BiPO4 nanorods anchored in biomass-based carbonaceous aerogel skeleton: A 2D-3D heterojunction composite as an energy-efficient photocatalyst

The Journal of Supercritical Fluids 147 (2019) 33–41

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BiPO4 nanorods anchored in biomass-based carbonaceous aerogel skeleton: A 2D-3D heterojunction composite as an energy-efficient photocatalyst

T

Wei Weia, , Huihui Hua, Zhiye Huanga, Zhifeng Jianga,b, Xiaomeng Lva, Jimin Xiea, Lirong Konga, ⁎

a b

⁎

School of Chemistry & Chemical Engineering, Jiangsu University, Zhenjiang, 212013, PR China School of Life Sciences, The Chinese University of Hong Kong, Hong Kong, PR China

GRAPHICAL ABSTRACT

ARTICLE INFO

ABSTRACT

Keywords: Biomass-based aerogel skeleton structure In situ grown 2D-3D heterojunction Energy-efficient photocatalyst

An ultra-light 2D-3D heterojunction composite is successfully designed and fabricated by introducing biomassbased carbonaceous aerogel as the skeleton structure and embedded 2D BiPO4 nanorods. The biomass-based carbonaceous aerogel is not only provided a large and continuous surface with porous for photocatalytic nanoparticles, but also supported the synergistic enhancement between substrates and BiPO4 nanorods, which enhanced the electron − hole separation and the solar energy collecting efficiency. In consequence, the BiPO4/ biomass-based carbonaceous aerogel (BP/BCA) composite showed the outstanding activity for degradation of simulated dye (Methylene blue, MB) under visible light irradiation, which was almost 7.12 times as high as that of pure BiPO4. Furthermore, the improved photocatalytic activity was achieved attribute to the efficient production of catalytically active species (O2%− and %OH), which were demonstrated by means of extensive mechanism research. The ultra-light 2D-3D heterojunction composite offers opportunities for future energy-efficient photocatalyst in this photocatalytic system.

1. Introduction Aerogel is a type of gels consisting of three-dimensional (3D) nanostructural solid networks with air as infilling mediums in the interspaces [1]. The continuous nanopores in aerogels are constructed into an open 3D skeleton, which is conducive to diffusion/mass transfer on the substrates [23]. Hitherto, there’s a tendency to prepare carbonbased carbonaceous aerogel (BCA) from biomass. The preparation process of BCA is totally green synthetic method by using cheap and

⁎

ubiquitous biomass as the carbon source, which can increase the biocompatibility and broaden their applicability and the prepared BCA showed extraordinary flexibility as well as high chemical reactivity, which endow them great application potential [4–7]. The prominent property of BCA, such as hierarchical three-dimensional (3D) network structure, high specific surface area and porosity, potential active sites, abundant surface functional groups, mineral compositions, particularly excellent heat and acid/alkali resistance, endows them with superior adsorption capacity [89]. Furthermore, the BCA was intrinsically

Corresponding authors. E-mail addresses: [email protected] (W. Wei), [email protected] (L. Kong).

https://doi.org/10.1016/j.supflu.2019.02.012 Received 28 November 2018; Received in revised form 17 January 2019; Accepted 11 February 2019 Available online 14 February 2019 0896-8446/ © 2019 Elsevier B.V. All rights reserved.

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suitable for binding to nanoparticles of metal and metal oxide, by their enhanced adsorption of capability in contaminants treatment, due to pore structure, abundant groups and potential active sites [10–13]. For decades, BCA materials have been advanced as energy-efficient and ideal catalyst carriers on account of their inherent superior properties of the above statement [14]. Distinctly, many BCA supported nanoparticle photocatalyst materials had shown enhanced catalytic efficiency toward the mineralization of pollutants (e.g. dyes, antibiotics, surfactant, etc.) that are of current environmental issues [10,1516]. BiPO4, as a two-dimensional (2D) inorganic non-metal salt of oxy-acid photocatalyst, was used in the past several years [17–20]. It has become a promising photocatalyst due to its better UV-responsive than TiO2 (P25) to some extent [21]. BiPO4 has a wide band gap and a high valence band, so it can provide a powerful oxidation capacity to shorten the mineralization time for converting pollutants into inorganic substance [22]. Though, the wide band gap of BiPO4 and quantum efficiency limited practical application of BiPO4 [23]. To compensate for these deficiencies, some effective methods of modification have been used by researchers for enhancing the activity. For example, the introduction of nonmetal or metal ions were into the structure, the F/P doped BiPO4 and Eu3+, Ga3+ or Ln3+ doped BiPO4 [22,24–27]. In addition, forming heterojunctions between BiPO4 and noble metal (Ag) [28] or semiconductor (Bi2SiO5 [29], Bi2MoO6 [30], CdS [31], Ag3PO4 [32–34]) is also an effective method to improve the catalytic activity. Recently, some researchers focus on carbon materials, such as grapheme [35], reduced graphene oxide(rGO) [3637], carbon [38], carbon nitride(C3N4) [39–41], a series of the π-conjugated photocatalyst junctions was reported to enhance the separation efficiency of the photogenerated electron and hole (e−-h+) pairs and degradation rate [42–44]. Zhang et al. reported that BiPO4/reduced graphene oxide (rGO) composites showed higher photocatalytic activity on the degradation of MB dye under visible light, compared to the pure BiPO4 [45]. BiPO4/carbon nanotubes (CNT) composite were prepared by Vadivel et al. via one-step solvothermal approach showed the degradation of MO could get 95% due to effective separation of photoinduced hole-electron pairs between host BiPO4 and CNT [46]. She et al. prepared BiPO4@glucose-based C (BiPO4@C) core-shell nanorod heterojunction photocatalyst and it showed greater than 94% photodegradation of dye under UV light, and its degradation rate was 4.1 times higher than that of commercial TiO2 and 2.3 times higher than that of pure BiPO4 [47]. However, some remarkable problems of CNT and graphene aerogel, such as the harmful and expensive precursors, and complicated equipment process, dramatically hamper their largescale production for industry applications, which further highlights the advantages of using BCA as a catalytic carrier [48–54]. Unsurprisingly, nanoparticles embedded in BCA, as sustainable resources and renewable energy, had special research significance in pollutant degradation. The aim of this study was to construct 2D-3D heterojunctions and seek to bridge the synergy between BiPO4 and biomass-based carbon material in order to optimize photocatalysis under visible light illumination. Herein, we prepared two-dimensional (2D) BiPO4 nanorods (BP)/ three-dimensional (3D) biomass-based carbonaceous aerogel (BCA) heterojunctions as a photocatalyst material for degradation of methylene blue (MB) by hydrothermal-carbonization method. The BiPO4 nanorods are in situ grown on biomass-based carbonaceous aerogel to ensure the strong interfacial bonding between the BiPO4 nanorods and carbon matrix. In the construction of the 2D-3D architecture the biomass-based carbonaceous material effectively anchored with BiPO4 nanorods provide the pore channel for electron transfer during photodegradation process under visible light. Such enhanced properties provide important prospects for BP/BCA composite revealed excellent photocatalyst material for environmental remediation.

2. Experimental sections 2.1. Materials Bismuth nitrate pentahydrate (Bi(NO3)3·5H2O), sodium dihydrogen Phosphate dihydrate (NaH2PO4·2H2O), anhydrous ethanol (EtOH, C2H5OH), tert-butanol (t-BuOH), 1,4-benzoquinone (BQ), ammonium oxalate (AO) were purchased from Sinopharm Chemical Reagent Co. Ltd., China. Wintermelon was purchased from the local fruit supermarket and thoroughly washed with deionized water before used. The methylene blue (MB, C16H18ClN3S) was obtained from Public-private joint venture XinZhong Chemical Factory, Shanghai, China. All reagents used in the experiments were of the analytical grade and used without further purification. Deionized (DI) water was made from laboratory. 2.2. Preparation of photocatalysts 2.2.1. Fabrication of biomass-based carbonaceous aerogel (BCA) The monolithic BCA was prepared through a hydrothermal method according to the research group's previous report [55]. The fresh winter melon was removed the rind, soft pulp and seeds, then cut it into a block of uniform size. Subsequently, added them into a Teflon-lined stainless steel autoclave and hydrothermal reaction was conducted at 180 °C for 12 h. After cooling down to room temperature, the as-obtained black carbonaceous hydrogel was washed by a mixed solution of water and ethanol with the volume ratio of 1:1 for removal of soluble impurities at 40 ℃ for 24 h. Finally, ultra-light BCA was obtained by vacuum freeze drying at -56℃ for 24 h (The vacuum freeze drier, FD1C-50, Shanghai Bilon Instrument Manufacturing Co., Ltd.). 2.2.2. Fabrication of BiPO4 anchored in biomass-based carbonaceous aerogel (BP/BCA) The BP/BCA composites were synthesized by the facile hydrothermal process in Fig. 1. A certain amount of BCA was immersed into 30 mL mixed solution of water and ethanol with the volume ratio of 1:2 under stirring for 1 h. Then 1 mmol Bi(NO3)3·5H2O by added under stirring for 6 h to ensure Bi3+ can adhere equably to the surface of carbonaceous. At that point, added 1 mmol NaH2PO4·2H2O and stirring until it completely dissolved. The mixture was subsequently transferred into a 50 mL Teflon-lined stainless steel autoclave filled to 80% capacity and conducted at 180 °C for 12 h. In the following, the BP/BCA composite was washed with deionized water and ethanol several times in turn and subsequently dried it in a vacuum oven at 60℃ for 12 h. Herein, the mole ratio of Bi(NO3)3·5H2O and NaH2PO4·2H2O was set at 1:1, prepared a series of BP/BCA with quality fraction of samples were 5%, 10%, 20%, 30% and 40% by adjusting the quality of BiPO4 and BCA. As a comparison, pure BiPO4 was synthesized by a similar solvothermal method without BCA as a support, marked as BP. 2.3. Characterization Various properties characterizing techniques have been performed in order to understand the properties of synthetic materials. X-ray diffraction (XRD) patterns of materials were performed by D/Max-γA x-ray diffractometer (D8 Advance, Bruker, Germany) at 40 kV and 100 mA with monochromatic Cu (scan 10-80°, scanning velocity 7° min−1). The surface properties of samples were analyzed by Fourier transform infrared (FTIR) spectra (Nicolet Nexus470, Thermo Electron, USA). The Raman spectra were performed using a Laser Raman spectrometer with a 532 nm laser source (DXR, Thermo Fisher Corporation, USA). The morphology of the prepared samples was measured by the field emission electron microscope (JSM-7001 F, JEOL, Japan). The chemical elements of the samples were determined by Energy Dispersive X-ray Spectroscopy (SAIMADZUSSX, JEOL, Japan). Transmission electron microscopy (TEM) patterns of samples were obtained by TEM electron microscope at an accelerating voltage of 200 kV (TECNAI12, Philips, 34

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Fig. 1. Schematic illustrations of the preparation process of BP/BCA composites.

Netherlands). X-ray photoelectron spectroscopy (XPS) was acquired on Thermo ESCALAB 250XI spectrometer (Thermo Fisher Corporation, USA). UV–vis diffuse reflectance spectra (UV–vis DRS) were collected on UV-2450 spectrophotometer (Shimadzu Corporation, Japan), a white standard of BaSO4 as a reference. Photocurrent measurements were achieved on the electrochemical analyzer (CHI 760B, Chenhua Instruments Company, China). The photoluminescence (PL) spectra were utilized by a QuantaMaster40 (Photon Technology International, USA). The electron spin resonance (ESR) analysis was conducted with an electron paramagnetic resonance A300-10/12 spectrometer (AXS, Bruker, Germany). 2.4. Photoactivity measurements The photocatalytic activity of the prepared composites was evaluated using MB solution as a simulated contaminant. Briefly, 20 mg of the BP/BCA composite was dispersed in the 10 mg/L MB aqueous solution. Before the photodegradation reaction, the suspension was stirred in dark for 30 min to reach the adsorption/desorption equilibrium, and then the photodegradation reaction was initiated by irradiating the reactor with Xenon arc lamp (λ > 400 nm, 350 W). In a 20 min interval, approximately 3 mL suspensions were withdrawn and centrifuged to remove impurities. The MB concentration variations were determined by UV-2450 spectrophotometer. Air is pumped in throughout the reaction and a cooling water circulator to keep the whole reaction system at a constant temperature.

Fig. 2. The XRD patterns of as-prepared BCA, BP and BP/BCA composites.

increased as the mass fraction of BP increased, which indicated that the between of BP and BCA had acting force at the composite interface. In addition, no other significant diffraction peaks were observed in the figure, which indicated that the BP/BCA complexes were pure twophase complexes. The morphology and dispersion states of the BP/BCA composite were ascertained by SEM, TEM, EDS and mapping. In Fig. 3a, the BP nanorods showed regular rod structure and relatively uniform size and densely anchored onto the BCA after the solvothermal reaction. In Fig. 3b, the TEM image of the composite could clearly distinguish the two components between the BP and BCA in the composite. The length of BP nanorods was about 50 nm–200 nm, and the width is about 50 nm. The biomass-based aerogels as a skeleton could firmly supported the BP nanorods, marked them evenly distributed on the BCA surface without obvious agglomeration. In Fig. 3c and d, the elementary composition of BP/BCA was further determined by EDS elemental mapping attached on the SEM. The EDS elemental mapping of the BP/BCA presented the distribution of Bi, P, O, and C elements, which indicated that BP nanorods were homogeneous distributed on the surface of the BCA. Interestingly, BP nanorods were still tightly attached on the surface of BCA during the long time ultrasonication during the TEM

3. Results and discussion The X-ray diffraction (XRD) pattern analyses of the samples were presented to identify the crystallinity and phases. The XRD patterns of BCA and BP/BCA composites were given in Fig. 2. The broad characteristic peaks located at around 22° indicated that the BCA consists of the graphitized carbon, consistent with the previous report [12]. The diffraction peaks at 20.1°, 29.5°, 37.9°, 41.9°, 48.7° and 53.5°correspond to BP with hexagonal phase spinel structure (JCPDS No.15-0766) [5657]. Furthermore, no other characteristic peaks of impurities were observed, which demonstrated the high purity of the BP/BCA. Compared with the XRD pattern of pure BP, all the characteristic diffraction peaks of BP/BCA were consistent with the BP and their characteristic diffraction peak intensity was relatively large without any impurities, which indicated the high crystallization of the sample. It could also be seen from the Fig. 2 that the intensity of the diffraction peak gradually 35

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Fig. 3. (a) SEM images of the as-prepared 20 wt% BP/BCA and (b) TEM images of the 20 wt% BP/BCA. (c) EDS image and (d) the corresponding element mappings of the 20 wt% BP/BCA.

sample preparation, which revealed a good interfacial contact between BP nanorods and BCA. In consequence, combined with SEM, TEM, EDS and mapping results, it was obvious that BP nanorods were successfully distributed on the surface of BCA. FTIR spectroscopy allows the identification of the different functional groups of the BP/BCA. Fig. 4 showed the FTIR spectra of BCA, BP and different percentage composition BP/BCA [17,58]. For pure BCA, the absorption peak at 890 cm−1 was mainly attributed to the plane bending vibration of C–H. The broad and strong band at 3450 cm-1 belongs to the stretching vibration of OeH. The peaks at 1597 cm-1 was stretching vibrations of C]O and C]C, which indicated the presence of aromatic and furanic groups. The main reason at 1120 cm−1 is the bending vibration of CeOeC or the skeleton vibration of CeC, the absorption peak at 1023 cm−1 belongs to the asymmetric expansion vibration of ν3(PO4). In the low wavenumber region, 593 and 540 cm−1 could be assigned to the symmetrical stretching vibration of δ(O-P-O) and ν4(PO4), respectively [17,58]. These results illustrated that the surface of BCA is rich in substantial oxygen-containing functional groups, which was beneficial to conducting further reaction with other materials. Further, only the characteristic peaks of BP and BCA existed in the composite material, indicating that the composite material was BP/BCA. As shown in Fig. 5, Raman spectra of CA, BP and 20 wt% BP/CA

Fig. 4. FTIR spectra of as-prepared BP and BP/BCA composites.

36

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extended photo-response range so that the photocatalyst could absorb more photons, which is favorable for the visible light photocatalytic reaction. Fig. 8 showed the fluorescence spectrum of BP and 5/10/20/30/ 40 wt% BP/BCA at the excitation wavelength of 417 nm. It could be seen from the figure that BP had a significant characteristic peak at 540 nm, which was consistent with previous relevant reports [35]. Photocatalytic materials produced fluorescence when photogenic electrons and holes were combined. In addition, the weaker the fluorescence intensity indicated it generated more electrons, holes and active substances participate in the reaction and the weaker the ability of electron-hole recombination. It is obvious from the figure that the fluorescence intensity of BP/BCA decreased obviously compared with BP, which indicated that after the compounding, the e−-h+ pairs in the excited BiPO4 could be efficiently separated through the injection of electrons from the BiPO4 to BCA. This might also show better photocatalytic activity of composite material in photocatalytic experiments. The photocatalytic activities of the BP/BCA composites were evaluated by the degradation of MB under visible-light irradiation. The photocatalytic degradation of MB by BP and 5, 10, 20, 30, 40 wt% BP/ BCA was studied by UV–vis spectroscopy. The 10 mg/L MB solution was selected as the simulated pollutant to investigate the photocatalytic activity of BP and BP/BCA composite materials under visible light irradiation. As shown in Fig. 9a, the blank test results showed the self photodegradation of MB was negligible. In addition, it could be seen that the introduction of BCA could improve the photocatalytic activity of pure BP. After 180 min of light irradiation, the degradation rate of MB solution was significantly improved by the prepared BP/BCA composite. We could see form the Fig. 9 that 20 wt% BP/BCA had the highest photocatalytic activity, while the degradation rate of MB could reach 93.51%. In comparison, the degradation rate of pure BP to MB in the same time interval was about 63.03%. In order to further quantitatively analyze the degradation process of MB, the pseudo-first-order reaction kinetics model was used:

Fig. 5. Raman spectra of BP, BCA, 20 wt%BP/BCA composite.

were further performed to affirm the intimate interaction between BP and BCA in the 20 wt% BP/BCA. BCA showed two typical characteristic peaks of D-band at 1386 cm−1 and G-band at 1595 cm−1, respectively [23]. The D-band was caused by the sp3 hybridized CeC bond and the vibrational absorption of surface defects, while the G-band was derived from the vibrational absorption of sp2 hybridized C]C bond, which was a characteristic peak of carbon materials. In the 20 wt% BP/BCA, the absorption peak at 212 cm−1 was attributed to the symmetry bending peak of OeBieO, and the 397 cm−1 peak belonged to the bending vibration peak of ν2 (PO4). The peaks of 460, 565 and 596 cm−1 belonged to the bending vibration peak of ν3(PO4), and the characteristic peak at 965 cm−1 was attributed to the symmetric stretching peak of ν1(PO4), and 1041 cm−1 was attributed to the antisymmetric stretching peak of ν3(PO4) [24,58]. Raman spectra further validated that the prepared sample was BP/BCA, which was consistent with previous related reports. In the Raman spectrum of the 20 wt% BP/BCA, only the characteristic peaks of the BP and BCA were observed, further indicated that the coexistence of BP and BCA, which was consistent with the XRD analysis. The chemical composition and valence states of the surface elements of 20 wt% BP/BCA were analyzed by X-ray photoelectron spectroscopy (XPS), as shown in Fig. 6. It could be seen from the spectrum that the atoms in the 20 wt% BP/BCA were mainly composed of C 1 s, Bi 4f, O 1 s and P 2p [17]. Fig. 6a showed the high-resolution XPS spectrum of Bi 4f. The signal peaks at the binding energies of 159.4 eV and 164.7 eV mainly correspond to the two atomic orbitals of Bi 4f 5/2 and Bi 4f 7/2, which was mainly attributable to the Bi valence of Bi3+ in BP [59]. Fig. 6b showed the high-resolution XPS spectrum of P 2p, in 132.8 eV was attributed to the valence state of P in PO43− [60]. As shown in Fig. 6c, the characteristic peaks of C 1 s were found at the binding energies of 284.8 eV, 286.4 eV, and 288.4 eV, respectively, corresponding to the presence of different groups of C]C, C–O and C]C–O in the carbon components. Fig. 6d shows the high resolution XPS spectrum of O 1s, which was peaked at 531.1 eV and 532.7 eV, which can be assigned to the OH group [20]. UV-vis diffuser reflectance was normally utilized to characterize the electronic states of the semiconductor photocatalyst, wherein the electronic structure had a crucial role in the photocatalytic process. Fig. 7 showed the UV–vis diffuse reflectance spectra of BP and BP/BCA composites with different mass ration. It could be seen that the pure BP shows a strong absorption in the ultraviolet region at between 200 and 290 nm [23,28]. When BP was combined with BCA, the intensity of light absorption of BP/BCA with different ratios was enhanced in the wavelength range of 300–400 nm. The interaction between BP and BCA

ln(Ct/C0) = -kt −1

(1) −1

where k (h ) is the pseudo-first-order rate constant, Ct (mg·L ) is the concentration of the MB solution at time (t), C0 (mg·L−1) is the original concentration of the MB solution. As shown in Fig. 9b, the change of MB concentration with reaction time was very consistent with the pseudofirst-order reaction kinetics model. The determined k of BP and BP/BCA was calculated by the slope corresponding to the fitting line. The pseudo-first-order reaction rate constant (k) and correlation coefficient (R2) were shown in Table 1. It was indicated that the photocatalytic reactivity order is 20% BP/BCA (0.719) > 10 wt% BP/BCA (0.528) > 5 wt% BP/BCA (0.461) > 30 wt% BP/BCA (0.425) > 40 wt% BP/BCA (0.404) > BP (0.101). Therein, 20 wt% BP/BCA has the largest rate constant, which was even 7 times as much as pure BP. Electrons are transferred from the conduction band of the semiconductor to the electrode to generate light current. Therefore, a high intensity of light current often means a high efficiency of optoelectronic hole separation. Fig. 10 was the light current response diagram of four open-close cycle experiments when BP and 20 wt% BP/BCA composite materials were modified into electrode materials under visible light irradiation. Compared with the pure BP, 20 wt% BP/BCA showed significantly enhanced photocurrent under visible light irradiation intensity, almost 3 times was pure BP photocurrent intensity, and the intensity of photocurrent was stable and repeatable. These results showed that the combination of BP and BCA can effectively restrain the recombination of photogenic electron hole pairs. These test results of photoelectric current response were consistent with those of PL analysis. It is further confirmed that the introduction of BCA was an effective method to improve photocatalytic performance. In order to study the mechanism of improving the photocatalytic activity of 20 wt% BP/BCA composite materials, free radical and hole 37

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Fig. 6. XPS spectra of 20 wt% BP/BCA composite: (a) Bi 4f, (b) P 2p (c) C 1 s and (d) O 1 s.

Fig. 7. UV–vis diffuse reflectance spectra of BP and BP/BCA composites. Fig. 8. PL spectra of BP and BP/BCA composites.

capture experiments were conducted to detect the main active species during the photocatalytic reaction process. The photocatalystic mechanism could be studied by trapping experiments of radicals and an electron spin resonance (ESR) spin-trap technique. 1,4-benzoquinone (BQ), ammonium oxalate (AO), and tert-butanol (t-BuOH) were used as electron hole (h+) scavengers, superoxide free radical (%O2−) scavengers and hydroxyl free radical (%OH) scavengers, respectively. As shown in Fig. 11, the degradation rate of MB decreased from 93.51% to 15.84% when BQ was added, indicating that superoxide free radicals played a major role in photodegradation. When 1 mmol of AO was

added, the degradation rate of methylene blue solution did not change significantly. The degradation rate is less inhibited when t-BuOH was added, which indicated that hydroxyl radicals play a secondary role in photocatalytic degradation. The electron spin resonance test was used to determine whether hyperoxy free radicals and hydroxyl free radicals existed in photocatalytic reaction. Dissolved 5,5-dimethyl-1-pyrrolidine oxide (DMPO) in aqueous or methanol solution to detect hydroxyl radicals as shown in 38

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Fig. 9. (a) Photodegradation performance of MB over the as-prepared photocatalysts; (b) the Kinetic curves of -ln(Ct/C0) versus reaction time for degradation of MB over the as-prepared photocatalysts. Table 1 Pseudo-first-order rate constant for MB over the as-prepared photocatalysts. series

photocatalyst

The first order kinetic equation

k (h−1)

R2

1 2 3 4 5 6

BiPO4 5 wt% BP/BCA 10 wt% BP/BCA 20 wt% BP/BCA 30 wt% BP/BCA 40 wt% BP/BCA

ln(C0/C) = 0.10098 t ln(C0/C) = 0.46039 t ln(C0/C) = 0.52781 t ln(C0/C) = 0.71982 t ln(C0/C) = 0.42496 t ln(C0/C) = 0.40412 t

0.10098 0.46039 0.52781 0.71982 0.42496 0.40412

0.97012 0.99749 0.99450 0.99314 0.99724 0.99699

Fig. 11. Degradation ratio of the 20 wt% BP/BCA composite with different scavengers.

were used for the oxidation reaction. A number of electrons migrate to the surface of BiPO4 NPs and BCA and react with O2 to form %O2−, which can further oxidize dye molecules. Likewise, the holes are generated to oxidize H2O to %OH, which can also act as an active species for the degradation of dye. The photocatalytic degradation mechanism can be summarized as follows: (1) the 3D structure of BCA as building blocks could provide more adsorption sites and photocatalytic reaction sites, which was beneficial to improve the photocatalytic activity of BP/ BCA composite materials; (2) the carbon skeleton contains sp2, sp3 and other hybrid carbon atoms and a large number of oxygen-containing bonds on sp3 hybrid carbon atoms. But beyond that, BCA can be approximated as p-type semiconductor materials and BP was a kind of ntype semiconductor material, a type-II (p/n) heterostructure was formed after combined BCA, this ultra-light 2D/3D BP/BCA heterostructure shortens the transport time and distance for the photo-generated charge carriers and thus decreases the recombination possibility of charge carriers during migration.

Fig. 10. Transient photocurrent response for BP and 20 wt% BP/BCA composite.

Fig. 12 [[17], 62]. Fig. 12a was the test result in methanol solution, and the characteristic peak of %O2− cannot be detected in the absence of light. When exposed to visible light, four characteristic peaks of %O2− were detected, and their intensity ratios were about 1:1:1:1. In Fig. 12b, characteristic peaks of were detected during illumination in aqueous solution and their peak intensity ratios were about 1:2:2:1. The experimental results showed that %O2− and %OH were two main active species in photocatalytic reaction. Based on the above experimental results and discussion, the possible photocatalytic mechanisms of the 2D/3D BP/BCA heterostructure were proposed, as shown in Fig. 13. In this work, BP and BCA can be synergistically excited to generate electron-hole pairs under visible-light irradiation, promoting the migration of electrons from valence band to the edge of conduction band which can be used for the reduction reaction, and the generation of holes in the valence band retains which

4. Conclusion A 2D-3D BiPO4 nanorods/biomass-based carbonaceous aerogel (BP/ BCA) heterojunction had been successfully prepared via the well-established hydrothermal method. The uniform BiPO4 nanorods particle were anchored in biomass-based carbonaceous aerogel skeletons. Compared with BiPO4, the BP/BCA composites showed an enhanced photocatalytic activity toward the degradation of dye. The analysis of reactive species generated by indirect scavenger method and electron spin resonance method indicates that the •OH and •O2− were the 39

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Fig. 12. ESR spectra of the 20 wt% BP/BCA composite with visible light irradiation. (a)DMPO - %O2− in methanol dispersion, (b) DMPO - %OH in aqueous dispersion.

[7] [8] [9]

[10] [11] [12] [13] [14]

Fig. 13. Schematic diagram of the photocatalytic degradation mechanism of MB with BP/BCA composite.

[15]

critical species for the degradation of pollutant. This type-II (p/n) heterostructure of BP/BCA composite was accelerated the transfer and separation of the photo-generated charge carriers during the photodegradation process. This study constructed 2D-3D heterojunctions and seeked to bridge the synergy between BiPO4 and biomass-based carbon material provides a new insight into the design for photochemical reactions.

[16] [17] [18]

Acknowledgements

[19]

This work was supported by the National Natural Science Foundation of China (21607063), China Postdoctoral Science Foundation (2018M630530).

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