Introduction

Mineral water production and degradation of organic impurities in water using the advanced oxidation process (AOP) semiconductor photocatalysts accept the much consideration and are in the front line of central studies and technological approaches in recent years [1, 2]. The industrial wastewater contains a small number of dye molecules, which stimulate water contamination as many pollutants are non-reactive towards light, acids, bases and O2, thus, the colour of the material turns permanent. Dye degradation comprises action in which the macroscopic dye molecule is split up into lower molecules like carbon dioxide and purified by-products [3, 4]. From many methods for water remediation, reported adsorption, coagulation, filtration [5] and photocatalysis [6], the latter technique is an exceptionally favourable route for water cleaning from harmful contaminants owing to the simplicity of equipment and complete degradation of pollutants [7].

Photocatalysis represents an advanced oxidation process (AOP) in which the interaction of light to a semiconducting material normally metallic elemental oxides, which give electron–hole pairs (ehp) and leads to successful proceeding of electrons from valence band to conduction band [8]. The electron interacts with O2 and hole with H2O molecules to form superoxide anions and ·OH radical, which is highly reducing and oxidizing agent for the organic dye compounds used in many industries [9]. The primary mechanism of AOP is to increase the adsorption capacity of semiconductor material towards organic contaminants through using low consumption source such as in visible light [10, 11].

Zinc Oxide (ZnO) nanomaterials have been studied extensively in the past years and presented unique properties for multidisciplinary applications such as sensors [12], catalyst [13], high effective and functional devices [14] and photoelectron devices [15, 16]. ZnO has a wide direct energy bandgap (3.1–3.4 eV) semiconductor at ambient conditions [17]. ZnO materials with different nanostructured are utilized for photocatalytic, photochemical, optoelectronic and electrical applications [6, 18]. However, the photocatalytic application of native ZnO has been constructed to the UV spectrum of light attributable to the broad bandgap. Since ultraviolet illumination just accounts for 5% of the solar spectrum, besides, its stableness and fast recombination of charge carriers (ehps) under illumination are receded that are necessary to be addressed [19, 20].

Likewise, the interaction between bacteria and nanomaterials surface is a versatile tool to access the antibacterial activity of the materials. Till date, different ZnO nanostructures such as one-dimensional, two-dimensional, spherical, and hierarchical nanostructures have been prepared and utilized for photocatalytic degradation of organic pollutants and drug-resistant bacteria. The advantage of ZnO nanomaterial usage as antibacterial purposes is that they can be promptly blended with wound dressing stuff. So this study is an attempt to enhance the photocatalytic and antimicrobial activity of the ZnO-Nps.

Synthesis of ZnO nanomaterials have been reported in literature using different protocols such as combustion method [21, 22], thermal decomposition [23], sol–gel method [24], hydrothermal method [25], Laser ablation [26], electrochemical [27], ultrasound [28], microwave-assisted method [29], co-precipitation method [30, 31], two-step mecheno-chemical thermal synthesis [32], biological methods [33,34,35,36,37] and electrophoretic deposition [38]. Among these different methods, the hydrothermal method is a versatile synthesis method and favourable than many other synthesis processes, imputable energy reduction, low handling temperatures, more crystallinity, controlled growth, the adeptness to recycle wastes, the secure and appropriate disposal of rubbishes [39, 40]. Therefore, we used hydrothermal method for the synthesis of ZnO catalyst.

From the literature survey [2, 4, 41,42,43,44,45,46,47] and previous discussion, we have come to know that for photocatalytic and antimicrobial purposes the catalyst morphology, the synthesis route, and specific surface area have significant effects. Therefore, stimulated from these concepts, in the present report, ZnO-Nps were prepared, as ZnO with novel nanoplate morphology is rarely reported using a hydrothermal method which gives small grain size with improved crystallinity and larger surface area as compared to other methods. The attractive features of ZnO and its extensive applications motivate scientists to explore further bright aspects of this material. Different properties of the ZnO catalyst were quantified by XRPD, XPS, SEM, TEM, EDS, FT-IR, BET, PL, and EIS. The photocatalytic activity of ZnO-Nps photocatalyst was inquired by the degradation of Methylene orange (MO) dye under visible light. Antimicrobial properties of the ZnO-Nps were also investigated. This work reveals the potential of ZnO nanopalates for environmental remediation as well as in Antimicrobial effect.

Materials and Methods

Zinc acetate, cetyl trimethyl ammonium bromide (CTAB), and MO were purchased from Merck Company. In all tests double distilled water was used. In a typical procedure, 6.30 g zinc acetate [Zn(CH3COO)2⋅2H2O] and 0.70 g CTAB was dissolved in 70 mL deionized water, respectively under vigorous stirring for 2 h. Hydrochloric acid (HCl) and sodium hydroxide (NaOH) were exploited to adjust the pH value of solution around eight as and if required. The solution so formed was transferred into 100 mL Teflon lined autoclave. The autoclave was kept in the oven at 180 °C for 24 h. Subsequently, the autoclave was cooled to room temperature naturally. The obtained precipitates were filtered, washed several times with deionized water and ethanol alternatively and dried at 60 °C in oven for 8 h [48, 49]. The as-synthesized ZnO sample was then characterized for different physical and chemical conformations, i.e., X-ray powder diffraction, SEM, TEM, EDX, XPS, FTIR spectroscopy, EIS, BET surface area analysis and photoluminescence techniques. The proposed mechanism digram for the synthesis of ZnO-Nps by hydrothermal method is illustrated in Fig. 1.

Fig. 1
figure 1

Schematic illustration of the proposed hydrothermal synthesis of ZnO-Nps and its application

Full size image

Photocatalytic Activity

The photocatalytic activity of the ZnO photocatalyst was investigated over the Methylene orange (MO) dye under the visible light. A Xenon lamp with accumulative 300 W was used as a visible light source. A Perkin Elmer λ 35 UV–Vis spectrophotometer is used to notify the absorbance spectra of the solution. For experimentations, 100 mg of the ZnO catalyst was first dispersed to 100 mL of an aqueous solution containing 100 mg MO dye. The aqueous solution was stirred vigorously for 2 h in the dark to reach adsorption–desorption equilibrium between MO dye and ZnO catalyst. To avoid any thermal degradation of MO in solution, an ice bath was used throughout the experimentation and constant stirring was maintained. After appropriate sessions of irradiation, a small amount of samples ~ 3 mL suspensions were periodically removed after each 30 min and centrifuged at 5000 rpm and tested for the residual concentration of MO in the solution which has a maximum absorbance at a maximum wavelength of 464 nm [50, 51]. The degradation efficiency is calculated by following the equation;

$${\text{Degradation efficiency}} = \left( {\text{C}_{0}}-\text{C}_{\text{t}}\right)/{\text{C}}_{{0}}$$
(1)

where C0 is the initial concentration of dye at adsorption–desorption equilibrium, while Ct is the concentration at a later time ‘t’ [52, 53].

Antimicrobial Activity

The antibacterial activity of ZnO-Nps was identified by time kill method against clinical isolates of Methicillin-resistant Staphylococcus aureus (MRSA) and Escherichia coli (E. coli). In time kill method, optical density (OD) of bacterial culture (MRSA and E. coli) containg ZnO-Nps was recored after different interval of time [54]. There were 100 µg ZnO-Nps added into 3 mL of LB broth with 100 µL of each bacteria inoculum prepared with McFarland concentration inubated along with positive control containing no ZnO-Nps for comparison. The optical density was checked after different intervals of time (2, 4, 8, 21 and 24 h) [55].

The minimal inhibitory concentration (MIC) was identified by micro-dilution method, including 100 µL of LB broth added in each well of micro-titration plates with 10 µL of each bacterial strains with twofold dilution of ZnO-Nps (100, 50, 25, 12.5, 6.25 and 3.125 µg) and recorded O.D600 after the incubation at 37 °C for 16–20 h [56]. For identification of ZnO-Nps are bactericidal or bacteriostatic, minimal bactericidal concentration (MBC) was used, 10 µL of culture form each well of MICs finding and grown LB agar medium by the incubation at 37 °C for overnight [57].

Results and Discussion

For phase purity and crystallinity, the ZnO nano powder was characterized by XRPD technique as illustrated in Fig. 2. The sharp peaks indicating intense crystal quality of ZnO with the sharpest peak positioned at 38.39°. All the peaks are indexed with JCPDS card No. 01-076-0704 with a lattice parameter of a = 0.325 nm and c = 0.521 nm and well-matched from the previously reported results [58, 59]. It can be seen from Fig. 2 that no other phase of ZnO is present, consequently confirms the purity of the output product.

Fig. 2
figure 2

The XRD pattern for the as-synthesized ZnO-Nps by hydrothermal method

Full size image

For morphology and topographical purposes, SEM characterization was used and the micrographs of as-synthesized ZnO are shown in Fig. 3a–c. The SEM images were taken at high magnification, indicating the plate type morphology with width 150–300 nm and thickness 10–60 nm estimated using ImageJ software. It is revealed, the formation of Nps is owed to chelation of metal cations in the suspension [47], which is also confirmed by TEM analysis (Fig. 3e). A plate-like morphology indicates the feasibility of formed nanostructures for the photocatalytic application. Although it is difficult to take the idea about the particle thickness in TEM image porous surfaces can be witnessed. To confirm the chemical composition of the as-synthesized sample, the Energy-dispersive X-ray spectroscopic (EDX) technique was applied, the result of EDX indicate that the sample is composed of Zn and O elements, no other impurity or contamination was noticed within the detection limit, the C and Pt peaks are raised due to background carbon tape and Pt coating as displayed in Fig. 3d [59, 60].

Fig. 3
figure 3

SEM image at a low magnification, and b, c high magnification, d EDX spectra of as-synthesised ZnO-Nps, e TEM image of ZnO-Nps synthesized hydrothermally at 180 °C

Full size image

ZnO Nps have been further characterized by X-ray photoelectron spectroscopy (XPS) to probe chemical composition, as XPS is a flexible tool for the chemical conformation investigation and surface state purity of the samples, Fig. 4 is showing the XPS results which confirmed that the as-synthesized material is composed of pure Zn and O elements. The binding energies of Zn for core levels 2p are 1027.40 eV for 2p3/2 and 1014.14 eV for 2p1/2 as shown in high resolution XPS spectra (Fig. 4, inset) and for O core levels 2s, the typical binding energy 530.4 eV is noticed [60, 61]. The FTIR spectroscopy of ZnO Nps provides information about the attached functional groups (Fig. 5). The absorbance from 400 to 480 cm−1 is owed to Zn–O stretching mode, the peak at 1630 cm−1 is indicating O–H bending band and the broadband at 3480 cm−1 is the O–H stretching mode indicating the absorbance of water at the ZnO-Nps surface [61, 62]. Thus, the FTIR result is also confirming the formation of pure ZnO catalyst. XRD, EDX, XPS, and FTIR results confirm that the product is pure ZnO.

Fig. 4
figure 4

Wide scan total XPS spectrum of the ZnO-Nps photocatalyst, (inset) High resolution spectra of Zn 2p and O 1s

Full size image
Fig. 5
figure 5

FTIR spectra of hydrothermally synthesized ZnO nanoplates

Full size image

It is well known that photocatalytic phenomena are surface area dependent, higher the surface area, greater will be the absorption and consequently raise the degradation of the pollutants. The specific Brunauer–Emmett–Teller (BET) surface area of the ZnO-Nps was evaluated by adsorption–desorption isotherms of nitrogen gas in the pressure range 0.05–1 (Fig. 6). The BET area was found 126 cm2 g−1 which is sufficiently large and indicating high crystalline nature and regular placement of pores within the structures. The pore range ensued by Barret–Joyner–Halender (BJH) method and was found the average pore size of 10 nm (Fig. 6, inset). The pore size intense peak distribution is mostly from 3 to 9 nm indicating the nano-porous within the ZnO-Nps. This type of nanostructures is considered well suited for photocatalysis as it provides effective cites for reactants and products. The ZnO-Nps isotherm followed a typical type III isotherm presenting the hysteresis of H3 type indicate the formation of wide frame group of atoms and reveal the presence of mesopores within the structure and strong adsorbate–adsorbate interaction [63, 64]. So, it is confirming that ZnO-Nps is highly crystalline mesoporous material.

Fig. 6
figure 6

N2 absorption–desorption curve for BET surface area measurements of ZnO-Nps

Full size image

PL spectrum for ZnO-Nps in Fig. 7a depicts a strong broad emission band in visible-range. This emission is indicating the potential of ZnO-Nps in the visible light region; it means that visible PL bands correspond to a deep level in ZnO-Nps that can be excited by the visible light source. From photocatalytic degradation point of view, this particular behaviour to absorb visible light is very promising for the photocatalysis process. The Photoluminance (PL) result is matching inconsistent with the previously reported results [65, 66]. Electrochemical impedance spectroscopy (EIS) provides a versatile method for the psychoanalysis of the electron transfer, conduction, and surface area of materials. Figure 7b exhibits the comparability of the bare electrode and modified ZnO GCE graphical record in 0.1 M of LiSO4 solution. The spectrum of the ZnO GCE is indicated by a semicircle at the higher frequency and represents nearly a straight line at low frequency. The results exhibit that the ZnO GCE is an electron channel moderated at high frequency and diffusion operated at low frequencies. The final result shows that the ZnO GCE brings down the electron transportation impedance and turn into controlled electron transmits activity, suggesting a significantly minimized recombination rate.

Fig. 7
figure 7

a Photoluminescence spectra of hydrothermally synthesized ZnO nanoplates, b EIS (in Nyquist plots) of modified ZnO GCE in 0.1 M of LiSO4 showed the reduce charge transfer impedance and recombination rate

Full size image

Photocatalytic Analysis of Zno-Nps Prepared By Hydrothermal Method

For a photocatalytic response, ZnO-Nps were used for the degradation of MO dye solution under the visible light irradiation; for comparison, the photocatalytic response without ZnO catalyst just under visible light and ZnO-Nps without visible light were investigated. The absorption spectra of MO dye solution after particular irradiation of visible light intervals over the ZnO-Nps is depicted in Fig. 8a. The typical absorption peak of MO is detected at 464 nm. The strength of the absorption peak decreased noticeably with prolong of the visible irradiation time [67]. MO dye concentration decreases to 0.09 mg L−1 upon visible light irradiation for 4 h. The colour of MO dye solution changes from yellow to colourless. The C/C0 ratio from Fig. 8b indicates that the ZnO-Nps conduct ordinal photocatalytic response for MO in the absence of visible light. Similarly, no photocatalytic activity was observed with only visible light illumination. This exhibits that degradation of MO over the ZnO-Nps is light-dependent [68, 69]. The photocatalytic experiment of MO dye soluation over the ZnO-Nps was repeated three times and the calculated error-bar for efficiency is shown in Fig. 8c, which also confirm the recyclability and stability of ZnO-Nps photocatalyst. The photocatalytic decomposition process is the absorption of pollutant dye molecules on ZnO-Nps surface. The diffusion of electron–hole charge carriers on the ZnO-Np surface and the photocatalytic reaction is explained in the following equations.

$${\text{ZnO-Nps}} + {\text{h}}\upnu \to {}^{*}{\text{ZnO-Nps}}\left( {{\text{e}}^{-} + {\text{h}}^{+} } \right)$$
(2)

Photo-oxidation reaction

$${\text{H}}_{{2}} {\text{O}} + {\text{h}}^{ + } \to {\text{OH}}^{ - } + {\text{H}}^{ + }$$
(3)

And photo reduction reaction

$${\text{O}}_{{2}} + {\text{e}}^{ - } \to {\text{O}}_{2}^{ - }$$
(4)
$${\text{O}}_{{2}}^{ - } + {\text{H}}^{ + } \to {}^{ \cdot }{\text{OH}}$$
(5)

Overall reaction

$${\text{OH}}^{ \cdot } + {\text{O}}_{2}^{ - } + {\text{H}}^{ + } + {\text{MO}}\left( {{\text{dye}}} \right) \to {\text{CO}}_{{2}} + {\text{H}}_{{2}} {\text{O}} + {\text{Degradation by-products}}$$
(6)
Fig. 8
figure 8

a Follow up of MO dye solution degradation overtime under visible light using UV–vis spectrophotometer, b MO dye concentration with respect to initial concentration for different irradiation time, c recyclability for the photocatalytic performance of dye solution for three cycles (error bar shows the stability of the nanomaterials)

Full size image

When a photon of appropriate energy i.e. equal or greater than the energy bandgap of the ZnO-Nps interact at the surface, an electron–hole pair is generated, the electron transfers to the conduction band and interact with the oxygen molecule to make superoxide ion. This ion interacts with a water molecule and creates a series of hydroxyl radical, meanwhile, the positive hole created in the valance band interact with hydrogen ion to create hydroxyl radical [70]. These series of hydroxyl radicals from both sides being a strong and powerful oxidizing agent that degrade the MO dye molecules into the harmless form or minerals as proposed in Eq. (6) [71]. The involved propose mechanism for photocatalytic activity of ZnO-Nps is illustrated in Fig. 9.

Fig. 9
figure 9

Proposed photocatalytic mechanism of the as-synthesized ZnO-Nps by hydrothermal method

Full size image

Evaluation of Antimicrobial Activity of ZnO-Nps Prepared By Hydrothermal Method

ZnO is non-toxic, bio-safe and has higher antibacterial potency, used as healing wounds and take advantage as nanomedicine against treating different bacteria and diseases including cancer. It has well established anti-microbial activity as studied by Farzana [72]. An investigation has been made against the effect of ZnO-NPs on the antimicrobial inhibition rate of MRSA and E. coli. MRSA and E. coli are harmful bacteria which are commonly found in food, water, intestinal, and throat infection diseases. The exact mechanism of interaction of the ZnO-Nps with bacterial cells is even arguable and unclear. Thus effective and cheap antibacterial agents should be developed to inhibit the bacterial growths.

The analyzed results are obtained at different incubation intervals (24 h) of strains with and without NPs at 37 °C in the incubation chamber. Absorption spectrometer recorded the results at OD of 600 nm for the variation of time of incubation. The bacteria growth curve to time-kill assay has been shown in Fig. 10a, b. Untreated bacterial strains are controls in this experiment. ZnO-NPs have shown antimicrobial effect towards all the strains.

Fig. 10
figure 10

Illustration of the effect of a ZnO-Nps on MRSA, b ZnO-Nps on E. coil bacterial strains at the various time intervals

Full size image

Growth trend of bacteria inhibited by ZnO-Nps has been observed to a significant rate as shown in graphs and inset. The rate of MRSA bacteria has been inhibited up to 7.44, 45.10, 82.00, 100.51, and 219.5% for 2, 4, 8, 21 and 24 h of incubation respectively against 265% of bacterial growth at 24 h, as shown in Fig. 10a. Similarly, Fig. 10b illustrated the rate of E. coli has been inhibited by ZnO-NPs up to 26.48, 34.85, 46.56, 98.51, and 107.6% for 2, 4, 8, 21 and 24 h of incubation against 313.3% of bacterial growth at 24 h. Effect of ZnO-Nps on bacterial growth inhibition has been reported by Nicole Jones for antibacterial activity of ZnO nanoparticles [73]. Raghupathi has reported size-dependent bacterial inhibition growth and mechanism of ZnO nanoparticles [74].

There are several theories on the mechanism of loss in cell viability of bacteria. Recent research has provided the reason for charge unbalancing caused by the contact of Nps with the membrane of the cell (bacteria). Due to electrostatic interaction caused between the positively charged (electron acceptors) ZnO-Nps and negatively charged cell membrane of bacteria resulted in the transfer of charges from bacteria membrane to ZnO-Nps, thus caused the enveloping the cell membrane and inducing the production of reactive oxygen species into the cells. The ZnO-Nps released Zn2+ ions, and these positive metal ions of Zn2+ penetrate the cells resulting in bacterial cell death [75].

The MICs result revealed that the 12.5 and 6.25 µg of ZnO-NPs are minimal concentration which inhibited the growth of MRSA and E. coli respectively as shown in Fig. 11. In case of MBCs there is no growth was observed in those plate streaked from MICs well with 25 and 12.5 µg ZnO-Nps against MRSA and E. coli respectively. MRSA and E. coli are clinically significant bacteria which are commonly found in food and water also responsible for causing intestinal, and throat infection diseases which lead to bacteremia [76]. Due to evolution in these bacteria, modifying themselves by developing various antibiotics resistance mechanism. Thus effective and cheap antibacterial agents those are alternatives of these antibiotics should be developed to combat these multidrug resistant bacteria. Further studies can be conducted by varying the doses of ZnO-Nps into bacteria and also conjugating it with other therapies and drug for the synergistic effect of treating different clinically significant pathogens.

Fig. 11
figure 11

MICs of ZnO-NPs against MRSA and E. coli

Full size image

Conclusion

In summary, the ZnO-Nps were successfully synthesized by facile hydrothermal method. The XRD, EDX, XPS and FTIR confirm the formation of pure ZnO. The SEM, TEM micrographs of hydrothermally prepared ZnO show the morphology with nanoplates type with 150–300 nm width and thickness 10–60 nm. The ZnO-Nps comprises a sufficiently large surface area 126 cm2 g−1 and indicating the nanopores within the ZnO-Nps, which enhanced the photocatalysis and antimicrobial activity. The obtained PL results also reveal the ZnO visible activity in the visible regions. These results indicate that the as-synthesized ZnO is a promising candidate for photocatalytic activity as well as in antimicrobial discipline.