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Green-fuel-mediated synthesis of self-assembled NiO nano-sticks for dual

applications—photocatalytic activity on Rose Bengal dye and antimicrobial action on bacterial

strains

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2017 Mater. Res. Express 4 085030

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1. Introduction

Nano-scale structures with distinct length scale, size and morphology have potential multifunctional applications in numerous technological areas [1]. Nano-scale nickel oxide (NiO) is a promising inorganic functional material. It has a large exciton binding energy and p-type semiconducting behavior with a wide band gap of 3.6–4.0 eV [2] and has many potential industrial applications in catalysis, drug delivery systems, gas sensors, magnetic appliances and as an outstanding anode material for lithium ion batteries or supercapacitors, transparent conductor films, dye-sensitized solar cells and solid oxide fuel cells (SOFC) [3, 4]. The macroscopic quantum tunneling effect along with the volume, quantum size and surface effects of nano-scaled NiO are key to its superior applications in the above-mentioned fields, rather than micro-scaled NiO particles [1]. Huge attempts have been made to synthesize nano-scaled NiO material with different morphologies, such as particles, rings, sheets and ribbons [1, 5, 6]. The available physical and chemical synthesis methods utilize chemical reagents; the synthesized nano-scaled materials are thereby not suitable for biomedical applications owing to the adsorption of harmful chemicals employed during synthesis. Nano-scaled NiO has been synthesized by several approaches, including sol–gel [7], electron beam evaporation [8], thermal evaporation [9], micro-emulsion [10], sputtering [11], hydrothermal [12], microwave combustion [13], spray pyrolysis [14], pulsed laser deposition [15], chemical deposition [16], co-precipitation [17] and solid state reactions [18]. However, green-fuel-mediated hot-combustion synthesis is realized to be simple, eco-friendly and cost-effective method. Green fuels are a very good choice as reducing agents to replace toxic reducing agents such as sodium borohydride (NaBH4) and hydrazine (N2H4) [19]. Therefore, in recent years, green synthesis of nano-scaled materials has become increasingly important compared with other methods [20, 21]. In the present paper we have carried out 100% green-fuel-mediated synthesis of nano-scaled NiO; this in turn was evaluated for its dual roles in photocatalysis and as an antibacterial.

The motivation for this greener approach came from a literature review about the utilization of plant extracts in synthesis methods, thus making the products more suitable for pharmaceutical applications and waste water treatment. In addition, the phytochemicals which are present in the plant extracts can be employed as excellent

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Green-fuel-mediated synthesis of self-assembled NiO nano-sticks for dual applications—photocatalytic activity on Rose Bengal dye and antimicrobial action on bacterial strains

P Iyyappa Rajan1, J Judith Vijaya1, S K Jesudoss1, K Kaviyarasu2,3, L John Kennedy4, R Jothiramalingam5, Hamad A Al-Lohedan5 and Mansoor-Ali Vaali-Mohammed6

1 Catalysis and Nanomaterials Research Laboratory, Department of Chemistry, Loyola College (Autonomous), Chennai 600 034, India2 UNESCO-UNISA Africa Chair in Nanosciences/Nanotechnology Laboratories, College of Graduate Studies, University of South Africa

(UNISA), Muckleneuk Ridge, PO Box 392, Pretoria, South Africa3 Nanosciences African Network (NANOAFNET), Materials Research Group (MRG), iThemba LABS-National Research Foundation

(NRF), 1 Old Faure Road, 7129, PO Box 722, Somerset West, Western Cape Province, South Africa4 Materials Division, School of Advanced Sciences, Vellore Institute of Technology (VIT) University, Chennai Campus, Chennai 600 127,

India5 Surfactant Research Chair, Chemistry Department, College of Science, King Saud University, Riyadh 11451, Saudi Arabia6 College of Medicine, King Khalid University Hospital, King Saud University, Riyadh 11451, Saudi Arabia

E-mail: jjvijaya78@gmail.com (J Judith Vijaya)

Keywords: nano-scale, NiO nano-sticks, hot plate, Rose Bengal dye

AbstractWith aim of promoting the employability of green fuels in the synthesis of nano-scaled materials with new kinds of morphologies for multiple applications, successful synthesis of self-assembled NiO nano-sticks was achieved through a 100% green-fuel-mediated hot-plate combustion reaction. The synthesized NiO nano-sticks show excellent photocatalytic activity on Rose Bengal dye and superior antibacterial potential towards both Gram-positive and Gram-negative bacteria.

PAPER2017

RECEIVED 1 June 2017

REVISED

3 July 2017

ACCEPTED FOR PUBLICATION

7 July 2017

PUBLISHED 17 August 2017

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fuels in the oxidative combustion reaction of metal oxide synthesis [22]. The photocatalytic activity of nano-scaled NiO was evaluated by means of heterogeneous photocatalytic degradation, an advanced oxidation process which is a well-established and successful technique for environmental remediation.

In the present study we have chosen Rose Bengal dye, an anionic dye which is widely present in textile dye effluents. This dye belongs to the class of xanthene derivatives and is water soluble [23, 24]. The current anti-bacterial agents employed in the textile industry, medical applications, food packaging and water disinfection are mostly toxic organics [25], and we were interested in promoting the use of nano-scaled NiO material as an antibacterial agent which can destroy bacteria without affecting other tissues. In this work we successfully syn-thesized nano-scaled NiO in a 100% green-fuel-mediated process; we describe the synthesis methodology and the characterization and applications of the synthesized nano-scaled NiO particles.

2. Experimental

The principle behind the green-mediated synthesis of nano-scaled NiO is based on the hot-plate combustion method. 1 M solution of Ni (NO3)2·6H2O (Merck, 98%) was taken as the precursor solution A and 2 g of graviola leaf extract dissolved in 25 ml of water, 5 g of pomegranate dissolved in 25 ml of water and 5 g of dates dissolved in 25 ml of water were taken as precursor solution B. Prior to adding B to A, the fruits and leaves were crushed in a mortar and pestle and centrifuged at 10000–11000 rpm for 10 min at 4–5 °C. Then the supernatant solution was filtered by Whatman filter paper and the resultant filtrate added to precursor solution A with continuous stirring in a magnetic stirrer for 30 min at 40 °C. The obtained mixture was placed on a hot-plate for the combustion reaction for about 30 min at 100 °C until a gel-like appearance was obtained. Finally, the gel like mixture was kept in the furnace for thermal treatment for 5 h at 250 °C and characterized in order to account for the phase formation, morphology, magnetic behavior and bonding of the final product.

The secondary metabolites present in the green fuel extracted from the plants have the tendency to reduce metal ions to a zero oxidation state. The higher reduction potential of metal salts will allow metal ions to be dis-placed from its anionic part of the metal salt and reduction of metal ions to a zero oxidation state can occur easily when using green fuel extracted from plants. Secondary metabolites such as alkaloids, flavonoids, polyphenols and terpenoids can behave as a chelating ligands and bind to metal ions, which can then reduce the metal ions. The major coordinating site is –OH groups largely present in flavonoids and polyphenols. Therefore, the synthesis mechanism involves the following three key steps: (1) metal ion precursors are added to green fuel and reduced to a zero oxidation state followed by nucleation; (2) the neighboring nanoparticles (NPs) undergo spontaneous coa-lescence, and an Ostwald ripening like process occurs to increase the thermodynamic stability (these first two steps occur through the capping agents present in the green fuel); (3) metal oxide (NiO) is formed by furnace treatment of capped NPs at high temperature. A pictorial representation of these steps is shown in figure 1.

3. Results and discussion

The x-ray diffractogram (XRD) of nano-scaled NiO clearly demonstrates the existence of a high crystalline phase with well-structured high-intensity peaks. The diffractogram was indexed with JCPDS card number 89-7130, which belongs to the face-centered cubic structure shown in figure 2; we report major Bragg reflections at 2θ values of 37.02°, 43.2°, 62.6°, 75.2° and 79.2°, which were allocated to (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) lattice planes, respectively. The absence of impurity peaks confirmed the purity of the NiO phase.

Magnetic hysteresis measurements of nano-scaled NiO material were recorded at room temperature, and it is clear from figure 3 that the room temperature magnetization (M–H) curve shows apparent linear behavior with respect to the applied magnetic field with no coercivity or remanence. Also, it is no surprise that saturation is not attained, even in a high applied magnetic field of 3 kOe. This is certainly due to the antiferromagnetic exchange interaction between the Ni2+ in the NiO crystal lattice and results in zero net magnetization as a consequence of complete magnetic spin compensation in magnetic sublattices. This observation evidently anticipates that there is no appearance of superparamagnetism and no occurrence of magnetic impurities in the nano-scaled NiO material.

The Fourier transform infrared spectrum of nano-scaled NiO is shown in figure 4: the narrow band at 3674 cm−1 is distinctive for chemically bonded hydroxyl groups [2] and bands at 3357 and 1656 cm−1 are assigned to stretching and bending vibrations of absorbed water molecules [26]. The weak bands at 1370, 1278, 1046 cm−1 correspond to weaker adsorption of atmospheric CO2 molecules [1] and Ni–O vibration was observed at 464 cm−1 [27, 28].

High-resolution transmission electron microscope images of nano-scaled NiO are shown in figures 5(a)–(e) at various magnifications. Self-assembled lengthy nano-sticks of NiO particles are clearly observed in figures 5(a)–(c), and are composed of a number of discrete levels. The stronger antiferromagnetic interactions between Ni2+ ions, heat energy produced during the hot-plate combustion process and significant electrostatic

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Figure 1. Mechanism of green-fuel-mediated synthesis of self-assembled NiO nano-sticks (NP, nanoparticles).

Figure 2. X-ray diffraction pattern of NiO nano-sticks.

Figure 3. Magnetic hysteresis loops of NiO nano-sticks.

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interactions between Ni2+ and intercalated −CO32 ions in the layers of the NiO crystal lattice are the factors respon-

sible for the self-assembly of NiO nano-sticks. The value of the fringe spacing with respect to the (1 1 1) lattice plane was 0.24 nm. As a next step, the characterized NiO nano-sticks were evaluated for their photocatalytic and antibacterial potential.

A pilot reaction was carried out before our main experiments to justify the photocatalytic activity of NiO nano-sticks. After the pilot reaction, we examined the optimum experimental parameters, such as catalyst con-centration, dye concentration and pH of the medium. A catalyst concentration of 50 mg/100 ml gave the highest degradation performance and the photocatalytic activity decreased for higher and lower catalyst concentrations, as shown in figure 6. The higher concentration of catalyst results in agglomeration and prevents effective illumi-nation by UV light directed on the surface of the catalyst, decreasing the rate of production of hydroxyl (∙OH) radicals, which are known to be primary oxidants in photocatalytic degradation [29]. Experiments were also performed in which the concentration of Rose Bengal dye was increased: we observed a decrease in the rate of

Figure 4. Fourier transform infrared spectrum of NiO nano-sticks.

Figure 5. (a)–(e) High-resolution transmission electron micrographs with different scales of magnification. The inset to (b) is the energy dispersive x-ray spectrum and the inset to (e) the selected area electron diffraction pattern of NiO nano-sticks.

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degradation from 10 to 40 ppm, as shown in figure 7. This can be attributed to the preferential adsorption of dye molecules, which increases with increasing concentration; however, the increased adsorption does not balance with the rate of production of ∙OH and superoxide ( −O ·2 ) radicals [30]. The irradiation time is also constant as are the chances of photons being interrupted on reaching the catalyst surface can also decrease the rate of photodeg-radation [31]. The rate of production of ∙OH is influenced by the pH of the medium in the degradation reaction. To determine the role of pH in photocatalytic activity, we performed degradation experiments in both acidic and basic pH ranges. The maximum rate of degradation was observed at pH 8, due to the increased production of ∙OH in a basic medium; this can enhance the rate of degradation, and the degradation efficiency increases with increasing pH, as shown in figure 8. At a very high pH value (pH 10) the observed decrease in photocatalytic activity is due to the repulsive force between the anionic Rose Bengal dye molecules and the highly negatively charged NiO photocatalyst surface. After the optimization of experimental parameters, we investigated the rate and order of the reaction for reaction time t = 100 min, initial Rose Bengal dye concentration 10 ppm, catalyst dosage 50 mg, wavelength of light 365 nm and pH 8. Finally we obtained a pseudo first-order rate constant of 2.76 × 10−2 min−1 with an excellent regression correlation coefficient (R2) value of 0.9999 from the slope of the linear plot of ln(Ct/C0) versus irradiation time as displayed in figure 9, Ct is the concentration of Rose Bengal dye after t minutes and C0 is the initial concentration of Rose Bengal dye.. The mechanism of photocatalytic activity of NiO nano-sticks in the degradation of Rose Bengal (RB) dye is given below and images of dye degradation at various time intervals are shown in figure 10

+ ++ −hvNiO h e  → (1)

  →+ ++ +H O h H OH·2 (2)

+ − −O e ·O22  →   (3)

+ −RB OH· ·O degradation products2( / ) → (4)

+ ∗hvRB RB  → (5)

Figure 6. The photocatalytic degradation (PCD) of Rose Bengal in the presence of NiO nano-sticks: the effect of the catalyst concentration.

Figure 7. The photocatalytic degradation (PCD) of Rose Bengal (RB) in the presence of NiO nano-sticks: effect of the initial RB dye concentration.

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+ +∗ +RB NiO RB NiO e→ ( ) (6)

+ +−NiO e O ·O NiO22( )   →   (7)

where h is the Planck constant, ν is frequency, RB is Rose Bengal, e– is an electron and h+ is a positive hole.The variation in tolerance of oxidative stress is due to the variation in susceptibility of the different bacterial

strains and the factors such as particle size, specific surface area and morphology of the nano-scaled materials that play an influential role in the determination of antibacterial potential [30, 32]. As stated in our previous reports [30, 33], the nano-scaled materials will undergo an electrostatic interaction with bacterial strains, and as a result of this interaction reactive oxygen species will be generated which in turn destroy the bacterial cells. In the cur rent investigation of nano-scaled NiO as an antibacterial agent we found that this material displayed an excel-lent antagonistic effect when evaluated on both Gram-positive and Gram-negative bacterial strains. The diam-eters of the inhibition zones are shown in figure 11. In the current evaluation we chose two Gram-positive (Strep-tococcus pneumoniae, Bacillus anthracis) and two Gram-negative (Klebsiella pneumoniae, Enterobacter aerogenes) bacterial strains and chloramphenicol was taken as a standard for the comparison of our results, as shown in table 1. The mechanism of antibacterial activity of nano-scaled materials is still unclear, but some suggestions were given in our previous investigations [30, 33]. Two reactions are possible when the nano-scaled materials are used to treat the bacterial strains, as shown in figure 12. The first reaction is proposed to be a strong interaction of positive cations like Ni2+ with negatively charged parts of bacterial cells, resulting in the collapse of bacterial strains. The second possible reaction will happen when the surface of the NiO particles is irradiated with light, which ultimately results in excitation of electrons from the valence to the conduction band. Further reaction of excited electrons with O2 molecules generates −O ·2 radicals followed by production of H2O2. The production of ∙OH occurs when a h+ reacts with water. The −O ·2 and ∙OH species cannot penetrate inside the bacterial cell membrane, but they will have a significant effect by breaking down any protein and lipid molecules present in the outer surface of the bacterial cell.

Figure 8. The photocatalytic degradation (PCD) of Rose Bengal in the presence of NiO nano-sticks: the effect of pH.

××××

Figure 9. The photocatalytic degradation (PCD) of Rose Bengal in the presence of NiO nano-sticks: kinetic studies of ln(Ct/C0) versus reaction time (t, min) (experimental conditions: t = 100 min, λ = 365 nm).

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Figure 10. Rose Bengal in the presence of NiO nano-sticks at various time intervals.

Figure 11. Zone of inhibition produced by NiO nano-sticks against the Gram-positive (S. pneumoniae, B. anthracis) and Gram-negative (K. pneumoniae, E. aerogenes) bacterial strains and a comparison with standard chloramphenicol.

Table 1. Antibacterial activity of the NiO nano-sticks compared with the antibiotic chloramphenicol.

Bacteria

Zone of inhibition (mm)

NiO Chloramphenicol

S. pneumoniae 22.8 22.4

B. anthracis 25.4 23.5

K. pneumoniae 23.2 13.4

E. aerogenes 27.5 25.3

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4. Conclusion

In summary, self-assembled NiO nano-sticks were synthesized for the first time via a 100% green-fuel-mediated hot-plate combustion reaction. They showed excellent photocatalytic activity for the photodegradation of Rose Bengal dye and promising antibacterial activity against both Gram-positive and Gram-negative bacteria.

Acknowledgments

This work was financially supported by Loyola College, Tamil Nadu, India through a Loyola College–Times of India (LC-TOI) Major Research Project scheme (project code 2LCTOI14CHM003, dated 25.11.2014). Authors RJ and HAA also thank the Deanship of Scientific Research, King Saud University for funding through the Vice Deanship of Scientific Research Chair.

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