Carbonate doped Bi2MoO6 hierarchical nanostructure with enhanced transformation of active radicals for efficient photocatalytic removal of NO

Journal of Colloid and Interface Science 557 (2019) 816–824

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Carbonate doped Bi2MoO6 hierarchical nanostructure with enhanced transformation of active radicals for efficient photocatalytic removal of NO Wangchen Huo a,b, Weina Xu c, Tong Cao a, Ziyang Guo a, Xiaoying Liu d, Guangxu Ge a, Nan Li e, Tian Lan e, Hong-Chang Yao f, Yuxin Zhang a,⇑, Fan Dong b,⇑ a

State Key Laboratory of Mechanical Transmissions, College of Materials Science and Engineering, Chongqing University, Chongqing 400044, PR China Research Center for Environmental Science & Technology, Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 611731, PR China c Department of Physics, Chongqing University, Chongqing 401331, PR China d Engineering Research Center for Waste Oil Recovery Technology and Equipment of Ministry of Education, Chongqing Key Laboratory of Catalysis and New Environmental Materials, Chongqing Technology and Business University, Chongqing 400067, PR China e Aerospace Institute of Advanced Materials & Processing Technology, Beijing 100074, PR China f College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou City, Henan Province 450001, PR China b

g r a p h i c a l a b s t r a c t Carbonate dopants could modulate the electron states and promote generation of radicals for high-efficiency photocatalytic NO removal.

a r t i c l e

i n f o

Article history: Received 10 August 2019 Revised 23 September 2019 Accepted 24 September 2019 Available online 25 September 2019

a b s t r a c t Doping heteroatoms in photocatalyst is an effective strategy to signally enhance the photocatalytic activity. Herein, we have favorably fabricated the carbonate doped Bi2MoO6 via a facile one-pot solvothermal method, which was verified by structure and constituent characterization analysis. In addition, the NO removal efficiency of carbonate-intercalated Bi2MoO6 is ~34%, far-exceeding that of the pure Bi2MoO6 (~13%), whilst exhibits a good stability and durability, owing to that the dopants could modulate the electron states of the Bi2MoO6, thus stimulating charge separation and migration, incenting transformation of

⇑ Corresponding authors. E-mail addresses: [email protected] (Y. Zhang), [email protected] (F. Dong). https://doi.org/10.1016/j.jcis.2019.09.089 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.

W. Huo et al. / Journal of Colloid and Interface Science 557 (2019) 816–824

Keywords: Doping carbonate Bi2MoO6 photocatalyst Oxidation process In situ DRIFTS DFT calculation

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reactive oxygen species and facilitating reactants activation, which are synthetically investigated by experimental characterization coupled with DFT calculation. Significantly, the in situ DRIFTS measurement was employed to dynamic monitor the NO oxidation process and clarify the photocatalytic mechanism under visible light irradiation. This work provides an efficient strategy to design photocatalysts with tunable motivating charge conversion and reactants activation towards NO photooxidation. Ó 2019 Elsevier Inc. All rights reserved.

1. Introduction Solar-driven catalytic reactions have attracted growing interest due to their enormous potential application in producing renewable energy and purifying the contaminant, such as CO2 reduction [1], water splitting [2], synthesis of nitrate [3] and NOx removal [4] et al. Over the past decades, a great deal of effort has been devoted to developing the photocatalysts with cost-efficient, eco-friendly and highly active features [5,6]. Among various photocatalysts, Bi-based semiconductor materials, such as Bi2WO6 [7], Bi2O2CO3 [6], BiOX (X = Cl, Br) [8,9] and Bi2MoO6 [10] et al. have been regarded as one of the most promising candidates for highly photocatalytic efficiency, owing to the good light absorption ability, excellent stability and superior potential engineering application [11,12]. Significantly, orthorhombic koechlinite (Bi2MoO6) material shows a great potential to meet the requirement for realizing efficient photocatalysis, which can be ascribed to the individual alternating layer structure composed of Bi-O and Mo-O layers, favoring of charge separation and transfer [11]. However, the inferior quantum yield, rapid recombination rate of electron-hole pairs, depressed conversion rate of reactive radicals and sluggish surface reaction kinetics are severe downgrading the photocatalytic activity and impeding the commercial applications of Bi2MoO6 [10–13]. To overcome these deficiencies and promote the photocatalytic efficiency, various advanced strategies have been developed, e.g. tuning morphology, constructing heterojunction, and doping heteroatoms [10–15]. Specifically, Qiao et al. [10] had successfully synthesized the Bi2MoO6&Bi2S3 heterojunctions by utilizing a facile in situ routes, and the hybrids shown a promising photocatalytic performance of removal hexavalent chromium under visible light irradiation, owing to the formation of type II junction between the Bi2MoO6 and Bi2S3, which could accelerate the separation and migration of electron-hole pairs, thus improving the photocatalytic activity. Xing et al. [15] reported that the carbon-doped Bi2MoO6 photocatalysts were favorably fabricated via employing hydrothermal-calcination routes for high-efficiency photocatalytic activity, owning to the carbon dopants promoting the conversion of reactive oxygen species. Although these examples could certify the effectivity of the strategies as mentioned above, the promotion performance method via regulating morphology and constructing heterojunction could not exceed the intrinsic activity of the materials, while the doping heteroatoms could dramatically improve the activity, even surpass the intrinsic activity [16]. This is because of the dopants can modulate the electronic states and surface physicochemical properties, which may in turn motivate migration and transformation of electron-hole pairs, and heighten abilities of trapping electrons on the surface. In addition, the study about electron transfer mechanism and photocatalytic reaction process, including conversion of reactive radicals, activation of reactants and elementary reactions, which are of far-reaching significance for developing efficient solar-driven catalytic technologies and improving its commercial applications, is still deficiency and challenge. Herein, we applied the means of doping heteroatoms in photocatalyst to enhance the photocatalytic activity of NO removal. The

carbonate dopants have favorably introduced in orthorhombic Bi2MoO6 layer structure by a facile one-pot solvothermal routine, which are confirmed by the structure characterization analysis, e.g. XRD, TEM and elemental analysis. In addition, the carbonate doped Bi2MoO6 shows a high photocatalytic activity, NO removal efficiency reaches ~34%, and excellent stability and durability (no obvious recession after five repetitive test). The good performance can be ascribed to the introduction of carbonate dopants, favoring of the separation and migration of electron-hole pairs, conversion of reactive oxygen species, activation of reactants and surface reaction kinetics process, which are certified by experimental characterization and density functional theory (DFT) calculation. Importantly, the photocatalytic NO oxidation mechanism and elementary reaction steps were visually investigated by applying the in situ DRIFTS measurement. This work provides an effective strategy to promote photocatalytic activity and profoundly insight into the promotion mechanism of NO oxidation. 2. Experimental section 2.1. Chemical materials Bismuth nitrate pentahydrate (Bi(NO3)3
5H2O, 98.0%), sodium carbonate (Na2CO3, 98.0%), and sodium molybdate dehydrate (Na2MoO4
2H2O, 99.0%) were purchased from Alfa Aesar. Ethanol (C2H5OH, Grade AR 96.0%) and ethylene glycol (EG, 99.0%) were purchased from Chongqing Chuandong Chemical (Group) co. LTD, China. All chemical reagents are the analytical purity without any further purification during the applying process. 2.2. Synthesis of carbonate-intercalated Bi2MoO6 (CO3-BMO) The carbonate-intercalated Bi2MoO6 (CO3-BMO) has been successfully fabricated by utilizing the facile one-step solvothermal method. In the typically, A solution: 3 mmol Bi(NO3)3
5H2O was dissolved in 20 mL EG solution; B solution: 1. 2 mmol Na2MoO4
2H2O and 0.3 mmol Na2CO3 were added in 10 mL EG solution. After forming the homogeneous and clear solutions, the B solution was added into A solution under drastic stirring for 1 h. Then the mixed solution was transferred into the 50 mL Teflon-lined autoclave and heated to 160 °C for 18 h. After the reaction finished, the autoclaves were naturally cooled down, and the products were washed several times and dried at 60 °C for 12 h. The obtained product was named as 20%-CO3-BMO. In addition, the different proportions of Na2CO3 and Na2MoO4
2H2O (0: 1.5, 0.15: 1.35, 0.45: 1.05) were used to synthesize the Bi2MoO6 with different doping concentration of carbonate by repeating the above process. The collected products were marked with BMO, 10%-CO3-BMO and 30%-CO3BMO, respectively. 2.3. Characterization analysis The focused ion beam scanning electron microscope (Zeiss Auriga FIB/SEM), transmission electron microscope (TEM, ThermoFisher Scientific, Talos F200S), the X-ray diffraction (Cu Ka,

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Rigaku D/max 2500), Raman spectrum (LabRAM HR Evolution) and elemental analyzer (Vario EL cube, analytical precision of C element < 0.2%) were used to investigate the morphology, crystal structures and composition of the as-prepared samples. The Xray photoelectron spectroscopy (XPS, ESCALAB250Xi, Al Ka) was employed to analyze the surface chemical state of the products. The UV–vis spectra (UV-2700, Japan) and fluorescence spectrophotometer (PL, Hitachi F4600) were applied to investigate the light absorption and carrier recombination, and BaSO4 as the reference. The electron spin resonance (ESR) measurement (Bruker JES FA200) was used to detect the reaction activity species, including 0 hydroxyl (OH) and superoxide radicals (O 2 ), with DMPO (5, 5 dimethyl-1-pirroline-N-oxide) as a spin-trap reagent. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurement [4,7] was employed to dynamic monitor the reaction intermediate and final products during the photocatalytic NO oxidation process. In details, the mixture gas (50% NO and 50% O2) consecutively passes the detection chamber of In situ DRIFTS measurement during the adsorption and oxidization process, where the adsorption process is before light on while the oxidization process is after light on, which method could dynamic detect the reaction process and investigate the accumulation of intermediate and final products on the surface of photocatalysts. 2.4. Photocatalytic efficiency Removing NO was employed to evaluate the photocatalytic activity of the prepared samples, which methods and equipment are the same with our previous reports [17]. In details, the asprepared samples (0.2 g) was ultrasonically dispersed and uniformly covered on two 12.0 cm diameter glass culture dishes, and then dried at 60 °C. After naturally cooling down, the dishes were placed into the 4.5 L (30  15  10 cm) rectangular reactor with continuous stream. The initial NO (100 ppm, balance N2) offered by gas cylinder was diluted to ~550 ppb by utilizing air stream. The tungsten halogen lamp provided visible light (150 W, UV cutoff filter, k  420 nm) was light on after reaching adsorption-desorption equilibrium. The variation of NO concentration was detected by the NOx analyzer (Thermo Scientific, model 42c-TL), which recorded a data every minute. In addition, the NO removal efficiency (g) was calculated from g = (1  C/C0)  100%, where C0 and C are the NO concentrations in the inlet and outlet, respectively. 2.5. DFT calculations Density functional theory (DFT) calculations were performed on the Vienna ab-initio simulation package (VASP5.4) with projector augmented wave (PAW) pseudopotential [18–21], which was used to describe the interaction of ions and valence electrons. Generalized gradient approximation (GGA) method performed for exchange correlation functional to carry out the periodic calculations [22,23]. A plane wave cutoff of 500 eV, the 8  3  8 Monkhorst-Pack mesh for Brillouin zones, and the force of atom convergence (0.01 eV/Å) were employed [4]. The DFT + U method was applied to calculate strong correlation systems, and the U values were set to 5 and 5 eV corresponding the Bi and Mo, respectively [24]. A 1  1  2 Bi2MoO6 supercell was first optimized, and then one carbon atom substituted a Mo atom (Schematic S1, Supplementary Information). Moreover, three layers slabs (108 atoms) with 15 Å vacuum thickness cut along the (131) facets. The adsorption energy (Eads ) is defined as

Eads ¼

1 ðEtot  Eslab  Emol Þ A

ð1Þ

where A, Etot , Eslab and Emol refers the surface area of the Bi2MoO6 slab, total energy of the complex, the Bi2MoO6 slab, and the isolated molecule species, respectively. 3. Results and discussions The carbonate-intercalated Bi2MoO6 was favorably fabricated via a facile one-step solvothermal routes with ethylene glycol assistant. The concentration of dope carbonate in Bi2MoO6 structure is well controllable. A detailed synthesis process described in the Experimental Section, and the schematic diagram of synthesis process and design philosophy were put forward (Schematic S2). The structure and component nature of the prepared catalysts are comprehensively and systematically analyzed by utilizing various characterization methods. Firstly, the morphologies and structural characteristics of the prepared samples were investigated by the SEM and TEM. As shown in the SEM images (Fig. 1), the uniformly Bi2MoO6 micro/nano-sphere structures are composed of the nanosheets. The thickness of nanosheets is decreased with the increasing of carbonate doping concentration by comparing the BMO, 10%-CO3-BMO and 20%-CO3-BMO. Differently, the 30%CO3-BMO is made up of the extremely tiny nanosheets or nanoparticles. Additionally, some impure sheets are appeared in the 30%-CO3BMO, suggesting the formation of impure phase. The TEM images of the BMO (Fig. 2a) and 20%-CO3-BMO (Fig. 2d) are further certified the nanosheets. From the HRTEM image of BMO (Fig. 2b), and the calibration details of the lattice spacing were shown in Fig. S1 (Supplementary Information), the lattice spacing is 0.315 nm, indexing the (1 3 1) lattice planes of orthorhombic phase of Bi2MoO6. For 20%-CO3-BMO (Fig. 2e), the lattice spacing (0.320 nm) is slightly larger than the BMO, which could be ascribed to the distortion of the Bi2MoO6 crystal structure caused by carbonate intercalation. Moreover, the corresponding SAED information of BMO (Fig. 2c) and 20%-CO3-BMO (Fig. 2f) present the ring shape, implying the polycrystal nature of the prepared samples, and each ring are assigned to the (1 3 1), (2 0 0), (2 1 2) and (3 3 1) facets of the Bi2MoO6 with orthorhombic phase, respectively. Secondly, the crystal structures and composition information of the synthesized photocatalysts were characterized by the XRD, Raman spectrums and elemental analyzer. Then, the phase structures of as-prepared products are shown in the Fig. 3a, and all the diffraction peaks of BMO are well agreement with the Bi2MoO6 pertained of orthorhombic phase (PDF#77-1246), implying the favorably synthesis of Bi2MoO6 photocatalysts. More significantly, the several main diffraction peaks, such as the (131) peak (Fig. S2), have a little bit left shift with the increasing of the doping concentration of carbonate, suggesting the increasing of lattice spacing with the increased carbonate, which can be attributed to the introducing of carbonate in Bi2MoO6 phase resulted in the lattice deformation. Meanwhile, the peaks indexed to Bi2O2CO3 are appeared in 30%-CO3-BMO, suggesting the emergence of Bi2O2CO3 phase with adding excess carbonate. These results are very consistent with the aforesaid results of SEM and TEM. In addition, the phonon modes of the samples were recorded, as presented in Fig. 3b, which are consistent with the vibration model of orthorhombic Bi2MoO6, certifying the constitution of Bi2MoO6 [25,26]. Specifically, the Raman bands center at 352 and 358 cm1 could be assigned to the bending vibrations of OAMoAO bonds [25]. The bands at 719, 787, 799 and 845 cm1 could be ascribed to stretching motions of MoO6 octahedra [25,26]. And the bands at 895 cm1 could be attributed to the stretching variations of CAO bonds.[27,28] Noticeably, the Raman peaks are

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Fig. 1. SEM images of the BMO (a), 10%-CO3-BMO (b), 20%-CO3-BMO (c) and 30%-CO3-BMO (d).

Fig. 2. TEM (a and d), HRTEM (b and e) and the corresponding SAED (c and f) images of the BMO (a–c) and 20%-CO3-BMO (d–f).

weakened with the enhancing of carbonate, which could be owned to the distortion of lattice caused by doping carbonate [1,29]. To further and accurately confirm the intercalating concentration of carbonate, quantitative analysis of carbon elements displayed in Fig. 3c was measured by elemental analyzer. It can be easily seen that there are existent the 0.96% (wt.) carbon element in BMO, which can be ascribed to the adsorption of carbon from the ambient. Moreover, the carbon content is increased with the order

of BMO < 10-CO3-BMO < 20%-CO3-BMO < 30%-CO3-BMO. Even though the carbon in 10%-CO3-BMO, 20%-CO3-BMO and 30%-CO3BMO could be partially owned to the adsorption of carbon, it still verifies that the doping carbon is increased following the order. Finally, the surface characters of the as-samples were surveyed by employing the XPS measurement. From the full scan spectrum of all samples (Fig. 3d), the Bi, Mo, C and O elements are clearly observed, confirming the existence of these elements. Based on

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Fig. 3. XRD patterns (a), Raman spectrums (b), quantitative analysis of carbon elements (c), the full XPS survey (d) and high-resolution XPS spectra of C1s (e) and O 1s (f) of the BMO, 10%-CO3-BMO, 20%-CO3-BMO and 30%-CO3-BMO.

the high-resolution spectra of C 1s (Fig. 3e), the peaks at 284.8 eV can be assigned to the CAC bonds [30]. And the peaks at 285.7–286.4 eV and 288.2–288.5 eV can be ascribed to the CAO bonds and carbonate species, respectively [30,31]. More importantly, the fingerprint peak areas of carbonate species are enlarged with the increasing of introduced carbonate, corresponding with the quantitative analysis of carbon elements. The increased peak areas of CAO bonds are also observed in the high-resolution spectra of O 1s, accompanying the decreased peak areas of Mo-O bonds (Fig. 3f), whilst the relative content about carbonate species of different samples were exhibited in Table S1 (Supplementary Information). The O 1s plots can be deconvolved to three peaks. The peaks central at 529.6–529.9, 530.2–530.6 and 531.3–531.8 eV are consistent with the Bi-O, Mo-O and CAO bonds, respectively [7,30]. In addition, the all peaks in O 1s curves have a little bit right shift, which could be owned to the bond distortion caused by increasing of doping carbonate content, accompanying the charge redistribution [26]. For the high-resolution spectrum of Bi 4f (Fig. S3a) and Mo 3d (Fig. S3b), the Bi 4f5/2 and Bi 4f7/2 of Bi3+ state are corresponded to 164.6–164.8 and 159.3–159.5 eV, respectively [7,10]. The peaks at 235.4–235.8 and 232.3–232.7 eV could be assigned to the Mo 3d3/2 and Mo 3d5/2 spin states of Mo6+ in octahedral MoO6, and the slight decrease of binding energies for Mo 3d peak indicated that a little of electrons transfer to Mo atoms from O atoms [10]. The lower binding energy at 233.8–234.5 and 230.5– 231.5 eV in Mo 3d plots could be attributed to the chemical state in MoAOAC bonds [30]. From the above, the carbonate has been favorably intercalated Bi2MoO6 layer structure, and effecting the crystal structure and surface chemical state, which would be favor of the photocatalytic NO oxidation on the surface of as-prepared products. Based on this, the photocatalytic activity of NO removal is significantly promoted after introducing carbonate in Bi2MoO6 structure, as presented in Fig. 4a. Specifically, the highest photocatalytic efficiency is up to 34% on the surface of 20%-CO3-BMO under visible light irradiation, while that of the BMO is just 13%. Moreover, the 20%-CO3-BMO photocatalysts exhibit an excellent repeatability

and stability after continuous five repetitive cycles (Fig. 4b), where the catalysts are not washed with water to remove the final products on the surface after each cycle. Due to the significance of promotion mechanism in photocatalytic technologies, the comprehensive methods were used to analyze the promotion mechanism, including the light absorption, charge recombination, the formation of reactive radicals and the activation of reactants. For the light absorption, the 20%-CO3-BMO possess the preferable light harvesting ability than others, as demonstrated in UV–vis spectra (Fig. 5a). In addition, the estimated band gap was calculated g=2

following the formula ahm ¼ Aðhm  Eg Þ , where a, h, A and Eg refer the reflect coefficient, Planck constant, a constant and bandgap energy, respectively [7,30]. In this system, g = 4 owning to the indirect semiconductor [13,30]. As shown in Fig. 5b, the estimated band gaps of photocatalysts with different carbonate content are decreased in the order of: BMO (2.70 eV) < 10%-CO3-BMO (2.67 eV) < 30%-CO3-BMO (2.59 eV) < 20%-CO3-BMO (2.55 eV), suggesting that the doping carbonate could narrow the ban gap and enhance the light absorption ability. As well, the electron state was investigated by utilizing the DFT + U methods. From the Density of state (DOS) shown in Fig. 5c, the conduction band (CB) edge is composed of O and Bi, while the value band (VB) edge is contributed by Mo, implying that the electrons provided by O and Bi atoms would transfer to Mo atoms under light illumination and accompany the generation of electron-hole pairs. Additionally, the band gap of CO3-BMO calculated by DFT are lower than that of the BMO, according to the band gap estimated by UV–vis spectra. The recombination rate of photoinduced electron-holes pairs is one of the main effect photocatalytic efficiency [32]. The charge recombination characters of the as-synthesized photocatalysts were investigated by PL spectra (Fig. 5d). Apparently, the 20%-CO3-BMO has the lowest recombination rate, while the BMO is the highest, suggesting that the carbonate intercalated in Bi2MoO6 layer structure would inhibit the recombination of carriers. In addition, the ESR spectra with DMPO as the spin-trap reagent was applied to analyze the active species in photocatalytic reaction (Fig. 5e and f). As expected, the DMPO-OH and DMPO-O 2

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Fig. 4. NO removal curves of the BMO, 10%-CO3-BMO, 20%-CO3-BMO and 30%-CO3-BMO under irradiation of visible light (a). Cycling runs for the photocatalytic NO removal over 20%-CO3-BMO under visible light irradiation (b), the photcatalysts was not washed to remove the nitrate on the surface after each cycle.

Fig. 5. UV–vis diffuse reflectance spectra (a), the estimated band gaps (b) and PL spectra (d) of the BMO, 10%-CO3-BMO, 20%-CO3-BMO and 30%-CO3-BMO. Density of state (DOS) of BMO and CO3-BMO (c) was calculated by utilizing the DFT + U methods. ESR spectra of DMPO-OH (e) and DMPO-O 2 (f) for the BMO and 20%-CO3-BMO in darkness and under visible light irradiation.

signals of the 20%-CO3-BMO are stronger than the BMO under visible light irradiation, which elucidate the rapidly increasing of the OH and O 2 radicals with the extension of light time, suggesting that the carbonate doped in Bi2MoO6 phase can improve the activation of H2O and O2 molecules, spatial charge separation and transformation to generate the OH and O radicals, 2 respectively. To further and intuitively survey the activation of the reactants (H2O and O2), the surface adsorption and the Bader charge based on DFT simulation were employed and shown in the Fig. 6. For the H2O molecules adsorbed on the surface of photocatalysts (Fig. 6a and b), the bond lengths of H2O adsorbed on CO3-BMO are larger than that of BMO, accompanying the stronger surface adsorption energy, signaling that the doping state could improve the activation of H2O molecules. In addition, the calculated Bader charge difference was used to investigate the charge transformation and electrons redistribution between the reactants and photocatalysts, following the computational formula: Dq ¼ qads  qiso , where the qads and qiso are the Bader charge of the reactant

adsorbed on photocatalyst and isolated molecules, respectively. Meanwhile, the positive Dq manifests that the reactants would receive electrons from the surface of catalysts, while the negative would provide electrons. In there, the Dq of H2O molecules on the CO3-BMO is 0.018 e, while it is 0.023 e on BMO, suggesting that the surface of CO3-BMO would accelerate the oxidation of H2O molecules to produce OH radicals. Similarly, as shown in Fig. 6(c and d), the O2 species adsorbed on CO3-BMO has a longer bond length (1.257 Å) and stronger adsorption energy (0.071 J m2) than that of BMO, as well the Bader charge difference of CO3-BMO (0.227 e) is larger than the BMO (0.048 e), which reveal that the O2 molecules as electron acceptor would more easily obtain electrons from the CO3-BMO and generate the   O2 radicals. These results are in accord with the ESR analysis, confirming that the carbonate intercalated in Bi2MoO6 layer structure can facilitate the activation and conversion of H2O and O2 species to form active radicals (OH and O 2 ). Furthermore, in order to intuitively and dynamically reveal the photocatalytic NO oxidation mechanism, the In situ DRIFTS

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Fig. 6. The adsorption nature, Bader charge difference and charge density deference of the H2O (a and b) and O2 (c and d) on the surface of the BMO (a and c) and CO3-BMO (b and d). In the figures, yellow and blue region refer electron accumulation and depletion, respectively (isosurface level: 0.002 e Å3). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 7. In situ DRIFTS spectra of NO adsorption process (a and c) under dark ambient and reaction processes (b and d) under visible light irradiation over the BMO (a and b) and 20%-CO3-BMO (c and d). The relative content variation of NO (e) and NO 3 (f) on the surface of BMO and 20%-CO3-BMO, which information come from the In situ DRIFTS spectra.

measurement was performed to detect the reaction process with the function of the time. As shown in the Fig. 7a, when the mixture gases (NO + O2) are continuously flowed the reaction chamber and adsorbed on the surface of BMO under dark ambient, bands appear and enhance over time at 1161 and 1436 cm1, which can be

attributed to the NO and adsorbed NO2 species, respectively [33,34]. After reaching adsorption-desorption equilibrium and light on (Fig. 7b), the bands at 1278 and 1503 cm1 can be obviously observed and rapidly increased with the time, corresponding to the monodentate nitrate (NO 3 ) [35,36]. In contrast, the adsorp-

W. Huo et al. / Journal of Colloid and Interface Science 557 (2019) 816–824 Table 1 Assignment of the In situ DRIFTS bands observed on the surface of BMO and 20%-CO3BMO. Wavenumbers (cm1)

Assignment

Refs.

1054, 1003, 1013 1085 1161 1275, 1278, 1491, 1492, 1503 1317, 1320 1333, 1337, 1338 1366

Bidentate nitrate, m(NO3) Line nitrites, m(NO2) NO Monodentate nitrate, m (NO3) Bridge nitrate, m(NO3) Bridged nitrite, m(NO2) Monodentate nitrites, m (NO2) NO2 N2O4

[39,40] [41] [33] [35,36,38,39,42]

1436, 1443 1677

[41] [36,41] [41] [34] [36]

Photocatalysts þ hm ! h þ e þ

þ

823

ð2Þ

h þ H2 O ! Hþ þ
OH

ð3Þ

e þ O2 !
O2

ð4Þ

NO þ
O2 ! NO2

ð5Þ

NO þ
OH ! HNO2

ð6Þ

NO2 þ
HO ! HNO3

ð7Þ

HNO2 þ
O2 ! HNO3

ð8Þ

4. Conclusions

Fig. 8. The schematic diagram of promotion photocatalytic oxidation mechanism.

tion and reaction process on the 20%-CO3-BMO are recorded in the Fig. 7(c and d). The similar bands at 1160 and 1443 cm1 respectively assigned to NO and NO2 are also observed during the adsorption process, and the bands at 1275 and 1491 cm1 indexed to monodentate nitrate raised in oxidation process [37,38]. In addition, the assignments of other bands are shown in the Table 1. Comparing the reaction process over the surface of BMO and 20%-CO3-BMO monitored by In situ DRIFTS, they have the similar  intermediate (NO, NO2 and NO 2 ) and final (NO3 ) products. However, the band intensity (Fig. 7a–d) and relative content (Fig. 7e and f) of NO and NO 3 on the 20%-CO3-BMO are stronger than that of BMO, which suggest that the carbonate induced in the Bi2MoO6 layer structure can improve the photocatalytic NO oxidation, including the formation of intermediates and finals. Based on the aforementioned analysis, it could be concluded that the carbonate inserted into layer structure affects the electron structure and photocatalytic activity of Bi2MoO6. Carbonateintercalating not only narrows the band gap to improve the light absorption, but also inhibits the recombination of carriers to enhance the transfer and conversion electron-hole pairs. Significantly, the surface structure of Bi2MoO6 with carbonate dopants can accelerate H2O molecules, as electron donor, providing electrons to form hydroxyl radicals, whilst facilitate O2 species, as electron acceptor, capturing electrons to generate superoxide radicals, which are the reactive oxygen species, thus promoting the NO oxidation. In addition, the photocatalytic NO oxidation process recorded by in situ measurement also verifies that the carbonate dopants could positive effect elementary reactions. Visually, the elementary reactions of photocatalytic NO oxidation could be described by the following equations (2 ~ 8), as well the promotion photocatalytic oxidation mechanism is illustrated in Fig. 8.

In summary, we have successfully synthesized the carbonate substituted the molybdate in Bi2MoO6 layer structure for efficient photocatalytic NO oxidation by a mild solvothermal process, which photocatalyst possesses a promising NO removal efficiency and good durability. To reveal the role of carbonate dopants, the characterization analysis and DFT simulation were used, which results demonstrate that the existence of carbonate in Bi2MoO6 layer structure can facilitate charge separation and transformation to produce reactive radicals, accelerate reactants activation and expedite surface reaction kinetics. Moreover, the DRIFTS measurement was employed to dynamic investigate the NO oxidation process under visible light irradiation, suggesting that the dopants could positively promote the reaction and enhance the photocatalytic activity. This study not only systematically explores the positive effects of intercalary carbonate dopants, but also provides an intuitively method to understand the promotion mechanism of NO oxidation. Declaration of Competing Interest There are no conflicts to declare. Acknowledgments This work received the financial support of the Fundamental Research Funds for the Central Universities (2018CDYJSY0055), the National Natural Science Foundation of China (Grant no. 21576034, 51908092), Joint Funds of the National Natural Science Foundation of China-Guangdong (Grant no. U1801254), the project funded by Chongqing Special Postdoctoral Science Foundation (XmT2018043), Technological projects of Chongqing Municipal Education Commission (KJZDK201800801), and the Innovative Research Team of Chongqing (CXTDG201602014). The authors thank the Electron Microscopy Center of Chongqing University for Materials Characterizations. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.09.089. References [1] J. Di, C. Zhu, M. Ji, M. Duan, R. Long, C. Yan, K. Gu, J. Xiong, Y. She, J. Xia, H. Li, Z. Liu, Defect-rich Bi12O17Cl2 nanotubes self-accelerating charge separation for boosting photocatalytic CO2 reduction, Angew. Chem. Int. Ed. Engl. 57 (45) (2018) 14847–14851. [2] P. Zhang, L. Yu, X.W.D. Lou, Construction of heterostructured Fe2O3-TiO2 microdumbbells for photoelectrochemical water oxidation, Angew. Chem. Int. Ed. Engl. 57 (46) (2018) 15076–15080.

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