Recent Developments in Lead and Lead-Free Halide Perovskite Nanostructures towards Photocatalytic CO2 Reduction

Perovskite materials have been widely considered as emerging photocatalysts for CO2 reduction due to their extraordinary physicochemical and optical properties. Perovskites offer a wide range of benefits compared to conventional semiconductors, including tunable bandgap, high surface energy, high charge carrier lifetime, and flexible crystal structure, making them ideal for high-performance photocatalytic CO2 reduction. Notably, defect-induced perovskites, for example, crystallographic defects in perovskites, have given excellent opportunities to tune perovskites’ catalytic properties. Recently, lead (Pb) halide perovskite and their composites or heterojunction with other semiconductors, metal nanoparticles (NPs), metal complexes, graphene, and metal-organic frameworks (MOFs) have been well established for CO2 conversion. Besides, various halide perovskites have come under focus to avoid the toxicity of lead-based materials. Therefore, we reviewed the recent progress made by Pb and Pb-free halide perovskites in photo-assisted CO2 reduction into useful chemicals. We also discussed the importance of various factors like change in solvent, structure defects, and compositions in the fabrication of halide perovskites to efficiently convert CO2 into value-added products.

Typically, perovskite materials are indicated with the chemical formula ABX 3 [26,27]; here, A site occupies large size cation (e.g., Cs + , Rb + , methylammonium), and B sites are occupied by small size cation (e.g., Pb 2+ , Sn 2+ ). At the same time, X (e.g., O 2− , Br − , Cl − , I − ) holds an anion that bonds to both A and B. According to the crystallographic perspective, the ideal perovskite structure is cubic and unbending; however, most perovskites are generally distorted. Based on the perovskite elements, the properties such as chemical stability, bandgap energies, optical stability, and crystal structure

Lead Halide Perovskites for Photocatalytic CO 2 Reduction
So far, organic-inorganic lead halide perovskites (LHPs) have been well established for photovoltaic applications with high power conversion proficiency [25]. Later, they have been successfully used for various applications, including photodetector, laser, LED, thermoelectric, and piezoelectric [51,52]. Recently, Zhu et al. reported LHPs for organic synthesis that have fundamental significance in drug production [53]. The high efficiency of LHPs was mainly attributed to the properties that are very suitable for photocatalytic applications, such as high absorption coefficient, greater defect tolerance, superior photogenerated charge-carrier lifetime, and high carrier mobility [54]. In recent years, LHPs have also been used as photocatalytic materials for various applications like hydrogen evolution reaction [55,56], organic pollutant degradation [57,58], and alkylation of aldehyde [41,51]. In addition, they have been proved the best materials for oxygen evolution reactions; thus, water can be used as an electron source for the photoreduction of CO 2 by preventing the use of sacrificial agents [59]. Thus, the introduction of halide perovskites for CO 2 photoreduction has made substantial progress in the field of catalysis. These materials possess poor water stability due to the ionic nature of the LHPs. Numerous attempts have been made to boost the catalytic activity and stability of LHPs towards photocatalytic CO 2 reduction. In this contribution, Wu et al. encapsulated methylammonium lead iodide quantum dots (QDs) in iron-based metal-organic frameworks (MOFs) and successfully utilized them for CO 2 photoreduction [60]. They have used Fe-porphyrin-based MOFs to increase the water stability of perovskite. Fe act as an active catalytic site for CO 2 photoreduction, which also suppresses the charge recombination and effectively enhances the charge transportation. The optimized samples of MAPbI 3 with Fe-porphyrin MOF PCN-221(Fe x ) exhibited a total hydrocarbon yield of 1559 µmol g −1 combined for CO and CH 4 production. As the concentration of Fe in PCN-221 (Fe x ) increases, the improvement in CO and CH 4 formation was observed. The catalyst showed excellent stability over 80 h, which was much higher than pristine halide perovskite QDs. Remarkably, water was used as a sacrificial agent with ethyl acetate, which acts as an electron source for the CO 2 reduction reaction. The light-harvesting efficiency of MAPbI 3 and fast electron transfer from MAPbI 3 to Fe were attributed to high catalytic activity.
Apart from this exceptional research, most methylammonium halides have been documented for hydrogen production. Previous studies have shown that MAPbI 3 is unstable in humid conditions; thus, other halide perovskites have been intended for CO 2 photoreduction [61]. Subsequently, major advances have been made in the field of research with cesium lead halide perovskites as a novel and most common catalyst for CO 2  into CH 4 , CO, and H 2 [61]. Using less costly inorganic precursors and oleylamine/oleic acid as a surface ligand, the QDs were synthesized using a simple solution process. These colloidal QDs display the tunable particle size due to the quantum confinement effect at different temperatures, as shown in Figure 1a-d. The optical properties of CsPbBr 3 showed a significant impact on photocatalytic efficiency (Figure 1e,f). During the photocatalytic reaction, the average electron yield for CO 2 reduction was 20.9 µmol g −1 .
A Time-resolved PL study signifies that an optimized catalyst has the most extended lifetime responsible for enhanced catalytic activity (Figure 1g). The band alignment of the valence and conduction band were well matched for CO 2 reduction and water oxidation leading to the formation of CO, CH 4 , and H 2 ( Figure 1h). Therefore, such QDs can be used as inexpensive catalysts for catalytic applications. Nanomaterials 2020, 10, x FOR PEER REVIEW 4 of 26 dependent CO2 photoreduction into CH4, CO, and H2 [61]. Using less costly inorganic precursors and oleylamine/oleic acid as a surface ligand, the QDs were synthesized using a simple solution process. These colloidal QDs display the tunable particle size due to the quantum confinement effect at different temperatures, as shown in Figure 1a-d. The optical properties of CsPbBr3 showed a significant impact on photocatalytic efficiency (Figure 1e,f). During the photocatalytic reaction, the average electron yield for CO2 reduction was 20.9 µmol g −1 . A Time-resolved PL study signifies that an optimized catalyst has the most extended lifetime responsible for enhanced catalytic activity ( Figure 1g). The band alignment of the valence and conduction band were well matched for CO2 reduction and water oxidation leading to the formation of CO, CH4, and H2 ( Figure 1h). Therefore, such QDs can be used as inexpensive catalysts for catalytic applications.   Mixed halide perovskites, i.e., a mixture of the cation (MA + , Cs + ) and anion (Cl − , I − , Br − ), can further increase the efficiency of catalytic reaction towards CO 2 reduction. Tuning the halide ratio can change the structural combination of perovskites that significantly impact catalytic behavior. Thus, Su and co-workers reported a low-cost, cubic phase CsPb(Br 0.5 /Cl 0.5 ) 3 perovskite with varied ratios of Br and Cl and studied for photocatalytic CO 2 reduction [62]. The controlled synthesis of mixed halide perovskite was carried out by the hot injection method. Mixed halide perovskite, i.e., CsPb(Br x /Cl 1−x ) 3, offers excellent absorption in the visible region ranging from 400-700 nm, relatively broader than distinctive semiconductors like TiO 2 and ZnO. The calculated band edges were quite suitable for CO 2 reduction potentials of CO and CH 4 , with the bandgap ranging from 2.33 to 2.98 eV for CsPb(Br x /Cl 1−x ) 3 samples. While in the emission spectra, the gradual shifting and quenching of pristine CsPbBr 3 from 517 to 413 nm was observed as the amount of Cl increases, which is attributed to the improved electron separation. The authors performed a CO 2 reduction test in ethyl acetate. The increased CO and CH 4 formation was found with an increase in the Cl concentration, and the optimized sample achieved much higher catalytic activity than the pristine CsPbBr 3 and CsPbCl 3 perovskites. The time-dependent CO 2 reduction rate for the optimized catalyst demonstrated strong stability over the 9 h for CH 4 and CO evolution. The increased activity and stability of mixed halide perovskite were attributed to the controlled ratios of Br and Cl.
It is well known that pristine semiconductor/perovskites suffer from poor catalytic activity and stability due to the rapid recombination of photogenerated charges or lack of suitable optical absorption, or ineffective CO 2 adsorption. Therefore, several studies have been documented with metal doping for cesium halide perovskites to boost catalytic activity, stability, and selectivity. In order to attain these distinct characteristics, it is crucial to consider the mechanism of product formation and the pathway of CO 2 reduction over the catalyst. Tang et al. theoretically studied the effect of metal doping in CsPbBr 3 on product selectivity by employing DFT calculations ( Figure 2) [63]. The model used for this analysis has considered the chemical potential of an electron and proton is equal to half of the hydrogen gas phase at standard pressure. The first step of CO 2 reduction formed the HCOO*, and further adsorption of the excessive proton on the oxygen atom leads to HCO*OH formation. The HCO*OH is a spontaneous reaction; however, dissociation of HCO*OH to CO and H 2 O is the nonspontaneous reaction in the case of pristine-CsPbBr 3 . After Co and Fe doping, the catalytic activity was improved and showed a downhill reaction. The numerical values of Gibbs free energy indicated that Co and Fe doping increases the rate of chemical reaction approximately two times for catalytic activity than pristine-CsPbBr 3 material. Pristine CsPbBr 3 was initially examined to reduce CO 2 , and the findings of the free barrier energy showed that CsPbBr 3 is inactive for the evolution of any hydrocarbons. While Fe and Co-doped CsPbBr 3 were investigated for selective formation of CH 4 , these metals have great potential to break the O-C-O bond. The resultant activity and selectivity towards CH 4 formation were ascribed to the successful adsorption and activation of CO 2 • on the doped CsPbBr 3 . After this, Pradhan and co-workers practically explored Fe (II)-CsPbBr 3 for selective CH 4 evolution via CO 2 photoreduction [64]. Doping of Fe (II) replaces Pb (II) in the CsPbBr 3 lattice, and an increased CH 4 selectivity was observed over the increment of Fe (II) concentration; however, pristine CsPbBr 3 forms CO. Later, another study by Su et al. showed that Mn 2+ substitution to perovskite could significantly improve the optical and thermal properties of halide perovskites [65]. They observed that Mn-doped cesium lead halide (Br/Cl) perovskite showed more than 14 times improvement in catalytic performance than pristine catalysts. The Mn-CsPb(Br/Cl) exhibited the catalytic towards CO and CH 4 formation with a yield of 1917 µmol g −1 and 82 µmol g −1 , respectively. Other metals such as Fe and Co have been reported as ideal dopants with cesium perovskite, and improved water sustainability was reported [63]. For instance, Fe (II) doped CsPbBr 3 and Co-doped CsPbBr 3  Afterward, more outcomes of metal confined halide perovskites are reported. Lu and coworkers' study showed that Co-doped CsPbBr3/Cs4PbBr6 combination improved catalytic performance for CH4 and CO evolution [59]. Co doping to perovskite has two benefits; first, it produces surface trap states due to the presence of Co 2+ and extends the lifetime of photogenerated charges. Second, it broadens the adsorption of CO2* intermediate to the catalytic surface. Remarkably, they have employed hexafluorobutyl methacrylate to improve NC's stability and dispersity in an aqueous medium. Then, in this class of materials, Zhang and coworkers reported Co-doped CsPbBr3 embedded in the matrix of Cs4PbBr6 (a surface protector) [66]. The reason behind such protection of Cs4PbBr6 was to improve the stability of ligand-free CsPbBr3 perovskite. As a result, the optimized Co-doped perovskite composite exhibited excellent CO2 reduction towards CO formation with 1835 µmol g −1 in 15 h. In combination with the acetonitrile/water solution, the methanol was used as a hole scavenger to improve the catalytic activity towards CO2 reduction. Interestingly, Cs4PbBr6 does not take part in the redox reaction due to its unsuitable band potentials/alignment; however, CsPbBr3 takes part in the CO2 reduction process and triggers the catalytic activity towards CO formation. After these reports, Pt has also been successfully used as a co-catalyst for CsPbBr3 [67]. This study stated the solvent effect, where it was observed that acetate is the most potent solvent, which provides a stable atmosphere for perovskite to conduct the CO2 reduction process. The optimized Pt loaded catalyst demonstrated an electron consumption rate of 5.6 µmol g −1 h −1 in ethyl acetate. In another study of Zhu and co-workers, the water-stable CsPbCl3 was documented with Mn and Ni doping. They illustrated the significance of the Pb-rich surface, which extends the lifetime of PL to increase the catalytic activity [68]. By interpenetrating solid-liquid, the synthesis of surface Pb enriched CsPbCl3 was accomplished by allowing the water to drain through the CsPbCl3 layer originating with Cs + and Cl − . These Cs + and Cl − inhibit the decomposition of CsPbCl3 and increase the PL lifetime. Therefore, Ni-doped Pb-rich CsPbCl3 QDs displayed superior CO2 reduction behavior against CO evolution with a rate of 169.37 µmol g −1 h −1 .
In addition to metal doping/deposition, perovskites are combined with supporting material to increase light absorption and charge separation. Among various materials, graphene can be a good choice as a support material for perovskites due to its well-known surface, optoelectronic, and physicochemical properties [69,70], which ultimately prolongs the electron/hole pair's lifetime and makes perovskite ideal for photocatalytic CO2 conversion. Hence, Xu et al. fabricated CsPbBr3 QDs/graphene oxide (GO) composite by a simple precipitation process for photoconversion of CO2 in a nonaqueous medium [71]. The CO2 reduction reaction was conducted in a Pyrex bottle using ethyl acetate as a solvent. The use of ethyl acetate has two benefits; (i) it stabilizes CsPbBr3 due to its moderate polarity; and (ii) it increases the solubility of CO2 even more than in water. The presence of Afterward, more outcomes of metal confined halide perovskites are reported. Lu and co-workers' study showed that Co-doped CsPbBr 3 /Cs 4 PbBr 6 combination improved catalytic performance for CH 4 and CO evolution [59]. Co doping to perovskite has two benefits; first, it produces surface trap states due to the presence of Co 2+ and extends the lifetime of photogenerated charges. Second, it broadens the adsorption of CO 2 * intermediate to the catalytic surface. Remarkably, they have employed hexafluorobutyl methacrylate to improve NC's stability and dispersity in an aqueous medium. Then, in this class of materials, Zhang and coworkers reported Co-doped CsPbBr 3 embedded in the matrix of Cs 4 PbBr 6 (a surface protector) [66]. The reason behind such protection of Cs 4 PbBr 6 was to improve the stability of ligand-free CsPbBr 3 perovskite. As a result, the optimized Co-doped perovskite composite exhibited excellent CO 2 reduction towards CO formation with 1835 µmol g −1 in 15 h. In combination with the acetonitrile/water solution, the methanol was used as a hole scavenger to improve the catalytic activity towards CO 2 reduction. Interestingly, Cs 4 PbBr 6 does not take part in the redox reaction due to its unsuitable band potentials/alignment; however, CsPbBr 3 takes part in the CO 2 reduction process and triggers the catalytic activity towards CO formation. After these reports, Pt has also been successfully used as a co-catalyst for CsPbBr 3 [67]. This study stated the solvent effect, where it was observed that acetate is the most potent solvent, which provides a stable atmosphere for perovskite to conduct the CO 2 reduction process. The optimized Pt loaded catalyst demonstrated an electron consumption rate of 5.6 µmol g −1 h −1 in ethyl acetate. In another study of Zhu and co-workers, the water-stable CsPbCl 3 was documented with Mn and Ni doping. They illustrated the significance of the Pb-rich surface, which extends the lifetime of PL to increase the catalytic activity [68]. By interpenetrating solid-liquid, the synthesis of surface Pb enriched CsPbCl 3 was accomplished by allowing the water to drain through the CsPbCl 3 layer originating with Cs + and Cl − . These Cs + and Cl − inhibit the decomposition of CsPbCl 3 and increase the PL lifetime. Therefore, Ni-doped Pb-rich CsPbCl 3 QDs displayed superior CO 2 reduction behavior against CO evolution with a rate of 169.37 µmol g −1 h −1 .
In addition to metal doping/deposition, perovskites are combined with supporting material to increase light absorption and charge separation. Among various materials, graphene can be a good choice as a support material for perovskites due to its well-known surface, optoelectronic, and physicochemical properties [69,70], which ultimately prolongs the electron/hole pair's lifetime and makes perovskite ideal for photocatalytic CO 2 conversion. Hence, Xu et al. fabricated CsPbBr 3 QDs/graphene oxide (GO) composite by a simple precipitation process for photoconversion of CO 2 in a nonaqueous medium [71]. The CO 2 reduction reaction was conducted in a Pyrex bottle using ethyl acetate as a solvent. The use of ethyl acetate has two benefits; (i) it stabilizes CsPbBr 3 due to its moderate polarity; and (ii) it increases the solubility of CO 2 even more than in water. The presence of GO provides additional electron transfer from CsPbBr 3 to GO; therefore, it was found that the CsPbBr 3 /GO composite exhibited improved catalytic activity compared to pristine CsPbBr 3 QDs. The catalyst displayed stability over 12 h, and no phase change of CsPbBr 3 was observed during the catalytic reaction. Excited electrons from CsPbBr 3 quickly transfer to the GO sheet, suppress the electron-hole pair's recombination, and increase catalytic activity.
Shortly after, few more studies have been reported in combination with graphene. For instance, Eslava and the group reported a surfactant-free synthesis of CsPbBr 3 NCs [72]. They have synthesized CsPbBr 3 on a gram scale by employing the simple mechanochemical process to get the different morphology of NCs, including nanorods, nanosheets, and nanospheres. To improve the catalytic efficiency, CsPbBr 3 was further combined with Cu-RGO by the mechanochemical process. The CsPbBr 3 nanosheets with Cu-RGO achieved 12.7, 0.46, and 0.27 µmol g −1 h −1 of CH 4 , CO, and H 2 evolution rates after CO 2 reduction. The catalyst composed of CsPbBr 3 -Cu-RGO achieved 1.10% apparent quantum efficiency and showed excellent stability over three consecutive runs. Such an alternative method of large-scale synthesis is notable and essential for advancing the photocatalytic technology practically. Later, Wang and co-workers demonstrated the effectiveness of RGO sheets combined with Cs 4 PbBr 6 for CO 2 reduction [73]. They revealed that the dual nature of the RGO is responsible for increased catalytic activity and stability, (i) the defect-induced RGO efficiently traps the electron excited by Cs 4 PbBr 6 , and (ii) oxygen-deficient RGO adsorbs and stimulates CO 2 molecule. In addition to these studies, Mu and co-workers described the ultrathin small-sized graphene oxide (USGO) as an electron mediator between CsPbBr 3 /α-Fe 2 O 3 Z-scheme photocatalyst [74]. Intense contact between CaPbBr 3 /USGO through the Br-O-C bond; and USGO/α-Fe 2 O 3 via the C-O-Fe bond accelerates the transfer of electrons through the Z-scheme. As a result, such a Z-scheme combination achieved 9 times greater catalytic efficiency compared to CsPbBr 3 NCs. Later on, such a Z-scheme heterojunction of CsPbBr 3 QDs was also reported with Bi 2 WO 6 nanosheet [75]. Therefore, the Z-scheme combination then performs efficient charge separation across the closely connected interface and enhances the catalytic activity.
Recently, 2D materials have grabbed huge attention as the best supporting materials in many applications. A halide perovskites/2D composite materials strategy can achieve an efficient charge transfer and reduced electron-hole recombination among the perovskite-based photocatalysts. Xu et al. produced the CsPbBr 3 /Pd Schottky junction in 2018 and studied the enhanced consumption rate of electrons for photoreduction of CO 2 [76]. For this analysis, CsPbBr 3 NCs were deposited on Pd nanosheets under atmospheric conditions using a simple technique on a glass substrate, and the photocatalytic reduction of CO 2 to CO/CH 4 was studied. The charge transport and charge carrier dynamics among the composite were studied by PL and fs-TAS (femtosecond transient absorption spectroscopy). The PL quenching of 0.5-8.6% in the CsPbBr 3 /Pd composite was observed compared to the pristine sample ( Figure 3a). Likewise, the decreased PL decay (Figure 3b) of the composite was observed in TRPL analysis with an average lifetime of 2.71-13.38 ns; however, such a decay lifetime for CsPbBr 3 was measured to be 52.03 ns. Furthermore, a similar trend was observed in the fs-TAS analysis (Figure 3c), where a large decrease in peak intensity of composite than the pristine sample was observed. These findings indicate that the recombination rate of the electron-hole in CsPbBr 3 was reduced by integrating it over Pd nanosheets, which consciously promotes the process of CO 2 reduction to form CH 4 and CO ( Figure 3d). Remarkably, the Schottky contact between CsPbBr 3 /Pd composite increases the electron consumption rate up to 33.80 µmol g −1 (for CH 4 and CO evolution), 2.43 times higher than the pristine CsPbBr 3 with the improved quantum efficiency (Figure 3e,f). Therefore, a combination of halide perovskite with 2D materials proved to be a good strategy for the photocatalytic CO 2 reduction reaction. As expected, the PL and TRPL quenching in composite compared to pristine perovskite confirm the efficient charge transfer among the interface of CsPbBr3/MXene. The photocatalytic CO2 reduction test was carried out under light irradiation using ethyl acetate solvent towards CO and CH4 formation with a rate of 26.32 and 7.25 µmol g −1 h −1 , respectively. Therefore, such perovskite/2D composites can be used as an efficient catalyst for photocatalytic applications.
Construction of perovskites/semiconductor heterojunction is advantageous to improve the optoelectronic or photochemical properties of catalytic systems. As well known, when two semiconductors with different band potentials combine, heterojunction forms at the interface of particular semiconductors and facilitate the charge separation process. Among various candidates, TiO2 was proven to be an excellent semiconducting material all over the years for photocatalytic applications. In this aspect, Xu et al. in 2018 reported an amorphous TiO2 encapsulated CsPbBr3 Later on, Liu et al. introduced a functional CsPbBr 3 /MXene nanocomposite for CO 2 reduction to CO and CH 4 under visible light [77]. The consistent growth of CsPbBr 3 on exfoliated MXene −n (n = 10, 20, 30, 40, and 50, a different amount of MXene) nanosheet was achieved by an in-situ method, as shown in Figure 4a. The etching of Ti 3 AlC 2 was carried out by using the HCl-HF solution to form a Ti 3 C 2 T x nanosheet. Then, the final nanocomposite was obtained by the exfoliation of multilayered Ti 3 C 2 T x and in-situ growth of CsPbBr 3 on Ti 3 C 2 T x nanosheets. The dispersion of cubic CsPbBr 3 with an average size of 25 nm on MXene nanosheets was observed in the TEM images (Figure 4b-d), and the presence of Ti, Pb, Br, and Cs confirms the constant growth of perovskite NCs on MXene nanosheet (Figure 4e-i). As expected, the PL and TRPL quenching in composite compared to pristine perovskite confirm the efficient charge transfer among the interface of CsPbBr 3 /MXene. The photocatalytic CO 2 reduction test was carried out under light irradiation using ethyl acetate solvent towards CO and CH 4 formation with a rate of 26.32 and 7.25 µmol g −1 h −1 , respectively. Therefore, such perovskite/2D composites can be used as an efficient catalyst for photocatalytic applications. result, the CsPbBr3/amorphous TiO2 composite demonstrated 6.5 times better photoelectron intake during the CO2 photoreduction and stability over 30 h. The high selectivity towards CH4 formation was attributed to the perfect combination of CsPbBr3 and amorphous TiO2. The electron generated by CsPbBr3 accumulates on TiO2 to break the dynamic barrier and therefore fast-track the rate of CH4 formation. Thus, such a study helps to understand the surface modification of halide perovskites for various applications. Afterward, in 2020 Yu and co-workers developed a self-assembled CsPbBr3 QDs/TiO2 nanofibers, an S-scheme heterojunction hybrid for CO2 reduction under the irradiation of UV-Vis light [79]. The electron microscopy results revealed that CsPbBr3 QDs were uniformly distributed on TiO2 nanofibers. Density functional theory (DFT) and experimental studies were combined to comprehend the interfacial charge transfer among the composite. The chemical states of pristine and composite samples were explored by in-situ and ex-situ XPS analysis. As shown in Figure 5a,b, the Construction of perovskites/semiconductor heterojunction is advantageous to improve the optoelectronic or photochemical properties of catalytic systems. As well known, when two semiconductors with different band potentials combine, heterojunction forms at the interface of particular semiconductors and facilitate the charge separation process. Among various candidates, TiO 2 was proven to be an excellent semiconducting material all over the years for photocatalytic applications. In this aspect, Xu et al. in 2018 reported an amorphous TiO 2 encapsulated CsPbBr 3 composite with enhanced catalytic efficiency [78]. The improved separation of the charge between CsPbBr 3 and TiO 2 was shown by the decay of the PL and TRPL, where dramatic quenching of radiative recombination was observed. Moreover, the fs-TAS analysis revealed that the decreased electron-hole recombination improves the charge separation efficiently among the composite. As a result, the CsPbBr 3 /amorphous TiO 2 composite demonstrated 6.5 times better photoelectron intake during the CO 2 photoreduction and stability over 30 h. The high selectivity towards CH 4 formation was attributed to the perfect combination of CsPbBr 3 and amorphous TiO 2 . The electron generated by CsPbBr 3 accumulates on TiO 2 to break the dynamic barrier and therefore fast-track the rate of CH 4 formation. Thus, such a study helps to understand the surface modification of halide perovskites for various applications.
Afterward, in 2020 Yu and co-workers developed a self-assembled CsPbBr 3 QDs/TiO 2 nanofibers, an S-scheme heterojunction hybrid for CO 2 reduction under the irradiation of UV-Vis light [79]. The electron microscopy results revealed that CsPbBr 3 QDs were uniformly distributed on TiO 2 nanofibers. Density functional theory (DFT) and experimental studies were combined to comprehend the interfacial charge transfer among the composite. The chemical states of pristine and composite samples were explored by in-situ and ex-situ XPS analysis. As shown in Figure 5a,b, the Ti 2p peak of Ti 4+ ions, O 1s of lattice oxygen, and surface -OH group are present in all the samples. However, in the in-situ measurement, the binding energies (BE) of Ti 2p and O 1s peaks shifted towards higher BE than ex-situ spectra. A similar observation but opposite peak shift was observed in the Br 2d peak, mainly attributed to an electron transfer from CsPbBr 3 to TiO 2 (Figure 5c). Such electron transfer is responsible for constructing the S-scheme heterojunction among TiO 2 /CsPbBr 3 , which efficiently separates the photogenerated charges to promote CO 2 reduction. The electron transfer from CsPbBr 3 to TiO 2 was further validated by work function values calculated from the energy difference of vacuum and fermi levels, as shown in Figure 5d-f. Due to the lower Fermi level of TiO 2 than CsPbBr 3 QDs (work function (φ) of CsPbBr 3 , 5.79 eV and TiO 2 , 7.18 or 7.08 eV), the electron flow would be favorable from CsPbBr 3 to TiO 2 for enabling the phases at the similar Fermi level and created an internal electric field at the interface of TiO 2 /CsPbBr 3 . A similar observation was observed in DFT calculations. Therefore, such an improved electron transfer is responsible for the enhanced catalytic performance of hybrid (9.02 µmol g −1 h −1 ) than the pristine CsPbBr 3 and TiO 2 (4.94 and 4.68 µmol g −1 h −1 ), respectively, towards CO formation. The improved CO 2 adsorption on CsPbBr 3 QDs and S-scheme heterojunction formation was ascribed to superior photocatalytic activity. Nanomaterials 2020, 10, x FOR PEER REVIEW 10 of 26 Ti 2p peak of Ti 4+ ions, O 1s of lattice oxygen, and surface -OH group are present in all the samples. However, in the in-situ measurement, the binding energies (BE) of Ti 2p and O 1s peaks shifted towards higher BE than ex-situ spectra. A similar observation but opposite peak shift was observed in the Br 2d peak, mainly attributed to an electron transfer from CsPbBr3 to TiO2 (Figure 5c). Such electron transfer is responsible for constructing the S-scheme heterojunction among TiO2/CsPbBr3, which efficiently separates the photogenerated charges to promote CO2 reduction. The electron transfer from CsPbBr3 to TiO2 was further validated by work function values calculated from the energy difference of vacuum and fermi levels, as shown in Figure 5d-f. Due to the lower Fermi level of TiO2 than CsPbBr3 QDs (work function (ϕ) of CsPbBr3, 5.79 eV and TiO2, 7.18 or 7.08 eV), the electron flow would be favorable from CsPbBr3 to TiO2 for enabling the phases at the similar Fermi level and created an internal electric field at the interface of TiO2/CsPbBr3. A similar observation was observed in DFT calculations. Therefore, such an improved electron transfer is responsible for the enhanced catalytic performance of hybrid (9.02 µmol g −1 h −1 ) than the pristine CsPbBr3 and TiO2 (4.94 and 4.68 µmol g −1 h −1 ), respectively, towards CO formation. The improved CO2 adsorption on CsPbBr3 QDs and S-scheme heterojunction formation was ascribed to superior photocatalytic activity. Next, 3D structures, e.g., 3D microporous graphene, exhibited possible support material for catalytic reactions [80,81]. Such a 3D network structure can provide more CO2 reactive sites and provide fast charge transport across multidimensional networks. Kaung and co-workers fabricated the hierarchical ternary nanocomposite of CsPbBr3 with ZnO nanowire/3D graphene through a multistep process for photocatalytic CO2 reduction [81]. First, in-situ 1D ZnO/2D RGO macropores with a high specific surface area were fabricated on a film. Then, as-prepared CsPbBr3 was used for the synthesis of ternary composite through the centrifugation cast method. SEM and TEM images revealed that the CsPbBr3 NCs are well decorated in the ZnO nanowires over RGO. Similarly, optoelectronic and surface properties showed improved light harvesting in the visible region and improved CO2 adsorption on the catalyst's surface, respectively. As a result, the ternary composite Next, 3D structures, e.g., 3D microporous graphene, exhibited possible support material for catalytic reactions [80,81]. Such a 3D network structure can provide more CO 2 reactive sites and provide fast charge transport across multidimensional networks. Kaung and co-workers fabricated the hierarchical ternary nanocomposite of CsPbBr 3 with ZnO nanowire/3D graphene through a multi-step process for photocatalytic CO 2 reduction [81]. First, in-situ 1D ZnO/2D RGO macropores with a high specific surface area were fabricated on a film. Then, as-prepared CsPbBr 3 was used for the synthesis of ternary composite through the centrifugation cast method. SEM and TEM images revealed that the CsPbBr 3 NCs are well decorated in the ZnO nanowires over RGO. Similarly, optoelectronic and surface properties showed improved light harvesting in the visible region and improved CO 2 adsorption on the catalyst's surface, respectively. As a result, the ternary composite exhibited 52.02 µmol g −1 h −1 of Nanomaterials 2020, 10, 2569 11 of 24 CH 4 evolution (96.7% selectivity) along with CO formation. The electron pathway for CO 2 reduction was achieved via CsPbBr 3 to 1D ZnO to 3D RGO.
Apart from metal oxides, CsPbBr 3 can also be anchored with g-C 3 N 4 due to its superior properties, such as visible-light active, tunable band potentials, and rich active surface area [82,83]. The combination of CsPbBr 3 and g-C 3 N 4 facilitates efficient charge transport through their closely connected interface. In this regard, Xu et al. attached CsPbBr 3 QDs to amino-functionalized g-C 3 N 4 nanosheet through N-Br bonding [83]. The 20 wt.% contained QDs anchored on g-C 3 N 4 achieved a superior photocatalytic CO 2 reduction towards CO formation with a rate of 149 µmol h −1 g −1 in acetonitrile/water solvent. The surface functionalization of g-C 3 N 4 with abundant NH x was shown to help build a bridge between g-C 3 N 4 and CsPbBr 3 . Such N-Br bonding was confirmed by XPS analysis, which is responsible for the enhanced charge separation and decreased electron-hole recombination rate. Resulting, the composite showed 15-and 3-fold improved catalytic activity than pristine CsPbBr 3 QDs and g-C 3 N 4 . Later, in 2019 a similar study was reported by Zhang and co-workers for CsPbBr 3 /g-C 3 N 4 containing TiO species (TiO-CN) [84]. The well-defined composite of CsPbBr 3 @TiO-CN was able to undergo CO 2 reduction to produce a 129 µmol g −1 of CO under 10 h visible light irradiation. The interaction among CsPbBr 3 and g-C 3 N 4 was established via N-Br and O-Br, reducing the recombination of the electron/hole pair. The electrons generated in the conduction band of CsPbBr 3 transfer to TiO-CN nanosheet and react with adsorbed CO 2 molecules. At the same time, water oxidation was carried out by holes accumulated at the valence band of CsPbBr 3 .
The core-shell combinations have been reported as an alternative option to improve halide perovskite's stability and activity towards CO 2 reduction, where coating the surface of perovskite may also upsurge water stability. Hence, various materials, including metal oxides, polymers, silica, zeolites, and metal-organic framework (MOF), have been successfully utilized. These days, MOFs are widely employed for photocatalytic applications due to their unique properties such as high specific surface area, more catalytic active sites, and tunable structural flexibility. In this way, CsPbBr 3 @ZIF (zeolitic imidazolate framework) was reported for an efficient CO 2 photoreduction [85]. In this study, CsPbBr 3 was coated with the Zn-based metal-organic system ZIF-8 and the Co-based ZIF-67 by an in-situ approach that activates the CO 2 molecule, as shown in Figure 6a. The HAADF-STEM images and elemental mapping confirmed the formation of the CsPbBr 3 @ZIF core-shell structure, as shown in Figure 6b-e. It has been reported that ZIF coating increases the stability of CsPbBr 3 due to its weak hydrophobic nature. The gas-phase photocatalytic CO 2 reduction with water vapor showed CH 4 and CO evolution; and, the CH 4 formation was increased with the increase in irradiation time and achieved 100% selectivity. The increased catalytic activity was demonstrated by composite with ZIF-67 achieving 10.53 µmol g -1 of CH 4 evolution (Figure 6f). The electron consumption rate for ZIF-8 and ZIF-67 composites was 15.498 and 29.630 µmol g -1 h -1 . Moreover, the CsPbBr 3 @ZIF catalyst showed stability for six consecutive cycles, which proves its excellent proficiency (Figure 6g). Therefore, such studies could lead to fabricated highly stable hybrid composites of perovskite materials. Later on, few more studies have been reported on a similar class of hybrid perovskites. For instance, Wang and co-workers successfully developed CsPbBr 3 QDs/UiO-66(NH 2 ) nano junction and employed it for visible-light-active CO 2 reduction [86]. TEM images confirmed the construction of a nano junction between CsPbBr 3 and UiO-66(NH 2 ). The optimized catalyst was able to produce 98.57 µmol g −1 of CO and 3.08 µmol g −1 of CH 4 . Interestingly, the specific surface of pristine UiO-66(NH 2 ) was 709.02 m 2 g −1 , which was more than the nanocomposite (465.68 m 2 g −1 ); the catalyst exhibited much more catalytic activity than the bare samples. The catalyst's reusability was reported for three cycles, proving its high chemical stability and photo resistivity. The suitable VB and CB potentials of CsPbBr 3 and HOMO-LUMO of UiO-66(NH 2 ) were well matched for CO 2 reduction and water oxidation to generate H + and O 2 , resulting to form CO. The photocatalytic reactions were carried out in ethyl acetate/H 2 O combination; therefore, H 2 evolution could be possible. However, the absence of any co-catalyst restricts the H 2 evolution and selectively produces CO more efficiently along with CH 4 . Nanomaterials 2020, 10, x FOR PEER REVIEW 12 of 26 Metal complexes have long been recognized for CO2 photoreduction due to their exciting characteristics such as high selectivity and CO2 conversion activity, and structural flexibility [87][88][89][90]. The conjugated structures of metal complexes are commonly employed as a multi-electron transporter in the catalytic process. Moreover, the structural flexibility of these materials proved beneficial for tailoring catalytic activity and product selectivity. Previously, metal complexes were employed for visible-light-driven CO2 reduction combined with various organic photosensitizers [91]. In the study of Kaung and co-workers in 2020, similar class of material anchoring CsPbBr3 with Re(CO)3Br(dcbpy) (dcbpy ¼ 4,4′-dicarboxy-2,2′-bipyridine) complex has been reported [92]. The interface between Re-complex and CsPbBr3 was established through the carboxyl group, which is responsible for the fast electron transfer to boost the catalytic activity towards CO2 reduction. Hence, the optimized catalyst showed 23 times higher electron consumption rate than CsPbBr3 towards CO evolution. However, arduous synthetic procedures of photosensitizer or the use of precious metals restrict them for large-scale applications. Later, the combination of CsPbBr3 perovskite NCs with (Ni(tpy)), a hybrid transition metal complex, was developed by Gaponik et al. and cast-off for the conversion of photocatalytic CO2 into CO/CH4 [93]. The synthesis of the hybrid composite includes multi-steps, synthesis of (i) organic ligands of CsPbBr3, (ii) ligand exchange, and (iii) assembly of CsPbBr3-Ni(tpy) by immobilization. The charge transfer between the composite was confirmed by TRPL decay and transient absorption spectroscopy, where the electron transfer from CsPbBr3 to Ni(tpy) was observed. Thus, under the light irradiation, the catalyst undergoes CO2 reduction and achieved 1724 µmol g −1 of CO/CH4 formation. The catalytic activity was shown to be 26 times higher than the pristine CsPbBr3 and AQE of 0.23% for CO and CH4 evolution under monochromatic light (450 nm). Also, the catalyst showed stability over 16 h, and post catalytic analysis confirmed its high Metal complexes have long been recognized for CO 2 photoreduction due to their exciting characteristics such as high selectivity and CO 2 conversion activity, and structural flexibility [87][88][89][90]. The conjugated structures of metal complexes are commonly employed as a multi-electron transporter in the catalytic process. Moreover, the structural flexibility of these materials proved beneficial for tailoring catalytic activity and product selectivity. Previously, metal complexes were employed for visible-light-driven CO 2 reduction combined with various organic photosensitizers [91]. In the study of Kaung and co-workers in 2020, similar class of material anchoring CsPbBr 3 with Re(CO) 3 Br(dcbpy) (dcbpy 1 4 4,4 -dicarboxy-2,2 -bipyridine) complex has been reported [92]. The interface between Re-complex and CsPbBr 3 was established through the carboxyl group, which is responsible for the fast electron transfer to boost the catalytic activity towards CO 2 reduction. Hence, the optimized catalyst showed 23 times higher electron consumption rate than CsPbBr 3 towards CO evolution. However, arduous synthetic procedures of photosensitizer or the use of precious metals restrict them for large-scale applications. Later, the combination of CsPbBr 3 perovskite NCs with (Ni(tpy)), a hybrid transition metal complex, was developed by Gaponik et al. and cast-off for the conversion of photocatalytic CO 2 into CO/CH 4 [93]. The synthesis of the hybrid composite includes multi-steps, synthesis of (i) organic ligands of CsPbBr 3 , (ii) ligand exchange, and (iii) assembly of CsPbBr 3 -Ni(tpy) by immobilization. The charge transfer between the composite was confirmed by TRPL decay and transient absorption spectroscopy, where the electron transfer from CsPbBr 3 to Ni(tpy) was observed. Thus, under the light irradiation, the catalyst undergoes CO 2 reduction and achieved 1724 µmol g −1 of CO/CH 4 formation. The catalytic activity was shown to be 26 times higher than the pristine CsPbBr 3 and AQE of 0.23% for CO and CH 4 evolution under monochromatic light (450 nm). Also, the catalyst showed stability over 16 h, and post catalytic analysis confirmed its high stability. Therefore, Ni(tpy) offers more catalytic sites for the CO 2 molecule and improves the catalytic performance.
After MAPbBr x and CsPbBr x , Que and co-workers introduced a novel FAPbBr 3 as an alternative option for traditional perovskites [94]. The synthesis of FAPbBr 3 was carried out by a hot injection method, and the results were compared with CsPbBr 3 synthesized similarly. As shown in Figure 7a-e, the XRD patterns and optical properties of as-prepared FAPbBr 3 are almost identical to that of CsPbBr 3 , and they possess identical morphology with cubic shape. Despite, FAPbBr 3 showed an enormous improvement in the CO evolution (main product) under the CO 2 photoreduction compared to CsPbBr 3 , achieving 181.25 µmol g −1 h −1 , which was ≈17 times greater than CsPbBr 3 (Figure 7f). The significant cyclic stability was observed in FAPbBr 3 , preserving more than 165 µmol g −1 h −1 of CO evolution after three cycles (Figure 7g). Such high catalytic efficiency in FAPbBr 3 was due to the improved lifetime of 7003 ps compared to CsPbBr 3 with 956 ps. Nanomaterials 2020, 10, x FOR PEER REVIEW 13 of 26 stability. Therefore, Ni(tpy) offers more catalytic sites for the CO2 molecule and improves the catalytic performance. After MAPbBrx and CsPbBrx, Que and co-workers introduced a novel FAPbBr3 as an alternative option for traditional perovskites [94]. The synthesis of FAPbBr3 was carried out by a hot injection method, and the results were compared with CsPbBr3 synthesized similarly. As shown in Figure 7ae, the XRD patterns and optical properties of as-prepared FAPbBr3 are almost identical to that of CsPbBr3, and they possess identical morphology with cubic shape. Despite, FAPbBr3 showed an enormous improvement in the CO evolution (main product) under the CO2 photoreduction compared to CsPbBr3, achieving 181.25 µmol g −1 h −1 , which was ≈17 times greater than CsPbBr3 (Figure 7f). The significant cyclic stability was observed in FAPbBr3, preserving more than 165 µmol g −1 h −1 of CO evolution after three cycles (Figure 7g). Such high catalytic efficiency in FAPbBr3 was due to the improved lifetime of 7003 ps compared to CsPbBr3 with 956 ps.

Lead-Free Halide Perovskites
Throughout the years, Pb-based perovskites have been proved as the most efficient materials for photocatalytic CO2 reduction applications due to their excellent photophysical properties. Nevertheless, Pb perovskites' high toxicity may restrict large-scale applications in the near future [95]. Li and colleagues' recent research revealed that the biological impact of Pb-perovskite is unsafe, which shows that Pb could reach the human food chain by plants from perovskites leakage into the ground [96]. Lead exposure can cause a severe problem to human health, including nausea, clumsiness, muscle weakness, and clouded consciousness [97,98]. Therefore, eliminating Pb from perovskite structure should be the primary concern to use them for long-term applications [99,100]. To this end, numerous attempts are being made to replace Pb from halide perovskite structure with other potential candidates, including Sn, Sb, Bi, Cu, In, and Pd. In this contribution, Chu et al. published a review article on lead-free halide double perovskites for various applications covering photodetector, X-ray detector, LEDs, solar cells, and photocatalysis [98]. To date, a limited number of studies were carried out on Pb-free halide perovskites for photocatalytic CO2 reduction [101].
Recently, halide double perovskite materials have been recognized as the ideal alternative for toxic lead halide perovskites [98]. Numerous experiments have demonstrated excellent

Lead-Free Halide Perovskites
Throughout the years, Pb-based perovskites have been proved as the most efficient materials for photocatalytic CO 2 reduction applications due to their excellent photophysical properties. Nevertheless, Pb perovskites' high toxicity may restrict large-scale applications in the near future [95]. Li and colleagues' recent research revealed that the biological impact of Pb-perovskite is unsafe, which shows that Pb could reach the human food chain by plants from perovskites leakage into the ground [96]. Lead exposure can cause a severe problem to human health, including nausea, clumsiness, muscle weakness, and clouded consciousness [97,98]. Therefore, eliminating Pb from perovskite structure should be the primary concern to use them for long-term applications [99,100]. To this end, numerous attempts are being made to replace Pb from halide perovskite structure with other potential candidates, including Sn, Sb, Bi, Cu, In, and Pd. In this contribution, Chu et al. published a review article on lead-free halide double perovskites for various applications covering photodetector, X-ray detector, LEDs, solar cells, and photocatalysis [98]. To date, a limited number of studies were carried out on Pb-free halide perovskites for photocatalytic CO 2 reduction [101].
Recently, halide double perovskite materials have been recognized as the ideal alternative for toxic lead halide perovskites [98]. Numerous experiments have demonstrated excellent optoelectronic features of halide double perovskites, which are also suitable for photocatalytic CO 2 applications [102,103]. In 2018, Zhou et al. demonstrated a highly crystalline Cs 2 AgBiBr 6 double perovskite NCs synthesized through a hot injection process [104]. To acquire the highly crystalline Cs 2 AgBiBr 6 , the temperature was optimized, and it was observed that 200 • C is a suitable temperature to get a pure form of double perovskite. The significant role of OLA and OA ligands towards the formation of crystalline Cs 2 AgBiBr 6 was studied, where it was observed that in the absence of these ligands, bulk Cs 2 AgBiBr 6 was formed. The bandgap was calculated to be 2.52 eV by Tauc's plot, and band potentials were measured by combining the results of VB-XPS and bandgap values. The enlarged band gap was observed in NCs as compared to bulk Cs 2 AgBiBr 6 , which was attributed to its quantum confinement effect. The stability of the NCs was studied in different solvents ranging from polar, partial polar, non-polar, and protonic solvents. Results revealed that the NCs were quickly decomposed in polar solvents like DMF or acetone and highly stable in mild/non-polar solvents for 3 weeks. The high ligand density on the surface of NCs may block electron/hole transportation and decrease the catalytic activity. Therefore, to decline the ligand density, the NCs were washed with absolute ethanol, which was supposed to improve the catalytic activity. The XPS and FTIR revealed that the surface ligands were wholly removed by the washing process (Figure 8a, b), while TGA results further confirm the removal of organic residues (Figure 8c, d). As shown in Figure 9a, b, the catalytic activity of CO 2 reduction towards CO and CH 4 evolution was much higher in the NCs washed by absolute ethanol than the Cs 2 AgBiBr 6 NCs without the washed one in 6 h. The band potentials of the NCs are well suitable for the reduction of CO 2 to produce CO/CH 4 , as shown in Figure 9c. The stability of catalysts was examined by post catalytic analysis using TEM, XRD, and XPS analysis. The results revealed that the catalyst's structure and the surface did not differ from those of fresh samples. The further extension for the development of the Z-scheme Cs 2 AgBiBr 6 @g-C 3 N 4 was carried out by Wang and coworkers [105]. Nanomaterials 2020, 10, x FOR PEER REVIEW 14 of 26 optoelectronic features of halide double perovskites, which are also suitable for photocatalytic CO2 applications [102,103]. In 2018, Zhou et al. demonstrated a highly crystalline Cs2AgBiBr6 double perovskite NCs synthesized through a hot injection process [104]. To acquire the highly crystalline Cs2AgBiBr6, the temperature was optimized, and it was observed that 200 °C is a suitable temperature to get a pure form of double perovskite. The significant role of OLA and OA ligands towards the formation of crystalline Cs2AgBiBr6 was studied, where it was observed that in the absence of these ligands, bulk Cs2AgBiBr6 was formed. The bandgap was calculated to be 2.52 eV by Tauc's plot, and band potentials were measured by combining the results of VB-XPS and bandgap values. The enlarged band gap was observed in NCs as compared to bulk Cs2AgBiBr6, which was attributed to its quantum confinement effect. The stability of the NCs was studied in different solvents ranging from polar, partial polar, non-polar, and protonic solvents. Results revealed that the NCs were quickly decomposed in polar solvents like DMF or acetone and highly stable in mild/non-polar solvents for 3 weeks. The high ligand density on the surface of NCs may block electron/hole transportation and decrease the catalytic activity. Therefore, to decline the ligand density, the NCs were washed with absolute ethanol, which was supposed to improve the catalytic activity. The XPS and FTIR revealed that the surface ligands were wholly removed by the washing process (Figure 8a, b), while TGA results further confirm the removal of organic residues (Figure 8c, d). As shown in Figure 9a, b, the catalytic activity of CO2 reduction towards CO and CH4 evolution was much higher in the NCs washed by absolute ethanol than the Cs2AgBiBr6 NCs without the washed one in 6 h. The band potentials of the NCs are well suitable for the reduction of CO2 to produce CO/CH4, as shown in Figure 9c. The stability of catalysts was examined by post catalytic analysis using TEM, XRD, and XPS analysis. The results revealed that the catalyst's structure and the surface did not differ from those of fresh samples. The further extension for the development of the Z-scheme Cs2AgBiBr6@g-C3N4 was carried out by Wang and coworkers [105].  The Z-scheme combination was achieved by the in-situ method, mixing g-C3N4 precursor to Cs2AgBiBr6 nanoparticles in dichloromethane/toluene. The optimized catalyst achieved 2.0 µmol g −1 h −1 of activity for CO and CH4 production with CH4 selectivity over 70%. The construction of Zscheme among perovskite and g-C3N4 improves the redox ability of the system. After that, Sn-based halide perovskites fell into the spotlight as Pb-free materials. Wang et al. successfully developed the novel Cs2SnI6/SnS2 nanosheet combination in 2019 [106]. Such a heterojunction mixture of perovskite NCs with metal dichalcogenide (SnS2) nanosheets greatly increases the lifetime of photogenerated electrons from 1290 to 3080 ps, observed from transient absorption measurements. DFT studies confirmed the type-II band alignment in Cs2SnI6/SnS2 heterojunction, which was further supported by UPS measurement. Such a heterojunction was responsible for improved electron transportation through Cs2SnI6 and SnS2 interface, and hole extraction by Cs2SnI6 from SnS2, defeating the electronhole recombination. As a result, the 5.4 times improved activity was observed in the Cs2SnI6(1.0)/SnS2 sample (CH4, 6.09 µmol g −1 ) compared to the pristine SnS2 and the stability of 3 cycles. No changes in the XRD pattern and UV-Vis-NIR spectra were observed in the samples tested after CO2 reduction. Apart from Sn-based perovskites, Bi-based materials are considered the best replacement for Pbmaterials [107]. In this regard, Bhosale et al. developed a system anchored series of non-toxic, Bibased halide perovskites, such as Rb3Bi2I9, Cs3Bi2I9, and MA3Bi2I9 by an ultrasonic, top-down method (Figure 10a). The catalyst showed 12 h of stability after seven days of aging under UV illumination, confirmed by XRD patterns. The catalytic responses were acquired at a gas-solid interface under UV irradiation. The time-dependent CH4 evolution was measured for 10 h illumination, and increased CH4 production was observed in all the samples (Figure 10b). The comparative catalyst activity for Bi-based perovskite was observed in the order of Cs3Bi2I9 > Rb3Bi2I9 > MA3Bi2I9 towards CO and Rb3Bi2I9 > Cs3Bi2I9 > MA3Bi2I9 for CH4 evolution after 10 h UV illumination (Figure 10c). The Z-scheme combination was achieved by the in-situ method, mixing g-C 3 N 4 precursor to Cs 2 AgBiBr 6 nanoparticles in dichloromethane/toluene. The optimized catalyst achieved 2.0 µmol g −1 h −1 of activity for CO and CH 4 production with CH 4 selectivity over 70%. The construction of Z-scheme among perovskite and g-C 3 N 4 improves the redox ability of the system. After that, Sn-based halide perovskites fell into the spotlight as Pb-free materials. Wang et al. successfully developed the novel Cs 2 SnI 6 /SnS 2 nanosheet combination in 2019 [106]. Such a heterojunction mixture of perovskite NCs with metal dichalcogenide (SnS 2 ) nanosheets greatly increases the lifetime of photogenerated electrons from 1290 to 3080 ps, observed from transient absorption measurements. DFT studies confirmed the type-II band alignment in Cs 2 SnI 6 /SnS 2 heterojunction, which was further supported by UPS measurement. Such a heterojunction was responsible for improved electron transportation through Cs 2 SnI 6 and SnS 2 interface, and hole extraction by Cs 2 SnI 6 from SnS 2 , defeating the electron-hole recombination. As a result, the 5.4 times improved activity was observed in the Cs 2 SnI 6 (1.0)/SnS 2 sample (CH 4 , 6.09 µmol g −1 ) compared to the pristine SnS 2 and the stability of 3 cycles. No changes in the XRD pattern and UV-Vis-NIR spectra were observed in the samples tested after CO 2 reduction. Apart from Sn-based perovskites, Bi-based materials are considered the best replacement for Pb-materials [107]. In this regard, Bhosale et al. developed a system anchored series of non-toxic, Bi-based halide perovskites, such as Rb 3 Bi 2 I 9 , Cs 3 Bi 2 I 9 , and MA 3 Bi 2 I 9 by an ultrasonic, top-down method (Figure 10a). The catalyst showed 12 h of stability after seven days of aging under UV illumination, confirmed by XRD patterns. The catalytic responses were acquired at a gas-solid interface under UV irradiation. The time-dependent CH 4 evolution was measured for 10 h illumination, and increased CH 4 production was observed in all the samples (Figure 10b). The comparative catalyst activity for Bi-based perovskite was observed in the order of Cs 3 Bi 2 I 9 > Rb 3 Bi 2 I 9 > MA 3 Bi 2 I 9 towards CO and Rb 3 Bi 2 I 9 > Cs 3 Bi 2 I 9 > MA 3 Bi 2 I 9 for CH 4 evolution after 10 h UV illumination (Figure 10c). Later, halide perovskite confined with Sb metal center (i.e., Cs3Sb2Br9) was developed by Lu et al. and showed 10 times better activity than CsPbBr3 NCs [108]. The effect of the ligand in the hot injection synthesis of Cs3Sb2Br9 from CsPbX3 was studied. It was revealed that the use of saturated octanoic acid by replacing unsaturated oleic acid produces pure Cs3Sb2Br9 NCs due to the temperature expansion up to 230 °C. Moreover, these ligands were purified/removed by simple hexane/acetone washings before CO2 reduction tests. Resulting, the photocatalytic CO2 reduction of Cs3Sb2Br9 was carried out in the presence of dried octadecene solvent. The octadecene plays an important role; (i) it has low volatility and (ii) increases the solubility of CO2 compared to typical solvents like acetonitrile or ethyl acetate. After 4 h light illumination, Cs3Sb2Br9 generated 510 µmol g −1 of CO, which was over 10 times greater than various halide perovskites. The DFT calculations revealed that the sites Cs3Sb2Br9 on the (1000) and (0001) surfaces play an essential role in forming COOH* and CO* intermediates. Hence, such studies may help establish a practical, large-scale, Pbfree photocatalyst for the CO2 reduction process in the near future. The summary of all the studied catalysts has been presented in Table 2.  Later, halide perovskite confined with Sb metal center (i.e., Cs 3 Sb 2 Br 9 ) was developed by Lu et al. and showed 10 times better activity than CsPbBr 3 NCs [108]. The effect of the ligand in the hot injection synthesis of Cs 3 Sb 2 Br 9 from CsPbX 3 was studied. It was revealed that the use of saturated octanoic acid by replacing unsaturated oleic acid produces pure Cs 3 Sb 2 Br 9 NCs due to the temperature expansion up to 230 • C. Moreover, these ligands were purified/removed by simple hexane/acetone washings before CO 2 reduction tests. Resulting, the photocatalytic CO 2 reduction of Cs 3 Sb 2 Br 9 was carried out in the presence of dried octadecene solvent. The octadecene plays an important role; (i) it has low volatility and (ii) increases the solubility of CO 2 compared to typical solvents like acetonitrile or ethyl acetate. After 4 h light illumination, Cs 3 Sb 2 Br 9 generated 510 µmol g −1 of CO, which was over 10 times greater than various halide perovskites. The DFT calculations revealed that the sites Cs 3 Sb 2 Br 9 on the (1000) and (0001) surfaces play an essential role in forming COOH* and CO* intermediates. Hence, such studies may help establish a practical, large-scale, Pb-free photocatalyst for the CO 2 reduction process in the near future. The summary of all the studied catalysts has been presented in Table 2.

Summary and Outlook
Halide perovskites have been considered an advanced and high-performance material for many applications over the last few years. Their superior optoelectronic properties make them suitable for photocatalytic CO 2 reduction. Pb-halide perovskites have proven to be the finest materials for CO 2 conversion due to their high catalytic activity, high stability towards humidity, and long-term photostability. However, due to Pb's high toxicity, the research focus is shifting towards the development of non-toxic, Pb-free halide perovskites. Therefore, in this review, the Pb and Pb-free halide perovskite's recent progress towards photocatalytic CO 2 reduction has been involved. We have covered the halide perovskites and their hybrid heterostructures/composites formed with metal co-catalyst, graphene, metal complexes, MOFs, and other 0D or 2D semiconductors. Although halide perovskites achieved significant success for photoreduction of CO 2 ; still the catalytic efficiency is restricted in the µmol range, which keeps them away from large-scale use. Almost all the reported halide perovskites undergo CO 2 reduction to form C 1 products like CO and CH 4 . Therefore, more focus should be given to generate higher hydrocarbons, which are industrially important. Therefore, understanding the reaction mechanism towards the formation of higher-ordered organic chemicals via CO 2 reduction of perovskite structure is crucial. Moreover, it is necessary to emphasize improving the structural and chemical stability of these materials. Improved catalytic efficiency and stability can be achieved by combining halide perovskite with other efficient semiconductors, improving the optical behavior, and charge separation ability. Therefore, developing sustainable, scalable, and low-cost halide perovskites will be a tremendous challenge for real applications. We believe that the process of unlocking more perovskite materials with improved optoelectronic features should continue to make them perfect for better CO 2 reduction performance.