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Original article
13 (
4
); 5137-5150
doi:
10.1016/j.arabjc.2020.02.012

Enhanced UV–Visible triggered photocatalytic degradation of Brilliant green by reduced graphene oxide based NiO and CuO ternary nanocomposite and their antimicrobial activity

, , ,
a Dept of Chemistry, Adikavi Nannaya University, Rajamahendravaram 533296, India
b National Centre for Polar and Ocean Research, Vasco Da Gama, Goa 403804, India

⁎Corresponding author. drbjaganreddy@gmail.com (Boggu Jagan Mohan Reddy)

Received: , Accepted: ,
© The Author(s) 2020
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

In this manuscript, the synthesis of ternary nanocomposites NiO-CuO-RGO (NCR) via hydrothermal route by the different GO loading amounts resulting in preparation of NCR-5% and NCR-10% composites were discussed. The prepared composites were excessively studied by X-ray diffraction, Fourier transform infrared spectroscopy, Raman, Field emission scanning electron microscopy, energy dispersive X-ray, transmission electron microscopy, Photoluminescence and UV–Visible spectrophotometer to execute their electronic, optical and spectroscopic properties. However, the XRD and EDX results confirmed the pure phase formation of NCR composites without impurities. Later, the photocatalytic efficiency of prepared samples was examined over model organic pollutant Brilliant green (BG) degradation under UV–Visible light irradiations. Interestingly, the tested composites possess better photocatalytic efficiency than previous reports, predominantly, NCR-10% was most effective for the degradation percentage rate of BG dye (91%) in 60 min under visible light irradiation. This catalyst shown greater stability and was recycled for five successive runs. Unambiguously as we expected, NCR-5% composite was effectively degraded BG dye (80%) under UV light than NCR-10%composite which revealed less degradation rate. In addition, the indolence of pathogens also examined against two different pathogens and these results clearly stated that NCR-10% composite was acted as benign antimicrobial agent.

Keywords

NiO-CuO-RGO
Ternary nanocomposite
Photocatalysis
Brilliant green
Antimicrobial activity
1

1 Introduction

Dyeing matter discharges pollutants into water and creates the severe environmental threat to the world. However, the extensive amount of research stage especially in the field of sustainable water treatment processes which are capable of improving the water quality. The inaccessibility of potable water is a critical problem predominantly where the treatment systems have been failed to eliminate the toxic metal ions, aquatic pathogens and industrial wastes. The research and development in this domain leads to the rise of a new processes class called as advanced oxidation processes (AOPs), particularly in the form of heterogeneous photocatalysis in which the light energy is converted into chemical energy (Ahmed and Haider, 2018; Botsa and Basavaiah, 2019). Photocatalysis is not just fascinating tool for the conversion of energy and also for environmental decontamination owing to its conspicuous nature (Abhilash et al., 2019). Using metal-semiconductors as photocatalyst is an eco-friendly which simulates that photosynthesis approach to speed up the chemical reactions with the involvement of light (Zhang et al., 2012).

In this connection, Nickel oxide (NiO), a p-type oxide semiconductor with cubic lattice structure, is used in versatile applications like catalysis, battery cathodes, gas sensors, magnetic materials and electrochromic films depending on the effect of quantum size, volume, surface area which implies anodic electrochromism, terrific durability, electrochemical stability, and huge spin optical density (El-Kemaryn et al., 2013). The former reports state that NiO has a wide bandgap energy range of 3.6–4.0 eV, translucent to UV, visible and near infrared radiation (El-Kemaryn et al., 2013; Karunakaran et al., 2012).

Due to the wider bandgap, NiO requires a UV-C light (280 nm) for excitation and create electron-hole pairs (Karunakaran et al., 2012). To overwhelm the bandgap issue, metal oxides can be utilized as a photocatalyst, which is exposure to UV or visible light or both, to endorse the photo-excited electrons from the valence (VB) to conduction band (CB) to accelerate the recombination of electron and hole pair (Ahmed and Haider, 2018; Ramos et al., 2015). A strategy to conquer this is through doping NiO with various transition metal oxides or their composites by coupling with other dissimilar band edge metal oxides for obtaining better physicochemical properties than the pure metal oxides (Hussein and Shende, 2014). In general, the composites improve photocatalytic activity (Aghabeygi and Khademi-Shamami, 2018; Fan et al., 2020), renders new crystallographic phases which leads to produce defect energy levels in the bandgap area (Shirmardi et al., 2018), that alters the surface distinctiveness of the pure oxides owing to the development of new locations in the interface among the components (Tomar and Chakrabarty, 2013) and also enhance the stability of a photocatalyst crystalline phase (Pirzada et al., 2015).

However, for example, CuO was used as eco-catalyst with ZnO to enhance the photocatalytic activity of ZnO under sunlight irradiation due to its narrow bandgap. The copper oxide (CuO) a p-type semiconductor which possesses direct bandgap almost the range is 1.2–1.79 eV and absorbs a larger fraction of the solar spectrum which enhances the electron-hole pair improves the photocatalytic efficiency (Botsa et al., 2019; Hassanpour et al., 2015; Botsa et al., 2018). Both metal oxide semiconductors undergo illumination simultaneously and excited the electrons from VB to CB then form hole and electrons respectively, resulting in better photocatalytic efficiency (Sherly et al., 2014). Yet, for instance, the ZnO/CuO nanocomposites also suffer from the electron-hole pair recombination rates. To resolve this issue for improving the degradation efficacy of the ZnO/CuO binary nanocomposites, reduced graphene oxide (RGO) was used to form the (ZnO/CuO)/RGO ternary nanocomposites (Zhao et al., 2019). RGO is one of the members of the carbon family 2D one atom thick monolayer of sp2 bonded carbon atoms which show high thermal conductivity, electron mobility and specific surface area. RGO is excellent for metal oxides as supporting material and forms hybrid nanocomposites, finally, stifle the electron-hole pair recombination. Therefore, the researchers showed their interest in preparing the RGO based on nanocomposites with semiconductor metal oxides to improve the electron transport rates and to avoid the charge recombination rates that may use to develop the photodegradation efficiency of the catalysts (Anjaneyulu et al., 2018). RGO was obtained from Graphene oxide (GO) which can behave as an excellent electron transporting bridge because of its great electron mobility and enlarged p-electron conjugation that can stabilize the extraneous electron (Mohan et al., 2019). This property of GO plays a key role in decreasing the electron-hole recombination rate that renders better photocatalytic efficiency of the graphene supported composites with metal oxides, predominantly with semiconductors than pure ones. So there is a necessity to progress simple, scalable, economical, and green chemical methods for synthesizing massively active, reusable and stable heterogeneous nanomaterials.

Hence, the present work aim is to prepare the NiO/CuO/RGO ternary nanocomposites by a simple, facile hydrothermal route and studies their optical, structural and morphological properties along with the photocatalytic efficiency of prepared samples over the decolourisation of model organic pollutant Brilliant green under UV and Visible light irradiations. In addition, the bactericidal assay of synthesized materials also examined against different pathogens.

2

2 Experimental section

2.1

2.1 Materials

Nickel nitrate (NiNO3)2, sodium hydroxide (NaOH), sodium borohydride (NaBH4), copper acetate Cu(CH3COO)2, Graphite flakes, sulphuric acid (H2SO4), potassium permanganate (KMnO4), sodium nitrate (NaNO3)2, hydrogen peroxide (H2O2) were purchase from Sigma Aldrich Company, India and utilized with no further purification. The BG dye was received from Merck chemical Ltd, India and the Milli Q water is used in the preparation of solutions.

2.2

2.2 Synthesis of NiO-CuO-RGO ternary nanocomposite

For the synthesis of the desired ternary nanocomposite, initially a pre-calculated amount of GO was placed into a 100 ml beaker and was further sonicated for 20 min to homogeneity. Equimolar concentrations of (NiNO3)2 and Cu(CH3COO)2 with ten times higher molar ratio of NaOH were taken into a Teflon lined stainless steel beaker. The sonicated GO was added to the above mixture solution followed by a little pinch of NaBH4 and sealed the beaker of the hydrothermal reactor which was kept in hot air stabilizer for 8 h at controlled 200 °C temperature. After completion of the reaction, this bomb was authorized to cool to the room temperature attained and the produced yield was centrifuged, washed with water several times followed by ethanol and at last dried in a hot air sterilizer at 70 °C for 10 h. The similar procedure was implemented for the synthesis of different ternary nanocomposites with a change in GO loading amount (Eqs. (1)–(3)).

(1)
(NiNO3)2 + NaOH → Ni(OH)2
(2)
Cu(CH3COO)2 + NaOH → Cu(OH)2
(3)
Ni OH 2 + Cu OH 2 + GO → NaBH 4 NiO/CuO/RGO

2.3

2.3 Synthesis of binary nanocomposites

2.3.1

2.3.1 NiO-CuO

In a typical synthesis, equal amount of 0.01 M (NiNO3)2, 0.01 M Cu(CH3COO)2and one molar of NaOH and a pinch of NaBH4 were placed in a beaker and stirred for 60 min. Then this mixture solution shifted into a Teflon stainless steel beaker of the hydrothermal reactor which was kept in hot air stabilizer and maintained the temperature at 180 °C for 10 h. This bomb was cooled to attain the room temperature. Finally, the acquired yield was washed with Milli Q water then ethanol and lastly dried in hot air oven at 60 °C for the overnight. The same process was adopted for the preparation of NiO in absence of Cu(CH3COO)2.

2.3.2

2.3.2 CuO-RGO

In a simple synthesis (Botsa and Basavaiah, 2019), the prepared GO (50 mg) was sonicated for 10 min. Then, equal amount of Cu(CH3COO)2(0.01 M), NaOH (1 M) and one ml of NaBH4 aqueous solutions were poured in Teflon beaker and sealed which was kept in muffle furnace at 180 °C for 3 h. After completion of the reaction, the blend was cooled to the room temperature and then washed with water promptly with ethanol. Finally, the product was filtered and dried at 70 °C in an oven overnight. The CuO nanoparticles were synthesized by adsorbing the same procedure in the absence of GO.

2.3.3

2.3.3 Synthesis of GO

GO was synthesized by a modified Hummer's process (Hummers and Offeman, 1958). Initially, graphite flakes (3 g) were undergone sonication for 30 min prior to oxidation. The modified graphite residue was dried at 60 °C for 120 min. Then, 1 g of sonicated graphite and 0.5 g of NaNO3 were dissolved in 70 ml concentrated H2SO4 in a 500 ml round bottom flask under an ice bath (maintain temperature below 5 °C) with gentle magnetic stirring for 20 min. Then, 3 g of KMnO4 was added continuously pinch by pinch by keeping the temperature of the solution did not exceed than 20 °C. Then, the ice bath was removed and the solution was stirred at 35 °C under a reflux condenser for 3 h. After that, 130 ml of Milli Q water was added and stirring continued for another one hour. Excess unreacted KMnO4 was removed by 10 ml of 30% H2O2. The complete removal of KMnO4 was confirmed by changing the colour from dark to yellow. As prepared GO was carefully washed with Milli Q water three times and dried the whole night in the hot air sterilizer at 70 °C.

2.4

2.4 Characterization of prepared catalysts

The SEM depicts were taken with a JEOL JSM-6610LV Field Emission Scanning Electron Microscope with an energy dispersive x-ray spectrometer for the morphology and confirmation of elemental analysis. The average diameters of synthesized compounds were measured by J Image software. FTIR spectra from 500 to 4000 cm−1 of dried samples were obtained from IR Prestige 21, Shimadzu, Japan with KBr pellets for identifying the functional groups. To support FTIR, Raman spectra at 632.8 nm excitation radiation were acquired on a BTC111-Raman 785 spectrometer. Pure phase formation confirmed using XRD (D8 Bruker AXS) with Cu-Kα radiation at a scan speed of 0.017 sec−1. The UV–vis absorption spectra of prepared samples obtained at the wavelength of 200–800 nm (Shimadzu 2600R). To study the ROS species, the PL spectra of the prepared composite were taken using Flouromax-4, HORIBA, Japan.

2.5

2.5 Appraisement of photocatalytic activity

The photocatalytic efficiency of prepared samples was studied by the decolourisation of dye in aqueous suspension. Direct photolysis employing visible light source cannot mineralize the dyes, it was done by using a self-constructed photoreactor which consists of a quartz tube with 350 W metal halide lamp for the visible light source and UV lamp 12 W for the UV source. Initially, 50 mg of sample was added to the desired concentration of dye solution, which was taken in a beaker and then stirred (30 min) in dark room to attain the adsorption-desorption thermal equilibrium between the catalyst surface and dye molecule. Later, this beaker undergoes light irradiation and collected 4 ml aliquot at regular time intervals. The strength of dye aqueous solution was determined by measuring absorbance at particular wavelength with UV–Vis spectrophotometer in the range of 200–800 nm. The photodegradation efficiency (DE) was measured by the following Eq. (4).

(4)
DE % = Co - C t Co × 100 where ‘Co’ and ‘Ct’ are the respective concentrations at the initial and final time of dye solution. Finally, the plot has drawn between ‘C/Co’ and irradiation time has shown the degradation efficiency. Langmuir-Hinshelwood kinetics model discuss the photocatalytic rate constant (k) was measured by the subsequent empirical formula (5). From this rate-kinetics model, the changes in concentration of BG dye before and after light irradiation (C/C0) with the time is noticed, where C0 and C are the initial and final concentrations of the dye solution.
(5)
C = C 0 e - kt

2.6

2.6 Appraisement of antimicrobial activity

The antimicrobial assay of hydrothermally prepared samples was carried out over the two different bacterial strains such as Escherichia coli (MTCC-443) and Staphylococcus aureus (MTCC-3260); agar well diffusion method was employed for the appraisement of antimicrobial activity. In detail, dissolved the nutrient agar medium (High media) in demineralised water taken into a 100 ml conical flask and sterilized in an autoclave at normalized conditions (120 °C with 15 lbp) for 15 min. After completion of autoclave, the media was poured into the purified petriplates. Inoculums were dispersing over the surface of agar plates with sterile glass spreader and created four wells at equal distance using sterile cork borer. Prepared the concentrations such as 40, 60, 80 µg/mL of extract that is poured in individual well along with one positive control (chloramphenicol) to study the antibactericidal assay of synthesized samples against aforementioned strains and kept for incubation at 37 °C for 24 h. After incubation is over, the diameter (mm) of the inhibitory zone formed around the wall has been measured.

3

3 Results and discussion

3.1

3.1 Characterization study of NCR composites

3.1.1

3.1.1 UV–Vis absorption spectra

The UV–Vis absorption spectra of hydrothermally synthesized composites are depicted in Fig. 1. The broad absorption peak of pure NiO illustrates at 229 nm in the ultraviolet (UV) region owing to the wide bandgap semiconductor NiO. But, this peak was shifted to 311 nm because of the formation of a composite with a narrow bandgap of CuO. However, for the NCR composites, the curve displays increased greater absorption intensity which in the visible region that means these composites can make superb usage of sunlight as the presence of zero bandgap RGO (Zhang et al., 2016).

Uv–vis absorption spectra of (a) NiO, (b) CuO, (c) NiO-CuO, (d) NCR-10% and (e) NCR-5% composite materials.
Fig. 1 Uv–vis absorption spectra of (a) NiO, (b) CuO, (c) NiO-CuO, (d) NCR-10% and (e) NCR-5% composite materials.

3.1.2

3.1.2 XRD analysis

The XRD patterns of synthesized materials by hydrothermal presented in Fig. 2. GO implies a sharp peak at 10.86owhich belongs to (0 0 1) plane and the XRD patterns of NiO and its composites presented at 37.62°, 43.54°, 63.15°, 75.6°, 79.8° corresponds to the respective planes (1 1 1), (2 0 0) and (2 2 0), consensus with standard JCPDS file (No: 47-1049) (Yan et al., 2014). No apparent change is noticed in the position of the peaks of NiO and its composites stated that NiO phase is still constant without any disturbance. The synthesized CuO and CuO-RGO composite possess the XRD patterns at 2θ = 35.73, 39.4 and 61.69°, which are corresponding to their respective planes (0 0 2), (2 0 0) and (1 1 3) for monoclinic CuO (standard JCPDS 45-0397) (Botsa and Basavaiah, 2019).

XRD patterns of prepared samples.
Fig. 2 XRD patterns of prepared samples.

From the XRD patterns of prepared samples, the average crystalline size (S) was estimated using Debey Scherer equation (6).

(6)
S = 0.94 λ / β cos θ where λ is the X-radiation wavelength, β is the full width at half maximum (FWHM) and θ the diffraction angle. The calculated results showed the NCR-10% composite has lesser in size (20.72 nm) followed by NCR-5% (21.43 nm), NiO-CuO composite (24.43 nm) and NiO was found to be 27.53 nm (Table 1). The previous reports stated that the following parameters are nature, doping or composite, morphology, size and surface chemistry of catalyst shown great impact on the efficiency of the photocatalytic assay. However, the crystalline size is a key factor for the efficient photocatalytic activity of the catalyst. By the alteration of particle size, the surface area may be varies. Indeed, the smaller particles posses greater surface area which resulting in better photocatalytic efficiency (Nandiyanto et al., 2020). Being smaller in size nanoparticles have shown great photocatalytic applications, hence, NCR composites are attentive for catalytic applications.
Table 1 X-ray crystallographic properties of prepared samples.
Sample FWHM D spacing Average crystalline size (nm)
NiO 0.7303 1.68 27.53
CuO 0.642 1.7631 29.6
NiO-CuO 0.5576 1.903925 24.43
CuO-RGO 0.6122 1.9763 19.3
NiO-CuO-RGO 5% 0.47685 1.78193 21.43
NiO-CuO-RGO 10% 0.4284 2.5024 20.72

3.1.3

3.1.3 FTIR spectral analysis

FTIR analysis was carried out for the investigation of organic species on the surface of synthesized materials. Fig. 3 illustrates the FTIR spectra of all the prepared samples. The FTIR spectra of GO displays a broad peak at 3377.16 cm−1 represents the strong stretching mode of OH group. Besides that, other peaks are at 1031.85 and 1708.83 cm−1 attributes C—O stretching group, similarly the peak at 1573.82 cm−1 contributes C—OH stretching mode. In binary and ternary nanocomposites (RGO containing composites) the broad peak located at 3377.16 cm−1 (as shown in GO spectra) is not observed that means no stretching mode of OH group after the reduction confirms the formation of RGO in NCR composites (Gong and Lia, 2015). These results indicate the removal of the partial functional groups of GO during the hydrothermal approach. The absorption bands in NiO and its composites at 491.84 cm−1 contributed to the Ni—O stretching mode and another peak at 1124.5 cm−1 is assigned to the C—O group.

FTIR spectra of synthesized materials.
Fig. 3 FTIR spectra of synthesized materials.

3.1.4

3.1.4 Raman analysis

The Raman analysis was used to characterize the further FTIR analysis of GO structural variation during GO reduced to RGO and the obtained results as shown in Fig. 4. In general, the single layer graphene Raman spectra shows two peaks at 1350 cm−1 and 1596 cm−1corresponds to D (disorder) and G (in-phase vibration) bands respectively (Anjaneyulu et al., 2018). But a slight variation was perceived in Raman spectra of synthesized NCs, where the G and D bands are at 1611 cm−1 and 1372 cm−1 respectively, this signifies the reduction of GO to RGO by hydrothermal synthesis comprises hydrazine hydrate (reducing agent) exhibits the restoration of the isolated double bonds formed in GO to the conjugated double bonds in RGO (Fig. 4). During the conversion of GO to RGO by the reduction, the Raman peaks imply change in band position and shape that means the presence of double bonds that resonate at higher wavenumber (Ambrosi et al., 2011). This is a good evidence for the RGO formation in a prepared NCR-10% composite. The ternary composites with RGO supported materials possess excellence in photocatalytic activity with light irradiation. The intensity ratio of ID/IG of NCR-10% composite is 1.14 higher than pure GO (0.8), authorizes the formation of graphene based NiO and CuO composite (Anjaneyulu et al., 2018; Yan et al., 2012). The obtained Raman results are consensus with the FTIR results.

Raman spectra of hydrothermally synthesized materials.
Fig. 4 Raman spectra of hydrothermally synthesized materials.

3.1.5

3.1.5 Morphology study

The morphological studies were carried out for the preparation of samples with the help of SEM and TEM. The SEM micrographs stated that it decreased the tendency to the state of particle agglomeration. Fig. 5 illustrates the SEM images of prepared NCR composites shown agglomeration in different magnifications and rod-like shapes due to the presence of CuO in ternary composite as shown in ESI (Fig. S1).

FESEM micrographs of (a–b) NCR-5% and (c–d) NCR-10%.
Fig. 5 FESEM micrographs of (a–b) NCR-5% and (c–d) NCR-10%.

Further, the morphology of synthesized NCR-10% composite studied using TEM and SAED shows the small particles with grey, but the agglomeration was clearly seen in prepared nanocomposite as it contains oxides of nickel, copper and RGO. Histogram distribution curve illustrates that the size of particles was found to be 14 nm which is in close covenant with the XRD results (Fig. 6).

(a–b) TEM (c) SAED and (d) Histogram images of NCR-10% composite.
Fig. 6 (a–b) TEM (c) SAED and (d) Histogram images of NCR-10% composite.

In support of XRD results, the purity of the product was estimated by EDX analysis as shown in Fig. 7, which implies the homogeneous distribution of NiO and/or CuO upon RGO substrate. The elemental analysis of synthesized materials displays major peaks for Ni, C, O and Cu in the spectrum without impurity peaks in the ternary composite. Fig. S2 (ESI) shows the EDX images of prepared NiO, CuO and NiO-CuO composite formed without impurities in their chemical composition.

EDX images of prepared samples.
Fig. 7 EDX images of prepared samples.

3.2

3.2 Catalytic applications of NCR composites:

3.2.1

3.2.1 Photocatalytic activity

Firstly, the visible light enhanced the photocatalytic degradation of BG dye solution by studying all the synthesized samples under the experimental conditions such as 100 ml of 10 ppm BG dye solution with pH-8 and 50 mg catalyst dosage. Remarkably, the absorption peak of BG dye solution positioned at 625 nm that weakens gradually with an increase in the irradiation time under the visible light. The photodegradation of BG dye suspension is done by hydrothermally synthesized metal oxides and their composites with RGO. The degradation percentage of BG dye follows the order as NiO (49.4%) in 180 min < CuO (66.4%) in 150 min < NiO-CuO (68.9%) in 90 min < CuO-RGO (72.9%) in 90 min as depicted in Fig. 8, further analysis is required to accomplish the decolourisation of BG dye aqueous solution. Hence, we have been attempted with the RGO based ternary composites.

Photocatalytic degradation of BG dye by (a) NiO, (b) CuO, (c) NiO-CuO and (d) CuO-RGO.
Fig. 8 Photocatalytic degradation of BG dye by (a) NiO, (b) CuO, (c) NiO-CuO and (d) CuO-RGO.

3.2.2

3.2.2 Effect of RGO on photocatalytic degradation of BG dye

Investigations on photocatalytic activities of the RGO supported metal oxide composites stated that both NCR composites exhibited excellence in photodegradation of BG dye under the visible light irradiation (Fig. 9) RGO reduces the recombination rate of charge carriers separation because of RGO provides the electrons which are responsible for the superoxide radicals formation, hence grander photocatalytic activity was noticed (Lu et al., 2008). In addition, the results declared that the particle size plays a vital role in the photocatalytic experiment. However, too much catalyst loading has a negative impact on the increases in the turbity that reduce the light transmission through the solution. Thus, the optimum initial catalyst loading must be investigated independently.

Photocatalytic degradation of BG dye by (a) NCR-5% and (b) NCR-10%.
Fig. 9 Photocatalytic degradation of BG dye by (a) NCR-5% and (b) NCR-10%.

The obtained results show a path to optimise the photocatalyst NCR-10% as exhibited higher photocatalytic efficiency (91%) in 60 min among all tested. Similarly, the NCR-5% composite was degraded 88.7% of BG dye solution under visible light irradiation in 60 min as shown in Fig. 9. The optimised conditions for the photocatalytic degradation of BG dye under visible light irradiations are effect of pH, catalyst dosage amount and concentration of BG dye solution. BG consists of azo group, sulphate group, and an aromatic ring. The complete mineralization of BG depends on the oxidation of these groups and ring by ternary composites.

3.2.3

3.2.3 Effect of pH of dye solution

Three different pH of dye solution such as 4, 7 and 10 were selected to optimise the pH of dye solution at constant catalyst dose (50 mg) and dye concentration (10 ppm). The BG dye solution of pH-10 was declined gradually with increasing time when irradiated with the visible light. Besides that, when pH increased from 4 to 10, the degradation efficiency of BG dye also increased (Fig. 10a). Interestingly, after reach the pH-10further increased the pH but degradation rate was declined, this may reduce the coulombic attraction between dye molecule and catalyst surface, as dye does not in cationic form due to more Hydroxyl ions (OH−), hence the degradation rate was increased at pH-10 (Poulios and Tsachpinis, 1999). Hydroxyl radicals tend to be generating easier by oxidizing, more hydroxyl groups available and the degradation process preferable in alkaline media (pH-10).

Optimised conditions: effect of pH (a), catalyst dose (b), concentration of dye solution (c) and rate order kinetics of prepared samples (d).
Fig. 10 Optimised conditions: effect of pH (a), catalyst dose (b), concentration of dye solution (c) and rate order kinetics of prepared samples (d).

3.2.4

3.2.4 Effect of catalyst dosage

A series of trials were accompanied to attain the optimum catalyst dosage by changing the designated catalyst amount such as 50, 40, 30 and 20 mg at concentration of 10 ppm BG dye solution with pH-10. Remarkably, in this study, the maximum catalyst amount (50 mg) of NCR-10% exhibited better photocatalytic degradation rate as shown in Fig. 10b, then gradually declined with decrease in catalyst loading amount, owes to the large surface can be accommodate more number of dye molecules resulting in the enhancement of the photons. We have been tried with increase photocatalyst (more than 50 mg), catalyst dose has not shown any effective photocatalytic degradation of BG dye because of overdose the catalyst surface cannot accommodate the dye molecules (Kahan et al., 2008).

3.2.5

3.2.5 Effect of dye concentration

Fig. 10c illustrates the effect of BG dye aqueous solution concentration (10, 15 and 20 ppm) over NCR-10% photocatalyst under visible light irradiations at the similar conditions (pH-10 and 50 mg photocatalyst). The degradation rates order of tested concentration dye solutions is 10 ppm followed by 15 ppm and 20 ppm, the results declared that NCR-10% composite acted as a benign photocatalyst for the degradation of BG dye solution in 60 min time under visible light irradiations.

However, the rate order kinetics can be obtained from photocatalytic reaction and tabulated the results in ESI (Table S1). The linear relationship of ln(C/C0) with time demonstrates the following reaction (Eq. (7)).

(7)
H = kt = ln C / C 0 where ‘C/C0’ is normalized initial concentration, ‘t’ is time and ‘k’ is rate constant (min−1) of reaction. The experimental results declared these follows pseudo first order kinetic reaction. It is acknowledged that the BG dye degradation rate follows the order as NCR-10% > NCR-5% > NiO-CuO > CuO > NiO after the similar irradiation time (Fig. 10d). These results signify the photocatalytic degradation efficiency of BG dye suspension can be improved in the presence of RGO as compared to pure NiO or CuO, remarkably improved with the NCR-10% composite.

3.3

3.3 Statistical analysis

The statistical analysis was carried out for the optimised degradation factors such as effect of pH, catalyst dose and concentration of dye solution to study the significance. Table 2 discussed the covariance analysis which confirmed that 10 mg catalyst dosage, and 10 ppm dye solution of pH-10 were the optimised conditions for the degradation of BG dye using prepared NCR-10% composite as the highest covariance was noticed (9 3 3). ANOVA (analysis of variance) showed the less than 0.05 of significant F value (0.0044) and p value (0.047) that means three variables are significant and also R2 values were obtained and determination coefficient (R2 0.83) is much closer to adjusted R2 (0.793) of three variables (Table 3). It signifies that this model is appropriate as the adjusted R2 value is smaller than R2 value.

Table 2 Covariance analysis of photocatalytic degradation of BG dye.
50 mg 40 mg 30 mg 20 mg pH-4 pH-7 pH-10 10 ppm 15 ppm 20 ppm
50 mg 933
40 mg 843 776
30 mg 784 686 758
20 mg 618 536 627 532
pH-4 488 425 453 368 281
pH-7 671 607 605 495 363 512
pH-10 933 843 784 618 488 671 933
10 ppm 933 843 784 618 488 671 933 933
15 ppm 683 615 626 512 375 523 683 683 537
20 ppm 596 527 570 471 339 461 596 596 476 431
Table 3 Analysis of variance statistics for optimised conditions of BG degradation.
Regression Statistics
Multiple R 0.909792442
R2 0.827722287
Adjusted R2 0.793266744
ANOVA
df SS MS F Significance F
Regression 1 5406.334135 5406.334135 24.02291 0.004469348
Residual 5 1125.245624 225.0491248
Coefficients Standard Error t Stat P-value Lower 95% Upper 95% Lower 95.0% Upper 95.0%
Intercept 21.6369 10.22189286 2.116 0.047 −4.6392 47.913 −4.63928 47.91314
Time 1.3895 0.283504299 4.901 0.004 0.6607 2.1183 0.660773 2.118315

3.3.1

3.3.1 Photocatalytic degradation plausible mechanism

Here, a probable mechanism is proposed for the degradation of BG dye suspension by both synthesized NCR composite in presence of light (UV or visible irradiation) is schematically explained in Fig. 11. Briefly, the metal oxides (NiO and/or CuO) were irradiated first with the light (UV and/or visible), then the electrons undergo ejected from valence band (VB) to conduction band (CB) of metal oxide, meanwhile the positive holes (h+) in VB and electrons (e−) in CB are formed respectively. These h+ and e− acts as good oxidizing and reducing agents respectively, they fascinates to the generation of radicals (•OH and •O2−) formed in respective VB and CB of NiO/CuO. Importantly, RGO can trap the photogenerated electrons and enhanced the ROS such as •OH and •O2− radicals of NCR photocatalyst which is responsible for the BG dye degradation under light illumination. As the NCR composite has lower bandgap among the prepared samples that can enhance the photocatalytic degradation of BG dye by capturing the light (UV or visible). Because it decrease the recombination rate of electron and hole pair in NCR composite. The greater conductivity of RGO enriches the charge separation expressively and it is crucial for electron-dominated reduction reaction (Kahan et al., 2008; Yu and Chuang, 2007). Finally, the ROS reacts with dye molecule and produce degradable products such as water and CO2 (Eqs. (8)–(13)).

(8)
NiO + light → NiO (h+) + NiO (e−)
(9)
CuO + light → CuO (h+) + CuO (e−)
(10)
NiO/CuO + light → RGO (e−)s
(11)
(e−) + O2 → •O2−
(12)
(h+) + OH → •OH
(13)
•OH + •O2− + BG → Degradation products
Possible mechanism for the degradation of BG dye solution by NCR composite under light irradiation.
Fig. 11 Possible mechanism for the degradation of BG dye solution by NCR composite under light irradiation.

3.3.2

3.3.2 Reactive oxygen species assay

The reactive oxygen species (ROS) (h+, •O2−, and •OH) are the most important in the photodegradation and ROS that serve as associates for photocatalysts under light radiations. In general, these ROS reacts with the dye molecule and delivers the degradation yields such as H2O and CO2 (Amiri et al., 2019). The 2-propanol is used for the confirmation of •OH radicals, similarly Benzoquinone is for O2− radicals and potassium iodide (KI) is for the both holes and •OH radicals. Remarkably, in the absence of scavenger, 97% removal was observed as shown in Fig. 12a but in the presence of scavengers the photodegradation quantity is very less. In the addition of RGO to the pure metal oxides enlarges the surface area of NCR composites which results in the increase of the more active sites that lies on the catalyst surface, thus NCR composites exhibit superior photocatalytic efficiency. Moreover, NCR-10% composite has stronger absorptions in the visible region than individual metal oxides so it enhances the photodegradation rate under visible light irradiation. This is proven through PL experiment in which catching the charge-carrier, migrate and transfer in photocatalyst due to electron-hole annihilation by the impact of RGO. Meanwhile, the PL emission peak was significantly quenched, signifying a substantial separation of photogenerated electrons and holes takes place (Kahan et al., 2008). The maximum excitation peak at 609 nm of NCR composite was further declined due to the presence of RGO (Fig. 12b) in composite that signified the NCR-10% contributes a great role in photocatalysis process by the radicals development for the interaction with BG dye molecules. NCR-10% composite having slighter electron and hole pair recombination rate, so it exhibits greater photocatalytic efficiency.

(a) ROS assay and (b) PL spectra of NCR-10% composite.
Fig. 12 (a) ROS assay and (b) PL spectra of NCR-10% composite.

3.3.3

3.3.3 Recyclability and stability of NCR-10%

The suspended solid (photocatalyst) in BG dye suspension was collected and washed with Milli Q water followed by ethanol for the examination of the recyclability of hydrothermally prepared NCR-10% composite. Therefore, the same photocatalyst was utilized for five successive trials or runs in photocatalytic degradation of BG dye solution under the similar conditions, the results clearly said that the NCR-10% composite has great potentiality for the degradation up to five successive runs without loss of more catalyst (Fig. 13a). This depletion of photodegradation may be due to the loss of quantity of catalyst during washes. After examining the recyclability of tested composite, collected and checked the stability by characterizing with XRD. The XRD patterns are same even after the completion of five successive runs without loss of more catalyst (Fig. 13b). To conclude that synthesized NCR-10% composite is stable photocatalyst and recyclable for successive five runs.

(a) Reusability and (b) stability of NCR-10% composite.
Fig. 13 (a) Reusability and (b) stability of NCR-10% composite.

3.3.4

3.3.4 UV lamp assisted degradation

Unambiguously as we expected, NCR-5% composite was degraded more BG dye solution (80%) compared to NCR-10% composite which, exhibited less degradation rate and showing the fluctuate values after 10 min (Fig. 14), for this, few factors affected on this abnormality, initially adsorption followed by slow desorption takes place due to the overdose of GO loading in preparation of the NCR ternary nanocomposites. This may also be of the wide bandgap that NiO has that falls in the UV region, as RGO reduced the bandgap which leads to fall in the visible region, thus lesser RGO contained material (NCR-5% composite) degraded BG dye more in UV region.

UV lamp assisted photodegradation of BG dye by (a) NCR-5% and (b) NCR-10% ternary composites.
Fig. 14 UV lamp assisted photodegradation of BG dye by (a) NCR-5% and (b) NCR-10% ternary composites.

Predominantly, the BG dye can be removed from the contaminated water using various technologies and materials, but this composite shown greater photocatalytic efficiency over the degradation of BG dye solution under visible light irradiation in a short time (Table 4).

Table 4 Literature survey on degradation of BG dye using various nanomaterials.
Photocatalyst Degradation (%) Time (min) Dye concentration Catalyst dose (mg) Ref
Sr-doped TiO2 96 60 25 ppm 500 (Yu and Chuang, 2007)
BiVO4 100 90 10 ppm 100 (Amiri et al., 2019)
Lead Chromate 100 45 1.0 × 10–3 M 482 (Sood et al., 2015)
ZnO 95 120 20.8 μM 2 g/L (Umabala et al., 2016)
GO/CuO 98 90 1 × 10−5 M 70 (Rawal et al., 2015)
TiO2 98 180 4 ppm 300 (Gole et al., 2017)
Zn doped SnO2 98 105 5 ppm 100 (Bhanu, 2019)
NiO-CuO-RGO 92 60 10 ppm 50 Present study

3.3.5

3.3.5 Appraisement of antimicrobial assay

The antibactericidal assay of hydrothermally prepared samples was executed all gram negative (E. coli) and gram positive (S. aereus) bacterial strains at the desired concentration of 40, 60 and 80 µg/mL. Initially, the same test was carried out with NiO but there was no significant result. Based on these results, further antimicrobial study was carried out for the synthesized composites. The synthesized NCR composites were used for antibacterial assay, these composites were effectively inhibited the bacterial growth and showed excellent results for gram positive than gram negative bacteria. The obtained results stated that S. aereus is the most effective (19 µg/mL) at the concentration of 80 µg/mL by NCR-10% composite. The measured zone of inhibition of prepared NCR composites against aforementioned bacterial strains as shown in Fig. 15 and tabulated the results in Table 5.

Inactivation of pathogens by (a–b) NCR-5% and (c–d) NCR-10%.
Fig. 15 Inactivation of pathogens by (a–b) NCR-5% and (c–d) NCR-10%.
Table 5 Bacterial zone of inhibition by prepared NCR composites.
Tested sample Organism Zone of inhibition (mm)
40 µg/ml 60 µg/ml 80 µg/ml Standard (Chloramphenicol) 30 µg/ml
NCR-5% E. coli 10 12 14 16
S. aureus 11 13 15 18
NCR-10% E. coli 14 16 18 20
S. aureus 16 18 19 22

Fig. S3 (ESI) has confirmed the improved antibacterial activity for NCR than prepared binary composites and pure metal oxides and the results are listed in ESI (Table S2) due to the presence of CuO and RGO. The mechanism of antibacterial activity of nanomaterials not yet reported. However, two steps are involved in this process such as adsorption and penetration. In adsorption, the hydrothermally synthesized materials adsorb on the bacterial cell wall due to the electrostatic interactions. Thus, NCR composites display a significant antimicrobial activity due to the discharge of Cu2+ from NCR composite (Cioffi et al., 2005); this is because of the electrostatic forces among copper ions and bacteria opposite charges, held to cause bioactivity and adhesion. Based on results, it is assumed that binding of Cu2+ ions from NiO-CuO-RGO nanocomposites to surface of the bacteria cell is a key factor in antibacterial activity. Later it penetrates into the cell and damages the cell structures are DNA, nucleus and intracellular organisms. For example, the gram negative bacterium E. coli may let more copper ions (Cu2+) to attain the plasma membrane of cell but is generally considered less vulnerable to antibacterial agents than gram positive bacteria (Mirzaei and Darroudi, 2017). Moreover the gram positive bacterium shows greater inhibition growth rate than gram negative due to they possess variances in their membrane structure that means differences in cell structure, metabolism, degree of interaction of nanoparticles with microorganisms and the most distinctive in thickness of peptidoglycan layer.

4

4 Conclusion

In conclusion, the most environmental hazards such as dye and microorganisms successfully removed from the contaminated water by prepared materials. Brilliant green dye is almost completely decolourized by prepared NCR composites under both Visible and UV light irradiations. The NCR-10% composite has great attention in photocatalytic degradation of BG dye under visible light irradiations in 60 min, alike that, NCR-5% composite exhibited high photocatalytic efficiency over BG dye solution under UV light irradiations in 60 min. Moreover, the NCR composites are more effective for inactivation of the bacterial strains such as E. coli and S. aereus, predominantly, S. aereus was inhibited more. However, the RGO contributes great role in photocatalytic degradation of BG dye solution and antibacterial activity due to its narrow bandgap energy, large surface area and generating more ROS.

Acknowledgement

The author G Satya Sree acknowledges the Andhra Pradesh Pollution Control Board (APPCB), Vijayawada for the financial assistance under APPCB Research fellowship (APPCB/RF/2018-19/07).

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Appendix A

Supplementary material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.arabjc.2020.02.012.

Appendix A

Supplementary material

The following are the Supplementary data to this article:

Supplementary data 1

Supplementary data 1


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