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Reusable and Flexible g-C3N4/Ag3PO4/Polyacrylonitrile Heterojunction Nanofibers for the Photocatalytic Dye Degradation and Oxygen Evolution Ran Tao, Shu Yang, Changlu Shao, Xinghua Li, Xiaowei Li, Shuai Liu, Jian Zhang, and Yichun Liu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00428 • Publication Date (Web): 03 May 2019 Downloaded from http://pubs.acs.org on May 5, 2019

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ACS Applied Nano Materials

Reusable

and

Flexible

g-C3N4/Ag3PO4/Polyacrylonitrile

Heterojunction Nanofibers for the Photocatalytic Dye Degradation and Oxygen Evolution

Ran Tao, Shu Yang, Changlu Shao*, Xinghua Li*, Xiaowei Li, Shuai Liu, Jian Zhang, Yichun Liu

Centre for Advanced Optoelectronic Functional Materials Research, and Key Laboratory of UV-Emitting Materials and Technology (Northeast Normal University), Ministry of Education Changchun 130024 (P.R. China)

a

Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] Tel: +86 43185098803

1

ACS Paragon Plus Environment

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Flexible photocatalysts stands out from numerous photocatalysts, owing to their foldable, reusable, and mechanically stable properties. Here, g-C3N4/Ag3PO4/PAN nanofibers were prepared by immobilizing Ag3PO4 nanoparticles on electrospun gC3N4/PAN nanofibers through room-temperature in situ synthesis method. Compared with g-C3N4/PAN and Ag3PO4/PAN, the g-C3N4/Ag3PO4/PAN nanofibers exhibited better photocatalytic performance. The degradation rates of Rhodamine B and tetracycline hydrochloride of g-C3N4/Ag3PO4/PAN nanofibers were 8.8 and 12.9 times higher than those of g-C3N4/PAN nanofibers, while 3.1 and 6.5 times higher than those of Ag3PO4/PAN nanofibers, respectively. In addition, the oxygen evolution rate of gC3N4/Ag3PO4/PAN nanofibers was 1.9 times better than that of g-C3N4/PAN and 5.8 times better than that of Ag3PO4/PAN. The improved photocatalytic performance of gC3N4/Ag3PO4/PAN was likely because of the existence of Z-scheme g-C3N4/Ag3PO4 heterojunction with efficient charge separation. Furthermore, g-C3N4/Ag3PO4/PAN nanofibers had a better photochemical stability than Ag3PO4/PAN, which was probably due to the inhibited of Ag3PO4 photocorrosion by the transfer of electrons from Ag3PO4 to g-C3N4. These photocatalysts could be easily separated and reused due to their extralong nanofibrous mat structures and flexible properties. This work provided a new road to design and fabricate flexible self-supporting photocatalysts for pollutants degradation and energy conversion. KEYWORDS: g-C3N4/Ag3PO4 heterojunction; nanofibers; flexible photocatalysts; 2


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pollutant degradation; oxygen evolution.

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1. INTRODUCTION Semiconductor photocatalysts have recently become a popular research topic.1–6 As a visible light catalytic material, silver phosphate (Ag3PO4) hasble a narrow band gap of 2.36 eV and can absorb light of 530 nm and less.7,8 It is widely studied for organic pollutant decomposition and oxygen evolution.9–11 It has been discovered that Ag3PO4 has a built-in electric field between PO43− and Ag+ ions, which is ascribed to the separation of photoelectronhole pairs, leading to a quantum efficiency about 90% from 400 to 480 nm.12,13 However, a major problem associated with Ag3PO4 is its poor photochemical stability because the photocatalyst is irreversibly reduced to metallic silver in the absence of a suitable electron scavenger in aqueous solutions. Several attempts have been made to solve this problem by introducing electron cocatalysts,

adding

heterojunctions.13–17

electron Of

these

capturers, methods,

and the

constructing

construction

of

Ag3PO4-based Ag3PO4-based

heterojunctions, such as NiFe2O4, WO3, MoS2, and g-C3N4, is a simple and effective method to inhibit the photocorrosion of Ag3PO4.17–20 Among these, graphite carbon nitride (g-C3N4) is known to be a photocatalyst that uses visible light.21–25 The efficient separation of photoelectronhole pairs of the g-C3N4/Ag3PO4 heterojunction not only results in good photochemical stability but also provides benefits for designing a high performance visible light photocatalyst.2,26–29 In particular, due to their large specific surface areas, higher photocatalytic activities have been found for nanostructured gC3N4/Ag3PO4 heterojunctions compared to bulk materials.9,20,30 However, small-sized 4


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g-C3N4/Ag3PO4 heterojunctions can readily agglomerate, which can decrease photocatalytic activity by reducing the effective surface area. In addition, separation and recycling from the solution during the reaction can be adversely affected by the small size of the nanostructure. Thus, there is a need to create supports with large surface areas and macrostructures that can provide enough sites for the photocatalytic reaction and be readily separated. Electrospun nanofibers are good supports for immobilizing photocatalysts because of their extra-long one-dimensional structures, nanosized diameters, and macroscopic mat structures.31–33 In particular, electrospun polymer nanofibers with flexible properties could be more conducive to separation and reuse during the photocatalytic reaction. In the present study, we fabricated g-C3N4/Ag3PO4/PAN nanofibers by growing Ag3PO4 nanoparticles uniformly on electrospun g-C3N4/PAN by room-temperature in situ synthesis. Our novel g-C3N4/Ag3PO4/PAN nanofibers have two main advantages: (1) they contain high specific surface areas that provide more active sites to maximize contact with the solution and increase the photocatalytic activity; and (2) their structures, consisting of extra-long one-dimensional mats, along with their flexibilities can result in their good reusability. This study provides an effective paradigm for designing and fabricating self-supporting, flexible photocatalysts for energy production and environmental remediation.

2. EXPERIMENTAL SECTION 5



2.1. Fabrication of g-C3N4/NaH2PO4/PAN nanofibers We first grounded urea powder (10 g) with a mortar for 30 min and transferred the powder to a covered alumina crucible, which was heated to 550°C at 5°C min−1 and held at this temperature in a semi-closed environment for 2 h. The obtained g-C3N4 powder had a faint-yellow color. The detailed characterization results for the g-C3N4 powder are shown in Figure S1. NaH2PO4·2H2O (400 mg) and g-C3N4 powder (500 mg) were dispersed in N, N-dimethylformamide (DMF) (13 mL) using ultrasonication for 2 h. We subsequently added polyacrylonitrile (PAN) powder (1.5 g) to the mixture with continuous stirring for 24 h. We inserted the above solution into a plastic injector (10 mL) for electrospinning. We applied a 10 kV positive voltage to the tip and set the distance between the injector tip and the collector to 15 cm. The g-C3N4/NaH2PO4/PAN nanofibers were deposited on aluminum foil. The NaH2PO4/PAN and g-C3N4/PAN nanofibers were similarly prepared. 2.2. Fabrication of g-C3N4/Ag3PO4/PAN nanofibers As illustrated in Scheme 1, 50 mg of g-C3N4/NaH2PO4/PAN nanofibers were immersed into 250 mL of an AgNO3 aqueous solution (2 mM) for 30 min. The gC3N4/NaH2PO4/PAN color changed from faint-yellow to yellow, indicating that Ag3PO4 was immobilized on the nanofibers in situ. After three washes with deionized water, the resulting nanofibers were dried for 10 h at 50°C. We fabricated the photocatalysts using different NaH2PO4 amounts of 0.2, 0.4, and 0.8 g, denoting the resulting samples as g-C3N4/Ag3PO4/PAN-1, g-C3N4/Ag3PO4/PAN-2, and g6


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C3N4/Ag3PO4/PAN-3, respectively. As a control, we generated Ag3PO4/PAN nanofibers by immersing NaH2PO4/PAN in aqueous Ag3PO4. The g-C3N4/Ag3PO4 nanoparticles (NPS) were prepared by immersing a mixture of g-C3N4 and NaH2PO4 in aqueous Ag3PO4 (Supporting Information).

Scheme 1 Schematic for g-C3N4/Ag3PO4/PAN nanofiber synthesis.

3. RESULTS AND DISCUSSION 3.1. Morphologies and structures of the nanofibers The electrospun g-C3N4/PAN nanofibers possessed raised surfaces (Figure 1a), and the PAN nanofibers possessed smooth surfaces (Figure S3a), illustrating that the gC3N4 nanostructures were successfully fixed on the PAN nanofibers. The surfaces of the g-C3N4/NaH2PO4/PAN nanofibers showed no significant change compared to g7



C3N4/PAN, indicating that NaH2PO4 was uniformly dispersed in the nanofibers of gC3N4/NaH2PO4/PAN (Figure S3b). When the g-C3N4/NaH2PO4/PAN was immersed in an AgNO3 solution for 30 min, the Ag3PO4 nanoparticles were uniformly dispersed on the nanofibers, as shown in Figure 1bd. As the NaH2PO4 content increased in the gC3N4/NaH2PO4/PAN nanofibers, the loading amount of the Ag3PO4 nanoparticles on the nanofibers also increased. The yellow color of the g-C3N4/NaH2PO4/PAN nanofibers obviously changed (Figure 1bd inset) compared to the g-C3N4/PAN nanofibers (Figure 1a inset). Moreover, the SEM image of the Ag3PO4/PAN nanofibers is shown in Figure S3c. This showed that the g-C3N4/Ag3PO4/PAN nanofibers could be fabricated at room temperature using a basic impregnation method. The g-C3N4/Ag3PO4/PAN-1 nanofiber morphology was also observed using transmission electron microscopy (TEM) imaging, which showed raised g-C3N4 structures on the nanofiber surfaces. Many small black dots appeared on the gC3N4/Ag3PO4/PAN nanofibers (Figure 1e); these represented that Ag3PO4 nanoparticles were dispersed uniformly among the nanofibers. Two types of clear lattice fringes at 0.325 and 0.245 nm were observed by high-resolution TEM (HRTEM) imaging; these corresponded to the (2 0 0) plane of g-C3N4 and the (2 1 1) plane of the Ag3PO4, respectively (Figure 1f).34,35 The TEM imaging confirmed that the Ag3PO4 successfully grew on the g-C3N4/PAN nanofibers and corroborated our SEM results.

8


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Figure 1 SEM images of (a) g-C3N4/PAN, (b) g-C3N4/Ag3PO4/PAN-1, (c) gC3N4/Ag3PO4/PAN-2, and (d) g-C3N4/Ag3PO4/PAN-3 nanofibers. (e) TEM and (f) corresponding HRTEM images of g-C3N4/Ag3PO4/PAN-1 nanofiber.

We used X-ray diffraction (XRD) patterns to assess the sample phase purity. Figure S5 shows that g-C3N4/PAN had a characteristic peak at 27.4°, which was indicative of aromatic interplanar stacking and perfectly matched the diffraction of the g-C3N4 (0 0 2) plane.2 After loading the Ag3PO4 nanoparticles, g-C3N4/Ag3PO4/PAN-1, g9



C3N4/Ag3PO4/PAN-2, and g-C3N4/Ag3PO4/PAN-3 produced additional diffraction peaks with 2θ values of 20.9°, 29.7°, 33.3°, 36.6°, 47.8°, 52.7°, 55.0°, and 57.3°. These could be well indexed to the (1 1 0), (2 0 0), (2 1 0), (2 1 1), (3 1 0), (2 2 2), (3 2 0), and (3 2 1) crystal planes of the Ag3PO4 cubic phase (JCPDS, 06-0505).36 No characteristic peaks were seen that indicated impurities. Additionally, Ag3PO4/PAN nanofibers matched the cubic phase of Ag3PO4, (JCPDS, 06-0505).36 To further validate the above results, we performed Fourier-transform infrared (FTIR) spectroscopy. The characteristic peak at 809 cm−1 corresponded to the s-triazine ring unit of g-C3N4 (Figure S6a).26 The characteristic peaks at 1322, 1559, and 1635 cm−1 represented the CN heterocycles of g-C3N4.37 Furthermore, the characteristic peaks at 1237, 1453, 1736, and 2245 cm−1 represented vibrations of C-C, C-H, C-O, and C≡N bonds/groups derived from the PAN, respectively.38 Moreover, the peak at 547 cm−1 for the Ag3PO4 nanoparticles may be characteristic of PO43−.36 We also observed that the PO43− vibration peak in the g-C3N4/Ag3PO4/PAN had a notable blue shift (Figure S6b). This may have been due to interfacial interactions between Ag3PO4 and g-C3N4. The FTIR analysis implied that the g-C3N4/Ag3PO4/PAN nanofiber composites were not a mere physical mixture. The BET nitrogen adsorption/desorption isotherms and thermogravimetric (TG) curves of the samples are shown in Figures S7 and S8. The mass percentages of g-C3N4, Ag3PO4, and PAN in the g-C3N4/Ag3PO4/PAN nanofibers were determined by TG analysis, as discussed in the Supporting Information. Moreover, the BET specific surface areas of the samples are shown in Table S1. 10


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3.2. Optical properties and energy band position calculation The optical properties of g-C3N4/PAN, g-C3N4/Ag3PO4/PAN, and Ag3PO4/PAN were characterized by UV-Vis diffuse reflection spectra (DRS) (Figure 2a). The gC3N4/PAN sample exhibited an optical absorption edge at ~450 nm, but Ag3PO4/PAN exhibited better light adsorption. The absorption intensity of g-C3N4/Ag3PO4/PAN also gradually increased as the Ag3PO4 content increased, which could improve the utilization of solar light and enhance the photocatalysis. Additionally, we calculated the g-C3N4/PAN and Ag3PO4/PAN nanofiber band gaps as follows: αℎ𝑣 = 𝐴(ℎ𝑣 ― 𝐸𝑔)𝑛/2,

(1)

where v, a, A, and Eg are the light frequency, absorption coefficient, a constant, and band gap, respectively. For g-C3N4 and Ag3PO4, n = 1 (indirect transition).17,39 Thus, we determined the band gap energies of g-C3N4 and Ag3PO4 to be 2.70 and 2.37 eV, respectively (Figure 2b).

11



Figure 2 (a) UVVis diffuse reflection spectra of g-C3N4/PAN, g-C3N4/Ag3PO4/PAN1, g-C3N4/Ag3PO4/PAN-2, g-C3N4/Ag3PO4/PAN-3, and Ag3PO4/PAN nanofibers. (b) Ag3PO4 and g-C3N4 optical band gaps in PAN nanofibers. MottSchottky plots of (c) Ag3PO4 and (d) g-C3N4 in PAN nanofibers.

We also measured MottSchottky curves to determine the valence band (VB) and conduction band (CB) positions of Ag3PO4 and g-C3N4 (Figure 2c and d). The curve slopes indicated that Ag3PO4 was a p-type semiconductor and g-C3N4 was an n-type semiconductor. Additionally, the flat band potentials (Vfb) were defined via extending tangent to 1/C2= 0. The Vfb of Ag3PO4 and g-C3N4 were +2.38 eV and -1.33 eV vs. Ag/AgCl (at pH = 6.8), respectively.1 Thus, the Vfb of Ag3PO4 and g-C3N4 were +2.58 eV and -1.13 eV vs. a normal hydrogen electrode (NHE), respectively.1 A previous 12


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study reported that the Vfb of n-type semiconductors were close to the CB potential (ECB) and the Vfb of p-type semiconductors were close to the VB potential (EVB).40 Therefore, the values of the Ag3PO4 EVB and g-C3N4 ECB were +2.58 eV and -1.13 eV, respectively. In addition, the value of the Ag3PO4 ECB and g-C3N4 EVB can be calculated by the following equation: 𝐸𝐶𝐵 = 𝐸𝑉𝐵 ―𝐸𝑔.

(2)

The pure Ag3PO4 and g-C3N4 Eg values were approximately 2.37 and 2.70 eV. Thus, the ECB value of Ag3PO4 was around +0.21 eV, and the EVB value of g-C3N4 was around +1.57 eV. The band positions of Ag3PO4 and g-C3N4 were very close to those reported previously.2,30 3.3. X-ray photoelectron spectroscopy analysis and mechanisms We performed an in-depth study of the interfacial interactions in gC3N4/Ag3PO4/PAN between the g-C3N4 and Ag3PO4 using XPS spectroscopy. The full XPS spectra of the samples are shown in Figure 3a. The characteristic peaks of C and N 1s in g-C3N4/PAN corresponded to sp2 C-C bonds (284.6 eV), sp3-coordinated carbon bonds (285.7 eV), and C-N=C (397.6 eV) from the PAN (Figure 3b and c). In addition, peaks at 289.2 and 398.1 corresponded to the N=C-N and C-N=C, respectively, from g-C3N4.41,42 The spectra of Ag 3d, P 2p, and O 1s of Ag3PO4/PAN are also clearly seen in Figure 3df. The O 1s characteristic peak at 531.3 eV corresponded to the O-H bond.30 After adding Ag3PO4, the N=C-N and C-N=C binding energies of the g-C3N4 in the resulting g-C3N4/Ag3PO4/PAN were higher than those of g-C3N4/PAN. The 13



Ag3PO4 Ag 3d, P 2p, and O 1s binding energies in g-C3N4/Ag3PO4/PAN were also shifted lower compared to those of Ag3PO4/PAN.

Figure 3 (a) Full XPS spectra. (b) C 1s and (c) N 1s core-level spectra for g-C3N4/PAN, g-C3N4/Ag3PO4/PAN-2, and Ag3PO4/PAN nanofibers. (d) Ag 3d, (e) P 2p, and (f) O 1s core-level spectra for g-C3N4/Ag3PO4/PAN-2 and Ag3PO4/PAN nanofibers.

The binding energy shifts can be explained as electrons being transferred from the gC3N4 to Ag3PO4 (dashed line in Scheme 2a).43 The g-C3N4 Fermi level was higher than the Ag3PO4 level, and the CB was much higher (Scheme 2a). When Ag3PO4 contacted g-C3N4 to form a heterojunction, electrons should have transferred from the g-C3N4 to Ag3PO4. As a result, the g-C3N4 would have a decreased electron density, and Ag3PO4 would have an increased electron density in the g-C3N4/Ag3PO4 heterojunction. Thus, 14


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the interface of the g-C3N4 and Ag3PO4 formed an internal electric field (Scheme 2b).43 The C and N 1s binding energies of g-C3N4 were increased by the positive potentials, while the Ag 3d, P 2p, and O 1s binding energies of Ag3PO4 were decreased by the negative potential.

Scheme 2 (a) Ag3PO4 and g-C3N4 electronic band structures prior to contacting each other. (b) Schematic for Ag3PO4 and g-C3N4 band bending after contact to form a heterojunction. (c) Photoelectronhole transmission path under visible light.

The internal electric field can also cause a positive shift in the Ef of g-C3N4 and a negative shift in the Ef of Ag3PO4.43 These Ef shifts can lead to positive and negative energy band bending for g-C3N4 and Ag3PO4, respectively (Scheme 2b).43 The electrons in the Ag3PO4 CB would easily transfer to the heterointerface, and the holes in the Ag3PO4 VB would easily transfer to the surface due to the existence of an internal electric field. In addition, the electrons in the CB of g-C3N4 would easily transfer to the surface, and the photogenerated holes in the VB of g-C3N4 would easily transfer to the heterointerface. There was fast recombination between the g-C3N4 valence band holes 15



and Ag3PO4 conduction band electrons at the heterointerface (Scheme 2c). Thus, this analysis indicated that the g-C3N4/Ag3PO4 heterojunction was a direct Z-scheme heterojunction. 3.4. Photocurrent and photoluminescence To determine the g-C3N4/Ag3PO4/PAN-2 photogenerated charge excitation and transfer, we measured the photocurrent spectra along with the steady- and transientstate photoluminescence (PL) spectra. Figure 4a shows the photocurrent spectra for gC3N4/Ag3PO4/PAN-2, Ag3PO4/PAN, and g-C3N4/PAN. As expected, the highest photocurrent intensity among the three was found for the g-C3N4/Ag3PO4/PAN-2 nanofibers. The greater g-C3N4/Ag3PO4/PAN-2 photocurrent indicated that the photoelectronhole separation was effective, perhaps due to the g-C3N4/Ag3PO4 heterojunction formation.36 As shown in Figure 4b, g-C3N4/PAN produced a strong emission peak at ~450 nm by a 337 nm excitation. The g-C3N4/Ag3PO4/PAN-2 produced a much lower emission peak intensity compared to g-C3N4/PAN, indicating that the g-C3N4/Ag3PO4 heterojunction strongly inhibited the radiative recombination.16,36

Figure 4 (a) g-C3N4/PAN, g-C3N4/Ag3PO4/PAN-2, and Ag3PO4/PAN photocurrents 16


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under visible light (400 nm < λ < 800 nm). (b) Steady-state PL and (c) time-resolved transient PL decay spectra of g-C3N4/PAN and g-C3N4/Ag3PO4/PAN-2 nanofibers.

We performed time-resolved transient PL spectroscopy using a 337 nm excitation wavelength, and the emission decay data fit a double-exponential formula with two lifetime components (Figure 4c). The g-C3N4/PAN average fluorescent lifetime increased from 2.12 to 3.87 ns after Ag3PO4 was deposited, calculated as follows: 𝐴1𝜏21 + 𝐴2𝜏22

< τ >= 𝐴1𝜏1 + 𝐴2𝜏2.

(3)

This test proved that there was good photogenerated charge transfer in this heterojunction, which enhanced the photocatalytic activity. 3.5. Photocatalytic degradation performance We evaluated g-C3N4/Ag3PO4/PAN nanofiber photocatalytic activities by observing the Rhodamine B (RhB) degradation under visible light (400 nm < λ < 800 nm). The compositions of the Ag3PO4 nanoparticles and g-C3N4/PAN nanofibers in the physical mixture sample are discussed in the Supporting Information. The preparation method and detailed characterization of g-C3N4/Ag3PO4 nanoparticles (NPS) are shown in the Supporting Information. Figure S9a shows that all of the samples reached adsorption desorption equilibrium for 30 min in the dark. Additionally, self-degradation of the RhB under visible light was negligible (Figure 5a). After a 120-min reaction, the RhB photodegradation efficiencies were 37%, 67%, 45%, and 95% for the g-C3N4/PAN, Ag3PO4/PAN, physical mixture nanofibers, and g-C3N4/Ag3PO4 nanoparticles (NPS), 17



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respectively. However, the RhB degradation efficiencies were 92%, 96%, and 95% after a 120-min visible light reaction for the g-C3N4/Ag3PO4/PAN-1, gC3N4/Ag3PO4/PAN-2,

and

g-C3N4/Ag3PO4/PAN-3,

respectively.

The

g-

C3N4/Ag3PO4/PAN exhibited a far better photocatalytic efficiency than the gC3N4/PAN or Ag3PO4/PAN.

Figure 5 (a) Degradation and (b) kinetic linear simulation curves of RhB with different photocatalysts (400 nm < λ < 800 nm). (c) TOC removal of RhB by different photocatalysts after the reaction (120 min). (d) Degradation and (e) kinetic linear simulation curves of TC with different photocatalysts (400 nm < λ < 800 nm). (f) 18


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Degradation rates of RhB and TC with different photocatalysts. S1: g-C3N4/PAN, S2: physical

mixture,

S3:

Ag3PO4/PAN,

S4:

g-C3N4/Ag3PO4

NPS,

S5:

g-

C3N4/Ag3PO4/PAN-1, S6: g-C3N4/Ag3PO4/PAN-2, and S7: g-C3N4/Ag3PO4/PAN-3.

We further compared the photocatalysis via kinetic analysis of the degradation reaction. The reaction followed a Langmuir-Hinshelwood apparent first-order kinetic model is described as follows:43 r = 𝑑𝐶 𝑑𝑡 = 𝑘𝐾𝑐 (1 + 𝐾𝐶),

(4)

where r is the degradation rate of RhB (mg·L-1·min-1), C is the RhB concentration (mg·L-1), t is the reaction time (min), k is the reaction rate constant (mg·L-1·min-1), and K is the RhB absorption coefficient (L·mg-1). With a low initial RhB concentration (10 mg·L-1), this can be simplified as follows:43 In𝐶 𝐶0 = ―𝑘𝐾𝑡 = ― 𝑘𝑎𝑝𝑝𝑡,

(5)

where kapp is the apparent first-order rate constant (min−1). We determined the kapp values for the RhB degradation as 0.0030, 0.0085, 0.0044, 0.0259, 0.0206, 0.0263, and 0.0245 min-1 for the g-C3N4/PAN, Ag3PO4/PAN, physical mixture, g-C3N4/Ag3PO4 NPS, g-C3N4/Ag3PO4/PAN-1, g-C3N4/Ag3PO4/PAN-2, and g-C3N4/Ag3PO4/PAN-3, respectively (Figure 5b and f). The kapp of the g-C3N4/Ag3PO4/PAN-2 was larger than those of the g-C3N4/Ag3PO4/PAN-1 and g-C3N4/Ag3PO4/PAN-3, and the gC3N4/Ag3PO4/PAN-2 degradation rate was remarkably 8.8 and 3.1 times higher than those of the g-C3N4/PAN and Ag3PO4/PAN, respectively. 19



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The amount of carbon in an organic compound can be determined via the total organic carbon (TOC) analysis, which can track the degradation of dye molecules during photocatalysis.44,45 The TOC removal indicated that a 120-min photocatalytic reaction mineralized RhB by 35.51%, 61.52%, 44.14%, 93.08%, 88.62%, 95.39%, and 88.70% for the g-C3N4/PAN, Ag3PO4/PAN, physical mixture, g-C3N4/Ag3PO4 NPS, gC3N4/Ag3PO4/PAN-1,


and


respectively (Figure 5c). The g-C3N4/Ag3PO4/PAN-2 was the best at mineralizing the dye. Thus, the g-C3N4/Ag3PO4/PAN nanofibers are potentially superior catalysts for mineralizing organic dyes into inorganic substances, such as water and carbon dioxide. To exclude the potential for dye-sensitization, we used the samples to degrade the colorless pollutant tetracycline hydrochloride (TC) with visible light (400 nm < λ < 800 nm). As illustrated in Figure S9b, adsorption/desorption equilibrium were attained by all the samples after 30 min in the dark. TC removal of the g-C3N4/Ag3PO4/PAN-2 was 94% with 8 min of visible light irradiation. The values were 22%, 45%, 35%, 92%, 79%, and 87% for the g-C3N4/PAN, Ag3PO4/PAN, physical mixture, g-C3N4/Ag3PO4 NPS, g-C3N4/Ag3PO4/PAN-1, and g-C3N4/Ag3PO4/PAN-3, respectively (Figure 5d). The kapp values for TC degradation were 0.0278, 0.0554, 0.0469, 0.2792, 0.1585, 0.3593, and 0.2081 min-1 for the g-C3N4/PAN, Ag3PO4/PAN, physical mixture, gC3N4/Ag3PO4

NPS,

C3N4/Ag3PO4/PAN-3,

g-C3N4/Ag3PO4/PAN-1, respectively

(Figure


and

g-

5e–f).

the

g-

Interestingly,

C3N4/Ag3PO4/PAN-2 photocatalytic activity was again higher than those of the g20


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C3N4/Ag3PO4/PAN-1 and g-C3N4/Ag3PO4/PAN-3, similar to the RhB degradation. As shown above, the g-C3N4/Ag3PO4/PAN-2 exhibited a higher photocatalytic activity than those of the g-C3N4/Ag3PO4/PAN-1 or g-C3N4/Ag3PO4/PAN-3. The heterojunction extent could increase with increased Ag3PO4 loading contents in the gC3N4/Ag3PO4 heterojunction. Therefore, the g-C3N4/Ag3PO4/PAN-2 exhibited a higher activity than the g-C3N4/Ag3PO4/PAN-1 due to better electronhole pair separation. In addition, the SEM images of the g-C3N4/Ag3PO4/PAN-3 showed that the Ag3PO4 nanoparticles fully covered the surfaces of the g-C3N4/PAN nanoﬁbers. The transmission channel of the photogenerated electrons and holes was prolonged by the higher Ag3PO4 coating thickness, enhancing their recombination rate within the catalyst.10,46 For g-C3N4/Ag3PO4/PAN-3, separation of the electron-hole pairs might have been somewhat restrained. In addition, there were fewer g-C3N4/Ag3PO4/PAN-3 surface active sites because of the smaller specific surface area,34 also resulting in a decreased photocatalysis rate. Moreover, the g-C3N4/Ag3PO4/PAN-2 nanofibers exhibited a higher photocatalytic efficiency than the g-C3N4/Ag3PO4 NPS. This was because the g-C3N4/Ag3PO4 heterojunctions were uniformly dispersed on the surfaces of the PAN nanofibers in the g-C3N4/Ag3PO4/PAN-2, which prevented the aggregation of the g-C3N4/Ag3PO4 heterojunctions and increased the number of active sites for the reaction. These results showed that the number of heterojunctions, their surface morphologies, and the specific surface areas could influence the photocatalytic activity. 21



3.6. ESR analysis and trapping experiment We next employed an electron spin resonance (ESR) spin-trap method during irradiation to observe the active radicals and provide insight into the photoreaction mechanism. There were no ESR signals when the two samples were not illuminated (Figure 6a and b), indicating that visible light was indispensable for generating reactive species. With visible light, DMPO-•O2− signals of the Ag3PO4/PAN were weaker than those of the g-C3N4/Ag3PO4/PAN-2, suggesting that the photogenerated electrons of the Ag3PO4/PAN did not reduce O2 to •O2−, but those of g-C3N4/Ag3PO4/PAN-2 did. The DMPO-•OH ESR spectra for the Ag3PO4/PAN and g-C3N4/Ag3PO4/PAN-2 also showed four antisymmetric peak sets with intensities of 1:2:2:1, which were DMPO•OH adduct characteristic signals.47 Thus, •OH radicals were the active species of the Ag3PO4/PAN and g-C3N4/Ag3PO4/PAN-2 during photooxidation via visible light. To further explore the photocatalytic mechanism, we also performed trapping experiments with the active species. Figure 6c shows that the photogenerated holes, •O2− and •OH took part in the photocatalytic reaction of the g-C3N4/Ag3PO4/PAN-2. However, only photogenerated holes and •OH took part in the photocatalytic reaction of the Ag3PO4/PAN (Figure 6d). The details of the trapping experiments are described in the Supporting Information.

22


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Figure 6 ESR spectra of (a) DMPO-•O2− and (b) DMPO-•OH with Ag3PO4/PAN and g-C3N4/Ag3PO4/PAN-2 before and after visible light irradiation. Active species were subject to trapping experiments during the degradation of RhB with (c) gC3N4/Ag3PO4/PAN-2 and (d) Ag3PO4/PAN.

The Ag3PO4 had a lower CB than the g-C3N4, but its VB was higher (Scheme 2a). Given that the Ag3PO4 CB potential (+0.21 eV vs. NHE) was higher than the O2/•O2− standard reduction potential (+0.13 eV vs. NHE),31 the electrons generated in the Ag3PO4 CB were unable to reduce O2 to •O2−. In addition, the trapping results and ESR demonstrated that more •O2− radicals were generated by the g-C3N4/Ag3PO4/PAN photocatalysis than by that of the Ag3PO4/PAN. This suggested that the participating 23



photoelectrons were derived from the g-C3N4 during the g-C3N4/Ag3PO4/PAN photocatalysis reaction. It also indicated that the g-C3N4/Ag3PO4 heterojunction mechanism was a direct Z-scheme charge transfer rather than a traditional band-band transfer under visible light. 3.7. Photocatalytic oxygen evolution performance These nanofibers exhibited not only good degradation performances but also good O2 evolution performances. As shown in Figure 7a, the g-C3N4/Ag3PO4/PAN nanofibers exhibited a good photocatalytic O2 evolution performance. The evolution of photocatalytic O2 from the g-C3N4/PAN, Ag3PO4/PAN, g-C3N4/Ag3PO4/PAN-2, physical mixture, and g-C3N4/Ag3PO4 NPS was measured using artificial sunlight. The g-C3N4/Ag3PO4/PAN-2 nanofibers clearly exhibited the best photocatalytic O2 evolution. The reaction rates of the g-C3N4/PAN, Ag3PO4/PAN, physical mixture, and g-C3N4/Ag3PO4 NPS were 1490, 480, 1020, and 2208 μmol g−1 h−1, respectively (Figure 7b). However, the reaction rate of the g-C3N4/Ag3PO4/PAN-2, was 2776 μmol g−1 h−1, roughly 1.9, 5.8, 2.7, and 1.3 times those of the g-C3N4/PAN, Ag3PO4/PAN, physical mixture, and g-C3N4/Ag3PO4 NPS, respectively. We believe that the increased O2 evolution was due to the effective photoelectron–hole separation at the interfaces of the Z-scheme heterojunctions between the Ag3PO4 and g-C3N4 (Figure S10). Table S2 shows the photocatalytic O2 evolution experimental conditions and O2 evolution rate compared to some different photocatalytic materials from previous reports. Compared with other oxygen evolution materials, the g-C3N4/Ag3PO4/PAN nanofibers exhibited 24


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good oxygen evolution performances and are promising photocatalysts for oxygen evolution.

Figure 7 Photocatalytic O2 evolution (a) amounts and (b) rates with the g-C3N4/PAN, Ag3PO4/PAN, g-C3N4/Ag3PO4/PAN-2, physical mixture, and g-C3N4/Ag3PO4 NPS under artificial sunlight.

3.8. Stability properties and reusable properties Catalyst stability and separability are extremely important for the practical use of photocatalysts. The photochemical stabilities of the g-C3N4/Ag3PO4/PAN-2 and Ag3PO4/PAN nanofibers were investigated via the cycling photocatalytic degradation of RhB experiments (Figure 8a). The g-C3N4/Ag3PO4/PAN-2 exhibited a higher photochemical stability compared to the Ag3PO4/PAN at 400 nm < λ < 800 nm. This was because the photoelectrons in the Ag3PO4 CB of the g-C3N4/Ag3PO4/PAN-2 could transfer to g-C3N4, which prevented the photocorrosion of Ag3PO4. However, the photoelectrons in the Ag3PO4 CB of the Ag3PO4/PAN could not transfer, which caused the photocorrosion of Ag3PO4. Moreover, the g-C3N4/Ag3PO4/PAN-2 stability worsened when the irradiation wavelength was 450 nm < λ < 800 nm. This was because 25



wavelengths in this range could not excite the g-C3N4 to generate photoelectron-hole pairs. The g-C3N4 VB possessed no photogenerated holes that could recombine with the electrons in the Ag3PO4 CB, which may have increased the Ag3PO4 photocorrosion. To prove that the stability of the g-C3N4/Ag3PO4/PAN-2 was derived from the formation of g-C3N4/Ag3PO4 Z-scheme heterojunctions, the photocurrent spectra were tested using excitation wavelengths of 450 nm < λ < 800 nm (Figure 8b). The photocurrent spectrum of the g-C3N4/Ag3PO4/PAN-2 under 450 nm < λ < 800 nm exhibited a weak downward trend compared to the g-C3N4/Ag3PO4/PAN-2 under 400 nm < λ < 800 nm (Figure 4a). This downward trend of the g-C3N4/Ag3PO4/PAN-2 was similar to that of the Ag3PO4/PAN (Figure 8b), demonstrating the poor photochemical stability of the g-C3N4/Ag3PO4/PAN-2. This result was consistent with the above photocatalytic cycling experiments. Moreover, the XRD and XPS Ag 3d core-level spectra for the g-C3N4/Ag3PO4/PAN2 and Ag3PO4/PAN nanofibers after 5 cycles of degradation of RhB are shown in Figure S11. As shown in Figure S11a, a new XRD diffraction peak at 38.1° was detected after the photocatalysis, corresponding to the (1 1 1) phase of metallic silver (JCPDS NO. 65-2871). This indicated that Ag+ may have been reduced into metallic Ag during the photocatalysis. The peak intensity of Ag in the Ag3PO4/PAN was stronger than that of the g-C3N4/Ag3PO4/PAN-2 after 5 cycles, indicating the more metallic Ag was obtained by reduction in the Ag3PO4/PAN. In addition, the XPS spectra showed that the peak area ratio of the Ag0/Ag+ of Ag3PO4/PAN was larger than that of the g26


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C3N4/Ag3PO4/PAN-2 (Figure S11b). This indicated that more Ag+ in the Ag3PO4/PAN was converted to Ag0 compared with the g-C3N4/Ag3PO4/PAN-2 after 5 cycles. The XRD and XPS results proved that the g-C3N4/Ag3PO4/PAN-2 had a better photochemical stability than the Ag3PO4/PAN. Additionally, the TC degradation and O2 evolution performance of the gC3N4/Ag3PO4/PAN did not decrease significantly after 5 cycling experiments (Figure 8c and d), indicating the good stability of the g-C3N4/Ag3PO4/PAN-2.

Figure 8 (a) RhB degradation by g-C3N4/Ag3PO4/PAN-2 and Ag3PO4/PAN under 27



visible light (400 nm < λ < 800 nm) and g-C3N4/Ag3PO4/PAN-2 under visible light (450 nm < λ < 800 nm) for five cycles. (b) g-C3N4/Ag3PO4/PAN-2 and Ag3PO4/PAN photocurrent spectra under visible light (450 nm < λ < 800 nm). (c) Degradation cycling test for TC by g-C3N4/Ag3PO4/PAN-2 under visible light (400 nm < λ < 800 nm). (d) Photocatalytic O2 evolution cycling test for g-C3N4/Ag3PO4/PAN-2 under artificial sunlight. (e) Separation demonstration of g-C3N4/Ag3PO4/PAN-2 nanofibers and gC3N4/Ag3PO4 nanoparticles after TC degradation.

As shown in Figure 8e, the g-C3N4/Ag3PO4/PAN nanofibers, with a nanofibrous matlike structure, could be easily separated from the solutions after photocatalysis. However, it required 20 min for the g-C3N4/Ag3PO4 nanoparticles to be separated by sedimentation. Using PAN as a support of the g-C3N4/Ag3PO4 heterojunction had the following two advantages. (1) The g-C3N4/Ag3PO4 heterojunction could uniformly disperse on the surface of the PAN nanofibers without aggregation, which was beneficial for enhancing the photocatalyst activity. (2) Due to the nanofibrous mat-like structure of the PAN, the g-C3N4/Ag3PO4/PAN nanofibers could be easily removed and recycled for the next use, thereby avoiding problems with sedimentation. This flexible g-C3N4/Ag3PO4/PAN photocatalyst could extend the range of applications beyond powder photocatalysts, including wearable photocatalyst devices.

4. CONCLUSION 28


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Ag3PO4 nanoparticles were evenly fabricated on flexible electrospun g-C3N4/PAN nanofibers using a room-temperature in situ synthesis method. As comparison to pure g-C3N4/PAN or Ag3PO4/PAN, the resulting g-C3N4/Ag3PO4/PAN nanofibers showed higher photocatalytic degradation performance against RhB and TC, and exhibited better O2 evolution performance. The improved photocatalytic performance was likely owing to effective photoelectronhole separation at the interfaces of the Z-scheme heterojunctions between Ag3PO4 and g-C3N4. The g-C3N4/Ag3PO4/PAN nanofibers also showed enhanced photochemical stability versus Ag3PO4/PAN, which is because of the inhibition of Ag3PO4 photocorrosion by transferring photoelectron from Ag3PO4 to g-C3N4. Moreover, these photocatalysts can be easily separated and reused due to their extralong one-dimensional mats structures and flexible properties. The present research provided a new way to design and fabricate self-supporting and flexible photocatalysts for practical applications.

ASSOCIATED CONTENT Supporting Information Characterization; fabrication of g-C3N4/Ag3PO4 nanoparticles; photocatalytic test, electrochemical measurements; discussion of the BET nitrogen adsorption/desorption isotherms,

TG

curves,

trapping

experiments;

component

contents,

g-

C3N4:Ag3PO4:PAN weight ratios calculated as Rc and BET specific surface areas of the samples (Table S1); O2 evolution rate compared to some previously reported 29



photocatalytic materials (Table S2); SEM image, XRD pattern, FTIR spectrum, UVVis diffuse reflection spectrum, XPS spectra of g-C3N4 (Figure S1); SEM images and XRD pattern of the g-C3N4/Ag3PO4 nanoparticles (Figure S2); SEM images of the PAN, g-C3N4/NaH2PO4/PAN and Ag3PO4/PAN nanofibers (Figure S3); EDX spectra of the g-C3N4/PAN and g-C3N4/Ag3PO4/PAN-1 nanofibers (Figure S4); XRD patterns of the g-C3N4/PAN, g-C3N4/Ag3PO4/PAN, and Ag3PO4/PAN nanofibers (Figure S5); FTIR spectra of different photocatalysts (Figure S6); BET nitrogen adsorption/desorption isotherms of different photocatalysts (Figure S7); TG curves of the g-C3N4/PAN, gC3N4/Ag3PO4/PAN, and Ag3PO4/PAN nanofibers (Figure S8); degradation profiles of the different photocatalysts in the dark (Figure S9); photocatalytic O2 evolution mechanism (Figure S10); XRD and XPS spectra of the g-C3N4/Ag3PO4/PAN-2 and Ag3PO4/PAN nanofibers after 5 cycles of degradation (Figure S11).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Changlu Shao). Fax: +86 43185098803. Tel: +86 431 85098803. *E-mail: [email protected] (Xinghua Li). Fax: +86 43185098803. Tel: +86 431 85098803. ORCID Changlu Shao: 0000-0002-5024-3268 30


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Xinghua Li: 0000-0002-9685-8727 Xiaowei Li: 0000-0002-9719-1629 Ran Tao: 0000-0001-7127-2116 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Nos. 51572045, 51732003, 61803080, and 91233204), the 111 Project (No. B13013), the China Postdoctoral Science Foundation (Nos. 2017M610188 and 2018T110240), the Science and Technology Development Program of Jilin Province (20180520192JH), the "13th five-year plan" science and technology research project of the education department of Jilin Province (No. JJKH20180018KJ).

31



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Table of Contents

41


Polyacrylonitrile

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