Fabrication of Graphene Quantum Dots Modified BiOI/PAN Flexible Fiber with Enhanced Photocatalytic Activity

Rongan He, Rong Chen, Jinhua Luo, Shiying Zhang, Difa Xu

Hunan Province Key Laboratory of Applied Environmental Photocatalysis, Changsha University, Changsha 410022, China.

Abstract: In environment remediation, photocatalytic oxidation is a promising technique for removing organic pollutants. Compared to adsorption, biodegradation, and chemical oxidation, photocatalytic oxidation can eliminate organic pollutants completely, conveniently, and cheaply in an environmentally friendly manner. Visible-light-driven photocatalytic oxidation is particularly advisable because of the high proportion of visible light energy in solar energy. Bismuth oxyiodide (BiOI)is a promising visible-light-driven photocatalyst for the oxidization of pollutants, not only because of its narrow band gap, but also for its relatively low valence band (VB), which is adequate for photogenerated holes to oxidize a variety of organic compounds. However, the shortcomings of BiOI powder, such the difficulty of recycling it, its low surface area, and fast carrier recombination, limit its practical applications. Meanwhile, the flexibility and hierarchical structure of photocatalysts are particularly advisable because these properties are beneficial for the convenient operation, recycling, and performance improvement of these materials. Herein, based on an electro-spun polyacrylonitrile(PAN) nanofiber substrate, a hierarchical BiOI/PAN fiber was prepared through an in situ reaction. In the as-prepared BiOI/PAN fibers, BiOI flakes were aligned vertically and uniformly around the PAN fibers. BiOI nuclei generated from preintroduced Bi(III) in the PAN fiber act as seeds for the growth of BiOI nanoplates, which is crucial for the formation of a hierarchical structure. Such a hierarchical structure can improve both the light absorption and carrier generation of the BiOI/PAN fibers, as demonstrated by UV-Vis diffuse reflectance spectra and photoluminescence emission. Therefore, the BiOI/PAN fibers exhibited higher photocatalytic activity than BiOI powder. When the BiOI/PAN fibers were decorated with pre-prepared graphene quantum dots (GQDs), a GQD-modified BiOI/PAN fibrous composite (GQD-BiOI/PAN) was fabricated. The morphology of the obtained GQD-BiOI/PAN fibers was nearly the same as that of the BiOI/PAN fibers. A step-scheme (S-scheme) heterojunction was formed between the GQDs and BiOI, which was confirmed by the fabrication method, photoluminescence emission, reactive radical tests, and XPS analysis. This kind of S-scheme heterojunction can not only effectively suppress the recombination of photogenerated holes, but can also reserve the more reductive electrons on the lowest unoccupied molecular orbital of GQDs and the more oxidative holes on the VB of BiOI, for the photocatalytic degradation of phenol. Because of the fibrous hierarchical structure and S-scheme heterojunction, GQD-BiOI/PAN outperformed BiOI nanoparticles and BiOI/PAN nanofibers in the photocatalytic oxidation of phenol under visible light. In addition, because of tight bonding, GQD-BiOI/PAN can be tailored and operated by hand, which is convenient for recycling.During recycling, no obvious loss of sample or decrease in photocatalytic activity was observed. This work provides a new pathway for the fabrication of flexible photocatalysts and a new insight into the enhancement of photocatalysts.

Key Words: Hierarchical; Photocatalytic oxidization; Phenol; S-scheme heterojunction; Visible light

1 Introduction

In environment remediation, photocatalytic oxidation is a promising technique for removing organic pollutants. As compared with other processes, such as adsorption,biodegradation, and chemical oxidation, photocatalytic oxidation can eliminate organic pollutants completely,conveniently, and cheaply in an environmentally friendly manner1–6. Visible-light-driven photocatalytic oxidation is particularly advisable because visible light energy constitutes bout 43% of solar energy, and is readily available both indoors and outdoors. Up to now, many visible-light-driven photocatalysts have been prepared and investigated, including WO3, Bi2O3, CdS, MoS2, BiVO4, Bi2WO6, g-C3N4, BiOI, and Fe2O37–11.

Among them, BiOI is an attractive visible-light-driven photocatalyst for the oxidization of pollutants. This is because it has not only a narrow band gap, but also a relatively low valence band (VB), adequate for the photogenerated holes to oxidize a variety of organic compounds12–17. However, it is not easily recycled and exhibits fast carrier recombination, which limit its practical application. Like most photocatalysts, BiOI nanopowders is difficult to be extracted from the reaction solution, which not only results in loss of the photocatalyst, but also increases the cost of recycling. Moreover, nano-sized BiOI powders tend to aggregate together to reducing surface energy,leading to a decrease in surface area. Furthermore, fast recombination of carriers suppresses the photocatalytic activity of BiOI. Thus, numerous attempts have been made to improve the photocatalytic performance and recyclability of BiOI.

As compared with powder photocatalysts, flexible or soft photocatalysts possess many advantages, such as separation and operation convenience, and well-dispersed reactive sites. As a result, more and more efforts have been devoted to the preparation of flexible photocatalysts18,19. One of the most widely adopted preparation method for these photocatalysts is immobilization of semiconductor material on flexible supporting materials, including SiO2fiber20, graphene foam21, active carbon fiber22, Cu nets23, and PAN fiber24. Among these substrate materials, PAN fiber is considered to be the most effective substrate, because it is transparent and highly flexible,and its structure can be easily controlled24–28. Therefore,immobilization of BiOI on PAN fibers can increase the performance and recycling convenience of the composite.

Beside immobilization of BiOI, coupling BiOI with a nonmetal co-catalyst to construct an S-scheme heterojunction is also an effective way to improve its performance11,29–37. Usually,semiconductors with narrow band gap are employed as the cocatalyst in building S-scheme heterojunctions, such as CdS,MoS2, C3N4, and Bi2O338–41. In fact, graphene quantum dots(GQDs) can also act as the co-catalyst in constructing S-scheme heterojunctions. Unlike graphene and graphene oxide, GQDs possess a finite band gap because of the quantum confinement effect42–44. Therefore, GQDs can act as a semiconductor in a composite44–47. In addition, the Fermi level of GQDs is higher than that of BiOI48. These characteristics favor the formation of S-scheme heterojunctions between GQDs and BiOI. What is more, GQDs are suitable for the coating of flexible catalysts because they can spontaneously attach to the surface of semiconductors. The fabrications of GQDs are mainly carried out through top-down and bottom-up strategies42,49. In a topdown way, the carbon materials in large size are etched and fractionated till the quantum sized graphene formed, such as oxidative cutting of anthracite coal, carbon fiber with concentrated nitric acid and sulfuric acid48,50. In a bottom-up way, GQDs were formed through the polycondensation of small organic molecular, such as methylbenzene, hexabromobenzene,citric acid, and 1,3,6-trinitropyrene. Among these methods, the polycondensation of citric acid is a more desirable way for its safety, low cost, and convenience44,51–54.

Herein, we prepared GQD-modified BiOI/PAN fibrous composite by way ofin situfabrication followed by a selfassembly process, and investigated its photocatalytic activity on removal of phenol.

2 Experimental section

2.1 Synthesis of flexible photocatalysts

All reagents were analytically pure, including Bi(NO3)3·5H2O(Guangdong Chemical Reagent Engineering Technological Research and Development Center, Guangzhou, China), KI(Guangdong Guanghua Chemical Factory Co. Ltd., Guangzhou,China), polyacrylonitrile (PAN,Mw= 150000, DuPont Co., Ltd.,Shenzhen, China),N,N'-dimethylformamide (DMF, Sinopharm Co. Ltd., Shanghai, China), citric acid, ethylene glycol (EG),ethanol, and phenol (Tianjin Guangcheng Chemical Reagent Co.Ltd., Tianjin, China). These reagents were used without further treatment.

The PAN precursor nanofiber (PAN-fNF) were prepared previouslyviaelectrospinning. 1 g of PAN powder and 50 mg Bi(NO3)3·5H2O were dissolved in 10 mL of DMF. Then the solution was electro-spun under a potential of 20 kV at a feeding rate of 0.5 mL·h−1, to obtain PAN-NF.

To fabricate BiOI/PAN nanofibers mats (BiOI/PAN), the asprepared PAN-NF was immersed in a reaction solution, which was prepared by dissolving 0.6 g Bi(NO3)3·5H2O and 0.2 g KI into a mixed solvent of 50 mL EG and 20 mL distilled water.The immersion was carried out for 3 h until BiOI/PAN formed.Finally, BiOI/PAN was poached several times with distilled water and ethanol, and dried at 80 °C overnight.

The GQDs were prepared by thermal polymerization of citric acid (CA) with some adjustment44,55. 2.0 g CA was thermally treated at 200 °C for 30 min in a 50 mL flask, until it turned dark orange. Then, the product was dispersed with 50 mL NaOH (0.1 mol·L−1) and dialyzed for 24 h to obtain the GQDs hydrosol.

To fabricate the GQDs-modified BiOI/PAN nanofibers(GQD-BiOI/PAN), a simple self-assembly method was employed. Briefly, the pre-prepared GQDs hydrosol was sprayed onto the surface of BiOI/PAN, and then BiOI/PAN was thermally treated at 160 °C for 3 h in a tubular furnace to obtain the final GQD-BiOI/PAN.

Hierarchical BiOI particles were synthesized for comparisonviaa hydrolysis method, in which 1.5 g Bi(NO3)3·5H2O and 0.50 g KI were dissolved in a mixed solvent of 50 mL EG and 20 mL distilled water. The solution was stirred at ambient temperature for 2 h until a red powder formed. Then, the red powder (BiOI)was washed and dried.

2.2 Characterization

Morphology analysis was conducted using a Helios NanoLab G3 field emission scanning electron microscope (Helios NanoLab G3 FEI, USA), and a transmission electron microscope(TEM, Tecnai G2 F20, USA). X-ray diffraction (XRD) patterns were measured using a Bruker D8 Advance X-ray diffractometer(Germany). The surface states of samples were analyzed by an X-ray photo-electron spectrometer (XPS, EscaLab Xi+, Thermo Fisher Scientific, USA). The diffuse reflectance spectra (DRS)analysis was carried out on a UV-2600 spectrophotometer(Shimadzu, Japan). A F-7000 fluorescence spectrophotometer(Hitachi, Japan) was employed to record the photoluminescence(PL) spectra. The specific surface area and pore size distribution analysis was performed using a gas sorption analyzer (Autosorb iQ, Quantachrome, USA).

2.3 Photocatalytic performance test

To evaluate the performance of the as-prepared samples, the removal efficiency of phenol in aqueous solution was studied. 50 mg of the samples was added to a Petri dish and mixed with 15 mL of phenol solution (0.1 g·L−1). Then, the mixture was irradiated under a 60 W LED lamp with a wavelength above 400 nm. The UV-Vis spectrum of the phenol solution was subtracted at the given time interval and recorded using a UV-Vis spectrophotometer (Shimadzu UVmini-1280). The temporal change in the concentration of phenol was estimated with the intensity at 269 nm.

3 Results and discussion

3.1 Crystal structure

The XRD analysis results are shown in Fig. 1. In the XRD pattern of the PAN-NF, two peaks can be observed. The one at 16.8° is the characteristic peak of orthorhombic PAN, the other broad peak at about 29.0° arises from the poor crystallization of the nanofibers24,28. In the XRD pattern of BiOI/PAN, these two peaks can also be seen. Furthermore, new peaks are visible at 29.7°, 31.7°, 45.4° and 55.1°, which can be attributed to the tetragonal BiOI phase (JCPDS No. 10-0445). BiOI/PAN comprises of PAN and BiOI. The pristine BiOI is still in a tetragonal phase, but its characteristic peaks are much sharper than that of BiOI/PAN, indicating that it possesses larger crystalline grains. The XRD pattern of GQD-BiOI/PAN is similar to that of BiOI/PAN, but slightly broader than those of BiOI/PAN. This can be ascribed to the shield effect of GQDs on the surface. No obvious peaks of GQDs can be found because of the low concentration. Nevertheless, by spraying and subsequent thermal treatment, GQDs will definitely deposit on the surface.

Fig. 1 XRD patterns of BiOI, PAN-NF, BiOI/PAN and GQD-BiOI/PAN.

3.2 Morphologies

The morphologies of BiOI, PAN-NF, BiOI/PAN, and GQDBiOI/PAN are shown in Fig. 2. Pristine BiOI exhibited a hierarchical structure composed of aggregated nanoplates. PANNF is composed of uniform nanofibers with a smooth surface and a diameter of 0.2–0.3 μm. The optical photo of PAN-NF(inset of Fig. 2b) presents a flexible white mat. BiOI/PAN and GQD-BiOI/PAN exhibited obvious hierarchical structures,while their fibrous shape remains unchanged (Fig. 2c,d). The whole composite fiber appears like a hemp rope. In such a hierarchical structure, the BiOI flakes stand uniformly around the PAN fiber, and are aligned in the same direction as the fibers.As compared with pristine BiOI, the dispersion of BiOI flakes is significantly improved, and their agglomeration is effectively inhibited. Furthermore, the hierarchical morphology of BiOI/PAN and GQD-BiOI/PAN is much more significant than those reported BiOI/PAN fibers28,56–58. In some of those reported preparations, Bi(NO3)3was not introduced in PAN nanofibers,BiOI nuclei can only be generated from the adsorbed Bi3+. As a result, the formation of BiOI nuclei became very difficult because of their size are much smaller. As is well known, smaller the particles, higher the solubility (Oswald ripening). In other fabrications, Bi(III) compounds were introduced in PAN fibers.however, no more Bi(III) compound was in the reaction solution.Thus the growth of BiOI was limited, and the hierarchical formed structure is still not obvious. In this work, Bi(NO3)3was added previously in PAN fibers, when the PAN fibers were immerged in the reaction solution, the introduced Bi(NO3)3reacted with KI, and transformed into larger BiOI nuclei and dispersed uniformly on the surface of PAN fibers. These BiOI nuclei can act as the seeds for the growth of BiOI flakes, favoring the growth of BiOI nanoplates. Therefore, large amount of BiOI flakes grew uniformly on the exterior of the PAN nanofibers,forming the unique hierarchical morphology of BiOI/PAN. In the formation of the fibrous hierarchical structure, the introduced Bi(NO3)3in PAN-NF played an important role.

Fig. 2 Morphologies of (a) BiOI, (b) PAN-NF, (c) BiOI/PAN, and (d) CQD-BiOI/PAN.

Fig. 3 (a) TEM and (b) HRTEM images of GQDs.

In the morphology image of GQD-BiOI/PAN, GQDs are not visible because of their small size. The morphology of pure GQDs is shown in Fig. 3a, in which there are many dark dots with a diameter of 2–4 nm. In Fig. 3b, the image of GQDs with distinct lattice fringes can be seen. The interplanar spacing of lattice fringes was estimated to be 0.203 nm, consistent with the reported data of GQDs46.

3.3 Specific surface area and pore structure

The N2adsorption-desorption isotherms and pore size distributions (inset) of BiOI, BiOI/PAN, and GQD-BiOI/PAN are shown in Fig. 4. Based on the desorption isotherms, the specific surface area of BiOI, BiOI/PAN, and GQD-BiOI/PAN were estimated to be 8, 36, and 32 m2·g−1, respectively. In the isotherms of BiOI/PAN and GQD-BiOI/PAN, a large hysteresis loop indicates a large mesopore volume, which can be seen more clearly in its pore size distribution. This is in agreement with their SEM images. The higher specific surface areas of BiOI/PAN and GQD-BiOI/PAN are mainly ascribed to their unique fibrous hierarchical structure. The specific surface area of GQD-BiOI/PAN is slightly smaller than that of BiOI/PAN.This is ascribed to the decrease of mesopores during the thermal treatment of GQD-BiOI/PAN.

Fig. 4 N2 adsorption-desorption isotherms and pore size distribution (inset) of BiOI, BiOI/PAN and GQD-BiOI/PAN.

Fig. 5 (a) UV-Vis diffuse reflectance spectra (DRS) of BiOI,PAN-NF, BiOI/PAN, and GQD-BiOI/PAN, (b) UV-Vis spectra of GQDs suspension.

3.4 Optical properties

The DRS spectra of BiOI, PAN-NF, BiOI/PAN, and GQDBIOI/PAN are shown in Fig. 5a. According to the absorption edges of BiOI (665 nm) and BiOI/PAN (626 nm), the band gap of BiOI and BiOI/PAN are estimated to be 1.88 and 1.98 eV,respectively. The larger band gap of BiOI/PAN is because of its smaller grain size, as evident from the XRD results. The light absorption of BiOI/PAN is stronger than pristine BiOI because BiOI/PAN possesses a well-arranged hierarchical structure (see Fig. 2c). The weak light absorption of PAN-NF is also beneficial for the light absorption of the BiOI flakes on BiOI/PAN. The light absorption characteristics of GQD-BiOI/PAN are similar to that of BiOI/PAN, except for a slight decrease in intensity within the wavelength range of 300–600 nm, suggesting that the decoration of BiOI/PAN with GQDs does not significantly affect its light absorption. The UV-Vis absorption spectrum of a GQD sol (Fig. 5b) looks similar to the DRS spectra of the photocatalyst, but the light absorption edge is not very clear and can only be roughly estimated to be at about 530 nm(corresponding band gap of 2.34 eV).

Under the irradiation of excitation light of 510 nm, the PL spectra of BiOI, BiOI/PAN, and GQD-BiOI/PAN were obtained,as shown in Fig. 6. The PL emission of BiOI/PAN was much stronger than that of BiOI, indicating the higher carrier generation of BiOI/PAN. Given its higher specific surface area,BiOI/PAN can absorb more irradiation and generate more carriers. The PL emission of GQD-BiOI/PAN is much weaker than that of BiOI/PAN, demonstrating the effective separation of photo-generated holes and electrons. This is reasonable because the light absorption of GQD-BiOI/PAN is close to that of BiOI/PAN, which will result in a similar carrier generation.Although the light absorption of GQD-BiOI/PAN is slightly weaker than that of BiOI/PAN, it could not cause such a decrease on carrier generation, given the similar morphologies and crystal structures. Moreover, the slight light decrease of GQDBiOI/PAN in UV-Vis light range demonstrates that the shield effect of GQDs is not significant. Therefore, the decreased PL emission of GQD-BiOI/PAN should be attributed to the inhibited carrier recombination.

Fig. 6 Photoluminescence (PL) spectra of BiOI, BiOI/PAN, and GQD-BiOI/PAN excited at 510 nm.

3.5 XPS analysis

Fig. 7 shows the recorded XPS spectra of BiOI/PAN and GQD-BiOI/PAN under light irradiation. The Bi 4f5/2and Bi 4f7/2binding energies of BiOI/PAN are at 164.2 and 158.9 eV,respectively (see Fig. 7b). In the spectrum of GQD-BiOI/PAN,these two binding energies blue-shift to 164.5 and 159.2 eV,respectively, revealing an outward transfer of electrons from Bi.Such a phenomenon is similar to that of BiOI/g-C3N4prepared in our previous work30. This also demonstrates a strong bond between the GQDs and BiOI.During the thermal treatment, the surface energy of GQD and BiOI was minimized by forming a close contact interface, leading to the tightly bonded GQDs and BiOI. The C 1sspectra of BiOI/PAN and GQD-BiOI/PAN are illustrated in Fig. 7c. In the spectrum of BiOI/PAN, the strong peak at 284.8 eV is assigned to C―H, C―C, C＝C in PAN,while the weak peak at 288.7 eV is attributed to O―C＝O from the attached DMF24. The spectrum of GQD-BiOI/PAN is similar to that of carbon-dot-decorated SiO2/PAN composite,reported in reference59. The peak at 284.8 eV is mainly attributed to C in PAN, which is slightly stronger than that of BiOI/PAN. This might be a signal from the deposited GQDs.The peak at 288.7 eV decreases because DMF was removed during the thermal treatment. The I 3dspectra of GQDBiOI/PAN is nearly the same as that of BiOI/PAN, no obvious shift can be found (see Fig. 7d).

To gain a better understanding of the carrier migration mechanism between BiOI and the GQDs, their Fermi levels were compared. The Fermi level of the (001) surface of BiOI has been estimated to be 0.42 eV in our previous work30. Therefore, in this work, only the Fermi level of GQDs was calculated,employing the absolute vacuum scale of 0 eV and the normal hydrogen electrode scale (Ee) of 4.6 eV, as used in Ref. 8. The calculated Fermi level of the GQDs was shown in Fig. 8. The work function (Ф) of the GQDs was estimated to be 4.32 eV,and the corresponding Fermi level was −0.28 eV, which was higher than that of BiOI. When the GQDs and BiOI are attached,the electrons within the GQDs will migrate to BiOI until their Fermi energy is balanced. As a result, the electron density on the GQDs will decrease while the electron density on the BiOI side will increase, leading to the formation of an internal electric field(IEF) pointing toward the BiOI side. Thus, an S-scheme heterojunction can form.

Fig. 7 XPS spectra of BiOI/PAN and GQD-BiOI/PAN: (a) survey, (b) Bi 4f, (c) C 1s, and (d) I 3d.

Fig. 8 Calculated Fermi levels of GQDs.

3.6 Photocatalytic performance

The photocatalytic performances of BiOI, BiOI/PAN, and GQD-BiOI/PAN are compared in Fig. 9a, using phenol as the chemical probe. When the photocatalyst is not added, theC/C0value of phenol does not change during the irradiation of an LED lamp, demonstrating that phenol dose not decompose under visible light. The main reason for this is that phenol does not absorb visible light and is not easily excited by it. The concentration decrease of phenol is much more significant in the presence of photocatalysts. The performance of BiOI/PAN suppresses that of BiOI, while GQD-BiOI/PAN has an even higher performance. The rapid decrease of the characteristic peak at 269 nm demonstrates a quick degradation of phenol on GQD-BiOI/PAN (Fig. 9b). Considering that the band gap of BiOI/PAN is larger than that of BiOI, the improved activity of BiOI/PAN can be ascribed to the increased light absorption and specific surface area. As for GQD-BiOI/PAN, the decoration of GQDs further enhanced the photocatalytic activity.

To investigate the role of GQDs, the reactive radicals in the photocatalytic reaction were evaluated through electron spin resonance (ESR) analysis, with 5,5-dimethyl-1-pyrroline-Noxide (DMPO) from Sigma Co. Ltd. as trapping agent. The ESR spectra of BiOI/PAN and GQD-BiOI/PAN are compared in Fig.10. As shown in Fig. 10a, both DMPO-·OH signals of BiOI/PAN and GQD-BiOI/PAN are weak, because their VB is not low enough to oxidize H2O to ·OH. In Fig. 10b, the DMPO-·O2−signal of BiOI/PAN is rather weak, indicating that there is very low generation of ·O2−. This is because the conduction band (CB)of BiOI is not negative enough to reduce O2to ·O2−. By contrast,the DMPO-·O2−signal of GQD-BiOI/PAN is much stronger,indicating that more ·O2−is produced. The increased ·O2−should be generated from the O2reduced by the electrons on the LUMO of the GQDs. Considering the higher LUMO of GQDs, the carrier migration between BiOI and the GQDs should follow the postulated S-scheme mechanism. If a type II heterojunction forms, a decrease in ·O2−generation should emerge, which is not in agreement with the DMPO-·O2−signal results. Combining the result of XPS, the formation of an S-scheme heterojunction can be further verified. In such a heterojunction, the holes in GQDs are consumed by the electrons from BiOI, resulting in more available holes on BiOI and electrons on GQDs for the phenol oxidization and ·O2−production, respectively.

Fig. 9 (a) Degradation curves for phenol on BiOI, BiOI/PAN and GQD-BiOI/PAN under LED lamp irradiation (λ > 400 nm) and(b) temporal evolution of the UV spectra of the phenol solution at presence of GQD-BiOI/PAN.

Fig. 10 ESR results of (a) DMPO-·OH and (b) DMPO-·O2− over BiOI/PAN and GQD-BiOI/PAN.

Fig. 11 Sketch of the hierarchical fibrous structure and S-scheme charge migration of GQD-BiOI/PAN.

Based on the above analysis, the photocatalysis mechanism of GQD-BiOI/PAN is proposed, as shown in Fig. 11. The unique hierarchical fibrous structure of GQD-BiOI/PAN increased light absorption and surface accessibility, resulting in improved photocatalytic activity as compared with BiOI. When the GQDs were deposited on the surface of BiOI, the electrons on BiOI migrated to the GQDs until their Fermi energy reached the same level, resulting in the formation of an IEF and bent bands. Upon photoexcitation, the photo-generated electrons on the CB of BiOI transferred to the highest occupied molecular orbital(HOMO) of the GQDs under the driving force of the IEF, and combined with the holes on the HOMO of the GQDs.This kind of S-scheme pattern can reserve electrons on the LUMO of the GQDs and holes on the VB of BiOI, for the photocatalytic degradation of phenol. Thus the formed S-scheme heterojunction can further enhance the activity of GQDBiOI/PAN.

4 Conclusions

A hierarchical GQDs-modified BiOI/PAN fiber (GQDBIOI/PAN) was prepared and exhibited improved photocatalytic activity. The hierarchical fibrous structure and the S-scheme heterojunction of GQD-BiOI/PAN are crucial for its improved performance. By increasing the photogenerated carriers and utilizing them more effectively, GQD-BiOI/PAN remarkably outperformed BiOI nanoparticles in the photocatalytic degradation of phenol under visible light. This finding has the potential to provide an alternative design route for photocatalysts.

Supporting Information:available free of chargeviathe internet at http://www.whxb.pku.edu.cn.