Accepted Manuscript Structural, optical, photoluminescence and antibacterial properties of copperdoped silver sulfide nanoparticles Ali Fakhri, Melika Pourmand, Reza Khakpour, Sajjad Behrouz PII: DOI: Reference:
S1011-1344(15)00170-0 http://dx.doi.org/10.1016/j.jphotobiol.2015.05.013 JPB 10044
To appear in:
Journal of Photochemistry and Photobiology B: Biology
Received Date: Accepted Date:
26 March 2015 25 May 2015
Please cite this article as: A. Fakhri, M. Pourmand, R. Khakpour, S. Behrouz, Structural, optical, photoluminescence and antibacterial properties of copper-doped silver sulfide nanoparticles, Journal of Photochemistry and Photobiology B: Biology (2015), doi: http://dx.doi.org/10.1016/j.jphotobiol.2015.05.013
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Structural, optical, photoluminescence and antibacterial properties of copper-doped silver sulfide nanoparticles
Ali Fakhria,*, Melika Pourmandb, Reza Khakpour c, and Sajjad Behrouzb
a
Young Researchers and Elites Club, Science and Research Branch, Islamic Azad University, Tehran, Iran b
Department of Biology, Central Tehran Branch, Islamic Azad University, Tehran, Iran c
Department of Physics, Tehran North Branch, Islamic Azad University, Tehran, Iran
* Corresponding author. Tel.: +98(12)4271296; fax: +98(21)22873079. E-mail: [email protected]
1
Abstract The Ag2S and Cu doped Ag2S nanoparticles were prepared by simple chemical coprecipitation method and characterized by XRD, SEM, EDX, TEM, PL and UV–vis spectra. The photocatalytic activity of Ag2S and Cu doped Ag2S nanoparticles were investigated with Ofloxacin antibiotic, which is part of the fluoroquinolone family. The morphological study indicated that the products were spherical shape in with diameter size of 30 nm. The photocatalytic results demonstrated that the Cu doping increased the photocatalytic efficiency of Ag2S nanoparticles. The outcome of antibacterial experiment under visible light irradiation indicate that the Cu doped Ag2S nanoparticles represent increased antibacterial performance compared with un-doped Ag2S nanoparticles.
Keywords: Cu doped Ag2S; Photoluminescence; Photocatalytic degradation; Antibacterial activity
2
Introduction Due to broad diversity of major efficiency in biological field and optoelectronic, nanotechnology is currently a severe area of scientific research. Nowadays, the study of semiconductor nanoparticles has much consideration because of their novel electronic and optical properties [1]. Nanoparticles (NPs) such as PbS, CdSe and CdS have thin band gap semiconductor quantum dots which have been used as photocatalysts in recently. Among all, silver sulfide nanoparticles are major materials as photocatalysts. Ag2S is a direct, thinband gap semiconductor with good optical limiting and great chemical stability properties [2-4]. Ag2S has a direct band gap (0.9–1.05 eV), and large absorption coefficient which demonstrates a performance semiconductor material for photovoltaic application [2]. The unparalleled properties of Ag2S NPs led to diversity of applications, such as, in light emitting diodes and switches, photo-detectors, solar cells, magnetic field sensors, IR detectors, optical filters, photoconductors and room temperature oxygen sensors [1,3,5,6]. Doping is a commonly applied method to treat magnetic, electrical and optical properties of semiconductor compounds, used in making most optoelectronic and electronic devices. Among transition metal ions, copper (Cu) is the major promising dopant, because of its catalytic activity [7-9]. Therefore, the characterization of transition metal ion states is very important. In the present work, synthesis of un-doped and Cu2+ doped Ag2S nanoparticles, and morphological, spectroscopic, structural properties were studied in detail. These materials may have potential application in lighting devices and solar cells.
3
Material and methods In this study, raw materials were purchased from Sigma-Aldrich Ltd. Synthesis of un-doped and Cu doped Ag2S NPs Ag2S NPs was synthesized by a co-precipitation method. Briefly, 50 mL of Na2S solution (0.001 M) was added dropwise to 50 mL of AgNO3 solution (0.001 M) at 80°C. PVP solution was added to the above mixture, once it attained 60°C. Then solutions were stirred under magnetic stirrer for 3 h until it turned into a light brown color with black precipitates on the surface and then calcined at 100°C for 30 min. The Cu doped Ag2S Nanoparticles (Ag1-xCuxS where x=0.00, 0.05, and 0.10) were synthesized according to the following procedure: Freshly prepared 50ml of aqueous solution of 0.1M Na2S was added dropwise to 50ml of 0.1M solution of AgNO3 and 50ml of 0.1M solution of CuSO4.5H2O using vigorous stirring and then added 0.05g of PVP capping agent. After the completion of reaction, the solution was aged for 45min. The residual solution was filtered and washed several times with distilled water. The wet precipitate was dried in an oven at 80°C for 2h and then calcined at 100°C for 30 min (heating rate :10°C/min). Characterization and analytic instruments A scanning electron microscope (SEM); JEOL JSM-5600 Digital Scanning Electron Microscope, transmission electron microscopy (TEM, JEM- 2100F HR, 200 kV), and X-ray diffractometer (XRD) Philips X’Pert were used for samples characterization. UV-Vis DRS mensuration was performed in a double beam spectrophotometer (JASCO V-550). The SEM (JSM 6701F—6701) was using for examination of surface morphology. Photoluminescence studies were performed using TEC Avaspec 2048 Spectrophotometer (excitation source=Xenon arc lamp 450W). The catalyst compositions were analyzed with an energy dispersive X-ray spectrometer (EDX-700HS, SHIMADZU). Photocatalytic experiments The photocatalytic activity was investigated by measuring the degradation of Ofloxacin antibiotic solution (10 mg/L at pH 6.5) under UV (125W UV lamp at 365nm)
4
and visible (1000W halogen lamp) light illumination. The reactor system made of double walled reaction chamber of glass tubes was applied for photo degradation tests. The solution for photo degradation measurement was prepared by adding pure or 6 and 10mol% Cu doped Ag2S (1g/L) to 50ml aqueous solution of Ofloxacin antibiotic (10mg/L at natural pH=6.5). The mixture was stirred for 10min, and kept in the dark for 1h in order to achieve equilibrium of adsorption reaction. The Ofloxacin antibiotic concentration was distinguished with the aid of a two dimensional Gas Chromatogharphy (GC*GC) (Kimia Shangarf Pars Research Co., Iran). Measurement of antimicrobial property Enterococcus faecalis (ATCC 29212) and Staphylococcus aureus (ATCC 6538) were chosen as the standard bacterium. The shaking flask method was used for the antibacterial experiments. The tests were performed under visible light and in the dark. The shaking flask method consists of adding 0.2g of each sample into 20mL of 0.75mM saline water containing about 108 CFU/mL of bacteria and then shake under visible light. 0.5 mL of the suspension was separated after a 3h contact, and diluted to an appointed volume by tenfold dilution. The diluted solution was plated (in nutrient agar plates) in incubated at 40±1 °C for 24 h.
5
Results and discussion X-ray diffraction analysis The X-ray diffraction patterns of the Ag2S NPs are shown in Fig. 1, and all the characteristic peaks can be observed (JCPDS no. 65-2356). Some of the sharp peaks labeled to as (111), (112), (121), (103), (031), (200), (213) and (134) are attributed to Ag2S NPs. The NPs size at the mentioned 2θ was calculated using the Debye–Scherrer formula, D=0.9λ/βcosθ, where D is the crystalline average diameter, θ is the Bragg angle, β is the excess line width of the diffraction peak in radians and, λ is the wavelength of X-ray [10]. The Full width at half maximum (FWHM) of the diffraction peaks showed some changing upon doping with ions. The average size of un-doped and 6 and 10 mol% Cu doped Ag2S NPs was found to be around 30, 25, and 21 nm, respectively. Fig. 1. XRD patterns of un-doped (A), 6 (B) and 10 mol% (C) Cu doped Ag2S nanoparticles. Morphological studies The prepared NPs were subjected to SEM for the morphological investigation. Fig. 2A and B indicate the surface structure and morphology of un-doped and 10 mol% Cu doped Ag2S nanoparticles. Spherical to ellipsoid shaped particles were formed. The Fig. 2C and D demonstrate the EDX spectrum of the un-doped and Cu doped and the peaks corresponding to Cu observed in the EDX spectrum of the doped instances confirms the incorporation of Cu ions into Ag2S nanoparticles lattice network. Fig. 2. SEM image and EDX spectrum of the un-doped (A,C) and 10 mol% Cu doped Ag2S nanoparticles (B,D) Fig. 3A and B show the TEM images of un-doped and Cu-doped Ag2S nanoparticles from the typical tests. It can be seen that these nanoparticles are nearly spherical in shape and clearly well dispersed. Fig. 3C and D indicate the size distribution of un-doped and Cudoped Ag2S nanoparticles; the samples average diameter are 31.00±0.1497nm (undoped Ag2S), and 22.00±0.1221nm (Cu-doped Ag2S), respectively. The large nanoparticle sizes are in great agreement with the results of XRD characterization.
6
Fig. 3. TEM image and size distribution of un-doped (A,C) and 10 mol% Cu doped Ag2S nanoparticles (B,D)
Optical characterization Optical absorption spectroscopy of synthesized un-doped and Cu (6 and 10%) doped Ag2S NPs was carried out by UV– Visible absorption spectroscopy (25ᵒC). Fig. 4(A) indicates that there is a strong excitonic absorption peak at 415 nm for both Cu doped Ag2S NPs samples. These peaks have good optical quality and large exciton binding energy. From Fig. 4(A), it can be seen that all Cu doped NPs demonstrate better absorption of visible light as compared to un-doped NPs. The band gap energies of the un-doped and doped Ag2S nanoparticles were evaluated by the Kubelka–Munk function F(R)2 vs. energy in electron volts and extrapolation of the linear portion of the curve to F(R)2=0 (Fig. 3(B)) [11]. The optical absorption factor ( ) computed by F(R) =
= (1 - R)2/2R, where R is the percentage
of reflected light. The incident photon energy (
) and the optical band gap energy (Eg)
correspond to the transformed Kubelka–Munk function, [F(R)
]p = A(
- Eg), where, p is
the power index and A is a constant related to transition probability, for the optical absorption process. p equals to 2 or 1/2 for a direct or an indirect allowed transition, respectively. The band-gap of the samples enhances with Cu concentration and was 3.22, 3.31 and 3.38 eV for un-doped Ag2S and 6 and 1 mol% Cu-doped Ag2S, respectively.
Fig. 4. UV–Visible absorption spectra (A) and Kubelka–Munk plots (B) and band gap energy estimation for un-doped (a), 6 (b) and 10 mol% (c) Cu doped Ag2S nanoparticles.
Photoluminescence (PL) analysis Photoluminescence (Pl) emission spectra were applied to investigate the transfer of holes and photo-generated electrons, as well as to gain an understanding of the recombination and separation of photo-generated charge carriers [12,13]. To study the photoelectric properties of the synthesized samples, Pl spectra obtained for different 7
samples are shown in Fig. 5. The spectrum exhibits four emission peaks at around 456, 463, 475nm and 456, 463, 471 nm, respectively, corresponding to blue emission for un-doped and two doped samples of Ag2S nanoparticles. The strong PL peaks may correspond to crystalline defects induced during the growth. Visible emissions are referred as deep-level emission and are due to the recombination of electrons deeply trapped in silver interstitials and oxygen vacancies, with photo-generated holes [14]. A slight shift is seen in PL spectra towards higher wavelength after doping Cu into Ag2S lattice and intensity of luminescence is also reduced, when compared to the un-doped sample. Fig. 5. The PL spectra for un-doped (a), 6 (b) and 10 mol% (c) Cu doped Ag2S nanoparticles
Peruse of photocatalyst activity In this research, the photocatalytic activity of Ag2S NPs was distinguished based on the ability of the Ag2S NPs to decompose Ofloxacin antibiotic under UV and visible light. Since the band gap energy of Ag2S NPs is much larger than the energy of UV irradiation, the generation of reactive species like •OH radicals might take place due to the native defects. Fig. 6 indicates the influence of un-doped and doped Ag2S NPs nanoparticles on decomposing of Ofloxacin antibiotic under UV and visible light illumination [15-19]. The amount of doping Cu atom did not attain the maximum activity, so the light absorption and activity are not 100%. The catalyst has the highest photocatalyst activity for a higher Cu doping.
Fig. 6. Influence of doping on the activity of different Ag2S nanoparticles for degradation of Ofloxacin antibiotic. Process and Mechanism of photocatalytic activity The complexities of the role of transition metal dopant ion are such that it can participate in all of these processes. Acting as electron and/or hole traps is the major function of dopant. The trap of charge carrier scan reduces the recombination rate of
8
electron–hole pairs and consequently, enhances the life time of charge carriers. The process of charge trapping is as follows: Ag2S + hv → h++ eMn+ + e- → M(n-1)+ M(n-1)+ + O2 → M(n-1+1)+ + O2Mn+ + h+ → M(n+1)+ M(n+1)+ + OH- → M(n+1-1)+ + •OH where M is the Cu dopant and n is the valency of Cu ion. The energy level of Mn+/M(n-1)+ lies below the conduction band edge and the energy level of Mn+/M(n+1)+ lies above the valence band edge. Thus, the energy level of Cu ion affects the trapping efficiency. The trapping of electrons facilitates holes to transference onto the surface of semiconductor and react with OH- in the Ofloxacin antibiotic solution and form active •OH to participate the destruction of Ofloxacin antibiotic. Hydroxyl radicals have been deemed to be major active species during the photocatalytic oxidation reaction [20]. Antibacterial activity The prepared un-doped and doped Ag2S nanomaterials were tested for their antibacterial activities under visible light irradiation and in the dark [21]. The antibacterial results are given in Fig. 7A and C. Fig. 7B and D show the SEM images of E. faecalis and S. aureus. It can be seen that the E. faecalis and S. aureus are ovoid and cocci shape. All of the experiments were repeated twice, and average values are shown by using un-doped and doped Ag2S, the surviving bacteria were reduced under visible light irradiation; however, the surviving bacteria of Cu-Ag2S materials were significantly reduced, compared to Ag2S materials. For the less surviving bacteria, antibacterial activities are stronger. This shows that copper synergistic effect is responsible for its efficient photocatalytic antibacterial activity of Cu-Ag2S. In the dark, their antibacterial activities are weaker than those of under the visible light irradiation. Fig. 7. The antibacterial effect of all Ag2S Nanoparticles samples against E. faecalis (A) and S. aureus (C); SEM images of the cultured E. faecalis (B) and S. aureus (D) 9
Conclusions The un-doped and doped Ag2S nanoparticles were successfully synthesized by a simple chemical co-precipitation method. The analysis of SEM and XRD showed that the particles of all samples were at the nano-scale. UV-vis spectra indicated that, as the doping Ag2S increased, the band gap shifted to higher wavelengths. The PL outcome demonstrated that the tuning range of emission wavelength is nearly between 456 and 477 nm in the present one type doped Ag2S nanoparticles. The doping Cu had an influence on the degradation of Ofloxacin antibiotic the Ag2S nanoparticles. The comparison of antibacterial activity of different synthesized Ag2S samples, clearly indicated that the Cu-Ag2S nanoparticles are the active antibacterial for inhibition properties over the growth of Staphylococcus aureus and Enterococcus faecalis.
Acknowledgment The author acknowledges Materials & Energy Research Center and Razi Metallurgical Research Center (RMRC) from Iran for performance support through the project.
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Fig. 1
Fig. 2 13
Fig. 3.
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Fig. 4
Fig. 5
15
100
Degradation of ofloxacin (%)
90
UV light
Ag2S 6% Cu-Ag2S 10% Cu-Ag2S
80 70 60 50 40 30 20 10 0
30
60 90 Irradiation time (min)
100
Degradation of ofloxacin (%)
90
120
Ag2S 6% Cu-Ag2S 10% Cu-Ag2S
Visible light
80 70 60 50 40 30 20 10 0
20
50
80 110 Irradiation time (min)
Fig. 6
16
140
Fig. 7
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Research Highlights
•
Silver sulfide nanoparticles were synthesized by chemical co-precipitation method.
•
Silver sulfide and copper-doped nanoparticles exhibited photocatalytic property under UV and visible light.
• The particles exhibited excellent antimicrobial property.
19
S1011-1344(15)00170-0 http://dx.doi.org/10.1016/j.jphotobiol.2015.05.013 JPB 10044
To appear in:
Journal of Photochemistry and Photobiology B: Biology
Received Date: Accepted Date:
26 March 2015 25 May 2015
Please cite this article as: A. Fakhri, M. Pourmand, R. Khakpour, S. Behrouz, Structural, optical, photoluminescence and antibacterial properties of copper-doped silver sulfide nanoparticles, Journal of Photochemistry and Photobiology B: Biology (2015), doi: http://dx.doi.org/10.1016/j.jphotobiol.2015.05.013
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Structural, optical, photoluminescence and antibacterial properties of copper-doped silver sulfide nanoparticles
Ali Fakhria,*, Melika Pourmandb, Reza Khakpour c, and Sajjad Behrouzb
a
Young Researchers and Elites Club, Science and Research Branch, Islamic Azad University, Tehran, Iran b
Department of Biology, Central Tehran Branch, Islamic Azad University, Tehran, Iran c
Department of Physics, Tehran North Branch, Islamic Azad University, Tehran, Iran
* Corresponding author. Tel.: +98(12)4271296; fax: +98(21)22873079. E-mail: [email protected]
1
Abstract The Ag2S and Cu doped Ag2S nanoparticles were prepared by simple chemical coprecipitation method and characterized by XRD, SEM, EDX, TEM, PL and UV–vis spectra. The photocatalytic activity of Ag2S and Cu doped Ag2S nanoparticles were investigated with Ofloxacin antibiotic, which is part of the fluoroquinolone family. The morphological study indicated that the products were spherical shape in with diameter size of 30 nm. The photocatalytic results demonstrated that the Cu doping increased the photocatalytic efficiency of Ag2S nanoparticles. The outcome of antibacterial experiment under visible light irradiation indicate that the Cu doped Ag2S nanoparticles represent increased antibacterial performance compared with un-doped Ag2S nanoparticles.
Keywords: Cu doped Ag2S; Photoluminescence; Photocatalytic degradation; Antibacterial activity
2
Introduction Due to broad diversity of major efficiency in biological field and optoelectronic, nanotechnology is currently a severe area of scientific research. Nowadays, the study of semiconductor nanoparticles has much consideration because of their novel electronic and optical properties [1]. Nanoparticles (NPs) such as PbS, CdSe and CdS have thin band gap semiconductor quantum dots which have been used as photocatalysts in recently. Among all, silver sulfide nanoparticles are major materials as photocatalysts. Ag2S is a direct, thinband gap semiconductor with good optical limiting and great chemical stability properties [2-4]. Ag2S has a direct band gap (0.9–1.05 eV), and large absorption coefficient which demonstrates a performance semiconductor material for photovoltaic application [2]. The unparalleled properties of Ag2S NPs led to diversity of applications, such as, in light emitting diodes and switches, photo-detectors, solar cells, magnetic field sensors, IR detectors, optical filters, photoconductors and room temperature oxygen sensors [1,3,5,6]. Doping is a commonly applied method to treat magnetic, electrical and optical properties of semiconductor compounds, used in making most optoelectronic and electronic devices. Among transition metal ions, copper (Cu) is the major promising dopant, because of its catalytic activity [7-9]. Therefore, the characterization of transition metal ion states is very important. In the present work, synthesis of un-doped and Cu2+ doped Ag2S nanoparticles, and morphological, spectroscopic, structural properties were studied in detail. These materials may have potential application in lighting devices and solar cells.
3
Material and methods In this study, raw materials were purchased from Sigma-Aldrich Ltd. Synthesis of un-doped and Cu doped Ag2S NPs Ag2S NPs was synthesized by a co-precipitation method. Briefly, 50 mL of Na2S solution (0.001 M) was added dropwise to 50 mL of AgNO3 solution (0.001 M) at 80°C. PVP solution was added to the above mixture, once it attained 60°C. Then solutions were stirred under magnetic stirrer for 3 h until it turned into a light brown color with black precipitates on the surface and then calcined at 100°C for 30 min. The Cu doped Ag2S Nanoparticles (Ag1-xCuxS where x=0.00, 0.05, and 0.10) were synthesized according to the following procedure: Freshly prepared 50ml of aqueous solution of 0.1M Na2S was added dropwise to 50ml of 0.1M solution of AgNO3 and 50ml of 0.1M solution of CuSO4.5H2O using vigorous stirring and then added 0.05g of PVP capping agent. After the completion of reaction, the solution was aged for 45min. The residual solution was filtered and washed several times with distilled water. The wet precipitate was dried in an oven at 80°C for 2h and then calcined at 100°C for 30 min (heating rate :10°C/min). Characterization and analytic instruments A scanning electron microscope (SEM); JEOL JSM-5600 Digital Scanning Electron Microscope, transmission electron microscopy (TEM, JEM- 2100F HR, 200 kV), and X-ray diffractometer (XRD) Philips X’Pert were used for samples characterization. UV-Vis DRS mensuration was performed in a double beam spectrophotometer (JASCO V-550). The SEM (JSM 6701F—6701) was using for examination of surface morphology. Photoluminescence studies were performed using TEC Avaspec 2048 Spectrophotometer (excitation source=Xenon arc lamp 450W). The catalyst compositions were analyzed with an energy dispersive X-ray spectrometer (EDX-700HS, SHIMADZU). Photocatalytic experiments The photocatalytic activity was investigated by measuring the degradation of Ofloxacin antibiotic solution (10 mg/L at pH 6.5) under UV (125W UV lamp at 365nm)
4
and visible (1000W halogen lamp) light illumination. The reactor system made of double walled reaction chamber of glass tubes was applied for photo degradation tests. The solution for photo degradation measurement was prepared by adding pure or 6 and 10mol% Cu doped Ag2S (1g/L) to 50ml aqueous solution of Ofloxacin antibiotic (10mg/L at natural pH=6.5). The mixture was stirred for 10min, and kept in the dark for 1h in order to achieve equilibrium of adsorption reaction. The Ofloxacin antibiotic concentration was distinguished with the aid of a two dimensional Gas Chromatogharphy (GC*GC) (Kimia Shangarf Pars Research Co., Iran). Measurement of antimicrobial property Enterococcus faecalis (ATCC 29212) and Staphylococcus aureus (ATCC 6538) were chosen as the standard bacterium. The shaking flask method was used for the antibacterial experiments. The tests were performed under visible light and in the dark. The shaking flask method consists of adding 0.2g of each sample into 20mL of 0.75mM saline water containing about 108 CFU/mL of bacteria and then shake under visible light. 0.5 mL of the suspension was separated after a 3h contact, and diluted to an appointed volume by tenfold dilution. The diluted solution was plated (in nutrient agar plates) in incubated at 40±1 °C for 24 h.
5
Results and discussion X-ray diffraction analysis The X-ray diffraction patterns of the Ag2S NPs are shown in Fig. 1, and all the characteristic peaks can be observed (JCPDS no. 65-2356). Some of the sharp peaks labeled to as (111), (112), (121), (103), (031), (200), (213) and (134) are attributed to Ag2S NPs. The NPs size at the mentioned 2θ was calculated using the Debye–Scherrer formula, D=0.9λ/βcosθ, where D is the crystalline average diameter, θ is the Bragg angle, β is the excess line width of the diffraction peak in radians and, λ is the wavelength of X-ray [10]. The Full width at half maximum (FWHM) of the diffraction peaks showed some changing upon doping with ions. The average size of un-doped and 6 and 10 mol% Cu doped Ag2S NPs was found to be around 30, 25, and 21 nm, respectively. Fig. 1. XRD patterns of un-doped (A), 6 (B) and 10 mol% (C) Cu doped Ag2S nanoparticles. Morphological studies The prepared NPs were subjected to SEM for the morphological investigation. Fig. 2A and B indicate the surface structure and morphology of un-doped and 10 mol% Cu doped Ag2S nanoparticles. Spherical to ellipsoid shaped particles were formed. The Fig. 2C and D demonstrate the EDX spectrum of the un-doped and Cu doped and the peaks corresponding to Cu observed in the EDX spectrum of the doped instances confirms the incorporation of Cu ions into Ag2S nanoparticles lattice network. Fig. 2. SEM image and EDX spectrum of the un-doped (A,C) and 10 mol% Cu doped Ag2S nanoparticles (B,D) Fig. 3A and B show the TEM images of un-doped and Cu-doped Ag2S nanoparticles from the typical tests. It can be seen that these nanoparticles are nearly spherical in shape and clearly well dispersed. Fig. 3C and D indicate the size distribution of un-doped and Cudoped Ag2S nanoparticles; the samples average diameter are 31.00±0.1497nm (undoped Ag2S), and 22.00±0.1221nm (Cu-doped Ag2S), respectively. The large nanoparticle sizes are in great agreement with the results of XRD characterization.
6
Fig. 3. TEM image and size distribution of un-doped (A,C) and 10 mol% Cu doped Ag2S nanoparticles (B,D)
Optical characterization Optical absorption spectroscopy of synthesized un-doped and Cu (6 and 10%) doped Ag2S NPs was carried out by UV– Visible absorption spectroscopy (25ᵒC). Fig. 4(A) indicates that there is a strong excitonic absorption peak at 415 nm for both Cu doped Ag2S NPs samples. These peaks have good optical quality and large exciton binding energy. From Fig. 4(A), it can be seen that all Cu doped NPs demonstrate better absorption of visible light as compared to un-doped NPs. The band gap energies of the un-doped and doped Ag2S nanoparticles were evaluated by the Kubelka–Munk function F(R)2 vs. energy in electron volts and extrapolation of the linear portion of the curve to F(R)2=0 (Fig. 3(B)) [11]. The optical absorption factor ( ) computed by F(R) =
= (1 - R)2/2R, where R is the percentage
of reflected light. The incident photon energy (
) and the optical band gap energy (Eg)
correspond to the transformed Kubelka–Munk function, [F(R)
]p = A(
- Eg), where, p is
the power index and A is a constant related to transition probability, for the optical absorption process. p equals to 2 or 1/2 for a direct or an indirect allowed transition, respectively. The band-gap of the samples enhances with Cu concentration and was 3.22, 3.31 and 3.38 eV for un-doped Ag2S and 6 and 1 mol% Cu-doped Ag2S, respectively.
Fig. 4. UV–Visible absorption spectra (A) and Kubelka–Munk plots (B) and band gap energy estimation for un-doped (a), 6 (b) and 10 mol% (c) Cu doped Ag2S nanoparticles.
Photoluminescence (PL) analysis Photoluminescence (Pl) emission spectra were applied to investigate the transfer of holes and photo-generated electrons, as well as to gain an understanding of the recombination and separation of photo-generated charge carriers [12,13]. To study the photoelectric properties of the synthesized samples, Pl spectra obtained for different 7
samples are shown in Fig. 5. The spectrum exhibits four emission peaks at around 456, 463, 475nm and 456, 463, 471 nm, respectively, corresponding to blue emission for un-doped and two doped samples of Ag2S nanoparticles. The strong PL peaks may correspond to crystalline defects induced during the growth. Visible emissions are referred as deep-level emission and are due to the recombination of electrons deeply trapped in silver interstitials and oxygen vacancies, with photo-generated holes [14]. A slight shift is seen in PL spectra towards higher wavelength after doping Cu into Ag2S lattice and intensity of luminescence is also reduced, when compared to the un-doped sample. Fig. 5. The PL spectra for un-doped (a), 6 (b) and 10 mol% (c) Cu doped Ag2S nanoparticles
Peruse of photocatalyst activity In this research, the photocatalytic activity of Ag2S NPs was distinguished based on the ability of the Ag2S NPs to decompose Ofloxacin antibiotic under UV and visible light. Since the band gap energy of Ag2S NPs is much larger than the energy of UV irradiation, the generation of reactive species like •OH radicals might take place due to the native defects. Fig. 6 indicates the influence of un-doped and doped Ag2S NPs nanoparticles on decomposing of Ofloxacin antibiotic under UV and visible light illumination [15-19]. The amount of doping Cu atom did not attain the maximum activity, so the light absorption and activity are not 100%. The catalyst has the highest photocatalyst activity for a higher Cu doping.
Fig. 6. Influence of doping on the activity of different Ag2S nanoparticles for degradation of Ofloxacin antibiotic. Process and Mechanism of photocatalytic activity The complexities of the role of transition metal dopant ion are such that it can participate in all of these processes. Acting as electron and/or hole traps is the major function of dopant. The trap of charge carrier scan reduces the recombination rate of
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electron–hole pairs and consequently, enhances the life time of charge carriers. The process of charge trapping is as follows: Ag2S + hv → h++ eMn+ + e- → M(n-1)+ M(n-1)+ + O2 → M(n-1+1)+ + O2Mn+ + h+ → M(n+1)+ M(n+1)+ + OH- → M(n+1-1)+ + •OH where M is the Cu dopant and n is the valency of Cu ion. The energy level of Mn+/M(n-1)+ lies below the conduction band edge and the energy level of Mn+/M(n+1)+ lies above the valence band edge. Thus, the energy level of Cu ion affects the trapping efficiency. The trapping of electrons facilitates holes to transference onto the surface of semiconductor and react with OH- in the Ofloxacin antibiotic solution and form active •OH to participate the destruction of Ofloxacin antibiotic. Hydroxyl radicals have been deemed to be major active species during the photocatalytic oxidation reaction [20]. Antibacterial activity The prepared un-doped and doped Ag2S nanomaterials were tested for their antibacterial activities under visible light irradiation and in the dark [21]. The antibacterial results are given in Fig. 7A and C. Fig. 7B and D show the SEM images of E. faecalis and S. aureus. It can be seen that the E. faecalis and S. aureus are ovoid and cocci shape. All of the experiments were repeated twice, and average values are shown by using un-doped and doped Ag2S, the surviving bacteria were reduced under visible light irradiation; however, the surviving bacteria of Cu-Ag2S materials were significantly reduced, compared to Ag2S materials. For the less surviving bacteria, antibacterial activities are stronger. This shows that copper synergistic effect is responsible for its efficient photocatalytic antibacterial activity of Cu-Ag2S. In the dark, their antibacterial activities are weaker than those of under the visible light irradiation. Fig. 7. The antibacterial effect of all Ag2S Nanoparticles samples against E. faecalis (A) and S. aureus (C); SEM images of the cultured E. faecalis (B) and S. aureus (D) 9
Conclusions The un-doped and doped Ag2S nanoparticles were successfully synthesized by a simple chemical co-precipitation method. The analysis of SEM and XRD showed that the particles of all samples were at the nano-scale. UV-vis spectra indicated that, as the doping Ag2S increased, the band gap shifted to higher wavelengths. The PL outcome demonstrated that the tuning range of emission wavelength is nearly between 456 and 477 nm in the present one type doped Ag2S nanoparticles. The doping Cu had an influence on the degradation of Ofloxacin antibiotic the Ag2S nanoparticles. The comparison of antibacterial activity of different synthesized Ag2S samples, clearly indicated that the Cu-Ag2S nanoparticles are the active antibacterial for inhibition properties over the growth of Staphylococcus aureus and Enterococcus faecalis.
Acknowledgment The author acknowledges Materials & Energy Research Center and Razi Metallurgical Research Center (RMRC) from Iran for performance support through the project.
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Fig. 1
Fig. 2 13
Fig. 3.
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Fig. 4
Fig. 5
15
100
Degradation of ofloxacin (%)
90
UV light
Ag2S 6% Cu-Ag2S 10% Cu-Ag2S
80 70 60 50 40 30 20 10 0
30
60 90 Irradiation time (min)
100
Degradation of ofloxacin (%)
90
120
Ag2S 6% Cu-Ag2S 10% Cu-Ag2S
Visible light
80 70 60 50 40 30 20 10 0
20
50
80 110 Irradiation time (min)
Fig. 6
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140
Fig. 7
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Research Highlights
•
Silver sulfide nanoparticles were synthesized by chemical co-precipitation method.
•
Silver sulfide and copper-doped nanoparticles exhibited photocatalytic property under UV and visible light.
• The particles exhibited excellent antimicrobial property.
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