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Bi2O2[BO2(OH)] heterojunction for enhanced photocatalytic CO2 reduction
Journal Pre-proofs Z-Scheme g-C3N4/Bi2O2[BO2(OH)] Heterojunction for Enhanced Photocatalytic CO2 Reduction Lina Guo, Yong You, Hongwei Huang, Na Tian, Tianyi Ma, Yihe Zhang PII: DOI: Reference:
S0021-9797(20)30170-3 https://doi.org/10.1016/j.jcis.2020.02.025 YJCIS 26020
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
19 November 2019 31 December 2019 9 February 2020
Please cite this article as: L. Guo, Y. You, H. Huang, N. Tian, T. Ma, Y. Zhang, Z-Scheme g-C3N4/ Bi2O2[BO2(OH)] Heterojunction for Enhanced Photocatalytic CO2 Reduction, Journal of Colloid and Interface Science (2020), doi: https://doi.org/10.1016/j.jcis.2020.02.025
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Z-Scheme g-C3N4/Bi2O2[BO2(OH)] Heterojunction for Enhanced Photocatalytic CO2 Reduction Lina Guo†, Yong You†, Hongwei Huang,*, † Na Tian,*, † Tianyi Ma‡, Yihe Zhang† †Beijing
Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid
Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing 100083, China ‡School
of Environmental & Life Sciences, The University of Newcastle (UON),
Callaghan, NSW 2308 Australia
*Corresponding author: [email protected] (H.W. Huang), [email protected] (N. Tian)
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ABSTRACT: Construction of Z-scheme heterojunction photocatalyst for CO2 photoreduction shows great significance as it holds strong redox ability and high charge separation efficiency. In this work, we developed a Z-scheme heterojunction photocatalyst graphitic carbon nitride (g-C3N4)/basic bismuth borate (Bi2O2[BO2(OH)]) by a simple high-energy ball milling method. The structure, surface element distribution and morphology of the composite samples were systematically analyzed. The photocatalytic performance of the samples was surveyed by CO2 reduction experiment
under
the
simulated
solar
light
irradiation.
Almost
all
the
g-C3N4/Bi2O2[BO2(OH)] composites show enhanced photocatalytic activity for converting CO2 into CO, and the highest CO production rate observed for g-C3N4/Bi2O2[BO2(OH)] (CNBB-3) among all the samples was determined to be approximately 6.09 µmolg-1h-1, which is 2.78 times higher that of pristine g-C3N4. The largely strengthened photocatalytic CO2 reduction activity mainly originates from the formation of Z-scheme band structures between g-C3N4 and Bi2O2[BO2(OH)] benefiting for the efficient charge separation, which were confirmed by the photoeletrochemical, photoluminescence and ESR spectra. This study provides a new reference for fabrication of high-performance Z-scheme photocatalysts for CO2 reduction. Keywords: CO2 reduction; g-C3N4; Bi2O2[BO2(OH)]; Z-scheme; charge separation
2
1. Introduction With the continuous development of industrialization and urbanization, the rapid growth of carbon dioxide emission and the deterioration of the environment have seriously threatened the survival and development of mankind [1-3]. With the joint efforts of countless predecessors, semiconductor photocatalysis has been proven to be one of the attractive strategies for the serious environmental and energy crisis [4-6]. By utilizing the clean and inexhaustible solar energy, it can catalytically decompose water into hydrogen, reduce carbon dioxide into carbon-containing fuels and degrade organic pollutants to non-toxic products [7,8]. Whereas, low efficiency, high cost and toxicity are the major drawbacks of most reported photocatalysts [9,10]. In particular, the insufficient light absorption and the high recombination rate of electron-hole pairs severely limit the photocatalytic activity [11-13]. Therefore, many strategies have been proposed to solve these problems, such as band gap engineering by doping metal or non-metal elements, depositing of noble metals or constructing heterojunctions, etc. [14,15].
Fabrication
of
Z-scheme
heterojunction
photocatalyst,
where
the
photogenerated electrons from the conduction band (CB) of one semiconductor are injected into the valence band (VB) of the other semiconductor and recombined with its holes, receives considerable attention as it allows highly-efficient charge separation and meanwhile maintains strong reduction with negative CB position and oxidation abilities with positive VB position, thereby highly improving the photocatalytic activity, like organic pollutants degradation, water splitting and CO2 photoreduction [16-20].
3
Nonmetallic polymer semiconductor graphite carbon nitride (g-C3N4) is demonstrated to be an excellent visible-light-responsive photocatalyst. Owing to the merits of abundant resources, the simple synthetic process, suitable band gap (about 2.7 eV), high physicochemical stability, low cost, etc., g-C3N4 has drawn great attention in the field of photocatalysis [21]. However, the bulk g-C3N4 has small specific surface area, limited active sites and severe recombination rate of photoinduced electron-hole pairs, hindering its photocatalytic activity seriously [22,23]. Continuous attempts have been made, such as nanostructure regulation [24-26], metal or non-metal doping [27], noble metal deposition [28-31], and heterojunction construction [32-34] to overcome the above shortcomings of bulk g-C3N4. Recently, our group reported that Bi2O2[BO2(OH)] is a promising layered bismuth-based photocatalyst, and the crystal structure consists of alternatively stacked [BO2(OH)] triangles and [Bi2O2]2+ layers [35,36]. The internal electric field derived from the layered structure, dispersive energy bands, and large negatively charged (BO3)3- ions collectively contribute to the high photocatalytic activity of Bi2O2[BO2(OH)]. Nonetheless, Bi2O2[BO2(OH)] is only responsive to UV light, and its charge separation efficiency needs further improvement. Therefore, construction of Z-scheme heterojunction between g-C3N4 and Bi2O2[BO2(OH)] should be an attractive route to promote their photocatalytic activity. In this work, we prepared a series of g-C3N4/Bi2O2[BO2(OH)] composite photocatalysts by the high-energy ball milling method. Compared to g-C3N4 and Bi2O2[BO2(OH)], the g-C3N4/Bi2O2[BO2(OH)] composites exhibit highly enhanced
4
photocatalytic activity for reduction of CO2 into CO. It is attributed to the formation of Z-scheme heterojunction between g-C3N4 and Bi2O2[BO2(OH)], which results in efficient separation of photogenerated electron-hole pairs due to the matchable band structures and intimate interfacial interaction. And the photocatalytic mechanism was verified by ESR.
2. Experimental section 2.1. Materials All the raw materials here are analytically pure and used without further purification. Melamine and Bi(NO3)•5H2O were purchased from Sinopharm Chemical Reagent Co., Ltd.. H3BO3 and NaOH were obtained from Aldrich company. 2.2. Synthesis. Graphitic carbon nitride (g-C3N4) was prepared as follows: The melamine powder (4 g) was put into a crucible and heated to 520 ℃ for 4 hours in a muffle furnace with a heating rate of 2 ℃/min. Then it was cooled down to room temperature and ground thoroughly in an agate mortar for the following experiments [37]. The synthesis process for Bi2O2[BO2(OH)] was shown in the following. First, 0.979 g of Bi(NO3)•5H2O and 0.989 g of H3BO3 were separately dissolved into 20 mL deionized water, and stirred for 30 minutes to achieve an even solution. Then the H3BO3 solution was dropwise added into the Bi(NO3)•5H2O solution, and the pH value of the solution was adjusted to 7.5 by sodium hydroxide. It was then transferred to a hydrothermal autoclave of PTFE, and reacted in an oven at 180 ℃ for 12 hours. After cooling naturally to room temperature, the samples were washed by deionized
5
water and ethanol for three times, and finally they were dried in an oven for 6 hours at 60 ℃ [36]. The g-C3N4/Bi2O2[BO2(OH)] composites were prepared by a high-energy wet ball-milling method [38]. The above as-obtained Bi2O2[BO2(OH)] and g-C3N4 with different mass ratios (6:1, 3:1, 1:1, 1:3) were dissolved in 10 mL deionized water, and then added into ball mill tanks. Then, they are milled with a rotating speed of 560 rpm for 5 h. The obtained samples were washed with deionized water for 6 times, and dried for 6 hours at 60 ℃. The final composite materials with the g-C3N4 : Bi2O2[BO2(OH)] mass ratios of 6:1, 3:1, 1:1 and 1:3 were named as CNBB-1, CNBB-2, CNBB-3 and CNBB-4, respectively. In addition, mechanically-mixed g-C3N4/Bi2O2[BO2(OH)] sample with mass ratio of 1:1 was prepared only by simply mixing and named as CNBB-3-MM. 2.3. Characterization. Powder X-ray diffraction (XRD) was applied for the crystalline structure analysis on a Bruker D8 focus Advance diffractometer (Cu Kα radiation, 40 kV/40 mA). The Fourier-transform infrared (FTIR) spectra were recorded by a Bruker spectrometer in the frequency range of 4000-450 cm-1. X-ray photoelectron spectroscopy (XPS, PerkinElmer PHI 5000C) was used for testing the surface element of samples. Scanning electron microscopy (SEM) was used to analyze the general morphology of the photocatalysts on a Hitachi S4800 (Japan) instrument in Japan. Transmission electron microscopy (TEM) and EDX-mapping were used to characterize the morphology and element distribution of samples. UV-diffuse reflectance spectra (DRS) 6
were obtained from the Varian Cary5000 UV-vis spectrophotometer and recorded with an integrator sphere. The photoluminescence (PL) spectra were measured by a Hitachi F4600 fluorescence spectrophotometer made in Japan, using a 150W Xe lamp at 400 V as the excitation lamp (λex = 260 nm ） . The time-resolved fluorescence decay spectra were recorded by transient steady-state fluorescence spectrometer (FLS 980). The reactive oxygen species were detected by JES-FA200 ESR spectrometer (Japan). 2.4. Photocatalytic CO2 Reduction. The experiment of photocatalytic CO2 reduction was conducted in a Pyrex vacuumed sealed glass reactor (Figure S1). First, 20 mg of sample and 2 mL of deionized water were added into a glass petri dish under ultrasound treatment for 10 min, and then were put in an oven at 60 ℃ for 6 hours. Secondly, the dish was placed on a tripod in the photoreactor containing 1.7 g of Na2CO3 at the bottom. Thirdly, the reactor was sealed with vacuum grease and vacuumed by air pump. Fourthly, 15mL of H2SO4 (0.1mol/L) was injected into the reactor to react with the Na2CO3, and meantime the glass reactor was illuminated by a 300 W Xe lamp providing simulated sunlight. At every one hour, 3 mL of gas in the reactor was sampled and analyzed by a Gas Chromatograph (GC7806, Perfect Light, China). 2.5. Photoelectrochemical Measurements. The photoelectrochemical measurements were carried out by a standard three electrode system with using an electrochemical workstation (CHI660E, Chenhua Instruments Co., China). In the three electrode system, a saturated calomel electrode (SCE) was used as the reference electrode, a platinum wire as the counter electrode,
7
and 0.1 M Na2SO4 solution as the electrolyte solution. The working electrode was prepared as following: 10 mg of powder sample was dissolved into 1m of ethanol and coated on a ITO film (2 cm×4 cm), and dried at room temperature. The photocurrent (PC) and electrochemical impedance spectra (EIS) were measured with irradiation of a 300W Xe lamp.
3. Results and discussion 3.1 Structure, Composition and Microstructure. The
crystalline
structure
of
pure
g-C3N4,
Bi2O2[BO2(OH)]
and
the
g-C3N4/Bi2O2[BO2(OH)] composites was characterized by the X-ray diffraction (XRD), as shown in Fig. 1. It is obvious that the XRD pattern of g-C3N4 shows two main wide diffraction peaks at 13.0°and 27.4°, which are corresponding to the (100) and (002) crystal plane of g-C3N4 (JCPDS 87-1526), respectively. In contrast, the XRD pattern of Bi2O2[BO2(OH)] displays sharp peaks, which means the high crystallinity. The characteristic peaks of Bi2O2[BO2(OH)] located at 12.3°, 23.1°, 29.7°, 32.9° and 41.8° are assigned to the (020), (-111), (-131), (130) and (041) crystal planes, respectively [39]. XRD patterns of g-C3N4 and Bi2O2[BO2(OH)] are in good consistence
with
the
previously
reported
results.
For
the
series
of
g-C3N4/Bi2O2[BO2(OH)] composites, the characteristic peaks of Bi2O2[BO2(OH)] can be obviously seen, while not for g-C3N4, which should be due to the low intensity of diffraction peaks of g-C3N4 in the composites. In order to further determine the composition of the sample, the FT-IR spectra of the samples were measured. As seen from Fig. 2, the absorption bands of g-C3N4 in 8
the range of 900~1200 cm-1, 810 cm-1 and 3000~3420 cm-1 can be attributed to the stretching vibration of C-N, C=N or outside the C-N heterocycle, the bending vibration characteristic peaks of the triazine unit, and the stretching vibration of -NH/-NH2 groups on the broken aromatic ring of g-C3N4 edge or the stretching vibration of water molecules adsorbed on the surface, respectively [40]. The absorption bands of Bi2O2[BO2(OH)] in the range of 1500-1100 cm-1 are related to the vibration of the BO3 group [36]. When g-C3N4 was combined with Bi2O2[BO2(OH)], the composites exhibit both the vibration bands of g-C3N4 and Bi2O2[BO2(OH)], confirming that the g-C3N4/Bi2O2[BO2(OH)] composites have been successfully constructed. X-ray photoelectron spectroscopy (XPS) was conducted over g-C3N4, Bi2O2[BO2(OH)] and the g-C3N4/Bi2O2[BO2(OH)] composite (CNBB-3) to analyze the surface chemical state and formation of bonds in CNBB-3. As it can be seen from Fig. 3a, the C, N, Bi and O elements were detected in CNBB-3 compared with pure g-C3N4 and Bi2O2[BO2(OH)], suggesting the successful fabrication of CNBB-3 composite. The high-resolution C 1s XPS spectra of CNBB-3 and g-C3N4 (Fig. 3b) could be resolved into three peaks, respectively. The peak at 284.76 eV was assigned to amorphous carbon adsorbed on the surface of catalysts (C-C bonding), while the other two peaks were identified as C-(N)3. Fig. 3c is the high-resolution spectrum of Bi 4f. The peaks at 164.13 and 158.83 eV correspond to Bi 4f5/2 and Bi 4f7/2, respectively, which are consistent with the Bi3+ characteristic peaks [41]. The O 1s peaks of CNBB-3 and pure Bi2O2[BO2(OH)] (Fig. 3d) can be fitted by three peaks. 9
The peaks at 530.05 and 529.87 eV correspond to the Bi-O bond in the (Bi2O2)2+ layer, and peaks at 531.23 and 530.87 eV are attributed to the O-H bond, and peaks at 532.60 and 532.51 eV are owing to H2O adsorbed on the surface of samples [42]. Notably, the O and C of C-(N)3 peaks in CNBB-3 sample are shifted to higher-energy position compared with that in pristine g-C3N4 and Bi2O2[BO2(OH)]. It demonstrates the electron interaction between g-C3N4 and Bi2O2[BO2(OH)], which indicates formation of bonds between two substances. The microstructure and morphology of samples were characterized by SEM, TEM and EDX mapping. As seen from Fig. 4a, Bi2O2[BO2(OH)] shows nanosheet structure, which may effectively promote the separation of electron-hole pairs. As depicted in Fig. 4b, the morphology of g-C3N4 is irregular block structure in micrometer size. Fig. 4c shows that both irregular block and thin layered structures were observed in the CNBB-3
sample,
confirming
the
successful
combination
of
g-C3N4
and
Bi2O2[BO2(OH)]. Fig. 4d and e display the TEM images and EDX mapping of CNBB-3. The small Bi2O2[BO2(OH)] nanosheets are attached on the surface of g-C3N4, which is consistent with the SEM result. EDX mapping images of CNBB-3 exhibit the coexistence of C, N, Bi and B elements, further verifying the formation of g-C3N4/Bi2O2[BO2(OH)] composites. 3.2. Optical Properties. UV-Vis diffuse reflectance spectroscopy (DRS) was applied to investigate the optical absorption of g-C3N4, Bi2O2[BO2(OH)], and g-C3N4/Bi2O2[BO2(OH)] composites. As shown in Fig. 5a, the absorption edges of g-C3N4 and Bi2O2[BO2(OH)] 10
are about 453 and 394 nm, respectively. With the increase of g-C3N4 content, the absorption edge of the g-C3N4/Bi2O2[BO2(OH)] photocatalysts shows obvious red-shift compared with the pure Bi2O2[BO2(OH)]. The band gaps (Eg) of semiconductors can be calculated by the plots of (αhv)1/2 versus (hv) (Fig. 5b) [43]. Considering
that
both
g-C3N4 and
Bi2O2[BO2(OH)]
are
indirect-band-gap
semiconductors, the Eg of g-C3N4 and Bi2O2[BO2(OH)] is estimated to be 2.71 eV and 2.87 eV, respectively, in consistent with the literatures [44,45]. From our previous work, the ECB of g-C3N4 and Bi2O2[BO2(OH)] was separately determined to be -1.27 eV and 0.41 eV by Mott-Schottky method [46,47]. According to the band gap from DRS, the EVB of g-C3N4 and Bi2O2[BO2(OH) ] is 1.44 eV and 3.28 eV, respectively. 3.3. Photocatalytic CO2 Reduction Performance. The photocatalytic CO2 reduction activity of as-prepared samples was investigated under simulated solar light irradiation in a gas-solid reaction system, and the reductive products were detected by gas chromatography (GC). As seen from Fig. 6a-d, the controlled experiments demonstrated that the as-produced CO can be neglected when the photoreactions were conducted without photocatalysts or in the darkness. However, a slight amount of CO was produced under the Ar atmosphere, which should be ascribed to the organics attached on the surface of materials. The actual CO production rate for all the samples was determined combined with the results from controlled experiments. The CO generation rate of g-C3N4 is 2.19 µmolg-1h-1, and that of Bi2O2[BO2(OH)] is 0.21 µmolg-1h-1, indicating that it almost cannot reduce CO2. Compared with pure Bi2O2[BO2(OH)] and g-C3N4, the CNBB-2,
11
CNBB-3 and CNBB-4 composites all display improved CO2 photoreduction performance, while CNBB-1 has a lower activity than g-C3N4, but higher than Bi2O2[BO2(OH)]. The highest CO generation rate obtained for CNBB-3 was determined to be approximately 6.09 µmolg-1h-1, which is 2.78 times that of g-C3N4. For comparison, the mechanically-mixed g-C3N4 and Bi2O2[BO2(OH)] with mass ratio of 1:1 was synthesized, and named as CNBB-3-MM. As seen from Fig. 6c, CNBB-3-MM shows an apparently lower CO2 reduction activity than CNBB-3, indicating that the ball-milling process allows an intimate interface interaction between Bi2O2[BO2(OH)] and g-C3N4, and thus CNBB-3 exhibits more efficient photocatalytic performance. 3.4. Mechanism Investigation on Photocatalytic Performance Improvement. It is well known that the photocatalytic activity of materials is mainly dominated by the separation and transfer efficiencies of photoelectron-hole pairs in addition to light absorbing ability. The transient photocurrent response, electrochemical impedance spectroscopy (EIS), photoluminescence spectroscopy (PL) and fluorescence decay curve were used to investigate the separation, transfer and recombination rates of the photocatalysts. In Fig.7a, the photocurrent of all samples is rapidly generated with the light turning on and the g-C3N4/Bi2O2[BO2(OH)] composites show higher intensities than the pure Bi2O2[BO2(OH)] and g-C3N4. The CNBB-3 composite photocatalyst has the strongest photocurrent intensity in all the samples, indicating the highest charge separation efficiency [48]. Fig. 7b shows the Nyquist plots of Bi2O2[BO2(OH)], g-C3N4 and CNBB-3. Obviously, the arc radius of the CNBB-3 curve is smaller than 12
that of g-C3N4 and Bi2O2[BO2(OH)]. It is reported that a smaller radius of the arc indicates a higher efficiency of interfacial photo-charge transfer [49]. The electrons and holes in the quasi-equilibrium state emit light through recombination to form a difference spectrum of intensity or energy distribution of wavelength light [50]. PL spectroscopy is an effective means for evaluating the recombination rate of photogenerated electrons and holes in photocatalysts. A lower PL intensity indicates a lower recombination rate of the carriers, which is more favorable for the carriers to participate in the photocatalytic reaction. Fig. 7c depicts the PL spectra of Bi2O2[BO2(OH)], g-C3N4 and g-C3N4/Bi2O2[BO2(OH)] composites with the excitation wavelength of 260 nm. All the samples show a broad emission peak around 465 nm, and the peak intensities of g-C3N4/Bi2O2[BO2(OH)] composites are significantly reduced compared with that of g-C3N4, meaning that the charge recombination can be effectively suppressed by fabrication of g-C3N4/Bi2O2[BO2(OH)] composites. To determine the lifetime of photoexcited charge carries, the time-resolved fluorescence decay spectra of g-C3N4, Bi2O2[BO2(OH)], and CNBB-3 were recorded as shown in Fig. 7d. Apparently, both the short lifetime (τ1) and long lifetime (τ2) of CNBB-3 are much longer than those of g-C3N4 and Bi2O2[BO2(OH)], indicating that the charge carriers generated in CNBB-3 have more time to transfer to the surface of the photocatalyst and react with the reactants. To reveal the photocatalytic mechanism, the reactive oxygen species generated from CNBB-3, including hydroxyl radicals (·OH), superoxide radicals (·O2-) and singlet oxygen (1O2), are surveyed via DMPO or TEMP assisted ESR measurements under
13
simulated sunlight irradiation. As shown in Fig. 8a-c, Bi2O2[BO2(OH)] and CNBB-3 show similar seven characteristic peaks with intensity of 1:2:1:2:1:2:1, which stand for DMPOX derived from the oxidation of DMPO by two ·OH [51]. Fig. 8d and f demonstrated that obvious characteristic peaks of the DMPO-·O2- adducts can be detected for g-C3N4 and CNBB-3, while not for Bi2O2[BO2(OH)] (Fig. 8e), implying that ·O2- was generated from g-C3N4 and CNBB-3. The similar phenomenon with ·O2was also observed for 1O2. Both g-C3N4 and CNBB-3 produce obvious signal peaks of TEMP-1O2 adducts (Fig. 8g-i). With regard to CNBB-3, all the three kinds of signals (DMPO-·OH, DMPO-·O2- and TEMP-1O2) can be detected. It discloses the successful construction of Z-scheme junction of g-C3N4/Bi2O2[BO2(OH)] [52]. Based on the above analyses, the enhancement on photocatalytic activity of CNBB-3 is mainly resulted from the promoted charge separation and transfer due to the formation of efficient Z-scheme heterojunction. As illustrated in Fig. 9, under simulated solar light illumination, g-C3N4 and Bi2O2[BO2(OH)] generate electrons and transit from VB to CB, leaving equivalent holes in VB. Owing to the construction of Z-scheme junction between g-C3N4 and Bi2O2[BO2(OH)], the photogenerated electrons from the CB of Bi2O2[BO2(OH)] are more prone to recombine with the holes in the VB of g-C3N4, allowing the accumulation of holes in the VB of Bi2O2[BO2(OH)] and photogenerated electrons in the CB of g-C3N4 [16-20]. This process effectively inhibits the recombination of the photoexcited electrons and holes [38] In addition, the electrons in CB of g-C3N4 has a stronger reductive ability, and the holes in VB of Bi2O2[BO2(OH)] has a higher oxidative potential. Therefore, the abundant survival 14
electrons with strong reductive driving force efficiently reduce CO2 into CO, resulting in
remarkably
enhanced
photocatalytic
activity
compared
to
g-C3N4 and
Bi2O2[BO2(OH)].
4. Conclusion In summary, a Z-scheme photocatalyst g-C3N4/Bi2O2[BO2(OH)] was prepared by a facile high-energy ball milling method, which provides close interfacial interaction between g-C3N4 and Bi2O2[BO2(OH)]. The photocatalytic experiment demonstrated that this heterojunction shows much higher photocatalytic performance for CO2 reduction, and the highest CO production rate of g-C3N4/Bi2O2[BO2(OH)] is ~2.78 times higher than that of g-C3N4. It reveals that the remarkably enhanced photocatalytic activity is mainly attributed to the largely facilitated separation and depressed recombination of charge carriers due to the formation of Z-scheme band structure, which was verified by ESR. This work demonstrates a facile route for preparing Z-scheme photocatalytic system for energy applications.
Acknowledgements This work was jointly supported by the National Natural Science Foundations of China (No. 51972288 and 51672258), the Fundamental Research Funds for the Central Universities (2652018290), Key Laboratory of Functional Crystals and Laser Technology, TIPC, CAS.
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References [1] W. Yu, L. Zou, H. Wang, X. Wang, Novel Bi12TiO20/g-C3N4 composite with enhanced photocatalytic performance through Z-scheme mechanism, J. Materi. Sci. 53 (2018) 10039-10048. [2] X.Wu, C. Wang, Y. Wei, J. Xiong, Y. Zhao, Z. Zhao, J. Liu, J. Li, Multifunctional photocatalysts of Pt-decorated
3DOM
perovskite-type SrTiO3 with enhanced
CO2 adsorption and photoelectron enrichment for selective CO2 reduction with H2O to CH4. Journal of Catalysis. 377 (2019) 309-321. [3] Y. Song, J. Gu, K. Xia, J. Yi, H. Chen, X. She, Z. Chen, C. Ding, H. Li, H. Xu, Construction of 2D SnS2/g-C3N4 Z-scheme composite with superior visible-light photocatalytic performance. Applied Surface Science. 467–468 (2019) 56–64. [4] H. Wang, S. Chen, D. Yong, X. Zhang, S. Li, W. Shao, B. Pan, Y. Xie, Giant electron–hole interactions in confined layered structures for molecular Oxygen activation. Journal of the American Chemical Society. 139 (2017) 4737-4742. [5] Y. Wei, X. Wu, Y. Zhao, L. Wang, Z. Zhao, X. Huang, J. Liu, J. Li, Efficient photocatalysts of TiO2 nanocrystals-supported PtRu alloy nanoparticles for CO2 reduction with H2O: Synergistic effect of Pt-Ru. Applied Catalysis B: Environmental. 236 (2018) 445-457. [6] F. Chen, H. Huang, L. Guo, Y. Zhang, T. Ma, The role of polarization in photocatalysis. Angew. Chem. Int. Ed. 58 (2019) 10061-10073.
16
[7] Y. Wei, X. Wu, Y. Zhao, L. Wang, Z. Zhao, X. Huang, J. Liu, J. Li, Efficient photocatalysts of TiO2 nanocrystals-supported PtRu alloy nanoparticles for CO2 reduction with H2O: Synergistic effect of Pt-Ru. Applied Catalysis B: Environmental. 236 (2018) 445-457. [8] J. Liu, S. Xie, Z. Geng, K. Huang, L. Fan, W. Zhou, L. Qiu, D. Gao, L. Ji, L. Duan, L. Lu, W. Li, S. Bai, Z. Liu, W. Chen, S. Feng, Y. Zhang, Carbon nitride supramolecular hybrid material enabled high effciency photocatalytic water treatments. Nano Letters. 16 (10) (2016) 6568 -6575. [9] M. Li, S. Yu, Y. Zhang, X. Li, Y. Feng, C. Wang, T. Ma, H. Huang, Unprecedented Eighteen-faceted BiOCl with Ternary Facet Junction Boosting Cascade Charge Flow and Photo-redox. Angew. Chem. Int. Ed. 58 (2019) 9517-9521. [10] F. Chen, H. Huang, L. Ye, T. Zhang, Y. Zhang, X. Han, T. Ma, Thickness-dependent facet junction control of layered BiOIO3 single crystals for highly efficient CO2 photoreduction. Adv. Funct. Mater. 28 (2018) 1804284. [11] Z. Lu, L. Zeng, W. Song, Z. Qin, D. Zeng, C. Xie, In situ synthesis of C-TiO2/g-C3N4, heterojunction nanocomposite as highly visible light active photocatalyst originated from effective interfacial charge transfer. Applied Catalysis B Environmental. 202 (2017) 489-499. [12] H. Yu, J. Li, Y. Zhang, S. Yang, K. Han, F. Dong, T. Ma, H. Huang, Three-in-one oxygen vacancy: Whole visible-spectrum absorption, efficient charge separation and surface site activation for robust CO2 photoreduction. Angew. Chem. Int. Ed. 58 (2019) 3880-3884. 17
[13] N. Tian, H. Huang, C. Liu, F. Dong, T. Zhang, X. Du, S. Yu, Y. Zhang, In situ copyrolysis fabrication of CeO2/g-C3N4 n–n type heterojunction for synchronously promoting photo-induced oxidation and reduction properties, J. Mater. Chem. A. 3 (2015) 17120. [14] Y. Bai, L. Ye, L. Wang, X. Shi, P. Wang, W. Bai, P. Wong, g-C3N4 /Bi4O5I2 heterojunction with I3-/I- redox mediator for enhanced photocatalytic CO2 conversion. Applied Catalysis B: Environmental. 194 (2016) 98-104. [15] Z. Ma, P. Li, L. Ye, Y. Zhou, F. Su, C. Ding, Y. Bai, H. Xie, P. Wong, Oxygen vacancies induced exciton dissociation of flexible BiOCl nanosheets for effective photocatalytic CO2 Conversion, J. Mater. Chem. A. 5 (47) (2017) 200. [16] Y. He, L. Zhang, X. Wang, Y. Wu, H. Lin, L. Zhao, W. Weng, H. Wan, M. Fan, Enhanced photodegradation activity of methyl orange over Z-scheme type MoO3/g-C3N4 composite under visible light irradiation, RSC Adv. 4 (2014) 13610-13619. [17] Y. Bai, P. Wang, J. Liu, X. Liu, Enhanced photocatalytic performance of direct Z-scheme BiOCl/g-C3N4 photocatalysts, RSC Adv. 4 (2014) 19456-19461. [18] A. Meng, B. Zhu, B. Zhong, L. Zhang, B. Cheng, Direct Z-scheme TiO2/CdS hierarchical photocatalyst for enhanced photocatalytic H2-production activity, Applied Surface Science. 422 (2017) 518-527. [19] J. Jin, J. Yu, D. Guo, C. Cui, W. Ho, Hierarchical Z-Scheme CdS-WO3 Photocatalyst with Enhanced CO2 Reduction Activity, Small. 11 (2015) 5262-5271.
18
[20] W. Yu, J. Chen, T. Shang, L. Chen, L. Gu, T. Peng, Direct Z-scheme g-C3N4/WO3 photocatalyst with atomically defined junction for H2 production, Applied Catalysis B: Environmental, 219 (2017) 693-704. [21] Y. Guo, Li, J.; Gao, Z.; Zhu, X.; Liu, Y.; Wei, Z.; Zhao, W.; Sun. C. A simple and effective method for fabricating novel p-n heterojunction photocatalyst g-C3N4/Bi4Ti3O12 and its photocatalytic performances. Applied Catalysis B. Environmental. 5 (2016) 57-71. [22] X. Zhang, H. Wang, H. Wang, Q. Zhang, J. Xie, Y. Tian, J. Wang, Y. Xie, Single - layered Graphitic-C3N4 quantum dots for two-photon fluorescence imaging of cellular nucleus. Adv. Mater. 26 (26) (2014) 4438-4443. [23] L. Cui, X. Ding, Y. Wang, H. Shi, L. Huang, Y. Zuo, S. Kang, Facile preparation of Z-scheme WO3/g-C3N4, composite photocatalyst with enhanced photocatalytic performance under visible light. Applied Surface Science. 391 (2017) 202-210. [24] W. Shan, Y. Zhao, G. Bai, M. Zheng, C. Wei, In situ preparation of g-C3N4/bismuth-based oxide nanocomposites with enhanced photocatalytic activity. Applied Catalysis B, Environmental. 188 (2016) 1-12. [25] H. Huang, K. Xiao, N. Tian, F. Dong, T. Zhang, X. Du, Y. Zhang, Template-free precursor-surface-etching route to porous, thin g-C3N4 nanosheets for enhancing photocatalytic reduction and oxidation activity. J. Mater. Chem. A. 5 (2017) 17452-17463. [26] J. Zhang, M. Zhang, C. Yang, X. Wang, Nanospherical carbon nitride frameworks with sharp edges accelerating charge collection and separation at a soft
19
photocatalytic interface. Advanced Materials. 26 (24) (2014) 4121-4126. [27] J. Wen, J. Xie, X. Chen, L. Xin, A review on g-C3N4-based photocatalysts. Applied Surface Science. 391 (2017) 72-123. [28] X. She, J. Wu, J. Zhong, X. Hui, Y. Yang, R. Vajtai, Oxygenated monolayer carbon nitride for excellent photocatalytic hydrogen evolution and external quantum efficiency. Nano Energy. 27 (2016) 138-146. [29] L. Hu, H. He, D. Xia, Y. Huang, J. Xu, H. Li, C. He, W. Yang, D. Shu, P. Wong, Highly efficient performance and conversion pathway of photocatalytic CH3SH oxidation on self-stabilized indirect Z-scheme g-C3N4/I3–-BiOI. ACS Applied Materials & Interfaces. 22 (10) (2018) 18693-18708. [30] L. Hao, L. Kang, H. Huang, L. Ye, K. Han, S. Yang, H. Yu, Munkhbayar. Batmunkh, Y. Zhang, T. Ma, Surface halogenation induced atomic site activation and local charge separation for superb CO2 photoreduction. Adv. Mater. 31 (2019) 1900546. [31] R. Baig, S. Verma, R. Varma, M. Nadagouda, Magnetic Fe@g-C3N4: A photoactive catalyst for the hydrogenation of Alkenes and Alkynes. ACS Sustainable Chem. Eng. 4 (2016) 1661-1664. [32] J. Wang, L. Tang, G. Zeng, Y. Liu, Y. Zhou, Y. Deng, J. Wang, B. Peng, Plasmonic Bi metal deposition and g-C3N4 coating on Bi2WO6 microspheres for efficient visible light photocatalysis. ACS Sustainable Chem. Eng. 5 (2017) 1062-1072. [33] N. Tian, H. Huang, Y. He, Y. Guo, Y. Zhang, Organic-inorganic hybrid
20
photocatalyst g-C3N4/Ag2CO3 with highly efficient visible-light-active photocatalytic activity. Colloids & Surfaces A Physicochemical & Engineering Aspects. 467 (2015) 188-194. [34] Z. Chen, P. Sun, B. Fan, Z. Zhang, X. Fang, In situ template-free ion-exchange process to prepare visible-light-active g-C3N4\NiS Hybrid Photocatalysts with enhanced hydrogen evolution activity. The Journal of Physical Chemistry C. 15 (118) (2014) 7801-7807. [35] N. Tian, H. Huang, Y. He, Y. Guo, Y. Zhang, Novel g-C3N4/BiIO4 heterojunction photocatalysts: synthesis, characterization and enhanced visible-light-responsive photocatalytic activity. Rsc Advances. 80 (4) (2014) 42716-42722. [36] R. Cong, J. Sun, T. Yang, M. Li, F. Liao, Y. Wang, J. Lin, Syntheses and crystal structures of two new bismuth hydroxyl borates containing [Bi2O2]2+ Layers: Bi2O2[B3O5(OH)] and Bi2O2[BO2(OH)]. Inorg Chem. 11 (50) (2011) 5098-5104. [37] Y. Bai, P.Q. Wang, J.Y. Liu, X.J. Liu, Enhanced photocatalytic performance of direct Z-scheme BiOCl-g-C3N4 photocatalysts, RSC Adv. 4 (2014) 19456-19461. [38] Y. You, S. Wang, K. Xiao, T. Ma, Y. Zhang, and H. Huang, Z-Scheme g-C3N4/Bi4NbO8Cl Heterojunction for Enhanced Photocatalytic Hydrogen Production, ACS Sustainable Chem. Eng. 6 (2018) 16219−16227. [39] H. Huang, Y. He, Z. Lin, L. Kang, Y. Zhang, Two novel Bi-based borate photocatalysts: crystal structure, electronic structure, photoelectrochemical properties, and photocatalytic activity under simulated solar light irradiation. The Journal of Physical Chemistry C. 44 (117) (2013) 22986-22994.
21
[40] S. Yang, Y. Gong, J. Zhang, L. Zhan, L. Ma, Z. Fang, R. Vajtain, X. Wang, PM. Ajayan, Exfoliated graphitic carbon nitride nanosheets as efficient catalysts for hydrogen evolution under visible light. Advanced Materials. 17 (25) (2013) 2452-2456. [41] H. Huang, Y. He, Z. Lin, L. Kang, Y. Zhang, Two novel Bi-based borate photocatalysts: crystal structure, electronic structure, photoelectrochemical properties, and photocatalytic activity under simulated solar light irradiation. The Journal of Physical Chemistry C. 44 (117) (2013) 22986-22994. [42] S. Yang, Y. Gong, J. Zhang, L. Zhan, L. Ma, Z. Fang, R. Vajtain, X. Wang, PM. Ajayan, Exfoliated graphitic carbon nitride nanosheets as efficient catalysts for hydrogen evolution under visible light. Advanced Materials. 17 (25) (2013) 2452-2456. [43] J. Wang, H. Yao, Z. Fan, L. Zhang, J. Wang, S. Zang, Z. Li, X. Sun, Indirect Z-scheme BiOI/g-C3N4 photocatalysts with enhanced photoreduction CO2 activity under visible light irradiation. ACS Appl. Mater. Interfaces. 8 (2016) 3765−3775. [44] C. Liu, H. Huang, L. Ye, S. Yu, N. Tian, X. Du, T. Zhang, Y. Zhang, Intermediate-mediated strategy to horn-like hollow mesoporous ultrathin g-C3N4 tube with spatial anisotropic charge separation for superiorphotocatalytic H2 evolution. Nano Energy. 41 (2017) 738-748. [45] R. Zhang, Y. Dai, Z. Lou, Z. Li, Z. Wang, Y. Yang, X. Qin, X. Zhang, B. Huang, Layered photocatalyst Bi2O2[BO2(OH)] nanosheets with internal polar field enhanced photocatalytic activity. Cryst. Eng. Comm. 23 (16) (2014) 4931. 22
[46] C. Wang, Y. Zhao, H. Xu, Y. Li, Y. Wei, J. Liu, Z. Zhao, Efficient Z-scheme photocatalysts of ultrathin g-C3N4-wrapped Au/TiO2-nanocrystals for enhanced visible-light-driven conversion of CO2 with H2O. Applied Catalysis B. 263 (2020) 118314. [47] D. Yu, F. Teng, L. Yang, L. Pang, Q. Xu, Q. Zhang, A initiative intercalating strategy for conversion of Bi2O3 to Bi2O2[BO2(OH)] and double internal electric field dependent
photoelectrochemistry
properties.
Chemistryselect.
14
(1)
(2016)
4209-4217. [48] D. Ramakrishna, B. Rajkumar, G. Reddy, G. Veerabhadram, Construction of In-situ
self-assembled
FeWO4/g-C3N4
nanosheet
heterostructured
Z-scheme
photocatalysts for enhanced photocatalytic degradation of Rhodamine B and tetracycline. Nanoscale Advances. 1 (2019) 322-333. [49] N. Tian, H. Huang, X. Du, F. Dong, Y. Zhang. Rational nanostructure design of graphitic carbon nitride for photocatalytic applications. J. Mater. Chem. A. 7 (2019) 11584. [50] Z. Ma, P. Li, L. Ye, Y. Zhou, F. Su, C. Ding, Y. Bai, H. Xie, P. Wong, Oxygen vacancies induced exciton dissociation of flexible BiOCl nanosheets for effective photocatalytic CO2 conversion. J. Mater. Chem. A. 5 (2017) 24995-25004. [51] H. Huang, S. Tu, C. Zeng, T. Zhang, AliH Reshak, Y. Zhang, Macroscopic polarization enhancement promoting photo and piezoelectric-induced charge separation and molecular oxygen activation. Angew Chem Int Ed. 39 (56) (2017) 11860-11864. 23
[52] N. Tian, H. Huang, Y. He, Y. Guo, Y. Zhang, A g-C3N4 /Bi2O2CO3 composite with high visible light-driven photocatalytic activity for rhodamine B degradation. Applied Surface Science. 322 (2014) 249-254.
24
27.4 13.0
g-C3N4
intensity (a.u.)
12.3 23.1 29.7
32.9
41.8
CNBB-4
(041)
(130)
(-131)
(020)
(-111)
CNBB-3 CNBB-2 CNBB-1 Bi2O2[BO2(OH)]
10
20
30
40
2 theta (degree)
50
60
Figure 1. XRD patterns of Bi2O2[BO2(OH)], g-C3N4 and g-C3N4/Bi2O2[BO2(OH)]
Transmittence (a.u.)
composites.
Bi2O2[BO2(OH)] CNBB-4 CNBB-3
CNBB-2 CNBB-1 g-C3N4
4000
3500
3000
2500
2000
1500
Wavenumber (cm-1)
1000
Figure 2. FT-IR spectra of g-C3N4, Bi2O2[BO2(OH)] and g-C3N4/Bi2O2[BO2(OH)] composites.
25
Figure 3. XPS spectra of g-C3N4 , Bi2O2[BO2(OH)] and the g-C3N4/Bi2O2[BO2(OH)] composite (CNBB-3). (a) Survey, (b) N 1s, (c) Bi 4f and (d) O 1s.
26
Figure 4. SEM images of (a) Bi2O2[BO2(OH)], (b) g-C3N4 and (c) CNBB-3. (d) TEM images and (e) EDX-mapping of CNBB-3.
27
Figure 5. (a) UV-vis diffuse reflectance spectra of g-C3N4, Bi2O2[BO2(OH)] and g-C3N4/Bi2O2[BO2(OH)] composites. (b) Band gaps of g-C3N4 and Bi2O2[BO2(OH)].
28
Figure 6. (a) Photocatalytic production and (b) evolution rate of CO over different photocatalysts under simulated sunlight irradiation. (c) CO production over g-C3N4, Bi2O2[BO2(OH)], CNBB-3 and CNBB-3-MM. (d) CO evolution rate under the different reaction conditions with CNBB-3.
29
Figure 7. (a) Transient photocurrent densities of g-C3N4, Bi2O2[BO2(OH)] and CNBB-3. (b) EIS Nynquist plots of g-C3N4, Bi2O2[BO2(OH)] and CNBB-3 (under simulated sunlight, [Na2SO4] = 0.1 M). (c) Photoluminescence (PL) spectra of g-C3N4, Bi2O2[BO2(OH)] and g-C3N4/Bi2O2[BO2(OH)] composites. (d) Time-resolved photoluminescence decay spectra of g-C3N4, Bi2O2[BO2(OH)] and CNBB-3.
30
Figure 8. ESR signals of (a, b, c) DMPO-·OH, (d, e, f) DMPO-·O2- and TEMPO-1O2 (g, h, i) adducts in the presence of g-C3N4, Bi2O2[BO2(OH)] and CNBB-3 under simulated sunlight, respectively.
31
Figure 9. Schematic diagram of charge separation and the possible reaction mechanism over g-C3N4/Bi2O2[BO2(OH)] composite under simulated sunlight irradiation.
32
Graphical Abstract
Z-scheme photocatalyst g-C3N4/Bi2O2[BO2(OH)] was prepared via a facile high-energy ball-milling method, which exhibits a promoted photocatalytic activity for CO2 reduction into CO.
33
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
34
Credit Author Statement Lina Guo: Methodology, Software, Writing- Original draft preparation. Yong You: Data curation, Software Hongwei Huang: Conceptualization, Writing- Reviewing and Editing Na Tian: Writing- Reviewing and Editing Tianyi Ma: Validation Yihe Zhang: Supervision
35
S0021-9797(20)30170-3 https://doi.org/10.1016/j.jcis.2020.02.025 YJCIS 26020
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
19 November 2019 31 December 2019 9 February 2020
Please cite this article as: L. Guo, Y. You, H. Huang, N. Tian, T. Ma, Y. Zhang, Z-Scheme g-C3N4/ Bi2O2[BO2(OH)] Heterojunction for Enhanced Photocatalytic CO2 Reduction, Journal of Colloid and Interface Science (2020), doi: https://doi.org/10.1016/j.jcis.2020.02.025
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© 2020 Published by Elsevier Inc.
Z-Scheme g-C3N4/Bi2O2[BO2(OH)] Heterojunction for Enhanced Photocatalytic CO2 Reduction Lina Guo†, Yong You†, Hongwei Huang,*, † Na Tian,*, † Tianyi Ma‡, Yihe Zhang† †Beijing
Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid
Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing 100083, China ‡School
of Environmental & Life Sciences, The University of Newcastle (UON),
Callaghan, NSW 2308 Australia
*Corresponding author: [email protected] (H.W. Huang), [email protected] (N. Tian)
1
ABSTRACT: Construction of Z-scheme heterojunction photocatalyst for CO2 photoreduction shows great significance as it holds strong redox ability and high charge separation efficiency. In this work, we developed a Z-scheme heterojunction photocatalyst graphitic carbon nitride (g-C3N4)/basic bismuth borate (Bi2O2[BO2(OH)]) by a simple high-energy ball milling method. The structure, surface element distribution and morphology of the composite samples were systematically analyzed. The photocatalytic performance of the samples was surveyed by CO2 reduction experiment
under
the
simulated
solar
light
irradiation.
Almost
all
the
g-C3N4/Bi2O2[BO2(OH)] composites show enhanced photocatalytic activity for converting CO2 into CO, and the highest CO production rate observed for g-C3N4/Bi2O2[BO2(OH)] (CNBB-3) among all the samples was determined to be approximately 6.09 µmolg-1h-1, which is 2.78 times higher that of pristine g-C3N4. The largely strengthened photocatalytic CO2 reduction activity mainly originates from the formation of Z-scheme band structures between g-C3N4 and Bi2O2[BO2(OH)] benefiting for the efficient charge separation, which were confirmed by the photoeletrochemical, photoluminescence and ESR spectra. This study provides a new reference for fabrication of high-performance Z-scheme photocatalysts for CO2 reduction. Keywords: CO2 reduction; g-C3N4; Bi2O2[BO2(OH)]; Z-scheme; charge separation
2
1. Introduction With the continuous development of industrialization and urbanization, the rapid growth of carbon dioxide emission and the deterioration of the environment have seriously threatened the survival and development of mankind [1-3]. With the joint efforts of countless predecessors, semiconductor photocatalysis has been proven to be one of the attractive strategies for the serious environmental and energy crisis [4-6]. By utilizing the clean and inexhaustible solar energy, it can catalytically decompose water into hydrogen, reduce carbon dioxide into carbon-containing fuels and degrade organic pollutants to non-toxic products [7,8]. Whereas, low efficiency, high cost and toxicity are the major drawbacks of most reported photocatalysts [9,10]. In particular, the insufficient light absorption and the high recombination rate of electron-hole pairs severely limit the photocatalytic activity [11-13]. Therefore, many strategies have been proposed to solve these problems, such as band gap engineering by doping metal or non-metal elements, depositing of noble metals or constructing heterojunctions, etc. [14,15].
Fabrication
of
Z-scheme
heterojunction
photocatalyst,
where
the
photogenerated electrons from the conduction band (CB) of one semiconductor are injected into the valence band (VB) of the other semiconductor and recombined with its holes, receives considerable attention as it allows highly-efficient charge separation and meanwhile maintains strong reduction with negative CB position and oxidation abilities with positive VB position, thereby highly improving the photocatalytic activity, like organic pollutants degradation, water splitting and CO2 photoreduction [16-20].
3
Nonmetallic polymer semiconductor graphite carbon nitride (g-C3N4) is demonstrated to be an excellent visible-light-responsive photocatalyst. Owing to the merits of abundant resources, the simple synthetic process, suitable band gap (about 2.7 eV), high physicochemical stability, low cost, etc., g-C3N4 has drawn great attention in the field of photocatalysis [21]. However, the bulk g-C3N4 has small specific surface area, limited active sites and severe recombination rate of photoinduced electron-hole pairs, hindering its photocatalytic activity seriously [22,23]. Continuous attempts have been made, such as nanostructure regulation [24-26], metal or non-metal doping [27], noble metal deposition [28-31], and heterojunction construction [32-34] to overcome the above shortcomings of bulk g-C3N4. Recently, our group reported that Bi2O2[BO2(OH)] is a promising layered bismuth-based photocatalyst, and the crystal structure consists of alternatively stacked [BO2(OH)] triangles and [Bi2O2]2+ layers [35,36]. The internal electric field derived from the layered structure, dispersive energy bands, and large negatively charged (BO3)3- ions collectively contribute to the high photocatalytic activity of Bi2O2[BO2(OH)]. Nonetheless, Bi2O2[BO2(OH)] is only responsive to UV light, and its charge separation efficiency needs further improvement. Therefore, construction of Z-scheme heterojunction between g-C3N4 and Bi2O2[BO2(OH)] should be an attractive route to promote their photocatalytic activity. In this work, we prepared a series of g-C3N4/Bi2O2[BO2(OH)] composite photocatalysts by the high-energy ball milling method. Compared to g-C3N4 and Bi2O2[BO2(OH)], the g-C3N4/Bi2O2[BO2(OH)] composites exhibit highly enhanced
4
photocatalytic activity for reduction of CO2 into CO. It is attributed to the formation of Z-scheme heterojunction between g-C3N4 and Bi2O2[BO2(OH)], which results in efficient separation of photogenerated electron-hole pairs due to the matchable band structures and intimate interfacial interaction. And the photocatalytic mechanism was verified by ESR.
2. Experimental section 2.1. Materials All the raw materials here are analytically pure and used without further purification. Melamine and Bi(NO3)•5H2O were purchased from Sinopharm Chemical Reagent Co., Ltd.. H3BO3 and NaOH were obtained from Aldrich company. 2.2. Synthesis. Graphitic carbon nitride (g-C3N4) was prepared as follows: The melamine powder (4 g) was put into a crucible and heated to 520 ℃ for 4 hours in a muffle furnace with a heating rate of 2 ℃/min. Then it was cooled down to room temperature and ground thoroughly in an agate mortar for the following experiments [37]. The synthesis process for Bi2O2[BO2(OH)] was shown in the following. First, 0.979 g of Bi(NO3)•5H2O and 0.989 g of H3BO3 were separately dissolved into 20 mL deionized water, and stirred for 30 minutes to achieve an even solution. Then the H3BO3 solution was dropwise added into the Bi(NO3)•5H2O solution, and the pH value of the solution was adjusted to 7.5 by sodium hydroxide. It was then transferred to a hydrothermal autoclave of PTFE, and reacted in an oven at 180 ℃ for 12 hours. After cooling naturally to room temperature, the samples were washed by deionized
5
water and ethanol for three times, and finally they were dried in an oven for 6 hours at 60 ℃ [36]. The g-C3N4/Bi2O2[BO2(OH)] composites were prepared by a high-energy wet ball-milling method [38]. The above as-obtained Bi2O2[BO2(OH)] and g-C3N4 with different mass ratios (6:1, 3:1, 1:1, 1:3) were dissolved in 10 mL deionized water, and then added into ball mill tanks. Then, they are milled with a rotating speed of 560 rpm for 5 h. The obtained samples were washed with deionized water for 6 times, and dried for 6 hours at 60 ℃. The final composite materials with the g-C3N4 : Bi2O2[BO2(OH)] mass ratios of 6:1, 3:1, 1:1 and 1:3 were named as CNBB-1, CNBB-2, CNBB-3 and CNBB-4, respectively. In addition, mechanically-mixed g-C3N4/Bi2O2[BO2(OH)] sample with mass ratio of 1:1 was prepared only by simply mixing and named as CNBB-3-MM. 2.3. Characterization. Powder X-ray diffraction (XRD) was applied for the crystalline structure analysis on a Bruker D8 focus Advance diffractometer (Cu Kα radiation, 40 kV/40 mA). The Fourier-transform infrared (FTIR) spectra were recorded by a Bruker spectrometer in the frequency range of 4000-450 cm-1. X-ray photoelectron spectroscopy (XPS, PerkinElmer PHI 5000C) was used for testing the surface element of samples. Scanning electron microscopy (SEM) was used to analyze the general morphology of the photocatalysts on a Hitachi S4800 (Japan) instrument in Japan. Transmission electron microscopy (TEM) and EDX-mapping were used to characterize the morphology and element distribution of samples. UV-diffuse reflectance spectra (DRS) 6
were obtained from the Varian Cary5000 UV-vis spectrophotometer and recorded with an integrator sphere. The photoluminescence (PL) spectra were measured by a Hitachi F4600 fluorescence spectrophotometer made in Japan, using a 150W Xe lamp at 400 V as the excitation lamp (λex = 260 nm ） . The time-resolved fluorescence decay spectra were recorded by transient steady-state fluorescence spectrometer (FLS 980). The reactive oxygen species were detected by JES-FA200 ESR spectrometer (Japan). 2.4. Photocatalytic CO2 Reduction. The experiment of photocatalytic CO2 reduction was conducted in a Pyrex vacuumed sealed glass reactor (Figure S1). First, 20 mg of sample and 2 mL of deionized water were added into a glass petri dish under ultrasound treatment for 10 min, and then were put in an oven at 60 ℃ for 6 hours. Secondly, the dish was placed on a tripod in the photoreactor containing 1.7 g of Na2CO3 at the bottom. Thirdly, the reactor was sealed with vacuum grease and vacuumed by air pump. Fourthly, 15mL of H2SO4 (0.1mol/L) was injected into the reactor to react with the Na2CO3, and meantime the glass reactor was illuminated by a 300 W Xe lamp providing simulated sunlight. At every one hour, 3 mL of gas in the reactor was sampled and analyzed by a Gas Chromatograph (GC7806, Perfect Light, China). 2.5. Photoelectrochemical Measurements. The photoelectrochemical measurements were carried out by a standard three electrode system with using an electrochemical workstation (CHI660E, Chenhua Instruments Co., China). In the three electrode system, a saturated calomel electrode (SCE) was used as the reference electrode, a platinum wire as the counter electrode,
7
and 0.1 M Na2SO4 solution as the electrolyte solution. The working electrode was prepared as following: 10 mg of powder sample was dissolved into 1m of ethanol and coated on a ITO film (2 cm×4 cm), and dried at room temperature. The photocurrent (PC) and electrochemical impedance spectra (EIS) were measured with irradiation of a 300W Xe lamp.
3. Results and discussion 3.1 Structure, Composition and Microstructure. The
crystalline
structure
of
pure
g-C3N4,
Bi2O2[BO2(OH)]
and
the
g-C3N4/Bi2O2[BO2(OH)] composites was characterized by the X-ray diffraction (XRD), as shown in Fig. 1. It is obvious that the XRD pattern of g-C3N4 shows two main wide diffraction peaks at 13.0°and 27.4°, which are corresponding to the (100) and (002) crystal plane of g-C3N4 (JCPDS 87-1526), respectively. In contrast, the XRD pattern of Bi2O2[BO2(OH)] displays sharp peaks, which means the high crystallinity. The characteristic peaks of Bi2O2[BO2(OH)] located at 12.3°, 23.1°, 29.7°, 32.9° and 41.8° are assigned to the (020), (-111), (-131), (130) and (041) crystal planes, respectively [39]. XRD patterns of g-C3N4 and Bi2O2[BO2(OH)] are in good consistence
with
the
previously
reported
results.
For
the
series
of
g-C3N4/Bi2O2[BO2(OH)] composites, the characteristic peaks of Bi2O2[BO2(OH)] can be obviously seen, while not for g-C3N4, which should be due to the low intensity of diffraction peaks of g-C3N4 in the composites. In order to further determine the composition of the sample, the FT-IR spectra of the samples were measured. As seen from Fig. 2, the absorption bands of g-C3N4 in 8
the range of 900~1200 cm-1, 810 cm-1 and 3000~3420 cm-1 can be attributed to the stretching vibration of C-N, C=N or outside the C-N heterocycle, the bending vibration characteristic peaks of the triazine unit, and the stretching vibration of -NH/-NH2 groups on the broken aromatic ring of g-C3N4 edge or the stretching vibration of water molecules adsorbed on the surface, respectively [40]. The absorption bands of Bi2O2[BO2(OH)] in the range of 1500-1100 cm-1 are related to the vibration of the BO3 group [36]. When g-C3N4 was combined with Bi2O2[BO2(OH)], the composites exhibit both the vibration bands of g-C3N4 and Bi2O2[BO2(OH)], confirming that the g-C3N4/Bi2O2[BO2(OH)] composites have been successfully constructed. X-ray photoelectron spectroscopy (XPS) was conducted over g-C3N4, Bi2O2[BO2(OH)] and the g-C3N4/Bi2O2[BO2(OH)] composite (CNBB-3) to analyze the surface chemical state and formation of bonds in CNBB-3. As it can be seen from Fig. 3a, the C, N, Bi and O elements were detected in CNBB-3 compared with pure g-C3N4 and Bi2O2[BO2(OH)], suggesting the successful fabrication of CNBB-3 composite. The high-resolution C 1s XPS spectra of CNBB-3 and g-C3N4 (Fig. 3b) could be resolved into three peaks, respectively. The peak at 284.76 eV was assigned to amorphous carbon adsorbed on the surface of catalysts (C-C bonding), while the other two peaks were identified as C-(N)3. Fig. 3c is the high-resolution spectrum of Bi 4f. The peaks at 164.13 and 158.83 eV correspond to Bi 4f5/2 and Bi 4f7/2, respectively, which are consistent with the Bi3+ characteristic peaks [41]. The O 1s peaks of CNBB-3 and pure Bi2O2[BO2(OH)] (Fig. 3d) can be fitted by three peaks. 9
The peaks at 530.05 and 529.87 eV correspond to the Bi-O bond in the (Bi2O2)2+ layer, and peaks at 531.23 and 530.87 eV are attributed to the O-H bond, and peaks at 532.60 and 532.51 eV are owing to H2O adsorbed on the surface of samples [42]. Notably, the O and C of C-(N)3 peaks in CNBB-3 sample are shifted to higher-energy position compared with that in pristine g-C3N4 and Bi2O2[BO2(OH)]. It demonstrates the electron interaction between g-C3N4 and Bi2O2[BO2(OH)], which indicates formation of bonds between two substances. The microstructure and morphology of samples were characterized by SEM, TEM and EDX mapping. As seen from Fig. 4a, Bi2O2[BO2(OH)] shows nanosheet structure, which may effectively promote the separation of electron-hole pairs. As depicted in Fig. 4b, the morphology of g-C3N4 is irregular block structure in micrometer size. Fig. 4c shows that both irregular block and thin layered structures were observed in the CNBB-3
sample,
confirming
the
successful
combination
of
g-C3N4
and
Bi2O2[BO2(OH)]. Fig. 4d and e display the TEM images and EDX mapping of CNBB-3. The small Bi2O2[BO2(OH)] nanosheets are attached on the surface of g-C3N4, which is consistent with the SEM result. EDX mapping images of CNBB-3 exhibit the coexistence of C, N, Bi and B elements, further verifying the formation of g-C3N4/Bi2O2[BO2(OH)] composites. 3.2. Optical Properties. UV-Vis diffuse reflectance spectroscopy (DRS) was applied to investigate the optical absorption of g-C3N4, Bi2O2[BO2(OH)], and g-C3N4/Bi2O2[BO2(OH)] composites. As shown in Fig. 5a, the absorption edges of g-C3N4 and Bi2O2[BO2(OH)] 10
are about 453 and 394 nm, respectively. With the increase of g-C3N4 content, the absorption edge of the g-C3N4/Bi2O2[BO2(OH)] photocatalysts shows obvious red-shift compared with the pure Bi2O2[BO2(OH)]. The band gaps (Eg) of semiconductors can be calculated by the plots of (αhv)1/2 versus (hv) (Fig. 5b) [43]. Considering
that
both
g-C3N4 and
Bi2O2[BO2(OH)]
are
indirect-band-gap
semiconductors, the Eg of g-C3N4 and Bi2O2[BO2(OH)] is estimated to be 2.71 eV and 2.87 eV, respectively, in consistent with the literatures [44,45]. From our previous work, the ECB of g-C3N4 and Bi2O2[BO2(OH)] was separately determined to be -1.27 eV and 0.41 eV by Mott-Schottky method [46,47]. According to the band gap from DRS, the EVB of g-C3N4 and Bi2O2[BO2(OH) ] is 1.44 eV and 3.28 eV, respectively. 3.3. Photocatalytic CO2 Reduction Performance. The photocatalytic CO2 reduction activity of as-prepared samples was investigated under simulated solar light irradiation in a gas-solid reaction system, and the reductive products were detected by gas chromatography (GC). As seen from Fig. 6a-d, the controlled experiments demonstrated that the as-produced CO can be neglected when the photoreactions were conducted without photocatalysts or in the darkness. However, a slight amount of CO was produced under the Ar atmosphere, which should be ascribed to the organics attached on the surface of materials. The actual CO production rate for all the samples was determined combined with the results from controlled experiments. The CO generation rate of g-C3N4 is 2.19 µmolg-1h-1, and that of Bi2O2[BO2(OH)] is 0.21 µmolg-1h-1, indicating that it almost cannot reduce CO2. Compared with pure Bi2O2[BO2(OH)] and g-C3N4, the CNBB-2,
11
CNBB-3 and CNBB-4 composites all display improved CO2 photoreduction performance, while CNBB-1 has a lower activity than g-C3N4, but higher than Bi2O2[BO2(OH)]. The highest CO generation rate obtained for CNBB-3 was determined to be approximately 6.09 µmolg-1h-1, which is 2.78 times that of g-C3N4. For comparison, the mechanically-mixed g-C3N4 and Bi2O2[BO2(OH)] with mass ratio of 1:1 was synthesized, and named as CNBB-3-MM. As seen from Fig. 6c, CNBB-3-MM shows an apparently lower CO2 reduction activity than CNBB-3, indicating that the ball-milling process allows an intimate interface interaction between Bi2O2[BO2(OH)] and g-C3N4, and thus CNBB-3 exhibits more efficient photocatalytic performance. 3.4. Mechanism Investigation on Photocatalytic Performance Improvement. It is well known that the photocatalytic activity of materials is mainly dominated by the separation and transfer efficiencies of photoelectron-hole pairs in addition to light absorbing ability. The transient photocurrent response, electrochemical impedance spectroscopy (EIS), photoluminescence spectroscopy (PL) and fluorescence decay curve were used to investigate the separation, transfer and recombination rates of the photocatalysts. In Fig.7a, the photocurrent of all samples is rapidly generated with the light turning on and the g-C3N4/Bi2O2[BO2(OH)] composites show higher intensities than the pure Bi2O2[BO2(OH)] and g-C3N4. The CNBB-3 composite photocatalyst has the strongest photocurrent intensity in all the samples, indicating the highest charge separation efficiency [48]. Fig. 7b shows the Nyquist plots of Bi2O2[BO2(OH)], g-C3N4 and CNBB-3. Obviously, the arc radius of the CNBB-3 curve is smaller than 12
that of g-C3N4 and Bi2O2[BO2(OH)]. It is reported that a smaller radius of the arc indicates a higher efficiency of interfacial photo-charge transfer [49]. The electrons and holes in the quasi-equilibrium state emit light through recombination to form a difference spectrum of intensity or energy distribution of wavelength light [50]. PL spectroscopy is an effective means for evaluating the recombination rate of photogenerated electrons and holes in photocatalysts. A lower PL intensity indicates a lower recombination rate of the carriers, which is more favorable for the carriers to participate in the photocatalytic reaction. Fig. 7c depicts the PL spectra of Bi2O2[BO2(OH)], g-C3N4 and g-C3N4/Bi2O2[BO2(OH)] composites with the excitation wavelength of 260 nm. All the samples show a broad emission peak around 465 nm, and the peak intensities of g-C3N4/Bi2O2[BO2(OH)] composites are significantly reduced compared with that of g-C3N4, meaning that the charge recombination can be effectively suppressed by fabrication of g-C3N4/Bi2O2[BO2(OH)] composites. To determine the lifetime of photoexcited charge carries, the time-resolved fluorescence decay spectra of g-C3N4, Bi2O2[BO2(OH)], and CNBB-3 were recorded as shown in Fig. 7d. Apparently, both the short lifetime (τ1) and long lifetime (τ2) of CNBB-3 are much longer than those of g-C3N4 and Bi2O2[BO2(OH)], indicating that the charge carriers generated in CNBB-3 have more time to transfer to the surface of the photocatalyst and react with the reactants. To reveal the photocatalytic mechanism, the reactive oxygen species generated from CNBB-3, including hydroxyl radicals (·OH), superoxide radicals (·O2-) and singlet oxygen (1O2), are surveyed via DMPO or TEMP assisted ESR measurements under
13
simulated sunlight irradiation. As shown in Fig. 8a-c, Bi2O2[BO2(OH)] and CNBB-3 show similar seven characteristic peaks with intensity of 1:2:1:2:1:2:1, which stand for DMPOX derived from the oxidation of DMPO by two ·OH [51]. Fig. 8d and f demonstrated that obvious characteristic peaks of the DMPO-·O2- adducts can be detected for g-C3N4 and CNBB-3, while not for Bi2O2[BO2(OH)] (Fig. 8e), implying that ·O2- was generated from g-C3N4 and CNBB-3. The similar phenomenon with ·O2was also observed for 1O2. Both g-C3N4 and CNBB-3 produce obvious signal peaks of TEMP-1O2 adducts (Fig. 8g-i). With regard to CNBB-3, all the three kinds of signals (DMPO-·OH, DMPO-·O2- and TEMP-1O2) can be detected. It discloses the successful construction of Z-scheme junction of g-C3N4/Bi2O2[BO2(OH)] [52]. Based on the above analyses, the enhancement on photocatalytic activity of CNBB-3 is mainly resulted from the promoted charge separation and transfer due to the formation of efficient Z-scheme heterojunction. As illustrated in Fig. 9, under simulated solar light illumination, g-C3N4 and Bi2O2[BO2(OH)] generate electrons and transit from VB to CB, leaving equivalent holes in VB. Owing to the construction of Z-scheme junction between g-C3N4 and Bi2O2[BO2(OH)], the photogenerated electrons from the CB of Bi2O2[BO2(OH)] are more prone to recombine with the holes in the VB of g-C3N4, allowing the accumulation of holes in the VB of Bi2O2[BO2(OH)] and photogenerated electrons in the CB of g-C3N4 [16-20]. This process effectively inhibits the recombination of the photoexcited electrons and holes [38] In addition, the electrons in CB of g-C3N4 has a stronger reductive ability, and the holes in VB of Bi2O2[BO2(OH)] has a higher oxidative potential. Therefore, the abundant survival 14
electrons with strong reductive driving force efficiently reduce CO2 into CO, resulting in
remarkably
enhanced
photocatalytic
activity
compared
to
g-C3N4 and
Bi2O2[BO2(OH)].
4. Conclusion In summary, a Z-scheme photocatalyst g-C3N4/Bi2O2[BO2(OH)] was prepared by a facile high-energy ball milling method, which provides close interfacial interaction between g-C3N4 and Bi2O2[BO2(OH)]. The photocatalytic experiment demonstrated that this heterojunction shows much higher photocatalytic performance for CO2 reduction, and the highest CO production rate of g-C3N4/Bi2O2[BO2(OH)] is ~2.78 times higher than that of g-C3N4. It reveals that the remarkably enhanced photocatalytic activity is mainly attributed to the largely facilitated separation and depressed recombination of charge carriers due to the formation of Z-scheme band structure, which was verified by ESR. This work demonstrates a facile route for preparing Z-scheme photocatalytic system for energy applications.
Acknowledgements This work was jointly supported by the National Natural Science Foundations of China (No. 51972288 and 51672258), the Fundamental Research Funds for the Central Universities (2652018290), Key Laboratory of Functional Crystals and Laser Technology, TIPC, CAS.
15
References [1] W. Yu, L. Zou, H. Wang, X. Wang, Novel Bi12TiO20/g-C3N4 composite with enhanced photocatalytic performance through Z-scheme mechanism, J. Materi. Sci. 53 (2018) 10039-10048. [2] X.Wu, C. Wang, Y. Wei, J. Xiong, Y. Zhao, Z. Zhao, J. Liu, J. Li, Multifunctional photocatalysts of Pt-decorated
3DOM
perovskite-type SrTiO3 with enhanced
CO2 adsorption and photoelectron enrichment for selective CO2 reduction with H2O to CH4. Journal of Catalysis. 377 (2019) 309-321. [3] Y. Song, J. Gu, K. Xia, J. Yi, H. Chen, X. She, Z. Chen, C. Ding, H. Li, H. Xu, Construction of 2D SnS2/g-C3N4 Z-scheme composite with superior visible-light photocatalytic performance. Applied Surface Science. 467–468 (2019) 56–64. [4] H. Wang, S. Chen, D. Yong, X. Zhang, S. Li, W. Shao, B. Pan, Y. Xie, Giant electron–hole interactions in confined layered structures for molecular Oxygen activation. Journal of the American Chemical Society. 139 (2017) 4737-4742. [5] Y. Wei, X. Wu, Y. Zhao, L. Wang, Z. Zhao, X. Huang, J. Liu, J. Li, Efficient photocatalysts of TiO2 nanocrystals-supported PtRu alloy nanoparticles for CO2 reduction with H2O: Synergistic effect of Pt-Ru. Applied Catalysis B: Environmental. 236 (2018) 445-457. [6] F. Chen, H. Huang, L. Guo, Y. Zhang, T. Ma, The role of polarization in photocatalysis. Angew. Chem. Int. Ed. 58 (2019) 10061-10073.
16
[7] Y. Wei, X. Wu, Y. Zhao, L. Wang, Z. Zhao, X. Huang, J. Liu, J. Li, Efficient photocatalysts of TiO2 nanocrystals-supported PtRu alloy nanoparticles for CO2 reduction with H2O: Synergistic effect of Pt-Ru. Applied Catalysis B: Environmental. 236 (2018) 445-457. [8] J. Liu, S. Xie, Z. Geng, K. Huang, L. Fan, W. Zhou, L. Qiu, D. Gao, L. Ji, L. Duan, L. Lu, W. Li, S. Bai, Z. Liu, W. Chen, S. Feng, Y. Zhang, Carbon nitride supramolecular hybrid material enabled high effciency photocatalytic water treatments. Nano Letters. 16 (10) (2016) 6568 -6575. [9] M. Li, S. Yu, Y. Zhang, X. Li, Y. Feng, C. Wang, T. Ma, H. Huang, Unprecedented Eighteen-faceted BiOCl with Ternary Facet Junction Boosting Cascade Charge Flow and Photo-redox. Angew. Chem. Int. Ed. 58 (2019) 9517-9521. [10] F. Chen, H. Huang, L. Ye, T. Zhang, Y. Zhang, X. Han, T. Ma, Thickness-dependent facet junction control of layered BiOIO3 single crystals for highly efficient CO2 photoreduction. Adv. Funct. Mater. 28 (2018) 1804284. [11] Z. Lu, L. Zeng, W. Song, Z. Qin, D. Zeng, C. Xie, In situ synthesis of C-TiO2/g-C3N4, heterojunction nanocomposite as highly visible light active photocatalyst originated from effective interfacial charge transfer. Applied Catalysis B Environmental. 202 (2017) 489-499. [12] H. Yu, J. Li, Y. Zhang, S. Yang, K. Han, F. Dong, T. Ma, H. Huang, Three-in-one oxygen vacancy: Whole visible-spectrum absorption, efficient charge separation and surface site activation for robust CO2 photoreduction. Angew. Chem. Int. Ed. 58 (2019) 3880-3884. 17
[13] N. Tian, H. Huang, C. Liu, F. Dong, T. Zhang, X. Du, S. Yu, Y. Zhang, In situ copyrolysis fabrication of CeO2/g-C3N4 n–n type heterojunction for synchronously promoting photo-induced oxidation and reduction properties, J. Mater. Chem. A. 3 (2015) 17120. [14] Y. Bai, L. Ye, L. Wang, X. Shi, P. Wang, W. Bai, P. Wong, g-C3N4 /Bi4O5I2 heterojunction with I3-/I- redox mediator for enhanced photocatalytic CO2 conversion. Applied Catalysis B: Environmental. 194 (2016) 98-104. [15] Z. Ma, P. Li, L. Ye, Y. Zhou, F. Su, C. Ding, Y. Bai, H. Xie, P. Wong, Oxygen vacancies induced exciton dissociation of flexible BiOCl nanosheets for effective photocatalytic CO2 Conversion, J. Mater. Chem. A. 5 (47) (2017) 200. [16] Y. He, L. Zhang, X. Wang, Y. Wu, H. Lin, L. Zhao, W. Weng, H. Wan, M. Fan, Enhanced photodegradation activity of methyl orange over Z-scheme type MoO3/g-C3N4 composite under visible light irradiation, RSC Adv. 4 (2014) 13610-13619. [17] Y. Bai, P. Wang, J. Liu, X. Liu, Enhanced photocatalytic performance of direct Z-scheme BiOCl/g-C3N4 photocatalysts, RSC Adv. 4 (2014) 19456-19461. [18] A. Meng, B. Zhu, B. Zhong, L. Zhang, B. Cheng, Direct Z-scheme TiO2/CdS hierarchical photocatalyst for enhanced photocatalytic H2-production activity, Applied Surface Science. 422 (2017) 518-527. [19] J. Jin, J. Yu, D. Guo, C. Cui, W. Ho, Hierarchical Z-Scheme CdS-WO3 Photocatalyst with Enhanced CO2 Reduction Activity, Small. 11 (2015) 5262-5271.
18
[20] W. Yu, J. Chen, T. Shang, L. Chen, L. Gu, T. Peng, Direct Z-scheme g-C3N4/WO3 photocatalyst with atomically defined junction for H2 production, Applied Catalysis B: Environmental, 219 (2017) 693-704. [21] Y. Guo, Li, J.; Gao, Z.; Zhu, X.; Liu, Y.; Wei, Z.; Zhao, W.; Sun. C. A simple and effective method for fabricating novel p-n heterojunction photocatalyst g-C3N4/Bi4Ti3O12 and its photocatalytic performances. Applied Catalysis B. Environmental. 5 (2016) 57-71. [22] X. Zhang, H. Wang, H. Wang, Q. Zhang, J. Xie, Y. Tian, J. Wang, Y. Xie, Single - layered Graphitic-C3N4 quantum dots for two-photon fluorescence imaging of cellular nucleus. Adv. Mater. 26 (26) (2014) 4438-4443. [23] L. Cui, X. Ding, Y. Wang, H. Shi, L. Huang, Y. Zuo, S. Kang, Facile preparation of Z-scheme WO3/g-C3N4, composite photocatalyst with enhanced photocatalytic performance under visible light. Applied Surface Science. 391 (2017) 202-210. [24] W. Shan, Y. Zhao, G. Bai, M. Zheng, C. Wei, In situ preparation of g-C3N4/bismuth-based oxide nanocomposites with enhanced photocatalytic activity. Applied Catalysis B, Environmental. 188 (2016) 1-12. [25] H. Huang, K. Xiao, N. Tian, F. Dong, T. Zhang, X. Du, Y. Zhang, Template-free precursor-surface-etching route to porous, thin g-C3N4 nanosheets for enhancing photocatalytic reduction and oxidation activity. J. Mater. Chem. A. 5 (2017) 17452-17463. [26] J. Zhang, M. Zhang, C. Yang, X. Wang, Nanospherical carbon nitride frameworks with sharp edges accelerating charge collection and separation at a soft
19
photocatalytic interface. Advanced Materials. 26 (24) (2014) 4121-4126. [27] J. Wen, J. Xie, X. Chen, L. Xin, A review on g-C3N4-based photocatalysts. Applied Surface Science. 391 (2017) 72-123. [28] X. She, J. Wu, J. Zhong, X. Hui, Y. Yang, R. Vajtai, Oxygenated monolayer carbon nitride for excellent photocatalytic hydrogen evolution and external quantum efficiency. Nano Energy. 27 (2016) 138-146. [29] L. Hu, H. He, D. Xia, Y. Huang, J. Xu, H. Li, C. He, W. Yang, D. Shu, P. Wong, Highly efficient performance and conversion pathway of photocatalytic CH3SH oxidation on self-stabilized indirect Z-scheme g-C3N4/I3–-BiOI. ACS Applied Materials & Interfaces. 22 (10) (2018) 18693-18708. [30] L. Hao, L. Kang, H. Huang, L. Ye, K. Han, S. Yang, H. Yu, Munkhbayar. Batmunkh, Y. Zhang, T. Ma, Surface halogenation induced atomic site activation and local charge separation for superb CO2 photoreduction. Adv. Mater. 31 (2019) 1900546. [31] R. Baig, S. Verma, R. Varma, M. Nadagouda, Magnetic Fe@g-C3N4: A photoactive catalyst for the hydrogenation of Alkenes and Alkynes. ACS Sustainable Chem. Eng. 4 (2016) 1661-1664. [32] J. Wang, L. Tang, G. Zeng, Y. Liu, Y. Zhou, Y. Deng, J. Wang, B. Peng, Plasmonic Bi metal deposition and g-C3N4 coating on Bi2WO6 microspheres for efficient visible light photocatalysis. ACS Sustainable Chem. Eng. 5 (2017) 1062-1072. [33] N. Tian, H. Huang, Y. He, Y. Guo, Y. Zhang, Organic-inorganic hybrid
20
photocatalyst g-C3N4/Ag2CO3 with highly efficient visible-light-active photocatalytic activity. Colloids & Surfaces A Physicochemical & Engineering Aspects. 467 (2015) 188-194. [34] Z. Chen, P. Sun, B. Fan, Z. Zhang, X. Fang, In situ template-free ion-exchange process to prepare visible-light-active g-C3N4\NiS Hybrid Photocatalysts with enhanced hydrogen evolution activity. The Journal of Physical Chemistry C. 15 (118) (2014) 7801-7807. [35] N. Tian, H. Huang, Y. He, Y. Guo, Y. Zhang, Novel g-C3N4/BiIO4 heterojunction photocatalysts: synthesis, characterization and enhanced visible-light-responsive photocatalytic activity. Rsc Advances. 80 (4) (2014) 42716-42722. [36] R. Cong, J. Sun, T. Yang, M. Li, F. Liao, Y. Wang, J. Lin, Syntheses and crystal structures of two new bismuth hydroxyl borates containing [Bi2O2]2+ Layers: Bi2O2[B3O5(OH)] and Bi2O2[BO2(OH)]. Inorg Chem. 11 (50) (2011) 5098-5104. [37] Y. Bai, P.Q. Wang, J.Y. Liu, X.J. Liu, Enhanced photocatalytic performance of direct Z-scheme BiOCl-g-C3N4 photocatalysts, RSC Adv. 4 (2014) 19456-19461. [38] Y. You, S. Wang, K. Xiao, T. Ma, Y. Zhang, and H. Huang, Z-Scheme g-C3N4/Bi4NbO8Cl Heterojunction for Enhanced Photocatalytic Hydrogen Production, ACS Sustainable Chem. Eng. 6 (2018) 16219−16227. [39] H. Huang, Y. He, Z. Lin, L. Kang, Y. Zhang, Two novel Bi-based borate photocatalysts: crystal structure, electronic structure, photoelectrochemical properties, and photocatalytic activity under simulated solar light irradiation. The Journal of Physical Chemistry C. 44 (117) (2013) 22986-22994.
21
[40] S. Yang, Y. Gong, J. Zhang, L. Zhan, L. Ma, Z. Fang, R. Vajtain, X. Wang, PM. Ajayan, Exfoliated graphitic carbon nitride nanosheets as efficient catalysts for hydrogen evolution under visible light. Advanced Materials. 17 (25) (2013) 2452-2456. [41] H. Huang, Y. He, Z. Lin, L. Kang, Y. Zhang, Two novel Bi-based borate photocatalysts: crystal structure, electronic structure, photoelectrochemical properties, and photocatalytic activity under simulated solar light irradiation. The Journal of Physical Chemistry C. 44 (117) (2013) 22986-22994. [42] S. Yang, Y. Gong, J. Zhang, L. Zhan, L. Ma, Z. Fang, R. Vajtain, X. Wang, PM. Ajayan, Exfoliated graphitic carbon nitride nanosheets as efficient catalysts for hydrogen evolution under visible light. Advanced Materials. 17 (25) (2013) 2452-2456. [43] J. Wang, H. Yao, Z. Fan, L. Zhang, J. Wang, S. Zang, Z. Li, X. Sun, Indirect Z-scheme BiOI/g-C3N4 photocatalysts with enhanced photoreduction CO2 activity under visible light irradiation. ACS Appl. Mater. Interfaces. 8 (2016) 3765−3775. [44] C. Liu, H. Huang, L. Ye, S. Yu, N. Tian, X. Du, T. Zhang, Y. Zhang, Intermediate-mediated strategy to horn-like hollow mesoporous ultrathin g-C3N4 tube with spatial anisotropic charge separation for superiorphotocatalytic H2 evolution. Nano Energy. 41 (2017) 738-748. [45] R. Zhang, Y. Dai, Z. Lou, Z. Li, Z. Wang, Y. Yang, X. Qin, X. Zhang, B. Huang, Layered photocatalyst Bi2O2[BO2(OH)] nanosheets with internal polar field enhanced photocatalytic activity. Cryst. Eng. Comm. 23 (16) (2014) 4931. 22
[46] C. Wang, Y. Zhao, H. Xu, Y. Li, Y. Wei, J. Liu, Z. Zhao, Efficient Z-scheme photocatalysts of ultrathin g-C3N4-wrapped Au/TiO2-nanocrystals for enhanced visible-light-driven conversion of CO2 with H2O. Applied Catalysis B. 263 (2020) 118314. [47] D. Yu, F. Teng, L. Yang, L. Pang, Q. Xu, Q. Zhang, A initiative intercalating strategy for conversion of Bi2O3 to Bi2O2[BO2(OH)] and double internal electric field dependent
photoelectrochemistry
properties.
Chemistryselect.
14
(1)
(2016)
4209-4217. [48] D. Ramakrishna, B. Rajkumar, G. Reddy, G. Veerabhadram, Construction of In-situ
self-assembled
FeWO4/g-C3N4
nanosheet
heterostructured
Z-scheme
photocatalysts for enhanced photocatalytic degradation of Rhodamine B and tetracycline. Nanoscale Advances. 1 (2019) 322-333. [49] N. Tian, H. Huang, X. Du, F. Dong, Y. Zhang. Rational nanostructure design of graphitic carbon nitride for photocatalytic applications. J. Mater. Chem. A. 7 (2019) 11584. [50] Z. Ma, P. Li, L. Ye, Y. Zhou, F. Su, C. Ding, Y. Bai, H. Xie, P. Wong, Oxygen vacancies induced exciton dissociation of flexible BiOCl nanosheets for effective photocatalytic CO2 conversion. J. Mater. Chem. A. 5 (2017) 24995-25004. [51] H. Huang, S. Tu, C. Zeng, T. Zhang, AliH Reshak, Y. Zhang, Macroscopic polarization enhancement promoting photo and piezoelectric-induced charge separation and molecular oxygen activation. Angew Chem Int Ed. 39 (56) (2017) 11860-11864. 23
[52] N. Tian, H. Huang, Y. He, Y. Guo, Y. Zhang, A g-C3N4 /Bi2O2CO3 composite with high visible light-driven photocatalytic activity for rhodamine B degradation. Applied Surface Science. 322 (2014) 249-254.
24
27.4 13.0
g-C3N4
intensity (a.u.)
12.3 23.1 29.7
32.9
41.8
CNBB-4
(041)
(130)
(-131)
(020)
(-111)
CNBB-3 CNBB-2 CNBB-1 Bi2O2[BO2(OH)]
10
20
30
40
2 theta (degree)
50
60
Figure 1. XRD patterns of Bi2O2[BO2(OH)], g-C3N4 and g-C3N4/Bi2O2[BO2(OH)]
Transmittence (a.u.)
composites.
Bi2O2[BO2(OH)] CNBB-4 CNBB-3
CNBB-2 CNBB-1 g-C3N4
4000
3500
3000
2500
2000
1500
Wavenumber (cm-1)
1000
Figure 2. FT-IR spectra of g-C3N4, Bi2O2[BO2(OH)] and g-C3N4/Bi2O2[BO2(OH)] composites.
25
Figure 3. XPS spectra of g-C3N4 , Bi2O2[BO2(OH)] and the g-C3N4/Bi2O2[BO2(OH)] composite (CNBB-3). (a) Survey, (b) N 1s, (c) Bi 4f and (d) O 1s.
26
Figure 4. SEM images of (a) Bi2O2[BO2(OH)], (b) g-C3N4 and (c) CNBB-3. (d) TEM images and (e) EDX-mapping of CNBB-3.
27
Figure 5. (a) UV-vis diffuse reflectance spectra of g-C3N4, Bi2O2[BO2(OH)] and g-C3N4/Bi2O2[BO2(OH)] composites. (b) Band gaps of g-C3N4 and Bi2O2[BO2(OH)].
28
Figure 6. (a) Photocatalytic production and (b) evolution rate of CO over different photocatalysts under simulated sunlight irradiation. (c) CO production over g-C3N4, Bi2O2[BO2(OH)], CNBB-3 and CNBB-3-MM. (d) CO evolution rate under the different reaction conditions with CNBB-3.
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Figure 7. (a) Transient photocurrent densities of g-C3N4, Bi2O2[BO2(OH)] and CNBB-3. (b) EIS Nynquist plots of g-C3N4, Bi2O2[BO2(OH)] and CNBB-3 (under simulated sunlight, [Na2SO4] = 0.1 M). (c) Photoluminescence (PL) spectra of g-C3N4, Bi2O2[BO2(OH)] and g-C3N4/Bi2O2[BO2(OH)] composites. (d) Time-resolved photoluminescence decay spectra of g-C3N4, Bi2O2[BO2(OH)] and CNBB-3.
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Figure 8. ESR signals of (a, b, c) DMPO-·OH, (d, e, f) DMPO-·O2- and TEMPO-1O2 (g, h, i) adducts in the presence of g-C3N4, Bi2O2[BO2(OH)] and CNBB-3 under simulated sunlight, respectively.
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Figure 9. Schematic diagram of charge separation and the possible reaction mechanism over g-C3N4/Bi2O2[BO2(OH)] composite under simulated sunlight irradiation.
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Graphical Abstract
Z-scheme photocatalyst g-C3N4/Bi2O2[BO2(OH)] was prepared via a facile high-energy ball-milling method, which exhibits a promoted photocatalytic activity for CO2 reduction into CO.
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Credit Author Statement Lina Guo: Methodology, Software, Writing- Original draft preparation. Yong You: Data curation, Software Hongwei Huang: Conceptualization, Writing- Reviewing and Editing Na Tian: Writing- Reviewing and Editing Tianyi Ma: Validation Yihe Zhang: Supervision
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