Environmentally Sustainable Fabrication of Ag@g

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Environmentally Sustainable Fabrication of Ag@g-C3N4 Nanostructures and their Multifunctional Efficacy as Antibacterial agents and Photocatalysts Mohammad Ehtisham Khan, Thi Hiep Han, Mohammad Mansoob Khan, Md Rezaul Karim, and Moo Hwan Cho ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00548 • Publication Date (Web): 11 May 2018 Downloaded from http://pubs.acs.org on May 14, 2018

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ACS Applied Nano Materials

Environmentally Sustainable Fabrication of Ag@g-C3N4 Nanostructures and their Multifunctional Efficacy as Antibacterial agents and Photocatalysts Mohammad Ehtisham Khan1*, Thi Hiep Han1, Mohammad Mansoob Khan2*, Md Rezaul Karim1, and Moo Hwan Cho1* 1

School of Chemical Engineering, Yeungnam University, Gyeongsan-si, Gyeongbuk 38541, South Korea. Phone: +82-53-810-2517, Fax: +82-53- 810-4631. 2 Chemical Sciences, Faculty of Science, Universiti Brunei Darussalam, Jalan Tungku Link, Gadong, BE 1410, Brunei Darussalam ABSTRACT Noble-metal silver (Ag) nanoparticles (NPs) anchored/decorated onto polymeric graphitic carbon nitride (g-C3N4) as nanostructures (NSs) were prepared using modest and environment-friendly synthesis method with a developed-single-strain biofilm as a reducing implement. The as-fabricated NSs were characterized using standard characterization techniques. The nanosized and uniform AgNPs were well deposited on to the sheet-like matrix of g-C3N4 and exhibited good antimicrobial activity and superior photodegradation of dyes methylene blue (MB) and Rhodamine B (RhB) dyes under visible-light illumination. The Ag@g-C3N4 NSs exhibited active and effective bactericidal performance and a survival test in counter to Escherichia coli, Staphylococcus aureus, and Pseudomonas aeruginosa. The as-fabricated NSs also exhibited superior visible light photodegradation of MB and RhB in much less time as compared to other reports. Ag@g-C3N4 NSs (3 mM) showed superior photocatalytic measurements under visible-light irradiation: ~100% MB degradation and ~89% of RhB degradation in 210 and 250 min, respectively. The obtained results recommend that the AgNPs were well deposited onto the g-C3N4 structure, which decreases the charge recombination rate of photogenerated electrons and holes, and extends the performance of pure g-C3N4 under visible-light. In conclusion, the as-fabricated Ag@g-C3N4 NSs are keen nanostructured materials that can be applied as antimicrobial materials and visible lightinduced photocatalysts. Keywords: Biogenic synthesis, biofilm, AgNPs, polymeric g-C3N4, Antimicrobial activity, 1

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Photocatalysis INTRODUCTION Traditional/conventional approaches for the fabrication of metal nanoparticles (NPs) typically contain the use of extremely toxic chemicals, expensive solvents, wrapping agents, and risky controlled conditions that increase the cost and cause of environmental pollution1-2. The biogenic/green synthesis of NPs and nanostructures (NSs) using a green chemistry approach can be the best option for reducing the cost and improving it environmental friendliness2-3. Metal-metal oxides NPs have been shaped using physical and chemical approaches for an extended time, nevertheless the latest advances highlight the serious role of micro-organism systems in the biogenic fabrication of metal-metal oxides NPs4. The utilization of micro-organisms for this aim is emerging rapidly due to the simplicity of biogenic formation of NPs with different shapes and sizes. Furthermore, the biogenic synthesis of metal NPs is an ecologically friendly approach deprived of the need for harsh, toxic, and expensive chemicals4-8. For example, the formation of silver (Ag) NPs by a chemical reduction approach (e.g., hydrazine hydrate, sodium borohydride, DMF, and ethylene glycol) might prime to the absorption of severe chemicals on the exteriors of the NPs with potential toxicity and environmental pollution6. Among the metal NPs, AgNPs have concerned

substantial

consideration

for

their

probable

applications

in

catalysis,

biotechnology9, bioengineering, electronics, and optics10-12. In present years, NSs of polymeric graphitic carbon nitride (g-C3N4) have become some of the utmost exciting sustainable materials because of their unique properties and hopeful applications as a homogeneous/heterogeneous catalyst in model dye degradation13. The NSs of g-C3N4 are novel visible-light influence photocatalysts with an appropriate band gap value of 2.7 eV14-16. These NSs area favorable high-performance sheet-like material with good stiffness, lightweight, richness, fabrication from simply available preliminary nanostructures, and better constancy under ambient conditions15, 2


17-18

. The significant

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practical applications of polymeric g-C3N4 have been attributed to the better electron movement and precise surface area, which could be utilize as an effective electron acceptor to advance the photoinduced charge transfer and obstruct the backward reaction by unraveling the evolution sites for better photocatalytic dye degradation performance19-22. This novel sheet-like material has numerous possible applications, such as, water purification, energy conversion and noble performance in the photodegradation of model dyes. The material is also used as a base or supporting material for decoration with metal/metal oxide NPs and as well a stable photocatalyst meant for H2 evolution by splitting water under visible-light irradiation23-24. Although polymeric g-C3N4 has been accepted as a material by means of encouraging possible in the matter of photocatalysis, the photocatalytic ability of wellordered (mesopores structure by means of a defined band gap value of 2.7 eV) polymeric gC3N4 is still in the initial stages because of the high electron–hole charge separation rate, due to the absorbance of blue light from the solar spectrum (λ = 450 nm). Several approaches have been used to develop significant efficacy and promote the photocatalytic ability, such as transitional metal combination, non-metal doping, conjugated polymer adjustment, and pairing with other semiconductors24-25. The synergistic effect and combination of two πconjugated schemes not only steadies the hybrid materials, however improves the consumption of visible light from the spectra through prolonging the optical absorption to an extended wavelength region26-27. To resolve the rapid charge recombination problem, several approaches have been realistic to advance the photocatalytic ability and photo-response of polymeric g-C3N4, for example coupling to other semiconducting materials. Whereas, the doping of elements is considered one of the simplest and most efficient ways to inhibit the recombination of photogenerated charge carriers, and the doped material possibly will be observed as an electron trap27. Furthermore, novel metal NPs are simply in interaction through the semiconductor medium. Furthermore, the experimental increase in the photocatalytic performance has been ascribed to a synergistic effects of the localized surface 3



plasmon resonance (LSPR)28. Superior biogenic/green synthesis strategy can advance the catalytic performance of nanostructures significantly. The metal-polymeric graphitic NSs have been developed as plasmonic photocatalysts with many advantages. First, biogenic synthesis can avoid the need for harsh oxidizing and reducing chemicals and protects the environment. Second, it makes the most of the metal–support interactions via threedimensional interaction among the metal and sheet-like structure of polymeric graphitic materials. Finally, the local electromagnetic field of the LSPR enters the shell, which could be used to adjust the focused wavelength of LSPR28-29. First time, Ag@g-C3N4 NSs was fabricated by an innovative and simple biogenic approach. The effect of a small amount of AgNPs decorated on to the sheet-like structure of g-C3N4 on the superior photocatalytic performance and antibacterial effect were investigated. The effects of the Ag content (1 mM and 3 mM) as a dopant to polymeric g-C3N4 were examined by observing the photodegradation of methylene blue (MB, 10 mg/L) and Rhodamine B (RhB, 10 mg/L) under visible-light illumination (λ > 500 nm). The probable mechanism for the improved photocatalytic performance over the Ag@g-C3N4 NSs photocatalysts was also suggested. The antibacterial performance of the biosynthesized Ag@g-C3N4 NSs were investigated using the agar diffusion method. As expected, the larger inhibition zone of Ag@g-C3N4 NSs (3 mM) compare to Ag@g-C3N4 NSs (1 mM) showed that a higher content of AgNPs results in higher antibacterial activity. To the finest of the author’s information, this report is the leading of the fabrication of Ag@g-C3N4 NSs by a developed-single-strain biofilm and their superior visible light-induced photodegradation and antibacterial performance studies were also investigated systematically. EXPERIMENTAL SECTION Chemicals and Instrumentation Silver nitrate (AgNO3, 99%), MB, and RhB were obtained from Sigma–Aldrich. Urea (98.0%) and sodium acetate (CH3COONa) were attained from KANTO Chemical Co., Japan. The 4


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bacterial culture medium was obtained from the Becton-Dickinson Company (NJ, USA). Altogether chemicals were of analytical grade and utilized as received. The chemical reaction suspension was formed through double-deionized water acquired from a PURE ROUP 30 water distillation arrangement throughout the experiment. X-ray photoelectron spectroscopy (XPS, ESCALAB 250 XPS System, Thermo Fisher Scientific U.K.) was conducted at the Center for Research Facilities, Yeungnam University, South

Korea30

following

same

method

as

described

in

previous

study.

The

absorbance/reflectance (DRS) measurement was carried out using an ultraviolet–visible–near infrared (UV–VIS–NIR) double beam spectrophotometer (VARIAN, Cary 5000, USA)30. The photoluminescence (PL, Kimon, 1 K, Japan) of the as fabricated nanostructures was carried out with the scanning range, 200–800 nm, at an excitation of λ = 325 nm31. Structural analysis were carried out using X-ray diffraction (XRD, PANalytical, X’pert-PRO MPD) using Cu Kɑ radiation (λ = 0.15405 nm)30. The specific surface area analysis were measured using Brunauer, Emmett, and Teller (BET) with specific Belsorp II-mini (BEL, Japan Inc.)32. The morphological and elemental compositional analysis were examined by high resolution transmission electron microscopy (FE-TEM, Tecnai G2 F20, FEI, USA)32. The photocatalytic experiments were conducted using a 400 W lamp (3 M, USA) with λ > 500 nm and an irradiating intensity of 31 mW cm2. The dye degradation ability of the model dye pollutants was examined by evaluating the absorption

value

of

the

model

pollutants

through

UV–Vis

spectrophotometry

(OPTIZEN2120UV)32. Development of single strain biofilm The biofilm on the carbon foam was developed in anode chamber of a microbial fuel cell (MFC) through the procedure reported elsewhere30, 33. Carbon foam (2.5 × 4.5 cm2) in size, were fabricated by melamine sponge template calcination method used as the anode electrode. In anode chamber was filled with, Luria Broth (LB) medium which was inoculated 5



with an overnight culture of Shewanella oneidensis MR-1 at a ratio of 1:100. Before being autoclaved the LB media was sparged with N2 gas for 15 min to eliminate the available oxygen and sustain the anaerobic surroundings. The complete developed biofilm on to the carbon foam was confirmed when the MFC reached at the appropriate voltage after 2 weeks. The obtained living biofilm on the carbon foam was used to fabricate a sequence of Ag@gC3N4 NSs. Developed single-strain biofilm refereed fabrication of Ag@g-C3N4 NSs The polymeric g-C3N4 NSs was prepared via a simplistic single step process using the modest heating of urea at 550 °C in a muffle furnace for 4 h through a ramping rate of 20 °C min-1 under flowing air conditions. The extracted powder remained in light yellow colour subsequently cooling down at room temperature called as polymeric g-C3N4 NSs. Two setup arrangements of 200 mL, 1 mM and 3 mM aqueous suspensions of Ag+ ions were prepared. Consequently, the ideal quantity of sodium acetate (0.2 g) was included separately to the reaction suspension of carbon source. The suspensions were purged by nitrogen (N2) gas for 15 min to ensure the anaerobic surroundings. Fully developed single strain biofilm was dangled into both reaction chamber and the reaction chamber were wrapped and stirred at 30 °C. The reaction mixture systems were stirred for additional 8 h to far-reaching the reaction. In the both cases, the early white color initiated varying to a light off-white then grey color within 30 min. Finally, grayish colored precipitates were attained in the 1 mM and 3 mM AgNO3 cases. The suspensions were centrifuged and washed number of times with ethanol and DI water. The powdered Ag@g-C3N4 NSs were isolated for additional analysis, assessment of applications, and antibacterial studies. The color changes were considered as confirmation of the successful fabrication of Ag@g-C3N4 NSs (Supporting Information Fig. S1). The two accurate reaction mixtures were completed to observe the part of the developed single strain biofilm and addition of sodium acetate. The two sets of 5 mM g-C3N4 6


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aqueous suspensions (200 mL) were organized. In the first precise synthesis, an aqueous suspension comprising a 0.2 g of sodium acetate and 1 mM of AgNO3 was added. In the next second precise synthesis, individually, 3 mM AgNO3 aqueous suspension was added. Separately reaction suspensions were purged with N2 gas for 10 min to sustain the oxygenfree atmosphere. The fully developed single-strain biofilm was dangled in another precise synthesis. Together the systems were taped up and stirred at 30 °C. No differences were identified, even after 48 h. These recognized reaction phases confirmed that the developed single strain biofilms and addition of sodium acetate are needed to complete the fabrication of Ag@g-C3N4 NSs. Antibacterial tests of the Ag@g-C3N4 NSs Determination of Minimum inhibitory concentration The minimum inhibitory concentration (MIC) is well-defined as the minimum concentration of the antibacterial agent that completely hinders the growing stage of bacteria in the medium. The MIC of the as-fabricated Ag@g-C3N4 NSs and polymeric g-C3N4 were resolute using the tube double dilution method34. E. coli was chosen as the index bacteria, and incubated overnight in LB medium at 37 °C. The culture was attuned to 106–107 CFU/mL, and serial concentrations of polymeric g-C3N4 and Ag@g-C3N4 were then introduced to the solution. The growth of bacteria was resolute by evaluating the optical density at 600 nm (OD600) of the bacterial culture solution after 24 h of incubation at 37 °C using a UV-Vis Spectrophotometer (VARIAN, Cary 5000, USA). All the experiments were conducted in triplicate. The antibacterial activity of AgNPs at the similar content mixed with polymeric gC3N4 was examined according to the content of AgNPs in the Ag@g-C3N4 NSs. Growth kinetic test The effects of the as-fabricated NSs on the growth of bacteria and its bacterial activity were examined using E. coli as the model bacteria. The bacterial suspension (106–107 CFU/mL) was treated with 10 µg/mL of either Ag@g-C3N4 or AgNPs + g-C3N4 at 37 °C with 7



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150 rpm. The growth rates of bacterial were assessed by the spectrophotometric absorbance at 600 nm at regular intervals. The LB medium without adding material and polymeric gC3N4 was used as the control. Antibacterial test by dish-diffusion method The antibacterial activities of the as-fabricated NSs against some pathogenic bacteria (Escherichia coli, Staphylococcus aureus and Pseudomonas aeruginosa) were tested using the agar disk-diffusion method35. Briefly, 100 µL of bacteria (106-107 CFU) was spread over an agar plate. Filter paper discs with a diameter of 6 mm were then placed on the agar surface and 30 ml of the solutions at a concentration of 50 µg/mL was then added. Finally, the petri dishes were incubated at 37 °C overnight. The zone of inhibition was then analyzed. Photocatalytic degradation performance of MB and RhB dyes using Ag@g-C3N4 NSs under visible-light irradiation The photocatalytic dye degradation performance of the as-fabricated 1 mM and 3 mM Ag@g-C3N4 NSs and polymeric g-C3N4 samples of 10 mg/L MB and RhB dyes under visiblelight irradiation was studied. The rate of dye degradation was evaluated by the reduction in the absorbance of the reacted solutions30, 33, 36. For the photodecomposition tests, 2 mg of each photocatalyst was added in a 20 mL of aqueous suspension of MB and RhB dyes. In the dark the solutions were sonicated for 5 min, then stirred for 30 min to complete the adsorption and desorption equilibrium stage for the Ag@g-C3N4 NSs and polymeric g-C3N4. Visible-light irradiations of the MB and RhB solutions were conducted

for 210 min and 250

min, respectively, using a 3M 400 W lamp (λ>500 nm). The rate of dyes degradation calculated by observing the UV-Vis spectrum of 1.7 mL of reaction samples each 1 h after removing the catalyst by centrifugation. The scavenger studies were also performed using the similar procedure of the photodegradation of dyes experiment excluding the adding of the isopropyl alcohol (IA), and sodium bicarbonate (SB) and the representative data is shown in Supporting Information Fig. S5 (a and b). 8


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Stability and reusability of the 1 mM and 3 mM Ag@g-C3N4 NSs and polymeric g-C3N4 The initial tests for the stability were achieved by suspending the Ag@g-C3N4 NSs in water and sonicated them for 1 h. To determine the stability of the Ag@g-C3N4 NSs and assess their probable use as a photocatalyst, the level of AgNPs leached out in the solution was observed by UV–Vis spectrophotometry. The recyclability of the Ag@g-C3N4 NSs was conducted by harvesting and reusing the catalyst for 3 cycles under the same conditions. After each cycle, the used catalyst from the MB and RhB dyes solution was collected for centrifugation, washed out with DI water and dried in an oven at 90 °C. RESULTS AND DISCUSSION 3.1. Biogenic fabrication and the projected mechanism for Ag@g-C3N4 NSs The Ag@g-C3N4 NSs were prepared by a unique biogenic/green scheme, which is environment-friendly. The single strain developed biofilms were used as a sustainable, environmentally friendly and surfactant-free method, which could be suitable for this purpose. Fig. 1 describes the fabrication of the Ag@g-C3N4 NSs consuming the biofilm as a reducing agent; the NSs delivered an excess of electrons through decaying the sodium acetate naturally. These electrons support to reduce the Ag+ ions to Ag0 NPs and anchor them to the surface of the g-C3N4. The single-strain biofilm decays the sodium acetate as a carbon source biologically, as expressed in the following equation30, 33, 36-37:

CH3COO¯ + 4H2O + Biofilm → 8e¯ + 9H+ + 2HCO3¯

9


(i)


Figure 1. Representative diagram of the biogenic/green fabrication process for Ag@g-C3N4 NSs. The main benefit of this biogenic procedure, that it does not require any exterior energy involvement or harsh oxidizing/reducing chemicals, which marks this synthesis exceedingly beneficial and effective in the biogenic syntheses of NSs. This is a preliminary report on the biogenic fabrication of Ag@g-C3N4 NSs by a single strain developed biofilm. This study is a best part of the design for a new synthetic path intended to novel photocatalysts using the LSPR of Ag and the visible light-induced behavior of polymeric gC3N4. The g-C3N4 turns not simply as a base material for the AgNPs, but the developed biofilm producing electrons by decomposing the sodium acetate. These electrons can be easily trapped by the mesopores structure of carbon based materials (g-C3N4). These trapped electrons help in the reduction of silver ions to Ag0 significantly at the surface of g-C3N4. 3.2. Standard characterization of bare g-C3N4 and Ag@g-C3N4 NSs 3.2.1. Purity, surface chemical composition and fitted spectral analysis of as-fabricated NSs X-ray photoelectron spectroscopy The purity and surface chemical conformation of the as-fabricated polymeric g-C3N4 and Ag@g-C3N4 (1 mM and 3 mM) NSs were analyzed by XPS, as revealed in Fig. 2(a). The 10


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combined survey spectra (200 – 800 eV) of the polymeric g-C3N4 and Ag@g-C3N4 (1 mM and 3 mM) NSs revealed clear peaks of C, N, Ag, and small intensity peak of O38-39. These peaks provide clear evidence of the successful fabrication of Ag@g-C3N4 (1 mM and 3 mM) NSs. The combined spectra of C 1s showed two C 1s peaks at 284.7 eV and 287.9 eV corresponding to the sp2 C–C bonds (Fig. 2 (b)). These peaks were assigned to extrinsic impurity and defects comprising sp2-hybridized carbon atoms existing in the graphitic domains39. Fig. 2(c) shows the combined N 1s spectra. The main peak at 398.4 eV resembles to sp2-hybridized aromatic N bonded to C atoms (C=N-C). The peak at 399.3 eV was corresponds to the tertiary N bonded to C atoms in the form of N-(C)3. The hump at 400.7 eV was assigned to the N-H structure corresponding to the graphitic structure of C3N4 decorated with AgNPs. In this case, the bonding between carbon and nitrogen in the form of C=N-C shows the sheet like arrangement of two-dimensional structure which exhibit distinctly different electronic and optical properties. Fig. 2(d) displays the high resolution XP spectra of Ag 3d peaks for the Ag@g-C3N4 (1 mM and 3 mM) NSs. Binary peaks at the Ag (3d5/2) and Ag (3d3/2) core levels highlighted at binding energies of 367.4 eV and 373.5 eV were ascribed to the metal Ag0.

300

(c)

400 500 600 Binding energy (eV)

398.4 eV

Combined N 1s spectra

394

700

400.7 eV

396

398 400 402 Binding energy (eV)

404

Pure-g-C3N4 Ag@g-C3N4 (1 mM) Ag@g-C3N4 (3 mM)

287.9 eV

Combined C1s spectra

284.7 eV

800 280 282 284 286 288 290 292 294 296 298 Binding energy (eV)


399.3 eV

(b) Relative Intensity (a.u.)

O 1s

Ag 3d5/2 Ag 3d3/2

C 1s

200

Ag 3p3/2 Ag 3p1/2


(d) Relative Intensity (a.u.)

Combined Survey scan spectra

N 1s

Relative Intensity (a.u.)

(a)

Relative Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ag 3d (1 mM) Ag 3d (3 mM)

Ag 3d5/2

Combined Ag 3d spectra

Ag 3d3/2 0

Ag

0

Ag

406 360 362 364 366 368 370 372 374 376 378 380 Binding energy (eV) 11



Figure 2. XPS (a) survey scans spectra, high resolution spectra (b) C 1s (c) N 1s and (d) fitted spectra of Ag 3d peaks. The atomic weight percentage of the element such as, carbon, nitrogen and silver are revealed in the Supporting Information Table S1, which is attained from XPS analysis. The higher atomic weight% of carbon and nitrogen indicate the existence of carbon and nitrogen rich sheet of g-C3N4. In instance of Ag@g-C3N4 (1 mM and 3 mM) small amount of AgNPs 1.08 and 3.06 weight% exhibited in the as fabricated nanostructures. This is attributed the combined effect of AgNPs and g-C3N4 which made an improved antibacterial and photocatalytic performance. Diffuse-reflectance spectroscopy analysis Fig. 3(a) presents the UV-Vis diffuse reflectance spectra of the as-fabricated polymeric g-C3N4 and Ag@g-C3N4 (1 mM and 3 mM) NSs. Polymeric g-C3N4 has an absorption edge at approximately 460 nm, which generally initiates from its band gap value of 2.7 eV40-41. In the case of Ag@g-C3N4 (1 mM and 3 mM) NSs, the absorption peaks clearly show the LSPR effect of AgNPs42. The absorption edge of the Ag@g-C3N4 (1 mM and 3 mM) NSs as a photocatalyst was moved to a wavelength of 440 nm (known as the higher energy region) from 460 nm for the bare g-C3N4. Whenever higher energy absorption takes place due to plasmonic effect, it leads to shift towards smaller wavelength i.e. blue shift43. This higher absorption value confirmed the successful decoration of g-C3N4 sheets with AgNPs. (a)

10

Pure-g-C3N4 Ag-g-C3N4 (1 mM) Ag-g-C3N4 (3 mM)

Absorbance (a.u.)

1/2

SPR effect

(b)

Pure-g-C3N4 Ag-g-C3N4 (1 mM) Ag-g-C3N4 (3 mM)

8 6

*

(F(R) hv)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4 *Band gap values Pure-g-C3N4 = 2.70 eV Ag-g-C3N4 (1 mM) = 2.50 eV Ag-g-C3N4 (3 mM) = 2.40 3V

2

200

300

400 500 600 Wavelength (nm)

700

0 1.5

800

12

2.0

2.5


3.0 3.5 hv (eV)

4.0

4.5

5.0

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Figure 3. UV–Vis spectra of the as-fabricated samples (a), Tauc plots of polymeric g-C3N4 and Ag@g-C3N4 (1 mM and 3 mM) NSs (b). The optical band gaps were calculated using the Kubleka-Munk equation, as shown in Fig. 3(b)7, 42, as follows:

F R ∞ =

∞

∞

=

λ

λ

∝ α =

ν

ν

(ii)

where F(R∞) is the K-M function or re-emission function; R∞ is the diffuse reflectance of an noticeably dense sample; K(λ) is the absorption coefficient; s(λ) is the scattering coefficient; and hν is the photon energy. The band gap (Eg) value was resolute by extrapolating the linear portion (indicated by drawn-out line in Fig. 3(b)) of the plot attained among [(F(R∞)hν)1/2] vs. hν44. The considered band gaps of the polymeric g-C3N4 and asfabricated Ag@g-C3N4 (1 mM and 3 mM) NSs are as follows: 2.70, 2.50, and 2.40 eV, respectively. The small modification was attributed to the existence of AgNPs on the g-C3N4 sheet. Photoluminescence analysis Fig. 4 shows the photoluminescence (PL) spectra of the polymeric g-C3N4 and Ag@g-C3N4 NSs. The spectra are precise supportive of the migration, charge transfer of carriers, separation of electron-hole charge recombination processes for the photogenerated electron–hole pairs24, 45. The PL intensity is dependent on the recombination processes of electron–hole pair. The as-fabricated polymeric g-C3N4 and Ag@g-C3N4 NSs showed only one PL intensity at 455 nm, which was allocated to the band–band PL occurrence through light energy roughly equivalent to the band gap value of the polymeric g-C3N4 and Ag@gC3N4 NSs for the photo based performance45. The PL intensity is in reverse associated to the charge recombination concerning the photogenerated electron–hole pairs. The decoration of the AgNPs on to the sheet-like arrangement of polymeric g-C3N4 possibly will prevent the charge recombination amid the opposite charge carriers, important to amended photocatalytic

13



performance45-46. Pure-g-C3N4 Ag@g-C3N4 (1 mM) Ag@g-C3N4 (3 mM) Pure-g-C3N4 Ag@g-C3N4 (1 mM) Ag@g-C3N4 (3 mM)

Intensity (a.u.)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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420 440 460 480 500 520 540 560 Wavelength (nm)

300

400

500 600 Wavelength (nm)

700

800

Figure 4. Photoluminescence spectra of the as-fabricated polymeric g-C3N4 and Ag@g-C3N4 (1 mM and 3 mM) NSs. The intense emission peak of the polymeric g-C3N4 highlighted at 455 nm recommends a high recombination possibility of the photogenerated electron-hole pairs, whereas in the instance of Ag@g-C3N4 (1 mM and 3 mM) NSs, significant PL quenching was experiential related to that of polymeric g-C3N4 (Fig. 4). This clearly shows that the lower recombination rate of photogenerated electrons and holes of these NSs were acquired. Hence, after the creation of a heterojunction among g-C3N4 and Ag@g-C3N4 NSs, the recombination rate of photogenerated charge carriers is significantly suppressed. Consequently, the photogenerated electron-hole pairs of the Ag@g-C3N4 (1 mM and 3 mM) NSs can be transferred efficiently at the interface of the heterostructures, resulting in improved photocatalytic performance compared to bare g-C3N4. The expanded graph of Fig. 4 shows the suppressed peak of photogenerated charge carriers between 1 mM and 3 mM of Ag@gC3N4 NSs.

14


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X-ray diffraction The crystal structure, phase and purity of the as-fabricated nanostructures were analyzed by XRD. Fig. 5 (a-d) shows the XRD patterns of the polymeric g-C3N4 and Ag@gC3N4 (1 mM and 3 mM) NSs. Two broad peaks at approximately 13.1°2θ and 27.3° 2θ in the XRD pattern for the polymeric g-C3N4 were resembles to the (100) and (002) planes with d = 0.676 nm and 0.324 nm, respectively. The strong peak at 27.3° 2θ was attributed to the longrange interplanar stacking of the aromatic arrangement which documented as the (002) plane of polymeric g-C3N4 (JCPDS 87-1526)47. (b)

Pure-g-C3N4

(002) g-C3N4

(002)

Ag@g-C3N4 (1 mM)

(111)

(JCPDS 87-1526) Intensity (a.u.)

(100) g-C3N4

Intensity (a.u.)

(a)

(JCPDS - 04-0783)

(200)

(100) (142) (220) (311)

10

20

30

40 50 60 2 Theta (degree)

80 10

70

(111)

20

30


(d)

Ag@g-C3N4 (3 mM)

(c)

80


(111)

(JCPDS - 04-0783)

70

(002)

(200)

(220) 20

30


(002) (200)

(100)

(142)

(100) 10

Intensity (a.u.)

(JCPDS - 04-0783)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


70

(142)

(311) 80 10

(220) 20

30


70

(311) 80

Figure 5. Representative XRD patterns of (a) polymeric g-C3N4, (b and c) 1 mM and 3 mM of Ag@g-C3N4 NSs, and (d) showing the combined XRD patterns.

In 1 mM and 3 mM Ag@g-C3N4 NSs samples (Fig. 5 (b and c)), XRD revealed five separate reflections as well as two peaks for polymeric g-C3N4. The XRD peaks at 32.2°, 46.1°, 54.55°, 67.74°, and 76.84° 2θ were assigned to 111, 200, 142, 220, and 311 planes for AgNPs. 15



Page 16 of 33

The observed reflections were fit coordinated with the AgNPs in the as-fabricated NSs, conforming to the known JCPDS values (04-0783)48-50. The peak intensity intended for the AgNPs improved progressively with cumulative loading of Ag+ ions on to the sheet-like arrangement of polymeric g-C3N4. The peaks confirmed the anchoring of the AgNPs onto the g-C3N4 surface, which were evidently absent in the XRD array of the polymeric g-C3N4 sample. The intensity of the peaks of polymeric g-C3N4 decreased with increasing Ag+ ion concentration and the most intense of peak of AgNPs increased. The combined spectra (Fig. 5 d) also showed the same behavior of the peaks for 1 mM and 3 mM AgNPs with polymeric gC3N4. The existence of both Ag planes and polymeric g-C3N4 confirmed the effective fabrication of the Ag@g-C3N4 NSs by the green synthesis methodology. The mean crystallite size of the polymeric g-C3N4 and Ag@g-C3N4 NSs were calculated using the Scherer’s formula51, D = κλ/βcosθ

……………………(ii)

where κ is the shape factor with a characteristic value of ~ 0.9; λ is the wavelength (Cu Kα = 0.15405 nm); β is the full width at half maximum of the maximum strong peak (in radians); and θ is the main peak of g-C3N4, which is obtained at 27.43° 2θ. The evaluated crystallite size of polymeric g-C3N4 and 1 and 3 mM Ag@g-C3N4 NSs from the maximum intense peak at 27.43° 2θ was 6.6 nm, 24.6 nm, and 31.1 nm, correspondingly. This indicates the crystallite size of the Ag@g-C3N4 NSs increased owing to the anchoring of AgNPs on to the g-C3N4 sheets. These crystallite values confirmed the effective fabrication of the Ag@g-C3N4 NSs. BET specific surface area analysis of the polymeric g-C3N4 and Ag@g-C3N4 NSs The N2-BET (Nitrogen adsorption Brunauer-Emmett-Teller) surface area was examined to identify the variations in the specific surface area of the prepared nanostructures. The precise specific surface areas of the polymeric g-C3N4 and Ag@g-C3N4 NSs (1 mM and 3 mM) were 16.5990 m²/g, 23.7982 m²/g and 38.0923 m²/g, respectively (Supporting Information Table S2). The specific surface area of Ag@g-C3N4 NSs is increases, and the 16


Page 17 of 33

specific surface area is increased slowing with increasing amount of AgNPs, which is owning to the infinitesimal particle size of AgNPs. Moreover, according to the Barret-Joyner-Halenda (BJH) pore size distribution of samples (1 mM and 3 mM). As expected, the 3 mM Ag@gC3N4 NSs revealed considerably higher specific surface area than that of polymeric g-C3N4, and possibly will provide rich reactive sites and a short bulk diffusion length for decreasing the recombination possibility of photoexcited charge carriers52-53. 35 -1

Pure g-C3N4 Ag@g-C3N4 (1 mM) Ag@g-C3N4 (3 mM)

30

-1 3

96 88 80 72 64 56 48 40 32 24 16 8 0 0.0

Pore volume (cm g nm )

3 -1

Volume adsorbed (cm g STP)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


25 20 15 10 5 0 0

150

300

450 600 750 900 1050 1200 Pore diameter (nm)

Pure g-C3N4 Ag@g-C3N4 (1 mM) Ag@g-C3N4 (3 mM)

0.2 0.4 0.6 0.8 Relative pressure (P/P0)

1.0

Figure 6. Nitrogen adsorption-desorption isotherm with the consistent pore size distribution curve (inset) of the polymeric g-C3N4 and Ag@g-C3N4 NSs.

The inset in Fig. 6 displays the pore size distributions of the polymeric g-C3N4 and Ag@g-C3N4 NSs. In the case of 3 mM, the pore volume was three times superior to that of polymeric g-C3N4. The data show that 3 mM of AgNPs is sufficient to construct mesopores structures with a significantly higher surface area to promote the photocatalytic performance54. This may possibly offer more active sites for adsorption in photocatalytic reaction to enhance the photocatalytic performances.

17



High resolution transmission electron microscopy (HR-TEM) analysis The surface analysis, morphology, particle shape, and size of the as-prepared samples, Ag@g-C3N4 NSs, and polymeric g-C3N4 were examined through TEM and HR-TEM. As revealed in Fig. 7 (a and b), the black spots with a shady color were allocated to AgNPs and the sheet-like gray color area was corresponds to the sheet-like morphology of polymeric gC3N4. The sheets of g-C3N4 consisted mostly of graphitic planes through a conjugated aromatic system24. The flat surface of the polymeric g-C3N4 sheet turns as a visible-light absorber. TEM (Fig 7 (a and b)) showed that the number of AgNPs increases through increasing concentration of the Ag precursor, and 3 mM Ag precursor was the optimal amount for loading AgNPs onto the g-C3N4 sheet. SAED (inset of Fig. 7 (a and b)) of the nanostructure revealed a sequence of bright concentric rings, signifying that the as-prepared nanostructures is polycrystalline in nature.

Figure 7. (a and b) TEM images of the 1 mM and 3 mM of Ag@g-C3N4 NSs showing the uniform existence of AgNPs on the g-C3N4 sheet, inset showing the SAED crystal ring pattern, (c) HR-TEM image display the boundary between the smaller and spherical shaped AgNPs 18


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on g-C3N4 with the inset displaying the d spacing from the lattice fringes of AgNPs, (d) selected area of drift correlated spectrum for elemental mapping, (e, f and g), showing elemental maps of C (red color), N (orange color), and Ag (green color), (h) showing the elemental composition of the Ag@g-C3N4 NSs. Fig. 7 (c) clearly shows the interfacial contact of AgNPs with the sheet-like arrangement of g-C3N4, which likewise enclosed the complete surface area of the sheet consistently. The lattice fringes of the Ag (111) plane for metallic Ag specified the crystalline manners of the samples, which further confirmed the existence of AgNPs and the good interaction at the interface of the g-C3N4 sheet. The elemental mapping in Fig. 7 (e, f and g) displays C (red), N (orange), and Au (green), which will be responsible for strong evidence and presence of carbon, nitrogen, and AgNPs attached effectively onto the sheet-like arrangement of polymeric g-C3N4. Fig. 7 (h) presents the elemental composition of the Ag@g-C3N4 NSs without any other elemental peaks. Supporting Information Fig. S2 (a and b) HR-TEM image display the interface and lattice fringes of AgNPs between the smaller and spherical shaped AgNPs on to g-C3N4. Applications of the Ag@g-C3N4 NSs Antibacterial tests using polymeric g-C3N4 and Ag@g-C3N4 NSs The antibacterial activity can be determined by the minimum inhibitory concentration (MIC)55-56. Supporting Information Table S3 lists the MIC of AgNPs+g-C3N4, bare g-C3N4, and Ag@g-C3N4 NSs. The obtained results suggest that the Ag@g-C3N4 NSs (3 mM) had higher antibacterial activity than (1 mM) of Ag@g-C3N4 NSs against E. coli. The MIC of Ag@g-C3N4 NSs (3 mM) was 37.5 µg/mL, whereas the MIC of Ag@g-C3N4 NSs (1 mM) was 75 µg/mL. Bare polymeric g-C3N4 did not show any antibacterial activity; E. coli grew well even at 150µg/mL g-C3N4. The antibacterial activity of the mixture of pure AgNPs and g-C3N4 with the same AgNPs content in the Ag@g-C3N4 NSs (3 mM) was much lower than that of the NSs. This suggests that the as-fabricated Ag@g-C3N4 NSs exhibit combination of 19



accelerated antimicrobial activity, which can reduce the viability of tested bacteria dramatically, due to the decoration of AgNPs on to the surface of g-C3N4. The antibacterial activity of the biosynthesized Ag@g-C3N4 NSs was investigated further using the agar diffusion method. As shown in Fig. 8, the pure g-C3N4 and AgNPs + gC3N4 have practically no noticeable inhibition zone, whereas the as-fabricated Ag@g-C3N4 NSs and streptomycin showed clear inhibition zone compared to all the pathogens tested. As expected, the larger inhibition zone of Ag@g-C3N4 NSs (3 mM) equated to that of Ag@gC3N4 NSs (1 mM) shows that Ag@g-C3N4 with a higher Ag content has higher antibacterial activity. In general, the agar diffusion method, by observing the inhibition zones, is well correlated with the MIC test. The sample with the larger inhibition zone had a smaller MIC. The antibacterial activity of the as-fabricated Ag@g-C3N4 NSs was compared to that of streptomycin, a common antibiotic used to treat numeral of bacterial contaminations, at the same concentration. The antibacterial activity of Ag@g-C3N4 NSs against E. coli and S. aureus was lower than that of streptomycin. In case of P. aeruginosa, which has an advanced antibiotic resistance mechanism, the antibacterial activity of Ag@g-C3N4 NSs was much better than that of streptomycin. This shows that the as-fabricated NSs can be an alternative to traditional antibiotics to treat serious diseases associated with P. aeruginosa. The advanced antibiotic resistance mechanism of P. aeruginosa was caused by the low antibiotic susceptibility and the low permeability of the bacterial cellular envelopes57. The Ag@g-C3N4 NSs with a size on the nanoscale have better permeability through the bacteria cell wall and become more efficient than the antibiotic.

20


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Figure 8. Antimicrobial effects tests on three different microorganisms (Escherichia coli, Staphylococcus aureus and Pseudomonas aeruginosa). (1). Ag@g-C3N4 (1 mM) (2). AgNPs + g-C3N4 (3). Pure g-C3N4 (4). Streptomycin (5). Ag@gC3N4 (3 mM) The optical densities at 600 nm were measured to observe the bacterial growth rate and define the growth curve in the existence of different types of AgNPs. As shown in Supporting Information Fig. S3, The OD value at 600 nm for a mixture of AgNPs+g-C3N4 and 10 µg/mL g-C3N4 increased gradually with increasing incubation time. The OD600 value of a mixture of AgNPs+g-C3N4 was slightly smaller than the control (no material), indicating that the growth of E. coli. is inhibited slightly. The higher OD600 of the bare polymeric gC3N4 than the control was caused by the turbidity of that material. When 10 µg/mL of Ag@gC3N4 NSs (1 mM) was added to the bacterial solution, the maximum absorbance (at 600 nm) was quite low and increased steadily after 100 min. When the bacteria were treated with 10 µg/mL of Ag@g-C3N4 NSs (3 mM), the OD600 was almost constant with time, indicating that the growth of E. coli had been inhibited completely. These results confirm that the as-fabricated Ag@g-C3N4 NSs have powerful antibacterial activity, with a smaller MIC and larger inhibition zone for almost all bacteria studied compared to the other as-fabricated materials. The antibacterial activity of Ag@gC3N4 has been explained by two mechanisms. (1) Ag@g-C3N4 NSs can produce highly reactive oxygen species (ROS) that can oxidize the nucleic acids, proteins, and 21



polysaccharides of the biofilms58. (2) Moreover, the AgNPs can react with DNA, cell membrane, and cellular proteins, leading to cell death59. The mechanisms of the antibacterial activity of Ag@g-C3N4 were reported elsewhere58. Initially, Ag@g-C3N4 NSs can produce highly reactive oxygen species (ROS) that can oxidize the nucleic acids, proteins, and polysaccharides of the biofilms. Secondly, the AgNPs can react with DNA, cell membrane, and cellular proteins, leading to cell death60. Because the antibacterial activity of the mixture with bare polymeric g-C3N4 at the similar AgNPs content in the Ag@g-C3N4 NSs (3 mM) was much lower than that of the NSs, the former mechanism possibly will be more correct for the as-fabricated Ag@g-C3N4 NSs. In this study the physical mixture of AgNPs and bare polymeric g-C3N4, the AgNPs content may be too small (1.39 %) to kill the bacteria. On the other hand, when the AgNPs are deposited on to the surface of polymeric g-C3N4, the AgNPs are dispersed simply, avoiding the accumulation of AgNPs and prominent to improved bacterial activity61. The constancy of AgNPs was also improved by decorating them on to the surface of polymeric g-C3N4, which also helps to accelerate the antimicrobial activity of Ag@g-C3N4 NSs13. Photocatalytic evaluation of model dyes pollutant using polymeric g-C3N4 and Ag@gC3N4 NSs From the point view of practical applications, the Ag@g-C3N4 NSs were tested as a photocatalyst under visible-light irradiation. The results revealed exceedingly enhanced performance equated to the polymeric g-C3N4 because of the higher electron mobility and better speciﬁc surface area, and the sheet-like structure of polymeric g-C3N4 makes it further efficient. Polymeric g-C3N4 plays a significant part as a capable electron generator to improve photoinduced charge transfer for developed photocatalytic performance62. The spatial LSPR effect of AgNPs might make a partial contribution to the improved photocatalytic ability with g-C3N4. The synergistic outcome on the photocatalytic degradation activity of the asfabricated nanostructure was estimated by the photodegradation of MB and RhB as organic 22


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dye pollutants in presence of visible-light illumination. Fig. 9 (a and b) displays the photocatalytic dye degradation kinetics of MB and RhB as a purpose of the irradiation time. Here, C relates to the absorption of the dyes suspension at the time t, and C0 corresponds to the initial absorption (time 0).

Figure 9. (a and b) C/C0 vs. time (h) plots for the photodegradation of MB and RhB (c) stability test, and (d) reusability tests of polymeric g-C3N4 and Ag@g-C3N4 NSs (1 mM and 3 mM). Fig. 9 (a and b) present the photodegradation ability of the dye pollutants, MB and RhB, which were used to examine the photocatalytic degradation capability of polymeric gC3N4 and Ag@g-C3N4 NSs (1 mM and 3 mM) in the dark and under visible-light illumination using a 400 W lamp (λ> 500 nm). The reaction kinetics was plotted as ln (C/C0) vs. time (t)8, 63

. To examine the effect of the AgNPs loading on the sheet-like structure of polymeric g-

C3N4 for the photocatalytic degradation of MB, the bare g-C3N4 degraded ~43% of the MB dye because of the visible-light active behavior of g-C3N4 but the high charge recombination rate was the main factor impeding the degradation rate of g-C3N4. In contrast, the 23



photocatalytic degradation ability of Ag@g-C3N4 NSs (1 mM) was increased to ~89% compared to pure g-C3N4 within 210 min. After the same time, 3 mM of Ag@g-C3N4 NSs showed ~100% degradation, which was attributed to the electronic effects of the AgNPs and the visible light active behavior of g-C3N4 to enhance the photocatalytic performance. Fig. 9 (b) shows the photocatalytic degradation performance of one of the ideal pollutant dyes, RhB, using pure g-C3N4, and Ag@g-C3N4 NSs (1 mM and 3 mM) under visible-light irradiation. A low degradation rate (~23%) were achieved for RhB degradation in the existence of pure gC3N4 after visible-light irradiation, which recommends that the RhB dye is relatively more steady under visible light irradiation64,65. In Ag@g-C3N4 NSs (1 mM and 3 mM), significantly improved photocatalytic ability was observed equated to the bare g-C3N4, which suggests that AgNPs decoration of the sheet-like structure of g-C3N4 can improve the photodegradation ability. The Ag@g-C3N4 NSs (1 mM and 3 mM), showed high photodegradation effectiveness: able to ~63% and ~89% within 250 min under visible-light irradiation66. This was ascribed to the high surface to volume ratio of the small AgNPs decorating the sheet-like structure of polymeric g-C3N4, which assisted to improve the number of photocatalytic sites and g-C3N4 helped decline the charge recombination rate. The photocatalytic dye degradation results indicated that as matched to bare g-C3N4 the fabricated Ag@g-C3N4 NSs (1 mM and 3 mM) showed improved performance. The as fabricated nanostructures are strongly influenced by the ratio of AgNPs 1.08% atomic weight for 1 mM and 3.06% atomic weight for 3 mM (Supporting Information Table S-1) ratio of AgNPs shows the maximum photocatalytic efficacy, which practically two times higher than the bare g-C3N4. This improved photocatalytic performance could be attributed to the synergistic effect of both materials for better visible light utilization. From the viewpoint of practical applications67, the photocatalytic constancy of the sample was evaluated by suspending Ag@g-C3N4 NSs in water and sonicated it for 1 h. As shown in Fig. 9 (c), no AgNPs leached out in the suspension, which defines the stability of 24


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the Ag@g-C3N4 NSs and the probability of using them as a better catalyst. The polymeric gC3N4 and as-fabricated Ag@g-C3N4 NSs showed good reusability tests results, maintaining similar reactivity after three cycles under the same conditions (Fig. 9 (d)). The slight decrease observed originated from the expected loss of catalyst for the duration of the recycling process. Supporting Information Fig. S4 (a, b, c and d) showing the clear adsorption/desorption in the dark and photodegradation of MB and RhB under visible light irradiation to further support the claim of adsorption/desorption equilibrium and rate of degradation with decreasing absorbance value of dyes. Schematic projected photocatalytic mechanism of the Ag@g-C3N4 NSs Fig. 10 displays the photodegradation process using Ag@g-C3N4 NSs under visiblelight illumination for organic pollutant degradation.

Figure 10. Charge transfer mechanism in Ag@g-C3N4 NSs under visible-light irradiation for organic pollutant degradation. Upon visible-light (λ>500 nm) irradiation of the Ag@g-C3N4 NSs, the e- on g-C3N4 can excited easily and move from the VB to CB (eCB-), leaving behind h+ on the VB (hVB+) of g-C3N4. At this point, the excited electrons can transfer simply from the CB of g-C3N4 to the CB of AgNPs. Subsequently, the spatial SPR effect of the AgNPs involved in electrons on 25



Page 26 of 33

the CB of the AgNPs react with O2 and create superoxide radicals (•O2-), which additional produce hydroxyl radicals (•OH)32, 68-69. These created species were confirmed through the addition of different trapping chemical reagents. Isopropyl alcohol (IA) and sodium bicarbonate (NaHCO3) were used as superoxide radicals and hydroxyl radicals scavenger respectively. As revealed in Supporting Information Fig. S5 (a and b), the rate constant Ag@g-C3N4 nanostructures in the existence of IA and NaHCO3 were estimated less than 10% with these scavenger chemical reagents. Therefore, the results display that the above stated species plays a key role in the photodegradation of MB and RhB dyes under visible light irradiation. These •OH radicals are accountable for the MB and RhB degradation. In different, h+ in the VB of g-C3N4 is similarly accountable for the creation of •OH radicals from OH- ions, which are the active species in dye degradation69-70. The overall electron transfer and dye degradation reactions in Ag@g-C3N4 NSs are as follows:

g-C3N4 hʋ

g-C3N4 (eCB- + hVB+)

AgNPs (eCB-) + O2 O2- + 2H+

SPR effect of AgNPs

Movement of e-

AgNPs (eCB-)

(iii)

O2-

•

(iv)

•

2•OH

(v)

h+ + OH-

•

OH

(vi)

OH + MB/RhB mineralized products

(vii)

•

These reactions show that species, such as e-, •O2-, •OH, and h+, are related directly to the photocatalytic degradation process69. Therefore, the formation of •O2-, and •OH radicals actlike major concern in the photocatalytic degradation of dyes under visible-light irradiation. Conclusion A biogenic, green, simple, and cost effective methodology was developed for the fabrication of small spherical AgNPs, which were deposited over sheet-like structure of polymeric gC3N4. The spherical AgNPs enhanced the antibacterial activity, extended the response of the NSs to visible light, suppressed charge recombination, and finally generated reactive oxygen 26


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species, such as e-, •O2-, •OH, and h+, which are accountable for the improved photocatalytic degradation of dyes. The antibacterial test showed that 3 mM of Ag@g-C3N4 NSs have a substantial outcome against E. coli compared to S. aureus and P. aeruginosa. The photocatalytic degradation performance Ag@g-C3N4 NSs (3 mM) was superior to that of 1 mM and polymeric g-C3N4. Approximately ~100% MB degradation and ~89% RhB degradation were achieved within 210 and 250 min, respectively, under visible-light illumination. This exceptional photocatalytic performance may owe the spherical AgNPs and conjugated π structure of polymeric g-C3N4, which have been confirmed to efficiently suppress photocorrosion in duration of the photocatalytic reaction and improve the photodegradation process. Overall, these results provide a novel visible light-induced environment-friendly photocatalyst with high constancy and show a novel way for the fabrication of effective smart NSs for multiple applications. Associated content Author contribution: M. E. Khan planned the work, performed the experiments, analyzed the results and wrote the manuscript. M. E. Khan, T. H. Han equally contributed to this work. M. M. Khan helps in the interpretation of the results and double-checks the English grammar of manuscript. Md. R. Karim helps to develop the biofilm. M. H. Cho approved the submission. All authors have given approval to the final version of the manuscript. Supporting Information The effect of biofilm mediated synthesis for Ag+ ions to Ag0 by changing of colors; atomic weight percent (%) of elements; specific surface area measured from BET analysis of the pure g-C3N4 and Ag@g- C3N4 NSs; MIC tests of the synthesized AgNPs and Ag@g-C3N4; HR-TEM image showing the interface between the smaller and spherical shaped AgNPs on g-C3N4 lattice fringes of AgNPs; growth curve of E. coli treated with 10µg/mL Ag@g-C3N4 NSs; the degradation of MB and RhB dyes under visible light irradiation; the C/C0 plot for the degradation of MB and RhB in the presence of different scavenger reagents (PDF). 27



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Acknowledgements This study was supported by Priority Research

Centers Program

(Grant No.

2014R1A6A1031189) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education. Corresponding Author information Prof. Dr. Moo Hwan Cho Email: [email protected] Orcid ID: https://orcid.org/0000-0002-5350-6035

Notes The authors declare no competing ﬁnancial interest.

28


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TOC Graphic

Proposed schematic of charge transfer mechanism for Ag@g-C3N4 nanostructures under visible light-irradiation.

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Environmentally Sustainable Fabrication of Ag@g

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