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Article

Encapsulating Halide Perovskite Quantum Dots in Metal–Organic Frameworks for Efficient Photocatalytic CO2 Reduction

by
Jingwen Zhang
,
Wentian Zhou
,
Junying Chen
* and
Yingwei Li
*
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China
*
Authors to whom correspondence should be addressed.
Catalysts 2024, 14(9), 590; https://doi.org/10.3390/catal14090590
Submission received: 9 August 2024 / Revised: 24 August 2024 / Accepted: 28 August 2024 / Published: 3 September 2024
(This article belongs to the Section Photocatalysis)
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Abstract

:
Halide perovskite has shown great potential in photocatalysis owing to its diversity, suitable energy band alignment, rapid charge transfer, and excellent optical properties. However, poor stability, especially under humid conditions, hinders their practical application in photocatalysis. In this work, we report the encapsulation of inorganic–organic hybrid perovskite QDs into MIL-101(Cr) through an in situ growth strategy to prepare a series of MAPbBr3@MIL-101(Cr) (MA = CH3NH3+) composites. The perovskite precursors, i.e., MABr and PbBr2, were successively introduced into the pores of MOF, where the perovskite quantum dots were self-assembled in the confined environment. In photocatalytic CO2 reduction, 11%MAPbBr3@MIL-101(Cr) composite displayed the best performance among the composites with a total CO and CH4 yield of 875 μmol g−1 in 9 h, which was 8 times higher than that of the pure MAPbBr3. Such high gas production efficiency could be maintained for 78 h at least without structural and morphologic decomposition. The remarkable stability and catalytic activity of composites are mainly due to the synergistic effect and improved electron transfer between MAPbBr3 and MIL-101(Cr). Moreover, these composites revealed a novel mechanism, showing switched CH4 selectivity with the controlling of the perovskite location and contents. Those with perovskites encapsulated in the mesopores of MIL-101(Cr) were more preferential for CO production, while those with perovskites encapsulated in both meso- and micropores could produce CH4 dominantly.

1. Introduction

CO2 is the highest oxidation state of carbon, which plays a key role in affecting the carbon cycle in nature. Photocatalytic CO2 reduction into value-added hydrocarbon fuels including CH4, CH3OH, and C2 products, as well as CO, which is one of the most important resources for industrial carbonization and an efficient reducing reagent, is of great promise in clean energy conversion to solve the energy shortage crisis and severe environmental issues [1,2]. However, its C=O bonds are extremely stable thermodynamically for breaking, leading to low conversion efficiency and selectivity to the desired product. Various materials have been developed to improve photocatalytic CO2 conversion efficiency, including metals, metal oxides, nitrides, sulfides, chalcogenides, organic polymers, and perovskite materials [3,4,5,6,7,8,9,10,11]. Many of them have shown limits, such as high cost, lengthy and complex synthesis process, long-term instability, and low catalytic activity. Recently, perovskite materials have been widely considered as rising photocatalysts for photocatalysis due to a wide range of benefits over conventional semiconductors, including their diversity, suitable energy band alignment, rapid charge transfer, and excellent optical properties [12,13]. Among the various perovskite materials, halide perovskite has been invoked as an emerging class of semiconducting photocatalyst, due to its extraordinary properties like cost-effectiveness, easy synthesis process, visible light absorption, high CO2 adsorption surface area, surface disorders for charge trapping, and tunable structure [14,15,16,17,18]. However, due to the ionic properties of halide perovskite, these materials usually have poor stability. The structural features of perovskites experience swift degradation under different stress conditions such as moisture, UV light, oxygen, and high temperature, hindering their practical application in photocatalysis. Various approaches including the integration of multiple capping agents/stabilizers and encapsulation into porous materials have been developed to tackle this limit [19,20,21,22]. Moreover, quantum dots (QDs) refer to the particles with the uniform size distribution in the range of 2–10 nm, which have shown significantly improved performances over the bulk materials in catalysis, especially in solar energy conversion, due to the unique advantages including tunable and size-dependent electronic properties, extended light-harvesting region, and quantum confinement effect. However, the small particle size also leads to high energy of its surface atoms so that the aggregation of those QDs is inevitable.
In recent years, metal–organic frameworks (MOFs) have shown specific merits as the host to incorporate and stabilize perovskite materials owing to their unique properties including robust frameworks, high specific surface area, adjustable pore size and structure, and massive catalytic active sites [23,24,25], rendering the resulting perovskite/MOF composites with improved stability and reduced perovskite particle sizes [26,27,28]. Chang et al. reported the first example of incorporating MOF with perovskite to enhance the crystallinity of the perovskite in 2015 using a one-step deposition method [29]. The perovskite crystals were formed both on the surface and inside the pores of the MOF. Early works about the perovskite/MOF composites in photocatalysis were reported in the late 2010s. Su and co-workers grew the zinc/cobalt-based imidazolate framework (ZIF) on the surface of CsPbBr3 QDs, resulting in a largely enhanced moisture stability for up to 10 days [30]. The CsPbBr3@ZIF-67 composite could produce CO and CH4 with yields of 2.301 µmol and 10.537 µmol g–1, respectively, with an electron consumption rate (Relectron) of 29.630 µmol g–1 h–1, while the pure CsPbBr3 gave an Relectron of 11.14 µmol g–1 h−1. The recycling test of CsPbBr3@ZIF-67 was monitored for 18 h without significant decay. Later in 2019, Lu and co-workers reported the encapsulation of MAPbI3 QDs in the pores of a Fe-porphyrin-based MOF of PCN-221(Fex) [27]. The highest CO2 photoreduction was detected for MAPbI3@PCN-221(Fe0.2), showing a high yield of 1559 µmol g–1 in 80 h including 34% CO and 66% CH4, and Relectron of 129.0 µmol g–1 h−1. Zhong and co-workers prepared a CsPbBr3 QDs/UiO-66(NH2) nanojunction which could catalyze CO2 conversion under visible light irradiation, giving the highest CO yield of 98.57 μmol g−1 and CH4 yield of 3.08 μmol g−1 in 12 h, which is much higher than that of pure CsPbBr3 QDs and UiO-66(NH2) [31]. Its Relectron was 18.5 μmol g−1 h−1. More recently, Su and co-workers reported a ternary CsPbBr3/Au/PCN-333(Al) hybrid, showing a 100% CO selectivity with a yield of 186.15 μmol g−1 in a 3 h reaction [32]. Relectron was calculated to be 124.1 μmol g−1 h−1. As listed, the conversion efficiency and the stability and reusability of photocatalytic CO2 reduction catalyzed by perovskite/MOF composite is still relatively low. The research on the development of perovskite/MOF composite towards its application in CO2 photoreduction is still at an early stage and needs epic improvement. In addition, the precise control of the perovskite location in the MOF host is still challenging with random occasions neither inside the pore nor on the surface, leading to low photocatalysis efficiency and product selectivity.
In this work, we report a facile in situ growth method to encapsulate the perovskite quantum dots (QDs) into the pores of MOFs. A series of xMAPbBr3@MIL-101(Cr) (MA = CH3NH3+, x = 8%, 9%, 11%, 14%) composites were prepared. The confined environment limits the growth of the perovskite QDs, resulting in an average size of 2–3 nm, which showed superiority for catalysis. The composites also display improved light harvesting, charge separation, and transfer, as well as enhanced lifetime of excited electrons. As a result, in photocatalytic CO2 reduction, 11%MAPbBr3@MIL-101(Cr) composite displayed an extraordinary performance with a total CO and CH4 yield of 875 μmol g−1 in 9 h, and impressive stability in an as long as 78 h test without structural and morphologic decomposition compared to that of the pure MAPbBr3. Moreover, we also reveal a novel mechanism for the photocatalytic CO2 reduction catalyzed by these perovskite@MOF composites that, through the precise control of the location and amount of the encapsulated perovskite QDs in the MOF pores, the product selectivity could be switched from a CO dominant mode in those with perovskites encapsulated in the mesopores of MIL-101(Cr) to a CH4 dominant one for those with perovskites encapsulated in both meso- and micropores.

2. Results and Discussion

MIL-101(Cr), constructed via the reaction between the Cr3+ node and the terephthalate ligand, was chosen as the host MOF in this study owing to its robust property and massive meso- and micropores which are accessible for the perovskite precursors [33,34,35,36,37]. The confined growth of the perovskite QDs could be protected by the MOF matrix from leaching and decomposition. As depicted in Scheme 1, MAPbBr3@MIL-101(Cr) composites were prepared using a two-step deposition strategy, during which the pristine species for the in situ nucleation of MAPbBr3 QDs were sequentially introduced and self-assembled in the pores of MIL-101(Cr). By adjusting the usage of PbBr2 and MABr, the MAPbBr3 content in the composites could be tuned. For example, in the xMAPbBr3@MIL-101(Cr) (x = 8%, 9%, 11%, 14%) composites, the percentage numbers represent the actual weight percent of MAPbBr3 determined by inductively coupled plasma optical emission spectrometer (ICP-OES) analysis and calculated by Equation (S1), which are 8.29%, 9.02%, 11.17%, and 13.95%, respectively (Table S1).
From the wide-angle PXRD patterns of MIL-101(Cr), xMAPbBr3@MIL-101(Cr), and MAPbBr3 bulk composites in Figure 1a, it is clear that the characteristic peaks of MIL-101(Cr) and MAPbBr3 match well with the simulated patterns prepared from the crystal structures (CCDC 605510 and 1446529), indicating the good purity and crystallinity of the prepared samples. After the introduction of MAPbBr3 QDs, the typical patterns of MIL-101(Cr) remain unchanged, suggesting the undisturbed integrity of the host MOF. In addition, the characteristic peaks of MAPbBr3 QDs are not observed in all xMAPbBr3@MIL-101(Cr) composites, probably owing to the small sizes confined in the pores of MIL-101(Cr). Figure 1b shows the small-angle (1~10°) XRD patterns of MIL-101(Cr) and xMAPbBr3@MIL-101(Cr) samples, in which the intense peaks demonstrate the mesoporous structure of these samples. The characteristic diffraction peaks of MIL-101(Cr) shift slightly to larger angles with increasing the MAPbBr3 encapsulation content; while the peak intensity decreases. It is due to the influence of the occupation of QDs in the MOF pore to the crystal structures [37]. These observations imply the encapsulation of MAPbBr3 QDs in the mesopores of MIL-101(Cr) while keeping intact the high crystallinity of MIL-101 [33,34,37].
Subsequently, the nitrogen adsorption–desorption isotherms and the pore size distribution of MIL-101(Cr) and xMAPbBr3@MIL-101(Cr) recorded at 77 K and 1 atm are shown in Figure 2. The N2 adsorption–desorption isotherms of xMAPbBr3@MIL-101(Cr) display type I and type IV isotherms with narrow H4 hysteresis loops similar to that of the pristine MIL-101(Cr), indicating the co-existence of micro- and mesopores in these samples (Figure 2a). The BET-specific surface areas of MIL-101(Cr) and xMAPbBr3@MIL-101(Cr) (x = 8%, 9%, 11%, 14%) composites calculated by Brunauer–Emmett–Teller equation are 2714.0, 1426.4, 1319.5, 975.5, and 936.8 m2 g−1, respectively. The total pore volume values of MIL-101(Cr) and xMAPbBr3@MIL-101(Cr) composites determined by using the adsorption branch of N2 adsorption–desorption isotherm at P/P0 = 0.95 are 1.28, 0.72, 0.63, 0.49, and 0.47 cm3 g−1, respectively. The results indicate that upon the encapsulation of MAPbBr3 QDs, both the N2 uptake capacity and the total pore volume decreased largely with increasing the perovskite content, but a saturation phenomenon occurred after 14% MAPbBr3 QDs were encapsulated in MIL-101(Cr), which gave about 65% and 63% decreases for the specific surface area and total pore volume, respectively. Moreover, it could be observed from the corresponding micro- and mesopore volumes in Table S2 that the major part of MAPbBr3 QDs first fulfill the mesopores as the QDs contents increased to 9% to reach a maximum and then started to occupy the micropores as the QDs content further increased to 14% with a satisfactory encapsulation. The CO2 adsorption isotherms of these samples in Figure S1 also show a decreasing trend upon increasing the content of perovskite QDs in the MIL-101(Cr) pores that are consistent with the N2 adsorption results, confirming the location of the QDs.
The encapsulation of MAPbBr3 QDs inside the MIL-101(Cr) pores was further evidenced by IR spectra. As compared with the spectrum for MIL-101(Cr), the new peaks around 1257 nm−1 appeared for all halide perovskite-based samples, which can be assigned to the CH3-NH3+ rock vibration (Figure S2) [38]. These results demonstrate that the targeted xMAPbBr3@MIL-101(Cr) composite system has been successfully constructed.
11%MAPbBr3@MIL-101(Cr) was chosen as a typical sample to evaluate the influence of encapsulating perovskite QDs into the MOF pores on the microstructure and morphology based on the following reactivity study, showing the best photocatalytic CO2 reduction activity and stability. SEM and TEM images of the pristine MIL-101(Cr) show a classical octahedral morphology with smooth surfaces and a diameter of ca. 700 nm, consistent with the previous reports [33,34]. After encapsulation, the well-maintained octahedron indicates structural integrity (Figure 3a). No obvious particles could be detected either on the surfaces or inside the octahedron, probably due to the short detection time since MIL-101 is highly sensitive to the electron beam which would cause the collapse of the structure. The HAADF-STEM image clearly shows white bright spots evenly distributed in octahedral particles with an average particle size of about 2–3 nm, which matches the mesoporous pore sizes (29 Å and 34 Å) of MIL-101(Cr) (Figure 3b) [37]. In addition, the elemental mapping images of 11%MAPbBr3@MIL-101(Cr) show that Br, Pb, N, C, and Cr elements are evenly distributed (Figure 3c–h). These results confirmed the successful encapsulation of MAPbBr3 QDs inside the pores of MIL-101(Cr).
X-ray photoelectron spectroscopy (XPS) analysis was employed to investigate the surface properties of the composite and the interfacial interaction between MIL-101(Cr) and MAPbBr3 QDs. The total survey spectra of the samples in Figure 4a demonstrate the coexistence of Pb, Br, and N elements in all materials. The high-resolution Br 3d XPS spectra are shown in Figure 4b. The curves for the pure MAPbBr3 QDs could be deconvoluted into two separated peaks centered at 69.0 and 67.9 eV for the Br 3d5/2 and 3d3/2 hybrid orbits, respectively. Positive shifts to higher binding energies of 69.2 and 68.1 eV, respectively, could be found for 9%MAPbBr3@MIL-101(Cr), which are further shifted to 69.3 eV and 68.2 eV, respectively, for those in 11%MAPbBr3@MIL-101(Cr). In the Pb 4f XPS spectra of MAPbBr3 QDs, 9% and 11%MAPbBr3@MIL-101(Cr) (Figure 4c), the peaks located at 143.0 and 138.1 eV could be attributed to the binding energies of Pb 4f5/2 and Pb 4f7/2 in the pure MAPbBr3 QDs, which also shifts to higher bind energies of 143.5 eV and 138.5 eV for 9%MAPbBr3@MIL-101(Cr) and 143.7 eV and 138.7 eV for 11%MAPbBr3@MIL-101(Cr), respectively. Meanwhile, a contrary shift to lower binding energies of 0.1 and 0.3 eV could be observed in the Cr 2p1/2 and 2p2/3 XPS spectra of the 9% and 11%MAPbBr3@MIL-101(Cr) samples, respectively, in comparison with those of the pure MIL-101(Cr) sample (Figure 4d). These XPS results confirm the positive shifts of the Br 3d and Pb 4f binding energies, as well as the negative shift of the Cr 2p upon incorporation of MIL-101(Cr) and MAPbBr3 QDs compared to the single phase. The systematic shifts vary with the gradual increase of MAPbBr3 QDs contents in the composites. Therefore, the increased electron cloud density of Cr and reduced electron cloud density of Pb and Br in MAPbBr3 recommend an electron transfer pathway from Br− to Cr3+, leading to the strong coordination between Cr3+ and Br- ions and subsequent interaction between MAPbBr3 and chromium oxide SBU of MIL-101(Cr).
The photocatalytic CO2 reduction performances of the samples were evaluated subsequently via a gaseous reaction mode by placing the photocatalysts in a sealed Pyrex bottle. This kind of gas–solid reaction system could greatly avoid the leaching issue of the toxic Pb and Cr elements to the environment and the competition reaction of water splitting to produce H2. The reactor was vacuumed to remove any air inside and filled with CO2 and a tiny amount of H2O (10 μL), which would provide vapor under light irradiation. Such a gas–solid reaction system is useful to prohibit the product from the comparative water-splitting reaction [37]. Figure 5a shows the photocatalytic reduction ability of MAPbBr3 and xMAPbBr3@MIL-101(Cr) in a 9 h reaction, which has been normalized by the content of reactive sites, i.e., the actual halide perovskite content for the composites and the weight for neat MIL-101(Cr). For all samples, the photocatalytic CO2 reduction products were CO and CH4. Neither H2 nor other liquid products including CH3OH and HCOOH could be detected. Control experiments using N2 to replace CO2 gas under the same condition revealed no product. As depicted in Figure 5a, pure MIL-101(Cr) cannot serve as a photocatalyst in CO2 reduction with merely no reactivity, while for pure MAPbBr3, the total production of CO and CH4 was 107 μmol g−1. These results indicated that both single-phase samples were inefficient photocatalysts. However, after incorporation, the photocatalytic activity increased significantly even with a small encapsulation amount of MAPbBr3, i.e., 5% (144 μmol g−1), as compared with that of MIL-101(Cr). The total gas yields and the CH4 selectivity increase with increasing halide perovskite contents in xMAPbBr3@MIL-101(Cr). The xMAPbBr3@MIL-101(Cr) composites (x = 8%, 9%, 11%, and 14%) displayed a total production of CO and CH4 6–8 times higher than that of pure MAPbBr3, among which 11%MAPbBr3@MIL-101(Cr) obtained the highest total yield of 875 μmol g−1 in 9 h with the CO and CH4 formation rates of 16.9 and 80.3 μmol g−1 h−1, respectively. Notably, CO dominates in the product when the perovskite contents in the composites are less than 11%; while those with perovskite contents above 11% are more preferential for CH4 production. Interestingly, the selectivity switch happens when the amount and the location of the encapsulated at the number of 11%, which has been demonstrated to include the perovskite QDs in both meso- and micropores of MIL-101(Cr) by N2 adsorption–desorption and XPS analysis results. In the CO2 photoreduction of 11%MAPbBr3@MIL-101(Cr), the CH4 selectivity is 82.6%, which is a little bit higher than 75.7% for that of pure MAPbBr3 and much higher than 9.0% for the 5%MAPbBr3@MIL-101(Cr). Moreover, the surface area of the material normally plays a critical role in catalysis, displaying a positive correlation with the catalysis efficiency. However, in our case, MIL-101(Cr) is inactive although it has the highest surface area. Incorporation with MAPbBr3 would make the surface area of MIL-101(Cr) less, but the catalytic efficiency of the composites increased a lot. In addition, as for the xMAPbBr3@MIL-101(Cr) composites, the surface areas were systematically decreased when increasing the number x, although their photocatalytic activities showed a volcano-type relationship with the highest CO2 conversion at the number of 11%.
The electron consumption rate (Relectron) calculated by Formula (S2) is normally utilized to compare the efficiency in photocatalytic CO2 reduction. As shown in Figure S3, 11%MAPbBr3@MIL-101(Cr) obtained the highest Relectron value of 676.4 μmol g−1 h−1, which is 8.6 and 17-fold higher than the pure MAPbBr3 (77.8 μmol g−1 h−1) and 5%MAPbBr3@MIL-101(Cr) (40 μmol g−1 h−1), respectively. The enhancement of Relectron is higher than the total gas yield enhancement (8-fold) due to the largely enhanced selectivity toward CH4, which normally requires 8H+/8e- to proceed and should be much harder than the generation of CO thermodynamically. Interestingly, although pure perovskite is more preferential to CH4 production, the encapsulated composites clearly display a selectivity shift from CO to CH4 at the dividing point of 9% encapsulation of perovskite, from which the encapsulated perovskite sufficiently occupies the mesopores and start to enter the micropores of MIL-101(Cr), according to the analysis of the N2 adsorption–desorption isotherms and the pore size distribution.
With an extended reaction time of 27 h, the xMAPbBr3@MIL-101(Cr) photocatalyst not only shows well-maintained catalytic activity but also demonstrates high selectivity for CH4 (Figure 5b). However, the CH4 yield decreased slightly, leading to a decrease in the electron consumption rate. In addition, no photosensitizer or sacrificial agent is added to the reaction system, which brings a breakthrough and possibility for the practical application of CO2 photoreduction. To the best of our knowledge, this result is among the highest values for CO2 conversion and electron consumption rate of CO2 photoreduction catalyzed by perovskite-based photocatalytic materials under similar reaction conditions reported so far (Table S3).
Further, by increasing the reaction time to 78 h, the CO production increased linearly with a relatively consistent rate of 16.9 μmol g−1 h−1, while the CH4 production rate decreased continuously, as shown in the gas yield versus time curves (Figure S4). The catalyst collected after photocatalysis was characterized by XRD and SEM, showing well-maintained crystallinity and octahedral morphology (Figure S5). In addition, such a long runtime with impressive stability is rare in the reported studies of perovskite-involved photocatalysis, probably due to the use of MIL-101(Cr) as the host, which could not only confine the self-growth of the perovskite QDs but also serves as a physical barrier to protect the perovskite under photocatalysis conditions.
To further explore the reason for the remarkably enhanced photocatalytic performance, the negative effect of the surface areas on the catalytic activity, and the switched product selectivity of xMAPbBr3@MIL-101(Cr) composites in the CO2 photoreduction, their light-harvesting ability, energy band structure, electron-charge separation, and transfer efficiency were studied and compared. Figure 6a shows the light absorption behaviors of xMAPbBr3@MIL-101(Cr), MIL-101(Cr), and MAPbBr3 via UV−vis absorption spectroscopy. The absorption band edge is located at ~550 nm in the UV spectrum of MAPbBr3 QDs, corresponding to an energy band (Eg) of 2.25 eV, which is similar to the reported value [39]. The xMAPbBr3@MIL-101(Cr) exhibits similar light-absorption behaviors as compared to the pristine MIL-101(Cr) with strong absorption in the range of 200–400 nm. A slight, but systematic enhancement of the absorption at ~540 nm was observed with increasing MAPbBr3 contents, probably due to the small ratio of encapsulated perovskite QDs inside the pores of MIL-101(Cr). This result suggests that the photoelectrons in xMAPbBr3@MIL-101(Cr) could be easily excited upon visible light irradiation, which benefits the utilization of solar energy. The bandgaps (Eg) of MIL-101(Cr) and MAPbBr3 are estimated to be 3.63 and 2.2 eV from the Tauc plots, respectively (Figure S6a,b); while those for the composites are between 3.4 and 3.6 eV (Figure S6c–f). Furthermore, the conduction band edge for MIL-101(Cr) and MAPbBr3 obtained from the Mott–Schottky plots are −1.62 V and −1.00 V vs. NHE, respectively (Figure S7a,b). Those positive slopes suggest the n-type semiconductor nature of the samples. These results indicate that the photoexcited electrons in MAPbBr3 have enough driving force to reduce CO2 to CO (−0.53 V vs. NHE) or CH4 (−0.24 V vs. NHE) [17,19,40,41]. Moreover, steady-state PL spectra can also verify the electron transfer property by a much lower PL intensity of the 11%MAPbBr3@MIL-101(Cr) nanocomposite as compared with MAPbBr3 QDs (Figure 6b), suggesting the promoted electron transfer between the single component and thereby improved charge separation of the photoinduced electron–hole pairs.
Time-resolved photoluminescence (TRPL) was conducted to reveal the acceleration of the electron separation and transfer. The decay plots fitted via a multiexponential function show a much longer lifetime of excited electrons for the 11% MAPbBr3@MIL-101(Cr) (16.64 ns) than the bare MAPbBr3 (3.35 ns) (Figure 6c and Table S2), indicating the greatly suppressed nonradiative recombination upon the encapsulation of MAPbBr3 QDs inside the MOF pores. The enhanced carrier separation efficiency values of MIL-101(Cr), MAPbBr3, and xMAPbBr3@MIL-101(Cr) nanocomposites were further confirmed by other photoelectrochemical properties. The photocurrent density shows the generation of photocurrent upon switching the light-on and -off mode (Figure 6d). The current densities of xMAPbBr3@MIL-101(Cr) nanocomposites are all much higher than MAPbBr3 QDs and MIL-101(Cr) due to the high charge separation efficiency in those nanocomposites, which elucidates the interaction between MIL-101(Cr) and MAPbBr3. Among the xMAPbBr3@MIL-101(Cr), 11%MAPbBr3@MIL-101(Cr) reveals the highest photocurrent density. Similarly, electrochemical impedance spectra (EIS) also illustrate the fast kinetic charge transfer behavior of xMAPbBr3@MIL-101(Cr) upon illumination. The EIS Nyquist plots of all xMAPbBr3@MIL-101(Cr) show a much smaller semicircle arc at high frequencies than the two single components (Figure 6e). In addition, the semicircle arc of 11%MAPbBr3@MIL-101(Cr) in the EIS plots is not the smallest, but its slope of the linear line at low frequencies is the highest among all xMAPbBr3@MIL-101(Cr) composites, indicating the most efficient ion diffusion during the reactions. These results evidently reveal the efficient charge transfer between MAPbBr3 QDs and MIL-101(Cr), which offers a brand-new opportunity for perovskite material in photo/optical-related applications.
A similar preparation method was also used for the synthesis of a series of xMAPbI3@MIL-101(Cr) composites (x = 9%, 13%, 14%, and 15%, indicating the real weight percentage of MAPbI3 in the composites). Their physical, morphological, optical, and electrochemical properties were also characterized (Figures S8–15, Tables S1 and S2). In photocatalytic CO2 reduction reactions, they were relatively inefficient as compared with those for xMAPbBr3@MIL-101(Cr) (Figures S16–S18).
The band alignment of MAPbBr3 and MIL-101(Cr) determined from the UV–vis spectra and Mott–Schottky plots is given in Figure 7. The CB and VB of MaPbBr3 are in between those for the MIL-101(Cr), suggesting a possible Type I junction formed at the interface of MAPbBr3 and MIL-101(Cr), in which photogenerated electrons and holes should transfer from MIL-101(Cr) to MAPbBr3. However, the XPS results reveal the electron transfer pathway from the perovskite to the Cr3+ of the MOF, which is contrary to the Type I junction character. Therefore, electron paramagnetic resonance (EPR) recorded at 77 K was utilized to monitor the chemical state change of Cr under different conditions. As shown in Figure 6f, a broad and intense signal at g = 1.98 confirmed the existence of a high-spin S = 3/2 Cr(III) species [37,42]. An obvious intensity decrease could be detected for the perovskite-encapsulated sample, probably due to the formation of the EPR silent Cr2+ from the reduction of Cr3+ species. Upon irradiation of 11%MAPbBr3@MIL-101(Cr), the signal assigning to Cr3+ further decreases, consistent with the electron transfer pathway to form Cr2+.
In combination with the CO2 photoreduction results that MIL-101(Cr) with a more negative CB band showed no activity, it could be deduced that the incorporation of the two components does not form a Type I heterojunction (Figure 7). Electrons on the CB of MAPbBr3 transfer to the SBU of MIL-101(Cr) first, where Cr3+ could be reduced to Cr2+ (Ered = −0.55 V), and then to the adsorbed CO2 molecular, leading to the CH4 (Ered = −0.24 V) and/or CO (Ered = −0.53 V) production. On this condition, the SBUs of MOF serve as the functional catalytic sites, which could also unveil the interesting selectivity switch for those xMAPbBr3@MIL-101(Cr) composites. When the amount of encapsulated perovskite is less than 9%, their location is mainly in the mesopores of MIL-101(Cr). Upon illumination, the photogenerated electrons transferred from the perovskite QDs to the SBU of MIL-101(Cr) and then participated in CO2 reduction. Under this condition, CO is the main product since it is dynamically more favorable compared to the formation of CH4; both the CO2 conversion and CH4 selectivity increased with further increasing the encapsulated perovskite content. Up to an 11% encapsulation, the mesopores of MIL-101(Cr) were fully occupied by the QDs; the excess amount of these QDs started to enter the micropores of MIL-101(Cr), as shown in N2 and CO2 adsorption analysis results, leading to the lack of space for CO2 adsorption and its contact with the catalytic active sites, i.e., SBU of MIL-101(Cr). In that case, the perovskite itself functions as the catalytic site for CO2 photoreduction, revealing the high CH4 selectivity comparable to that of the pure perovskite material.

3. Experimental

3.1. Materials and Methods

Chromium nitrate (Cr(NO3)3·9H2O, Aladdin, Shanghai, China, 99%), terephthalic acid (H2BDC, C6H4(CO2H)2, Energy Chemical, Shanghai, China, 98%), hydrofluoric acid (HF, Macklin, Shanghai, China, 40%), CH3NH3Br (MABr, Macklin, China 99.5%), lead bromide (PbBr2, Macklin, China, 99.9%), DMF (N, N-dimethylformamide, Guangdong Guanghua Sci-Tech Co, Ltd., Shantou, China, AR), toluene (C7H8, Guangzhou Chemical Reagent Factory, Guangzhou, China, AR), and cyclohexane (C6H12, Aladdin, Shanghai, China, 99.5%) were used without further purification.

3.1.1. Preparation of MIL-101(Cr)

MIL-101(Cr) was synthesized according to the previous reports with slight modifications [33,34]. Typically, H2BDC (0.823 g, 5 mmol), Cr(NO3)3·9H2O (2 g, 5 mmol), HF(40 wt%, 5 mmol, 250 μL), and 24 mL of deionized water were added into an 80 mL Teflon-lined stainless-steel autoclave. After sonication for 30 min, the mixture was heated in an oven with a temperature programming rate of 1 °C/min until 220 °C, under which it was kept for 8 h. After that, the mixture was cooled down to 150 °C first within 1 h and then slowly to room temperature, followed by filtration with a G2 funnel to remove any unreacted H2BDC, and then a G4 funnel to collect the green powder, which was sequentially washed with deionized water and ethanol, and finally soaked in ethanol (95% ethanol with 5% water) at 80 °C for 24 h. The final product was obtained by isolation from the solution and dried for 12 h at 150 °C under a vacuum.

3.1.2. Preparation of MAPbBr3@MIL-101(Cr)

Firstly, the activated MIL-101(0.1 g) was dispersed in a PbBr2 solution (0.02 g) of DMF (1 mL) for 72 h to ensure enough PbBr2 to be adsorbed in the MIL-101 pores. Then, PbBr2@MIL-101(Cr) was obtained after rinsing off unadsorbed PbBr2 with DMF. An equivalent mole of MABr was dissolved in ethanol and mixed with the as-prepared PbBr2@MIL-101(Cr) solution of toluene (1 mL). After stirring for 10 min, the 11%MAPbBr3@MIL-101(Cr) sample could be obtained by concentration, rinsing with cyclohexane and ethanol, and drying in a 50 °C oven. By adjusting the usage of PbBr2 and the corresponding MABr, xMAPbBr3@MIL-101(Cr) with different MAPbBr3 contents can be obtained, i.e., 0.02, 0.03, 0.04, and 0.05 g of PbBr2 for xMAPbBr3@MIL-101(Cr) (x = 8%, 9, 11%, and 14%, respectively).

3.2. Characterization

Crystal structures of the synthesis samples were characterized by powder X-ray diffraction (XRD) patterns recorded on Bruker D8 ADVANCE using Cu Kα radiation (40 kV, 40 mA, λ = 1.543 Å). The MAPbBr3 content was determined by an inductively coupled plasma optical emission spectrometer (ICP-OES, Agilent 720ES, Agilent, Santa Clara, CA, USA). The size, morphology, and elemental distribution of samples were determined by high-resolution transmission electron microscopy (HRTEM, JEOL, Tokyo, Japan, JEM-2100F) with an energy-dispersive X-ray spectroscopy (EDX) analysis system (Bruker, Karlsruhe, Germany, X flash 5030T) at 200 kV. X-ray photoelectron spectroscopy (XPS) analysis was performed on a Thermo Scientific (Waltham, MA, USA) K-Alpha photoelectron spectrometer with an Al Kα (1486.6 eV) microfocus monochromatic source. Fourier transform infrared spectra (FTIR) were recorded on a Thermo Fisher iS10 spectrometer with a deuterated triglycine sulfate detector and potassium bromide pellets. Brunauer−Emmett−Teller (BET) surface area and pore size measurements were determined by the N2 adsorption–desorption isotherms measured on a Micromeritics (Norcross, GA, USA) ASAP 2020M instrument at 77 K. Before the analyses, the samples were degassed at 50 °C for 6 h. UV–vis diffuse reflectance spectra (UV–vis DRS) were recorded on a HITACHI (Tokyo, Japan) U-3010 spectrophotometer using a BaSO4 plate as a reference. Photoluminescence spectra (PL) were studied by Edinburgh (Edinburgh, UK) FLS1000. The time-resolved PL decay curves were fitted with a multiexponential decay kinetic.
All the electrochemical measurements were recorded on the CHI760E electrochemical workstation (Bee Cave, TX, USA) with a three-electrode system with Pt foil and Ag/AgCl being the counter and reference electrode, respectively. Ethyl acetate solution containing 0.1 M of tetrabutylammonium hexafluorophosphate was used as the electrolyte. The working electrode was prepared as follows: 2 mg sample powder was added to a mixture of 950 μL of ethanol and 50 μL of naphthol to form a uniform suspension, which was then dip-coated on the conductive side of FTO glass (1 cm × 1 cm) and dried at 60 °C for 12 h. Photocurrent density curves were measured under the irradiation of an Xe lamp at a bias potential of 0.2 V with a baffle to control the light on and off. Electrochemical impedance spectroscopy (EIS) measurements measured over frequencies ranging from 0.1 Hz to 100 kHz with an amplitude of 5 mV. Mott–Schottky (M–S) plots were measured using the same mode with frequencies of 1.0, 1.5, and 2.0 kHz.

3.3. Photocatalysis Measurements

The photocatalytic CO2 reduction activity of the samples was evaluated in a solid–gas system with a 50 mL Pyrex bottle. Specifically, 5 mg photocatalysts were put into a sealed reactor which was degassed repeatedly to remove air to ensure an anaerobic condition and then filled with CO2 and 10 μL water. A 300 W Xe lamp (PLS-SXE300CUV, Perfect Light. Co., Ltd., Beijing, China) was utilized as the light source, whose light intensity was calibrated to 120 mW cm−2 by a PL-MW2000 spectroradiometer (Perfect Light, Beijing, China). The gaseous product was collected every 1 h by a syringe and quantified by a gas chromatograph (TECHCOMP, Shanghai, China, GC 7920) equipped with a thermal conductivity detector (TCD) for H2 and a flame ionization detector (FID) for CO and CH4. High-purity Ar was used as the carrier gas.

4. Conclusions

In summary, a facile synthetic method to incorporate organic–inorganic MAPbBr3 perovskite QDs into MOFs is presented. This method constitutes two sequential steps to introduce PbBr2 and MABr into the MIL-101(Cr) pores, where MAPbBr3 quantum dots could be self-assembled with an average size of 2–3 nm, and protected for the photocatalysis application. As a result, the xMAPbBr3@MIL-101(Cr) composites reveal remarkably enhanced catalytic activity in CO2 photoreduction and excellent stability. Specifically, 11%MAPbBr3@MIL-101(Cr) composite displays the best performance among the composites with a total CO and CH4 yield of 875 μmol g−1 in 9 h, and a recorded high electron consumption rate of 676.4 μmol g−1 h−1, as well as excellent stability for at least 78 h without structural and morphologic decomposition, ranking among the best of reported performances for perovoskite@MOF-based photocatalysts. A novel switched mechanism indicates that the precisely controlled location and content of encapsulated perovskite determine the switched product selectivity. CO dominates when the amount of perovskite encapsulation is less than 11%, which shifts to the CH4 dominant mode for those higher than 11%. This work might provide new insights into the design and construction of halide perovskite/MOF composites for advanced energy conversion with high selectivity toward desired products.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal14090590/s1. Refs. [27,30,31,43,44,45,46,47] are cited in Supplementary Materials.

Author Contributions

J.Z.: conceptualization, data curation, formal analysis, writing—original draft. W.Z.: data curation, formal analysis. J.C.: investigation, writing—review and editing, supervision, funding acquisition. Y.L.: writing—review and editing, investigation, funding acquisition, project administration. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (22378140 and 22138003), the State Key Laboratory of Pulp and Paper Engineering (202214, 2022C04, 2022ZD05, 2023PY06), the Fundamental Research Funds for the Central Universities (2022ZYGXZR108), the Guangdong Natural Science Foundation (2023B1515040005, 2023A1515011665), and the Science and Technology Program of Qingyuan City (2021YFJH01002).

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Acknowledgments

We thank the School of Chemistry and Chemical Engineering of South China University of Technology for their generous support.

Conflicts of Interest

There are no conflicts to declare.

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Scheme 1. Schematic illustration of the fabrication process of MAPbBr3@MIL-101(Cr).
Scheme 1. Schematic illustration of the fabrication process of MAPbBr3@MIL-101(Cr).
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Figure 1. (a) PXRD patterns and (b) Small-angle XRD patterns of MIL-101(Cr), MAPbBr3, and xMAPbBr3@MIL-101(Cr) samples.
Figure 1. (a) PXRD patterns and (b) Small-angle XRD patterns of MIL-101(Cr), MAPbBr3, and xMAPbBr3@MIL-101(Cr) samples.
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Figure 2. (a) N2 adsorption–desorption isotherms at 77 K and (b) Pore size distribution of MIL-101(Cr) and xMAPbBr3@MIL-101(Cr).
Figure 2. (a) N2 adsorption–desorption isotherms at 77 K and (b) Pore size distribution of MIL-101(Cr) and xMAPbBr3@MIL-101(Cr).
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Figure 3. (a) TEM, (b) HAADF-STEM, and (c–h) Elemental mapping images of 11%MAPbBr3@MIL-101(Cr).
Figure 3. (a) TEM, (b) HAADF-STEM, and (c–h) Elemental mapping images of 11%MAPbBr3@MIL-101(Cr).
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Figure 4. (a) XPS survey spectra and (b–d) high-resolution XPS Br 3d (b), Pb 4f (c) and Cr 2p (d) spectra of 9%MAPbBr3@MIL-101(Cr), 11%MAPbBr3@MIL-101(Cr), MAPbBr3 and MIL-101(Cr).
Figure 4. (a) XPS survey spectra and (b–d) high-resolution XPS Br 3d (b), Pb 4f (c) and Cr 2p (d) spectra of 9%MAPbBr3@MIL-101(Cr), 11%MAPbBr3@MIL-101(Cr), MAPbBr3 and MIL-101(Cr).
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Figure 5. (a) The yields of CH4 and CO in a 9 h photocatalytic CO2 reduction with xMAPbBr3@MIL-101(Cr) and MAPbBr3. (b) Reactivity comparison of 11%MAPbBr3@MIL-101(Cr) and MAPbBr3 under 27 h irradiation.
Figure 5. (a) The yields of CH4 and CO in a 9 h photocatalytic CO2 reduction with xMAPbBr3@MIL-101(Cr) and MAPbBr3. (b) Reactivity comparison of 11%MAPbBr3@MIL-101(Cr) and MAPbBr3 under 27 h irradiation.
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Figure 6. (a) UV–vis spectra, (b) Photoluminescence spectra excited at 340 nm, (c) Time-resolved photoluminescence spectra excited at 340 nm, (d) Photocurrent density curves plotted at 0.2 V (vs. Ag/AgCl) under light and dark, (e) Electrochemical impedance spectra of and (f) Electro paramagnetic resonance spectra of MIL-101(Cr), MAPbBr3, and xMAPbBr3@MIL-101(Cr).
Figure 6. (a) UV–vis spectra, (b) Photoluminescence spectra excited at 340 nm, (c) Time-resolved photoluminescence spectra excited at 340 nm, (d) Photocurrent density curves plotted at 0.2 V (vs. Ag/AgCl) under light and dark, (e) Electrochemical impedance spectra of and (f) Electro paramagnetic resonance spectra of MIL-101(Cr), MAPbBr3, and xMAPbBr3@MIL-101(Cr).
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Figure 7. Energy band alignment of the materials and schematic illustration of the possible photocatalytic mechanism of CO2 reduction.
Figure 7. Energy band alignment of the materials and schematic illustration of the possible photocatalytic mechanism of CO2 reduction.
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Zhang, J.; Zhou, W.; Chen, J.; Li, Y. Encapsulating Halide Perovskite Quantum Dots in Metal–Organic Frameworks for Efficient Photocatalytic CO2 Reduction. Catalysts 2024, 14, 590. https://doi.org/10.3390/catal14090590

AMA Style

Zhang J, Zhou W, Chen J, Li Y. Encapsulating Halide Perovskite Quantum Dots in Metal–Organic Frameworks for Efficient Photocatalytic CO2 Reduction. Catalysts. 2024; 14(9):590. https://doi.org/10.3390/catal14090590

Chicago/Turabian Style

Zhang, Jingwen, Wentian Zhou, Junying Chen, and Yingwei Li. 2024. "Encapsulating Halide Perovskite Quantum Dots in Metal–Organic Frameworks for Efficient Photocatalytic CO2 Reduction" Catalysts 14, no. 9: 590. https://doi.org/10.3390/catal14090590

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