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Plasma-treatment induced H2O dissociation for the enhancement of photocatalytic CO2 reduction to CH4 over graphitic carbon nitride
Applied Surface Science 508 (2020) 145173
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Plasma-treatment induced H2O dissociation for the enhancement of photocatalytic CO2 reduction to CH4 over graphitic carbon nitride
T
Kexin Jianga, Li Zhub, Zihua Wangb, Kang Liub, Hongmei Lib, Junhua Huc, Hao Pand, ⁎ ⁎ ⁎ Junwei Fub, , Ning Zhange, , Xiaoqing Qiua, , Min Liub a
College of Chemistry and Chemical Engineering, Central South University, 932 South Lushan Road, Changsha 410083, Hunan, PR China School of Physics and Electronics, Central South University, 932 South Lushan Road, Changsha 410083, Hunan, PR China c School of Materials Science and Engineering, Zhengzhou University, 100 Kexue Avenue, Zhengzhou 450001, Henan, PR China d Department of Periodontics & Oral Mucosal Section, Xiangya Stomatological Hospital, Central South University, 72 Xiangya Road, Changsha 410008, Hunan, PR China e School of Materials Science and Engineering, Central South University, 932 South Lushan Road, Changsha 410083, Hunan, PR China b
ARTICLE INFO
ABSTRACT
Keywords: Photocatalysis Carbon nitride CO2 reduction H2O dissociation Product selectivity
Though graphitic carbon nitride (g-C3N4) is a star photocatalyst for CO2 reduction, its unsatisfactory efficiency and lower reduced-state product (primary product is CO) greatly limit the further application. Dissociation of H2O is known as key step to provide abundant protons for CO2 reduction. The sluggish kinetic of H2O dissociation on g-C3N4 restricts the generation of higher reduced-state hydrocarbon products. Herein, we designed holey g-C3N4 nanosheets with numerous surface defects by Ar plasma treatment. Density functional theoretical (DFT) calculations prove the Ar plasma-treated g-C3N4 (P-x-CN) exhibits better H2O adsorption and dissociation abilities than pure g-C3N4. The separation of photogenerated charge carriers in P-x-CN is also more efficient than pure g-C3N4, which offers higher density of surface photogenerated electrons. The probability of multiple electron reduction reactions to hydrocarbon products greatly increases. As a result, the optimal Ar plasmatreated g-C3N4 (P-80-CN) shows a 40 times higher efficiency of CO2 reduction to CH4 than the pure g-C3N4. This work demonstrates the important role of H2O adsorption and dissociation in tuning product selectivity of CO2 reduction reactions, and provides an effective plasma treatment to modify the surface structure of photocatalysts.
1. Introduction Global energy crisis and carbon emission issue seriously threaten the sustainable development of human society [1–3]. Direct use of solar light to achieve photocatalytic conversation of CO2 into solar fuels is a promising way to simultaneously solve the energy crisis and carbon emissions [4–7]. Since Wang et al. [8] firstly reported the g-C3N4 as photocatalyst to undergo water splitting in 2009, g-C3N4 has attracted widespread attention due to its extraordinary features, including environmental friendliness [9,10], easy preparation [11,12], metal-free composition [13,14], good thermal and chemical stability [15,16]. Extensive researches have proved that g-C3N4 is active in photocatalytic CO2 reduction reactions [17–23]. However, the overall efficiency of gC3N4-based photocatalyst remains unsatisfactory [24]. Moreover, the primary CO2 reduction product of pure g-C3N4 photocatalyst is CO [25,26], which is a typical two-electron and low value-added product. High value-added hydrocarbon products require more protons to
⁎
participate in the CO2 reduction reactions [27]. The H2O adsorption and dissociation to obtain effective protons on the surface of photocatalyst are extremely important for generation of high value-added hydrocarbon products [28,29]. Improving the H2O adsorption and dissociation ability of g-C3N4 is a critical factor for tuning the product selectivity of CO2 reduction reactions. The role of H2O adsorption and dissociation on CO2 reduction reactions has been concerned [30]. Yin et al. [31] applied the first-principles calculations to explore the co-adsorption effect of H2O and CO2 on TiO2 for CO2 reduction. The energy barrier of bicarbonate and CH4 greatly decreased due to the co-adsorption of H2O and CO2, which brought more adsorbed protons from H2O dissociation, and changed the adsorption configuration of CO2. Fu et al. [32] studied the co-adsorption of H2O and CO2 on the surface of P defects modified CoP/carbon. They found that both the P defects and partially adsorbed H2O facilitated proton (from H2O dissociation) capture by CO2. Wu et al. [33] used density functional theory (DFT) and molecular dynamics (MD) to
Corresponding authors. E-mail addresses: [email protected] (J. Fu), [email protected] (N. Zhang), [email protected] (X. Qiu).
https://doi.org/10.1016/j.apsusc.2019.145173 Received 11 September 2019; Received in revised form 16 December 2019; Accepted 23 December 2019 Available online 27 December 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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study the dynamics of H2O adsorption and dissociation on g-C3N4. The results indicated that H2O dissociation is more likely to occur at the defective g-C3N4 with s-triazine vacancy, while cannot occur on pure gC3N4. Therefore, the appropriate defects in g-C3N4 play a crucial role in increasing H2O dissociation ability, and then change product selectivity of CO2 reduction reactions [33]. Surface plasma treatment is an efficient tool to introduce surface defects on photocatalysts. As the fourth state of matter (besides solid, liquid, and gas state), plasma is a partially ionized gas [34,35], consisting of electrons, ions, molecules, radicals, photons, and excited species [34,36]. In this work, we use argon (Ar) plasma to modify the surface of g-C3N4. Experimental results indicate that Ar-plasma treatment breaks the tri-coordination nitrogen–carbon bonds (NeC) in the tri-s-triazine structure of g-C3N4. Compared with pure g-C3N4, these surface defects in plasma-treated g-C3N4 can improve the separation efficiency of photogenerated charge carriers [37], providing higher density of surface photogenerated electrons for multi-electron reduction reactions. Density functional theoretical (DFT) calculations and FTIR spectra prove that the plasma-treated g-C3N4 exhibits better H2O adsorption and dissociation abilities than pure g-C3N4, which is key step to provide abundant protons for CO2 reduction to hydrocarbon products. As a result, the optimal plasma-treated g-C3N4 shows a greatly improved CH4 yield, which is about 40 times higher than pure g-C3N4. This work provides new insight for constructing surface defects by plasma method, and further proves the important role of H2O adsorption and dissociation in CO2 reduction reactions.
obtained on Micromeritics ASAP 2020 nitrogen adsorption apparatus. Brunauer–Emmett–Teller (BET) surface areas were calculated by a multipoint BET method using adsorption data at the relative pressure (P/P0) range 0.05–0.3. Pore size distributions were determined by the Barrett–Joyner–Halenda (BJH) method using adsorption data. The photoluminescence (PL) measurements were performed on Horiba LabRAM HREVO with the excitation wavelength 365 nm. The time correlated single photon counting (TCSPC) system (Picoquant “Timeharp 300”) was used to measure PL lifetime at 420 nm emission wavelength. CO temperature program desorption (TPD) curves were measured on Micromeritics AutoChem 2920. 2.3. Photocatalytic activity measurements The photocatalytic CO2 reduction tests were carried out in a 200 mL sealed double-neck flask with a groove in one neck. A 300 W Xe lamp (Pefectlight, Beijing, China) was used as the light source and positioned ~24 cm above the photocatalytic reactor. Typically, 50 mg of photocatalyst was dispersed in the double-neck flask, and NaHCO3 (84 mg) was filled in the groove. Nitrogen gas was used to replace the air in the reactor before illumination for anaerobic environment. H2SO4 (2 M, 0.6 mL) was added into the groove of reactor by a syringe. CO2 and H2O vapor were obtained by the reaction of NaHCO3 and H2SO4. The flask was sealed and kept under xenon lamp irradiation. 1 mL of mixed gas was taken from the reactor at designed intervals (1 h) during the irradiation and detected by gas chromatograph (GC-2014c, Shimadzu, Japan). All the gas products were calibrated with a standard mixture gas and determined by the retention time. The selectivity of CH4 was calculated by the formula below:
2. Experimental section 2.1. Synthesis of samples
S
CH 4 =
Pure g-C3N4 was prepared via thermal-condensation method using urea as precursor. Typically, urea (10 g) was loaded in a quartz boat and heated to 550 °C for 2 h with a 5 °C/min heating rate. After the product naturally cooled, the obtained yellow sample was milled into fine powder, and denoted as pure g-C3N4. Ar-plasma treatment: 0.2 g of pure g-C3N4 was placed in a quartz boat and transferred into the quartz tube of furnace. The Ar-plasma treatment was performed on a plasma-enhanced chemical vapor deposition furnace (Plasma-Chemical Vapor Deposition System, Tianjin Zhonghuan, China) with input power of 200 W, Ar flow rate of 50 mL s−1, and pressure of 40 Pa. The treatment durations were designed as 20, 40, 60, 80, 100, 120 min, respectively. After cooling down, the treated samples were collected and denoted as P-x-CN (x represents the different treatment duration). The entire preparation process is shown in Scheme 1.
the yeild of CH4 100% the yeild of CH4 + the yeild of CO
2.4. Photoelectrochemical measurements The transient photocurrents (I-T curves) and electrochemical impedance spectroscopy (EIS) measurements were performed on a CHI 660D electrochemical workstation (Chenhua Instrument, Shanghai, China) with a three-electrode configuration. The preparation of working electrodes and the setting of operating parameters were consistent with the previous work [17]. 2.5. Details of DFT calculations To explore the adsorption and dissociation process of H2O on the surface of samples, (2 × 2 × 1) bi-layer perfect and defective g-C3N4 supercells were built. All the DFT calculations were employed by VASP [38]. The interaction between the electron and ion was represented by projector-augmented wave [39]. The exchange and correlation functional were described by the generalized gradient approximation of Perdew-Burke-Ernzerhof (PBE) [40]. The energy cutoff of plane wave was set as 400 eV, and only Gamma point was used for Brillouin zone integration. All atomic fully relaxed to the convergence criteria of energy and force with 10−5 eV/atom and 0.02 eV/Å, respectively. The transition states and activation barriers were obtained using the climbing image nudged elastic band (CI-NEB) method [41,42].
2.2. Characterizations Transmission electron microscopy (TEM) was conducted on a Titan G2 60–300 microscope with probe corrector. Powder X-ray diffraction (XRD) patterns were collected using a D8 advance X-ray diffractometer (Rigaku, Japan) with Cu Kα radiation (λ = 0.15406 nm) at a scan rate (2θ) of 0.05° s−1. The electron spin resonance (ESR) signals were carried out at room temperature on a Bruker A 300 spectrometer. X-ray photoelectron spectroscopy (XPS) measurements were performed on Thermo Fisher Scientific-Escalab 250 XI, and all the binding energies were calibrated by the C 1s peak at 284.8 eV. The Fourier transform infrared (FT-IR) spectra were collected on a Nicolet 6700 FT-IR spectrometer with the wavenumber ranged from 400 to 4000 cm−1. The infrared spectra of samples exposed to water vapor were tested with wet sample, and the infrared signal of pure water was served as the background. Raman spectra were recorded with a Thermo IS50. UV–visible diffuse reflection spectra were obtained on a UV–visible spectrophotometer (UV-2600, Shimadzu, Japan) with BaSO4 as the reflectance sample. Nitrogen adsorption desorption isotherms were
3. Results and discussion Fig. 1a and b exhibit the TEM images of pure g-C3N4 and plasmatreated g-C3N4 samples (P-80-CN), respectively. Clearly, typical lamellar structure with several μm size can be observed in pure g-C3N4, [43]. After plasma treatment, the P-80-CN keeps the lamellar structure, while numerous holes appear on the nanosheets. This result proves that plasma treatment can etch the surfaces of g-C3N4, obtaining more surface defects and holes. Fig. 1c compares the XRD patterns of the as2
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Scheme 1. Sample preparation process (The red dotted circles indicate surface defects before and after Ar plasma treatment. The yellow atoms represent the CeN layer below). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
prepared samples. As for pure g-C3N4, obvious characteristic peaks located at 13.0° and 27.5° can be assigned to g-C3N4. The former is related to the periodic arrangement of in-plane tri-s-triazine motif, the latter reflects the interlayer stacking of g-C3N4 structure [44]. With the increasing of plasma treatment time, the characteristic peaks located at 27.5° become weaker, which further demonstrates the emergence of more surface defects. Moreover, the characteristic peaks located at 13.0° gradually disappear, indicating the change of in-plane periodic arrangement in tri-s-triazine motif. The XRD peaks located at 27.5°
exhibit slight negative shifts compared with that of pure g-C3N4. This slight negative shift can be attributed to a larger interlayer spacing [45], which proves the etching and thinning effect caused by plasma treatment. To further investigate the structure changes, Fig. 1d compares the room-temperature electron paramagnetic resonance (EPR) spectra of pure g-C3N4 and P-80-CN. EPR signal intensity is an indication of the concentration of unpaired electrons in flat π-conjugated gC3N4 [17]. Clearly, the P-80-CN exhibits higher EPR signal intensity than pure g-C3N4, which can be ascribed to the more unpaired
Fig. 1. TEM images of (a) pure g-C3N4 and (b) P-80-CN. (c) XRD patterns of the as-prepared samples. (d) Room-temperature EPR spectra of pure g-C3N4 and P-80-CN. 3
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Fig. 2. High-resolution XPS (a) C 1s and (b) N 1s spectra of pure g-C3N4 and P-80-CN. The possible structure diagram of (c) pure g-C3N4 and (d) P-80-CN (The red dotted circles indicate surface defects before and after Ar plasma treatment. The yellow atoms are the CeN layer below). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
electrons. This result indirectly reflects the existence of more surface structure defects in P-80-CN. XPS is a strong tool to measure the chemical states and structure defects in plasma-treated g-C3N4. All the binding energies were calibrated by the C 1s peak at 284.8 eV. As shown in Fig. 2a, the highresolution C 1s spectra of pure g-C3N4 and P-80-CN samples show two peaks located at 284.8 eV and 288.0 eV. The former peak (284.8 eV) is ascribed to sp2 carbon, and the latter one (288.0 eV) corresponds to the carbon in N]CeN2 structure [46]. The similar C 1s XPS spectra of gC3N4 and P-80-CN samples indicate that the Ar plasma treatment did not change the main chemical state of C species. Fig. 2b compares the high-resolution N 1s spectra of pure g-C3N4 and P-80-CN. Both spectra can be fitted into three peaks located at 398.5, 399.8 and 400.9 eV, which can be assigned to nitrogen in two-coordinated N (NeC2), threecoordinated N (NeC3) and surface amino (eNH2 species), respectively [5]. According to the fitting data, the ratio of NeC2 to NeC3 in pure gC3N4 is 36.8%, while the ratio decreased to 21.5% in P-80-CN. This result further proves the obvious structure change of N species after plasma treatment. As shown in Fig. 2c, the planar structure of pure gC3N4 is composed of tri-s-triazine units, and these units are connected by three-coordinated N. Herein, we assume that plasma treatment break the three-coordinated N bonds between tri-s-triazine units. This model can explain the decreased ratio of the two-coordinated N to three-coordinated N (NeC2 to NeC3). Fig. 2d exhibits the possible planar structure of plasma-treated g-C3N4. Obvious s-triazine vacancy defects can be observed. This defective structure is consistent with the abundant holes in TEM image of plasma-treated g-C3N4 (shown in Fig. 1b). The structure change of plasma-treated g-C3N4 was further revealed by FTIR and Raman spectra. As shown in Fig. 3a, similar characteristic peaks of g-C3N4 can be observed in pure g-C3N4 and plasma-treated gC3N4 samples. The sharp peaks at 808 cm−1 correspond to a breathing vibration mode of the tri-s-triazine units. The main absorption bands in the range of 1150–1650 cm−1 are attributed to the characteristic CeN and C]N stretching vibration of the skeletal tri-s-triazine units. Moreover, the broad bands range from 2900 to 3600 cm−1 can be assigned to the stretching vibrations of surface amino group and adsorbed
eOH group [47]. This result indicates good retention of s-triazine ring and melon units after plasma treatment. Fig. 3b compares the Raman spectra of pure g-C3N4 and P-80-CN. All the characteristic peaks can be ascribed to the resonances of melon [48]. No significant new characteristic peak was detected. Both the FTIR and Raman results suggest good retention of s-triazine units in plasma-treated g-C3N4 samples. Fig. 4a shows the UV–vis diffuse reflection spectra of the as-prepared samples. Clearly, all the samples exhibit typical absorption edges around 450 nm, which are characteristic absorption edges of g-C3N4 [18]. This result proves that the plasma treatment has no significant effect on light absorption. Higher specific surface area can bring more surface catalytic active sites, thus improving catalytic performance. Fig. 4b compares the N2 adsorption–desorption isotherms of pure gC3N4 and P-80-CN. In the region of low relative pressure (P/P0 < 0.6), the two curves almost can coincide. The BET specific surface areas were calculated using adsorption data at P/P0 = 0.05–0.35, so the BET specific surface area of the two samples did not change much. The value is 46 m2/g for pure g-C3N4 and 43 m2/g for P-80-CN. It is worth noting that the P-80-CN exhibits a higher adsorption capacity than pure g-C3N4 at a higher relative pressure (P/P0 > 0.8). This result indicates more macroporous in P-80-CN sample than pure g-C3N4 [47], which is consistent with the morphology observed by TEM. Both the light absorption and specific surface area did not change significantly. These two factors were not the key impact point of better catalytic performance in this experiment. The separation efficiency of photogenerated electrons and holes is one of the important factors for photocatalytic reactions [49]. Fig. 5a exhibits the photoluminescence (PL) spectra of pure g-C3N4 and P-80CN. Clearly, the PL emission intensity at 465 nm greatly decreases in P80-CN, which means a better separation efficiency of photogenerated charge carriers after Ar-plasma treatment [50]. Furthermore, time-resolved photoluminescence (TRPL) spectra with normalized intensities are shown in Fig. 5b. The slight reduced lifetime of photogenerated charge carriers in P-80-CN can be observed. The shorter lifetime might be related to the promoted nonradioactive intersystem transfer process, indicating a better separation efficiency of photogenerated charge carriers. Moreover, electrochemical measurements were also performed 4
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Fig. 3. (a) FTIR and (b) Raman spectra of the as-prepared samples.
to study the seperation efficiency of photogenerated charge carriers. As depicted in Fig. 5c, the measured semicircular Nyquist plots of the pure g-C3N4 exhibited bigger diameter than that of P-80-CN, suggesting that the plasma-treated samples possess lower interface resistances for charge transfer [51]. The photocurrent response curves shown in Fig. 5d prove the higher photocurrent density of P-80-CN. The lower recombination rates, faster transfer process of the photogenerated charge carriers, lower interface resistances, and higher photocurrent density can result in high density of photogenerated electron on the surface of photocatalyst, which are beneficial for generating higher reduced-state hydrocarbon products [49]. Fig. 6a and 6b exhibit the photocatalytic CO2 reduction activities of the as-prepared samples. Obviously, pure g-C3N4 shows ultralow CH4 production rate, which has also been reported by others [52–54]. After Ar-plasma treatment, the CH4 production rates greatly improve. The optimized P-80-CN sample can obtain 0.42 μmol/g/h yield, which is about 40 times higher than the pure g-C3N4. Fig. 6b compares the CO yield of as-prepared samples. The CO production rates slightly decreased after Ar-plasma treatment. The Ar-plasma treated samples possess higher CH4 selectivity (shown in Fig. 6c). Fig. 6d compares the CO temperature program desorption (TPD) curves of pure g-C3N4 and P-80-CN. Clearly, the CO desorption on pure g-C3N4 is much easier than P-80-CN, which is consistent with the main CO product of pure g-C3N4. As we known, the reduction potentials of CO2/CO and CO2/CH4 are −0.53 and −0.24 V (vs. SHE, pH = 7), respectively [17]. In thermodynamically, it’s easier to obtain CH4 than CO. Herein, the ultralow CH4 yield of pure g-C3N4 can be ascribed to the low H2O adsorption and dissociation ability. The inadequate source of protons makes it difficult to obtain CH4 in pure g-C3N4. As described in the introduction, the defects in g-C3N4 can greatly improve the H2O adsorption and dissociation ability. We used DFT calculation to confirm the change of H2O adsorption and dissociation ability after Ar-plasma treatment. As shown in Fig. 7a and b, we used
two-layer models to simulate the actual structure of samples [55]. From the side view, H2O molecules can easily enter the cavity of P-x-CN, while H2O just adsorbed on the surface of pure g-C3N4. As evaluated by the reaction barriers in Fig. 7c, the preferred reaction process of H2O adsorption and dissociation on the P-x-CN exhibits a barrier of 0.93 eV, while the pure g-C3N4 is 2.98 eV. Fig. 7d exhibits the detailed adsorption and dissociation process of H2O on the surface of samples in initial (I.S.), transition (T.S.) and final states (F.S.). We can infer that P-x-CN can reduce the potential energy barrier of H2O adsorption and dissociation, and finally provide more efficient protons for CO2 reduction to CH4. The FTIR spectra of samples exposed to water vapor were tested to study the H2O adsorption and dissociation process. As shown in Fig. 8, P-80-CN shows a stronger hydroxyl signal (2900–3600 cm−1) than the pure g-C3N4 in water exposure environment. This result can be attributed to the fact that the plasma-treated samples have better H2O dissociation performance. Moreover, Zeta potential measurement were performed to study the H2O dissociation process on the surface of samples. As shown in Fig. 9, the zeta potential value (pH = 7) of P-80CN was measured to be +32.7 mV as compared to that of −6.9 mV for pure g-C3N4. The charged states of the samples can be ascribed to the absorption of H+ or OH− from H2O dissociation process. Generally, the higher absolute value of Zeta potential indicates the higher ability of H2O dissociation. Fig. 10 exhibits the polarization curves of pure g-C3N4 and P-80-CN in 1 M KOH. As we known, the H2O dissociation process is the rate determining step for hydrogen evolution reaction under alkaline conditions. Clearly, the P-80-CN shows higher current density than pure g-C3N4, which indirectly reflects a better H2O dissociation ability of P-80-CN than pure g-C3N4. 4. Conclusion In summary, we obtain holey g-C3N4 nanosheets with numerous
Fig. 4. (a) UV–vis diffuse reflection spectra of the as-prepared samples. (b) N2 adsorption–desorption isotherms of pure g-C3N4 and P-80-CN. 5
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Fig. 5. (a) PL, (b) TRPL, (c) Nyquist plots of EIS and (d) photocurrent response curves of pure g-C3N4 and P-80-CN.
surface defects by Ar-plasma etching of g-C3N4. TEM, XPS, FTIR, and Raman spectra prove the possible structure of Ar-plasma treated gC3N4. Experimental results indicate the higher separation efficiency of photogenerated charge in plasma-treated g-C3N4. Density functional theoretical (DFT) calculations suggest that the Ar-plasma treated gC3N4 can realize better H2O adsorption and dissociation than pure gC3N4. The better H2O adsorption and dissociation ability provide more
efficient protons for CO2 reduction to CH4. As a result, the optimal P-xCN shows a greatly improved photocatalytic efficiency of CO2 reduction to CH4, which is about 40 times higher than pure g-C3N4. This work demonstrates the important role of H2O adsorption and dissociation in photocatalytic CO2 reduction, and provides a new idea for tuning surface property of g-C3N4 through plasma treatment.
Fig. 6. (a) Photocatalytic CO2 reduction to CH4 with the as-prepared samples. (b) Temperature program desorption (TPD) curves of CO on pure g-C3N4, and P-80-CN. (c) Photocatalytic CO2 reduction to CO with the as-prepared samples. (d) The CH4 selectivity of photocatalytic CO2 reduction reactions. 6
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Fig. 7. The adsorption models of H2O on the surface of (a) pure g-C3N4 and (b) P-80-CN from the top view and side view. (c) Reaction coordinate for H2O adsorption and dissociation on the surface of pure g- C3N4 and P-x-C3N4 respectively. (d) Detailed adsorption and dissociation process of H2O on the surface of samples in initial (I.S.), transition (T.S.) and final states (F.S).
Fig. 8. FTIR spectra of the pure g-C3N4 and P-80-CN exposed to water vapor.
Fig. 10. Polarization curves of pure g-C3N4 and P-80-CN in 1M KOH.
Author contribution J. Fu, N. Zhang and X. Qiu supervised the project. K. Jiang and J. Fu designed and carried out all the experiments. L. Zhu and K. Liu carried out the DFT simulation. All authors discussed the results and assisted during manuscript preparation. Acknowledgements This work was supported by China Postdoc Innovation Talent Support Program, Postdoctoral Science Foundation (2018M640759), National Natural Science Foundation of China (Grant No. 21872174), Project of Innovation-Driven Plan in Central South University (2017CX003), State Key Laboratory of Powder Metallurgy, Shenzhen Science and Technology Innovation Project (JCYJ20180307151313532), Thousand Youth Talents Plan of China and Hundred Youth Talents Program of Hunan.
Fig. 9. Zeta potentials of pure g-C3N4 and P-80-CN at pH = 7.
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Declaration of Competing Interest [27]
The authors declare no conflict of interest.
[28]
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Full Length Article
Plasma-treatment induced H2O dissociation for the enhancement of photocatalytic CO2 reduction to CH4 over graphitic carbon nitride
T
Kexin Jianga, Li Zhub, Zihua Wangb, Kang Liub, Hongmei Lib, Junhua Huc, Hao Pand, ⁎ ⁎ ⁎ Junwei Fub, , Ning Zhange, , Xiaoqing Qiua, , Min Liub a
College of Chemistry and Chemical Engineering, Central South University, 932 South Lushan Road, Changsha 410083, Hunan, PR China School of Physics and Electronics, Central South University, 932 South Lushan Road, Changsha 410083, Hunan, PR China c School of Materials Science and Engineering, Zhengzhou University, 100 Kexue Avenue, Zhengzhou 450001, Henan, PR China d Department of Periodontics & Oral Mucosal Section, Xiangya Stomatological Hospital, Central South University, 72 Xiangya Road, Changsha 410008, Hunan, PR China e School of Materials Science and Engineering, Central South University, 932 South Lushan Road, Changsha 410083, Hunan, PR China b
ARTICLE INFO
ABSTRACT
Keywords: Photocatalysis Carbon nitride CO2 reduction H2O dissociation Product selectivity
Though graphitic carbon nitride (g-C3N4) is a star photocatalyst for CO2 reduction, its unsatisfactory efficiency and lower reduced-state product (primary product is CO) greatly limit the further application. Dissociation of H2O is known as key step to provide abundant protons for CO2 reduction. The sluggish kinetic of H2O dissociation on g-C3N4 restricts the generation of higher reduced-state hydrocarbon products. Herein, we designed holey g-C3N4 nanosheets with numerous surface defects by Ar plasma treatment. Density functional theoretical (DFT) calculations prove the Ar plasma-treated g-C3N4 (P-x-CN) exhibits better H2O adsorption and dissociation abilities than pure g-C3N4. The separation of photogenerated charge carriers in P-x-CN is also more efficient than pure g-C3N4, which offers higher density of surface photogenerated electrons. The probability of multiple electron reduction reactions to hydrocarbon products greatly increases. As a result, the optimal Ar plasmatreated g-C3N4 (P-80-CN) shows a 40 times higher efficiency of CO2 reduction to CH4 than the pure g-C3N4. This work demonstrates the important role of H2O adsorption and dissociation in tuning product selectivity of CO2 reduction reactions, and provides an effective plasma treatment to modify the surface structure of photocatalysts.
1. Introduction Global energy crisis and carbon emission issue seriously threaten the sustainable development of human society [1–3]. Direct use of solar light to achieve photocatalytic conversation of CO2 into solar fuels is a promising way to simultaneously solve the energy crisis and carbon emissions [4–7]. Since Wang et al. [8] firstly reported the g-C3N4 as photocatalyst to undergo water splitting in 2009, g-C3N4 has attracted widespread attention due to its extraordinary features, including environmental friendliness [9,10], easy preparation [11,12], metal-free composition [13,14], good thermal and chemical stability [15,16]. Extensive researches have proved that g-C3N4 is active in photocatalytic CO2 reduction reactions [17–23]. However, the overall efficiency of gC3N4-based photocatalyst remains unsatisfactory [24]. Moreover, the primary CO2 reduction product of pure g-C3N4 photocatalyst is CO [25,26], which is a typical two-electron and low value-added product. High value-added hydrocarbon products require more protons to
⁎
participate in the CO2 reduction reactions [27]. The H2O adsorption and dissociation to obtain effective protons on the surface of photocatalyst are extremely important for generation of high value-added hydrocarbon products [28,29]. Improving the H2O adsorption and dissociation ability of g-C3N4 is a critical factor for tuning the product selectivity of CO2 reduction reactions. The role of H2O adsorption and dissociation on CO2 reduction reactions has been concerned [30]. Yin et al. [31] applied the first-principles calculations to explore the co-adsorption effect of H2O and CO2 on TiO2 for CO2 reduction. The energy barrier of bicarbonate and CH4 greatly decreased due to the co-adsorption of H2O and CO2, which brought more adsorbed protons from H2O dissociation, and changed the adsorption configuration of CO2. Fu et al. [32] studied the co-adsorption of H2O and CO2 on the surface of P defects modified CoP/carbon. They found that both the P defects and partially adsorbed H2O facilitated proton (from H2O dissociation) capture by CO2. Wu et al. [33] used density functional theory (DFT) and molecular dynamics (MD) to
Corresponding authors. E-mail addresses: [email protected] (J. Fu), [email protected] (N. Zhang), [email protected] (X. Qiu).
https://doi.org/10.1016/j.apsusc.2019.145173 Received 11 September 2019; Received in revised form 16 December 2019; Accepted 23 December 2019 Available online 27 December 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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study the dynamics of H2O adsorption and dissociation on g-C3N4. The results indicated that H2O dissociation is more likely to occur at the defective g-C3N4 with s-triazine vacancy, while cannot occur on pure gC3N4. Therefore, the appropriate defects in g-C3N4 play a crucial role in increasing H2O dissociation ability, and then change product selectivity of CO2 reduction reactions [33]. Surface plasma treatment is an efficient tool to introduce surface defects on photocatalysts. As the fourth state of matter (besides solid, liquid, and gas state), plasma is a partially ionized gas [34,35], consisting of electrons, ions, molecules, radicals, photons, and excited species [34,36]. In this work, we use argon (Ar) plasma to modify the surface of g-C3N4. Experimental results indicate that Ar-plasma treatment breaks the tri-coordination nitrogen–carbon bonds (NeC) in the tri-s-triazine structure of g-C3N4. Compared with pure g-C3N4, these surface defects in plasma-treated g-C3N4 can improve the separation efficiency of photogenerated charge carriers [37], providing higher density of surface photogenerated electrons for multi-electron reduction reactions. Density functional theoretical (DFT) calculations and FTIR spectra prove that the plasma-treated g-C3N4 exhibits better H2O adsorption and dissociation abilities than pure g-C3N4, which is key step to provide abundant protons for CO2 reduction to hydrocarbon products. As a result, the optimal plasma-treated g-C3N4 shows a greatly improved CH4 yield, which is about 40 times higher than pure g-C3N4. This work provides new insight for constructing surface defects by plasma method, and further proves the important role of H2O adsorption and dissociation in CO2 reduction reactions.
obtained on Micromeritics ASAP 2020 nitrogen adsorption apparatus. Brunauer–Emmett–Teller (BET) surface areas were calculated by a multipoint BET method using adsorption data at the relative pressure (P/P0) range 0.05–0.3. Pore size distributions were determined by the Barrett–Joyner–Halenda (BJH) method using adsorption data. The photoluminescence (PL) measurements were performed on Horiba LabRAM HREVO with the excitation wavelength 365 nm. The time correlated single photon counting (TCSPC) system (Picoquant “Timeharp 300”) was used to measure PL lifetime at 420 nm emission wavelength. CO temperature program desorption (TPD) curves were measured on Micromeritics AutoChem 2920. 2.3. Photocatalytic activity measurements The photocatalytic CO2 reduction tests were carried out in a 200 mL sealed double-neck flask with a groove in one neck. A 300 W Xe lamp (Pefectlight, Beijing, China) was used as the light source and positioned ~24 cm above the photocatalytic reactor. Typically, 50 mg of photocatalyst was dispersed in the double-neck flask, and NaHCO3 (84 mg) was filled in the groove. Nitrogen gas was used to replace the air in the reactor before illumination for anaerobic environment. H2SO4 (2 M, 0.6 mL) was added into the groove of reactor by a syringe. CO2 and H2O vapor were obtained by the reaction of NaHCO3 and H2SO4. The flask was sealed and kept under xenon lamp irradiation. 1 mL of mixed gas was taken from the reactor at designed intervals (1 h) during the irradiation and detected by gas chromatograph (GC-2014c, Shimadzu, Japan). All the gas products were calibrated with a standard mixture gas and determined by the retention time. The selectivity of CH4 was calculated by the formula below:
2. Experimental section 2.1. Synthesis of samples
S
CH 4 =
Pure g-C3N4 was prepared via thermal-condensation method using urea as precursor. Typically, urea (10 g) was loaded in a quartz boat and heated to 550 °C for 2 h with a 5 °C/min heating rate. After the product naturally cooled, the obtained yellow sample was milled into fine powder, and denoted as pure g-C3N4. Ar-plasma treatment: 0.2 g of pure g-C3N4 was placed in a quartz boat and transferred into the quartz tube of furnace. The Ar-plasma treatment was performed on a plasma-enhanced chemical vapor deposition furnace (Plasma-Chemical Vapor Deposition System, Tianjin Zhonghuan, China) with input power of 200 W, Ar flow rate of 50 mL s−1, and pressure of 40 Pa. The treatment durations were designed as 20, 40, 60, 80, 100, 120 min, respectively. After cooling down, the treated samples were collected and denoted as P-x-CN (x represents the different treatment duration). The entire preparation process is shown in Scheme 1.
the yeild of CH4 100% the yeild of CH4 + the yeild of CO
2.4. Photoelectrochemical measurements The transient photocurrents (I-T curves) and electrochemical impedance spectroscopy (EIS) measurements were performed on a CHI 660D electrochemical workstation (Chenhua Instrument, Shanghai, China) with a three-electrode configuration. The preparation of working electrodes and the setting of operating parameters were consistent with the previous work [17]. 2.5. Details of DFT calculations To explore the adsorption and dissociation process of H2O on the surface of samples, (2 × 2 × 1) bi-layer perfect and defective g-C3N4 supercells were built. All the DFT calculations were employed by VASP [38]. The interaction between the electron and ion was represented by projector-augmented wave [39]. The exchange and correlation functional were described by the generalized gradient approximation of Perdew-Burke-Ernzerhof (PBE) [40]. The energy cutoff of plane wave was set as 400 eV, and only Gamma point was used for Brillouin zone integration. All atomic fully relaxed to the convergence criteria of energy and force with 10−5 eV/atom and 0.02 eV/Å, respectively. The transition states and activation barriers were obtained using the climbing image nudged elastic band (CI-NEB) method [41,42].
2.2. Characterizations Transmission electron microscopy (TEM) was conducted on a Titan G2 60–300 microscope with probe corrector. Powder X-ray diffraction (XRD) patterns were collected using a D8 advance X-ray diffractometer (Rigaku, Japan) with Cu Kα radiation (λ = 0.15406 nm) at a scan rate (2θ) of 0.05° s−1. The electron spin resonance (ESR) signals were carried out at room temperature on a Bruker A 300 spectrometer. X-ray photoelectron spectroscopy (XPS) measurements were performed on Thermo Fisher Scientific-Escalab 250 XI, and all the binding energies were calibrated by the C 1s peak at 284.8 eV. The Fourier transform infrared (FT-IR) spectra were collected on a Nicolet 6700 FT-IR spectrometer with the wavenumber ranged from 400 to 4000 cm−1. The infrared spectra of samples exposed to water vapor were tested with wet sample, and the infrared signal of pure water was served as the background. Raman spectra were recorded with a Thermo IS50. UV–visible diffuse reflection spectra were obtained on a UV–visible spectrophotometer (UV-2600, Shimadzu, Japan) with BaSO4 as the reflectance sample. Nitrogen adsorption desorption isotherms were
3. Results and discussion Fig. 1a and b exhibit the TEM images of pure g-C3N4 and plasmatreated g-C3N4 samples (P-80-CN), respectively. Clearly, typical lamellar structure with several μm size can be observed in pure g-C3N4, [43]. After plasma treatment, the P-80-CN keeps the lamellar structure, while numerous holes appear on the nanosheets. This result proves that plasma treatment can etch the surfaces of g-C3N4, obtaining more surface defects and holes. Fig. 1c compares the XRD patterns of the as2
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Scheme 1. Sample preparation process (The red dotted circles indicate surface defects before and after Ar plasma treatment. The yellow atoms represent the CeN layer below). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
prepared samples. As for pure g-C3N4, obvious characteristic peaks located at 13.0° and 27.5° can be assigned to g-C3N4. The former is related to the periodic arrangement of in-plane tri-s-triazine motif, the latter reflects the interlayer stacking of g-C3N4 structure [44]. With the increasing of plasma treatment time, the characteristic peaks located at 27.5° become weaker, which further demonstrates the emergence of more surface defects. Moreover, the characteristic peaks located at 13.0° gradually disappear, indicating the change of in-plane periodic arrangement in tri-s-triazine motif. The XRD peaks located at 27.5°
exhibit slight negative shifts compared with that of pure g-C3N4. This slight negative shift can be attributed to a larger interlayer spacing [45], which proves the etching and thinning effect caused by plasma treatment. To further investigate the structure changes, Fig. 1d compares the room-temperature electron paramagnetic resonance (EPR) spectra of pure g-C3N4 and P-80-CN. EPR signal intensity is an indication of the concentration of unpaired electrons in flat π-conjugated gC3N4 [17]. Clearly, the P-80-CN exhibits higher EPR signal intensity than pure g-C3N4, which can be ascribed to the more unpaired
Fig. 1. TEM images of (a) pure g-C3N4 and (b) P-80-CN. (c) XRD patterns of the as-prepared samples. (d) Room-temperature EPR spectra of pure g-C3N4 and P-80-CN. 3
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Fig. 2. High-resolution XPS (a) C 1s and (b) N 1s spectra of pure g-C3N4 and P-80-CN. The possible structure diagram of (c) pure g-C3N4 and (d) P-80-CN (The red dotted circles indicate surface defects before and after Ar plasma treatment. The yellow atoms are the CeN layer below). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
electrons. This result indirectly reflects the existence of more surface structure defects in P-80-CN. XPS is a strong tool to measure the chemical states and structure defects in plasma-treated g-C3N4. All the binding energies were calibrated by the C 1s peak at 284.8 eV. As shown in Fig. 2a, the highresolution C 1s spectra of pure g-C3N4 and P-80-CN samples show two peaks located at 284.8 eV and 288.0 eV. The former peak (284.8 eV) is ascribed to sp2 carbon, and the latter one (288.0 eV) corresponds to the carbon in N]CeN2 structure [46]. The similar C 1s XPS spectra of gC3N4 and P-80-CN samples indicate that the Ar plasma treatment did not change the main chemical state of C species. Fig. 2b compares the high-resolution N 1s spectra of pure g-C3N4 and P-80-CN. Both spectra can be fitted into three peaks located at 398.5, 399.8 and 400.9 eV, which can be assigned to nitrogen in two-coordinated N (NeC2), threecoordinated N (NeC3) and surface amino (eNH2 species), respectively [5]. According to the fitting data, the ratio of NeC2 to NeC3 in pure gC3N4 is 36.8%, while the ratio decreased to 21.5% in P-80-CN. This result further proves the obvious structure change of N species after plasma treatment. As shown in Fig. 2c, the planar structure of pure gC3N4 is composed of tri-s-triazine units, and these units are connected by three-coordinated N. Herein, we assume that plasma treatment break the three-coordinated N bonds between tri-s-triazine units. This model can explain the decreased ratio of the two-coordinated N to three-coordinated N (NeC2 to NeC3). Fig. 2d exhibits the possible planar structure of plasma-treated g-C3N4. Obvious s-triazine vacancy defects can be observed. This defective structure is consistent with the abundant holes in TEM image of plasma-treated g-C3N4 (shown in Fig. 1b). The structure change of plasma-treated g-C3N4 was further revealed by FTIR and Raman spectra. As shown in Fig. 3a, similar characteristic peaks of g-C3N4 can be observed in pure g-C3N4 and plasma-treated gC3N4 samples. The sharp peaks at 808 cm−1 correspond to a breathing vibration mode of the tri-s-triazine units. The main absorption bands in the range of 1150–1650 cm−1 are attributed to the characteristic CeN and C]N stretching vibration of the skeletal tri-s-triazine units. Moreover, the broad bands range from 2900 to 3600 cm−1 can be assigned to the stretching vibrations of surface amino group and adsorbed
eOH group [47]. This result indicates good retention of s-triazine ring and melon units after plasma treatment. Fig. 3b compares the Raman spectra of pure g-C3N4 and P-80-CN. All the characteristic peaks can be ascribed to the resonances of melon [48]. No significant new characteristic peak was detected. Both the FTIR and Raman results suggest good retention of s-triazine units in plasma-treated g-C3N4 samples. Fig. 4a shows the UV–vis diffuse reflection spectra of the as-prepared samples. Clearly, all the samples exhibit typical absorption edges around 450 nm, which are characteristic absorption edges of g-C3N4 [18]. This result proves that the plasma treatment has no significant effect on light absorption. Higher specific surface area can bring more surface catalytic active sites, thus improving catalytic performance. Fig. 4b compares the N2 adsorption–desorption isotherms of pure gC3N4 and P-80-CN. In the region of low relative pressure (P/P0 < 0.6), the two curves almost can coincide. The BET specific surface areas were calculated using adsorption data at P/P0 = 0.05–0.35, so the BET specific surface area of the two samples did not change much. The value is 46 m2/g for pure g-C3N4 and 43 m2/g for P-80-CN. It is worth noting that the P-80-CN exhibits a higher adsorption capacity than pure g-C3N4 at a higher relative pressure (P/P0 > 0.8). This result indicates more macroporous in P-80-CN sample than pure g-C3N4 [47], which is consistent with the morphology observed by TEM. Both the light absorption and specific surface area did not change significantly. These two factors were not the key impact point of better catalytic performance in this experiment. The separation efficiency of photogenerated electrons and holes is one of the important factors for photocatalytic reactions [49]. Fig. 5a exhibits the photoluminescence (PL) spectra of pure g-C3N4 and P-80CN. Clearly, the PL emission intensity at 465 nm greatly decreases in P80-CN, which means a better separation efficiency of photogenerated charge carriers after Ar-plasma treatment [50]. Furthermore, time-resolved photoluminescence (TRPL) spectra with normalized intensities are shown in Fig. 5b. The slight reduced lifetime of photogenerated charge carriers in P-80-CN can be observed. The shorter lifetime might be related to the promoted nonradioactive intersystem transfer process, indicating a better separation efficiency of photogenerated charge carriers. Moreover, electrochemical measurements were also performed 4
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Fig. 3. (a) FTIR and (b) Raman spectra of the as-prepared samples.
to study the seperation efficiency of photogenerated charge carriers. As depicted in Fig. 5c, the measured semicircular Nyquist plots of the pure g-C3N4 exhibited bigger diameter than that of P-80-CN, suggesting that the plasma-treated samples possess lower interface resistances for charge transfer [51]. The photocurrent response curves shown in Fig. 5d prove the higher photocurrent density of P-80-CN. The lower recombination rates, faster transfer process of the photogenerated charge carriers, lower interface resistances, and higher photocurrent density can result in high density of photogenerated electron on the surface of photocatalyst, which are beneficial for generating higher reduced-state hydrocarbon products [49]. Fig. 6a and 6b exhibit the photocatalytic CO2 reduction activities of the as-prepared samples. Obviously, pure g-C3N4 shows ultralow CH4 production rate, which has also been reported by others [52–54]. After Ar-plasma treatment, the CH4 production rates greatly improve. The optimized P-80-CN sample can obtain 0.42 μmol/g/h yield, which is about 40 times higher than the pure g-C3N4. Fig. 6b compares the CO yield of as-prepared samples. The CO production rates slightly decreased after Ar-plasma treatment. The Ar-plasma treated samples possess higher CH4 selectivity (shown in Fig. 6c). Fig. 6d compares the CO temperature program desorption (TPD) curves of pure g-C3N4 and P-80-CN. Clearly, the CO desorption on pure g-C3N4 is much easier than P-80-CN, which is consistent with the main CO product of pure g-C3N4. As we known, the reduction potentials of CO2/CO and CO2/CH4 are −0.53 and −0.24 V (vs. SHE, pH = 7), respectively [17]. In thermodynamically, it’s easier to obtain CH4 than CO. Herein, the ultralow CH4 yield of pure g-C3N4 can be ascribed to the low H2O adsorption and dissociation ability. The inadequate source of protons makes it difficult to obtain CH4 in pure g-C3N4. As described in the introduction, the defects in g-C3N4 can greatly improve the H2O adsorption and dissociation ability. We used DFT calculation to confirm the change of H2O adsorption and dissociation ability after Ar-plasma treatment. As shown in Fig. 7a and b, we used
two-layer models to simulate the actual structure of samples [55]. From the side view, H2O molecules can easily enter the cavity of P-x-CN, while H2O just adsorbed on the surface of pure g-C3N4. As evaluated by the reaction barriers in Fig. 7c, the preferred reaction process of H2O adsorption and dissociation on the P-x-CN exhibits a barrier of 0.93 eV, while the pure g-C3N4 is 2.98 eV. Fig. 7d exhibits the detailed adsorption and dissociation process of H2O on the surface of samples in initial (I.S.), transition (T.S.) and final states (F.S.). We can infer that P-x-CN can reduce the potential energy barrier of H2O adsorption and dissociation, and finally provide more efficient protons for CO2 reduction to CH4. The FTIR spectra of samples exposed to water vapor were tested to study the H2O adsorption and dissociation process. As shown in Fig. 8, P-80-CN shows a stronger hydroxyl signal (2900–3600 cm−1) than the pure g-C3N4 in water exposure environment. This result can be attributed to the fact that the plasma-treated samples have better H2O dissociation performance. Moreover, Zeta potential measurement were performed to study the H2O dissociation process on the surface of samples. As shown in Fig. 9, the zeta potential value (pH = 7) of P-80CN was measured to be +32.7 mV as compared to that of −6.9 mV for pure g-C3N4. The charged states of the samples can be ascribed to the absorption of H+ or OH− from H2O dissociation process. Generally, the higher absolute value of Zeta potential indicates the higher ability of H2O dissociation. Fig. 10 exhibits the polarization curves of pure g-C3N4 and P-80-CN in 1 M KOH. As we known, the H2O dissociation process is the rate determining step for hydrogen evolution reaction under alkaline conditions. Clearly, the P-80-CN shows higher current density than pure g-C3N4, which indirectly reflects a better H2O dissociation ability of P-80-CN than pure g-C3N4. 4. Conclusion In summary, we obtain holey g-C3N4 nanosheets with numerous
Fig. 4. (a) UV–vis diffuse reflection spectra of the as-prepared samples. (b) N2 adsorption–desorption isotherms of pure g-C3N4 and P-80-CN. 5
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Fig. 5. (a) PL, (b) TRPL, (c) Nyquist plots of EIS and (d) photocurrent response curves of pure g-C3N4 and P-80-CN.
surface defects by Ar-plasma etching of g-C3N4. TEM, XPS, FTIR, and Raman spectra prove the possible structure of Ar-plasma treated gC3N4. Experimental results indicate the higher separation efficiency of photogenerated charge in plasma-treated g-C3N4. Density functional theoretical (DFT) calculations suggest that the Ar-plasma treated gC3N4 can realize better H2O adsorption and dissociation than pure gC3N4. The better H2O adsorption and dissociation ability provide more
efficient protons for CO2 reduction to CH4. As a result, the optimal P-xCN shows a greatly improved photocatalytic efficiency of CO2 reduction to CH4, which is about 40 times higher than pure g-C3N4. This work demonstrates the important role of H2O adsorption and dissociation in photocatalytic CO2 reduction, and provides a new idea for tuning surface property of g-C3N4 through plasma treatment.
Fig. 6. (a) Photocatalytic CO2 reduction to CH4 with the as-prepared samples. (b) Temperature program desorption (TPD) curves of CO on pure g-C3N4, and P-80-CN. (c) Photocatalytic CO2 reduction to CO with the as-prepared samples. (d) The CH4 selectivity of photocatalytic CO2 reduction reactions. 6
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Fig. 7. The adsorption models of H2O on the surface of (a) pure g-C3N4 and (b) P-80-CN from the top view and side view. (c) Reaction coordinate for H2O adsorption and dissociation on the surface of pure g- C3N4 and P-x-C3N4 respectively. (d) Detailed adsorption and dissociation process of H2O on the surface of samples in initial (I.S.), transition (T.S.) and final states (F.S).
Fig. 8. FTIR spectra of the pure g-C3N4 and P-80-CN exposed to water vapor.
Fig. 10. Polarization curves of pure g-C3N4 and P-80-CN in 1M KOH.
Author contribution J. Fu, N. Zhang and X. Qiu supervised the project. K. Jiang and J. Fu designed and carried out all the experiments. L. Zhu and K. Liu carried out the DFT simulation. All authors discussed the results and assisted during manuscript preparation. Acknowledgements This work was supported by China Postdoc Innovation Talent Support Program, Postdoctoral Science Foundation (2018M640759), National Natural Science Foundation of China (Grant No. 21872174), Project of Innovation-Driven Plan in Central South University (2017CX003), State Key Laboratory of Powder Metallurgy, Shenzhen Science and Technology Innovation Project (JCYJ20180307151313532), Thousand Youth Talents Plan of China and Hundred Youth Talents Program of Hunan.
Fig. 9. Zeta potentials of pure g-C3N4 and P-80-CN at pH = 7.
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Declaration of Competing Interest [27]
The authors declare no conflict of interest.
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