Lattice interconversion of 1D ferrocene-based perovskite induced by solvent molecules for selective photocatalytic toluene oxidation

Abstract

Metal halide perovskites (MHPs) have attracted enormous attention for potential applications in the fields of optoelectronics and photocatalysis due to their excellent optoelectronic characteristics, such as broad light absorption range, high charge mobility and long carrier diffusion length. However, using MHPs as photocatalysts for photocatalytic toluene oxidation is rare, and the underlying mechanisms affecting toluene oxidation are still unclear. In this study, we constructed two novel one dimensional (1D) ferrocene-based perovskite catalysts, (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF, of which (C₁₃H₁₇FeNH)PbI₃ can photocatalyze the oxidation of toluene to benzaldehyde effectively. It is interesting that the crystal lattices of (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF can be converted to each other through the gain and loss of solvent molecules, which not only regulates the electronic band structure but also increases the separation efficiency of the photogenerated carriers. These results were confirmed by steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) spectra, transient photocurrent response measurements and density functional theory (DFT) calculations. In addition, the lower exciton binding energy (43.2 meV) of (C₁₃H₁₇FeNH)PbI₃ further demonstrated its effective carrier separation efficiency. Furthermore, the selective adsorption of (C₁₃H₁₇FeNH)PbI₃ on toluene and benzaldehyde provided a prerequisite for the efficient selective oxidation of toluene. Finally, (C₁₃H₁₇FeNH)PbI₃ exhibited excellent catalytic activity for the photocatalytic oxidation of toluene to benzaldehyde with a conversion of 28.5% and selectivity (95.3%) towards benzaldehyde.

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Introduction

Selective oxidation of toluene to benzaldehyde for the production of highly value-added chemicals is important in the chemical industry.^1–4 Benzaldehyde, as an aromatic carbonyl product, is a versatile stepping stone in the manufacturing of perfumes, solvents, dyes, plasticizers, flame retardants, and pharmaceuticals.^5–9 However, to activate the thermodynamically strong and kinetically inert C(sp³)–H bonds, thermochemical processes are most frequently employed with the use of expensive metal catalysts or hazardous oxidants under harsh reaction conditions (high temperature and/or high pressure).^10,11 Such processes significantly suffer from the difficulty of controlling product selectivity and the simultaneous emission of large amount of waste, making their operation costly, energy-intensive, and environmentally unfriendly. Recently, the photocatalytic oxidation of hydrocarbons using clean and inexhaustible solar energy has garnered wide attention.^12–16 It is well known that the active radicals (e.g., ˙O₂⁻ and ˙OH) generated by photocatalysis are extremely reactive with a high redox ability, which enables some challenging reactions to take place under green and mild conditions.¹⁷ Thus, several photocatalytic systems for the activation of C(sp³)–H bonds under solar or visible light illumination have been reported.

Over the past decade, metal halide perovskites (MHPs) have caused interdisciplinary interest and revolutionized optoelectronic fields due to their outstanding properties, such as excellent light absorption capability, large charge mobility, long carrier diffusion length, and low-cost solution processing.^18–23 Inspired by these merits, in recent years, the utilization of MHPs for solar-to-chemical energy conversion has also received widespread attention, especially in the field of photocatalysis.^24–27 More importantly, MHPs exhibit excellent catalytic performance as photocatalysts since they have high extinction coefficients, narrow band gaps with highly negative conduction band minimums (CBMs), low exciton binding energies (E_b), and fast charge transport.^28,29 In recent years, some MHPs have become emerging catalysts for the photocatalytic oxidation of toluene. For instance, in 2020, Xu et al. employed A₃Sb₂Br₉ perovskite nanoparticles (NPs) as a new photocatalyst in the visible-light driven photocatalytic oxidation of toluene, indicating that the photocatalytic activity is strongly related to the distortion of the [SbBr₆] octahedron affected by A-site cations.³⁰ In 2023, Chen et al. reported a layered all-inorganic lead-free MHP, Cs₄ZnSb₂Cl₁₂, which exhibits excellent photocatalytic performance in the transformation of toluene into benzaldehyde with a conversion rate of 1893 μmol g⁻¹ h⁻¹ and 95% selectivity.³¹ Although some progress has been made in the photocatalytic oxidation of toluene by MHPs, the underlying mechanisms affecting toluene oxidation are still unclear. In addition, lattice changes in MHPs may affect their semiconductor properties, such as photogenerated carrier dynamics, thereby affecting their photocatalytic activity. However, due to the lack of suitable systems, exploring the effect of lattice changes of MHPs on photocatalytic toluene oxidation has not been reported and remains a huge challenge.

In this work, we designed and synthesized two novel one dimensional (1D) ferrocene-based perovskites, (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF, of which (C₁₃H₁₇FeNH)PbI₃ can selectively and effectively photocatalyze the oxidation of toluene to benzaldehyde. Of note, the crystal lattices of (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF can be converted to each other through the gain and loss of DEF molecules, which not only regulates the electronic band structure, but also increases the separation efficiency of photogenerated carriers. These results were confirmed by steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) spectra, transient photocurrent response measurements and density functional theory (DFT) calculations. In addition, the E_b of (C₁₃H₁₇FeNH)PbI₃ was calculated to be 43.2 meV, much lower than that of (C₁₃H₁₇FeNH)PbI₃·DEF (76.8 meV), further suggesting the effective carrier separation efficiency of (C₁₃H₁₇FeNH)PbI₃. Furthermore, the selective adsorption of (C₁₃H₁₇FeNH)PbI₃ on toluene and benzaldehyde provides a prerequisite for the efficient selective oxidation of toluene. Finally, (C₁₃H₁₇FeNH)PbI₃ exhibited excellent catalytic activity for the photocatalytic oxidation of toluene to benzaldehyde with a conversion of 28.5% and selectivity (95.3%) towards benzaldehyde.

Results and discussion

Crystal structural analysis of (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF

Although (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF exhibit similar topological structures, their synthetic methods are vastly different. The N,N-dimethylaminomethylferrocene-based 1D perovskite single crystal with DEF solvents, (C₁₃H₁₇FeNH)PbI₃·DEF, was grown by a conventional solvent evaporation method, while (C₁₃H₁₇FeNH)PbI₃ was obtained using an anti-solvent process. Single-crystal X-ray diffraction analysis indicated that (C₁₃H₁₇FeNH)PbI₃ crystallizes in the monoclinic P112₁ space group, featuring a 1D chain-based three-dimensional (3D) supramolecular structure. As shown in Fig. S1,† the basic structural motif of (C₁₃H₁₇FeNH)PbI₃ is composed of one [PbI₃]⁻ and one protonated N,N-dimethylaminomethylferrocene (C₁₃H₁₇FeNH). Adjacent [PbI₃]⁻ groups are connected together through coordination bonds between I⁻ ions and Pb²⁺ ions, forming a 1D anionic chain (Fig. 1a). C₁₃H₁₇FeNH is arranged neatly around this chain as a counter cation to balance the charge. Finally, one 1D chain-based 3D supramolecular structure, (C₁₃H₁₇FeNH) PbI₃, is obtained through supramolecular interactions between adjacent 1D chains (Fig. 1b). Of note, as shown in Fig. 1b, the distance of adjacent 1D anionic chains at the (002) crystal planes is 13.2 Å. Interestingly, the C–N bond of C₁₃H₁₇FeNH can rotate around the C atom as the center (Fig. 1c), which provides a prerequisite for the lattice change of (C₁₃H₁₇FeNH)PbI₃.

Fig. 1 1D chain structure of (C₁₃H₁₇FeNH)PbI₃ (a); 1D chain-based 3D supramolecular structure of (C₁₃H₁₇FeNH)PbI₃ (b); rotation diagram of the C–N bond of C₁₃H₁₇FeNH (c); 1D chain structure of (C₁₃H₁₇FeNH)PbI₃·DEF (d); 3D supramolecular structure of (C₁₃H₁₇FeNH)PbI₃·DEF (e).

As shown in Fig. 1d, DEF is embedded into the lattice of (C₁₃H₁₇FeNH)PbI₃·DEF in the form of molecules through hydrogen bonding interactions (Fig. S2†), which affects the rearrangement of C₁₃H₁₇FeNH in the lattice, resulting in the number of layers of C₁₃H₁₇FeNH in the lattice of these two perovskites changing from a single layer of (C₁₃H₁₇FeNH)PbI₃ to double layers of (C₁₃H₁₇FeNH)PbI₃·DEF. This leads to a longer distance between the adjacent 1D anionic chains at the (002) crystal planes (17.3 Å) of (C₁₃H₁₇FeNH)PbI₃·DEF compared to (C₁₃H₁₇FeNH)PbI₃. This lattice change induced by solvent molecules may cause differences in the semiconductor properties of these two perovskites, thereby affecting the photocatalytic activity. Pertinent crystal data and relevant refinement parameters for (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF are listed in Table S1.†

Lattice interconversion of (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF

Firstly, powder X-ray diffraction (PXRD) was performed to confirm the successful synthesis of (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF. As shown in Fig. 2a, the PXRD patterns of the synthesized (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF match well with their simulated ones from the simulated single crystal data, illustrating their good phase purities. It should be noted that the diffraction peak position of the (002) crystal plane of (C₁₃H₁₇FeNH)PbI₃ (7.2°) is obviously greater than that (C₁₃H₁₇FeNH)PbI₃·DEF (5.3°), which further indicates that the lattice of (C₁₃H₁₇FeNH)PbI₃·DEF is larger than that of (C₁₃H₁₇FeNH)PbI₃, consistent with their crystallographic data. The (002) crystal planes of the two perovskites represented in the crystal structure can be found in Fig. S3.† Due to the weak interaction between the DEF molecule and C₁₃H₁₇FeNH in the lattice of (C₁₃H₁₇FeNH)PbI₃·DEF, we found that (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF can be converted to each other through the gain and loss of solvent molecules. As illustrated in Fig. 2b, (C₁₃H₁₇FeNH)PbI₃ can be transformed into (C₁₃H₁₇FeNH)PbI₃·DEF by adding DEF dropwise, while, by heating, (C₁₃H₁₇FeNH)PbI₃·DEF can be converted into (C₁₃H₁₇FeNH)PbI₃ gradually. This structural interconversion was also revealed by changes in the PXRD patterns. As shown in Fig. 2c, when (C₁₃H₁₇FeNH)PbI₃·DEF was heated at 80 °C for 20 min, the diffraction peak position of the (002) crystal plane after heating shifted towards a larger angle by 1.89°. It's interesting that the diffraction peak position of the (002) crystal plane after adding DEF returned to the original position of (C₁₃H₁₇FeNH)PbI₃·DEF. This also demonstrates that the lattice interconversion between (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF can be achieved by controlling the gain and loss of solvent molecules. As shown in Fig. 2d, this lattice interconversion may lead to differences in the photoelectric properties of these two perovskites, further affecting their photocatalytic activities. The effect of DEF molecules on the thermal stabilities of these two perovskites was investigated by thermogravimetric analysis (TGA). As show in Fig. S4,† for (C₁₃H₁₇FeNH)PbI₃·DEF, the weight loss of about 10.4% at 25–85 °C corresponds to DEF molecules. Then, its framework starts to decompose at approximately 180 °C. (C₁₃H₁₇FeNH)PbI₃, due to the lack of DEF molecules in the crystal lattice, does not possess a weight loss process at 25–85 °C and maintains excellent thermal stability until 180 °C.

Fig. 2 As-synthesized and simulated PXRD patterns of (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF single crystal powders (a); photos of (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF powders and schematic of their structural interconversion (b); PXRD pattern changes of (C₁₃H₁₇FeNH)PbI₃·DEF after heating and adding DEF (c); schematic of lattice transformation between (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF (d); UV-vis diffuse reflectance spectra of (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF samples (e); electronic band structure diagram of (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF (f).

The electronic band structures of (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF were then investigated.

The UV-vis diffuse reflectance spectra (DRS) of (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF powder samples were obtained first to evaluate their bandgaps (E_g) (Fig. 2e). E_g can be defined as the intersection point between the energy axis and the line extrapolated from the linear portion of the adsorption edge in the plot of the Kubelka–Munk function F against E. F = (1 − R)²/2R was transformed from the measured diffuse reflectance data, in which R presents the reflectance of an infinitely thick layer at a given wavelength.^32,33 As shown in Fig. S5a and b,† the E_g of (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF were calculated to be 2.23 eV and 2.20 eV, respectively. Then, we calculated the conduction band minima (CBM) of (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF through Mott-Schottky plots. As illustrated in Fig. S6,† the flat band potential (E_fb) of (C₁₃H₁₇FeNH)PbI₃ was calculated to be −0.67 eV vs. SCE, corresponding to −0.43 eV vs. NHE. Considering the CBM could be 0–0.1 eV more negative than the E_fb, the CBM of (C₁₃H₁₇FeNH)PbI₃ equals −0.53 eV vs. NHE.³⁴ As shown in Fig. S7,† the CBM of (C₁₃H₁₇FeNH)PbI₃·DEF was calculated to be −0.59 eV vs. NHE. The position of the valence band maxima (VBM) of (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF were calculated to be 1.70 eV and 1.61 eV, respectively, based on the E_g and CBM. Thus, the electronic band structure diagrams of (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF are shown in Fig. 2f. Due to the lattice change induced by DEF molecules, the electronic band structures between (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF can be effectively regulated.

Low exciton binding energy and excellent photoelectrochemical properties

For photocatalysts, the separation efficiency of photogenerated carriers is a key factor affecting the photocatalytic performance. The PL and TRPL spectra were thus investigated. As shown in Fig. 3a, the relatively lower PL emission for (C₁₃H₁₇FeNH)PbI₃ compared to that of (C₁₃H₁₇FeNH)PbI₃·DEF is due to the recombination of the photogenerated electron-hole being effectively suppressed, indicating low exciton binding energy and favorable charge transport. As shown in Fig. 3b, the TRPL curves of (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF can be well fitted by a double-exponential function³⁵where τ₁ and τ₂ are the decay times of the first and second decay components, respectively, and A₁ and A₂ are the amplitudes of the first and second decay components, respectively. The amplitudes of the decay components reflect the total contribution of each lifetime component toward the average lifetime (τ_ave), which can be calculated as follows.^36,37

Fig. 3 PL (a) and TRPL (b) spectra of (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF, respectively; temperature-dependent PL intensity plots of (C₁₃H₁₇FeNH)PbI₃ (c); exciton binding energies from fitting the integrated PL intensity to temperature (d); transient photocurrent responses of (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF under 0.5 V bias voltage (e); electrochemical impedance spectroscopy of (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF (f); PDOS plots of (C₁₃H₁₇FeNH)PbI₃ (g); HOMO and LUMO of (C₁₃H₁₇FeNH)PbI₃ (h); schematic of charge transition process of (C₁₃H₁₇FeNH)PbI₃ (i).

All fitting parameters are tabulated in Table S2,† and τ_ave (0.352 ns) of (C₁₃H₁₇FeNH)PbI₃ is shorter than that of (C₁₃H₁₇FeNH)PbI₃·DEF (0.541 ns), which further illustrates the higher separation efficiency and transfer ability of photogenerated charges of (C₁₃H₁₇FeNH)PbI₃. Temperature-dependent photoluminescence was employed to explore the exciton binding energies of both perovskites. Fig. 3c and Fig. S8† show the temperature-dependent PL intensity plots of (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF, respectively. The PL intensity decreases with the increase of temperature. The exciton binding energy is calculated by fitting the relation curve between PL integral intensity and temperature through the following equation^38,39

where I is PL intensity, A and B are the fitting parameters, E_b represents exciton binding energy, k_b is the Boltzmann constant, and T is temperature. From Fig. 3d, the E_b of (C₁₃H₁₇FeNH)PbI₃ was calculated to be 43.2 meV, which is much lower than that of (C₁₃H₁₇FeNH)PbI₃·DEF (76.8 meV), further indicating the greater separation efficiency of photogenerated carriers of (C₁₃H₁₇FeNH)PbI₃. Transient photocurrent response measurements can directly reflect the separation efficiency of photogenerated carriers. As shown in Fig. 3e, the photocurrent density of (C₁₃H₁₇FeNH)PbI₃ can reach as high as 9.1 μA cm⁻², about 3.3 times more than that of (C₁₃H₁₇FeNH)PbI₃·DEF (2.7 μA cm⁻²). The higher photocurrent density of (C₁₃H₁₇FeNH)PbI₃ illustrates its higher separation efficiency of photogenerated carriers.

The electrochemical impedance spectroscopy (EIS) Nyquist plots of (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF were measured to illustrate their capacities of charge separation and transfer (Fig. 3f). The equivalent circuit diagram is inserted. From fitting the EIS data, the charge transfer resistance values (R_CT) of (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF electrodes were found to be 4.58 × 10⁴ Ω and 7.82 × 10⁴ Ω, respectively. In general, the smaller the R_CT, the smaller the radius of the impedance spectrum, which means a higher efficiency of charge separation and transfer. Thus, the charge transfer capacity of (C₁₃H₁₇FeNH)PbI₃ is larger than that of (C₁₃H₁₇FeNH)PbI₃·DEF. To further reveal the contributions of different atoms of these two perovskites to the LUMO and HOMO orbitals, as well as the differences of the charge transfer process of these two perovskites, DFT calculations were carried out. As shown in Fig. 3g, the electron density of the LUMO orbital of (C₁₃H₁₇FeNH)PbI₃ mainly lies on the I 5p and Pb 6s orbitals of the [PbI₃]⁻ group. The HOMO orbital of (C₁₃H₁₇FeNH)PbI₃ is mostly concentrated on the C 2p and Fe 3d orbitals of the C₁₃H₁₇FeNH group. These results show that the electrons of (C₁₃H₁₇FeNH)PbI₃ primarily transition from the C 2p and Fe 3d orbitals of C₁₃H₁₇FeNH to the I 5p and Pb 6s orbitals of the [PbI₃]⁻ group (Fig. 3h and i), indicating the efficient separation of photogenerated carriers. However, in contrast to (C₁₃H₁₇FeNH)PbI₃, the HOMO and LLUMO orbitals of (C₁₃H₁₇FeNH)PbI₃·DEF are both contributed by the C₁₃H₁₇FeNH groups (Fig. S9†), illustrating that the presence of DEF molecules in the crystal lattice is not conducive to the separation of photogenerated carriers, which is consistent with the PL, TRPL, and transient photocurrent response measurements.

Photocatalytic oxidation of toluene

To explore the influence of lattice changes induced by DEF on photocatalytic activities, the photocatalytic C(sp³)–H bond activation in toluene molecules was investigated under visible light irradiation (λ > 420 nm) using these two perovskites as catalysts at room temperature (Fig. 4a). As shown in Fig. 4b, both (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF can photocatalyze the oxidation of toluene, but the yield of benzaldehyde over (C₁₃H₁₇FeNH)PbI₃ is higher than the yield using (C₁₃H₁₇FeNH)PbI₃·DEF as catalyst within all reaction times. Using (C₁₃H₁₇FeNH)PbI₃ as photocatalyst, the amount of benzaldehyde after 4 h of reaction can reach as high as 251 μmol, which is about 1.8 times higher than that of (C₁₃H₁₇FeNH)PbI₃·DEF (140 μmol). As shown in Fig. 4c, the selectivity of (C₁₃H₁₇FeNH)PbI₃ towards BA is as high as 95.3%, which is larger than that of (C₁₃H₁₇FeNH)PbI₃·DEF (73%). In addition, both (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF have very low selectivity for BY production, with 3.2% and 12%, respectively. Of note, the conversion rate of toluene using (C₁₃H₁₇FeNH)PbI₃ as catalyst can reach 28.5%, which is also larger than that of (C₁₃H₁₇FeNH)PbI₃·DEF (26%). These results indicate that the DEF molecules in the crystal lattice of (C₁₃H₁₇FeNH)PbI₃·DEF are not conducive to its photocatalytic activity for toluene oxidation. We further compared the catalytic performance of (C₁₃H₁₇FeNH)PbI₃ with those of some other reported catalysts, as shown in Table S3.† The results indicate that (C₁₃H₁₇FeNH)PbI₃ possesses excellent catalytic performance for photocatalytic toluene oxidation. As shown in Fig. 4d, the apparent quantum efficiency (AQE) value for the oxidation of toluene to benzaldehyde on (C₁₃H₁₇FeNH)PbI₃·DEF is 2.5%. (C₁₃H₁₇FeNH)PbI₃ presents the significantly increased AQE of 4.3%, further demonstrating the excellent photocatalytic performances of (C₁₃H₁₇FeNH)PbI₃. Reusability and stability are important figures of merit to evaluate a catalyst. As shown in Fig. 4e, only a slight decrease in the yield of benzaldehyde (from 251 μmol to 223 μmol) was found after five cycling experiments, indicating that the structural framework of (C₁₃H₁₇FeNH)PbI₃ is basically stable after five cycles.

Fig. 4 Photocatalytic selective oxidation of toluene; conditions: photocatalyst (50 mg), toluene (1 mmol), MeCN (3 mL), O₂ (1 atm), λ > 420 nm, and irradiation time (4 h) (a); effect of reaction time on BA amount over (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF catalysts (b); selectivity of BA and By as well as the conversion of toluene over (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF catalysts (c); AQE of toluene oxidation for (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF (λ = 365 nm) (d); photocatalytic cycling experiment for toluene oxidation (e).

For the purpose of further exploring and verifying the catalytic potential of (C₁₃H₁₇FeNH)PbI₃, the reactivities of various toluene derivatives were examined under the same conditions (Table 1). When p-xylene as a substrate was added into this reaction system, a substrate conversion rate of 37.3% was achieved with a corresponding aldehyde selectivity of 88.4%. In addition, o-xylene and m-xylene as substrates also present higher conversion than toluene, and the corresponding aldehyde selectivities are around 80%. Several electron-withdrawing para-position-substituted toluenes, including –F, –Cl and –Br, were subjected to the optimal conditions, and all can also convert into their corresponding aldehydes. However, compared to toluene, electron-withdrawing substituted toluenes show slightly lower conversion, and electron-donating substituted toluenes show relatively higher conversion. These results indicate that (C₁₃H₁₇FeNH)PbI₃ can serve as an effective photocatalyst for photocatalytic oxidation of toluene derivatives.

Table 1 Substrate scope on (C₁₃H₁₇FeNH)PbI₃ ^a

Entry	R	Conv. (%)	Sel.(%)
			Aldehyde	Alcohol
a Reaction conditions: substrate (1 mmol), MeCN (3 mL), O2 (1 atm), photocatalyst (50 mg), λ > 420 nm, and reaction time (4 h).
1	H	28.5	95.3	3.2
2	o-CH3	33.7	81.5	8.7
3	m-CH3	32.1	83.2	9.5
4	p-CH3	37.3	88.4	5.2
5	p-F	17.7	82.1	9.4
6	p-Cl	21.2	89.6	6.6
7	p-Br	24.8	93.0	4.6

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Photocatalytic mechanism analysis

To clarify the reaction mechanism of photocatalytic toluene oxidation over (C₁₃H₁₇FeNH)PbI₃, a series of free-radical-capture experiments were performed to reveal the key reactive species induced by light (Fig. 5a). The introduction of tetramethylpiperidine N-oxide (TEMPO) as the quenching agent for all radicals in the reaction completely suppresses the conversion of toluene, indicating the key role of the radicals for toluene oxidation. The addition of tert-butanol (TBA) as an ˙OH scavenger has no noticeable effect on toluene conversion, indicating that ˙OH is not involved in the reaction. The addition of butylated hydroxytoluene (BHT) as the quenching agent of carbon-centered radicals in the reaction produces an apparent decline in toluene conversion, indicating that the carbon-centered radicals are important intermediates for the oxidation of toluene to benzaldehyde. In Fig. 5a, it can be seen that the conversion of toluene was significantly blocked after ammonium oxalate (AO) was added as a hole (h⁺) scavenger, suggesting that holes play a key role for toluene conversion. Furthermore, there was a significant decline in toluene conversion when either benzoquinone (BQ) or K₂S₂O₈ was added, which are scavengers for superoxide radicals (˙O₂⁻) and photogenerated electrons (e⁻), respectively. Since the formation of ˙O₂⁻ is through the combination of molecular oxygen with photogenerated electrons, the quenching of photogenerated electrons would result in the decline of ˙O₂⁻ formation. The results strongly suggest that the ˙O₂⁻ play a critical role in the photocatalytic toluene oxidation.

Fig. 5 Control experiments of radical quenching for the photocatalytic oxidation of toluene (a); adsorption models for toluene on (C₁₃H₁₇FeNH)PbI₃ (b) and (C₁₃H₁₇FeNH)PbI₃·DEF (d); adsorption models for benzaldehyde on (C13H17FeNH)PbI3 (c) and (C13H17FeNH)PbI3·DEF (e).

DFT calculations were further employed to investigate the difference in adsorption capacities of these two perovskites for toluene and benzaldehyde. As shown in Fig. 5b, the calculated adsorption energy (E_ad) of toluene molecules on (C₁₃H₁₇FeNH)PbI₃ was −1.20 eV, which is substantially lower than that of (C₁₃H₁₇FeNH)PbI₃·DEF (−0.27 eV), confirming that the adsorption of toluene on (C₁₃H₁₇FeNH)PbI₃ is energy favorable. This is a prerequisite for (C₁₃H₁₇FeNH)PbI₃ to exhibit a higher conversion rate to toluene. In addition, considering that a large adsorption energy of the catalyst to the substrate is not conducive to the removal of the substrate from the catalyst, which will affect the selectivity of the catalyst towards the production of substrate, we further calculated the E_ad of (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF for benzaldehyde. As shown in Fig. 5e, the E_ad of benzaldehyde on (C₁₃H₁₇FeNH)PbI₃·DEF is as low as −3.13 eV, which is much lower than that of (C₁₃H₁₇FeNH)PbI₃ (−0.82 eV). This indicates that benzaldehyde is more easily removed from (C₁₃H₁₇FeNH)PbI₃, hindering its further oxidation and improving the selectivity of benzaldehyde.

Based on experimental and DFT calculation results, a plausible reaction mechanism for the selective photocatalytic oxidation of toluene to benzaldehyde over (C₁₃H₁₇FeNH)PbI₃ can be proposed as follows (Fig. 6): (1) (C₁₃H₁₇FeNH)PbI₃ is excited to generate photoinduced electrons and holes under visible light irradiation; (2) the accumulated holes at the VBM of (C₁₃H₁₇FeNH)PbI₃ oxidize toluene to yield benzyl radicals and the accumulated electrons at the CBM of (C₁₃H₁₇FeNH)PbI₃ react with O₂ to generate ˙O₂⁻; (3) benzyl radicals couple with ˙O₂⁻ and a proton to form the hydroperoxide intermediate; (4) the hydroperoxide species can be dehydrated to form the benzaldehyde species, which desorbs from (C₁₃H₁₇FeNH)PbI₃ quickly due to its low adsorption energy to produce benzaldehyde, avoiding the overoxidation process.

Fig. 6 Possible photocatalytic reaction mechanism of toluene oxidation over (C₁₃H₁₇FeNH)PbI₃.

Conclusions

In summary, two novel 1D ferrocene-based perovskites, (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF, were constructed for the selective photocatalytic oxidation of toluene to benzaldehyde. Impressively, we found that the crystal lattices of (C₁₃H₁₇FeNH)PbI₃ and (C₁₃H₁₇FeNH)PbI₃·DEF can be converted to each other through the gain and loss of DEF molecules, which not only regulates the electronic band structure, but also increases the separation efficiency of photogenerated carriers. These results were confirmed by PL, TRPL, and transient photocurrent response measurements. In addition, the significantly lower exciton binding energy of (C₁₃H₁₇FeNH)PbI₃ further demonstrates its favorable charge separation. Finally, (C₁₃H₁₇FeNH)PbI₃ exhibited excellent catalytic activity for the photocatalytic oxidation of toluene to benzaldehyde with a conversion of 28.5% and selectivity (95.3%) towards benzaldehyde. This work reveals the influence of perovskite lattice changes on their optoelectronic properties and photocatalytic performances, which may provide new ideas and insights for designing perovskite-based photocatalysts with highly efficient separation efficiencies of photogenerated carriers and selectivity towards substrates.

Experimental

Materials and characterization

All reagents utilized in this study were of analytical purity and used without additional processing. Additional information regarding the reagents and instrument characterizations can be found in the ESI.†

Synthesis of C₁₃H₁₇FeNHI

6.8 mL of HI was dissolved in 15 mL ethyl alcohol, and then 10 mL of C₁₃H₁₇FeN was slowly added dropwise to the above solution. After the mixed solution was stirred in an ice bath for 2 h, C₁₃H₁₇FeNHI powder was obtained using a rotary evaporator and purified by washing with dichloromethane. The C₁₃H₁₇FeNHI powder was dried in a vacuum oven overnight at 55 °C before use.

Synthesis of (C₁₃H₁₇FeNH)PbI₃

A mixture of C₁₃H₁₇FeNHI (0.01 mol, 3.71 g) and PbI₂ (0.01 mmol, 4.61 g) was dissolved in 3 mL of DEF in a 10 ml beaker at 50 °C. After stirring at 100 °C for 2 h, the solution was immediately filtered through a 0.45 μm poly(tetrafluoroethylene) filter to obtain a clear perovskite precursor solution. Then, the (C₁₃H₁₇FeNH)PbI₃ single crystal can be obtained by placing above solution in a closed container with a dichloromethane atmosphere for 7 days.

Synthesis of (C₁₃H₁₇FeNH)PbI₃·DEF

A mixture of C₁₃H₁₇FeNHI (0.01 mol, 3.71 g) and PbI₂ (0.01 mmol, 4.61 g) was dissolved in 3 mL of DEF in a 10 ml beaker at 50 °C. After stirring at 100 °C for 2 h, the solution was immediately filtered through a 0.45 μm poly(tetrafluoroethylene) filter to obtain a clear perovskite precursor solution. Then, the (C₁₃H₁₇FeNH)PbI₃·DEF single crystal was obtained after the solution slowly evaporated at 55 °C for 3 days.

Photocatalytic oxidation of toluene experiments

The photocatalytic oxidation of toluene to benzaldehyde was performed in a 10 mL quartz reactor with water-cooling system under visible light irradiation (λ > 420 nm, 100 mW cm⁻²). A 300 W xenon lamp (Perfect Light Company, Beijing) was chosen as the light source. For each photocatalytic reaction, 1 mmol of toluene, 50 mg of photocatalyst and 3 mL of acetonitrile (CH₃CN) were uniformly mixed in the quartz reactor. O₂ was bubbled through the mixture at a rate of 1.0 mL min⁻¹ for 1 h. Then, the quartz reactor was sealed with a balloon to keep the oxygen gas at one atmosphere pressure. The suspension was stirred in the dark for 30 min before illumination. Then, the reaction was carried out under visible light irradiation. After the reaction, the catalyst was removed by centrifugation and the products were directly analyzed with a gas chromatograph (Agilent GC 7820A). The conversion (con.) of toluene and selectivity (sel.) of the products were calculated according to the following equations.^40,41

Data availability

The data supporting the findings of this study are available from the corresponding authors upon reasonable request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (No. 22071019 and 22172022).

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2379877 for (C₁₃H₁₇FeNH)PbI₃ and 2416757 for (C₁₃H₁₇FeNH)PbI₃·DEF. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5qi00148j

‡ These authors contributed equally to this work.

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