Bismuth Subcarbonate with Designer Defects for Broad Spectrum

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Bismuth Subcarbonate with Designer Defects for Broad Spectrum Photocatalytic Nitrogen Fixation Chenmin Xu, Pengxiang Qiu, Liyuan Li, Huan Chen, Fang Jiang, and Xin Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05925 • Publication Date (Web): 03 Jul 2018 Downloaded from http://pubs.acs.org on July 9, 2018

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ACS Applied Materials & Interfaces

Bismuth Subcarbonate with Designer Defects for Broad Spectrum Photocatalytic Nitrogen Fixation Chenmin Xu1†, Pengxiang Qiu1†, Liyuan Li§, Huan Chen†*, Fang Jiang†*, Xin Wang‡ †

Key Laboratory of Jiangsu Province for Chemical Pollution Control and Resources Reuse,

School of Environmental and Biological Engineering, Nanjing University of Science and Technology, Nanjing 210094, China. ‡

Key Laboratory of Soft Chemistry and Functional Materials, Nanjing University of Science and

Technology, Ministry of Education, Nanjing 210094, China § Shanghai Research Institute of Petrochemical Technology, Sinopec, Shanghai, 201208, China KEYWORDS: photocatalytic nitrogen fixation, bismuth subcarbonate, oxygen vacancies, controllable defect density, defect level

ABSTRACT

A facial hydrothermal method is applied to synthesize bismuth subcarbonate (Bi2O2CO3, BOC) with controllable defect density (named BOC−X) using sodium bismuthate (NaBiO3) and graphitic carbon nitride as precursors. The defects of BOC−X may originate from the extremely slow decomposition of graphitic carbon nitride during the hydrothermal process. The BOC−X

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with optimal defect density shows a photocatalytic nitrogen fixation amount of 957 µmol L-1 under simulated sunlight irradiation within 4 hours, which is 9.4 times as high as that of pristine BOC. This superior photocatalytic performance of BOC−X is attributed to the surface defect sites. These defects in BOC−X contribute to a defect level in the forbidden band, which extends the light−harvest region of photocatalyst from ultraviolet to visible−light region. Besides, surface defects prevent electron−hole recombination by accommodating photo−generated electrons in the defect level to promote the separation efficiency of charge−carrier pairs. This work not only demonstrates a novel and scalable strategy to synthesize defective Bi2O2CO3, but also presents a new perspective for the synthesis of photocatalysts with controllable defect density.

INTRODUCTION Nitrogen (N) is an indispensable element for all life forms on earth to build biomolecules. N element exists in atmosphere as dinitrogen gas (N2) plentifully; however, N2 cannot be metabolized directly by most of the organisms unless it is “fixed”. In nature, nitrogen fixation occurs through geochemical processes (such as lightning) and biological catalysis on nitrogenase; additionally, the reduction of nitrogen is accomplished by Haber−Bosch process in industry.1-3 Though nearly half of the population exist on the nitrogen fertilizer produced by Haber−Bosch process, the process is still subject to harsh reaction conditions (300–550 °C, 15– 25 MPa) and high energy consumption. Therefore, searching for an environmentally friendly and cost effective method for nitrogen fixation is still an active research field as well as a challenge. Photocatalysis shows great potential in nitrogen fixation with mild reaction conditions and sustainable energy source.4-7


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As a typical Bi−based semiconductor, bismuth subcarbonate (Bi2O2CO3) is widely applied as antibacterial, sensors, super capacitors, and photocatalysts.8-11 The photocatalytic activity of Bi2O2CO3 benefits from its large internal electric field and asymmetrical polarization effect but suffers from large band gap (3.3 eV).12 Thus, various strategies were adopted to improve the photocatalytic activity of Bi2O2CO3, including doping,12-14 morphological modulation,15,16 coupling with heterogeneous semiconductors,17-20 as well as defect engineering21. Defect engineering is an effective measure to improve the catalytic performance of semiconductor by tuning the electronic structure and charge transport.22-25 Among the defects, oxygen vacancies have been reported to promote light harvesting through the formation of defect level and also improve the carrier separation efficiency.26 In photocatalytic nitrogen fixation reaction, oxygen vacancies are capable of the adsorption and activation of N2, which create lower energy molecular steps to overcome the kinetic inertia.27 Li et al. reported the oxygen vacancies on BiOBr nanosheets could activate N2 and accelerate the transfer of photo−excited electrons from BiOBr nanosheets to adsorbed N2, which led to the enhancement of nitrogen photofixation.28 Previously, different strategies were applied to form oxygen vacancies on the surface of Bi2O2CO3, such as thermal treatment in vacuum21, cation doping12, and dominant exposing facets tuning29. However, finding an easy and controllable strategy to introduce surface defects (such as oxygen vacancies) to Bi2O2CO3 remains to be a challenge. In this work, we synthesized defective Bi2O2CO3 (BOC−X) from sodium bismuthate (NaBiO3) and graphitic carbon nitride (GCN) through hydrothermal treatment. The defects may be formed through the cleavage of Bi−O bonds, as GCN decomposed extremely slowly during the hydrothermal process. The photocatalytic nitrogen fixation rate of BOC−X is much higher than that of pristine Bi2O2CO3, since the defects extended the light absorption range and inhibited the



recombination of photogenerated charge carriers via the formation of defect level in the forbidden band. EXPERIMENTAL SECTION Materials. Melamine (99%) was purchased from Tokyo Chemical Industry. Sodium bismuthate (NaBiO3, 85%) was obtained from Acros Organics. Nessler's reagent was acquired from Merck Chemicals (Shanghai) Co., Ltd. Methanol, ethanol, and urea were purchased from Chengdu Kelong Chemical Reagent Company. All the reagents were used without further purification. Preparation of Bi2O2CO3 with defects Defective Bi2O2CO3 samples were synthesized from sodium bismuthate (NaBiO3) and graphitic carbon nitride (GCN) through hydrothermal treatment. Typically, graphitic carbon nitride (GCN) was prepared by polymerization of melamine at 550 °C for 5 h with a heating rate of 5 °C min−1 in a muffle furnace. 1.12 g of NaBiO3 and a certain amount (1.50 g, 1.00 g, 0.75 g and 0.25 g) of GCN were added into 40 mL of water and kept under stirring for 2 h. Then the suspension was transferred to a Teflon−lined steel autoclave. The autoclave was heated to 160 °C and kept for 24 h. The obtained product was washed with deionized water and ethanol several times and dried at 60 °C. As-prepared defective Bi2O2CO3 samples were labelled as BOC−X. BOC−1 stands for the sample prepared with 1.50 g GCN, BOC−2 stands for the sample prepared with 1.00g GCN, and so on. Preparation of pristine Bi2O2CO3


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Pristine Bi2O2CO3 sample was prepared using hydrothermal method as described in the literature.30 Typically, 0.5 g of urea was dissolved in 40 mL of deionized water. Then 2.45 g of Bi(NO3)3 was added to the solution mentioned above and kept under magnetically stirring for 30 min. The resultant suspension was transferred to a Teflon−lined steel autoclave and kept in an oven at 200 °C for 12 h. The product was washed with deionized water and ethanol several times and dried at 60 °C. The obtained pristine Bi2O2CO3 was named as BOC. Characterization. Powder X−ray diffraction (XRD) analysis was performed on a Bruker D8 Advanced X-ray diffractometer with Cu Kα radiation at ambient temperature. Transmission electron microscopy (TEM) images and high−resolution transmission electron microscopy (HRTEM) images were taken on a JEOL JEM2100 microscope with an electron acceleration energy of 200 kV. Electron paramagnetic resonance (EPR) spectrum was recorded on a Bruker EMX 10/12 spectrometer. X−ray photoelectron spectrum (XPS) and valence band X−ray photoelectron spectrum (VBXPS) were recorded on a Thermo-VG Escalab 250 spectrometer with Al Kα radiation. UV–vis diffuse reflectance spectrum was taken on a Thermo Fisher Evolution 220 UV–vis spectrometer in air with BaSO4 as a reference. Dielectric constant was obtained on an Agilent N5244A PNA−X microwave network analyzer. Time-resolved photoluminescence measurement was carried out on an Edinburgh FLSP920 spectrophotometer with a Xe lamp (λ = 310 nm). The Mott-Schottky curve and transient photocurrent response were analyzed on a CHI-660E electrochemical workstation with a standard three−electrode cell (a platinum wire as the counter electrode and an Ag/AgCl electrode as the reference electrode) in 0.5 mol L-1 NaSO4 solution. Preparation of electrodes



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The working electrodes in transient photocurrent response and Mott-Schottky experiments were prepared by dip-coating method: 5 mg of photocatalyst was mixed with 5 mL of ethanol to form a homogeneous photocatalyst–ethanol suspension in an ultrasonic bath. Then the suspension was dropped onto a rectangle indium tin oxides (ITO) glass (10mm×50 mm) within an area of 10mm×10 mm at 60 °C. The resulting ITO glass was heated at 160 °C for 30 min with N2 flow. Photocatalytic nitrogen fixation Photocatalysis experiments were carried out on a photochemical reactor (XPA-7, Nanjing Xujiang Electromechanical Plant) equipped with a 500 W Xe lamp. For nitrogen fixation measurements 30 mg of the photocatalyst was dispersed in 50 mL of methanol-water solution (5%, v/v). Nitrogen flow was kept bubbling through at a rate of 100 mL min-1. Before irradiation, the suspension was stirred in the dark for 30 minutes. About 2 mL of resultant suspension was withdrawn and centrifuged after 4 h. The concentration of ammonia in the supernatant was analyzed by Nessler's reagent spectrophotometry. The reaction conditions of recycle experiments were same as aforesaid processes. After each cycle, the photocatalyst was collected by centrifugation and washed totally. The apparent quantum efficiencies (AQE) were obtained with the same conditions, except for different monochromatic light wavelengths (365 nm, 420 nm, 475 nm, 500 nm, 550 nm, 650 nm, and 700 nm) irradiation. The values of light intensities were measured. The NH4+ yields were measured after 4 h photoreaction, and the AQE was calculated on the basis of the following equation: =

(1)

Calculation Methods


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The structural optimization and density functional theory (DFT) calculations were conducted using DMol3 code.31,32 The generalized gradient approximation (GGA) in the Perdew–Burke– Ernzerhof functional revised for solids (PBEsol) form33 was employed with an all−electron double numerical plus polarization (DNP) as basis set. The real−space global orbital cutoff radius was set to 5.1 Å. The convergence tolerance of the energy was set to 1.0×10−5 Ha (1 Ha = 27.21 eV), and the maximum allowed force is 2.0×10-3 Ha Å-1. The k−point meshes of 5×5×1 were applied for the calculations. All the atoms in the models were relaxed to the minimum in enthalpy without any constraints. The coplanar of all the atoms with each other was confirmed by the test of corrugation effects. RESULTS AND DISCUSSION Crystal structure and morphology The XRD patterns were obtained to analyze the component and crystallization of BOC−X. As shown in Figure 1b, the diffraction peaks conformed the phase of Bi2O2CO3 in BOC−X and pristine Bi2O2CO3 according to JCPDS No. 41−1488.34 Moreover, the diffraction peaks of BOC−X were the same as that of BOC, which suggested that there were no residual GCN, NaBiO3 or other compounds in BOC−X. TEM image of BOC−2 (Figure 1c) showed a plate−like structure. HRTEM image of BOC-2 is presented in Figure 1d. BOC−2 had lattice fringes with d spacing of 0.271 nm, which could be indexed to the (110) lattice planes of Bi2O2CO3.12 Same structure could be observed on the TEM and HRTEM images of BOC (Figure S-1).



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Figure 1. a) Crystal structure of pristine Bi2O2CO3, b) XRD patterns of BOC and BOC−X. c) TEM image and d) HRTEM image of BOC−2. The possible formation process is illustrated in Scheme 1. Firstly, NaBiO3 decomposed into NaOH, Bi2O22+, and O2 under high temperature and pressure. GCN, known as a layered metal−free polymer with high chemical and physical stability, was difficult to be decomposed in a hydrothermal process. However, the alkaline environment in the reactor accelerated the decomposition of GCN into CO32- and NH4+. During this process, BOC−X nuclei were constructed by Bi2O22+ layers interleaved with slabs of CO32- (Figure 1a), and further grew to be larger particles. This process could be described by the following equations35-38: 2NaBiO3 + H2O → Bi2O3 + 2OH- + 2Na+ +O2

(2)

Bi2O3 + H2O → Bi2O22+ + 2OH-

(3)

graphitic carbon nitride (C3N4) + 2HO- + 7H2O → 3CO32- + 4NH4+

(4)


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Bi2O22+ + CO32- → Bi2O2CO3

(5)

Scheme 1. Schematic illustration of the possible formation route for defective Bi2O2CO3. Chemical states and oxygen vacancies XPS was applied to explore the elemental compositions and chemical states of photocatalysts. No signal ascribed to N atoms could be observed in XPS survey spectra (Figure 2a) and N1s core−level spectra (Figure S-2a) of BOC and BOC−X, suggesting that introduction of N−containing impurities from GCN into BOC−X was avoided during the synthesis process. A peak of CO32- could be observed at 288.77 eV in the C1s spectra (Figure S-2b) of BOC and BOC−X. Besides, the peak at 284.80 eV was the surface adventitious carbon from the instrument. As shown in Figure 2b, the O1s spectrum of BOC exhibited a strong peak at 531.68 eV, while the peaks at O1s spectra of BOC−X shifted to higher binding energies compared with that of BOC. The chemical shift might result from the reduction of electrostatic shielding of the nuclear charge with the decreasing outer electron density of the O atoms. The Bi4f spectra of BOC and BOC−X are shown in Figure 2c. The peaks at 159.04 and 164.36 eV were attributed to 4f7/2 and 4f5/2 of Bi3+, respectively.39 To the opposite of O1s spectra, the peaks at Bi4f spectra of BOC−X shifted to lower binding energies as compared to that of BOC, which might be caused



by the increase of outer electron density. The offsets of binding energies in O1s and Bi4f spectra suggested the cleavage of Bi−O bonds in Bi2O22+, since the outer electrons of Bi formerly attracted by O nucleus returned to the orbitals of Bi. This cleavage might be originated from the shortage of CO32- on account of the extremely slow decomposition of GCN during the generation of BOC−X. For the further study of the cleavage of Bi−O bonds in Bi2O22+ layers, EPR was conducted to analyze the unpaired electrons in photocatalysts. As shown in Figure 2d, a peak appeared at 1800 G in the EPR spectra of BOC and BOC−X photocatalysts, which corresponded to the unpaired electrons in Bi atoms. The signal intensity followed the order of BOC−4 > BOC−3 > BOC−2 > BOC−1 > BOC, indicating that the content of unpaired electrons in Bi atoms increased with the decreased dosage of GCN. Besides, it is worth noting that a strong peak at 3480 G could be observed from the EPR spectra of BOC−X, which was ascribed to single ionized oxygen vacancies,21 while no signal could be observed on the spectrum of BOC, suggesting the successful introduction of oxygen vacancies to BOC−X. The paramagnetic spin concentration (NS) of oxygen vacancies was measured with an ER 4105 double resonator and evaluated according to integral area calculated with Bruker Winepr. As presented in Table 1, the NS of BOC−X increased from 2.1×1019 spin g-1 to 4.8×1019 spin g-1 with the GCN amount decreased from 1.50 g (BOC−1) to 0.25 g (BOC−4), indicating the negative correlation between oxygen vacancy density and dosage of GCN in BOC−X. Based on the points discussed above, it is reasonable to infer that the cleavage of Bi−O bonds in Bi2O22+ layers during the preparation process could result in the formation of oxygen vacancies on the surface of BOC−X with GCN as precursor in the synthesis process. As compared to defective Bi2O2CO3 reported before, the BOC−X photocatalysts investigated here have many advantages: (1) the synthesis method is


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facial and available for scalable fabrication; (2) the defect density is controllable through the variation of GCN amount; (3) BOC−X samples are free of any incorporated heteromaterials. Table 1. The paramagnetic spin concentration (NS), band gap energy (Eg), transition energy (Et) from VB to defect level, and donor density (ND) of BOC and BOC−X.

a)

Samples

NS (spin/g)

Eg (eV)

Et (eV)

ND (cm-3)

BOC

−a

3.42±0.04

−

7.14×1019

BOC−1

2.1×1019

3.21±0.05

2.07±0.07

1.70×1020

BOC−2

2.5×1019

3.21±0.07

1.89±0.06

2.90×1020

BOC−3

3.7×1019

3.24±0.05

1.82±0.04

3.05×1020

BOC−4

4.8×1019

3.24±0.03

1.74±0.02

3.77×1020

No EPR signal of oxygen vacancies was detected on BOC.

Figure 2. a) Survey, b) O1s, and c) Bi4f XPS spectra of BOC and BOC−X. d) EPR spectra of BOC and BOC−X.



Band structure of BOC−X UV-vis diffuse reflectance spectra, VBXPS spectra, and density functional theory (DFT) calculations were obtained to study the band structure of BOC and BOC-X. As shown in Figure 3a, BOC presented the light−harvesting capability in UV light region (420 nm), while BOC−2 showed comparatively high AQE under visible light, i.e. 1.05% at 420 nm, 0.81% at 475 nm, 0.72% at 500 nm, 0.42% at 550 nm, and even 0.12% at 650 nm. These results proved that the photocatalytic nitrogen fixation could not carried out on BOC under visible light irradiation, while the reaction could occur on BOC−2 under irradiation of visible light. The transient photocurrent responses of BOC and BOC-2 at 475 nm were measured to confirm the extended light absorption. As shown in Figure 4b, BOC-2 presented noticeable photocurrent with the light on, in contrast, BOC did not response to the light of 475 nm, which demonstrated the light absorption range was widened with the appearance of defect level owing to the surface defects on photocatalyst. With the appearance of defect level, there are two possible channels for electrons to be excited in BOC−2: direct transition from VB to CB and from VB to defect level. Mott−Schottky analysis (Figure 4c) were performed to determine the flat band potential (Efb). The Efb of BOC and BOC−X were calculated and listed in Table S1, in this case, E (vs. NHE) =


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E (vs. Ag/AgCl) + 0.197 eV. The donor density (ND) can also be estimated from Mott−Schottky plots with equations 6 and 741:

=

$% =

− −

!"

#

(6)

. ( # ) ' & *+,

(7)

Here, C is the differential capacitance of the Helmholtz layer, e is the electron charge (1.602×1019

C), ε is the dielectric of pristine Bi2O2CO3 (ε=34), ε0 is the permittivity of vacuum (8.85×10-14

F cm-1), E is the applied bias at the electrode, κ is the Boltzmann constant (1.38×10-23 J K-1), and T is the absolute temperature. The ND values of BOC and BOC−X (X=1−4) were calculated to be 7.14×1019, 1.70×1020, 2.90×1020, 3.05×1020, and 3.77×1020 cm-3, respectively, which suggested that the electron transfer efficiency of BOC−X is higher than that of BOC.42,43 Time-resolved PL spectra (Figure 4d) were applied to investigate the transfer of photogenerated charge carriers. The lifetime of charge carriers in BOC-2 (25.6 ns,) was longer than that in BOC (19.8 ns), suggesting that BOC-2 exhibited an improved separation efficiency of charge carriers.44 Besides, the prolonged lifetime implied that photogenerated charge carriers might have higher possibility in participating in reaction before recombination, which was favorable for the photocatalytic activity. The promoted charge separation on BOC-X might also result from the surface defects, since the defect level can separate photogenerated electron–hole pairs through temporarily trapping photo−induced electrons from the CB. On the basis of above experiments, it is rational to reckon that the light−harvesting capacity and the electron transfer efficiency were significantly enhanced with the introduction of defect level originating from the surface defects on BOC−X. Therefore, it can be inferred that the defect 15 Environment ACS Paragon Plus


level resulting from the surface defects plays a key role in extending the absorption range and promoting charge separation on BOC-X.

Figure 4. a) Quantum efficiencies of BOC and BOC−2 at 365 nm, 420 nm, 475 nm, 500 nm, 550 nm, 650nm and 700 nm. b) Transient photocurrent response of BOC and BOC−2 under visible light (λ = 475 nm) illumination. c) Mott−Schottky curves of BOC and BOC-X. d) time-resolved PL spectra of BOC and BOC-2. Photocatalytic nitrogen fixation and mechanism The photocatalytic nitrogen fixation efficiencies under UV light (420 nm, 13.25 mW cm-2) were tested respectively. As shown in Figure 5a, the UV light driven photocatalytic performance of BOC−X was slightly better than


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that of BOC, among which BOC−2 was the best. Under visible−near infrared light irradiation, BOC showed little activity, while the photocatalytic nitrogen fixation amounts of BOC−X (X=1−4) were as high as 591, 704, 627, and 412 µmol L-1 respectively (Figure 5b). The photocatalytic nitrogen fixation activity under simulated sunlight (14.73 mW cm-2) is present in Figure 5c. Pristine BOC exhibited a very low photocatalytic nitrogen fixation amount of 102 µmol L-1 within 4 hours, while the nitrogen fixation amounts of BOC−X (X=1−4) were 795, 957, 863, and 594 µmol L-1 respectively. Among them, the nitrogen fixation rate of BOC−2 was 9.4 times as high as that of BOC. As shown in Figure S-5a, BOC-2 demonstrated good reusability with the NH4+ concentration still reaching 882 µmol L-1 by 4 hours at the 5th cycle. The slight decrease in ammonia production might come from the loss of photocatalyst during recycling. Besides, the TEM image (Figure S-5b) as well as Bi4f and O1s XPS spectra (Figure S-5c-d) of five times used BOC-2 was almost the same as those of the fresh one. The superior nitrogen fixation activity of BOC−X mainly ascribed to two electrochemical aspects: the enhanced light−harvesting capacity and the improved carrier separation efficiency. With the introduction of surface defects, electrons could be excited from the VB to defect level of BOC−X, which conduced to the absorption of photons with energies smaller than the band gap. Thus, the BOC−X samples with higher defect density exhibited wider absorption range in the UV−vis diffuse reflectance spectra. Besides, the defect level could be the centers to capture photogenerated electrons during photoreactions. Therefore, the photogenerated electrons on CB preferred to transfer to defect level rather than recombining with the holes on VB, which greatly promoted the separation efficiency of photogenerated electron−hole pairs. In addition, surface defects, such as oxygen vacancies, also played an important role in the photocatalytic nitrogen fixation. As previously presented in Figure 3e, the multi-electron



reduction processes such as N2 + 4H+ + 4e- → N2H4 (-0.36 V vs. NHE) and N2 + 6H+ + 6e- → 2NH3 (0.55 V vs. NHE) could be achieved on BOC-X regarding of reaction potential; however, in view of kinetics, there was slight possibility in the occurrence of these two reactions.45 Though the potential of first proton-coupled electron transfer process in nitrogen fixation via N2 + H+ + e- → N2H was as negative as -3.20 V vs NHE, which was more negative than the CBM of BOC−X, the nitrogen fixation was still expected to occur on the surface of photocatalyst since the oxygen vacancies could adsorb and activate N2 as catalytic centers to provide a feasible pathway.27,28 Different gas flows were bubbled into electrolyte solution when the transient photocurrent response experiments were conducted to explore the electron transfer pathway. As shown in Figure 5d, the photocurrents of BOC under the N2 and Ar atmosphere were similar, while BOC-2 exhibited a pronounced decreased photocurrent under the N2 atmosphere compared with under the Ar atmosphere. With the presence of surface defects, the electrons were inclined to transfer to N2 molecules, which suggested the surface defects on BOC-2 may act as reaction sites during the photocatalytic nitrogen fixation process.


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Figure 5. The photocatalytic nitrogen fixation activity of BOC and BOC−X. Comparison of the photocatalytic activity for nitrogen fixation under a) UV light (λ < 365 nm), b) visible light (λ > 420 nm) illumination, and c) simulated sunlight. d) Transient photocurrent response of BOC and BOC-2 under the N2 or Ar atmosphere. Light source: Xe lamp, 300W. e) Mechanism of photocatalytic nitrogen fixation on defective Bi2O2CO3. Specifically, the photocatalytic mechanism of nitrogen fixation on BOC−X is presented in Figure 5e. If the photon energy is higher than the band gap, the VB electrons of BOC−X will be excited to the CB, and react with the adsorbed N2 to form NH3. In addition, part of the VB



electrons excited to CB will participate in the reaction directly, and part of them will transfer to defect level and react with N2 or intermediates. If the photon energy is lower than the band gap, the VB electrons of neither BOC nor BOC−X will be excited to CB; however, the electrons in BOC−X will be excited from VB to defect level, and then take part in the nitrogen fixation. BOC−2 exhibited a higher photocatalytic activity than other BOC−X photocatalysts due to the proper defect level position. If the position of defect level is too high, higher photon energy (shorter wavelength) will be required for VB electrons to be excited to defect level, which will reduce the utilization of light. On the contrary, if the position of defect level is too low, the reduction ability of excited electrons will not be enough for nitrogen fixation. The presence of surface defects modulated the band structure of BOC−X and resulted in the extensive light absorption range and accelerated carrier transport, leading to a remarkable enhancement in photocatalytic nitrogen fixation activity. Conclusions Defect−controllable Bi2O2CO3 (BOC−X) was prepared through a facile hydrothermal method using NaBiO3 and GCN as precursors. EPR spectra indicated the presence of oxygen vacancies on the surface of Bi2O2CO3, and the defect density was found to be negatively correlated with the amount of GCN. A defect level attributed to the surface defects appeared in the forbidden band of BOC−X. The defect level in BOC−X not only extended the light absorption range but also inhibited the recombinaition of photogenerated charge carrier pairs, which promoted its activity in photocatalytic nitrogen fixation. The photocatalytic nitrogen fixation amount of BOC−2 was as high as 957 µmol L-1 under simulated sunlight irradiation within 4 hours, while that of pristine Bi2O2CO3 was only 102 µmol L-1. An apparent quantum efficiency as high as


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1.05% was reached by BOC−2 at 420 nm. This study provides a novel strategy to synthesize defective Bi2O2CO3 and nominates a highly active photocatalyst for nitrogen fixation. ASSOCIATED CONTENT Supporting Information. The flat band potential of BOC and BOC-X. TEM image and HRTEM image of pristine Bi2O2CO3. The high-resolution XPS spectra of N1s and C1s for BOC and BOC-X. The band gap energy and transition energy from VB to the defect level for BOC and BOC-X. Photocatalytic nitrogen fixation on BOC and BOC-X: experimental group and control groups. Reusability of BOC-2. AUTHOR INFORMATION Corresponding Author * [email protected] *[email protected] Author Contributions 1 These authors contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT



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The financial supports from the National Natural Science Foundation of China (Nos. 51778295, 51678306

and

51478223),

China

Postdoctoral

Science

Foundation

(2017T100372,

2016M590458 and 2013M541677) are gratefully acknowledged. REFERENCES (1) Hoffman, B. M.; Lukoyanov, D.; Yang, Z. Y.; Dean, D. R.; Seefeldt, L. C. Mechanism of nitrogen fixation by nitrogenase: the next stage. Chem. Rev. 2014, 114, 4041-4062. (2) Bao, D.; Zhang, Q.; Meng, F.; Zhong, H.; Shi, M.; Zhang, Y.; Yan, J.; Jiang, Q.; Zhang, X. Electrochemical reduction of N2 under ambient conditions for artificial N2 fixation and renewable energy storage using N2/NH3 cycle. Adv. Mater. 2017, 29, 1604799. (3) Shi, M. M.; Bao, D.; Wulan, B. R.; Li, Y. H.; Zhang, Y. F.; Yan, J. M.; Jiang, Q. Au subnanoclusters on TiO2 toward highly efficient and selective electrocatalyst for N2 Conversion to NH3 at ambient conditions. Adv. Mater. 2017, 29, 1606550. (4) Ali, M.; Zhou, F.; Chen, K.; Kotzur, C.; Xiao, C.; Bourgeois, L.; Zhang, X.; MacFarlane, D. R. Nanostructured photoelectrochemical solar cell for nitrogen reduction using plasmonenhanced black silicon. Nat. Commun. 2016, 7, 11335. (5) Medford, A. J.; Hatzell, M. C. Photon-driven nitrogen fixation: current progress, thermodynamic considerations, and future outlook. ACS Catal. 2017, 7, 2624-2643. (6) Oshikiri, T.; Ueno, K.; Misawa, H. Selective dinitrogen conversion to ammonia using water and visible light through plasmon-induced charge separation. Angew. Chem. Int. Ed. 2016, 128, 4010-4014. (7) Liu, J.; Kelley, M. S.; Wu, W. Q.; Banerjee, A.; Douvalis, A. P.; Wu, J. S.; Zhang, Y. B.; Schatz, G. C.; Kanatzidis, M. G. Nitrogenase-mimic iron-containing chalcogels for


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Bismuth Subcarbonate with Designer Defects for Broad Spectrum

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