Antibacterial, magnetic, optical and humidity sensor studies of β-CoMoO4 - Co3O4 nanocomposites and its synthesis and characterization

Accepted Manuscript Antibacterial, magnetic, optical and humidity sensor studies of β-CoMoO4 - Co3O4 nanocomposites and its synthesis and characterization

A. Mobeen Amanulla, S.K. Jasmine Shahina, R. Sundaram, C. Maria Magdalane, K. Kaviyarasu, Douglas Letsholathebe, S.B. Mohamed, J. Kennedy, M. Maaza PII: DOI: Reference:

S1011-1344(18)30364-6 doi:10.1016/j.jphotobiol.2018.04.034 JPB 11217

To appear in:

Journal of Photochemistry & Photobiology, B: Biology

Received date: Revised date: Accepted date:

5 April 2018 17 April 2018 17 April 2018

Please cite this article as: A. Mobeen Amanulla, S.K. Jasmine Shahina, R. Sundaram, C. Maria Magdalane, K. Kaviyarasu, Douglas Letsholathebe, S.B. Mohamed, J. Kennedy, M. Maaza , Antibacterial, magnetic, optical and humidity sensor studies of β-CoMoO4 - Co3O4 nanocomposites and its synthesis and characterization. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Jpb(2018), doi:10.1016/j.jphotobiol.2018.04.034

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ACCEPTED MANUSCRIPT Antibacterial, Magnetic, Optical and Humidity sensor studies of β-CoMoO4 - Co3 O4 nanocomposites and its synthesis and characterization A. Mobeen Amanulla1 , S.K. Jasmine Shahina2 , R. Sundaram1,* , C. Maria Magdalane3 , K. Kaviyarasu4,5,* , Douglas Letsholathebe6 , S.B. Mohamed7 , J. Kennedy4,8 , M. Maaza4,5

Department of Chemistry, Presidency College (Autonomous), Chennai 600 005, India

2

Department of Microbiology, J.B.A.S College for women (Autonomous), Chennai 600018,

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1

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India

Department of Chemistry, St. Xavier's College (Autonomous), Tirunelveli - 627002, India

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UNESCO-UNISA Africa Chair in Nanosciences/Nanotechnology Laboratories, College of

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3

Graduate Studies, University of South Africa (UNISA), Muckleneuk Ridge, P O Box 392, 5

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Pretoria, South Africa

Nanosciences African network (NANOAFNET), Materials Research Department (MRD),

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iThemba LABS-National Research Foundation (NRF), 1 Old Faure Road, 7129, P O Box 722, Somerset West, Western Cape Province, South Africa Department of Physics, University of Botswana, Private Bag 0022, Gaborone, Botswana

7

Department of Materials Science, Central University of Tamil Nadu, Neelakudi, Thiruvarur -

610 005, Tamil Nadu, India

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National Isotope Centre, GNS Science, Lower Hutt, New Zealand

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Corresponding author: [email protected] ; [email protected]

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ACCEPTED MANUSCRIPT Abstract

Cobalt Molybdate (β-CoMoO4 ) and Cobalt Oxide (Co3 O4 ) nanocomposite was prepared via co-precipitation and solid-state methods. Various techniques like powder XRD, FESEM, HRTEM, FTIR, VSM, UV-Vis and PL spectroscopy were used to investigate the structure and morphology of as prepared samples. Powder X-ray diffraction (XRD) reveals monoclinic

observed

using

field

emission

electron

microscopy

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and cubic structure for β-CoMoO 4 and Co3 O4 respectively. The surface morphology was (FESEM)

and

high-resolution

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transmission electron microscopy (HRTEM), which shows the formation of nanocomposites at nanoscale range, the presence of elements were determined by energy dispersive x-ray

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spectroscopy (EDX). FTIR analysis confirms the formation and bonding nature of the samples. The anti-ferromagnetic behavior of CMCO64 composite was determined by

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vibrating sample magnetometer (VSM). The bandgap values were calculated by extrapolating the straight line on the energy axis (hν), and the values of β-CoMoO 4, CO3 O 4 and β-CoMoO 4 -

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CO3 O4 composites were determined to be 2.20, 2.09 eV and 1.54 - 2.44 eV respectively. The weak blue emission peak observed at 489 nm is corresponds to crystal defects only observed in CMCO01 and CMCO64 composite, for CMCO10 the peak shifted to green region.

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Antibacterial studies illustrate good result for the CMCO64 composite against both Gramnegative and Gram-positive bacteria. The sensor studies were measured at different humidity

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environment (RH5% to RH98%). It was found that the increase in relative humidity leads to increase in the sensitivity factor of the samples. Among the samples CMCO64 composite possess highest sensitivity factor of (S f = 4851) with response time of 60 s and recovery time

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of 230 s respectively.

Keywords: Cobalt Molybdate; Cobalt Oxide; Antibacterial activity; Sensitivity factor

ACCEPTED MANUSCRIPT 1. Introduction

Inorganic nanomaterials such as semiconductors and magnetic materials possess growing interest due to its exclusive physical and chemical properties. Nanomaterials at nanoscale range enrich the microbial activity owing to their large surface area than volume ratio [1]. The new aspect of nanotechnology is its ability to develop new anti-bacterial agent through

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the synthesis of NPs [2]. The inorganic antibacterial agents exhibit long self-life and good stability at high pressure and temperature when compared to organic antibacterial agents [3].

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The sizes of nanomaterials are smaller than the bacterial species, which encourage the NPs to easily enter the bacterial cell and restrict its growth [4]. Magnetic nanoparticles (MNs) paid

supercapacitors

[7],

and

sensors

[8].

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much attention owed to its huge applications like magnetic storage devices [5], catalysis [6], Combination

of

weak

ferromagnetic

and

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antiferromagnetic behaviour of NPs gained much attention due to the existence of the multifunctional properties, which makes them potential applicant in wide range of application

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such as energy storage devices [7], lithium batteries [9] and magnetic tunnel devices [10]. Humidity sensors not only used to measure humidity environment, however it is also used in automobile industries [11], weather stations [12], and industrial applications [13] etc. The

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preferred characteristic of sensing humidity is easy for fabrication, high sensitivity, good response time and good thermal stability [14]. Transition metal molybdates (AMoO 4 , A=Co,

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Ni, Mn etc.) found to be important semi-conducting materials due to its favourable properties for instance low cost, high electrical and thermal stability, eco-friendly and abundant resources [15-17]. This properties of metal molybdates raise them in wide spread applications

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for example symmetric super capacitors [16], supercapacitors [17], pigments [18] etc. Among the molybdates, cobalt molybdate CoMoO 4 has paid great interest due to its outstanding catalytic [15], hybrid capacitor applications [19] and lithium batteries [20].

Recently, nanostructured metal oxides (ZnO, Co3 O4, MnO, MoO3 etc.) paid much attention in sensor applications owing to their extraordinary properties such as low cost, good compatibility and semi-conductivity [4, 5, 17, 18]. Particularly cobalt oxide Co3 O4 is a p-type antiferromagnetic semiconductor, it exhibits electronic, electro-catalysis, bio-sensing and gas sensing ability [8, 9, 11, 21]. The presence of normal spinel structure enhances the transfer of electrons between Co2+ and Co3+. Nowadays, Co3 O4 and Co3 O4 based nano composites have great potential application in biosensors [21], electrical and dielectric properties [22],

ACCEPTED MANUSCRIPT supercapacitors [23], detection of biological toxicity [24], oxygen reduction electro catalysis [25]. To prepare CoMoO 4 and Co3 O4 different approaches were adopted they are simple precipitation method [26], combustion method [27], hydrothermal synthesis method [8], coprecipitation method [28] and so on. Among these methods co-precipitation is one of the best method to prepare the samples because it yields high product with uniform distribution of

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chemical combustion, easy to control, low calcination temperature and eco- friendly.

However, the related literature reports focused on the incorporation of Co 3 O4 with other

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metals, metal oxides and polymers only. To the best of our knowledge, the incorporation of Co3 O4 with metal molybdates for the application of humidity sensor and antibacterial activity

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has not yet reported. The aim of the present study is to prepare β- CoMoO 4 , Co3 O4 and its composites β-CoMoO 4 - Co3 O4 at different mole ratio through co-precipitation and solid-state

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method followed by characterization techniques. The foremost objective of this study is to find the structure, morphology, optical, humidity sensor and anti-bacterial studies of as

2. Experimental

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2.1. Materials and methods

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prepared β-CoMoO 4 - Co3 O4 composites.

The chemicals used to prepare the pure β-CoMoO 4 , CO3 O4 were analytical grade and used without further purification. Cobalt acetate [Co(C2 H3 O2 )2 ], Ammonium hepta molybdate tetra

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hydrate [(NH4 )6 Mo6 O27 .4H2 O], Ammonium hydroxide [NH4 OH], cobaltous nitrate [Co (NO 3 )2 .6H2 O], ethanol [C2 H2 OH] and deionized water was used for all the experiments. 2.2. Preparation of samples

In a typical procedure 2M of [(NH4 )6 Mo6 O27 .4H2 O] and 2M of [Co(C2 H3 O 2 )2 ] was dissolved separately in 20 mL of distilled water and the solutions were mixed together under magnetic stirring for 30 minutes to form homogeneous mixture (pH=9). The obtained precipitate was washed with ethanol and deionized water to remove impurities then dried, cooled subsequently the obtained purple colour powder was calcined at 250 o C for 3h. To synthesis Co3 O4 NPs, 0.1M of [Co (NO 3 )2 .6H2 O] was dissolved in 100 mL of deionized water and 10%

ACCEPTED MANUSCRIPT of glycerol was added under vigorous stirring at 50 o C for 20 minutes after that 10 mL of 1M ammonium hydroxide was added drop wise as well as stirring was continued for 2 h. The obtained precipitate was washed several times to remove impurities and calcined at 500 o C for 2h. The β-CoMoO 4 - CO3 O4 composites were prepare via solid state method, as prepared pure samples of β-CoMoO 4 , CO3 O4 was assorted at different mole ratio (80:20, 60:40, 40:60, 20:80) using ethanol as binding agent by grinding process for 2 h and calcined at 500 o C for

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4h. The mole ratio of the composites and its respective band energy is shown in Table.1.

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2.3. Characterization Studies

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The above synthesized β-CoMoO 4 - CO3 O4 composites were structurally and morphologically characterized using various techniques such as Powder X-ray diffraction analysis was

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performed at room temperature through PAN alytical X’PERT-PRO X-ray diffractometer operated at 40 kV and 30 mA with CuKα radiation (λ=1.5406 Å) at the range of 2 = 10⁰ -

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70⁰ . Field emission scanning electron microscopy (FESEM, Hitachi S-3400) was recorded to analyse the morphology of the samples. The elemental composition was evaluated by energy dispersive X-ray spectroscopy (EDX). The magnetic properties of the sample were

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examined using vibrating sample magnetometer (VSM) under an applied field sweeping between ±15,000 Oe at room temperature. The Fourier transform infra-red spectrum (FTIR)

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was measured using KBr as standard reference in the range 500 - 4000 cm-1 . UV-Visible spectroscopy was detected at the range 200-800 nm and the emission spectra were recorded at

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the range 350-600 nm using Cary Eclipseversion1.1 (132) instrument.

2.4 Antimicrobial analysis

Agar well diffusion method was used to investigate the antimicrobial activity of CMCO64 composite against the three bacterial strains Escherichia coli, Pseudomonous aeruginosa and Staphylococcus aureus. The stock bacterial cultures were stored at 4 o C afterward the active cultures were prepared by transferring a loop full of culture to nutrient broth tube incubated at 37 o C for 24 hrs. Mac Farland standard 0.5 was used to match the turbidity. Muller-Hinton agar was prepared and allowed to solidify. By using sterile cotton swab the lawns were prepared and the wells were made with the help of sterile cork borer (6 mm). The sample

ACCEPTED MANUSCRIPT extracts at different concentrations were poured into the well. The diameter of zone of inhibition was measured around the well which was incubated aerobically at 37 o C for 24 hrs.

2.5 Humidity sensor measurement

The sensor element made up of fabricated pellet (10 mm diameter and 1 mm thickness) fixed

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with copper wire on both sides used to measure the humidity subsequently the samples were dispensed by dissolving in ethanol then dried at room temperature. This fabricated pellet was

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placed in different desiccators under different humidity environments such as CH3 COOK potassium acetate (20%), calcium chloride hexa hydrate [CaCl2 .6H2 O] (31%), zinc nitrate [NH4 Cl]

(66%),

barium chloride

dehydrate

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hexahydrate [Zn(NO 3 )2 .6H2 O] (42%), sodium nitrite [NaNO 2 ] (51%), ammonium chloride [BaCl2 .2H2 O]

(79%),

copper sulphate

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pentahydrate [CuSO 4 .5H2 O] (98%) and anhydrous phosphorous pent oxide [P2 O5 ] (5%) under room temperature. The humidity was measured by change in dc resistance with relative

3. Results and discussion

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humidity using bargio hygrometer.

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3.1 Crystalline and phase identification analysis Fig. 1 shows Powder XRD pattern of the pure β-CoMoO 4 , CO3 O4 and β-CoMoO 4 - CO3 O4

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composites at different mole ratio were to analyse the crystal structure and phase purity. All the diffraction peaks shown in Fig. 1(a) are exactly indexed to monoclinic phase of βCoMoO 4 having cell parameters a=10.21Å, b=9.268Å, c=7.022Å and β=106.9⁰ belong to c2/m space group which is good agreement with (JCPDS card no. 021-0868) and the reported values [26, 19]. The spectrum exhibits the peaks located at 2 = 23.27⁰ , 25.47⁰ , 26.49⁰ , 28.49⁰ and 33.69⁰ are corresponds to (021), (201), (002), ( 11) and ( 22) planes with d space value of 3.82Å, 3.49Å, 3.36Å, 3.13Å and 2.65Å respectively. The formation of βCoMoO 4 raised over dehydration of samples at calcination process [29] and no other peaks corresponds to α-CoMoO 4 was found in XRD pattern, indicating the purity of β -CoMoO 4 . Fig. 1(b) reveals the diffraction peaks at 2=31.28⁰ , 36.85⁰ , 44.84⁰ , 59.46⁰ and 65.26⁰ are indexed the planes of (220), (311), (400), (511) and (440) with d space values of 2.85Å,

ACCEPTED MANUSCRIPT 2.43Å, 2.02Å, 1.55Å and 1.42Å and Fd m (227) space group of cubic C O3 O4 . It is well matched with the (JCPDS card no.42-1467) and reported value [30] and found no other oxides of cobalt. The crystallite sizes of the samples were calculated by Scherer’s formula which is given as D= 0.9λ/βhklcos; Where, D = crystallite size, β = full width half maximum (FWHM),  = diffraction angle and λ=X-ray wavelength [31]. The calculated crystallite size values are 20.3 nm and 33.81 nm for most predominant peaks at 26.49⁰ (002) and 36.85⁰

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(311) planes of β-CoMoO 4 and CO3 O4 respectively. In the case of β-CoMoO 4 - CO3 O4 nanocomposites the presence of peaks are completely depends on the ratio of both β-CoMoO 4

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and CO3 O4 as shown in Fig. 1(c-f). It was observed when increasing the ratio of C O3 O 4 the diffraction peaks situated at 26.49⁰ of β-CoMoO 4 was reduced gradually and leads to

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increase the characteristic peak of C O3 O4 at 36.85⁰ . Finally, the presence of both peaks of β-

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CoMoO 4 and CO3 O4 are evident for the formation of the β-CoMoO 4 - CO3 O 4 nanocomposites.

Fig. 1. XRD pattern of the pure β-CoMoO 4 , CO3 O4 and β-CoMoO 4 - CO3 O4 composites

ACCEPTED MANUSCRIPT 3.2 Functional group analysis FTIR spectroscopy was attained to characterize the bonding nature of the β – CoMoO 4 – CO3 O4 composites at different mole ratio as shown in Fig. 2(a-d). The bands around 3317– 3375 cm-1 and 1629-1737 cm-1 are ascribed to stretching and bending vibrations of -OH group present on the surface of the composites. The bands located at 931-943 cm-1 are

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attributed to the stretching vibrations of t-MoO4 group present in CoMoO 4 . The bands near 783-852 cm-1 are assigned to Mo-O [32]. The bands around the interval of 543-559 cm-1 and

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651-663 cm-1 are ascribed to the stretching vibration of Co 3+-O in an octahedral position and Co2+ -O in a tetrahedral position of Co3 O4 [30]. From the observed data the formation of the β

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– CoMoO 4 – CO3 O4 composites are confirmed by the existence of the two bands of Co 3 O4 on

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the surface of β – CoMoO 4 .

Fig. 2. FTIR spectrum of β-CoMoO 4 - CO3 O 4 composites

ACCEPTED MANUSCRIPT 3.3 Morphology analysis

FESEM analysis was used to identify the size and morphology of the CMCO64 nanocomposites as shown in Fig. 3(a), which illustrates the nanorod structure and the size of the nanorods were found to be less than 500 nm. Rico et al., observed similar morphology for CoMoO 4 and reported that the calcinated power was purple in colour, the width and length of

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the rods were around 100 nm and 1-5 m respectively [33]. We observed the existence of sphere like structure on the surface of the nanorods which may be the presence of CO3 O4

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shown in Fig. 3(a) inset (b). The red dotted line indicates the development of pores, which enhances the sensitivity of the CMCO64 composite. The elemental composition of the β-

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CoMoO 4 - CO3 O4 nanocomposites was determined using EDX analysis shown in Fig. 3(b), it reveals the presence of Co, Mo and O. The absence of other peaks concludes the purity of the

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CMCO64 composite. Fig. 3(c) insert reveals that the percentage of the elements present in the

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CMCO64 composite is in the order of Co > Mo > O.

Fig. 3(a) FESEM images of CMCO64 composite, (b) EDX data of the sample, the percentage of elemental composition shown in inset Fig. 3(c)

ACCEPTED MANUSCRIPT 3.4 HRTEM analysis

The microstructure and morphology of the CMCO64 nanocomposites was further analysed by HRTEM as shown in Fig. 4. The images clearly displays that the composite consist of both nanorod structure for β-CoMoO 4 and sphere like structure for Co3 O4 at 20 nm Fig. 4(b & c). The corresponding selected area electron diffraction (SAED) of both β-CoMoO 4 and

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Co3 O4 taken from the single crystals of composite indicates that the obtained nanocrystals are monoclinic phase for β-CoMoO4 and cubic phase for Co3 O4 as shown in Fig. 4(d-e). The

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HRTEM lattice fringes (Fig. 3a upper and lower) shows the inter planner spacing of d=3.363Å from the plane (002) of monoclinic phase of β-CoMoO 4 as well as d=2.438Å from

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plane of (311) cubic phase of Co3 O4 respectively. The HRTEM results confirm the XRD and

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SEM analysis in the terms of both structure and shape.

Fig. 4(a) HRTEM image of CMCO64 composite, (b) selected area of rod β-CoMoO 4 and (c) sphere like structure of Co3 O4 at 20 nm, inset of Fig. 4(a) right top shows fringe image of βCoMoO 4 and right down shows fringe image of Co3 O4 at 5 nm. Fig. 4(d) and (e) displays SAED pattern of β-CoMoO 4 and Co3 O4 respectively

ACCEPTED MANUSCRIPT 3.5 UV-Vis absorption spectra analysis Fig. 5(a) depicts the UV-Vis absorption spectra of β-CoMoO 4, CO3 O4 and β-CoMoO 4 - CO3 O4 composites. From the spectrum we observed that the β-CoMoO 4 - CO3 O4 composites rise the absorption intensity and moves towards the red shift region, which indicates the composites are very narrow bandgap when compared to the bandgap of pure β-CoMoO 4 and CO3 O 4 . The

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red shift is due to the presence of electron transition takes place between β-CoMoO 4 and CO3 O4 . Using the absorption data, the energy gap of the samples were examined by Tauc’s

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equation. The graph between (αhν)2 and hν, is shown in Fig. 5(b). The bandgap values were calculated by extrapolating the straight line on the energy axis (hν), and the values of β-

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CoMoO 4, CO3 O4 and β-CoMoO4 - CO3 O4 composites were determined to be 2.20, 2.09 eV and 1.54 - 2.44 eV respectively. Among the samples the bandgap of CMCO64 composite is most

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appreciable, which suggested that CMCO64 composite has strong absorption capacity in visible region that enhance luminescence efficiency. The bandgap values of the samples are

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stated in the Table 1.

Fig. 5(a) UV-Vis absorption spectra of the pure β-CoMoO 4 , CO3 O4 and β-CoMoO 4 - CO3 O4 composites

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Fig. 5(b) Energy bandgap plot of the pure β-CoMoO4 , CO3 O4 and β-CoMoO 4 - CO3 O 4 composites

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Table. 1 Preparation of the samples at different mole ratio and corresponding energy band

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gap

ACCEPTED MANUSCRIPT 3.6 PL emission spectra analysis Fig. 6(a-c) displays the PL emission spectra of β-CoMoO 4, CO3 O4 and β-CoMoO 4 - CO3 O4 Bandgap CoMoO4

Co3 O4

Sample code

1

100

0

CMCO10

2.20 eV

2

80

20

CMCO82

2.44 eV

3

60

40

CMCO64

1.54 eV

4

40

60

CMCO46

1.80 eV

5

20

80

CMCO28

2.01 eV

6

0

100

CMCO01

2.09 eV

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S. No.

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Energy

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composites recorded at room temperature (RT). It reveals the multiple emission peaks centred at 360, 489, 520, 542 and 587 nm with excitation wavelength located at 350, 470, 507 and 587 nm respectively. This type of multiple emissions was observed by Kwona et al., for

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CO3 O4 layers on SnO 2 nanowires and reported the outer electronic configuration of both cobalt and oxygen plays a major role to create oxygen vacancies, which is important in the process of recombination between VB and CB [34, 35]. In this work the PL emission was divided into three emission regions, (i) Weak blue emission at 489 nm, (ii) Broad green emission at 520-542 nm and (iii) Red emission at above 587 nm the spectrum shows the emission peak centred at 360 nm ascribed to the band edge emission of free exciton [36]. The weak blue emission peak observed at 489 nm is corresponds to crystal defects only observed in CMCO01 and CMCO64 composite, for CMCO10 the peak shifted to green region. At the same time the intensity of CMCO64 composite is higher than CMCO01, that represents the addition of CO 2+ in the composite is responsible to enhance the blue emission [37, 38]. All the samples exhibits green emission at 520 nm under excitation at 507 nm with addition peak

ACCEPTED MANUSCRIPT situated at 542 nm for CMCO10 and CMCO64 composite which is due to charge – transfer transition takes place within MoO 4 2-. Usually, the concentration of free electrons or surface defects formed during heating process enhanced the green emission [39]. Furthermore, the spectrum shows the red emission peak at 594 nm at the excitation of 587 nm. The emission intensity in all the three samples were observed at broad green emission and its order of intensity is reported as follows: CMCO64 > CMCO01 > CMCO10. The enhancement of PL

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intensity in the CMCO64 composite is owing to the dropping in the particle size and characteristic emission is produced by the disappearance of a self-trapped exciton in MoO 4 2-

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[40]. Hereafter, we assumed that the CMCO64 composite is used to investigate other

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application due to its narrow size, higher in surface defects and extraordinary superficial area.

Fig. 6. PL emission spectra of the pure β-CoMoO4 , CO3 O 4 and CMCO64 composite 3.7 Magnetic properties studies

The magnetic properties of the CMCO64 nanocomposites was recorded at room temperature is shown in Fig. 7(a). A typical M-H curve illustrates the interesting dual properties of the composite, which exhibits antiferromagnetic behaviour at applied field and ferromagnetic

ACCEPTED MANUSCRIPT behaviour at low applied field [10, 38]. From the VSM analysis we found the values of saturation magnetization Ms = 12.063 emu, remenent magnetization Mr = 28.303 emu and the coercive field Hc = 32.951 Oe. The squareness ratio of the CMCO64 composite was calculated as Mr /Ms

=

2.346. These values are in good contract with reported value [41].

Moreover, the ferromagnetic behaviour of the composite was not found upto 15 kOe of applied magnetic field due to the domination of antiferromagnetic behaviour present in the

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composite, in which super exchange interaction takes place between cubic structured Co 3 O4 [42]. At low applied field shown in Fig. 7(b), the change in ferromagnetic behaviour is

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attributed to the increase in uncompensated spins on the surface or finite size effects of the particle [43, 44]. This type of magnetic behaviour encourages our sample to act as a simple

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core-shell [45]. Furthermore, the magnetic properties of CMCO64 composite is extremely

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determined by the size, shape, magnetization direction and so on.

Fig. 7(a) VSM image of CMCO64 composite exhibits antiferromagnetic behaviour

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Fig. 7(b) displays ferromagnetic behaviour of CMCO64 composite at lower applied field

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3.8 Antibacterial activity of CMCO64 composite

The antibacterial activity of CMCO64 nanocomposites against three bacterial pathogens

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Escherichia coli, Pseudomonuas aeruginosa and Staphylococcus aureus were assayed by agar well diffusion method. The antibacterial activity was observed in six different concentration of the composite on three bacterial species as shown in Fig. 8(a). The result

Gram-positive

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shows that the CMCO64 composite posseses high impact of antibacterial activity against Staphylococcus

aureus

than

Gram-negative

Escherichia

coli

and

Pseudomonuas aeruginosa. At high concentration 50 mg/ml Pseudomonuas aeruginosa

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posses higher (19 mm) zone of inhibition but was observed compartively low in E.coli (17 mm). At lower concentration Staphylococcus aureus exhibits higher zone of inhibition (14 mm) followed by 12 mm in other two bacterial species as shown in Table. 2. The augmentations of zone of inhibition increased with concentration of the sample, moreover the solvent (DMSO) used in this study didn’t implicate the zone of inhibition of the

micro-

organism. In fact, the nanocomposites are smaller in size and higher surface area to volume than the bacterial species, which support the micro-organism to undisturbed absorption of nanoparticles [46]. Yumei Kong et al., reported that the antibacterial activity of the rodCoMoO 4 in nanoscale range possess good result against E.coli whereas absent in bulk CoMoO 4 [33]. Usually inorganic nanoparticles destroy the bacterial growth, in which Grampositive bacteria has the thick layer of peptidoglycan in the cell wall and it is responsible for

ACCEPTED MANUSCRIPT the electrostatic attraction between positively charged metal ions with -SH group of intracellular proteins [47]. This interaction not only restrict the bacterial growth, but also it encourages the formation of reactive oxygen species and leads to bacterial death [48]. The above result conclude that the sample concentration plays major role in inhibition of bacterial growth owing to the requirement of strong electrostatic force binding with micro-organism [49]. Fig. 8(b) shows the diameter of zone of inhibition is in the order of the micro-

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organisms, Staphylococcus aureus > Pseudomonuas aeruginosa > Escherichia coli.

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Fig. 8(a) Antibacterial activity images of the CMCO64 composite

Fig. 8(b) Diameter of the zone of inhibition of the micro-organisms at different concentration of sample

ACCEPTED MANUSCRIPT Table. 2 Sensitivity factor (Sf) calculations of the samples

Sample code

CoMoO 4 : Co3 O4

Resistance

Resistance

R5% (Ω)

R98% (Ω)

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Mole%

Sf = (R5%/R98%)

CMCO10

6.58  107

2.07  105

317

80:20

CMCO82

8.0  108

2.617  106

307

60:40

CMCO64

1.0325  107

2.128X 103

4351

40:60

CMCO46

4.025  106

1.81  103

2224

20:80

CMCO28

2.3325  106

1.717  103

1358

0:100

CMCO01

7.57 109

4.35103

1740

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3.9 Humidity sensing properties

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100:0

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The change in the resistance as the function of relative humidity of the pure β-CoMoO 4 , Co3 O4 and its composites at different mole ratio was measured at ambient temperature. The

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plots of log R vs RH % of all the samples are shown in Fig. 9(a). The proposed mechanism of the change in resistance of the samples are owed to the combination of chemisorption, physisorption and capillary condensation of water molecules present on the surface of the

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sample [50]. Among the samples CMCO64 composite exhibits the linear decrease in resistance with increase in relative humidity (RH). The humidity sensing factor S f = R5%/R98%, where R5% and R

98%

are resistance at 5% and 98% of relative humidity. The

calculated Sf values of all the samples are shown in Table. 3, wherein we observed the greater the value of Sf indicates the higher the sensitivity of samples towards a moisture [51]. In the composites, the calculated humidity sensing factor was in the order of CMCO64 (4851) > CMCO46 (2224) > CMCO28 (1358) > CMCO82 (307). Among the samples CMCO64 composite exhibits favourable sensing ability to moisture due to the high porosity and smaller in grain size than other samples. Consequently, the presence of higher surface area agrees more sites for adsorption of water molecules, finally increases high electrical conduction. The response and recovery time was studied for CMCO64 composite. The plot of log R vs Time

ACCEPTED MANUSCRIPT was measured for 5% RH and 98% RH shown in Fig. 9(b). The response and recovery time was found to be 60 s and 230 s respectively. The result concludes that the stability of the

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CMCO64 composite very good to use as humidity sensor.

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Fig. 9(a) Humidity sensor measurements of the samples, a plot of log R Vs RH

Fig. 9(b) Response and recovery measurement of CMCO64 composite, a plot of log R Vs Time (sec)

ACCEPTED MANUSCRIPT Table. 3 Diameter of zone of inhibition in mm

Diameter of Zone of inhibition (mm) Microorganism 12.5mg/ml

6.75mg/ml

18

16

15

15

17

14

14

19

16

14

3.12mg/ml

1.56mg/ml

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25mg/ml

15

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13

12

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Staphylococcus

50mg/ml

Escherichia

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Pseudomonuas

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aureus

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aeruginosa

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4. Conclusion

We conclude that the samples were successfully prepared by simple co-precipitation method. The XRD pattern determined the formation of both monoclinic β-CoMoO4 and cubic Co3 O4

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with crystallite size of 20.3 nm and 33.81 nm respectively. The CMCO64 composite exhibits rod like morphology with high porosity and it has the combination of Ferro and antiferromagnetic behaviour. All the samples exhibits the multiple emission peaks centred at 360, 489, 520, 542 and 587 nm and the calculated bandgap was lies between 1.54-2.44 eV. The

humidity

sensing

measurement

illustrates

CMCO64

composite

possess

highest

sensitivity factor Sf = 4851 with response time of 60 s and recovery time of 230 s respectively.

The

antibacterial activity

of CMCO64

composite

reveals

antibacterial activity against both Gram- positive and Gram-negative bacteria.

encouraging

ACCEPTED MANUSCRIPT Acknowledgement

This work is supported by Maulana Azad National Fellowship (MANF), UGC-MHRD, Govt. of India, New Delhi through JRF grant No.F.17.1/2016/MANF-2015-17-TAM-62835/ (SAIII/website).

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Graphical abstract

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Combination of Ferro and antiferromagnetic behaviour Exhibits broad green and red emission peaks Nanorod structure for β-CoMoO4 and sphere like structure for Co 3O4 Shows highest sensitivity factor of Sf = 4851 Promising antibacterial activity against both Gram-negative and Gram-positive bacteria

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