Realizing cost-effective ZnO:Sr nanoparticles@graphene nanospreads for improved photocatalytic and antibacterial activities

Abstract

Strontium doped ZnO nanoparticles@graphene nanospreads were synthesized using a facile cost effective wet chemical method. Their structural, surface morphological, photocatalytic and antibacterial properties were studied and compared with those of the ZnO:Sr and bare ZnO particles. X-ray diffraction analysis confirmed that all the synthesized samples have hexagonal wurtzite structure. The Field Emission Scanning Electron Microscope (FESEM) study displayed a platelet like morphology and EDX mapping confirmed the presence of the elements involved in the components of the synthesized composite material. Transmission Electron Microscope (TEM) analysis revealed that the ZnO:Sr nanoparticles are anchored on graphene sheets. The presence of the constituents of the composite is confirmed by Fast Fourier Transform (FFT) analysis. ZnO:Sr/G nanocomposite sample exhibits enhanced photocatalytic activity against methylene blue dye compared to the ZnO:Sr and bare ZnO samples. Similarly, an improved antibacterial activity against two Gram-positive as well as two Gram-negative bacteria was also observed for the ZnO:Sr/G nanocomposite.

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1. Introduction

Degradation of organic pollutants, purification of industrial effluents and disinfection of pathogenic bacteria are some of the most important environmental related challenges that should be given high priority. Photocatalysis with semiconductors like TiO₂, ZnO and SnO₂ is one of the green technologies suitable for addressing these environmental issues. But a major drawback in utilizing these semiconductors as photocatalysts is fast recombination of photogenerated electron–hole pairs, which greatly affect the efficiency of the photocatalytic activity. Further, visible light response of large bandgap semiconductors is also a limiting factor in photocatalytic degradation of effluents. Development of multi component nanocomposites, is one of the best ways for the utilization of larger portion of solar energy spectrum and suppression of the photogenerated electron–hole pair recombination.

ZnO is one of the well known photocatalysts having several unique features viz. low cost, abundance, non-toxicity and large exciton binding energy (60 meV).¹ However, because of its wide bandgap energy, photoexcitation is possible only with ultraviolet (UV) radiation. Moreover the quantum efficiency of ZnO is much affected by instantaneous recombination of photogenerated electron–hole pairs. Therefore it is necessary to modify the properties of ZnO suitably, so that it can act as an efficient photocatalyst. Doping ZnO with transition metals,² alkaline elements, rare earth elements,³ noble metals,⁴ graphene oxide,⁵ graphene⁶ and graphdiyne⁷ can enhance the photocatalytic efficiency of ZnO.

Literature report reveals that doping of alkaline earth metals with ZnO leads to the creation of lattice defects due to the charge compensation and ionic radius mismatch between the alkaline earth metal ions (Mg²⁺, Ba²⁺, Sr²⁺ etc.) and Zn²⁺, which can improve the photocatalytic activities.⁸ It is demonstrated that the strontium doped ZnO nanocrystallites is a potential candidate for the degradation of Rhodamine B dye.⁹

Hybridization of ZnO with carbon materials is an effective strategy to improve the photocatalytic performance and stability of ZnO as a photocatalyst.¹⁰ Among carbon involved materials, graphene is a promising candidate for photoredox reactions. Graphene is an allotrope of carbon, which is composed of layers of carbon atoms packed into a honeycomb network, having remarkable properties such as atom-thick 2D structure, excellent transparency (∼97.7%), theoretically high specific surface area (2630 m² g⁻¹), superior mobility of charge carriers (∼10 000 cm² V⁻¹ s⁻¹), and high electrochemical stability.¹¹ Further, graphene posses exceptional Young's modulus (∼1.0 TPa), large spring constants (1–5 Nm⁻¹) and excellent thermal conductivity (∼5000 W m⁻¹ K⁻¹).¹² These superior qualities of graphene induce enormous interest in the scientific and industrial communities. Several researchers reported the enhanced photocatalytic activities of the graphene hybridized ZnO composite photocatalysts in which semiconductor components act as the light harvesters while the role of graphene is co-catalyst.

To the best of our knowledge, reports on multicomponent nanocomposite system involving ZnO:Sr with graphene for photocatalytic applications is hardly available in the literature. Similarly, the bactericidal activity of the graphene-semiconductor with metal system is not much explored. Considering the above mentioned points, in the present work, we have synthesized a new photocatalytic material ZnO:Sr/G nanocomposite by adopting a facile wet chemical method. Pristine ZnO and strontium doped ZnO nanoparticles were also synthesized for comparison.

The combination of graphene-single semiconductor with metal composites leads to the multinary composites with multilevel/route electron transfer system.¹³ Enhanced photocatalytic performance can be expected from nanocomposites with multilevel electron transfer system because of their integrative combination of the individual components contributing desirable properties suitable for various steps involved in the photocatalytic activity.

The photocatalytic activity of the synthesized samples was tested for the degradation of Methylene Blue (MB) dye. Along with photocatalytic activities, the antibacterial properties of the synthesized nanocomposite samples were also tested against two Gram-positive (Bacillus subtilis, Staphylococcus aureus) and two Gram-negative (Escherichia coli, Klebsiella pneumoniae) bacteria. The effect of Sr and graphene incorporation is investigated on the antibacterial efficiencies is studied and reported.

2. Experimental

2.1. Preparation of ZnO, ZnO:Sr and ZnO:Sr/graphene samples

Graphene Oxide (GO) was prepared from graphite powder by modified Hummers method.¹⁴ For the ZnO nanoparticle synthesis, 2.194 g of zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O) was dissolved in 50 mL of deionised water to obtain a solution having concentration 0.2 M. The solution was stirred continuously using magnetic stirrer until a homogeneous solution was obtained. Then 1 g of polyvinylpyrrolidone (PVP M.W. 40 000) was added into the zinc precursor solution, as a capping agent. Finally, 1.6 g of NaOH was dissolved in 50 mL of deionised water (0.8 M) and this solution was slowly added into the PVP modified zinc precursor solution. Stirring was continued for 2 h and the white precipitate thus obtained was rinsed with deionised water several times and filtered. The resultant product was dried at 60 °C for 2 h. For the ZnO:Sr nanoparticle synthesis, required amount of strontium nitrate (Sr(NO₃)₂) was added to keep 5 wt% (0.01 M) of Sr in the starting solution and the same procedure was followed as in the case of bare ZnO. For the ZnO:Sr/G sample synthesis, 50 mg of graphene oxide was dispersed in 50 mL of deionized water and sonicated for 30 min. Then 50 mL of PVP modified zinc precursor solution with 5 wt% of Sr was added with the GO solution under magnetic stirring followed by addition of 4 mL of 0.0008 M NaBH₄ solution. Here, NaBH₄ solution acts as a reducing agent. The remaining synthesis procedures were the same as that of the bare ZnO synthesis. All the products were calcinated at 400 °C for 3 h.

2.2. Characterization techniques

The crystal structure of the synthesized samples was studied using X-ray diffractometer (PANalytical-PW 340/60 X' pert PRO) with Cu-K_α radiation (λ = 1.5406 Å). The absorption spectra were measured by the UV-vis Spectrophotometer (Perkin Elmer LAMBDA-35). Photoluminescence (PL) spectra were obtained using spectro-fluorometer (VARIAN – CARY Eclipse). Fourier transform infrared (FTIR) spectra were observed using spectrophotometer (Bruker, Alpha T, Germany). The surface morphology was examined by Field Emission Scanning Electron Microscope (FESEM – CarlZeiss, Neon 40). HR-TEM images and Selected Area Electron Diffraction (SAED) pattern was obtained by JEOL-2010 transmission electron microscope.

2.3. Photocatalytic test

The photocatalytic activity of the samples was evaluated by the photo degradation of Methylene Blue (MB) dye using a visible annular type photoreactor equipped with 500 W tungsten lamp as the visible light source. 50 mg of the synthesized sample was dispersed in 150 mL of MB solution (1 × 10⁻⁵ M). The mixed solution was sonicated for 30 min in dark to reach an adsorption–desorption equilibrium. Under ambient conditions, the photocatalyst mixed MB solution was exposed to visible light irradiation for 75 min. The test solution was sampled at intervals of 15 min, centrifuged and filtered to remove the photocatalyst. The dye degradation was observed by measuring the absorbance of MB at 664 nm using a UV-vis Spectrophotometer (Perkin Elmer LAMBDA-35).

2.4. Antibacterial activity evaluation

The antibacterial activity of the synthesized samples was tested against two Gram-positive bacteria (Bacillus subtilis, Staphylococcus aureus) and two Gram-negative bacteria (Escherichia coli, Klebsiella pneumoniae) by adopting disc diffusion method using Mueller–Hinton agar. The prepared medium was poured into Petriplates and fresh bacterial cultures were spread over the plates by spread plate technique. Discs of 6 mm diameter containing samples (20 μg) were dispensed on to the Petriplates and pressed gently in order to ensure that the discs were attached to the agar. For comparison, a disc containing Gentamicin, a standard antibiotic was also placed on each plate. The plates were incubated at 37 °C for 24 h. After the incubation period, the diameter of the inhibition zone formed around the paper discs was measured and expressed in mm.

3. Results and discussion

3.1. XRD studies

Fig. 1 shows the XRD patterns of ZnO, ZnO:Sr nanoparticles and ZnO:Sr/G nanocomposite. All the diffraction peaks recorded for bare ZnO nanoparticles are consistent with the hexagonal wurtzite phase of ZnO (JCPDS no. 36-1451). However for the ZnO:Sr sample, new weak peaks at 2θ = 25.2°, 44.25° and 49.94° correspond to SrCO₃ (JCPDS no. 84-1778) are observed. The formation of SrCO₃ may be due to the decomposition of Sr (NO₃)₂ into SrO on the surface of ZnO, and the subsequent absorption of atmospheric CO₂ during the calcination process at higher temperature. The as formed SrO generally absorbs the CO₂ resulting in alkaline metal carbonate formation.¹⁵

Fig. 1 (a) XRD patterns of ZnO, ZnO:Sr and ZnO:Sr/G samples. (b) Enlarged region for 2θ values between 30° and 40° of (a).

In the case of ZnO:Sr/G nanocomposite also a new peak arises at 2θ = 25.5°. But here it is difficult to elucidate to which phase this peak is associated with, because the reduced graphene oxide phase also has a diffraction peak close to this diffraction angle. This reflection peak at 25.5° with the d-spacing value 3.5 Å may correspond to the (002) plane of graphene or the (111) plane of SrCO₃. Even though the observed results cannot confirm the presence of graphene, the SEM and TEM images of this sample act as strong evidences for the presence of graphene.

The crystallite sizes (D) of the synthesized samples were calculated by measuring the full width half maximum of the intense diffraction peak corresponds to (101) facet in all the cases, using Scherrer's formula,

where λ is the wavelength of the X-rays (1.5406 Å for Cu K_α radiation), β is the FWHM in radians on the 2θ scale and θ is the Bragg's angle.

The lattice parameters a and c are calculated using the formula,

The volume of the unit cell (V) is determined using the relation,

The calculated crystallite sizes and lattice parameters are listed in the Table 1. The calculated crystallite sizes are 34, 30 and 21 nm for the ZnO, ZnO:Sr, ZnO:Sr/G samples, respectively. The observed values of V do not vary significantly which indicates that the lattice structure of ZnO is not altered after the addition of strontium and graphene. The broadening of the diffraction peak in the case of ZnO:Sr/G sample is a clear indication for the reduction in the crystallite size as depicted in the enlarged portion (Fig. 1(b)) of the diffraction pattern. The reduction in crystallite size with the graphene addition may be due to one of the salient features of graphene – it restricts the growth of grains/crystallites or limits the agglomeration of particles formed on it. Several reports reveal that during the in situ formation of graphene involved nanocomposites, the graphene sheets act as ultrathin support base for the particles of the coexisting components and keeps them in dispersed form on it as elaborated in the surface morphological studies.¹⁶

Table 1 Structural parameters of ZnO, ZnO:Sr and ZnO:Sr/G samples

Sample	Lattice constants^a (Å)		Crystallite size D (nm)	c/a	Volume of the unit cell V (Å^3)
	a	c
a Standard values: JCPDS card no. 36-1451, data: a = 3.2498 Å and c = 5.2066 Å.
ZnO	3.2609	5.2106	34	1.5979	47.9822
ZnO:Sr	3.2600	5.2007	30	1.5953	47.8646
ZnO:Sr/G	3.2608	5.2142	21	1.5990	48.0124

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3.2. UV-vis absorption studies

The absorption spectra of the synthesized samples are shown in Fig. 2. All the samples show a sharp characteristic absorption peak at 376 nm which is due to the intrinsic band gap absorption of ZnO. It is observed that the peak positions of the ZnO:Sr and ZnO:Sr/G samples are not much affected with the introduction of strontium and strontium/graphene, respectively. However, the absorbance of the ZnO:Sr/G sample increases over the entire spectrum. In this case, the carbon content was free graphitic carbon, which leads to an increase in the absorbance without altering the band gap energy. The enhancement of light absorption intensity and range with the graphene content in the ZnO:Sr/G sample might be due to the increase of surface electric charge of the oxides in the nanocomposite, leading to modifications of the fundamental process of electron/hole pair formation while irradiation.¹⁷ The enhanced light absorption in the visible region is beneficial for the photocatalytic degradation process.

Fig. 2 UV-vis absorption spectra of ZnO, ZnO:Sr and ZnO:Sr/G samples.

3.3. Photoluminescence studies

In order to investigate the charge separation efficiency of the synthesized samples the PL emission spectra were recorded at room temperature which is shown in Fig. 3. All the samples show a strong UV fluorescence emission at 392 nm which is due to the free exciton recombination of photogenerated electron–hole pairs near the band edge of wide band gap ZnO.¹⁸ The peaks at 412 and 462 nm are generally occurring due to the surface defects of ZnO lattice. The blue emission at 444 nm corresponds to Zn interstitials.¹⁹ Another blue emission peak appears at 490 nm is originated due to the oxygen vacancy defects.²⁰ The green emission at 520 nm is related to the singly ionized oxygen vacancies in ZnO, which appears due to the recombination of photogenerated hole with the singly ionized charge state of the particular defect.²¹ It is interesting to note that the PL intensity at 390 nm corresponds to the near band edge emission decreases significantly after the inclusion of strontium and graphene, indicating the effective inhibition of recombination of photoinduced charge carriers. This suggests that there are additional routes for the transfer of photogenerated charge carriers because of the interactions between the excited semiconductor:metal (ZnO:Sr) nanoparticles and the graphene. This observed hindering of photogenerated electron–hole pair recombination can favors the enhanced photocatalytic and antibacterial activities as discussed in Sections 3.6. and 3.7., respectively.

Fig. 3 Photoluminescence spectra of ZnO, ZnO:Sr and ZnO:Sr/G samples.

3.4. FT-IR studies

The Fourier transform infrared (FTIR) spectra of the synthesized samples are shown in Fig. 4. The absorption at 430 cm⁻¹ can be assigned to the Zn–O stretching vibrations of ZnO.²² The absorption at 567 cm⁻¹ is attributed to the O–H bending of the hydroxyl group.²³ The absorption at 1482 cm⁻¹ is assigned to the asymmetric stretching vibrations of C–O due to the CO₃ group as confirmed by XRD studies and the absorption at 857 cm⁻¹ can be ascribed to the in-plane bending of CO₃.²⁴ The decreased absorption at 1482 cm⁻¹ and 857 cm⁻¹ indicating the reduction of graphene oxide into reduced graphene oxide during the synthesis of ZnO:Sr/G sample. The broad absorption band at 3425 cm⁻¹ is assigned to the OH stretching vibrations of absorbed H₂O molecules.²⁵

Fig. 4 FT-IR spectra of ZnO, ZnO:Sr and ZnO:Sr/G samples.

3.5. Morphological studies

The surface morphology of the synthesized nanostructures was studied using SEM and TEM. Fig. 5 shows the SEM micrograph, elemental composition mapping and EDX spectrum of (a) ZnO, (b) ZnO:Sr and (c) ZnO:Sr/G samples. All the nanostructures show platelet like morphology with irregular shapes. It is clearly seen from Fig. 5(a) and (b) that the size of the ZnO nanoparticle reduces with Sr doping and reduces further when graphene is added as shown in Fig. 5(c). The presence of graphene sheets with strontium doped ZnO nanoparticles are identified in SEM micrograph. To confirm the presence of graphene in ZnO:Sr/G system we have carried out the TEM measurements. Fig. 6(a) depicts the TEM micrograph of ZnO:Sr/G nanocomposite. From the figure it is obvious that the Sr doped ZnO nanoparticles having platelet like morphology reside on graphene sheets, as already confirmed from SEM micrograph. Fig. 6(b) shows the high resolution TEM image of ZnO:Sr/G nanocomposite. From the image, it is clearly seen that the ZnO:Sr nanoparticles are covered with graphene. The SAED pattern of ZnO:Sr/G nanoparticles are indexed and presented as an inset of Fig. 6(c). The SAED pattern confirms the hexagonal ZnO:Sr nanostructure which is consistent with the XRD results. To obtain another supporting evidence for the presence of graphene, we have done the Fast Fourier Transform (FFT) analysis with the HRTEM for ZnO:Sr/G as shown in Fig. 6(d). FFT patterns were taken at five different places which are denoted in the image as A, B, C, D and E. The FFT at point A shows the pattern of carbon layer present in the TEM grid. Point B shows the blurred ring pattern correspond to the graphene on the ZnO:Sr nanoparticles. Points C, D and E show the FFT dot patterns from the lattice fringes of ZnO:Sr nanoparticles with different orientations.

Fig. 5 (a) SEM micrograph, elemental composition mapping and EDX spectrum of ZnO. (b) SEM micrograph, elemental composition mapping and EDX spectrum of ZnO:Sr. (c) SEM micrograph, elemental composition mapping and EDX spectrum of ZnO:Sr/G.

Fig. 6 ZnO:Sr/G nanocomposite – TEM micrograph (a), HR-TEM micrographs (b and c). Inset of (c) shows SAED pattern of ZnO:Sr, and HR-TEM micrograph with FFT patterns (d).

3.6. Photocatalysis – degradation of methylene blue

Fig. 7(a–c) illustrate the absorption spectra of Methylene Blue (MB) dye with ZnO, ZnO:Sr and ZnO:Sr/G samples respectively, which reveal the photocatalytic degradation patterns under visible light irradiation. The characteristic peak of the MB dye is centered at 664 nm. It is observed that the intensity of this characteristic peak value gradually decreases with time, indicating a decrease in the concentration of the MB in the test solution. As the optical absorbance (A) at any time is directly proportional to the temporal concentration of the dye, the ratio between the concentration at any time and the initial concentration (C/C_o) can be determined by finding the ratio between the corresponding absorbance (A/A_o) values. Fig. 7(d) shows the photodegradation of MB dye as a function of time for all the three samples. Fig. 7(a–c) reveal that the change in concentration of the MB solution for ZnO:Sr is relatively high when compared with that of bare ZnO. Interestingly, a remarkable reduction in the concentration is evidenced when the dye solution is treated with ZnO:Sr/G.

Fig. 7 Absorption spectra of MB solution under visible light irradiation in the presence of ZnO (a), ZnO:Sr (b), ZnO:Sr/G (c), photodegradation of MB with photocatalysts (d) and plots of ln(C_o/C) versus irradiation time (e).

The photocatalytic degradation efficiency (η) was determined using the relation,

where, C/C_o is the temporal concentration ratio. The calculated photodegradation efficiency values are listed in Table 2.

Table 2 Photocatalytic degradation efficiency (in percentage)

Sample	Efficiency (%)
	15 min	30 min	45 min	60 min	75 min
ZnO	11.27	18.63	26.19	39.28	52.42
ZnO:Sr	32.04	54.80	66.07	80.10	89.50
ZnO:Sr/G	38.27	60.16	81.57	92.58	96.73

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The control experiment – the photodegradation of MB in the absence of catalyst – is conducted under visible light irradiation. It is found that the degradation is only minimal indicating that the photodegradation of MB dye is mainly due to the photocatalytic effect of the synthesized samples and not due to the photolysis of MB (see Fig. S1, ESI†). Further, the degradation of MB is slightly higher in the presence of graphene alone under similar experimental conditions (see Fig. S2, ESI†). The photodegradation efficiency of the bare ZnO after 75 min is only 52.42% where as the efficiency increases to 89.50% for the strontium doped ZnO and increases further to 96.73% for the ZnO:Sr/G sample. The observed degradation of MB by the ZnO:Sr sample synthesized in the present study using wet chemical method is much faster than the strontium-doped ZnO nanoparticles prepared using sol–gel method as reported by R. Yousefi et al.²⁶ M. Azarang et al. reported a 99.5% of MB degradation (180 min) in the presence of ZnO/rGO nanocomposite prepared by sol–gel method.²⁷ Fig. 7(e) shows the plots of ln(C_o/C) versus irradiation time (t). The apparent rate constant k is obtained by linear fitting with the equation,

The evaluated rate constant values are listed in Table 3. The high values of regression coefficient (R²) indicate the good linear relationship for the photo degradation of MB dye by the photocatalysts ZnO, ZnO:Sr and ZnO:Sr/G, following pseudo first-order kinetic model. The ZnO:Sr/G displays a highest photocatalytic activity with rate constant, k = 0.0461 min⁻¹ which is 1.5 times faster than that of ZnO:Sr and 5 times faster than that of bare ZnO. X. Zhou et al. investigated the MB degradation with hydrothermally synthesized ZnO-reduced graphene oxide hybrid, having an optimal rate constant of 0.117 min⁻¹.²⁸

Table 3 Apparent rate constants (k) of MB degradation and linear regression coefficients from plot of ln(C_o/C) = kt

Sample	Rate constant (min^−1)	R^2
ZnO	0.0094	0.9447
ZnO:Sr	0.0290	0.9798
ZnO:Sr/G	0.0461	0.9751

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Enhancement of photocatalytic activity of the nanomaterials generally depends on certain crucial features such as doping, particle size, morphology and extent of defects in the crystal structure.¹⁶ Doping of metal oxides with other elements creates energy states within the band gap of the metal oxide, which act as intermediate steps for electrons during their transitions between the valence band and conduction band, extending the light absorption to the visible region. The size reduction of metal oxide nanoparticles and suitable morphological changes can enhance the effective surface area which facilitates more active sites in the photocatalyst system. Presence of defects in metal oxide systems can act as exited electron trapping centers which favors the suppression of photo induced electron–hole pair recombination.²⁹

The enhanced photocatalytic activity of ZnO:Sr/G obtained in the present study can be understood as follows: the photocatalytic degradation of dyes under visible light irradiation generally involves two mechanisms as illustrated in Fig. 8. The first one is related on the excitation of dye itself.³⁰ Here, the dye itself acts as both a sensitizer and a pollutant. The dye molecules are excited by absorbing visible light and consequently there is an electron transfer from the excited dye to the conduction band of ZnO.²² The transferred electrons trapped by surface adsorbed O₂ create various Reactive Oxygen Species (ROS) which decomposes the dye molecules. Thus the dye molecules undergo self degradation. The work functions of excited MB, graphene and conduction band of ZnO are −3.60, −4.40 and −4.05 eve, respectively.³¹ Considering the potential of the conduction band of ZnO and graphene, direct electron transfer from excited MB to the graphene and semiconductor ZnO is thermodynamically favorable (route 1). This electron transfer is desirable for the enhanced photocatalytic activity because these electrons are responsible for the generation of ROSs.

Fig. 8 Proposed mechanism for the photodegradation of MB with ZnO:Sr/G photocatalyst.

The second mechanism is based on excitation of electrons from the valence band of ZnO to the conduction band, leaving holes in the valence band, by absorbing visible light of suitable energy (≥band gap energy of ZnO) (route 2). The photogenerated electron–hole pairs migrate to catalytically active sites at the semiconductor surface where they reduce the electron acceptors or oxidize the electron donor species, thereby driving photocatalytic degradation reactions. The electrons in the conduction band react with oxygen molecules generating superoxide anion radicals. Concomitantly, the holes in the valence band react with water molecules generating hydroxyl radicals. These ROSs generated from ZnO–graphene system are responsible for the oxidative decomposition of MB to CO₂, H₂O and other mineralization products. The results obtained from the control experiments reveal that the photodegradation process follows the second mechanism (route 2).

Literature studies highly recommend to assess the photocatalytic activity of new materials with standards such as phenol.^7,32 To study the dye sensitization effect of MB, the degradation of phenol (concentration 50 mg L⁻¹), a colorless pollutant was used to evaluate the visible light photocatalytic activity of the ZnO:Sr/G sample and the resultant absorption spectra are shown in Fig. 9(a). This spectra clearly show that the concentration of phenol decreases during the photocatalytic degradation due to presence of the ZnO:Sr/G sample. From Fig. 9(b), it is observed that nearly 50% of the phenol was degraded after 180 min of light irradiation. Therefore, the photocatalyst ZnO:Sr/G can be effectively used for the photodegradation of colorless pollutants like phenol.

Fig. 9 Absorption spectra of phenol solution under visible light irradiation with ZnO:Sr/G photocatalyst (a) and plot of (C/C_o) versus irradiation time (b).

3.7. Antibacterial studies

The ZnO, ZnO:Sr, ZnO:Sr/G samples are studied against two Gram-positive bacteria (Bacillus subtilis, Staphylococcus aureus) and two Gram-negative bacteria (Escherichia coli, Klebsiella pneumoniae) using disc diffusion method using Mueller–Hinton agar. Fig. 10 shows the zone of inhibition formed around each disc loaded with the synthesized samples along with the control disc and a disc loaded with standard antibiotic (Gentamicin) for comparison. The measured inhibition zone diameters are presented (see Table S3, ESI†) and represented graphically in Fig. 11.

Fig. 10 Antibacterial activity of the synthesized samples against Bacillus subtilis (a), Staphylococcus aureus (b), Escherichia coli (c) and Klebsiella pneumoniae (d).

Fig. 11 Bar diagram displaying inhibition zones of the synthesized samples against test pathogens.

No inhibition zone is observed around the control for all the organisms. It is worthy to mention that the observed antibacterial activity against the Gram-negative bacteria is higher than Gram-positive bacteria in all the cases except the standard (Gentamicin). The antibacterial activity of the nanostructures may be attributed to the two possible mechanisms viz., (i) the generation of increased levels of Reactive Oxygen Species (ROS) i.e. super oxide anion radical (O₂˙⁻), hydroxyl radical (˙OH) and hydrogen peroxide (H₂O₂) and (ii) the deposition of nanoparticles on the surface of the bacteria or accumulation of nanoparticles either in the cytoplasm or in the periplasmic region causing disruption in the cellular function.³³ The bactericidal activity of the ZnO sample may be explained as follows:

ZnO with crystal defects (as explained in PL analysis) can be activated by both UV and visible light. When a photon of suitable energy falls on ZnO, an electron from the valence band is excited to the conduction band leaving a hole in the valence band. The electrons in the conduction band react with the dissolved oxygen molecules creating superoxide anion radicals (O₂˙⁻). The hole in the valence band split water molecule in to OH⁻ and H⁺ ions. The superoxide anion radical (O₂˙⁻) further react with H⁺ generating HO₂˙ radicals. The HO₂˙ radical reacts with electron and H⁺ yielding H₂O₂ molecule. The H₂O₂ can easily penetrate the cell membrane and eventually cause the death of the bacteria. The generation of ROSs is possible even under ordinary room light with a light intensity of 10 μW cm⁻¹.³⁴ Moreover, the Zn²⁺ ions released from ZnO suspension attach negatively charged bacterial cell walls and rupture them, leading to protein denaturation and cell death.³⁵ It should be noted here that the differences in growth inhibition between Gram-positive and Gram-negative bacteria may be due to the differences in sensitivities of the cell membranes of the microorganisms towards the ROS.³⁶ The ZnO sample shows higher antibacterial activity against E. coli (Gram-negative). This may be due to the fact that cell wall of E. coli (because to negatively charged lipopolysaccharides) might allow ZnO nanoparticles to interact electro statically, leading to the breakdown of the membrane barrier of the E. coli, which in turn causes cell damage.³⁴ An enhanced antibacterial activity of E. coli is observed with ZnO:Sr sample and ZnO:Sr/G causes a further enhancement. Similar trend is observed for the K. pneumoniae bacterium also. It is noted that the morphology and shape of the nanoparticle play a vital role in antibacterial activities.³⁷ The SEM image of (Fig. 5(b)) ZnO:Sr shows the reduction of particle size (as discussed in morphological studies) with uneven surfaces and rough edges. The reduction in particle size leads larger specific area to volume ratio, providing more contact area during the interactions with the test pathogens and causes severe damage to the cell membrane. In the present study, the best antibacterial activity is observed with the ZnO:Sr/G sample against all the tested bacteria. It is reported that graphene nanosheets are a better antibacterial agents towards pathogenic bacteria.³⁸ The decreased crystallite size of the ZnO:Sr/G sample (as confirmed by XRD and SEM analyses) is also one of the reasons for the enhanced bactericidal activity. The sharp edges of graphene slice the cell membrane leading to the destruction of the cell integrity by draining out the cell contents resulting in eventual cell death.¹⁶ Thus the synergistic effect of ZnO:Sr nanoparticle and graphene clearly results in an enhanced antibacterial activities against the tested bacteria.

The lower bactericidal activities of the synthesized samples against the Gram-positive bacteria compared to the Gram-negative bacteria could be due to the presence of peptidoglycan, which is a complex structure that contains teichoic acids or lipoteichoic acids which have strong negative charges. Thus the Gram-positive bacteria may allow less Zn²⁺ ions released from ZnO suspension to reach the cytoplasmic membrane and therefore the less observed bactericidal activities.

4. Conclusions

ZnO:Sr/G nanocomposite prepared using a simple cost-effective wet chemical method exhibits an enhanced photocatalytic activity against methylene blue – a representative organic dye and an improved antibacterial efficacy against two Gram-positive bacteria (Bacillus subtilis and Staphylococcus aureus) and two Gram-negative bacteria (Escherichia coli and Klebsiella pneumoniae). The degradation rate constant of ZnO:Sr/G nanocomposite is observed as 0.0461 min⁻¹, which is 1.5 times greater than that of ZnO:Sr (0.0290 min⁻¹) and 5 times greater than that of bare ZnO (0.0094 min⁻¹). This enhancement in the photocatalytic efficiency is owing to the synergistic effect of strontium doping and graphene layers which can efficiently separate the photoinduced charge carriers and thereby delay the recombination. The results show that the synthesized ZnO:Sr/G nanocomposite is a potential candidate for the above mentioned environmental applications.

Acknowledgements

The authors gratefully acknowledge the Secretary and Correspondent, Principal, Dean of Sciences and Head of the Department of Physics, AVVM Sri Pushpam College (Autonomous), Poondi, Thanjavur – 613 503, Tamil Nadu, India, for their encouragement and support.

References

1. K. Ravichandran, K. Karthika, B. Sakthivel, N. J. Begum, S. Snega, K. Swaminathan and V. Senthamilselvi, Tuning the combined magnetic and antibacterial properties of ZnO nanopowders through Mn doping for biomedical applications, J. Magn. Magn. Mater., 2014, 358–359, 50–55.
2. Y. Lu, Y. Lin, D. Wang, L. Wang, T. Xie and T. Jiang, A high performance cobalt-doped ZnO visible light photocatalyst and its photogenerated charge transfer properties, Nano Res., 2011, 4, 1144–1152.
3. T. Jia, W. Wang, F. Long, Z. Fu, H. Wang and Q. Zhang, Fabrication, characterization and photocatalytic activity of La-doped ZnO nanowires, J. Alloys Compd., 2009, 484, 410–415.
4. M.-H. Hsu and C.-J. Chang, Ag-doped ZnO nanorods coated metal wire meshes as hierarchical photocatalysts with high visible-light driven photoactivity and photostability, J. Hazard. Mater., 2014, 278, 444–453.
5. B. Li, T. Liu, Y. Wang and Z. Wang, ZnO/graphene-oxide nanocomposite with remarkably enhanced visible-light-driven photocatalytic performance, J. Colloid Interface Sci., 2012, 377, 114–121.
6. Y. Yokomizo, S. Krishnamurthy and P. V. Kamat, Photoinduced electron charge and discharge of graphene–ZnO nanoparticle assembly, Catal. Today, 2013, 199, 36–41.
7. T. Sakthivel, K. Krishnamoorthy, V. Krishnaswamy, N. Raju, S.-J. Kim and V. Gunasekaran, Graphdiyne–ZnO nanohybrids as an advanced photocatalytic material, J. Phys. Chem. C, 2015, 119, 22057–22065.
8. N. Venkatachalam, M. Palanichamy and V. Murugesan, Sol–gel preparation and nanosize TiO₂: its photocatalytic performance, Mater. Chem. Phys., 2007, 15, 454–459.
9. D. Li, J.-F. Huang, L.-Y. Cao, J.-Y. Li, H.-B. OuYang and C.-Y. Yao, Microwave hydrothermal synthesis of Sr²⁺ doped ZnO crystallites with enhanced photocatalytic properties, Ceram. Int., 2014, 40, 2647–2653.
10. C. Han, M.-Q. Yang, B. Weng and Y.-J. Xu, Improving the photocatalytic activity and anti-photocorrosion of semiconductor ZnO by coupling with versatile carbon, Phys. Chem. Chem. Phys., 2014, 16, 16891–16903.
11. M.-Q. Yang and Y.-J. Xu, Selective photoredox using graphene-based composite photocatalyst, Phys. Chem. Chem. Phys., 2013, 15, 19102–19118.
12. N. Zhang, Y. Zhang and Y. Xu, Recent progress on graphene-based photocatalysts: current status and future perspectives, Nanoscale, 2012, 4, 5792–5813.
13. N. Zhang, M. Yang, S. Liu, Y. Sun and Y. Xu, Waltzing with the Versatile Platform of Graphene to Synthesize Composite Photocatalysts, Chem. Rev., 2015, 115, 10307–10377.
14. W. S. Hummers and R. E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc., 1958, 80, 1339.
15. A. S. H. Hameed, C. Karthikeyan, S. Sasikumar, V. S. Kumar, S. Kumaresan and G. Ravi, Impact of alkaline metal ions Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺ on the structural, optical, thermal and antibacterial properties of ZnO nanoparticles prepared by the co-precipitation method, J. Mater. Chem. B, 2013, 1, 5950–5962.
16. R. K. Upadhyay, N. Soin and S. S. Roy, Role of graphene/metal oxide composites as photocatalysts, adsorbents and disinfectants in water treatment: a review, RSC Adv., 2014, 4, 3823–3851.
17. T. G. Xu, L. W. Zhang, H. Y. Cheng and Y. F. Zhu, Significantly enhanced photocatalytic performance of ZnO via graphene hybridization and the mechanism study, Appl. Catal., B, 2011, 101, 382–387.
18. F. Wang, X. Qin, D. Zhu, Y. Meng, L. Yang and Y. Ming, PEG-assisted hydrothermal synthesis and photoluminescence of flower-like ZnO microstructures, Mater. Lett., 2014, 117, 131–133.
19. S. B. Rana, A. Singh and N. Kaur, Structural and optoelectronic characterization of prepared and Sb doped ZnO nanoparticles, J. Mater. Sci.: Mater. Electron., 2013, 24, 44–52.
20. L. Irimpan, V. P. N. Nampoori, P. Radhakrishnan, A. Deepthy and B. Krishnan, Size dependent fluorescence spectroscopy of nanocolloids of ZnO, J. Appl. Phys., 2007, 102, 063524.
21. B. Pal and P. K. Giri, High temperature ferromagnetism and optical properties of Co doped ZnO Nanoparticles, J. Appl. Phys., 2010, 108, 084322.
22. B. Li and H. Cao, ZnO@graphene composite with enhanced performance for the removal of dye from water, J. Mater. Chem., 2011, 21, 3346–3349.
23. T. Thilagavathi and D. Geetha, Nano ZnO structures synthesized in presence of anionic and cationic surfactant under hydrothermal process, Appl. Nanosci., 2014, 4, 127–132.
24. M. A. Alavi and A. Morsali, Syntheses and characterization of Sr(OH)₂ and SrCO₃ nanostructures by ultrasonic method, Ultrason. Sonochem., 2010, 17, 132–138.
25. K. Saravanakumar, K. Ravichandran, R. Chandramohan, S. Gobalakrishnan and M. Chavali, Investigation on simultaneous doping of Sn and F with ZnO nanopowders synthesized using a simple soft chemical route, Superlattices Microstruct., 2012, 52, 528–540.
26. R. Yousefi, F. J. Sheini, M. Cheraghizade, S. K. Gandomani, A. Sáaedi, N. M. Huang, W. J. Basirun and M. Azarang, Enhanced visible-light photocatalytic activity of strontium-doped zinc oxide nanoparticles, Mater. Sci. Semicond. Process., 2015, 32, 152–159.
27. M. Azarang, A. Shuhaimi, R. Yousefi, A. M. Golsheikh and M. Sookhakian, Synthesis and characterization of ZnO NPs/reduced graphene oxide nanocomposite prepared in gelatin medium as highly efficient photo-degradation of MB, Ceram. Int., 2014, 40, 10217–10221.
28. X. Zhou, T. Shi and H. Zhou, Hydrothermal preparation of ZnO-reduced graphene oxide hybrid with high performance in photocatalytic degradation, Appl. Surf. Sci., 2012, 258, 6204–6211.
29. D. Chen, Z. Wang, T. Ren, H. Ding, W. Yao, R. Zong and Y. Zhu, Influence of Defects on the Photocatalytic Activity of ZnO, J. Phys. Chem. C, 2014, 118(28), 15300–15307.
30. Z. Xiong, L. L. Zhang, J. Ma and X. S. Zhao, Photocatalytic degradation of dyes over graphene–gold nanocomposites under visible light irradiation, Chem. Commun., 2010, 46, 6099–6101.
31. J. S. Lee, K. H. You and C. B. Park, Highly Photoactive, Low Bandgap TiO₂ Nanoparticles Wrapped by Graphene, Adv. Mater., 2012, 24, 1084–1088.
32. J. M. Buriak, P. V. Kamat and K. S. Schanze, Best practices for reporting on heterogeneous photocatalysis, ACS Appl. Mater. Interfaces, 2014, 6, 11815–11816.
33. K. R. Raghupathi, R. T. Koodali and A. C. Manna, Size-Dependent Bacterial Growth Inhibition and Mechanism of Antibacterial Activity of Zinc Oxide Nanoparticles, Langmuir, 2011, 27(7), 4020–4028.
34. R. K. Dutta, B. P. Nenavathu, M. K. Gangishetty and A. V. R. Reddy, Studies on antibacterial activity of ZnO nanoparticles by ROS induced lipid peroxidation, Colloids Surf., B, 2012, 94, 143–150.
35. J. Cui and Y. Liu, Preparation of graphene oxide with silver nanowires to enhance antibacterial properties and cell compatibility, RSC Adv., 2015, 5, 85748–85755.
36. X. Wang, F. Yang, W. Yang and X. Yang, A study on the antibacterial activity of one-dimensional ZnO nanowire arrays: effects of the orientation and plane surface, Chem. Commun., 2007, 4419–4421.
37. K. Ravichandran, R. Rathi, M. Baneto, K. Karthika, P. V. Rajkumar, B. Sakthivel and R. Damodaran, Effect of Fe+F doping on the antibacterial activity of ZnO powder, Ceram. Int., 2015, 41, 3390–3395.
38. K. Krishnamoorthy, M. Veerapandian, L. H. Zhang, K. Yun and S.-J. Kim, Antibacterial efficiency of graphene nanosheets against pathogenic bacteria via lipid peroxidation, J. Phys. Chem. C, 2012, 115, 17280–17287.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra08697g

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