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KOH etching graphitic carbon nitride for simulated sunlight photocatalytic nitrogen fixation with cyano groups as defects
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KOH etching graphitic carbon nitride for simulated sunlight photocatalytic nitrogen fixation with cyano groups as defects Ning Zhou, Pengxiang Qiu, Huan Chen∗, Fang Jiang Key Laboratory of Jiangsu Province for Chemical Pollution Control and Resources Reuse, School of Environmental and Biological Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
a r t i c l e
i n f o
Article history: Received 13 September 2017 Revised 24 November 2017 Accepted 24 November 2017 Available online xxx Keywords: KOH etching graphitic carbon nitride Simulated sunlight Nitrogen photofixation Cyano defects
a b s t r a c t Cyano-deficient g-C3 N4 (graphitic carbon nitride, labeled GCN) was synthesized by KOH etching treatment of bulk g-C3 N4 . Characterization results indicated that KOH etching treatment had a certain effect on the morphology and structure of GCN, and successfully introduced the cyano groups into GCN framework. The obtained KOH etching g-C3 N4 catalysts, named ACN, exhibited the pore and/or ladder-like thin layered structure. Meanwhile, the introduction of cyano groups reduced the conduction band position of ACN, and effectively inhibited the recombination of photo-generated electron–hole pairs. In addition, ACN introduced more chemical adsorption sites to activate nitrogen, which was beneficial to the reaction of photocatalytic nitrogen fixation. The modification of morphology and electronic property, especially the introduction of cyano functional groups, remarkably promoted the activity of ACN on the simulated sunlight photocatalytic nitrogen fixation, which was 7.6-fold higher than that of GCN. © 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction The conversion of solar energy into chemical energy, such as hydrogen and ammonia, is an effective way to solve global energy and environmental problems. The photocatalytic fixation of nitrogen to ammonia has gradually attracted attention due to its advantages of mild conditions, low power consumption and low cost. Thus, it is considered to be a promising method to replace traditional industrial nitrogen fixation techniques. Schrauzer and Guth first reported the formation of ammonia over Fe-doped titanium dioxide under UV light irradiation in 1977 [1]. Since then, most of the researches on nitrogen photofixation are focused on the modification of TiO2 to improve the efficiency [2–4]. Other photocatalysts, such as modified SrTiO3 [5], Fe @ 3D Graphene [6] and BiOBr [7], etc., had also been found to be useful for nitrogen fixation under light irradiation at ambient temperature and pressure. Graphitic carbon nitride (g-C3 N4 , labeled GCN), which was used for photocatalytic hydrogen evolution experiments for the first time in 2009 [8], has been utilized as a low-cost, sustainable, metal-free, and visible-light-active photocatalyst in the field of solar energy conversion in recent years [9–11]. To further improve its photocatalytic performance, many approaches, such as morphology
∗
Corresponding author. E-mail addresses: [email protected] (H. Chen), [email protected] (F. Jiang).
control [12,13], band-gap engineering (including element doping [14,15] and copolymerization [16,17]), and coupling with other semiconductors [18,19], have been reported extensively. The introduction of defects in semiconductor materials is considered to be an effective way to improve the photocatalytic activity, because defects can modify the electronic structure and trap photo-generated electrons or holes to inhibit the recombination of photo-generated carriers. It has been reported that oxygen vacancies on TiO2 could trap electrons and enhance photocatalytic performance of TiO2 in the visible range [20,21]. Similarly, Niu et al. [22] has reported that the nitrogen vacancies were introduced into the framework of GCN by a simple temperature-controlling route and modified the electronic structure to improve the photocatalytic activity of GCN. Dong et al. [23] investigated the role of nitrogen vacancies in nitrogen heat-treated GCN for N2 photofixation under visible-light irradiation. The result indicated that the N2 was selectively adsorbed and activated on the nitrogen vacancies and photo-generated electrons could also be trapped to inhibit the recombination of photo-generated charge carriers. Hong et al. [24] developed a facile hydrothermal strategy using ammonium thiosulfate as a weak oxidant to prepare nitrogen-deficient GCN, and the results revealed that the deficiency of amino species on GCN was beneficial to light harvesting and separation of charge carriers. A template-free method using different solvents (such alcohols) to pretreat the precursor had been reported to synthesize sponge-like GCN with a large surface area and nitrogen vacancies [25]. In recent year, the effects of different defects on the
https://doi.org/10.1016/j.jtice.2017.11.028 1876-1070/© 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: N. Zhou et al., KOH etching graphitic carbon nitride for simulated sunlight photocatalytic nitrogen fixation with cyano groups as defects, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.11.028
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photocatalytic reaction, such as oxygen vacancies [26], carbon vacancies [27], sulfur vacancies [28], and defects caused by catalyst morphology or surface functional groups [29,30], have also been further studied. It is noteworthy that there are some reports on the impact of nitrogen vacancies on the activity of photocatalysts [22,24,25,31–34], but little about cyano defects. In theory, cyano groups as a strong electron acceptor may act as defects. Ou et al. [30] and Yu et al. [35] also have reported that the presence of cyano groups could effectively improve the photocatalytic hydrogen performance of carbon nitride. Based on those, cyano groups as a defect may have an effect on photocatalytic nitrogen fixation. In this work, we successfully prepared cyanide-deficient carbon nitride (ACN) via KOH etching treatment of bulk GCN. The simulated sunlight nitrogen photofixation ability was tested to evaluate the performance of photocatalysts. The results indicated that the cyano defects contributed to the separation of photo-generated carriers and significantly improved the nitrogen fixation capacity of ACN. This work reported a facile and effective way to introduce cyano groups into GCN and discussed in detail the impacts of cyano deficiency on the morphology, structure and optical properties of ACN. 2. Experiment 2.1. Material synthesis The pristine GCN was prepared by heating melamine in a muffle furnace at 550 °C for 4 h with a ramp rate of 2 °C/min in air. The KOH etching graphitic carbon nitride (ACN) was synthesized as follows: the as-prepared GCN powder was dispersed in anhydrous ethanol containing an appropriate amount of KOH. Solvents were removed by evaporation at 70 °C with vigorous stirring. The obtained powders were heated to 500 °C under flowing nitrogen and held there for 2 h in a horizontal tube furnace. After cooling down, the resultant samples were repeatedly washed by deionized water until neutral pH was reached. Finally, the product was dried at 60 °C overnight. The obtained catalysts were denoted as ACNx (x = 2%, 5%, 10%), where x stands for the mass ratios of KOH to GCN. 2.2. Characterization The X-ray diffraction (XRD) patterns of the photocatalysts were recorded on a Bruker D8 Advanced diffraction-meter with Cu Kα radiation and with the scanning angle ranging from 10° to 80°. The scanning electron microscopy (SEM) images were obtained on Quant 250FEG instrument and the transmission electron microscopy (TEM) images were measured on TECNAI G2 20 (LaB6). The nitrogen adsorption isotherms at 77 K were measured on Micromeritics ASAP 2200. The samples were degassed at 523 K prior to measurements. The Brunauer-Emmett-Teller (BET) specific surface areas were calculated based on the adsorption isotherm. The Fourier transform infrared (FT-IR) spectroscopy was carried on a Nexus 870 spectrometer. X-ray photoelectron spectroscopy (XPS) measurements were performed on a RBD upgraded PHI-50 0 0C ESCA system (Perkin Elmer) with Mg Kα radiation (hν = 1253.6 eV). Elemental analysis (EA) data were obtained by using EuroVector EA30 0 0. UV–vis diffuse reflectance spectroscopy was carried out on a Hitachi U-3010 UV–vis spectrometer. Photoluminescence (PL) spectra were measured at room temperature with a jobinYvon SPEX Fluorolog-3-P spectroscope. The photocurrents were measured using an electrochemical workstation (CHI 660E) in a standard three-electrode system under illumination using a 300 W Xe lamp. The catalyst coated ITO glass was used as the working electrode, a Pt foil was used as the counter-electrode, and an Ag/AgCl electrode was used as the reference electrode. A 0.5 M
Fig. 1. XRD patterns of GCN and ACN-x.
Na2 SO4 aqueous solution was used as the working electrolyte. N2 Temperature programmed desorption (N2 -TPD) was performed using a CHEMBET-30 0 0 (Quantachrome, U.S.A.) instrument in the temperature range of 313–1073 K. The Electron paramagnetic resonance (EPR) spectra were conducted on a Bruker EMX-10/12 EPR. 2.3. Photocatalytic experiment The photocatalytic nitrogen fixation experiments were performed in a double-walled quartz reactor in air. For these experiments, 0.1 g of the photocatalyst powder was suspended in 200 mL aqueous solution containing 10 vol % methanol as scavenger. During the reaction, a 500 W Xe lamp was used as simulated sunlight source (the light intensity on quartz tube was 7.66 mW/cm2 ) and air was used as N2 source. At given time intervals, 2.5 mL of the reaction solution was sampled and immediately centrifuged to separate the liquid samples from the solid catalyst. The concentration of ammonia was measured using the Nessler’s reagent spectrophotometry method (HJ 535–2009) with a VIS-7220 spectrophotometer. 3. Results and discussion 3.1. Characterization of catalysts The XRD patterns of GCN and ACN-x photocatalysts are shown in Fig. 1. The bulk GCN contains two pronounced diffraction peaks at around 27.6° and 13.0°, the former can be indexed as the (002) peak characteristic for interlayer stacking of aromatic systems, and the latter can be indexed as the (100) peak that corresponds to the in-plane repeating motif [36]. After KOH etching, the ACN-x photocatalysts have the same crystal phases as pristine GCN. However, in comparison with the pristine GCN, the intensities of the (002) peak of ACN-x are significantly reduced with increasing KOH usage, which suggests the deterioration of crystallinity after KOH etching. The (100) peak diffraction of ACN-2% and ACN-5% becomes weaker and broader, and that of ACN-10% even disappears, indicating that the ordered arrangement of tri-s-triazine units is broken or changed and some defects may exist in ACN-x framework [23,25]. The morphological structures of the samples were examined by SEM and TEM analysis. From the SEM images in Fig. 2a, the bulk
Please cite this article as: N. Zhou et al., KOH etching graphitic carbon nitride for simulated sunlight photocatalytic nitrogen fixation with cyano groups as defects, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.11.028
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Fig. 2. SEM images of GCN (a), ACN-2% (b), ACN-5% (c), ACN-10% (d), and TEM images of GCN (e), ACN-10% (f, g).
GCN shows a large particle construction and the typical layered structure that is similar to the analog graphite. Compared to bulk GCN, all the ACN-x samples have been corroded and the surface display irregular structure (Fig. 2b–d). Many irregular pores are observed on the surface of ACN-2% sample, which should be attributed to the fierce KOH etching during activation process [37,38]. For the ACN-10%, ladder-like thin layered structure in the verge
is observed, implying that the stacked layered structure is broken into smaller size. In the case of ACN-5%, both pores and ladder-like structure are observed. This indicates that the pore structure in the surface of carbon nitride is broken to form debris with the KOH usage increase. The BET specific surface area (SBET ) of GCN, ACN2%, ACN-5%, and ACN-10% are calculated to be 13.62, 29.34, 18.54, 11.76 cm2 /g, respectively (Fig. S1). The SBET of ACN-2% is higher
Please cite this article as: N. Zhou et al., KOH etching graphitic carbon nitride for simulated sunlight photocatalytic nitrogen fixation with cyano groups as defects, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.11.028
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Fig. 3. FT-IR spectra of GCN and ACN-x.
than that of GCN due to the formation of irregular pores. With the mass ratio of KOH to GCN increases, fragments are formed in GCN, rather than the pore structure, resulting in reduced SBET . The BET results are consistent with that of SEM. Fig. 2e and f displays the TEM images of GCN and ACN-10%, both of which consisted of stacked sheets. Compared with GCN, the ACN-10% has less dense stacking and produces thin sheet structure at the edges of the catalyst. In addition, the hole can be observed at the edge of the sheet (Fig. 2g), which is in agreement with the result of SEM analysis. Those changes in morphology are similar to previous reports, in which porous graphene consisting of micropores and wrinkled structure can be obtained by KOH activation of graphene [37–39]. Therefore, we speculate that KOH etched the melon unit of GCN in the verge during the thermal activation process, and then formed pore or fragment structure. The functional group information of GCN and ACN-x photocatalysts were confirmed by FT-IR spectroscopy as shown in Fig. 3. All samples show a spectrum with typical characteristic peaks located at 801 cm−1 , 120 0–170 0 cm−1 , 30 0 0–350 0 cm−1 , which are
contributed to the breathing mode of the triazine unit, the stretching modes of aromatic C–N heterocycles, and the stretching vibration of N–H groups, respectively [40]. It indicates that the ACN-x have the same backbone as the GCN. In particular, the FT-IR spectra of all ACN-x samples show a new vibration band at 2177/cm. This new peak is ascribed to the asymmetric stretching vibration of cyano groups [41–43]. The peak intensity of the C≡N groups increases gradually with the KOH usage increasing. This result clearly indicates that KOH activates the bulk g-C3 N4 and introduces the cyano C≡N functional groups into ACN-x framework. The C in the C≡N groups is positively charged since the electron pair between C≡N groups is strongly biased to N and considered to be strong electron acceptors. The formation of cyano groups may affect the photocatalytic activity of samples. XPS was used to further investigate the surface chemical compositions of the samples. For the N 1 s region (Fig. 4a), the main peaks at around 400.7 eV, 399.0 eV and 398.4 eV are ascribed to amino groups carrying hydrogen (C–NHx ), bridging N atoms (N3C , N–(C)3) and sp2 N atoms in triazine rings (N2C , C–N=C) separately [25,34,36,44]. Notably, the peak area ratio of N3C /N2C decreases from 0.67 for CN to 0.62 for ACN-10%, indicating that the tertiary N atoms were preferentially lost in the reaction [25,34]. In the C 1 s region (Fig. 4b), both GCN and ACN-10% contain three peaks at the binding energies of 288.0 eV, 286.1 eV and 284.6 eV. The main contribution peak centered at 288.0 eV corresponds to sp2 C atoms bonded to N in the triazine ring (N–C=N). The peak at 284.6 eV is typically assigned to C–C bonds, while the lowest peak at 286.1 eV is attributed to the C–NHx species [45,46]. Compared to the GCN, the intensity of the peak at 286.1 eV in ACN-10% is significantly enhanced. Combined with the FI-TR results, this can be taken as an additional evidence for the formation of cyano groups since C≡N groups possess similar C1s binding energies to C–NHx [35]. The chemical composition of the sample was further analyzed by EA (Table S1). The C/N molar ratios of ACN-x are similar to that of GCN, indicating that the formation of cyano groups does not affect the N content of the samples. In combination with the N 1 s result above, the cyano groups could originate from the tertiary N atoms. Combined with the alteration of the morphology and crystal structure of ACN-x photocatalysts, we propose the role of KOH in the chemical activation of bulk carbon nitride, as shown in Scheme 1. During the thermal activation process, the KOH may etch the structural unit of carbon nitride, tri-s-triazine ring, thus generating irregular pores and thin debris structure. KOH has been molten when the activation temperature reaches to 350–550 °C, and
Fig. 4. N 1 s (a) and C 1 s (b) XPS spectra of GCN and ACN-10%.
Please cite this article as: N. Zhou et al., KOH etching graphitic carbon nitride for simulated sunlight photocatalytic nitrogen fixation with cyano groups as defects, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.11.028
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Scheme 1. Schematic illustration of the formation of ACN from GCN by KOH etching.
Fig. 5. UV–vis light absorption spectra and band gap energies (inset) of GCN and ACN-x (a), the band structure alignments of GCN and ACN-10% (b).
terminal amine groups may react with the OH− to form cyano groups [35]. To gain more insight into C≡N formation process, another KOH etching g-C3 N4 sample synthesized at a rather low heating temperature (200 °C) was prepared and named as L-ACN. In the FT-IR spectra of L-ACN (Fig. S2), the peak at 2177 cm−1 almost disappears, meaning that the cyano groups did not form during thermal reaction process at 200 °C. This phenomenon indirectly proves that the OH− released upon melting react with carbon nitride to form C≡N groups at 500 °C. The optical property and electronic band structure of GCN and ACN-x photocatalysts were investigated by UV–visible diffuse reflectance absorption spectra and valence band XPS spectra. As seen in Fig. 5a, the absorption edge of ACN-x show a slight red shift in comparison with bulk GCN. The corresponding band gaps are estimated from the tangent lines in the plots of the square root of the Kubelka-Munk functions against the photon energy. The band gap of ACN-10% is approximately 2.70 eV, which is slightly lower than that of GCN (2.78 eV). This result is probably due to the presence of cyano defect in the ACN-x, leading to changes in the electronic structures and optical properties of these catalysts. Then the valence band (VB) edge potential of GCN and ACN10% were measured by the XPS valence band. The GCN and ACN10% have the same the VB potential of 1.56 eV versus NHE (Fig. S3). In combination with the band gap energies, the conduction band (CB) potentials of GCN and ACN-10% are determined to be −1.22
and −1.14 eV, respectively (Fig. 5b). The data suggests that the formation of cyano groups reduces the CB position compared to that of GCN, resulting in the narrowing of band gap. The CB potential of ACN-10% is still more negative than the redox potential of NH4 + /N2 , which can be used in nitrogen fixation to reduce N2 to NH4 + . The room temperature PL spectra were used to investigate the interfacial charge transfer effect of the samples. As shown in Fig. 6a, the bulk GCN shows a strong fluorescence emission peak which is attributed to the recombination of photo-generated electron– hole pairs. Compared to the bulk GCN, the ACN-x photocatalysts have a series of weaker emission peaks, suggesting that the recombination of the photo-generated carriers has been effectively inhibited. In the process of carrier recombination, since the cyano groups are relatively good electron acceptors, the excited electrons on the conduction band of ACN-x are preferentially trapped by C≡N groups, thereby improving the separation of photo-excited charge carriers. In addition, with the KOH usage increasing, the emission peaks of ACN-x show a gradual red shift. This result reflects the narrowing of band gaps for ACN-x, corresponding to the UV–vis result. Fig. 6b shows the photocurrent densities of GCN and ACN-10% with typical on–off cycles of visible light irradiation. An enhanced photocurrent response for ACN-10% is generated, which is nearly three times higher than that of the pristine GCN, strongly illustrating that the mobility of the photo-excited charge carriers is promoted. This is consistent with the PL results.
Please cite this article as: N. Zhou et al., KOH etching graphitic carbon nitride for simulated sunlight photocatalytic nitrogen fixation with cyano groups as defects, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.11.028
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Fig. 6. PL spectra of GCN and ACN-x (a), the photoelectrochemical responses of GCN and ACN-10% under visible-light irradiation (b).
The results reflected that ACN-10% is more susceptible to adsorption and activation of nitrogen [7, 26]. 3.2. Photocatalytic N2 fixation ability of ACN
Fig. 7. The N2 -TPD of GCN and ACN-10%.
N2 -TPD was used to determine the state of N2 adsorbed on the GCN and ACN-10%. In Fig. 7, two adsorbed N2 species in ACN-10% and only one adsorbed N2 species in GCN are observed. The peak at ∼104 °C is related to the physical adsorption. The peak at 250 °C, which is assigned to the strong chemisorption species of N2 is observed for ACN-10% but not for GCN. This result indicates cyano groups could introduce many chemical adsorption sites on the surface of ACN-10%. Because the chemisorption is generally associated with activation, these chemical adsorption sites will activate N2 for nitrogen photofixation. Thus it is speculated that the ACN-10% exhibit the higher photocatalytic N2 fixation ability than GCN. EPR analysis was showed in Fig. S4a. For the GCN, a weak signal at g = 2.003 is observed. After KOH etching treatment, ACN-10% possessed a much stronger signal compared to that of the GCN, which means a much higher delocalized electrons concentration on ACN-10% [47]. The Zeta analysis (Fig. S4b) also showed that the surface charge is more than that of GCN at the same pH conditions.
The photocatalytic activities of GCN and ACN-x were evaluated by photocatalytic nitrogen fixation experiments under the xenon lamp irradiation, as shown in Fig. 8a. After irradiation for 4 h, the GCN exhibits the total amount of ammonia of 6.77 mg/L. For the ACN-2%, ACN-5% and ACN-10%, the NH4 + concentration increase to 28.78, 41.97, 51.65 mg/L, which are approximately 4.2-fold, 6.2-fold and 7.6-fold higher than that of GCN, respectively. This demonstrates that the KOH etching treatment is an effective method to improve the photofixation nitrogen performance. For ACN-10% photocatalyst, the NH4 + generation rate of this catalytic is not obviously decreased after continuing irradiation for 12 h (Fig. 8b), which means that the nitrogen photofixation ability of the catalyst is stability. In addition, the XRD pattern, FT-IR spectra and N1s spectra of ACN-10% after irradiation for 12 h are consistent with that of ACN-10% before the reaction, further demonstrating the stability of the photocatalyst (Fig. S5). Fig. S6 presents the wavelength dependence of the AQY on N2 photofixation in the ACN-10%. The AQY decreased with increasing the wavelength of the monochromatic light, which matched with the UV–vis light absorption spectra of the ACN-10%. Under the 420 nm monochromatic light irradiation, the AQY was calculated to be about 1.03%. The addition of AgNO3 as electron scavenger sharply suppresses the nitrogen photofixation ability of ACN-10% (Fig. 8c), meaning that the electrons are the main active species. Methanol is an effective scavenger because it could quickly capture holes to exhibit the recombination with electrons. Fig. 8d exhibits the influence of methanol to ammonia formation using ACN-10%. Since the excited electrons were recombined with holes quickly and could not be trapped by activated nitrogen, the nitrogen photofixation reaction is significantly reduced when there is no methanol present. It is noteworthy that there is still a small amount of ammonia formed in the absence of methanol. In recent years, it has been reported that the photocatalysts still possess light-driven photocatalytic N2 fixation performance in water without any organic scavengers or noble co-catalysts [7,26]. As reported, the oxygen vacancies induced defect states might act as the initial acceptor of charge carriers to inhibit electron/hole recombination, and also promote the
Please cite this article as: N. Zhou et al., KOH etching graphitic carbon nitride for simulated sunlight photocatalytic nitrogen fixation with cyano groups as defects, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.11.028
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Fig. 8. Nitrogen photofixation performance over the prepared catalysts under the simulated sunlight irradiation (a) and the photocatalytic stability of ACN-10% (b), NH4 + production ability of ACN-10% using AgNO3 as the scavenger (c), the influence of methanol to ammonia formation using ACN-10% (d).
interfacial charge transfer from the excited BiOBr nanosheets to N2 on the oxygen vacancies. The role of cyano defect may be similar to that of oxygen vacancies, the part of excited electrons in nitrogen defect may act as electron donor and react with the activated N2 to produce NH3 . Although the SBET is reduced with the increase of KOH usage, the NH4 + production ability of ACN-x still gradually enhances. It indicates that morphology is not the main factor in the improvement of nitrogen photofixation ability of the ACN-x catalysts. Taking the optical property into consideration, the separation rate of photoexcited electron holes of ACN-x is obviously higher than that of GCN, and contributing to the enhancement of nitrogen photofixation ability. Since the cyano groups can effectively capture excited electrons, the cyano groups in ACN-x play an important role in suppressing the recombination of charge carriers. In addition, the existence of cyano groups introduces more chemical adsorption sites to activate N2 . Therefore, the formation of cyano groups by etching bulk GCN with KOH is the main reason
for the improved photocatalytic N2 fixation activity of the ACN-x catalysts. 4. Conclusions In conclusion, KOH etching is an effective way to improve the performance of graphitic carbon nitride (GCN) on simulated sunlight photocatalytic nitrogen fixation. The KOH etching process has a certain effect on the morphology and crystal structure of GCN, but still maintains the intrinsic layered structure of GCN. Importantly, the cyano groups are successfully introduced into the GCN structure edge, which is the main reason that the ACN-x show higher NH4 + generation rates than that of GCN. The cyano groups not only effectively trap the electrons to improve the separation of photo-generated carriers, but also introduce more chemical adsorption sites to adsorb and activate N2 molecules. This study presents a facial and effective way to introduce cyano defect groups and improve the nitrogen photofixation ability of GCN,
Please cite this article as: N. Zhou et al., KOH etching graphitic carbon nitride for simulated sunlight photocatalytic nitrogen fixation with cyano groups as defects, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.11.028
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making ACN-x a great potential in simulated sunlight photocatalytic nitrogen fixation reaction.
[20]
Acknowledgments [21]
The financial supports from the National Natural Science Foundation of China (Nos. 51678306 and 51478223), China Postdoctoral Science Foundation (2017T100372, 2013M541677 and 2016M590458), the Jiangsu Planned Projects for Postdoctoral Research Funds (1202007B), the Fundamental Research Funds for the Central University (30915011308) are gratefully acknowledged. Supplementary materials
[22] [23] [24]
[25]
Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2017.11.028.
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KOH etching graphitic carbon nitride for simulated sunlight photocatalytic nitrogen fixation with cyano groups as defects Ning Zhou, Pengxiang Qiu, Huan Chen∗, Fang Jiang Key Laboratory of Jiangsu Province for Chemical Pollution Control and Resources Reuse, School of Environmental and Biological Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
a r t i c l e
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Article history: Received 13 September 2017 Revised 24 November 2017 Accepted 24 November 2017 Available online xxx Keywords: KOH etching graphitic carbon nitride Simulated sunlight Nitrogen photofixation Cyano defects
a b s t r a c t Cyano-deficient g-C3 N4 (graphitic carbon nitride, labeled GCN) was synthesized by KOH etching treatment of bulk g-C3 N4 . Characterization results indicated that KOH etching treatment had a certain effect on the morphology and structure of GCN, and successfully introduced the cyano groups into GCN framework. The obtained KOH etching g-C3 N4 catalysts, named ACN, exhibited the pore and/or ladder-like thin layered structure. Meanwhile, the introduction of cyano groups reduced the conduction band position of ACN, and effectively inhibited the recombination of photo-generated electron–hole pairs. In addition, ACN introduced more chemical adsorption sites to activate nitrogen, which was beneficial to the reaction of photocatalytic nitrogen fixation. The modification of morphology and electronic property, especially the introduction of cyano functional groups, remarkably promoted the activity of ACN on the simulated sunlight photocatalytic nitrogen fixation, which was 7.6-fold higher than that of GCN. © 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction The conversion of solar energy into chemical energy, such as hydrogen and ammonia, is an effective way to solve global energy and environmental problems. The photocatalytic fixation of nitrogen to ammonia has gradually attracted attention due to its advantages of mild conditions, low power consumption and low cost. Thus, it is considered to be a promising method to replace traditional industrial nitrogen fixation techniques. Schrauzer and Guth first reported the formation of ammonia over Fe-doped titanium dioxide under UV light irradiation in 1977 [1]. Since then, most of the researches on nitrogen photofixation are focused on the modification of TiO2 to improve the efficiency [2–4]. Other photocatalysts, such as modified SrTiO3 [5], Fe @ 3D Graphene [6] and BiOBr [7], etc., had also been found to be useful for nitrogen fixation under light irradiation at ambient temperature and pressure. Graphitic carbon nitride (g-C3 N4 , labeled GCN), which was used for photocatalytic hydrogen evolution experiments for the first time in 2009 [8], has been utilized as a low-cost, sustainable, metal-free, and visible-light-active photocatalyst in the field of solar energy conversion in recent years [9–11]. To further improve its photocatalytic performance, many approaches, such as morphology
∗
Corresponding author. E-mail addresses: [email protected] (H. Chen), [email protected] (F. Jiang).
control [12,13], band-gap engineering (including element doping [14,15] and copolymerization [16,17]), and coupling with other semiconductors [18,19], have been reported extensively. The introduction of defects in semiconductor materials is considered to be an effective way to improve the photocatalytic activity, because defects can modify the electronic structure and trap photo-generated electrons or holes to inhibit the recombination of photo-generated carriers. It has been reported that oxygen vacancies on TiO2 could trap electrons and enhance photocatalytic performance of TiO2 in the visible range [20,21]. Similarly, Niu et al. [22] has reported that the nitrogen vacancies were introduced into the framework of GCN by a simple temperature-controlling route and modified the electronic structure to improve the photocatalytic activity of GCN. Dong et al. [23] investigated the role of nitrogen vacancies in nitrogen heat-treated GCN for N2 photofixation under visible-light irradiation. The result indicated that the N2 was selectively adsorbed and activated on the nitrogen vacancies and photo-generated electrons could also be trapped to inhibit the recombination of photo-generated charge carriers. Hong et al. [24] developed a facile hydrothermal strategy using ammonium thiosulfate as a weak oxidant to prepare nitrogen-deficient GCN, and the results revealed that the deficiency of amino species on GCN was beneficial to light harvesting and separation of charge carriers. A template-free method using different solvents (such alcohols) to pretreat the precursor had been reported to synthesize sponge-like GCN with a large surface area and nitrogen vacancies [25]. In recent year, the effects of different defects on the
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Please cite this article as: N. Zhou et al., KOH etching graphitic carbon nitride for simulated sunlight photocatalytic nitrogen fixation with cyano groups as defects, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.11.028
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photocatalytic reaction, such as oxygen vacancies [26], carbon vacancies [27], sulfur vacancies [28], and defects caused by catalyst morphology or surface functional groups [29,30], have also been further studied. It is noteworthy that there are some reports on the impact of nitrogen vacancies on the activity of photocatalysts [22,24,25,31–34], but little about cyano defects. In theory, cyano groups as a strong electron acceptor may act as defects. Ou et al. [30] and Yu et al. [35] also have reported that the presence of cyano groups could effectively improve the photocatalytic hydrogen performance of carbon nitride. Based on those, cyano groups as a defect may have an effect on photocatalytic nitrogen fixation. In this work, we successfully prepared cyanide-deficient carbon nitride (ACN) via KOH etching treatment of bulk GCN. The simulated sunlight nitrogen photofixation ability was tested to evaluate the performance of photocatalysts. The results indicated that the cyano defects contributed to the separation of photo-generated carriers and significantly improved the nitrogen fixation capacity of ACN. This work reported a facile and effective way to introduce cyano groups into GCN and discussed in detail the impacts of cyano deficiency on the morphology, structure and optical properties of ACN. 2. Experiment 2.1. Material synthesis The pristine GCN was prepared by heating melamine in a muffle furnace at 550 °C for 4 h with a ramp rate of 2 °C/min in air. The KOH etching graphitic carbon nitride (ACN) was synthesized as follows: the as-prepared GCN powder was dispersed in anhydrous ethanol containing an appropriate amount of KOH. Solvents were removed by evaporation at 70 °C with vigorous stirring. The obtained powders were heated to 500 °C under flowing nitrogen and held there for 2 h in a horizontal tube furnace. After cooling down, the resultant samples were repeatedly washed by deionized water until neutral pH was reached. Finally, the product was dried at 60 °C overnight. The obtained catalysts were denoted as ACNx (x = 2%, 5%, 10%), where x stands for the mass ratios of KOH to GCN. 2.2. Characterization The X-ray diffraction (XRD) patterns of the photocatalysts were recorded on a Bruker D8 Advanced diffraction-meter with Cu Kα radiation and with the scanning angle ranging from 10° to 80°. The scanning electron microscopy (SEM) images were obtained on Quant 250FEG instrument and the transmission electron microscopy (TEM) images were measured on TECNAI G2 20 (LaB6). The nitrogen adsorption isotherms at 77 K were measured on Micromeritics ASAP 2200. The samples were degassed at 523 K prior to measurements. The Brunauer-Emmett-Teller (BET) specific surface areas were calculated based on the adsorption isotherm. The Fourier transform infrared (FT-IR) spectroscopy was carried on a Nexus 870 spectrometer. X-ray photoelectron spectroscopy (XPS) measurements were performed on a RBD upgraded PHI-50 0 0C ESCA system (Perkin Elmer) with Mg Kα radiation (hν = 1253.6 eV). Elemental analysis (EA) data were obtained by using EuroVector EA30 0 0. UV–vis diffuse reflectance spectroscopy was carried out on a Hitachi U-3010 UV–vis spectrometer. Photoluminescence (PL) spectra were measured at room temperature with a jobinYvon SPEX Fluorolog-3-P spectroscope. The photocurrents were measured using an electrochemical workstation (CHI 660E) in a standard three-electrode system under illumination using a 300 W Xe lamp. The catalyst coated ITO glass was used as the working electrode, a Pt foil was used as the counter-electrode, and an Ag/AgCl electrode was used as the reference electrode. A 0.5 M
Fig. 1. XRD patterns of GCN and ACN-x.
Na2 SO4 aqueous solution was used as the working electrolyte. N2 Temperature programmed desorption (N2 -TPD) was performed using a CHEMBET-30 0 0 (Quantachrome, U.S.A.) instrument in the temperature range of 313–1073 K. The Electron paramagnetic resonance (EPR) spectra were conducted on a Bruker EMX-10/12 EPR. 2.3. Photocatalytic experiment The photocatalytic nitrogen fixation experiments were performed in a double-walled quartz reactor in air. For these experiments, 0.1 g of the photocatalyst powder was suspended in 200 mL aqueous solution containing 10 vol % methanol as scavenger. During the reaction, a 500 W Xe lamp was used as simulated sunlight source (the light intensity on quartz tube was 7.66 mW/cm2 ) and air was used as N2 source. At given time intervals, 2.5 mL of the reaction solution was sampled and immediately centrifuged to separate the liquid samples from the solid catalyst. The concentration of ammonia was measured using the Nessler’s reagent spectrophotometry method (HJ 535–2009) with a VIS-7220 spectrophotometer. 3. Results and discussion 3.1. Characterization of catalysts The XRD patterns of GCN and ACN-x photocatalysts are shown in Fig. 1. The bulk GCN contains two pronounced diffraction peaks at around 27.6° and 13.0°, the former can be indexed as the (002) peak characteristic for interlayer stacking of aromatic systems, and the latter can be indexed as the (100) peak that corresponds to the in-plane repeating motif [36]. After KOH etching, the ACN-x photocatalysts have the same crystal phases as pristine GCN. However, in comparison with the pristine GCN, the intensities of the (002) peak of ACN-x are significantly reduced with increasing KOH usage, which suggests the deterioration of crystallinity after KOH etching. The (100) peak diffraction of ACN-2% and ACN-5% becomes weaker and broader, and that of ACN-10% even disappears, indicating that the ordered arrangement of tri-s-triazine units is broken or changed and some defects may exist in ACN-x framework [23,25]. The morphological structures of the samples were examined by SEM and TEM analysis. From the SEM images in Fig. 2a, the bulk
Please cite this article as: N. Zhou et al., KOH etching graphitic carbon nitride for simulated sunlight photocatalytic nitrogen fixation with cyano groups as defects, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.11.028
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Fig. 2. SEM images of GCN (a), ACN-2% (b), ACN-5% (c), ACN-10% (d), and TEM images of GCN (e), ACN-10% (f, g).
GCN shows a large particle construction and the typical layered structure that is similar to the analog graphite. Compared to bulk GCN, all the ACN-x samples have been corroded and the surface display irregular structure (Fig. 2b–d). Many irregular pores are observed on the surface of ACN-2% sample, which should be attributed to the fierce KOH etching during activation process [37,38]. For the ACN-10%, ladder-like thin layered structure in the verge
is observed, implying that the stacked layered structure is broken into smaller size. In the case of ACN-5%, both pores and ladder-like structure are observed. This indicates that the pore structure in the surface of carbon nitride is broken to form debris with the KOH usage increase. The BET specific surface area (SBET ) of GCN, ACN2%, ACN-5%, and ACN-10% are calculated to be 13.62, 29.34, 18.54, 11.76 cm2 /g, respectively (Fig. S1). The SBET of ACN-2% is higher
Please cite this article as: N. Zhou et al., KOH etching graphitic carbon nitride for simulated sunlight photocatalytic nitrogen fixation with cyano groups as defects, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.11.028
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Fig. 3. FT-IR spectra of GCN and ACN-x.
than that of GCN due to the formation of irregular pores. With the mass ratio of KOH to GCN increases, fragments are formed in GCN, rather than the pore structure, resulting in reduced SBET . The BET results are consistent with that of SEM. Fig. 2e and f displays the TEM images of GCN and ACN-10%, both of which consisted of stacked sheets. Compared with GCN, the ACN-10% has less dense stacking and produces thin sheet structure at the edges of the catalyst. In addition, the hole can be observed at the edge of the sheet (Fig. 2g), which is in agreement with the result of SEM analysis. Those changes in morphology are similar to previous reports, in which porous graphene consisting of micropores and wrinkled structure can be obtained by KOH activation of graphene [37–39]. Therefore, we speculate that KOH etched the melon unit of GCN in the verge during the thermal activation process, and then formed pore or fragment structure. The functional group information of GCN and ACN-x photocatalysts were confirmed by FT-IR spectroscopy as shown in Fig. 3. All samples show a spectrum with typical characteristic peaks located at 801 cm−1 , 120 0–170 0 cm−1 , 30 0 0–350 0 cm−1 , which are
contributed to the breathing mode of the triazine unit, the stretching modes of aromatic C–N heterocycles, and the stretching vibration of N–H groups, respectively [40]. It indicates that the ACN-x have the same backbone as the GCN. In particular, the FT-IR spectra of all ACN-x samples show a new vibration band at 2177/cm. This new peak is ascribed to the asymmetric stretching vibration of cyano groups [41–43]. The peak intensity of the C≡N groups increases gradually with the KOH usage increasing. This result clearly indicates that KOH activates the bulk g-C3 N4 and introduces the cyano C≡N functional groups into ACN-x framework. The C in the C≡N groups is positively charged since the electron pair between C≡N groups is strongly biased to N and considered to be strong electron acceptors. The formation of cyano groups may affect the photocatalytic activity of samples. XPS was used to further investigate the surface chemical compositions of the samples. For the N 1 s region (Fig. 4a), the main peaks at around 400.7 eV, 399.0 eV and 398.4 eV are ascribed to amino groups carrying hydrogen (C–NHx ), bridging N atoms (N3C , N–(C)3) and sp2 N atoms in triazine rings (N2C , C–N=C) separately [25,34,36,44]. Notably, the peak area ratio of N3C /N2C decreases from 0.67 for CN to 0.62 for ACN-10%, indicating that the tertiary N atoms were preferentially lost in the reaction [25,34]. In the C 1 s region (Fig. 4b), both GCN and ACN-10% contain three peaks at the binding energies of 288.0 eV, 286.1 eV and 284.6 eV. The main contribution peak centered at 288.0 eV corresponds to sp2 C atoms bonded to N in the triazine ring (N–C=N). The peak at 284.6 eV is typically assigned to C–C bonds, while the lowest peak at 286.1 eV is attributed to the C–NHx species [45,46]. Compared to the GCN, the intensity of the peak at 286.1 eV in ACN-10% is significantly enhanced. Combined with the FI-TR results, this can be taken as an additional evidence for the formation of cyano groups since C≡N groups possess similar C1s binding energies to C–NHx [35]. The chemical composition of the sample was further analyzed by EA (Table S1). The C/N molar ratios of ACN-x are similar to that of GCN, indicating that the formation of cyano groups does not affect the N content of the samples. In combination with the N 1 s result above, the cyano groups could originate from the tertiary N atoms. Combined with the alteration of the morphology and crystal structure of ACN-x photocatalysts, we propose the role of KOH in the chemical activation of bulk carbon nitride, as shown in Scheme 1. During the thermal activation process, the KOH may etch the structural unit of carbon nitride, tri-s-triazine ring, thus generating irregular pores and thin debris structure. KOH has been molten when the activation temperature reaches to 350–550 °C, and
Fig. 4. N 1 s (a) and C 1 s (b) XPS spectra of GCN and ACN-10%.
Please cite this article as: N. Zhou et al., KOH etching graphitic carbon nitride for simulated sunlight photocatalytic nitrogen fixation with cyano groups as defects, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.11.028
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Scheme 1. Schematic illustration of the formation of ACN from GCN by KOH etching.
Fig. 5. UV–vis light absorption spectra and band gap energies (inset) of GCN and ACN-x (a), the band structure alignments of GCN and ACN-10% (b).
terminal amine groups may react with the OH− to form cyano groups [35]. To gain more insight into C≡N formation process, another KOH etching g-C3 N4 sample synthesized at a rather low heating temperature (200 °C) was prepared and named as L-ACN. In the FT-IR spectra of L-ACN (Fig. S2), the peak at 2177 cm−1 almost disappears, meaning that the cyano groups did not form during thermal reaction process at 200 °C. This phenomenon indirectly proves that the OH− released upon melting react with carbon nitride to form C≡N groups at 500 °C. The optical property and electronic band structure of GCN and ACN-x photocatalysts were investigated by UV–visible diffuse reflectance absorption spectra and valence band XPS spectra. As seen in Fig. 5a, the absorption edge of ACN-x show a slight red shift in comparison with bulk GCN. The corresponding band gaps are estimated from the tangent lines in the plots of the square root of the Kubelka-Munk functions against the photon energy. The band gap of ACN-10% is approximately 2.70 eV, which is slightly lower than that of GCN (2.78 eV). This result is probably due to the presence of cyano defect in the ACN-x, leading to changes in the electronic structures and optical properties of these catalysts. Then the valence band (VB) edge potential of GCN and ACN10% were measured by the XPS valence band. The GCN and ACN10% have the same the VB potential of 1.56 eV versus NHE (Fig. S3). In combination with the band gap energies, the conduction band (CB) potentials of GCN and ACN-10% are determined to be −1.22
and −1.14 eV, respectively (Fig. 5b). The data suggests that the formation of cyano groups reduces the CB position compared to that of GCN, resulting in the narrowing of band gap. The CB potential of ACN-10% is still more negative than the redox potential of NH4 + /N2 , which can be used in nitrogen fixation to reduce N2 to NH4 + . The room temperature PL spectra were used to investigate the interfacial charge transfer effect of the samples. As shown in Fig. 6a, the bulk GCN shows a strong fluorescence emission peak which is attributed to the recombination of photo-generated electron– hole pairs. Compared to the bulk GCN, the ACN-x photocatalysts have a series of weaker emission peaks, suggesting that the recombination of the photo-generated carriers has been effectively inhibited. In the process of carrier recombination, since the cyano groups are relatively good electron acceptors, the excited electrons on the conduction band of ACN-x are preferentially trapped by C≡N groups, thereby improving the separation of photo-excited charge carriers. In addition, with the KOH usage increasing, the emission peaks of ACN-x show a gradual red shift. This result reflects the narrowing of band gaps for ACN-x, corresponding to the UV–vis result. Fig. 6b shows the photocurrent densities of GCN and ACN-10% with typical on–off cycles of visible light irradiation. An enhanced photocurrent response for ACN-10% is generated, which is nearly three times higher than that of the pristine GCN, strongly illustrating that the mobility of the photo-excited charge carriers is promoted. This is consistent with the PL results.
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Fig. 6. PL spectra of GCN and ACN-x (a), the photoelectrochemical responses of GCN and ACN-10% under visible-light irradiation (b).
The results reflected that ACN-10% is more susceptible to adsorption and activation of nitrogen [7, 26]. 3.2. Photocatalytic N2 fixation ability of ACN
Fig. 7. The N2 -TPD of GCN and ACN-10%.
N2 -TPD was used to determine the state of N2 adsorbed on the GCN and ACN-10%. In Fig. 7, two adsorbed N2 species in ACN-10% and only one adsorbed N2 species in GCN are observed. The peak at ∼104 °C is related to the physical adsorption. The peak at 250 °C, which is assigned to the strong chemisorption species of N2 is observed for ACN-10% but not for GCN. This result indicates cyano groups could introduce many chemical adsorption sites on the surface of ACN-10%. Because the chemisorption is generally associated with activation, these chemical adsorption sites will activate N2 for nitrogen photofixation. Thus it is speculated that the ACN-10% exhibit the higher photocatalytic N2 fixation ability than GCN. EPR analysis was showed in Fig. S4a. For the GCN, a weak signal at g = 2.003 is observed. After KOH etching treatment, ACN-10% possessed a much stronger signal compared to that of the GCN, which means a much higher delocalized electrons concentration on ACN-10% [47]. The Zeta analysis (Fig. S4b) also showed that the surface charge is more than that of GCN at the same pH conditions.
The photocatalytic activities of GCN and ACN-x were evaluated by photocatalytic nitrogen fixation experiments under the xenon lamp irradiation, as shown in Fig. 8a. After irradiation for 4 h, the GCN exhibits the total amount of ammonia of 6.77 mg/L. For the ACN-2%, ACN-5% and ACN-10%, the NH4 + concentration increase to 28.78, 41.97, 51.65 mg/L, which are approximately 4.2-fold, 6.2-fold and 7.6-fold higher than that of GCN, respectively. This demonstrates that the KOH etching treatment is an effective method to improve the photofixation nitrogen performance. For ACN-10% photocatalyst, the NH4 + generation rate of this catalytic is not obviously decreased after continuing irradiation for 12 h (Fig. 8b), which means that the nitrogen photofixation ability of the catalyst is stability. In addition, the XRD pattern, FT-IR spectra and N1s spectra of ACN-10% after irradiation for 12 h are consistent with that of ACN-10% before the reaction, further demonstrating the stability of the photocatalyst (Fig. S5). Fig. S6 presents the wavelength dependence of the AQY on N2 photofixation in the ACN-10%. The AQY decreased with increasing the wavelength of the monochromatic light, which matched with the UV–vis light absorption spectra of the ACN-10%. Under the 420 nm monochromatic light irradiation, the AQY was calculated to be about 1.03%. The addition of AgNO3 as electron scavenger sharply suppresses the nitrogen photofixation ability of ACN-10% (Fig. 8c), meaning that the electrons are the main active species. Methanol is an effective scavenger because it could quickly capture holes to exhibit the recombination with electrons. Fig. 8d exhibits the influence of methanol to ammonia formation using ACN-10%. Since the excited electrons were recombined with holes quickly and could not be trapped by activated nitrogen, the nitrogen photofixation reaction is significantly reduced when there is no methanol present. It is noteworthy that there is still a small amount of ammonia formed in the absence of methanol. In recent years, it has been reported that the photocatalysts still possess light-driven photocatalytic N2 fixation performance in water without any organic scavengers or noble co-catalysts [7,26]. As reported, the oxygen vacancies induced defect states might act as the initial acceptor of charge carriers to inhibit electron/hole recombination, and also promote the
Please cite this article as: N. Zhou et al., KOH etching graphitic carbon nitride for simulated sunlight photocatalytic nitrogen fixation with cyano groups as defects, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.11.028
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Fig. 8. Nitrogen photofixation performance over the prepared catalysts under the simulated sunlight irradiation (a) and the photocatalytic stability of ACN-10% (b), NH4 + production ability of ACN-10% using AgNO3 as the scavenger (c), the influence of methanol to ammonia formation using ACN-10% (d).
interfacial charge transfer from the excited BiOBr nanosheets to N2 on the oxygen vacancies. The role of cyano defect may be similar to that of oxygen vacancies, the part of excited electrons in nitrogen defect may act as electron donor and react with the activated N2 to produce NH3 . Although the SBET is reduced with the increase of KOH usage, the NH4 + production ability of ACN-x still gradually enhances. It indicates that morphology is not the main factor in the improvement of nitrogen photofixation ability of the ACN-x catalysts. Taking the optical property into consideration, the separation rate of photoexcited electron holes of ACN-x is obviously higher than that of GCN, and contributing to the enhancement of nitrogen photofixation ability. Since the cyano groups can effectively capture excited electrons, the cyano groups in ACN-x play an important role in suppressing the recombination of charge carriers. In addition, the existence of cyano groups introduces more chemical adsorption sites to activate N2 . Therefore, the formation of cyano groups by etching bulk GCN with KOH is the main reason
for the improved photocatalytic N2 fixation activity of the ACN-x catalysts. 4. Conclusions In conclusion, KOH etching is an effective way to improve the performance of graphitic carbon nitride (GCN) on simulated sunlight photocatalytic nitrogen fixation. The KOH etching process has a certain effect on the morphology and crystal structure of GCN, but still maintains the intrinsic layered structure of GCN. Importantly, the cyano groups are successfully introduced into the GCN structure edge, which is the main reason that the ACN-x show higher NH4 + generation rates than that of GCN. The cyano groups not only effectively trap the electrons to improve the separation of photo-generated carriers, but also introduce more chemical adsorption sites to adsorb and activate N2 molecules. This study presents a facial and effective way to introduce cyano defect groups and improve the nitrogen photofixation ability of GCN,
Please cite this article as: N. Zhou et al., KOH etching graphitic carbon nitride for simulated sunlight photocatalytic nitrogen fixation with cyano groups as defects, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.11.028
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making ACN-x a great potential in simulated sunlight photocatalytic nitrogen fixation reaction.
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Acknowledgments [21]
The financial supports from the National Natural Science Foundation of China (Nos. 51678306 and 51478223), China Postdoctoral Science Foundation (2017T100372, 2013M541677 and 2016M590458), the Jiangsu Planned Projects for Postdoctoral Research Funds (1202007B), the Fundamental Research Funds for the Central University (30915011308) are gratefully acknowledged. Supplementary materials
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Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2017.11.028.
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Please cite this article as: N. Zhou et al., KOH etching graphitic carbon nitride for simulated sunlight photocatalytic nitrogen fixation with cyano groups as defects, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.11.028