Effect of Silver on Plasmonic, Photocatalytic, and Cytotoxicity of Gold

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Effect of Silver on Plasmonic, Photocatalytic and Cytotoxicity in AuAgZnO Bimetallic Nanocomposites Inakhunbi Chanu, Pattabiraman Krishnamurthi, and Periakaruppan Thangiah Manoharan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b02232 • Publication Date (Web): 06 Apr 2017 Downloaded from http://pubs.acs.org on April 10, 2017

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The Journal of Physical Chemistry

Effect of Ag on Plasmonic, Photocatalytic and Cytotoxicity of Au in AuAgZnO Nanocomposites Inakhunbi Chanub, Pattabiraman Krishnamurthia and Periakaruppan T. Manoharan*a a

Department of Chemistry, Indian Institute of Technology/Madras, Chennai 600036, India.

E-mail: [email protected] b

Centre for Material Science and Nanotechnology, Sikkim Manipal Institute of Technology,

Sikkim Manipal University, Sikkim, India

ABSTRACT

Four different nanocomposites of AuAgZnO with differing compositions of silver and gold were made by solution combustion method where the EDAX analysis gave the exact composition. Nanocomposites were formed as nanospheres. The composites contained bimetallic AuAg particles on the surface of ZnO semiconductor, the interface being silver. The deconvoluted UVDRS spectra revealed the presence of two plasmons due to Au and Ag. The amount of gold dominates the AuAg indicating the presence of only a thin layer of silver below gold causing a blue-shifted gold plasmon band and red-shifted silver plasmon band in comparison to their pure entity. The intensity of the former gets increased due to interparticle interaction with silver. Presence of excess gold in one sample of the starting materials tends to form amorphous Au2O3

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as proved by both EDAX and XPS. The number of oxygen vacancies in ZnO as seen in EPR depends on AuAg metal content effects the photocatalytic degradation of Rhodamine-B and the photocatalytic production of hydrogen. The interparticle interaction of Ag with Au and ZnO also plays a major role in photocatalytic process. An in vitro cytotoxicity study of AuAgZnO nanocomposites on MCF-7 cell lines is compared with the effect of gold alone in AuZnO. The current study revealed that the presence of silver in the bimetallic system suppresses the efficacy of Au and even ZnO. Gold by itself is a powerful cytotoxic nanoagent even with small amount of ZnO, the latter being known for its nontoxicity. It is interesting to note that Ag has a negative role in cytotoxicity but it contributes positively to photocatalysis.

1. Introduction ZnO is one of the most extensively investigated metal oxide semiconductor nanomaterials due to its low cost and non-toxic nature. ZnO is an n-type semiconductor having a band gap energy of ~3.37eV. It has been used in a wide range of applications such as sensors, optoelectronic devices etc.1-3 It also acts as an efficient photocatalyst for the degradation of organic pollutants and hydrogen evolution process.4-6 However, having a large band gap and high binding energy of 60 MeV, use of ZnO as a photocatalyst is restricted to UV region, which occupy only 5% of solar radiation. Therefore, it is important to enhance the efficiency of photocatalytic activity of ZnO. Many works have been reported on the modification of structure of semiconductor nanomaterial to enhance their optical properties and photocatalytic activity by doping with metal7-10 or by the formation of semiconductor nanocomposites such ZnO-CdS, TiO2-ZnO, CuO-ZnO.11-14 Based on the surface plasmon resonance (SPR) properties which arise from the collective oscillation of electrons on the surface of nanoparticles, doping with noble metals such as Ag, Au, Pt etc have


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received a considerable attention as they have the ability to absorb in the visible region. The augmented properties of the bimetallic NP’s over the monometallic ones have received a lot of attention in recent years. For example, the bimetallic NPs such as Au-Pt, Au-Ag, Au-Pd, etc., which normally exhibit two plasmons, have shown better catalytic activity than the monometallic ones.15-17 Depending on the nature of morphology and structural arrangement, the bimetallic NPs have shown unique optical properties.18 It is reported that bimetallic particles in the form of alloy NPs show a single SPR peak located between the expected two monometallic SPR peaks,19 whereas the core shell bimetallic NPs show two distinct SPR band arising from the two separate monometallic NPs20. Bimetallic NP’s can be tuned to create the size/shape of the core and thickness of shell. Hence it may be possible to study the relation between LSPR and intimate chemical structure. By modifying the semiconductor nanomaterial with metal or bimetallic NPs, the efficiency of their photocatalytic activity can be enhanced both in UV and visible region. Tsukamoto et al21 reported the double effects of Au-Ag bimetallic alloy nanoparticles loaded on TiO2. Enhanced H2O2 formation and suppressed decomposition of H2O2 are the double effects achieved using Au-Ag bimetallic alloy particles (AuAg/TiO2) for the photocatalytic production of H2O2 from ethanol/O2 system. The Ag-Au/TiO2 has shown better photocatalytic activity in photodegradation in the visible region than the monometallic doping.22 The Ag@Au@ZnO triple core-shell nanostructures and their photocatalytic activities in visible region have also been reported.23 In this study, degradation of methyl orange was greatly enhanced to about 93% from 53%, due to the SPR generated electrons from Au@Ag@ZnO than pure ZnO (8% degradation) and Au@ZnO (53% degradation). The authors attribute the efficiency of Au@Ag@ZnO due to better charge separation (electron-hole pair) compared to ZnO and Au@ZnO. Yan Li et al24 prepared Au-Ag alloy modified ZnO nanocomposite films by sol-gel and spin-coating method.


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They showed the there is a quasilinear relationship between the alloy precursor ratio to photocatalytic activity. They studied degradation of Rhodamine B using UV light irradiation for 2 hours and they achieved 79% degradation for the alloy Ag0.1Au0.2/ZnO which is higher than the monometallic system 57% - Ag0.3/ZnO and 54% - Au0.3/ZnO and 2 times higher than ZnO (34%). Synthesis of Au-Ag core-shell bimetallic nanoparticles were achieved sonochemically by S. Anandan et al.25 They used polyethylene glycol and ethylene glycol to co-reduce the noble metal ions to form the Au-Ag core-shell bimetallic nanoparticles. Lan chen et al26 studied the photodegradation of methyl orange using Au/Ag sensitized ZnO films. Their studies showed that the degradation efficiency of the AgZnO increased to 52.5% from 40% on the introduction of Au into the system (Au-Ag-ZnO system). The enhancement of photocurrent in Au-Ag-ZnO system is attributed to the higher separation efficiency of photoinduced electron-hole pairs.

Most

recently Mengtao Sun and his coworkers have shown the possibility of surface plasmons driven photocatalytic synthesis27-30 through hot-electron. A trilayered nanorods of Au/Ag/TiO2 were prepared and their photocatalytic activity was investigated for oxidation of 2-propanol by Horiguchi et al.31 They found that the Ag content and shell thickness of TiO2 play an important role in photo-oxidation of 2-propanol. TiO2 supported AuAg bimetallic composite materials32,33 have shown to greater photocatalytic degradation of Methylene blue than their individual counterparts. Hollow Au-Ag bimetallic nanoparticles prepared by galvanic replacement reaction exhibited higher catalytic reduction of p-nitrophenol to p-aminophenol.34 Plasmonic effects of Au-Ag bimetallic nanoparticles has largely been realised as a substrate for Surface Enhanced Raman Scattering (SERS) studies/sensors.35-36 However, the method of preparation such as coreduction of metal precursors,35 evaporation of metal films36 and multi-step synthetic procedure37 have yielded different composition and structures of Au-Ag bimetallic nanoparticles that has its


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own applications in SERS sensors, bio-sensors, bio-imaging, medical diagnostics., etc. Multispiked Au-Ag nanoparticles were used along with Si/PEDOT:PSS in solar cells as a plasmonic material to achieve an efficiency of 7.70%.38 However, the incorporation of bimetallic NPs into semiconductor needs to be explored further to bring out an in-depth understanding of their structural behaviour for efficient improvement in their optical and photocatalytic activities. Solution combustion synthesis (SCS)39-42 is a simple and straight forward synthetic route to synthesize nano-size materials. SCS is an exothermic self-propagating combustion synthesis of nano-materials with the aid of fuels such as glycine, polyethylene glycol, etc and solvents such as water, ethanol, hydrocarbons, etc., The synthesized nano materials by SCS methods have been applied as heterogenous catalyst, energetic materials, thin films, electro-optical and also as magnetic materials in various fields. Our group has earlier reported on the solution combustion synthesis of monometallic nanocomposites, AuZnO and AgZnO and their applications to photocatalysis, cytotoxicity and biocidal activity.43,44 The incorporation of Ag-Au into ZnO is greatly required to investigate their modified physicochemical properties and their applications. The size and morphology can be tuned using parameters such as concentration of metal precursors, semiconductor material and fuel ratio. The formation of AuAgZnO nanocomposites have been controlled just by changing a simple parameter like concentration, using a fixed fuel ratio on the basis of our earlier experience. The current findings brings to fore the role and importance of bimetallic systems in enhancing the photochemical activity in the visible region. The EPR of oxygen vacancy and conduction band due to ZnO has been very valuable in the understanding of the mechanisms of cytotoxicity and photocatalysis. A comparative study of in vitro cytotoxicity of these bimetallic nanoparticles along the lines of our earlier work on AuZnO43 reveals the negative role of silver.


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2. Experimental details 2.1 Materials and Methods 2.1.1 Materials All chemicals used in our experiments were of analytical grade and used without further purifications. Zn(NO3)2.6H2O (Qualigens), HAuCl4.3H2O, (laboratory reagent),methanol (SRL), Rhodamine B (RhB), polyethylene glycol (PEG) and AgNO3 from Merck, Sodium Sulphide (Rankem) and Sodium Sulphite (S.D. fine chemicals) were purchased and Millipore water was used for all analyses. 2.1.2 Synthesis of AuAgZnO nanocomposites Nanocrystalline pure ZnO was prepared by the solution combustion synthesis (SCS) as reported earlier by our group [27]. In brief, Zn(NO3)2 was used as the Zn source and Polyethylene glycol (PEG) as the fuel. It involves direct mixing of a desired molar ratio of Zn(NO3)2 and PEG. The ratio of Zn(NO3)2 to PEG was kept at 5. After mixing Zn(NO3)2 and PEG uniformly, the mixture was transferred to a muffle furnace preheated to 450◦C for ignition. The reaction lasted for less than ten minutes and produced dry ZnO nanocrystals. The synthesized material was further kept at the same temperature for another 20 min to complete the reaction. Different concentration of AuAgZnO nanocomposites were synthesized by the same procedure with HAuCl4 as the Au source and AgNO3 as Ag source. After completion of the reaction, the samples were washed with millipore water several times and then with methanol in order to remove the impurities. The synthesized materials were finally calcined at 500ºC for 30 min. The prepared samples have been designated as 1%Au4%AgZnO (S1), 2.5%Au2.5%AgZnO


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(S2), 4%Au1%AgZnO (S3), 5%Au5%AgZnO (S4) 5%AgZnO (S5) and 0.5%Au0.5%AgZnO (S6) respectively for the compositions Au 0.010 Ag 0.040 ZnO 0.950 (S1), Au 0.025 Ag 0.025 ZnO 0.950(S2), Au 0.040 Ag 0.010 ZnO 0.950 (S3), Au 0.050 Ag 0.050 ZnO 0.900 (S4), Ag 0.05 ZnO 0.950 (S5) and Au 0.005 Ag 0.005ZnO 0.990 (S6). Pure ZnO as prepared by the same method without the addition of other metals is termed as S7. 2.1.3 Instrumentation Structural characterization of samples were done using X-ray powder diffraction measurements recorded using X-ray diffractometer (Bruker D8 ADVANCE) using CuKα (l = 0.1548 nm) radiation to determine the constituent crystals and the metals present in the systems as identified by the JCPDS data taken from the literature. The morphology and composition of the samples were recorded using a field emission scanning electron microscope (FE-SEM, FEI quanta FEG 200 – high resolution scanning electron microscope) equipped with an energy dispersive X-ray analyser (EDAX). A high resolution transmission electron microscope (HRTEM, JEOL 3010) operated at 300 kV was used for determining the crystallite size and lattice fringes of the samples. Diffuse Reflectance Spectroscopy (DRS) UV-visible spectra of all samples were performed on a Jasco 600 UV-vis spectrophotometer using BaSO4 as a reference to determine the band gap and plasmon frequencies. UV-visible absorption spectra of all the samples for photodegradation measurements were performed with the use of Shimadzu 1800 UV-Vis spectrophotometer. Balzers Thermostar and Newport Light source was utilized for measuring hydrogen ion current from the AuAgZnO samples. The experimental samples were exposed to UV-Visible light at 195 watts for 1 hour, 1.30 hours and 2 hours to measure the respective hydrogen ion current.


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2.2 Experimental Section 2.2.1 Photocatalytic Degradation of Rhodamine B For the photo catalytic studies, RhB dye was used as a representative organic pollutant to determine the photocatalytic activity of AuAgZnO nanocomposite samples. A 100 ml pyrex glass reactor surrounded by a circulating water jacket was used as a photo reactor and a visible light source was provided by 150 watt metal halide lamp of Philips make. In a typical procedure, a 50 ml solution containing 5 µM RhB and 0.05 g photocatalyst were taken in a reactor and stirred for 30 min. The reaction mixture was kept in dark for 1 h to attain equilibrium. Then the reaction mixture was exposed to visible light source. After regular interval, a 5 ml of the solution was taken and the photo catalysts were removed by centrifugation. The supernatant solution containing RhB concentration was monitored by UV-visible absorption spectroscopy at 554 nm with deionized water as the reference medium. The degradation efficiency of the photo catalysts was calculated using the following formula:

=

× - - - -

(1)

where C0 and Ct are the initial and time dependent concentrations of RhB. 2.2.2 Hydrogen Evolution Study of AuAgZnO nanocomposite Hydrogen evolution studies were conducted only for S1 and S3 samples. In a 30ml pyrex glass reactor about 30mg of the samples was dispersed in 15ml of 0.35M sodium Sulphide and 15ml of 0.25M Sodium Sulphite solutions. Then, the samples were ultrasonicated for 15 to 20 minutes. The samples were exposed to UV-Visible light (Newport Source Xe-Hg lamp) at 195 Watts for 1, 1 1/2 and 2 hours under magnetically stirring conditions with water circulation. The hydrogen


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gas evolved from the system was measured as hydrogen ion current using Balzers Thermostar. For the purpose of standardization 1000 µL of Hydrogen gas (99.9% pure) was injected separately into the system to measure the hydrogen ion current. This was taken as the standard value. From the measured hydrogen ion current values of the experimental samples and the hydrogen gas, hydrogen gas evolved from the samples was calculated in units of µmol/g for the respective time of exposure. 2.2.3 In vitro assay for cytotoxic behaviour (MTT assay). The MTT assay was done in a manner similar to our earlier work43 with some small modifications. The MTT solution was prepared by dissolving the MTT in Dulbecco’s Phosphate Buffered Saline, pH=7.4 (DPBS) to 5 mg/ml. Then the MTT solution was filtered using a 0.2 µm filter paper followed by sterilization. It was saved in a light protected container at 4°C for frequent use or at -20°C for long term storage. The solubilization solution was prepared in a solvent resistant container using 40% (V/V) Dimethylformamide and 20% (V/V) Glacial acetic acid. To this 16% (V/V) Sodium Dodecyl Sulfate was added, dissolved and the pH was adjusted to 4.7. Then the solution was stored at room temperature. The prepared cells along with the test compounds S1, S2, S3 and S5 were placed in 96 well plates containing a final volume of 100 µl/well. The same was incubated for 48 hours. 10 µl of MTT solution was added to each cell to achieve a final concentration of 0.45 mg/ml. Then they are incubated for 1 to 4 hours at 37ºC. Then 10 µl of solubilisation solution was added to each well to dissolve the formazan crystals and the resultant solution was recorded for absorbance at 570nmIn vitro assay for cytotoxic behaviour (MTT assay). 3. Results and Discussion


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3.1 Structure, Morphology and evidence for the formation of AuAgZnO nanocomposites Fig 1 shows the XRD pattern of pure ZnO, and AuAgZnO nanocomposites. For pure ZnO, all the diffraction patterns are assigned to the hexagonal wurtzite structure of ZnO(♦) according to JCPDS card no. 36-1451. For AuAgZnO nanocomposites, the separate XRD patterns of Au and Ag NPs cannot be distinguished due to their close lattice constants (lattice constants are Au: 4.0786 Å and Ag: 4.0862 Å). Accordingly, the peaks observed at 2θ = 38.2, 44.3, 64.7 and 77.4 are assigned

to both Au and Ag (♣ ) with miller indices (111), (200), (220) and (311)

respectively, for face centered cubic (fcc) structure of Au and Ag according to JCPDS Card No. 65-2870 and JCPDS Card 65-2871 respectively. There are three minor peaks at 27.8, 32.2 and 46.4 due to AgCl (♥) in the samples of S2,S3 and S4, corresponding to the JCPDS card No.851355.


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Figure 1 X-ray diffraction patterns of pure ZnO and Au@AgZnO nanocomposites; Note: ♦ ZnO; ♣ - Au & Ag; ♥ - AgCl. The average crystallite sizes in the ZnO and AuAgZnO nanocomposite samples were calculated using the Debye-Scherrer equation.

D

=

kλ β Cosθ

------- (2)

where k is the shape factor, λ is the incident X-ray wavelength of Cu Kα radiation (1.5408 Ǻ), θ is the Bragg diffraction angle, and β is the full width at half maximum (FWHM) of the (101) plane. The particle sizes of are calculated to be 21 nm, 23 nm, 24 nm, 39 nm, 45 nm and 45 nm for S7, S5, S1, S3, S2 and S4 respectively, suggesting the increase in particle size with increment in Au and Ag concentration. The relative ratio of Au and Ag (2θ = 38.2) to ZnO (2θ = 36.2) from the XRD data are calculated to be 0.182, 0.199, 0.245, 0.627 and 1.352 for S1, S3, S5, S2 and S4. However, an analysis of the atomic percentage from the EDAX data (vide infra) the values for Au, Ag, Zn and O for four composite materials are given in Table 1. Hence the relative ratio of ‘Au-Ag’ to ZnO from SEM/EDAX data turns out to be 0.031%, 0.029%, 0.053% and 0.393% are respectively for the experimental samples S1, S2, S3, and S4 indicating the true compositions in the prepared composite nanoparticles. The data from EDAX must be considered as real composition for further experimental interpretations. It is obvious from Table 1 that the percentage of Ag present in the above said four samples are approximately similar but low values indicate that it forms AuAg particles on ZnO (for more proof vide infra) for the nanocomposites of AuAgZnO. Also it indicates that most of the Au@Ag with varying amounts of Au but with a much smaller amount of Ag is an indication of a thin layer of Ag below Au,


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which forms on the surface of abundant ZnO. A similar observation of Au or Ag cluster on ZnO surface was earlier reported by our group.43,44

Table 1 Actual Atomic Weight Percentage as determined from SEM/EDAX analysis S.no

Sample (Experimental composition)

Au (At. Wt. %)

Ag (At. Wt. %)

Zn (At. Wt. %)

O (At. Wt. %)

Au/Ag

Ag/Au

1

S1 (1%Au4%AgZnO)

0.42

0.44

58.38

40.76

0.95

1.04

2

S2 (2.5%Au2.5%AgZnO)

1.70

0.11

62.61

35.38

15.5

0.07

3

S3 (4%Au1%AgZnO)

2.80

0.26

57.99

38.95

10.8

0.09

4

S4 (5%Au5%AgZnO)

12.76

0.15

32.86

54.22

85.0

0.01

Yet another interesting property from the data in Table 1 is that the atomic percentage of oxygen is less than expected to match with Zinc, suggesting the presence of a sizeable number of oxygen vacancies (vide infra EPR) except for the sample S4 where we suggest the presence of amorphous Au2O3 which will not make its appearance in XRD. This suggestion draws support from the fact that the gold content of the latter has a relatively higher atomic percentage compared to that of all other samples as shown in Table 1.


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Figure 2 SEM image of AuAgZnO nanocomposites The surface morphology of the nanocomposites has been analysed by SEM images (Fig. 2). It is clearly seen from the SEM image that they exhibit nanospherical structure. As the percentage of Au and Ag increases, the particles are accumulated forming a honey comb like structure (Fig. 2 S4). The EDAX analysis of the nanocomposites, S1, S2, S3 and S4 are shown in Fig.S1 (ESI).


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Figure 3 TEM image of sample S1 (a) and (b), the latter with lattice fringes. The TEM image of sample S1 (a) and its lattice fringes (b) in the region marked with nanocomposite particles are shown in Fig.3. Heavier atoms in a sample will scatter more electrons and are expected to have a smaller mean free path than a lighter atom in forming an image contrast in TEM analysis. Basically it depends upon the nature of the material and also the sample thickness. Comparing Ag and Au, the latter being a heavier atom, will lead to darker images. Hence the darker and lighter image part contrast is due to the difference in atomic density of Au (darker) and Ag (lighter).The sample S1 exhibits two different shades of contrast in the TEM (Fig.3) as marked with yellow circles. The circle 1 with bottom lighter shade represents ZnO and the upper grey part may be due to only silver i.e. forming AgZnO or a poor concentration of Au on the top of Ag i.e. AuAgZnO nanocomposite while yellow circle 2 clearly represents AuAgZnO nanocomposites as further evidenced from the UV-DRS results (vide infra). The presence of lattice fringes shown in Fig S2 (b,c,d) (ESI) clearly proves the material to be crystalline. The HRTEM of S4 shows d spacing of 2.48 Å corresponding to ZnO (101) plane and 2.0 Å for Au/Ag (200) plane.


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3.2 Optical Property: Double plasmon formation and interparticle interactions in AuAgZnO nanoparticles with enhanced Au plasmon band intensity To determine the plasmonic nature of AuAgZnO nanocomposite, the UV-vis diffuse reflectance spectra (DRS) of pure and AuAgZnO samples were recorded which is shown in fig. 4. The optical properties of metal NPs are determined by the nature of SPR bands. It is known that the pure Au NPs shows SPR band maxima around 520 nm and pure Ag NPs at 420nm.45 However, there is a red shift in plasmon bands when they are on the surface of ZnO to 550nm for Au/ZnO43 and 522.3nm for Ag/ZnO44.

Figure 4 (a) DRS UV-Vis spectra of pure ZnO, Ag@ZnO and nanocomposites; (b) DRS UVVis spectrum sample S4 nanocomposites showing clarity of the SP band. It can be seen from fig 4a that there is a broad absorbance in visible region due to the SPR band of Au and Ag in ZnO, whereas there are no changes in the band gap energy of ZnO in the presence of AuAg except for its continuous decrease in intensity on change of metals though the


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total experimental metal content remains the same (exception S4, viz. 5%Au5%AgZnO in Fig 4b). It must be noted that Fig.4 (b) shows the presence of the conduction band due to ZnO and a two band system marked at 474nm and 526 nm due to the SPR’s of Ag and Au respectively for S4. For all other samples, the Ag-Au region is broadened with a clear band due to Au and a shoulder due to Ag plasmons, making it necessary to carry out careful deconvolution. Depending on the mass ratios of the constituent metals, say, Ag and Au the plasmon bands will undergo shifts.46 In the currently studied NP’s, Au does not see any shift while Ag undergoes a large shift as proved by simulation (Fig.5). One should compare the present results with the earlier observations of a broad and strong SPR band for 5%AgZnO44 and a broad but much weaker SPR band in 5%AuZnO43. In the present work a narrower SPR band with a maxima at 560 nm along with a weak shoulder at lower wavelength is observed for S1 (see fig.4a). While a clear two plasmonic band system is found in S4 (see fig.4b), the SPR bands in other samples gets broadened on increasing the % of Au and Ag. It has been reported that nature of metal alloy NPs is determined by the presence of single SPR band and the bimetallic NPs by the presence of two maxima in the SPR band.47 We did not observe a single SPR band revealing the lack of alloy formation. However, in order to correctly estimate the plasmonic band positions, their relative intensities and their origin, we subtracted the ZnO component of each spectral feature and deconvoluted the

plasmonic bands after

conversion of wavelength spectra into energy (eV).


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Figure 5 Deconvoluted Plasmonic Bands in four experimental Samples. Deconvoluted Silver plasmon (blue) and gold plasmon (Green); experimental (black) and fitted (red) The most important observations are: (i) There is no single SPR band due to any possible alloy formation; (ii) All experimental samples exhibited two bands, with shoulders on the intense bands in some cases due to Au (lower energy) and Ag (higher energy); (iii) From table 2, it can be said that the Au plasmonic band position in the experimental composites is almost consistently similar (2.25 ±0.05 eV ≈ 551 nm) as in the case of 5% AuZnO (2.25 eV) and is red shifted with respect to pure gold in the absence of ZnO.29 This clearly indicates that there is no Au-ZnO interparticle interaction in all the experimental samples.


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(iv) A look at the plasmonic band position of Ag shows a change with varying amounts of Ag about 0.4-0.5 eV higher than the original 5% AgZnO as reported earlier in our work44 and the bands in the present work is blue shifted as observed in this case clearly indicating that Ag interacts with ZnO by transferring part of its electrons into the Fermi levels of ZnO.43,48 (v) The band edge intensity of ZnO continuously decreases with increase in silver content (see Fig.4a) revealing increased interparticle interaction between Ag and ZnO as well as the formation of AuAg particles on the surface of ZnO. (Vi) As the experimental Au to Ag ratio increases in the nanocomposites (S1 & S3), the absorption peak broadens and shifts towards blue side. This trend was reflected remarkably in photocatalytic activity (vide infra)

Table 2 Results from deconvolution of the experimental Plasmonic UV-DRS

Au Sample

Ag

Plasmonic Band peak (eV)

FWHM (eV)

Relative Intensity

Plasmonic Band peak (eV)

FWHM (eV)

Relative Intensity

5% (AuZnO) and 5% (AgZnO)

2.25*

--

100

2.37

--

100

S1 (1%Au4%AgZnO)

2.25

0.61

87.27

2.77

0.46

12.73

S2 (2.5%Au2.5%AgZnO)

2.25

0.62

48.8

2.91

0.85

51.2

S3 (4%Au1%AgZnO)

2.23

0.61

36.92

2.88

1.02

63.08

S4 (5%Au5%AgZnO)

2.30

0.61

39.12

2.88

0.94

60.88

The observation of consistently similar plasmonic band energy for Au in all AuAgZnO samples at 2.25±0.05 eV is a clear indication of Au being on the top while Ag with considerably upward changes in plasmon energy due to its interparticle interaction with Au and ZnO is sandwiched


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between Au and ZnO. This is in accordance with generally reported observation that the SPR electron and energy transfer from metal surface to semiconductor material will modify the optical properties.43,44,48-51 Also noted earlier, the actual amount of Ag in all experimental samples (see Table 1) being much smaller leads to its poor intensity in XPS (vide infra). Another important observation is the dramatically enhanced intensity of gold plasmon band in the deconvoluted UV-DRS spectra of all our nanocomposites (Fig.5) when compared to that of a very weak and broad band in 5%AuZnO.43 This observation could be due to the presence of Ag on top of Au since a similar observation of increase in SPR intensity of Au has been made by Tokonami et al29 on the addition of AgClO4 in an aqueous dispersion of Au nanoparticles. However, under the hypothesis of core-shell structure52, the formation of Au core surrounded by a silver shell is more favoured because of the larger surface tension of Au (1138 mN/m) than that of silver (960mN/m)53,54 and hence the observed enhanced intensity of Au55,56. The formation of core shell structure being Au in core and Ag occupying shell can also be explained according to the reduction rate of the individual metal ions. However, our present study reveals the formation of nanocomposites with no evidence for core-shell formation.


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Figure 6 XPS Spectra of Au 4f – Zn 3p and Ag 3d regions of the experimental sample S4 (5%Au5%AgZnO). A point of much importance must be said about the last sample of 5%Au5%AgZnO. A very high value for the actual amount of gold from EDAX (Table 1) at 12.76 atomic percentage coupled with large excess of oxygen suggests the presence of amorphous Au2O3. The presence of this Au2O3 is confirmed by XPS which shows the presence of Zn2+, Au0 and Au3+ from the deconvolution of XPS spectra of Au 4f–Zn 3p shown in Fig. 6 (left). It was resolved into six peaks at 83.93, 87.81, 85.61, 89.73, 88.92 and 91.75 eV respectively, corresponding to Au0 4f7/2, Au0 4f5/2, Au3+ 4f7/2, Au3+ 4f5/, Zn 3p3/2 and Zn p1/2.57,58 Similarly we observed the presence of silver through their XPS peaks at 368.2 and 374.2 eV respectively for Ag 3d5/2 and 3d3/2 shown again in Fig. 6 (right) in agreement with Xianga et al.59 All these prove that the experimental sample S4 has amorphous Au2O3 causing the increase in the atomic percentage of Au as in EDAX measurement. Moreover, the amount of Au2O3 is not


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only small but it is also amorphous which cannot be detected by XRD but could be detected by XPS; however, the amount of Ag is small and present as a very thin layer and possibly not uniformly packed on the abundant part of Au which would allow the penetration of X-ray to enable the detection of rather noisy XPS signals for Ag in its zero oxidation state. The XPS survey spectrum of S4 sample is shown in Fig.S3 (ESI). 3.3 Oxygen Vacancies detected in AuAgZnO nanocomposites by EPR spectroscopy

Figure 7 EPR spectra of experimental samples showing peaks corresponding to oxygen vacancy at high field region and conduction band electrons at low field region.


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The presence of oxygen vacancies in ZnO containing composites is quite common as we have observed in some of our earlier work43,44 as well as many other uncited references. However, EPR spectroscopy detects the oxygen vacancies as well as ZnO conduction band electrons (or any other shallow donor) in these materials at g values 1.95 and 1.999 respectively. There are two elegant works confirming g=1.95 as that of VO+.60,61 In the first work, the oxygen vacancies are found to disappear after the material was heated in O2 atmosphere and in the latter work the native oxygen vacancies have been found to be filled up by heat treatment in presence of oxygen. The increase in actual silver content in atomic percentage (Table 1) as measured by EDAX analysis: 0.44 (S1) > 0.26 (S3) > 0.15 (S4) > 0.11 ( S2) seems to match with the increased EPR intensities of g values 1.95 (corresponding to VO+) of the samples, 1310 > 943.2 > 935.5 > 869 , suggesting that silver in contact with ZnO in the AuAgZnO influences the formation of oxygen vacancies. In fact, the same sample with the highest VO+ i.e. S1 gives the best photocatalytic effect as shown below in the section 3.4 (vide infra). There is also a small but broad EPR line at a g-value slightly higher than that for the conduction electrons and it is due to both Ag and Au since their electronic structure is very similar in the d10s1 configuration. The intensities of the broad line increase in the ratio of the actual Ag + Au content as shown in Table 1. , i.e 0.86, 1.81, 3.06 and 12.91 for the four spectra depicted in Fig. 7. 3.4 Photocatalytic studies 3.4.1 Effective Photocatalytic degradation of RhB in Visible light region To determine the photocatalytic activity of AuAgZnO nanocomposite, the photodegradation of RhB dye in aqueous media has been investigated using visible light source. As referred earlier in the introduction many researchers have exploited this aspect by looking at the degradation of methyl orange23, 26, RhB 624,36, some alcohols62,63, reduction of Ag+ and photocatalytic evolution


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of hydrogen4-6. We have directed our attention on degradation of RhB by the use of 5 samples, S1 to S5 where S5 is pure Ag@ZnO while S1-S4 are AuAgZnO with differing concentration of Au and Ag in which the latter plays an important role because of its direct contact with ZnO. Fig. 8a shows the UV-Visible absorption spectra for the degradation of the dye as a function of irradiation time with S1 sample. The two points that deserve attention are: (i) the radiation used comes from the most intense region of solar radiation and (ii) this radiation is responsible for the generation of plasmons from Ag and Au and is not connected to the bandgap region of ZnO. The absorption peak height of the dye at 554 nm is found to decrease on increasing the irradiation time. A look at Fig. 8b reveals that about 97% and 95% of RhB is degraded in 180 min of irradiation for S1 and S5, and only 48%, 46% and 40% of RhB degradation are respectively observed for S4, S2 and S3 (the latter three may be bracketed together). This is in line with the fact that Ag in AuAg plays a major role on their photocatalytic activity and also depends on the actual amount of Ag as in Table 1. An interesting property of the nanocomposite S1 is also to be noted here that from Fig 4a, the absorption peak of sample S1 is sharper in comparison to other nanocomposites and found to be lying between 540 to 560nm which is also the wavelength region where RhB degrades. This fact confirms why sample S1 is more effective in degrading RhB than other nanocomposites. Our group has earlier reported the efficient photocatalytic activity of AgZnO in visible region.44 Another factor is the amount of Ag in all these samples, which decreases in the order S1 ~ S5 >> S2 >S4>S3. Hence Ag in contact with ZnO plays the major role in the degradation property of Rh B and the role of Au though present in decreasing order S4 >S3>S2>S1 >S5(nil) does play a minimum role. Hence we propose Ag plasmons formed during irradiation releases electrons which are then transported to Au which in turn degrades RhB after trapping oxygen to form superoxide.

However, presence of Au in little


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amount in sample S1 seems to confine the plasmon band towards red side and enhances the photocatalytic activity more than S5. Au has a limited role of getting itself negatively charged, which derives the support from the results of Tsukuda et al.62-64 However, it is also possible that part of the electrons from irradiated silver can get transferred to ZnO because of their interfacial interaction, which turn may initiate the superoxide formation and subsequent degradation of RhB. Our arguments derive support from the earlier works of Tsukuda et al.62-64 The specific schematic diagram of RhB degradation using AuAgZnO nanocomposite sample is illustrated in Scheme 1 while the mechanism is summarized in Scheme 2.

Figure 8 (a) Photocatalytic degradation of RhB in the presence of S1 and (b) percentage of degradation.


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Scheme 1 Schematic diagram for photocatalytic mechanism of AuAgZnO nanocomposite on degradation of RhB upon visible light irradiation.


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Scheme 2 Schematic representation of photocatalytic mechanism of degradation of dyes using AuAgZnO nanocomposite upon visible light irradiation. 3.4.2 Hydrogen Evolution Studies Table 3 Hydrogen Evolution values of 1%Au4%AgZnO (S1) and 4%Au1%AgZnO (S3) when exposed to UVVisible light at 195 Watts using Xe-Hg lamp. Experimental samples 5%AgZnO

1h (µmol/g) 90.89

1 h 30 min (µmol/g) 847.05

2 hrs (µmol/g) 2043.95

1%Au4%AgZnO (S1) 4%Au1%AgZnO (S3)

109.94

502.57

997.94

59.31

306.08

817.32

Table 3 shows the values for hydrogen production in the decreasing order, S5 >S1 > S3. It may be noted that S5 does not have any gold at all while Table 2 shows that Ag and Au are present in equal amounts in S1 while in S3, the amount of Ag present is much less than that of Au which is dominantly present. The sample of pure 5% AgZnO (S5) with no Au performs much better in hydrogen evolution reaction than either of the bimetallic nanoparticles. A reason for the better performance of S5 seems to be due to the involvement of ZnO band gap excitation by UVVisible radiation. Moreover, the published literature60,65 indicates that oxygen vacancies do contribute to enhanced photocatalytic activity. This supports the fact that the relative intensities of EPR lines due to oxygen vacancies in comparison to those of conduction band decreases in the order of S5>>S1>S3 noting all these excitations involve more of ZnO and Ag since the radiation used is UV-Visible rather than the visible radiation used for the degradation of Rh B supporting only the plasmonic excitation of Ag/Au. On UV-Visible light activation of AuAgZnO, the


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following two mechanisms are suggested drawing support from the work of Balaji et al60 and Zhang et al65. (1)The photo-generated electrons from the valence band ZnO will move towards the conduction band of ZnO leaving a photo-generated hole. The photo-generated holes are trapped by the surface oxygen vacancies thereby reducing the electron-hole pair recombination. Surface oxygen vacancies trapped hole sites act as active catalyst site for electron donors. It is to be noted that decreasing silver content from Table 2 i.e. (S5) > 0.44 (S1) > 0.26 (S3) seems to match with the increased EPR intensities of g values 1.95 (corresponding to VO+) of the samples, large (S5) >1310(S1) > 935.5(S3), and EPR intensity ratio I(VO+) /I(CB) decreases in the order S5>S1>S3 suggesting that silver in contact with ZnO in the AuAgZnO influences the formation of oxygen vacancies which in turn promotes hydrogen production. The sample with the highest silver content with no formation of conduction band and only Vo+ at g=1.959 i.e. S5 gives the best photocatalytic effect and among the two Au containing AuAgZnO systems (S1 and S3), S1 with more silver is better than S3 with less silver. To reemphasize, AgZnO (S5) contains only silver having total contact with ZnO and hence has the highest hydrogen generating capability through photocatalysis. Another important rationale comes from the particle sizes of the three samples S5(19.7nm)< S1(24nm)< S3(39nm) contributing to the understanding of differential photocatalytic activity. (2) The SPR generated electrons in Au and Ag get transferred to the conduction band of ZnO. The electrons rapidly combine with the holes in the valence band leading to a band gap emission. This band emission also enhances the photocatalytic activity.


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The above two mechanisms act simultaneously to give enhanced photocatalytic water splitting. Furthermore, another surprising observation was the increase in the rate of hydrogen production at increased time of exposure; this probably suggests that the agglomerated catalyst breaks down into smaller particles increasing the surface area for better catalytic activity. It is an accepted fact that the actual values for hydrogen production in this work are relatively smaller than what has been reported earlier66 for other systems in table 2 of the latter. Hence it is possible to improve the rate of formation of hydrogen through finding methods of creating particles of smaller size with the increased total surface area. We have therefore planned to modify the present catalyst systems to improve their photocatalytic performance in future. Scheme 3: Schematic diagram for photocatalytic mechanism of Au@AgZnO nanocomposite for hydrogen generation upon UV-Visible light irradiation.


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Scheme 4: Schematic representation of photocatalytic mechanism of Hydrogen generation using AuAgZnO nanocomposite upon UV-Visible light irradiation

3.4.3 In vitro cytotoxic studies by the bimetallic nanoparticles/clusters reveals a negative effect of silver on the efficacy of Au.

Figure 9 MCF – 7 cell lines % Viability due to anticancer activity of AuAgZnO nanoagents.


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Figure 10 A comparative account of anticancer activity of AuZnO, AgZnO and AuAgZnO nanoagents on MCF – 7 cell lines In our earlier work43 we made some significant observations on the in vitro cytotoxicity of the gold nanoparticles/clusters on the surface of zinc oxide nanocrystals, i.e., AuZnO. The nontoxic property of gold nanoparticles makes them powerful agents both for cancer detection and cancer treatment. In our earlier work we highlighted the anticancer activity of gold nanoparticles in ZnO in terms of cell viability towards the MCF-7cell lines. However, in the current report we have compared the anticancer activity of AuZnO NPs with that of the bimetallic nanoparticles, with one constituent being Au and the other being silver Ag, i.e., AuAgZnO. It may be noted that AgZnO, AuZnO and AuAgZnO were made with the same Zn/PEG ratio of 5. Another point to note is that ZnO not only does not affect the normal cells67 but also has limited anticancer properties with poor cell viability67-70. Given that the fuel ratio is common to all the above-said systems, the parameters that will play a role in destruction of the cells would be: (i) the percentage of gold vs silver, (ii) the quantity of anticancer agents, (iii) particle size and the charge on the NP interacting site. Table 4 gives the results of the anticancer activities of AuAgZnO nanoagents on the MCF-7 cell lines. The results of AuAgZnO of four samples


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including AgZnO without any gold studied are given in Fig. 9 and comparative account with earlier work on AuZnO is given Fig. 10 and Table 4, one can see that silver presence basically suppresses anticancer activity. This is very clear from a comparison of cell viability of ZnO and S5 NPs. In fact, pure ZnO with its smallest particle size has a lower % of cell viability, meaning better cell-destruction efficacy, than that for 5%AgZnO despite the fact that the latter also has ZnO. The mechanism of action of AuAgZnO nanocomposite on the MCF cell lines is exactly similar to that described in our earlier paper using AuZnO.43 At this instant it is necessary to point out that the toxicity of nanocomposite towards cancer cells follows the order 71,72 positively charged NPs > negatively charged NPs > neutral NPs. We infer that the presence of silver in these nanoparticles seems to work against cell destruction, probably because AgZnO unit being more or less neutral is unable to gain entry into the cells. The neutral nature of AgZnO is made possible by a tight binding of Ag with ZnO. A mechanism can be suggested to explain the experimental cytotoxic behaviour of bimetallic particle in a manner similar to the proposal of Song et al 73 for glucose capped Au nanoparticles as shown in Scheme 5. Here endosome takes up the gold particles and undergoes enzymolysis, leading to mitochondrial destruction. While it is much easier for Au in AuZnO to directly enter the cell it is more difficult for the bimetallic nanoparticle with silver as in AuAgZnO , i.e. silver plays a negative role by not easily allowing Au to directly enter the cell.


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Scheme 5 Schematic mechanism of MCF-7 cell line destruction by AuAgZnO as adapted from Song et al 73.

Table 4 % Cell Viability of AuAgZnO nanocomposites Conc. (µg/ml)

% of cell viability 10µg 25µg 50µg 100µg 47.9±5.7 41.9±1.1 36.3±1.9 1%AuZnO N/A 59.7±0.6 42.8±1.4 29.9±1.5 5%AuZnO N/A 79.7±5.3 65.9±1.0 59.6±3.2 40.18±2.7 S6 77.9±5.6 56.8±1.4 50.9±2.1 41.89±1.7 S2 N/A 68.7±1.7 60.8±3.6 51.9±0.1 S7 80.9±2.2 76.1±1.3 72.8±0.7 63.35±1.7 S5 * - Interpolated data for S2, S5 & S6

200µg* 12.3±2.9 18.2±1.9 29.3±0.8 36.3±0.9 46.4±1.3 54.9±3.1

Furthermore, the systems such as 5%AuZnO and 1%AuZnO as in our earlier work 43 shows that a lower concentration of gold in zinc oxide is preferred for cell destruction, probably due to the presence of a thinner film in the latter than in former. Similarly, the current experiments in


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bimetallic AuAgZnO nanocomposites also reveal a better efficacy of lower gold content as well as reduced activity of the same in presence of Ag. 4. Conclusions In conclusion, AuAgZnO nanocomposites were synthesized by solution combustion method by varying the percentage of Au and Ag doping on ZnO and also keeping the Zn(NO3)2/PEG = 5 fixed. The deconvolution procedure used on UV-DRS spectra reveals that the intensity of plasmon band due to Au gets considerably enhanced when in contact with Ag. About 97% of Rh B was degraded under visible light irradiation by 1%Au4%AgZnO. Similarly hydrogen evolution experiments suggest the need for further improvement by modifying the catalyst nanoparticles though the importance of Ag along with the non-importance of Au is indicated. Oxygen vacancies, related to greater silver presence, play a very important role. The interparticle interaction of Ag with Au and ZnO play a major role in photocatalytic process. On the other hand for cancer cell destruction the role of Au is positive while that of Ag is negative. ASSOCIATED CONTENT Supporting Information Supporting Information contains Fig S1 (EDAX) and Fig S2 (TEM) images of S1, S2, S3 and S4 samples; Fig S3 XPS survey Spectrum of sample S4 The following files are available free of charge. AUTHOR INFORMATION Corresponding Author Periakaruppan T. Manoharan


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Present Addresses † aDepartment of Chemistry, Indian Institute of Technology/Madras, Chennai 600036, India. E-mail: [email protected] ACKNOWLEDGMENT T.I Chanu acknowledges Department of Science & Technology, Government of India for financial support vide reference no SR/WOS-A/CS-120/2013 under Women Scientist Scheme and Dr S. Chatterjee, CMSNT, SMIT for his kind support. PTM acknowledges the SERB for funding the projects SR/S1/IC-53/2012 as well as EMR/2016/000745, and INSA (project no. SP/HIS/2016) for the award of Honorary Scientist position and PK for support from the earlier project. Professor T. Pradeep is thanked for the use of his DST unit of nanoscience. Thanks are also due to the SAIF, Department of Chemistry, and NCCR of this Institute. REFERENCES 1.

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“TOC Graphic”


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Two plasmon bands with increase in intensity for Au due to interparticle interaction of Ag are found in AuAgZnO nanocomposites made by solution combustion synthesis and photocatalytic studies performed. In AuAgZnO nanocomposite, Ag strongly enhances Photocatalytic activity whereas Au has good efficacy over cytotoxicity in which Ag has negative role.


Effect of Silver on Plasmonic, Photocatalytic, and Cytotoxicity of Gold

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