Citation

Ning-Na Guo, Zhao-Lei Liu, Yan-Fei Mu, Meng-Ran Zhang, Yuan Yao, Min Zhang, Tong-Bu Lu. In-situ growth of PbI₂ on ligand-free FAPbBr₃ nanocrystals to significantly ameliorate the stability of CO₂ photoreduction[J]. Chin. Chem. Lett., 2022, 33(6): 3039-3042  

Ning-Na Guo^a, Zhao-Lei Liu^a, Yan-Fei Mu^a, Meng-Ran Zhang^a, Yuan Yao^b,*, Min Zhang^a,*, Tong-Bu Lu^a     

Using clean solar energy to directly transform CO₂ into value-added fuels and chemicals through artificial photosynthesis, known as photocatalytic CO₂ reduction reaction [1], is one of the most promising strategies to settle the global ever-growing energy crisis and environmental problems [2]. To promote the economic efficiency of this technology, a vast variety of semiconductor photocatalysts [3-8] have been explored during the last decade. In this regard, increasing the light-harvesting capacity and improving the separation efficiency of photogenerated carriers are two essential motivations for most of the studies [9], which play critical roles in the final photocatalytic performance. In this context, low-cost metal halide perovskite (MHP) nanocrystals (NCs) have been considered as fascinating candidates for photocatalytic CO₂ reduction recently [10-14], on primary account of their strong visible light response and superior carrier transport. The photocatalytic CO₂ reduction activity of MHP NCs has been greatly improved through surface modification [15-18], morphology control [19-22] and heterojunction engineering [23-34] strategies in the past few years.

Unfortunately, when the photocatalytic reaction is carried out in a water-contained system, the stability of MHP NCs is poor owing to their highly ionic nature [35], especially in the absence of the protection of long alkyl chain ligands, which seriously limits their photocatalytic performance for a long time. Moreover, the long alkyl chain ligand capping on the surface of the perovskites will hinder the separation of surface photogenerated carriers as confirmed in our previously reported work [36, 37]. Therefore, the development of ligand-free MHP NCs is beneficial for improving the photocatalytic activity by increasing the probability of contact between the reaction substrate and the surface of MHP NCs. Nevertheless, the problem of how to improve the stability of MHP NCs without long alkyl chain ligands has not been well solved. In order to conquer this dilemma, herein we prepared a novel ligand-free formamidinium lead bromide (FAPbBr₃) based composite by in-situ forming PbI₂ on the surface of FAPbBr₃ NCs (FAPbBr₃/PbI₂) via the strategy of ZnI₂ isopropanol (IPA) solution assistance. Owing to the high moisture resistivity of PbI₂, the resultant FAPbBr₃/PbI₂ composite exhibits significantly improved stability and activity in comparison with pristine FAPbBr₃ NCs for photocatalytic CO₂ reduction.

The ligand-free FAPbBr₃ NCs were prepared via one-step spin coating process on the filter paper substrate (Fig. S1 in Supporting information). The powder X-ray diffraction (XRD) pattern of as-prepared FAPbBr₃ (Fig. S2 in Supporting information) verifies the successful synthesis of cubic phase FAPbBr₃ with high purity, displaying distinct characteristic diffraction peaks corresponding to cubic phase FAPbBr₃ (Pm3m, No. 221). In addition, high-resolution transmission electron microscopy (HRTEM) image of FAPbBr₃ NCs (Fig. S3a in Supporting information) displays the well-defined interplanar distance of 0.42 nm and 0.59 nm, corresponding to the (011) and (001) crystal planes of cubic phase FAPbBr₃ [38], respectively, further confirming the successful generation of cubic phase FAPbBr₃. Moreover, a sequence of ordered bright spots can be clearly observed in the corresponding fast Fourier transform (FFT) pattern of FAPbBr₃ (Fig. S3b in Supporting information), proving once again the well-crystallinity of FAPbBr₃ NCs. The composite of ligand-free FAPbBr₃/PbI₂ was prepared by in-situ constructing PbI₂ on the surface of FAPbBr₃ NCs via the strategy of ZnI₂ IPA solution assisted method as illustrated in Fig. S1, and the details are provided in Supporting information. Briefly, the dried filter paper coated with FAPbBr₃ was immersed in ZnI₂ IPA solution and annealed at 80 ℃ for 10 min to remove the residual IPA. It is well known that IPA can be used as a weak extractant to extract formamidinium halide from formamidinium lead halide perovskite [39, 40], and its extraction ability can be enhanced by introducing ZnI₂ through the reaction of ZnI₂ with halide ions to form ZnX₃^- and ZnX₄^2- [41], where the X denotes the halide ions. Meanwhile, the residual PbI₂ could crystallize and retain on the surface of FAPbBr₃ owing to its insolubility in IPA. The mass ratio of PbI₂ in composite can be determined based on X-ray photoelectron spectroscopy (XPS) analysis, being 2.6%.

To further explore the structure of the FAPbBr₃/PbI₂ composite, we first carried out the XRD measurement to analyze the composition of the as-prepared composite. Compared to pristine FAPbBr₃ NCs, the main diffraction peaks of cubic phase FAPbBr₃ were well maintained on the XRD pattern of FAPbBr₃/PbI₂ (Fig. S2), while no XRD characteristic peaks of PbI₂ can be observed in FAPbBr₃/PbI₂, which may be due to the low content and small size of PbI₂ on the surface of FAPbBr₃. While the transmission electron microscopy (TEM) (Fig. 1a) and HRTEM (Fig. 1b) images of FAPbBr₃/PbI₂ composite display the lattice spacings of 0.42 nm and 0.24 nm for (011) lattice plane of FAPbBr₃ and (109) lattice plane of PbI₂ (JCPDS-PbI₂: 01–073–9472), respectively, implying the existence of PbI₂ on the surface of FAPbBr₃. High-resolution scanning electron microscopy (HRSEM) measurements revealed that the as-prepared FAPbBr₃ sample is irregular nanoparticle with the average size of ~ 20 nm (Fig. 1c), and there are obvious agglomerations due to the lack of surface ligands. ZnI₂ IPA solution treatment leads to further increase in reunion (Fig. 1d).

Figure 1

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Fig. 1. (a) TEM and (b) HRTEM images of FAPbBr3/PbI2. HRSEM images of (c) FAPbBr3 and (d) FAPbBr3/PbI2.

The influence of ZnI₂ IPA solution treatment on the interfacial electron structure of halide perovskite was further investigated by recording the XPS spectra of FAPbBr₃ NCs and FAPbBr₃/PbI₂ composite. As shown in Fig. 2a, C, N, Pb and Br co-exist in both as-prepared FAPbBr₃ NCs and FAPbBr₃/PbI₂ composite. Two new peaks at 630.60 and 619.10 eV corresponding to I 3d_3/2 and I 3d_5/2 can be clearly observed in the FAPbBr₃/PbI₂ composite as presented in Fig. 2b, which could be attributed to the formation of PbI₂ on the surface of FAPbBr₃ NCs. It is noted that the characteristic peaks of Br 3d for Br 3d_5/2 and Br 3d_3/2 in FAPbBr₃/PbI₂ composite are located 68.25 and 69.25 eV, respectively, which are perceivably lower than those of Br 3d (68.40 and 69.40 eV) in pristine FAPbBr₃ NCs as shown in Fig. 2c, indicating the presence of effective electron coupling between FAPbBr₃ and PbI₂ owing to the close contact between them. This inference can be further confirmed by scrutinizing the changes of Pb 4f signals in FAPbBr₃ NCs and FAPbBr₃/PbI₂ composite (Fig. 2d), which displays the same trend as Br 3d. Moreover, the Fourier transform infrared (FTIR) spectra of as-prepared samples were further measured to scrutinize the interaction between FAPbBr₃ and PbI₂. With respect to pristine FAPbBr₃, there is a new peak occurring near 3550 cm^-1 for FAPbBr₃/PbI₂ (Fig. S4 in Supporting information), which can vest in the Pb-Ⅰ stretching vibration mode [42], indicating the formation of PbI₂ on FAPbBr₃ nanocrystals. In addition, compared with pristine PbI₂, a perceptible shift is occurred for the Pb-Ⅰ vibration of FAPbBr₃/PbI₂, suggesting a strong interaction between FAPbBr₃ and PbI₂. This effective electron coupling should be beneficial to the interfacial electron transfer between FAPbBr₃ and PbI₂.

Figure 2

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Fig. 2. XPS spectra for FAPbBr3 and FAPbBr3/PbI2: (a) full spectra, (b) I 3d, (c) Br 3d and (d) Pb 4f.

In order to inspect the possible electron transfer orientation between the interface of FAPbBr₃/PbI₂ composite, we first performed the ultraviolet-visible diffuse reflectance spectroscopy (UV–vis DRS) and electrochemical measurements to obtain the thermodynamic information of components. As presented in Fig. S5 (Supporting information), the absorption spectrum of FAPbBr₃/PbI₂ shows a perceivable red-shift compared with pristine FAPbBr₃, indicating the existence of strong interaction between FAPbBr₃ and PbI₂ [43], which is consistent with the results of XPS measurements. The corresponding Tauc plots of FAPbBr₃ and FAPbBr₃/PbI₂ are presented in Fig. S6 (Supporting information), where the band gaps (E_g) of FAPbBr₃ NCs and FAPbBr₃/PbI₂ composite can be determined to be 2.25 and 2.22 eV, respectively. The values of conduction band edge potentials (E_CB) can be derived from the Mott-Schottky curves (Fig. S7 in Supporting information), being −1.10 and −0.98 V vs. the normal hydrogen electrode (NHE) for FAPbBr₃ and FAPbBr₃/PbI₂, respectively. In combination with the values of E_g, the values of valance band edge potentials (E_VB) for FAPbBr₃ and FAPbBr₃/PbI₂ can be obtained as 1.15 and 1.24 V vs. NHE, respectively. In addition, according to the previously reported values [41], the values of E_CB and E_VB for PbI₂ are −1.00 and 1.17 V vs. NHE, respectively. Therefore, a type-Ⅱ band alignment could be formed between the interface of FAPbBr₃ and PbI₂, as illuminated in Fig. S8 (Supporting information), indicating that the photogenerated electron transfer from FAPbBr₃ to PbI₂ is thermodynamically feasible. In addition, ultraviolet photoelectron spectra (UPS) of FAPbBr₃ and FAPbBr₃/PbI₂ were further measured to obtain the information of Fermi levels (E_F), which also play an important role in the interfacial free electron transfer process. As depicted in Fig. S9 (Supporting information), the calculated value of E_F for FAPbBr₃ (−4.14 eV vs. vacuum) is lower than that of FAPbBr₃/PbI₂ composite (−3.93 eV vs. vacuum), indicating that the E_F of PbI₂ on the FAPbBr₃ surface is higher than that of FAPbBr₃. When FAPbBr₃ and PbI₂ are in contact, the free electrons in PbI₂ will transfer into FAPbBr₃ to realize the E_F equilibrium of composite. This is in line with the XPS results in Fig. 2, in which the binding energies of Pb 4f and Br 3d in composite shift towards lower values compared to FAPbBr₃, which can be attributed to Pb and Br in composite exposing to a more negative chemical environment. A built-in electric field pointing from PbI₂ to FAPbBr₃ will be constructed by free electron movement at the interface between FAPbBr₃ and PbI₂, which also facilitates the photogenerated electron transfer from FAPbBr₃ to PbI₂.

The detailed charge transfer dynamics between FAPbBr₃ and PbI₂ were further evaluated by monitoring the steady state photoluminescence (PL) and time-resolved PL (TRPL) decay curves of FAPbBr₃ NCs and FAPbBr₃/PbI₂ composite. As depicted in Fig. 3a, pristine FAPbBr₃ NCs displayed a strong emission peak at 525 nm, while the PL emission intensity was sharply quenched to 12% after in-situ conversion of PbI₂ on the surface of FAPbBr₃. This observation was in accordance with the photographs of FAPbBr₃ NCs and FAPbBr₃/PbI₂ composite under UV illumination (Fig. S10 in Supporting information), revealing the occurrence of swift charge transfer between FAPbBr₃ and PbI₂. Fig. 3b displays the TRPL decay curves of FAPbBr₃ NCs and FAPbBr₃/PbI₂ composite, where an obviously accelerated PL decay for FAPbBr₃/PbI₂ can be identified compared with pristine FAPbBr₃ NCs. The corresponding fitting parameters based on a triple-exponential function were summarized in Table S1 (Supporting information). The PL average lifetimes for FAPbBr₃ NCs and FAPbBr₃/PbI₂ composite are calculated as 39.2 and 18.0 ns, respectively, further demonstrating the swift charge transfer between FAPbBr₃ and PbI₂. In addition, the photocurrent response (I-t) and electrochemical impedance spectroscopy (EIS) measurements further confirmed that in-situ formation of PbI₂ on the surface of FAPbBr₃ is beneficial to the charge separation. FAPbBr₃/PbI₂ composite exhibits larger photocurrent intensity (Fig. 3c) and smaller semicircular radius in Nyquist plots (Fig. 3d) with respect to individual FAPbBr₃ NCs.

Figure 3

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Fig. 3. (a) Steady state PL spectra, (b) time-resolved PL decay curves, (c) photocurrent response, and (d) photoelectrochemical impedancespectroscopy (EIS) plots of FAPbBr3 and FAPbBr3/PbI2.

The photocatalytic performance for CO₂ reduction was evaluated in a gas-solid system (see details in Supporting information). For FAPbBr₃ NCs and FAPbBr₃/PbI₂ composite, the major products of CO₂ reduction are CO and CH₄. As presented in Fig. 4a, the in-situ formation of PbI₂ on the surface of FAPbBr₃ can significantly enhance the photocatalytic activity and stability of FAPbBr₃. The yields of CO and CH₄ for FAPbBr₃/PbI₂ composite achieve impressive values of 720 and 76.5 µmol/g, respectively, after 70 h of irradiation. The corresponding electron consumption (R_electron = 2n_product (CO) + 8n_product (CH₄), n_product denotes the yield of product) yield is 2053 µmol/g for FAPbBr₃/PbI₂ composite (Fig. 4b), which is about 7-fold over that of pristine FAPbBr₃ NCs. What is more, there is no obvious decrease in CO evolution for FAPbBr₃/PbI₂ composite after 70 h of illumination, while the pristine FAPbBr₃ loses its photocatalytic activity after 20 h, owing to the destroyed structure during photoreaction as identified by the XRD measurements (Fig. S11a in Supporting information). In contrast, FAPbBr₃/PbI₂ composite can maintain its crystal structure well after photocatalytic reaction (Fig. S11b in Supporting information). Moreover, after the photocatalytic reaction, there is no change in XPS characteristic peaks (Fig. S12 in Supporting information) of FAPbBr₃/PbI₂ composite, further suggesting the good stability of FAPbBr₃/PbI₂ in this gas-solid reaction, which can be further confirmed by photocatalytic cycling experiment (Fig. S13 in Supporting information). The good stability of FAPbBr₃/PbI₂ should be attributed to the high moisture resistivity of PbI₂, which endows FAPbBr₃/PbI₂ with a larger water contact angle of 82.3° (Fig. 4c) than that of 36.4° (Fig. 4d) for pristine FAPbBr₃ NCs, protecting FAPbBr₃ NCs from water corrosion. In addition, a sequence of control experiments was conducted with FAPbBr₃/PbI₂ as catalyst. As shown in Fig. S14 (Supporting information), without catalyst, CO₂ or H₂O vapor, there are negligible CO and CH₄ can be detected, indicating that the products of CO and CH₄ come from the photocatalytic reduction of CO₂ over catalyst. According to the test result of ¹³CO₂ labeling experiment (Fig. S15 in Supporting information), the major reaction products ¹³CO with m/z value of 29 and ¹³CH₄ with m/z value of 17 can be obviously observed, further confirming that the C in CO and CH₄ products originate from the reduction of CO₂ by photogenerated electrons in FAPbBr₃/PbI₂. In addition, H₂¹⁸O labeling experiment was also performed (Fig. S16 in Supporting information), and ¹⁸O₂ with m/z value of 36 can be also obviously observed, indicating that water is the reductant.

Figure 4

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Fig. 4. (a) The evolution of CO and CH4 yield and (b) corresponding Relectron with FAPbBr3, FAPbBr3/PbI2 as photocatalysts. Images of water droplets between water and (c) FAPbBr3/PbI2 or (d) FAPbBr3 on the silicon substrate.

To summarize, we have demonstrated a facile strategy to improve the water tolerance of ligand-free MHP NCs in a water-contained photocatalytic reaction system, by in-situ forming PbI₂ on the surface of ligand-free FAPbBr₃ NCs with the assistance of ZnI₂ IPA solution. The formation of PbI₂ layer not only significantly ameliorates the stability of MHP NCs for photocatalytic CO₂ reduction with H₂O vapor as electron source, but also brings forth an accelerated interfacial charge separation as demonstrated by the photophysical and electrochemical measurements. The improved stability and charge separation endow the FAPbBr₃/PbI₂ composite with a remarkable improvement of photocatalytic performance for CO₂ reduction, achieving an inspiring electron consumption yield of 2053 µmol/g without any organic sacrificial agents, over 7-fold higher than that of individual FAPbBr₃ NCs.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

This work was supported by the Natural Science Foundation of Tianjin City (No. 17JCJQJC43800).

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2021.09.033.