The photocatalytic reduction of U(VI) by Ag-doped SnS₂ materials under visible light

With the boom in nuclear industry activity, the environmental problems caused by uranium(VI) (U(VI)) have attracted more and more attention. There are two main forms of uranium in the water environment, namely U(VI) with high mobility and U(IV) with low chemical toxicity (Liang et al. 2020). Compared to U(IV), U(VI) not only has a higher solubility in aqueous solutions, but also can accumulate in biological tissues, causing serious kidney, liver damage, and even death (Sadeghian et al. 2021). Therefore, the separation and recycling of uranium from sewage water has become an incredibly urgent problem. Adsorption is supposed to be an effective technology in removing uranium contamination. However, due to the material adsorption saturation and environmental changes, uranium is highly possible to go back into the water, resulting in secondary contamination (Jiang et al. 2018). Therefore, reducing U(VI) to insoluble U(IV) is regarded as the most effective method to remove uranium contamination (Deng et al. 2019). Photocatalytic technology has high properties such as efficiency, safety, and low cost. Among them, semiconductor photocatalytic technology is particularly emphasized, such as TiO₂ (Chu et al. 2022), CdS (Wu et al. 2020), and Bi₂MoO₆ (Ding et al. 2017). It has received wide attention in the research on U(VI) reduction (Li et al. 2020). When a single semiconductor is used as a catalyst, the photocatalytic efficiency is limited by the combination of photogenerated electrons and holes (Kuang et al. 2020). Consequently, it is necessary to improve the utilization rate of photogenerated carriers to promote the photoreduction efficiency of uranium.

Semiconductor metal sulfides are promising visible-driven photocatalysts or wide-bandgap semiconductor sensitizers because they have absorption properties in visible and short-wave near-infrared regions (Wang et al. 2010). SnS₂, as an N-type semiconductor, has widespread applications in the photocatalytic field. Because of the narrow band gap (about 2.0 eV), strong visible light response, good chemical stability, and non-toxicity, SnS₂ has been widely used in the field of photocatalysis (Wang et al. 2020). Nevertheless, single SnS₂ is limited in practical application due to the high photogenerated e⁻/h⁺ pair recombination rate and low charge transfer rate (Han et al. 2019). So far, to enhance the photocatalysis capacity of SnS₂, researchers have conducted a large number of studies. Among all the modification ways, chemical doping is considered to be a simple and effective way to adjust the photoelectric properties of semiconductors. An et al. studied Cu²⁺ doping with SnS₂, which significantly increased the photocatalytic activity of SnS₂ (An et al. 2014). Zhang et al. reported that SnS₂ doped with Au significantly improved the photocatalytic reduction efficiency of Cr⁶⁺ compared with SnS₂ (Zhang et al. 2021). Park et al. constructed a 3D flower-like photocatalyst of SnS₂ doped with In³⁺, which had excellent photocatalytic degradation efficiency (Park et al. 2017). Studies have shown that many foreign impurities can effectively narrow the bandgap, reduce the particle size, and act as the trap center of e⁻/h⁺ (Wang et al. 2022a; Ahmad et al. 2023; Xue et al. 2023). In recent years, silver as a doping agent of photocatalyst has gained extensive concern because of its antibacterial properties (Gao et al. 2020; Rosario et al. 2020). Wang et al. prepared Ag-doped CuS composite materials, and the results showed that the doping of silver ions greatly reduced the electron–hole composite rate, thus improving the photocatalytic ability under visible light (Wang et al. 2021). Li et al. found that the absorption spectrum of Ag-doped MoS₂ nanomaterials increased with the increase in the amount of Ag-doped, and the band gap width decreased to 0.76 eV, thereby improving the photocatalytic activity (Li et al. 2019). Therefore, partial substitution of Sn with Ag is expected to improve the photocatalytic activity during the U(Ⅵ) reduction process. The doping of Ag elements can narrow the band gap and enhance the utilization of visible light. The Ag_0.5–SnS₂ exhibited a prominent photocatalytic reduction efficiency of UO₂²⁺ about 86.4% under optical light for 75 min. So, Ag_x–SnS₂ composite might be a promising catalyst during the photocatalytic application in the future.

In this paper, SnS₂ with different Ag doping amounts was prepared by a simple one-step hydrothermal method. The structural and photoelectric properties of the Ag_x–SnS₂ were characterized by various kinds of analytical methods. The photocatalytic properties of the prepared Ag-doped SnS₂ nanosheets were studied by photoreduction of U(VI) with visible light. Meanwhile, the reasons for enhanced photocatalytic activity and catalytic reaction mechanism were discussed, and the ability and stability of material recycling were investigated.

Uranyl nitrate hexahydrate was purchased from Hubei Chemical Inc., the purity was 99%. The other chemicals used in this experiment were purchased from Aladdin Industrial Inc., all reagents were analytically pure and not further purified.

The vacuum drying model was DZF-6020 from Shanghai Scientific Instrument Inc. The Teflon-lined autoclave model was D076107 from Lichen Technology.

In this experiment, the preparation method of materials was modified according to the literature (Zhang et al. 2011; Liu et al. 2013; Yuan et al. 2015). Ag-doped SnS₂ was prepared by the hydrothermal method. In our work, the materials were mixed at room temperature (about 26 °C). A certain molar mass of AgNO₃, 15 mmol thioacetamide, and 3.75 mmol SnCl₄•5H₂O was dissolved in 60 ml of deionized water. After continuous stirring for 60 min, the mixture solution was sealed into a Teflon-lined autoclave and reacted for 12 h at 180 °C. Repeated washing was done with deionized water and anhydrous ethanol. Also, the products were collected by centrifugation at 6,000 rpm for 5 min. Finally, it was vacuum dried at 60°C for 12 h.

The obtained samples were labeled as SnS₂, Ag_0.1–SnS₂, Ag_0.5–SnS₂, Ag_0.9–SnS₂, and Ag_1.3–SnS₂, representing the molar mass of AgNO₃ as 0, 0.1, 0.5, 0.9, and 1.3 mmol.

The chemical constitution and valent states of materials were measured by ESCALAB 250XI X-ray photoelectron spectrometer (XPS) with Al-Kα radiation. The crystalline structure and crystallinity of the samples were analyzed by Bruker D8 Advance X-ray diffractometer (Cu-Kα radiation, λ = 1.5406 Å). The Bruker X-Flash 6130 X-ray energy dispersive instrument (EDX) was used to determine the elemental composition of the materials. The microstructure and morphology were observed by scanning electron microscopy (SEM, Zeiss; Merlin). The UV–vis diffuse reflectance spectra were obtained by using Hitachi UH-4150 UV–vis spectrophotometer. The absorption was obtained at 300–800 nm. Photoluminescence spectroscopy (PL) was determined using an Edinburgh FLS980 optical spectrometer.

Generally, the 0.02 g photocatalyst was dispersed into a quartz tube containing 100 mL of 50 mg/L U(VI) solution (pH was regulated by 0.1 M HNO₃ and NaOH solution). The solution pH was 6 during the photocatalytic process. Before being placed under a 450-W Xe lamp, N₂ was introduced to eliminate O₂ in the system. Then, the solution in the quartz tube was magnetically mixed in the darkness for 90 min to achieve the adsorption–desorption balance. The concentration of U(VI) was measured by UV–visible spectrophotometry after the reaction solution was filtered by a 0.22-μm filter. The average value of data was acquired from three parallel tests.

The Ag_0.5–SnS₂ photocatalytic reduction of U(VI) had the best removal efficiency among the various ratios of Ag-doped SnS₂ materials. Hence, in order to determine the reasons of the enhanced photocatalytic activity of Ag_x–SnS₂, we mainly characterized pure SnS₂ and Ag_0.5–SnS₂, then analyzed and compared them.

PL spectra are commonly used to explore the mobility, transferability, and recombination property of photocarrier in semiconductors. In general, weaker emission intensity indicates a slower compounding rate of photoinduced electron–hole pairs. It means that more photoelectrons and vacancies can participate in the whole reaction process, which helps to improve photocatalytic activity. The result in Figure 4(d) illustrated the PL spectra of pure SnS₂ and modified Ag_0.5–SnS₂ materials, which displayed that the emission peaks of two samples were mainly focused on 468, 481, and 618 nm. In contrast, the PL peak intensity of the Ag_0.5–SnS₂ sample was much weaker than that of SnS₂. This indicated that the photocatalytic activity of the sample would be improved as the carrier recombination rate decreased. The existent potential difference between the Fermi level of Ag and the semiconductor CB could promote the electron transfer between the semiconductor matrix and the doped metal (Ma et al. 2010). When the photocatalyst was illuminated by visible light, the doped silver acted as electron acceptor to capture photoexcited electrons from SnS₂, and the photogenerated electrons could be transferred from the CB to the VB. Clearly, the effective segregation of electrons and holes improved the photoactivity.

The influence of different pH values on photodegradation of U(VI) is shown in Figure 5(b). When pH increased from 4.0 to 6.0, U(VI) reduction rate increased. When pH was 6, the photoreduction rate by Ag_0.5–SnS₂ was the highest. The U(VI) removal efficiency reduced when pH value was increased further. As shown in Figure 5(c), the zeta potential of Ag_0.5–SnS₂ was decreased with the increase in pH. The zero potential of Zeta was at pH = 2.47. It was worth noting that when pH = 4.0, the photoreduction reaction on Ag_0.5–SnS₂ was almost completely prohibited. The hydrolysates of different mononuclear and polynuclear U(VI) existed in the form of [(UO₂)_P(OH)_q]^(2p−q)+ at different pH values (Li et al. 2012). UO₂²⁺ was the dominant species in the uranium solution at pH ≤ 4.0 (Kaynar et al. 2014). Considering the competition of H⁺, the number of binding sites on the catalyst surface was greatly reduced. It was difficult for uranium to be adsorbed on the Ag_0.5–SnS₂ surface, which was unfavorable for the photocatalytic reaction. U(VI) existed in solution as positively charged polynuclear products at pH 4.0–6.0 (e.g., UO₂(OH)⁺, (UO₂)₂(OH)₂²⁺, (UO₂)₃(OH)₄²⁺, (UO₂)₃(OH)⁵⁺), which would be adsorbed by a catalyst and easily accepts electrons from the photocatalyst (Shakur et al. 2016). A large number of negatively charged uranium ions predominate at pH ≥ 7.0(e.g., UO₂(OH)³⁻, UO₃(OH)⁷⁻). So the electrostatic repulsion between photocatalyst and photocatalyst led to the decrease of photoreduction efficiency (Zou et al. 2017).

In conclusion, compared with the single SnS₂, Ag–SnS₂ could effectively narrow the band gap, increase the carrier density, and inhibit the electron–hole pair recombination. As a result, Ag–SnS₂ had a good photocatalytic activity and was able to reduce U(VI) to U(IV).

In summary, Ag_x–SnS₂ materials with different Ag doping amounts were prepared by a one-step hydrothermal method. Compared with single SnS₂, the U(VI) removal rate of Ag_x–SnS₂ materials was obviously improved. Among them, the Ag_0.5–SnS₂ had the best photocatalytic removal performance under illumination. The removal rate of U(VI) was achieved up to 86.4%. The excellent photocatalytic activity of Ag_0.5–SnS₂ was due to the narrowing of band gap, the enhancement of visible light absorption, and the effective separation of photoelectron–hole pairs. The Ag_0.5–SnS₂ complex has also shown good stability and renderability. Therefore, Ag_0.5–SnS₂ composite might be a promising catalyst during the photocatalytic application in the future.

This work was funded by financial support from National Natural Science Foundation of China (No. 51778239).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.