bismuth oxybromide heterojunction for enhanced photocatalytic air purification and mechanism exploration

Chemical Engineering Journal 379 (2020) 122380

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Lanthanum orthovanadate/bismuth oxybromide heterojunction for enhanced photocatalytic air purification and mechanism exploration ⁎

T

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Xuejun Zoua, Chenyu Yuana, Yuying Donga, , Hui Gea, Jun Keb, , Yubo Cuia a b

Department of Environmental Science and Technology, Dalian Minzu University, Dalian 116600, China School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan 430073, China

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

LaVO /BiOBr heterojunction is con• structed through a facile hydrothermal 4

route.

conversions of VOCs are enhanced • The due to formation of p-n heterojunction.

mechanism • Reaction based on in-situ FTIR.

are identified

A R T I C LE I N FO

A B S T R A C T

Keywords: Air purification Advanced oxidation process Volatile organic compounds Photocatalysis Heterojunction

In this work, Lanthanum orthovanadate (LaVO4)/bismuth oxybromide (BiOBr) hybridized heterojunction was fabricated through a facile hydrothermal method for enhancing separation and transferring efficiency of photogenerated charge carriers in photocatalytic advanced oxidation process. The photoelectrochemical data demonstrate that the introduction of LaVO4 not only improves the solar energy harvesting efficiency due to decreasing of bandgap energies from 2.71 eV to 2.17 eV, but also reduces the transfer resistance of charge carriers in the hybridized system. Furthermore, the LaVO4/BiOBr heterojunctions exhibit enhanced photocatalytic degradation for gaseous air pollutants under visible light irradiation in comparison with bare LaVO4 and BiOBr. Among them, 3% LaVO4/BiOBr displays the best photocatalytic performance and degradation conversions of acetone and toluene achieve to 95.4% for 3 h and 87.1% for 4 h, respectively, which are 2.6 and 5.3 times higher than that of pristine BiOBr sample, respectively. Moreover, the OH% and O2%− were identified as the main radicals during the photo-degradation process. Based on the results of in-situ infrared spectra, a series of intermediates at different oxidation stages, including alcohols, aldehydes, and acids are captured, strongly supporting the proposed photoactive mechanism at gas/solid interface, which is beneficial for understanding the whole reaction process and pursuing new path to eliminating indoor air pollution.

1. Introduction Commonly, volatile organic compounds (VOCs) are viewed as hazardous contaminants because of their toxicity, carcinogenicity,

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persistence, bioconcentration and high C/N ratios in wastewater treatment [1–3]. Thereby, efficient and mild removing VOCs from the gaseous environment have become a hot topic in the field of environment pollution control [4,5]. Meanwhile, the VOCs also at present, the

Corresponding authors. E-mail addresses: [email protected] (Y. Dong), [email protected] (J. Ke).

https://doi.org/10.1016/j.cej.2019.122380 Received 15 May 2019; Received in revised form 25 July 2019; Accepted 29 July 2019 Available online 30 July 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.

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constructing LaVO4/BiOBr heterojunction system. In comparison with the bare LaVO4 and BiOBr, the synthesized LaVO4/BiOBr hybrid system exhibited enhanced photocatalytic activities for gaseous toluene and acetone. Furthermore, the photoelectrochemical data displayed that the introduction of LaVO4 not only improved the solar energy harvesting efficiency but also reduced the transfer resistance of charge carriers in the hybridized system.

well-studied technologies for removal of VOCs are direct oxidation methods, including direct combustion and catalytic oxidation, in the past decades [2,6]. However, for above methods, high energy consumption and investment still limit large-scale usage. In contrast, photocatalytic oxidation, being an advanced oxidation process, are widely used in environmental remediation field as it can take advantage of solar energy to decompose organic pollutants into CO2 and H2O by producing strong oxidizing radicals at mild conditions [7–10]. Titanium dioxide (TiO2) is a general photocatalyst and has been widely investigated by the researchers over the world. For example, Szanyi et al. investigated the photooxidation of acetone on commercial TiO2 (P25) under UV irradiation at ambient temperature by in-situ infrared spectroscopy [11]. Tasbihi et al reported the photocatalytic degradation of toluene on TiO2 nanoparticles immobilized on fiberglass cloth under UV irradiation [12]. Caudillo-Flores et al. synthesized series of anatasebased tungsten doped and composite materials and used to degrade the toluene and styrene [13]. However, TiO2 has a wide bandgap (about 3.2 eV), thereby, its utilization is only limited in ultraviolet part of solar energy. On the other hand, bare TiO2 shows low photocatalytic activity caused by rapid recombination rate of photoexcited electrons and holes. Therefore, nowadays, a series of new and efficient photocatalysts have been developed for not only improving solar energy harvesting efficiency but also enhancing separation and transferring efficiency of photoinduced electrons and holes [14–16]. Recent years, bismuth oxyhalide-based photocatalysts, which is as a potential photocatalyst, has been intensively concerned in potential application fields [17–19]. Among them, BiOBr is one of the most studied bismuth oxyhalide-based photocatalysts because of its high chemical stability and proper bandgap. BiOBr as a V–VI–VII ternary compound is composed of double slabs of [Br] and one slab of [Bi2O2]. This unique layered crystal structure can result in formation of strong interior electric field between positive [Bi2O2] layer and negative [Br] layer, which can spatially separate photoinduced electron-hole pairs, thereby increasing the capability of active species [20–22]. Nonetheless, the individual BiOBr photocatalyst is still limited utilization due to low light absorption and rapid recombination of electrons and holes. Therefore, several modification methods, including nanostructure engineering and constructing heterojunction, are often used to adjust the charge movement in semiconductor photocatalysts. Various micro or nano scale morphologies, for example, microspheres, nanotubes and nanospheres, have already been prepared by controlling the synthesization conditions [23–26]. In these morphological feature, microspheres have been paid more attention because of hierarchical structure and large surface area, which could provide more adsorption sites for catalytic reactions [27]. In addition, construction of heterojunction by coupling BiOBr with other materials together is viewed as an another effective way to achieve the enhanced photocatalytic activity of BiOBr, which can effectively extend the charge separation and transfer by forming an additional internal electric field [28–30]. Until now, some heterojunction materials have been successfully synthesized, such as, BiOI/CeO2 [31], BiPO4/BiOBr [32], CuInS2/Bi2WO6 [33] and so on. The results evidently demonstrate that the multiple-component heterojunctions exhibit much higher photocatalytic activities than the single-component photocatalysts. Recently, LaVO4 as a semiconductor photocatalyst is a promising choice due to its unique catalytic properties and the preferable harvesting ability to visible light [34–36]. When other semiconductors were combined with LaVO4 together, their photocatalytic activities were greatly improved through the formation of heterojunction [35,37–39]. For example, Fang et al. reported that Pt/ LaVO4/TiO2 hybrid system was synthesized and performed enhanced photocatalytic degradation for benzene irradiated by simulated solar light [40]. Meanwhile, Ma et al. demonstrated that the introduction of LaVO4 enhanced the photoctalytic ability of bare BiOBr for degradation of organic dyes [41]. Based this strategy, in this work, the flower-like BiOBr nanospheres was combined with LaVO4 by in-situ hydrothermal method for

2. Experimental 2.1. Fabrication of LaVO4 nanoparticles The LaVO4 nanoparticles were fabricated according to our report [39]. In typical, equimolar NaOH and NH4VO3 (6.5 mmol) were mixed and dissolved into 10 mL of deionized water, forming an ivory-white NaVO3 solution. After that, 13 mL of La(NO3)3 solution (0.5 M) was slowly dropped into the above mixed solution and then the color quickly changed to yellow. Then, the mixed suspension was transferred into a 100 mL Teflon-lined autoclave (Anhui Kemi Machinery Technology Co., Ltd.), maintained at 200 °C for 48 h. The obtained samples were repeatedly washed several times by using distilled water and absolute ethanol. Finally, a green LaVO4 was dried at 100 °C for overnight. 2.2. Synthesis of flower-like LaVO4/BiOBr heterojunction nanocomposites The flower-like LaVO4/BiOBr were synthesized through a facile solvothermal method. Typically, 0.5 mmol of CTAB and 0.5 mmol of Bi (NO3)3·5H2O were mixed and dissolved into 5 mL of ethylene glycol. Subsequently, the value of pH was adjusted to 3 using Na2CO3 solution. After stirring for 30 min, the above prepared LaVO4 nanoparticles (the different initial mass ratios of LaVO4 to BiOBr denoted 1%, 2%, 3%, 5% and 10%) was added into the above mixture. Then, the obtained mixed solution was transferred into Teflon-lined autoclave, maintained at 130 °C for 6 h. After cooling, the precipitate was collected and washed by using distilled water and absolute ethanol for several times, respectively, and then dried at 60 °C for overnight. The pristine BiOBr was also fabricated by the same method in the absence of LaVO4. 2.3. Characterization of flower-like heterojunction LaVO4/BiOBr nanocomposites Scanning electron microscope (SEM, JSM-5600LV, JEOL) and transmission electron microscope (TEM, JEM-2100, JEOL) were utilized to characterize morphologies and nanostructures of the produced samples. The X-ray powder diffraction patterns of the samples were measured on a D8 X-ray diffractometer (Bruker) equipped with Cu Kα radiation (λ = 1.5406 Å). XPS data were recorded on a VG Multilab 2000 (VG Inc.) photoelectron spectrometer using Al Kα radiation as the excitation source under vacuum at 2 × 10−6 Pa using C 1s as energy reference (284.6 eV). Pore and surface areas of the obtained sample were measured by using gas sorption analyzer (Quantachrome). UV–vis diffuse reflectance spectra (DRS) were taken on UV-2600 (Shimadzu). Photoluminescence (PL) spectra were measured with 380 nm excitation wavelength on a RF-6000 spectro fluorophotometer (Shimadzu). Electron spin resonance (ESR) was conducted on E500 spectrometer (Bruker), where the radicals were trapped by 5, 5-dimethyl-1-pyrroline N-oxide (DMPO). 2.4. Photocatalytic activity evaluation The photocatalytic activities of the synthesized samples for gaseous toluene and acetone were evaluated over in a quartz reaction cell. Briefly, the wafer was formed by 0.2 g of photocatalyst and fixed to the holder of the reactor. After that, liquid toluene or acetone (5 μL) was injected into the reactor with a micro syringe. During the process of photocatalytic reaction, the concentration of toluene or acetone was 2

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Besides, two strong peaks at 1620 cm−1 and 3470 cm−1 could be assigned to the adsorbed H2O. For bare BiOBr, the sharp peak at 516 cm−1 was ascribed to the stretching mode of Bi–O bond [43]. In the case of LaVO4/BiOBr, it was found that the main characteristic peaks of BiOBr and LaVO4 were also appeared in the LaVO4/BiOBr composites. Furthermore, the strong peaks at 2800–3000 cm−1 and 1000–1200 cm−1 are attributed to the stretching vibration of C–H in methyl group and C–O in alcohol group, respectively, which results from the residual of organic solvent in the heterojunction samples. Apparently, according to the above results, it is concluded that these heterojunctions are consisting of LaVO4 and BiOBr. In order to analyze oxidation state and surface chemical composition of the samples, XPS measurement was tested on pristine BiOBr and 3% LaVO4/BiOBr. In Fig. 2a, XPS spectra of 3% LaVO4/BiOBr and bare BiOBr samples are displayed, where these characteristic peaks of Bi, Br, V, O, La, and C are observed clearly. In Fig. 2b, for the pristine BiOBr, the binding energy peaks at 159.43 eV and 164.73 eV are ascribed to Bi 4f 7/2 and Bi 4f 5/2, respectively, indicating that the Bi3+ exists in BiOBr sample [44]. However, the Bi 4f peaks in 3% LaVO4/BiOBr shifted to 163.96 eV and 158.64 eV, respectively, which is due to the formation of heterojunction between LaVO4 and BiOBr. In Fig. 2c, the binding energy at 68.57 eV and 69.58 eV are detected, which are ascribed to the Br 3d 5/2 and Br 3d 3/2 of the pristine BiOBr, respectively [45], while the Br 3d peaks in 3% LaVO4/BiOBr shift to 67.81 eV and 68.78 eV, respectively. In Fig. 2d, binding energies at 516.37 eV and 523.68 eV were found, resulting from V 2p3/2 and V 2p5/2 in 3% LaVO4/BiOBr, meaning that V element exists as the V5+ oxidation state [46]. Compared with the V 2p3/2 and V 2p5/2 at 517.16 eV and 524.63 eV in pure LaVO4, respectively, the peaks of V 2p in hybrid sample shifted to lower binding energies, which was attributed to the formation of heterojunction between LaVO4 and BiOBr. In Fig. 2e, binding energies at 834.20 eV and 851.11 eV were ascribed to La 3d5/2 and La 2d3/2 in 3% LaVO4/BiOBr, meaning that La element exists as the La3+ [47]. Compared with the La 3d in pure LaVO4, in which the binding energies were located at 834.67 eV and 851.52 eV, respectively, the binding energies of the hybrid sample also decreased. The XPS of O 1s were fitted into two peaks at 531.28 eV and 530.15 eV in the pristine BiOBr, as shown in Fig. 2f, which derived from the hydroxyl groups and the lattice oxygen, respectively. The binding energy at 530.15 eV can be ascribed to the lattice oxygen in the pure LaVO4. Meanwhile, three characteristic peaks of O 1s at 529.48 eV, 531.43 and 531.84 eV (Fig. 2f) in 3% LaVO4/BiOBr were observed, which were related to lattice oxygen of LaVO4 and BiOBr and hydroxyl groups, respectively [48]. These results demonstrated that the introduced LaVO4 leads to the formation of heterojunction between LaVO4 and BiOBr.

Scheme 1. A schematic of the photocatalytic reactor.

detected at certain intervals by the GC2010 plus (Shimadzu) with a FID detector. The reactor is the cylinder with 40 mm of the diameter and 194 mm of the length, as shown in Scheme 1. The transmission IR spectra were measured ranging from 4000 to 500 cm−1 by Tracer-100 FTIR spectroscopy (Shimadzu) with a resolution of 1 cm−1. Visible light source chose A 500 W Xenon lamp equipped with a 400 nm cut-off filter. 2.5. Photoelectrochemical test The photoelectrochemical tests were measured on CHI660E electrochemistry workstation (Shanghai Chenhua Instrument Co. Ltd) with a three-electrode system. The platinum foil and saturated calomel electrode (SCE) were as counter electrode and reference electrode, respectively. As working electrode, Indium-Tin Oxide (ITO) conductive glass with 1 mm × 2 mm was chosen after ultrasonically washing by acetone. Then, the catalysts were mixed with Nafion (10%) solution to form slurry solution and then dropped to the pretreated ITO and dried at 60 °C. In addition, Na2SO4 solution (0.5 M) was chosen as electrolyte. Surface photovoltage (SPV) were obtained through a lock in-based system equipped with a monochromator and amplifier, where the phase spectra were synchronously taken with the SPV spectra. 3. Results and discussion

3.2. Morphologic characterization

3.1. Structure characterization

In Fig. 3a, it is found that the size of bare LaVO4 nanoparticles is about 100 nm whilst the pristine BiOBr is flower-like hierarchical structure with an average diameter of 2–3 μm (Fig. 3b and inset). In the case of LaVO4/BiOBr hybrid system, it is apparently observed that LaVO4 nanoparticles are anchored at the surface of BiOBr microsphere (Fig. 3c and inset). Fig. 3d–h shows the elemental maps of 3% LaVO4/ BiOBr hybrid system, which further demonstrates the homogeneous distribution of LaVO4 on the surface of BiOBr. TEM and HRTEM are employed to further investigate the morphology and confirm the heterojunction structure between LaVO4 and BiOBr. The TEM and HRTEM images of pristine BiOBr and 3% LaVO4/BiOBr are shown in Fig. 4. It is demonstrated that the BiOBr is likely flowers, which is consistent with the result of SEM. Besides, in Fig. 4b, it is observed that the LaVO4 nanoparticles closely contact with the BiOBr nanospheres, which is beneficial for rapid transferring of photoexcited charge carriers. Furthermore, in Fig. 4c and d, two lattice fringes of 0.278 nm and 0.292 nm are found, which are consistent with the values of (1 1 0) structure of the BiOBr (JCPDS card no. 09-0393) and (0 1 2) plane of monoclinic

The crystal phase of LaVO4, BiOBr and LaVO4/BiOBr were shown in Fig. 1a. For bare LaVO4, the diffraction peaks of LaVO4 are indexed to monoclinic phase (JCPDS No. 50-0367), where these peaks are very strong and sharp, indicating that m-LaVO4 has high crystallization. For the pristine BiOBr, the (1 0 1), (1 1 0), (2 0 0) and (2 0 0) crystal planes are corresponding to the peaks at 25.3°, 32.2°, 46.3° and 57.2°, respectively, which is assigned to tetragonal BiOBr, corresponding to JCPDS No. 09-0393 [42]. For the LaVO4/BiOBr heterojunctions, the characteristic diffraction peaks attributed to BiOBr can be observed, whereas it is obvious that the ones gradually moved to higher degree. Meanwhile, the characteristic peaks of LaVO4 are detected in 10% LaVO4/BiOBr composites, as shown in Fig. S1, while the peaks of LaVO4 disappear in rest of composites due to the lower amount of LaVO4. FTIR spectra of these synthesized samples are shown in Fig. 1b. In the case of LaVO4, the two characteristic peaks centered at 791 and 438 cm−1 could be ascribed to vibration of VO43− and La-O, respectively [37]. 3

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Fig. 1. (a) X-ray diffraction patterns and (b) FTIR spectra of the LaVO4, BiOBr and (1%, 2%, 3%, 4% and 5%) LaVO4/BiOBr.

Fig. 2. XPS spectra of survey (a), Bi 4f (b), Br 3d (c), V 2p (d), La 3d (e), and O 1s (f) of 3% LaVO4/BiOBr, pure BiOBr and pure LaVO4.

the introduction of LaVO4 has weak influence on surface area and the difference of photocatalytic performance is assigned to other factors [51].

LaVO4 (JCPDS card no. 50-0367), respectively [49,50]. Based on the above analysis, it indicates that the introduced LaVO4 nanoparticles are tightly anchored at the surface of the BiOBr nanospheres through a facile hydrothermal route, inferring the formation of heterojunction between LaVO4 and BiOBr. In Fig. 5, N2 adsorption-desorption isotherms of the obtained samples are displayed, where the hysteresis loops of the pristine BiOBr and 3% LaVO4/BiOBr belong to IV-type isotherms. Table S1 also displays the surface area, pore volume and pore diameter of the pristine BiOBr and 3% LaVO4/BiOBr composites. The surface area of the pristine BiOBr and 3% LaVO4/BiOBr heterojunction is 9.78 m2 g−1 and 9.17 m2 g−1, respectively, which is almost unchanged and indicates that the incorporation of LaVO4 almost has slightly effect on the specific surface area of pristine BiOBr. Meanwhile, as shown in the inset of Fig. 5, the pore sizes and pore volumes of the pristine BiOBr and 3% LaVO4/BiOBr heterojunctions show slightly different, which means that

3.3. The optical properties The photoresponse properties of photocatalysts generally have significant effects on photocatalytic activities because strong photoresponse ability infers the high quantum efficiency, which can provide more photogenerated electrons and holes, producing higher concentration of active species, thereby exhibiting better photocatalytic performance [52,53]. Therefore, DRS analyses of the LaVO4/BiOBr composites were conducted, as shown in Fig. 6a. It is observed that the pristine BiOBr displays efficient absorption with the onset at 457 nm. Compared with BiOBr, LaVO4 exhibits a wider band gap. For LaVO4/ BiOBr composites, it exhibits stronger visible light absorption ability 4

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was measured for better comparison. As shown in Fig. S4, the absorption edge of 3% LaVO4/BiOBr by hydrothermal method displays redshift compared with the pure BiOBr due to the introduction of LaVO4. However, 3% LaVO4/BiOBr sample prepared by mixing method displays no significant redshift and independent absorption feature, which is attributed to no formation of heterojunction and the weak interaction between LaVO4 and BiOBr. To investigate separation process of photogenerated charge carriers, surface photovoltage (SPV) is used generally, where higher SPV signal indicates lower recombination efficiency of photo-induced charge carriers [56,57]. Herein, the SPV and corresponding phase spectra of the prepared BiOBr and LaVO4/BiOBr hybrids are displayed in Fig. 7. For all the samples, SPV response can be obviously found ranging from 300 to 500 nm, which is consistent with the DRS results. In Fig. 7a, it is observable that the SPV response intensity of LaVO4/BiOBr heterojunction is higher than that of bare BiOBr nanospheres, which is ascribed to the introduction of LaVO4, thus producing more photo-induced charge carriers. The SPV response of 3% LaVO4/BiOBr hybrids is the strongest, which indicates that recombination efficiency of photoinduced electrons and holes in 3% LaVO4/BiOBr hybrids is the weakest. On the other hand, the corresponding SPV phase value of the pristine BiOBr is from −180° to +180°, which means that the photoexcited charge carriers are movement disorder in hybrid system, as shown in Fig. 7b. It could cause severe recombination of electron and hole. Furthermore, it is found that for LaVO4/BiOBr composites, the corresponding SPV phase values cover from 0 to 90° or −90°. This result indicates that the photogenerated charge carriers can orientationally move inside these samples. Therefore, it demonstrates that the formation of heterojunction could apparently affect the transfer behavior of photoinduced charge carriers, thus suppressing recombination probability of photogenerated electrons and holes in the LaVO4/BiOBr composites. Photoluminescence is another indicator of recombination of photogenerated electrons and holes, where low PL intensity suggests low recombination of electrons and holes [58,59]. Fig. 8 illustrates the PL spectra of the pristine BiOBr and LaVO4/BiOBr hybrid system. A peak centered at 489 nm is attributed to the intrinsic emission of BiOBr while the PL intensities of LaVO4/BiOBr decrease gradually due to the suppressed recombination of the photoinduced charge carriers resulting from the formation of heterojunctions. Combined with the results of the DRS and SPV, the LaVO4/BiOBr heterojunctions possess stronger absorption abilities to visible light and lower recombination rate of charge carriers, which could be beneficial for enhancing photocatalytic activities.

Fig. 3. SEM images of LaVO4 (a), BiOBr (b) and 3% LaVO4/BiOBr (c); the elemental mapping of O (d), Bi (e), Br (f), V (g) and La (h) of the prepared 3% LaVO4/BiOBr hetereojunctions.

3.4. Photoelectrochemical properties

than the pristine BiOBr. Clearly, absorption edges display a redshift with the increasing amount of LaVO4, which is ascribed to formation of the heterojunction between LaVO4 and BiOBr, thereby effective improving the absorption capacity in region of visible light. Furthermore, the bandgap energies of the samples are estimated by using the following equation [54]:

(αhν ) = k (hν − Eg )n/2

The photoelectrochemical properties of the pristine BiOBr and 3% LaVO4/BiOBr heterostructure composites have been measured to provide efficiently evidence for demonstrating the separation and transfer efficiency of photo-induced charge carriers [60,61]. The photocurrent response of the pristine BiOBr and 3% LaVO4/BiOBr heterostructure are displayed in Fig. 9a. It is observable that the photocurrent intensity of the 3% LaVO4/BiOBr heterostructure is higher than that of the pristine BiOBr, indicating that the formation of the heterojunction between LaVO4 and BiOBr efficiently improves the solar energy harvesting efficiency and suppresses the recombination of photo generated electrons and holes. To further investigate the photoelectrochemical nature of the as-prepared samples, flat band potential and carrier density were measured by taking Mott-Schottky (M-S) curves. For better comparison, the electrode potential vs. SCE was converted to reversible hydrogen electrode (RHE) according to the following Nernst equation (2):

(1)

where Eg is the calculated band gap, hν is the discrete photon energy, k is a constant, and n is 1 and 4 for direct and indirect semiconductor, α is the absorption coefficient, respectively. In Fig. 6b, the estimated bandgap energies of bare LaVO4 and BiOBr are equal to 2.06 eV and 2.71 eV, respectively, which are agreement with the previous reports [34,55]. Furthermore, the bandgap energies of LaVO4/BiOBr heterojunctions with different loadings of LaVO4 are calculated to be 2.67 eV, 2.63 eV, 2.57 eV, 2.50 eV and 2.17 eV, respectively. It is found that the introduction of LaVO4 can effectively reduce the apparent bandgap energy leading to the enhancement of solar harvesting and utilization efficiency. Furthermore, DRS of 3% LaVO4/ BiOBr by mixing method

0 VRHE = VSCE + 0.059pH + V SCE

(2)

where VRHE, VSCE and V0SCE are the converted potential vs. RHE, the 5

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Fig. 4. (a, b) TEM images of pristine BiOBr and 3% LaVO4/BiOBr, respectively and (c, d) HRTEM image of pristine BiOBr and 3% LaVO4/BiOBr, respectively.

observed, indicating two different electronic behaviors coexist in heterojunction sample, which demonstrates that the introduction of LaVO4 results in formation of heterojunction [62]. The flat band potential of BiOBr shifted to −0.15 V, indicating the realignment of band energy level after formation of heterojunction. Besides, the electrochemical impedance spectroscopy (EIS) in the manner of Nyquis diagram is shown in Fig. S3. Generally, a smaller arc radius corresponds to a lower charge-transfer resistance, which indicates that the photoexcited charge carriers can easily transfer at the interface. In Fig. S3, it is observed that the 3% LaVO4/BiOBr heterojunctions exhibits smaller resistance arc radius than the pristine BiOBr, which suggests that 3% LaVO4/BiOBr hybrid system displays better performance in the charge-transfer process. It is expected that the formation of heterojunction between LaVO4 and BiOBr is beneficial for movement of the photogenerated charge carriers at the interface. 3.5. Photocatalytic degradation of gaseous acetone and toluene Fig. 5. N2 adsorption-desorption isotherms and the pore size distribution (the inset) of the bare BiOBr and 3% LaVO4/BiOBr.

The photocatalytic performance of the obtained samples was evaluated through photooxidation degradation of gaseous toluene and acetone under visible light illumination. As shown in Fig. 10a, for the LaVO4 and BiOBr, the photocatalytic conversion efficiencies of acetone are just 6.8% and 36.1% irradiated by visible light for 3 h, respectively. While the conversion efficiencies of acetone are significantly improved, reaching to 75.5% even 95.4% over LaVO4/BiOBr with different amounts of LaVO4. It is observed that the photocatalytic ability of 3% LaVO4/BiOBr is much higher than the other samples. As shown in Fig. 10c, the photocatalytic efficiency of toluene was only 6.0% and 16.3% over pure LaVO4 and BiOBr under visible light irradiation after 4 h, respectively. The conversion efficiencies of toluene were from 65.2% to 87.1% over LaVO4/BiOBr with different amounts of LaVO4, respectively. It is also observed that the photocatalytic ability of 3% LaVO4/BiOBr is much higher than the other samples. The enhanced photocatalytic activities of these LaVO4/BiOBr hybrid systems are ascribed to the following reasons: (1) the formation of heterojunctions between LaVO4 and BiOBr could harvest more visible light because absorption edges occurred to red shift with the increasing amount of LaVO4; (2) The formation of heterojunctions between LaVO4 and BiOBr

measured experimental potential against the SCE and 0.245 V at pH 6.8, 25 °C, respectively. Furthermore, the potential at the electrode-electrolyte interface was calculated according to the following equation:

1 1 ⎞ ⎡ (V − Vfb) − kT ⎤ =⎛ 2 ⎢ ⎥ C2 ε ε eN A e ⎝ r 0 d ⎠⎣ ⎦ ⎜

⎟

(3)

where C, εr, Nd, V, Vfb, T and A are specific capacity, dielectric constant, carrier density, applied potential, flat band potential, the absolute temperature and efficient area of electrode, respectively. ε0, e, and k are electric permittivity of vacuum (8.85 × 10−12 N−1 C2 m−2), free electron charge (1.602 × 10−19 C), and Boltzmann constant, respectively. In Fig. 9b, it is found that the M-S curve of BiOBr has positive slope, which indicates that the BiOBr is n-type semiconductor. Meanwhile, the flat band potential of the BiOBr is −0.24 V vs RHE by using the x intercept of the liner region. Moreover, for the LaVO4/BiOBr heterojunction, a ‘V-shape’ curve consisting of positive and negative slopes is 6

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Fig. 6. (a) UV–vis spectra and (b) bandgap energies of the prepared samples.

Fig. 7. (a) SPV spectra and (b) corresponding phase spectra of the prepared samples.

Fig. 10b and d and Table S2, it is seen that the kinetic constants of acetone and toluene photodegradation over 3% LaVO4/BiOBr (0.888 h−1 and 0.496 h−1) is 6.0 and 11.5 times as high as the pristine BiOBr (0.147 h−1 and 0.043 h−1), respectively, implying that the coupling BiOBr with LaVO4 significantly enhances degradation efficiency of gaseous pollutions over these fabricated photocatalysts. Furthermore, the photocatalytic activities of commercial TiO2 (P25) and previous reported BiPO4/BiOBr composites have been added for better comparison. In Fig. S4, it is observed that the P25 and BiPO4/BiOBr composites display 8% and 45% degradation conversion of toluene for 4 h under visible light irradiation, respectively, which are weaker than that of the 3% LaVO4/BiOBr heterojunction in this work. The results mainly are attributed to the wide bandgap energies of P25 and BiPO4/ BiOBr composites. The photocatalyst stability of the catalysts is another important issue from the point of practical application. To investigate the stability of pristine BiOBr and 3% LaVO4/BiOBr, four runs of cycling acetone photodegradation experiments under identical conditions were evaluated (Fig. S5). The photocatalytic efficiency of the pristine BiOBr decreases with the increasing cycling. In contrast, the photocatalytic degradation efficiency of 3% LaVO4/BiOBr composite remained at 92.5% after the fourth runs, which was much more stable than that of the pristine BiOBr.

Fig. 8. PL spectra of the prepared samples.

would efficiently suppress recombination possibility of photo-induced charge carriers. Furthermore, the Langmuir-Hinshelwood model is used to match reaction kinetics based on the following equation [63,64]:

ln(C0 /Ct ) = kt

(4) 3.6. In situ FTIR

where Ct, C0 and t are the final concentration, the initial concentration and reaction time, respectively and k is the kinetic constant. From

In situ FTIR can provide real-time information about the surface 7

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Fig. 9. (a) Photocurrent of the pristine BiOBr and 3% LaVO4/BiOBr and (b) Mott-Schottky curves of the pristine BiOBr and 3% LaVO4/BiOBr.

probably ascribed to acetaldehyde or formaldehyde, which results from the initial photooxidation of acetone [11]. Meanwhile, the bands at 2801, 2703, 1547, 1418 and 1307 cm−1 are attributed to further photooxidation products, such as acetic acid or formic acid [66]. It can be observed that these peaks increase at interval of first 1.5 h and then decreased after 1.5 h, which demonstrates that the gaseous acetone is first oxidized to aldehydes or ketones and further carboxylic acids. Finally, the carboxylic group is peeled to form CO2. The formation of CO2 and H2O species can be testified by the appearance of bands at 2361, 2341 and 3728–3598 cm−1, respectively [67], indicating the total mineralization of organic pollutants, which is consistent with the previous

adsorbed species during the reaction, which strongly supports the proposed foundational mineralization reaction mechanism. Herein, in situ FTIR transmittance spectra of the photocatalytic oxidation of acetone and toluene over 3% LaVO4/BiOBr heterojunctions were taken to investigate the photooxidation reaction path. As shown in Fig. 11a and b, before the lamp is lightened, strong peaks appeared at 2973, 1739, 1367, 1227 and 1225 cm−1 result from the characteristic stretching vibration of gas-phase acetone [65]. After that, it is found that these vibration peaks decrease progressively with the continuous irradiation while some new feature peaks appeared gradually. The bands located at 2843, 2822, 2731, 1757, 1439 and 1124 cm−1 are

Fig. 10. Photocatalytic conversion of gaseous acetone (a) and toluene (c) and kinetic curves of gaseous acetone (b) and toluene (d) over the LaVO4, BiOBr and LaVO4/BiOBr. 8

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Fig. 11. In situ FTIR spectra over 3% LaVO4/BiOBr during the photooxidation of acetone.

3.7. Mechanism exploration

reports. Meanwhile, in Fig. 11c and d, it is found that the increasing of characteristic stretching vibration peaks of CO2 and H2O is not synchronous with that of the ketones and carboxylic acids, which means that the thorough mineralization of acetone is achieved gradually from ketones to carboxylic acids to CO2 and H2O. These bands and corresponding assignments are listed in Table S3. As shown in Fig. 12a, for the toluene degradation, strong peaks at 3078, 3039, 2937 and 2880 cm−1 originate from stretching vibration of C–H in benzene ring and methyl group, respectively [67]. In Fig. 12b, four bands at 1799, 1611 and 1500 cm−1 are attributed to the characteristic vibrations of aromatic ring. The vibration intensities of bands decrease gradually when the system is irradiated, which indicates the toluene is gradually degraded over the LaVO4/BiOBr heterojunctions [68]. In contrast, the intensities of bands at 2361 and 2341 cm−1 corresponding to CO2 increase with the photodegradation of toluene [69], as the shown in Fig. 12e. At the same time, the region of 3600–3800 cm−1 corresponding to hydroxyl bonding also increases (Fig. 12f), which means the formation of H2O. Furthermore, some new peaks are detected, as shown in Fig. 12b, where the bands at 1362 and 1339 cm−1 is attributed to the vibration of benzyl alcohol [70] and the bands located at 1548 and 1510 cm−1 is ascribed to the vibration stretching of formed C]O bond resulting from benzaldehyde species [71], which derives from the attacking of reactive oxygen species, for example, OH% and %O2−. Furthermore, the bands situated at 1510 and 1419 cm−1 belong to the asymmetric stretching vibration of COO– while the bands at 1548 cm−1 are assigned to the stretching vibration of C]C [72], which clearly demonstrates the opening of aromatic ring due to the strong oxidative ability of reactive oxygen species formed at the surface of LaVO4/BiOBr heterojunctions. These bands and corresponding assignments are displayed in Table S4.

During photocatalytic oxidation process, hydroxyl radicals (OH%), superoxide radicals (O2%−), and active holes (h+) often separately or together act as the main radicals for destroying and mineralizing pollutants [73]. Herein, electron spin resonance (ESR) is utilized to detect the involved active species during the photocatalytic course, the. In Fig. 13a, it is observed that no characteristic peaks attributed to any active species are detected in dark condition while strong characteristic four peaks ascribed to DMPO-OH· with intensity ratio of 1:2:2:1 are observed in LaVO4/BiOBr heterojunction under visible light irradiation. Meanwhile, six characteristic peaks were also stronger, which were ascribed to the DMPO-O2%−, as shown in Fig. 13b. The results of ESR demonstrated that the OH% and O2%− radical species are produced as the predominant species, which is crucial to efficient photocatalytic activity of 3% LaVO4/BiOBr. Meanwhile, although the pristine BiOBr also are found to produce characteristic peaks resulting from OH% and O2%− radical, as shown in Fig. 13, the intensities is obvious less than that of the 3% LaVO4/BiOBr hybrids, which indicates the heterojunctions can produce more active species, thereby exhibiting better photocatalytic performance. For better ananlyzing photocatalytic mechanism separation, herein, the conduction band (CB) and valence band (VB) of LaVO4 and BiOBr are calculated by using the following equation [74–76]:

EVB = X− Ee + 1/2Eg

(5)

ECB = EVB − Eg

(6)

where ECB, EVB are the positions of CB and VB, respectively, and Eg is the band gap. Ee is the electronegativity of free electrons, 4.5 eV. X is Mulliken’s electronegativity of the samples, where the X values of LaVO4 and BiOBr are 5.53 eV and 6.45 eV, respectively. The VB top of 9

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Fig. 12. In situ FTIR spectra over 3% LaVO4/BiOBr during the photooxidation of toluene.

and electrons. The accumulated holes could capture the adsorbed H2O to form OH· and participate in photocatalytic oxidation reaction. Meanwhile, the enriched electrons at the CB of BiOBr could react with the adsorbed O2 to produce O2·−. Subsequently, the generated radicals attack acetone and toluene and produce a series of intermediates to the final products including CO2 and H2O.

LaVO4 and BiOBr are equal to 2.01 eV and 3.31 eV, respectively, and the CB bottom of LaVO4 and BiOBr locate at −0.05 eV and 0.60 eV, respectively (in Table S5). Integrating the above results together, the probable photocatalytic mechanism is proposed and displayed in Scheme 2. When the LaVO4 was introduced and coupled with BiOBr, the heterojunction is formed at the interface between LaVO4 and BiOBr, where the Fermi levels of BiOBr and LaVO4 will trend to reach an equilibrium state, and then an inner electric field is formed. When the LaVO4/BiOBr heterojunction is excited by visible light, electrons are produced at the CB of LaVO4 and BiOBr and holes are created at the VB of LaVO4 and BiOBr. After that, these holes at the VB of BiOBr tend to move to the VB of LaVO4. In contrast, the excited electrons at the CB of LaVO4 transfer to the CB of BiOBr. Furthermore, the inner electric field at junction interface will further push the holes on the VB of BiOBr toward that of LaVO4, whilst the electrons will move from CB of LaVO4 to the CB of BiOBr. This type II band alignment could significantly improve the separation of holes

4. Conclusions In this work, flowers-like LaVO4/BiOBr heterojunctions were fabricated through a facile solvothermal method to obtain enhanced photocatalytic degradation for gaseous pollutants. Photoelectrochemical results demonstrate that the introduction of LaVO4 can not only significantly enhance the separation of photo-induced electrons and holes but also promot the solar energy harvesting efficiency. In comparison with pristine BiOBr and LaVO4, a series of LaVO4/BiOBr samples exhibit enhanced photocatalytic activity. The 3%

Fig. 13. ESR tests of DMPO-OH· (a) and DMPO-O2%− (b) in bare BiOBr and 3% LaVO4/BiOBr heterojunction. 10

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Scheme 2. A schematic of band gap match of the 3% LaVO4/BiOBr and degradation pathway of gaseous acetone or toluene.

LaVO4/BiOBr displayed the best photoactivity, where conversions of toluene and acetone achieved to 95.4% for 3 h and 87.1% for 4 h, respectively. Based on the results of in-situ FTIR, a series of intermediates, including a series of alcohols, aldehydes, and carboxylic acids were detected and certified in the photooxidation process of acetone and toluene. Furthermore, the main active species including OH% and O2%− are confirmed involving in the photocatalytic course of acetone or toluene. Such novel catalysts could be promising candidate to promote the photodegradation of VOCs in the ambient environment.

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