Mechanisms on the enhanced sterilization performance of fluorocarbon resin composite coatings modified by g-C3N4/Bi2MoO6 under the visible-light

Article abstract of DOI:10.1016/j.jphotochem.2017.09.043

Photocatalysts, which are produced by strong oxidizing groups ([rad]OH, h⁺, [rad]O₂^?), could solve marine biofouling problems efficiently. A number of studies showed that the microbial attachment was the original of marine biological attachment, and oxidizing groups inhibit the bacterial activity. In present work, the hybrid g-C₃N₄/Bi₂MoO₆ photocatalysts were synthesized via a sonochemical method. The successful combination of g-C₃N₄ and Bi₂MoO₆ was characterized by various physicochemical techniques, such as XRD, SEM, TEM, UV–vis and EIS test. As the results showed, the sterilization performance of the PEVE composite coatings was enhanced after adding g-C₃N₄/Bi₂MoO₆ under the visible-light irradiation. When the content of g-C₃N₄ in Bi₂MoO₆ was 7%, the sterilization performance of composite coatings was prominent under visible light irradiation for 4 h. This finding was attributed to the increased separation efficiency of the photogenerated electron-hole pairs of Bi₂MoO₆ by g-C₃N₄, which contributed to the generation of superoxide radicals ([rad]O₂^?). Meanwhile, we also tested the stability and sterilization performance of 7% g-C₃N₄/Bi₂MoO₆/PEVE coating, the results demonstrated that the g-C₃N₄/Bi₂MoO₆/PEVE coating has good stability and cycle performance. This work has great significance in the photocatalyst technology as a coating for killing marine bacteria to address marine biofouling problems.

Full text of DOI:10.1016/j.jphotochem.2017.09.043

Journal of Photochemistry and Photobiology A: Chemistry 350 (2018) 10–16
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Invited feature article
Mechanisms on the enhanced sterilization performance of
fluorocarbon resin composite coatings modified by g-C3N4/Bi2MoO6
under the visible-light
*
Yu Tian, Feng Zhou , Su Zhan, Zhenyu Zhu, Qiuchen He
Key Laboratory of Ship-Machinery Maintenance and Manufacture for Ministry of Transport, Dalian Maritime University, Dalian 116026, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 31 May 2017
Received in revised form 18 September 2017
Accepted 19 September 2017
Available online 22 September 2017
Photocatalysts, which are produced by strong oxidizing groups (ꢀOH, h+, ꢀO2ꢁ), could solve marine
biofouling problems efficiently. A number of studies showed that the microbial attachment was the
original of marine biological attachment, and oxidizing groups inhibit the bacterial activity. In present
work, the hybrid g-C3N4/Bi2MoO6 photocatalysts were synthesized via a sonochemical method. The
successful combination of g-C3N4 and Bi2MoO6 was characterized by various physicochemical
techniques, such as XRD, SEM, TEM, UV–vis and EIS test. As the results showed, the sterilization
performance of the PEVE composite coatings was enhanced after adding g-C3N4/Bi2MoO6 under the
visible-light irradiation. When the content of g-C3N4 in Bi2MoO6 was 7%, the sterilization performance of
composite coatings was prominent under visible light irradiation for 4 h. This finding was attributed to
the increased separation efficiency of the photogenerated electron-hole pairs of Bi2MoO6 by g-C3N4,
which contributed to the generation of superoxide radicals (ꢀO2ꢁ). Meanwhile, we also tested the stability
and sterilization performance of 7% g-C3N4/Bi2MoO6/PEVE coating, the results demonstrated that the
g-C3N4/Bi2MoO6/PEVE coating has good stability and cycle performance. This work has great significance
in the photocatalyst technology as a coating for killing marine bacteria to address marine biofouling
problems.
Keywords:
g-C3N4
Bi2MoO6
Fluorocarbon resin coatings
Sterilization
Visible-light
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
performance for Escherichia coli, and Staphylococcus aureus [4].
Under light irradiation, the electrons (eꢁ) and holes (h+) generated
With the development of internal trade, the ships play
indispensable roles in trade contracts. However, the marine
biofouling is one of the worldwide problems in marine systems,
which results from the accumulation of algae, marine plants, and
colonization of marine microorganisms, causing a series of serious
economical and safety problems [1]. Traditionally, the application
of antifouling coatings for releasing toxic substances and polluting
marine environment has been banned [2], thus, many researches
are focusing on novel, efficient methods and technologies to solve
the marine problems.
Semiconductor-based photocatalysis technology is a novel and
green technique, which uses sunlight as the energy source to
degrade pollutants and kill bacteria [3]. Existing studies showed
that many researches focus on evaluating the photocatalytic
antibacterial activity of TiO2, confirming its effective antibacterial
by the semiconductor will react with water or air to generate active
oxidizing radicals (h+, ꢀOH, ꢀO2ꢁ), which directly contact and
decompose the pollutants or bacteria [5]. However, TiO2 can only
absorb ultraviolet light because of its large band gap (Eg = 3.2 eV)
[6–8], resulting in low photocatalytic activity under sunlight
conditions. Hence, its application is limited. Therefore, numerous
kinds of visible light active photocatalysts have been developed.
Bismuth molybdate compounds are catalysis materials, and
among the various bismuth-based materials, recent research
studies have shown that Bi2MoO6 possess excellent visible light
photocatalytic properties because of its small band gap
(Eg = 2.5–2.8 eV) [9,10]. Ma et al. reported that the Bi2MoO6
photocatalysts were synthesized by a facile impregnation method
for the highly improved photoelectrochemical (PEC) property
related to water oxidation [11]. Li et al. found that Bi2MoO6 could
be used to kill bacteria under visible light [12]. However, compared
with other semiconductor materials, the photocatalytic perfor-
mance of Bi2MoO6 is limited by the high recombination rate of
photoinduced electrons and holes and by the relatively poor
* Corresponding author.
E-mail address: zhoufeng99@mails.tsinghua.edu.cn (F. Zhou).
https://doi.org/10.1016/j.jphotochem.2017.09.043
1010-6030/© 2017 Elsevier B.V. All rights reserved.

Y. Tian et al. / Journal of Photochemistry and Photobiology A: Chemistry 350 (2018) 10–16
11
quantum yield [11]. Among the numerous methods of improving
the photocatalytic performance of Bi2MoO6, the use of structural
as 5% CN/BMO). Following the above method, other
g-C3N4/Bi2MoO6 composite photocatalytic samples with different
weight content of g-C3N4 in composite of 3%, 7% and 15% were
prepared (noted as 5% CN/BMO, 7% CN/BMO, and 15% CN/BMO).
p
material to modify Bi2MoO6 is efficient [11,12]. The g-C3N4 has a
narrow band gap (Eg = 2.7 eV), thus, it can absorb visible light,
thereby increasing the separation efficiency of electron-hole pairs
by forming electronic interactions with photocatalysts during the
catalytic reaction [13–15]. Liang et al. reported that the Bi2MoO6-g-
C3N4 composite photocatalysts were synthesized by a facile
impregnation method [16]. Lv et al. reported that g-C3N4/Bi2MoO6
composite photocatalysts have the significant photocatalytic
performance than that of g-C3N4 and Bi2MoO6 photocatalyst,
respectively [17]. Ma et al. synthesized g-C3N4/Bi2MoO6 composite
photocatalysts by a hydrothermal deposition method and the
composite photocatalysts photocatalytic activity was improved
[18]. Yan et al. Used the template-free solvothermal method to
synthesize the g-C3N4/Bi2MoO6 photocatalysts, and they got the
results that the as-prepared samples exhibited superior photo-
catalytic activity towards the degradation of dyes (Rhodamine B
and Methyl blue) under visible light irradiation [19]. But, in
addition, almost no studies on g-C3N4/Bi2MoO6 composites for
photocatalytic disinfection of bacteria have been reported so far.
Hence, we proceeded a successful attempt to synthesize the
hybrid g-C3N4/Bi2MoO6 photocatalysts via a sonochemical method
[20], to enhance the sterilization performance of fluorocarbon
resin (PEVE) coatings. The photocatalytic sterilization test was
performed to evaluate the antibacterial activity of
2.3. Preparation of g-C3N4/Bi2MoO6/PEVE coatings
Initially, the g-C3N4/Bi2MoO6 sample prepared by the above-
mentioned method was mixed with fluorocarbon resin (PEVE)
paint at a ratio of 3:100 (mass fraction) [23]. In addition, the
polyvinylpyrrolidone (PVP) was blended with the g-C3N4/Bi2MoO6
at a ratio of 3:10 [25]. Then, the mixed solution was subjected to
ultrasound for 1 h (40 kHz, 150 W), and the composite paints were
uniformly coated on previously prepared sheet of glass (25 mm
ꢄ 76 mm ꢄ 1 mm) and dried in air. Following the above methods,
different g-C3N4/Bi2MoO6/PEVE coatings were prepared (noted as
BMO/PEVE, 3% CNBP, 5% CNBP, 7% CNBP, and 15% CNBP).
2.4. Photocatalytic antifouling experiments
The photocatalytic antifouling experiments were performed
using a 500 W Xe lamp (l> 420 nm) as the light source, and the
antibacterial activities of the coatings were tested by counting
colony-forming units (CFUs) on the culture dishes [26]. 2216E Solid
medium (peptone, 5 g; yeast extract, 1 g; FePO4, 0.01 g; agar, 18 g;
sea water, 700 ml; pH 7.4–7.8) was prepared before the experiment
[5]. The process of bacterial cultivation under visible light
irradiation is as follows. First, the prepared CNBP coatings were
sterilized under UV light. Then, the coatings were immersed in
certain amount of seawater and lit for 4 h [26]. The bacterial
membrane was scraped from the coating surface using a sterilized
cotton sign and diluted the sample in 50 ml sterilized seawater
every once in a while. Then, we inoculated 0.1 ml of the resulting
suspension to the solid medium by sterile pipettes [27]. The
process was accomplished in a biological sterilization box. Finally,
we maintained the vaccinal solid medium in the box at constant
temperature box (28 ꢃC) for 24 h and observed the colony
distribution on the medium.
g-C3N4/Bi2MoO6/PEVE coatings under visible light (l> 420 nm)
irradiation. As the results showed, the sterilization performance of
PEVE coatings was enhanced after adding g-C3N4/Bi2MoO6 under
visible-light irradiation. Meanwhile, we also tested the stability
and sterilization performance of g-C3N4/Bi2MoO6/PEVE coatings,
the results demonstrated that the g-C3N4/Bi2MoO6/PEVE coatings
have good stability and cycle performance. Moreover, possible
mechanisms for the enhanced sterilization performance of
fluorocarbon resin (PEVE) coatings was discussed. This work has
great significance in the photocatalysis technology as a coating for
killing marine bacteria to address marine biofouling problems.
2. Experiments
2.1. Synthesis of Bi2MoO6 and g-C3N4
2.5. Characterizations
All chemicals used were of analytical grade and were not
purified further. Pure Bi2MoO6 was synthesized through
The antibacterial activities of the coatings were tested by
counting colony-forming units (CFUs) on the culture dishes.
Crystallographic properties were characterized by powder X-ray
diffraction (XRD, Rigaku DMAX- Ultima+ diffractometer) with Co
target in the range of 10ꢃ–90ꢃ. Fourier transform infrared
spectroscopy spectra (FT-IR) was performed on Perkin Elmer
Frontier with a range of 4000–400 cmꢁ1, and infrared transmission
spectra were recorded for KBr discs containing the powder sample.
The microstructures were acquired by scanning electron micro-
scope (SEM) with SUPRA 55 SAPPHHIRE and field emission on
high-resolution transmission electron microscopy (HRTEM) with
JEOL JEM-2100 instrument, and X-ray spectroscopy (EDX) was also
performed. The UV–vis diffuse reflectance spectra (DRS) were
measured with TU-1901 UV–vis spectrophotometer.
hydrothermal method [21,22]. First, 0.004 mol of Bi(NO3)3
ꢂ
5H2O
and 0.002 mol of Na2MoO4 2H2O were dissolved in 50 ml of
ꢂ
deionized water and stirred at room temperature for 1 h with a
speed of 500 rpm. Then, the resultant mixture was processed
hydrothermal at 180 ꢃC for 24 h. After funnel separation, the
Bi2MoO6 powder (noted as BMO) was rinsed with distilled water
and ethanol, and then dried at 80 ꢃC in a vacuum oven.
The g-C3N4 was synthesized through melamine thermolysis
[23]. A certain amount of melamine was added in a crucible and
then heated to 550 ꢃC at a rate of 3 ꢃC/min for 4 h in air [24]. Then
the obtained solid was ground into powder, to produce g-C3N4.
2.2. Preparation of g-C3N4/Bi2MoO6 composites
Moreover, to test the transfer efficiency of the photogenerated
electrons of the samples, photoelectrochemical measurements
(EIS) were carried out in three electrode quartz cell, the reference
and the counter electrodes were calomel electrode (SCE saturated
with KCl) and platinum electrodes, respectively. The working
electron was selected (Bi2MoO6 and g-C3N4/Bi2MoO6). The
electrolyte was 0.1 mol/L NaSO4 solution, and all the photo-
electrochemical measurements were carried out under visible
light irradiation from a 500 W Xe lamp with 420 nm cutoff filters.
The g-C3N4/Bi2MoO6 composites were prepared as follows:
0.046 g g-C3N4 was dissolved into 30 ml of methanol solution and
was subjected to ultrasound for 30 min (40 kHz, 150 W). Then
0.915 g Bi2MoO6 was added into the g-C3N4 solution, and the
suspension was stirred for 6 h to ensure better performance after
ultrasonication for 30 min (40 kHz, 150 W). Finally, the composites
were centrifuged, washed with water and ethanol, and dried at
80 ꢃC for 6 h to obtain the 5% g-C3N4/Bi2MoO6 photocatalyst (noted

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Y. Tian et al. / Journal of Photochemistry and Photobiology A: Chemistry 350 (2018) 10–16
3. Results and discussion
3.1. Photocatalytic antibacterial activity under visible light irradiation
The photocatalytic sterilization of CNBP coatings was investi-
gated. The PEVE and BMO/PEVE coatings were chosen for
comparison by counting colony-forming units (CFUs) method.
Then, the photocatalytic sterilization experiment was carried out
under visible light irradiation. The results of the experiment are
shown in Fig. 1. The PEVE coating did not exhibit conspicuous
sterilization effect. However, it was enhanced with the addition of
Bi2MoO6 and g-C3N4/Bi2MoO6 under visible light irradiation. With
the increasing content of g-C3N4 in Bi2MoO6, the sterilization
performance of PEVE composite coatings was enhanced gradually.
Furthermore, when the content of g-C3N4 in Bi2MoO6 was 7%, the
ratio of sterilization of composite coating reached 100% under
visible light within 4 h. Moreover, the ratio of sterilization of PEVE
and BMO/PEVE coating only reached 5% and 40% under the same
experiment conditions. The ratio of sterilization of 7% CNBP
coating was double than that of the PEVE coating. When the
content of g-C3N4 in Bi2MoO6 was 15%, the sterilization perfor-
mance of CNBP coatings decreased, which may be caused by the
weak absorption of light because of the excessive g-C3N4 generated
that covered layers on Bi2MoO6 [5]. To explore the stability and
sterilization performance of 7% CNBP coating, it was retested of the
photocatalytic antibacterial activity experiment for four cycles
under visible light for 3 h, and the results shown in Fig. 2. The
recycling test results showed that the ratio of sterilization of 7%
CNBP coating also reached around 90% although the test was done
for four cycles, and this result corresponded the photocatalytic
sterilization of 7% CNBP coating under visible light within 4 h. The
recycling test demonstrated that the CNBP coating has good
stability and sterilization performance. There was no extreme
decrease in the photocatalytic antibacterial activity although the
test was done for four cycles.
Fig. 2. The sterilization rate of the reused 7% CNBP coating for four cycles under
visible light irradiation for 3 h.
3.2. Characterization of g-C3N4/Bi2MoO6 composite photocatalysts
The crystalline structure and purity of as-prepared photo-
catalysts were investigated by XRD. Fig. 3 shows the XRD patterns
of g-C3N4 and g-C3N4/Bi2MoO6 composites. For the XRD patterns of
pure g-C3N4, two pronounced diffraction peaks were observed at
2u
= 16.9ꢃ and 31.2ꢃ, which were in good agreement with the (100)
and (002) planes of the g-C3N4 respectively. In the XRD patterns of
pure Bi2MoO6, diffraction peaks at 2
u
= 12.6ꢃ, 27.3ꢃ, 32.9ꢃ, 38.5ꢃ,
38.6ꢃ, 42.8ꢃ, 54.8ꢃ, 55.2ꢃ, 65.3ꢃ, 66.4ꢃ and 69.1ꢃ corresponded to the
(020), (111), (131), (002), (060), (151), (202), (260), (331), (191), and
(262) planes, respectively (PDF card NO.21-0102). No impurity
peaks were observed and the above results indicated the high
purity of the as-samples. The diffraction peaks of g-C3N4 were not
observed in g-C3N4/Bi2MoO6 composites, which may be attributed
to the low intensity of the g-C3N4. However, the narrow sharp
peaks suggest that the g-C3N4/Bi2MoO6 was highly crystalline. To
further improve the g-C3N4/Bi2MoO6, we conducted the composite
samples by FTIR.
The sterilization experiments of CNBP composite coatings
under visible light confirmed that the sterilization performance of
PEVE coating was enhanced by the addition of Bi2MoO6 and
g-C3N4/Bi2MoO6. When the content of g-C3N4 in Bi2MoO6 was 7%,
the ratio of sterilization of composite coating reached the best. To
elucidate the mechanisms of enhanced sterilization performance
of CNBP coatings, the following characterizations and tests were
conducted for the 7% CN/BMO sample.
The FTIR spectras of g-C3N4, Bi2MoO6 and g-C3N4/Bi2MoO6
composites are shown as Fig. 4. In the spectrum of g-C3N4, the peak
at 808 cmꢁ1 can be attributed to the breathing mode of s-triazine
units [28], and the peaks at 1243 cmꢁ1–1647 cmꢁ1 corresponded to
the CꢁꢁN and C¼N stretching modes [28]. For the spectrum of
Fig. 1. The sterilization rate of composite coatings under visible light irradiation for
Fig. 3. XRD patterns of g-C3N4, Bi2MoO6 and g-C3N4/Bi2MoO6 with different
4 h.
hybridization ratio.

Y. Tian et al. / Journal of Photochemistry and Photobiology A: Chemistry 350 (2018) 10–16
13
5%, 7% and 15%, and with the increase of g-C3N4 in Bi2MoO6, the
Mo-O absorption peaks appeared apparently red shift from
570 cmꢁ1 to 568, 565, 562, and 560 cmꢁ1. This phenomenon
may be caused by the interaction between the crystal surface of
g-C3N4 and Bi2MoO6, further demonstrating the successful
synthesis of g-C3N4/Bi2MoO6 composites.
To further demonstrate the combination of g-C3N4 and
Bi2MoO6, the morphology of g-C3N4, Bi2MoO6 and 7% CN/BMO
composites were tested by SEM analysis and EDX pattern. Fig. 5(a)
clearly showed the morphology of g-C3N4 with a layered structure
and numerous stacking layers. The morphology of Bi2MoO6 sample
displayed a surface smooth and a plate-like structure. After
hybridization, the plate-like structure of Bi2MoO6 was not changed
and the two types of materials were all found in 7% CN/BMO
composites. Moreover, the EDX also confirmed that the composites
were constituted by C, N, Bi, Mo and O elements, the EDX elements
mapping images showed obviously that the C, N, Bi, Mo, and O
elements were homogenously distributed in the bulk structure,
confirming that a relatively homogeneous distribution of Bi2MoO6
on g-C3N4 surface, and the large Bi2MoO6 in the enclose area in
Fig. 5(c), the EDX and the mapping tests also illustrating that the
composites were composed of g-C3N4 and Bi2MoO6.
Fig. 4. FTIR spectra of g-C3N4, Bi2MoO6 and g-C3N4/Bi2MoO6 composites.
Bi2MoO6, the absorption peaks at 570, 734, 797 and 843 cmꢁ1 were
attributed to the MoꢁꢁO stretching vibrations, BiꢁꢁO stretching
and deformation vibrations [29]. All the absorption spectrums of
g-C3N4 and Bi2MoO6 were observed apparently in the spectrum of
g-C3N4/Bi2MoO6 composites with different weight content of 3%,
TEM and HRTEM were also examined to further confirm the
morphology of Bi2MoO6 and g-C3N4/Bi2MoO6. The morphology of
g-C3N4/Bi2MoO6 composite was unchanged compared with that of
Fig. 5. FE-SEM images of (a) g-C3N4 (b) Bi2MoO6 (c) g-C3N4/Bi2MoO6 and (d) EDX of 7% CN/BMO composite (e–i) EDX elements mapping of 7% CN/BMO composite.

14
Y. Tian et al. / Journal of Photochemistry and Photobiology A: Chemistry 350 (2018) 10–16
the pure Bi2MoO6 as shown in Fig. 6(a). The lattice spacing was
approximately 0.315 nm, corresponding to the (131) plane of
Bi2MoO6 as observed from the HRTEM image (Fig. 6(b)). This result
corresponded to the XRD patterns shown in Fig. 3, and surrounding
Bi2MoO6, where the lattice fringes disappeared and surrounded by
g-C3N4, which demonstrated the formation between g-C3N4 and
Bi2MoO6. To explore the interaction of the relationship between
g-C3N4 and Bi2MoO6 on the photocatalytic performance, we tested
the surface spectral properties of the g-C3N4/Bi2MoO6 photo-
catalysts.
The UV–vis diffuse reflectance spectra of the as-prepared
samples are shown in Fig. 7. The absorption of the pure g-C3N4 was
visible light with the edge of 460 nm, and the absorption of
Bi2MoO6 was roughly estimated to range from 450 nm to 470 nm.
After the hybridization of g-C3N4 and Bi2MoO6, the
g-C3N4/Bi2MoO6 samples showed higher absorbance compared
to purity of Bi2MoO6 and g-C3N4, and the absorbance was
significant when the content of the g-C3N4 in Bi2MoO6 was 7%.
This was because of the presence of g-C3N4 promoted the
corresponding Bi2MoO6 to the light absorption. All the absorption
of the composites showed slightly red-shift after the hybridization
of g-C3N4 and Bi2MoO6, which demonstrated that the visible light
response of the composite samples were successfully extended
[11]. The band gap (Eg) can be evaluated by the formula [30,31].
Fig. 7. UV–vis diffuse reflectance spectra of g-C3N4, Bi2MoO6 and g-C3N4/Bi2MoO6
composites with different g-C3N4 content of 3%, 5%, 7%, 15%.
a
hv = A(hv-Eg)n/2
(1)
where
a
, h, A, v and Eg are molar extinction coefficient, planck
constant, light frequency, a constant, and band gap energy,
respectively. Among them, n is determined by the optical transition
of semiconductor (n = 1 for director transition and n = 4 for
indirector transition) [5]. As existing researches reported, the n
values of g-C3N4 and Bi2MoO6 were 4 and 1, respectively. From
Eq. (1), the Eg values of g-C3N4 and Bi2MoO6 were estimated to be
2.72 and 2.59 eV, respectively.
The migration and transfer process of photogenerated charge
carries in a semicionductor can be investigated via electrochemical
impedance spectra (EIS) [32,33]. Under the visible light irradiation
condition, the arc in the EIS usually indicates the interface layer
resistance occurred at the surface of electrode [21]. In Fig. 8, the arc
radius of 7% CN/BMO composite was smaller than pure g-C3N4 and
Bi2MoO6, with the addition of g-C3N4, the charge distribution on
the surface of the g-C3N4/Bi2MoO6 composite photocatalyst was
changed under the visible light conditions, and the total space
resistance of the composite photocatalyst samples were smaller
than that of Bi2MoO6. The EIS test results demonstrated that a
higher transfer rate of electrons occurred in g-C3N4/Bi2MoO6
composite electrode.
Fig. 8. EIS Nynquist plots of g-C3N4, Bi2MoO6 and 7% CN/BMO electrodes under the
irradiation of visible light (l> 420 nm, [Na2SO4] = 0.1 M).
Fig. 6. (a) TEM images of 7% CN/BMO composite photocatalyst (b) HRTEM images of 7% CN/BMO composite photocatalyst.

Y. Tian et al. / Journal of Photochemistry and Photobiology A: Chemistry 350 (2018) 10–16
15
3.3. Photocatalytic antimicrobial mechanism of g-C3N4/Bi2MoO6/PEVE
composite coatings
Based on the above results, the enhanced photocatalytic
antimicrobial mechanism for PEVE coatings using g-C3N4/Bi2MoO6
composites was discussed. As shown in Fig. 9, both g-C3N4 and
Bi2MoO6 are n-type semiconductors [34], and the mechanism of
sterilization as well as the electron-hole separation and transport
at the composite photocatalyst interface were discussed as
follows: the redox potentials of the conduction band (Fcb) and
the valence band (Fvb) are positioned at ca. ꢁ0.32 and 2.27 V vs.
NHE, respectively. And the CB and VB edge potentials of g-C3N4
were at ꢁ1.12 and 1.6 V vs. NHE, respectively [40].
Before hybridization, the conduction band (CB) edge of Bi2MoO6
is lower than that of g-C3N4, and the Fermi level (EF) of Bi2MoO6 is
also lower than that of g-C3N4. After hybridization, the electrons
diffuse from g-C3N4 to Bi2MoO6, and the EF of Bi2MoO6 increases
while the EF of g-C3N4 decreases until an equilibrium state is
achieved [34]. The mechanisms of sterilization performance of
CNBP coating is shown in Fig. 9, both g-C3N4 and Bi2MoO6 are all
excited to generate eꢁ and h+ pairs under visible light irradiation.
Due to the interfacial interactions between g-C3N4 and Bi2MoO6, as
well as the lower CB potential of g-C3N4 than that of Bi2MoO6, the
photogenerated electrons (eꢁ) in the CB of g-C3N4 can transfer to
Bi2MoO6, and the electrons (eꢁ) in the CB of Bi2MoO6 (ꢁ0.32 V) can
ꢁ
Fig. 10. The mechanisms of ꢀO2 and h+ damaging of the DNA of bacteria under
visible light irradiation.
with the O2 and H+ in the air to form R-OOH and generate the
8-hydroxy-20-deoxyguanosine (8-OHdG) [38], With the effect of
ꢀO2ꢁ and h+, the 8-OHdG is oxidized to CO2 and H2O (Fig. 9), and the
bacteria lose activity [39]. The probable energy band structure of
the equilibrium state g-C3N4/Bi2MoO6 heterojunction is shown in
Fig. 10.
further reduce O2 to generate ꢀO2 (E(O2/ꢀO2ꢁ) = ꢁ0.33 V). The
ꢁ
holes in the VB of the g-C3N4 (1.6 V) and the Bi2MoO6 (2.27 V) have
no ability to oxidize H2O to ꢀOH (E(ꢀOH/H2O) = 2.38 V)
[3,12,16,17,36], so the holes in the VB of the g-C3N4 and the
Bi2MoO6 also played a dominant role in disinfection of bacteria
directly. The holes in the VB of Bi2MoO6 also would inject to that of
g-C3N4. Thus, the electrons (eꢁ) and holes (h+) were effectively
separated in the g-C3N4/Bi2MoO6 composites. Meanwhile, the
oxidation and reduction sites were located in the surface and edge
of layered structure of Bi2MoO6, the photogenerated h+ can easily
g-C3N4/Bi2MoO6 + hv !eꢁ + h+
(4)
ꢁ
e
ꢁ + O2 ! ꢀO2
(5)
transfer to the surface of Bi2MoO6. The generated
O2ꢁ and h+ held
ꢂ
(6)ꢀO2ꢁ, h+ + bacteria ! inactivated bacteria
strong oxydic ability and can induce DNA damage of bacteria,
reduciꢁng bacteria activity. Meanwhile, guanine is the primary site
of ꢀO2 and h+ oxidative damage in the bacterial DNA structure
[37]. The guanine is prone to oxidize and transfer a ꢁꢁH+ group to
form the guanine group, which is unstable and easy to combine
In brief, g-C3N4 accelerated the transfer and separation
efficiency of electron-hole pairs of Bi2MoO6, and contribute to
generate more strong oxidizing groups (ꢀO2ꢁ, h+) under visible
light irradiation through the energy-level transition between
Fig. 9. The mechanisms of g-C3N4/Bi2MoO6 photocatalysts effect on bacteria under visible light irradiation.

16
Y. Tian et al. / Journal of Photochemistry and Photobiology A: Chemistry 350 (2018) 10–16
g-C3N4 and Bi2MoO6. The highly active ꢀO2 and h+ greatly
inhibited bacterial activity.
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ꢁ
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4. Conclusions
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In summary, the g-C3N4/Bi2MoO6 composite photocatalysts
were prepared by sonochemical method. The sterilization perfor-
mance of PEVE coatings was enhanced with the addition of
g-C3N4/Bi2MoO6 composite photocatalysts, and the sterilization
performance was prominent when the hybridization content of
g-C3N4 in Bi2MoO6 was 7%. Meanwhile, the results of the stability
and sterilization performance experiment of 7% CNBP coating have
demonstrated that the CNBP coatings have good stability and cycle
performance. The successful preparation of g-C3N4/Bi2MoO6
composite photocatalysts was demonstrated by XRD, FTIR, SEM,
EDX, TEM and UV–vis methods. Furthermore, the g-C3N4
accelerated the electron-hole pair transfer efficiency of Bi2MoO6
under visible light, and contributed to the production ꢁof more
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strong oxidizing groups (ꢀO2ꢁ, h+). The highly active O2 and h+
ꢂ
greatly inhibited bacterial activity. This work has great significance
in the photocatalyst technology for the production of coatings that
will address the marine biofouling problems.
Acknowledgements
This work is supported by the National Nature Science
Foundation of China (No. 21676040), National Science Foundation
of Liaoning Province (2015020184), Program for Liaoning Excellent
Talents in University (LJQ2015013) and the Fundamental Research
Funds for the Central Universities (No. 3132016065 and No.
3132016341).
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