 10) and PANI-ZnO compo-
sites (Figures 11 and 12) and MB (10 mg/L) for irradiation periods of 30 min and 60 min.
Initial surface coverage extents and degradation of MB was expressed by decolorization
as A664.
Figure 9. Photocatalytic degradation of MB upon use of PANI-ES and PANI-EB: Dose effect.
From a general perspective, dose effect of both PANI specimens reflected the same
increasing trend with respect to increase from 0.125 g/L to 0.25 g/L followed by a decrease
upon use of 0.50 g/L. Lower doses such as 0.125 g/L exerted specimen specific and irradi-
ation time dependent variations, such as decolorization attained by use of PANI-ES in 60
min which were slightly higher than PANI-EB in 30 min. Decolorization efficiencies at-
tained by using PANI-EB were higher compared to PANI-ES (<15%) irrespective of the
dose amount most possibly due to the enhanced electrostatic attractions under both irra-
diation periods. The lowest decolorization was attained upon use of 0.125 g/L PANI-ES in
30 min and the highest decolorization was observed upon use of 0.25 g/L PANI-EB.
Water 2021, 13, 1025 21 of 31
Figure 10. Photocatalytic degradation of MB upon use of sole ZnO: Dose effect.
Surface coverage of MB onto either PANI-ES or PANI-EB (mg/g) with respect to in-
creasing dose reflected the following inconsistent trend as:
PANI-ES: 1.99 > 1.14 > 0.394 for 0.125 g/L, 0.50 g/L and 0.25 g/L, respectively.
PANI-EB: 3.28 > 2.14 > 0.460 for 0.125 g/L, 0.50 g/L and 0.25 g/L, respectively.
The reason could be attributed to the irregular dandelion shape of the PANI speci-
mens (Figures 5 and 6).
Surface coverage of MB onto ZnO (mg/g) with respect to increasing dose reflected as
an inconsistent trend; 6.04 > 5.77 > 2.50 for 0.25 g/L, 0.125 g/L and 0.50 g/L, respectively.
Even though increasing ZnO dose on initial adsorption followed a remarkable increase
(7–15%) then a slight decrease (12.5%), a continuous increase in decolorization efficiencies
were attained for irradiation periods of 30 min and 60 min irrespective of the applied ZnO
dose. The lowest decolorization efficiency was attained as 85% for ZnO dose of 0.125 g/L
however almost similar 97% decolorization efficiency was attained upon use of both 0.25
g/L and 0.50 g/L. Therefore, it could be demonstrated that 0.25 g/L dose could be regarded
as suitable for photocatalytic treatment of MB using sole ZnO under BLF light irradiation.
Upon use of all PZI specimens, an inconsistent trend was observed with respect to
initial adsorption extents (Figure 11). Lower dose as 0.125 g/L displayed sample specific
initial adsorption extents in an order as PAZI-3 > PZI-1 > PZI-6 > PZI-9. On the other hand,
high dose as 0.50 g/L reflected the effect of increasing PANI to ZnO ratio in PZI specimens.
All PZI specimens followed a similar trend as 0.25 g/L, 0.125 g/L and 0.50 g/L except for
PZI-3 as a decreasing trend with respect to increasing dose.
PZI-1 specimen:7.12 > 4.63 > 1.68 for 0.25 g/L, 0.125 g/L and 0.50 g/L, respectively.
PZI-3 specimen: 5.96 > 5.64 > 2.04 for 0.125 g/L, 0.25 g/L and 0.50 g/L, respectively.
PZI-6 specimen: 6.08 > 4.06 > 2.76 for 0.25 g/L, 0.125 g/L and 0.50 g/L, respectively.
PZI-9 specimen: 6.00 > 3.12 > 2.96 for 0.25 g/L, 0.125 g/L and 0.50 g/L, respectively.
During photocatalysis, an increase in decolorization efficiency was more remarkable
by doubling from 0.125 g/L to 0.25 g/L in comparison to increase to 0.50 g/L irrespective
of the PZI specimen type and initial surface coverage effects. On the other hand, upon
longer irradiation period of 60 min, an average increase in decolorization percentages of
MB from ~70% to ~90% was attained with increasing the dosage from 0.125 g/L to 0.50 g/L.
The reason of this could be related to the possibility of the enhanced light harvesting ca-
pacity due to the increase in exposed surface area. However, working dose of 0.25 mg/mL
was selected as evidenced by high removal efficiencies.
From a general perspective, upon use of all PZS specimens, a continuous increasing
trend was observed in initial adsorption extents with respect to increasing dose covering
Water 2021, 13, 1025 22 of 31
sample specific differences within each dose (Figure 12). Lower dose as 0.125 g/L dis-
played more significant variations in surface coverage extents in an order as PZS-6 > PZS-
9 > PZS-3 > PZS-1 (<10%). However, high dose as 0.50 g/L reflected a different trend as
PZS-9 > PZS-1 ≈ PZS-3 > PZS-6 with a very slight insignificant change as 1%.
Figure 11. Photocatalytic degradation of MB upon use of PZI specimens: Dose effect.
Water 2021, 13, 1025 23 of 31
Figure 12. Photocatalytic degradation of MB upon use of PZS specimens: Dose effect.
All PZS specimens followed a similar trend 0.25 g/L, 0.125 g/L and 0.50 g/L except for
PZS-6 as a decreasing trend with respect to increasing dose.
PZS-1 specimen: 5.76 > 2.55 ≈ 2.50 for 0.25 g/L, 0.125 g/L and 0.50 g/L, respectively.
PZS-3 specimen: 6.68 > 3.11 > 2.46 for 0.25 g/L, 0.125 g/L and 0.50 g/L, respectively.
PZS-6 specimen: 6.24 > 4.04 > 2.30 for 0.125 g/L, 0.25 g/L and 0.50 g/L, respectively.
PZS-9 specimen: 6.92 > 3.78 > 2.76 for 0.25 g/L, 0.125 g/L and 0.50 g/L, respectively.
Upon photocatalysis, increase in decolorization efficiency was more remarkable by
doubling the photocatalyst from 0.125 g/L to 0.25 g/L in comparison to increase to 0.50 g/L
Water 2021, 13, 1025 24 of 31
with minor variations with respect to PZS specimen. Dose effect was not found to be in
correlation to PANI/ZnO ratio.
3.2.6. Effect of Initial Dye Concentration
Effect of initial MB concentration was investigated in the range of 5–30 mg/L (5 mg/L,
10 mg/L, 20 mg/L, 30 mg/L) in the presence of all photocatalyst specimens and expressed
by decolorization percentages (A664).
Initial dye concentration exerted a negligible effect on the surface adsorption of MB
onto both PANI-ES and PANI-EB samples (<10%) under dark conditions from 10 mg/L
onwards (Figure 13). Due to prevailing attractive forces, PANI-EB was more effective than
PANI-ES, however the effect was almost equalized for 30 mg/L due to higher MB concen-
tration with respect to exposed surface area. As a comparison, adsorbed amount/adsor-
bent (mg/g) could be simply expressed as:
PANI-ES specimen: 3.13 > 2.26 > 0.816 > 0.394 for 30 mg/L, 20 mg/L, 5 mg/L and 10
mg/L, respectively.
PANI-EB specimen: 3.54 > 3.43 > 1.31 > 0.460 for 20 mg/L, 30 mg/L, 5 mg/L and 10
mg/L, respectively.
Figure 13. Photocatalytic degradation of MB upon use of PANI-ES and PANI-EB: Initial MB con-
centration effect.
In this respect, upon irradiation, increasing MB concentration resulted in an incre-
mental decreasing decolorization trend although remarkable increases were attained due
to increased exposure periods being more significant for PANI-EB for lower MB concen-
trations.
Considering the very low surface area of ZnO (5 m2/g), initial surface coverage extent
decreased from 21% to 8.5% upon increase from 5 mg/L to 30 mg/L MB possibly due to
the decreased surface area with respect to the amount of substrate (Figure 14). Adsorbed
amount of MB/ZnO (mg/g) could be simply expressed as; 10.3 ≈ 10.0 > 6.04 > 4.12 for 30
mg/L, 20 mg/L, 10 mg/L and 5 mg/L, respectively.
Accordingly, the decolorization efficiencies also decreased being more significant for
higher amounts of MB. Even though higher MB decolorization efficiency was attained for
lower concentration as 5 mg/L that could be related to the availability of the exposed area.
Water 2021, 13, 1025 25 of 31
Figure 14. Photocatalytic degradation of MB upon use of sole ZnO: Initial MB concentration effect.
Upon use of PZI specimens, prior to initiation of light exposure, all samples dis-
played variations in surface coverage extents (A664 < 20%) irrespective of MB concentration
and specimen type (Figure 15). The highest adsorption of MB onto PZI-1 was 18% and the
lowest was 5% onto PZI-9. In a comparative manner, the initial adsorbed amount of MB
onto PZI specimens were expressed as follows;
PZI-1 specimen: 7.95 > 7.12 > 6.86 > 2.86 for 20 mg/L, 10 mg/L, 30 mg/L and 5 mg/L,
respectively.
PZI-3 specimen: 9.30 > 6.90 > 5.64 > 2.42 for 30 mg/L, 20 mg/L, 10 mg/L and 5 mg/L,
respectively.
PZI-6 specimen: 8.81 > 6.38 > 6.08 > 2.18 for 30 mg/L, 20 mg/L, 10 mg/L and 5 mg/L,
respectively.
PZI-9 specimen: 6.49 > 6.00 > 5.60 > 2.66 for 20 mg/L, 10 mg/L, 30 mg/L and 5 mg/L,
respectively.
Accordingly, upon photocatalysis, efficiencies of decolorization were lower in the
presence of 30 mg/L MB both for 30 min and 60 min irradiation periods. Even though 5
mg/L MB displayed highest removal efficiencies as the working MB concentration 10
mg/L could be visualized as the highest concentration revealing decolorization efficiencies
>50 % irrespective of the PZI type.
On the other hand, use of PZS specimens expressed considerably similar inconsistent
initial adsorption extent trend (A664 < 20%) (Figure 16). The highest adsorption of MB onto
PZS-9 was 18% and the lowest was 3.4% onto PZS-6. On the other hand, the adsorbed
amount of MB onto PZS specimens were presented in the following manner in terms as
mg/g;
PZS-1 specimen: 6.28 > 5.76 > 4.84 > 2.10 for 20 mg/L, 10 mg/L, 30 mg/L and 5 mg/L,
respectively.
PZS-3 specimen: 8.48 > 6.68 > 5.52 > 2.78 for 20 mg/L, 10 mg/L, 30 mg/L and 5 mg/L,
respectively.
PZS-6 specimen: 6.85 > 4.49 > 4.04 > 1.95 for 20 mg/L, 30 mg/L, 10 mg/L and 5 mg/L,
respectively.
PZS-9 specimen: 6.92 > 6.51 > 4.74 > 2.26 for 10 mg/L, 20 mg/L, 30 mg/L and 5 mg/L,
respectively.
Water 2021, 13, 1025 26 of 31
Figure 15. Photocatalytic degradation of MB upon use of PZI specimens: Initial MB concentration
effect.
Consequently, upon irradiation during photocatalysis, decolorization efficiencies
were found to be lower in the presence of 30 mg/L MB both for 30 min (<25%) and 60 min
(<50%) irradiation periods. The reason could be attributed to the masking effect resulting
in diminished light intensity thereby reducing the capacity of generation of reactive oxy-
gen species especially hydroxyl radicals [58]. In a similar fashion to PZI samples, although
Water 2021, 13, 1025 27 of 31
5 mg/L MB displayed highest removal efficiencies, 10 mg/L MB concentration could be
regarded as the working concentration revealing decolorization efficiencies >80 % irre-
spective of the PZS type.
Figure 16. Photocatalytic degradation of MB upon use of PZS specimens: Initial MB concentration
effect.
Comparison of PZI and PZS specimens demonstrated that both preparation methods
could be considered as successful in case that MB as a cationic dye was used as the model
Water 2021, 13, 1025 28 of 31
compound and the light source was specially chosen to exclude any direct photolysis of
the substrate. Under all conditions including PANI-ES, PANI-EB and even ZnO, surface
adsorption extents were very low due to surface-oriented nature of photocatalysis. More-
over, since uniform encapsulating of ZnO was not visualized by SEM images, partial sur-
face area of ZnO could be directly exposed to irradiation. The role of PANI could possibly
be related to the “harvesting of MB” to the close proximity of ZnO core during photoca-
talysis. On the other hand, very high efficiencies were attained as expressed by UV-vis
parameters (A664, A292 and A264) expressing both decolorization and removal of aromatic
structures thereby decomposition and degradation of MB.
Based on the above presented information, the mechanism and kinetics of MB deg-
radation upon use of PANI-ZnO composites deserve to be investigated under simulated
solar light conditions and by using different model dye compounds.
Furthermore, as a novel approach the release of these “emerging nanoparticles” such
as PANI-ZnO composites also deserve to be investigated with respect to their possible
ecotoxicological impacts on the environment [63,64].
4. Discussion
The core part of this study is the preparation of PANI-modified ZnO photocatalysts
by using two different methods. To achieve this goal, PANI-ZnO composites were suc-
cessfully synthesized by (i), in situ polymerization in neutral media and (ii), hybridization
(physical blending) methods to overcome the dissolution problem of ZnO. PANI/ZnO
composites were prepared in various ratios 1%, 3%, 3% and 9%. As intermediates thus
formed PANI-ES and, PANI-EB, sole ZnO as the base photocatalyst and PANI-ZnO com-
posites were all characterized by FT-IR, Raman spectroscopy, XRD and SEM-EDAX tech-
niques. The presence of PANI in PANI-ZnO composites were proven with the determina-
tion of functional groups by FT-IR and Raman Spectroscopy. The surface defects and the
interactions between PANI and ZnO were observed by Raman Spectroscopy. A remarka-
ble change was not observed on the crystallite sizes of ZnO and PANI-ZnO composites.
The flower like shape of PANI was seen with a deformation in composites and the ag-
glomeration was enhanced with an increase in PANI concentration. Besides, the existence
of PANI in composites was confirmed by EDAX spectra. In SEM images of PZI compo-
sites, more aggregation was detected compared to PZS samples. In addition, characteri-
zation techniques revealed the structural and morphological differences of PANI-ES and
PANI-EB.
The synergic effect between PANI and ZnO was examined by the photocatalytic deg-
radation of MB. As prepared by using either way, PANI modified photocatalyst speci-
mens exhibited a comparatively better photocatalytic activity than ZnO. By comparing
the calculated rate constants, the prepared composites by PZS method were more success-
ful to degrade MB compared to the ones prepared by PZI method. It was observed that
the dose effect of photocatalyst efficiency did not play a consistent effect on the degrada-
tion of MB. In a similar manner, initial concentration effect also displayed variations with
respect to experimental conditions. As an outcome of this study, it could be emphasized
that the synthesized composite photocatalysts especially PZS-3 could function better than
the bare ZnO photocatalyst under the specified experimental conditions. The functionality
expected by PANI modification should be related to the system specifications and opera-
tional conditions. Due to the applied sequential filtration process to obtain clear sample
for spectroscopic analysis, reusability experiments were not considered. Therefore, devel-
opment of one-step removal of PANI-ZnO specimens from the reaction medium it is
highly recommended. This study presents the variations in performance of PANI-ZnO
photocatalyst with respect to the preparation methods through a systematical approach
to fulfill baseline study requirements. Further research is highly recommended to eluci-
date the effect of solar light using various model dyes.
Water 2021, 13, 1025 29 of 31
Supplementary Materials: The following are available online at www.mdpi.com/2073-
4441/13/8/1025/s1, Figure S1: A schematic of PANI-ZnO preparation method including the ways
preventing ZnO dissolution composites. Figure S2: Time dependent UV-vis spectra of MB under
irradiation and BLF lamp output spectra. Figure S3: Dark interactions between MB and photocata-
lyst specimens. Figure S4: Chemical structures of MB and related compounds. Figure S5: Irradiation
time dependent UV-vis absorption spectra of MB using (a) PANI-ES and (b) PANI-EB. Figure S6:
Photocatalytic degradation of MB in the presence of PANI-ES and PANI-EB in comparison to pho-
tolysis of MB. (a) A664, (b) A292, and (c) A246. Figure S7: Decolorization A664 kinetic plots of MB for (a)
ZnO and PZI composites, and (b) ZnO and PZS composites. Figure S8: Decolorization A292 kinetic
plots of MB for (a) ZnO and PZI composites, and (b) ZnO and PZS composites. Figure S9: Decolor-
ization A246 kinetic plots of MB for (a) ZnO and PZI composites, and (b) ZnO and PZS composites.
Author Contributions: Conceptualization, Y.K., M.B. and N.T.; methodology, Y.K. and N.T.; inves-
tigation, N.T.; writing—original draft preparation, N.T.; writing—review and editing, M.B. and
Y.K.; project administration, Y.K. and N.T.; funding acquisition, Y.K. and N.T. All authors have read
and agreed to the published version of the manuscript.
Funding: Financial support provided by Research Fund of Kirsehir Ahi Evran University through
Project FEF.A4.20.008 and FEF.A4.20.009 is gratefully acknowledged.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The data presented in the study is available on request from the cor-
responding author.
Acknowledgments: The authors are thankful to Bogazici University Advanced Technologies Re-
search & Development Center for SEM-EDAX and XRD measurements.
Conflicts of Interest: The authors declare that they have no known competing financial interests or
personal relationships that could have appeared to influence the work reported in this paper.
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