nanosheets on electrospun p-CuAl

Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 10714−10723

Assembling n‑Bi2MoO6 Nanosheets on Electrospun p‑CuAl2O4 Hollow Nanofibers: Enhanced Photocatalytic Activity Based on Highly Efficient Charge Separation and Transfer Jian Zhang, Changlu Shao,* Xinghua Li,* Jiayu Xin, Ran Tao, and Yichun Liu Center for Advanced Optoelectronic Functional Materials Research, Key Laboratory of UV-Emitting Materials and Technology (Northeast Normal University), Ministry of Education, 5268 Renmin Street, Changchun 130024, People’s Republic of China

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S Supporting Information *

ABSTRACT: Rational construction of heterostructures (especially p-n heterostructures) is an excellent strategy for efficient charge separation and desirable photocatalytic performance. Regretfully, the measurements of charge separation efficiency of p-n heterojunction are often monotonous. In this paper, a well-designed p-CuAl2O4/n-Bi2MoO6 heterojunction was successfully synthesized by assembling Bi2MoO6 nanosheets (NSs) on the electrospun CuAl2O4 hollow nanofibers (HNFs) through a solvothermal method. The Bi2MoO6 NSs tightly connect with CuAl2O4 HNFs via strong chemical bonding, and the loading amount can be easily controlled by the concentration of precursor. Significantly, for the first time the charge separation efficiency of the heterojunction is systematically investigated via its photoelectrochemical responses under visible light irradiation with and without trapping agents, and the highly efficient charge separation and transfer of the heterojunction is tightly confirmed. Photocatalytic experiments show that the highly efficient charge separation and transfer of the heterojunction results in the enhanced photocatalytic performance for degrading all pollutant models (Rhodamine B, Methyl Orange, Cr(VI), and 4-nitrophenol), whose reaction rate is 1 order of magnitude higher than the reference samples. The work may be useful for rational constructing p-n heterojunctions and provide novel insights to investigate the photoelectrochemical and photocatalytic performance. KEYWORDS: p-n heterostructure, Electrospinning, Charge separation, Photocatalysis, Recycling stability

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INTRODUCTION

In recent years, Aurivillius oxide semiconductors with general formula Bi2Xn−1YnO3n+3 (X = Ca, Sr, Ba, Pb, Na and Y = Ti, Nb, Ta, Mo, W) have attracted extensive interest owing to their layered structures and outstanding photoelectrical properties.7,8 As the simplest member (n = 1), bismuth molybdate (Bi2MoO6) consists of alternately ranged [Bi2O2]2+ and perovskite slabs (MoO42−) layers,9,10 which endow it suitable band gap (∼2.85 eV) for visible light absorption and controllable morphology.4 More attractively, Bi2MoO6 has been proven to be a valuable n-type catalyst in photodegradation,11,12 water oxidation,13,14 and other photoelectrochemical fields.15,16 To reduce the charge recombination and improve the quantum yield, Cu-based semiconductors have been chosen as p-type materials to combine with Bi2MoO6 forming a novel p-n heterojunction due to their special physicochemical properties.17−20 Among them, copper aluminum (CuAl2O4), as a promising semiconductor with spinel structure, has been proven to be a valuable visible light

Solar energy conversion for degradation of organic pollutants in wastewater with an efficient and eco-friendly photocatalyst has been considered as a promising technique for environmental remediation for a long time.1−4 How to find an excellent photocatalyst with good photocatalytic performance has become a very important issue. A desirable photocatalyst should have the vital properties of wide optical absorption range (i), highly efficient charge separation (ii), enough redox power (iii), and long-term stability (iv). It is, however, very difficult for individual material content to meet the comprehensive requirements. A possible solution is constructing heterostructures, especially p-n heterostructures, which will generate an interfacial electric field and restrain the mutual transfer of photoexcited electron−hole pairs to decrease the recombination rate,5,6 leading to a highly efficient charge separation and transfer. Besides, constructing heterojunctions with visible light driven (VLD) materials may broaden the optical absorption range (meet (i) to generate more charge carriers for better activity. Therefore, it is highly desirable to develop novel p-n heterostructure VLD photocatalysts. © 2018 American Chemical Society

Received: May 4, 2018 Revised: May 29, 2018 Published: June 19, 2018 10714

DOI: 10.1021/acssuschemeng.8b02040 ACS Sustainable Chem. Eng. 2018, 6, 10714−10723

Research Article

ACS Sustainable Chemistry & Engineering

pure Bi2MoO6 was synthesized by the same method except for the absence of CuAl2O4 HNFs. Detailed experimental information is listed in Table S1. More detailed characterizations of the samples can be found in the Supporting Information.

photocatalyst because of its appropriate band gap (1.7−2.5 eV),21−23 mechanical strength,24 physicochemical stability,25−27 resistance to acid or alkali,28 and low cost. Importantly, the band gap potentials of CuAl2O4 and Bi2MoO6 are perfectly matched to form p-n heterojunction facilitating the charge separation (meet (ii)) and suitable for degrading most organic pollutants (meet (iii)). On the other hand, material size is another important factor to affect its photocatalytic performance. Nanostructured materials, especially the nanoparticles with much smaller size, will exhibit higher photocatalytic activity due to their larger specific surface area and shorter migration distance for charge transfer.29 However, although nanoparticles with large surface area exhibit good photocatalytic performance, they usually suspend in the water after reactions and are hard to be separated,30,31 which may limit their practical applications. Thus, constructing nanostructured materials with both desirable photocatalytic activity and good separable property is essential. Based on the above considerations, in this paper, we synthesize a 3D hierarchical CuAl2O4/Bi2MoO6 p-n heterojunction by assembling n-Bi2MoO6 nanosheets (NSs) on the electrospun p-CuAl2O4 hollow nanofibers (HNFs) through a solvothermal method. The as prepared heterojunction has an overlong 1D structure and a macroscopically 3D nonwoven web framework, which may provide a large surface area for better photocatalytic performance and will be easily separated by sedimentation and filtration at the same time (meet (iv)). Significantly, for the first time the photoelectrochemical responses of the heterojunction are contrastively detected with and without trapping agents to investigate the charge separation efficiency. The photocatalytic performance and long-term stability of the heterojunction are systematically and quantificationally investigated by degrading various pollutant models (Rhodamine B, Methyl Orange, Cr(VI), and 4nitrophenol), and a possible photocatalytic mechanism is also proposed.

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RESULTS AND DISCUSSION Crystallographic Structure. XRD patterns were recorded to precisely identify the crystallographic structure of the samples. As displayed in Figure 1, the diffraction peaks at

Figure 1. XRD patterns of the samples: CuAl2O4 HNFs (a), CB1 (b), CB2 (c), CB3 (d), and pure Bi2MoO6 nanoflowers (e).

31.3°, 36.87°, 44.86°, 55.7°, 59.42°, 65.29°, and 77.47° in Figure 1a and 28.31°, 32.53°, 46.74°, 55.44°, and 58.48° in Figure 1e can be perfectly indexed as (220), (311), (400), (422), (511), (440), and (533) planes of spinel-type CuAl2O4 (JCPDS No. 33-0448) and (131), (200), (202), (331), and (262) planes of orthorhombic Bi2MoO6 (JCPDS No. 210102), respectively. From Figure 1b, c, and d it can be seen that the characteristic peaks of spinel-type CuAl2O4 and orthorhombic Bi2MoO6 are all present. However, with the concentration of precursor increasing, the characteristic peaks of Bi2MoO6 become intense and sharp, confirming the existence of the hybrid phase in CB1, CB2, and CB3 and the increasing loading amount of the Bi2MoO6 phase. Morphologies and Loading Amount of the Samples. The morphologies of the samples were detected by SEM. From Figure S1A, it can be observed that CuAl2O4 nanofibers with diameters of 150−200 nm present hollow structure (from the inset and Figure S1B) with nonwoven web 3D framework, which may provide large specific surface area for better photocatalytic performance and be easy to separate and recycle for practical applications. In Figure 2A, Bi2MoO6 nanoparticles are grown on the surface of CuAl2O4 HNFs with the size of 15−20 nm, confirming the existence of active sites for growing Bi2MoO6 outside the CuAl2O4 HNFs. When the concentration of the precursor doubled, Bi2MoO6 nanoparticles directionally grow and become nanosheets (Figure 2C). As a contrast, SEM images of pure Bi2MoO6 are displayed in Figure S1C; it is obvious that without CuAl2O4 HNFs as substrate, pure Bi2MoO6 NSs aggregate by self-assembling and become nanoflowers. Thus, it can be inferred that the CuAl2O4 HNFs can not only build heterojunctions with Bi2MoO6 but also act as a support to grow highly dispersive Bi2MoO6 NSs to achieve heterogeneous structural fibrosis. However, with the concentration of the precursor tripled, CuAl2O4 HNFs is fully covered (Figure 2E) and the diameter of the heterojunction nanofibers dramatically increases. Interestingly, from the insets it can be seen that the thickness and morphology of Bi2MoO6 NSs do not present much difference except for the loading

EXPERIMENTAL SECTION

Synthesis of CuAl2O4 Hollow Nanofibers. The process for preparing CuAl2O4 HNFs is similar to our previous work.28 0.52 g of Cu(NO3)2·3H2O and 1.774 g of Al(NO3)3·9H2O were dissolved in 20 mL of DMF under magnetic stirring. Then 2.5 g of PVP was added into the above solution, followed by continuous stirring for 12 h at room temperature to form transparent precursor solution. The precursor solution was transferred into a syringe to be electrospun by TEADFS-101 (Tech Nova). Aluminum foil was used as a collector with a distance of 10−15 cm to the syringe needle and the voltage was set at about 10 kV. After that, the dense web of electrospun nanofiber was collected and calcined at 800 °C for 4 h with the heating rate of 5 °C/min to obtain CuAl2O4 HNFs. Fabrication of CuAl2O4/Bi2MoO6 Hierarchical p-n Heterojunction. Hierarchical p-CuAl2O4/n-Bi2MoO6 heterojunctions were fabricated via solvothermal reactions. In a typical procedure, 315.75 mg of Bi(NO3)3·5H2O and 78.75 mg of Na2MoO4 were dissolved in the mixture of 10 mL ethylene glycol and 30 mL of ethanol under magnetic stirring. Fifteen milligrams of CuAl2O4 HNFs was dispersed in the above solution with stirring for another 2 h. Then the suspension was transferred into a 50 mL Teflon-lined stainless steel autoclave and heated at 160 °C for 24 h. After cooling down to room temperature, the sample was washed with deionized water and ethanol several times to remove residual ions and dried at 80 °C overnight. The sample was denoted as CB1. By doubling and tripling the concentration of Bi(NO3)3·5H2O and Na2MoO4, the obtained samples were denoted as CB2 and CB3, respectively. Furthermore, 10715


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amount, implying the chemical stability of Bi2MoO6. The loading amount of Bi2MoO6 was calculated by EDX. In Figure 2B, D and F, it is clear that the atomic ratio (At %) of Bi/Cu is increasing, confirming the loading amount of Bi2MoO6 with the growing concentration of precursor. To make further observation of the CuAl2O4/Bi2MoO6 heterostructure, TEM was also employed. As shown in Figure 3A, a large number of Bi2MoO6 NSs were uniformly lying flat and vertically on the surface of CuAl2O4 HNFs. From the inset, the HRTEM image shows the distance of 0.465 and 0.318 nm, which correspond to the (1 1 1) plane of CuAl2O4 and the (1 3 1) plane of Bi2MoO6, respectively. In addition, the elemental mappings (Figure 3B−F) from the dotted box in Figure 3A identify the spatial distributions of Cu, Al, Bi, Mo, and O in the hierarchical heterostructure. In the center of the nanostructure, Cu and Al signals are strongly detected, while Bi and Mo are not clear. On the contrary, Bi and Mo present much out of the nanostructure, but Cu and Al are barely observed, suggesting the CuAl2O4 HNFs are coated by Bi2MoO6 NSs. More TEM images of the other samples can be found in Figure S1D−F. SEM and TEM results reveal that the growth of Bi2MoO6 will not destroy the nanofibrous framework and the 3D hierarchical p-CuAl2O4/n-Bi2MoO6 heterostructure is formed. XPS Analysis. Detailed information on chemical states was studied by XPS. Figure 4A shows the fully scanned spectrum of CB2, which consists of C, Cu, Al, Bi, Mo, and O elements. The C element at about 284.6 eV is owing to extraneous Ccontained impurities, which can be used as the reference to calibrate. The other elements belong to CuAl2O4 and Bi2MoO6, and no other peaks are detected, which perfectly

Figure 2. SEM images of CB1 (A), CB2 (C), and CB3 (E) (Insets are highly magnified vision). EDX spectra of CB1 (B), CB2 (D), and CB3 (E).

Figure 3. TEM image of CB2 (A) (the inset is the corresponding HRTEM image of CB2 at the interface region). Elemental mappings from the region noted by the dotted box in (A): Bi (B), Mo (C), Cu (D), Al (E), O (F). 10716


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232.4 ± 0.02 eV and 235.5 ± 0.02 eV are ascribed to Mo 3d5/2 and Mo 3d3/2 of Bi2MoO6, respectively.34 However, Mo peaks of CB2 shift about 0.28 ± 0.02 eV toward higher binding energy. On the contrary, in Figure 4D and E, the Cu 2p and Al 2p peaks of CuAl2O4 locating at 934.8 ± 0.02 eV, 954.6 ± 0.02 eV, and 74.6 ± 0.02 eV belong to Cu 2p3/2, Cu 2p1/2, and Al 2p28 in CuAl2O4, respectively. Nevertheless, the peaks of Cu 2p and Al 2p in CB2 move 0.8 ± 0.02 eV and 0.7 ± 0.02 eV to lower binding energy. According to the literature,35−37 shifts of binding energy in XPS spectra are ascribed to the strong interaction (electron transfer) between two or more nanoscale semiconductors with different Fermi energy levels. When ntype Bi2MoO6 bond with p-type CuAl2O4, electrons transfer from Bi2MoO6 to CuAl2O4, adjusting their Fermi energy levels in the same value. As a result, the electron concentration of Bi2MoO6 decreases, while the electron concentration of CuAl2O4 increases. The decrease of electron concentration can restrain the electron screening effect, resulting in the enhancement of binding energy. Therefore, with the charge transfer in the CuAl2O4/Bi2MoO6 heterojunction, the peaks of Bi 4f and Mo 3d shift toward higher binding energy, while the peaks of Cu 2p and Al 2p move to lower binding energy. XPS results indicate that the interfaces of the heterostructure are well connected by strong chemical bonding, which may provide effective channels for charge transfer. Optical Absorption and Band Gap Positions. Optical absorption of the samples was detected by UV−vis diffuse reflection spectra (DRS), which converted from the corresponding absorption spectra by means of the Kubelka−Munk equation:38,39

Figure 4. XPS spectra of full scanned (A), Bi 4f core-level (B), Mo 3d core-level (C), Cu 2p core-level (D), Al 2p core-level (E).

match the EDX results above. Subsequently, the Bi 4f corelevel spectra of CB2 and pure Bi2MoO6 are shown in Figure 4B. Obviously, pure Bi2MoO6 presents two symmetric peaks at 159.1 ± 0.02 eV and 164.4 ± 0.02 eV, which belong to Bi 4f7/2 and Bi 4f5/2 of Bi2MoO6, respectively.32,33 However, the Bi peaks of CB2 shift about 0.3 ± 0.02 eV toward higher binding energy due to the reduction of electron cloud density. It is noteworthy that the spin−orbit splittings between Bi 4f7/2 and Bi 4f5/2 for both pure Bi2MoO6 and CB2 are about 5.3 ± 0.01 eV, implying that the valence of Bi is +3 without changing. Similarly, two peaks of pure Bi2MoO6 in Figure 4C locating at

F(R ) = (1 − R )2 /2R = α /S

(1)

R = R sample/RBaSO4

(2)

where R, α, and S are reflectance, absorption, and scattering coefficient, respectively. As shown in Figure S2A, pure Bi2MoO6 has an intrinsic absorption edge at 443 nm. While interestingly, the CuAl2O4 HNFs has two absorption edges at

Figure 5. Transient photocurrent responses of CB2, CuAl2O4 HNFs, Bi2MoO6 NSs, and their mechanical mixture (A). Diagram of the parameters for calculation of transient kinetic curves (B). Anodic transient kinetic curves under visible light with 80 mW cm−2 (C). Open circuit voltage of CB2, CuAl2O4 HNFs, Bi2MoO6 NSs and their mechanical mixture (D). Ideal OCVD waveform and the formula of the response time (E) (ΔV is the voltage drop across the diode after removing the light resource, which results from the internal series resistance of the heterojunction). dv(t)/ d(t) versus time, which converted from the open circuit voltage curve of CB2 (F). 10717


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Figure 6. Transient photocurrent response of CB2, CuAl2O4 HNFs, Bi2MoO6 NSs, and their mechanical mixture with and without ammonium oxalate (A) and Cr(VI) (B) under visible light with 80 mW cm−2 (The current intensity of the samples after adding AO/Cr(VI) was doubled to compensate the intensity loss due to the decreased carrier concentration). Recombination ratio (C) and separation efficiency (D) of charge carriers in CB2, CuAl2O4 HNFs, Bi2MoO6 NSs, and their mechanical mixture.

564 and 365 nm, which results from different optical transitions (different bottoms of conduction band and tops of valence band). From the curves of CB1, CB2, and CB3, it can be clearly seen that the optical absorption is broadened at 450−600 nm compared to Bi2MoO6 NSs. In addition, the band gaps of the CuAl2O4 HNFs and Bi2MoO6 NSs are calculated via the formula:

D=

It − Ist Iin − Ist

(5)

where It, Ist, and Iin are transient-state, steady-state, and initial photocurrent, respectively (Figure 5B). It is known that photoexcited charge carriers usually follow a first order kinetic behavior during the photoresponse process, which means

( ) (τ t

αhv = A(hv − Eg )n /2

D ∼ exp − τ

D

(3)

refers to the average lifetime of charge

carriers), or in other forms: ln D ∼ −

where α, v, Eg, and A are the absorption coefficient, light frequency, band gap, and a constant, respectively. For CuAl2O4 and Bi2MoO6, n = 1 (direct transition semiconductors). Therefore, the band gap of as prepared CuAl2O4 HNFs and Bi2MoO6 NSs can be estimated via the plot of (αhv)2 vs photon energy hv. As displayed in Figure S2B, the band gaps of CuAl2O4 and Bi2MoO6 are calculated to be 2.2 eV/3.4 and 2.8 eV, which are close to the former reports.28,40 In addition, to investigate the band gap positions of CuAl2O4 and Bi2MoO6, XPS-VB spectra were introduced. As shown in Figure S2C and D, the intersection of the tangents reveals that the VB positions of CuAl2O4 and Bi2MoO6 are 1.5 and 2.5 eV, respectively. Then, according to the following equation: ECB = E VB − Eg

D

t . τD

Thus, the transient

time (τ) can be defined as the time when ln D = −1 in the normalized plots of ln D ∼ t (Figure 5C), which is proportional to the average lifetime of charge carriers (τD).41−43 The transient time of CB2 is estimated to be 2.8 s, which is almost 7-fold of CuAl2O4 and 5-fold of Bi2MoO6, confirming the prolonged lifetime of charge carriers induced by highly efficient charge separation. Furthermore, open circuit voltage decay (OCVD) is also employed as another measurement to determine the recombination kinetics of the samples. OCVD measurement is monitoring the subsequent decay of photovoltage (v(t)) after turning off the illumination. The response time (τn) can be calculated by the normalized equation:44−46 kT i dv(t ) yzz τn = − jjjj z e k d(t ) z{

−1

(4)

the conduction band positions (ECB) of CuAl2O4 and Bi2MoO6 are −0.7 eV and −0.3 eV, respectively. Photoelectrochemical Response. Transient photocurrent responses were measured to understand the kinetic behaviors of the photoexcited charge carriers in different samples (Figure 5A). The anodic current of CB2 is much stronger and the recovery is slower than the others, implying the internal electric field in CB2 facilitates the charge separation. To quantitatively investigate the charge separation behaviors, the normalized parameter (D) is introduced:41

(6)

where k, T, e, and dv(t)/d(t) are the Boltzmann constant, temperature in Kelvin, elementary charge, and open circuit voltage transient, respectively, as shown in Figure 5E. However, it is important to notice that the above equation could only be appropriate when the voltage decay is linear with a first-order dependence on time, which means the recombination of charge carriers with electrolyte is dominating, rather than the recombination of photoexcited electrons and holes within samples.47 As seen in Figure 5D, the voltage 10718


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Figure 7. Photodegradation of RhB under visible light (A). Pseudo-first-order kinetic analysis of degradation (B). Absorption spectra of filtrates degraded by CB2 (C) (the inset is the photographs of the filtrates). Kinetic constant for degradation of various pollutants with different samples (D).

and electrolyte). From Figure 6B, similar results can be clearly inferred except the decreased current intensity. Furthermore, to quantifiably investigate the charge separation and transfer, the recombination ratio is calculated via the formula:

intensity of CB2 is much higher and the voltage decay is significantly slower compared to the other samples, which perfectly matches the above results of photocurrent response. It is noteworthy that during the periods of voltage decay, the plot of CB2 is linear while the others are not, implying that the charge separation in CB2 is highly efficient and the recombination can be negligible, which means the OCVD calculation is available. Figure 5F displays the plot of dv(t)/ d(t) versus time, which can be fitted by the horizontal line at −0.008. Then, response time (τn) is calculated to be 3.16 s. To further investigate the kinetic behavior of charge carriers, trapping reagent (ammonium oxalate (AO) for scavenging photoexcited holes and Cr(VI) for scavenging photoexcited electrons) is introduced during the photocurrent measurement to enhance the charge separation. As shown in Figure 6A, after adding AO, the photocurrent response of CuAl2O4 dramatically improves while Bi2MoO6 improves a little, because CuAl2O4 has better absorption of visible light compared to Bi2MoO6, which provides more charge carriers, leading to higher response intensity. Meantime, the corresponding response enhancement of the mixture is at the medium level. Interestingly, there is almost no response enhancement of CB2 with the presence of AO, revealing that the p-n heterostructure indeed provides highly efficient charge separation and the adding of trapping reagent does not further improve it in essence. More attractively, after adding AO, the response intensity of CB2 is still a little higher and the photocurrent decay is slower compared to the mixture, suggesting that the pCuAl2O4/n-Bi2MoO6 heterostructure can extremely facilitate charge separation and transfer even with the presence of the trapping reagent. To strengthen the evidence, photoexcited electron scavenger (Cr(VI)) is also employed. It is noteworthy that Cr(VI) has a little optical absorption at 350−370 nm, so the photocurrent responses of the samples without scavengers are detected by inserting Cr(VI) as another optical filter (10 mg/L Cr(VI) solution placed in the middle of the light source

ηrec =

Isca − Ist Isca

(7)

where ηrec is recombination ratio and Isca and Ist are the current intensity of different samples with and without the scavengers, respectively. As shown in Figure 6C, the calculations with the presence of two scavengers show almost the same ratio, confirming the applicability of the method to value the charge separation. The contrastive calculation results show that there is serious charge recombination in pure CuAl2O4 (95.63%) and Bi2MoO6 (40.82%). After constructing the heterostructure, the heterojunction exhibits only 2.95% recombination ratio, which is lower than the other samples by 1 order of magnitude, demonstrating the highly efficient charge separation and transfer in the CuAl2O4/Bi2MoO6 heterojunction. In the meantime, the separation efficiency is given in Figure 6D. Moreover, it can be further detected by electrochemical impedance spectroscopy (EIS) and photoluminescence (PL) spectra. As displayed in Figure S3A, the equivalent circuit can be used for fitting the electrochemical process. Rs is the resistance of electrolyte between working and counter electrode. C is the capacitance between working electrode and electrolyte. Rct is charge transfer resistance, which is associated with the radius of the arc in EIS.48 Clearly, CuAl2O4 and Bi2MoO6 have large semicircles with the resistance of 34820 Ω and 57300 Ω, respectively. While after building the pn heterostructure, the charge transfer resistance dramatically decreases to 5643 Ω, which reduces by an order of magnitude, confirming faster charge transfer and better separation efficiency. The other fitted parameters of the samples can be found in Table. S2. On the other hand, the PL spectrum is another technique to detect the charge recombination. Figure 10719


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ACS Sustainable Chemistry & Engineering S3B shows the comparison of PL spectra of CuAl2O4 HNFs, Bi2MoO6 NSs, and CB2 with exciting light at 325 nm. Bi2MoO6 has an emission peak at 420−500 nm,49 while the emission intensity of CB2 diminishes a lot, implying the charge recombination is strongly restrained by the internal electric field. It is necessary to point out there is no emission peak of CuAl2O4 HNFs because the energy of charge recombination in CuAl2O4 may transform in other ways (lattice vibration, for example). From the above results, the highly efficient charge separation and transfer provided by the construction of the p-n heterostructure is tightly confirmed. Photocatalytic Performance. The photocatalytic performance of the samples was measured by degrading RhB, MO, Cr(VI), and 4-NP under visible light (λ ≥ 420 nm) irradiation. We take RhB as an example, and the other model pollutants can be found in the Supporting Information (Figure S4). As shown in Figure 7A, C/C0 is used to characterize the degradation ratio, where C0 is the initial concentration of RhB solution (10 mg/L) and C is the filtrates we take during the reactions. Obviously, C/C0 is almost invariant during −10−0 min, implying the absorption−desorption equilibrium has been achieved in the first 20 min. After turning on the light, CB2 exhibits the best photocatalytic activity, which degrades 98.59% of RhB solution in 90 min. As a contrast, other samples show relatively poor photocatalytic activity. In particular, pure CuAl2O4 HNFs, pure Bi2MoO6 nanoflowers, and their mechanical mixture based on the loading amount of CB2 degrade only 28.17%, 75.87%, and 61.88% of RhB solution, respectively. To make further comparison of photocatalytic performance, the kinetic analysis of the photodegradation was employed. When the initial concentration of dyes is low, the degradation rates follow a Langmuir−Hinshelwood apparent first-order kinetic model, which can be explained as the formula:50 r = dC /dt = kKC /(1 + KC)

Meanwhile, the de-ethylationed RhB is desorbed and readsorbed over and over again to reached adsorption−desorption dynamic equilibrium. After 60 min, the absorption peak is stable at λ = 498 nm with decrease in intensity, owing to the decomposition of aromatic rings.53 From the inset, the color change can be seen more obviously. As an important factor for photocatalytic performance, the specific surface area of the samples was measured, but the results (Figure S5) show that there is not much difference of the samples, implying that it is not the main reason. Therefore, the higher photocatalytic activity of CB2 over the reference samples is ascribed to the highly efficient charge separation and transfer, as discussed above. Moreover, an appropriate loading amount may also make great contributions to the photocatalytic performance. On the one hand, compared to nanoparticles (CB1) on CuAl2O4 HNFs, the appropriate space among nanosheets (CB2) provides multiple reflections of visible light,54 resulting in more efficient absorption of visible light for better activity. On the other hand, overgrowth of nanosheets (CB3) also leads to poor photocatalytic performance because32 (1) excessive Bi2MoO6 NSs cover the CuAl2O4 HNFs and restrain the light scattering in CuAl2O4 HNFs, which prevent CuAl2O4 from taking advantage of its hollow structure; (2) more light is absorbed by Bi2MoO6 NSs, making it hard to induce more photoexcited electrons in the composite (light absorption competition),55,56 which is unfavorable for charge separation. Similar results can be obtained from the other pollutant models in the Supporting Information (Figure S4), and the kinetic constants of the reactions are summarized in Figure 7D. The figure compares the degradation rate of RhB, MO, Cr(VI), and 4-NP at the fixed concentration (10 mg/L) with different samples. Compared to pure CuAl2O4, Bi2MoO6, and their mechanical mixture, the heterojunction increases the photocatalytic activity by more than an order of magnitude, which may be ascribed to the highly efficient charge separation. It is noteworthy that all samples have the same tendency to degrade the four model pollutants except for CuAl2O4, because CuAl2O4 exhibits better activity for degrading MO rather than RhB. As we know that RhB has the absorption peak of 553 nm and the emission peak of 600 nm, while the absorption and emission peak of MO locates at 464 and 520 nm. The emission light of MO can be reabsorbed by CuAl2O4 (Figure S2A) while the emission light of RhB cannot, leading to more optical utilization and better activity for MO degradation. Photocatalytic Mechanism. It is known that active radicals play the most important roles during photocatalytic reactions. To further discuss the photocatalytic process, trapping reagents are often used to scavenge the active radicals during the reactions. K2Cr2O7, ammonium oxalate (AO), benzoquinone (BQ), and tert-butyl alcohol (TBA) are employed to trap photoexcited electrons (e−), holes (h+), superoxide radicals (•O2−), and hydroxyl radicals (•OH), respectively. Results are shown in Figure S6. It is obvious that photocatalytic performance is not influenced much after adding TBA, which means •OH are not the main active radicals, while with the presence of K2Cr2O7 and BQ, especially for the AO, the photocatalytic activity dramatically decreases, confirming the vital role of holes during the reactions. Based on the above trapping experiments, a possible synergetic mechanism is proposed in Figure S7. With the irradiation of visible light, both CuAl2O4 and Bi2MoO6 can be excited to generate electrons and holes. The photoexcited electrons tend to transfer to the conduction band (CB) of

(8)

where r is the degradation rate of the dyes (mg·L−1·min−1), C is the concentration of dyes (mg·L−1), t is the reaction time (min), k is the reaction rate constant (mg·L−1·min−1), and K is the absorption coefficient of the dyes (L·mg−1), respectively. With the low initial concentration of dyes (10 mg·L−1), eq 8 can be simplified as follows:51 ln(C0/C) = kKt = kappt

(9) −1

where kapp is the apparent first-order rate constant (min ). As shown in Figure 7B, it is obvious that the reaction rate of all the samples can be well fitted by a straight line, implying the degradation of RhB and MO belong to the apparent first-order kinetic reaction. Meanwhile, the corresponding apparent firstorder rate constants (kapp) of all samples are calculated and displayed. Comparing the reaction rate constants we can see that CB2 exhibits the best photocatalytic performance, which is 17.8 times of pure CuAl2O4 HNFs, 3.44 times of pure Bi2MoO6 NSs, and 5.0 times of their mechanical mixture. Moreover, absorption spectra of filtrates are shown in Figure 7C, in which we can see that the absorption peak of the initial RhB solution is at λ = 553 nm and the location does not change before irradiation, which means the decrease in intensity is caused by the adsorption of RhB. After turning on the light, a blue-shift of peaks can be clearly observed, indicating that RhB is degraded step by step to small species, as displayed in Scheme S1. Ethyl groups of RhB are removed one by one as confirmed by the gradual blue-shift in Figure 7C.52 10720


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Figure 8. Recycling runs on photodegradation of RhB with CB2 (A). Sedimentation property of CB2 compared to pure Bi2MoO6 (B) after reactions. TEM image after 5 times of photodegradation (C) and XRD patterns before/after the reactions (D) of CB2.

phase of the sample are not destroyed, confirming the well recycling stability of the p-CuAl2O4/n-Bi2MoO6 heterojunction.

Bi2MoO6, while holes tend to move to the valence band (VB) of CuAl2O4 via interfacial chemical bonding due to more positive CB potential of Bi2MoO6 and more negative VB potential of CuAl2O4. According to Figure S6, holes can directly attack RhB, which makes the most contribution in the degradation, while electrons react with the adsorbed O2 to generate ·O2− and then participate in the reactions. Only a small part of •O2− protonize in water and gradually become • OH degrading RhB, which is negligible. Major reaction equations involved in the process are displayed as follows:

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CONCLUSION In summary, 3D hierarchical p-CuAl2O4/n-Bi2MoO6 heterojunction was successfully synthesized by assembling Bi2MoO6 NSs on the electrospun CuAl 2 O 4 HNFs through a solvothermal method. Photoelectrochemical measurements reveal that the p-n heterojunction has much higher photoresponse intensity and slower decay rate, confirming the highly efficient charge separation and transfer induced by the strong internal electric field, which also make the heterojunction exhibit higher photocatalytic activity than the reference samples by 1 order of magnitude for degrading all pollutant models. In addition, the heterojunction can be easily separated by natural sedimentation in a very short time due to its overlong 1D structure and macroscopically 3D nonwoven web framework, which can be recycled many times without decrease in photocatalytic activity. Therefore, it is expected that the 3D hierarchical p-CuAl2O4/n-Bi2MoO6 heterojunction with excellent photocatalytic performance and long-term stability will meet the comprehensive requirements for practical application in water purification and environment cleaning. Moreover, we believe that the p-n heterojunction may exhibit efficient photocatalytic water oxidation because of its appropriate band gap potentials, which we will study later.

CuAl 2O4 /Bi 2MoO6 + hv → CuAl 2O4 (e− + h+) + Bi 2MoO6 (e− + h+)

(10)

CuAl 2O4 (e− + h+) + Bi 2MoO6 (e− + h+) → CuAl 2O4 (h+) + Bi 2MoO6 (e−)

(11)

O2 + e− → •O−2

(12)

• − O2

(13)

•

+ H 2O → • HO2 + OH−

HO2 + H 2O → H 2O2 + •OH

h+/ •O−2 / •OH + RhB → degraded products

(14) (15)

Recycling Stability and Separable Property. It is known that recycling stability and separable property are very important in practical photocatalytic applications. Recycling experiments on photodegradation in the same conditions are carried out and results are given in Figure 8A. It is obvious that the photocatalytic performance is barely diminished after 5 runs. On the other hand, the separable property can be investigated by natural sedimentation. As shown in Figure 8B, pure Bi2MoO6 nanoflowers suspend in the solution after reaction without sedimentation, which may be only separated by centrifugation. On the contrary, the p-n heterojunction with macroscopically 3D nonwoven web framework can be easily separated by natural sedimentation in only 5 min, which has excellent separable property for reusing in practical applications. Furthermore, Figure 8C and D further demonstrate that morphology, structure, and crystal

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b02040. Materials and methods, dosages and sample morphologies, parameters fitted by the equivalent circuit of the samples, SEM images, UV−vis spectra, EIS Nyquist plots, photodegradation under visible light illumination, nitrogen absorption−desorption isotherms, possible mechanism of photodegradation, and model of possible degradative process (PDF) 10721


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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]; Tel. 8643185098803. *E-mail: [email protected]; Tel. 8643185098803. ORCID

Changlu Shao: 0000-0002-5024-3268 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work is supported financially by the National Natural Science Foundation of China (Nos. 51572045, 51272041, 61201107, 11604044, and 91233204), the National Basic Research Program of China (973 Program) (No. 2012CB933703), the 111 Project (No. B13013), the Natural Science Foundation of Jilin Province of China (20160101313JC), the Science and Technology Development Program of Jilin Province (20180520192JH), the Fundamental Research Funds for the Central Universities (2412017FZ009, 2412017QD007, 2412016KJ017), and the China Postdoctoral Science Foundation (No. 2017M610188).

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nanosheets on electrospun p-CuAl

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