Water-stable perovskite nanotube array with enhanced transport of charge carriers induced by functionalized polyoxometalate for the highly efficient photoreduction of uranium(VI)

Abstract

Since metal halide perovskites (MHPs) possess excellent optoelectronic performances, constructing MHP-based photocatalysts is a promising strategy to promote photocatalytic uranium(VI) reduction. However, the instability of MHPs in water limits their practical application, which is still a major issue and challenge. In this work, we constructed a perovskite nanotube array-based catalyst encapsulated by a functionalized POM, (HMTA)₃Pb₂Br₇@STA-PW₁₂, which can maintain stability in water for 10 hours under stirring conditions. It is noteworthy that considering the “electron-sponge” property of POMs, STA-PW₁₂ acting as an electronic transfer medium not only increases the stability of the catalyst in water due to the hydrophobic long-chain STA but also contributes to the separation of photogenerated carriers and enhances charge transfer from (HMTA)₃Pb₂Br₇ to PW₁₂, which significantly enhances the photocatalytic activity. The enhanced electron carrier mobility (μ_e) (1.1 cm² V⁻¹ s⁻¹) and carrier diffusion length (245 nm) of (HMTA)₃Pb₂Br₇@STA-PW₁₂ further illustrate its effective charge carrier transfer. DFT calculations further indicate the transition of electrons from (HMTA)₃Pb₂Br₇ to PW₁₂, which greatly inhibits the recombination of photogenerated carriers, thereby advancing electron transfer. Finally, the synthesized catalyst exhibits an excellent performance in the photocatalytic removal of U(VI) with a removal rate of 99.3% at a U(VI) concentration of 40 ppm after 40 min under simulated sunlight.

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Introduction

Nuclear power is the energy of the future owing to its inherent merits of low carbon emissions, high power densities, and low power generation costs.^1–4 Uranium is one of the essential components in nuclear fuel and can be easily released into the environment during nuclear activities.^5–7 In radioactive wastewater, uranium exists in the form of stable and dissolved U(VI), which possesses the characteristics of strong migration capacity and chemical toxicity. It can cause serious harm to the ecosystem and human beings if it enters the natural environment through natural infiltration and water migration.^8–11 Therefore, effectively removing U(VI) from uranium-containing wastewater is an issue that needs to be solved. In recent years, reducing soluble U(VI) to insoluble U(IV) using photocatalytic technology, which have the advantages of possessing extensive energy sources and sustainable development, has provided a new approach for treating uranium-containing wastewater.^12–15 For instance, in 2019, Wang et al. designed and synthesized a polyoxometalate (POM)–organic framework material (SCU-19), which exhibits advanced removal performance toward U(VI) radionuclides under visible light irradiation.¹⁶ In 2024, Yu et al. reported a TTh-COF-TiO₂ catalyst, which shows a superior U(VI) removal rate of 99.8% within 40 min under visible light irradiation.¹⁷ In the photocatalytic reaction system, solar-driven energy conversion is a promising technology for a sustainable energy future and environmental remediation. An efficient catalyst with effective separation of photogenerated electron–hole pairs and electron transfer is the key to further improve the efficiency of U(VI) reduction.

Over the past decade, metal halide perovskites (MHPs) have triggered interdisciplinary interest and revolutionized optoelectronic fields due to their fantastic properties, such as broad light absorption range, high charge mobility, long carrier diffusion length, and low-cost solution processing.^18–21 Inspired by these merits, the utilization of MHPs for solar-to-chemical energy conversion has also gained much attention in recent years, especially in the field of photocatalysis. Since the first work reported by Park et al. in 2016, who used CH₃NH₃PbI₃ (MAPbI₃) as a photocatalyst for H₂ generation,²² MHPs have been successfully adopted for photocatalytic H₂ evolution, organic synthesis, CO₂ reduction and pollutant degradation.^23–25 Despite these encouraging achievements, their stability issue has been considered the main bottleneck that limits their further applications, especially their instability against water since most MHPs are very sensitive to water and tend to be degraded rapidly when in contact with water molecules. To improve the stability of MHPs in aqueous media, some strategies modifying MHPs with water-proof material have been recently developed, such as designing a core–shell structure, crafting functional layers, and constructing encapsulation layers to screen the MHPs from water molecules.^26–28 For instance, in 2019, Lu et al. encapsulated MAPbI₃ perovskite quantum dots (QDs) in the pores of metal–organic framework (MOF) PCN-221(Fe_x) to construct a series of composite photocatalysts of MAPbI₃@PCN-221(Fe_x) (x = 0–1), which exhibits an excellent performance for photocatalytic CO₂ reduction using water as an electron source.²⁹ However, as shown in Scheme 1, although modifying MHPs with water-proof material can enhance their stability in water, characteristics of these materials being insensitive to electrons may hinder the transfer of photogenerated carriers, which is not conducive to the progress of the reaction. Thus, we believe that constructing MHPs composite catalyst modified by water-proof materials combining media with excellent electronic transfer capability may promote the charge transfer, which is beneficial for the photocatalytic reduction of U(VI) in water.

Scheme 1 Schematic of constructing MHPs with enhanced water stability and excellent carrier transfer capability.

Considering the “electron-sponge” property of polyoxometalates (POMs), which enables POMs to maintain structural integrity when they gain or lose electrons, POMs can effectively promote the separation and transfer of photogenerated carriers.^30–34 In this work, we designed and synthesized a perovskite nanotube array-based catalyst encapsulated by functionalized POMs as a water-proof material, (HMTA)₃Pb₂Br₇@STA-PW₁₂, which can maintain stability in water for 10 hours under stirring conditions. It is noteworthy that STA-PW₁₂ not only increases the stability of the catalyst in water due to the hydrophobic long-chain STA but also contributes to the separation of photogenerated carriers and enhances charge transfer from (HMTA)₃Pb₂Br₇ to the [PW₁₂O₄₀]³⁻ anionic cluster, which acts as a charge transfer medium to improve the photocatalytic performance. The enhanced electron carrier mobility (μ_e) (1.1 cm² V⁻¹ s⁻¹) and carrier diffusion length (245 nm) of (HMTA)₃Pb₂Br₇@STA-PW₁₂ further illustrate its effective charge carrier transfer, which is also confirmed through transient photocurrent response measurements and density functional theory (DFT) calculations. Finally, the synthesized catalyst exhibits excellent performance in the photocatalytic removal of U(VI) with a removal rate of 99.3% at a U(VI) concentration of 40 ppm after 40 min under simulated sunlight.

Results and discussion

Structure and characterization of the (HMTA)₃Pb₂Br₇ nanotube array and composite (HMTA)₃Pb₂Br₇@STA-PW₁₂

Nanoscale (HMTA)₃Pb₂Br₇ MHPs were obtained by conventional solvent method according to a previous literature procedure with some modifications.³⁵ In brief, certain amounts of lead(II) bromide and hexamethylenetetramine hydrobromide were mixed and dissolved in DMF to form a clear precursor solution, and then a large amount of DCM acting as a poor solution was quickly added to the aforementioned solution and supersonically dispersed for 30 minutes. Nanoscale and high-quality (HMTA)₃Pb₂Br₇ MHPs could be harvested. The detailed synthetic process of 1D nanotube (HMTA)₃Pb₂Br₇ and composite (HMTA)₃Pb₂Br₇@STA-PW₁₂ material can be obtained from the Experimental section. As shown in Fig. S1,† the basic structural motif of (HMTA)₃Pb₂Br₇ is composed of one lead bromide dimer [Pb₂Br₇]³⁻ and three protonated HMTA (C₆H₁₃N₄⁺). The six lead bromide dimers connect at the corners to form rugged rings with an inner diameter of about 11.04 Å (Fig. S2†), which extends in one-dimensional direction (Fig. 1a). The protonated HMTA cations acting as the counter cation are anchored by hydrogen bonding and coulombic interactions. It is worth noting that (HMTA)₃Pb₂Br₇ is a very rare one-dimensional nanotube structure (Fig. 1c) in perovskite materials, which may have better carrier transport capability compared to traditional 1D MHPs. Furthermore, adjacent 1D single nanotube (HMTA)₃Pb₂Br₇ forms fascinating 1D nanotube array (Fig. 1b and d) through the supramolecular interaction of hydrogen bonding between HMTA cations and [Pb₂Br₇]³⁻ anions (Fig. S3†). Considering that stearyltrimethylammonium bromide (STAB) has excellent hydrophobic property and “electron-sponge” property of H₃PW₁₂O₄₀ (PW₁₂), we constructed composite (HMTA)₃Pb₂Br₇@STA-PW₁₂ materials using functional STA-PW₁₂, modifying the 1D nanotube array (HMTA)₃Pb₂Br₇ MHPs. As shown in Fig. 1e, covering the surface of (HMTA)₃Pb₂Br₇ with STA-PW₁₂ through the supramolecular interactions of electrostatic and hydrogen bonding interactions may enhance its stability in water. In addition, the PW₁₂ in (HMTA)₃Pb₂Br₇@STA-PW₁₂ acting as the electronic medium may be conducive to the separation of photogenerated carriers and electron transfer.

Fig. 1 1D nanotube structure of (HMTA)₃Pb₂Br₇ from the front (a) and flank (c) directions; nanotube array structure of (HMTA)₃Pb₂Br₇ from the front (b) and flank (d) directions; (e) schematic for the preparation of (HMTA)₃Pb₂Br₇@STA-PW₁₂.

Powder X-ray diffraction (PXRD) was performed to confirm the successful synthesis of 1D nanotube (HMTA)₃Pb₂Br₇ MHPs and composite (HMTA)₃Pb₂Br₇@STA-PW₁₂ material. As shown in Fig. 2a, the PXRD pattern of the synthesized 1D nanotube (HMTA)₃Pb₂Br₇ MHPs matches well with its simulated one from the simulated single crystal data, illustrating its good phase purity. It should be noted that due to the amorphous characteristics of STAB and rapid crystallization process of STA-PW₁₂, only one strong peak at 8.60° can be observed, which demonstrates its imperfect crystallinity. Notably, 1D nanotube (HMTA)₃Pb₂Br₇ MHPs has a diffraction peak at 8.33° for the (200) crystal plane, which can be distinguished easily from STA-PW₁₂. In addition, during the synthetic process, the molar ratio of PW₁₂ and (HMTA)₃Pb₂Br₇ plays a crucial role in the successful synthesis of (HMTA)₃Pb₂Br₇@STA-PW₁₂. As shown in Fig. 2a, when the molar ratio of PW₁₂ and (HMTA)₃Pb₂Br₇ is 1/3, (HMTA)₃Pb₂Br₇@STA-PW₁₂ can be harvested successfully. We further investigated the stability of (HMTA)₃Pb₂Br₇@STA-PW₁₂ in water. As illustrated in Fig. 2b, the state of (HMTA)₃Pb₂Br₇@STA-PW₁₂ can maintain stability for about 10 hours under stirring conditions, which leaves the operation window for the photocatalytic reduction of U(VI). To further confirm its stability in water, we performed Fourier transform infrared (FTIR) spectra tests on STA-PW₁₂ powder, (HMTA)₃Pb₂Br₇ samples and (HMTA)₃Pb₂Br₇@STA-PW₁₂ powder samples, respectively. From Fig. 2c, it can be seen that some characteristic absorption bands of (HMTA)₃Pb₂Br₇ can also be found in (HMTA)₃Pb₂Br₇@STA-PW₁₂ after immersing them in water for 10 hours under stirring conditions. A strong peak located at 1254 cm⁻¹ in the (HMTA)₃Pb₂Br₇ powder samples, which is attributed to the C–N bonds’ stretching vibration of HMTA (Fig. 2c), is slightly blue-shifted to 1263 cm⁻¹ after modification by STA-PW₁₂, which indicates that their supramolecular molecular interaction of hydrogen bonding. In addition, the characteristic peaks at 1076 cm⁻¹, 972 cm⁻¹ and 889 cm⁻¹ of PW₁₂ can also be observed in the (HMTA)₃Pb₂Br₇@STA-PW₁₂ powder samples after stirring them for 10 hours in water. These results demonstrate that (HMTA)₃Pb₂Br₇@STA-PW₁₂ can maintain stability in water for 10 hours under stirring conditions. We further compared the durability of (HMTA)₃Pb₂Br₇@STA-PW₁₂ in water with some other perovskites, as shown in Table S1.† The results indicate that (HMTA)₃Pb₂Br₇@STA-PW₁₂ possesses excellent stability in water. In order to evaluate the acid–base tolerance of (HMTA)₃Pb₂Br₇@STA-PW₁₂, PXRD patterns were measured at various pH values (2–12). When (HMTA)₃Pb₂Br₇@STA-PW₁₂ was immersed into aqueous solutions with different pH values for 1 h at 25 °C, no obvious peak changes were observed in the PXRD patterns (Fig. S4†), indicating that (HMTA)₃Pb₂Br₇@STA-PW₁₂ possesses excellent acid–base tolerance. The morphologies of the 1D nanotube (HMTA)₃Pb₂Br₇ MHPs and (HMTA)₃Pb₂Br₇@STA-PW₁₂ composites were investigated using SEM. It can be clearly seen that (HMTA)₃Pb₂Br₇ exhibits a smooth surface and sharp fringe with a size of 100–400 nm (Fig. 2d). However, as shown in Fig. 2e, STA-PW₁₂ shows a rough, irregular surface and bulk morphology with a size of 0.5–3 μm, which is consistent with its imperfect crystallinity. The morphology of (HMTA)₃Pb₂Br₇@STA-PW₁₂ composites is significantly different from that of 1D nanotube (HMTA)₃Pb₂Br₇ MHPs because of the wrapping of STA-PW₁₂, showing a rough and irregular surface (Fig. 2f), which is consistent with the transmission electron microscopy (TEM) images (Fig. S5†). The nanosizing of 1D nanotube (HMTA)₃Pb₂Br₇ MHPs is a prerequisite for it to be encapsulated by the micron level STA-PW₁₂. In addition, EDS mapping of (HMTA)₃Pb₂Br₇@STA-PW₁₂ indicates that P, W, Br and Pb are uniformly dispersed in the composites (Fig. 2g), which is consistent with the PXRD and FTIR measurements. Fig. S6† displays the high-resolution X-ray photoelectron spectroscopy (XPS) spectra of P 2p, W 4f, Br 3d and Pb 46 of (HMTA)₃Pb₂Br₇@STA-PW₁₂. As shown in Fig. S6a and S6b,† the binding energy peak at 134.1 eV can be attributed to the characteristic peak of P 2p of P⁵⁺, and peaks at 37.7 and 35.6 eV are characteristic peaks of W 4f_5/2 and W 4f_7/2 of W⁶⁺. Fig. S6c and S6d† show that Br⁻ and Pb²⁺ exist in single valence state, respectively. All these results demonstrate that the (HMTA)₃Pb₂Br₇@STA-PW₁₂ composite catalyst was successfully synthesized.

Fig. 2 (a) PXRD patterns of the synthesized (HMTA)₃Pb₂Br₇ and its simulated one from the single crystal data as well as patterns of (HMTA)₃Pb₂Br₇@STA-PW₁₂ under different molar ratios of raw materials; (b) PXRD patterns of (HMTA)₃Pb₂Br₇@STA-PW₁₂ after immersing in water; (c) FTIR spectra of STA-PW₁₂, (HMTA)₃Pb₂Br₇ and (HMTA)₃Pb₂Br₇@STA-PW₁₂, respectively; (d–f) SEM images of (HMTA)₃Pb₂Br₇, STA-PW₁₂, and (HMTA)₃Pb₂Br₇@STA-PW₁₂, respectively; (g) EDS mapping of (HMTA)₃Pb₂Br₇@STA-PW₁₂ composites (including P, W, Br, and Pb elements).

Excellent optical and photoelectrochemical properties

To reveal the intrinsic semiconductor properties of (HMTA)₃Pb₂Br₇@STA-PW₁₂, the transient photocurrent responses was firstly explored to evaluate the separation efficiency of photogenerated carriers. As shown in Fig. 3a, the photocurrent density of (HMTA)₃Pb₂Br₇@STA-PW₁₂ can reach as high as 15.4 μA cm⁻², which is 3 times more than that of (HMTA)₃Pb₂Br₇ (4.8 μA cm⁻²). The higher current density illustrates the improved separation efficiency of photogenerated carriers and enhanced charges transfer capability of (HMTA)₃Pb₂Br₇@STA-PW₁₂. The Nyquist plots obtained from the electrochemical impedance spectra (EIS) of (HMTA)₃Pb₂Br₇@STA-PW₁₂ and (HMTA)₃Pb₂Br₇ were measured to evaluate the interfacial charge transfer (Fig. 3b). The radius of (HMTA)₃Pb₂Br₇@STA-PW₁₂ is smaller than that of (HMTA)₃Pb₂Br₇, which means that the charge transfer of (HMTA)₃Pb₂Br₇@STA-PW₁₂ is faster than that of (HMTA)₃Pb₂Br₇. The electronic band structures of (HMTA)₃Pb₂Br₇ and STA-PW₁₂ were then identified to investigate the charges carriers’ separation and transfer mechanism for the (HMTA)₃Pb₂Br₇@STA-PW₁₂. The UV-vis diffuse reflectance spectra of (HMTA)₃Pb₂Br₇ and STA-PW₁₂ powder samples were measured to evaluate their bandgap (E_g) (Fig. 3c). 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 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.³⁶ The bandgaps of (HMTA)₃Pb₂Br₇ and STA-PW₁₂ were calculated to be 3.20 eV and 2.87 eV, respectively (Fig. 3d and Fig. S7†). Then, Mott–Schottky plots were recorded to evaluate the conduction band minimum (CBM). As shown in Fig. 3e, the flat band potential (E_fb) of (HMTA)₃Pb₂Br₇ was calculated to be −0.55 eV vs. SCE, corresponding to −0.31 eV vs. NHE. Considering that the CBM could be 0–0.1 eV more negative than the E_fb, the CBM of (HMTA)₃Pb₂Br₇ equals −0.41 eV vs. NHE. Also, STA-PW₁₂ was calculated to be −0.06 eV vs. NHE (Fig. S8†). The position of the valence band maximum (VBM) of (HMTA)₃Pb₂Br₇ and STA-PW₁₂ were 2.79 eV and 2.81 eV, respectively, based on the bandgaps and CBM. According to the results of Tauc plots and Mott–Schottky plots, the electronic band structure diagram of (HMTA)₃Pb₂Br₇@STA-PW₁₂ was drawn (Fig. 3f). Due to the well-matched energy level structure between (HMTA)₃Pb₂Br₇ and STA-PW₁₂, the photogenerated carriers’ separation efficiency and transfer capacity can be significantly enhanced, which is also consistent with the measurements of transient photocurrent responses.

Fig. 3 (a) Transient photocurrent responses of (HMTA)₃Pb₂Br₇@STA-PW₁₂ and (HMTA)₃Pb₂Br₇ under 0.5 V bias voltage; (b) EIS curves of (HMTA)₃Pb₂Br₇@STA-PW₁₂ and (HMTA)₃Pb₂Br₇; (c) UV-vis diffuse reflectance spectra of STA-PW₁₂ and (HMTA)₃Pb₂Br₇ samples; (d) Tauc plot of (HMTA)₃Pb₂Br₇. (e) Mott–Schottky plot of (HMTA)₃Pb₂Br₇; (f) electronic band structure diagram of (HMTA)₃Pb₂Br₇@STA-PW₁₂.

Enhancing the carrier transport capability

To further reveal the separation efficiency of photogenerated carriers and carrier diffusion lengths of (HMTA)₃Pb₂Br₇ and (HMTA)₃Pb₂Br₇@STA-PW₁₂, respectively, the steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) spectra were firstly measured. As shown in Fig. 4a, the relatively lower steady-state photoluminescence (PL) emission for (HMTA)₃Pb₂Br₇@STA-PW₁₂ compared to that of (HMTA)₃Pb₂Br₇ is due to the recombination of photogenerated carriers being effectively suppressed, indicating the low exciton binding energy and favorable charges transport, which is consistent with the transient photocurrent measurements. To affirm this result, the time-resolved photoluminescence (TRPL) spectra of (HMTA)₃Pb₂Br₇@STA-PW₁₂ and (HMTA)₃Pb₂Br₇ were further measured. Fig. 4b shows that (HMTA)₃Pb₂Br₇@STA-PW₁₂ displays shorter lifetimes (21.2 ns) compared to (HMTA)₃Pb₂Br₇ (98.5 ns), which illustrates the higher separation efficiency and transfer ability of photogenerated charges of (HMTA)₃Pb₂Br₇@STA-PW₁₂. The BCP 8 nm and Cu 20 nm were deposited on two surfaces of (HMTA)₃Pb₂Br₇ and (HMTA)₃Pb₂Br₇@STA-PW₁₂ wafers, respectively, by thermal evaporation under 5 × 10⁻⁴ Pa. Then, the two electrodes of the wafers are respectively connected with thin copper wires (Fig. S9†). Carrier mobility (μ) is an important quality factor that measures the transfer ability of electrons or holes in semiconducting materials. We quantitatively harvested the electrons carrier mobility μ_e of (HMTA)₃Pb₂Br₇@STA-PW₁₂ and (HMTA)₃Pb₂Br₇ wafers using a space charge-limited current (SCLC) method. As illustrated in Fig. 4c and d, the current increased from the linear ohmic region, through a trap-filled limited (TFL) region, and finally to the quadratic Child's region with increasing bias voltage. μ_e can be calculated from the Child's region using the equation³⁷, where J_D is the current density, V represents the bias voltage, and ε₀ is the vacuum permittivity. The μ_e of the (HMTA)₃Pb₂Br₇@STA-PW₁₂ wafer was calculated to be 1.10 cm² V⁻¹ s⁻¹, which is 6 times higher than that of (HMTA)₃Pb₂Br₇ (0.17 cm² V⁻¹ s⁻¹), indicating that the transport capacity of charge carriers in (HMTA)₃Pb₂Br₇@STA-PW₁₂ is enhanced compared to that of (HMTA)₃Pb₂Br₇. Carrier diffusion lengths (L_D) can be evaluated from the PL lifetimes (τ) and the values of carrier mobility (μ) using the relation³⁸, where k_B is the Boltzmann constant, T is the temperature in Kelvin, and e is the electron charge. The calculated L_D of (HMTA)₃Pb₂Br₇@STA-PW₁₂ is 245 nm (Fig. 4f), which is obviously higher than that of (HMTA)₃Pb₂Br₇ (207 nm) (Fig. 4e), further indicating that photogenerated carriers are more easily separated and diffused in (HMTA)₃Pb₂Br₇@STA-PW₁₂.

Fig. 4 (a) PL and (b) TRPL spectra of (HMTA)₃Pb₂Br₇@STA-PW₁₂ and (HMTA)₃Pb₂Br₇, respectively; current–voltage curves for electron-only wafer devices of (c) (HMTA)₃Pb₂Br₇ and (d) (HMTA)₃Pb₂Br₇@STA-PW₁₂, respectively; (e) schematic of carrier diffusion length of (HMTA)₃Pb₂Br₇ samples; (f) schematic of carrier diffusion length of the (HMTA)₃Pb₂Br₇@STA-PW₁₂ sample.

Photocatalytic removal of U(VI)

Considering that (HMTA)₃Pb₂Br₇@STA-PW₁₂ possesses excellent stability in water and efficient carriers’ transport capability, the performance of (HMTA)₃Pb₂Br₇@STA-PW₁₂ samples for the photocatalytic reduction of U(VI) under simulated sunlight was systematically studied. The relationship between the U(VI) removal rate and the solid–liquid ratio over the (HMTA)₃Pb₂Br₇@STA-PW₁₂ catalyst is depicted in Fig. 5a. It is clear that the adsorption rate of (HMTA)₃Pb₂Br₇@STA-PW₁₂ on U(VI) increases from 16.1% to 36.9% at equilibrium period in dark with the solid–liquid ratio increasing from 0.2 to 0.8 g L⁻¹, which is a prerequisite for achieving highly efficient photoreduction of U(VI). After the light is turned on, the best removal rate of U(VI) can reach as high as 99.3% at a solid–liquid ratio of 0.6 g L⁻¹, which is higher than that the removal rate at other solid–liquid ratios. Furthermore, the reaction rate constant (k) was calculated by pseudo first order kinetics equation³⁹ ln(C_t/C₀) = −k_t, where C_t and C₀ represent the residual U(VI) and initial U(VI) concentrations at time t after the photocatalytic process, respectively. From Fig. S10,† it can be seen that the k value at a solid–liquid ratio of 0.6 g L⁻¹ (0.114 min⁻¹) is obviously larger than that at 0.2 g L⁻¹ (0.008 min⁻¹), 0.4 g L⁻¹ (0.019 min⁻¹) and 0.8 g L⁻¹ (0.066 min⁻¹), which further indicates that using (HMTA)₃Pb₂Br₇@STA-PW₁₂ as a catalyst for the photoreduction of U(VI), the ideal solid–liquid ratio is 0.6 g L⁻¹. Generally, the pH of a solution is always a key factor for the photocatalytic reduction of metal ions due to its close correlation with the surface charge of catalysts. Thus, the effect of pH of the solution on the photocatalytic reduction of U(VI) was evaluated by changing the pH values from 2 to 12. Zeta potential curve (Fig. S11†) indicates that the surface of the (HMTA)₃Pb₂Br₇@STA-PW₁₂ catalyst possesses negative charges in the pH range from 2 to 6 and positive charges in the pH range of 8 to 12. As shown in Fig. 5b, with a decrease in the pH value, the adsorption ability for U(VI) over (HMTA)₃Pb₂Br₇@STA-PW₁₂ gradually increases, which is mainly because the electrostatic attraction between (HMTA)₃Pb₂Br₇@STA-PW₁₂ and U(VI) is gradually strengthening as the surface charges of (HMTA)₃Pb₂Br₇@STA-PW₁₂ gradually become more negative, which is consistent with the zeta potentials. At pH 3, U(VI) can be almost completely removed with a removal rate of 99.3%, which is larger than that of other pH values. Notably, (HMTA)₃Pb₂Br₇@STA-PW₁₂ exhibits negligible catalytic activity for U(VI) extraction due to the electrostatic repulsion between (HMTA)₃Pb₂Br₇@STA-PW₁₂ and U(VI). Moreover, the k value at pH 3 (0.114 min⁻¹) is also larger than that at other pH values (Fig. S12†). Notably, although (HMTA)₃Pb₂Br₇@STA-PW₁₂ possesses stronger adsorption capacity at pH 2, the removal rate and the reaction rate constant (k = 0.045 min⁻¹ at pH 2) are lower than that at pH 3. Thus, pH 3 was selected as the optimal pH. Hole sacrificial reagents in a photocatalytic system can consume holes, which prevents the recombination of photogenerated electron–hole pairs on the semiconductor surface, thus enhancing the photocatalytic efficiency. Fig. 5c shows the effect of different hole sacrificial reagents including methanol, ethanol, triethanolamine and formic acid on the photocatalytic performance. Under the same conditions, methanol and ethanol prove to be more effective hole sacrificial reagents than formic acid and triethanolamine, which is probably because the –COOH or –NH₂ groups of formic acid and triethanolamine may coordinate with U(VI) ions, which hinders U(VI) adsorption on the catalyst and obstructs the photocatalytic progress. In addition, the photocatalytic performance using methanol as the hole sacrificial reagent is higher than that of ethanol. Thus, methanol was selected as the hole sacrificial reagent for the photocatalytic reduction of U(VI) in this system.

Fig. 5 Influence of solid–liquid ratio (a) on catalytic performance, (b) pH values of solution, (c) different hole sacrificial reagents on photocatalytic performance; (d) effect of (HMTA)₃Pb₂Br₇@STA-PW₁₂ on the U(VI) removal rate under different competitive metal ions and multiple competing ions; (e) adsorption capacity of (HMTA)₃Pb₂Br₇@STA-PW₁₂ for different metal ions. (f) XPS spectrum of (HMTA)₃Pb₂Br₇@STA-PW₁₂ after photocatalysis; (g) high-resolution XPS spectrum of U 4f after photocatalysis; (h) PXRD patterns of (HMTA)₃Pb₂Br₇@STA-PW₁₂ before and after photocatalysis; (i) photocatalytic cycling experiment for U(VI) reduction.

Considering that the actual radioactive wastewater contains different metal ions, which may compete with U(VI) in the process of adsorption or photocatalysis, equivalent different amounts of competitive ions were added to investigate the adsorption and photocatalytic performance of the catalyst. As shown in Fig. 5d, the solution containing La³⁺, Sm³⁺, Gd³⁺, Tb³⁺, Yb³⁺, Co²⁺, Co²⁺, Ni²⁺, Zn²⁺ and multiple competing ions was selected to explore the effect of different competitive ions on the performance of photocatalytic removal of U(VI). The results show that these competing ions in this photocatalytic system have negligible effect on the removal rate of U(VI), while the removal rate in a solution containing multiple competing ions displays a slight decrease, which may be due to the combined effect of different competitive metal ions. Considering that the good adsorption capacity for reaction substrates is a prerequisite for the photocatalytic process, we further investigated the discrepancies in the adsorption capacity for different competitive metal ions using (HMTA)₃Pb₂Br₇@STA-PW₁₂ catalyst. As shown in Fig. 5e, the Q_e of U(VI) at equilibrium period in dark can reach as high as 61 mg g⁻¹, which is significantly higher than that of competitive metal ions, further indicating that these competitive metal ions have negligible effect on the removal rate of U(VI). To demonstrate the successful photocatalytic reduction of U(VI) to U(IV), XPS and PXRD measurements were carried out to investigate the (HMTA)₃Pb₂Br₇@STA-PW₁₂ catalyst after the photocatalytic process. The full spectrum scanning of (HMTA)₃Pb₂Br₇@STA-PW₁₂ samples after photocatalysis is shown in Fig. 5f. It can be clearly seen that there are spectral lines of W, Pb, Br and U elements in the spectrum of (HMTA)₃Pb₂Br₇@STA-PW₁₂, which illustrates that U exists on the surface of the catalyst. Fig. 5g displays the high-resolution spectrum of U 4f, and the binding energy peaks at 393.2 eV and 382.4 eV are characteristic peaks of U⁶⁺, while the peaks at 391.4 eV and 380.8 eV can be attributed to U⁴⁺,⁴⁰ which demonstrates that U(VI) was successfully reduced in the reaction. As shown in Fig. 5h, some new peaks for the PXRD pattern of (HMTA)₃Pb₂Br₇@STA-PW₁₂ after photocatalysis at 28.1°, 32.4°, 47°, 47.8° were observed, which can be attributed to the diffraction peaks of U₃O₇ (PDF# 42-1215) (Fig. 5h and Fig. S13†). This indicates that U(VI) was successfully reduced to products with a structure similar to U₃O₇, which is consistent with the XPS results. Reusability and stability are important figure-of-merits to evaluate a catalyst. As shown in Fig. 5i, only a slight decrease in the efficiency for the photocatalytic removal of U(VI) (from 99.3% to 93.1%) was found after three cycling experiments, indicating that the structural framework of (HMTA)₃Pb₂Br₇@STA-PW₁₂ is basically stable after three cycles.

Photocatalytic mechanism analysis

To reveal the charge transfer process and mechanism of photocatalytic reduction of U(VI) using (HMTA)₃Pb₂Br₇@STA-PW₁₂ catalyst, DFT calculations are conducted. As illustrated in Fig. 6a, the electron density of the HOMO orbital of (HMTA)₃Pb₂Br₇@STA-PW₁₂ is mostly concentrated on the Br 4p and Pb 6s orbitals of 1D (HMTA)₃Pb₂Br₇. The LUMO orbitals of (HMTA)₃Pb₂Br₇@STA-PW₁₂ mainly lie on the W 5d and O 2p orbitals of the {PW₁₂} cluster. Calculated results show that the electrons of the STA-PW₁₂ chains-functionalized 1D (HMTA)₃Pb₂Br₇ primarily transition from the Br 4p and Pb 6s orbitals of the 1D (HMTA)₃Pb₂Br₇ to the W 5d and O 2p orbitals of the {PW₁₂} clusters (Fig. 6b and c), indicating the efficient separation of photogenerated carriers. Furthermore, we investigated potential adsorption models for UO₂²⁺ on (HMTA)₃Pb₂Br₇@STA-PW₁₂ and calculated the adsorption energy (E_ad). The bridging oxygen atom (O_b) of PW₁₂ exhibits stronger adsorption capacity for UO₂²⁺ than the terminal oxygen atom (O_t), with adsorption energy of 0.26 eV (Fig. S14†). In addition, the adsorption capacity of O_t for UO₂²⁺ at different positions is basically the same (0.16 eV for E_ad(i); 0.17 eV for E_ad(ii)). These results indicate that O_t and O_b of (PW₁₂) can all serve as binding sites for uranyl ions. It is noteworthy that the CMB of STA-PW₁₂ is more negative than the redox potential of UO₂²⁺/UO₂ (0.411 V vs. NHE); thus,⁴¹ theoretically, (HMTA)₃Pb₂Br₇@STA-PW₁₂ can photocatalytically reduce U(VI). Generally, because the redox potential of O₂/˙O₂²⁻ (−0.33 eV vs. NHE)⁴² is less negative than the CBM of (HMTA)₃Pb₂Br₇@STA-PW₁₂ (−0.41 eV vs. NHE), O₂ may capture photogenerated electrons to produce the superoxide radical (˙O₂²⁻) in the reaction, which reduces the utilization rate of electrons, thus not being conducive to the photocatalytic reduction of U(VI). However, as shown in Fig. S15,† the removal rates of U(VI) with O₂ and N₂ atmosphere in this system are basically the same, which may be due to the effective and fast transfer of photogenerated electrons from the CBM of (HMTA)₃Pb₂Br₇ to the CBM of STA-PW₁₂. The CBM of STA-PW₁₂ (−0.06 eV vs. NHE) is negative compared to the redox potential of O₂/˙O₂²⁻ while positive compared to the redox potential of UO₂²⁺/UO₂, which is consistent with the result that there is negligible effect of O₂ on the photocatalytic performance.

Fig. 6 (a) PDOS plots of (HMTA)₃Pb₂Br₇@STA-PW₁₂; (b) HOMO and (c) LUMO of (HMTA)₃Pb₂Br₇@STA-PW₁₂.

Based on experimental and DFT calculation results, we propose a plausible mechanism for the photocatalytic reduction of U(VI) using (HMTA)₃Pb₂Br₇@STA-PW₁₂ catalyst (Fig. 7). First, due to the hydrophobic long-chain STA in the catalyst and effective encapsulation of (HMTA)₃Pb₂Br₇ by STA-PW₁₂, the (HMTA)₃Pb₂Br₇@STA-PW₁₂ catalyst exhibits enhanced stability in water for 10 hours, which is the operation window for the photocatalytic reduction of U(VI). When the reaction system was irradiated by simulated sunlight, the photogenerated electrons transition occurs from the VBM to the CBM, leaving holes in the VBM. Considering the “electron-sponge” properties of POM, which can store and transfer electrons effectively, the photogenerated electrons in the CBM of (HMTA)₃Pb₂Br₇ transfer quickly to the CBM of STA-PW₁₂ (O_b and O_t) due to the well-matched energy level structure. Then, methanol as a hole sacrificial agent quenched the photogenerated holes. The accumulated electrons in the O_b and O_t of STA-PW₁₂ can effectively transfer to the U(VI), achieving complete U(VI)-to-U(IV) photocatalytic reduction.

Fig. 7 Possible mechanism for the photocatalytic reduction of U(VI) using (HMTA)₃Pb₂Br₇@STA-PW₁₂ catalyst.

Conclusions

In summary, a novel perovskite nanotube array-based catalyst encapsulated by a functionalized polyoxometalate (HMTA)₃Pb₂Br₇@STA-PW₁₂ was constructed for the photocatalytic reduction of U(VI). Impressively, STA-PW₁₂ acting as the charge transfer medium not only increases the stability of the catalyst in water due to the hydrophobic long-chain STA but also contributes to the separation of photogenerated carriers and enhances the charge transfer from (HMTA)₃Pb₂Br₇ to PW₁₂, which significantly improves the photocatalytic activities. The removal rate of U(VI) can reach as high as 99.3% at a U(VI) concentration of 40 ppm after 40 minutes under simulated sunlight irradiation using (HMTA)₃Pb₂Br₇@STA-PW₁₂ catalyst. Given the intrinsic advantages of MHPs including designable architectures and chemical compositions as well as excellent optoelectronic properties, combined with the “electron-sponge” property of POMs, more versatile candidates based on perovskite catalysts modified by POMs for photocatalytic applications are visible in the future.

Experimental

Materials and characterization

All reagents utilized in this study were analytically purity and used without additional processing. Additional information regarding the reagents and instrumental characterization can be found in the ESI.†

Synthesis of 1D nanotube (HMTA)₃Pb₂Br₇ and composite (HMTA)₃Pb₂Br₇@STA-PW₁₂

For the synthesis of nanosized (HMTA)₃Pb₂Br₇, a mixture of lead(II) bromide (PbBr₂) (2.72 mmol, 1.0 g) and hexamethylenetetramine hydrobromide (HMTA) (4.09 mmol, 0.9 g) was dissolved in 30 mL of DMF and stirred for 30 minutes to form a clear precursor solution. Then, a large amount of DCM solvent (200 mL) was added to the above solution quickly and supersonically dispersed for 30 minutes. The nanosized (HMTA)₃Pb₂Br₇ was collected by filtration, cleaned alternately with anhydrous ethanol three times, and then dried under vacuum at 50 °C. For the synthesis of (HMTA)₃Pb₂Br₇@STA-PW₁₂, firstly, the synthesized nanosized (HMTA)₃Pb₂Br₇ (0.3 mmol, 0. 42 g) was dispersed in a 20 mL ethanol solution with stearyltrimethylammonium bromide (STAB, 0.9 mmol, 0.35 g) dissolved in it. Then, 20 mL ethanol solution containing 0.9 mmol of H₃PW₁₂O₄₀ (2.6 g) was added to the above solution slowly with stirring for 1 hour. The obtained (HMTA)₃Pb₂Br₇@STA-PW₁₂ was collected by filtration and then dried under vacuum at 50 °C.

Photocatalytic reduction experiments of uranium(VI)

The photocatalytic experiments were conducted in a quartz beaker photoreactor with a water-cooling system under simulated sunlight (300 W xenon lamp, AM 1.5 G, 100 mW cm⁻²), and the reaction temperature was maintained at room temperature (25 ± 0.2 °C). Typically, 30 mg catalyst was added to 50 mL of 40 ppm U(VI) aqueous solutions with 0.5 M CH₃OH as the sacrificial reagent under continuous stirring. The pH of the solution was tuned by 0.1 M HNO₃ or NaOH. The suspension solution was stirred for 1 h in the dark to reach a balance of adsorption–desorption. Then, photocatalytic reduction of U(VI) under simulated sunlight irradiation was determined. To analyze the uranium concentration, 1 mL of the reaction solution was sucked out using a syringe every 30 min and filtered with 0.44 μm polyether sulfone (PES) filters. After the reaction, the concentration of residual U(VI) was determined by an inductively coupled plasma-mass spectrometer (ICP-MS).⁴³ Then, the removal rate (R) of U(VI) was determined as follows: R = [(C₀ − C_t)/C₀] × 100%, where C₀ represents the initial concentration of U(VI) and C_t is the concentration of U(VI) at time t. The adsorption capacity for U(VI) and competitive metal ions at adsorption equilibrium time (Q_e, mg g⁻¹) was calculated using the equation Q_e = (C₀ − C_e)/m × V, where V represents the volume of the treated solution (L), m is the amount of used adsorbent (g), C₀ is the initial concentration of metal ions, and C_e is the concentration of metal ions at adsorption equilibrium time.

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. See DOI: https://doi.org/10.1039/d4qi02393e

‡ These authors contributed equally to this work.

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