Tailoring Structure: Current Design Strategies and Emerging Trends to Hierarchical Catalysts

Abstract

Nature mimicking implies the design of nanostructured materials, which can be assembled into a hierarchical structure, thus outperforming the features of the neat components because of their multiple length scale organization. This approach can be effectively exploited for the design of advanced photocatalysts with superior catalytic activity for energy and environment applications with considerable development in the recent six years. In this context, we propose a review on the state of the art for hierarchical photocatalyst production. Particularly, different synthesis strategies are presented, including template-free structuring, and organic, inorganic, and hybrid templating. Furthermore, emerging approaches based on hybrid and bio-waste templating are also highlighted. Finally, a critical comparison among available methods is carried out based on the envisaged application.

1. Introduction

“Learning from Nature” is always a winning strategy for materials scientists. Indeed, natural, and biological systems exhibit a multitude of properties and functions, often tailored for a specific application. Thus, they may be source of imitation for the design of high performance materials, able to meet rising ambitious industrial needs, following a biomimetic approach [1,2,3,4]. What makes bioavailable materials so interesting is that they often share a hierarchical structure made by a self-organization of molecular building blocks that are assembled with other phases, which in turn are self- organized at increasing size levels ranging from the nanoscale up to the macroscale [2,5,6,7]. The complex interplay between structure, morphology, and surface chemistry leads to a wide range of outstanding properties including superhydrophobicity (ex-lotus leaves [8,9]), improved mechanical performances (ex-bird feathers [10,11]), and unique optical behavior (ex-butterfly wings [12,13]). The appealing perspective to translate nature examples into industrial and technological devices steers research towards the synthesis of hierarchical nanomaterials. Indeed, intriguing features of nanostructured systems can be further improved if they are built up into hierarchically structured materials (HSPM), which exhibit a porous architecture, consisting of interconnected pores of different length scales from micro- (<2 nm), meso- (2–50 nm), to macropores (>50 nm), following bimodal or even trimodal pore size distributions [14]. Morphological features such as interconnected multilevel hierarchical porosity, high surface area, and large accessible space are responsible for enhanced light harvesting, electron and ion transport, mass loading, and diffusion [14,15].

Such outstanding properties confer to HSPM a pivotal role in a great number of technological fields including energy storage and conversion [16], catalysis [17] and photocatalysis [18], water remediation [19], separation [20], sensing [21], and biomedicine [22]. Indeed, the variety of physico-chemical characteristics exhibited by these nature-inspired materials can be modified by tuning surface chemistry and morphological properties such as surface area and pore distribution. Notably, a three-dimensional multimodal extended pore structure would be ideal for application in heterogeneous catalysis. The combination of different levels of intraparticle porosity, as well as the presence of mesopores, improve the diffusion of reactants and products towards the active sites and the external surface of the catalyst, respectively, and reduce the length of pore channels, resulting in decreased diffusion barriers in the solid volume [23,24,25,26]. Moreover, the large surface area (from 100 to thousands m²/g [26,27]) allows for better accessibility and uniformity of active sites in the catalysts, causing an improvement in the surface reaction kinetics [15]. Building up a deeply extended multimodal porous architecture in semiconductors paves the way to the design of more efficient photocatalysts [28] by overcoming the typical limits that obstacle their performances such as short lifetime of photogenerated electron-hole pairs [29], partial utilization of solar light [30], low safety, high cost, and poor chemical stability [31]. Widespread application of photocatalysis accounts for huge technological potential of hierarchical photocatalysts, prompting scientific interest towards the design and use of these systems. Accordingly, a great number of outstanding studies has been published on this subject in the last years, which have been overviewed in different reviews [15,32,33,34,35,36,37,38].

Indeed, the number of articles on hierarchical photocatalysts have been constantly growing so far and so has the sensitivity towards environmental energetic issues and sustainability [39,40,41,42,43].

This has prompted the flourishing of several systems produced according to sustainable logics, some of them with exotic structures and unusual compositions, in the attempt to improve photocatalytic performance compared to state of the art [43,44,45,46].

Thus, we believe that a review summarizing the most recent advances and strategies in the design of hierarchical photocatalysts is timely and could enable more conscious development of these systems. First, the advantages of hierarchical photocatalysts are shortly illustrated. Then, the most promising strategies for their fabrication are systematically reviewed focusing on the most outstanding studies published in the period between 2015 and 2021. Particularly, as a strong element of novelty with respect to previous reviews published on this subject, emerging manufacturing approaches are highlighted, including the use of hybrid templates, such as metallorganic frameworks (MOFs). Furthermore, special emphasis is placed on the use of bioavailable structure directing agents, even derived from bio-waste valorization, according to a circular approach. Notably, the mechanism accounting for improved photocatalytic activity has been described to highlight the strength of each synthesis approach and provide scientists involved in this field with clear indication on the key parameters that can be easily manipulated to optimize catalytic performance. To this purpose, modification strategies, including doping and co-doping, the use of co-catalysts, and heterostructures formation have been illustrated. In addition, the use of emerging non-conventional energy sources in the synthesis of hierarchical nanostructures has been shortly overviewed. Furthermore, main applications in environmental and energetic fields, devoted to pollutants removal and hydrogen production, are reviewed and tables reporting collected results on photocatalytic performance were created to enable straight comparison among systems produced using similar or different synthesis strategies. These have been analyzed highlighting pros and cons to get interested scientists more conscious with the opportunities offered by each approach. Moreover, main unaddressed issues and future challenges for both fundamental investigation and technological application have been envisaged. We hope that the review can be useful to stimulate more focused and fruitful research in this field and can open new routes in the development of more effective photocatalytic systems.

2. Hierarchical Photocatalysts

The expression hierarchical photocatalysts usually refers to nanostructured semiconductors having multidimensional domains at different size levels and multimodal pore structure [16].

Since photocatalysis involves a series of complicated interconnected phenomena including light absorption, charge excitation/separation, charge migration, transport and recombination, and charge utilization, the overall efficiency of a photocatalyst is calculated as the product of the individual efficiency of each phenomenon [15]:

η_c = η_abs ∙η_cs ∙η_cmt ∙η_cu,

(1)

It appears evident that a decrease in any singular efficiency would contribute to the decay of the overall efficiency. Hierarchical photocatalysts exhibit a series of advantages with respect to traditional photocatalysts, which are summarized in Figure 1 and include higher surface area, enhanced molecular diffusion/transfer, and improved light absorption, among others (Figure 1).

Indeed, properly tuning the pore hierarchy can increase the number of light traveling paths within the catalyst, resulting in multiple reflections as in hollow structures or enhanced light scattering phenomena, thus improving the absorption efficiency (Figure 2) [15,47].

In addition, high surface area enables heterojunction formation and promotes surface reactions as well as charge separation. Furthermore, nanosized building blocks reduce migration distance of charge carriers (Figure 1). To date, pristine and doped TiO₂ [28,48,49,50,51,52,53,54,55], ZnO [56,57,58,59,60,61], CeO₂ [62,63], and graphene-blended hierarchical photocatalysts [64,65,66] have been synthesized in multiple shapes and morphologies such as nanospheres [67,68], nanoflowers [69,70,71,72,73,74,75,76], nanosheets [77,78,79,80,81], urchin-like nanostructures [68,82,83,84,85,86,87], and nanoflakes [88,89]. The effort to produce efficient hierarchical nanostructured photocatalysts materialized into a heterogeneous framework of synthesis strategies so far, all of them groupable into two macro-categories: templating strategies and template-free methods. Herein, we present an overview of the latest produced hierarchically nanostructured photocatalysts, focusing on the different synthesis strategies which have been implemented to develop them. Our sincere opinion is that this review can shed light on the most promising approaches that could be exploited to design cutting-edge photocatalysts with impressive performance in a huge range of energy and environmental applications, including H₂ production, water remediation, and organic pollutant degradation.

Synthesis Approaches to Hierarchical Structures

The main synthesis strategies to hierarchical semiconductors are overviewed in Figure 3 and can be grouped into three wide categories: templating approaches, templated free methods, and post-synthesis approaches.

Templated approaches are the most employed methods to obtain hierarchical materials, offering good reproducibility, large scale application, as well as tailored structures, due to the huge range of available templates. These approaches can be further distinguished into hard templating, soft templating, and bio-templating strategies.

In hard templating methods, also known as nanocasting processes, either inorganic or organic structures with rigid shape are employed as sacrificial templates to build up hierarchical framework [90]. These are subsequently removed through either selective leaching or thermal decomposition to achieve the final porous structure.

As major drawbacks, the use of corrosive and harmful leaching agents or high energy demand hamper large scale applications of these processes.

Contrarily to hard templates, soft templates are highly deformable and more easily removable. They include block copolymers (e.g., Pluronics P123, F127, PEG), microemulsions (micelles and vesicles), ionic liquids [91] hydrogels, surfactants (e.g., sodium dedecyl sulfate (SDS) and cetyl trimethylammonium bromide (CTAB)), and even gas bubbles [92]. On the other hand, bioavailable moieties are emerging as a cheaper and more sustainable alternative.

3. Inorganic Templates

Inorganic template-assisted synthesis is widely used for generation of multi-modal and hierarchical pores in the desired products. Generally, during this synthesis strategy, the catalyst precursors are filled into preexisting templates with suitable pore size, followed by subsequent removal of sacrificial framework thus generating a stable porous structure with uniform pore distribution. The final porous architecture can be controlled tuning the pore size and the wall thickness of hard templates. Most used templates include silica and ice as hard pore-directing agents (

Table 1. Hierarchical catalysts produced by using inorganic template(s).

Composition	Synthesis Strategy	Morphology	SSA (m2/g)	Application	Activity	Irradiation	Template(s)	Reference
SrTiO3/TiO2@carbon	hydrothermal	heterostructure with tri-modal (micro-, meso-, macro-) pores	93–417	photocatalytic hydrogen production from water splitting, methylene blue degradation	2.52 mmol h−1 g−1 100% Degradation	UV	KOH	[93]
PbTiO3/TiO2/carbon	dual template	quasi-1D nanoneedle	277–374	photocatalytic and photoelectrochemical performances	MB degradation r 100% Under UV, 75% Under Visible+ ultrasound radiation the rates of hydrogen generation are 2360 and 9.6 μmol−1 g−1	UV-Vis	ice/silica hard templates	[94]
N–TiO2/C	dropping	flower-like in hierarchical porous structure	217–407	MB degradation and hydrogen production	2.0 and 48 times of benchmark P25 for MB degradation: 95% under UV and 92% under Vis irradiation and the hydrogen production rates are as high as 2.832 and 0.038 mmol g−1 h−1	UV-Vis	ice/silica hard templates	[95]
Metal oxide/C	Dual-template method followed by heat treatment	hollow “dragon-bone” structure	14–375	methylene blue degradation	100% (UV) 90% (Vis)	UV-Vis	ice/silica hard templates	[96]
TiO2	hydrothermal	3D hierarchical porous core-shell	13.5–42.3	Adsorption and mineralization of tetracycline hydrochloride	up to 70%	UV-Vis	silica	[97]

). Worthy of note is a bi-templated method, proposed to produce both SrTiO₃/TiO₂/C (STC) (Figure 4) [93] and PbTiO₃/TiO₂/carbon (PTC) [94] heterostructures with a tri-modal micro-/meso-/macro-porosity. This was obtained by combining freeze casting process with the use of silica colloid as the hard template, then followed by an appropriate pyrolysis treatment. Particularly, ice and silica templates were used to obtain macro- and meso-pores, while pyrolysis introduced micropores. The proposed strategy was successful in tuning microstructure of both synthesized STC and PTC architectures leading to excellent photocatalytic performances in the degradation of methylene blue under UV light irradiation. In addition to the organic dye degradation, the photochemical reactivity of the STC samples was further investigated in the photocatalytic hydrogen production from water splitting resulting in a H₂ production rate as high as 2.52 mmol h⁻¹ g⁻¹ under UV irradiation.

A similar procedure was also employed to fabricate porous N doped TiO₂/C nanocomposites [95]. In this case, the ice/silica hard templates and urea pyrolysis were used to obtain flower-like hierarchical pores at micro-, meso-, and macro-scale and the resulting materials showed highly improved photocatalytic activity for both methylene blue degradation and hydrogen production under both UV and visible light irradiation. Furthermore, this approach was exploited to synthetize other metal oxide systems (TiO₂ and ZrO₂) [96], and in all cases the corresponding hierarchical carbon-based composites showed superior photocatalytic activity towards methylene blue degradation compared with the control samples. Silica, as mesopore structuring agent, was also exploited to produce hierarchical structure of porous core-shell homojunction constructed by crystalline and amorphous TiO₂ [97]. Combining the amorphous TiO₂ shell with the mesoporous rutile crystals resulted in a simultaneously enhancement of adsorption ability, removal rate, and mineralization efficiency, under UV irradiation, of Tetracycline hydrochloride (TCH) chosen as antibiotic model. The systems developed are shown in Table 1.

4. Organic Templates

A great number of different templating agents have been proposed for the synthesis of hierarchically porous materials and a widespread attention has been recently focused on the use of organic (Table 2) and/or bioavailable soft templates (Table 3). Biogenic-templated catalyst which owns intrinsic hierarchical, multi-dimensional, and multi-level structure holds promising potency to enhance light-harvesting and photocatalytic performance.

Hierarchically ordered macro–mesoporous anatase TiO₂ was successfully produced by Zhao et al. [98] using triblock copolymer P123 and natural pearl oyster shell as dual templates. Inexpensive and environment-friendly pearl oyster shell is exploited as bio-template for the fabrication of macroporous structures and at the same time P123 is employed as a mesopore-directing agent to produce highly ordered mesopores. The authors investigated the obtained materials after calcination at different temperatures and demonstrated that the resultant crystalline anatase structure is made of both macropores and mesopores which are well-preserved after calcination at 350 °C or 450 °C. The photocatalytic activities of the produced samples were monitored through the photodegradation of an aqueous Rhodamine B (RhB) solution under UV irradiation. The highest photocatalytic activity was obtained in the case of TiO₂ sample calcined at 450 °C, with a degradation of RhB of about 90%, because of the presence of hierarchical macro–mesoporous structures, high specific surface areas, and the anatase phase. Among bioavailable sources, bio-wastes are emerging as abundant and cheap templates to build up photoactive hierarchical structures, following a waste to wealth approach. Indeed, because of intrinsic lightness, some bio-wastes can drive formation of water floating photocatalysts, which exhibit remarkable advantages over powder suspensions, in terms of efficient recycling as well as high light harvesting efficiency due to their proximity to air-solution interface [99].

As a proof of concept, bio-waste poplar catkin was combined with triazine-based porous organic polymer to obtain a water-floatable photocatalyst for water remediation, exhibiting relevant sunlight activity towards Cr(VI) and methylene blue decontamination (Figure 5) [100].

Among biowastes, rice husk has attracted great research interest because of its unique and hierarchically porous structure with excellent mechanical property and extremely high light-harvesting and exploitation efficiency.

Rice husk was used by Chen et al. [101] as main raw material to form hybrid silica-carbon bio-template to fabricate porous TiO₂ anatase.

A detailed study was reported on the influence of calcination temperature and impregnation times on the hierarchical porous structures and photocatalytic activity of the synthesized materials. Porous structures with 1D nanostructures network connection showed efficient light-harvesting and photocatalytic degradation of Rh B (120 mg L⁻¹) which was completely removed in just 180 min. Furthermore, the observed high thermal stability opened new perspectives in the use of these materials in a wide range of applications including photocatalysis, catalysis, solar-cell, separation, and purification processes.

Biotemplated synthesis of hierarchical α-Fe₂O₃ fibers was proposed by Chen et al. [102], using cotton as biological template. The original structure of cotton was preserved in the final iron oxide fibers which also exhibit certain unexpected magnetic behavior; in fact, common α-Fe2O3 is not magnetic. In addition, superior photocatalytic decolorating performances were observed under visible light irradiation, up to 96.8% methylene blue degraded in only 200 min. Moreover, no loss of catalytic activity was appreciated after three uses, proving high stability. Indeed, this study evidenced the key role of hierarchical porosity in catalytic activity, with fibrous structures acting as effective catalytic centers for organic dye elimination. Recently, electrospinning fiber formation technology has emerged as a powerful and straightforward tool for tailoring material structures and assembling hierarchical architectures [103,104]. There is plenty of scope for improving photocatalytic properties if this methodology is combined with more conventional processes including solvothermal and microemulsion synthesis.

In this context, hierarchical titanium dioxide nanofibers with distinctive microstructures exhibiting mixed rutile/anatase crystalline phase were successfully fabricated via microemulsion electrospinning approach by Zhang et al. [105]. More in detail, in order to make the microemulsion, paraffin was used as oil phase, while tetrabutyl titanate was dissolved in an alcoholic acid solution (containing a mixture of CTAB and PVP) as continuous phase.

The structure of porous nanofibers was regulated by changing the ratio of continuous phase and oil phase. Different morphologies such as multi-channel, hollow, irregular hollow TiO₂ nanofibers were obtained by varying the ratio between the TiO₂ precursor, tetrabutyl titanate (TBT), and paraffin oil [105]. In addition, the authors demonstrated that brittle nanostructure with abundant mesopores were formed when the ratio of TBT/paraffin oil decreased to 1. Compared with solid TiO₂ nanofibers, all samples prepared by microemulsion electrospinning had improved photocatalytic performances towards the MB degradation UV irradiation. These results can be attributed to a combination of multiple structure dependent effects: first, porosity plays a vital role in absorbing photoelectron transition from rutile to anatase. In addition, photocatalyst with higher porosity can absorb more oxygen moieties on the surface which react with photoelectron, thus avoiding recombination and resulting in more electron and holes available for photocatalytic process (Figure 6). Finally, mixed anatase and rutile crystalline structure are expected to enhance charge separation ultimately improving photocatalytic performance

A soft template approach was exploited by Zhou et al. [106] to produce three-dimensional (3D) flower-like β-Bi₂O₃/Bi₂O₂CO₃ heterojunction photocatalyst. A composite soft template composed of DL-aspartic acid (DLAA) and Pluronic F123 was employed to fabricate a 3D flower-like Bi containing micro/nanomaterial, in which DLAA acted as both coordination- and structure-directing agent to control the hierarchical structure, while F127 acted as a capping agent.

For the first time, the β-Bi₂O₃/Bi₂O₂CO₃ p–n heterojunction photocatalyst was applied in the simulated-sunlight-driven photodegradation of antibiotic agent tetracycline (TC). The photocatalyst obtained after calcination at 290 °C showed relevant photocatalytic performance with 98.79% TC degradation being achieved within 60 min of irradiation. The obtained results confirm that the combination between the narrow band gap, heterojunction structure, and 3D hierarchical structure results in excellent photocatalytic performances. In particular, •OH and h+ proved to be the main active species in TC photodegradation process. Finally, the β-Bi₂O₃/Bi₂O₂CO₃ heterojunction catalyst was not photo-corroded after six consecutive cycles, suggesting very high photostability (Figure 7).

Among bioavailable polymers, cellulose appears to be a promising green bio-template for porous hierarchical structure synthesis, because of its intrinsic hierarchical structure [107,108]. It was successfully exploited to produce H₃PW₁₂O₄₀/TiO₂ nanocomposite with remarkable activity towards organic pollutant removal under sunlight [107].

A non-solvent induced phase separation (NIPS) technique was explored by Sun et al. [108]. One-pot route towards active TiO₂ doped hierarchically porous cellulose provided for highly efficient photocatalysts for methylene blue degradation [108]. Ethyl acetate was chosen as the non-solvent with the aim to induce the phase separation of cellulose into a cellulose/LiCl/N,N-dimethylacetamide solution containing TiO₂ nanoparticles. The subsequent solvent exchange/freeze-drying treatment allowed to obtain cellulose/TiO₂ composite monoliths featuring large surface area and hierarchically porous structures with two kinds of interconnected macropores. The cellulose/TiO₂ monoliths showed high efficiency of photocatalytic activity in the decomposition of methylene blue dye up to 99% within 60 min under UV light. This behavior is probably due to high adsorption properties of the material itself because of the hierarchical porous structure which improves the contact frequency between photocatalysts and MB. In addition, after 10 cycles, the monoliths retained 90% of the photodegradation efficiency.

Table 2. Hierarchical catalysts produced by using synthesis templates.

Composition	Synthesis Strategy	Morphology	SSA (m2/g)	Application	Activity	Irradiation	Template(s)	Template Nature	Reference
Anatase TiO2	Sol-gel	Hierarchical ordered macro-meso	125.91	photodegradation of aqueous Rhodamine B (RhB)	90%	365 nm	copolymer P12/oyster shell	Soft/hard	[98]
Dimethylglyoxime (DMG)/TiO2/polyacrylonitrile (PAN) nanofiber	Electrospinning	Nanofibers	50–60	Photocatalytic MB degradation efficiency	97%	Visible Light	PAN/PVP	Soft	[103]
Bi2WO6/WO3/PAN	Electrospinning and solvothermal process	Nanofibrous membrane	-	Degradation of cationic pollutants	85% of RhB, 87.8% for BQ and 95.7% for IPA, 96% of MB, 77.4% of chlortetracycline hydrochloride and 90.37% of tetracycline hydrochloride	UV-Vis	Triton-X	Soft	[104]
Anatase/rutile TiO2	microemulsion electrospinning (ME-ES)/pyrolysis	multi-channel irregular mesoporous TiO2 nanofibers (hundreds of nm in diameter)	/	Photodegradation of aqueous methylene Blue	100%	365 nm	CTAB	soft	[105]
β-Bi2O3/Bi2O2CO3	Sol-gel with reflux	Mesoporous Micrometric flower-like structures	27.78	Degradation of tetracycline	98.79%	Sunlight	DL-aspartic acid (DLAA)/Pluronic F123	soft	[106]

Table 2. Hierarchical catalysts produced by using synthesis templates.

Composition	Synthesis Strategy	Morphology	SSA (m2/g)	Application	Activity	Irradiation	Template(s)	Template Nature	Reference
Anatase TiO2	Sol-gel	Hierarchical ordered macro-meso	125.91	photodegradation of aqueous Rhodamine B (RhB)	90%	365 nm	copolymer P12/oyster shell	Soft/hard	[98]
Dimethylglyoxime (DMG)/TiO2/polyacrylonitrile (PAN) nanofiber	Electrospinning	Nanofibers	50–60	Photocatalytic MB degradation efficiency	97%	Visible Light	PAN/PVP	Soft	[103]
Bi2WO6/WO3/PAN	Electrospinning and solvothermal process	Nanofibrous membrane	-	Degradation of cationic pollutants	85% of RhB, 87.8% for BQ and 95.7% for IPA, 96% of MB, 77.4% of chlortetracycline hydrochloride and 90.37% of tetracycline hydrochloride	UV-Vis	Triton-X	Soft	[104]
Anatase/rutile TiO2	microemulsion electrospinning (ME-ES)/pyrolysis	multi-channel irregular mesoporous TiO2 nanofibers (hundreds of nm in diameter)	/	Photodegradation of aqueous methylene Blue	100%	365 nm	CTAB	soft	[105]
β-Bi2O3/Bi2O2CO3	Sol-gel with reflux	Mesoporous Micrometric flower-like structures	27.78	Degradation of tetracycline	98.79%	Sunlight	DL-aspartic acid (DLAA)/Pluronic F123	soft	[106]

Table 3. Hierarchical catalysts produced by using bioavailable templates (biowaste).

Composition	Synthesis Strategy	Morphology	SSA (m2/g)	Application	Activity	Irradiation	Template(s)	Template Nature	Reference
PC/POP	hydrothermal	Self-supporting film-like structures	122.5	Cr(IV) photoreduction/methylene blue photodegradation	100% Cr (IV) reduction/100% methylen blue photodegradation	Visible light	Poplar catckins	Hard	[100]
α-Fe2O3	Sol-gel	hierarchical porous and fibrous structures (15 μm diameter, hundreds μm length)	51.3	Photodegradation of aqueous methylene Blue	96.8%	Visible light	Cotton	hard	[102]
H3PW12O40/TiO2	Sol-gel & calcination	nanotubes	80.7	Methylene blue photodegradation	95% methylene blue degradation	UV light	Cellulose	soft	[107]
Cellulose-TiO2	non-solvent induced phase separation (NIPS) technique	Macro-Mesoporous composite monoliths	16.96	photodegradation of aqueous methylene Blue	99%	UV lamp	Cellulose	soft	[108]
	-	-

Table 3. Hierarchical catalysts produced by using bioavailable templates (biowaste).

Composition	Synthesis Strategy	Morphology	SSA (m2/g)	Application	Activity	Irradiation	Template(s)	Template Nature	Reference
PC/POP	hydrothermal	Self-supporting film-like structures	122.5	Cr(IV) photoreduction/methylene blue photodegradation	100% Cr (IV) reduction/100% methylen blue photodegradation	Visible light	Poplar catckins	Hard	[100]
α-Fe2O3	Sol-gel	hierarchical porous and fibrous structures (15 μm diameter, hundreds μm length)	51.3	Photodegradation of aqueous methylene Blue	96.8%	Visible light	Cotton	hard	[102]
H3PW12O40/TiO2	Sol-gel & calcination	nanotubes	80.7	Methylene blue photodegradation	95% methylene blue degradation	UV light	Cellulose	soft	[107]
Cellulose-TiO2	non-solvent induced phase separation (NIPS) technique	Macro-Mesoporous composite monoliths	16.96	photodegradation of aqueous methylene Blue	99%	UV lamp	Cellulose	soft	[108]
	-	-

5. Hybrid Templates

At the boundary between organic and inorganic structure directing agents, metal-organic frameworks (MOF) are hugely porous crystalline hybrid structures made of metal clusters and organic binders. They are emerging as powerful and versatile hard templates to obtain hierarchical photocatalysts with tailored structure as well as porosity and function [109] (Table 4).

Indeed, upon decomposition of organic moieties, obtained materials inherit MOF large mesoporous structure allowing high active site exposure and enabling reactant and product exchange as well as fast charge migration to exposed reactive positions, with limited recombination phenomena [109,110]. As a result, MOF-templated photocatalysts are more active than their bulk counterparts and can be exploited in a huge range of energy and environmental applications, including H₂ production [111,112], CO₂ reduction [110,113,114], as well as organic pollutant degradation [115,116,117,118,119]. The main explored technological uses of MOF derived hierarchical photocatalysts are reported in Table 4.

MOF templated approach is extremely versatile and can be exploited to produce different semiconductor compositions. Porous metal oxides are easily obtained by MOF thermal decomposition [109]. Particularly, a Ti-MOF upon calcination leads to TiO₂, composed by both anatase and rutile nanostructured phases, whose intimate interaction suppresses electron-hole recombination and promote photocatalytic activity, which results even better than P25 [120].

Following a similar strategy, Co MOF with 1,4-naphtalenedicarboxylic acid as ligand has been decomposed to obtain Co₃O₄ nanosheets with uniform size distribution and significant CO₂ reduction conversion to CO, with high selectivity (77%) [110]. Moreover, metal oxides produced by MOF decomposition can be easily converted into metal sulfides by sulphuration process [121,122]. This strategy was exploited to obtain hierarchical CdS structures, with higher photocatalytic activity than both nanostructured and bulk counterparts, towards H₂ production by water splitting [109]. Following a similar procedure, ZnCoS solid solution was fabricated, which exhibited high stability and activity because of a wide light absorption range, a great number of catalytic sites, and fast electron migration [122] (Table 4).

However, MOF templated method is time consuming since it is built on two steps, synthesis and calcination, which are difficult to be carried out on a large scale. Furthermore, the criteria to adequately choose MOF precursors must be further investigated to create the desired composition exhibiting hierarchical structure [109]. Indeed, selection of MOFs based on the same metallic cation, but with different structures, can deeply affect the morphology, structure, and overall performance of obtained catalysts. As a general rule, thermal stability is strictly demanded to avoid structure collapse during thermal treatment. Among available composition, MIL(53) meets this requirement, thus it holds huge promise to build controlled multilevel porous structures [109].

A promising strategy to obtain unique hierarchical structures relies on the use of defect-rich MOF [111]. This approach was effectively exploited to obtain hollow C doped CuO structures through pyrolytic degradation of Cu-MOF. In this study, not only did MOF act as a template to tune the structure of the newborn catalytic phase, but if pyrolysis occurred in reducing atmosphere, they could also supply for carbon atoms to modify the catalyst lattice. Both C-doped and C-containing hybrid composites can be produced following this approach and exhibiting relevant photocatalytic activity towards H₂ production as well as pollutants degradation, due to peculiar porous and electronic structure [123,124] (Table 4).

Indeed, due to the controlled high surface area, MOF derived hierarchical structures are suitable platforms to be loaded with dopants that act as co-catalysts and can further improve photocatalytic properties. Apart from doping with C atoms, modification options include metal nanoparticles, encompassing noble metals (Pd, Pt) [114], and also cheaper transition metals (Ni, W, Cu) [125,126,127] as well as metal sulfides [128].

Indeed, doping can be carried out through “extra-situ” approach; in that case, MOF thermal decomposition can provide for either the catalyst lattice (Figure 8) or doping moieties for the preformed catalytic network. This can be further decorated with a proper dopant through hydrothermal treatment (Figure 8) [122,129,130].

Alternatively, in-situ methodology can be also explored, wherein the formation of catalyst backbone from MOF decomposition and dopants growth in the lattice concurrently occur during solvothermal process [126]. Because of MOF huge surface area and controlled porosity, high dopant dispersion and tunable morphology are usually afforded, producing a significant improvement of both catalytic performance and stability [14] thanks to the presence of multifunctional sites and the improved charge carrier separation [127]. Furthermore, concurrent modification with several co-catalysts can be easily carried out to achieve high photocatalytic performance. In this regard, TiO₂ was combined with conductive bimetallic NiCoS-porous carbon shell, which afforded fast charge transport and prompt reaction for hydrogen production [129].

Moreover, MOF templates offer a simple, straightforward, and largely effective strategy in creating heterojunctions within hierarchical structures.

This approach is also successful for sulfide compounds, which usually require multi-step preparation (Figure 8). The large number of heterojunctions promote charge separation and migration and consequently much higher photocatalytic activity than conventional catalysts [131] for a huge range of applications spanning from organic pollutant removal to hydrogen production through water splitting [112,132]. MOF tunable composition and porosity offer the chance to prepare a plethora of hybrid catalysts combining hierarchical structures with p-n heterojunctions for photocatalytic processes (Table 4) [133]. Following this approach, creative and unique morphologies and structures can be achieved, including hierarchical hollow heterostructural cages combined with 2d nanosheets (Figure 9), hollow caps [134], boxes, polyhedra [135], and capsules which can be further modified through hydrothermal approach, reaching outstanding catalytic performance even under visible light.

To exploit advantages of combining different semiconductors, interface must be accurately designed to match band gaps and enable fast charge migrations as well as their easy transfer to reactants. To this purpose an appropriate distribution of reactive sites must be achieved. All these requirements can be easily met through MOF templated approach [136]. As a proof of concept for the efficacy of this strategy, hepitaxially grown MOF-on-MOF heterostructure was exploited as a precursor to produce N-doped C encapsulated pagoda-like CuO–In₂O₃ (In₂O₃/CuO@N-C) micro-rods. They exhibited improved light-absorption efficacy due to peculiar lamellar structure, enhanced separation efficacy of charge carriers thanks to CuO–In₂O₃ p–n heterojunction, and separated reduction and oxidation sites to promote charge carrier transfer to reactants. These features confer outstanding activity for cross-dehydrogenative coupling (CDC) reaction [136].

Due to high surface and reactivity, combination of MOF obtained photocatalysts into heterojunctions can be easily carried out through solvothermal synthesis [137,138]. This strategy was exploited to produce p-n heterostructure NiFe₂O₄/CuInSe₂ catalysts with high activity towards bisphenol-A and resorcinol degradation [139].

Indeed, solvothermal approach appears as a straightforward production route, since moderate temperature during the process get organic ligands degraded, thus straightly leading to the final desired p-n semiconductor [137]. Following this method, even Z-scheme heterojunctions can be easily obtained [138], such as CoSx@CdS polyhedrons, provoking a marked improvement of charge transfer and enabling a high sensitivity toward Hg²⁺ ions detection (Figure 10) [138]. As an alternative, p-n heterojunction can be obtained by thermal treatment of bi-metallic MOF [112].

Moreover, one of the highest strengths of MOF templated approach lies in the opportunity to design complex hierarchical architectures even with multi-level hierarchy. To this regard, MOF can be effectively used to obtain 2D hierarchical nanoarrays, by assembling 2D structures and conductive sheets, which exhibit outstanding catalytic activity because of high catalytic site exposure and fast charge transport [113,117]. As a proof of concept, in-situ MOF derived approach was successfully employed to produce hierarchical Co-Co layered double hydroxide/Ti₃C₂TX nanosheets (Co-Co LDH/TNS) nanoarray through solvothermal process and evidencing relevant activity towards CO₂ photoreduction under visible light (Figure 11) [113].

Highly effective heterojunctions can be also obtained by combining MOF and C₃N₄ sheets via thermal treatment. Following this route, N doped carbon C₃N₄ composite was produced exhibiting relevant activity towards bisphenol A Degradation (Table 4) [117]. Finally, MOF derived multilevel composites exhibit multiple heterojunctions and consequently enhanced catalytic performance even in selective oxidation [116]. Similarly, one-pot calcination process of Zn-Fe mixed MOF produced ZnO/ZnFe₂O₄ hierarchical heterostructures with high photocatalytic activity towards dye degradation and which can be easily recovered from solution because of its ferromagnetic features [119].

Indeed, due to the high versatility of MOF templates, they provide the opportunity to engineer the photocatalyst structure, by combining heterostructure formation with co-catalyst surface modification [121,140]. This occurred in TiO₂ based photocatalyst, which was modified by Co₃O₄ and Ni to promote oxidation and reduction, respectively. Obtained ternary TiO₂/Co₃O₄/Ni photocatalyst disclosed an impressive hydrogen production 8.7 times higher than that of neat TiO₂ under UV-visible irradiation [140]. Because of the high versatility of MOF templated approach, tuning of electronic properties, through heterojunction formation and co-catalyst introduction, is accompanied by fine morphology control. In this regard, hollow CdS/TiO₂ nanohybrids modified with NiS cocatalyst exhibited very good photocatalytic performance towards H₂ production under visible light conditions (Figure 12) [121].

Table 4. Hierarchical catalysts produced by using hybrid templates.

Composition	Synthesis Strategy	Morphology	SSA (m2/g)	Application	Activity	Irradiation	Template(s)	Reference
Rutile/anatase TiO2	Sol-gel	Submicrometric parallelepiped with rounded corners	19	Photocatalytic reduction of water	1394 μmol h−1 g−1 H2 production rate	UV-vis	MIL-125	[120]
Co3O4	Oil bath method	2D hierarchical nanosheets	24.96	Efficient photocatalytic conversion of CO2	39.70 μmol h−1 CO generation	Visible light	Co/1,4-H2NDC	[110]
NiS/CdS/TiO2	Hydrolysis + sulfidation	Porous/hollow structure		Photocatalytic reduction of water	H2 production rate of 2149.15 µmol g−1 h−1	Visible light	NH2-MIL-125	[121]
CdS	Impregnation + pyrolysis	Microporous nanoparticles	119	Photocatalytic reduction of water	634.0 µmol g−1 h−1	Visible light	MIL-53 (Al)	[109]
C-CuO	pyrolysis	Hollow spheres	72.8	Photocatalytic reduction of water	67.3 mmol/g/h H2 production	Visible light	Cu/benzoic acid/1,4 dicarboxybenzene	[111]
C-ZnO	Pyrolysis under N2 atmosphere	ZnO crystals embedded within a porous carbonaceous matrix	500	Adsorption and photodegradation of RhB	100% adsorption efficiency	//	MOF-5	[123]
Pd-Anatase TiO2	pyrolysis	Submicron TiO2 tablets		Photocatalytic reduction of water	979.7/112.7 mol h−1 H2 production	UV/solar light	NH2-MIL-125	[114]
Ni/g-C3N4	Pyrolysis under Ar atmosphere	layered platelets with curled edges	64.9	Photocatalytic reduction of water	H2 production rate of 2989.5 μmol g−1 h−1	Visible light	2D Co-MOF	[125]
W/Co3S4	Hydrothermal treatment	10 mm square sheet arrangement	10.93	Photocatalytic reduction of water	85.7 μmol/h H2 production rate	Visible light	Co-ZIF-9	[126]
Cu/TiO2	Photolysis under N2 atmosphere	Mesoporous core-shell Cu/TiO2 hybrid nanoplatforms	-	Photocatalytic reduction of water	334 μmol g−1 h−1 H2 production	Simulated light	Cu-MOF	[127]
TiO2–NiCoS-PC	Annealing under Ar atmpsphere	Spherical porous carbon shell Embedding TiO2	93.5	Photocatalytic reduction of water	1.29 mmol h−1 g−1	Visible light	NiCo-ZIF	[129]
MoS2−Zn0.5Co0.5S	Hydrothermal	Hollow rhombic dodecahedra	57	Photocatalytic reduction of water	15.47 mmol h−1 g−1	UV-Vis	ZnxCo1−x-ZIF	[122]
MnS/In2S3	Solvothermal	Sub-micro rods	/	photocatalytic CO2 reduction	58 μmol g−1 h−1 CO production rate	Xenon lamp 300 W (credo UV-vis)	MIL-68 (In)	[131]
NiO/CeO2	Hydrothermal and calcination	Porous microsphere	31.1	Dye photodegradation/water splitting	100% MO and MB degradation/71.5 μmol g−1 H2 production rate	UV	Ni/Ce mixed-metal MOF	[112]
Cu/CuO-TiO2	Sol-gel + calcination	nanoparticles	45.3	Photocatalytic reduction of water	286 mmol g−1 h−1 H2 production rate	Solar (Sun)	Cu-MOF	[132]
CdS/ ZnxCo3-xO4	Hydrothermal method	Hollow polyhedra	29.6	Photocatalytic reduction of water	3978.6 μmol g−1 h−1 H2 production rate	Visible	ZnCo-ZIF	[135]
In2O3/CuO@N-C	calcination	Pagoda-like heteroepitaxial micro-rods	147.1	cross-dehydrogenative coupling (CDC)	88–99% yield of reaction	Blue light	MIL-68-In	[136]
CuInSe2/NiFe2O4	hydrothermal	Irregular-shaped nanoparticles	27.54	Bisphenol-A and resorcinol degradation	95–95% removal rate	Visible light	Ni–Fe MOF	[137]
CoSx/CdS	solvothermal	CdS NPs on the surface of CoSx polyhedrons	/	Hg2+ detection	0.010 to 1000 nM Hg2+ detection	Visible light	ZIF-67	[138]
Co-Co LDH/Ti3C2TX	solvothermal	3D nanosheets nanoarray	/	CO2 photoreduction	1.25 × 104 µmol h−1 g−1 CO production rate	Visible light	ZIF-67	[113]
ZIF-NC/g-C3N4	Thermal treatment	ZIF-NC dodecahedra deposited onto g-C3N4 layers	/	bisphenol A degradation	95% removal	Visible light	ZIF-8	[117]
MoS2@Cu/Cu2O@C	Pyrolysis	Hierarchical rough polyhedra	94	Selective oxidation of cyclohexane to KA oil	1.31% conversion rate; 98% selectivity	Visible light	Cu-MOF	[116]
ZnO/ZnFe2O4	calcination	Spherical assemble of flake-like nanosheets	87.74	RhB and MB degradation	100% degradation	UV-vis	Zn(Fe)-MOF	[119]

Table 4. Hierarchical catalysts produced by using hybrid templates.

Composition	Synthesis Strategy	Morphology	SSA (m2/g)	Application	Activity	Irradiation	Template(s)	Reference
Rutile/anatase TiO2	Sol-gel	Submicrometric parallelepiped with rounded corners	19	Photocatalytic reduction of water	1394 μmol h−1 g−1 H2 production rate	UV-vis	MIL-125	[120]
Co3O4	Oil bath method	2D hierarchical nanosheets	24.96	Efficient photocatalytic conversion of CO2	39.70 μmol h−1 CO generation	Visible light	Co/1,4-H2NDC	[110]
NiS/CdS/TiO2	Hydrolysis + sulfidation	Porous/hollow structure		Photocatalytic reduction of water	H2 production rate of 2149.15 µmol g−1 h−1	Visible light	NH2-MIL-125	[121]
CdS	Impregnation + pyrolysis	Microporous nanoparticles	119	Photocatalytic reduction of water	634.0 µmol g−1 h−1	Visible light	MIL-53 (Al)	[109]
C-CuO	pyrolysis	Hollow spheres	72.8	Photocatalytic reduction of water	67.3 mmol/g/h H2 production	Visible light	Cu/benzoic acid/1,4 dicarboxybenzene	[111]
C-ZnO	Pyrolysis under N2 atmosphere	ZnO crystals embedded within a porous carbonaceous matrix	500	Adsorption and photodegradation of RhB	100% adsorption efficiency	//	MOF-5	[123]
Pd-Anatase TiO2	pyrolysis	Submicron TiO2 tablets		Photocatalytic reduction of water	979.7/112.7 mol h−1 H2 production	UV/solar light	NH2-MIL-125	[114]
Ni/g-C3N4	Pyrolysis under Ar atmosphere	layered platelets with curled edges	64.9	Photocatalytic reduction of water	H2 production rate of 2989.5 μmol g−1 h−1	Visible light	2D Co-MOF	[125]
W/Co3S4	Hydrothermal treatment	10 mm square sheet arrangement	10.93	Photocatalytic reduction of water	85.7 μmol/h H2 production rate	Visible light	Co-ZIF-9	[126]
Cu/TiO2	Photolysis under N2 atmosphere	Mesoporous core-shell Cu/TiO2 hybrid nanoplatforms	-	Photocatalytic reduction of water	334 μmol g−1 h−1 H2 production	Simulated light	Cu-MOF	[127]
TiO2–NiCoS-PC	Annealing under Ar atmpsphere	Spherical porous carbon shell Embedding TiO2	93.5	Photocatalytic reduction of water	1.29 mmol h−1 g−1	Visible light	NiCo-ZIF	[129]
MoS2−Zn0.5Co0.5S	Hydrothermal	Hollow rhombic dodecahedra	57	Photocatalytic reduction of water	15.47 mmol h−1 g−1	UV-Vis	ZnxCo1−x-ZIF	[122]
MnS/In2S3	Solvothermal	Sub-micro rods	/	photocatalytic CO2 reduction	58 μmol g−1 h−1 CO production rate	Xenon lamp 300 W (credo UV-vis)	MIL-68 (In)	[131]
NiO/CeO2	Hydrothermal and calcination	Porous microsphere	31.1	Dye photodegradation/water splitting	100% MO and MB degradation/71.5 μmol g−1 H2 production rate	UV	Ni/Ce mixed-metal MOF	[112]
Cu/CuO-TiO2	Sol-gel + calcination	nanoparticles	45.3	Photocatalytic reduction of water	286 mmol g−1 h−1 H2 production rate	Solar (Sun)	Cu-MOF	[132]
CdS/ ZnxCo3-xO4	Hydrothermal method	Hollow polyhedra	29.6	Photocatalytic reduction of water	3978.6 μmol g−1 h−1 H2 production rate	Visible	ZnCo-ZIF	[135]
In2O3/CuO@N-C	calcination	Pagoda-like heteroepitaxial micro-rods	147.1	cross-dehydrogenative coupling (CDC)	88–99% yield of reaction	Blue light	MIL-68-In	[136]
CuInSe2/NiFe2O4	hydrothermal	Irregular-shaped nanoparticles	27.54	Bisphenol-A and resorcinol degradation	95–95% removal rate	Visible light	Ni–Fe MOF	[137]
CoSx/CdS	solvothermal	CdS NPs on the surface of CoSx polyhedrons	/	Hg2+ detection	0.010 to 1000 nM Hg2+ detection	Visible light	ZIF-67	[138]
Co-Co LDH/Ti3C2TX	solvothermal	3D nanosheets nanoarray	/	CO2 photoreduction	1.25 × 104 µmol h−1 g−1 CO production rate	Visible light	ZIF-67	[113]
ZIF-NC/g-C3N4	Thermal treatment	ZIF-NC dodecahedra deposited onto g-C3N4 layers	/	bisphenol A degradation	95% removal	Visible light	ZIF-8	[117]
MoS2@Cu/Cu2O@C	Pyrolysis	Hierarchical rough polyhedra	94	Selective oxidation of cyclohexane to KA oil	1.31% conversion rate; 98% selectivity	Visible light	Cu-MOF	[116]
ZnO/ZnFe2O4	calcination	Spherical assemble of flake-like nanosheets	87.74	RhB and MB degradation	100% degradation	UV-vis	Zn(Fe)-MOF	[119]

6. Template Free Approach

Using a templating agent proved to be effective in building up hierarchical structures, yet it is a labor consuming approach, because of the operations required to remove the backbone.

On the other hand, self-assembly of primary particles into larger structures by oriented aggregation occurring during solvothermal processes can be a more straightforward and greener strategy, since it can potentially avoid toxic chemicals [141]. In these processes, it appears that solvents play a key role in hierarchical morphology formation [142]; indeed, their molecules or, in the case of alcohols, polymers obtained by condensation reactions can adsorb on the particle surface and tune their growth [143]. Solvothermal route has been successfully exploited to obtain hierarchically structured conventional semiconductor oxides including ZnO [60,142] TiO₂ [144,145,146], CeO₂ [147], Cu₂O [148], and more unusual compositions, such as WO₃ [149] and mesoporous Bi₂WO₆ [150]. Furthermore, doping of the main lattice with various ions can be carried out concurrently with its formation [151]. Moreover, a relevant number of heterostructured composites has been easily produced so far through one pot solvothermal treatment; these include combinations of semiconductor oxides (TiO₂, ZnO, CeO₂) with other semiconductors [62] including metal sulfides (SnS₂) [152], carbon nitride [133], or with carbonaceous species (CS). This class encompasses ZnO-graphene [141,153] and TIO₂-CS particles [154] as well as TiO₂ nanotubes wrapped by carbon coatings [155], among others. Furthermore, CoS/CdS heterojunctions [156] and even less conventional compositions, including BiVO₄/Bi₂WO₆ [157] systems, can be easily fabricated. Indeed, the presence of different morphology nanostructures, such as g-C₃N₄ nanosheets and ZnO nanoparticles, in the same batch during the solvothermal treatment appears to be a good strategy promoting self-assembly into hierarchical structures to decrease surface energy [133]. Moreover, synthesis parameters and particularly reactant molar ratio, solvent and precursor nature, as well as temperature and treatment time [145] can tune morphological and microstructural features. Accordingly, a significant range of morphologies can be easily fabricated, including hollow microspheres [141,142,149,155,158,159], nanoplates [156], as well as more exotic arrangements, such as broom-like [151], urchin-like [160], and nest-like [149] structures (Figure 13).

As a matter of fact, photocatalytic activity of hierarchical ZnO obtained by solvothermal process decrease according to the following order of employed precursor: zinc sulfate > zinc nitrate > zinc acetate > zinc chloride [60]. Furthermore, the use of pure ethanol as solvent during solvothermal treatment of TiOSO₄ precursor produces nanoparticle morphology, whereas the addition of ethyl ether and ethanol mixture has morphology changed to microrods with huge surface area (244.4 m² g⁻¹), promoting light scattering and reactivity [155].

Solvothermal hierarchical structures usually feature an interconnected porosity with proper pore size to enable mass transfer as well as improved light harvesting and even fast charge carrier mobility; these properties concur to achieve higher photocatalytic performance than conventional systems [150]. Obtained systems exhibit relevant activity as well as photoelectrochemical performances under UV [141,146] and even visible [145,148,156,157] and full solar irradiation [155] for energy and environmental photocatalysis. Applications range from environmental to energetic field, spanning from pollutant degradation in water and gas phase [144], including dye [62,133,145,146,157] and nitric oxide removal (Table 5) [150,161] to H₂ production [156].

The use of small organic molecules as structure directing agents can aid self-assembly and significant enhance photocatalytic performance, due to the formation of peculiar morphologies, with high degree of crystallinity, high light-harvesting ability, as well as efficient separation of photogenerated carriers [159,161,162]. At the same time, their decomposition can provide a source of doping species for the catalyst including carbon atoms which can also give rise to organized porous frameworks hosting the photoactive phase and contributing to a significant improvement of its performance under UV-irradiation [163]. To this regard, citric acid has been exploited to synthetize urchin-like LaPO₄ hollow spheres, exhibiting marked activity towards CO₂ reduction under near UV [159].

Similarly, the use of N-Acetyl-d-Proline amino acid during hydrothermal treatment produced multi-shelled ZnO microspheres more active for NO oxidation under UV irradiation than other hierarchical ZnO samples [161]. Indeed, low molecular weight compounds can play multiple roles during the hydrothermal process, behaving not only as morphological agents but even as reducing compounds [164,165]. As a matter of fact, L-cysteine and glucose were employed during the hydrothermal treatment to obtain a TiO₂-graphene-MoS₂ composite as a high performance and stable catalyst for CO₂ reduction (Figure 14). Notably, L-cysteine acted as a reducing agent for graphene oxide, whereas glucose played a key role to control morphology, since it formed amorphous carbon which inhibited the TiO₂ and MoS₂ growth (Table 5 and Figure 14) [165].

Template free solvothermal processes can be carried out sequentially [162] or straightforwardly combined with wet impregnation or deposition procedures to have surface of produced catalyst decorated with other active moieties [161,163]. These often include plasmonic metal nanoparticles [163,166] as well as other semiconductors including both inorganic and organic systems [162,167]. The former improve catalytic activity through surface plasmon resonance [163]. The latter enable formation of p-n [161] as well as Z scheme heterostructures (Figure 15) [156,162,167,168], which exhibit superior visible light photocatalytic activity coupled with relevant stability and reusability [168,169].

Among wet chemistry routes, precipitation represents a facile low temperature approach to make hierarchical structures [159,170]. This method was effectively employed to produce CdMoO₄ hollow microspheres which exhibited higher photocatalytic activity towards Rhodamine-B aqueous solution under the UV irradiation than commercial TiO₂ [170].

In some cases, self-assembly into a hierarchical architecture can be promoted by electrostatic aggregation [171]. To this regard, g-C₃N₄/ZnO microspheres were easily produced starting from a physical dispersion of the two components and exploiting their opposite surface charge. Z-scheme obtained by band matching between the two semiconductors as well as enhanced light absorption because of multiple scattering were responsible for a marked improvement of photocatalytic performance towards CO₂ reduction under visible light (Figure 16) [171].

Other template free chemistry routes than solvothermal synthesis can be exploited to obtain hierarchical architectures. These include solid phase processes based on either solid or solid-gas reactions. Notably, solid-state reactions followed by hydrolysis at room temperature proved to be a simple route to make bismuth oxoiodide (BiO) with surface heterojunction between (0 0 1) and (1 1 0) surfaces promoting photocatalytic performance under visible light [172]. Solid-gas phase reactions usually involve pyrolysis [173], decomposition [174], or oxidation [175] and occur at moderate temperatures (higher than 150 °C). In these processes, temperature plays a key role in defining the final structure and overall photocatalytic performance, accordingly [174]. Moreover, either metal or non-metal heteroatom doping [175,176] as well as semiconductor heterostructures [171] can be easily achieved during the thermal process leading to superior photocatalytic performances [175,176]. Following this approach, nanosheet flower-like SnS₂ nanostructures have been obtained and used for Cr(IV) photocatalytic reduction under visible light with high performance as stability [173], even though these methods have been more extensively applied to make C₃N₄ catalysts. As a matter of fact, a facile pyrolysis process with urea was exploited to produce Yttrium-doped graphitic carbon nitride (Y/g-C₃N₄) catalysts, which feature high surface area, poor recombination phenomena, as well as wide spectral absorption accounting for enhanced photocatalytic activity towards rhodamine B degradation under visible light [176]. Alternatively, thermal oxidation and exfoliation, followed by curling condensation were carried out on bulk C₃N₄. Obtained O-doped carbon nitride (g-C₃N₄) nanotubes exhibit outstanding photocatalytic activity towards CO₂ reduction to synthetic fuels, sensitively higher than massive g-C₃N₄ [175].

Indeed, obtaining porous g-C₃N₄ is a challenging task, most of available processes to fabricate being time-consuming and energy intensive. A green and facile alternative consists in a fast quenching from high temperature, which produces an 3D interconnected porous assembly of g-C₃N₄ nano-scrolls, featuring high light absorption, poor recombination phenomena, and enhanced carrier mobility, accounting for improved catalytic performance toward CO₂ reduction [177].

Hierarchical heterostructures between semiconductors can be straightforwardly obtained by simultaneous decomposition of their precursors [171]. Following this route, calcination of tetrabutyl titanate and melamine precursors produced Z-scheme TiO₂/g-C₃N₄ heterojunction with unique features even due to concurrent C and N doping [171].

Template free processes also include combustion synthesis, a simple one-pot method exploiting fuel-oxidizer solutions, such as those containing sorbitol-nitrate mixtures, to fabricate mixed phase hierarchical structures, including bismuth rich-bismuth oxychlorides nanocatalysts, which exhibited high photocatalytic activity towards antibiotic ofloxacin degradation under solar light irradiation [178].

Moreover, non-conventional energy sources can be exploited to build self-templated hierarchical structures. To this regard, pulsed laser deposition (PLD) followed by thermal treatment had hierarchical nanostructured hematite coated onto glass surface and exhibiting enhanced and stable catalytic performance because of high availability of active sites, improved interaction with reactants as well as limited recombination phenomena [179].

Alternatively, ultrasound microwave and ultrasound irradiation can assist wet chemistry processes to promote hierarchical structure formation. Notably, microwave irradiation of copper acetate and thiourea aqueous solutions led to hierarchical CuS structures with tunable porosity and morphology, which evidenced higher activity than commercial CuS catalyst in dye degradation processes under visible light [180,181]. As an alternative, ultrasound irradiation is understood to be a facile and fast yet extremely efficient method to assist particles self-assembly processes into hierarchical architectures [160]. It proved effective in producing Urchin-shaped iron oxide nanoparticles, with huge surface area, up to 282.7 m²/g and which evidenced 25 times faster removal of As(V) and Cr(VI) than commercial Fe₂O₃ (Figure 17) [160].

Similarly, sonochemical treatment of Melamine in water and its further calcination produced hierarchical graphitic-C₃N₄, which was tested for rhodamine B degradation and H₂ production and exhibited 16.7 and 8.7 higher visible light photo-catalytic activity than bulk homologous composition [182].

On the other hand, reactive magnetron sputtering deposition appears to be a facile methodology for large scale fabrication of hierarchical films [183]. This approach was exploited to fabricate two level porosity AgO nanorod arrays, which exhibited excellent photocatalytic properties under UV and visible-light conditions [183].

Table 5. Hierarchical catalysts produced without templates.

Composition	Synthesis Strategy	Morphology	SSA (m2/g)	Application	Activity	Irradiation	Ref
ZnCuCo layered double hydroxide	Hydrothermal	3D flower-like hierarchical morphologies with in- terlaced petal-like nanosheet	86–179	photocatalytic H2 production and degradation of SMZ	H2 production rate (3700 μmol g−1 h−1, 95% sulfamethazine (SMZ) degradation	Visible light	[43]
Ti3C2/Bi2WO6	hydrothermal	2D/2D hetero junction i	33.5–58.3	Photocatalysis degradation of tetracycline hydrochloride (TC-HCl)	0.430 min−1	Visible light	[44]
ZnFe2O4 modified Cu2S	in-situ self-assembly method	Dendritic fractal structure similar to a snowflake	-	photocatalytic reduction of nitrobenzene with and degradation of methyl orange and methylene blue dyes proficiently	98% yield of degradation of nitrobenzene, 94.3 and 86% fordegradation of methyl orange and methylene blue, respectively	Visible light	[45]
ZnO-graphene nanocomposite	Solution route	hollow microspheres	29.7–37.6	Adsorption/photocatalytic activity towards degradation of water-soluble organic pollutants (such as Rhodamine B, methyl orange, phenol)	90% adsorption capacity	UV (254 nm)	[141]
ZnO	Solvothermal route	Monodisperse microspheres	18	Photocatalytic activity for degradation of methylene blue	100%	UV-Vis	[142]
ZnO	Hydrothermal and calcination	Hierarchically porous microspheres composed of nanosheets	46–91	Photocatalytic RhB degradation activity	Up to 100%	solar	[60]
TiO2	Hydrothermal route	(i) long and well oriented macrochannels (ii) surface macropores of 0.8–9.3 μm in size, and (iii) porous walls with pores mostly smaller than 0.5 μm.	82.9–216.1	Decomposition of methyl ethyl ketone (MEK) in a continuous flow photoreactor	Up to 37%	UV (254 nm)	[144]
TiO2	Hydrothermal route	Partial spherical like structure	30–43.7	RhB photodegradation	Up to 98%	Visible light	[145]
TiO2	Hydrothermal route	Nanoflowers with a spherical hierarchical structure	20–80	Aqueous methylene blue photo-oxidation	Up to 50%	solar	[146]
CeO2	Hydrothermal route	Mesoporous nanosphere	42.1–68.2	photocatalytic activity of rhodamine B (RhB) dye degradation	97.8–92% RhB dye degradation	UV-Vis and acidic condition	[147]
Cu2O	Low temperature route	Spherical, cuboctahedral or cubic nanoparticles	7–13	Photocatalytic degradation of the antibiotic trimethoprim	Up to 48%	Visible light	[148]
WO3	Hydrothermal route	Regular-shaped nanosheets with an average thickness of approximately 30–40 nm	2–18	Photocatalytic activity towards an aqueous solution of tetracycline (TC) and possess good stability and reusability	Up to 94%	Visible light	[149]
Bi2WO6	Hydrothermal method and calcining process	Mesoporous nanoplate multi-directional	53.5	Photocatalytic oxidation of NO	Up to 90%	Solar	[150]
Sm, Y, La and Nd-doped CeO2	hydrothermal	broom-like hierarchical structure	-	BPA degradation and on CO2 evolution from CH3CHO decomposition	Up to 99%	UV	[151]
Yttria (Y2O3) nanosphere decorated ceria (CeO2)	Hydrothermal route	Yttrium nanoparticles are anchored on the surface of CeO2 nanorod with a particle size of 10 nm	-	Photocatalytic decomposition of aqueous Rhodamine B	Up to 96%	Solar light	[62]
SnS2/TiO2	Ultrasonic treatment	Ordered channels with a size about 1–3 μm were formed in the particles, and lots of holes appeared on the wall of the channels.	28	Photocatalytic degradation of Methyl Orange (MO)	Up to 90.9%	Solar light	[152]
ZnO/g-C3N4	Solvothermal	Porous microsphere with a size of about 700nm.	9.9–32-7	Photocatalytic degradation on rhodamine B and phenol	Up to 100%	Solar	[133]
ZnO/graphene	Solvothermal	Core-shell structure	65–201	Photocatalytic degradation of rhodamine B	Up to 98.5%	Visible light	[153]
TiO2 Microspheres with Carbonaceous Species	Solvothermal	Porous structure	337	Photodegradation of rhodamine B	Up to 100%	Visible light	[154]
Carbon-coated TiO2	solvothermal	Hierarchical nanotubes	Up to 244.4	Photocatalysts activity for water oxidation	Up to 705 µmol h−1 g−1	Solar light	[155]
S-deficient CoS/CdS	Solvothermal	Hexagonal nanoplates	Up to 84.32	Photocatalytic water-splitting	Up to 14.5%	Visible light	[156]
BiVO4/Bi2WO6	Solvothermal	Self-assembled hierarchical BiVO4/Bi2WO6 heterostructured composites	5.16	Photocatalytic activities for methylene blue (MB) degradation and photoelectrochemical performance	Up to 50%	Visible light	[157]
3D LaPO4	Solution route using citric acid (CA)	Urchin-like hollow sphere	51–124	Photocatalytic CO2-reduction performance	6.8-fold enhancement of the AQY	UV	[159]
ZnO	Hydrothermal-calcinatio n	Mesoporous multi-shelled ZnO microspheres	3–20	Photocatalysis for NO oxidation	Up to 77%	UV	[161]
ZnO/CeO2	Hydrothermal/calcination	Spherically hierarchical structure	41–56	photocatalytic rhodamine B (RhB)	Up to 96%	Visible light	[162]
TiO2/graphene/MoS2	Hydrothermal	2D rGO sheets assembled into macroporous 3D structures	124	CO2 Reduction Photocatalyst	Up to 97%	UV-Vis	[165]
Ag,Au/TiO2	Solvothermal	Mesoporous spherical shape	153–173	Degradation of Textile Dyes	Up to 85%	Solar and visible light	[163]
Au-H-ZnO	Low temperature aqueous reaction and heat treatment	Nanosheets	-	Gas sensing and photocatalytic properties	Up to 94.8%	Visible light	[166]
PDA-modified Zn	Self-assembly	Rough microstructures	32–38	Photocatalytic CO2 reduction	Up to 0.95 µmol h−1 g−1 of CH3OH	UV	[167]
ZnIn2S4 marigold flower/Bi2WO6 (ZIS/BW)	Hydrothermal method followed by wet-impregnation	Hierarchical marigold flower and flower-like morphologies	14–73	Decomposition of metronidazole	Up to 56%	Visible light	[168]
BiOCl/BiVO4	Coprecipitation-hydrothermal method	Micro-nanosheet	1.53–2.83	Photo- degradation rate of rhodamine B (RhB)	Up to 96%	Visible light	[169]
CuS	Microwave-assisted wet chemical process	Spherical monodispersed submicron particles	17.8–26.4	Decomposition of organic dyes including rhodamine B, methylene blue and malachite green	Up to 100%	Visible light	[180]
CuS	Microwave irradiation	Aggregates of roughly spherical nanoparticles	14.74	Degradation of methylene blue, methyl orange, and 4- chlorophenol	Up to 100%	Solar light	[181]
3D- Fe2O3	Ultrasound irradiation method	Large quantity of 3D sea urchin-like structures combining a 1D rod-like structure on spherical support	129.4–282.7	Adsorption for heavy metals and photocatalytic activities toward the dyes (methylene blue and phenol)	Up to 100%	Solar light	[160]
g-C3N4	Ultrasound-assisted molecular rearrangement strategy	Hierarchical Rodlike	88.6	RhB degradation and H2 evolution	Up to 23%	Visible light	[182]
CdMoO4	Low temperature oil bath method	Uniform and porous spheres	-	Photocatalytic removal of mixed dye aqueous solutions	Up to 100%	UV-Vis	[170]
g-C3N4/ZnO	Electrostatic self-assembly method	nanosheets	156	Photocatalytic CO2 reduction activity	0.64 µmol h−1 g−1	Solar light	[171]
BiOI	Solid-state reaction with subsequent hydrolysis at room temperature.	Hierarchical microspheres assembled by nanoplates	3.1–13.8	Photocatalytic activity for phenol degradation.	Up to 100%	Visible light	[172]
SnS2	Heating the mixture of SnCl2·2H2O and thiourea in air at 170 °C for 2	Porous flower-like hierarchical nanostructure	36.15–82.4	Adsorption and photocatalytic reduction of aqueous Cr(VI)	Up to 79.4%	Visible light	[173]
ZnO	Annealing of zinc oxalate.	Mesoporous nanostructured microlumps	1.7–29−9	Discoloration of the Methyl violet 2B	Up to 100%	UV	[174]
O-Doped g-C3N4	Successive thermal oxidation exfoliation and curling-condensation of bulk g-C3N4	Uniform porous network	36	Photocatalytic CO2 Reduction Activity	Up to 0.88 μmol g−1 h−1	Visible light	[175]
Yttrium-doped g-C3N4	Pyrolysis method	Sheet-like morphology with worm-like pores	39–106	Photocatalytic performance in rhodamine B degradation	Up to 100%	Visible light	[176]
3D g-C3N4	Cold quenching	Self-assembled nanoscrolls	76	Photocatalytic CO2 reduction	Up to 11.2 μmol g−1 h−1	UV-Vis	[177]
Bismuth oxychlorides (BOC)	One-pot sorbitol-nitrate solution auto-combustion method	Mesoporous-mixed-phase of grain-like	93	Photocatalytic application in treatment of antibiotic effluents	Up to 80%	Visible light	[178]
Fe2O3	Pulsed Laser Deposition and thermal oxidation	Spherical particulates to an urchin-like struc- ture with evolution of nanowires	-	Photocatalytic water purification	Up to 100%	H2O2 and visible light	[179]
AgO	Oxidizing solid Ag films in an environment of reactive magnetron sputtering deposition of NiO	Porous AgO nanorod films.	-	Photocatalyst and all solid-state thin film battery	Up to 100%	UV-Vis	[183]

Table 5. Hierarchical catalysts produced without templates.

Composition	Synthesis Strategy	Morphology	SSA (m2/g)	Application	Activity	Irradiation	Ref
ZnCuCo layered double hydroxide	Hydrothermal	3D flower-like hierarchical morphologies with in- terlaced petal-like nanosheet	86–179	photocatalytic H2 production and degradation of SMZ	H2 production rate (3700 μmol g−1 h−1, 95% sulfamethazine (SMZ) degradation	Visible light	[43]
Ti3C2/Bi2WO6	hydrothermal	2D/2D hetero junction i	33.5–58.3	Photocatalysis degradation of tetracycline hydrochloride (TC-HCl)	0.430 min−1	Visible light	[44]
ZnFe2O4 modified Cu2S	in-situ self-assembly method	Dendritic fractal structure similar to a snowflake	-	photocatalytic reduction of nitrobenzene with and degradation of methyl orange and methylene blue dyes proficiently	98% yield of degradation of nitrobenzene, 94.3 and 86% fordegradation of methyl orange and methylene blue, respectively	Visible light	[45]
ZnO-graphene nanocomposite	Solution route	hollow microspheres	29.7–37.6	Adsorption/photocatalytic activity towards degradation of water-soluble organic pollutants (such as Rhodamine B, methyl orange, phenol)	90% adsorption capacity	UV (254 nm)	[141]
ZnO	Solvothermal route	Monodisperse microspheres	18	Photocatalytic activity for degradation of methylene blue	100%	UV-Vis	[142]
ZnO	Hydrothermal and calcination	Hierarchically porous microspheres composed of nanosheets	46–91	Photocatalytic RhB degradation activity	Up to 100%	solar	[60]
TiO2	Hydrothermal route	(i) long and well oriented macrochannels (ii) surface macropores of 0.8–9.3 μm in size, and (iii) porous walls with pores mostly smaller than 0.5 μm.	82.9–216.1	Decomposition of methyl ethyl ketone (MEK) in a continuous flow photoreactor	Up to 37%	UV (254 nm)	[144]
TiO2	Hydrothermal route	Partial spherical like structure	30–43.7	RhB photodegradation	Up to 98%	Visible light	[145]
TiO2	Hydrothermal route	Nanoflowers with a spherical hierarchical structure	20–80	Aqueous methylene blue photo-oxidation	Up to 50%	solar	[146]
CeO2	Hydrothermal route	Mesoporous nanosphere	42.1–68.2	photocatalytic activity of rhodamine B (RhB) dye degradation	97.8–92% RhB dye degradation	UV-Vis and acidic condition	[147]
Cu2O	Low temperature route	Spherical, cuboctahedral or cubic nanoparticles	7–13	Photocatalytic degradation of the antibiotic trimethoprim	Up to 48%	Visible light	[148]
WO3	Hydrothermal route	Regular-shaped nanosheets with an average thickness of approximately 30–40 nm	2–18	Photocatalytic activity towards an aqueous solution of tetracycline (TC) and possess good stability and reusability	Up to 94%	Visible light	[149]
Bi2WO6	Hydrothermal method and calcining process	Mesoporous nanoplate multi-directional	53.5	Photocatalytic oxidation of NO	Up to 90%	Solar	[150]
Sm, Y, La and Nd-doped CeO2	hydrothermal	broom-like hierarchical structure	-	BPA degradation and on CO2 evolution from CH3CHO decomposition	Up to 99%	UV	[151]
Yttria (Y2O3) nanosphere decorated ceria (CeO2)	Hydrothermal route	Yttrium nanoparticles are anchored on the surface of CeO2 nanorod with a particle size of 10 nm	-	Photocatalytic decomposition of aqueous Rhodamine B	Up to 96%	Solar light	[62]
SnS2/TiO2	Ultrasonic treatment	Ordered channels with a size about 1–3 μm were formed in the particles, and lots of holes appeared on the wall of the channels.	28	Photocatalytic degradation of Methyl Orange (MO)	Up to 90.9%	Solar light	[152]
ZnO/g-C3N4	Solvothermal	Porous microsphere with a size of about 700nm.	9.9–32-7	Photocatalytic degradation on rhodamine B and phenol	Up to 100%	Solar	[133]
ZnO/graphene	Solvothermal	Core-shell structure	65–201	Photocatalytic degradation of rhodamine B	Up to 98.5%	Visible light	[153]
TiO2 Microspheres with Carbonaceous Species	Solvothermal	Porous structure	337	Photodegradation of rhodamine B	Up to 100%	Visible light	[154]
Carbon-coated TiO2	solvothermal	Hierarchical nanotubes	Up to 244.4	Photocatalysts activity for water oxidation	Up to 705 µmol h−1 g−1	Solar light	[155]
S-deficient CoS/CdS	Solvothermal	Hexagonal nanoplates	Up to 84.32	Photocatalytic water-splitting	Up to 14.5%	Visible light	[156]
BiVO4/Bi2WO6	Solvothermal	Self-assembled hierarchical BiVO4/Bi2WO6 heterostructured composites	5.16	Photocatalytic activities for methylene blue (MB) degradation and photoelectrochemical performance	Up to 50%	Visible light	[157]
3D LaPO4	Solution route using citric acid (CA)	Urchin-like hollow sphere	51–124	Photocatalytic CO2-reduction performance	6.8-fold enhancement of the AQY	UV	[159]
ZnO	Hydrothermal-calcinatio n	Mesoporous multi-shelled ZnO microspheres	3–20	Photocatalysis for NO oxidation	Up to 77%	UV	[161]
ZnO/CeO2	Hydrothermal/calcination	Spherically hierarchical structure	41–56	photocatalytic rhodamine B (RhB)	Up to 96%	Visible light	[162]
TiO2/graphene/MoS2	Hydrothermal	2D rGO sheets assembled into macroporous 3D structures	124	CO2 Reduction Photocatalyst	Up to 97%	UV-Vis	[165]
Ag,Au/TiO2	Solvothermal	Mesoporous spherical shape	153–173	Degradation of Textile Dyes	Up to 85%	Solar and visible light	[163]
Au-H-ZnO	Low temperature aqueous reaction and heat treatment	Nanosheets	-	Gas sensing and photocatalytic properties	Up to 94.8%	Visible light	[166]
PDA-modified Zn	Self-assembly	Rough microstructures	32–38	Photocatalytic CO2 reduction	Up to 0.95 µmol h−1 g−1 of CH3OH	UV	[167]
ZnIn2S4 marigold flower/Bi2WO6 (ZIS/BW)	Hydrothermal method followed by wet-impregnation	Hierarchical marigold flower and flower-like morphologies	14–73	Decomposition of metronidazole	Up to 56%	Visible light	[168]
BiOCl/BiVO4	Coprecipitation-hydrothermal method	Micro-nanosheet	1.53–2.83	Photo- degradation rate of rhodamine B (RhB)	Up to 96%	Visible light	[169]
CuS	Microwave-assisted wet chemical process	Spherical monodispersed submicron particles	17.8–26.4	Decomposition of organic dyes including rhodamine B, methylene blue and malachite green	Up to 100%	Visible light	[180]
CuS	Microwave irradiation	Aggregates of roughly spherical nanoparticles	14.74	Degradation of methylene blue, methyl orange, and 4- chlorophenol	Up to 100%	Solar light	[181]
3D- Fe2O3	Ultrasound irradiation method	Large quantity of 3D sea urchin-like structures combining a 1D rod-like structure on spherical support	129.4–282.7	Adsorption for heavy metals and photocatalytic activities toward the dyes (methylene blue and phenol)	Up to 100%	Solar light	[160]
g-C3N4	Ultrasound-assisted molecular rearrangement strategy	Hierarchical Rodlike	88.6	RhB degradation and H2 evolution	Up to 23%	Visible light	[182]
CdMoO4	Low temperature oil bath method	Uniform and porous spheres	-	Photocatalytic removal of mixed dye aqueous solutions	Up to 100%	UV-Vis	[170]
g-C3N4/ZnO	Electrostatic self-assembly method	nanosheets	156	Photocatalytic CO2 reduction activity	0.64 µmol h−1 g−1	Solar light	[171]
BiOI	Solid-state reaction with subsequent hydrolysis at room temperature.	Hierarchical microspheres assembled by nanoplates	3.1–13.8	Photocatalytic activity for phenol degradation.	Up to 100%	Visible light	[172]
SnS2	Heating the mixture of SnCl2·2H2O and thiourea in air at 170 °C for 2	Porous flower-like hierarchical nanostructure	36.15–82.4	Adsorption and photocatalytic reduction of aqueous Cr(VI)	Up to 79.4%	Visible light	[173]
ZnO	Annealing of zinc oxalate.	Mesoporous nanostructured microlumps	1.7–29−9	Discoloration of the Methyl violet 2B	Up to 100%	UV	[174]
O-Doped g-C3N4	Successive thermal oxidation exfoliation and curling-condensation of bulk g-C3N4	Uniform porous network	36	Photocatalytic CO2 Reduction Activity	Up to 0.88 μmol g−1 h−1	Visible light	[175]
Yttrium-doped g-C3N4	Pyrolysis method	Sheet-like morphology with worm-like pores	39–106	Photocatalytic performance in rhodamine B degradation	Up to 100%	Visible light	[176]
3D g-C3N4	Cold quenching	Self-assembled nanoscrolls	76	Photocatalytic CO2 reduction	Up to 11.2 μmol g−1 h−1	UV-Vis	[177]
Bismuth oxychlorides (BOC)	One-pot sorbitol-nitrate solution auto-combustion method	Mesoporous-mixed-phase of grain-like	93	Photocatalytic application in treatment of antibiotic effluents	Up to 80%	Visible light	[178]
Fe2O3	Pulsed Laser Deposition and thermal oxidation	Spherical particulates to an urchin-like struc- ture with evolution of nanowires	-	Photocatalytic water purification	Up to 100%	H2O2 and visible light	[179]
AgO	Oxidizing solid Ag films in an environment of reactive magnetron sputtering deposition of NiO	Porous AgO nanorod films.	-	Photocatalyst and all solid-state thin film battery	Up to 100%	UV-Vis	[183]

7. Conclusions

Hierarchical structures have raised great interest for different photocatalytic applications, because of improved light harvesting, charge separation, as well as mass transport and adsorption efficacy, resulting in superior catalytic properties. Even though different fabrication strategies have been explored, as discussed in this review, some major challenges must be still addressed to allow easy synthesis of hierarchical photocatalysts and help to scale up process from laboratory to industrial production.

Among available methods, templated free wet synthesis approaches based on self- aggregation processes offer a facile route to obtain a wide choice of compositions and structures. Although most of them feature lower average surface area than systems obtained using inorganic templates, they usually exhibit comparable photocatalytic performance especially towards pollutant degradation, which is complete under UV irradiation and, for some composition reaches even 95% under visible light. Yet, some issues must be addressed mainly related to the difficulty to carry out large scale and high yield synthesis. Furthermore, post-process modification is usually required to further improve photocatalytic properties. Among templating approaches, inorganic backbones lead to the largest surface area, as high as 400 m²/g, even though this feature does not necessarily result in the best photocatalytic performance, while organic templated systems exhibit comparable activity, allowing for organic pollutant complete degradation even under visible light. On the other hand, hybrid templating strategies, based on MOFs, are extremely versatile since they provide for a huge number of semiconductor compositions, with tunable morphology and structure. In addition, post-modification is usually not required, since catalyst functional modification including doping, hetero-junction formation, and loading with co-catalyst moieties can be easily carried out in-situ upon thermal treatments in dry or solvothermal conditions, concurrently with template decomposition. Carbonaceous residues resulting from this process can act themselves as doping species and sensitively improve photocatalytic properties. However, MOF templated method is expensive, time consuming, and difficult to be carried out on a large scale. Moreover, hierarchical structures obtained through this approach often show comparable performance to those obtained through self-templated methods, particularly for decontamination processes, thus the latter are usually preferred for these applications. On the other hand, more advanced technological routes involving hydrogen production are worth the effort of using MOF templated structures, which outperform with respect to conventional templates because of their extremely versatile features.

Nevertheless, the criteria to adequately choose MOF precursors must be further investigated to create the desired composition exhibiting hierarchical structure.

Bio-templates offer a sustainable and simple opportunity to design hierarchical photocatalytic nanomaterials. Among bioavailable compounds, bio-wastes are attracting great interest as abundant and cheap templates. However, their heterogeneity and poor knowledge of growth mechanisms make the process difficult and poor reproducible. Further investigation on building blocks growth and assembly is highly demanded to overcome these limitations and achieve rational control of the final hierarchical structure. More efforts should be taken towards the investigation of structure/function relationships and the mechanism underlying photocatalytic behavior.

Finally, a combination of the described strategies appears to be a promising way to combine benefits of each single approach. In this view, very recent studies evidence that integrating bio-templated design with MOF derived nanoparticles provide for a unique hierarchical structure, with efficient light absorption, superior charge separation efficacy, accounting for potent photocatalytic activity [184]. To this aim, this study could provide useful information to move towards more performant hierarchical nanostructures for advanced environmental and energy applications.