Graphene Quantum Dot Composite

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N-doped BiOCO/Graphene Quantum Dot Composite Photocatalyst: Enhanced Visible-Light Photocatalytic NO Oxidation and In Situ DRIFTS Studies Yang Liu, Shan Yu, Ziyan Zhao, Fan Dong, Xing’an Dong, and Ying Zhou J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 10 May 2017 Downloaded from http://pubs.acs.org on May 13, 2017

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The Journal of Physical Chemistry

N-doped

Bi2O2CO3/Graphene

Quantum

Dot

Composite

Photocatalyst: Enhanced Visible-Light Photocatalytic NO Oxidation and In situ DRIFTS studies Yang Liu†,‡, Shan Yu†,‡, Ziyan Zhao‡, Fan Dong§, Xing An Dong§, Ying Zhou*,‡,

‡

The Center of New Energy Materials and Technology, School of Materials Science and

Engineering, Southwest Petroleum University, Chengdu 610500, China §

Chongqing Key Laboratory Catalysis and New Environmental Materials, college of Environment

and Resources, Chongqing Technology and Business University, Chongqing 400067, China

*E-mail: [email protected] Tel: +86-28-83037411; Fax: +86-28-83037406; †

Y. Liu. and S. Yu contributed equally in this paper.

ABSTRACT: In the present work, we have synthesized N-doped Bi2O2CO3/graphene quantum dots composite (N-BOC/GQDs) under ambient environment for the first time. The as-prepared samples show notably improvement for visible light photocatalytic removal of indoor air pollutant NO in ppb level with comparison to the pristine N-BOC. X-ray diffraction (XRD) and transmission electron microscopy (TEM) characterization show that GQDs has no obvious influence on the structure and morphology of N-BOC; but GQDs could modify the surface of N-BOC to some extent, judging from the X-ray photoelectron spectroscopy (XPS). Moreover, results from UV-Vis diffuse and reflectance spectroscopy (DRS), photoluminescence spectroscopy and photo-electrochemical experiments altogether manifest that the efficiency of both light harvesting and charge separation of N-BOC/GQDs during the

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photocatalytic process is enhanced. Electron paramagnetic resonance (EPR) spectroscopy and thermodynamic analysis shows that superoxide radicals and should be the main active species during the photocatalytic process. In addition, in situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) reveals us the specific changes of NOx related species during the whole reaction process. We believe that the present work could offer a new way to improve the phtocatalytic efficiency of bismuth compounds.


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1. INTRODUCTION Photocatalysis is a promising technique which is expected to apply in the field of water splitting, water decontamination, air purification and so on.1 For the last 40 years, various kinds of photocatalysts have been prepared and one important kind is bismuth compounds due to their special electronic structures. Usually, the 6s2 orbital of Bi3+ in the compound could contribute to the valance band and hence narrow its band gap. Also, many bismuth compounds possess special layered structure.2 A lot of bismuth compounds such as Bi2O3, Bi2S3, BiVO4, Bi2WO6, Bi2MoO6, BiOX (X = Cl, Br and I) and Bi2Ti2O7 have been reported in the field of photocatalyis,3-5 and so is the case for Bi2O2CO3. Bi2O2CO3 is a typical Sillén phase compound, which is consisted of [Bi2O2]2+ layers with orthogonal intercalated CO32− layers.6 In 2010, Zhang and Xie et al. separately synthesized flower-like and nanosheet Bi2O2CO3 and applied them for photodegradation of organic dyes like methyl orange (MO) and Rhodamine B (RhB) for the first time.6-8 However, the relatively wide band gap (ca. 3.4 eV) of Bi2O2CO3 makes its response to visible light very weak. To solve this problem, a series of work has been reported, mainly focusing on doping of Bi2O2CO3 with nonmetal elements (S, N and O-vacancy)9-10 or formation of Bi2O2CO3 composites with Ag,11 Bi2O3,12 Bi2S3,13 Fe2O3,14 Bi2MoO6,15 Bi2WO6,16 BiOCl,17 g-C3N418 and so on.19 Recently, CO32− self-doped Bi2O2CO3 was also reported to be a good catalyst for photocatalytic removal of NO.20 For the past years, our group has also done a series of work on Bi2O2CO3 for photocatalysis. In 2014, we first synthesized polyaniline (PANI) decorated Bi2O2CO3 (BOC) nanosheets with {001} facets. The interactions between PANI and BOC have significant influences on the band structure and charge separation efficiency of system,



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which results in a notably enhanced photocatalytic activity in the visible light region.21 Later, with the assistance of cetyltrimethylammonium bromide (CTAB), we have successfully prepared N-doped Bi2O2CO3 (N-BOC), in which N atoms are interstitially doped on the surface of BOC. This also endows N-BOC the improved efficiency for photocatalytic removal of NO under visible light irradiation.22 However, the efficiency of the system is still limited, which urges us to further optimize the N-BOC system. GQDs are recently emerged zero dimensional carbon materials consisting of several layers of sp2 hybridized carbon structure with honeycomb, and they have got considerable attention in photocatalysis.23 For instance, there are several works that combines GQDs with TiO2 for improved photocatalytic efficiency.24-29 Besides, composites of GQDs with CdS, Bi2MoO6 and g-C3N4 have also been reported.30-32 It is generally accepted that GQDs could be beneficial for photocatalysis by improving visible light responsibility or enhancing charge separation efficiency in the system. Nevertheless, to the best of our knowledge, there’s no relevant work on the composites of GQDs and BOC for photocatalysis before. Our experimental results show that such a composite could be easily formed under ambient environment, and the as-prepared heterostructure (N-BOC/GQDs) has exhibited significantly improved photocatalytic efficiency in contrast to pristine N-BOC. Structure and morphology characterizations as well as the mechanism behind the efficiency promotion effect of N-BOC/GQDs were carefully studied in the present work. In addition, we also investigated the specific photocatalytic process for removal of NO by in situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS), which would help us understand this photocatalytic reaction deeply.


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2. EXPERIMENTAL SECTION All the chemicals were purchased from Chengdu Kelong Co. Ltd. with analytical grade and used without further purification. Deionized water was used in all experiments. 2.1 Synthesis of N-doped Bi2O2CO3 (N-BOC) nanosheets Two-dimensional N-BOC nanosheets were synthesized according to the previous work.22 In a typical procedure, 48.5 g of Bi(NO3)3·5H2O was dissolved in 100 mL of HNO3 (1 M) as solution A, while 84.5 g of Na2CO3 and 10 g of cetyltrimethylammonium bromide (CTAB) were dissolved into 900 mL of water as solution B. Then, solution B was drop wise added into solution A within 30 min under stirring at 30 °C. After that, the precipitate was filtrated and washed several times by isopropanol and deionized water. Finally, the solid was dried at 60 °C in air and collected as the final product. 2.2 Synthesis of graphene quantum dots (GQDs) GQDs were synthesized via an alkali-mediated hydrothermal method.29, 33 First, 0.75 g of 1,3,6-trinitropyrene was dispersed in 150 mL of NaOH aqueous solution (0.2 M), which was then ultrasonicated (300 W, 40 kHz) for 2 h to yield a homogeneous suspension. This suspension was transferred into a 250 mL of Teflon-lined stainless-steel autoclave and heated at 200 °C for 10 h. After it cooled to room temperature, the insoluble product in the system was removed by filtration with a 0.22 µm membrane. The remaining solution was then purified with dialysis bag (with retained molecular weight: 3500 Da). Finally, GQDs was obtained as a deep-brown aqueous solution, which could be further collected as solid crystals by rotary evaporation.



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2.3 Synthesis of N-BOC/GQDs composite Typically, 180 mg of the as-prepared BOC powder and 1.8 mg of GQDs were dispersed in 100 mL of water. After ultra-sonication for 30 min and stirring for 5 h, the suspension was evaporated to get the solid composite. To study the possible influence of the different content of GQDs in the composite on the photocatalytic activity, a series of N-BOC/GQDs composites with a weight content of 0.5%, 1% and 2% for GDQs were synthesized, which corresponds to 0.9, 1.8 and 3.6 mg of GQDs added into the system before reaction. The relevant composites are noted as N-BOC/GQDs (0.5%), N-BOC/GQDs (1%) and N-BOC/GQDs (2%), respectively. 2.4 Characterization Powder X-ray diffraction (PXRD) was performed with a PANalytical X'pert diffractometer operated at 40 kV and 40 mA using Cu Kα radiation. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) images were recorded on FEI Tecnai G2 20 microscope operated at 200 kV. UV-Vis absorption spectra and diffuse reflection spectra (DRS) were recorded from the Shimadzu 2600 UV-Vis spectrophotometer, and BaSO4 was used as the reflectance standard sample. Fourier transform infrared (FT-IR) spectra were performed on a Nicolet 6700 spectrometer with KBr pellets as the substrate. Photoluminescence spectra were measured using Hitachi F-7000 with the light source of MVL-210 (Mejiro Genossen Inc.). X-ray photoelectron spectroscopy (XPS) was investigated on a Thermo ESCALAB 250Xi spectrometer with Al Kα emission at 1486.6 eV. Nitrogen adsorption-desorption isotherm were recorded on a nitrogen adsorption apparatus (Thermo Scientific Model 42i-TL) and all samples were dried by evaporation. Electron paramagnetic resonance (EPR) signals were detected by JESFA200


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spectrometer at room temperature, during which methanol and dissolved oxygen in water were used as the trapping agent for hydroxyl radical(•OH) and superoxide radical(•O2-) respectively. BET surface areas were measured on a nitrogen adsorption apparatus (Quantachrome) and all the samples were degassed at 150 °C for 4 h. In situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) measurements for NO degradation were based on homemade apparatus, in which MIR-TR-MCT spectrometer (Bruker Tensor II) was equipped with an in situ diffuse reflectance cell (Harrick). The cell is consisted of two KBr windows and one quartz window. MVL-210 and MUA-165 lamp from Mejiro Genossen Inc. were used for visible-light and UV-light irradiation during the test, respectively (cf. Supporting Information Fig.S1). Before reaction, He gas was purged into the cell with a flow rate of 100 mL/min at 200 °C to remove impurities like H2O and CO2. After the temperature was cooled to room temperature naturally, the reaction mixture gas (50 mL/min O2 and 50 mL/min NO) was introduced into the cell. The scan range during the tests was from with 4000 to 600 cm-1 and KBr was used as the substrate. 2.5 Photoelectrochemical and photocatalytic measurements Photo-electrochemical (PEC) and electrochemical measurements were conducted on CHI660D electrochemical work station in a conventional three-electrode system with saturated calomel electrode (SCE) and Pt wire as the reference and counter electrode, and Na2SO4 aqueous solution (0.5 M) as the electrolyte. The working electrode was based on fluorine-doped tin oxide (FTO) conducting glass and prepared by doctor-blade method. The corresponding slurry was made from the mixture of dimethylformamide (DMF) and N-BOC (or N-BOC/GQDs). The deposition area was controlled to be 1 cm × 1 cm. For photo-electrochemical measurements, Xe lamp (PLS-SXE300/UV, Beijing Perfect light Technology Co., Ltd.) was used as the light



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source. Photocatalytic removal of NO at ppb level was based on our home-made testing system, which has been described in detail before.34 In our experiment, the flow rate of air and NO are 2.0 L min-1 and 12.0 mL min-1 respectively. The removal ratio (η) of NO was calculated by the equation η (%) = (1 - C/C0) × 100%, where C is the concentration of NO during the reaction and C0 represents the initial concentration of NO before photocatalytic reaction. Besides, the generation ratio (ω) of NO2 during reaction was calculated by ω(%) = [C(NOx) - C(NO)] / C(NOx) × 100%, where C(NOx) is the concentration of whole nitric oxide (NO2 and NO).

3. RESULT AND DISCUSSION 3.1 Photocatalytic activity

Figure 1. (a) Photocatalytic removal ratio of NO and (b) generation of NO2 in the presence of N-BOC and N-BOC/GDQs under visible light irradiation (λ > 420 nm). (c) Photocatalytic recycling tests on N-BOC/GQDs (1%) under visible light and (d) that under UV-Visible light.


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Photocatalytic activity of the as-prepared samples was evaluated by the degradation of air pollutant NO at an indoor ppb level. In specific, the efficiency of the system was assessed from two aspects. On the one hand, the removal ratio of NO is the intuitive evaluation standard. On the other hand, the NO2 selectivity during the photocatalytic process is also very important because of the secondary pollution from NO2.35 Figure 1a and b show the removal ratio of NO and fraction of NO2 generated over N-BOC and N-BOC/GQDs, respectively. Before illumination, all the samples were exposed to the flowing reaction gas for 30 min to guarantee their surface adsorption-desorption equilibrium with NO. After the switch-on of the visible light, the ratio of NO in all the reaction systems first dropped immediately and then became steady approximately. In specific, the removal ratio of NO with N-BOC is around 21% within 30 min, which is consistent with our previous report.22 After the deposition of GQDs on N-BOC, the removal ratio of NO was largely improved. It increases to 46%, 53% and 35% for N-BOC/GQDs (0.5%), N-BOC/GQDs (1%) and N-BOC/GQDs (2%), respectively. Usually, GQDs could promote the photocatalytic process by improvement of the visible light responsibility or enhancement of the charge separation efficiency of the system, and an excessive amount of GQDs may block the active sites of N-BOC, which deteriorates the efficiency of the system instead.27, 36-37 These factors together make N-BOC/GQDs (0.5%) and N-BOC/GQDs (1%) superior to N-BOC and N-BOC/GQDs (2%) for removal of NO. Nevertheless, a further study of the fraction of NO2 shows that N-BOC/GQDs (1%) is a much better photocatalyst than N-BOC/GQDs (0.5%): the fraction of NO2 from system with N-BOC/GQDs (1%) is about 4% within 30 min, which is the lowest among all the samples, but that with N-BOC/GQDs (0.5%) is as high as 11%. Therefore, N-BOC/GQDs (1%) is overall the best photocatalyst among all the investigated samples and we then select it for the



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photocatalytic

recycling

test.

Furthermore,

control

experiment

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shows

the

photocatalytic activity of SiO2/GQDs material. With same loading amount of GQDs and reaction condition, the removal ratio is only 5% in the presence of SiO2/GQDs (1%). This is much lower than that of N-BOC/GQDs (1%) which clearly demonstrate that N-BOC is more than a supporting material in the composite, and there’s indeed a synergistic effect between N-BOC and GQDs (Figure.S2). Figure 1c shows that the activity of the sample under visible light irradiation gradually decreased within 3 cycles and this should mainly be attributed to the accumulation of degradation product NO3− on the surface of the sample, which possibly blocks the active sites for NO.38 After the removal of these adsorbed NO3− by washing with water, the activity of the sample could be effectively recovered. Surprisingly, we find out that under UV-Vis light irradiation, the activity of N-BOC/GQDs (1%) was well maintained even after 5 consecutive cycles (Figure 1d). This indicates that UV light is more beneficial for the desorption of NO3− in contrast to visible light. Further information about this will be discussed in the DRIFTS test. 3.2 Structure and morphology of N-BOC/GQDs

Figure 2. XRD patterns of N-BOC and N-BOC/GQDs and the corresponding PDF card information.


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XRD patterns of N-BOC and N-BOC/GQDs shows that the diffraction peaks for all the samples are almost the same and could be well indexed to tetragonal Bi2O2CO3 (JCPDS card No. 41-1448, Figure 2). The peaks at 12.9˚, 23.9˚, 30.3˚, 32.7˚, 42.3˚, 47.0˚, 53.4˚ and 56.9˚could be assigned to the (002), (011), (013), (110), (114), (020), (121) and (123) facets, respectively. This indicates that the structure of N-BOC was preserved well after the introduction of GQDs. This is within our expectation due to the mild synthetic conditions of N-BOC/GQDs. No diffraction peaks relevant to GQDs could be observed, which is probably resulted from the low content as well as the casual distribution of GQDs on the surface of N-BOC. The existence of GQDs in the system was then confirmed by FT-IR spectra (Figure S3). For GQDs only, the peaks at 1640 and 1386 cm-1 are attributed to the deformation and stretching vibration of O-H, and the peak at 1120 cm-1 is attributed to stretching vibration of C-O-C.29 For pristine N-BOC, these peaks at 846, 1068 and 1471 cm−1 are assigned to the out-of-plane bending mode of CO32− and ant-symmetric vibration mode of CO32−, respectively.39 All the above peaks are discernable in the spectrum of N-BOC/GQDs (1%), suggesting that the composite of N-BOC and GQDs has been successfully synthesized.



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Figure 3. (a) and (c) TEM images of N-BOC and N-BOC/GQDs (1%) composite, and (b) and (d) HRTEM images of N-BOC and N-BOC/GQDs (1%) composites, respectively.

SEM images of N-BOC/GQDs (1%) show that it presents in the morphology of 2D nanosheets with a size of hundreds of nanometers (Figure S4), which resembles that of pristine N-BOC as we reported before.22 This was also proved by the TEM images (Figure 3a and c). Especially, these nanosheets are very thin and mostly transparent under the irradiation of electron beam. Two crystal facets with the same lattice distance of 0.276 nm and a crossing angle of 90° could be further observed from high resolution TEM (HRTEM) images (Figure 3b), which coincide well with the 2 theta angle of 32.7° in the XRD data and correspond to the (110) and (110) facet of N-BOC, respectively. This manifests that there’s no obvious change on the morphology and crystal structure of N-BOC in the composite. Most importantly, GQDs around 5 nm could be recognized in the composite (Figure 3d), demonstrating the simple but effective method for introduction of GDQs in our system.


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Figure 4. High-resolution XPS spectra of Bi 4f (a), O 1s (b), N 1s (c) and C 1s (d) of N-BOC and N-BOC/GQDs (1%).

The chemical state of the elements in N-BOC/GQDs and the possible interactions between N-BOC and GQDs were then investigated by XPS. Figure 4 shows the high-resolution spectra of Bi 4f, O 1s, N 1s and C 1s of our samples. For N-BOC, the two peaks with binding energy at 164.4 eV and 159.1 eV and separated by 5.3 eV were characteristic of Bi 4f5/2 and Bi 4f7/2 with Bi3+ oxidation state. Compare to the pristine N-BOC, these peaks for N-BOC/GQDs positively shift to 164.7 and 159.3 eV by 0.2-0.3 eV. The peaks of the O 1s spectrum of N-BOC could be split into three peaks at 531.0, 530.5 and 529.7 eV, respectively. The peak at 530.1 eV was ascribed to the Bi-O bond in Bi2O2CO3, while the other two peaks may originate from the carbonate species on the surface of N-BOC.40 Similarly, these peaks positively shift to 531.7, 531.1 and 530.1 eV by 0.4-0.7 eV for N-BOC/GQDs. In contrast, the N 1s peak at 403.3 eV resulted from the surface doped interstitial N



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atoms on N-BOC has negatively shifted to 402.7 eV by 0.6 eV after the introduction of GDQs, which indicates the increased electron density on N atom in the composite. Due to the interference of the adventitious organics, the information of element C in GQDs are hard to capture. However, a small change of C 1s peak assigned to carbonate in Bi2O2CO3 from 288.7 eV to 289.0 eV could still be observed. All the changes of these peaks indicate that though the introduction of GQDs doesn’t affect the crystal structure and morphology of N-BOC obviously, it does have some influence on the surface of N-BOC. We conjecture that this should be caused by the interaction between the remnant functional group like hydroxyl and carboxyl groups on GQDs and the dangling bonds of N-BOC.41 Moreover, the positive shifts of Bi, O and C in N-BOC/GQDs were very similar to the results of our previous reported Bi2O2CO3 sample with enhanced photocatalytic efficiency;21 therefore, we temporarily speculate that the decreased electron density on Bi, O and C in BOC is beneficial for photocatalysis. 3.3 Mechanism for efficiency enhancement in N-BOC/GQDs

Figure 5. Diffuse and reflectance spectra of N-BOC/GQDs composites. Inset is the absorption spectrum of GQDs dispersed in water.

To understand the mechanism behind the enhancement of the photocatalytic activity


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of N-BOC/GQDs, we first studied the changes of the surface area of these samples. Figure S5 is N2 adsorption-desorption isotherms and the corresponding pore size distribution curves of four samples. The all samples exhibit the representative isotherms of type IV and hysteresis loop of type H3 at high relative pressure (P/P0). The pore distribution of the four samples ranged widely from 3 to 100 nm. Besides, BET tests also show that the surface area of N-BOC, N-BOC/GQDs (0.5%), N-BOC/GQDs (1%) and N-BOC/GQDs (2%) is 24.81, 22.10, 20.91 and 25.62 m2/g, respectively. Such small difference of these samples shows that the influence of surface area change should not be the main factor for the enhanced photocatalytic activity. We then studied the composite in more details, starting from the light harvesting abilities of N-BOC/GQDs. As shown in Figure 5, pristine N-BOC has an absorption edge at around 380 nm, which is from the intrinsic band edge absorption of N-BOC and indicates its band gap of 3.4 eV. In contrast, besides of this intrinsic absorption of N-BOC, a typical absorption peak at 482 nm could be clearly seen for all the N-BOC/GQDs samples. The intensity of this peak increases gradually as the amount of GQDs increases from 0.5% to 2%. This clearly demonstrates that the absorption peak is related to GQDs. A comparison of the results with the absorption spectrum of GQDs further confirms this (inset of Figure 5). Due to the wide absorption of GQDs in the whole visible light region, the introduction of GQDs could effectively improve the light harvesting ability of the composites. Notably, though the absorption of the composites is largely improved in the visible light region, the intrinsic absorption edge of N-BOC in the UV region barely shifts. This illustrates that the band gap of N-BOC is not significantly affected by GQDs.



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Figure 6. Photoluminescence spectra of N-BOC and N-BOC/GQDs (1%). Excitation wavelength: 280 nm.

Photoluminescence spectra were then investigated to check out the irradiative recombination changes of the samples, which could be seen as an indication of the charge separation efficiency (Figure 6). The shapes of the spectra are almost the same for N-BOC and N-BOC/GQDs (1%), with a broad emission from 350 nm to 520 nm; however, the intensity for the latter decreases strongly. This suggests that less photo generated charge carriers are involved with the radiative recombination process; hence, the efficiency of the charge separation may be improved in N-BOC/GDQs (1%). Besides, the emission spectrum of pure GQDs was also given in Figure S6 Transient emission spectra of the two samples were also recorded which showed in Figure S7; however, due to the limitation of the apparatus (with a timescale of nanoseconds), no useful signals relevant to the dynamical process could be captured. This hints the probably ultrafast charge transition processes in N-BOC and N-BOC/GQDs.


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Figure 7. (a) Nyquist plot and (b) transient photocurrent response of N-BOC and N-BOC/GQDs (1%) under visible light illumination and in darkness.

Furthermore, we have carried out the electrochemical impedance spectra (EIS) measurement with and without the visible light illumination. Under dark conditions, the Nyquist plot of N-BOC exhibits a much larger arc radius than that of N-BOC/GDQs (1%) (Figure 7a); this suggests that the intrinsic charges recombination processes occurring on the surface of N-BOC is relatively serious and the introduction of a small amount of GQDs in the composite could effectively facilitate the charge separation process. This is also the case for the samples under light illumination. The transient photocurrent responses of N-BOC and N-BOC/GQDs (1%) were also studied (Figure 7b). For N-BOC, the photocurrent density is weak (less than 0.1 mA cm-2) and it decreases gradually during the 5 light on and off cycle, which should be caused by its weak responsibility to visible light and the serious charge recombination. In contrast, the photocurrent density for N-BOC/GQDs (1%) reaches 0.7 mA cm-2 after light-on and this value is well maintained for 5 on and off cycle, demonstrating the higher efficiency and longer stability of the system. A more careful examination of the photocurrent curve shows that the rise and fall of photocurrent for



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N-BOC/GQDs (1%) with light on and off is a gradual process, and we believe this is due to the influence of GQDs as a sink for charge carriers. This result, together with the EIS data has emphasized the importance of GQDs to improve the charge separation efficiency of the sample.

Figure 8. DMPO spin-trapping EPR spectra of N-BOC and N-BOC/GQDs (1%) in methanol dispersion for DMPO-•O2− (a and b) and in aqueous dispersion for DMPO-•OH (c and d) with or without visible light irradiation.

In addition, the possible active radicals in the reaction process were investigated by -

EPR measurement. Generally, radicals like ·O2 and ·OH are believed to involve in photocatalytic oxidative reactions.42-43 This is also the case for the photocatalytic removal of NO. 5, 5-Dimethyl-1-pyrroline N-oxide (DMPO) here was used to spin traps the active radicals in the system for detection. As shown in Figure 8a and c, no EPR signals relevant to DMPO adducts could be observed for N-BOC, whether it is in darkness or under the illumination of visible light. No signals could be observed for


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N-BOC/GQDs in darkness, either. Nevertheless, after illumination, a signal of four peaks with nearly the same intensity could be observed for N-BOC/GQDs in the -

presence of oxygen, which comes from the DMPO/·O2 adduct (Figure 8b). Similarly, a typical four-line signal with a relative intensity ratio close to 1:2:2:1 could be observed for N-BOC/GQDs aqueous dispersion as well, which is characteristic of the DMPO/·OH adduct (Figure 8d). Both the intensity of these two signals becomes -

stronger with the increasing irradiation time, illustrating that the generation of ·O2

and ·OH is assisted with irradiation. According to literature, the standard electrode -

-

reduction potential of O2/·O2 , H2O/·OH and OH /·OH couples is -0.28 V, 2.37 V and 1.99 V (vs. NHE), respectively.44 In our present system, the band gap of N-BOC was unchanged in N-BOC/GQDs and results from XPS valance band spectra further show that the valence band edge of N-BOC and N-BOC/GQDs are exactly the same (2.02 eV) (Figure S8), so the conduction band edge of both N-BOC and N-BOC/GQDs should be -1.38 V (vs. NHE). Therefore, photo generated electrons in N-BOC/GQDs -

can react with the adsorbed O2 to form ·O2 , whereas the photo generated holes cannot oxidize H2O to form ·OH thermodynamically. Nevertheless, ·OH could be generated from the following equation: 2e- + ·O2 + 2H+ → ·OH + OH-. 34, 44-45 Both •O2- and • -

OH are active species for the oxidation of NO (Eq. 4 and Eqs. 6-7). In addition, photogenerated holes could direct oxidize NO as well (Eq. 2). The particular reaction process should be as follows: Eq.1: N-BOC/GQDs + hν → e- + h+ Eq.2: 3h+ + NO + 2H2O → NO3- + 4H+ Eq.3: e- + O2 →•O2Eq.4: NO + •O2- → NO3-



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Eq.5: •O2- + 2H+ + 2e- →•OH + OHEq.6: 2•OH + NO → NO2 + H2O Eq.7: NO2 + •OH → NO3- + H+ After light excitation of the sample (Eq. 1), photogenerated holes could directly react with NO to yield NO3- (Eq. 2), whereas photogenerated electrons are prone to reaction with O2 in the air to form superoxide radical •O2- (Eq. 3). The •O2- could react with NO directly to generate NO3- (Eq. 4) as well, or it can accept electrons again to form hydroxyl radicals •OH and hydroxide OH- in the presence of protons (Eq. 5). The as-generated •OH could further react with NO to form NO2 or NO3- (Eqs. 6 and 7). These reaction details have been widely accepted for photocatalytic removal of NO. 18, 45-46

Overall, the net chemical reaction should be Eq.8 or Eq.9 (see below), depending

on the final product including NO2 or not. Eq.8: 4NO + 3O2 + 2H2O → 4NO3- + 4H+ Eq.9: 3NO + 2O2 + H2O → NO2 + 2NO3- + 2H+ 3.4 In situ DRIFTS investigations

Figure 9. In situ DRIFTS of NO adsorption on N-BOC/GQDs (1%) under an atmosphere with 50 ppm of NO and 50 ppm of O2 before (a), during (b) and after (c) the visible light illumination..

In situ DRIFTS was then used to study the NO related species on the surface of N-BOC/GQDs throughout the photocatalytic process. As can be seen from Figure 9a,


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after exposure of N-BOC/GQDs to the mixed gas of NO and O2, several peaks in the region from 900-1300 emerge immediately. Peaks at 988, 1032, 1045 and 1094 cm-1 are originated from HNO2, dentate or monodentate nitrite and monodentate nitrite, respectively (see Table 1 for clarity), which could be generated from NO directly with or without the presence of oxygen;47-48 while the peak at 1176 cm-1 can be attributed to NO−. 44-46 In addition, a weak peak at 1260 cm-1 could also be observed, which may come from monodentate nitrate.49,

52

These indicate that NO could be efficiently

adsorbed on the surface of N-BOC/GQDs. After the light was turned on, the peak centered at 1274 cm-1 increased significantly, suggesting the formation of nitrate during the photocatalytic process. Peaks at 1095 and 1056 cm-1 increased as well, indicating there is an accumulation of nitrite, which is in consistent with the photo catalytic result. Note that the increase degree of the peak intensity is more obvious at 1274 cm-1, and this demonstrates that the majority of NO is probably completely oxidized into nitrate. After illumination, we kept monitoring the spectral changes of N-BOC/GQDs for another 20 min. In contrast to the quickly increasing during illumination, all peaks decrease very slowly in the spectrum. This indicates that some of the photocatalytic products are hard to leave from the surface of N-BOC/GQDs, which is detrimental for the long-time efficiency of the system. This has already been confirmed by photocatalytic test, during which the activity of N-BOC/GQDs gradually decreased under visible light illumination. Nevertheless, the decrease of these peaks is more obvious after illumination of UV light (Figure S9). We conjecture that the reaction products are more easily to desorb from N-BOC/GQDs under this situation and therefore the activity of the samples would be barely affected with the increase of reaction time, which is highly consistent with the experimental result.



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Table1. Assignments of the IR bands observed during NO adsorption over N-BOC/GQDs.

Wavenumber (cm-1)

Assignment

Reference

988-962

ν(N-O) from HNO2

47-48

1032-1056

Dentate or monodentate nitrite

22

1094-1104

Monodentate nitrite

52

-

1176

NO

49-51

1260-1274

Monodentate nitrate

49, 51

4. CONCLUSION In summary, we have prepared N-BOC/GQDs composite with a facile method here for the first time. The structure and morphology of N-BOC were well maintained in the composite, and the possible interactions between GQDs and N-BOC were studied. The existence of GQDs in the composite improves the efficiency of light harvesting and charge separation of the system, which significantly promotes its efficiency for photocatalytic removal of NO in contrast to that of pristine N-BOC. We believe this work could render a novel way for the optimization of bismuth compounds systems for photocatalysis applications. Furthermore, in situ DRIFTS test has been adopted in the present study to monitor the NOx species changes throughout the reaction, which helps us better recognize the photocatalytic process of NO removal.

ASSOCIATED CONTENT Supporting Information Picture of the reaction cell for in situ DRIFTS test, control experiment of NO in the presence of SiO2/GQDs (1%) and N-BOC samples under visible light irradiation, N2 adsorption-desorption isotherms, FT-IR spectra of GQDs, Photoluminescence spectra of pure GQDs, transient emission spectra in the presence of N-BOC and


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N-BOC/GQDs (1%), SEM images of N-BOC/GQDs (1%), XPS valence band spectra of N-BOC and N-BOC/GQDs (1%) and in situ DRIFTS of NO adsorption on N-BOC/GQDs after the UV light illumination. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Author Contributions † Y. Liu. and S. Yu contributed equally in this paper. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We acknowledge the National Natural Science Foundation of China (51102245), the Sichuan Youth Science and Technology Foundation (2014JQ0017), the Sichuan Provincial International Cooperation Project (2017HH0030), the Scientific Research Starting Project of SWPU (2014QHZ021) and the Innovative Research Team of Sichuan Province (2016TD0011).

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TOC Graphic


Graphene Quantum Dot Composite

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