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near-infrared light
Solar Energy Materials & Solar Cells 144 (2016) 732–739
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Synthesis of olive-green few-layered BiOI for efficient photoreduction of CO2 into solar fuels under visible/near-infrared light Liqun Ye a,n, Hui Wang b, Xiaoli Jin a, Yurong Su a, Dongqi Wang b,n, Haiquan Xie a,n, Xiaodi Liu a, Xinxin Liu a a
College of Chemistry and Pharmaceutical Engineering, Nanyang Normal University, Nanyang 473061, China CAS Key Laboratory of Nuclear Radiation and Nuclear Energy Techniques, and Multidisciplinary Initiative Center, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
b
art ic l e i nf o
a b s t r a c t
Article history: Received 8 August 2015 Received in revised form 10 October 2015 Accepted 17 October 2015 Available online 11 November 2015
Olive-green few-layered BiOI with expanded spacing of the (001) facets and oxygen vacancy is successfully synthesized and characterized. The experimental analysis and theoretical calculation demonstrate that the expanded facets spacing and oxygen vacancy of few-layered BiOI result in enhanced separation efficiency of photoinduced carriers and photon absorption efficiency. Therefore, few-layered BiOI shows much higher photocatalytic activity for CO2 conversion than that of bulk BiOI under visible or near-infrared (NIR) light. And the apparent quantum yields (AQY) are 0.14% and 0.02% for few-layered BiOI under 420 and 700 nm monochromatic light irradiation, respectively. These findings deep our understanding of few-layered BiOX (X ¼Cl,Br,I) photocatalysts, and propose a new route to design high efficient photocatalysts for energy and environmental photocatalysis. & 2015 Elsevier B.V. All rights reserved.
Keywords: Bismuth oxyiodide Few-layer Photocatalysis Oxygen vacancy Carbon dioxide Solar fuels
1. Introduction Photon absorption efficiency and carrier separation efficiency are the two key factors to affect the activity of semiconductor photocatalysts [1–3]. In the past fifty years, various approaches, such as cocatalyst utilization, element doping, coupling, surface plasmon resonance and dye sensitization, were employed to enhance the photon absorption efficiency and photoexcited carrier separation efficiency, further leading to improved photocatalytic activity [4,5]. However, all these modifications need assistance of external materials and cannot enhance the photon absorption efficiency and carrier separation efficiency simultaneously. Recently, layered two-dimensional (2D) nano-materials have greatly developed due to their potential applications in photocatalysis [6,7]. Compared with bulk materials, 2D nano-materials have more dangling bond atoms and higher specific surface areas. Therefore, 2D nano-materials usually display more vacancies than bulk materials [6,7]. The vacancies may significantly affect the physical and chemical properties of photocatalysts. For instance, few-layered graphene and MoS2 can be used as deriving-electron n
Corresponding authors. Tel.: þ 86 377 63525056. E-mail addresses: [email protected] (L. Ye), [email protected] (D. Wang), [email protected] (H. Xie). http://dx.doi.org/10.1016/j.solmat.2015.10.022 0927-0248/& 2015 Elsevier B.V. All rights reserved.
types cocatalysts in photocatalysis [8], and ultrathin g-C3N4 [9,10] and In2O3 nanosheets as photocatalyst for water splitting [11]. These pioneering work have stimulated extensive studies on the synthesis of single-layer and multilayer nanosheets in order to expand their applications. As a series of layered bismuth-based semiconductor photocatalyst, BiOI exposed with dominated {001} facets has been demonstrated to exhibit excellent photocatalytic activity due to their open and layered crystalline structure [12–14]. As shown in Fig. S1, the layered structure of BiOI consists of [Bi2O2]2 þ layers placed between two slabs of iodine ions along z axis ([001] direction). Therefore, it is feasible to synthesize single layer or fewlayered BiOI nanosheets with exposed dominated {001} active facets. More importantly, the increasing defects concentration, small size effect, and enhanced internal electric field intensity of few-layered BiOI will result in improved efficiency of photon absorption and carrier separation. The escalating environment and energy crises, partially due to the increasing atmospheric CO2 due to the consumption of fossil fuels [1,2], threaten human existence and development. Thus, more and more researchers focus on conversion of CO2 to fuels [15–18]. Various approaches, such as catalytic hydrogenation [19], electro-catalysis [20], and photocatalytic reduction [21] were used to convert CO2 to fuels. Among these methods, semiconductor
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photocatalysis technology has attracted intensive attention at fundamental application levels owing to its low-cost and green nature to solve environment and energy crises [1,2,22]. However, until now, much photoreduction of CO2 were done only under UV light irradiation, limiting efficient utilization of the solar energy. In this paper, few-layered BiOI with expanded spacing of the (001) facets and oxygen vacancy was successfully synthesized. The thickness of few-layered BiOI nanosheets was about 2.8 nm. And the expanded spacing of the (001) facets was proved by theoretical simulation. The photocatalytic results showed that few-layered BiOI can photocatalytic convert CO2 into solar fuels effectively under visible/near-infrared-light. To the best of our knowledge, it is the first time to synthesize few-layered BiOI, and also the first time to employ the as-prepared photocatalyst for CO2 conversion under near-infrared (NIR) light.
2. Experimental sections 2.1. Synthesis of few-layered BiOI 20 mL of KI (0.5 M) ethylene glycol solution was added dropwise into 20 mL of Bi(NO3)3 5H2O (0.5 M) ethylene glycol solution under continuous magnetic stirring. After stirring for another 1 h, 1 mL benzaldehyde was added to the above mixture. Then the above solution was transferred into to Teflon-lined stainless steel autoclave (50 mL) and kept at 160 °C for 24 h. The olive-green precipitate was centrifuged, washed successively with ethanol and distilled water three times, and finally dried at 80 °C for 12 h. The synthesis of bulk BiOI followed the same procedure to that of few-layered BiOI, except that the solvent was distilled water instead of ethylene glycol without benzaldehyde addition. 2.2. Characterization X-ray diffraction patterns (XRD) of samples were tested by a Bruker D8 advance X-ray diffractometer at room temperature using Cu Kα radiation. Scanning electron microscope (SEM) images were recorded by a JEOL JEM-7600F Field Emission Scanning Electron Microscope. Transmission electron microscopy (TEM) and high-resolution TEM images were analyzed by a JEOL JEM-2100 (RH) Field Emission Electron Microscope. GBC UV/VIS 916 spectrophotometer (GBC Company, Australia) was used to record the UV–vis diffuse reflectance spectra (DRS) with BaSO4 as a reference. Photoluminescence (PL) spectra of samples were obtained by a cary eclipse spectrophotometer (λexc ¼320 nm). The Brunauer– Emmett–Teller (BET) surface areas of samples were measured using quantachrome autosorb-1 automated gas sorption systems at 77 K. X-Ray photoelectron spectroscopy (XPS) measurements were carried out using a ESCALAB 250XI X spectrometer (Thermo Electron Corporation) with an Ar Ka X-ray source, and the spectra calibrated to the C 1s peak at 284.6 eV. Electron spin resonance (ESR) spectra were obtained on JEOL JES-FA300 electron spin resonance spectrometer at room temperature with micro frequency at 8982 MHz. 2.3. Photocatalytic reduction of CO2 The photocatalytic reduction activities of the samples for CO2 conversion was conducted in Labsolar-IIIAG closed gas system (PLS-SXE300, Beijing Perfectlight Technology Co., Ltd., China, Scheme 1) with the total reaction volume of 500 mL. 1.712 g of NaHCO3 was added firstly. Then 0.15 g of the obtained sample was uniformly dispersed onto a watch-glass with an area of 28.26 cm2 before putting into a watch-glass equipped reaction cell. Prior to the light irradiation, the above system was thoroughly vacuum-
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Scheme 1. Closed gas photocatalytic system for reduction of CO2.
treated to remove the air completely. Subsequently, 5 mL H2SO4 (4 M) was injected into the reactor to react with NaHCO3 to achieve 1 atm CO2 gas. After that, the reactor was irradiated from the top by a 300 W high pressure xenon lamp (PLS-SXE300, Beijing Perfectlight Technology Co., Ltd., China) maintaining the photoreaction temperature at 20 °C with DC-0506 low-temperature thermostat bath (Shanghai Sunny Hengping Scientific Instrument Co., Ltd., China). The visible light and near-infrared light were achieved using 420 and 700 nm cutoff filters, respectively. At each time interval, about 0.15 mL of gas was taken from the reaction cell with subsequent qualitative analysis by GC9790II gas chromatography (GC, Zhejiang Fuli Analytical Instrument Co., Ltd., China) equipped with a flame ionization detector (FID, GDX-502 columns) and a thermal conductivity detector (TCD, TDX-01 columns). The quantification of the production yield was based on a calibration curve. The outlet gases were determined to be CO, CH4 and CO2. The apparent quantum yields (AQY) was measured under the same photocatalytic reaction condition, except for the incident light wavelength. The solar fuels (CO and CH4) yields of 1 h photoreaction under different monochromatic light wavelengths (420, 450, 475, 500, 525, 550, 600, 650 and 700 nm) were measured. The band-pass and cutoff filters were used in the above measurement. AQY at different wavelengths is calculated by the following equation: AQYð%Þ ¼ number of reacted electrons=number of incident photonsÞ 100% ¼ ð2 number of evolved CO molecules þ8 number of evolved CH4 moleculesÞ=number of incident photons 100% 2.4. Theoretical calculation method Both the thin and thick BiOI (001) nanosheets were studied by the supercell model combined a plane-wave method as implemented in the Vienna ab initio simulation package (VASP) [23]. The ion-electron interaction was described by the Perdew–Burke– Ernzerhof (PBE) function [24] with the generalized gradient approximation (GGA) and the core electrons were described by the full-potential projector augmented wave (PAW) method [25,26] with an energy cutoff of 520 eV for the plane-wave expansion. The geometry optimization ended until the force on the relax atoms less than 0.02 eV/Å and all calculations utilized the spinpolarization. Integrations in the Brillouin zone were performed using k-point grid generated with the Monkhorst-Pack grid. The BiOI (001) surface was modeled by a periodic slab and the (1 1) surface was applied. The thin one was modeled by 11 atom layers with O atoms exposed on the top and bottom; the thick one was modeled by 16 atom layers with the bottom 5 atom layers
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Fig. 1. The simulated spacing of the (001) crystal plane and charge density contour plots for bulk BiOI (a) and few-layered BiOI (b) along [001] direction; and total DOS and PDOS of bulk BiOI (c) and few-layered BiOI (d).
fixed [13]. The slabs were separated by a vacuum region of 20 Å. The k-point grid 4 4 4 was used for the BiOI bulk, and 4 4 1 for the 001 surface.
3. Results and discussion 3.1. Theoretical calculations To compare the electronic structure of few-layered BiOI and bulk BiOI, theoretical calculations were carried out to simulate spacing of the (001) crystal plane and the charge density contour plots along the [001] direction. The total density of sates (DOS) and partial DOS (PDOS) of bulk BiOI and few-layered BiOI were also computationally obtained. As shown in Fig. 1a, the spacing of (001) crystal plane of bulk BiOI is 0.915 nm. When the thickness reduced to few layers, the spacing of the (001) crystal plane expands to 0.950 nm (Fig. 1b, Fig. S2). This phenomenon implies that the interlayer distance between [Bi2O2] and double iodine slabs can increase as the thickness of BiOI nanosheets decreases. Additionally, the charge density surrounding [Bi2O2] layer is higher than that of double iodine slabs for bulk BiOI and 3L-BiOI. This is due to the inhomogeneous charge distribution between [Bi2O2] and double iodine slabs, which results in static electric field along z axis by polarizing the related orbitals and atoms. It should be noted that the increased (001) crystal plane spacing will induced a greater difference of charge density between [Bi2O2] and double iodine slabs. Accordingly, the internal electric field intensity of BiOI can be enhanced by decreasing the thickness. Moreover, due to the size effect, few-layered BiOI shows different DOS comparing with bulk BiOI (Fig. 1c and d). The band gap (Eg) of few-layered BiOI is found to be much larger than that of bulk BiOI. And the
Fig. 2. XRD patterns of few-layered BiOI and bulk BiOI.
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conduction band minimum (CBM) and valence band maximum (VBM) potentials of few-layered BiOI were higher and lower than that of bulk BiOI, respectively. As the CBM and VBM potentials of photocatalyst are highly related to the reducing and oxidizing ability of photo-induced carriers, the result indicates that the photoexcited electrons and holes of few-layered BiOI can respectively display higher reducing power and oxidizing power than those of bulk BiOI. Based on above theoretical calculation results, it can be concluded that few-layered BiOI should exhibit higher photocatalytic activity than that of bulk BiOI. 3.2. Material characterization The chemical compositions of the samples were analyzed by XPS. As shown in Fig. S3, the few-layered BiOI consists of Bi, O, I elements, and the atomic ratio of Bi/I is 49:46. The XRD pattern of bulk BiOI and fewlayered BiOI (Fig. 2a) exhibits all diffraction peaks from 5° to 60°. Obviously, as-synthesized bulk BiOI and few-layered BiOI can be well indexed to the standard tetragonal BiOI structure (JPCDS no. 01-0732063). Noted that the diffraction peak around at 2θ ¼10° corresponds to the (001) plane of BiOI samples. The (001) diffraction peak (9.35°) of the few-layered BiOI is smaller than that of bulk BiOI (9.69°). According to the Bragg equation, the interlayer distance of (001) facets (d001) should be expanded from 0.915 nm to 0.945 nm. This interesting phenomenon may be resulted from the enhanced surface tension as the thickness reduces to atomic scale. In addition, the 2θ angle of (012) diffraction peak of few-layered BiOI also shifts to a smaller angle. As shown in Fig. 2b, for bulk BiOI, the interlayer distance of (002), (010) and (012) planes was 0.456, 0.398 and 0.303 nm, respectively. In parallel, the interlayer distances for few-layered BiOI was 0.473, 0.398 and 0.309 nm, respectively (Fig. S4). Therefore, the interlayer distance of (012) plane of few-layered BiOI is larger than that of bulk BiOI. Namely, the 2θ of (012) diffraction peak of few-layered BiOI is smaller than bulk BiOI. Fig. 3a shows the TEM image of the few-layered BiOI. It can be seen that the morphology of the as-prepared sample is nanosheetshaped with an average diameter of about 300 nm. A clear SAED pattern is observed in Fig. 3b, demonstrating a single crystal structure of the few-layered BiOI. Moreover, the diffraction spots can be indexed to (110), (1–10) and (200), respectively. In addition,
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the angle between {110} and {200} facets is about 45.0°, corresponding to the theoretical value (45.0°). Thus, the exposed facets (top and bottom surfaces) of few-layered BiOI was proved to be {001} facets. Comparatively, the bulk BiOI is also with layer shape (Figs. S5 and S6). Interestingly, A top view along [001] orientation in the HRTEM image of few-layered BiOI shows that the distances between two vertical interlayer are both 0.28 nm (Fig. 3c), which can be indexed to {110} and {1–10} facets, respectively. In parallel, the thickness of few-layered BiOI in side view is found to be about 2.8 nm with four layers (Fig. 3d). Accordingly, the clear lattice spacings of {001} facets is 0.95 nm, in agreement with the results of computative value (0.950 nm) and XRD analysis (0.945 nm). In contrast, for bulk BiOI, the interlayer distance of (001) plane is calculated to be 0.912 nm according to the Bragg equation (Fig. S4), which is also in agreement with the computative value (0.915 nm). The expansion of the interlayer spacing may be due to the stronger interaction between BiOI surface and water molecules. As mentioned above, defect is propitious to enhance the photon absorption efficiency. The defects of as-prepared samples are examined by ESR and high resolution XPS measurements. As shown in Fig. 4a, the ESR spectra of few-layered BiOI exhibits a signal at g¼2.001, which can be attributable to the presence of oxygen vacancies in BiOX photocatalysts [27–29]. This indicates that benzaldehyde assisted solvothermal method is very efficient to prepare few-layered BiOI with oxygen vacancies, since benzaldehyde can be used as redactor to create oxygen vacancies. XPS analysis is conducted to further confirm the existence of oxygen vacancies of fewlayered BiOI (Fig. 4b). The two peaks at 166.6 and 161.2 eV of Bi 4f spectrum of bulk BiOI indicates that all Bi atoms in BiOI are Bi3 þ [30,31]. However, few-layered BiOI has two additional peaks with lower binding energies at 164.3 and 158.9 eV, respectively. This suggests the existence of Bi( þ 3 x) due to the presence of oxygen vacancies in bismuth-based semiconductor photocatalysts [27,28]. Based on the above results, it is reasonably concluded that few-layered BiOI possesses increased vacancy concentration and expanded spacing of the (001) crystal plane. In order to examine whether these geometry variations affect photon absorption efficiency and carrier separation efficiency, UV–vis DRS and PL, spectra of few-layered BiOI and bulk BiOI were obtained. As
Fig. 3. (a) TEM image, (b) SAED pattern, (c) top view HRTEM image, and (d) top side view HRTEM image of few-layered BiOI.
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Fig. 4. (a) Electron spin resonance spectra, (b) high resolution X-ray photoelectron spectroscopy of Bi 4f of few-layered BiOI and bulk BiOI.
shown in Fig. 5a, the absorption band edge of few-layered BiOI locates at 557 nm, which is lower than that of bulk BiOI (660 nm), indicating a larger energy gap of few-layered BiOI (2.23 eV) compared to that of bulk BiOI (1.82 eV). Notably, due to the existence of oxygen vacancies in few-layered BiOI, the color of few-layered BiOI was olive-green and the absorption range of few-layered BiOI covers not only all UV–vis light but also NIR light. In contrast, bulk BiOI was red-colored and can only absorb partial visible light (λ o 660 nm). The result implies that fewlayered BiOI can display photocatalytic activity under NIR light rather than bulk BiOI. PL analysis is accepted as an useful method to evaluate the separation efficiency of photo-generated electrons and holes [32,33]. The lower PL peak intensity reflects the higher separation efficiency. As shown in Fig. 5b, with an excitation wavelength at 360 nm, bulk BiOI shows an emission peak around 430 nm owing to the direct recombination between photo-generated carriers. However, few-layered BiOI presents lower PL intensity at 430 nm. This is because the expanded spacing of the (001) crystal plane and atomic size thickness of few-layered BiOI result in higher internal field electric field intensity. The result demonstrates that the separation efficiency of photo-generated electrons and holes of few-layered BiOI is better than that of bulk BiOI. 3.3. Photocatalytic CO2 conversion The photocatalytic performances of few-layered BiOI and bulk BiOI were evaluated by CO2 reduction. Control experiments show that no detectable carbon-related products are found without photocatalyst or
Fig. 5. (a) UV–vis DRS spectra and (b) PL spectra of few-layered BiOI and bulk BiOI with λexc ¼ 320 nm. (The inset image is the photograph of samples). (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
light irradiation. Therefore, the photocatalyst and light energy are the prerequisites for the photocatalytic CO2 conversion. The quantity of CO2 converted CO and CH4 under visible or NIR light irradiation over few-layered BiOI and bulk BiOI is compared in Fig. 6. When λ Z420 nm (visible light), the generation of CO (CO2 þ 2H þ þ 2e COþH2O) and CH4 (CO2 þ 8H þ þ8e -CH4 þ 2H2O) over few-layered BiOI is about 0.615 and 0.063 umol h 1 respectively (Fig. 6a), which are much more than the generation of CO (0.077 umol h 1) and CH4 (0.029 umol h 1) over bulk BiOI (Fig. 6b). This indicates that the thickness decreasing can significantly improve the photocatalytic activity of BiOI. After 4 h hibernation, both few-layered BiOI and bulk BiOI maintain their good photocatalytic activities for CO2 conversion under irradiation, suggesting that BiOI is a stable photocatalyst for photoreduction of CO2. Surprisingly, the few-layered BiOI is found to selective photoreduce CO2 to more CO than CH4. The selectivity of fewlayered BiOI was much obvious than that of bulk BiOI, which can be ascribed to the few-layered structure and oxygen vacancy in fewlayered BiOI [37]. According to the optical spectra (Fig. 5a), few-layered BiOI should display photocatalytic activity under NIR light irradiation. Here, the CO2 photoreduction activity of bismuth-based photocatalysts under NIR light (λ Z700 nm) is explored. As shown in Fig. 6c, the generated CO and CH4 over few-layered BiOI is respectively about 0.119 and 0.021 umol h 1 under NIR light irradiation. However, few CH4 and CO was detected over bulk BiOI. It can be inferred that the oxygen vacancy of few-layered BiOI induced the NIR light responsive photocatalytic activity. To the best of my knowledge, it is the first time to report NIR light responsive photocatalytic activity of bismuth-based semiconductor photocatalysts.
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Fig. 6. The quantity of gas (CO and CH4) under irradiation over few-layered BiOI and bulk BiOI.
Fig. 7 shows the DRS spectrum of few-layered BiOI along with the AQY of solar fuels (CO and CH4) productions as a function of the incident monochromatic light wavelength. The AQY value shows decreasing trend upon increasing the wavelength of monochromatic light, which is consistent with the change tendency of DRS spectrum. The AQY for the solar fuels (CO and CH4) productions over few-layered BiOI reaches a maximum of 0.14% under 420 nm monochromatic light irradiation. When the wavelength of irradiated monochromatic light prolongs to 700 nm, the AQY of few-layered BiOI still reaches 0.02%. Thus, few-layered BiOI with expanded spacing of the (001) facets and oxygen vacancy can efficiently photoreduce CO2 into solar fuels under visible/NIR light. The theoretical calculation demonstrates that the CBM position raises (Fig. 1d) when the thickness of BiOI reduce to atom scale. The CBM positions of the few-layered BiOI and bulk BiOI were further validated by valence-band XPS spectra. As shown in Fig. 8, the VBM positions of few-layered BiOI and bulk BiOI were at 1.59 and 1.41 eV, respectively. Accordingly, the CBM positions of few-layered BiOI and bulk BiOI are calculated to be 0.64 and 0.47 eV, respectively. Obviously, the result is in agreement with the DOS result (Fig. 1c and d). The higher CBM position and lower VBM position endow few-
Fig. 7. Wavelength-dependent AQY and DRS spectra of few-layered BiOI.
layered BiOI with enhanced photocatalytic reduction and oxidation activity, respectively. Therefore, few-layered BiOI displays higher photocatalytic activity than bulk BiOI for photoreduction of CO2.
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characterizations. These unique properties of few-layered BiOI result in superior photocatalytic performance for the photoreduction of CO2 into solar fuels under visible/NIR light over the bulk counterpart. These findings could deepen the understanding of single layer or few-layered BiOX (X ¼Cl, Br, I) photocatalysts and their applications in energy and environmental photocatalysis.
Acknowledgments
Fig. 8. Valence-band XPS spectra of the few-layered BiOI and bulk BiOI.
This work was supported by the National Natural Science Foundation of China (No. U1404506 and 51502146), Natural Science Foundation of Henan Department of Science & Technology (No. 142102210477), Natural Science Foundation of Henan Department of Education (No. 14A150021), Natural Science Foundation of Nanyang Normal University (No. ZX2014039) and Graduate Innovation Foundation of Nanyang Normal University (No. 2015CX005). Prof. Ye also thanks Dr Dan Wu for revising the manuscript.
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.solmat.2015.10.022.
References
Fig. 9. The photocatalytic mechanism of few-layered BiOI for efficient photoreduction of CO2 into solar fuels under visible/NIR light.
Fig. 9 displays the photocatalytic mechanism of few-layered BiOI for efficient photoreduction of CO2 into solar fuels under visible/NIR light. As mentioned above, when the thickness of BiOI reduce to atom scale, the CBM position should be raised about 0.17 eV. Consequently, few-layered BiOI displays higher CBM position and photocatalytic reduction activity for CO2 conversion, leading to higher photocatalytic activity for CO2 reduction to generate solar fuels, especially for CO generation. On the other hand, the existence of oxygen vacancy also can affect the photocatalytic reduction activity and product type of CO2 over BiOI. In our previous work, the oxygen vacancies have been proved to favor the separation of carriers [27,34,35]. Furthermore, Wang et al. reported that oxygen vacancies on BiOCl nanoplates surface can improve the photoreduction of CO2 [36]. Zhang et al. also suggested that oxygen vacancies of BiOCl nanoplates can improve the photoreduction of water for hydrogen generation [37]. More importantly, the oxygen vacancies expand the range of light absorption, leading to the near-infrared light responsive CO2 photoreduction activity of few-layered BiOI. As a result, fewlayered BiOI with oxygen vacancies can photoreduce CO2 to CO more effectively. Therefore, few-layered BiOI nanosheets were achieved with high specific surface area (138 m2/g), fully exposed {001} facets, extended d-spacing of (001) facets and oxygen vacancy. Those unique properties result in surprising photocatalytic properties of few-layered BiOI.
4. Conclusion In conclusion, few-layered BiOI is demonstrated to possess expanded spacing of the (001) facets, oxygen vacancy and larger band gaps by theoretical calculations and detailed experimental
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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Synthesis of olive-green few-layered BiOI for efficient photoreduction of CO2 into solar fuels under visible/near-infrared light Liqun Ye a,n, Hui Wang b, Xiaoli Jin a, Yurong Su a, Dongqi Wang b,n, Haiquan Xie a,n, Xiaodi Liu a, Xinxin Liu a a
College of Chemistry and Pharmaceutical Engineering, Nanyang Normal University, Nanyang 473061, China CAS Key Laboratory of Nuclear Radiation and Nuclear Energy Techniques, and Multidisciplinary Initiative Center, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
b
art ic l e i nf o
a b s t r a c t
Article history: Received 8 August 2015 Received in revised form 10 October 2015 Accepted 17 October 2015 Available online 11 November 2015
Olive-green few-layered BiOI with expanded spacing of the (001) facets and oxygen vacancy is successfully synthesized and characterized. The experimental analysis and theoretical calculation demonstrate that the expanded facets spacing and oxygen vacancy of few-layered BiOI result in enhanced separation efficiency of photoinduced carriers and photon absorption efficiency. Therefore, few-layered BiOI shows much higher photocatalytic activity for CO2 conversion than that of bulk BiOI under visible or near-infrared (NIR) light. And the apparent quantum yields (AQY) are 0.14% and 0.02% for few-layered BiOI under 420 and 700 nm monochromatic light irradiation, respectively. These findings deep our understanding of few-layered BiOX (X ¼Cl,Br,I) photocatalysts, and propose a new route to design high efficient photocatalysts for energy and environmental photocatalysis. & 2015 Elsevier B.V. All rights reserved.
Keywords: Bismuth oxyiodide Few-layer Photocatalysis Oxygen vacancy Carbon dioxide Solar fuels
1. Introduction Photon absorption efficiency and carrier separation efficiency are the two key factors to affect the activity of semiconductor photocatalysts [1–3]. In the past fifty years, various approaches, such as cocatalyst utilization, element doping, coupling, surface plasmon resonance and dye sensitization, were employed to enhance the photon absorption efficiency and photoexcited carrier separation efficiency, further leading to improved photocatalytic activity [4,5]. However, all these modifications need assistance of external materials and cannot enhance the photon absorption efficiency and carrier separation efficiency simultaneously. Recently, layered two-dimensional (2D) nano-materials have greatly developed due to their potential applications in photocatalysis [6,7]. Compared with bulk materials, 2D nano-materials have more dangling bond atoms and higher specific surface areas. Therefore, 2D nano-materials usually display more vacancies than bulk materials [6,7]. The vacancies may significantly affect the physical and chemical properties of photocatalysts. For instance, few-layered graphene and MoS2 can be used as deriving-electron n
Corresponding authors. Tel.: þ 86 377 63525056. E-mail addresses: [email protected] (L. Ye), [email protected] (D. Wang), [email protected] (H. Xie). http://dx.doi.org/10.1016/j.solmat.2015.10.022 0927-0248/& 2015 Elsevier B.V. All rights reserved.
types cocatalysts in photocatalysis [8], and ultrathin g-C3N4 [9,10] and In2O3 nanosheets as photocatalyst for water splitting [11]. These pioneering work have stimulated extensive studies on the synthesis of single-layer and multilayer nanosheets in order to expand their applications. As a series of layered bismuth-based semiconductor photocatalyst, BiOI exposed with dominated {001} facets has been demonstrated to exhibit excellent photocatalytic activity due to their open and layered crystalline structure [12–14]. As shown in Fig. S1, the layered structure of BiOI consists of [Bi2O2]2 þ layers placed between two slabs of iodine ions along z axis ([001] direction). Therefore, it is feasible to synthesize single layer or fewlayered BiOI nanosheets with exposed dominated {001} active facets. More importantly, the increasing defects concentration, small size effect, and enhanced internal electric field intensity of few-layered BiOI will result in improved efficiency of photon absorption and carrier separation. The escalating environment and energy crises, partially due to the increasing atmospheric CO2 due to the consumption of fossil fuels [1,2], threaten human existence and development. Thus, more and more researchers focus on conversion of CO2 to fuels [15–18]. Various approaches, such as catalytic hydrogenation [19], electro-catalysis [20], and photocatalytic reduction [21] were used to convert CO2 to fuels. Among these methods, semiconductor
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photocatalysis technology has attracted intensive attention at fundamental application levels owing to its low-cost and green nature to solve environment and energy crises [1,2,22]. However, until now, much photoreduction of CO2 were done only under UV light irradiation, limiting efficient utilization of the solar energy. In this paper, few-layered BiOI with expanded spacing of the (001) facets and oxygen vacancy was successfully synthesized. The thickness of few-layered BiOI nanosheets was about 2.8 nm. And the expanded spacing of the (001) facets was proved by theoretical simulation. The photocatalytic results showed that few-layered BiOI can photocatalytic convert CO2 into solar fuels effectively under visible/near-infrared-light. To the best of our knowledge, it is the first time to synthesize few-layered BiOI, and also the first time to employ the as-prepared photocatalyst for CO2 conversion under near-infrared (NIR) light.
2. Experimental sections 2.1. Synthesis of few-layered BiOI 20 mL of KI (0.5 M) ethylene glycol solution was added dropwise into 20 mL of Bi(NO3)3 5H2O (0.5 M) ethylene glycol solution under continuous magnetic stirring. After stirring for another 1 h, 1 mL benzaldehyde was added to the above mixture. Then the above solution was transferred into to Teflon-lined stainless steel autoclave (50 mL) and kept at 160 °C for 24 h. The olive-green precipitate was centrifuged, washed successively with ethanol and distilled water three times, and finally dried at 80 °C for 12 h. The synthesis of bulk BiOI followed the same procedure to that of few-layered BiOI, except that the solvent was distilled water instead of ethylene glycol without benzaldehyde addition. 2.2. Characterization X-ray diffraction patterns (XRD) of samples were tested by a Bruker D8 advance X-ray diffractometer at room temperature using Cu Kα radiation. Scanning electron microscope (SEM) images were recorded by a JEOL JEM-7600F Field Emission Scanning Electron Microscope. Transmission electron microscopy (TEM) and high-resolution TEM images were analyzed by a JEOL JEM-2100 (RH) Field Emission Electron Microscope. GBC UV/VIS 916 spectrophotometer (GBC Company, Australia) was used to record the UV–vis diffuse reflectance spectra (DRS) with BaSO4 as a reference. Photoluminescence (PL) spectra of samples were obtained by a cary eclipse spectrophotometer (λexc ¼320 nm). The Brunauer– Emmett–Teller (BET) surface areas of samples were measured using quantachrome autosorb-1 automated gas sorption systems at 77 K. X-Ray photoelectron spectroscopy (XPS) measurements were carried out using a ESCALAB 250XI X spectrometer (Thermo Electron Corporation) with an Ar Ka X-ray source, and the spectra calibrated to the C 1s peak at 284.6 eV. Electron spin resonance (ESR) spectra were obtained on JEOL JES-FA300 electron spin resonance spectrometer at room temperature with micro frequency at 8982 MHz. 2.3. Photocatalytic reduction of CO2 The photocatalytic reduction activities of the samples for CO2 conversion was conducted in Labsolar-IIIAG closed gas system (PLS-SXE300, Beijing Perfectlight Technology Co., Ltd., China, Scheme 1) with the total reaction volume of 500 mL. 1.712 g of NaHCO3 was added firstly. Then 0.15 g of the obtained sample was uniformly dispersed onto a watch-glass with an area of 28.26 cm2 before putting into a watch-glass equipped reaction cell. Prior to the light irradiation, the above system was thoroughly vacuum-
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Scheme 1. Closed gas photocatalytic system for reduction of CO2.
treated to remove the air completely. Subsequently, 5 mL H2SO4 (4 M) was injected into the reactor to react with NaHCO3 to achieve 1 atm CO2 gas. After that, the reactor was irradiated from the top by a 300 W high pressure xenon lamp (PLS-SXE300, Beijing Perfectlight Technology Co., Ltd., China) maintaining the photoreaction temperature at 20 °C with DC-0506 low-temperature thermostat bath (Shanghai Sunny Hengping Scientific Instrument Co., Ltd., China). The visible light and near-infrared light were achieved using 420 and 700 nm cutoff filters, respectively. At each time interval, about 0.15 mL of gas was taken from the reaction cell with subsequent qualitative analysis by GC9790II gas chromatography (GC, Zhejiang Fuli Analytical Instrument Co., Ltd., China) equipped with a flame ionization detector (FID, GDX-502 columns) and a thermal conductivity detector (TCD, TDX-01 columns). The quantification of the production yield was based on a calibration curve. The outlet gases were determined to be CO, CH4 and CO2. The apparent quantum yields (AQY) was measured under the same photocatalytic reaction condition, except for the incident light wavelength. The solar fuels (CO and CH4) yields of 1 h photoreaction under different monochromatic light wavelengths (420, 450, 475, 500, 525, 550, 600, 650 and 700 nm) were measured. The band-pass and cutoff filters were used in the above measurement. AQY at different wavelengths is calculated by the following equation: AQYð%Þ ¼ number of reacted electrons=number of incident photonsÞ 100% ¼ ð2 number of evolved CO molecules þ8 number of evolved CH4 moleculesÞ=number of incident photons 100% 2.4. Theoretical calculation method Both the thin and thick BiOI (001) nanosheets were studied by the supercell model combined a plane-wave method as implemented in the Vienna ab initio simulation package (VASP) [23]. The ion-electron interaction was described by the Perdew–Burke– Ernzerhof (PBE) function [24] with the generalized gradient approximation (GGA) and the core electrons were described by the full-potential projector augmented wave (PAW) method [25,26] with an energy cutoff of 520 eV for the plane-wave expansion. The geometry optimization ended until the force on the relax atoms less than 0.02 eV/Å and all calculations utilized the spinpolarization. Integrations in the Brillouin zone were performed using k-point grid generated with the Monkhorst-Pack grid. The BiOI (001) surface was modeled by a periodic slab and the (1 1) surface was applied. The thin one was modeled by 11 atom layers with O atoms exposed on the top and bottom; the thick one was modeled by 16 atom layers with the bottom 5 atom layers
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Fig. 1. The simulated spacing of the (001) crystal plane and charge density contour plots for bulk BiOI (a) and few-layered BiOI (b) along [001] direction; and total DOS and PDOS of bulk BiOI (c) and few-layered BiOI (d).
fixed [13]. The slabs were separated by a vacuum region of 20 Å. The k-point grid 4 4 4 was used for the BiOI bulk, and 4 4 1 for the 001 surface.
3. Results and discussion 3.1. Theoretical calculations To compare the electronic structure of few-layered BiOI and bulk BiOI, theoretical calculations were carried out to simulate spacing of the (001) crystal plane and the charge density contour plots along the [001] direction. The total density of sates (DOS) and partial DOS (PDOS) of bulk BiOI and few-layered BiOI were also computationally obtained. As shown in Fig. 1a, the spacing of (001) crystal plane of bulk BiOI is 0.915 nm. When the thickness reduced to few layers, the spacing of the (001) crystal plane expands to 0.950 nm (Fig. 1b, Fig. S2). This phenomenon implies that the interlayer distance between [Bi2O2] and double iodine slabs can increase as the thickness of BiOI nanosheets decreases. Additionally, the charge density surrounding [Bi2O2] layer is higher than that of double iodine slabs for bulk BiOI and 3L-BiOI. This is due to the inhomogeneous charge distribution between [Bi2O2] and double iodine slabs, which results in static electric field along z axis by polarizing the related orbitals and atoms. It should be noted that the increased (001) crystal plane spacing will induced a greater difference of charge density between [Bi2O2] and double iodine slabs. Accordingly, the internal electric field intensity of BiOI can be enhanced by decreasing the thickness. Moreover, due to the size effect, few-layered BiOI shows different DOS comparing with bulk BiOI (Fig. 1c and d). The band gap (Eg) of few-layered BiOI is found to be much larger than that of bulk BiOI. And the
Fig. 2. XRD patterns of few-layered BiOI and bulk BiOI.
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conduction band minimum (CBM) and valence band maximum (VBM) potentials of few-layered BiOI were higher and lower than that of bulk BiOI, respectively. As the CBM and VBM potentials of photocatalyst are highly related to the reducing and oxidizing ability of photo-induced carriers, the result indicates that the photoexcited electrons and holes of few-layered BiOI can respectively display higher reducing power and oxidizing power than those of bulk BiOI. Based on above theoretical calculation results, it can be concluded that few-layered BiOI should exhibit higher photocatalytic activity than that of bulk BiOI. 3.2. Material characterization The chemical compositions of the samples were analyzed by XPS. As shown in Fig. S3, the few-layered BiOI consists of Bi, O, I elements, and the atomic ratio of Bi/I is 49:46. The XRD pattern of bulk BiOI and fewlayered BiOI (Fig. 2a) exhibits all diffraction peaks from 5° to 60°. Obviously, as-synthesized bulk BiOI and few-layered BiOI can be well indexed to the standard tetragonal BiOI structure (JPCDS no. 01-0732063). Noted that the diffraction peak around at 2θ ¼10° corresponds to the (001) plane of BiOI samples. The (001) diffraction peak (9.35°) of the few-layered BiOI is smaller than that of bulk BiOI (9.69°). According to the Bragg equation, the interlayer distance of (001) facets (d001) should be expanded from 0.915 nm to 0.945 nm. This interesting phenomenon may be resulted from the enhanced surface tension as the thickness reduces to atomic scale. In addition, the 2θ angle of (012) diffraction peak of few-layered BiOI also shifts to a smaller angle. As shown in Fig. 2b, for bulk BiOI, the interlayer distance of (002), (010) and (012) planes was 0.456, 0.398 and 0.303 nm, respectively. In parallel, the interlayer distances for few-layered BiOI was 0.473, 0.398 and 0.309 nm, respectively (Fig. S4). Therefore, the interlayer distance of (012) plane of few-layered BiOI is larger than that of bulk BiOI. Namely, the 2θ of (012) diffraction peak of few-layered BiOI is smaller than bulk BiOI. Fig. 3a shows the TEM image of the few-layered BiOI. It can be seen that the morphology of the as-prepared sample is nanosheetshaped with an average diameter of about 300 nm. A clear SAED pattern is observed in Fig. 3b, demonstrating a single crystal structure of the few-layered BiOI. Moreover, the diffraction spots can be indexed to (110), (1–10) and (200), respectively. In addition,
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the angle between {110} and {200} facets is about 45.0°, corresponding to the theoretical value (45.0°). Thus, the exposed facets (top and bottom surfaces) of few-layered BiOI was proved to be {001} facets. Comparatively, the bulk BiOI is also with layer shape (Figs. S5 and S6). Interestingly, A top view along [001] orientation in the HRTEM image of few-layered BiOI shows that the distances between two vertical interlayer are both 0.28 nm (Fig. 3c), which can be indexed to {110} and {1–10} facets, respectively. In parallel, the thickness of few-layered BiOI in side view is found to be about 2.8 nm with four layers (Fig. 3d). Accordingly, the clear lattice spacings of {001} facets is 0.95 nm, in agreement with the results of computative value (0.950 nm) and XRD analysis (0.945 nm). In contrast, for bulk BiOI, the interlayer distance of (001) plane is calculated to be 0.912 nm according to the Bragg equation (Fig. S4), which is also in agreement with the computative value (0.915 nm). The expansion of the interlayer spacing may be due to the stronger interaction between BiOI surface and water molecules. As mentioned above, defect is propitious to enhance the photon absorption efficiency. The defects of as-prepared samples are examined by ESR and high resolution XPS measurements. As shown in Fig. 4a, the ESR spectra of few-layered BiOI exhibits a signal at g¼2.001, which can be attributable to the presence of oxygen vacancies in BiOX photocatalysts [27–29]. This indicates that benzaldehyde assisted solvothermal method is very efficient to prepare few-layered BiOI with oxygen vacancies, since benzaldehyde can be used as redactor to create oxygen vacancies. XPS analysis is conducted to further confirm the existence of oxygen vacancies of fewlayered BiOI (Fig. 4b). The two peaks at 166.6 and 161.2 eV of Bi 4f spectrum of bulk BiOI indicates that all Bi atoms in BiOI are Bi3 þ [30,31]. However, few-layered BiOI has two additional peaks with lower binding energies at 164.3 and 158.9 eV, respectively. This suggests the existence of Bi( þ 3 x) due to the presence of oxygen vacancies in bismuth-based semiconductor photocatalysts [27,28]. Based on the above results, it is reasonably concluded that few-layered BiOI possesses increased vacancy concentration and expanded spacing of the (001) crystal plane. In order to examine whether these geometry variations affect photon absorption efficiency and carrier separation efficiency, UV–vis DRS and PL, spectra of few-layered BiOI and bulk BiOI were obtained. As
Fig. 3. (a) TEM image, (b) SAED pattern, (c) top view HRTEM image, and (d) top side view HRTEM image of few-layered BiOI.
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Fig. 4. (a) Electron spin resonance spectra, (b) high resolution X-ray photoelectron spectroscopy of Bi 4f of few-layered BiOI and bulk BiOI.
shown in Fig. 5a, the absorption band edge of few-layered BiOI locates at 557 nm, which is lower than that of bulk BiOI (660 nm), indicating a larger energy gap of few-layered BiOI (2.23 eV) compared to that of bulk BiOI (1.82 eV). Notably, due to the existence of oxygen vacancies in few-layered BiOI, the color of few-layered BiOI was olive-green and the absorption range of few-layered BiOI covers not only all UV–vis light but also NIR light. In contrast, bulk BiOI was red-colored and can only absorb partial visible light (λ o 660 nm). The result implies that fewlayered BiOI can display photocatalytic activity under NIR light rather than bulk BiOI. PL analysis is accepted as an useful method to evaluate the separation efficiency of photo-generated electrons and holes [32,33]. The lower PL peak intensity reflects the higher separation efficiency. As shown in Fig. 5b, with an excitation wavelength at 360 nm, bulk BiOI shows an emission peak around 430 nm owing to the direct recombination between photo-generated carriers. However, few-layered BiOI presents lower PL intensity at 430 nm. This is because the expanded spacing of the (001) crystal plane and atomic size thickness of few-layered BiOI result in higher internal field electric field intensity. The result demonstrates that the separation efficiency of photo-generated electrons and holes of few-layered BiOI is better than that of bulk BiOI. 3.3. Photocatalytic CO2 conversion The photocatalytic performances of few-layered BiOI and bulk BiOI were evaluated by CO2 reduction. Control experiments show that no detectable carbon-related products are found without photocatalyst or
Fig. 5. (a) UV–vis DRS spectra and (b) PL spectra of few-layered BiOI and bulk BiOI with λexc ¼ 320 nm. (The inset image is the photograph of samples). (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
light irradiation. Therefore, the photocatalyst and light energy are the prerequisites for the photocatalytic CO2 conversion. The quantity of CO2 converted CO and CH4 under visible or NIR light irradiation over few-layered BiOI and bulk BiOI is compared in Fig. 6. When λ Z420 nm (visible light), the generation of CO (CO2 þ 2H þ þ 2e COþH2O) and CH4 (CO2 þ 8H þ þ8e -CH4 þ 2H2O) over few-layered BiOI is about 0.615 and 0.063 umol h 1 respectively (Fig. 6a), which are much more than the generation of CO (0.077 umol h 1) and CH4 (0.029 umol h 1) over bulk BiOI (Fig. 6b). This indicates that the thickness decreasing can significantly improve the photocatalytic activity of BiOI. After 4 h hibernation, both few-layered BiOI and bulk BiOI maintain their good photocatalytic activities for CO2 conversion under irradiation, suggesting that BiOI is a stable photocatalyst for photoreduction of CO2. Surprisingly, the few-layered BiOI is found to selective photoreduce CO2 to more CO than CH4. The selectivity of fewlayered BiOI was much obvious than that of bulk BiOI, which can be ascribed to the few-layered structure and oxygen vacancy in fewlayered BiOI [37]. According to the optical spectra (Fig. 5a), few-layered BiOI should display photocatalytic activity under NIR light irradiation. Here, the CO2 photoreduction activity of bismuth-based photocatalysts under NIR light (λ Z700 nm) is explored. As shown in Fig. 6c, the generated CO and CH4 over few-layered BiOI is respectively about 0.119 and 0.021 umol h 1 under NIR light irradiation. However, few CH4 and CO was detected over bulk BiOI. It can be inferred that the oxygen vacancy of few-layered BiOI induced the NIR light responsive photocatalytic activity. To the best of my knowledge, it is the first time to report NIR light responsive photocatalytic activity of bismuth-based semiconductor photocatalysts.
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Fig. 6. The quantity of gas (CO and CH4) under irradiation over few-layered BiOI and bulk BiOI.
Fig. 7 shows the DRS spectrum of few-layered BiOI along with the AQY of solar fuels (CO and CH4) productions as a function of the incident monochromatic light wavelength. The AQY value shows decreasing trend upon increasing the wavelength of monochromatic light, which is consistent with the change tendency of DRS spectrum. The AQY for the solar fuels (CO and CH4) productions over few-layered BiOI reaches a maximum of 0.14% under 420 nm monochromatic light irradiation. When the wavelength of irradiated monochromatic light prolongs to 700 nm, the AQY of few-layered BiOI still reaches 0.02%. Thus, few-layered BiOI with expanded spacing of the (001) facets and oxygen vacancy can efficiently photoreduce CO2 into solar fuels under visible/NIR light. The theoretical calculation demonstrates that the CBM position raises (Fig. 1d) when the thickness of BiOI reduce to atom scale. The CBM positions of the few-layered BiOI and bulk BiOI were further validated by valence-band XPS spectra. As shown in Fig. 8, the VBM positions of few-layered BiOI and bulk BiOI were at 1.59 and 1.41 eV, respectively. Accordingly, the CBM positions of few-layered BiOI and bulk BiOI are calculated to be 0.64 and 0.47 eV, respectively. Obviously, the result is in agreement with the DOS result (Fig. 1c and d). The higher CBM position and lower VBM position endow few-
Fig. 7. Wavelength-dependent AQY and DRS spectra of few-layered BiOI.
layered BiOI with enhanced photocatalytic reduction and oxidation activity, respectively. Therefore, few-layered BiOI displays higher photocatalytic activity than bulk BiOI for photoreduction of CO2.
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characterizations. These unique properties of few-layered BiOI result in superior photocatalytic performance for the photoreduction of CO2 into solar fuels under visible/NIR light over the bulk counterpart. These findings could deepen the understanding of single layer or few-layered BiOX (X ¼Cl, Br, I) photocatalysts and their applications in energy and environmental photocatalysis.
Acknowledgments
Fig. 8. Valence-band XPS spectra of the few-layered BiOI and bulk BiOI.
This work was supported by the National Natural Science Foundation of China (No. U1404506 and 51502146), Natural Science Foundation of Henan Department of Science & Technology (No. 142102210477), Natural Science Foundation of Henan Department of Education (No. 14A150021), Natural Science Foundation of Nanyang Normal University (No. ZX2014039) and Graduate Innovation Foundation of Nanyang Normal University (No. 2015CX005). Prof. Ye also thanks Dr Dan Wu for revising the manuscript.
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.solmat.2015.10.022.
References
Fig. 9. The photocatalytic mechanism of few-layered BiOI for efficient photoreduction of CO2 into solar fuels under visible/NIR light.
Fig. 9 displays the photocatalytic mechanism of few-layered BiOI for efficient photoreduction of CO2 into solar fuels under visible/NIR light. As mentioned above, when the thickness of BiOI reduce to atom scale, the CBM position should be raised about 0.17 eV. Consequently, few-layered BiOI displays higher CBM position and photocatalytic reduction activity for CO2 conversion, leading to higher photocatalytic activity for CO2 reduction to generate solar fuels, especially for CO generation. On the other hand, the existence of oxygen vacancy also can affect the photocatalytic reduction activity and product type of CO2 over BiOI. In our previous work, the oxygen vacancies have been proved to favor the separation of carriers [27,34,35]. Furthermore, Wang et al. reported that oxygen vacancies on BiOCl nanoplates surface can improve the photoreduction of CO2 [36]. Zhang et al. also suggested that oxygen vacancies of BiOCl nanoplates can improve the photoreduction of water for hydrogen generation [37]. More importantly, the oxygen vacancies expand the range of light absorption, leading to the near-infrared light responsive CO2 photoreduction activity of few-layered BiOI. As a result, fewlayered BiOI with oxygen vacancies can photoreduce CO2 to CO more effectively. Therefore, few-layered BiOI nanosheets were achieved with high specific surface area (138 m2/g), fully exposed {001} facets, extended d-spacing of (001) facets and oxygen vacancy. Those unique properties result in surprising photocatalytic properties of few-layered BiOI.
4. Conclusion In conclusion, few-layered BiOI is demonstrated to possess expanded spacing of the (001) facets, oxygen vacancy and larger band gaps by theoretical calculations and detailed experimental
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