Efficient Photocatalytic CO2 Reduction with MIL-100(Fe)-CsPbBr3 Composites

Bromide-based metal halide perovskites (MHPs) are promising photocatalysts with strong blue-green light absorption. Composite photocatalysts of MHPs with MIL-100(Fe), as a powerful photocatalyst itself, have been investigated to extend the responsiveness towards red light. The composites, with a high specific surface area, display an enhanced solar light response, and the improved charge carrier separation in the heterojunctions is employed to maximize the photocatalytic performance. Optimization of the relative composition, with the formation of a dual-phase CsPbBr3 to CsPb2Br5 perovskite composite, shows an excellent photocatalytic performance with 20.4 μmol CO produced per gram of photocatalyst during one hour of visible light irradiation.


Introduction
The capture of CO 2 and further conversion of this greenhouse gas into chemical fuels (CO, CH 4 , CH 3 OH, etc.) has been a hot topic during the past decades [1]. The reduction of CO 2 is complicated by its inherent chemical stability [2,3]. Despite this issue, photocatalytic CO 2 reduction is still seen as one of the promising ways to sustainably produce chemicals [4]. To date, a wide variety of photocatalysts such as TiO 2 , ZnO, CdS, perovskite oxides and their composites, and metal (complex) functionalized derivatives, have been tested. But the obtained performances under solar irradiation are still limited, mainly due to the low efficiency of using visible light.
Over the last decades, metal-organic frameworks (MOFs) have been one of the fastest developing materials, exhibiting unique properties such as structural flexibility, large specific surface area, tunable but uniform cavities, and easy ligand functionalization [5,6]. MOF-5 was the first MOF reported to have photocatalytic activity in UV-induced phenol photodegradation [7]. Fu et al. successfully employed an amine-functionalized Ti-based MOF, NH 2 -MIL-125(Ti), in CO 2 photoreduction [8]. Ever since, MOFs are not only used as matrix for other semiconductors or noble metals but are also seen as promising photocatalysts on their own [9][10][11].
Recently, all-inorganic CsPbBr 3 perovskites have emerged as promising photocatalysts in various applications ranging from dye degradation [12] to selective organic reactions [13] and renewable fuel generation from CO 2 and water [14]. These materials combine the excellent optoelectronic properties of organic-inorganic hybrid perovskites, such as a bandgap largely overlapping with the visible part of the electromagnetic spectrum, a high absorption coefficient, and long-range charge transport, with good temperature stability [15,16]. However, the performances of MHPs are limited by their poor structural stability in a humid atmosphere. Heterojunction formation with materials such as graphene oxide,   Table 1. Weight ratio of MIL-100(Fe) in the composites.

Sample
p-30Fe p-60Fe p-90Fe p-120Fe p-180Fe MIL-100(Fe) content/wt% 9 14 18 27 53 The surface chemical composition and chemical states of the as-synthesized composites were further revealed by X-ray photoelectron spectroscopy (XPS). The XPS spectrum (Figure 3a) of p-90Fe shows distinct peaks from both of MIL-100(Fe) and CsPbBr3. Relevant high-resolution spectra of   The surface chemical composition and chemical states of the as-synthesized composites were further revealed by X-ray photoelectron spectroscopy (XPS). The XPS spectrum (Figure 3a) of p-90Fe shows distinct peaks from both of MIL-100(Fe) and CsPbBr3. Relevant high-resolution spectra of  The surface chemical composition and chemical states of the as-synthesized composites were further revealed by X-ray photoelectron spectroscopy (XPS). The XPS spectrum (Figure 3a) of p-90Fe shows distinct peaks from both of MIL-100(Fe) and CsPbBr 3 . Relevant high-resolution spectra of p-90Fe ( Figure S1) display peaks of Cs 3d, Pb 4f, and Br 3d in the range of 720-745, 134-148, and 64-74 eV, respectively, which are well-matched with those of CsPbBr 3 [31,32]. Two dominant peaks of Fe ( Figure 3b) at 724.6 and 711.7 eV are attributed to Fe 2p 1/2 and Fe 2p 3/2 , respectively. Additional satellite peak appearing at 717.2 eV corresponds to Fe 3+ in MIL-100(Fe) [33].
Catalysts 2020, 10, x FOR PEER REVIEW 4 of 13 p-90Fe ( Figure S1) display peaks of Cs 3d, Pb 4f, and Br 3d in the range of 720-745, 134-148, and 64-74 eV, respectively, which are well-matched with those of CsPbBr3 [31,32]. Two dominant peaks of Fe ( Figure 3b) at 724.6 and 711.7 eV are attributed to Fe 2p1/2 and Fe 2p3/2, respectively. Additional satellite peak appearing at 717.2 eV corresponds to Fe 3+ in MIL-100(Fe) [33]. The response of MIL-100(Fe) in the visible light region arises from the direct excitation of the Fe-O clusters. As shown in the absorption spectra in Figure 4a (as obtained by UV-vis diffuse reflectance, see Materials and Methods), p-30Fe, p-60Fe, and p-90Fe basically maintain the pattern of CsPbBr3. p-120Fe and p-180Fe exhibit a pattern similar to MIL-100(Fe), due to the larger amount of MIL-100(Fe) included. The addition of MIL-100(Fe) enhances the composites' optical response in the visible light region, especially above 550 nm. The absorption edges around 550 nm gradually blue shift, corresponding to the increased bandgaps (Eg) (calculated by the Tauc plots, Figure 4b) of the composites consisting of more MIL-100(Fe) ( Table 2). This blue-shift arises from the phase transformation from CsPbBr3 (Eg = 2.3 eV) to CsPb2Br5 (Eg = 3.0 eV) and the more dominant role of MIL-100(Fe) in the composite photocatalysts upon increasing the Fe content.  Table 2, were estimated following Kubelka-Munk transformation. Each bandgap is determined by the intersection point of the corresponding tangent line and horizontal axis.  The response of MIL-100(Fe) in the visible light region arises from the direct excitation of the Fe-O clusters. As shown in the absorption spectra in Figure 4a (as obtained by UV-vis diffuse reflectance, see Section 3), p-30Fe, p-60Fe, and p-90Fe basically maintain the pattern of CsPbBr 3 . p-120Fe and p-180Fe exhibit a pattern similar to MIL-100(Fe), due to the larger amount of MIL-100(Fe) included. The addition of MIL-100(Fe) enhances the composites' optical response in the visible light region, especially above 550 nm. The absorption edges around 550 nm gradually blue shift, corresponding to the increased bandgaps (E g ) (calculated by the Tauc plots, Figure 4b) of the composites consisting of more MIL-100(Fe) ( Table 2). This blue-shift arises from the phase transformation from CsPbBr 3 (E g = 2.3 eV) to CsPb 2 Br 5 (E g = 3.0 eV) and the more dominant role of MIL-100(Fe) in the composite photocatalysts upon increasing the Fe content.
Catalysts 2020, 10, x FOR PEER REVIEW 4 of 13 p-90Fe ( Figure S1) display peaks of Cs 3d, Pb 4f, and Br 3d in the range of 720-745, 134-148, and 64-74 eV, respectively, which are well-matched with those of CsPbBr3 [31,32]. Two dominant peaks of Fe ( Figure 3b) at 724.6 and 711.7 eV are attributed to Fe 2p1/2 and Fe 2p3/2, respectively. Additional satellite peak appearing at 717.2 eV corresponds to Fe 3+ in MIL-100(Fe) [33]. The response of MIL-100(Fe) in the visible light region arises from the direct excitation of the Fe-O clusters. As shown in the absorption spectra in Figure 4a (as obtained by UV-vis diffuse reflectance, see Materials and Methods), p-30Fe, p-60Fe, and p-90Fe basically maintain the pattern of CsPbBr3. p-120Fe and p-180Fe exhibit a pattern similar to MIL-100(Fe), due to the larger amount of MIL-100(Fe) included. The addition of MIL-100(Fe) enhances the composites' optical response in the visible light region, especially above 550 nm. The absorption edges around 550 nm gradually blue shift, corresponding to the increased bandgaps (Eg) (calculated by the Tauc plots, Figure 4b) of the composites consisting of more MIL-100(Fe) ( Table 2). This blue-shift arises from the phase transformation from CsPbBr3 (Eg = 2.3 eV) to CsPb2Br5 (Eg = 3.0 eV) and the more dominant role of MIL-100(Fe) in the composite photocatalysts upon increasing the Fe content.  Table 2, were estimated following Kubelka-Munk transformation. Each bandgap is determined by the intersection point of the corresponding tangent line and horizontal axis.   Table 2, were estimated following Kubelka-Munk transformation. Each bandgap is determined by the intersection point of the corresponding tangent line and horizontal axis.  Figure 5 and Figure S2 show the steady-state photoluminescence (PL) spectra of the photocatalysts at an excitation wavelength of 380 nm. As shown in Figure S2, CsPbBr 3 has significantly higher peak intensity than the composites. The broad emission peak from 400 to 450 nm can be attributed to the 1,3,5-benzene tricarboxylic acid linkers in the MIL-100(Fe) structure [34]. The PL emission spectra of p-30Fe, p-60Fe and p-90Fe show a gradual blue shift and weakening intensity of the peak at around 530 nm. Different from 3D CsPbBr 3 , Cs + ions separate the layers of 2D CsPb 2 Br 5 , and the excitons are slowed down by the layered structure [35]. Thus, with more CsPb 2 Br 5 formed, the PL peaks of p-120Fe and p-180Fe get more blue shifted [36,37]. p-90Fe possesses the lowest PL intensity among all as-synthesized CsPbBr 3 /MIL-100(Fe) samples.
Catalysts 2020, 10, x FOR PEER REVIEW 5 of 13 Figure 5 and Figure S2 show the steady-state photoluminescence (PL) spectra of the photocatalysts at an excitation wavelength of 380 nm. As shown in Figure S2, CsPbBr3 has significantly higher peak intensity than the composites. The broad emission peak from 400 to 450 nm can be attributed to the 1,3,5-benzene tricarboxylic acid linkers in the MIL-100(Fe) structure [34]. The PL emission spectra of p-30Fe, p-60Fe and p-90Fe show a gradual blue shift and weakening intensity of the peak at around 530 nm. Different from 3D CsPbBr3, Cs + ions separate the layers of 2D CsPb2Br5, and the excitons are slowed down by the layered structure [35]. Thus, with more CsPb2Br5 formed, the PL peaks of p-120Fe and p-180Fe get more blue shifted [36,37]. p-90Fe possesses the lowest PL intensity among all as-synthesized CsPbBr3/MIL-100(Fe) samples.  Figure S3a, the pure CsPbBr3 sample displays nanoparticles with a uniform cubic shape. MIL-100(Fe) synthesized in HF-free conditions crystallizes into small nanoparticles with no clearly visible shape compared to the classic octahedral shapes ( Figure S3b); this is in line with previous reports [38]. In the composite materials, e.g., p-90Fe, the cubic CsPbBr3 morphology is lost and only agglomerated particles are observed ( Figure S3c). Selected location elemental analysis by energy-dispersive X-ray spectroscopy (EDS) confirms the co-existence of both CsPbBr3 and MIL-100(Fe) ( Figure S3d).

As shown in
The components' dispersion and interaction were studied by confocal laser scanning fluorescence measurements on p-90Fe, using a 375 nm laser. Figure 6d showed the wide field image of p-90Fe. Based on the PL spectra mentioned above, two emission channels at 430-470 nm and 505-540 nm were chosen to visualize the MIL-100(Fe) (red colored) ( Figure 6a) and CsPbBr3 (green colored) (Figure 6b) distribution, respectively. Both channels were acquired with the same excitation power. As CsPbBr3 has a significantly stronger signal than MIL-100(Fe) upon excitation ( Figure S2), Figure 6a was adjusted to be 25% brighter than Figure 6b.  Figure S3a, the pure CsPbBr 3 sample displays nanoparticles with a uniform cubic shape. MIL-100(Fe) synthesized in HF-free conditions crystallizes into small nanoparticles with no clearly visible shape compared to the classic octahedral shapes ( Figure S3b); this is in line with previous reports [38]. In the composite materials, e.g., p-90Fe, the cubic CsPbBr 3 morphology is lost and only agglomerated particles are observed ( Figure S3c). Selected location elemental analysis by energy-dispersive X-ray spectroscopy (EDS) confirms the co-existence of both CsPbBr 3 and MIL-100(Fe) ( Figure S3d).

As shown in
The components' dispersion and interaction were studied by confocal laser scanning fluorescence measurements on p-90Fe, using a 375 nm laser. Figure 6d showed the wide field image of p-90Fe. Based on the PL spectra mentioned above, two emission channels at 430-470 nm and 505-540 nm were chosen to visualize the MIL-100(Fe) (red colored) ( Figure 6a) and CsPbBr 3 (green colored) ( Figure 6b) distribution, respectively. Both channels were acquired with the same excitation power. As CsPbBr 3 has a significantly stronger signal than MIL-100(Fe) upon excitation ( Figure S2), Figure 6a was adjusted to be 25% brighter than Figure 6b.
As shown in Figure 6c, yellow shaded areas appear where both signals overlap. Based on particle count, ca. 80% particles were yellowish-green. These uniformly dispersed yellow shades reflect the close contact between MOF and MHP parts in the composite.
The complementary light absorption by CsPbBr 3 and MIL-100(Fe) should result in an efficient photocatalytic activity under simulated solar irradiation. Here, CO 2 photoreduction under visible light irradiation was chosen to test the photoactivity of the different catalysts. The reaction was performed under 1 bar hydrated CO 2 atmosphere, at ambient temperature. CO was found as the only photoreduction product generated from CO 2 , and no other carbonaceous products was detected. A series of control experiments were also conducted. No appreciable amounts of CO or other hydrocarbons were detected in the absence of light irradiation, or photocatalyst, or CO 2 (under wet He atmosphere). energy-dispersive X-ray spectroscopy (EDS) confirms the co-existence of both CsPbBr3 and MIL-100(Fe) ( Figure S3d).
The components' dispersion and interaction were studied by confocal laser scanning fluorescence measurements on p-90Fe, using a 375 nm laser. Figure 6d showed the wide field image of p-90Fe. Based on the PL spectra mentioned above, two emission channels at 430-470 nm and 505-540 nm were chosen to visualize the MIL-100(Fe) (red colored) ( Figure 6a) and CsPbBr3 (green colored) (Figure 6b) distribution, respectively. Both channels were acquired with the same excitation power. As CsPbBr3 has a significantly stronger signal than MIL-100(Fe) upon excitation ( Figure S2), Figure 6a was adjusted to be 25% brighter than Figure 6b. As shown in Figure 6c, yellow shaded areas appear where both signals overlap. Based on particle count, ca. 80% particles were yellowish-green. These uniformly dispersed yellow shades reflect the close contact between MOF and MHP parts in the composite.
The complementary light absorption by CsPbBr3 and MIL-100(Fe) should result in an efficient photocatalytic activity under simulated solar irradiation. Here, CO2 photoreduction under visible light irradiation was chosen to test the photoactivity of the different catalysts. The reaction was performed under 1 bar hydrated CO2 atmosphere, at ambient temperature. CO was found as the only photoreduction product generated from CO2, and no other carbonaceous products was detected. A series of control experiments were also conducted. No appreciable amounts of CO or other hydrocarbons were detected in the absence of light irradiation, or photocatalyst, or CO2 (under wet He atmosphere). Figure 7a shows the time-dependent CO production on the as-synthesized samples during a 4 h experiment. The CO production rate over pure CsPbBr3 and MIL-100(Fe) are similar, about 4.5 μmol g −1 h −1 . The composite materials show significantly higher activity, and p-90Fe exhibits a maximum CO production of 20.4 μmol g −1 h −1 , which is about 4.5 times higher than the pure constituents. Upon increasing the load of MIL-100(Fe), lower photocatalytic activity is obtained. The decrease in photocatalytic activity may be caused by an increasing amount of CsPb2Br5 in the composites, which is not active upon visible light irradiation. The critical role of H2O was revealed by a test reaction on p-90Fe in high purity CO2 gas without H2O. The CO yield over p-90Fe dropped from 20.4 to 5.3 μmol g −1 h −1 . The composites' stability was evaluated by four consecutive runs (4 h each, in total 16 h) on p-90Fe. As shown in Figure 7b, the composite exhibits no significant deactivation, and its crystal structure is well maintained after the 16 h photocatalytic reaction ( Figure S4).  Figure 7a shows the time-dependent CO production on the as-synthesized samples during a 4 h experiment. The CO production rate over pure CsPbBr 3 and MIL-100(Fe) are similar, about 4.5 µmol g −1 h −1 . The composite materials show significantly higher activity, and p-90Fe exhibits a maximum CO production of 20.4 µmol g −1 h −1 , which is about 4.5 times higher than the pure constituents. Upon increasing the load of MIL-100(Fe), lower photocatalytic activity is obtained. The decrease in photocatalytic activity may be caused by an increasing amount of CsPb 2 Br 5 in the composites, which is not active upon visible light irradiation. The critical role of H 2 O was revealed by a test reaction on p-90Fe in high purity CO 2 gas without H 2 O. The CO yield over p-90Fe dropped from 20.4 to 5.3 µmol g −1 h −1 . The composites' stability was evaluated by four consecutive runs (4 h each, in total 16 h) on p-90Fe. As shown in Figure 7b, the composite exhibits no significant deactivation, and its crystal structure is well maintained after the 16 h photocatalytic reaction ( Figure S4).
Two reference samples, p-post and p-mix, with the same amount of MIL-100(Fe) loaded as p-90Fe, were constructed. p-post was synthesized by the anti-solvent deposition of CsPbBr 3 onto MIL-100(Fe) [39]. p-mix was prepared through ultrasonically mixing the CsPbBr 3 and MIL-100(Fe) powders. Under visible light irradiation, the CO production rates over p-post and p-mix are only 8.7 µmol g −1 h −1 and 6.8 µmol g −1 h −1 , nearly one-third of that obtained via the newly introduced in situ growth route. A summary of the reported photocatalytic CO 2 reduction performance on perovskite-based and traditional photocatalysts under various illumination conditions is listed in Table S1. is not active upon visible light irradiation. The critical role of H2O was revealed by a test reaction on p-90Fe in high purity CO2 gas without H2O. The CO yield over p-90Fe dropped from 20.4 to 5.3 μmol g −1 h −1 . The composites' stability was evaluated by four consecutive runs (4 h each, in total 16 h) on p-90Fe. As shown in Figure 7b, the composite exhibits no significant deactivation, and its crystal structure is well maintained after the 16 h photocatalytic reaction ( Figure S4). Two reference samples, p-post and p-mix, with the same amount of MIL-100(Fe) loaded as p-90Fe, were constructed. p-post was synthesized by the anti-solvent deposition of CsPbBr3 onto MIL-100(Fe) [39]. p-mix was prepared through ultrasonically mixing the CsPbBr3 and MIL-100(Fe) powders. Under visible light irradiation, the CO production rates over p-post and p-mix are only 8.7 μmol g −1 h −1 and 6.8 μmol g −1 h −1 , nearly one-third of that obtained via the newly introduced in situ growth route. A summary of the reported photocatalytic CO2 reduction performance on perovskite-based and traditional photocatalysts under various illumination conditions is listed in Table S1.
It is observed that CsPbBr3 gradually transforms to CsPb2Br5 in the composites, and CsPb2Br5 is a large bandgap material without visible light response. Therefore, full-spectrum (300-800 nm) measurements were performed on the selected samples to investigate the photocatalytic contribution of CsPb2Br5 in the composites. As shown in Figure 7c, only slight improvements in CO generation were found on CsPbBr3, p-post, and p-mix, compared to that under visible light irradiation (420-800 nm). The CO generation significantly enhanced from 20.4 to 29.6 μmol g −1 h −1 on p-90Fe and from 13.6 to 16.6 μmol g −1 h −1 on p-120Fe. Hence, CsPb2Br5, used to be seen as the undesirable byproduct of CsPbBr3, can contribute to the composites' photoactivity.
It is acknowledged that the photocatalysts' performance can be influenced by the specific surface area. First, gas sorption measurements were performed ( Figure S5a). N2 physisorption revealed a negligible surface area for the pure CsPbBr3 microcrystals. The composite materials show a tremendously increased specific surface area between 130 and 400 m 2 /g (Table 3); the specific surface area increases with the amount of MIL-100(Fe). This surface area enhancement favors the exposure of active sites and the adsorption of CO2 molecules. For the composites, a type IV isotherm is observed related to the existence of a mesoporous structure. Further, Figure S5b shows the CO2 adsorption isotherms of the as-obtained samples. The CO2 uptakes in the composites are 15 to 30 times higher than that in pure CsPbBr3, which benefits the photocatalytic efficiency. Figure S6 shows the N2 physisorption isotherms and related pore size distribution of p-90Fe before and after the reaction. After the reaction, the surface area reduced from 201 m 2 /g to 155 m 2 /g, the average pore size decreased from 4.5 nm to 3.7 nm, and the pore volume dropped from 0.21 cm 3 /g to 0.11 cm 3 /g. As can be seen in Figure S6b, p-90Fe has dominant peaks at 1.2 and 2.0 nm, which is the typical pore size distribution of MIL-100(Fe) [40]. After the reaction, the portion of the mesopores around 10 nm decreased, which may be due to the influence of H2O on the particle size and shape during the reaction. It is observed that CsPbBr 3 gradually transforms to CsPb 2 Br 5 in the composites, and CsPb 2 Br 5 is a large bandgap material without visible light response. Therefore, full-spectrum (300-800 nm) measurements were performed on the selected samples to investigate the photocatalytic contribution of CsPb 2 Br 5 in the composites. As shown in Figure 7c, only slight improvements in CO generation were found on CsPbBr 3 , p-post, and p-mix, compared to that under visible light irradiation (420-800 nm). The CO generation significantly enhanced from 20.4 to 29.6 µmol g −1 h −1 on p-90Fe and from 13.6 to 16.6 µmol g −1 h −1 on p-120Fe. Hence, CsPb 2 Br 5 , used to be seen as the undesirable byproduct of CsPbBr 3 , can contribute to the composites' photoactivity.
It is acknowledged that the photocatalysts' performance can be influenced by the specific surface area. First, gas sorption measurements were performed ( Figure S5a). N 2 physisorption revealed a negligible surface area for the pure CsPbBr 3 microcrystals. The composite materials show a tremendously increased specific surface area between 130 and 400 m 2 /g (Table 3); the specific surface area increases with the amount of MIL-100(Fe). This surface area enhancement favors the exposure of active sites and the adsorption of CO 2 molecules. For the composites, a type IV isotherm is observed related to the existence of a mesoporous structure. Further, Figure S5b shows the CO 2 adsorption isotherms of the as-obtained samples. The CO 2 uptakes in the composites are 15 to 30 times higher than that in pure CsPbBr 3 , which benefits the photocatalytic efficiency. Figure S6 shows the N 2 physisorption isotherms and related pore size distribution of p-90Fe before and after the reaction. After the reaction, the surface area reduced from 201 m 2 /g to 155 m 2 /g, the average pore size decreased from 4.5 nm to 3.7 nm, and the pore volume dropped from 0.21 cm 3 /g to 0.11 cm 3 /g. As can be seen in Figure S6b, p-90Fe has dominant peaks at 1.2 and 2.0 nm, which is the typical pore size distribution of MIL-100(Fe) [40]. After the reaction, the portion of the mesopores around 10 nm decreased, which may be due to the influence of H 2 O on the particle size and shape during the reaction. As shown in Figure 8, the PL decay plots of CsPbBr 3 and composite p-90Fe are fitted with a biexponential decay function. The short and long PL lifetimes can be assigned to two different physical origins. The short (τ 1 ) and long (τ 2 ) lifetimes are related to the trap-assisted and exciton recombination, respectively [13]. The average lifetime (τ) of CsPbBr 3 exhibits an obvious decrease from 4.4 to 1.6 ns after adding MOF, resulting from the suppressed exciton recombination. MIL-100(Fe) here functions as a quencher of CsPbBr 3 , endowing the composite with a more effective electron extraction [41]. origins. The short (τ1) and long (τ2) lifetimes are related to the trap-assisted and exciton recombination, respectively [13]. The average lifetime (τ) of CsPbBr3 exhibits an obvious decrease from 4.4 to 1.6 ns after adding MOF, resulting from the suppressed exciton recombination. MIL-100(Fe) here functions as a quencher of CsPbBr3, endowing the composite with a more effective electron extraction [41]. The photocatalytic activity of MIL-100(Fe) mainly originates from the direct excitation of the Fe3-μ3-oχo clusters inside the structure. Under light irradiation, Fe-O clusters in MIL-100(Fe) can be excited, transferring an electron from the O 2− to Fe 3+ for the formation of Fe 2+ , which is responsible for CO2 reduction over pure MIL-100(Fe) [22,42]. The combination of the bandgap data with valence band measurements ( Figure S7 and Table 2) allow to determine the VB and CB edge potentials of CsPbBr3 to be 0.82 and −1.45 eV, respectively. The VB and CB edge potentials of MIL-100(Fe) are calculated to be 1.75 and −0.93 eV, respectively. Therefore, a typical type II heterojunction was formed by the perfect band structure matching of CsPbBr3 and MIL-100(Fe) [42].
A possible mechanism for the visible-light driven photocatalytic CO2 reduction over the composite is proposed, as shown in Figure 9. Photo-induced electron and hole pairs are generated on CsPbBr3 and MIL-100(Fe) and tend to transfer. The electrons in the conduction band of CsPbBr3 will transfer to that of MIL-100(Fe), where CO2 would be reduced to CO. The holes on the valence band of MIL-100(Fe) would migrate to that of CsPbBr3, where H2O will be trapped to generate O2. With H2O involved in the synthesis, CsPbBr3 gradually converts to CsPb2Br5, which has no visible light response. Revealed by the XRD and UV-vis absorption spectra, p-90Fe has the highest amount of MIL-100(Fe) in the composite while maximally retaining CsPbBr3. MIL-100(Fe) greatly increases the surface area and enhances the visible light absorption ability of CsPbBr3. Upon visible The photocatalytic activity of MIL-100(Fe) mainly originates from the direct excitation of the Fe 3 -µ 3 -oχo clusters inside the structure. Under light irradiation, Fe-O clusters in MIL-100(Fe) can be excited, transferring an electron from the O 2− to Fe 3+ for the formation of Fe 2+ , which is responsible for CO 2 reduction over pure MIL-100(Fe) [22,42]. The combination of the bandgap data with valence band measurements ( Figure S7 and Table 2) allow to determine the VB and CB edge potentials of CsPbBr 3 to be 0.82 and −1.45 eV, respectively. The VB and CB edge potentials of MIL-100(Fe) are calculated to be 1.75 and −0.93 eV, respectively. Therefore, a typical type II heterojunction was formed by the perfect band structure matching of CsPbBr 3 and MIL-100(Fe) [42].
A possible mechanism for the visible-light driven photocatalytic CO 2 reduction over the composite is proposed, as shown in Figure 9. Photo-induced electron and hole pairs are generated on CsPbBr 3 and MIL-100(Fe) and tend to transfer. The electrons in the conduction band of CsPbBr 3 will transfer to that of MIL-100(Fe), where CO 2 would be reduced to CO. The holes on the valence band of MIL-100(Fe) would migrate to that of CsPbBr 3 , where H 2 O will be trapped to generate O 2 .
Catalysts 2020, 10, x FOR PEER REVIEW 8 of 13 origins. The short (τ1) and long (τ2) lifetimes are related to the trap-assisted and exciton recombination, respectively [13]. The average lifetime (τ) of CsPbBr3 exhibits an obvious decrease from 4.4 to 1.6 ns after adding MOF, resulting from the suppressed exciton recombination. MIL-100(Fe) here functions as a quencher of CsPbBr3, endowing the composite with a more effective electron extraction [41]. The photocatalytic activity of MIL-100(Fe) mainly originates from the direct excitation of the Fe3-μ3-oχo clusters inside the structure. Under light irradiation, Fe-O clusters in MIL-100(Fe) can be excited, transferring an electron from the O 2− to Fe 3+ for the formation of Fe 2+ , which is responsible for CO2 reduction over pure MIL-100(Fe) [22,42]. The combination of the bandgap data with valence band measurements ( Figure S7 and Table 2) allow to determine the VB and CB edge potentials of CsPbBr3 to be 0.82 and −1.45 eV, respectively. The VB and CB edge potentials of MIL-100(Fe) are calculated to be 1.75 and −0.93 eV, respectively. Therefore, a typical type II heterojunction was formed by the perfect band structure matching of CsPbBr3 and MIL-100(Fe) [42].
A possible mechanism for the visible-light driven photocatalytic CO2 reduction over the composite is proposed, as shown in Figure 9. Photo-induced electron and hole pairs are generated on CsPbBr3 and MIL-100(Fe) and tend to transfer. The electrons in the conduction band of CsPbBr3 will transfer to that of MIL-100(Fe), where CO2 would be reduced to CO. The holes on the valence band of MIL-100(Fe) would migrate to that of CsPbBr3, where H2O will be trapped to generate O2. With H2O involved in the synthesis, CsPbBr3 gradually converts to CsPb2Br5, which has no visible light response. Revealed by the XRD and UV-vis absorption spectra, p-90Fe has the highest amount of MIL-100(Fe) in the composite while maximally retaining CsPbBr3. MIL-100(Fe) greatly increases the surface area and enhances the visible light absorption ability of CsPbBr3. Upon visible With H 2 O involved in the synthesis, CsPbBr 3 gradually converts to CsPb 2 Br 5 , which has no visible light response. Revealed by the XRD and UV-vis absorption spectra, p-90Fe has the highest amount of MIL-100(Fe) in the composite while maximally retaining CsPbBr 3 . MIL-100(Fe) greatly increases the surface area and enhances the visible light absorption ability of CsPbBr 3 . Upon visible light irradiation, the separation and transfer of the photogenerated charge carriers is promoted in the composites, resulting in enhanced photocatalytic performance. Further increasing the amount of MIL-100(Fe) increases the specific surface area and CO 2 uptake, but at the expense of CsPbBr 3 . In a typical procedure, 2.02 g iron (III) nitrate nonahydrate (Fe(NO 3 ) 3 ·9H 2 O)(99%, ACROS Organics, Geel, Belgium), and 0.7 g 1,3,5-benzenetricarboxylic acid (H 3 BTC, 95%, Sigma-Aldrich, Overijse, Belgium) were added to 5 mL H 2 O and stirred for 30 min at RT. This mixture was then heated to 95 • C and maintained at this temperature for 12 h. After cooling down the mixture, the obtained orange solid product was collected by centrifugation, washed with distilled water, and dried at 80 • C.

Synthesis of the CsPbBr 3 /MIL-100(Fe) Composites
The CsPbBr 3 /MIL-100(Fe) composites were obtained by in situ growth. The Fe precursor solution for MIL-100(Fe) was made first, by adding 30, 60, 90, 120, and 180 mg Fe(NO 3 ) 3 ·9H 2 O into 3 mL 1-propanol (99.5%, ACS agent, Fisher Chemical, Merelbeke, Belgium). Next, 0.1 g CsPbBr 3 was added to this Fe precursor solution and the mixture was stirred for 12 h at RT. The H 3 BTC powder was then added with a 3:1 molar ratio of H 3 BTC to Fe(NO 3 ) 3 ·9H 2 O. The mixture was heated to 95 • C and kept at this temperature for 12 h. The resulting orange solid product was collected by centrifugation, washed by toluene, and dried at 100 • C in a vacuum oven. The obtained samples were named as p-xFe (x = the weight of Fe(NO 3 ) 3 ·9H 2 O added).
For comparison, CsPbBr 3 was loaded onto MIL-100(Fe) by anti-solvent deposition [39]. This sample is named as p-post. Furthermore, a physical mixture of the pure CsPbBr 3 and MIL-100(Fe) was prepared by ultrasonically mixing the powders, named as p-mix. The weight ratios of MIL-100(Fe) in both p-post and p-mix are 18 wt%. Finally, 90 mg Fe(NO 3 ) 3 ·9H 2 O was dehydrated in a vacuum oven at 80 • C for 4 h before the composite synthesis, and the obtained sample was named as p-90Fe-dehydrated.

Characterization
X-ray diffraction (XRD) patterns were recorded on a Malvern PANalytical Empyrean system (Malvern PANalytical, Hoeilaart, Belgium). Scanning electron microscopy (SEM) images and the corresponding energy dispersive X-ray spectroscopy (EDS) data were taken with a FEI-Q FEG250 system (Thermo Fisher Scientific, Zaventem, Belgium). X-ray photoelectron spectroscopy (XPS) data were recorded on an ESCALAB 250 spectrometer (Thermo Fisher Scientific, Beijing, China) with a monochromatized Al Ka X-ray as the excitation source, and the binding energies were calibrated by the C1s peak at 284.8 eV. Gas sorption measurements were performed on a 3Flex Surface Analyzer (Micromeritics, Unterschleissheim, Germany) where nitrogen (N 2 ) adsorption-desorption isotherms were acquired at −196 • C and CO 2 adsorption isotherm were acquired at 0 • C. Before the measurement, all the samples were degassed at 150 • C for 8 h under flowing N 2 . Thermogravimetric analysis (TGA) was performed from room temperature to 800 • C at a heating rate of 10 • C min −1 in O 2 on a TA-TGA Q500 (TA Instruments, Antwerp, Belgium). Fourier transformed infrared (FTIR) spectra were collected using a Bruker FTIR spectrophotometer (Bruker, Kontich, Belgium). UV-Vis diffuse reflectance spectra were recorded with a Lambda-950 UV-Vis spectrometer (PerkinElmer, Mechelen, Belgium). The steady-state photoluminescence (PL) spectra were acquired on an Edinburgh FLS980 instrument (Edinburgh Instruments Ltd, Livingston, UK). The samples were prepared by filling a thin quartz cuvette (Macro cell 110-QS, 1 mm light path, Hellma, Kruibeke, Belgium) with the same volume of powders. Time-resolved PL spectra were acquired by a home-built confocal FLIM microscope, equipped with a single-photon counting device (Picoquant, Berlin, Belgium) Powders of CsPbBr 3 and p-90Fe were dispersed in toluene and sonicated for 15 min. The suspension was then dropped onto a 20 × 20 mm cover slide and dried in the vacuum oven at 80 • C overnight. The excitation source was a 5 MHz pulsed 485 nm laser diode, and the emission was filtered by a 530 ± 25 nm band pass filter. Confocal fluorescence images were acquired on a Fluoview FV1000 confocal microscope (Olympus, Tokyo, Japan). An Olympus 20 × 0.75 NA air immersion objective lens was used. A 375 nm laser was used as the excitation source, and the detectors for fluorescence emission were set at 430-470 and 505-540 nm. The image size was 1024 × 768 pixels with a pixel dwell time of 4 µs.

Photocatalytic CO 2 Reduction Measurement
The photocatalytic reduction of CO 2 was performed in a homemade Pyrex reactor (volume: 150 mL). Visible light was generated by a 300 W Xe lamp with a 420 cut-off filter (Newport, Darmstadt, Germany) and positioned 5 cm away from the photocatalytic reactor. In a typical sample preparation, 20 mg photocatalyst was uniformly dispersed on a flat glass plate with an area of 4 cm 2 . The as-prepared sample plate was left in the vacuum oven at 80 • C overnight to remove the residual solvent. Before the reaction, helium flowed through the reactor for about 20 min to eliminate the air inside. Then, a mixture of CO 2 and water vapor, generated by passing high purity CO 2 (99.99%) gas through a water bubbler, flowed through the reactor for another 40 min in the dark. Afterwards, the reactor was closed off and light irradiation was started. The gas sample was evaluated every 1 h by gas chromatography (GC-2014, Shimadzu, Tokyo, Japan) equipped with a ShinCarbon packed column with a flame ionization detector (FID) and a thermal conductivity detector (TCD). The carrier gas used in the GC-2014 was high purity helium. After the 16 h stability test, the sample was collected and treated at 80 • C in the vacuum oven overnight.

Conclusions
In summary, we have reported the in situ synthesis of CsPbBr 3 /MIL-100(Fe) for CO 2 photoreduction under visible light irradiation. Compared to anti-solvent deposition and physical mixing methods, the composites obtained by this new route show a significantly improved CO 2 photoreduction'activity. H 2 O is necessary for the in situ growth of the MOF structure, but excessive H 2 O initiates the transformation of CsPbBr 3 to CsPb 2 Br 5 . Through optimization of the composite material composition, the highest CO production rate is 20.4 µmol g −1 h −1 , which is about 4.5 times that of the parent materials. The composite showed good stability in a 16 h photocatalytic reaction, where H 2 O is involved as the reactant. The introduction of MIL-100(Fe) endowed the composites with a largely increased surface area and enhanced light-harvesting capability in the visible light region. The perfect band matching between CsPbBr 3 and MIL-100(Fe) attributes to better electron-hole separation and transfer. The findings here could serve as a steppingstone for further developing MHP photocatalysts, involving MOF-based heterojunctions.
Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4344/10/11/1352/s1, Figure S1: XPS spectra of p-90Fe, Figure S2: Photoluminescence spectra of the as-prepared samples at an excitation wavelength of 380 nm, Figure S3: Typical SEM images of CsPbBr 3 , MIL-100(Fe) and p-90Fe, and EDS pattern of the selected region in p-90Fe, Figure S4: XRD pattern of p-90Fe before and after a 16 h photocatalytic reaction, Figure S5: N 2 adsorption-desorption isotherms and CO 2 adsorption isotherms of the photocatalysts, Figure S6: N 2 adsorption-desorption isotherms and pore size distribution curves of the p-90Fe before and after reaction, Figure S7: Valence band XPS spectra of MIL-100(Fe), CsPbBr 3 and p-90Fe, Table S1: Summary of the reported photocatalytic CO 2 reduction performance of perovskite-based and traditional photocatalysts under various illumination conditions.

Conflicts of Interest:
The authors declare no conflict of interest.