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Capture of atmospheric CO2 into (BiO)2 CO3 /graphene or graphene oxide nanocomposites with enhanced photocatalytic performance Wendong Zhang a , Fan Dong b,∗ , Wei Zhang c,∗ a
Department of Scientific Research Management, Chongqing Normal University, Chongqing, 401331, China Chongqing Key Laboratory of Catalysis and Functional Organic Molecules, College of Environment and Resources, Chongqing Technology and Business University, Chongqing, 400067, China c Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, China b
a r t i c l e
i n f o
Article history: Received 28 July 2015 Received in revised form 19 August 2015 Accepted 21 August 2015 Available online xxx Keywords: Atmospheric CO2 (BiO)2 CO3 /graphene (BiO)2 CO3 /graphene oxide Photocatalytic performance Charge separation
a b s t r a c t Self-assembly of (BiO)2 CO3 nanoflakes on graphene (Ge) and graphene oxide (GO) nanosheets, as an effective strategy to improve the photocatalytic performance of two-dimensional (2D) nanostructured materials, were realized by a one-pot efficient capture of atmospheric CO2 at room temperature. The as-synthesized samples were characterized by XRD, SEM, TEM, XPS, UV–vis DRS, Time-resolved ns-level PL and BET-BJH measurement. The photocatalytic activity of the obtained samples was evaluated by the removal of NO at the indoor air level under simulated solar-light irradiation. Compared with pure (BiO)2 CO3 , (BiO)2 CO3 /Ge and (BiO)2 CO3 /GO nanocomposites exhibited enhanced photocatalytic activity due to their large surface areas and pore volume, and efficient charge separation and transfer. The present work could provide a simple method to construct 2D nanocomposites by efficient utilization of CO2 in green synthetic strategy. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Bismuth subcarbonate (BiO)2 CO3 as a promising photocatalyst has received wide attention because of its typical physical and chemical properties, such as abundant, nontoxicity, high photoactivity, various morphologies, and amenability to chemical modification, etc. [1–4]. Recently, various micro/nanostructures of (BiO)2 CO3 including nanoparticles, nanotubes, nanosheets, nanoplates, nanobars, nanoflowers and hollow microspheres have been synthesized by hydrothermal process [5]. However, the high temperature and pressure, and long time reaction of hydrothermal treatment limited their practical large-scale application. Recently, some studies showed that the two dimensional (2D) bismuthcontaining nanostructures could facilitate effective separation and transfer of the photo-generated electrons and holes, which can improve the photocatalytic activity [6–8]. Thus, it is highly desirable to develop cost-effective and facile method to synthesize 2D (BiO)2 CO3 with high photoactivity. Very recently, we developed a new strategy to synthesize (BiO)2 CO3 nanoflakes by utilization of pure CO2 or atmospheric CO2
∗ Corresponding authors. Tel.: +86 23 62769785 605; fax: +86 23 62769785 605. E-mail addresses: [email protected] (F. Dong), [email protected] (W. Zhang).
at room temperature [9]. However, there are two main drawbacks, which restrict their large scale applications. First, the large band gap (3.1–3.5 eV) of (BiO)2 CO3 can only be activated with ultraviolet (<5% fraction of solar light). Second, the high recombination of photo-generated electrons and holes results in relatively low photocatalytic activity. In order to address these problems, various strategies have been employed, such as in situ N-doped (BiO)2 CO3 [10], (BiO)2 CO3 -BiOI nanocomposites [11] and BiVO4 -(BiO)2 CO3 heterojunctions [12]. In particular, the construction of 2D nanojunctions is a promising route to improve the photoactivity in matrix photocatalysts. The closely contacted interfaces between two components could facilitate the separation and transfer of photo-generated charges and accelerate the surface photocatalytic reactions, which could exhibit higher photocatalytic activity in composites compared with the individual photocatalyst. Graphene (Ge) and graphene oxide (GO) as typical carbonbased nanomaterials have attracted increasingly attention owing to their unique physical–chemical features as follows [13,14]. First, owing to the 2D layered structure, Ge and GO possessing theoretic large specific surface area could provide more active sites for reactants absorption. Second, Ge and GO with sp2 hybridized carbon single atomic nanosheets could act as an excellent supporting matrix, which allows for good contact with some supported nanomaterials. Third, Ge and GO have excellent electronic conductivity because of the two 2D -conjugated structure, which can serve
http://dx.doi.org/10.1016/j.apsusc.2015.08.172 0169-4332/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: W. Zhang, et al., Capture of atmospheric CO2 into (BiO)2 CO3 /graphene or graphene oxide nanocomposites with enhanced photocatalytic performance, Appl. Surf. Sci. (2015), http://dx.doi.org/10.1016/j.apsusc.2015.08.172
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as a good acceptor of electrons. Therefore, these properties enable Ge and GO to be considered as ideal nanomaterials for supporting various semiconductor photocatalysts, such as TiO2 -Ge [15], ZnWO4 -Ge [16], BiOI-GO [17], g-C3 N4 -GO nanocomposites [18]. Recently, (BiO)2 CO3 /graphene nanocomposite with enhanced photocatalytic activity has been prepared by hydrothermal method. However, self-assembly of (BiO)2 CO3 nanoflakes on Ge and GO nanosheets by a one-pot efficient capture of atmospheric CO2 at room temperature have not been reported. In this work, (BiO)2 CO3 /Ge and (BiO)2 CO3 /GO nanocomposites were synthesized by utilization of atmospheric CO2 as carbonate source at room temperature. (BiO)2 CO3 /Ge and (BiO)2 CO3 /GO nanocomposites exhibited enhanced photocatalytic activity compared with pure (BiO)2 CO3 . Furthermore, it is interesting to note that the (BiO)2 CO3 /Ge showed higher photocatalytic activity than that of (BiO)2 CO3 -GO nanocomposites. The mechanism of enhanced photocatalytic performance of (BiO)2 CO3 /Ge and (BiO)2 CO3 /GO nanocomposites were investigated. The present work could not only provide a facile method to construct 2D nanocomposites, but also favor the efficient utilization of CO2 in green synthetic chemistry.
2. Experimental 2.1. Construction of (BiO)2 CO3 /Ge and (BiO)2 CO3 /GO nanocomposites All chemicals used in this study were analytical grade. The preparation process was conducted at room temperature. In a typical synthesis, 2.425 g of Bi(NO3 )3 ·5H2 O was dissolved into 100 mL of aqueous solution containing 4 mL (22.2 mol/L) nitric acid and vigorously stirred for 1 h (Solution A). Then, Ge (0.0128 g) was added into the solution A and the mixture was ultrasonicated for 1 h (Suspension B). 15 mL of concentrated NH3 ·H2 O was added dropwise into the suspension B and pumped air stream at a flow rate of 1.0 L/min for 3 h. The precipitates of (BiO)2 CO3 -Ge was collected by filtration, washed thoroughly four times with distilled water and ethanol, and then dried at 60 ◦ C overnight to get the final samples. (BiO)2 CO3 /GO was synthesized under the same conditions with adding of appropriate amount of GO. The mass ratio of Ge to (BiO)2 CO3 and GO to (BiO)2 CO3 was 1 wt%, respectively. The pure (BiO)2 CO3 was synthesized under the same conditions without adding Ge or GO. The (BiO)2 CO3 , (BiO)2 CO3 -Ge and (BiO)2 CO3 GO nanocomposites were labeled as BOC, BOC-Ge and BOC-GO, respectively.
2.2. Characterization X-ray diffraction with Cu K␣ radiation (XRD: model D/max RA, Rigaku Co., Japan) were used to analyze the crystal phases of the sample. Raman spectra were recorded at room temperature using a micro-Raman spectrometer (Raman: RAMANLOG 6, USA) with a 514.5 nm Ar+ laser as the excitation source in a backscattering geometry. A scanning electron microscope (SEM, JEOL model JSM-6490, Japan) was used to characterize the morphology of the samples. The morphological structure of the samples was examined by transmission electron microscopy (TEM: JEM-2010, Japan). Xray photoelectron spectroscopy with Al K␣ X-rays (h = 1486.6 eV) radiation operated at 150 W (XPS: Thermo ESCALAB 250, USA) was used to investigate the surface properties. The shift of the binding energy due to relative surface charging was corrected using the C1s level at 284.8 eV as an internal standard. The UV–vis diffuse reflection spectra were obtained for the dry-pressed disk samples using a Scan UV–vis spectrophotometer (UV–vis DRS: UV-2450, shimadzu, Japan) with an integrating sphere assembly and BaSO4 as a reflectance. The nitrogen adsorption–desorption isotherms were measured by the BET method (BET-BJH: ASAP 2020, USA), from which the surface area, pore volume, and average pore diameter were calculated by using the BJH method. Time-resolved photoluminescence (PL) spectroscopy was recorded on FLsp920 Fluorescence spectrometer (Edinburgh Instruments) with excitation at 350 nm. All the samples were degassed at 90 ◦ C prior to measurements. 2.3. Solar light photocatalytic performance The photocatalytic activity was tested by removal of NO at ppb levels in a continuous flow reactor. The volume of the rectangular reactor, made of polymethyl methacrylate plastics and covered with Saint-Glass, was 4.5 L (30 cm × 15 cm × 10 cm). A 100 W commercial tungsten halogen lamp was vertically placed outside and above the reactor. Four mini-fans were used to cool the reactor system to control the temperature. For each photocatalytic test, two sample dishes (with a diameter of 12.0 cm) containing photocatalyst powder were placed in the center of reactor. The photocatalyst samples were prepared by coating aqueous suspension of the samples onto the glass dishes. The weight of the photocatalyst for each dish was kept at 0.10 g. The coated dish was pretreated at 60 ◦ C to remove the water in the suspension. The NO gas was acquired from a compressed standard gas cylinder at a concentration of 100 ppm of NO (N2 balance). The initial concentration of NO was diluted to about 600 ppb by the air stream
Fig. 1. XRD diffraction patterns (a) and Raman absorption spectra (b) of BOC, BOC-Ge and BOC-GO.
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supplied by standard air. The relative humidity (RH) level of the NO flow was controlled at 70% by passing the zero air streams. The gas streams were premixed completely by a gas blender with flowing rate of 2.4 L/min. After the adsorption–desorption equilibrium was achieved, the lamp was turned on. The concentration of NO was continuously measured by a chemiluminescence NO analyzer (Thermo Environmental Instruments Inc., model 42c-TL) with a sampling rate of 1.0 L/min. The removal ratio () of NO was calculated as (%) = (1 − C/C0 ) × 100%, where C and C0 are concentrations of NO in the outlet steam and the feeding stream, respectively. 3. Results and discussion 3.1. XRD and Raman Fig. 1a shows the XRD patterns of BOC, BOC-Ge and BOC-GO. All the diffraction peaks can be perfectly indexed to orthorhombic BOC crystallites (JCPDS–ICDD card No. 25-1464) [19], suggesting the formation of BOC by this simple method. However, no obvious typical diffraction peaks of Ge and GO can be detected, which can be ascribed to the shielding effect of strong diffraction signals of the BOC. To further confirm the presence of Ge and GO, Raman
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spectroscopy, as an effective tool to characterize carbonaceous materials, were used. As illustrated in Fig. 1b, the Raman bands at 162 and 355 cm−1 can be attributed to the external vibration of BOC. The band at 512 cm−1 is indexed to the Bi O stretching. The bands at 1070 and 1050 cm−1 are assigned to the carbonate ion CO3 2− and the residual nitrate ion NO3 − in the samples, respectively. Two prominent peaks at 1351 and 1597 cm−1 can be observed in BOCGe nanocomposites, which correspond to the D band and G band of Ge [16], respectively. Furthermore, two intense peaks at 1355 and 1592 cm−1 can also be detected in BOC-GO nanocomposites corresponding to the D band and G band of GO [18], respectively. The results indicate that the Ge and GO are successfully coupled with BOC, respectively. According to Raman spectra as shown in Fig. 1, both of the intense peaks of D and G bands of BOC-Ge were weaker than those of BOC-GO. The fact can be ascribed to the surface interaction of functional groups (e.g. hydroxyl, carboxyl, carbonyl) in GO.
3.2. Morphology The low-magnification SEM image in Fig. 2a implies clearly that the BOC nanoflakes are randomly stacked together. As shown in
Fig. 2. SEM (a, b), TEM (c, d), HRTEM (e) and SAED patterns (f) of the as-synthesized BOC.
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Fig. 2b, the magnified image reveals that the BOC nanoflakes have an irregular morphology with a thickness of about 25 nm. Moreover, the TEM image (Fig. 2c, d) further confirms that the BOC are composed of large amount of ultrathin nanoflakes varying from 25 to 120 nm in size. The HRTEM image (Fig. 2e) demonstrates clear lattice fringes of a single nanoflake with d-spacing of 0.27 nm, which is consistent with the (1 1 0) crystal facet of BOC. The SAED pattern (Fig. 2f) confirms that BOC is single-crystal nanoflake with well crystallized structure.
Figs. 3 and 4 display that the BOC nanoflakes are decorated with the near-transparent Ge and GO nanosheets, respectively. Furthermore, the EDX spectra in Fig. S1 and Fig. S2 suggest that both BOC-Ge and BOC-GO are composed of C, O and Bi elements. However, it is hard to identify Ge and GO in the BOC-Ge and BOC-GO nanocomposites. In addition, EDS elemental mapping indicate that the C, O and Bi elements are uniformly distributed in the BOC-Ge and BOC-GO, respectively. All these results can be ascribed to the fact that the 2D Ge and GO
Fig. 3. SEM (a, b) and TEM (c, d) of the as-synthesized BOC-Ge. EDS elemental mapping (e–h) of the same region, indicating the spatial distribution of C, O and Bi, respectively.
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are intimately loaded on the BOC nanoflakes in a face-to-face way. 3.3. Chemical composition The XPS measurements were employed to determine the surface chemical compositions and chemical state in BOC, BOC-Ge and BOC-GO. The XPS survey spectra in Fig. 5a illustrate that BOC, BOCGe and BOC-GO samples contain Bi, C and O elements. As shown
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in Fig. 5b, two peaks at 159.0 and 164.3 eV are indexed to Bi4f7/2 and Bi4f5/2 in BOC, BOC-Ge and BOC-GO, respectively. Fig. 5c–e shows the C1s spectra of the BOC, BOC-Ge and BOC-GO, respectively. The C peaks at 284.8, 286.9, 287.8 and 288.3 eV are identified as the adventitious carbon species, C O, C O and O C O bonds in BOC, respectively. The C peaks at 284.9, 287.2 and 289.8 eV can be assigned to the C C (adventitious carbon and sp2 hybridized carbon) bond, C O bond and carbonate ion in BOC-Ge, respectively. The C peaks at 284.8, 287.3 and 288.2 eV corresponded to the C C
Fig. 4. SEM (a, b) and TEM (c, d) of the as-synthesized BOC-GO. EDS elemental mapping (e–h) of the same region, indicating the spatial distribution of C, O and Bi, respectively.
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Fig. 5. XPS spectra of BOC, BOC-Ge and BOC-GO. (a) Survey, (b) Bi4f, (c–e) C1s and (f–h) O1s.
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Table 1 The kinetics of emission decay parameters of BOC, BOC-GO and BOC-Ge samples. Samples
Component
Life time (ns)
Relative percentage (%)
2
BOC
1 2
0.8686 –
100 –
1.064
BOC-GO
1 2
0.6458 3.674
91.72 8.28
1.182
BOC-Ge
1 2
0.6757 4.003
90.40 9.60
1.192
(adventitious carbon), C O and O C O in BOC-GO, respectively. Fig. 5f–h shows the O1s spectra of the BOC, BOC-Ge and BOC-GO, respectively. The O peaks at 530.1, 531.1 and 532.3 eV are the characteristic of Bi–O binding energy, carbonate species and adsorbed H2 O on the surface in BOC, respectively. Additionally, the O peaks in BOC-Ge are similar to the BOC. The O peaks at 530.1, 531.3, 532.4 and 533.4 eV can be indexed to Bi O, C O, C O (O H) and O C O bonds in BOC-GO, respectively. The results demonstrate that BOCGe and BOC-GO nanocomposites can be successfully synthesized by the facile method. 3.4. Optical properties Fig. 6 shows that BOC, BOC-Ge and BOC-GO have very close absorption edges in the UV region. Comparison with that of the pure BOC, the light absorption intensity was significantly improved after coupling with Ge and GO in the visible region ( >400 nm), respectively. The result demonstrates that Ge and GO can affect the optical property of the BOC-Ge and BOC-GO nanocomposites, which can be ascribed to the typical interface interaction between BOC and Ge/GO [16–19]. To investigate the carriers transfer kinetics of the samples under irradiation, the time-resolved fluorescence decay spectra at nslevel were displayed as shown in Fig. S3 and Table 1. The short lifetime ( 1 ) of BOC is 0.8686 ns. By coupling with GO (Ge), the 1 is decreased to 0.6458 and 0.6757 ns. The long lifetime ( 2 ) of BOCGO and BOC-Ge is 3.674 and 4.003 ns, respectively. These results suggest that the GO (Ge) could store and shuttle electrons from photoexcited BOC, leading to accelerated electron transfer rate. 3.5. BET-BJH analysis The nitrogen adsorption–desorption isotherms and Barrett–Joyner–Halender (BJH) pore size distribution cures of BOC, BOC-Ge and BOC-GO are shown in Fig. 7. The adsorption–desorption isotherms of BOC-Ge and BOC-GO samples
Fig. 6. UV–vis diffuse reflectance spectra of BOC, BOC-Ge and BOC-GO.
can be indexed to type IV with a typical H3 hysteresis loop (Fig. 7a), which illustrates the presence of slit-like pores [9]. However, the BOC sample displays type IV isotherms, which characteristically indicate weak interaction between N2 molecules and BOC sample, and the presence of mesopores. The specific surface areas and pore volumes of BOC-Ge (69 m2 /g and 0.29 cm3 /g) and BOC-GO (52 m2 /g and 0.26 cm3 /g) are apparently higher than those of pure BOC (15 m2 /g and 0.07 cm3 /g). Furthermore, BOC-GO has large mesopores (∼32 nm) (Fig. 7b). The data implies that both the Ge and GO play a key role in increasing the specific surface area and enlarging the pore volume in the nanocomposites. 3.6. Photocatalytic activity In the present work, the photocatalytic activities of BOC, BOCGe and BOC-GO were evaluated by removal of NO in gas phase under simulated solar light irradiation. Fig. 8 shows the variation of NO concentration (C/C0 %) with irradiation time over the samples under solar light irradiation. In the presence of the as-synthesized photocatalytic materials, the NO reacted with the photo-generated reactive radicals and produced products of HNO2 , which involved four reactions displayed in Eqs. (1)–(3) [20,21]. NO + 2• OH → NO2 + H2 O
(1)
NO2 + • OH → NO3 − + H+
(2)
NO + NO2 + H2 O → 2HNO2
(3)
Fig. 7. N2 adsorption–desorption isotherms (a) and the corresponding pore size distribution curves (b) of BOC, BOC-Ge and BOC-GO.
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[19–23]. First, as illustrated in Fig. 9a and b, Ge and GO can serve as a good acceptor of photo-generated electrons in the nanocomposites, which could facilitate the separation of photo-generated electron–hole pairs in BOC-Ge and BOC-GO nanocomposites. Second, BOC-Ge and BOC-GO possess larger specific surface areas and pore volumes compared with the pure BOC, which could provide much more active sites for the given reactions. Third, the accelerated transfer of charge carriers could facilitate the formation of reactive radicals in photocatalysis, which is favorable for the photocatalytic degradation of NOx. Finally, BOC-Ge and BOC-GO nanocomposites show the enhanced visible light absorption, which was also favorable for the efficient solar light utilization. In addition, it is interesting to find that BOC-Ge shows higher photocatalytic activity than that of BOC-GO, as BOC-Ge possesses larger surface areas and pore volume than those of BOC-GO.
Fig. 8. Photocatalytic activity of the as-synthesized BOC, BOC-Ge and BOC-GO nanocomposites for removal of NO under solar light irradiation at room temperature.
The NO concentration for all samples decreased rapidly in 5 min. The reaction intermediates and final products generated during irradiation may occupy the active sites of photocatalysts, which results in the slight decrease in activity during 5–15 min [10]. When the reaction reached equilibrium, the activity was kept constant. After 30 min irradiation, the NO removal ratio of BOC, BOCGe and BOC-GO are 54.8%, 63.5% and 61.6%, respectively. The enhancement of photocatalytic activity of BOC-Ge and BOC-GO nanocomposites can be ascribed to the following synergistic factors
4. Conclusion In summary, BOC-Ge and BOC-GO nanocomposites have been synthesized by the facile one-pot method by capture of atmospheric CO2 at room temperature. In comparison with pure BOC, BOC-Ge and BOC-GO nanocomposites show improved photocatalytic activity under solar light irradiation. The Ge and GO, as good electron collector, can facilitate the separation and transfer of photo-generate electrons as well as enhance the utilization of solar light. Moreover, Ge and GO could increase the surface areas and pore volumes of nanocomposites. The present preparation method could also be extended to synthesize other carbonate photocatalysts coupled with Ge and GO for enhancing photocatalysis.
Fig. 9. Photocatalytic removal mechanism of NO over BOC-Ge (a) and BOC-GO (b) nanocomposites.
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Capture of atmospheric CO2 into (BiO)2 CO3 /graphene or graphene oxide nanocomposites with enhanced photocatalytic performance Wendong Zhang a , Fan Dong b,∗ , Wei Zhang c,∗ a
Department of Scientific Research Management, Chongqing Normal University, Chongqing, 401331, China Chongqing Key Laboratory of Catalysis and Functional Organic Molecules, College of Environment and Resources, Chongqing Technology and Business University, Chongqing, 400067, China c Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, China b
a r t i c l e
i n f o
Article history: Received 28 July 2015 Received in revised form 19 August 2015 Accepted 21 August 2015 Available online xxx Keywords: Atmospheric CO2 (BiO)2 CO3 /graphene (BiO)2 CO3 /graphene oxide Photocatalytic performance Charge separation
a b s t r a c t Self-assembly of (BiO)2 CO3 nanoflakes on graphene (Ge) and graphene oxide (GO) nanosheets, as an effective strategy to improve the photocatalytic performance of two-dimensional (2D) nanostructured materials, were realized by a one-pot efficient capture of atmospheric CO2 at room temperature. The as-synthesized samples were characterized by XRD, SEM, TEM, XPS, UV–vis DRS, Time-resolved ns-level PL and BET-BJH measurement. The photocatalytic activity of the obtained samples was evaluated by the removal of NO at the indoor air level under simulated solar-light irradiation. Compared with pure (BiO)2 CO3 , (BiO)2 CO3 /Ge and (BiO)2 CO3 /GO nanocomposites exhibited enhanced photocatalytic activity due to their large surface areas and pore volume, and efficient charge separation and transfer. The present work could provide a simple method to construct 2D nanocomposites by efficient utilization of CO2 in green synthetic strategy. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Bismuth subcarbonate (BiO)2 CO3 as a promising photocatalyst has received wide attention because of its typical physical and chemical properties, such as abundant, nontoxicity, high photoactivity, various morphologies, and amenability to chemical modification, etc. [1–4]. Recently, various micro/nanostructures of (BiO)2 CO3 including nanoparticles, nanotubes, nanosheets, nanoplates, nanobars, nanoflowers and hollow microspheres have been synthesized by hydrothermal process [5]. However, the high temperature and pressure, and long time reaction of hydrothermal treatment limited their practical large-scale application. Recently, some studies showed that the two dimensional (2D) bismuthcontaining nanostructures could facilitate effective separation and transfer of the photo-generated electrons and holes, which can improve the photocatalytic activity [6–8]. Thus, it is highly desirable to develop cost-effective and facile method to synthesize 2D (BiO)2 CO3 with high photoactivity. Very recently, we developed a new strategy to synthesize (BiO)2 CO3 nanoflakes by utilization of pure CO2 or atmospheric CO2
∗ Corresponding authors. Tel.: +86 23 62769785 605; fax: +86 23 62769785 605. E-mail addresses: [email protected] (F. Dong), [email protected] (W. Zhang).
at room temperature [9]. However, there are two main drawbacks, which restrict their large scale applications. First, the large band gap (3.1–3.5 eV) of (BiO)2 CO3 can only be activated with ultraviolet (<5% fraction of solar light). Second, the high recombination of photo-generated electrons and holes results in relatively low photocatalytic activity. In order to address these problems, various strategies have been employed, such as in situ N-doped (BiO)2 CO3 [10], (BiO)2 CO3 -BiOI nanocomposites [11] and BiVO4 -(BiO)2 CO3 heterojunctions [12]. In particular, the construction of 2D nanojunctions is a promising route to improve the photoactivity in matrix photocatalysts. The closely contacted interfaces between two components could facilitate the separation and transfer of photo-generated charges and accelerate the surface photocatalytic reactions, which could exhibit higher photocatalytic activity in composites compared with the individual photocatalyst. Graphene (Ge) and graphene oxide (GO) as typical carbonbased nanomaterials have attracted increasingly attention owing to their unique physical–chemical features as follows [13,14]. First, owing to the 2D layered structure, Ge and GO possessing theoretic large specific surface area could provide more active sites for reactants absorption. Second, Ge and GO with sp2 hybridized carbon single atomic nanosheets could act as an excellent supporting matrix, which allows for good contact with some supported nanomaterials. Third, Ge and GO have excellent electronic conductivity because of the two 2D -conjugated structure, which can serve
http://dx.doi.org/10.1016/j.apsusc.2015.08.172 0169-4332/© 2015 Elsevier B.V. All rights reserved.
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as a good acceptor of electrons. Therefore, these properties enable Ge and GO to be considered as ideal nanomaterials for supporting various semiconductor photocatalysts, such as TiO2 -Ge [15], ZnWO4 -Ge [16], BiOI-GO [17], g-C3 N4 -GO nanocomposites [18]. Recently, (BiO)2 CO3 /graphene nanocomposite with enhanced photocatalytic activity has been prepared by hydrothermal method. However, self-assembly of (BiO)2 CO3 nanoflakes on Ge and GO nanosheets by a one-pot efficient capture of atmospheric CO2 at room temperature have not been reported. In this work, (BiO)2 CO3 /Ge and (BiO)2 CO3 /GO nanocomposites were synthesized by utilization of atmospheric CO2 as carbonate source at room temperature. (BiO)2 CO3 /Ge and (BiO)2 CO3 /GO nanocomposites exhibited enhanced photocatalytic activity compared with pure (BiO)2 CO3 . Furthermore, it is interesting to note that the (BiO)2 CO3 /Ge showed higher photocatalytic activity than that of (BiO)2 CO3 -GO nanocomposites. The mechanism of enhanced photocatalytic performance of (BiO)2 CO3 /Ge and (BiO)2 CO3 /GO nanocomposites were investigated. The present work could not only provide a facile method to construct 2D nanocomposites, but also favor the efficient utilization of CO2 in green synthetic chemistry.
2. Experimental 2.1. Construction of (BiO)2 CO3 /Ge and (BiO)2 CO3 /GO nanocomposites All chemicals used in this study were analytical grade. The preparation process was conducted at room temperature. In a typical synthesis, 2.425 g of Bi(NO3 )3 ·5H2 O was dissolved into 100 mL of aqueous solution containing 4 mL (22.2 mol/L) nitric acid and vigorously stirred for 1 h (Solution A). Then, Ge (0.0128 g) was added into the solution A and the mixture was ultrasonicated for 1 h (Suspension B). 15 mL of concentrated NH3 ·H2 O was added dropwise into the suspension B and pumped air stream at a flow rate of 1.0 L/min for 3 h. The precipitates of (BiO)2 CO3 -Ge was collected by filtration, washed thoroughly four times with distilled water and ethanol, and then dried at 60 ◦ C overnight to get the final samples. (BiO)2 CO3 /GO was synthesized under the same conditions with adding of appropriate amount of GO. The mass ratio of Ge to (BiO)2 CO3 and GO to (BiO)2 CO3 was 1 wt%, respectively. The pure (BiO)2 CO3 was synthesized under the same conditions without adding Ge or GO. The (BiO)2 CO3 , (BiO)2 CO3 -Ge and (BiO)2 CO3 GO nanocomposites were labeled as BOC, BOC-Ge and BOC-GO, respectively.
2.2. Characterization X-ray diffraction with Cu K␣ radiation (XRD: model D/max RA, Rigaku Co., Japan) were used to analyze the crystal phases of the sample. Raman spectra were recorded at room temperature using a micro-Raman spectrometer (Raman: RAMANLOG 6, USA) with a 514.5 nm Ar+ laser as the excitation source in a backscattering geometry. A scanning electron microscope (SEM, JEOL model JSM-6490, Japan) was used to characterize the morphology of the samples. The morphological structure of the samples was examined by transmission electron microscopy (TEM: JEM-2010, Japan). Xray photoelectron spectroscopy with Al K␣ X-rays (h = 1486.6 eV) radiation operated at 150 W (XPS: Thermo ESCALAB 250, USA) was used to investigate the surface properties. The shift of the binding energy due to relative surface charging was corrected using the C1s level at 284.8 eV as an internal standard. The UV–vis diffuse reflection spectra were obtained for the dry-pressed disk samples using a Scan UV–vis spectrophotometer (UV–vis DRS: UV-2450, shimadzu, Japan) with an integrating sphere assembly and BaSO4 as a reflectance. The nitrogen adsorption–desorption isotherms were measured by the BET method (BET-BJH: ASAP 2020, USA), from which the surface area, pore volume, and average pore diameter were calculated by using the BJH method. Time-resolved photoluminescence (PL) spectroscopy was recorded on FLsp920 Fluorescence spectrometer (Edinburgh Instruments) with excitation at 350 nm. All the samples were degassed at 90 ◦ C prior to measurements. 2.3. Solar light photocatalytic performance The photocatalytic activity was tested by removal of NO at ppb levels in a continuous flow reactor. The volume of the rectangular reactor, made of polymethyl methacrylate plastics and covered with Saint-Glass, was 4.5 L (30 cm × 15 cm × 10 cm). A 100 W commercial tungsten halogen lamp was vertically placed outside and above the reactor. Four mini-fans were used to cool the reactor system to control the temperature. For each photocatalytic test, two sample dishes (with a diameter of 12.0 cm) containing photocatalyst powder were placed in the center of reactor. The photocatalyst samples were prepared by coating aqueous suspension of the samples onto the glass dishes. The weight of the photocatalyst for each dish was kept at 0.10 g. The coated dish was pretreated at 60 ◦ C to remove the water in the suspension. The NO gas was acquired from a compressed standard gas cylinder at a concentration of 100 ppm of NO (N2 balance). The initial concentration of NO was diluted to about 600 ppb by the air stream
Fig. 1. XRD diffraction patterns (a) and Raman absorption spectra (b) of BOC, BOC-Ge and BOC-GO.
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supplied by standard air. The relative humidity (RH) level of the NO flow was controlled at 70% by passing the zero air streams. The gas streams were premixed completely by a gas blender with flowing rate of 2.4 L/min. After the adsorption–desorption equilibrium was achieved, the lamp was turned on. The concentration of NO was continuously measured by a chemiluminescence NO analyzer (Thermo Environmental Instruments Inc., model 42c-TL) with a sampling rate of 1.0 L/min. The removal ratio () of NO was calculated as (%) = (1 − C/C0 ) × 100%, where C and C0 are concentrations of NO in the outlet steam and the feeding stream, respectively. 3. Results and discussion 3.1. XRD and Raman Fig. 1a shows the XRD patterns of BOC, BOC-Ge and BOC-GO. All the diffraction peaks can be perfectly indexed to orthorhombic BOC crystallites (JCPDS–ICDD card No. 25-1464) [19], suggesting the formation of BOC by this simple method. However, no obvious typical diffraction peaks of Ge and GO can be detected, which can be ascribed to the shielding effect of strong diffraction signals of the BOC. To further confirm the presence of Ge and GO, Raman
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spectroscopy, as an effective tool to characterize carbonaceous materials, were used. As illustrated in Fig. 1b, the Raman bands at 162 and 355 cm−1 can be attributed to the external vibration of BOC. The band at 512 cm−1 is indexed to the Bi O stretching. The bands at 1070 and 1050 cm−1 are assigned to the carbonate ion CO3 2− and the residual nitrate ion NO3 − in the samples, respectively. Two prominent peaks at 1351 and 1597 cm−1 can be observed in BOCGe nanocomposites, which correspond to the D band and G band of Ge [16], respectively. Furthermore, two intense peaks at 1355 and 1592 cm−1 can also be detected in BOC-GO nanocomposites corresponding to the D band and G band of GO [18], respectively. The results indicate that the Ge and GO are successfully coupled with BOC, respectively. According to Raman spectra as shown in Fig. 1, both of the intense peaks of D and G bands of BOC-Ge were weaker than those of BOC-GO. The fact can be ascribed to the surface interaction of functional groups (e.g. hydroxyl, carboxyl, carbonyl) in GO.
3.2. Morphology The low-magnification SEM image in Fig. 2a implies clearly that the BOC nanoflakes are randomly stacked together. As shown in
Fig. 2. SEM (a, b), TEM (c, d), HRTEM (e) and SAED patterns (f) of the as-synthesized BOC.
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Fig. 2b, the magnified image reveals that the BOC nanoflakes have an irregular morphology with a thickness of about 25 nm. Moreover, the TEM image (Fig. 2c, d) further confirms that the BOC are composed of large amount of ultrathin nanoflakes varying from 25 to 120 nm in size. The HRTEM image (Fig. 2e) demonstrates clear lattice fringes of a single nanoflake with d-spacing of 0.27 nm, which is consistent with the (1 1 0) crystal facet of BOC. The SAED pattern (Fig. 2f) confirms that BOC is single-crystal nanoflake with well crystallized structure.
Figs. 3 and 4 display that the BOC nanoflakes are decorated with the near-transparent Ge and GO nanosheets, respectively. Furthermore, the EDX spectra in Fig. S1 and Fig. S2 suggest that both BOC-Ge and BOC-GO are composed of C, O and Bi elements. However, it is hard to identify Ge and GO in the BOC-Ge and BOC-GO nanocomposites. In addition, EDS elemental mapping indicate that the C, O and Bi elements are uniformly distributed in the BOC-Ge and BOC-GO, respectively. All these results can be ascribed to the fact that the 2D Ge and GO
Fig. 3. SEM (a, b) and TEM (c, d) of the as-synthesized BOC-Ge. EDS elemental mapping (e–h) of the same region, indicating the spatial distribution of C, O and Bi, respectively.
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are intimately loaded on the BOC nanoflakes in a face-to-face way. 3.3. Chemical composition The XPS measurements were employed to determine the surface chemical compositions and chemical state in BOC, BOC-Ge and BOC-GO. The XPS survey spectra in Fig. 5a illustrate that BOC, BOCGe and BOC-GO samples contain Bi, C and O elements. As shown
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in Fig. 5b, two peaks at 159.0 and 164.3 eV are indexed to Bi4f7/2 and Bi4f5/2 in BOC, BOC-Ge and BOC-GO, respectively. Fig. 5c–e shows the C1s spectra of the BOC, BOC-Ge and BOC-GO, respectively. The C peaks at 284.8, 286.9, 287.8 and 288.3 eV are identified as the adventitious carbon species, C O, C O and O C O bonds in BOC, respectively. The C peaks at 284.9, 287.2 and 289.8 eV can be assigned to the C C (adventitious carbon and sp2 hybridized carbon) bond, C O bond and carbonate ion in BOC-Ge, respectively. The C peaks at 284.8, 287.3 and 288.2 eV corresponded to the C C
Fig. 4. SEM (a, b) and TEM (c, d) of the as-synthesized BOC-GO. EDS elemental mapping (e–h) of the same region, indicating the spatial distribution of C, O and Bi, respectively.
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Fig. 5. XPS spectra of BOC, BOC-Ge and BOC-GO. (a) Survey, (b) Bi4f, (c–e) C1s and (f–h) O1s.
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Table 1 The kinetics of emission decay parameters of BOC, BOC-GO and BOC-Ge samples. Samples
Component
Life time (ns)
Relative percentage (%)
2
BOC
1 2
0.8686 –
100 –
1.064
BOC-GO
1 2
0.6458 3.674
91.72 8.28
1.182
BOC-Ge
1 2
0.6757 4.003
90.40 9.60
1.192
(adventitious carbon), C O and O C O in BOC-GO, respectively. Fig. 5f–h shows the O1s spectra of the BOC, BOC-Ge and BOC-GO, respectively. The O peaks at 530.1, 531.1 and 532.3 eV are the characteristic of Bi–O binding energy, carbonate species and adsorbed H2 O on the surface in BOC, respectively. Additionally, the O peaks in BOC-Ge are similar to the BOC. The O peaks at 530.1, 531.3, 532.4 and 533.4 eV can be indexed to Bi O, C O, C O (O H) and O C O bonds in BOC-GO, respectively. The results demonstrate that BOCGe and BOC-GO nanocomposites can be successfully synthesized by the facile method. 3.4. Optical properties Fig. 6 shows that BOC, BOC-Ge and BOC-GO have very close absorption edges in the UV region. Comparison with that of the pure BOC, the light absorption intensity was significantly improved after coupling with Ge and GO in the visible region ( >400 nm), respectively. The result demonstrates that Ge and GO can affect the optical property of the BOC-Ge and BOC-GO nanocomposites, which can be ascribed to the typical interface interaction between BOC and Ge/GO [16–19]. To investigate the carriers transfer kinetics of the samples under irradiation, the time-resolved fluorescence decay spectra at nslevel were displayed as shown in Fig. S3 and Table 1. The short lifetime ( 1 ) of BOC is 0.8686 ns. By coupling with GO (Ge), the 1 is decreased to 0.6458 and 0.6757 ns. The long lifetime ( 2 ) of BOCGO and BOC-Ge is 3.674 and 4.003 ns, respectively. These results suggest that the GO (Ge) could store and shuttle electrons from photoexcited BOC, leading to accelerated electron transfer rate. 3.5. BET-BJH analysis The nitrogen adsorption–desorption isotherms and Barrett–Joyner–Halender (BJH) pore size distribution cures of BOC, BOC-Ge and BOC-GO are shown in Fig. 7. The adsorption–desorption isotherms of BOC-Ge and BOC-GO samples
Fig. 6. UV–vis diffuse reflectance spectra of BOC, BOC-Ge and BOC-GO.
can be indexed to type IV with a typical H3 hysteresis loop (Fig. 7a), which illustrates the presence of slit-like pores [9]. However, the BOC sample displays type IV isotherms, which characteristically indicate weak interaction between N2 molecules and BOC sample, and the presence of mesopores. The specific surface areas and pore volumes of BOC-Ge (69 m2 /g and 0.29 cm3 /g) and BOC-GO (52 m2 /g and 0.26 cm3 /g) are apparently higher than those of pure BOC (15 m2 /g and 0.07 cm3 /g). Furthermore, BOC-GO has large mesopores (∼32 nm) (Fig. 7b). The data implies that both the Ge and GO play a key role in increasing the specific surface area and enlarging the pore volume in the nanocomposites. 3.6. Photocatalytic activity In the present work, the photocatalytic activities of BOC, BOCGe and BOC-GO were evaluated by removal of NO in gas phase under simulated solar light irradiation. Fig. 8 shows the variation of NO concentration (C/C0 %) with irradiation time over the samples under solar light irradiation. In the presence of the as-synthesized photocatalytic materials, the NO reacted with the photo-generated reactive radicals and produced products of HNO2 , which involved four reactions displayed in Eqs. (1)–(3) [20,21]. NO + 2• OH → NO2 + H2 O
(1)
NO2 + • OH → NO3 − + H+
(2)
NO + NO2 + H2 O → 2HNO2
(3)
Fig. 7. N2 adsorption–desorption isotherms (a) and the corresponding pore size distribution curves (b) of BOC, BOC-Ge and BOC-GO.
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[19–23]. First, as illustrated in Fig. 9a and b, Ge and GO can serve as a good acceptor of photo-generated electrons in the nanocomposites, which could facilitate the separation of photo-generated electron–hole pairs in BOC-Ge and BOC-GO nanocomposites. Second, BOC-Ge and BOC-GO possess larger specific surface areas and pore volumes compared with the pure BOC, which could provide much more active sites for the given reactions. Third, the accelerated transfer of charge carriers could facilitate the formation of reactive radicals in photocatalysis, which is favorable for the photocatalytic degradation of NOx. Finally, BOC-Ge and BOC-GO nanocomposites show the enhanced visible light absorption, which was also favorable for the efficient solar light utilization. In addition, it is interesting to find that BOC-Ge shows higher photocatalytic activity than that of BOC-GO, as BOC-Ge possesses larger surface areas and pore volume than those of BOC-GO.
Fig. 8. Photocatalytic activity of the as-synthesized BOC, BOC-Ge and BOC-GO nanocomposites for removal of NO under solar light irradiation at room temperature.
The NO concentration for all samples decreased rapidly in 5 min. The reaction intermediates and final products generated during irradiation may occupy the active sites of photocatalysts, which results in the slight decrease in activity during 5–15 min [10]. When the reaction reached equilibrium, the activity was kept constant. After 30 min irradiation, the NO removal ratio of BOC, BOCGe and BOC-GO are 54.8%, 63.5% and 61.6%, respectively. The enhancement of photocatalytic activity of BOC-Ge and BOC-GO nanocomposites can be ascribed to the following synergistic factors
4. Conclusion In summary, BOC-Ge and BOC-GO nanocomposites have been synthesized by the facile one-pot method by capture of atmospheric CO2 at room temperature. In comparison with pure BOC, BOC-Ge and BOC-GO nanocomposites show improved photocatalytic activity under solar light irradiation. The Ge and GO, as good electron collector, can facilitate the separation and transfer of photo-generate electrons as well as enhance the utilization of solar light. Moreover, Ge and GO could increase the surface areas and pore volumes of nanocomposites. The present preparation method could also be extended to synthesize other carbonate photocatalysts coupled with Ge and GO for enhancing photocatalysis.
Fig. 9. Photocatalytic removal mechanism of NO over BOC-Ge (a) and BOC-GO (b) nanocomposites.
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Please cite this article in press as: W. Zhang, et al., Capture of atmospheric CO2 into (BiO)2 CO3 /graphene or graphene oxide nanocomposites with enhanced photocatalytic performance, Appl. Surf. Sci. (2015), http://dx.doi.org/10.1016/j.apsusc.2015.08.172