Construction of g-C₃N₄/Ag/TiO₂ Z-scheme photocatalyst and Its improved photocatalytic U(VI) reduction application in water

With the growth of the global energy crisis, nuclear energy is widely exploited and utilized. As a crucial nuclear energy fuel, uranium is widely applied in nuclear power generation, and it has grown over the century constantly (Carrara 2020). While nuclear energy is widely used, a large quantity of uranium-containing wastewater is created in the mining process, nuclear power plant operation process and nuclear accident leakage. Uranium-containing wastewater brings a severe risk to the environment because of its chemical and radiological toxicity (Liu et al. 2017a; Han et al. 2019). The uranium element in water is mainly in the form of hexavalent U(VI), which has good solubility and easy migration. The practical means to remove U(VI) from uranium-containing wastewater has become the research focus. Traditional methods (such as adsorption, evaporation, membrane separation, ion exchange) have some limitations and drawbacks (Heshmati et al. 2014; Guo et al. 2017; Torkabad et al. 2017; Taghipour et al. 2019; Yuan et al. 2020). Compared with these methods mentioned above, the reduction of soluble U(VI) to insoluble U(IV) has been considered as one of the valid approaches to solve this problem. Among many reduction methods, photocatalysis is widely concerned because of its efficiency, green and no secondary pollution (Xu et al., 2016, Khorsandi et al. 2019).

Some photocatalysts have been used to reduce U(VI). Such as TiO₂, graphitic carbon nitride (g-C₃N₄), iron oxide (α-Fe₃O₄), ZnO and so on (Guo et al. 2016; Guo et al. 2017; Wen et al. 2017, Lassoued et al. 2018; Liu et al. 2019; Liu et al. 2020). As the most classical photocatalyst, TiO₂ has been widely concerned because of its high activity and stability (Yu et al. 2014). Zhang et al. synthesized rutile TiO₂ nanorods for the photocatalytic reduction of U(VI), which obtained gratifying results (Zhang et al. 2013). Li et al. extracted uranium from seawater by photocatalytic reaction of TiO₂ (Li et al. 2019). However, the unsatisfactory visible light responsiveness of TiO₂ restricted its further applications (Almeida et al. 2022). g-C₃N₄ is a new research focus in photocatalysis due to its advantages such as proper band gap, high photocatalytic activity and no environmental contamination. Lu et al. has achieved satisfactory results in U(VI) reduction with a photocatalyst based on g-C₃N₄ (Lu et al. 2016; Lu et al. 2017). Nevertheless, the photocatalytic activity of pristine g-C₃N₄ is unsatisfactory because of its high recombination of photo-produced electrons and holes (Wen et al. 2017; Wang et al. 2018). Construction of heterojunction is considered as one of the effective ways to solve the above shortcomings of single component photocatalyst (Wang et al. 2020). Z-scheme heterojunction photocatalysts showed excellent photocatalytic activity because they could effectively separate electrons and holes produced by light, thus inhibiting their recombination (Tian et al. 2015; Ma et al. 2017; Liu et al. 2017b; ; Mao et al. 2018). There are a large number of reports about Z-scheme photocatalysts used to treat pollutants in aqueous (Zhu et al. 2016; Chen et al. 2019). In recent years, some literatures have reported the photocatalytic reduction of U(VI). However, this research is still at its primary stage compared with other applications, such as photocatalytic CO₂ reduction, water splitting, heavy metals treatment and organic pollutants degradation (Ping et al. 2019). Recently, Z-scheme g-C₃N₄/TiO₂ photocatalyst has been applied in reduction of U(VI), which has achieved gratifying results, Jiang et al. reported that g-C₃N₄/TiO₂ showed excellent photocatalytic property of U(VI) reduction and As(III) oxidation under simulated sunlight irradiation (Jiang et al. 2018). The preliminary study of our research group found that g-C₃N₄/TiO₂ had significant photocatalytic effect on U(VI) reduction under ultraviolet light irradiation (Liu et al. 2021). Dai et al. reported that ZnFe₂O₄/g-C₃N₄ could efficient removal of U(VI) via simultaneous adsorption and photoreduction under visible LED light irradiation (Dai et al. 2021). Adding a noble metal into a semiconductor is one of the effective means to improve their photocatalytic performance. The Fermi level of a noble metal is usually less than that of semiconductor photocatalytic material, when the noble metal is deposited on the photocatalytic material, electrons can be rapidly transferred from the semiconductor photocatalytic material to the noble metal material, so that the overall Fermi level of the material can reach the level of noble metal (Link et al. 2011; Guo et al. 2013; Bian et al. 2014). Tada et al. modified TiO₂ by Ag deposition, and the results showed that supporting a small amount of Ag could significantly improve the activity of TiO₂ photocatalytic reduction of nitrobenzene (Tada et al. 2004). According to the current reports, photocatalytic reduction of U(VI) employing Z-scheme photocatalysts is still an innovative method. In addition, it is also necessary to further study this photocatalytic process and its corresponding mechanism in reduction of U(VI).

Herein, g-C₃N₄/Ag/TiO₂ nanocomposites were synthesized and characterized by scanning electron microscope (SEM), X-ray diffraction and X-ray electron spectroscopy (XPS) in this work. The as-synthesized g-C₃N₄/Ag/TiO₂ was applied for efficient reduction of U(VI) via photochemical reaction under visible light irradiation. The Z-scheme photocatalytic mechanism of g-C₃N₄/Ag/TiO₂ has been researched, which could cut down the recombination of photogenerated electrons and holes. More significantly, the role of Ag has been investigated, which could increase the light absorption range of photocatalysts. In this study, an all-solid Z-scheme photocatalytic system was constructed, and the principle of fast separation of carriers was used to promote the participation of electrons on the surface of g-C₃N₄ in U(VI) reduction and improve the photocatalytic efficiency.

The g-C₃N₄/Ag/TiO₂ was synthesized reference to patent (Wu 2013) and some adjustments were made to the synthesis process.

All chemical reagents were domestic reagents, which were at analytic grade without further purification. P25 was used as the source of TiO₂, and it was purchased from King chemical. U₃O₈ (99%) was dissolved by HCl and H₂O₂ under heating conditions for standard U(VI) solution. An appropriate amount of melamine was weighed into a porcelain crucible and placed in a muffle roaster. The temperature of the muffle furnace was kept at 540 °C for 2 h and so cooled down naturally. After cooling to room temperature, the sample was weighed and ground into powder form to obtain the light yellow photocatalyst g-C₃N₄.

100 g g-C₃N₄ powder and 4 g P25 (TiO₂) was added into anhydrous ethanol solution and then sonicated (40,000 Hz) in a water bath for 15 min until the powder was utterly dispersed (keep stirring throughout the process). The g-C₃N₄ powder was evaporated by vacuum drying at 80 °C for 5 h. After grinding, the powder was forged in a muffle furnace at 400 °C for 2 h to cool down naturally. After cooling to room temperature and removing, the sample was weighed and ground into powder form to obtain binary composite photocatalyst g-C₃N₄/TiO₂.

0.1 g g-C₃N₄/TiO₂ and 0.5 g aminopropyl trimethoxysilane were dispersed in 20 ml ethanol and stirred for 8 h (the amount of ethanol does not need to be strictly controlled, as long as g-C₃N₄/TiO₂ could be utterly dispersed). The separation was washed for three times after it was separated by centrifugation. The resulting solid was dispersed in 20 ml ethanol again. 0.5 g AgNO₃ was added and stirred for 2 h. After centrifugation, it was dispersed in water. A small amount of NaBH₄ was added. The above suspension was dried at 80 °C for 5 h, the photocatalyst g-C₃N₄/Ag/TiO₂ was prepared.

The surface morphology of the samples was analyzed and measured using a Sigma 300 high-resolution scanning electron microscope (acceleration voltage 0.2–30 KV, INLENS and ET secondary electron detector imaging and EDS energy spectrometer, electromagnetic/electrostatic compound lens for objective lens, resolution 1.0 nm@15 kV, 1.6 nm@1 kV) from Zeiss, Germany.

The crystal structure and surface chemical composition of the samples were determined using a D/MAX-RB X-ray diffractometer (Cu Kα excitation source, scanning range 5°–90°, scanning rate 10°/min) from Bruker, Germany.

An X-ray electron spectrometer (ESCALAB 250Xi, Thermo Fischer, USA) with a vacuum of 8xl0-10 Pa, an Al ka-ray excitation source (hv = 1486.6 eV), an operating voltage of 12.5 kV, a filament current of 16 mA, and 10 cycles of signal accumulation was used. The test passing energy was 50 ev for the full spectrum, 20 eV for the narrow spectrum with a step of 0.05 eV, dwell time of 40–50 ms, and charge correction with C1 s = 284.80 eV binding energy as the energy standard) was used to analyze the surface elements and valence states of the samples.

The UV-vis diffuse reflectance spectrometer UV-3600 (with integrating sphere device) of Shimadzu Corporation, Japan, was used to measure the light absorption properties of the samples.

The fluorescence spectra of the samples were measured using a fluorescence spectrophotometer of Edinburgh, UK, model FLS1000 with a xenon lamp as the excitation source (375 nm).

R_U(VI) are removal ratios of U(VI), C_t and C₀ are concentration of U(VI) at 0 and t time, A₀ and A_t are absorbance intensity corresponding to 0 and t time, respectively. The stability of g-C₃N₄/Ag/TiO₂ was evaluated by reusing the photocatalyst in five recurrent tests for reduction of U(VI) underneath constant conditions.

The morphologies of g-C₃N₄, TiO₂, g-C₃N₄/TiO₂, and g-C₃N₄/Ag/TiO₂ were characterized by SEM, and the results are shown in Figure 1. From Figure 1(a), it can be seen that the g-C₃N₄ is presented with a bulk structure consisting of many irregularly shaped wrinkled sheets aggregated. A magnified SEM image of g-C₃N₄ is shown in Figure 1(b), in which it can be clearly seen that the g-C₃N₄ has a block structure with holes. These cavities can effectively increase the specific surface area of g-C₃N₄ and provide more reactive sites for photocatalytic reactions. The SEM images of pure TiO₂(P25) are shown in Figure 1(c) and 1(d). It can be seen from the images that TiO₂ is an irregular spherical particle with particle size less than 100 nm, which is much smaller than g-C₃N₄. The SEM images of the binary composite g-C₃N₄/TiO₂ are shown in Figure 1(e) and 1(f). It can be seen in the images that TiO₂ nanoparticles are uniformly attached to the surface of g-C₃N₄. There is no obvious exposed g-C₃N₄ surface in these SEM images. Figures 1(g) and 1(h) show the SEM images of the ternary composite g-C₃N_4/Ag/TiO₂. In Figure 1(g), it can be seen that a large number of irregular spherical particles are uniformly attached to the surface of the bulk g-C₃N₄, and no obvious exposed outer surface of g-C₃N₄ is seen in the images. The magnified g-C₃N₄/Ag/TiO₂ SEM image is shown in Figure 1(h); it is evident that there are two particles of different sizes present on the surface of g-C₃N₄. From the analysis of the particle number ratio and the size of TiO₂ particles in Figure 1(f), it is inferred that the larger particle on the outer surface of g-C₃N₄ in Figure 1(h) is Ag nanoparticles. The SEM results indicate that the ternary g-C₃N₄/Ag/TiO₂ composite photocatalyst was successfully prepared.

The crystal structures of TiO₂, g-C₃N₄, g-C₃N₄/TiO₂, and g-C₃N₄/Ag/TiO₂ were characterized by XRD, and the results are shown in Figure 2. The red line in the figure is the XRD pattern of g-C₃N₄, and it can be seen that the significant characteristic peaks appear at two positions, 13.1° and 27.3°, comparable to the (100) and (002) crystal planes of g-C₃N₄, severally. This result indicates that the prepared g-C₃N₄ has multiple honeycombs and stacked structures within the facets. The weak diffraction peak signal at 13.1° indicates that the prepared g-C₃N₄ has a graphene structure. The blue line is the XRD pattern of g-C₃N₄/TiO₂. The intensity of the significant peak of g-C₃N₄ is reduced due to the high proportion of TiO₂ in g-C₃N₄/TiO₂. The diffraction peaks of both anatase TiO₂ and rutile TiO₂ appear in the nanostructure of g-C₃N₄/TiO₂. Most of the diffraction peaks are characteristic peaks of anatase TiO₂ (Liu et al. 2015), including (101), (004), (200), (105), (211), (204), (116), (220) and (215). The remaining (110), (101), (111) and (220) are characteristic peaks of rutile TiO₂, and the corresponding characteristic peaks are consistent with those of pure P25 (black lines). The pink line in the XRD pattern is belonging to g-C₃N₄/Ag/TiO₂. It can be seen that the pink line is almost the same as the blue line of g-C₃N₄/TiO₂ sample, and no significant characteristic peak of Ag is shown. This is probably because the proportion of Ag in g-C₃N₄/Ag/TiO₂ is too low to detect. A similar phenomenon was appeared in the XRD characterization of the photocatalyst Ag/2D-C₃N₄ prepared by Yi et al. (2016).

In order to further verify the presentation of Ag in g-C₃N₄/Ag/TiO₂, the elemental analysis of g-C₃N₄/Ag/TiO₂ was performed by XPS characterization, and the results are shown in Figure 3. Figure 3(a) reveals the XPS full spectrum of g-C₃N₄/Ag/TiO₂, which contains the characteristic peaks of C 1 s, N 1 s, Ti 2p, O 1 s and Ag 3d. Figure 3(b) shows the high-resolution XPS spectrum of Ti 2p in g-C₃N₄/Ag/TiO₂, and the positions of the two characteristic peaks of Ti 2p3/2 and Ti 2p1/2 in.the spectrum indicate that the element Ti exists in g-C₃N₄/Ag/TiO₂ in the form of Ti⁴⁺. Figure 3(c) shows the high-resolution XPS spectra of C1 s in g-C₃N₄/Ag/TiO₂. The characteristic peaks of C1 s at two positions, 284.8 eV and 288.0 eV, can be seen due to the coordination bonds of aromatic hydrocarbon carbon atoms and C-N-C, respectively. Figure 3(d) shows two high-resolution XPS peaks at 398.5 eV and 400.4 eV of N 1 s in g-C₃N₄/Ag/TiO₂, which could be ascribed to the coordination bonds of C-N = C and C-N-H, respectively. Figure 3(e) shows the high-resolution XPS spectrum of O 1 s in g-C₃N₄/Ag/TiO₂, which shows the characteristic peak of O 1 s formed only at 530.1 eV, indicating that O exists only in the valence state of O²⁻ in g-C₃N₄/Ag/TiO_2. This result matches the morphology of O²⁻ in TiO₂, indicating that both Ti and O elements exist in the sample in the form of TiO₂. Figure 3(f) shows the high-resolution XPS spectrum of Ag 3d in g-C₃N₄/Ag/TiO₂, in which it can be seen that Ag 3d appears as characteristic peaks at 367.8 eV and 373.8 eV, respectively. An energy difference of ∼6.0 eV between these two peaks, indicating the presence of Ag in g-C₃N₄/Ag/TiO₂ in the form of singlet silver. The XPS results further indicate that the composite photocatalyst g-C₃N₄/Ag/TiO₂ was synthesized successfully, in which Ag exists in the form of elemental silver.

The light absorption properties of TiO₂, g-C₃N₄, g-C₃N₄/TiO₂, and g-C₃N₄/Ag/TiO₂ were analyzed by UV-vis diffuse reflection factor spectroscopic analysis (DRS). The results are shown in Figure 4(a). In this figure, the light absorption area of TiO₂ was represented by the red line, which is mainly concentrated in the UV region, with the absorption sideband at ∼400 nm. The blue line represents g-C₃N₄ which is able to response light at lower frequency wavelengths compared to TiO₂, with the absorption sideband at ∼450 nm, indicating that g-C₃N₄ has a certain responsiveness to visible light at wavelengths above 420 nm. The yellow line represents the absorption sideband of g-C₃N₄/TiO₂, which is very close to that of g-C₃N₄ also at ∼450 nm. g-C₃N₄/TiO₂ has a lower absorption ability than TiO₂ in the UV region but higher than g-C₃N₄, indicating that the composite g-C₃N₄/TiO₂ photocatalyst was synthesized successfully. The absorption ability of g-C₃N₄/Ag/TiO₂ in the UV region represented by the green line is higher than that of the other three materials. An obvious surface plasmon resonance absorption peak of silver nanoparticles located at ∼450 nm appears, which makes the absorption intensity of g-C₃N₄/Ag/TiO₂ in the visible area significantly enhanced (red arrow in the figure). In addition, the bandgap energies of TiO₂, g-C₃N₄, g-C₃N₄/TiO₂, and g-C₃N₄/Ag/TiO₂ were extrapolated based on the plot of (ahv)² versus hv, and the results are shown in Figure 4(b). The band gap energies (E_g) of g-C₃N₄, TiO₂, g-C₃N₄/TiO₂, and g-C₃N₄/Ag/TiO₂ were calculated to be 2.7 eV, 3.2 eV, 2.8 eV and 2.25 eV respectively. Figure 4 illustrated that the absorption performance of g-C₃N₄/Ag/TiO₂ for light is enhanced after loading with Ag, especially in the visible region.

The efficiency of photogenerated electron-hole separation in g-C₃N₄, TiO₂, g-C₃N₄/TiO₂, and g-C₃N₄/Ag/TiO₂ under illumination conditions was examined by PL. The photogenerated electron-hole recombination releases energy in the form of PL emission. Therefore, the PL intensity is directly related to the recombination rate of photogenerated electrons with holes. PL spectra of g-C₃N₄, TiO₂, g-C₃N₄/TiO₂, and g-C₃N₄/Ag/TiO₂ are shown in Figure 5. Among them, for g-C₃N₄ and TiO₂ (the black line and pink line), the PL spectrum has a strong emission peak at ∼460 nm, indicating the presence of a large number of photo-generated electrons recombined with holes. A large amount of energy was discharged throughout the recombination process may well be detected by the PL spectrum. For g-C₃N₄/TiO₂ represented by the red line, there is a weaker emission peak at ∼430 nm in the PL spectrum, indicating the presence of a small number combination of photogenerated electrons and holes, because the Z-scheme structure of the composite photocatalyst g-C₃N₄/TiO₂ effectively reduces the photogenerated electron-hole recombination rate. For the g-C₃N₄/Ag/TiO₂ represented by the blue line, there is no obvious emission peak in the PL spectrum, indicating that the Ag-loaded g-C₃N₄/Ag/TiO₂ further reduces the photogenerated electron-hole recombination rate because the good conductivity of Ag makes the photogenerated electrons migrate in a directional manner. Meanwhile, the space charge layer formed between Ag, g-C₃N₄ and TiO₂ also plays a role in suppressing the recombination of photogenerated electrons and holes.

The results of the photocatalytic reduction test are shown in Figure 6. The concentration of U(VI) did not change significantly under the light conditions without the injection of the photocatalyst, indicating that U(VI) was stable under light and the self-photocatalytic process could be ignored. Under the conditions of photocatalysts (g-C₃N₄, TiO₂, g-C₃N₄/TiO₂, g-C₃N₄/Ag/TiO₂) injection, the dark reaction was first allowed to reach adsorption equilibrium for 30 min, at which time the highest residual rate of U(VI) of the test group with g-C₃N₄ injection was close to 90%; the test group with TiO₂, g-C₃N₄/TiO₂, g-C₃N₄/Ag/TiO₂ were all-around 67% of the residual rate of U(VI). After the light source was turned on for the photoreaction, the remaining concentration of U(VI) in the test group with g-C₃N₄ injection decreased slowly, and the remaining rate of U(VI) was 80.8% after 30 min of photoreaction. The residual concentration of U(VI) in the test group with TiO₂ injection was decreased slowly, and the residual rate of U(VI) was 58.4% after 30 min of photoreaction. In the g-C₃N₄/TiO₂ group, the residual concentration of U(VI) decreased slightly faster than that of the TiO₂ group as the photoreaction proceeded, and the residual rate of U(VI) was 52.8% after 30 min of photoreaction. The concentration of U(VI) in the test group with g-C₃N₄/Ag/TiO₂ was decreased rapidly at the first 5 min of photoreaction. The concentration of U(VI) was also decreased gradually during the subsequent 25 min of photoreaction, and the residual rate of U(VI) was less than 1% at the end of the reaction (i.e., the removal rate was more than 99%).

The XPS spectra of U elements deposited on the g-C₃N₄/Ag/TiO₂ surface before and after the photocatalytic reaction were used to analyze the valence change of U elements during the photocatalytic reaction. Figure 7 shows the high-resolution XPS spectra of U4f before and after the photocatalytic reaction. It can be seen from the figure that the peak positions of U4f5/2 and 4f7/2 changed when U(VI) was reduced to U(IV). At the bottom of the red curve, four peaks were obtained at 379.6 eV, 381.9 eV, 390.4 eV and 391.3 eV, respectively, using the peak differentiation technique. Among them, the peaks at 381.9 eV and 391.3 eV belong to the U(VI) characteristic position, and the peaks at 379.6 eV and 390.4 eV belong to the U(IV) characteristic position. This indicates that the majority of the U(VI) on the g-C₃N₄/Ag/TiO₂ surface is reduced to U(IV) during the photoreaction. The process of U(VI) removal is not only the adsorption of the material, but also the process of valence change exists.

The stability of g-C₃N₄/Ag/TiO₂ photocatalytic reduction of U(VI) was investigated by a cycling test. After each cycle, the photocatalyst was separated from the suspension by centrifugation. The photocatalyst was stripped of its surface deposits by ultrasonic shaking and rinsed with deionized water. Subsequently, the photocatalyst was dried at 60 °C before the next cycle for the next use. The results of the cycling tests are shown in Figure 8. After five rounds of cycling tests, the photocatalytic activity of g-C₃N₄/Ag/TiO₂ did not decrease significantly. The final removal rate was still above 99%, which indicates that the photocatalytic activity of g-C₃N₄/Ag/TiO₂ remained stable after several cycles of reaction.

The main mechanism for the enhanced photocatalytic performance of g-C₃N₄/Ag/TiO₂ composite photocatalysts is the Z-scheme photocatalytic mechanism assisted by the LSPR effect. Combining noble metals with semiconductors could significantly promote the photocatalytic activity of semiconductors. Firstly, the LSPR effect of noble metal effectively promotes the separation of carriers, which is a kind of absorption effect of noble metals on photons: when the incident photon frequency matches the vibration frequency of conduction electrons of noble metal nanoparticles, the nanoparticles can produce strong absorption of photon energy (Tong et al. 2011). A part of such electrons injected from noble metal to semiconductor still retransmits to the Fermi energy level of the noble metal to compound with holes, which reduces the photocatalytic activity. In addition, too many of the electrons injected into the semiconductor do not have time to participate in the reaction before they were combined with the holes on the semiconductor, which is also an important factor in reducing the photocatalytic activity (Ingram & Linic 2011). Therefore, the appropriate amount of noble metal loading is required to improve the photocatalytic activity of semiconductors.

Figure 9 demonstrates the photocatalytic mechanism of g-C₃N₄/Ag/TiO₂ nanostructure under simulated solar (xenon lamp) irradiation. The bandgap energies of g-C₃N₄ and TiO₂ are 2.7 and 3.2 eV, respectively, relative to the standard hydrogen potential (Li & Ding 2010; Linic et al. 2011). The work function of Ag is 4.8 eV, and its position can be calculated as 0.3 eV relative to the standard hydrogen potential (Zhou et al. 2013). Under simulated sunlight irradiation, as shown in Figure 9, g-C₃N₄, TiO₂, and metallic Ag can all absorb light and generate photogenerated electron-hole pairs. Among them, the metal Ag can absorb visible light and thus be activated by the near-field enhancement effect of LSPR. In addition, the noble metal nanoparticles could generate a strong in situ surface electric field, which excites g-C₃N₄ and TiO₂ to produce more electron-hole pairs. The photogenerated electrons generated by metallic Ag move to the VB of TiO₂ for carrier combination with the holes. The excited electrons on the surface of g-C₃N₄ move to the metal Ag for the second carrier combination. The electrons on the CB of g-C₃N₄ can finally participate directly in the reduction of U(VI); while the holes left on the VB of TiO₂ can be trapped by methanol to produce CO₂ and H₂O. In this way, metal Ag served as a combination site for electrons and holes from different semiconductors. It is the main driving force for carrier separation on g-C₃N₄ and TiO₂. In addition, the noble metal Ag is a conductive material that can directly act as a centre to combine the electrons on the surface of TiO₂ with the holes on g-C₃N₄, ultimately forming a Z-scheme photocatalytic system.

In this way, the redox reaction happened on each semiconductor by their photogenerated electrons (g-C₃N₄) and holes (TiO₂). The recombination of photogenerated electrons and holes is effectively avoided, and therefore the photocatalytic performance is improved. Based on Marta's previous work (Litter 2017), the mechanism of U(VI) photocatalytic reduction is mainly as follows: in the first step, U(VI) is reduced to U(V) by photogenerated electrons. U(V) reacts spontaneously with U(VI) and U(IV), following the electron balance principle. Meanwhile U(V) can be further reduced to U(IV) by photogenerated electrons. And according to the one-step reduction theory of U(VI) (Lu et al. 2016), the reduction potential E₀ = 0.411 eV is required when U(VI) is directly reduced to U(IV) by one step in the presence of photogenerated electrons. As shown in Figure 9, the reduction potential of CB on g-C₃N₄ (E₀ = −1.3 eV) is more negative than the reduction potential required for a series of reactions. In theory, the photocatalytic reduction reaction can be carried out. Although g-C₃N₄ responds to visible light, as presented in Figure 5, the combination of photogenerated electron and hole was rather high. TiO₂ and g-C₃N₄/TiO₂ have a poor response to visible light (Figure 4). Therefore, the g-C₃N₄/Ag/TiO₂ exhibited better photocatalytic activity under the irradiation of visible light compared to pure g-C₃N₄, TiO₂ and g-C₃N₄/TiO₂ composition.

The ternary composite photocatalyst g-C₃N₄/Ag/TiO₂ was prepared by loading a small amount of the noble metal Ag. The characterization results showed that the ternary composite photocatalyst g-C₃N₄/Ag/TiO₂ was successfully prepared. g-C₃N_4/Ag/TiO₂ exhibited the best photocatalytic reduction performance for U(VI) under visible light irradiation. After 30 min irradiation, the removal rate of U(VI) was above 99%. By the XPS analysis of U elements deposited on the surface of g-C₃N₄/Ag/TiO₂ before and after the photocatalytic reaction, most of the U(VI) was reduced to U(IV). In addition, the photocatalytic activity of g-C₃N₄/Ag/TiO₂ did not decrease significantly after five rounds of experiments, showing its good stability. g-C₃N₄/Ag/TiO₂ exhibited better photocatalytic reduction of U(VI) under visible light irradiation mainly by the Z-scheme photocatalytic mechanism assisted by the LSPR effect. Although the introduction of noble metals will cause the price of photocatalyst preparation to increase, the increase in cost is not significant because of the small loading amount, and the overall cost performance is high. This idea has some significance in designing other photocatalytic materials.

The authors are grateful to the Laboratory of Pollution Control and Resource Technology and the Department of Municipal Engineering at the University of South China for providing the necessary facilities for the research. This research was funded by the National Natural Science Foundation of China (NO.11475080). Natural Science Foundation of Hunan Province (No.2019JJ50127).

Yuelin Liu contributed to the conception and design of the study. Yilei Yuan and Shangyuan Ni contributed to the acquisition of data. Yuelin Liu and Jun Liu contributed to analysis and interpretation of data. Yuelin Liu contributed to drafting the manuscript. Shuibo Xie and Jun Liu contributed to revising it critically for important intellectual content. Shuibo Xie and Yingjiu Liu contributed to the reagents/materials/analysis tools.

The authors declare no conflict of interest.

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