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Development of ZnO and ZrO2 nanoparticles: Their photocatalytic and bactericidal activity
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JECE 577 1–6 Journal of Environmental Chemical Engineering xxx (2015) xxx–xxx
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Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Development of ZnO and ZrO2 nanoparticles: Their photocatalytic and bactericidal activity
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Saima Sultana a,1, Rafiuddin a,2 , Mohammad Zain Khan a,3 , Mohammad Shahadat b, *
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a b
Physical Chemistry Division, Department of Chemistry, Aligarh Muslim University, Aligarh, UP 202002, India School of Distance Education, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia
A R T I C L E I N F O
A B S T R A C T
Article history: Received 7 October 2014 Accepted 20 February 2015
Zinc oxide and zirconia nanoparticles were synthesized by using a very simple sol–gel approach in order to study their thermal, photocatalytic and bactericidal properties. The nanoparticles were characterized by X-ray diffraction, transmission electron microscopy as well as scanning electron microscopy to ensure the crystallite size, geometry and surface morphology of the nanoparticles. The average crystallite size was found to be around 18 and 30 nm for ZnO and ZrO2 NPs, respectively. Thermal gravimetric analysis revealed that ZrO2 NPs are thermally more stable and can sustain up to 1000 C with less than 10% weight loss only. The photodegradation experiments revealed that ZnO NPs have better photocatalytic activity than ZrO2 NPs. Further, the photodegradation was found to be dose dependent phenomenon while the change in pH does not significantly effects the rate of photodegradation in the present case. The removal efficiency increases, with decrease in pH and increase in photocatalyst dosage. Additionally, the nanomaterials showed remarkable antibacterial activity against two gram negative (Pseudomonas aeruginosa, Escherichia coli) and two gram positive (Staphylococcus aureus, Bacillus subtilis) strains. The studies showed that the materials can be successfully used for water decontamination, antibacterial agents and high temperature application. ã 2015 Published by Elsevier Ltd.
Keywords: Nanoparticles Sol–gel Characterization Thermal stability Photocatalyst Azo-dyes Antibacterial activity
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Introduction
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Nowadays, nanomaterials are considered as next generation materials that have attracted recent attention due to their novel properties differing from those of the bulk materials [1–3]. Research on the synthesis and application of nanomaterials has experienced tremendous growth in recent years, owing to their unique properties making them suitable for applications in almost every field of science [4–8]. They can even be used as building blocks in nano-scale optical, electronic and optoelectronic devices in near future [9,10]. Nanoparticles possess high chemical activity because of their crystallographic surface structure together with large surface to volume ratio [11]. They have unique electrical, optical, magnetic, photocatalytic and electronic properties that lead to an intense attention to design cost effective nano-devices [12].
* Corresponding author. Tel.: +60 1112400516. E-mail address: [email protected] (M. Shahadat). 1 Tel.: +91 9760702088. 2 Tel.: +91 9997005502. 3 Tel.: +91 8266057545.
During the past decades, nanoparticles (NPs) have attracted a great deal in biological and pharmaceutical applications as bactericidal agents [13]. The antibacterial studies of nanomaterials become more and more important due to the increased resistance of new strains of bacteria against most potent antibiotics [14]. The bactericidal property of nanoparticles depends on their stability in the growth medium, which imparts greater retention time for bacterium nanoparticle interaction [15]. Silver nanoparticles are the most comprehensively studied for their bactericidal effects against many gram negative as well as positive bacteria, hence, find use in the field of medical science to treat burns and other infections [16–18]. In addition to silver and its compounds, few other metallic oxide nanoparticles show adequate antibacterial as well as antifungal activities and have been successfully used as potent bactericidal agents [19–21]. However, the studies relating to antibacterial activity of metallic oxide nanomaterials are still limited. That is why the present study focuses possible significant application of metallic oxides nanomaterials in diverse field. Metals and metal oxides have been widely studied for potential applications such as heterogeneous catalysts, surfaces of heat exchangers, ultraviolet laser, photo-detectors and solar cells due to their high photosensitivity, non-toxic nature, low cost, and environment friendliness [22,23]. They are also believed to exhibit
http://dx.doi.org/10.1016/j.jece.2015.02.024 2213-3437/ ã 2015 Published by Elsevier Ltd.
Please cite this article in press as: S. Sultana, et al., Development of ZnO and ZrO2 nanoparticles: Their photocatalytic and bactericidal activity, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.02.024
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photocatalytic activities which render their application in the decomposition and destruction of organic pollutants, thereby resulting in the decontamination of water [24]. The properties will be more pronounced if the size of the metallic oxide particles is in the nano range due to their large grain boundaries relative to their grain size, i.e., large surface to volume ratio [11,25,26]. Among the various metallic oxides, titania (TiO2) has been widely studied as photocatalyst and sometimes composite materials involving titania and ZrO2 or CuO or Al2O3 were also studied for possible activity [27–30]. In the present study, zinc oxide (ZnO) and zirconia (ZrO2) were selected for feasibility studies. ZrO2 has wide Eg value and the high negative value of the conduction band potential [31,32]. Its band gap ranges from 3.25 to 5.1 eV, depending upon its method of preparation [33]. Zinc oxide, on the other hand is an environmental friendly n-type semiconductor with a fairly wide band gap (3.4 eV) and large binding energy value (60 meV) [34]. It has high UV absorptivity and can absorb more quanta of light in UV spectrum than TiO2 semiconductors [35]. The present work deals with the synthesis and characterization of ZnO and ZrO2 NPs followed by a comparative study on their thermal, photocatalytic as well as bactericidal behavior which may open up new prospective towards a wide range of applications. The materials can be mixed together in suitable proportions to incorporate some very unique properties in the resulting nanocomposites.
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Experimental
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Materials and methods
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All the reagents (ZnCl22H2O, ZrOCl28H2O, NaOH and HCl) were of analytical grade, procured from Fisher Scientific, India and were used as received. The ZnO and ZrO2 sols were prepared in dilute solution of HCl (0.05 M).
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Synthesis of ZnO and ZrO2 nanoparticles
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ZnO and ZrO2 nanoparticles were synthesized by using sol–gel approach since the technique is a promising low-temperature route that provides good homogeneity and chemical purity of the resulting nano-materials sample over other techniques such as impregnation and inert gas evaporation [36]. In a typical synthesis, 50 mL each of 0.1 M solution of ZnCl22H2O and ZrOCl28H2O in 0.05 M HCl was taken in a separate beaker and 50 mL NaOH (0.5 M) was added drop wise to this solution using standardized burette under constant stirring to undergo the hydrolysis of ZnCl2 and ZrOCl2 [3]. The formed gels were again constantly stirred for 2 h and kept for 12 h. The obtained white colloid was filtered and washed repeatedly with double distilled water to remove any traces of any precursor ions (e.g., Cl ions). The gels were heated to 120 C and held there until dried. Finally, the the dried gels were heated up to calcination temperature (400 C), aged for 2 h, and then allowed to cool naturally. The final product was nanostructured ZnO and ZrO2, respectively.
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Characterization
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Characterization of the prepared materials was done with the help X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy. X-ray diffractograms for the materials were recorded using a ‘Bruker Advanced D8’ diffractometer with Cu-Ka radiations. The average crystallite size of the two nanoparticles was calculated from the XRD peaks using Scherer’s formula [11] shown below:
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D¼
0:9l b cos u
(1)
where D is the crystallite size, l is the X-ray wavelength, b is the broadening of the diffraction peak at half maximum and u is the diffraction angle. TEM images of the nanoparticles were taken using ‘Philips CM-10’ Transmission Electron Microscope while SEM images were taken by using ‘JEOL JSM-6510’ Scanning Electron Microscope. Samples were coated with gold prior to the SEM analysis.
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Thermal stability
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Thermal gravimetric analysis (TGA) was done by heating the samples in an inert atmosphere in the temperature range 25–1000 C using a ‘PerkinElmer’ instrument with alumina powder as reference. The heating rate was kept constant at 10 C/min during the TGA measurements.
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Photocatalytic activity
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The photocatalytic activity of the synthesized nanomaterials was tested against a textile azo-dye (Acid Blue 25) in a UV photocatalytic reactor made up of borosilicate glass with an outer cooling water jacket to control the reaction temperature. Air was bubbled into the reactor at the rate of 50–60 mL/s for maintaining the required oxygen level and material transfer. The UV source (light intensity 1.50 mW/cm2) of the photoreactor was surrounded by an outer borosilicate glass chamber where the dye solution was continuously stirred with the help of a magnetic stirrer (Fig. 1). The UV–visible spectra of the influent dye samples (0.2 mM Acid Blue 25) were taken and the decolorization rate of the azo-dye in presence of ZnO and ZrO2 NPs photocatalyst was (0.1 g/L) recorded by a UV–visible spectrophotometer (Shimadzu-1601, Japan). A catalyst concentration of 0.1 g/L was used during the photodegradation experiments. Effect of pH and photocatalyst dosage on photocatalytic activity was also studied and presented.
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Antibacterial activity
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The antibacterial activity of the synthesized nanoparticles were tested in-vitro against two gram positive (Staphylococcus aureus MSSA-22 and Bacillus subtilis ATCC-6051) and two gram negative (Escherichia coli K-12 and Pseudomonas aeruginosa MTCC-2488) bacterial strains using disk diffusion method [37,38]. The test
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Fig. 1. Simple sketch of the photoreactor used during the study.
Please cite this article in press as: S. Sultana, et al., Development of ZnO and ZrO2 nanoparticles: Their photocatalytic and bactericidal activity, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.02.024
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strains for possible antibacterial activity were cultured in Nutrient Broth: Lab-Lemco powder 0.1% (w/v), yeast extract 0.2% (w/v), peptone 0.5% (w/v) and sodium chloride 0.5% (w/v). The medium pH value was checked after addition of each agent and the resulting pH was not altered. Bacteria were maintained on Mueller Hinton Agar: meat infusion 30% (w/v); casein hydrolysate 1.75% (w/v); starch 0.15% (w/v); agar–agar 1.7% (w/v). The screening for antibacterial activities were carried out using sterilized disks (5 mm) previously soaked in 0.2 mM concentration of the test samples then dried at 70 C for 6 h. All the tested strains were inoculated into Nutrient Broth and incubated at 37 C for 48 h. Inoculums containing 107–108 CFU of bacterial cells were spread on Mueller-Hinton Agar plates (100 mL inoculum for each plate). The disks soaked with ZnO and ZrO2 NPs solution were placed on the inoculated agar, pressed slightly and incubated at 37 C for 24 h. Tetracycline (30 mg/disk, Hi-Media) was used as control. The zone of inhibition was measured (in mm) after 24 h. Each experiment was repeated thrice.
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Results and discussion
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X-ray diffraction analysis
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The room temperature powder XRD patterns of ZnO and ZrO2 NPs are shown in Fig. 2. The diffraction peaks of both the samples are well-defined. The diffraction peaks centered at 32 and 55 in Fig. 2a with corresponding crystal plane parameters (0 0 2) (1 0 3) represents ZrO2. ZrO2 NPs show the standard pattern previously explained by Zhang et al. [39]. However, the broad diffraction peak centered at 2u value 37 and 43 in Fig. 2b with corresponding crystal planes parameters (11 0) (2 0 0) are the characteristic peak of the ZnO, with all diffraction peaks well indexed to the standard diffraction pattern of hexagonal phase ZnO (JCPDS No. 36-1451),
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3
Fig. 2. XRD spectra of (a) ZrO2 and (b) ZnO nanoparticles prepared by sol–gel approach.
indicating a wurtzite structured with high crystallinity. The corresponding h k l (miller indices) values are (11 0) (2 0 0). The average crystallite size of ZrO2 and ZnO NPs as calculated from diffraction peaks using Scherer’s formula was found to be 30 and 18 nm, respectively.
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Microscopic analysis
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Microscopic characterization was done with the help of TEM and SEM techniques and the images are presented in Fig. 3,
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Fig. 3. Microscopic images of ZnO and ZrO2 nanoparticles obtained after calcination at 400 C for 2 h.
Please cite this article in press as: S. Sultana, et al., Development of ZnO and ZrO2 nanoparticles: Their photocatalytic and bactericidal activity, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.02.024
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providing imperative information regarding the size, morphology and nature of the metallic oxide NPs. The microscopic images demonstrate the fine particle size nature of the NPs. The nanoparticles are much smaller than the magnified area of 50 nm under the transmission electron micrographs (Fig. 3a and b) which demonstrate the fine particle size of the as-prepared materials ZnO nano-flowers with high crystallinity can be seen clearly in Fig. 3c . However, the microscopic images of ZrO2 nanoparticles (Fig. 3b and d) did not show significant crystallinity but rather exhibit agglomeration of nanoparticles, which may be ascribed to their high surface energy [40]. The information provided by the microscopic images is consistent with the XRD data. The aggregated structure of ZrO2 NPs suggested a reduction in the total active surface area of nanoparticles (lowering the surface to volume ratio) thereby reducing the number of free valences which in turn affects the photocatalytic activity of the samples.
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Thermal stability (thermal gravimetric analysis)
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Fig. 5. UV absorption spectra of Acid Blue 25 at different time intervals during the photodegradation (spectra taken at T = 0–90 min, at 15 min interval).
Plots shown in Fig. 4 represent the thermal gravimetric curves of ZnO and ZrO2 NPs. It is evident from the plots that both the nanoparticles have sufficiently high thermal stability with less than 25% weight loss in the temperature range 25–1000 C. Moreover, the stability of ZnO NPs is slightly lower than ZrO2, and is being attributed to the low thermal conductivity and high inertness of zirconium, thus, making it useful for refractory applications. The weight loss at around 100 C is attributed to desorption (removal) of water physically bonded to the nanomaterials while the second gradual weight loss starting at around 450 C represents the loss of hydroxyl (—OH) attached to the surface of Q4 the nanomaterials (Zhao et al., 2004). Till 1000 C, ZnO NPs showed nearly 25% weight loss while ZrO2 experienced less than 15% weight loss only. Furthermore, both the NPs show gradual weight losses up to 700 C, thereafter, ZnO NPs shows a sudden weight loss. Thus, thermal gravimetric analysis demonstrates superior thermal stability of ZrO2 NPs over ZnO NPs.
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Photocatalytic activity – photodegradation of Acid Blue 25
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The photocatalytic degradation involves the generation of electrons (in conduction band) and holes (in valence bands) upon irradiation with UV or solar light [34]. These photogenerated electrons and holes react with water and oxygen to generate hydroxyl (OH) and hydroperoxyl (OOH) radicals which are very reactive and can easily oxidize the substrate. A fraction of these
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Ct ¼ ekt C0
where C0 is the initial concentration while Ct is the concentration at any time ‘t’ in mg/L, and k is rate constant in min1. It was observed that ZnO NPs have better photocatalytic activity (nearly 80% removal efficiency) than ZrO2 NPs (Fig. 6). This may be
1.2
105 ZnO
100
1.0
ZrO2
95
0.8
90 85
ZrO2 ZnO No UV Light
0.6
80
0.4
75 70
0.2
0
200
400
600
800
1000
Temperature ( C) Fig. 4. Thermal gravimetric plots of ZnO and ZrO2 nanoparticles.
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(2)
Ct/C0
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Per cent weight loss (w/w0)
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radicals react with the adsorbed molecules resulting in to photocatalysis [41]. The photocatalytic degradation (or oxidation) is rapid, easy and very effective method of substrate degradation [42]. The photocatalytic activity of the materials was tested by studying the decolorization rate of Acid Blue 25 dye in presence of UV illuminating source in a quartz jacketed photoreactor. The azo-dye gives a characteristic peak at 600 nm [43]. The spectra shown in Fig. 5 validate the photodegradation of dye in presence of ZnO and ZrO2 catalyst. In order to quantify the photodegradation of Acid Blue 25 dye, initial concentrations (0.2 mM) of the dye were fed to the quartz jacketed photoreactor and continuously stirred in the dark for about 75 min to ensure adsorption/desorption equilibrium. Thereafter, the contents were irradiated with UV light to study the degradation of Acid Blue 25. The reaction was started and the samples (5 mL) were withdrawn at an interval of 15 min and the absorbance was monitored. The decay curve of the azo-dye followed first order kinetics as per the equation given below [3]:
0
20
40
60
80
t (mins) Fig. 6. Photodegradation of textile azo-dye Acid Blue 25 using ZnO and ZrO2 NPs (photocatalytic activity).
Please cite this article in press as: S. Sultana, et al., Development of ZnO and ZrO2 nanoparticles: Their photocatalytic and bactericidal activity, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.02.024
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Table 1 Effect of photocatalyst dosage on removal efficiency.
Table 2 Antibacterial activity of the synthesized nanoparticles.
Photocatalyst dosage (mg L1) % removal efficiency of AB 25 (1 C/C0) 100
50 100 200 300
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ZnO
ZrO2
45 70 82 95
20 33 52 65
254
attributed to finer particle size and higher surface area of ZnO NPs. The results are consistent with the XRD and TEM data that supports a lower particle size for ZnO NPs. The value of rate constant k (calculated from the line fittings) for ZnO and ZrO2 catalyzed photodegradation reaction was 0.0181 (0.009) and 0.0028 (0.0005) min1 with R2 value 0.9937 and 0.899, respectively. Further, the photocatalytic activity was found to be a dose related response, i.e., with increase in the concentration of photocatalyst, the removal efficiency increases owing to increase in the number of active sites. However, the removal efficiency of the dye increases less rapidly in case of ZrO2 NPs (Table 1). The study demonstrates the practical application of ZnO and ZrO2 NPs in removing toxic azo-dyes from natural water streams.
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Effect of pH on photodegradation of dyes
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Experiments were carried out to determine the pH dependency of the rate of photodegradation. Fig. 7 shows the effect of pH on photodegradation of dye in presence of ZnO and ZrO2 NPs. The experiments were carried out at six different pH values ranging from highly acidic to extremely basic (2.0, 3.5, 5.8, 7.5, 9.0 and 11.5). Although the photocatalytic activity of both the NPs increases with increase in pH till 9.0 and thereafter decreases, the effect was not very significant. The photocatalytic activity was highest at pH 9.0, due to the increase of hydroxyl ions, which induces more hydroxyl radical formation [44]. Additionally, the photocatalytic activity of ZnO NPs is better than ZrO2 NPs at all pH values.
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Antibacterial activity
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The antibacterial studies on the synthetic nanoparticles – ZnO and ZrO2, have been performed in order to investigate their bactericidal action employing disk diffusion method. Two gram positive (S. aureus MSSA-22 and B. subtilis ATCC-6051) and two gram negative (E. coli K-12 and P. aeruginosa MTCC-2488) bacterial strain were selected for feasibility studies. Tetracycline was used as standard drug for comparison of the antibacterial property of ZnO
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8 Photocatalytic activity C0/C
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ZnO
7
ZrO2
6 5 4 3 2 1 0
2
3.5
5.8
7.5
5
9
11.5
pH of the medium Fig. 7. pH dependency of the photocatalytic activity of the nanoparticles.
Compound
Zone inhibition (mm)
Synthesized nanoparticle ZnO ZrO2 Tetracycline
S. aureus 17 10 9
B. substilis 16 11 8
E. coli 14 9 8
P. aeruginosa 15 7 6
and ZrO2 and the results are provided in Table 2. The newly synthesized nanoparticles have shown remarkable inhibitory effects (better than tetracycline) against the growth of all the tested bacterial strains (Fig. 7). The antimicrobial activity of several other synthetic molecules (e.g., 1,6-bis(benzimidazol-2-yl)-3,4dithiahexane ligand) were investigated by the researchers in a similar manner [45]. The data indicate that the synthetic ZnO and ZrO2 nanoparticles exhibit greater activity against S. aureus and B. subtilis as compared to the E. coli and P. aeruginosa. Furthermore, it is also clear that ZnO NPs shows better antibacterial activity than ZrO2 NPs against the tested bacterial strains due to the stability of ZnO NPs in the growth medium which imparts greater bacterium-nanoparticle interactions [18]. The results clearly evidenced that the synthesized nanoparticles are more effective than tetracycline against all tested strains and can be used as potent antibacterial agents.
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Conclusions
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Nanostructured ZnO and ZrO2 with exceptional photocatalytic and antibacterial activity were successfully prepared using sol–gel method. XRD, TEM and SEM provide useful information regarding the size (average crystallite size of ZnO NPs = 18 nm and ZrO2 NPs = 30 nm) geometries and surface morphology of the nanoparticles. The as-prepared nanoparticles possess high thermal stability, which renders their application in high temperature fields. Although, ZrO2 NPs shows better thermal stability, the photocatalytic and antibacterial activity was found to be superior for ZnO NPs. The photocatalytic activity increases with increasing the amount of photocatalyst, however, the effect was not very significant. The photocatalytic activity allows the use of ZnO NPs for treating toxic waste water while antibacterial activity renders their application in as disinfecting agents for killing pathogens. An important point which cannot be neglected is that, a compromise could be made by mixing different proportions of ZnO and ZrO2 nanoparticles in order to incorporate some unique properties in to the resulting nanocomposite.
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Acknowledgements
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Authors are thankful to Aligarh Muslim University for providing necessary research facilities. Department of Applied Physics is duly acknowledged for the XRD facility. CSIR is to be thanked for Q5 providing financial support to this project.
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Contents lists available at ScienceDirect
Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Development of ZnO and ZrO2 nanoparticles: Their photocatalytic and bactericidal activity
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Saima Sultana a,1, Rafiuddin a,2 , Mohammad Zain Khan a,3 , Mohammad Shahadat b, *
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a b
Physical Chemistry Division, Department of Chemistry, Aligarh Muslim University, Aligarh, UP 202002, India School of Distance Education, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia
A R T I C L E I N F O
A B S T R A C T
Article history: Received 7 October 2014 Accepted 20 February 2015
Zinc oxide and zirconia nanoparticles were synthesized by using a very simple sol–gel approach in order to study their thermal, photocatalytic and bactericidal properties. The nanoparticles were characterized by X-ray diffraction, transmission electron microscopy as well as scanning electron microscopy to ensure the crystallite size, geometry and surface morphology of the nanoparticles. The average crystallite size was found to be around 18 and 30 nm for ZnO and ZrO2 NPs, respectively. Thermal gravimetric analysis revealed that ZrO2 NPs are thermally more stable and can sustain up to 1000 C with less than 10% weight loss only. The photodegradation experiments revealed that ZnO NPs have better photocatalytic activity than ZrO2 NPs. Further, the photodegradation was found to be dose dependent phenomenon while the change in pH does not significantly effects the rate of photodegradation in the present case. The removal efficiency increases, with decrease in pH and increase in photocatalyst dosage. Additionally, the nanomaterials showed remarkable antibacterial activity against two gram negative (Pseudomonas aeruginosa, Escherichia coli) and two gram positive (Staphylococcus aureus, Bacillus subtilis) strains. The studies showed that the materials can be successfully used for water decontamination, antibacterial agents and high temperature application. ã 2015 Published by Elsevier Ltd.
Keywords: Nanoparticles Sol–gel Characterization Thermal stability Photocatalyst Azo-dyes Antibacterial activity
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Introduction
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Nowadays, nanomaterials are considered as next generation materials that have attracted recent attention due to their novel properties differing from those of the bulk materials [1–3]. Research on the synthesis and application of nanomaterials has experienced tremendous growth in recent years, owing to their unique properties making them suitable for applications in almost every field of science [4–8]. They can even be used as building blocks in nano-scale optical, electronic and optoelectronic devices in near future [9,10]. Nanoparticles possess high chemical activity because of their crystallographic surface structure together with large surface to volume ratio [11]. They have unique electrical, optical, magnetic, photocatalytic and electronic properties that lead to an intense attention to design cost effective nano-devices [12].
* Corresponding author. Tel.: +60 1112400516. E-mail address: [email protected] (M. Shahadat). 1 Tel.: +91 9760702088. 2 Tel.: +91 9997005502. 3 Tel.: +91 8266057545.
During the past decades, nanoparticles (NPs) have attracted a great deal in biological and pharmaceutical applications as bactericidal agents [13]. The antibacterial studies of nanomaterials become more and more important due to the increased resistance of new strains of bacteria against most potent antibiotics [14]. The bactericidal property of nanoparticles depends on their stability in the growth medium, which imparts greater retention time for bacterium nanoparticle interaction [15]. Silver nanoparticles are the most comprehensively studied for their bactericidal effects against many gram negative as well as positive bacteria, hence, find use in the field of medical science to treat burns and other infections [16–18]. In addition to silver and its compounds, few other metallic oxide nanoparticles show adequate antibacterial as well as antifungal activities and have been successfully used as potent bactericidal agents [19–21]. However, the studies relating to antibacterial activity of metallic oxide nanomaterials are still limited. That is why the present study focuses possible significant application of metallic oxides nanomaterials in diverse field. Metals and metal oxides have been widely studied for potential applications such as heterogeneous catalysts, surfaces of heat exchangers, ultraviolet laser, photo-detectors and solar cells due to their high photosensitivity, non-toxic nature, low cost, and environment friendliness [22,23]. They are also believed to exhibit
http://dx.doi.org/10.1016/j.jece.2015.02.024 2213-3437/ ã 2015 Published by Elsevier Ltd.
Please cite this article in press as: S. Sultana, et al., Development of ZnO and ZrO2 nanoparticles: Their photocatalytic and bactericidal activity, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.02.024
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photocatalytic activities which render their application in the decomposition and destruction of organic pollutants, thereby resulting in the decontamination of water [24]. The properties will be more pronounced if the size of the metallic oxide particles is in the nano range due to their large grain boundaries relative to their grain size, i.e., large surface to volume ratio [11,25,26]. Among the various metallic oxides, titania (TiO2) has been widely studied as photocatalyst and sometimes composite materials involving titania and ZrO2 or CuO or Al2O3 were also studied for possible activity [27–30]. In the present study, zinc oxide (ZnO) and zirconia (ZrO2) were selected for feasibility studies. ZrO2 has wide Eg value and the high negative value of the conduction band potential [31,32]. Its band gap ranges from 3.25 to 5.1 eV, depending upon its method of preparation [33]. Zinc oxide, on the other hand is an environmental friendly n-type semiconductor with a fairly wide band gap (3.4 eV) and large binding energy value (60 meV) [34]. It has high UV absorptivity and can absorb more quanta of light in UV spectrum than TiO2 semiconductors [35]. The present work deals with the synthesis and characterization of ZnO and ZrO2 NPs followed by a comparative study on their thermal, photocatalytic as well as bactericidal behavior which may open up new prospective towards a wide range of applications. The materials can be mixed together in suitable proportions to incorporate some very unique properties in the resulting nanocomposites.
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Experimental
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Materials and methods
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All the reagents (ZnCl22H2O, ZrOCl28H2O, NaOH and HCl) were of analytical grade, procured from Fisher Scientific, India and were used as received. The ZnO and ZrO2 sols were prepared in dilute solution of HCl (0.05 M).
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Synthesis of ZnO and ZrO2 nanoparticles
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ZnO and ZrO2 nanoparticles were synthesized by using sol–gel approach since the technique is a promising low-temperature route that provides good homogeneity and chemical purity of the resulting nano-materials sample over other techniques such as impregnation and inert gas evaporation [36]. In a typical synthesis, 50 mL each of 0.1 M solution of ZnCl22H2O and ZrOCl28H2O in 0.05 M HCl was taken in a separate beaker and 50 mL NaOH (0.5 M) was added drop wise to this solution using standardized burette under constant stirring to undergo the hydrolysis of ZnCl2 and ZrOCl2 [3]. The formed gels were again constantly stirred for 2 h and kept for 12 h. The obtained white colloid was filtered and washed repeatedly with double distilled water to remove any traces of any precursor ions (e.g., Cl ions). The gels were heated to 120 C and held there until dried. Finally, the the dried gels were heated up to calcination temperature (400 C), aged for 2 h, and then allowed to cool naturally. The final product was nanostructured ZnO and ZrO2, respectively.
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Characterization
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Characterization of the prepared materials was done with the help X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy. X-ray diffractograms for the materials were recorded using a ‘Bruker Advanced D8’ diffractometer with Cu-Ka radiations. The average crystallite size of the two nanoparticles was calculated from the XRD peaks using Scherer’s formula [11] shown below:
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D¼
0:9l b cos u
(1)
where D is the crystallite size, l is the X-ray wavelength, b is the broadening of the diffraction peak at half maximum and u is the diffraction angle. TEM images of the nanoparticles were taken using ‘Philips CM-10’ Transmission Electron Microscope while SEM images were taken by using ‘JEOL JSM-6510’ Scanning Electron Microscope. Samples were coated with gold prior to the SEM analysis.
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Thermal stability
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Thermal gravimetric analysis (TGA) was done by heating the samples in an inert atmosphere in the temperature range 25–1000 C using a ‘PerkinElmer’ instrument with alumina powder as reference. The heating rate was kept constant at 10 C/min during the TGA measurements.
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Photocatalytic activity
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The photocatalytic activity of the synthesized nanomaterials was tested against a textile azo-dye (Acid Blue 25) in a UV photocatalytic reactor made up of borosilicate glass with an outer cooling water jacket to control the reaction temperature. Air was bubbled into the reactor at the rate of 50–60 mL/s for maintaining the required oxygen level and material transfer. The UV source (light intensity 1.50 mW/cm2) of the photoreactor was surrounded by an outer borosilicate glass chamber where the dye solution was continuously stirred with the help of a magnetic stirrer (Fig. 1). The UV–visible spectra of the influent dye samples (0.2 mM Acid Blue 25) were taken and the decolorization rate of the azo-dye in presence of ZnO and ZrO2 NPs photocatalyst was (0.1 g/L) recorded by a UV–visible spectrophotometer (Shimadzu-1601, Japan). A catalyst concentration of 0.1 g/L was used during the photodegradation experiments. Effect of pH and photocatalyst dosage on photocatalytic activity was also studied and presented.
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Antibacterial activity
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The antibacterial activity of the synthesized nanoparticles were tested in-vitro against two gram positive (Staphylococcus aureus MSSA-22 and Bacillus subtilis ATCC-6051) and two gram negative (Escherichia coli K-12 and Pseudomonas aeruginosa MTCC-2488) bacterial strains using disk diffusion method [37,38]. The test
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Fig. 1. Simple sketch of the photoreactor used during the study.
Please cite this article in press as: S. Sultana, et al., Development of ZnO and ZrO2 nanoparticles: Their photocatalytic and bactericidal activity, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.02.024
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strains for possible antibacterial activity were cultured in Nutrient Broth: Lab-Lemco powder 0.1% (w/v), yeast extract 0.2% (w/v), peptone 0.5% (w/v) and sodium chloride 0.5% (w/v). The medium pH value was checked after addition of each agent and the resulting pH was not altered. Bacteria were maintained on Mueller Hinton Agar: meat infusion 30% (w/v); casein hydrolysate 1.75% (w/v); starch 0.15% (w/v); agar–agar 1.7% (w/v). The screening for antibacterial activities were carried out using sterilized disks (5 mm) previously soaked in 0.2 mM concentration of the test samples then dried at 70 C for 6 h. All the tested strains were inoculated into Nutrient Broth and incubated at 37 C for 48 h. Inoculums containing 107–108 CFU of bacterial cells were spread on Mueller-Hinton Agar plates (100 mL inoculum for each plate). The disks soaked with ZnO and ZrO2 NPs solution were placed on the inoculated agar, pressed slightly and incubated at 37 C for 24 h. Tetracycline (30 mg/disk, Hi-Media) was used as control. The zone of inhibition was measured (in mm) after 24 h. Each experiment was repeated thrice.
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Results and discussion
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X-ray diffraction analysis
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The room temperature powder XRD patterns of ZnO and ZrO2 NPs are shown in Fig. 2. The diffraction peaks of both the samples are well-defined. The diffraction peaks centered at 32 and 55 in Fig. 2a with corresponding crystal plane parameters (0 0 2) (1 0 3) represents ZrO2. ZrO2 NPs show the standard pattern previously explained by Zhang et al. [39]. However, the broad diffraction peak centered at 2u value 37 and 43 in Fig. 2b with corresponding crystal planes parameters (11 0) (2 0 0) are the characteristic peak of the ZnO, with all diffraction peaks well indexed to the standard diffraction pattern of hexagonal phase ZnO (JCPDS No. 36-1451),
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Fig. 2. XRD spectra of (a) ZrO2 and (b) ZnO nanoparticles prepared by sol–gel approach.
indicating a wurtzite structured with high crystallinity. The corresponding h k l (miller indices) values are (11 0) (2 0 0). The average crystallite size of ZrO2 and ZnO NPs as calculated from diffraction peaks using Scherer’s formula was found to be 30 and 18 nm, respectively.
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Microscopic analysis
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Microscopic characterization was done with the help of TEM and SEM techniques and the images are presented in Fig. 3,
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Fig. 3. Microscopic images of ZnO and ZrO2 nanoparticles obtained after calcination at 400 C for 2 h.
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providing imperative information regarding the size, morphology and nature of the metallic oxide NPs. The microscopic images demonstrate the fine particle size nature of the NPs. The nanoparticles are much smaller than the magnified area of 50 nm under the transmission electron micrographs (Fig. 3a and b) which demonstrate the fine particle size of the as-prepared materials ZnO nano-flowers with high crystallinity can be seen clearly in Fig. 3c . However, the microscopic images of ZrO2 nanoparticles (Fig. 3b and d) did not show significant crystallinity but rather exhibit agglomeration of nanoparticles, which may be ascribed to their high surface energy [40]. The information provided by the microscopic images is consistent with the XRD data. The aggregated structure of ZrO2 NPs suggested a reduction in the total active surface area of nanoparticles (lowering the surface to volume ratio) thereby reducing the number of free valences which in turn affects the photocatalytic activity of the samples.
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Thermal stability (thermal gravimetric analysis)
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Fig. 5. UV absorption spectra of Acid Blue 25 at different time intervals during the photodegradation (spectra taken at T = 0–90 min, at 15 min interval).
Plots shown in Fig. 4 represent the thermal gravimetric curves of ZnO and ZrO2 NPs. It is evident from the plots that both the nanoparticles have sufficiently high thermal stability with less than 25% weight loss in the temperature range 25–1000 C. Moreover, the stability of ZnO NPs is slightly lower than ZrO2, and is being attributed to the low thermal conductivity and high inertness of zirconium, thus, making it useful for refractory applications. The weight loss at around 100 C is attributed to desorption (removal) of water physically bonded to the nanomaterials while the second gradual weight loss starting at around 450 C represents the loss of hydroxyl (—OH) attached to the surface of Q4 the nanomaterials (Zhao et al., 2004). Till 1000 C, ZnO NPs showed nearly 25% weight loss while ZrO2 experienced less than 15% weight loss only. Furthermore, both the NPs show gradual weight losses up to 700 C, thereafter, ZnO NPs shows a sudden weight loss. Thus, thermal gravimetric analysis demonstrates superior thermal stability of ZrO2 NPs over ZnO NPs.
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Photocatalytic activity – photodegradation of Acid Blue 25
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The photocatalytic degradation involves the generation of electrons (in conduction band) and holes (in valence bands) upon irradiation with UV or solar light [34]. These photogenerated electrons and holes react with water and oxygen to generate hydroxyl (OH) and hydroperoxyl (OOH) radicals which are very reactive and can easily oxidize the substrate. A fraction of these
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Ct ¼ ekt C0
where C0 is the initial concentration while Ct is the concentration at any time ‘t’ in mg/L, and k is rate constant in min1. It was observed that ZnO NPs have better photocatalytic activity (nearly 80% removal efficiency) than ZrO2 NPs (Fig. 6). This may be
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Temperature ( C) Fig. 4. Thermal gravimetric plots of ZnO and ZrO2 nanoparticles.
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(2)
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Per cent weight loss (w/w0)
215
radicals react with the adsorbed molecules resulting in to photocatalysis [41]. The photocatalytic degradation (or oxidation) is rapid, easy and very effective method of substrate degradation [42]. The photocatalytic activity of the materials was tested by studying the decolorization rate of Acid Blue 25 dye in presence of UV illuminating source in a quartz jacketed photoreactor. The azo-dye gives a characteristic peak at 600 nm [43]. The spectra shown in Fig. 5 validate the photodegradation of dye in presence of ZnO and ZrO2 catalyst. In order to quantify the photodegradation of Acid Blue 25 dye, initial concentrations (0.2 mM) of the dye were fed to the quartz jacketed photoreactor and continuously stirred in the dark for about 75 min to ensure adsorption/desorption equilibrium. Thereafter, the contents were irradiated with UV light to study the degradation of Acid Blue 25. The reaction was started and the samples (5 mL) were withdrawn at an interval of 15 min and the absorbance was monitored. The decay curve of the azo-dye followed first order kinetics as per the equation given below [3]:
0
20
40
60
80
t (mins) Fig. 6. Photodegradation of textile azo-dye Acid Blue 25 using ZnO and ZrO2 NPs (photocatalytic activity).
Please cite this article in press as: S. Sultana, et al., Development of ZnO and ZrO2 nanoparticles: Their photocatalytic and bactericidal activity, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.02.024
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Table 1 Effect of photocatalyst dosage on removal efficiency.
Table 2 Antibacterial activity of the synthesized nanoparticles.
Photocatalyst dosage (mg L1) % removal efficiency of AB 25 (1 C/C0) 100
50 100 200 300
242
ZnO
ZrO2
45 70 82 95
20 33 52 65
254
attributed to finer particle size and higher surface area of ZnO NPs. The results are consistent with the XRD and TEM data that supports a lower particle size for ZnO NPs. The value of rate constant k (calculated from the line fittings) for ZnO and ZrO2 catalyzed photodegradation reaction was 0.0181 (0.009) and 0.0028 (0.0005) min1 with R2 value 0.9937 and 0.899, respectively. Further, the photocatalytic activity was found to be a dose related response, i.e., with increase in the concentration of photocatalyst, the removal efficiency increases owing to increase in the number of active sites. However, the removal efficiency of the dye increases less rapidly in case of ZrO2 NPs (Table 1). The study demonstrates the practical application of ZnO and ZrO2 NPs in removing toxic azo-dyes from natural water streams.
255
Effect of pH on photodegradation of dyes
256
Experiments were carried out to determine the pH dependency of the rate of photodegradation. Fig. 7 shows the effect of pH on photodegradation of dye in presence of ZnO and ZrO2 NPs. The experiments were carried out at six different pH values ranging from highly acidic to extremely basic (2.0, 3.5, 5.8, 7.5, 9.0 and 11.5). Although the photocatalytic activity of both the NPs increases with increase in pH till 9.0 and thereafter decreases, the effect was not very significant. The photocatalytic activity was highest at pH 9.0, due to the increase of hydroxyl ions, which induces more hydroxyl radical formation [44]. Additionally, the photocatalytic activity of ZnO NPs is better than ZrO2 NPs at all pH values.
243 244 245 246 247 248 249 250 251 252 253
257 258 259 260 261 262 263 264 265 266 267
Antibacterial activity
268
The antibacterial studies on the synthetic nanoparticles – ZnO and ZrO2, have been performed in order to investigate their bactericidal action employing disk diffusion method. Two gram positive (S. aureus MSSA-22 and B. subtilis ATCC-6051) and two gram negative (E. coli K-12 and P. aeruginosa MTCC-2488) bacterial strain were selected for feasibility studies. Tetracycline was used as standard drug for comparison of the antibacterial property of ZnO
270 271 272 273 274
8 Photocatalytic activity C0/C
269
ZnO
7
ZrO2
6 5 4 3 2 1 0
2
3.5
5.8
7.5
5
9
11.5
pH of the medium Fig. 7. pH dependency of the photocatalytic activity of the nanoparticles.
Compound
Zone inhibition (mm)
Synthesized nanoparticle ZnO ZrO2 Tetracycline
S. aureus 17 10 9
B. substilis 16 11 8
E. coli 14 9 8
P. aeruginosa 15 7 6
and ZrO2 and the results are provided in Table 2. The newly synthesized nanoparticles have shown remarkable inhibitory effects (better than tetracycline) against the growth of all the tested bacterial strains (Fig. 7). The antimicrobial activity of several other synthetic molecules (e.g., 1,6-bis(benzimidazol-2-yl)-3,4dithiahexane ligand) were investigated by the researchers in a similar manner [45]. The data indicate that the synthetic ZnO and ZrO2 nanoparticles exhibit greater activity against S. aureus and B. subtilis as compared to the E. coli and P. aeruginosa. Furthermore, it is also clear that ZnO NPs shows better antibacterial activity than ZrO2 NPs against the tested bacterial strains due to the stability of ZnO NPs in the growth medium which imparts greater bacterium-nanoparticle interactions [18]. The results clearly evidenced that the synthesized nanoparticles are more effective than tetracycline against all tested strains and can be used as potent antibacterial agents.
275
Conclusions
291
Nanostructured ZnO and ZrO2 with exceptional photocatalytic and antibacterial activity were successfully prepared using sol–gel method. XRD, TEM and SEM provide useful information regarding the size (average crystallite size of ZnO NPs = 18 nm and ZrO2 NPs = 30 nm) geometries and surface morphology of the nanoparticles. The as-prepared nanoparticles possess high thermal stability, which renders their application in high temperature fields. Although, ZrO2 NPs shows better thermal stability, the photocatalytic and antibacterial activity was found to be superior for ZnO NPs. The photocatalytic activity increases with increasing the amount of photocatalyst, however, the effect was not very significant. The photocatalytic activity allows the use of ZnO NPs for treating toxic waste water while antibacterial activity renders their application in as disinfecting agents for killing pathogens. An important point which cannot be neglected is that, a compromise could be made by mixing different proportions of ZnO and ZrO2 nanoparticles in order to incorporate some unique properties in to the resulting nanocomposite.
292
Acknowledgements
310
Authors are thankful to Aligarh Muslim University for providing necessary research facilities. Department of Applied Physics is duly acknowledged for the XRD facility. CSIR is to be thanked for Q5 providing financial support to this project.
311
References
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