- Email: [email protected]
Morphology, photocatalytic and antibacterial activities of radial spherical ZnO nanorods controlled with a diblock copolymer
Superlattices and Microstructures 51 (2012) 103–113
Contents lists available at SciVerse ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Morphology, photocatalytic and antibacterial activities of radial spherical ZnO nanorods controlled with a diblock copolymer Pongsaton Amornpitoksuk a,e,⇑, Sumetha Suwanboon b,e, Suthinee Sangkanu c, Ampaitip Sukhoom c, Nantakan Muensit d,e a Department of Chemistry and Center for Innovation in Chemistry, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand b Department of Materials Science and Technology, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand c Department of Microbiology, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand d Department of Physics, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand e NANOTEC Center of Excellence at Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand
a r t i c l e
i n f o
Article history: Received 23 July 2011 Received in revised form 17 October 2011 Accepted 2 November 2011 Available online 10 November 2011 Keywords: ZnO Photocatalytic degradation Diblock copolymer
a b s t r a c t Radial spherical ZnO nanorods were synthesized directly from an aqueous zinc acetate dihydrate solution in the presence of the poly(ethylene oxide)-b-poly(propylene oxide) copolymer at a mole ratio of Zn2+:OH = 1:10. The diameter of the hexagonal facet and the length of each rod decreased with an increase of the copolymer concentrations. The blue-shift in the optical band gap was caused by an increase of the compressed lattice. The efficiency of photocatalytic degradation of methylene blue in aqueous solution increased with an increase of their surface areas. However, the decrease of their sizes did not improve their antibacterial activities. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Zinc oxide (ZnO) is an n-type semiconductor with a wide direct band gap (3.37 eV) and a large exciton binding energy (60 meV). It is well known that the morphology of ZnO has a big effect on its properties and corresponding potential applications. A one dimensional (1D) ZnO nanostructure such as a nanorod, nanowire, nanobelt and nanotube has received increasing attention in both academic research and industrial applications because of its potential as the building block for other structures ⇑ Corresponding author at: Department of Chemistry and Center for Innovation in Chemistry, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand. Tel.: +66 74 28 84 38; fax: +66 74 55 88 41. E-mail address: [email protected] (P. Amornpitoksuk). 0749-6036/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.spmi.2011.11.002
104
P. Amornpitoksuk et al. / Superlattices and Microstructures 51 (2012) 103–113
[1]. It has also attracted attention for its electrical and optical applications, such as for light-emitting diodes, because this 1D structure reduced the carrier scattering due to the carriers being confined in a certain direction. This improved the optical and electrical properties [2,3]. Furthermore, it can be used as a photocatalyst for degradation and complete elimination of environmental pollutants. There are many reports that the 1D ZnO nanostructure can also improve this activity [4,5]. Currently, there are numerous methods in the literature for preparing the 1D ZnO nanostructure including both wet and dry methods [6,7]. Precipitation is one of the most effective methods because it provides a simple and effective way to control the various shapes and sizes of 1D ZnO nanostructure by starting materials and/or stabilizers without the need for high temperatures and sophisticated equipment. Examples of the stabilizers used are ceryltrimethylammonium bromide (CTAB) [8], sodium dodecyl sulfate (SDS) [5], polyethylene glycol (PEG) [9] and so on. In recent years, the morphological control of 1D ZnO nanostructure by use of a triblock copolymer has been extensively investigated but there have been no reports prepared on the structure obtained using a diblock copolymer. In general, the triblock copolymer used such as poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) or PEO-b-PPO-b-PEO is a water-soluble amphiphilic block copolymer. In aqueous solution, the hydrophobic PPO part forms the inner core while the hydrophilic PEO chains are oriented outward. This generates the macromolecular micelles that enwrap the growth species of ZnðOHÞ2 4 [10]. The size of the formed micelle by this triblock copolymer is very large because there are two chains of PEO in the structure. However, the size of the micelle could be decreased if the number and length of the PEO parts in the structure were reduced. In this present work, we have investigated the effect of using poly(ethylene oxide)-b-poly(propylene oxide) (PEO)19-b-(PPO)3 as a diblock copolymer on the morphological control of radial spherical ZnO nanorods and also report the optical, photocatalytic and antibacterial properties of the ZnO powders prepared from the (PEO)19-b-(PPO)3-assisted Zn2+ solution by the precipitation method.
2. Experimental In a typical procedure, 7.50, 11.25, 13.50 and 15.00 mmol PEO-b-PPO (MW 1000, Huntsman, USA) were added to each aqueous Zn(C2H3O2)22H2O (Fluka) solution so the mole ratios of Zn2+:PEO-b-PPO were 1:0.5, 1:0.75, 1:0.9 and 1:1, respectively. Finally, a 15 mmol NaOH solution was added to the PEO-b-PPO-modified Zn2+ precursor solution then moderately stirred at 80 °C for 1 h. After being cooled to room temperature, these precipitates were filtered, rinsed with distilled water several times, then collected and dried at 100 °C for 1 h in air. The structural identification of ZnO powders was carried out using an X-ray diffractometer (XRD, X’Pert MPD, Philips) with Cu Ka radiation at a wavelength (k) of 0.15406 nm. Morphological studies were conducted using a scanning electron microscope (SEM, JSM-5800, JEOL) and transmission electron microscope (TEM, JEM 2010, JEOL). For the TEM, the sample suspended in a solution was dropped onto a carbon-coated copper grid and air-dried for 1 day. The UV–Vis diffuse reflectance spectra were recorded on a UV–Vis spectrophotometer (UV-2450, Shimadzu) using BaSO4 as a reflectance standard. Room temperature photoluminescence (PL) measurements were performed by a luminescence spectrophotometer (LS/55, Perkin–Elmer). A photocatalytic testing of the as-prepared ZnO powders was performed by measuring the decomposition of methylene blue (MB) under 3 15 W blacklight fluorescence tubes. ZnO powders (150 mg) were added into 150 mL of 1 105 M methylene blue aqueous solution and allowed to equilibrium for 30 min in dark conditions. After irradiation for 0.5, 1, 1.5, 2 and 3 h, 3 mL of MB solution was collected and centrifuged. The remaining dye concentration was determined using a UV–Vis spectrophotometer (Lambda 25, Perkin–Elmer). The minimum inhibitory concentration (MIC) of as-prepared ZnO powders was determined by a broth microdilution method. Triplicate 50 lL samples of ZnO (400 lg/mL) were placed into sterile 96-well microtiter plates. The bacterial inoculum (0.5 MF) was diluted 1:200 (5 105 CFU/mL) using Mueller–Hinton broth (MHB) and 50 lL was added to the top well in each row so the final concentration of ZnO was 200 lg/mL. Plates were incubated at 35 °C for 15 h then 10 lL of resazurin indicator (0.18%) was added to each well and examined after incubation for 2–3 h at 35 °C. After incubation
105
P. Amornpitoksuk et al. / Superlattices and Microstructures 51 (2012) 103–113
under appropriate conditions, the lowest concentration of ZnO that inhibited growth (blue color) was recorded as the MIC. Gentamicin and vancomycin at a final concentration of 10 lg/mL were used as standard antibacterial agents for positive control of Escherichia coli (E. coli) ATCC 25922 and Staphylococcus aureus (S. aureus) ATCC 25923, respectively. 3. Results and discussion 3.1. Structure and morphology The as-prepared ZnO powders synthesized from the solutions containing mole ratios of Zn2+:PEOb-PPO, in the range of 1:0–1:1 at the mole ratio of Zn2+:OH = 1:10 were characterized by XRD and the results are displayed in Fig. 1. It is noteworthy to mention that only a single phase of ZnO was formed and there were no characteristic peaks for any impurity phase such as Zn(OH)2 even though no calcination process was introduced. All diffraction peaks were identified as belonging to the hexagonal or würtzite ZnO structure in accordance with the JCPDS card number 36-1451. Table 1 shows the crystallographic data of as-prepared ZnO powders. It is obvious that the lattice parameter c tended to decrease with an increase of the stabilizer concentrations and this corresponded to a reduction of the length of ZnO rods as shown in Fig. 2. SEM images of as-prepared ZnO powders precipitated from the aqueous solution at the mole ratios of Zn2+:PEO-b-PPO = 1:0–1:1 are presented in Fig. 2. The ZnO shape tended to form a radial spherical rod-like structure at the mole ratio of Zn2+:OH = 1:10 without any copolymer. On increasing the PEO-b-PPO concentrations, the diameter of the hexagonal facet for each ZnO rod decreased from
Fig. 1. X-ray diffraction patterns of as-prepared ZnO powders precipitated at the mole ratios of Zn2+:PEO-b-PPO; (a) 1:0, (b) 1:0.5, (c) 1:0.75, (d) 1:0.9 and (e) 1:1.
Table 1 BET surface area and the data of structural and optical properties as-prepared ZnO powders precipitated at various mole ratios of Zn2+:PEO-b-PPO. Mole ratio of Zn2+:PEO-b-PPO
BET surface area (m2/g)
Lattice parameter a (Å)
c (Å)
V (Å)
1:0 1:0.5 1:0.75 1:0.9 1:1
6.10 8.61 9.22 10.15 11.32
3.2518 3.2517 3.2515 3.2511 3.2511
5.2099 5.2073 5.2070 5.2064 5.2055
47.70889 47.68361 47.67568 47.65636 47.64378
Eg (eV)
E0 (eV)
3.178 3.184 3.190 3.212 3.216
0.108 0.109 0.114 0.119 0.126
3
106
P. Amornpitoksuk et al. / Superlattices and Microstructures 51 (2012) 103–113
Fig. 2. SEM images of as-prepared ZnO powders precipitated at various mole ratios of Zn2+:PEO-b-PPO.
180 to 90 nm and its length also reduced from 1300 to 600 nm. It was classified as a 1D nanostructure because there was two dimensions less than 100 nm. Based on many proposed mechanisms, Zn2+ ions react with OH to form ZnðOHÞ2 4 as a growth specie in a strong alkaline solution [11,12]. At an elevated temperature, ZnO nuclei were formed from the dehydration of ZnðOHÞ2 4 and followed the growth step [11,12]. The growth units of ZnðOHÞ2 4 begin to be incorporated into ZnO along the c-axis and the growth along the [0 0 0 1] direction proceeds continuously as the velocity of the ZnO crystal growth in different directions is reported to be: V(0 0 0 1) > V(1 0 1 1Þ > Vð1 0 1 0Þ > Vð1 0 1 1Þ > Vð0 0 0 1Þ in solution method [13]. To reduce the total surface energy of the system [9,14,15], the growth in this direction is favored and a radial spherical rod shape is obtained. In the presence of PEO-b-PPO, these
P. Amornpitoksuk et al. / Superlattices and Microstructures 51 (2012) 103–113
107
Fig. 3. TEM and HRTEM (inset b, top) images and SAED pattern (inset b, bottom) of as-prepared ZnO powders precipitated at the mole ratio of Zn2+:PEO-b-PPO = 1:1.
tiny ZnO nuclei became enwrapped by micelles generated from the copolymer. The diameter of its micelle probably inhibited the growth velocity of the ZnO nanorods in the (0 0 0 1) plane or controlled the size of radial spherical ZnO nanorods as was reported by Zhang and Mu [10]. At low concentrations of PEO-b-PPO, the copolymer could not form a complete micelle then the growth in the c-axis still occurred to produce a large ZnO rod. The degree of the suppressed growth along the c-axis increased as a function of the diblock copolymer concentrations and this was indicated by the decrease of the diameter of the radial spherical ZnO nanorods. Fig. 3 shows the TEM image of ZnO prepared from the aqueous solution at the mole ratio of Zn2+:PEO-b-PPO = 1:1. The insets in Fig. 3(b) are the selected area electron diffraction patterns (SAED) and the high resolution TEM image. The SAED indicates that the as-prepared ZnO nanorods were single crystalline. Furthermore, the interplanar spacing obtained was approximately 0.52 nm and corresponded to the d spacing of the (0 0 0 1) plane of ZnO in its würtzite structure. This confirmed that the growth direction was along the [0 0 0 1] direction or c-axis. 3.2. Optical properties The optical band gap of ZnO powders was estimated by the Kubelka–Munk model [16] using the relation:
ahv ¼ Aðhv Eg Þ1=n where a is the absorption coefficient, A is a constant, h is the Plank’s constant, v is the photon frequency, Eg is the optical band gap and n = 2 for the direct allowed transition. An estimation of the value of the band gap according to this model was calculated from a plot of (ahv)2 vs. photon energy (hv) as shown in Fig. 4. Extrapolation of the linear portion to the energy axis at (ahv)2 = 0 gives the Eg value. In this investigation, the Eg values increased from 3.178 to 3.216 eV. This was achieved as the mole ratio of Zn2+:PEO-b-PPO copolymer was increased from 1:0 to 1:1 as shown in Table 1. In the case when the size was much larger than the Bohr radius for ZnO, the blue-shift in the band gap could arise from the
108
P. Amornpitoksuk et al. / Superlattices and Microstructures 51 (2012) 103–113
Fig. 4. Evolution of (ahv)2 vs. hv of as-prepared ZnO powders precipitated at the mole ratios of Zn2+:PEO-b-PPO = 1:0 (h), 1:0.5 (), 1:0.75 (), 1:0.9 (⁄) and 1:1 (4), respectively.
Fig. 5. Photoluminescence spectra of as-prepared ZnO powders precipitated at various mole ratios of Zn2+:PEO-b-PPO.
compressive strain or the lattice contraction caused by an increased repulsion between the oxygen 2p and the zinc 4s band [17–19]. In this work, the unit cell volume decreased with an increase of the copolymer concentrations as shown in Table 1 then the compression strain could affect the band gap as mentioned above. Fig. 5 shows the room temperature PL spectra of as-prepared ZnO powders. The emission peak centered at about 390–400 nm in the UV region is well understood as it is the nearband-edge emission. As the concentration of PEO-b-PPO was increased, a blue-shift of these emissions was achieved as shown in the inset of Fig. 5. This evidence corresponds with the increase of the band gap of ZnO as seen in Table 1. The broad visible emission originated from the structural defects, e.g. oxygen vacancies and zinc interstitials. In fact, ZnO has a lot of defects on or near the surface which can adsorb the O2 and O ions to form the oxygen vacancies (VO, V O and V O ). The photogenerated holes are trapped into surface defects and can tunnel back inside the ZnO. When it combines with a V O to form V O center, it leads to the transition responsible for the visible transition. This tunneling rate depends on the surface area per unit volume then small radial spherical ZnO nanorods have the
P. Amornpitoksuk et al. / Superlattices and Microstructures 51 (2012) 103–113
109
Fig. 6. Plot of ln(a) vs. hv of as-prepared ZnO powders precipitated at the mole ratios of Zn2+:PEO-b-PPO = 1:0 (h), 1:0.5 (), 1:0.75 (), 1:0.9 (⁄) and 1:1 (4), respectively.
highest surface area (Table 1) and this shows the maximum PL intensity. Moreover, the concentration of the oxygen vacancies can be estimated by the band tails that alter the absorption edge calculated by the Pankove’s expression [20]. The plot between the natural logarithm of a and photon energy (hv) as shown in Fig. 6, and the lists of the E0 values are presented in Table 1. The E0 is a parameter that describes the width of the localized state in the band gap resulting from all defects and it is difficult to pinpoint the nature of the variation in E0 [21]. It is possible that the oxygen defect, as an intrinsic defect, increases when the total defect concentrations are increased. Then, their values of E0 (Table 1) are in agreement with the PL intensity in the visible region. 3.3. Photocatalytic activity The efficiency of photodegradation of MB was indicated by the decrease of the ratio between the remaining dye in the solution after irradiation at time t (Ct) and the initial dye concentration (C0) as shown in Fig. 7. In dark conditions, the dyes were adsorbed onto the surface of ZnO powders and this reduced the ratio of Ct/C0 before irradiation. A small decrease of Ct/C0 for the MB solution occurred with the ZnO powders in the dark and without ZnO powders under UV illumination as shown in Fig. 7. This confirms that the adsorption and self-photodegradation effects can be negligible compared to the photocatalytic effect of the ZnO powders. Under UV-light, the reactive species such as the superoxide radical and hydroxyl radical, generated by the ZnO powders can oxidize the dye molecules to the degraded products then the ratio of Ct/C0 decreased with the irradiation time [22,23]. The photocatalytic discoloration of dye is believed to take place according to the following mechanism: þ
ZnO þ hv ! ZnOðhvb þ ecb Þ þ
hvb þ H2 O ! OH þ Hþ ecb þ O2 ! O2
O2 þ H2 O ! H2 O2 ! 2 OH
OH þ dye ! dyeox ! intermediates ! CO2 þ H2 O
110
P. Amornpitoksuk et al. / Superlattices and Microstructures 51 (2012) 103–113
Fig. 7. Relative concentration of methylene blue solution without ZnO powder under UV light (a), with ZnO powders precipitated at the mole ratio of Zn2+:PEO-b-PPO = 1:1 in the dark (b) and with ZnO powders precipitated at the mole ratios of Zn2+:PEO-b-PPO; (c) 1:0, (d) 1:0.5 and (e) 1:1 subjected to blacklight fluorescence tubes.
The photocatalytic activities are then suppressed if the photo-induced electrons are recombined with holes as follows: þ
hvb þ ecb ! heat This process is possibly inhibited by oxygen vacancies [4]. The surface area is one of the important parameters that can improve the photocatalytic degradation of dye molecules. From SEM images, the ZnO powder prepared at the mole ratio of Zn2+:PEO-b-PPO = 1:1 has the smallest particle size or the highest surface area (Table 1). This might help improve the photocatalytic degradation as seen in Fig. 7. Furthermore, it shows the strongest PL emission in the visible region and has the highest E0 as mentioned above, and these are associated with oxygen vacancies [24]. It implies that high oxygen
Fig. 8. The durability of (a) commercial ZnO and (b) as-prepared ZnO powders for photodegradation of MB solution after irradiation for 1 h under UV light. The degradation (%) calculated from [(Ct = 0 Ct = 1)/Ct = 0] 100 where Ct = 0 and Ct = 1 are the remaining concentrations of MB before and after irradiation for 1 h, respectively.
111
P. Amornpitoksuk et al. / Superlattices and Microstructures 51 (2012) 103–113
Table 2 Comparison of the broth dilution MIC values for as-prepared ZnO powders in this work and previous reports against Escherichia coli and Staphylococcus aureus. MIC (lg/mL)
E. coli S. aureus
Gentamicin
Vancomycin
ZnO (this work)
ZnO Ref. [25]
ZnO Ref. [26]
ZnO Ref. [27]
2 –
– 0.25
512 256
– 312.5
400 400
3500 1000
vacancies might inhibit the electron-hole pair recombination. Because of the high surface area and high content of the oxygen vacancies, this sample shows the highest efficiency for MB degradation [4,25]. Fig. 8 compares the efficiencies of photocatalytic degradation of MB catalyzed by the commercial ZnO and as-prepared ZnO at the mole ratio of Zn2+:PEO-b-PPO = 1:1 during a recycling process. Although the initial photocatalytic efficiency of commercial ZnO was still better than the as-prepared ZnO in this work, its degradation efficiency decreased with the number of reuses and was less than the as-prepared ZnO after being recycled for 10 times. In fact, the commercial ZnO powders are very light, small in size and difficult to sediment and this can result in a loss of catalyst in the step of changing between the photocatalyzed MB solution and the untreated MB solution in each cycle. For applications in wastewater treatment, the catalyst not only shows a high efficiency but also can be easily separated in order to reduce the running costs.
3.4. Antibacterial activity For demonstrating the antibacterial activity of as-prepared ZnO powders, S. aureus and E. coli were selected as the Gram-positive and Gram-negative bacteria, respectively. For all as-prepared ZnO powders, the MIC values obtained from the broth dilution test for E. coli and S. aureus were 512 and 256 lg/mL, respectively and neither depended on the size of ZnO powders. However, these values were better than others reported by the same technique as shown in Table 2. Because there is not a recommended standard method for testing the inhibiting activities for inorganic antibacterial agents, different methods such as a broth dilution minimal inhibitory concentration, minimum bactericidal concentration, agar well diffusion assay and plate counting technique have been applied to determine their activities and this makes it difficult to provide direct comparisons. The difference in activity against these two types of bacteria could be attributed to the structural and compositional differences of the cell membrane. In general, Gram-negative bacteria have a lipopolysaccharide (LPS) which protects the cytoplasmic membrane from external chemicals. An antibacterial mechanism of ZnO in dark conditions has been proposed to occur in several ways [28,29] but the precise mechanism is still not clearly understood. The mechanical destruction of cell membrane by reactive species such as the O 2 and OH, and H2O2 has been usually mentioned in many publications [26,30]. These species are generally obtained from the results of the photocatalytic reaction when ZnO absorbs UV light. In dark conditions, the superoxide radical ( O 2 ) can still be generated from the surface of ZnO powders and has been detected by electron spin resonance spectroscopy [31]. This confirms the possibility of achieving the destruction of cell membranes by reactive species generated by ZnO in the dark.
4. Conclusions The radial spherical ZnO nanorods can be prepared by the precipitation method at a low temperature. The PEO-b-PPO copolymer displays good potential to suppress ZnO growth along the c-axis by micellization. The increase of the band gap with the mole ratio of Zn2+:PEO-b-PPO results from a compression of the lattice contraction. At the mole ratio of Zn2+:PEO-b-PPO = 1:1, the as-prepared ZnO powders have the highest surface area and highest oxygen vacancies causing high photocatalytic properties. This investigation did not detect any effect of the size of ZnO on the antibacterial activities.
112
P. Amornpitoksuk et al. / Superlattices and Microstructures 51 (2012) 103–113
Although the MIC for E. coli and S. aureus are excessively high, the antibacterial activities of as-prepared ZnO powders are still better than those obtained in previous reports using the same technique.
Acknowledgments This research is supported by Thailand Research Fund (TRF) under contract number MRG5480071. The authors also acknowledge the Center for Innovation in Chemistry (PERCH-CIC), Commission on Higher Education, Ministry of Education and the National Nanotechnology Center (NANOTEC), NSTDA, Ministry of Science and Technology, through its program of Center of Excellence Network at PSU. We would like to thank Dr. Brian Hodgson for assistance with the English.
References [1] B. Weintraub, Z. Zhou, Y. Li, Y. Deng, Solution synthesis of one-dimensional ZnO nanomaterials and their applications, Nanoscale 2 (2010) 1573–1587. [2] J.H. Lee, K.H. Ko, B.O. Park, Electrical and optical properties of ZnO transparent conducting films by the sol–gel method, J. Cry. Growth 247 (2003) 119–125. [3] X. Ma, J. Pan, P. Chen, D. Li, H. Zhang, Y. Yang, D. Yang, Room temperature electrically pumped ultraviolet random lasing from ZnO nanorod arrays on Si, Opt. Express 17 (2009) 14426–14433. [4] Y. Wang, X. Li, N. Wang, X. Quan, Y. Chen, Controllable synthesis of ZnO nanoflowers and their morphology-dependent photocatalytic activities, Sep. Purif. Technol. 62 (2008) 727–732. [5] H. Usui, Surfactant concentration dependence of structure and photocatalytic properties of zinc oxide rods prepared using chemical synthesis in aqueous solutions, J. Colloid Interface Sci. 336 (2009) 667–678. [6] Y.J. Gao, W.C. Zhang, X.L. Wu, Y. Xia, G.S. Huang, L.L. Xu, J.C. Shen, G.G. Siu, P.K. Chu, Hydrothermal self-assembling of ZnO nanorods into sphere-like superstructures and their optical characteristics, Appl. Surf. Sci. 255 (2008) 1982–1987. [7] P.X. Gao, C.S. Lao, W.L. Hughes, Z.L. Wang, Three-dimensional interconnected nanowire networks of ZnO, Chem. Phys. Lett. 408 (2005) 174–178. [8] X.M. Sun, X. Chen, Z.X. Dang, Y.D. Li, A CTAB-assisted hydrothermal orientation growth of ZnO nanorods, Chem. Phys. 78 (2002) 99–104. [9] X.F. Zhou, Z.L. Hu, Y. Chen, H.Y. Shang, Microscale sphere assembly of ZnO nanotubes, Mater. Res. Bull. 43 (2008) 2790– 2798. [10] Z. Zhang, J. Mu, Hydrothermal synthesis of ZnO nanobundles controlled by PEO–PPO–PEO block copolymers, J. Colloid Interface Sci. 307 (2007) 79–82. [11] S. Music, D. Dragcevic, S. Popovic, Influence of synthesis route on the formation of ZnO particles and their morphologies, J. Alloys Compd. 429 (2007) 242–249. [12] R. Wahab, S.G. Ansari, Y.S. Kim, M. Song, H.S. Shin, The role of pH variation on the growth of zinc oxide nanostructure, Appl. Surf. Sci. 255 (2009) 4891–4896. [13] P. Li, Y. Wei, H. Liu, X.K. Wang, Growth of well-defined ZnO microparticles with additives from aqueous solution, J. Solid State Chem. 178 (2005) 855–860. [14] D. Wu, Z. Bai, K. Jiang, Temperature induced hierarchical growth of ZnO microcrystal, Mater. Lett. 63 (2009) 1057–1060. [15] P. Li, H. Liu, Y.F. Zhang, Y. Wei, X.K. Wang, Synthesis of flower-like ZnO microstructures via a simple solution route, Mater. Chem. Phys. 106 (2007) 63–69. [16] J. Yu, C. Li, S. Liu, Effect of PSS on morphology and optical properties of ZnO, J. Colloid Interface Sci. 326 (2008) 433–438. [17] K.M. McPeak, J.B. Baxter, ZnO nonowire grown by chemical bath deposition in a continuous flow microreactor, Cry. Growth Design 9 (2009) 4538–4545. [18] T.P. Rao, M.C.S. Kumar, S.A. Angayarkanni, M. Ashok, Effect of stress on optical band gap of ZnO thin films with substrate temperature by spay pyrolysis, J. Alloys Compd. 485 (2009) 413–417. [19] R. Ghosh, D. Basak, S. Fujihara, Effect of substrate-induced strain on the structure, electrical, and optical properties of polycrystalline ZnO thin films, J. Appl. Phys. 96 (2004) 2689–2692. [20] B.Y. Chen, X.L. Xu, G.H. Zhang, H. Xue, S.Y. Ma, A comparative study of the microstructures and optical properties of Cu- and Ag-doped ZnO thin films, Physica B 404 (2009) 3645–3649. [21] Z.B. Bahsi, A.Y. Oral, Effects of Mn and Cu doping on the microstructures and optical properties of sol–gel derived ZnO thin films, Opt. Mater. 29 (2007) 672–678. [22] Y. Zhang, J. Mu, One-pot synthesis, photoluminescence, and photocatalysis of Ag/ZnO composites, J. Colloid Interface Sci. 309 (2007) 478–484. [23] M.A. Behnajady, N. Modirshahla, R. Hamzavi, Kinetic study on photocatalytic degradation of C.I. acid yellow 23 by ZnO photocatalyst, J. Hazard. Mater. 133 (2006) 226–232. [24] N. Samaele, P. Amornpitoksuk, S. Suwanboon, Effect of pH on the morphology and optical properties of modified ZnO particles by SDS via a precipitation method, Powder Technol. 203 (2010) 243–247. [25] J. Xu, Y. Ao, D. Fu, C. Yuan, Low-temperature preparation of F-doped TiO2 film and its photocatalytic activity under solar light, Appl. Surf. Sci. 254 (2008) 3033–3038. [26] M. Fang, J.H. Chen, X.L. Xu, P.H. Yang, H.F. Hildebrand, Antibacterial activities of inorganic agents on six bacteria associated with oral infections by two susceptibility tests, Inter. J. Antimicrob. Agents 27 (2006) 513–517. [27] L. Yang, J. Mao, X. Zhang, T. Xue, T. Hou, L. Wang, M. Tu, Preparation and characteristics of Ag/nano-ZnO composite antimicrobial agent, Nanoscience 11 (2006) 44–48.
P. Amornpitoksuk et al. / Superlattices and Microstructures 51 (2012) 103–113
113
[28] W. Lu, G. Liu, S. Gao, S. Xing, J. Wang, Tyrosin-assisted preparation of Ag/ZnO nanocomposites with enhanced photocatalytic performance and synergistic antibacterial activities, Nanotechnology 19 (2008) 445711. [29] Q. Li, S. Mahendra, D.Y. Lyon, L. Brunet, M.V. Liga, D. Li, P.J.J. Alvarez, Antimicrobial nanomaterials for water disinfection and microbial control: potential applications and implications, Water Res. 42 (2008) 4591–4602. [30] N. Padmavathy, R. Vijayaraghavan, Enhanced bioactivity of ZnO nanoparticles – An antimicrobial study, Sci. Technol. Adv. Mater. 9 (2008) 035004. [31] K. Hirota, M. Sugimoto, M. Kato, K. Tsukagoshi, T. Tanigawa, H. Sugimoto, Preparation of zinc oxide ceramics with a sustainable antibacterial activity under dark conditions, Cert. Int. 36 (2010) 497–506.
Contents lists available at SciVerse ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Morphology, photocatalytic and antibacterial activities of radial spherical ZnO nanorods controlled with a diblock copolymer Pongsaton Amornpitoksuk a,e,⇑, Sumetha Suwanboon b,e, Suthinee Sangkanu c, Ampaitip Sukhoom c, Nantakan Muensit d,e a Department of Chemistry and Center for Innovation in Chemistry, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand b Department of Materials Science and Technology, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand c Department of Microbiology, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand d Department of Physics, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand e NANOTEC Center of Excellence at Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand
a r t i c l e
i n f o
Article history: Received 23 July 2011 Received in revised form 17 October 2011 Accepted 2 November 2011 Available online 10 November 2011 Keywords: ZnO Photocatalytic degradation Diblock copolymer
a b s t r a c t Radial spherical ZnO nanorods were synthesized directly from an aqueous zinc acetate dihydrate solution in the presence of the poly(ethylene oxide)-b-poly(propylene oxide) copolymer at a mole ratio of Zn2+:OH = 1:10. The diameter of the hexagonal facet and the length of each rod decreased with an increase of the copolymer concentrations. The blue-shift in the optical band gap was caused by an increase of the compressed lattice. The efficiency of photocatalytic degradation of methylene blue in aqueous solution increased with an increase of their surface areas. However, the decrease of their sizes did not improve their antibacterial activities. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Zinc oxide (ZnO) is an n-type semiconductor with a wide direct band gap (3.37 eV) and a large exciton binding energy (60 meV). It is well known that the morphology of ZnO has a big effect on its properties and corresponding potential applications. A one dimensional (1D) ZnO nanostructure such as a nanorod, nanowire, nanobelt and nanotube has received increasing attention in both academic research and industrial applications because of its potential as the building block for other structures ⇑ Corresponding author at: Department of Chemistry and Center for Innovation in Chemistry, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand. Tel.: +66 74 28 84 38; fax: +66 74 55 88 41. E-mail address: [email protected] (P. Amornpitoksuk). 0749-6036/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.spmi.2011.11.002
104
P. Amornpitoksuk et al. / Superlattices and Microstructures 51 (2012) 103–113
[1]. It has also attracted attention for its electrical and optical applications, such as for light-emitting diodes, because this 1D structure reduced the carrier scattering due to the carriers being confined in a certain direction. This improved the optical and electrical properties [2,3]. Furthermore, it can be used as a photocatalyst for degradation and complete elimination of environmental pollutants. There are many reports that the 1D ZnO nanostructure can also improve this activity [4,5]. Currently, there are numerous methods in the literature for preparing the 1D ZnO nanostructure including both wet and dry methods [6,7]. Precipitation is one of the most effective methods because it provides a simple and effective way to control the various shapes and sizes of 1D ZnO nanostructure by starting materials and/or stabilizers without the need for high temperatures and sophisticated equipment. Examples of the stabilizers used are ceryltrimethylammonium bromide (CTAB) [8], sodium dodecyl sulfate (SDS) [5], polyethylene glycol (PEG) [9] and so on. In recent years, the morphological control of 1D ZnO nanostructure by use of a triblock copolymer has been extensively investigated but there have been no reports prepared on the structure obtained using a diblock copolymer. In general, the triblock copolymer used such as poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) or PEO-b-PPO-b-PEO is a water-soluble amphiphilic block copolymer. In aqueous solution, the hydrophobic PPO part forms the inner core while the hydrophilic PEO chains are oriented outward. This generates the macromolecular micelles that enwrap the growth species of ZnðOHÞ2 4 [10]. The size of the formed micelle by this triblock copolymer is very large because there are two chains of PEO in the structure. However, the size of the micelle could be decreased if the number and length of the PEO parts in the structure were reduced. In this present work, we have investigated the effect of using poly(ethylene oxide)-b-poly(propylene oxide) (PEO)19-b-(PPO)3 as a diblock copolymer on the morphological control of radial spherical ZnO nanorods and also report the optical, photocatalytic and antibacterial properties of the ZnO powders prepared from the (PEO)19-b-(PPO)3-assisted Zn2+ solution by the precipitation method.
2. Experimental In a typical procedure, 7.50, 11.25, 13.50 and 15.00 mmol PEO-b-PPO (MW 1000, Huntsman, USA) were added to each aqueous Zn(C2H3O2)22H2O (Fluka) solution so the mole ratios of Zn2+:PEO-b-PPO were 1:0.5, 1:0.75, 1:0.9 and 1:1, respectively. Finally, a 15 mmol NaOH solution was added to the PEO-b-PPO-modified Zn2+ precursor solution then moderately stirred at 80 °C for 1 h. After being cooled to room temperature, these precipitates were filtered, rinsed with distilled water several times, then collected and dried at 100 °C for 1 h in air. The structural identification of ZnO powders was carried out using an X-ray diffractometer (XRD, X’Pert MPD, Philips) with Cu Ka radiation at a wavelength (k) of 0.15406 nm. Morphological studies were conducted using a scanning electron microscope (SEM, JSM-5800, JEOL) and transmission electron microscope (TEM, JEM 2010, JEOL). For the TEM, the sample suspended in a solution was dropped onto a carbon-coated copper grid and air-dried for 1 day. The UV–Vis diffuse reflectance spectra were recorded on a UV–Vis spectrophotometer (UV-2450, Shimadzu) using BaSO4 as a reflectance standard. Room temperature photoluminescence (PL) measurements were performed by a luminescence spectrophotometer (LS/55, Perkin–Elmer). A photocatalytic testing of the as-prepared ZnO powders was performed by measuring the decomposition of methylene blue (MB) under 3 15 W blacklight fluorescence tubes. ZnO powders (150 mg) were added into 150 mL of 1 105 M methylene blue aqueous solution and allowed to equilibrium for 30 min in dark conditions. After irradiation for 0.5, 1, 1.5, 2 and 3 h, 3 mL of MB solution was collected and centrifuged. The remaining dye concentration was determined using a UV–Vis spectrophotometer (Lambda 25, Perkin–Elmer). The minimum inhibitory concentration (MIC) of as-prepared ZnO powders was determined by a broth microdilution method. Triplicate 50 lL samples of ZnO (400 lg/mL) were placed into sterile 96-well microtiter plates. The bacterial inoculum (0.5 MF) was diluted 1:200 (5 105 CFU/mL) using Mueller–Hinton broth (MHB) and 50 lL was added to the top well in each row so the final concentration of ZnO was 200 lg/mL. Plates were incubated at 35 °C for 15 h then 10 lL of resazurin indicator (0.18%) was added to each well and examined after incubation for 2–3 h at 35 °C. After incubation
105
P. Amornpitoksuk et al. / Superlattices and Microstructures 51 (2012) 103–113
under appropriate conditions, the lowest concentration of ZnO that inhibited growth (blue color) was recorded as the MIC. Gentamicin and vancomycin at a final concentration of 10 lg/mL were used as standard antibacterial agents for positive control of Escherichia coli (E. coli) ATCC 25922 and Staphylococcus aureus (S. aureus) ATCC 25923, respectively. 3. Results and discussion 3.1. Structure and morphology The as-prepared ZnO powders synthesized from the solutions containing mole ratios of Zn2+:PEOb-PPO, in the range of 1:0–1:1 at the mole ratio of Zn2+:OH = 1:10 were characterized by XRD and the results are displayed in Fig. 1. It is noteworthy to mention that only a single phase of ZnO was formed and there were no characteristic peaks for any impurity phase such as Zn(OH)2 even though no calcination process was introduced. All diffraction peaks were identified as belonging to the hexagonal or würtzite ZnO structure in accordance with the JCPDS card number 36-1451. Table 1 shows the crystallographic data of as-prepared ZnO powders. It is obvious that the lattice parameter c tended to decrease with an increase of the stabilizer concentrations and this corresponded to a reduction of the length of ZnO rods as shown in Fig. 2. SEM images of as-prepared ZnO powders precipitated from the aqueous solution at the mole ratios of Zn2+:PEO-b-PPO = 1:0–1:1 are presented in Fig. 2. The ZnO shape tended to form a radial spherical rod-like structure at the mole ratio of Zn2+:OH = 1:10 without any copolymer. On increasing the PEO-b-PPO concentrations, the diameter of the hexagonal facet for each ZnO rod decreased from
Fig. 1. X-ray diffraction patterns of as-prepared ZnO powders precipitated at the mole ratios of Zn2+:PEO-b-PPO; (a) 1:0, (b) 1:0.5, (c) 1:0.75, (d) 1:0.9 and (e) 1:1.
Table 1 BET surface area and the data of structural and optical properties as-prepared ZnO powders precipitated at various mole ratios of Zn2+:PEO-b-PPO. Mole ratio of Zn2+:PEO-b-PPO
BET surface area (m2/g)
Lattice parameter a (Å)
c (Å)
V (Å)
1:0 1:0.5 1:0.75 1:0.9 1:1
6.10 8.61 9.22 10.15 11.32
3.2518 3.2517 3.2515 3.2511 3.2511
5.2099 5.2073 5.2070 5.2064 5.2055
47.70889 47.68361 47.67568 47.65636 47.64378
Eg (eV)
E0 (eV)
3.178 3.184 3.190 3.212 3.216
0.108 0.109 0.114 0.119 0.126
3
106
P. Amornpitoksuk et al. / Superlattices and Microstructures 51 (2012) 103–113
Fig. 2. SEM images of as-prepared ZnO powders precipitated at various mole ratios of Zn2+:PEO-b-PPO.
180 to 90 nm and its length also reduced from 1300 to 600 nm. It was classified as a 1D nanostructure because there was two dimensions less than 100 nm. Based on many proposed mechanisms, Zn2+ ions react with OH to form ZnðOHÞ2 4 as a growth specie in a strong alkaline solution [11,12]. At an elevated temperature, ZnO nuclei were formed from the dehydration of ZnðOHÞ2 4 and followed the growth step [11,12]. The growth units of ZnðOHÞ2 4 begin to be incorporated into ZnO along the c-axis and the growth along the [0 0 0 1] direction proceeds continuously as the velocity of the ZnO crystal growth in different directions is reported to be: V(0 0 0 1) > V(1 0 1 1Þ > Vð1 0 1 0Þ > Vð1 0 1 1Þ > Vð0 0 0 1Þ in solution method [13]. To reduce the total surface energy of the system [9,14,15], the growth in this direction is favored and a radial spherical rod shape is obtained. In the presence of PEO-b-PPO, these
P. Amornpitoksuk et al. / Superlattices and Microstructures 51 (2012) 103–113
107
Fig. 3. TEM and HRTEM (inset b, top) images and SAED pattern (inset b, bottom) of as-prepared ZnO powders precipitated at the mole ratio of Zn2+:PEO-b-PPO = 1:1.
tiny ZnO nuclei became enwrapped by micelles generated from the copolymer. The diameter of its micelle probably inhibited the growth velocity of the ZnO nanorods in the (0 0 0 1) plane or controlled the size of radial spherical ZnO nanorods as was reported by Zhang and Mu [10]. At low concentrations of PEO-b-PPO, the copolymer could not form a complete micelle then the growth in the c-axis still occurred to produce a large ZnO rod. The degree of the suppressed growth along the c-axis increased as a function of the diblock copolymer concentrations and this was indicated by the decrease of the diameter of the radial spherical ZnO nanorods. Fig. 3 shows the TEM image of ZnO prepared from the aqueous solution at the mole ratio of Zn2+:PEO-b-PPO = 1:1. The insets in Fig. 3(b) are the selected area electron diffraction patterns (SAED) and the high resolution TEM image. The SAED indicates that the as-prepared ZnO nanorods were single crystalline. Furthermore, the interplanar spacing obtained was approximately 0.52 nm and corresponded to the d spacing of the (0 0 0 1) plane of ZnO in its würtzite structure. This confirmed that the growth direction was along the [0 0 0 1] direction or c-axis. 3.2. Optical properties The optical band gap of ZnO powders was estimated by the Kubelka–Munk model [16] using the relation:
ahv ¼ Aðhv Eg Þ1=n where a is the absorption coefficient, A is a constant, h is the Plank’s constant, v is the photon frequency, Eg is the optical band gap and n = 2 for the direct allowed transition. An estimation of the value of the band gap according to this model was calculated from a plot of (ahv)2 vs. photon energy (hv) as shown in Fig. 4. Extrapolation of the linear portion to the energy axis at (ahv)2 = 0 gives the Eg value. In this investigation, the Eg values increased from 3.178 to 3.216 eV. This was achieved as the mole ratio of Zn2+:PEO-b-PPO copolymer was increased from 1:0 to 1:1 as shown in Table 1. In the case when the size was much larger than the Bohr radius for ZnO, the blue-shift in the band gap could arise from the
108
P. Amornpitoksuk et al. / Superlattices and Microstructures 51 (2012) 103–113
Fig. 4. Evolution of (ahv)2 vs. hv of as-prepared ZnO powders precipitated at the mole ratios of Zn2+:PEO-b-PPO = 1:0 (h), 1:0.5 (), 1:0.75 (), 1:0.9 (⁄) and 1:1 (4), respectively.
Fig. 5. Photoluminescence spectra of as-prepared ZnO powders precipitated at various mole ratios of Zn2+:PEO-b-PPO.
compressive strain or the lattice contraction caused by an increased repulsion between the oxygen 2p and the zinc 4s band [17–19]. In this work, the unit cell volume decreased with an increase of the copolymer concentrations as shown in Table 1 then the compression strain could affect the band gap as mentioned above. Fig. 5 shows the room temperature PL spectra of as-prepared ZnO powders. The emission peak centered at about 390–400 nm in the UV region is well understood as it is the nearband-edge emission. As the concentration of PEO-b-PPO was increased, a blue-shift of these emissions was achieved as shown in the inset of Fig. 5. This evidence corresponds with the increase of the band gap of ZnO as seen in Table 1. The broad visible emission originated from the structural defects, e.g. oxygen vacancies and zinc interstitials. In fact, ZnO has a lot of defects on or near the surface which can adsorb the O2 and O ions to form the oxygen vacancies (VO, V O and V O ). The photogenerated holes are trapped into surface defects and can tunnel back inside the ZnO. When it combines with a V O to form V O center, it leads to the transition responsible for the visible transition. This tunneling rate depends on the surface area per unit volume then small radial spherical ZnO nanorods have the
P. Amornpitoksuk et al. / Superlattices and Microstructures 51 (2012) 103–113
109
Fig. 6. Plot of ln(a) vs. hv of as-prepared ZnO powders precipitated at the mole ratios of Zn2+:PEO-b-PPO = 1:0 (h), 1:0.5 (), 1:0.75 (), 1:0.9 (⁄) and 1:1 (4), respectively.
highest surface area (Table 1) and this shows the maximum PL intensity. Moreover, the concentration of the oxygen vacancies can be estimated by the band tails that alter the absorption edge calculated by the Pankove’s expression [20]. The plot between the natural logarithm of a and photon energy (hv) as shown in Fig. 6, and the lists of the E0 values are presented in Table 1. The E0 is a parameter that describes the width of the localized state in the band gap resulting from all defects and it is difficult to pinpoint the nature of the variation in E0 [21]. It is possible that the oxygen defect, as an intrinsic defect, increases when the total defect concentrations are increased. Then, their values of E0 (Table 1) are in agreement with the PL intensity in the visible region. 3.3. Photocatalytic activity The efficiency of photodegradation of MB was indicated by the decrease of the ratio between the remaining dye in the solution after irradiation at time t (Ct) and the initial dye concentration (C0) as shown in Fig. 7. In dark conditions, the dyes were adsorbed onto the surface of ZnO powders and this reduced the ratio of Ct/C0 before irradiation. A small decrease of Ct/C0 for the MB solution occurred with the ZnO powders in the dark and without ZnO powders under UV illumination as shown in Fig. 7. This confirms that the adsorption and self-photodegradation effects can be negligible compared to the photocatalytic effect of the ZnO powders. Under UV-light, the reactive species such as the superoxide radical and hydroxyl radical, generated by the ZnO powders can oxidize the dye molecules to the degraded products then the ratio of Ct/C0 decreased with the irradiation time [22,23]. The photocatalytic discoloration of dye is believed to take place according to the following mechanism: þ
ZnO þ hv ! ZnOðhvb þ ecb Þ þ
hvb þ H2 O ! OH þ Hþ ecb þ O2 ! O2
O2 þ H2 O ! H2 O2 ! 2 OH
OH þ dye ! dyeox ! intermediates ! CO2 þ H2 O
110
P. Amornpitoksuk et al. / Superlattices and Microstructures 51 (2012) 103–113
Fig. 7. Relative concentration of methylene blue solution without ZnO powder under UV light (a), with ZnO powders precipitated at the mole ratio of Zn2+:PEO-b-PPO = 1:1 in the dark (b) and with ZnO powders precipitated at the mole ratios of Zn2+:PEO-b-PPO; (c) 1:0, (d) 1:0.5 and (e) 1:1 subjected to blacklight fluorescence tubes.
The photocatalytic activities are then suppressed if the photo-induced electrons are recombined with holes as follows: þ
hvb þ ecb ! heat This process is possibly inhibited by oxygen vacancies [4]. The surface area is one of the important parameters that can improve the photocatalytic degradation of dye molecules. From SEM images, the ZnO powder prepared at the mole ratio of Zn2+:PEO-b-PPO = 1:1 has the smallest particle size or the highest surface area (Table 1). This might help improve the photocatalytic degradation as seen in Fig. 7. Furthermore, it shows the strongest PL emission in the visible region and has the highest E0 as mentioned above, and these are associated with oxygen vacancies [24]. It implies that high oxygen
Fig. 8. The durability of (a) commercial ZnO and (b) as-prepared ZnO powders for photodegradation of MB solution after irradiation for 1 h under UV light. The degradation (%) calculated from [(Ct = 0 Ct = 1)/Ct = 0] 100 where Ct = 0 and Ct = 1 are the remaining concentrations of MB before and after irradiation for 1 h, respectively.
111
P. Amornpitoksuk et al. / Superlattices and Microstructures 51 (2012) 103–113
Table 2 Comparison of the broth dilution MIC values for as-prepared ZnO powders in this work and previous reports against Escherichia coli and Staphylococcus aureus. MIC (lg/mL)
E. coli S. aureus
Gentamicin
Vancomycin
ZnO (this work)
ZnO Ref. [25]
ZnO Ref. [26]
ZnO Ref. [27]
2 –
– 0.25
512 256
– 312.5
400 400
3500 1000
vacancies might inhibit the electron-hole pair recombination. Because of the high surface area and high content of the oxygen vacancies, this sample shows the highest efficiency for MB degradation [4,25]. Fig. 8 compares the efficiencies of photocatalytic degradation of MB catalyzed by the commercial ZnO and as-prepared ZnO at the mole ratio of Zn2+:PEO-b-PPO = 1:1 during a recycling process. Although the initial photocatalytic efficiency of commercial ZnO was still better than the as-prepared ZnO in this work, its degradation efficiency decreased with the number of reuses and was less than the as-prepared ZnO after being recycled for 10 times. In fact, the commercial ZnO powders are very light, small in size and difficult to sediment and this can result in a loss of catalyst in the step of changing between the photocatalyzed MB solution and the untreated MB solution in each cycle. For applications in wastewater treatment, the catalyst not only shows a high efficiency but also can be easily separated in order to reduce the running costs.
3.4. Antibacterial activity For demonstrating the antibacterial activity of as-prepared ZnO powders, S. aureus and E. coli were selected as the Gram-positive and Gram-negative bacteria, respectively. For all as-prepared ZnO powders, the MIC values obtained from the broth dilution test for E. coli and S. aureus were 512 and 256 lg/mL, respectively and neither depended on the size of ZnO powders. However, these values were better than others reported by the same technique as shown in Table 2. Because there is not a recommended standard method for testing the inhibiting activities for inorganic antibacterial agents, different methods such as a broth dilution minimal inhibitory concentration, minimum bactericidal concentration, agar well diffusion assay and plate counting technique have been applied to determine their activities and this makes it difficult to provide direct comparisons. The difference in activity against these two types of bacteria could be attributed to the structural and compositional differences of the cell membrane. In general, Gram-negative bacteria have a lipopolysaccharide (LPS) which protects the cytoplasmic membrane from external chemicals. An antibacterial mechanism of ZnO in dark conditions has been proposed to occur in several ways [28,29] but the precise mechanism is still not clearly understood. The mechanical destruction of cell membrane by reactive species such as the O 2 and OH, and H2O2 has been usually mentioned in many publications [26,30]. These species are generally obtained from the results of the photocatalytic reaction when ZnO absorbs UV light. In dark conditions, the superoxide radical ( O 2 ) can still be generated from the surface of ZnO powders and has been detected by electron spin resonance spectroscopy [31]. This confirms the possibility of achieving the destruction of cell membranes by reactive species generated by ZnO in the dark.
4. Conclusions The radial spherical ZnO nanorods can be prepared by the precipitation method at a low temperature. The PEO-b-PPO copolymer displays good potential to suppress ZnO growth along the c-axis by micellization. The increase of the band gap with the mole ratio of Zn2+:PEO-b-PPO results from a compression of the lattice contraction. At the mole ratio of Zn2+:PEO-b-PPO = 1:1, the as-prepared ZnO powders have the highest surface area and highest oxygen vacancies causing high photocatalytic properties. This investigation did not detect any effect of the size of ZnO on the antibacterial activities.
112
P. Amornpitoksuk et al. / Superlattices and Microstructures 51 (2012) 103–113
Although the MIC for E. coli and S. aureus are excessively high, the antibacterial activities of as-prepared ZnO powders are still better than those obtained in previous reports using the same technique.
Acknowledgments This research is supported by Thailand Research Fund (TRF) under contract number MRG5480071. The authors also acknowledge the Center for Innovation in Chemistry (PERCH-CIC), Commission on Higher Education, Ministry of Education and the National Nanotechnology Center (NANOTEC), NSTDA, Ministry of Science and Technology, through its program of Center of Excellence Network at PSU. We would like to thank Dr. Brian Hodgson for assistance with the English.
References [1] B. Weintraub, Z. Zhou, Y. Li, Y. Deng, Solution synthesis of one-dimensional ZnO nanomaterials and their applications, Nanoscale 2 (2010) 1573–1587. [2] J.H. Lee, K.H. Ko, B.O. Park, Electrical and optical properties of ZnO transparent conducting films by the sol–gel method, J. Cry. Growth 247 (2003) 119–125. [3] X. Ma, J. Pan, P. Chen, D. Li, H. Zhang, Y. Yang, D. Yang, Room temperature electrically pumped ultraviolet random lasing from ZnO nanorod arrays on Si, Opt. Express 17 (2009) 14426–14433. [4] Y. Wang, X. Li, N. Wang, X. Quan, Y. Chen, Controllable synthesis of ZnO nanoflowers and their morphology-dependent photocatalytic activities, Sep. Purif. Technol. 62 (2008) 727–732. [5] H. Usui, Surfactant concentration dependence of structure and photocatalytic properties of zinc oxide rods prepared using chemical synthesis in aqueous solutions, J. Colloid Interface Sci. 336 (2009) 667–678. [6] Y.J. Gao, W.C. Zhang, X.L. Wu, Y. Xia, G.S. Huang, L.L. Xu, J.C. Shen, G.G. Siu, P.K. Chu, Hydrothermal self-assembling of ZnO nanorods into sphere-like superstructures and their optical characteristics, Appl. Surf. Sci. 255 (2008) 1982–1987. [7] P.X. Gao, C.S. Lao, W.L. Hughes, Z.L. Wang, Three-dimensional interconnected nanowire networks of ZnO, Chem. Phys. Lett. 408 (2005) 174–178. [8] X.M. Sun, X. Chen, Z.X. Dang, Y.D. Li, A CTAB-assisted hydrothermal orientation growth of ZnO nanorods, Chem. Phys. 78 (2002) 99–104. [9] X.F. Zhou, Z.L. Hu, Y. Chen, H.Y. Shang, Microscale sphere assembly of ZnO nanotubes, Mater. Res. Bull. 43 (2008) 2790– 2798. [10] Z. Zhang, J. Mu, Hydrothermal synthesis of ZnO nanobundles controlled by PEO–PPO–PEO block copolymers, J. Colloid Interface Sci. 307 (2007) 79–82. [11] S. Music, D. Dragcevic, S. Popovic, Influence of synthesis route on the formation of ZnO particles and their morphologies, J. Alloys Compd. 429 (2007) 242–249. [12] R. Wahab, S.G. Ansari, Y.S. Kim, M. Song, H.S. Shin, The role of pH variation on the growth of zinc oxide nanostructure, Appl. Surf. Sci. 255 (2009) 4891–4896. [13] P. Li, Y. Wei, H. Liu, X.K. Wang, Growth of well-defined ZnO microparticles with additives from aqueous solution, J. Solid State Chem. 178 (2005) 855–860. [14] D. Wu, Z. Bai, K. Jiang, Temperature induced hierarchical growth of ZnO microcrystal, Mater. Lett. 63 (2009) 1057–1060. [15] P. Li, H. Liu, Y.F. Zhang, Y. Wei, X.K. Wang, Synthesis of flower-like ZnO microstructures via a simple solution route, Mater. Chem. Phys. 106 (2007) 63–69. [16] J. Yu, C. Li, S. Liu, Effect of PSS on morphology and optical properties of ZnO, J. Colloid Interface Sci. 326 (2008) 433–438. [17] K.M. McPeak, J.B. Baxter, ZnO nonowire grown by chemical bath deposition in a continuous flow microreactor, Cry. Growth Design 9 (2009) 4538–4545. [18] T.P. Rao, M.C.S. Kumar, S.A. Angayarkanni, M. Ashok, Effect of stress on optical band gap of ZnO thin films with substrate temperature by spay pyrolysis, J. Alloys Compd. 485 (2009) 413–417. [19] R. Ghosh, D. Basak, S. Fujihara, Effect of substrate-induced strain on the structure, electrical, and optical properties of polycrystalline ZnO thin films, J. Appl. Phys. 96 (2004) 2689–2692. [20] B.Y. Chen, X.L. Xu, G.H. Zhang, H. Xue, S.Y. Ma, A comparative study of the microstructures and optical properties of Cu- and Ag-doped ZnO thin films, Physica B 404 (2009) 3645–3649. [21] Z.B. Bahsi, A.Y. Oral, Effects of Mn and Cu doping on the microstructures and optical properties of sol–gel derived ZnO thin films, Opt. Mater. 29 (2007) 672–678. [22] Y. Zhang, J. Mu, One-pot synthesis, photoluminescence, and photocatalysis of Ag/ZnO composites, J. Colloid Interface Sci. 309 (2007) 478–484. [23] M.A. Behnajady, N. Modirshahla, R. Hamzavi, Kinetic study on photocatalytic degradation of C.I. acid yellow 23 by ZnO photocatalyst, J. Hazard. Mater. 133 (2006) 226–232. [24] N. Samaele, P. Amornpitoksuk, S. Suwanboon, Effect of pH on the morphology and optical properties of modified ZnO particles by SDS via a precipitation method, Powder Technol. 203 (2010) 243–247. [25] J. Xu, Y. Ao, D. Fu, C. Yuan, Low-temperature preparation of F-doped TiO2 film and its photocatalytic activity under solar light, Appl. Surf. Sci. 254 (2008) 3033–3038. [26] M. Fang, J.H. Chen, X.L. Xu, P.H. Yang, H.F. Hildebrand, Antibacterial activities of inorganic agents on six bacteria associated with oral infections by two susceptibility tests, Inter. J. Antimicrob. Agents 27 (2006) 513–517. [27] L. Yang, J. Mao, X. Zhang, T. Xue, T. Hou, L. Wang, M. Tu, Preparation and characteristics of Ag/nano-ZnO composite antimicrobial agent, Nanoscience 11 (2006) 44–48.
P. Amornpitoksuk et al. / Superlattices and Microstructures 51 (2012) 103–113
113
[28] W. Lu, G. Liu, S. Gao, S. Xing, J. Wang, Tyrosin-assisted preparation of Ag/ZnO nanocomposites with enhanced photocatalytic performance and synergistic antibacterial activities, Nanotechnology 19 (2008) 445711. [29] Q. Li, S. Mahendra, D.Y. Lyon, L. Brunet, M.V. Liga, D. Li, P.J.J. Alvarez, Antimicrobial nanomaterials for water disinfection and microbial control: potential applications and implications, Water Res. 42 (2008) 4591–4602. [30] N. Padmavathy, R. Vijayaraghavan, Enhanced bioactivity of ZnO nanoparticles – An antimicrobial study, Sci. Technol. Adv. Mater. 9 (2008) 035004. [31] K. Hirota, M. Sugimoto, M. Kato, K. Tsukagoshi, T. Tanigawa, H. Sugimoto, Preparation of zinc oxide ceramics with a sustainable antibacterial activity under dark conditions, Cert. Int. 36 (2010) 497–506.