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Ag-AgCl for efficient photocatalytic inactivation of bacteria
Journal Pre-proofs Plasmonic Ag as electron-transfer mediators in Bi2MoO6/Ag-AgCl for efficient photocatalytic inactivation of bacteria Mingmei Li, Deguan Li, Zhiruo Zhou, Pengfei Wang, Xueyue Mi, Yuguo Xia, Haitao Wang, Sihui Zhan, Yi Li, Liumin Li PII: DOI: Reference:
S1385-8947(19)32172-2 https://doi.org/10.1016/j.cej.2019.122762 CEJ 122762
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
31 May 2019 20 August 2019 6 September 2019
Please cite this article as: M. Li, D. Li, Z. Zhou, P. Wang, X. Mi, Y. Xia, H. Wang, S. Zhan, Y. Li, L. Li, Plasmonic Ag as electron-transfer mediators in Bi2MoO6/Ag-AgCl for efficient photocatalytic inactivation of bacteria, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.122762
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Plasmonic Ag as electron-transfer mediators in Bi2MoO6/Ag-AgCl for efficient photocatalytic inactivation of bacteria
Mingmei Li,a,1 Deguan Li,c,1 Zhiruo Zhou,a Pengfei Wang,a,b,* Xueyue Mi,a Yuguo Xia,d Haitao Wang,a Sihui Zhana,*, Yi Lie and Liumin Lif
a
MOE Key Laboratory of Pollution Processes and Environmental Criteria / Tianjin
Key Laboratory of Environmental Remediation and Pollution Control, College of Environmental Science and Engineering, Nankai University, Tianjin 300350, P. R. China. E-mail: [email protected], [email protected]. b State c d
Key Laboratory of Mineral Processing, Beijing 102628, P. R. China.
Institute of Radiation Medicine, PUMC & CAMS, Tianjin 300192, P. R. China. School of Chemistry & Chemical Engineering, National Engineering Research
Center for Colloidal Materials, Shandong University, Jinan 250100, P. R. China. e
Department of Chemistry, Tianjin University, Tianjin 300072, P. R. China.
f
China Municipal Engineering North China Commercial Research Institute Co., Ltd,
Tianjin 300074, P. R. China. 1
These authors contributed equally to this work.
ABSTRACT Solar-driven photocatalysis undoubtedly represents one of the most promising alternative water disinfection technologies, however, its practical application is still restrained by the fast recombination of carriers. To overcome this limitation, plasmon-induced photocatalytic processes have gained attention due to their extended optical absorption and enhanced charge separation when metal nanoparticles act as co-catalysts. In this work, to enhance the photocatalytic disinfection of Bi2MoO6/AgCl, the formation of plasmonic Ag nanoparticles as electron transfer mediators was promoted on the surface of Bi2MoO6 photocatalysts. Specifically, the co-modified Bi2MoO6/Ag-AgCl photocatalysts were synthesized via a two-step process involving the precipitation of AgCl and photo-reduction of Ag on the Bi2MoO6 surface. It was found that Bi2MoO6/Ag-AgCl photocatalysts exhibited higher photocatalytic disinfection activity under visible light irradiation than those of Bi2MoO6 and Bi2MoO6/AgCl. The photocatalytic mechanism of Bi2MoO6/AgCl could be attributed to the excellent synergistic effect of Ag and AgCl, where plasmonic Ag nanoparticles promote light absorption and work as electron-transfer mediators to transfer electrons from Bi2MoO6 to AgCl, meanwhile AgCl work as interfacial catalytic active sites to product free radicals based on the powerful characterizations, such as PL spectra, photoelectrochemical methodology and theoretical calculations. This work may provide new insights to employ synergistic effect of heterogeneous photocatalysts for improving the catalytic activities in water purification.
Keywords: Bi2MoO6 microspheres, plasmon effect, Ag, charge separation and transfer, photocatalytic disinfection
1. Introduction Modern society is facing a serious problem of water pollution, particularly the microbial contamination has been paid more and more attention, which pose a huge threat to human health [1]. Every year, millions of people died from serious water-borne diseases [2, 3]. Although the traditional disinfection methods are effective against most pathogens, they may be too costly to implement in developing
regions [4]. Hence, it is necessary to develop effective and environmentally methods to control or eradicate microbial contamination [5]. In recent decades, with the inexhaustible and inexhaustible solar energy, photocatalysis has been widely used in the removal of pollutants, and is expected to be devoted into practical applications [6]. However, due to the lower solar-energy utilization efficiency, high recombination rate of electron-hole pairs and poor selective adsorption, the further commercial development of the technology is still restricted [7]. The activity of photocatalytic reaction has long relied on facilitating charge separation and surface redox reaction [8]. Particularly, the recombination of electron-hole pairs arisen from the random charge transfer dramatically thwart the photocatalytic reaction efficiency [9]. Furthermore, the electron and hole may transfer to the surface on the picosecond timescale, thus the carriers should be separated as far as possible, or as long as possible to achieve optimal efficiency. [10]. In this regard, plasmon-induced photocatalytic processes have attracted widespread attention due to their enhanced charge separation lifetimes and extended optical absorption in a wide range of the visible spectrum when metal nanoparticles act as co-catalysts [11]. Among the metal nanoparticles usually used in photocatalytic pollution degradation, silver has been regarded as one of the most effective co-catalysts, which has shown an enhancement of the interfacial charge transfer and the increasement of the visible light excitation [12]. And several research groups have successfully incorporated Ag into photocatalysts, which confirms the enhancement of photocatalytic sterilization [13-15]. Bi2MoO6, as a typical Aurivillius oxide, has aroused wide interest recently due to its dielectric, ion-conductive, luminescent and catalytic properties [16]. Bi2MoO6 is a layered bismuth oxide, which is formed by [Bi2O2]2+ layers and MoO42- slabs. Due to its relatively small band gap (2.5-2.8eV), Bi2MoO6 is able to capture visible light irradiation [17, 18]. More and more studies have shown that Bi2MO6 can be used as an excellent photocatalyst and solar conversion material for the decomposition and degradation of organic compounds under visible light irradiation [19, 20]. However, due to the low quantum yield of pure Bi2MoO6, it is still a challenge to improve its
photocatalytic efficiency to satisfy the practical applications. Furthermore, the photocatalytic antibacterial effects of the Bi2MoO6 have not been systematically investigated. Herein, highly efficient visible light active Bi2MoO6 photocatalysts co-modified with Ag nanoparticles as electron-transfer mediators and AgCl as interfacial catalytic active sites (denoted as Bi2MoO6/Ag-AgCl) were synthesized, in which AgCl was first deposited on Bi2MoO6, thereafter AgCl decomposed into Ag by photo-reduction and Ag could demonstrate strong plasmon resonance to transfer electrons from Bi2MoO6 to AgCl. It was found that the Bi2MoO6/Ag-AgCl photocatalyst exhibited enhanced
photocatalytic
disinfection
effect
compared
with
Bi2MoO6 and
Bi2MoO6/AgCl. Furthermore, both theoretical and experimental studies were performed to investigate the origin of plasmonic Ag on enhanced photocatalytic performance. This work may provide new insights for rational design and development of highly efficient photocatalysts for practical applications.
2. Experimental section 2.1 Preparation of flower-like Bi2MoO6 microspheres All chemicals were of analytical grade and were used directly without further purification. Flower-like Bi2MoO6 microspheres were synthesized according to reported method with minor modifications [18]. Typically, 3.47 mmol Bi(NO3)3∙5H2O and 1.74 mmol Na2MoO4∙2H2O were dissolved in 10 mL ethylene glycol under stirring. Then, 20 mL ethanol was added slowly to the solution and stirred for another 10 min. The solution was transferred to a 100 mL Teflon-lined autoclave and the autoclave was sealed and maintained at 160 °С for 24 h in an electric oven. After natural cooling at room temperature, the resulting samples were collected and washed with water and ethanol several times, and then dried at 80 °С for 8 h. Finally, the samples were calcinated at 400 °С for 2h in air to obtain Bi2MoO6 microspheres. 2.2 Preparation of Bi2MoO6/AgCl composites All deposited samples were synthesized by deposition and precipitation [18]. Typically, 0.4g Bi2MoO6 was dissolved in 60 ml ultra-pure water and sonicated for 20
min to obtain solution A. Then, 0.18 g NaCl were added into solution A and stirred for 30 min to get solution B. 0.0714 g AgNO3 (mass ratio of AgCl to Bi2MoO6 was 15%) were dissolved in 20ml ultra-pure water named as solution C.
Solution C was
slowly dripped into solution B with magnetically stirring under dark condition. After stirring for 60 min, the precipitate was collected and washed with ultra-pure water for several times, and then dried at 60°С to obtain Bi2MoO6/AgCl (denoted as BA). 2.3 Preparation of Bi2MoO6/Ag-AgCl composites Bi2MoO6/Ag-AgCl photocatalysts were synthesized according to previous method with minor modifications [18]. Briefly, 0.3g Bi2MoO6/AgCl were dispersed into 60 ml ultra-pure water, after stirring for 20 min, the suspension was irradiated by solar simulator 300 W Xe lamp (CEL-HXF300, Ceaulight, Beijing) for 60 min. Then, the product was centrifuged, washed with ultra-pure water and ethyl alcohol; the final sample was dried at 60 °С and named as Bi2MoO6/Ag(60)-AgCl (BAA-60). Similarly, Bi2MoO6/Ag(30)-AgCl (BAA-30), Bi2MoO6/Ag(90)-AgCl (BAA-90) composites were prepared by different illumination time, for 30min, 90min respectively. 2.4 Characterizations Powder X-ray diffraction (XRD) was recorded on Rigaku D/Max 2200PC X-ray diffractometer using Cu Kα radiation (λ=0.15418 nm). Scanning electron microscopy (SEM) was carried out on a Hitachi SU8010 scanning electron microscope operated at an accelerating voltage of 3 kV. High-resolution transmission electron microscopy (TEM and HRTEM) analyses and elemental mapping images were performed on a JEOL JEM-2100F electron microscope operated at an accelerating voltage of 200 kV. XPS measurements were obtained on a Thermal ESCALAB 250 electron spectrometer. The UV-vis diffused reflectance spectra were measured using a Shimadzu Corporation UV-vis spectrophotometer. Fourier transform infrared spectra (FTIR) of the samples were obtained from Thermo Nicolet iS5 FTIR spectrometer. Photoluminescence spectra (PL) were performed on an Edinburgh Instruments FLS920P spectrophotometer at an excitation of 395 nm. 2.5 Theoretical calculation
The exchange correlation functions derived from Perdew, Burke and Ernzerof (PBE) were calculated by using density functional theory (DFT) and generalized gradient approximation (GGA). The valence electron-ion interaction was modeled by the projector augmented wave (PAW) potential as implemented in the Vienna ab initio simulation pakage (VASP) [21,22]. For all the calculations, both the lattice constant and the positions of all atoms are relaxed until the force is less than 0.22 eV/ Å. The total energy standard is 1 × 10−5 eV. 2.6 Photocatalytic inactivation of E. coli. Gram-negative E. coli and Gram-positive S. aureus were used as model bacteria. The bacteria were cultured in a bed temperature incubator at 37 °С for 15 hours to yield a cell count of 109 cfu/mL. Afterwards, the bacteria were centrifuged and washed with sterile 0.85% (wt/vol) saline solution for 3 times. The final concentration of bacteria in the disinfection process was diluted to 107 cfu/mL using 0.85% (wt/vol) saline solution. Samples were dispersed in sterilized water to form 4 mg/mL suspension. The application of prepared samples for disinfection was investigated under visible-light irradiation. Typically, 250 μL of photocatalysts suspension was added into a sterilized beaker which contained 10 mL of E. coli solution (107 cfu/mL) and shaked evenly. Then the mixture was irradiated under a 300 W Xenon arc lamp (CEL-HXF300) with a UV cutoff (λ< 420 nm) and timed simultaneously. The solution was carefully pipetted out at planed time and the bacterial density was calculated by the standard plate count method. Similarly, the dark control groups were carried out in the dark at the same condition. The light control group was conducted without any photocatalyst. All the disinfection experiments were repeated three times. 2.6 Photoelectrochemical measurements The 4 mg sample, 1 ml ethanol and 20 μl electrolyte were mixed by sonication and then dropped onto an indium-tin oxide (ITO) glass to obtain the photoelectric anode. The photoanodes were dried at 60 °С for 6 h after ethanol was completely evaporated. Photoelectrochemical properties of the prepared photoanodes were tested on an
electrochemical workstation (CHI660B, Shanghai Chenhua) with a three-electrode quartz battery system (a saturated calomel electrode reference electrode and a platinum plate counter electrode). 0.1 M Na2SO4 solution was used as the electrolyte. The visible light irradiation was provided using a 300w xenon lamp (CEL-HXF300, Ceaulight, Beijing). Usually, the polarization curves were recorded at a scanning rate of 5 mV s-1 and the transient photocurrent was detected by chopping irradiation.
3. Results and discussion 3.1 Morphologies and microstructures of Bi2MoO6/Ag-AgCl photocatalysts To verify the successful preparation of Bi2MoO6/Ag-AgCl photocatalysts, the as-prepared samples were characterized by XRD, FT-IR, SEM, TEM and XPS. The crystallinity and purity of the synthesized photocatalysts were analyzed by XRD patterns and the results were displayed in Fig. 1a. Intense diffraction peaks of pure Bi2MoO6 samples at 2θ= 28.1°, 32.3°, 46.9°, 55.5° and 58.5° were attributed to (131), (200)/(002), (260)/(212), (331)/(062) and (262) crystal planes of Bi2MoO6, respectively, which can be indexed to Bi2MoO6 (JCPDS No. 76-2388) [17]. In addition to Bi2MoO6, diffraction peaks of AgCl ((JCPDS No.85-1355) [18] were also observed in Bi2MoO6/AgCl and Bi2MoO6/Ag-AgCl photocatalysts, suggesting that the prepared samples were composed of well-crystallized Bi2MoO6 and AgCl. Moreover, the weak diffraction peaks of BAA-30, BAA-60 and BAA-90 samples at 2θ = 38.1° (Fig. 1a, inset) could be attributed to Ag0 (JCPDS No.87-717) [18]. Furthermore, no characteristic peaks corresponding to other phases was discovered, indicating the high purity of the prepared samples.
Fig.1. (a) XRD patterns and (b) FT-IR spectra of as-fabricated samples.
FT-IR can be used to determine the functional groups of photocatalysts. The FT-IR spectra of Bi2MoO6, BA, BAA-30, BAA-60 and BAA-90 were exhibited in Fig.1b. In the spectrum of Bi2MoO6, the peaks at 3425 and 1637 cm−1 could be assigned to O-H vibrations. The absorption bands at 930-630 cm−1 could be ascribed to Mo-O stretching vibrations [23]. The peaks at 571 cm−1 and447 cm−1 were attributed to Bi-O stretching vibration and deformation vibration, respectively [24]. After introduced Ag/AgCl, the Mo-O stretching vibrations bands at 930-630 cm−1 shifted to 900-610 cm−1, suggesting the evident interactions between the Ag-AgCl and Bi2MoO6. This result demonstrated the successful hybridization between Ag-AgCl and Bi2MoO6 rather than simple mixing. To further confirm the morphology and composition of the photocatalysts, SEM and TEM were carried out. The SEM images of Bi2MoO6 were shown in Fig. 2a and Fig. S1a, it was clear that the flower-like Bi2MoO6 microspheres (1-4 μm in diameter) were actually assembled by amounts of flakes with an average thickness of about 10 nm. After the introduction of Ag-AgCl, it can be observed that many irregular Ag-AgCl particles attached on the surfaces of flower-like Bi2MoO6. (Fig. 2b and Fig. S1b). A clear contrast between the dark edges and the pale center on the surface of the composite displayed in TEM image (Fig. 2c) suggested that the interior of the Bi2MoO6 sphere was hollow and the Ag-AgCl on the surface of Bi2MoO6 contained rough and porous surfaces [25]. In the elemental map (Fig. 2d), different colors represented different elements to identify their position in the photocatalyst. Obviously, both the Ag and Cl elements were well distributed on the surface of Bi2MoO6. The TEM images of BAA-30, BAA-60 and BAA-90 in Fig. S2 can further confirm the fabrication of Bi2MoO6/Ag-AgCl hybrid photocatalyst. Furthermore, the clear lattice fringes with the spacing of 0.236, 0.167 and 0.276 nm in the HRTEM images (Fig. 2e) corresponded to the (111) facet of Ag, (311) facet of AgCl and (002) facet of Bi2MoO6, respectively [18, 19]. According to the above results, metallic Ag and
AgCl
were
successfully
loaded
on
the
Bi2MoO6 surface
to
form
Bi2MoO6/Ag-AgCl photocatalyst by the present facile strategy. And the plasmonic Ag may act as the electron-transfer mediators to promote the transfer of photogenerated
electrons and holes and enhance the photocatalytic activity [18].
Fig.2. SEM images of (a) Bi2MoO6 and (b) BAA-60; TEM (c), corresponding element mapping for Ag, Cl, Mo, Bi (d) and HRTEM (e) images of BAA-60. The successful loading of Ag and AgCl on the Bi2MoO6 surface was verified by XPS, which revealed the component elements and their chemical states in the Bi2MoO6/Ag-AgCl photocatalyst. The survey spectrum displayed in Fig. S3 demonstrated that AAB-60 was mainly composed of Bi, Mo, O, Ag and Cl. Two peaks located at about 164.4 eV and 159.1 eV were observed in the XPS spectrum of Bi 4f (Fig. 3a), which could be attributed to the Bi 4f5/2 and Bi 4f7/2 binding energies, respectively. Fig. 3b showed the Mo 3d spectrum of AAB-60, two peaks were observed at around 235.5 eV and 232.4 eV, which could be indexed to the Mo 3d3/2 and Mo 3d5/2 binding energies, respectively. Fig. 3c displayed the Cl 2p1/2 and Cl 2p3/2 peaks of AAB-60 sample, which were located at 199.6 eV and 198.0 eV, respectively [18]. Fig. 3d showed the Ag 3d spectrum of BAA-30, BAA-60 and BAA-90. For BAA-60, the Ag 3d peak consisted of two peaks at 372.9 and 367.0 eV, which could be ascribed to Ag 3d3/2 and Ag 3d5/2 binding energies, respectively. Moreover, each of the peak can be further divided into two different peaks at 373.6 (Ag0), 372.9 (Ag+) and 368.3 (Ag0), 367.0 eV (Ag+), respectively [18]. The XPS results of Ag 3d confirmed the presence of Ag NPs, which was consistent with the results of XRD analysis (Fig. 1a). Notably, the molar ratio of Ag0 to total Ag was 26.34% based on
the XPS spectra. In contrast, the molar ratio of Ag0 to total Ag in BAA-30 and BAA-90 was calculated to be 6.23% and 44.53%, respectively (Table S1). The XPS results confirmed that the prepared composite photocatalysts were consisted of metallic Ag, AgCl and Bi2MoO6, furthermore, the amount of metallic Ag increased with the photo-reduction time extension.
Fig. 3. The high-resolution XPS spectra of BAA-60: (a) Bi 4f, (b) Mo 3d, (c) Cl 2p; (d) Ag 3d peaks of BAA-30, BAA-60 and BAA-90 3.2 Photocatalytic disinfection performance of Bi2MoO6/Ag-AgCl photocatalysts
Fig. 4. Disinfection efficiencies of the as-synthesized samples (100 μg/mL) toward E. coli (107 cfu/mL) (a) in the dark, (b) under visible light irradiation. Error bars represent standard deviations from triplicate experiments (n = 3). The antibacterial activities of Bi2MoO6, BA, Ag/AgCl, BAA-30, BAA-60 and BAA-90 samples were evaluated using Gram-negative E. coli and Gram-positive S. aureus as bacteria organism. As shown in Fig. 4a, no obvious viable cell density
decrease of E. coli cells was observed under dark condition, indicating that the as-synthesized photocatalysts exhibited negligible cytotoxicity to E. coli without visible light illumination. Fig. 4b illustrated the disinfection results of as-prepared photocatalysts towards E. coli with visible light irradiation. It can be observed that almost no E. coli cells was inactivated in the light control, demonstrating that the saline solution or visible light had no adverse impact on the cell viability. Among all fabricated photocatalysts, Bi2MoO6/Ag-AgCl (BAA) composites displayed higher activity than that of Bi2MoO6, Bi2MoO6/AgCl (BA) and Ag/AgCl. After 30 min of visible light irradiation, only about 0.16, 4.13 and 2.33 log CFU/mL of viable cells were inactivated by Bi2MoO6, BA and Ag/AgCl respectively. The enhanced disinfection performance of Bi2MoO6/Ag-AgCl may be due to the SPR effect of metallic Ag, which can promote the separation of electrons and holes and improve the photocatalytic activity. Notably, as for BAA samples, the BAA-60 showed the best disinfection efficiency, which completely inactivated E. coli in 30 min. In addition, 4.95 and 5.29 log CFU/mL of viable cells were inactivated by BAA-30 and BAA-90, respectively. The results indicated that photo-reduction time affected the photocatalytic activity of the composite photocatalysts by adjusting the amounts of Ag/AgCl. Fig. S4 exhibited the disinfection results of the prepared photocatalysts on S. aureus. It can be observed that B composites also exhibited excellent sterilization effect toward S. aureus under visible light irradiation, and BAA-60 can completely inactivate S. aureus within 50 min. To avoid the waste and shortage of catalyst, we tested the bacteriostatic performance of BAA-60 with different concentrations (50 μg/mL, 80 μg/mL, 100 μg/mL, 120 μg/mL, 150 μg/mL) toward E. coli. As shown in fig. S5, the bacteriostatic activity of BAA-60 increased with the increase of catalyst concentration within 100 μg/mL. However, when the catalyst concentration exceeded 100 μg/mL, the sterilization effects were no longer improved. This results indicated that the optimum amount of catalyst was 100 μg/mL.
Fig. 5. The removal efficiencies of BAA-60 (100 μg/mL) against E. coli (107 CFU/mL) in four cycles under visible light. The reusability and stability of photocatalysts are crucial for their practical application in water remediation. To investigate the reusability of BAA-60, the materials were collected after photocatalytic disinfection to the next cycle. As shown in Fig. 5, all the E. coli cells were inactivated in the first two cycles. After four cycles, only 1.25 log of E. coli cells survived, which may be due to the insufficient cleaning of bacterial residue on the surface of the materials [26]. The SEM image of BAA-60 composites with ultrasonication after antibacterial application was shown in Fig. S6, it can be observed that there were a large number of bacterial residues on the surface of BAA-60 (as indicated by the red circles). This result indicated that the as-synthesized BAA-60 heterojunction photocatalysts exhibited excellent stability in the VLD photocatalytic disinfection process. 3.3 Photocatalytic mechanism of Bi2MoO6/Ag-AgCl photocatalysts
Fig. 6. (a) UV-vis diffuse reflectance, (b) PL spectra, (c) Photocurrent responses and (d) EIS spectra of as-synthesized photocatalysts. To verify the photocatalytic mechanism, the UV-vis diffuse reflectance, PL, Photoelectrochemical measurements and DFT calculations were conducted. Firstly, the absorption capacity of the as-prepared materials for visible light was studied by UV–vis DRS. As exhibited in Fig. 6a, the absorption edges of pure Bi2MoO6 and BA was determined to be 480nm and 486nm, respectively. After the introduction of Ag nanoparticles, the absorption abilities in visible light region were obviously enhanced and an apparent red shift was observed, indicating that the presence of Ag nanoparticles expended the range of visible light spectrum available for utilization comparing with pure Bi2MoO6 and BA. This phenomenon may be due to the surface plasmon resonance (SPR) effect of silver nanoparticles which was derived from the collective oscillation of free electrons excited by the matching photon energy [19, 27, 28]. Specifically, the absorption edge of BAA-60 was determined to be 580 nm, which was wider than that of BAA-30 (564 nm) and BAA-90 (556 nm). This result suggested that the amount of metallic Ag affected the absorption range of photocatalysts for visible light. However, for BAA-90, too much of AgCl would be photo-reduced to Ag-AgCl nanoclusters, which may weaken the SPR effect of Ag nanoparticles as well as shaded the active sties on the surface of Bi2MoO6 [29, 30]. Therefore, the photocatalytic activity of the composite photocatalysts would be
reduced. Based on the above results, the BAA-60 photocatalysts exhibited the best photocatalytic performance which was consistent with the result of photocatalytic sterilization experiments (Fig. 4b). In addition to the optical absorption properties of materials, the separation and recombination efficiency of photo-generated electron-hole pairs also played a key role on the photocatalytic activity [17, 31]. To investigate the recombination efficiency of photo-generated electrons and holes in the photocatalysts, PL spectra characterizing the radiative charge recombination was conducted (Fig. 6b). It can be observed that the emission peak intensities in PL spectra exhibited the following order: Bi2MoO6 ＞ BAA-90＞BAA-30＞BA＞BAA-60. It is believed that the weaker the intensity of the emission peak, the lower the recombination efficiency of charge carriers [17]. Consequently, the introduction of Ag-AgCl on the surface of Bi2MoO6 microspheres can effectively prevent the recombination of electron-hole pairs. Furthermore, BAA-60 composite photocatalysts exhibited the lowest recombination rate of photogenerated charge carriers among all samples. Transient photocurrent response was used to research the separation process of photogenerated charge carriers [1, 32]. Fig. 6c exhibited the transient photocurrent response results of Bi2MoO6, BA, BAA-30, BAA-60 and BAA-90 with several on-off cycles of intermittent visible light irradiation. The intensity of photocurrent responses under visible light irradiation exhibited the following order: BAA-60＞BAA-30＞BA＞BAA-90＞Bi2MoO6 and the photocurrent responses were significantly enhanced after introduced Ag-AgCl. The results suggested that compared with pure Bi2MoO6, the recombination efficiency of the photogenerated electron-hole pairs was reduced greatly and the lifetime of the free charge carriers was extended after introduced Ag-AgCl. Notably, BAA-60 exhibited the highest intensity of photocurrent responses. EIS tests of the as-fabricated samples were conducted to determine their charge migration efficiency [1, 32]. Fig. 6d showed the Nyquist plots under visible light irradiation. It can be observed that, the radian of BAA-60 on the EIS plots was the smallest of all test samples, suggesting that BAA-60 exhibited the highest separation and migration efficiency of
photogenerated charge carriers. To further explore the underlying mechanisms, O2 activation tests were conducted with 3,3′,5,5′-tetramethylbenzidine (TMB) as an indicator molecule by monitoring the absorbance change in the different sample solutions [33]. As illustrated in Fig. S7, BAA-60 exhibited the highest O2 activation ability for ROS generation, which further explains the optimal sterilization efficiency of BAA-60. Based on above results, the optical absorption capabilities for visible light of synthesized samples as well as the separation and transfer efficiency of photo-generated electron-hole pairs were enhanced greatly after the introduction of A-AgCl. Moreover, BAA-60 exhibited the best optical absorption capabilities for visible light, photoelectrochemical performance, and highest O2 activation ability indicating that the illumination time played a significant role in improving the photocatalytic performances, with too long or too short irradiation time should be avoided.
Fig. 7. (a) Mott-Schottky plots of Bi2MoO6 and BAA-60. (b) Constructed cluster model and (c) electron difference density analysis of Bi2MoO6/Ag. Furthermore, to further understand and retrieve quantitative insight about the charge carrier density of BAA-60, the capacitance of electrode/electrolyte was measured. Fig. 7a showed the Mott-Schottky plots as 1/C2 vs potential, the slopes of all samples were positive, indicating that they were n-type semiconductors with electrons as majority carriers. The linear parts of the curves are extrapolated to 1/C2 = 0, the values of Efb are estimated to be -0.19 eV and -0.09 eV for Bi2MoO6 and
BAA-60, respectively. Then, the calculated carrier densities [34] of Bi2MoO6 and BAA-60 were 3.328×1030 and 4.596×1030 cm-3, respectively. The electron density of BAA-60 was about 1.5 times higher than Bi2MoO6, suggesting a much faster carrier transfer in BAA-60. Moreover, the simulated charge distributions and charge difference analysis (Fig. 7b and 7c) showed that Ag nanoparticles in Bi2MoO6/Ag were gaining electrons, which provide intuitive proof of the charge transfer from Bi2MoO6 to Ag nanoparticles. Therefore, combined with the above measurements, it was considered that the Bi2MoO6/Ag-AgCl photocatalyst optimized the vital processes in photocatalytic disinfection. In the following parts, BAA-60 was selected to further investigate the disinfection process, the changes of bacterial morphology as well as the effect of pH and humic acid on the photocatalytic disinfection activities. 3.4 Photocatalytic disinfection process of Bi2MoO6/Ag-AgCl photocatalysts
Fig. 8. Confocal fluorescence images of live and dead E. coli (109 cfu/mL) during the photocatalytic inactivation process. The scale bar is 50 μm. To explore the disinfection progress of BAA-60 towards E. coli, fluorescent-based cell live/dead test was carried out after exposed to visible light for different time. The DNAs of E. coli were stained with a mixture of two fluorescent nucleic acid dyes, SYTO9 and PI. SYTO9 is a cell-permeable green-fluorescent nucleic acid dye marking both live and dead bacteria, while PI is a cell-impermeable red fluorescent nucleic acid dye which only marks membrane-compromised cell. As shown in Fig. 8, before the beginning of the experiment, almost no dead cells were observed. After 5 min, a small number of cells displayed red fluorescence, indicting the integrity of some cells were damaged. Ten minutes later, the number of dead E. coli cells increased (exhibiting red fluorescence). After 20 minutes, most of the E. coli cells
were inactivated, while some of cells in free state still survived. The result implied that the absorption of bacteria on the photocatalysts was the first step in the sterilization process. As time extended to 30 min, almost all E. coli cells were agglomerated and dead. All the above results demonstrated that the E. coli cells were most likely to be inactivated by the active species on the surface of photocatalysts.
Fig. 9. SEM images of E. coli treated with AAB-60 (100 μg/mL) at different experimental duration: (a) 0 min, (b) 5min, (c) 10 min, (d) 15min, (e) 20min (f) 30min.
In order to better understand the destruction progress of E. col cells, we also examined the morphology of bacteria at different stages of disinfection using SEM technology (Fig. 9). Prior to the visible-light irradiation, E. coli cells exhibited a regular rod-like structure with an average length of about 1 μm and the cell membrane was smooth and complete. (Fig. 9a). After 5 minutes of visible light illumination, the E. coli cells were absorbed on the photocatalysts and the cell wall wrinkled with small pits (Fig. 9b), indicating that the reactive substance rapidly generated and began to destroy the cell membrane of the E. coli. After 10 min of irradiation, obvious holes were observed on the surface of the membrane, which may be caused by the oxidation of different active species (Fig. 9c). Notably, the E. coli cells were about twice as long as the normal one (as indicated by the yellow double arrow). Cell elongation is a typical SOS response when cell is exposed to fungicides and UV irradiation [1, 35]. With the extension of photocatalytic sterilization time, large cracked cell walls and membranes were observed due to the destruction of the photocatalysts, which could
lead to the leakage of cellular contents (Fig. 9d). After 20 min, the cell morphology was severely distorted and the cell membrane was seriously broken, which promoted the entry of active substances and the degradation of intracellular components (Fig. 8e). At the end of the photocatalytic disinfection experiment, the morphology of E. coli cells was completely destructed and some cells were flat due to the loss of cellular contents (Fig. 9f). The above results indicated that the destruction of E. coli cells began with the cell membrane, and over time, the intracellular components were destroyed, eventually leading to the death of E. coli.
Fig. 10. Effects of (a) pH and (b) humic acid on the photocatalytic disinfection efficiency toward E. coli (107cfu/mL) with AAB-60 (100 μg/mL). Due to the short lifetime and easy recombination of free charge carriers, the better the adsorption efficiency, the higher the degradation efficiency. The pH of the solution is generally considered to be a vital factor affecting the adsorption of bacteria on the photocatalysts. Furthermore, the pH of wastewater polluted by bacteria may change on account of the bacterial metabolism [1]. Therefore, the effect of pH on the photocatalytic disinfection performance was investigated by adjusting the pH of the mixture while maintaining other experimental conditions. As shown in Fig. 10a, the sterilization rates of BAA-60 increased slightly with the increase of pH at different pH (5.4, 6.4, 7.4, 8.4), but all of the E. coli cells were completely inactivated within 30 minutes. The above results suggested a better sterilization efficiency of BAA-60 could be obtained in alkaline conditions. Humic acid (HA) is a major component of natural organic matter and is widely found in nature water. Coming from the decomposition of animals and plants, HA is a kind of chemically heterogeneous compound including a great deal of functional
groups such as alcohol, hydroxyl, carbonyl, amine, and carboxyl. It has been reported that HA may influence the disinfection activity by retarding the collision between E. coli cells and photocatalysts [1, 36]. The cell membrane and cell wall of E. coli contain carboxyl, amidogen, and hydroxyl groups, in addition, humic acid has aromaticity. So the E. coli and HA may be combined by hydrogen bond and π-π stacking interactions [1, 37, 38, 39]. If the humic acid substances are absorbed on the surface of E. coli, the disinfection efficiency is quite possible to be influenced since the cell membrane and cell wall are the foremost targets to be attacked by reactive oxygen species [1, 37]. In this study, we explored the effect of HA with different concentrations (3mg/L,5 mg/L,10 mg/L) on the photocatalytic sterilization activity. As exhibited in Fig. 10b, HA showed negligible effect on photocatalytic bactericidal activity indicating the high adaptability of BAA-60 under different HA conditions. Based on the above experiments, the synthetic photocatalysts exhibited potential application in the actual disinfection of microbial contaminated wastewater. 3.5 Photocatalytic disinfection mechanism of Bi2MoO6/Ag-AgCl photocatalysts
Fig. 11. (a) The bactericidal efficiency with different concentrations of Ag+ (2 mg/L, 4 mg/L,6 mg/L), (b) the photocatalytic inactivation efficiencies of E. coli by BAA-60 (100 μg/mL) in the presence of different scavengers (0.05 mmol//L Cr(VI), 0.1 mmol/L EDTA-2Na, 0.5 mmol/L isopropanol, 2 mmol/L TEMPOL) under visible light irradiation. According to previous report, Ag+ exhibits disinfection performance toward a
variety of bacteria [17, 40]. Therefore, it was necessary to detect the concentration of Ag irons during the photocatalytic sterilization process by ICP and test the bactericidal performance (under dark) with different concentrations of Ag+. The concentration of Ag+ leaking from AAB-60 within the experimental duration was detected below 6 mg/L (Fig. S8). According to the sterilization effect with different concentrations of Ag+, the toxicity of Ag+ was negligible under the experimental condition (Fig. 11a).
Hence, the Ag+ leaking from AAB-60 had little contribution to
the sterilization performance. The photo-generated reactive species produced by the photocatalysts could be mainly responsible for their outstanding disinfection performance. As we all know, the reactive oxidative species including e-, •O2−, h+, and •OH play significant roles in the photocatalytic process [1, 41]. Generally speaking, the mechanism of photocatalytic sterilization mainly includes three aspects. Specifically, because the cell wall and membrane of bacteria are semi-permeable, the active species produced by photocatalysis will destroy them, leading to the leakage of potassium ions and other macromolecules in the cells, and finally inactivating bacteria. In addition, coenzyme A plays an important role in cell metabolism. h+ can directly participate in the oxidation of coenzyme A, which can inhibit its reaction in cells and inactivate bacteria. Furthermore, ·OH can directly destroy the structure of DNA and change the protein capsid of RNA to inactivate bacteria [1,41,42]. Since clarifying the roles of different active species in the VLD disinfection process was very important to improve the inactivation performance, sterilization experiments with addition of different
scavengers
into
the
reaction
system
were
conducted.
Cr(VI),
Ethylenediamine tetra-acetic acid disodium salt (EDTA-2Na), isopropanol and TEMPOL were used as scavengers for e-, hole, •OH and •O2−, respectively [17, 32, 41, 43]. As shown in Fig. 11b, conspicuous decrease of sterilization effect was observed after the introduction of EDTA-2Na, Cr(VI) and isopropanol, suggesting the significant role of h+, e− and •OH in the photocatalytic disinfection process. In addition, the sterilization effect of AAB-60 was slightly inhibited after the addition of TEMPOL, indicating that a small amount of •O2− was produced during the
sterilization process. Moreover, no obvious characteristic peak of •O2− was detected in the ESR test (Fig. S9a), which suggested the weak role of •O2−. However, obvious signal of •OH was produced by BAA-60 under visible light irradiation (Fig. S9b). Based on the above results, the main active species generated by BAA-60 in the VLD inactivation system were determined to be h+, e− and •OH.
Fig. 12. Schematic diagram of the photocatalytic mechanism of Bi2MoO6 loaded with Ag/AgCl. Based on the above results and analysis, a possible mechanism for the photocatalytic activity enhancement of the Bi2MoO6/Ag-AgCl photocatalyst was proposed in Fig. 12, in which the band gaps of Bi2MoO6 and AgCl was divided from the Tauc plots (Fig. S10). The layered nanostructure of Bi2MoO6/Ag-AgCl can provide a lot of interfaces between the Ag-AgCl and Bi2MoO6, which were considered to provide a large number of active sites for photocatalytic disinfection. Under visible light irradiation, more light can be absorbed by Bi2MoO6 with the assistance of plasmonic Ag, resulting in more electron-hole pairs. Then, Ag nanoparticles in direct contact with the semiconductor can efficiently capture the photo-generated electrons from the Bi2MoO6 conduction band, while the holes accumulated at the valence band can directly inactivate bacteria. Meanwhile, since AgCl cannot be excited by visible light due to its wide band gap, some electrons can further transfer from Ag to the conduction band of AgCl. Subsequently, the electrons in AgCl will be trapped by absorbed O2 and H2O to form •OH, which can further
inactivated bacteria.
4. Conclusions Bi2MoO6/Ag-AgCl photocatalysts were synthesized by a two-step process including the precipitation of AgCl and photo-reduction of Ag on the Bi2MoO6 surface. The resultant Bi2MoO6/Ag-AgCl sample achieved greater photocatalytic activity than that of Bi2MoO6 and Bi2MoO6/AgCl. Under visible light, the Bi2MoO6/Ag (60)-AgCl can photocatalytic disinfection and absolutely inactivate E. coli (107 CFU/mL) in 30 minutes. The improved photocatalytic disinfection activity was attributed to the excellent synergistic effect of Ag and AgCl, where the plasmonic Ag nanoparticles not only enhance the light absorption, but also functioned as an effective intermediary to transfer electrons from Bi2MoO6 conduction band to AgCl conduction band, which can effectively restrain the recombination of electron-hole pairs. This study provided the promising idea of using plasmonic metallic nanoparticles, and promoted the application of photocatalysts in water pollution.
Acknowledgements The authors gratefully acknowledge the financially support by the Ministry of Education, People’s Republic of China as an innovation team rolling project (grant No. IRT-17R58), the Natural Science Foundation of China as general projects (grant Nos. 21377061, 81270041, and 21677080), the Tianjin Commission of Science and Technology as key technologies R&D projects (grant Nos. 15JCYBJC48400, 14ZCZDSF00001, 15JCZDJC41200 and 16YFZCSF00300), Investigation and emergency management of hazardous chemical accidents, research and demonstration of investigation technology and equipment (grant Nos. 2016JSYJD04) and Open Foundation
of
State
Key
Laboratory
of
Mineral
Processing
(BGRIMM-KJSKL-2019-20).
References [1] S.L. Ma, S.H. Zhan, Y.N. Jia, Q. Shi, Q.X. Zhou, Enhanced disinfection application of Ag-modified g-C3N4 composite under visible light, Appl. Catal. B 186 (2016) 77-87.
[2] S.H. Zhan, D.D. Zhu, S.L. Ma, W.C. Yu, Y.N Jia, Y. Li, H.B. Yu, Z.Q. Shen, Highly efficient removal of pathogenic bacteria with magnetic graphene composite, Acs Appl. Mater. Interfaces 7 (2015) 4290-4298. [3]S.F. Zhou, S.B. Huang, X.H. Li, I. Angelidaki, Y.F. Zhang, Microbial electrolytic disinfection process for highly efficient Escherichia coli inactivation, Chem. Eng. J. 342 (2018) 220-227. [4] D.M.A. Alrousan, P.S.M. Dunlop, T.A. Mcmurray, J.A. Byrne, Photocatalytic Inactivation of E. Coli in Surface Water Using Immobilised Nanoparticle TiO2 Films ,Water Res. 43 (2009) 47-54. [5] J. Podporska-Carroll, E. Panaitescu, B. Quilty, L.L. Wang, L. Menon, S.C. Pillai, Antimicrobial properties of highly efficient photocatalytic TiO2 nanotubes, Appl. Catal. B 176 (2015) 70-75. [6] Y.Y. Fan, W.G. Ma, D.X. Han, S.Y. Gan, X.D. Dong, L. Niu, Convenient recycling of 3D AgX/graphene aerogels (X = Br, Cl) for efficient photocatalytic degradation of water pollutants, Adv. Mater. 27 (2015) 3767-3773. [7] H.B. Yu, L.B. Jiang, H. Wang, B.B. Huang, X.Z. Yuan, J.H. Huang, J. Zhang, G.M. Zeng, Modulation of Bi2MoO6-based materials for photocatalytic water splitting and environmental application: a critical review, Chem. Eng. J. 15 (2019) 782-802. M.C. Liu, Y.B. Chen, J.Z. Su, J.W. Shi, X.X. Wang, L.J. Guo, Photocatalytic hydrogen production using twinned nanocrystals and an unanchored NiSx co-catalyst, Nat. Energy 1 (2016) 16151. [8] T. Hisatomi, J. Kubota, K. Domen, Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting , Chem. Soc. Rev. 43 (2014) 7520-7535. [9] J. Li, G.M. Zhan, Y. Yu, L.Z. Zhang, Superior visible light hydrogen evolution of Janus bilayer junctions via atomic-level charge flow steering, Nat. Commun. 7 (2016) 11480. [10] S.J.A. Moniz, S.A. Shevlin, D.J. Martin, Z.X. Guo, J.W. Tang, Visible-light driven heterojunction photocatalysts for water splitting - a critical review, Energy
Environ. Sci. 8 (2015) 731-759. [11] L. Collado, A. Reynal, J.M. Coronado, D.P. Serrano, J.R. Durrant, V.A.D.L.P. O'Shea, Effect of Au surface plasmon nanoparticles on the selective CO2 photoreduction to CH4, Appl. Catal. B 178 (2015) 177-185. [12] R.V. Grieken, J. Marugán, C. Sordo, P. Martínez, C. Pablos, Photocatalytic Inactivation of Bacteria in Water Using Suspended and Immobilized Silver-TiO2, Appl. Catal. B 93 (2009) 112-118. [13] H.X. Shi, G.Y. Li, H.W. Sun, T.C. An, H.J. Zhao, P.K. Wong, Visible-light-driven photocatalytic inactivation of E. coli by Ag/AgX-CNTs (X= Cl, Br, I) plasmonic photocatalysts: bacterial performance and deactivation mechanism, Appl. Catal. B 158 (2014) 301-307. [14]
T.S.
Wu,
K.X.
Wang,
G.D.
Li,
S.Y.
Sun,
J.
Sun,
J.S.
Chen,
Montmorillonite-supported Ag/TiO2 nanoparticles: an efficient visible-light bacteria photodegradation material, ACS Appl. Mater. Interfaces 2 (2010) 544-550. [15] A.Y. Booshehri, R. Wang, R. Xu, The effect of re-generable silver nanoparticles/multi-walled carbon nanotubes coating on the antibacterial performance of hollow fiber membrane, Chem. Eng. J. 230 (2013) 251-259. [16] L.W. Zhang, T.G. Xu, Z. Xu, Y.F. Zhu, Controllable synthesis of Bi2MoO6 and effect of morphology and variation in local structure on photocatalytic activities, Appl. Catal. B 98 (2010) 138-146. [17] J.L. Liang, F.Y. Liu, J. Deng, M. Li, M.P. Tong, Efficient bacterial inactivation with Z-scheme AgI/Bi2MoO6 under visible light irradiation, Water Res. 123 (2017) 632-641. [18] X.X. Li, S.M. Fang, L. Ge, C.C. Han, P. Qiu, W.L. Liu, Synthesis of flower-like Ag/AgCl-Bi2MoO6 plasmonic photocatalysts with enhanced visible-light photocatalytic performance, Appl. Catal, B 176 (2015) 62-69. [19] Y.H. Liang, S.L. Lin, L. Li, J.S. Hu, W.Q. Cui, Oil-in-water self-assembled Ag@AgCl QDs sensitized Bi2WO6: Enhanced photocatalytic degradation under visible light irradiation, Appl. Catal. B 164 (2015) 192-203.
[20] J. Tian, P. Hao, N. Wei, H.Z. Cui, H. Liu, 3D Bi2MoO6 nanosheet/TiO2 nanobelt heterostructure: enhanced photocatalytic activities and photoelectochemistry performance, 5 (2015) 4530-4536. [21] G. Kresse, J. Hafner, Ab initio molecular dynamics for liquid metals, Phys. Rev. B 47 (1993) 558. [22] G. Kresse, J. Furthmiiller, efficiency of Ab-initio total energy calculations for metals and semiconductors using a plane-wave basis Set, Comput. Mater. Sci. 6 (1996) 15-50 [23] H.P. Li, J.Y. Liu, W.G. Hou, N. Du, R.J. Zhang, X.T. Tao, Synthesis and characterization of g-C3N4/Bi2MoO6 heterojunctions with enhanced visible light photocatalytic activity, Appl. Catal. B 160-161 (2014) 89-97. [24] J. Wu, Y.Y. Sun, C.H. Gu, T. Wang, Y.J. Xin, C. Chai, C.Y. Cui, D. Ma, Pt supported
and
carbon
coated
Bi2MoO6
composite
for
enhanced
2,4-dibromophenol degradation under visible-light irradiation: insight into band gap structure and photocatalytic mechanism, Appl. Catal. B 237 (2018) 622-632. [25] G.H. Tian, Y.J. Chen, W. Zhou, K. Pan, Y.Z. Dong, C.G. Tian, H.G. Fu, Facile solvothermal synthesis of hierarchical flower-like Bi2MoO6 hollow spheres as high performance visible-light driven photocatalysts, J. Mater. Chem. 21 (2011) 887-892. [26] S.L. Ma, S.H. Zhan, Y.N. Jia, Q.X. Zhou, Highly efficient antibacterial and Pb(II) removal effects of AgCoFe2O4-GO nanocomposite, Appl. Mater. Interfaces 7
(2015) 10576-10586.
[27] C.Y. Li, M. Meng, S.C. Huang, L. Li, S.R. Huang, S. Chen, L.Y. Meng, P. Rajapandiyan,
S.J.
Zhang,
B.
Ren, “ Smart”
Ag
nanostructures
for
plasmon-enhanced spectroscopies, J. Am. Chem. Soc. 137 (2015) 13784-13787. [28] M.R. Elahifard, S. Rahimnejad, S. Haghighi, M.R. Gholami, Apatite-coated Ag/AgBr/TiO2 visible-light photocatalyst for destruction of bacteria, J. Am. Chem. Soc. 129 (2007) 9552-9553. [29] J.D. Zhuang, W.X. Dai, Q.F. Tian, Z.H. Li, L.Y. Xie, J.X. Wang, P. Liu, X.C.
Shi, D.H. Wang, Photocatalytic degradation of RhB over TiO2 bilayer films: effect of defects and their location, Langmuir 26 (2010) 9686–9694. [30] Q. Yan, M. Sun, T. Yan, M.M. Li, L.G. Yan, D. Wei, B. Du, Fabrication of a heterostructured Ag/AgCl/Bi2MoO6 plasmonic photocatalyst with efficient visible light activity towards dyes, RSC Adv. 5 (2015) 17245-17252. [31] H.P. Li, T.X. Hu, R.J. Zhang, J.Q. Liu, W. G.Hou, Preparation of solid-state Z-scheme Bi2MoO6/MO (M=Cu, Co-3/4, or Ni) heterojunctions with internal electric field-improved performance in photocatalysis, Appl. Catal. B 188 (2016) 313-323. [32] W. Yu, X.J. Liu, L.K. Pan, J.L. Li, J.Y. Liu, J. Zhang, P. Li, C. Chen, Z. Sun, Enhanced visible light photocatalytic degradation of methylene blue by F-doped TiO2, Appl. Surf. Sci. 319 (2014) 107-112. [33] X.S. Sun, X. Lou, X.D. Zhang, J.F. Xie, S. Jin, H. Wang, X.S. Zheng, X.J. Wu, Y. Xie, Enhanced superoxide generation on defective surfaces for selective photooxidation, J. Am. Chem. Soc. 141 (2019) 3797-3801. [34] Z.H. Zhang, Y.J. Yu, P. Wang, Hierarchical Top-porous/bottom-tubular TiO2 nanostructures decorated with Pd nanoparticles for efficient photoelectrocatalytic decomposition of synergistic pollutants, Acs Appl. Mater. 4 (2012) 990-996. [35] G. Applerot, A. Lipovsky, R. Dror, N. Perkas, Y. Nitzan, R. Lubart, A. Gedanken, Enhanced antibacterial activity of nanocrystalline ZnO due to increased ROS-mediated cell injury, Adv. Funct. Mater. 19 (2010) 842-852. [36] J. Li, S.W. Zhang, C.L. Chen, G.X. Zhao, X. Yang, J.X. Li, X.K. Wang, Removal of Cu(II) and fulvic acid by graphene oxide nanosheets decorated with Fe3O4 nanoparticles, Acs Appl. Mater. 4 (2012) 4991-5000. [37] O.K. Dalrymple, E. Stefanakos, M.A. Trotz, D.Y. Goswami, A review of the mechanisms and modeling of photocatalytic disinfection，Appl. Catal. B 98 (2010) 27-38. [38] J.L. Weishaar, G.R. Aiken, B.A. Bergamaschi, M.S. Fram, R. Fujii, K. Mopper, Evaluation of specific ultraviolet absorbance as an indicator of the chemical
composition and reactivity of dissolved organic carbon，Environ. Sci. Technol. 37 (2003) 4702-4708. [39] M.N. Moura, M.J. Martin, F.J. Burguillo, A comparative study of the adsorption of humic acid, fulvic acid and phenol onto Bacillus subtilis and activated sludge, J. Hazard. Mater. 149 (2007) 42-48 [40] R. Goei, T.T. Lim, Ag-decorated TiO2 photocatalytic membrane with hierarchical architecture: photocatalytic and anti-bacterial activities, Water Res. 59 (2014) 207-218. [41] Y.M. Chen, A.H. Lu, Y. Li, L.S. Zhang, H.Y. Yip, H.J. Zhao, T.C. An, P.K. Wong, Naturally occurring sphalerite as a novel cost-effective photocatalyst for bacterial disinfection under visible light, Environ. Sci. Technol. 45 (2011) 5689-5695. [42] S. Lee, K. Nishida, M. Otaki, S. Ohgaki, Photocatalytic inactivation of phage Q-beta by immobilized titanium dioxide mediated photocatalyst, Water Sci. Technol. 35 (1997) 101-106. [43] W.J. Wang, T.W. Ng, W.K. Ho, J.H. Huang, S.J. Liang, T.C. An, G.Y. Li, J.C. Yu, P.K. Wong, CdIn2S4 microsphere as an efficient visible-light-driven photocatalyst for bacterial inactivation: synthesis, characterizations and photocatalytic inactivation mechanisms, Appl. Catal. B 129 (2013) 482-490.
Plasmonic Ag was used as the electron-transfer mediators to transfer electrons from Bi2MoO6 to AgCl.
The Bi2MoO6/Ag-AgCl photocatalyst has excellent activity in photocatalytic inactivation of bacterial with good stability.
Both theoretical and experimental studies were performed to investigate the origin of plasmonic Ag on enhanced photocatalytic performance.
S1385-8947(19)32172-2 https://doi.org/10.1016/j.cej.2019.122762 CEJ 122762
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Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
31 May 2019 20 August 2019 6 September 2019
Please cite this article as: M. Li, D. Li, Z. Zhou, P. Wang, X. Mi, Y. Xia, H. Wang, S. Zhan, Y. Li, L. Li, Plasmonic Ag as electron-transfer mediators in Bi2MoO6/Ag-AgCl for efficient photocatalytic inactivation of bacteria, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.122762
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Plasmonic Ag as electron-transfer mediators in Bi2MoO6/Ag-AgCl for efficient photocatalytic inactivation of bacteria
Mingmei Li,a,1 Deguan Li,c,1 Zhiruo Zhou,a Pengfei Wang,a,b,* Xueyue Mi,a Yuguo Xia,d Haitao Wang,a Sihui Zhana,*, Yi Lie and Liumin Lif
a
MOE Key Laboratory of Pollution Processes and Environmental Criteria / Tianjin
Key Laboratory of Environmental Remediation and Pollution Control, College of Environmental Science and Engineering, Nankai University, Tianjin 300350, P. R. China. E-mail: [email protected], [email protected]. b State c d
Key Laboratory of Mineral Processing, Beijing 102628, P. R. China.
Institute of Radiation Medicine, PUMC & CAMS, Tianjin 300192, P. R. China. School of Chemistry & Chemical Engineering, National Engineering Research
Center for Colloidal Materials, Shandong University, Jinan 250100, P. R. China. e
Department of Chemistry, Tianjin University, Tianjin 300072, P. R. China.
f
China Municipal Engineering North China Commercial Research Institute Co., Ltd,
Tianjin 300074, P. R. China. 1
These authors contributed equally to this work.
ABSTRACT Solar-driven photocatalysis undoubtedly represents one of the most promising alternative water disinfection technologies, however, its practical application is still restrained by the fast recombination of carriers. To overcome this limitation, plasmon-induced photocatalytic processes have gained attention due to their extended optical absorption and enhanced charge separation when metal nanoparticles act as co-catalysts. In this work, to enhance the photocatalytic disinfection of Bi2MoO6/AgCl, the formation of plasmonic Ag nanoparticles as electron transfer mediators was promoted on the surface of Bi2MoO6 photocatalysts. Specifically, the co-modified Bi2MoO6/Ag-AgCl photocatalysts were synthesized via a two-step process involving the precipitation of AgCl and photo-reduction of Ag on the Bi2MoO6 surface. It was found that Bi2MoO6/Ag-AgCl photocatalysts exhibited higher photocatalytic disinfection activity under visible light irradiation than those of Bi2MoO6 and Bi2MoO6/AgCl. The photocatalytic mechanism of Bi2MoO6/AgCl could be attributed to the excellent synergistic effect of Ag and AgCl, where plasmonic Ag nanoparticles promote light absorption and work as electron-transfer mediators to transfer electrons from Bi2MoO6 to AgCl, meanwhile AgCl work as interfacial catalytic active sites to product free radicals based on the powerful characterizations, such as PL spectra, photoelectrochemical methodology and theoretical calculations. This work may provide new insights to employ synergistic effect of heterogeneous photocatalysts for improving the catalytic activities in water purification.
Keywords: Bi2MoO6 microspheres, plasmon effect, Ag, charge separation and transfer, photocatalytic disinfection
1. Introduction Modern society is facing a serious problem of water pollution, particularly the microbial contamination has been paid more and more attention, which pose a huge threat to human health [1]. Every year, millions of people died from serious water-borne diseases [2, 3]. Although the traditional disinfection methods are effective against most pathogens, they may be too costly to implement in developing
regions [4]. Hence, it is necessary to develop effective and environmentally methods to control or eradicate microbial contamination [5]. In recent decades, with the inexhaustible and inexhaustible solar energy, photocatalysis has been widely used in the removal of pollutants, and is expected to be devoted into practical applications [6]. However, due to the lower solar-energy utilization efficiency, high recombination rate of electron-hole pairs and poor selective adsorption, the further commercial development of the technology is still restricted [7]. The activity of photocatalytic reaction has long relied on facilitating charge separation and surface redox reaction [8]. Particularly, the recombination of electron-hole pairs arisen from the random charge transfer dramatically thwart the photocatalytic reaction efficiency [9]. Furthermore, the electron and hole may transfer to the surface on the picosecond timescale, thus the carriers should be separated as far as possible, or as long as possible to achieve optimal efficiency. [10]. In this regard, plasmon-induced photocatalytic processes have attracted widespread attention due to their enhanced charge separation lifetimes and extended optical absorption in a wide range of the visible spectrum when metal nanoparticles act as co-catalysts [11]. Among the metal nanoparticles usually used in photocatalytic pollution degradation, silver has been regarded as one of the most effective co-catalysts, which has shown an enhancement of the interfacial charge transfer and the increasement of the visible light excitation [12]. And several research groups have successfully incorporated Ag into photocatalysts, which confirms the enhancement of photocatalytic sterilization [13-15]. Bi2MoO6, as a typical Aurivillius oxide, has aroused wide interest recently due to its dielectric, ion-conductive, luminescent and catalytic properties [16]. Bi2MoO6 is a layered bismuth oxide, which is formed by [Bi2O2]2+ layers and MoO42- slabs. Due to its relatively small band gap (2.5-2.8eV), Bi2MoO6 is able to capture visible light irradiation [17, 18]. More and more studies have shown that Bi2MO6 can be used as an excellent photocatalyst and solar conversion material for the decomposition and degradation of organic compounds under visible light irradiation [19, 20]. However, due to the low quantum yield of pure Bi2MoO6, it is still a challenge to improve its
photocatalytic efficiency to satisfy the practical applications. Furthermore, the photocatalytic antibacterial effects of the Bi2MoO6 have not been systematically investigated. Herein, highly efficient visible light active Bi2MoO6 photocatalysts co-modified with Ag nanoparticles as electron-transfer mediators and AgCl as interfacial catalytic active sites (denoted as Bi2MoO6/Ag-AgCl) were synthesized, in which AgCl was first deposited on Bi2MoO6, thereafter AgCl decomposed into Ag by photo-reduction and Ag could demonstrate strong plasmon resonance to transfer electrons from Bi2MoO6 to AgCl. It was found that the Bi2MoO6/Ag-AgCl photocatalyst exhibited enhanced
photocatalytic
disinfection
effect
compared
with
Bi2MoO6 and
Bi2MoO6/AgCl. Furthermore, both theoretical and experimental studies were performed to investigate the origin of plasmonic Ag on enhanced photocatalytic performance. This work may provide new insights for rational design and development of highly efficient photocatalysts for practical applications.
2. Experimental section 2.1 Preparation of flower-like Bi2MoO6 microspheres All chemicals were of analytical grade and were used directly without further purification. Flower-like Bi2MoO6 microspheres were synthesized according to reported method with minor modifications [18]. Typically, 3.47 mmol Bi(NO3)3∙5H2O and 1.74 mmol Na2MoO4∙2H2O were dissolved in 10 mL ethylene glycol under stirring. Then, 20 mL ethanol was added slowly to the solution and stirred for another 10 min. The solution was transferred to a 100 mL Teflon-lined autoclave and the autoclave was sealed and maintained at 160 °С for 24 h in an electric oven. After natural cooling at room temperature, the resulting samples were collected and washed with water and ethanol several times, and then dried at 80 °С for 8 h. Finally, the samples were calcinated at 400 °С for 2h in air to obtain Bi2MoO6 microspheres. 2.2 Preparation of Bi2MoO6/AgCl composites All deposited samples were synthesized by deposition and precipitation [18]. Typically, 0.4g Bi2MoO6 was dissolved in 60 ml ultra-pure water and sonicated for 20
min to obtain solution A. Then, 0.18 g NaCl were added into solution A and stirred for 30 min to get solution B. 0.0714 g AgNO3 (mass ratio of AgCl to Bi2MoO6 was 15%) were dissolved in 20ml ultra-pure water named as solution C.
Solution C was
slowly dripped into solution B with magnetically stirring under dark condition. After stirring for 60 min, the precipitate was collected and washed with ultra-pure water for several times, and then dried at 60°С to obtain Bi2MoO6/AgCl (denoted as BA). 2.3 Preparation of Bi2MoO6/Ag-AgCl composites Bi2MoO6/Ag-AgCl photocatalysts were synthesized according to previous method with minor modifications [18]. Briefly, 0.3g Bi2MoO6/AgCl were dispersed into 60 ml ultra-pure water, after stirring for 20 min, the suspension was irradiated by solar simulator 300 W Xe lamp (CEL-HXF300, Ceaulight, Beijing) for 60 min. Then, the product was centrifuged, washed with ultra-pure water and ethyl alcohol; the final sample was dried at 60 °С and named as Bi2MoO6/Ag(60)-AgCl (BAA-60). Similarly, Bi2MoO6/Ag(30)-AgCl (BAA-30), Bi2MoO6/Ag(90)-AgCl (BAA-90) composites were prepared by different illumination time, for 30min, 90min respectively. 2.4 Characterizations Powder X-ray diffraction (XRD) was recorded on Rigaku D/Max 2200PC X-ray diffractometer using Cu Kα radiation (λ=0.15418 nm). Scanning electron microscopy (SEM) was carried out on a Hitachi SU8010 scanning electron microscope operated at an accelerating voltage of 3 kV. High-resolution transmission electron microscopy (TEM and HRTEM) analyses and elemental mapping images were performed on a JEOL JEM-2100F electron microscope operated at an accelerating voltage of 200 kV. XPS measurements were obtained on a Thermal ESCALAB 250 electron spectrometer. The UV-vis diffused reflectance spectra were measured using a Shimadzu Corporation UV-vis spectrophotometer. Fourier transform infrared spectra (FTIR) of the samples were obtained from Thermo Nicolet iS5 FTIR spectrometer. Photoluminescence spectra (PL) were performed on an Edinburgh Instruments FLS920P spectrophotometer at an excitation of 395 nm. 2.5 Theoretical calculation
The exchange correlation functions derived from Perdew, Burke and Ernzerof (PBE) were calculated by using density functional theory (DFT) and generalized gradient approximation (GGA). The valence electron-ion interaction was modeled by the projector augmented wave (PAW) potential as implemented in the Vienna ab initio simulation pakage (VASP) [21,22]. For all the calculations, both the lattice constant and the positions of all atoms are relaxed until the force is less than 0.22 eV/ Å. The total energy standard is 1 × 10−5 eV. 2.6 Photocatalytic inactivation of E. coli. Gram-negative E. coli and Gram-positive S. aureus were used as model bacteria. The bacteria were cultured in a bed temperature incubator at 37 °С for 15 hours to yield a cell count of 109 cfu/mL. Afterwards, the bacteria were centrifuged and washed with sterile 0.85% (wt/vol) saline solution for 3 times. The final concentration of bacteria in the disinfection process was diluted to 107 cfu/mL using 0.85% (wt/vol) saline solution. Samples were dispersed in sterilized water to form 4 mg/mL suspension. The application of prepared samples for disinfection was investigated under visible-light irradiation. Typically, 250 μL of photocatalysts suspension was added into a sterilized beaker which contained 10 mL of E. coli solution (107 cfu/mL) and shaked evenly. Then the mixture was irradiated under a 300 W Xenon arc lamp (CEL-HXF300) with a UV cutoff (λ< 420 nm) and timed simultaneously. The solution was carefully pipetted out at planed time and the bacterial density was calculated by the standard plate count method. Similarly, the dark control groups were carried out in the dark at the same condition. The light control group was conducted without any photocatalyst. All the disinfection experiments were repeated three times. 2.6 Photoelectrochemical measurements The 4 mg sample, 1 ml ethanol and 20 μl electrolyte were mixed by sonication and then dropped onto an indium-tin oxide (ITO) glass to obtain the photoelectric anode. The photoanodes were dried at 60 °С for 6 h after ethanol was completely evaporated. Photoelectrochemical properties of the prepared photoanodes were tested on an
electrochemical workstation (CHI660B, Shanghai Chenhua) with a three-electrode quartz battery system (a saturated calomel electrode reference electrode and a platinum plate counter electrode). 0.1 M Na2SO4 solution was used as the electrolyte. The visible light irradiation was provided using a 300w xenon lamp (CEL-HXF300, Ceaulight, Beijing). Usually, the polarization curves were recorded at a scanning rate of 5 mV s-1 and the transient photocurrent was detected by chopping irradiation.
3. Results and discussion 3.1 Morphologies and microstructures of Bi2MoO6/Ag-AgCl photocatalysts To verify the successful preparation of Bi2MoO6/Ag-AgCl photocatalysts, the as-prepared samples were characterized by XRD, FT-IR, SEM, TEM and XPS. The crystallinity and purity of the synthesized photocatalysts were analyzed by XRD patterns and the results were displayed in Fig. 1a. Intense diffraction peaks of pure Bi2MoO6 samples at 2θ= 28.1°, 32.3°, 46.9°, 55.5° and 58.5° were attributed to (131), (200)/(002), (260)/(212), (331)/(062) and (262) crystal planes of Bi2MoO6, respectively, which can be indexed to Bi2MoO6 (JCPDS No. 76-2388) [17]. In addition to Bi2MoO6, diffraction peaks of AgCl ((JCPDS No.85-1355) [18] were also observed in Bi2MoO6/AgCl and Bi2MoO6/Ag-AgCl photocatalysts, suggesting that the prepared samples were composed of well-crystallized Bi2MoO6 and AgCl. Moreover, the weak diffraction peaks of BAA-30, BAA-60 and BAA-90 samples at 2θ = 38.1° (Fig. 1a, inset) could be attributed to Ag0 (JCPDS No.87-717) [18]. Furthermore, no characteristic peaks corresponding to other phases was discovered, indicating the high purity of the prepared samples.
Fig.1. (a) XRD patterns and (b) FT-IR spectra of as-fabricated samples.
FT-IR can be used to determine the functional groups of photocatalysts. The FT-IR spectra of Bi2MoO6, BA, BAA-30, BAA-60 and BAA-90 were exhibited in Fig.1b. In the spectrum of Bi2MoO6, the peaks at 3425 and 1637 cm−1 could be assigned to O-H vibrations. The absorption bands at 930-630 cm−1 could be ascribed to Mo-O stretching vibrations [23]. The peaks at 571 cm−1 and447 cm−1 were attributed to Bi-O stretching vibration and deformation vibration, respectively [24]. After introduced Ag/AgCl, the Mo-O stretching vibrations bands at 930-630 cm−1 shifted to 900-610 cm−1, suggesting the evident interactions between the Ag-AgCl and Bi2MoO6. This result demonstrated the successful hybridization between Ag-AgCl and Bi2MoO6 rather than simple mixing. To further confirm the morphology and composition of the photocatalysts, SEM and TEM were carried out. The SEM images of Bi2MoO6 were shown in Fig. 2a and Fig. S1a, it was clear that the flower-like Bi2MoO6 microspheres (1-4 μm in diameter) were actually assembled by amounts of flakes with an average thickness of about 10 nm. After the introduction of Ag-AgCl, it can be observed that many irregular Ag-AgCl particles attached on the surfaces of flower-like Bi2MoO6. (Fig. 2b and Fig. S1b). A clear contrast between the dark edges and the pale center on the surface of the composite displayed in TEM image (Fig. 2c) suggested that the interior of the Bi2MoO6 sphere was hollow and the Ag-AgCl on the surface of Bi2MoO6 contained rough and porous surfaces [25]. In the elemental map (Fig. 2d), different colors represented different elements to identify their position in the photocatalyst. Obviously, both the Ag and Cl elements were well distributed on the surface of Bi2MoO6. The TEM images of BAA-30, BAA-60 and BAA-90 in Fig. S2 can further confirm the fabrication of Bi2MoO6/Ag-AgCl hybrid photocatalyst. Furthermore, the clear lattice fringes with the spacing of 0.236, 0.167 and 0.276 nm in the HRTEM images (Fig. 2e) corresponded to the (111) facet of Ag, (311) facet of AgCl and (002) facet of Bi2MoO6, respectively [18, 19]. According to the above results, metallic Ag and
AgCl
were
successfully
loaded
on
the
Bi2MoO6 surface
to
form
Bi2MoO6/Ag-AgCl photocatalyst by the present facile strategy. And the plasmonic Ag may act as the electron-transfer mediators to promote the transfer of photogenerated
electrons and holes and enhance the photocatalytic activity [18].
Fig.2. SEM images of (a) Bi2MoO6 and (b) BAA-60; TEM (c), corresponding element mapping for Ag, Cl, Mo, Bi (d) and HRTEM (e) images of BAA-60. The successful loading of Ag and AgCl on the Bi2MoO6 surface was verified by XPS, which revealed the component elements and their chemical states in the Bi2MoO6/Ag-AgCl photocatalyst. The survey spectrum displayed in Fig. S3 demonstrated that AAB-60 was mainly composed of Bi, Mo, O, Ag and Cl. Two peaks located at about 164.4 eV and 159.1 eV were observed in the XPS spectrum of Bi 4f (Fig. 3a), which could be attributed to the Bi 4f5/2 and Bi 4f7/2 binding energies, respectively. Fig. 3b showed the Mo 3d spectrum of AAB-60, two peaks were observed at around 235.5 eV and 232.4 eV, which could be indexed to the Mo 3d3/2 and Mo 3d5/2 binding energies, respectively. Fig. 3c displayed the Cl 2p1/2 and Cl 2p3/2 peaks of AAB-60 sample, which were located at 199.6 eV and 198.0 eV, respectively [18]. Fig. 3d showed the Ag 3d spectrum of BAA-30, BAA-60 and BAA-90. For BAA-60, the Ag 3d peak consisted of two peaks at 372.9 and 367.0 eV, which could be ascribed to Ag 3d3/2 and Ag 3d5/2 binding energies, respectively. Moreover, each of the peak can be further divided into two different peaks at 373.6 (Ag0), 372.9 (Ag+) and 368.3 (Ag0), 367.0 eV (Ag+), respectively [18]. The XPS results of Ag 3d confirmed the presence of Ag NPs, which was consistent with the results of XRD analysis (Fig. 1a). Notably, the molar ratio of Ag0 to total Ag was 26.34% based on
the XPS spectra. In contrast, the molar ratio of Ag0 to total Ag in BAA-30 and BAA-90 was calculated to be 6.23% and 44.53%, respectively (Table S1). The XPS results confirmed that the prepared composite photocatalysts were consisted of metallic Ag, AgCl and Bi2MoO6, furthermore, the amount of metallic Ag increased with the photo-reduction time extension.
Fig. 3. The high-resolution XPS spectra of BAA-60: (a) Bi 4f, (b) Mo 3d, (c) Cl 2p; (d) Ag 3d peaks of BAA-30, BAA-60 and BAA-90 3.2 Photocatalytic disinfection performance of Bi2MoO6/Ag-AgCl photocatalysts
Fig. 4. Disinfection efficiencies of the as-synthesized samples (100 μg/mL) toward E. coli (107 cfu/mL) (a) in the dark, (b) under visible light irradiation. Error bars represent standard deviations from triplicate experiments (n = 3). The antibacterial activities of Bi2MoO6, BA, Ag/AgCl, BAA-30, BAA-60 and BAA-90 samples were evaluated using Gram-negative E. coli and Gram-positive S. aureus as bacteria organism. As shown in Fig. 4a, no obvious viable cell density
decrease of E. coli cells was observed under dark condition, indicating that the as-synthesized photocatalysts exhibited negligible cytotoxicity to E. coli without visible light illumination. Fig. 4b illustrated the disinfection results of as-prepared photocatalysts towards E. coli with visible light irradiation. It can be observed that almost no E. coli cells was inactivated in the light control, demonstrating that the saline solution or visible light had no adverse impact on the cell viability. Among all fabricated photocatalysts, Bi2MoO6/Ag-AgCl (BAA) composites displayed higher activity than that of Bi2MoO6, Bi2MoO6/AgCl (BA) and Ag/AgCl. After 30 min of visible light irradiation, only about 0.16, 4.13 and 2.33 log CFU/mL of viable cells were inactivated by Bi2MoO6, BA and Ag/AgCl respectively. The enhanced disinfection performance of Bi2MoO6/Ag-AgCl may be due to the SPR effect of metallic Ag, which can promote the separation of electrons and holes and improve the photocatalytic activity. Notably, as for BAA samples, the BAA-60 showed the best disinfection efficiency, which completely inactivated E. coli in 30 min. In addition, 4.95 and 5.29 log CFU/mL of viable cells were inactivated by BAA-30 and BAA-90, respectively. The results indicated that photo-reduction time affected the photocatalytic activity of the composite photocatalysts by adjusting the amounts of Ag/AgCl. Fig. S4 exhibited the disinfection results of the prepared photocatalysts on S. aureus. It can be observed that B composites also exhibited excellent sterilization effect toward S. aureus under visible light irradiation, and BAA-60 can completely inactivate S. aureus within 50 min. To avoid the waste and shortage of catalyst, we tested the bacteriostatic performance of BAA-60 with different concentrations (50 μg/mL, 80 μg/mL, 100 μg/mL, 120 μg/mL, 150 μg/mL) toward E. coli. As shown in fig. S5, the bacteriostatic activity of BAA-60 increased with the increase of catalyst concentration within 100 μg/mL. However, when the catalyst concentration exceeded 100 μg/mL, the sterilization effects were no longer improved. This results indicated that the optimum amount of catalyst was 100 μg/mL.
Fig. 5. The removal efficiencies of BAA-60 (100 μg/mL) against E. coli (107 CFU/mL) in four cycles under visible light. The reusability and stability of photocatalysts are crucial for their practical application in water remediation. To investigate the reusability of BAA-60, the materials were collected after photocatalytic disinfection to the next cycle. As shown in Fig. 5, all the E. coli cells were inactivated in the first two cycles. After four cycles, only 1.25 log of E. coli cells survived, which may be due to the insufficient cleaning of bacterial residue on the surface of the materials [26]. The SEM image of BAA-60 composites with ultrasonication after antibacterial application was shown in Fig. S6, it can be observed that there were a large number of bacterial residues on the surface of BAA-60 (as indicated by the red circles). This result indicated that the as-synthesized BAA-60 heterojunction photocatalysts exhibited excellent stability in the VLD photocatalytic disinfection process. 3.3 Photocatalytic mechanism of Bi2MoO6/Ag-AgCl photocatalysts
Fig. 6. (a) UV-vis diffuse reflectance, (b) PL spectra, (c) Photocurrent responses and (d) EIS spectra of as-synthesized photocatalysts. To verify the photocatalytic mechanism, the UV-vis diffuse reflectance, PL, Photoelectrochemical measurements and DFT calculations were conducted. Firstly, the absorption capacity of the as-prepared materials for visible light was studied by UV–vis DRS. As exhibited in Fig. 6a, the absorption edges of pure Bi2MoO6 and BA was determined to be 480nm and 486nm, respectively. After the introduction of Ag nanoparticles, the absorption abilities in visible light region were obviously enhanced and an apparent red shift was observed, indicating that the presence of Ag nanoparticles expended the range of visible light spectrum available for utilization comparing with pure Bi2MoO6 and BA. This phenomenon may be due to the surface plasmon resonance (SPR) effect of silver nanoparticles which was derived from the collective oscillation of free electrons excited by the matching photon energy [19, 27, 28]. Specifically, the absorption edge of BAA-60 was determined to be 580 nm, which was wider than that of BAA-30 (564 nm) and BAA-90 (556 nm). This result suggested that the amount of metallic Ag affected the absorption range of photocatalysts for visible light. However, for BAA-90, too much of AgCl would be photo-reduced to Ag-AgCl nanoclusters, which may weaken the SPR effect of Ag nanoparticles as well as shaded the active sties on the surface of Bi2MoO6 [29, 30]. Therefore, the photocatalytic activity of the composite photocatalysts would be
reduced. Based on the above results, the BAA-60 photocatalysts exhibited the best photocatalytic performance which was consistent with the result of photocatalytic sterilization experiments (Fig. 4b). In addition to the optical absorption properties of materials, the separation and recombination efficiency of photo-generated electron-hole pairs also played a key role on the photocatalytic activity [17, 31]. To investigate the recombination efficiency of photo-generated electrons and holes in the photocatalysts, PL spectra characterizing the radiative charge recombination was conducted (Fig. 6b). It can be observed that the emission peak intensities in PL spectra exhibited the following order: Bi2MoO6 ＞ BAA-90＞BAA-30＞BA＞BAA-60. It is believed that the weaker the intensity of the emission peak, the lower the recombination efficiency of charge carriers [17]. Consequently, the introduction of Ag-AgCl on the surface of Bi2MoO6 microspheres can effectively prevent the recombination of electron-hole pairs. Furthermore, BAA-60 composite photocatalysts exhibited the lowest recombination rate of photogenerated charge carriers among all samples. Transient photocurrent response was used to research the separation process of photogenerated charge carriers [1, 32]. Fig. 6c exhibited the transient photocurrent response results of Bi2MoO6, BA, BAA-30, BAA-60 and BAA-90 with several on-off cycles of intermittent visible light irradiation. The intensity of photocurrent responses under visible light irradiation exhibited the following order: BAA-60＞BAA-30＞BA＞BAA-90＞Bi2MoO6 and the photocurrent responses were significantly enhanced after introduced Ag-AgCl. The results suggested that compared with pure Bi2MoO6, the recombination efficiency of the photogenerated electron-hole pairs was reduced greatly and the lifetime of the free charge carriers was extended after introduced Ag-AgCl. Notably, BAA-60 exhibited the highest intensity of photocurrent responses. EIS tests of the as-fabricated samples were conducted to determine their charge migration efficiency [1, 32]. Fig. 6d showed the Nyquist plots under visible light irradiation. It can be observed that, the radian of BAA-60 on the EIS plots was the smallest of all test samples, suggesting that BAA-60 exhibited the highest separation and migration efficiency of
photogenerated charge carriers. To further explore the underlying mechanisms, O2 activation tests were conducted with 3,3′,5,5′-tetramethylbenzidine (TMB) as an indicator molecule by monitoring the absorbance change in the different sample solutions [33]. As illustrated in Fig. S7, BAA-60 exhibited the highest O2 activation ability for ROS generation, which further explains the optimal sterilization efficiency of BAA-60. Based on above results, the optical absorption capabilities for visible light of synthesized samples as well as the separation and transfer efficiency of photo-generated electron-hole pairs were enhanced greatly after the introduction of A-AgCl. Moreover, BAA-60 exhibited the best optical absorption capabilities for visible light, photoelectrochemical performance, and highest O2 activation ability indicating that the illumination time played a significant role in improving the photocatalytic performances, with too long or too short irradiation time should be avoided.
Fig. 7. (a) Mott-Schottky plots of Bi2MoO6 and BAA-60. (b) Constructed cluster model and (c) electron difference density analysis of Bi2MoO6/Ag. Furthermore, to further understand and retrieve quantitative insight about the charge carrier density of BAA-60, the capacitance of electrode/electrolyte was measured. Fig. 7a showed the Mott-Schottky plots as 1/C2 vs potential, the slopes of all samples were positive, indicating that they were n-type semiconductors with electrons as majority carriers. The linear parts of the curves are extrapolated to 1/C2 = 0, the values of Efb are estimated to be -0.19 eV and -0.09 eV for Bi2MoO6 and
BAA-60, respectively. Then, the calculated carrier densities [34] of Bi2MoO6 and BAA-60 were 3.328×1030 and 4.596×1030 cm-3, respectively. The electron density of BAA-60 was about 1.5 times higher than Bi2MoO6, suggesting a much faster carrier transfer in BAA-60. Moreover, the simulated charge distributions and charge difference analysis (Fig. 7b and 7c) showed that Ag nanoparticles in Bi2MoO6/Ag were gaining electrons, which provide intuitive proof of the charge transfer from Bi2MoO6 to Ag nanoparticles. Therefore, combined with the above measurements, it was considered that the Bi2MoO6/Ag-AgCl photocatalyst optimized the vital processes in photocatalytic disinfection. In the following parts, BAA-60 was selected to further investigate the disinfection process, the changes of bacterial morphology as well as the effect of pH and humic acid on the photocatalytic disinfection activities. 3.4 Photocatalytic disinfection process of Bi2MoO6/Ag-AgCl photocatalysts
Fig. 8. Confocal fluorescence images of live and dead E. coli (109 cfu/mL) during the photocatalytic inactivation process. The scale bar is 50 μm. To explore the disinfection progress of BAA-60 towards E. coli, fluorescent-based cell live/dead test was carried out after exposed to visible light for different time. The DNAs of E. coli were stained with a mixture of two fluorescent nucleic acid dyes, SYTO9 and PI. SYTO9 is a cell-permeable green-fluorescent nucleic acid dye marking both live and dead bacteria, while PI is a cell-impermeable red fluorescent nucleic acid dye which only marks membrane-compromised cell. As shown in Fig. 8, before the beginning of the experiment, almost no dead cells were observed. After 5 min, a small number of cells displayed red fluorescence, indicting the integrity of some cells were damaged. Ten minutes later, the number of dead E. coli cells increased (exhibiting red fluorescence). After 20 minutes, most of the E. coli cells
were inactivated, while some of cells in free state still survived. The result implied that the absorption of bacteria on the photocatalysts was the first step in the sterilization process. As time extended to 30 min, almost all E. coli cells were agglomerated and dead. All the above results demonstrated that the E. coli cells were most likely to be inactivated by the active species on the surface of photocatalysts.
Fig. 9. SEM images of E. coli treated with AAB-60 (100 μg/mL) at different experimental duration: (a) 0 min, (b) 5min, (c) 10 min, (d) 15min, (e) 20min (f) 30min.
In order to better understand the destruction progress of E. col cells, we also examined the morphology of bacteria at different stages of disinfection using SEM technology (Fig. 9). Prior to the visible-light irradiation, E. coli cells exhibited a regular rod-like structure with an average length of about 1 μm and the cell membrane was smooth and complete. (Fig. 9a). After 5 minutes of visible light illumination, the E. coli cells were absorbed on the photocatalysts and the cell wall wrinkled with small pits (Fig. 9b), indicating that the reactive substance rapidly generated and began to destroy the cell membrane of the E. coli. After 10 min of irradiation, obvious holes were observed on the surface of the membrane, which may be caused by the oxidation of different active species (Fig. 9c). Notably, the E. coli cells were about twice as long as the normal one (as indicated by the yellow double arrow). Cell elongation is a typical SOS response when cell is exposed to fungicides and UV irradiation [1, 35]. With the extension of photocatalytic sterilization time, large cracked cell walls and membranes were observed due to the destruction of the photocatalysts, which could
lead to the leakage of cellular contents (Fig. 9d). After 20 min, the cell morphology was severely distorted and the cell membrane was seriously broken, which promoted the entry of active substances and the degradation of intracellular components (Fig. 8e). At the end of the photocatalytic disinfection experiment, the morphology of E. coli cells was completely destructed and some cells were flat due to the loss of cellular contents (Fig. 9f). The above results indicated that the destruction of E. coli cells began with the cell membrane, and over time, the intracellular components were destroyed, eventually leading to the death of E. coli.
Fig. 10. Effects of (a) pH and (b) humic acid on the photocatalytic disinfection efficiency toward E. coli (107cfu/mL) with AAB-60 (100 μg/mL). Due to the short lifetime and easy recombination of free charge carriers, the better the adsorption efficiency, the higher the degradation efficiency. The pH of the solution is generally considered to be a vital factor affecting the adsorption of bacteria on the photocatalysts. Furthermore, the pH of wastewater polluted by bacteria may change on account of the bacterial metabolism [1]. Therefore, the effect of pH on the photocatalytic disinfection performance was investigated by adjusting the pH of the mixture while maintaining other experimental conditions. As shown in Fig. 10a, the sterilization rates of BAA-60 increased slightly with the increase of pH at different pH (5.4, 6.4, 7.4, 8.4), but all of the E. coli cells were completely inactivated within 30 minutes. The above results suggested a better sterilization efficiency of BAA-60 could be obtained in alkaline conditions. Humic acid (HA) is a major component of natural organic matter and is widely found in nature water. Coming from the decomposition of animals and plants, HA is a kind of chemically heterogeneous compound including a great deal of functional
groups such as alcohol, hydroxyl, carbonyl, amine, and carboxyl. It has been reported that HA may influence the disinfection activity by retarding the collision between E. coli cells and photocatalysts [1, 36]. The cell membrane and cell wall of E. coli contain carboxyl, amidogen, and hydroxyl groups, in addition, humic acid has aromaticity. So the E. coli and HA may be combined by hydrogen bond and π-π stacking interactions [1, 37, 38, 39]. If the humic acid substances are absorbed on the surface of E. coli, the disinfection efficiency is quite possible to be influenced since the cell membrane and cell wall are the foremost targets to be attacked by reactive oxygen species [1, 37]. In this study, we explored the effect of HA with different concentrations (3mg/L,5 mg/L,10 mg/L) on the photocatalytic sterilization activity. As exhibited in Fig. 10b, HA showed negligible effect on photocatalytic bactericidal activity indicating the high adaptability of BAA-60 under different HA conditions. Based on the above experiments, the synthetic photocatalysts exhibited potential application in the actual disinfection of microbial contaminated wastewater. 3.5 Photocatalytic disinfection mechanism of Bi2MoO6/Ag-AgCl photocatalysts
Fig. 11. (a) The bactericidal efficiency with different concentrations of Ag+ (2 mg/L, 4 mg/L,6 mg/L), (b) the photocatalytic inactivation efficiencies of E. coli by BAA-60 (100 μg/mL) in the presence of different scavengers (0.05 mmol//L Cr(VI), 0.1 mmol/L EDTA-2Na, 0.5 mmol/L isopropanol, 2 mmol/L TEMPOL) under visible light irradiation. According to previous report, Ag+ exhibits disinfection performance toward a
variety of bacteria [17, 40]. Therefore, it was necessary to detect the concentration of Ag irons during the photocatalytic sterilization process by ICP and test the bactericidal performance (under dark) with different concentrations of Ag+. The concentration of Ag+ leaking from AAB-60 within the experimental duration was detected below 6 mg/L (Fig. S8). According to the sterilization effect with different concentrations of Ag+, the toxicity of Ag+ was negligible under the experimental condition (Fig. 11a).
Hence, the Ag+ leaking from AAB-60 had little contribution to
the sterilization performance. The photo-generated reactive species produced by the photocatalysts could be mainly responsible for their outstanding disinfection performance. As we all know, the reactive oxidative species including e-, •O2−, h+, and •OH play significant roles in the photocatalytic process [1, 41]. Generally speaking, the mechanism of photocatalytic sterilization mainly includes three aspects. Specifically, because the cell wall and membrane of bacteria are semi-permeable, the active species produced by photocatalysis will destroy them, leading to the leakage of potassium ions and other macromolecules in the cells, and finally inactivating bacteria. In addition, coenzyme A plays an important role in cell metabolism. h+ can directly participate in the oxidation of coenzyme A, which can inhibit its reaction in cells and inactivate bacteria. Furthermore, ·OH can directly destroy the structure of DNA and change the protein capsid of RNA to inactivate bacteria [1,41,42]. Since clarifying the roles of different active species in the VLD disinfection process was very important to improve the inactivation performance, sterilization experiments with addition of different
scavengers
into
the
reaction
system
were
conducted.
Cr(VI),
Ethylenediamine tetra-acetic acid disodium salt (EDTA-2Na), isopropanol and TEMPOL were used as scavengers for e-, hole, •OH and •O2−, respectively [17, 32, 41, 43]. As shown in Fig. 11b, conspicuous decrease of sterilization effect was observed after the introduction of EDTA-2Na, Cr(VI) and isopropanol, suggesting the significant role of h+, e− and •OH in the photocatalytic disinfection process. In addition, the sterilization effect of AAB-60 was slightly inhibited after the addition of TEMPOL, indicating that a small amount of •O2− was produced during the
sterilization process. Moreover, no obvious characteristic peak of •O2− was detected in the ESR test (Fig. S9a), which suggested the weak role of •O2−. However, obvious signal of •OH was produced by BAA-60 under visible light irradiation (Fig. S9b). Based on the above results, the main active species generated by BAA-60 in the VLD inactivation system were determined to be h+, e− and •OH.
Fig. 12. Schematic diagram of the photocatalytic mechanism of Bi2MoO6 loaded with Ag/AgCl. Based on the above results and analysis, a possible mechanism for the photocatalytic activity enhancement of the Bi2MoO6/Ag-AgCl photocatalyst was proposed in Fig. 12, in which the band gaps of Bi2MoO6 and AgCl was divided from the Tauc plots (Fig. S10). The layered nanostructure of Bi2MoO6/Ag-AgCl can provide a lot of interfaces between the Ag-AgCl and Bi2MoO6, which were considered to provide a large number of active sites for photocatalytic disinfection. Under visible light irradiation, more light can be absorbed by Bi2MoO6 with the assistance of plasmonic Ag, resulting in more electron-hole pairs. Then, Ag nanoparticles in direct contact with the semiconductor can efficiently capture the photo-generated electrons from the Bi2MoO6 conduction band, while the holes accumulated at the valence band can directly inactivate bacteria. Meanwhile, since AgCl cannot be excited by visible light due to its wide band gap, some electrons can further transfer from Ag to the conduction band of AgCl. Subsequently, the electrons in AgCl will be trapped by absorbed O2 and H2O to form •OH, which can further
inactivated bacteria.
4. Conclusions Bi2MoO6/Ag-AgCl photocatalysts were synthesized by a two-step process including the precipitation of AgCl and photo-reduction of Ag on the Bi2MoO6 surface. The resultant Bi2MoO6/Ag-AgCl sample achieved greater photocatalytic activity than that of Bi2MoO6 and Bi2MoO6/AgCl. Under visible light, the Bi2MoO6/Ag (60)-AgCl can photocatalytic disinfection and absolutely inactivate E. coli (107 CFU/mL) in 30 minutes. The improved photocatalytic disinfection activity was attributed to the excellent synergistic effect of Ag and AgCl, where the plasmonic Ag nanoparticles not only enhance the light absorption, but also functioned as an effective intermediary to transfer electrons from Bi2MoO6 conduction band to AgCl conduction band, which can effectively restrain the recombination of electron-hole pairs. This study provided the promising idea of using plasmonic metallic nanoparticles, and promoted the application of photocatalysts in water pollution.
Acknowledgements The authors gratefully acknowledge the financially support by the Ministry of Education, People’s Republic of China as an innovation team rolling project (grant No. IRT-17R58), the Natural Science Foundation of China as general projects (grant Nos. 21377061, 81270041, and 21677080), the Tianjin Commission of Science and Technology as key technologies R&D projects (grant Nos. 15JCYBJC48400, 14ZCZDSF00001, 15JCZDJC41200 and 16YFZCSF00300), Investigation and emergency management of hazardous chemical accidents, research and demonstration of investigation technology and equipment (grant Nos. 2016JSYJD04) and Open Foundation
of
State
Key
Laboratory
of
Mineral
Processing
(BGRIMM-KJSKL-2019-20).
References [1] S.L. Ma, S.H. Zhan, Y.N. Jia, Q. Shi, Q.X. Zhou, Enhanced disinfection application of Ag-modified g-C3N4 composite under visible light, Appl. Catal. B 186 (2016) 77-87.
[2] S.H. Zhan, D.D. Zhu, S.L. Ma, W.C. Yu, Y.N Jia, Y. Li, H.B. Yu, Z.Q. Shen, Highly efficient removal of pathogenic bacteria with magnetic graphene composite, Acs Appl. Mater. Interfaces 7 (2015) 4290-4298. [3]S.F. Zhou, S.B. Huang, X.H. Li, I. Angelidaki, Y.F. Zhang, Microbial electrolytic disinfection process for highly efficient Escherichia coli inactivation, Chem. Eng. J. 342 (2018) 220-227. [4] D.M.A. Alrousan, P.S.M. Dunlop, T.A. Mcmurray, J.A. Byrne, Photocatalytic Inactivation of E. Coli in Surface Water Using Immobilised Nanoparticle TiO2 Films ,Water Res. 43 (2009) 47-54. [5] J. Podporska-Carroll, E. Panaitescu, B. Quilty, L.L. Wang, L. Menon, S.C. Pillai, Antimicrobial properties of highly efficient photocatalytic TiO2 nanotubes, Appl. Catal. B 176 (2015) 70-75. [6] Y.Y. Fan, W.G. Ma, D.X. Han, S.Y. Gan, X.D. Dong, L. Niu, Convenient recycling of 3D AgX/graphene aerogels (X = Br, Cl) for efficient photocatalytic degradation of water pollutants, Adv. Mater. 27 (2015) 3767-3773. [7] H.B. Yu, L.B. Jiang, H. Wang, B.B. Huang, X.Z. Yuan, J.H. Huang, J. Zhang, G.M. Zeng, Modulation of Bi2MoO6-based materials for photocatalytic water splitting and environmental application: a critical review, Chem. Eng. J. 15 (2019) 782-802. M.C. Liu, Y.B. Chen, J.Z. Su, J.W. Shi, X.X. Wang, L.J. Guo, Photocatalytic hydrogen production using twinned nanocrystals and an unanchored NiSx co-catalyst, Nat. Energy 1 (2016) 16151. [8] T. Hisatomi, J. Kubota, K. Domen, Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting , Chem. Soc. Rev. 43 (2014) 7520-7535. [9] J. Li, G.M. Zhan, Y. Yu, L.Z. Zhang, Superior visible light hydrogen evolution of Janus bilayer junctions via atomic-level charge flow steering, Nat. Commun. 7 (2016) 11480. [10] S.J.A. Moniz, S.A. Shevlin, D.J. Martin, Z.X. Guo, J.W. Tang, Visible-light driven heterojunction photocatalysts for water splitting - a critical review, Energy
Environ. Sci. 8 (2015) 731-759. [11] L. Collado, A. Reynal, J.M. Coronado, D.P. Serrano, J.R. Durrant, V.A.D.L.P. O'Shea, Effect of Au surface plasmon nanoparticles on the selective CO2 photoreduction to CH4, Appl. Catal. B 178 (2015) 177-185. [12] R.V. Grieken, J. Marugán, C. Sordo, P. Martínez, C. Pablos, Photocatalytic Inactivation of Bacteria in Water Using Suspended and Immobilized Silver-TiO2, Appl. Catal. B 93 (2009) 112-118. [13] H.X. Shi, G.Y. Li, H.W. Sun, T.C. An, H.J. Zhao, P.K. Wong, Visible-light-driven photocatalytic inactivation of E. coli by Ag/AgX-CNTs (X= Cl, Br, I) plasmonic photocatalysts: bacterial performance and deactivation mechanism, Appl. Catal. B 158 (2014) 301-307. [14]
T.S.
Wu,
K.X.
Wang,
G.D.
Li,
S.Y.
Sun,
J.
Sun,
J.S.
Chen,
Montmorillonite-supported Ag/TiO2 nanoparticles: an efficient visible-light bacteria photodegradation material, ACS Appl. Mater. Interfaces 2 (2010) 544-550. [15] A.Y. Booshehri, R. Wang, R. Xu, The effect of re-generable silver nanoparticles/multi-walled carbon nanotubes coating on the antibacterial performance of hollow fiber membrane, Chem. Eng. J. 230 (2013) 251-259. [16] L.W. Zhang, T.G. Xu, Z. Xu, Y.F. Zhu, Controllable synthesis of Bi2MoO6 and effect of morphology and variation in local structure on photocatalytic activities, Appl. Catal. B 98 (2010) 138-146. [17] J.L. Liang, F.Y. Liu, J. Deng, M. Li, M.P. Tong, Efficient bacterial inactivation with Z-scheme AgI/Bi2MoO6 under visible light irradiation, Water Res. 123 (2017) 632-641. [18] X.X. Li, S.M. Fang, L. Ge, C.C. Han, P. Qiu, W.L. Liu, Synthesis of flower-like Ag/AgCl-Bi2MoO6 plasmonic photocatalysts with enhanced visible-light photocatalytic performance, Appl. Catal, B 176 (2015) 62-69. [19] Y.H. Liang, S.L. Lin, L. Li, J.S. Hu, W.Q. Cui, Oil-in-water self-assembled Ag@AgCl QDs sensitized Bi2WO6: Enhanced photocatalytic degradation under visible light irradiation, Appl. Catal. B 164 (2015) 192-203.
[20] J. Tian, P. Hao, N. Wei, H.Z. Cui, H. Liu, 3D Bi2MoO6 nanosheet/TiO2 nanobelt heterostructure: enhanced photocatalytic activities and photoelectochemistry performance, 5 (2015) 4530-4536. [21] G. Kresse, J. Hafner, Ab initio molecular dynamics for liquid metals, Phys. Rev. B 47 (1993) 558. [22] G. Kresse, J. Furthmiiller, efficiency of Ab-initio total energy calculations for metals and semiconductors using a plane-wave basis Set, Comput. Mater. Sci. 6 (1996) 15-50 [23] H.P. Li, J.Y. Liu, W.G. Hou, N. Du, R.J. Zhang, X.T. Tao, Synthesis and characterization of g-C3N4/Bi2MoO6 heterojunctions with enhanced visible light photocatalytic activity, Appl. Catal. B 160-161 (2014) 89-97. [24] J. Wu, Y.Y. Sun, C.H. Gu, T. Wang, Y.J. Xin, C. Chai, C.Y. Cui, D. Ma, Pt supported
and
carbon
coated
Bi2MoO6
composite
for
enhanced
2,4-dibromophenol degradation under visible-light irradiation: insight into band gap structure and photocatalytic mechanism, Appl. Catal. B 237 (2018) 622-632. [25] G.H. Tian, Y.J. Chen, W. Zhou, K. Pan, Y.Z. Dong, C.G. Tian, H.G. Fu, Facile solvothermal synthesis of hierarchical flower-like Bi2MoO6 hollow spheres as high performance visible-light driven photocatalysts, J. Mater. Chem. 21 (2011) 887-892. [26] S.L. Ma, S.H. Zhan, Y.N. Jia, Q.X. Zhou, Highly efficient antibacterial and Pb(II) removal effects of AgCoFe2O4-GO nanocomposite, Appl. Mater. Interfaces 7
(2015) 10576-10586.
[27] C.Y. Li, M. Meng, S.C. Huang, L. Li, S.R. Huang, S. Chen, L.Y. Meng, P. Rajapandiyan,
S.J.
Zhang,
B.
Ren, “ Smart”
Ag
nanostructures
for
plasmon-enhanced spectroscopies, J. Am. Chem. Soc. 137 (2015) 13784-13787. [28] M.R. Elahifard, S. Rahimnejad, S. Haghighi, M.R. Gholami, Apatite-coated Ag/AgBr/TiO2 visible-light photocatalyst for destruction of bacteria, J. Am. Chem. Soc. 129 (2007) 9552-9553. [29] J.D. Zhuang, W.X. Dai, Q.F. Tian, Z.H. Li, L.Y. Xie, J.X. Wang, P. Liu, X.C.
Shi, D.H. Wang, Photocatalytic degradation of RhB over TiO2 bilayer films: effect of defects and their location, Langmuir 26 (2010) 9686–9694. [30] Q. Yan, M. Sun, T. Yan, M.M. Li, L.G. Yan, D. Wei, B. Du, Fabrication of a heterostructured Ag/AgCl/Bi2MoO6 plasmonic photocatalyst with efficient visible light activity towards dyes, RSC Adv. 5 (2015) 17245-17252. [31] H.P. Li, T.X. Hu, R.J. Zhang, J.Q. Liu, W. G.Hou, Preparation of solid-state Z-scheme Bi2MoO6/MO (M=Cu, Co-3/4, or Ni) heterojunctions with internal electric field-improved performance in photocatalysis, Appl. Catal. B 188 (2016) 313-323. [32] W. Yu, X.J. Liu, L.K. Pan, J.L. Li, J.Y. Liu, J. Zhang, P. Li, C. Chen, Z. Sun, Enhanced visible light photocatalytic degradation of methylene blue by F-doped TiO2, Appl. Surf. Sci. 319 (2014) 107-112. [33] X.S. Sun, X. Lou, X.D. Zhang, J.F. Xie, S. Jin, H. Wang, X.S. Zheng, X.J. Wu, Y. Xie, Enhanced superoxide generation on defective surfaces for selective photooxidation, J. Am. Chem. Soc. 141 (2019) 3797-3801. [34] Z.H. Zhang, Y.J. Yu, P. Wang, Hierarchical Top-porous/bottom-tubular TiO2 nanostructures decorated with Pd nanoparticles for efficient photoelectrocatalytic decomposition of synergistic pollutants, Acs Appl. Mater. 4 (2012) 990-996. [35] G. Applerot, A. Lipovsky, R. Dror, N. Perkas, Y. Nitzan, R. Lubart, A. Gedanken, Enhanced antibacterial activity of nanocrystalline ZnO due to increased ROS-mediated cell injury, Adv. Funct. Mater. 19 (2010) 842-852. [36] J. Li, S.W. Zhang, C.L. Chen, G.X. Zhao, X. Yang, J.X. Li, X.K. Wang, Removal of Cu(II) and fulvic acid by graphene oxide nanosheets decorated with Fe3O4 nanoparticles, Acs Appl. Mater. 4 (2012) 4991-5000. [37] O.K. Dalrymple, E. Stefanakos, M.A. Trotz, D.Y. Goswami, A review of the mechanisms and modeling of photocatalytic disinfection，Appl. Catal. B 98 (2010) 27-38. [38] J.L. Weishaar, G.R. Aiken, B.A. Bergamaschi, M.S. Fram, R. Fujii, K. Mopper, Evaluation of specific ultraviolet absorbance as an indicator of the chemical
composition and reactivity of dissolved organic carbon，Environ. Sci. Technol. 37 (2003) 4702-4708. [39] M.N. Moura, M.J. Martin, F.J. Burguillo, A comparative study of the adsorption of humic acid, fulvic acid and phenol onto Bacillus subtilis and activated sludge, J. Hazard. Mater. 149 (2007) 42-48 [40] R. Goei, T.T. Lim, Ag-decorated TiO2 photocatalytic membrane with hierarchical architecture: photocatalytic and anti-bacterial activities, Water Res. 59 (2014) 207-218. [41] Y.M. Chen, A.H. Lu, Y. Li, L.S. Zhang, H.Y. Yip, H.J. Zhao, T.C. An, P.K. Wong, Naturally occurring sphalerite as a novel cost-effective photocatalyst for bacterial disinfection under visible light, Environ. Sci. Technol. 45 (2011) 5689-5695. [42] S. Lee, K. Nishida, M. Otaki, S. Ohgaki, Photocatalytic inactivation of phage Q-beta by immobilized titanium dioxide mediated photocatalyst, Water Sci. Technol. 35 (1997) 101-106. [43] W.J. Wang, T.W. Ng, W.K. Ho, J.H. Huang, S.J. Liang, T.C. An, G.Y. Li, J.C. Yu, P.K. Wong, CdIn2S4 microsphere as an efficient visible-light-driven photocatalyst for bacterial inactivation: synthesis, characterizations and photocatalytic inactivation mechanisms, Appl. Catal. B 129 (2013) 482-490.
Plasmonic Ag was used as the electron-transfer mediators to transfer electrons from Bi2MoO6 to AgCl.
The Bi2MoO6/Ag-AgCl photocatalyst has excellent activity in photocatalytic inactivation of bacterial with good stability.
Both theoretical and experimental studies were performed to investigate the origin of plasmonic Ag on enhanced photocatalytic performance.