Review on the improvement of the photocatalytic and antibacterial activities of ZnO

Journal of Alloys and Compounds 727 (2017) 792e820

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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Review

Review on the improvement of the photocatalytic and antibacterial activities of ZnO Kezhen Qi a, b, Bei Cheng a, Jiaguo Yu a, d, *, Wingkei Ho c, ** a

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Luoshi Road 122, Wuhan, 430070, PR China b Institute of Catalysis for Energy and Environment, College of Chemistry and Chemical Engineering, Shenyang Normal University, Shenyang, 110034, PR China c Department of Science and Environmental Studies, The Hong Kong Education University of Hong Kong, Tai Po, N. T. Hong Kong, PR China d Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah, 21589, Saudi Arabia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 July 2017 Received in revised form 15 August 2017 Accepted 16 August 2017 Available online 16 August 2017

Photocatalytic degradation is an effective method to alleviate environmental pollution caused by organic pollutants. In this work, research progress on the application of photocatalytic degradation and the antibacterial properties of zinc oxide (ZnO) nanomaterials is reviewed. The visible-light photo-response of ZnO has been expanded by employing various strategies, such as enhancing the photocatalytic activity of ZnO through modification of its electronic and optical properties, doping metal/nonmetal atoms, depositing noble metals, constructing heterojunctions, and coupling carbon materials, because the wide band gap of ZnO likely restricts its applications in photocatalysis. Although ZnO nanomaterials are commonly used for antibacterial applications, our understanding on the toxicity mechanisms of ZnO is limited. Some of the main toxicity mechanisms of this compound include reactive oxygen species generation, Zn2þ release, membrane dysfunction, and nanoparticle internalization into cells. Some of the main methods that improve antibacterial activities are coating inorganic or organic antimicrobial agents, doping ZnO, and tuning the size, morphological characteristics, and concentration of ZnO nanomaterials. This review aims to examine the current research progress on ZnO-based nanomaterials developed for the photocatalysis of organic contaminant degradation and antibacterial applications. © 2017 Elsevier B.V. All rights reserved.

Keywords: ZnO Modification Photocatalytic degradation Antibacterial

Contents 1. 2.

3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 793 Photocatalysis applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 793 2.1. Mechanisms of ZnO photocatalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 793 2.2. Methods to enhance ZnO photocatalytic activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 794 2.2.1. Doping metals or nonmetals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 794 2.2.2. Deposition of noble metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 797 2.2.3. Constructing heterojunctions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 798 2.2.4. Coupling carbon materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 804 Antibacterial application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 807 3.1. Mechanisms of ZnO antibacterium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 807 3.2. Strategies for enhancing ZnO antibacterial activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 808 3.2.1. Coating inorganic photocatalyst promoters and/or antimicrobial agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 808 3.2.2. Loading organic antimicrobial agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 810 3.2.3. Doping metals or nonmetals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 810

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Yu), [email protected] (W. Ho). http://dx.doi.org/10.1016/j.jallcom.2017.08.142 0925-8388/© 2017 Elsevier B.V. All rights reserved.

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3.2.4. Size, morphological characteristics, and concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 811 Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 813 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 814 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 814

1. Introduction With industrial development, environmental pollution exacerbates and yields high amounts of organic contaminants, and this phenomenon is a serious environmental problem [1,2]. Large amounts of air and land organic and toxic pollutants also reach wastewater stream. Various chemical and physical methods have been applied to treat these contaminants [3]. In 1972, Fujishima and Honda observed photoelectrochemical water splitting on semiconductor-based photocatalysts [4]. The use of solar power with semiconductor-based heterogeneous photocatalysts has been extensively investigated and the potential applications of sunlight have been presented to remove organic contaminants and noxious bacteria [5e7]. The efficiency of photocatalysts, such as ZnO [8,9], TiO2 [10e14], CdS [15,16], ZnS [17], SrTiO3 [18,19], C3N4 [20e23], and BiOI [24,25], is generally determined by light absorption, which excites electrons (e) in the valence band (VB) to the conduction band (CB), leaves a hole (hþ) in the VB, and immediately triggers photoredox reactions. As an important semiconductor photocatalyst, zinc oxide (ZnO) has been widely examined because of its excellent properties, including low cost, high redox potential, nontoxicity, and environmentally friendly features [26e30]. Although ZnO yields a wide band gap (3.37 eV) and a high exciton binding energy (60 meV) [31], this compound can absorb a larger fraction of the UV spectrum and exhibit a greater photocatalytic performance than TiO2 in the photodegradation of organic pollutants [32e36] because the electron mobility (200e300 cm2 V1 s1) of ZnO is much higher than that of TiO2 (0.1e4.0 cm2 V1 s1), which accelerates electron transfer and thus contributes to high quantum efficiency [37,38]. The position of the VB of ZnO is lower than that of the VB of TiO2; as such, the oxidation potential of hydroxyl radical generated by ZnO is higher than that of hydroxyl radical produced by TiO2 [39]. Thus, the photocatalytic performance of ZnO in degrading pollutants is usually higher than that of TiO2 [23e25]. However, the big band gap allows ZnO only to absorb UV light (~4% of solar spectrum) but not visible light (~43% of solar spectrum) [40]. Moreover, the fast recombination of photogenerated e-hþ pairs also diminishes the performance of ZnO as photocatalysts. Therefore, ZnO has been modified to extend its visible-light response and restrain the recombination of e-hþ pairs [41,42] by applying various methods, such as doping metals/nonmetals [43e46], depositing noble metals [47e50], constructing heterojunctions [51e55], and coupling carbon materials [56e61]. ZnO is a promising antibacterial agent because of its good thermal stability, high antimicrobial activity, and excellent biocompatibility [62e65]. ZnO demonstrates excellent antibacterial activity in a broad spectrum of bacteria [48e50], such as Staphylococcus aureus and Escherichia coli [66,67]. The interactions between ZnO nanoparticles (NPs) and bacteria are more effective when ZnO particles are reduced to a nanoscale size, and bacteria are mostly toxic to die. ZnO NPs are also non-toxic and biocompatible with human cells [68e70]. With these advantages, ZnO NPs can be used as antibacterial agents. Some strategies, such as coupling with inorganic/organic antimicrobial agents and tuning particle size, morphological characteristics, and concentration of

ZnO NPs, aim to enhance the antibacterial performance of ZnO. Various antibacterial mechanisms, such as photocatalytic generation of reactive oxygen species (ROS) and release of Zn2þ, have been proposed because these processes can destroy the bacterial cytomembrane [71,72]. However, our understanding of the precise antibacterial mechanisms of ZnO remains limited. This paper reviews, in the first part, the recent developments on the photocatalytic applications of ZnO nanomaterials. Particularly, different modification strategies successfully used to improve the photodegradation of organic dyes are discussed. And the emphasis is focused on the better understanding on the fundamental mechanism relevant to the enhancement of photocatalysis of these surface modification strategies. Then, this review also summarizes current researches on ZnO nanomaterials for antibacterial applications, especially those that focus on the mechanisms and improving strategies for enhancing the antibacterial performance. Finally, summary and future perspectives of ZnO-based nanomaterials using for photocatalysts and antibacterial agents are presented and discussed. 2. Photocatalysis applications 2.1. Mechanisms of ZnO photocatalysis The photodegradation of pollutants by using ZnO is illustrated in Fig. 1. Photocatalysis occurs as ZnO photocatalyst is irradiated by light with energy larger than its bandgap energy. Light energy absorption triggers charge separation and excites electrons from the VB to the CB, and holes are left in the VB [73]. The photogenerated e/hþ carriers then move to the surface of ZnO photocatalysts. Simultaneously, e and hþ undergo recombination, which reduces quantum yield. This recombination rate is affected by many factors related to photocatalyst structures and surface modifications [60,61]. Reactive e and hþ arrive at the surface of ZnO to facilitate oxidation and reduction reactions that generate excess ROS,   including superoxide anion (O 2 ) and hydroxyl radical ( OH). The bottom of the CB of ZnO (0.5 V vs. normal hydrogen electrode,  NHE) is more negative than the redox potential of O2/O 2 (0.33 V  vs. NHE). As such, these excited electrons can produce O 2 . At the same time, the top of the VB of ZnO (þ2.7 V vs. NHE) is more

Fig. 1. Basic mechanism of ZnO photocatalysis. Ehv is the irradiated photon energy, R is the electron acceptor, and D is the electron donor.

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Fig. 2. Schematic of the comparison of the band structures of pure, metal-doped ZnO, and nonmetal-doped ZnO.

positive than the redox potential of OH/H2O (þ2.53 V vs. NHE). Consequently, H2O molecules can be oxidized by these holes to form hydroxyl radicals. These highly reactive radical groups (OH,  O 2 ) directly oxidize organic pollutant molecules in solutions. 2.2. Methods to enhance ZnO photocatalytic activity The main drawback of ZnO photocatalysts is their large band gap, which is active only under UV light irradiation but is not active to absorb visible light; as a result, the efficiency of utilizing sunlight is limited [74]. To overcome this drawback, researchers developed several methods, such as doping of metal/nonmetal atoms, depositing noble metals, and coupling with other semiconductors or carbon materials [75e82]. 2.2.1. Doping metals or nonmetals Doping metal or nonmetal atoms can change the photoelectric properties of ZnO to extend its spectral response to the visible light region because it can effectively reduce the band gap of semiconductors [83]. The doped metal/nonmetal atoms change the coordination environment of Zn atom and tune the electronic structure of ZnO by adding localized electronic energy levels in the band gap (Fig. 2). The dopant energy level is temporally located below the CB, where photogenerated charge carriers are trapped and photocatalytic activities are consequently increased [84e86].

For example, doping Mn can change the optical properties of ZnO and improve its photocatalytic performance [96,104]. Mn2þ easily replaces Zn2þ in the ZnO lattice because the radius of Mn2þ is almost the same as that of Zn2þ (radius of Mn2þ is 0.080 nm, radius of Zn2þ is 0.074 nm) [105], and both ions carry the same amount of cationic charge. Lu et al. [106] successfully prepared pure ZnO and Mn-doped ZnO photocatalysts through simple alcoholysis method (Fig. 3), and both are uniform nanorods (Fig. 3A). Compared with pure ZnO, Mn-doped ZnO displays stronger visible light absorption and thus induces a gradual change in the color of samples from white to orange (Fig. 3B and C). The visible-light photocatalytic activity of 2,4-dichlorophenol (DCP) photodegradation by Mndoped ZnO is higher than that by undoped ZnO (Fig. 3D). This phenomenon suggests that Mn2þ doping to the ZnO lattice may elicits the following effects: excited charges transfer from the VB of ZnO to the adsorbed DCP on the surface and destroy DCP molecules. The formed holes simultaneously react with water molecules and consequently produce OH radicals that attack other DCP molecules and enhance degradation. Doping Mn2þ enhances the separation efficiency of photogenerated charges. This phenomenon is verified by photocurrent measurements (Fig. 3E), in which the photocurrent of Mn-doped ZnO is higher than that of pure ZnO. Similar results have been observed in Ni, Co, and Cu doping [107]. An optimal concentration of these doping atoms should be present to increase photocatalytic activities [96]. (b) Doping rare earth metals

2.2.1.1. Doping metals. Doping metal atoms, such as alkali metals, rare earth metals, transition metals, and noble metals, to ZnO crystal lattice has been extensively investigated [87e90]. Metals doped to ZnO likely expand the visible light response of ZnO and increase its quantum efficiency [91,92]. Thus, doping ZnO with metals possibly improves the photocatalytic activity of ZnO. (a) Doping transition metals Transition metals and their cations possess an unfulfilled d subshell. Transition metals with similar atomic radii to the atomic radius of Zn can be easily doped into the ZnO lattice. Many transition metals, including Fe [93], Co [94], Ni [95], Mn [96], Cr [97], V [98], Cu [99], and Zr [100], have been doped to ZnO to narrow the band gap of this compound [91]. The red shift is caused by the emerging charge transfer between d electrons of transition metals and the CB or VB of ZnO [101,102]. Furthermore, a metal atom under this condition possibly creates a new electron state in the band gap of ZnO, which subsequently traps excited electrons and inhibits the recombination of the e/hþ pair. Thus, the type and concentration of these doping transition metals are the main factors affecting the photocatalytic performance of ZnO [103].

Rare earth (RE) metals are good doping elements that can be used to change the electronic structure of ZnO and expand visible light absorption. Doping RE metals forms a localized impurity level in the band structure of ZnO and alters the band of ZnO. The electronic structure of ZnO is also affected by the charge transfer between the VB or CB of ZnO and the 4f or 5d electrons of RE metals [108e110]. Although RE doping provides some advantages, this technique is limited by the low doping saturation of RE ions in the ZnO crystal lattice because of the difference in ionic radii and a slight mismatch between the energy level positions of RE ions and ZnO [111]. Ce doping can improve the photocatalytic performance of ZnO [112e115]. Ce doping promotes crystallite formation to decrease particle size and increase the surface area, which helps enhance the photodegradation efficiency [116]. After RE doping is performed, RE-ZnO elicits either a blue shift [113] or a red shift [116]. After doping is completed, impure states may be created below the VB of ZnO to fulfill the compensation of electrical neutrality. Thus, a number of additional electron transitions may be induced between the VB of ZnO and the new energy states and may improve the visible-light response [117]. Sin et al. successfully obtained Ce-

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Fig. 3. (A) FESEM images of 5% Mn/ZnO sample, (B) UVeVis diffuse reflectance spectra, and (C) corresponding colors of as-prepared ZnO and Mn/ZnO with varying Mn/Zn molar ratios. (D) Photodegradation of 2,4-dichlorophenol (DCP) by Mn/ZnO with varying Mn/Zn molar ratios under visible-light (l > 420 nm) irradiation. (E) Surface photocurrent (SPC) of the as-prepared ZnO and Mn/ZnO with varying Mn/Zn molar ratios illuminated at 540 nm. Inset: schematic setup of the SPC measurement. Reprinted with permission from Ref. [106]. Copyright 2012 Royal Society of Chemistry.

doped ZnO microspheres through a simple chemical precipitation method and enhanced the photocatalytic performance of ZnO (Fig. 4) [118]. The structure of Ce-doped ZnO microsphere is fabricated by numerous interleaving nanosheets (Fig. 4A). The reaction rate constant (k) of Ce (1.5 at%)-doped ZnO is 1.9-fold higher than that of pure ZnO during phenol photodegradation (Fig. 4B). The photoluminescence (PL) intensities of Ce-doped ZnO are remarkably weaker than those of undoped ZnO (Fig. 4C) possibly because of the electron trapping effect of Ce dopant and thus prevent charge carrier recombination. Fig. 4D shows the suggested mechanisms of the photocatalytic degradation of phenol by using Ce-doped ZnO [113]. Upon irradiation by sunlight, the incorporated Ce4þ is reduced by photo-excited electrons, and Ce3þ is consequently 4þ  formed. Ce3þ then reacts with O2. Thus, O is 2 is produced and Ce  generated. hþ simultaneously reacts with H2O to generate OH. O 2 and OH subsequently degrade various pollutants. Similar results have been observed in Dy- and Eu-doped ZnO photocatalysts. Khataee et al. [119] synthesized Dy-doped ZnO NPs to photodegrade C. I. acid red 17. The increased photocatalytic activity of Dy-doped ZnO is attributed to the reversible reaction between Dy4þ and Dy3þ, that is, Dy3þ transfers electrons to O2 to 4þ 4þ  generate O transforms to Dy3þ by receiving 2 and Dy , and Dy photo-generated electrons from the CB of ZnO. These reaction cycles prevent e/hþ recombination and consequently improve photocatalytic perfomance. The photocatalytic activity of Eu-doped ZnO for dye degradation is higher than that of pure ZnO [120]. Eu doped in a ZnO lattice forms a EueOeZn bond, and doping

decreases the e/hþ recombination rate because Eu is in the form of Euþ3 [121]. It possesses a high photocatalytic activity because of  OH formation during the photocatalytic reactions of Eu-doped ZnO, and the photocatalytic mechanism of Eu-doped ZnO is proposed [121]. Eu3þ is a strong Lewis acid because of its partially filled 2þ 2þ f-orbital; thus, it can trap e beCB to produce Eu . However, Eu  comes unstable and easily follows the trapped eCB, which is    transferred to O2 and generate O 2 . O2 species then change to OH. These active species then participate in the catalytic reaction and improve photocatalytic activities. (c) Doping alkali metals Alkali metal-doped ZnO can enhance the photocatalytic activities for organic pollutant photodegradation. For example, the photocatalytic activity of Na-doped ZnO is higher than that of pure ZnO in dye photodegradation [122,123]. Photodegradation efficiency is also affected by the doping amount of alkali metals. For instance, the photocatalytic performance of low-concentration Kdoped ZnO is high during rhodamine B (RhB) degradation [124]. Various types of doping elements elicit different effects on photocatalytic activities. The photocatalytic activity of Li-doped ZnO in 4NP degradation is higher than that of Na- or K-doped ZnO because Liþ can trap excited electrons [115,116]. The visible-light photocatalytic activity of Mg-doped ZnO in MB degradation is enhanced because of band gap narrowing and efficient charge carrier transfer [125]. The band gap widening by Mg2þ doping is attributed to

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Fig. 4. (A) FESEM image of Ce(2.0 at%)-doped ZnO. (B) Kinetic study of the photodegradation of phenol by undoped and Ce-doped ZnO. (C) PL spectra of pure ZnO and Ce/ZnO with different Ce doping contents. (D) Illustration of the mechanism of phenol photodegradation by Ce-doped ZnO. Reprinted with permission from Refs. [113,118]. Copyright 2014, 2015 Elsevier.

MosseBurstein effect caused by electrons trapped in oxygen vacancies. Mg2þ doping improves the ability of oxygen vacancies to trap more excited electrons and thus increase electron concentrations as a consequence of differences in the ionic radii and electronegativities between Zn2þ and Mg2þ. However, doped Mg2þ can serve either as the lattice substitution of Zn2þ or the interstitial occupation entering the ZnO lattice that also affects photocatalytic performance (Fig. 5) [126].

2.2.1.2. Doping nonmetals. The substitution of lattice oxygen atoms by doping nonmetals to the ZnO lattice has been widely observed [119,120]. Doping elements should not only require lower electronegativity than oxygen, it should also have a similar atomic radius as the O atom; both conditions are necessary factors for effective doping [127]. Dopants, such as C, N, and S, can form intermediate energy levels in the band gap and hence increase visible-light photocatalytic activities [128,129]. N-doped ZnO has been the most studied form. The advantages of N include the similarity between N and O in terms of their ionic radii and 2p energy states, high solubility in ZnO, and low formation energy of O vacancies. The edge of ZnO VB increases and the band gap narrows through the hybridization of N 2p and O 2p states [130e133]. In Fig. 6, first-principle calculations indicate that N atom substitutional doping can be responsible for the 0.25 eV narrowing of Eg Ref. [134]. Thus, photoelectric transformation efficiency is
nchez et al. [135] synthesized N-doped ZnO enhanced. Macías-Sa with different N/Zn ratios and used this photocatalyst to investigate

2,4-D degradation under visible light irradiation. Different specific surface areas, crystallite sizes, and Eg are obtained with various amounts of N doping, and the controlled nucleation and growth of crystallites can be obtained in the presence of nitrogen [136,137]. Sun et al. prepared N-doped ZnO NPs for methyl orange degradation under simulated daylight. The photocatalytic efficiency of Ndoped ZnO is nearly 3.8 times higher than that of pure ZnO [138]. Rajbongshi et al. [139] suggested that the formation of defect states account for the high activity of N-doped ZnO materials. C-doped ZnO has also been examined. Liu et al. [140] synthesized C-doped ZnO nanoflowers to photodegrade RhB and 4chlorophenol (Fig. 7). Flower-like C-doped ZnO hierarchical structures are produced through the pyrolysis of morphologically similar precursors Zn5(CO3)2(OH)6 (Fig. 7A). These ZnO hierarchical structures are porous materials (Fig. 7B). Under visible light irradiation, ZnO flowers can highly generate hydroxyl radicals and exhibit an excellent photocatalytic RhB degradation (Fig. 7C). DFT calculation results (Fig. 7D) indicate that a new band forms at slightly lower part of the top of the VB after C is doped. As a result, the whole energy band shifts downward. Thus, less energy is required to transfer electrons from the gap state to the vacant states under light irradiation. Compared with pure ZnO, the absorption edge is redshifted [140]. The enhanced photocatalytic activity of C-doped ZnO is related to the organization of hierarchical nanosheets, that is, the dense voids between and within porous nanosheets, and C doping accounts for the enhanced absorption of light. Similarly, Cho et al. [141] prepared C-doped ZnO nanomaterials and demonstrated

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Fig. 5. Local density of states for (A) pure ZnO and (B) ZnO with 1/8 Mg interstitial doping every ZnO unit cell. The vertical dash line represents Fermi level. (C) Relationships between (ahn)2 and photon energy of Zn1-xMgxO samples. (D) Relative photocatalytic activity of the samples, Degussa P-25, and pure anatase TiO2 under UV light irradiation. Reprinted with permission from Ref. [126]. Copyright 2008 American Chemical Society.

high visible light photodegradation of orange II dye. Interestingly, a lattice expansion in C-doped ZnO can be observed because of the  difference in ionic radius, that is, large C4 and small O 2 at 260 and 140 pm, respectively [142,143]. The lattice constant of C-doped ZnO is smaller because the doped C is C4þ with an ionic radius of 16 pm and the replaced Zn2þ possesses a much larger ionic radius of 74 pm [144]. As S doped in ZnO, the larger Bohr radius and the smaller electronegativity of the S atom than the O atom can change the photoelectric properties of ZnO. The band gap of ZnO reportedly broadened after S doping due to the BursteineMoss effect [145,146]. In contrast, a narrowing band gap has been reported too [147,148]. The above two contradictory results on S-doped ZnO should be addressed, and the details may lie on some specific doping concentrations and methods, which may be interesting to view from a theoretical calculation. Zhang et al. [149] studied how sulfur doping affects the photocatalytic performance by DFT calculation. The DFT result shows that the substitution sites of sulfur atoms in ZnO lattice affects the properties of S-doped ZnO. In addition, the S 3p states will locate above the VB and will hybrid with O 2p states, thereby narrowing the band gap. They suggested that the main factors affecting the photocatalytic activity of Sdoped ZnO are all correlated with the introduction of VO (O vacancies) and IZn (Zn interstitials) in the ZnO crystal lattice or with the retardation of e/hþ recombination. 2.2.2. Deposition of noble metals The deposited noble metal NPs not only modify the reactive sites of ZnO NPs, but also serve as a cocatalyst for photodegradation of organic pollutants [150]. The noble metal (precisely, the

plasmonic metal, Cu, Ag, Au) islands can capture the photogenerated electrons and enhance the light absorption by ZnO via surface plasmon resonance (SPR). Both effects evidently facilitate the redox reactions that enhance photocatalytic performance [151]. However, some precious metal NPs with proper work function can promote the separation of photoinduced electronehole pairs, too. Moreover, a double charge layer surrounds the metal NPs, which benefits their electron storage [152]. Therefore, these metal NPs act as charge sinks [153] for the photo-induced electrons in the ZnO NPs to prevent the recombination of charge carriers [154,155]. ZnO is reported to combine with either noble metals with SPR such as Au and Ag, or other precious metals with proper work function such as Pt and Pd [146e150]. Among them, Ag is the cheapest [156]. ZnO photocatalyst modified by Ag will improve its photocatalytic activity and impose the photostability of ZnO [157e159]. Ag loading ZnO can be prepared using a solvothermal route [156e158] or the photo-deposition method via the photoreduction of Ag2þ [160]. Chen et al. [161] suggested that the oxygen defects formed due to appearance of metal composite nanorods benefit the separation of electron-hole pair, which increases the rate constant of 1.9  101 min1 for dye degradation. Ren et al. [162] revealed that the addition of Ag beyond the optimum concentration will decrease the degradation rate due to the excess Ag on ZnO surface blocking UV light. In the example of ZnO@Ag heterostructures, among the samples that included pure Ag nanowires, pure ZnO, and composites containing 3 or 11 at.% Ag, those containing 8 at.% Ag possessed the highest photocatalytic activity [157]. A discussion is offered in Fig. 8 on the manner in which Ag promotes the charge transfer in Ag/ZnO system under UV and visible light irradiation. Fig. 8A shows the band structure of the Ag

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Fig. 6. (A) Calculated density of states: (a) undoped ZnO and (b) N-doped ZnO. Fermi level denoted by the dashed line shifts to the top of the valence band. The bottom of the conduction band is indicated by the solid line. (B) Diffuse reflectance spectra of (a) commercial ZnO, (b) N-ZnO prepared by nitriding commercial ZnO in NH3 gas flow at 973 K for 5 h, and (cee) N-ZnO prepared by heating ZnOHF in NH3 gas flow at (c) 873 K, (d) 973 K, and (e) 1073 K for 2 h. (C) Photoelectrochemical water splitting on electrodes coated with NZnO nanobundles prepared by calcining ZnOHF precursor in NH3 gas flow at (a) 873 K, (b) 973 K, and (c) 1073 K for 2 h. Reprinted with permission from Ref. [134]. Copyright 2013 American Chemical Society.

and ZnO junction including the Fermi energy level (Ef) without irradiation. Considering the larger work function of ZnO, the Fermi energy level of Ag (Efm) is higher than that of ZnO (Efs); thus, e will be transferred from Ag to ZnO until the two systems attain equilibrium and a new and unique Ef arrives [163]. In this process, Ag NPs act as electron sinks, decreases the recombination of photoinduced e and hþ, and enhances the photocatalytic activity of ZnO. However, under UV irradiation [163], (Fig. 8B), the lower edge of the CB of ZnO is higher than the unique Ef of Ag/ZnO, inducing the photoexcited e on the CB transfers from ZnO to the Ag NPs. As the electrons in the Ag sinks can react with the chemisorbed O2, the holes can react with the surface hydroxyl. As mentioned, the surface plasma resonance of Ag-NPs also benefits the light absorption. The work function of metal also plays an important role in determining the e-hþ separation efficiency across interface between metal and ZnO. For example, Zhang and co-workers successfully synthesized high efficiency microreactor with Pt/ZnO nanorods [164]. The presence of Pt NPs on the surfaces of ZnO nanorods promoted the separation of photoinduced e-hþ pairs and thus enhanced the photocatalytic activity. By the way, the recyclable property of the microcreator was investigated. It was found that the microreactor displayed higher durability during the continuous photocatalytic process. The overlap between the SPR absorption window of Au and the visible emission band of ZnO benefits the excited electron transfer from Au to ZnO CB, which enhances UV emission and quenches the visible emission of ZnO [165,166]. Bora et al. synthesized Au/ZnO

composite which can absorb a wide portion of the solar spectrum with a potential application in photocatalysis [167]. Based on the TEM image of AueZnO NRs, the prepared Au NPs are sphericals and attached on the ZnO rods (Fig. 9A). Compared with the bare ZnO NRs which adsorb a small part of visible light (l > 400 nm), the AueZnO NRs showed a strong absorption at ~525 nm, which is characteristic of the SPR absorption of Au NPs [168]. Under visible light irradiation, AueZnO shows a higher reaction rate for photocatalytic degradation of MB than that of pure ZnO. Thus, the SPR induced local electric field on Au accelerates the generation of eehþ pairs in ZnO and suppresses their recombination. As shown in Fig. 9, the electron injection process can occur from photoexcited Au NPs to ZnO CB at the AueZnO interface under visible light irradiation. At the interface, Au forms a Schottky junction with ZnO due to its higher work function (FAu ~ 5.3 eV) than the electron affinity of ZnO (~4.2 eV) [169], which provides a higher gate toward the recombination of photogenerated charges; this recombination efficiently separates the photo-generated charges across the AueZnO Schottky interface [170]. The radical formation reactions then participate in the photocatalytic degradation of pollutants (Fig. 9).

2.2.3. Constructing heterojunctions The coupling of other semiconductors to form heterojunctions with ZnO is also interesting because varying interfacial interactions provide new properties, which do not belong to any individual nanomaterial [171,172]. A prolonged carrier lifetime and an

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Fig. 7. (A) SEM images of C-doped ZnO superstructures. (B) Nitrogen sorption isotherms for ZnO flowers calcined at varying calcination temperatures. (C) Plots of PL intensity at 425 nm against irradiation time for ZnO flowers calcined at various temperatures. (D) DOS of the C-doped ZnO. Reprinted with permission from Ref. [140]. Copyright 2011 Royal Society of Chemistry.

Fig. 8. (A) Band structures of the Ag and ZnO junction and the Fermi energy level equilibrium without UV irradiation. (B) The proposed charge separation process and the photocatalytic mechanism of as-prepared Ag/ZnO samples under UV irradiation. The electrons in the Ag sinks can be trapped by the chemisorbed O2, and the hole can be captured by the surface hydroxyl. Reprinted with permission from Ref. [163]. Copyright 2008 American Chemical Society.

enhancing interfacial charge transfer can be obtained from such heterostructures [173,174]. Generally, the ZnO-based heterojunction for photocatalytic reaction can be categorized into two different types depending on the charge carrier separation mechanism, namely, (1) conventional type-II and p-n junction, and (2) direct Z-scheme. In this section, the basic principles and applications of these three heterojunctions for photocatalytic reaction are discussed.

(a) Conventional type-II heterojunction An ordinary type-II heterojunction is the most typical heterojunction system created to enhance the photocatalytic performance of ZnO. A conventional type-II heterojunction is usually formed by combining semiconductors I and II with proper band location, that is, the CB of one and the VB of another can satisfy the transfer of ehþ pair as plotted in Fig. 10 [175]. Thus, the recombination

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Fig. 9. (A) TEM micrographs of AueZnO NRs showing the Au NPs deposited on the surface of the NRs. Inset in (A) displays the size distribution of the as-deposited Au NPs. (B) Steady state room temperature UV/Vis optical absorption spectra of bare ZnO and AueZnO NRs. Inset in (B) presents the UV/Vis optical absorption spectrum of colloidal Au NPs demonstrating the SPR peak at 520 nm. (C) Course of photocatalytic degradation of MB under visible light irradiation using bare ZnO and AueZnO NRs as photocatalysts. (D) Energy band diagram of the AueZnO interface and the SPR effect on ZnO for photocatalytic reaction. Reprinted with permission from Ref. [167]. Copyright 2015 Royal Society of Chemistry.

Fig. 10. Schematic of the conventional type-II heterojunction.

probability of photogenerated eehþ pairs is reduced, and the reduction and oxidation reactions are carried out on different semiconductors without interfering. Coupling with a narrow band-gap semiconductor is a means of enhancing the charge separation and improving the sunlight absorption of ZnO and others, including CdS [176], ZnSe [177], In2O3 [178], and CuO [179]. For example, Xu et al. reported the ZnO/CdS composites presented higher activity for photocatalytic degradation of RhB than that of pure ZnO under sunlight irradiation (Fig. 11)

[180]. ZnO microspheres are composed of ultrathin sheets (Fig. 11A). The ZnO/CdS composite is synthesized via the ultrasound agitation. In Fig. 11B, the ZnO/CdS heterostructure is composed of ZnO hierarchical microspheres covered by highly dispersed CdS nanoparticles. Under solar light, all ZnO/CdS heterostructures show higher activity for photocatalytic degradation of RhB than that of the pure ZnO and CdS, respectively (Fig. 11C). The type-II heterojunction can be produced between CdS and ZnO, and the bandgap diagram and eehþ pair separation of CdS-ZnO system is shown in Fig. 11D [181]. When the CdS-ZnO heterostructure is radiated by visible light, the generated excited-electrons transfer from CdS CB to ZnO CB, and the holes transfer from ZnO VB to CdS VB. Compared to pure ZnO, the optical absorption edge of the ZnO-CdS system is extended into visible light range [182e185]. The enhanced photocatalytic activity of ZnO/CdS heterostructures is attributed to the type-II heterojunction formation between ZnO and CdS, which accelerated electron-hole separation and enhanced visible light absorption capability. Other examples of type II heterostructure photocatalysts are as follows. Lee et al. synthesized ZnSe/ZnO heterostructures through simple solution-based reactions [177]. The ZnSe/ZnO heterostructures possessed higher activity than that of pure ZnO in visible-light photodegradation of orange-II [177]. Wang et al. synthesized In2O3/ZnO heterostructures using a coprecipitation method [178]. The In2O3/ZnO heterostructures demonstrated higher photocatalytic activity than that of ZnO because of the enhanced charge separation and transport of ZnO via the type II

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Fig. 11. TEM image of pure ZnO (A) and ZnO/CdS hierarchical heterostructure for S3 (B). (C) Degradation rate of RhB under solar radiation in the presence of pure ZnO, CdS, and ZnO/ CdS heterostructures. (D) Schematic profile exhibiting the energy band structure and eehþ pair separation in ZnO/CdS heterostructures. Reprinted with permission from Ref. [180]. Copyright 2012 Royal Society of Chemistry.

heterostructure. The highest photocatalytic activity is reached when Zn/In ratio of 1:1. As for the CuOeZnO system [186], the SEM images of the bare and CuO-modified ZnO nanowire (NW) arrays are plotted in Fig. 12. The uniform ZnO NW arrays utilized are synthesized on the mesh substrate, which is webbed with roundshaped wires (Fig. 12A). The CuO nanomaterials are flower-like and uniformly deposited on the tips of ZnO NWs (Fig. 12B). The CuOeZnO system is demonstrated using a photo responding system because the narrow band gap of CuO properly modified the ZnO NWs, which can be activated by visible light (Fig. 12C). The CuOeZnO NW is a stable type II heterostructure that efficiently separates the photogenerated eehþ pairs (Fig. 12D). (b) P-N junction A clear picture of p-n junction in the photocatalysts can be found in Fig. 13, in which the formation and mechanism of the junction in photocatalysis are illustrated [187]. From the plot, the migration route of the charge carriers can be picturesquely described as follows: In the region close to the p-n junction, the electrons in TiO2 will migrate to ZnO without irradiation, while the holes in ZnO will migrate to TiO2 to achieve the Fermi level equilibrium for the system, causing the internal electric field build up. However, under light irradiation, both n-type TiO2 and p-type ZnO will be excited, and charge carrier pairs will be generated. They will be then effectively separated and transferred to conduct the required reactions under the influence of the internal electric field. This step is also thermodynamically suitable because of the band locations. Therefore, the synergistic effect of the internal electric field and band alignment render the p-n heterojunction more efficient than

the conventional type-II heterojunction for enhancing the photocatalysis of TiO2. ZnO/TiO2 composites showed better light harvesting and higher photocatalytic performance as observed in the photodegradation of MO under UV light irradiation compared with pure ZnO and TiO2, respectively (Fig. 14) [188]. Nanostructured ZnO/TiO2 hedgehogs are prepared via a solvothermal route (Fig. 14A). The simultaneous presence of crystal lattices of crystallized ZnO and TiO2 in hedgehogs can been seen from the high resolution transmission electron microscopy (HRTEM) observations (Fig. 14B). ZnO/TiO2 heterojunctions exhibited superior photocatalytic activity in the decomposition of adsorbates by UV-light irradiation to the NPs of TiO2, the ZnO, mechanically mixed TiO2 and ZnO, and the commercial Degussa-P25 (Fig. 14C). This finding can be attributed to the increased quantum efficiency of the system due to the effect of TiO2 coupled with ZnO in hedgehogs-like composites. Owing to the pen junction formation, the efficient charge separation appears to prolong the lifetime of photogenerated electron-hole pair and to reduce its recombination in the composites. With these information, the model and the diagram of the e-hþ separation process for the pen junction formation are illustrated in Fig. 14D. ZnO/SnO2 composites show a higher activity for the photodegradation of methyl orange than that of SnO2 and ZnO, respectively [189,190], because of the easier interfacial charge transfer across the SnO2-ZnO pen junction with the more effective separation of the photogenerated electron-hole pair [191,192]. SnO2 is a direct bandgap semiconductor with a bandgap of ~3.6 eV [193]. As shown in Fig. 15, the CB of SnO2 is lower than the CB of TiO2 and serves as a sink for photogenerated electrons [191] as the holes move in opposite directions and produce more carriers available for

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Fig. 12. High-magnification SEM images of a bare ZnO NW array (A), a CuOeZnO NW array (B), and UVeVis absorption spectra of ZnO and CuOeZnO heterostructured NWs (C). Diagram showing the energy band structure and eehþ pair separation in CuOeZnO heterostructures (D). Reprinted with permission from Ref. [186]. Copyright 2011 Royal Society of Chemistry.

Fig. 13. pen Junction formation model and schematic of the eehþ separation process. Reprinted with permission from Ref. [187]. Copyright 2008 Elsevier.

free radical generation [194]. Thus, the construction of p-n heterojunction is one of the most promising strategies to enhance the photocatalytic activity of ZnO by combining the effects of the builtin electric field and the band alignment between two semiconductors. (c) Z-scheme heterojunction The Z-scheme heterojunction photocatalytic system is different from that of the conventional heterojunction photocatalytic system [195e206]. Under light irradiation, the photogenerated electrons on Semiconductor II will migrate to semiconductor I, which has a higher reduction potential (Fig. 16), whereas the photogenerated holes will remain in Semiconductor II, which has a higher oxidation potential, resulting in the spatial separation of charge carriers; furthermore, the e and hþ accumulate in the semiconductor with higher reduction and oxidation potential, respectively. Therefore, the redox ability of the Z-scheme heterojunction photocatalyst can

be intentionally magnified. Charge separation across the Z-scheme heterojunction is physically more feasible than that across the conventional type-II heterojunction due to the difference in the electrostatic forces associated with these two junctions. In the Zscheme heterojunction, the electrostatic attraction force between the photogenerated e on the CB of Semiconductor II and the photogenerated hþ on the VB of Semiconductor I will facilitate the migration of photogenerated e from Semiconductor II to I. In contrast, in the conventional type-II heterojunction, the electrostatic repulsion force between the photogenerated electrons of Semiconductors I and II will block the migration of electrons from Semiconductors I to II. Therefore, the Z-scheme heterojunction evidently has more advantages for achieving more efficient photocatalytic reduction compared with the conventional type-II heterojunction. Recently, Zhang et al. [207] reported an all-solid-state vectorial Z-scheme photosynthetic system composed of three functional components, i.e., vertically aligned ZnOeAu@CdS coreeshell nanorod arrays, prepared via a heteroepitaxial growth process as illustrated in Fig. 17. Owing to the synergistic effects of the three functional components integrated in this nanoarray, the photocatalytic efficiency of the optimal ternary ZnOeAu@CdS hybrid is ca. 79 and 28 times higher than the photocatalytic efficiency of ZnO and ZnOeAu counterparts, respectively, toward the selective reduction of aromatic nitro compounds in water under simulated sunlight irradiation. This photocatalytic efficiency is even 1.5 times as high as that of the direct Z-scheme featured ZnO@CdS system because of the effective vectorial Z-scheme electron transfer process. Furthermore, the ZnOeAu@CdS nanorod arrays with a film structure are not only readily recycled but also highly stable under the present reaction conditions. This work demonstrates a

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Fig. 14. (A) SEM image. (B) HRTEM image of hedgehogs. (C) Degradation profiles of MO over photocatalysts. (D) Schematic illustrating the formation of the pen junction and the proposed charge transfer and the separation process of ZnO/TiO2 heterojunctions under UV light irradiation. Reprinted with permission from Ref. [188]. Copyright 2015 Royal Society of Chemistry.

paradigm for constructing an all-solid-state Z-scheme artificial photosynthetic system with several advantages, such as enhancing light harvesting, efficient charge separation and transfer, easy recycling, and good photostability when irradiated by sunlight. The direct Z-scheme ZnO/g-C3N4 composite photocatalyst has been synthesized for CO2 reduction by Yu et al. [208]. The g-C3N4/

Fig. 15. Energy-band diagram and photocatalytic mechanism of SnO2/ZnO heterojunction, where vac is the vacuum level. Reprinted with permission from Ref. [191]. Copyright 2009 American Chemical Society.

ZnO composite shows a more extended visible light absorption than that of pure g-C3N4, because of more defect states in the gC3N4 phase generated by the interaction with ZnO NPs. The photocatalytic CO2 reduction efficiency of ZnO/g-C3N4 can clearly show it is supreme among commercial ZnO, lab-synthesized ZnO and pure g-C3N4 in Fig. 18A [208]. And the band structure of g-C3N4 and ZnO together with the standard potentials of relevant redox couples is presented in Fig. 18B. For the sake of elucidation, Fig. 18 also lists the electron-hole pairs separation inhibition mechanism for the type-II heterojunction (Fig. 18C) and the direct Z-scheme

Fig. 16. Schematic of the ZnO-based Z-scheme heterojunction photocatalyst.

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Fig. 17. Schematic of (a) the direct Z-scheme competing with type-II charge transfer in the binary ZnO@CdS composite and (b) vectorial Z-scheme charge transfer pathways in the ternary ZnOeAu@CdS all-solid-state system. Reprinted with permission from Ref. [207]. Copyright 2016 Royal Society of Chemistry.

(Fig. 18D). The ZnO/g-C3N4 possesses the direct Z-scheme heterojunction between g-C3N4 and ZnO, which can promote electronhole separation and optimize redox potential. Besides, DFT theoretical calculation results also confirm the formation of direct Zscheme heterojunction between g-C3N4 and ZnO. Moreover, the relevant effective masses of the photogenerated e and hþ of gC3N4 and ZnO were estimated, and found the relevant effective masses of the photogenerated electrons in G-Z direction of ZnO (0.034) is much lighter than those of the g-C3N4 (3.9), indicating the migration of electrons from ZnO to g-C3N4 is more favorable than that from g-C3N4 to ZnO. Therefore, Both OH radical trapping test and DFT calculation results all indicate that the formation of direct Z-scheme heterojunction causes the enhanced photocatalytic performance of ZnO/g-C3N4. 2.2.4. Coupling carbon materials Massive conjugated carbon materials, including fullerene, carbon tube, and graphene (GR), can be coupled with ZnO, and the photocatalytic activity of ZnO can be dramatically enhanced. This condition is due to these carbon materials can act as a photoelectron reservoir, which stores and shuttles the photogenerated electrons from ZnO to substrates, or perform as a photosensitizer, which improves the light absorption of photocatalyst as an organic dye [209e212]. Fullerene (C60) consists of spherical conjugated p orbitals, which benefit the photocatalytic process [213]. C60 has been used in photon energy conversion since the superior electron

conductivity of fullerenes [213e216]. In Fig. 19 [58], the photocurrent of C60-ZnO under the same bias potential is higher than that of pure ZnO film, which indicates that C60-ZnO has a high separation efficiency for photoinduced carriers via the interaction between C60 and ZnO. However, the photocurrent of the ZnO film cuts off sharply after 20 h irradiation by UV light. When the UV light irradiation is prolonged to 35 h, its electrochemical response declined dramatically as illustrated in Fig. 19A. In comparison, the C60-ZnO can be exposed to UV light extensively (Fig. 19B) without a noticeable difference in its electrochemical response. This phenomenon may be attributed to the improved photostability of ZnO by covering with the C60 layer. Moreover, the ZnO/C60 composites can extend the absorbance of ZnO to the visible light region; meanwhile, the enhancement of the absorption intensity of the ZnO/C60 composites is increased with the increase of the C60 loading amount as shown by the degradation of MB (Fig. 19C) [58]. The best loading amount of C60 is 1.5%, at which the rate constant is 0.0569 min1 or nearly three times as that of pure ZnO (0.0188 min1). The enhancement is due to the high migration efficiency of photoexcited charge carriers across the interface of C60 and ZnO via the strong interaction between ZnO and C60 due to the conjugative p orbital system of C60. When irradiated by sunlight, the electrons in the C60 2p orbitals at the ZnO-C60 heterojunctions are excited into the Zn 3D states, which follows the mechanism of ligand-to-metal charge transfer [217,218]. Hence, the C60-based sensitization is a single-step process, in which the complex forms between C60 and ZnO under UV light irradiation,

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Fig. 18. (A) Comparison of the photocatalytic CO2 reduction performance of commercial ZnO, lab-synthesized ZnO, ZnO/g-C3N4, and g-C3N4 for CH3OH production. (B) Schematic comparison of band-edge positions of g-C3N4 and ZnO, together with the standard potentials of relevant redox couples. Also, schematics of the eehþ pair separation mechanism of the type-II heterojunction (C) and of the direct Z-scheme photocatalyst (D). Reprinted with permission from Ref. [208]. Copyright 2015 Royal Society of Chemistry.

Fig. 19. Currentepotential plots recorded for ZnO film (A) and C60-hybridized ZnO film (B). (C) Photocatalytic degradation of MB over ZnO/C60 composites with different loading amounts of C60 under UV light illumination (l ¼ 254 nm) (inset: effect of C60 loading on the apparent rate constant, kapp). (D) Illustration of the photocatalytic degradation mechanism of MB over ZnO/C60 composite. Reprinted with permission from Ref. [58]. Copyright 2008 American Chemical Society.

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Fig. 20. Variation of Ln(C0/C) versus photoirradiation time for photodegradation of MB by using ZnO/MWCNT composites under visible light (A) and under UV light on pure ZnO and ZnO/MWCNT composites (B). (D) MWCNT inhibition of eehþ recombination mechanism in the ZnO/MWCNT composites under UV light (C) and photosensitized mechanism (under visible light) of MWCNT in the ZnO/MWCNT composites (D). Reprinted with permission from Ref. [56]. Copyright 2012 Elsevier.

and the electrons are directly injected from the C60 to the CB of ZnO. The superoxide O 2 should be generated because of electron transfer from the CB to O2 in the reaction solution, which is responsible for the degradation of MB (Fig. 19D). Carbon nanotube (CNT) with 1D structure is considered to construct highly efficient CNT-based photocatalysts with semiconductors [219e223]. Some studies have proven that combining CNT with ZnO can strengthen the photocatalytic activity of ZnO [202e205]. The 1D CNT has an extending 1D cylindrical conjugated molecular orbital with roomy, ultrafast, and long distance electron delivery capacity for reserving and shuttling electrons, which will block the recombination of photogenerated eehþ pairs [219,224]. Additionally, the enhanced photocatalytic activity is considered because of the improving dispersion of photocatalysts and the enhanced exposure of active sites [225]. The attached CNT changes the band structure of ZnO, behaving as a photosensitizer to sensitize ZnO; for example, the multi-walled CNT (MWCNT)-modified ZnO exhibits visible light photoactivity for photodegradation of MB [226]. Fig. 20A illustrates that the rate constant of the degradation of MB is 0.00387 mol min1 for ZnO/MWCNT composites but zero for pure ZnO. The rate constant of ZnO/MWCNT composites under UV irradiation is much higher (0.01445 min1) than that of pure ZnO (0.00286 min1) (Fig. 20B). In Fig. 20C and D, the photocatalytic mechanism is proposed [56]. The MWCNT under visible light irradiation can absorb the visible irradiation and transfer the photogenerated e into the CB of ZnO to react with O2 to degrade the organic pollutant while the positively charged CNTs remove an electron from the ZnO VB, leaving a hole to react with OH. However, when the ZnO/MWCNT composites are irradiated by UV light, the photogenerated electrons from ZnO are directed to the MWCNT and leave holes in the ZnO VB, resulting in reducing the probability of eehþ pair recombination and subsequently increasing the

photocatalytic activity of the degradation of MB. The formation of a composite with CNT may be an effective method for enhancing the photocatalytic activity of ZnO [227e229]. GR sheets with 2D structure are also utilized in this realm [230e236]. An extensive range of studies has been conducted on the hybridization of GR with ZnO to reinforce the photocatalytic performance of ZnO because of the vast 2D conjugation GR [77,237]. The loading of GR can reinforce the photocatalytic activity of ZnO in four aspects: (1) speeding up the eehþ separation and holding them apart, (2) improving the specific surface field because of the SPR, (3) enhancing the adsorption of dye molecules through pep combination between dye molecules and GR, and (4) enhancing the light utilization capacity [238]. Li et al. used facile chemical deposition means of preparing ZnO/GR oxide (ZnO/GO) composites, revealing a remarkable performance for photodegrading dyes from water under visible light [239]. The fabrication progress and formation mechanism of ZnO/GO composite is explained in Fig. 21A, i.e., ZnO uniformly grabs onto the GR nanosheets in solution. Then, the reaction solution is removed by thermal treatment in air, and the product is the GO sheet bonding with Zn2þ ions. Fig. 21B illustrates the comparison for photodegradation of MB over a photocatalyst-free solution among GO sheets, flowerlike ZnO particles, ZnO/GO nanocomposite, and annealed ZnO/GO under UV light irradiation and confirms that ZnO/GO nanocomposite and annealed ZnO/GO have the higher photocatalytic activity (Fig. 21B) [239]. Given that the GR sheets are wrinkled, the tight adsorption between ZnO and GR strengthens the electronic interaction, resulting in the improved separation efficiency of photoinduced electrons and holes (Fig. 21C). This result is reflected in the photocurrent measurement; the photocurrent of the GRhybridized ZnO is approximately 3.5 times higher than that of pure ZnO [57]. Moreover, the presence of GR in the ZnO/GR

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Fig. 21. (A) Illustration of the fabrication process and formation mechanism for ZnO/GO nanocomposite. (1) Oxidation of graphite to obtain graphite oxide (GO) with large interlayer spacing. (2) GO sheets formed from the ultrasonic exfoliation of GO. (3) Adsorption and bonding of Zn2þ ions with the GO sheets. (4) The nucleation and growth of ZnO crystallites on the GO sheet forming ZnO/GO nanocomposite. (B) Reaction rates of the photocatalytic degradation of MB over photocatalyst-free solution (blank), GO sheets, flower-like ZnO particles, ZnO/GO nanocomposite, and annealed ZnO/GO. (C) Schematic of the GR-loaded ZnO working during photocatalytic decomposition of dye. Reprinted with permission from Ref. [239]. Copyright 2012 Elsevier.

composite could intensify the adsorption affinity toward MB molecules, which aim to improve the photocatalytic activity of ZnO/GR composite [240e243]. Therefore, many factors contribute to enhancing the photocatalytic performance of ZnO/GR composites, such as strengthening the adsorption affinity toward MB molecules, increasing the specific surface area of reactive sites, and boosting the light harvesting [234,244e250]. ZnO/GR nanocomposites demonstrate a good photocatalytic activity for photodegradation of RhB under UV light [224e227]. 3. Antibacterial application ZnO is a promising antibacterial agent because of its excellent antimicrobial property and good biocompatibility. Particularly, ZnO NPs show good bactericidal performances on Gram-positive and Gram-negative bacteria [251]. However, the precise antibacterial mechanism of ZnO is not fully understood, thereby restricting the full application of ZnO as an antibacterial material. Investigations on the antibacterial mechanism of ZnO materials would enhance the research of and extend the medical usages for nanomaterials in general. 3.1. Mechanisms of ZnO antibacterium Currently, the suggested antibacterial mechanism of ZnO nanomaterials mainly aims at four aspects [252], namely, ROS generation [253], Zn2þ ion release [254], membrane dysfunction [255,256], and NP internalization [257] as illustrated in Fig. 22. However, the relative importance or the exact antibacterial mechanism of ZnO nanomaterial remains debatable.

Fig. 22. Illustration of possible toxicity mechanisms toward bacteria cells. Reprinted with permission from Ref. [252]. Copyright 2015 John Wiley and Sons.

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(a) ROS Generation Most researchers highlight that the toxicity of ZnO to bacteria attributes to ROS generation (Fig. 22) [240e242]. The toxicity of ROS to cells is due to the damage to cellular constituents, such as DNA, lipids, and proteins [258]. ROS generation is considered the main antimicrobial factor associated with the photoexcited ZnO on terminating the bacterial cells [259,260]. When ZnO is irradiated by light with proper energy, e will be activated and transferred from VB to CB for reduction reaction, while the simultaneously formed  hole (hþ) will transfer to VB for oxidation [261,262]. O 2 is generated when e reacts with O2 through a reductive process [263]. The hþ will react to either the e from water and/or the hydroxyl ions (OH) to generate OH through an oxidative process [264]. Singlet oxygen (1O2) is another strong oxidant and can be produced indi  rectly from the aqueous reactions of O 2 [265e267]. OH is the most reactive oxygen radical specie, because it reacts quickly with organic biomolecules, including nucleic acids, lipids, carbohydrates, proteins, DNA, and amino acids [268]. Hydrogen peroxide (H2O2) is also an oxidant and can be produced by recombining two OH radicals. The chemical interactions between bacteria and H2O2 generated from ZnO powder slurry are the predominant mechanism underlying the antibacterial activity of ZnO [269]. As mentioned above, 1O2 can damage tissues [270], causing bio membrane oxidation and degradation [271]. Finally, O 2 induces   significant biological effects, although O 2 is the precursor of OH 1 1  and O2 and is not an equally strong oxidant as OH and O2 [272]. (b) Zn2þ ion Release The toxicity of ZnO, which can be partially soluble, is often attributed to the Zn2þ ions released into the solution (Fig. 22). The Zn2þ ions can be adsorbed on the bacteria surface; the interaction between Zn2þ ions and bacterial cell membrane will lead to the Zn2þ ions easily entering bacteria and interacting with the functional groups on bioactive proteases, such as sulfhydryl, amino, and hydroxyl groups, to change the structure and performance of proteases that terminate the bacteria because of unbalanced metabolism [273]. However, the antibacterial mechanism of Zn2þ ions remains unclear. For example, the antibacterial mechanism of Zn2þ released may be studied by comparing the toxicity of metal oxide NP with that of a soluble metal salt [260,261]. Bellanger et al. [254] suggested that ZnO quantum dots (QDs) and ZnCl2 were more toxic toward E. coli MG1655 and C. metallidurans CH34 (the IC50 is 1.6  105 M and 2.7  105 M for ZnCl2 and ZnO QDs, respectively) than toward C. metallidurans CH34 (the IC50 is 2.8  104 M and 1.2  103 M for ZnCl2 and ZnO QDs, correspondingly). However, no quantitative relationship occurs between Zn2þ ions released from the ZnO NPs and the antibacterial activity of ZnO NPs used [274]. Naturally, different organisms may have different sensitivity to Zn2þ ions, and ZnO NP solubility and/or Zn2þ release may also be influenced by irradiation, i.e., photodissolution or photocorrosion [275]. In particular, although numerous factors reported can affect the antimicrobial activity of ZnO NPs [264e267], the main antibacterial mechanisms of ZnO NPs can be different in different media, because all the processes cannot avoid being influenced and altered by medium components. Therefore, comprehensive and thorough studies are still required. (c) Membrane dysfunction ZnO NPs with a positive charge will be electrostatically attracted to the negatively charged bacterial membranes [276], thus ZnO NPs can strongly adsorb on the bacteria membrane [277]. The interaction between ZnO and bacteria can disturb the charge balance of

cell surface, leading to a severe deformation of the cell and killing the bacteria via bacteriolysis (Fig. 22) [68,278,279]. Therefore, the toxicity originates from the interaction between ZnO NPs and cell wall [277,280,281]. This interaction is not only the electrostatic attraction mentioned above but also involving some other weak interactions, such as Van der Waals forces, hydrophobic interactions, or receptoreligand interactions [282]. The selection of the receptoreligand interaction instead of the weak electrostatic repulsion has been proposed as the origin of the attachment of ZnO NPs with negative zeta potential onto the bacteria membrane [282]. However, some previous works suggested that the toxicity does not directly come from the NP adsorption onto the cell surface [275,276], and the antibacterial effect can be obtained for the bacteria from oxygen-involved species penetrating inside the cells, although the bacteria and NPs are separated by a membrane [283]. Stoimenov et al. reported that the adsorption was accomplished through electrostatic forces, and then the bacteria were terminated [284], the root of which was assumed to be due to the abrasiveness of the cell membrane by the solid ZnO NPs initiated the disorganization of the cell membrane, and was responsible for the antibacterial performance of ZnO NPs [285]. The adsorption of the ZnO NPs on the cell membrane also affects the membrane viscosity and the cross-membrane transport [286]. (d) NP internalization Once ZnO NPs have internalized the bacteria, they will inhibit and shut off the vital and vulnerable exchange of matter and energy metabolism with the environment for bacteria [287,288] as depicted in Fig. 22. After ZnO NPs have disrupted and dysfunctioned the cell membrane, they are internalized into the bacteria. Disturbing the membrane can cause the loss of membrane integrity, resulting in the malfunction of the permeability barrier [284,289,290]. In addition, the large specific surface area and high surface energy of ZnO NPs will adsorb and terminate additional bacteria. Brayner et al. reported that ZnO NPs hinder E. coli growth based on this mechanism [290]. Huang et al. observed that the ZnO NPs can penetrate S. agalactiae cells for the same reason [288]. 3.2. Strategies for enhancing ZnO antibacterial activity The antibacterial mechanisms of ZnO NPs mainly include ROS generation [291,292], Zn2þ ion release, membrane dysfunction [255,256], and NP internalization [289]. Based on these antibacterial mechanisms, some strategies have been developed to improve the antibacterial performance of ZnO, including coating/loading/ modifying ZnO with inorganic/organic photocatalyst promoters and/or antimicrobial agents, tuning the particle size, morphology, and concentration of ZnO NPs. 3.2.1. Coating inorganic photocatalyst promoters and/or antimicrobial agents ROS is generated at the surface of ZnO NPs via the photocatalytic reactions. The photoexcited e and hþ recombination can occur simultaneously as they are generated, reducing the ROS generation and decreasing the antimicrobial activity. The surface modification of ZnO NPs with proper materials is an effective method to decrease this recombination effect [293e296]. Meanwhile, some of the noble metals, such as Ag, Au, and Cu, are good broad-spectrum antimicrobial agents [291e294]. ZnO is coated with Ag [297,298]. The yield rate of ROS is determined mainly by the coating amount and the dispersion degree of Ag NPs on the ZnO surface. When the dispersity of Ag NPs in Ag/ZnO is high, the photocatalytic activity of Ag/ZnO composites is also high [162,299]. The photocatalytic activity is enhanced,

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because the modification of ZnO with an appropriate amount of coated Ag NPs can not only enhance the separation efficiency of photogenerated eehþ pairs on the ZnO surface but also improve the ZnO photostability because of reducing ZnO surface defects by the coated Ag NPs [160]. Lu et al. reported that Ag NPs coated on ZnO microspheres possess the best coating ratio, i.e., 1.62 at.% Ag, among the coated amounts of 0, 0.83, 1.62, 3.30, and 6.54 at.% Ag [163]. This trend is also reflected in Ag/ZnO composites with the ZnO possessing nanostructures other than microsphere [300,301]. The electrostatic forces between Ag NPs and bacteria also play an important role in bactericidal application [302e304]. Changing the bacterial adhesion properties may occur after Ag NPs are introduced onto ZnO NPs. Lu et al. prepared Ag/ZnO composites, which exhibit antibacterial activity on E. coli [305]. The electroneutral Ag particles become highly positively charged after they adhere to the surface of ZnO NPs. This phenomenon may be attributed to the transfer of electrons from the Ag NPs to ZnO nanorods, which creates a strong electrostatic attraction interaction between the positively charged Ag particles and the negatively charged bacteria, increasing the bactericidal activity.

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Similarly, coating Au NPs on ZnO enhances the photoelectric conversion efficiency of ZnO NPs and the antimicrobial performance [306e310] for the same reason as coating Ag [311,312]. The Au/ZnO composites can absorb a wide range of solar spectrum, thus having a high potential application in photocatalysis and antibacterial [313]. Another important factor is attributed to enhancing the SPR, which induces the local electric field on Au NPs and thus accelerates the generation of eehþ pairs in ZnO NPs with the suppressed eehþ recombination [310]. Au/ZnO composites with various ZnO:Au molar ratios have been successfully synthesized to enhance the ROS generation in photodynamic therapy applications [314]. Among these applications, the composite with a ZnO:Au molar ratio of 20:1 generates the highest amount of ROS, leading to the lowest survival rate for HeLa and C2C12 cells [314]. Several researchers highlighted that the Au/ZnO NPs stick on cellular surfaces of B. subtilis via the electrostatic interactions between Au/ZnO composites and bacterial cells, and a series of the oxidative reactions will occur on the cell wall and practice the antibacterial effect [315]. He et al. prepared Au/ZnO composites by reflecting the heterogeneous nucleation and growing of the ZnO particles on the

  Fig. 23. (A) Position of the Fermi level of Au and the energy bands of ZnO compared with the redox potential of O 2 /O2 and H2O/ OH, (B) Expected reaction mechanism about the enhancement effect on ROS generation and photocatalytic activity. The deposition of Au into ZnO increases the charge carrier separation and transport efficiency in photoexcited ZnO NPs. The capability of ZnO NPs and ZnO/Au hybrid nanostructures in terminating S. aureus (C) and E. coli (D) under simulated sunlight for 10 min. Control 1 represents bacteria exposed to neither NPs nor light. Control 2 represents bacteria exposed to simulated sunlight for 10 min but without NPs. The bacteria grouped under ZnO were exposed to 0.1 mg/ mL ZnO alone or were exposed to 10 min of solar-simulated light and either 0.05 mg/mL or 0.1 mg/mL ZnO. Similarly, the bacteria grouped under ZnO/Au4% were exposed to 0.1 mg/ mL ZnO/Au4% alone or were exposed to 10 min of solar-simulated light and either 0.05 mg/mL or 0.1 mg/mL ZnO/Au4%. Reprinted with permission from Ref. [313]. Copyright 2014 American Chemical Society.

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presynthesized Au NP seeds. They found that these Au/ZnO composites had a higher photocatalytic activity than that of pure ZnO NPs [306]. Therefore, they proposed that the alteration of the bandgap of ZnO under UV irradiation and the enhanced efficacy of electron transfer in the Au/ZnO composites were responsible for the ROS generation and the observed improved photocatalytic performance (Fig. 23A) [313]. For terminating S. aureus and E. coli under the simulated sunlight, they found that the antibacterial activity of ZnO/Au NPs at a concentration of 0.05 mg mL1 was approximately two and three times higher, respectively, than that of the pure ZnO NPs; moreover, when the particle concentration was increased to 0.1 mg mL1, the corresponding values were increased by approximately 3.5 and 4.5 times, correspondingly (Fig. 23B), [313]. The Cu/ZnO system can also remarkably improve its solar spectrum absorbance, while 1.5 mol% Cu NPs coated on the ZnO surface is most active under visible and UV light irradiation [297]. Coating Cu NPs on ZnO to build Cu/ZnO composites takes advantage of ZnO NPs and Cu NPs in enhancing its antimicrobial performance. The result shows that the survival rate of E. coli treated with Cu/ZnO composites (3.12%) is lower than either that of pure Cu (76.12%) or ZnO (58.02%), respectively [316]. Hence, the Cu/ZnO composite possesses a synergic effect of Cu and ZnO, enhancing its antimicrobial activity. In addition, the other properties, including larger specific surface area and mesoporous property, were beneficial to the adsorption and termination of E. coli. Cu/ZnO composites improved the antibacterial activity by increasing the utilization rate of light. The report claimed that approximately no E. coli, which is treated by the ZnO/Cu composites, survived after 5 min of visible light irradiation; however, the survival rate of the E. coli treated by pristine Cu and ZnO NPs was 75% and 51.30%, respectively, and this high activity of ZnO/Cu composites was attributed to the ROS generation [316].

than Gram-positive bacteria by CS. Kumar et al. produced a bandage, which was composed of CS and ZnO NPs (CZBs). This CZB shows a high antimicrobial activity against E. coli growth [318]. Samzadeh-Kermani et al. used ZnO NPs and CS together by grafting them with polyaniline and montmorillonite. The bactericidal activities against S. aureus and E. coli showed that the 1.0% and 1.5% ZnO NPs, respectively, had high activities because of the reduced interaction with the cell membrane and thus create a narrow inhibition zone [319]. CS exhibits bacteriostatic and fungistatic activities; it is a linear homopolymer of 1,4b-linked N-acetyl-D-glucosamine and a partially N-deacetylated chitin [320,321]. The co-antimicrobial effect of CS and ZnO has been noticed, and the mechanism has been proposed as follows [322]: The CS, ZnO NPs, and released Zn2þ ions are adsorbed on the surface of the microbial cell membrane under the electrostatic attraction; thus, the membrane proteins are denatured, and the membrane permeability is altered; finally, the membrane structure is destroyed [287]. Petkova et al. observed that the growth of S. aureus at 60 min of incubation with ZnO/CS has been cut off by 98%, whereas ZnO NPs and CS alone only reduce bacterial growth by 61% and 31%, respectively; similarly, the viability of E. coli after 15 min is reduced by more than 96% with ZnO/CS [323]. CS possesses high viscosity and low solubility, thus prolonging the antimicrobial activity of ZnO/CS [323]. The bacterial infection and inflammation will be mitigated, because ZnO NPs can be immobilized on the surface of biomedical devices. The positively charged CS molecules and negatively charged microbial cell membranes will interact and lead to the proteinaceous and other intracellular constituents leaking out [324e326]. CS can penetrate the microorganisms toward the nuclei and interfere with the synthesis of mRNA, proteins, and RNA [327,328]. In summary, the synergistic effect of composited ZnO with CS should be noticed for designing antimicrobial agents.

3.2.2. Loading organic antimicrobial agents The composed organic antimicrobial agents with ZnO NPs have exhibited greater antibacterial activity than the pure ZnO NPs [316e318,320e322]. The organic antimicrobial agents are usually immobilized or embedded on the ZnO surface. For example, chitosan (CS) can strongly react with ZnO NPs because of its free amino functional groups. The inhibitory efficiency of CS against different microorganisms remains under debate. In previous studies, the antimicrobial activity of CS is higher against Gram-negative bacteria than against Gram-positive bacteria [317]. Gram-negative bacteria are believed to be more absorbed and strongly inhibited

3.2.3. Doping metals or nonmetals Impurity atoms doping into ZnO host lattice can tune physicochemical properties of ZnO and enhance its antibacterial activity [329e332]. Mn has been doped into ZnO NPs and found that the Mn-doped ZnO NPs can increase the antibacterial activity against bacteria compared with the undoped ZnO NPs. Ravichandran et al. synthesized Mn-doped ZnO and the prepared ZnO:Mn NPs with 10 wt% doping amount, exhibiting high antibacterial efficiency against E. coli bacteria [333]. Khatir et al. reported doping ZnO with Fe to produce magnetic NPs with antibacterial activity [334]. Their results indicated that the antibacterial activity of ZnO/Fe depends

Fig. 24. (A) Growth curves for the S. aureus bacterial strain in the presence of undoped and Mg-doped ZnO nanomaterials. (B) Growth curves for the Pseudomonas aeruginosa bacterial strain with undoped and Mg-doped ZnO nanomaterials. Reprinted with permission from Ref. [343]. Copyright 2014 Elsevier.

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on the Fe/Zn ratio, in which a high ratio is good. Dutta et al. [335] demonstrated that Fe- or Co-doped ZnO NPs can improve its antibacterial performance against E. coli. Undoped and Co-doped ZnO NPs with the Co ratio of 0.5, 1.0, 1.5, and 2.0 wt% were prepared through a microwave-assisted method, and their antibacterial studies showed that the 2.0 wt% Co-doped ZnO NPs were better against E. coli [336]. Ag-doped ZnO nanorods were investigated against the Gram-positive (M. leutus) and Gram-negative (K. pneumonia) bacteria [337]. The observed antibacterial activity of ZnO NPs could either be because of a direct interaction with the microbial cells (e.g., interrupt the transmembrane electron transfer, destroy/penetrate the cell envelope, and oxidize the cell components) or the production of secondary products (e.g., ROS) [338,339]. Cu is another important doping element. Cu2þ ion can enter the ZnO lattice and alter its physical properties [340,341]. Cu2þ ions are embedded in the ZnO lattice and join the ZnO particles to hinder bacterial growth [297,342]. Liang et al. [342] prepared polyaniline/ Cu0.05Zn0.95O composite and showed that ZnO/polyaniline and polyaniline/Cu0.05Zn0.95O composites can be used to terminate S. aureus, E. coli, and Candida albicans; however, the polyaniline/ Cu0.05Zn0.95O composite exhibited more antibacterial activities than that of ZnO/polyaniline. Iqbal et al. [343] studied the antibacterial activity of Mg-doped ZnO nanostructures toward S. aureus and Pseudomonas aeruginosa bacteria as depicted in Fig. 24. The results indicate that the synthesized nanostructures inhibit approximately 90% growth rate of both bacterial strains. The result of the in vitro test clearly suggests that the prepared nanostructures show antibacterial activity toward both bacterial strains. In the case of S. aureus, either undoped or Mg-doped ZnO nanostructures will assign a zone of inhibition about the same size, i.e., 10 mm, but bacterial strain will quickly regrow in the case of undoped ZnO. Interestingly, the regrowth will slow down with the doping of Mg and will even vanish as 10% Mg is doped into ZnO nanostructures [343]. N-doped ZnO NPs have been examined for antibacterial activity under the photoirradiation. The antibacterial effect of the NPs on E. coli and S. aureus has been examined. When N-doped ZnO is added, the remaining bacteria are hardly detected, while a considerable amount of bacteria are alive in the reference test [344].

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3.2.4. Size, morphological characteristics, and concentration (a) Particle size The antimicrobial activity of ZnO NPs exhibits size dependency, that is, small ZnO NPs yield high antibacterial activities [345e347]. The antibacterial activity of ZnO is attributed to ROS, which is generated through reactions between the ZnO surface and O2. Therefore, a small NP with a high surface area allows additional target bacteria to attach and then perish. Raghupathi et al. examined the antibacterial activity of different sizes of ZnO NPs [253]. In Fig. 25A, ZnO NPs induce a size-dependent inhibition of S. aureus growth at 6 mM. The survival rate of S. aureus cells decreases remarkably as the size of ZnO NPs applied at 6 mM decreases (Fig. 25B). Furthermore, they carefully provided evidence to eliminate the possible action provided by free Zn2þ in a ZnO colloidal solution [253]. Yamamoto et al. [346] investigated a related property regarding ZnO NPs with different sizes from 0.1 mm to 0.8 mm and found a similar result. Applerot et al. [277] investigated the same characteristic by using E. coli and S. aureus and observed that S. aureus is more sensitive to the size of NPs than E. coli, but the latter is less evident in deed. Furthermore, they excluded the possible action of Zn2þ and increased Zn2þ concentration to more than five times the aqueous solubility of ZnO NPs by dissolving Zn(Ac)2$2H2O in water, but they did not find any influence on the survival rate of E. coli or S. aureus [277]. Therefore, ZnO NP size is related to their antibacterial activity but is associated with two other factors: the amount of cellular internalization that appeared and the number of hydroxyl radicals generated. (b) Particle morphology The antibacterial activity is remarkably influenced by the morphological characteristics of ZnO NPs [348e350]. Proper synthesis and growth techniques may control these characteristics and hold added active facets of the ZnO NPs. Several methods have been developed to synthesize ZnO nanomaterials with various morphologies, such as nanorods [348], nanosheets [351], microspheres [352], QDs [353], and NPs [354]. The facet dependence of ZnO NPs on antibacterial activity has been assessed, and their antibacterial activity is enhanced when these NPs possess additional active facets [355]. One mechanism has been suggested that ZnO rods and

Fig. 25. Effects of different sizes of ZnO NPs on the growth of methicillin-sensitive S. aureus strain. (A) Growth analysis curves measured by monitoring the optical density at 600 nm and (B) viable S. aureus 6390 recovered from TSA plates after 24 h of incubation at 37  C. Colonies were counted, and the percentage of growth inhibition was calculated and plotted against particle sizes. Reprinted with permission from Ref. [253]. Copyright 2011 American Chemical Society.

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wires can penetrate the cell walls of bacteria more easily than spherical ones do because of morphology [356]. The antibacterial activity of flower-shaped ZnO nanomaterials against S. aureus and E. coli is higher than that of spherical ones [348]. In addition to internalization, antibacterial activity is enhanced because their polar surfaces contain numerous oxygen vacancies that may contribute to their high antibacterial activity because these vacancies generate ROS [357]. Therefore, antibacterial performance can be improved by using ZnO particles with more exposed (0001) polar facets [349]. Fig. 26 represents the viable

growth of Gram-negative (E. coli) and Gram-positive (S. aureus) bacterial strains with ZnO nanorods, nanospheres micro-flowers, and blank control under UV irradiation or under dark conditions [348]. The photocatalytic inactivation of ZnO nanostructures with flower-like morphology is better than that of nanorods and nanospheres. The inactivation efficiencies against E. coli and S. aureus under UV light irradiation are also higher than those under dark conditions. Moreover, the surface interstitial defects of ZnO NPs may influence the antibacterial activities of flower-like ZnO NPs [348].

Fig. 26. Comparison of colony-forming units for Gram-negative (E. coli) and Gram-positive (S. aureus) bacteria strains in the presence of ZnO nanorods (Sample-A), ZnO spheres (Sample-B), ZnO flowers (Sample-C), and control (Blank) under different conditions. Reprinted with permission from Ref. [348]. Copyright 2013 Elsevier.

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(c) Concentration The antibacterial activity of ZnO NPs is correlated with their concentration, that is, high concentrations correspond to high antibacterial activities [358,359]. Thus, the dissimilarity of testing media and cell types may be accounted for varying results. Li et al. examined the influence of the types of bacterial cells, namely, one Gram-positive bacterium (B. subtilis) and two Gram-negative bacteria (Pseudomonas putida and E. coli), on the fundamental dissimilarity of reaction media and cell types [360]. These bacteria behave differently, although their viability rates decrease when high ZnO NP concentrations are administered. The effect of ZnO concentration is confirmed, although P. putida and E. coli survive longer than B. subtilis does) [360]. Brayner et al. [290] investigated the toxicological effects on E. coli and found that the presence of 102 and 3.4  103 M ZnO NPs inhibits bacterial growth by nearly 100% but slightly inhibits bacterial growth at low ZnO NP concentrations. Several suggestions have been offered to explain this concentration-dependent phenomenon [361]. The field emission scanning electron microscopy (FESEM) images (Fig. 27AeC) of the mixture of ZnO NPs with E. coli can be observed: ZnO NPs are on bacterial surfaces and inhibit their growth [361]. The high amount of ROS generated from the lattice defects of ZnO NPs and the UV illumination further exacerbate the damage on bacteria (Fig. 27D and E) [361]. Therefore, the improved ZnO antibacterial performance should be attributed to the increased ROS generation, especially when the comparison is clear that the inhibitory effects of ZnOeO2 (ZnO NPs were annealed with oxygen) on bacterial growth are greater than those of ZnOeAP (ZnO NPs is used as purchased) because the high occurrence of surface defect is likely induced by annealing.

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4. Conclusions and outlook Modified ZnO nanomaterials have been recommended for their potential use as photocatalysts to degrade organic pollutants in solutions. This paper reviews various methods to modify ZnO nanomaterials and improve the photocatalytic degradation of organic pollutants. These modification methods include 1) doping with metals or nonmetals, 2) coupling with semiconductors, 3) depositing noble metals, and 4) coupling with carbon materials. Furthermore, 1) the doping amounts of metals or nonmetals, 2) the types of coupling semiconductors, and 3) the relative position of band gaps are crucial factors in the design of optimal ZnO hybrid NPs. This paper also describes the applications in organic pollutant photodegradation, and the applications of various modified ZnO NPs should be further elucidated in future studies. This review also aims to summarize the recent studies on the use of ZnO NPs in antibacterial applications. This review focuses on various mechanisms and conflicting observations in the antibacterial properties of ZnO NPs. In summary, 1) ROS generated by ZnO NPs can kill bacteria and have been identified as the major advantage ofthese materials; 2) the released Zn2þ can attach to the cell membrane and cause mechanical damage to the cell wall; 3) the interaction between ZnO and bacteria can disrupt the balance of charges on the cell surface and consequently kill bacteria; 4) NP internalization also induces bacterial death. However, the toxicity mechanism of ZnO remains poorly understood. Studies should promote the use of antibacterial ZnO nanomaterials because their mechanisms are related to the medical usage of nanostructured materials. Further theoretical experiments and quantum chemistry calculations should be performed to supplement basic and routine toxicity experiments, such as simple assays for oxidative stress and measurements of metal ion release and ROS generation. This

Fig. 27. (A) FESEM micrographs of E. coli exposed to ZnO; the arrows show ZnO particles on the bacteria surface, (B) untreated bacteria cells, (C) E. coli treated with ZnO, and (D, E) the percentage inhibition of E. coli treated with ZnOeAP and ZnOeO2 at the different concentrations with and without UVA illumination, respectively. Reprinted with permission from Ref. [361]. Copyright 2015 Springer.

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review also summarizes the following synthesis strategies, which have been developed to improve the antibacterial performance of ZnO: 1) modifying inorganic/organic antimicrobial agents, 2) doping ZnO with other elements, and 3) tuning the particle size, morphological characteristics, and concentration of ZnO NPs. Nevertheless, ZnO NP applications are limited by their low toxicity to some bacterial species. Moreover, new modification techniques, such as 1) ion doping and 2) NP polymer conjugation, have been proposed. The development of ZnO-based antimicrobial agents as an alternative to traditional antibiotics may be promising materials for future applications in pharmaceutics and medicine. Acknowledgments This work was partially supported by the National Basic Research Program of China (973 Program, 2013CB632402), National Natural Science Foundation of China (51602207, 51320105001, 51372190, 21573170, and 21433007), NSFHB (No. 2015CFA001), Self-determined and Innovative Research Funds of SKLWUT (2017ZD-4). This work was also partially supported by the Internal Research Grant (R3754) in The Education University of Hong Kong. References [1] P.R. Gogate, A.B. Pandit, A review of imperative technologies for wastewater treatment I: oxidation technologies at ambient conditions, Adv. Environ. Res. 8 (2004) 501e551. [2] J. Low, J. Yu, W. Ho, Graphene-based photocatalysts for CO2 reduction to solar fuel, J. Phys. Chem. Lett. 6 (2015) 4244e4251. [3] M. Liu, J. Xu, B. Cheng, W. Ho, J. Yu, Synthesis and adsorption performance of Mg(OH)2 hexagonal nanosheetegraphene oxide composites, Appl. Surf. Sci. 332 (2015) 121e129. [4] R.R. Kumar, R. Ramesh, Synthesis, molecular structure and electrochemical properties of nickel(II) benzhydrazone complexes influence of ligand substitution on DNA/protein interaction, antioxidant activity and cytotoxicity, RSC Adv. 5 (2015) 101932e101948. [5] X. Li, J. Yu, M. Jaroniec, Hierarchical photocatalysts, Chem. Soc. Rev. 45 (2016) 2603e2636. [6] S. Wang, X. Yang, X. Zhang, X. Ding, Z. Yang, K. Dai, H. Chen, A plate-on-plate sandwiched Z-scheme heterojunction photocatalyst: BiOBr-Bi2MoO6 with enhanced photocatalytic performance, Appl. Surf. Sci. 391 (2017) 194e201. [7] Y. Zhang, G. Zhu, M. Hojamberdiev, J. Gao, J. Hao, J. Zhou, P. Liu, Synergistic effect of oxygen vacancy and nitrogen doping on enhancing the photocatalytic activity of Bi2O2CO3 nanosheets with exposed {001} facets for the degradation of organic pollutants, Appl. Surf. Sci. 371 (2016) 231e241. [8] Y. Guo, S. Lin, X. Li, Y. Liu, Amino acids assisted hydrothermal synthesis of hierarchically structured ZnO with enhanced photocatalytic activities, Appl. Surf. Sci. 384 (2016) 83e91. [9] S. Duo, Y. Li, Z. Liu, R. Zhong, T. Liu, H. Xu, Preparation of ZnO from 2 D nanosheets to diverse 1 D nanorods and their structure, surface area, photocurrent, optical and photocatalytic properties by simple hydrothermal synthesis, J. Alloys Compd. 695 (2017) 2563e2579. [10] J. Schneider, M. Matsuoka, M. Takeuchi, J. Zhang, Y. Horiuchi, M. Anpo, D.W. Bahnemann, Understanding TiO2 photocatalysis: mechanisms and materials, Chem. Rev. 114 (2014) 9919e9986. [11] K. Qi, F. Zasada, W. Piskorz, P. Indyka, J. Grybo
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