Research Article | Open Access
Jinxing Yu, Xiaoxiang Xu, "Expediting H2 Evolution over MAPbI3 with a Nonnoble Metal Cocatalyst Mo2C under Visible Light", Energy Material Advances, vol. 2022, Article ID 9836095, 10 pages, 2022. https://doi.org/10.34133/2022/9836095
Expediting H2 Evolution over MAPbI3 with a Nonnoble Metal Cocatalyst Mo2C under Visible Light
Halide perovskites have been emerging as promising photocatalytic materials for H2 evolution from water due to their outstanding photoelectric properties. However, the lack of proper surface reactive sites greatly hinders the photocatalytic potential of these fascinating compounds. Here, Mo2C nanoparticles have been anchored onto methylammonium lead iodide (MAPbI3) as a nonnoble metal cocatalyst to promote H2 evolution reactions. The Mo2C nanoparticles have opposite zeta potential with MAPbI3 thereby electrostatically assembled onto the MAPbI3 surface, i.e., Mo2C@MAPbI3. Our results show that the anchored Mo2C nanoparticles have a strong interplay with MAPbI3 substrate so that photogenerated electrons of MAPbI3 can be rapidly separated and transferred into Mo2C for further H2 evolution reactions. Under optimal conditions, Mo2C@MAPbI3 delivers exceptionally high photocatalytic performance for visible light-driven H2 evolution that clearly outperforms pristine MAPbI3 and Pt-deposited MAPbI3. An apparent quantum efficiency as high as 12.65% at has been attained for H2 evolution, surpassing most of the MAPbI3-based photocatalyst reported. These results signify the usefulness and applicability of Mo2C as a new nonnoble metal-based cocatalyst in solar water splitting.
Hydrogen is an ideal substitute for traditional fossil fuels because of its zero-carbon emissions and high energy density [1, 2]. Photocatalytic water splitting has been considered as a promising route to store solar energy into hydrogen energy . Ever since photocatalysis over TiO2 was reported , numerous photocatalysts have been explored for H2 production from water, including SrTiO3 , ZnIn2S4 , g-C3N4 , MOFs , and COFs  [10–16]. Recently, halide perovskites with chemical formula ABX3 ( or organic cation, or Sn2+, and , Br-, or I-) have gained great interest as photocatalysts for H2 evolution reactions [17, 18]. This has been ascribed to their superior visible light absorption and high charge mobility which are strongly desired for photocatalytic reactions [19, 20]. The latter is extremely useful to inhibit charge recombination which is commonly encountered by conventional semiconductors. Although halide perovskites own many promising photoelectric properties, they are normally deficient in surface reactive sites where photocarriers cannot be promptly transferred for surface redox reactions . In addition, the extremely acidic environment that needed to stabilize halide perovskites in an aqueous solution restrains the choices of H2 evolution cocatalyst that can be deposited. In this regard, the noble metal cocatalyst Pt has been introduced to promote H2 evolution reactions but is still unsatisfactory probably due to the poor Pt/halide perovskite interfaces. Lately, some nonnoble metal-based cocatalysts have gained serious attention as alternatives to noble metal cocatalyst such as NiCoB , MoC , Ni3C , MoS2 [25, 26], BP , and CoP . However, their connections with halide perovskites are generally very weak due to structural mismatch thereby preventing fast charge collections from halide perovskites to these cocatalysts.
Mo2C, a promising low-cost electrocatalyst for H2 evolution reaction, exhibits excellent electrocatalytic activity over a wide pH range thereby serving as a potential candidate cocatalyst for photocatalytic H2 evolution [29–31]. More importantly, Mo2C is one of the few compounds that are stable in strong acids, rendering it an excellent alternative cocatalyst for halide perovskites which are stabilized in strong acid during photocatalytic reactions, e.g., HBr and HI aqueous solution [32–36].
Here, take methylammonium lead iodide (MAPbI3) as an example; we deposit Mo2C onto MAPbI3 as a nonnoble metal cocatalyst for photocatalytic water reduction into H2. The opposite zeta potentials between Mo2C and MAPbI3 ensure firm interconnections between these two materials that favor fast charge dissociation and transfer. It is shown that the photocatalytic activities of MAPbI3 are significantly boosted upon deposition of Mo2C nanoparticles with an optimal H2 evolution rate as high as 1.05 mmol h-1 g−1, being almost 66 times higher than pristine MAPbI3.
2.1. Material Synthesis
Synthesis of Mo2C NPs: 1.0000 g of (NH4)6Mo7O24·4H2O (Aladdin, 99.0%) and 2.9154 g of glucose (Aladdin, 99.5%) were dispersed into deionized water (c.a. 80 mL) under magnetic stirring. The so-formed admixtures were sealed into a stainless autoclave (Teflon-lined, 100 mL) for hydrothermal reaction at 453 K for 12 h. A solid power precursor can be collected after centrifugation and dried in a vacuum oven. The precursor was then calcined in 5% H2/Ar at 1123 K for 3 h. The resultant product was rinsed with deionized water and desiccated for further analysis.
Synthesis of MAPbI3 and MAPbI3-saturated HI/H3PO2 solution: 4.6146 g PbI2 (Aladdin, 99.9%) was dissolved in 25 mL HI/H3PO2 () aqueous solution. After being heated to 373 K, 1.6221 g MAI (Aladdin, 98%) was added. The so-formed solution was cooled naturally to ambient temperature as a saturation solution for MAPbI3. Black MAPbI3 precipitates appeared gradually and were centrifuged and dried in a vacuum oven overnight. The saturation solution was stored for further use.
Synthesis of Mo2C@MAPbI3 composites: the Mo2C@MAPbI3 composites were prepared by electrostatic self-assembling: in brief, 100 mg of MAPbI3 and proper amounts of Mo2C NPs (5, 10, 15, 20, and 25 mg) were added into 10 mL MAPbI3-saturated HI/H3PO2 solution, respectively. The so-formed suspensions were heated to 373 K under magnetic stirring for 30 min and cooled naturally. The precipitants obtained were denoted according to the amounts of Mo2C relative to MAPbI3 as 5% Mo2C@MAPbI3, 10% Mo2C@MAPbI3, 15% Mo2C@MAPbI3, 20% Mo2C@MAPbI3, and 25% Mo2C@MAPbI3, respectively.
ynthesis of Pt/MAPbI3: appropriate amounts of H2PtCl6 and 20 mg MAPbI3 were added into the MAPbI3-saturated HI/H3PO2 solution under magnetic stirring. The suspensions were subsequently irradiated under visible light illumination (Perfect Light, PLX-SXE300, ) for 30 min. The precipitants were centrifuged and collected for further analysis.
2.2. Material Characterization
All sample powders were analyzed by X-ray powder diffraction (XRD) techniques (Bruker D8 Focus diffractometer) for phase identifications. The morphologies of sample particles were inspected using a field-emission scanning electron microscope (FE-SEM, Hitachi S4800) and transmission electron microscope (TEM, JEOL JEM-2100). UV-vis diffuse reflectance spectra of sample powders were acquired using a UV-vis spectrophotometer (JASCO-V750). Photoluminescence (PL) spectra were acquired on a Hitachi F-7000 fluorescence spectrometer (excitation wavelength: 520 nm). X-ray photoelectron spectroscopy (XPS) data was collected on Thermo Escalab 250. Brunauer-Emmett-Teller (BET) surface areas were analyzed using TriStar 3020. Zeta potentials were measured with a Litesizer 500 Particle Analyzer.
2.3. Photocatalytic Activity
The photocatalytic hydrogen evolution reactions of prepared samples were assessed in a Labsolar-6A system (Perfect Light, China) at 293 K. Typically, 20 mg as-prepared catalyst was dispersed in 20 mL MAPbI3-saturated HI/H3PO2 solution. The suspensions were sealed and evacuated (100 Pa) for 60 minutes before light illumination. A 300 W Xenon lamp coupled with a UV cutoff filter () (Perfect Light, PLX-SXE300) was used as a light source. The gas evolution during photocatalytic reaction was monitored by an online gas chromatograph (TECHCOMP, GC7900). The carrier gas is ultrapure Ar (99.999%). The photon flux of the light source was analyzed by a quantum meter (Apogee MP-300) for the determination of the apparent quantum efficiency (AQE) according to the following equation:
2.4. Photoelectrochemical Measurements
Photoelectrodes of MAPbI3 and 10% Mo2C@MAPbI3 were prepared by a drop-casting method . 10 mg of catalysts was dispersed into 10 mL absolute diethyl ether under magnetic stirring to form suspensions. The suspensions were then dropped onto fluorine-doped tin oxide (FTO) glass and dried naturally. The deposited FTO glass was calcined at 353 K for 30 min under N2 atmosphere to improve the adhesion between sample powders and glass. The so-formed electrode was used as the working electrode. Photoelectrochemical (PEC) measurements were carried out in a three-electrode configuration which was controlled by a Zahner electrochemical workstation. The counter and reference electrodes were Pt foil and Ag/AgCl electrodes, respectively. MAPbI3-saturated aqueous HI/H3PO2 solution was used as the electrolyte which was degassed with nitrogen atmosphere for 30 min before the use. The light source was the same as photocatalytic experiment.
3. Results and Discussion
3.1. Phase Compositions
The as-prepared Mo2C, MAPbI3, and Mo2C@MAPbI3 powders were analyzed by XRD as shown in Figure 1(a). The as-prepared Mo2C and MAPbI3 powders have identical XRD patterns with those of standard ones, indicating the successful formation of single-phase compounds [26, 33]. The Mo2C@MAPbI3 powders contain both reflections from Mo2C and MAPbI3, suggesting the coexistence of both compounds. For instance, the main reflection of Mo2C around 39° can be easily identified for Mo2C@MAPbI3 powders whose intensity monotonically increased with the amounts of Mo2C introduced. The facile formation of Mo2C@MAPbI3 composites can be rationalized by the opposite zeta potentials of Mo2C and MAPbI3 , as shown in Figure 1(b). The strong coulombic force between Mo2C and MAPbI3 would enable firm interconnections among their particles which are further investigated by microscopic analysis.
The FE-SEM images of Mo2C, MAPbI3, and Mo2C@MAPbI3 are displayed in Figure S1, Figure S2, and Figure 2. As shown in Figure S1, the as-prepared Mo2C comprises spherical nanoparticles with a size of about 50 nm. The pristine MAPbI3, however, is composed of bulky particles of 10~20 micrometers with a smooth surface (Figure S2). Further loading Mo2C onto MAPbI3 results in intriguing microstructures where small Mo2C granules adhere tightly to the surface of MAPbI3 (Figures 2(a) and 2(b)). The dispersion of Mo2C at the surface of MAPbI3 is homogeneous as indicated by EDS elemental mapping analysis (Figure 2(c)). These observations are fully consistent with previous deductions that there are strong interconnections between Mo2C and MAPbI3. Such a strong interplay is highly beneficial for charge migration across the interfaces and highly desired for photocatalytic reactions [38, 39]. The corresponding EDS spectra of 10% Mo2C@MAPbI3 composites are shown in Figure S3 in which the content of Pb, I, and Mo is consistent with nominal compositions (Table S1). The Mo2C@MAPbI3 composites were further inspected under TEM conditions. Figure S4a illustrates the TEM image of the 10% Mo2C@MAPbI3 composites. Unlike MAPbI3 which normally has a smooth surface, 10% Mo2C@MAPbI3 clearly has small nanoparticles on its surfaces. HRTEM image suggests that these nanoparticles belong to Mo2C according to the lattice fringe of 0.228 nm, which correspond well with the (101) planes of hexagonal Mo2C (Figure S4b) . It is worth noting that Mo2C nanoparticles attached to MAPbI3 in a face-to-face manner, confirming the strong linkage between Mo2C and MAPbI3 [41, 42]. Thereby, we have successfully fabricated Mo2C@MAPbI3 composites with intimate contact between different moieties. As a result, the specific surface area of Mo2C@MAPbI3 composites is much higher than that of pristine MAPbI3 (0.1 m2/g) due to the presence of Mo2C nanoparticles with high specific surface area (12.11 m2/g) (Figure S5, Table S2) .
3.3. UV-vis DRS Spectra
The light absorption properties of pristine Mo2C, MAPbI3, and Mo2C@MAPbI3 composites were further studied by the UV-vis diffuse reflectance spectra (UV-vis DRS). As shown in Figure 3, bare Mo2C nanoparticles demonstrate excellent light absorption capacity from 200 to 900 nm, being consistent with the black color of Mo2C . MAPbI3 also maintains significant visible light absorption with a sharp absorption edge approaching 840 nm. Thanks to the optical properties of Mo2C and MAPbI3, Mo2C@MAPbI3 composites all have intense absorption in the visible light region (400 nm~800 nm). Their absorption tails above 850 nm increase along with the content of Mo2C in the samples, as frequently observed in semiconductor composites .
3.4. XPS Spectra
To further explore the interplay between Mo2C and MAPbI3, the surface state of Mo2C@MAPbI3 composites was investigated by XPS technique and was compared with pristine Mo2C and MAPbI3. Figure 4(a) shows the survey scan of all samples in which signals of constituent elements of I, Pb, Mo, and C can be clearly identified. The Pb 4f state of pristine MAPbI3 contains two peaks around 142.34 eV and 137.44 eV, corresponding to Pb 4f5/2 and Pb 4f7/2 state of Pb2+ species, respectively (Figure 4(b)) . These peaks, however, are clearly shifted to higher binding energy when attaching Mo2C to MAPbI3, suggesting MAPbI3 loses electron after Mo2C anchorage. A similar phenomenon is also observed in the I 3d state where spin-orbital pair of I- species blue shifts approximately 0.31 eV after the formation of Mo2C@MAPbI3 composites (Figure 4(c)). In contrast, the Mo 3d state of Mo2C@MAPbI3 composites red shifts about 0.45 eV compared with pristine Mo2C (Figure 4(d)). These results consistently suggest that there are strong interconnections between Mo2C and MAPbI3, and Mo2C can accept electrons from MAPbI3 . It is noteworthy that the Mo 3d state contains two distinct spin-orbital pairs. The pair at a higher binding energy side, i.e., 233.04 and 235.95 eV, is assignable to Mo 3d5/2 and Mo 3d3/2 states of Mo6+ in MoO3 while the one at the lower binding energy side belongs to Mo2+ of Mo2C. These results imply that Mo2C is slightly oxidized at the surface which is also noticed in the previous reports [48, 49].
3.5. Photocatalytic Properties
The photocatalytic activity of as-prepared samples was evaluated by comparing their H2 evolution in MAPbI3-saturated HI/H3PO2 solution under visible light illumination (). The system for the experiments was examined first by removing one of the following components including photocatalyst, light, and water during photocatalytic experiments. No H2 signal can be detected under these circumstances; therefore, the system is free of spontaneous H2 evolution reactions. However, H2 evolution was recorded upon illuminating sample powders with MAPbI3-saturated HI/H3PO2 solution, affirming true photocatalytic processes. The H2 evolution of all samples as a function of illumination time is summarized in Figure 5(a). Pristine MAPbI3 is characterized by a poor H2 evolution activity and produces only 40.14 μmol/g H2 for 2.5 h. This can be rationalized by the lack of proper surface reaction sites so that photogenerated electrons cannot be promptly transferred at the surface for water reduction reactions . Strikingly, H2 evolution is considerably boosted when Mo2C nanoparticles are loaded onto MAPbI3. Among all Mo2C@MAPbI3 composites, 10% Mo2C@MAPbI3 exhibited the highest H2 evolution activity (1054 μmol g−1 h−1), nearly 66-fold higher than that of pristine MAPbI3 (16 μmol g−1 h−1). It is worth noting that pristine Mo2C is completely inactive for H2 evolution under identical conditions, indicating that Mo2C is a cocatalyst to promote water reduction reactions . The average H2 evolution rates of all samples are illustrated in Figure 5(b). A volcano-type profile can be noticed for the H2 evolution rates vs. Mo2C content in the Mo2C@MAPbI3 composites. This can be attributed to the aggregation of Mo2C that decreases the active surface sites and/or blocks light penetration . The optimal Mo2C content is found to be 10% and is adopted for the measurement of apparent quantum efficiency (AQE). The AQE of 10% Mo2C@MAPbI3 is summarized in Figure 5(c). It can be seen from Figure 5(c) that 10% Mo2C@MAPbI3 maintains a high AQE (>10%) at all measurement points, verifying the exceptionally high photocatalytic activity for H2 evolution from water. On the other hand, the superior performance of Mo2C@MAPbI3 composites can also be realized by comparing their H2 evolution activity with those of Pt-deposited MAPbI3 (Figure S6). It can be seen from Figure S6 that the Pt-deposited MAPbI3 has a much lower H2 evolution activity than Mo2C@MAPbI3 even though the Pt content has been optimized. In addition, Mo2C@MAPbI3 composites also deliver a much better H2 evolution activity than most of the MAPbI3-based photocatalysts reported in the literatures as summarized in Table S3. After continuous six cycles of usage, Mo2C@MAPbI3 can still maintain most of its initial activity, indicative of good stability (Figure 5(d)). Besides, XRD analysis suggests that the crystal structure and the compositions of Mo2C@MAPbI3 composites have no discernable change before and after photocatalytic experiments, confirming again its good stability (Figure S7).
3.6. Photoluminescence Spectra and Photoelectrochemical Analysis
Given the superior photocatalytic activity of Mo2C@MAPbI3 composites, the charge separation conditions of MAPbI3 in response to Mo2C loading have been investigated. Firstly, the steady-state photoluminescence (PL) spectra of 10% Mo2C@MAPbI3 and MAPbI3 have been collected. Both samples show an intense PL signal around 790 nm, corresponding to the band edge emission of radiative-type charge recombination . It is clear from Figure 6(a) that this PL signal is considerably reduced when loading Mo2C onto MAPbI3, indicating reduced radiative-type charge recombination in the composites [53–55]. In addition, photocurrent measurements suggest that 10% Mo2C@MAPbI3 owns a much higher photocurrent than pristine MAPbI3 under the same electric bias, confirming the ameliorated charge separation conditions in Mo2C@MAPbI3 composites (Figure 6(b)) . On the other hand, electrochemical impedance spectra (EIS) suggest that the interfacial charge transfer resistance is significantly reduced after loading Mo2C onto MAPbI3, verifying the role of Mo2C as a cocatalyst to expedite interfacial charge transfer (Figure 6(c)) [57, 58]. These results consistently suggest that Mo2C serves as a good cocatalyst for MAPbI3 which not only accelerates charge separation in the bulk of MAPbI3 but also promotes interfacial charge transfer at the surface of MAPbI3. The flat-band potential of MAPbI3 is determined to be −0.48 V (vs. NHE) via a Mott-Schottky analysis (Figure S8b) [59, 60]. Combining the bandgap value (~1.51 eV) from Tauc plot analysis of MAPbI3 (Figure S8a), the band edge positions of MAPbI3 can be roughly deduced. Figure 6(d) schematically illustrates the charge migration and reaction pathways in Mo2C@MAPbI3 composites. MAPbI3 can be facilely excited by visible light photons to generate electron-hole pairs [61, 62]. The presence of Mo2C at the surface of MAPbI3 promotes the dissociation of electron-hole pairs and collects electrons from MAPbI3 [63, 64]. The spatially separated charges can then participate in the surface redox reactions, i.e., water reduction into H2 over Mo2C and I− oxidation into I3− over MAPbI3. The detailed mechanism of how charges are separated and transferred can be explored by theoretical calculations (e.g., DFT) and will be our future work.
We have successfully loaded Mo2C nanoparticles onto MAPbI3 by an electrostatic assembly method to fabricate Mo2C@MAPbI3 composites. The Mo2C nanoparticles are homogeneously and tightly anchored at the surface of MAPbI3. Thanks to the strong interconnections between Mo2C and MAPbI3, Mo2C@MAPbI3 composites exhibit superior photocatalytic activity for H2 evolution from water which clearly surpasses pristine MAPbI3 and Pt-deposited MAPbI3 under the same testing conditions. Under optimal conditions, Mo2C@MAPbI3 composites achieve a high AQE for H2 evolution (>10%) from 420 nm to 600 nm. Further analysis suggests that Mo2C nanoparticles not only facilitate charge separation in MAPbI3 but also substantially expedite interfacial charge transfer for water reduction reactions. These findings justify the Mo2C as an efficient nonnoble metal cocatalyst for halide perovskite photocatalysts that work under a highly acidic environment.
The data that support the findings of this study are available from the corresponding author upon reasonable request.
Conflicts of Interest
The authors declare that there is no conflict of interest regarding the publication of this article.
J.Y. conducted the experiments and analyzed the data. X.X. wrote the manuscript and supervised the project.
We are grateful for the financial support from the National Natural Science Foundation of China (Grant Nos. 51972233 and 52172225), the Natural Science Foundation of Shanghai (Grant No. 19ZR1459200), the Science and Technology Commission of Shanghai Municipality (19DZ2271500), and the Fundamental Research Funds for the Central Universities.
SEM images of Mo2C and MAPbI3; EDX spectrum of 10% Mo2C@MAPbI3; TEM and HRTEM image of 10% Mo2C@MAPbI3; nitrogen adsorption-desorption isotherm and pore size distribution of as-prepared samples; temporal photocatalytic hydrogen evolution for MAPbI3 loaded with different amounts of Pt under visible light () illumination; XRD patterns of 10% Mo2C@MAPbI3 before and after the photocatalytic cyclic test; the EDS results of Mo, Pb, and I in the 10% Mo2C@MAPbI3; BET surface area of as-prepared samples; the comparison of photocatalytic H2 evolution performances over recently reported metal halide perovskite. (Supplementary Materials)
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