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CuO/Cu(OH)2 hierarchical nanostructures as bactericidal photocatalysts O. Akhavan,ab R. Azimirad,*c S. Safad and E. Hasanie Received 14th December 2010, Accepted 6th April 2011 DOI: 10.1039/c0jm04364h Various morphologies of CuO/Cu(OH)2 nanostructures with different adsorbed –OH contents were synthesized on an acid-treated Cu foil through variation of NaOH concentration from 0 to 50 mM with an in situ oxidation method. X-ray diffractometry and X-ray photoelectron spectroscopy (XPS) indicated formation of CuO on the Cu(OH)2 crystalline phase at a growth temperature of 60 C for 20 h. Antibacterial activity of the nanostructures against Escherichia coli bacteria was studied in the dark and under light irradiation. The nanostructures grown at a NaOH concentration of 30 mM showed the highest surface area and the strongest antibacterial activity among the samples. After elimination of the contribution of the effective surface area of the nanostructures to the antibacterial activity, it was found that the surface morphology and chemical composition of the nanostructures were the other most important parameters in the antibacterial activity of the nanostructures. Using XPS analysis, the better photocatalytic activity per surface area of the nanostructures prepared at higher NaOH concentrations was substantially attributed to the amount of adsorbed OH bonds on the surface of the nanostructures.
1. Introduction In recent years, copper oxide nanostructures have attracted great interest because of their fundamental importance and promising applications. In fact, CuO, as a versatile p-type transition metal oxide semiconductor with a narrow band gap ranging from 1.2–1.6 eV,1–4 and a monoclinic crystalline structure, has been extensively investigated in various fields and applications, such as electrochromic devices,2,5 optical switching,6 solar cells,7,8 heterogeneous catalysis,9,10 photocatalysis,11–16 gas sensing,17 field emission,18,19 lithium batteries20 and superconducting materials.21 Recently, much interest has been drawn to superhydrophobic copper-based hierarchical micro/nanostructures by which a beaded water droplet can remove dirt as it rolls off the surface of such structures.22 This kind of self-cleaning characteristic is known as the ‘‘lotus effect’’, originating from the waterrepellent properties of lotus leaves. In recent years, because of the evolution of resistant bacteria strains against common antibiotics,23 the use of nanostructurebased materials with antibacterial properties, such as silver24–32 and copper,12,33–35 titanium dioxide,36–42 carbon nanotubes
a Department of Physics, Sharif University of Technology, P.O. Box 11155-9161, Tehran, Iran b Institute for Nanoscience and Nanotechnology, Sharif University of Technology, P.O. Box 14588-89694, Tehran, Iran c Malek–Ashtar University of Technology, Tehran, Iran. E-mail:
[email protected]; Fax: +98-21-22974546 d Department of Nanotechnology, Faculty of Engineering, Tarbiat Modares University, P.O. Box 14115-143, Tehran, Iran e Department of Physics, Science faculty, Karaj Branch, Islamic Azad University, Karaj, Iran
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(CNTs) and CNT-based materials43–46 and, very recently, graphene nanowalls47 has received considerable attention. Although copper oxide materials are known as effective (photo)catalysts, there have not been many investigations on (photo)inactivation of bacteria by copper and/or copper oxide nanostructures. Concerning the antibacterial property of copper oxide-based materials, Trapalis et al.48 observed total inactivation of Escherichia coli (E. coli) bacteria in 12 h on the surface of a CuO/ SiO2 composite thin film. Abou Neel et al.49 also reported bactericidal properties of CuO-doped phosphate glass fibres. The toxicity of nanosized and bulk CuO to some bacteria, including V. fischeri, D. magna, and T. platyurus, was investigated and compared to the toxicity of corresponding ZnO and TiO2 materials by Heinlaan et al.50 Perelshtein et al.51 observed total inactivation of E. coli and S. aureus bacteria in 3 h by CuO (1.4 wt.%)–cotton nanocomposite. Recently, antibacterial activity of crystallized Cu2O particles with various shapes was also studied by Pang et al.52 They found that all the morphologies of the Cu2O crystalline particles were antibacterial materials. Meanwhile, by changing the morphology from cubic to octahedral, the bactericidal properties were altered from general bacteriostasis to high selectivity.52 Concerning the photocatalytic activity of copper and copper oxide-based materials in the photoinactivation of bacteria, Gao et al.33 reported that copper-based nanostructures can not only show antibacterial properties, but also they (CuO and also Cu nanostructures) can act as effective photocatalysts for the degradation of methyl orange. Paschoalino et al.16 also studied the inactivation of E. coli bacteria by CuO powders in a photocatalytic process. Recently, (photo)inactivation of E. coli bacteria by CuO and Cu nanoparticle photocatalysts This journal is ª The Royal Society of Chemistry 2011
immobilized on the surface of a SiO2 thin film was investigated.12 But, so far, (photo)inactivation of bacteria by copper oxide nanostructures with various physical (e.g., morphological) and chemical (e.g., surface OH bonding) characteristics has not been reported. In this work, CuO/Cu(OH)2 hierarchical nanostructures as nano-photocatalysts were prepared on a Cu foil by its oxidation under alkaline conditions. The antibacterial activity of the CuO/Cu(OH)2 nanostructures with various morphologies (including various surface densities of the nanostructures and various shapes) and various amounts of surface OH bonds was studied against E. coli bacteria in the dark and under light irradiation. The relationship between the photoinactivation of the bacteria and surface chemical composition of the nanostructures was studied by X-ray photoelectron spectroscopy (XPS). By elimination of the positive effect of the surface area of the nanostructures on inactivation of the bacteria, the effect of surface morphology and surface –OH bonds on the (photo) inactivation was investigated.
2. Experimental details 2.1.
Synthesis of copper oxide nanostructures
The CuO nanostructures were prepared by in situ crystallization of copper oxide on Cu foils with dimensions of 2 2 cm. At first, to remove the surface impurities and oxide layers of the Cu foils, they were washed in 3 M HCl aqueous solution for 10 min. In addition, they were rinsed with absolute ethanol and distilled water several times. Then, the cleaned Cu foils were immersed into a sealed glass containing NaOH solution with concentrations of 10, 20, 30, 40 and 50 mM at a temperature of 60 C for 20 h. After that, the foils were washed with distilled water several times, and dried in air at 60 C. Uniform films of copper oxides with a black color were distinguishable on the Cu foils. A similar method to synthesize hierarchical nanostructures of cupric oxide was previously applied by Liu et al.53 In addition, the time evolution of the surface morphology of the nanostructures was also investigated by them for a NaOH concentration of 30 mM. In this work, we used the same experimental conditions, but different concentrations of NaOH were used to obtain various morphologies of the copper oxide nanostructures for application as antibacterial nanomaterials in the dark and under light irradiation. 2.2.
Characterization of samples
The surface morphology of the copper oxide nanostructures was characterized by a Hitachi SE4160 field emission scanning electron microscopy (SEM) apparatus at a voltage of 15–30 kV. The surface topography of the copper oxide nanostructures was studied by Thermo Microscopes Autoprobe CP-Research atomic force microscopy (AFM) in air with a silicon tip of 10 nm in radius and in contact mode. Phase formation and crystalline properties of the samples were examined by X-ray diffraction (XRD) obtained by using a PW1800 Philips diffractometer with a Cu-Ka radiation source. XPS was utilized to investigate the chemical states of the surface of the copper oxide nanostructures. In this regard, a hemispherical analyzer (Specs EA 10 Plus) with an Al-Ka X-ray source (hn ¼ 1486.6 eV) operating in a vacuum This journal is ª The Royal Society of Chemistry 2011
better than 107 Pa was applied. The Cu(2p) and O(1s) core levels were deconvoluted by using Gaussian components, after a Shirley background subtraction. All binding energy values were determined by calibration and fixing the C(1s) line to 285.0 eV. The water contact angle of the nanostructures was measured by a contact angle meter (KSV’s CAM 200). In each measurement, a droplet of deionized water was injected on the surface using a 2 ml micro-injector. 2.3.
Antimicrobial test
The antibacterial activity of the copper oxide nanostructures was checked against E. coli ATCC 25922 bacteria by using the socalled antimicrobial drop test. Before each microbiological test, all glassware and samples were sterilized at 120 C for 10 min. At first, the bacteria were cultured on a nutrient agar plate at 37 C for 24 h. Then, the cultured microorganisms were added in 10 mL saline solution (150 mM NaCl in distilled water) to alter the concentration of bacteria to 108 colony forming units per millilitre (CFU mL1) corresponding to the MacFarland scale. A portion of the saline solution containing the bacteria was diluted to 106 CFU mL1. For the antimicrobial drop test, each sample was put into a sterilized Petri dish. Then, 100 ml of the diluted saline solution containing the bacteria was spread onto the surface of the sample and incubated in the dark or under light irradiation of a 110 mW cm2 mercury lamp (peak wavelengths at 275, 350, and 660 nm). During the irradiation, the Petri dish containing the sample was contacted to a cool water bath to control the temperature of the sample in the range of 24–37 C. After the desired period of time elapsed, the bacteria were washed from the surface of the sample using 5 mL phosphate buffer solution (prepared by using K2HPO4 and KH2PO4) in the sterilized Petri dish. Then, 100 ml of each bacterial suspension was spread on the surface of a nutrient agar plate and incubated at 37 C for 24 h for counting the surviving bacterial colonies by using an optical microscope. The reported data were the average values of three separate runs. The dispersion of each data point was also about 50% of that point.
3. Results and discussion Variation in the surface morphology of the grown copper oxides nanostructures was examined by SEM. Fig. 1a presents the morphology of the copper oxide micro-nanostructures grown on the Cu foil after oxidation at a temperature of 60 C for 20 h (without using a NaOH solution). It shows uniform growth of some micro-features on the substrate (in the wide window image) and constitution of the features by some nano-thickness plates (in the close up image). Fig. 1b indicates that by adding the NaOH solution to the oxidation process, the surface density of the nanosheets began to increase (see the close up image of Fig. 1b). Fig. 1c shows that although at a NaOH concentration of 20 mM the surface density of the nanosheets increased, the surface morphology was not as uniform. By increasing the concentration of the NaOH solution to 30 and 40 mM (Fig. 1d and 1e), the surface density of the nanosheets greatly increased and they formed uniformly on the surface. It was found that increasing the NaOH concentration to 50 mM could not change the surface density and the surface morphology of the J. Mater. Chem., 2011, 21, 9634–9640 | 9635
Fig. 2 XRD patterns of the copper oxide nanostructures grown at NaOH concentration of 30 mM.
confirmed based on the XPS analysis (shown in the following). In fact, since the interaction of the surface of the prepared samples with bacteria is one of the subjects of this work, XPS (for surface characterization) can yield useful information about the surface chemical compositions of the prepared samples. Fig. 3 presents the XPS Cu(2p) spectrum of the copper oxide nanostructures synthesized at a NaOH concentration of 30 mM. The Cu(2p3/2) and Cu(2p1/2) peaks centered at 934.2 and 954.1 eV (with splitting of 19.9 eV), respectively, were attributed to the presence of the Cu2+ chemical state as an indication for formation of CuO and/or Cu(OH)2 on the surface of the oxidized sample. Moreover, the shake-up satellite peaks of the Cu(2p3/2) and Cu(2p1/2) at 942.4 and 962.6 eV, respectively (9 eV greater than the corresponding major peaks), confirmed formation of Cu2+ on the surface.54,55 According to the literature, the position of the Cu(2p3/2) peak for CuO and Cu(OH)2 chemical
Fig. 1 SEM images of the copper oxide nanostructures grown on copper foils at NaOH concentrations of a) 0, b) 10, c) 20, d) 30, and e) 40 mM.
nanostructures, as compared to those of the nanostructures obtained at NaOH concentration of 30 and 40 mM. Fig. 2 shows XRD patterns of the copper oxide nanostructures synthesized at a NaOH concentration of 30 mM. The survey pattern shows strong diffraction peaks pertaining to Cu (200), (220) and (311) planes of the metallic substrate. The inset figure shows the diffraction peaks relating to the formation of CuO, Cu2O and Cu(OH)2 crystalline phases. The weak intensity of the peaks related to the CuO and Cu(OH)2 crystalline phases can be attributed to the growth of such phases on the surface of the prepared samples. The stronger intensity of the peak related to Cu2O can be assigned to formation of this phase between the metallic substrate and the phases grown on the surface, as also 9636 | J. Mater. Chem., 2011, 21, 9634–9640
Fig. 3 The XPS spectrum of the Cu(2p) core level of the copper oxide nanostructures grown at NaOH concentration of 30 mM.
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compositions is at the binding energies of 934.2 and 935.0 eV, respectively.54 Hence, to distinguish the contribution of the Cu2+ chemical state in these chemical compositions, the Cu(2p3/2) peak was deconvoluted, as shown in Fig. 3. On the basis of the peak area of the deconvoluted peaks, the Cu(OH)2/CuO molar ratio was calculated as 0.13. Therefore, the CuO chemical composition was the main composition on surface of the copper oxide nanostructures synthesized at a NaOH concentration of 30 mM. The peak deconvolution also showed that only Cu2+ (neither Cu1+ nor Cu0 with a binding energy of 932.8 eV) chemical state existed on the surface of the prepared sample. To study the variation of OH bonds on the surface of the different samples in more detail, the O(1s) core level of the XPS was analyzed. Deconvolution of O(1s) XPS core levels of the copper oxide nanostructures grown in different NaOH concentrations can be used to determine the contribution of OH bonds on the surface of the samples. Fig. 4 presents peak deconvolution of the O(1s) core levels by using three Gaussian peaks, after a Shirley background subtraction. The deconvoluted peaks at binding energies of 529.3–529.4, 531.0–531.2 and 532.3–532.5 eV were attributed to oxygen in a lattice (CuO), –OH groups and H2O, respectively (see, e.g., ref. 56 and 57). On the basis of the peak area of the deconvoluted peaks, the relative surface concentration of OH bonds to CuO (O(in –OH groups)/O(in lattice) ratio) was
calculated and listed in Table 1. It was found that by increasing the NaOH concentration in the oxidation process, the O(in –OH groups)/O(in lattice) ratio on the surface of the copper oxide nanostructures increased, too. However, the O(in –OH groups)/ O(in lattice) ratios of the copper oxide nanostructures grown at NaOH concentrations greater than 30 mM (up to 50 mM) showed no considerable increase relative to the ratio of the nanostructures grown at a NaOH concentration of 30 mM. Hence, the amount of OH bonds on the surface of the nanostructures grown in NaOH concentrations of 30–50 mM approached a saturated value in our experimental conditions. In addition, based on the value of the Cu(OH)2/CuO ratio (0.13) calculated from the deconvoluted Cu(2p3/2) peak and the value of O(in –OH groups)/O(in lattice) ratio (0.96) calculated from the deconvoluted O(1s) peak, nearly 73% of the –OH bonds on the surface of the nanostructures prepared at 30 mM NaOH can be assigned to the adsorbed OH bonds on the surface of the CuO/Cu(OH)2 nanostructures rather than the OH of the Cu (OH)2 crystalline structure. Nevertheless, a portion of the –OH bonds (27%) can be assigned to the capability of the surface Cu (OH)2 in adsorption of the OH ions of the solution (Cu(OH)2 + 2OH / [Cu(OH)4]2). Similar results were obtained for the samples prepared at the other NaOH concentrations. The oxidation of copper in air and/or humid environments is a standard reaction, but in alkaline conditions, this spontaneous reaction can be accelerated. In presence of the NaOH solution (as an alkaline solution), Cu2+ ions are released from the Cu foil into the NaOH solution. Then, the released Cu2+ ions can be captured by OH in the solution to form Cu(OH)2 crystalline units, according to the following reaction: Cu2+ + 2OH / Cu(OH)2. At relatively low growth temperatures (e.g., 60 C, also used in this work), these units preserve their shapes during the thermal dehydration and so can act as templates for the growth of CuO,53,58 based on the following reaction: 60+ C
CuðOHÞ2 ! CuO þ H2 O:
Fig. 4 Deconvolution of the O(1s) XPS spectra of the copper oxide nanostructures grown at NaOH concentrations of a) 0, b) 10, c) 20, d) 30, and e) 40 mM.
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In fact, the growth temperature is very important in the formation of CuO. If the temperature is lower than 30 C, Cu (OH)2 rather than CuO is achieved.53,59 However, Cu(OH)2 can be transformed into CuO hierarchical nanostructures in a strong alkaline solution at 60 C and in a long reaction time (e.g., 20 h).53,60,61 In this work, at the beginning of the reaction, the concentration of OH ions was so high that the formation of Cu (OH)2 nuclei rather than CuO, which needs a longer reaction time, was further promoted. By increasing the reaction time, the concentration of OH ions gradually decreased (due to their consumption in formation of the Cu(OH)2 units), and so, formation of CuO on the surface of the Cu(OH)2 templates could be promoted, consistent with the results obtained from the XRD (formation of Cu(OH)2 crystalline phase at depth of the sample, i.e., at the initial stage of the reaction) and XPS (formation of CuO on surface of the sample, i.e., at the final stage of the reaction) analyses. The antibacterial activity of the copper oxide nanostructures grown at the different NaOH concentrations was studied against E. coli bacteria in the dark and under light irradiation, as shown J. Mater. Chem., 2011, 21, 9634–9640 | 9637
Table 1 The chemical, antibacterial, topographical, and hydrophobic characteristics of the copper oxide nanostructures grown on Cu foils at different NaOH concentrations, as compared to those of the Cu foil
NaOH concentration (mM) XPS Antibacterial activity (103 min1) AFM Contact angle ( )
O (in –OH groups)/O (in lattice) In the dark Under light irradiation Sr Surface roughness (nm)
in Fig. 4. It was found that the ratio of the number of viable bacteria to the initial number of bacteria on the surface of the copper oxide nanostructures exponentially decreased in terms of the contact time of the bacteria with surface of the nanostructures. Thus, on a logarithmic scale, the slope of the fitted lines, i.e., relative rate of reduction of the number of viable bacteria on the surface of a sample was defined as the antibacterial activity of that sample. In this regard, the antibacterial activity of the copper oxide nanostructures grown at the different NaOH concentrations was calculated and is given in Table 1. The acid-treated Cu foil, as a benchmark, showed a weak antibacterial activity of 1.6 103 min1 in the dark. For the copper oxide nanostructures grown on the Cu foil, without using the NaOH solution, the antibacterial activity sharply increased by a factor of about 4.6 in the dark. By adding NaOH solution of 10 mM in the oxidation process, the antibacterial activity further increased to 12.0 103 min1. In fact, it was found that by increasing the NaOH concentration from 10 to 30 mM, the antibacterial activity of the copper oxide nanostructures was multiplied by 2.4 (and by factor of 3.9 relative to the activity of the sample prepared without NaOH solution). However, the nanostructures grown at a NaOH concentration of 40 mM showed a lower antibacterial activity than the nanostructures prepared at a concentration of 30 mM (see Table 1). We also observed that light irradiation can increase the antibacterial activity of the copper oxide nanostructures, as shown in Fig. 5. Therefore the copper oxide nanostructures can work as photocatalysts for photoinactivation of bacteria. It was found that antibacterial activity of the nanostructures grown at NaOH concentrations of 0, 10, 20, 30 and 40 mM was increased by about 23, 46, 69, 88 and 89% by the light irradiation, respectively. These results indicated that the copper oxide nanostructures grown at the higher NaOH concentrations were better photocatalysts for photoinactivation of the bacteria. Although by using light irradiation, the same changes in the antibacterial activity of the nanostructures prepared at NaOH concentrations of 30 and 40 mM were observed, the best antibacterial activity was still found for the nanostructures synthesized at a concentration of 30 mM (see Table 1). Since the surface morphology and the amount of OH bonds of the nanostructures synthesized at NaOH concentrations of 30 and 40 mM were similar, the difference in the antibacterial activity of the two samples may be assigned to the difference in effective surface area of the samples, as studied in the following. Nanostructured antibacterial materials can present higher antibacterial activities than the corresponding bulk materials, because they have a higher surface area and because it is more 9638 | J. Mater. Chem., 2011, 21, 9634–9640
Cu foil
Copper oxide nanostructures
— — 1.6 1.9 1.009 2.74 85
0 0.20 7.4 9.1 1.901 91.8 113
10 0.40 12.0 17.6 2.754 103.3 132
20 0.65 21.7 36.8 4.219 112.9 131
30 0.96 28.8 54.2 4.881 97.6 121
40 1.00 20.8 39.3 3.546 101.2 118
difficult for them to be completely covered by surface contaminants. In this work, the surface area ratio (Sr) of the nanostructures was evaluated by AFM images shown in Fig. 6 (see also the values of Sr given in Table 1). The copper oxide nanostructures with the greatest Sr showed the strongest antibacterial activity among the other synthesized samples. On the other hand, the surface area may not be the only important parameter in the antibacterial activity of the copper oxide nanostructures. Hence, to eliminate the positive effect of the surface area of the nanostructures on the antibacterial activity, the antibacterial activity per surface area ratio was defined (see also ref. 62) and studied, as presented in Fig. 7. In the dark, by increasing the NaOH concentration, the antibacterial activity per surface area of the nanostructures only slightly increased (maximally 50%), which indicated the substantial contribution of the surface area in the antibacterial activity of the nanostructures in dark conditions. The slight increase of the antibacterial activity per surface area (up to 50%) can be assigned to the increase of the surface density of the nanoflakes, which could provide further release of Cu2+ ions as antibacterial agents. Moreover, the sharp edges of the vertical nanoflakes could damage the cell wall membrane of
Fig. 5 The ratio of viable E. coli bacteria on the surface of the copper oxide nanostructures grown on the Cu foil treated at NaOH concentrations of a) 0, b) 10, c) 20 and d) 30, as compared to e) the acid treated Cu foil (as a control sample), in the dark (C) and under light irradiation (B).
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Fig. 6 AFM images of the copper oxide nanostructures grown on copper foils at NaOH concentrations of a) 0, b) 10, c) 30 and d) 40 mM.
contact interaction of the bacteria with the edge of the nanostructures (graphene nanowalls) than the Gram-positive Staphylococcus aureus lacking the outer membrane. It is know that when metal oxide semiconductor photocatalysts are exposed to light, photoexcited electron–hole pairs are generated. The holes can be trapped by water or adsorbed –OH groups to produce hydroxyl radicals (OH_), which are strong oxidizing agents for the degradation of a wide range of organic pollutants.63–65 Related to this, the role played by –OH species in the photodegradation of dyes66 and photoinactivation of bacteria39,62,67 by using some different metal oxide semiconductor photocatalysts was investigated in more detail. In this work, under light irradiation, the antibacterial activity per surface area of the nanostructures was significantly enhanced (maximally 130%) by increasing the NaOH concentration, as shown in Fig. 7. This implied the significant effect of other parameters, excluding the surface area and surface morphology, on such enhanced antibacterial activity. On the basis of the XPS analysis (see Fig. 4 and Table 1), the surface concentration of the adsorbed OH bonds (which can act as the source of the OH_ radicals produced through interaction of the photoexcited hole of the CuO with the surface OH bonds) can be considered as one of the important parameters in photoinactivation of the bacteria. In this regard, the same antibacterial activity per surface area of the nanostructures grown at NaOH concentrations of 30 and 40 mM is consistent with the similarity of the surface morphology and the surface OH concentration of both samples. In fact, the decrease in the antibacterial activity of the nanostructures after increasing the NaOH concentration from 30 to 40 mM can be assigned to reduction of the effective surface area of the nanostructures at the higher NaOH concentration.
4. Conclusions
Fig. 7 Antibacterial activity per surface area ratio of the copper oxide nanostructures grown on the Cu foil treated at different concentrations of NaOH, as compared to the activity per surface area of the untreated Cu foil.
the bacteria, as previously studied for CNTs43,44 and graphene nanowalls47 with higher sharpness. In this work, it was found that all the synthesized copper oxide nanostructures exhibited hydrophobic properties (see Table 1). Hence, it was possible that the bacteria interact with the sharp edges of the vertical nanoflakes and this interaction could partially contribute to the bactericidal activity of the copper oxide nanostructures. In this regard, elsewhere,47 we have shown that the Gram-negative Escherichia coli bacteria, with an outer membrane, were more resistant to the cell membrane damage caused by the direct This journal is ª The Royal Society of Chemistry 2011
Variation of NaOH concentration in the in situ oxidation of the acid treated Cu foil resulted in the formation of various morphologies of CuO/Cu(OH)2 nanostructures with various contents of adsorbed –OH species. Increasing the NaOH concentration up to 50 mM resulted in the formation of further Cu(OH)2 nucleation units on the surface of the Cu foil, and so, formation of a higher surface density of the nanostructures. The optimum concentration of 30 mM yielded the nanostructures with the highest surface area and the strongest antibacterial activity among the nanostructures prepared in this work. The elimination of the positive effect of the surface area of the nanostructures on the antibacterial activity indicated that the surface density and chemical composition of the nanostructures were the other most important factors affecting the antibacterial activity of the CuO/Cu(OH)2 nanostructures in the dark and under light irradiation. The better photocatalytic activity per surface area of the nanostructures synthesized at higher NaOH concentrations was assigned to the amount of –OH bonds adsorbed on the nanostructures. A portion of this adsorption was assigned to the partial presence of surface Cu(OH)2 with the capability of adsorbing OH ions. These results showed that the hydrophobic CuO/Cu(OH)2 nanostructures synthesized at a NaOH concentration of 30 mM can be used as promising selfcleaning photocatalysts in the photoinactivation of bacteria, as well as in water purification. J. Mater. Chem., 2011, 21, 9634–9640 | 9639
Acknowledgements OA would like to thank the Research Council of Sharif University of Technology for financial support of the work.
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