Effect of metal ion doping on ZnO nanopowders for bacterial inactivation under visible-light irradiation

Powder Technology 315 (2017) 73–80

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Effect of metal ion doping on ZnO nanopowders for bacterial inactivation under visible-light irradiation Ziling Peng, Dan Wu, Wei Wang ⁎, Fatang Tan, Xinyun Wang, Jianguo Chen, Xueliang Qiao State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, Hubei, PR China

a r t i c l e

i n f o

Article history: Received 28 November 2016 Received in revised form 26 February 2017 Accepted 21 March 2017 Available online 22 March 2017 Keywords: Zinc oxide Metal ion doping Visible-light photocatalysis Bacterial inactivation

a b s t r a c t Among metal oxide photocatalysts, zinc oxide (ZnO) has attracted extensive attention due to its advantages of low toxicity and relatively low cost of production. In this work, ZnO nanopowders doped with different metal ions (Li+, Mg2+, Al3+ and Ti4+) were synthesized by a sol-gel method. Multiple techniques such as X-ray diffraction (XRD), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), UV-vis diffused reflectance spectra (UV-vis DRS), photoluminescence (PL) spectra and Brunauer-Emmett-Teller (BET) measurements were employed to study the structures, morphologies and physicochemical properties of the photocatalysts. The influence of metal ion doping on the photocatalytic activity of ZnO was assessed by inactivating a typical Gram-negative bacterium, Escherichia coli K-12 under visible-light irradiation. It was found that Al doping and Ti doping could promote the photocatalytic bacterial inactivation activity of ZnO, while Li doping and Mg doping hindered the bacterial inactivation activity of ZnO photocatalysts. Moreover, Al-doped ZnO exhibited the best visible-light-driven (VLD) photocatalytic activity among these samples, with 7-log of E. coli K-12 cells being completely inactivated within 4 h. The large percentage of absorbed oxygen, narrow band gap and extended visible light absorption were considered to contribute to the powerful VLD photocatalytic activity of Al-doped ZnO. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Our society is now troubled with environmental issues due to the aggravation of ecological damage. As a result, water sources are contaminated by pathogenic bacteria, which causes diseases even death in human beings [1]. Thus, it is necessary to develop efficient disinfection techniques for inactivation of infectious pathogens to improve environmental sanitation [2]. Compared with traditional disinfection methods, such as ozone and chlorination, which have the drawbacks of high cost and carcinogenicity, semiconductor photocatalysis has been regarded as a promising water purification approach to remove pathogenic microorganisms [3,4]. Zinc oxide (ZnO) has been one of the most widely used photocatalysts due to the advantages of low toxicity and relatively low cost of production [5–9]. Moreover, it is well established that reactive oxygen species (ROS) play a significant role in the photocatalytic activity of ZnO during photoexcitation [10,11], which can lead to the decomposition of bacteria cells through various actions [4].

⁎ Corresponding author. E-mail address: [email protected] (W. Wang).

http://dx.doi.org/10.1016/j.powtec.2017.03.052 0032-5910/© 2017 Elsevier B.V. All rights reserved.

Nevertheless, pure ZnO as a photocatalyst has some practical problems including irradiation by UV light, undesired recombination of photogenerated electron-hole pair and photocorrosion effect [12]. Thus, much attention has been drawn on improving the photocatalytic activity of ZnO by suitable modification, among which metal ion doping is considered to be an effective method. Ganesh et al. [13] proposed that Li doping could significantly improve the photocatalytic activity of ZnO nanopowders for methylene blue degradation. Etacheri et al. [14] reported that Mg-doped ZnO had enhanced sunlight-driven photocatalytic activity, owing to the combined effect of superior textural properties and more efficient electron-hole separation. Ahmadet et al. [15] concluded that the enhanced photocatalytic activity of Al-doped ZnO was due to the extended visible light absorption and enhanced adsorption of methyl orange (MO) dye molecule on the surface of Al-doped ZnO nanopowders. Blohet et al. [16] studied the effect of Ti doping on the photocatalytic activity of ZnO and gave the optimal doping ratio to maximize the photonic efficiency for acetaldehyde degradation under UVlight illumination. However, the preparation processes and conditions of these doped ZnO were different from each other, which may affect the physical and chemical properties of final products. To the best of our knowledge, a systematic study on the influence of different metal ion doping on the visible-light-driven (VLD) photocatalytic activity of

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ZnO has never been explored. Moreover, most previous studies about the photocatalytic performance of doped ZnO mainly focused on dye degradation and hydrogen production, there were few studies involving VLD photocatalytic bacterial inactivation by doped ZnO. In this work, the different metal ions of Li+, Mg2+, Al3+ and Ti4+ were selected to incorporate into ZnO crystal lattice. These four metal ions with different valence states had close ionic radius with that of 2+ 3+ 4+ Zn2+ (R+ Li = 0.076 nm, RMg = 0.072 nm, RAl = 0.054 nm, RTi = = 0.074 nm), so they were considered to be easily 0.060 nm and R2+ Zn doped into the ZnO matrix, resulting in the substitutional doping to generate different types of defect sites (oxygen vacancies, substitutional ions and metal vacancies). Photocatalytic activity of the doped ZnO samples was assessed by inactivating a typical Gram-negative bacterium, Escherichia coli K-12 under visible-light irradiation. Also, the influence of different metal ion doping on the structures, morphologies and physicochemical properties of ZnO was explored in the study. 2. Experimental 2.1. Materials Zinc nitrate hexahydrate (Zn(NO3)2·6H2O), aluminum nitrate nonahydrate (Al(NO3)3·9H2O), tetrabutyl titanate (Ti(OC4H9)4) and citric acid monohydrate (C6H8O7·H2O) were purchased from Sinopharm Chemical Reagent Co., Ltd. Lithium nitrate (LiNO3) was bought from Aladdin Chemistry Reagent Chemistry Co., Ltd. Magnesium nitrate hexahydrate (Mg(NO3)2·6H2O) was obtained from Shanghai Experimental Reagent Co., Ltd. All the chemical reagents were of analytical grade and used without further purification in this study.

of metal ions from the samples during bacterial inactivation was also determined by microwave plasma-atomic emission spectrometry (4100 MP-AES, Agilent Technologies). 2.4. Photocatalytic bacterial inactivation The photocatalytic bacterial inactivation performance of asprepared doped ZnO samples was evaluated by the inactivation of the representative Gram-negative bacterium Escherichia coli K-12 (E. coli). A 300 W xenon lamp with a UV cutoff filter (λ b 400 nm) was used as the visible light source, and the light intensity was fixed at ~200 mw/cm2. Firstly, E. coli K-12 was cultured in 50 mL of nutrient broth at 37 °C for 15 h in a shaking incubator. The bacteria cells were harvested by centrifugation for 1 min and washed twice with sterilized saline (0.9 wt% NaCl) solution and then resuspended in sterilized saline solution. The suspension (50 mL) containing ~107 colony forming unit (cfu) mL−1 of E. coli K-12 and 25 mg of photocatalyst was stirred for 30 min in dark to reach the adsorption equilibrium before light irradiation. Then the xenon lamp was turned on to start the photocatalytic reaction. At different time intervals, an aliquot of reaction solution was sampled and spread on Nutrient Agar (Oxoid, England) plates after a series of 10-fold dilution. The survival number of cells was determined after incubating at 37 °C for 24 h. For comparison, dark control (photocatalysts and bacterial cells without light irradiation) and light

2.2. Synthesis of the doped ZnO samples A citric acid-assist sol-gel method was applied for the preparation of the doped ZnO samples, and the molar ratios of doped metals to zinc were controlled at 5 mol%. Firstly, 0.019 mol of Zn(NO3)2·6H2O and 0.001 mol of the dopant (LiNO3, Mg(NO3)2·6H2O, Al(NO3)3·9H2O or Ti(OC4H9)4) were dissolved in 20 mL of distilled water. At the same time, 0.02 mol of C6H8O7·H2O was dissolved in 20 mL of distilled water, and subsequently added to the above solution under stirring until a homogeneous solution was obtained. Secondly, the mixed solution was placed in a water bath of 80 °C under continuous stirring. After a period of time, the mixture turned into a gel, which was then put in a drying oven at 150 °C to obtain a fluffy precursor. At last, the dried precursor was heated from room temperature to 600 °C in 5 h and held at 600 °C for 1 h to get the doped ZnO samples. For comparison, the undoped ZnO sample was also prepared using a similar procedure as mentioned above, without the addition of any metal ion dopant. 2.3. Characterizations The crystal structure of the as-prepared samples was characterized by X-ray diffraction (XRD) (Philips/X' Pert PRO). Morphology of the samples was observed using transmission electron microscope (TEM) (FEI Tecnai G220) with an acceleration voltage of 200 kV. The samples were suspended in ethanol and placed a drop on a carbon-coated Cu TEM grid for observation. Brunauer-Emmett-Teller (BET) surface area measurement was carried out on a Micromeritics ASAP 2020M surface area analyzer with nitrogen adsorption at 77 K. X-ray photoelectron spectroscopy (XPS) analysis was acquired on a Kratos/Axis Ultra DLD600W spectrometer, and the spectra were calibrated referring to the C 1s peak (284.6 eV). UV-vis diffuse reflectance spectra (UV-vis DRS) was conducted on a Varian Cary 500 UV-vis spectrophotometer equipped with a Labsphere diffuse reflectance accessory. Room temperature photoluminescence (PL) spectra were recorded using a fluorescence spectrometer (Jasco, Japan). The excitation source was a Xe lamp and the excitation wavelength was 325 nm. Besides, the leakage

Fig. 1. XRD spectra of the ZnO and doped ZnO samples.

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3. Results and discussion

Table 1 XRD data, BET surface areas and model kinetics parameters of the samples. Samples

Al\ \ZnO Ti\ \ZnO ZnO Mg\ \ZnO Li\ \ZnO

2θ (°)

36.266 36.353 36.300 36.271 36.275

FWHM (°)

1.255 1.370 0.286 0.306 0.132

Crystallite size (nm)

6.8 6.2 31.3 29.1 74.0

BET surface area (m2/g)

43.3 53.4 2.4 6.8 2.1

75

Kinetics parameters n

p

1.60 1.77 2.11 3.17 3.23

1.87 1.77 1.58 2.75 2.36

control (bacterial cells and light irradiation without photocatalysts) were also conducted. All of the above experiments were conducted in triplicates to ensure the reliability of experimental data.

3.1. Phase structures and morphologies of doped ZnO The XRD patterns of the prepared samples are shown in Fig. 1. For all samples, all diffraction peaks can be well indexed to a hexagonal wurtzite structure ZnO (JCPDS 36-1451). The peaks at 2θ = 31.8°, 34.4°, 36.3°,47.5°, 56.6°, 62.9°, 66.4°, 68.0°, 69.1°, 72.6° and 77.0° corresponded to the (100), (002), (101), (102), (110), (103), (200), (112), (201), (004) and (202) planes of ZnO, respectively, indicating that the wurtzite structure was not affected by metal ion doping. It was evident that no other peaks related to the doped metal components such as Li2O, MgO, Al2O3, and TiO2 were detected, which suggested that the dopant atoms (Li, Mg, Al or Ti) may substitute the Zn atoms in ZnO

Fig. 2. Typical TEM images of ZnO (a); Li-doped ZnO (b); Mg-doped ZnO (c); Al-doped ZnO (d) and Ti-doped ZnO (e).

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lattice although the doping amounts were 5 mol%. Notably, there were differences in the intensity and the full width at half maximum (FWHM) of diffraction peaks from the five samples. The intensity decreased in the following order: Li\\ZnO NZnO NMg\\ZnO NAl-ZnO NTi\\ZnO, and the FWHM decreased in the following order: Ti\\ZnO NAl\\ZnO NMg\\ZnO NZnO NLi\\ZnO. In addition, according to the FWHM of the strongest (101) diffraction peak, the crystallite sizes of the samples were calculated by Scherrer equation and shown in Table 1. Li-doped ZnO possessed the largest crystallite size (74 nm), while Ti-doped ZnO had the smallest crystallite size (6.2 nm). The crystallite size decreased in the following order: Li\\ZnONZnON Mg\\ZnOAl\\ZnONTi\\ZnO. The result demonstrated that Li doping had a role of facilitating the grain growth and crystallization of ZnO particles, Ti doping, Al doping and Mg doping exhibited an inhibitory action. Fig. 2 displays the typical morphologies of ZnO (a), Li-doped ZnO (b), Mg-doped ZnO (c), Al-doped ZnO (d), and Ti-doped ZnO (e). Obviously, the metal ion doping had different influences on the size of ZnO particles. The particle sizes of both Al-doped ZnO and Ti-doped ZnO were small (about 10 nm), while Li-doped ZnO possessed the biggest size (80–120 nm). Obviously, the particle sizes of samples were higher than the corresponding crystallite sizes calculated from XRD analysis, which was due to the fact that a particle may be made up of several primary crystallites. The particle size decreased in the following order: Li\\ZnON ZnONMg\\ZnONAl\\ZnONTi\\ZnO. The TEM results strongly demonstrated that Mg doping, Al doping and Ti doping inhibited the grain growth and crystallization of ZnO particles, while Li-doping promoted, which was consistent with the results of XRD analysis. Additionally, N2 sorption measurement was conducted to investigate the BET surface areas of the prepared samples. BET surface areas calculated from N2 adsorption-desorption isotherms were 2.1, 2.4, 6.8, 43.3 and 53.4 m2/g for Li-doped ZnO, ZnO, Mg-doped ZnO, Al-doped ZnO and

Ti-doped ZnO, respectively (Table 1). The surface areas followed an order: Ti\\ZnONAl\\ZnON Mg\\ZnON ZnON Li\\ZnO, which was opposite to the order of particle size. The result indirectly illustrated that Al-doped ZnO and Ti-doped ZnO had more surface active sites.

3.2. XPS analysis of doped ZnO The element composition and chemical bond of the doped ZnO samples were further analyzed by XPS technique, shown in Fig. 3. Fig. 3a indicated that the principal elements of Li-doped ZnO were Zn, O and Li, and the binding energy peak at 55.2 eV could be attributed to LiZn\\O bond [17,18]. From the XPS survey spectrum of Mg-doped ZnO (Fig. 3b), the signals from Zn, O and Mg elements were observed, and the Mg 1s XPS peak at the binding energy of 1304.4 eV could be assigned to the existence of Mg2+ replacing Zn2+ [19]. Fig. 3c demonstrated the presence of Zn and O in Al-doped ZnO, and the peak of Al 2p was located at 74.5 eV, corresponding to the Al\\O bonding [20,21]. Since the peaks of Al2O3 could not be found in the XRD spectrum, the Al\\O bonding was considered to be attributed to the substitutional doping of Al into the ZnO matrix. The XPS survey spectrum of Ti-doped ZnO (Fig. 3d) confirmed the existence of Ti, Zn and O in the sample. It can be seen from the insert that the Ti 2p spectrum consisted of two peaks at about 458.7 eV and 464.4 eV, which were attributed to Ti 2p3/2 and 2p1/2, respectively. The observed spin orbit splitting between the Ti 2p3/2 and Ti 2p1/2 was 5.7 eV, in good agreement with the values of Ti4+ oxidation state [22,23]. Fig. 4 shows the O1s high-resolution XPS spectra of ZnO and doped ZnO. Obviously, these O1s peaks were asymmetric, and each could be deconvoluted into two subpeaks. The subpeak located at low binding energy of 530.2 eV could be attributed to the lattice oxygen (OL) of ZnO, while the subpeak centered at high binding energy of 531.8 eV

Fig. 3. XPS spectra of Li-doped ZnO (a); Mg-doped ZnO (b); Al-doped ZnO (c) and Ti-doped ZnO (d).

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Fig. 4. The O 1s XPS spectra of ZnO (a); Li-doped ZnO (b); Mg-doped ZnO (c); Al-doped ZnO (d) and Ti-doped ZnO (e).

could be assigned to the adsorbed oxygen (OA) species on the surface of the samples. Interestingly, the calculated molar percentages of OA in O content for ZnO, Li-doped ZnO, Mg-doped ZnO, Al-doped ZnO and Tidoped ZnO were 34.58%, 23.75%, 30.47%, 50.98% and 41.07%, respectively. Clearly, a high percentage of OA was detected in the Al-doped ZnO and Ti-doped ZnO samples. It was worth noting that the chemisorbed oxygen depended strongly on the surface defects of the samples. Oxygen vacancies as one kind of defects had strong adsorption ability toward oxygen and were one of main factors for chemisorbed oxygen [24,25]. In addition, the particle size of the samples may influence the percentage of OA, as oxides with smaller particle size had more surface defects (oxygen vacancies) [26,27], which could absorb more oxygen [28,29]. The above XRD, TEM and BET studies suggested that Al-doped ZnO and Ti-doped ZnO had small particle sizes and high surface areas, while Li-doped ZnO possessed large particle size and low surface area. Therefore, more surface defects (oxygen vacancies) existed in Al-

doped ZnO and Ti-doped ZnO, leading to a higher percentage of OA in the two samples. 3.3. Optical properties of doped ZnO The UV-visible diffuse reflectance spectra of ZnO and doped ZnO samples were also obtained and illustrated in Fig. 5. There were some differences in the absorption onset wavelength among the samples, probably resulting from the different grain size, morphology, and surface defect among the samples [30,31]. The absorption onset wavelengths of Li-doped ZnO, Mg-doped ZnO, ZnO, Ti-doped ZnO and Al-doped ZnO were 405.3, 412.5, 414.8, 427.2 and 433.3 nm, respectively. The band gap energy (Eg) of them could be estimated according to the Kubelka-Munk function [32], and the corresponding band gap values were calculated to be 3.06, 3.01, 2.99, 2.90 and 2.86 eV for Li-doped ZnO, Mg-doped ZnO, ZnO, Ti-doped

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ZnO and Al-doped ZnO, respectively. Obviously, Al-doped ZnO could use visible light most efficiently compared with the other samples. It was well known that the existence of surface oxygen vacancies could lead to the narrowing of energy band gap due to the rise of valance band maximum [5,33]. Here, the band gap value decreased in the following order: Li\\ZnO N Mg\\ZnO N ZnO N Ti\\ZnO NAl\\ZnO, which was opposite to the order of OA percentage. Photoluminescence (PL) study is an effective tool to investigate the electronic structure of materials and detect the presence of surface states in semiconductor microstructures [14,15]. As the doped ZnO samples were expected to have different optical properties compared with undoped ZnO, the room temperature photoluminescence measurements were carried out and the PL spectra of the doped and undoped ZnO at an excitation wavelength of 325 nm was shown in Fig. 6. There were two main emission bands in the spectra. The strong UV emission at about 380 nm was attributed to free excitonic emission near-bandedge (NBE) [34–36], and the green emission at 550–570 nm was mainly due to point-like structural defects related to deep-level emission (DLE), such as oxygen vacancies, zinc vacancies, interstitial zinc and interstitial oxygen [34]. From the spectra, the lowest PL emission was found in the Al-doped ZnO sample, this could be attributed to the presence of abundant surface defect sites that acted as luminescence quenchers [37,38]. Since more surface defects (oxygen vacancies, zinc interstitials, zinc vacancies and oxygen interstitials) on Al-doped ZnO photocatalyst can act as electron traps or hole traps, which prevented the recombination of photogenerated electrons and holes [39], consequently causing a relatively weak PL intensity [40]. It should be noted that some shifts in the UV emission peak and a blue shift in the green emission peak were observed. These shifts were probably related to the band gap energy variation due to the effect of metal ions doping [35]. Additionally, the shifts in NBE peak of doped ZnO indirectly verified the existence of doping states of ZnO [35,41].

Fig. 6. PL spectra of ZnO and doped ZnO at an excitation wavelength of 325 nm.

The bacterial inactivation efficiencies of E. coli K-12 by ZnO and doped ZnO are shown in Fig. 7. In the light control experiment, almost no reduction of the bacterial cells can be observed within 4 h, indicating that no photolysis of bacterial cells under visible light irradiation happened. The photocatalytic bacterial inactivation activity trend followed the order: Al\\ZnON Ti\\ZnON ZnON Mg\\ZnONLi\\ZnO. Remarkably, The Al-doped ZnO sample exhibited the most powerful photocatalytic activity, with 7-log of E. coli K-12 cells being completely inactivated

within 4 h under visible light irradiation. However, only slight decrease (about 0.5-log) of bacterial cells was observed in dark condition, which indicated almost no toxic effect to bacterial cells by Al-doped ZnO without light irradiation. The bacterial inactivation kinetics can be well fitted by the Weibull model proposed by Mafart et al. [42], and the two characteristic parameters for the scale parameter n (time necessary to inactivate the first Log10 cycle of the microbial population) and the shape parameter p are also listed in Table 1. The shape parameter p of all the photocatalysts are N1, which means the inactivation curve shows downward concavity, indicating that remaining cells become increasingly damaged. Meanwhile, the scale parameter n for Al-doped ZnO is the smallest among all the samples, indicating that Al-doped ZnO exhibits the best photocatalytic bacterial inactivation activity. It was known that the photocorrosion effect of ZnO photocatalysts can lead to a decrease in their stability under light irradiation, which severely limited their applications [43]. Moreover, the released Zn2+ from ZnO can also inactivate bacterial cells [44]. Here, the leakages of metal ions from ZnO and Al-doped ZnO during irradiation were measured and shown in Fig. 8. The concentration of Zn2+ leached from ZnO increased as the illumination time went on. After irradiation for 4 h, the concentration of Zn2+ in solution reached 5.23 mg/L. However, the concentration of Zn2+ released from Al-doped ZnO stabilized at 0.76 mg/L

Fig. 5. UV-vis diffuse reflectance spectra (DRS) of ZnO and doped ZnO.

Fig. 7. Photocatalytic inactivation efficiencies of ZnO and doped ZnO against E. coli K-12.

3.4. Photocatalytic bacterial disinfection

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inhibit the recombination of electron-hole pairs. Moreover, the analysis of the leakage of metal ions revealed that the doping of Al3+ into ZnO matrix can efficiently prohibit the photocorrosion of ZnO during the photocatalytic process. The present results indicate that Al-doped ZnO is a promising VLD photocatalytic material for bacterial inactivation. Acknowledgment The authors gratefully acknowledge the supports from the National Natural Science Foundation of China (No. 50902054). The authors also acknowledge the technical support from Analytical and Testing Center of Huazhong University of Science & Technology. References

Fig. 8. The leakages of metal ions (Zn2+ and Al3+) from ZnO and Al-doped ZnO.

after 2 h. Given that the toxic concentrations of Zn2+ typically exceed 10 mg/L [45], the Zn2+ released from ZnO and Al-doped ZnO could not inactivate E. coli directly. In addition, only 0.03 mg/L Al3+ released from Al-doped ZnO even after 4 h. The results showed that Al-doped ZnO was much more stable than ZnO. It can be concluded that the bacterial inactivation activity of Al-doped ZnO photocatalyst was not due to the released metal ions (Zn2+ and Al3+). It was worth noting that ZnO semiconductor suffered from the photoinduced dissolution, which led to a decrease in photocatalytic activity [46,47]. This study revealed that the doping of Al3 + into ZnO matrix can efficiently suppress the photocorrosion of ZnO during the photocatalytic process. On the basis of the above characterization and analyses, the bacterial inactivation mechanisms of Al-doped ZnO can be explained as follows. On the one hand, given that the photocatalysis takes place on the surface of semiconductors, the oxygen species adsorbed on the surface of photocatalysts are easily captured by photogenerated electrons to form reactive species, resulting in the enhanced photocatalytic performance of Al-doped ZnO [2,30]. On the other hand, given that oxygen vacancies have strong adsorption ability toward oxygen and are one of main factors for chemisorbed oxygen, Al-doped ZnO also possesses more oxygen defects than other samples. Surface oxygen defects not only lead to the narrowing of the band gap which increases the transport rate of photogeneration carriers and extends the visible light absorption of the photocatalysts, but also work as electron traps to accept the photogenerated electrons which effectively prohibits the recombination of electron-hole pairs, contributing to the enhanced photocatalytic activity of Al-doped ZnO [5,33,46]. In addition, small particle size and large surface area were also favorable for the photocatalytic activity of Al-doped ZnO. More significantly, the doping of Al3+ into ZnO matrix can efficiently suppress the photocorrosion of ZnO, which effectively increases its stability under light irradiation. 4. Conclusions In summary, ZnO doped with different metal ions (Li+, Mg2+, Al3+ and Ti4+) were synthesized by a sol-gel method. The study systematically discussed the influence of different metal ions doping on the structures, morphologies and VLD photocatalytic activity of ZnO photocatalyst. It was proved that Mg doping, Al doping and Ti doping inhibited the grain growth and crystallization of ZnO particles, while Li-doping promoted. The results of VLD photocatalysis demonstrated that Al-doped ZnO exhibited the best photocatalytic bacterial inactivation activity against Escherichia coli K-12, due to an increase in defect concentration (oxygen vacancy), a narrow band gap and extended visible-light absorption, which can facilitate the generation of ROS and

[1] Y. Hou, X. Li, Q. Zhao, G. Chen, C.L. Raston, Role of hydroxyl radicals and mechanism of Escherichia coli inactivation on Ag/AgBr/TiO2 nanotube array electrode under visible light irradiation, Environ. Sci. Technol. 46 (2012) 4042–4050. [2] D. Wu, W. Wang, T.W. Ng, G. Huang, D. Xia, H.Y. Yip, H.K. Lee, G. Li, T. An, P.K. Wong, Visible-light-driven photocatalytic bacterial inactivation and the mechanism of zinc oxysulfide under LED light irradiation, J. Mater. Chem. A 4 (2016) 1052–1059. [3] R. Kumar, S. Anandan, K. Hembram, T.N. Rao, Efficient ZnO-based visible-lightdriven photocatalyst for antibacterial applications, ACS Appl. Mater. Interfaces 6 (2014) 13138–13148. [4] J. Podporska-Carroll, A. Myles, B. Quilty, D.E. McCormack, R. Fagan, S.J. Hinder, D.D. Dionysiou, S.C. Pillai, Antibacterial properties of F-doped ZnO visible light photocatalyst, J. Hazard. Mater. 59 (2015) 669–678. [5] Y. Lv, C. Pan, X. Ma, R. Zong, X. Bai, Y. Zhu, Production of visible activity and UV performance enhancement of ZnO photocatalyst via vacuum deoxidation, Appl. Catal. B 138-139 (2013) 26–32. [6] L. Shi, L. Liang, J. Ma, Y. Meng, S. Zhong, F. Wang, J. Sun, Highly efficient visible lightdriven Ag/AgBr/ZnO composite photocatalyst for degrading rhodamine B, Ceram. Int. 40 (2014) 3495–3502. [7] S. Duo, Y. Li, H. Zhang, T. Liu, K. Wu, Z. Li, A facile salicylic acid assisted hydrothermal synthesis of different flower-like ZnO hierarchical architectures with optical and concentration-dependent photocatalytic properties, Mater. Charact. 114 (2016) 185–196. [8] V. Štengl, J. Henych, M. Slušná, J. Tolasz, K. Zetková, ZnO/Bi2O3 nanowire composites as a new family of photocatalysts, Powder Technol. 270 (2015) 83–91. [9] F. Wang, L. Liang, L. Shi, M. Liu, J. Sun, CO2-assisted synthesis of mesoporous carbon/ C-doped ZnO composites for enhanced photocatalytic performance under visible light, Dalton Trans. 43 (2014) 16441–16449. [10] W. He, H. Wu, W.G. Wamer, H.K. Kim, J. Zheng, H. Jia, Z. Zheng, J.J. Yin, Unraveling the enhanced photocatalytic activity and phototoxicity of ZnO/metal hybrid nanostructures from generation of reactive oxygen species and charge carriers, ACS Appl. Mater. Interfaces 6 (2014) 15527–15535. [11] D.V. Dao, M. van den Bremt, Z. Koeller, T.K. Le, Effect of metal ion doping on the optical properties and the deactivation of photocatalytic activity of ZnO nanopowder for application in sunscreens, Powder Technol. 288 (2016) 366–370. [12] L. Shi, L. Liang, J. Ma, J. Sun, Improved photocatalytic performance over AgBr/ZnO under visible light, Superlattice. Microst. 62 (2013) 128–139. [13] I. Ganesh, P.S.C. Sekhar, G. Padmanabham, G. Sundararajan, Influence of Li-doping on structural characteristics and photocatalytic activity of ZnO nano-powder formed in a novel solution pyro-hydrolysis route, Appl. Surf. Sci. 259 (2012) 524–537. [14] V. Etacheri, R. Roshan, V. Kumar, Mg-doped ZnO nanoparticles for efficient sunlightdriven photocatalysis, ACS Appl. Mater. Interfaces 4 (2012) 2717–2725. [15] M. Ahmad, E. Ahmed, Y. Zhang, N.R. Khalid, J. Xu, M. Ullah, Z. Hong, Preparation of highly efficient Al-doped ZnO photocatalyst by combustion synthesis, Curr. Appl. Phys. 13 (2013) 697–704. [16] J.Z. Bloh, R. Dillert, D.W. Bahnemann, Zinc oxide photocatalysis: influence of iron and titanium doping and origin of the optimal doping ratio, ChemCatChem 5 (2013) 774–778. [17] J.G. Lu, Y.Z. Zhang, Z.Z. Ye, Y.J. Zeng, H.P. He, L.P. Zhu, J.Y. Huang, L. Wang, J. Yuan, B.H. Zhao, X.H. Li, Control of p- and n-type conductivities in Li-doped ZnO thin films, Appl. Phys. Lett. 89 (2006) 112113. [18] L. Qian, Z. Liu, Y. Mo, H. Yuan, D. Xiao, Large scale preparation of urchin like Li doped ZnO using simple radio frequency chemical vapor synthesis, Mater. Lett. 100 (2013) 124–126. [19] J. Dong, J. Shi, D. Li, Y. Luo, Q. Meng, Controlling the conduction band offset for highly efficient ZnO nanorods based perovskite solar cell, Appl. Phys. Lett. 107 (2015) 073507. [20] F. Zhang, K. Saito, T. Tanaka, M. Nishio, M. Arita, Q. Guo, Wide bandgap engineering of (AlGa)2O3 films, Appl. Phys. Lett. 105 (2014) 162107. [21] S.-F. Wang, Y.-T. Tsai, J.P. Chu, Resistive switching characteristics of a spinel ZnAl2O4 thin film prepared by radio frequency sputtering, Ceram. Int. 42 (2016) 17673–17679. [22] J. Fu, S. Cao, J. Yu, J. Low, Y. Lei, Enhanced photocatalytic CO2-reduction activity of electrospun mesoporous TiO2 nanofibers by solvothermal treatment, Dalton Trans. 43 (2014) 9158–9165. [23] S. Liu, K. Zhu, J. Tian, W. Zhang, S. Bai, Z. Shan, Submicron-sized mesoporous anatase TiO2 beads with trapped SnO2 for long-term, high-rate lithium storage, J. Alloys Compd. 639 (2015) 60–67.

80

Z. Peng et al. / Powder Technology 315 (2017) 73–80

[24] Y. Rao, W. Wang, F. Tan, Y. Cai, J. Lu, X. Qiao, Influence of different ions doping on the antibacterial properties of MgO nanopowders, Appl. Surf. Sci. 284 (2013) 726–731. [25] L.M. Liu, B. McAllister, H.Q. Ye, P. Hu, Identifying an O2 supply pathway in CO oxidation on Au/TiO2 (110): a density functional theory study on the intrinsic role of water, J. Am. Chem. Soc. 128 (2006) 4017–4022. [26] S. Biswas, A.S. Poyraz, Y. Meng, C.-H. Kuo, C. Guild, H. Tripp, S.L. Suib, Ion induced promotion of activity enhancement of mesoporous manganese oxides for aerobic oxidation reactions, Appl. Catal. B Environ. 165 (2015) 731–741. [27] Y. Liu, W. Zhuang, Y. Hu, W. Gao, J. Hao, Synthesis and luminescence of sub-micron sized Ca3Sc2Si3O12:Ce green phosphors for white light-emitting diode and fieldemission display applications, J. Alloys Compd. 504 (2010) 488–492. [28] W. Kim, M. Choi, K. Yong, Generation of oxygen vacancies in ZnO nanorods/films and their effects on gas sensing properties, Sensors Actuators B Chem. 209 (2015) 989–996. [29] Z. Zhang, W. Wang, E. Gao, M. Shang, J. Xu, Enhanced photocatalytic activity of Bi2WO6 with oxygen vacancies by zirconium doping, J. Hazard. Mater. 196 (2011) 255–262. [30] D. Hou, W. Luo, Y. Huang, J.C. Yu, X. Hu, Synthesis of porous Bi4Ti3O12 nanofibers by electrospinning and their enhanced visible-light-driven photocatalytic properties, Nanoscale 5 (2013) 2028–2035. [31] G. Liu, L. Wang, H.G. Yang, H.-M. Cheng, G.Q. Lu, Titania-based photocatalysts-crystal growth, doping and heterostructuring, J. Mater. Chem. 20 (2010) 831–843. [32] P. Kubelka, F. Munk, An article on optics of paint layers, Z. Tech. Phys. 12 (1931) 886–892. [33] Y. Tang, H. Zhou, K. Zhang, J. Ding, T. Fan, D. Zhang, Visible-light-active ZnO via oxygen vacancy manipulation for efficient formaldehyde photodegradation, Chem. Eng. J. 262 (2015) 260–267. [34] C. Abed, C. Bouzidi, H. Elhouichet, B. Gelloz, M. Ferid, Mg doping induced high structural quality of sol-gel ZnO nanocrystals: application in photocatalysis, Appl. Surf. Sci. 349 (2015) 855–863. [35] P.S. Shewale, Y.S. Yu, H2S gas sensing properties of undoped and Ti doped ZnO thin films deposited by chemical spray pyrolysis, J. Alloys Compd. 684 (2016) 428–437. [36] J. Wang, J. Yang, X. Li, B. Feng, B. Wei, D. Wang, H. Zhai, H. Song, Effect of surfactant on the morphology of ZnO nanopowders and their application for photodegradation of rhodamine B, Powder Technol. 286 (2015) 269–275.

[37] A. Nohara, S. Takeshita, Y. Iso, T. Isobe, Solvothermal synthesis of YBO3: Ce3+, Tb3+ nanophosphor: influence of B/(Y + Ce + Tb) ratio on particle size and photoluminescence intensity, J. Mater. Sci. 51 (2015) 3311–3317. [38] J. Hong, S. Yin, Y. Pan, J. Han, T. Zhou, R. Xu, Porous carbon nitride nanosheets for enhanced photocatalytic activities, Nanoscale 6 (2014) 14984–14990. [39] M.Y. Guo, A.M.C. Ng, F. Liu, A.B. Djurišić, W.K. Chan, H. Su, K.S. Wong, Effect of native defects on photocatalytic properties of ZnO, J. Phys. Chem. C 115 (2011) 11095–11101. [40] Y. Sun, T. Xiong, Z. Ni, J. Liu, F. Dong, W. Zhang, W.-K. Ho, Improving g-C3N4 photocatalysis for NOx removal by Ag nanoparticles decoration, Appl. Surf. Sci. 358 (2015) 356–362. [41] T.M. Hammad, J.K. Salem, R.G. Harrison, R. Hempelmann, N.K. Hejazy, Optical and magnetic properties of Cu-doped ZnO nanoparticles, J. Mater. Sci. Mater. Electron. 24 (2013) 2846–2852. [42] P. Mafart, O. Couvert, S. Gaillard, I. Leguerinel, On calculating sterility in thermal preservation methods: application of the Weibull frequency distribution model, Int. J. Food Microbiol. 72 (2002) 107–113. [43] C. Han, M.Q. Yang, B. Weng, Y.J. Xu, Improving the photocatalytic activity and antiphotocorrosion of semiconductor ZnO by coupling with versatile carbon, Phys. Chem. Chem. Phys. 16 (2014) 16891–16903. [44] Y.W. Wang, A. Cao, Y. Jiang, X. Zhang, J.H. Liu, Y. Liu, H. Wang, Superior antibacterial activity of zinc oxide/graphene oxide composites originating from high zinc concentration localized around bacteria, ACS Appl. Mater. Interfaces 6 (2014) 2791–2798. [45] N.M. Franklin, N.J. Rogers, S.C. Apte, G.E. Batley, G.E. Gadd, P.S. Casey, Comparative toxicity of nanoparticulate ZnO, bulk ZnO, and ZnCl2 to a freshwater microalga (Pseudokirchneriella subcapitata): the importance of particle solubility, Environ. Sci. Technol. 41 (2007) 8484–8490. [46] X. Bai, L. Wang, R. Zong, Y. Lv, Y. Sun, Y. Zhu, Performance enhancement of ZnO photocatalyst via synergic effect of surface oxygen defect and graphene hybridization, Langmuir 29 (2013) 3097–3105. [47] Y. Wang, R. Shi, J. Lin, Y. Zhu, Enhancement of photocurrent and photocatalytic activity of ZnO hybridized with graphite-like C3N4, Energy Environ. Sci. 4 (2011) 2922.