Synthesis of g-C₃N₄/TiO₂ nanostructures for enhanced photocatalytic reduction of U(VI) in water

Abstract

Photocatalytic technology is a valid solution for the remediation of wastewater containing uranium. In this study, the synthesis of Z-scheme g-C₃N₄/TiO₂ catalysts was made by a thermal synthetic approach for photocatalytic U(VI) reduction. The characterization results revealed the successful synthesis of g-C₃N₄/TiO₂ nanostructures. The g-C₃N₄ surface was uniformly coated with TiO₂ nanoparticles. The depletion of U(VI) in water evaluated the photocatalytic activity of g-C₃N₄/TiO₂ under UV light irradiation. The photocatalytic tests showed that g-C₃N₄/TiO₂ exhibited more effective photocatalytic activity than the raw materials (1.64 and 56.97 times higher than TiO₂(P25) and g-C₃N₄, respectively). Besides, a pseudo-first-order model was followed by the experimental kinetic data for the photocatalytic process. Moreover, g-C₃N₄/TiO₂ still presented high photocatalytic activity after four reacting cycles. Based on these experiment results, the improved photocatalytic activity could be attributed to the Z-scheme mechanism, which decreased the recombination of photo-produced electrons and holes. The synthesis of these g-C₃N₄/TiO₂ nanomaterials provides a facile and inexpensive method for treating wastewater containing U(VI).

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1. Introduction

As nuclear energy develops, uranium increasingly has entered the water environment, which seriously threatens human health.¹ Therefore, the treatment of wastewater containing uranium has become a research focus. The primary idea to solve this problem is to convert soluble U(VI) into insoluble U(IV) to separate it from the water.² Under light illumination, photocatalysts can produce electrons and holes for redox reactions. It has been proven to be an effective approach of treating wastewater containing U(VI). Compared with traditional treatment methods, photocatalysis has the advantages of simple operation, low energy consumption and no secondary pollution.³

The photocatalyst is the main point in a photocatalytic reaction process. As the most classic photocatalyst, TiO₂ has attracted the attention of researchers for its high activity and stability.⁴ TiO₂ commodity P25 is the only photocatalyst used in commercials currently. However, the poor visible light responsiveness limits its further applications.^5,6 Recently, various new photocatalysts have been developed such as graphitic carbon nitride (g-C₃N₄), iron oxide (α-Fe₃O₄), ZnO and noble metals.^7–12 Among them, g-C₃N₄ has been a new focus issue in the photocatalytic field because of its narrow bandgap, which can increase the range of light absorption. Moreover, researchers have tried to improve the photocatalytic performance by modifying the photocatalysts. At present, there are three main modification methods for catalysts, such as morphology control, element doping and a variety of materials for composites.^13,14 Among these modification methods, composite materials could avoid many disadvantages and limitations of a single semiconductor in a photocatalytic process. In many mechanisms of composite, the Z-scheme photocatalytic mechanism was proposed on a composite, whereby the photo-produced electrons (with less potential energy) of a semiconductor photocatalyst and the holes (with less potential energy) of another semiconductor were recombined through an interface connection, thus leaving the photo-produced electrons with higher potential energy and holes on their respective semiconductors to form active sites.^15,16 It could efficiently separate photo-produced electrons and holes, thereby inhibiting their recombination.^17,18 According to an authoritative report, there are no more than 40 studies about the photocatalytic reduction of U(VI).¹⁹ Among these reports are even fewer reports on the application of Z-scheme photocatalyst to reduce U(VI).^20–23 However, there have been numerous reports about the application of Z-scheme photocatalysts to remove other pollutants in water.^24,25 Therefore, the research on the application of Z-scheme photocatalysts for photocatalytic reduction of U(VI) is innovative. Moreover, the research on the mechanism of U(VI) reduction by the Z-scheme photocatalyst would help increase photocatalytic efficiency.

In this paper, g-C₃N₄/TiO₂ composites were presented for photocatalytic U(VI) reduction application. TiO2 (P25) nanoparticles were coated on g-C₃N₄ to form a Z-scheme photocatalyst by a thermal synthetic method. The morphology and structure characterizations were presented subsequently. U(VI) reduction in aqueous was employed for evaluating the photocatalytic activities of this g-C₃N₄/TiO₂. Moreover, the Z-scheme photocatalytic mechanism of g-C₃N₄/TiO₂ was investigated, which could decrease the recombination of photo-produced electrons and holes.

2. Experimental

2.1 Preparation of material

Chemical reagents applied in this research were all at analytical grade without further purification. All the reagents were domestic. The TiO₂ particle used in this study was P25, which was purchased from King Chemical. Standard U(VI) solution was prepared by dissolving U₃O₈(99%) with HCl and H₂O₂ under heating conditions. The porcelain crucible containing some urea was placed in a muffle furnace, and the temperature was raised from room temperature to 600 °C for 2 h, and then cooled down naturally. The ceramic crucible was removed, and its content was grounded into powder to obtain the yellow g-C₃N₄ photocatalyst. A certain amount of TiO₂ and g-C₃N₄ were placed into three beakers. The surface-mass ratio of g-C₃N₄/TiO₂ was maintained at 1 : 1, 2 : 1 and 3 : 1, marked as g-C₃N₄/TiO₂-1, g-C₃N₄/TiO₂-2 and g-C₃N₄/TiO₂-3, respectively. The raw materials were then added into deionized water, stirred evenly, and ultrasonic mixed for 15 min until the powder had completely dispersed. The suspension was transferred into a ceramic crucible and heated to powder form at 80 °C for 5 h. Then, the ceramic crucible was calcined at 400 °C for 2 h in a muffle furnace. After being cooled to indoor temperature, the ceramic crucible was removed and its content grounded into powder to obtain the composite photocatalyst.

2.2 Characterization

The characterization was performed on the g-C₃N₄/TiO₂-1 sample. X-ray diffraction (XRD) characterized the crystal structure of the photocatalyst by applying a D/MAX-RB diffractometer (Bruker, D8 advance, Germany) and adopting Cu Ka radiation (ranging from 5° to 95°, Step length: 0.01°, each step 0.1 s). Scan electron microscopy (SEM), which was tested by a SUPRATM55 high-resolution field emission scanning electron microscope (operating voltage of 2 kV, standard aperture diaphragm of 30 μm, SE2 secondary electron detector imaging) (Zeiss, Germany), analyzed the surface morphology of the photocatalyst. X-ray photoelectron spectroscopy (XPS, Thermo Scientific, Escalab 250Xi, UK) was used to perform the surface elemental analysis of the photocatalyst by applying an ESCALAB II XPS system. UV-vis Diffuse Reflectance Spectra (DRS) was examined to obtain the optical absorption property of the photocatalyst (Shimadzu, UV-3600, Japan, equipped with an integrating sphere). The specific surface area was calculated by the Brunauer–Emmett–Teller (BET) method (Micromeritics ASAP 2460 instrument at 77 K with N₂ as absorbate). A Xe lamp, as an excitation source, was used to record the PL spectra on a fluorescence spectrophotometer (FS5, Edinburgh Instruments Ltd., UK).

2.3 Photocatalytic activities of photocatalysts

The photocatalytic reduction of U(VI) solution evaluated the photocatalytic activities of the photocatalysts prepared. In the photocatalytic process, 0.02 g of as-prepared photocatalysts was added into 50 mL of U(VI) solution (initial concentration of 10 mg L⁻¹). Moreover, 2 mL of methanol was added into the suspension, which was used as the hole scavenger in the reaction. The mixture was ultrasonic oscillated for 30 min to disperse the photocatalysts. The pH of the suspension was adjusted to about 5.0 by hydrochloric acid and sodium hydroxide. For the elimination of the dissolved oxygen and realization of the adsorption–desorption equilibrium under anaerobic conditions, the suspension was bubbled with nitrogen (99.99%) for 30 min in the dark. Then, the mixture was illuminated by an ultraviolet light of 500 W. The photocatalytic process was carried out in the photocatalytic apparatus (GG-GHX-V, Shanghai Guigo Industrial Co. Ltd). Arsenazo III Spectrophotometric Method, used after centrifugation at regular intervals, determined the U(VI) concentration of the mixture. The following expression was used to calculate the removal ratios (R_U(VI)) of U(VI):

R_U(VI) = 1 − C_t/C₀ = (A₀ − A_t)/A₀ × 100%

where R_U(VI) is the removal ratios of U(VI); C_t and C₀ are the concentration of U(VI) at 0 and t time; and A₀ and A_t are the absorbance intensity corresponding to 0 and t time, respectively. The stability of g-C₃N₄/TiO₂ was assessed by reusing the photocatalyst in four repeated tests for the decrease of U(VI) under the same conditions.

3. Results and discussion

3.1 Structural characterization of g-C₃N₄/TiO₂ nanostructures

The characterization of the as-obtained g-C₃N₄, TiO₂ (P25) and g-C₃N₄/TiO₂ nanostructures were made by XRD and XPS techniques. The XRD patterns for these nanostructures are presented in Fig. 1a. In the XRD pattern of pure g-C₃N₄ (blue line), the significant diffraction peaks appeared at positions of 13.1° and 27.3°, and are indexed to (100) and (002) planes of g-C₃N₄. These are attributed to the planar, which repeats cell structure and sandwich stack reflection of g-C₃N₄.^26,27 The average interlayer distances were 0.675 nm and 0.326 nm, respectively, calculated by the Scherrer equation. After compositing with TiO₂ (P25), the significant peaks of g-C₃N₄ still had decreased peak intensity. The apparent diffraction peaks of anatase TiO₂ and rutile TiO₂ are shown in the g-C₃N₄/TiO₂ nanostructures (red line), where most of these peaks displayed (101), (004), (200), (105), (211), (204), (116), (220) and (215) planes of anatase TiO₂,²⁸ while other peaks were attributed to (110), (101), (111) and (220) planes of rutile TiO₂. These peaks are according to the XRD results of pure P25 (black line).

Fig. 1 (a) XRD patterns of g-C₃N₄, TiO₂ and g-C₃N₄/TiO₂ prepared, the labels of ♥ and ♣ are the corresponding peaks of anatase TiO₂ and rutile TiO₂, respectively; (b) the complete XPS spectra of g-C₃N₄/TiO₂, main peaks of Ti 2p (c), C 1s (d), N 1s (e) and O 1s (f) for g-C₃N₄/TiO₂.

Moreover, Fig. 1b–f show the XPS spectra of g-C₃N₄/TiO₂ nanostructures. In Fig. 1b, the full XPS spectrum presented all the peaks (C 1s, N 1s, Ti 2p, O 1s) of g-C₃N₄ plus TiO₂ nanomaterials. Fig. 1c–f showed high-resolution XPS spectra of Ti 2p, C 1s, N 1s and O 1s. After employing the XPS-peak-differentiation-imitating analysis method, Ti 2p_1/2 and Ti 2p_3/2 peaks appeared in Fig. 1c. C 1s and N 1s further confirmed the successful synthesis of g-C₃N₄ (Fig. 1d and e). The two C 1s that peaked at 284.8 and 288.0 eV, respectively, were caused by an aromatic carbon atom and C–N–C coordination (Fig. 1d). The O 1s identified that the TiO₂ existed in these nanostructures (Fig. 1f). Thus, the generation of g-C₃N₄/TiO₂ nanostructures can be confirmed by these structural characterizations.

3.2 Morphology of g-C₃N₄/TiO₂ nanostructures

The SEM images of as-synthesized pure g-C₃N₄, TiO₂ (P25) and g-C₃N₄/TiO₂ nanostructures are presented in Fig. 2. The g-C₃N₄ polymer with a blocky structure was gathered by many irregularly shaped wrinkled sheets, as displayed in Fig. 2a. These blocky structures with holes of g-C₃N₄ polymers could be reported as per the magnified SEM image of g-C₃N₄ (Fig. 2b). These holes may give the g-C₃N₄ many active reaction sites for various applications. The SEM images of the pure TiO₂ (P25) are shown in Fig. 2c and d. As shown in these images, the particle size of TiO₂ (P25) was too small (less than 100 nm) compared with g-C₃N₄. It was more favorable for TiO₂ to disperse on the surface of g-C₃N₄. After compositing TiO₂ with g-C₃N₄, TiO₂ nanoparticles uniformly adhered to g-C₃N₄ (Fig. 2e and f). This combination could promote the charge transfer between g-C₃N₄ and TiO₂ by the Z-scheme route. However, as presented in Fig. 2e, there still certain parts of g-C₃N₄ that were uncovered due to the imprecise control of the hydrothermal method. This result further displays the successful synthesis of the g-C₃N₄/TiO₂ nanostructures.

Fig. 2 SEM images of pure g-C₃N₄ (a and b), TiO₂ (P25, c and d) and g-C₃N₄/TiO₂ nanostructures (e and f).

3.3 Surface area of g-C₃N₄/TiO₂ nanostructures

Adsorption and desorption experiments by N₂ were carried out at 77 K, and N₂ isotherms have been applied to calculate the specific surface area by the multi-point BET method. As shown in Fig. 3a, the shape of the isotherm with two capillary condensation steps is like a type IV isotherm according to the IUPAC classification, indicating the presence of mesopores. The shape of the hysteresis loop was of the type H3, and the hysteresis loop shifts at high relative pressures (P/P₀) between 0.8 and 1.0, which suggests a mesoporous structure. Mesoporosity is typical of wrinkled, sheet-like particles, and this result is consistent with SEM images. This mesopore size distribution was confirmed by the pore size distributions shown in the inset in Fig. 3a. The average pore diameter of g-C₃N₄/TiO₂ nanostructures is estimated as 37 Å, which suggests the existence of mesopores. The BET specific area of the g-C₃N₄/TiO₂ nanostructures was 36.2 m² g⁻¹, which is between g-C₃N₄ (29.4 m² g⁻¹) and TiO₂ (51.6 m² g⁻¹).

Fig. 3 (a) N₂ adsorption–desorption isotherm of g-C₃N₄/TiO₂ nanostructures. Inset: pore size distribution of g-C₃N₄/TiO₂ nanostructures; (b) UV-vis diffuse reflectance spectra of pure g-C₃N₄, TiO₂ (P25) and g-C₃N₄/TiO₂ nanostructures; (c) plots of the (Ahv)² versus (hv) for pure g-C₃N₄, TiO₂ (P25) and g-C₃N₄/TiO₂ nanostructures, (d) PL spectra of g-C₃N₄, TiO₂ (P25) and g-C₃N₄/TiO₂ nanostructures.

3.4 Optical properties of g-C₃N₄/TiO₂ nanostructures

Firstly, UV-vis diffuse reflectance spectroscopy (DRS) was used to investigate the optical properties of pure g-C₃N₄, TiO₂ (P25) and g-C₃N₄/TiO₂ nanostructures. As shown in Fig. 3b, the significant absorption peak of g-C₃N₄ was at ∼398 nm, the absorption end of g-C₃N₄ was ∼450 nm (black line). The pure TiO₂ (P25) showed a high UV-light absorption, and the absorption edge was ∼400 nm (red line). After compositing TiO₂ with g-C₃N₄, the absorption edge was located on the range between 400 and 450 nm, and the absorption intensity in the UV region was lower than pure TiO₂ but higher than g-C₃N₄ (blue line), which reveals a successful synthesis of the g-C₃N₄/TiO₂ nanostructures. According to the plot of (Ahv)² versus (hv), as shown in Fig. 3c, the bandgap energies (E_g) of pure g-C₃N₄, TiO₂ (P25) and g-C₃N₄/TiO₂ have been estimated to be 2.7 eV, 3.2 eV and 2.75 eV, respectively. The calculation results show that the bandgap width of g-C₃N₄/TiO₂ nanostructures was very close to that of g-C₃N₄, which is narrower than that of TiO₂ (P25).

PL spectra were the principal approach to test the efficiency of charge separation during the photocatalytic process.¹⁶ The release of energy in the form of PL emission was made while recombining electron–hole pairs. Therefore, greater PL intensity shows a greater recombination rate of charge carriers. Hence, the PL spectra of pure g-C₃N₄, TiO₂ (P25) and g-C₃N₄/TiO₂ nanostructures were performed in Fig. 3d. For pure g-C₃N₄ and TiO₂ (P25), there is a relatively strong emission peak at ∼452 nm, which is displayed in the PL spectra (black line and red line), indicating that there are many photo-produced electrons combined with holes. A large amount of energy is released, which is detected by the PL spectra. For g-C₃N₄/TiO₂, there is no prominent emission peak in PL spectra (blue line). This phenomenon indicates the decreased recombination efficiency of photo-produced electrons and holes, which enhanced the photocatalytic performance of g-C₃N₄/TiO₂ because of its Z-scheme structure.²⁹

3.5 Photocatalytic reduction of U(VI)

U(VI) reduction was used to evaluate the photocatalytic activity of the photocatalysts. Fig. 4a and b show the results. Without photocatalysts, there is no obvious change of U(VI) concentration, showing U(VI) stability under light and negligible self-photocatalysis process. Under the condition of adding photocatalyst, the U(VI) concentration reduces slightly after the dark reaction reaches adsorption equilibrium in the first 30 min. It is indicated that photocatalyst has a certain adsorption ability for U(VI). For pure g-C₃N₄, U(VI) concentration does not decline obviously after the light is turned on, which shows that the photocatalytic effect of pure g-C₃N₄ for U(VI) is greatly restricted. For TiO₂, g-C₃N₄/TiO₂-1, g-C₃N₄/TiO₂-2 and g-C₃N₄/TiO₂-3, the U(VI) concentration drop rapidly in the first 5 min after the light is turned on. The U(VI) concentration decreases gradually in the following 20 min. As the most active photocatalyst, g-C₃N₄/TiO₂-1 removed >99% of U(VI) after illumination for 25 min. The reaction kinetics for the removal of U(VI) quantitatively was researched by pseudo-first-order kinetics model. As expressed in the formula −ln(C₀/C_t) = kt, where k stands for the apparent reaction rate constant, C₀ and C_t are the concentrations of aqueous U(VI) at reaction time t₀ and t, respectively. As displayed in Fig. 4b, the apparent rate constants k for g-C₃N₄, TiO₂, g-C₃N₄/TiO₂-1, g-C₃N₄/TiO₂-2 and g-C₃N₄/TiO₂-3 were calculated to be 0.0033, 0.1359, 0.2222, 0.1020 and 0.1073 min⁻¹, respectively. As the maximum in the series photocatalysts, the k values of g-C₃N₄/TiO₂-1 was 1.64 times of TiO₂ (the maximum of raw material), indicating that the photocatalytic performance has been significantly improved through the material composite.

Fig. 4 (a) The variation of U(VI) during the reaction; (b) the rate constant (k) of U(VI) reduction; (c) cycling runs in the photocatalytic reduction of U(VI); (d) XPS spectrum of U 4f for the U element on the surface of g-C₃N₄/TiO₂ before (top) and after (bottom) reduction process.

The long-time stability of a photocatalyst during the photocatalytic reaction is significant for practical use. Fig. 4c shows the recycling usability of four times for g-C₃N₄/TiO₂-1 in the photocatalytic process. After each cycle, centrifugation collected and separated the photocatalysts from the suspension aqueous. Photocatalysts were ultrasonic oscillated to peel off surface sediments and then washed by deionized water. Subsequently, the drying of photocatalysts was carried out at 60 °C before the next recycle. As shown in Fig. 4c, the curves for each cycle were very similar. The final removal rate of every cycle was more than 99%, which indicates the stable photocatalytic activity of g-C₃N₄/TiO₂ after several recycles.

XPS spectra of U element on g-C₃N₄/TiO₂ nanostructures were performed for verification of the reduction process. Fig. 4d shows higher resolution XPS spectra of U 4f (after and pre-reduction). After the reduction of U(VI) into U(IV), the peak positions of U 4f_5/2 and 4f_7/2 were changed. In the bottom red curve, the peak-differentiating technique was used to obtain four peaks located at 379.6, 381.9, 390.4, and 391.3 eV. The two peaks at 381.9 and 391.3 eV originated from U(VI), these positions were matched well with that of U(VI) sample without reduction (up to black line), and peaks at 379.6 and 390.4 eV belong to U(IV). The above results showed that most of the U(VI) on the surface of g-C₃N₄/TiO₂ nanostructures were reduced into U(IV).³⁰

3.6 Reduction mechanisms of U(VI)

After the above results and discussion, Fig. 5 proposed the possible mechanism of g-C₃N₄/TiO₂ photocatalytic reduction of U(VI). g-C₃N₄ and TiO₂ formed their photo-produced electrons and holes, respectively, in their internal parts under light irradiation. According to the relative position of VB and CB in g-C₃N₄ and TiO₂, the photo-produced electrons produced by TiO₂ migrate to the interface of g-C₃N₄/TiO₂ and integrate holes in the part of g-C₃N₄. The photo-produced electrons produced by g-C₃N₄ are used as a reduction site to reduce the U(VI) into U(IV). The holes in the VB of TiO₂ oxidize methanol into CO₂ and H₂O as oxidation sites. In this way, the photo-produced electrons and holes involved in the redox reaction are reacted on their respective semiconductors. The recombination of photo-produced electrons and holes is effectively avoided, increasing photocatalytic efficiency.³¹ At present, there are two mainstream opinions about the reduction process of U(VI).¹⁹ In the first opinion, the reduction process of U(VI) consists of two steps,³² which is shown as eqn (1)–(3) in Scheme 1. U(VI) is reduced to U(V) by photo-produced electrons in the first step (eqn (1)). U(V) can spontaneously react to U(VI) and U(IV) (eqn (2)), which follows the principle of electronic equilibrium. Moreover, U(V) can be further reduced to U(IV) by photo-produced electrons (eqn (3)). Another opinion indicated that U(VI) could be reduced to U(IV) directly just as eqn (4) in Scheme 1.³³ As displayed in Fig. 5, the reduction potential of CB on g-C₃N₄ (E⁰ = −1.3 eV) was more passive than the decreased potential for a series of reactions. Theoretically, the reduction reaction can proceed smoothly.

Fig. 5 The diagram of the proposed mechanism of g-C₃N₄/TiO₂ nanostructures under simulated sunlight irradiation.

Scheme 1 Proposed mechanistic steps for U(VI) photocatalytic reduction.

4. Conclusions

A thermal synthetic approach was proposed for the synthesis of Z-scheme catalyst g-C₃N₄/TiO₂. According to the results of characterization, a successful synthesis of g-C₃N₄/TiO₂ nanostructures was revealed. By comparing with raw materials, g-C₃N₄/TiO₂ displayed much greater photocatalytic activities for the reduction of radioactive U(VI) under the condition of UV light illumination (1.64 times higher of TiO₂ and 56.97 times of g-C₃N₄). More than 99% of U(VI) was removed by g-C₃N₄/TiO₂ after illumination for 25 min. XPS analyses showed that most of the U(VI) on the surface of g-C₃N₄/TiO₂ were reduced into U(IV). After the reaction kinetics analysis, the photocatalytic reaction follows pseudo-first-order kinetics. Moreover, the g-C₃N₄/TiO₂ displayed good stability after four reused tests. Its Z-scheme structure, which decreased recombination efficiency of photo-produced electrons and holes, could cause the enhancement of the photocatalytic activities of g-C₃N₄/TiO₂. In summary, the g-C₃N₄/TiO₂ practically represent a suitable in U(VI) photocatalytic reduction.

Author contributions

Yuelin Liu contributed to the conception and design of the study. Shanshan Wu contributed to the acquisition of data. Yuelin Liu and Jun Liu contributed to the 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.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

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).

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