rGO nanocomposites for enhanced photocatalytic and antibacterial activities

Author’s Accepted Manuscript Synthesis of ZnO:Co/rGO nanocomposites for enhanced photocatalytic and antibacterial activities K. Ravichandran, K. Nithiyadevi, B. Sakthivel, T. Arun, E. Sindhuja, G. Muruganandam www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(16)31373-6 http://dx.doi.org/10.1016/j.ceramint.2016.08.067 CERI13519

To appear in: Ceramics International Received date: 19 July 2016 Revised date: 30 July 2016 Accepted date: 10 August 2016 Cite this article as: K. Ravichandran, K. Nithiyadevi, B. Sakthivel, T. Arun, E. Sindhuja and G. Muruganandam, Synthesis of ZnO:Co/rGO nanocomposites for enhanced photocatalytic and antibacterial activities, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.08.067 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis of ZnO:Co/rGO nanocomposites for enhanced photocatalytic and antibacterial activities K. Ravichandran1*, K. Nithiyadevi1,2, B.Sakthivel1, T. Arun3, E. Sindhuja1, G. Muruganandam4 1

P.G. & Research Department of Physics, AVVM Sri Pushpam College (Autonomous), Poondi, Thanjavur 613503, Tamil Nadu, India. 2

Department of Physics, Bharathidasan University Constituent College for Women, Orathanadu- 614 625,Tamil Nadu, India. 3

Institute of Physics, Bhubaneswar, 751 005, Odisha, India.

4

P.G. & Research Department of Chemistry, AVVM Sri Pushpam College (Autonomous), Poondi, Thanjavur 613503, Tamil Nadu, India. *Corresponding author. Associate Professor, Post Graduate & Research Department of Physics, AVVM Sri Pushpam College (Autonomous), Poondi, Thanjavur-613 503, Tamil Nadu, India. Mobile: +919443524180; Land line: +04362 278602; Fax : +91 4374 239328. [email protected] Abstract Cobalt activated ZnO nanocomposites have been successfully grown on reduced graphene oxide (rGO) nanosheets via a cost-effective and simple soft chemical method to obtain ZnO:Co/rGO nanocomposites for the simultaneous enhancement of photocatalytic and antibacterial activities. The obtained samples were characterized for their structural, optical, surface morphological, photocatalytic and antibacterial properties. XRD profiles confirmed that the synthesized material is nanocrystalline ZnO with hexagonal wurtzite structure. The photocatalytic activities of the synthesized samples were evaluated by observing the degradation of methylene blue (MB), a representative organic dye, under visible light irradiation. The photo degradation of MB was found to follow pseudo-first order kinetics. Compared with bare ZnO, the Co +rGO activated ZnO nanocomposites exhibits significantly enhanced photocatalytic and antibacterial activities due to the synergetic effects of various mechanisms related to the Co and rGO incorporation. These

mechanisms are elaborated in detail with the help of appropriate schematic diagram along with the support of XRD, photoluminescence, UV-visible absorption, FTIR, FESEM , and TEM results. Keywords: ZnO; Graphene oxide; Nanocomposites; Photocatalysis; Antibacterial 1. Introduction Organic dyes and their industrial effluents have become one of the main sources of water pollution because of the increasing use of dyes in various industries [1]. The effluents, mainly from textile, plastic and paper industries contaminate the environment by releasing toxic, carcinogenic and coloured chemicals. For the eradication of such contaminants, photo degradation is one of the best ways, as this process can employ low cost, non-toxic and abundantly available semiconductors as photocatalytic materials. Unlike ordinary absorbent method of purification, where the toxic chemicals are only replaced from water but not decomposed into non-toxic entities, photo catalysis leads to a complete decomposition of organic compounds into water and carbon di oxide [2, 3]. Among the photocatalytic materials, semiconductors such as TiO 2, ZnO, and ZnS have attracted the attention of the material scientists owing to their tuneable photocatalytic properties and their potentials to the degradation of harmful contents in contaminated water and industrial effluents [4-7]. At present, TiO2 is the most popular photocatalytic material with industrial and commercial viability. Considering the factors like cost, abundance in nature, biocompatibility and oxidative nature, ZnO is the best alternative to TiO 2 for photocatalytic applications [8]. ZnO can exhibit comparable or even better prerequisite properties of an ideal photo catalyst. It is a non-toxic material with long shelf life having thermal and electrical stability. Another important point which has to be mentioned here is that ZnO is Generally Recognized As Safe (GRAS) by the U.S food and drug administration. Owing to their significant chemical, optical and electrical properties, zinc

oxide nanoparticles have been widely used in gas sensors, solar cells and electrochromic devices, in addition to photocatalytic and antibacterial applications [9]. Even though ZnO offers several advantages as a photocatalyst, its photocatalytic efficiency under solar energy spectrum is limited due to its wide band gap. Because of this wide band gap, it is operative only within a small portion of the solar energy. In other words, the band gap of ZnO confines its phocatalytic activity within the UV light range. As a result, it can utilize only 4% of the incident solar radiation. To tackle this problem, transition metals such as silver, gold, platinum, cobalt, nickel and manganese have been appropriately added to ZnO which can help in tuning the band gap energy suitably and thereby extend its photocatalytic response from the UV region to the visible light region [10-14]. It is well known that the proportion of visible light in the solar spectrum is as large as 46 % [15]. Among the above mentioned transition metals, cobalt is one of the most effective dopants that can help enhance the photocatalytic activity of ZnO [16-19]. When ZnO is doped with Co , the band gap of ZnO decreases considerably due to the spd exchange interaction between the band electrons and the localized electrons of the doped Co2+ ions which substitute the host Zn2+ions of the ZnO matrix. As Co is one of the best compatible dopants for ZnO, as established by the Lattice Compatibility Theory [20], in the present work, Co is selected for tuning the band gap of ZnO, in order to make it an efficient visible light responsive photocatalyst. This band gap tuning is also helpful for the antibacterial applications of ZnO. Moreover, when ZnO is doped with Co, Co2+ ions can readily occupy the regular zinc sites of ZnO matrix and thereby facilitate the release of more number of Zn2+ ions from the ZnO system. These released Zn2+ ions have the ability to penetrate into the cell membranes of micro-organisms and eventually destroy the cells [21]. Thus Co doping can enhance the antibacterial efficacy of ZnO. Considering all these factors, cobalt is added as one of the components of the prepared nanocomposite in the present work with the main material ZnO. As photo-generated electron-hole pairs play the key role in determining the photocatalytic activity of a material, the reduction of electron- hole recombination rate by

some means is very much essential to enhance the photocatalytic efficiency of a material. It has been established that ZnO + carbon derivatives can exhibit promising characteristics as visible responsive photocatalysts. Literature survey elucidates that graphene activated ZnO can have desirably enhanced photocatalytic as well as antibacterial performance [22,23].Especially, reduced graphene oxide has the ability to absorb the organic dye molecules easily thereby facilitate their fast decomposition. It is well known that the inclusion of graphene related materials can increase the generation of reactive oxygen species (ROS) which are responsible for photocatalytic degradation of organic pollutants [24]. In this context, reduced graphene oxide (rGO) have been added as another constituent material to obtain ZnO:Co/rGO nanocomposite in the present work. To the best of our knowledge, this is the first report on the photocatalytic and antibacterial studies of ZnO:Co/rGO nanocomposite prepared using a cost effective simple soft chemical method. 2. Experimental details 2.1. Synthesis process Nanopowders of ZnO and ZnO:Co, and ZnO:Co/rGO nanocomposite

were

synthesized using a simple soft chemical method. It is a facile and inexpensive method which does not require any sophisticated equipment. It is a scalable, commercially and industrially viable technique which enables easy doping and gives good yield. The process temperature is as low as 85ºC and hence it is called as soft chemical method. Most importantly, by appropriately varying the process parameters, one can control the properties of the final product as required by the application concerned. Schematic diagram of various steps involved in the synthesis of ZnO, ZnO:Co and ZnO:CO/rGO nano materials is shown in Fig.1. Zinc acetate dihydrate (Zn(CH3COO)2_2H2O - Sigma –Aldrich Chemicals) was used as the precursor for the preparation of ZnO nanopowder. For the synthesis of bare ZnO nanopowder, 8.77g of zinc acetate dehydrate (molecular weight 219.49 mol/g) was

dissolved in 200 mL of doubly deionized water to obtain the starting solution having molarity 0.2 M. Then NaOH solution was added drop by drop with the starting solution to keep the value of pH at eight [25]. This pH value is selected because the formation of ZnO bond with good crystalline quality has been observed when the value of the pH is 8. The prepared solution was stirred using magnetic stirrer for 2 h keeping the temperature of the solution at 85 ˚C. During the stirring process, the mouth of the solution bath was covered with a aluminium foil. The solution is then allowed to cool to room temperature and kept undisturbed for 3 h to get the required precipitate. The precipitate was filtered and then rinsed with a mixture of water and ethanol in the ratio of 3:1. Then it was dried in air ambient at room temperature for 1 hour. Finally, it was calcinated at 300 ˚C for 2 h to obtain the final bare ZnO nanopowder. For the synthesis of ZnO:Co nanopowders, cobalt acetate tetra hydrate (C 4H6COO4 4H2O- Merck Specialities Private Limited) of required amount was added with the starting solution to keep 2 at.% of Co in the starting solution. Similarly, for the synthesis of ZnO:Co/rGO nanocomposites, cobalt acetate tetrahydrate(2 at.%) + graphene oxide (5 wt.%) were added with the starting solution and NaBH4 was added as reducing agent. The same procedure adopted for the preparation of bare ZnO is followed for the preparation of ZnO:Co nanopowder and ZnO:Co/rGO nanocomposites also. The graphene oxide used in this synthesis was prepared using the well-known modified Hummer’s method [26]. Photographic images of (a) ZnO nanopowder (b) ZnO:Co nanopowder and (c)ZnO:Co/rGO nanocomposite synthesized in the present work are shown in Fig. 2. 2.2 Characterization techniques The crystalline structure of the powders were observed using powder diffraction method (PANalytical-PW 340/60 X’pert PRO) with Cu-Kα (1.5406 Å) radiation. The photoluminescence spectra of the powders were observed using spectro-fluorometer (Jobin Yvon-FLUROLOG-FL3-11) with xenon lamp (450W) as the excitation source of wavelength of 325 nm at room temperature. The optical absorbance measurements were

recorded using UV-vis-NIR double beam spectrophotometry (Perkin Elmer LAMBDA 35).The Fourier transform infrared (FTIR) spectra were recorded using PerkinElmer RX-I FTIR spectrophotometer. The surface morphology was studied using field emission electron microscope (FE-SEM) and transmission electron microscope (TEM). 2.3 Evaluation of photocatalytic performance of synthesized samples The photocatalytic activities of the synthesized samples were tested for the degradation of methylene blue (MB), an organic dye, using 300 W tungsten visible-lamp as light source. The temperature was kept constant (under room temperature) by making arrangements for circulating water in a jacket surrounding the photo reactor. 100 mL of MB dye solution of concentration 1×10-5M is taken in a beaker and 50 mg of the synthesized phtocatalyst was added with it and the resultant solution was stirred well. The absorption

spectrum

is

recorded

for

the

initial

solution

using

UV-vis-NIR

spectrophotometer before the start of the reaction process. Then the solution was taken in the reactor and the light source was turned on. During the reaction process, 5 mL of the sample solution was taken out at a time interval 20 min up to 80 min. The taken solution was centrifuged to remove the catalyst if any in the sample and optical absorbance was recorded for each time. The photodegradation of the dye was evaluated by measuring the absorbance values of the sample at the characteristic wavelength of 663 nm of MB. 2.4 Evaluation of antibacterial activity of the samples The antibacterial activities of the synthesized samples were tested against Escherichia coli (E.coli), a Gram-negative bacteria and Staphylococcus aureus (S.aureus), a Gram- positive bacteria by measuring the zones of inhibition using disc diffusion method. The above mentioned bacteria were grown individually. Nutrient agar medium was used for the bacterial growth which was poured onto the Petri plates. Fresh bacterial cultures of both organisms were swabbed on to the agar medium.

Standard paper disks

were impregnated with 200 µg/ml of stock solution of the samples and the discs were placed on the surface of the agar using sterilized forceps. These plates were incubated at

37ºC for 24 h. The antibacterial activity was evaluated by measuring the zone of inhibition. 3. Results and discussion 3.1. Structural studies The synthesized ZnO and ZnO:Co nanopowders, and ZnO:Co/rGO nanocomposite are subjected to the X-ray diffraction studies. The observed XRD patterns are shown in Fig. 3. The diffraction peaks recorded for all the three samples are related to the lattice planes (100), (002), (101), (102), (110), (103), (200), (112), (201) and (202), according to JCPDS no. 36-1451. It is found that the diffraction peaks are matched well with those of the standard data of hexagonal wurtzite structure of ZnO. The absence of any signature for secondary phases confirms that all the prepared samples have single phase. The diffraction peak intensities follow an order: (101)> (100)> (002)> (110)> (103)> (112) > (102)> (201)> (200), indicating that the crystal faces (101), (100) and (002) are dominant in the exposed faces of nanopowders. There are no peaks for rGO in the XRD pattern of ZnO:Co nanopowder, which is consistant with several reports [27-31]. Moreover, no secondary phases corresponding to cobalt are observed suggesting the proper incorporation of Co2+ions into the regular Zn2+ sites of the ZnO lattice. According to the Lattice Compatibility Theory (LCT), formulated by Boubaker and Petkova, [20] cobalt is one of the best dopants that can comfortably fit into the regular zinc sites of ZnO matrix. It is observed that the intensities of the prominent XRD peaks (101), (100) and (002) increase with cobalt doping which may be due to the compensation of zinc vacancies by the cobalt incorporation which can enhance the periodicity of the ZnO lattice. This enhancement in the intensities of the diffraction peaks suggests the proper incorporation Co2+ions into the Zn2+ sites of the ZnO lattice. However, the peak intensities of ZnO:Co/rGO nanocomposite are found to be suppressed compared to those of ZnO and ZnO:Co nanopowders. This reduction in diffraction intensities may be due to the hindrance to the stacking of crystalline layers and

the consequent degradation in the growth of the crystallites caused by rGO. In general, graphene derivatives resist the metal oxide particles to grow larger and facilitate the dispersed growth of the particles, thereby limiting the agglomeration [32] The mean crystallite size of the synthesized samples were calculated from the fullwidth at half-maximum intensities (FWHM) of the diffraction peaks using the well known Scherrer’s formula [33]:

where

radiation, θ is the Bragg’s angle and

is the FWHM.

(1.5406 A°) is the wavelength of Cu- Kα

The lattice parameters a and c are estimated from the relation [34,35]: (

)

Where d is the inter-planer distance and (hkl) are Miller indices. The volume of the unit cell (v), the volume of the crystallites (V) and the number of unit cells in a crystallite (Nu) are calculated using the following relation [36-38] √

V=D3 Nu = V/v The calculated values of the structural parameters are given in Table 1. The data clearly show that the lattice constants and the related structural parameters are not altered appreciably after Co doping, which may be considered as a good evidence for the above discussed compatibility between the ZnO lattice and the dopant Co. 3.2. Photoluminescence (PL) studies The photoluminescence spectrum of materials is widely used to investigate crystal defects and efficiency of charge carrier trapping, migration and transfer.

The

PL

spectra of synthesized samples are shown in Fig. 4. The emission spectra consist of three prominent UV-visible emission peaks centered at 396, 425 and 468 nm along with some other weak peaks. It is observed that the broad emission peak at UV region centered at 396nm is associated with the near band edge emission (NBE) of ZnO which results in due to the electronic transitions from the conduction band to the valence band [39]. The strongest peak at 425 nm corresponds to the zinc interstitial (Zni) [40]. The blue green emission peak observed at 468 nm is originated due to the singly ionized oxygen vacancies (vo+ ) [41].The intensity of this peak is suppressed significantly in the case of ZnO:Co/rGO nanocomposite as seen from Fig. 4. This suppression, as believed by us, may be due to the retardation of electron-hole recombination and the resultant increase in the lifetimes of the photo-generated charge carriers caused by the rGO [42].This hindrance offered by the rGO to the charge carrier recombination is the most significant factor that leads to an enhancement of the photocatalytic efficiency of the ZnO:Co/rGO nanocomposite which is discussed in detail in the photocatalysis studies (section 3.6).

3.3. Absorption studies The absorption edges of the UV-vis spectra of ZnO and ZnO:Co nanopowders, and ZnO:Co/rGO nanocomposite are observed (Fig.5) at 375, 378 and 380 nm, respectively, as can be seen from the plots drawn for the optical absorption as a function of wavelength. The red shift in the absorption edge clearly indicates the reduction in the band gap caused by the Co doping. The band gap energy (Eg) is calculated using the relation [43-45] : Eg = hc/λ where λ is the wavelength in nm. The calculated band gap for the bare ZnO is 3.30 eV. The band gap decreases to 3.28 eV and 3.26 eV for ZnO:Co nanopowder and ZnO:Co/rGO nanocomposite, respectively. The influence of this reduction in the band gap on the photocatalytic and antibacterial activities is discussed in the respective sections (section 3.6 and 3.7).

3.4. Surface morphological studies The surface morphologies of the synthesized samples are shown in the FESEM images (Fig. 6). The grains are mainly in the form of hexagonal nano bar structure. The size of the grains (diagonal of the hexagon) for ZnO nanopowders ranges from 60 to 90 nm. The grain size decreases to the range of 40 to 60 nm when Co is doped. When rGO is added along with Co, the size reduces further to 30 nm as observed from the images. This result is an expected one, because, as reported by several researchers, graphene related materials always hinder the growth of the grains of the co- existing components of the nanocomposite during the synthesis process. This reduction in grain size leads to a desirable increase in the exposed surface area of the grains. Therefore, the reactive contact area of the particles to interact with the pollutant molecules in the case of photocatalysis and interact with the cell walls of the microorganisms in the case of antibacterial activity increases. As a result, the efficiency of the ZnO:Co/rGO nanocomposite in

both the applications is found to be enhanced

considerably as observed in the respective studies. Fig. 7 shows the TEM micrographs of (a) bare ZnO (b) ZnO:Co (c) ZnO:Co/rGO nanostructures. The particle size values are in consistent with those observed from SEM images. It is observed that the particles are having hexagonal bar and hexagonal plate morphology which is consistent with the SEM measurement. A singled out image of a particle shown in Fig.d clearly depicts the hexagonal shape of the particles. The reduction in particle size with the addition of Co and Co-rGO can be observed from the TEM measurements which are also reflected in the SEM measurements. The presence of rGO as nano sheets beneath the ZnO:Co particles is obviously seen in Fig. 7 (c) even though it could not be identified from the XRD and

SEM analyses. The Co doped ZnO

nanoparticles decorated the rGO homogeneously throughout the two dimensional sheets. These distributed growth of Co doped ZnO nanoparticles on the rGO sheets is one of the main causes for the enhanced photocatalytic and antibacterial efficiency of the ZnO:Co/rGO nano composites.

The elemental composition of the materials is analysed using energy dispersive Xray analysis (EDAX). The EDAX spectra of the and ZnO:Co/rGO nanocomposite are shown in Fig.8(a and b) .The EDAX profiles confirm the presence of the expected elements Zn, O, C and Co in the respective samples. The elemental proportions in terms of atomic percentage are presented as insets in the EDAX spectra in Fig. 8(a and b). 3.5. FTIR studies The FTIR spectra were used to analyze the functional group of the materials. FTIR spectra of bare ZnO and ZnO:Co nanopowders and ZnO:Co/rGO nanocomposite recorded in the range of 400-4000 cm-1 are shown in Fig. 9. The peak appears in the range of ~ 450 cm-1 can be assigned to the stretching vibrations of Zn–O [32, 46]. The bands at ~1396 cm-1correspond to the C-OH group of vibrations [47].The absorption band in the region of 3400-3450 cm-1is attributed to the hydroxyl stretching mode of O-H[48],which confirms the presence of hydroxyl group that is mainly responsible for the photocatalytic activity of synthesized samples. 3.6. Photocatalytic activity The photocatalytic activities of the synthesized nanopowders and nanocomposite were investigated through the photo degradation of a representative organic dye, methylene blue (MB), under visible- light irradiation. Its absorption peak is in the visible range and its degradation can be easily monitored by observing the variation in the intensity of the absorption peak through optical absorption spectroscopy.

Fig.10. shows

the absorption spectra of MB dye without the catalyst. The figure clearly shows that no noticeable reduction in the absorbance is observed in the absence of a catalyst. Fig. 11. a, b and c show the UV-visible absorption spectra of MB recorded at the irradiation intervals of 20 min with the presence of bare ZnO and ZnO:Co nanopowders, and ZnO:Co:rGO nanocomposite, respectively. From the spectra, it is found that the intensity of the absorption peak of MB appear at 663 nm decreases gradually with the increase in the irradiation time, indicating that the concentration of MB in the solution reduces gradually.

The spectra clearly depict that ZnO:Co/rGO nanocomposite sample exhibits superior photocatalytic activity compared to the bare ZnO and ZnO:Co nanopowders. The possible delay in the recombination of photogenerated electrons and holes by the transfer of electrons from ZnO to RGO and the consequent generation of additional reactive oxidative species (ROS) like superoxide anions and hydroxyl radicals may be the reason for this enhanced photocatalytic efficiency of ZnO:Co:rGO nanopowder. These ROS react with the organic pollutants and as a result, the organic molecules are decomposed into CO 2 and H2O [49,50] as elaborated in the following reaction mechanism and the possible corresponding sequential reactions.

The reaction mechanism for the photocatalytic

decolorization of the MB solution in the presence of the synthesized photocatalytic material is illustrated in the schematic diagram shown in Fig. 12. As the valence band and conduction band energies of ZnO are -7.25 and -4.05 eV [16], respectively, when a photon of energy greater than or atleast equal to the band gap (3.2 eV) is incident on the ZnO system, an electron in the valence band absorbs the photon energy and transits into the conduction band creating a hole in the valence band. This reaction for the generation of electron - hole pair can be written as ZnO+hν → ZnO (

) + ZnO (

)

The separated electrons and holes involve themselves in the following reactions which lead to the generation of super oxide anions ( ZnO (

)+

ZnO (

)+

→

+ ZnO (

→ ZnO (

) and hydroxyl radicals (

).

)

)

The consecutive reactions take place in the photodegradation process of MB can be represented as follows: ZnO(

)+ →

→ +

(hydrogen peroxide)

(

)→(

+ MB→ C + MB→ C

)+(

)

+ +

Thus, eventually, the methylene blue molecule is decomposed into CO2 and H2O by photocatalysis. The photocatalytic degradation efficiencies of the synthesized samples in the present work were calculated from the formula [51- 53] ; η= (C0-C)/C0× 100 = (A0-A)/A0× 100 % where C0 and C are initial concentration of the dye and the concentration after the irradiation time t, respectively and, A0 and A are the corresponding absorbance values. The calculated degradation efficiency values are presented in Table 2. From the Table.2, it is noticed that the degradation efficiency of bare ZnO is 49 % after 80 min of photocatalysis process. The efficiency is 65% for ZnO:Co and 86% for ZnO:Co/rGO nanocomposite. It is obvious from the Fig.13, that at all degradation times, ZnO:Co/rGO nanocomposite and ZnO:Co nanopowder show high degradation efficiency compared to the bare ZnO nanopowder. The plots drawn between log C/C0 and degradation time (Fig.14) depict that relatively good linear relationship is obtained (Fig. 15) indicating that all the reactions obey pseudo- first- order kinetics [54],represented by the equation ln(C0/C) = kt where k is the reaction rate constant and t is the reaction time. The reaction rate constants were estimated as 0.00564 min -1, 0.01390 min-1 and 0.0214 min-1 for undoped ZnO and ZnO:Co nanopowders, and ZnO:Co/rGO nanocomposite, respectively . All these results clearly show that the photocatalytic efficiency increases after Co addition and enhances further with the addition of rGO. We believe that the improvement in the photocatalytic efficiency of ZnO:Co is mainly due to the decrease in the band energy gap of ZnO, caused by the sp-d exchange interaction between the band electrons of ZnO and the localized electrons of the substituted Co2+ ions [55,56]. It is a known fact that the reduction in the band gap of a

photocatalytic material can extend the utilization range of solar energy into the visible region. The release Zn2+ ions caused by the Co doping is another important reason for the enhancement of photocatalytic efficiency of ZnO:Co nanoparticles. Furthermore, the possible factors which are responsible for the observed remarkable enhancement in the photocatalytic efficiency of ZnO:Co/rGO nanocomposite are: i) The important characteristic feature of rGO by which it spreads as sheet beneath the ZnO particles which enables the particles to provide large reactive surface area to interact with the pollutant molecules [57] ii) The higher adsorption ability of rGO [58,59] iii) The ability of rGO to hinder the recombination of photogenerated electron-hole pairs by transferring the photo generated electrons from the conduction band of ZnO to the rGO sheet. The various possible factors that determine the photocatalytic and antibacterial activities of all the three samples are presented in Table 3. During the synthesis of ZnO:Co/rGO, the rGO which is in the form of ultra thin sheets, allow the ZnO particles to grow on it in a distributed manner, so that they cannot agglomerate among themselves [60]. As the ZnO particles are spread on the rGO sheets, the effective reactive surface area of the particles increases [61]. As a result, the degradation rate of the pollutant molecules increases and hence the photocatalytic efficiency enhances. This claimed spreading of ZnO nanoparticles on the rGO surface in the present work is clearly evident in the SEM and TEM images shown in Fig. 6 and 7. The higher adsorption ability of rGO is another important factor which can also determine the enhancement of the photocatalytic efficiency. The rGO surface can readily adsorb the pollutant molecules thereby facilitate the easy photo degradation of the molecules [62].

Moreover, as the work function of rGO is - 4.42 eV, it favors the

transfer of photoelectrons from the conduction band of ZnO and consequently inhibits the recombination of photo-generated electron–hole pairs. Graphene related material has been reported to be good acceptors due to the unique 2D π-conjugation structure. It is worth mentioning here that the prevention of photo-generated electro-hole pair is the key factor for the improvement of photo degradation/ de-colorization of organic dyes. As reported in

the literature, the electron transport ability of rGO has been proved by measuring the photocurrent generation in bare TiO2 and TiO2/rGO nanocomposite. It has been observed that the photocurrent increases nearly 6 times after the introduction of rGO beneath the semiconductor nanoparticles [63]. In addition to the above mentioned factors, it is believed that the oxygen vacancy defects which can facilitate the light absorption ability of metal oxide [64,65] and surface hydroxyl groups also cause an improvement in the photocatalytic performance of the prepared samples, which may be the reason for the photocatalytic activity of bare ZnO. In the present work, the presence of oxygen vacancies and hydroxyl groups is confirmed by the PL and FTIR analyses, respectively. The photocatalytic activity of Co and graphene derivatives activated ZnO nanostructures reported in terms of degradation efficiency and rate constant are presented in table 4. As the different research groups adopt different methods of synthesis, process parameters, type of dye, irradiation source and irradiation intervals, it is difficult to compare the reported experimental results directly with ours. However, it is obvious that the results obtained in the present work are seemed to be very much comparable or even better than many of the reported counterparts. 3.7 Antibacterial activity The antibacterial activities of the prepared samples were tested against S. aureus and E. coli bacteria which are generally considered as standard test strains for gram positive and gram negative bacteria, respectively. Disc diffusion method is adopted for the antibacterial studies and the resultant zone of inhibition obtained for the samples are shown in Fig .16.The results obtained in the test are listed in Table 5. The zone of inhibition around a test sample indicates the area within which the antibacterial activity of the sample is effective which prevents the bacterial growth in this area. The zone of inhibition was estimated by measuring the mean diameter around the test sample in mm. The zone of inhibition for bare ZnO against S. aureus and E. coli bacteria are 21 and 20

mm, respectively. From the Table 5, we can see that the zone diameter increases due to Co doping and increases further with rGO incorporation. In other words, ZnO:Co/rGO nanocomposite exhibits superior bactericidal activity against both gram positive and gram negative bacteria compared to Co doped and bare ZnO. Several mechanisms are proposed in the literature for the antibacterial activity of semiconductor materials and are supported with the obtained experimental results and convincing interpretations. The most discussed mechanism that determines the efficiency of antibacterial materials is the oxidative stress induced by reactive oxygen species (ROS) which are generated by photogenerated electron hole pairs [74,75]. The ROS include highly reactive chemical agents like hydroxyl radicals, super oxide anion and hydrogen peroxide. These chemical agents can react with bacterial cell membranes and consequently cause damage to nucleic acids which eventually leads to cell death. We believe that in the present study, in addition to the ROS, the Zn2+ ions that are released from the ZnO samples also cause damage to bacterial cells [76]. These Zn2+ ions are adsorbed on the surface of the bacterial cell membranes or penetrate through the membrane and directly interact with functional groups of proteins and nucleic acids. This interaction disrupts enzyme activity, disorganizes the structure of cells and consequently affects the bacterial growth. Even though this mechanism is valid for bare ZnO sample itself, it should be more pronounced when Co is doped. This is because of the fact that when Co is doped with ZnO, Co2+ions can readily substitute Zn2+ions as this dopant is compatible with the host ZnO lattice and thereby facilitates the release of Zn2+ ions from the ZnO. This compatibility between the host ZnO and dopant Co can be understood from the lattice compatibility theory (LCT) mentioned in the structural studies (section 3.1). This proper substitution of Co into the ZnO lattice and the resultant release of more number of Zn2+ions may be one of the reasons for the enhanced antibacterial efficacy of ZnO:Co nanopowders and ZnO:Co/rGO nanocomposites. The reduction in grain size due to the rGO incorporation may be another strong reason for the observed further enhancement in the antibacterial activity of the

ZnO:Co/rGO nanocomposites, as this reduction in grain size increases the effective reactive surface area as mentioned in the surface morphological studies. The narrowing of band gap as established in section 3.3 also facilitates enhanced charge carrier transitions from the valence band to the conduction band. Therefore more electron hole pairs are created which is also responsible for the pronounced antibacterial activity of Co doped ZnO naopowders and ZnO:Co/rGO nanocomposites. The prevention of recombination of electron hole pairs by the rGO sheets is one of the most significant advantages of using rGO as one of components of the antibacterial nanocomposites, as this retardation of recombination can improve the antibacterial activity to a greater extent. The various factors that influence the antibacterial efficacy of the materials studied in this work are listed in Table 3. 4. Conclusions ZnO:Co/rGO nanocomposite synthesized using a cost-effective and simple soft chemical method exhibits enhanced photocatalytic as well as antibacterial activities compared to bare ZnO and ZnO:Co nanopowders. The rate constant of ZnO:Co/rGO nanocomposite is 0.0214, which is nearly four times greater than that of bare ZnO nanopowder. Similarly, the ZnO:Co/rGO nanocomposite shows improved antibacterial activity against both the gram-positive(S.aureus) and Gram-negative (E.coli) bacteria tested in this study. The results obtained from the analytical studies viz. XRD, FESEM, TEM, PL, FTIR and UV-vis absorption correlate well with the outcome of the photocatalytic and antibacterial studies. From the analyses, it is found that the experimental results convincingly support the proposed mechanisms related to the photocatalytic and antibacterial activities of the synthesized ZnO:Co/rGO nanocomposite. The synergetic effect of the following four factors makes the synthesized ZnO:Co/rGO nanocomposite a good photocatalytic and antibacterial agent simultaneously.

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Fig. 1 Schematic diagram of various steps involved in the synthesis of ZnO, ZnO:Co and ZnO:CO/rGO nano materials. Fig. 2 Photographic images of (a) ZnO nanopowder (b) ZnO:Co nanopowder and (c)ZnO:Co/rGO nanocomposite synthesized in the present work. Fig. 3 XRD patterns of bare ZnO, ZnO: Co nanopowders and ZnO:Co/rGO nanocomposite. Fig. 4 Photoluminescence spectra of bare ZnO and ZnO:Co nanopowders, and ZnO:Co/rGO nanocomposite. Fig. 5 UV absorbance spectra of bare ZnO, ZnO:Co

nanopowders and ZnO:Co/rGO

nanocomposite. Fig. 6 SEM images of synthesized samples. (Inset in the third image is an enlarged portion showing a ZnO:Co nanoparticle grown on rGO nanosheet). Fig. 7 TEM images of a) ZnO, b)ZnO:Co and c)ZnO:Co/rGO d)image showing a particle having hexagonal plate shape.

Fig. 8 (a) EDAX result of ZnO:Co nanopowder and (b)EDAX result of ZnO:Co/rGO nanocomposite. Fig.9 FTIR spectra of ZnO, ZnO:Co, ZnO:Co/rGO samples. Fig.10 Absorbance spectra of MB dye without the catalyst. Fig.11. Absorbance spectra of Methylene Blue (MB) dye solution containing (b) bare ZnO nanopowder, (c) ZnO:Co nanopowder, (d) ZnO:Co/rGO nanocomposite. Fig. 12. Schematic diagram depicting various mechanisms involved in the photocatalytic activity of ZnO, ZnO:Co, and ZnO:Co/rGO sample. Fig.13 Comparison of degradation efficiency of bare ZnO and ZnO:Co nanopowders, and ZnO:Co/rGO nanocomposite. Fig.14 Plots between C/C0 vs degradation time for bare

ZnO, ZnO:Co nanopowder

and ZnO:Co/rGO nanocomposite. Fig.15 Plots of ln (Co/C) vs irradiation time for

bare ZnO and ZnO:Co nanopowders,

and ZnO:Co/rGO nanocomposite. Fig. 16 Inhibition zones of bare ZnO,

ZnO:Co nanopowder and ZnO:Co/rGO

nanocomposite against S. aureus and E. Coli. Table 1 Structural parameters of the synthesized samples. Sample

D(nm)

Lattice constants a(Å)

c(Å)

c/a

ν(Å)3

V X103

Nu

(nm)3

X106

ZnO

30.5

3.2521

5.2098

1.601

47.72

28.364

0.59

ZnO:Co

28.5

3.2553

5.2148

1.601

47.86

23.132

0.48

ZnO:Co

27.4

3.2626

5.2247

1.601

48.16

20.765

0.43

/rGO Standard values: a=3.2498 Å, c= 5.2066 Å, c/a=1.6021

Table 2 photocatalytic degradation efficiencies of the synthesized samples against MB. Catalyst

Degradation efficiency (%) 20 min

40 min

60 min

80 min

ZnO

8.19

10.57

26.04

49.47

ZnO:Co

12.44

34.79

51.61

65.22

ZnO:Co/rGO

32.85

53.61

68.84

86.17

Table 3 Possible factors that determine the photocatalytic and antibacterial activities of the ZnO nanopowder, ZnO:Co nanopowder and ZnO:Co/rGO nanocomposite. Material

Causes for photocatalytic activity

Causes for antibacterial activity

studied i. Photo generated electron - hole ZnO

pairs →ROS generation

i. Release of Zn 2+ ions ii. Photo generated electron - hole pair →ROS generation

ZnO:Co

i. shrinkage of Eg→ increased generation of electrons-hole pairs

i. Release of more number of Zn2+ due to Co2+ substitution ii. shrinkage of Eg→ increased generation of electrons-hole pairs

ZnO:Co/ rGO

i. shrinkage of Eg→ increased

i. Release of more number of Zn2+

generation of electrons-hole pairs

due to Co2+ substitution

ii. Adsorption of pollutants iii. Dispersed growth of particles

ii. shrinkage of Eg→ increased

→enhanced reactive surface area

generation of electrons-hole pairs

iv. Retardation of recombination of electron –hole pairs.

iii. Adsorption of micro organisms iv. Reduced particle size.

Table 4 Photocatalytic efficiency values of Co and graphene activated ZnO nanostructures reported by several researchers. Material

Method

ZnO:Co

Source

Efficiency

hydrothermal Ar (alizarin) hydrothermal MO Soft MB chemical reduction MB oxidation route Wet RhB chemical method solvothermal MB

visible light UV UV light

93% (60 min)

[16]

78% (240 min) K=0.006 min-1

[18] [66]

visible light

95.4% (8h)

[67]

UV light Visible light visible light

100% (60 min) 100% (38 h)

[68]

[69]

ZnO/G

electro chemical

MB

UV light

pure ZnO (58%). ZnO/G 100% (90 min) ZnO 60% (3h) ZnO/G 100% (3h)

GO/ZnO

Hydrothemal MB

UV-LED light

98% (140 min)

[71]

ZnO/rGO

Microwave assisted method

MB

UV light

91.7% (260 min)

[72]

ZnOnano

Microwave

MG

UV light

ZnO-49%(90

[73]

ZnO:Co ZnO, ZnO:Co ZnO :Co

ZnO:Co

ZnO/graphene Composites

Dye

References

[70]

pyramids/rGO sheet

irradiation

min) ZnORGO78%(90min)

Table 5 Zone of inhibition caused by ZnO, ZnO:Co, ZnO:Co/rGO nanocomposite against the tested bacteria. S. No.

Zone of Inhibition (diameter in mm ) Bacteria

ZnO

ZnO: Co

ZnO:Co /rGO

1

Escherichia coli

21

22

24

2

Staphylococcus aureus

20

23

26

Fig. 1

Fig. 2

Fig. 3

Fig. 4

Fig. 5

Fig. 6

Fig. 7

Fig. 8 (a)

Fig. 8 (b)

Fig. 9

Fig. 10

Fig. 11

Fig. 12

Fig.13

Fig.14

Fig.15

Fig. 16