A Review on the Fabrication of Hierarchical ZnO Nanostructures for Photocatalysis Application

Semiconductor photocatalysis provides potential solutions for many energy and environmental-related issues. Recently, various semiconductors with hierarchical nanostructures have been fabricated to achieve efficient photocatalysts owing to their multiple advantages, such as high surface area, porous structures, as well as enhanced light harvesting. ZnO has been widely investigated and considered as the most promising alternative photocatalyst to TiO2. Herein, we present a review on the fabrication methods, growth mechanisms and photocatalytic applications of hierarchical ZnO nanostructures. Various synthetic strategies and growth mechanisms, including multistep sequential growth routes, template-based synthesis, template-free self-organization and precursor or self-templating strategies, are highlighted. In addition, the fabrication of multicomponent ZnO-based nanocomposites with hierarchical structures is also included. Finally, the application of hierarchical ZnO nanostructures and nanocomposites in typical photocatalytic reactions, such as pollutant degradation and H2 evolution, is reviewed.


Introduction
Various semiconductor nanomaterials have been widely used to solve energy and environment problems with their excellent photocatalytic properties [1][2][3]. In recent years, many researchers have focused on the fabrication of hierarchical semiconductor nanostructures and nanocomposites owing to their advantages, including high surface area, porous structures, as well as enhanced light harvesting, etc. [4]. Particularly, zinc oxide (ZnO) has attracted considerable attention as an efficient and promising candidate in photocatalytic applications because of its low cost, non-toxicity and high quantum yields [5][6][7].
Up to now, several reviews have been published on the fabrication strategy of hierarchical photocatalysts with various morphologies. However, only a small part mentioned the synthesis of hierarchical ZnO materials in photocatalytic application [4,[8][9][10]. In addition, especially in the photocatalytic application of ZnO, most of the previous reviews are limited to low dimension (0D, 1D and 2D) ZnO nanostructures, where reviews about hierarchical ZnO nanostructures are still insufficient.
Herein, we present a review on the fabrication of hierarchical ZnO nanostructures for photocatalytic application. First, the advantages of ZnO hierarchical photocatalysts are discussed. Then, various synthetic strategies towards different hierarchical ZnO nanostructures and nanocomposites dimensional structures were formed, followed by a further chemical reaction process to obtain the final hierarchical structures. For example, Xu et al. [32] reported a two-step synthesis process to prepare hierarchical ZnO nanowire-nanosheet arrays that referred to two processes: (1) the preparation of ZnO nanosheet arrays on conductive glass substrates; and (2) the aqueous chemical growth of ZnO nanowires on the surfaces of the primary ZnO nanosheets (Figure 2). Cheng et al. [33] produced branched ZnO nanowires on conductive glass substrates via a similar strategy. Such hierarchical ZnO nanostructures can also be produced on other substrates, such as ITO, FTO, silicon, etc. [34][35][36][37].  Despite multi-step self-assembly with multi-discontinuous processes having been widely used in the fabrication of hierarchical ZnO structures, problems such as long reaction times and tedious procedures could limit the application for large-scale production. To deal with such problems, multi-step self-assembly completed in one-pot solution synthesis has been developed for efficient fabrication of hierarchical ZnO nanostructures. Using such a strategy, Lu et al. [38] reported a two-step continuous approach in one-pot reaction to construct ZnO 3D superstructures, in which the crystallization and the assembly process were controlled by a steady gas/liquid between H2O2 and dimethylsulfoxide (DMSO) organic solvent. The first step was the assembling of microspheres from ZnO nanorod building blocks, followed by the formation of the 3D superstructure via the connection within those microspheres' units side-by-side in a secondary assembly process. In other cases, hierarchical ZnO nanostructures composed of ZnO nanosheets [39] or ZnO nanorods [40] could also be obtained.
Recently, we reported hierarchical ZnO architectures assembled by nanosheets and nanorods via a facile solution method [41]. In this reaction system, the critical morphology controller is the concentration of OH − . When OH − was contained at 0.33 mol/L, the ZnO nanosheets were first formed as the substrates for the heterogeneous nucleation and growth, and then, ZnO nanorod arrays that were oriented grew on the nanosheets in a secondary assembly process to generate the ZnO nanosheet-nanorod structures in which γ-Zn(OH)2 and ε-Zn(OH)2 acted as the zinc sources for the growth of nanorod arrays ( Figure 3). The above developed approaches allowed the stepwise control of experimental conditions and provided an opportunity for the rational design and synthesis of controlled architectures in nanostructures. Recently, we reported hierarchical ZnO architectures assembled by nanosheets and nanorods via a facile solution method [41]. In this reaction system, the critical morphology controller is the concentration of OH − . When OH − was contained at 0.33 mol/L, the ZnO nanosheets were first formed as the substrates for the heterogeneous nucleation and growth, and then, ZnO nanorod arrays that were oriented grew on the nanosheets in a secondary assembly process to generate the ZnO nanosheet-nanorod structures in which γ-Zn(OH) 2 and ε-Zn(OH) 2 acted as the zinc sources for the growth of nanorod arrays ( Figure 3). The above developed approaches allowed the stepwise control of experimental conditions and provided an opportunity for the rational design and synthesis of controlled architectures in nanostructures. Despite multi-step self-assembly with multi-discontinuous processes having been widely used in the fabrication of hierarchical ZnO structures, problems such as long reaction times and tedious procedures could limit the application for large-scale production. To deal with such problems, multi-step self-assembly completed in one-pot solution synthesis has been developed for efficient fabrication of hierarchical ZnO nanostructures. Using such a strategy, Lu et al. [38] reported a two-step continuous approach in one-pot reaction to construct ZnO 3D superstructures, in which the crystallization and the assembly process were controlled by a steady gas/liquid between H2O2 and dimethylsulfoxide (DMSO) organic solvent. The first step was the assembling of microspheres from ZnO nanorod building blocks, followed by the formation of the 3D superstructure via the connection within those microspheres' units side-by-side in a secondary assembly process. In other cases, hierarchical ZnO nanostructures composed of ZnO nanosheets [39] or ZnO nanorods [40] could also be obtained.
Recently, we reported hierarchical ZnO architectures assembled by nanosheets and nanorods via a facile solution method [41]. In this reaction system, the critical morphology controller is the concentration of OH − . When OH − was contained at 0.33 mol/L, the ZnO nanosheets were first formed as the substrates for the heterogeneous nucleation and growth, and then, ZnO nanorod arrays that were oriented grew on the nanosheets in a secondary assembly process to generate the ZnO nanosheet-nanorod structures in which γ-Zn(OH)2 and ε-Zn(OH)2 acted as the zinc sources for the growth of nanorod arrays ( Figure 3). The above developed approaches allowed the stepwise control of experimental conditions and provided an opportunity for the rational design and synthesis of controlled architectures in nanostructures.

Template-Based Synthesis
Template-based syntheses were most widely used in fabricating hollow structures, porous structures, bio-inspired structures, etc., due to the advantages of well-controlled morphology,

Template-Based Synthesis
Template-based syntheses were most widely used in fabricating hollow structures, porous structures, bio-inspired structures, etc., due to the advantages of well-controlled morphology, large-scale production and diverse templates [42]. Generally, the formation process involves the growth of desired materials on hard templates, such as polymer, silica and carbon, which can be removed by chemical etching or thermal decomposition, or soft templates, such as emulsion micelles and even bubbles [43][44][45][46][47][48][49].

Hierarchical ZnO Hollow Structures
The ZnO hollow structures can be constructed in many forms, such as various hollow spheres, including core-shell, yolk-shell and some other interesting morphologies. For example, Dilger et al. [50] reported yolk-shell and hollow ZnO spheres synthesized by gas phase treatment at different temperatures in which the SiO 2 hard templates could be partially or fully removed. Soft templates can also be selected to assist the formation of hollow structures. Sinha et al. [25] reported that ZnO hollow spheroids could be obtained by using a soft template as water bubbles via a modified hydrothermal method under tungsten light irradiation. In addition, a co-surfactant template, including a triblock copolymer of complex polyethylene oxide-poly-propylene oxide-polyethylene oxide (PEO 20 -PPO 70 -PEO 20 , P 123 ) and absolute ethanol (EtOH), was employed to assist the formation of ZnO hollow spheres by Sun et al. [51]. In another case, Yin et al. [52] proposed a water-soluble biopolymer as soft templates used in producing hollow cage-like superstructures assembled by ZnO nanorods constructed with, and the number of the hollow cages could be adjusted by verifying the reaction time. Typically, the morphology of ZnO double-cage structures is as shown in Figure 4. large-scale production and diverse templates [42]. Generally, the formation process involves the growth of desired materials on hard templates, such as polymer, silica and carbon, which can be removed by chemical etching or thermal decomposition, or soft templates, such as emulsion micelles and even bubbles [43][44][45][46][47][48][49].

Hierarchical ZnO Hollow Structures
The ZnO hollow structures can be constructed in many forms, such as various hollow spheres, including core-shell, yolk-shell and some other interesting morphologies. For example, Dilger et al. [50] reported yolk-shell and hollow ZnO spheres synthesized by gas phase treatment at different temperatures in which the SiO2 hard templates could be partially or fully removed. Soft templates can also be selected to assist the formation of hollow structures. Sinha et al. [25] reported that ZnO hollow spheroids could be obtained by using a soft template as water bubbles via a modified hydrothermal method under tungsten light irradiation. In addition, a co-surfactant template, including a triblock copolymer of complex polyethylene oxide-poly-propylene oxide-polyethylene oxide (PEO20-PPO70-PEO20, P123) and absolute ethanol (EtOH), was employed to assist the formation of ZnO hollow spheres by Sun et al. [51]. In another case, Yin et al. [52] proposed a water-soluble biopolymer as soft templates used in producing hollow cage-like superstructures assembled by ZnO nanorods constructed with, and the number of the hollow cages could be adjusted by verifying the reaction time. Typically, the morphology of ZnO double-cage structures is as shown in Figure 4.

Porous Hierarchical ZnO Nanostructures
Owing to the large active surface areas favoring the diffusion of guest molecules, porous structures have attracted increasing attention, especially the three-dimensionally-ordered macroporous (3DOM) nanostructures [53]. Wang et al. [54] synthesized In-doped ZnO 3DOM structures using PMMA microspheres as templates through a one-step colloidal crystal templating (CCT) approach ( Figure 5). The PMMA hard templates could be easily removed by calcination. In another case, ZnO 3DOM structures were synthesized directly inside the microreactor using opals as the template [55]. The opals were first self-assembled on the channels of the microreactor to generate channels for the Zn source and finally removed by calcination. A similar synthesis strategy was also used to fabricate other ZnO porous structures, such as C-doped ZnO hierarchical structures on an ITO substrate [26]. In addition, through a one-step CCT route, Kim et al. [56] reported surfactant-templated methods to prepare hierarchical ZnO mesoporous structures from a coupling reaction of lauroyl chloride with different amino acids.

Porous Hierarchical ZnO Nanostructures
Owing to the large active surface areas favoring the diffusion of guest molecules, porous structures have attracted increasing attention, especially the three-dimensionally-ordered macroporous (3DOM) nanostructures [53]. Wang et al. [54] synthesized In-doped ZnO 3DOM structures using PMMA microspheres as templates through a one-step colloidal crystal templating (CCT) approach ( Figure 5). The PMMA hard templates could be easily removed by calcination. In another case, ZnO 3DOM structures were synthesized directly inside the microreactor using opals as the template [55]. The opals were first self-assembled on the channels of the microreactor to generate channels for the Zn source and finally removed by calcination. A similar synthesis strategy was also used to fabricate other ZnO porous structures, such as C-doped ZnO hierarchical structures on an ITO substrate [26]. In addition, through a one-step CCT route, Kim et al. [56] reported surfactant-templated methods to prepare hierarchical ZnO mesoporous structures from a coupling reaction of lauroyl chloride with different amino acids.

Bio-Inspired ZnO Hierarchical Structures
Inspired by nature, researchers have developed biomimetic routes to fabricate some interesting artificial structures with certain functions. Using butterfly wings as templates, a biomorphic ZnO replica with hierarchical periodic raster structures was synthesized under a three-step process [57]. Instead of using biological templates, biomimetic ZnO plate twin-crystal periodical arrays were deposited on a patterned substrate that was gelatin assisted under mild conditions [58]. Yin et al. [59] reported a bio-inspired photoelectrode of a ZnO-modified graphene honeycomb film fabricated by a two-step process involving self-assembly of graphene oxide/dimethyldioctadecylammonium (GO/DODA), followed by reduction of GO and in situ growth of ZnO nanorods under hydrothermal treatment. Sun et al. [60] described fish-scale bio-inspired multifunctional ZnO nanostructures that have a similar morphology and structure to the cycloid scales of the Asian Arowana using templates such as PEO20-PPO70-PEO20, P123 and EG co-surfactant. A similar strategy was used for rhabdom-like ZnO microspheres bio-inspired by fly eyes [61].

Template-Free Self-Organization
Template-based methods have been widely used in the fabrication of hierarchical ZnO structures. However, some typical disadvantages, such as a long reaction process, the high cost of templates, some uncontrollable morphological changes during template removal and the presence of heterogeneous impurities, remain [3]. To deal with those drawbacks, hence, convenient and efficient routes such as template-free methods have been developed for the fabrication of hierarchical nanostructures [68,69]. Among them, template-free self-organization is one of the most simple and effective routes to fabricate various hierarchical ZnO structures.
One of the most effective ways to fabricate a ZnO hollow structure is the Ostwald ripening process (ORP), which involves the coarsening of smaller particles into bigger particles [70,71]. Wang and his co-workers produced hollow ZnO microspheres via a one-pot template-free hydrothermal synthesis and found that the hollowness of these microspheres could be controlled by adjusting the zinc source concentration [72] (Figure 6). Ji et al. [73] reported hierarchical nanostructured ZnO dandelion-like hollow spheres synthesized by a one-step solvothermal method. Using a similar route, hierarchical ZnO hollow spheres consisting of nanoparticles were prepared by a microwave-assisted solvothermal method [74].

Bio-Inspired ZnO Hierarchical Structures
Inspired by nature, researchers have developed biomimetic routes to fabricate some interesting artificial structures with certain functions. Using butterfly wings as templates, a biomorphic ZnO replica with hierarchical periodic raster structures was synthesized under a three-step process [57]. Instead of using biological templates, biomimetic ZnO plate twin-crystal periodical arrays were deposited on a patterned substrate that was gelatin assisted under mild conditions [58]. Yin et al. [59] reported a bio-inspired photoelectrode of a ZnO-modified graphene honeycomb film fabricated by a two-step process involving self-assembly of graphene oxide/dimethyldioctadecylammonium (GO/DODA), followed by reduction of GO and in situ growth of ZnO nanorods under hydrothermal treatment. Sun et al. [60] described fish-scale bio-inspired multifunctional ZnO nanostructures that have a similar morphology and structure to the cycloid scales of the Asian Arowana using templates such as PEO 20 -PPO 70 -PEO 20 , P123 and EG co-surfactant. A similar strategy was used for rhabdom-like ZnO microspheres bio-inspired by fly eyes [61].

Template-Free Self-Organization
Template-based methods have been widely used in the fabrication of hierarchical ZnO structures. However, some typical disadvantages, such as a long reaction process, the high cost of templates, some uncontrollable morphological changes during template removal and the presence of heterogeneous impurities, remain [3]. To deal with those drawbacks, hence, convenient and efficient routes such as template-free methods have been developed for the fabrication of hierarchical nanostructures [68,69]. Among them, template-free self-organization is one of the most simple and effective routes to fabricate various hierarchical ZnO structures.
One of the most effective ways to fabricate a ZnO hollow structure is the Ostwald ripening process (ORP), which involves the coarsening of smaller particles into bigger particles [70,71]. Wang and his co-workers produced hollow ZnO microspheres via a one-pot template-free hydrothermal synthesis and found that the hollowness of these microspheres could be controlled by adjusting the zinc source concentration [72] (Figure 6). Ji et al. [73] reported hierarchical nanostructured ZnO dandelion-like hollow spheres synthesized by a one-step solvothermal method. Using a similar route, hierarchical ZnO hollow spheres consisting of nanoparticles were prepared by a microwave-assisted solvothermal method [74]. The self-organization strategy can be also used to fabricate ZnO hierarchical structures assembled by tunable building blocks. For example, ZnO hierarchical structures composed of interconnected and monocrystalline nanosheets can be constructed in an aqueous solution system without any templates under sonochemical treatment at room temperature [75]. The formation of the hierarchical structures is based on oriented attachment and reconstruction. In the reaction synthesis system, thicker, porous and coarse crystallized ZnO sheets were first constructed via oriented attachment of small-sized nanocrystals. After reconstruction, ultrathin, integrated and monocrystalline nanosheets were obtained (Figure 7). Based on the policy, various hierarchical ZnO structures, such as flower-like [76][77][78], comb-like [79], pompon-like [80], nanocrystallite aggregates [81], twin-sphere [82], hyperbranched array [83], hexagonal-pyramid-like microcrystals [84,85], etc., could also be obtained. Pachauri et al. [86] demonstrated various hierarchical structures, including flower-like, viscous-fingers-like and rolling-pin-like, using nanoplatelets as basic building blocks and Yagi-Uda-antenna-like ZnO hierarchical structures deploying nanowires as building blocks obtained under a simple low-temperature chemical bath-based growth procedure. The morphology of such hierarchical structures could be adjusted by the regulation of the precursors.  The self-organization strategy can be also used to fabricate ZnO hierarchical structures assembled by tunable building blocks. For example, ZnO hierarchical structures composed of interconnected and monocrystalline nanosheets can be constructed in an aqueous solution system without any templates under sonochemical treatment at room temperature [75]. The formation of the hierarchical structures is based on oriented attachment and reconstruction. In the reaction synthesis system, thicker, porous and coarse crystallized ZnO sheets were first constructed via oriented attachment of small-sized nanocrystals. After reconstruction, ultrathin, integrated and monocrystalline nanosheets were obtained (Figure 7). Based on the policy, various hierarchical ZnO structures, such as flower-like [76][77][78], comb-like [79], pompon-like [80], nanocrystallite aggregates [81], twin-sphere [82], hyperbranched array [83], hexagonal-pyramid-like microcrystals [84,85], etc., could also be obtained. Pachauri et al. [86] demonstrated various hierarchical structures, including flower-like, viscous-fingers-like and rolling-pin-like, using nanoplatelets as basic building blocks and Yagi-Uda-antenna-like ZnO hierarchical structures deploying nanowires as building blocks obtained under a simple low-temperature chemical bath-based growth procedure. The morphology of such hierarchical structures could be adjusted by the regulation of the precursors. The self-organization strategy can be also used to fabricate ZnO hierarchical structures assembled by tunable building blocks. For example, ZnO hierarchical structures composed of interconnected and monocrystalline nanosheets can be constructed in an aqueous solution system without any templates under sonochemical treatment at room temperature [75]. The formation of the hierarchical structures is based on oriented attachment and reconstruction. In the reaction synthesis system, thicker, porous and coarse crystallized ZnO sheets were first constructed via oriented attachment of small-sized nanocrystals. After reconstruction, ultrathin, integrated and monocrystalline nanosheets were obtained (Figure 7). Based on the policy, various hierarchical ZnO structures, such as flower-like [76][77][78], comb-like [79], pompon-like [80], nanocrystallite aggregates [81], twin-sphere [82], hyperbranched array [83], hexagonal-pyramid-like microcrystals [84,85], etc., could also be obtained. Pachauri et al. [86] demonstrated various hierarchical structures, including flower-like, viscous-fingers-like and rolling-pin-like, using nanoplatelets as basic building blocks and Yagi-Uda-antenna-like ZnO hierarchical structures deploying nanowires as building blocks obtained under a simple low-temperature chemical bath-based growth procedure. The morphology of such hierarchical structures could be adjusted by the regulation of the precursors.   Controllable hierarchical structures can also be prepared by regulating the pH [87] or the concentration of precursors [88]. Recently, we reported the ultra-rapid formation of various hierarchical ZnO structures, such as nanorods-based micro flowers, nanosheet-based microspheres and nanoparticles in star-like assemblies, via a facile solution method [89]. The shape of the nanoscale building blocks of the hierarchical structures could be easily controlled by adjusting the supersaturation depending on the variation of dilution ratios or the [OH − ] to [Zn 2+ ] ratios. With the increasing of supersaturation from 1.26, to 2.34, to 3.51, the morphology of hierarchical ZnO nanostructures transformed from nanorods-based to nanosheets-based and, finally, to nanoparticle-based assemblies, and the formation time of hierarchical ZnO structures deceased from 2 h to 30 s (Figure 8). Owing to the superiorities of template-free methods, further efforts should be made to achieve the large-scale synthesis of various hierarchical structures. Controllable hierarchical structures can also be prepared by regulating the pH [87] or the concentration of precursors [88]. Recently, we reported the ultra-rapid formation of various hierarchical ZnO structures, such as nanorods-based micro flowers, nanosheet-based microspheres and nanoparticles in star-like assemblies, via a facile solution method [89]. The shape of the nanoscale building blocks of the hierarchical structures could be easily controlled by adjusting the supersaturation depending on the variation of dilution ratios or the [OH − ] to [Zn 2+ ] ratios. With the increasing of supersaturation from 1.26, to 2.34, to 3.51, the morphology of hierarchical ZnO nanostructures transformed from nanorods-based to nanosheets-based and, finally, to nanoparticle-based assemblies, and the formation time of hierarchical ZnO structures deceased from 2 h to 30 s (Figure 8). Owing to the superiorities of template-free methods, further efforts should be made to achieve the large-scale synthesis of various hierarchical structures.

Precursor or Self-Templating Strategies
Another typical template-free route is the so-called precursor-derived or self-templating route. The synthesis process is followed by two main steps including the formation of precursors containing specific hierarchical structures followed by the calcination of the precursors to obtain the final products. The precursors used to prepare hierarchical structures are usually classified into inorganic and organic precursors.
Zinc-containing inorganic salts with a specific hierarchical morphology were usually selected to synthesize various hierarchical ZnO nanostructures. For instance, using a layered basic zinc carbonate (LBZC)-containing multi-layered structure as the precursor, mesoporous hierarchical ZnO nanostructures can be obtained as annealing products keeping the morphology, shape and sizes of the precursor LBZC [21]. Since the morphology of materials depends on the selection of precursors, many researchers focused on the facile formation of precursors. Liu et al. [90] introduced a PEG-mediated organic-inorganic interface cooperative self-organization strategy applied to achieve the self-assembly of Zn5(CO3)2(OH)6 nanosheets into flower-like 3D superstructures, then the 3D structures composed of nanosheets transformed into porous ZnO nanosheet-based hierarchical structures without morphology change. Furthermore, using the same precursors, Sinhamahapatra et al. [91] reported 3D hierarchically-porous ZnO architectures constructed of two-dimensional (2D) nanosheets through the calcination of the hydrozincite intermediate. Such flower-like ZnO hierarchical structures were also obtained by using Zn4(OH)6SO4·4H2O as the precursor [92]. For another different morphology of ZnO hierarchical structures, Wang et al. [93] prepared nest-like 3D ZnO porous structures through annealing the zinc hydroxide carbonate precursor, which was obtained by a one-pot hydrothermal process (Figure 9). A three-dimensional hierarchical porous ZnO with a tubular structure has been prepared by calcining a tubular hierarchical hydrozincite precursor [94]. A bladed bundle-like architecture has been fabricated from

Precursor or Self-Templating Strategies
Another typical template-free route is the so-called precursor-derived or self-templating route. The synthesis process is followed by two main steps including the formation of precursors containing specific hierarchical structures followed by the calcination of the precursors to obtain the final products. The precursors used to prepare hierarchical structures are usually classified into inorganic and organic precursors.
Zinc-containing inorganic salts with a specific hierarchical morphology were usually selected to synthesize various hierarchical ZnO nanostructures. For instance, using a layered basic zinc carbonate (LBZC)-containing multi-layered structure as the precursor, mesoporous hierarchical ZnO nanostructures can be obtained as annealing products keeping the morphology, shape and sizes of the precursor LBZC [21]. Since the morphology of materials depends on the selection of precursors, many researchers focused on the facile formation of precursors. Liu et al. [90] introduced a PEG-mediated organic-inorganic interface cooperative self-organization strategy applied to achieve the self-assembly of Zn 5 (CO 3 ) 2 (OH) 6 nanosheets into flower-like 3D superstructures, then the 3D structures composed of nanosheets transformed into porous ZnO nanosheet-based hierarchical structures without morphology change. Furthermore, using the same precursors, Sinhamahapatra et al. [91] reported 3D hierarchically-porous ZnO architectures constructed of two-dimensional (2D) nanosheets through the calcination of the hydrozincite intermediate. Such flower-like ZnO hierarchical structures were also obtained by using Zn 4 (OH) 6 SO 4 ·4H 2 O as the precursor [92]. For another different morphology of ZnO hierarchical structures, Wang et al. [93] prepared nest-like 3D ZnO porous structures through annealing the zinc hydroxide carbonate precursor, which was obtained by a one-pot hydrothermal process (Figure 9). A three-dimensional hierarchical porous ZnO with a tubular structure has been prepared by calcining a tubular hierarchical hydrozincite precursor [94]. A bladed bundle-like architecture has been fabricated from many precursors, such as ZnO·0.33ZnBr 2 ·1.74H 2 O, zinc glycerol and Zn-based hydroxide double salts (Zn-HDS) [95][96][97][98].  Besides those mentioned ZnO hierarchical structures formed from inorganic zincite-based salts, some organic zinc-bearing precursors were also employed. For example, Hong et al. [97] demonstrated a facile approach to prepare sheet-like hierarchical ZnO structures by treating zinc glycerol precursor with a calcination process. Moreover, Yang et al. prepared hierarchical aggregates of ZnO nanoparticles with 3D cubic morphologies via simple heat treatment of MOF-5 under different atmospheric conditions [20]. Furthermore, from the decomposition of the MOF-5 precursor, Li et al. reported hierarchical ZnO parallelepipeds [99]. Thus, it is desirable to obtain more and more interesting hierarchical structures via precursor or self-templating strategies in future research.

Fabrication of Hierarchical ZnO-Based Nanocomposites
To improve the photocatalytic efficiency of ZnO, hierarchical ZnO-based composites have been widely developed by combining semiconductors, metals and carbon materials with ZnO owing to the synergistic effects between the components.
One efficient way to improve the photocatalytic activity of hierarchical ZnO nanostructures is constructing ZnO-based composites with either wide band or narrow band semiconductors. Combining this with the former could efficiently prolong the life span of photoexcited electron-hole pairs and enhance the anti-photocorrosion ability, owing to the synergistic contribution of each unit in the composites [100][101][102]. For example, Xiao et al. [103] prepared branched hierarchical ZnO nanorod-TiO2 nanotube array heterostructures (ZnO NRs/NP-TNTAs) via a two-step assembly method. Compared with ZnO nanorods, the enhanced separation efficiency of the photogenerated electron-hole charge of ZnO NRs/NP-TNTAs was confirmed under photoelectrochemical studies, which led to the enhancement of RhB degradation performance. Various combinations of wide band semiconductors, such as a TiO2 nanobelt/ZnO nanorod hierarchical nanostructure [104], branched hierarchical TiO2/ZnO hierarchical nanostructures [105], ZnO-SnO2 hollow spheres [106] and SnO2-ZnO hierarchical structures with SnO2 back bones and ZnO branches [107] (Figure 10), were developed, which all showed an enhanced photocatalytic property. Despite a certain improvement having been achieved by combining ZnO with other wide band semiconductors, problem such as low efficiency of visible-light absorption for such semiconductors, remain. Hence, to improve the overall energy conversion, narrow band semiconductors were introduced to combine with ZnO hierarchical structures. For, example, Liu et al. [108] reported nanotree-like CdS/ZnO hierarchical composites, where the ZnO nanoseeds were first coated on the surface of CdS via a surface adsorption process, and then, ZnO nanowires were grown on CdS to form the branched assemblies after a reflux process. Under irradiation of visible light, The CdS/ZnO hierarchical composites exhibited enhanced photocatalytic ability compared with both ZnO and CdS nanowires. Besides those mentioned ZnO hierarchical structures formed from inorganic zincite-based salts, some organic zinc-bearing precursors were also employed. For example, Hong et al. [97] demonstrated a facile approach to prepare sheet-like hierarchical ZnO structures by treating zinc glycerol precursor with a calcination process. Moreover, Yang et al. prepared hierarchical aggregates of ZnO nanoparticles with 3D cubic morphologies via simple heat treatment of MOF-5 under different atmospheric conditions [20]. Furthermore, from the decomposition of the MOF-5 precursor, Li et al. reported hierarchical ZnO parallelepipeds [99]. Thus, it is desirable to obtain more and more interesting hierarchical structures via precursor or self-templating strategies in future research.

Fabrication of Hierarchical ZnO-Based Nanocomposites
To improve the photocatalytic efficiency of ZnO, hierarchical ZnO-based composites have been widely developed by combining semiconductors, metals and carbon materials with ZnO owing to the synergistic effects between the components.
One efficient way to improve the photocatalytic activity of hierarchical ZnO nanostructures is constructing ZnO-based composites with either wide band or narrow band semiconductors. Combining this with the former could efficiently prolong the life span of photoexcited electron-hole pairs and enhance the anti-photocorrosion ability, owing to the synergistic contribution of each unit in the composites [100][101][102]. For example, Xiao et al. [103] prepared branched hierarchical ZnO nanorod-TiO 2 nanotube array heterostructures (ZnO NRs/NP-TNTAs) via a two-step assembly method. Compared with ZnO nanorods, the enhanced separation efficiency of the photogenerated electron-hole charge of ZnO NRs/NP-TNTAs was confirmed under photoelectrochemical studies, which led to the enhancement of RhB degradation performance. Various combinations of wide band semiconductors, such as a TiO 2 nanobelt/ZnO nanorod hierarchical nanostructure [104], branched hierarchical TiO 2 /ZnO hierarchical nanostructures [105], ZnO-SnO 2 hollow spheres [106] and SnO 2 -ZnO hierarchical structures with SnO 2 back bones and ZnO branches [107] (Figure 10), were developed, which all showed an enhanced photocatalytic property. Despite a certain improvement having been achieved by combining ZnO with other wide band semiconductors, problem such as low efficiency of visible-light absorption for such semiconductors, remain. Hence, to improve the overall energy conversion, narrow band semiconductors were introduced to combine with ZnO hierarchical structures. For, example, Liu et al. [108] reported nanotree-like CdS/ZnO hierarchical composites, where the ZnO nanoseeds were first coated on the surface of CdS via a surface adsorption process, and then, ZnO nanowires were grown on CdS to form the branched assemblies after a reflux process. Under irradiation of visible light, The CdS/ZnO hierarchical composites exhibited enhanced photocatalytic ability compared with both ZnO and CdS nanowires. Noble metals are also promised to be combined with ZnO hierarchical structures to achieve photocatalytic functionality improvements due to noble metals being able to act as electron-scavenging centers to allow for effective electron-hole pair separation [109,110]. Ahmad et al. [111] synthesized hierarchical flower-like ZnO-Au nanostructures where Au was deposited on ZnO via an electrochemical method. Nanoplate-built ZnO hollow microspheres decorated with Au nanoparticles with enhanced photocatalytic activity were produced by Xia et al. [112] through a facile solvothermal route.
Besides those mentioned, recently, carbon-based materials, such as reduced graphene oxide (rGO) and carbon nanotube (CNT), have been utilized in conjunction with ZnO hierarchical structures owing to their good conductivity and large surface area. For instance, to enhance the photocurrent and photocatalytic activity, Luo et al. [113] reported rGO-hierarchical ZnO hollow sphere composites through a 15-min ultrasonic treatment in which the conjunction between rGO and ZnO could be attributed to electronic interaction between the components. Zhang et al. [114] have prepared a ZnO-CNT heterostructure via a hydrothermal route. ZnO nanowires were grown on modified well-aligned carbon nanotube (CNT) arrays where the pre-deposited ZnO grains on the CNTs served as the nucleation sites for the growth of the ZnO nanowires. Thus, it is expected to develop more efficient and eco-friendly methods to fabricate hierarchical ZnO-based composites in the future.

Photocatalytic Applications of Hierarchical ZnO Nanostructures and Nanocomposites
ZnO has received much attention in many fields of photocatalysis application, such as the degradation and environmental pollutants and H2 generation, owing to its lower cost, non-toxic and efficient photoelectrocatalytic performance [115][116][117][118][119][120][121][122]. Since ZnO (3.37 eV) has almost the same band gap energy as TiO2 (3.2 eV), its photocatalytic capability is anticipated to be similar to that of TiO2. Moreover, ZnO is relatively less expensive compared to TiO2, whereby the usage of TiO2 is uneconomical for photocatalytic application [123].
The simplified photocatalysis degradation or process of semiconductors, such as ZnO, is as follows: (1) excitation of ZnO by UV irradiation; (2) generation of excitons; (3) formation of various reactive oxidative species (ROSs); and (4) oxidation of the organic compounds or reduction of the water by ROSs [124][125][126]. H2 generation is a photoelechemical process in which water can be spilt. However, the wide band gap ZnO (3.37 eV) could be only activated in the ultraviolet (UV) region, which accounts for less than 5% of the total energy of the solar spectrum [127,128]. Moreover, the rapid recombination of photogenerated electron-hole pairs in ZnO often leads to decreased photocatalytic activity [129]. Therefore, various elements, such as N and C, have been doped into the wide-band-gap ZnO hierarchical structures to enhance the solar energy utilization [130]. For example, N-and C-doped ZnO hierarchical photocatalysts have been found to exhibit better Noble metals are also promised to be combined with ZnO hierarchical structures to achieve photocatalytic functionality improvements due to noble metals being able to act as electron-scavenging centers to allow for effective electron-hole pair separation [109,110]. Ahmad et al. [111] synthesized hierarchical flower-like ZnO-Au nanostructures where Au was deposited on ZnO via an electrochemical method. Nanoplate-built ZnO hollow microspheres decorated with Au nanoparticles with enhanced photocatalytic activity were produced by Xia et al. [112] through a facile solvothermal route.
Besides those mentioned, recently, carbon-based materials, such as reduced graphene oxide (rGO) and carbon nanotube (CNT), have been utilized in conjunction with ZnO hierarchical structures owing to their good conductivity and large surface area. For instance, to enhance the photocurrent and photocatalytic activity, Luo et al. [113] reported rGO-hierarchical ZnO hollow sphere composites through a 15-min ultrasonic treatment in which the conjunction between rGO and ZnO could be attributed to electronic interaction between the components. Zhang et al. [114] have prepared a ZnO-CNT heterostructure via a hydrothermal route. ZnO nanowires were grown on modified well-aligned carbon nanotube (CNT) arrays where the pre-deposited ZnO grains on the CNTs served as the nucleation sites for the growth of the ZnO nanowires. Thus, it is expected to develop more efficient and eco-friendly methods to fabricate hierarchical ZnO-based composites in the future.

Photocatalytic Applications of Hierarchical ZnO Nanostructures and Nanocomposites
ZnO has received much attention in many fields of photocatalysis application, such as the degradation and environmental pollutants and H 2 generation, owing to its lower cost, non-toxic and efficient photoelectrocatalytic performance [115][116][117][118][119][120][121][122]. Since ZnO (3.37 eV) has almost the same band gap energy as TiO 2 (3.2 eV), its photocatalytic capability is anticipated to be similar to that of TiO 2 . Moreover, ZnO is relatively less expensive compared to TiO 2 , whereby the usage of TiO 2 is uneconomical for photocatalytic application [123].
The simplified photocatalysis degradation or process of semiconductors, such as ZnO, is as follows: (1) excitation of ZnO by UV irradiation; (2) generation of excitons; (3) formation of various reactive oxidative species (ROSs); and (4) oxidation of the organic compounds or reduction of the water by ROSs [124][125][126]. H 2 generation is a photoelechemical process in which water can be spilt. However, the wide band gap ZnO (3.37 eV) could be only activated in the ultraviolet (UV) region, which accounts for less than 5% of the total energy of the solar spectrum [127,128]. Moreover, the rapid recombination of photogenerated electron-hole pairs in ZnO often leads to decreased photocatalytic activity [129]. Therefore, various elements, such as N and C, have been doped into the wide-band-gap ZnO hierarchical structures to enhance the solar energy utilization [130]. For example, N-and C-doped ZnO hierarchical photocatalysts have been found to exhibit better absorption of light in both visible and ultraviolet regions due to their smaller band gaps [130].
Especially, Liu et al. [90] reported hierarchical flower-like C-doped ZnO superstructures (ZnO flowers) assembled from porous nanosheets, which showed better photocatalytic decomposition of the RhB dye in aqueous solutions than ZnO due to the enhanced light absorption over a wide range of wavelengths. Semiconductors, such as CdS, are usually combined with ZnO to form hierarchical heterostructures for visible light photocatalytic degradation due to their narrow band gaps [108,[131][132][133]. For example, nanoscale tree-like CdS/ZnO nanocomposites with a hierarchical architecture showed the ability for selective oxidation of thioanisole and anaerobic reduction of nitro compound 4-nitroaniline under irradiation of artificial solar light [108]. In addition, hierarchical CdS-decorated 1D ZnO nanorod-2D graphene hybrids exhibit enhanced photocatalytic activity and recycling performance toward selective reduction process under visible light irradiation [29]. In the other case, we recently reported a highly efficient direct Z-scheme Ag 3 PO 4 -ZnO hierarchical photocatalysis by depositing Ag 3 PO4 particles on defect-rich ZnO hierarchical nanosheets used for degeneration of RhB under visible light irradiation [19]. The prepared heterostructured Ag 3 PO 4 -ZnO samples showed higher visible light photocatalytic activity than single-phase ZnO or Ag 3 PO 4 photocatalysts, which was attributed to the efficient charge transfer between ZnO and Ag 3 PO 4 through a synergistic effect of surface oxygen vacancies and Ag 3 PO 4 coupling. Recently, many researchers have focused on the localized surface plasmon resonance (LSPR) effect of metal nanoparticles (NPs) for visible-light-driven plasmonic photocatalysts application [134][135][136][137][138][139][140][141]. For instance, sandwiched ZnO@Au@Cu 2 O nanorod films were synthesized on steel mesh substrates via a simple three-step approach and showed an efficient visible-light photocatalytic performance for the degradation of the MO solution [142].
Those non-doped ZnO hierarchical composites could also be used in the field of H 2 generation [118,122,[143][144][145][146][147][148][149][150]. Barpuzary et al. [143] prepared urchin-like CdS@ZnO hetero-arrays via a template-based method. The core-shell CdS@ZnO nano-urchins showed enhanced hydrogen generation with apparent quantum yields of 15% ( Figure 11). Yu and his co-workers [147] fabricated a ternary heterostructure of CdS/Au/ZnO, through a two-step self-assembly process. The heterostructure of CdS/Au/ZnO showed improved photocatalytic hydrogen evolution rate with 60.8 mmol h −1 , which was 4.5-times higher than the CdS/ZnO heterostructure. Furthermore, Hsu et al. [146] reported that a hierarchical Ag 2 S-coupled ZnO@ZnS core−shell nanorod-decorated metal wire mesh showed higher H 2 production rates reaching 5870 and 168 µmol g −1 h −1 under UV and visible light irradiation than ZnO/metal mesh. Thus, the fabrication of hierarchical ZnO-based heterostructures is a promising strategy to enhance both photocatalytic degradation and H 2 generation. absorption of light in both visible and ultraviolet regions due to their smaller band gaps [130]. Especially, Liu et al. [90] reported hierarchical flower-like C-doped ZnO superstructures (ZnO flowers) assembled from porous nanosheets, which showed better photocatalytic decomposition of the RhB dye in aqueous solutions than ZnO due to the enhanced light absorption over a wide range of wavelengths. Semiconductors, such as CdS, are usually combined with ZnO to form hierarchical heterostructures for visible light photocatalytic degradation due to their narrow band gaps [108,[131][132][133]. For example, nanoscale tree-like CdS/ZnO nanocomposites with a hierarchical architecture showed the ability for selective oxidation of thioanisole and anaerobic reduction of nitro compound 4-nitroaniline under irradiation of artificial solar light [108]. In addition, hierarchical CdS-decorated 1D ZnO nanorod-2D graphene hybrids exhibit enhanced photocatalytic activity and recycling performance toward selective reduction process under visible light irradiation [29]. In the other case, we recently reported a highly efficient direct Z-scheme Ag3PO4-ZnO hierarchical photocatalysis by depositing Ag3PO4 particles on defect-rich ZnO hierarchical nanosheets used for degeneration of RhB under visible light irradiation [19]. The prepared heterostructured Ag3PO4-ZnO samples showed higher visible light photocatalytic activity than single-phase ZnO or Ag3PO4 photocatalysts, which was attributed to the efficient charge transfer between ZnO and Ag3PO4 through a synergistic effect of surface oxygen vacancies and Ag3PO4 coupling. Recently, many researchers have focused on the localized surface plasmon resonance (LSPR) effect of metal nanoparticles (NPs) for visible-light-driven plasmonic photocatalysts application [134][135][136][137][138][139][140][141]. For instance, sandwiched ZnO@Au@Cu2O nanorod films were synthesized on steel mesh substrates via a simple three-step approach and showed an efficient visible-light photocatalytic performance for the degradation of the MO solution [142]. Those non-doped ZnO hierarchical composites could also be used in the field of H2 generation [118,122,[143][144][145][146][147][148][149][150]. Barpuzary et al. [143] prepared urchin-like CdS@ZnO hetero-arrays via a template-based method. The core-shell CdS@ZnO nano-urchins showed enhanced hydrogen generation with apparent quantum yields of 15% ( Figure 11). Yu and his co-workers [147] fabricated a ternary heterostructure of CdS/Au/ZnO, through a two-step self-assembly process. The heterostructure of CdS/Au/ZnO showed improved photocatalytic hydrogen evolution rate with 60.8 mmol h −1 , which was 4.5-times higher than the CdS/ZnO heterostructure. Furthermore, Hsu et al. [146] reported that a hierarchical Ag2S-coupled ZnO@ZnS core−shell nanorod-decorated metal wire mesh showed higher H2 production rates reaching 5870 and 168 μmol g -1 h −1 under UV and visible light irradiation than ZnO/metal mesh. Thus, the fabrication of hierarchical ZnO-based heterostructures is a promising strategy to enhance both photocatalytic degradation and H2 generation.

Conclusions and Outlook
In this review, we comprehensively discussed the recent development in the synthesis routes of hierarchical ZnO nanostructures, as well as their photocatalytic potentiality. A variety of tailored hierarchical nanostructures were studied by researchers with logical design of experiment procedures. Especially, rational establishment of facile template-free synthesis technologies could be highly efficient and environmentally benign for the construction of hierarchical ZnO-based nanostructures. In addition, as photocatalysts, hierarchical ZnO-based composites combined with narrow band semiconductors or noble metals exhibited enhanced photocatalytic performance under visible light irradiation owing to their superiorities of high surface area, porosity and the synergisms among them, which led to the improvement of light utilization and charge-transfer properties. Therefore, such composites efficiently obtained under a proper template-free approach play important roles in environment and energy applications.
Although many significant achievements have been made in the synthesis of hierarchical ZnO-based nanostructures, further efforts are required to solve problems, such as small-scale, low-yield production of hierarchical ZnO-based materials, the unclear interaction mechanisms between building units and the low solar light utilization. In the future, the green, cost-effective and industry-scale template-free synthesis of hierarchical ZnO-based photocatalysts would be highly desirable. Furthermore, besides photocatalysis applications, the development of the novel syntheses of hierarchical ZnO nanostructures is also expected to lead to multiple potential applications in the fields of sensors, solar cells, electronic or photoelectrochemical devices.