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Ag and CuO impregnated on Fe doped ZnO for bacterial inactivation under visible light
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Ag and CuO impregnated on Fe doped ZnO for bacterial inactivation under visible light Rimzhim Guptaa, Neerugatti KrishnaRao Eswarb, Jayant M. Modaka, Giridhar Madrasa, a b
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Department of Chemical Engineering, Indian Institute of Science, Bangalore, India Centre for Nanoscience and Engineering, Indian Institute of Science, Bangalore, India
A R T I C L E I N F O
A B S T R A C T
Keywords: E. coli Antibacterial Photocatalysis Metal impregnation Transition metal doping
Interfacial coupling of semiconductor with metal has been demonstrated for inactivation of E. coli. Fe doped ZnO was synthesized by sol–gel method that resulted in enhanced absorbance in visible region. Various concentrations of Cu were impregnated on Fe doped ZnO that eventually turned into copper oxide and the photocatalytic activity of this material was compared with noble metal (Ag) impregnated on Fe doped ZnO. The obtained materials were characterized by various techniques. The crystal structures were determined by XRD and XPS was used to identify the oxidation states of the elements present in the photocatalyst. The morphologies and microstructures were determined by SEM. The optical absorbance of the photocatalysts was characterized by diffused reflectance spectra. Photocatalytic experiments were conducted for inactivation of E. coli using various catalysts. The rate constants obtained for 3 wt.% Cu impregnated Fe doped ZnO was higher than 1 wt.% Ag impregnated Fe doped ZnO. The higher photoactivity of these materials compared to pristine ZnO can be attributed to decreased recombination of the excitons in the synthesized photocatalysts that was validated by photoluminescence. This study indicates the possible employment of copper as a viable substitute for silver for anti-bacterial applications.
1. Introduction Bacterial inactivation has been extensively studied since the first reports [1,2] using TiO2 by photocatalysis. TiO2 and ZnO are well established photocatalysts as UV light active semiconductors [3,4]. The wide band gaps of TiO2 and ZnO, however, create a limitation on their usage in visible light irradiation. This limitation can be overcome via band engineering either by metal/non-metal doping or by forming composites with lower band gap materials. Metal or non-metal doping introduces sub bands (impurity levels) between the metal oxide energy levels. These inter-bands serve as trapping sites for charge carriers resulting in the lower band gap of the material [5]. The formation of composites modifies the optical properties by increasing the absorbance and lifetime of the excitons due to the incorporation of lower band gap material such as CdS, Fe2O3, BiOI, V2O5 etc. [6–9]. Transition metal doping on metal oxide has several advantages such as introduction of inter-bands below conduction band and enhancement in the formation of oxygen vacancies in the lattice [10]. The induced orbitals of the metal ion tend to readjust the orbitals of the parent semiconductor, thus introduces impurity levels which in turn reduces the band gap [5]. Oxygen vacancies play an important role in
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modifying the photo-induced behaviour of the semiconductor [10] by serving as trap sites for the photo-excited electrons that suppress the recombination. Among the transition metals, Fe is a better dopant because of the half-filled electronic configuration (d-orbital) that provides symmetrical distribution of electrons and increases the energy exchange of the system that provides stability [11,12]. The incorporation of Fe ions in the lattice provides shallow traps for photogenerated electrons that result in enhanced charge separation[13] and narrows the band gap by providing a red shift in the absorption edge[14]. Transition metal doping such as Co [15], Ni [16], Fe [17] into ZnO has proved to impart several benefits. Co metal doping into ZnO resulted in a decrease of band gap. This is possible due to electron- transitions between “dCo” orbital to “dZn” transitions and also improved interactions between sp and d bands [15,16]. Silver shows excellent activity for antibacterial applications. Impregnation of silver on TiO2 [18] and ZnO [19] has been shown to possess excellent antibacterial activity. Ag3PO4 and its composites with AgBr/CeO2 nanoflakes and TiO2 have been used for antibacterial applications [20,21]. AgCl/SiO2 [22], AgCl/W18O49 nanorods [23], Ag–TiO2/Ag/a-TiO2 [24] have also been reported as efficient visible light active photocatalysts. The leaching of silver into water from these
Corresponding author. E-mail address: [email protected] (G. Madras).
http://dx.doi.org/10.1016/j.cattod.2017.05.032 Received 12 December 2016; Received in revised form 1 April 2017; Accepted 8 May 2017 0920-5861/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Gupta, R., Catalysis Today (2017), http://dx.doi.org/10.1016/j.cattod.2017.05.032
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respectively.
materials limits their viable usage. To overcome some of these difficulties, various metals were impregnated on doped ZnO because these metals increase the photo-response resulting in higher activity and also give better stability than their respective oxides. Often noble metals like Ag, Au, Pt are impregnated on metal oxide semiconductors. However, considering their cost, in this work transition metals such as Cu was impregnated on Fe doped ZnO. This is compared with Ag impregnation on the Fe doped ZnO because Ag is known for its antibacterial properties. Besides better antibacterial properties, Ag and Cu exhibit surface plasmonic resonance (SPR). There are several beneficial effects of plasmonic resonance in the field of photocatalysis as it enhances visible light absorption and excitation of energetic charge carriers [25]. This results in enhanced redox reactions and enhanced rate of generation of reactive oxygen species [26]. In order to achieve better charge separation[27], a proper design of metal − semiconductor combination needs to be proposed that enhances the photocatalytic activity by suppressing the recombination. The objective of this study was to develop novel antibacterial materials wherein a transition metal is impregnated on doped ZnO and compare its photocatalytic antibacterial activity with the conventional Ag-based materials. In the present work, Fe doped ZnO was synthesized by the sol-gel process with different doping concentrations. Impregnation of different metals (Ag, Cu) was carried out on Fe doped ZnO. This wet impregnation was carried out for metal reduction on the semiconductor surface. However, Cu was converted to CuO in the process of impregnation and CuO impregnated on Fe doped ZnO was obtained. The photocatalytic activities of synthesized photocatalysts were studied under visible light irradiation for the inactivation of E. coli. This study opens a new avenue in the area of cost effective metal impregnated materials compared to Ag based materials for antibacterial activity.
2.2. Photocatalytic reactor The photochemical reactor used in the current study consists of a jacketed quartz tube and a glass reactor. Experiments were carried out using a metal halide lamp of intensity 68,000 Lux. Ballast with capacitor was connected in the series with the lamp to ensure constant input voltage. Cold water was circulated through the annulus of the quartz tube to maintain the reactor temperature at 30 °C. The source assembly was kept behind the glass reactor and placed in a wooden box. A filter with cut off wavelength < 400 nm was placed between the lamp and the reactor to ensure only visible light reaches the reactor. All experiments were conducted in this reactor with a bacterial solution and appropriate catalyst loading, as described later. 2.3. Culture and microorganism growth The growth culture for E. coli was prepared by inoculation of 100 μL inoculum/mother culture in 300 mL of sterilized 2 wt.% of LuriaBertani broth (HiMedia, India) in DI water. Mother culture was prepared using glycerol stock stored at −80 °C. 50 μL of glycerol stock was transferred by pipette to small volume of Luria-Bertani broth and kept for 9 h at 37 °C. The culture was kept at 200 rpm at 37 °C in shaking incubator (Orbitek, India) providing aerobic conditions for 6 h. Bacterial colony count was carried out by spreading the serial diluted samples using 2.8 % nutrient agar (HiMedia, India) plates. 2.4. Bacterial inactivation Required accessories such as pipette tips, Eppendorf tubes, beaker, DI water etc. were autoclaved at 120 °C for 90 min. Firstly, bacterial pellet was obtained after centrifuging the 30 mL culture at 4000 rpm. The pellet was washed twice with sterilized DI water to eliminate the nutrients completely. The final obtained pellet was re-suspended in 30 mL of sterilized DI water. The experiments were performed in 30 mL bacterial solution with 0.25 g/L catalyst loading. The solution was kept under visible light irradiation for 1 h. Samples were collected after every 15 min and diluted till the bacterial colony count reaches 30–300 CFU/mL. To obtain the colony count, 100 μL of the diluted samples were spread over nutrient agar plates for bacterial colony count.
2. Experimental 2.1. Materials Zinc nitrate hexahydrate (Zn(NO3)2·6H2O), hydrazine hydrate, silver nitrate (AgNO3), oxalic acid (C2H2O4), copper nitrate trihydrate (Cu(NO3)2·3H2O), ferric nitrate nonahydrate (Fe(NO3)3·9H2O), methanol were purchased from Merck (India). 2.1.1. Synthesis of ZnO and Fe doped ZnO ZnO was prepared by sol-gel technique, as described elsewhere [28]. Ferric nitrate nonahydrate and zinc nitrate hexahydrate were taken as precursors. 2.0 mol of zinc nitrate hexahydrate was dissolved in 100 mL of methanol and continuously stirred under reflux condition. Then the required wt.% of iron nitrate (dissolved in 20 mL of methanol) was added to the above zinc nitrate solution. Subsequently, oxalic acid solution was prepared in 50 mL of methanol and added to the above solution such that the molar ratio of zinc nitrate hexahydrate and oxalic acid was 1:1. Stirring was continued till a white gel was obtained and it was dried at 70 °C for 12 h. Finally, the powder was calcined at 400 °C for 3 h to remove all the impurities. 1, 2 and 5 wt.% Fe doped ZnO are named as Fe1Z, Fe2Z and Fe5Z, respectively.
2.5. Characterization The crystal structure was determined by X-ray diffraction using Rigaku Diffractometer at the scan rate of 1° min−1 and scan range of 5°–100°. The calculation of crystallite size was calculated using Debye˙ ) was used as the target material Scherrer’s formula. Cu-Kα (λ = 1.54 A for X-ray generation. Morphological and microscopic studies were carried out by SEM and TEM. ULTRA55 FESEM, Carl Zeiss was used for scanning electron microscopy. The catalyst was sonicated in ethanol/iso-propanol for 5 min and drop casted on the silicon wafer for SEM imaging. The sample was kept under vacuum to prevent undesirable interactions. Later, the SEM sample was sputtered with gold using Quorum sputtering to avoid charging. TEM was carried out using Tecnai F30 operated at 180 kV. The catalyst was sonicated in ethanol/iso-propanol for 10 min and drop casted on Cu TEM grid carefully and dried in oven for complete removal of the solvent. The sample was kept at vacuum for 12 h. In order to determine the band gap of the synthesized materials, diffused reflectance spectra was acquired using UV–vis spectrophotometer (PerkinElmer, Lambda 35) in the wavelength range of 300–700 nm. The reduced exciton recombination was validated by photoluminescence studies using Perkin-Elmer at excitation wavelength of 300 nm. The surface area analysis was performed by nitrogen adsorption using Nova-1000 Quantachrome.
2.1.2. Synthesis of metal (Ag, Cu) impregnated Fe doped ZnO 100 mg of prepared Fe doped ZnO was dispersed in 100 mL of deionized (DI) water. Required amount of metal precursor (for 1 wt.% Ag and 1, 2, 3 wt.% Cu) was then added. The reaction was allowed to occur in dark. Under vigorous mixing, 0.05 M of hydrazine hydrate solution was added drop wise to the above solution to reduce the metal ions. The reaction was continued for 1 h and then the suspension was washed with DI water, ethanol twice and dried for 12 h at 70 °C. 1 wt.% Ag impregnated Fe2Z is named Ag1Fe2Z; 1, 2 and 3 wt.% of Cu impregnated Fe2Z are named as Cu1Fe2Z, Cu2Fe2Z and Cu3Fe2Z, 2
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3.1. XRD analysis Fig. 1(a) shows the X-ray diffraction patterns of ZnO, 1, 2 and 5 wt. % Fe doped ZnO. Fig. 1(b) shows the diffraction spectra of 1 wt.% Ag impregnated Fe2Z, and 1–3 wt.% Cu impregnated Fe2Z. All the synthesized materials possess hexagonal structure. The crystal structures and the diffraction planes of ZnO were validated by JCPDF- 00-03089. The absence of Fe, Fe2O3 peaks in the patterns suggest the presence of Fe at substitutional sites of Zn. Fig. 1(b) shows the XRD pattern of Ag1Fe2Z. The peak at 37° corresponds to the plane (4 0 0) and is validated by JCPDF-00-001-1164. Similarly, Fig. 1(b) shows the XRD pattern of Cu impregnated Fe doped ZnO (Cu1Fe2Z, Cu2Fe2Z, and Cu3Fe2Z). Impregnation of Cu in metal form is not possible due to lower oxidation potential of copper. This causes the conversion of Cu metal into copper oxide. Many peaks of CuO are in the similar region of ZnO. A clear peak at 9.7° corresponding to CuO is present in the patterns (Fig. 1(b)), that matches with the peak of CuO (JCPDF-00-0021041). The synthesis was performed such that Fe is doped in the lattice. Lattice parameters obtained from Rietveld refinement are added in the manuscript in Table 1. This clearly shows a decrease in lattice parameters after doping of Fe in the lattice of ZnO. However, no change in the lattice parameters is observed after CuO impregnation on the Fe doped ZnO. This is consistent with our synthesis procedure where Fe is doped in ZnO while CuO and Ag are impregnated. 3.2. Photoluminescence Fig. 2(a) shows the photoluminescence spectra for ZnO, Fe1Z, Fe2Z, Fe5Z, Ag1Fe2Z and Cu3Fe2Z, respectively. The photoluminescence spectrum indicates the recombination of photogenerated electrons and holes. PL spectra of all the catalysts showed significant variations based on the recombination of excited charge carriers. The peaks at 430–440 nm denote the recombination of excitons at the band edges of the corresponding bands of ZnO. Other shoulder peaks were also observed at 480–490 and 530 nm [29,30]. The peaks at 480–490 nm correspond to recombination of excitons present in the shallow traps that emit at longer wavelengths. The presence of shallow traps may be due to inter-bands generated as the result of transition metal i.e. Fe doping. The peak at 530 nm corresponds to the deeper traps of the ionic core due to enhanced oxygen vacancies [31]. The PL intensity of Cu impregnated Fe doped ZnO is lower than other catalysts studied here.
Fig. 1. X-ray diffraction patterns of (a) ZnO, Fe1Z, Fe2Z and Fe5Z (b) Ag1Fe2Z, Cu1Fe2Z, Cu2Fe2Z, Cu3Fe2Z.
3. Results and discussion ZnO is the base semiconductor and Fe2Z is the parent material for all impregnated photocatalysts. After obtaining and characterizing the pure phase of photocatalysts using XRD, photocatalytic experiments were performed. Due to superior phtocatalytic activity of Fe2Z and Cu3Fe2Z in comparison to Fe1Z, Fe5Z, and Cu1Fe2Z, Cu2Fe2Z, respectively, other characterizations such as XPS, SEM and TEM were performed only for these photocatalysts.
3.3. Diffuse reflectance spectra Fig. 2(b) shows the absorbance spectra of undoped ZnO, 1, 2 and 5 wt.% Fe doped ZnO, 1 wt.% Ag impregnated Fe2Z, 3 wt.% Cu impregnated Fe2Z, respectively. Except ZnO all the materials absorb UV light. Incorporation of Fe in ZnO increases the absorption in visible region. The band gap is calculated by the Kubelka Munk Function and is 3.23, 3.21, 3.19 and 3.18 eV for ZnO and 1, 2 and 5 wt.% Fe doped ZnO, respectively, as shown in Fig. 2(c). Similar values (in range of
Table 1 Specific surface area, crystallite size, rate constants and lattice parameters for bacterial inactivation for all the photocatalysts under visible light. Photocatalysts
Specific surface area (m2/g) ( ± 1)
Crystallite size (nm)
Rate constant (h−1)
a (A)
b (A)
c (A)
Photolysis ZnO Fe1Z Fe2Z Fe5Z Ag1Fe2Z Cu1Fe2Z Cu2Fe2Z Cu3Fe2Z
– 44 38 63 68 47 58 57 55
– 17 15 11 10 – – – –
3.3 ± 0.2 7.9 ± 0.4 8.3 ± 0.4 9.1 ± 0.2 7.8 ± 0.4 12.5 ± 0.3 11.8 ± 0.5 15.5 ± 0.6 18.2 ± 0.6
– 3.2573 3.2522 3.2500 3.2476 3.2500 3.2500 3.2500 3.2500
– 3.2573 3.2522 3.2500 3.2476 3.2500 3.2500 3.2500 3.2500
– 5.2215 5.2126 5.2079 5.2040 5.2080 5.2080 5.2080 5.2080
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Fig. 2. (a) Photoluminescence spectra for ZnO, Fe1Z, Fe2Z, Fe5Z, Ag1Fe2Z, Cu3Fe2Z (b) Absorbance for ZnO, Fe1Z, Fe2Z, Fe5Z, Ag1Fe2Z, Cu3Fe2Z (c) Tauc plot for ZnO, Fe1Z, Fe2Z and Fe5Z.
observed clearly. The diffraction pattern of Cu3Fe2Z (Fig. 3(h)) shows the high nano-crystallinity of the photocatalyst.
3.3–3.33 eV) of band gaps have been reported elsewhere [32]. Impregnation of metals results in a significant increase in the absorbance of the photocatalysts in the visible region. The enhancement in the absorbance of Ag1Fe2Z and Cu3Fe2Z is due to surface plasmon behaviour due to the presence of silver nanoparticles [33]. The increase in absorbance in the visible region may be attributed to the presence of copper oxide in Cu3Fe2Z. Copper oxide possess lower band gap and absorbs visible light. The presence of CuO along with ZnO tunes the absorbance in such a way that the composite becomes visible active semiconductor with enhanced absorbance.
3.5. XPS analysis XPS studies were performed to determine the oxidation states of the elements present in the photocatalyst. Elements present in the photocatalysts were C, O, Zn, Fe, Cu. Fig. 4(a) shows the peak at 284.8 eV that denotes the presence of C–1 s carbon in the catalyst. In Fig. 4(b), the peak at 530 eV shows the lattice oxygen and the extended peak denote the oxygen vacancies in the Fe2Z the peak at lower binding energy shows the presence of hydroxyl group. Fig. 4(c) represents the O–1 s in Cu3Fe2Z. The highest peak denotes the lattice oxygen and the extended peaks at higher binding energy denote the oxygen vacancies and the chemisorbed oxygen [34]. The absence of any short peak at lower binding energy denotes the absence of the hydroxyl group due to the presence of CuO. Fig. 4(d) shows the doublets of Zn in Fe2Z. Peak at 1022 eV show the 2p3/2 orbital and peak at 1045 eV show the 2p1/2 orbital of zinc [29]. In Fig. 4(e), peaks denote 2p3/2 and 2p1/2 doublets of Fe that shows the presence of iron in Fe3+ form in the Fe doped ZnO [35,36]. Fig. 4(f) shows the presence of Cu in ionic form in the Cu3Fe2Z photocatalyst [37]. Presence of satellite peak between the doublets of Cu-2p indicates the presence of Cu in ionic form (Cu2+). This signifies the formation of copper oxide while synthesis. Impregnation of Cu reduces copper nitrate into Cu but lower oxidation potential of Cu results in the oxidation Cu into copper oxide. The difference in 2p3/2 and 2p1/2 peak was found to be 19.75 eV. Fig. 4(g) shows the XPS of Ag in 3d orbital [30]. This shows the presence of silver in metallic form. Earlier reports state that the peak exposition of 3d5/2 at 368.2 and 3d3/2 at 373.9 eV corresponds to the silver metal [38]. Fig. 4(h) show the wide spectra for Fe2Z, Ag1Fe2Z and Cu3Fe2Z in the range of
3.4. Microscopic analysis and BET surface area analyses In order to determine the morphology of the synthesized photocatalysts, scanning electron microscopic analyses were performed. Fig. 3(a)–(d) show the field emission SEM images of ZnO, Fe2Z, Ag1Fe2Z and Cu3Fe2Z impregnated Fe2Z, respectively. ZnO and Fe doped ZnO possess similar morphology that contributed to the high surface area that allows active interaction of the catalyst with the bacteria. The presence of metal particles/metal oxide nanoparticles at the surface of Fe doped ZnO leads to the reduction in surface area of the material due to firm attachment of the metal. As the percentage of impregnation increases, surface area decreases. The specific surface areas for all the photocatalysts are tabulated in Table 1. The TEM results show that the particle sizes of Fe2Z were around 10–15 nm, as shown in Fig. 3(e). Fig. 3(f) shows the bright field image of Ag1Fe2Z. A significant contrast between copper oxide particles and ZnO was observed from Fig. 3(g) where 3 wt.% Cu was impregnated on Fe2Z that proves the successful formation of heterojunction between Cu and Fe doped ZnO. Similarly, impregnation of Cu on Fe2Z has not increased the particle size but the firm attachment of CuOS can be 4
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Fig. 3. SEM images of (a) ZnO (b) Fe2Z (c) Ag1Fe2Z (d) Cu3Fe2Z TEM images of (e) Fe2Z (f) Ag1Fe2Z (g) Cu3Fe2Z and (h) diffraction pattern of Cu3Fe2Z.
3.6. Photocatalysis
0–1200 eV. The background of XPS is generally attributed to the scattering during photoelectron ejection to the surface between electron-electron, electron-ion and electron- plasmon. The rise in the background of Ag1Fe2Z can be attributed to the scattering of plasmon of Ag nanoparticles present at the semiconductor surface [39]. This was supported by the enhanced absorption of Ag1Fe2Z as seen in Fig. 2(a) indicating the existence of surface plasmon resonance (SPR) of Ag nanoparticles.
3.6.1. Effect of Fe doping on ZnO Optical properties were significantly found to be enhanced by doping Fe into ZnO. Recombination of electrons and holes were proven to be reduced compared to pristine ZnO [17]. Therefore, in the current study, Fe was doped into ZnO as 1, 2 and 5 wt.%, respectively. The photocatalytic activities of all concentrations of Fe doped ZnO samples have been tested by bacterial inactivation. As the Fe doping concentra5
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Fig. 4. XPS spectra for (a) C–1s (b) O–1s in Fe2Z (c) O–1s in Cu3Fe2Z (d) Zn-2p (e) Fe-2p in Fe2Z (f) Cu-2p in Cu3Fe2Z (g) Ag-3d in Ag1Fe2Z (h) wide spectra of Fe2Z, Ag1Fe2Z, and Cu3Fe2Z.
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3.6.2. Effect of metal (Ag, Cu) impregnation on Fe doped ZnO in dark Catalytic hetero-junctions such as semiconductor-semiconductor, metal-semiconductor were subjected to dark experiments. This is because, while synthesising the composites, not all the nucleation processes happen heterogeneously on the substrate (Fe2Z in this case) and homogeneous nucleation can also occur. Therefore, in order to nullify the effect of these homogeneously nucleated particles, reactions are performed under dark conditions to quantify the activity of impregnated nanoparticles alone. Initially, Cu was reduced on Fe2Z with varying wt.% between 1–3 wt.% and bacterial inactivation experiments were conducted in dark. The reduced Cu turns into copper oxide in the impregnation process. It is found that as the concentration of CuO increases, bacterial log reduction was increased. Fig. 5(a) shows bacterial inactivation plots for various catalysts under dark conditions. Similarly, Ag impregnation was limited to 1 wt.% as optimized elsewhere [3]. Generation of reactive oxygen species is common to silver nanoparticles and these ROS can actively participate in reduction of bacterial count [41]. These results suggest that 1–3 wt.% Cu impregnation show lesser antibacterial activity than 1 wt.% Ag in dark. Lower activity of Cu1Fe2Z, Cu2Fe2Z and Cu3Fe2Z may be attributed to the formation of copper oxide due to the oxidation of Cu metal while synthesis. The mechanism of inactivation by CuO has been widely reported [42,43]. Small concentration of copper ions leached out from copper oxide may disrupt the function of bacteria by binding itself to the DNA and thus disordering the double helical structure [43]. The other plausible reason for the inactivation of bacteria in the presence of CuO is ROS generation. These ROS after attaching to the bacterial surfaces enhances the intracellular oxidative stress and leads to the lipid peroxidation and results in cell death [42]. All the inactivation experiments under dark follow zero order reaction kinetics. The rate of inactivation under dark reactions suggests that silver has shown better inactivation. Silver, copper and metals possess capacity to adsorb oxygen. Adsorbed nascent oxygen specifically at the silver surface (due to silver affinity towards oxygen) reacts with the lipid content (-SH) present in the cell wall of bacteria and forms R-S-S-R bonds. This causes a rupture in the cell membranes that leads to death of the bacteria [44]. Copper and other metals instead form oxides and offer a barrier that retards the rate of inactivation in comparison to silver [44]. Metal nanoparticles rupture the plasma membrane that reduces the ATP content inside the cell [45]. Amines and carboxylic groups present on the surface of bacteria possess high affinity towards metal nanoparticles such as copper etc. Inactivation of the bacteria may occur either by the attack due to the nanoparticle itself or by the ions released by nanoparticles. Fig. 5. Degradation profile of E. coli with initial concentration ∼ 107 CFU/mL (a) under dark (b) photolysis, ZnO, Fe1Z, Fe2Z, Fe5Z under visible light irradiation (c) Ag1Fe2Z, Cu1Fe2Z, Cu1Fe2Z and Cu1Fe2Z under visible light irradiation.
3.6.3. Effect of metal (Ag, Cu) impregnation on Fe doped ZnO in the presence of light Fig. 5(b) shows the photocatalytic degradation of E. coli under visible light using various metal (Ag, Cu) impregnated photocatalysts. Among, copper impregnated catalysts, 3 wt.% Cu Fe2Z shows better photocatalytic activity compared to other two compositions such as 2 wt.% Cu and 1 wt.%. The rate constant of Cu3Fe2Z (18.2 ± 0.6 h−1) is higher than Ag1Fe2Z (12.5 ± 0.3 h−1). This proves that copper oxide can be used in place of silver in order to achieve enhanced photocatalytic activity than 1 wt.% Ag impregnation. Silver impregnation allows dissolved oxygen to couple with metal and form organo-metallic bonds preventing cell respiration [46]. Silver can inhibit proton motive force when pore proteins of bacteria interact with metal–metal oxide junction [45]. Charge species can effectively be transferred between semiconductor and metal due to the metal − semiconductor heterojunction that can easily destabilize the cell wall and plasma membrane [45]. The presence of intrinsic oxygen vacancies, in n-type semiconductors such as ZnO, the Fermi level is close to the conduction band due to the presence of majority charge carriers as electrons. Therefore, in
tion increases from 1 wt.% to 2 wt.% (as in Fe1Z and Fe2Z) as shown in Fig. 5(b), the bacterial count decreased. However, increasing the concentration to 5 wt.% reduced the photoactivity of the catalyst (Fe5Z) compared to the photoactivity obtained with 2 wt.% Fe doped ZnO. The addition of Fe atoms will induce several traps near the conduction band of ZnO [40]. Fe atom consists of vacant d-orbital. Therefore, excited electrons will be trapped in those vacant sites of Fe atom and prevent recombination. Though there is an increase in visible light absorption by Fe doped ZnO as in Fig. 2(a), the photoluminescence (Fig. 2(b)) spectra of pristine ZnO and Fe-doped ZnO shows only slight decrease in intensity. Further, 2 wt.% Fe doped ZnO shows higher photocatalytic activity compared to 1 and 5 wt.% indicating 2 wt.% Fe doping as a threshold limit. 2 wt.% Fe doping (Fe2Z) showed better results compared to other doping concentration and thus Fe2Z has been taken as base material for further studies. 7
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Fig. 6. Schematic of photocatalytic mechanism mediated by plasmonic resonance of silver.
order to maintain the charge equilibrium in the metal − semiconductor heterojunction, appropriate band bending occurs between the semiconductor Fermi level and the metal causing free electron movement during excitation. This is due to the position of Fermi level of silver and copper that lie below the Fermi level of ZnO. Downward band bending occurs in case of silver coupled with semiconductor. The electron transfer occurs until the Fermi level becomes equal [47]. Work function of ZnO is 4.65 eV which is higher than the work function of Ag i.e. 4.26 eV. The availability of electrons at the semiconductor conduction band in the case of silver facilitates the reduction reaction of oxygen to form superoxide radical. Conversely, in case of copper the charge transfer from semiconductor to metal enhances the charge separation and suppresses the recombination [47]. This mechanism is beneficial in efficient electron transfer and hydroxyl radical generation. Both contributes in the enhancement of photocatalytic activity of the semiconductor [27,48]. Fig. 6 shows the schematic of photocatalytic mechanism mediated by plasmonic resonance in case of silver as metal nanoparticle. If a semiconductor is coupled with noble metal, the free electrons are confined at the local spots. As the material is exposed to the irradiation of appropriate frequency, the electron charge density of the free electrons is redistributed and thus an electric field is generated. A coulombic charge is generated due to the presence of positive charge at the metal surface and hence downward band bending and transfer of electrons from metal to semiconductor is induced. This feature enhances the absorption of light significantly in the coupled metal semiconductor photocatalysts [26,49]. Formation of stable heterojunctions and generation of reactive radical species are responsible for better photoactivity exhibited by metal impregnates on zinc oxide. Therefore, coupling least expensive metals like copper to semiconductors has shown better photocatalytic activity compared Ag. This could be a sustainable and cost effective solution in the field of water purification. Fig. 7 shows the photocatalytic mechanism of Fe doped ZnO and copper oxide. Copper oxide possess band gap of (1.5–1.7 eV) which absorbs visible light. Conduction band of copper oxide lies above the conduction band of Fe2Z [50]. Potential of conduction band of Fe2Z and copper oxide crosses the potential of generation of superoxide radical. Similarly, Fe2Z valence band crosses the potential of hydroxyl radical formation potential. After absorbing the visible light, excitation of electrons occur leaving the vacant sites i.e. holes of Fe2Z and CuO separately. Electrons from the CuO conduction band can either move to the conduction band of Fe2Z or react with the dissolved oxygen and form superoxide radical [51]. Similarly, holes in the Fe2Z can react with the hydroxyl ions to form hydroxyl radicals and can be transferred to valence band of the copper oxide. After reaction of electrons with the dissolved oxygen and holes with the hydroxyl ion, superoxide radicals
Fig. 7. Schematic of photocatalytic mechanism Fe2Z-copper oxide composite.
and hydroxyl radical are generated, respectively. These radicals oxidize the impurities or rupture the cell wall after lipid peroxidation and this causes the inactivation of microorganism [52]. The holes present at the CuO do not possess the potential to form hydroxyl radical however they can directly attack the bacteria and oxidize the outer membrane [53–55]. Band edge positions of CuO ((in vacuum scale) −3.66 eV (C.B.) and −5.22 eV (V.B.)) and ZnO (−4.20 eV (C.B.) and −7.39 eV (V.B.)) are taken from elsewhere [56,30]. 3.6.4. Scavenger studies In order to detect to major responsible radicals for the inactivation of E. coli, scavenger experiments were performed using KI for hydroxyl radical scavanger and Ethylenediaminetetraacetic acid (EDTA) as hole (h+) scavengers and 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxy (TEMPO) as superoxide radical scavenger [29]. 2 mmol/L of TEMPO and 1 mmol/L of EDTA and 5 mmol/L of KI were added along with 0.25 g/L of catalysts Fe2Z and Cu2Fe2Z in the bacterial solution while conducting the experiments. 3.6.4.1. Scavenger study with Fe2Z. Fig. 8(a) shows the degradation profile of E. coli in the presence of TEMPO, KI, TEMPO in the presence of catalyst and KI in the presence of catalyst (Fe2Z) [57]. Degradation after KI, and TEMPO was observed to be equal to photolysis. This signifies that KI and TEMPO alone are not responsible for degradation of the bacteria. Inactivation using scavengers KI and TEMPO in the presence of catalysts showed a reduction in comparison to the inactivation in the presence of only catalyst. The reduction in the inactivation rate denotes the plausible radical responsible for inactivation of bacteria. Superoxide radical was observed to be major oxidation species for the killing mechanism of E. coli in the presence of Fe2Z. 3.6.4.2. Scavenger study with Cu2Fe2Z. Fig. 8(b) shows the degradation profile of E. coli in the presence of TEMPO and EDTA that are used as superoxide radicals and hole scavengers, respectively. Control studies were performed using only TEMPO and EDTA in the bacterial solution. Formation of AgI (Eg = 2.82 eV) and CuI (Eg = 2.73 eV) photocatalyst is the reason for not using KI as hydroxyl radical scavenger in presence of Cu2Fe2Z (impregnated catalysts). Formation of AgI and CuI results in further inactivation of E. coli and does not reveal any information about the responsible radical for inactivation. However, in Fig. 8(b), a significant reduction in the inactivation was observed when TEMPO was used in presence of catalyst (Cu2Fe2Z). The mechanism of charge transfer enhances the rate of generation of superoxide radical due to the presence of electrons at the conduction band of CuO and Fe2Z. This 8
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%
% −
Fig. 8. Degradation profile of E. coli under visible light with initial concentration of ≈ 2 × 107 CFU/mL (a) using KI ((OH ) scavenger) and TEMPO ((O2 ) scavenger) with catalyst Fe2Z (b) using EDTA ((h+ ) scavenger) and TEMPO
% − ((O2 )
scavenger) with catalyst Cu2Fe2Z. [2] J.C. Ireland, P. Klostermann, E.W. Rice, R.M. Clark, Inactivation of Escherichia coli by titanium dioxide photocatalytic oxidation, Appl. Environ. Microbiol. 59 (1993) 1668–1670. [3] S. Sontakke, J. Modak, G. Madras, Photocatalytic inactivation of Escherischia coli and Pichia pastoris with combustion synthesized titanium dioxide, Chem. Eng. J. 165 (2010) 225–233. [4] H.-L. Liu, T.C.-K. Yang, Photocatalytic inactivation of Escherichia coli and Lactobacillus helveticus by ZnO and TiO2 activated with ultraviolet light, Process Biochem. 39 (2003) 475–481. [5] S.G. Kumar, L.G. Devi, Review on modified TiO2 photocatalysis under UV/visible light: selected results and related mechanisms on interfacial charge carrier transfer dynamics, J. Phys. Chem. A 115 (2011) 13211–13241. [6] P. Kundu, P.A. Deshpande, G. Madras, N. Ravishankar, Nanoscale ZnO/CdS heterostructures with engineered interfaces for high photocatalytic activity under solar radiation, J. Mater. Chem. 21 (2011) 4209–4216. [7] A. Hernández, L. Maya, E. Sánchez-Mora, E.M. Sánchez, Sol-gel synthesis, characterization and photocatalytic activity of mixed oxide ZnO-Fe2O3, J. Sol-Gel Sci. Technol. 42 (2007) 71–78. [8] J. Jiang, X. Zhang, P. Sun, L. Zhang, ZnO/BiOI heterostructures: photoinduced charge-transfer property and enhanced visible-light photocatalytic activity, J. Phys. Chem. C 115 (2011) 20555–20564. [9] J. Liu, R. Yang, S. Li, Preparation and characterization of the TiO2-V2O5 photocatalyst with visible-light activity, Rare Met. 25 (2006) 636–642. [10] L. Jing, B. Xin, F. Yuan, L. Xue, B. Wang, H. Fu, Effects of surface oxygen vacancies on photophysical and photochemical processes of Zn-doped TiO2 nanoparticles and their relationships, J. Phys. Chem. B 110 (2006) 17860–17865. [11] M. Zhou, J. Yu, B. Cheng, Effects of Fe-doping on the photocatalytic activity of mesoporous TiO2 powders prepared by an ultrasonic method, J. Hazard. Mater. 137 (2006) 1838–1847. [12] M. Zhou, J. Yu, B. Cheng, H. Yu, Preparation and photocatalytic activity of Fedoped mesoporous titanium dioxide nanocrystalline photocatalysts, Mater. Chem. Phys. 93 (2005) 159–163. [13] T. Tong, J. Zhang, B. Tian, F. Chen, D. He, Preparation of Fe3+-doped TiO2 catalysts by controlled hydrolysis of titanium alkoxide and study on their photocatalytic activity for methyl orange degradation, J. Hazard. Mater. 155 (2008) 572–579. [14] J. Yu, Q. Xiang, M. Zhou, Preparation, characterization and visible-light-driven photocatalytic activity of Fe-doped titania nanorods and first-principles study for electronic structures, Appl. Catal. B: Environ. 90 (2009) 595–602. [15] X.-C. Liu, E.-W. Shi, Z.-Z. Chen, H.-W. Zhang, L.-X. Song, H. Wang, S.-D. Yao, Structural, optical and magnetic properties of Co-doped ZnO films, J. Cryst. Growth 296 (2006) 135–140. [16] E. Liu, P. Xiao, J.S. Chen, B.C. Lim, L. Li, Ni doped ZnO thin films for diluted magnetic semiconductor materials, Curr. Appl. Phys. 8 (2008) 408–411. [17] Y.Q. Wang, S.L. Yuan, L. Liu, P. Li, X.X. Lan, Z.M. Tian, J.H. He, S.Y. Yin, Ferromagnetism in Fe-doped ZnO bulk samples, J. Magn. Magn. Mater. 320 (2008) 1423–1426. [18] L. Shang, B. Li, W. Dong, B. Chen, C. Li, W. Tang, G. Wang, J. Wu, Y. Ying, Heteronanostructure of Ag particle on titanate nanowire membrane with enhanced photocatalytic properties and bactericidal activities, J. Hazard. Mater. 178 (2010) 1109–1114. [19] A. Adhikari, N.K. Banerjee, D. Eswar, G. Sarkar, Photocatalytic inactivation of E. coli by ZnO–Ag nanoparticles under solar radiation, RSC Adv. 5 (2015) 51067–51077. [20] N.K. Eswar, P.C. Ramamurthy, G. Madras, Enhanced sunlight photocatalytic activity of Ag3PO4 decorated novel combustion synthesis derived TiO2 nanobelts for dye and bacterial degradation, Photochem. Photobiol. Sci. 14 (2015) 1227–1237. [21] S.G. Kumar, K.K. Rao, Zinc oxide based photocatalysis: tailoring surface-bulk structure and related interfacial charge carrier dynamics for better environmental applications, RSC Adv. 5 (2015) 3306–3351. [22] P. Tan, Y.-H. Li, X.-Q. Liu, Y. Jiang, L.-B. Sun, Coreshell AgCl@SiO2n–anoparticles:
indicates that the major responsible species for inactivation of bacteria is superoxide radical. Degradation using EDTA in the presence of catalyst showed a decrease to some extent initially in comparison to catalyst alone but later the inactivation rate reduces. This indicates that initially direct hole attack leads to inactivate the bacteria but later superoxide radical formation dominates and inactivation occurs due to superoxide radical. The holes present at the valence band of Fe2Z moves to the valence band of CuO and lead to the interaction of holes directly to the bacterial surface and reduces the permeability of the membrane thus responsible for the inactivation of bacteria. EDTA and KI act as electron donor species that provides an ease in oxidation of impurity present [58]. TEMPO itself is a radical that forms a complex when it reacts with superoxide radical. This complex is important for the metabolism of bacteria that results in reduction of the rate of inactivation of bacteria. 4. Conclusions The combined effect of transition metal doping and metal coupling on Fe doped ZnO was examined. Fe doped ZnO and metal (Ag, Cu) impregnated Fe doped ZnO were synthesized successfully by sol-gel route and the metal precursor was reduced into metal in case of silver. Cu turned into CuO after the impregnation process. Extended absorption in visible range was achieved as a result of doping and metal impregnation. High charge separation i.e. reduced recombination was also achieved. This metal heterojunction also leads to surface bandbending in the bands of the semiconductor and results in high photoresponse of the photocatalyst. Plasmonic behaviour and charge carrier transportation due to metals (Ag) with semiconductor has resulted in the enhanced absorption of visible light of the material. The photocatalytic activity of the synthesized photocatalysts was examined for inactivation of E. coli under visible light. 3 wt.% Cu coupled Fe doped ZnO demonstrated higher photocatalytic activity than 1 wt.% Ag impregnated Fe doped ZnO. Acknowledgements The authors were thankful to Department of Science and Technology (DST), India for the financial support. The authors thank IPC, AFMM, MNCF (CeNSE), IISc for characterization facilities. Giridhar Madras thanks DST for the J.C. Bose fellowship. References [1] R. Cai, K. Hashimoto, K. Itoh, Y. Kubota, A. Fujishima, Photokilling of malignant cells with ultrafine TiO2powder, Bull. Chem. Soc. Jpn. 64 (1991) 1268–1273.
9
Catalysis Today xxx (xxxx) xxx–xxx
R. Gupta et al.
[23]
[24]
[25] [26] [27] [28]
[29]
[30]
[31]
[32] [33]
[34]
[35]
[36]
[37]
[38] [39] [40]
[41]
[42]
route of induced oxidative stress, Small 8 (2012) 3326–3337. [43] T. Pandiyarajan, R. Udayabhaskar, S. Vignesh, R.A. James, B. Karthikeyan, Synthesis and concentration dependent antibacterial activities of CuO nanoflakes, Mater. Sci. Eng.: C 33 (2013) 2020–2024. [44] V.S. Kumar, B.M. Nagaraja, V. Shashikala, A.H. Padmasri, S.S. Madhavendra, B.D. Raju, K.S.R. Rao, Highly efficient Ag/C catalyst prepared by electro-chemical deposition method in controlling microorganisms in water, J. Mol. Catal. A: Chem. 223 (2004) 313–319. [45] C.-N. Lok, C.-M. Ho, R. Chen, Q.-Y. He, W.-Y. Yu, H. Sun, P.K.-H. Tam, J.-F. Chiu, C.M. Che, Proteomic analysis of the mode of antibacterial action of silver nanoparticles, J. Proteome Res. 5 (2006) 916–924. [46] V.S. Kumar, B. Nagaraja, V. Shashikala, A. Padmasri, S.S. Madhavendra, B.D. Raju, K.R. Rao, Highly efficient Ag/C catalyst prepared by electro-chemical deposition method in controlling microorganisms in water, J. Mol. Catal. A: Chem. 223 (2004) 313–319. [47] W. He, H. Wu, W.G. Wamer, H.-K. Kim, J. Zheng, H. Jia, Z. Zheng, J.-J. Yin, Unraveling the enhanced photocatalytic activity and phototoxicity of ZnO/Metal hybrid nanostructures from generation of reactive oxygen species and charge carriers, ACS Appl. Mater. Interfaces 6 (2014) 15527–15535. [48] S. Ghosh, V. Goudar, K. Padmalekha, S. Bhat, S. Indi, H. Vasan, ZnO/Ag nanohybrid: synthesis, characterization, synergistic antibacterial activity and its mechanism, RSC Adv. 2 (2012) 930–940. [49] X. Zhou, G. Liu, J. Yu, W. Fan, Surface plasmon resonance-mediated photocatalysis by noble metal-based composites under visible light, J. Mater. Chem. 22 (2012) 21337–21354. [50] Y. Xu, M.A. Schoonen, The absolute energy positions of conduction and valence bands of selected semiconducting minerals, Am. Mineral. 85 (2000) 543–556. [51] S. Qin, F. Xin, Y. Liu, X. Yin, W. Ma, Photocatalytic reduction of CO2 in methanol to methyl formate over CuO–TiO2 composite catalysts, J. Colloid Interface Sci. 356 (2011) 257–261. [52] K. Sunada, Y. Kikuchi, K. Hashimoto, A. Fujishima, Bactericidal and detoxification effects of TiO2 thin film photocatalysts, Environ. Sci. Technol. 32 (1998) 726–728. [53] D. Wu, W. Wang, T.W. Ng, G. Huang, D. Xia, H.Y. Yip, H.K. Lee, G. Li, T. An, P.K. Wong, Visible-light-driven photocatalytic bacterial inactivation and the mechanism of zinc oxysulfide under LED light irradiation, J. Mater. Chem. A 4 (2016) 1052–1059. [54] X. Niea, G. Li, M. Gaoc, H. Suna, X. Liub, H. Zhao, P.K. Wongc, T. An, Comparative study on the photoelectrocatalytic inactivation of Escherichia coli K-12 and its mutant Escherichia coli BW25113 using TiO2 nanotubes as a photoanode, Appl. Catal. B: Environ. 147 (2014) 562–570. [55] D. Wu, B. Wang, W. Wang, T. An, G. Li, T.W. Ng, H.Y. Yip, C. Xiong, H.K. Leed, P.K. Wong, Visible-light-driven BiOBr nanosheets for highly facet-dependent photocatalytic inactivation of Escherichia coli, J. Mater. Chem. A 3 (2015) 15148–15155. [56] K. Nakaoka, J. Ueyama, K. Ogura, Photoelectrochemical Behavior of Electrodeposited CuO and Cu2O Thin Films on Conducting Substrates, J. Electrochem. Soc. 151 (10) (2004) C661–C665. [57] H. Gan, G. Zhang, H. Huang, Enhanced visible-light-driven photocatalytic inactivation of Escherichia coli by Bi2O2CO3/Bi3NbO7 composites, J. Hazard. Mater. 250–251 (2013) 131–137. [58] M. Yin, Z. Li, J. Kou, Z. Zou, Mechanism investigation of visible light-induced degradation in a heterogeneous TiO2/eosin Y/rhodamine B system, Environ. Sci. Technol. 43 (2009) 8361–8366.
Ag(I)-Based antibacterial materials with enhanced stability, ACS Sustain. Chem. Eng. 4 (2016) 3268–3275. S. Sun, X. Zhai, D. Gu, X. Chang, X. Hu, T. Liu, L. Dong, Y. Yin, Strongly-coupled silver chloride–tungsten oxide hybrid nanocomposite with excellent antibacterial effect, Adv. Powder Technol. 27 (2016) 1295–1300. O. Akhavan, Lasting antibacterial activities of Ag–TiO2/Ag/a-TiO2 nanocomposite thin film photocatalysts under solar light irradiation, J. Colloid Interface Sci. 336 (2009) 117–124. X. Zhang, Y.L. Chen, R.-S. Liu, D.P. Tsai, Plasmonic photocatalysis, Rep. Prog. Phys. 76 (2013) 046401. W. Fan, M.K. Leung, Recent development of plasmonic resonance-based photocatalysis and photovoltaics for solar utilization, Molecules 21 (2016) 180. Z. Zhang, J.T. Yates Jr., Band bending in semiconductors: chemical and physical consequences at surfaces and interfaces, Chem. Rev. 112 (2012) 5520–5551. A.A.H. Ba-Abbad, A.B. Kadhum, M.S. Mohamad, K. Takriff, Visible light photocatalytic activity of Fe3+-doped ZnO nanoparticle prepared via sol–gel technique, Chemosphere 91 (2013) 1604–1611. R. Gupta, N.K. Eswar, J.M. Modak, G. Madras, Visible light driven efficient N and Cu co-doped ZnO for photoinactivation of Escherichia coli, RSC Adv. 6 (2016) 85675–85687. R. Gupta, N.K. Eswar, J.M. Modak, G. Madras, Effect of morphology of zinc oxide in ZnO-CdS-Ag ternary nanocomposite towards photocatalytic inactivation of E. coli under UV and visible light, Chem. Eng. J. 307 (2017) 966–980. B. Choudhury, M. Dey, A. Choudhury, Shallow and deep trap emission and luminescence quenching of TiO2 nanoparticles on Cu doping, Appl. Nanosci. 4 (2014) 499–506. L. Xu, X. Li, Influence of Fe-doping on the structural and optical properties of ZnO thin films prepared by sol–gel method, J. Cryst. Growth 312 (2010) 851–855. P. Wang, B. Huang, Y. Dai, M.-H. Whangbo, Plasmonic photocatalysts: harvesting visible light with noble metal nanoparticles, Phys. Chem. Chem. Phys. 14 (2012) 9813–9825. N.K. Eswar, P.C. Ramamurthy, G. Madras, High photoconductive combustion synthesized TiO2 derived nanobelts for photocatalytic water purification under solar irradiation, New J. Chem. 39 (2015) 6040–6051. S. Karamat, R.S. Rawat, P. Lee, T.L. Tan, R.V. Ramanujan, Structural elemental, optical and magnetic study of Fe doped ZnO and impurity phase formation, Prog. Nat. Sci. Mater. Int. 24 (2014) 142–149. S. Kumar, R. Prakash, R.J. Choudhary, D.M. Phase, Structural, XPS and magnetic studies of pulsed laser deposited Fe doped Eu2O3 thin film, Mater. Res. Bull. 70 (2015) 392–396. M.C. Biesinger, L.W. Lau, A.R. Gerson, R.S.C. Smart, Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Sc, Ti, V, Cu and Zn, Appl. Surf. Sci. 257 (2010) 887–898. P.A. Thrower, Chemistry & Physics of Carbon, CRC Press, Florida, 1996. T. Ghodselahi, M.A. Vesaghi, A. Shafiekhani, A. Baghizadeh, M. Lameii, XPS study of the Cu@Cu2O core-shell nanoparticles, Appl. Surf. Sci. 255 (2008) 2730–2734. Y. Zhang, M.K. Ram, E.K. Stefanakos, D.Y. Goswami, Enhanced photocatalytic activity of iron doped zinc oxide nanowires for water decontamination, Surf. Coat. Technol. 217 (2013) 119–123. M. Shi, H.S. Kwon, Z. Peng, A. Elder, H. Yang, Effects of surface chemistry on the generation of reactive oxygen species by copper nanoparticles, ACS Nano 6 (2012) 2157–2164. G. Applerot, J. Lellouche, A. Lipovsky, Y. Nitzan, R. Lubart, A. Gedanken, E. Banin, Understanding the antibacterial mechanism of CuO nanoparticles: revealing the
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Ag and CuO impregnated on Fe doped ZnO for bacterial inactivation under visible light Rimzhim Guptaa, Neerugatti KrishnaRao Eswarb, Jayant M. Modaka, Giridhar Madrasa, a b
⁎
Department of Chemical Engineering, Indian Institute of Science, Bangalore, India Centre for Nanoscience and Engineering, Indian Institute of Science, Bangalore, India
A R T I C L E I N F O
A B S T R A C T
Keywords: E. coli Antibacterial Photocatalysis Metal impregnation Transition metal doping
Interfacial coupling of semiconductor with metal has been demonstrated for inactivation of E. coli. Fe doped ZnO was synthesized by sol–gel method that resulted in enhanced absorbance in visible region. Various concentrations of Cu were impregnated on Fe doped ZnO that eventually turned into copper oxide and the photocatalytic activity of this material was compared with noble metal (Ag) impregnated on Fe doped ZnO. The obtained materials were characterized by various techniques. The crystal structures were determined by XRD and XPS was used to identify the oxidation states of the elements present in the photocatalyst. The morphologies and microstructures were determined by SEM. The optical absorbance of the photocatalysts was characterized by diffused reflectance spectra. Photocatalytic experiments were conducted for inactivation of E. coli using various catalysts. The rate constants obtained for 3 wt.% Cu impregnated Fe doped ZnO was higher than 1 wt.% Ag impregnated Fe doped ZnO. The higher photoactivity of these materials compared to pristine ZnO can be attributed to decreased recombination of the excitons in the synthesized photocatalysts that was validated by photoluminescence. This study indicates the possible employment of copper as a viable substitute for silver for anti-bacterial applications.
1. Introduction Bacterial inactivation has been extensively studied since the first reports [1,2] using TiO2 by photocatalysis. TiO2 and ZnO are well established photocatalysts as UV light active semiconductors [3,4]. The wide band gaps of TiO2 and ZnO, however, create a limitation on their usage in visible light irradiation. This limitation can be overcome via band engineering either by metal/non-metal doping or by forming composites with lower band gap materials. Metal or non-metal doping introduces sub bands (impurity levels) between the metal oxide energy levels. These inter-bands serve as trapping sites for charge carriers resulting in the lower band gap of the material [5]. The formation of composites modifies the optical properties by increasing the absorbance and lifetime of the excitons due to the incorporation of lower band gap material such as CdS, Fe2O3, BiOI, V2O5 etc. [6–9]. Transition metal doping on metal oxide has several advantages such as introduction of inter-bands below conduction band and enhancement in the formation of oxygen vacancies in the lattice [10]. The induced orbitals of the metal ion tend to readjust the orbitals of the parent semiconductor, thus introduces impurity levels which in turn reduces the band gap [5]. Oxygen vacancies play an important role in
⁎
modifying the photo-induced behaviour of the semiconductor [10] by serving as trap sites for the photo-excited electrons that suppress the recombination. Among the transition metals, Fe is a better dopant because of the half-filled electronic configuration (d-orbital) that provides symmetrical distribution of electrons and increases the energy exchange of the system that provides stability [11,12]. The incorporation of Fe ions in the lattice provides shallow traps for photogenerated electrons that result in enhanced charge separation[13] and narrows the band gap by providing a red shift in the absorption edge[14]. Transition metal doping such as Co [15], Ni [16], Fe [17] into ZnO has proved to impart several benefits. Co metal doping into ZnO resulted in a decrease of band gap. This is possible due to electron- transitions between “dCo” orbital to “dZn” transitions and also improved interactions between sp and d bands [15,16]. Silver shows excellent activity for antibacterial applications. Impregnation of silver on TiO2 [18] and ZnO [19] has been shown to possess excellent antibacterial activity. Ag3PO4 and its composites with AgBr/CeO2 nanoflakes and TiO2 have been used for antibacterial applications [20,21]. AgCl/SiO2 [22], AgCl/W18O49 nanorods [23], Ag–TiO2/Ag/a-TiO2 [24] have also been reported as efficient visible light active photocatalysts. The leaching of silver into water from these
Corresponding author. E-mail address: [email protected] (G. Madras).
http://dx.doi.org/10.1016/j.cattod.2017.05.032 Received 12 December 2016; Received in revised form 1 April 2017; Accepted 8 May 2017 0920-5861/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Gupta, R., Catalysis Today (2017), http://dx.doi.org/10.1016/j.cattod.2017.05.032
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materials limits their viable usage. To overcome some of these difficulties, various metals were impregnated on doped ZnO because these metals increase the photo-response resulting in higher activity and also give better stability than their respective oxides. Often noble metals like Ag, Au, Pt are impregnated on metal oxide semiconductors. However, considering their cost, in this work transition metals such as Cu was impregnated on Fe doped ZnO. This is compared with Ag impregnation on the Fe doped ZnO because Ag is known for its antibacterial properties. Besides better antibacterial properties, Ag and Cu exhibit surface plasmonic resonance (SPR). There are several beneficial effects of plasmonic resonance in the field of photocatalysis as it enhances visible light absorption and excitation of energetic charge carriers [25]. This results in enhanced redox reactions and enhanced rate of generation of reactive oxygen species [26]. In order to achieve better charge separation[27], a proper design of metal − semiconductor combination needs to be proposed that enhances the photocatalytic activity by suppressing the recombination. The objective of this study was to develop novel antibacterial materials wherein a transition metal is impregnated on doped ZnO and compare its photocatalytic antibacterial activity with the conventional Ag-based materials. In the present work, Fe doped ZnO was synthesized by the sol-gel process with different doping concentrations. Impregnation of different metals (Ag, Cu) was carried out on Fe doped ZnO. This wet impregnation was carried out for metal reduction on the semiconductor surface. However, Cu was converted to CuO in the process of impregnation and CuO impregnated on Fe doped ZnO was obtained. The photocatalytic activities of synthesized photocatalysts were studied under visible light irradiation for the inactivation of E. coli. This study opens a new avenue in the area of cost effective metal impregnated materials compared to Ag based materials for antibacterial activity.
2.2. Photocatalytic reactor The photochemical reactor used in the current study consists of a jacketed quartz tube and a glass reactor. Experiments were carried out using a metal halide lamp of intensity 68,000 Lux. Ballast with capacitor was connected in the series with the lamp to ensure constant input voltage. Cold water was circulated through the annulus of the quartz tube to maintain the reactor temperature at 30 °C. The source assembly was kept behind the glass reactor and placed in a wooden box. A filter with cut off wavelength < 400 nm was placed between the lamp and the reactor to ensure only visible light reaches the reactor. All experiments were conducted in this reactor with a bacterial solution and appropriate catalyst loading, as described later. 2.3. Culture and microorganism growth The growth culture for E. coli was prepared by inoculation of 100 μL inoculum/mother culture in 300 mL of sterilized 2 wt.% of LuriaBertani broth (HiMedia, India) in DI water. Mother culture was prepared using glycerol stock stored at −80 °C. 50 μL of glycerol stock was transferred by pipette to small volume of Luria-Bertani broth and kept for 9 h at 37 °C. The culture was kept at 200 rpm at 37 °C in shaking incubator (Orbitek, India) providing aerobic conditions for 6 h. Bacterial colony count was carried out by spreading the serial diluted samples using 2.8 % nutrient agar (HiMedia, India) plates. 2.4. Bacterial inactivation Required accessories such as pipette tips, Eppendorf tubes, beaker, DI water etc. were autoclaved at 120 °C for 90 min. Firstly, bacterial pellet was obtained after centrifuging the 30 mL culture at 4000 rpm. The pellet was washed twice with sterilized DI water to eliminate the nutrients completely. The final obtained pellet was re-suspended in 30 mL of sterilized DI water. The experiments were performed in 30 mL bacterial solution with 0.25 g/L catalyst loading. The solution was kept under visible light irradiation for 1 h. Samples were collected after every 15 min and diluted till the bacterial colony count reaches 30–300 CFU/mL. To obtain the colony count, 100 μL of the diluted samples were spread over nutrient agar plates for bacterial colony count.
2. Experimental 2.1. Materials Zinc nitrate hexahydrate (Zn(NO3)2·6H2O), hydrazine hydrate, silver nitrate (AgNO3), oxalic acid (C2H2O4), copper nitrate trihydrate (Cu(NO3)2·3H2O), ferric nitrate nonahydrate (Fe(NO3)3·9H2O), methanol were purchased from Merck (India). 2.1.1. Synthesis of ZnO and Fe doped ZnO ZnO was prepared by sol-gel technique, as described elsewhere [28]. Ferric nitrate nonahydrate and zinc nitrate hexahydrate were taken as precursors. 2.0 mol of zinc nitrate hexahydrate was dissolved in 100 mL of methanol and continuously stirred under reflux condition. Then the required wt.% of iron nitrate (dissolved in 20 mL of methanol) was added to the above zinc nitrate solution. Subsequently, oxalic acid solution was prepared in 50 mL of methanol and added to the above solution such that the molar ratio of zinc nitrate hexahydrate and oxalic acid was 1:1. Stirring was continued till a white gel was obtained and it was dried at 70 °C for 12 h. Finally, the powder was calcined at 400 °C for 3 h to remove all the impurities. 1, 2 and 5 wt.% Fe doped ZnO are named as Fe1Z, Fe2Z and Fe5Z, respectively.
2.5. Characterization The crystal structure was determined by X-ray diffraction using Rigaku Diffractometer at the scan rate of 1° min−1 and scan range of 5°–100°. The calculation of crystallite size was calculated using Debye˙ ) was used as the target material Scherrer’s formula. Cu-Kα (λ = 1.54 A for X-ray generation. Morphological and microscopic studies were carried out by SEM and TEM. ULTRA55 FESEM, Carl Zeiss was used for scanning electron microscopy. The catalyst was sonicated in ethanol/iso-propanol for 5 min and drop casted on the silicon wafer for SEM imaging. The sample was kept under vacuum to prevent undesirable interactions. Later, the SEM sample was sputtered with gold using Quorum sputtering to avoid charging. TEM was carried out using Tecnai F30 operated at 180 kV. The catalyst was sonicated in ethanol/iso-propanol for 10 min and drop casted on Cu TEM grid carefully and dried in oven for complete removal of the solvent. The sample was kept at vacuum for 12 h. In order to determine the band gap of the synthesized materials, diffused reflectance spectra was acquired using UV–vis spectrophotometer (PerkinElmer, Lambda 35) in the wavelength range of 300–700 nm. The reduced exciton recombination was validated by photoluminescence studies using Perkin-Elmer at excitation wavelength of 300 nm. The surface area analysis was performed by nitrogen adsorption using Nova-1000 Quantachrome.
2.1.2. Synthesis of metal (Ag, Cu) impregnated Fe doped ZnO 100 mg of prepared Fe doped ZnO was dispersed in 100 mL of deionized (DI) water. Required amount of metal precursor (for 1 wt.% Ag and 1, 2, 3 wt.% Cu) was then added. The reaction was allowed to occur in dark. Under vigorous mixing, 0.05 M of hydrazine hydrate solution was added drop wise to the above solution to reduce the metal ions. The reaction was continued for 1 h and then the suspension was washed with DI water, ethanol twice and dried for 12 h at 70 °C. 1 wt.% Ag impregnated Fe2Z is named Ag1Fe2Z; 1, 2 and 3 wt.% of Cu impregnated Fe2Z are named as Cu1Fe2Z, Cu2Fe2Z and Cu3Fe2Z, 2
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3.1. XRD analysis Fig. 1(a) shows the X-ray diffraction patterns of ZnO, 1, 2 and 5 wt. % Fe doped ZnO. Fig. 1(b) shows the diffraction spectra of 1 wt.% Ag impregnated Fe2Z, and 1–3 wt.% Cu impregnated Fe2Z. All the synthesized materials possess hexagonal structure. The crystal structures and the diffraction planes of ZnO were validated by JCPDF- 00-03089. The absence of Fe, Fe2O3 peaks in the patterns suggest the presence of Fe at substitutional sites of Zn. Fig. 1(b) shows the XRD pattern of Ag1Fe2Z. The peak at 37° corresponds to the plane (4 0 0) and is validated by JCPDF-00-001-1164. Similarly, Fig. 1(b) shows the XRD pattern of Cu impregnated Fe doped ZnO (Cu1Fe2Z, Cu2Fe2Z, and Cu3Fe2Z). Impregnation of Cu in metal form is not possible due to lower oxidation potential of copper. This causes the conversion of Cu metal into copper oxide. Many peaks of CuO are in the similar region of ZnO. A clear peak at 9.7° corresponding to CuO is present in the patterns (Fig. 1(b)), that matches with the peak of CuO (JCPDF-00-0021041). The synthesis was performed such that Fe is doped in the lattice. Lattice parameters obtained from Rietveld refinement are added in the manuscript in Table 1. This clearly shows a decrease in lattice parameters after doping of Fe in the lattice of ZnO. However, no change in the lattice parameters is observed after CuO impregnation on the Fe doped ZnO. This is consistent with our synthesis procedure where Fe is doped in ZnO while CuO and Ag are impregnated. 3.2. Photoluminescence Fig. 2(a) shows the photoluminescence spectra for ZnO, Fe1Z, Fe2Z, Fe5Z, Ag1Fe2Z and Cu3Fe2Z, respectively. The photoluminescence spectrum indicates the recombination of photogenerated electrons and holes. PL spectra of all the catalysts showed significant variations based on the recombination of excited charge carriers. The peaks at 430–440 nm denote the recombination of excitons at the band edges of the corresponding bands of ZnO. Other shoulder peaks were also observed at 480–490 and 530 nm [29,30]. The peaks at 480–490 nm correspond to recombination of excitons present in the shallow traps that emit at longer wavelengths. The presence of shallow traps may be due to inter-bands generated as the result of transition metal i.e. Fe doping. The peak at 530 nm corresponds to the deeper traps of the ionic core due to enhanced oxygen vacancies [31]. The PL intensity of Cu impregnated Fe doped ZnO is lower than other catalysts studied here.
Fig. 1. X-ray diffraction patterns of (a) ZnO, Fe1Z, Fe2Z and Fe5Z (b) Ag1Fe2Z, Cu1Fe2Z, Cu2Fe2Z, Cu3Fe2Z.
3. Results and discussion ZnO is the base semiconductor and Fe2Z is the parent material for all impregnated photocatalysts. After obtaining and characterizing the pure phase of photocatalysts using XRD, photocatalytic experiments were performed. Due to superior phtocatalytic activity of Fe2Z and Cu3Fe2Z in comparison to Fe1Z, Fe5Z, and Cu1Fe2Z, Cu2Fe2Z, respectively, other characterizations such as XPS, SEM and TEM were performed only for these photocatalysts.
3.3. Diffuse reflectance spectra Fig. 2(b) shows the absorbance spectra of undoped ZnO, 1, 2 and 5 wt.% Fe doped ZnO, 1 wt.% Ag impregnated Fe2Z, 3 wt.% Cu impregnated Fe2Z, respectively. Except ZnO all the materials absorb UV light. Incorporation of Fe in ZnO increases the absorption in visible region. The band gap is calculated by the Kubelka Munk Function and is 3.23, 3.21, 3.19 and 3.18 eV for ZnO and 1, 2 and 5 wt.% Fe doped ZnO, respectively, as shown in Fig. 2(c). Similar values (in range of
Table 1 Specific surface area, crystallite size, rate constants and lattice parameters for bacterial inactivation for all the photocatalysts under visible light. Photocatalysts
Specific surface area (m2/g) ( ± 1)
Crystallite size (nm)
Rate constant (h−1)
a (A)
b (A)
c (A)
Photolysis ZnO Fe1Z Fe2Z Fe5Z Ag1Fe2Z Cu1Fe2Z Cu2Fe2Z Cu3Fe2Z
– 44 38 63 68 47 58 57 55
– 17 15 11 10 – – – –
3.3 ± 0.2 7.9 ± 0.4 8.3 ± 0.4 9.1 ± 0.2 7.8 ± 0.4 12.5 ± 0.3 11.8 ± 0.5 15.5 ± 0.6 18.2 ± 0.6
– 3.2573 3.2522 3.2500 3.2476 3.2500 3.2500 3.2500 3.2500
– 3.2573 3.2522 3.2500 3.2476 3.2500 3.2500 3.2500 3.2500
– 5.2215 5.2126 5.2079 5.2040 5.2080 5.2080 5.2080 5.2080
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Fig. 2. (a) Photoluminescence spectra for ZnO, Fe1Z, Fe2Z, Fe5Z, Ag1Fe2Z, Cu3Fe2Z (b) Absorbance for ZnO, Fe1Z, Fe2Z, Fe5Z, Ag1Fe2Z, Cu3Fe2Z (c) Tauc plot for ZnO, Fe1Z, Fe2Z and Fe5Z.
observed clearly. The diffraction pattern of Cu3Fe2Z (Fig. 3(h)) shows the high nano-crystallinity of the photocatalyst.
3.3–3.33 eV) of band gaps have been reported elsewhere [32]. Impregnation of metals results in a significant increase in the absorbance of the photocatalysts in the visible region. The enhancement in the absorbance of Ag1Fe2Z and Cu3Fe2Z is due to surface plasmon behaviour due to the presence of silver nanoparticles [33]. The increase in absorbance in the visible region may be attributed to the presence of copper oxide in Cu3Fe2Z. Copper oxide possess lower band gap and absorbs visible light. The presence of CuO along with ZnO tunes the absorbance in such a way that the composite becomes visible active semiconductor with enhanced absorbance.
3.5. XPS analysis XPS studies were performed to determine the oxidation states of the elements present in the photocatalyst. Elements present in the photocatalysts were C, O, Zn, Fe, Cu. Fig. 4(a) shows the peak at 284.8 eV that denotes the presence of C–1 s carbon in the catalyst. In Fig. 4(b), the peak at 530 eV shows the lattice oxygen and the extended peak denote the oxygen vacancies in the Fe2Z the peak at lower binding energy shows the presence of hydroxyl group. Fig. 4(c) represents the O–1 s in Cu3Fe2Z. The highest peak denotes the lattice oxygen and the extended peaks at higher binding energy denote the oxygen vacancies and the chemisorbed oxygen [34]. The absence of any short peak at lower binding energy denotes the absence of the hydroxyl group due to the presence of CuO. Fig. 4(d) shows the doublets of Zn in Fe2Z. Peak at 1022 eV show the 2p3/2 orbital and peak at 1045 eV show the 2p1/2 orbital of zinc [29]. In Fig. 4(e), peaks denote 2p3/2 and 2p1/2 doublets of Fe that shows the presence of iron in Fe3+ form in the Fe doped ZnO [35,36]. Fig. 4(f) shows the presence of Cu in ionic form in the Cu3Fe2Z photocatalyst [37]. Presence of satellite peak between the doublets of Cu-2p indicates the presence of Cu in ionic form (Cu2+). This signifies the formation of copper oxide while synthesis. Impregnation of Cu reduces copper nitrate into Cu but lower oxidation potential of Cu results in the oxidation Cu into copper oxide. The difference in 2p3/2 and 2p1/2 peak was found to be 19.75 eV. Fig. 4(g) shows the XPS of Ag in 3d orbital [30]. This shows the presence of silver in metallic form. Earlier reports state that the peak exposition of 3d5/2 at 368.2 and 3d3/2 at 373.9 eV corresponds to the silver metal [38]. Fig. 4(h) show the wide spectra for Fe2Z, Ag1Fe2Z and Cu3Fe2Z in the range of
3.4. Microscopic analysis and BET surface area analyses In order to determine the morphology of the synthesized photocatalysts, scanning electron microscopic analyses were performed. Fig. 3(a)–(d) show the field emission SEM images of ZnO, Fe2Z, Ag1Fe2Z and Cu3Fe2Z impregnated Fe2Z, respectively. ZnO and Fe doped ZnO possess similar morphology that contributed to the high surface area that allows active interaction of the catalyst with the bacteria. The presence of metal particles/metal oxide nanoparticles at the surface of Fe doped ZnO leads to the reduction in surface area of the material due to firm attachment of the metal. As the percentage of impregnation increases, surface area decreases. The specific surface areas for all the photocatalysts are tabulated in Table 1. The TEM results show that the particle sizes of Fe2Z were around 10–15 nm, as shown in Fig. 3(e). Fig. 3(f) shows the bright field image of Ag1Fe2Z. A significant contrast between copper oxide particles and ZnO was observed from Fig. 3(g) where 3 wt.% Cu was impregnated on Fe2Z that proves the successful formation of heterojunction between Cu and Fe doped ZnO. Similarly, impregnation of Cu on Fe2Z has not increased the particle size but the firm attachment of CuOS can be 4
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Fig. 3. SEM images of (a) ZnO (b) Fe2Z (c) Ag1Fe2Z (d) Cu3Fe2Z TEM images of (e) Fe2Z (f) Ag1Fe2Z (g) Cu3Fe2Z and (h) diffraction pattern of Cu3Fe2Z.
3.6. Photocatalysis
0–1200 eV. The background of XPS is generally attributed to the scattering during photoelectron ejection to the surface between electron-electron, electron-ion and electron- plasmon. The rise in the background of Ag1Fe2Z can be attributed to the scattering of plasmon of Ag nanoparticles present at the semiconductor surface [39]. This was supported by the enhanced absorption of Ag1Fe2Z as seen in Fig. 2(a) indicating the existence of surface plasmon resonance (SPR) of Ag nanoparticles.
3.6.1. Effect of Fe doping on ZnO Optical properties were significantly found to be enhanced by doping Fe into ZnO. Recombination of electrons and holes were proven to be reduced compared to pristine ZnO [17]. Therefore, in the current study, Fe was doped into ZnO as 1, 2 and 5 wt.%, respectively. The photocatalytic activities of all concentrations of Fe doped ZnO samples have been tested by bacterial inactivation. As the Fe doping concentra5
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Fig. 4. XPS spectra for (a) C–1s (b) O–1s in Fe2Z (c) O–1s in Cu3Fe2Z (d) Zn-2p (e) Fe-2p in Fe2Z (f) Cu-2p in Cu3Fe2Z (g) Ag-3d in Ag1Fe2Z (h) wide spectra of Fe2Z, Ag1Fe2Z, and Cu3Fe2Z.
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3.6.2. Effect of metal (Ag, Cu) impregnation on Fe doped ZnO in dark Catalytic hetero-junctions such as semiconductor-semiconductor, metal-semiconductor were subjected to dark experiments. This is because, while synthesising the composites, not all the nucleation processes happen heterogeneously on the substrate (Fe2Z in this case) and homogeneous nucleation can also occur. Therefore, in order to nullify the effect of these homogeneously nucleated particles, reactions are performed under dark conditions to quantify the activity of impregnated nanoparticles alone. Initially, Cu was reduced on Fe2Z with varying wt.% between 1–3 wt.% and bacterial inactivation experiments were conducted in dark. The reduced Cu turns into copper oxide in the impregnation process. It is found that as the concentration of CuO increases, bacterial log reduction was increased. Fig. 5(a) shows bacterial inactivation plots for various catalysts under dark conditions. Similarly, Ag impregnation was limited to 1 wt.% as optimized elsewhere [3]. Generation of reactive oxygen species is common to silver nanoparticles and these ROS can actively participate in reduction of bacterial count [41]. These results suggest that 1–3 wt.% Cu impregnation show lesser antibacterial activity than 1 wt.% Ag in dark. Lower activity of Cu1Fe2Z, Cu2Fe2Z and Cu3Fe2Z may be attributed to the formation of copper oxide due to the oxidation of Cu metal while synthesis. The mechanism of inactivation by CuO has been widely reported [42,43]. Small concentration of copper ions leached out from copper oxide may disrupt the function of bacteria by binding itself to the DNA and thus disordering the double helical structure [43]. The other plausible reason for the inactivation of bacteria in the presence of CuO is ROS generation. These ROS after attaching to the bacterial surfaces enhances the intracellular oxidative stress and leads to the lipid peroxidation and results in cell death [42]. All the inactivation experiments under dark follow zero order reaction kinetics. The rate of inactivation under dark reactions suggests that silver has shown better inactivation. Silver, copper and metals possess capacity to adsorb oxygen. Adsorbed nascent oxygen specifically at the silver surface (due to silver affinity towards oxygen) reacts with the lipid content (-SH) present in the cell wall of bacteria and forms R-S-S-R bonds. This causes a rupture in the cell membranes that leads to death of the bacteria [44]. Copper and other metals instead form oxides and offer a barrier that retards the rate of inactivation in comparison to silver [44]. Metal nanoparticles rupture the plasma membrane that reduces the ATP content inside the cell [45]. Amines and carboxylic groups present on the surface of bacteria possess high affinity towards metal nanoparticles such as copper etc. Inactivation of the bacteria may occur either by the attack due to the nanoparticle itself or by the ions released by nanoparticles. Fig. 5. Degradation profile of E. coli with initial concentration ∼ 107 CFU/mL (a) under dark (b) photolysis, ZnO, Fe1Z, Fe2Z, Fe5Z under visible light irradiation (c) Ag1Fe2Z, Cu1Fe2Z, Cu1Fe2Z and Cu1Fe2Z under visible light irradiation.
3.6.3. Effect of metal (Ag, Cu) impregnation on Fe doped ZnO in the presence of light Fig. 5(b) shows the photocatalytic degradation of E. coli under visible light using various metal (Ag, Cu) impregnated photocatalysts. Among, copper impregnated catalysts, 3 wt.% Cu Fe2Z shows better photocatalytic activity compared to other two compositions such as 2 wt.% Cu and 1 wt.%. The rate constant of Cu3Fe2Z (18.2 ± 0.6 h−1) is higher than Ag1Fe2Z (12.5 ± 0.3 h−1). This proves that copper oxide can be used in place of silver in order to achieve enhanced photocatalytic activity than 1 wt.% Ag impregnation. Silver impregnation allows dissolved oxygen to couple with metal and form organo-metallic bonds preventing cell respiration [46]. Silver can inhibit proton motive force when pore proteins of bacteria interact with metal–metal oxide junction [45]. Charge species can effectively be transferred between semiconductor and metal due to the metal − semiconductor heterojunction that can easily destabilize the cell wall and plasma membrane [45]. The presence of intrinsic oxygen vacancies, in n-type semiconductors such as ZnO, the Fermi level is close to the conduction band due to the presence of majority charge carriers as electrons. Therefore, in
tion increases from 1 wt.% to 2 wt.% (as in Fe1Z and Fe2Z) as shown in Fig. 5(b), the bacterial count decreased. However, increasing the concentration to 5 wt.% reduced the photoactivity of the catalyst (Fe5Z) compared to the photoactivity obtained with 2 wt.% Fe doped ZnO. The addition of Fe atoms will induce several traps near the conduction band of ZnO [40]. Fe atom consists of vacant d-orbital. Therefore, excited electrons will be trapped in those vacant sites of Fe atom and prevent recombination. Though there is an increase in visible light absorption by Fe doped ZnO as in Fig. 2(a), the photoluminescence (Fig. 2(b)) spectra of pristine ZnO and Fe-doped ZnO shows only slight decrease in intensity. Further, 2 wt.% Fe doped ZnO shows higher photocatalytic activity compared to 1 and 5 wt.% indicating 2 wt.% Fe doping as a threshold limit. 2 wt.% Fe doping (Fe2Z) showed better results compared to other doping concentration and thus Fe2Z has been taken as base material for further studies. 7
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Fig. 6. Schematic of photocatalytic mechanism mediated by plasmonic resonance of silver.
order to maintain the charge equilibrium in the metal − semiconductor heterojunction, appropriate band bending occurs between the semiconductor Fermi level and the metal causing free electron movement during excitation. This is due to the position of Fermi level of silver and copper that lie below the Fermi level of ZnO. Downward band bending occurs in case of silver coupled with semiconductor. The electron transfer occurs until the Fermi level becomes equal [47]. Work function of ZnO is 4.65 eV which is higher than the work function of Ag i.e. 4.26 eV. The availability of electrons at the semiconductor conduction band in the case of silver facilitates the reduction reaction of oxygen to form superoxide radical. Conversely, in case of copper the charge transfer from semiconductor to metal enhances the charge separation and suppresses the recombination [47]. This mechanism is beneficial in efficient electron transfer and hydroxyl radical generation. Both contributes in the enhancement of photocatalytic activity of the semiconductor [27,48]. Fig. 6 shows the schematic of photocatalytic mechanism mediated by plasmonic resonance in case of silver as metal nanoparticle. If a semiconductor is coupled with noble metal, the free electrons are confined at the local spots. As the material is exposed to the irradiation of appropriate frequency, the electron charge density of the free electrons is redistributed and thus an electric field is generated. A coulombic charge is generated due to the presence of positive charge at the metal surface and hence downward band bending and transfer of electrons from metal to semiconductor is induced. This feature enhances the absorption of light significantly in the coupled metal semiconductor photocatalysts [26,49]. Formation of stable heterojunctions and generation of reactive radical species are responsible for better photoactivity exhibited by metal impregnates on zinc oxide. Therefore, coupling least expensive metals like copper to semiconductors has shown better photocatalytic activity compared Ag. This could be a sustainable and cost effective solution in the field of water purification. Fig. 7 shows the photocatalytic mechanism of Fe doped ZnO and copper oxide. Copper oxide possess band gap of (1.5–1.7 eV) which absorbs visible light. Conduction band of copper oxide lies above the conduction band of Fe2Z [50]. Potential of conduction band of Fe2Z and copper oxide crosses the potential of generation of superoxide radical. Similarly, Fe2Z valence band crosses the potential of hydroxyl radical formation potential. After absorbing the visible light, excitation of electrons occur leaving the vacant sites i.e. holes of Fe2Z and CuO separately. Electrons from the CuO conduction band can either move to the conduction band of Fe2Z or react with the dissolved oxygen and form superoxide radical [51]. Similarly, holes in the Fe2Z can react with the hydroxyl ions to form hydroxyl radicals and can be transferred to valence band of the copper oxide. After reaction of electrons with the dissolved oxygen and holes with the hydroxyl ion, superoxide radicals
Fig. 7. Schematic of photocatalytic mechanism Fe2Z-copper oxide composite.
and hydroxyl radical are generated, respectively. These radicals oxidize the impurities or rupture the cell wall after lipid peroxidation and this causes the inactivation of microorganism [52]. The holes present at the CuO do not possess the potential to form hydroxyl radical however they can directly attack the bacteria and oxidize the outer membrane [53–55]. Band edge positions of CuO ((in vacuum scale) −3.66 eV (C.B.) and −5.22 eV (V.B.)) and ZnO (−4.20 eV (C.B.) and −7.39 eV (V.B.)) are taken from elsewhere [56,30]. 3.6.4. Scavenger studies In order to detect to major responsible radicals for the inactivation of E. coli, scavenger experiments were performed using KI for hydroxyl radical scavanger and Ethylenediaminetetraacetic acid (EDTA) as hole (h+) scavengers and 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxy (TEMPO) as superoxide radical scavenger [29]. 2 mmol/L of TEMPO and 1 mmol/L of EDTA and 5 mmol/L of KI were added along with 0.25 g/L of catalysts Fe2Z and Cu2Fe2Z in the bacterial solution while conducting the experiments. 3.6.4.1. Scavenger study with Fe2Z. Fig. 8(a) shows the degradation profile of E. coli in the presence of TEMPO, KI, TEMPO in the presence of catalyst and KI in the presence of catalyst (Fe2Z) [57]. Degradation after KI, and TEMPO was observed to be equal to photolysis. This signifies that KI and TEMPO alone are not responsible for degradation of the bacteria. Inactivation using scavengers KI and TEMPO in the presence of catalysts showed a reduction in comparison to the inactivation in the presence of only catalyst. The reduction in the inactivation rate denotes the plausible radical responsible for inactivation of bacteria. Superoxide radical was observed to be major oxidation species for the killing mechanism of E. coli in the presence of Fe2Z. 3.6.4.2. Scavenger study with Cu2Fe2Z. Fig. 8(b) shows the degradation profile of E. coli in the presence of TEMPO and EDTA that are used as superoxide radicals and hole scavengers, respectively. Control studies were performed using only TEMPO and EDTA in the bacterial solution. Formation of AgI (Eg = 2.82 eV) and CuI (Eg = 2.73 eV) photocatalyst is the reason for not using KI as hydroxyl radical scavenger in presence of Cu2Fe2Z (impregnated catalysts). Formation of AgI and CuI results in further inactivation of E. coli and does not reveal any information about the responsible radical for inactivation. However, in Fig. 8(b), a significant reduction in the inactivation was observed when TEMPO was used in presence of catalyst (Cu2Fe2Z). The mechanism of charge transfer enhances the rate of generation of superoxide radical due to the presence of electrons at the conduction band of CuO and Fe2Z. This 8
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%
% −
Fig. 8. Degradation profile of E. coli under visible light with initial concentration of ≈ 2 × 107 CFU/mL (a) using KI ((OH ) scavenger) and TEMPO ((O2 ) scavenger) with catalyst Fe2Z (b) using EDTA ((h+ ) scavenger) and TEMPO
% − ((O2 )
scavenger) with catalyst Cu2Fe2Z. [2] J.C. Ireland, P. Klostermann, E.W. Rice, R.M. Clark, Inactivation of Escherichia coli by titanium dioxide photocatalytic oxidation, Appl. Environ. Microbiol. 59 (1993) 1668–1670. [3] S. Sontakke, J. Modak, G. Madras, Photocatalytic inactivation of Escherischia coli and Pichia pastoris with combustion synthesized titanium dioxide, Chem. Eng. J. 165 (2010) 225–233. [4] H.-L. Liu, T.C.-K. Yang, Photocatalytic inactivation of Escherichia coli and Lactobacillus helveticus by ZnO and TiO2 activated with ultraviolet light, Process Biochem. 39 (2003) 475–481. [5] S.G. Kumar, L.G. Devi, Review on modified TiO2 photocatalysis under UV/visible light: selected results and related mechanisms on interfacial charge carrier transfer dynamics, J. Phys. Chem. A 115 (2011) 13211–13241. [6] P. Kundu, P.A. Deshpande, G. Madras, N. Ravishankar, Nanoscale ZnO/CdS heterostructures with engineered interfaces for high photocatalytic activity under solar radiation, J. Mater. Chem. 21 (2011) 4209–4216. [7] A. Hernández, L. Maya, E. Sánchez-Mora, E.M. Sánchez, Sol-gel synthesis, characterization and photocatalytic activity of mixed oxide ZnO-Fe2O3, J. Sol-Gel Sci. Technol. 42 (2007) 71–78. [8] J. Jiang, X. Zhang, P. Sun, L. Zhang, ZnO/BiOI heterostructures: photoinduced charge-transfer property and enhanced visible-light photocatalytic activity, J. Phys. Chem. C 115 (2011) 20555–20564. [9] J. Liu, R. Yang, S. Li, Preparation and characterization of the TiO2-V2O5 photocatalyst with visible-light activity, Rare Met. 25 (2006) 636–642. [10] L. Jing, B. Xin, F. Yuan, L. Xue, B. Wang, H. Fu, Effects of surface oxygen vacancies on photophysical and photochemical processes of Zn-doped TiO2 nanoparticles and their relationships, J. Phys. Chem. B 110 (2006) 17860–17865. [11] M. Zhou, J. Yu, B. Cheng, Effects of Fe-doping on the photocatalytic activity of mesoporous TiO2 powders prepared by an ultrasonic method, J. Hazard. Mater. 137 (2006) 1838–1847. [12] M. Zhou, J. Yu, B. Cheng, H. Yu, Preparation and photocatalytic activity of Fedoped mesoporous titanium dioxide nanocrystalline photocatalysts, Mater. Chem. Phys. 93 (2005) 159–163. [13] T. Tong, J. Zhang, B. Tian, F. Chen, D. He, Preparation of Fe3+-doped TiO2 catalysts by controlled hydrolysis of titanium alkoxide and study on their photocatalytic activity for methyl orange degradation, J. Hazard. Mater. 155 (2008) 572–579. [14] J. Yu, Q. Xiang, M. Zhou, Preparation, characterization and visible-light-driven photocatalytic activity of Fe-doped titania nanorods and first-principles study for electronic structures, Appl. Catal. B: Environ. 90 (2009) 595–602. [15] X.-C. Liu, E.-W. Shi, Z.-Z. Chen, H.-W. Zhang, L.-X. Song, H. Wang, S.-D. Yao, Structural, optical and magnetic properties of Co-doped ZnO films, J. Cryst. Growth 296 (2006) 135–140. [16] E. Liu, P. Xiao, J.S. Chen, B.C. Lim, L. Li, Ni doped ZnO thin films for diluted magnetic semiconductor materials, Curr. Appl. Phys. 8 (2008) 408–411. [17] Y.Q. Wang, S.L. Yuan, L. Liu, P. Li, X.X. Lan, Z.M. Tian, J.H. He, S.Y. Yin, Ferromagnetism in Fe-doped ZnO bulk samples, J. Magn. Magn. Mater. 320 (2008) 1423–1426. [18] L. Shang, B. Li, W. Dong, B. Chen, C. Li, W. Tang, G. Wang, J. Wu, Y. Ying, Heteronanostructure of Ag particle on titanate nanowire membrane with enhanced photocatalytic properties and bactericidal activities, J. Hazard. Mater. 178 (2010) 1109–1114. [19] A. Adhikari, N.K. Banerjee, D. Eswar, G. Sarkar, Photocatalytic inactivation of E. coli by ZnO–Ag nanoparticles under solar radiation, RSC Adv. 5 (2015) 51067–51077. [20] N.K. Eswar, P.C. Ramamurthy, G. Madras, Enhanced sunlight photocatalytic activity of Ag3PO4 decorated novel combustion synthesis derived TiO2 nanobelts for dye and bacterial degradation, Photochem. Photobiol. Sci. 14 (2015) 1227–1237. [21] S.G. Kumar, K.K. Rao, Zinc oxide based photocatalysis: tailoring surface-bulk structure and related interfacial charge carrier dynamics for better environmental applications, RSC Adv. 5 (2015) 3306–3351. [22] P. Tan, Y.-H. Li, X.-Q. Liu, Y. Jiang, L.-B. Sun, Coreshell AgCl@SiO2n–anoparticles:
indicates that the major responsible species for inactivation of bacteria is superoxide radical. Degradation using EDTA in the presence of catalyst showed a decrease to some extent initially in comparison to catalyst alone but later the inactivation rate reduces. This indicates that initially direct hole attack leads to inactivate the bacteria but later superoxide radical formation dominates and inactivation occurs due to superoxide radical. The holes present at the valence band of Fe2Z moves to the valence band of CuO and lead to the interaction of holes directly to the bacterial surface and reduces the permeability of the membrane thus responsible for the inactivation of bacteria. EDTA and KI act as electron donor species that provides an ease in oxidation of impurity present [58]. TEMPO itself is a radical that forms a complex when it reacts with superoxide radical. This complex is important for the metabolism of bacteria that results in reduction of the rate of inactivation of bacteria. 4. Conclusions The combined effect of transition metal doping and metal coupling on Fe doped ZnO was examined. Fe doped ZnO and metal (Ag, Cu) impregnated Fe doped ZnO were synthesized successfully by sol-gel route and the metal precursor was reduced into metal in case of silver. Cu turned into CuO after the impregnation process. Extended absorption in visible range was achieved as a result of doping and metal impregnation. High charge separation i.e. reduced recombination was also achieved. This metal heterojunction also leads to surface bandbending in the bands of the semiconductor and results in high photoresponse of the photocatalyst. Plasmonic behaviour and charge carrier transportation due to metals (Ag) with semiconductor has resulted in the enhanced absorption of visible light of the material. The photocatalytic activity of the synthesized photocatalysts was examined for inactivation of E. coli under visible light. 3 wt.% Cu coupled Fe doped ZnO demonstrated higher photocatalytic activity than 1 wt.% Ag impregnated Fe doped ZnO. Acknowledgements The authors were thankful to Department of Science and Technology (DST), India for the financial support. The authors thank IPC, AFMM, MNCF (CeNSE), IISc for characterization facilities. Giridhar Madras thanks DST for the J.C. Bose fellowship. References [1] R. Cai, K. Hashimoto, K. Itoh, Y. Kubota, A. Fujishima, Photokilling of malignant cells with ultrafine TiO2powder, Bull. Chem. Soc. Jpn. 64 (1991) 1268–1273.
9
Catalysis Today xxx (xxxx) xxx–xxx
R. Gupta et al.
[23]
[24]
[25] [26] [27] [28]
[29]
[30]
[31]
[32] [33]
[34]
[35]
[36]
[37]
[38] [39] [40]
[41]
[42]
route of induced oxidative stress, Small 8 (2012) 3326–3337. [43] T. Pandiyarajan, R. Udayabhaskar, S. Vignesh, R.A. James, B. Karthikeyan, Synthesis and concentration dependent antibacterial activities of CuO nanoflakes, Mater. Sci. Eng.: C 33 (2013) 2020–2024. [44] V.S. Kumar, B.M. Nagaraja, V. Shashikala, A.H. Padmasri, S.S. Madhavendra, B.D. Raju, K.S.R. Rao, Highly efficient Ag/C catalyst prepared by electro-chemical deposition method in controlling microorganisms in water, J. Mol. Catal. A: Chem. 223 (2004) 313–319. [45] C.-N. Lok, C.-M. Ho, R. Chen, Q.-Y. He, W.-Y. Yu, H. Sun, P.K.-H. Tam, J.-F. Chiu, C.M. Che, Proteomic analysis of the mode of antibacterial action of silver nanoparticles, J. Proteome Res. 5 (2006) 916–924. [46] V.S. Kumar, B. Nagaraja, V. Shashikala, A. Padmasri, S.S. Madhavendra, B.D. Raju, K.R. Rao, Highly efficient Ag/C catalyst prepared by electro-chemical deposition method in controlling microorganisms in water, J. Mol. Catal. A: Chem. 223 (2004) 313–319. [47] W. He, H. Wu, W.G. Wamer, H.-K. Kim, J. Zheng, H. Jia, Z. Zheng, J.-J. Yin, Unraveling the enhanced photocatalytic activity and phototoxicity of ZnO/Metal hybrid nanostructures from generation of reactive oxygen species and charge carriers, ACS Appl. Mater. Interfaces 6 (2014) 15527–15535. [48] S. Ghosh, V. Goudar, K. Padmalekha, S. Bhat, S. Indi, H. Vasan, ZnO/Ag nanohybrid: synthesis, characterization, synergistic antibacterial activity and its mechanism, RSC Adv. 2 (2012) 930–940. [49] X. Zhou, G. Liu, J. Yu, W. Fan, Surface plasmon resonance-mediated photocatalysis by noble metal-based composites under visible light, J. Mater. Chem. 22 (2012) 21337–21354. [50] Y. Xu, M.A. Schoonen, The absolute energy positions of conduction and valence bands of selected semiconducting minerals, Am. Mineral. 85 (2000) 543–556. [51] S. Qin, F. Xin, Y. Liu, X. Yin, W. Ma, Photocatalytic reduction of CO2 in methanol to methyl formate over CuO–TiO2 composite catalysts, J. Colloid Interface Sci. 356 (2011) 257–261. [52] K. Sunada, Y. Kikuchi, K. Hashimoto, A. Fujishima, Bactericidal and detoxification effects of TiO2 thin film photocatalysts, Environ. Sci. Technol. 32 (1998) 726–728. [53] D. Wu, W. Wang, T.W. Ng, G. Huang, D. Xia, H.Y. Yip, H.K. Lee, G. Li, T. An, P.K. Wong, Visible-light-driven photocatalytic bacterial inactivation and the mechanism of zinc oxysulfide under LED light irradiation, J. Mater. Chem. A 4 (2016) 1052–1059. [54] X. Niea, G. Li, M. Gaoc, H. Suna, X. Liub, H. Zhao, P.K. Wongc, T. An, Comparative study on the photoelectrocatalytic inactivation of Escherichia coli K-12 and its mutant Escherichia coli BW25113 using TiO2 nanotubes as a photoanode, Appl. Catal. B: Environ. 147 (2014) 562–570. [55] D. Wu, B. Wang, W. Wang, T. An, G. Li, T.W. Ng, H.Y. Yip, C. Xiong, H.K. Leed, P.K. Wong, Visible-light-driven BiOBr nanosheets for highly facet-dependent photocatalytic inactivation of Escherichia coli, J. Mater. Chem. A 3 (2015) 15148–15155. [56] K. Nakaoka, J. Ueyama, K. Ogura, Photoelectrochemical Behavior of Electrodeposited CuO and Cu2O Thin Films on Conducting Substrates, J. Electrochem. Soc. 151 (10) (2004) C661–C665. [57] H. Gan, G. Zhang, H. Huang, Enhanced visible-light-driven photocatalytic inactivation of Escherichia coli by Bi2O2CO3/Bi3NbO7 composites, J. Hazard. Mater. 250–251 (2013) 131–137. [58] M. Yin, Z. Li, J. Kou, Z. Zou, Mechanism investigation of visible light-induced degradation in a heterogeneous TiO2/eosin Y/rhodamine B system, Environ. Sci. Technol. 43 (2009) 8361–8366.
Ag(I)-Based antibacterial materials with enhanced stability, ACS Sustain. Chem. Eng. 4 (2016) 3268–3275. S. Sun, X. Zhai, D. Gu, X. Chang, X. Hu, T. Liu, L. Dong, Y. Yin, Strongly-coupled silver chloride–tungsten oxide hybrid nanocomposite with excellent antibacterial effect, Adv. Powder Technol. 27 (2016) 1295–1300. O. Akhavan, Lasting antibacterial activities of Ag–TiO2/Ag/a-TiO2 nanocomposite thin film photocatalysts under solar light irradiation, J. Colloid Interface Sci. 336 (2009) 117–124. X. Zhang, Y.L. Chen, R.-S. Liu, D.P. Tsai, Plasmonic photocatalysis, Rep. Prog. Phys. 76 (2013) 046401. W. Fan, M.K. Leung, Recent development of plasmonic resonance-based photocatalysis and photovoltaics for solar utilization, Molecules 21 (2016) 180. Z. Zhang, J.T. Yates Jr., Band bending in semiconductors: chemical and physical consequences at surfaces and interfaces, Chem. Rev. 112 (2012) 5520–5551. A.A.H. Ba-Abbad, A.B. Kadhum, M.S. Mohamad, K. Takriff, Visible light photocatalytic activity of Fe3+-doped ZnO nanoparticle prepared via sol–gel technique, Chemosphere 91 (2013) 1604–1611. R. Gupta, N.K. Eswar, J.M. Modak, G. Madras, Visible light driven efficient N and Cu co-doped ZnO for photoinactivation of Escherichia coli, RSC Adv. 6 (2016) 85675–85687. R. Gupta, N.K. Eswar, J.M. Modak, G. Madras, Effect of morphology of zinc oxide in ZnO-CdS-Ag ternary nanocomposite towards photocatalytic inactivation of E. coli under UV and visible light, Chem. Eng. J. 307 (2017) 966–980. B. Choudhury, M. Dey, A. Choudhury, Shallow and deep trap emission and luminescence quenching of TiO2 nanoparticles on Cu doping, Appl. Nanosci. 4 (2014) 499–506. L. Xu, X. Li, Influence of Fe-doping on the structural and optical properties of ZnO thin films prepared by sol–gel method, J. Cryst. Growth 312 (2010) 851–855. P. Wang, B. Huang, Y. Dai, M.-H. Whangbo, Plasmonic photocatalysts: harvesting visible light with noble metal nanoparticles, Phys. Chem. Chem. Phys. 14 (2012) 9813–9825. N.K. Eswar, P.C. Ramamurthy, G. Madras, High photoconductive combustion synthesized TiO2 derived nanobelts for photocatalytic water purification under solar irradiation, New J. Chem. 39 (2015) 6040–6051. S. Karamat, R.S. Rawat, P. Lee, T.L. Tan, R.V. Ramanujan, Structural elemental, optical and magnetic study of Fe doped ZnO and impurity phase formation, Prog. Nat. Sci. Mater. Int. 24 (2014) 142–149. S. Kumar, R. Prakash, R.J. Choudhary, D.M. Phase, Structural, XPS and magnetic studies of pulsed laser deposited Fe doped Eu2O3 thin film, Mater. Res. Bull. 70 (2015) 392–396. M.C. Biesinger, L.W. Lau, A.R. Gerson, R.S.C. Smart, Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Sc, Ti, V, Cu and Zn, Appl. Surf. Sci. 257 (2010) 887–898. P.A. Thrower, Chemistry & Physics of Carbon, CRC Press, Florida, 1996. T. Ghodselahi, M.A. Vesaghi, A. Shafiekhani, A. Baghizadeh, M. Lameii, XPS study of the Cu@Cu2O core-shell nanoparticles, Appl. Surf. Sci. 255 (2008) 2730–2734. Y. Zhang, M.K. Ram, E.K. Stefanakos, D.Y. Goswami, Enhanced photocatalytic activity of iron doped zinc oxide nanowires for water decontamination, Surf. Coat. Technol. 217 (2013) 119–123. M. Shi, H.S. Kwon, Z. Peng, A. Elder, H. Yang, Effects of surface chemistry on the generation of reactive oxygen species by copper nanoparticles, ACS Nano 6 (2012) 2157–2164. G. Applerot, J. Lellouche, A. Lipovsky, Y. Nitzan, R. Lubart, A. Gedanken, E. Banin, Understanding the antibacterial mechanism of CuO nanoparticles: revealing the
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