Nanosheet and nanosphere morphology dominated photocatalytic & antibacterial properties of ZnO nanostructures

Solid State Sciences 89 (2019) 1–14

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Nanosheet and nanosphere morphology dominated photocatalytic & antibacterial properties of ZnO nanostructures

T

Rajender Singha,b,∗, Karan Vermac, Anoop Patyalb, Indresh Sharmad, P.B. Barmana, Dheeraj Sharmaa a

Jaypee University of Information Technology, Department of Physics & Materials Science, Waknaghat, Solan, Himachal Pradesh, 173234, India Department of Central Instrumentation Laboratory (CIL), Panjab University, Chandigarh, India c Forevision Instruments (India) Pvt. Ltd., Hyderabad, Telangana, India d Department of Microbial Biotechnology, Panjab University, Chandigarh, India b

A R T I C LE I N FO

A B S T R A C T

Keywords: Nanostructures Antibacterial Photocatalytic Morphology

To address the issue of water contamination with the usage of dyes and undesired bacterial growth on food products, different ZnO nanostructures have been studied in present manuscript. XRD technique has investigated the crystal purity of ZnO nanostructures. The alteration in chemical structural parameters (bond vibration and spring constant etc.) confirms their structure level modification by fourier transform infrared spectroscopy (FTIR). The maximum efficiency of methylene blue dye degradation has been observed in nanosheet (∼96.42%) as compared to nanosphere (∼95.45%) & nanorod (∼87.12%) morphology. In addition, the antibacterial activity of ZnO nanospheres for E. coli and S. aureus bacteria shows inhibition diameter of 10 mm & 12 mm is credited due to generation of active oxidizing agents as compared to other reported morphologies. Present studied ZnO nanomaterials embolden researchers to investigate the unexplored aspects of ZnO nanomaterials to ameliorated efficiency in photocatalytic and antibacterial applications.

1. Introduction Safety of human health is under constant threat due to accumulation of hazardous pollutants in environment which disturbs whole ecosystem. This problem directs the researchers to develops the different innovative techniques for waste materials treatment. For example the drainage of organic pollutants into water and growth of bacterial infectious species on different food items are the major challenges in order to maintain healthy environment in our surroundings. The removal of organic pollutants from water through advanced oxidative process (AOP) will be a major approach towards the water purification under green technology approach [1–10]. Due to uncontrolled bacterial infection in environment leads to economic and social threats which direct us for development of active antibacterial agents in different food packaging materials, bioelectronics and photonics applications [11–15]. For example, due to infection of Shigella flexneri a type of bacteria claims 1.5 million deaths annually through contamination in food and drinks [16]. In order to tackle the challenges towards the humans' health with environment safety, search for the active agents

keep on going. The desired material should work for removal of organic pollutants from water and restrict the growth of different bacterial species at unwanted places [17–19]. For all above requirements, the ZnO semiconductor found to be the best candidate due to its abundant nature, photostability [20,21] and bactericidal mechanism with non toxic, biocompatible nature to human cells [22–24]. These features directs ZnO nanomaterial to be the best alternative towards photocatalysis and antibacterial applications. ZnO nanomaterials' direct wide band gap (3.37 eV), high exciton binding energy (60 meV) based features have attracted its applications in optoelectronics [25–31], dye sensitized solar cell [32], photocatalysis for removal of different pollutants from water [33,34] and as a biocidal agent [35]. There were few studies have been performed and interpreted critically in respect to morphology of ZnO for their applications in photocatalytic and antibacterial phenomenon [36]. The wide range of different morphology of ZnO nanoparticles (NPs) have made it distinctive candidate to further study. Till now different morphologies of ZnO NPs have been developed viz. wires, tube, rods, sheet, ribbons etc. [37–41] There are different existing synthesis methods for ZnO

∗

Corresponding author. Jaypee University of Information Technology, Department of Physics & Materials Science, Waknaghat, Solan, Himachal Pradesh, 173234, India. E-mail addresses: [email protected] (R. Singh), [email protected] (K. Verma), [email protected] (I. Sharma), [email protected] (D. Sharma). https://doi.org/10.1016/j.solidstatesciences.2018.12.011 Received 3 October 2018; Received in revised form 1 December 2018; Accepted 17 December 2018 Available online 21 December 2018 1293-2558/ © 2018 Published by Elsevier Masson SAS.

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(EDS) (Bruker X Flash 6100), Germany. Fourier transform infrared spectrometer (FTIR) of samples has been analyzed by using PerkinElmer 400 Nicolet FTIR. The UV–visible (UV–Vis.) absorption spectra of NPs were obtained with help of JASCO V530 (USA). Hitachi F7000 based fluorescence spectrometer (Japan) was used to analyse the photoluminescence (PL) behavior of samples. Electron spray ionizationmass spectrometry (ESI-MS) Waters Make technique has been used to study the mass fragments of degraded dye solutions.

nanomaterial viz. thermal evaporation, chemical vapor deposition, electro-deposition, and hydrothermal methods [42–45], which are very costly due their high temperature and working setup requirements. So in order to curtail the synthesis cost at minimum level, a simple and easy process based co-precipitation method [46] has been chosen for synthesis. In present study we have synthesized ZnO nanoparticles with different morphologies having no use of surfactant or high temperature [47]. It is very important to define the desired shape and size of nanoparticles with sufficient yield and quality through suitable synthesis technique. The factors like pH, concentration of precursor, reaction temperature etc. are the crucial factors for the development in morphologies with their other properties. Furthermore, the structural, optical properties of NPs also have been investigated to get insight of synthesized ZnO nanomaterial. In addition, the dye degradation of methylene blue dye and antibacterial activity over E. coli & S. aureus bacterial species with ZnO NPs have been studied.

2.3. Preparation of bacterial culture and determination of antibacterial activity by agar well diffusion assay The bacterial strains used in the study were obtained from the Institute of Microbial Technology (MTCC-IMTECH), Chandigarh, India. The bacterial strains Escherichia coli (E. coli) MTCC 2961, Staphylococcus aureus (S. aureus) MTCC 3160 were cultured in Mullar Hinton Agar (MHA, HiMedia, India). For agar plates, bacteriological agar (2.5% w/ v, HiMedia, India) was added to the medium. The bacterial cells were stored at −80 °C as frozen stocks with 15% glycerol containing growth media. Further, the bacterial cells were freshly revived on agar plates from the stock before agar well diffusion experiment. Antibacterial activity was performed against bacterial strains (S. aureus and E. coli) by agar well diffusion assay. The bacterial cells were grown overnight and diluted in MHB to cell density 105 CFU/mL (colony forming unit). Further the bacterial cells were spread on the MHA plate with the help of cotton swab and left for 30 min for drying. The wells in the agar well plate were made by punching holes by using 200 μL tips. After adding ZnO nanoparticles (5 mg/ml), the agar plates were incubated at 37 °C for 24 h.

2. Materials and methods Analytical grade reagent Zinc nitrate hexahydrate (99.0%) and sodium hydroxide (98.56%) purchased from Higmedia have been used for synthesis of ZnO NPs. All reagents were prepared using double distilled water. 2.1. Synthesis of zinc anchored oxygen (ZnO) nanostructures Zinc anchored oxygen ZnO NPs are synthesized by co-precipitation method. The one molar (1M) concentration of zinc nitrate precursor was dissolved in 100 ml distilled water and mixed properly using magnetic stirrer at room temperature. Subsequently, the 0.5M NaOH solution was added dropwise to precursor solution with continuous magnetic stirring to obtain various pH of solutions viz. 7, 12 & 13. Precipitated solution was washed properly with distilled water and ethanol (five time each) followed by centrifugation at 3000 rpm. Dried the washed precipitates at 90 °C for 4 h in vacuum oven. The schematic representation of synthesis of ZnO NPs at pH 7, 12 and 13 has been depicted in Fig. 1 and samples were named as ZpH7, ZpH12 & ZpH13 respectively.

3. Results and discussion 3.1. XRD The XRD pattern of synthesized ZnO nanoparticles at different pH values (7, 12 & 13) are shown in Fig. 2. All reported samples revealed the well defined and sharp diffraction peaks at 31.8°, 34.4°, 36.3°, 47.6°, 56.6°, 62.8°, 66.4° and 67.9° corresponding to (100), (002), (101), (102), (110), (103), (112) and (201) hkl planes (with their corresponding facets) matches with standard wurtzite phase of ZnO having JCPDS no. 36–1451 as shown in Fig. 2i(a, b, c). With the change in pH of samples prominent change in intensity of diffraction pattern of nanomaterials without any major variation in their position of diffraction angle has been observed. This will predicts the significant modification in the properties of ZnO nanostructures [48]. It is to be noted that except main diffraction peaks of ZnO NPs, no impurity phase peak has been observed for ZnO NPs. The average crystallite size (D) of different ZnO NPs samples is calculated for (101) hkl plane by Debye Scherrer's formula [49],

2.2. Characterization The synthesized ZnO NPs were studied by using different techniques. The crystal structure of NPs were identified at λ = 1.54 Å by XPERT PRO diffractometer, Netherlands (45 KV voltage and 40 mA current). Surface morphology and elemental information of NPs were studied using field emission scanning electron microscopy (FESEM) (Hitachi SU8010) Japan with energy dispersive X-ray spectroscopy

D=

0.9λ βCos (θ )

(1)

where λ is wavelength of X-ray used, β is angular full width half maxima (in radian), θ is Bragg's diffraction angle. The sample ZpH7, ZpH12 & ZpH13 have crystallite size of 41.79 nm, 21.43 nm & 29.47 nm respectively. This predict (i) presence of nano size particles, (ii) distortion in the ZnO host lattice by alternation in the nucleation and growth rate mechanism. As the extent of hydrolysis increases with increased content of NaOH into precursor solution from pH = 7 to 13, have restricted the growth of crystal initially with decrease in crystal size (from 41.79 nm to 21.43 nm) [50] and then further increase the crystallite size to 29.47 nm due to enhanced OH− ions in solutions [51] respectively. The effect of hydroxyl ions concentration on crystal structure can be understood by studying the variation in lattice parameters (a, b & c) from equation (2) using their inter-planner spacing (d) and (hkl) values of synthesized

Fig. 1. Schematic representation of ZnO NPs synthesis at pH = 7, 12 and 13 by co-precipitation method. 2

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Fig. 2. (i) XRD diffraction pattern of ZnO nanoparticles at (a) pH = 7, (b) pH = 12, (c) pH = 13 and (ii) depicts the enlarged view of 101 (hkl) planes of different synthesized NPs respectively.

materials.

1 4 h2 + hk + l 2 ⎫ l2 + 2 = ⎧ 2 ⎬ d2 3⎨ a c ⎭ ⎩

(2)

3.2. Dislocation density and micro-strain calculation From the XRD diffraction data, the dislocation density (δ) & microstrain (ɛ) were determined using equations (3) and (4) and their detail values are summarized in Table 1.

1 δ = ⎛ 2⎞ ⎝d ⎠

(3)

βCos θ ⎞ ε=⎛ ⎝ 4 ⎠

(4)

From Table 1, it is found that the dislocation density (δ) and micro strain (ɛ) values increases from 5.72 × 10−4 & 4.65 × 10−2 for ZpH7 to 2.18 × 10−3 & 9.07 × 10−2 for ZpH12 respectively. In continuation, the ZpH13 sample shows the decrease in above studied parameters as compared to ZpH12 values to 1.15 × 10−3 & 8.69 × 10−2. The variation in the above parameters may be due to change in nanostructures shape, size and defects level etc. [52]. The relative intensity (I10-10/I0002) of (10-10) to (0002) facets peaks for ZpH7, ZpH12 and ZpH13 samples have been calculated which are about 1.29, 1.00 & 1.20 respectively. Higher value of intensity ratio of I10-10/I0002 predicts presence of the larger amount of polar plane {0001} on sample surface for ZpH7 sample [53,54]. Further decrease in above calculated intensity ratio from ZpH7 to ZpH13 predicts the decrease in the ratio of {0001} polar facets on the samples surfaces. As a part of ZnO nanostructure (hexagonal close packing with tetrahedral coordinated zinc and oxygen atom), it is consist of positively charged Zn terminated {0001} facets with negative charge oxygen terminated {000–1} facets along c-axis. The {0001} plane has high surface energy with metastable state as compared to non polar (10-10) and (2-1-10)

Fig. 3. Graphical representation of FWHM, crystallite size (D) and micro strain of ZnO NPs synthesized at pH = 7, 12 & 13 respectively.

planes parallel to c-axis [55]. The detail of calculated values are summarized in Table 1. Fig. 3 depicted the relation between FWHM, crystallite size (D) and micro strain of synthesized ZnO NPs.

3.3. FESEM study The surface morphology of ZnO NPs synthesized at different pH were studied using FESEM in Fig. 4(a–d). It is clearly visualized from Fig. 4 that the morphology of ZnO nanostructures are directly dependent on the concentration of NaOH in the solution as variation in pH leads to develops different shapes of nanostructures. Hexagonal rod morphology has been obtained at pH = 7, possibly

Table 1 Detail values of particles size (D), micro strain (ɛ), dislocation density (δ), lattice parameters (a, b, c), and full width half maxima (FWHM) corresponding to ZpH7, ZpH12 & ZpH13 samples respectively. Sample

pH = 7 pH = 12 pH = 13

2θ(°)

36.23 36.23 36.23

(D) (nm)

41.79 21.43 29.47

(ɛ)

4.65 × 10−2 9.07 × 10−2 8.69 × 10−2

I10-10/I0002

1.29 1.00 1.20

(δ)

Lattice parameter (Å)

5.72 × 10−4 2.18 × 10−3 1.15 × 10−3

3

FWHM (β) (°)

a = b (100 peak)

c (002 peak)

c/a

3.256 3.249 3.254

5.208 5.207 5.206

1.599 1.603 1.599

0.200 0.390 0.370

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Fig. 4. Surface morphology of ZnO nanoparticles by FESEM for (a) ZpH7, (b) ZpH12, (c) ZpH13 and (d) depicts the schematic representation of different shapes with change in pH respectively.

Fig. 5. FTIR spectra of (i) (a) ZpH7, (b) ZpH12, (c) ZpH13 and (ii) depicts the zoom view of wavenumber from 400 to 700 cm−1 to observe ZneO vibration respectively.

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Fig. 6. UV–vis. absorption spectra of (i) (a) ZpH7, (b) ZpH12, (c) ZpH13 and (ii) corresponding optical band gap (Eopt) plots respectively. Table 2 Consist of absorption maxima, optical band gap (Eopt) by tauc plot for ZnO nanostructures. Sample

Absorption maxima (nm)

Optical band gap (Eopt) (eV)

ZpH7 ZpH12 ZpH13

366.59 369.08 366.58

3.34 3.33 3.37

Fig. 8. Band diagram of ZnO NPs emission at (a), (b), (c) and (d) energy levels corresponding to ZpH7, ZpH12 & ZpH13 samples respectively.

Fig. 9. Physical appearance of ZnO NPs in dark (a) just after addition of NPs, (b) after 3 h of interaction time respectively. Fig. 7. PL spectra of ZpH7, ZpH12 & ZpH13 nanostructures respectively.

3.4. Mechanism behind the different morphologies due to preferential growth along 002 (c-axis) {0001 facets} of ZnO crystal [56] (Fig. 4(a)). The nanorods have average length and width of 432.94 nm & 155.0 nm respectively. Further increasing the NaOH concentration into zinc precursor bring the transformation from nanorod into nanosheet morphology. The observed nanosheets have average dimension of two sides viz. 214.38 nm × 178.22 nm. The higher concentration of basic medium in reacting solution bring drastic change in morphology from nanosheet to spherical shape. This may be due to the excess of OH− via intermediate units like Zn(OH)4(ONa)x− as shown in reaction 6. The average particle size of spherical entities in ZpH13 is observed 53.99 nm.

The transformation of ZnO NPs into different morphologies can be explained by following equations:

NaOH + H2 O ↔ Na+OH−

(5)

Zn2 + + 2OH− + xNaOH → Zn(OH)2 (ONa)2x− + H2 O

(6)

Zn(OH)2 → ZnO + H2 O

(7)

Zn(OH)2 + 2OH− → [Zn(OH) 4]2 −

(8)

ZnO + H2 O + 2OH− → [Zn(OH) 4]2 −

(9)

Before the growth of ZnO crystal lattice in dehydration reaction in 5

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Fig. 10. Absorption spectral changes of MB with (i) ZpH7 (ii) ZpH12(iii) ZpH13 & (iv) physical appearance of different samples at different timings respectively.

various directions of ZnO crystal get restricted [57]. This stated reason may produce the growth of ZnO nanorod to nanosheet morphology. At pH = 13 solution further bring transformation of nanosheet morphology into spherical particles due to increased potential of OH− ions around the nanoparticles surface (Fig. 4(c) [21]. Growth of crystal along different facets have surface energy as a prominent factor for their growth in particular direction. According Gibbs-Wulff's theory [58].

Table 3 Depicts the values related to % dye degradation, rate constants and R2 values for different studied samples. ZpH12

ZpH7

Interaction time (minute) 30 min. 60 min. 90 min. 120 min. 150 min. 180 min.

MB % dye degradation

ZpH7

ZpH12

ZpH13

34.09 51.52 68.94 78.03 84.85 87.12

0.011 (0.9918)

0.018 (0.9852)

0.016 (0.9932)

46.21 68.94 84.85 90.15 94.70 96.52

ZpH13

Rate constant (min−1) (R2)

Sample

47.73 67.42 81.06 89.39 93.94 95.45

γ γ1 γ = 2 = 3 = … =constant h1 h2 h3

(10)

where γn is surface energy of facets n and hn is distance between facets n and point in crystal called Wulff's point (point to minimize the surface energy). The crystal facets which have minimum surface energy grow slowly as compared to maximum surface energy based crystal. This phenomenon leads to disappearance of growing facets into different

equation (7), the ONa− ions present in intermediate species in equation (6) get replaced with OH− ion. The replacement kinetics of above reaction have absorbs the heat of reaction through which the growth in 6

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∨¯ =

1 2πc

k μ

(11)

where ⊽, k, μ & c represent the wavenumber, bonds' force constant, reduced mass & velocity of light (3 × 108 cm/s) respectively. The reduced mass represented as:

1 1 1 = + μ m1 m2

(12)

where m1 & m2 represent the mass of bonded atoms (Zn and oxygen). The value of spring constant (k) for ZneO vibration have values viz. 1.14 × 104 dyne/cm, 1.11 × 104 dyne/cm & 1.48 × 104 dyne/cm for ZpH7, ZpH12 & ZpH13 respectively. Generally, bond spring constant (k) is direct indicative of change in the bond properties. 3.6. UV–visible spectroscopy The UV–visible absorption (abs.) and optical band gap (Eopt) pattern of synthesized ZnO NPs were examined by UV–vis. spectroscopy (Fig. 6). The sharp absorption maxima below 400 nm has been observed for synthesized ZnO NPs (Fig. 6 (i), which is characteristic of wurtzite structure of ZnO. The absorption maxima of ZpH7, ZpH12 & ZpH13 has been shifted from 366.59 nm to 369.08 nm & 366.58 nm respectively. The direct optical band gap for samples has been calculated from equation (13) [66,67].

Fig. 11. Rate constant observation for (a) ZpH7 (b) ZpH12(c) ZpH13 samples & inset view for their corresponding linear fit respectively.

morphologies for synthesized material at different conditions. Generally, ZnO grows along c-axis to form nanorods shape, which further in order to achieve minimum surface energy transformed into different morphologies (like nanosheet or nanoparticle). ZpH7 samples' crystallite size was 41.79 nm which reduces to 21.43 nm for ZpH12. This decrease in the size may be due to higher nucleation rate at lower pH get dominated at higher pH [59] with growth aspect. Further, ZpH13 has observed increase in crystallite size to 29.47 nm due to domination of nucleation part of ZnO crystal. In addition, the nanorods morphology having c-axis growth have different charge distribution along {0001} facets. Nanorods morphology have also possibility of stacking along (10-10) plane for crystal plane coupling [60], due to which nanorods get assembled into nanosheet morphology with (2-110) plane. In continuation observed nanosheets shape get transformed into nanosphere may be due to aggregation of ZnO nanocrystal [61,62] in growth reaction. The complete evolution in the morphologies of ZnO NPs are due to variation of NaOH concentration. The detail has been depicted in Fig. 4(d).

αhν = A (hν − Eopt )n

(13)

where α is absorption coefficient, A is constant, hν is photon energy, Eopt is optical band gap & ‘n’ is possible electronic transition. The graph between (αhν)2 vs hν (eV) has been plotted in Fig. 6(ii). The estimated optical band gap observed for ZnO NPs at pH = 7, 12 & 13 are 3.34 eV, 3.33 eV, 3.37 eV respectively. The observed alternation in optical band gap values is due to reorganization of valance and conduction band edges which indicates change in structural and morphological properties of ZnO nanostructures. In ZpH12, the optical band gap edge gets a blue shift as compared to ZpH7 sample. This is due to its prominent quantum confinement effect of ZnO nanocrystal with decrease in their crystallite size [68]as shown in Fig. 6 (ii). Further, with change in morphology to nanospheres for ZpH13 shows a red shift in optical band gap. The observed values of absorption maxima, optical band gap (Eopt) by tauc plot are summarized in Table 2.

3.5. FTIR

3.7. Photoluminescence (PL)

FTIR spectra of ZnO NPs synthesized at different pH are depicted in Fig. 5 (i & ii). In the range of (3200–3600) cm−1, a broad band has been observed at ∼ 3432 cm−1 for OeH vibration. With the change in the pH of synthesized samples from 7 to 12 & 13, we have clearly observed the drastic change in width of eOH vibration band. This observation has predicted the major transformation in crystal structure and morphology of NPs [63], which has been already confirmed by XRD and FESEM studies. The absorption band observed at 1641 cm−1 and 894 cm−1 wavenumber is due to bending modes of eOH vibration and stretching vibrational modes of nitrate ion (NO3−) [64]. The shifting of ZneO band vibration to lower wavenumber i.e. from 441 cm−1 to 432 cm−1 (Fig. 5-ii shows the zoomed view of ZneO vibration) with increase in pH from 7 to 12 is due to transformation in ZnO NPs morphology [63]. Further, on increasing the pH of solution to 13, the ZneO vibration band transformed into new band at 501 cm−1. This will predicts major alternation in the ZnO NPs structural arrangement [63]. Moreover to confirm the structural alternation with change in the pH of synthesis reaction, we have calculated the force constant for ZnO vibration for wavenumbers at positions viz. 441 cm−1, 432 cm−1 and 501 cm−1 for ZpH7, ZpH12 & ZpH13 with the help of equation (11) [65].

Fig. 7 depicts the PL spectra of ZpH7, ZpH12 & ZpH13 ZnO nanostructures. Fig. 8 depicts the proposed band diagram of ZnO NPs with their different emission energy levels. With the change in pH of the ZnO NPs, there is clear change in the intensity of emission peaks has been observed (Fig. 7 (i)). Generally, ZnO NPs have mainly two type of emissions: UV and visible emission, which are due to band to band emission (BBE) and deep level defect emission (DLE) respectively [11]. ZnO has mainly six type of defects viz. oxygen vacancy (Vo), oxygen interstitial (Oi), oxygen antisite (Zno), zinc vacancy (Vzn), zinc interstitial (Zni) and zinc antisite (Ozn) [69]. The emission band in UV spectral range has been observed at 425 nm for ZpH7, ZpH12 and 422 nm for ZpH13 respectively as shown in Fig. 8(i, ii & iii). The observed shifting and decrease in the intensity of emissive peak is due to alternation in the near band edges (NBE) through free exciton recombination [70]. Furthermore, emission at 440 nm for ZpH7, 455 nm for ZpH12 & ZpH13 has been observed and their corresponding energy level are denoted by number iv and v respectively in Fig. 8. In addition, DLE has been observed at 469 nm, 468 nm and 472 nm for ZpH7, ZpH12 and ZpH13 samples respectively (denoted by vi, vii and viii number). The electron recombination in oxygen vacancy through holes could be possible reason for DLE peaks [71]. 7

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Fig. 12. Mass spectrums in positive ion mode for (i) Methylene blue (stock), (ii) ZpH7, (iii) ZpH12 & (iv) ZpH13 respectively.

calculated using equation (14), where Co, Ao & Ct, At represents dye concentration, absorbance at time t = 0 and t = t respectively [74]. For dye degradation calculation, absorbance of different solutions is considered equivalent to their concentration. The observed data has been applied to pseudo first order kinetics using equation (15), in which graph between ln (C/Co) versus time has been plotted, which gives values of rate constant and linear regression (R2):

Broadly the above emission spectra in the range 408–490 nm are under the radiative recombination mechanism attributed due to blue emission from shallow donor centers with hole at the VB [72]. Furthermore, in green emission at 522 nm, 528 nm and 532 nm has been observed for ZpH7, ZpH12 and ZpH13 respectively (denoted by symbol ix, x and xi). There is prominent variation in shape and the intensity of ZpH12 emissive peaks, may be due to its smallest crystallite size i.e. 21.43 nm (by XRD) and nanosheet morphology (FESEM). In continuation, the ZpH13 sample shows broadness in the emission peaks as compared to ZpH12 which may be due to enhanced defects states on NPs surface [73]. It is noteworthy that all emission peaks for ZpH12 and ZpH13 samples shows the effective emission as compared to ZpH7.

% degradation = C Ln ⎛ ⎞ = kt ⎝ Co ⎠

3.8. Photo catalytic dye degradation

CO − Ct X 100 = CO

AO − At X 100 AO

(14)

(15)

In typical photocatalytic experiment, 25 mg of ZnO NPs were mixed with 50 ml of 20 ppm MB dye solution. The above solutions mixed properly by magnetic stirring followed by ultrasonication for 20 min. To observe the adsorption effect of NPs with dye, solutions were placed in dark for 3 h. No prominent effect of NPs on dye discoloration has been observed after three hours of interaction which has confirmed the

The photocatalytic performances of different ZnO NPs synthesized at different pH (7, 12 & 13) have been studied using degradation behavior of methylene blue (MB) dye in presence of sun light. The degree of photocatalytic degradation of MB dye as a function of time, was 8

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Fig. 13. Proposed scheme for mass fragmentation of pure dye and different ZnO nanostructure treated solution (ZpH7, ZpH12, ZpH13) under ESI-MS technique.

minimum absorption effect of NPs on dye solution. The physical appearance of MB dye with NPs just at initial time of mixing of NPs with dye and after three hours of interaction has been shown in Fig. 9. The solutions of dye with ZnO NPs have been exposed under sun light for three hours. The degraded dye solutions extracted after every 30 min of interaction time to observe the effect of NPs (photocatalyst) on dye. The detail of dye degradation effect have been depicted in Fig. 10 (i, ii & iii). Fig. 10 (iv) shows the physical appearance of dye solution with NPs under the exposure of sun light. Table 3 has summarized all the values related to % dye degradation, rate constants and R2 values for different studied samples. The dye degradation behavior has been observed for different samples with respect to their absorbance at λmax = 663 nm. Fig. 10 (i, ii, iii), synthesized ZnO sample at pH = 12 (ZpH12) (Fig. 10- ii) shows maximum efficiency for MB dye degradation followed by ZpH13 and ZpH7 samples (Fig. 10- i & iii) respectively. The detail values of % dye degradation are summarized in Table 3. In order to authenticate the observed values of dye degradation, we have calculated the rate

Fig. 14. Mechanism of MB dye degradation with ZnO NPs in presence of sun light.

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Fig. 15. i (a, b & c): XRD patterns (ii) (a, b, c) FTIR spectrums of ZnO nanostructures after photocatalytic dye degradation activity for ZpH7, ZpH12 & ZpH13 respectively.

Fig. 16. (a, b & c): FESEM images of ZnO nanostructure after photocatalytic dye degradation activity for ZpH7, ZpH12 & ZpH13 respectively.

Fig. 17. (a & b): Antibacterial activity of ZnO against E. coli and S. aureus bacteria.

constant using equation (15), whose values are 0.011 min−1, 0.018 min−1 & 0.016 min−1 for ZpH7, ZpH12 & ZpH13 respectively. The rate constant of different synthesized samples are depicted in Fig. 11, (inset view shows their linear fit). The slope of best linear fit gives the corresponding values of rate constant. Highest values of ZpH12 rate constant confirms its highest dye degradation efficiency. In addition to different MB dye degradation with ZnO nanostructures, ESI-MS technique has been used to analyse the dye structure after 180 min interaction time. Fig. 12(i-iv) gives mass spectrum of stock dye and dye after interaction with different NPs (ZpH7, ZpH12 & ZpH13). Fig. 13(i-ix) depicts the possible scheme of fragmentation of dye structure in degradation reaction. In Fig. 12(i), signal at m/z = 284 corresponds to MB molecular ion peak. After effective reaction of different ZnO NPs (ZpH7, ZpH12 & ZpH13) with dye solution results in

loss of intensity of m/z = 284 molecular ion peak. This will confirms major degradation of dye structure. During fragmentation process, the dye structure gets transformed into smaller mass fragments having m/ z = 149, 305, 309, 324, 330 by addition of hydroxyl group or removal of eCH3 group [75,76]. The proposed scheme of dye structure fragmentation has been depicted in Fig. 13. As in Table 3, the ZpH12 shows maximum dye degradation (96.52%) followed by ZpH13 (95.45%) & ZpH7 (87.12%) respectively. The reaction scheme in Fig. 13 also proposed the important role of hydroxyl ion in dye degradation mechanism.

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Fig. 18. (a, b & c): EDS spectrum of ZpH7, ZpH12 and ZpH13 respectively.

3.10. Proposed mechanism of dye degradation reaction In order to understand the dye degradation behavior of MB dye with ZnO NPs, we need to understand their interaction mechanism which is depicted in Fig. 14. Under irradiation of incident light energy which is higher than the band gap energy (Eopt) of ZnO NPs (which act as a photocatalyst), leads to generation of electrons (e−)/holes (h+) in conduction band and valance band. The generated electrons/holes can possibly get recombine or trapped at metastable surface or get adsorbed at the NPs surface. The generated electrons and hole may be captured by species like O2 and OH− which directs to generation of oxidizing species like O2. or OH. radicals [78]. In addition to observed band gap energies of different studied ZnO nanostructures, their particle size, morphology and enrich defects states factors have improved the dye degradation to higher efficiency [79]. In present case, the dye degradation efficiency has been found in order ZpH12 > ZpH13 > ZpH7. The effective degradation of any dye can be facilitated by diffusion and mass transportation of NPs on its surfaces. The enhancement in maximum generation of e−/h+ with suitable morphological, structural and defects state properties collectively responsible for the most effective MB dye degradation for ZpH12 [80].

Fig. 19. Graphical relation between oxygen/Zinc atomic % values with ZpH7, ZpH12 & ZpH13.

3.9. Possible reasons behind the variable MB dye degradation efficiency with different ZnO nanostructures

3.11. Reusability of ZnO NPs In order to reuse the ZnO NPs after the completion of dye degradation reaction, can be collected through proper centrifugation and washing steps. The quality of ZnO nanomaterials prepared at different pH shows their intact crystal, chemical structure and their respective morphology after degradation reaction as shown in Fig. 15 &16. Fig. 15-i(a,b,c) depicts the purity of ZnO crystal structure with respect to initially observed structure by XRD study as depicted in Fig. 2 (before dye degradation). Similar results have been observed under FTIR study (Fig. 15-ii(a,b,c)) with some minor shifting in peak positions may be due to some adsorption of dye on nanoparticles surface. Similar trend in morphology of studied nanostructures has been found in Fig. 16(a,b,c). One more inference from Fig. 16(a) is that the agglomerated nanorods morphology have been observed that could be one of reason of low dye degradation efficiency of ZpH7 as compared to other ZnO nanostructures.

The variable MB dye degradation efficiency for above synthesized ZnO NPs is due to many factors which have been used to explore the observed behavior. Highest dye degradation efficiency of ZpH12 sample can be correlated with nanosheet morphology, maximum defects levels and the lowest optical band gap value (3.33 eV) respectively as compared to other synthesized nanostructures. The nanosheet morphology offer maximum surface area for dye degradation reaction which in turns produces maximum electrons (e−)/holes (h+) which enhances the overall dye degradation efficiency [77]. Moreover, the values of dislocation density (δ) and micro strain (ɛ) (as tabulated in Table 1), are in following ascending order i.e ZpH7 < ZpH13 < ZpH12. This will directly generates the active sites (as confirmed by PL study) for the interaction of dye with ZnO NPs that will affect the dye degradation efficiencies. The lowest dye degradation efficiency by ZpH7 can be correlated with their lowest and broad luminescence pattern. This can be due to its biggest crystallite size (XRD) which leads to minimum interaction between dye and NPs. Nanosheet morphology (ZpH12) of ZnO NPs have prominent effect for dye degradation ability incomparison to ZpH13 and ZpH7 nanostructures.

3.12. Antibacterial activity of ZnO nanostructures The antibacterial activity of ZnO NPs against S. aureus and E. Coli were investigated by studying the zone of inhibition in Fig. 17. ZpH13 nanostructure shows significant increase in diameter of inhibition zone as compared to ZpH7 & ZpH12. From Fig. 17(aand b), inactivity 11

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Fig. 20. EDS mapping images from (a–c) for ZpH7, (d–f) for ZpH12 and (g–i) for ZpH13 respectively. Table 4 Depicts the detail of different morphology of ZnO NPs with their size (nm) and antibacterial activity with E. coli and S. aureus bacteria respectively. Sample

ZpH7

ZpH12

ZpH13

Morphology Size (nm) E. coli Inhibition diameter (mm) S. Aureus Inhibition diameter (mm)

Nanorods 432.94 nm × 155 nm Not detected (ND) ND

Nanosheet 214.38 nm × 178.22 nm ND ND

Nanospheres 53.99 nm 10 mm 12 mm

the homogeneity of zinc and oxygen element for studied nanomaterials. ZnO NPs synthesized at pH = 13 have shown activity against both species viz. E. coli and S. aureus with inhibition diameter i.e 10 mm and 12 mm (17 (a & b)) and detail of values are summarized in Table 4 respectively. Since activity of ZpH13 shows effective results against studied bacterial species, a further effort has been made to explore the mechanism behind the activity. The overall activity by S. aureus was found to be more effective than the E. coli bacteria. This can be possible due to high sensitivity of S. aureus with ZnO as earlier many researchers have reported the same [82–84]. The Gram negative bacteria (E. coli) has less sensitivity towards reactive oxygen species (ROS) of ZnO NPs, has structural difference in the bacterial membrane as compared to S. aureus. [85] The basic chemical structure of Gram positive and Gram

towards the inhibition of bacterial growth has been observed for ZpH7 and ZpH12, may be due to factors like inappropriate morphology, volume of oxygen defects & solubility of Zn+2 etc. The highest crystalline nature of ZnO NPs synthesized at pH = 7 (ZpH7) (average length and width of nanorods was 432.94 nm & 155 nm) has overcome their preferential polar face growth with lowest emission level in PL study [81]. Further, the nanosheet morphology of ZnO at pH = 12 (ZpH12) has average value of dimension was 214.38 nm × 178.22 nm [81]. The EDS study in Fig. 18 (a, b & c) shows the maximum atomic % ratio of oxygen/Zinc for ZpH13 followed by ZpH12 and ZpH7 respectively. Fig. 19 depicts the graphical relation of atomic % of oxygen/Zinc for above studied samples. In addition, EDS mapping of ZpH7, ZpH12 & ZpH13 has been performed to study their distribution of content present which is shown in Fig. 20(a–i). Fig. 20(a–c), (d-f), (g-i) confirms 12

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Fig. 21. Structural features of membrane of Grams positive and Grams negative bacteria.

nanosphere morphology of ZnO have effective antibacterial activity for E. coli and S. aureus as compared to nanorod and nanosheet morphology. The Gram positive bacteria (S. aureus) have cell wall thickness of ∼7–8 nm as compared to Gram negative bacteria's (E. coli) ∼20–80 nm. This is one major reason for less inhibition diameter of ZnO nanospheres with latter bacteria due to large cell wall thickness. Further, the nanorods and nanosheets morphologies of ZnO NPs are not able to penetrate the cell wall of both the bacteria as compared to nanospheres morphology [98,99]. However, ZnO nanorods have more active facets (111), (100) as compared to nanospheres (100) facets. Generally, (111) nanorods facets morphology has active antibacterial activity [100], but in present case nanorods' bigger particle size overcome the active facets feature, which further lead to inactivity towards antibacterial activity.

negative bacteria has been depicted in Fig. 21 in order to understand their variable inhibition behavior with NPs. The presence of additional outer membrane of lipopolysaccharide (LPS) layer in Gram negative in comparison to Gram positive bacteria acts as barrier for the interaction between ROS and bacterial surface [85]. The lower cell membrane polarity of S. aureus as compared to E. coli [86], could also be factor behind the fast penetration of ROS (hydroxyl radical, superoxide and peroxide ion) on its surface which enhances their inhibition diameter [87]. 3.13. Mechanism of antibacterial activity The insight behind the antibacterial activity for ZnO NPs has been explained on the basis following factors: 3.13.1. Reactive oxygen species (ROS) High reactivity, oxidizing property and toxicity of reactive oxygen species (ROS) on the ZnO surface are responsible for their antibacterial activity [88,89]. The reactive species like OH−, O22 − , OH. etc. on ZnO surface leads to produce H2O2 which has ability to penetrate the bacteria membrane followed by their killing. The continuous release of H2O2 from ZnO surface is the major reason for the efficacy of antibacterial activity of ZnO nanostructure for ZpH13 sample without any light illumination as compared to inactive behavior of ZpH7 & ZpH12 nanostructures respectively.

4. Conclusion XRD and FESEM techniques reveal the crystal purity and morphological information of synthesized ZnO nanostructures. The shifting of ZneO band vibration with spring constant (k) from lower to higher values have confirmed their chemical bondage variation in synthesized ZnO nanostructures. Optical band gap (Eopt) values of samples has been observed as 3.34 eV, 3.33 eV, and 3.37 eV for ZpH7, ZpH12, ZpH13 respectively. ZpH12 shows maximum MB dye degradation efficiency of 96.52%, followed by ZpH13 (nanosphere) with 95.45% and minimum with ZpH7 (nanorod) of values 87.12% respectively. Maximum dye degradation efficiency of ZpH12 is due to contribution of factor viz. nanosheet morphology, presence of defects states. ZpH13 (spherical morphology) shows activeness towards the inhibition in the growth of E. Coli and S. aureus bacteria. The smallest particle size, suited morphology (nanosphere), reactive oxygen sites and sufficient release of zinc ions etc. features of ZpH13 put together the penetration of cell wall of bacterial species. Finally, as per the demand of present market situations related to water contamination from different dyes and unwanted bacterial growth in different food items, the present synthesized ZnO NPs can proven to be best candidate for their prevention and removal with minimum hazard to environment.

3.13.2. Zn+2 ions release Another proposed reason for the antibacterial activity of ZnO NPs is release of Zn+2 ions in the solution containing NPs and bacteria. The optimization on the dissolution of nanostructures will control and modify their toxicity towards different bacterial growth [90–92] The limited solubility of ZnO will restrict the homogenous distribution of Zn+2 ions in medium leads to limited antibacterial effect [93]. Earlier people have studied the release of Zn+2 ions in reaction solution which is affected by factors like particle size, morphology of NPs, pH of solutions etc. [94] In present case ZpH7, ZpH12 sample have nil antibacterial activity may be due to low release of Zn+2 ions in medium whereas ZpH13 sample shown effective activity due to maximum solubility and high release of corresponding ion in reaction medium [95,96].

Acknowledgement

3.13.3. Morphology & particle size The shape and size of nanostructures will affect the antibacterial activity in terms of presence of active facets [97]. In present case, the

The author would like to thank SAIF, Panjab University, Chandigarh for providing different characterization (XRD, FESEM) facilities. 13

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Appendix A. Supplementary data

[46] K. Jeyasubramanian, G.S. Hikku, R. Krishna Sharma, J. Water Process Eng. 8 (2015) 35. [47] Yuxin Wang, Xinyong Li, Ning Wang, Quan Xie, Yongying Chen, Separ. Purif. Technol. 62 (2008) 727. [48] S.H. Jung, E. Oh, K.H. Lee, Y. Yang, C.G. Park, W. Park, S.H. Jeong, Cryst. Growth Des. 8 (2008) 265. [49] R. Chauhan, A. Kumar, R.P. Chaudharya, J. Chem. Pharmaceut. Res. 2 (2010) 178. [50] N. Samaele, P. Amornpitoksuk, S. Suwanboon, Powder Technol. 203 (2010) 243. [51] M. Jay Chithra, M. Sathya, K. Pushpanathan, Acta Metall. Sin. (Engl. Lett.) 28 (2015) 394. [52] N. Rajamanickam, S. Rajashabala, K. Ramachandran, J. Lumin. 146 (2014) 226. [53] A.L. Juez, F. Uines, O.L. Garcia, M.F. Garcia, F. Illas, J. Mater. Chem. A 3 (2015) 8782. [54] M. Wang, Y. Zhang, Y. Zhou, F. Yang, E.J. Kim, S.H. Hahn, S.G. Seong, CrystEngComm 15 (2013) 754. [55] L. Vyassieres, K. Keis, A. Hagfeldt, S.E. Lindquist, Chem. Mater. 13 (2001) 4395. [56] N. Talebian, S.M. Amininezhad, M. Doudi, J. Photochem. Photobiol. B Biol. 120 (2013) 66. [57] Xiao Qi, Suping Huang, Jiang Zhang, Chong Xiao, Xiaoke Tan, J. Alloy. Comp. 459 (2008) L18. [58] L. Zhang, W. Wang, D. Jiang, E. Gao, S. Sun, Nano. Res. 8 (2015) 812. [59] Z. Zhang, J. Mu, J. Colloid Interface Sci. 307 (2007) 79. [60] Q. Yue, M. Wang, Z. Sun, C. Wang, C. Wang, Y. Deng, D. Zhao, J. Mater. Chem. B. 1 (2013) 6085. [61] B. Li, Y. Wang, J. Phys. Chem. C 114 (2014) 890. [62] S. Cho, J.W. Jang, J.S. Lee, K.H. Lee, Langmuir 26 (2010) 14255. [63] R. Wahab, Y.S. Kim, H.S. Shin, Mater. Trans. 50 (2009) 2092. [64] K. Akhtar, N. Zubair, S. Ikram, Z. Ullah Khan, H. Khalid, Bull. Mater. Sci. 40 (2017) 459. [65] S. Lanfredi, D.H.M. Genova, I.A.O. Brito, A.R.F. Lima, M.A.L. Nobre, J. Solid State Chem. 184 (2011) 990. [66] R. Singh, P.B. Barman, D. Sharma, J. Mater. Sci. Mater. Electron. 28 (2017) 5705. [67] J. Kennedy, P.P. Murmu, J. Leveneur, A. Markwitz, J. Futter, Appl. Surf. Sci. 367 (2016) 52. [68] X. Liu, L. Ye, S. Liu, Y. Li, X. Ji, Sci. Rep. 6 (2016) 38474. [69] K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallant, J.A. Voigt, B.E. Gnade, J. Appl. Phys. 79 (1996) 7983. [70] Y. Jin, Q. Cui, K. Wang, J. Hao, Q. Wang, J. Zhang, J. Appl. Phys. 109 (2011) 053521. [71] C. Karunakaran, J. Jayabharathi, K. Jayamoorthy, P. Vinayagamoorthy, J. Photochem. Photobiol. Chem. 247 (2012) 16. [72] D. Liu, Y. Lv, M. Zhang, Y. Liu, Y. Zhu, R. Zong, Y. Zhu, J. Mater. Chem. A 2 (2014) 15377. [73] T. Andelman, Y. Gong, M. Polking, M. Yin, I. Kuskovsky, G. Neumark, S. O'Brien, J. Phys. Chem. 109 (2005) 14314. [74] F. Davar, A. Majedi, A. Mirzaei, J. Am. Ceram. Soc. 98 (2015) 1739. [75] A.P.L. Batista, H.W.P. Carvalho, G.H.P. Luz, P.F.Q. Martins, M. Goncalves, L.C.A. Oliveira, Environ. Chem. Lett. 8 (2010) 63. [76] A.A. Tireli, I.D.R.Guimaraes, J.C.D.S.Terra, R.R.D.Silva, M.C.Guerreiro, Environ. Sci. Pollut. Res., https://doi.org/10.1007/s11356-014-2973-x. [77] Y.J. Jose, U. Manjunathan, S.J. Selvaraj, J. Nanostruct. Chem. 7 (2017) 259. [78] X. Zhao, F. Lou, M. Li, X.D. Lou, Z. Li, J. Zhou, Ceram. Int. 40 (2014) 5507. [79] D. Scott Bohle, Carla J. Spin, J. Am. Chem. Soc. 131 (2009) 4397. [80] U. Pal, P. Santiago, J. Phys. Chem. B 109 (2005) 15317. [81] G.X. Tong, F.-F. Du, Y. Liang, Q. Hu, R.-N. Wu, J.-G. Guan, X. Hu, J. Mater. Chem. B. 1 (2013) 454. [82] L.K. Adams, D.Y. Lyon, P.J.J. Alvarez, Water Res. 40 (2006) 3527. [83] M. Premanathan, K. Karthikeyan, K. Jeyasubramanian, G. Manivannan, Nanomed. Nanotechnol. Biol. Med. 7 (2011) 184. [84] J. Sawai, J. Microbiol. Methods 54 (2003) 177. [85] A.D. Russell, J. Antimicrob. Chemother. 52 (2003) 750. [86] R. Sonohara, N. Muramatsu, H. Ohshima, T. Kondo, Biophys. Chem. 55 (1995) 273. [87] T. Gordon, B. Perlstein, O. Houbara, I. Felner, E. Banin, S. Margel, Colloid. Surface. Physicochem. Eng. Aspect. 374 (2011) 1. [88] H. Zhang, X. Lv, Y. Li, Y. Wang, J. Li, ACS Nano 4 (2009) 380. [89] L.K. Adams, D.Y. Lyon, P.J. Alvarez, Water Res. 40 (2006) 3527. [90] M. Heinlaan, A. Ivask, I. Blinova, H.C. Dubourguier, A. Kahru, Chemosphere 71 (2008) 1308. [91] S.W. Wong, P.T. Leung, A. Djurisic, K.M. Leung, Anal. Bioanal. Chem. 396 (2010) 609. [92] B. Wu, Y. Wang, Y.H. Lee, A. Horst, Z. Wang, D.R. Chen, R. Sureshkumar, Y.J. Tang, Environ. Sci. Technol. 44 (2010) 1484. [93] B.A. Sevinc¸, L. Hanley, J. Biomed. Mater. Res. B. 94 (2010) 22. [94] J. Pasquet, Y. Chevalier, J. Pelletier, E. Couval, D. Bouvier, M.A. Bolzinger, Colloids Surf., A 457 (2014) 263. [95] X. Peng, S. Palma, N.S. Fisher, S.S. Wong, Aquat. Toxicol. 102 (2011) 186. [96] X. Wang, H.F. Wu, Q. Kuang, R.B. Huang, Z.X. Xie, L.S. Zheng, Langmuir 26 (2009) 2774. [97] S. Pal, Y.K. Tak, J.M. Song, Appl. Environ. Microbiol. 73 (2007) 1712. [98] A. Sirelkhatim, S. Mahmud, A. Seeni, N.H.M. Kaus, L.C. Ann, S.K.M. Bakhori, H. Hasan, D. Mohamad, Nano-Micro Lett. 7 (2015) 219. [99] P.J.P. Espitia, N.D.F.F. Soares, J.S.D.D. Coimbra, N.J.D. Andrade, R.S. Cruz, A.A. Medriros, Food Bioprocess. Technol. 5 (2012) 1447. [100] G. Li, T. Hu, G. Pan, T. Yan, X. Gao, H. Zhu, J. Phys. Chem. C 112 (2008) 11859.

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solidstatesciences.2018.12.011. References [1] M.R. Hoffman, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69. [2] A.K. Bin, S.S. Madej, Sci. Eng. 34 (2012) 136. [3] B. Ning, N. Graham, Y. Zhang, M. Nakonechny, M.G. El-Din, Sci. Eng. 29 (2007) 153. [4] M.M. Momeni, Appl. Phys. A 119 (2015) 1413. [5] M.M. Momeni, Y. Ghayeb, Z. Ghonchegi, Ceram. Int. 41 (2015) 8735. [6] M.M. Momeni, Y. Ghayeb, Ceram. Int. 42 (2016) 7014. [7] M.M. Momeni, M. Mirhosseini, M. Chavoshi, A. Hakimizade, J. Mater. Sci. Mater. Electron. 27 (2016) 3941. [8] M.M. Momeni, Y. Ghayeb, J. Mater. Sci. Mater. Electron. 26 (2015) 5509. [9] M.M. Momeni, Y. Ghayeb, Z. Ghonchegi, J. Solid State Electrochem. 19 (2015) 1359. [10] M.M. Momeni, M. Hakimian, A. Kazempour, Ceram. Int. 41 (2015) 13692. [11] A.B. Amanulla, S.K.J. Shahina, R. Sundaram, C.M. Magdalane, K. Kaviyarasu, D. Letsholathebe, S.B. Mohamed, J. Kennedy, M. Maaza, J. Photochem. Photobiol., B (2018), https://doi.org/10.1016/j.jphotobiol.2018.04.034. [12] C.M. Magdalane, K. Kaviyarasu, J.J. Vijaya, C. Jayakumar, M. Maaza, B. Jeyaraj, J. Photochem. Photobiol., B 169 (2017) 110. [13] K. Kaviyarasu, N. Geetha, S. Sivaranjani, A. Ayeshamariam, J. Kennedy, R. Ladchumannandasiivam, U.U. Gomes, M. Maaza, Mater. Sci. Eng. C (2016), https://doi.org/10.1016/j.msec.2016.12.024. [14] K. Kaviyarasu, K. Kanimozhi, N. Matinise, C.M. Magdalane, G.T. Mola, J. Kennedy, M. Maaza, Mater. Sci. Eng. C (2017), https://doi.org/10.1016/j.msec.2017.03.210. [15] C.M. Magdalane, K. Kaviyarasu, J.J. Vijaya, B. Siddhardha, B. Jeyaraj, Photochem. Photobiol. B (2017), https://doi.org/10.1016/j.jphotobiol.2017.05.024. [16] K. Kotloff, J. Winickoff, B. Ivanoff, J.D. Clemens, D. Swerdlow, P. Sansonetti, G. Adak, M. Levine, Bull. World Health Organ. 77 (1999) 651. [17] M.M. Momeni, Y. Ghayeb, S. Gheibee, Ceram. Int. (2016), https://doi.org/10.1016/ j.ceramint.2016.09.195. [18] K. Kaviyarasu, L. Kotsedi, A. Simo, X. Fuku, G.T. Mola, J. Kennedy, M. Maaza, Appl. Surf. Sci. (2016), https://doi.org/10.1016/j.apsusc.2016.11.149. [19] C.M. Magdalane, K. Kaviyarasu, J.J. Vijaya, B. Siddhardha, B. Jeyaraj, M. Maaza, J. Alloy. Comp. 727 (2017) 1324. [20] C. Lizama, J. Freer, J. Baeza, H.D. Mansilla, Catal. Today 76 (2002) 235. [21] X. Pian, B. Lin, Y.L. Chen, J.D. Kuang, K.Z. Zhang, L.M. Fu, J. Phys. Chem. C 115 (2011) 6531. [22] J.T. Seil, T.J. Webster, Int. J. Nanomed. 7 (2012) 2767. [23] G. Colon, B.C. Ward, T.J. Webster, J. Biomed. Mater. Res. 78 (2006) 595. [24] N. Padmavathy, R. Vijayaraghavan, Sci. Technol. Adv. Mater. 9 (2008) 035004. [25] X.D. Wang, C.J. Summers, Z.L. Wang, Nano Lett. 4 (2004) 423. [26] J. Kennedy, A. Markwitz, Z. Li, W. Gao, C. Kendrick, S.M. Durbin, R. Reeves, Curr. Appl. Phys. 6 (2006) 495. [27] J. Kennedy, F. Fang, J. Flutter, J. Leveneur, P.P. Murmu, G.N. Panin, T.W. Kang, E. Manikandan, Diam. Relat. Mater. 71 (2017) 79. [28] J. Kennedy, P.P. Murmu, J. Leveneur, G.V.M. Williams, R.L. Moody, T. Maity, S.V. Chong, J. Nanosci. Nanotechnol. 18 (2018) 1384. [29] R.L. Mendelsberg, J. Kennedy, S.M. Durbin, R. Reeves, Curr. Appl. Phys. 8 (2008) 283. [30] A. Markwitz, J. Kennedy, Int. J. Nanotechnol. 6 (2009) 369. [31] J. Kennedy, P.P. Murmu, E. Manikandan, S.Y. Lee, J. Alloy. Comp. (2014), https:// doi.org/10.1016/j.jallcom.2014.07.179. [32] M. Law, L.E. Greene, J.C. Johnson, R. Saykally, P.D. Yang, Nat. Mater. 4 (2005) 455. [33] C. Hariharan, Appl. Catal. Gen. 304 (2006) 55. [34] K. Kaviyarasu, C.M. Magdalane, K. Kanimozhi, J. Kennedy, B. Siddhardha, E.B. Reddy, N.K. Rotte, C.S. Sharma, F.T. Thema, D. Letsholathebe, G.T. Mola, M. Maaza, J. Photochem. Photobiol., B (2017), https://doi.org/10.1016/j. jphotobiol.2017.06.026. [35] R. Tankhiwale, S.K. Bajpai, Colloids Surfaces B Biointerfaces 90 (2012) 16. [36] K. Kaviyarsu, C.M. Magdalane, K. Kanimizhi, J. Kennedy, B. Siddardha, E.S. Reddy, N.K. Rotte, C.S. Sharma, F.T. Thema, D. Letsholathebe, G.T. Mola, M. Maaza, J. Photochem. Photobiol., B 173 (2017) 466. [37] L. Guo, Y.L. Ji, H.B. Xu, P. Simon, Z.Y. Wu, J. Am. Chem. Soc. 124 (2002) 14864. [38] M.H. Huang, S. Mao, H. Feick, H.Q. Yan, Y.Y. Wu, H. Kind, E. Weber, R. Russo, P.D. Yang, Science 292 (2001) 1897. [39] S.J. Chen, Y.C. Liu, C.L. Shao, R. Mu, Y.M. Lu, J.Y. Zhang, D.Z. Shen, X.W. Fan, Adv. Mater. 17 (2005) 586. [40] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. [41] M.S. Wang, E.J. Kim, E.W. Shin, J.S. Chung, S.H. Hahn, C. Park, J. Phys. Chem. C 112 (2008) 1920. [42] F. Liu, P.J. Cao, H.R. Zhang, J.Q. Li, H.J. Gao, Nanotechnology 15 (2004) 949. [43] X.H. Sun, S. Lam, T.K. Sham, F. Heigl, A. Jurgensen, N.B. Wong, J. Phys. Chem. B 109 (2005) 3120 109. [44] B.Q. Cao, W.P. Cai, G.T. Duan, Y. Li, Q. Zhao, D.P. Yu, Nanotechnology 16 (2005) 2567–2574. [45] B. Liu, H.C. Zeng, J. Am. Chem. Soc. 125 (2003) 4430.

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