Hierarchical Nanostructured Photocatalysts for CO₂ Photoreduction

Abstract

Practical implementation of CO₂ photoreduction technologies requires low-cost, highly efficient, and robust photocatalysts. High surface area photocatalysts are notable in that they offer abundant active sites and enhanced light harvesting. Here we summarize the progress in CO₂ photoreduction with respect to synthesis and application of hierarchical nanostructured photocatalysts.

1. Introduction

The deleterious environmental impacts of fossil fuel [1] combustion are now well recognized [2,3,4,5,6,7,8,9,10,11,12], making clear the critical need to develop renewable energy technologies. In this regard, the conversion of CO₂ into fuel has received considerable attention in the past few years [13,14,15,16,17,18,19,20,21,22,23,24]. The utilization of solar energy for conversion of CO₂ into hydrocarbon fuels, compatible with the current energy infrastructure, is an intriguing idea for solving both energy and pollution issues [25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43]. However, to date CO₂ photoreduction efficiencies, and the intrinsic stability of the photocatalysts, remain too low for commercial application [44,45,46,47].

When semiconductors are exposed to photons possessing energy equal to or higher than the band gap energy, electron–hole pairs are generated. Some of these photogenerated charges undergo redox reactions, while others simply recombine thus limiting photocatalytic efficiency [48]. In this regard, the design of an efficient, highly stable and cost-effective photocatalytic system is necessary for practical implementation. While a number of semiconductor photocatalysts have been extensively studied for CO₂ photoreduction, such as, for example, ZnO [49,50,51], TiO₂ [52,53,54,55,56,57,58], CdS [59,60,61], Fe₂O₃ [62,63,64], g-C₃N₄ [65,66,67,68,69], Cu₂O [70,71,72], and Bi₂WO₆ [73,74,75], their performance is not yet sufficient for practical implementation due to constraints such as band gap energy that is too large, high recombination rate, and cost. To overcome such difficulties band gap engineering of semiconductor photocatalysts through co-catalyst insertion, synthesis of different morphologies, heterojunction and Z-scheme construction are of ongoing interest [76]. Hierarchical nanostructured materials offer the opportunity for readily modifying crystal structure, band gap alignment, and composition [52,77,78,79]. The hierarchical materials are typically composed of low-dimensional sub-units arranged in a well-ordered manner [80]. Compared to bulk materials these complex hierarchical nanostructures possess high surface area, enable enhanced light harvesting, and have high CO₂ adsorption and activation properties that correspond to higher reaction rates and photocatalytic activity [81,82]. In this review we examine progress in the synthesis and application of different hierarchical CO₂ photoreduction photocatalysts, elucidating pathways by which higher performance might be achieved.

2. CO₂ Photoreduction Mechanism

CO₂ reduction begins with breaking of the O=C=O bond to form new carbon bonds [48]. Photocatalytic CO₂ reduction is considered a “proton-assisted multi-electron reduction process” involving different intermediate steps as indicated in

Table 1. The main products of CO₂ reduction with the corresponding reduction potentials (pH = 7).

Reaction	Product	E°redox	Equation
CO2 + e− → CO2• −	CO2• − anion radical	−1.90 V	(1)
CO2 + 2H+ + 2e− → HCOOH	Formic acid	−0.61 V	(2)
CO2 + 2H+ + 2e− → CO + H2O	Carbon monoxide	−0.53 V	(3)
CO2 + 4H+ + 4e− → HCHO + H2O	Formaldehyde	−0.48 V	(4)
CO2 + 6H+ + 6e− → CH3OH + H2O	Methanol	−0.38 V	(5)
CO2 + 8H+ + 8e− → CH4 + 2H2O	Methane	−0.24 V	(6)
2H+ + 2e− → H2	Hydrogen	−0.41 V	(7)

, with intermediate products dependent upon the availability (density) of photogenerated electrons [83]. When a single electron is transferred to an adsorbed CO₂ molecule on the surface of a photocatalyst it results in an unstable CO₂^•− radical formation that has a redox potential of −1.90 V versus normal hydrogen electrode (NHE) as shown in Table 1, equation 1. The relative conduction band (CB) edges of most of semiconducting materials have lower reduction potential than that of CO₂^•− radical, therefore this reaction is unfeasible without a high over potential. The proton assisted transfer of multiple electrons is considered more favorable for CO₂ reduction reaction [48]. The as-produced CO₂^•− anion radical further counters with electrons (e⁻) and protons (H⁺), giving rise to different products such as CO, HCHO, HCOOH, CH₃OH and CH₄ depending upon their relative redox potential. When surface-absorbed CO₂ reacts with two protons and two electrons, it produces carbon monoxide (CO), with the possible formation of formic acid (HCOOH). Furthermore, the formaldehyde can be produced by reacting CO₂ with 4e⁻ and 4H⁺ ions. However, higher hydrocarbon like CH₄/CH₃OH can be produced by contributing 8/6 electrons and protons, respectively. One can tune the yield and selectivity of the product by manipulating factors like the band gap of the semiconductor, photo excited charge carriers, conductivity and engineering of surface morphology. This kind of engineering can also be helpful for production of higher hydrocarbon fuels from CO₂ photoreduction.

3. Benefits of Hierarchical Nanostructures in Photocatalysis

Hierarchical nanostructures offer a promising avenue for achieving enhanced photocatalytic activity due to their special features as illustrated in Figure 1.

In heterogeneous photocatalysis it is recognized that high surface area corresponds to increased overall photocatalytic activity. Jiao et al., for example, reported that hollow and mesoporous TiO₂ shows improved CO₂ conversion into CH₄ as compared with solid, low surface area, TiO₂ crystals [84]. Xiao et al. fabricated Bi₂WO₆ nanosheets which displayed 58 times higher specific surface area than that of bulk Bi₂WO₆ [85], 42.87 m² g⁻¹ versus 0.73 m² g⁻¹. Such high specific surface area of hollow Bi₂WO₆ has greatly improved photocatalytic CO₂ reduction into CH₄. On the other hand, the carbon allotrope-based hierarchical materials reached 800–2800 m² g⁻¹ specific surface area [76]. These studies imply a significant synergistic effect of high specific surface area of hierarchical nanostructures resulting in enhanced CO₂ adsorption and photoreduction. To achieve large CO₂ adsorption, various strategies have been successfully employed including: (i) augmentation of surface area, (ii) improvement in mass transfer, and (iii) expansion of the basic sites. Hierarchical materials with single/multicomponent mesoporous structures possess high CO₂ adsorption capacity due to their porous and hollow morphology. For instance, Pan et al. synthesized LaPO₄ hierarchical hollow spheres with enhanced photocatalytic CO₂ reduction activity and selectivity for CH₄ production was attributed to higher CO₂ adsorption ability [86]. Similarly, Fu et al. reported that O-doped g-C₃N₄ porous material having a higher CO₂ uptake ability than that of bulk g-C₃N₄, which resulted in enhanced photocatalytic CO₂ reduction [87]. It is well known that increased light-harvesting ability can greatly boost photocatalytic performance [88,89]. Hollow and mesoporous nanostructures, for example, have the ability to scatter light inside the pores hence improving light absorption efficiency. Recently Zhang et al. fabricated hierarchical CdS multi-cavity hollow nanoparticles that demonstrated high rates of CO₂ to CO conversion, a behavior attributed to the hollow and porous morphology. Like other properties, enhanced molecular diffusion is one of the prime features of hierarchical structures, decreasing the bulk to surface diffusion length thus facilitating charge separation.

4. Hierarchical Nanostructured Photocatalysts for CO₂ Reduction

In recent years hierarchical materials designed with either multiple components or intrinsic geometric complexity have drawn great interest for photocatalytic applications. These nanostructures can be categorized based on their structures ranging from one-dimensional to multi-dimensional. It is widely known that the activity of the photocatalysts is dependent upon the interrelated factors of morphology, crystalline size, defects and other surface properties [90]. In the following section, we review the synthesis of hierarchical nanostructures with respect to special morphologies targeted for promoting CO₂ photoreduction.

4.1. Hierarchical Nanostructures with Leaf-Like Morphology

Zhang et al. designed a stable and effective, 3D hierarchical Ru–polypyridine incorporated metal organic framework (MOF) with flower-like morphology for visible-light driven CO₂ photoreduction [91]. In their study, the monodispersed, surfactant less Ru-MOF were synthesized by a simple solvothermal method with controlled morphology tuned with sub-millimeter scale fakes to micro-scale nanoflowers as shown in Figure 2. As per scanning electron microscope (SEM) and transmission electron microscope (TEM) investigations (Figure 2a–f), the as-prepared nanoflowers have diameters in the range of 10–20 µm. While at higher magnification it can be clearly observed that, proper nanoflower morphology constructed with various petals was obtained. Furthermore, these nanoflowers were utilized for CO₂ photoreduction in the liquid phase, along with triethanolamine as a sacrificial agent in 100 mL Schlenk tube under a 500 W Xe lamp irradiation. These Ru-MOF nanoflowers were utilized for heterogeneous photocatalytic CO₂ reduction which produced 24.7 µmol g⁻¹ of formate anion (HCOO⁻) in 8 h irradiation time with quantum yield of 0.67%. The production of HCOO⁻ was 1.5-fold higher than those of controlled samples such as bulk crystals and micro-fakes. The enhanced activity was attributed to (i) large surface area (surface to volume ratio), (ii) better CO₂ adsorption, and (iii) activation by self-assembled nanoflowers. This study significantly provides some new ideas for fabrication of highly efficient metal organic framework-based photocatalysts for CO₂ reduction. In another example, Dai et al., designed a simple, efficient Bi₂MoO₆ hierarchical nanostructure with three-dimensional (3D) flower like morphology via a hydrothermal synthesis approach. Here, polyvinylpyrrolidone (PVP) was used as a crystal growth modifier [92]. It is believed that polymer molecules adsorb on nanoflakes and act as a crystal phase inhibitor in the system; therefore, PVP plays a crucial role in fabrication of proper flower-like morphology. Furthermore, the resulting hierarchical nanoflowers were successfully utilized for visible-light-driven CO₂ photoreduction for generation of methanol and ethanol with yield of 24.8 and 18.8 µmol g⁻¹ respectively. Here also, the high CO₂ photoreduction activity was ascribed to the high surface area of Bi₂MoO₆. In another study, stable CeO₂/Bi₂MoO₆ heterostructures with flower morphology were prepared via a simple solvothermal route with different weight ratios of CeO₂ to Bi₂MoO₆ [93]. The nitrogen adsorption-desorption technique was used to determine structural properties along with their texture and the results revealed a characteristic mesoporous isotherm. Furthermore, CO₂ photoreduction tests were carried out in water, saturated with CO₂, in presence of visible light (≥420 nm) which resulted in equal amount of methanol and ethanol (58.4 µmol gcat⁻¹) as main products for CeO₂/Bi₂MoO₆ (5:100 w/w ratio) sample. The product yield was 1.9 and 4.1 folds higher as compared to that of pure Bi₂MoO₆ and pure CeO₂ respectively. The improved photocatalytic activity of as-obtained heterostructure was referred to high charge carrier separation efficiency of hierarchical nanostructure which was well supported by transient photocurrent responses under visible light irradiation. Furthermore, apart from flower like morphology, Zhou et al. designed the leaf like bio-templated 3D hierarchical perovskite titanates system for CO₂ photoreduction to hydrocarbons fuels, CO and CH₄ [94]. The natural leaf itself contains various components for photosynthesis which efficiently utilize solar light to produce carbohydrates. In this study, perovskite titanates (ATiO₃, A= Sr, Ca, and Pb) were designed by preserving the fine morphological details of leaves. In the typical synthesis, green leaves of cherry blossoms were washed and immersed in HCl (5%) solution to get rid of various ions and washed several times with water and ethanol. These leaves were immersed in precursor solutions for 8 h and again washed with ethanol 4 times and dried for one day at 100 °C followed by sintering at 600 °C for 10 h. Here, three types of artificial photocatalysts were synthesized such as SrTiO₃ (STO), CaTiO₃ (CTO) and PbTiO₃ (PTO) respectively by changing the precursors. Furthermore, the loading of co-catalysts (such as Pt, Au, Ag and Cu), RuO₂ and NiO_x was also carried out for comparing CO₂ photoreduction activity. Bare STO and CTO generated CO and CH₄ as main solar fuels in the absence of any sacrificial agents. While, STO and CTO constructed with leaf architecture hierarchical templates revealed enhanced activity of CO and CH₄ to that of bare samples. Such improvement in photocatalytic activity was attributed to escalated gas diffusion and light-harvesting capacity of hierarchical titanates perovskites. Furthermore, loading of co-catalysts such as Au, Ag and Cu on STO-improved CO₂ photoreduction activity, highest with Au (loaded via precipitation), for both CO and CH₄. The study clearly indicates, large variety of plants and biological systems are capable for designing a new class of hybrid photocatalysts that can be applied for diverse energy applications. In another study, CdS hierarchical multi-cavity hollow particles (HMCHPs) possessing fruit (raspberry) like morphology were fabricated for efficient CO₂ photoreduction [95]. Here, CdS HMCHPs were designed by complex nano-architecture with stepwise sequence starting from solution growth, followed by sulfidation, and a cation-exchange approach. As shown in Figure 3a, solid spheres of cobalt glycerate (Co-G) were synthesized by simple mixing of cobalt nitrate and glycerol in isopropanol solution at 130 °C and synthesis of Co-G@ZIF-8 was carried out by growth of a Zn-grounded zeolitic imidazolate framework (i.e., ZIF-8) onto Co-G solid spheres. After that, the Co-G@ZIF-8 composite gained was treated with thioacetamide leading to the formation of CoS_x@ZnS HMCHPs, which later undergoes hydrothermal cation-exchange reaction at 40 °C for 5 h to obtain raspberry morphological-featured CdS HMCHPs (Figure 3a). The SEM images revealed that as-formed CdS HMCHPs clearly shows a raspberry morphology (Figure 3b,c) with a central large cavity containing hollow particles on the shell (Figure 3d). The TEM images (Figure 3e–g), also acknowledged the well-defined HMCHPs were constructed with ultrafine nanoparticles of 0.33 nm crystal lattice spacing (Figure 3g) and selected area (electron) diffraction (SAED) pattern as well further suggest that the as formed CdS HMCHPs are constructed from crystallized nanoparticles. Further CdS HMCHPs were utilized for CO₂ photoreduction along with two references i.e., CdS solid spheres (SSs) and CdS hierarchical structures (HSs). The photoreduction tests were carried out in a typical system containing a mixture of H₂O and acetonitrile with Co(bpy)₃²⁺ (bpy = 2, 2′-bipyridine) as co-catalyst and triethanolamine (TEOA) as an electron donor, respectively. The CdS HMCHPs exhibited highest CO₂ photoreduction into CO with 1337 µmol h⁻¹ g⁻¹ yield which was much higher than CdS SSs and HSs samples (Figure 3h). Further selectivity and photocatalytic activity were increased by loading small amount of Au (co-catalyst) with 2.8-fold increased CO generation rate (3758 µmol h⁻¹ g⁻¹ for 0.25 wt% Au) than that of bare CdS HMCHPs (Figure 3i) with stability of four cycles (Figure 3j). Along with CO generation, a small amount of H₂ was also generated in all the samples. The study of wavelength-dependent yield with appropriate long-pass cutoff filters (such as 400, 420, 455, and 495 nm) clearly revealed that CO₂ photoreduction is encouraged by photoexcitation of Au(25)@CdS HMCHPs and maximum evolution with respect to 400 nm cutoff filter (Figure 3k). The overall high yield of CO was credited to high specific surface area of hollow structures which efficiently promotes large amount of CO₂ adsorption and light-scattering effect. Herein, Au acts as charge trap which restricts the recombination of electrons and holes and effectively promotes charge separation for better photocatalytic activity. Therefore, these examples of hierarchical nanostructures containing flower-, leaf- and fruit-like morphologies proved as potential materials for CO₂ photoreduction.

4.2. Hierarchical Nanosphere Morphologies

Among the diverse structures, another way to attain the desirable photocatalytic behavior is the fabrication of spherical hierarchical nanostructures due to their immense features such as high porosity, high surface area, low bulk density and reflection of light in the interior cavities [96]. In this regard, Fang et al. reported mesosphere of TiO₂ hierarchical nanostructure for photocatalytic CO₂ reduction [97]. The synthesis was carried out by employing the sol-gel technique. In detail, the desired amount of C₂H₅OH was mixed with deionized water (H₂O) followed by the addition of concentrated HCl and titanium isopropoxide. After that, aqueous solution of chitosan glacial acetic acid along with NH₄OH were added and calcinated at 450 °C for 3 h to get resulting TiO₂ spheres. Morphological features from the digital (Figure 4a) as well as SEM images (Figure 4b) reveal that as-formed dense TiO₂ particles have spherical morphology. At high magnification, the surface was observed to be tightly packed with small particles comprising mesoporous morphology. Furthermore, CO₂ photoreduction investigation was carried out using a batch reactor under Hg UV lamp (40 W, 254 nm) irradiation for 24 h. The results showed that, as-prepared spherical TiO₂ yielded 0.94, 2.32 and 2.03 μmol g⁻¹ h⁻¹ of CH₄, CO, and H₂, respectively which is higher than bare P25. The Pt-loaded TiO₂ spheres with various Pt contents were synthesized. The results clearly revealed that, with 0.6% loading of Pt there is 20-times increment in CH₄ generation than pure P25. The improved catalytic activity and selectivity (CH₄) of TiO₂ spheres over bare P25 was attributed to the enhanced multiple scattering, mesoporous volume, hierarchical porosity and large hollow channels that facilitates the fast mass transport within the special structure. In another similar study, porous TiO₂ microspheres were developed by Di et al. employing a microwave assisted solvothermal method in combination with heat treatment in air [98].

The morphological study reveals that as formed TiO₂ hierarchical microspheres were well arranged into nanosheets with porous features. The high surface area of the TiO₂ hierarchical nanostructure, i.e., 142 m² g⁻¹ was confirmed by N₂ adsorption-desorption along with enhanced pore size distribution as compared to anatase TiO₂. Similarly, CO₂ uptake capacity of TiO₂ hierarchical photocatalyst was 0.42 µmol g⁻¹ which was almost double that of anatase TiO₂, 0.25 µmol g⁻¹. These collective features such as high specific surface area, large pore volume and enhanced CO₂ uptake ability of spherical-shaped TiO₂ hierarchical nanostructures play an important role in boosting CO₂ photoreduction activity. A CO₂ photoreduction test was carried out in a batch reactor employing Hg UV lamp (40 W) as a light source for 1 h illumination time. The results showed that, CH₄ and CH₃OH were obtained as main products with yields of 0.23 and 0.08 µmol respectively which were much higher than pure P25 and anatase TiO₂. The band structure of TiO₂ hierarchical nanostructure was analyzed using the Mott–Schottky measurement and the result showed that the band gap potential obtained is more negative than the redox potential of photocatalytic reactions which endows stronger reduction ability. Therefore, the improved photocatalytic activity was attributed to ultrathin microspheres with large specific surface area which eventually creates many active sites for CO₂ adsorption and further helps to improve the overall CO₂ photoreduction yield. In 2016, Pan et al. reported the self-assembled hollow spheres of LaPO₄ photocatalysts for enhanced CO₂ photoreduction into hydrocarbon fuels [86]. In their study, citric acid was utilized as a structure-directing agent for the fabrication of LaPO₄ hollow spheres via a facile solution route and growth mechanism of as prepared hierarchical structures and was studied in a solution-phase. Typical synthesis of LaPO₄ spheres began with the formation of a lanthanum metal citrate complex by mixing lanthanum nitrate and the desired amount of aqueous solution of citric acid followed by metal complex dissolution in H₃PO₄ aqueous solution to form the desired hierarchical structure. The uniformly distributed monodispersed microsphere morphology was confirmed from low magnified SEM images (Figure 5a). While at higher magnifications, spines like LaPO₄ 3D microspheres were observed with diameters of 600-800 nm. The results were well supported by TEM images where it can be clearly seen that individual microspheres were densely arranged by nanorods with diameters of 10-20 nm. The citric acid ratio in synthesis of LaPO₄ was varied from 0, 10, 100 and 500 mg for comparing the results with respect to their photocatalytic activity. Here, citric acid plays a crucial role in the formation of the hierarchical structure (Figure 6). It can be clearly seen that, in absence of citric acid, the isolated nanorods (Figure 6a) were obtained while with introduction of 10 mg citric acid, the 3D microspheres with diameter of 1.28 μm appeared (Figure 6b). In addition to this, at 100 mg citric acid (Figure 6c), the morphology remained unchanged with only modification in the diameter (600–800 nm).

At 500 mg concentration, half-baked microspheres with 300-400 nm diameter were formed as shown in Figure 6d. Therefore, in such a solution phase reaction, coordination of H₃PO₄ and citric acid is very important to obtain the desired spherical morphology. The CO₂ photoreduction results revealed that CH₄ (10.5 μmol) was obtained as a main product along with generation of small amount of H₂ (6.3 μmol) after 5 h irradiation. The as-synthesized 3D LaPO₄ hierarchical mesosphere structures exhibited a 6.2-fold increased photocatalytic activity compared with 1D LaPO₄ nanorods. The outcome of better catalytic activity was mainly credited to: the (i) hollow structure, (ii) enhanced light-harvesting ability, and (iii) remarkable charge-carrier separation capability of spheres.

In 2015, Ho’s group reported Z-scheme CdS-WO₃ photocatalyst for CO₂ photoreduction into CH₄ in the presence of visible light [99]. Here, WO₃ hierarchical hollow spheres were first synthesized and CdS nanoparticles were grown on hollow spheres to obtain Z-scheme heterostructure. The maximum photocatalytic activity for CO₂ reduction into CH₄, 1.02 µmol h⁻¹ g⁻¹ was reported for the optimized CdS–WO₃ sample (with 5 mol% CdS content) which was 100 and 10 times higher than that of bare WO₃ and CdS, respectively. The BET specific surface area, calculated by employing N₂ adsorption-desorption isotherms, confirmed the mesoporosity with the range of 2-50 nm while pores shape was confirmed by using pore (0.8-1) structure study. In another study, Lin et al. developed hybrid complex/semiconductor (spherical shaped) heterophotocatalyst by employing feasible hydrothermal method [100]. In this study, spherical TiO₂ hybridization with a cobalt complex i.e., [Co(bipy)₃]²⁺ structure was fabricated. The photocatalytic CO₂ reduction results showed that the photocatalyst possessing spherical morphology has a high surface area which exhibits a superior CO₂ photoreduction performance compared to that of bulk TiO₂. The morphological features obtained from SEM (Figure 7a) and TEM (Figure 7b,c) images clearly indicate that, the as prepared TiO₂ nanostructures were spherical in shape, and well-arranged into nanosheets. Furthermore, the N₂ adsorption-desorption isotherm study revealed that as-prepared TiO₂ having a mesoporous structure represented a type-IV isotherm with a hysteresis loop of type H₃ as shown in Figure 7d. Due to such porous morphology, the BET surface area was coming out as 136 m² g⁻¹, a much higher than bulk TiO₂ (14 m² g⁻¹). The H₂ and CO were obtained as the main products with yield of 6.6 and 16.8 µmol respectively using TiO₂ containing Co(bipy)₃²⁺ hybrid system. Therefore, a hollow spherical TiO₂ hierarchical structure plays an important role in CO₂ photoreduction which allows efficient charge transfer to cobalt complex onto the surface and acts as a photon trap and eventually enhances the light absorption. Henceforth, all these examples proved that, such spherical morphology acquired hierarchical nanostructures can be considered as an efficient and potential CO₂ photoreduction photocatalysts in future prospective.

4.3. Fiber-Like Hierarchical Materials

The properly designed 1D structures such as fibers/nanofibers have also been utilized for solar light harvesting due to their efficient photochemical properties. While 1D nanomaterials were considered as one of the ideal material to fabricate hierarchical heterostructures as a function of high surface area along with enhanced charge transfer ability [101,102]. Numerous synthesis approaches have been developed to acquire ideal nanofiber morphology with controlled reaction systems. For instance, Renones et al. in 2016 fabricated nanofibers of TiO₂ by combining electrospinning and sol–gel techniques and studied the photocatalytic behavior [103]. The TiO₂ nanofibers were synthesized by the electrospinning method while for comparison with respect to photocatalytic performance, TiO₂ nanoparticles were prepared by employing sol-gel technique. Morphological features using SEM (Figure 8a,b) and TEM (Figure 8c,d) clearly depicted the formation of nanofibers. The pore size of the as-prepared hierarchical fibers was reported to be 20 nm which was much higher than normal TiO₂ (4.7 nm). Further, these 1D hierarchical nanofibers were utilized for CO₂ photoreduction in gas phase using continuous flow reactor under UV light (λ_max = 365 nm) irradiation. After 20 h irradiation CO, CH₄, CH₃OH and H₂ were obtained as main products with yield of 23.91, 26.88, 5.04 and 398.84 µmol g⁻¹ catalyst, respectively. The yield and selectivity of the hierarchical nanofibers was much higher than that of regular TiO₂ nanoparticles with 0.030% apparent quantum yield (AQY).

In another study, Meng et al. synthesized a hierarchical TiO₂/Ni(OH)₂ composite by loading Ni(OH)₂ on TiO₂ fibers [26]. In typical synthesis (Figure 9a), TiO₂ nanofibers were first prepared using the electrospinning technique followed by calcination and precipitation to get the desired product i.e., TiO₂/Ni(OH)₂ composite with varied weight ratios of Ni(OH)₂ (i.e., 0.5, 1, 1.5, 2 or 15%). Field emission SEM (FESEM) (Figure 9b), and TEM images (Figure 9d,e) revealed the existence of nanofiber morphology. Here, TiO₂ nanofibers acted as backbone for the growth of Ni(OH)₂ as shown in Figure 9b. Further, CO₂ uptake ability of as-prepared samples was estimated by employing CO₂ adsorption isotherms. The results demonstrated that, sample of TiO₂/Ni(OH)₂ containing 15% weight ratio of Ni(OH)₂ exhibited almost two-fold higher CO₂ adsorption capability than bare TiO₂ nanofibers.

Therefore, Ni(OH)₂ acts as an CO₂ adsorbent which further plays an important role in enhancing the CO₂ photoreduction activity. Indeed, solar light illumination (300 W Xe lamp) of as-prepared TiO₂ fibers showed that CH₄ and CO were the main products for all the samples (Figure 9f). Also, after increasing Ni(OH)₂ content of 0.5 wt%, the notable amount of CH₃OH and C₂H₅OH also achieved. Maximum yields for CH₃OH, 0.58 µmol h⁻¹ g⁻¹ and C₂H₅OH, 0.37 µmol h⁻¹ g⁻¹ were appeared with 15 wt% loading. However, time-coursed photocatalytic fuel production over different samples was also carried out as depicted in Figure 9g–i and results proved that photocatalytic CO₂ conversion to various fuels can be greatly enhanced by using such hybridized hierarchical nanofibers. In addition to such hybrid hierarchical nanofibers, CuInS₂ sensitized TiO₂ hybrid nanofibers were successfully synthesized by Xu et al. by employing a Z-scheme heterojunction [104]. Such nanofibers were synthesized using a simple electrospinning setup and further CuInS₂ were grown via hydrothermal synthesis approach with varied concentration of precursor. The resulting CuInS₂ sensitized TiO₂ hybrid nanofibers outperformed for CO₂ photoreduction to obtain CH₄ and CH₃OH as the main products under a 350 W solar Xe lamp irradiation using a homemade Pyrex reactor. After 1 h irradiation, the hybrid composite with 2.5 wt% CuInS₂ loading generated CH₄ and CH₃OH as the main products through production of 2.5 and 0.85 μmol h^–1 g^–1, respectively. Enhanced CO₂ photoreduction performance was attributed to the extended light absorption, increased surface area of hierarchical nanostructure and generation of Z-scheme heterojunction. Hierarchical, 1D/2D, TiO₂/MoS₂ hybrid nanostructures containing TiO₂ fibers were constructed via a hydrothermal transformation method by Xu et al. [105]. The typical synthesis involved, in situ growing of MoS₂ (2D) nanosheets on the nanofibers of TiO₂ (1D) to obtain 1D/2D hybrid hierarchical nanostructure which enhanced optical absorption along with increased CO₂ adsorption compared to those of pristine TiO₂ and MoS₂. The formation of TiO₂ nanofibers was confirmed by SEM images with a diameter of about 200 nm (Figure 10a). While after insertion of MoS₂ on the surface of TiO₂ nanofibers, the surface of fibers became larger and denser upon increasing the concentration of MoS₂ from 10 wt% to 25 wt% (Figure 10b,c). The microstructures in 10% MoS₂/TiO₂ were clearly observed in TEM and HRTEM images comprising lattice d spacing of 0.352, 0.325 and 0.62 nm corresponding to anatase (101) TiO₂, rutile (110) TiO₂ and MoS₂ facets respectively (Figure 10d,e). The EDX mapping images of 10% MoS₂/TiO₂ confirmed the presence of Ti, O, Mo and S elementals which represent the successful hybridization of TiO₂ and MoS₂ hierarchical composite (Figure 10f). The N₂ adsorption-desorption isotherms of pristine TiO₂, pristine MoS₂ and hybrid 1D/2D nanostructures were measured and the results revealed that all samples exhibit a type-IV isotherm and type-H₂ hysteresis loop with a 0.45–0.9 P/P₀ range. The results clearly illustrate the mesoporous nature of TiO₂ nanofibers which could be due to MoS₂ nanosheets. Hybrid MoS₂/TiO₂ hierarchical photocatalyst with 10 wt% MoS₂ loading exhibited enhanced performance compared to pristine TiO₂ and MoS₂. The photocatalytic CO₂ reduction results showed CH₄ and CH₃OH were the main products yielding 2.86 and 2.55 μmol g^–1 h^–1, respectively, along with trace amounts of CO formation. Such improved catalytic activity of the fiber nanostructure was attributed to high surface area, improved light absorption after hybridization, increased CO₂ adsorption capacity and the presence of MoS₂ nanosheets which improved charge separation. Referring to the aforementioned discussion it can be established that the photocatalysts with fiber-like hierarchical hybrid composites can be employed for sustainable and efficient CO₂ photoreduction to solar fuel generation.

4.4. Hierarchical Tube/Rod Morphologies

In general, an ideal photocatalyst should possess better photochemical properties, enhanced surface activity therefore optimization and fabrication of photocatalysts with the desired size and shape is prime objective of photocatalysis in CO₂ photoreduction systems. However, to maximize the performance of photocatalysts, the design with proper nanostructure is also indispensable. The 1D nanotubes with hollow and porous morphology also proved to be effective for CO₂ photoreduction. These hierarchical nanotubes can facilitate separation and migration of photogenerated charges and enhance the adsorption of the CO₂ molecule which offers active sites for redox reaction on the surface of the catalyst [106].

Fu et al. fabricated O-doped graphitic carbon nitride (O-C₃N₄) hierarchical nanotubes by employing thermal oxidation exfoliation and curling-condensation of bulk g-C₃N₄ for CO₂ reduction [87]. Bulk g-C₃N₄ was prepared by using melamine as starting material and employing thermal polycondensation followed by exfoliation to get g-C₃N₄ nanosheets. After condensation of g-C₃N₄ nanosheets the one dimensional (1D), g-C₃N₄ nanotubes were obtained. Meanwhile, O-doping was carried out by substituting C or N atoms during high-temperature oxidation process in airflow to get desired photocatalysts (OCN-tubes).

The uniform porous network structure was well observed from SEM images without bulk agglomerate while in case of bulk g-C₃N₄, the agglomerated morphology was noticed. While distinct TEM images confirmed the 1D tubular nanostructure of typical multi-walled carbon nanotubes (MWCNTs). Thus, such hierarchical structures due to their distinct features such as: (i) high specific surface area, (ii) multiple scattering in between channels, (iii) exposed active edges, (iv) excellent CO₂ uptake ability, and (v) enhanced molecular diffusion kinetics, can be designated as promising CO₂ reduction photocatalysts. As a result, hierarchical OCN-tube displayed CH₃OH formation of 0.88 µmolg⁻¹ h⁻¹, which exhibited five-fold higher photocatalytic performance compared to that of bulk g-C₃N₄ (0.17 µmol g⁻¹ h⁻¹). These results are coherent to N₂ adsorption–desorption isotherms with corresponding pore size distribution curves (Figure 11a). The meso and macroporous nature of 1D nanotubes was detected with 0.128 cm³ g⁻¹ pore volume higher than that of bulk g-C₃N₄ (0.037 cm³ g⁻¹). The enhanced photo absorption of the OCN-tube in the visible region was confirmed by ultraviolet–visible (UV–Vis) diffusion reflectance spectra (DRS) as shown in Figure 11b. The increased absorption was attributed to the multiple reflection of light inside the hierarchical multi-walled nanotubes. Considering the tubular hierarchical architectures with their own special properties, Wang et al. in 2017 fabricated novel, self-templated, In₂S₃-CdIn₂S₄ heterostructures for efficient CO₂ reduction accounting for solar fuel production [105]. In short, MOF (MIL-68) hexagonal prisms were first prepared and hierarchical In₂S₃ nanotubes were prepared by a liquid phase sulfidation approach and finally hierarchical In₂S₃-CdIn₂S₄ were collected by cation exchange reaction. The FESEM and TEM images revealed 1D tubular morphology with ultrathin sheet-shaped subunits as shown in Figure 12a–f. After evaluation for CO₂ photoreduction, it was observed that optimized tubular In₂S₃-CdIn₂S₄ sample generated a noticeable amount of CO (825 μmol h⁻¹ g⁻¹) in addition to a small amount of H₂ (Figure 12g). While the efficiency of CO was 12-fold higher than pristine In₂S₃ along with an enhanced selectivity and stability. Such high performance was attributed to unique tubular morphology displaying reduced diffusion length, large surface area, enhanced CO₂ adsorption ability and compositional features of the photocatalyst.

The CO₂ photoreduction results were further supported by N₂ adsorption isotherms (Figure 12h) of a hierarchical nanotube possessing a high BET surface area (68 m² g⁻¹) and exhibiting a high CO₂ uptake of ca. 25 cm³ g⁻¹ at 760 mmHg. The as-prepared photocatalyst exhibited selectivity for 6 cycles without any deactivation which proves its superiority as compared to other traditional photocatalysts. In addition to this work, Wang et al. in 2018, designed a sandwich-like ZnIn₂S₄−In₂O₃ hierarchical heterostructures acquired tubular morphology owing high stability and superior CO₂ photoreduction performance into deoxygenation to form CO as the main product [107]. In this study, ZnIn₂S₄ nanosheets were well assembled on the inside as well as on the outer surfaces of In₂O₃ to form a tubular sandwich-like ZnIn₂S₄−In₂O₃ double heterojunction. SEM images (Figure 13a–c) demonstrated uniform coverage of ZnIn₂S₄ on the surface of In₂O₃ which had a well-maintained 1D tubular morphology with open ends. At higher magnification (Figure 13c), ZnIn₂S₄ layers composed of nanosheets were grown on a In₂O₃ microtubular surface to form sandwich like heterostructure. Afterwards, such highly porous, hollow interiored nanotubes were utilized for CO₂ photoreduction and results exhibited the formation of CO as a main product in addition with noticeable amount of H₂ (Figure 13d,e). Here, the mixture of H₂O/acetonitrile with Co(bpy)₃²⁺ (bpy = 2, 2′-bipyridine) was used as co-catalyst and triethanolamine (TEOA) was used as an electron donor to conduct the CO₂ photoreduction test. The optimized sample produced CO with yield of 3075 μmol h⁻¹ g⁻¹ which was recorded much higher than bare ZnIn₂S₄ particles (875 μmol h⁻¹ g⁻¹) and ZnIn₂S₄ NSs (1275 μmol h⁻¹ g⁻¹) as shown in Figure 13d. The high activity was due to the large specific surface area (128 m² g⁻¹) of ZnIn₂S₄-In₂O₃ exhibiting porous features possessing the maximum CO₂ uptake ability of ca. 20 cm³ g⁻¹ at 0 °C. The transient photocurrent spectra also evidenced that as-prepared hierarchical structure exhibited enhanced photocurrent which was due to transfer of photogenerated charges in heterojunction system (Figure 13f). Therefore, such types of efficient materials possessing special morphology may inspire the development of artificial photocatalytic system for real and large-scale application.

Xiao et al. constructed hierarchical assemblies of Bi₂WO₆ acquiring a hollow and rod-shaped appearance [85]. To fabricate hierarchical Bi₂WO₆ structures, a solid Bi precursor was prepared and employed as a template to obtain a hollow morphology of Bi₂WO₆ because of the Kirkendall effect. The as-obtained hierarchical Bi₂WO₆ structures were composed of numerous nanosheets on the surface of hollow micro-rods as shown in Figure 14a. When the surface was practically broken, it was observed that nanosheets are uniformly decorated around the particles with a hollow interior of microrods (Figure 14b). In addition, the TEM image (Figure 14c) of Bi₂WO₆ hierarchical rods further confirmed its hollow feature with average shell thickness of about 300 nm. This study reveals that CO₂ photoreduction activity of as-obtained Bi₂WO₆ hierarchical rods is much higher as compared to bulk Bi₂WO₆. The CO₂ reduction was carried out under Xe lamp irradiation and 2.6 μmol g⁻¹ h⁻¹ CH₄ yield was achieved which was 8-fold higher than bulk Bi₂WO₆ along with a high recycling stability. The band gap investigation was carried out by using UV-vis DRS and a Mott–Schottky plot and outcomes indicated that the band gap of hierarchical Bi₂WO₆ (2.92 eV) is larger than bulk Bi₂WO₆. While the conduction band (CB) edge of the hierarchical Bi₂WO₆ rod was lower than redox potentials of CO₂/CH₄ (−0.24 V vs. reversible hydrogen electrode (RHE)), making it thermodynamically feasible for CO₂ reduction to CH₄. The N₂ adsorption-desorption isotherm was recorded to understand the nature of the material and results confirmed that as-prepared material possesses meso-/macro-porous morphology with district hysteresis loop in between 0.5-1.0 P/P₀. Henceforth, such hierarchical tubes/rods are considered as potential photocatalysts for advanced practical application in CO₂ photoreduction to various solar fuels.

4.5. Sheet-Like Hierarchical Materials

In addition to photocatalysts morphologies, 2D nanosheets hierarchical structures have also garnered enormous attention in CO₂ photoreduction process field. Hierarchical structured ZnV₂O₆ nanosheets, synthesized by one-step solvothermal method, when used for CO₂ photoreduction, showed extraordinary catalytic activity [108]. As-prepared ZnV₂O₆ nanosheets possess high surface area (BET) of 11.57 m² g⁻¹ and smaller pore diameter of 17.3 nm due to its hierarchical structure which played important role in minimizing mass transfer limitations and eventually increases the overall catalytic performance. In UV-Vis spectra, the ZnV₂O₆ nanosheets displayed absorption in the visible region with band gap of 2.02 eV. The SEM images revealed surface morphology of a hierarchical microstructure composed of uniform-sized nanosheets while the SAED pattern showed a ring of polycrystalline due to good crystalline nature of ZnV₂O₆. The CO₂ photoreduction performance of as-prepared ZnV₂O₆ nanosheets was measured using a slurry type photoreactor with quartz glass under visible light irradiation (35 W high-intensity discharge (HID) Xe lamp) at room temperature. Employing ZnV₂O₆, the CH₃OH, HCOOH and CH₃COOH were obtained as resulting solar products with a yield of 3253.84, 2886.9 and 530.1 μmol g⁻¹ respectively. While 48.78% selectivity of ZnV₂O₆ towards CH₃OH was achieved in current photocatalytic system which was attributed to its hierarchical structure possessing enhanced charge separation. Apart from the role of the main catalyst, 2D nanosheeets were also used as co-catalysts in photocatalytic processes. Jung et al. designed a ternary hierarchical hybrid structure containing mesoporous TiO₂ on graphene containing few layered 2D MoS₂ sheets via a one pot hydrothermal route [109]. The graphene played an important role to reduce the channel length of 3D structure and shorten the diffusion length for photogenerated carriers. However, 2D MoS₂ sheets were used as co-catalyst as it possesses high robustness and layer-dependent photoactivity. Therefore, in a typical hierarchical structure 2–3 layered MoS₂ were formed simultaneously with TiO₂ and graphene oxide (GO) during a one-pot hydrothermal process. The SEM and TEM images revealed that, 2D rGO sheets were assembled with a microporous nature while TiO₂ particles and MoS₂ sheets were uniformly covered over the graphene surface to form a hierarchical porous structure. The decreased photoluminescence intensity revealed the effective charge transfer of photogenerated carriers and delayed the recombination rate which played an important role in CO₂ photoreduction. The CO₂ reduction results exhibited that, CO was main product with selectivity of 97%. While, CO formation rate in optimized sample of TiO₂-graphene-MoS₂ hierarchical structure was 92.33 μmol g⁻¹ h⁻¹ which was 14.5-fold higher than the bare TiO₂. The photo-excited electrons generated from TiO₂ and graphene were accommodated on the surface of the MoS₂ where the CO₂ molecules were reduced to form CO as a solar product. Here, combination of microporous 3D graphene along with microporous TiO₂ and 2D-MoS₂ co-catalyst hierarchical nanostructured system helped to enhance the overall CO formation rate. Therefore, such studies highlight the promising ways of fabrication of efficient and high-performance photocatalysts for CO₂ photoreduction.

4.6. Hierarchical Nanoboxes

The prevention of unwanted aggregation and improvement in light absorption can be achieved by highly porous, hollow, three-dimensional (3D) nanoboxed hierarchical structures. For example, Qiu et al. successfully designed CdS/Co₉S₈ hollow cubes containing a cubic (box) structure and employed for photocatalytic water splitting [110]. They observed that hollow cubes exhibited stronger light absorption due to multi-light scattering/reflection inside the cavities of hollow architecture which enhanced overall photocatalytic performance. In this regard, Wang et al. in 2018, constructed complex hierarchical nanoboxes of nitrogen-doped carbon@NiCo₂O₄ (NC@NiCo₂O₄) for efficient visible light CO₂ reduction [82]. The as-prepared hybrid hierarchical nanoboxes containing hollow features possess both morphological and functional advantages such as enhancement in charge separation and increased CO₂ adsorption on the surface which offers more active sites for photocatalytic reaction. Figure 15a presents stepwise synthetic illustration of complex hierarchical NC@NiCo₂O₄ nanobox including, (i) Fe₂O₃@PDA core-shell formation via sol-gel followed by (ii) N-doped carbon formation after heating in a N₂ atmosphere. Finally, (iii) NiCo₂O₄ nanosheets were grown on NC nanoboxes through a hydrothermal approach followed by thermal treatment to obtain NC@NiCo₂O₄ double-shelled nanobox hierarchical structure. The FESEM images of nanoboxes revealed its cubic nature (Figure 15b,c) while at higher magnification (single nanobox) it can be observed that, NC@NiCo₂O₄ nanobox is enclosed with NiCo₂O₄ ultrathin sheets subunits (Figure 15d). The TEM and HRTEM images also acknowledged: (i) hollow nanostructured morphology and (ii) formation of NiCo₂O₄ nanosheets with interlayer distance of 0.28 and 0.46 nm corresponding to (220) and (111) crystal planes of NiCo₂O₄ (Figure 15e–g). While the SAED pattern gives information about the polycrystalline nature of NiCo₂O₄ nanosheets, elemental mapping calculated from EDX spectra revealed the presence of necessary elements (Figure 15h). The obtained nanobox contained a hollow architecture with high specific surface area of 142 m² g⁻¹. In addition, the CO₂ adsorption isotherm calculation exhibited higher CO₂ uptake of 60 cm³ g⁻¹ (at 0 °C) which was higher compared to bulk NiCo₂O₄ particles. Henceforth, these features of hierarchical nanoboxes actively promote deoxygenation of CO₂ to form the useful solar fuel, CO. The CO₂ photoreduction tests were carried out under visible light irradiation in H₂O/acetonitrile mixture using [Ru(bpy)₃]Cl₂.6H₂O as photosensitizer and triethanolamine (TEOA) as electron donor. As a result, hierarchical NC@NiCo₂O₄ nanoboxes exhibited 26.2 µmol h⁻¹ of CO which was higher than bulk NiCo₂O₄ (Figure 15i). To understand the effects of nanobox morphology, CO₂ photoreduction was carried out under different reaction conditions and the results are illustrated in Figure 15i. In particular, NC@NiCo₂O₄ nanoboxes exhibited excellent selectivity (86.6%) towards CO production along with the formation of a small amount of H₂. Additionally, the resulting CO₂ photoreduction system demonstrated the maximum visible-light AQY of 1.07% (at 420 nm) for production of CO. The improved photocatalytic performance was credited to, (i) the compositional features with reduced diffusion length, (ii) hollow morphology, (iii) high specific surface area, (iv) more active sites, and (v) high CO₂ adsorption ability. Thus, excellent features of nanoboxed hierarchical structures led to significantly enhanced photocatalytic activity, which offers a new viewpoint for constructing highly active photocatalysts for visible light-driven CO₂ reduction.

4.7. Hierarchical Natural Photocatalysts

Sakimoto et al. fabricated a self-augmented biological system by combining the non-photosynthetic bacterium Moorella thermoacetica with cadmium sulfide and utilized it for the production of acetic acid [111]. The results showed that, such semi-artificial system undergo CO₂ reduction to produce acetic acid continuously over several days with excellent quantum yield which provides a new route towards CO₂ reduction into useful solar fuels. Therefore, such systems have generated great interest among researchers as an alternative solar-to-fuel CO₂ conversion pathway.

Jiang et al. recently fabricated such a semi-artificial, nature-based visible light-driven hierarchical photocatalyst for CO₂ reduction exhibiting superior activity [112]. In their study, hierarchical treated rape pollen (TRP) was successfully employed for CO₂ photoreduction into CO with yield of 488.4 µmol g⁻¹ h⁻¹. Prior to use it for CO₂ photoreduction, the surface of rape pollen was washed with ethanol and morphology was fixed by treating with ethanol and formaldehyde (1:1) followed by treatment with H₂SO₄ to get final product i.e., TRP. FESEM images revealed its oval shape morphology incorporated with porous network structure (Figure 16a,b).The pore channels in the porous structure were beneficial for maximum light harvesting as shown in Figure 16c. At higher magnification in the TEM images (Figure 16d–f), it can be seen that, the channels are uniformly distributed in porous and hollow structure. However, elemental mapping analysis informed that the TRP is collectively composed of C, O, N and S elements which were further confirmed by XPS. The bandgap of 1.08 eV was calculated by using UV-vis DRS technique. It was observed that, the TRP absorbs light ranging from the UV to visible and NIR region. Therefore all these results confirmed its importance in CO₂ photoreduction reaction. Finally, CO₂ reduction test was carried out at room temperature under irradiation of Xenon lamp (300 W). The results revealed CO was main product of CO₂ reduction along with trace amount of CH₄ formation. Under the full spectrum (i.e., UV-Vis-NIR) the CO evolution rate was 845.7 µmol h⁻¹ g⁻¹ while under visible light region it was 488.4 µmol h⁻¹ g⁻¹ (selectivity, 98.3%). It is worth noting that the yield was much higher than common photocatalysts such as P25, g-C₃N₄ and other carbon-based materials at 420 nm along with photostability over three cycles. Such high CO production was attributed to porous and hollow structure containing large surface area of TRP which was further well supported by CO₂ adsorption capacity of 17.2 cm³ g⁻¹. Therefore, such nature-inspired biomaterials with a unique hierarchical, porous and hollow structure may open a promising avenue for solar light-driven CO₂ photoreduction to produce sustainable solar fuels.

Table 2. Summary of hierarchical morphologies with specific examples of photocatalysts, CO₂ reduction parameters, and efficiency evaluation of different products along with their yield and apparent quantum yield (AQY).

Hierarchical Morphology	Photocatalyst	Light Source and Reactor Type	CO2 Photoreduction Results; Yield and AQY	Ref.
Flower-, leaf- and fruit-like structures	Ru-MOF	A 500 W Xe lamp, liquid phase, in a 100 mL Schlenk tube (triethanolamine as a sacrificial agent)	HCOO−, 24.7 µmol g−1 (8 h), AQY 0.67%	[89]
	Bi2MoO6	A 300 W Xe arc lamp, liquid phase, in closed vessel	CH3OH, 24.8 and C2H5OH, 18.8 µmol g−1 (4 h)	[90]
	CeO2@Bi2MoO6	A 300 W Xe arc lamp (PLS-SXE300) with a 420 nm, a closed 200 mL quartz glass reactor containing 50 mL of ultrapure water	CH3OH and C2H5OH, 58.4 µmol g−1 (4 h)	[91]
	Perovskite Titanates	A 300 W Xe arc lamp, A gas closed circulation system with an upside window with λ > 420 nm cut-off-filter	CO, 349 and CH4, 231 nmol g−1 h−1	[94]
	Au@CdS HMCHPs (Au, 0.25%)	A 300 W Xe lamp with long-pass cutoff filter, a gas-closed glass reactor (80 mL)	CO, 3758 µmol g−1 h−1, AQY, 0.61%	[95]
Spheres (nano/micro)	Pt doped TiO2 spheres	A 40 W Hg UV lamp, a batch reactor (diameter of 39 mm and depth of 9 mm)	CO, 19 and CH4, 3.5 µmol g−1 h−1, AQY, CO, 1.632% CH4, 1.315%	[97]
	Porous TiO2	A 300 W Xe lamp, homemade Pyrex reactor (200 mL)	CH4, 0.23 μmol h−1 and CH3OH, 0.08 μmol h−1	[98]
	LaPO4	A 125-W high-pressure Hg lamp, Inner-irradiation quartz reactor (200-mL), Liquid phase	CH4, 10.5 μmol, AQY of 0.54%	[86]
	CdS-WO3	A 300 W Xe arc lamp with a UV cut-off- filter (λ ≥ 420 nm), home-made Pyrex reactor (200 mL)	CH4, 1.02 µmol g−1 h−1	[99]
	Hollow TiO2	A non-focused 6 W UV lights (Hitachi F6T5, 365nm), Schlenk flask (80 mL)	CO, 16.8 and H2, 6.6 µmol (2h), AQY, 0.66%	[100]
Nanofibers	Mesoporous TiO2	A 6 W UV lamp (λmax = 365 nm), gas-phase stainless steel photoreactor (190 mL)	CO, 203.91, CH4, 26.88, CH3OH, 5.04 and H2, 398.84 µmol g−1 AQY, 0.030%	[103]
	TiO2/Ni(OH)2	A 350 W Xe lamp (40 mW cm−2), self-designed reactor	CH4, 2.20, CO, 0.71 and CH3OH, 0.11 µmol g−1 h−1	[26]
	2.5% CuInS2/ TiO2	A 50 W Xe lamp, home-made Pyrex reactor (200 mL)	CH4, 2.5 and CH3OH, 0.86 μmol g–1 h–1	[104]
	10% MoS2/TiO2	A 350 W Xe lamp, home-made reactor	CH4, 2.86 and CH3OH, 2.55 μmol g–1 h–1, AQY, 0.16%	[105]
Nanotubes	O-g-C3N4	A 350 W Xe lamp along with a 420 nm cut-off-filter, Double-neck cylindrical home-made flask (200 mL)	CH3OH, 0.88 μmol g–1 h–1	[87]
	In2S3-CdIn2S4	A 300 W Xe lamp (400 nm cut-off-filter), An 80 mL reactor	CO, 825 μmol h−1 g−1	[106]
	ZnIn2S4-In2O3	A 300 W Xe lamp along with a 400 nm cut-off-filter, A 80 mL glass reactor (with gas-closed)	CO, 3075 μmol h−1 g−1	[107]
	Bi2WO6	A 300 W Xe lamp, A 100 mL quartz reactor	CH4, 2.6 μmol g−1 h−1	[85]
Sheets (and platelets)	ZnV2O6	A 35 W HID Xe lamp, A slurry type photoreactor with quartz glass	CH3OH, 3253.84, HCOOH, 2886.9 and CH3COOH, 530.1 μmol g−1	[108]
	TiO2-graphene-MoS2	A 300 W Xe lamp, stainless-steel reactor	2.33 μmol g−1 h−1	[109]
Nanoboxes	N-carbon@NiCO2O4	A 300W Xe lamp (420 nm cut-off-filter), 80 mL glass reactor	26.2 μmol h−1	[82]
Nature based materials
Oval	Treated rape pollen	300 W Xenon lamp, closed gas system	CO, 488.4 μmol h−1 g−1, Quantum Efficiency (QE), 6.7%	[112]

summarises the latest state of the art CO₂ photoreduction results specifically focussing upon hierarchical nanostructures.

In this account, the necessity of solar energy capture for sustainable fuel production has motivated the development of efficient photocatalysts. Hierarchical nanostructures are considered to be potential candidates due to their immense features aimed at conversion of solar energy to chemical energy. Even though various synthetic approaches have been established, it is still challenging to achieve a cost-effective, large-scale fabrication of these hierarchical nanostructures along with high-yield and stability. The state-of-the-art designs for sustainable photocatalysts are always related to the continuous exploitation of fabrication procedures. Therefore, to overcome these issues, discovering novel synthetic approaches could be significant. Refining the current synthesis methods of these hierarchical nanotemplates is also of equal importance. The light scattering phenomenon in the cavities of flower/fruit possess maximum light harvesting ability. For instance, Au@CdS HMCHPs due to its hollow and porous morphology yielded 3758 µmol g⁻¹ h⁻¹ of CO because of the light-scattering effect. Overall, we believe that the sophisticated hollow and porous structures can provide treasured opportunities in the near future with improvements in cost, yield, processing, and stability.

5. Summary and Outlook

Herein, we have surveyed recent progress in various hierarchical nanostructures with application to CO₂ photoreduction. The use of solar energy for sustainable fuel production has motivated the development of efficient photocatalysts for CO₂ reduction. In conversion of CO₂ reactions efficient adsorption of CO₂ molecule on the surface of photocatalyst is quite important. The high surface area self-assembled hierarchical nanostructured photocatalysts show great potential for promoting CO₂ adsorption due to their abundant active sites and, furthermore, ability to promote enhanced light harvesting. Clearly, advanced fabrication methods affording precise manipulation of surface, structural properties, and a hierarchical pore network are critical for the rational design of high-performance photocatalysts.