Layered-Defect Perovskite K 3 Bi 2 X 9 (X = I, Br, and Cl) Thin Films for CO 2 Photoreduction: An Analysis of Their Pseudocatalytic Behavior

: Lead-free layered-defect perovskite K 3 Bi 2 X 9 (X = I, Br, and Cl) ﬁlms were proposed as efﬁcient photocatalysts for the CO 2 reduction reaction (CO2RR) to obtain clean and sustainable formic acid (HCOOH), a widely used feedstock in the industry. The ﬁlms exhibited high crystallinity, hexagonal morphologies, and visible light absorption, which were modiﬁed by proportionally increasing the diameter of the X anion. The obtained photocatalytic activities showed values of 299 µ mol h − 1 (K 3 Bi 2 Br 9 ), 283 µ mol h − 1 (K 3 Bi 2 I 9 ), and 91 µ mol h − 1 (K 3 Bi 2 Cl 9 ). However, the stability of the ﬁlms is an important parameter that must be solved; therefore, three strategies were implemented—one with an intrinsic approach (solvent engineering) and two others with an extrinsic focus (substrate modiﬁcation and heterojunction engineering). These modiﬁcations favored yields of up to 738 µ mol h − 1 and constant production over 6 h, demonstrating that the perovskite maintains continuous HCOOH generation. The analysis of the reaction medium showed the degradation of the material structure to BiOI and K + , which could have enhanced its afﬁnity towards CO 2 . In this manner, the degraded perovskite (K 3 Bi 2 I 9 /BiOI) might still react with the CO 2 to generate HCOOH in an aqueous medium under visible light, showing pseudocatalytic behavior.


Introduction
The ever-increasing presence of CO 2 in the atmosphere has raised many sustainability concerns worldwide and has led to an extenuating evolution in climate politics, as this pollutant is the main component of the greenhouse effect gases that consequently cause climate change [1].CO 2 is also the principal exhaust gas in processes that involve fossil fuel combustion.Its two high-energy C=O bonds (of approximately 750 kJ mol −1 ) render this compound thermodynamically stable, and it is this high stability level that makes anthropogenic CO 2 mitigation such a difficult task [2].Mainly, the sustainable policy-making has been focused on (i) environmental legislation to reduce the emission of greenhouse gases [3]; (ii) CO 2 absorption and storage strategies that prevent this pollutant from leaking to the atmosphere [4]; (iii) the utilization of alternative energy sources, which could help to diminish the need for fossil fuel combustion processes [5]; and (iv) the use of CO 2 as a chemical feedstock to obtain various industrial products [6].Most of these strategies are partially effective at attenuating this global-scale pollution crisis.Regardless, the most promising solution strategies rely on sustainable technologies that can effectively terminate the presence of the most ubiquitous greenhouse gas in the environment.For this matter, the photocatalytic CO 2 reduction reaction (CO2RR) remains one of the most attractive mitigation strategies due to the possibility to utilize CO 2 as a source to produce clean and renewable fuels such as methane (CH 4 ), methanol (CH 3 OH), carbon monoxide (CO), and formic acid (HCOOH) [7] from the conversion of this waste product [8,9].These sustainable fuels can be produced with outstanding efficiency if the physical and chemical properties of the semiconductor materials (photocatalysts) are adequate for this purpose.For instance, the transport and separation of charge carriers can impact photocatalytic reactions and modulate the selectivity towards valuable products.Moreover, a high quantum yield is necessary for the emission of sufficient electrons for carrying out photocatalysis, and adequate energy band potentials are required for the reduction and oxidation reactions.Taking these factors into consideration, there are many suitable materials that have been extensively used, such as the layered double hydroxides (LDH) [10], g-C 3 N 4 [11], and SrTiO 3 [12].However, despite the good results obtained with these materials, there is a constant need for the exploration of new functional materials with beneficial properties to carry out an outstanding photocatalytic CO2RR at the industrial level, contemplating both environmental and economic sustainability.This is the case for perovskite-structured photocatalysts, which have been increasingly employed in successful photocatalytic systems in recent times.
The general formula for perovskites is ABX 3 , where A and B are organic or inorganic cations (FA + , MA + , Pb 2+ , Ti 4+ , etc.) with different radii (A > B), while X comprises oxide (O 2 − ) or halide (I − , Br − , Cl − or F − ) anions.This variation results in the distortion of the original cubic lattice to diminished symmetries, such as orthorhombic, hexagonal, triclinic, or monoclinic [13].Additionally, this diversity of lattices give rise to variations in the properties of the perovskites (e.g., electronic structure, dipole moment), which consequentially alter the generation and transport of charge carriers for the photocatalytic process [14].
There are two major subgroups into which perovskites can be classified-metal oxide perovskites (MOPs) and metal halide perovskites (MHPs).Therefore, due to the perovskites' remarkable structure-property relationship, it is to be expected that there will be significant changes between the characteristics of these subgroups.MOPs are formed when the X anion in the perovskite's ABX 3 structure is occupied by an O 2 − ion, while the A and B cations have oxidation state values that sum a total of 6+ (being either 2+ and 4+ or 1+ and 5+ for A and B, respectively).A notable aspect of these materials is that they have previously shown photocatalytic activity towards the CO2RR.For instance, Sr-doped NaTaO 3 was proposed to produce 352 µmol of CO g −1 h −1 with 2% Ag as a cocatalyst and 0.1 M NaHCO 3 as a hole scavenger [15].Moreover, SrTiO 3 had a production rate of 369 µmol g −1 h −1 using Au and Rh in a heterojunction [16], while C-doped LaCoO 3 generated 95 µmol g −1 h −1 with Na 2 CO 3 under UV-visible light [17].As for MHPs, these materials might show additional tilting in their structure (compared to MOPs) due to variations in the X anion.However, when the Pb 2+ cation occupies the B site, the obtention of stable cubic-structured MHPs becomes theoretically feasible [18].There are reports that showcase the good properties of lead-based MHPs that result in remarkable performances in the photocatalytic CO2RR [19].Most of these reports are focused specifically on the CsPbBr 3 perovskite, due to its adequate reduction potentials [20].However, the highly volatile ionic interaction of MHPs also favors their transformation into other phases [21], which can be beneficial for the exploration of new materials for photocatalysis, resulting in the efforts made to render these materials more stable and to resolve the toxicity issues derived from the development of the most promising MHPs-lead halide perovskites [22].Among the attempts to replace lead in the structure of halide perovskites for this matter, the most notable reconfigurations rely on the use of elements of similar ionic radii to produce minimum alterations to its lattice, thereby retaining the valuable properties of lead halide perovskites without the toxicity and stability disadvantages.These alternative materials are commonly referred to as lead-free halide perovskites (LFHPs).These can be formed with different elements that achieve low toxicity while maintaining low costs of production.In this sense, the use of trivalent atoms (e.g., Bi 3+ , As 3+ , Sb 3+ ) can accomplish this requirement, as they can be used to replace the divalent Pb 2+ in the B site of the ABX 3 formula, resulting in a different configuration-A 3 B 2 X 9 .Of these trivalent atoms, bismuth has been shown to improve visible light absorption [23], which has led to good results in the CO2RR [24].Additionally, the use of bismuth-based LFHPs for this application has not yet been extensively explored, and of these materials Cs 3 Bi 2 X 9 (X = I, Br, Cl) has been the most utilized, as the Cs + cation has previously shown good integration in the lattice of this configuration, as opposed to organic cations [25].The highest generation of solar fuels using this perovskite has been obtained with a Cs 3 Bi 2 I 9 /CeO 2 heterojunction, reaching production rates as high as 135 µmol h −1 [26].Another cation with the possibility to form the A 3 Bi 2 X 9 configuration is potassium (K).This is a widely abundant element with relatively insignificant effects on the environment.However, the use of K 3 Bi 2 I 9 for the CO2RR has not been investigated due to its low resistance to environmental factors, such as moisture, high temperatures, or the presence of atmospheric oxygen [27].For this reason, the K 3 Bi 2 I 9 perovskite has only been tested for solar cells [28].Therefore, in this study, the use of the K 3 Bi 2 X 9 (X = I, Br, Cl) LFHPs for the photocatalytic CO2RR with different stability strategies was investigated.
Theoretically, K 3 Bi 2 I 9 crystallizes in the monoclinic C2/c space group (Figure 1) [27].The atomic arrangement of the potassium cations and the iodide anions can be described as a sequence of densely packed, slightly distorted KI 3 −2 layers.There are two inequivalent K + sites and five inequivalent I − sites.Meanwhile, Bi 3+ is bonded to six I − atoms to form corrugated incomplete face-sharing BiI 6 octahedra, which only allow for an oversized irregular gap for the K + site in the center, altering the symmetry of this material.
Sustainability 2023, 15, x FOR PEER REVIEW 3 of 20 materials are commonly referred to as lead-free halide perovskites (LFHPs).These can be formed with different elements that achieve low toxicity while maintaining low costs of production.In this sense, the use of trivalent atoms (e.g., Bi 3+ , As 3+ , Sb 3+ ) can accomplish this requirement, as they can be used to replace the divalent Pb 2+ in the B site of the ABX3 formula, resulting in a different configuration-A3B2X9.Of these trivalent atoms, bismuth has been shown to improve visible light absorption [23], which has led to good results in the CO2RR [24].Additionally, the use of bismuth-based LFHPs for this application has not yet been extensively explored, and of these materials Cs3Bi2X9 (X = I, Br, Cl) has been the most utilized, as the Cs + cation has previously shown good integration in the lattice of this configuration, as opposed to organic cations [25].The highest generation of solar fuels using this perovskite has been obtained with a Cs3Bi2I9/CeO2 heterojunction, reaching production rates as high as 135 µmol h −1 [26].Another cation with the possibility to form the A3Bi2X9 configuration is potassium (K).This is a widely abundant element with relatively insignificant effects on the environment.However, the use of K3Bi2I9 for the CO2RR has not been investigated due to its low resistance to environmental factors, such as moisture, high temperatures, or the presence of atmospheric oxygen [27].For this reason, the K3Bi2I9 perovskite has only been tested for solar cells [28].Therefore, in this study, the use of the K3Bi2X9 (X = I, Br, Cl) LFHPs for the photocatalytic CO2RR with different stability strategies was investigated.
Theoretically, K3Bi2I9 crystallizes in the monoclinic C2/c space group (Figure 1) [27].The atomic arrangement of the potassium cations and the iodide anions can be described as a sequence of densely packed, slightly distorted KI3 −2 layers.There are two inequivalent K⁺ sites and five inequivalent I − sites.Meanwhile, Bi 3 ⁺ is bonded to six I⁻ atoms to form corrugated incomplete face-sharing BiI6 octahedra, which only allow for an oversized irregular gap for the K + site in the center, altering the symmetry of this material.Similar to other halide perovskites (ABiX3; where A = organic and inorganic cations), K3Bi2I9 may have low resistance to various environmental factors (e.g., moisture, oxygen, irradiation, etc.), mainly due to the ionic interactions that connect the atoms of MHPs, which lead to a facilitated interaction of water or oxygen to disrupt their lattice and spontaneously form vacancies upon irradiation [29].There are two different approaches for tackling these issues-intrinsic and extrinsic [30].The intrinsic approach relies on the modification of the internal structure of materials to strengthen its resistance to degradation.Some examples of these modifications are surface passivation [31], recrystallization [32], and cation exchange [33].On the other hand, the extrinsic approach Similar to other halide perovskites (ABiX 3 ; where A = organic and inorganic cations), K 3 Bi 2 I 9 may have low resistance to various environmental factors (e.g., moisture, oxygen, irradiation, etc.), mainly due to the ionic interactions that connect the atoms of MHPs, which lead to a facilitated interaction of water or oxygen to disrupt their lattice and spontaneously form vacancies upon irradiation [29].There are two different approaches for tackling these issues-intrinsic and extrinsic [30].The intrinsic approach relies on the modification of the internal structure of materials to strengthen its resistance to degradation.Some examples of these modifications are surface passivation [31], recrystallization [32], and cation exchange [33].On the other hand, the extrinsic approach focuses on the interface of a material with its reaction medium.For this, some of the most common strategies are heterojunction engineering [34] and surface encapsulation [35].However, it is not clear how these strategies impact the structural and optical properties after several evaluation cycles.Considering this, here, layered-defect LFHP perovskite K 3 Bi 2 X 9 (X = I, Br, Cl) films were synthesized through a reproducible method and evaluated for the photocatalytic CO2RR.The stability of the perovskites was investigated through the implementation of three strategies following intrinsic and extrinsic approaches, with the aim of guaranteeing their long-term implementation.The results could serve as the basis for the development of low-cost, stable, and efficient LFHP photocatalysts with inconsequential environmental impact for sustainable applications.
2.2.Synthesis of K 3 Bi 2 X 9 (X = I, Br, and Cl) LFHP perovskites with a formula of K 3 Bi 2 X 9 (X = I, Br, Cl) were synthesized by mixing 1.5 M KX and 1 M BiX 3 (as shown in Equation ( 1)) in 1 mL of a mixture of N,Ndimethylformamide (DMF) and dimethyl sulfoxide (DMSO) in a ratio of 7:3 v/v.The integration of the halide reactants generated the following reaction in the precursor solution: The solutions were sonicated for 15 min and stirred in a magnetic stirrer at 70 • C for 2 h.Subsequently, the mixture was cooled to room temperature and then filtered using a PTFE syringe (pore size = 0.2 mm) to obtain a light red (I), greenish-yellow (Br), or white (Cl) solution.The glass substrates used for the deposition of the solution were previously cleaned by immersing them in a piranha solution, which consisted of concentrated H 2 SO 4 and H 2 O 2 in a 4:1 ratio.The substrates were then rinsed with distilled water and dried at room temperature.Prior to the spin-coating deposition step, the filtered solution and the glass substrates were heated to 70 • C for 10 min.As a final step, the solution was deposited on the substrates at 2000 rpm for 30 s on the spin-coater and then annealed on a hot plate to 100 • C for 2 min.

Application of Stability Strategies
It is essential to develop stability enhancements to the structure of perovskites to guarantee their sustainable long-term implementation.As previously stated, different intrinsic and extrinsic stability strategies were implemented in this study, with the aim of ensuring a consistent performance during several photocatalytic cycles.These methodologies were carried out in the following manner.

(a) Intrinsic approach
To eliminate impurities in the film and strengthen the structure of the synthesized materials, a recrystallization method was performed.For the correct elimination of impurities, the solvent must be miscible with the precursor solution's solvent (and non-miscible with the crystallized material) to preserve the deposited perovskite film.For this matter, the solvent must also possess low polarity and viscosity.The properties of the chosen solvents for the recrystallization of the K 3 Bi 2 X 9 perovskites are shown in Table 1.Isopropyl alcohol and diethyl ether have useful characteristics for recrystallization, as they are both miscible with DMF, the main component of the perovskite precursor solution.These solvents also have low polarity rates and adequate viscosity.Therefore, in a final spin-coating step (under the same conditions of 2000 rpm and 30 s), the K 3 Bi 2 I 9 films were washed with IPA and DE separately.The films were then heated on the griddle for 20 min and stored.The annealing times were reduced based on our observations.In this study, two stability strategies with an extrinsic approach were considered to assure good stability in the perovskite film.The first one consisted of changing the film's substrate and growing the K 3 Bi 2 I 9 layers on flexible earth-abundant mica substrates via spin-coating deposition.Conversely, the second one involved the design of a heterostructure with BiOI, which would grant the perovskite with higher stability and photocatalytic activity in visible light.It is worth mentioning that the determination of an adequate number of layers of the deposit was carried out on the film prior to this modification, with the purpose of maximizing the photocatalytic activity of K 3 Bi 2 I 9 before adding the effect of any cocatalyst.

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Growth of K 3 Bi 2 I 9 in earth-abundant mica substrates: The adherence and stability of the films was improved via the substitution of the substrates using flexible (and natural) mica.These substrates were cut to the same size as the previously used glass substrates (7.6-1.3 cm), and their frames were covered with mildly adhesive tape to prevent leakage through the edges.After these preparation steps, the spin-coating method was performed as stated above (2000 rpm; 30 s).

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Formation of m-BiOI/K 3 Bi 2 I 9 heterostructures: Based on the hypothesis that a heterostructure of BiOI with the K 3 Bi 2 I 9 perovskite would cause self-assembled co-sharing of the I − ions in both materials (thereby enhancing the film adhesion), a microwaveassisted methodology was carried out.For this purpose, the BiOI film was grown on mica substrates using a microwave-hydrothermal method, as previously reported by our research group [36].The utilized equipment was a Mars 6 ® synthesis research microwave into which a Teflon reactor was introduced.This reactor contained a single mica film, BiI 3 and Bi(NO 3 ) 3 as reactants, and a solvent mixture of isopropyl alcohol and glycerol.The solvents and reactants were added separately in a 2:1 v/v ratio to each other.The reaction conditions were 600 W of power, a temperature of 180 • C, a reaction time of 30 min, and a heating step of 10 • C/min.Finally, the K 3 Bi 2 I 9 spin-coating deposition procedure was performed on this film, as described previously.
A summary of these experiments is schematically represented in Figure 2.

CO 2 Photoreduction Experiments
The photocatalytic activity of the materials was evaluated at room temperature utilizing the experimental arrangement shown in Figure 3.In short, a Pyrex glass reactor was employed, into which the perovskite film was introduced along with 100 mL of water stirred at 200 rpm.In addition, an LED lamp (µ > 400 nm) placed on the side of the reactor was used as a source of visible light irradiation.Prior to each experiment, the reactor was saturated with a continuous flow of CO 2 to eliminate the presence of environmental air.This gaseous flow was maintained throughout the experimental runs.

CO2 Photoreduction Experiments
The photocatalytic activity of the materials was evaluated at room temperature utilizing the experimental arrangement shown in Figure 3.In short, a Pyrex glass reactor was employed, into which the perovskite film was introduced along with 100 mL of water stirred at 200 rpm.In addition, an LED lamp (µ > 400 nm) placed on the side of the reactor was used as a source of visible light irradiation.Prior to each experiment, the reactor was saturated with a continuous flow of CO2 to eliminate the presence of environmental air.This gaseous flow was maintained throughout the experimental runs.

Characterization Techniques
For X-ray diffraction (XRD) characterization, the employed equipment was a Malvern Panalytical Empyrear Dy1681 model (Malvern, Worcestershire, UK) with Cu Kα radiation (4 kV, 100 mA) and an angular resolution of 0.26° (FWHM on LaB6).The measurements were carried out over a range of 2θ = 5-70°, with a step size of 0.02° and a time of 0.3 s for each step.The morphological characterization was achieved using scanning electron microscopy (SEM) in a JEOL JSM-6490LV microscope (JEOL USA,

CO2 Photoreduction Experiments
The photocatalytic activity of the materials was evaluated at room temperature utilizing the experimental arrangement shown in Figure 3.In short, a Pyrex glass reactor was employed, into which the perovskite film was introduced along with 100 mL of water stirred at 200 rpm.In addition, an LED lamp (µ > 400 nm) placed on the side of the reactor was used as a source of visible light irradiation.Prior to each experiment, the reactor was saturated with a continuous flow of CO2 to eliminate the presence of environmental air.This gaseous flow was maintained throughout the experimental runs.

Characterization Techniques
For X-ray diffraction (XRD) characterization, the employed equipment was a Malvern Panalytical Empyrear Dy1681 model (Malvern, Worcestershire, UK) with Cu Kα radiation (4 kV, 100 mA) and an angular resolution of 0.26° (FWHM on LaB6).The measurements were carried out over a range of 2θ = 5-70°, with a step size of 0.02° and a time of 0.3 s for each step.The morphological characterization was achieved using scanning electron microscopy (SEM) in a JEOL JSM-6490LV microscope (JEOL USA,

Characterization Techniques
For X-ray diffraction (XRD) characterization, the employed equipment was a Malvern Panalytical Empyrear Dy1681 model (Malvern, Worcestershire, UK) with Cu Kα radiation (4 kV, 100 mA) and an angular resolution of 0.26 • (FWHM on LaB6).The measurements were carried out over a range of 2θ = 5-70 • , with a step size of 0.02 • and a time of 0.3 s for each step.The morphological characterization was achieved using scanning electron microscopy (SEM) in a JEOL JSM-6490LV microscope (JEOL USA, Peabody, MA, USA) coupled with an Oxford model energy-dispersive detector (EDS).On the other hand, diffuse reflectance spectroscopy (DRS) was employed to calculate the bandgap energy (Eg) of the materials through optical characterization with a wavelength range of 200-800 nm using an Agilent Technologies UV-Vis-NIR Cary 5000 spectrophotometer equipped with an integration sphere (Agilent Technologies, Santa Clara, CA, USA).The material was placed in a quartz sample holder and a barium sulfate (BaSO 4 ) standard was used as the reference.A tangent line was drawn in the region where a change in absorbance was appreciated.The gaseous reaction products were measured using GC gas chromatography using a Thermo Scientific Trace 1310 model (Thermo Fisher Scientific, Waltham, MA, USA), while the liquid products were examined through HPLC by utilizing a Shimadzu Prominence-i LC-2030C model (Shimadzu Scientific Instruments, Columbus, MD, USA).

Results and Discussion
3.1.Characterization of K 3 Bi 2 X 9 (X = I, Br, and Cl) The crystalline phases of the perovskite materials were investigated through XRD, and the results are shown in Figure 4.All samples presented intense and defined reflections, which can be attributed to their high crystallinity.The K 3 Bi 2 I 9 film showed similarities with the reflections of the theoretical card of K 3 Bi 2 I 9 (mp-1120792).The conventionally synthesized sample showed reflections at 2θ = 12 • , 26 • , 39 • , and 53 • .Regarding the K 3 Bi 2 Br 9 and K 3 Bi 2 Cl 9 films, there were analogous reflections to K 3 Bi 2 I 9 with a slight shift towards higher diffraction angles, which is consistent with previous reports that state that the size of the X anion in the A 3 B 2 X 9 crystal structure is inversely proportional to the angles of the XRD reflections [37].Likewise, the three films presented similar reflections to each other, although with a slight displacement, which was proportional to the increase in the ionic radius of the structure's anion.Moreover, based on Scherrer's equation [38], the crystallite size of these materials was calculated, showing values of 108 nm (K 3 Bi 2 I 9 ), 100 nm (K 3 Bi 2 Br 9 ), and 83 nm (K 3 Bi 2 Cl 9 ).range of 200-800 nm using an Agilent Technologies UV-Vis-NIR Cary 5000 spectrophotometer equipped with an integration sphere (Agilent Technologies, Santa Clara, CA, USA).The material was placed in a quartz sample holder and a barium sulfate (BaSO4) standard was used as the reference.A tangent line was drawn in the region where a change in absorbance was appreciated.The gaseous reaction products were measured using GC gas chromatography using a Thermo Scientific Trace 1310 model (Thermo Fisher Scientific, Waltham, MA, USA), while the liquid products were examined through HPLC by utilizing a Shimadzu Prominence-i LC-2030C model (Shimadzu Scientific Instruments, Columbus, MD, USA).

Characterization of K3Bi2X9 (X = I, Br, and Cl)
The crystalline phases of the perovskite materials were investigated through XRD, and the results are shown in Figure 4.All samples presented intense and defined reflections, which can be attributed to their high crystallinity.The K3Bi2I9 film showed similarities with the reflections of the theoretical card of K3Bi2I9 (mp-1120792).The conventionally synthesized sample showed reflections at 2θ = 12°, 26°, 39°, and 53°.Regarding the K3Bi2Br9 and K3Bi2Cl9 films, there were analogous reflections to K3Bi2I9 with a slight shift towards higher diffraction angles, which is consistent with previous reports that state that the size of the X anion in the A3B2X9 crystal structure is inversely proportional to the angles of the XRD reflections [37].Likewise, the three films presented similar reflections to each other, although with a slight displacement, which was proportional to the increase in the ionic radius of the structure's anion.Moreover, based on Scherrer's equation [38], the crystallite size of these materials was calculated, showing values of 108 nm (K3Bi2I9), 100 nm (K3Bi2Br9), and 83 nm (K3Bi2Cl9).The absorption spectra of the K 3 Bi 2 X 9 materials were obtained using DRS (Figure 5).These showed similarities to each other, although K 3 Bi 2 I 9 displayed additional absorption bands in the visible spectrum from 500 to 600 nm, which were attributed to the excitonic peak of these perovskites.These results are consistent with previous reports stating that this compound could have multiple gap states within its bandgap energy range [39].The energy values in all K 3 Bi 2 X 9 perovskites displayed the following tendency: 1.8 eV (K 3 Bi 2 I 9 ) < 2.7 eV (K 3 Bi 2 Br 9 ) < 3.2 eV (K 3 Bi 2 Cl 9 ).As can be seen, the bandgap energy values apparently increase proportionally to the size of the X-anion in the K 3 Bi 2 X 9 structure.The electronegativity difference between the anions and cations of these perovskites might be a defining factor in these phenomena, since the electron sharing involved in compound electronegativity has been shown to be related to the bandgap energy required for the electron transfer process from the VB to the CB [40].The obtained bandgap energy values were comparable to those previously reported for these compounds, apart from K 3 Bi 2 I 9 , which presented a lower energy value (1.8 eV).
electronegativity has been shown to be related to the bandgap energy required for the electron transfer process from the VB to the CB [40].The obtained bandgap energy values were comparable to those previously reported for these compounds, apart from K3Bi2I9, which presented a lower energy value (1.8 eV).
The morphologies of the perovskite samples were characterized through SEM, as shown in Figure 6.All three samples showed distinct morphologies.It was observed that the K3Bi2I9 film presented a heterogeneous morphology, composed of elongated particles larger than 5 µm (Figure 6a).On the other hand, K3Bi2Cl9 exhibited an amorphous layer composed of clusters of particles with indistinct shapes, whereby a mildly hexagonal morphology was visible in the larger grains (<7.5 µm) and several sharp nanoflakes were present throughout (Figure 6b).In the case of K3Bi2Br9, an amorphous clustered layer was also visible (Figure 6c).Nonetheless, there was a higher degree of homogeneity in the semi-hexagonal particles of smaller sizes (<3.8 µm) that composed its morphology.The morphologies of the perovskite samples were characterized through SEM, as shown in Figure 6.All three samples showed distinct morphologies.It was observed that the K 3 Bi 2 I 9 film presented a heterogeneous morphology, composed of elongated particles larger than 5 µm (Figure 6a).On the other hand, K 3 Bi 2 Cl 9 exhibited an amorphous layer composed of clusters of particles with indistinct shapes, whereby a mildly hexagonal morphology was visible in the larger grains (<7.5 µm) and several sharp nanoflakes were present throughout (Figure 6b).In the case of K 3 Bi 2 Br 9 , an amorphous clustered layer was also visible (Figure 6c).Nonetheless, there was a higher degree of homogeneity in the semi-hexagonal particles of smaller sizes (<3.8 µm) that composed its morphology.

Preliminary Study of the Photocatalytic Activity of K3Bi2X9 (X = I, Br, and Cl)
Once the films were characterized, they were evaluated as photocatalysts in the photocatalytic reduction of CO2.The main product obtained was formic acid, which is one of the solar fuels with a greater value in the market (1767-3135 €/ton), as shown in Figure 7.As can be seen, there were comparable HCOOH production rates between K3Bi2I9 (283 µmol h −1 ) and K3Bi2Br9 (299 µmol h −1 ), while K3Bi2Cl9 presented a lower production rate for this sustainable solar fuel (91 µmol h −1 ).
The photocatalytic activities of each perovskite may have been influenced by their visible light absorption.The DRS spectrum of K3Bi2Br9 shows absorbance bands in the short range of the visible spectrum (380-450 nm).Hence, the lower HCOOH production  Once the films were characterized, they were evaluated as photocatalysts in the photocatalytic reduction of CO 2 .The main product obtained was formic acid, which is one of the solar fuels with a greater value in the market (1767-3135 €/ton), as shown in Figure 7.As can be seen, there were comparable HCOOH production rates between K 3 Bi 2 I 9 Sustainability 2023, 15, 16835 9 of 19 (283 µmol h −1 ) and K 3 Bi 2 Br 9 (299 µmol h −1 ), while K 3 Bi 2 Cl 9 presented a lower production rate for this sustainable solar fuel (91 µmol h −1 ).
short range of the visible spectrum (380-450 nm).Hence, the lower HCOOH production observed in perovskite K3Bi2Cl9 could be related to its lower activation in the visible range.The high performance of K3Bi2Br9 can also be explained by the relatively higher uniformity of its thin film, which has been linked to higher photocatalytic activity [41] and electric conductivity rates [42].In addition, according to the XRD analyses, this perovskite presented intense reflections related to its high crystallinity, which could have improved its photocatalytic performance.Even though the perovskites K3Bi2I9 and K3Bi2Br9 displayed similar performances, the following stages of this study were carried out by utilizing K3Bi2I9 films, since this material also had a higher absorption rate in the visible light spectrum.The photocatalytic activities of each perovskite may have been influenced by their visible light absorption.The DRS spectrum of K 3 Bi 2 Br 9 shows absorbance bands in the short range of the visible spectrum (380-450 nm).Hence, the lower HCOOH production observed in perovskite K 3 Bi 2 Cl 9 could be related to its lower activation in the visible range.The high performance of K 3 Bi 2 Br 9 can also be explained by the relatively higher uniformity of its thin film, which has been linked to higher photocatalytic activity [41] and electric conductivity rates [42].In addition, according to the XRD analyses, this perovskite presented intense reflections related to its high crystallinity, which could have improved its photocatalytic performance.Even though the perovskites K 3 Bi 2 I 9 and K 3 Bi 2 Br 9 displayed similar performances, the following stages of this study were carried out by utilizing K 3 Bi 2 I 9 films, since this material also had a higher absorption rate in the visible light spectrum.
However, according to the literature, there are concerns about the stability of these materials [47].For this purpose, the stability of K 3 Bi 2 I 9 was analyzed after three consecutive tests, the results of which are shown in Figure S1.These results indicated that after the first photocatalytic evaluation, the film exhibited significant decay (up to 20 times) in its activity to reduce CO 2 .This result could be explained by its unfavorable estimated formation energy (−113.95kJ mol −1 ) [48], which also promotes dissociation to its components (i.e., metal halides KX and BiX 3 ) under external environmental factors (e.g., light, moisture, oxygen) due to the perovskites' thermodynamic instability [49].However, in order to provide solutions to this instability issue, three strategies were implemented, the results of which are discussed in the following sections.

Implementation of Intrinsic and Extrinsic Strategies for the K 3 Bi 2 I 9 Perovskite
As shown, the K 3 Bi 2 I 9 films gave an outstanding performance in the photoreduction of CO 2 .However, to ensure their economic sustainability, it was mandatory to improve their stability, so efforts were made to amend this aspect.Two different approaches were used-extrinsic and intrinsic.In this study, the growth of K 3 Bi 2 I 9 on mica substrates was the extrinsic methodology of choice.This adjustment provided more flexibility and greater adhesive properties, as well as allowing the simplification of the film deposition method, as these substrates do not require previous treatments (as opposed to glass).Parallel to this technique, an intrinsic recrystallization method was also implemented, with the aim of eliminating the presence of impurities and enhancing the stability of the K 3 Bi 2 I 9 films.This strategy consisted of depositing DE and IPA as anti-solvents using a final spin-coating step following the deposition of K 3 Bi 2 I 9 .The films were then characterized using XRD, SEM, and DRS techniques based on their crystal structure, morphology, and optical properties.

Strategies 1 and 2: Recrystallization and Substrate Modification
Figure S2 reveals that the diffractogram of the recrystallized films grown on mica substrates show similar reflections to the mica by itself.According to the theoretical K 3 Bi 2 I 9 reference, the prominent reflections of this substrate could be overlapped with most of the perovskite reflections, except for 2θ = 12 • , for which slightly different intensity levels can be observed for the two types of anti-solvents used for recrystallization-IPA and DE.These results differ from as-synthesized perovskites on glass, since this substrate is amorphous and t is not visible in a diffractogram, as opposed to mica.
The recrystallized three-layered K 3 Bi 2 I 9 mica film was also characterized via DRS to determine its optical properties.As is shown in Figure S3, there was an increased absorbance intensity in the recrystallized materials' spectra compared to the as-synthesized film.This was possibly caused by the better adhesion of the film on the mica substrate, as well as improved formation of the films during the recrystallization process with the anti-solvents.The spectra also show decreased absorbance in the 200-300 nm band in the recrystallized films, which could be associated with a lower number of impurities in these samples, especially when using IPA as an anti-solvent.Moreover, the bandgap energy values of these samples were calculated from the DRS spectra.The as-synthesized K 3 Bi 2 I 9 mica film had an energy value of 1.67 eV, whereas the two recrystallized films showed a decreased value (1.49 eV).
The morphology of the recrystallized perovskite was analyzed using scanning electron microscopy (Figure 8).Regarding the as-synthesized film in the mica substrate (without recrystallization; Figure 8a,b), there was an apparent formation of large grains, forming a heterogeneous layer with incomplete coverage of the substrate.After the implementation of a recrystallization technique using DE as an anti-solvent (Figure 8c,d), it was possible to achieve better coverage of the substrate, in addition to the formation of a better-defined lamellar morphology with elongated shapes.However, the recrystallization process also resulted in the conglomeration of the deposition layers on the substrate (Figure 8c).On the contrary, by using IPA in this process (Figure 8e,f), it was possible to achieve high-quality crystallization of the film, obtaining rectangular morphologies.
Based on the observations obtained from the characterization process, IPA was chosen as the anti-solvent, with which the experiments resumed.The effect of the number of deposit layers was investigated, with the purpose of determining the optimal quantity of perovskite photocatalyst present on the mica substrate.Firstly, XRD characterization was performed on K 3 Bi 2 I 9 films with 3, 5, and 10 layers of deposit (Figure S4).By depositing 3 layers, a prominent reflection was detected at 2θ = 8 • , which was associated with the presence of the perovskite.As the number of layers increased to 5, this reflection showed a higher intensity due to the greater amount of film deposited on the substrate.However, the intensity decreased when the number of layers was increased to 10, which could be associated with a greater thickness on the film that probably led to greater compaction and lower crystallinity.
The morphology of the recrystallized perovskite was analyzed using scanning electron microscopy (Figure 8).Regarding the as-synthesized film in the mica substrate (without recrystallization; Figure 8a,b), there was an apparent formation of large grains, forming a heterogeneous layer with incomplete coverage of the substrate.After the implementation of a recrystallization technique using DE as an anti-solvent (Figure 8c,d), it was possible to achieve better coverage of the substrate, in addition to the formation of a better-defined lamellar morphology with elongated shapes.However, the recrystallization process also resulted in the conglomeration of the deposition layers on the substrate (Figure 8c).On the contrary, by using IPA in this process (Figure 8e,f), it was possible to achieve high-quality crystallization of the film, obtaining rectangular morphologies.Based on the observations obtained from the characterization process, IPA was chosen as the anti-solvent, with which the experiments resumed.The effect of the number of deposit layers was investigated, with the purpose of determining the optimal quantity of perovskite photocatalyst present on the mica substrate.Firstly, XRD characterization was performed on K3Bi2I9 films with 3, 5, and 10 layers of deposit (Figure S4).By depositing 3 layers, a prominent reflection was detected at 2θ = 8°, which was associated with the presence of the perovskite.As the number of layers increased to 5, this reflection showed a higher intensity due to the greater amount of film deposited on the substrate.However, the intensity decreased when the number of layers was increased to 10, which could be associated with a greater thickness on the film that probably led to greater compaction and lower crystallinity.
The effect of the number of layers was also investigated via a DRS analysis (Figure S5).The results showed that when modifying the number of layers of the photocatalyst, the bandgap energy values were 1.49 and 1.56 eV for the three and five-layered films, respectively.It should be noted that samples with 5 and 10 layers showed no significant differences in their bandgap values.Recrystallized K3Bi2I9 was evaluated in the photocatalytic CO2 reduction to determine the effect of employing different amounts of the photocatalyst by modifying the number of deposited layers (Figure 9).As expected, it was found that a greater number of deposited layers allowed us to obtain higher production rates of HCOOH, reaching a maximum production rate of 210 µmol h −1 with 10 layers of deposit.Taking the superficial area into consideration, this production The effect of the number of layers was also investigated via a DRS analysis (Figure S5).The results showed that when modifying the number of layers of the photocatalyst, the bandgap energy values were 1.49 and 1.56 eV for the three and five-layered films, respectively.It should be noted that samples with 5 and 10 layers showed no significant differences in their bandgap values.Recrystallized K 3 Bi 2 I 9 was evaluated in the photocatalytic CO 2 reduction to determine the effect of employing different amounts of the photocatalyst by modifying the number of deposited layers (Figure 9).As expected, it was found that a greater number of deposited layers allowed us to obtain higher production rates of HCOOH, reaching a maximum production rate of 210 µmol h −1 with 10 layers of deposit.Taking the superficial area into consideration, this production resulted in a 2.2-fold increment with respect to the initial K 3 Bi 2 I 9 sample on glass substrates (Figure S6), owing to improved superficial adhesion and the removal of impurities due to the substrate exchange and recrystallization, respectively.In contrast, when 15 and 20 layers were tested, the generation of HCOOH decreased, possibly because the irradiated light was not able to be absorbed by the film during the photocatalytic evaluations.Therefore, 10-layered films of K 3 Bi 2 I 9 were used for the photocatalytic evaluations that followed in this study.

Stability
To determine the stability of the 10-layered K 3 Bi 2 I 9 film recrystallized with IPA, its photocatalytic activity was evaluated in three consecutive runs, in which two different scenarios were used: (i) a test with the same reaction liquid (Figure 10a) and (ii) another one where the liquid was changed (Figure 10b).This helped determine whether the photocatalyst could still be activated after its desorption from the substrate.As seen in Figure 10a, under the first scenario, the generation of HCOOH was drastically lowered after the first run, which was likely due to the dissociation of the perovskite to its components (KI and BiI 3 ) upon contact with water.This is consistent with previous reports, which stated that metal halide perovskites are prone to having low resistance to environmental factors, such as water.On the other hand, Figure 10b shows that by retaining the reaction liquid (the second scenario), there is a stark difference in the performance of this perovskite, as it retains its high HCOOH production rate (<500 µmol).Thus, according to the results, the liquid reaction medium appears to hold the photocatalyst when diluted in the water, which remains activated by the irradiated light in the reactor.
Sustainability 2023, 15, x FOR PEER REVIEW 12 of 20 resulted in a 2.2-fold increment with respect to the initial K3Bi2I9 sample on glass substrates (Figure S6), owing to improved superficial adhesion and the removal of impurities due to the substrate exchange and recrystallization, respectively.In contrast, when 15 and 20 layers were tested, the generation of HCOOH decreased, possibly because the irradiated light was not able to be absorbed by the film during the photocatalytic evaluations.Therefore, 10-layered films of K3Bi2I9 were used for the photocatalytic evaluations that followed in this study.

Stability
To determine the stability of the 10-layered K3Bi2I9 film recrystallized with IPA, its photocatalytic activity was evaluated in three consecutive runs, in which two different scenarios were used: (i) a test with the same reaction liquid (Figure 10a) and (ii) another one where the liquid was changed (Figure 10b).This helped determine whether the photocatalyst could still be activated after its desorption from the substrate.As seen in Figure 10a, under the first scenario, the generation of HCOOH was drastically lowered after the first run, which was likely due to the dissociation of the perovskite to its components (KI and BiI3) upon contact with water.This is consistent with previous reports, which stated that metal halide perovskites are prone to having low resistance to environmental factors, such as water.On the other hand, Figure 10b shows that by retaining the reaction liquid (the second scenario), there is a stark difference in the performance of this perovskite, as it retains its high HCOOH production rate (<500 µmol).Thus, according to the results, the liquid reaction medium appears to hold the photocatalyst when diluted in the water, which remains activated by the irradiated light in the reactor.

Discussion of the Photocatalytic Mechanism
According to the results presented in Figure 10, the contact of the K3Bi2I9 perovskite with water seems to activate a hydrolyzation reaction in which this material is dissociated into its former reactants, as seen in Equation (2) [50].Subsequently, the formation of BiOI (another visible light photocatalyst) seems to occur from the interaction of BiI3 with water (Equation

Discussion of the Photocatalytic Mechanism
According to the results presented in Figure 10, the contact of the K 3 Bi 2 I 9 perovskite with water seems to activate a hydrolyzation reaction in which this material is dissociated into its former reactants, as seen in Equation (2) [50].Subsequently, the formation of BiOI (another visible light photocatalyst) seems to occur from the interaction of BiI 3 with water (Equation ( 3)), which can explain the fact that the HCOOH generation remains constant within the photocatalytic reaction after several cycles without changing the reaction medium: K 3 Bi 2 I 9 → 3KI + 2BiI 3 (2) To corroborate this hypothesis, the presence of K + ions in the liquid medium was confirmed via ion exchange chromatography (Figure 11b).It is evident that the concentration of potassium rises after only one cycle of reaction, which suggests that the perovskite starts to decompose and its components (i.e., metal ions) dissolve in the reaction medium.This process is shown in Figure 11(a3) for the case of potassium.The interaction of potassium iodide with water generates potassium hydroxide (KOH), and hydrogen iodide (HI) can be also produced.Then, KOH can dissociate in the presence of water to hydroxyls and potassium ions, which could explain the chromatography results we obtained.hydrogen iodide (HI) can be also produced.Then, KOH can dissociate in the presence of water to hydroxyls and potassium ions, which could explain the chromatography results we obtained.

Strategy 3: Heterojunction Engineering
Based on the assumption that K3Bi2I9 is ultimately converted to BiOI after its interaction with water during the reaction (Equation ( 3)), it is reasonable to infer that the synthesis of this compound as a cocatalyst would lead to a beneficial interaction in the case of the degradation of the perovskite.Regarding bismuth-based compound heterojunctions, a co-sharing of the Bi 3+ ion between layers has been previously reported for Cs3Bi2I9/Bi2WO6 [51].It was found that such co-sharing can promote intimate contact and effective electron coupling, which is advantageous for interfacial charge transfer.The implementation of a heterojunction would, thus, strengthen the bond between the perovskite and BiOI.Therefore, for the enhancement of the stability of the perovskite film, a single layer BiOI was deposited as a cocatalyst on a mica substrate, after which 10 layers of K3Bi2I9 film were deposited.A representation of this procedure is shown in Figure 12.After photocatalytic evaluations, the BiOI layer is expected to thicken as a consequence of the degradation of K3Bi2I9 (as previously shown in Figure 11(a2)).The mechanism with which this reaction takes place in the perovskite can be described as "pseudocatalysis" [52], since this material effectively facilitates the photocatalytic reduction of CO2, although it simultaneously converts into other products (contradicting the principle of a catalyst).Additionally, one of these products, BiOI, is the material expected to continue with the

Strategy 3: Heterojunction Engineering
Based on the assumption that K 3 Bi 2 I 9 is ultimately converted to BiOI after its interaction with water during the reaction (Equation ( 3)), it is reasonable to infer that the synthesis of this compound as a cocatalyst would lead to a beneficial interaction in the case of the degradation of the perovskite.Regarding bismuth-based compound heterojunctions, a co-sharing of the Bi 3+ ion between layers has been previously reported for Cs 3 Bi 2 I 9 /Bi 2 WO 6 [51].It was found that such co-sharing can promote intimate contact and effective electron coupling, which is advantageous for interfacial charge transfer.The implementation of a heterojunction would, thus, strengthen the bond between the perovskite and BiOI.Therefore, for the enhancement of the stability of the perovskite film, a single layer BiOI was deposited as a cocatalyst on a mica substrate, after which 10 layers of K 3 Bi 2 I 9 film were deposited.A representation of this procedure is shown in Figure 12.After photocatalytic evaluations, the BiOI layer is expected to thicken as a consequence of the degradation of K 3 Bi 2 I 9 (as previously shown in Figure 11(a2)).The mechanism with which this reaction takes place in the perovskite can be described as "pseudocatalysis" [52], since this material effectively facilitates the photocatalytic reduction of CO 2 , although it simultaneously converts into other products (contradicting the principle of a catalyst).Additionally, one of these products, BiOI, is the material expected to continue with the reduction process, resulting in uninterrupted CO2RR product generation, even when the perovskite is subject to instability.Regarding the characterization of this heterojunction, its optical properties show a discernible change compared to pure BiOI (Figure 13a), as the formation of an excitonic peak with an additional band takes place (450-600 nm), similarly to the K3Bi2I9 film (Figure 5).There is also another band at 250-350 nm, which corresponds to this perovskite.The BiOI film appears to have a band at 450-600 nm.It is worth noting that a band was observed at 200 nm in all samples, corresponding to the mica substrate.From these spectra, the bandgap energy values were obtained, which were 1.79 eV (K3Bi2I9/BiOI) and 1.61 eV (BiOI).These values were higher than those of recrystallized K3Bi2I9 (1.49 eV).The K3Bi2I9/BiOI heterojunction films were analyzed using SEM (Figure 13b-e).The formation of a lamellar morphology was apparent in these films (Figure 13b), consistent with SEM images for the slightly compressed perovskite film recrystallized with DE.Likewise, a slight compaction could have ensued after 10 coatings of K3Bi2I9 during recrystallization with IPA.In the heterojunction, the presence of BiOI is evident due to the characteristic nanosheet morphology that can be seen in the gaps of the perovskite layer (Figure 13c).The exposure of BiOI could lead to the conversion of CO2 on the surfaces of both the perovskite and this photocatalyst.The K3Bi2I9/BiOI heterojunction was evaluated in the CO2RR over 6 h of the reaction, Regarding the characterization of this heterojunction, its optical properties show a discernible change compared to pure BiOI (Figure 13a), as the formation of an excitonic peak with an additional band takes place (450-600 nm), similarly to the K 3 Bi 2 I 9 film (Figure 5).There is also another band at 250-350 nm, which corresponds to this perovskite.The BiOI film appears to have a band at 450-600 nm.It is worth noting that a band was observed at 200 nm in all samples, corresponding to the mica substrate.From these spectra, the bandgap energy values were obtained, which were 1.79 eV (K 3 Bi 2 I 9 /BiOI) and 1.61 eV (BiOI).These values were higher than those of recrystallized K 3 Bi 2 I 9 (1.49eV).The K 3 Bi 2 I 9 /BiOI heterojunction films were analyzed using SEM (Figure 13b-e).The formation of a lamellar morphology was apparent in these films (Figure 13b), consistent with SEM images for the slightly compressed perovskite film recrystallized with DE.Likewise, a slight compaction could have ensued after 10 coatings of K 3 Bi 2 I 9 during recrystallization with IPA.In the heterojunction, the presence of BiOI is evident due to the characteristic nanosheet morphology that can be seen in the gaps of the perovskite layer (Figure 13c).The exposure of BiOI could lead to the conversion of CO 2 on the surfaces of both the perovskite and this photocatalyst.
The K 3 Bi 2 I 9 /BiOI heterojunction was evaluated in the CO2RR over 6 h of the reaction, as depicted in Figure 14a, along with a schematic representation of the basis of this heterojunction and its co-sharing of I − ions (Figure 14b).Compared to the production rate displayed by the K 3 Bi 2 I 9 film in the same conditions (Figure 10), the heterojunction displayed an important increase in its photocatalytic activity (38%; 738 µmol h −1 HCOOH), which may have been caused by an improved photogenerated charge transfer (due to adequate band alignment; Figure 14c), as well as better retention of the catalyst in the mica substrate during the reaction (as is visible in Figure 14d).The BiOI nanosheet morphology is present in the heterojunction after one photocatalytic cycle (previously shown in Figure 13d,e), along with nanoflowers that cover the substrate in a thin layer, caused by the migration of the material to the reaction medium (i.e., water).The outstanding efficiency with which the CO2RR takes place in the K 3 Bi 2 I 9 /BiOI heterojunction confirms the aforementioned pseudocatalytic behavior, which may also have been enhanced by its morphology, since there are reports that nanoflower morphologies allow for a greater number of adsorption sites to be obtained, in which CO 2 could be converted to HCOOH [53].Furthermore, an improved affinity for CO 2 could have been achieved, owing to the forceful interaction between the K + ions and CO 2 [54].This was also evident in the remarkable production of HCOOH observed in the perovskite when potassium ions were not removed from the reaction liquid, as previously displayed in Figure 10.The strength of this interaction is such that it has previously led to the development of many potassium-based materials for CO 2 adsorption [55].Adequate interfacial contact with CO 2 has also been reported for Bi 3+ ions [56], which are readily available on the surface of BiOI and could have been the determinant in the obtained photocatalytic activity.Regarding the characterization of this heterojunction, its optical properties show a discernible change compared to pure BiOI (Figure 13a), as the formation of an excitonic peak with an additional band takes place (450-600 nm), similarly to the K3Bi2I9 film (Figure 5).There is also another band at 250-350 nm, which corresponds to this perovskite.The BiOI film appears to have a band at 450-600 nm.It is worth noting that a band was observed at 200 nm in all samples, corresponding to the mica substrate.From these spectra, the bandgap energy values were obtained, which were 1.79 eV (K3Bi2I9/BiOI) and 1.61 eV (BiOI).These values were higher than those of recrystallized K3Bi2I9 (1.49 eV).The K3Bi2I9/BiOI heterojunction films were analyzed using SEM (Figure 13b-e).The formation of a lamellar morphology was apparent in these films (Figure 13b), consistent with SEM images for the slightly compressed perovskite film recrystallized with DE.Likewise, a slight compaction could have ensued after 10 coatings of K3Bi2I9 during recrystallization with IPA.In the heterojunction, the presence of BiOI is evident due to the characteristic nanosheet morphology that can be seen in the gaps of the perovskite layer (Figure 13c).The exposure of BiOI could lead to the conversion of CO2 on the surfaces of both the perovskite and this photocatalyst.The K3Bi2I9/BiOI heterojunction was evaluated in the CO2RR over 6 h of the reaction, as depicted in Figure 14a, along with a schematic representation of the basis of this heterojunction and its co-sharing of I − ions (Figure 14b).Compared to the production rate displayed by the K3Bi2I9 film in the same conditions (Figure 10), the heterojunction displayed an important increase in its photocatalytic activity (38%; 738 µmol h −1 HCOOH), which may have been caused by an improved photogenerated charge transfer (due to adequate band alignment; Figure 14c), as well as better retention of the catalyst in the mica substrate during the reaction (as is visible in Figure 14d).The BiOI nanosheet morphology is present in the heterojunction after one photocatalytic cycle (previously shown in Figure 13d,e), along with nanoflowers that cover the substrate in a thin layer, caused by the migration of the material to the reaction medium (i.e., water).The outstanding efficiency with which the CO2RR takes place in the K3Bi2I9/BiOI heterojunction confirms the aforementioned pseudocatalytic behavior, which may also have been enhanced by its morphology, since there are reports that nanoflower morphologies allow for a greater number of adsorption sites to be obtained, in which CO2 could be converted to HCOOH [53].Furthermore, an improved affinity for CO2 could have been achieved, owing to the forceful interaction between the K + ions and CO2 [54].This was also evident in the remarkable production of HCOOH observed in the perovskite when potassium ions were not removed from the reaction liquid, as previously displayed in Figure 10.The strength of this interaction is such that it has previously led to the development of many potassiumbased materials for CO2 adsorption [55].Adequate interfacial contact with CO2 has also been reported for Bi 3+ ions [56], which are readily available on the surface of BiOI and could have been the determinant in the obtained photocatalytic activity.

Conclusions
Lead-free bismuth halide perovskites of the K3Bi2X9 family (X = I, Br, Cl) were synthesized via a reproducible one-pot method at room temperature.These materials displayed an outstanding performance (up to 299 µmol h −1 using K3Bi2Br9) in photocatalytic CO2 reduction for the obtention of formic acid, a clean alternative fuel.The

Conclusions
Lead-free bismuth halide perovskites of the K 3 Bi 2 X 9 family (X = I, Br, Cl) were synthesized via a reproducible one-pot method at room temperature.These materials displayed an outstanding performance (up to 299 µmol h −1 using K 3 Bi 2 Br 9 ) in photocatalytic CO 2 reduction for the obtention of formic acid, a clean alternative fuel.The anions' electronegativity seems to influence these materials in terms of their morphologies, light absorption rates, and bandgap energy values.Additionally, a variety of modification methods (intrinsic and extrinsic) were carried out on the K 3 Bi 2 I 9 perovskite to ensure a reliable performance.The implementation of these strategies led to the obtention of more efficient functional films, with enhanced adhesion and flexibility, well-defined morphologies, and improved photocatalytic activity (2.2-fold; 535 µmol h −1 during 12 h of reaction).An analysis of the reaction medium and the characterization of the sample suggested a pseudocatalytic behavior in the K 3 Bi 2 I 9 perovskite, where newly formed BiOI and K + enhanced the photocatalytic processes via more efficient charge transfer, improved light absorption, and an increase in the CO 2 affinity of the K + ions.Thus, it is in this manner that this material can provide reliable performance in the CO2RR, with the possibility of sustainable industrial applications.For its part, the implementation of a heterojunction engineering approach allowed for remarkable efficiency rates (738 µmol h −1 ) that were up to 38% higher than the improved K 3 Bi 2 I 9 perovskite during 6 h of constant photocatalytic evaluation.This was probably caused by the junction of the energy bands of these catalysts, which resulted in an improved photogenerated charge transfer, as well as better retention of the film in the substrate due to the co-sharing I − ion binding.The obtained results demonstrated that the design and implementation stability strategies promoted an efficiency enhancement for the CO2RR in the lead-free K 3 Bi 2 I 9 perovskite, providing sustainable alternatives to scale this product up at the following engineering levels.

Figure 1 .
Figure 1.Plane (001) in the monoclinic structure of perovskite K3Bi2I9 (a).Arrangement of the most densely packed layers in K3Bi2I9 along the plane (b).The shaded area highlights the absence of an octahedron within the structure (i.e., a vacancy).The a, b, and c axis of the plane are denoted.Adapted from [27].

Figure 1 .
Figure 1.Plane (001) in the monoclinic structure of perovskite K 3 Bi 2 I 9 (a).Arrangement of the most densely packed layers in K 3 Bi 2 I 9 along the plane (b).The shaded area highlights the absence of an octahedron within the structure (i.e., a vacancy).The a, b, and c axis of the plane are denoted.Adapted from [27].

Figure 2 .
Figure 2. Summary of the proposed methodology in this work.

Figure 3 .
Figure 3. Experimental arrangement utilized in this work for the evaluation of the CO2RR.

Figure 2 .
Figure 2. Summary of the proposed methodology in this work.

Figure 2 .
Figure 2. Summary of the proposed methodology in this work.

Figure 3 .
Figure 3. Experimental arrangement utilized in this work for the evaluation of the CO2RR.

Figure 3 .
Figure 3. Experimental arrangement utilized in this work for the evaluation of the CO2RR.

3. 2 .
Preliminary Study of the Photocatalytic Activity of K 3 Bi 2 X 9 (X = I, Br, and Cl)

Figure 8 .
Figure 8. SEM characterization of as-synthesized K3Bi2I9 perovskite (a,d) and the effect of its recrystallization with DE (b,e) and IPA (c,f) on its crystal structure.

Figure 8 .
Figure 8. SEM characterization of as-synthesized K 3 Bi 2 I 9 perovskite (a,d) and the effect of its recrystallization with DE (b,e) and IPA (c,f) on its crystal structure.

Figure 9 .
Figure 9.Effect of the number of layers of K3Bi2I9 in the recrystallized mica films on the HCOOH production under visible light throughout 1 h of reaction (a) and post-reaction (b).

Figure 9 . 20 Figure 10 .
Figure 9.Effect of the number of layers of K 3 Bi 2 I 9 in the recrystallized mica films on the HCOOH production under visible light throughout 1 h of reaction (a) and post-reaction (b).Sustainability 2023, 15, x FOR PEER REVIEW 13 of 20

3 Figure 10 .
Figure 10.Stability of the recrystallized K 3 Bi 2 I 9 film over three runs, with (a) and without (b) changing the liquid reaction medium.

Figure 11 .
Figure 11.Possible reaction mechanism of the degradation of the K3Bi2I9 perovskite (a) and potassium detected by ICP in the aqueous medium (b).Equations (1)-(3) indicate the steps in the degradations process.

Figure 13 .
Figure 13.DRS spectra of the K3Bi2I9 perovskite with BiOI as a cocatalyst (a), as well as its SEM characterization before (b,c) and after (d,e) 6 h of reaction.

Figure 13 .
Figure 13.DRS spectra of the K3Bi2I9 perovskite with BiOI as a cocatalyst (a), as well as its SEM characterization before (b,c) and after (d,e) 6 h of reaction.

Figure 13 .
Figure 13.DRS spectra of the K 3 Bi 2 I 9 perovskite with BiOI as a cocatalyst (a), as well as its SEM characterization before (b,c) and after (d,e) 6 h of reaction.

Figure 14 .
Figure 14.Photocatalytic activity of K3Bi2I9/BiOI towards the generation of HCOOH (a), as well as a schematic representation of the ion co-sharing of this heterojunction (b) and its band alignment (c).Moreover, an image of K3Bi2I9/BiOI deposited on flexible mica substrates is shown (d).

Figure 14 .
Figure 14.Photocatalytic activity of K 3 Bi 2 I 9 /BiOI towards the generation of HCOOH (a), as well as a schematic representation of the ion co-sharing of this heterojunction (b) and its band alignment (c).Moreover, an image of K 3 Bi 2 I 9 /BiOI deposited on flexible mica substrates is shown (d).

Table 1 .
Miscibility and properties of various commonly used solvents.